Temperature-Programmed Desorption Study of CO on Pt-Reforming

Chapter 23

Temperature-Programmed Desorption Study of CO on Pt-Reforming Catalysts R. L . Mieville and M. G . Reichmann

1

Downloaded by UNIV LAVAL on November 5, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch023

Amoco O i l Company, Amoco Research Center, Naperville, I L 60566

The characterization technique of CO TemperatureProgrammed Desorption has been studied with Pt reforming catalysts. C r i t i c a l factors i n the experimental procedure and the catalyst pretreatment conditions were examined. The CO desorption spectrum consists mainly of two peaks which are probably combinations of other peaks and the result of various binding energy states of CO to Pt. These i n turn could be due either to the interaction between Pt and the alumina support or the results of high and low coordination s i t e s on the Pt c r y s t a l l i t e s . No s i g n i f i c a n t relationship between the character of the CO desorption p r o f i l e and the a c t i v i t y of commercial catalysts was observed.

The r o u t i n e c h a r a c t e r i z a t i o n o f P t r e f o r m i n g c a t a l y s t s has depended m a i n l y on t h e t e c h n i q u e s o f X-ray d i f f r a c t i o n and CO and c h e m i s o r p t i o n f o r the s t a t e o f the P t , and N^ BET s u r f a c e a r e a measurements f o r the s u p p o r t . A l t h o u g h t h e s e t e c h n i q u e s can s i g n a l g r o s s changes i n the performance o f the c a t a l y s t , i n s t a n c e s occur when decreases i n a c t i v i t y and s e l e c t i v i t y are not r e f l e c t e d i n measurements made by the above methods. E v i d e n t l y a more d e t a i l e d a n a l y s i s o f P t i s r e q u i r e d . I n t h e o r y t h i s c o u l d be p r o v i d e d by a CO TPD method, but as y e t no r e l a t i o n s h i p between c a t a l y s t p e r f o r m ance and the CO d e s o r p t i o n spectrum has been e s t a b l i s h e d . I n f a c t , the l i t e r a t u r e i s s u r p r i s i n g l y s p a r s e on CO TPD s t u d i e s o f s u p p o r t e d P t . P a r t l y t h i s may be due t o the r e a l i z a t i o n t h a t the d e s o r p t i o n p r o c e s s from porous c a t a l y s t s o c c u r s p r i m a r i l y under e q u i l i b r i u m c o n d i t i o n s ( 1 ) . A l s o the v a r i e t y o f CO p r o f i l e s r e p o r t e d suggest t h a t e x p e r i m e n t a l procedures may be d i f f i c u l t t o reproduce ( 2 - 7 ) . I n o r d e r t o overcome some p o t e n t i a l e x p e r i m e n t a l problems, s p e c i a l c a r e was t a k e n t o t o e l i m i n a t e c o n t a m i n a t i o n . The r e s u l t s o f t h i s and o t h e r i n v e s t i g a t i o n s a r e p r e s e n t e d i n t h i s paper. 1

Current address: Amoco Chemicals, Naperville, IL 60566 0097-6156/89/0411-0243$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.

244

CHARACTERIZATION AND CATALYST DEVELOPMENT

Experimental The TPD a p p a r a t u s c o n s i s t e d o f a s t a i n l e s s s t e e l f l o w system connected t o a t h e r m a l c o n d u c t i v i t y c e l l . C a t a l y s t samples o f 0.1 g were p l a c e d i n one arm o f an L-shaped, 6 mm Vycor t u b e . A d u a l a d s o r p t i o n bed c o n t a i n i n g a l u m i n a and Oxy-Trap ( A l l t e c h ) was p l a c e d i n t h e o t h e r arm t o p r e v e n t c o n t a m i n a t i o n by w a t e r and 0^» r e s p e c t i v e l y . Frequent r e g e n e r a t i o n i n H and He was r e q u i r e d . T h i s i n - s i t u a d s o r p t i o n bed was found n e c e s s a r y d e s p i t e p u r i f i c a t i o n t r a p s on a l l gas l i n e s coming i n t o t h e f l o w system. P u l s e s o f 0.25 c c o f a 10% m i x t u r e o f CO i n He were i n j e c t e d i n t o t h e He c a r r i e r gas and passed o v e r t h e p r e t r e a t e d c a t a l y s t a t room t e m p e r a t u r e . A l l runs were programmed h e a t e d a t a r a t e o f 20 K min . The P t c a t a l y s t s , e i t h e r commercial o r l a b o r a t o r y produced, were p r e p a r e d by t h e i m p r e g n a t i o n o f c h l o r o p l a t i n i c a c i d on Cyanamid's Aero 1000 a l u m i n a , except f o r two c a t a l y s t s which were p r e p a r e d by p l a t i n u m diamino d i n i t r i t e impregnation.


2

R e s u l t s and D i s c u s s i o n Some p r e l i m i n a r y runs were made on t h e AlÔ^ s u p p o r t i n t h e absence of P t . H e a t i n g t h i s alumina w i t h o u t CO p u l s i n g showed a s m a l l peak a p p e a r i n g near 100°C. T h i s was i d e n t i f i e d as adsorbed as an i m p u r i t y from t h e c a r r i e r gas. CO c o m p l e t e l y d i s p l a c e s t h e on p u l s i n g a t room temperature and, as p r e v i o u s l y r e p o r t e d , p r o b a b l y r e s u l t s from the presence o f Lewis a c i d - b a s e s i t e s gn t h e alumina (8). The amount o f t h i s peak was a p p r o x i m a t e l y 10 moles p e r g o f AlÔ^ w h i c h r e p r e s e n t s l e s s than 5% o f t h e CO d e s o r b i n g from a w e l l d i s p e r s e d P t c a t a l y s t o f 0.4 w t % l o a d i n g . Runs w i t h o u t t h e i n - s i t u HÔ and 0« t r a p had shown t h a t CO^ desorbed w i t h t h e CO. However, w i t h t h e I n - s i t u t r a p m i n i m a l CO^ was observed. T h i s was c o n f i r m e d by a p a i r o f runs shown i n F i g u r e 1 w i t h and w i t h o u t A ^ O ^ ( C 0 t r a p ) p l a c e d a t t h e e x i t o f t h e d e s o r p t i o n tube. One o f t h e most i m p o r t a n t v a r i a b l e s i n t h e TPD o f CO from a s u p p o r t e d P t c a t a l y s t i s t h e sample p r e t r e a t m e n t . C a l c i n a t i o n a t 500°C f o r one hour f o l l o w e d by r e d u c t i o n i s t h e c o n v e n t i o n a l method to o b t a i n t h e maximum exposed P t and t h i s f o l l o w s c l o s e l y t o r e f i n e r y p r a c t i c e f o r s t a r t - u p and r e g e n e r a t i o n o f commercial c a t a l y s t s . The f i n a l s t e p i n our case was a 600°C He sweep f o r 30 m i n u t e s t o ensure a f u l l y d e h y d r a t e d c a t a l y s t up t o t h i s temperature so t h a t no w a t e r e v o l v e d d u r i n g t h e subsequent TPD. We had p r e v i o u s l y observed t h a t a h i g h temperature He sweep c o u l d reduce t h e P t c a t a l y s t w i t h o u t a p r i o r H r e d u c t i o n presumably by t h e d e c o m p o s i t i o n o f t h e P t o x i d e . F i g u r e 2 shows t h e e f f e c t i v e n e s s o f t h i s procedure as a f u n c t i o n of t h e p r e o x i d a t i o n t e m p e r a t u r e . A t 500°C p r e o x i d a t i o n no d i f f e r e n c e i s observed between e i t h e r a He o r H^ r e d u c t i o n . A f t e r a 600°C c a l c i n a t i o n t h e He r e d u c t i o n was l e s s e f f e c t i v e , e s p e c i a l l y f o r h i g h e r temperature p e a k s , and t h i s d i f f e r e n c e was more pronounced a f t e r a 700°C c a l c i n a t i o n t e m p e r a t u r e . I t i s i m p o r t a n t t o note t h a t more t h a n o x i d a t i o n o c c u r s a t h i g h t e m p e r a t u r e s , s i n t e r i n g o f t h e P t and d e h y d r a t i o n o f a l u m i n a a l s o o c c u r . However, i t i s n o t s u r p r i s i n g t h a t h a r d t o reduce P t o c c u r s a t t h e h i g h e s t temperature o f p r e o x i d a t i o n . A l l t h e s e f a c t o r s c o n t r i b u t e t o t h e change i n t h e 2

2


23.

Desorption ofCO on Pt-Reforming Catalysts

MIEVILLE & REICHMANN


o

500

°C

F i g u r e 1. TPD S p e c t r a o f CO from 0.6 wt% P t / A l 0 3 a) Without C 0 t r a p b) With C 0 t r a p . 2

2

2

1

1

1

r"

1

1—

700°

/

Q. O

1

\

600°C^

o CJ 500°C^^

/ i

100

200

300

400

500

F i g u r e 2. E f f e c t o f C a l c i n a t i o n Temperature W i t h D i f f e r e n t Reductions CO TPD from 0.4 wt% P t / A l 0 1) 1 Hr H a t 500°C than He a t 600°C (30 min.) 2) 1 Hr He a t 600°C . 2

3

2


245


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amount o f CO desorbed and t h e c h a r a c t e r o f t h e d e s o r p t i o n p r o f i l e w i t h the c a l c i n a t i o n t e m p e r a t u r e . F i g u r e 3 shows t h r e e TPD s p e c t r a r a n g i n g from 0.1 t o 0.3 g o f c a t a l y s t sample. B e s i d e s t h e amount o f CO d e s o r b e d , no major change i n t h e s p e c t r a was o b s e r v e d . However, a s m a l l d i s c e r n i b l e s h i f t t o i n c r e a s i n g temperatures i s seen on i n c r e a s i n g sample amount w h i c h suggests a s m a l l i n t e r p a r t i c l e r e a d s o r p t i o n e f f e c t . Pore d i f f u s i o n e f f e c t s on t h e CO TPD s p e c t r a were i n v e s t i g a t e d by v a r y i n g the p a r t i c l e s i z e o f t h e c a t a l y s t . F i g u r e 4 shows t h r e e s p e c t r a r a n g i n g i n p a r t i c l e s i z e from 60-80 mesh t o 14-20 mesh, and e s s e n t i a l l y no d i f f e r e n c e i n t h e s p e c t r a was o b s e r v e d . The e f f e c t o f t i t r a t i n g t h e P t w i t h s m a l l p u l s e s o f CO below t h e t o t a l s a t u r a t i o n coverage was examined. F i g u r e 5 shows t h e r e s u l t s of t h e d e s o r p t i o n s o f p r e v i o u s l y adsorbed i n t e g r a l amounts o f 0.036cc of CO on a 0.3 g sample o f 0.6 w t % P t / A ^ O ^ . CO desorbs s t e p w i s e from the h i g h t o t h e l o w temperature range w h i c h p r o b a b l e r e s u l t s n o t from s e l e c t i v e a d s o r p t i o n b u t from movement o f CO t o h i g h e r b i n d i n g s i t e s on d e s o r p t i o n i n d i c a t i n g t h a t r e a d s o r p t i o n i s o c c u r r i n g f r e e l y . F i g u r e 6 compares t h r e e TPD s p e c t r a , r a n g i n g from 0.3 t o 2.0 w t % P t , and shows t h a t t h e r e i s an o v e r a l l i n c r e a s e i n t h e s i z e o f t h e h i g h , r e l a t i v e t o t h e l o w temperature peak a s t h e P t l o a d i n g i n c r e a s e s . A t t h e same t i m e , t h e r e i s an upward s h i f t i n temperature f o r t h i s low temperature peak. I t might be expected t h a t t h e l a r g e r c r y s t a l l i t e s formed a t h i g h e r P t l o a d i n g s would have more t e r r a c e s i t e s , w h i l e s m a l l e r c r y s t a l l i t e s have more s t e p s and k i n k s w h i c h a r e a s s o c i a t e d w i t h the h i g h e r b i n d i n g s t a t e s ( 9 - 1 2 ) . However, t h i s i s c o n t r a r y t o t h e observed peak s h i f t . One p o s s i b l e e x p l a n a t i o n w h i c h i s c o n s i s t e n t w i t h t h e observed peak s h i f t i n v o l v e s t h e p l a t i n u m -support i n t e r a c t i o n . I n t h i s e x p l a n a t i o n , there i s a l i m i t t o the AlÔ^ s i t e s which c a n i n t e r a c t s t r o n g l y w i t h P t (low temperature peak; and on f u r t h e r a d d i t i o n , P t moves t o l e s s i n t e r a c t i n g s i t e s . However, t h i s t h e o r y i s o n l y p a r t i a l l y c o n s i s t e n t w i t h t h e s p e c t r a i n F i g u r e 7 where two samples o f t h e same P t l o a d i n g (0.2 wt%) b u t o f d i f f e r e n t d i s p e r s i o n ( H / P t o f 0.40 and 1.07) a r e shown. A l t h o u g h t h e sample w i t h the h i g h e r d i s p e r s i o n d i d show a l a r g e r l o w temperature peak, i t a l s o appears t o c o n t a i n h i g h e r b i n d i n g s t a t e s w h i c h a r e c o m p l e t e l y absent i n t h e l o w d i s p e r s i o n c a s e . On t h e b a s i s o f the p l a t i n u m s u p p o r t i n t e r a c t i o n t h e o r y , we would e x p e c t a s m a l l e r h i g h temperature peak f o r t h e l e s s h i g h l y d i s p e r s e d sample ( 1 3 ) . I t s h o u l d be noted t h a t d e c r e a s e i n d i s p e r s i o n cannot always be e x p l a i n e d by c r y s t a l l i t e f o r m a t i o n . E v i d e n t a l l y f u r t h e r study and development i s needed. The main g o a l o f t h i s work was t o d e f i n e t h a t p a r t o f the TPD spectrum w h i c h c o n t r i b u t e s most t o t h e a c t i v i t y o f t h e c a t a l y s t . Four used r e f o r m i n g c a t a l y s t s w i t h r e l a t i v e a c t i v i t i e s r a n g i n g from 0.49 t o 1.00 (on the b a s i s o f performance i n a p i l o t p l a n t ) were examined. The r e s u l t s o f t h e s e r u n s a r e shown i n F i g u r e 8 and no o b v i o u s c o r r e l a t i o n seems t o e x i s t between t h e s p e c t r a and t h e corresponding a c t i v i t i e s . An attempt was made t o l e a r n whether i n c r e a s e d i n t e r a c t i o n o f p l a t i n u m w i t h exposed A l i o n s c o u l d be seen on more h i g h l y d r i e d a l u m i n a . The p o p u l a t i o n o f such i o n s i n c r e a s e s w i t h temperature o f d e h y d r a t i o n o f t h e a l u m i n a (8) . A P t / A l ^ c a t a l y s t was p r e d r i e d a t v a r i o u s t e m p e r a t u r e s i n He. The r e s u l t s i n F i g u r e 9 show a 2



Desorption oj CO on Pt-Reforming Catalysts


23.

F i g u r e 5. Coverage V a r i a t i o n CO added i n 0.036cc p u l s e s . Number o f p u l s e s as i n d i c a t e d (0.3 g, 0.5 wt% P t / A l 0 3 ) . 2


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F i g u r e 6. E f f e c t o f P t L o a d i n g Pt wt% a) 2.00 b) 0.70 c)

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F i g u r e 7. E f f e c t o f P t D i s p e r s i o n 0.2 wt% P t / A l 2 0 3 ( d i a m i n o - d i n i t r i t e Dispersion a) 107% b) 40%.

t

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impregnation)

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F i g u r e 8. CO TPD S p e c t r a o f P t / A l 2 0 3 C a t a l y s t With D i f f e r e n t Activities Relative A c t i v i t i e s a) 1.0 b) 0.83 c ) 0.49 d) 0.56.


23.

Desorption of CO on Pt-Reforming Catalysts


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p r o g r e s s i v e i n c r e a s e i n t h e r e l a t i v e s i z e o f t h e low temperature peak as t h e p r e d r y i n g t e m p e r a t u r e i s i n c r e a s e d b u t t h e d e c r e a s e i n h i g h temperature peak, a s might be e x p e c t e d , i s n o t c o n s i s t e n t l y seen here. S i m i l a r e x p e r i m e n t s were performed i n an IR s p e c t r o m e t e r . In these e x p e r i m e n t s t h e same P t c a t a l y s t was p r e d r i e d by e v a c u a t i o n a t 500° and 700°C a f t e r i n i t i a l c a l c i n a t i o n a t 500°C, b u t i n t h i s case before reduction i n a t 500°C. CO was added a t room temperature and t h e n p r o g r e s s i v e l y removed a t h i g h e r t e m p e r a t u r e s . T h i s r e s u l t s , as seen i n F i g u r e 10, i n a s e r i e s o f d e c r e a s i n g s i z e bands w i t h a f r e q u e n c y s h i f t t o lower wave numbers. Such s h i f t s i n t h e IR f r e quency w i t h CO coverage c o u l d p o s s i b l y o c c u r by changes i n

t

I

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F i g u r e 9. E f f e c t o f P r e d r y i n g Temperature 0.74 wt% P t / A l 0 a) 500°C b) 600°C c)700°C. 2

3

1950 F i g u r e 10. E f f e c t s o f P r e d r y i n g and P a r t i a l D e s o r p t i o n IR S p e c t r a o f Absorbed CO 1. C a l c i n e d a t 500°C 2. C a l c i n e d a t 700°C D e s o r p t i o n Temperature A) A f t e r CO added B) 60°C D)250°C E) 350°C F) 400°C.

on t h e

C)150°C



250


d i p o l e - d i p o l e c o u p l i n g . However, i t has been p r e v i o u s l y observed t h a t no changes i n f r e q u e n c y o c c u r d u r i n g the t i t r a t i o n o f a 0.6% P t / A l ^ O ^ sample w i t h s m a l l known amounts o f CO up t o a coverage o f 0. 7 CO m o l e c u l e s / P t atom ( p r e s s u r e ~ 7 x l 0 ~ T o r r ) . .At h i g h e r p r e s s u r e s , the band s h i f t e d from 2060 t o 2070 cm" ( 1 4 ) . This e q u i v a l e n t s h i f t i s seen i n F i g u r e 10 between s p e c t r a A and B. A l l f u r t h e r s h i f t s a r e then l i k e l y caused by the removal o f weaker h e l d CO by t h e r m a l d e s o r p t i o n w h i c h r e s u l t s i n the subsequent i n c r e a s e i n the s t r e n g t h the CO t o P t bond w h i l e d e c r e a s i n g the bond s t r e n g t h o f the C t o 0. The major l o s s o f P t s u r f a c e a r e a seen a f t e r the 700°C p r e t r e a t m e n t was e x p e c t e d on the b a s i s o f p r e v i o u s work showing t h a t h e a t i n g p r e o x i d i z e d P t / a l u m i n a i n vacuum gave e s s e n t i a l l y the same l o s s i n a r e a as h e a t i n g i n oxygen ( 1 4 ) . F i g u r e 10 a l s o shows t h a t the s p e c t r a o b t a i n e d from the more s t r o n g l y d r i e d c a t a l y s t (the 700°C t r e a t m e n t ) s h i f t s i n the band maxima t o f r e q u e n c i e s h i g h e r t h a n seen on the l e s s s t r o n g l y d r i e d c a t a l y s t (the 500°C t r e a t m e n t ) . T h i s peak s h i f t i n d i c a t e s t h a t t h e Pt i n t e r a c t s more w i t h the more h i g h l y d r i e d a l u m i n a s u r f a c e and t h e r e f o r e c o n f i r m s the TPD r e s u l t s . I n c o n c l u s i o n , the TPD s p e c t r a can be c h a r a c t e r i z e d by two broad peaks, one a t 100° and the o t h e r a t 450°. However, these c o u l d be made up o f a c o m b i n a t i o n o f s e v e r a l d i f f e r e n t peaks. Assignments o f these i n d i v i d u a l peaks t o s p e c i f i c P t s t a t e s are h a r d l y p o s s i b l e , b u t the broad TPD p r o f i l e a r i s e s out o f two p o s s i b l e c a u s e s : e i t h e r t o s t r u c t u r a l changes i n the P t c r y s t a l l i t e forms o r t o v a r i a t i o n o f P t - a l u m i n a i n t e r a c t i o n s due t o the h e t e r o g e n e i t y o f the a l u m i n a surface. We would l i k e t o acknowledge the two 0.2 wt% P t c a t a l y s t samples g i v e n t o us by P r o f e s s o r R. L. B u r w e l l and the I.R. s p e c t r a o b t a i n e d from Dr. J . B. P e r i . We s h o u l d a l s o l i k e t o thank them b o t h f o r many helpful discussions. LITERATURE CITED 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Herz, R. K., K i e l a , J . B. and Marin, S. P., J . Catal. 1982, 73, 66. Darling, T. A., and Moss, R. L., J . Catal. 1967, 7, 378. Hoang-Van, C., Ghorbel, A., Pommier, B., and Teichner, S. J . B u l l . Soc. Chim. 1976, 355. Bain, F. T., Jackson, S. D., Thomson, S. J . , Webb, G., and Willocks, E., J . Chem. Soc. (Faraday). 1976, 72, 2516. Foger, K., Anderson, J . R., Appl. Surface S c i . 1979, 2, 335. Zhang, X. Q., Xue, H. and Xiexian, G., Cuihua Xueba. 1981, 2, 100. Saveleva, G. A., Galeev, T. K., Popova, N. M., Vozdvizhenskii, V. F., and Mishchenko, V. M., Kinet. Katal. 1981, 22, 1253. M i e v i l l e , R. L., J . Catal. 105, 1987, 536. Morgan, A. E., and Somorjai, G. A., Surface S c i . 1968,12,405. McCabe, R. W., and Schmidt,L. D., Surface S c i . 1977, 66, 101. Altman, E. I., and Gorte, R. J., Surface Sci. 1986, 172, 71. Gdowski, G. E., and Madix, R. J . , Surface S c i . 1982, 115, 524. Herz, R. K., and McCready, D. F., J . Catal. 1983, 81, 358. P e r i , J . B., J . Phys Chem. 1978, 52, 144.

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January 10, 1989


Temperature-Programmed Desorption Study of CO on Pt-Reforming

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