A Pilot Plant Reactor-Surface Analysis System for Catalyst Studies

2 A Pilot Plant Reactor-Surface Analysis System for Catalyst Studies T. H. Fleisch Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch002

Amoco Research Center, Standard Oil Company (Indiana), Naperville, IL 60566

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled in series to a minireactor which is connected, via a high vacuum sample transfer system, to a surface analysis instrument equipped with XPS, AES, SAM, and SIMS. When interesting kinetic data are observed, the reaction i s stopped and the test sample is transferred from the minireactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system w i l l be illustrated with a study of surface chemical changes of a CuO/ZnO/Al2O3 catalyst during activation and methanol synthesis. Metallic Cu was identified by XPS as the only Cu surface site during methanol synthesis.

The development o f modern s u r f a c e c h a r a c t e r i z a t i o n t e c h n i q u e s has p r o v i d e d means t o s t u d y t h e r e l a t i o n s h i p between t h e c h e m i c a l a c t i v i t y and t h e p h y s i c a l o r s t r u c t u r a l p r o p e r t i e s o f a c a t a l y s t s u r f a c e . E x p e r i m e n t a l work t o u n d e r s t a n d t h i s r e a c t i v i t y / s t r u c t u r e r e l a t i o n s h i p has been o f two t y p e s : f u n d a m e n t a l s t u d i e s on model c a t a l y s t systems (1,2) and postmortem a n a l y s e s o f c a t a l y s t s w h i c h have been removed from r e a c t o r s ( 3 , 4 ) . Experimental a p p a r a t u s f o r t h e s e s t u d i e s have i n v o l v e d s m a l l volume r e a c t o r s mounted w i t h i n (1) o r appended t o (5) vacuum chambers c o n t a i n i n g analysis instrumentation. A l t e r n a t e l y , c a t a l y s t samples have been removed f r o m remote r e a c t o r s v i a t r a n s f e r a b l e sample mounts (6) o r an i n e r t gas g l o v e box ( 3 , 4 ) . Very l i t t l e r e s e a r c h has attempted t o r e l a t e r e a c t i o n k i n e t i c s t o c a t a l y s t s u r f a c e p r o p e r t i e s a s a f u n c t i o n o f time under a c t u a l r e a c t o r o p e r a t i n g c o n d i t i o n s . The g o a l o f t h e 0097-6156/85/0288-0015$06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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p r e s e n t d e s i g n was t o c o u p l e a p i l o t p l a n t r e a c t o r t o a s u r f a c e a n a l y s i s i n s t r u m e n t i n a way w h i c h would a l l o w t h e r e a c t i o n k i n e t i c s o f a l a r g e c a t a l y s t volume t o be r e l a t e d t o c a t a l y s t s u r f a c e c h e m i c a l p r o p e r t i e s a s determined by X-ray P h o t o e l e c t r o n Spectroscopy (XPS o r ESCA) and b o t h S t a t i c and Scanning Auger M i c r o s c o p y (AES/SAM).

Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch002

D e s c r i p t i o n o f Apparatus G e n e r a l D e s c r i p t i o n . The major subsystems o f t h e p i l o t p l a n t / s u r f a c e a n a l y s i s system a r e a c o m p u t e r - c o n t r o l l e d p i l o t p l a n t r e a c t o r and a sample i n t r o d u c t i o n t r a n s f e r system m o d i f i e d to i n c l u d e a m i n i r e a c t o r w h i c h a t t a c h e s t o a P e r k i n - E l m e r PHI Model 550 ESCA/SAM i n s t r u m e n t ( F i g u r e 1 ) . F o r a g i v e n e x p e r i m e n t , a p p r o x i m a t e l y 50 grams o f t h e c a t a l y s t o f i n t e r e s t a r e l o a d e d i n t o a plug flow reactor i n the p i l o t p l a n t . A pressed d i s c of the same c a t a l y s t ( a p p r o x i m a t e l y 0.1 gram) mounted i n a s p e c i a l sample h o l d e r i s i n s e r t e d i n t o t h e ESCA/SAM system f o r s u r f a c e c h a r a c t e r i z a t i o n . The sample h o l d e r w i t h c a t a l y s t d i s c i s then t r a n s f e r r e d under h i g h vacuum c o n d i t i o n s from t h e i n t r o d u c t i o n system i n t o t h e m i n i r e a c t o r . A m e t a l / m e t a l s e a l s e p a r a t e s t h e h i g h p r e s s u r e r e a c t o r s i d e from t h e h i g h vacuum sample t r a n s f e r s i d e . A f t e r t h e p l u g f l o w r e a c t o r , m i n i r e a c t o r , and c o n n e c t i n g t u b i n g a r e a t o p e r a t i n g t e m p e r a t u r e , r e a c t a n t gas f l o w i s s t a r t e d . R e a c t i o n p r o d u c t s a r e m o n i t o r e d by a gas chromatograph ( F i g u r e 1 ) . When an i n t e r e s t i n g k i n e t i c b e h a v i o r i s o b s e r v e d , the r e a c t a n t f l o w i s stopped and t h e c a t a l y s t d i s c i s t r a n s f e r r e d back i n t o t h e ESCA/SAM system f o r s u r f a c e c h a r a c t e r i z a t i o n . The c y c l e o f exposure t o r e a c t i o n c o n d i t i o n s f o l l o w e d by s u r f a c e c h a r a c t e r i z a t i o n i s c o n t i n u e d u n t i l t h e experiment i s completed. P i l o t P l a n t . The p i l o t p l a n t c o n s i s t s o f two gas i n l e t systems, a p l u g f l o w r e a c t o r , and a gas chromatograph. The p l u g f l o w r e a c t o r i s f a b r i c a t e d from an 86 cm s t a i n l e s s s t e e l tube (3.2 cm O.D. and 2.3 cm I.D.). An A n a l o g D e v i c e s MACSYM 2 computer c o n t r o l s p r e s s u r e , f l o w r a t e , and p i l o t p l a n t r e a c t o r and m i n i r e a c t o r t e m p e r a t u r e s . T h i s computer c o n t r o l i n c o n j u n c t i o n w i t h a number o f a l a r m s t h a t t r i g g e r r e a c t i o n shutdown p e r m i t s u n a t t e n d e d o p e r a t i o n . Gas chromatographic a n a l y s i s o f r e a c t i o n p r o d u c t s i s automated w i t h a gas sampling v a l v e and an HP 5880A l e v e l f o u r t e r m i n a l . Sample I n t r o d u c t i o n and T r a n s f e r System. The sample i n t r o d u c t i o n and sample t r a n s f e r system i s a lengthened v e r s i o n o f t h e PHI Model 15-720B i n t r o d u c t i o n system w h i c h c o n s i s t s o f a polymer b e l l o w s - c o v e r e d h e a t i n g and c o o l i n g p r o b e , a t r a n s f e r a b l e sample h o l d e r , an e i g h t - p o r t d u a l - a x i s c r o s s , and the m i n i r e a c t o r i n t e r f a c e p o r t and t r a n s f e r probe ( F i g u r e 2 ) . There i s a l s o a t r a n s f e r v e s s e l p o r t w i t h t h e n e c e s s a r y t r a n s f e r probe f o r i n t r o d u c t i o n o f a i r s e n s i t i v e samples. They a r e n o t p a r t o f t h e r e a c t o r / s u r f a c e a n a l y s i s system. The d u a l c r o s s and a t t a c h e d hardware a r e supported by t h e probe d r i v e mechanism w h i c h f l o a t s on a b l o c k d r i v e n v e r t i c a l l y and t r a n s v e r s e l y by two m i c r o m e t e r s . These m i c r o m e t e r s p l u s t h e probe d r i v e mechanism a l l o w X-Y-Z

Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

FLEISCH

Pilot Plant Reactor-Surface Analysis System

ESCA/SAM system Sample transfer Gas

Pilot plant reactor I

^—ι


Pressure

Minireactor

J Flow

I r

Temperature Gas chromatograph

Computer

F i g u r e 1. B l o c k diagram showing t h e subsystems o f t h e combined r e a c t o r and s u r f a c e a n a l y s i s system.

Minireactor and interface chamber

jjjj

Hot/cold probe

Gas lines to pilot plant reactor and GC I^

Transfer vessel port

Analysis chamber

Transfer probe (for minireactor)

Transfer probe (for transfer vessel)

F i g u r e 2. Sample i n t r o d u c t i o n and sample t r a n s f e r system. (Reproduced w i t h p e r m i s s i o n from R e f . 7. C o p y r i g h t 1984, Academic P r e s s . )



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sample p o s i t i o n i n g i n t h e a n a l y s i s chamber. Pumping f o r the i n t r o d u c t i o n system i s p r o v i d e d by a B a l z e r s Model TSUI10 t u r b o m o l e c u l a r pump backed by a m e c h a n i c a l pump. T h i s l a t t e r pump a l s o p r o v i d e s d i f f e r e n t i a l pumping f o r f o u r s e t s o f T e f l o n sliding seals. F i g u r e 3 shows an e x p l o d e d v i e w o f the t r a n s f e r a b l e sample h o l d e r on w h i c h a 1.2 cm d i a m e t e r p r e s s e d c a t a l y s t d i s c i s mounted. W i t h the sample h o l d e r engaged i n the h o t / c o l d probe and t h e d u a l c r o s s e v a c u a t e d , t h e h o l d e r t e m p e r a t u r e can be v a r i e d from -170°C t o +650°C. When t r a n s f e r r i n g the sample h o l d e r from the h o t / c o l d p r o b e , a s p l i t p i n on t h e t r a n s f e r probe i s p r e s s e d i n t o t h e h o l e i n t h e s i d e o f t h e sample h o l d e r . The h o t / c o l d probe i s t h e n r e t r a c t e d t o d i s e n g a g e the h o l d e r and p r o v i d e c l e a r a n c e t o move the h o l d e r i n t o the i n t e r f a c e chamber. By t r a n s f e r r i n g the sample from the h o t / c o l d probe i n t o a r e a c t o r r a t h e r t h a n r e a c t i n g the sample w h i l e a t t a c h e d t o the p r o b e , the sample i n t r o d u c t i o n system can be used v e r y f l e x i b l y . Day-to-day a n a l y t i c a l work can be performed w h i l e l o n g t e r m r e a c t i o n experiments are i n progress. R e a c t i o n I n t e r f a c e and M i n i r e a c t o r . The r e a c t i o n i n t e r f a c e i s a f i v e p o r t s t a i n l e s s s t e e l vacuum chamber w i t h t h e m i n i r e a c t o r and sample d r i v e mechanism mounted on the upper and l o w e r 11.4 cm d i a m e t e r f l a n g e s , r e s p e c t i v e l y ( F i g u r e s 4a,4b). At r i g h t a n g l e s t o t h e v e r t i c a l a x i s a r e 7 cm d i a m e t e r p o r t s f o r sample a c c e s s , a v i e w p o r t , and a pumping p o r t p l u s p r e s s u r e r e l i e f v a l v e . A g a t e v a l v e s e p a r a t e s the r e a c t i o n i n t e r f a c e chamber from the d u a l c r o s s . I n the i n t e r f a c e chamber the sample h o l d e r i s t r a n s f e r r e d from the t r a n s f e r probe i n t o a s t a i n l e s s s t e e l cup as shown i n F i g u r e 4a· V e r t i c a l m o t i o n o f the sample h o l d e r i n t o the m i n i r e a c t o r i s p r o v i d e d by a b e l l o w s - s e a l e d p l a t e a t t a c h e d t o a s c r e w - d r i v e n frame. A t o r q u e wrench i s used t o a p p l y a s e a l i n g f o r c e o f 300 pounds t o a S i e r r a c i n - H a r r i s o n type 24105 g o l d - c o a t e d m i n i - s e a l w h i c h s e a l s t h e h i g h p r e s s u r e m i n i r e a c t o r from the h i g h vacuum i n t e r f a c e chamber ( F i g u r e 4 b ) . H e l i u m p r e s s u r e t e s t i n g u s i n g a UTI 100C q u a d r u p o l e mass s p e c t r o m e t e r showed no d e t e c t a b l e h e l i u m l e a k a g e a t 100 p s i p r e s s u r e and a r e a c t o r t e m p e r a t u r e o f 600°C. The m i n i r e a c t o r i t s e l f i s a s t a i n l e s s s t e e l c y l i n d e r w i t h a 5.6 cm o u t s i d e d i a m e t e r and 2.24 cm i n s i d e d i a m e t e r w h i c h i s t h e r m a l l y i s o l a t e d from i t s 11.4 cm mounting f l a n g e by a t h i n - w a l l e d c o n i c a l s e c t i o n ( F i g u r e 4)· A gas i n l e t and o u t l e t p o r t i s s i t u a t e d d i r e c t l y above the sample. S i x 150 w a t t c a r t r i d g e h e a t e r s i n the m i n i r e a c t o r w a l l p r o v i d e adequate power t o h e a t the r e a c t o r f r o m room t e m p e r a t u r e t o 600°C i n 20 m i n u t e s . The m i n i r e a c t o r t e m p e r a t u r e i s c o n t r o l l e d by the MACSYM 2 computer u s i n g e i t h e r o f two t h e r m o c o u p l e s , one embedded i n t h e r e a c t o r w a l l o r one i n p r e s s u r e c o n t a c t w i t h a l e d g e machined i n t o the sample h o l d e r ( F i g u r e s 3,4). In steady s t a t e o p e r a t i o n , t e m p e r a t u r e r e a d i n g s f r o m the two thermocouples a g r e e w i t h i n 2°C. Operating temperature s t a b i l i t y i s w i t h i n ±1°C and the t e m p e r a t u r e d i f f e r e n c e between p i l o t p l a n t r e a c t o r and m i n i r e a c t o r i s a l w a y s l e s s than 2 C. e



FLEISCH


transfer pin

F i g u r e 3. E x p l o d e d v i e w o f t h e sample h o l d e r and c a t a l y s t d i (Reproduced w i t h p e r m i s s i o n from Réf. 7. Copyright 198U, Academic P r e s s . )




FLEISCH


Study of M e t h a n o l S y n t h e s i s C a t a l y s t A s t u d y o f a methanol s y n t h e s i s c a t a l y s t i s used t o demonstrate the u s e f u l n e s s o f the new p i l o t p l a n t / m i n i r e a c t o r / s u r f a c e a n a l y s i s system. T h i s work i s d e s c r i b e d i n more d e t a i l e l s e w h e r e (7)· I n d u s t r i a l methanol s y n t h e s i s c a t a l y s t s a r e based on CuO/ZnO/Al 0 o r CuO/ZnO/Cr 0 c o m p o s i t i o n s . R. G. Herman e t a l . (8) s t u d i e d these c a t a l y s t systems i n g r e a t d e t a i l and suggested a Cu+1 s o l u t i o n i n ZnO as a c t i v e phase where Cu+1 n o n - d i s s o c i a t i v e l y chemisorbs and a c t i v a t e s CO and ZnO a c t i v a t e s H . I n the range of 15 t o 85% CuO i n the c a t a l y s t , up t o 16% Cu+1 became d i s s o l v e d i n the ZnO (9) and Cu+1 has been w i d e l y a c c e p t e d as a c t i v e s i t e ( 1 0 ) . Recently, however, Raney Cu-Zn c a t a l y s t s have been shown t o be v e r y a c t i v e methanol s y n t h e s i s c a t a l y s t s ( 1 1 ) . The a c t i v e component f o r t h e s e Raney c a t a l y s t s was found t o be m e t a l l i c Cu w i t h an a c t i v i t y maximum a t 97 wt% Cu ( 1 2 ) . The c a t a l y s t i n t h i s study was a commercial c a t a l y s t (C18HC, U n i t e d C a t a l y s t , Inc.) w i t h 42% CuO, 47% ZnO and 10% A 1 0 . The f e e d gas c o n s i s t e d of 73% H , 25% CO and 2% C 0 . The c h e m i c a l s t a t e o f Cu was s t u d i e d by XPS ( A l Κα e x c i t a t i o n , hv«1486.6eV) u s i n g the Cu 2p and Cu (Ι^Μι^Μι^) Auger l i n e . With those two l i n e s Cu+2, Cu+1 and Cu can e a s i l y be d i s t i n g u i s h e d employing a s o - c a l l e d c h e m i c a l s t a t e p l o t (CSP) w i t h the Cu 2 p , b i n d i n g energy on the a b s c i s s a ( d e c r e a s i n g from l e f t t o r i g h t ) and the Cu (Ι^Μι^Μι^) k i n e t i c energy on the o r d i n a t e as shown i n F i g u r e 5 (13,14). The p o s i t i o n of Cu o f the u n t r e a t e d c a t a l y s t i n the CSP ( p o i n t 1) c l e a r l y i d e n t i f i e s i t as CuO. After s y n t h e s i s gas c o n v e r s i o n a t 250°C o n l y m e t a l l i c Cu i s seen on the c a t a l y s t s u r f a c e ( p o i n t 3 ) . A t 100°C, Cu+2 becomes reduced t o Cu+1 but no methanol f o r m a t i o n i s o b s e r v e d . Z i n c o x i d e does not become reduced d u r i n g methanol s y n t h e s i s . The s m a l l changes i n the CSP ( F i g u r e 5) a r e due t o d r y i n g . Thus, the w o r k i n g c a t a l y s t s u r f a c e i s suggested t o c o n s i s t o f m e t a l l i c Cu, ZnO, and A 1 0 , S p e c i a l a t t e n t i o n was p a i d t o the d e t e c t i o n o f r e s i d u a l Cu+1 q u a n t i t i e s accompanying the m e t a l l i c Cu. The r e l a t i v e amounts of Cu+1 and Cu were determined by c u r v e - f i t t i n g the Cu (LMM) s p e c t r a u s i n g the P h y s i c a l E l e c t r o n i c s V e r s i o n 6 c u r v e - f i t t i n g program. The c a t a l y s t showed r e d u c t i o n of Cu+2 i n t o a m i x t u r e of Cu+1 and Cu a f t e r r e d u c t i o n i n H a t 250°C f o r one hour ( F i g u r e 6) as e v i d e n c e d by the two r e s o l v e d peaks i n the Cu (LMM) spectrum a t 568.0 and 570.3 eV w h i c h a r e c h a r a c t e r i s t i c o f Cu and Cu+1, r e s p e c t i v e l y , and by the d i s a p p e a r a n c e of the Cu+2 2p s a t e l l i t e s t r u c t u r e . I t c o u l d be shown t h a t l e s s t h a n 2%, i f any, o f the t o t a l Cu c o u l d be p r e s e n t i n the +1 o x i d a t i o n s t a t e d u r i n g methanol f o r m a t i o n . However, when the c a t a l y s t was b r i e f l y exposed t o a i r (1 m i n u t e ) , a few p e r c e n t o f Cu+1 r e a d i l y formed ( 7 ) . Thus, any k i n d of o x i d a t i o n environment has t o be a v o i d e d between methanol s y n t h e s i s and c a t a l y s t a n a l y s i s . O t h e r w i s e , a p p r e c i a b l e amounts o f Cu+1 w i l l be d e t e c t e d . 2

3

2

3


2

2

3

2

2

3

2

3

2


2


CO

c

0)

0)

ο c

>

as received

/

': /

;

A Β C

Cu

2

3

r.u + 2 J

/ 11·/

/ Cu

•/Cu + K

2

3

/\

1

935 934 933 932 931

/

Cu

CU2Û

CuO

2p /2 binding energy, eV

938 937 936

913

914

915

916

917

α> c α> 919 ο c 918

J?

2

921

922

> E> 920

>

923

F i g u r e 5. C h e m i c a l s t a t e changes o f Cu and Zn i n a commercial C u 0 / Z n 0 / A l 0 c a t a l y s t i n f e e d gas (H /C0/C0 = 73/25/2, 2 atm).

2p3/2 binding energy, eV

1026 1025 1024 10231022 1021 1020 1019

2 100°C, 1 hr 3 250°C, 10 hr

1


η m η m

C/3

δ

Ν

2

m

5

Η η as >

C/3

π


2.

FLEISCH


23

F i g u r e 6. Changes i n Cu 2p and C u i l ^ M ^ M ^ ) e l e c t r o n s p e c t r a upon i n s i t u reduction i n H . (Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1984, Academic P r e s s . ) 2


CATALYST C H A R A C T E R I Z A T I O N SCIENCE

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Discussion Two major q u e s t i o n s r e g a r d i n g t h e a p p l i c a b i l i t y o f t h i s system remain t o be d i s c u s s e d . F i r s t , how r e p r e s e n t a t i v e i s t h e c a t a l y s t sample i n t h e m i n i r e a c t o r o f t h e c a t a l y s t i n t h e p i l o t p l a n t ? And second, how does t h e i n t e r r u p t i o n o f t h e r e a c t i o n i n f l u e n c e k i n e t i c behavior? The f i r s t q u e s t i o n can be answered e a s i l y . A t l o w c o n v e r s i o n and i s o t h e r m a l c o n d i t i o n s no s i g n i f i c a n t d i f f e r e n c e s i n t h e c a t a l y s t c o m p o s i t i o n as f u n c t i o n o f c a t a l y s t bed p o s i t i o n a r e u s u a l l y o b s e r v e d . Thus, t h e s u r f a c e a n a l y s i s d a t a from t h e c a t a l y s t i n t h e m i n i r e a c t o r s h o u l d be r e p r e s e n t a t i v e n o t o n l y o f the bottom s e c t i o n b u t o f a l l t h e c a t a l y s t i n t h e p l u g f l o w r e a c t o r . I n t h e case o f t h e methanol f o r m a t i o n s t u d i e s r e p o r t e d i n t h i s p a p e r , c o n v e r s i o n was low (

A Pilot Plant Reactor-Surface Analysis System for Catalyst Studies

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