Chapter 3
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Surface Oxide-Support Interactions in the Molecular Design of Supported Metal Oxide Selective Oxidation Catalysts Goutam Deo and Israel E. Wachs Department of Chemical Engineering, Zettlemoyer Center for Surface Studies, Lehigh University, Bethlehem, PA 18015
A series of metal oxides were deposited on the surface of different oxide supports to study the surface oxide support interactions. The dehydrated Raman spectra of the supported metal oxide catalysts reveal the presence and structure of the supported metal oxide phases. The same surface metal oxide species were found on the different oxide supports for each of the supported metal oxide systems. The reactivity of the surface metal oxide species, however, depends on the specific oxide support (TiO ~ZrO >Nb O >Al O ~SiO ). For a given oxide support, the reactivity depends on the specific surface metal oxide species (e.g. VO > MoO ). The redox activation energy for a l l the surface metal oxide phases l i e in the range of 18-22 kcal/mole. The similar activation energies suggests that the number of active sites and/or the a c t i v i t y per site is responsible for the difference in reactivity. The redox TON for the methanol oxidation reaction correlates with the reduction temperature during TPR experiments, which suggests that the bridging M-O-Support bond controls the activity during redox reactions. 2
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Supported metal oxide catalysts are formed when one metal oxide component ( i . e . , Re O , CrO , MoO , WO , V O , Nb O , etc.), the supported metal oxide phase, is deposited on a second metal oxide substrate ( i . e . , A l O , T i O , SiO , e t c . ) , the oxide support [1]. The supported metal oxide phase is present on the oxide support as a surface metal oxide species. The reactivity of these 2
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0097-6156/93/0523-0031$06.00/0 © 1993 American Chemical Society In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
CATALYTIC SELECTIVE OXIDATION
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supported metal o x i d e c a t a l y s t s have been intensivelyi n v e s t i g a t e d o v e r t h e p a s t d e c a d e i n numerous c a t a l y t i c a p p l i c a t i o n s and t h e main e m p h a s i s h a s been t o r e l a t e the r e a c t i v i t y with t h e s t r u c t u r e o f t h e s u r f a c e metal oxide species [1,2] . In d e t e r m i n i n g t h e s t r u c t u r e o f t h e s e s u r f a c e m e t a l o x i d e s p e c i e s Raman s p e c t r o s c o p y h a s proven t o be i n d i s p e n s a b l e b e c a u s e o f t h e a b i l i t y o f Raman spectroscopy t o d i s c r i m i n a t e between different m e t a l o x i d e s p e c i e s t h a t may s i m u l t a n e o u s l y be p r e s e n t in the catalyst. The reactivity studies have demonstrated t h a t these s u r f a c e metal oxide s p e c i e s a r e t h e a c t i v e s i t e s f o r many c a t a l y t i c r e a c t i o n s [3] . The combined s t r u c t u r a l and r e a c t i v i t y i n f o r m a t i o n c u r r e n t l y a v a i l a b l e about these oxide c a t a l y s t s i s beginning t o allow us t o d e v e l o p an u n d e r s t a n d i n g of the surface o x i d e s u p p o r t i n t e r a c t i o n s and t o a p p l y t h i s knowledge for the molecular design of supported metal oxide catalysts. The molecular design of supported metal oxide c a t a l y s t s r e q u i r e s t h a t we s p e c i f y t h e s y n t h e s i s method, oxide support, catalyst composition, calcination temperature, location of the surface metal oxide s p e c i e s , as w e l l as i t s r e a c t i v i t y . Consequently, t h e influence o f each o f t h e above parameters upon t h e s t r u c t u r e and c a t a l y t i c p r o p e r t i e s of supported metal o x i d e c a t a l y s t s n e e d s t o be e x a m i n e d . The p r e s e n t s t u d y p r i m a r i l y f o c u s e s on t h e m o l e c u l a r d e s i g n a s p e c t s o f supported vanadium oxide catalysts because these catalysts constitute a very important class of heterogeneous o x i d e c a t a l y s t s . However, c o m p a r i s o n with other supported metal oxide systems (Mo0 , Re 0 ,and C r 0 ) w i l l a l s o be made. 3
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Experimental The Ti0 Si0
o x i d e s u p p o r t s employed i n t h e p r e s e n t s t u d y were: (Degussa, - 5 5 m^/g) , A 1 0 (Harshaw, - 1 8 0 m /g) , (Cabot, -300 m /g) , Z r 0 (Degussa, - 3 9 m /g) and 2^5 ( N i o b i u m P r o d u c t s Co., — 5 0 m /g) . Many d i f f e r e n t s y n t h e s i s methods have been u s e d t o p r e p a r e supported m e t a l o x i d e c a t a l y s t s . In t h e c a s e o f s u p p o r t e d v a n a d i u m o x i d e c a t a l y s t s , t h e c a t a l y s t s were p r e p a r e d by v a p o r phase grafting with V0C1 , nonaqueous impregnation (vanadium alkoxides), aqueous impregnation (vanadium oxalate), as well as spontaneous dispersion with crystalline V 0 [ 4 ] . No d r a s t i c reduction of surface a r e a o f t h e c a t a l y s t s was o b s e r v e d . Structural characterization of the surface metal o x i d e s p e c i e s was o b t a i n e d by l a s e r Raman s p e c t r o s c o p y u n d e r a m b i e n t and d e h y d r a t e d c o n d i t i o n s . The l a s e r Raman spectroscope consists of a Spectra Physics A r laser p r o d u c i n g 1-100 mW o f power measured a t t h e s a m p l e . The s c a t t e r e d r a d i a t i o n was f o c u s e d i n t o a Spex T r i p l e m a t e s p e c t r o m e t e r c o u p l e d t o a P r i n c e t o n A p p l i e d R e s e a r c h 0ΜΑ I I I o p t i c a l m u l t i c h a n n e l a n a l y z e r . About 100-200 mg o f 2
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In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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3. DEOANDWACHS
Surface Oxide-Support Interactions
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the pure catalysts were pelletized and used for o b t a i n i n g t h e Raman s p e c t r a i n t h e d e h y d r a t e d mode. F o r a m b i e n t s p e c t r a 5-20 mg o f c a t a l y s t s was p l a c e d on a K B r backing. The s u p p o r t e d m e t a l o x i d e c a t a l y s t s were e x a m i n e d f o r t h e i r r e a c t i v i t y i n t h e m e t h a n o l o x i d a t i o n r e a c t i o n . The reactor was operated using milligram amounts of c a t a l y s t s that provide d i f f e r e n t i a l reaction conditions by keeping conversions below 10%. A methano1/oxygen/he1iurn m i x t u r e o f ~6/13/81 (mole %) a t 1 atm p r e s s u r e was used a s t h e r e a c t a n t g a s f o r a l 1 t h e data presented. The analysis was performed with an o n l i n e g a s c h r o m a t o g r a p h (GC) (HP 5840A) c o n t a i n i n g two c o l u m n s ( P o r o p a k R and C a r b o s i e v e S I I ) and two d e t e c t o r s (FID and TCD) . R e a c t i o n d a t a a t 230 °C a r e p r e s e n t e d i n the form of turnover number (TON) - d e f i n e d a s t h e number o f moles o f methanol converted p e r mole o f vanadium per second. The reaction data f o r some c a t a l y s t s were a l s o o b t a i n e d a t 200, 230, and 240 'C t o calculate the activation energy and check for d i f f u s i o n a l l i m i t a t i o n s i n t h e r e a c t o r . No mass and h e a t t r a n s f e r l i m i t a t i o n s were o b s e r v e d . R e s u l t s and D i s c u s s i o n The v a n a d i u m o x i d e s p e c i e s i s f o r m e d on t h e s u r f a c e o f the oxide support during the p r e p a r a t i o n of supported vanadium oxide catalysts. This i s evident by the consumption of surface hydroxyIs (OH) [5] and t h e s t r u c t u r a l t r a n s f o r m a t i o n o f t h e supported metal oxide phase that takes place during hydration-dehydration s t u d i e s and c h e m i s o r p t i o n o f r e a c t a n t g a s m o l e c u l e s [ 6 ] . Recently, a number o f s t u d i e s have shown that the s t r u c t u r e o f t h e s u r f a c e vanadium o x i d e s p e c i e s depends on t h e s p e c i f i c c o n d i t i o n s t h a t t h e y a r e o b s e r v e d u n d e r . F o r example, u n d e r a m b i e n t c o n d i t i o n s t h e s u r f a c e o f t h e oxide supports possesses a t h i n l a y e r o f moisture which p r o v i d e s an aqueous e n v i r o n m e n t o f a c e r t a i n pH a t p o i n t of z e r o charge (pH a t p z c ) f o r t h e s u r f a c e v a n a d i u m o x i d e s p e c i e s and c o n t r o l s t h e s t r u c t u r e o f t h e v a n a d i u m o x i d e phase [7] . Under r e a c t i o n c o n d i t i o n s (300-500 °C) , m o i s t u r e desorbs from t h e s u r f a c e o f t h e o x i d e support and t h e vanadium oxide s p e c i e s i s f o r c e d t o d i r e c t l y interact with the oxide support which results in a different structure [8] . These structural transformations taking place during hydration and dehydration c o n d i t i o n s o f the oxide support suggest t h a t the correlation of the s t r u c t u r e - r e a c t i v i t y data s h o u l d be p e r f o r m e d with the s t r u c t u r a l data obtained under dehydration conditions, and correlating such s t r u c t u r a l information with the r e a c t i v i t y data. A s e r i e s o f ~ 1 % ^2^5 c a t a l y s t s was p r e p a r e d by non aqueous i m p r e g n a t i o n o f v a n a d i u m t r i - i s o p r o p o x i d e oxide ( f i n a l c a l c i n a t i o n i n oxygen a t 450/500 °C) i n o r d e r t o investigate the influence of d i f f e r e n t oxide supports
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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CATALYTIC SELECTIVE OXIDATION
upon t h e d e h y d r a t e d m o l e c u l a r s t r u c t u r e a n d r e a c t i v i t y o f t h e s u r f a c e vanadium o x i d e s p e c i e s . Under ambient conditions Raman bands due to orthovanadate, p y r o v a n a d a t e , m e t a v a n a d a t e , and d e c a v a n a d a t e s p e c i e s a r e observed. Assignments of t h e Raman bands t o the different s p e c i e s a r e made e l s e w h e r e [7, 9] . A t t h e s e surface coverages a single surface vanadium oxide s p e c i e s i s p r e d o m i n a n t l y p r e s e n t on t h e d i f f e r e n t o x i d e s u p p o r t s a s i s e v i d e n t by t h e p r e s e n c e o f a dominant 1015-1039 cm" band i n t h e d e h y d r a t e d Raman s p e c t r a a n d potential complication due to additional surface v a n a d i u m o x i d e s p e c i e s a r e e l i m i n a t e d . The d e h y d r a t e d Raman band due t o t h e V=0 bond was f o u n d t o v a r y f r o m 1015-1039 cm" as a f u n c t i o n of the different oxide s u p p o r t s a s shown i n F i g u r e 1. The s l i g h t d i f f e r e n c e i n band position i s due t o s l i g h t l y different V=0 bond l e n g t h s o f t h e i s o l a t e d s u r f a c e vanadium o x i d e s p e c i e s on t h e d i f f e r e n t o x i d e s u p p o r t s and c o r r e l a t e s w i t h t h e d i f f e r e n t oxygen c o o r d i n a t i o n o f t h e o x i d e s u p p o r t s . The Raman s p e c t r a r e v e a l t h a t e s s e n t i a l l y t h e same s u r f a c e v a n a d i u m o x i d e s p e c i e s i s p r e s e n t on a l l t h e d i f f e r e n t o x i d e s u p p o r t s . T h i s s u r f a c e vanadium o x i d e s p e c i e s i s described as a distorted four coordinated species p o s s e s s i n g a s i n g l e t e r m i n a l bond (V=0) and t h r e e bonds t o t h e s u p p o r t (V-O-S) . The s°ame c o n c l u s i o n i s r e a c h e d from s o l i d s t a t e V NMR s t u d i e s o f t h e s e c a t a l y s t s [ 9 ] . Low s u r f a c e c o v e r a g e s , — 1 % metal oxide, of supported molybdenum oxide (Mo0 ) [10] , r h e n i u m oxide (Re 0 ) [ 1 1 , 1 2 ] , and chromium o x i d e ( C r 0 ) [11,12] a l s o i n d i c a t e the presence o f a s i n g l e s u r f a c e metal o x i d e s p e c i e s . The similar Raman band positions of the supported vanadium-oxygen (V=0) stretching frequency during d e h y d r a t e d c o n d i t i o n s a r e g i v e n i n T a b l e I . In a d d i t i o n , the structural transformation taking place due t o d e h y d r a t i o n i s o b s e r v e d by c o m p a r i n g columns 2 and 3 o f T a b l e I . T h u s , a t low c o v e r a g e s t h e d e h y d r a t e d s u r f a c e vanadium oxide and related metal oxide molecular s t r u c t u r e s (Re 07, C r 0 , and Mo0 ) a r e i n d e p e n d e n t o f t h e s p e c i f i c oxide support.
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T a b l e I . D e h y d r a t e d Raman band p o s i t i o n f o r t h e V=0 t e r m i n a l s t r e t c h i n g v i b r a t i o n s f o r 1% V 0 on d i f f e r e n t o x i d e s u p p o r t s a l o n g w i t h t h e h i g h e s t a m b i e n t Raman band position 2
Oxide Support Si0 Nb 0 Ti0 Zr0 Α1 0* 2
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H i g h e s t band (cm" ) ambient c o n d i t i o n s 1000 970-980 940-950 960 920-930
In Catalytic Selective Oxidation; Oyama, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch003
3.
DEOANDWACHS
Surface Oxide-Support Interactions
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The r e a c t i v i t y o f t h e s u r f a c e v a n a d i u m o x i d e s p e c i e s ( — 17c V2O5) on "the d i f f e r e n t o x i d e s u p p o r t s was p r o b e d by t h e m e t h a n o l o x i d a t i o n r e a c t i o n . The m e t h a n o l o x i d a t i o n reaction i s very sensitive t o the nature of surface s i t e s p r e s e n t . S u r f a c e redox s i t e s form formaldehyde, methyl formate, and d i m e t h o x y methane a s t h e r e a c t i o n products. Formaldehyde being formed as the first oxidation product from the methoxy intermediate. Subsequent reactions of the methoxy intermediate produces methyl formate and d i m e t h o x y methane. S u r f a c e a c i d s i t e s , Lewis as w e l l as Bronsted, result i n the formation of dimethyl ether. Surface basic s i t e s yield C0/C0 as t h e r e a c t i o n products [9] . F o r a l l t h e s u p p o r t e d vanadium o x i d e c a t a l y s t s , w i t h t h e e x c e p t i o n of alumina, the surface vanadia redox s i t e s produced formaldehyde almost exclusively. On alumina, only a trace o f f o r m a l d e h y d e was f o r m e d b e c a u s e t h e s u r f a c e acid sites produced dimethyl ether. Thus, f o r the V2O5/AI2O3 s y s t e m t h e f o r m a l d e h y d e p r o d u c e d was t a k e n a s representative of the r e a c t i v i t y of the surface vanadia redox sites. The reactivity of the surface vanadia s p e c i e s on d i f f e r e n t o x i d e s u p p o r t s was f o u n d t o depend d r a m a t i c a l l y on t h e s p e c i f i c o x i d e s u p p o r t a s shown i n Table I I . As t h e s u p p o r t was changed f r o m silica to zirconia the turnover number (TON) f o r t h e s u r f a c e v a n a d i u m o x i d e s p e c i e s was f o u n d t o i n c r e a s e by t h r e e o r d e r s o f m a g n i t u d e . S i m i l a r t r e n d s were a l s o observed f o r s u p p o r t e d molybdenum o x i d e [ 1 0 ] , r h e n i u m o x i d e [ 1 2 ] , and chromium o x i d e [12] . 2
T a b l e I I . The TON and s e l e c t i v i t y t o f o r m a l d e h y d e f o r t h e m e t h a n o l o x i d a t i o n r e a c t i o n on v a r i o u s 1% s u p p o r t e d vanadium o x i d e c a t a l y s t s - 1
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Si0 A1 0 Nb 0 Ti0 Zr0
2.0*10" 2.0*10~ 7.0*10 1.8*10° 2.3*10°
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