Characterization and Catalyst Development

Chapter 21

Catalyst Characterization by Infrared Spectroscopic Methods

Downloaded by UCSF LIB CKM RSCS MGMT on November 21, 2014 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch021

J . B. Peri Department of Chemical Engineering, Northeastern University, Boston, M A 02115

Infrared spectroscopic methods are highly e f f e c t i v e for characterization of catalyst surfaces. Improved understanding r e s u l t i n g from i n f r a r e d studies has contributed s i g n i f i c a n t l y to catalyst development i n the past and has the p o t e n t i a l for even greater future contributions. Improvements i n spectrometers and techniques have heightened i n t e r e s t i n i n f r a r e d characterization of c a t a l y s t s , but possible problems, both experimental and t h e o r e t i c a l , should be recognized. Simplest subjects for routine characterization are surface groups, acid s i t e s , and "strained" bonds. Supported metals, although highly important, are usually more difficult to characterize.

A l t h o u g h i n f r a r e d s p e c t r o s c o p y has been w i d e l y used f o r c h a r a c t e r i z a t i o n o f c a t a l y s t s u r f a c e s f o r o v e r t h i r t y y e a r s , many r e v i e w s ( 1 - 4 ) , books ( 5 - 7 ) and a r t i c l e s a t t e s t t o i t s c o n t i n u e d p o p u l a r i t y . Past a p p l i c a t i o n s o f t h i s technique i n s t u d i e s o f c o m m e r c i a l l y i m p o r t a n t c a t a l y s t s have i n some i n s t a n c e s (e.g. r e f s . 7 » l 6 ) l e d t o improved u n d e r s t a n d i n g o f t h e s e c a t a l y s t s w h i c h h a s a i d e d c a t a l y s t development o r l e d t o b e t t e r p r e t r e a t m e n t p r o c e d u r e s . The p r o s p e c t s f o r i m p o r t a n t c o n t r i b u t i o n s t o c a t a l y s t development from f u t u r e i n f r a r e d s t u d i e s a r e v e r y b r i g h t . U n d e r s t a n d i n g remains i m p e r f e c t , however, and f a c t o r s o t h e r t h a n s u r f a c e c h e m i s t r y a r e o f t e n v i t a l l y i m p o r t a n t i n p r a c t i c a l c a t a l y s t s . Improvements i n s p e c t r o m e t e r s and computer t r e a t m e n t o f s p e c t r a l d a t a have i n r e c e n t y e a r s g r e a t l y i n c r e a s e d t h e convenience and s e n s i t i v i t y o f i n f r a r e d c h a r a c t e r i z a t i o n and enhanced i n t e r e s t i n t h e u s e o f i n f r a r e d f o r r o u t i n e a n a l y t i c a l c h a r a c t e r i z a t i o n o f c a t a l y s t s . I n f r a r e d can, o f c o u r s e , g i v e t h e same i n f o r m a t i o n on b u l k c o m p o s i t i o n o f s o l i d c a t a l y s t s as t h a t o b t a i n a b l e f o r o t h e r s o l i d s . I t c a n a l s o be a h i g h l y s e n s i t i v e and g e n t l e , n o n - d e s t r u c t i v e t e c h n i q u e f o r s u r f a c e

0097-6156/89/0411-0222$06.00/0 c 1989 American Chemical Society

In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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c h a r a c t e r i z a t i o n , o c c a s i o n a l l y under r e a c t i o n c o n d i t i o n s . Owing t o s t r o n g a b s o r p t i o n o f i n f r a r e d by the s u p p o r t , however, t y p i c a l transmission i n f r a r e d studies of oxide-supported c a t a l y s t s are u s u a l l y l i m i t e d t o the r e g i o n above 1000 cm , and c o n d u c t i n g s u p p o r t s , such as c a r b o n , may make i n f r a r e d s t u d y i m p r a c t i c a l . Raman s p e c t r o s c o p y can p r o v i d e a d d i t i o n a l i n f o r m a t i o n i n f a v o r a b l e cases. D i f f u s e - r e f l e c t a n c e , e m i s s i o n , p h o t o a c o u s t i c , and o t h e r r e l a t e d s p e c t r o s c o p i c methods a l s o o f f e r p o t e n t i a l a d v a n t a g e s , p a r t i c u l a r l y i n ease o f sample p r e p a r a t i o n , but t r a n s m i s s i o n methods a r e p r e f e r r e d when t h e y can be u s e d and q u a n t i t a t i v e comparisons a r e sought (2_,_4) .


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Considerations

Use o f t r a n s m i s s i o n i n f r a r e d ( e i t h e r F o u r i e r t r a n s f o r m o r d i s p e r s i v e ) i n s u r f a c e s t u d i e s u s u a l l y imposes s p e c i a l r e q u i r e m e n t s on sample p r e p a r a t i o n . I n f r a r e d i s i n h e r e n t l y a bulk technique, and the s u r f a c e s p e c i e s o f i n t e r e s t u s u a l l y r e p r e s e n t o n l y a s m a l l f r a c t i o n o f t h e m a t e r i a l i n the i n f r a r e d beam. Samples must n o r m a l l y be f a i r l y t h i c k , t o p r o v i d e s u f f i c i e n t s u r f a c e , u n i f o r m t h i c k n e s s , and adequate m e c h a n i c a l s t r e n g t h . High s c a t t e r i n g l o s s e s w i t h t y p i c a l commercial c a t a l y s t s r e s u l t i n low s i g n a l - t o - n o i s e ratios i n spectra. KBr d i s k s o r o i l m u l l t e c h n i q u e s cannot be u s e d t o r e d u c e s c a t t e r i n g l o s s e s w i t h o u t m o d i f y i n g the s u r f a c e and rendering i t inaccessible f o r further adsorption studies. Signal a v e r a g i n g , o r o t h e r t e c h n i q u e s , can l a r g e l y o f f s e t energy l o s s e s c a u s e d by s c a t t e r i n g , but improved methods o f p r e p a r i n g t h i n samples a r e s t i l l needed, a t p r e s e n t , samples a r e commonly p r e p a r e d by the " p r e s s e d d i s k " method, i n w h i c h a powder i s p r e s s e d i n a s t a i n l e s s s t e e l d i e , without a b i n d e r , to produce a s e l f - s u p p o r t i n g d i s k . T y p i c a l " t h i c k n e s s e s " range between 5 and 20 mg/cm . T h i n n e r d i s k s would u s u a l l y be p r e f e r a b l e . A d h e s i o n o f samples t o the d i e can be t r o u b l e s o m e , but use o f g o l d - p l a t e d d i e s can m i n i m i z e t h i s problem. P r e s s i n g samples between s h e e t s o f p a p e r o r p r e c a l c i n i n g b e f o r e p r e s s i n g can sometimes a l s o p r o v e h e l p f u l . O t h e r p r o c e d u r e s can be used. S p r a y i n g a s l u r r y , o r d e p o s i t i n g a t h i n powder l a y e r , on a i n f r a r e d - t r a n s p a r e n t window can p r o v i d e t h i n n e r samples and b e t t e r t e m p e r a t u r e c o n t r o l t h a n the p r e s s e d d i s k method ( 8 ) , but p o t e n t i a l problems i n c l u d e i n t e r a c t i o n o f the c a t a l y s t w i t h the s u p p o r t i n g window d u r i n g h i g h - t e m p e r a t u r e t r e a t m e n t s and n o n - u n i f o r m , irreproducible thickness. Samples have a l s o been s u c c e s s f u l l y p r e p a r e d by p r e s s i n g powders i n t o m e t a l s c r e e n s and i n o t h e r ways. 2

A s u i t a b l e i n f r a r e d c e l l must be u s e d f o r s t u d y o f c a t a l y s t s u r f a c e s under c o n t r o l l e d c o n d i t i o n s . Many c e l l d e s i g n s have been p u b l i s h e d , b u t no c e l l d e s i g n i s i d e a l . C e l l s made o f P y r e x and fused quartz are u s u a l . A c e l l f o r high-pressure i n f r a r e d study must be made o f m e t a l , w h i c h e n t a i l s p o s s i b l e problems w i t h m e t a l contamination. Even when a c e l l does n o t p r o d u c e c o n t a m i n a n t s , m e t a l s from c a t a l y s t samples can be d e p o s i t e d on c o o l e r p o r t i o n s o f the c e l l v i a f o r m a t i o n o f v o l a t i l e o x i d e s , c a r b o n y l s , e t c . . These m e t a l s may a g a i n form c a r b o n y l s o r o t h e r v o l a t i l e compounds d u r i n g



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subsequent e x p e r i m e n t s and c o n t a m i n a t e l a t e r samples. B l a n k s must be r u n , and c e l l s must be c l e a n e d o r r e p l a c e d i f m e t a l c o n t a m i n a t i o n i s l i k e l y . Sample movement o u t o f t h e i n f r a r e d beam i s i m p o r t a n t f o r o b t a i n i n g s p e c t r a o f t h e gas phase o r m a t e r i a l on c e l l windows, b u t r e p r o d u c i b l e sample p o s i t i o n i n g i s v i t a l , p a r t i c u l a r l y i n F o u r i e r t r a n s f o r m i n f r a r e d (FTIR) s t u d i e s . C o m b i n a t i o n o f i n f r a r e d w i t h g r a v i m e t r i c o r o t h e r t e c h n i q u e s i s d e s i r a b l e , b u t t h e added c o m p l e x i t y o f c e l l s p e r m i t t i n g g r a v i m e t r i c measurements has l i m i t e d t h e i r use. A major need e x i s t s f o r b e t t e r g a s k e t i n g m a t e r i a l s and cements b a k e a b l e t o a t l e a s t 500 C f o r a t t a c h i n g c e l l windows. C e l l s can p e r m i t c h a r a c t e r i z a t i o n under high-vacuum o r h i g h p r e s s u r e and under s t a t i c o r f l o w c o n d i t i o n s . U l t r a h i g h vacuum i s seldom needed i n s t u d i e s o f h i g h - a r e a c a t a l y s t s , b u t t r a c e i m p u r i t i e s i n gases used i n f l o w e x p e r i m e n t s c a n l e a d t o p r o b l e m s . E i t h e r FTIR o r c o m p u t e r i z e d d i s p e r s i v e s p e c t r o m e t e r s c a n be e f f e c t i v e f o r i n f r a r e d s u r f a c e c h a r a c t e r i z a t i o n . (Even noncomputerized d i s p e r s i v e spectrometers a r e o f t e n s t i l l u s e f u l . ) T h e i r r e l a t i v e m e r i t s depend h e a v i l y on t h e p a r t i c u l a r a p p l i c a t i o n and, o f c o u r s e , on p r a c t i c a l d i f f i c u l t i e s i n o b t a i n i n g e c o n o m i c a l s e r v i c e on " o b s o l e t e " i n s t r u m e n t s . FTIR i s n o t a panacea. FTIR s p e c t r o m e t e r s g i v e m o r e - a c c u r a t e f r e q u e n c i e s and, i n p r i n c i p l e , major advantages i n energy t h r o u g h p u t , p e r m i t t i n g f a s t e r r e c o r d i n g o f a spectrum, o f major i m p o r t a n c e i n k i n e t i c s t u d i e s (3»9). F a s t r e c o r d i n g o f a complete spectrum i s seldom needed i n s u r f a c e c h a r a c t e r i z a t i o n , however. O t h e r f a c t o r s c a n be more i m p o r t a n t . FTIR i s a s i n g l e beam t e c h n i q u e , and an a b i l i t y t o a c c u r a t e l y s u b t r a c t ( o r " r a t i o " ) a background c a n be e x t r e m e l y i m p o r t a n t i n c a t a l y s t s t u d i e s . Problems w i t h " s t r a y l i g h t " , once thought t o be n o n - e x i s t e n t w i t h FTIR, have i n some i n s t a n c e s been q u i t e s e r i o u s , c a u s i n g a c t u a l i n v e r s i o n o f a b s o r p t i o n bands i n t h e O H - s t r e t c h i n g r e g i o n . O t h e r problems c a n be caused by a c h a n g i n g background. T h i s can be p a r t i c u l a r l y troublesome i n i n - s i t u i n f r a r e d s t u d i e s o f c a t a l y t i c r e a c t i o n s . A q u i c k l y - o b t a i n e d spectrum may n o t be u s e f u l i f a c h a n g i n g background must be s u b t r a c t e d b e f o r e u s e f u l information i s obtained. The problem becomes worse i f many s c a n s must be averaged t o produce good s p e c t r a . The n o i s e l e v e l o f FTIR s p e c t r o m e t e r s i s much worse a t t h e ends o f t h e spectrum ( i n c l u d i n g the i m p o r t a n t 0-H and C-H s t r e t c h i n g r e g i o n s ) t h a n i n t h e c e n t e r , and t h e u s e o f MCT ( m e r c u r y - c a d m i u m - t e l l u r i d e ) l i q u i d - n i t r o g e n cooled detectors o r c u t o f f f i l t e r s i s often e s s e n t i a l f o r studies above 2500 cm" . Computer t r e a t m e n t o f s p e c t r a has s i m p l i f i e d q u a n t i t a t i v e a n a l y s e s , b u t p i t f a l l s e x i s t f o r t h e unwary i n s u b t r a c t i o n o f backgrounds and when d e a l i n g w i t h broadened o r s h i f t e d bands. Adsorbed m o l e c u l e s have l o n g been used as " p r o b e s " t o p r o v i d e i n f o r m a t i o n on s u r f a c e s i t e s i n a d s o r p t i o n s t u d i e s , b u t p o t e n t i a l problems e x i s t i n t h e u s e o f such "probes". P u r e r e a g e n t s a r e o b v i o u s l y e s s e n t i a l , b u t i t i s easy t o u n d e r e s t i m a t e p o s s i b l e e f f e c t s o f t r a c e c o n t a m i n a n t s i n probe gases o r v a p o r s . A t y p i c a l e x p e r i m e n t c a n expose a c a t a l y s t sample t o a l a r g e e x c e s s o f a probe. A f t e r a p p r e c i a b l e contact time, t r a c e i m p u r i t i e s can adsorb to a h i g h l y observable extent. Such i m p u r i t i e s i n c l u d e m e t a l 1



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c a r b o n y l s , commonly found i n "pure" CO, and w a t e r , o f t e n p r e s e n t i n p y r i d i n e and o t h e r n i t r o g e n b a s e s . Problems w i t h t r a c e i m p u r i t i e s have become more s e r i o u s as t h e s e n s i t i v i t y o f i n f r a r e d c h a r a c t e r i z a t i o n has i n c r e a s e d . Pretreatment i s extremely important i n determining surface c h e m i s t r y . Samples must be c a r e f u l l y p r e p a r e d , d r i e d , o x i d i z e d , r e d u c e d , and k e p t i n t h e d e s i r e d f i n a l s t a t e u n t i l a d m i s s i o n o f probe m o l e c u l e s and r e c o r d i n g o f s p e c t r a . S e e m i n g l y m i n o r d i f f e r e n c e s i n p r e p a r a t i o n o r p r e t r e a t m e n t c a n produce major d i f f e r e n c e s i n t h e s u r f a c e c h e m i s t r y o f many c a t a l y s t s . Adsorbable i m p u r i t i e s i n c o n v e n t i o n a l h i g h vacuum systems do n o t u s u a l l y s e r i o u s l y c o n t a m i n a t e t y p i c a l h i g h - a r e a c a t a l y s t samples. I f , however, samples s t a n d f o r many hours s i g n i f i c a n t changes c a n o f t e n be o b s e r v e d , p a r t i c u l a r l y w i t h reduced s u p p o r t e d m e t a l s . T y p i c a l Important A p p l i c a t i o n s C h a r a c t e r i z a t i o n o f S u r f a c e Groups on S u p p o r t s and C a t a l y s t s . S u r f a c e g r o u p s , such as -OH, -NH , C 0 , e t c . , w h i c h g i v e bands i n an a c c e s s i b l e s p e c t r a l r e g i o n a r e u s u a l l y r e a d i l y d e t e c t a b l e and i d e n t i f i a b l e . P r e t r e a t m e n t may m e r e l y c o n s i s t o f e v a c u a t i o n a t d e s i r e d t e m p e r a t u r e s ( u s i n g a l i q u i d - n i t r o g e n - c o o l e d t r a p t o remove desorbed water). D e c o n v o l u t i o n and d e t a i l e d i n t e r p r e t a t i o n o f s p e c t r a c a n p r e s e n t major problems, however. As i l l u s t r a t e d i n F i g u r e 1, many bands c a n appear i n h y d r o x y l s p e c t r a o f z e o l i t e s and o t h e r c a t a l y s t s . D e c o n v o l u t i o n c a n be a i d e d by p r o g r e s s i v e d r y i n g and s e l e c t i v e exchange o f h y d r o x y l groups w i t h d e u t e r a t e d compounds. Broad u n d e r l y i n g bands c a n c o m p l i c a t e d e c o n v o l u t i o n and l e a d t o e r r o n e o u s c o n c l u s i o n s as t o r e l a t i v e r a t e s o f d i s a p p e a r a n c e o f d i f f e r e n t types o f h y d r o x y l g r o u p s . Even a f t e r s a t i s f a c t o r y d e c o n v o l u t i o n , d e t e r m i n i n g t h e n a t u r e and o r i g i n o f t h e d i f f e r e n t t y p e s o f h y d r o x y l groups r e s p o n s i b l e f o r t h e bands u s u a l l y s t i l l poses problems. =

2

3

C h a r a c t e r i z a t i o n o f Surface Acid S i t e s . C h a r a c t e r i z a t i o n o f surface a c i d i t y was one o f t h e f i r s t s u c c e s s f u l a p p l i c a t i o n s o f i n f r a r e d i n c a t a l y s t studies (1). Catalyst a c i d i t y i s t y p i c a l l y a function o f s u r f a c e h y d r a t i o n , and w a t e r i s r e l a t i v e l y easy t o remove. D e s o r p t i o n o f w a t e r o f t e n c o n v e r t s B r o n s t e d t o L e w i s a c i d s , and r e a d s o r p t i o n o f w a t e r c a n r e s t o r e B r o n s t e d a c i d i t y . Probe m o l e c u l e s , s u c h as ammonia, p y r i d i n e , e t c . , a r e used t o e v a l u a t e B r o n s t e d and L e w i s a c i d i t y . These compounds may c o n t a i n w a t e r as an i m p u r i t y , however. Water produced by r e d u c t i o n o f m e t a l o x i d e s c a n a l s o be r e a d s o r b e d on a c i d s i t e s . Probe m o l e c u l e s c a n i n some c a s e s r e a c t on s u r f a c e a c i d s i t e s , g i v i n g m i s l e a d i n g i n f o r m a t i o n on t h e n a t u r e o f the o r i g i n a l s i t e . A c i d i t y , and a c c e s s i b i l i t y , o f h y d r o x y l groups o r adsorbed w a t e r on z e o l i t e s and a c i d i c o x i d e s c a n v a r y w i d e l y . Study o f a d s o r b e d n i t r o g e n bases i s v e r y u s e f u l i n c h a r a c t e r i z a t i o n o f s u r f a c e a c i d s i t e s , b u t p o t e n t i a l problems i n t h e u s e o f t h e s e p r o b e s s h o u l d be k e p t i n mind. F i g u r e 2A shows h y d r o x y l s p e c t r a ( a f t e r background s u b t r a c t i o n ) t y p i c a l o f h i g h l y - d r i e d ZSM-5 z e o l i t e . The a c i d i t i e s o f t h e



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F i g u r e 1. H y d r o x y l and D e u t e r o x y l bands f o r a "US-Y" z e o l i t e . A. US S i e v e steamed 65 h r a t 500 C and d r i e d i n vacuum a t 500 C. B.- A. a f t e r exchange w i t h D 0 and r e d r y i n g a t 500 C. 2

3800

3700

3600

3500

3050

2950

Wavenumber (cm ~

2850

2750

1

F i g u r e 2 . A. H y d r o x y l bands on a ZSM-5 z e o l i t e (1.352 A l 0 ) . P r e d r i e d : a. 700 C, l h r . ; b. 700 C, 2 h r . ; c. 800 C, 1.5 h r . ; d. 800 C, 3 h r . B. O H bands produced a f t e r 1 h r . c o n t a c t w i t h e t h y l e n e (2 T o r r ) f o l l o w i n g s p e c t r a o f A. a


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h y d r o x y l groups r e s p o n s i b l e f o r the two bands seen have been shown t o be q u i t e d i f f e r e n t by a d s o r p t i o n o f ammonia, p y r i d i n e , o r o t h e r b a s e s . Changes i n the r e l a t i v e numbers o f t h e s e groups on f u r t h e r d e h y d r a t i o n can a l s o be e v a l u a t e d r a t h e r e a s i l y . As d e s c r i b e d e l s e w h e r e (1, 4 - 7 ) , f o r m a t i o n o f ammonium o r p y r i d i n i u m i o n s g i v e s bands i n the 1400-1600 cm" r e g i o n w h i c h p e r m i t assessment o f B r o n s t e d a c i d i t y . The p r o d u c t i o n o f such bands i s u s u a l l y accompanied by d i s a p p e a r a n c e o f a band o r bands i n the h y d r o x y l s t r e t c h i n g r e g i o n . A l t h o u g h n o t shown i n F i g u r e 2, a d s o r p t i o n o f ammonia would s e l e c t i v e l y e l i m i n a t e the band n e a r 3605 cm" and produce a band n e a r 1470 cm" a r i s i n g from ammonium i o n s . The 3605 cm" band e v i d e n t l y a r i s e s from h y d r o x y l groups w h i c h a r e s t r o n g Bronsted a c i d s . F i g u r e 2B shows C-H s t r e t c h i n g bands r e p r e s e n t i n g o l i g o m e r p r o d u c e d on the same c a t a l y s t , d r i e d as i n F i g u r e 2A, a f t e r 1 h o u r c o n t a c t w i t h e t h y l e n e a t 50 C. The amount o f o l i g o m e r p r o d u c e d i n c r e a s e d as the a c i d i c h y d r o x y l band d e c r e a s e d , s u g g e s t i n g t h a t o l i g o m e r i z a t i o n r e q u i r e d Lewis r a t h e r than Bronsted a c i d s . A d s o r p t i o n o f ammonia o r p y r i d i n e a l s o p r o v i d e s i n f o r m a t i o n on L e w i s a c i d s , b u t t h e s e probes tend t o adsorb u n s e l e c t i v e l y on f a r more s i t e s t h a n appear c a t a l y t i c a l l y i m p o r t a n t . A d s o r p t i o n o f CO o r C0 i s o f t e n more u s e f u l i n i d e n t i f y i n g important Lewis a c i d s i t e s . F i g u r e 3 shows FTIR s p e c t r a o b t a i n e d f o r C0 adsorbed on Na-Y and Na-H Y z e o l i t e s . The band n e a r 2355 cm" a p p a r e n t l y r e s u l t s from a d s o r p t i o n on Na i o n s , w h i l e t h a t a t 2370 shows a d s o r p t i o n on exposed A l i o n - o x i d e i o n s i t e s where b o t h c a t i o n and a n i o n a r e c o o r d i n a t i v e l y u n s a t u r a t e d . S i m i l a r s i t e s can be seen on many o t h e r z e o l i t e s and on h i g h l y d r i e d amorphous s i l i c a - a l u m i n a . F i g u r e 4 shows t h a t s i m i l a r A l i o n s i t e s can be produced on a ZSM-5 z e o l i t e by h i g h - t e m p e r a t u r e d r y i n g and t h a t a c t i v i t y f o r e t h y l e n e o l i g o m e r i z a t i o n accompanies f o r m a t i o n o f s u c h s i t e s . These s i t e s a r e e v i d e n t l y c r e a t e d on z e o l i t e s by removal o f A l from t h e c r y s t a l l i n e framework a t h i g h t e m p e r a t u r e s , l e a d i n g t o the exposure o f A l i o n s , p o s s i b l y as A10 , o u t s i d e the framework. A l t h o u g h no o l i g o m e r i z a t i o n was seen on the o r i g i n a l B r o n s t e d a c i d i c ZSM-5, and o l i g o m e r i z a t i o n a c t i v i t y i n c r e a s e d as the c a t a l y s t was p r o g r e s s i v e l y d r i e d , subsequent r e a d d i t i o n o f w a t e r , w h i c h e n t i r e l y removed the s i t e s w h i c h h e l d C0 , d i d n o t e l i m i n a t e , but i n s t e a d somewhat i n c r e a s e d t h i s a c t i v i t y . F i g u r e 5 shows (on ZSM-5 c o m p l e t e l y exchanged w i t h d e u t e r i u m o x i d e and d r i e d a t 800 C) t h a t r e a d d i t i o n o f w a t e r (D 0) gave two new bands, p o s s i b l y a r i s i n g from h y d r a t i o n o f A10 i o n s t o form A 1 ( 0 D ) i o n s . T h i s s u g g e s t s t h a t e i t h e r h y d r a t e d o r anhydrous complex A l c a t i o n s , h e l d o u t s i d e the z e o l i t i c framework can be a c t i v e f o r ethylene oligomerization. 1

1


1

1

2

2

1

+

+

2

2

+

+

2

S t r a i n e d Bonds on Oxides and Z e o l i t e s . Many d r y o x i d e s and s i l i c a t e s , i n c l u d i n g z e o l i t e s , c o n t a i n oxide l i n k a g e s which are h i g h l y " s t r a i n e d " . Such s t r a i n can be r e l i e v e d by c h e m i s o r p t i o n o f w a t e r , ammonia, methanol, e t c . , w h i c h opens s t r a i n e d l i n k a g e s by




F i g u r e 3 . S p e c t r a o f C 0 on Na-Y and H-Na Y z e o l i t e s , a. Na-Y d r i e d a t 500 C; b. H-Na Y d r i e d a t 300 C; c. H-Na Y d r i e d a t 500 C; d. H-Na Y d r i e d a t 700 C. 2

2450

2400

2350

2300

3050

2950

2850

-1

Wavenumber ( c m )

F i g u r e 4 . S p e c t r a o f C 0 and o f o l i g o m e r s u b s e q u e n t l y formed on ZSM-5 (1.352 A1 0 ) a f t e r 1 h r c o n t a c t w i t h e t h y l e n e (2 T o r r ) . D r i e d : a. 500 C; b. 600 C; c. 800C; d. 800 C, 3 h r . 2

2

3



21.


PERI

229

h y d r o l y s i s , ammonolysis, m e t h a n o l y s i s , e t c . . S p e c t r a o b t a i n e d b e f o r e and a f t e r a d s o r p t i o n o f methanol on h i g h - s i l i c a m o l e c u l a r s i e v e s p r e d r i e d a t 500 C show, w i t h computer s u b t r a c t i o n , t h a t a l t h o u g h some methoxy groups a r e a t t a c h e d by replacement o f OH groups, many o t h e r s a r e a t t a c h e d by m e t h a n o l y s i s o f s t r a i n e d s i l o x a n e l i n k a g e s . S i m i l a r c h e m i s o r p t i o n o f o t h e r m o l e c u l e s can a l s o be demonstrated. F i g u r e 6 p r e s e n t s s p e c t r a o f methanol adsorbed on s i l i c a l i t e (13 ppm A l ) p r e d r i e d a t 500 and 700 C. The background has i n a l l c a s e s been s u b t r a c t e d . A l t h o u g h on 500 C - p r e d r i e d s i l i c a l i t e some methanol was a t t a c h e d by replacement o f e x i s t i n g OH groups w i t h methoxy groups (as shown by the n e g a t i v e OH b a n d s ) , more methanol was a p p a r e n t l y a t t a c h e d by m e t h a n o l y s i s o f s t r a i n e d s i l o x a n e bonds, f o r m i n g methoxy (bands between 2800 and 3000 cm" ) and new h y d r o x y l (3^00 cm' ) groups. On 700 C - p r e d r i e d S i l i c a l i t e n e a r l y a l l t h e methanol was chemisorbed by m e t h a n o l y s i s o f s i l o x a n e bonds. 1

1

C h a r a c t e r i z a t i o n o f M e t a l S i t e s on Supported M e t a l C a t a l y s t s . C h a r a c t e r i z a t i o n o f s u p p o r t e d m e t a l s i s u s u a l l y more d i f f i c u l t . C o n s i d e r a b l e v a r i a t i o n can f r e q u e n t l y be found i n t h e s t a t e o f the reduced m e t a l as a r e s u l t o f a p p a r e n t l y minor d i f f e r e n c e s i n pretreatment, i m p u r i t i e s i n the support, o r r e s i d u a l water o r o t h e r contaminants. The problem i s most s e v e r e w i t h r e a d i l y o x i d i z a b l e m e t a l s . N i (10), Mo (11) , Re (12) and o t h e r m e t a l s can a l l show major v a r i a t i o n s depending on sample p r e t r e a t m e n t and r e d u c t i o n p r o c e d u r e s . Even i n the case o f p l a t i n u m group m e t a l s many c o m p l i c a t i o n s e x i s t . The f r e q u e n c i e s o f bands o b s e r v e d when CO i s adsorbed i n a g i v e n manner (e.g. " l i n e a r " o r " b r i d g e d " ) can s h i f t by up t o 100 cm" w i t h coverage by CO o r between d i f f e r e n t samples. T h i s r e s u l t s i n l a r g e p a r t from d i p o l e - d i p o l e i n t e r a c t i o n s between n e i g h b o r i n g adsorbed CO m o l e c u l e s (13-1*0, b u t changes i n t h e t y p e s o f s i t e s a v a i l a b l e f o r CO a d s o r p t i o n on one o r more exposed c r y s t a l f a c e s o r e l e c t r o n i c m o d i f i c a t i o n o f t h e s i t e s r e m a i n i n g as a r e s u l t o f i n i t i a l a d s o r p t i o n o f a p a r t i a l monolayer may a l s o l e a d t o f r e q u e n c y s h i f t s . Supported m e t a l s can be i n f l u e n c e d by n e i g h b o r i n g c a t i o n s o r a n i o n s , w h i c h cause changes i n the e l e c t r o n i c n a t u r e o f exposed m e t a l atoms and i n the f r e q u e n c i e s o f CO o r o t h e r m o l e c u l e s adsorbed on t h e s e atoms. Such e l e c t r o n i c s u p p o r t e f f e c t s s h o u l d o n l y become s i g n i f i c a n t f o r v e r y h i g h l y d i s p e r s e d m e t a l s , however. Even v e r y s m a l l c r y s t a l s o f c l e a n s u p p o r t e d m e t a l s p r o b a b l y d i f f e r l i t t l e from l a r g e c r y s t a l s i n t h e i r e l e c t r o n i c p r o p e r t i e s e x c e p t i n s o f a r as s i z e i n f l u e n c e s f a c e exposure and edge and c o r n e r atom abundance. S p e c t r a a p p a r e n t l y showing e f f e c t s o f P t c r y s t a l s i z e on t h e f r e q u e n c y o f adsorbed NO have been p u b l i s h e d (1^). D e s p i t e much work, doubt s t i l l e x i s t s on the p r o p e r i n t e r p r e t a t i o n o f s h i f t s seen i n t h e f r e q u e n c i e s o f adsorbed CO and NO on s u p p o r t e d m e t a l s . This s i t u a t i o n a r i s e s p a r t l y because d i f f e r e n t i n v e s t i g a t o r s have l o o k e d a t samples d i f f e r i n g i n m e t a l l o a d i n g , p r e p a r a t i o n , p r e t r e a t m e n t , and c l e a n l i n e s s . Use o f t h e most e f f e c t i v e t e c h n i q u e s a v a i l a b l e cannot l e a d t o agreement when d i f f e r e n t i n v e s t i g a t o r s l o o k at d i f f e r e n t s u r f a c e s . 1


230



.10

(OH band frequencies corresponding to OD bands are given in parenthesis.)

2800

2750 2700 2650 Wavenumber (cm )

2600

-1

F i g u r e 5» E f f e c t s o f r e h y d r a t i o n ( w i t h d e u t e r i u m o x i d e ) o n d e u t e r o x y l bands o f ZSM-5 (1.352 A1 0 ) . P r e t r e a t m e n t : a. D r i e d a t 800 C; b. R e h y d r a t e d w i t h D 0 and d r i e d a t 300 C f o l l o w i n g a. 2

3

2

F i g u r e 6. S p e c t r a o f methanol chemisorbed on S i l i c a l i t e s u b t r a c t i o n o f o r i g i n a l background). S i l i c a l i t e p r e d r i e d a t A: 5 0 0 C ; B: 700 C. S p e c t r a shown a r e a f t e r a d s o r p t i o n o f e x c e s s methanol and d e s o r p t i o n a t a., 150 C; b., 300 C; c , 400 C.


(after

21.

PERI


231

F i g u r e 7 i l l u s t r a t e s the marked v a r i a t i o n s o b s e r v e d i n s p e c t r a o f NO adsorbed on 1% I r / p u r e gamma-alumina reduced by t h r e e d i f f e r e n t methods. R e d u c t i o n i n s t a t i c hydrogen e v i d e n t l y l e f t more unreduced I r (as i n d i c a t e d by the band a t 1895 cm" ) t h a n d i d r e d u c t i o n i n f l o w i n g hydrogen, a l t h o u g h r e d u c t i o n a t 700 C was f a i r l y e f f e c t i v e even i n s t a t i c hydrogen. The r e d u c t i o n a t 300-500 C i n f l o w i n g hydrogen may have produced a m o r e - c r y s t a l l i n e i r i d i u m , however, because the NO band f r e q u e n c y (1805) i s s u b s t a n t i a l l y l o w e r t h a n t h a t seen on t h e c a t a l y s t reduced a t 700 C. I n t h e case o f a l u m i n a s u p p o r t e d rhenium (12), t h e n a t u r e o f t h e s u p p o r t e d m e t a l a l s o seems h i g h l y dependent on p r e t r e a t m e n t o f t h e c a t a l y s t . I n c r e a s i n g t h e p r e c a l c i n a t i o n t e m p e r a t u r e from 500 t o 700 C, e v i d e n t l y i n c r e a s e d t h e amount o f exposed, f u l l y reduced Re shown s u b s e q u e n t l y ( a f t e r r e d u c t i o n i n hydrogen a t 500 C) by a d s o r p t i o n o f CO. A d d i t i o n a l t y p e s o f reduced Re a d s o r p t i o n s i t e s were a l s o a p p a r e n t l y p r e s e n t on the sample p r e c a l c i n e d a t 700 C. As shown i n F i g u r e 8, p r e t r e a t m e n t o f Re/alumina w i t h d i m e t h y l s u l f i d e p r i o r t o r e d u c t i o n can e l i m i n a t e a t l e a s t one o f t h e two c h a r a c t e r i s t i c bands t y p i c a l l y seen f o r NO adsorbed on u n s u l f i d e d samples. The e l i m i n a t i o n o f the s i t e s r e p o n s i b l e f o r t h i s band appears t o c o r r e l a t e w i t h the e l i m i n a t i o n o f t h e e x c e s s i v e h y d o g e n o l y s i s a c t i v i t y shown by u n s u l f i d e d f r e s h P t - R e / a l u m i n a c a t a l y s t s . The p r o b a b l e n a t u r e o f the s i t e s r e s p o n s i b l e f o r t h e two NO bands has been d i s c u s s e d e l s e w h e r e (16). I t i s l i k e l y t h a t t h e band a t the h i g h e r f r e q u e n c y a r i s e s from NO on exposed Re i o n s . R e g a r d l e s s o f t h e o r i g i n o f the two bands, however, some o f t h e exposed Re atoms have c l e a r l y been r a t h e r p e r m a n e n t l y a l t e r e d by s u l f i d i n g , w i t h i m p o r t a n t c a t a l y t i c consequences. A w e a l t h o f d e t a i l e d e v i d e n c e on t h e n a t u r e o f s u p p o r t e d m e t a l s can r e a d i l y be o b t a i n e d from i n f r a r e d c h a r a c t e r i z a t i o n s t u d i e s , b u t c o r r e c t i n t e r p r e t a t i o n o f much o f t h i s e v i d e n c e i s s t i l l f a r from c l e a r . The s u r f a c e c h e m i s t r y o f s u p p o r t e d m e t a l s i s g e n e r a l l y v e r y complex, and a s s e r t i o n s as t o the o r i g i n s o f v a r i o u s band s h i f t s and t h e e x a c t n a t u r e o f a d s o r p t i o n s i t e s s h o u l d be t a k e n w i t h some c a u t i o n a t p r e s e n t . C l e a r l y , however, b e t t e r u n d e r s t a n d i n g o f t h e complex n a t u r e o f s u p p o r t e d m e t a l c a t a l y s t s s h o u l d c o n t r i b u t e g r e a t l y t o t h e development o f more e f f i c i e n t c a t a l y s t s f o r many i m p o r t a n t i n d u s t r i a l p r o c e s s e s and t o more e f f i c i e n t p r e t r e a t m e n t and r e g e n e r a t i o n p r o c e d u r e s .


1

C o n c l u s i o n s and F u t u r e P r o s p e c t s The r o l e o f i n f r a r e d s p e c t r o s c o p i c c h a r a c t e r i z a t i o n o f c a t a l y s t s s h o u l d become i n c r e a s i n g l y i m p o r t a n t i n c a t a l y s t development, b u t p r e s e n t problems, b o t h e x p e r i m e n t a l and t h e o r e t i c a l must be r e c o g n i z e d and overcome b e f o r e r e l i a b l e i n f o r m a t i o n can be r o u t i n e l y o b t a i n e d u s i n g i n f r a r e d methods. Techniques and u n d e r s t a n d i n g w i l l improve as more i n f o r m a t i o n becomes a v a i l a b l e , b u t d e t a i l e d i n t e r p r e t a t i o n o f s p e c t r a w i l l c o n t i n u e t o p r e s e n t problems f o r some t i m e . P a r t i a l i n t e r p r e t a t i o n and " f i n g e r p r i n t i n g " can s t i l l be



232


O'

1 1 L 1700

1

1

1900

1

1

1900

1 1 I 1700

1

1

1

1900 1700

WAVENUMBER (CM~ > 1

F i g u r e 7« S p e c t r a o f NO adsorbed on 1% I r / p u r e gamma-alumina p r e r e d u c e d u s i n g t h r e e d i f f e r e n t p r o c e d u r e s . Dashed s p e c t r a — background a f t e r r e d u c t i o n ; s o l i d — a f t e r a d d i t i o n o f NO; d o t dash — a f t e r 5 min. e v a c u a t i o n .

0 I 2000

i

i

1

1800

1

1600

1

•

1400

WAVENUMBER (CM*)

1

1

1200

F i g u r e 8. E f f e c t o f s u l f i d e t r e a t m e n t on i n f r a r e d bands o f NO a d s o r b e d on r e d u c e d 0.62 Re/alumina. a., NO added a f t e r p r e r e d u c t i o n a t 500 C; b., NO added a f t e r e x p o s u r e t o d i m e t h y l s u l f i d e , h e a t i n g i n oxygen, and r e d u c t i o n a t 500 C; c. NO readded a f t e r r e d u c t i o n a t 600 C f o l l o w i n g t h e above.


21.

PERI


233

h i g h l y u s e f u l a t p r e s e n t i n c a t a l y s t development, q u a l i t y c o n t r o l o r p a t e n t m a t t e r s , however.


Acknowledgments The examples g i v e n o f a p p l i c a t i o n s o f i n f r a r e d t o c a t a l y s t c h a r a c t e r i z a t i o n have been t a k e n from my p a s t work i n t h e Amoco O i l Company Research and Development Department. Amoco O i l Company's c o n s i s t e n t support f o r n e a r l y t h i r t y years, the a s s i s t a n c e o f a s e r i e s o f a b l e t e c h n i c i a n s , and t h e a d v i c e and encouragement o f many o t h e r c o l l e a g u e s a r e g r a t e f u l l y acknowledged. Literature Cited 1. Eischens, R. P.; P l i s k i n , W. A. Advances i n C a t a l y s i s 1958, 10, 1. 2. Haller, G. L. In Spectroscopy i n Heterogeneous Catalysis; Delgass, W. N.; Haller, G. L.; Kellerman, R.; Lunsford, J . H., Eds.; Academic Press: New York, 1979; pp 19-57. 3. B e l l , A. T. In Vibrational Spectroscopies for Adsorbed Species; B e l l , A. T.; Hair, M. L., Eds.; American Chemical Society: Washington, DC, 1980; pp 13-35. 4. P e r i , J . B. In Catalysis; Anderson, J . R.; Boudart, M., Eds.; Springer-Verlag: Berlin-New York, 1984; Vol. 5, pp 172-219. 5. L i t t l e , L. H. Infrared Spectra of Adsorbed Species; Academic Press: New York, 1966. 6. Hair, M. L. Infrared Spectroscopy i n Surface Chemistry; Marcel Dekker: New York, 1967. 7. Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; John Wiley & Sons: New York-Toronto, 1975. 8. Yates, J . T., J r . ; Gelin, P.; Beebe, T. In Catalyst Characterization Science; Deviney, M. L.; Gland, J . L., Eds.; ACS Symposium Series No. 288; American Chemical Society: Washington, DC, 1985; pp 404-421. 9. Perkins, W. D. J . Chemical Education 1987, 64, A269-271. 10. P e r i , J . B. J . Catal. 1984, 86, 84. 11. P e r i , J . B. J . Phys. Chem. 1982, 86, 1615. 12. P e r i , J . B.; M i e v i l l e , R. L. PREPRINTS, Div. of P e t r o l . Chem., ACS 1987, 32, 3, 737. 13. Hammaker, R. M.; Francis, S. A.; Eischens, R. P. Spectrochim. Acta 1965, 21, 1295. 14. Hendrickx, H. A. C. M.; Des Bouvrie, C.; Ponec, V. J . C a t a l . 1988, 109, 120. 15. Primet, M.; Basset, J . M.; Garbowski, E.; Mathieu, M. V. J . Am. Chem. Soc. 1975, 97, 3655. 16. P e r i , J . B. J . Catal. 1978, 52, 144. R E C E I V E D January 26, 1989


Characterization and Catalyst Development

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