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Electrochemical behaviour of platinum-iridium anodes Wensley, Donald Arthur 1973

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cl ELECTROCHEMICAL BEHAVIOUR OF PLATINUM-IRIDIUM ANODES  BY  DONALD ARTHUR WENSLEY B . A . S c , U n i v e r s i t y o f B r i t i s h Columbia, 1970  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n t h e Department of METALLURGY .  We a c c e p t t h i s t h e s i s as conforming t o t h e required standard  THE UNIVERSITY OF BRITISH COLUMBIA June, 1973  In  presenting  this  an a d v a n c e d d e g r e e the I  Library  further  for  agree  in p a r t i a l  fulfilment  of  at  University  of  Columbia,  the  make  it  freely  that permission for  this  representatives. thesis  for  It  financial  of  gain  Metallurgy  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, C a n a d a  Date  O c t o b e r 4,  1973  for  extensive by  the  Columbia  shall  not  the  requirements  reference copying of  Head o f  is understood that  written permission.  Department  British  available  s c h o l a r l y p u r p o s e s may be g r a n t e d  by h i s of  shall  thesis  I  agree  and this  that  study. thesis  my D e p a r t m e n t  copying or  for  or  publication  be a l l o w e d w i t h o u t my  ii  ABSTRACT  This thesis considers the electrochemistry of platinumiridium electrodes i n both sulphate- and chloride-containing e l e c t r o lytes at 20 - 25°C.  Both wire electrodes of appropriate a l l o y composi-  tions and titanium-substrate electrodes were employed. P o l a r i z a t i o n curves were obtained, and a technique f o r measuring the surface area of the electrodes was employed i n order to determine the e f f e c t of potentiostatic e l e c t r o l y s i s on the electrochemically a c t i v e area. The wire a l l o y electrodes showed p o l a r i z a t i o n behaviour i n 1M NaCl; pH 2 i d e n t i c a l to that of platinum electrodes, indicating that iridium i s not e f f e c t i v e i n reducing the passivation of these electrodes even with up to 25% a l l o y  content.  The coated electrodes showed i r r e v e r s i b l e surface area losses i n both sulphate and chloride e l e c t r o l y t e s , with the l a t t e r producing s i g n i f i c a n t reductions after very short p o l a r i z a t i o n times.  It  i s suggested that oxidation of the substrate leading to e l e c t r i c a l i s o l a t i o n of coating plates i s responsible for the area decay.  iii TABLE OF CONTENTS  Page 1.  INTRODUCTION  1  2.  LITERATURE SURVEY AND THEORETICAL CONSIDERATION  5  2.1  ELECTRODE PRETREATMENT AND ACTIVATION  6  2.1.1  Nature o f the Problem  6  2.1.2  Methods o f Pretreatment  2.1.3  Mechanisms o f A c t i v a t i o n o f Noble M e t a l  2.2  and A c t i v a t i o n  ....  9  Electrodes  11  2.1.3.1  S u r f a c e Area I n c r e a s e s  11  2.1.3.2  A c t i v e Surface S t r u c t u r e  14  2.1.3.3  A c t i v e Oxidized Surface  18  2.1.3.4  A c t i v e Reduced S u r f a c e  20  2.1.3.5  A c t i v i t y Induced by  Dermasorption  of Oxygen  24  2.1.3.6  A l t e r e d E l e c t r o n i c P r o p e r t i e s ....  27  2.1.3.7  Impurity Removal  29  2.1.4 Summary SURFACE AREA OF NOBLE METAL ELECTRODES 2.2.1 Bases f o r E l e c t r o c h e m i c a l S u r f a c e A r e a Measurement 2.2.1.1  2.2.1.2  C o m p l i c a t i o n s due t o Processes  32 33 33  Simultaneous 34  Compensation f o r Double L a y e r Charging  36  2.2.1.3  Monolayer Formation  37  2.2.1.4  Absorption  38  iv Page 2.2.1.5 2.2.2  2.2.3 2.3  42  2.2.2.1  Potential Sweep Techniques  42  2.2.2.2 \Galvanostatic Charge Techniques ..  44  Summary  46  ELECTROLYSIS OF CHLORIDE SOLUTIONS  48  2.3.1  P o l a r i z a t i o n of Smooth Noble Metal Anodes .  50  2.3.2  P o l a r i z a t i o n of Titanium-Substrate Anodes .  58  2.3.2.1  Behaviour of Titanium  59  2.3.2.2  Coupling of Platinum Metals with  2.3.3  3.  Titanium  61  P o l a r i z a t i o n Characteristics  63  Summary  64  DISSOLUTION OF THE NOBLE METALS  65  2.4.1  The Active Dissolution of the Noble Metals  66  2.4.2  Dissolution with Oxygen P a r t i c i p a t i o n  72  2.4.3  Dissolution During A c t i v a t i o n  78  2.4.4  Degradation of Noble Metal Coatings 2.4.4.1 Degradation as a Result of Coating Undermining  83 83  2.4.4.2  86  2.4.5 2.5  40  Procedures f o r Surface Area Measurement ...  2.3.2.3  2.4  Surface Atom Density  Other Causes of Coating Loss  Summary  RELATION TO AIMS OF PRESENT WORK  87 88  EXPERIMENTAL  93  3.1  93  ELECTRODES  V  Page  4.  3.2  ELECTROLYTES  95  3.3  CELLS  96  3.4  PROCEDURES  6.  3.4.1  Anodic  G a l v a n o s t a t i c Measurements  100  3.4.2  Anodic  P o t e n t i o s t a t i c Measurements  101  3.4.3  S u r f a c e Area Measurements  102  3.4.4  Observation  105  of E l e c t r o d e S u r f a c e s  RESULTS  106  4.1  GALVANOSTATIC POLARIZATION CURVES  106  4.2  POTENTIOSTATIC POLARIZATION CURVES  119  4.3  CHANGE OF SURFACE AREA WITH POTENTIOSTATIC  4.4 5.  100  ANODIZATION  122  OBSERVATIONS OF ELECTRODE SURFACES  132  DISCUSSION  140  5.1  ANODIC GALVANOSTATIC MEASUREMENTS  140  5.2  ANODIC POTENTIOSTATIC MEASUREMENTS  142  5.3  SURFACE AREA CHANGES  143  PROPOSALS FOR FUTURE WORK  148  BIBLIOGRAPHY  153  APPENDIXES  163  APPENDIX I APPENDIX I I  E l e c t r o d e Surface Conditions Surface Area Measurement  APPENDIX I I I X-ray D i f f r a c t i o n R e s u l t s  163 167 170  vi  LIST OF TABLES TABLE 1.  2.  3.  4.  5.  6.  7.  8.  9.  10.  11.  12.  13.  Page Hydrogen and oxygen monolayer charges f o r p l a t i n u m and i r i d i u m e l e c t r o d e s  41  E l e c t r o d e a r e a s measured a f t e r d e t e r m i n a t i o n of the p o l a r i z a t i o n curves  109  T a f e l parameters f o r Pt and P t / I r w i r e e l e c t r o d e s f o r the lower T a f e l r e g i o n of the p o l a r i z a t i o n c u r v e s i n 1M NaCl; pH 2  I l l  T a f e l parameters f o r Pt and P t / I r w i r e e l e c t r o d e s f o r the a s c e n d i n g and descending upper T a f e l r e g i o n s of the p o l a r i z a t i o n c u r v e i n 1M NaCl; pH 2  I l l  P a s s i v a t i o n data f o r P t and P t / I r w i r e e l e c t r o d e s from p o l a r i z a t i o n c u r v e s i n 1M NaCl; pH 2  113  S u r f a c e a r e a changes as a r e s u l t of p o t e n t i o s t a t i c p o l a r i z a t i o n i n c h l o r i d e e l e c t r o l y t e s w i t h Pt/30 I r T i electrodes  118  E f f e c t of p o t e n t i o s t a t i c p o l a r i z a t i o n i n 1M ^ S O ^ the s u r f a c e a r e a of Pt/30 I r - T i e l e c t r o d e s  119  on  E f f e c t of treatment i n aqua r e g i a on the s u r f a c e a r e a of Pt/30 I r - T i e l e c t r o d e s  120  E f f e c t s o f " a c t i v a t i o n " procedures on the s u r f a c e a r e a of Pt/30 I r - T i e l e c t r o d e s  126  S u r f a c e c o n d i t i o n s of w i r e e l e c t r o d e s used i n g a l v a n o s t a t i c p o l a r i z a t i o n experiments  163  S u r f a c e c o n d i t i o n s o f c o a t e d e l e c t r o d e s used i n p o t e n t i o s t a t i c p o l a r i z a t i o n and s u r f a c e a r e a d e t e r m i n a t i o n s ..  165  I d e n t i f i c a t i o n o f X-ray d i f f r a c t i o n peaks f o r a titanium substrate electrode  170  new  I d e n t i f i c a t i o n of X-ray d i f f r a c t i o n peaks f o r a used t i t a n i u m s u b s t r a t e e l e c t r o d e (3 weeks i n 1M H„S0, a t .2 A / f t . and 40°C) 2  171  vii LIST OF FIGURES  FIGURE  Page  1.  Galvanostatic c e l l  98  2.  Cell  99  3.  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r p l a t i n u m w i r e e l e c t r o d e i n h e l i u m - s a t u r a t e d 1M NaCl; pH 2  109  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r p l a t i n u m / 5 % i r i d i u m w i r e e l e c t r o d e i n h e l i u m - s a t u r a t e d 1M N a C l ; pH 2  110  4.  5.  6.  7.  8.  f o r s u r f a c e a r e a measurement  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r platinum/10% w i r e e l e c t r o d e i n h e l i u m - s a t u r a t e d 1M NaCl; pH 2  iridium  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r platinum/20% w i r e e l e c t r o d e i n h e l i u m - s a t u r a t e d 1M NaCl; pH 2  iridium  G a l v a n o s t a t i c p o l a r i z a t i o n c u r v e f o r platinum/25% w i r e e l e c t r o d e i n h e l i u m - s a t u r a t e d 1M NaCl; pH 2  iridium  I l l  112  113  P o t e n t i o s t a t i c p o l a r i z a t i o n curve f o r Pt/30 I r - T i i n u n s t i r r e d 1M l^SO^  120  P o t e n t i o s t a t i c p o l a r i z a t i o n curve f o r Pt/30 I r - T i i n u n s t i r r e d 1M NaCl; pH 2  121  Current/time r e l a t i o n s f o r p o t e n t i o s t a t i c p o l a r i z a t i o n w i t h Pt/30 I r - T i e l e c t r o d e s a t 1800 mV (S.C.E.) i n various electrolytes  127  C u r r e n t / t i m e r e l a t i o n s f o r a Pt/30 I r - T i e l e c t r o d e f o r p o t e n t i o s t a t i c e l e c t r o l y s i s of IM H^SO^ a t 1800 mV and 25°C, a f t e r v a r i o u s pretreatment times i n aqua r e g i a ..  128  C u r r e n t / t i m e r e l a t i o n s f o r a Pt/30 I r - T i e l e c t r o d e f o r p o t e n t i o s t a t i c e l e c t r o l y s i s o f 1M H^SO^ a t 2000 mV and 25°C, b e f o r e and a f t e r p o t e n t i o s t a t x c e l e c t r o l y s i s o f 1M NaCl; pH 2  129  13.  S.E.M. O b s e r v a t i o n o f Pt/30 I r - T i s u r f a c e s  134  14.  S.E.M. O b s e r v a t i o n o f used Pt/30 I r - T i e l e c t r o d e s  135  9.  10.  11.  12.  viii  FIGURE  Page  15.  S.E.M. O b s e r v a t i o n o f p l a t i n u m sheet  136  16.  S.E.M. O b s e r v a t i o n of p l a t i n u m w i r e e l e c t r o d e s  137  17.  S.E.M. O b s e r v a t i o n o f Pt/25 I r w i r e e l e c t r o d e s  138  18.  E.P. O b s e r v a t i o n o f new Pt/30 I r - T i e l e c t r o d e  139  19.  Schematic r e p r e s e n t a t i o n o f t h e p o t e n t i a l h i s t o r y o f a w i r e e l e c t r o d e used i n g a l v a n o s t a t i c p o l a r i z a t i o n experiments ;  164  Schematic r e p r e s e n t a t i o n o f t h e p o t e n t i a l h o s t o r y of a coated e l e c t r o d e used i n p o t e n t i o s t a t i c p o l a r i z a t i o n experiments  166  R e p r e s e n t a t i o n of a t y p i c a l a n o d i c charge c u r v e i n dea e r a t e d 1M I^SO^ a t 20°C, showing c o n s t r u c t i o n s f o r d e t e r m i n i n g oxygen d e p o s i t i o n charge  169  20.  21.  I wish to thank Dr. I.H. Warren f o r h i s guidance throughout the course of this research, the s t a f f of the Science D i v i s i o n , Main Library, "U.B.C. f o r their invaluable a i d , and my wife, Darlene, f o r her enduring patience.  F i n a n c i a l support from  the National Research Council and International Nickel Company i s also acknowledged.  1.  INTRODUCTION  Of a l l the possible electrodes for oxidation reactions in aqueous solutions, those utilizing metals of the platinum group (platinum, palladium, iridium, osmium, ruthenium, and rhodium) have been found to be the most suitable.  The platinum group metals are a l l  effective "electrocatalysts" - for example, the activation energy for a given electrochemical reaction may be lower on a platinum group metal due to the facilitated formation of a proper activated complex.  This  property, coupled with the profound inertness of these metals, is the reason for the widespread interest and use of anodes composed of one or more of these metals (or their oxides).  In practical systems, the  low-overvoltage characteristics of platinum metal electrodes make them very attractive for use in electrochemical oxidation reactions (for example, the production of hypochlorites, chlorates, perchlorates, etc. from chloride solutions) or as auxiliary electrodes in metal plating applications.  Their particular suitability for use as anodes i s due  to the resistance of the metals to oxidation, and hence, dissolution and/or passivation when made anodic to industrially useful potentials. Platinum itself i s the most commonly utilized material, due more to i t s relative abundance than to some relatively superior combination of electrochemical properties.  Recently, however, the application  of binary noble metal alloy electrode systems - as well as the use of the rarer platinum metals by themselves - has been under intense investigation.  For the case of alloy electrodes, the choices of the particular  2 platinum metals i s governed by: 1)  the d i f f e r e n t corrosion-resistant properties of the i n d i v i d u a l metals toward the medium i n question.  2)  the a v a i l a b i l i t y and cost of the i n d i v i d u a l metals.  3)  the crystallographic systems of the metals w i l l r e s t r i c t the compositions of homogeneous a l l o y s and w i l l l i m i t the a l l o y s which can be worked. The extreme cost of the platinum metals, together with the  problems associated with producing desirable a l l o y systems, has led to the concept of coating a suitably inert substrate material with a t h i n precious metal (or metal oxide) cover.  Such an electrode exhibits  s i m i l a r electrochemical properties to those of the pure noble metal or metals comprising the coating.  The choice of a suitable substrate i s  limited by the following: 1)  Cost and a v a i l a b i l i t y .  The material must be both r e l a t i v e l y  inexpensive and e a s i l y formed to desired shapes. 2)  Inertness.  If the electrode i s to be subjected to anodic  p o l a r i z a t i o n , the substrate must form an insulating oxide f i l m wherever i t contacts the e l e c t r o l y t e . 3)  Breakdown voltage.  In addition, the non-conducting areas of  the anode must be able to withstand the electrochemical " s t r e s s " imposed by high anodic p o t e n t i a l s . 4)  Conductivity.  The substrate metal must possess adequate  elect-  r i c a l conductivity i n order to permit current to pass through  3 the body of the electrode.  This i s of p a r t i c u l a r concern with  large anodes, whose extremities are far-removed from the e l e c trical 5)  contact.  Resistance to deformation i n service.  It i s desirable  to  maintain close control of anode/cathode separations. 6)  Adherence of the coating.  The material must be able to accept  a strongly adherent precious metal coating after a r e l a t i v e l y simple pretreatment of the substrate. The metals found to be most suitable for substrates i n precious metal coated anodes are the refractory metals (titanium, alum, tungsten, zirconium, niobium).  tant-  These metals share the property  of formation of a protective oxide f i l m on being made anodic.  As t i t -  anium i s r e a d i l y available commercially, i t has been u t i l i z e d extensively. The r e s i d u a l current for titanium d i s s o l u t i o n through i t s oxide f i l m is extremely low.  Its  down voltage are a l l acceptable, coatings i s obtained.  protective  strength, conductivity and oxide breakand good adherance of precious metal  Although other metals such as niobium or tantalum  (higher breakdown voltage) or a l l o y s such as titanium/molybdenum (better coating adherence) are even more suitable,  t h e i r cost precludes the wide-  spread a p p l i c a t i o n of such alternative substrate materials. The s t a b i l i t y of precious metal anodes which are used i n certain systems i s of concern, where the term " s t a b i l i t y " i s taken to refer  to: 1)  Loss of metal from the electrode.  4  2)  Constancy of overvoltage  (or current density) at a given  applied current density (or p o t e n t i a l ) . 3)  Reproducibility of electrochemical data.  I t has been found that, apart from inherent differences i n the e l e c t r o chemical behaviour of the i n d i v i d u a l platinum metals, the above phenomena are affected - indeed, even determined - by i t s treatment before use as an anode.  That i s , the h i s t o r y of the electrode must be taken  into consideration i f differences i n electrochemical a c t i v i t y are to be resolved. Before attempting  meaningful experimental  work on the elec-  trochemical behaviour of noble metal anodes (either i n a pure form or as a coating over a suitable substrate), i t i s necessary to e s t a b l i s h the parameters which a t t a i n to the p a r t i c u l a r system(s) of i n t e r e s t . These include: 1)  Surface condition p r i o r to use as anode, and any changes i n this condition as a r e s u l t of anodic circumstances.  This  may  be taken to include changes i n the electrochemically active surface area, formation of oxide or other films, and  formation  of d i f f e r e n t surface structures as the r e s u l t of d i s s o l u t i o n . 2)  Possible anodic reactions. material, the composition state of the electrode.  These are determined by the anode of the e l e c t r o l y t e , and the surface  5  2.  LITERATURE  Available the n o b l e metals entire often the  field little  field.  films  of  For example,  the  of  ively  yet  researched,  anodic  ysis  metals.  i n the  this  thesis. of  the  yet  effect  these  electrode  other  deals  surface  researchers  areas  still  the  subject.  survey  only with  and t h e o r e t i c a l those  topics  the  above f i e l d s ,  of  surface  are  also  certainly  be  possible.  been  extens-  roughness  of  inconsistent  consideration  which are  the behaviour and t h e  topics  of  author  has  sections  of  of  electrode  anodes  could easily  i n the  within  (namely  pretreatelectrolthe  be d e a l t the  undertaken to w r i t e which  contain-  immediate  d i s s o l u t i o n of  t h a n c o u l d be r e p r e s e n t e d  Indeed,  areas  electrode  still  list  is  possibility  assumed  An endless  the  constructed.  aforementioned  length  the  has  use  of  fields  e x p e r i m e n t a l work c o n t a i n e d w i t h i n -  Each of  of  on the  of  there  anodic  w h i c h may n o n e t h e l e s s  chloride-containing solutions,  much g r e a t e r  formation of  processes,  a r e a measurement,  at  surveys  true  literature  thesis  surface of  in  problem,  a b o v e m e n t i o n e d w o r k may n e g l e c t  work c o u l d be  The  ment,  literature  T o compound t h e  r e s e a r c h on the  when c o n s i d e r i n g t h i s  electrochemical  interest  body o f  the work done by o t h e r s  on v a r i o u s  Again,  in this  electrochemical behaviour  largest  electrochemistry.  The e s t a b l i s h m e n t  ed  the  anodic d i s s o l u t i o n reactions  factors  on the  i n t e r r e l a t i o n among w o r k p e r f o r m e d i n d i f f e r e n t  pretreatment  of  literature  constitutes  may n e g l e c t  related.  SURVEY AND THEORETICAL CONSIDERATION  the  noble with  confines extensive results,  6  mainly from a mechanistic point of view) are reproduced i n t h i s thesis with appropriate c o r r e l a t i o n with other topics.  Such work has a high  degree of r e p e t i t i o n as a r e s u l t of the c o r r e l a t i o n , but i t i s f e l t that t h i s i s necessary i n order f o r the topics dealt with herein to be successfully applied to experimental determinations.  2.1  ELECTRODE PRETREATMENT AND ACTIVATION 2.1.1  Nature of the Problem  A l l electrodes are subjected to some form of pretreatment p r i o r to experimental use (usually some combination of i n t e n t i o n a l and unintentional procedures) which may have a s i g n i f i c a n t effect on the experimental r e s u l t s .  For noble metal electrodes i n p a r t i c u l a r , pre-  treatments are deliberately applied i n order to " a c t i v a t e " such e l e c t rodes f o r some electrochemical process.  However, researchers present  c o n f l i c t i n g views as to what i s the state of an active electrode and as to the properties which define an active electrode. Electrode a c t i v i t y i s best understood when comparing e l e c t rodes of d i f f e r e n t materials with respect to their behaviour i n a given electrochemical s i t u a t i o n .  I t i s less clear, however, to understand  the changes i n a c t i v i t y of a single electrode material such as platinum. Hence, a profusion of mechanisms can be found i n the l i t e r a t u r e to describe the active state of the l a t t e r , which may or may not be related depending on the d e f i n i t i o n of this state.  B r e i t e r ^ defines an active  7  electrode as one having a reproducible surface state of high r e a c t i v i t y for electrochemical reactions.  However, i t i s apparent that the r e a c t -  2 ion of i n t e r e s t be s p e c i f i e d , as i t may  belong to either of two  (limit-  3 4 ing) classes *  where the reactants may  or may  not form bonds with the  electrode surface (and hence, the substrate-dependence of the reaction is different).  Parsons"* states that the v a r i a t i o n i n electrode reaction  k i n e t i c s with the substrate can be attributed to the v a r i a t i o n i n bond 6 7  strength between the adsorbed r a d i c a l s and the substrate.  8  Bockris ' '  says that the presence of vacant o r b i t a l s i n the d-band i s the c o n t r o l l ing e l e c t r o n i c factor i n determining  activity.  For example, ruthenium  (which has the largest number of unpaired d-electrons of the metals) interacts strongly with electron-donating  platinum  species and consequ-  ently shows the largest coverage with adsorbed oxygen.^  The development  of e f f e c t i v e " e l e c t r o c a t a l y s t s " i s i n a c t u a l i t y a study of the a c t i v i t y of these materials, and the r e s u l t s can consequently be applied to other work concerning  the e f f e c t s of electrode pretreatment on electrode act-  9  ivity.  Woods  has made a d i s t i n c t i o n between " f a c i l e " and "demanding"  reactions, where a f a c i l e reaction has an a c t i v i t y v i r t u a l l y independent of the mode of preparation, and a demanding reaction i s much more sensit i v e to the non-uniformities of the electrode surface. Most workers agree that the surface condition of noble metal electrodes should be reproducibly produced by some method of surface preparation.  Most theories of adsorption or c a t a l y s i s assume a d i s t r i -  bution of "active centers" over the electrode surface.  These may  involve  8  i n d i v i d u a l atoms, atomic groups, c r y s t a l planes or angles, or edges and l a t t i c e defects, f o r example, and may located.  I t may  or may  not be randomly  be expected that the production and d i s t r i b u t i o n  of such s i t e s i s a r e s u l t of electrode pretreatment.  I t i s necessary  to e s t a b l i s h some form of measurement i n order to compare the a c t i v i -  12 t i e s of d i f f e r e n t l y prepared electrodes.  Hammett  recommended the  measurement of the hydrogen (evolution or ionization) current at a  13 given overpotential. p o t e n t i a l rose sharply  Rius  noted the current density at which the  (that i s , the electrode became passivated)  2 14 i n chloride e l e c t r o l y s i s .  Warner and Schuldiner  '  measured the  decay time of a monolayer of adsorbed oxygen i n hydrogen-saturated solution.  Biegler"''^ used the shape of the hydrogen "peaks" i n c y c l i c  voltammetry as an i n d i c a t i o n of a c t i v i t y .  Unfortunately,  such methods  show only that r e l a t i v e differences i n a c t i v i t y do occur, and  these  must be interpreted i n conjunction with theories f o r the d i s t r i b u t i o n of active centers, electronic properties, and i n terms of the reaction of i n t e r e s t .  9  2.1.2  Methods of Pretreatment and A c t i v a t i o n  There i s a wide range of pretreatment procedures, any combination of which may be used to clean the electrode and produce some form of i n i t i a l surface condition.  A c t i v a t i o n procedures are  not generally separated from other pretreatment processes.  Mechanical  treatments such as polishing are not generally used as a f i n a l pro^ cedure i n electrode pretreatment ( i t i s the h i s t o r y of those steps immediately prior to experimental use of the electrode which most c r i t i c a l l y affect i t s behaviour) due to the tendency f o r f o r e i g n matter to be incorporated into the electrode surface.  Of course,  d i f f e r e n t mechanically treated forms can be obtained as a r e s u l t of the process of manufacture  ( r o l l i n g , drawing, e t c . ) .  Electrochemical pretreatments involve anodization, cathodi z a t i o n , or a combination of both, by a p p l i c a t i o n of d i r e c t or a l t e r nating currents, p o t e n t i o s t a t i c or potentiodynamic p o l a r i z a t i o n s , or by pulses of constant current or p o t e n t i a l .  Such procedures are often  performed with the electrode i n s i t u i n the experimental system. Thermal pretreatments must necessarily include any e f f e c t s due to the manufacture of the electrode, as w e l l as any treatments.  subsequent  These generally involve holding the electrode i n a c e r t a i n  atmosphere at a given temperature for a desired period of time.  10  Chemical p r e t r e a t m e n t s can range  i n s e v e r i t y from mere  c l e a n i n g o f the e l e c t r o d e t o s u r f a c e e t c h i n g o r o x i d a t i o n . cedures a r e performed  o u t s i d e the e x p e r i m e n t a l  Such p r o -  system.  A comprehensive survey of p r e t r e a t m e n t and  activation  procedures, i n c l u d i n g t h e i r e f f e c t s on s u r f a c e s t r u c t u r e , m e t a l s o l u t i o n , s u r f a c e a r e a , and on the subsequent made by the  author.^  dis-  experimental r e s u l t s  has  11 2.1.3  Mechanisms of A c t i v a t i o n df Noble Metal Electrodes 2.1.3.1  Surface Area  Increases  It i s evident that i n many cases the reported  increases  i n electrode a c t i v i t y as a r e s u l t of c e r t a i n pretreatments can be  ex-  plained by an increase i n the surface area of the electrode as a r e s u l t of these operations.  That i s , the " s p e c i f i c " a c t i v i t y ( a c t i v i t y per  true unit of area) may  not be altered at a l l . 18  W i l l and Knorr  found that anodic pretreatment at 2 V  (R.H.E.) for periods of time ranging from one second to ten minutes produced "intense" increases i n surface roughness. 19 Sawyer and Seo described a procedure for producing  an  activated platinum surface (pf limited l i f e ) i n which the electrode  was  plated b r i e f l y at low current density i n a chloroplatinate s o l u t i o n . I t i s obvious that such a treatment merely increases the surface area of the electrode, due to the roughness of the deposit. 20 Hoare  found that alternating current treatment produced  v i s i b l e darkening of a platinum electrode surface, with a s i g n i f i c a n t increase i n surface area.  As he observed only i n s i g n i f i c a n t v a r i a t i o n s  i n area as a r e s u l t of anodic/cathodic into the hydrogen region, he suggested  treatment which did not extend 20 21 22 23 '  '  '  that the mechanism  for surface disruption must be connected with the presence of hydrogen (rather than oxygen, as other researchers have suggested).  Repeated  penetration (hydrogen i s able to d i f f u s e into the body of the metal during the cathodic h a l f - c y c l e ) and removal of hydrogen, with consequent  12 changes i n l a t t i c e parameters, may break up the metal surface.  He  further suggested that, i n the presence of impurities, fewer hydrogen atoms would combine on the surface and more penetrate the skin, r e s u l t ing i n a more quickly broken-up surface.  In support f o r Hoare's mech-  anism, i t was found that only those noble metals which were capable of d i s s o l v i n g appreciable amounts of hydrogen (platinum and palladium) could be transformed into metal-black electrodes with a l t e r n a t i n g current p o l a r i z a t i o n alone. 24 Gilman  disputed the hydrogen adsorption/absorption rough-  ening mechanism, as he observed surface roughening even under conditions where hydrogen was not present at any stage of the experimental program 21 [that i s , potentials no lower than .4 V (R.H.E.)].  Hoare  countered,  however, with the claim that, under the high-temperature conditions used by Gilman (120°C), hydrogen may s t i l l be expected to be adsorbed. Gilman offered two explanations f o r his observed roughening e f f e c t s . 1)  Rapid reduction may not allow s u f f i c i e n t time f o r the surface to "anneal".  2)  Slight solution of the "oxide" may occur and t h i s , or subsequent reduction of dissolved platinum, may  lead to roughening.  Biegler"'"^ did not observe roughening with anodic/cathodic treatments which extended into the hydrogen region, and even goes so far as to suggest that the presence of hydrogen on the surface i n h i b i t e d the roughening process.  This i n h i b i t i o n was attributed to the replace-  ment of adsorbed oxygen with adsorbed hydrogen, so that at no time i s  13 the m e t a l s u r f a c e p e r m i t t e d  t o r i s e t o the energy l e v e l i t has i n t h e  absence o f chemisorbed s p e c i e s . 25 V o l o d i n and T y u r i n electrode surface during  a t t r i b u t e d the break-up o f a  platinum  c y c l i c o x i d a t i o n and r e d u c t i o n t o m e t a l d i s -  s o l u t i o n d u r i n g o x i d a t i o n , and a l s o t o the i n s e r t i o n of hydrogen and oxygen atoms i n t o d e f e c t s which appeared i n the c r y s t a l s t r u c t u r e as a r e s u l t of the d i s s o l u t i o n .  They d i d n o t b e l i e v e t h a t a change i n  roughness f a c t o r a l t e r e d the a d s o r p t i o n respect  vated  with  t o oxygen and hydrogen. 26 Kronenberg  theory  capacity per u n i t surface  d i d not subscribe  to the surface area  o f a c t i v a t i o n , and c i t e d e x p e r i m e n t a l  increase  e v i d e n c e i n which an a c t i -  e l e c t r o d e was produced by treatment i n a hydrogen flame, where  the e l e c t r o d e s u r f a c e would be expected t o become annealed  (smoothed).  27 Shibata,  commenting on t h e p r e c e d i n g  remark, s t a t e s t h a t an u n s t a b l e  ( d i s r u p t e d o r o t h e r w i s e ) s u r f a c e l a y e r would have been produced due t o the r a p i d c o o l i n g o f the e l e c t r o d e a f t e r such treatment. 28 Shibata non-multilayer  found t h a t s u c c e s s i v e f o r m a t i o n  and r e d u c t i o n o f  oxygen coverages d i d n o t a l t e r the t r u e s u r f a c e area o f  the e l e c t r o d e , b u t t h a t when m u l t i l a y e r f o r m a t i o n  occurred,  the a r e a  increased. I t must be n o t e d , f u r t h e r , t h a t the method used i n d e t e r mining the s u r f a c e a r e a may not n e c e s s a r i l y determine the a c t u a l  area  of the e l e c t r o d e which i s a v a i l a b l e f o r the e l e c t r o c h e m i c a l r e a c t i o n of i n t e r e s t .  This  i s e s p e c i a l l y of concern when "demanding" r a t h e r  than  14  " f a c i l e " r e a c t i o n s are c o n s i d e r e d ,  as the former one  expected to be  more s p e c i f i c as to s i t e s , t h a t i s , more s e n s i t i v e t o the of the s u r f a c e .  For example, Bagotzky e t a l ^ found t h a t an  decrease i n a c t i v i t y by  uniformity  f o r the o x i d a t i o n of methanol c o u l d be  apparent explained  the i n a c c e s s i b i l i t y of m i c r o c a v i t i e s on the e l e c t r o d e s u r f a c e t o  l a r g e methanol m o l e c u l e .  However, hydrogen atoms, used i n t h e i r method  of s u r f a c e a r e a d e t e r m i n a t i o n , c a v i t i e s and  were c a p a b l e of a d s o r p t i o n  i n these m i c r o -  c o n s e q u e n t l y the r e s u l t a n t s p e c i f i c a c t i v i t y appeared  2.1.3.2  the  A c t i v e Surface  low.  Structure  A l t h o u g h i n c r e a s e s i n e l e c t r o d e s u r f a c e a r e a have been duced by means of s e v e r a l p r e t r e a t m e n t p r o c e d u r e s , many  pro-  researchers  b e l i e v e t h a t an a c t i v a t e d e l e c t r o d e does not n e c e s s a r i l y have t o possess a g r e a t e r s u r f a c e area than a l e s s - a c t i v e one. 29 Shibata,  W i l l and  Knorr,  18  30 and  French and  Kuwana  proposed the concept of a " s t r u c t u r -  a l l y d i s t i n c t " a c t i v e s u r f a c e l a y e r , which has  a c t i v e s i t e s of enhanced  a c t i v i t y r a t h e r than j u s t more s i t e s per u n i t a r e a . p r o b a b l e t h a t the r e p o r t e d c e r t a i n p r e t r e a t m e n t s may f a c t o r s of the e l e c t r o d e s .  I t i s also quite  " a c t i v a t i o n " of e l e c t r o d e s as a r e s u l t be due  e n t i r e l y to i n c r e a s e i n the  roughness  B i e g l e r " ^ noted the c l o s e c o n n e c t i o n  the roughening of e l e c t r o d e s and  the a n o d i c / c a t h o d i c  ures commonly used by many o t h e r workers. l y d i s t i n g u i s h between an " a c t i v a t e d " and both i n v o l v e s u r f a c e s t r u c t u r e a l t e r a t i o n .  He  of  between  a c t i v a t i o n proced-  i s a b l e , however, to c l e a r -  "roughened" e l e c t r o d e , a l t h o u g h B i e g l e r found t h a t roughened  15 electrodes did not necessarily exhibit high a c t i v i t y (as indicated by a high hydrogen peak height i n c y c l i c voltammograms), and  conversely,  that activated electrodes could be produced by means of "non-roughening" 29 anodic/cathodic treatments.  Shibata  postulates that an active p l a t -  inum electrode i s one which possesses an unstable surface layer whose atoms have been rearranged by oxygen atoms, giving r i s e to s t r a i n . Repeated formation and reduction of the oxygen layer would cause s u f f i cient accumulation  of this s t r a i n to produce roughening.  believes that a freshly-manufactured  Biegler^  platinum surface contains a large  number of high-energy surface platinum atoms, overlying atoms of low coordination.  The strong platinum-oxygen chemisorption band further  weakens the coordination of the high-energy atoms causing e i t h e r atomic rearrangement or complete separation of platinum-oxygen species from the surface.  A (non-roughening) a c t i v a t i o n procedure w i l l r e s u l t i n  eventual removal of most or a l l of the high-energy platinum atoms, leaving a stable surface where low-index, high-coordination faces are predominant. It i s generally accepted that the surface of a noble metal electrode i s e n e r g e t i c a l l y heterogeneous, exhibiting centers of varying degrees of electrochemical a c t i v i t y . may  Indeed, d i f f e r e n t c r y s t a l planes  exhibit d i f f e r e n t a c t i v i t i e s with respect to c e r t a i n electrochemical 3  reactions. Damjanovic (cf. Bockris ) said that d i f f e r e n t c r y s t a l planes may be the source of d i f f e r i n g r e a c t i v i t i e s as a r e s u l t of their having 3  unequal work functions.  Schuldiner et a l  derived s i g n i f i c a n t l y d i f f e r e n t  16  atom d e n s i t i e s f o r p e r f e c t l y smooth p l a t i n u m c r y s t a l f a c e s , 3.9231 angstroms as t h e l a t t i c e parameter. has  using  B i e g l e r ^ says t h a t p l a t i n u m  a l a r g e s p r e a d o f e n e r g e t i c a l l y d i f f e r e n t s i t e s , and t h a t some p r e -  t r e a t m e n t s may f a v o u r t h e p r o d u c t i o n cathodic  of c e r t a i n c r y s t a l faces.  t r e a t m e n t may a l t e r t h e s u r f a c e  Anodic/  s t r u c t u r e o f an e l e c t r o d e by  means o f p r e f e r e n t i a l a t t a c k o f s p e c i f i c c r y s t a l f a c e s .  I n f a c t , he  observed a n g u l a r p i t s o f i n c r e a s i n g s i z e s on e l e c t r o d e s w h i c h has been subjected  t o i n c r e a s i n g l y severe roughening treatments.  The p o s t u l a t i o n  of t h e appearance o r d i s a p p e a r a n c e o f c e r t a i n c r y s t a l p l a n e s as a cause f o r changes i n e l e c t r o d e  a c t i v i t y has n o t been found t o be adequate t o  e x p l a i n a l l o f t h e observed phenomena.  In general,  the differences i n  the e l e c t r o c h e m i c a l b e h a v i o u r o f p l a t i n u m s i n g l e - c r y s t a l e l e c t r o d e s o f various  o r i e n t a t i o n s has been found t o be s m a l l o r n o n - e x i s t e n t  and do  not conform t o l o g i c a l sequences such as atom d e n s i t y o r s p a c i n g , 32  a l t h o u g h o t h e r s such as Pyshnograeva graphic  10  and Bagotzky  believe  crystallo-  o r i e n t a t i o n t o b e t h e major s t r u c t u r e f a c t o r a f f e c t i n g e l e c t r o -  chemical a c t i v i t y .  Many b e l i e v e , l i k e B i e g l e r , t h a t s u r f a c e  platinum  atoms may e x i s t i n s t r u c t u r e s w h i c h may n o t n e c e s s a r i l y c o r r e s p o n d t o those d e r i v e d  from t h e b u l k u n i t c e l l .  O t h e r s , l i k e Damjanovic and  34  Brusic,  do n o t l i k e the t h e o r y o f t h e appearance o f d e f i n i t e c r y s t a l  p l a n e s as they a r e u n a b l e t o f i n d a r e a s o n as t o why t h e mechanism o f the r e a c t i o n they c o n s i d e r e d different  (oxygen r e d u c t i o n )  s h o u l d be d i f f e r e n t on  planes. 35  Appleby  has s a i d t h a t a p l a t i n u m atom a t a d e f e c t has  17 extra energy available for bonding, which permits an increase i n bond strength f o r the reaction intermediates at such s i t e s .  31 36 Schuldiner et a l '  reported that the presence of grain-  boundaries or surface layer stresses i n p o l y c r y s t a l l i n e bead electrodes, or even the presence of impurities i n the bead, greatly enhances the process of passivation over pure platinum single c r y s t a l electrodes. These factors appear to be of much greater importance than atomic geometry i n determining  the c a t a l y t i c a c t i v i t y .  Bagotzky et a l  observed no e f f e c t of mechanical treatment  on the c a t a l y t i c a c t i v i t y of his electrodes other than that due to an increase i n the true electrode area.  That i s , they found most c r y s t a l  defects do not act as active centers governing the properties of the platinum  surface.  27 Shibata and Sumino  found that annealed electrodes  (reduced  surface s t r a i n , fewer defects) showed the same high i n i t i a l a c t i v i t y as f r e s h l y prepared electrodeposited electrodes (highly strained surface, many defects).  They consider that the short exposure to the a i r exper-  ienced by the electrodes during transfer from the furnace to their c e l l was s u f f i c i e n t to re-activate the electrode.  18 2.1.3.3  Active Oxidized  Surface  An oxygen-covered electrode i s generally described as passivated, where such processes as metal d i s s o l u t i o n are i n h i b i t e d . Furthermore, the potentials required for a given process chlorine evolution) may metal.  (for example,  be higher on a passive surface than on the bare  The presence of oxygen on the electrode surface may,  f a c i l i t a t e or even be necessary for other  however,  processes.  37 F l i s and Bynyaeva  found that the amount and bonding  strength of oxygen produced (chemically) on a platinum electrode surface depended on the oxidant used.  They further said that the quantity  and energy state of the oxygen probably depends on the structure and state of the electrode surface, leading to d i f f e r e n t types of i n t e r actions between the oxygen and the metal.  The "nature and conditions"  of these interactions were said to be responsible for the r e v e r s i b i l i t y or i r r e v e r s i b i l i t y of potentials measured at these electrodes.  38 Mayell and Langer  hypothesised  that platinum black derived  i t s c h a r a c t e r i s t i c c a t a l y t i c properties due to i t s i n a b i l i t y to form a " t i g h t " PtO structure such as smooth platinum can.  The  crystalline  disarray of platinum black prevents the formation of this structure, leaving many platinum atoms with unshared d-electrons which would be available for bonding o r b i t a l s . 22 23 Hoare  '  said that the formation of an oxygen layer on a  platinum electrode w i l l cause r e p r o d u c i b i l i t y of r e s u l t s , as the c a t a l y t i c surface on which the electrode reaction surface takes place would be stable.  19  39 Pospelova et a l say that a " c e r t a i n type" of passivating f i l m i s necessary i f the r e a c t i v i t y of c e r t a i n adsorbed p a r t i c l e s i s to be enhanced.  They point out the d i f f e r e n t changes i n adsorption  properties of certain noble metal oxygen films caused by s i m i l a r anodic treatments as evidence f o r t h e i r hypothesis. 40 Hoare  41 and Rand  say that the r e v e r s i b l e oxygen p o t e n t i a l  i s attainable only at a completely oxygen film-covered platinum surface, due to the suppression of other reactions which contribute to a mixed potential (such as metal d i s s o l u t i o n from the bare metal). 42 Afon'shin et a l  suggests that d i f f e r e n t pretreatments  produce d i f f e r e n t forms of surface oxide which can account for the d i f f e r e n t a c t i v i t i e s (for the oxygen evolution reaction) produced by the pretreatments.  At potentials s l i g h t l y above 1.5 V (R.H.E.) the  oxygen f i l m i s not continuous but occupies only the active portions of the surface.  On increasing the p o t e n t i a l , these isolated regions i n t e r -  lock, forming a f i l m which i s passive with respect to oxygen evolution. At about 1.9 V the monolayer oxide contracts as a result of reorientat i o n or r e c r y s t a l l i z a t i o n , l i b e r a t i n g part of the surface.  Subsequent  f i l l i n g of these regions forms a "dense monolayer" which i s considered to be active with respect to oxygen evolution. 43 Velter and Schultz suggest that currentless rearrangement processes occur after the oxide layer has been formed, r e s u l t i n g i n a stronger and more heterogeneous  layer.  44 Davis  explained the improved c a t a l y t i c behaviour of  20 f r e s h l y anodized platinum by means of f a c i l i t a t e d electron transfer from the oxidized surface v i a an oxygen "bridging" mechanism.  This  theory r e l i e s on the existence of platinum oxide at the g r a i n boundaries and other active s i t e s .  Other researchers  45 46 ' favoured t h i s theory  on the grounds that pre-anodization seemed necessary to a c t i v a t e an electrode.  47 James considered the oxide bridge theory to be  untenable  because he could produce long-lived a c t i v a t i o n of the hydrogen evolution 22 23 reaction, where no surface oxide e x i s t s .  Hoare  '  r e j e c t s the oxide  bridge theory for the reason that, i n a redox system, the reducing agent would be expected to remove oxygen bridges (the electrochemical oxidat i o n of a reducing agent i n solution has been shown to occur mainly through oxidation by the adsorbed  oxygen) and hence a l t e r the oxidant/  reductant r a t i o of the redox system.  That i s , an indicator electrode  would not be expected to function.  2.1.3.4  Active Reduced Surface  Many a c t i v a t i o n procedures  involve the reduction of a pre-  v i o u s l y oxidized electrode, r e s u l t i n g i n an "active reduced" surface. I t i s clear that a non-passivated electrode w i l l show a superior a c t i v i t y f o r processes such as metal dissolution, and f o r such cases any treatment which removes s u p e r f i c i a l oxygen w i l l appear to activate the electrode.  In addition, however, several researchers have proposed  that some kind of d i s t i n c t active structure i s produced by the reduction  21 of a previously oxidized surface. Anson  48 49 19 ' and Sawyer suggested that a platinum  electrode  which has been oxidized and subsequently reduced i s covered by a specia l l y active thin p l a t i n i z e d l a y e r .  This active layer i s derived from  30 the thin oxide f i l m generated by pre-anodization.  French and Kuwana  stated that the formation of the a c t i v e surface state must involve the formation and subsequent reduction of oxide, that i s , bond-rupture must occur. Conway"^ has stated that the presence of adsorbed oxygen atoms on platinum could cause metal atom rearrangement at potentials above 1 V (H.E.). Biegler"^ found that the alternate formation and reduction of the oxygen layer was responsible for a c t i v a t i o n , where a r e d i s t r i b u tion of surface platinum atoms occurs due to periodic formation and breaking of platinum-oxygen bonds.  A c y c l i c program of oxidation and  reduction or heat-treatment i n an oxidizing flame w i l l r e s u l t i n r e moval of platinum atoms i n high-energy s i t e s (by atom rearrangement or separation from the surface). Appleby"*"*" observed that the a c t i v i t y of his electrodes increased i f the maximum p o t e n t i a l obtained i n his anodic/cathodic treatment was above 1.4 V (R.H.E.).  pre-  He attributed this to an increase  i n the surface energy due to an increase i n surface defect density when the phase oxide monolayer formed at these potentials i s reduced. 52 Kravchenko et a l suggests that the reduction of a  22 previously formed thick oxide coating on platinum w i l l produce a layer of platinum black on the surface.  53 Feldberg  attributed electrode a c t i v a t i o n to the production  of a "half-reduced" state, which corresponded layer.  The formation of this state was  to an adsorbed Pt(OH)  x  said to involve a two-stage  mechanism involving: 1)  a slow one-electron transfer step [Pt(OH) reduce] Pt + x H 0 2  2)  •>  Pt(OH) + x H  +  x  x  i s d i f f i c u l t to  + e  a fast one-electron transfer step l P t ( 0 ) i s e a s i l y reduced] x  Pt(OH) x  ->- Pt(0)  x  + x H  +  + e  Rapid cycling of the electrode would produce a surface covered only with Pt(OH) > x  Consequently, the r a t i o of the charges consumed i n the  anodic and cathodic processes would change from 2 to 1.  (The charge  imbalance of oxygen monolayer oxidation and reduction has been reported  v  . . . .  by many investigators.  53,54,55,56. )  _  Hoare  22,23 ,  _  , ^.  .  has stated that he finds u  i t d i f f i c u l t to accept that the removal of adsorbed hydroxyl r a d i c a l s should be a slow process during reduction of the electrode.  Adams^  noted that there i s not any s p e c i f i c property of the "half-reduced" state which would f a c i l i t a t e  an electron-transfer process.  Chodos and  Meites"^ re-define the "half-reduced" state to correspond to an electrode having a higher concentration of oxygen atoms just beneath the surface than on the surface i t s e l f .  This concept i s discussed i n the following  35 section.  Appleby  does not accept Peldberg's  theory of a c t i v a t i o n as  23 he f i n d s t h a t an a c t i v e s u r f a c e produced by hydrogen r e d u c t i o n a t 500°C (where o x i d a t i o n of the s u r f a c e i s v e r y u n l i k e l y ) i s k i n e t i c a l l y s i m i l a r t o those produced by F e l d b e r g ' s theory  anodic/cathodic  cycling.  He  furthermore r e j e c t s  on the grounds t h a t he f i n d s i t d i f f i c u l t  t h a t an o x i d e f i l m can be  e x a c t l y reduced t o a P t ( O H )  t h i c k n e s s , and because the observed a d s o r p t i o n  x  to b e l i e v e  f i l m of the same  i s not L a n g m u i r i a n as i s 47  expected f o r s p e c i e s whose coverage i s c l o s e t o u n i t y . ered F e l d b e r g ' s theory ity  a t 0.8  stable.  s t a t e i s supposedly  the f a c t t h a t the hydrogen e v o l u t i o n r e a c t i o n can  a c t i v a t e d f o r long times makes i t u n l i k e l y t h a t the e x i s t e n c e half-reduced  consid-  untenable as he observed a r a p i d decay i n a c t i v -  V (N.H.E.), where the " h a l f - r e d u c e d "  Further,  James  of  the  s t a t e i s r e s p o n s i b l e f o r the a c t i v a t i o n . _ 27,28,29,58,59 , „ . ^. . „. ,, Shibata favours the platimzation concept,  where the r e d u c t i o n of an o x i d i z e d p l a t i n u m s u r f a c e r e s u l t s i n the t i o n of a very  t h i n l a y e r of u n s t a b l e  p l a t i n u m , and h a v i n g a h i g h He  atoms, s i m i l a r to  a c t i v i t y and  low  that stable surfaces  long p r e - a n o d i z a t i o n s  f i e d the e l e c t r o d e  r e c r y s t a l l i z a t i o n rate.  rode s u r f a c e  He  oxide which modi-  says t h a t oxygen adsorbed on the e l e c t -  causes rearrangement of s u r f a c e p l a t i n u m atoms and  a f t e r oxygen removal, the p l a t i n u m atoms are l e f t His  pre-  of h i g h a c t i v i t y were produced  (which produced t h i c k s u r f a c e  surface).  forma-  electrodeposited  found t h a t the l i f e t i m e of an a c t i v e s u r f a c e depended on the  o x i d a t i o n time, and by  be  i n unstable  that,  positions.  concept i s based on the assumption t h a t m u l t i l a y e r oxide i s produced  on p l a t i n u m under c e r t a i n c o n d i t i o n s .  I t i s i n t e r e s t i n g to note t h a t  24 James^ f a i l e d to produce an active electrode a f t e r severe anodization and that he rejects a c t i v a t i o n mechanisms involving the formation of 28 oxygen coverages above a monolayer as a necessary countered  step.  Shibata  that a too-severe anodization w i l l drive the electrode i n t o  a "passive" state where the multilayer oxide i s not formed. i f a protective passive f i l m was  That i s ,  allowed to form, the surface would 59  have been protected against further oxidation.  Shibata  also found  that multilayer oxide could not be developed on a w e l l pre-annealed electrode.  He assumes electrode o x i d i z a b i l i t y to be proportional to  the coverage of disordered platinum atoms, and that o x i d i z a b i l i t y decays as the disordered atoms reassume a c r y s t a l l i n e structure.  Measurement  of the o x i d i z a b i l t i y of electrodes annealed at d i f f e r e n t temperatures gave an a c t i v a t i o n energy for the rearrangement of unstable atoms to stable l a t t i c e positions of 11 ± 1 k cal/mole, where both vacancy migration and ad-atom migration can be postulated as mechanisms f o r the s e l f diffusion.  As mentioned before, he a t t r i b u t e s electrode roughening to  the accumulation  of r e s i d u a l s t r a i n created by the disarranged atoms.  2.1.3.5  A c t i v i t y Induced by Dermasorption of Oxygen  The "dermasorption" of oxygen refers to the incorporation of oxygen i n the surface layers of the electrode material.  The concept  was f i r s t introduced i n order to explain the anodic and cathodic charge imbalance f o r the formation and reduction of the oxygen monolayer,^^'^^  25 and to account f o r the thickening of the oxygen layer above monolayer proportions.  Other investigators r e j e c t this notion, saying that the  oxygen:platinum r a t i o could be increased by allowing the surface p l a t inum atoms to undergo a progressive valence change or by permitting  63 them to adsorb more than one oxygen atom.  64 Bagotskii et a l  found that the phenomenon of the penetra-  t i o n of oxygen into the bulk of platinum metal i s much more pronounced on degassed platinum than on cathodically reduced platinum, indicating that absorbed  oxygen  p e r s i s t s at cathodic potentials and may be cap-  able of d i f f u s i n g to the surface and a f f e c t i n g the a c t i v i t y . believes dermasorbed  Chemodanov'  oxygen to be responsible f o r a c t i v a t i o n of the  hydrogen evolution reaction.  Moruet and P e t r i i ^ stated that gas d i s -  solved i n the metal caused d i s t o r t i o n s i n their slow charging curves  2 used to construct the hydrogen adsorption isotherm.  Warner et a l  have  said that oxygen dermasorption, the amount of which determined c a t a l y t i c a c t i v i t y , could occur even when less than a monolayer  of adsorbed oxygen  was present.  35 Appleby  attributes the active platinum surface to the  presence of dissolved oxygen.  He reasons that the high energy of the  platinum l a t t i c e containing i n t e r s t i t i a l oxygen makes i t energetically d i f f i c u l t f o r oxygen to d i f f u s e out through the outer one or two p l a t inum layers which have been reduced.  These surface platinum layers,  after oxide reduction, are thus above a disordered and highly strained platinum-oxygen " a l l o y " l a t t i c e , with the consequence  that the surface  26 i s a high-energy, randomly-oriented platinum layer containing numerous defects.  He also suggests that l a t t i c e oxygen may help reduce induced  heterogeneity e f f e c t interactions, permitting a more Langmuirian adsorpt i o n isotherm to operate than on an annealed surface. Hoare  21,22,23,40,67  . . _ . , . _ , a t t r i b u t e s the enhanced a c t i v i t y of prefc  t  Jt  anodized platinum towards various redox systems to the presence of oxygen dissolved i n the surface layers of the metal. of platinum above 1000 mV  Anodic p o l a r i z a t i o n  (N.H.E.) causes a saturation of the f i r s t  2 or 3 atom layers with oxygen,^8,69,70  subsequent rapid reduction  w i l l leave a bare metal surface containing dermasorbed  oxygen.  He has  termed this a "platinum-oxygen a l l o y " electrode, and finds t h i s " a l l o y " structure to be very r e s i s t a n t to heating to red heat i n a hydrogen flame or to cathodic p o l a r i z a t i o n .  (Electrodes passivated i n concent-  rated n i t r i c acid show high rest potentials i n oxygen-saturated solutions. When reduced, the rest potential drops, but not to values as low as those for untreated electrodes.) Hoare considers that oxygen removed from the surface (layers) must be replaced by oxygen dissolved i n the metal i n t e r ior.  Indeed, once the metal i n t e r i o r i s loaded with oxygen (for example,  by repeated heating followed by quenching i n concentrated n i t r i c acid),, severe cathodization or melting of the l a t t i c e by heating to white heat in a hydrogen flame i s necessary to return the electrode to the untreated condition.  He contends that the " a l l o y " a l t e r s the electronic structure  of platinum with a r e s u l t i n g increase i n the number of holes i n the dband, and that dermasorbed  oxygen e f f e c t i v e l y lowers the electronic work  27 function of the surface, thus f a c i l i t a t i n g electron t r a n s f e r .  He con-  siders that an electrode covered with an adsorbed layer of oxygen i n h i b i t s electron transfer due to an e f f e c t i v e increase i n the work funct i o n caused by the negative dipoles of the metal-oxygen bonds.  2.1.3.6  Altered E l e c t r o n i c Properties  Research has been performed with regard to the r e l a t i o n s h i p between the e l e c t r o n i c properties and the electrochemical behaviour of d i f f e r e n t metal electrodes.  When differences i n the c a t a l y t i c a c t i v i t y  of a single material such as platinum are considered, however, there i s l i t t l e relevant l i t e r a t u r e . t i o n concerning  This i s due to the lack of precise informa-  the e l e c t r o n i c properties of this metal and i t s a l l o y s ,  due i n turn to the incomplete knowledge of the state of the metal surface. Rao  et a l ^ compared the adsorption of oxygen on noble metal  electrodes as a function of the number of unpaired and concluded that the unpaired  d-electrons per atom,  d-electrons participated d i r e c t l y i n  the bonding of oxygen to the metal (each oxygen required two electrons 38 from the d-band).  Mayell and Langer  said that, for a platinum  black  structure, many of the platinum atoms i n corners and cracks would have unshared d-electrons which would thus be a v a i l a b l e for bonding o r b i t a l s . Bockris and Wroblowa^ altered the a c t i v i t y of noble metal electrodes by 40 means of a l l o y i n g i n order to change the e l e c t r o n i c properties. Hoare  28 considers that the presence of dissolved oxygen i n the surface layers of a noble metal electrode w i l l modify the e l e c t r o n i c structure of the metal much i n the same way  as a l l o y i n g with another noble metal.  d i s s o l u t i o n of oxygen into the metal l a t t i c e gives r i s e to a  The  platinum-  oxygen " a l l o y " with an increased number of holes i n the d-band. 72 Srinivasan et a l  f e e l s that the influence of the number of holes i n  the d-band i s most s i g n i f i c a n t i n cases where chemisorbed oxygen i s an intermediate i n the reaction, as the metal-oxygen bond strength v a r i e s with the percent d-vacancy.  For example, i n oxygen reduction, gold  (which has few unpaired d-electrons) shows a s i g n i f i c a n t l y lower exchange current than the platinum metals. Other researchers have compared e l e c t r o c a t a l y t i c properties with the work function of the electrode material.  The e l e c t r o n i c work  function corresponds to the energy required to remove an electron from the metal surface, and i s a s i g n i f i c a n t determining 8 overvoltage.  Bockris et a l  factor i n a c t i v a t i o n  obtained an approximately l i n e a r r e l a t i o n -  ship between the surface energy and the logarithm of the r a t i o of the exchange currents obtained for hydrogen evolution on electrochemically and thermally pretreated electrodes, respectively, for a number of noble metals.  They explained the electrochemical a c t i v a t i o n of pure metals  by allowing the work function at. or near defects to be lower.  An expon-  e n t i a l dependence of the number of defects formed (as a r e s u l t of a given pretreatment procedure) on surface energy would give the observed 73 l i n e a r r e l a t i o n . Bockris et a l also stated that species absorbed i n  29 platinum  could influence the work function and hence i t s electrochemical 35  properties.  Appleby  says the work function i s lower at defects which  reduces the a c t i v a t i o n energy necessary for reactions such as oxygen 74 reduction.  The induced heterogeneity model of chemisorption  associated with the effect of the adsorbed dipole double-layer work function of the metal.  75 '  is  on the  For this model, the adsorption s i t e s  be equivalent, but heterogeneity  may  i s "induced" due to a progressive  change i n the work function (which influences the energy of chemisorption) with increasing adsorption. Recent work by Bagotzky et a l " ^ revealed that neither v a l ency unsaturated surface atoms nor c r y s t a l defects acted as active centers f o r the adsorptive and c a t a l y t i c properties of the platinum  2.1.3.7  surface.  Impurity Removal  While many investigators a t t r i b u t e electrode a c t i v a t i o n to the creation of some d i s t i n c t surface structures, others f e e l that the removal of impurities from the electrode surface i s more l i k e l y to be the mechanism of a c t i v a t i o n .  Impurities on the electrode may  zed, reduced, desorbed, or removed by mechanical means.  be o x i d i -  I t has been  well established that the presence of even trace amounts of impurities can seriously d i s t o r t electrochemical measurements. that the working potential of the electrode may effect of adsorbed impurities, as they may  G i l m a n ^ notes  determine the degree of  be adsorbed over only a  30 s p e c i f i c range of p o t e n t i a l s and may adsorption with species  be f o r c e d o f f due  such as oxygen.  to  competitive  B r e i t e r ^ has  found t h a t  the  oxygen l a y e r formed by chromic a c i d p r e t r e a t m e n t a c t e d  to p r o t e c t  the  s u r f a c e a g a i n s t hydrocarbon and  other  impurity  adsorption.  78 Khazova e t a l  s a i d t h a t a c t i v a t i o n may  s o r p t i o n of adsorbed f o r e i g n p a r t i c l e s . p o t e n t i a l range 0.3  to 0.9  V  be due  t o the  They suggested t h a t , i n  (R.H.E.), a p l a t i n u m  electrode  p o i s o n e d by a n i o n s and  neutral molecules.  Anodizing  w i l l adsorb oxygen and  d i s p l a c e adsorbed o r g a n i c s .  de-  the  is partially  t o , say,  1.1  R e d u c t i o n of  V the  adsorbed oxygen w i l l thus f r e e the s u r f a c e f o r the e l e c t r o c h e m i c a l  re-  a c t i o n of i n t e r e s t . 26  Kronenberg  a t t r i b u t e d a c t i v a t i o n phenomena t o the  of t r a c e m e t a l i m p u r i t i e s from the e l e c t r o d e  surface.  He  noted  o n l y those p r e t r e a t m e n t s which were c a p a b l e of o x i d i z i n g and deposited  m e t a l s produced a c t i v a t i o n .  p o s s i b i l i t y of the f o r m a t i o n  that  removing  Kronenberg f u r t h e r a c c e p t s  of some d i s t i n c t  as the r e s u l t of p r e t r e a t m e n t s , but  removal  considers  type of s u r f a c e  the  structure  such e f f e c t s o n l y  second-  a r y to t h a t of the presence of m e t a l l i c i m p u r i t i e s . B r i e t e r ^ has  s t a t e d t h a t the r e a s o n f o r the charge imbalance  between oxygen monolayer f o r m a t i o n charge i n c l u d e s an o r g a n i c ued  anodic/cathodic  impurity  and  reduction  i s t h a t the  oxidation contribution.  c y c l i n g , the a n o d i c / c a t h o d i c  anodic With c o n t i n -  charge r a t i o approaches  u n i t y as the i m p u r i t i e s are removed. 79 Damjanovic e t a l found t h a t p u r i f i c a t i o n of the working  e l e c t r o l y t e (with or without  the experimental working electrode)  affected the a c t i v i t y of the working electrode. the impurity effect i s at least a r e a l  This i s evidence  that  one.  80 Gilman  attributed the enhancement of anodic currents (for  organic oxidation reactions) a f t e r reduction of a passivated  platinum  surface to either a smaller surface concentration of i n a c t i v e species, or to a more favourable coverage with active intermediates. 2 Warner et a l  said that improvements i n the a c t i v i t y of an  electrode can only occur i n cases where sorbed impurities are present, as they found that conventional anodization methods to enhance a c t i v i t y had no effect i f the electrodes were kept i n a clean system. 47  James  produced a c t i v a t i o n by means of mild anodizations  (surface rearrangement u n l i k e l y ) and through solution p u r i f i c a t i o n . Further, he found anodic/cathodic  treatment produced (temporary) a c t i v a -  t i o n i n impure systems only. 81 Petrii  found that anodic p o l a r i z a t i o n was not  to produce an active electrode, i n which case  necessary  the impurity-desorption  mechanism appears operative. 82 Lu et a l  consider that the r e a c t i v a t i o n of anodes by means  of a superimposed a c t i v a t i n g pulse i s due to the current-dependent r e moval rate of impurities. 83 Shibata cayed i f i t was was maintained  found the a c t i v i t y of an activated electrode de-  stored i n hydrogen- or nitrogen-stirred solutions, but i f stored i n an air-saturated s o l u t i o n . He says i t i s  32  difficult  t o a c c e p t an i m p u r i t y d e a c t i v a t i o n mechanism, and t h a t t h e  a c t i v e s t a t e i s i n f l u e n c e d by t h e a d s o r p t i o n o f oxygen, which m a i n t a i n s the m e t a l atoms i n u n s t a b l e ( a c t i v e )  2.1.4  positions.  Summary  The e s t a b l i s h m e n t o f t h e " a c t i v e " e l e c t r o d e c o n d i t i o n  shall  be taken t o i n v o l v e the p r o d u c t i o n o f a r e p r o d u c i b l e s u r f a c e c o n d i t i o n which produces  a h i g h degree o f e x p e r i m e n t a l r e p r o d u c i b i l i t y .  No d i s -  t i n c t i o n w i l l be made c o n c e r n i n g the enhancement o f r e v e r s i b l e b e h a v i o u r as a r e s u l t o f p r e t r e a t m e n t , as i t i s b e l i e v e d t h a t i t i s n o t p o s s i b l e to a d e q u a t e l y s e p a r a t e those procedures which promote " a c t i v a t i o n " other procedures.  from  I t i s c o n s i d e r e d t h a t the p r o d u c t i o n o f the r e p r o d u c -  i b l e surface i s a s u f f i c i e n t  task i n order to provide meaningful  sons among i n d i v i d u a l experiments.  compari-  I t i s u s e f u l , however, t o take  account o f the p o s s i b l e e f f e c t s o f t h e v a r i o u s p r e t r e a t m e n t s and t h e experiments  themselves, on the e l e c t r o d e s .  These a r e o u t l i n e d i n  Appendix I f o r r e l e v a n t procedures and t h e e l e c t r o d e s o f i n t e r e s t .  33 2.2  SURFACE AREA OF NOBLE METAL ELECTRODES  I t i s not r e a l i s t i c  to presume t h a t the geometric area  an e l e c t r o d e i s e q u i v a l e n t to i t s t r u e s u r f a c e a r e a . ished surfaces contain i r r e g u l a r i t i e s ,  probable  value.  to c o n v e r t  Experience  a r e a of an e l e c t r o d e may electrodes  has  Even h i g h l y p o l -  such as s c r a t c h e s , which w i l l  c o n t r i b u t e to an i n c r e a s e i n the a r e a . i s proposed i n o r d e r  G e n e r a l l y , a "roughness f a c t o r "  the apparent s u r f a c e a r e a  to a more  shown, however, t h a t the t r u e  not have a c o n s t a n t v a l u e .  ( f o r example, p l a t i n u m w i r e s ) may  surface  Furthermore,  have g r e a t l y v a r y i n g  of s u r f a c e a r e a , depending on t h e i r pretreatment  histories.  2.2.1  similar values  There a r e ,  however, s e v e r a l e l e c t r o c h e m i c a l procedures a v a i l a b l e which can accurate, reproducible surface  of  give  areas.  Bases f o r E l e c t r o c h e m i c a l S u r f a c e Area Measurement  E a r l y research i n t o charging determined t h a t , when an a n o d i c  phenomena on p l a t i n u m  p u l s e was  a p p l i e d to an e l e c t r o d e at  c a t h o d i c p o t e n t i a l s , the r e s u l t i n g charge curve f o r i o n i z a t i o n of adsorbed hydrogen and L a t e r , p o t e n t i a l sweeping techniques  electrodes  showed s e p a r a t e  a d s o r p t i o n of oxygen.  regions  83 84 85 ' '  were developed which a l s o r e v e a l e d 18  separate I f i t was  oxygen and hydrogen r e g i o n s on the c u r r e n t / p o t e n t i a l c u r v e s . assumed t h a t the s o l e process  o c c u r r i n g i n the  r e g i o n s of the charge or sweep curves was removal) or hydrogen i o n i z a t i o n  one  '  appropriate  of oxygen a d s o r p t i o n  (or d e p o s i t i o n ) , and  86  t h a t such  (or  processes  34 i n v o l v e d the f o r m a t i o n (or removal) of complete monolayers  o f these  s p e c i e s (with a one-to-one correspondence w i t h s u r f a c e m e t a l  atoms),  then knowledge of the number of p l a t i n u m atoms per square c e n t i m e t e r ( c a l c u l a t e d from c r y s t a l l o g r a p h i c d a t a ) , the charge o f an e l e c t r o n ,  and  the v a l e n c e of the a d s o r b i n g s p e c i e s i s a l l t h a t i s n e c e s s a r y t o a r r i v e a t a v a l u e of the t r u e s u r f a c e a r e a .  Measured v a l u e s o f the a c t u a l  charge consumed i n the oxygen o r hydrogen converted i n t o s u r f a c e areas. encountered  p r o c e s s e s can thus be  readily  However, t h e r e a r e s e v e r a l problems  i n such d e t e r m i n a t i o n s which render the measurements much  more d i f f i c u l t .  2.2.1.1  C o m p l i c a t i o n s Due  t o Simultaneous  Processes  U n f o r t u n a t e l y , i t i s not p o s s i b l e to s e p a r a t e o t h e r chargeconsuming p r o c e s s e s from the r e a c t i o n of i n t e r e s t . presence of an unknown o x i d i z a b l e i m p u r i t y may  F o r example, the  cause the q u a n t i t y of  charge consumed i n the anodic d e p o s i t i o n of an oxygen monolayer t o be increased.  S c h a l d i n e r and W a r n e r ^ and Thacker and H o a r e ^  studied  the e f f e c t of the presence of i m p u r i t i e s on the shape of the oxygen 24 r e g i o n of the charge c u r v e .  Gilman  found t h a t s o l u t i o n a n i o n a d s o r p -  t i o n i n phosphoric a c i d e l e c t r o l y t e p a r t i a l l y blocked surface o x i d a t i o n . 88 — Gilman a l s o found t h a t adsorbed i m p u r i t y a n i o n s (such as C l ) d i d not significantly affect  the hydrogen  24 electrode.  Gilman  adsorption capacity of a platinum  88 '  has developed s u i t a b l e p r e t r e a t m e n t  to remove i m p u r i t i e s from the e l e c t r o d e s u r f a c e .  procedures  In a d d i t i o n , he  has  35 found that the use of f a s t techniques  (high current density  charging,  high sweep rates) could help i n avoiding contaminant e f f e c t s .  He  has  calculated that the times f o r contamination of a previously clean e l e c trode surface (1% coverage with impurities), assuming d i f f u s i o n control, are 50 seconds and 2 seconds i n unstirred and s t i r r e d solutions respect62 ively.  G i l r o y and Conway  also recommend surface area estimation be  attempted as soon as possible a f t e r cleaning the electrode surface. The p o s s i b i l i t y also e x i s t s that the processes of hydrogen i o n i z a t i o n and oxygen adsorption, oxygen adsorption and evolution, oxygen reduction and hydrogen deposition, hydrogen deposition and hydrogen evolution, and hydrogen evolution and i o n i z a t i o n may simultaneously.  occur  The good separation of the hydrogen and oxygen regions  on the anodic charge curves i n acid solution i s good evidence that these processes are indeed separate. the oxygen layer on platinum  On cathodic reduction curves, however,  i s reduced at potentials much closer to  the hydrogen deposition p o t e n t i a l s . Good separation i s s t i l l found for 89 this case i n acid s o l u t i o n . Giner found that he could detect hydrogen evolution (on a PTFE-bonded platinum electrode) at up to 200 mV above 33 the r e v e r s i b l e hydrogen p o t e n t i a l . evolution above 0.10  Biegler et a l  found hydrogen  V (R.H.E.) to be i n s i g n i f i c a n t on both smooth and  p l a t i n i z e d electrodes.  The hydrogen deposition and evolution regions  overlap, however, which can lead to some d i f f i c u l t y i n separating the 88 33 respective charges. ' An analogous s i t u a t i o n can also be found with 88 oxygen adsorption and evolution. Depending on the measurement scheme  36  employed, i t i s n e c e s s a r y s u i t a b l e pretreatments  to make a l l o w a n c e s f o r - or t o e l i m i n a t e  - any  by  c o m p l i c a t i n g simultaneous e l e c t r o c h e m i c a l  reactions.  2.2.1.2  The  Compensation f o r Double Layer  Charging  double l a y e r i s charged when a c u r r e n t i s passed  the s o l u t i o n / e l e c t r o d e boundary.  I f no  across  e l e c t r o n t r a n s f e r occurs,  a l l the c u r r e n t i s used i n t h i s c h a r g i n g .  83  The  development of  then  the  84 double l a y e r o c c u r s a t a l l p o t e n t i a l s ,  but  i s most obvious on  or sweep curves between the hydrogen and  oxygen r e g i o n s .  charge  Some workers  have attempted to use measurements of the c a p a c i t y of the double l a y e r as i n d i c a t o r s of t r u e e l e c t r o d e s u r f a c e a r e a . ^ ' ^ ' ^ ' ^  However,  due  to the g r e a t d i f f i c u l t y i n s e p a r a t i n g the double l a y e r charge from  other  charge-consuming p r o c e s s e s , the r e s u l t s a r e s u b j e c t to g r e a t u n c e r t a i n t y . 84 93 S l y g i n and Frumkin and S c h u l d i n e r and Roe noted the p r e s e n c e of hydrogen on the e l e c t r o d e i n the s o - c a l l e d " d o u b l e - l a y e r " r e g i o n of charge c u r v e .  The  l a t t e r authors  suggest t h a t o n l y about a t h i r d  the c u r r e n t i n t h i s r e g i o n a c t u a l l y goes to c h a r g i n g Formaro and  Trasatti  94  also find  and  Schuldiner  of  of the double l a y e r .  t h a t adsorbed oxygen i n t e r f e r e s  c a p a c i t a n c e measurements a t p o t e n t i a l s as low as 500 mV 95  the  with  (R.H.E.).  Rosen  96 '  found t h a t r e l i a b l e double l a y e r charge v a l u e s  could  be o b t a i n e d o n l y w i t h c h a r g i n g p u l s e s of extremely s h o r t (<200 nsec.) d u r a t i o n . T h i s work can be assumed t o supercede s i m i l a r i n v e s t i g a t i o n s w i t h p u l s e s of l o n g  (5 ysec.) d u r a t i o n ,  93  although  the e a r l i e r work  may  contain valuable information concerning the charge consumed i n unknown (impurity) processes.  2.2.1.3  Monolayer Formation  Charging or sweep methods presume that monolayer formation takes place from a state of zero surface coverage to one of monolayer coverage (and reduction or i o n i z a t i o n processes involve the reverse), and that points on the curves can be i d e n t i f i e d with these states, making c a l c u l a t i o n of the charge consumption an easy task.  The area  determination techniques based on hydrogen or oxygen processes r e l y on the assumption that these species form a f u l l monolayer coverage (that i s , a one-to-one correspondence between the adsorbed species and surface metal atoms).  Rao et a l ^ found that the coverage of noble  metal electrodes with oxygen-containing species at open-circuit i n oxygen-saturated  IN l^SO^ solution was f a r from complete.  and iridium, the coverages were 27% and 84%, respectively.  For platinum Thus, the  use of measurement techniques which involve oxygen deposition at openc i r c u i t i s not recommended.  The problem s t i l l exists, however, as to  whether or not the coverage obtained during charge or sweep methods actually can be considered to correspond to a monolayer. sorption stoichiometry may  While chemi-  suggest this r e l a t i o n , i t has yet to be  88 proved conclusively.  Perhaps the best evidence i s the observation  that the hydrogen monolayer charge i s found to be very close to half the oxygen monolayer charge, as would be expected for a one-electron  38 76  88  hydrogen process and a two-electron oxygen process. Gilman ' found the hydrogen monolayer charge to be independent of sweep speed, but that a slow v a r i a t i o n of the oxygen monolayer charge with sweep speed existed, i n d i c a t i n g that possibly gas evolution occurred before mono97 layer oxygen coverage was monolayer deposition was  obtained.  Schuldiner  98 '  found that oxygen  complete when the p o t e n t i a l reached about  1.5 V (N.H.E.). 2.2.1.4  Absorption  The possible absorption of oxygen into the i n t e r i o r of a 99 platinum  electrode was  first  suggested by Kalish and Burshtein.  100 '  Oxygen absorption has been proposed as an explanation for the difference between the charges measured for oxygen layer formation and 22 23 (see Hoare  '  reduction  68 ).  Hoare  showed that oxygen could d i f f u s e through a  platinum bi-electrode (oxygen evolution on one side, hydrogen on the other; the bi-electrode was  a platinum  f o i l which completely  the two c e l l halves) and a f f e c t the hydrogen overpotential. bi-electrode configuration was  separated I f the  changed such that the f o i l acted as a  diaphragm, the e f f e c t s of the oxygen dissolved i n the metal persisted for  long times afterward.  Thacker and H o a r e ^ found that "dermasorbed"  oxygen could be resolved as a high-overpotential reduction wave on a galvanostatic s t r i p p i n g curve.  They also point out that, i f an e l e c t -  rode i s so-stripped and then l e f t at open-circuit, the p o t e n t i a l r a p i d l y returns to a high anodic value.  A second s t r i p p i n g curve resulted i n 1  39 the d e t e c t i o n of b o t h s u r f a c e and dermasorbed oxygen.  Only w i t h r e -  peated p u l s i n g i n t h i s manner c o u l d a l l t h e d i s s o l v e d oxygen be removed. It  i s c l e a r , however, t h a t s u r f a c e s i t e s can be f i l l e d  d i f f u s i o n from t h e i n t e r i o r o f t h e m e t a l .  Schuldiner  from oxygen e t a l " * " ^ says  t h a t as many as t h r e e o r f o u r monolayers o f oxygen atoms c a n be dermasorbed  a t h i g h p o t e n t i a l s , and t h a t dermasorbed oxygen appears a t p o t -  e n t i a l s above 1.2 - 1.3 V (N.H.E.).  The a b s o r p t i o n o f hydrogen has been 102  less extensively considered.  S c h u l d i n e r and Warner  d i s c u s s both  hydrogen and oxygen a b s o r p t i o n a s p o s s i b l e causes f o r excess charge consumption i n t h e hydrogen and oxygen r e g i o n s . Other workers suggest t h a t , r a t h e r than a b s o r p t i o n , t h e f o r m a t i o n o f o x i d e s o f d i f f e r i n g t h i c k n e s s e s and/or changes i n t h e formal valences  can e x p l a i n ( o r produce)  monolayer c h a r g e . ^ * ^ b l i s h e d concerning  d e v i a t i o n s from t h e d e s i r e d  To date, no c o n c l u s i v e proof has been e s t a -  the existence of surface oxides of platinum  the c o n d i t i o n s e x p e r i e n c e d  d u r i n g a r e a measurements.  under 103  Bockris et a l  notes t h a t X-ray d i f f r a c t i o n does n o t d e t e c t p a t t e r n s f o r o x i d e s ,  even  a f t e r h i g h - p o t e n t i a l a n o d i z a t i o n s . While t h i s c o u l d be due t o the o f a c t t h a t o n l y s m a l l (<50 A) patches o f o x i d e a r e formed, o r t h a t the oxide l a t t i c e i s so h i g h l y d i s t o r t e d as t o become amorphous, t h e p o s s i 33 63 bility  that chemisorption  occurs  i s more l i k e l y .  B i e g l e r and Woods  '  say t h a t d e r m a s o r p t i o n e f f e c t s c o u l d a l s o be produced by a l l o w i n g t h e s u r f a c e t o be capable inum atom.  o f a d s o r b i n g more than one oxygen atom per p l a t -  They a l s o r e j e c t t h e p o s s i b i l i t y o f phase o x i d e  formation  40 as they f i n d a l i m i t i n g value of the oxygen charge with p o t e n t i a l .  104 Vetter and Schultz  oppose this assumption,  saying that the surface  layer i s an oxide which grows continuously with time and p o t e n t i a l . 2.2.1.5  Surface Atom Density  In order to compare surface area values among d i f f e r e n t electrodes, one must assume that their surface c r y s t a l l i n e states or combinations of c r y s t a l planes are the same.  Indeed, i t i s necessary  to know the surface c r y s t a l l i n e state i n order to determine the true electrode area, as the packing densities of the i n d i v i d u a l atom planes are s i g n i f i c a n t l y d i f f e r e n t .  The atom densities of the major planes  of the f . c . c . l a t t i c e are:  (100)  1.30 x 1 0  (110)  0.93 x 1 0  (111)  . „ 15 1.51 x 10  15  Pt atoms/cm.  2  15  3  8  3 8  38  in  in.  i  i  or  1.31 x 1 0  0.92 x 1 0 . _ 1.5 * i  lfV  Pt atoms/cm.  15  2  15  15 85 105 *  have been used.  1  3 1  L5  31,101  x 10  m!5 31,70,84,93,101  For p o l y c r y s t a l l i n e platinum, values of 1.31 x 10 1.6 x 10  3  92  Feltham and Spiro  .  and  note that there  i s some d i f f i c u l t y i n determining the correct packing density.  Biegler  33 et a l  say that the d e f i n i t i o n of surface atoms f o r the (110) plane  and f o r higher-index planes i s not c l e a r , depending on whether surface atoms are defined as those with coordination 7, or those with coordination 11 as well as 7.  41 The charge r e q u i r e d t o form a complete monolayer gen  of hydro-  (or oxygen) can i n t u r n be computed from t h e v a l u e o f the e l e c t r o n -19  charge (1.6 x 10  coul.).  I f i t i s assumed t h a t one  single-valence  hydrogen atom (or one d o u b l e - v a l e n c e oxygen atom) adsorbs per  site,  then the r e s u l t a n t charges a r e as r e p r e s e n t e d i n T a b l e 1. TABLE 1 Hydrogen  and oxygen monolayer  and i r i d i u m e l e c t r o d e s .  C r y s t a l Plane Pt  Pt  Pt  (100)  (110)  (111)  Average of the 3 planes Polycrystalline  charges f o r p l a t i n u m  [] denote c a l c u l a t e d  Hydrogen Monolayer Charge ycoul./cm.^  value.  Oxygen Monolayer Charge ycoul./cm.^  Reference  [209]  418  38  [210]  420  31  208  [416]  33  [149]  298  38  [147]  294  31  147 o r 295*  [294 or 590]*  33  [242]  484  241  [482]  199 or 248*  [398 or  101,38,31  496]  33 33  210  [420]  24,33,18,86  [250]  500  7,103  [210]  420  70,93,101  [256]  512  Polycrystalline  220  [440]  86  iridium  [262]  525  7,103  platinum  105  The coverage of a l l o y e l e c t r o d e s can be expected to v a r y l i n e a r l y w i t h , . ^103 atomic p e r c e n t .  * depending on d e f i n i t i o n o f s u r f a c e atoms.  42  B i e g l e r and Woods  63  have p o i n t e d out t h a t t h e r e are no s t e r i c  l i m i t i n g oxygen c h e m i s o r p t i o n . s u r f a c e atom i s about 7.5  The  average a r e a o c c u p i e d by  °2 A , whereas the diameter o  oxygen i s no more than 1.5  A.  L a s t l y , i t s h o u l d be  factors a platinum  of c o v a l e n t l y bonded  c o n s i d e r e d t h a t sur-  f a c e atoms may  r e s i d e i n s t r u c t u r e s which need not correspond 33 c a l c u l a t e d from the b u l k u n i t c e l l .  2.2.2  Procedures  2.2.2.1  to  those  f o r S u r f a c e A r e a Measurement  P o t e n t i a l Sweep Techniques  By v a r y i n g the p o t e n t i a l of an e l e c t r o d e i n a l i n e a r manner w i t h time, a r e s u l t a n t t r a c e of c u r r e n t v s . p o t e n t i a l i s o b t a i n e d . a r e a under t h i s curve i s e q u i v a l e n t t o the charge consumed d u r i n g trace.  As  charge.  8 8  and T h a c k e r ^ ^ ' h a v e  can be e l u c i d a t e d .  c o n s i d e r e d the e s t i m a t i o n of the oxygen  U n f o r t u n a t e l y , w i t h the sweep technique  a d s o r p t i o n i s obscured by  the endpoint  of oxygen  the commencement of oxygen gas e v o l u t i o n .  Hence, e x t r a p o l a t i o n s must be and gas  this  the hydrogen and oxygen r e g i o n s are w e l l - s e p a r a t e d i n a c i d i c  e l e c t r o l y t e s , hydrogen or oxygen monolayer charges Gilman  The  evolution currents.  c o n s i d e r e d f o r the monolayer  formation  Most p o t e n t i a l sweep techniques  f o r surface  a r e a measurement e x p l o i t the hydrogen d e p o s i t i o n r e g i o n of the sweep curve.  24,33,41,76,86,88,106 » » » » » »  T  i  t  . . . . . «. t h i s case, however, the endpoint  n  hydrogen d e p o s i t i o n i s masked by  of  the onset of m o l e c u l a r hydrogen e v o l u -  t i o n as the sweep proceeds from anodic to c a t h o d i c p o t e n t i a l s .  The  43  assumption of an a r b i t r a r y  ( f i x e d p o t e n t i a l ) endpoint  i s n o t recoramend-  33 ed  as t h i s g i v e s d i f f e r e n t monolayer charge v a l u e s f o r smooth p l a t i n u m  as opposed to p l a t i n i z e d p l a t i n u m  (whose sweep curves v a r y somewhat).  S e v e r a l c o n s t r u c t i o n s have been a p p l i e d t o sweep curves i n o r d e r t o determine some r e p r o d u c i b l e s o r t of endpoint  2A  o r t o determine the 88 33 e x t r a p o l a t e d hydrogen d e p o s i t i o n and e v o l u t i o n c u r r e n t s . ' Such c o n s t r u c t i o n s , along w i t h the measurement of areas under the c u r v e s , make t h i s method somewhat t e d i o u s . 24 Gilman  62  88 '  recommends t h a t f a s t sweep r a t e s be  employed  ( f o r example, 50 v o l t s per second f o r hydrogen d e p o s i t i o n ; 1000  volts  per second f o r oxygen a d s o r p t i o n ) i n o r d e r to reduce contaminant e f f e c t s . The h i g h e r sweep speed f o r the oxygen case i s n e c e s s a r y and oxygen monolayer charges coincide.  i f the hydrogen  ( c o r r e c t e d f o r v a l e n c e d i f f e r e n c e s ) are to  An analogy w i t h the g a l v a n o s t a t i c c h a r g i n g curve case becomes  apparent, where h i g h c u r r e n t d e n s i t y p u l s e s are employed i n o r d e r to prevent  a b s o r p t i o n of oxygen i n t o the e l e c t r o d e . 24 Gilman  has  developed  remove s u r f a c e i m p u r i t i e s and determination.  pretreatment  A p o t e n t i a l s t e p sequence i s employed whereby the  p r i o r to sweep a p p l i c a t i o n  ( e i t h e r anodic  V - 1.55  to  V - 0.4  V  elect-  (R.H.E.)  or c a t h o d i c sweeps) f o r d e f i n -  In t h i s manner, i m p u r i t i e s are desorbed  c u l a r oxygen i s swept away, and state.  designed  to create a reproducible surface f o r area  rode i s exposed to the p o t e n t i a l s 0 V - 1.8  i t e times.  procedures  or o x i d i z e d , mole-  the s u r f a c e i s reduced  to a reproducible  44  2.2.2.2  Galvanostatic Charge Technique  Application of an anodic galvanostatic pulse to an electrode whose p o t e n t i a l i s i n i t i a l l y at the hydrogen p o t e n t i a l w i l l r e s u l t i n a p o t e n t i a l vs. time trace showing four regions:  hydrogen i o n i z a t i o n ,  double layer charging, oxygen adsorption, and oxygen evolution.  An  example of t h i s type of curve, with the d e t a i l s of i t s analysis, i s found i n Appendix I I . Application of a cathodic galvanostatic pulse to an electrode at anodic potentials w i l l show s i m i l a r regions for the r e verse processes.  Measurement of hydrogen monolayer deposition or r e -  moval charges i s not considered r e l i a b l e due to problems i n determining 31. 33  the end potentials of hydrogen i o n i z a t i o n and deposition,  '  84  '  93  *  the oxidation of hydrogen dissolved i n the metal or present on the sur102  face,  the p o s s i b i l i t y of molecular hydrogen evolution at potentials 89  anodic to the R.H.E.,  92  '  and the interference of dermasorbed oxygen  reduction. Most galvanostatic pulse methods are employed to determine „,  ,  i  ,2,9,31,70,85,92,93,97,98,101,102  the oxygen adsorption charge (anodic pulse) _  _  ,  .  _  i  *  * j  .  i  % 38,70,89,92,  or the charge for removal of the oxygen layer (cathodic pulse). 104,107  obtaining the " t r a n s i t i o n times" for oxygen monolayer  formation  or d i s s o l u t i o n , i t i s then a simple matter to calculate the charge consumed, and hence a r r i v e at an estimation of the active surface area. 2 14 101 102 Schuldiner, Warner et a l ' ' ' f i n d that for high current density 2  anodic pulses (1-7 A/cm.  ) only surface-adsorbed oxygen i s formed.  Thacker and H o a r e ^ f i n d smaller current pulses (4.1 mA) w i l l give s a t i s f a c t o r y r e s u l t s provided the electrode i s pre-saturated with dermasorged oxygen. In order to obtain an anodic  (or cathodic) charging  curve,  i t i s required that the i n i t i a l electrode p o t e n t i a l be more cathodic (or anodic) than the range of potentials of i n t e r e s t .  The p o t e n t i a l  can be lowered to cathodic values by operating i n hydrogen-saturated 70 93  solution  '  or by potentiostatting the electrode to the desired pot-  e n ti i a l 31,97,98 followed by fast (e.g., 1 ysec.) switching to the anodic current pulse. Strong pretreatments  are sometimes employed to saturate the  metal with dissolved oxygen, such as pre-anodization at 1.8 v (N.H.E.) for times of 5 minutes or more.^''''^  Thacker and Hoare found that pre-  anodization at about 2000 mv (N.H.E.) was s u f f i c i e n t to cause a separation of arrests ( i n the cathodic stripping curve) between surface- and derma-sorbed oxygen.  The slow removal of dermasorbed oxygen can be  exploited by the use of hydrogen bubbling after strong anodization i n order to carry the electrode to the desired i n i t i a l cathodic p o t e n t i a l . The oxygen subsequently  deposited during the anodic charging pulse w i l l  remain on the surface as the dermasorbed layers are f i l l e d with oxygen from the metal i n t e r i o r . ^ Double layer charge compensation i s a rather d i f f i c u l t problem with the galvanostatic charge technique.  A fixed value of the  capacitative charge can be assumed and subtracted from the o v e r a l l  46  measured charge.  14  Rosen and Schuldiner  95  have proposed that double  layer charge values obtained from long-pulse measurements be used when i t i s desired to include unknown adsorption/desorption processes,  and  that the more-reliable short-pulse values be used at p o t e n t i a l s where such unknown processes are considered u n l i k e l y . The slope of the "double layer region" i n the charge curve may 31 70 92 double layer charge. cribed i n Appendix I I .  '  '  be used to obtain a value of the  107 '  The appropriate constructions are des-  I t must be remembered, however, that this "double  layer slope" contains contributions form hydrogen i o n i z a t i o n , oxygen 93 adsorption, and unknown charge-consuming processes.  Schuldiner and  Roe  found that the hydrogen i o n i z a t i o n complication could be removed by prepotentiostatting at potentials above 0.35  v (N.H.E.), r e s u l t i n g i n a  "double layer region" with a much steeper slope, indicating a smaller amount of charge being consumed i n t h i s region. 2.2.3  Summary It i s c l e a r that, regardless of the method chosen for deter-  mination of the surface area, the value obtained does not a c t u a l l y r e present a true measure of the electrochemically active surface area. It i s nevertheless, quite meaningful, and i f the assumptions made i n the computation of this value are correct, then the surface area measurement could correspond  to that which a c t u a l l y e x i s t s .  In actual fact,  the measured surface areas can be termed "determined surface areas" (the  47 notation "apparent  surface area" i s assumed to apply to measurements  of the area from the geometry of the electrode) and i t i s i n this r e gard with which the surface area values reported i n this thesis must be taken.  Consequently,  i t should be remembered that any reported sur-  face area values are s t r i c t l y dependent on the assumptions made and on the method employed.  Correlation among the r e s u l t s of the various r e -  searchers who have used d i f f e r e n t assumptions and/or methods can only be v a l i d when these are known.  48 2.3  ELECTROLYSIS OF CHLORIDE SOLUTIONS  In t h e e l e c t r o l y s i s o f c h l o r i d e - c o n t a i n i n g  .  A  <  108,109,110,111,112  primary anode r e a c t i o n i s :  '  t  2C1  The  s o l u t i o n s , the  '  '  '  C l + 2e 2  a n o d i c f o r m a t i o n o f oxygen H„0  ±  2H* + ~ 0„ + 2e  i s n e g l i g i b l e , except i n the case o f v e r y d i l u t e b r i n e s . However, the study o f the a n o d i c p r o c e s s e s i n c h l o r i d e e l e c t r o l y l e s i s c o m p l i c a t e d by t h e p o s s i b i l i t y o f s e v e r a l r e a c t i o n s i n the e l e c t r o l y t e .  secondary  These i n c l u d e , i n n e u t r a l o r a c i d e l e c t -  113 r o l y t e , c h l o r i n e h y d r o l y s i s which may o c c u r i n the d i f f u s i o n l a y e r :  C 1  Further,  2  hypochlorite  +  H  2°  H  C  1  °  +  H  +  +  C  1  i o n s may be generated:  HC10  t  H  + CIO"  +  Hypochlorite  p r o d u c t i o n i s f a v o u r e d i n e l e c t r o l y t e s of n e u t r a l pH.  hypochlorous  a c i d can be n e u t r a l i z e d by OH  t  H0C1 + OH  Hypochlorite  CIO  +  ions  6C10" + 3H„0  t  2  possible  2C10 " + 4C1~ + 6H o  (produced a t the cathode)  H 0  d i s c h a r g e a t the anode becomes  The  + # 0  o  + 6e  49 Thus, a steady-state s i t u a t i o n can be v i s u a l i z e d where hypochlorite formed from the secondary solution reactions i s balanced by the amount discharged at the anode.  In more-acid e l e c t r o l y t e s , chlorate ion  (ClO^ ) can be produced by means of the chemical decomposition chlorite.  The addition of H  of hypo-  to a neutral (CIO -containing e l e c t r o l y t e )  +  results i n the formation of H0C1 which then oxidizes the remaining  CIO  to C10 ~: 3  2H0C1 + 0C1~ The H  +  t  c  l° " +  +  2 H +  3  2 C  1~  so-produced leads to the production of further. H0C1,  i s formed and repeated u n t i l a l l the CIO can be suppressed  at low temperatures.  i s exhausted.  and a cycle  This reaction  The presence of ClO^  ions  (either from hypochlorite discharge or decomposition) leads to a further possible anode reaction: CIO ~ + H.O •3 2  t  CIO ~ + 2H 4  +  + 2e .  This reaction occurs, however, with considerable overpotential and be ignored as long as the chloride content i s s i g n i f i c a n t .  can  Thus, ClO^  can be considered to b u i l d up i n concentration during the course of electrolysis. In hydrochloric acid solutions, the formation of the C l ^ -, ion must also be  , - 114,115 considered: Cl„ + C l " t  Cl ~  50  In a d d i t i o n , itself  the p o s s i b i l i t y of d i s s o l u t i o n of the anode  e x i s t s , and i s i n f a c t , more l i k e l y t o o c c u r i n complexing  e l e c t r o l y t e s such as those c o n t a i n i n g I t i s c l e a r , thus, that  chloride.  c a r e must be taken t o e i t h e r p r e -  v e n t o r account f o r the p o s s i b i l i t y o f secondary c h e m i c a l and e l e c t r o chemical r e a c t i o n s  i f m e a n i n g f u l r e s u l t s a r e t o be o b t a i n e d from  i e s o f the p o l a r i z a t i o n of n o b l e m e t a l e l e c t r o d e s  2.3.1  stud-  i n c h l o r i d e media.  P o l a r i z a t i o n of Smooth Noble M e t a l Anodes  The a n o d i c c u r r e n t / p o t e n t i a l r e l a t i o n i n c h l o r i d e media i s characterized  by a " p o t e n t i a l jump" s e p a r a t i n g  slower p o l a r i z a t i o n .  The p o t e n t i a l jump i s s h i f t e d t o h i g h e r  densities with increasing perature.  two r e g i o n s o f much  chloride i o n concentration  current  a c i d i t y , and tem-  Since 1966, the b e h a v i o u r o f noble m e t a l anodes i n such  e l e c t r o l y t e s has been the s u b j e c t oxygen e v o l u t i o n  2  E  =  t  0  2  + 4H  +  TOO  reaction:  2.3 RT „ 2.3 RT _ 1.23 - • pH + — r | ; — § P°2 »  2C1~ =  Although  + 4e  l o  the predominant r e a c t i o n i s t h a t of c h l o r i n e  E  study.  i s the thermodynamically f a v o u r e d 2H 0  _  of a c c e l e r a t e d  t  1.358 +  evolution:  C l + 2e 2  log C 1 P  2  - i ^ J T ^  g  ^  51 Thermodynamics would also favour platinum d i s s o l u t i o n i n chloride media, but the reactions are k i n e t i c a l l y n e g l i g i b l e (comparatively) due to the high a c t i v a t i o n energy.  In chloride-free media (such as 1M I^SO^),  the only anodic product at normally encountered potentials i s oxygen. The " p o t e n t i a l jump" i s not a c h a r a c t e r i s t i c of the p o l a r i z a t i o n curve i n such media. Van L a e r ^ ^ attributed the i n f l e c t i o n i n the current/potenti a l r e l a t i o n to the transformation of Pt 0^ to Pt 0^, which was said to involve an increase i n the overpotential f o r chloride oxidation. conclusion was derived from the potential-pH  This  diagram f o r platinum,  where: Pt 0 + H 0 2  E Pt 0  Q  =  + H0  2  2  E  Q  =  =  Pt 0  + 2H + 2e~ +  2  1.045 - .059 pH =  Pt 0  + 2H + 2e~ +  3  2.000 - .059 pH  At the pH's found i n synthetic sea water (pH 6-7), the Pt O^/^t 0^ equilibrium p o t e n t i a l i s thus E  Q  = 1.336 - 1.395, which corresponds  nearly exactly to the p o t e n t i a l of the i n f l e c t i o n i n the p o l a r i z a t i o n curve. Suzuki et al''"'^ studied the p o l a r i z a t i o n behaviour of p l a t inum i n saturated  chloride solution of pH 3.  They found that the oxide  f i l m tended to dissolve when the electrode was kept standing i n the e l e c t r o l y t e after anodization.  They explained  t h i s by means of a  52  " l o c a l c e l l " s i t u a t i o n whereby the o x i d e f i l m was inum o r a lower o x i d e . e l e c t r o d e was  T h i s theory was  cathodic to bare  plat-  confirmed when the a n o d i z e d  s h o r t - c i r c u i t e d w i t h an u n o x i d i z e d p l a t i n u m e l e c t r o d e i n  the s o l u t i o n , where the r e s t p o t e n t i a l decreased remarkably w i t h  time.  T h i s l e d t o the c o n c l u s i o n t h a t the oxide f i l m formed by o x i d a t i o n does not c o m p l e t e l y cover the p l a t i n u m s u r f a c e .  They a t t r i b u t e d  the  two  o v e r p o t e n t i a l r e g i o n s of the p o l a r i z a t i o n curve t o the f a c t t h a t  the  s u r f a c e i s Pt 0 f o r the lower curve and Pt 0^ f o r the upper c u r v e .  118 L i t t a u e r and  Shrier  p o i n t e d out t h a t c h l o r i d e and oxygen  s p e c i e s undergo c o m p e t i t i v e a d s o r p t i o n , w i t h oxygen e v e n t u a l l y r e p l a c i n g c h l o r i d e on the e l e c t r o d e s u r f a c e w i t h i n c r e a s i n g a n o d i c The  l o w - o v e r p o t e n t i a l r e g i o n of the p o l a r i z a t i o n curve was  to i n v o l v e C l  At the p o t e n t i a l jump, the  of oxide f o r m a t i o n or oxygen a d s o r p t i o n was  evolution  o x i d a t i o n of platinum.  suggested  The k i n e t i c s of the  , + Cl" ad  ->  overcome, chlorine  Cl„ + e 2  would thus be d i f f e r e n t on the oxygen-covered of the "jump", i t was  suggested  surface.  The  e f f e c t of the oxide f i l m was  the a c t i v e s u r f a c e or complete  After  t h a t the s u r f a c e developed  l a y e r and/or c o n s o l i d a t i o n of a p r e v i o u s l y chemisorbed  riers  to be  inhibition  reaction: Cl  layer.  considered  d i s c h a r g e on a c l e a n s u r f a c e , or one h a v i n g o n l y a chemi-  sorbed Pt-0 and/or P t - C l l a y e r .  w i t h subsequent  potential.  coverage  completion  an oxide  Pt-0 or P t - C l  assumed to i n v o l v e b l o c k i n g such t h a t the f r e e energy  c o n t r o l l i n g e l e c t r o n t r a n s f e r a t the i n t e r f a c e were a l t e r e d .  bar-  53 119 Toshima and Okanlwa suggested t h a t below 1.4 - 1.6 V (N.H.E.) o n l y c h l o r i d e i o n o x i d a t i o n o c c u r r e d , whereas above t h i s o t h e r potential-determining the p r o d u c t i o n  processes occurred  of o x y - c h l o r i d e  such as oxygen e v o l u t i o n o r  species.  The p o t e n t i a l jump i t s e l f  was  determined t o correspond t o a l i m i t i n g d i f f u s i o n c u r r e n t . Takahashi and Odashima  120  a t t r i b u t e d the change i n c h l o r i n e  o v e r p o t e n t i a l t o a t r a n s i t i o n from P t 0 t o P t 0^ as the s u r f a c e  species.  121 B i t t l e s and L i t t a u e r  i n v e s t i g a t e d the p o l a r i z a t i o n be-  h a v i o u r of p l a t i n u m i n b o t h N a C l and H C l e l e c t r o l y t e s .  They  t h a t the p o t e n t i a l a t which the " p o t e n t i a l jump" began was potential.  Below t h i s , c h l o r i n e e v o l u t i o n was  f r e e l y d i s s o l v i n g platinum surface. was  considered  a passivation  assumed t o o c c u r on a  The p a s s i v a t i o n p o t e n t i a l , Ep,  found t o be independent o f pH and c h l o r i d e i o n a c t i v i t y f o r the  range o f c o n d i t i o n s  studied  missed the p o s s i b i l i t y as such e v o l u t i o n c o u l d  (-2 Z pH 1 +6,  1 i a  t h a t oxygen e v o l u t i o n was  Pt + H 0 2  E  =  ->  Pt 0 + 2H  /-> r> r> i 2.3 RT 0.99 + § H 0 l o  a  2  t h a t s e v e r a l types of s u r f a c e  +  + 2e 2.3 RT — F ~ "  ,  p  H  I t was  then  considered  s i t e s c o u l d e x i s t on the p l a t i n u m  b e f o r e p a s s i v a t i o n , which d i d n o t i n c o r p o r a t e 4 +  the cause of the jump,  The r e a c t i o n +  was a l s o r u l e d o u t , due to i t s pH-dependence.  included Pt, P t  They d i s -  o n l y o c c u r a t 2 - 2.4 V (N.H.E.) - s i g n i f i c a n t l y  h i g h e r p o t e n t i a l s than the p o t e n t i a l jump.  r-i  - < 90).  Pt-Cl~, Pt-Cl, P t C l  3 +  oxygen s p e c i e s .  , and PtCl*" ". 1  surface These  The s u r f a c e would  54 aquire an oxygen coating above the passivation p o t e n t i a l though a complex series of reactions involving these s i t e s . 122 Helber  determined the p o l a r i z a t i o n curves on platinum  i n saturated NaCl e l e c t r o l y t e f o r temperatures from 25 to 100°C. this he was  able to p l o t log ip vs. 1/T, where i p was  From  the current den-  s i t y corresponding to the p o t e n t i a l jump, and obtain an a c t i v a t i o n energy of 24.4  k cal/mole for the passivation process.  Potentiostatic  e l e c t r o l y s i s at potentials below, within, and above the p o t e n t i a l jump showed behaviour c h a r a c t e r i s t i c of non-passivating  chemisorbed layers  i n the f i r s t case, and of chemical passivation of the l a s t (only s l i g h t decay of current i n the former case, sharp decrease i n the  latter).  Potentiostatic anodization at a p o t e n t i a l within the jump did not show as great a rate of current decay as that at potentials above the jump, i n d i c a t i n g that the conversion  of adsorbed species on the electrode  a f a i r l y stable oxide and/or chloride layer occurred  to  only at the top  of jump. 123 Kokoulina et a l  12A *  used a p o t e n t i a l sweep method to  determine the oxygen coverage on a platinum anode i n acid chloride media. They attributed the cathodic s h i f t of the reduction peak with increasing anodic sweep-end p o t e n t i a l to an increase of Pt-0 bond strength with p o t e n t i a l . The f a c t that no new peaks were found i n sweep curves i n chloride media below the chlorine evolution p o t e n t i a l , as opposed to IM H„SO,, indicated that the f i l m consisted only of adsorbed oxygen and 2 4 was  not, say, Pt-Cl„.  At more p o s i t i v e p o t e n t i a l s , the presence of  55 strongly adsorbed chloride ions or chlorine atoms i n the surface f i l m was  considered to be l i k e l y .  1M E^SO^  The coverage/potential r e l a t i o n s i n  containing various amounts of chloride (from 10  a l l showed increasing coverage with p o t e n t i a l .  to IN  KC1)  In chloride-free solu-  t i o n , the amount of adsorbed oxygen corresponded to a monolayer at 1.5 (N.H.E.) and to two monolayers at 2.0 V.  The amount of oxygen was  inished with increasing chloride content.  V  dim-  For IN chloride, the curve  showed monolayer coverage only at about 2.0 V, and the coverage/potential curve showed a s l i g h t i n f l e c t i o n at 1.6 V. surface was  found to be free from adsorbed oxygen up to 1.1 - 1.15  Chloride i n h i b i t i o n of oxygen adsorption was this  In .IN and IN chloride, the V.  proposed to account f o r  behaviour. Kokoulina et a l noted that the " p o t e n t i a l jump" occurred  at about 1.5 V (N.H.E.), independent of chloride i o n concentration, but that the extent of oxygen coverage at this p o t e n t i a l varied s i g n i f i c a n t l y with chloride ion concentration (for example, 6 = .25 f o r [ C l ] = ' oxygen r  IN).  They concluded  that the bottom segment of the p o l a r i z a t i o n curve  corresponded to chlorine evolution on a free platinum surface, and that the upper one corresponded to the same reaction on a mixed surface f i l m which was more than monomolecular. oxygen adsorption. was  Passivation, then, was  The formation of anode products  attributed to  other than chlorine  suggested to occur at potentials above 2.2 V, on an electrode with  a nearly complete bimolecular f i l m .  Chlorate formation i s favoured i n  high chloride-containing e l e c t r o l y t e s , where an appreciable quantity of  56 c h l o r i n e would be be  contained  i n the f i l m .  Perchlorate  formation  would  f a v o u r e d i n more d i l u t e s o l u t i o n s where the f i l m i s m o s t l y oxygen. Blanchi^""^ suggested t h a t the p a s s i v a t i o n phenomenon on  platinum  c o u l d be  diffuses  to the e l e c t r o d e s u r f a c e , d i r e c t l y forming o x y c h l o r i d e  or d i s c h a r g e s  due  t o the presence of h y p o c h l o r i t e , which e i t h e r  according 6C10"  to:  + 60H"  [ r e v e r s i b l e p o t e n t i a l = 0.7  -> V  2C10 " + 4C1~ 3  + f °  +  3  2  H 2  °  chemisorbed on the p l a t i n u m  surface.  He  6  e  (not expected f o r OH  Although h i s work was  r o d e s , the proposed mechanism i s not  remain  f i n d s the p a s s i v a t i o n to - d i s c h a r g e ) and  (the p a s s i v a t i o n i s d e l a y e d at lower pH's, a t lower pH's).  +  (N.H.E.)]  which r e s u l t s i n the e v o l u t i o n of oxygen, some of which may  stirring-dependent  species,  CIO  be  pH-dependent  production  i s reduced  with titanium-substrate  elect-  i n a p p l i c a b l e f o r the case of smooth  electrodes. Kuhn and W r i g h t ^ ^ ' " ^ " ' found the p o t e n t i a l / c o v e r a g e on p l a t i n u m ions.  For  L)  GP  anodes i n NaCl s o l u t i o n to have t h r e e 2M NaCl at pH  1:  E = 1.4  2,  to E = 1.7  G = 1 and 2)  GP  2:  E = 1.7 .9  3)  =  the three  6 =  - 1.8  characteristic sect-  s e c t i o n s were d e s c r i b e d  - 1.8  V  relation  as  follows:  (N.H.E.)', coverage between  2. to E = 2.3  V,  coverage between 0 = 2  4.  Constant coverage at a l l p o t e n t i a l s above 2.3  V.  and  Both GP 1 and GP 2 sections showed l i n e a r dependence or p o t e n t i a l , with GP 1 being slower than GP 2.  To account f o r this behaviour, they pro-  posed the following reaction scheme:  Pt + C l " -> Pt C l " , ads Pt C l " + C l " •+ ads ( P t  C 1  Ids  )  C1  ads  Pt 0, , + H.O is; 2  +  H  (Pt C l " ) C l . + e ads ads  2° -  P t  °(s)  +  2 H C 1 +  6  -> Pt 2(0) + 2H + 2e s +  They speculated that the existence of two growth sections may only be c h a r a c t e r i s t i c of those e l e c t r o l y t e s whose ions enter into competitive adsorption with oxygen.  The "potential jump" was considered to be  caused by development of GP 1. P o t e n t i a l sweep curves on platinum show that there i s no evidence f o r oxygen adsorption below the chlorine evolution p o t e n t i a l . If the anodic l i m i t i s raised above the aforementioned  potential, a  reduction peak appears which could either be due to C l ^ a  charge transfer to become C l j , or C l ^ a  g  g  g  undergoing  formation on top of s p e c i f i -  c a l l y adsorbed C l . The authors favour the l a t t e r . 125 Kuhn and Wright  also studied the p o t e n t i a l sweep behavi-  our of iridium i n sulphuric acid and sodium chloride e l e c t r o l y t e s . Iridium shows reversible behaviour towards oxygen adsoption, unlike platinum.  However, i f the anodic sweep l i m i t i s extended  into the  oxygen evolution region i n .5M ^SO^, formation of a formal iridium  oxide i s indicated - which i s more i r r e v e r s i b l e than s i m i l a r oxides on platinum.  Unlike platinum, sweep curves i n NaCl e l e c t r o l y t e show  c h a r a c t e r i s t i c s s i m i l a r to those i n sulphuric acid.  No evidence for  the s p e c i f i c adsorption of chloride ions i s found. No work has been reported i n the l i t e r a t u r e on the p o l a r i zation curves f o r smooth Pt/Ir a l l o y s i n chloride solution.  F a i t a et  a l " ^ ^ and Kuhn and W r i g h t , h o w e v e r , say that Pt/Ir a l l o y s should allow a greater range of working electrode p o t e n t i a l before the r i s e i n overpotential takes place, compared with pure platinum.  Blanchi"*"^  found no p a s s i v i t y phenomena on I r / T i electrodes, and suggested that i n the case of Ir/Pt a l l o y s , the p a s s i v i t y phenomena should be attenuated with increasing iridium percentage.  This was  confirmed with titanium-  substrate electrodes.  2.3.2  P o l a r i z a t i o n of Titanium-Substrate Anodes Two  factors contribute to the d i f f e r e n t p o l a r i z a t i o n behav-  iour of titanium substrate electrodes as compared with smooth noble metal electrodes. F i r s t , the high surface area of the coated anodes results i n correspondingly lower-overvoltage behaviour at given apparent current d e n s i t i e s .  Secondly, the noble metal coatings applied to  electrodes of this class are porous. may be exposed to the e l e c t r o l y t e .  That i s , the titanium substrate For the l a t t e r reason, i t i s of  interest to review the electrochemical behaviour of the base metal.  59 2.3.2.1  Behaviour of  Cotton exposed t o any  128  >  129  >  131  s t a t e s t h a t whenever t i t a n i u m i s  The  oxygen, a t h i n t e n a c i o u s  formation  or n i t r i c chlorine.  of t h i s f i l m i s the r e a s o n f o r  a c i d s o l u t i o n s , m i x t u r e s of s t r o n g n i t r i c  and h y d r o c h l o r i c a c i d s , or h y d r o c h l o r i c  Only i n environments where the f o r m a t i o n  sulphuric free  of t h i s f i l m i s  f a v o u r e d , such as i n s u l p h u r i c of h y d r o c h l o r i c a c i d s , where hydrogen  use  significant.  of t i t a n i u m as an anode r e s u l t s i n the induced f o r m a t i o n  p r o t e c t i v e anodic f i l m .  The  p o l a r i z a t i o n curve f o r t i t a n i u m i n  u r i c a c i d shows a c o r r o s i o n maximum a t p o t e n t i a l s n e g a t i v e and  and  acid containing  i s produced on the m e t a l , i s the c o r r o s i o n r a t e o f t i t a n i u m The  surface  c h e m i c a l r e s i s t a n c e of t i t a n i u m to such c o r r o s i v e environments  as s t r o n g n i t r i c  not  130  environment c o n t a i n i n g  f i l m of o x i d e i s formed. the h i g h  >  Titanium  complete p r o t e c t i o n i s o b t a i n e d  above about 1.0  V  i n non-oxidizing  of  a  sulph-  to 0 V  (S.C.E.)  acids at p o t e n t i a l s  (S.C.E.).  Yakimenko et a l  132  reported  t i t a n i u m i n c r e a s e d w i t h an i n c r e a s e  t h a t the r a t e of c o r r o s i o n of  i n c o n d u c t i v i t y of the o x i d e  film.  133 Dugdale and of t i t a n i u m was  Cotton  considerably  sulphuric acid solution.  noted t h a t the "breakdown p o t e n t i a l "  lower i n c h l o r i d e e l e c t r o l y t e s than i n  They proposed t h a t o n l y i n s o l u t i o n s of  ions  w i t h a s u f f i c i e n t l y h i g h " p o l a r i z i n g power" (charge to a r e a r a t i o ) d i d formation  of the p r o t e c t i v e o x i d e o c c u r .  were supposed to be at  a i d e d by  (The  surface  t i t a n i u m atoms  the e l e c t r o s t a t i c e f f e c t of i o n s  adsorbed  the o x i d e / s o l u t i o n i n t e r f a c e i n a t t a i n i n g the n e c e s s a r y 4-valent  state  60 for protective oxide f i l m formation.)  Chloride ions were said to have  i n s u f f i c i e n t p o l a r i z i n g power, thus promoting breakdown. 134 Van Laer  determined the threshold p o t e n t i a l of the dan-  ger of intense anodic corrosion of titanium i n s y t h e t i c sea water (27 gpl NaCl; pH 6.32) was +7.0 V (S.C.E.).  Since titanium oxide i s  e l e c t r i c a l l y very r e s i s t a n t to current passage i n an e l e c t r o l y t e , an attempt to force high currents through i t w i l l lead to a rapid increase i n p o t e n t i a l to f i l m breakdown. 2 above about 20 mA/cm.  I t i s not possible to pass currents  through non-alloyed  titanium.  135 Mazza titanium.  found that breakdown occurred i n crevices i n the  Aqueous chloride solutions and anodic current were said to  produce the following phenomena inside crevices where d i f f u s i o n or convection were hindered: the accumulation of titanium corrosion products due to metal d i s s o l u t i o n and the accumulation of chlorine from chloride ion discharge.  Secondary reactions were postulated which could  reduce the s t a b i l i t y of the passive f i l m .  Among these was the p o s s i b i l -  i t y of production of free acid due to hydrolysis of corrosion products or chlorine. 136 Thomas and Nobe  observed that on immersion of titanium  i n de-aerated IN H^SO^, s e l f - p a s s i v a t i o n occurred, but that i n many cases s e l f - a c t i v a t i o n occurred after several hours.  Anodic p o l a r i z a t i o n  of titanium i n the pH range 0.25 - 2.0 produced a passive electrode, but the active corrosion p o t e n t i a l could be regained by cathodic p o l a r i z a tion.  In l e s s - a c i d e l e c t r o l y t e s , however, the active corrosion potentials  c o u l d n o t be  regained. 137 Cerny  ide s o l u t i o n s .  studied the anodic behaviour of t i t a n i u m i n c h l o r -  He n o t e d t h a t p i t t i n g o c c u r r e d  b u b b l e s were a t t c h e d  t o the s u r f a c e , s u g g e s t i n g  c o u l d p o s s i b l y have been d e s t r o y e d 138 Brieter  a t areas where c h l o r i n e t h a t the p a s s i v e  film  by c h l o r i n e .  suggested t h a t t h e r e a s o n f o r p i t t i n g  corrosion  was t h a t o x i d e formed i n s u r f a c e wedges and grooves had s m a l l e r mecha n i c a l s t r e n g t h , and t h u s b r o k e down p r e f e r e n t i a l l y . 127 Kuhn and W r i g h t i s favoured by increased  s t a t e t h a t the d i s s o l u t i o n o f t i t a n i u m  a c i d i t y and t e m p e r a t u r e .  In c r e v i c e s , condi-  t i o n s were p o s t u l a t e d t o e x i s t where t i t a n i u m i s no l o n g e r  passive.  As temperatures above 140°C a r e r e q u i r e d f o r t h i s , i t was suggested t h a t t h i s type of a t t a c k may be due t o l o c a l o v e r h e a t i n g tance  connections. 2.3.2.2 C o u p l i n g  of P l a t i n u m M e t a l s w i t h  at high-resis-  Titanium  128 129 130 Cotton  '  '  w i t h a m e t a l i n the p l a t i n u m  found t h a t g a l v a n i c c o u p l i n g o f t i t a n i u m group c o u l d r a i s e t h e p o t e n t i a l o f t h e  t i t a n i u m i n t o t h e p r o t e c t i v e range.  I t was a l s o seen t h a t the p r o t e c t -  i v e s u r f a c e f i l m on t i t a n i u m had low r e s i s t i v i t y when i n c o n t a c t  with  another m e t a l , b u t would n o t a c c e p t e l e c t r o n s from p o s i t i v e l y - c h a r g e d ions i n s o l u t i o n .  C o n s e q u e n t l y , most of t h e c u r r e n t was c a r r i e d by  the n o b l e m e t a l , w i t h t h e p r o p o r t i o n c a r r i e d by t h e t i t a n i u m b e i n g 139 s u f f i c i e n t t o ensure c o n t i n u o u s maintenance of t h e o x i d e f i l m . Lowe  s a i d t h i s " l e a k a g e c u r r e n t " was about 1 mA/cm.  i n chloride solutions  under n o r m a l c o n d i t i o n s . 134 Van L a e r  s a i d that the platinum  d e p o s i t on t i t a n i u m may  be p o r o u s , f i s s u r e d , o r i n c o m p l e t e , as a p r o t e c t i v e o x i d e f i l m would form a t t h e b a r e m e t a l .  P i t t i n g c o r r o s i o n may o c c u r , however, i f t h e  d i s t a n c e from the platinum  coating i s too great. 140  Warne and H a y f i e l d  s a i d the r e s i d u a l current  the t i t a n i u m was p a r t l y e l e c t r o n f l o w  (from i o n d i s c h a r g e )  through and p a r t l y  i o n i c (due t o slow t i t a n i u m d i s s o l u t i o n ) . As t h e a r e a of exposed titanium with respect comes more n e g a t i v e ,  to platinum  i n c r e a s e d , t h e mixed p o t e n t i a l b e -  l e a d i n g t o an i n c r e a s e d  Immersion i n c o n c e n t r a t e d  chance f o r d i s s o l u t i o n .  h y d o r c h l o r i c a c i d s o l u t i o n s was found t o p r o -  duce undermining o f the p l a t i n u m  c o a t i n g f o l l o w e d by e s t a b l i s h m e n t  the t i t a n i u m c o r r o s i o n p o t e n t i a l .  In less-concentrated  of  s o l u t i o n s , the  p o t e n t i a l s remained w i t h i n t h e zone o f p a s s i v e f i l m f o r m a t i o n ,  although  i n d e - a e r a t e d s o l u t i o n s t h e p o t e n t i a l s were much c l o s e r t o t h e F l a d e potential. 141 Khodkevich e t a l through t h e t i t a n i u m was h i g h e r  142 '  found t h a t t h e l e a k a g e  current  i n a l k a l i n e than i n a c i d s o l u t i o n , i n -  d i c a t i n g t h a t t h e s u b s t r a t e takes a g r e a t e r p a r t i n t h e a n o d i c p r o c e s s ( t h r o u g h pores i n t h e c o a t i n g )  i n t h e former c a s e .  They d e v e l o p e d a  method f o r d e t e r m i n i n g  the p o r o s i t y of p l a t i n i z e d t i t a n i u m  by means o f m o n i t o r i n g  the a c t i v a t i o n of the titanium i n hot,  hydrochloric acid solution.  electrodes strong  63  Many authors b e l i e v e t h a t the d e s t r u c t i o n of t i t a n i u m s t r a t e e l e c t r o d e s as a r e s u l t of a n o d i z a t i o n the s u b s t r a t e . . . contact  l e a d i n g to undermining and  r~ i • or f l a k x n g  eventual  to o x i d a t i o n  l o s s of  electrical  Polarization Characteristics  F a i t a et a ^ ^ , 146,148 £ as low as 0.5%  o u n (  j  t  n  a  t  i  r  i d i  u  contents  m  were e f f e c t i v e i n d e c r e a s i n g  coated anodes to p a s s i v a t e .  They e x p l a i n e d  Mixed o x i d e coated e l e c t r o d e s  showed no  t h i s i n terms of the  long times, and  i s enhanced by r a p i d r o t a t i o n of the e l e c t r o d e .  t h a t the He  be r e s p o n s i b l e f o r p a s s i v a t i o n where s u r f a c e  oxychlorides  or oxygen produced by  i t was  specualted  t e n t of the  i t s discharge not  may  electrodes  passivation  suggests t h a t hypo-  c h l o r i t e may  P a s s i v a t i o n was  oxygen.  composition.  Bianchi^"'"^ n o t e s t h a t the p a s s i v a t i o n of c o a t e d  on the e l e c t r o d e .  rever-  hysteresis i n their polarization  i n d i c a t i n g the s t a b i l i t y of the s u r f a c e  i s a slow p r o c e s s i n v o l v i n g v e r y  (in Pt/Ir  the tendency of  s i b l e b e h a v i o u r of i r i d i u m towards the p a s s i v a t i n g s p e c i e s ,  curves,  of  „. 127,140,141,143-7 o f f of the n o b l e m e t a l coatxng.  2.3.2.3  coatings)  i s due  sub-  formation  of  remain chemisorbed  found f o r I r - T i e l e c t r o d e s ,  and  t h a t p a s s i v a t i o n would be a f u n c t i o n of i r i d i u m c o n -  coating. 149 Landolt  and  Ibl  coated e l e c t r o d e s , which was ment, s u g g e s t i n g  found a " p o t e n t i a l jump" phenomenon w i t h s t r o n g l y dependent on e l e c t r o d e  a non-mass t r a n s f e r l i m i t e d mechanism.  pretreat-  64  Shembel et a l  found that oxygen was  present  gas i n increasing amounts with increasing p o t e n t i a l . curve i n sulphuric acid solution was  considered  i n the anode  The p o l a r i z a t i o n  to be an extreme case  where the passivation process occurred at a f a i r l y high rate.  147 Weber and P o s i r i l  observed s i g n i f i c a n t decreases i n the  electrochemically active surface area a f t e r potential-sweep experiments i n 0.5M  H^SO^, yet found no platinum  loss.  They attributed these losses  to either platinum r e c r y s t a l l i z a t i o n or interference with the conductive connection  between the substrate and coating caused by a formation  layer of titanium 2.3.3  of a  oxide.  Summary  It i s apparent that meaningful experimentation i n chloride solutions must involve an appreciation for the p o s s i b i l i t y of eous reactions.  simultan-  Many can be made n e g l i g i b l e by s t r i c t control of elec-  t r o l y s i s conditions, but the phenomenon of electrode passivation appears to be the major obstacle i n the attempt to understand electrode processes in chloride media.  There i s much speculation about the nature of the  passivation, and l i t t l e work has been done concerning of mixed-metal electrodes.  the passivation  I t i s also apparent that the substrate mat-  e r i a l , while r e l a t i v e l y i n e r t , plays an important role i n determining the p o l a r i z a t i o n behaviour of coated electrodes, and that the i s much more "environment-sensitive"  than i s the coating.  substrate  As with s o l i d  noble metal electrodes, l i t t l e work has been done concerning  the e f f e c t s  65  of  p a s s i v a t i o n o f mixed-metal c o a t i n g s .  Furthermore,  i t must be noted  that the b e h a v i o u r o f noble m e t a l e l e c t r o d e s i n c h l o r i d e media i s strongly  2.4  pretreatment-dependent.  DISSOLUTION OF THE NOBLE METALS  S i n c e Llopis"'""'"*" p u b l i s h e d an e x t e n s i v e r e v i e w o f the c o r r o s i o n of the p l a t i n u m m e t a l s i n 1968,  much a d d i t i o n a l work has been done.  As the aim o f t h i s t h e s i s i s the p r e p a r a t i o n and c h a r a c t e r i z a t i o n of systems which w i l l to for  be used i n the study of c o r r o s i o n , i t i s a p p r o p r i a t e  i n c l u d e a summary of r e c e n t i n v e s t i g a t i o n s c o n c e r n i n g the mechanism c o r r o s i o n of the n o b l e m e t a l s .  Mechanisms f o r a n o d i c d i s s o l u t i o n  can be c o n s i d e r e d f o r b o t h " a c t i v e " and " p a s s i v e " c a s e s .  That i s , mech-  anisms not i n v o l v i n g oxygen and those i n which oxygen p a r t i c i p a t e s i n some manner.  The e f f e c t s of o t h e r s p e c i e s , such as c h l o r i d e i o n s ,  can  then be c o n s i d e r e d l i k e w i s e under these two g e n e r a l c l a s s i f i c a t i o n s . I t i s important to r e a l i z e t h a t the " c u r r e n t e f f i c i e n c y " f o r n o b l e m e t a l d i s s o l u t i o n i s e x t r e m e l y low f o r p r a c t i c a l e l e c t r o l y s i s c o n d i t i o n s , t h a t the s e p a r a t i o n of t h i s p a r t i a l p r o c e s s from the main ing  reactions  and  current-consum-  (such as c h l o r i n e and oxygen e v o l u t i o n ) has made r e s e a r c h  i n t h i s f i e l d very  difficult.  66  The  d i s s o l u t i o n of n o b l e metals  d u r i n g a c t i v a t i o n or s i m i l a r  (such as a n o d i c / c a t h o d i c treatment must be may  procedures  or a l t e r n a t i n g current e l e c t r o l y s i s )  t r e a t e d s e p a r a t e l y as b o t h a c t i v e and p a s s i v e a n o d i c mechanisms  be i n v o l v e d , as w e l l as r e d u c t i o n mechanisms.  s i o n of p l a t i n u m m e t a l s i d e r e d by i t s e l f ,  c o a t i n g s on v a l u e m e t a l  s u b s t r a t e s must be  as o t h e r mechanisms o f d e g r a d a t i o n  i n v o l v e the noble m e t a l ) may  2.4.1  F u r t h e r , the c o r r o con-  (which do not  operate.  The A c t i v e D i s s o l u t i o n of the Noble M e t a l s 152 Chemodanov  s t a t e s t h a t the a c t i v e d i s s o l u t i o n of p l a t i n u m  occurs o n l y when the e l e c t r o l y t e i s a b l e to form s t a b l e complexes w i t h the m e t a l iting  c a t i o n s and i s capable of s p e c i f i c a l l y a d s o r b i n g , thus  the f o r m a t i o n of an oxide f i l m .  i n the a c t i v e r e g i o n have been found passive  inhib-  The mechanisms f o r d i s s o l u t i o n t o be d i f f e r e n t from those i n the  state. Llopis"'"^"'' c o n s i d e r s such a mechansim as:  M  -e H0  ->  M  n  2  which i n v o l v e the f o r m a t i o n of aquo-complexes i s o n l y of i n t e r e s t f o r the p a r t i c u l a r case of p a l l a d i u m .  The  standard h a l f - c e l l r e a c t i o n  has  153 been d i r e c t l y determined  f o r t h i s case by  Izatt.  For the  other  p l a t i n u m m e t a l s , the v a l u e s of the s t a n d a r d p o t e n t i a l s i n a c i d media 154 have been c a l c u l a t e d from thermodynamic d a t a by Goldberg and H e p l e r :  67 Rh/Rh  E ° = 0.7  V  Pd/Pd" " "  E ° = 0.92 V  Pt/Pt"^  E ° = 1.2  1  1  V  155 Goodridge and K i n g palladium  have suggested  ( i n 2N R^SO^ Pd  solution) i s : surface  Pd  _ surface +  H  Pd  t h a t the c o r r o s i o n mechanism f o r  _ surface  ->  Pd . + e surface  ->  Pd" " _ + e surface  +  1-4  2° -> Pd  Of more p r a c t i c a l concern a r e those e l e c t r o l y t e s whose  156 anions are capable of i n t e r a c t i n g w i t h the p l a t i n u m m e t a l s . has noted  t h a t t h e r e are s e v e r a l i n t e r p r e t a t i o n s c o n c e r n i n g the r o l e o f  s o l u t i o n anions i n d e t e r m i n i n g 1)  Kolotyrkin  the c o r r o s i o n o f m e t a l s .  These i n c l u d e :  the f o r m a t i o n o f simple m e t a l i o n s w i t h subsequent  interaction  w i t h the a n i o n s , 2)  d i r e c t f o r m a t i o n of complexes of the m e t a l ,  3)  i n t e r a c t i o n between the anions and o t h e r s o l u t i o n c o n s t i t u e n t s ,  4)  p a r t i c i p a t i o n of anions i n cases where  the f i n a l p r o d u c t s a r e  simple metal ions or t h e i r h y d r o l y s i s products. The a c t i v e c o r r o s i o n of a l l the p l a t i n u m metals found  t o be g r e a t e r i n the presence  i n a c i d i c media has been  of h a l i d e i o n s (than i n o t h e r common  e l e c t r o l y t e s ) due to the g r e a t e r s t a b i l i t y  of the h a l i d e - c o m p l e x e s  with  68 respect to the aquo-complexes.^^  Halide ions s p e c i f i c a l l y adsorb on  the noble metals at potentials much more cathodic than oxygen, and the coverage i s almost complete at potentials approaching that f o r oxygen adsorption. The formation of halide-complexes on the platinum 158 surface has been considered thus to delay the onset of passivation, as competitive adsorption between oxygen and halide ions would then involve the replacement of an already-adsorbed halide ion.  (However,  the halides would not remain adsorbed, as they form soluble species.) Indeed, the passivation of platinum does not occur i n such a concentr121 ated e l e c t r o l y t e as 8M HCl. Standard electrode potentials f o r couples involving solution 154 anions have been determined by many authors. Goldberg and Hepler have summarized most of these.  Of p a r t i c u l a r i n t e r e s t are the reactions:  Pt(c) '+ 4C1 (aq) t  Pt C l ^ a q ) + 2e  E° = 0.75 V  Pt(c)  + 6Cl"(aq)  t  Pt Cl"(aq) + 4e~ o  E° = 0.76 V  Ir(c) + 6Cl~(aq)  t  I r ClJ(aq) + 3e~  E° = 0.86 V  Ir(c) + 6Cl~(aq)  t  I r Cl^(aq) + 4e~  E° = 0.86 V  Llopis"'""'"'"'"''"^ has proposed a general mechanism for active platinum metal d i s s o l u t i o n i n halide media.  This involves the chemi-  sorption of the halogen on the anoxidized metal surface, followed by concurrent formation of complexes and halogen evolution (due to the high overpotential for the former):  69 M  ~ -> e  X-  (M....X)  IMX  l  . X - y  (  n  _  Y  )  +  X-  'X  M  +  X  2  -e  or  T i t  CM... .X~) ^ — -  [MX j  X-  Y  132 Chemodanov  ( n _ Y ) +  160 *  found that the active d i s s o l u t i o n rate  of platinum to increase with the chloride concentration, rate was  f i r s t order with respect to the chloride ion  and that t h i s  concentration.  That i s , i n the active region of p o t e n t i a l s , chloride ions p a r t i c i p a t e d d i r e c t l y i n the f i r s t one-electron step of anodic platinum d i s s o l u t i o n (perhaps chloride ion discharge on a platinum surface atom).  Chemodanov  noted the rates of platinum d i s s o l u t i o n and halide discharge were not d i r e c t l y connected with each other, and hence metal d i s s o l u t i o n could not be approximately determined by an extrapolation from the halide discharge rate.  Indeed, the active d i s s o l u t i o n rates of platinum are  comparable i n either 3N HCl or 3N HBr, whereas the halogen evolution rates d i f f e r by orders of magnitude.  Llopis'*""'"'' has extrapolated  the  linear active region of the p o l a r i z a t i o n curves for d i s s o l u t i o n and deposition of Pd Br^ and Pd C l ^ , and has obtained v i r t u a l l y i d e n t i c a l exchange current densities.  This indicates that the d i s s o l u t i o n and  deposition processes i n these systems are likewise nearly Z e l e n s k i i and K r a v t s o v ^ ' ^ 1  to C l  1  1  2  identical.  found the orders with respect  of e l e c t r o d i s s o l u t i o n and electrodeposition of palladium to be  70 different. product  The  c a t h o d i c r e a c t i o n order was  -2,  formed from the s o l u t i o n i o n Pd C l ^ .  order was  1.33,  which they e x p l a i n e d by  i n d i c a t i n g a Pd Clr,  The  anodic r e a c t i o n  assuming an i n c r e a s e i n C l  c o n c e n t r a t i o n i n the l a y e r near the e l e c t r o d e .  They suggest t h a t i n  c h l o r i d e e l e c t r o l y t e s of s u f f i c i e n t l y h i g h c o n c e n t r a t i o n , the  surface  of a p a l l a d i u m  Pd  anode i s f i l l e d w i t h  s u r f a c e complexes.  adsorbed c h l o r i d e forming  I n t u r n , a c t i v a t i n g complexes c o n t a i n i n g two  i d e i o n s each are formed  J.Pd  ^1  ^  w  * *i:  cn  (Cl^^) chlor-  immediately i o n i z e s .  Such a mechanism would g i v e a d i s s o l u t i o n r e a c t i o n o r d e r p r o p o r t i o n a l to the square of the b u l k  c o n c e n t r a t i o n of h a l i d e i o n s . 121  B i t t l e s and L i t t a u e r distinct  types  speculate  t h a t there a r e s e v e r a l  of a d s o r p t i o n s i t e s on a p l a t i n u m  surface i n chloride  s o l u t i o n , i n c l u d i n g bare metal,  o x i d i z e d s u r f a c e atoms, adsorbed c h l o r 3+ i d e s p e c i e s i n c l u d i n g C l and C l , s u r f a c e complexes such as Pt C l 4+ 4and Pt C l , and i n v o l v i n g o x y - h a l i d e s p e c i e s such as Pt C l C l ^ O ) The  processes  of c h l o r i n e e v o l u t i o n and m e t a l i o n i z a t i o n may  have sev-  e r a l of these s i t e s i n common d u r i n g t h e i r r e a c t i o n sequences. Helber with be  122  suspects  chloride ion concentration)  favoured  due  enhanced, or even be Kuhn and  r a t e of p l a t i n u m i s due  i n the form of s u r f a c e complexes  to the much h i g h e r  s u r f a c e than i n the b u l k . be  t h a t the presence of C l ^ (which i n c r e a s e s  The  c h l o r i d e i o n c o n c e n t r a t i o n at  a c t i v e d i s s o l u t i o n of p l a t i n u m may  caused by,  C l ^ s u r f a c e complex  may the thus  formation.  Wright"*"^ suggest t h a t the i n c r e a s i n g c o r r o s i o n  i n solutions containing increasing chloride  t o the i n c r e a s e d r e t a r d a t i o n of c o m p e t i t i v e  oxygen  contents  adsorption.  71 That i s , surface coverage by  (Pt 0 1 j ) ^ 1 ^ ^ a<  g  type species predominates.  Like B i t t l e s and L i t t a u e r , they support the idea that corrosion i s associated with adsorbed oxy-halide  or halide complexes.  Other, not necessarily anionic, species may  enhance the 163  active d i s s o l u t i o n of the platinum metals. that a pronounced blackening  of t h e i r palladium  ing the anodic oxidation of ethylene. was  Blake et a l  observed  electrode occurred dur-  They suspected this phenomenon  due to the deposition of f i n e l y - d i v i d e d metal, and suggested the  mechanism: Pd + C_H.  t  PdC H. 0  2 4 solution P d C  2 4ads H  2 P d C  t  P d C  2 4 ads H  *  +  2 4 ads  2 4 ads H  Pd(C  +  + e  Vr ° + Pd  2  Further, s o l u t i o n reactions were also considered ++ itated palladium:  H  2°  Pd^H^  +  + CH  3  • CHO + 2H  mental data indicated that the charge-transfer rium.  as a source of precip-  + Pd.  step was  The model also accounts for the zero-order  Their experi-  i n pre-equilib-  pH-dependence.  Goodridge and K i n g ^ ^ observed the same phenomenon, and i n addition to the mechanism of Blake et a l , considered: Pd -> P d . + e surface +  Pd  . surface  +  Pd^ , surface  -  +  Pd"*" + C_H. 2 4 -1  PdC„H, + H 0 2 4 2 +  o  +  Pd"^ . + e surface Pd^ -*• PdC-H^ 2 4 Pd° + CH CH0 + J o  2H  +  72  Napp and Bruckenstein  164  produced d i s s o l u t i o n of bare p l a t -  —6  inum by the addition of 10  "M Cu(II) to t h e i r s o l u t i o n .  this to the following reaction: Pt + Cu(II) -> Pt(II) + Pt(IV) + Cu(I)  They attributed  . ^ ads + aq  Organic species have been found to accelerate noble metal 140  dissolution. noble metals  Warne and Hayfield  attributed enhanced corrosion of  (as coatings or wrought material) to some anodic complex165  ing reaction with organic addition agents.  Juchniewicz  reported  that the presence of sucrose, glucose, and s i m i l a r organic compounds led to rapid destruction of coated anodes. 154  Chemodanov  says certain organic compounds have a strong  i n h i b i t i n g e f f e c t on the process of active platinum d i s s o l u t i o n .  Llopis  found that sulphur-containing species were e f f e c t i v e i n h i b i t o r s .  The  strong adsorption of i n h i b i t o r s w i l l block the metal surface from the adsorption of other solution ions and molecules, thus preventing them from reaching the active electrode surface s i t e s . 2.4.2  Dissolution with Oxygen P a r t i c i p a t i o n Oxygen adsorption on a nobel metal electrode i n h i b i t s the  active d i s s o l u t i o n of the metal.  At low chloride ion contents, a high  surface oxygen coverage i s formed which suppresses the action of the halide ions."'""' '"'""^ S i m i l a r l y , a reduction i n the a c i d i t y of the 2  152  e l e c t r o l y t e f a c i l i t a t e s oxygen chemisorption.  159  Llopis  suggests  73  t h a t a maximum m e t a l c o r r o s i o n  (as found i n the p o l a r i z a t i o n curve)  i n d i c a t e s t h a t the p a s s i v a t i n g f i l m undergoes a m o d i f i c a t i o n w i t h i n c r e a s i n g p o t e n t i a l s , such t h a t t h e r e t r a n s p o r t across  the f i l m .  i s a higher  resistance to cation  He a l s o says t h a t the e s t a b l i s h m e n t  true p a s s i v a t i n g f i l m coincides with the establishment onic conductivity. film itself  of h i g h  I n such a c a s e , t h e p o t e n t i a l g r a d i e n t  of a electr-  a c r o s s the  i s s m a l l , and c o n s e q u e n t l y the m e t a l i o n s have l i t t l e d r i v 158  ing  f o r c e t o m i g r a t e t o the o x i d e / s o l u t i o n i n t e r f a c e .  Hoar  says  t h a t the r a t e o f d i s s o l u t i o n o f a p a s s i v e m e t a l may become l i m i t e d by the r a t e o f c h e m i c a l d i s s o l u t i o n of the m a t e r i a l comprising 160 vating film.  Chemodanov  of the s u r f a c e  the p a s s i -  a t t r i b u t e s t h e c o m p o s i t i o n and p r o p e r t i e s  oxygen l a y e r on p l a t i n u m  as r e s p o n s i b l e  f o r the absence  of a p o t e n t i a l e f f e c t on the d i s s o l u t i o n r a t e between 1.5 and 2.8 V 41 (R.H.E.).  Rand and Woods  speculate  r e v e r s i b l e oxygen p o t e n t i a l r e q u i r e s metal d i s s o l u t i o n currents p o t e n t i a l mechanism The s i o n can o c c u r .  a f u l l oxygen coverage, such t h a t  a r e c o m p l e t e l y i n h i b i t e d (and hence no mixed  operates).  p o s s i b i l i t y e x i s t s , however, t h a t p a s s i v e m e t a l c o r r o Indeed, i n a l k a l i n e media t h e p l a t i n u m  to form s t a b l e s u r f a c e  complexes w i t h OH  which a r e s o l u b l e , or form s u r f a c e soluble species  t h a t the m a i n t a i n e n c e o f the  (platinum,  metals a r e a b l e  palladium,  o x i d e s which r e a c t w i t h OH  (ruthenium, osmium), o r r e a c t d i r e c t l y w i t h OH  rhodium) t o form t o form  s o l u b l e s p e c i e s (osmium, rhodium). I n a c i d media, ruthenium, osmium , , ' . . ,; 151,154,157,166,167,168 and rhodxum a r e c a p a b l e o f forming s o l u b l e o x i d e s .  74  L l o p i s ^ " ' ' has suggested t h a t t h e p o s s i b l e d i s s o l u t i o n mechanism f o r n o b l e m e t a l s i n non-complexing a c i d m e d i a i s : " -> M...0 e  M + mHLO 2  -* M (H_0) 2 m M...0  ->  + H  +  MO 168  O t h e r s , such as Burke and O'Meara a noble metal surface i s incomplete  p o s t u l a t e that the p a s s i v a t i o n of o r t h a t new s u r f a c e i s exposed when  s u r f a c e s t r e s s e s a t h e a v i l y o x i d i z e d s u r f a c e s cause t h e o x i d e t o c r a c k . M e t a l d i s s o l u t i o n thus a r i s e s from a c t i v e d i s s o l u t i o n o f an u n o x i d i z e d surface.  Chemodanov^ says t h a t f o r a p l a t i n u m e l e c t r o d e a t p o t e n t i a l s  corresponding constant  to e l e c t r o d e o x i d a t i o n , the d i s s o l u t i o n r a t e i s r e l a t i v e l y  as s u r f a c e o x i d e s a r e c o n t i n u o u s l y b e i n g g e n e r a t e d and a r e 169  thus always p r e s e n t f o r d i s s o l u t i o n .  Others,  such as K o l o m i e t s  are  a l s o of t h e o p i n i o n t h a t the f o r m a t i o n o f s u r f a c e oxygen s p e c i e s i s necessary  f o r t h e d i s s o l u t i o n of p l a t i n u m a t h i g h a n o d i c p o t e n t i a l s .  K o l o t y r i k i n ' ' " ^ s a i d t h a t p l a t i n u m c o r r o s i o n i n a c e t a t e systems under conditions corresponding composition  t o t h e K o l b e s y n t h e s i s depended o n l y on t h e  and p r o p e r t i e s o f t h e s u r f a c e o x i d e s , and t h a t t h e a n i o n i c 65 152  c o m p o s i t i o n was n o t a f a c t o r .  Chemodanov  '  123 and K o k o u l i n a  attri-  bute the increase i n the d i s s o l u t i o n r a t e of platinum a t p o t e n t i a l s above 1.8 V (N.H.E.) t o a change i n t h e p r o p e r t i e s of t h e s u r f a c e oxygen l a y e r .  I n p e r c h l o r i c a c i d , due t o t h e s p e c i f i c o x i d i z i n g p r o p e r t i e s  52 "ofa c tthe i v e e loxygen" o r r o s i owni tr he s a i s htiagnhc ec.o n tKravchenko e c t r o l y wt he i, c hanhas o x i dae reduced f i l m i sc formed ent of  75 observed t h a t the maximum i n the p l a t i n u m d i s s o l u t i o n r a t e i n p e r c h l o r i c a c i d s o l u t i o n corresponded w e l l w i t h the maximum i n o x i d e t h i c k n e s s i n t h a t medium. The presence of oxygen may  be r e s p o n s i b l e f o r c o r r o s i o n  phenomena a t p o t e n t i a l s w h i c h a r e n o t i n the range c o r r e s p o n d i n g oxide formation.  Ginstrup^"'" has s u s p e c t e d  to  t h a t oxygen d i s s o l v e d i n  the m e t a l or r e s i d u a l a i r i n the s o l u t i o n c o u l d be r e s p o n s i b l e f o r h i g h e r c o r r o s i o n r a t e s t h a n c o u l d be accounted f o r by an a c t i v e d i s s o l u t i o n  65 mechanism.  Chemodanov  e l e c t r o l y t e was polarization.  found t h a t f o r such p o t e n t i a l s the n a t u r e of  the  o f secondary i m p o r t a n c e w i t h r e s p e c t t o the l e n g t h of He s p e c u l a t e s t h a t dermasorbed oxygen, w h i c h d i f f u s e s t o  the e l e c t r o d e s u r f a c e a t p o t e n t i a l s c o r r e s p o n d i n g  t o the hydrogen or  double l a y e r r e g i o n s , can account f o r the observed a s y m p t o t i c  decay t o  z e r o of p l a t i n u m d i s s o l u t i o n under these c o n d i t i o n s . The  presence of " a g r e s s i v e " anions i n the e l e c t r o l y t e  may  158 a l t e r the d i s s o l u t i o n mechanisms of p a s s i v a t e d n o b l e m e t a l s .  Hoar  suggested a m e c h a n i c a l mechanism f o r a n i o n p e n e t r a t i o n and breakdown of passivating films.  C o n t i n u e d a n i o n a d s o r p t i o n may  u l t i m a t e l y reach  a s t a t e where the r e p u l s i v e f o r c e s among anions push them, and t o w h i c h they are s t r o n g l y a t t a c h e d , a p a r t .  the  such oxide  This then leads to f u r t h e r  156 and p r o g r e s s i v e p e n e t r a t i o n of a n i o n s .  Kolotyrkin  s i d e r i n g the a c c e l e r a t i n g e f f e c t of anions  s a i d t h a t , i n con-  on the d i s s o l u t i o n o f a p a s s -  i v e m e t a l , not o n l y must the p e n e t r a t i o n of a p a s s i v a t i n g l a y e r a t i n d i v i d u a l p o i n t s be taken i n t o a c c o u n t , b u t a l s o the d i r e c t p a r t i c i p a t i o n of  76 anions and s o l u t i o n molecules i n the d i s s o l u t i o n process.  Foley  172  discusses a " f i e l d e f f e c t " mechanism f o r chloride ions acting on a passive metal, where the strong e l e c t r i c f i e l d produced by chloride adsorption draws ions from the metal.  He also considers a c a t a l y t i c  e f f e c t whereby chloride catalyses the d i s s o l u t i o n reaction by forming some form of intermediate bridging  structure.  L l o p i s considers the  formation of chloro-complexes from the noble metal surface oxide layer as responsible f o r the corrosion of osmium and  ruthenium.'Llopis  suggests the mechansims:  ' • M O p - ' surface  X-^ ^ ^ y ^  [MO ] . p surface  HCl  J  [MX (OH) ]( a 'y-a  n -  Y)  +  The f i r s t includes transport of metal cations across the oxide layer, and the second i s chemical d i s s o l u t i o n of the oxide layer.  L l o p i s also  suggests that halide-complexes may not be formed d i r e c t l y as the r e s u l t of some electrochemical step, but rather by means of some secondary 152 160 chemical reaction.  Chemodanov  '  has found that the presence of  strong hydrofluoric acid sharply accelerates the d i s s o l u t i o n of passive platinum.  This was considered to be the r e s u l t of d i s s o l u t i o n of the 52  oxide f i l m .  Kravchenko et a l  say that anion e f f e c t s w i l l only be  s i g n i f i c a n t i n passive metal d i s s o l u t i o n under those conditions which favour anion i n c l u s i o n i n the oxide f i l m .  Conditions which produce  thin-layer films are thus considered favourable f o r reducing metal  77 dissolution.  Others m a i n t a i n t h a t t h e r e i s always c o m p e t i t i o n between  oxygen and c h l o r i d e f o r s u r f a c e a d s o r p t i o n s i t e s .  B i t t l e s and L i t t a u e r  121  have proposed a s e r i e s o f a d s o r p t i o n complexes, i n v o l v i n g b a t h w a t e r and 132  c h l o r i d e a d s o r p t i o n , f o r the passive e l e c t r o d e .  Chemodanov  173  '  b u t e s t h e a c c e l e r a t i n g e f f e c t of c h l o r i d e i o n s a t r e l a t i v e l y m i l d  attripass-  i v e p o t e n t i a l s t o t h e a b i l i t y o f c h l o r i d e t o adsorb on t h e b a r e m e t a l surface.  A t h i g h a n o d i c p o t e n t i a l s , however, t h e r e d u c t i o n i n e l e c t r o d e  s u r f a c e o x i d a t i o n due t o c h l o r i d e a d s o r p t i o n i s accompanied by a l o w e r i n g of platinum d i s s o l u t i o n .  Chemadanov s u g g e s t s t h a t p l a t i n u m  dissolution  a t such h i g h p o t e n t i a l s proceeds v i a a c t i v e s u r f a c e oxygen compounds and t h a t t h e c o n c e n t r a t i o n o f these compounds i s reduced b y c h l o r i d e adsorption.  Chemodanov,^®  Vijh,^^  and Hackerman"^"* found e l e c t r o d e  d i s s o l u t i o n t o be h i n d e r e d by i o d i d e and f l u o r i d e due t o t h e f o r m a t i o n o f p r o t e c t i v e f i l m s o f these s p e c i e s on t h e m e t a l . Tanaka and Fujisawa"*"^ suggested t h a t the a n o d i c r e a c t i o n scheme i n EDTA s o l u t i o n s was: P t + 2H 0 2  Pt(OH)  2  ->  + H Y 2  Pt(OH) =  ->  2  + 2H  PtY  =  +  + 2e  + 2H 0 2  T h e i r i n c l u s i o n of s u r f a c e oxygen i n t h e d i s s o l u t i o n mechanism was based 152 on o b s e r v a t i o n s  o f t h e l o s s o f s u r f a c e oxygen.  Chemodanov  says the  e f f e c t o f o r g a n i c compounds a t h i g h a n o d i c p o t e n t i a l s may be e i t h e r i n h i b i t o r y o r a c c e l e r a t i n g , depending on t h e n a t u r e and c o m p o s i t i o n o f the  solution.  78 2.4.3  D i s s o l u t i o n During A c t i v a t i o n  Under t h i s h e a d i n g t h e t e r m " a c t i v a t i o n " w i l l be used t o d e s c r i b e any c y c l i c o x i d a t i o n / r e d u c t i o n t r e a t m e n t , whether o r n o t t h e treatment was performed t o a c t i v a t e t h e e l e c t r o d e f o r some o t h e r chemical i n v e s t i g a t i o n .  electro-  The o x i d a t i o n / r e d u c t i o n p r o c e d u r e may i n v o l v e  a programmed p o t e n t i o d y n a m i c  sequence, a l t e r n a t i n g c u r r e n t  electrolysis,  a s i m p l e a n o d i z a t i o n / c a t h o d i z a t i o n o r even o n - o f f s w i t c h i n g . i d e n t i f i c a t i o n o f t h e problem o f m e t a l d i s s o l u t i o n d u r i n g  As t h e  conventional  a c t i v a t i o n p r o c e d u r e s has been r e l a t i v e l y r e c e n t , much o f t h e e a r l y work on a l t e r n a t i n g c u r r e n t e l e c t r o l y s i s has been used as t h e background f o r i n t e r p r e t a t i o n of the d i s s o l u t i o n . 22 23 Hoare  '  observed d a r k e n i n g  o f n o b l e m e t a l e l e c t r o d e s under  t h a a p p l i c a t i o n o f a l t e r n a t i n g c u r r e n t e l e c t r o l y s i s , accompanied w i t h s i g n i f i c a n t increases i n electrode area.  T h i s e f f e c t was e s p e c i a l l y  pronounced w i t h p a l l a d i u m , w h i c h absorbs hydrogen b e t t e r t h a n t h e o t h e r metals.  Hence, Hoare concluded  t h a t i t was t h e p e n e t r a t i o n o f hydrogen  i n t o t h e m e t a l d u r i n g the c a t h o d i c h a l f - c y c l e s w h i c h caused t h e break-up of the metal s u r f a c e .  F u r t h e r , the darkening  c o u l d be a r r e s t e d b y p o l a r -  i z i n g w i t h a d i r e c t c u r r e n t b i a s such t h a t t h e most c a t h o d i c p o t e n t i a l 140 reached was w e l l above t h e hydrogen p o t e n t i a l .  Warne and H a y f i e l d  s t a t e t h a t o n l y l o w - f r e q u e n c y C- 50 Hz) c u r r e n t r e v e r s a l s promoted platinum d i s s o l u t i o n .  Llopis,"'"^"'' i n c o n s i d e r i n g t h e a l t e r n a t i n g c u r r e n t -  d i s s o l u t i o n o f r u t h e n i u m i n 6N H C l , where t h e e l e c t r o d e i s k e p t i n t h e  79 p a s s i v e r e g i o n of p o t e n t i a l s a t a l l times,  a t t r i b u t e s metal d i s s o l u t i o n  t o the p a r t i a l removal of the p a s s i v e f i l m  (an u n d e f i n e d  outer i n t e r f a c e w i t h subsequent slow i n t e r a c t i o n w i t h  o x i d e ) at  the  the  electrolyte  177 to produce v a r i o u s complexes.  Yukhevich  a l s o found t h a t the  i o n of p l a t i n u m became l e s s i n t e n s e w i t h h i g h - f r e q u e n c y current. during  He  pointed  alternating  to the absence of hydrogen i n the gas  the c a t h o d i c h a l f - p e r i o d as evidence  the m e t a l d i d not have time to c c c u r .  evolved  t h a t the d e - p a s s i v a t i o n of  Yukhevich found t h a t  intense  c o r r o s i o n ensued i f an a l t e r n a t i n g c u r r e n t component of g r e a t e r 70.7%  corros-  of the f i x e d d i r e c t c u r r e n t v a l u e was  imposed.  He  than  suggested t h a t  the hydrogen adsorbed on the m e t a l d u r i n g the c a t h o d i c h a l f - c y c l e impeded the f o r m a t i o n  ide ions.  of an oxide 178  Kukushkin  l a y e r , hence a i d i n g a c t i v e c o r r o s i o n by  chlor-  a t t r i b u t e d the f a s t r a t e of p l a t i n u m d i s s o l u t i o n  w i t h a l t e r n a t i n g c u r r e n t to the d e p a s s i v a t i o n of the s u r f a c e d u r i n g cathodic cycle.  Ryazanov e t al^^  and Kadaner and  Bolko^^  the b a s i c problem i n a l t e r n a t i n g c u r r e n t c o r r o s i o n of the of the r e a c t i o n s d u r i n g a c y c l e and d u r i n g the r e a c t i o n s . considered  s t a t e that determination  the d i s t r i b u t i o n of c u r r e n t expended  Both the anodic  and  c a t h o d i c h a l f - c y c l e s may  as the sums of harmonic processes the sum  at the same frequency  example, the anodic h a l f - c y c l e may  be  and  For m e t a l d i s s o l u t i o n t o occur  chlorine evolution currents).  the  be (for  of the m e t a l d i s s o l u t i o n the  q u a n t i t y of e l e c t r i c i t y expended f o r d i s s o l u t i o n i n the p o s i t i v e h a l f c y c l e must be cycle.  g r e a t e r than t h a t f o r d i s s o l u t i o n i n the n e g a t i v e  half-  80 The  d i s s o l u t i o n o f n o b l e m e t a l e l e c t r o d e s as a r e s u l t o f  i n t e n t i o n a l or u n i n t e n t i o n a l o x i d a t i o n / r e d u c t i o n s u b j e c t o f much s p e c u l a t i o n . be  treatments has been  The mechansim f o r m e t a l l o s s has y e t t o  adequately confirmed. 140 Warne and H a y f i e l d  to o n - o f f platinum  suggested t h a t c u r r e n t r e v e r s a l s due  s w i t c h i n g may g i v e r i s e t o t h e e l e c t r o c h e m i c a l r e d u c t i o n o f s u r f a c e oxide  and thus g i v e r i s e t o the f o r m a t i o n  of a f i n e l y -  d i v i d e d m e t a l t h a t i s more capable o f passage i n t o s o l u t i o n .  72 Srinivasan et a l  proposed a p o s s i b l e a c t i v a t i o n mechanism  whereby the m e t a l i s d i s s o l v e d on the anodic  p o r t i o n during  h o d i c p u l s i n g , y i e l d i n g a f r e s h s u r f a c e o f constant  anodic/cat-  catalytic  activity.  34 Damjanovic and B r u s i c  a t t r i b u t e d the changes i n the a c t i v -  i t y of noble metal a l l o y electrodes with e i t h e r the f o r m a t i o n  anodic/cathodic  pulsing to  o f new c r y s t a l s on t h e e l e c t r o d e s u r f a c e o r the  escape i n t o s o l u t i o n of c a t i o n s which formed t h e o x i d e s . Llopis"''"'"'' found t h a t the c o r r o s i o n o f rhodium by a squarewave c u r r e n t o c c u r r e d  o n l y i f the p o t e n t i a l on t h e p o s i t i v e h a l f - c y c l e  reached a v a l u e above 0.88 V (S.C.E.). as due t o the r e p e t i t i v e f o r m a t i o n  He thus e x p l a i n e d  the c o r r o s i o n  and r e d u c t i o n o f a s u r f a c e  oxide  l a y e r on t h e e l e c t r o d e .  163 Blake e t a l  stated that a f i n e l y - d i v i d e d palladium  pre-  c i p i t a t e on a smooth e l e c t r o d e s u r f a c e which had a lower a c t i v a t i o n energy f o r d i s s o l u t i o n than the s u b s t r a t e ,  c o u l d account f o r o r d e r - o f -  magnitude d i f f e r e n c e s i n r e a c t i o n r a t e s a t a g i v e n imposed p o t e n t i a l .  81 Biegler  15  suggested  t h a t a c t i v a t i o n by  anodic/cathodic  c y c l i n g p r o b a b l y i n v o l v e d s u r f a c e s t r u c t u r a l changes i n v o l v i n g a r e d i s t r i b u t i o n o f s u r f a c e p l a t i n u m atoms, and t h a t t h i s freedom o f movement a r o s e f r o m the p e r i o d i c f o r m a t i o n and b r e a k a g e o f bonds.  Unstable high-energy  platinum-oxygen  p l a t i n u m atoms a r e r e a r r a n g e d o r removed  c o m p l e t e l y i n t h i s manner, l e a v i n g a s t a b l e e l e c t r o d e s u r f a c e . f u r t h e r suggested imbalance  t h a t p l a t i n u m d i s s o l u t i o n may  Biegler  account f o r the charge  i n the o x i d a t i o n and r e d u c t i o n of a p l a t i n u m s u r f a c e and  o b s e r v a t i o n t h a t these charges  the  converge w i t h r e p e a t e d c y c l i n g as a de-  c r e a s i n g f r a c t i o n o f atoms l e a v e the s u r f a c e . 181 M a y e l l and Barber  proposed a d i s s o l u t i o n / r e d e p o s i t i o n  mechanism i n o r d e r t o e x p l a i n the changes i n a c t i v i t y of p l a t i n u m rhodium a l l o y e l e c t r o d e s (porous and smooth) w i t h c y c l i c v o l t a m m e t r y . F o r the smooth a l l o y s , the p a r t l y - s o l u b l e rhodium o x i d e s formed d u r i n g the a n o d i c c y c l e a r e a b l e t o d i f f u s e away and a r e not d e p o s i t e d i n the r e d u c t i o n h a l f of the c y c l e .  Hence, more p l a t i n u m becomes exposed.  For the porous a l l o y s , the rhodium o x i d e s formed w i t h i n the s t r u c t u r e a r e not a b l e t o c o m p l e t e l y d i f f u s e away b e f o r e they a r e r e - d e p o s i t e d . On d e p o s i t i o n , some of the p l a t i n u m i s covered and t h e e l e c t r o d e assumes more r h o d i u m - l i k e b e h a v i o u r .  Only a f t e r l o n g p e r i o d s o f c y c l i n g i s  s i g n i f i c a n t rhodium l o s t such t h a t p l a t i n u m - l i k e b e h a v i o u r  reappears.  182 Rand and Woods  l a t e r c o n c u r r e d w i t h these o b s e r v a t i o n s . 65  Chemodanov p l a t i n u m t o the presence  a t t r i b u t e d enhanced d i s s o l u t i o n b e h a v i o u r  of  o f oxygen on t h e s u r f a c e and i n the s u r f a c e  82 layers of the metal.  A t p o t e n t i a l s n e a r t h a t o f t h e hydrogen e l e c t r o d e ,  the d i s s o l u t i o n r a t e decays as t h e m e t a l o x i d e s g r a d u a l l y go i n t o s o l u tion. 52  Kravchenko e t a l  support the i d e a that platinum d i s s o l u -  t i o n as a r e s u l t o f a c t i v a t i o n i s due t o t h e o x i d a t i o n o f p l a t i n u m b l a c k formed by t h e r e d u c t i o n o f t h i c k o x i d e s . 183 Johnson e t a l  s t a t e d that the d i s s o l u t i o n of platinum  oxide i s thermodynamically p o s s i b l e during cathodic p o t e n t i a l scanning i n p e r c h l o r i c a c i d b y means o f : P t 0 + 4 H + 2e P t ( I I ) + 2H 0  E° = .64 V (S.C.E.)  +  2  2  I n c o n t r a s t t o t h e f i n d i n g s o f Johnson e t a l , Rand and Woods "'" r e p o r t e d t h a t p l a t i n u m d i s s o l v e s m a i n l y 4  as P t ( I V ) d u r i n g  cyclic  v o l t a m m e t r y , b u t a t t r i b u t e t h i s t o t h e f o r m a t i o n o f a more s t a b l e comp l e x w i t h t h i s s p e c i e s and t h e i r s u l p h u r i c a c i d e l e c t r o l y t e .  They con-  s i d e r a c a t h o d i c d i s s o l u t i o n mechanism such as t h a t suggested by Johnson e t a l was u n l i k e l y as i t would n o t e x p l a i n the observed charge imbalance between a n o d i c and c a t h o d i c c h a r g i n g .  Rand and Woods thus suggest t h a t  the d i s s o l u t i o n r e a c t i o n s must be a n o d i c ,  and i n v o l v e e i t h e r d i r e c t  d i s s o l u t i o n o r t h e i n v o l v e m e n t o f s u r f a c e oxygen s p e c i e s . Vetter  1 0 4  has r e c e n t l y n o t e d t h a t t h e d i f f e r e n c e i n t h e  v a l u e s f o r t h e a n o d i c and c a t h o d i c charges has y e t t o be e x p l a i n e d by metal corrosion.  That i s , o n l y i n cases where t h e s e v a l u e s have been  found t o be v e r y c l o s e can t h e i r d i f f e r e n c e be a t t r i b u t e d t o c o r r o s i o n .  83 Chao e t a l  184  found t h a t the passage of p l a t i n u m  into  s o l u t i o n d u r i n g p o t e n t i a l c y c l i n g d i d not p l a y a s i g n i f i c a n t r o l e i n the observed s u r f a c e r e s t r u e t u r a t i o n .  2.4.4  D e g r a d a t i o n o f Noble M e t a l  Coatings  Mechanisms f o r t h e d e g r a d a t i o n f o r the most p a r t , m e c h a n i c a l .  of n o b l e m e t a l c o a t i n g s  That i s , the l o s s of m e t a l i s a r e s u l t  of a l o s s of c o a t i n g adherence which a r i s e s from a t t a c k o f the material.  substrate  Coated e l e c t r o d e s w i t h v e r y t h i c k n o b l e m e t a l l a y e r s show  s i m i l a r behaviour t o massive p l a t i n u m , Burghardt  are,  185  and  Okamura  186  but a r e not  economic.  r e p o r t t h a t t h i c k c o a t i n g s may  low a t t a c k r a t e q u a l i t i e s of c o a t i n g s of l e s s e r t h i c k n e s s . however, t h a t as c o a t i n g s become so t h i n  Indeed,  not have the It is clear,  (or porous) t h a t the  substrate  i s exposed, the chances f o r a t t a c k of the s u b s t r a t e become much g r e a t e r . 140 144 On the other hand, t h i c k e r c o a t i n g s prevent s u b s t r a t e a t t a c k . '  2.4.4.1  D e g r a d a t i o n as a R e s u l t of C o a t i n g  Undermining  140 Warne and H a y f i e l d flows media. and  d i s c u s s the g a l v a n i c c u r r e n t which  as a r e s u l t of immersion of p l a t i n i z e d The main p r o c e s s e s  titanium i n agressive acid  would be hydrogen e v o l u t i o n from the  c o v e r i n g of the t i t a n i u m w i t h a p r o t e c t i v e f i l m .  t i o n of the c u r r e n t would be due l e a d to undermining and  platinum  However, a  propor-  to t i t a n i u m d i s s o l u t i o n w h i c h c o u l d  subsequent detachment of the c o a t i n g .  As  the  143  84  r e l a t i v e area of exposed titanium increases, the chances f o r substrate d i s s o l u t i o n increase.  While the steady-state p o t e n t i a l i n aerated con-  centrated hydrochloric acid l i e s well within the region of potentials for formation of a passive f i l m on titanium, the p o t e n t i a l i n de-aerated solution l i e s very close to the Flade p o t e n t i a l , with the chances f o r substrate d i s s o l u t i o n thus being  greater.  144  Antler and Butler  postulated that metal loss i n acid  chloride media involved substrate corrosion through the pores i n the coating, whereby the corrosion f i l m spreads l a t e r a l l y and undercuts the coating. 145  Khodkevich  attributed the d i s r i p t i o n of adhesion of the  coating i n a l k a l i n e carbonate solutions to the deposition of hydrates of platinum, titanium, and solution impurities i n the pores of the coating as a r e s u l t of OH  discharge, where the layer near the anode  becomes a c i d i f i e d leading to the deposition of hydrates which were s o l 141 142  uble i n excess a l k a l i .  Khodkevich  '  also considered  oxidation of the substrate was the main reason f o r coating  the state of degradation.  Certain oxide films showed a high leakage current, i n d i c a t i n g that the films were porous and l e s s - p r o t e c t i v e . 121  B i t t l e s and Littauer  said that rapid d e t e r i o r a t i o n of  p l a t i n i z e d valve metals may be due to the morphology of the coating, where the detachment of the coating was the r e s u l t of substrate oxidation at deposit imperfections. 127  Kuhn  noted that the r e s i d u a l d i s s o l u t i o n of exposed t i t a n -  ium increased with higher a c i d concentrations  and temperature.  Operation  85 under stagnant a c i d i t y , and  c o n d i t i o n s may  g i v e r i s e to l o c a l r e g i o n s of  hence o f s u b s t r a t e a t t a c k .  c o n d i t i o n s may  higher  He p o s t u l a t e d t h a t , i n c r e v i c e s ,  e x i s t where the s u b s t r a t e was  a t t a c k must i n v o l v e l o c a l i z e d o v e r h e a t i n g .  not p a s s i v e , a l t h o u g h  such  Yukhevichattributed  the  a c i d i t y o f the s o l u t i o n i n such c r e v i c e s as the r e s u l t of secondary r e a c t i o n s such as:  Cl  2  + H0  TiCl  Z  2  4  + 2H 0 2  HCIO + H  +  %  + H  Ti0  2  + Cl" +  + Cl"  146 F a i t a et a l about by  suggested t h a t i n c r e a s e d a c i d i t y i n the p o r e s was  brought  the e v o l u t i o n of some oxygen: 2H 0 2  t  0  2  + 4H  +  +  4e  I t i s i n t e r e s t i n g to note t h a t improved e l e c t r o d e l i f e  has  been o b t a i n e d by means of s u b s t r a t e treatments which improve c o a t i n g adherence.  Some treatments i n v o l v e e t c h i n g which l e a v e s the  w i t h many deep c r e v i c e s , of oxides  187  vacuum treatment to prevent  or impure compounds between the c o a t i n g and  the  substrate formation  substrate,  186  and  188 the f o r m a t i o n  of a t i t a n i u m h y d r i d e  surface l a y e r .  In a d d i t i o n , the  use of s u b s t r a t e s which form more p r o t e c t i v e f i l m s (such as niobium, and undermining.  tantalum,  some t i t a n i u m a l l o y s ) w i l l a l s o reduce m e t a l l o s s due  to  86  2.4.4.2  Coating  Other Causes o f C o a t i n g  Loss  l o s s may a r i s e as a r e s u l t o f t h e normal d i s s o l u t i o n  of the m e t a l , which may be enhanced by t h e presence o f c e r t a i n o r g a n i c 140 additives,  the f a c t t h a t t h e n o b l e m e t a l c r y s t a l l i t e s may be l o o s e l y 143  bound t o t h e s u r f a c e ,  t h e promotion o f h i g h l y - c o r r o s i v e  conditions  i n pores , o r the use o f h i g h o v e r p o t e n t i a l s ., i n t h e c o a t i.n g127,189 peroxidation processes). Others c o n s i d e r  that the production  (as i n  o f gas may a f f e c t c o a t -  143 i n g adherence.  Haley  non-porous e l e c t r o d e s anodization.  found t h a t hydrogen d i s s o l v e d i n caused the c o a t i n g  to l i f t  pre-cathodized,  o f f t h e base m e t a l on  Yukhevich"'"^ s t a t e d t h a t the presence of hydrogen  could  impede o x i d e l a y e r f o r m a t i o n , l e a d i n g t o f i s s u r e c o r r o s i o n due t o c h l o r 146 ide ions. Faita et a l s a i d the e v o l u t i o n o f gas i n p o r e s a f f e c t s t h e 190 m e c h a n i c a l s t a b i l i t y of t h e e l e c t r o d e .  Bianchi et a l  s a i d the b l a n -  k e t i n g e f f e c t of r i s i n g gas bubbles c o u l d aggravate c u r r e n t i t y on l a r g e c o a t e d  Dreyman  non-uniform-  anodes. 191 f i n d s t h a t improving t h e e l e c t r i c a l  of the s u b s t r a t e r e s u l t s i n improved anode, and lowered c o a t i n g  temperature c o n d i t i o n s  consumption.  resistivity f o r the  87  2.4.5  Summary  This section on platinum d i s s o l u t i o n was a complete survey of the mechanisms of corrosion or degradation which have been suggested over the past half-decade.  I t i s intended to supplement the e a r l i e r  review made by L l o p i s , a n d i s i n fact an extraction from a much more extensive review of the author's.  I t i s f e l t , however, that this recent  work e f f e c t i v e l y summarizes most situations for noble metal d i s s o l u t i o n . As with other electrochemical processes, that of d i s s o l u t i o n suffers from the occurrence of obscuring, simultaneous reactions.  In addition, there  are a multitude of possible d i s s o l u t i o n reactions to choose from involving the noble metal alone, not to mention those i n which the substrate of a coated electrode takes some part.  F i n a l l y , consideration must be  given to the effects of electrode pretreatment on the d i s s o l u t i o n of noble metal electrodes.  88  2.5  RELATION TO AIMS OF PRESENT WORK  In this thesis, the anodic electrochemical behaviour of platinum/iridium a l l o y s i s considered - f o r both wire and coated  (tit-  anium substrate) electrodes - i n sulphuric acid and chloride solutions. Within the l i m i t a t i o n s of this experimental framework, the topics which have been dealt with extensively i n the preceding sections can be d i s cussed with respect to t h e i r bearing on the present work. The effects of electrode pretreatment have been discussed from mainly a mechanistic point of view.  I t appears to be necessary to  have at least an appreciation f o r the possible e f f e c t s of c e r t a i n procedures on electrodes, as an aid to understanding other electrochemical phenomena such as metal d i s s o l u t i o n and passivation, or to comprehending sources of error or interference i n electrochemical measurements.  It  i s not the purpose of t h i s thesis to test the various alternative theories proposed f o r electrode a c t i v a t i o n , but to recognize the possible effects of t h i s phenomenon.  From the r e s u l t s of past workers, the pro-  cedures used i n this thesis for preparing the electrodes have been s e l ected i n order to provide a reproducible, understandable, surface cond i t i o n (see Appendix I ) . Furthermore, from these past r e s u l t s , a number of phenomena can be expected to occur i n the present experimental systems: 1)  Severe anodic treatment may be expected to cause some disruption of the noble metal surface.  89  2)  C y c l i c procedures involving anodization followed by mild r e duction would not be expected to cause an increase i n surface area and may,  conversely, lead to a decrease i n roughness  factor. 3)  For the purposes of this thesis, i t i s not c r i t i c a l to be able to d i s t i n g u i s h between "activated" states of the electrode, but only to ensure that a constant surface state i s produced i n the electrodes i n this work.  Thus, such concepts as "active  surface structure" or other " a c t i v e " surfaces need not be considered. 4)  The fact that d i f f e r e n t amounts of oxygen can be produced withi n , and on, noble metal electrodes i s the most important  con-  cept to be r e a l i z e d from the discussion on electrode pretreatment. 5)  Impurity  effects must be minimized i n order to obtain meaning-  f u l r e s u l t s , as their presence can mask other electrochemical processes. The section on surface area determination was  included to  provide a key for ready comparison with the surface area figures quoted by other authors. section 3.4.3  The procedure u t i l i z e d i n this work (discussed i n  and i n Appendix II) i s i n fact a synthesis of several  methods to be found i n the l i t e r a t u r e , and was i l y because i t was  arrived at not  the most r e l i a b l e , but because i t was  necessar-  the most read-  i l y applied with the equipment a v a i l a b l e . The surface area values quoted  90  i n this thesis are, i n r e a l i t y , "determined  surface areas", and are  comparable to other r e s u l t s mentioned i n the l i t e r a t u r e i f consideration i s given the assumptions behind such procedures.  The present sys-  tem takes advantage of the galvanostatic pulse technique. For e l e c t r o l y s i s i n chloride solutions, the past l i t e r a t u r e indicated that much work had taken place with platinum electrodes, and that there were several interpretations of the phenomena encountered during p o l a r i z a t i o n i n such solutions.  I t was decided that an i n v e s t i -  gation of noble metal a l l o y systems i n such media would help expand the f i e l d of knowledge i n this area and extend the v a l i d i t y of the r e s u l t s of other workers to such systems.  For the a l l o y wire electrodes, i t  was expected that:  1)  p o l a r i z a t i o n c h a r a c t e r i s t i c s would be similar to platinum showing a three-part p o l a r i z a t i o n curve.  2)  the "potential jump" would be expected to show some r e l a t i o n to a l l o y content, i f the phenomenon was related to the format i o n of platinum surface oxides, or to differences i n the r e l a t i v e adsorption of oxygen and chloride between platinum and iridium.  3)  passivation tendency should be reduced with higher iridiumcontent electrodes due to the more r e v e r s i b l e behaviour of this metal toward oxygen adsorption.  For the case of titanium substrate electrodes, i t was not possible to  r e l y extensively on the r e s u l t s of other workers i n order to predict the behaviour of the present electrodes.  The reason for this i s the  myriad possible methods f o r construction of such electrodes which lead to c h a r a c t e r i s t i c s which d i f f e r l i t t l e from that of a s o l i d  platinum  electrode to high degrees of dispersion and porosity of the coatings. Hence, i t was necessary to obtain p o l a r i z a t i o n curves i n the present work which must be regarded as the only r e l i a b l e source of p o l a r i z a t i o n data for the p a r t i c u l a r electrodes used here.  As the present  electrodes  show a high degree of porosity, i t i s important to consider the following: 1)  the substrate i s expected to be oxidized during pretreatment, and to remain that way  throughout experimentation  (that i s ,  the presence of bare titanium i s u n l i k e l y ) . 2)  the "breakdown" p o t e n t i a l of t h i s oxide f i l m i s lowered i n the presence of chloride ions.  3)  the existance of crevices and pores over the electrode surface may  be s i t e s for conditions of l o c a l i z e d corrosion of the sub-  strate. 4)  the substrate i s not wholly inert and, form of progressive degradation  i n f a c t , undergoes some  during anodic p o l a r i z a t i o n .  Anodic d i s s o l u t i o n i s not investigated quantitatively i n this thesis, but i t i s nevertheless necessary to obtain an appreciation for d i s s o l u t i o n phenomena both with an aim to future work on this topic and i n order to summarize the e f f e c t s (or lack of them) which may  be  92  related  1)  t o e l e c t r o d e c o r r o s i o n i n the p r e s e n t work.  These a r e :  A c t i v e d i s s o l u t i o n o f n o b l e m e t a l anodes i s s i g n i f i c a n t o n l y i n complexing media, and a t p o t e n t i a l s where complexing a r e a b l e t o s u c c e s s f u l l y adsorb on the m e t a l s u r f a c e .  ions The  "lower T a f e l r e g i o n " o f the p o l a r i z a t i o n c u r v e s i n c h l o r i d e media d e f i n e s a r e g i o n o f simultaneous m e t a l d i s s o l u t i o n and chlorine 2)  evolution.  The n a t u r e o f the p a s s i v a t i n g l a y e r formed  on n o b l e m e t a l anodes  at h i g h e r p o t e n t i a l s has y e t to be c l e a r l y e s t a b l i s h e d , and hence the mechanism o f d i s s o l u t i o n of p a s s i v a t e d anodes i s not i e n t l y known.  suffic-  P o l a r i z a t i o n a t h i g h e r p o t e n t i a l s does, however,  i n v o l v e simultaneous m e t a l d i s s o l u t i o n  ( p r o b a b l y through s e v e r a l  mechanisms) a l o n g w i t h c h l o r i n e and oxygen e v o l u t i o n . 3)  The c u r r e n t e f f i c i e n c y o f n o b l e m e t a l d i s s o l u t i o n i s low and  can  be made v i r t u a l l y n e g l i g i b l e by employing anode m a t e r i a l s o f known h i g h c o r r o s i o n r e s i s t a n c e , and e l e c t r o l y t e s which a r e n o t s t r o n g l y a c i d i c or 4)  complexing.  T i t a n i u m s u b s t r a t e anodes w i t h porous c o a t i n g s may  be expected  to show d i f f e r e n t behaviour owing to the p o s s i b i l i t y of a t t a c k of the s u b s t r a t e which may  l e a d to the undermining  (and hence  m e c h a n i c a l detachment) of the c o a t i n g or to i n t e r f e r e n c e from c o r r o s i o n p r o d u c t s of t h e s u b s t r a t e . 5)  C o n d i t i o n s which promote the c o r r o s i o n o f the s u b s t r a t e m e t a l would be expected t o cause a c c e l e r a t e d d e g r a d a t i o n of the c o a t e d anodes.  93 3.  3.1  EXPERIMENTAL  ELECTRODES  Two concern was  t y p e s of working e l e c t r o d e were employed.  the behaviour of an e l e c t r o d e m a t e r i a l having  s u b s t r a t e , w i t h a c o a t i n g of p l a t i n u m  c o n t a i n i n g 30%  Of a  prime  titanium  iridium.  The 192  coated  e l e c t r o d e s were p r e p a r e d by I m p e r i a l M e t a l I n d u s t r i e s L i m i t e d  by a t h e r m a l d e c o m p o s i t i o n method.  I t was  b e h a v i o u r of smooth w i r e e l e c t r o d e s  (of p l a t i n u m  a l l o y s ) would be l e s s d i f f i c u l t e l e c t r o d e c o n s t r u c t i o n and  f e l t , however, t h a t and  to i n v e s t i g a t e (due  simpler  the i n t e r p r e t a t i o n of r e s u l t s ) .  of 0.020 i n c h diameter were o b t a i n e d  coated  the e l e c t r o d e i t s e l f  e l e c t r o d e s were made and  hole i n a t e f l o n holder.  to the ease of  electrodes  Hence, w i r e  contacted  electrodes  the s o l u t i o n .  these were p r e s s e d  E l e c t r i c a l c o n t a c t was  D i s c s of  I n order  e l e c t r o l y t e behind the e l e c t r o d e and  provided  by means of a  p l a c e , the e n t i r e assembly was were c o n s t r u c t e d  through  a l s o pressed  to p r e v e n t leakage of  to f i r m l y hold  f i l l e d w i t h epoxy.  the  into a similarly sized  through a pyrex tube which was  a h o l e on the top of the h o l d e r .  Limited.  such t h a t o n l y pyrex,  w i r e spot-welded to the back of the e l e c t r o d e which passed up the t e f l o n h o l d e r and  the  created  from Johnson Matthey M e t a l s  Working e l e c t r o d e s were c o n s t r u c t e d t e f l o n , and  platinum-iridium  apparatus assembly, as w e l l as  f a c t t h a t the enormous s u r f a c e a r e a of the coated problems w i t h  the  into the  the e l e c t r o d e i n Wire working  electrodes  by f i r s t l y s o l d e r i n g a copper w i r e to about a two-inch  length of noble metal wire.  I t was hoped that any e f f e c t s of heat t r e a t -  ment caused by the soldering would not reach the extreme h a l f - i n c h of the wire which would ultimately be exposed to the solution.  (The wire  electrodes were used i n their as-received condition, with no additional heat treatments applied.)  The wire was then pushed through a small plug  of t e f l o n which i n turn f i t snugly i n the end of a pyrex tube.  As be-  fore, the assembly was then f i l l e d with epoxy i n order to prevent any leakage and to f i r m l y f i x the position of the wire electrode.  Prior  to assembly, a l l pyrex and t e l f o n parts were thoroughly cleaned i n chromic/sulphuric a c i d .  After construction, and before use, the e l e c t -  rodes were soaked i n chromic/sulphuric acid for a few minutes, followed by washing and storage i n t w i c e - d i s t i l l e d water.  Additional pretreat-  ments were made according to the nature of the i n d i v i d u a l experiments. The reference electrodes used were Fisher Calomel Reference Electrodes of the porous plug type.  Such electrodes served to minimize  l i q u i d junction potentials i n the chloride e l e c t r o l y t e s , where p o t e n t i a l measurements were made.  Calomel electrodes were also used i n sulphuric  acid e l e c t r o l y t e s (for surface area measurements) where precise knowledge of the electrode p o t e n t i a l was not important.  Reproducibility was checked  from run to run by measuring the p o t e n t i a l established i n the (same) electrolyte when hydrogen was bubbled through the system.  Values within a  few m i l l i v o l t s of -0.249 mV (S.C.E.) were always obtained. Liquid junction potentials were not considered to be important i n measurements i n chloride media.  Reported values of these potentials  95 i n the l i t e r a t u r e show t h a t , f o r t h e s a t u r a t e d K C l - d i l u t e KC1 boundary, the p o t e n t i a l i s l e s s than 5 mV f o r s i m i l a r c o n d i t i o n s t o those  encoun-  193 194 t e r e d i n t h i s work.  '  The v a l u e s f o r KC1 and NaCl o f s i m i l a r a c t -  i v i t i e s can be assumed t o be comparable due t o t h e c l o s e n e s s o f t h e t r a n s f e r e n c e numbers o f t h e N a  +  and K  +  ions.  The use o f t h e s a t u r a t e d c a l o m e l e l e c t r o d e i n s u l p h a t e 195 e l e c t r o l y t e s i s not a d v i s a b l e , a t e contamination)  but no adverse  effects  (due t o s u l p h -  were n o t i c e d , as t h e e l e c t r o d e was of t h e porous  p l u g type and t h e l e v e l was always m a i n t a i n e d s o l u t i o n , t h i s hindering contamination.  above t h a t o f t h e s u l p h a t e  Moreover, t h e e l e c t r o d e was  o n l y used i n s u l p h a t e s o l u t i o n f o r t h e n e c e s s a r y p e r i o d o f time t o p e r form t h e experiments i n q u e s t i o n and was then removed.  The s u l p h u r i c  a c i d / K C l s o l u t i o n l i q u i d j u n c t i o n p o t e n t i a l v a r i e s w i t h the c o n c e n t r a t i o n of t h e s a l t and f o r t u i t o u s l y , f o r 1M H^SO^, t h e v a l u e o f t h i s p o t e n t i a l passes  from a r e g i o n o f h i g h p o s i t i v e v a l u e s t o n e g a t i v e  val-  ues a t t h e s a t u r a t e d KC1 c o n c e n t r a t i o n , i . e . , the l i q u i d j u n c t i o n p o t e n tial  i s c l o s e to z e r o .  3.2  ELECTROLYTES  Reagent grade c h e m i c a l s were employed i n a l l c a s e s , the d e s i r e d c o n c e n t r a t i o n s b e i n g o b t a i n e d by d i l u t i o n w i t h water.  with  twice-distilled  G a l v a n o s t a t i c p o l a r i z a t i o n c u r v e s were o b t a i n e d i n 1M NaCl w i t h  a pH o f 2 ( t h e pH used i n the e l e c t r o l y t i c p r o d u c t i o n of c h l o r i n e ) . A l s o , pH 2 has been r e p o r t e d as the lowest pH v a l u e where P t i s stable.  96 i n the p r e s e n c e of c h l o r i n e and  chlorides.  146  Potentiostatic polariza-  t i o n curves were o b t a i n e d i n the c h l o r i d e e l e c t r o l y t e and H^SO^.  Potentiostatic anodization  t r o l y t e s : IM H S 0 , IM NaCl; pH 2  elec-  IM N a C l ; n e u t r a l  are  used i n commercial c h l o r a t e p r o d u c t i o n ) , ments were made i n IM ^SO^, erent  electrolyte.  4M H C l .  ( n e u t r a l pH's  S u r f a c e a r e a measure-  which i s a s u i t a b l e non-complexing  In a d d i t i o n ,  the oxygen a d s o r p t i o n  separated from the hydrogen r e g i o n  region  3.3  i n such an a c i d i c s o l u t i o n .  i n order to remove d i s s o l v e d oxygen  (Fig. 1).  p o l a r i z a t i o n measurements, an The  anode and  by a f i n e g l a s s f r i t which was  i n t o the  compartments ( c e l l c a p a c i t y :  sure and  spent e l e c t r o l y t e could  gas  The could  to c o n c e n t r a t i o n  con-  two  compartments  by  These j o i n t s were then wrapped w i t h The  2 liters)  e l e c t r o l y t e was  fed  by means of h e l i u m p r e s -  be removed t o a s t o r a g e v e s s e l by means  of a vacuum hose a t t a c h e d to t h i s v e s s e l .  avoided.  "H-type" c e l l was  i n s e r t e d between the  t e f l o n tape to p r e v e n t l o s s of e l e c t r o l y t e .  e f f e c t s due  investigations.  cathode compartments were s e p a r a t e d  means of ground g l a s s t a p e r j o i n t s .  and  before  gas.  k i n d s o f c e l l were employed i n these  galvanostatic  structed  any  All  CELLS  Two For  indiff-  i s well^-  e l e c t r o l y t e s were purged w i t h commercial ( b o t t l e d ) h e l i u m gas use  IM  runs were performed i n v a r i o u s  2,  4  also i n  I n t h i s way,  i t was  hoped  changes i n the e l e c t r o l y t e c o u l d  c e l l compartments were f u r t h e r f i t t e d  w i t h gas  that  be  bubblers,  a l s o be bubbled through the p r i m a r y f r e s h e l e c t r o l y t e  97  storage v e s s e l .  The f r e s h e l e c t r o l y t e and t h e a n o l y t e c o u l d f u r t h e r -  more be m a i n t a i n e d under a h e l i u m gas b l a n k e t . M a g n e t i c s t i r r e r s were used i n b o t h compartments.  The L u g g i n c a p i l l a r y was mounted i n a t e f l o n  t a p e r j o i n t w h i c h passed t h r o u g h the s i d e o f t h e c e l l , w i t h t h e t i p of the  c a p i l l a r y r e a c h i n g t o w i t h i n a m i l l i m e t e r of t h e w o r k i n g e l e c t r o d e .  A l t h o u g h t h e c e l l i t s e l f and t h e s t o p c o c k s were c o n s t r u c t e d o f t e f l o n or  g l a s s , PVC  electrolyte. employed.  t u b i n g was used f o r gas l i n e s and f o r t r a n s f e r of the None of t h e s e m a t e r i a l s were a t t a c k e d by t h e s o l u t i o n s  The c e l l was p o s i t i o n e d i n a c o n s t a n t - t e m p e r a t u r e b a t h o f  a n o n c o n d u c t i n g o i l (Mentor 2 9 ) . A second c e l l was  employed f o r measurement of s u r f a c e a r e a ,  for  t h e d e t e r m i n a t i o n of some p o t e n t i o s t a t i c p o l a r i z a t i o n c u r v e s , and  for  potentiostatic electrolysis  (Fig. 2).  The w o r k i n g and  e l e c t r o d e s were not s e p a r a t e d by any means.  When a w i r e w o r k i n g e l e c t -  rode was used, a c y l i n d r i c a l gauze a u x i l i a r y e l e c t r o d e was around i t .  auxiliary  positioned  When a f l a t - s u r f a c e d w o r k i n g e l e c t r o d e was u s e d , the gauze  was r e p l a c e d by a l o n g l e n g t h of bent p l a t i n u m w i r e p o s i t i o n e d a c r o s s the  c e l l and f a c i n g t h e w o r k i n g e l e c t r o d e .  The c e l l i t s e l f was  constr-  u c t e d from a 600 m l . round-topped b e a k e r , and t h e l i d was made from t e f l o n sheet w h i c h c o u l d be clamped down on t h e b e a k e r .  Gas b u b b l i n g  and b l a n k e t i n g c o u l d be employed, and s t i r r i n g was a c c o m p l i s h e d by means of a m a g n e t i c s t i r r i n g b a r .  A. B. C. D. E. F. G. H. I. J. K. L. M.  Figure I. Galvaoostatic Cell  Anode compartment Cathode compartment R e f e r e n c e e l e c t r o d e (S.C.E.) Luggen c a p i l l a r y Gas o u t l e t s Gas b u b b l e r s E l e c t r o l y t e feed i n l e t s B l a n k e t i n g gas i n l e t Working e l e c t r o d e Gauze a u x i l i a r y e l e c t r o d e Spent e l e c t r o l y t e o u t l e t s Sintered glass f r i t M a g n e t i c s t i r r i n g bars  oo  A. B. C. D. E. F. G. H. I. J.  Working e l e c t r o d e Gauze a u x i l i a r y e l e c t r o d e R e f e r e n c e e l e c t r o d e (S.C.E.) Luggen c a p i l l a r y B l a n k e t i n g gas i n l e t Gas o u t l e t Gas b u b b l e r T e f l o n top 600 m l . beaker M a g n e t i c s t i r r i n g bar  Figure 2. Cell for surface area measurement  vO vo  100  3.4  PROCEDURES  3.4.1  Anodic  G a l v a n o s t a t i c Measurements  G a l v a n o s t a t i c p o l a r i z a t i o n curves were o b t a i n e d f o r w i r e e l e c t r o d e s of p l a t i n u m and p l a t i n u m - i r i d i u m a l l o y s c o n t a i n i n g 5, 10, 20, and  25% i r i d i u m .  C u r r e n t was s u p p l i e d by a Beckman E l e c t r o s c a n 30,  w h i c h c o u l d p r o v i d e up to 100 mA.  P r i o r t o the i n i t i a l  t e s t , the H - c e l l  was c l e a n e d w i t h c h r o m i c - s u l p h u r i c a c i d and then washed w i t h d i s t i l l e d water.  twice-  A f t e r c o m p l e t i o n o f subsequent experiments,  was f l u s h e d w i t h f r e s h e l e c t r o l y t e .  Rigorous  the c e l l  d e t e r m i n a t i o n s were p e r -  formed i n a IM N a C l e l e c t r o l y t e whose pH was a d j u s t e d t o a v a l u e of 2 by H C l a d d i t i o n . A f t e r i n t r o d u c t i o n o f t h e working e l e c t r o d e i n t o t h e c e l l , and  alignment  o f t h e Luggen c a p i l l a r y , t h e system was m a i n t a i n e d a t  o p e n - c i r c u i t w h i l e t h e e l e c t r o l y t e was purged w i t h h e l i u m .  The working  e l e c t r o d e was then s u b j e c t e d t o c a t h o d i c p o l a r i z a t i o n a t 100 mA f o r 5 minutes.  T h i s treatment was n e c e s s a r y  t o remove any o x i d e s p r e s e n t on  the e l e t r o d e s , and t o render t h e i r i n i t i a l o p e n - c i r c u i t p o t e n t i a l s of un-cathodized F o l l o w i n g the c a t h o d i c treatment, to  drift  s u r f a c e s r e p r o d u c i b l e , as t h e  electrodes varied  widely.  t h e e l e c t r o d e p o t e n t i a l was p e r m i t t e d  a t o p e n - c i r c u i t w h i l e t h e c e l l was d r a i n e d , f l u s h e d , and r e -  f i l l e d with fresh e l e c t r o l y t e .  T h i s e l e c t r o l y t e was a l s o purged w i t h  helium. When t h e e l e c t r o d e p o t e n t i a l reached  a stable value, the  101  t r a c i n g of the g a l v a n o s t a t i c p o l a r i z a t i o n curve was b u b b l i n g and order  b l a n k e t i n g of the e l e c t r o l y t e was  to sweep away the generated gases and  tamination.  maintained  t o prevent  Mild  helium  throughout i n  atmospheric con-  S t a r t i n g w i t h c u r r e n t s of o n l y a few micro-amperes i n t e n s -  i t y , c u r r e n t s were i n c r e a s e d i n r e g u l a r increments. over which a g i v e n c u r r e n t was of the p o t e n t i a l . had  begun.  a p p l i e d was  The  time p e r i o d  determined by the  G e n e r a l l y , the c u r r e n t was  stability  i n c r e a s e d i f the p o t e n t i a l  remained i n v a r i a n t f o r a minute a f t e r i m p o s i t i o n of t h e c u r r e n t .  some cases, 10 mV  however, the p o t e n t i a l was  over 300  observed to r i s e s l o w l y  seconds) and much l o n g e r  t h a t the p o t e n t i a l had  indeed  times were n e c e s s a r y  become s t a b l e .  In  (by about  t o ensure  I n the r e g i o n of  the  " p o t e n t i a l jump" the p o t e n t i a l r o s e s l o w l y , but p r o g r e s s i v e l y , over time p e r i o d s up  to 40. hours l o n g .  The  experiments were concluded  after  t a i n i n g a T a f e l s l o p e f o r the upper branch of t h e p o l a r i z a t i o n Runs were performed i n d u p l i c a t e , and served, a t h i r d r u n was  3.4.2  ob-  made.  A n o d i c P o t e n t i o s t a t i c Measurements  i r i d i u m coated  model 68 FR  curve.  i f s i g n i f i c a n t v a r i a t i o n was  P o t e n t i o s t a t i c p o l a r i z a t i o n curves were o b t a i n e d 30%  ob-  0.5  was  titanium electrodes. used as the c o n s t a n t  for  platinum-  A Wenking F a s t R i s e P o t e n t i o s t a t p o t e n t i a l supply.  s t a t i c measurements i n v o l v e d measuring the change i n c u r r e n t w i t h time upon i m p o s i t i o n of the p o t e n t i a l of i n t e r e s t .  Potentioflowing  I n t h i s manner,  102  p o l a r i z a t i o n c u r v e s c o u l d be c o n s t r u c t e d from t h e r e s u l t s of many d i f f e r e n t experiments.  I n d e t e r m i n i n g t h e p o l a r i z a t i o n c u r v e s , t h e same  e l e c t r o d e was used throughout.  Between e x p e r i m e n t a l d e t e r m i n a t i o n s ,  the e l e c t r o d e was s u b j e c t e d to a m i l d r e d u c i n g treatment  (hydrogen gas  b u b b l i n g a t o p e n - c i r c u i t ) i n o r d e r t o remove any a n o d i c p r o d u c t s formed during the preceeding anodization.  P o t e n t i o s t a t i c p o l a r i z a t i o n curves  were generated a t v a r i o u s a n o d i z a t i o n time i n t e r v a l s from 10 - 600 s e c onds.  The s o l u t i o n s were purged w i t h h e l i u m b e f o r e each r u n , b u t were  u n s t i r r e d d u r i n g t h e experiments.  Helium gas b l a n k e t i n g h e l p e d t o p r o -  t e c t the system from atmospheric c o n t a m i n a t i o n and t o h e l p sweep away generated gases. P o t e n t i o s t a t i c a n o d i z a t i o n s were a l s o performed  i n sulphuric  a c i d and h a l i d e e l e c t r o l y t e s f o r times as s h o r t as 30 seconds t o p e r i o d s of n e a r l y a day.  These runs were performed  "batch"-wise, w i t h no r e p l e n -  ishment or replacement of t h e e l e c t r o l y t e attempted.  Such runs were  i n t e r r u p t e d i n o r d e r to determine t h e change ( i f any) i n the e l e c t r o chemically active surface area.  3.4.3  S u r f a c e Area Measurements  A f t e r c o n s i d e r i n g the t e c h n i q u e s d e s c r i b e d i n the l i t e r a t u r e f o r measuring  s u r f a c e a r e a , and a s s e s s i n g the a v a i l a b l e a p p a r a t u s , an  a c c u r a t e , r e p r o d u c i b l e , and f a i r l y such measurement.  simple procedure was developed f o r  An a c i d i c e l e c t r o l y t e  ClH H„S0,) was employed i n  103  o r d e r t o p r o v i d e good s e p a r a t i o n of the hydrogen and oxygen r e g i o n s , and  the l a t t e r was  pulse techniques.  chosen as i t was  b e s t s u i t e d to g a l v a n o s t a t i c a n o d i c  The Wenking F a s t R i s e P o t e n t i o s t a t was  used  i n both  p o t e n t i o s t a t i c and g a l v a n o s t a t i c modes, i n c o n j u n c t i o n w i t h a T e k t r o n i x Type 564 B O s c i l l o s c o p e w i t h a camera a t t a c h e d t o m o n i t o r  the p o t e n t i a l  of the working e l e c t r o d e d u r i n g c h a r g i n g .  E l e c t r o d e s were s u b j e c t e d to  s t r o n g a n o d i z a t i o n s (1 - 5 m i n u t e s a t 1800  - 2000 mV  electrodes; 5 - 1 0  minutes a t 1800  e l e c t r o d e s ) i n de-aerated  - 2000 mV  S.C.E. f o r w i r e  S.C.E. f o r coated t i t a n i u m  (He-purged) e l e c t r o l y t e , f o l l o w e d by a  i n g treatment w i t h hydrogen gas bubbled  i n t o the s o l u t i o n .  reduc-  This bubbl-  i n g , accompanied w i t h v i g o r o u s s t i r r i n g w i t h a t e f l o n - c o a t e d magnetic s t i r r e r , caused potential.  the e l e c t r o d e p o t e n t i a l t o drop  (The time r e q u i r e d f o r the p o t e n t i a l t o drop was  about 3 minutes.)  Such treatment  s o l v e d oxygen.  i n t e r i o r saturated with  w i t h no a d d i t i o n a l charge g o i n g t o  A f t e r hydrogen b u b b l i n g , the s o l u t i o n was  purged w i t h  i n order to remove the p o s s i b i l i t y of m o l e c u l a r  o x i d a t i o n w i t h the r e l a t i v e l y l o w - c u r r e n t p u l s e s used e l e c t r o d e s ; 20.5  mA  f o r coated e l e c t r o d e s ) .  (5 mA  t h a t the replacement  hydrogen f o r wire  The p o t e n t i a l r o s e o n l y  s l i g h t l y toward a n o d i c p o t e n t i a l s d u r i n g i n e r t - g a s p u r g i n g ,  slight.  dis-  Subsequent a n o d i c c h a r g i n g would thus o n l y d e p o s i t s u r -  f a c e oxygen ( i . e . , a monolayer), dermasorption.  usually  has been a p p l i e d ^ t o remove o n l y  s u r f a c e - a d s o r b e d oxygen, l e a v i n g the metal  h e l i u m gas  t o the hydrogen e l e c t r o d e  of s u r f a c e oxygen from the m e t a l  indicating  i n t e r i o r was  very  About one minute b e f o r e a p p l i c a t i o n of the c h a r g i n g p u l s e , t h e  104  gas purge was solution.  stopped,  w i t h the f l o w d i v e r t e d t o m e r e l y b l a n k e t  A p p r o p r i a t e o s c i l l o s c o p e scan speeds ranged from  the  5-50  m s e c . / d i v i s i o n f o r w i r e e l e c t r o d e s to .5 - 1 s e c . / d i v i s i o n f o r coated electrodes. l a y e r " , and  The r e s u l t a n t t r a c e s showed w e l l - d e f i n e d hydrogen, oxygen r e g i o n s .  "double-  Compensation f o r d o u b l e - l a y e r c h a r g i n g  was  made by u s i n g an e x t r a p o l a t i o n o f the s l o p e of the " d o u b l e - l a y e r " r e g i o n to the oxygen e v o l u t i o n p o t e n t i a l flattening  of  (which was  t h e charge c u r v e ) .  r e a d i l y apparent  due  The p l a t i n u m and p l a t i n u m - i r i d i u m s u r -  f a c e s were assumed t o have the f o l l o w i n g charges  associated with  oxygen monolayer, assuming a l i n e a r v a r i a t i o n of t h i s charge w i t h percent composition  the atomic  i n the a l l o y s :  Electrode  Oxygen Monolayer Charge  Ir  440  Pt  420  Pt/5 % I r  421  Pt/10% I r  422  Pt/20% I r  424  Pt/25% I r  425  Pt/30% I r  426  The  to a  ucoul/sm.  a r e a s o f e l e c t r o d e s w h i c h were p o t e n t i o s t a t i c a l l y  i z e d i n c h l o r i d e e l e c t r o l y t e s were a l s o d e t e r m i n e d w i t h l i t t l e t i o n to the procedure.  The  oxygen f i l m formed  r e a d i l y r e d u c e d a t o p e n - c i r c u i t , and  anod-  modifica-  by such t r e a t m e n t i s n o t  t h e e l e c t r o d e can i n f a c t be removed  105  f r o m one age.  cell  and t r a n s f e r e d  Indeed,  gas  was  acid  bubbling  indicative  encountered  electrolyte  and o b t a i n i n g  Observation  3.4.4  of  probe  materials. jected of  to  time.  were  electron s u c h as  several  were  observed probe  kinds  study  platinum,  the  charging  Electrode  scanning to  of  e v e n when s t o r e d  long periods  of  time.  curve  surface  for  with  monolayer  of  into  the  electron microscope the  i n order  iridium,  treatments with  S.E.M., to  also  the  surfaces  for  S.E.M.  and they  determine  titanium,  a  hydrogen  deposition,  Surfaces  observe  chemical  at  Hence,  such an e l e c t r o d e the  coverthe  (S.E.M.) of  were  the  also  sub-  lengths  electrodes the  subject  d i s t r i b u t i o n of  and oxygen.  investigated  different Used  and  electrode  t i t a n i u m and smooth p l a t i n u m s h e e t s were  p l a t i n u m and t i t a n i u m were scans.  for  loss  w i l l maintain  coverage  in transferring  then observed  using  oxygen  water  techniques  Both coated  These  also  (E.P.)  electrodes  and t h e n r e d u c i n g  U s e was made o f electron  of  in twice-distilled  no d i f f i c u l t y sulphuric  a n o t h e r w i t h no a p p a r e n t  such p r e - p o t e n t i o s t a t t e d  high rest-potentials open-circuit  to  elements  The d i s t r i b u t i o n s  b y means  of X - r a y  of  line-  of  106 4.  4.1  RESULTS  GALVANOSTATIC POLARIZATION CURVES  The p o l a r i z a t i o n curves f o r platinum and platinum/iridium wire electrodes i n helium-stirred 1M NaCl; pH 2 are shown i n Figures 3-7.  For a l l cases, two overpotential regions were found,  by a " p o t e n t i a l jump".  separated  At low currents, the lower overpotential region  was not linear (not shown i n the f i g u r e s ) , probably due to the i o n i z a t i o n of hydrogen produced during the cathodic pretreatment.  The l i n e a r -  i t y of the lower overpotential region was found to exist over a large range of current density, with a progressive p o s i t i v e deviation from l i n e a r i t y appearing at current densities approaching that of the potenti a l jump.  For the Pt/5 Ir and Pt/25 Ir electrodes, quite large devia-  tions from l i n e a r i t y were observed before the p o t e n t i a l jump.  In a l l  cases, at least 48 hr. of p o l a r i z a t i o n at the passivation current den121 sity ip  was required i n order for the p o t e n t i a l to reach values  responding to the upper overpotential region.  cor-  I t was noticed that i n t r o -  duction of fresh e l e c t r o l y t e to the anode compartment produced an immedi a t e , rapid r i s e i n p o t e n t i a l (of the order of 20 mV), which was i r r e v e r sible.  D i f f i c u l t y was encountered i n obtaining the actual i p value, as  many hours were required f o r steady-state to a t t a i n at current densities approaching i p .  The upper overpotential region was not anywhere near  as r e l i a b l y reproducible or as l i n e a r as the lower overpotential region,  107  probably  due  to the combined e f f e c t s o f c o n c e n t r a t i o n p o l a r i z a t i o n  i n t e r f e r e n c e due  to the h i g h r a t e of gas  T r a c i n g the curve  e v o l u t i o n from the anode.  i n the r e v e r s e d i r e c t i o n  a marked h y s t e r e s i s , w i t h no  and  ( d e c r e a s i n g c u r r e n t s ) showed  tendency f o r t h e e l e c t r o d e to r e t u r n to i t s  o r i g i n a l s t a t e even a f t e r l o n g times a t c u r r e n t d e n s i t i e s l e s s than i p . On  s h u t t i n g o f f the c u r r e n t , the p o t e n t i a l dropped t o a r e s t p o t e n t i a l  of 1.1  v o l t s , which was  potential.  Cathodic  as a f t e r c h a r g i n g  c o n s i d e r a b l y more a n o d i c  charging f a i l e d  than the o r i g i n a l  t o a f f e c t the new  the p o t e n t i a l returned  rest  to a n o d i c v a l u e s .  potential, A f t e r remain-  i n g a t o p e n - c i r c u i t i n the e l e c t r o l y t e f o r a t l e a s t 24 h o u r s , the rode r e g a i n e d  rest  the c h a r a c t e r i s t e i c s o f an " a c t i v e " e l e c t r o d e .  the p o l a r i z a t i o n curve r e s u l t e d i n n e a r - d u p l i c a t i o n of most  elect-  Re-tracing experimental  p o i n t s , e s p e c i a l l y f o r the lower o v e r p o t e n t i a l r e g i o n . The  current densities i n Figures 3 - 7  areas of the e l e c t r o d e s . ves on one  I t was  refer  to the  geometric  seen t h a t s u p e r p o s i t i o n of a l l the  graph r e s u l t e d i n an e x c e l l e n t f i t of a l l the d a t a , w i t h  e x c e p t i o n of the cases of the Pt/5 showed h i g h e r  i  values  i n good agreement).  curthe  I r and Pt/25 I r e l e c t r o d e s , which  (but whose p o l a r i z a t i o n curves  P l o t t i n g the p o l a r i z a t i o n curves  otherwise  were  on the b a s i s of  the a r e a measured by oxygen a d s o r p t i o n l e d to a random assortment of i p v a l u e s , w i t h poor r e p r o d u c i b i l i t y among curves electrodes.  It is felt  t i o n technique 2, due  t h a t the areas  f o r the  individual  determined by the oxygen a d s o r p -  were u n r e l i a b l e , a l t h o u g h  they a r e reproduced i n T a b l e  to the p o s s i b i l i t y of v a r i a t i o n of the a r e a d u r i n g  anodization  108 at high p o t e n t i a l s .  I t was l a t e r confirmed that electrode roughening  occurred when anodization at potentials above about 2.0 V (S.C.E.) was performed, and that the electrode became progressively rougher with increasing anodization time.  As electrodes used i n the p o l a r i z a t i o n  experiments were anodized at such high potentials for s i g n i f i c a n t l y d i f f e r e n t periods of time, i t i s f e l t that they received comparatively d i f f e r e n t degrees of roughening, and that comparison of true electrode areas i s not v a l i d after t h i s has occurred. T a f e l parameters were obtained from each curve, f o r the ascending upper and lower overpotential regions and f o r the descending overpotential, where this was measured, using the T a f e l equation: n  =  a + b log I 2  where n i s the overpotential, a i s the intercept at log lA/cm. , and b i s the slope per decade.  The transfer c o e f f i c i e n t can be obtained  from a  2.303 RT b F '  =  and the exchange current density can be obtained from the value obtained at zero overpotential.  io  =  1  The standard electrode p o t e n t i a l of the chlorine electrode CE£^) was determined  197 by F a i t a et a l to be 1.3583 v o l t s on the absolute scale.  log i,(npA/cm ) 2  F i g u r e 3.  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r p l a t i n u m w i r e e l e c t r o d e i n h e l i u m - s a t u r a t e d IM NaCl; pH 2.  Temperature 25°C.  F i g u r e 4.  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r p l a t i n u m / 5 % i r i d i u m w i r e e l e c t r o d e i n h e l i u m - s a t u r a t e d IM NaCl; pH 2.  Temperature 25°C.  o  F i g u r e 5.  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r platinum/10% i n helium-saturated  i r i d i u m wire  1M NaCl; pH 2 a t 25°C.  electrode  F i g u r e 6.  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r platinum/20% i n helium-saturated  i r i d i u m wire  IM NaCl; pH 2 a t 25°C.  electrode  F i g u r e 7.  G a l v a n o s t a t i c p o l a r i z a t i o n curve f o r p l a t i n u m / 2 5 % i n helium-saturated  IM N a C l ;  iridium wire  pH 2 a t 25°C.  electrode  114  TABLE 2 Electrode areas measured after determination of the p o l a r i z a t i o n curves, as compared to their measured geometric areas.  Area from oxygen adso ptiom (cm. )  Geometric Area (cm. )  Roughness Factor  1.127  0.246  4.6  Ir  0.844  0.235  3.6  Pt/10 I r  0.976  0.251  3.8  Pt/20 I r  1.125  0.246  4.6  Pt/25 Ir  1.233  0.252  4.9  Electrode  2  Pt Pt/5  2  115  The Nernst e q u a t i o n f o r the r e a c t i o n  t  2C1 can thus be expressed E  The  second  =  Cl  2  +  2e  as: 1.358  + .0295 l o g p  c l  - .0591  log  a~ Q1  term on the r i g h t hand s i d e can be n e g l e c t e d i n the case of  c h l o r i n e - s t i r r e d s o l u t i o n . C h l o r i d e i o n a c t i v i t i e s were o b t a i n e d 196 Harned and Owen.  from  The p o t e n t i a l of the s a t u r a t e d c a l o m e l e l e c t r o d e  «. i 198 taken to be k  was  E  f o r the purposes electrode.  S  C  E  =  0.2490 - .00065 (°C-20)  of c o n v e r t i n g to p o t e n t i a l s r e l a t i v e to t h i s  T a f e l parameters a r e r e c o r d e d i n T a b l e s 3 and As the s o l u t i o n was  t h i s gas throughout  standard  4.  purged w i t h helium, and s t i r r e d  the experiment  with  - as w e l l as the e l e c t r o l y t e b e i n g  p e r i o d i c a l l y replaced with fresh solution - i t i s d i f f i c u l t  to d e t e r -  mine a v a l u e f o r p^-  to be  .  I n a d d i t i o n , p_..  would be expected  d i f f e r e n t , depending on the c u r r e n t d e n s i t y , due  to the v i r t u a l  i b l e e v o l u t i o n a t s m a l l c u r r e n t s and v i g o r o u s e v o l u t i o n w i t h  neglig-  larger  c u r r e n t s , which would tend to produce a r e v e r s i b l e p o t e n t i a l t h a t  tends  to become more anodic w i t h r e a d i n g s a t h i g h e r c u r r e n t d e n s i t i e s .  If a  _3 v a l u e of p . i s obtained.  = 10  i s assumed, a Nernst p o t e n t i a l of 1.033  V  (S.C.E.)  116  TABLE 3  Tafel parameters for Pt and Pt/Ir wire electrodes for the lower Tafel region of the polarization curves in IM NaCl; pH 2 at 25°C. a  Electrode  a  decade  i  0  (A/cm.2)  Pt  0.20  0.045  1.31  3.6  x  10"  Pt/5 % I r  0.16  0.035  1.69  2.1  x  IO  - 5  Pt/10% I r  0.20  0.041  1.44  1.3  x  10  - 6  Pt/20% I r  0.19  0.040  1.48  1.8  x  IO  - 5  Pt/25% I r  0.19  0.0375  1.57  0.85  x  10~  5  5  TABLE 4 T a f e l parameters f o r Pt and P t / I r w i r e e l e c t r o d e s f o r the a s c e n d i n g and descending upper T a f e l  regions  of the p o l a r i z a t i o n c u r v e i n IM NaCl; pH 2 a t  Electrode  Ascending currents a  b  a  i  Q  25°C.  Descending  (A/cm. )  Pt  1.19  0.27  0. 22  5.9 x  10  Pt/5 % I r  1.17  0.31  0. 190  6.6 x  10"  1  Pt/10% I r  1.23  0.30  0. 20  5.7 x  10"  1  Pt/20% I r  1.13  0.32  0. 19  5.7 x  10  _ 1  Pt/25% I r  1.18  0.29  0. 20  6.9 x  10  - 1  a  b  a  currents i  (A/cm. ) 2  Q  - 1  1.14  0.27  0.22  6.4  x  10  - 1  1.16  0.26  0.22  6.9  x  10  - 1  117  Table 5 records  the passivation current d e n s i t i e s , i p ,  corresponding to the potential jump, and the passivation potentials, Ep, as obtained from projecting the lower T a f e l slope u n t i l i t i n t e r sected ip."'" "'" 2  Experiments i n other e l e c t r o l y t e s were not considered r e l i a b l e , and curves are not reproduced here.  However, i t was clear  that the passivation current density was increasingly higher f o r p l a t inum electrodes i n solutions of higher a c i d i t y and/or chloride ion concentration  - an observation already reported by Littauer and co-  118,121 workers.  TABLE 5 Passivation data for Pt and P t / I r wire electrodes from p o l a r i z a t i o n curves i n 1M NaCl; pH 2 at Electrode Pt  i  p  (mA/cm.z)  8.2 -  Pt/5 % Ir  10.3  *  Pt/10% Ir  66 8 - 10  Pt/20% Ir  8 - 10  Pt/25% Ir  16 - 20  25°C.  Ep (V, S . C . E . ) 1.14  A  1.16 1.14 1.14 A  *  1.15  It i s believed that these numbers are not the true steady state values for t h i s particular electrode. See section 5.1 for discussion.  119  4.2  POTENTIOSTATIC POLARIZATION CURVES  The p o t e n t i o s t a t i c p o l a r i z a t i o n c u r v e s i n b o t h 1M -^SO^ and 1M NaCl; pH 2 showed s i m i l a r forms, w i t h no d i s t i n g u i s h a b l e slopes.  Tafel  The l o c a t i o n o f the c u r v e s was a f u n c t i o n o f a n o d i z a t i o n time,  and s t e a d y - s t a t e c o n d i t i o n s were never a t t a i n e d .  The a r e a o f the  e l e c t r o d e used i n t h e c h l o r i d e e l e c t r o l y t e was found to d e c r e a s e by more than 50% over t h e c o u r s e o f p l o t t i n g t h e c u r v e , whereas t h e e l e c t r o d e used i n 1M l ^ S O ^ showed no s i g n i f i c a n t s u r f a c e a r e a change. Curves a r e reproduced i n F i g u r e s 8 and 9.  J-  I  J-  I  ]  log I,(mA) 10  F i g u r e 8. P o t e n t i o s t a t i c a t 20°C.  p o l a r i z a t i o n curve f o r Pt/30 I r - T i i n u n s t i r r e d IM H„SO, 2 E l e c t r o d e a r e a , measured p r i o r t o r u n was 148 cm. . R.F.=76.  F i g u r e 9.  P o t e n t i o s t a t i c p o l a r i z a t i o n c u r v e f o r Pt/30 I r - T i i n u n s t i r r e d 1M N a C l ; pH 2  a t 20 C.  E l e c t r o d e area decreased from 148 t o 71 cm.  over the c o u r s e of the r u n .  122  4.3  CHANGE OF SURFACE AREA WITH POTENTIOSTATIC  ANODIZATION  The o b s e r v a t i o n t h a t the s u r f a c e a r e a o f coated e l e c t r o d e s was decreased  i n the c o u r s e o f c o n s t r u c t i n g t h e p o t e n t i o s t a t i c  z a t i o n c u r v e l e d t o more i n t e n s i v e i n v e s t i g a t i o n o f t h i s The  e f f e c t s of p o l a r i z a t i o n a t h i g h anodic p o t e n t i a l s  polari-  phenomenon.  (1800 and 2000 mV,  S.C.E.) i n v a r i o u s e l e c t r o l y t e s , as w e l l as t h e e f f e c t o f c h e m i c a l p r e treatment were i n v e s t i g a t e d , and a r e r e p o r t e d i n T a b l e s 6,7, and 8. The a c c u r a c y o f t h e s u r f a c e a r e a t e c h n i q u e was checked many times, and s u c c e s s i v e d u p l i c a t e runs were always found o f each o t h e r .  t o be a c c u r a t e w i t h i n 3%  However, i n d i v i d u a l experiments  with different electrodes  i n c h l o r i d e e l e c t r o l y t e s d i d n o t show such r e p r o d u c i b i l i t y , o t h e r the g e n e r a l tendency  t o l o s e i n c r e a s i n g amounts o f s u r f a c e a r e a w i t h  i n c r e a s i n g a n o d i z a t i o n times.  I n f a c t , no e f f e c t o f c h l o r i d e i o n con-  c e n t r a t i o n o r a c i d i t y was noted among t h e 1M NaCl, 4M H C l e l e c t r o l y t e s . ^SO^)  than  1M NaCl; pH 2, and  E l e c t r o l y s i s i n c h l o r i d e - f r e e e l e c t r o l y t e (1M  caused no area changes over p e r i o d s o f a n o d i z a t i o n commonly  used  i n d e t e r m i n i n g t h e s u r f a c e a r e a , but a long-term r u n showed an a r e a decrease of n e a r l y 10% (which was s t i l l n o t as g r e a t as t h a t caused by a n o d i z a t i o n s o f o n l y 30 seconds i n the c h l o r i d e e l e c t r o l y t e s ) . containing  1M  ^SO^  .1M NaCl showed an i n t e r m e d i a t e b e h a v i o u r , w i t h some l o s s o f  s u r f a c e a r e a noted, but nowhere approaching the h i g h e r c h l o r i d e - c o n t a i n i n g e l e c t r o l y t e s .  t h e magnitude observed  with  TABLE 6 Surface area changes as a result of potentiostatic p o l a r i z a t i o n i n chloride electrolytes with Pt/30 I r - T i electrodes.  Electrolyte  a  +  ci~  0.1M NaCl + IM H S0, 2 4 o  IM NaCl  0.67  IM NaCl; pH 2  4M HCl  Anodization time t (sec.)  1800  120  120  148  148  1800  10  10  154  30  30  90  1800  9.50  1800  Af-Ai Ai  Af-Ao Ao  144  -.027  -.027  154  137  -.11  -.11  161  161  127  -.21  -.21  120  161  127  113  -.11  -.30  110  120  154  137  118  -.14  -.23  880  1000  161  113  106  -.062  -.34  880  1000  154  118  94  -.20  -.39  60  60  162  162  103  -.36  -.36  60  120  162  99  89  -.10  -.45  4880  5000  162  89  76  -.15  -.53  30  30  155  155  134  -.14  -.14  90  120  155  134  112  -.16  -.28  1000  1000  159  159  83  -.48  -.48  A  Cumulative time t (sec.)  Area Original prior to area anodization Ao(cm. ) Ai(cm. )  Anodization potential (mV S.C.E.)  c  2  2  Area after anodization Af(cm. ) 2  TABLE 7 E f f e c t o f p o t e n t i o s t a t i c p o l a r i z a t i o n i n 1M I^SO^ on t h e s u r f a c e area of Pt/30 I r - T i e l e c t r o d e s .  Anodization potential (mV S.C.E.)  Anodization time t (sec.) A  Cumulative time t (sec.) c  Original area Ao(cm. )  Area p r i o r t o anodization Ai(cm,2)  Area a f t e r anodization Af(cm.2)  Af-Ai Ai  Af-Ao Ao  2  300  900  148  148  148  .00  .00  300  900  162  162  161  -.006  -.006  83620  83620  162  161  148  -.081  -.086  .00  1800  300  *  113  113  300  *  71  73  +.028  300  *  58  59  +.017  3000  *  73  72  -.014  17500  *  72  72  .00  2000  Cumulative a n o d i z a t i o n time not g i v e n because e l e c t r o d e was a l s o p o l a r i z e d i n c h l o r i d e electrolyte.  TABLE 8 Effect of treatment i n aqua regia on the surface area of Pt/30 I r - T i electrodes.  Immersion time (sec.)  Cumulative time (sec.)  Original area Ao(cm. )  Area prior to treatment Ai(cm. )  Area after treatment Af(cm. )  2  2  2  Af-Ai Ai  Af-Ao Ao  30  30  151  151  151  .00  .00  270  300  151  151  151  .00  .00  3700  4000  151  151  111  -.26  -.26  168300  172300  151  111  78  -.30  -.48  126 In every case considered, the current density was to decay with time of potentiostatic anodization.  observed  Interruption of  e l e c t r o l y s i s (for purposes of surface area measurement) led to temporary " r e - a c t i v a t i o n " of electrodes, where the current density assumed i t s o r i g i n a l high values.  However, the current density quickly decayed  to previous l e v e l s .  Comparative curves for various e l e c t r o l y t e s are  shown i n Figure 10.  No steady-state readings were obtained, even  after anodizing for periods up to a day long i n some cases. In addition to the current/time curves determined f o r each potentiostatic anodization, the electrodes were removed from the v a r i ous e l e c t r o l y t e s a f t e r several of the runs, and their history recorded i n a standard e l e c t r o l y t e (IM ^SO^) s t a t i c anodization at 1800  or 2000 mV  (S.C.E.).  current/time with potentio-  Figures 11 and 12 show  the e f f e c t s of immersion i n aqua regia and potentiostatic anodization i n LM NaCl; pH 2.  In both cases, the current/time curve has been  shifted towards lower current values. polarized only i n IM ^SO^,  If however, the electrode i s  no s i g n i f i c a n t difference can be found  among the current/time curves determined before, during, and after such treatment.  Furthermore, an "aged" electrode (one which has l o s t apprec-  iable surface area) shows reproducible current/time curves regardless of the treatment given i t . pH 2 at 2000 mV  for 11000  (Even potentiostatic anodization of IM NaCl; seconds f a i l e d to a l t e r the current/time  haviour of such an electrode.)  be-  300 -US"  I  (mA) _ 200  D  -o— -o—  100  A B  c  4 M HC\ A 1 5 5 c m . , A a o ' l 3 4 c m , A " 112 cm lMNaCUpH2 A o l&l cm * A « o 103 cm ,A.2o» 8 9 c m * lMMaCI Ao= 161 cm , A <r 12? Cm*,Ai20 = • I 3 cm* S  2  2  0  8  2  s  2  2  3  jMH SQ,MMNaa A o * 148cm D iMH2S0 A * 162cm? E.  1  ?  4  A 12c 144 em *  0  20 F i g u r e 10.  2  l 2 0  40  t, (sec)  Current/time r e l a t i o n s f o r p o t e n t i o s t a t i c  60  80  p o l a r i z a t i o n w i t h Pt/30 I r - T i  e l e c t r o d e s a t 1800 mV (S.C.E.) i n v a r i o u s e l e c t r o l y t e s a t 25°C. 2 Apparent a r e a o f a l l e l e c t r o d e s 1.95 cm. .  200  o No treatment Aj =151 cm 2 + 300s®c. immersion Af =151 cmr A 48 hr. immersion  160  I  Af = 78c  (mA) 120  80  40  0  10  50.  + /  T (seconds)  100  500  1000  7  F i g u r e 11.  C u r r e n t / t i m e r e l a t i o n s f o r a Pt/30 I r - T i e l e c t r o d e f o r p o t e n t i o s t a t i c e l e c t r o l y s i s of 1M H-SO,  a t 1800 mV  oo  and 25°C, a f t e r v a r i o u s 2  p r e t r e a t m e n t times i n aqua r e g i a .  Apparent e l e c t r o d e a r e a 1.95  cm.  .  300 Before , A: =148cm  2  I h mA) After, A = 71 c m f  200  100 1 0  F i g u r e 12.  t/(seconds)  100  1000  C u r r e n t / t i m e r e l a t i o n s f o r a Pt/30 I r - T i e l e c t r o d e f o r p o t e n t i o s t a t i c e l e c t r o l y s i s of IM R^SO^ a t 2000 mV and 25°C, b e f o r e and a f t e r p o t e n t i o s t a t i c e l e c t r o l y s i s o f IM NaCl; pH 2 a t 1800 mV f o r 17,000 2 seconds. Apparent e l e c t r o d e a r e a 1.95 cm. .  2  130 In order to determine whether the surface area changes were r e v e r s i b l e , various " a c t i v a t i o n " procedures were applied i n a few cases of severe area-loss. Table 9.  These procedures are summarized i n  Severe cathodic treatment and anodic/cathodic sweeping d i d  not produce s i g n i f i c a n t changes i n the electrode area, and t h e i r e f f e c t s could not be found to be d i f f e r e n t from anodization i n the same (1M I^SO^) e l e c t r o l y t e . A simple test was performed to determine i f the spent electrolytes contained any dissolved titanium, and none was  indicated.  131  TABLE 9 E f f e c t s o f " a c t i v a t i o n " p r o c e d u r e s on t h e s u r f a c e a r e a of Pt/30 I r - T i e l e c t r o d e s .  Treatment  (Apparent a r e a 1.95  2  2  Surface area b e f o r e treatment (cm. )  Surface area a f t e r treatment (cm. )  72  69  91  93  103  99  2  Potentiostatic cathodization i n 1M H S 0 a t - 1000 mV (S.C.E.) f o r 3 minutes. Average c u r r e n t : 320 mA.  cm. )  2  4  P o t e n t i a l c y c l e d a t 220 mV/sec. between 0 and 2000 mV (S.C.E.) and back t o 0. Repeated twelve times i n 1M H2SO4. Potentiostatic cathodization i n 1M H2SO4 a t - 1000 mV (S.C.E.) f o r 1000 seconds. Average c u r r e n t : 240 mA.  132  4.4  OBSERVATIONS OF ELECTRODE SURFACES  Scanning e l e c t r o n m i c r o s c o p y the s u r f a c e s of both coated chemical  pretreatments  and  (S.E.M.) was  smooth e l e c t r o d e s .  used to observe  For  the former,  were shown to have no v i s i b l e e f f e c t .  Figure  13 shows t h a t even treatments such as immersion i n hot aqua r e g i a f o r an hour l e a v e the e l e c t r o d e v i s i b l y unchanged. show l i t t l e d i f f e r e n c e .  However, an e l e c t r o d e which had been  to long-term a n o d i z a t i o n i n IM NaCl; pH nodular  protruberances  Smooth p l a t i n u m was  Used e l e c t r o d e s a l s o  2 a t 2000 mV  (S.C.E.) developed  a t v a r i o u s p l a c e s over i t s s u r f a c e  g r e a t l y a f f e c t e d by c h e m i c a l  subjected  (Figure  pretreatment.  Immer-  s i o n i n hot aqua r e g i a produced a p r o g r e s s i v e d e s t r u c t i o n of the rode s u r f a c e .  A f t e r l e s s than a minute, e t c h i n g of s u r f a c e  was  F i g u r e 15 shows the s u r f a c e s t a t e a f t e r 5 and  apparent.  immersion.  S i m i l a r e f f e c t s were o b t a i n e d  l o n g e r times to be produced.  elect-  scratches 60 minutes  i n c o l d aqua r e g i a , but  Immersion i n c o n c e n t r a t e d  14).  took  hydrochloric  a c i d produced no v i s i b l e changes except a f t e r l o n g times, where s u r f a c e s c r a t c h e t c h i n g appeared a f t e r one hour. r e v e a l e d t h a t the i n i t i a l  s u r f a c e s t a t e was  a t e d by s u r f a c e a r e a measurements. of new  and used  (after determination  curves) platinum  and  the smooth p l a t i n u m d u r i n g the course  Observation  of w i r e  electrodes  not a t a l l smooth, as  F i g u r e s 16 and  17 show the  indic-  surfaces  of the g a l v a n o s t a t i c p o l a r i z a t i o n  platinum/iridium electrodes.  I t i s apparent t h a t  e l e c t r o d e undergoes some s u p e r f i c i a l m o d i f i c a t i o n  of p l o t t i n g the g a l v a n o s t a t i c c u r v e s , but  the  effect  133  on the a l l o y electrode i s not obvious. Electron probe analysis revealed no s i g n i f i c a n t difference between new  and used coated electrodes.  Figure 18 shows the d i s t r i b u -  t i o n of platinum and titanium over the surface of a new Pt/30 I r - T i electrode.  Similar determinations  f o r electrodes subjected to anodi-  zation i n sulphuric acid, sodium chloride, or hydrochloric acid s o l u tions showed no v i s i b l e changes, even though the measured surface areas were s i g n i f i c a n t l y lower for the l a t t e r two  cases.  X-ray linescans, however, showed remarkable differences between new  and used electrodes.  areas of high titanium-content trode anodized i n 1M H^O^  For a new  electrode, r e l a t i v e l y  were found on the surface.  for 84220 seconds at 1800 mV  few  For an e l e c -  (S.C.E.),  a  s l i g h t increase i n the number of high-titanium peaks was  noted.  An  electrode anodized i n 4M HCl for 1000  showed high  seconds at 1800 mV  titanium content over v i r t u a l l y the whole surface. X-ray d i f f r a c t i o n analysis of the surfaces of new and used coated electrodes revealed only the presence of titanium and on a new  electrode.  p o l a r i z a t i o n , new  As an electrode was  platinum  subjected to progressive  anodic  peaks appeared which could be attributed to the devel-  opment of oxides of titanium.  X-ray data are presented  i n Appendix I I I .  13A  S.E.M. o b s e r v a t i o n o f Pt/30 Ir-Ti s u r f a c e s (400X; specimen t i l t e d a t 45°) (a) New e l e c t r o d e (b) Immersed 1 h r . i n h o t aqua r e g i a  F i g u r e 13.  F i g u r e 14.  S.E.M. o b s e r v a t i o n of used Pt/30 I r - T i e l e c t r o d e  F i g u r e 15.  S.E.M. o b s e r v a t i o n of p l a t i n u m sheet (400X) (a) Immersed 5 min. i n hot aqua r e g i a (b) Immersed 60 min. i n hot aqua r e g i a  137  (b) F i g u r e 16.  S.E.M. o b s e r v a t i o n of p l a t i n u m w i r e e l e c t r o d e s (a) New e l e c t r o d e (b) Used e l e c t r o d e  (1300X)  138  (b) F i g u r e 17.  S.E.M. o b s e r v a t i o n of P t / 2 5 % I r w i r e e l e c t r o d e s (a) New e l e c t r o d e (b) Used e l e c t r o d e  (1300X)  139  F i g u r e 18.  E.P. o b s e r v a t i o n of new Pt/30 I r - T i e l e c t r o d e (a) Absorbed e l e c t r o n image (b) X-ray image; P t (c) X-ray image; T i .  (500X)  140  5.  5.1  DISCUSSION  ANODIC GALVANOSTATIC "MEASUREMENTS  As i r i d i u m wire  e l e c t r o d e s was  and Ep v a l u e s to be  expected, the p o l a r i z a t i o n b e h a v i o u r of the  f o r the Pt/5  s i g n i f i c a n t l y higher  platinum/  s i m i l a r to t h a t of pure p l a t i n u m .  The  i  I r and Pt/25 I r a l l o y s were found, however, than t h a t f o r the o t h e r a l l o y s .  t h a t , on i n s p e c t i o n of the p o l a r i z a t i o n curves  I t i s noted  f o r the former, t h e r e i s  c o n s i d e r a b l e d e v i a t i o n from the lower T a f e l r e l a t i o n b e f o r e p a s s i v a t i o n (i.e.,  the p o t e n t i a l jump) o c c u r s .  I t i s suggested t h a t , f o r such  the t r u e p a s s i v a t i o n parameters were not o b t a i n e d allowance f o r the e s t a b l i s h m e n t p o t e n t i a l jump.  of steady  due  to  cases,  insufficient  s t a t e conditions before  the  That i s , the curve which d e v i a t e s from l i n e a r i t y i s  i n f a c t an i n d i c a t i o n t h a t the t r u e p a s s i v a t i o n c u r r e n t d e n s i t y has surpassed,  but  taken p l a c e . competitive  t h a t the complete p a s s i v a t i o n of the e l e c t r o d e has T h i s i s to be an expected phenomenon i n any  a d s o r p t i o n between two  not  c a s e where  s p e c i f i c a l l y - a d s o r b i n g species  when the p o t e n t i a l - r a n g e s of t h e i r a d s o r p t i o n o v e r l a p .  been  exists,  From the work  121 of B i t t l e s and L i t t a u e r  on p l a t i n u m  e l e c t r o d e s , v i r t u a l l y no  t i o n from l i n e a r i t y i s found i n the lower T a f e l r e g i o n s of curves  obtained  i n s i m i l a r media.  case f o r P t , Pt/10  I r , and  T h i s was  t i o n parameters f o r the Pt/5  polarization  a l s o observed i n the  Pt/20 I r e l e c t r o d e s .  I f , then,  devia-  present  the p a s s i v a -  I r and Pt/25 I r anodes a r e r e - d e t e r m i n e d ,  t a k i n g the p o i n t a t which the lower o v e r v o l t a g e  r e g i o n d e v i a t e s from  141  linearity,  the i  and  Ep v a l u e s  f o r these e l e c t r o d e s  those a l r e a d y determined f o r the other  i  P  Ep  electrodes,  =  8-10  2 mA/cm.  =  1.14  V  coincide with  namely:  (S.C.E.)  Such r e s u l t s i n d i c a t e t h a t the p o l a r i z a t i o n c u r v e s f o r smooth and  platinum/iridium  a l l o y s have no  f o r i r i d i u m c o n t e n t s of up  substantial differences, at least  to 25%.  These f i n d i n g s are not  of F a i t a et i 126,146,148 ^  presence of even 0.5%  Ir i n their Pt/Ir alloy-coated titanium  9  decreased the tendency of the e l e c t r o d e s shown to be r e v e r s i b l e w i t h r e s p e c t but  q  n o  ted  i n agreement  w i t h the o b s e r v a t i o n s  a  platinum  to p a s s i v a t e .  of a f o r m a l  the electrodes  I r i d i u m has  to oxygen a d s o r p t i o n  the i r r e v e r s i b l e a n o d i c f o r m a t i o n  that  and  been  removal,  o x i d e of i r i d i u m  22  *  has  125 been r e p o r t e d . i r i d i u m may  ( I t seems r e a s o n a b l e to suggest t h a t the presence of  not r e t a r d the p a s s i v a t i o n of P t / I r a l l o y anodes, and  i n f a c t even c o n t r i b u t e to i t s p a s s i v a t i o n .  Thus, the p r e s e n c e of a  " p o t e n t i a l jump" phenomenon would s t i l l be expected on ing electrodes.)  The  the upper o v e r v o l t a g e of the s u b s t r a t e pH.  may  iridium-contain-  mechanism f o r the t r a n s i t i o n from the lower to r e g i o n has  y e t to be  (as found i n the p r e s e n t  found.  Ep  i s independent  work), c h l o r i d e c o n t e n t ,  Bianchi^''"^ suggested t h a t h y p o c h l o r i t e  dicharge  may  be  the  or  cause  149 of p a s s i v a t i o n .  However, L a n d o l t  and  Ibl  r e p o r t t h a t the  phenomenon a l s o o c c u r s i n h y p o c h l o r i t e - f r e e s o l u t i o n s . proposed by B i t t l e s and Littauer"'" "'' and 2  volve  a progressive  Kuhn and  The  passivation mechanisms  Wright,which in-  s e r i e s of i r r e v e r s i b l e r e a c t i o n s whereby  the  23  142  electrode surface obtains an oxygen coverage, appear to best explain the process.  5.2  ANODIC POTENTIOSTATIC MEASUREMENTS  Continued anodization of the coated electrodes produced a progressive passivation of the electrodes (that i s , the current density decayed with time) for which no c l e a r steady state was attained.  This  passivation can be attributed to the i r r e v e r s i b l e build-up of oxides of the noble metal coating.  Simultaneously,  progressive oxidation of  the substrate was observed (which was of importance only i n c h l o r i d e containing s o l u t i o n s ) . On cessation of anodic treatment, followed by mild (hydrogen) reduction, the noble metal coating could be " r e - a c t i v a ted" inasmuch as the noble metal oxides were removed.  The r e - a c t i v a t i o n  did not extend to the substrate, however, which remained i r r e v e r s i b l y oxidized and contributed to an i r r e v e r s i b l e loss of a c t i v e surface area. It i s an unwise procedure to u t i l i z e the same coated electrode for determination of p o l a r i z a t i o n curves i n e l e c t r o l y t e s where this progressive attack of the substrate (and hence, progressive decay of the surface area) i s possible.  In such cases, i t would be advisable to obtain the  current/time behaviour of as many d i f f e r e n t electrodes as there are potentials to investigate. This progressive passivation and substrate degradation are suggested as the reasons for the poor r e p r o d u c i b i l i t y and i n a p p l i c a b i l i t y of p o l a r i z a t i o n curves for coated electrodes which have been reported i n the l i t e r a t u r e ( i n addition to other f a c t o r s ,  143  such as t h e d i f f e r e n c e s I n t h e n o b l e m e t a l d e p o s i t s ) . If  i t i s assumed t h a t the oxygen e v o l u t i o n r e l a t i o n s h i p  (as determined by t h e p o l a r i z a t i o n c u r v e i n 1M H^SO^) i s a l s o obeyed during  the e l e c t r o l y s i s o f 1M NaCl s o l u t i o n , and t h a t b o t h oxygen- and  c h l o r i n e - e v o l u t i o n occur i n d e p e n d e n t l y o f one another  (that i s , the  p o l a r i z a t i o n curve i n c h l o r i d e s o l u t i o n a c t u a l l y r e p r e s e n t s the two p r o c e s s e s ) ,  then t h e comparative c u r r e n t  t h e sum o f  d e n s i t i e s at a given  p o t e n t i a l can be used to determine t h e c u r r e n t  efficiency for chlorine  e v o l u t i o n , a t any g i v e n  C a l c u l a t i o n s show t h a t ,  time o f e l e c t r o l y s i s .  a f t e r 5 minutes, the c u r r e n t  e f f i c i e n c i e s a t 1.3 V, 1.5V, and 1.8V  (S.C.E.) a r e 99.8%, 76%, and 66%, r e s p e c t i v e l y .  I t seems p r o b a b l e  that  the p r o c e s s o f p a s s i v a t i o n i s r e l a t e d t o t h e tendency of t h e e l e c t r o d e to generate oxygen w i t h i n c r e a s i n g p o t e n t i a l s .  5.3  SURFACE AREA CHANGES  S u r f a c e a r e a changes as a r e s u l t o f p o l a r i z a t i o n o f c o a t e d electrodes  have been r e p o r t e d  i n t h e l i t e r a t u r e p r e v i o u s l y by Weber and  147 Posiril,  who used e l e c t r o d e p o s i t e d  mmetry i n s u l p h u r i c a c i d s o l u t i o n . extreme case of c o a t i n g substrate  P t - T i electrodes  for cyclic volta-  Other workers have r e p o r t e d  l o s s as a r e s u l t o f d e g r a d a t i o n o f t h e t i t a n i u m  as a r e s u l t o f p r o l o n g e d a n o d i c p o l a r i z a t i o n .  cal  t o expect t h a t , p r i o r t o m a n i f e s t a t i o n  ing  l o s s , there  metal contact  the more  i s a progressive  I t seems l o g i -  o f t h i s d e g r a d a t i o n by c o a t -  d e t e r i o r a t i o n o f the  substrate/noble  caused by the growth o f some c o r r o s i o n product o f t h e  144  substrate.  I t would a l s o appear l i k e l y t h a t l o s s o f e l e c t r i c a l  of the c o a t i n g  ( e s p e c i a l l y f o r s m a l l or i s o l a t e d " p l a t e s " ) would be  p r e l i m i n a r y step i n the  s u r f a c e a r e a was  a  degradation.  In the case of the p r e s e n t  experiments, the " d e t e r m i n e d "  found to d e c r e a s e as a r e s u l t of a n o d i c  The m i l d r e d u c t i o n treatment was face area  contact  found not  (at most w i t h i n the a c c u r a c y  T h i s would suggest t h a t i t i s indeed  to a l t e r  treatment  the measured  of the d e t e r m i n a t i o n  only.  sur-  itself).  the o x i d a t i o n of the  substrate  which i s r e s p o n s i b l e f o r the l o s s of a c t i v e s u r f a c e a r e a .  Scanning  electron microscopic  electrode  surfaces confirm  and  X-ray d i f f r a c t i o n s t u d i e s of used  the p r o g r e s s i v e a t t a c k of the s u b s t r a t e , w i t h  appearances of p r o t r u b e r a n c e s the development of p i t s , and  over the e l e c t r o d e s u r f a c e the f o r m a t i o n  of o x i d e s  the  (Figure  14),  of t i t a n i u m  being  indicated. S u b s t a n t i a l l o s s e s of s u r f a c e a r e a were found to occur  only  w i t h c h l o r i d e s o l u t i o n s , which i n d i c a t e s t h a t i n such media c o n d i t i o n s i n s u r f a c e pores and substrate.  c r e v i c e s can l e a d to r a p i d d e t e r i o r a t i o n o f  These c o n d i t i o n s are p r o b a b l y  the  such phenomena as l o c a l i n -  135 creases  to h y d r o l y s i s of c h l o r i n e or other p r o d u c t s , c h l o r 137 138 127 i n e bubble f o r m a t i o n , i m p e r f e c t oxide growth, or d e p a s s i v a t i o n .  The  i n pH  spreading  due  of these r e g i o n s of a t t a c k to the s u b s t r a t e beneath  noble m e t a l c o a t i n g  (where c r e v i c e - l i k e c o n d i t i o n s would be m a i n t a i n e d )  would l e a d to a r e d u c t i o n i n the e l e c t r i c a l c o n t a c t between the and  the c o a t i n g .  the  It is realized  substrate  t h a t complete undermining o f i n d i v i d u a l  145  coating "plates" would be required to break the e l e c t r i c a l contact f o r the consequent reduction i n electrochemically active surface area to occur.  I t may also be possible that the substrate i s oxidized beneath  the coating by means of d i f f u s i o n of oxygen through the coating, but i t i s expected that t h i s process would be s u b s t a n t i a l l y slower, and that an "induction time" for the process would e x i s t .  This i s u n l i k e l y  when the slow d i f f u s i o n rates i n s o l i d s at ambient temperatures are considered  i n conjunction with the observations  occur r a p i d l y .  that surface area losses  Growth of an i n s u l a t i n g oxide layer over the electrode  surface may also be a possible mechanism, although t h i s has not been observed. I r r e v e r s i b l e oxidation of the coating metal could also r e sult i n a loss of active surface area, insofar as the electrode would be p a r t i a l l y filmed with oxygen p r i o r to oxygen deposition, which would result i n a smaller value for the charge Qo.  However, severe cathodic  treatment or even leaving the electrode at open-circuit for several hours (which w i l l remove any passivation from a wire electrode) do not produce a " r e - a c t i v a t i o n " to the o r i g i n a l surface area (indeed, they have no effect at a l l on the determined surface area).  Hence, i t i s  u n l i k e l y that the mechanism of area loss could be attributed to coating oxidation. Electrochemical d i s s o l u t i o n of the noble metal coating i s also not l i k e l y to be responsible for the substantial decreases i n the determined surface area.  Although the d i s s o l u t i o n rate of the platinum  146  m e t a l s i n c r e a s e s i n c h l o r i d e media, i t i s s t i l l v i r t u a l l y  negligible  over t h e s h o r t time-spans d u r i n g which the most s i g n i f i c a n t  surface  area d e c r e a s e s o c c u r .  present  A t the p o t e n t i a l s encountered i n the  experiments, p l a t i n u m m e t a l d i s s o l u t i o n would be expected t o occur a t a r a t e o f about 10 ^ mA/cm. ,"'"^ assuming d i r e c t e l e c t r o c h e m i c a l s o l u 2  t i o n as a c h l o r i d e complex. i s f u r t h e r considered mining i s achieved.  I n view o f t h e s h o r t times i n v o l v e d , i t  u n l i k e l y t h a t c o a t i n g detachment through underT h i s i s supported  by o b s e r v a t i o n s  that the e l e c t -  rode s u r f a c e s remain v i s u a l l y u n a l t e r e d over t h e time p e r i o d s f o r t h e most s u b s t a n t i a l determined s u r f a c e a r e a  decreases.  Another p o s s i b l e mechanism f o r the r e d u c t i o n i n a c t i v e s u r face area  i n v o l v e s t h e rearrangement o f s u r f a c e p l a t i n u m  t h a t the roughness f a c t o r o f the c o a t i n g i s d e c r e a s e d .  atoms, such Other workers  have r e p o r t e d s u b s t a n t i a l i n c r e a s e s i n the s u r f a c e a r e a o f smooth p l a t inum e l e c t r o d e s w i t h p o t e n t i a l c y c l i n g , and i t has been observed smoothening o f rough e l e c t r o d e s c a n occur was  a r e l a t i v e l y slow process."'""'  that  i f the r e d u c t i o n h a l f - c y c l e  Rearrangement o f s u r f a c e  atoms i n t o l e s s - a c t i v e p o s i t i o n s i s i n d i c a t e d .  platinum  The phenomenon o f "age-  i n g " o f d i s p e r s e d e l e c t r o d e s has been observed, where the a c t i v e s u r f a c e area diminishes  a f t e r prolonged  use a t e l e v a t e d  temperatures.^'"'"^"'  199 Stonehart c o u l d occur  has c o n s i d e r e d  that the s i n t e r i n g of platinum  a t lower temperatures, p r o v i d e d  crystallites  t h e i r p a r t i c l e s i z e was  o sufficiently  small  (20-200 A ) .  He suggested t h a t the a c t i v a t i o n energy  f o r " s u r f a c e rearrangement" i s 33 k c a l / m o l e .  I t i s not l i k e l y  that,  147  f o r t h e p r e s e n t c a s e , s u r f a c e atom rearrangement  i s responsible for  t h e s u b s t a n t i a l d e c r e a s e s i n the determined s u r f a c e a r e a .  Indeed,  t h e r a t h e r v i g o u r o u s a n o d i c t r e a t m e n t r e c e i v e d by t h e c o a t e d e l e c t r o d e s would appear t o be a cause o f e l e c t r o d e roughening r a t h e r t h a n the converse.  148  6.  PROPOSALS TOR  FUTURE WORK  The r e s u l t s presented i n t h i s thesis must be considered i n the l i g h t of future work to be performed on the electrochemical behaviour of platinum/iridium a l l o y s .  The range of experimental work cover-  ed has permitted the author to gain f a m i l i a r i t y with several methods of electrochemical investigation, and many of the experiments performed may be regarded as indicators of the paths i n which the future work must be directed. The r e s u l t s presented f o r the work with the wire anodes have shown that the behaviour of the a l l o y electrodes i s i d e n t i c a l to that f o r pure platinum, insofar as the p o l a r i z a t i o n c h a r a c t e r i s t i c s i n IM NaCl; pH 2 are concerned.  Further work i n this f i e l d should involve  the a p p l i c a t i o n of: systems where the pH and/or chloride ion a c t i v i t y vary over a wide range, d i f f e r e n t temperatures, d i f f e r e n t  atmospheres.  In this manner, the s i m i l a r i t y of the p o l a r i z a t i o n behaviour for the a l l o y wire electrodes could be confirmed over a wide range of experimental conditions.  Improvements i n c e l l design to f a c i l i t a t e e l e c t r o l -  yte introduction and removal (or to establish a constant flow rate) would ensure that the anodic behaviour was not influenced by the d e t r i mental build-up of electrode products.  This i s especially essential  for long-term e l e c t r o l y s i s where the r e s u l t s are obscured by the production of d i f f e r e n t solution species. It i s i n the applications of the coated electrodes that  149  most of the future work w i l l be d i r e c t e d . The determination of anodic p o l a r i z a t i o n curves i n chloride media should be re-done i n order to eliminate the e f f e c t s of decreasing active surface area (that i s , a fresh electrode should be used f o r each p o t e n t i a l point) and to estab l i s h a d e f i n i t e framework against which future work with long term potentiostatic polarizations are employed (for determining surface area changes and corrosion). As with the wire electrodes, the p o l a r i z a t i o n behaviour should be extended to systems of d i f f e r e n t concentrations (including i n d u s t r i a l e l e c t r o l y t e s ) , temperatures,  and atmospheres.  In addition, since the problems of generation of electrode products i s more severe for these high surface area electrodes, improvements i n c e l l design to permit more constant e l e c t r o l y t e concentrations are indicated. The r e s u l t s concerning the determined  surface areas of the  coated electrodes are the most promising f o r the a p p l i c a t i o n of future work.  It has been found that, i n chloride-containing e l e c t r o l y t e s , the  determined  surface areas decrease quickly and substantially with potent-  i o s t a t i c anodization.  If the mechanism of surface area loss i s indeed  through some method of coating undermining, then i t i s of interest to extend the present work to longer time periods i n order to determine i f electrode coating loss i s related to the decrease i n active surface area.  In addition, the observation of the electrode surfaces could be  extended f o r the case of these long-term anodizations i n order to estab l i s h the changes i n surface structure, i f any, which accompany anode degradation.  150  Confirmation of the observed decreases i n determined  sur-  face area by another electrochemical method, such as cathodic charging could strengthen the arguments presented i n this thesis with respect to the mechanism f o r active surface area decrease.  Further, the cath-  odic charging method would enable the determination of the amount of oxygen associated with the noble metal surface as a function of anodizat i o n time, which would lead to conclusions concerning the k i n e t i c s of passivation of these electrodes. Establishment of the " a c t i v a t i o n " of the coated electrodes i s also of i n t e r e s t .  In the present work, the current/time behaviour  was used as an indicator of the a c t i v i t y of a given coated electrode during potentiostatic anodization.  This method i s questionable, how-  ever, due to the p o s s i b i l i t y of concentration p o l a r i z a t i o n e f f e c t s and surface blockage with gas bubbles during vigourous gas evolution, which could mask the true electrode surface.  That i s , the method employed i s  not s u f f i c i e n t l y sensitive to differences i n electrode a c t i v i t y .  Of  more concern i s the p o s s i b i l i t y of " r e - a c t i v a t i o n " of the coated e l e c t rodes, which was not achieved i n the present work.  The extension of  the present r e - a c t i v a t i o n work to other methods (including chemical and heat treatment)  i s indicated. Also, the a p p l i c a t i o n of electrochemical  procedures which are known to produce increases i n the roughness factor of noble metals would be of interest to see i f the decay i n active surface area can be counteracted.  These include anodic/cathodic cycling  with v a r i a t i o n s i n the amplitude, frequency, and "shape" of the cycles.  151  The employment of coated electrodes of similar manufacture, hut with d i f f e r e n t substrates or coating compositions would be of i n t e r est.  For example, tantalum and niobium substrates have been shown to  be much more r e s i s t a n t to degradation, and i t i s f e l t that the measurement of the dependence of the active surface areas (of anodes based with these materials) on anodization time would provide some information on the degradation mechanism.  I t would be expected that, due to the super-  ior corrosion-resistance of these metals, the decay of active surface area would be slower.  The use of coatings involving, say, palladium  (which i s the least corrosion-resistant of the platinum metals) or d i f f erent amounts of platinum and iridium (or other platinum metals) would show i f there i s any composition-dependence of the p o l a r i z a t i o n or surface area c h a r a c t e r i s t i c s .  I t i s expected that the s u s c e p t i b i l i t y of  d i f f e r e n t coatings to passivation would a f f e c t the current/time behaviour for potentiostatic e l e c t r o l y s i s , but not necessarily the steady state conditions ( i . e . , the ultimate form of the p o l a r i z a t i o n curves).  The  r e l a t i v e a b i l i t i e s of the various platinum metals to sorb oxygen may also be a factor i n the oxidation of the substrate. It i s possible, then, to outline a program for future work. This would include: 1.  Extension of present short-term anodic p o l a r i z a t i o n studies to longer times i n order to establish: (a)  p o l a r i z a t i o n curves for t > 1 day  (b)  the determined  surface area/time  relation  152  (c) 2.  3.  4.  anode d e g r a d a t i o n  E x p a n s i o n o f systems t o i n c l u d e : (a)  a range o f pH and c h l o r i d e c o n c e n t r a t i o n s  (b)  industrial  (c)  a range of temperatures  (d)  a range of atmospheres  electrolytes  I n v e s t i g a t i o n o f the e l e c t r o d e  surfaces:  (a)  by e l e c t r o n m i c r o s c o p y , e l e c t r o n probe, X - r a y  (b)  by new methods such a s e l l i p s o m e t r y  (c)  w i t h d i f f e r e n t s u r f a c e a r e a measuring  diffraction  techniques  I n v e s t i g a t i o n of e l e c t r o d e d i s s o l u t i o n by: (a)  d e t e r m i n a t i o n s o f m e t a l s i n s o l u t i o n and d e p o s i t e d on the cathode.  (The use o f some form o f t r a c e r t e c h n i q u e  appears t o be most h e l p f u l . ) (b)  c o r r e l a t i o n between t h e l o s s o f a c t i v e s u r f a c e a r e a and the l o s s of c o a t i n g m e t a l .  5.  I n v e s t i g a t i o n o f t h e a c t i v a t i o n and r e - a c t i v a t i o n o f t h e electrodes: (a)  by employing known a c t i v a t i n g p r o c e d u r e s i n an attempt to r e v e r s e observed s u r f a c e a r e a l o s s e s .  (b)  by employing known roughening t e c h n i q u e s t o c o u n t e r a c t i r r e v e r s i b l e surface area l o s s e s .  153 BIBLIOGRAPHY  1.  B r e i t e r , M.W.,  E l e c t r o c h i m . A c t a , 11, 905 (1966).  2.  Warner, T.B.; S c h u l d i n e r , S.; P i e r s m a , B . J . , J . E l e c t r o c h e m . S o c , 114, 1120 (1967).  3.  B o c k r i s , J.O'M.; Mannan, R . J . ; D a m j a n o v i c , A., J . Chem. Phys., 48, 1898 (1968).  4.  P a r s o n s , R., S u r f a c e S c i e n c e , 2_, 418 (1964).  5.  P a r s o n s , R., S u r f a c e S c i e n c e , 18, 28 (1969).  6.  B o c k r i s , J.O'M.; Wroblowa, H., J . E l e c t r o a n a l . Chem., 7_» 428 (1964).  7.  Rao, M.L.B.; Damjanovic, A.; B o c k r i s , J.O'M., J . P h y s . Chem., 67, 2508 (1963).  8.  B o c k r i s , J.O'M.; Damjanovic, A.; Mannan, R . J . , J . E l e c t r o a n a l . Chem., 18, 349 (1968).  9.  Woods, R., E l e c t r o c h i m . A c t a , 13, 1967 (1968).  10.  B a g o t z k y , V.S.; V a s s i l i e v , Yu.B., Pyshnograeva, I . I . , E l e c t r o c h i m . A c t a , 16, 2141 (1971).  11.  C a d l e , S.H.; B r u c k e n s t e i n , S., A n a l . Chem., 43, 1858 (1971).  12.  Hammett, L.P., J . Amer. Chem. S o c , 46, 7 (1924).  13.  R i u s , A.; L l o o i s , J . ; T o r d e s i l l a s , I.M., A n a l e s Soc. F i s . y Quim., 48B, 35 (1952).  14.  Warner, T.B.; S c h u l d i n e r , S., J . E l e c t r o c h e m . S o c , 115, 28 (1968).  15.  B i e g l e r , T., J . E l e c t r o c h e m . S o c , 116, 1131 (1969).  16.  Adams, R.N., Ed., " E l e c t r o c h e m i s t r y a t S o l i d S u r f a c e s " , Chapter 7, M a r c e l Dekker (1969).  17.  Wensley, D.A., A Review o f E l e c t r o d e A c t i v a t i o n , u n p u b l i s h e d .  18.  W i l l , F.G.; K n o r r , C.A., Z e i t . E l e k t r o c h e m . , 64, 258 (1960).  19.  Sawyer, D.T.; Seo, E.T., J . E l e c t r o a n a l . Chem., .5, 23 (1963).  154 20.  Hoare, J.P., E l e c t r o c h i m . A c t a ,  599 (1964).  21.  Hoare, J.P., E l e c t r o a n a l . Chem., 12, 260 (1966).  22.  Hoare,.J.P., 201 (1967).  23.  Hoare, J.P., "The E l e c t r o c h e m i s t r y o f Oxygen", I n t e r s c i e n c e (1968).  24.  Gilman, S., J . E l e c t r o a n a l . Chem., 9_, 276 (1965).  25.  V o l o d i n , G.F.; T y u r i n , Yu.M., E l e k t r o k h i m . , 7_> 233 (1971).  26.  Kronenberg, M.L., J . E l e c t r o a n a l . Chem., 12, 122 (1966).  27.  S h i b a t a , S.; Sumino, M.P., E l e c t r o c h i m . A c t a , 16, 1511 (1971).  28.  S h i b a t a , S.; Sumino, M.P., E l e c t r o c h i m . A c t a , 16, 1089 (1971).  29.  S h i b a t a , S., B u l l . Chem. Soc. Japan, 33, 1635 (1960).  30.  F r e n c h , W.G.;  31.  S c h u l d i n e r , S.; Rosen, M.; F l i n n , D.R., J . E l e c t r o c h e m . S o c , 117, 1251 (1970).  32.  Pyshnograeva, I . I . ; Skundin, A.M.; V a s i l e v , Yu.B.; B a g o t s k i i , V.S., E l e k t r o k h i m . , 6, 142 (1970).  33.  B i e g l e r , T.; Rand, D.; Woods, R., J . E l e c t r o a n a l . Chem., 29, 269 (1971).  34.  Damjanovic,  35.  A p p l e b y , A . J . , J . E l e c t r o c h e m . S o c , 117, 641 (1970).  36.  S c h u l d i n e r , S.; Rosen, M., J . E l e c t r o c h e m . S o c , 118, 1138 (1971).  37.  F l i s , I.K.; Bynyaeva, M.K., Zhu. F i z . Khim., 37, 2621 (1963).  38.  M a y e l l , J.S.; Langer, S.H., J . E l e c t r o c h e m . S o c , 111, 438 (1964).  39.  P o s p e l o v a , N.V.; Rakov, A.A.; V e s e l o v s k i i , V . I . , E l e k t r o k h i m . , J5, 722 (1970).  40.  Hoare, J.P.; Thacker, R.; Wise, C.R., J . E l e c t r o a n a l . Chem., 30, 15 (1971).  Adv. E l e c t r o c h e m i s t r y and E l e c t r o c h e m i c a l Eng., _6,  Kuwana, T., J . Phys. Chem., 68, 1279 (1964).  A.; B r u s i c , V., E l e c t r o c h i m . A c t a , 12, 1171 (1967).  155  41.  Rand, D.A.J.; Woods, R., J . E l e c t r o a n a l . Chem., 35, 209 (1972).  42.  A f o n ' s h i n , G.N.; V o l o d i n , G.F.; T y u r i n , Yu.M., E l e k t r o k h i m . , 7_» 1338 (1971).  43.  V e t t e r , K . J . ; S c h u l t z e , J.W., J . E l e c t r o a n a l . Chem., 34, 141 (1972).  44.  D a v i s , D.J., T a l a n t a , 3, 335 (I960).  45.  L i n g a n e , J . J . , J . E l e c t r o a n a l . Chem., 2, 296 (1961).  46.  Sawyer, D.T.; I n t e r r a n t e , L.V., J . E l e c t r o a n a l . Chem., 2> 310 (1961).  47.  James, S.D., J . E l e c t r o c h e m .  48.  Anson, F.C., A n a l . Chem., 33, 934 (1961).  49.  Anson, F.C.; K i n g , D.M., A n a l . Chem., 34, 362 (1962).  50.  Conway, B.E.; M a r i n c i c , N.; G i l r o y , D.; Rudd, E., J . E l e c t r o c h e m . S o c , 113, 1144 (1966).  51.  Appleby, A.J., J . Electrochem.  52.  Kravchenko, N.Ya.; Fomichev, V.G.; S e r y s h e v , G.A., Z a s h c h i t a M e t a l l o v , 7_, 475 (1971).  53.  F e l d b e r g , W.G.; Enke, C.G.; B r i c k e r , C.E., J . E l e c t r o c h e m . S o c , 110, 826 (1963).  54.  L a i t i n e n , H.A.; Enke, C.G., J . E l e c t r o c h e m .  55.  Luk'yanycheva, V . I . ; B a g o t s k i i , V.S., D o k l . Akad. Nauk SSSR, 155, 160 (1964).  56.  T i k h o m i r o v a , V . I . ; Oshe, A . I . ; B a g o t s k i i , V.S.; L u k y a n y c h e v a , V . I . , D o k l . Akad. Nauk SSSR, 159, 644 (1964).  57.  Chodos, A.A.; M e i t e s , L., A n a l . Chem., 41, 846 (1969).  58.  S h i b a t a , S., E l e c t r o c h i m . A c t a , 17_, 395 (1972).  59.  S h i b a t a , S.; Sumino, M.P., E l e c t r o c h i m . A c t a , 17, 2215 (1972).  60.  James, S.D., J . E l e c t r o c h e m .  •'I.  Luk'yanycheva, V . I . ; T i k h o m i r o v a , 1, 262 (1965).  Soc., 114, 1113  (1967).  S o c , 117, 328 (1970).  S o c , 107, 773 (1960).  1  S o c , 116, 1681 (1969). V . I . ; B a g o t s k i i , V.S., E l e k t r o k h i m . ,  156  62.  G i l r o y , D.; Conway, B.E., Can. J . Chem., 46, 875 (1968).  63.  B i e g l e r , T.; Woods, R., J . E l e c t r o a n a l . Chem., 20, 73 (1969).  64.  B a g o t s k i i , V.S.; Nekrasov, 34, 1697 (1965).  65.  Chemodanov, A.N.; K o l o t y r k i n , Ya.M.; D e m b r o v s k i i , M.A., E l e k t r o k h i m . , 6, 460 (1970).  66.  Marvet, R.V.; P e t r i i ,  67.  Hoare, J.P., E l e c t r o c h i m . A c t a , 17, 1907 (1972).  68.  Hoare, J.P., J . E l e c t r o c h e m . Soc., 116, 612 (1969).  69.  Hoare, J.P., J . E l e c t r o c h e m . S o c , 116, 1390 (1969).  70.  Thacker, R.; Hoare, J.P., J . E l e c t r o a n a l . Chem., 30, 1 (1971).  71.  Rao, M.L.B.; Damjanovic, 2508 (1963).  72.  S r i n i v a s a n , S.; Wroblowa, H., B o c k r i s , J.O'M., Adv. C a t a l y s i s , 17, 351 (1967).  73.  B o c k r i s , J.O'M.; Argade, 1259 (1969).  74.  Brummer, S.B.; C a h i l l ,  75.  Conway, B.E.; G i l e a d i , E., T r a n s . F a r a d . S o c , 58, 2493 (1962).  76.  Gilman,  77.  B r e i t e r , M.W.,  78.  Khazova, O.A.; V a s i l e v , Yu.B.; B a g o t s k i i , V.S., E l e k t r o k h i m . , _1, 439 (1965).  79.  Damjanovic, A.; Genshaw, M.A.; B o c k r i s , J.O'M., J . E l e c t r o c h e m . S o c , 114, 466 (1967).  80.  Gilman,  81.  P e t r i i , O.A.; Shchigorev, I.G., E l e k t r o k h i m . , 4_, 370 (1968).  L.N.; Shumilova, N.A., U s p e k h i K h i m i i ,  O.A., E l e k t r o k h i m . , 3, 591 (1967).  A.; B o c k r i s , J.O'M., J . Phys. Chem., 67,  S.D.; G i l e a d i , E., E l e c t r o c h i m . A c t a , 14,  K., D i s c . F a r a d . S o c , 45, 67 (1968).  S., E l e c t r o c h i m . A c t a , £ , 1025 (1964). J . E l e c t r o a n a l . Chem., 8_, 230 (1964).  S., i n d i s c u s s i o n o f r e f e r e n c e number 50 (1966).  157  82.  L u , C C ; A s a k u r a , S.; F u e k i , K.; Makaibo, T., D e n k i Kagaku, 38, 213 (1970).  83.  B u t l e r , J.A.V.; A r m s t r o n g ,  84.  S l y g i n , A.; F r u m k i n , A., A c t a P h y s i c o c h i m . URSS, 3, 791 ( 1 9 3 5 ) .  85.  H i c k l i n g , A., T r a n s . F a r a d . S o c , 41, 333 (1945).  86.  W i l l , F.G.; K n o r r , C.A., Z e i t . E l e k t r o c h e m . , 64, 270 (1960).  87.  S c h u l d i n e r , S.; Warner, T.B., J.Phys. Chem., 68, 1223 (1964).  88.  G i l m a n , S., E l e c t r o a n a l y t i c a l C h e m i s t r y , 2, 111 (1967).  89.  G i n e r , J . ; P a r r y , J.M.; Smith, S.; Turchan, M., J . E l e c t r o c h e m . S o c , 116, 1692 (1969).  90.  Brodd, R . J . ; Hackerman, N., J . E l e c t r o c h e m . S o c , 104, 704 (1957).  91.  Thacker, R., "Hydrocarbon F u e l C e l l Technology", p. 525, Academic P r e s s (1965).  92.  F e l t h a m , A.M.; S p i r o , M., Chem. Rev., 71, 177 (1971).  93.  S c h u l d i n e r , S.; Roe, R.M., J . E l e c t r o c h e m . S o c , 110, 332 (1963).  94.  Formaro, L.; T r a s a t t i , S., E l e c t r o c h i m . A c t a , 12^, 1457 (1967).  95.  Rosen, M.; S c h u l d i n e r , S., J . E l e c t r o c h e m . S o c , 117, 35 (1970).  96.  Rosen, M.; F l i n n , D.R.; 1112 (1969).  97.  S c h u l d i n e r , S., NRL Rept. 6703 ( C a t . no. 68-16381) (1968).  98.  S c h u l d i n e r , S., J . E l e c t r o c h e m . S o c , 115, 897 (1968).  99.  K a l i s h , T.V.; B u r s h t e i n , R.Kh., D o k l . Akad. Nauk SSSR, 81, 1093 (1951).  C , P r o c . Roy. Soc. A, 137, 604 (1932).  Ed. B.S. Baker,  S c h u l d i n e r , S., J . E l e c t r o c h e m . S o c , 116,  100.  K a l i s h , T.V.; B u r s h t e i n , R.Kh., D o k l . Akad. Nauk SSSR, 88, 863 (1953) .  101.  S c h u l d i n e r , S.; Rosen, M.; F l i n n , D.R., (1973) .  102.  S c h u l d i n e r , S.; Warner, T.B., J . E l e c t r o c h e m . S o c , 112, 212 (1965).  E l e c t r o c h i m . A c t a , 18, 19  158  103.  B o c k r i s , J.O'M.; Damjanovic, A.; McHardy, J . , Journees I n t . Etude P i l e s Combust., C.R., B r u s s e l s , p. 15 (1969).  104.  V e t t e r , K . J . ; S c h u l t z e , J.W., J . E l e c t r o a n a l . Chem., 34, 131 (1972).  105.  T h a c k e r , R., N a t u r e , 212, 182 (1966).  106.  B i e g l e r , T., J . E l e c t r o c h e m . S o c , 114, 1261 (1967).  107.  Kuhn, A.T.; W r i g h t , P.M., J . E l e c t r o a n a l . Chem., 38, 291 (1972).  108.  F o e r s t e r , F., T r a n s . Amer. E l e c t r o c h e m . S o c , 46, 23 (1924).  109.  K n i b b s , N.V.S.; P a l f r e e m a n , H., T r a n s . F a r a d . Soc., 16, 402 (1921).  110.  Regner, A., " E l e c t r o c h e m i c a l P r o c e s s e s i n C h e m i c a l C o n s t a b l e and Co. (1957).  111.  D e V a l e r a , V., T r a n s . F a r a d . S o c , 49, 1338 (1953).  112.  D e V a l e r a , V., T r a n s . F a r a d . S o c , 52, 250 (1956).  113.  L a n d o l t , D.; I b l , N., E l e c t r o c h i m . A c t a , 15, 1165 (1970).  114.  S h e r r i l l , M.S.; I z a r d , E.F., J . Amer. Chem. S o c , 53, 1667 (1931).  115.  B i a n c h i , G., J . A p p l . E l e c t r o c h e m . , 1, 231 (1971).  116.  Van L a e r , P., CEBELCOR Tech. Rept. 143 (1966).  117.  S u z u k i , C ; Y o s h i d a , M.; Onoue, H.; Matsuno, T., D e n k i Kagaku, 34, 165 (1966).  118.  L i t t a u e r , E.F.; S h r i e r , L.L., E l e c t r o c h i m . A c t a , 1 1 , 527 (1966).  119.  Toshima, S.; Okaniwa, H., D e n k i Kagaku, 34_, 958 (1966).  120.  T a k a h a s h i , M.; Odashima, T., D e n k i Kagaku, 35, 805 ( 1 9 6 7 ) .  121.  B i t t l e s , J.A.; L i t t a u e r , E.L., C o r r o s i o n S c i . ,  12.2.  H e l b e r , H., M.S. T h e s i s , C a l i f o r n i a S t a t e C o l l e g e (1970).  123.  K o k o u l i n a , D.V.; K r a s o v i t s k a y a , Y u . I . ; K r i s h t a l i k , L . I . , E l e k t r o k h i m . , 7_, 1218 (1971).  124.  K o k o u l i n a , D.V.; K r a s o v i t s k a y a , Y u . I . ; K r i s h t a l i k , L . I . , E l e k t r o k h i m . , 7, 11.54 (1971).  Industries",  10, 29 (1970).  159  125.  Kuhn, A.T.; W r i g h t , P.M., J . E l e c t r o a n a l . Chem., 41, 329 (1973).  126.  F a i t a , G.; F i o r i , G.; A u g u s t y n s k i , J.W., J . E l e c t r o c h e m . S o c , 116, 928 (1969).  127.  Kuhn, A.T.; W r i g h t , P.M., " E l e c t r o d e s f o r I n d u s t r i a l P r o c e s s e s " , Ed. A.T. Kuhn, Chapter 14, p i 525 (1971).  128.  C o t t o n , J.B., Chem. and I n d . , 68 (1958).  129.  C o t t o n , J.B., Chem. and I n d . , 492 (1958).  130.  C o t t o n , J.B., P l a t i n u m Met. Rev., 2, 45 (1958).  131.  C o t t o n , J.B., " C o r r o s i o n " , Ed. L.L. S h r i e r , S e c t i o n 5.4, George Newnes L t d . (1963).  132.  Yakimenko, L.M.; Kokhanov, G.N.; V e s e l o v s k a y a , I . E . , Dzhagatspanyan, R.V., Khim. Prom., 1, 43 (1962).  133.  Dugdale,  134.  Van L a e r , P., CEBELCOR Tech. Rept. 142 (1966).  135.  Mazza, F., C o r r o s i o n , 23 223 (1967).  136.  Thomas, N.T.; Nobe, K., J . E l e c t r o c h e m . S o c , 116, 1748 (1969).  137.  Cerny, M., W e r k s t o f f e u. K o r r o s i o n , 21, 610 (1970).  138.  B r e i t e r , M.W.,  139.  Lowe, R.A., M a t e r i a l s P r o t . , 5_, 23 (1966).  140.  Warne, M.A.; H a y f i e l d , P.C.S., T r a n s . I n s t . Met. F i n i s h i n g , 4v5, 83 (1967) .  141.  K h o d k e v i c h , S.D., V e s e l o v s k a y a , I . E . ; Yakimenko, L.M.; Guskova, L.A., E l e k t r o k h i m . , 6, 135 (1970).  142.  K h o d k e v i c h , S.D.; V e s e l o v s k a y a , I.E.; Yakimenko, L.M.; D a n i l o v a , O.L., E l e k t r o k h i m . , 7_, 357 (1971).  1-13.  H a l e y , A . J . , E n g e l h a r d I n d . Tech. B u l l . , 7_, 157 (1967).  144.  A n t l e r , M.; B u l t l e r , C.A., E l e c t r o c h e m . T e c h n o l . , _5, 126 (1967).  1*>. . . Z  I . ; C o t t o n , J.B., C o r r o s i o n S c i . , 4_, 397 (1964).  E l e c t r o c h i m . A c t a , 15, 1195 (1970).  !Nholkev:ch, S.D. ; Veselovskaya, E l e k t r o k h i m . , 5, 1332 (1969).  I . E . ; Yakimenko, L.M.; Uzbekov, A.A.,  160 146.  F a i t a , G.; F i o r i , G.; N i d o l a , A., J . E l e c t r o c h e m . Soc., 117, 1333 (1970).  147.  Weber, J . ; P o s l r i l , B., J . E l e c t r o a n a l . Chem., 38, 417 (1972).  148.  F a i t a , G.; F i o r i , G., J . A p p l . E l e c t r o c h e m . , 2, 31 (1972).  149.  L a n d o l t , D.; I b l , N., J . A p p l . E l e c t r o c h e m . , 2, 201 (1972).  150.  Shembel, E.M.; K a l i n o v s k i i , E.A.; S h u k s t i l l n s k a y a , T.G., E l e k t r o k h i m . , 8, 1433 (1972).  151.  L l o p i s , J . , C a t a l y s i s Rev., 2, 161 (1968).  152.  Chemodanov, A.N.; K o l o t y r k i n , Ya.M., Ann. U n i v . F e r r a r a , S e z . 5, 49 (1970).  153.  I z a t t , R.M.; Eatough, D.; C h r i s t e n s e n , J . J . , J . Chem. Soc. ( A ) , 1301 (1967).  154.  G o l d b e r g , R.N.; H e p l e r , L.G., Chem. Rev., 68, 229 (1968).  155.  Goodridge, F.; K i n g , C.J.H., T r a n s . F a r a d . S o c , 66, 2889 (1970).  156.  K o l o t y r k i n , Ya.M., Z a s h c h i t a M e t a l l o r , 3, 131 (1967).  157.  L l o p i s , J.F.; Gamboa, J.M.; V i c t o r i , 2225 (1972).  158.  Hoar, T.P., C o r r o s i o n S c i . ,  159.  L l o p i s , J . ; Gamboa, J.M.; A l f a y a t e , J.M., E l e c t r o c h i m . A c t a , 12, 57 (1967).  160.  Chemodanov, A.N.; K o l o t y r k i n , Ya.M.; Kosmaty, V.E.; D e m b r o v s k i i , M.A., E l e k t r o k h i m . , 4, 1466 (1968).  161.  Z e l a n s k i i , M.A.; K r a u t s o v , V . I . , V e s t . L e n i n g r a d U n i v . , 23, 118 (1968).  162.  K r a u t s o v , V . I . ; Z e l e n s k i i , M.I., E l e k t r o k h i m . , 5_, 247 (1969).  163.  B l a k e , A.R.; Kuhn, A.T.; Sunderland, J . Chem. Soc. ( A ) , 3015 (1969).  164.  Napp, D.T.; B r u c k e n s t e i n , S., A n a l . Chem., 40, 1036 (1968).  165.  J u c h n i e w i c z , R.; W a l a s z k o w s k i , J . ; Widuchowski, A.; Bohdanowicz, D a n z i g P o l i t . Z e s z . Nauk. Chem., 20, 119 (1970).  L., E l e c t r o c h i m . A c t a , 17,  7_> 341 (1967).  W.,  161 166.  L l o p i s , J . ; Vazquez, M., A n a l e s R e a l . Soc. P i s . y Quim., 63B, 273 (1967).  167.  Zotov, G.N.; K o l o t y r k i n , Ya.M.; Bune, N.Ya., E l e k t r o k h i m . , 6_, 857 (1970).  168.  Burke, L.D.; O'Meara, T.O., F a r a d . T r a n s . I , 68_, 839 (1972).  169.  K o l o m i e t s , B.S.; Kovsman, E.P.; Chemodanov, A.N.; G i l m a n , V.A.; K o l o t y r k i n , Ya.M.; F i o s h i n , M.Ya., Z a s h c h i t a M e t a l l o v , 6_, 678 ( 1 9 7 0 ) .  170.  K o l o t y r k i n , Ya.M.; Gilman, V.A.; K o l o m i e t s , V.S.; Chemodanov, A.N.; Kousman, E.P.; M i r o n o v , Yu.M.; D e m b r o v s k i i , M.A.; F i o s h i n , M.Ya., Z a s h c h i t a M e t a l l o v , .5, 660 (1969).  171.  G i n s t r u p , 0.; Leden, I . , A c t a . Chem. Scand., _26, 2689 (1967).  172.  F o l e y , R.T., C o r r o s i o n , 26, 58 (1970).  173.  Chemodanov, A.N.; K o l o t y r k i n , Ya.M.; D e m b r o v s k i i , M.A., E l e k t r o k h i m . , 5, 578 (1969).  174.  V i j h , A.K., J . E l e c t r o c h e m . S o c , 115, 1096 (1968).  175.  Hackerman, N.; S n a v e l y , E.S.; F i e l , L.D., C o r r o s i o n S c i . , ]_, 39 (1967).  176.  Tanaka, N.; F u j i s a w a , T., B u l l . Chem. Soc. Japan, 42, 2300 (1969).  177.  Y u k h e v i c h , R.; Bogdanovich, V . I . , Z a s h c h i t a M e t a l l o v , 5, 259 (1969).  178.  K u k u s h k i n , Yu.N.; M a l s o v , E . I . ; Simanova, S.A., A l a s h k e v i c h , V.P.; V o r o t n i k o v , M.V., Zhu. P r i k l . Khim., 44^, 244 (1971).  179.  Ryaznov, A . I . ; P e t r e n k o , G.D.; Domanova, E.G., Zhu. P r i k l . Khim., 43_, 838 (1970).  180.  Kadaner, L . I . , B o i k o , A.V., E l e k t r o k h i m . , 7_>  181.  M a y e l l , J.S.; B a r b e r , W.A.,  182.  Rand, D.A.J.; Woods, R., J . E l e c t r o a n a l . Chem., 36, 57 (1972).  183.  Johnson, D.C.; Napp, D.T.; 3 493 (1970).  184.  C l a o , P.; C o s t a , M.; T a d j e d d i n e , A., Comptes Rendus Acad. S c i . P a r i s , 274C, 16L3 (1972).  1 4 8 3  (1971).  J . E l e c t r o c h e m . S o c , 116, 1333 (1969).  B r u c k e n s t e i n , S., E l e c t r o c h i m . A c t a , 15,  162  185.  B u r g h a r d t , S . I . , Canadian P a t e n t 771, 140 (1970).  186.  Okamura, T., U.S. P a t e n t 3, 497, 426 (1970).  187.  A o k i , K., D e n k i Kagaku, 35, 136 (1967).  188.  H e n z i , R.; G i o r i a , J.M.; Meyer, A., Ger. O f f e n . 2,200,527 (1972).  189.  M a r s h a l l , C ; M i l l i n g t o n , J.P., J . A p p l . Chem., 19_, 298 (1969).  190.  B i a n c h i , G.; G a l l o n e , P.; N i d o l a , A.E., U.S. P a t e n t 3,491,014 (1970).  191.  Drayman, E.W., M a t e r i a l s P r o t . P e r f . , 11, 17 (1972).  192.  A n g e l l , C.H.; D e r i a z , M.G., B r i t . P a t . 984,973 (1965).  193.  Bianchi, G.; F a i t a , G.; G a l l i , R.; M u s s i n e , T., E l e c t r o c h i m . A c t a , 12, 439 (1967).  194.  P i c K n e t t , R.G., T r a n s . F a r a d . S o c , 64, 1059 (1968).  195.  E v e r y , R.L., E l e c t r o c h e m . T e c h n o l . , 4_, 275 (1966).  196.  Earned, H.S.; Owen, B.B., "The P h y s i c a l C h e m i s t r y o f E l e c t r o l y t i c S o l u t i o n s " , A.C.S. Monograph S e r i e s (1958).  197.  F a i t a , G.; L o n g h i , P.; M u s s i n i , T., J . E l e c t r o c h e m . S o c , 114, 340 (1967) .  198.  Toshima, S.; Okaniwa, H., D e n k i Kagaku, 34, 958 (1966).  199.  S t o n e h a r t , P.; Zucks, P.A., E l e c t r o c h i m . A c t a , 17_, 2333 (1972).  163 APPENDIXES  APPENDIX I  Electrode Surface  Conditions  TABLE 10 Surface  c o n d i t i o n s o f w i r e e l e c t r o d e s used i n  g a l v a n o s t a t i c p o l a r i z a t i o n experiments  Procedure  1.  soaked i n chromic/ sulphuric acid  2.  washed i n t w i c e d i s t i l l e d water  Aspects about 3 m i n u t e s  Remarks s u r f a c e i m p u r i t i e s removed, s u r f a c e oxygen f i l m p a r t l y formed. traces of cleaning s o l u t i o n removed.  stored i n twiced i s t i l l e d water p r i o r to use introduction into c e l l  open-circuit, helium-purging  p o s s i b l e r e m o v a l o f some s u r f a c e oxygen.  5.  cathodic  100 mA f o r 5 minutes  sorbed oxygen and i m p u r i t i e s removed, hydrogen f i l m formed.  6.  e l e c t r o l y t e changed  open-circuit, helium-purging  hydrogen f i l m removed ( p o t e n t i a l d r i f t s to anodic v a l u e s ) .  7.  experiment begun  smallest anodic current applied  i n i t i a l electrode condition reproducible.  8.  experiment  current increased i n anodic i n c r e ments  electrode surface condition changes u n t i l i t i s u l t i m a t e l y c o m p l e t e l y f i l m e d w i t h oxygen.  pretreatment  In  F i g u r e 19.  Schematic r e p r e s e n t a t i o n of the p o t e n t i a l h i s t o r y of a w i r e e l e c t r o d e used i n g a l v a n o s t a t i c p o l a r i z a t i o n experiments. (Numbers c o r r e s p o n d to those of T a b l e 10.)  4  165 TABLE 11 S u r f a c e c o n d i t i o n s of coated p o t e n t i o s t a t i c p o l a r i z a t i o n and  Procedure  e l e c t r o d e s used i n  surface area  Aspects  determinations.  Remarks  1.  soaked i n c h r o m i c / sulphuric acid  2.  washed i n t w i c e d i s t i l l e d water  3.  stored i n twiced i s t i l l e d water p r i o r t o use  4.  introduction into c e l l (1M H S 0 )  open-circuit, helium-purging  p o s s i b l e r e m o v a l of some surf a c e oxygen from c o a t i n g .  5.  anodization  5-10 m i n u t e s a t 1800-2000 mv (S.C.E.)  sorbed i n and on c o a t i n g , f u r t h e r o x i d a t i o n of s u b s t r a t e .  6.  reduction  open-circuit, hydrogen-purging u n t i l hydrogen potential attained  removal of oxygen f r o m s u r f a c e of n o b l e m e t a l c o a t i n g , h y d r o gen d e p o s i t e d on s u r f a c e .  about 1-2 m i n u t e s f o l l o w e d by about 1 minute to p e r m i t s o l u t i o n t o become quiescent  s u r f a c e hydrogen removed, s o l u t i o n purged of d i s s o l v e d hydrogen.  20.5 mA f o r about 20 seconds  monolayer of oxygen adsorbed on c o a t i n g s u r f a c e .  2  7.  8.  4  helium-purging  anodic pulse (surface area determination)  about 3 m i n u t e s  s u r f a c e i m p u r i t i e s removed, oxygen f i l m p a r t l y formed on coating, substrate i r r e v e r s ibly oxidized. t r a c e s of c l e a n i n g s o l u t i o n removed.  (Steps 5 t h r o u g h 8 may be r e p e a t e d s e v e r a l t i m e s d u r i n g c o n s t r u c t i o n of a p o l a r i z a t i o n curve, w i t h anodizations at given p o t e n t i a l s f o r given p e r i o d s of t i m e . I n a d d i t i o n , the a n o d i z a t i o n s may be performed i n d i f f e r e n t e l e c t r o l y t e s , w h i c h r e q u i r e s that, a p p r o p r i a t e washing s t e p s be i n c l u d e d p r i o r to and a f t e r such t r e a t m e n t . )  166  SC.E  I  -4  0 -  0  Figure 2 0 .  5  t, (minutes)  10  Schematic r e p r e s e n t a t i o n of the p o t e n t i a l h i s t o r y of a c o a t e d e l e c t r o d e used i n p o t e n t i o s t a t i c p o l a r i z a t i o n experiments. (Numbers correspond to those of T a b l e 11.)  15  167  APPENDIX I I  S u r f a c e A r e a Measurement  I n o r d e r to p r o v i d e a more thorough u n d e r s t a n d i n g technique employed f o r d e t e r m i n i n g  the s u r f a c e a r e a s of  e l e c t r o d e s , a d e t a i l e d example i s p r e s e n t e d the s u r f a c e a r e a f o r a coated e l e c t r o d e .  of  the  noble-metal  showing the c a l c u l a t i o n of  The assumptions on w h i c h the  d e t e r m i n a t i o n i s based a r e : 1.  P r e l i m i n a r y anodic  treatment  produces s a t u r a t i o n of  e l e c t r o d e s u r f a c e and upper atomic  the  l a y e r s w i t h oxygen.  The  oxygen i n the m e t a l i n t e r i o r i s so s l o w l y removed t h a t i t does not d i f f u s e outward and produce new w i t h i n the times employed d u r i n g the 2.  s u r f a c e coverage  experiment.  Hydrogen b u b b l i n g removes a l l s u r f a c e adsorbed oxygen and subsequent h e l i u m b u b b l i n g removes the o x i d i z a b l e hydrogen, l e a v i n g a v i r t u a l l y bare s u r f a c e .  3.  A n o d i c p u l s i n g r e s u l t s i n o n l y two e l e c t r o d e r e a c t i o n s i n the r e g i o n of p o t e n t i a l s c o r r e s p o n d i n g l y d o u b l e - l a y e r c h a r g i n g and  4.  t o oxygen d e p o s i t i o n , name-  oxygen d e p o s i t i o n .  The d o u b l e - l a y e r charge can be d e t e r m i n e d d i r e c t l y from the s l o p e of the " d o u b l e - l a y e r r e g i o n " and  thus can be  readily  s u b s t r a c t e d from the t r a n s i t i o n time measured f o r the oxygen deposition region.  168  5.  A monolayer  o f oxygen i s d e p o s i t e d , w i t h a one-to-one  corres-  pondence w i t h s u r f a c e n o b l e m e t a l atoms. 6.  The oxygen monolayer  charge f o r t h e c o a t e d e l e c t r o d e s i s 426  i / ycoul/cm. . 2  I n F i g u r e 21 i s shown a t y p i c a l p o t e n t i a l v s . time c u r v e where an a n o d i c g a l v a n o s t a t i c p u l s e o f 20.5 mA i s a p p l i e d t o an e l e c t rode which was i n i t i a l l y a t the hydrogen p o t e n t i a l .  In the f i g u r e are  r e p r e s e n t e d a h o r i z o n t a l c u r v e a t -0.25 V (S.C.E.) w h i c h i n d i c a t e s t h e i n i t i a l p o t e n t i a l o f t h e e l e c t r o d e , t h e t r a c e formed on a p p l i c a t i o n o f the a n o d i c charge (showing hydrogen i o n i z a t i o n , d o u b l e l a y e r c h a r g i n g , and oxygen monolayer  d e p o s i t i o n ) , f o l l o w e d by an upper h o r i z o n t a l s e c -  t i o n r e p r e s e n t i n g m o l e c u l a r oxygen e v o l u t i o n .  The t r a n s i t i o n t i m e f o r  oxygen e v o l u t i o n , T , i s d e r i v e d by means of t h e c o n s t r u c t i o n shown, where d.l.  o  o r , t h e o v e r a l l t r a n s i t i o n time i n t h e oxygen r e g i o n ( T ) i s t h e sum o f the t r a n s i t i o n t i m e s f o r double l a y e r c h a r g i n g and oxygen d e p o s i t i o n . With t h i s measured v a l u e , t h e "determined s u r f a c e a r e a " o f t h e c o a t e d e l e c t r o d e c a n be r e a d i l y e v a l u a t e d by means o f t h e e q u a t i o n :  ( I A  =  determined  charge ' ° .«, .. , 2 426 ycoul/cm. )  (  t  )  T y p i c a l t r a n s i t i o n times were o f t h e o r d e r o f 3 seconds o r l e s s f o r coated e l e c t r o d e s .  - 0.5'  1  0  F i g u r e 21.  1  '  1  1  1  1  1  i  5  T (sec.) R e p r e s e n t a t i o n of a t y p i c a l a n o d i c charge c u r v e i n d e - a e r a t e d IM H^SO^  a t 20 C, showing c o n s t r u c t i o n s  f o r d e t e r m i n i n g oxygen d e p o s i t i o n charge.  10  £  170 APPENDIX I I I  X-Ray D i f f r a c t i o n R e s u l t s  Cu-Ka r a d i a t i o n was u t i l i z e d , w i t h t h e beam a n g l e v a r i e d form 5° t o 80°.  Peak a n g l e s , c a l c u l a t e d d - s p a c i n g s and t h e p r o b a b l e  s p e c i e s a r e r e p o r t e d i n T a b l e s 12 and 13 w h i c h r e p r e s e n t b o t h new and used anodes.  TABLE 12 I d e n t i f i c a t i o n of X-ray d i f f r a c t i o n peaks f o r a new t i t a n i u m s u b s t r a t e e l e c t r o d e .  20  d  Species  38,3°  2.35  Ti  39.7  2.27  Pt  40.1  2.24  Ti  46.2  1.97  Pt  52.9  1.73  Ti  62.9  1.48  Ti  67.6  1.38  Pt  70.6  1.33  Ti  75.2  1.25  Ti  81.4  1.18  Pt  These may more p r o p e r l y be taken t o r e f e r to t h e p l a t i n u m - i r i d i u m a l l o y . The d i f f r a c t i o n peaks f o r t h e two s e p a r a t e s p e c i e s are o n l y s l i g h t l y d i s p l a c e d from one another.  171 TABLE 13 I d e n t i f i c a t i o n o f X - r a y d i f f r a c t i o n peaks f o r a used t i t a n i u m s u b s t r a t e e l e c t r o d e (3 weeks i n 1M H S 0 2  20  28.0°  4  a t .2 A / f t .  and 40°C).  2  d  3.18  Species  Ti 0 , Ti 0 , T1 0 , 3  Ti  1 0  5  5  0  1 9  ,  9  6  Ti0  Ti 0  U  7  2.65  Ti 0 , Ti 0 ,  34.7  2.58  TI,  35.1  2.55  Ti 0, Ti 0 , T i 0  35.6  2.52  TiO, T i 0 , T i 0 ,  37.4  2.39  TiO, T i 0 , T i 0 , T i 0  38.3  2.35  Ti,  38.9  2.31  T1  40.0  2.25  10°19 Pt  41.5  2.17  Ti 0 ,  46.4  1.95  Pt  53.0  1.73  Ti  54.1  1.69  Ti 0 , Ti 0 ,  63.0  1.47  Ti,  67.6  1.38  Pt  70.7  1.34  Ti  76.2  1.27  Ti  77.4  1.26  Ti  81.7  ] .17  Pt  86.2  1.13  Pt, ( T i 0 ,  3  5  Ti 0  9  6  8  1 5  ,  6  n  u  2  2  3  2  Ti 0  (Ti 0 )  2  3  ,  2  33.0  2  1 3  (Ti  5  3  3  4  5  0  1 Q  3  1 9  , Ti 0  1 3  8  Ti0  ?  5  Ti0  3  ?  9  6  1 5  ,  Ti  1 Q  0  0  i g  ,  1 9  2  i  Ti  r  1 0  Ti0  2  )  2  Ti0  5  2  ( T i 0 , TiO, T i 0 , T i 0 ) 2  2  2  2  Ti 0 ) 2  3  * f o r o x i d e s p e c i e s , i n c o m p l e t e s e t s of d i f f r a c t i o n l i n e s were found, and p e a k - h e i g h t s were obscured by the f a c t t h a t the d i f f e r e n t o x i d e s possessed  s i m i l a r d - s p a c i n g s . A l l o x i d e s must be r e g a r d e d as " p o s s i b l e " .  

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