Characterization of Catalysts by Scanning Transmission Electron

Chapter 31

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

Downloaded by COLUMBIA UNIV on February 26, 2013 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch031

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.


31.

TARGOS & BRADLEY

Characterization of Catalysts by STEM

343

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


344

CHARACTERIZATION AND CATALYST DEVELOPMENT


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


31.

TARGOS & BRADLEY


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


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.


345


346


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

2


31.

TARGOS & BRADLEY

347


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.


2

2

3

2

3

2

3

2

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.


348


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 .


axial


31.

TARGOS & BRADLEY

Figure 4 a . alumina.


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.


349

350


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.


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



31. TARGOS & BRADLEY

351


•

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


352


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.


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 ,

Characterization of Catalysts by Scanning Transmission Electron

Recommend Documents