Structural and Thermodynamic Basis for Catalytic Behavior of Bismuth

4 Structural and Thermodynamic Basis for Catalytic Behavior of Bismuth-Cerium Molybdate Selective Oxidation Catalysts 1

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J. F. BRAZDIL , R. G. TELLER , R. K. GRASSELLI , and E. KOSTINER 1

Sohio Research Center, The Standard Oil Company (Ohio), Cleveland, OH 44128 Department of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, CT 06268

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The Bi Ce Mo O two phase system has been examined for its activity in the catalytic oxidation of propylene to acrylonitrile. The two phases have been characterized as a solid solution of Bi in cerium molybdate and Ce in bismuth molybdate. Results of these oxidation studies have been correlated with structural results on the pure and doped end members. A plot of catalytic activity versus x shows three maxima which coincide with the maximum concentration of Ce in Bi Mo O , Bi in Ce Mo O and equal concentrations of cerium in bismuth molybdate and bismuth in cerium molybdate. These results suggest that selective propylene ammoxidation occurs in a trifunctional matrix which contains metals that; activate propylene to form an a l l y l intermediate (Bi), insert oxygen into the a l l y l i c intermediate, (Mo) and contain a redox couple (Ce). Aspects of phase cooperation in a multiphase catalyst are also discussed. 2-x

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Much of the understanding of the s o l i d state mechanism of heterogeneous c a t a l y s i s stems from fundamental studies of single phase model compounds (1-5). In many cases, the r o l e of a metal component i n a c a t a l y t i c process has been discerned through i t s incorporation into s o l i d solutions of r e l a t i v e l y inert host matrices (6). In the case of the selective oxidation and ammoxidation of olefins to unsaturated aldehydes and n i t r i l e s , respectively ( e . g . the ammoxidation of propylene to a c r y l o n i t r i l e ) , such studies have established several important tenets for the process. These include the need for the coexistence of key c a t a l y t i c elements with the proper electronic structure, redox chemistry, and metal-oxygen bond strength (_7). It i s however well recognized that the most effective c a t a l y s t s , be they mixed metals or mixed metal oxides, are usually multiphase in nature (8). Some progress has been made in understanding the source of the synergistic effects observed in these multiphase c a t a l y s t s . For example, S i n f e l t and coworkers 09) have been able to explain the

0097-6156/85/0279-0057$06.00/0 © 1985 American Chemical Society

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c a t a l y t i c b e h a v i o r o f b i m e t a l l i c , b i p h a s i c systems t h r o u g h the use of s t r u c t u r a l c h a r a c t e r i z a t i o n t o o l s such as EXAFS. In oxide c a t a l y s t s for s e l e c t i v e o x i d a t i o n , rigorous studies of multiphase c a t a l y s t s have been l i m i t e d p r i m a r i l y t o vanadium o x i d e s (10-12). I n t h i s c a s e , i t has been shown t h a t the c o e x i s t e n c e o f s t r u c t u r a l l y r e l a t e d o x i d i z e d and reduced vanadium o x i d e phases i s s t a b i l i z e d by the presence of s t r u c t u r a l l y coherent phase boundaries. Recently, significant efforts have been made to d e v e l o p a r a t i o n a l mechanism f o r the c a t a l y t i c b e h a v i o r o f multicomponent bismuth molybdate based selective oxidation c a t a l y s t s ( 1 3 - 1 5 ) . However, the c o m p l e x i t y o f the c a t a l y s t systems i n v e s t i g a t e d has prevented the development o f a r i g o r o u s model f o r c a t a l y t i c behavior. Our r e c e n t work on the b i s m u t h - c e r i u m molybdate c a t a l y s t system has shown t h a t i t can serve as a t r a c t a b l e model f o r the s t u d y o f the s o l i d s t a t e mechanism of s e l e c t i v e o l e f i n o x i d a t i o n by multicomponent molybdate c a t a l y s t s . A l t h o u g h c o m p o s i t i o n a l l y and s t r u c t u r a l l y q u i t e s i m p l e compared to o t h e r m u l t i p h a s e molybdate c a t a l y s t s y s t e m s , b i s m u t h - c e r i u m molybdate c a t a l y s t s are e x t r e m e l y effective for the selective ammoxidation of propylene to a c r y l o n i t r i l e (16). I n p a r t i c u l a r , we have found t h a t the a d d i t i o n of c e r i u m to b i s m u t h molybdate s i g n i f i c a n t l y enhances i t s c a t a l y t i c a c t i v i t y for the selective ammoxidation of propylene to acrylonitrile. Maximum c a t a l y t i c a c t i v i t y was observed for s p e c i f i c c o m p o s i t i o n s i n the s i n g l e phase and two phase r e g i o n s o f the phase diagram ( 1 7 ) . These c h a r a c t e r i s t i c s o f t h i s c a t a l y s t system a f f o r d the o p p o r t u n i t y t o understand the p h y s i c a l b a s i s f o r synergies i n multiphase c a t a l y s t s . I n a d d i t i o n to t h i s p r e v i o u s l y p u b l i s h e d work, we a l s o i n c l u d e some o f our most r e c e n t r e s u l t s on the b i s m u t h - c e r i u m molybdate s y s t e m . As s u c h , the p r e s e n t account r e p r e s e n t s a summary o f our i n t e r p r e t a t i o n s o f the d a t a on t h i s system. Experimental B i s m u t h c e r i u m molybdates were prepared by c o p r e c i p i t a t i o n u s i n g aqueous s o l u t i o n s o f ( N H ^ M o ^ ^ , (NH, K C e d K O , , and B i ( N 0 ~ ) ' 5Ô. The c a t a l y s t s were supported on SiO^ (20% by weight) u s i n g an ammonium s t a b i l i z e d s i l i c a s o l . Samples f o r d i f f r a c t i o n a n a l y s i s were u n s u p p o r t e d . Samples were c a l c i n e d i n a i r at 290 and 425°C f o r t h r e e hours each f o l l o w e d by 16 hours a t 500, 550, o r 6 0 0 ° C . X-ray powder p a t t e r n s were o b t a i n e d u s i n g a Rigaku D / M a x - I I A X - r a y diffTactometer u s i n g Cu Κ r a d i a t i o n . Powder n e u t r o n d i f f r a c t i o n d a t a f o r B i ^ gCe^ 2 ^ ° 3 ^ 1 2 c o l l e c t e d at Brookhaven N a t i o n a l L a b o r a t o r y ' s * Higfi F l u x Beam Reactor. D e t a i l s o f the e x p e r i m e n t a l procedure and R i e t v e l d refinement have been r e p o r t e d p r e v i o u s l y ( 1 8 ) . S t a r t i n g parameters f o r Ce doped and pure B i ^ M o ^ O ^ were taken from a s i n g l e c r y s t a l x - r a y d i f f r a c t i o n study o f B i 6 o ^ 0 by Van den E l z e n and R i e c k (19a). Time-of-flight (TOF) Powder n e u t r o n d i f f r a c t i o n d a t a for BiJfoÔ.2 c o l l e c t e d at Argonne N a t i o n a l L a b o r a t o r y s I n t e n s e P u l s e d Neutron Source (iPNijO u t i l i z i n g the S p e c i a l Environment Powder D i f f r a c t o m e t e r (SEPD). D e t a i l s o f the i n s t r u m e n t , d a t a c o l l e c t i o n software, and R i e t v e l d a n a l y s i s software have been 3

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previously published(20). TOF d a t a from the b a c k s c a t t e r i n g d a t a banks (20=150°) were used i n the a n a l y s i s and corresponded to TOF(min) of 7500msec (d(min)=.99 Â) and TOF(max) of 23500msec (d(max)=3.10 À ) . In a d d i t i o n to background (_5), h a l f w i d t h ( 3 ) , a b s o r b t i o n , e x t i n c t i o n , l a t t i c e ( 4 ) , and s c a l e p a r a m e t e r s , a l l p o s i t i o n a l and i s o t r o p i c t h e r m a l parameters were v a r i e d i n the l e a s t - s q u a r i n g p r o c e s s , f o r a t o t a l o f 84 v a r i a b l e s . The l e a s t squares p r o c e s s converged to R(p)=O.037, R(wp)=O.056, R(exp)=O.044. Refinements were a l s o c a r r i e d out f o r TOF(min) v a l u e s o f 7000 and 8000 s e c . There was no s i g n i f i c a n t d i f f e r e n c e between the r e s u l t s o f those r e f i n e m e n t s and those presented h e r e . Figure 1 displays the n e u t r o n d i f f r a c t o g r a m of B i ^ M o ^ O ^ S i n g l e c r y s t a l s o f B i doped c e r i u m molybdate were prepared i n s e a l e d tube e x p e r i m e n t s . A quantity of m a t e r i a l of composition B i C e M o ^ O ^ was packed i n t o a welded g o l d c a p s u l e pinched shut a t the_gpen end, p l a c e d i n a . q u a r t z . t u b e .and s e a l e d under vacuum (10 mm H g ) . The tube was h e l d i n a v e r t i c a l p o s i t i o n and kept a t 950°C f o r one week. S i n g l e c r y s t a l s c o r r e s p o n d i n g to two phases were i s o l a t e d from these e x p e r i m e n t s . Red c r y s t a l s h a r v e s t e d possessed c e l l parameters and c o m p o s i t i o n ( v i a EDX a n a l y s i s ) t h a t was c o n s i s t e n t w i t h Ce doped Bi^MoÔ ^ . * s i m i l a r f a s h i o n , the amber c r y s t a l s t h a t were a l s o i s o l a t e d from t h i s procedure were i d e n t i f i e d as h a v i n g the ^ e ^ M o ^ O ^ s t r u c t u r e w i t h some B i p r e s e n t . EDX a n a l y s i s o f t h r e e amber c r y s t a l s gave an average c o m p o s i t i o n o f Ce. g Q i 3 3 ° n Q ( t o 3 Mo atoms, o x y g e n ^ t o i c h i o m e t r y c a l c u l a t e d on t n e ' b a s i s o f charge b a l a n c e assuming Ce ) . S i n g l e c r y s t a l x - r a y d i f f r a c t i o n d a t a were c o l l e c t e d on one of the amber c r y s t a l s w i t h a FACS-I automated P i c k e r d i f f r a c t o m e t e r w i t h Zr f i l t e r e d Mo Κα r a d i a t i o n . L a t t i c e parameters ( a = 1 6 . 8 8 6 ( 5 ) , b = l l . 8 3 9 ( 3 ) , c=15.797(5)A, b = 1 0 8 . 6 4 ( l ) were determined by a l e a s t squares f i t of 24 r e f l e c t i o n s i n the a n g u l a r range 5 1 < 2 0 < 6 1 ° . D e t a i l s o f the i n s t r u m e n t , d a t a c o l l e c t i o n , d a t a r e d u c t i o n , and a n a l y s i s have been p u b l i s h e d ( 2 1 ) . S t a r t i n g parameters f o r the f u l l m a t r i x l e a s t squares refinement were taken from L a ^ M o ^ Q ^ d ^ b ) w i t h 90%Ce and 10%Bi o c c u p a t i o n f o r each L a s i t e . Parameters v a r i e d were: a l l p o s i t i o n a l p a r a m e t e r s , a n i s o t r o p i c temperature f a c t o r s f o r m e t a l atoms, i s o t r o p i c temperature f a c t o r s f o r the oxygen atoms, s c a l e and e x t i n c t i o n f a c t o r s and s c a t t e r i n g f a c t o r s f o r the C e / B i s i t e s . Data i n the a n g u l a r range 201.0 A) and the Β c a t i o n f a i r l y s m a l l (on the o r d e r o f O.6 A ) . The p r o t o t y p e o f t h i s s t r u c t u r e i s the m i n e r a l s c h e e l i t e , CaWO,, w h i c h c r y s t a l l i z e s ( 2 1 ) w i t h four formula u n i t s per u n i t c e l l i n tjjç t e t r a g o n a l space group 1 4 ^ / a (a_ = 5 . 2 4 3 , £ - 11.376 A ) . Each Ca ion is surrounded by eight oxygen atoms from different t e t r a h e d r a l l y coordinated Β i o n s , £our 2.44 A and f o u r at 2.48 A. The n e a r l y r e g u l a r d i s c r e t e WO. t e t r a h e d r a have f o u r e q u a l W-0 d i s t a n c e s ( 1 . 7 8 A ) . The i d e a l s t r u c t u r e i s i l l u s t r a t e d i n F i g u r e 2. One c h a r a c t e r i s t i c of the scheelite structure-type i s the number and e x t e n t o f c a t i o n i c o x i d a t i o n s t a t e s and d e f e c t ( c a t i o n d e f i c i e n t ) s t r u c t u r e s t h a t have been f o u n d . The s i n g l e g u i d e to the f o r m a t i o n o f s c h e e l i t e - t y p e structure seems t o be the a b i l i t y of A c a t i o n s to be e i g h t - c o o r d i n a t e d (i.e., r a t h e r l a r g e ) and Β i o n s ^ t o a t t a i n t e t r a h e d r a l c o o r d i n a t i o n ( n o t e , however, t h a t P0^ or S i O . c o n t a i n i n g s c h e e l i t e s a r e unknown). If we f i r s t c o n s i d e r the s i m p l e r cases o f heterovalent s u b s t i t u t i o n w i t h t h i s s t r u c t u r e - t y p e (22) we f i n d , i n a d d i t i o n to the 2+/6+ (A+/B+) v a l a n c e s typified by CaWO, , the valance c o m b i n a t i o n s 1+/7+ (KRu0 ), 3+/5+ ( G d M o 0 ) , and 4+/4+ ( C e G e 0 ) . E x t e n d i n g t h i s by d o u b l i n g ( o r t r i p l i n g ) the c h e m i c a l f o r m u l a we can o b t a i n , by c o u p l e d s u b s t i t u t i o n , mixed ( o r s u b s t i t u t e d ) i o n s on the A s i t e : ( l + , 3 + ) / 6 + [ N a L a ( M o O , ) ] , ( l + , 4 + ) / 6 + [Na Th(MoO, ) J , and (2+,4+)/5+ [ P b T h C V O ^ J . * S u b s t i t u t i o n on the anion" s i t e (the Β s i t e ) has a l s o been observed. S e v e r a l examples a r e : 3+/(4+,6+) [ L a ( S i O , ) ( W O , ) ] , 3+/(3+,6+) [ B i ( F e 0 ) ( M o 0 ) ] , and 3+/(2+,6+) [ B i ^ Z n O , ) ( M o O j ] . I f two o r more i o n s occupy e i t h e r the A o r the S s i t e , the p o s s i b i l i t y of having e i t h e r an ordered or a d i s o r d e r e d (random) s t r u c t u r e w i l l e x i s t . F o r example, o r d e r on the A s i t e s o c c u r s i n KEu(Mo0 ). The compound B i ^ F e O ^ M o O ^ e x i s t s i n b o t h an o r d e r e d and d i s o r d e r e d f o r m ( 2 3 ) , the ordered form r e s u l t i n g from the o r d e r i n g of the F e 0 and MoO, ( B - i o n ) t e t r a h e d r a . The c r y s t a l c h e m i s t r y o f s c h e e l i t e - t y p e s t r u c t u r e s i s f u r t h e r c o m p l i c a t e d by d e f e c t s t o i c h i o m e t r i e s w i t h extensive vacancies at cation sites. In g e n e r a l , random c a t i o n v a c a n c i e s a r e examples o f point defects. However, at h i g h c o n c e n t r a t i o n s , an o r d e r i n g o f v a c a n c i e s can o c c u r at which time the vacancy can no l o n g e r be c o n s i d e r e d as a p o i n t d e f e c t and a new p e r i o d i c l a t t i c e ( o r u n i t c e l l ) " w i l l have been g e n e r a t e d . As an example o f r a n d o m l y d i s o r d e r e d v a c a n c i e s , the coupled s u b s t i t u t i o n o f two B i i o n s and one vacancy f o r t h r e e Pb ions i n PbMo O. g i v e s r i s e (24) to a s e r i e s of s o l i d solutions a

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(l~3 ) 2x^x^ °4^3' Φ represents a c a t i o n vacancy. Depending on t e m p e r a t u r e , a fairly l a r g e range o f random c a t i o n v a c a n c i e s (up t o 15%) has been o b s e r v e d . At h i g h c o n c e n t r a t i o n o f c a t i o n v a c a n c i e s (Φ = O . 3 3 ) , new o r d e r e d c o m p o s i t i o n s are observed w i t h u n i t c e l l s c o r r e s p o n d i n g to s u p e r c e l l s o f the scheelite structure. The g e n e r a l f o r m u l a f o r

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Οο F i g u r e 2 . A r e p r e s e n t a t i o n o f the s c h e e l i t e (CaWO^) s t r u c t u r e . Reproduced by p e r m i s s i o n from A n n . Ν . Y . A c a d . S c i . , V o l . 272, p . 2 3 , a u t h o r s : A . S l e i g h t and W. L i n n .

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these c a t i o n d e f i c i e n t scheelites i s Α^ ^τΦ^ 3 3 4 » ( ~ i° v a c a n c y ) . An e q u i v a l e n t n o t a t i o n ( m u l t i p l y i n g " t h r o u g h by 3) would be A^ φ ( B O ^ ) - o r , more s i m p l y , A ( B O . ) ^ . Three d i f f e r e n t schemes for c a t i o n ordering i n defect s c h e e l i t e have been observed - the EuîWO^)^ s t r u c t u r e (25) ( w h i c h i s found f o r s e v e r a l r a r e e a r t h molybdates and t u n g s t a t e s ) , the LaîMoO^)^ s t r u c t u r e (26) (found for larger rare e a r t h molybdates o n l y ) , and B i ^ i M o O ^ ) ^ ( 1 8 ) , a unique s t r u c t u r e . The c a t i o n o r d e r i n g schemes f o r these t h r e e s t r u c t u r e s as d e r i v e d from the i d e a l s c h e e l i t e s t r u c t u r e i s shown i n F i g u r e 3 , w h i c h i s a p r o j e c t i o n o f each o f t h e s e s t r u c t u r e s down t h e i r r e s p e c t i v e m o n o c l i n i c b-axes ( f o r the d e f e c t compounds) w i t h r e f e r e n c e to the c o r r e s p o n d i n g £ - a x i s p r o j e c t i o n o f CaWO^. I t i s evident from t h i s schematic r e p r e s e n t a t i o n t h a t these s t r u c t u r e s not o n l y d i f f e r i n the manner i n w h i c h the v a c a n c i e s o r d e r (and t h e r e f o r e i n t h e i r u n i t c e l l m e t r i c s ) but i n d i s t o r t i o n s of the BO^ p o l y e d r a . Diffraction studies have shown t h a t these d i s t o r t i o n s , w h i c h a r e most pronounced i n the structure of B i ^ i M o O ^ ) ^ ( v i d a i n f r a ) , a l s o a f f e c t the Β i o n c o o r d i n a t i o n . C a t a l y t i c A c t i v i t y and Phase C o m p o s i t i o n All

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The v a r i a t i o n i n a c t i v i t y f o r a c r y l o n i t r i l e f o r m a t i o n as a f u n c t i o n o f c o m p o s i t i o n f o r the *2-x^ x °3^12 * * F i g u r e 4 . Maxima i n a c t i v i t y , as measured by f i r s t o r d e r r a t e constant f o r propylene disappearance, o c c u r at t h r e e c o m p o s i t i o n s . A p a r t i a l phase diagram f o r the s y s t e m , c o n s t r u c t e d w i t h powder x - r a y d i f f r a c t i o n i s d i s p l a y e d i n Figure 5. The phase diagram c o n s i s t s of t h r e e r e g i o n s , two of w h i c h are s i n g l e phase r e g i o n s . On the B i - r i c h s i d e , Ce i s s o l u b l e i n Bi2Mo 0^2« The maximum s o l u b i l i t y of Ce i n b i s m u t h molybdate i s a p p r o x i m a t e l y 10 mole p e r c e n t . The C e - r i c h s i d e c o n s i s t s o f a solid s o l u t i o n of B i i n C e M o 0 . , w h i c h has the La^Mo^^ structure. The maximum s o l u b i l i t y of B i i n t h i s phase Is a p p r o x i m a t e l y 50 mole p e r c e n t . The i n t e r m e d i a t e r e g i o n c o n s i s t s o f a m i x t u r e o f b o t h phases. Comparison o f the v a r i a t i o n i n c a t a l y t i c a c t i v i t y w i t h c o m p o s i t i o n as presented i n F i g u r e 4 w i t h the phase diagram r e v e a l s t h a t maxima i n c a t a l y t i c a c t i v i t y o c c u r s a t the s o l u b i l i t y l i m i t s o f the two s i n g l e phases. An u n d e r s t a n d i n g of the b a s i s f o r t h i s b e h a v i o r r e q u i r e s a b e t t e r d e f i n i t i o n o f the s o l i d s t a t e s t r u c t u r a l a s p e c t s o f the c a t a l y s t s y s t e m . B

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A n a l y s i s o f the S i n g l e Phase C a t a l y s t s I n an attempt to i d e n t i f y any B i / C e o r d e r i n g o r s i t e p r e f e r e n c e , d i f f r a c t i o n d a t a was c o l l e c t e d on two end members o f the s e r i e s . A powder n e u t r o n d i f f r a c t i o n d a t a set was c o l l e c t e d f o r a sample o f B i . gCe^ 2 ^ ° 3 ^ 1 2 composition. Additionally, since neutron d i f f r a c t i o n d a t a had not been taken on the the end member of the series, Bi Mo^0 , and comparisons to t h i s model were deemed 9

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Ο Ο

ο ο ο

Θ

Ο

Ο

ο

Ο

ο

Ο

ο

ο

Ο

Ο

ο

Ο

Ο Ο

F i g u r e 3 . I d e a l s c h e e l i t e s t r u c t u r e compared to o r d e r e d d e f e c t s t r u c t u r e s . P r o j e c t i o n i s down the c a x i s w i t h o n l y 1/2 the u n i t c e l l shown i n t h i s d i r e c t i o n . Shaded c i r c l e s and t e t r a h e d r a a r e a t the top o f l e v e l , unshaded 1/4 o f the way down the u n i t c e l l , (a) CaWO . (b) L a ( M o 0 ) . (c) E u ( W 0 ) . (d) B i ( M o 0 ) . Reproduced by p e r m i s s i o n from A n n . Ν. Y . A c a d . S c i . , V o l . 272, p . 2 3 , a u t h o r s : A . S l e i g h t and W. L i n n . 2

4

3

2

4

3

2

4

4.

BRAZDIL ET AL.

Bi- Ce Molybdate Selective Oxidation Catalysts

65

• • Ο - 600°C

οI 0

ι

O.2

ι O.4

ι O.6

I

ι

I

1

O.8

1.0

1.2

1.4

χ

F i g u r e 4 . C a t a l y t i c a c t i v i t y as a f u n c t i o n o f c o m p o s i t i o n and r e a c t i o n temperature f o r the B i C e ( M o 0 ) system. Catalytic a c t i v i t y i s e x p r e s s e d as k , the f i r s t o r d e r r a t e c o n s t a n t f o r propylene disappearance. Reproduced from R e f . 17. C o p y r i g h t 1983 American C h e m i c a l S o c i e t y . 2

x

4

3

F i g u r e 5 . P a r t i a l phase diagram f o r the B i _ C e ( M o 0 ) system. Reproduced w i t h p e r m i s s i o n from R e f . 16. C o p y r i g h t 1983, Academic Press, Inc. 2

x

x

4

3

66


s i g n i f i c a n t , powder n e u t r o n d i f f r a c t i o n d a t a were c o l l e c t e d on t h i s m a t e r i a l as w e l l . The structure of Bi^MoÔ (perhaps better written Βί^^Φ^/0M0O, ) , i s based on s c h e e l i t e . As d i s c u s s e d above, t h e r e i s c o n s i d e r a b l e d i s t o r t i o n from the i d e a l s c h e e l i t e s t r u c t u r e , t h i s i s p r o b a b l y due to the B i l o n e p a i r o f e l e c t r o n s ( v i d a i n f r a ) . As a r e s u l t o f t h e s e d i s t o r t i o n s i n the a n i o n p a c k i n g , the Mo atoms are p e n t a - c o o r d i n a t e (as opposed to t e t r a h e d r a l l y c o o r d i n a t e d i n CaWO.) and MoÔg " d i m e r s " are o b s e r v e d , w i t h M o . . . Mo s e p a r a t i o n s of 3.4 Â and two b r i d g i n g 0 atoms. A l i s t i n g of Mo-0 d i s t a n c e s d e r i v e d from the n e u t r o n d i f f r a c t i o n d a t a f o r B i ^ M o ^ O ^ given i n T a b l e 1. Note t h a t the 0 c o o r d i n a t i o n about each o f the t h r e e c r y s t a l l o g r a p h i c a l l y d i s t i n c t Mo atoms i s similar. Each Mo atom c o n t a i n s one s h o r t ( d o u b l e ) Mo-0 b o n d , one s i n g l y bound 0 atom, two oxygens w i t h i n t e r m e d i a t e bond o r d e r s and one weakly bound oxygen atom. The r e s u l t i s a v e r y asymmetric oxygen c o o r d i n a t i o n sphere about each Mo atom. i

Table I .

s

A Comparison o f I n t e r a t o m i c Mo-0 D i s t a n c e s i n Bi Mo 0 and Β 1 g C e ^ 2

3

1 2

χ

Bi Mo 0 2

a

3

1 2

#

Bi

K

8

Ce

0

>

2

MD O 3

Mo(l)-0(4) -0(5) -0(2) -0(10) -0(10)'

1.70(1) 1.78(1) 1.80(2) 1.89(1) 2.20(1)

1.78(3) 1.67(2) 1.92(3) 1.85(2) 2.27(2)

Mo(2)-0(l) -0(9) -0(6) -0(3) -0(8)

1.65(2) 1.75(2) 1.85(2) 1.97(2) 2.16(2)

1.63(3) 1.68(3) 1.98(4) 1.90(3) 2.23(3)

Mo(3)-0(12) -0(11) -0(8) -0(7) -0(6)

1.74(2) 1.63(2) 1.87(2) 1.90(1) 2.48(2)

1.72(2) 1.88(3) 1.93(3) 1.83(3) 2.34(3)

1

2

a) Bond d i s t a n c e s a r e g i v e n i n angstroms, w i t h e s t i m a t e d s t a n d a r d d e v i a t i o n o f the l a s t d i g i t g i v e n i n p a r e n t h e s i s .

R e s u l t s ( T a b l e I ) from r e f i n e m e n t o f the powder n e u t r o n d i f f r a c t i o n d a t a f o r the Ce doped m a t e r i a l , B i . g Q 2 3 1 2 ' significant structural alterations have r e s u l t e d from Ce i n c o r p o r a t i o n i n t o the solid. The t h r e e Mo atoms a r e no l o n g e r chemically equivalent. The c o o r d i n a t i o n about M o ( l ) i s unchanged from t h a t o f the parent compound. A second Mo atom c o n t a i n s two s h o r t ( d o u b l e ) bonds t o oxygen atoms. These molybdate d i o x o ( d i m o l y b d e n y l ) groups a r e b e l i e v e d to be an i m p o r t a n t f e a t u r e f o r s e l e c t i v e o x i d a t i o n c a t a l y s i s (_7). The t h i r d Mo c o o r d i n a t i o n sphere c o n t a i n s no Mo-0 b o n d . Bond o r d e r c a l c u l a t i o n s (27) about C e

Μ θ

0

i

n

d

i

c

a

t

e

t

n

a

t

4.

BRAZDIL ET AL.


67

t h i s l a t t e r Mo s i t e i n d i c a t e the t o t a l Mo-0 bond o r d e r to be 5 . T h i s apparent r e d u c t i o n of Mo from 6 t o 5 i s most l i k e l y accomp a n i e d by some Ce o x i d a t i o n to 4 a n d / o r oxygen removal from the solid. The d i s t r i b u t i o n o f the Ce dopant i s not u n i f o r m . Of the t h r e e c a t i o n s i t e s i n the parent compound, the f i r s t two are f u l l y o c c u p i e d by B i atoms, w h i l e the t h i r d i s empty. This r e s u l t s i n a sequence o f c a t i o n o c c u p a t i o n on the [010] face as shown below i n A and B . S i t e a i s 92% B i , 4% C e , s i t e b i s 88% B i , 12% C e , and s i t e c , n o r m a l l y v a c a n t , i s 4% Ce o c c u p i e d . E x a m i n a t i o n o f the o x i d e environment about s i t e c i n d i c a t e s t h a t t h e r e i s i n s u f f i c i e n t room f o r a Ce c a t i o n . O c c u p a t i o n o f the s i t e must t h e r e f o r e r e s u l t i n some l o c a l d i s o r d e r , the most l i k e l y m a n i f e s t a t i o n t h e r e o f b e i n g a vacancy i n s i t e a ( t h e d i s t a n c e between s i t e s a and c are too s h o r t to a l l o w s i m u l t a n e o u s o c c u p a t i o n ) . Note a l s o t h a t s i t e a i s not 100% o c c u p i e d . Consequently, about 96% o f the time the d i s t r i b u t i o n of cations (Bi/Ce) in B i , o Q 2 3°12 below ( n o r m a l l y found i n b i s m u t h m o l y b d a t e ; , and 4% o f the time i t i s as i n B . C e

A) B)

M M

M M

• Ce

M •

M M

• •

M M

M o

i

S

a

S

i

n

A

M • M • M=Bi/Ce

The d i s t r i b u t i o n o f c a t i o n s r e p r e s e n t e d i n B i s r e m i n i s c e n t o f t h a t of C e M o 0 on the [010] f a c e . A s t r u c t u r a l study o f a Ce r i c h c a t a l y s t was a l s o u n d e r t a k e n . A s i n g l e c r y s t a l x - r a y d i f f r a c t i o n a n a l y s i s of a c r y s t a l of c o m p o s i t i o n Ce^ 3^°3^12 P ^ > * r e s u l t s (along w i t h comparisons t o I s o s t r u c t u r a l L a ^ o ^ O ^ ) a r e presented i n T a b l e II. The r e s u l t s o f the experiment i n d i c a t e t h a t the B i s u b s t i t u t e s f o r Ce i n the structure. U n l i k e B ^ M o ^ O , the parent compound C e M o 0 - has a r e l a t i v e l y u n d i s t o r t e d s c n e e l i t e - b a s e d s t r u c t u r e w i t h 1/3 c a t i o n v a c a n c i e s . The d i f f e r e n c e between the b i s m u t h and c e r i u m molybdate s t r u c t u r e s l i e s i n the o r d e r i n g o f t h e s e v a c a n c i e s as i n d i c a t e d above. The comparison between L a ^ M o ^ O . ^ , * doped cerium molybdate i s q u i t e good. The Mo atoms are approximately t e t r a h e d r a l l y coordinated to f o u r oxygen atoms, and the C e / B i atoms a r e eight coordinate. Because the gross d i s t o r t i o n s i n b i s m u t h molybdate are a t t r i b u t e d to the B i l o n e pairs, this r e l a t i v e l a c k of scheelite d i s t o r t i o n i n cerium molybdate i s not u n e x p e c t e d . At s i g n i f i c a n t l y lower concentrations of B i atoms l o n e p a i r - l o n e p a i r i n t e r a c t i o n s a r e m i n i m i z e d . As i n the b i s m u t h r i c h structure discussed above, the B i atoms i n B i doped cerium molybdate are not randomly d i s t r i b u t e d on the Ce s i t e s . The c o m p o s i t i o n s o f M ( l ) , M ( 2 ) , and M(3) ( s e e T a b l e I I ) a r e 86%Ce and 14%Bi, 92%Ce and 8 % B i , 73%Ce and 2 7 % B i , respectively. This compositional difference i s r e f l e c t e d i n the M ( 3 ) - 0 d i s t a n c e s as w e l l . Note t h a t the d i f f e r e n c e between the maximum and minimum M ( 3 ) - 0 d i s t a n c e s i s l a r g e r f o r B i doped c e r i u m molybdate than lanthanum m o l y b d a t e . E v i d e n t l y , a 27% o c c u p a t i o n o f B i ( w i t h the a t t e n d e n t l o n e p a i r ) i s s u f f i c i e n t f o r a s m a l l but n o t i c a b l e d i s t o r t i o n i n the average l o c a l 0 environment. Summar i z i n g the structural results on Ce i n c o r p o r a t i o n i n t o b i s m u t h 2

3

1 2

W

a

S

e r

o m e d

a n (

s

o

m

e

2

2

3

2

a n c

t

n

e

B

i

1 2

1.73 1.75 1.78 1.82 1.75 1.76 1.76 1.82 1.73 1.75 1.79 1.81 1.76 1.76 1.77 1.81 1.75 1.81

3

1.73 1.74 1.82 1.85 1.74 1.77 1.78 1.82 1.75 1.74 1.82 1.79 1.74 1.75 1.77 1.82 1.76 1.80

î.yô.a

1M o

3°12

3

La(l)-0(4) -0(12) -0(7) -0(16) -0(5) -0(16) -0(17) -0(10) La(2)-0(14) -0(9) -0(3) -0(6) -0(2) -0(13) -0(18) -0(11) La(3)-0(15) -0(1) -0(2) -0(8) -0(11) -0(8) -0(17) -0(10)

2

La Mo 0 1 2

n

d

2.48 2.48 2.50 2.51 2.52 52 57 57 46 48 52 52 53 2.53 2.54 57 44 46 49 53 55 2.55 2.55 2.59

a

C e

C e

B i

1.7

M

3°12

2.45 2.46 2.51 2.47 2.45 2.48 2.53 2.53 2.44 2.44 2.53 2.47 2.47 2.52 2.52 2.50 2.38 2.38 2.40 2.54 2.48 2.54 2.53 2.62

0.3 °3°12

M o

B i

0.3

M(l)-0(4) -0(12) -0(7) -0(16) -0(5) -0(16) -0(17) -0(10) M(2)0(14) -0(9) -0(3) -0(6) 0(2) -0(13) -0(18) -0(11) M(3)-0(15) -0(1) -0(2) -0(8) -0(11) -0(8) -0(17) -0(10)

l.7

2

3

each Mo-0 d i s t a n c e i s .01. Distances for A) The e s t i m a t e d standard deviation for La Mo 0 a r e t a k e n from Reference 26. A l l bond d i s t a n c e s a r e g i v e n i n a n g s t r o m s .

Mo(l)-0(4) -0(3) -0(1) -0(2) Mo(2)-0(7) -0(5) -0(6) -0(8) Mo(3)-0(9) -0(12) -0(11) -0(10) Mo(4)-0(l4) -0(13) -0(15) -0(16) Mo(5)-O(18) -0(19)

2

1 2

A Comparison o f Mo-0 D i s t a n c e s i n I ^ M o ^ O ^

La Mo 0

Table I I .

2 n

H

g 53

m

2?

Ο

C/3

4.


BRAZDIL ET AL.

69

molybdate and B i i n c o r p o r a t i o n i n t o c e r i u m molybdate one f i n d s nonrandom dopant s u b s t i t u t i o n . A p p a r e n t l y the symmetry o f the different +3 catôji s i t e s , ^ c o u p l e d w i t h the d i f f e r e n t s t e r i c r e q u i r e m e n t s o f Ce and B i i o n s , p l a y s an i m p o r t a n t r o l e i n d e t e r m i n i n g the o c c u p a t i o n s o f the v a r i o u s s i t e s . Additionally, in the case o f b i s m u t h m o l y b d a t e , a r a d i c a l a l t e r a t i o n i n the Mo-0 c o o r d i n a t i o n spheres r e s u l t s upon Ce i n c o r p o r a t i o n . Coupled w i t h t h i s d i s t o r t i o n t h e r e i s an apparent r e d u c t i o n o f one Mo atom, and the c r e a t i o n o f a d i m o l y b d e n y l group on another Mo s i t e . I n comparing the two s t r u c t u r e s , one f a i l s to f i n d a s t r u c t u r e type o r d i s t o r t i o n ( s u c h as M o . . . M o i n t e r a c t i o n s o r g r o s s l y d i s t o r t e d m e t a l environments) common t o b o t h m a t e r i a l s i n the structures described here. Y e t , some commonality i s expected to e x i s t based on the c l o s e correspondence o f the c a t a l y t i c d a t a . There are s e v e r a l p o s s i b l e e x p l a n a t i o n s for t h i s : a) c o n s i d e r a b l y more d i s t o r t i o n i s found i n more B i r i c h c e r i u m molybdate compounds ( e . g . B i g ^Ce i ° 3 ^ i o ^ * analysis of t h i s material i s needeâ, D) l o c a l environments w i t h i n the two structures d e s c r i b e d here are s i m i l a r , but the average s t r u c t u r e s t h a t r e s u l t from d i f f r a c t i o n experiments do not r e v e a l this s i m i l a r i t y or c) i n c o r p o r a t i o n o f a n o t h e r m e t a l more g r e a t l y e f f e c t s the s u r f a c e of each m a t e r i a l not the b u l k , and c a t a l y t i c b e h a v i o r i s a r e f l e c t i o n of surface s t r u c t u r e . I n c r e a s e d p r o p y l e n e ammoxidation a c t i v i t y o f each phase upon a l t e r i o n doping i s due to the j u x t a p o s i t i o n o f a l l n e c e s s a r y elements f o r o x i d a t i o n c a t a l y s i s i n a s i n g l e phase. The r e q u i r e ments o f a good o x i d a t i o n c a t a l y s t are a) a c t i v a t i o n o f the s u b s t r a t e m o l e c u l e , b) o x i d a t i o n a c t i v i t y (oxygen i n s e r t i n g ) and c) f a c i l e redox c a p a b i l i t i e s to ease e l e c t r o n c o n d u c t i o n and s i t e r e c o n s t r u c t i o n . F o r reasons d i s c u s s e d e x t e n s i v e l y i n the l i t e r a t u r e (7), we a s s i g n these r o l e s to B i , Mo, and Ce i o n s i t e s r e s p e c t i v e l y i n the c a t a l y s t s described here. The s o l i d s o l u t i o n f o r m a t i o n observed i n t h e s e m a t e r i a l s e n a b l e s a l l o f these f u n c t i o n s t o be r e p r e s e n t e d i n one phase and on one s u r f a c e o f the c a t a l y s t . M

a n C

a

s

t

r

u

c

t

u

r

e

A n a l y s i s o f the M u l t i p h a s e C a t a l y s t The maximum i n c a t a l y t i c a c t i v i t y observed for the m u l t i p h a s e r e g i o n o f the phase diagram n e c e s s a r i l y a r i s e s from i n t e r a c t i o n s between the s e p a r a t e phases. The b i s m u t h r i c h and c e r i u m r i c h s o l i d s o l u t i o n s can r e a d i l y form coherent i n t e r f a c e s a t the phase b o u n d a r i e s due to the structural s i m i l a r i t i e s between the two phases w h i c h can permit e p i t a x i a l n u c l e a t i o n and g r o w t h . A good l a t t i c e match e x i s t s between the [010] faces o f the compounds, t h i s match i s d i s p l a y e d i n F i g u r e 6. We have a l s o shown t h a t r e g i o n s o f an [010] face o f a Ce doped b i s m u t h molybdate c r y s t a l resembles c e r i u m molybdate c o m p o s i t i o n a l l y . T h i s means t h a t the i n t e r f a c e between the two compounds need not have sharp c o m p o s i t i o n gradients. It i s s t r u c t u r a l l y p o s s i b l e f o r the B i - r i c h phase to possess a m e t a l s t i o c h i o m e t r y at the s u r f a c e t h a t matches t h a t o f the C e - r i c h phase. I n o r d e r to a s s e s s the n a t u r e o f t h i s i n t e r f a c i a l r e g i o n , the thermodynamic treatment of Cahn and H i l l i a r d (28) was employed (17) to d e r i v e the f o l l o w i n g f r e e energy f u n c t i o n :

70


Bi

Ce

BiCe(Mo0 )

MoO

1.8 0.2(

4)3

4

3

(010) CONTACT PLANE

F i g u r e 6. the

Bi^

C r y s t a I l o g r a p h i c match between t h e [010] f a c e s o f QCCQ

2^°^4^3

Reproduced from R e f .

an(

17.

*

B'CeiMoO^)^ Copyright

solid solutions.

1983 A m e r i c a n C h e m i c a l

Society.

4.

BRAZDIL ET AL.

Af

= RT X ^ l n


[a

B i

/a^]

+ X^ln

71

[ a ^ / a ^ ]

where a and a are the a c t i v i t i e s o f b i s m u t h and c e r i u m , r e s p e c t i v e l y , i n the i n t e r f a c i a l r e g i o n and a and a ^ are the a c t i v i t i e s o f b i s m u t h and c e r i u m , r e s p e c t i v e l y , i n the e q u i l i b r i u m s o l i d s o l u t i o n . The f u n c t i o n can be regarded as the f r e e energy o f an e q u i l i b r i u m m i x t u r e o f the two s o l i d s o l u t i o n phases. The f r e e energy f u n c t i o n A f was c a l c u l a t e d as a f u n c t i o n o f c o m p o s i t i o n i n the two phase r e g i o n o f the Bt^^Ce^HoO^)^ system (Figure 7). Comparison of F i g u r e s 5 and 7 r e v e a l s t h a t energy o f an e q u i l i b r i u m m i x t u r e o f the phases i s m i n i m i z e d at the c o m p o s i t i o n w h i c h g i v e s maximum c a t a l y t i c a c t i v i t y . I t i s apparent t h a t a t c o m p o s i t i o n s where A f i s a minimum, the d i f f e r e n c e i n the c h e m i c a l p o t e n t i a l s o f the components i n the i n t e r f a c i a l r e g i o n and the e q u i l i b r i u m s o l i d s o l u t i o n s i s m i n i m i z e d . Thus, an i n t e r f a c i a l r e g i o n w h i c h i s c h e m i c a l l y s i m i l a r to the s a t u r a t e d s o l i d s o l u t i o n s appears optimum f o r maximum c a t a l y t i c e f f i c i e n c y . P h y s i c a l l y , the r e l a t i o n s h i p between c a t a l y t i c a c t i v i t y and A f can be understood from a s t u d y o f s i n g l e phase b i s m u t h c e r i u m molybdate s o l i d s o l u t i o n s . The r e s u l t s show t h a t maximum a c t i v i t y i s a c h i e v e d when t h e r e e x i s t s a maximum number and o p t i m a l d i s t r i b u t i o n of a l l the key c a t a l y t i c components; bismuth, molybdenum and c e r i u m i n the solid. Therefore, i t reasonably f o l l o w s t h a t the low c a t a l y t i c a c t i v i t y observed f o r the two phase c o m p o s i t i o n s where Af ¥ A f ( m i n ) results from the presence o f interfacial regions i n the catalysts where the compositional u n i f o r m i t y d e v i a t e s s i g n i f i c a n t l y from the e q u i l i b r i u m d i s t r i b u t i o n o f b i s m u t h and cerium c a t i o n s p r e s e n t i n the s o l i d s o l u t i o n s . These c o m p o s i t i o n s may c o n t a i n areas i n the i n t e r f a c i a l r e g i o n w h i c h a r e more b i s m u t h - r i c h o r c e r i u m - r i c h than the s a t u r a t e d s o l i d solutions. C o n v e r s e l y , at A f ( m i n ) , the c a t a l y s t i s s i m i l a r to an i d e a l mixture of the two optimal solid solutions. The c o m p o s i t i o n a l homogeneity o f the i n t e r f a c i a l r e g i o n approaches t h a t of the saturated s o l i d solutions. Therefore, the catalytic b e h a v i o r o f c o m p o s i t i o n s at A f ( m i n ) is s i m i l a r to t h a t o f the saturated s o l i d s o l u t i o n s . Summary and C o n c l u s i o n s S e v e r a l c o n c l u s i o n s can be drawn about the s o l i d s t a t e mechanism o f s e l e c t i v e o l e f i n ammoxidation by b o t h s i n g l e phase and m u l t i p h a s e oxide c a t a l y s t . F i r s t l y , optimum c a t a l y t i c performance i s a c h i e v e d when t h e r e i s maximum i n t e r a c t i o n between key c a t a l y t i c components i n a s o l i d oxide m a t r i x . Maximum i n t e r a c t i o n o c c u r s i n a s i n g l e phase s a t u r a t e d s o l i d solutions s i n c e t h e s e c o n t a i n the maximum number and d i s p e r s i o n o f the c a t a l y t i c a l l y important c o - e x i s t i n g elements. S e c o n d l y , these key c a t a l y t i c components f o r s e l e c t i v e o l e f i n ammoxidation are i d e n t i f i e d a s : an a-H a b s t r a c t i n g element ( B i ) , an o l e f i n c h e m i s o r p t i o n and n i t r o g e n i n s e r t i o n element (Mo) and a m u l t i v a l e n t redox c o u p l e (Ce Ce ) . The m u l t i v a l e n t redox c o u p l e enhances oxygen i o n , e l e c t r o n and a n i o n vacancy t r a n s p o r t i n the s o l i d w h i c h enhances c a t a l y t i c a c t i v i t y by i n c r e a s i n g the r e c o n s t r u c t i o n / r e o x i d a t i o n r a t e o f the a c t i v e B i and Mo c o n t a i n i n g s i t e s . This r e s u l t s i n an e f f e c t i v e i n c r e a s e o f the

72


1500 L-

900 h

O.4 O.6 O.8 X in B i 2 . x C e x ( M o 0 4 ) 3

1.2

Figure 7. Free energy function Af for Β'ο-τχ^χ^ ^^ function of composition. Reproduced from R e f . 1 7 . C o p y r i g h t 1983 American Chemical 0

Society.

as

a

4.

BRAZDIL ET AL.


73

number of active s i t e s available at the surface at any given time. T h i r d l y , we have found that the chemical and structural nature of the interfacial region between co-existing phases in Bi2_ Ce (MoO, )^ catalysts has a profound effect on c a t a l y t i c behavior. Thermodynamic calculations show that compositions which give maximum c a t a l y t i c a c t i v i t y also give minima in the free energy of mixing of the two phases r e l a t i v e to the saturated s o l i d solutions. This can be explained on the basis that at the free energy minimum the chemical and compositional s i m i l a r i t y between the i n t e r f a c i a l region and the equilibrium s o l i d solutions i s greatest. The simultaneous existence of coherent phase boundaries between the separate phases of multicomponent catalyst i s also an important c r i t e r o n for maximum c a t a l y t i c activity. In bismuthcerium molybdates, close structural s i m i l a r i t y between the two saturated solid solutions permits mutual e p i t a x i a l growth which produces a "pseudo" single phase c a t a l y s t . As a consequence, oxygen i o n , anion vacancy and electron transport between the phases can r e a d i l y occur. In addition, oxygen ion and electron transfer between the individual phases w i l l be f a c i l i t a t e d when the compositional nonuniformity of the region at the interface i s minimized. A compositionally non-uniform region in which the redox couple (cerium in this case) i s not properly distributed w i l l exhibit diminished oxygen ion mobility across the coherent phase boundaries and the overall c a t a l y t i c a c t i v i t y for selective o l e f i n oxidation w i l l be less than optimum. x

x

Acknowledgment s We g r a t e f u l l y acknowledge M. H. Rapposch for c o l l e c t i n g the x-ray data and a s s i s t i n g i n the structure s o l u t i o n , and L . C. Glaeser for catalyst preparation and t e s t i n g . The authors thank the US department of energy for supporting IPNS at Argonne as a national users f a c i l i t y , and the Standard O i l Company (Sohio) for permission to publish this work. Literature 1. 2. 3. 4. 5. 6. 7. 8. 9.

Cited

Grasselli, R. K.; Suresh, D. D. J. Catal., 1972, 25, 273. Bart, J. C. J.; Giordano, N. J. Catal., 1980, 64, 356 Brazdil, J. F.; Suresh, D. D.; Grasselli, R.K. J. Catal., 1980, 66, 347. Aso, I.; Furukawa, S.; Yamazoe, N.; Seiyama, T. 1980, J. Catal., 64, 29. Aso, I.; Amamoto, T.; Yamazoe, N.; Seiyama, T. Chem. Lett.,1980, 365. Stone, F.S. J. Solid State Chem. 1975, 12, 271. Grasselli, R. K.; Burrington, J. D.; Brazdil, J. F. Farad. Disc.,1982, 72, 203 and references therein. Schuit, G. C. Α.; Gates, B. C. Chem. Tech. 1983, November 693. Sinfelt, J. H.; Via, G. H.; Lytle, F. W. J. Chem. Phys., 1980, 72, 4832.

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Vejux, Α.; Courtine, P. J. Solid State Chem.,1978, 23, 93. 11. Bordes, Ε.; Courtine, P. J. Catal.,1979, 57, 236 12. Eon, J. G.; Courtine, P. J. Solid State Chem. 1980, 32, 67. 13. Carson, D.; Coudurier, G.; Forissier, M.; Vedrine, J. C. J. Chem. Soc., Farad Trans. 1, 1983, 79, 1921 1983. 14. Ueda, W.; Moro-oka, Y.; Ikawa, T. J. Chem Soc., Farad, Trans I.,1982, 78, 495. 15. Chaze, A. M.; Courtine, P. J. Chem. Research, 1983, 96. 16. Brazdil, J. F.; Grasselli, R. K., J. Catal. 1983, 79, 104. 17. Brazdil, J. F.; Glaeser, L. C.; Grasselli, R. K. J. Phys. Chem. 1983, 87, 5485. 18. Teller, R. G.; Brazdil, J. F.; Grasselli, R. K.; Thomas, R.; Corliss, L.; Hastings J. J. Solid State Chem., 1984, 52,313. 19a. Van den Elzen, A. F.; Rieck, G. D. Acta. 1973, Cryst., B29,2433. 19b. Zalkin, Α.; Templeton, D. H. J. Chem. Phys. 1964,40, 501. 20. Von Dreele, R. B.; Jorgensen, J. D.; Windsor, C. G. J. Appl. Cryst., 1982, 15, 581 and Jorgensen, J. D.; Faber, J. ICANS-II, proc. VIth Int. Collab. Adv. Neutron Sources, Argonne Natl. Lab., June 28-July 2, 1982, 1983 (ANL-82-80. 21. Rapposch, M. H.; Anderson, J. B.; Kostiner, E. Inorg. Chem., 1980, 19, 3531. 22. Sleight, A. W. In "Advanced Materials in Catalysis"; Ed. Burton and Garten, Academic Press, NY 1977,181. 23. Jeitschko, W.; Sleight, A. W.; McClellan, W. R.; Weiher, J. F. Acta Cryst. 1976, B32, 1163. 24. Sleight, A. W.; Aykan, K.; Rogers, D. B. J. Solid State Chem., 1975, 13, 231. 25. Templeton, D. H.; Zalkin, A. Acta Cryst. 1963, 16, 762. 26. Jeitschko, W. Acta Cryst., 1973, B29, 2074. 27. Brown, I. D.; Wu, Κ. K. Acta. Cryst., 1976, B32, 1957. 28. Cahn, J. W.; Hilliard, J. E. J. Chem. Phys., 1958, 28, 258. RECEIVED

February 26, 1985

Structural and Thermodynamic Basis for Catalytic Behavior of Bismuth

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