Characterization of Catalysts by Scanning Transmission Electron

Oct 3, 1989 - The dedicated scanning transmission electron microscope (STEM) is an integral tool for characterizing catalysts because of its unique ab...
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Chapter 31

Characterization of Catalysts by Scanning Transmission Electron Microscopy William M . Targos and Steven A. Bradley

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U O P Research Center, Des Plaines, IL 60017

The dedicated scanning transmission electron microscope (STEM) is an integral tool for characterizing catalysts because of its unique ability to image and analyze nano­ sized volumes. This information is valuable in optimiz­ ing catalyst formulations and determining causes for reduced catalyst performance. For many commercial catal­ ysts direct correlations between structural features of metal crystallites and catalytic performance are not attainable. When these instances occur, determination of elemental distribution may be the only information avail­ able. In this paper we will discuss some of the techni­ ques employed and limitations associated with charac­ terizing commercial catalysts.

The e l e c t r o n m i c r o s c o p e o f f e r s a unique approach f o r measuring i n d i v i d u a l n a n o - s i z e d volumes w h i c h may be c a t a l y t i c a l l y a c t i v e as opposed t o t h e a v e r a g i n g method employed by s p e c t r o s c o p i c t e c h n i ques. I t i s j u s t t h i s a b i l i t y o f b e i n g a b l e t o observe and measure d i r e c t l y s m a l l c r y s t a l l i t e s o r nano-volumes o f a c a t a l y s t s u p p o r t t h a t s e t s t h e m i c r o s c o p e a p a r t from o t h e r a n a l y s e s . There have been many s t u d i e s r e p o r t e d i n t h e l i t e r a t u r e over t h e p a s t f i f t e e n y e a r s w h i c h emphasize t h e use o f a n a l y t i c a l and t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y i n t h e 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 . Reviews ( 1 - 5 ) o f t h e s e s t u d i e s emphasize t h e r e l a t i o n s h i p between t h e s t r u c t u r e o f the s i t e and c a t a l y t i c a c t i v i t y and s e l e c t i v i t y . Most commercial c a t a l y s t s do n o t r e a d i l y p e r m i t such c l e a r d i s t i n c t i o n o f p h y s i c a l p r o p e r t i e s w i t h performance. The importance o f e s t a b l i s h i n g t h e p r o x i m i t y o f elements, e l e m e n t a l d i s t r i b u t i o n and component p a r t i c l e s i z e i s o f t e n o v e r l o o k e d as v i t a l i n f o r m a t i o n i n t h e d e s i g n and evaluation of catalysts. F o r example, t h i s i n t e r a c t i v e approach was s u c c e s s f u l l y u s e d i n t h e development o f a F i s c h e r - T r o p s c h c a t a l y s t (6). A l t h o u g h some measurements on commercial c a t a l y s t s c a n be made r o u t i n e l y w i t h a STEM, t h e r e a r e complex c a t a l y s t s w h i c h r e q u i r e 0097-6156/89/0411-0342$06.00/0 © 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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Characterization of Catalysts by STEM

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t e d i o u s a n a l y t i c a l approaches. The methodology and l i m i t a t i o n s as they a p p l y t o commercial c a t a l y s t s w i l l be d i s c u s s e d i n t h i s paper. Most o f t h e s t u d i e s r e p o r t e d i n t h e l i t e r a t u r e use t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e s , TEM o r a m o d i f i e d TEM w i t h s c a n n i n g f e a t u r e s c a l l e d a s c a n n i n g t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e , STEM. These m o d i f i e d m i c r o s c o p e s a r e r e f e r r e d t o as TEM/STEM. There i s a l s o a n o t h e r type o f m i c r o s c o p e , a d e d i c a t e d s c a n n i n g t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e (STEM), w h i c h o f f e r s unique advantages w i t h r e g a r d t o t h e a n a l y t i c a l a s p e c t s o f t h e measurements made i n c h a r a c t e r i z i n g c a t a l y s t s . The p r i m a r y advantage t h e STEM has o v e r a c o n v e n t i o n a l TEM i s t h a t a more i n t e n s e e l e c t r o n beam c a n be r o u t i n e l y f o c u s e d i n t o an l.Onm beam s i z e . The r e a s o n f o r t h i s advantage i s t h a t a f i e l d e m i s s i o n e l e c t r o n s o u r c e i s used t o g e n e r a t e t h e e l e c t r o n beam. Thus n a n o - s i z e d c r y s t a l l i t e s c a n be e a s i l y i d e n t i f i e d and c h a r a c t e r i z e d . I n g e n e r a l , t h e two i n s t r u m e n t s c o m p l i ment each o t h e r and s h o u l d be used t o g e t h e r when c h a r a c t e r i z i n g c a t a l y s t s . Most o f t h e comments made i n t h i s paper w i l l be from t h e p o i n t o f v i e w o f u s i n g t h e d e d i c a t e d STEM. F i n a l l y , as w i t h most problems e n c o u n t e r e d i n i n d u s t r y , a m u l t i p l e t e c h n i q u e approach s h o u l d be implemented when p o s s i b l e t o a c h i e v e a b e t t e r u n d e r s t a n d i n g o f t h e problem. Experimental The m i c r o s c o p e used i n o b t a i n i n g t h e r e s u l t s p r e s e n t e d i n t h i s paper was a Vacuum G e n e r a t o r s HB-5 STEM. A Kevex energy d i s p e r s i v e x - r a y s p e c t r o m e t e r , EDS, w i t h 10mm Be window was used f o r t h e e l e m e n t a l microanalysis. Specimen p r e p a r a t i o n f o r c a t a l y s t s i s n o t as s e v e r e a problem as i s commonly found f o r m e t a l l u r g i c a l o r c e r a m i c samples. F o r example, r e a s o n a b l y good specimens c a n be p r e p a r e d by s i m p l y g r i n d i n g a c a t a l y s t i n t o a f i n e powder, s u s p e n d i n g t h e powder i n i s o p r o p a n o l , and d e p o s i t i n g t h e s u s p e n s i o n on a c a r b o n c o a t e d n y l o n o r copper g r i d . T h i s approach r e s u l t s i n a wide range o f p a r t i c l e s i z e s from w h i c h t o observe unique m o r p h o l o g i c a l f e a t u r e s and d e t e r m i n e t h e c o m p o s i t i o n and s t r u c t u r a l p r o p e r t i e s o f t h e c a t a l y s t . U l t r a m i c r o t o m i n g a c a t a l y s t can a l s o p r o v i d e unique informat i o n t o the a n a l y s t . Since the thickness i s uniform throughout the specimen, e l e m e n t a l p a r t i c l e i n f o r m a t i o n , m e t a l s d i s t r i b u t i o n and s t r u c t u r a l i n f o r m a t i o n n o t o b t a i n e d by t h e g r i n d i n g method c a n be o b t a i n e d (7-8). The l i m i t a t i o n s a r e t h a t i t i s more time consuming and c o s t l y t h a n t h e g r i n d i n g o r s c r a p i n g method. A l s o specimens may n o t n e c e s s a r i l y be o b t a i n e d from t h e e x a c t r e g i o n o f i n t e r e s t . I n b o t h p r e p a r a t i v e methods t h e i n t e g r i t y o f t h e c a t a l y s t i s compromised. Exposure t o ambient c o n d i t i o n s , e x t e r i o r c h e m i c a l e n v i r o n m e n t s , and exposure t o h i g h vacuum i n t h e m i c r o s c o p e may a l t e r t h e c a t a l y s t p r o p e r t i e s . Employment o f s p e c i a l chambers f o r redox p r e t r e a t m e n t s and/or e n v i r o n m e n t a l m i c r o s c o p e s p r o v i d e s some a s s i s t a n c e i n overcoming t h e s e problems (9). I t appears t h a t r e g a r d l e s s o f t h e approach t a k e n , compromises i n specimen p r e p a r a t i o n o r i n s t r u m e n t a l performance must be made. These f a c t o r s must be k e p t i n mind when i n t e r p r e t i n g r e s u l t s o b t a i n e d from e l e c t r o n m i c r o s c o p i c measurements. 2

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

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Commercial and E x p e r i m e n t a l C a t a l y s t s Many o f t h e TEM s t u d i e s o f c a t a l y s t s r e p o r t e d i n t h e l i t e r a t u r e u s e model c a t a l y s t s w i t h v i s i b l e m e t a l c r y s t a l l i t e s s u p p o r t e d on a m e t a l o x i d e s u p p o r t , w h i c h u s u a l l y i s s e l e c t e d because i t o f f e r s m i n i m a l e l e c t r o n s c a t t e r i n g i n t e r f e r e n c e . These model c a t a l y s t s a r e s t u d i e d because d i r e c t measurements r e g a r d i n g c r y s t a l l i t e s i z e and s t r u c t u r e can be made. E x t r a p o l a t i o n o f t h i s i n f o r m a t i o n t o s m a l l e r m e t a l c l u s t e r s i z e s u s u a l l y p r e s e n t i n commercial c a t a l y s t s i s a n t i c i p a t e d t o p r o v i d e i n s i g h t i n t o t h e r e l a t i o n s h i p between s t r u c t u r e and chemical r e a c t i v i t y (10). Commercial f r e s h , s p e n t , and r e g e n e r a t e d c a t a l y s t s e x h i b i t a wide range o f m e t a l d i s t r i b u t i o n s and d i s p e r s i o n s . I d e a l l y , the c a t a l y t i c c h e m i s t would l i k e t o know i n d e t a i l t h e s t r u c t u r e , quant i t y , and d i s t r i b u t i o n o f t h e c a t a l y t i c s i t e s . There a r e a number o f e x p e r i m e n t a l l i m i t a t i o n s w h i c h i n h i b i t such a complete s u r v e y o f the p r o p e r t i e s o f t h e c a t a l y s t . O f t e n commercial c a t a l y s t s w i l l n o t have r e a d i l y i d e n t i f i a b l e s i t e s , i . e . , c r y s t a l l i t e s w h i c h c a n be o b s e r v e d and a n a l y z e d f o r s t r u c t u r a l and c o m p o s i t i o n a l p r o p e r t i e s . T h i s i s u s u a l l y t h e case e n c o u n t e r e d f o r f r e s h c a t a l y s t s w h i c h have m e t a l l o a d i n g s below 0.5 wt.% and on s u p p o r t s w h i c h o f f e r a h i g h degree o f i n t e r f e r e n c e from e l e c t r o n s c a t t e r . On a c a t a l y s t o f t h i s t y p e , o f t e n o n l y i n f o r m a t i o n on the u n i f o r m i t y o f t h e e l e m e n t a l d i s t r i b u t i o n c a n be determined. I n g e n e r a l , t h e d e d i c a t e d STEM w i l l a l l o w the m i c r o s c o p i s t t o determine c o m p o s i t i o n a l a n a l y s e s on volumes o f a p p r o x i m a t e l y 40 nm . An example o f two s u p p o r t s s y n t h e s i z e d by s i m i l a r p r o c e d u r e s and a n a l y z e d f o r u n i f o r m i t y i s shown i n T a b l e I . I t i s o b v i o u s from the s t a n d a r d d e v i a t i o n t h a t s u p p o r t A was n o t made as w e l l as B. T h i s problem was r e s o l v e d by a l t e r i n g mixing variables. S u b t l e v a r i a t i o n s o f t h i s type o f t e n have a d r a m a t i c i n f l u e n c e on c a t a l y s t performance. Catalytically a c t i v e m e t a l s on s u p p o r t s c a n a l s o be m o n i t o r e d i n t h i s manner ( 1 1 ) . Many commercial c a t a l y s t s employ m i x t u r e s o f s e v e r a l m e t a l o x i d e s . The advantages o f t h e s e mixed o x i d e systems v a r i e s from t h e r m a l s t a b i l i t y t o enhanced a c i d i c p r o p e r t i e s and/or s u p p o r t m e t a l i n t e r a c t i o n s . The d e d i c a t e d STEM does an e x c e l l e n t j o b o f i d e n t i f y i n g t h e i n d i v i d u a l mixed o x i d e components and d e t e r m i n i n g l o c a t i o n of the c a t a l y t i c metals. B e i n g a b l e t o o b t a i n i n f o r m a t i o n on the m e t a l s d i s t r i b u t i o n and d i s p e r s i o n as a f u n c t i o n o f p r e p a r a t i v e , o p e r a t i o n a l , and regeneration c o n d i t i o n s i s v i t a l l y important i n h e l p i n g d i r e c t the o p t i m i z a t i o n o f new c a t a l y s t f o r m u l a t i o n s . A f t e r a c a t a l y s t has been exposed t o o p e r a t i n g and/or carbon b u r n c o n d i t i o n s , m e t a l s a g g l o m e r a t i o n may o c c u r . A l s o t h e r e may be c i r c u m s t a n c e s w h e r e i n the f r e s h c a t a l y s t might have s p e c i f i e d s i z e m e t a l c r y s t a l l i t e s . I n t h e s e c a s e s , t h e c h a r a c t e r i z a t i o n by STEM becomes more complex. Thorough c h a r a c t e r i z a t i o n r e q u i r e s c o m p o s i t i o n a l and s t r u c t u r a l i n f o r m a t i o n o f t h e d i f f e r e n t s i z e c r y s t a l l i t e s , and t o what e x t e n t a l l o f t h e c r y s t a l l i t e s comprise t h e t o t a l amount o f m e t a l p r e s e n t . T h i s i s a d i f f i c u l t i f n o t i m p o s s i b l e t a s k t o c a r r y o u t i n some cases because o f t h e i n t e r f e r e n c e t h e s u p p o r t l e n d s t o t h e a n a l y s i s problem. I n F i g u r e 1, a m i c r o g r a p h o f 2-3 nm p l a t i n u m - c r y s t a l l i t e s on 7 - A l 0 i s shown. A l t h o u g h t h e r e a r e many c o n t r a s t i n g f e a t u r e s , o n l y those i n d i c a t e d by arrows were p l a t i n u m . T h i s i n f o r m a t i o n 3

2

3

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

31.

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Characterization of Catalysts by STEM

Comparison o f Two D i f f e r e n t l y P r e p a r e d S i l i c a - A l u m i n a Supports

Support

A

Support

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Weight %

B

Weight %

Al

Si

Al

Si

47,.0 33..0 21..1 40..9 20,.1* 26..2 50,.2 59,.3 60,.0 30.,4 38.,8±14.9

53,.0 67,.0 78,.9 59,.1 79,.9 73,.8 49,.8 40,.8 40,.0 68,.6 61,.2±14.9

35,.1 30..5 36,.6 34..1 34,.9 33,.8 34..0 32.,3 36..7 28.,6 33.,9 37.,0 34.,0±2.5

64,.9 69,.5 63,.4 65,.9 65,.1 66,.2 66,.0 67,.7 63,.3 71,.4 66,.1 63,.0 66..0±2.5

F i g u r e 1.

P l a t i n u m c r y s t a l l i t e s on 7-alumina.

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

345

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c o u l d o n l y be v e r i f i e d by a p o i n t by p o i n t m i c r o a n a l y s i s u s i n g EDS. X - r a y mapping u s u a l l y does n o t p r o v i d e s u f f i c i e n t s i g n a l t o n o i s e f o r d i s t i n g u i s h i n g 2-3nm c r y s t a l l i t e s from t h e s u p p o r t ( 1 2 ) . I n c r e a s e d sweep times w i t h s t a t e o f t h e a r t EDS d e t e c t o r s may i n some cases p r o v i d e s u f f i c i e n t s e n s i t i v i t y t o make t h i s measurement. C o n s e q u e n t l y , t h e e l e c t r o n beam must be d i r e c t e d t o a v e r y s m a l l volume o f c a t a l y s t by o p e r a t i n g a t h i g h m a g n i f i c a t i o n s , i . e . , 20 m i l l i o n times i n a r a s t e r o r s p o t t e d beam mode. O f t e n count t i m e s o f up t o 100 seconds o r more a r e r e q u i r e d t o determine i f an a r e a o f h i g h c o n t r a s t c o n t a i n s a c a t a l y t i c m e t a l . T h i s experiment i s t e d i o u s , b u t i n many i n s t a n c e s i s t h e o n l y p r a c t i c a l approach. There a r e s i t u a t i o n s i n w h i c h c r y s t a l l i t e s a r e r e a d i l y v i s i b l e , e s p e c i a l l y on s u p p o r t s w h i c h do n o t o f f e r e x c e s s i v e e l e c t r o n scatter. I n these c a s e s , m e t a l c o n t e n t c a n be q u a n t i t a t i v e l y d e t e r m i n e d f o r areas w h i c h have h i g h l y d i s p e r s e d m e t a l and agglomerated metal. This information i n conjunction w i t h the c r y s t a l l i t e s i z e d i s t r i b u t i o n provides the m i c r o s c o p i s t w i t h the i n f o r m a t i o n r e q u i r e d t o make an e s t i m a t e o f m e t a l d i s p e r s i o n ( 1 3 ) . These e s t i m a t e s a r e v a l u a b l e e s p e c i a l l y i n s i t u a t i o n s where c o n v e n t i o n a l gas a d s o r p t i o n measurements cannot be made on t h e m e t a l , i . e . , when the c r y s t a l l i t e s a r e contaminated, have m u l t i p l e o x i d a t i o n s t a t e s , or are poisoned. One o f t h e d i f f i c u l t i e s i n d i r e c t l y a n a l y z i n g s t r u c t u r e s o f c r y s t a l l i t e s w i t h a 1-3 run s i z e range i s t h e i r beam s e n s i t i v i t y . Some c r y s t a l l i t e s w i l l d i s s o c i a t e i n t h e beam w h i l e o t h e r s t e n d t o agglomerate ( 1 4 ) . The mass o f t h e c r y s t a l l i t e , s u p p o r t - m e t a l i n t e r a c t i o n , c h e m i c a l environment, o x i d a t i o n s t a t e o f t h e m e t a l , e t c . , a l l have an i n f l u e n c e on how t h e c r y s t a l l i t e and e l e c t r o n beam interact. I n o r d e r t o f o r m u l a t e a c o r r e l a t i o n o f these v a r i a b l e s w i t h c r y s t a l l i t e r e a c t i v i t y w i t h t h e beam, t h e c r y s t a l l i t e s i t e c h e m i s t r y i s r e q u i r e d . T h i s i s d i f f i c u l t i f n o t i m p o s s i b l e t o do because t h e s i t e c h e m i s t r y i s a l t e r e d d u r i n g m i c r o s c o p i c examinat i o n . W i t h p a r a l l e l EELS d e t e c t i o n t h e time may be s u f f i c i e n t l y reduced t h a t u s e f u l c h e m i c a l i n f o r m a t i o n c a n be o b t a i n e d and c o r r e l a t i o n s o f t h e type p r e v i o u s l y d e s c r i b e d c a n be made. The s e n s i t i v i t y o f these s m a l l c r y s t a l l i t e s t o t h e e l e c t r o n beam makes o b t a i n i n g m i c r o d i f f r a c t i o n p a t t e r n s v e r y d i f f i c u l t . U s u a l l y a p p a r a t u s w h i c h i n v o l v e s s p e c i a l i z e d o p t i c s and e l e c t r o n i c m o n i t o r i n g d e v i c e s such as those developed b y Cowley (15) c a n reduce beam exposure time and thus p r o v i d e some m e a n i n g f u l d a t a w h i c h might be u s e f u l i n s t u d y i n g t h e s u p p o r t - m e t a l i n t e r a c t i o n o r s t r u c t u r e reactivity relationship. There a r e two approaches w h i c h enhance o b s e r v a t i o n o f 1-2 nm supported metal c r y s t a l l i t e s . One i s a n n u l a r dark f i e l d o r Z-cont r a s t d e t e c t i o n (16) and t h e o t h e r i s c a l l e d a x i a l dark f i e l d ( 1 7 ) . These t e c h n i q u e s c a n be e a s i l y u t i l i z e d f o r s i m p l e c a t a l y s t composit i o n s . The a n n u l a r dark f i e l d d e t e c t i o n method a n a l y z e s e l a s t i c a l l y s c a t t e r e d e l e c t r o n s a t wide a n g l e s from t h e c e n t r a l beam. The s c a t t e r i n g a n g l e i s a f u n c t i o n o f atomic number. Consequently, elements such as p l a t i n u m a r e e f f e c t i v e l y o b s e r v e d i n low atomic number s u p p o r t s such as A l 0 o r S i 0 (18) . The method i s a l s o e f f e c t i v e i n d e t e c t i n g l a r g e r c r y s t a l l i t e s i n support p a r t i c l e s 2

3

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u s u a l l y too t h i c k f o r e l e c t r o n beam t r a n s m i s s i o n . Such a n example f o r 10-60 nm p l a t i n u m c r y s t a l l i t e s on a l u m i n a i s shown i n F i g u r e 2. V a r y i n g the b l a c k l e v e l o f the p h o t o m u l t i p l i e r c a n enhance imaging of the c r y s t a l l i t e s by making them l o o k l i g h t e r o r d a r k e r w i t h r e s p e c t t o the s u p p o r t . The a x i a l dark f i e l d i m a g i n g i s a c h i e v e d b y obtaining a m i c r o - d i f f r a c t i o n pattern o f a c r y s t a l l i t e , t i l t i n g the e l e c t r o n beam so as t o move one o f the d i f f r a c t i o n s p o t s t o t h e o p t i c a x i s , and t h e n imaging w i t h t h i s d i f f r a c t e d beam. I f t h e c r y s t a l l i t e s p r e s e n t have s u f f i c i e n t l y d i f f e r e n t l a t t i c e s p a c i n g s from t h e s u p p o r t , the c r y s t a l l i t e s w i l l be h i g h l i g h t e d on the d a r k f i e l d image. For example, i n F i g u r e 3, c r y s t a l l i t e s o n c a r b o n a r e shown u s i n g t h i s t e c h n i q u e . Cowley and G a r c i a (19) demonstrated t h i s t e c h n i q u e on the more c o m p l i c a t e d Rh/Ce0 system. A t t e m p t s t o use t h i s approach f o r p l a t i n u m on 7 - A l 0 w i l l n o t work, because t h e major d i f f r a c t i o n s p o t s f o r P t and 7 - A l 0 cannot be d i f f e r e n t i a t e d . The ease o f a n a l y s i s depends on how w e l l some o f t h e s e t e c h n i ques a r e i n t e g r a t e d w i t h one a n o t h e r t o e s t a b l i s h how the c a t a l y s t i s designed. For monometallic supported c a t a l y s t s p r e v i o u s l y d e s c r i b e d , more o f t h e s e t e c h n i q u e s are a p p l i c a b l e because o f t h e r e d u c e d i n t e r f e r e n c e from the s u p p o r t . The o p t i o n s a r e r e d u c e d as the s u p p o r t c o m p o s i t i o n becomes more complex. There a r e some commercial c a t a l y s t s w h i c h c o n t a i n m u l t i m e t a l l i c s i t e s on complex m e t a l o x i d e s u p p o r t s . D i r e c t m i c r o a n a l y s i s o f a r e a s b y EDS i s sometimes the o n l y approach t h a t can be t a k e n , e s p e c i a l l y i f t h e metals are h i g h l y d i s p e r s e d . I f c r y s t a l l i t e s are p r e s e n t , x-ray mapping (20) might be a f e a s i b l e approach f o r q u i c k l y e s t a b l i s h i n g the e x t e n t o f b i m e t a l l i c f o r m a t i o n . Sometimes b i m e t a l l i c c r y s t a l l i t e s have s p e c i f i c m o r p h o l o g i c a l c h a r a c t e r i s t i c s w h i c h e n a b l e t h e m i c r o s c o p i s t t o i d e n t i f y the b i m e t a l l i c c r y s t a l l i t e s . I n F i g u r e s 4a and b, a c a t a l y s t c o m p r i s e d o f R u / I r on 7 - A l 0 and Ru on 7 - A l 0 , r e s p e c t i v e l y a r e shown. The R u / I r c r y s t a l l i t e s a r e e l o n g a t e d whereas the Ru c r y s t a l l i t e s a r e s y m m e t r i c a l . EDS shows t h e s e R u / I r c r y s t a l l i t e s t o have a c o m p o s i t i o n o f 80 wt.% Ru and 20 wt.% I r . Thus b y v i s u a l o b s e r v a t i o n a r a p i d e v a l u a t i o n o f the e x t e n t o f b i m e t a l l i c f o r m a t i o n can be made f o r t h i s c a t a l y s t . However, most of the time the d a t a a r e n o t e a s i l y o b t a i n e d and i n d i v i d u a l a n a l y s e s of a r e a s o f i n t e r e s t must be made.

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2

2

3

2

3

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3

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3

There a r e a l s o c a t a l y s t f o r m u l a t i o n s w h i c h have h i g h l y d i s p e r s e d m e t a l s w h i c h are d e l i b e r a t e l y h e t e r o g e n e o u s l y d i s t r i b u t e d on a s u p p o r t . I f the m i c r o s c o p i s t i s aware o f the s i t u a t i o n , he c a n t a k e p r e c a u t i o n s i n the sample p r e p a r a t i o n . T h i s t y p e o f sample i s the w o r s t p o s s i b l e case t o a n a l y z e because n o t o n l y does the a n a l y s t have a complex m i x t u r e o f components t o s o r t o u t , b u t the a n a l y s i s s t a t i s t i c s a r e v e r y poor. C o n s e q u e n t l y , a d d i t i o n a l time i s u s u a l l y r e q u i r e d t o s u r v e y the c a t a l y s t p a r t i c l e s i n o r d e r t o e s t a b l i s h a consensus o f how i t was c o n s t r u c t e d . S p e c i a l i z e d specimen p r e p a r a t i o n such as u l t r a m i c r o t o m i n g and s c r a p i n g the e x t e r i o r o f a sphere or e x t r u d a t e may a l l e v i a t e some o f the i n t e r p r e t a t i o n problems. A d d i t i o n a l a i d may be s o l i c i t e d from a s c a n n i n g e l e c t r o n m i c r o s c o p e w h e r e i n an e l e m e n t a l d i s t r i b u t i o n o f a p o l i s h e d c r o s s s e c t i o n o f t h e c a t a l y s t sphere o r e x t r u d a t e can be made.

American Chemical Society Library 1155 15th St., N.W.

In Characterization and Catalyst Development; Bradley, S., et al.; Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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F i g u r e 2. P l a t i n u m c r y s t a l l i t e s on t h i c k p a r t i c l e s o f 7-alumina u s i n g annular dark f i e l d .

F i g u r e 3. P l a t i n u m c r y s t a l l i t e s on a c a r b o n f i l m u s i n g dark f i e l d .

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

axial

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Figure 4 a . alumina.

Characterization of Catalysts by STEM

Ruthenium - i r i d i u m e l o n g a t e d c r y s t a l l i t e s on 7-

Figure 4b.

Ruthenium on

7-alumina.

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C a t a l y s t Poisons Any c a t a l y s t exposed t o r e a c t i o n c o n d i t i o n s may have been s u b j e c t e d t o p o i s o n s . I n some c a s e s , a n a l y s i s b y STEM o f i n d i v i d u a l c r y s t a l l i t e s w i l l d e t e c t t h e p o i s o n ( 1 2 ) . T h i s i s p r i m a r i l y because t h e a c t i v e c a t a l y s t s i t e may s e l e c t i v e l y scavenge t h e p o i s o n thus concentrating i t to a level of detectability. I n many c a s e s , however, the p o i s o n s a r e a t l o w c o n c e n t r a t i o n s and may remain u n d e t e c t e d by STEM.

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C a t a l y s t S t a b i l i t y Under Beam

Exposure

I t may be t a k e n f o r g r a n t e d t h a t a n a l y s i s f o r a l l elements on a c a t a l y s t s u p p o r t work e q u a l l y w e l l . We have performed experiments on many d i f f e r e n t c a t a l y s t s and have found t h a t elements such as C I , K, Na, and S a r e v e r y s e n s i t i v e t o t h e e l e c t r o n beam. C I and S appear t o v o l a t i l i z e i n t h e vacuum w h i l e K and Na move away from t h e i n c i d e n t beam. T h i s i s e s p e c i a l l y t r u e when t h e beam i s s p o t t e d d i r e c t l y onto t h e p a r t i c l e v e r s u s u s i n g t h e l e s s damaging r a s t e r mode. Examples o f how e l e m e n t a l a n a l y s e s o f a BaS0 , z e o l i t e , and NaCl p a r t i c l e s v a r y as a f u n c t i o n o f time a r e shown i n F i g u r e 5. O f t e n c a t a l y s t m a t e r i a l s a r e viewed as b e i n g e x t r e m e l y s t a b l e because t h e y a r e exposed t o s e v e r e o x i d a t i o n and r e d u c t i o n c o n d i t i o n s a t v e r y h i g h temperature and a r e c o n s e q u e n t l y s t a b i l i z e d b y such t r e a t m e n t s . I n a n a l y z i n g a c a t a l y s t w i t h an e l e c t r o n beam many i n t e r e s t i n g c h e m i c a l r e a c t i o n s may o c c u r w h i c h r e n d e r t h e s u p p o r t m e t a s t a b l e . Many examples have been s i g h t e d i n t h e l i t e r a t u r e (21) w h i c h show s u p p o r t s such as z e o l i t e s , a l u m i n a , and s i l i c a degrade under an e l e c t r o n beam. I t i s o f i n t e r e s t t o note t h a t a l t h o u g h beam damage may o c c u r i n a l l c a s e s , t h e e x t e n t o f d e g r a d a t i o n appears t o be h i g h l y v a r i a b l e . We have o b s e r v e d i n s t a n c e s i n w h i c h z e o l i t e s and/or a l u m i n a have shown v a r i a b l e r e a c t i v i t y i n an e l e c t r o n beam. Sometimes p a r t i c l e s o b t a i n e d from t h e same c a t a l y s t may e x h i b i t d i f f e r e n t beam s e n s i t i v i t y . I t i s possible that the catal y s t was n o t c o m p l e t e l y t r a n s f o r m e d t o a s t a b l e c o n d i t i o n d u r i n g preparation. Atomic rearrangement and/or f u r t h e r r e d u c t i o n o f c r y s t a l l i t e s w i t h mixed o x i d a t i o n s t a t e s (22) as a f u n c t i o n o f e l e c t r o n beam exposure have been observed. We have n o t e d t h a t m e t a l c r y s t a l l i t e s l e s s t h a n 4 nm i n s i z e a r e p a r t i c u l a r l y s u s c e p t i b l e t o beam i r r a d i a t i o n damage. Another phenomenon commonly observed i n t h e a n a l y s i s o f c a t a l y s t s i s t h e d e p o s i t i o n o f carbon i n t h e a r e a o f beam c o n c e n t r a t i o n ( 2 3 ) . I n many i n s t a n c e s t h i s c a n be a s s o c i a t e d w i t h r e s i d u a l o r g a n i c m a t e r i a l l e f t on t h e c a r b o n c o a t e d g r i d s . We have a l s o found t h a t t h e c a t a l y s t p a r t i c l e s themselves may have o r g a n i c d e b r i s on them w h i c h s u b s e q u e n t l y r e a c t i n t h e e l e c t r o n beam c r e a t i n g carbonaceous m a t e r i a l . U n f o r t u n a t e l y , t h i s may d i s r u p t o b s e r v a t i o n o f s m a l l c r y s t a l l i t e s and/or t h e q u a n t i t a t i v e a n a l y s i s o f t h e particle. F o r example, STEM e x a m i n a t i o n o f a F i s c h e r - T r o p s c h c a t a l y s t w h i c h h a d wax d e p o s i t e d on t h e a l u m i n a p a r t i c l e s c o n t a i n i n g Ru c r y s t a l l i t e s was made. Due t o t h e r e a c t i v i t y o f t h e wax i n t h e e l e c t r o n beam t h e p a r t i c l e s t u r n e d p r o g r e s s i v e l y d a r k e r as a f u n c t i o n o f exposure t i m e , u n t i l f i n a l l y t h e Ru c r y s t a l l i t e s were n o t A

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

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ZEOLITE K/SI

A Cl/Na • S/Ba

A



• A

• •X100

*A 0

0

i

40

80

120

160

200

TIME (SEC) F i g u r e 5. E f f e c t o f the e l e c t r o n beam on the K / S i r a t i o f o r a z e o l i t e , C l / N a r a t i o f o r N a C l , and S/Ba r a t i o f o r BaS0 . 4

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observable. C a t a l y s t s w h i c h c o n t a i n c a r b o n from o t h e r r e a c t i o n s i n w h i c h t h e carbon i s h i g h l y u n s a t u r a t e d show l i t t l e beam i n t e r a c t i o n . Sometimes t h e o r i g i n o f t h e carbon d e p o s i t i o n and/or o t h e r c o n t a m i n a t i o n e f f e c t s may n o t be r e a d i l y i d e n t i f i a b l e . A d d i t i o n a l p r e p a r a t i o n may be r e q u i r e d t o o b t a i n a specimen w h i c h c a n be analyzed. F o r example, p l a c i n g t h e c a t a l y s t p a r t i c l e s on d i f f e r e n t c a r b o n c o a t e d g r i d s , u s i n g a d i f f e r e n t method f o r d e p o s i t i n g t h e c a t a l y s t p a r t i c l e s , o r u s i n g a d i f f e r e n t g r i d f i l m such as s i l i c a may o f f e r a l t e r n a t i v e approaches f o r o b t a i n i n g u s e f u l e l e c t r o n micrographs.

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A n a l y s i s o f L i g h t Elements A means o f p r o b i n g carbon and t h e l i g h t elements may be more e f f e c t i v e l y a c h i e v e d w i t h e l e c t r o n energy l o s s s p e c t r o s c o p y , EELS. With the i m p l e m e n t a t i o n o f the p a r a l l e l EELS d e t e c t o r ( 2 4 ) , t h e s e n s i t i v i t y o f t h i s t e c h n i q u e has i n c r e a s e d d r a m a t i c a l l y , maybe 1000 f o l d . S m a l l c l u s t e r s o f atoms may be d e t e c t a b l e ,