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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.Sc, U n i v e r s i t y of 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 the Department of METALLURGY . We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1973 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r 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 the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f M e t a l l u r g y The U n i v e r s i t y o f B r i t i s h Co lumbia V a n c o u v e r 8, Canada Date O c t o b e r 4 , 1973 i i ABSTRACT This thesis considers the electrochemistry of platinum-iridium electrodes in both sulphate- and chloride-containing electro-lytes at 20 - 25°C. Both wire electrodes of appropriate alloy composi-tions and titanium-substrate electrodes were employed. Polarization curves were obtained, and a technique for measuring the surface area of the electrodes was employed in order to determine the effect of potentiostatic electrolysis on the electrochemically active area. The wire alloy electrodes showed polarization behaviour in 1M NaCl; pH 2 identical to that of platinum electrodes, indicating that iridium is not effective in reducing the passivation of these electrodes even with up to 25% alloy content. The coated electrodes showed irreversible surface area losses in both sulphate and chloride electrolytes, with the latter pro-ducing significant reductions after very short polarization times. It is suggested that oxidation of the substrate leading to ele c t r i c a l iso-lation of coating plates i s responsible for the area decay. i i i 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 of the Problem 6 2.1.2 Methods of Pretreatment and A c t i v a t i o n .... 9 2.1.3 Mechanisms of A c t i v a t i o n of Noble Metal Electrodes 11 2.1.3.1 Surface Area Increases 11 2.1.3.2 Active Surface Structure 14 2.1.3.3 Active Oxidized Surface 18 2.1.3.4 Active Reduced Surface 20 2.1.3.5 A c t i v i t y Induced by Dermasorption of Oxygen 24 2.1.3.6 Alter e d E l e c t r o n i c Properties .... 27 2.1.3.7 Impurity Removal 29 2.1.4 Summary 32 2.2 SURFACE AREA OF NOBLE METAL ELECTRODES 33 2.2.1 Bases f o r Electrochemical Surface Area Measurement 33 2.2.1.1 Complications due to Simultaneous Processes 34 2.2.1.2 Compensation f o r Double Layer Charging 36 2.2.1.3 Monolayer Formation 37 2.2.1.4 Absorption 38 i v Page 2.2.1.5 Surface Atom Density 40 2.2.2 Procedures for Surface Area Measurement ... 42 2.2.2.1 Potential Sweep Techniques 42 2.2.2.2 \Galvanostatic Charge Techniques .. 44 2.2.3 Summary 46 2.3 ELECTROLYSIS OF CHLORIDE SOLUTIONS 48 2.3.1 Polarization of Smooth Noble Metal Anodes . 50 2.3.2 Polarization of Titanium-Substrate Anodes . 58 2.3.2.1 Behaviour of Titanium 59 2.3.2.2 Coupling of Platinum Metals with Titanium 61 2.3.2.3 Polarization Characteristics 63 2.3.3 Summary 64 2.4 DISSOLUTION OF THE NOBLE METALS 65 2.4.1 The Active Dissolution of the Noble Metals 66 2.4.2 Dissolution with Oxygen Participation 72 2.4.3 Dissolution During Activation 78 2.4.4 Degradation of Noble Metal Coatings 83 2.4.4.1 Degradation as a Result of Coat-ing Undermining 83 2.4.4.2 Other Causes of Coating Loss 86 2.4.5 Summary 87 2.5 RELATION TO AIMS OF PRESENT WORK 88 3. EXPERIMENTAL 93 3.1 ELECTRODES 93 V Page 3.2 ELECTROLYTES 95 3.3 CELLS 96 3.4 PROCEDURES 100 3.4.1 Anodic Galvanostatic 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 Surface Area Measurements 102 3.4.4 Observation of Electrode Surfaces 105 4. RESULTS 106 4.1 GALVANOSTATIC POLARIZATION CURVES 106 4.2 POTENTIOSTATIC POLARIZATION CURVES 119 4.3 CHANGE OF SURFACE AREA WITH POTENTIOSTATIC ANODIZATION 122 4.4 OBSERVATIONS OF ELECTRODE SURFACES 132 5. DISCUSSION 140 5.1 ANODIC GALVANOSTATIC MEASUREMENTS 140 5.2 ANODIC POTENTIOSTATIC MEASUREMENTS 142 5.3 SURFACE AREA CHANGES 143 6. PROPOSALS FOR FUTURE WORK 148 BIBLIOGRAPHY 153 APPENDIXES 163 APPENDIX I Electrode Surface Conditions 163 APPENDIX II Surface Area Measurement 167 APPENDIX III X-ray D i f f r a c t i o n Results 170 v i LIST OF TABLES TABLE Page 1. Hydrogen and oxygen monolayer charges f o r platinum and i r i d i u m electrodes 41 2. Electrode areas measured a f t e r determination of the p o l a r i z a t i o n curves 109 3. T a f e l parameters f o r Pt and Pt/ I r wire electrodes f o r the lower T a f e l region of the p o l a r i z a t i o n curves i n 1M NaCl; pH 2 I l l 4. T a f e l parameters f o r Pt and P t / I r wire electrodes f o r the ascending and descending upper T a f e l regions of the p o l a r i z a t i o n curve i n 1M NaCl; pH 2 I l l 5. Passivation data f o r 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 2 113 6. Surface area 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 chloride e l e c t r o l y t e s with Pt/30 I r -T i electrodes 118 7. 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 ^SO^ on the surface area of Pt/30 I r - T i electrodes 119 8. E f f e c t of treatment i n aqua r e g i a on the surface area of Pt/30 I r - T i electrodes 120 9. E f f e c t s of " a c t i v a t i o n " procedures on the surface area of Pt/30 I r - T i electrodes 126 10. Surface conditions of wire electrodes used i n galvano-s t a t i c p o l a r i z a t i o n experiments 163 11. Surface conditions of coated electrodes used i n potent-i o s t a t i c p o l a r i z a t i o n and surface area determinations .. 165 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 for a new titanium substrate electrode 170 13. 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 titanium substrate electrode (3 weeks i n 1M H„S0, at .2 A / f t . 2 and 40°C) 171 v i i LIST OF FIGURES FIGURE Page 1. Galvanostatic c e l l 98 2. C e l l f o r surface area measurement 99 3. Galvanostatic p o l a r i z a t i o n curve for platinum wire electrode i n helium-saturated 1M NaCl; pH 2 109 4. Galvanostatic p o l a r i z a t i o n curve f o r platinum/5% i r i d i u m wire electrode i n helium-saturated 1M NaCl; pH 2 110 5. Galvanostatic p o l a r i z a t i o n curve f o r platinum/10% i r i d i u m wire electrode i n helium-saturated 1M NaCl; pH 2 I l l 6. Galvanostatic p o l a r i z a t i o n curve f o r platinum/20% i r i d i u m wire electrode i n helium-saturated 1M NaCl; pH 2 112 7. Galvanostatic p o l a r i z a t i o n curve f o r platinum/25% i r i d i u m wire electrode i n helium-saturated 1M NaCl; pH 2 113 8. 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 un s t i r r e d 1M l^SO^ 120 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 curve f o r Pt/30 I r - T i i n un s t i r r e d 1M NaCl; pH 2 121 10. 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 with Pt/30 I r - T i electrodes at 1800 mV (S.C.E.) i n various e l e c t r o l y t e s 127 11. Current/time r e l a t i o n s f or a Pt/30 I r - T i electrode 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^ at 1800 mV and 25°C, a f t e r various pretreatment times i n aqua regia .. 128 12. Current/time r e l a t i o n s for a Pt/30 I r - T i electrode for 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^ at 2000 mV and 25°C, before and a f t e r potentiostatxc e l e c t r o l y s i s of 1M NaCl; pH 2 129 13. S.E.M. Observation of Pt/30 I r - T i surfaces 134 14. S.E.M. Observation of used Pt/30 I r - T i electrodes 135 v i i i FIGURE Page 15. S.E.M. Observation of platinum sheet 136 16. S.E.M. Observation of platinum wire electrodes 137 17. S.E.M. Observation of Pt/25 I r wire electrodes 138 18. E.P. Observation of new Pt/30 I r - T i electrode 139 19. Schematic representation of the p o t e n t i a l h i s t o r y of a wire electrode used i n galvanostatic p o l a r i z a t i o n experiments ; 164 20. Schematic representation of the p o t e n t i a l hostory of a coated electrode 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 21. Representation of a t y p i c a l anodic charge curve i n de-aerated 1M I^SO^ at 20°C, showing constructions for determining oxygen deposition charge 169 I wish to thank Dr. I.H. Warren for his guidance throughout the course of this research, the staff of the Science Division, Main Library, "U.B.C. for their invaluable aid, and my wife, Darlene, for her enduring patience. Financial support from the National Research Council and International Nickel Company is also acknowledged. 1. INTRODUCTION Of a l l the possible electrodes for oxidation reactions in aqueous solutions, those u t i l i z i n g metals of the platinum group (plat-inum, 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 i t s e l f is the most commonly utilized material, due more to i t s relative abundance than to some relatively superior combina-tion 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 invest-igation. For the case of alloy electrodes, the choices of the particular 2 platinum metals is governed by: 1) the different corrosion-resistant properties of the individual metals toward the medium i n question. 2) the avai l a b i l i t y and cost of the individual metals. 3) the crystallographic systems of the metals w i l l r e s t r i c t the compositions of homogeneous alloys and w i l l limit the alloys which can be worked. The extreme cost of the platinum metals, together with the problems associated with producing desirable alloy systems, has led to the concept of coating a suitably inert substrate material with a thin precious metal (or metal oxide) cover. Such an electrode exhibits similar 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 av a i l a b i l i t y . The material must be both relatively inexpensive and easily formed to desired shapes. 2) Inertness. If the electrode i s to be subjected to anodic polarization, the substrate must form an insulating oxide film wherever i t contacts the electrolyte. 3) Breakdown voltage. In addition, the non-conducting areas of the anode must be able to withstand the electrochemical "stress" imposed by high anodic potentials. 4) Conductivity. The substrate metal must possess adequate elect-r i c a l conductivity in order to permit current to pass through 3 the body of the electrode. This is of particular concern with large anodes, whose extremities are far-removed from the elec-t r i c a l contact. 5) Resistance to deformation in 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 re la t ive ly simple pretreatment of the substrate. The metals found to be most suitable for substrates in precious metal coated anodes are the refractory metals (titanium, tant-alum, tungsten, zirconium, niobium). These metals share the property of formation of a protective oxide f i lm on being made anodic. As t i t -anium is readily available commercially, i t has been ut i l i zed extensively. The residual current for titanium dissolution through i t s protective oxide f i lm is extremely low. Its strength, conductivity and oxide break-down voltage are a l l acceptable, and good adherance of precious metal coatings is obtained. Although other metals such as niobium or tantalum (higher breakdown voltage) or alloys such as titanium/molybdenum (better coating adherence) are even more suitable, their cost precludes the wide-spread application of such alternative substrate materials. The s tab i l i ty of precious metal anodes which are used in certain systems is of concern, where the term " s t ab i l i ty " 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 potential). 3) Reproducibility of electrochemical data. It has been found that, apart from inherent differences in the electro-chemical behaviour of the individual platinum metals, the above pheno-mena are affected - indeed, even determined - by i t s treatment before use as an anode. That i s , the history of the electrode must be taken into consideration i f differences in electrochemical activity are to be resolved. Before attempting meaningful experimental work on the elec-trochemical behaviour of noble metal anodes (either in a pure form or as a coating over a suitable substrate), i t is necessary to establish the parameters which attain to the particular system(s) of interest. These include: 1) Surface condition prior to use as anode, and any changes in this condition as a result of anodic circumstances. This may be taken to include changes in the electrochemically active surface area, formation of oxide or other films, and formation of different surface structures as the result of dissolution. 2) Possible anodic reactions. These are determined by the anode material, the composition of the electrolyte, and the surface state of the electrode. 5 2. LITERATURE SURVEY AND THEORETICAL CONSIDERATION A v a i l a b l e l i t e r a t u r e o n t h e 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 t h e n o b l e m e t a l s c o n s t i t u t e s t h e l a r g e s t b o d y o f l i t e r a t u r e i n t h e e n t i r e f i e l d o f e l e c t r o c h e m i s t r y . To compound t h e p r o b l e m , t h e r e i s o f t e n l i t t l e 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 a r e a s o f t h e f i e l d . F o r e x a m p l e , r e s e a r c h o n t h e f o r m a t i o n o f a n o d i c s u r f a c e f i l m s may n e g l e c t t h e work d o n e b y o t h e r s o n t h e e f f e c t o f e l e c t r o d e p r e t r e a t m e n t on v a r i o u s a n o d i c p r o c e s s e s , y e t t h e s e f i e l d s a r e c e r t a i n l y r e l a t e d . A g a i n , t h e 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 t h e p o s s i b i l i t y o f a n o d i c d i s s o l u t i o n r e a c t i o n s w h i c h may n o n e t h e l e s s s t i l l b e p o s s i b l e . T h e e s t a b l i s h m e n t o f t r u e e l e c t r o d e s u r f a c e a r e a s h a s a l s o b e e n e x t e n s -i v e l y r e s e a r c h e d , y e t o t h e r r e s e a r c h e r s s t i l l u s e assumed r o u g h n e s s f a c t o r s when c o n s i d e r i n g t h i s s u b j e c t . A n e n d l e s s l i s t o f i n c o n s i s t e n t e l e c t r o c h e m i c a l w o r k c o u l d be c o n s t r u c t e d . The l i t e r a t u r e s u r v e y and t h e o r e t i c a l c o n s i d e r a t i o n c o n t a i n -ed i n t h i s t h e s i s d e a l s o n l y w i t h t h o s e t o p i c s w h i c h a r e o f i m m e d i a t e i n t e r e s t i n t h e e x p e r i m e n t a l w o r k c o n t a i n e d w i t h i n - e l e c t r o d e p r e t r e a t -m e n t , s u r f a c e a r e a m e a s u r e m e n t , t h e b e h a v i o u r o f a n o d e s i n 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 s o l u t i o n s , and t h e d i s s o l u t i o n o f t h e n o b l e m e t a l s . E a c h o f t h e a f o r e m e n t i o n e d t o p i c s c o u l d e a s i l y b e d e a l t w i t h a t much g r e a t e r l e n g t h t h a n c o u l d b e r e p r e s e n t e d w i t h i n t h e c o n f i n e s o f t h i s t h e s i s . I n d e e d , t h e a u t h o r h a s u n d e r t a k e n t o w r i t e e x t e n s i v e s u r v e y s o f t h e a b o v e f i e l d s , s e c t i o n s o f w h i c h ( n a m e l y t h e r e s u l t s , 6 mainly from a mechanistic point of view) are reproduced i n this thesis with appropriate correlation with other topics. Such work has a high degree of repetition as a result of the correlation, but i t i s f e l t that this i s necessary in order for 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 prior to experimental use (usually some combination of intentional and unintentional procedures) which may have a significant effect on the experimental results. For noble metal electrodes in particular, pre-treatments are deliberately applied in order to "activate" such elect-rodes for some electrochemical process. However, researchers present conflicting views as to what i s the state of an active electrode and as to the properties which define an active electrode. Electrode activity is best understood when comparing elect-rodes of different materials with respect to their behaviour in a given electrochemical situation. It i s less clear, however, to understand the changes in activity of a single electrode material such as platinum. Hence, a profusion of mechanisms can be found in the literature to des-cribe the active state of the latter, which may or may not be related depending on the definition of this state. Breiter^ defines an active 7 electrode as one having a reproducible surface state of high reactivity for electrochemical reactions. However, i t i s apparent that the react-2 ion of interest be specified, 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 variation in electrode reaction kinetics with the substrate can be attributed to the variation in bond 6 7 8 strength between the adsorbed radicals and the substrate. Bockris ' ' says that the presence of vacant orbitals in the d-band i s the controll-ing electronic factor in determining activity. For example, ruthenium (which has the largest number of unpaired d-electrons of the platinum metals) interacts strongly with electron-donating species and consequ-ently shows the largest coverage with adsorbed oxygen.^ The development of effective "electrocatalysts" is in actuality a study of the activity of these materials, and the results can consequently be applied to other work concerning the effects of electrode pretreatment on electrode act-9 i v i t y . Woods has made a distinction between " f a c i l e " and "demanding" reactions, where a f a c i l e reaction has an activity v i r t u a l l y independent of the mode of preparation, and a demanding reaction is much more sensi-tive 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 catalysis assume a d i s t r i -bution of "active centers" over the electrode surface. These may involve 8 individual atoms, atomic groups, crystal planes or angles, or edges and l a t t i c e defects, for example, and may or may not be randomly located. It may be expected that the production and distribution of such sites is a result of electrode pretreatment. It i s necessary to establish some form of measurement in order to compare the a c t i v i -12 ties of differently prepared electrodes. Hammett recommended the measurement of the hydrogen (evolution or ionization) current at a 13 given overpotential. Rius noted the current density at which the potential rose sharply (that i s , the electrode became passivated) 2 14 in chloride electrolysis. Warner and Schuldiner ' measured the decay time of a monolayer of adsorbed oxygen in hydrogen-saturated solution. Biegler"''^ used the shape of the hydrogen "peaks" in cyclic voltammetry as an indication of activity. Unfortunately, such methods show only that relative differences in activity do occur, and these must be interpreted in conjunction with theories for the distribution of active centers, electronic properties, and in terms of the reaction of interest. 9 2.1.2 Methods of Pretreatment and Activation 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. Activation 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 in electrode pretreatment ( i t is the history 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 for foreign matter to be incorporated into the electrode surface. Of course, different mechanically treated forms can be obtained as a result of the process of manufacture (rolling, drawing, etc.). Electrochemical pretreatments involve anodization, cathod-ization, or a combination of both, by application of direct or alter-nating currents, potentiostatic or potentiodynamic polarizations, or by pulses of constant current or potential. Such procedures are often performed with the electrode in situ in the experimental system. Thermal pretreatments must necessarily include any effects due to the manufacture of the electrode, as well as any subsequent treatments. These generally involve holding the electrode i n a certain atmosphere at a given temperature for a desired period of time. 10 Chemical pretreatments can range i n s e v e r i t y from mere cleaning of the electrode to surface etching or oxidation. Such pro-cedures are performed outside the experimental system. A comprehensive survey of pretreatment and a c t i v a t i o n procedures, including t h e i r e f f e c t s on surface structure, metal d i s -s o l u t i o n , surface area, and on the subsequent experimental r e s u l t s has made by the a u t h o r . ^ 11 2.1.3 Mechanisms of Activation df Noble Metal Electrodes 2.1.3.1 Surface Area Increases It is evident that in many cases the reported increases in electrode activity as a result of certain pretreatments can be ex-plained by an increase in the surface area of the electrode as a result of these operations. That i s , the "specific" activity (activity per true unit of area) may not be altered at a l l . 18 Will 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 in surface roughness. 19 Sawyer and Seo described a procedure for producing an activated platinum surface (pf limited l i f e ) in which the electrode was plated b r i e f l y at low current density in a chloroplatinate solution. It is 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 vi s i b l e darkening of a platinum electrode surface, with a significant increase in surface area. As he observed only insignificant variations in area as a result of anodic/cathodic treatment which did not extend 20 21 22 23 into the hydrogen region, he suggested ' ' ' 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 is able to diffuse into the body of the metal during the cathodic half-cycle) and removal of hydrogen, with consequent 12 changes in l a t t i c e parameters, may break up the metal surface. He further suggested that, in the presence of impurities, fewer hydrogen atoms would combine on the surface and more penetrate the skin, result-ing in a more quickly broken-up surface. In support for Hoare's mech-anism, i t was found that only those noble metals which were capable of dissolving appreciable amounts of hydrogen (platinum and palladium) could be transformed into metal-black electrodes with alternating-current polarization 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 for his observed roughening effects. 1) Rapid reduction may not allow sufficient time for the surface to "anneal". 2) Slight solution of the "oxide" may occur and this, or subsequ-ent 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 inhibited the roughening process. This inhibition was attributed to the replace-ment of adsorbed oxygen with adsorbed hydrogen, so that at no time is 13 the metal surface permitted to r i s e to the energy l e v e l i t has i n the absence of chemisorbed species. 25 Volodin and Tyurin a t t r i b u t e d the break-up of a platinum electrode surface during c y c l i c oxidation and reduction to metal d i s -s o l u t i o n during oxidation, and also to the i n s e r t i o n of hydrogen and oxygen atoms in t o defects which appeared i n the c r y s t a l structure as a r e s u l t of the d i s s o l u t i o n . They did not be l i e v e that a change i n roughness f a c t o r a l t e r e d the adsorption capacity per un i t surface with respect to oxygen and hydrogen. 26 Kronenberg did not subscribe to the surface area increase theory of a c t i v a t i o n , and c i t e d experimental evidence i n which an a c t i -vated electrode was produced by treatment i n a hydrogen flame, where the electrode surface would be expected to become annealed (smoothed). 27 Shibata, commenting on the preceding remark, states that an unstable (disrupted or otherwise) surface layer would have been produced due to the rapid cooling of the electrode a f t e r such treatment. 28 Shibata found that successive formation and reduction of non-multilayer oxygen coverages did not a l t e r the true surface area of the electrode, but that when multilayer formation occurred, the area increased. I t must be noted, f u r t h e r , that the method used i n deter-mining the surface area may not n e c e s s a r i l y determine the act u a l area of the electrode which i s ava i l a b l e f o r the electrochemical 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" rather than 14 " f a c i l e " reactions are considered, as the former one expected to be more s p e c i f i c as to s i t e s , that i s , more s e n s i t i v e to the uniformity of the surface. For example, Bagotzky et a l ^ found that an apparent decrease i n a c t i v i t y for the oxidation of methanol could be explained by the i n a c c e s s i b i l i t y of microcavities on the electrode surface to the large methanol molecule. However, hydrogen atoms, used i n t h e i r method of surface area determination, were capable of adsorption i n these micro-c a v i t i e s and consequently the resultant s p e c i f i c a c t i v i t y appeared low. 2.1.3.2 Active Surface Structure Although increases i n electrode surface area have been pro-duced by means of several pretreatment procedures, many researchers b e l i e v e that an activated electrode does not n e c e s s a r i l y have to possess 18 a greater surface area than a l e s s - a c t i v e one. W i l l and Knorr, 29 30 Shibata, 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 surface layer, which has active s i t e s of enhanced a c t i v i t y rather than j u s t more s i t e s per unit area. I t i s also quite probable that the reported " a c t i v a t i o n " of electrodes as a r e s u l t of c e r t a i n pretreatments may be due e n t i r e l y to increase i n the roughness factors of the electrodes. B i e g l e r " ^ noted the close connection between the roughening of electrodes and the anodic/cathodic a c t i v a t i o n proced-ures commonly used by many other workers. He i s able, however, to c l e a r -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 "roughened" electrode, although both involve surface structure a l t e r a t i o n . Biegler found that roughened 15 electrodes did not necessarily exhibit high activity (as indicated by a high hydrogen peak height i n cyclic voltammograms), and conversely, that activated electrodes could be produced by means of "non-roughening" 29 anodic/cathodic treatments. Shibata postulates that an active plat-inum electrode is one which possesses an unstable surface layer whose atoms have been rearranged by oxygen atoms, giving rise to strain. Repeated formation and reduction of the oxygen layer would cause s u f f i -cient accumulation of this strain to produce roughening. B i e g l e r ^ believes that a freshly-manufactured 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 either atomic rearrangement or complete separation of platinum-oxygen species from the surface. A (non-roughening) activation procedure w i l l result in 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 is generally accepted that the surface of a noble metal electrode is energetically heterogeneous, exhibiting centers of varying degrees of electrochemical activity. Indeed, different crystal planes may exhibit different activities with respect to certain electrochemical 3 reactions. Damjanovic (cf. Bockris ) said that different crystal planes may be the source of differing reactivities as a result of their having 3 unequal work functions. Schuldiner et a l derived significantly different 16 atom d e n s i t i e s f o r p e r f e c t l y smooth platinum c r y s t a l f a c e s , u s i n g 3.9231 angstroms as the l a t t i c e parameter. B i e g l e r ^ says that platinum has a l a r g e spread of 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 that some pre-treatments may favour the production of c e r t a i n c r y s t a l f a c e s . Anodic/ cathodic treatment may a l t e r the surface s t r u c t u r e of an e l e c t r o d e by means of p r e f e r e n t i a l a t t a c k of 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 angular p i t s of i n c r e a s i n g s i z e s on el e c t r o d e s which has been subjected to 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 the appearance or disappearance of c e r t a i n c r y s t a l planes as a cause f o r changes i n ele c t r o d e a c t i v i t y has not been found to be adequate to e x p l a i n a l l of the observed phenomena. In g e n e r a l , the d i f f e r e n c e s i n the e l e c t r o c h e m i c a l behaviour of platinum s i n g l e - c r y s t a l e l e c t r o d e s of va r i o u s o r i e n t a t i o n s has been found to be sm a l l or non-existent and do not conform to l o g i c a l sequences such as atom den s i t y or spacing, 32 10 although others such as Pyshnograeva and Bagotzky b e l i e v e c r y s t a l l o -graphic o r i e n t a t i o n to be the 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 surface platinum atoms may e x i s t i n s t r u c t u r e s which may not n e c e s s a r i l y correspond to those derived from the bulk u n i t c e l l . Others, l i k e Damjanovic and 34 B r u s i c , do not l i k e the theory of the appearance of d e f i n i t e c r y s t a l planes as they are unable to f i n d a reason as to why the mechanism of the r e a c t i o n they considered (oxygen reduction) should be d i f f e r e n t on d i f f e r e n t planes. 35 Appleby has s a i d that a platinum atom at a defect has 17 extra energy available for bonding, which permits an increase i n bond strength for the reaction intermediates at such sites. 31 36 Schuldiner et a l ' reported that the presence of grain-boundaries or surface layer stresses i n polycrystalline bead electrodes, or even the presence of impurities in the bead, greatly enhances the process of passivation over pure platinum single crystal electrodes. These factors appear to be of much greater importance than atomic geo-metry in determining the catalytic activity. Bagotzky et a l observed no effect of mechanical treatment on the catalytic activity of his electrodes other than that due to an increase i n the true electrode area. That i s , they found most crystal defects do not act as active centers governing the properties of the platinum surface. 27 Shibata and Sumino found that annealed electrodes (reduced surface strain, fewer defects) showed the same high i n i t i a l activity as freshly prepared electrodeposited electrodes (highly strained surface, many defects). They consider that the short exposure to the air exper-ienced by the electrodes during transfer from the furnace to their c e l l was sufficient to re-activate the electrode. 18 2.1.3.3 Active Oxidized Surface An oxygen-covered electrode is generally described as passivated, where such processes as metal dissolution are inhibited. Furthermore, the potentials required for a given process (for example, chlorine evolution) may be higher on a passive surface than on the bare metal. The presence of oxygen on the electrode surface may, however, f a c i l i t a t e or even be necessary for other processes. 37 F l i s and Bynyaeva found that the amount and bonding strength of oxygen produced (chemically) on a platinum electrode sur-face 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 different types of inter-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 characteristic catalytic properties due to i t s i n a b i l i t y to form a "tight" 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 orbitals. 22 23 Hoare ' said that the formation of an oxygen layer on a platinum electrode w i l l cause reproducibility of results, as the cataly-t i c surface on which the electrode reaction surface takes place would be stable. 19 39 Pospelova et a l say that a "certain type" of passivating film i s necessary i f the reactivity of certain adsorbed particles i s to be enhanced. They point out the different changes in adsorption properties of certain noble metal oxygen films caused by similar anodic treatments as evidence for their hypothesis. 40 41 Hoare and Rand say that the reversible oxygen potential is 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 dissolution from the bare metal). 42 Afon'shin et a l suggests that different pretreatments produce different forms of surface oxide which can account for the different a c t i v i t i e s (for the oxygen evolution reaction) produced by the pretreatments. At potentials slightly above 1.5 V (R.H.E.) the oxygen film is not continuous but occupies only the active portions of the surface. On increasing the potential, these isolated regions inter-lock, forming a film which i s passive with respect to oxygen evolution. At about 1.9 V the monolayer oxide contracts as a result of reorienta-tion or recrystallization, liberating part of the surface. Subsequent f i l l i n g of these regions forms a "dense monolayer" which is 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, resulting in a stronger and more heterogeneous layer. 44 Davis explained the improved catalytic behaviour of 20 freshly anodized platinum by means of f a c i l i t a t e d electron transfer from the oxidized surface via an oxygen "bridging" mechanism. This theory relies on the existence of platinum oxide at the grain boundar-45 46 ies and other active sites. Other researchers ' favoured this theory on the grounds that pre-anodization seemed necessary to activate an 47 electrode. James considered the oxide bridge theory to be untenable because he could produce long-lived activation of the hydrogen evolution 22 23 reaction, where no surface oxide exists. Hoare ' rejects the oxide bridge theory for the reason that, in a redox system, the reducing agent would be expected to remove oxygen bridges (the electrochemical oxida-tion of a reducing agent in solution has been shown to occur mainly through oxidation by the adsorbed oxygen) and hence alter the oxidant/ reductant ratio of the redox system. That i s , an indicator electrode would not be expected to function. 2.1.3.4 Active Reduced Surface Many activation procedures involve the reduction of a pre-viously oxidized electrode, resulting in an "active reduced" surface. It is clear that a non-passivated electrode w i l l show a superior activ-i t y for processes such as metal dissolution, and for such cases any treatment which removes superficial oxygen w i l l appear to activate the electrode. In addition, however, several researchers have proposed that some kind of distinct active structure is produced by the reduction 21 of a previously oxidized surface. 48 49 19 Anson ' and Sawyer suggested that a platinum electrode which has been oxidized and subsequently reduced i s covered by a speci-a l l y active thin platinized layer. This active layer is derived from 30 the thin oxide film generated by pre-anodization. French and Kuwana stated that the formation of the active 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 activation, where a redistribu-tion of surface platinum atoms occurs due to periodic formation and breaking of platinum-oxygen bonds. A cyclic program of oxidation and reduction or heat-treatment in an oxidizing flame w i l l result in re-moval of platinum atoms in high-energy sites (by atom rearrangement or separation from the surface). Appleby"*"*" observed that the activity of his electrodes increased i f the maximum potential obtained in his anodic/cathodic pre-treatment was above 1.4 V (R.H.E.). He attributed this to an increase in the surface energy due to an increase in surface defect density when the phase oxide monolayer formed at these potentials is 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 activation to the production of a "half-reduced" state, which corresponded to an adsorbed Pt(OH) x layer. The formation of this state was said to involve a two-stage mechanism involving: 1) a slow one-electron transfer step [Pt(OH) x is d i f f i c u l t to reduce] Pt + x H20 •> Pt(OH) x + x H + + e 2) a fast one-electron transfer step lPt(0) x i s easily reduced] Pt(OH) ->- Pt(0) + x H + + e x x Rapid cycling of the electrode would produce a surface covered only with Pt(OH)x> Consequently, the ratio of the charges consumed in the anodic and cathodic processes would change from 2 to 1. (The charge imbalance of oxygen monolayer oxidation and reduction has been reported v . . . . 53,54,55,56. _ 22,23 , _ , .^ u . by many investigators. ) Hoare has stated that he finds i t d i f f i c u l t to accept that the removal of adsorbed hydroxyl radicals should be a slow process during reduction of the electrode. Adams^ noted that there is not any specific 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 is discussed i n the following 35 section. Appleby does not accept Peldberg's theory of activation as 23 he finds that an active surface produced by hydrogen reduction at 500°C (where oxidation of the surface i s very 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 to those produced by anodic/cathodic c y c l i n g . He furthermore r e j e c t s Feldberg's theory on the grounds that he finds i t d i f f i c u l t to b e l i e v e that an oxide f i l m can be exactly reduced to a Pt(OH) x f i l m of the same thickness, and because the observed adsorption i s not Langmuirian as i s 47 expected f o r species whose coverage i s close to unity. James consid-ered Feldberg's theory untenable as he observed a rapid decay i n a c t i v -i t y at 0.8 V (N.H.E.), where the "half-reduced" state i s supposedly stable. Further, the f a c t that the hydrogen evolution r e a c t i o n can be activated f o r long times makes i t u n l i k e l y that the existence of the half-reduced state i s responsible f o r the a c t i v a t i o n . _ 27,28,29,58,59 , „ . ^. . „. ,, Shibata favours the p l a t i m z a t i o n concept, where the reduction of an oxidized platinum surface r e s u l t s i n the forma-t i o n of a very t h i n layer of unstable atoms, s i m i l a r to electrodeposited platinum, and having a high a c t i v i t y and low r e c r y s t a l l i z a t i o n rate. He found that the l i f e t i m e of an active surface depended on the pre-oxidation time, and that stable surfaces of high a c t i v i t y were produced by long pre-anodizations (which produced thick surface oxide which modi-f i e d the electrode surface). He says that oxygen adsorbed on the e l e c t -rode surface causes rearrangement of surface platinum atoms and that, a f t e r oxygen removal, the platinum atoms are l e f t i n unstable p o s i t i o n s . His concept i s based on the assumption that m u l t i l a y e r oxide i s produced on platinum under c e r t a i n conditions. I t i s i n t e r e s t i n g to note that 24 James^ failed to produce an active electrode after severe anodization and that he rejects activation mechanisms involving the formation of 28 oxygen coverages above a monolayer as a necessary step. Shibata countered that a too-severe anodization w i l l drive the electrode into a "passive" state where the multilayer oxide is not formed. That i s , i f a protective passive film was 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 well pre-annealed electrode. He assumes electrode oxidizability to be proportional to the coverage of disordered platinum atoms, and that oxidizability decays as the disordered atoms reassume a crystalline structure. Measurement of the oxidizabiltiy of electrodes annealed at different temperatures gave an activation 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 migra-tion and ad-atom migration can be postulated as mechanisms for the s e l f -diffusion. As mentioned before, he attributes electrode roughening to the accumulation of residual strain created by the disarranged atoms. 2.1.3.5 Activity Induced by Dermasorption of Oxygen The "dermasorption" of oxygen refers to the incorporation of oxygen in 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 for the formation and reduction of the oxygen monolayer,^^'^^ 25 and to account for the thickening of the oxygen layer above monolayer proportions. Other investigators reject this notion, saying that the oxygen:platinum ratio could be increased by allowing the surface plat-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-tion of oxygen into the bulk of platinum metal is much more pronounced on degassed platinum than on cathodically reduced platinum, indicating that absorbed oxygen persists at cathodic potentials and may be cap-able of diffusing to the surface and affecting the activity. Chemodanov' believes dermasorbed oxygen to be responsible for activation of the hydrogen evolution reaction. Moruet and P e t r i i ^ stated that gas dis-solved i n the metal caused distortions in 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 catalytic activity, 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 lattice 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 for oxygen to diffuse out through the outer one or two plat-inum layers which have been reduced. These surface platinum layers, after oxide reduction, are thus above a disordered and highly strained platinum-oxygen "alloy" l a t t i c e , with the consequence that the surface 26 is 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 effect interactions, permitting a more Langmuirian adsorp-tion isotherm to operate than on an annealed surface. 21,22,23,40,67 . . fc _ . , t . Jt_ , Hoare attributes the enhanced activity of pre-anodized platinum towards various redox systems to the presence of oxy-gen dissolved in the surface layers of the metal. Anodic polarization of platinum above 1000 mV (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 alloy" electrode, and finds this "alloy" structure to be very resistant to heating to red heat in a hydrogen flame or to cathodic polarization. (Electrodes passivated in concent-rated n i t r i c acid show high rest potentials in 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 in the metal inter-ior. Indeed, once the metal interior i s loaded with oxygen (for example, by repeated heating followed by quenching in concentrated n i t r i c acid),, severe cathodization or melting of the la t t i c e by heating to white heat in a hydrogen flame is necessary to return the electrode to the untreated condition. He contends that the "alloy" alters the electronic structure of platinum with a resulting increase in the number of holes in the d-band, and that dermasorbed oxygen effectively lowers the electronic work 27 function of the surface, thus f a c i l i t a t i n g electron transfer. He con-siders that an electrode covered with an adsorbed layer of oxygen i n -hibits electron transfer due to an effective increase in the work func-tion caused by the negative dipoles of the metal-oxygen bonds. 2.1.3.6 Altered Electronic Properties Research has been performed with regard to the relationship between the electronic properties and the electrochemical behaviour of different metal electrodes. When differences in the catalytic activity of a single material such as platinum are considered, however, there i s l i t t l e relevant literature. This is due to the lack of precise informa-tion concerning the electronic properties of this metal and i t s alloys, due in turn to the incomplete knowledge of the state of the metal sur-face. Rao et a l ^ compared the adsorption of oxygen on noble metal electrodes as a function of the number of unpaired d-electrons per atom, and concluded that the unpaired d-electrons participated directly in 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 in corners and cracks would have unshared d-electrons which would thus be available for bonding orbitals. Bockris and Wroblowa^ altered the activity of noble metal electrodes by 40 means of alloying i n order to change the electronic properties. Hoare 28 considers that the presence of dissolved oxygen in the surface layers of a noble metal electrode w i l l modify the electronic structure of the metal much in the same way as alloying with another noble metal. The dissolution of oxygen into the metal l a t t i c e gives rise to a platinum-oxygen "alloy" with an increased number of holes in the d-band. 72 Srinivasan et a l feels that the influence of the number of holes in the d-band is most significant in cases where chemisorbed oxygen is an intermediate in the reaction, as the metal-oxygen bond strength varies with the percent d-vacancy. For example, in oxygen reduction, gold (which has few unpaired d-electrons) shows a significantly lower exchange current than the platinum metals. Other researchers have compared electrocatalytic properties with the work function of the electrode material. The electronic work function corresponds to the energy required to remove an electron from the metal surface, and is a significant determining factor i n activation 8 overvoltage. Bockris et a l obtained an approximately linear relation-ship between the surface energy and the logarithm of the ratio 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 activation of pure metals by allowing the work function at. or near defects to be lower. An expon-ential dependence of the number of defects formed (as a result of a given pretreatment procedure) on surface energy would give the observed 73 linear relation. Bockris et a l also stated that species absorbed in 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 activation energy necessary for reactions such as oxygen 74 75 reduction. The induced heterogeneity model of chemisorption ' is associated with the effect of the adsorbed dipole double-layer on the work function of the metal. For this model, the adsorption sites may be equivalent, but heterogeneity is "induced" due to a progressive change in the work function (which influences the energy of chemisorp-tion) with increasing adsorption. Recent work by Bagotzky et a l " ^ revealed that neither v a l -ency unsaturated surface atoms nor crystal defects acted as active cen-ters for the adsorptive and catalytic properties of the platinum surface. 2.1.3.7 Impurity Removal While many investigators attribute electrode activation to the creation of some distinct surface structures, others feel that the removal of impurities from the electrode surface is more l i k e l y to be the mechanism of activation. Impurities on the electrode may be oxidi-zed, reduced, desorbed, or removed by mechanical means. It has been well established that the presence of even trace amounts of impurities can seriously distort electrochemical measurements. Gilman^ notes that the working potential of the electrode may determine the degree of effect of adsorbed impurities, as they may 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 be forced o f f due to competitive adsorption with species such as oxygen. B r e i t e r ^ has found that the oxygen layer formed by chromic acid pretreatment acted to protect the surface against hydrocarbon and other impurity adsorption. 78 Khazova et a l s a i d that a c t i v a t i o n may be due to the de-sorption of adsorbed fo r e i g n p a r t i c l e s . They suggested that, i n the p o t e n t i a l range 0.3 to 0.9 V (R.H.E.), a platinum electrode i s p a r t i a l l y poisoned by anions and neutral molecules. Anodizing to, say, 1.1 V w i l l adsorb oxygen and displace adsorbed organics. Reduction of the adsorbed oxygen w i l l thus free the surface f o r the electrochemical r e -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 to the removal of trace metal impurities from the electrode surface. He noted that only those pretreatments which were capable of o x i d i z i n g and removing deposited metals produced a c t i v a t i o n . Kronenberg further accepts the p o s s i b i l i t y of the formation of some d i s t i n c t type of surface structure as the r e s u l t of pretreatments, but considers such e f f e c t s only second-ary to that of the presence of m e t a l l i c impurities. B r i e t e r ^ has stated that the reason for the charge imbalance between oxygen monolayer formation and reduction i s that the anodic charge includes an organic impurity oxidation c o n t r i b u t i o n . With contin-ued anodic/cathodic c y c l i n g , the anodic/cathodic charge r a t i o approaches unity as the impurities are removed. 79 Damjanovic et a l found that p u r i f i c a t i o n of the working electrolyte (with or without the experimental working electrode) affected the activity of the working electrode. This i s evidence that the impurity effect is at least a real one. 80 Gilman attributed the enhancement of anodic currents (for organic oxidation reactions) after reduction of a passivated platinum surface to either a smaller surface concentration of inactive species, or to a more favourable coverage with active intermediates. 2 Warner et a l said that improvements in the activity of an electrode can only occur in cases where sorbed impurities are present, as they found that conventional anodization methods to enhance activity had no effect i f the electrodes were kept in a clean system. 47 James produced activation by means of mild anodizations (surface rearrangement unlikely) and through solution purification. Further, he found anodic/cathodic treatment produced (temporary) activa-tion in impure systems only. 81 P e t r i i found that anodic polarization was not necessary to produce an active electrode, in which case the impurity-desorption mechanism appears operative. 82 Lu et a l consider that the reactivation of anodes by means of a superimposed activating pulse is due to the current-dependent re-moval rate of impurities. 83 Shibata found the activity of an activated electrode de-cayed i f i t was stored in hydrogen- or nitrogen-stirred solutions, but was maintained i f stored in an air-saturated solution. He says i t i s 32 d i f f i c u l t to accept an impurity deactivation mechanism, and that the acti v e state i s influenced by the adsorption of oxygen, which maintains the metal atoms i n unstable (active) p o s i t i o n s . 2.1.4 Summary The establishment of the " a c t i v e " electrode condition s h a l l be taken to involve the production of a reproducible surface condition which produces a high degree of experimental 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 concerning the enhancement of r e v e r s i b l e behaviour as a r e s u l t of pretreatment, as i t i s believed that i t i s not possible to adequately separate those procedures which promote " a c t i v a t i o n " from other procedures. I t i s considered that the production of the reproduc-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 compari-sons among i n d i v i d u a l experiments. I t i s us e f u l , however, to take account of the possible e f f e c t s of the various pretreatments and the experiments themselves, on the electrodes. These are outlined i n Appendix I for relevant procedures and the electrodes of i n t e r e s t . 33 2.2 SURFACE AREA OF NOBLE METAL ELECTRODES It i s not r e a l i s t i c to presume that the geometric area of an electrode i s equivalent to i t s true surface area. Even highly p o l -ished surfaces contain i r r e g u l a r i t i e s , such as scratches, which w i l l contribute to an increase i n the area. Generally, a "roughness f a c t o r " i s proposed i n order to convert the apparent surface area to a more probable value. Experience has shown, however, that the true surface area of an electrode may not have a constant value. Furthermore, s i m i l a r electrodes (for example, platinum wires) may have greatly varying values of surface area, depending on t h e i r pretreatment h i s t o r i e s . There are, however, several electrochemical procedures a v a i l a b l e which can give accurate, reproducible surface areas. 2.2.1 Bases for Electrochemical Surface Area Measurement Ear l y research into charging phenomena on platinum electrodes determined that, when an anodic pulse was applied to an electrode at cathodic p o t e n t i a l s , the r e s u l t i n g charge curve showed separate regions 83 84 85 for i o n i z a t i o n of adsorbed hydrogen and adsorption of oxygen. ' ' Later, p o t e n t i a l sweeping techniques were developed which also revealed 18 86 separate oxygen and hydrogen regions on the c u r r e n t / p o t e n t i a l curves. ' If i t was assumed that the sole process occurring i n the appropriate regions of the charge or sweep curves was one of oxygen adsorption (or removal) or hydrogen i o n i z a t i o n (or deposition), and that such processes 34 involved the formation (or removal) of complete monolayers of these species (with a one-to-one correspondence with surface metal atoms), then knowledge of the number of platinum atoms per square centimeter (calculated from c r y s t a l l o g r a p h i c data), the charge of an e l e c t r o n , and the valence of the adsorbing species i s a l l that i s necessary to a r r i v e at a value of the true surface area. Measured values of the ac t u a l charge consumed i n the oxygen or hydrogen processes can thus be r e a d i l y converted into surface areas. However, there are several problems encountered i n such determinations which render the measurements much more d i f f i c u l t . 2.2.1.1 Complications Due to Simultaneous Processes Unfortunately, i t i s not possible to separate other charge-consuming processes from the r e a c t i o n of i n t e r e s t . For example, the presence of an unknown oxidizable impurity may cause the quantity of charge consumed i n the anodic deposition of an oxygen monolayer to be increased. Schaldiner and Warner^ and Thacker and H o a r e ^ studied the e f f e c t of the presence of impurities on the shape of the oxygen 24 region of the charge curve. Gilman found that s o l u t i o n anion adsorp-t i o n i n phosphoric acid e l e c t r o l y t e p a r t i a l l y blocked surface oxidation. 88 — Gilman also found that adsorbed impurity anions (such as C l ) d i d not s i g n i f i c a n t l y a f f e c t the hydrogen adsorption capacity of a platinum 24 88 electrode. Gilman ' has developed s u i t a b l e pretreatment procedures to remove impurities from the electrode surface. In addition, he has 35 found that the use of fast techniques (high current density charging, high sweep rates) could help in avoiding contaminant effects. He has calculated that the times for contamination of a previously clean elec-trode surface (1% coverage with impurities), assuming diffusion control, are 50 seconds and 2 seconds in unstirred and stirred solutions respect-62 ively. Gilroy and Conway also recommend surface area estimation be attempted as soon as possible after cleaning the electrode surface. The possibility also exists that the processes of hydrogen ionization and oxygen adsorption, oxygen adsorption and evolution, oxygen reduction and hydrogen deposition, hydrogen deposition and hydrogen evolution, and hydrogen evolution and ionization may occur simultaneously. The good separation of the hydrogen and oxygen regions on the anodic charge curves in acid solution is good evidence that these processes are indeed separate. On cathodic reduction curves, however, the oxygen layer on platinum is reduced at potentials much closer to the hydrogen deposition potentials. Good separation is s t i l l found for 89 this case in acid solution. Giner found that he could detect hydrogen evolution (on a PTFE-bonded platinum electrode) at up to 200 mV above 33 the reversible hydrogen potential. Biegler et a l found hydrogen evolution above 0.10 V (R.H.E.) to be insignificant on both smooth and platinized electrodes. The hydrogen deposition and evolution regions overlap, however, which can lead to some d i f f i c u l t y in separating the 88 33 respective charges. ' An analogous situation can also be found with 88 oxygen adsorption and evolution. Depending on the measurement scheme 36 employed, i t i s necessary to make allowances for - or to eliminate by s u i t a b l e pretreatments - any complicating simultaneous electrochemical reactions. 2.2.1.2 Compensation f o r Double Layer Charging The double layer i s charged when a current i s passed across the solution/electrode boundary. I f no e l e c t r o n transfer occurs, then 83 a l l the current i s used i n t h i s charging. The development of the 84 double layer occurs at a l l p o t e n t i a l s , but i s most obvious on charge or sweep curves between the hydrogen and oxygen regions. Some workers have attempted to use measurements of the capacity of the double layer as i n d i c a t o r s of true electrode surface a r e a . ^ ' ^ ' ^ ' ^ However, due to the great d i f f i c u l t y i n separating the double layer charge from other charge-consuming processes, the r e s u l t s are subject to great uncertainty. 84 93 Slygin and Frumkin and Schuldiner and Roe noted the presence of hydrogen on the electrode i n the so-called "double-layer" region of the charge curve. The l a t t e r authors suggest that only about a t h i r d of the current i n t h i s region a c t u a l l y goes to charging of the double l a y e r . 94 Formaro and T r a s a t t i also f i n d that adsorbed oxygen i n t e r f e r e s with capacitance measurements at p o t e n t i a l s as low as 500 mV (R.H.E.). Rosen 95 96 and Schuldiner ' found that r e l i a b l e double layer charge values could be obtained only with charging pulses of extremely short (<200 nsec.) duration. This work can be assumed to supercede s i m i l a r investigations 93 with pulses of long (5 ysec.) duration, although the e a r l i e r work may contain valuable information concerning the charge consumed in 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 ionization processes involve the reverse), and that points on the curves can be identified with these states, making calculation of the charge consumption an easy task. The area determination techniques based on hydrogen or oxygen processes rely 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 in oxygen-saturated IN l^SO^ solution was far from complete. For platinum and iridium, the coverages were 27% and 84%, respectively. Thus, the use of measurement techniques which involve oxygen deposition at open-circ 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. While chemi-sorption stoichiometry may suggest this relation, i t has yet to be 88 proved conclusively. Perhaps the best evidence is 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 variation of the oxygen monolayer charge with sweep speed existed, indicating that possibly gas evolution occurred before mono-97 98 layer oxygen coverage was obtained. Schuldiner ' found that oxygen monolayer deposition was complete when the potential reached about 1.5 V (N.H.E.). 2.2.1.4 Absorption The possible absorption of oxygen into the interior of a 99 100 platinum electrode was f i r s t suggested by Kalish and Burshtein. ' Oxygen absorption has been proposed as an explanation for the difference between the charges measured for oxygen layer formation and reduction 22 23 68 (see Hoare ' ). Hoare showed that oxygen could diffuse 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 separated the two c e l l halves) and affect the hydrogen overpotential. If the bi-electrode configuration was changed such that the f o i l acted as a diaphragm, the effects of the oxygen dissolved in the metal persisted for long times afterward. Thacker and Hoare^ found that "dermasorbed" oxygen could be resolved as a high-overpotential reduction wave on a galvanostatic stripping curve. They also point out that, i f an elect-rode is so-stripped and then l e f t at open-circuit, the potential rapidly returns to a high anodic value. A second stripping curve 1 resulted in 39 the detection of both surface and dermasorbed oxygen. Only with r e -peated pulsing i n t h i s manner could a l l the dissolved oxygen be removed. I t i s c l e a r , however, that surface s i t e s can be f i l l e d from oxygen d i f f u s i o n from the i n t e r i o r of the metal. Schuldiner et a l " * " ^ says that as many as three or four monolayers of oxygen atoms can be derma-sorbed at high p o t e n t i a l s , and that dermasorbed oxygen appears at pot-e n t i a l s above 1.2 - 1.3 V (N.H.E.). The absorption of hydrogen has been 102 le s s extensively considered. Schuldiner and Warner discuss both hydrogen and oxygen absorption as possible causes f o r excess charge consumption i n the hydrogen and oxygen regions. Other workers suggest that, rather than absorption, the formation of oxides of d i f f e r i n g thicknesses and/or changes i n the formal valences can explain (or produce) deviations from the desired monolayer c h a r g e . ^ * ^ To date, no conclusive proof has been esta-bli s h e d concerning the existence of surface oxides of platinum under 103 the conditions experienced during area measurements. Bockris et a l notes that X-ray d i f f r a c t i o n does not detect patterns f o r oxides, even a f t e r high-potential anodizations. While t h i s could be due to the o f a c t that only small (<50 A) patches of oxide are formed, or that the oxide l a t t i c e i s so highly d i s t o r t e d as to become amorphous, the p o s s i -33 63 b i l i t y that chemisorption occurs i s more l i k e l y . B i e g l e r and Woods ' say that dermasorption e f f e c t s could also be produced by allowing the surface to be capable of adsorbing more than one oxygen atom per p l a t -inum atom. They also r e j e c t the p o s s i b i l i t y of phase oxide formation 40 as they find a limiting value of the oxygen charge with potential. 104 Vetter and Schultz oppose this assumption, saying that the surface layer is an oxide which grows continuously with time and potential. 2.2.1.5 Surface Atom Density In order to compare surface area values among different electrodes, one must assume that their surface crystalline states or combinations of crystal planes are the same. Indeed, i t is necessary to know the surface crystalline state in order to determine the true electrode area, as the packing densities of the individual atom planes are significantly different. The atom densities of the major planes of the f.c.c. l a t t i c e are: (100) 1.30 x 10 1 5 Pt atoms/cm.2 3 8 or 1.31 x 10 1 5 Pt atoms/cm.2 3 1 (110) 0.93 x 10 1 5 3 8 0.92 x 10 1 5 3 1 . „ i n15 38 . _ lfVL5 31,101 (111) 1.51 x 10 1.5 x 10 in. i i * i m!5 31,70,84,93,101 . For polycrystalline platinum, values of 1.31 x 10 and 15 85 105 92 1.6 x 10 * have been used. Feltham and Spiro note that there i s some d i f f i c u l t y in determining the correct packing density. Biegler 33 et a l say that the definition of surface atoms for the (110) plane and for higher-index planes is not clear, depending on whether surface atoms are defined as those with coordination 7, or those with coordina-tion 11 as well as 7. 41 The charge required to form a complete monolayer of hydro-gen (or oxygen) can i n turn be computed from the value of the el e c t r o n -19 charge (1.6 x 10 c o u l . ) . I f i t i s assumed that one single-valence hydrogen atom (or one double-valence oxygen atom) adsorbs per s i t e , then the r e s u l t a n t charges are as represented i n Table 1. TABLE 1 Hydrogen and oxygen monolayer charges f o r platinum and i r i d i u m electrodes. [] denote calculated value. C r y s t a l Plane Hydrogen Monolayer Charge ycoul./cm.^ Oxygen Monolayer Charge ycoul./cm.^ Reference Pt (100) [209] 418 38 [210] 420 31 208 [416] 33 Pt (110) [149] 298 38 [147] 147 or 295* 294 [294 or 590]* 31 33 Pt (111) [242] 484 101,38,31 241 [482] 33 Average of the 3 planes P o l y c r y s t a l l i n e 199 or 248* 210 [398 or 496] [420] 33 24,33,18,86 platinum [250] 500 7,103 [210] 420 70,93,101 [256] 512 105 P o l y c r y s t a l l i n e 220 [440] 86 i r i d i u m [262] 525 7,103 The coverage of a l l o y electrodes can be expected to vary l i n e a r l y with , . ^103 atomic percent. * depending on d e f i n i t i o n of surface atoms. 42 63 Biegler and Woods have pointed out that there are no s t e r i c factors l i m i t i n g oxygen chemisorption. The average area occupied by a platinum °2 surface atom i s about 7.5 A , whereas the diameter of covalently bonded o oxygen i s no more than 1.5 A. L a s t l y , i t should be considered that sur-face atoms may reside i n structures which need not correspond to those 33 calculated from the bulk u n i t c e l l . 2.2.2 Procedures f o r Surface Area Measurement 2.2.2.1 P o t e n t i a l Sweep Techniques By varying the p o t e n t i a l of an electrode i n a l i n e a r manner with time, a resultant trace of current vs. p o t e n t i a l i s obtained. The area under t h i s curve i s equivalent to the charge consumed during t h i s trace. As the hydrogen and oxygen regions are well-separated 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 can be elucidated. G i l m a n 8 8 and T h a c k e r ^ ^ ' h a v e considered the estimation of the oxygen charge. Unfortunately, with the sweep technique the endpoint of oxygen adsorption i s obscured by the commencement of oxygen gas evolution. Hence, extrapolations must be considered f o r the monolayer formation and gas evolution currents. Most p o t e n t i a l sweep techniques f o r surface area measurement e x p l o i t the hydrogen deposition region of the sweep 24,33,41,76,86,88,106 T t. . . . . «. -curve. » » » » » » i n t h i s case, however, the endpoint of hydrogen deposition i s masked by the onset of molecular hydrogen evolu-t i o n as the sweep proceeds from anodic to cathodic p o t e n t i a l s . The 43 assumption of an a r b i t r a r y (fixed p o t e n t i a l ) endpoint i s not recoramend-33 ed as t h i s gives d i f f e r e n t monolayer charge values for smooth platinum as opposed to p l a t i n i z e d platinum (whose sweep curves vary somewhat). Several constructions have been applied to sweep curves i n order to 2 A determine some reproducible sort of endpoint or to determine the 88 33 extrapolated hydrogen deposition and evolution currents. ' Such constructions, along with the measurement of areas under the curves, 62 make th i s method somewhat tedious. 24 88 Gilman ' recommends that f a s t sweep rates be employed (for example, 50 v o l t s per second f o r hydrogen deposition; 1000 v o l t s per second f o r oxygen adsorption) i n order to reduce contaminant e f f e c t s . The higher sweep speed f o r the oxygen case i s necessary i f the hydrogen and oxygen monolayer charges (corrected f o r valence differences) are to coincide. An analogy with the galvanostatic charging curve case becomes apparent, where high current density pulses are employed i n order to prevent absorption of oxygen in t o the electrode. 24 Gilman has developed pretreatment procedures designed to remove surface impurities and to create a reproducible surface f o r area determination. A p o t e n t i a l step sequence i s employed whereby the e l e c t -rode i s exposed to the p o t e n t i a l s 0 V - 1.8 V - 1.55 V - 0.4 V (R.H.E.) p r i o r to sweep a p p l i c a t i o n (either anodic or cathodic sweeps) f o r d e f i n -i t e times. In t h i s manner, impurities are desorbed or oxidized, mole-cular oxygen i s swept away, and the surface i s reduced to a reproducible s t a t e . 44 2.2.2.2 Galvanostatic Charge Technique Application of an anodic galvanostatic pulse to an electrode whose potential is i n i t i a l l y at the hydrogen potential w i l l result in a potential vs. time trace showing four regions: hydrogen ionization, double layer charging, oxygen adsorption, and oxygen evolution. An example of this type of curve, with the details of i t s analysis, is found in Appendix II. Application of a cathodic galvanostatic pulse to an electrode at anodic potentials w i l l show similar regions for the re-verse processes. Measurement of hydrogen monolayer deposition or re-moval charges is not considered reliable due to problems in determining 31. 33 84 93 the end potentials of hydrogen ionization and deposition, ' ' * the oxidation of hydrogen dissolved in the metal or present on the sur-102 face, the possibility of molecular hydrogen evolution at potentials 89 92 anodic to the R.H.E., ' 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 "transition times" for oxygen monolayer formation or dissolution, i t i s then a simple matter to calculate the charge con-sumed, and hence arrive at an estimation of the active surface area. 2 14 101 102 Schuldiner, Warner et a l ' ' ' find that for high current density 2 anodic pulses (1-7 A/cm. ) only surface-adsorbed oxygen i s formed. Thacker and Hoare^ find smaller current pulses (4.1 mA) w i l l give satisfactory results provided the electrode i s pre-saturated with der-masorged 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 potential be more cathodic (or anodic) than the range of potentials of interest. The potential can be lowered to cathodic values by operating in hydrogen-saturated 70 93 solution ' or by potentiostatting the electrode to the desired pot-ential followed by fast (e.g., 1 ysec.) switching to the anodic current i 31,97,98 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 sufficient to cause a separa-tion of arrests (in 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 in order to carry the electrode to the desired i n i t i a l cathodic potential. 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 pro-blem with the galvanostatic charge technique. A fixed value of the capacitative charge can be assumed and subtracted from the overall 46 14 95 measured charge. Rosen and Schuldiner 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 potentials where such unknown processes are considered unlikely. The slope of the "double layer region" in the charge curve may be used to obtain a value of the 31 70 92 107 double layer charge. ' ' ' The appropriate constructions are des-cribed in Appendix II. It must be remembered, however, that this "double layer slope" contains contributions form hydrogen ionization, oxygen 93 adsorption, and unknown charge-consuming processes. Schuldiner and Roe found that the hydrogen ionization complication could be removed by pre-potentiostatting at potentials above 0.35 v (N.H.E.), resulting in a "double layer region" with a much steeper slope, indicating a smaller amount of charge being consumed in this region. 2.2.3 Summary It is clear that, regardless of the method chosen for deter-mination of the surface area, the value obtained does not actually re-present a true measure of the electrochemically active surface area. It i s nevertheless, quite meaningful, and i f the assumptions made in the computation of this value are correct, then the surface area measure-ment could correspond to that which actually exists. 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 in this re-gard with which the surface area values reported in 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 results of the various re-searchers who have used different assumptions and/or methods can only be valid when these are known. 48 2.3 ELECTROLYSIS OF CHLORIDE SOLUTIONS In the e l e c t r o l y s i s of chloride-containing s o l u t i o n s , the . A < 108,109,110,111,112 primary anode rea c t i o n i s : ' ' ' ' 2C1 t C l 2 + 2e The anodic formation of oxygen H„0 ± 2H* + ~ 0„ + 2e i s n e g l i g i b l e , except i n the case of very d i l u t e b r i n e s . However, the study of the anodic processes i n chloride e l e c t r o l y l e s i s complicated by the p o s s i b i l i t y of several secondary reactions i n the e l e c t r o l y t e . These include, i n neutral or acid elect-113 r o l y t e , chlorine hydrolysis which may occur i n the d i f f u s i o n l a y e r : C 1 2 + H 2 ° H C 1 ° + H + + C 1 Further, hypochlorite ions may be generated: HC10 t H + + CIO" Hypochlorite production i s favoured i n e l e c t r o l y t e s of n e u t r a l pH. The hypochlorous acid can be neut r a l i z e d by OH ions (produced at the cathode) H0C1 + OH t CIO + H 20 Hypochlorite discharge at the anode becomes possible 6C10" + 3H„0 t 2C10 o" + 4C1~ + 6H + # 0 o + 6e 49 Thus, a steady-state situation can be visualized where hypochlorite formed from the secondary solution reactions i s balanced by the amount discharged at the anode. In more-acid electrolytes, chlorate ion (ClO^ ) can be produced by means of the chemical decomposition of hypo-chlorite. The addition of H + to a neutral (CIO -containing electrolyte) results i n the formation of H0C1 which then oxidizes the remaining CIO to C103~: 2H0C1 + 0C1~ t c l ° 3 " + 2 H + + 2 C 1 ~ The H + so-produced leads to the production of further. H0C1, and a cycle is formed and repeated u n t i l a l l the CIO is exhausted. This reaction can be suppressed at low temperatures. The presence of ClO^ ions (either from hypochlorite discharge or decomposition) leads to a further possible anode reaction: CIO ~ + H.O t CIO ~ + 2H+ + 2e •3 2 4 . This reaction occurs, however, with considerable overpotential and can be ignored as long as the chloride content is significant. Thus, ClO^ can be considered to build up in concentration during the course of electrolysis. In hydrochloric acid solutions, the formation of the C l ^ -, , - - 114,115 ion must also be considered: Cl„ + C l " t Cl ~ 50 In addition, 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 i t s e l f e x i s t s , and i s i n f a c t , more l i k e l y to occur i n complexing e l e c t r o l y t e s such as those containing c h l o r i d e . I t i s c l e a r , thus, that care must be taken to e i t h e r pre-vent or account f o r the p o s s i b i l i t y of secondary chemical and e l e c t r o -chemical reactions i f meaningful r e s u l t s are to be obtained from stud-i e s of the p o l a r i z a t i o n of noble metal electrodes i n chloride media. 2.3.1 P o l a r i z a t i o n of Smooth Noble Metal Anodes The anodic 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 chloride media i s characterized by a " p o t e n t i a l jump" separating two regions of much 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 to higher current d e n s i t i e s with increasing chloride i on concentration a c i d i t y , and tem-perature. Since 1966, the behaviour of noble metal anodes i n such e l e c t r o l y t e s has been the subject of accelerated study. Although oxygen evolution i s the thermodynamically favoured reaction: 2H20 t 0 2 + 4H + + 4e _ TOO 2.3 RT „ 2.3 RT _ -E = 1.23 - • pH + — r | ; — l o § P°2 » the predominant re a c t i o n i s that of chlorine evolution: 2C1~ t C l 2 + 2e E = 1.358 + log P C 1 2 - i ^ J T ^ g ^ 51 Thermodynamics would also favour platinum dissolution i n chloride media, but the reactions are kinetically negligible (comparatively) due to the high activation energy. In chloride-free media (such as 1M I^SO^), the only anodic product at normally encountered potentials i s oxygen. The "potential jump" is not a characteristic of the polarization curve in such media. Van Laer^^ attributed the inflection in the current/potent-i a l relation to the transformation of Pt 0^ to Pt 0^, which was said to involve an increase in the overpotential for chloride oxidation. This conclusion was derived from the potential-pH diagram for platinum, where: Pt 0 + H20 = Pt 0 2 + 2H+ + 2e~ E Q = 1.045 - .059 pH Pt 0 2 + H20 = Pt 0 3 + 2H + + 2e~ E Q = 2.000 - .059 pH At the pH's found i n synthetic sea water (pH 6-7), the Pt O^/^t 0^ equilibrium potential i s thus E Q = 1.336 - 1.395, which corresponds nearly exactly to the potential of the inflection in the polarization curve. Suzuki et al''"'^ studied the polarization behaviour of plat-inum i n saturated chloride solution of pH 3. They found that the oxide film tended to dissolve when the electrode was kept standing i n the electrolyte after anodization. They explained this 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 oxide f i l m was cathodic to bare p l a t -inum or a lower oxide. This theory was confirmed when the anodized electrode was s h o r t - c i r c u i t e d with an unoxidized platinum electrode 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 with time. This led to the conclusion that the oxide f i l m formed by oxidation does not completely cover the platinum surface. They a t t r i b u t e d the two overpotential regions of the p o l a r i z a t i o n curve to the f a c t that the surface i s Pt 0 f o r the lower curve and Pt 0^ f o r the upper curve. 118 L i t t a u e r and Shrier pointed out that chloride and oxygen species undergo competitive adsorption, with oxygen eventually re p l a c -ing chloride on the electrode surface with increasing anodic p o t e n t i a l . The low-overpotential region of the p o l a r i z a t i o n curve was considered to involve C l discharge on a clean surface, or one having only a chemi-sorbed Pt-0 and/or P t - C l layer. At the p o t e n t i a l jump, the i n h i b i t i o n of oxide formation or oxygen adsorption was suggested to be overcome, with subsequent oxidation of platinum. The k i n e t i c s of the chlorine evolution reaction: C l , + C l " -> Cl„ + e ad 2 would thus be d i f f e r e n t on the oxygen-covered surface. A f t e r completion of the "jump", i t was suggested that the surface developed an oxide layer and/or consolidation of a previously chemisorbed Pt-0 or P t - C l layer. The e f f e c t of the oxide f i l m was assumed to involve blocking the active surface or complete coverage such that the free energy bar-r i e r s c o n t r o l l i n g electron transfer at the i n t e r f a c e were a l t e r e d . 53 119 Toshima and Okanlwa suggested that below 1.4 - 1.6 V (N.H.E.) only chloride i on oxidation occurred, whereas above t h i s other potential-determining processes occurred such as oxygen evolution or the production of oxy-chloride species. The p o t e n t i a l jump i t s e l f was determined to correspond to a l i m i t i n g d i f f u s i o n current. 120 Takahashi and Odashima a t t r i b u t e d the change i n chlorine overpotential to a t r a n s i t i o n from Pt 0 to Pt 0^ as the surface species. 121 B i t t l e s and L i t t a u e r investigated the p o l a r i z a t i o n be-haviour of platinum i n both NaCl and HCl e l e c t r o l y t e s . They considered that the p o t e n t i a l at which the " p o t e n t i a l jump" began was a pa s s i v a t i o n p o t e n t i a l . Below t h i s , chlorine evolution was assumed to occur on a f r e e l y d i s s o l v i n g platinum surface. The pass i v a t i o n p o t e n t i a l , Ep, was found to be independent of pH and chloride i on a c t i v i t y f o r the range of conditions studied (-2 Z pH 1 +6, 1 i a - < 90). They d i s -missed the p o s s i b i l i t y that oxygen evolution was the cause of the jump, as such evolution could only occur at 2 - 2.4 V (N.H.E.) - s i g n i f i c a n t l y higher p o t e n t i a l s than the p o t e n t i a l jump. The rea c t i o n Pt + H 20 -> Pt 0 + 2H + + 2e r - i /-> r> r> i 2.3 RT - 2.3 RT E = 0.99 + l o § aH 20 + — F ~ " p H was also ruled out, due to i t s pH-dependence. It was then considered that several types of surface s i t e s could e x i s t on the platinum surface before passivation, which did not incorporate oxygen species. These included Pt, P t 4 + , P t - C l ~ , P t - C l , P t C l 3 + , and PtCl*" 1". The surface would 54 aquire an oxygen coating above the passivation potential though a com-plex series of reactions involving these sites. 122 Helber determined the polarization curves on platinum in saturated NaCl electrolyte for temperatures from 25 to 100°C. From this he was able to plot log ip vs. 1/T, where ip was the current den-sity corresponding to the potential jump, and obtain an activation energy of 24.4 k cal/mole for the passivation process. Potentiostatic electrolysis at potentials below, within, and above the potential jump showed behaviour characteristic of non-passivating chemisorbed layers in the f i r s t case, and of chemical passivation of the last (only slight decay of current i n the former case, sharp decrease in the l a t t e r ) . Potentiostatic anodization at a potential within the jump did not show as great a rate of current decay as that at potentials above the jump, indicating that the conversion of adsorbed species on the electrode to a f a i r l y stable oxide and/or chloride layer occurred only at the top of jump. 123 12A Kokoulina et a l * used a potential sweep method to determine the oxygen coverage on a platinum anode in acid chloride media. They attributed the cathodic shift of the reduction peak with increasing anodic sweep-end potential to an increase of Pt-0 bond strength with potential. The fact that no new peaks were found in sweep curves in chloride media below the chlorine evolution potential, as opposed to IM H„SO,, indicated that the film consisted only of adsorbed oxygen and 2 4 was not, say, Pt-Cl„. At more positive potentials, the presence of 55 strongly adsorbed chloride ions or chlorine atoms i n the surface film was considered to be l i k e l y . The coverage/potential relations i n 1M E^SO^ containing various amounts of chloride (from 10 to IN KC1) a l l showed increasing coverage with potential. In chloride-free solu-tion, the amount of adsorbed oxygen corresponded to a monolayer at 1.5 V (N.H.E.) and to two monolayers at 2.0 V. The amount of oxygen was dim-inished with increasing chloride content. For IN chloride, the curve showed monolayer coverage only at about 2.0 V, and the coverage/potential curve showed a slight inflection at 1.6 V. In .IN and IN chloride, the surface was found to be free from adsorbed oxygen up to 1.1 - 1.15 V. Chloride inhibition of oxygen adsorption was proposed to account for this behaviour. Kokoulina et a l noted that the "potential jump" occurred at about 1.5 V (N.H.E.), independent of chloride ion concentration, but that the extent of oxygen coverage at this potential varied significantly with chloride ion concentration (for example, 6 = .25 for [Cl ] = r ' oxygen IN). They concluded that the bottom segment of the polarization curve corresponded to chlorine evolution on a free platinum surface, and that the upper one corresponded to the same reaction on a mixed surface film which was more than monomolecular. Passivation, then, was attributed to oxygen adsorption. The formation of anode products other than chlorine was suggested to occur at potentials above 2.2 V, on an electrode with a nearly complete bimolecular film. Chlorate formation is favoured in high chloride-containing electrolytes, where an appreciable quantity of 56 chlorine would be contained i n the f i l m . Perchlorate formation would be favoured i n more d i l u t e solutions where the f i l m i s mostly oxygen. Blanchi^""^ suggested that the p a s s i v a t i o n phenomenon on platinum could be due to the presence of hypochlorite, which e i t h e r d i f f u s e s to the electrode surface, d i r e c t l y forming oxychloride species, or discharges according to: 6C10" + 60H" -> 2C10 3" + 4C1~ + f ° 2 + 3 H 2 ° + 6 e [ r e v e r s i b l e p o t e n t i a l = 0.7 V (N.H.E.)] which r e s u l t s i n the evolution of oxygen, some of which may remain chemisorbed on the platinum surface. He finds the p a s s i v a t i o n to be stirring-dependent (not expected for OH -discharge) and pH-dependent (the passivation i s delayed at lower pH's, CIO production i s reduced at lower pH's). Although his work was with titanium-substrate e l e c t -rodes, the proposed mechanism i s not i n a p p l i c a b l e f o r the case of smooth electrodes. Kuhn and Wright^^'"^"' found the potential/coverage r e l a t i o n on platinum anodes i n NaCl s o l u t i o n to have three c h a r a c t e r i s t i c sect-ions. For 2M NaCl at pH 2, the three sections were described as follows: L) GP 1: E = 1.4 to E = 1.7 - 1.8 V (N.H.E.)', coverage between G = 1 and 6 = 2. 2) GP 2: E = 1.7 - 1.8 to E = 2.3 V, coverage between 0 = 2 and . 9 = 4. 3) Constant coverage at a l l p o t e n t i a l s above 2.3 V. Both GP 1 and GP 2 sections showed linear dependence or potential, with GP 1 being slower than GP 2. To account for this behaviour, they pro-posed the following reaction scheme: Pt + C l " -> Pt C l " , ads Pt C l " + C l " •+ (Pt C l " ) Cl . + e ads ads ads ( P t C 1 I d s ) C 1ads + H2° - P t °(s) + 2 H C 1 + 6 Pt 0, , + H.O -> Pt 2(0) + 2H+ + 2e is; 2 s They speculated that the existence of two growth sections may only be characteristic of those electrolytes whose ions enter into competitive adsorption with oxygen. The "potential jump" was considered to be caused by development of GP 1. Potential sweep curves on platinum show that there i s no evidence for oxygen adsorption below the chlorine evolution potential. If the anodic limit is raised above the aforementioned potential, a reduction peak appears which could either be due to C l a ^ g undergoing charge transfer to become C l a j g , or Cl ^ g formation on top of spec i f i -cally adsorbed Cl . The authors favour the latter. 125 Kuhn and Wright also studied the potential sweep behavi-our of iridium in sulphuric acid and sodium chloride electrolytes. Iridium shows reversible behaviour towards oxygen adsoption, unlike platinum. However, i f the anodic sweep limit is extended into the oxygen evolution region i n .5M ^SO^, formation of a formal iridium oxide is indicated - which is more irreversible than similar oxides on platinum. Unlike platinum, sweep curves i n NaCl electrolyte show characteristics similar to those in sulphuric acid. No evidence for the specific adsorption of chloride ions i s found. No work has been reported i n the literature on the polari-zation curves for smooth Pt/Ir alloys i n chloride solution. Faita et a l " ^ ^ and Kuhn and Wright,however, say that Pt/Ir alloys should allow a greater range of working electrode potential before the rise in overpotential takes place, compared with pure platinum. Blanchi"*"^ found no passivity phenomena on Ir/Ti electrodes, and suggested that i n the case of Ir/Pt alloys, the passivity phenomena should be attenuated with increasing iridium percentage. This was confirmed with titanium-substrate electrodes. 2.3.2 Polarization of Titanium-Substrate Anodes Two factors contribute to the different polarization 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 appar-ent current densities. Secondly, the noble metal coatings applied to electrodes of this class are porous. That i s , the titanium substrate may be exposed to the electrolyte. For the latter reason, i t is of interest to review the electrochemical behaviour of the base metal. 59 2.3.2.1 Behaviour of Titanium 128 129 130 131 Cotton > > > states that whenever titanium i s exposed to any environment containing oxygen, a t h i n tenacious surface f i l m of oxide i s formed. The formation of t h i s f i l m i s the reason f o r the high chemical resistance of titanium to such corrosive environments as strong n i t r i c acid s o l u t i o n s , mixtures of strong n i t r i c and sulphuric or n i t r i c and hydrochloric acids, or hydrochloric acid containing free c h l o r i n e . Only i n environments where the formation of t h i s f i l m i s not favoured, such as i n sulphuric of hydrochloric acids, where hydrogen i s produced on the metal, i s the corrosion rate of titanium s i g n i f i c a n t . The use of titanium as an anode r e s u l t s i n the induced formation of a protective anodic f i l m . The p o l a r i z a t i o n curve for titanium i n sulph-u r i c a c i d shows a corrosion maximum at p o t e n t i a l s negative to 0 V (S.C.E.) and complete pr o t e c t i o n i s obtained i n non-oxidizing acids at p o t e n t i a l s above about 1.0 V (S.C.E.). 132 Yakimenko et a l reported that the rate of corrosion of titanium increased with an increase i n conductivity of the oxide f i l m . 133 Dugdale and Cotton noted that the "breakdown p o t e n t i a l " of titanium was considerably lower i n chloride e l e c t r o l y t e s than i n sulphuric acid s o l u t i o n . They proposed that only i n solutions of ions with a s u f f i c i e n t l y high " p o l a r i z i n g power" (charge to area r a t i o ) did formation of the protective oxide occur. (The surface titanium atoms were supposed to be aided by the e l e c t r o s t a t i c e f f e c t of ions adsorbed at the oxide/solution i n t e r f a c e i n a t t a i n i n g the necessary 4-valent state 60 for protective oxide film formation.) Chloride ions were said to have insufficient polarizing power, thus promoting breakdown. 134 Van Laer determined the threshold potential of the dan-ger of intense anodic corrosion of titanium i n sythetic sea water (27 gpl NaCl; pH 6.32) was +7.0 V (S.C.E.). Since titanium oxide i s ele c t r i c a l l y very resistant to current passage i n an electrolyte, an attempt to force high currents through i t w i l l lead to a rapid increase in potential to film breakdown. It is not possible to pass currents 2 above about 20 mA/cm. through non-alloyed titanium. 135 Mazza found that breakdown occurred in crevices i n the titanium. Aqueous chloride solutions and anodic current were said to produce the following phenomena inside crevices where diffusion or convection were hindered: the accumulation of titanium corrosion pro-ducts due to metal dissolution and the accumulation of chlorine from chloride ion discharge. Secondary reactions were postulated which could reduce the st a b i l i t y of the passive film. Among these was the possibil-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 in de-aerated IN H^ SO^ , self-passivation occurred, but that in many cases self-activation occurred after several hours. Anodic polarization of titanium in the pH range 0.25 - 2.0 produced a passive electrode, but the active corrosion potential could be regained by cathodic polariza-tion. In less-acid electrolytes, however, the active corrosion potentials could not be regained. 137 Cerny s t u d i e d the anodic behaviour of t i t a n i u m i n c h l o r -ide s o l u t i o n s . He noted that p i t t i n g occurred at areas where c h l o r i n e bubbles were attched to the s u r f a c e , suggesting that the passive f i l m could p o s s i b l y have been destroyed by c h l o r i n e . 138 B r i e t e r suggested t h a t the reason f o r p i t t i n g c o r r o s i o n was that oxide formed i n surface wedges and grooves had sm a l l e r mech-a n i c a l s t r e n g t h , and thus broke down p r e f e r e n t i a l l y . 127 Kuhn and Wright s t a t e that the d i s s o l u t i o n of t i t a n i u m i s favoured by increased a c i d i t y and temperature. 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 to e x i s t where t i t a n i u m i s no longer p a s s i v e . As temperatures above 140°C are r e q u i r e d f o r t h i s , i t was suggested that t h i s type of at t a c k may be due to l o c a l overheating at h i g h - r e s i s -tance connections. 2.3.2.2 Coupling of Platinum Metals w i t h Titanium 128 129 130 Cotton ' ' found that g a l v a n i c coupling of t i t a n i u m w i t h a metal i n the platinum group could r a i s e the p o t e n t i a l of the t i t a n i u m i n t o the p r o t e c t i v e range. I t was a l s o seen that the p r o t e c t -i v e surface 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 contact w i t h another metal, but would not accept 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 . Consequently, most of the current was c a r r i e d by the noble metal, w i t h the p r o p o r t i o n c a r r i e d by the t i t a n i u m being 139 s u f f i c i e n t to ensure continuous maintenance of the oxide f i l m . Lowe s a i d t h i s "leakage c u r r e n t " was about 1 mA/cm. i n c h l o r i d e s o l u t i o n s under normal c o n d i t i o n s . 134 Van Laer s a i d that the platinum deposit on t i t a n i u m may be porous, f i s s u r e d , or incomplete, as a p r o t e c t i v e oxide f i l m would form at the bare metal. P i t t i n g c o r r o s i o n may occur, however, i f the dis t a n c e from the platinum c o a t i n g 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 through the t i t a n i u m was p a r t l y e l e c t r o n flow (from i o n discharge) and p a r t l y i o n i c (due to slow t i t a n i u m d i s s o l u t i o n ) . As the area of exposed t i t a n i u m w i t h respect to platinum i n c r e a s e d , the mixed p o t e n t i a l be-comes more negative, l e a d i n g to an increased chance f o r d i s s o l u t i o n . Immersion i n concentrated 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 to pro-duce undermining of the platinum c o a t i n g followed by establishment of 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 - c o n c e n t r a t e d 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 the zone of passive f i l m formation, although i n de-aerated s o l u t i o n s the p o t e n t i a l s were much c l o s e r to the Flade p o t e n t i a l . 141 142 Khodkevich et a l ' found that the leakage current through the t i t a n i u m was higher 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 that the sub s t r a t e takes a greater p a r t i n the anodic process (through pores i n the coating) i n the former case. They developed a method f o r determining 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 e l e c t r o d e s by means of monitoring the a c t i v a t i o n of the t i t a n i u m i n hot, strong h y d r o c h l o r i c a c i d s o l u t i o n . 63 Many authors believe that the destruction of titanium sub-str a t e electrodes as a r e s u l t of anodization i s due to oxidation of the substrate leading to undermining and eventual l o s s of e l e c t r i c a l . . . r~ i • - „. 127,140,141,143-7 contact or flakxng off of the noble metal coatxng. 2.3.2.3 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 F a i t a et a ^ ^ , 146,148 £ o u n ( j t n a t i r i d i u m contents ( i n P t / I r coatings) as low as 0.5% were e f f e c t i v e i n decreasing the tendency of coated anodes to passivate. They explained t h i s i n terms of the rever-s i b l e behaviour of i r i d i u m towards the passivating species, oxygen. Mixed oxide coated electrodes showed no hysteresis i n t h e i r p o l a r i z a t i o n curves, i n d i c a t i n g the s t a b i l i t y of the surface composition. Bianchi^"'"^ notes that the passivation of coated electrodes i s a slow process involving very long times, and that the passivation i s enhanced by rapid r o t a t i o n of the electrode. He suggests that hypo-c h l o r i t e may be responsible for passivation where surface formation of oxychlorides or oxygen produced by i t s discharge may remain chemisorbed on the electrode. Passivation was not found for I r - T i electrodes, and i t was specualted that passivation would be a function of i r i d i u m con-tent of the coating. 149 Landolt and I b l found a " p o t e n t i a l jump" phenomenon with coated electrodes, which was strongly dependent on electrode p r e t r e a t -ment, suggesting a non-mass transfer l i m i t e d mechanism. 64 Shembel et a l found that oxygen was present in the anode gas in increasing amounts with increasing potential. The polarization curve in sulphuric acid solution was considered 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 significant decreases in the electrochemically active surface area after potential-sweep experiments in 0.5M H^ SO^ , yet found no platinum loss. They attributed these losses to either platinum recrystallization or interference with the conductive connection between the substrate and coating caused by a formation of a layer of titanium oxide. 2.3.3 Summary It i s apparent that meaningful experimentation in chloride solutions must involve an appreciation for the possibility of simultan-eous reactions. Many can be made negligible by s t r i c t control of elec-trolysis conditions, but the phenomenon of electrode passivation appears to be the major obstacle in the attempt to understand electrode processes in chloride media. There is much speculation about the nature of the passivation, and l i t t l e work has been done concerning the passivation of mixed-metal electrodes. It is also apparent that the substrate mat-er i a l , while relatively inert, plays an important role in determining the polarization behaviour of coated electrodes, and that the substrate is much more "environment-sensitive" than is the coating. As with solid noble metal electrodes, l i t t l e work has been done concerning the effects 65 of p a s s i v a t i o n of mixed-metal coatings. Furthermore, i t must be noted that the behaviour of noble metal electrodes i n chloride media i s strongly pretreatment-dependent. 2.4 DISSOLUTION OF THE NOBLE METALS Since Llopis"'""'"*" published an extensive review of the corro-sion of the platinum metals i n 1968, much a d d i t i o n a l work has been done. As the aim of t h i s thesis i s the preparation and c h a r a c t e r i z a t i o n of systems which w i l l be used i n the study of corrosion, i t i s appropriate to include a summary of recent investigations concerning the mechanism for corrosion of the noble metals. Mechanisms for anodic d i s s o l u t i o n can be considered f o r both " a c t i v e " and "passive" cases. That i s , mech-anisms not involving 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 other species, such as ch l o r i d e ions, can then be considered likewise under these two general 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 that the "current e f f i c i e n c y " for noble metal d i s s o l u t i o n i s extremely 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 conditions, and that the separation of t h i s p a r t i a l process from the main current-consum-ing reactions (such as c h l o r i n e and oxygen evolution) has made research i n t h i s f i e l d very d i f f i c u l t . 66 The d i s s o l u t i o n of noble metals during a c t i v a t i o n or s i m i l a r procedures (such as anodic/cathodic treatment 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 ) must be treated separately as both a c t i v e and passive anodic mechanisms may be involved, as w e l l as reduction mechanisms. Further, the corro-sion of platinum metal coatings on value metal substrates must be con-sidered by i t s e l f , as other mechanisms of degradation (which do not involve the noble metal) may operate. 2.4.1 The Active D i s s o l u t i o n of the Noble Metals 152 Chemodanov states that the a c t i v e d i s s o l u t i o n of platinum occurs only when the e l e c t r o l y t e i s able to form stable complexes with the metal cations and i s capable of s p e c i f i c a l l y adsorbing, thus i n h i b -i t i n g the formation of an oxide f i l m . The mechanisms f o r d i s s o l u t i o n i n the active region have been found to be d i f f e r e n t from those i n the passive s t a t e . Llopis"'"^"'' considers such a mechansim as: -e M -> M n H 20 which involve the formation of aquo-complexes i s only 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 palladium. The standard h a l f - c e l l r eaction has 153 been d i r e c t l y determined f o r t h i s case by I z a t t . For the other platinum metals, the values of the standard p o t e n t i a l s i n acid media 154 have been calculated from thermodynamic data by Goldberg and Hepler: 67 Rh/Rh E° = 0.7 V Pd/Pd"1"1" E° = 0.92 V Pt/Pt"^ E° = 1.2 V 155 Goodridge and King have suggested that the corrosion mechanism f o r palladium ( i n 2N R^SO^ solution) i s : Pd -> P d + . + e surface surface P d + _ -> Pd"1-4" _ + e surface surface H 2 ° Pd _ -> Pd surface Of more p r a c t i c a l concern are 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 with the platinum metals. K o l o t y r k i n has noted that there are several i n t e r p r e t a t i o n s concerning the r o l e of solu t i o n anions i n determining the corrosion of metals. These include: 1) the formation of simple metal ions with subsequent i n t e r a c t i o n with the anions, 2) d i r e c t formation of complexes of the metal, 3) i n t e r a c t i o n between the anions and other s o l u t i o n constituents, 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 products are simple metal ions or t h e i r hydrolysis products. The active corrosion of a l l the platinum metals i n a c i d i c media has been found to be greater i n the presence of halide ions (than i n other common el e c t r o l y t e s ) due to the greater s t a b i l i t y of the halide-complexes with 68 respect to the aquo-complexes.^^ Halide ions specifically adsorb on the noble metals at potentials much more cathodic than oxygen, and the coverage i s almost complete at potentials approaching that for 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 concentr-121 ated electrolyte as 8M HCl. Standard electrode potentials for couples involving solution 154 anions have been determined by many authors. Goldberg and Hepler have summarized most of these. Of particular interest are the reactions: Pt(c) '+ 4C1 (aq) t Pt Cl^aq) + 2e E° = 0.75 V Pt(c) + 6Cl"(aq) t Pt Cl"(aq) + 4e~ E° = 0.76 V o Ir(c) + 6Cl~(aq) t Ir ClJ(aq) + 3e~ E° = 0.86 V Ir(c) + 6Cl~(aq) t Ir Cl^(aq) + 4e~ E° = 0.86 V Llopis"'""'"'"'"''"^  has proposed a general mechanism for active platinum metal dissolution in 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-> (M....X) IMX l ( n _ Y ) + X- . X - y X-'X M + X2 -e or T i t CM... .X~) ^ — - [MX j ( n _ Y ) + X- Y 132 160 Chemodanov * found that the active dissolution rate of platinum to increase with the chloride concentration, and that this rate was f i r s t order with respect to the chloride ion concentration. That i s , i n the active region of potentials, chloride ions participated directly in the f i r s t one-electron step of anodic platinum dissolution (perhaps chloride ion discharge on a platinum surface atom). Chemodanov noted the rates of platinum dissolution and halide discharge were not directly connected with each other, and hence metal dissolution could not be approximately determined by an extrapolation from the halide discharge rate. Indeed, the active dissolution rates of platinum are comparable in either 3N HCl or 3N HBr, whereas the halogen evolution rates differ by orders of magnitude. Llopis'*""'"'' has extrapolated the linear active region of the polarization curves for dissolution and deposition of Pd Br^ and Pd C l ^ , and has obtained vi r t u a l l y identical exchange current densities. This indicates that the dissolution and deposition processes in these systems are likewise nearly identical. Zelenskii and Kravtsov 1^ 1' 1^ 2 found the orders with respect to Cl of electrodissolution and electrodeposition of palladium to be 70 d i f f e r e n t . The cathodic r e a c t i o n order was -2, i n d i c a t i n g a Pd Clr, product formed from the s o l u t i o n ion Pd C l ^ . The anodic r e a c t i o n order was 1.33, which they explained by assuming an increase i n C l concentration i n the layer near the electrode. They suggest that i n chloride 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 high concentration, the surface of a palladium anode i s f i l l e d with adsorbed chloride forming Pd ( C l ^ ^ ) surface complexes. In turn, a c t i v a t i n g complexes containing two chlor-ide ions each are formed J.Pd ^ 1 ^ w* i :*- c n immediately i o n i z e s . Such a mechanism would give a d i s s o l u t i o n r e a c t i o n order proportional to the square of the bulk concentration of halide ions. 121 B i t t l e s and L i t t a u e r speculate that there are several d i s t i n c t types of adsorption s i t e s on a platinum surface i n chloride s o l u t i o n , i n c l u d i n g bare metal, oxidized surface atoms, adsorbed chlor-3+ ide species i n c l u d i n g C l and C l , surface complexes such as Pt C l 4+ 4-and Pt C l , and i n v o l v i n g oxy-halide species such as Pt ClCl^O) The processes of chlorine evolution and metal 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 during t h e i r reaction sequences. 122 -Helber suspects that the presence of C l ^ (which increases with chloride ion concentration) i n the form of surface complexes may be favoured due to the much higher chloride ion concentration at the surface than i n the bulk. The a c t i v e d i s s o l u t i o n of platinum may thus be enhanced, or even be caused by, C l ^ surface complex formation. Kuhn and Wright"*"^ suggest that the increasing corrosion rate of platinum i n solutions containing increasing chloride contents i s due to the increased retardation of competitive oxygen adsorption. 71 That i s , surface coverage by (Pt 01 a <j g) ^1^^ type species predominates. Like Bittles and Littauer, they support the idea that corrosion is associated with adsorbed oxy-halide or halide complexes. Other, not necessarily anionic, species may enhance the 163 active dissolution of the platinum metals. Blake et a l observed that a pronounced blackening of their palladium electrode occurred dur-ing the anodic oxidation of ethylene. They suspected this phenomenon was due to the deposition of finely-divided metal, and suggested the mechanism: Pd + C_H. t PdC0H. 2 4 solution 2 4 ads P d C 2 H 4 a d s t P d C 2 H 4 + a d s + e 2 P d C 2 H 4 + a d s * P d ( C2Vr +Pd° Further, solution reactions were also considered as a source of precip-++ H2° + itated palladium: P d ^ H ^ + CH3 • CHO + 2H + Pd. Their experi-mental data indicated that the charge-transfer step was in pre-equilib-rium. The model also accounts for the zero-order pH-dependence. Goodridge and King^^ observed the same phenomenon, and i n addition to the mechanism of Blake et a l , considered: Pd -> Pd + . + e surface Pd + . + Pd"^ . + e surface surface P d ^ , - P d ^ surface Pd"*-1" + C_H. -*• PdC-H^ 2 4 2 4 PdC„H, + + Ho0 + Pd° + CHoCH0 + 2H+ 2 4 2 J 72 164 Napp and Bruckenstein produced dissolution of bare plat-—6 inum by the addition of 10 "M Cu(II) to their solution. They attributed this to the following reaction: Pt + Cu(II) -> Pt(II) + Pt(IV) + Cu(I) . ^ ads + aq Organic species have been found to accelerate noble metal 140 dissolution. Warne and Hayfield attributed enhanced corrosion of noble metals (as coatings or wrought material) to some anodic complex-165 ing reaction with organic addition agents. Juchniewicz reported that the presence of sucrose, glucose, and similar organic compounds led to rapid destruction of coated anodes. 154 Chemodanov says certain organic compounds have a strong inhibiting effect on the process of active platinum dissolution. Llopis found that sulphur-containing species were effective inhibitors. The strong adsorption of inhibitors 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 sites. 2 . 4 . 2 Dissolution with Oxygen Participation Oxygen adsorption on a nobel metal electrode inhibits the active dissolution of the metal. At low chloride ion contents, a high surface oxygen coverage is formed which suppresses the action of the halide ions."'""'2'"'""^ Similarly, a reduction in the acidity of the 152 159 electrolyte f a c i l i t a t e s oxygen chemisorption. Llopis suggests 73 that a maximum metal corrosion (as found i n the p o l a r i z a t i o n curve) indicates that the passivating f i l m undergoes a mod i f i c a t i o n with increasing p o t e n t i a l s , such that there i s a higher resistance to cation transport across the f i l m . He al s o says that the establishment of a true passivating f i l m coincides with the establishment of high e l e c t r -onic conductivity. In such a case, the p o t e n t i a l gradient across the f i l m i t s e l f i s small, and consequently the metal ions have l i t t l e d r i v -158 ing force to migrate to the oxide/solution i n t e r f a c e . Hoar says that the rate of d i s s o l u t i o n of a passive metal may become l i m i t e d by the rate of chemical d i s s o l u t i o n of the mat e r i a l comprising the p a s s i -160 vating f i l m . Chemodanov a t t r i b u t e s the composition and properties of the surface oxygen layer on platinum as responsible 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 rate between 1.5 and 2.8 V 41 (R.H.E.). Rand and Woods speculate that the maintainence of the re v e r s i b l e oxygen p o t e n t i a l requires a f u l l oxygen coverage, such that metal d i s s o l u t i o n currents are completely i n h i b i t e d (and hence no mixed p o t e n t i a l mechanism operates). The p o s s i b i l i t y e x i s t s , however, that passive metal corro-sion can occur. Indeed, i n a l k a l i n e media the platinum metals are able to form stable surface complexes with OH (platinum, palladium, rhodium) which are soluble, or form surface oxides which react with OH to form soluble species (ruthenium, osmium), or react d i r e c t l y with OH to form soluble species (osmium, rhodium). In acid media, ruthenium, osmium , , ' . . , ; 151,154,157,166,167,168 and rhodxum are capable of forming soluble oxides. 74 L l o p i s ^ " ' ' has suggested that the p o s s i b l e d i s s o l u t i o n mechanism f o r noble metals i n non-complexing a c i d media i s : " e + M + mHLO -* M (H_0) -> M...0 + H 2 2 m M...0 -> MO 168 Others, such as Burke and O'Meara p o s t u l a t e that the p a s s i v a t i o n of a noble metal surface i s incomplete or that new surface i s exposed when surface s t r e s s e s at h e a v i l y o x i d i z e d surfaces cause the oxide to crack. Metal 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 of an unoxidized s u r f a c e . Chemodanov^ says that f o r a platinum e l e c t r o d e at p o t e n t i a l s corresponding 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 constant as surface oxides are continuously being generated and are 169 thus always present f o r d i s s o l u t i o n . Others, such as Kolomiets are a l s o of the op i n i o n that the formation of surface oxygen species i s necessary f o r the d i s s o l u t i o n of platinum at high anodic 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 that platinum c o r r o s i o n i n acetate systems under c o n d i t i o n s corresponding to the Kolbe synthesis depended only on the composition and p r o p e r t i e s of the surface oxides, and that the a n i o n i c 65 152 123 composition was not a f a c t o r . Chemodanov ' and Kokoulina a t t r i -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.) to a change i n the p r o p e r t i e s of the surface oxy-gen l a y e r . In p e r c h l o r i c a c i d , due to the 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 of the e l e c t r o l y t e , an oxide f i l m i s formed w i t h a high content of 52 " a c t i v e oxygen" which has a reduced c o r r o s i o n r e s i s t a n c e . Kravchenko 75 observed that the maximum i n the platinum 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 oxide thickness i n that 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 at p o t e n t i a l s which are not i n the range corresponding to oxide formation. Ginstrup^"'" has suspected that oxygen d i s s o l v e d i n the metal or r e s i d u a l a i r i n the s o l u t i o n could be r e s p o n s i b l e f o r higher c o r r o s i o n r a t e s than could 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 found that f o r such p o t e n t i a l s the nature of the e l e c t r o l y t e was of secondary importance w i t h respect to the length of p o l a r i z a t i o n . He speculates that dermasorbed oxygen, which d i f f u s e s to the e l e c t r o d e surface at p o t e n t i a l s corresponding to the hydrogen or double l a y e r r e g i o n s , can account f o r the observed asymptotic decay to zero of platinum 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 noble metals. Hoar suggested a mechanical mechanism f o r anion p e n e t r a t i o n and breakdown of p a s s i v a t i n g f i l m s . Continued anion a d s o r p t i o n may u l t i m a t e l y reach such 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 the oxide to which they are s t r o n g l y attached, apart. 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 anions. K o l o t y r k i n s a i d t h a t , i n con-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 on the d i s s o l u t i o n of a pass-i v e metal, not only 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 at i n d i v i -dual p o i n t s be taken i n t o account, but 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 76 172 anions and solution molecules i n the dissolution process. Foley discusses a " f i e l d effect" mechanism for chloride ions acting on a passive metal, where the strong electric f i e l d produced by chloride adsorption draws ions from the metal. He also considers a catalytic effect whereby chloride catalyses the dissolution reaction by forming some form of intermediate bridging structure. Llopis considers the formation of chloro-complexes from the noble metal surface oxide layer as responsible for the corrosion of osmium and r u t h e n i u m . ' L l o p i s suggests the mechansims: ' • M O p - ' surface X-^ ^^y^ [MO ] . [MX (OH) ] ( n - Y ) + p Jsurface HCl a 'y-a The f i r s t includes transport of metal cations across the oxide layer, and the second is chemical dissolution of the oxide layer. Llopis also suggests that halide-complexes may not be formed directly as the result 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 dissolution of passive platinum. This was considered to be the result of dissolution of the 52 oxide film. Kravchenko et a l say that anion effects w i l l only be significant in passive metal dissolution under those conditions which favour anion inclusion in the oxide film. Conditions which produce thin-layer films are thus considered favourable for reducing metal 77 d i s s o l u t i o n . Others main t a i n that there i s always competition between 121 oxygen and c h l o r i d e f o r surface 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 have proposed a s e r i e s of ad s o r p t i o n complexes, i n v o l v i n g bath water and 132 173 c h l o r i d e a d s o rption, f o r the passive e l e c t r o d e . Chemodanov ' a t t r i -butes the 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 ions at r e l a t i v e l y m i l d pass-i v e p o t e n t i a l s to the a b i l i t y of c h l o r i d e to adsorb on the bare metal su r f a c e . At high anodic p o t e n t i a l s , however, the r e d u c t i o n i n el e c t r o d e surface o x i d a t i o n due to c h l o r i d e a d s o r p t i o n i s accompanied by a lowering of platinum d i s s o l u t i o n . Chemadanov suggests that platinum d i s s o l u t i o n at such hi g h p o t e n t i a l s proceeds v i a a c t i v e surface oxygen compounds and that the conc e n t r a t i o n of these compounds i s reduced by c h l o r i d e a d s o r p t i o n . Chemodanov,^® V i j h , ^ ^ and Hackerman"^"* found e l e c t r o d e d i s s o l u t i o n to be hindered by i o d i d e and f l u o r i d e due to the formation of p r o t e c t i v e f i l m s of these species on the metal. Tanaka and Fujisawa"*"^ suggested that the anodic r e a c t i o n scheme i n EDTA s o l u t i o n s was: Pt + 2H 20 -> Pt(OH) 2 + 2H + + 2e Pt(OH) 2 + H 2Y = -> P t Y = + 2H 20 Their i n c l u s i o n of surface oxygen i n the d i s s o l u t i o n mechanism was based 152 on observations of the l o s s of surface oxygen. Chemodanov says the e f f e c t of organic compounds at h i g h anodic 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 or a c c e l e r a t i n g , depending on the nature and composition of the s o l u t i o n . 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 heading the term " a c t i v a t i o n " w i l l be used to describe 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 treatment, whether or not the treatment was performed to a c t i v a t e the e l e c t r o d e f o r some other e l e c t r o -chemical i n v e s t i g a t i o n . The o x i d a t i o n / r e d u c t i o n procedure may i n v o l v e a programmed potentiodynamic sequence, 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 , a simple a n o d i z a t i o n / c a t h o d i z a t i o n or even on-off s w i t c h i n g . As the i d e n t i f i c a t i o n of the problem of metal d i s s o l u t i o n during conventional a c t i v a t i o n procedures has been r e l a t i v e l y r e c e n t , much of the e a r l y work on a l t e r n a t i n g current e l e c t r o l y s i s has been used as the 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 darkening of noble metal e l e c t r o d e s under tha a p p l i c a t i o n of a l t e r n a t i n g current 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 i n c r e a s e s i n el e c t r o d e area. This e f f e c t was e s p e c i a l l y pronounced w i t h palladium, which absorbs hydrogen b e t t e r than the other metals. Hence, Hoare concluded that i t was the p e n e t r a t i o n of hydrogen i n t o the metal during the cathodic h a l f - c y c l e s which caused the break-up of the metal s u r f a c e . F u r t h e r , the darkening could be a r r e s t e d by p o l a r -i z i n g w i t h a d i r e c t current b i a s such that the most cathodic p o t e n t i a l 140 reached was w e l l above the 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 that only low-frequency C- 50 Hz) current 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 the 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 of ruthenium i n 6N HCl, where the e l e c t r o d e i s kept i n the 79 passive region of p o t e n t i a l s at 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 to the p a r t i a l removal of the passive f i l m (an undefined oxide) at the outer i n t e r f a c e with subsequent slow i n t e r a c t i o n with the e l e c t r o l y t e 177 to produce various complexes. Yukhevich also found that the corros-ion of platinum became less intense with high-frequency a l t e r n a t i n g current. He pointed to the absence of hydrogen i n the gas evolved during the cathodic h a l f - p e r i o d as evidence that the de-passivation of the metal did not have time to cccur. Yukhevich found that intense corrosion ensued i f an a l t e r n a t i n g current component of greater than 70.7% of the f i x e d d i r e c t current value was imposed. He suggested that the hydrogen adsorbed on the metal during the cathodic h a l f - c y c l e impeded the formation of an oxide layer, hence aiding a c t i v e corrosion by chlor-178 ide ions. Kukushkin a t t r i b u t e d the f a s t rate of platinum d i s s o l u t i o n with a l t e r n a t i n g current to the depassivation of the surface during the cathodic c y c l e . Ryazanov et al^^ and Kadaner and B o l k o ^ ^ state that the b a s i c problem i n a l t e r n a t i n g current corrosion of the determination of the reactions during a cycle and the d i s t r i b u t i o n of current expended during the reactions. Both the anodic and cathodic h a l f - c y c l e s may be considered as the sums of harmonic processes at the same frequency (for example, the anodic h a l f - c y c l e may be the sum of the metal d i s s o l u t i o n and chlorine evolution currents). For metal d i s s o l u t i o n to occur the quantity 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 -cycle must be greater than that f o r d i s s o l u t i o n i n the negative h a l f -cycle. 80 The d i s s o l u t i o n of noble metal electrodes as a r e s u l t of i n t e n t i o n a l or unintentional oxidation/reduction treatments has been subject of much speculation. The mechansim f o r metal loss has yet to be adequately confirmed. 140 Warne and Ha y f i e l d suggested that current reversals due to on-off switching may give r i s e to the electrochemical reduction of platinum surface oxide and thus give r i s e to the formation of a f i n e l y -divided metal that i s more capable of passage i n t o s o l u t i o n . 72 Srinivasan et a l proposed a possible a c t i v a t i o n mechanism whereby the metal i s dissolved on the anodic portion during anodic/cat-hodic pulsing, y i e l d i n g a fresh surface of constant c a t a l y t i c a c t i v i t y . 34 Damjanovic and Brusic 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 anodic/cathodic pulsing to eith e r the formation of new c r y s t a l s on the electrode surface or the escape i n t o s o l u t i o n of cations which formed the oxides. Llopis"''"'"'' found that the corrosion of rhodium by a square-wave current occurred only i f the p o t e n t i a l on the p o s i t i v e h a l f - c y c l e reached a value above 0.88 V (S.C.E.). He thus explained the corrosion as due to the r e p e t i t i v e formation and reduction of a surface oxide layer on the electrode. 163 Blake et 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 electrode surface 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 substrate, could account f o r order-of-magnitude differences i n rea c t i o n rates at a given imposed p o t e n t i a l . 81 15 B i e g l e r 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 probably i n v o l v e d surface 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 of surface platinum atoms, and t h a t t h i s freedom of move-ment arose from the p e r i o d i c formation and breakage of platinum-oxygen bonds. Unstable high-energy platinum atoms are rearranged or removed completely 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 . B i e g l e r f u r t h e r suggested that platinum d i s s o l u t i o n may account f o r the charge imbalance i n the o x i d a t i o n and r e d u c t i o n of a platinum surface and the observation that these charges converge w i t h repeated c y c l i n g as a de-creasing f r a c t i o n of atoms leave 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 order to e x p l a i n the changes i n a c t i v i t y of platinum-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 voltammetry. For the smooth a l l o y s , the p a r t l y - s o l u b l e rhodium oxides formed during the anodic c y c l e are able to d i f f u s e away and are not deposited 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 platinum becomes exposed. For the porous a l l o y s , the rhodium oxides formed w i t h i n the s t r u c t u r e are not able to completely d i f f u s e away before they are re-deposited. On d e p o s i t i o n , some of the platinum i s covered and the e l e c t r o d e assumes more rhodium-like behaviour. Only a f t e r long periods of 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 that p l a t i n u m - l i k e behaviour reappears. 182 Rand and Woods l a t e r concurred w i t h these observations. 65 Chemodanov a t t r i b u t e d enhanced d i s s o l u t i o n behaviour of platinum to the presence of oxygen on the surface and i n the surface 82 l a y e r s of the metal. At p o t e n t i a l s near that of the 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 the metal oxides g r a d u a l l y go i n t o s o l u -t i o n . 52 Kravchenko et a l support the id e a that platinum d i s s o l u -t i o n as a r e s u l t of a c t i v a t i o n i s due to the o x i d a t i o n of platinum b l a c k formed by the r e d u c t i o n of t h i c k oxides. 183 Johnson et 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 by means of: Pt 0 2 + 4H + + 2e P t ( I I ) + 2H 20 E° = .64 V (S.C.E.) In c o n t r a s t to the f i n d i n g s of Johnson et a l , Rand and Woods4"'" reported that platinum d i s s o l v e s mainly as Pt(IV ) during c y c l i c voltammetry, but a t t r i b u t e t h i s to the formation of a more s t a b l e com-pl e x w i t h t h i s species 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 cathodic d i s s o l u t i o n mechanism such as that suggested by Johnson et a l was u n l i k e l y as i t would not e x p l a i n the observed charge imbalance between anodic and cathodic charging. Rand and Woods thus suggest that the d i s s o l u t i o n r e a c t i o n s must be anodic, 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 or the involvement of surface oxygen sp e c i e s . V e t t e r 1 0 4 has r e c e n t l y noted that the d i f f e r e n c e i n the values f o r the anodic and cathodic charges has yet to be explained by metal c o r r o s i o n . That i s , only i n cases where these values have been found to be very 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 to c o r r o s i o n . 83 184 Chao et a l found that the passage of platinum i n t o s o l u t i o n during p o t e n t i a l c y c l i n g d i d not play a s i g n i f i c a n t r o l e i n the observed surface r e s t r u e t u r a t i o n . 2.4.4 Degradation of Noble Metal Coatings Mechanisms f o r the degradation of noble metal coatings are, for the most part, mechanical. That i s , the l o s s of metal i s a r e s u l t of a l o s s of coating adherence which a r i s e s from attack of the substrate m a t e r i a l . Coated electrodes with very thick noble metal layers show s i m i l a r behaviour to massive platinum, but are not economic. Indeed, 185 186 Burghardt and Okamura report that thick coatings may not have the low attack rate q u a l i t i e s of coatings of lesser thickness. I t i s c l e a r , however, that as coatings become so t h i n (or porous) that the substrate 143 i s exposed, the chances for attack of the substrate become much greater. 140 144 On the other hand, thicker coatings prevent substrate attack. ' 2.4.4.1 Degradation as a Result of Coating Undermining 140 Warne and H a y f i e l d discuss the galvanic current which flows as a r e s u l t of immersion of p l a t i n i z e d titanium i n agressive a c i d media. The main processes would be hydrogen evolution from the platinum and covering of the titanium with a p r o t e c t i v e f i l m . However, a propor-t i o n of the current would be due to titanium d i s s o l u t i o n which could lead to undermining and subsequent detachment of the coating. As the 84 relative area of exposed titanium increases, the chances for substrate dissolution increase. While the steady-state potential in aerated con-centrated hydrochloric acid l i e s well within the region of potentials for formation of a passive film on titanium, the potential in de-aerated solution l i e s very close to the Flade potential, with the chances for substrate dissolution thus being greater. 144 Antler and Butler postulated that metal loss i n acid chloride media involved substrate corrosion through the pores in the coating, whereby the corrosion film spreads laterally and undercuts the coating. 145 Khodkevich attributed the disription of adhesion of the coating in alkaline carbonate solutions to the deposition of hydrates of platinum, titanium, and solution impurities in the pores of the coating as a result of OH discharge, where the layer near the anode becomes acidified leading to the deposition of hydrates which were sol-141 142 uble in excess a l k a l i . Khodkevich ' also considered the state of oxidation of the substrate was the main reason for coating degradation. Certain oxide films showed a high leakage current, indicating that the films were porous and less-protective. 121 Bittles and Littauer said that rapid deterioration of platinized valve metals may be due to the morphology of the coating, where the detachment of the coating was the result of substrate oxidation at deposit imperfections. 127 Kuhn noted that the residual dissolution of exposed titan-ium increased with higher acid concentrations and temperature. Operation 85 under stagnant conditions may give r i s e to l o c a l regions of higher a c i d i t y , and hence of substrate attack. He postulated that, i n c r e v i c e s , conditions may e x i s t where the substrate was not passive, although such attack must involve l o c a l i z e d overheating. Y u k h e v i c h a t t r i b u t e d the a c i d i t y of the s o l u t i o n i n such crevices as the r e s u l t of secondary reactions such as: C l 2 + H 20 Z HCIO + H + + C l " T i C l 4 + 2H 20 % T i 0 2 + H + + C l " 146 F a i t a et a l suggested that increased a c i d i t y i n the pores was brought about by the evolution of some oxygen: 2H20 t 0 2 + 4H + + 4e I t i s i n t e r e s t i n g to note that improved electrode l i f e has been obtained by means of substrate treatments which improve coating adherence. Some treatments involve etching which leaves the substrate 187 with many deep crevices, vacuum treatment to prevent the formation 186 of oxides or impure compounds between the coating and substrate, and 188 the formation of a titanium hydride surface l a y e r . In addition, the use of substrates which form more protective f i l m s (such as tantalum, niobium, and some titanium a l l o y s ) w i l l a lso reduce metal l o s s due to undermining. 86 2.4.4.2 Other Causes of Coating Loss Coating l o s s may a r i s e as a r e s u l t of the normal d i s s o l u t i o n of the metal, which may be enhanced by the presence of c e r t a i n organic 140 addi t i v e s , the f a c t that the noble metal c r y s t a l l i t e s may be l o o s e l y 143 bound to the surface, the promotion of highly-corrosive conditions i n pores i n the c o a t i n g , o r the use of high overpotentials (as i n ., . 127,189 peroxidation processes). Others consider that the production of gas may a f f e c t coat-143 ing adherence. Haley found that hydrogen dissolved i n pre-cathodized, non-porous electrodes caused the coating to l i f t o f f the base metal on anodization. Yukhevich"'"^ stated that the presence of hydrogen could impede oxide layer formation, leading to f i s s u r e corrosion due to c h l o r -146 ide ions. F a i t a et a l said the evolution of gas i n pores a f f e c t s the 190 mechanical s t a b i l i t y of the electrode. Bianchi et a l said the blan-keting e f f e c t of r i s i n g gas bubbles could aggravate current non-uniform-i t y on large coated anodes. 191 Dreyman finds that improving the e l e c t r i c a l r e s i s t i v i t y of the substrate r e s u l t s i n improved temperature conditions f o r the anode, and lowered coating consumption. 87 2.4.5 Summary This section on platinum dissolution was a complete survey of the mechanisms of corrosion or degradation which have been suggested over the past half-decade. It is intended to supplement the earlier review made by L l o p i s , a n d i s in fact an extraction from a much more extensive review of the author's. It i s f e l t , however, that this recent work effectively summarizes most situations for noble metal dissolution. As with other electrochemical processes, that of dissolution suffers from the occurrence of obscuring, simultaneous reactions. In addition, there are a multitude of possible dissolution reactions to choose from involv-ing the noble metal alone, not to mention those in which the substrate of a coated electrode takes some part. Finally, consideration must be given to the effects of electrode pretreatment on the dissolution of noble metal electrodes. 88 2.5 RELATION TO AIMS OF PRESENT WORK In this thesis, the anodic electrochemical behaviour of platinum/iridium alloys i s considered - for both wire and coated ( t i t -anium substrate) electrodes - in sulphuric acid and chloride solutions. Within the limitations of this experimental framework, the topics which have been dealt with extensively in the preceding sections can be dis-cussed with respect to their bearing on the present work. The effects of electrode pretreatment have been discussed from mainly a mechanistic point of view. It appears to be necessary to have at least an appreciation for the possible effects of certain pro-cedures on electrodes, as an aid to understanding other electrochemical phenomena such as metal dissolution and passivation, or to comprehending sources of error or interference in electrochemical measurements. It is not the purpose of this thesis to test the various alternative theor-ies proposed for electrode activation, but to recognize the possible effects of this phenomenon. From the results of past workers, the pro-cedures used in this thesis for preparing the electrodes have been se l -ected i n order to provide a reproducible, understandable, surface con-dition (see Appendix I). Furthermore, from these past results, a number of phenomena can be expected to occur in the present experimental systems: 1) Severe anodic treatment may be expected to cause some disruption of the noble metal surface. 89 2) Cyclic procedures involving anodization followed by mild re-duction would not be expected to cause an increase in surface area and may, conversely, lead to a decrease in 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 distinguish between "activated" states of the electrode, but only to ensure that a constant surface state i s produced in the electrodes in this work. Thus, such concepts as "active surface structure" or other "active" surfaces need not be con-sidered. 4) The fact that different amounts of oxygen can be produced with-in, and on, noble metal electrodes i s the most important con-cept to be realized from the discussion on electrode pretreat-ment. 5) Impurity effects must be minimized in order to obtain meaning-f u l results, 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. The procedure uti l i z e d in this work (discussed in section 3.4.3 and in Appendix II) is in fact a synthesis of several methods to be found in the literature, and was arrived at not necessar-i l y because i t was the most reliable, but because i t was the most read-i l y applied with the equipment available. The surface area values quoted 90 in this thesis are, in reality, "determined surface areas", and are comparable to other results mentioned in the literature i f considera-tion i s given the assumptions behind such procedures. The present sys-tem takes advantage of the galvanostatic pulse technique. For electrolysis in chloride solutions, the past literature indicated that much work had taken place with platinum electrodes, and that there were several interpretations of the phenomena encountered during polarization in such solutions. It was decided that an investi-gation of noble metal alloy systems in such media would help expand the f i e l d of knowledge in this area and extend the valid i t y of the results of other workers to such systems. For the alloy wire electrodes, i t was expected that: 1) polarization characteristics would be similar to platinum -showing a three-part polarization curve. 2) the "potential jump" would be expected to show some relation to alloy content, i f the phenomenon was related to the forma-tion of platinum surface oxides, or to differences in the relative adsorption of oxygen and chloride between platinum and iridium. 3) passivation tendency should be reduced with higher iridium-content electrodes due to the more reversible behaviour of this metal toward oxygen adsorption. For the case of titanium substrate electrodes, i t was not possible to rely extensively on the results of other workers in order to predict the behaviour of the present electrodes. The reason for this i s the myriad possible methods for construction of such electrodes which lead to characteristics which differ l i t t l e from that of a solid platinum electrode to high degrees of dispersion and porosity of the coatings. Hence, i t was necessary to obtain polarization curves in the present work which must be regarded as the only reliable source of polarization data for the particular electrodes used here. As the present electrodes show a high degree of porosity, i t is important to consider the follow-ing: 1) the substrate is expected to be oxidized during pretreatment, and to remain that way throughout experimentation (that i s , the presence of bare titanium is unlikely). 2) the "breakdown" potential of this oxide film i s lowered in the presence of chloride ions. 3) the existance of crevices and pores over the electrode surface may be sites for conditions of localized corrosion of the sub-strate. 4) the substrate is not wholly inert and, in fact, undergoes some form of progressive degradation during anodic polarization. Anodic dissolution is not investigated quantitatively in this thesis, but i t is nevertheless necessary to obtain an appreciation for dissolution phenomena both with an aim to future work on this topic and in order to summarize the effects (or lack of them) which may be 92 rel a t e d to electrode corrosion i n the present work. These are: 1) Ac t i v e d i s s o l u t i o n of noble metal anodes i s s i g n i f i c a n t only i n complexing media, and at p o t e n t i a l s where complexing ions are able to s u c c e s s f u l l y adsorb on the metal surface. The "lower T a f e l region" of the p o l a r i z a t i o n curves i n ch l o r i d e media defines a region of simultaneous metal d i s s o l u t i o n and ch l o r i n e evolution. 2) The nature of the passivating layer formed on noble metal anodes at higher p o t e n t i a l s has yet to be c l e a r l y established, and hence the mechanism of d i s s o l u t i o n of passivated anodes i s not s u f f i c -i e n t l y known. P o l a r i z a t i o n at higher p o t e n t i a l s does, however, involve simultaneous metal d i s s o l u t i o n (probably through several mechanisms) along with c h l o r i n e and oxygen evolution. 3) The current e f f i c i e n c y of noble metal 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 materials of known high corrosion resistance, and e l e c t r o l y t e s which are not strongly a c i d i c or complexing. 4) Titanium substrate anodes with porous coatings 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 attack of the substrate which may lead to the undermining (and hence mechanical detachment) of the coating or to interference from corrosion products of the substrate. 5) Conditions which promote the corrosion of the substrate metal would be expected to cause accelerated degradation of the coated anodes. 93 3. EXPERIMENTAL 3.1 ELECTRODES Two types of working electrode were employed. Of prime concern was the behaviour of an electrode m a t e r i a l having a titanium substrate, with a coating of platinum containing 30% i r i d i u m . The 192 coated electrodes were prepared by Imperial Metal Industries Limited by a thermal decomposition method. I t was f e l t , however, that the behaviour of smooth wire electrodes (of platinum and platinum-iridium a l l o y s ) would be l e s s d i f f i c u l t to investigate (due to the ease of electrode construction and simpler apparatus assembly, as w e l l as the f a c t that the enormous surface area of the coated electrodes created problems with the i n t e r p r e t a t i o n of r e s u l t s ) . Hence, wire electrodes of 0.020 inch diameter were obtained from Johnson Matthey Metals Limited. Working electrodes were constructed such that only pyrex, t e f l o n , and the electrode i t s e l f contacted the s o l u t i o n . Discs of the coated electrodes were made and these were pressed into a s i m i l a r l y sized hole i n a t e f l o n holder. E l e c t r i c a l contact was provided by means of a wire spot-welded to the back of the electrode which passed up through the t e f l o n holder and through a pyrex tube which was also pressed into a hole on the top of the holder. In order to prevent leakage of the e l e c t r o l y t e behind the electrode and to f i r m l y hold the electrode i n place, the e n t i r e assembly was f i l l e d with epoxy. Wire working electrodes were constructed by f i r s t l y soldering a copper wire to about a two-inch length of noble metal wire. It was hoped that any effects of heat treat-ment caused by the soldering would not reach the extreme half-inch 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 teflon which in 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 in order to prevent any leakage and to firmly f i x the position of the wire electrode. Prior to assembly, a l l pyrex and telfon parts were thoroughly cleaned i n chromic/sulphuric acid. After construction, and before use, the elect-rodes were soaked in chromic/sulphuric acid for a few minutes, followed by washing and storage in twice-distilled water. Additional pretreat-ments were made according to the nature of the individual experiments. The reference electrodes used were Fisher Calomel Reference Electrodes of the porous plug type. Such electrodes served to minimize liquid junction potentials in the chloride electrolytes, where potential measurements were made. Calomel electrodes were also used i n sulphuric acid electrolytes (for surface area measurements) where precise knowledge of the electrode potential was not important. Reproducibility was checked from run to run by measuring the potential established in the (same) elec-trolyte 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 import-ant in measurements in chloride media. Reported values of these potentials 95 i n the l i t e r a t u r e show that, f o r the saturated K C l - d i l u t e KC1 boundary, the p o t e n t i a l i s les s than 5 mV f o r s i m i l a r conditions to those encoun-193 194 tered i n t h i s work. ' The values f o r KC1 and NaCl of s i m i l a r a c t -i v i t i e s can be assumed to be comparable due to the closeness of the transference numbers of the Na + and K + ions. The use of the saturated calomel electrode i n sulphate 195 e l e c t r o l y t e s i s not advisable, but no adverse e f f e c t s (due to sulph-ate contamination) were noticed, as the electrode was of the porous plug type and the l e v e l was always maintained above that of the sulphate s o l u t i o n , t h i s hindering contamination. Moreover, the electrode was only used i n sulphate s o l u t i o n f o r the necessary period of time to per-form the experiments i n question and was then removed. The sulphuric acid/KCl 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 with the concentra-t i o n of the s a l t and f o r t u i t o u s l y , f o r 1M H^SO^, the value of t h i s p o t e n t i a l passes from a region of high p o s i t i v e values to negative v a l -ues at the saturated KC1 concentration, i . e . , the l i q u i d j u n c t i o n poten-t i a l i s close to zero. 3.2 ELECTROLYTES Reagent grade chemicals were employed i n a l l cases, with the desired concentrations being obtained by d i l u t i o n with t w i c e - d i s t i l l e d water. Galvanostatic p o l a r i z a t i o n curves were obtained i n 1M NaCl with a pH of 2 (the pH used i n the e l e c t r o l y t i c production of c h l o r i n e ) . Also, pH 2 has been reported as the lowest pH value where Pt i s stable. 96 146 i n the presence of chlorine and c h l o r i d e s . 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 obtained i n the c h l o r i d e e l e c t r o l y t e and also i n IM H^SO^. P o t e n t i o s t a t i c anodization runs were performed i n various e l e c -t r o l y t e s : IM H 2S0 4, IM NaCl; pH 2, IM NaCl; n e u t r a l (neutral pH's are used i n commercial chlorate production), 4M HCl. Surface area measure-ments were made i n IM ^SO^, which i s a s u i t a b l e non-complexing i n d i f f -erent e l e c t r o l y t e . In addition, the oxygen adsorption region i s well^-separated from the hydrogen region i n such an a c i d i c s o l u t i o n . A l l e l e c t r o l y t e s were purged with commercial (bottled) helium gas before use i n order to remove dissolved oxygen gas. 3.3 CELLS Two kinds of c e l l were employed i n these i n v e s t i g a t i o n s . For galvanostatic p o l a r i z a t i o n measurements, an "H-type" c e l l was con-structed (Fig. 1). The anode and cathode compartments were separated by a f i n e glass f r i t which was inserted between the two compartments by means of ground glass taper j o i n t s . These j o i n t s were then wrapped with t e f l o n tape to prevent los s of e l e c t r o l y t e . The e l e c t r o l y t e was fed i n t o the compartments ( c e l l capacity: 2 l i t e r s ) by means of helium pres-sure and spent e l e c t r o l y t e could be removed to a storage v e s s e l by means of a vacuum hose attached to t h i s v e s s e l . In t h i s way, i t was hoped that any e f f e c t s due to concentration changes i n the e l e c t r o l y t e could be avoided. The c e l l compartments were further f i t t e d with gas bubblers, and gas could also be bubbled through the primary 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 the a n o l y t e could f u r t h e r -more be maintained under a helium gas b l a n k e t . Magnetic s t i r r e r s were used i n both compartments. The Luggin c a p i l l a r y was mounted i n a t e f l o n taper j o i n t which passed through the s i d e of the c e l l , w i t h the t i p of the c a p i l l a r y reaching to w i t h i n a m i l l i m e t e r of the working e l e c t r o d e . Although the c e l l i t s e l f and the stopcocks were constructed of t e f l o n or g l a s s , PVC tubing 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 e l e c t r o l y t e . None of these m a t e r i a l s were attacked by the s o l u t i o n s employed. The c e l l was p o s i t i o n e d i n a constant-temperature bath of a nonconducting o i l (Mentor 29). A second c e l l was employed f o r measurement of surface area, f o r the determination 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 curves, and 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 ( F i g . 2). The working and a u x i l i a r y e l e c t r o d e s were not separated by any means. When a wire working 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 p o s i t i o n e d around i t . When a f l a t - s u r f a c e d working e l e c t r o d e was used, the gauze was replaced by a long length of bent platinum w i r e p o s i t i o n e d across the c e l l and f a c i n g the working e l e c t r o d e . The c e l l i t s e l f was c o n s t r -ucted from a 600 ml. round-topped beaker, and the l i d was made from t e f l o n sheet which could be clamped down on the beaker. Gas bubbling and b l a n k e t i n g could be employed, and s t i r r i n g was accomplished by means of a magnetic s t i r r i n g bar. Figure I. Galvaoostatic Cell A. Anode compartment B. Cathode compartment C. Reference e l e c t r o d e (S.C.E.) D. Luggen c a p i l l a r y E. Gas o u t l e t s F. Gas bubblers G. E l e c t r o l y t e feed i n l e t s H. Blanketing gas i n l e t I . Working e l e c t r o d e J . Gauze a u x i l i a r y e l e c t r o d e K. Spent e l e c t r o l y t e o u t l e t s L. S i n t e r e d glass f r i t M. Magnetic s t i r r i n g bars oo A. Working e l e c t r o d e B. Gauze a u x i l i a r y e l e c t r o d e C. Reference e l e c t r o d e (S.C.E.) D. Luggen c a p i l l a r y E. Bla n k e t i n g gas i n l e t F. Gas o u t l e t G. Gas bubbler H. T e f l o n top I . 600 ml. beaker J . Magnetic 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 Galvanostatic Measurements Galvanostatic p o l a r i z a t i o n curves were obtained f o r wire electrodes of platinum and platinum-iridium a l l o y s containing 5, 10, 20, and 25% ir i d i u m . Current was supplied by a Beckman Electroscan 30, which could provide up to 100 mA. P r i o r to the i n i t i a l t e s t , the H - c e l l was cleaned with chromic-sulphuric acid and then washed with twice-d i s t i l l e d water. A f t e r completion of subsequent experiments, the c e l l was flushed with fresh e l e c t r o l y t e . Rigorous determinations were per-formed i n a IM NaCl e l e c t r o l y t e whose pH was adjusted to a value of 2 by HCl addition. A f t e r introduction of the working electrode into the c e l l , and alignment of the Luggen c a p i l l a r y , the system was maintained at open-circuit while the e l e c t r o l y t e was purged with helium. The working electrode was then subjected to cathodic p o l a r i z a t i o n at 100 mA for 5 minutes. This treatment was necessary to remove any oxides present on the eletrodes, and to render t h e i r i n i t i a l surfaces reproducible, as the open-circuit potentials of un-cathodized electrodes varied widely. Following the cathodic treatment, the electrode p o t e n t i a l was permitted to d r i f t at open-circuit while the c e l l was drained, flushed, and r e -f i l l e d with f r e s h e l e c t r o l y t e . This e l e c t r o l y t e was also purged with helium. When the electrode p o t e n t i a l reached a stable value, the 101 t r a c i n g of the galvanostatic p o l a r i z a t i o n curve was begun. M i l d helium bubbling and blanketing of the e l e c t r o l y t e was maintained throughout i n order to sweep away the generated gases and to prevent atmospheric con-tamination. S t a r t i n g with currents of only a few micro-amperes intens-i t y , currents were increased i n regular increments. The time period over which a given current was applied was determined by the s t a b i l i t y of the p o t e n t i a l . Generally, the current was increased i f the p o t e n t i a l had remained i n v a r i a n t f o r a minute a f t e r imposition of the current. In some cases, however, the p o t e n t i a l was observed to r i s e slowly (by about 10 mV over 300 seconds) and much longer times were necessary to ensure that the p o t e n t i a l had indeed become stable. In the region of the " p o t e n t i a l jump" the p o t e n t i a l rose slowly, but progressively, over time periods up to 40. hours long. The experiments were concluded a f t e r ob-t a i n i n g a T a f e l slope f o r the upper branch of the p o l a r i z a t i o n curve. Runs were performed i n duplicate, and i f s i g n i f i c a n t v a r i a t i o n was ob-served, a t h i r d run was made. 3.4.2 Anodic P o t e n t i o s t a t i c Measurements 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 obtained for platinum-30% i r i d i u m coated titanium electrodes. A Wenking Fast Rise Potentiostat model 68 FR 0.5 was used as the constant p o t e n t i a l supply. Potentio-s t a t i c measurements involved measuring the change i n current flowing with time upon imposition of the p o t e n t i a l of i n t e r e s t . In t h i s manner, 102 p o l a r i z a t i o n curves could be constructed from the r e s u l t s of many d i f -ferent experiments. In determining the p o l a r i z a t i o n curves, the same electrode was used throughout. Between experimental determinations, the electrode was subjected to a mild reducing treatment (hydrogen gas bubbling at open-circuit) i n order to remove any anodic products 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 at various anodization time i n t e r v a l s from 10 - 600 sec-onds. The solutions were purged with helium before each run, but were u n s t i r r e d during the experiments. Helium gas blanketing helped to pro-tect the system from atmospheric contamination and to help sweep away generated gases. P o t e n t i o s t a t i c anodizations were also performed i n sulphuric a c i d and halide e l e c t r o l y t e s f o r times as short as 30 seconds to periods of nearly a day. These runs were performed "batch"-wise, with no replen-ishment or replacement of the e l e c t r o l y t e attempted. Such runs were interrupted i n order to determine the change ( i f any) i n the e l e c t r o -chemically a c t i v e surface area. 3.4.3 Surface Area Measurements Afte r considering the techniques described i n the l i t e r a t u r e for measuring surface area, and assessing the a v a i l a b l e apparatus, an accurate, reproducible, and f a i r l y simple procedure was developed f o r such measurement. 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 order to provide good separation of the hydrogen and oxygen regions, and the l a t t e r was chosen as i t was best suited to galvanostatic anodic pulse techniques. The Wenking Fast Rise Potentiostat was used i n both p o t e n t i o s t a t i c and galvanostatic modes, i n conjunction with a Tektronix Type 564 B Oscilloscope with a camera attached to monitor the p o t e n t i a l of the working electrode during charging. Electrodes were subjected to strong anodizations (1 - 5 minutes at 1800 - 2000 mV S.C.E. for wire electrodes; 5 - 1 0 minutes at 1800 - 2000 mV S.C.E. f o r coated titanium electrodes) i n de-aerated (He-purged) e l e c t r o l y t e , followed by a reduc-ing treatment with hydrogen gas bubbled into the s o l u t i o n . This bubbl-ing, accompanied with vigorous s t i r r i n g with a teflon-coated magnetic s t i r r e r , caused the electrode p o t e n t i a l to drop to the hydrogen electrode p o t e n t i a l . (The time required for the p o t e n t i a l to drop was u s u a l l y about 3 minutes.) Such treatment has been a p p l i e d ^ to remove only surface-adsorbed oxygen, leaving the metal i n t e r i o r saturated with d i s -solved oxygen. Subsequent anodic charging would thus only deposit sur-face oxygen ( i . e . , a monolayer), with no a d d i t i o n a l charge going to dermasorption. A f t e r hydrogen bubbling, the s o l u t i o n was purged with helium gas i n order to remove the p o s s i b i l i t y of molecular hydrogen oxidation with the r e l a t i v e l y low-current pulses used (5 mA for wire electrodes; 20.5 mA for coated ele c t r o d e s ) . The p o t e n t i a l rose only s l i g h t l y toward anodic p o t e n t i a l s during inert-gas purging, i n d i c a t i n g that the replacement of surface oxygen from the metal i n t e r i o r was very s l i g h t . About one minute before a p p l i c a t i o n of the charging pulse, the 104 gas purge was stopped, with the flow diverted to merely blanket the s o l u t i o n . Appropriate o s c i l l o s c o p e scan speeds ranged from 5 - 5 0 msec./division for wire electrodes to .5 - 1 s e c . / d i v i s i o n f o r coated electrodes. The r e s u l t a n t traces showed well-defined hydrogen, "double-layer", and oxygen regions. Compensation for double-layer charging was made by using an extrapolation of the slope of the "double-layer" region to the oxygen evolution p o t e n t i a l (which was r e a d i l y apparent due to a f l a t t e n i n g of the charge curve). The platinum and platinum-iridium sur-faces were assumed to have the following charges associated with the 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 with atomic percent composition i n the a l l o y s : Electrode Oxygen Monolayer Charge Ir 440 ucoul/sm. Pt 420 Pt/5 % I r 421 Pt/10% Ir 422 Pt/20% Ir 424 Pt/25% Ir 425 Pt/30% I r 426 The areas of e l e c t r o d e s which were p o t e n t i o s t a t i c a l l y anod-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 determined w i t h l i t t l e m o d i f i c a -t i o n to the procedure. The oxygen f i l m formed by such treatment i s not r e a d i l y reduced at o p e n - c i r c u i t , and the e l e c t r o d e can i n f a c t be removed 105 f r o m one c e l l a n d t r a n s f e r e d t o a n o t h e r w i t h no a p p a r e n t l o s s o f c o v e r -a g e . I n d e e d , s u c h p r e - p o t e n t i o s t a t t e d e l e c t r o d e s w i l l m a i n t a i n t h e h i g h r e s t - p o t e n t i a l s i n d i c a t i v e o f o x y g e n c o v e r a g e e v e n when s t o r e d a t o p e n - c i r c u i t i n t w i c e - d i s t i l l e d w a t e r f o r l o n g p e r i o d s o f t i m e . H e n c e , no d i f f i c u l t y was e n c o u n t e r e d i n t r a n s f e r r i n g s u c h a n e l e c t r o d e i n t o a s u l p h u r i c a c i d e l e c t r o l y t e a n d t h e n r e d u c i n g t h e s u r f a c e w i t h h y d r o g e n gas b u b b l i n g a n d o b t a i n i n g t h e c h a r g i n g c u r v e f o r m o n o l a y e r d e p o s i t i o n , 3.4.4 O b s e r v a t i o n o f E l e c t r o d e S u r f a c e s U s e was made o f s c a n n i n g e l e c t r o n m i c r o s c o p e ( S . E . M . ) a n d e l e c t r o n p r o b e ( E . P . ) t e c h n i q u e s t o o b s e r v e t h e s u r f a c e s o f e l e c t r o d e m a t e r i a l s . B o t h c o a t e d t i t a n i u m a n d s m o o t h p l a t i n u m s h e e t s w e r e s u b -j e c t e d t o s e v e r a l k i n d s o f c h e m i c a l t r e a t m e n t s f o r d i f f e r e n t l e n g t h s o f t i m e . T h e s e w e r e t h e n o b s e r v e d w i t h t h e S . E . M . U s e d e l e c t r o d e s were a l s o o b s e r v e d u s i n g t h e S . E . M . , a n d t h e y w e r e a l s o t h e s u b j e c t o f e l e c t r o n p r o b e s t u d y i n o r d e r t o d e t e r m i n e t h e d i s t r i b u t i o n o f e l e m e n t s s u c h as p l a t i n u m , i r i d i u m , t i t a n i u m , a n d o x y g e n . T h e d i s t r i b u t i o n s o f p l a t i n u m a n d t i t a n i u m w e r e a l s o i n v e s t i g a t e d b y means o f X - r a y l i n e -s c a n s . 106 4. RESULTS 4.1 GALVANOSTATIC POLARIZATION CURVES The polarization curves for platinum and platinum/iridium wire electrodes in helium-stirred 1M NaCl; pH 2 are shown in Figures 3 - 7 . For a l l cases, two overpotential regions were found, separated by a "potential jump". At low currents, the lower overpotential region was not linear (not shown in the figures), probably due to the ioniza-tion of hydrogen produced during the cathodic pretreatment. The linear-ity of the lower overpotential region was found to exist over a large range of current density, with a progressive positive deviation from linearity appearing at current densities approaching that of the potent-i a l jump. For the Pt/5 Ir and Pt/25 Ir electrodes, quite large devia-tions from linearity were observed before the potential jump. In a l l cases, at least 48 hr. of polarization at the passivation current den-121 sity ip was required in order for the potential to reach values cor-responding to the upper overpotential region. It was noticed that intro-duction of fresh electrolyte to the anode compartment produced an immed-iate, rapid rise in potential (of the order of 20 mV), which was irrever-sible. D i f f i c u l t y was encountered in obtaining the actual ip value, as many hours were required for steady-state to attain at current densities approaching ip. The upper overpotential region was not anywhere near as reliably reproducible or as linear as the lower overpotential region, 107 probably due to the combined e f f e c t s of concentration p o l a r i z a t i o n and interference due to the high r a t e of gas evolution from the anode. Tracing the curve i n the reverse d i r e c t i o n (decreasing currents) showed a marked h y s t e r e s i s , with no tendency f o r the electrode to return to i t s o r i g i n a l state even a f t e r long times at current d e n s i t i e s l e s s than i p . On shutting o f f the current, the p o t e n t i a l dropped to a r e s t p o t e n t i a l of 1.1 v o l t s , which was considerably more anodic than the o r i g i n a l r e s t p o t e n t i a l . Cathodic charging f a i l e d to a f f e c t the new r e s t p o t e n t i a l , as a f t e r charging the p o t e n t i a l returned to anodic values. A f t e r remain-ing at open-circuit i n the e l e c t r o l y t e f o r at l e a s t 24 hours, the e l e c t -rode regained the c h a r a c t e r i s t e i c s of an " a c t i v e " electrode. Re-tracing the p o l a r i z a t i o n curve r e s u l t e d i n near-duplication of most experimental points, e s p e c i a l l y for the lower overpotential region. The current d e n s i t i e s i n Figures 3 - 7 r e f e r to the geometric areas of the electrodes. I t was seen that superposition of a l l the cur-ves on one graph res u l t e d i n an excellent f i t of a l l the data, with the exception of the cases of the Pt/5 Ir and Pt/25 Ir electrodes, which showed higher i values (but whose p o l a r i z a t i o n curves otherwise were i n good agreement). P l o t t i n g the p o l a r i z a t i o n curves on the basis of the area measured by oxygen adsorption led to a random assortment of i p values, with poor r e p r o d u c i b i l i t y among curves f o r the i n d i v i d u a l electrodes. I t i s f e l t that the areas determined by the oxygen adsorp-t i o n technique were u n r e l i a b l e , although they are reproduced i n Table 2, due 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 area during anodization 108 at high potentials. It was later 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 in the polarization experiments were anodized at such high potentials for significantly different periods of time, i t is f e l t that they received comparatively different degrees of roughening, and that comparison of true electrode areas is not valid after this has occurred. Tafel parameters were obtained from each curve, for the ascending upper and lower overpotential regions and for the descending overpotential, where this was measured, using the Tafel equation: n = a + b log I 2 where n is the overpotential, a is the intercept at log lA/cm. , and b is the slope per decade. The transfer coefficient can be obtained from 2.303 RT  a = b F ' and the exchange current density can be obtained from the value obtained at zero overpotential. io = 1 The standard electrode potential of the chlorine electrode CE£^) was 197 determined by Faita et a l to be 1.3583 volts on the absolute scale. log i,(npA/cm2) Figure 3. Gal 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 wire e l e c t r o d e i n helium-saturated IM NaCl; pH 2. Temperature 25°C. Figure 4. Galvanostatic p o l a r i z a t i o n curve f o r platinum/5% i r i d i u m wire e l e c t r o d e i n helium-saturated IM NaCl; pH 2. Temperature 25°C. o Figure 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 r i d i u m wire e l e c t r o d e i n helium-saturated 1M NaCl; pH 2 at 25°C. F i g u r e 6. Galvanostatic p o l a r i z a t i o n curve f o r platinum/20% i r i d i u m wire e l e c t r o d e i n helium-saturated IM NaCl; pH 2 at 25°C. Figure 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 platinum/25% i r i d i u m wire e l e c t r o d e i n helium-saturated IM NaCl; pH 2 at 25°C. 114 TABLE 2 Electrode areas measured after determination of the polarization curves, as compared to their measured geometric areas. Area from oxygen Electrode adso ptiom Geometric Area Roughness Factor (cm.2) (cm.2) Pt 1.127 0.246 4.6 Pt/5 Ir 0.844 0.235 3.6 Pt/10 Ir 0.976 0.251 3.8 Pt/20 Ir 1.125 0.246 4.6 Pt/25 Ir 1.233 0.252 4.9 115 The Nernst equation f o r the r e a c t i o n 2C1 t C l 2 + 2e can thus be expressed as: E = 1.358 + .0295 log p c l - .0591 log aQ1~ The second term on the r i g h t hand side can be neglected 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 . Chloride ion a c t i v i t i e s were obtained from 196 Harned and Owen. The p o t e n t i a l of the saturated calomel electrode «. i k 198 was taken to be E S C E = 0.2490 - .00065 (°C-20) for the purposes of converting to p o t e n t i a l s r e l a t i v e to t h i s standard electrode. T a f e l parameters are recorded i n Tables 3 and 4. As the s o l u t i o n was purged with helium, and s t i r r e d with t h i s gas throughout the experiment - as w e l l as the e l e c t r o l y t e being p e r i o d i c a l l y replaced with fresh s o l u t i o n - i t i s d i f f i c u l t to deter-mine a value f o r p^- . In addition, p_.. would be expected to be d i f f e r e n t , depending on the current density, due to the v i r t u a l n e g l i g -i b l e evolution at small currents and vigorous evolution with larger currents, 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 that tends to become more anodic with readings at higher current d e n s i t i e s . I f a _3 value of p . = 10 i s assumed, a Nernst p o t e n t i a l of 1.033 V (S.C.E.) i s obtained. 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. Electrode a decade a i 0 (A/cm.2) Pt 0.20 0.045 1.31 3.6 x 10" 5 Pt/5 % Ir 0.16 0.035 1.69 2.1 x I O - 5 Pt/10% I r 0.20 0.041 1.44 1.3 x 1 0 - 6 Pt/20% I r 0.19 0.040 1.48 1.8 x I O - 5 Pt/25% Ir 0.19 0.0375 1.57 0.85 x 10~ 5 TABLE 4 T a f e l parameters for Pt and Pt/ I r wire electrodes f o r the ascending and descending upper T a f e l regions of the p o l a r i z a t i o n curve i n IM NaCl; pH 2 at 25°C. Electrode Ascending currents Descending currents a b a i Q (A/cm. ) a b a i Q (A/cm.2) Pt 1.19 0.27 0. 22 5.9 x 1 0 - 1 Pt/5 % I r 1.17 0.31 0. 190 6.6 x 10" 1 1.14 0.27 0.22 6.4 x 1 0 - 1 Pt/10% Ir 1.23 0.30 0. 20 5.7 x 10" 1 Pt/20% I r 1.13 0.32 0. 19 5.7 x 1 0 _ 1 Pt/25% Ir 1.18 0.29 0. 20 6.9 x 1 0 - 1 1.16 0.26 0.22 6.9 x 1 0 - 1 117 Table 5 records the passivation current densities, ip, corresponding to the potential jump, and the passivation potentials, Ep, as obtained from projecting the lower Tafel slope u n t i l i t inter-sected ip."'"2"'" Experiments in other electrolytes were not considered reliable, and curves are not reproduced here. However, i t was clear that the passivation current density was increasingly higher for plat-inum electrodes i n solutions of higher acidity and/or chloride ion concentration - an observation already reported by Littauer and co-118,121 workers. TABLE 5 Passivation data for Pt and Pt/Ir wire electrodes from polarization curves in 1M NaCl; pH 2 at 25 °C . Electrode i p (mA/cm.z) Ep (V, S.C.E.) Pt 8.2 - 10.3 1.14 Pt/5 % Ir * 66 A 1.16 Pt/10% Ir 8 - 10 1.14 Pt/20% Ir 8 - 10 1.14 Pt/25% Ir 16 A - 20 * 1.15 It i s believed that these numbers are not the true steady state values for this 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 curves i n both 1M -^SO^ and 1M NaCl; pH 2 showed s i m i l a r forms, with no di s t i n g u i s h a b l e T a f e l slopes. The l o c a t i o n of the curves was a function of anodization time, and steady-state conditions were never attained. The area of the electrode used i n the chl o r i d e e l e c t r o l y t e was found to decrease by more than 50% over the course of p l o t t i n g the curve, whereas the electrode used i n 1M l^SO^ showed no s i g n i f i c a n t surface area change. Curves are reproduced i n Figures 8 and 9. J- J - I I ] log10I,(mA) Figure 8. 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 IM H„SO, 2 at 20°C. Electrode area, measured p r i o r to run 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 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 at 20 C. Electrode area decreased from 148 to 71 cm. over the course of the run. 122 4.3 CHANGE OF SURFACE AREA WITH POTENTIOSTATIC ANODIZATION The observation that the surface area of coated electrodes was decreased i n the course of constructing the p o t e n t i o s t a t i c p o l a r i -zation curve led to more inte n s i v e i n v e s t i g a t i o n of t h i s phenomenon. The e f f e c t s of p o l a r i z a t i o n at high anodic p o t e n t i a l s (1800 and 2000 mV, S.C.E.) i n various e l e c t r o l y t e s , as w e l l as the e f f e c t of chemical pre-treatment were investigated, and are reported i n Tables 6,7, and 8. The accuracy of the surface area technique was checked many times, and successive duplicate runs were always found to be accurate w i t h i n 3% of each other. However, i n d i v i d u a l experiments with d i f f e r e n t electrodes i n c h l o r i d e e l e c t r o l y t e s did not show such r e p r o d u c i b i l i t y , other than the general tendency to lose increasing amounts of surface area with increasing anodization times. In f a c t , no e f f e c t of chl o r i d e i on con-centration or a c i d i t y was noted among the 1M NaCl, 1M NaCl; pH 2, and 4M HCl e l e c t r o l y t e s . 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 ^SO^) caused no area changes over periods of anodization commonly used i n determining the surface area, but a long-term run showed an area decrease of nearly 10% (which was s t i l l not as great as that caused by anodizations of only 30 seconds i n the chloride e l e c t r o l y t e s ) . 1M ^SO^ containing .1M NaCl showed an intermediate behaviour, with some loss of surface area noted, but nowhere approaching the magnitude observed with the higher chloride-containing e l e c t r o l y t e s . TABLE 6 Surface area changes as a result of potentiostatic polarization in chloride electrolytes with Pt/30 Ir-Ti electrodes. Area Anodization Anodization Cumulative Original prior to Area after Af-Ai Af-Ao Electrolyte a + ci~ potential time time area anodization anodization A i Ao (mV S.C.E.) t A(sec.) t c(sec.) Ao(cm.2) Ai(cm.2) Af(cm.2) 0.1M NaCl + IM HoS0, 2 4 IM NaCl IM NaCl; pH 2 4M HCl 0.67 1800 1800 9.50 1800 1800 120 10 30 90 110 880 880 60 60 4880 30 90 1000 120 10 30 120 120 1000 1000 60 120 5000 30 120 1000 148 154 161 161 154 161 154 162 162 162 155 155 159 148 154 161 127 137 113 118 162 99 89 155 134 159 144 137 127 113 118 106 94 103 89 76 134 112 83 -.027 -.027 -.11 -.21 -.11 -.14 -.062 -.20 -.36 -.10 -.15 -.14 -.16 -.48 -.11 -.21 -.30 -.23 -.34 -.39 -.36 -.45 -.53 -.14 -.28 -.48 TABLE 7 E f f e c t of po 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 the surface area of Pt/30 I r - T i electrodes. Anodization p o t e n t i a l (mV S.C.E.) Anodization time t A ( s e c . ) Cumulative time t c ( s e c . ) O r i g i n a l area Ao(cm. 2) Area p r i o r to anodization Ai(cm,2) Area a f t e r anodization Af(cm.2) A f - A i A i Af-Ao Ao 1800 300 900 148 148 148 .00 .00 300 900 162 162 161 -.006 -.006 83620 83620 162 161 148 -.081 -.086 2000 300 * 113 113 .00 300 * 71 73 +.028 300 * 58 59 +.017 3000 * 73 72 -.014 17500 * 72 72 .00 Cumulative anodization time not given because electrode was also polarized i n c h l o r i d e e l e c t r o l y t e . TABLE 8 Effect of treatment in aqua regia on the surface area of Pt/30 Ir-Ti electrodes. Immersion time (sec.) Cumulative time (sec.) Original area Ao(cm.2) Area prior to treatment Ai(cm. 2) Area after treatment Af(cm.2) Af-Ai Ai Af-Ao Ao 30 270 3700 168300 30 300 4000 172300 151 151 151 151 151 151 151 111 151 151 111 78 .00 .00 -.26 -.30 .00 .00 -.26 -.48 126 In every case considered, the current density was observed to decay with time of potentiostatic anodization. Interruption of electrolysis (for purposes of surface area measurement) led to tempor-ary "re-activation" of electrodes, where the current density assumed i t s original high values. However, the current density quickly decayed to previous levels. Comparative curves for various electrolytes are shown in Figure 10. No steady-state readings were obtained, even after anodizing for periods up to a day long in some cases. In addition to the current/time curves determined for each potentiostatic anodization, the electrodes were removed from the v a r i -ous electrolytes after several of the runs, and their current/time history recorded in a standard electrolyte (IM ^SO^) with potentio-static anodization at 1800 or 2000 mV (S.C.E.). Figures 11 and 12 show the effects of immersion in aqua regia and potentiostatic anodization in LM NaCl; pH 2. In both cases, the current/time curve has been shifted towards lower current values. If however, the electrode is polarized only in IM ^SO^, no significant difference can be found among the current/time curves determined before, during, and after such treatment. Furthermore, an "aged" electrode (one which has lost apprec-iable surface area) shows reproducible current/time curves regardless of the treatment given i t . (Even potentiostatic anodization of IM NaCl; pH 2 at 2000 mV for 11000 seconds failed to alter the current/time be-haviour of such an electrode.) 300 -US" I (mA) _ 200 100 A B c D E. -o— -o— 4 M HC\ A 0 S 155cm 2 . , A a o ' l34cm 2 ,A l 2 0 " 112 cm 2 lMNaCUpH2 A o 8 l&l cm 2* A«o s 103 cm 2 ,A .2o» 89 cm* lMMaCI Ao= 161 cm 2, A 3<r 12? Cm*,Ai20 = • I 3 cm* jMH?SQ,MMNaa A o * 148cm1 A 12c 144 em * iMH2S04 A 0 * 162cm? D 2 0 4 0 t, (sec) 60 80 F i g u r e 10. 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 ectrodes at 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 at 25°C. 2 Apparent area of a l l e l e c t r o d e s 1.95 cm. . 2 0 0 160 I (mA) 120 8 0 4 0 0 o No treatment Aj =151 cm 2 + 300s®c. immersion Af =151 cmr A 48 hr. immersion Af = 78c 10 + / 50. 100 T7(seconds) 5 0 0 1 0 0 0 Figure 11. Current/time 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, at 1800 mV and 25°C, a f t e r v a r i o u s 2 pretreatment times i n aqua r e g i a . Apparent e l e c t r o d e area 1.95 cm. . oo 300 I h mA) 200 100 Before , A: =148cm2 After, A f= 71 cm 2 1 0 t/(seconds)1 0 0 1000 Figure 12. Current/time 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^ at 2000 mV and 25°C, before 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 of IM NaCl; pH 2 at 1800 mV f o r 17,000 2 seconds. Apparent electrode area 1.95 cm. . 130 In order to determine whether the surface area changes were reversible, various "activation" procedures were applied i n a few cases of severe area-loss. These procedures are summarized in Table 9. Severe cathodic treatment and anodic/cathodic sweeping did not produce significant changes in the electrode area, and their effects could not be found to be different from anodization in the same (1M I^SO^) electrolyte. A simple test was performed to determine i f the spent elec-trolytes contained any dissolved titanium, and none was indicated. 131 TABLE 9 E f f e c t s of " a c t i v a t i o n " procedures on the surface area of Pt/30 I r - T i electrodes. (Apparent area 1.95 cm.2) Surface area Surface area Treatment before treatment a f t e r treatment (cm.2) (cm.2) Po t e n t i o s t a t i c cathodization i n 1M H 2S0 4 at - 1000 mV (S.C.E.) for 3 minutes. Average 72 69 current: 320 mA. Po t e n t i a l cycled at 220 mV/sec. between 0 and 2000 mV (S.C.E.) and back to 0. Repeated 91 93 twelve times i n 1M H2SO4. P o t e n t i o s t a t i c cathodization i n 1M H2SO4 at - 1000 mV (S.C.E.) for 1000 seconds. Average 103 99 current: 240 mA. 132 4.4 OBSERVATIONS OF ELECTRODE SURFACES Scanning electron microscopy (S.E.M.) was used to observe the surfaces of both coated and smooth electrodes. For the former, chemical pretreatments were shown to have no v i s i b l e e f f e c t . Figure 13 shows that even treatments such as immersion i n hot aqua r e g i a f o r an hour leave the electrode v i s i b l y unchanged. Used electrodes also show l i t t l e d i f f e r e n c e . However, an electrode which had been subjected to long-term anodization i n IM NaCl; pH 2 at 2000 mV (S.C.E.) developed nodular protruberances at various places over i t s surface (Figure 14). Smooth platinum was greatly a f f e c t e d by chemical pretreatment. Immer-sion i n hot aqua regia produced a progressive destruction of the e l e c t -rode surface. A f t e r less than a minute, etching of surface scratches was apparent. Figure 15 shows the surface state a f t e r 5 and 60 minutes immersion. Similar e f f e c t s were obtained i n cold aqua r e g i a , but took longer times to be produced. Immersion i n concentrated hydrochloric acid produced no v i s i b l e changes except a f t e r long times, where surface scratch etching appeared a f t e r one hour. Observation of wire electrodes revealed that the i n i t i a l surface state was not at a l l smooth, as i n d i c -ated by surface area measurements. Figures 16 and 17 show the surfaces of new and used (after determination of the galvanostatic p o l a r i z a t i o n curves) platinum and platinum/iridium electrodes. I t i s apparent that the smooth platinum electrode 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 during the course of p l o t t i n g the galvanostatic curves, but the e f f e c t 133 on the alloy electrode is not obvious. Electron probe analysis revealed no significant difference between new and used coated electrodes. Figure 18 shows the distribu-tion of platinum and titanium over the surface of a new Pt/30 Ir - T i electrode. Similar determinations for electrodes subjected to anodi-zation in sulphuric acid, sodium chloride, or hydrochloric acid solu-tions showed no v i s i b l e changes, even though the measured surface areas were significantly lower for the latter two cases. X-ray linescans, however, showed remarkable differences between new and used electrodes. For a new electrode, relatively few areas of high titanium-content were found on the surface. For an elec-trode anodized in 1M H^O^ for 84220 seconds at 1800 mV (S.C.E.), a slight increase i n the number of high-titanium peaks was noted. An electrode anodized in 4M HCl for 1000 seconds at 1800 mV showed high titanium content over v i r t u a l l y the whole surface. X-ray diffraction analysis of the surfaces of new and used coated electrodes revealed only the presence of titanium and platinum on a new electrode. As an electrode was subjected to progressive anodic polarization, new peaks appeared which could be attributed to the devel-opment of oxides of titanium. X-ray data are presented in Appendix III. 1 3 A Figure 13. S.E.M. observation of Pt/30 Ir-Ti surfaces (400X; specimen t i l t e d at 45°) (a) New ele c t r o d e (b) Immersed 1 hr. i n hot aqua r e g i a Figure 14. S.E.M. observation of used Pt/30 I r - T i e l e c t r o d e Figure 15. S.E.M. observation of platinum 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) Figure 16. S.E.M. observation of platinum wire e l e c t r o d e s (1300X) (a) New el e c t r o d e (b) Used e l e c t r o d e 1 3 8 (b) Figure 17. S.E.M. observation of Pt/25% I r wire e l e c t r o d e s (1300X) (a) New electrode (b) Used e l e c t r o d e 139 Figure 18. E.P. observation of new Pt/30 I r - T i e l e c t r o d e (500X) (a) Absorbed e l e c t r o n image (b) X-ray image; Pt (c) X-ray image; T i . 140 5. DISCUSSION 5.1 ANODIC GALVANOSTATIC "MEASUREMENTS As expected, the p o l a r i z a t i o n behaviour of the platinum/ i r i d i u m wire electrodes was s i m i l a r to that of pure platinum. The i and Ep values for the Pt/5 Ir and Pt/25 Ir a l l o y s were found, however, to be s i g n i f i c a n t l y higher than that f o r the other a l l o y s . I t i s noted that, on inspection of the p o l a r i z a t i o n curves for the former, there i s considerable deviation from the lower T a f e l r e l a t i o n before p a s s i v a t i o n ( i . e . , the p o t e n t i a l jump) occurs. I t i s suggested that, f o r such cases, the true passivation parameters were not obtained due to i n s u f f i c i e n t allowance f o r the establishment of steady state conditions before the p o t e n t i a l jump. That i s , the curve which deviates 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 that the true passivation current density has been surpassed, but that the complete passivation of the electrode has not taken place. This i s to be an expected phenomenon i n any case where competitive adsorption between two s p e c i f i c a l l y - a d s o r b i n g species e x i s t s , when the potential-ranges of t h e i r adsorption overlap. From the work 121 of B i t t l e s and L i t t a u e r on platinum electrodes, v i r t u a l l y no devia-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 regions of p o l a r i z a t i o n curves obtained i n s i m i l a r media. This was also observed i n the present case f o r Pt, Pt/10 I r , and Pt/20 Ir electrodes. I f , then, the passiva-t i o n parameters for the Pt/5 Ir and Pt/25 Ir anodes are re-determined, taking the point at which the lower overvoltage region deviates from 141 l i n e a r i t y , the i and Ep values f o r these electrodes coincide with those already determined f o r the other electrodes, namely: 2 i = 8-10 mA/cm. P Ep = 1.14 V (S.C.E.) Such r e s u l t s i n d i c a t e that the p o l a r i z a t i o n curves f o r smooth platinum and platinum/iridium a l l o y s have no s u b s t a n t i a l d i f f e r e n c e s , at l e a s t for i r i d i u m contents of up to 25%. These find i n g s are not i n agreement with the observations of F a i t a et ai 9126,146,148 ^ q n o t e d that the presence of even 0.5% Ir i n t h e i r P t / I r alloy-coated titanium electrodes decreased the tendency of the electrodes to passivate. Iridium has been 22 23 shown to be r e v e r s i b l e with respect to oxygen adsorption and removal, * but the i r r e v e r s i b l e anodic formation of a formal oxide of i r i d i u m has 125 been reported. (It seems reasonable to suggest that the presence of i r i d i u m may not retard the passivation of Pt/Ir a l l o y anodes, and may i n f a c t even contribute to i t s passivation. Thus, the presence of a " p o t e n t i a l jump" phenomenon would s t i l l be expected on iridium-contain-ing electrodes.) The mechanism f o r the t r a n s i t i o n from the lower to the upper overvoltage region has yet to be found. Ep i s independent of the substrate (as found i n the present work), chloride content, or pH. Bianchi^''"^ suggested that hypochlorite dicharge may be the cause 149 of passivation. However, Landolt and I b l report that the passivation phenomenon also occurs i n hypochlorite-free s o l u t i o n s . The mechanisms proposed by B i t t l e s and Littauer"'"2"'' and Kuhn and W r i g h t , w h i c h i n -volve a progressive s e r i e s of i r r e v e r s i b l e reactions whereby the 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 clear steady state was attained. This passivation can be attributed to the irreversible build-up of oxides of the noble metal coating. Simultaneously, progressive oxidation of the substrate was observed (which was of importance only in chloride-containing solutions). On cessation of anodic treatment, followed by mild (hydrogen) reduction, the noble metal coating could be "re-activa-ted" inasmuch as the noble metal oxides were removed. The re-activation did not extend to the substrate, however, which remained irreversibly oxidized and contributed to an irreversible loss of active surface area. It is an unwise procedure to u t i l i z e the same coated electrode for deter-mination of polarization curves in electrolytes 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 different electrodes as there are potentials to investigate. This progressive passivation and substrate degradation are suggested as the reasons for the poor reproducibility and inapplicability of polarization curves for coated electrodes which have been reported in the literature (in addition to other factors, 143 such as the diff e r e n c e s In the noble metal depo s i t s ) . I f i t i s assumed that the oxygen evolution r e l a t i o n s h i p (as determined by the p o l a r i z a t i o n curve i n 1M H^SO^) i s also obeyed during the e l e c t r o l y s i s of 1M NaCl s o l u t i o n , and that both oxygen- and chlo r i n e - e v o l u t i o n occur independently of one another (that i s , the p o l a r i z a t i o n curve i n chl o r i d e s o l u t i o n a c t u a l l y represents the sum of the two processes), then the comparative current 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 the current e f f i c i e n c y f o r ch l o r i n e evolution, at any given time of e l e c t r o l y s i s . C alculations show that, a f t e r 5 minutes, the current e f f i c i e n c i e s at 1.3 V, 1.5V, and 1.8V (S.C.E.) are 99.8%, 76%, and 66%, r e s p e c t i v e l y . I t seems probable that the process of passivation i s r e l a t e d to the tendency of the electrode to generate oxygen with increasing p o t e n t i a l s . 5.3 SURFACE AREA CHANGES Surface area changes as a r e s u l t of p o l a r i z a t i o n of coated electrodes have been reported i n the l i t e r a t u r e previously by Weber and 147 P o s i r i l , who used electrodeposited P t - T i electrodes f o r c y c l i c v o l t a -mmetry i n sulphuric a c i d s o l u t i o n . Other workers have reported the more extreme case of coating loss as a r e s u l t of degradation of the titanium substrate as a r e s u l t of prolonged anodic p o l a r i z a t i o n . I t seems l o g i -c a l to expect that, p r i o r to manifestation of t h i s degradation by coat-ing l o s s , there i s a progressive d e t e r i o r a t i o n of the substrate/noble metal contact caused by the growth of some corrosion product of the 144 substrate. I t would also appear l i k e l y that l o s s of e l e c t r i c a l contact of the coating ( e s p e c i a l l y for small or i s o l a t e d "plates") would be a preliminary step i n the degradation. In the case of the present experiments, the "determined" surface area was found to decrease as a r e s u l t of anodic treatment only. The mild reduction treatment was found not to a l t e r the measured sur-face area (at most within the accuracy of the determination i t s e l f ) . This would suggest that i t i s indeed the oxidation of the substrate which i s responsible f o r the l o s s of a c t i v e surface area. Scanning electron microscopic and X-ray d i f f r a c t i o n studies of used electrode surfaces confirm the progressive attack of the substrate, with the appearances of protruberances over the electrode surface (Figure 14), the development of p i t s , and the formation of oxides of titanium being i n d i c a t e d . Substantial losses of surface area were found to occur only with c h l o r i d e solutions, which indicates that i n such media conditions i n surface pores and crevices can lead to rapid d e t e r i o r a t i o n of the substrate. These conditions are probably such phenomena as l o c a l i n -135 creases i n pH due to hydrolysis of chlorine or other products, c h l o r -137 138 127 ine bubble formation, imperfect oxide growth, or depassivation. The spreading of these regions of attack to the substrate beneath the noble metal coating (where c r e v i c e - l i k e conditions would be maintained) would lead to a reduction i n the e l e c t r i c a l contact between the substrate and the coating. I t i s r e a l i z e d that complete undermining of i n d i v i d u a l 145 coating "plates" would be required to break the ele c t r i c a l contact for the consequent reduction in electrochemically active surface area to occur. It may also be possible that the substrate i s oxidized beneath the coating by means of diffusion of oxygen through the coating, but i t i s expected that this process would be substantially slower, and that an "induction time" for the process would exist. This i s unlikely when the slow diffusion rates in solids at ambient temperatures are considered in conjunction with the observations that surface area losses occur rapidly. Growth of an insulating oxide layer over the electrode surface may also be a possible mechanism, although this has not been observed. Irreversible oxidation of the coating metal could also re-sult in a loss of active surface area, insofar as the electrode would be partially filmed with oxygen prior to oxygen deposition, which would result in 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 "re-activation" to the original surface area (indeed, they have no effect at a l l on the determined surface area). Hence, i t i s unlikely that the mechanism of area loss could be attributed to coating oxidation. Electrochemical dissolution 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 dissolution rate of the platinum 146 metals increases 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 n e g l i g i b l e over the short time-spans during which the most s i g n i f i c a n t surface area decreases occur. At the p o t e n t i a l s encountered i n the present experiments, platinum metal d i s s o l u t i o n would be expected to occur at a rate of about 10 ^  mA/cm.2,"'"^ assuming d i r e c t electrochemical s o l u -t i o n as a chloride complex. In view of the short times involved, i t i s further considered u n l i k e l y that coating detachment through under-mining i s achieved. This i s supported by observations that the e l e c t -rode surfaces remain v i s u a l l y unaltered over the time periods for the most s u b s t a n t i a l determined surface area decreases. Another possible mechanism for the reduction i n a c t i v e sur-face area involves the rearrangement of surface platinum atoms, such that the roughness factor of the coating i s decreased. Other workers have reported s u b s t a n t i a l increases i n the surface area of smooth p l a t -inum electrodes with p o t e n t i a l c y c l i n g , and i t has been observed that smoothening of rough electrodes can occur i f the reduction h a l f - c y c l e was a r e l a t i v e l y slow process."'""' Rearrangement of surface platinum 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 indicated. The phenomenon of "age-ing" of dispersed electrodes has been observed, where the a c t i v e surface area diminishes a f t e r prolonged use at elevated temperatures.^'"'"^"' 199 Stonehart has considered that the s i n t e r i n g of platinum c r y s t a l l i t e s could occur at lower temperatures, provided t h e i r p a r t i c l e s i z e was o s u f f i c i e n t l y small (20-200 A). He suggested that the a c t i v a t i o n energy for "surface rearrangement" i s 33 k cal/mole. I t i s not l i k e l y that, 147 f o r the present case, surface atom rearrangement i s r e s p o n s i b l e f o r the s u b s t a n t i a l decreases i n the determined s u r f a c e area. Indeed, the r a t h e r vigourous anodic treatment r e c e i v e d by the coated e l e c t r o d e s would appear to be a cause of e l e c t r o d e roughening r a t h e r than the converse. 148 6. PROPOSALS TOR FUTURE WORK The results presented in this thesis must be considered in the light of future work to be performed on the electrochemical behav-iour of platinum/iridium alloys. The range of experimental work cover-ed has permitted the author to gain familiarity with several methods of electrochemical investigation, and many of the experiments performed may be regarded as indicators of the paths in which the future work must be directed. The results presented for the work with the wire anodes have shown that the behaviour of the alloy electrodes i s identical to that for pure platinum, insofar as the polarization characteristics in IM NaCl; pH 2 are concerned. Further work in this f i e l d should involve the application of: systems where the pH and/or chloride ion activity vary over a wide range, different temperatures, different atmospheres. In this manner, the similarity of the polarization behaviour for the alloy wire electrodes could be confirmed over a wide range of experi-mental conditions. Improvements in c e l l design to f a c i l i t a t e electrol-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 is especially essential for long-term electrolysis where the results are obscured by the pro-duction of different solution species. It i s in the applications of the coated electrodes that 149 most of the future work w i l l be directed. The determination of anodic polarization curves in chloride media should be re-done in order to eliminate the effects of decreasing active surface area (that i s , a fresh electrode should be used for each potential point) and to esta-blish a definite 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 polarization behaviour should be extended to systems of different concentrations (including industrial electrolytes), temperatures, and atmospheres. In addition, since the problems of generation of electrode products i s more severe for these high surface area electrodes, improvements in c e l l design to permit more constant electrolyte concentrations are indicated. The results concerning the determined surface areas of the coated electrodes are the most promising for the application of future work. It has been found that, in chloride-containing electrolytes, the determined surface areas decrease quickly and substantially with potent-iostatic anodization. If the mechanism of surface area loss i s indeed through some method of coating undermining, then i t is of interest to extend the present work to longer time periods in order to determine i f electrode coating loss i s related to the decrease in active surface area. In addition, the observation of the electrode surfaces could be extended for the case of these long-term anodizations in order to esta-blish the changes in 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 in this thesis with respect to the mechanism for 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 anodiza-tion time, which would lead to conclusions concerning the kinetics of passivation of these electrodes. Establishment of the "activation" of the coated electrodes is also of interest. In the present work, the current/time behaviour was used as an indicator of the activity of a given coated electrode during potentiostatic anodization. This method i s questionable, how-ever, due to the possibility of concentration polarization effects and surface blockage with gas bubbles during vigourous gas evolution, which could mask the true electrode surface. That i s , the method employed is not sufficiently sensitive to differences in electrode activity. Of more concern is the possibility of "re-activation" of the coated elect-rodes, which was not achieved in the present work. The extension of the present re-activation work to other methods (including chemical and heat treatment) is indicated. Also, the application of electrochemical procedures which are known to produce increases in the roughness factor of noble metals would be of interest to see i f the decay in active sur-face area can be counteracted. These include anodic/cathodic cycling with variations in the amplitude, frequency, and "shape" of the cycles. 151 The employment of coated electrodes of similar manufacture, hut with different substrates or coating compositions would be of inter-est. For example, tantalum and niobium substrates have been shown to be much more resistant to degradation, and i t i s f e l t that the measure-ment 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. It 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 is any composition-dependence of the polarization or sur-face area characteristics. It is expected that the susceptibility of different coatings to passivation would affect the current/time behaviour for potentiostatic electrolysis, but not necessarily the steady state conditions (i.e., the ultimate form of the polarization curves). The relative a b i l i t i e s of the various platinum metals to sorb oxygen may also be a factor in 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 polarization studies to longer times in order to establish: (a) polarization curves for t > 1 day (b) the determined surface area/time relation 152 (c) anode degradation 2. Expansion of systems to include: (a) a range of pH and ch l o r i d e concentrations (b) i n d u s t r i a l e l e c t r o l y t e s (c) a range of temperatures (d) a range of atmospheres 3. 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A c t a , 17_, 2333 (1972). 163 APPENDIXES APPENDIX I Ele c t r o d e Surface C o n d i t i o n s TABLE 10 Surface c o n d i t i o n s of wire 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 Aspects Remarks 1. soaked i n chromic/ s u l p h u r i c a c i d about 3 minutes surface i m p u r i t i e s removed, surface oxygen f i l m p a r t l y formed. 2. washed i n twice-d i s t i l l e d water t r a c e s of c l e a n i n g s o l u t i o n removed. stored i n twice-d i s t i l l e d water p r i o r to use i n t r o d u c t i o n i n t o c e l l 5. cathodic pretreatment 6. e l e c t r o l y t e changed 7. experiment begun 8. experiment o p e n - c i r c u i t , helium-purging 100 mA f o r 5 minutes o p e n - c i r c u i t , helium-purging smallest anodic current a p p l i e d current increased i n anodic i n c r e -ments p o s s i b l e removal of some sur-face oxygen. sorbed oxygen and i m p u r i t i e s removed, hydrogen f i l m formed. hydrogen f i l m removed (potent-i a l d r i f t s to anodic v a l u e s ) . i n i t i a l e l e c t r o d e c o n d i t i o n r e p r o d u c i b l e . e l e c t r o d e surface c o n d i t i o n changes u n t i l i t i s u l t i m a t e l y completely f i l m e d w i t h oxygen. In 4 Figure 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 wire electrode 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 correspond to those of Table 10.) 165 TABLE 11 Surface c o n d i t i o n s of coated 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 surface area determinations. Procedure Aspects Remarks 1. soaked i n chromic/ s u l p h u r i c a c i d 2. washed i n t w i c e -d i s t i l l e d water about 3 minutes surface i m p u r i t i e s removed, oxygen f i l m p a r t l y formed on c o a t i n g , s u b s t r a t e i r r e v e r s -i b l y o x i d i z e d . traces of c l e a n i n g s o l u t i o n removed. 3. stored i n twice-d i s t i l l e d water p r i o r to use 4. i n t r o d u c t i o n i n t o c e l l (1M H 2S0 4) o p e n - c i r c u i t , helium-purging p o s s i b l e removal of some sur-face oxygen from c o a t i n g . 5. a n o d i z a t i o n 6. r e d u c t i o n 7. helium-purging 8. anodic pulse (surface area determination) 5-10 minutes at 1800-2000 mv (S.C.E.) o p e n - c i r c u i t , hydrogen-purging u n t i l hydrogen p o t e n t i a l a t t a i n e d about 1-2 minutes followed by about 1 minute to permit s o l u t i o n to become quiescent 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 substrate. removal of oxygen from surface of noble metal c o a t i n g , hydro-gen deposited on surface. surface 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 monolayer of oxygen adsorbed 20 seconds on c o a t i n g s u r f a c e . (Steps 5 through 8 may be repeated s e v e r a l times during 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 periods of time. In a d d i t i o n , the anodizations 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 , which r e q u i r e s that, appropriate washing steps be included p r i o r to and a f t e r such treatment.) 166 SC.E 0 -I - 4 0 5 10 t, (minutes) 15 Figure 2 0 . 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 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. (Numbers correspond to those of Table 11.) 167 APPENDIX I I Surface Area Measurement In order to provide a more thorough understanding of the technique employed f o r determining the surface areas of noble-metal e l e c t r o d e s , a d e t a i l e d example i s presented showing the c a l c u l a t i o n of the surface area f o r a coated e l e c t r o d e . The assumptions on which the determination i s based are: 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 the electrode surface and upper atomic l a y e r s w i t h oxygen. The oxygen i n the metal i n t e r i o r i s so s l o w l y removed that i t does not d i f f u s e outward and produce new surface coverage w i t h i n the times employed during the experiment. 2. Hydrogen bubbling removes a l l surface adsorbed oxygen and subsequent helium bubbling 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 surface. 3. Anodic p u l s i n g r e s u l t s i n only 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 corresponding to oxygen d e p o s i t i o n , name-l y double-layer charging and oxygen d e p o s i t i o n . 4. The double-layer charge can be determined d i r e c t l y from the slope of the "double-layer r e g i o n " and thus can be r e a d i l y substracted from the t r a n s i t i o n time measured f o r the oxygen d e p o s i t i o n r e g i o n . 168 5. A monolayer of oxygen i s deposited, w i t h a one-to-one corres-pondence w i t h surface noble metal atoms. 6. The oxygen monolayer charge f o r the coated e l e c t r o d e s i s 426 i / 2 ycoul/cm. . In Figure 21 i s shown a t y p i c a l p o t e n t i a l v s . time curve where an anodic g a l v a n o s t a t i c pulse of 20.5 mA i s a p p l i e d to an e l e c t -rode which was i n i t i a l l y at the hydrogen p o t e n t i a l . In the f i g u r e are represented a h o r i z o n t a l curve at -0.25 V (S.C.E.) which i n d i c a t e s the i n i t i a l p o t e n t i a l of the e l e c t r o d e , the trace formed on a p p l i c a t i o n of the anodic charge (showing hydrogen i o n i z a t i o n , double l a y e r charging, and oxygen monolayer d e p o s i t i o n ) , followed by an upper h o r i z o n t a l sec-t i o n r e presenting molecular oxygen e v o l u t i o n . The t r a n s i t i o n time f o r oxygen e v o l u t i o n , T , i s derived by means of the c o n s t r u c t i o n shown, where d . l . o or, the o v e r a l l t r a n s i t i o n time i n the oxygen r e g i o n (T) i s the sum of the t r a n s i t i o n times f o r double l a y e r charging and oxygen d e p o s i t i o n . With t h i s measured v a l u e , the "determined surface area" of the coated electrode can be r e a d i l y evaluated by means of the equation: A = ( I c h a r g e ) ' ( t ° ) determined .«, .. , 2 426 ycoul/cm. T y p i c a l t r a n s i t i o n times were of the order of 3 seconds or l e s s f o r coated e l e c t r o d e s . - 0.5' 1 1 ' 1 1 1 1 1 i 0 5 10 T (sec.) Figure 21. Representation of a t y p i c a l anodic charge curve i n £ de-aerated IM H^SO^ at 20 C, showing c o n s t r u c t i o n s f o r determining oxygen d e p o s i t i o n charge. 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 the beam angle v a r i e d form 5° to 80°. Peak angles, c a l c u l a t e d d-spacings and the probable species are reported i n Tables 12 and 13 which represent both 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 substrate e l e c t r o d e . 20 d Species 38,3° 2.35 T i 39.7 2.27 Pt 40.1 2.24 T i 46.2 1.97 Pt 52.9 1.73 T i 62.9 1.48 T i 67.6 1.38 Pt 70.6 1.33 T i 75.2 1.25 T i 81.4 1.18 Pt These may more pr o p e r l y be taken to r e f e r to the 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 the two separate species are only 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 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 substrate e l e c t r o d e (3 weeks i n 1M H 2S0 4 at .2 A / f t . 2 and 40°C). 20 d Species 28.0° 3.18 T i 3 0 5 , T i 5 0 9 , T 1 6 0 U , T i 7 0 1 3 , T i 8 0 1 5 , T i 1 0 0 1 9 , T i 0 2 33.0 2.65 T i 2 0 3 , T i 5 0 9 , T i 6 0 n 34.7 2.58 TI, ( T i 6 0 u ) 35.1 2.55 T i 2 0 , T i 2 0 3 , T i ? 0 1 3 , T i 8 0 1 5 , T i 1 Q 0 1 9 35.6 2.52 TiO, T i 2 0 3 , T i 4 0 ? , T i 0 2 37.4 2.39 TiO, T i 3 0 5 , T i 5 0 9 , T i 6 0 i r T i 1 0 0 i g , T i 0 2 38.3 2.35 T i , ( T i 1 Q 0 1 9 ) 38.9 2.31 T 110°19 40.0 2.25 Pt 41.5 2.17 T i 3 0 5 , T i 0 2 46.4 1.95 Pt 53.0 1.73 T i 54.1 1.69 T i 2 0 3 , T i 3 0 5 , T i 0 2 63.0 1.47 T i , ( T i 2 0 , TiO, T i 2 0 , T i 0 2 ) 67.6 1.38 Pt 70.7 1.34 T i 76.2 1.27 T i 77.4 1.26 T i 81.7 ] .17 Pt 86.2 1.13 Pt , ( T i 2 0 , T i 2 0 3 ) * f o r oxide species, incomplete sets of d i f f r a c t i o n l i n e s were found, and peak-heights were obscured by the f a c t that the d i f f e r e n t oxides possessed s i m i l a r d-spacings. A l l oxides must be regarded as " p o s s i b l e " . 

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