Applications of Magnetic Resonance in Catalytic ... - ACS Publications

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 18, 2015 | http://pubs.acs.org Publication Date: June 3, 1983 | doi: 10.1021/bk-1983-0222.ch027

Applications of Magnetic Resonance in Catalytic Research W A L L A C E S. BREY University of Florida, Department of Chemistry, Gainesville, F L 32611

The early development of the use of magnetic resonance methods in catalyst research is surveyed. Various magnetic resonance parameters that may be utilized are introduced and the types of information that may he obtained by their measurement are described in relation to contributions by early -workers in the field. Some comments on the limitations of NMR and EPR methods as veil as indications of prospects for future progress are given, along with a brief mention of recent advances in the applications of specialized techniques of NMR. References are provided for a number of extensive and detailed review papers. I n t h i s a c c o u n t we w i l l a t t e m p t t o t r a c e some o f t h e d e v e l opments i n t h e a p p l i c a t i o n o f n u c l e a r a n d e l e c t r o n m a g n e t i c resonance techniques t o t h e study o f c a t a l y s t s and c a t a l y t i c p r o c e s s e s f r o m t h e p i o n e e r i n g s t u d i e s up t o t h e m i d d l e 1960 s. To p l a c e t h e s e d e v e l o p m e n t s i n p e r s p e c t i v e , some a c c o u n t w i l l b e g i v e n o f t h e i n v e n t i o n o f m a g n e t i c r e s o n a n c e methods f o r c o n d e n s e d m a t t e r a n d o f t h e v a r i o u s t y p e s o f a p p a r a t u s t h a t were a v a i l a b l e , e s p e c i a l l y f o r M R where t h e r e h a s b e e n s u b s t a n t i a l improvement i n i n s t r u m e n t a t i o n w i t h t i m e . (We s h a l l a b b r e v i a t e n u c l e a r m a g n e t i c r e s o n a n c e b y "NMR" a n d e l e c t r o n p a r a m a g n e t i c r e s o n a n c e b y "EPR". We p r e f e r t h e l a t t e r d e s i g n a t i o n t o e l e c t r o n s p i n r e s o n a n c e , b e c a u s e e l e c t r o n o r b i t a l a s w e l l as s p i n moments may b e i n v o l v e d . ) S i n c e many r e a d e r s may n o t b e f a m i l i a r w i t h the p r i n c i p l e s o f magnetic r e s o n a n c e , q u a l i t a t i v e e x p l a n a t i o n s of b a s i c aspects a r e i n c l u d e d . Emphasis w i l l b e on w o r k i n t h e U n i t e d S t a t e s , b u t some c o n t r i b u t i o n s f r o m e l s e w h e r e w i l l b e mentioned. ?

0097-6156/83/0222-0375$06.00/0 © 1983 American Chemical Society

In Heterogeneous Catalysis; Davis, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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HETEROGENEOUS CATALYSIS

The B e g i n n i n g s o f M a g n e t i c R e s o n a n c e E x p e r i m e n t s


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I n t h e 1920* s a n d t h e 1930 s, p h y s i c i s t s p e r f o r m i n g e x p e r i ments i n t h e gas p h a s e , u s u a l l y a t v e r y l o w p r e s s u r e , d e m o n s t r a t e d u n e q u i v o c a l l y t h a t e l e c t r o n s a n d some n u c l e i "behave a s m i n u t e m a g n e t i c d i p o l e s . P a u l i h a d p r o p o s e d i n 192k t h a t t h e h y p e r f i n e s t r u c t u r e observed i n atomic e l e c t r o n i c s p e c t r a might r e s u l t from a n u c l e a r a n g u l a r momentum a n d a n a s s o c i a t e d m a g n e t i c moment, a n d i n t h e n e x t y e a r U h l e n b e c k a n d Goudsmit showed t h a t f i n e s t r u c t u r e i n a t o m i c s p e c t r a c a n b e e x p l a i n e d on t h e b a s i s o f i n t e r a c t i o n s o f t h e e l e c t r o n i c o r b i t a l m a g n e t i c moment w i t h a n i n t r i n s i c e l e c t r o n m a g n e t i c moment, s u c h as m i g h t b e a s s o c i a t e d w i t h t h e a n g u l a r momentum o f a " s p i n n i n g " e l e c t r i c a l l y c h a r g e d p a r t i c l e , i n accordance w i t h t h e r e s u l t s o f t h e well-known S t e r n - G e r l a c h e x p e r i m e n t o f 192h. T h i s e x p e r i m e n t i n v o l v e d t h e d e f l e c t i o n o f a beam o f atoms c o n t a i n i n g u n p a i r e d e l e c t r o n s i n an inhomogeneous m a g n e t i c f i e l d , a n d R a b i e x t e n d e d t h e method t o t h e measurement o f n u c l e a r m a g n e t i c moments. H y p e r f i n e e f f e c t s f r o m n u c l e a r moments were a l s o o b s e r v e d as p e r t u r b a t i o n s on t h e r o t a t i o n a l energy l e v e l s o f s m a l l molecules (l_). I n o r d e r t o make measurements o f e l e c t r o n i c a n d s p i n magn e t i c moments i n c o n d e n s e d m a t t e r — i n s o l i d s o r l i q u i d s — w i t h o u t the n e c e s s i t y f o r v o l a t i l i z a t i o n i n a high-vacuum s y s t e m , i t was a t t r a c t i v e t o a t t e m p t r e s o n a n c e e x p e r i m e n t s , w h i c h w o u l d have t h e added a d v a n t a g e o f a s p e c i f i c d e t e r m i n a t i o n o f t h e e n e r g y l e v e l p a t t e r n as deduced f r o m t h e p a r t i c u l a r e n e r g i e s r e q u i r e d t o i n duce t r a n s i t i o n s . F o r e l e c t r o n i c s p i n s , t h i s type o f experiment i s n o t q u i t e so c r i t i c a l , because t h e magnitude o f t h e magnetic moment i s s u f f i c i e n t l y l a r g e t o g i v e s e n s i b l e c o n t r i b u t i o n s t o the b u l k m a g n e t i c s u s c e p t i b i l i t y , b u t t h e m a g n i t u d e s o f n u c l e a r m o m e n t s — a t h o u s a n d o r more t i m e s s m a l l e r t h a n t h a t f o r t h e e l e c t r o n — c a u s e t h e n u c l e a r c o n t r i b u t i o n t o t h e magnetic suscept i b i l i t y t o be l o s t i n t h e e x p e r i m e n t a l u n c e r t a i n t y o f t h e suscept i b i l i t y measurements. A p a r t i c u l a r i n c e n t i v e t o measure a c c u r a t e l y t h e m a g n e t i c moment o f a n u c l e a r s p e c i e s — m o r e s p e c i f i c a l l y , t h e n u c l e a r gyromagnetic o r magnetogyric r a t i o , equal t o t h e r a t i o o f t h e moment o f t h e n u c l e u s t o i t s a n g u l a r momentum—was t h e d e s i r e t o t e s t t h e o r i e s o f n u c l e a r s t r u c t u r e , w h i c h were u n d e r development i n t h e l a t e 30 s a n d e a r l y U0 s. I n H o l l a n d t h e p h y s i c i s t C. J . G o r t e r l e d a group w h i c h i n v e s t e d s u b s t a n t i a l e f f o r t i n t h e search f o r n u c l e a r resonance i n b u l k m a t t e r , o n l y t o f a i l because o f t h e u n f o r t u n a t e c h o i c e o f samples f o r w h i c h t h e r e l a x a t i o n t i m e was v e r y l o n g ( 2_). E l e c t r o n r e s o n a n c e was t o come f i r s t , w i t h a r e p o r t f r o m Z a v o i s k y i n R u s s a i n 19^5 o f s u c c e s s i n t h i s a r e a (3.). Success i n t h e o b s e r v a t i o n o f n u c l e a r magnetic resonance i n b u l k m a t t e r was a c h i e v e d a l m o s t s i m u l t a n e o u s l y b y two g r o u p s i n the U n i t e d S t a t e s : P u r c e l l , T o r r e y , a n d Pound a t H a r v a r d u s e d a s i n g l e - c o i l o r b r i d g e method w i t h p a r a f f i n as t h e sample ( h ) , 1

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and B l o c h , H a n s e n , and P a c k a r d a t S t a n f o r d employed an a p p a r a t u s w i t h s e p a r a t e t r a n s m i t t e r a n d r e c e i v e r c o i l s w i t h w a t e r as t h e sample (_5 ). B l o c h and P u r c e l l s h a r e d t h e N o b e l p r i z e i n r e c o g n i t i o n o f these achievements.


The F i r s t Decade o f A p p l i c a t i o n s t o C a t a l y s i s The a u t h o r f i r s t l e a r n e d o f t h e m a g n e t i c r e s o n a n c e e x p e r i ment i n 1951» "when P i e r c e S e l w o o d gave a t a l k on m a g n e t i c methods as a p p l i e d t o c a t a l y s t s as p a r t o f a s e r i e s o f l e c t u r e s on r e c e n t developments i n c a t a l y s i s sponsored by t h e P h i l a d e l p h i a S e c t i o n o f t h e A m e r i c a n C h e m i c a l S o c i e t y . A l t h o u g h S e l w o o d d i d n o t men t i o n n u c l e a r r e s o n a n c e i n t h e m a i n p a r t o f t h e l e c t u r e , t h e r e was c o n s i d e r a b l e e x c i t e m e n t a b o u t t h i s m y s t e r i o u s new t y p e o f e x p e r i ment among some o f t h e members o f t h e a u d i e n c e , w h i c h r e s u l t e d i n a l e n g t h y d i s c u s s i o n p e r i o d devoted almost e n t i r e l y t o an explana t i o n o f n u c l e a r i n d u c t i o n , as t h e e x p e r i m e n t was t h e n c a l l e d , with the a i dof a precessing pointer. A pioneer i n the applica t i o n s o f a l l s o r t s o f m a g n e t i c methods t o c a t a l y t i c s y s t e m s . S e l w o o d h a d p u b l i s h e d h i s f i r s t n u c l e a r i n d u c t i o n p a p e r i n 19^-9 ( 6 ) . W i t h R. B. S p o o n e r , he u t i l i z e d a d o u b l e - c o i l a p p a r a t u s w i t h 20 Hz m o d u l a t i o n , t o measure b y n u c l e a r r e s o n a n c e a b s o r p t i o n , a c c o r d i n g t o t h e t e c h n i q u e s o f B l o e m b e r g e n , P u r c e l l , a n d Pound (7) t h e e f f e c t s o f s u p p o r t e d p a r a m a g n e t i c c a t a l y s t s on t h e r e l a x a t i o n t i m e s o f t h e n u c l e i o f l i q u i d w a t e r . F o r MnO^ on T i O ^ , f o r e x a m p l e , i t was f o u n d t h a t a s m a l l e r f r a c t i o n o f t h e p a r a m a g n e t i c i o n s i s a v a i l a b l e t o t h e w a t e r when t h e c o n c e n t r a t i o n o f a c t i v e o x i d e on t h e s u p p o r t i s g r e a t e r , i n d i c a t i n g l a r g e r m i c r o c r y s t a l s of t h e a c t i v e o x i d e a t h i g h e r c o n c e n t r a t i o n s , a r e s u l t i n a c c o r d w i t h magnetic s u s c e p t i b i l i t y d a t a f o r t h i s system. F o r i r o n o x i d e on γ-alumina, h o w e v e r , t h e r e l a x a t i o n r e s u l t s were q u i t e d i f f e r e n t f r o m t h e s u s c e p t i b i l i t y r e s u l t s , a n d i t was c o n c l u d e d t h a t t h e i r o n atoms go i n t o s o l i d s o l u t i o n i n t h e a l u m i n a upon h e a t i n g , t h u s becoming i n a c c e s s i b l e t o t h e w a t e r as w e l l as u n a v a i l a b l e f o r c a t a l y t i c p r o c e s s e s . A comment i n t h i s p a p e r i s interesting: "The s i g n a l s o b t a i n e d a r e v e r y s m a l l ; t h e i r d e t e c t i o n i s a m a t t e r o f d i f f i c u l t y owing t o s p u r i o u s e f f e c t s . " How ever, t h e r e i s a l s o a note o f optimism i n t h e p r o p o s a l t h a t t h e method w o u l d be s u i t a b l e f o r " i n s i t u " measurements o f c a t a l y t i c systems. S e l w o o d s n e x t p u b l i c a t i o n on NMR a p p e a r s i n t h e p r o c e e d i n g s of a Faraday S o c i e t y D i s u c s s i o n h e l d i n L i v e r p o o l i n A p r i l o f I95O ( Q ) . I n c i d e n t a l l y , t h i s volume c o n t a i n s an e x t e n d e d i n t r o d u c t i o n b y S i r Hugh T a y l o r i n w h i c h he draws a t t e n t i o n t o two e a r l i e r F a r a d a y S o c i e t y G e n e r a l D i s c u s s i o n s , one i n 1922 i n w h i c h t h e Lindemann a n d L a n g m u i r t h e o r i e s were i n t r o d u c e d , a n d one i n 1932 i n w h i c h t h e d i s t i n c t i o n b e t w e e n c h e m i c a l a d s o r p t i o n a n d p h y s i c a l a d s o r p t i o n was c l e a r l y drawn. I n h i s p a p e r , S e l w o o d presented data f o r t h e a c c e s s i b i l i t y t o water o f t h e paramagnetic m a t e r i a l s i r o n o x i d e , copper o x i d e , and c h r o m i a , each s u p p o r t e d 5

1



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on a l u m i n a . The r e l a x a t i o n t i m e s o f w a t e r i n c o n t a c t w i t h t h e o x i d e were compared t o t h o s e f o r w a t e r i n s o l u t i o n s c o n t a i n i n g t h e same m e t a l i o n s . As i n t h e p r e v i o u s r e s u l t s , a c c e s s i b i l i t y tended t o decrease w i t h i n c r e a s i n g c o n c e n t r a t i o n o f paramagnetic i o n s on t h e s u r f a c e , b u t t h e r e i s now a l s o p o i n t e d o u t a d i r e c t p a r a l l e l o f t h e r e s u l t s f o r chromia w i t h t h e a c t i v i t y o f samples o f t h i s m a t e r i a l f o r t h e c o n v e r s i o n o f η-heptane t o t o l u e n e . To u n d e r s t a n d S e l w o o d ' s i n i t i a l work a b i t b e t t e r , i t i s h e l p f u l f o r one t o have a d e s c r i p t i o n o f s e v e r a l a s p e c t s o f nuclear "relaxation". S p i n - l a t t i c e o r T^ r e l a x a t i o n p r o c e s s e s are t r a n s i t i o n s f r o m one s p i n s t a t e t o a n o t h e r , t e n d i n g t o r e s t o r e i n time t h e s p i n s i n a system t o t h e r m a l e q u i l i b r i u m a c c o r d i n g t o a B o l t z m a n n d i s t r i b u t i o n , and i n d u c e d b y o c c u r r e n c e s i n t h e sample i n t h e v i c i n i t y o f t h e n u c l e u s r a t h e r t h a n b y r a d i o s i g n a l s a p p l i e d by t h e experimenter. F o r n u c l e i w i t h s p i n quantum number 1/2, t h e s e t r a n s i t i o n s a r e p r o d u c e d b y f l u c t u a t i o n s i n t h e magnetic f i e l d a t t h e l o c a t i o n o f t h e n u c l e u s , f l u c t u a t i o n s most e f f e c t i v e when t h e y have a maximum f r e q u e n c y component a t t h e n u c l e a r r e s o n a n c e f r e q u e n c y i n t h e f i x e d m a g n e t i c f i e l d o f t h e s p e c t r o m e t e r . They s h o u l d , i n o t h e r w o r d s , c o r r e spond f a i r l y c l o s e l y i n f r e q u e n c y t o t h e s p e c t r o m e t e r o p e r a t i n g f r e q u e n c y . S u c h f l u c t u a t i o n s i n f i e l d may o c c u r as t h e r e s u l t o f r o t a t i o n o f t h e m o l e c u l e c o n t a i n i n g t h e n u c l e u s , when t h e e f f e c t s of o t h e r magnetic n u c l e i i n t h e m o l e c u l e a r e f e l t , o r by d i f f u s i o n o f t h e molecule i n t h e presence o f magnetic f i e l d s from o t h e r m o l e c u l e s . R e l a x a t i o n i s p a r t i c u l a r l y r a p i d when t h e r e a r e p a r a m a g n e t i c s p e c i e s n e a r b y , b e c a u s e o f t h e l a r g e m a g n e t i c moment of t h e e l e c t r o n which generates magnetic f i e l d s e f f e c t i v e a t l o n g er r a n g e s ; any a p p r e c i a b l e c o n c e n t r a t i o n o f s p e c i e s w i t h u n p a i r e d e l e c t r o n s causes paramagnetic r e l a x a t i o n t o predominate over other types o f r e l a x a t i o n , except perhaps f o r n u c l e i w i t h s p i n g r e a t e r t h a n 1/2. N u c l e i w i t h s p i n s o f 1 o r more h a v e e l e c t r i c q u a d r u p o l e moments as w e l l as m a g n e t i c d i p o l e moments, a n d t h e r e f o r e undergo s p i n f l i p s caused by v a r y i n g e l e c t r i c f i e l d s a s s o c i ated w i t h motions o f e l e c t r o n s . I n 1956, S e l w o o d p u b l i s h e d t h e r e s u l t s o f s t u d i e s b y t h e s p i n - e c h o method, i n w h i c h t h e r e s p o n s e o f t h e n u c l e a r m a g n e t i z a t i o n t o a s e r i e s o f p u l s e s i s measured, f o r methanol, e t h a n o l , w a t e r , a n d n-hexane a d s o r b e d on γ-alumina, s i l i c a - a l u m i n a , MnO, and CuO, a l l p r e s u m a b l y d i a m a g n e t i c m a t e r i a l s (9.). He f o u n d f o r t h e p r o t o n r e l a x a t i o n t i m e marked r e d u c t i o n s over t h o s e i n t h e l i q u i d p h a s e , b u t p o i n t e d o u t t h a t many u n c e r t a i n t i e s r e m a i n e d , s u c h as t h e e f f e c t s o f t r a c e s o f p a r a m a g n e t i c i m p u r i t i e s a n d o f t h e shapes o f p o r e s i n w h i c h t h e a d s o r b a t e m o l e c u l e s were l o c a t e d . The y e a r 1956 saw t h e F i r s t I n t e r n a t i o n a l C o n g r e s s on C a t a l y s i s i n P h i l a d e l p h i a , f o r w h i c h t h e p r o c e e d i n g s were pub l i s h e d i n 1957 as Volume 9 i n t h e A d v a n c e s i n C a t a l y s i s s e r i e s . The a u t h o r was f o r t u n a t e i n b e i n g a b l e t o a t t e n d t h i s m e e t i n g ; held i n the Bellevue-Stratford hotel, i t represented a gathering of l e a d e r s i n c a t a l y t i c r e s e a r c h from throughout t h e w o r l d ,



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BREY


379

including Russia. O n l y two p a p e r s r e f e r r e d t o m a g n e t i c r e s o n a n c e , however. Selwood m e r e l y m e n t i o n e d t h e p r o m i s e o f n u c l e a r and e l e c t r o n p a r a m a g n e t i c r e s o n a n c e t e c h n i q u e s as a d j u n c t s t o magnetic s u s c e p t i b i l i t y methods, but John T u r k e v i c h d e s c r i b e d h i s work on t h e e f f e c t s o f t h e t e m p e r a t u r e o f h e a t i n g and o f t h e a d s o r p t i o n o f oxygen o r n i t r i c o x i d e on t h e i n t e n s i t y and b a n d w i d t h o f r e s o n a n c e s f r o m s u g a r c h a r c o a l (10_) · About t h i s t i m e , a number o f w o r k e r s were i n v e s t i g a t i n g t h e EPR s p e c t r a o f p o r o u s c a r b o n s p r e p a r e d i n v a r i o u s w a y s , and a p a p e r f r o m Ingram's l a b o r a t o r y i n E n g l a n d had b e e n e n t i t l e d , " P a r a m a g n e t i c R e s o n a n c e f r o m B r o k e n Carbon Bonds" ( l l ) . I n 1958 T u r k e v i c h showed t h a t t h e c a t a l y t i c a c t i v i t y o f c h a r c o a l i s r e l a t e d t o t h e c h a r a c t e r i s t i c s o f i t s EPR a b s o r p t i o n (12). For the o r t h o - p a r a h y d r o g e n c o n v e r s i o n a t - I 9 6 , t h e r a t e c o n s t a n t was a maximum f o r samples t r e a t e d a t 600°, w h i c h have a maximum s i g n a l i n t e n s i t y and minimum l i n e w i d t h . For hydrogen-deuterium exchange a t 50°, t h e r a t e c o n s t a n t i n c r e a s e s m o n o t o n i c a l l y w i t h p r e p a r a t i o n t e m p e r a t u r e up t o 950°, p a r a l l e l i n g an i n c r e a s e i n l i n e w i d t h w h i c h i s a t t r i b u t e d t o an i n c r e a s e i n e l e c t r o n mobility. I n t h e l a t t e r 50 s, s e v e r a l g r o u p s a p p l i e d NMR t o s t u d y a d s o r p t i o n on m e t a l o x i d e s , a l t h o u g h d i r e c t c o r r e l a t i o n s w i t h c a t a l y t i c a c t i v i t y were n o t made. A s t o n s g r o u p a t P e n n s y l v a n i a S t a t e l o o k e d a t w a t e r , CH^, and CF^ on T i O ^ ( l 3 - l 6 ) , and w o r k e r s at B e l l L a b o r a t o r i e s examined w a t e r on t h e same o x i d e (17.). A n a l y s i s was b a s e d p r i m a r i l y on l i n e w i d t h s : f o r l o n g c o r r e l a t i o n t i m e s , o r s o l i d - l i k e b e h a v i o r , one e x p e c t s v e r y b r o a d l i n e s , w h i l e h i g h m o b i l i t y i s a s s o c i a t e d w i t h narrow l i n e s . John Zimmerman and c o w o r k e r s a t M a g n o l i a P e t r o l e u m F i e l d R e s e a r c h L a b o r a t o r i e s i n D a l l a s b e g a n a s e r i e s o f more s o p h i s t i c a t e d r e l a x a t i o n measurements f o r w a t e r on s i l i c a g e l w h i c h w i l l be d e s c r i b e d f u r t h e r b e l o w , and W i n k l e r a t L e i p z i g was i n v o l v e d i n i n v e s t i g a t i o n s o f w a t e r on a l u m i n a w h i c h were i n t e r p r e t e d i n t e r m s o f m a c r o p o r e s and m i c r o p o r e s i n t h e a d s o r b e n t (l8_). 0

f

1

Developments

Through t h e E a r l y

I960's

The p e r i o d o f about f i v e y e a r s b e g i n n i n g i n 1958 r e p r e s e n t e d a s u b s t a n t i a l i n c r e a s e i n i n t e r e s t i n magnetic resonance i n cat a l y s i s as r e f l e c t e d by t h e number and d i v e r s i t y o f p u b l i c a t i o n s . I n 1958, t h e r e was a s u b s t a n t i a l l e a p f o r w a r d i n s e n s i t i v i t y and i n o p e r a t i n g c o n v e n i e n c e i n c o m m e r c i a l NMR i n s t r u m e n t a t i o n w i t h t h e i n t r o d u c t i o n by V a r i a n A s s o c i a t e s o f arli|,000 g a u s s s p e c t r o m e t e r o p e r a t i n g a t 60 MHz f o r p r o t o n s . T h i s i n s t r u m e n t i n c l u d e d an o p t i c a l l y a c t u a t e d f i e l d s t a b i l i z e r and a phase s e n s i t i v e detector. E a r l i e r V a r i a n i n s t r u m e n t s o p e r a t e d a t ho MHz f o r h y d r o gen and u s e d d i o d e d e t e c t i o n so t h a t n u l l i n g b a s e l i n e l e a k a g e was n o t e a s y . The a u t h o r was p r i v i l e g e d t o have one o f t h e f i r s t o f the new i n s t r u m e n t s i n h i s l a b o r a t o r y i n 1958; a "dual-purpose" or DP m o d e l , i t c o u l d p e r f o r m a u d i o - m o d u l a t e d w i d e - l i n e e x p e r i ments as w e l l as h i g h - r e s o l u t i o n e x p e r i m e n t s .



380

HETEROGENEOUS CATALYSTS

I n I960, t h e f i r s t r e v i e w o f m a g n e t i c r e s o n a n c e a p p l i e d t o c a t a l y s i s a p p e a r e d . W r i t t e n b y D. E. O ' R e i l l y o f G u l f R e s e a r c h and Development L a b o r a t o r i e s i n P i t t s b u r g h , i t was p u b l i s h e d as a s e c t i o n o f Volume 12 o f Advances i n C a t a l y s i s (19). I n a d d i t i o n t o a comprehensive r e v i e w o f r e s u l t s o f o t h e r workers i n t h e f i e l d and a t h o r o u g h d i s c u s s i o n o f p r i n c i p l e s and methods, O ' R e i l l y i n c l u d e d c o n s i d e r a b l e d e t a i l about v a r i o u s r e s u l t s f r o m t h e G u l f L a b o r a t o r i e s , i n c l u d i n g some m a t e r i a l n o t p r e v i o u s l y published. EPR s t u d i e s o f t h e o x i d a t i o n s t a t e s o f chromium i n c h r o m i a - a l u m i n a c a t a l y s t s a r e e x t e n s i v e l y d e s c r i b e d and t h e r e s u l t s a r e r e l a t e d t o a c t i v i t y f o r hydrogen p e r o x i d e decomposition and f o r d e h y d r o c y c l i z a t i o n o f η-heptane r e p o r t e d b y o t h e r s . Some o f O ' R e i l l y ' s NMR r e s u l t s r e p r e s e n t a change i n emphasis i n t h e s t u d y o f c a t a l y t i c phenomena--previous work c o n c e n t r a t e d on t h e resonances o f adsorbed s p e c i e s — t o w a r d d i r e c t examination o f t h e c a t a l y s t s t h e m s e l v e s . He examined c a r e f u l l y t h e l i n e s h a p e s o f the aluminum n u c l e a r t r a n s i t i o n s i n v a r i o u s f o r m s o f a l u m i n a a n d r e l a t e d t h e t r a n s i t i o n i n t e n s i t y t o BET a r e a f o r γ-alumina. The f l u o r i n e - 1 9 r e s o n a n c e i n f l u o r i n e doped a l u m i n a was i n v e s t i g a t e d , as were t h e r e l a x a t i o n t i m e s o f p r o t o n s i n p a r t i a l l y d e h y d r a t e d s i l i c a g e l a n d s i l i e a - a l u m i n a ; much o f t h e l a s t work h a d b e e n r e p o r t e d e a r l i e r i n a c l a s s i c p a p e r w i t h L e f t i n a n d H a l l i n 1958

(20). I n E u r o p e , r a d i o f r e q u e n c y s p e c t r o s c o p i s t s — m a n y o f them now m a g n e t i c r e s o n a n c e r e s e a r c h e r s — m e t i n L e i p z i g i n September 1961 at t h e T e n t h C o l l o q u e Ampère. The p r o c e e d i n g s o f t h i s m e e t i n g i n c l u d e d s e v e r a l p a p e r s d e a l i n g w i t h c a t a l y s t s y s t e m s (21). Work f r o m T u r k e v i c h ' s l a b o r a t o r y on t h e c a t a l y t i c p r o p e r t i e s o f L i A l H ^ , h e a t e d t o p r o d u c e a c t i v e c e n t e r s f o r H^-D^ e x c h a n g e , was d e s c r i b e d . A V a r i a n hO MHz s p e c t r o m e t e r was u s e d i n an a t t e m p t t o o b s e r v e L i and A l r e s o n a n c e ; a l t h o u g h s u c c e s s was l i m i t e d , t h i s was a n o t h e r e a r l y a t t e m p t t o l o o k d i r e c t l y a t a c a t a l y s t . From N i c o l a u ' s l a b o r a t o r y i n B e r l i n came a r e p o r t o f EPR s i g n a l s o f P t c a t a l y s t s s u p p o r t e d o n c a r b o n , where t h e e f f e c t o f h e a t i n g produced p a r a l l e l v a r i a t i o n o f s i g n a l c h a r a c t e r i s t i c s and a c t i v i t y f o r hydrogénation. A g r o u p i n G r e n o b l e o b s e r v e d d e u t e r i u m double-quantum t r a n s i t i o n s i n z e o l i t e s i n w h i c h t h e w a t e r o f c r y s t a l l i z a t i o n h a d b e e n exchanged b y D^O. A l t h o u g h i t h a d no p a r t i c u l a r r e f e r e n c e t o c a t a l y t i c a c t i v i t y , t h i s work i s i n t e r e s t ing because i t i s a v e r y e a r l y a p p l i c a t i o n o f a t e c h n i q u e used e x t e n s i v e l y i n much more r e c e n t s t u d i e s o f d e u t e r i u m . The f i r s t volume o f t h e J o u r n a l o f C a t a l y s i s a p p e a r e d i n 1962, and i n c l u d e d two s i g n i f i c a n t p a p e r s on t h e a p p l i c a t i o n o f EPR t o r e a l c a t a l y s t s y s t e m s . One b y P o o l e , K e h l , and M a c l v e r (22) e x t e n d e d EPR s t u d i e s o f c h r o m i a - a l u m i n a s y s t e m s . The i n v e s t i g a t i o n o f c h r o m i a - c o n t a i n i n g c a t a l y s t s c o n t i n u e d t o be a v e r y p r o d u c t i v e a n d w e l l - p l o u g h e d f i e l d and h a s b e e n d e s c r i b e d i n s e v e r a l p l a c e s (23). I n t h e o t h e r m a g n e t i c r e s o n a n c e p a p e r i n t h e Journal» K e i t h



27.

BREY

Applications

of

Magnetic

Resonance

381

Hall showed that several polynuclear hydrocarbons and phenylated amines react to form cation radicals on the surface of a s i l i c a alumina catalyst (2k). It was hoped to use hyperfine s p l i t t i n g to identify the radicals formed, but broadening of the lines by the surface obscured much of the hoped-for detail. Rough e s t i mates of the spin concentration could be made and a significant effect of oxygen in formation of radicals and in determination of their concentration was observed, although the precise role of oxygen could not be assigned. Hall's work was part of an effort to answer two related questions which continued to perplex catalyst researchers—and were sometimes the cause of controversy among them—questions as to whether the acid sites on the catalyst surface are Lewis or Bronsted in nature, and whether the intermediates in catalytic reactions are carbonium ions or radicals. The work followed closely upon preliminary results of Rooney and Pink in England (25.), who showed that perylene and anthracene when adsorbed on silica-alumina catalysts gave an EPR signal with hyperfine s p l i t t i n g similar to that found for the radicals in concentrated sulfuric acid, and who later concluded that cation-radical formation takes place at Lewis acid sites and showed that oxygen has a reversible broadening effect at room temperature (26). In addition, Fogo of Union O i l , in a short communication (27), had reported that the presence of oxygen on the surface f a c i l i t a t e d formation of radicals from anthracene, and that hydrogénation of a silica-alumina catalyst reduced the formation of radicals from perylene. Thus, as MR spectroscopists were turning some of their attention to the material of the catalyst, the EPR folks were beginning to look at the species that might be found on the surface. The effects of high-energy radiation, both in aiding the formation of radicals on the surface and in producing defect centers within the catalyst, were becoming of interest. In Russia, Kazanskii was working in this f i e l d (28, 29), and Paul Emmett collaborated with scientists at Oak Ridge to study the formation of hydrogen atoms in irradiated catalysts (30). It was argued that the mutual modification of constituent properties in silica-alumina catalysts was indicated by the fact that these materials gave more hydrogen atoms upon irradiation than did the separate components, and i t was pointed out that the number produced should depend both upon acid strength of the hydrogen source and upon the s t a b i l i t y of the trapping sites. It was suggested that, although NMR does not distinguish between the OH groups i n the mixed catalysts and those in the pure components, part of the water content of the mixed materials must be in a special form capable of yielding hydrogen atoms upon irradiation. We mentioned earlier the series of papers from Zimmerman's group i n Dallas; a series which was continued with important .contributions from Don Woessner and under the name of Socony Mobil O i l Company (31-37). The relaxation behavior of water on


382

HETEROGENEOUS

CATALYSIS

s i l i c a g e l v a s m e a s u r e d "by spin-r-echo methods and i t v a s e s t a b l i s h e d that the v a t e r e x i s t s i n t v o d i f f e r e n t environments, v i t h d i f f e r e n t r e s t r i c t i o n s on m o b i l i t y . W h e t h e r o r n o t t h e t v o p h a s e s c a n be d i s t i n g u i s h e d depends upon t h e r e l a t i o n b e t v e e n t h e l i f e t i m e i n each phase and t h e r e l a x a t i o n t i m e s i n t h e t v o phases. A t h e o r y r e l a t i n g t h e s e magnitudes vas developed f o r i s o t r o p i c m o t i o n o f t h e m o l e c u l e s . The t r e a t m e n t v a s t h e n e x t e n d e d t o a n i s o t r o p i c m o t i o n , a s s u m i n g random r e o r i e n t a t i o n o f t h e p r o t o n - p r o t o n v e c t o r about a normal t o t h e s u r f a c e , and t h e p r e d i c t i o n s v e r e f o u n d t o be c o n s i s t e n t v i t h t h e e x p e r i m e n t a l results. S t u d i e s o f t h e s i l i c a - v a t e r system over an extended t e m p e r a t u r e r a n g e p r o v e d v e r y p r o f i t a b l e a n d p e r m i t t e d a much more d e t a i l e d a n a l y s i s o f t h e m o t i o n s i n t h e t v o p h a s e s as v e i l as an e s t i m a t e o f a b o u t h . 9 k c a l / m o l f o r t h e a c t i v a t i o n e n e r g y for interphase transfer. I n 1966, Woessner a p p l i e d t h e s e t h e o r i e s t o an e x t e n s i v e s t u d y o f b e n z e n e a d s o r b e d on s i l i c a g e l . F u r t h e r d e v e l o p m e n t s i n t h e g e n e r a l a r e a have o v e d much t o t h e t h e o r e t i c a l analysis of the i n t e r r e l a t i o n s of motion, r e l a x a t i o n , and i n t e r p h a s e t r a n s f e r as v o r k e d o u t b y R e s i n g (3,8, 3 9 ) .


9

An a r e a o f c o n s i d e r a b l e p o t e n t i a l i n c a t a l y t i c s t u d i e s v h i c h v a s i n v e s t i g a t e d i n t h e l a t e 50's a n d e a r l y 60's a n d s t i l l h o l d s promise o f f u r t h e r a p p l i c a b i l i t y i n v o l v e s t h e t r a n s f e r o f p o l a r i z a t i o n f r o m one m a g n e t i c s p e c i e s t o a n o t h e r . U e b e r s f e l d publ i s h e d a v h o l e s e r i e s o f papers concerned v i t h t h e e f f e c t s obs e r v e d v h e n t h e EPR t r a n s i t i o n s i n a p a r a m a g n e t i c m a t e r i a l s u c h as c h a r c o a l a r e pumped, a n d p r o t o n s i n t h e v i c i n i t y o f t h e u n p a i r e d e l e c t r o n s a r e o b s e r v e d (UO, h i ) . I f t h e p r o t o n s a r e l i n k e d t o t h e e l e c t r o n s by h y p e r f i n e c o u p l i n g , t h e p r o t o n r e s o n ance i n t e n s i t y i s enhanced b y t h e O v e r h a u s e r e f f e c t ; i f t h e interaction i s of the direct dipolar type, the " s o l i d effect" l e a d s t o enhancement. K r e b s a l s o i n v e s t i g a t e d t h e s e e f f e c t s i n d e t a i l and o b t a i n e d i n f o r m a t i o n about t h e d e f e c t c e n t e r s i n c h a r c o a l t h r o u g h t h e enhancements b y t h e s o l i d e f f e c t o f t h e r e s o n a n c e s a s s o c i a t e d v i t h c h e m i c a l l y b o u n d p r o t o n s (h2). The g e n e r a l a p p r o a c h o f t r a n s f e r r i n g s p i n i n f o r m a t i o n f r o m one s p e c i e s t o a n o t h e r has c o n s i d e r a b l e p r o m i s e f o r p r o v i d i n g a d d i t i o n a l i n f o r m a t i o n a b o u t a t o m i c and m o l e c u l a r a r r a n g e m e n t s i n the v i c i n i t y of surfaces. The C h e m i c a l S h i f t By t h e e a r l y s i x t i e s , t h e v a r i o u s a s p e c t s o f m a g n e t i c r e s o n a n c e t h a t m i g h t be e x p l o i t e d i n t h e s t u d y o f a d s o r p t i o n a n d c a t a l y s i s h a d b e e n f a i r l y v e i l d e l i n e a t e d , v i t h one e x c e p t i o n . Improvements i n i n s t r u m e n t a t i o n , d e v e l o p m e n t s i n t h e o r y , and b e t t e r c o r r e l a t i o n s v i t h o t h e r e x p e r i m e n t s v e r e t o come, b u t , e x c e p t f o r t h e a p p l i c a t i o n s o f t h e NMR c h e m i c a l s h i f t , s u b s t a n t i a l b e g i n n i n g s h a d b e e n made. The d e s i r e o f p h y s i c i s t s t o measure p r e c i s e l y n u c l e a r m a g n e t o g y r i c r a t i o s v a s , as v e have s e e n , one o f t h e i m p e l l i n g



27.

BREY


383

reasons for the invention of the NMR Experiment. Very shortly, however, i t became evident that this effort was to be less f r u i t f u l than anticipated, when Knight observed that the resonance peak for cupric ion i n solution was accompanied by another peak, which he came to realize was produced by metallic copper i n the probe. The complication which entered the efforts at measurement of the nuclear properties i s that the electrons surrounding the nucleus get i n the way by altering the magnitude of the magnetic f i e l d which reaches the nucleus from that which i s produced by the external magnet. Called by the physicists, perhaps a b i t disdainfully, the "chemical s h i f t " , this feature of NMR was to become a keystone i n the structural application of NMR for molecules in the liquid phase. The magnitudes of chemical shift differences between nuclei in different environments are relatively small i n comparison to typical linewidths of resonances in the solid phase, and so nuclei of different shifts i n solids could not be distinguished by the techniques of the sixties. Adsorbed molecules f a l l i n a borderline region between solid and liquid; depending upon their mobility and consequent line narrowing, i t may or may not be possible to distinguish nuclei i n different chemical locations i n a molecule, although such distinctions would permit one to learn much about the relative restrictions on motion of different parts of a molecule as i t i s held on the surface. Wide-line NMR methods, using audio modulation and phase sensitive detection at the modulation frequency, gain sensiuvity at the expense of resolution, as the resonance lines are broadened by the modulation. And pulse methods, used without Fourier transformation to unravel the frequency content of the response, give only one averaged relaxation time result for a l l nuclei of one species, such as a l l the hydrogen atoms, i n a sample. Chemical shift distinctions would permit one to learn about the relative mobility of different parts of a molecule—and perhaps about the extent of the perturbation caused by the adsorption process—which might lead to a better model of the adsorbent-adsorbate interaction. It i s interesting to r e c a l l that, in the late 1950 s, Paul Emmett, v i s i t i n g the author's laboratories, told of his experience i n being persuaded to send a sample of an adsorbentadsorbate system to Varian for evaluation of NMR techniques. There came back an enthusiastic report of a substantial chemical shift difference between the adsorbate and the material i n the l i q u i d phase, to be followed shortly by another communication which stated that the difference disappeared when a magnetic susceptibility correction was applied to the results. This illustrates a problem which arises whenever one attempts to measure a chemical shift value i n a heterogeneous system, the problem of a suitable reference and of eliminating magnetic susceptibility effects which are particularly troublesome i n such systems of dubious geometry. Differences between chemical shifts i n the same molecule, i f they can be resolved, are !


384


s o u n d l y b a s e d , b u t a b s o l u t e v a l u e s may a

v e r y w e l l n o t be

attain-

l e

^ * A l t h o u g h t h e r e a r e some r e p o r t s t o t h e c o n t r a r y , i n s t r u m e n t a t i o n o f t h i s p e r i o d p r o b a b l y d i d not permit r e s o l u t i o n o f s h i f t d i f f e r e n c e s f o r hydrogen f o r s u r f a c e coverages of l e s s than a monolayer, those of c a t a l y t i c s i g n i f i c a n c e . However, i n 1 9 6 k K e n n e t h Lawson, i n t h e s e l a b o r a t o r i e s , was a b l e t o show by a d i f f e r e n t a p p r o a c h , u t i l i z i n g t e m p e r a t u r e dependence s t u d i e s , v a r i a t i o n s i n r e s t r i c t i o n s on b o n d i n g o f v a r i o u s p a r t s o f a l c o h o l o r amine m o l e c u l e s a d s o r b e d on h i g h - a r e a t h o r i u m o x i d e ( k 3 ) . F o r e x a m p l e , as t h e s y s t e m i s c o o l e d , t h e h y d r o x y l h y d r o g e n c o n t r i b u t i o n t o the p r o t o n resonance of methanol i s broadened at a b o u t -15°C t o t h e p o i n t where i t i s n o t o b s e r v e d , a l t h o u g h t h e m e t h y l r e s o n a n c e c o n t i n u e s as a r e l a t i v e l y n a r r o w l i n e down t o -115°. D e v e l o p m e n t s i n NMR methods i n r e c e n t y e a r s have made t h e p r o s p e c t s f o r e x t e n s i o n o f t h e use o f c h e m i c a l s h i f t d i f f e r e n c e s to characterize adsorption very exciting. F o r e x a m p l e , t h e use o f Waugh-type p u l s e s e q u e n c e s on h y d r o g e n has made i t p o s s i b l e t o a v e r a g e o u t most o f t h e d i p o l a r i n t e r a c t i o n s b e t w e e n n e i g h b o r i n g h y d r o g e n atoms, a l t h o u g h t h e e x p e r i m e n t i s r a t h e r d i f f i c u l t t o c a r r y out because o f t h e high-power, s h o r t d u r a t i o n p u l s e s w h i c h must be a p p l i e d t o t h e s p i n s y s t e m (kk). F o r o t h e r n u c l e i t h a n h y d r o g e n , w h i c h have much g r e a t e r chemical s h i f t ranges, behavior of d i f f e r e n t p a r t s of molecules may more r e a d i l y be d i s t i n g u i s h e d . I n r e s u l t s from our l a b o r a t o r i e s , f o r w h i c h we have n o t b e e n a b l e t o g e n e r a t e an u n a m b i g u ous m o d e l , r e s o n a n c e s f o r a l l f o u r n o n e q u i v a l e n t t y p e s o f f l u o r i n e i n p e r f l u o r o p r o p y l e n e a d s o r b e d on aluminum o x i d e were r e solved. The a p p e a r a n c e o f two d i f f e r e n t CF resonances at s l i g h t l y d i f f e r e n t c h e m i c a l s h i f t s s u g g e s t s t h e p r e s e n c e o f two d i s t i n c t arrangements o f t h e a d s o r b e n t - a d s o r b a t e complex. C a r b o n - 1 3 s p e c t r o s c o p y seems t o have g r e a t p o t e n t i a l f o r t h e s t u d y o f h e t e r o g e n e o u s s y s t e m s (k3_ k 6 ) . Not o n l y i s t h e c h e m i c a l s h i f t r a n g e s u b s t a n t i a l , b u t t h e use o f s t r o n g i r r a d i a t i o n o f h y d r o g e n s e l i m i n a t e s t h e d i p o l a r b r o a d e n i n g f r o m t h a t s o u r c e and m a g i c a n g l e s p i n n i n g a v e r a g e s o u t c h e m i c a l s h i f t a n i s o t r o p y (kj). C r o s s p o l a r i z a t i o n f r o m p r o t o n s (U8) can be u t i l i z e d t o enhance s e n s i t i v i t y , and t h e a v a i l a b i l i t y o f h i g h - f i e l d s u p e r c o n d u c t i n g s p e c t r o m e t e r s may be h e l p f u l . A l t h o u g h u n f a v o r a b l e e f f e c t s on r e l a x a t i o n t i m e s may u n d e r some c i r c u m s t a n c e s be a» d i s a d v a n t a g e f o r h i g h - f i e l d observation of spin-1/2 n u c l e i , high f i e l d s are b e n e f i c i a l f o r q u a d r u p o l e n u c l e i s u c h as aluminum. Magic-angle s p i n n i n g i s a l s o p a r t i c u l a r l y h e l p f u l i n removing f i r s t - o r d e r quadrupolar i n t e r a c t i o n s of these n u c l e i ( k 9 ) . A l t h o u g h we h a v e n o t y e t q u i t e a t t a i n e d S e l w o o d s g o a l o f f o l l o w i n g c a t a l y t i c r e a c t i o n s by magnetic resonance w h i l e t h e r e a c t i o n s a r e o c c u r r i n g , t h e r e i s s t i l l hope t h a t t h e f u t u r e w i l l see s u b s t a n t i a l p r o g r e s s i n e x p e r i m e n t a l t e c h n i q u e s s u c h t h a t h i s v i s i o n w i l l become a r e a l i t y . W h i l e we must c o n t i n u e t o a s k


9

9

1


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questions about the results of magnetic resonance—Are these results relevant to the catalytic process? Are the molecule or radicals giving rise to the spectra intermediates i n the reaction or do they represent deadend byways?—answers w i l l often be forth coming from consideration of circumstantial evidence and from comparison with other spectroscopic and kinetic results, and much of interest and value surely remains to be learned.


Reviews of the Literature For readers who may wish further details on the subject, we conclude by mentioning some reviews and collections of papers which may serve as a guide to the literature of the subject. In a recent volume on spectroscopic methods i n catalysis, there i s one chapter on NMR and one on EPR; both contain references to other earlier reviews ( 5 0 ) . EPR i n catalysis was reviewed by Kokes i n 1968 (51) and by Tanford i n 1972 (52). Nuclear relaxation studies of adsorbates were reviewed by Packer i n 1967 (53), and in 1972, Pfeifer ( 5 k ) provided a comprehensive account of the theory of relaxation and diffusion analysis for adsorbates together with a complete l i s t i n g of publications up to that time, arranged alphabetically by authors. In 1972, an extensive article on NMR written by the French-Belgian group of workers i n this f i e l d appeared i n Catalysis Reviews ( 5 5 ) . Finally, for listings of literature i n the last few years, one may consult recent volumes of the Specialist Periodical Reports published by the Royal Society (56, 57, 58). Literature Cited 1.

Kopfermann, H. "Nuclear Moments," translated by Ε. E. Schneider, Academic Press, New York, 1958. 2. Gorter, C. J.; Broer, L. J . F. Physica 1942, 9, 591. 3. Zavoisky, E. J . Phys. (U.S.S.R.) 1945, 9, 211. 4. Purcell, E. M.; Torrey, H. C.; Pound, R. V. Phys. Rev. 1946, 69, 37. 5. Bloch, F . ; Hansen, W. W.; Packard, M. E. Phys. Rev. 1946, 69, 127. 6. Spooner, R. B.; Selwood, P. W. J. Am. Chem. Soc. 1949, 71, 218U. 7. Bloembergen, N.; Purcell, Ε. M.; Pound, R. V. Phys. Rev. 1948, 73, 679. 8. Selwood, P. W.; Schroyer, F. K. Disc. Faraday Soc. 1950, 8, 337. 9. Hickmott, T. W.; Selwood, P. W. J . Phys. Chem. 1956, 60, 452. 10. "Advances in Catalysis", Vol. IX, Academic Press, New York, 1957. 11. Bennett, J . E . ; Ingram, D. J . E . ; Tapely, J . G. J . Chem. Phys. 1955, 23, 215. 12. Turkevich, J.; Laroche, J . Z. physik. Chem. 1958, 15, 399-


386

13. 14. 15. 16. 17.


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HETEROGENEOUS

CATALYSIS

Fuschillo, N.; Aston, J . G. J . Chem. Phys. 1956, 24, 1277Fuschillo, N.; Renton, C. A. Nature 1957, 180, 1063. Stottlemyer, Q. R.; Murray, G. R.; Aston, J . G. J . Am. Chem. Soc. I960, 82, 1284. Mays, J . M.; Brady, G. W. J . Chem. Phys. 1956, 25, 583. Aston, J . G.; Bernard, J . W. J. Am. Chem. Soc. 1963, 85, 1573. Winkler, H. Kolloid-Z., 1958, 161, 127. O'Reilly, D. E. "Advances in Catalysis," Vol. XII, Academic Press, New York, 1960; 31-116. O'Reilly, D. E . ; Leftin, H. P.; Hall, W. K. J . Chem. Phys. 1958, 29, 970. "Spectroscopy and Relaxation at Radiofrequencies," Proceed ings of the Tenth Colloque Ampere, Leipzig; North-Holland, Amsterdam, 1962. Poole, C. P.; Kehl, W. L.; MacIver, D. S. J . Catal. 1962, 1, 407. Kokes, R. J . "Experimental Methods in Catalytic Research," Anderson, R. Β . , Ed., Academic Press, New York, 1968; 467470. Hall, W. K. J . Catal. 1962, 1, 53. Rooney, J. J.; Pink, R. C. Proc. Chem. Soc. 1961, 70. Rooney, J. J.; Pink, R. C. Trans. Faraday Soc. 1962, 58, 1632. Fogo, J . K. J . Phys. Chem. 1961, 65, 1919Kazanskii, V. B.; Pariiskii, G. B.; Voevodskii, V. V. Disc. Farad. Soc. 196l, 31, 203. Kazanskii, V. B.; Pecherskaya, Y. I. Kinetic Catalysis (U.S.S.R.), Eng. trans. 1961, 2, 417. Eramett, P. H.; Livingston, R.; Zeldes, Η., Kokes, R. J . J. Phys. Chem. 1962, 66, 921. Zimmerman, J. R.; Holmes, B. G.; Lasater, J . A. J . Phys. Chem. 1956, 60, 1157. Zimmerman, J. R.; Brittin, W. E.; J. Phys. Chem. 1957, 6l, 1328. Zimmerman, J. R.; Lasater, J . A. J . Phys. Chem. 1958, 62, 1157. Woessner, D. E. J . Chem. Phys. 1961, 35, 41. Woessner, D. E . ; Zimmerman, J. R. J. Phys. Chem. 1963, 67, 1590. Woessner, D. E. J . Chem. Phys. 1963, 39, 2782. Woessner, D. E. J . Phys. Chem. 1966, 70, 1217. Resing, H. A. J . Chem. Phys. 1965, 43, 669. Resing, H. A. Advan. Mol. Relaxation Processes, 1968, 1, 109. Erb, E . ; Motchane, J . L . ; Uebersfeld, J . Compt. rend. 1958, 246, 2121. Jacubowicz, M.; Uebersfeld, J . Compt. rend. 1959, 249, 2743. Krebs, J. J.; Thompson, J. K. J. Chem. Phys. 1962, 36, 2509. Brey, W. S.; Lawson, K. D. J . Phys. Chem. 1964, 68, 1474.


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Kaplan, S.; Resing, Η. A.; Waugh, J . S. J . Chem. Phys. 1973, 59, 5681. Gay, I. D, J. Phys, Chem. 1974, 78, 38. Michel, D.; pfeifer, J.; Deljnau, J . J . Magn. Reson. 1981, 45, 30. Stejskal, E. O.; Schaefer, J.; Henis, J . M. S.; Tripodi, M. K. J . Chem. Phys. 1974, 61, 2351 Vaughan, R. W.; Schreiber, L. B.; Schwarz, J . A. "Magnetic Resonance in Colloid and Interface Science," Resing, Η. Α.; Wade, C. G., Eds., ACS Symposium Series 34, American Chemical Society, Washington, 1976; 275. Fyfe, C. Α.; Gobbi, G. C.; Hartman, J . S.; Lenkinski, R. E.; O'Brien, J . H.; Beange, E. R.; Smith, M. A. R. J . Magn. Reson. 1982, 47, 168. Delgass, W. N.; Haller, G. L . ; Kellerman, R.; Lunsford, J . H. "Spectroscopy in Heterogeneous Catalysis," Academic Press, New York, 1979. Kokes, R. J . "Experimental Methods in Catalytic Research," Anderson, R. B., Ed., Academic Press, New York, 1968; 436475. Lunsford, J . H. "Advances in Catalysis," Vol. 22, Academic Press, New York, 1972; 265-344. Packer, K. J . "Progress in Nuclear Magnetic Resonance Spectroscopy," Emsley, J . W.; Feeney, J.; Sutcliffe, L. Η., Eds., Vol. 3, Pergamon Press, Oxford, 1967; 87-128 Pfeifer, H. "NMR Basic Principles and Progress," Diehl, P.; Fluck, E . ; Kosfeld, R., Eds., Vol. 7, Springer, New York, 1972; 53-153. Derouane, E. G.; Fraissard, J.; Fripiat, J. J.; Stone, W. E. E. Catalysis Reviews, 1972, 7, 121-212. "Specialist Periodical Reports. Spectroscopic Properties of Inorganic and Organometallic Compounds," Vol. 14, Royal Society of Chemistry, London, 1981; 102-105. "Specialist Periodical Reports. Nuclear Magnetic Reso nance," Vol. 9, Royal Society of Chemistry, London, 1980; 143-146, 268-269. "Specialist Periodical Reports. Electron Spin Resonance," Vol. 6, Royal Society of Chemistry, London, 198l; 119-121, 232.

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