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Mossbauer investigation of Fe 57 in Linde L Zeolite Wedd, Robert William James 1969

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MOSSBAUER INVESTIGATION OF Fe LINDE  L ZEOLITE  by ROBERT WILLIAM B.Sc,  IN  University  ^ JAMES WEDD  of V i c t o r i a ,  1967  A T H E S I S SUBMITTED IN P A R T I A L FULFILMENT THE REQUIREMENTS FOR THE DEGREE^ OF MASTER! OF  SCIENCE  in t h e Department of Chemistry  We a c c e p t t h i s the required  t h e s i s as conforming t o  standard  THE U N I V E R S I T Y OF B R I T I S H COLUMBIA May,  1969  OF  In p r e s e n t i n g  this thesis  in p a r t i a l f u l f i l m e n t of the r e q u i r e m e n t s  an a d v a n c e d d e g r e e a t t h e 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 , t h e 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 I further agree that permission f o r s c h o l a r l y p u r p o s e s may by h i s r e p r e s e n t a t i v e s .  C L±~^>  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, C a n a d a Date  , j  U  t  A  l  <H~iA^i  Columbia  1*6?  thesis or  publication  g a i n s h a l l n o t be a l l o w e d w i t h o u t  t  that  Study.  Department  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  permission.  Department of  and  copying of t h i s  be g r a n t e d b y t h e Head o f my  of this thesis for f i n a n c i a l written  for extensive  I agree  for  my  ABSTRACT Two  I n d e p e n d e n t Fe  into Linde  L zeolite.  is the molecular  3+ One  s p e c i e s have been s i m u l t a n e o u s l y  i s an e x c h a n g e d Fe"^ s p e c i e s w h i l e t h e o t h e r  r^,  both  -species the  Fe  +  species FeCI^,  3+ exchanged  introduced  By s i m p I e , o u t g a s s i n g  a t 573°K, t h e  2+ i s r e d u c e d t o Fe  species are converted i s reduced  .  By s w e e p i n g t h e s y s t e m a t 523°K w i t h  t o f e r r o u s a n d , a t 573°K, t h e e x c h a n g e d  t o F e ^ and t h e F e C I ^  i s reduced t o F e C ^ .  Outgassing  l a t t e r s y s t e m a t 573°K o x i d i z e s t h e F e ^ t o Fe^O^ w h i l e t h e F e C ^  remains  intact.  Spectroscopy  These v a l e n c y  changes were examined u s i n g  and, using t h i s technique,  have been d e t e c t e d  i n the Fe^ species.  a Morin  transition  MCssbauer appears t o  -  ii  ACKNOWLEDGEMENTS I would assistance,  sincerely  advice,  l i k e t o t h a n k D r . J . R . Sams f o r h i s  patience  and encouragement  during  t h e t i m e I was  conducting t h i s  i n v e s t i g a t i o n , a l s o t o t h a n k Mr. J.C. S c o t t  of  program and h e l p f u l  h i s computer  discussion.  invaluable  f o r t h e use  Ml  T A B L E OF CONTENTS Page  ABSTRACT  1  ACKNOWLEDGEMENTS  j l  L I S T OF F I G U R E S  iv  L f S T OF T A B L E S  v  INTRODUCTION  I  MOSSBAUER SPECTROSCOPY  8  EXPERIMENTAL  18  R E S U L T S AND D I S C U S S I O N  33  BIBLIOGRAPHY  57  iv  L I S T OF F I G U R E S  Figure 1. 2. 3,  Page E n e r g y l e v e l d i a g r a m s f o r Isomer S h i f t , Q u a d r u p o l e S p l i t t i n g and Zeeman S p l i t t i n g , .MOssbauer C e l I .  12 23  MOssbauer Spectrometer (Schematic),  25  57 4,  Decay Scheme f o r Co  5,  Transducer-Analyser Cycle,  30  6. 7.  S p e c t r a o f o r i g i n a l and o u t g a s s e d s a m p l e s , S p e c t r a o f r e d u c e d s a m p l e and s u b s e q u e n t oxidation, S p e c t r a of samples reduced under v a r y i n g conditions.  42 49  8,  ,  28  51  V  L I S T OF T A B L E S  Table  Page  1,  D-spacings of zeolites treatments,  2,  MOssbauer Parameters of FeCI^-Ether exchanged z e o l i t e .  37  3,  MOssbauer parameters f o r outgassed p e l l e t i z e d samples  39  4,  MOssbauer parameters f o r p e l l e t i z e d samples treated with and outgassed  45  5,  MOssbauer parameters f o r p e l l e t i z e d s a m p l e s r e d u c e d w i t h H^.  47  6,  Quadrupole spectra.  55  interactions  under v a r i o u s  in Zeeman-split  35  I.  INTRODUCTI ON Zeolites  interesting  of a l u m i n o s i I i c a t e s which e x h i b i t  are a class  features.  The m o l e c u l a r s i e v e a c t i o n  known f o r some t i m e .  This process  guest molecules from e n t e r i n g molecules of the right through  shape and s i z e  This "sieving"  c h a n n e l s and t h u s t h e s i z e o f m o l e c u l e s by s i z e  cations  f o r other  altered.  the blocking of  abi'lity  certain  imbibed  of the z e o l i t e  by t h e z e o l i t e s i s  ability  of z e o l i t e s  f o r decationizing  guest molecules.  b e e n more r e c e n t l y  Apart studied  from t h e i r m o l e c u l a r s i e v i n g , as very  useful  I t h a s b e e n shown t h a t a c i d i c  alkylation  of aromatics  those of the f a u j a s i t e rigidity  ' ' .  family  for catalysis  been c a t a l y z e d  cracking, of  ethanol  alcohol  catalysts.  are very a c t i v e  catalysts  in  usually  w h i c h p o s s e s s v e r y open s t r u c t u r e , well-defined  metal  by a c i d i c  Small-pore z e o l i t e s  have  surface.  great  Unusual  a r e f o u n d t o e x i s t when t h e z e o l i t e s a r e b a s e -  e x c h a n g e d t o remove t h e a l k a l i  has a l s o  zeolites  The z e o l i t e s employed a r e  and a c r y s t a I l o g r a p h i c a I l y  opportunities  zeolites  accessible  34  2 the  and v a l u a b l e  c a n be  the zeolites  w h i c h e n l a r g e s t h e z e o l i t e c a v i t i e s a n d makes them much more to  transmitted  By i o n - e x c h a n g i n g t h e  the molecular sieving  A p r o c e s s has been d e v i s e d '  other  of z e o l i t e s i s  on t h e w a l l s  and v a l e n c y o f t h e c a t i o n s . ions,  has been  a r e f r e e l y and c o p i o u s l y  by t h e c a t i o n s w h i c h a r e l o c a t e d  affected  of z e o l i t e s  the channels of the z e o l i t e s while  t h e porous c r y s t a l .  inhibited  involves  a number o f  content. 5  zeolites  have been used  chloride  rearrangement  ,  i n m o l e c u l a r shape  d e h y d r a t i o n and h y d r o h a l o g e n  and HCI t o f o r m e t h y l  The Beckman  reactions^.  has been c a t a l y z e d  selective The  reaction  by z e o l i t e s .  Most of  these  reactions  use  a  rare  earth  exchanged  z e o l i t e as  the  catalyst^. Other  catalytic  conversion,  the  uses  exchange  of  z e o l i t e s are  in ortho  -  and  para  -  ht,  reaction  H  + D  2  2HD,  t  2  8  9  -oxidation  of  --2^3'^*  Very  -also  serve  r^S  as  The metal  give  open  of  or  through  between  Dehydration  of  common o x y g e n  the  tetrahedra  results  in  large  adsorption  remains  intact.  The  for  by  remains  new  valency  i n c l u s i o n of  cations  of  the  of  NO  of  to  f^O  faujasite family  effecting cross-Iinking silicone,  briefly  in  neoprene  arrays  atoms'"^. occupied  described  of  A10^  and  may  the  or  styrene  -  the  of  same o r  as  aluminum  cation.  potassium  -  tetrahedra  molecules.  water  long  of.the  S i 0^  by  of  as  crystalline  dehydration,  molecular  gases  and  as  P r i o r to  c a v i t i e s of  or  the  be  of  are  sodium  electrically  Linde a  the  is usually  other  those  in curing  consisting  sites for  cation  as  z e o l i t e s can  obvious  balanced  disproportionation  eIastamers''''^.  a IuminosiIicates  spaces  , and  carriers for  rubber,  structure  connected  sulfur  z e o l i t e s such  catalyst  •vulcanization butadiene  to  dimensions  the  In  cations  d i f f e r e n t valency  is  zeolites,  can  be  providing  are  structure  structure  synthetic  these  which  crystal  in the  the  this  exchanged the  structure  neutral.  z e o l i t e L,  the  faujasite synthetic  type  of  z e o l i t e used  z e o l i t e which  in t h i s  consists  of  a  investigation  is  tetrahedral  14 arrangement occupy joining  three the  of  truncated  sites. central  octahedra  Site  I  octahedra  is to  .  The  in the the  cations  centre  truncated  of  the  one.  in t h i s  zeolite  hexagonal Site  II  prisms i s on  the  hexagonal f a c e s of t h e t r u n c a t e d o c t a h e d r a and S i t e of t h e c h a n n e l s between t h e c u b e - o c t a h e d r a ,  III  i s on t h e  The l a r g e c a v i t i e s  walls which  a r e f i l l e d w i t h water p r i o r t o d e h y d r a t i o n a r e e l l i p t i c a l l y s h a p e d , approximately shaped,  I2A° i n l e n g t h , e n t e r e d by a p e r t u r e s of d i s t o r t e d c h a i r  12 membered r i n g s which have a d i a m e t e r of a p p r o x i m a t e l y 8A°  Variations t h e AI and Si  i n t h e Si/AI r a t i o may o c c u r from z e o l i t e t o z e o l i t e as  atoms can r e p l a c e each o t h e r 3  sites.  '.  However,  i f Al  +  4  isomorphousIy on t h e  lattice  +  r e p l a c e s Si  then a d d i t i o n a l c a t i o n s  are  15 i n t r o d u c e d s u f f i c i e n t t o b a l a n c e t h e framework change Na  +  + Al  3 +  Z Si  4 +  :  .  Such d o u b l e s u b s t i t u t i o n s cannot be a f f e c t e d d i r e c t l y s i n c e  it  i s the  r e s u l t of d i f f e r i n g c h e m i c a l e n v i r o n m e n t s d u r i n g t h e p r o c e s s of formation.  If  t h e p r o c e s s c o u l d go e n t i r e l y t o the r i g h t , no A l ^  no e x c h a n g e a b l e c a t i o n would remain and a n e u t r a l , p o r o u s , c r y s t a l l i n e s i l i c a would r e s u l t . However,  zeolite and  hydrated  No such p r o d u c t has y e t been p r e p a r e d .  i t has been shown t h a t t h e most s i l i c a  where a c i d s t a b i l i t y  +  is g r e a t e s t ' . 6  r i c h z e o l i t e s are  those  L i n d e z e o l i t e L, which has a Si/AI  17 r a t i o of a p p r o x i m a t e l y 4-5/1 zeolites  i s t h e most a c i d s t a b l e of t h e  ( Z e o l i t e Y has an Si/AI  i t was chosen f o r t h i s work.  synthetic  r a t i o of a p p r o x i m a t e l y 1-3/1) and t h u s  The chemical c o m p o s i t i o n of z e o l i t e L has  been r e p o r t e d ' t o be l.0+0.IM . 0:Al 0,:6.4±0.5S!0 :yrL0 2/n 2 3 2 ' / where M d e s i g n a t e s a t l e a s t one exchangeable c a t i o n , n r e p r e s e n t s o  o  o  the  v a l e n c e of t h a t . c a t ion and y may have a v a l u e r a n g i n g between 0 and 7 depending upon t h e degree of d e h y d r a t i o n .  The c r y s t a l  s t r u c t u r e of  z e o l i t e L u n f o r t u n a t e l y has not been p u b l i s h e d but w i l l be a v a i l a b l e 18 soon  , which w i l l make s t r u c t u r a l p r e d i c t i o n s much more r i g o r o u s .  There a r e s e v e r a l  methods f o r s t u d y i n g t h e s u r f a c e s and s t r u c t u r a l  groupings w i t h i n the z e o l i t e . I n f r a r e d s t u d i e s have been made of t h e r e s i d u a l OH groups which can 19 be seen a f t e r t h e e n c l o s e d water has been d r i v e n o u t by d e h y d r a t i o n The r e s u l t s show t h r e e t y p e s of r e s i d u a l OH g r o u p s , two o f which a r e associated with the a IuminosiIicate the z e o l i t i c c a t i o n .  s t r u c t u r e and t h e t h i r d c o u p l e d t o  P r i o r t o d e h y d r a t i o n , t h e i n f r a r e d spectrum of 20  the absorbed water i n s y n t h e t i c z e o l i t e s has been i n v e s t i g a t e d s p e c t r a show c o n s i d e r a b l e d i f f e r e n c e s  .  The  i n t h e s t a t e of t h e hydroxy I groups  as t h e e x c h a n g e a b l e c a t i o n i s v a r i e d and from one z e o l i t e t o a n o t h e r . A d s o r p t i o n of gases such as N^, 0 , CH^, A r , and Kr on. t h e i n n e r 2  s u r f a c e s of d e c a t i o n i z e d z e o l i t e s w i l l  result  in infrared s h i f t s  in the  OH s t r e t c h i n g f r e q u e n c i e s which can be a t t r i b u t e d t o s l i g h t changes in the s t r u c t u r a l  f e a t u r e s o f t h e z e o l i t e s and s h i f t s i n e l e c t r o n d e n s i t i e s  a t t h e OH s i t e .  A l s o , from a d s o r p t i o n s t u d i e s , one can o b t a i n an  e s t i m a t e o f t h e i n t e r n a l and e x t e r n a l s u r f a c e a r e a s of t h e z e o l i t e s and t h u s c a v i t y volumes can be d e t e r m i n e d .  However, t h e a c c u r a c y of t h e s e  measurements may be l i m i t e d by pore c o n d e n s a t i o n . Low Energy E l e c t r o n D i f f r a c t i o n (LEED)  i s a l s o a t e c h n i q u e which  21 might be used t o study e x t e r i o r s u r f a c e s technique i s that  .  One problem w i t h  this  i t i s sometimes d i f f i c u l t t o a s s e s s whether t h e  resultant d i f f r a c t i o n pattern s t r u c t u r e o r the actual has been shown t h a t  i s due t o adsorbed gases on t h e s u r f a c e  s u r f a c e of t h e a b s o r b e n t .  As an example,  i n t h e H„-Ni s y s t e m , some rearrangement o f t h e  it  5 s u r f a c e Ni atom t a k e s p l a c e on a d s o r p t i o n of \\^. z e o l i t e s u r f a c e would p r o b a b l y  A s i m i l a r study on a  involve considerable  experimental  d i f f i c u l t i e s due t o t h e problem of e l i m i n a t i n g t h e absorbed  material  p r i o r t o e x a m i n a t i o n and t o t h e very porous s t r u c t u r e of t h e z e o l i t e .  22 Electron Microscopy zeolite exterior surfaces.  i s a n o t h e r t o o l which c o u l d be used t o study In p r i n c i p l e , u s i n g t h i s t e c h n i q u e ,  one  c o u l d examine p a r t i c l e s of m o l e c u l a r d i m e n s i o n s which w o u l d , in t h e case of s u r f a c e a d s o r p t i o n i n z e o l i t e s , lead t o an i n s i g h t i n t o t h e p a c k i n g arrangement of an adsorbed m o n o l a y e r . difficulties limit  However, the e x p e r i m e n t a l  i n l o o k i n g a t a monolayer w i t h an e l e c t r o n m i c r o s c o p e might  i t s u s e f u l n e s s as p r e s s u r e s of the o r d e r of  10 '^mm Hg are  E l e c t r o n m i c r o s c o p y p r o b a b l y w o u l d , however, y i e l d v a l u a b l e about a g g r e g a t e s on t h e z e o l i t e  required.  information  surface.  Both LEED and e l e c t r o n m i c r o s c o p y , a l t h o u g h v e r y u s e f u l  for  s t u d y i n g t h e e x t e r i o r s u r f a c e s of z e o l i t e s , p r o v i d e no i n f o r m a t i o n about the  internal  cavities  and t h e changes which o c c u r t h e r e i n w i t h a d s o r p t i o n .  X-ray and e l e c t r o n d i f f r a c t i o n s t u d i e s w i l l  e l u c i d a t e the s t r u c t u r e of  z e o l i t e - pore s i z e s , cage d i m e n s i o n s , and the  l o c a t i o n of the  atoms - but a g a i n , no knowledge r e g a r d i n g t h e changes  the  different  incurred within  the  z e o l i t e w i t h a d s o r p t i o n , both e x t e r i o r and i n t e r i o r , can be g a i n e d u s i n g diffraction  data.  It appeared t h a t Mossbauer s p e c t r o s c o p y might p r o v i d e a tool  valuable  f o r s t u d y i n g the n a t u r e of the c a t i o n s i t e s and changes which o c c u r  when v a r i o u s gases are a d s o r b e d .  By s u i t a b l y exchanging the z e o l i t e so  t h a t t h i s t e c h n i q u e can be employed, one can determine s m a l l changes f i e l d g r a d i e n t s w i t h i n the cages as m o l e c u l e s are a d s o r b e d .  Also,  in one  can look a t s h i f t s  in e l e c t r o n d e n s i t y around t h e Mossbauer n u c l i d e ,  which i f embedded in the a I u m i n o s i I i c a f e framework, can y i e l d  valuable  i n f o r m a t i o n about s m a l l changes in bonding f o r c e s and s t r u c t u r a l  features  which c o u l d a l t e r t h e c a t a l y t i c and m o l e c u l a r s i e v i n g a b i l i t y o f  the  zeolite. The purpose of t h i s work was t o d e v i s e a method f o r p u t t i n g a Mossbauer n u c l i d e i n t o z e o l i t e s , and t o c h a r a c t e r i s e the system o b t a i n e d . In o r d e r t o make subsequent a d s o r p t i o n s t u d i e s (not a p a r t of  this  t h e s i s ) m e a n i n g f u l , one would need t o have i n f o r m a t i o n r e g a r d i n g t h e o x i d a t i o n s t a t e and chemical environment of t h e Mossbauer n u c l i d e , and if possible its  l o c a t i o n i n the z e o l i t e .  Moreover,  it  is important to  d e t e r m i n e whether t h e Mossbauer element r e s i d e s on the e x t e r i o r o r w i t h i n t h e z e o l i t e , and t h e e f f e c t s of such fundamental  surfaces  procedures  as o u t g a s s i n g and d e h y d r a t i o n . The f i r s t s t e p was t o develop a t e c h n i q u e f o r e x c h a n g i n g the original  cations  i n t h e z e o l i t e f o r a s u i t a b l e Mossbauer c a t i o n .  The  57 n u c l i d e chosen f o r t h i s work was Fe  , p r i m a r i l y because i t  i s the most  e a s i l y s t u d i e d of a l l Mossbauer elements and shows a r e a d i l y room t e m p e r a t u r e r e s o n a n c e .  A f t e r a d o p t i n g a method f o r i n s e r t i n g  i n t o t h e a n i o n i c framework o f the z e o l i t e , the type of  a molecular  iron  i t was n e c e s s a r y t o d e t e r m i n e  i r o n s p e c i e s p r e s e n t : whether i t  s p e c i e s merely e n t e r i n g t h e  measurable  is a molecularly  neutral  l a r g e c a v i t i e s arid not r e p l a c i n g any c a t i o n s ;  ion which has t o r e p l a c e an ion in t h e z e o l i t e in o r d e r  t h e system t o remain e l e c t r i c a l l y n e u t r a l ; o r as s e v e r a l  different  for species  F u r t h e r m o r e , t h e o x i d a t i o n s t a t e of the i r o n s p e c i e s p r e s e n t must be determined - t h i s  i s r e a d i l y o b t a i n a b l e from the Mossbauer p a r a m e t e r s .  7  F i n a l l y , the e f f e c t s of outgassing, dehydration,  r e h y d r a t i o n , and  p o s s i b i l i t y o f o x i d i z i n g a n d / o r r e d u c i n g t h e i r o n s p e c i e s had t o investigated.  the be  '  Before discussing • i t - m i g h t be h e l p f u l  the experimental  d e t a i l s and r e s u l t s  obtained,  t o r e v i e w t h e M O s s b a u e r e f f e c t and t h e t y p e s o f  . I n f o r m a t i o n w h i c h c a n be o b t a i n e d f r o m a M o s s b a u e r  spectrum.  I I.  MOSSBAUER  SPECTROSCOPY  M O s s b a u e r S p e c t r o s c o p y o r n u c l e a r gamma r a y a b s o r p t i o n , c o n s i s t s of n u c l e a r gamma r a y e m i s s i o n a n d r e s o n a n t a b s o r p t i o n .  In o r d e r f o r t h i s  p h e n o m e n o n t o o c c u r , t h e gamma r a y e m i s s i o n m u s t be a c c o m p a n i e d r e c o i I o f t h e e m i t t i n g o r t h e absorbing atoms. i s o l a t e d , e x c i t e d nucleus with energy  by no  If o n e c o n s i d e r s an  E , p r i o r t o gamma r a y e m i s s i o n ,  t h e n when t h e gamma r a y i s e m i t t e d , t h e n u c l e u s w i l l  r e c o i l due t o t h e  c o n s e r v a t i o n o f momentum a n d t h e e n e r g y o f t h e gamma r a y w i l l t h a t needed t o e x c i t e another s i m i l a r nucleus o f energy  be l e s s  than  E by t h e e n e r g y o f  recoiI, Er E  2  Er = 2Mc where E ^ i s t h e y-ray.energy, velocity of light. y-ray o f energy  M t h e mass o f t h e n u c l e u s (atom) and c t h e  A l s o , when a n u c l e u s o f e n e r g y  E , t h e n some o f t h e y - r a y e n e r g y ,  r e c o i l i n g t h a t nucleus and t h e r e f o r e t h e energy excitation The half  E i s b o m b a r d e d by a namely E r , w i l l  go i n t o  actually available for  i s n o t s u f f i c i e n t and t h e nucleus w i l l  s t a y i n i t s ground  state.  linewidth o f t h e emission (and absorption) line i s r e l a t e d t o t h e l i f e of t h e nucleus  i n i t s e x c i t e d s t a t e by t h e r e l a t i o n s h i p  r =  0.693tf  where r i s t h e l i n e w i d t h , H P l a n c k s c o n s t a n t and  the half-life ofthe  excited state. The linewidth f o r a t y p i c a l Mossbauer n u c l i d e i s o f t h e _9 o r d e r o f 10 eV w h i c h i s v e r y much s m a l l e r t h a n t h e r e c o i l e n e r g y o f t h e - 3 - 2 nucleus, E r , t y p i c a l l y  10  -10  eV,  Therefore, there will  b e no o v e r l a p  o f t h e e m i s s i o n and a b s o r p t i o n l i n e s whose s e p a r a t i o n i s t h e e n e r g y 2 E r  9 a n d t h u s no MOssbauer e f f e c t w i l l However,  i f t h e e m i t t i n g and a b s o r b i n g n u c l e i a r e each p a r t of a  l a t t i c e and t h e r e c o i l energy the  be observed f o r t h i s m o d e l .  i s l e s s than t h e c h a r a c t e r i s t i c energy of  l a t t i c e v i b r a t i o n , the s i t u a t i o n becomes q u i t e d i f f e r e n t .  lattice  i s a q u a n t i z e d s y s t e m , t h e r e c o i l energy  .excite the  S i n c e the  i s not s u f f i c i e n t t o  l a t t i c e phonon f r e q u e n c i e s and t h u s t h e r e w i l l  be no n u c l e a r  -reco i-l but-mere ly-a-sl-ight-"heating of the - l a t t i c e as a-whole.  Overlap  of t h e e m i s s i o n and a b s o r p t i o n l i n e s now o c c u r s about the c e n t r o i d of --energy E  q  and the MOssbauer e f f e c t  is observed.  If t h e s o u r c e and t h e a b s o r b e r of t h e y - r a d i a t i o n are  i n t h e same  c h e m i c a l e n v i r o n m e n t , t h e energy of the e m i s s i o n and a b s o r p t i o n w i l l t h e same, E . ' o  However,  '  i f the  immediate environments of the s o u r c e and  a b s o r b e r a r e d i f f e r e n t , as in d i f f e r e n t chemical compounds, the energy  levels w i l l  not be e x a c t l y the same.  from t h e s o u r c e w i l l  T h e r e f o r e , the  nuclear  yradiation  not e x c i t e the a b s o r b e r s i n c e the n u c l e a r t r a n s i t i o n s  from which t h e gamma rays o r i g i n a t e w i l l However,  be  s i n c e t h e y-ray  have s l i g h t l y d i f f e r e n t e n e r g i e s .  l i n e w i d t h i s about 10  of v e l o c i t y of  light,  t h e energy can be modulated by moving t h e s o u r c e r e l a t i v e t o the a b s o r b e r a t a few mm/sec. source w i l l  T h i s small range i n energy which i s imparted t o the  be wide enough t o e f f e c t an a b s o r p t i o n i n the a b s o r b e r .  T h e r e f o r e , by measuring t h e amount of e x t r a e n e r g y , p o s i t i v e o r  negative,  r e q u i r e d t o o b t a i n a n u c l e a r t r a n s i t i o n in t h e a b s o r b e r , one can determine a c c u r a t e l y the change in n u c l e a r energy  levels  in the a b s o r b e r  relative  to the source. The minimum o b s e r v a b l e  l i n e w i d t h t h a t can be o b t a i n e d in a  MOssbauer e x p e r i m e n t i s t w i c e the  l i n e w i d t h of the garnma ray e m i s s i o n .  10 T h i s f o l lows geometr i c a M y from e m i s s i o n and a b s o r p t i o n l i n e o v e r l a p . However, t h i s  l i n e w i d t h can be broadened c o n s i d e r a b l y by sample t h i c k n e s s ,  s o u r c e p r e p a r a t i o n and i n s t r u m e n t a l  effects.  The t h r e e parameters of p r i n c i p a l S p e c t r a a r e the  Isomer S h i f t ,  i n t e r e s t o b t a i n a b l e from MGssbauer  Quadrupole S p l i t t i n g , A, and N u c l e a r  6,  Zeeman o r M a g n e t i c H y p e r f i n e S p l i t t i n g .  The energy  l e v e l diagrams f o r  57  Isomer S h i f t and Quadrupole S p l i t t i n g f o r Fe  a r e shown in F i g . l a  while  the N u c l e a r Zeeman S p l i t t i n g i s i l l u s t r a t e d in F i g . l b . The Isomer S h i f t a r i s e s when the s o u r c e of the y r a d i a t i o n and absorber are c h e m i c a l l y d i f f e r e n t .  T h i s i s due t o the f a c t t h a t  the  i n t e r a c t i o n between the n u c l e a r charge and the s - e l e c t r o n d e n s i t y a t n u c l e u s i s d i f f e r e n t f o r d i f f e r e n t compounds.  The s - e l e c t r o n  the  density  has a f i n i t e p r o b a b i l i t y of b e i n g i n the v i c i n i t y of the n u c l e u s whereas t h e p- o r d - e l e c t r o n d e n s i t y f u n c t i o n s have e s s e n t i a l l y no p r o b a b i l i t y of b e i n g a t the n u c l e u s and t h e r e f o r e the  Isomer S h i f t w i l l  not change w i t h  c h a n g i n g p- o r d - e l e c t r o n d e n s i t y ( e x c e p t by s h i e l d i n g e f f e c t s ) .  The  Isomer S h i f t depends upon the f a c t t h a t the e x c i t e d and ground r a d i i s l i g h t l y d i f f e r e n t and t h e r e f o r e t h e i r e l e c t r o s t a t i c e n e r g i e s w i l l  are  differ.  The change in y-ray energy of an e m i t t i n g n u c l e u s may be expressed a s : 3 E = 2rr 2 | ^ | 2 2 _ 2 5 ex gd Y  Z e  ( o )  (  R  1  2  where Z i s t h e change on the n u c l e u s , e . t h e charge of an e l e c t r o n ,  |^(o)|  t h e p r o b a b i l i t y of f i n d i n g an e l e c t r o n a t the n u c l e u s ( R = 0 ) and R R ^ t h e e x c i t e d and ground s t a t e r a d i i r e s p e c t i v e l y . s o u r c e and a b s o r b e r a r e :  The energy c f  and the  FIGURE I  Energy level diagram  showing:  a.  Isomer S h i f t and C u a d r u p o l e  b.  N u c l e a r Zeeman S p l i t t i n g and t h e  Splitting.  thereon of Quadrupole Coupling,  effect  ISOMER  SHIFT  MAGNETIC HYPERFINE SPLITTING  MAGNETIC HYPERFINE SPLITTING 4- Q U A D R U P O L E SPLITTING  13  The Isomer S h i f t  E  s  E  a  (6)  = E  o  = E  o  + fjL 5  +  Z  e  Ze  |1 5  (o)|  |y  2  1y  2  s  U (o)| a  | r  [R  2  -  2  ex  1  2  1  CR  R  -  2  ex  2  gd  R  -1  ]  J  2  gd  i s d e f i n e d as t h e energy d i f f e r e n c e between  the  s o u r c e and a b s o r b e r : Ze  Ck (o)l  2  2  a  - l * < o ) | D CR^ 2  s  -  X  o r more c o n c i s e l y a s : «.«= ^ B  n« (o)i a  2  -  k(oi] 2  s  where A R i s t h e change i n t h e n u c l e a r r a d i u s f o l l o w i n g y - r a y e m i s s i o n , R t h e average n u c l e a r charge r a d i u s and B a c o n s t a n t determined by t h e gamma r a d i a t i o n of t h e p a r t i c u l a r t r a n s i t i o n . e x c i t e d s t a t e of Fe a positive  57  The r a d i u s of t h e 23  i s l e s s than t h a t of t h e ground s t a t e  Isomer S h i f t  first  and t h u s  i n d i c a t e s a r e d u c t i o n of e l e c t r o n i c charge d e n s i t y  a t t h e n u c l e u s of t h e a b s o r b e r . Quadrupole S p l i t t i n g i s a phenomenon dependent upon t h e  interaction  between t h e n u c l e a r q u a d r u p o l e moment and t h e g r a d i e n t of t h e  electric  f i e l d across the nucleus.  i s s e t up  The e l e c t r i c f i e l d g r a d i e n t (EFG)  by c h a r g e s  i n t h e immediate v i c i n i t y of t h e n u c l e u s .  1=0,  have no q u a d r u p o l e moment as i s t h e case in t h e ground s t a t e of  Fe  5*7  V2 .  However, t h e f i r s t e x c i t e d s t a t e of Fe  57  Nuclei with spin  has I = V2  and t h u s has 24  a q u a d r u p o l e moment. & 0  The h a m i l t o n i a n f o r the p e r t i n e n t  = vf/or^ 1 x LV (31 4 X ( 2 I — I) zz z  2  - KI  + I))  + (V  xx  interaction - V  yy  )(I  2  x  where eQ i s t h e q u a d r u p o l e moment of t h e n u c l e u s , I i s the n u c l e a r I. a r e t h e p r o j e c t i o n s of I on t h e r e s p e c t i v e a x e s , V . .  -  is I' )] y 2  spin,  are t h e d i a g o n a l  14 components o f t h e e l e c t r i c f i e l d g r a d i e n t t e n s o r directions.  in the  respective  The EFG can be e x p r e s s e d i n terms of two independent  parameters s i n c e t h e L a p l a c i a n c o n d i t i o n o f v a n i s h i n g charge d e n s i t y  is  s a t i s f i e d at the nucleus: V + V + V =0 xx yy zz .these two independent components can be c o n v e n i e n t l y  expressed:  eq = ..V  n  V - V xx yy V zz  =  (Asymmetry  parameter)  The e i g e n v a l u e s of t h e h a m i l t o n i a n can now be w r i t t e n a s :  V  4TT^T ^  - KI  "3 t!  +  +4>' 2/  ,where m^ i s t h e m a g n e t i c quantum number = I,  I - I,  ...  , I -  57 For Fe  , t h e Quadrupole S p l i t t i n g  between t h e I AE  m  j  * ^  =  a n  ^ "^  = ^ be i ng p a r t i a I Iy Q  ie  r n  j  =  is the d i f f e r e n c e  i n energy  levels  s t a t e s , t h e degeneracy of  ±  I i f t e d by t h e EFG.  Thus t h e QuadrupoIe  S p I i t t i ng,  is A  E  q  =  E  (|)  - E 4>  The t r a n s i t ion probab i I i t i e s  = i e  ±  2  q  ±  t h e a n g u l a r dependence of t h e two l i n e s crystalline orientation effects. s h o u l d average o u t on i n t e g r a t i o n  Q  [ .  and  +  i  .H|j /2 1  -> * — a r e equa I,  but  i s not always t h e same due t o  However,  i n powdered s a m p l e s , a l l  l e a d i n g t o two l i n e s of equal  angles  intensity.  15 The c r y s t a l  symmetry o f t h e m o l e c u l e i n q u e s t i o n can lead one t o  p r e d i c t i o n s about t h e asymmetry p a r a m e t e r , S p l i t t i n g , and v i c e - v e r s a . Quadrupole S p l i t t i n g w i l l nucleus.  n, and thus t o the Quadrupole  If a m o l e c u l e has c u b i c symmetry, be z e r o s i n c e t h e r e w i l l  be no EFG a c r o s s  F o r an a x i a l l y d i s t o r t e d c u b e , such as t h e  d i s t o r t i o n of an o c t a h e d r o n , V '  " S p l i t t i n g reduces t o : n t  Q  = V xx  the the  tetragonal  and t h u s n = 0 so t h e Quadrupole yy  2  Other t y p e s of d i s t o r t i o n s and symmetries have been t r e a t e d  mathematically  .and t h e r e s u l t s compared w i t h t h e e x p e r i m e n t a l l y determined Quadrupole Splittings  25  .  5 As an example, f o r f e r r i c h i g h s p i n compounds, 3d ,  h a l f - f i l l e d d shell  i s s p h e r i c a l l y symmetric and thus the EFG i s s e t up  by t h e arrangement and e l e c t r o n e g a t i v i t i e s of t h e v a r i o u s thus i f a l l the  the  Iigands are  no Quadrupole S p l i t t i n g .  I i g a n d s and  i d e n t i c a l and i n a c u b i c a r r a y , t h e r e w i l l However,  i f the  be  I i g a n d s a r e d i f f e r e n t , o r do 3+  not have c u b i c symmetry, and EFG w i l l and a s m a l l s p l i t t i n g w i l l  result.  be s e t up a c r o s s the Fe However,  nucleus  i f the c e n t r a l metal  ion  is  not surrounded by a s p h e r i c a l s h e l l of v a l e n c e e l e c t r o n s , a d i f f e r e n t situation occurs.  For h i g h s p i n f e r r o u s , 3 d ^ , f o r example the s i x t h d  e l e c t r o n c o n t r i b u t e s much more t o t h e EFG than do the s u r r o u n d i n g I igands and t h e  l a r g e Quadrupole S p l i t t i n g t h a t r e s u l t s i s due p r i m a r i l y t o  that  extra electron. The M a g n e t i c H y p e r f i n e  I n t e r a c t i o n o r Zeeman S p l i t t i n g i s the  r e s u l t of the i n t e r a c t i o n between t h e magnetic d i p o l e moment, u, and t h e magnetic f i e l d ,  H,  The magnetic f i e l d r e s u l t s from the magnetic p r o p e r t i e s  16 of t h e m a t e r i a l  under i n v e s t i g a t i o n .  As an example,  i r o n metal  c l u s t e r s which s e t up a magnetic f i e l d , H, which  leads t o the  splitting.  is:  The h a m i l t o n i a n f o r the i n t e r a c t i o n  m  and t h e energy  m  '  are:  = -uHrrv/I = -qu Hm l n I a  where g i s g y r o m a g n e t i c r a t i o , u  n  In t h e absence of any q u a d r u p o l e  i n t e r a c t i o n , t h e r e a r e 21 + I e q u a l l y spaced  l e v e l s 2gu H I .  This gives r i s e to s i x lines  be o b s e r v e d .  If  in the  The f a c t t h a t t h e r e a r e o n l y  i s d i c t a t e d by t h e s e l e c t i o n r u l e Am^. = 0, * I.  quadrupole i n t e r a c t i o n a s h i f t will  the  n  MOssbauer spectrum as shown i n F i g . l b . lines  l e v e l s , the s p l i t t i n g  l e v e l s b e i n g gv H> and the s p l i t t i n g between  lowest and h i g h e s t  six  r  t h e n u c l e a r magneton and the o t h e r  •.symbols have been d e f i n e d p r e v i o u s l y .  between a d j a c e n t  observed  = -u«H  levels obtained E  forms  When t h e r e  is  in t h e e n e r g i e s o f - t h e d i f f e r e n t m^. s t a t e s  t h e magnetic f i e l d  is p a r a l l e l  f i e l d g r a d i e n t , then the q u a d r u p o l e s p l i t t i n g i s  EFG i s a x i a l ly s y r n m e t r i c ^ ' . 2 6  |f |_| ;  s  I  to the  electric  2  e Qq p r o v i d i n g the  p e r p e n d i c u l a r t o an a x i a l ly 1 2  symmetric EFG, t h e Quadrupole S p l i t t i n g slightly different shifts  i n the  is  states.  e qQ, which i s due t o The M o r i n t r a n s i t i o n p o i n t  i s t h e t e m p e r a t u r e a t which t h e magnetic f i e l d changes from p a r a l l e l p e r p e n d i c u l a r a l i g n m e n t w i t h the EFG,  These two cases are  to  simple  examples of a wide range of p o s s i b l e c o m b i n a t i o n s of magnetic f i e l d s and EFG's which it  lead t o a l a r g e a s s o r t m e n t of d i f f e r e n t s o l u t i o n s .  However,  i s q u i t e s i m p l e t o d e t e r m i n e whether o r not a h y p e r f i n e p a t t e r n  any q u a d r u p o l e c o u p l i n g a s s o c i a t e d w i t h  it.  If the s p l i t t i n g of  has the  17 f i r s t two peaks i s s l i g h t l y d i f f e r e n t from t h e a quadrupole i n t e r a c t i o n .  l a s t two, then t h e r e  is  A M o r i n t r a n s i t i o n temperature can e a s i l y  be  d e t e r m i n e d s i n c e , a t t h a t t e m p e r a t u r e , t h e magnitudes of t h e s p l i t t i n g s of peaks I,  2 and 5, 6 wiI I r e v e r s e .  It  e x p e r i m e n t a l l y d e t e r m i n e whether one i s  i s sometimes d i f f i c u l t t o looking at a quadrupole  interaction  o r e x p e r i m e n t a l e r r o r as the v a l u e s of the quadrupole i n t e r a c t i o n may be very smaII. T h u s , u s i n g MOssbauer S p e c t r o s c o p y , the o x i d a t i o n s t a t e ,  electrical  environment and magnetic p r o p e r t i e s of an i r o n s p e c i e s w i t h i n t h e a IuminosiIicate  framework of t h e z e o l i t e can be a c c u r a t e l y s t u d i e d .  Ml.  EXPERIMENTAL Ion Exchanges - The s o l u t i o n t o the problem of e x c h a n g i n g t h e  p o t a s s i u m in z e o l i t e L f o r i r o n proved t o be r a t h e r d i f f i c u l t .  An  aqueous s o l u t i o n of f e r r i c c h l o r i d e was f i r s t used whose pH was about 2. The a c i d s t a b i l i t y of z e o l i t e L i s not a c c u r a t e l y t o decompose a t pH-2 as i t --the o r d e r of 3.  known but i t  i s thought  i s known t h a t Y z e o l i t e decomposes a t p H ' s of  A s t o c k s o l u t i o n of 400g/1 of F e C l y C h ^ O  i n water was  made up and t h e exchanges were a c c o m p l i s h e d by s t i r r i n g t h i s s o l u t i o n w i t h t h e z e o l i t e u n t i l exchange e q u i l i b r i u m had been e s t a b l i s h e d , u s u a l l y 3-4 h o u r s .  In o r d e r t o put t h e maximum amount of  i r o n i n t o the  z e o l i t e , m u l t i p l e exchanges were done where each exchanged p r o d u c t was t r e a t e d a g a i n w i t h a f r e s h batch of the s t o c k s o l u t i o n . was r e p e a t e d s e v e r a l  This process  t i m e s and the f i n a l p r o d u c t was f i l t e r e d , t h o r o u g h l y  washed w i t h water t o a n e g a t i v e KCNS t e s t f o r F e ^ , and d r i e d a t +  I05°C.  X-ray powder photographs i n d i c a t e d t h a t the p r o d u c t had l o s t i t s z e o l i t e s t r u c t u r e and t h u s the amorphous s i l i c a some f e r r i c s p e c i e s .  r e s i d u e was p r o b a b l y mixed w i t h  However, MOssbauer s p e c t r a were determined f o r  these products. N e x t , an anhydrous FeCI^ exchange was attempted u s i n g d i s t i l l e d methanol as t h e s o l v e n t .  The L z e o l i t e was added t o t h i s s o l u t i o n and  t h e r e s u l t i n g s l u r r e y was a g i t a t e d u s i n g a t e f l o n coated magnetic s t i r r i n g bar.  The p r o d u c t o b t a i n e d a f t e r t h i s m a t e r i a l  for a s u f f i c i e n t l y  had been mixed  long time f o r exchange e q u i l i b r i u m t o be a c h i e v e d ,  was  3+ washed t h o r o u g h l y w i t h d i s t i l l e d methanol u s i n g the KCNS t e s t f o r Fe a s c e r t a i n when washing s h o u l d be t e r m i n a t e d .  to  The X-ray powder photographs  19  •i  ' •  .  •,  • .  on t h e p r o d u c t i n d i c a t e d t h a t the s t r u c t u r a l p r o p e r t i e s of the z e o l i t e had been r e t a i n e d a f t e r t r e a t m e n t . indicated the w i t h i n the  However, t h e MGssbauer s p e c t r a  i r o n was a complexed s u r f a c e s p e c i e s and not e n c l o s e d  zeolite.  The c o n d i t i o n s adopted f i n a l l y f o r i n t r o d u c i n g Fe i n t o t h e z e o l i t e were m i x i n g t h e L z e o l i t e w i t h a s o l u t i o n of FeCI^»6H20 in e t h y l ether.  The mechanics of t h e exchange were t h e same as d e s c r i b e d  p r e v i o u s l y e x c e p t t h a t any f o r e i g n m a t e r i a l was f i l t e r e d o f f from t h e 33% w e i g h t t o volume F e C I y 6 H 2 0 - e t h e r introduced.  m i x t u r e b e f o r e the z e o l i t e was  The exchange was a l l o w e d t o proceed f o r one hour s i n c e  it  was found t h a t exchanges which proceeded f o r a longer p e r i o d of time d i d not a p p r e c i a b l y  i n c r e a s e the amount of  Washing t h e p r e c i p i t a t e w i t h remove any  i r o n i n the z e o l i t e .  Also,  l a r g e volumes of e t h e r d i d not seem t o  i r o n , t h e r e f o r e t h e i r o n was p r o b a b l y i n s i d e t h e z e o l i t e and  not on t h e s u r f a c e .  X-ray powder photographs on the p r o d u c t s showed t h a t  no l o s s of c r y s t a I I i n i t y  had o c c u r r e d .  X-ray Powder Photographs - The samples f o r powder photos were mounted i n 0.5 mm g l a s s c a p i l l a r i e s ,  A sample of  l e n g t h 5 mm i n t h e  c a p i l l a r y was s u f f i c i e n t , so powder photographs r e q u i r e d o n l y v e r y small amounts of m a t e r i a l . a General loading.  E l e c t r i c powder camera of  14.32 cm d i a m e t e r , w i t h  Straumanis  Cu K a , X = 1.54050 A ° , X - i r r a d i a t i o n was u s e d , w i t h a Ni  t o reduce KB r a d i a t i o n . base" cut  The X-ray powder photographs were o b t a i n e d u s i n g  The f i l m used was I I f o r d "I I f e x  filter  safety  i n t o s t r i p s 4 cm x 4 3 . 2 cm f o r use i n the powder camera.  For  f i l m s t o be used f o r i d e n t i f i c a t i o n purposes o n l y , when a s h o r t exposure time was d e s i r e d , a s l i t c o l l i m a t o r was used on the camera, but t o o b t a i n  20 photographs f o r measurements of  line p o s i t i o n s , a pinhole collimator  was u s e d , g i v i n g photographs w i t h s h a r p ,  low a n g l e  lines.  The l i n e s on t h e photographs were indexed on a l i g h t box p r o v i d e d w i t h a meter s t i c k , t o which was a t t a c h e d a measuring s l i d e assembly. . . c o n t a i n i n g a v e r n i e r and a m a g n i f i e d c r o s s - h a i r f o r diffraction  line.  l o c a t i o n of  ( " F i l m I l l u m i n a t o r and measuring d e v i c e " ,  no. 5 2 0 2 2 / 1 , P h i l i p s E l e c t r o n i c s  Inc.).  the  type  The d-spacings were o b t a i n e d  "from t h e s e measurements by a s i m p l e c a l c u l a t i o n , MOssbauer CeI Is - For powder s a m p l e s , t h e z e o l i t e was e v e n l y to a thickness  l e s s t h a n I mm i n a b r a s s c e l l  m y l a r windows.  For room t e m p e r a t u r e  1.25 cm i n d i a m e t e r  packed having  r u n s , t h e c e l l was clamped i n the  correctly aligned position d i r e c t l y  i n the y-ray path u s i n g an aluminum  Holder.  i t was i n s e r t e d  For  l i q u i d nitrogen runs,  c o p p e r c o l d - f i n g e r which was immersed i n a dewar of The n i t r o g e n  level  liquid nitrogen  i n t h e top of a liquid nitrogen.  was m a i n t a i n e d by a S u p e r i o r A i r P r o d u c t s Company  level  controller.  Styrofoam  i n s u l a t i o n was  placed  around the, a b s o r b e r t o m i n i m i z e heat t r a n s f e r t o t h e sample w h i l e t r a n s m i s s i o n of gamma r a d i a t i o n .  .With t h i s arrangement,  t e m p e r a t u r e c o u l d be m a i n t a i n e d a t 80  ±  the  sample  |°K,  The MOssbauer s p e c t r a of p e l l e t i z e d samples were run i n a novel t y p e of c e l l  allowing  rather  a l l o w i n g o u t g a s s i n g , sweeping w i t h d i f f e r e n t gases and  a d s o r p t i o n t o t a k e p l a c e p r i o r t o Mossbauer work. p e l l e t was heated t o t e m p e r a t u r e s  For o u t g a s s i n g , the  up t o 350°C, w h i l e m a i n t a i n i n g a  - 4 - 5 vacuum of  10  - 10  mm Hg.  T h i s p r e s s u r e was a c c o m p l i s h e d u s i n g a  c o m b i n a t i o n of r o t a r y and mercury d i f f u s i o n pumps w i t h the p r e s s u r e s e s t i m a t e d on a MacLeod gauge.  The t e m p e r a t u r e was c o n t r o l l e d a t  the  being  21 r e q u i r e d p o i n t w i t h a C o l e Parmer P r o p o r t i o N u I I 1300 s e r i e s c o n t r o l Ier connected t o a furnace c o n s t r u c t e d was a c c u r a t e t o  4  I0°C.  constantine Sim-Ply-Trol  in t h i s department.  This  arrangement  The t e m p e r a t u r e was measured u s i n g an pyrometer.  iron  For " s w e e p i n g " , o r p a s s i n g gases  •'such as 0^ o r F^ o v e r the z e o l i t e , t h e gas was f i r s t d r i e d by b u b b l i n g t h r o u g h two v e s s e l s c o n t a i n i n g 36N ^ S O ^ and s u b s e q u e n t l y  introduced  . i n t o t h e MCssbauer c e l l , t h e t e m p e r a t u r e b e i n g m a i n t a i n e d a t ^ r e q u i r e d p o i n t u s i n g t h e s e t up d e s c r i b e d above. - s t u d i e s were not done i n t h i s work, t h e c e l l  any  Although adsorption  and vacuum system c o u l d  --easily be adapted t o a c c o m p l i s h such measurements as t h e equipment i n c l u d e d a mercury manometer f o r measuring h i g h e r gas p r e s s u r e s . s c h e m a t i c diagram of t h e c e l l  used i s i l l u s t r a t e d  in F i g . 2 .  A  When o u t -  g a s s i n g o r " s w e e p i n g " , t h e p e l l e t h o l d e r and p e l l e t were jn t h e p o s i t i o n .shown, t h e p e l l e t h e l d t o i t s h o l d e r w i t h Pt g a u z e , and the sidearms of the c e l l  b e i n g used e i t h e r f o r o u t g a s s i n g o r f o r p e r m i t t i n g a f l o w of  over the p e l l e t .  When Mossbauer s p e c t r a were r u n , t h e c e l l was  so t h a t the p e l l e t f i t t e d  gas  inverted  i n t o a s l o t between t h e 0 . 0 1 0 " Be windows,  a l l o w i n g t r a n s m i s s i o n of y r a d i a t i o n t h r o u g h the a b s o r b e r so t h a t  the  s p e c t r a c o u l d be run w i t h o u t removing t h e p e l l e t from i t s c e l l .  The  p e l l e t s were made u s i n g a C a r v e r L a b o r a t o r y P r e s s Model B and t y p i c a l p r e s s u r e s were of t h e o r d e r of  15,000 p s i .  They were then shaped t o  fit  the holder using a razor blade, MBssbauer Spectrometer a spectrometer,  - The MOssbauer s p e c t r a were o b t a i n e d u s i n g  a b l o c k diagram of which i s shown in F i g , 3 .  of t h e s p e c t r o m e t e r c o n s i s t of a v e l o c i t y t r a n s d u c e r  The  elements  (TMC Model 305)  which  i s e s s e n t i a l l y a loud-speaker c o i l t h a t p r o v i d e s D o p p l e r M o d u l a t i o n t o  the  22  FIGURE 2 Diagram o f t h e MOssbauer Cel I used f o r p e l l e t i z e d s a m p l e s .  .23  PYREX  y STOPCOCK  STOPCOCK  Pt" G A U G E ZEOLITE  PELLET  24  FIGURE 3 B l o c k diagram showing e s s e n t i components of t h e MOssbauer Spectrometer.  25  MOSSBAUER  SPECTROMETER  SCOPE  TYPEWRITER  400  WORD  MEMORY  I  SYNC. SIGNAL  P U L S E  ANALYSER  WAVE  I I I  FORM  I I I  GENERATOR  7 ke V  SOURCE TRANSDUCER  (  ii—hT  0 ABSORBER  0  DETECTOR  (4-4 ke V  26 ?  '  '  -  •  •  . . .  '  •  -  57 y - r a y energy e m i t t e d from t h e r a d i o a c t i v e Co  source.  The decay  57 scheme f o r Co  i s given in F i g . 4 .  The s o u r c e , which i s embedded i n a  Pd m a t r i x i s mounted on t h e t r a n s d u c e r which i s d r i v e n a t a c c e l e r a t i o n by a TMC Model 306 wave form g e n e r a t o r  constant  (WFG).  Once per  c y c l e t h e WFG s u p p I i e s an 8 - v o l t s y n c h r o n i z a t i o n s i g n a l f o r phase l o c k i n g t h e RIDL Model 24-2 400 word m u l t i c h a n n e l a n a l y z e r (MCA).  This consists  •of t r i g g e r i n g t h e MCA a t a s p e c i f i c p o i n t t o s t a r t r e c e i v i n g c o u n t s from •the s i n g l e channel a n a l y z e r ( N u c l e a r C h i c a g o Model 3 3 - 1 5 ) , which ^ d i s c r i m i n a t e s p u l s e s from t h e p r o p o r t i o n a l c o u n t e r ( R e u t e r - S t o k e s Model -RSG-30A) a l l o w i n g o n l y t h o s e f a l l i n g w i t h i n a c e r t a i n energy range t o be 57 passed on t o t h e MCA. selected.  In t h i s c a s e , the  Thus in each of  14.4 KeV y r a y of Fe  i t s 400 c h a n n e l s , t h e MCA s t o r e s the number  of counts corresponding t o a p a r t i c u l a r v e l o c i t y motion of t h e t r a n s d u c e r . memory u s i n g a t y p e w r i t e r  was  This  increment from t h e  i n f o r m a t i o n can be tapped from the  (IBM Model 44-16) and the p r o g r e s s of  s p e c t r u m can be viewed v i a an o s c i l l o s c o p e ( H e w l e t t - P a c k a r d  the  Model  The r a t e a t which t h e MCA sweeps t h r o u g h each of the 400 c h a n n e l s d e t e r m i n e d by a N u c l e a r C h i c a g o Model 54-6 t i m e base g e n e r a t o r Fig.5  I20B). is  (TBG).  i l l u s t r a t e s the c y c l e g i v e n the t r a n s d u c e r by t h e WFG.  The  t i m e of t h e complete c y c l e can be a l t e r e d , and by a s u i t a b l e c h o i c e of capacitors  i n t h e WFG, one can s e t t h e p e r i o d t o be s l i g h t l y longer t h e n  t h e t i m e t h a t t h e MCA t a k e s t o sweep t h r o u g h i t s 400 c h a n n e l s w h i c h , t h e TBG i s s e t f o r 200 u s e c / c h a n n e I , count a t  100 u s e c / c h a n n e I ,  i s 8 2 . 5 msec.  If t h e TBG i s s e t t o  then the MCA w i l l o n l y s t o r e counts f o r a  l e s s than h a l f the wave form g i v e n by the WFG. next t r i g g e r i n g b e f o r e i t b e g i n s t o s t o r e a g a i n .  if  little  It w i l l then w a i t f o r the However,  i f t h e TBG  is  27  FIGURE 4 Diagram showing t h e decay  scheme  57 f o r Co  and t h e a s s o c i a t e d  parameters.  28  CO  7/2  5  270d  7  E.C. 99.84  %  I 5/2  137 k e V  123 k e V  9 %  91  %  5/2 !4.4keV  MOSSBAUER  y-  RAY  1/2 STABLE  HALF-LIFE ISOTOPIC  ( t I/2)  Fe  O F FIRST EXCITED  ABUNDANCE  LINEWIDTH  ( D  =  MINIMUM O B S E R V A B L E E r = 1.9567 x  IO eV - 3  57  (IA)  8  S T A T E = 9 . 7 7 x IO S E C .  =  2 . 19 4.6697  x IO  LINEWIDTH (Wo) = O.I9427  %  12,  ke V  MM/SEC  29  FIGURE 5 Diagram  showing  how t h e MCA  sweeps through I t s c h a n n e l s as d i c t a t e d by t h e t r i g g e r i n g the  WFG.  from  30  VELOCITY  T I M E  O  SYNC. SIGNAL  MEMORY CHANNEL NUMBER  COUNTS  O  +v  4-V  O  -v  C H A N N E L N6.= V E L O C I T Y  >-  31 s e t f o r 200 u s e c / c h a n n e I ,  t h e n n e a r l y the whole waveform i s used f o r  s t o r a g e of d a t a and two s p e c t r a per waveform a r e o b t a i n e d , each a m i r r o r image of t h e o t h e r .  Each spectrum w i l l t h u s c o n s i s t of 200 r a t h e r  400 channels^and t h u s t h e r e s o l u t i o n w i l l rate i s increased c o n s i d e r a b l y .  than  be c u t i n h a l f but the c o u n t i n g  The i n s t r u m e n t was o p e r a t e d i n t h e  l a t t e r f a s h i o n d u r i n g t h e c o u r s e of t h i s  investigation. •  N u c l e a r y - r a y c o u n t i n g i s a s t a t i s t i c a l p r o c e s s and t h u s a l l  peaks  o b s e r v e d s h o u l d have normal d i s t r i b u t i o n s and l o r e n z i a n l i n e s h a p e s . Thus c o n s i d e r a b l e t i m e has t o be a l l o w e d t o b u i l d up s u f f i c i e n t i n o r d e r t o o b t a i n a c c u r a t e v a l u e s f o r the peak p o s i t i o n s . t h e s p e c t r a r e q u i r e d 6-12 . h r s , t o become w e l l s q u a r e s computer program w r i t t e n by J , C ,  resolved.  statistics  In t h i s work, Using a  least-  S c o t t , t h e data were f i t t e d on  an IBM 7044 o r 360/67 computer t o l o r e n z i a n c u r v e s , and the peak p o s i t i o n s , isomer s h i f t s and q u a d r u p o l e s p I i f t i n g s g i v e n i n terms of channel In o r d e r t o g e t energy e q u i v a l e n t s f o r the channel s c a l e used has t o be c a l i b r a t e d a t f r e q u e n t  numbers, the  numbers.  velocity  i n t e r v a l s between r u n s .  s c a l e was c a l i b r a t e d a g a i n s t the q u a d r u p o l e s p l i t t i n g of an N . B . S . sodium n i t r o p r u s s i d e s i n g l e c r y s t a l 1.726 mm/sec.  in terms of mm/sec ch which  numbers g i v e s the energy which c a n , i f  be c o n v e r t e d t o eV u s i n g a c o n v e r s i o n f a c t o r , which f o r Fe -7  I mm/sec = 4 . 3 x 10 E r r o r s - The e r r o r s  standard  a b s o r b e r which has the v a l u e of  This gives a c a l i b r a t i o n factor  when m u l t i p l i e d by channel  The  57  ,  necessary,  is:  eV  i n c u r r e d i n a Mossbauer e x p e r i m e n t a r e p r i m a r i l y  statistical  in nature.  Thus i t  i s d i f f i c u l t t o quote a b s o l u t e e r r o r s as  such and i t  i s found much mere c o n v e n i e n t t o s e t up c o n f i d e n c e  These are e s t i m a t e d on the b a s i s of the s t a n d a r d d e v i a t i o n s of  limits. the  32 computer f i t and t h e r e p r o d u c i b i l i t y of the s p e c t r u m . limit  in these experiments is  ±  The c o n f i d e n c e  0,02 mm/sec f o r t h e isomer s h i f t and  q u a d r u p o l e s p l i t t i n g and a s l i g h t l y  larger v a r i a t i o n  in the  linewidths.  IV.  RESULTS AND DISCUSSION The breakdown i n s t r u c t u r e of t h e aqueous f e r r i c exchange was  undoubtedly due t o the low pH of t h e exchange s o l u t i o n .  This  lattice.  breakdown has a l s o been observed f o r attempted f e r r o u s and p r o l o n g e d 27 f e r r i c exchanges by o t h e r workers Al  3 +  .  S o l u t i o n s of  i n t t h e f o l l o w i n g way'^:  r  i  0 \ _ S i  /  low pH wiI I remove  1  0 — Al  I  0  H  0  / + \ S i — + 4 h L 0 -^Si  \  OH  /  3  HO  H  [  / Si-  \  3+ +AI  2  9  so t h a t t h e a I u m i n o s i I i c a t e  framework i s e s s e n t i a l l y d e s t r o y e d .  MGssbauer s p e c t r a on t h e r e s u l t i n g p r o d u c t show a magnetic pattern at  +4H„0  hyperfine  l i q u i d n i t r o g e n t e m p e r a t u r e and both the method of  and t h e MQssbauer s p e c t r a o b t a i n e d a r e  The  preparation  i n d i c a t i v e of 3 - FeOOH which  is  28 p r o b a b l y mixed w i t h t h e s i l i c a  residue  .  The method of  g - FeOOH from i t s MOssbauer h y p e r f i n e p a t t e r s  identifying this  i s by making a comparison  w i t h t h a t of a known s p e c i e s , s i n c e both the o r d e r i n g temperature and magnitude of t h e Zeeman s p l i t t i n g a r e c h a r a c t e r i s t i c of a g i v e n compound. For t h e exchanges which were attempted i n m e t h a n o l , a l t h o u g h the X-ray powder photographs i n d i c a t e d no s t r u c t u r a l were not e n c o u r a g i n g .  breakdown, t h e Mbssbauer  results  A h y p e r f i n e p a t t e r n of s i x l i n e s o r more was  o b s e r v e d a t both 80°K and 298°K.  T h i s was i n d i c a t i v e c f  large  clusters  34 (approximately  IO  3  atoms o r more) of some s p e c i e s of  magnetically ordered.  i r o n which was  From an e x a m i n a t i o n of the h y p e r f i n e p a t t e r n ,  s p e c i e s appeared t o be Fe20^.  the  P a r t i c l e s i z e s of the d i m e n s i o n s r e q u i r e d  t o o b t a i n a h y p e r f i n e p a t t e r n a r e t o o l a r g e t o be enveloped by the i n t e r n a l c a v i t i e s of t h e z e o l i t e s i n c e the maximum c a v i t y s i z e i s approx14 imately  I3A°  and t h u s t h e o n l y p o s s i b i l i t y i s t h a t the m a t e r i a l  d e p o s i t e d on t h e e x t e r i o r s u r f a c e s .  A t e s t was made of t h i s  was  hypothesis  by e v a p o r a t i n g a FeCI^-methanoI s o l u t i o n t o d r y n e s s and r u n n i n g the MOssbauer spectrum on the r e s u l t i n g s o l i d r e s i d u e . that,  The spectrum showed  i n d e e d , t h i s was t h e case and the s u r f a c e m a t e r i a l was some form of  Fe^Oj o r a m i x t u r e of the d i f f e r e n t  varieties.  The e t h e r exchanges were much more i n t e r e s t i n g as the i n d i c a t e d t h a t the lattice.  results  i r o n had gone i n t o the z e o l i t e w i t h o u t r u p t u r i n g t h e  The r e s u l t s of X-ray powder photography are g i v e n i n T a b l e I  a l o n g w i t h the v a l u e s o b t a i n e d f o r the z e o l i t e p r i o r t o any  treatment,  t h o s e r e p o r t e d f o r z e o l i t e L.J and the v a l u e s f o r a s u b s e q u e n t l y p e l l e t which w i l l  be d i s c u s s e d l a t e r .  treated  Most of t h e d - s p a c i n g s l i e w i t h i n  the range of t h e r e p o r t e d v a l u e s and any d i s c r e p a n c i e s o r o m i s s i o n s can be accounted f o r by the f a c t t h a t t h e photographs were taken u s i n g a p i n - h o l e c o l l i m a t o r and the indistinct. 295°K w i t h  l i n e s were thus r a t h e r weak and sometimes  The Mb'ssbauer r e s u l t s showed d o u b l e t s a t both 80°K and Isomer S h i f t s c h a r a c t e r i s t i c of f e r r i c s p e c i e s .  Isomer S h i f t s f o r the t h r e e t h i s work a r e F e  3 +  (Typical  i r o n o x i d a t i o n s t a t e s which were r e l e v a n t  - 0.6 mm/sec,• F e  r e l a t i v e t o sodium n i t r o p r u s s i d e ) ,  2 +  to  - 1.2 mrn/sec, and Fe^ - 0.26 mm/sec, The observed l i n e s were n o n - I o r e n z i a n  a t both t e m p e r a t u r e s and very broad a t 80°K; much broader than would be  35 TABLE  Literature VaIues 16.1  Neat Z e o l i t e  Fe-L-Zeolite  15.77  Treated Pel l e t  L-Zeolite  15.77  ±  .3  7.52  ±  .04  7,47  6.00  ±  .02  6,01  6,04  5.98  .03  4.58  4.58  4.56  4.57  15.77  I  7.42  4.35  4.35  ±  .04  4.38  3.91  ±  .02  3.90  3.90  3.88  3.65  3.63  3,63  3.47  ±  .02  3.45  3.47'  3.46  3.28  ±  .02  3.27  3.26  3.27  3.17  ±  .01  3.16  3.17  3.17  3.07  ±  .01  3.06  3.06  3.05  2.91  ±  .01  2.90  2.90  2.89  2.65  ±  .01  2.64  2.65  2.64  2.46  ±  .01  2.48  2.49  2.42  ±  .01  2.43  2.40  2.19  ±  ,01  2.19  See R e f e r e n c e  -.  i.  '2,19  2,18  36 e x p e c t e d merely from v i b r a t i o n s caused by l i q u i d n i t r o g e n b o i l - o f f .  The  e x p l a n a t i o n f o r t h i s i s t h a t t h e r e a r e p r o b a b l y two f e r r i c s p e c i e s i n t h e z e o l i t e with nearly  identical  Isomer S h i f t s and Quadrupole S p l i t t i n g s ,  which makes r e s o l u t i o n of t h e f o u r peaks i m p o s s i b l e .  However, the  fact  t h a t t h e peaks a r e much broader a t 80°K than a t 295°K s u g g e s t s t h a t s p e c t r a run a t even lower t e m p e r a t u r e s might e n a b l e r e s o l u t i o n of  the  peaks. The MOssbauer parameters f o r t h r e e  independent p r e p a r a t i o n s of  e t h e r exchanges u s i n g i d e n t i c a l c o n d i t i o n s a r e e x p e r i m e n t a l e r r o r here i s about * 0.04 mm/sec.  l i s t e d i n T a b l e 2.  the The  The l a r g e r e r r o r  in  t h e s e s p e c t r a i s due t o the poor q u a l i t y of the computer f i t caused by non-lorenzian l i n e shapes.  The v a l u e s f o r 6 and A l i e w i t h i n the  of e x p e r i m e n t a l e r r o r but the % e f f e c t ,  e,  range  i s g r e a t e r a t 295°K than 80°K.  T h i s i s r a t h e r d i f f i c u l t t o e x p l a i n as one would e x p e c t the r e v e r s e on the b a s i s of r e c o i l - f r e e f r a c t i o n arguments.  Some l o w e r i n g of c i s  e x p e c t e d due t o s t y r o f o a m i n s u l a t i o n , d i s t a n c e of s o u r c e from a b s o r b e r and " i c i n g up" which c o u l d lead t o b a c k - s c a t t e r , but the d i m i n u t i o n here i s g r e a t e r than u s u a l l y o b s e r v e d .  It s h o u l d be noted the v a r i a t i o n  in  e between d i f f e r e n t samples i s expected s i n c e e changes w i t h sample t h i c k n e s s as w e l l and t h u s comparison of e from one sample t o a n o t h e r  is  not v e r y s i g n i f i c a n t . In o r d e r t o d e t e r m i n e whether the s p e c t r a o b t a i n e d were due t o FeC I .j* 6H20-ether complexes on the s u r f a c e of the z e o l i t e , a p o r t i o n of  the  33$ weight t o volume s o l u t i o n was e v a p o r a t e d t o d r y n e s s and t h e MOssbauer spectrum was run on t h e r e s i d u e . in Table 2.  The parameters o b t a i n e d a r e a l s o g i v e n  They s u g g e s t t h a t the s p e c i e s b e i n g observed by MBssbauer  37 TABLE  Trial  No.  1.  Washed w i t h 20 - 100 ml p o r t i o n s of ether 2.  3.  Evap. E t h e r residue  a  Temp  (°K)  6  2  a  295  0.64  0.76  0.52  8,18  80  0.74  0.92  0.69  7.02  295.  0.65  0,74  0,47  14.18  80  0,74  0,91  0,69  4.93  295  0.65  0,76  0.48  1 1.26  80  0.73  0.90  0.76  9.17  295  0.64  0,77  0.50  10.15  80  0.72  0.85  0.67  10.13  295  0.66  0.93  0.46 '  9.90  80  0.76  0.90  0.51  5.80  AI I v a I u e s i n mm/sec and r e l a t i v e  .  t o sodium n i t r o p r u s s i d e  38 s p e c t r o s c o p y i s not an i r o n c h l o r i d e e t h e r r e s i d u e but by no means out the p o s s i b i l i t y .  rule  A l s o repeated washing t r e a t m e n t s of 20 r- 100 ml  p o r t i o n s of e t h e r d i d not seem t o remove any FeCI^ o r change t h e MCssbauer parameters as shown in T a b l e 2 which a l s o s u g g e s t s t h a t the s p e c i e s not on t h e s u r f a c e .  The computer f i t t o the l i n e s , however,  is  i s much  b e t t e r a t both 80°K and 295°K which s u g g e s t s o n l y one r a t h e r than two f e r r i c species.  F u r t h e r e v i d e n c e which lends i t s e l f t o t h e p r o p o s a l of  two f e r r i c s p e c i e s Pascher,  is the a n a l y t i c a l  d a t a , performed b y . D r s . F. & E.  Bonn, Germany, which g i v e an i r o n t o c h l o r i n e r a t i o of  2:3.  T h i s s u g g e s t s t h a t some of t h e i r o n has e n t e r e d the z e o l i t e as an i o n , Fe  3 +  ,  and some as the m o l e c u l a r s p e c i e s FeCI^.  No carbon was found  which i n d i c a t e s FeCI^ o r F e C l y X H ^ O r a t h e r than an FeCI^-ether complex in the z e o l i t e .  A comparison w i t h r e s u l t s o b t a i n e d by M o r i c e and Rees  ( t o be d i s c u s s e d more f u l l y  27  l a t e r ) shows t h a t some of t h e i r o n has e n t e r e d  3+ as Fe  , exchanging t h e c a t i o n .  It would t h e r e f o r e a p p e a r , on t h e  of t h i s e v i d e n c e t h a t a p p r o x i m a t e l y 50% of the p r e s e n t as F e  3 +  Iron in t h e z e o l i t e  and 50$ as FeCI-^; the p o s i t i o n of the  latter  As t h i s system w i t h two f e r r i c s p e c i e s appeared  interesting, all  is  i s thought  t o be i n s i d e the pores of t h e z e o l i t e but a t t h i s s t a g e , t h i s important.  basis  i s not v e r y rather  f u r t h e r work was c a r r i e d out on i t .  The sample was f i r s t p e l l e t i z e d by t h e method p r e v i o u s l y d e s c r i b e d and s u b s e q u e n t l y run in i t s c e l l on t h e MQssbauer s p e c t r o m e t e r .  Typical  r e s u l t s from a l a r g e number of e x p e r i m e n t s are g i v e n in T a b l e 3. i n s p e c t i o n of t h i s t a b l e and comparison w i t h T a b l e 2, v a l u e s of t h e  it  By  is clear  isomer s h i f t and' t h e q u a d r u p o l e s p l i t t i n g f o r the  have not changed a p p r e c i a b l y a t 295°K from t h o s e of the powder  that  pellet (assuming  39  TABLE  Temp ( ° K )  Treatment  6  a  3  A  a  r  a  e {%)  295  0.63  0,78  0,65  6.0  80  0.66  1.13  0,83  3.6  0/g @ 423°K  295  0.59  0,86  0.60  5.5  80  0,69  1 ,01  0,65  8.1  0/g § 498°K  295  0.59  1.07  0.65  5.8  80  0.68  1.17  0.81  6.3  295 295  0.61 1.27  0.99 2.31  295  0.63  0,84  None  b  0/g @ 573°K  I&2 I&3  Exposed t o atmosphere  In mm/sec and r e l a t i v e t o sodium n i t r o p r u s s i d e b  Outgassed  '  -  •  _  0.55  4.8 1.4 7.1  40 an e x p e r i m e n t a l e r r o r of slightly  ±  0,04 mm/sec), a l t h o u g h the  larger f o r the p e l l e t ,  linewidths are  However, a t 80°K, both 6 and A v a l u e s  a r e d i f f e r e n t from t h o s e f o r the powder.  Thermal motions which g i v e  rise  t o the second o r d e r Doppler S h i f t c o u l d e x p l a i n the change in 6.  The  i n c r e a s e i n A and r f o r t h e p e l l e t c o u l d be a t t r i b u t e d t o the f a c t  that  under p r e s s u r e , as i n the p e l l e t i z e d s a m p l e s , the water i s squeezed o u t thus  l o w e r i n g the  l o c a l symmetry about the i r o n which i n c r e a s e s A.  The r e s u l t s of v a r i o u s o u t g a s s i n g t r e a t m e n t s a r e a l s o g i v e n T a b l e 3.  The isomer s h i f t  in  i s e s s e n t i a l l y c o n s t a n t and independent of  o u t g a s s i n g t e m p e r a t u r e , but the quadrupole s p l i t t i n g i n c r e a s e s as the outgassing temperature  increases.  water c a u s i n g a d e c r e a s e E,  in l o c a l symmetry about the i r o n .  f o r o u t g a s s e d samples i s  a t 80°K, but t h e r e v e r s e  l o s s of  The % e f f e c t ,  l e s s f o r s p e c t r a run a t 298°K than t h o s e run  i s t r u e f o r powdered'samp I e s ,  of water.accompanied by an i n c r e a s e with  T h i s a g a i n i s p r o b a b l y due t o  Thus the removal  in e must be due t o the f a c t t h a t  l i t t l e o r no water cannot " i c e up" i n t e r n a l l y but the u n t r e a t e d  samples samples  can " i c e up" t h u s r e d u c i n g y-ray t r a n s m i s s i o n and i n c r e a s i n g b a c k - s c a t t e r i n g . The appearance of t h r e e as shown in F i g . 6 b i s r a t h e r  l i n e s when the sample i s outgassed a t 573°K interesting.  r e a d i n g from l e f t t o r i g h t , c l e a r l y  The asymmetry of peaks I and 2,  i n d i c a t e t h a t the o t h e r h a l f of  the  q u a d r u p o l e d o u b l e t a s s o c i a t e d w i t h peak 3 i s embedded in peak I,  Thus,  on t h e b a s i s of t h i s , one can a t t r i b u t e peaks I and 2 t o a f e r r i c  species  and I and 3 t o a f e r r o u s s p e c i e s , - t h i s parameters g i v e n i n T a b l e 3.  i s s u b s t a n t i a t e d by the Mossbauer  The appearance of a f e r r o u s peak when the  system i s o u t g a s s e d a t 573°K i s r a t h e r hard t o e x p l a i n .  M o r i c e and Rees  who have o b s e r v e d s i m i l a r r e d u c t i o n s w i t h exchanged Fe  i n Linde X and Y  57  27  41  FIGURE  MOssbauer s p e c t r a  of:  a.  Original  b.  Outgassed sample Fe  + Fe  Fe^  6  .  +  species. containing  43 z e o l i t e s and C l i n o p t i l o l i t e , have p o s t u l a t e d a mechanism; X - 0~ + F e 2 Fe  2 +  3 +  ( H 0 ) -»• X - OH + F e  - 0H^*>e  A l t h o u g h t h i s mechanism seems r a t h e r  2  2 +  + H 0 + i;0 2  involved,  it  2  The r e - o x i d a t i o n t o  i s o b s e r v e d in t h e  work and i n t h e work done by M o r i c e and R e e s .  As w e l l  2+ t h e Fe  - OH  i s hard t o c r i t i c i z e  w i t h o u t a more i n t i m a t e knowledge of t h e s y s t e m . f e r r i c r e q u i r e s both oxygen and water which  2 +  present  as e l i m i n a t i n g  peak on exposure t o t h e atmosphere,  parameters,  it  •  i s seen t h a t  the  6 and A, have gone back t o t h e i r p r e v i o u s v a l u e s p r i o r t o  o u t g a s s i n g - t h u s t h e r e - s o r p t i o n of water must c r e a t e an environment the, i r o n s i m i l a r t o t h a t p r e s e n t p r i o r t o d e h y d r a t i o n .  for  The c h a r a c t e r -  2+ i s t i c s of t h e Fe  peak in z e o l i t e s has a c o n s i d e r a b l e dependence on t h e  t y p e of z e o l i t e b e i n g t r e a t e d . c o n d i t i o n s were n e c e s s a r y different zeolites. some exchanged F e  3 +  M o r i c e and Rees found s e v e r a l  f o r the r e - o x i d a t i o n o f F e  However,  by c o m p a r i s o n , i t  2 +  -> F e  3 +  different for  is clear that there  in t h e z e o l i t e .  A l t h o u g h t h e appearance of t h e f e r r o u s peak was r e v e r s i b l e sense t h a t  is  in the  i t was e l i m i n a t e d upon exposure t o t h e atmosphere, t h e  ferrous  peak c o u l d not be produced a g a i n by s i m p l e o u t g a s s i n g .  T h i s c o u l d mean  t h a t t h e mechanism of M o r i c e and Rees may not be t o t a l l y  correct.  A t t h i s p o i n t i t was f e l t t h a t f u r t h e r o u t g a s s i n g t r e a t m e n t would not y i e l d any more r e v e a l i n g r e s u l t s so a d i f f e r e n t employed,  l i n e of a t t a c k  was  A f r e s h p e l l e t was o u t g a s s e d a t 5 7 3 ° K , y i e l d i n g a 3 - I i n e  MSssbauer spectrum as shown in F i g . 6 b ,  Exposure t o the atmosphere 2+  r e s u l t e d once more in d e s t r u c t i o n of t h e Fe  line.  The c e l l  was then  44 e v a c u a t e d , 300 t o r r o f 0^ a d m i t t e d , and t h e sample was heated a t 573°K for 8 hours.  The c e l l was r e - e v a c u a t e d , t h o r o u g h l y purged w i t h 0 , 2  heated f o r a f u r t h e r 20 hours a t 573°K i n 600 t o r r of 0 , and f i n a l l y 2  evacuated a g a i n .  As can be seen from T a b l e 4 , t h i s t r e a t m e n t y i e l d s t h e  same f e r r i c MOssbauer parameters as does s i m p l e o u t g a s s i n g , but t h e oxygen t r e a t m e n t does not a f f o r d a f e r r o u s s p e c i e s .  These r e s u l t s a l s o  make i t appear u n l i k e l y t h a t ^ © 2 ^ 3 i s produced i n t h i s t r e a t m e n t , t h i s cannot be r u l e d o u t w i t h c e r t a i n t y . that  i f F&2®3 —  P  r e s e n  +  It s h o u l d be n o t e d ,  must be paramagnetic  no e v i d e n c e f o r a magnetic h y p e r f i n e  '  although  however,  , there being  interaction.  F o l l o w i n g t h i s t r e a t m e n t , t h e same p e l l e t was heated i n  a t 573°K  f o r 6 hours and s u b s e q u e n t l y o u t g a s s e d a t t h e same t e m p e r a t u r e . r e s u l t i n g spectrum was most s u r p r i s i n g i n t h a t a h y p e r f i n e  The  pattern  r e s u l t e d w i t h a Zeeman s p l i t t i n g c o r r e s p o n d i n g t o F ^ O ^ , superimposed on a quadrupolar doublet. mined a c c u r a t e l y  The parameters of t h e l a t t e r c o u l d not be d e t e r -  because o f o v e r l a p w i t h t h e h y p e r f i n e s p e c t r u m , but  i n d i c a t e d t h e p r e s e n c e o f a f e r r o u s s p e c i e s r a t h e r than f e r r i c (see T a b l e 4).  T h i s seemed a v e r y s t r a n g e phenomenon t o o c c u r i n a r e d u c i n g  atmosphere. A r e p e a t o f t h e rL, p l u s o u t g a s s i n g t r e a t m e n t was done w i t h a new p e l l e t and t h e same spectrum r e s u l t e d , t h a t due t o F^O-j p l u s a c e n t r a l ferrous doublet.  These r e s u l t s showed t h r e e  important f e a t u r e s .  F i r s t l y , a t l e a s t p a r t of t h e i r o n p r e s e n t was b e i n g reduced from f e r r i c t o f e r r o u s , as was t o be e x p e c t e d . . S e c o n d l y , and q u i t e s u r p r i s i n g l y , t h e r e appeared t o be p r o d u c t i o n of F ^ O ^ '  n  a hydrogen atmosphere.  F i n a l l y , t h e f a c t t h a t t h e Fe„0^. was m a g n e t i c a l l y o r d e r e d a t room  45 TABLE  4  Zeeman S p I i t t i ng  Treatment  6°  A  None  0.65  0.76  0.49  0.61  0.93  0,57  0.61  0.95  0.62  ' A  1.9  1.3  0/g  @ 573°K  c  Heated i n 0^ 6 573°K  a, b  1  Heated i n H ':§ 573°K and 0/g @ 573°K Fe 0 2  3  (bulk)  0.60  15.83  e  16.70  0.62  In mm/sec, r e l a t i v e t o sodium n i t r o p r u s s i d e . 295°K I mm/sec = 30.96 KOe  b  c d  A l l r e s u l t s obtained at  Parameters f o r Fe 2+ 4 . Fe component  3+  component o n l y a r e q u o t e d .  c  e  Hyperfine f  component  "MSssbauer E f f e c t Data Index" ( E d . A . H . M u i r , K . J . I n t e r s c i e n c e , New Y o r k , 1966.  Ando, H.M. Coogan),  46 t e m p e r a t u r e showed c o n c l u s i v e l y t h a t t h i s s p e c i e s was a g g r e g a t i n g on t h e e x t e r i o r s u r f a c e s o f t h e z e o l i t e , and was no l o n g e r c o n f i n e d t o t h e interior zeolitic  cavities,  A new p e l l e t was made and heated  i n a stream o f hydrogen a t 573°K  f o r 6 h o u r s , and not s u b s e q u e n t l y o u t g a s s e d , m a t e r i a l was a c e n t r a l  The spectrum o f t h i s  f e r r o u s d o u b l e t superimposed on a m e t a l l i c  h y p e r f i n e p a t t e r n as shown i n F i g , 7 a .  The parameters f o r t h e Fe  d o u b l e t and t h e Fe^ h y p e r f i n e s p l i t t i n g appear t h i s pel l e t a t 573°K produced t h e r ^ O ^ 2+ Fe  s  P  e c  +  i n T a b l e 5,  r u m  iron 2+  Outgassing  superimposed on an  d o u b l e t , whose parameters a r e i n T a b l e 4 and t h e spectrum i s shown 2+  in F i g . 7 b .  The asymmetry of t h e Fe  d o u b l e t in t h e s e s p e c t r a a r e  i n d i c a t i v e of p r e f e r e n t i a l o r i e n t a t i o n .  I t was now c l e a r t h a t  i t was i n  f a c t t h e o u t g a s s i n g of t h e sample which led t o ^ 2 ^ 3 p r o d u c t i o n , and t h a t t h e hydrogen t r e a t m e n t r e s u l t e d i n complete r e d u c t i o n o f one f e r r i c s p e c i e s and p a r t i a l To i n v e s t i g a t e  r e d u c t i o n of the o t h e r . i n more d e t a i l t h e r e d u c t i o n p r o c e s s a new p e l l e t  was made and t r e a t e d w i t h hydrogen under l e s s s e v e r e c o n d i t i o n s . p e l l e t was i n i t i a l l y heated  i n hydrogen a t 473°K f o r 7 h o u r s .  The' The  spectrum ( F i g . 8 a ) was t h e same as f o r an u n t r e a t e d sample ( f e r r i c Next,  i t was heated f o r 8 hours i n  a t 523°K where o n l y a Fe  r e s u l t e d as shown i n F i g . 8 b and t h e a s s o c i a t e d parameters  Heating in  a t 573°K f o r 8 hours gave t h e Fe  0  2+  + Fe  2+  doublet), doublet  i n T a b l e 5.  spectrum as  shown i n F i g , 8 c w i t h parameters comparable t o t h o s e i n T a b l e 5 . .  Heating f o r a longer time in 2+ Fe  a t 573°K lessened t h e i n t e n s i t y o f t h e  peak but p r o l o n g e d t r e a t m e n t a t t h i s temperature d i d not a l t e r t h e  47 TABLE  Treatment  Temp ( ° K )  Heated i n H„ @ 523°K  Heated i n H .573°K  T  a  a  295  1.02  2,08  0.74  80  1,14  2.30  0.72  295  1.31, 0.26  2.13 -  0.78  1,44): 0.29  2.42 -  0.76  d  80 Fe metaI  6  5  295  0.26  Zeeman S p l i t t i n g ' 3  10.72 10.93 10.66  In mm/sec, r e l a t i v e t o sodium n i t r o p r u s s i d e . b  C  I mm/sec = 30.96 KOe Fe  2 +  component  ^ H y p e r f i n e component e  "MOssbauer E f f e c t Data Index" ( E d . D.H. M u i r , K . J . I n t e r s c i e n c e , New-York, 1966,  Ando, H.M, Co'ogan)  48  FIGURE 7 MOssbauer S p e c t r a a.  Fe  b.  F^O^  + Fe . 2+ +  Fe  of:  49  —i  -  1  1  2  -  1  6 DOPPLER  1  1  ••—i  O VELOCITY  1  1  6 ( M M  SEC" )  1—  12:  50  FIGURE 8 MOssbauer s p e c t r a o f : a .  O r i g i n a l Fe^  +  species.  2+ b.  Sample reduced t o Fe  c.  Sample reduced i n hours t o F e  d.  2 +  i n h^. for 6  + Fe^.  Sample reduced i n  a further  7 hours. e.  Sample reduced in H„ t o Fe^.  51  52  s p e c t r u m a p p r e c i a b l y f r o m t h a t shown i n F i g t 8 d .  0 2+ Heating t h e Fe /Fe 2+  s y s t e m t o a p p r o x i m a t e l y 825°K f o r 4 h o u r s r e m o v e s a l l F e h y p e r f i n e l i n e s o f F e ^ r e m a i n a s shown i n F i g , 8 e , o u t g a s s e d a t 573°K n o to  F e  2°3  '  s  produced.  and t h e 6  I f t h i s s y s t e m i s now  T h e m e t a l l i c i r o n now a p p e a r s  b e q u i t e s t a b l e , b o t h t o o u t g a s s i n g a t 573°K, a n d t o e x p o s u r e t o a i r a t  room t e m p e r a t u r e . T h e s e r e s u l t s may b e p l a u s i b l y e x p l a i n e d a s f o l l o w s .  Since there  i s v e r y good e v i d e n c e f o r t h e e x i s t e n c e o f two i r o n s p e c i e s , t h e i n i t i a l r e d u c t i o n t o f e r r o u s must be a t w o - p a r t p r o c e s s .  The exchanged  Fe^  +  is  2+ reduced t o Fe  , t h e charge n e u t r a l i t y o f t h e system presumably  made up b y p r o t o n s .  being  T h e c o n d i t i o n s a r e s u f f i c i e n t l y m i l d (523°K) s o  that further reduction i s apparently not favoured.  A t t h e same t i m e  t h e r e i s r e d u c t i o n o f FeCI-j t o F e C l v i a t h e e q u i l i b r i u m 2FeCI * 2FeCI + C l 2  3  the  2  2  s t r e a m o f h y d r o g e n s e r v i n g t o sweep o u t t h e c h l o r i n e a n d f o r c e t h e  equilibrium to the right. FeClygraphite  S i m i l a r r e d u c t i o n s have been o b s e r v e d f o r  systems'^ '.  When t h e t e m p e r a t u r e f u r t h e r reduced t o Fe^.  0  i s r a i s e d t o 573°K, t h e e x c h a n g e d  species i s  T h e i r o n atoms must t h e n d i f f u s e t o t h e s u r f a c e  ( t h e y s h o u l d be q u i t e m o b i l e a t t h i s t e m p e r a t u r e ) , where t h e y form a g g r e g a t e s o f s u f f i c i e n t s i z e t o e x h i b i t Zeeman s p l i t t i n g .  Again, the  c h a r g e n e u t r a l i t y w i t h i n t h e z e o l i t e i s p r e s u m a b l y m a i n t a i n e d by p r o t o n s . Under these c o n d i t i o n s t h e F e C I it appears u n l i k e l y that f u l l ed a t t h i s t e m p e r a t u r e .  2  s e e m s t o be a f f e c t e d o n l y s l i g h t l y , a n d  r e d u c t i o n t o m e t a l l i c i r o n c a n be a c c o m p l i s h -  H e a t i n g t h e s a m p l e a t much h i g h e r t e m p e r a t u r e s  (825°K)  53  i n hydrogen f i n a l l y causes t h e FeCI^ t o be reduced t o Fe^,  which  must a l s o d i f f u s e t o t h e s u r f a c e and a g g r e g a t e , s i n c e a t t h e end of treatment a l l the  i r o n i n the system i s f e r r o m a g n e t i c .  The mechanism f o r t h e o x i d a t i o n of Fe^ t o ^ ^3 e  i s not c o m p l e t e l y c l e a r .  by s i m p l e o u t g a s s i n g  S i n c e t h e r e a r e no oxygen atoms  e x c e p t t h o s e a s s o c i a t e d w i t h t h e Si and AI atoms of the framework, employed.  this  available  zeolitic  i t seems r e a s o n a b l e t o suppose t h a t t h e s e oxygen atoms a r e The s u r f a c e Fe^ p r o b a b l y e x i s t s as v e r y f i n e p a r t i c l e s  h i g h l y a c t i v a t e d s t a t e and t h u s a t e l e v a t e d t e m p e r a t u r e s  in a  i t could  c o n c e i v a b l y a b s t r a c t 0 atoms from t h e z e o l i t e s u r f a c e t o form r~e,£>^. The indexed X-ray powder photograph of t h e z e o l i t e w i t h s u r f a c e was not v e r y v a l u a b l e as t h e i n t h e neat z e o l i t e . Table  l i n e s were i n a l m o s t the same p o s i t i o n s as  However, t h e r e were some small s h i f t s as shown in  I possibly indicating slight structural alterations.  The small  amount of s u r f a c e f ^ O ^ ( a p p r o x i m a t e l y 3% of t h e t o t a l sample) the o b s e r v a t i o n of  F^O-j  l i n e s due t o t h i s s p e c i e s .  The p l u c k i n g of oxygen  atoms from t h e z e o l i t e s h o u l d not a l t e r the a I u m i n o s i I i c a t e a p p r e c i a b l y but i t w i l l  precludes  structure  leave a net p o s i t i v e charge on the donor atoms  s i n c e t h e c h a r g e produced i n t h e o x i d a t i o n of Fe^ t o F e d i s s i p a t e d by t h e n e g a t i v e charge of the o x y g e n s . c h a r g e c o u l d be removed i f t h e p r o t o n s , p r e s e n t  3 +  will  be  T h i s net p o s i t i v e  i n t h e c h a n n e l s when the  Fe^ i s d e p o s i t e d on t h e s u r f a c e , a r e reduced and pumped o f f as H . 2  would leave t h e a I u m i n o s i I i c a t e e x p l a n a t i o n , however, A f t e r a l l the not produce F e . , 0 , .  framework e l e c t r i c a l l y n e u t r a l ,  i s o n l y t e n t a t i v e and r e q u i r e s f u r t h e r  This This  investigation.  i r o n had been c o n v e r t e d t o Fe^ a t 825°K, o u t g a s s i n g d i d This  i s p r o b a b l y due t o t h e f a c t t h a t t h e i r o n has  54 formed l a r g e r p a r t i c l e s and become l e s s a c t i v e by a n n e a I i n g a n d , i n inactive state,  its  i s unable t o form F 2 - 3 by a b s t r a c t i n g s u r f a c e oxygens o r e  <  )  by any o t h e r p r o c e s s . The z e o l i t i c s u r f a c e  Is by no means r e g u l a r and t h u s i t seemed  r e a s o n a b l e t o suppose t h a t t h e r e might be an EFG s e t up by t h e s u r f a c e g i v e a q u a d r u p o l e i n t e r a c t i o n on t h e ^ 2^3* e  '  n  P  a r +  Icular,  if there  to  i r o n , p r e s e n t e i t h e r as Fe^ o r  i s a q u a d r u p o l e i n t e r a c t i o n , i t would be  I n t e r e s t i n g t o d e t e r m i n e whether a M o r i n t r a n s i t i o n e x i s t s between 80°K and 295°K.  B u l k Fe <D 2  3  shows a M o r i n t r a n s i t i o n a t 2 6 3 ° K , but as 3 2  yet  o n l y a spectrum a t 295°K on t h e F e ^ O ^ - z e o l i t e system has been o b t a i n e d . As shown i n T a b l e 6 , t h e t~&2®3  d  o  e  s  show a q u a d r u p o l e i n t e r a c t i o n .  Fe^ both i n t h e p r e s e n c e and absence of F e Interaction 295°K.  2 +  The  appears t o show a q u a d r u p o l e  a l s o , w i t h a M o r i n t r a n s i t i o n somewhere between 80°K and  As t h e d i f f e r e n c e  in t h e s p l i t t i n g s i s v e r y s m a l l , and may depend  a g r e a t deal on t h e a c c u r a c y of t h e f i t , t h e s e r e s u l t s a r e by no means c o n c l u s i v e r e g a r d i n g t h e p r e s e n c e o r absence of a M o r i n t r a n s i t i o n and q u a d r u p o l e i n t e r a c t i o n i n Fe^. interaction  However, t h e r e  i n Fe^O-^.  The systems s t u d i e d in t h i s f a i r l y complicated. before i t  is c l e a r l y a quadrupole  Further  i n v e s t i g a t i o n have been d e p i c t e d as  i n v e s t i g a t i o n s are d e f i n i t e l y  i s c o m p l i c a t e d more by a d s o r p t i o n work.  m i c r o s c o p y s t u d y may y i e l d  necessary  An e l e c t r o n  i n f o r m a t i o n about t h e s u r f a c e s p e c i e s and a  M o r i n t r a n s i t i o n t e m p e r a t u r e d e t e r m i n a t i o n on t h e Fe^j-^ p e l l e t  will  d e t e r m i n e whether t h e o b s e r v e d q u a d r u p o l e c o u p l i n g i s due t o t h e EFG  set  up in t h e ^ 2 ^ 3 i t s e l f o r by t h e z e o l i t e s u r f a c e s t r u c t u r e ,  Sweeping  the  the  i n i t i a l system w i t h a more i n e r t gas such as l\L w i l l t e s t  55 TABLE  Sample  6  Temp ( ° K )  A  1 0  - A  12 Fe 0 2  3  on z e o l i t e  F e ° on z e o l i t e Fe° <j>n z e o l i t e (Fe component present) 2  '  295  0.151  .295  -0,088  80  0,021  295  -0.043  80  0.048  a  '  56  In mm/sec S p l i t t i n g of t h e l a s t two peaks of the h y p e r f i n e p a t t e r n from t h e f i r s t t w o .  substracted  56  proposed mechanism f o r t h e p r o d u c t i o n of Fe^ and t h e subsequent o x i d a t i o n t o Fe^O^.-  More a n a l y t i c a l  f u l l y and, f i n a l l y , valuable  d a t a w i I 1 be needed t o d e s c r i b e t h e system  low t e m p e r a t u r e s t u d i e s , down t o 4°K s h o u l d g i v e  i n f o r m a t i o n about the magnetic p r o p e r t i e s of the s y s t e m .  BIBLIOGRAPHY 1.  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