Transient Low-Pressure Studies of Catalytic Carbon Monoxide Oxidation

2 Transient Low-Pressure Studies of Catalytic Carbon Monoxide Oxidation J. R. C R E I G H T O N and J . M . W H I T E

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: May 1, 1982 | doi: 10.1021/bk-1982-0178.ch002

University of Texas, Department of Chemistry, Austin, TX 78712

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation over transition metal substrates. Particular emphasis is given to the use of in-situ electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these w i l l be assessed with regard to kinetic insight and quantification. An attempt w i l l be made to identify questions that are ripe for investigation. Although not limited to i t , the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction (1) will be useful to readers who are interested in a more comprehensive view. The carbon monoxide oxidation reaction has been widely studied over the course of many years. The pioneering work of Langmuir (2) set the tone for much of the subsequent work and we continue to refine many of his ideas. Catalyzed carbon monoxide oxidation is perhaps better understood than any other heterogeneous reaction and, because i t shows l i t t l e variation with crystal plane (3), there is a good correlation of low area single crystal results with high area supported catalyst results. This is particularly true under conditions where CO accumulates and inhibits the rate of CO production (4,5). In the realm of applications, CO oxidation is a significant consideration in systems designed to limit undesirable emissions from combustion processes. At the outset, i t is worth noting how transient data fit into the overall picture of this reaction and what we expect to learn by doing such experiments. With the exception of oscillations (6), transients are induced by rapidly changing one or more of the independent variables of the system. For the case 2

0097-6156/82/0178-0033$06.50/0 © 1982 American Chemical Society Bell and Hegedus; Catalysis Under Transient Conditions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


34

C A T A L Y S I S

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o f CO o x i d a t i o n , c h a n g i n g t h e s u b s t r a t e t e m p e r a t u r e o r t h e p a r t i a l p r e s s u r e s o f CO o r O2 i n d u c e s s u c h transients. The i n f o r m a t i o n g e n e r a t e d comes f r o m measurement o f t h e time e v o l u t i o n o f s u r f a c e c o v e r a g e s and gas p h a s e c o m p o s i t i o n . The f o r m e r a r e , o f c o u r s e , c e n t r a l t o any k i n e t i c d e s c r i p t i o n i n v o l v i n g a d s o r b e d s p e c i e s ; measurements under w o r k i n g c o n d i t i o n s g i v e the g r e a t e s t i n s i g h t because they a r e d i r e c t l y r e l a t e d t o t h e CO2 p r o d u c t i o n r a t e s . The l a t t e r p r o v i d e p r o d u c t f o r m a t i o n and a d s o r p t i o n / d e s o r p t i o n r a t e s d u r i n g t r a n s i e n t s . I n a d d i t i o n , when u s e d i n c o n j u n c t i o n w i t h a m o d e l , c o v e r a g e s c a n be c a l c u l a t e d from p r e s s u r e versus time data. Taken t o g e t h e r t h e s e measurements i n p r i n c i p l e p r o v i d e t h e r a t e s and coverages needed f o r a f u l l k i n e t i c d e s c r i p t i o n . I n p r a c t i c e , t h e r e a r e s e r i o u s e x p e r i m e n t a l l i m i t s on the measurements. I n p a r t i c u l a r t h e dynamic range o v e r w h i c h s u r f a c e c o n c e n t r a t i o n s c a n be r e l i a b l y m e a s u r e d i s 0 . 0 1 t o 1 m o n o l a y e r ; t h e l o w e r end o f t h i s range i s p a r t i c u l a r l y troublesome. Moreover, because adsorbed s p e c i e s o f t e n occupy s i t e s of v a r i a b l e a c t i v i t y on a s u r f a c e , m e a s u r e m e n t o f t h e t o t a l c o n c e n t r a t i o n o f an a d s o r b e d s p e c i e s may be insufficient. I m p r o v e d r e s o l u t i o n and m e a s u r e m e n t of s m a l l c o n c e n t r a t i o n s of r e l a t i v e l y l a b i l e s u r f a c e species l i e a t the heart of experimental progress i n the s u r f a c e chemistry of heterogeneous c a t a l y s i s . S u c c i n c t l y p u t , the o l d c a t a l y t i c chemists' proverb, " i f you can see a c e r t a i n s u r f a c e s p e c i e s , i t i s n o t k i n e t i c a l l y i m p o r t a n t " , must be t a k e n s e r i o u s l y . H o w e v e r , i n t h e c a s e o f CO o x i d a t i o n , t h e k i n e t i c a l l y a c t i v e a d s o r b e d oxygen and c a r b o n monoxide s p e c i e s c a n i n f a c t be m e a s u r e d u n d e r many, b u t n o t a l l , conditions. Transients

I n d u c e d by T e m p e r a t u r e

Changes

One o f t h e s t a n d a r d s u r f a c e s c i e n c e m e t h o d s f o r a s s e s s i n g t h e c o n c e n t r a t i o n and s t a b i l i t y o f a chemisorbed species i s thermal desorption spectroscopy (TDS). An e a r l y p a p e r by R e d h e a d ( 7 ) d e v e l o p e d t h e c o n c e p t u a l framework f o r c e r t a i n cases. Many p a p e r s s i n c e then have expanded the a p p l i c a b i l i t y of t h i s method. Recent work of Madix ( 8 ) , Weinberg (9) Schmidt (10) i s p a r t i c u l a r l y noteworthy. Most o f t h i s w o r k f o c u s e s on t h e d e s o r p t i o n o f a s i n g l e m o l e c u l a r s p e c i e s a n d n o t on r e a c t i o n s i n d e s o r b i n g systems. However, q u a l i t a t i v e f e a t u r e s of t h e t e m p e r a t u r e d e p e n d e n c e o f r e a c t i o n s c a n be a s s e s s e d u s i n g t h i s method. F i g u r e s 1 and 2 t a k e n from t h e a

Bell and Hegedus; Catalysis Under Transient Conditions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

n

d

2.

CREiGHTON

A N D

1


-

Catalytic Carbon

W H I T E

1

1

1

Monoxide

1

1

χ

< «ΓΙ0.0

g

I

X

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ce

4.0

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I '

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35

Oxidation

'

\/

1073^

/

973

//

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A

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v

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Θ73

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M

^

\

773

ω

673

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573 f?I

/ ^

- ' V/

—

m

373 00

/

ι 40

ι 80

ι 120

TIME (s)

1 160

1 200

1 240 Journal of Catalysis

Figure 1. Variation of C0 pressure with time during the flash of a Pd substrate preexposed to oxygen at high temperatures. The total pressure was about 10' Pa and Ο /CO ~ 1.5. Key: , temperature variation; and , C0 pressure (11). 2

A

2

2


36


C A T A L Y S I S

10

20

30 T I M E

U N D E R

40

T R A N S I E N T

C O N D I T I O N S

50

(s) Journal of Catalysis

Figure 2. Non-steady state variation of 0 pressure with time during flashing of a stable poly crystalline Pd substrate. Initial Ο J CO = 1.5. The origin of the ordinate is defined as the 0 pressure at 300 K. Key: O, observed 0 ; V, calculated on the basis of C0 production; , difference between calculated and observed; and , temperature (11). 2

2

2

2


2.

CREiGHTON

A N D

W H I T E

Catalytic

Carbon

Monoxide

37

Oxidation

w o r k o f J o h n C l o s e (11) i n o u r l a b o r a t o r y adequately i l l u s t r a t e the p o i n t . F i g u r e 1 shows t h e v a r i a t i o n of t h e CO2 pressure d u r i n g t h e f l a s h and s u b s e q u e n t c o o l down o f a p o l y c r y s t a l l i n e Pd f o i l . The s a m p l e , m o u n t e d w i t h i n a c o n t i n u o u s l y pumped UHV s y s t e m , was e x p o s e d t o 10~^ t o r r o f a m i x t u r e c h a r a c t e r i z e d by C^/CO = 1.5. After s t a b i l i z a t i o n a t 300K. t h e s a m p l e was h e a t e d r e s i s t i v e l y a t a r a t e o f 12KS"" w h i l e t h e CO2 p a r t i a l p r e s s u r e was m o n i t o r e d mass s p e c t r o m e t r i c a l l y . The CO2 signal, w h i c h i s a m e a s u r e o f t h e CO2 p r o d u c t i o n r a t e , i s s t r o n g l y p e a k e d on b o t h t h e h e a t i n g and c o o l i n g c y c l e s . S i n c e t h e m a x i m a b o t h o c c u r n e a r 573K, we e x p e c t t o f i n d maxima i n t h e s t e a d y - s t a t e CO2 p r o d u c t i o n r a t e s n e a r t h e same t e m p e r a t u r e . T h i s d a t a was u s e d as a q u a l i t a t i v e guide only. The o n l y s i g n i f i c a n t d i f f e r e n c e s i n the r a t e s are the a m p l i t u d e , w h i c h i s h i g h e r d u r i n g c o o l - d o w n , and t h e p e a k t e m p e r a t u r e , w h i c h i s s l i g h t l y l o w e r d u r i n g c o o l down. These are a s c r i b e d t o two d i f f e r e n c e s : (1) t h e a b s o l u t e r a t e o f t e m p e r a t u r e c h a n g e and (2) t h e a m o u n t s o f a d s o r b e d CO and o x y g e n w h i c h a r e p r e s e n t d u r i n g t h e t r a n s i e n t s . F i g u r e 2 shows d a t a f r o m t h e v a r i a t i o n o f P t a k e n u n d e r c o n d i t i o n s s i m i l a r t o t h o s e o f F i g u r e 1. The o r i g i n o f t h e o r d i n a t e i s d e f i n e d as t h e s t e a d y s t a t e O2 p r e s s u r e m e a s u r e d a t 300K. As t h e s a m p l e i s h e a t e d t h e O2 p r e s s u r e d r o p s ( o p e n c i r c l e s ) g o e s t h r o u g h a l o c a l minimum and a maximum b e t w e e n 15 and 40s (500 and 8 0 0 K ) . The t r i a n g l e s show a p r e d i c t e d O2 p r e s s u r e c u r v e b a s e d on CO2 p r e s s u r e . Although one m i g h t a r g u e t h a t i n a c c u r a t e r e l a t i v e s e n s i t i v i t i e s c a n a c c o u n t f o r some o f t h e d i f f e r e n c e i n t h e s e two c u r v e s , i t i s c l e a r t h a t t h e l o c a l maximum i n t h e e x p e r i m e n t a l O2 c o n s u m p t i o n i s n o t r e f l e c t e d i n t h e c a l c u l a t e d c u r v e b a s e d on CO2 p r o d u c t i o n . As i n d i c a t e d by t h e d i f f e r e n c e s p e c t r u m ( s o l i d l i n e i n t h e u p p e r p o r t i o n o f F i g u r e 2) t h e r e i s n o t a s i n g l e s c a l i n g f a c t o r w h i c h w i l l b r i n g any s i g n i f i c a n t p o r t i o n o f t h e s e two c u r v e s i n t o c o i n c i d e n c e . From t h e s e r e s u l t s we c o n c l u d e t h a t 0 p e n e t r a t i o n i n t o t h e s u b s u r f a c e r e g i o n i s an i m p o r t a n t process, p a r t i c u l a r l y at h i g h t e m p e r a t u r e s . The s i g n i f i c a n c e o f t h i s p r o c e s s has b e e n c o n f i r m e d by w o r k i n o u r own (12) and o t h e r l a b o r a t o r i e s ( 1 3 ) . A c e n t r a l p o i n t t o be made i n c o n n e c t i o n w i t h F i g u r e s 1 and 2 i s t h a t a g r e a t d e a l o f q u a l i t a t i v e i n s i g h t c a n be g a i n e d f r o m r e l a t i v e l y s i m p l e e x p e r i ments. The r e s u l t s g u i d e t h e s e l e c t i o n o f t e m p e r a t u r e s f o r s t e a d y - s t a t e and t r a n s i e n t e x p e r i m e n t s w h i c h c a n be a n a l y z e d more q u a n t i t a t i v e l y .


1

Q


38

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C O N D I T I O N S

A n o t h e r k i n d of t h e r m a l l y i n d u c e d t r a n s i e n t i s i l l u s t r a t e d i n F i g u r e 3. T h i s d a t a , taken from work o f C h a r l e s C a m p b e l l and S h e i - K u n g S h i Ç14), i n v o l v e s t h e t r a n s i e n t p r o d u c t i o n o f CO2 w h i c h o c c u r s when a c o a d s o r b e d m i x t u r e o f CO m o l e c u l e s and 0 atoms on a Rh s u r f a c e i s h e a t e d . The v a r i o u s s p e c t r a a r e f o r d i f f e r e n t t i t r a t i o n t i m e s i n d i c a t e d on t h e f i g u r e ( 0 - 2 0 m i n ) ; a p o l y c r y s t a l l i n e Rh s u r f a c e p r e s a t u r a t e d w i t h o x y g e n was t i t r a t e d a t 360K f o r χ m i n . by 9.6 χ 10~ Pa (133 Pa = 1 T o r r ) o f CO a n d , a f t e r e v a c u a t i o n , was h e a t e d t o 750K o r h i g h e r . With t i t r a t i o n time ( i . e . p r i o r t o f l a s h i n g ) CO2 i s p r o d u c e d . This lowers t h e amount o f c h e m i s o r b e d o x y g e n and a l l o w s i n c r e a s i n g a m o u n t s o f CO t o a d s o r b . Thus, the areas beneath the curves i n F i g u r e 3 r e f l e c t q u a l i t a t i v e l y the mathe m a t i c a l p r o d u c t o f t h e c o v e r a g e o f CO t i m e s t h e coverage of oxygen p r e s e n t at the s t a r t of the h e a t i n g cycle. The f o r m e r i s s m a l l a t s h o r t t i t r a t i o n t i m e s w h i l e the l a t t e r i s s m a l l at long t i t r a t i o n times. I t i s i m p o r t a n t t o n o t e t h e s p e c t r a shown i n F i g u r e 3 w e r e g e n e r a t e d by h e a t i n g t h e s u r f a c e i n v a c u u m . The a r e a b e n e a t h t h e CO2 p r e s s u r e t r a n s i e n t v a r i e s s t r o n g l y w i t h t i t r a t i o n t i m e i n d i c a t i n g t h a t CO2 i s r e a d i l y p r o d u c e d by r e a c t i o n o f c o a d s o r b e d CO and 0 . In f a c t , c o m p a n i o n CO s p e c t r a show t h a t a l l t h e c h e m i s o r b e d CO r e a c t s t o make CO2 so l o n g as t h e r e i s e x c e s s oxygen. By a n a l y z i n g C 0 , CO and O2 t r a n s i e n t s l i k e t h o s e o f F i g u r e 3, t h e t e m p e r a t u r e d e p e n d e n c e o f t h e r a t e o f CO2 p r o d u c t i o n c a n be e v a l u a t e d f o r a r e l a t i v e l y w i d e r a n g e o f CO and 0 c o v e r a g e s ( 6 and 0 ). T h i s a n a l y s i s shows t h a t t h e r a t e i s n o t d e s c r i b a b l e by a s i m p l e LH e x p r e s s i o n o f t h e f o r m


7

2

c Q

Q

R

co

=

2

A

e x

P

E

{"

R T }

( L / H

Θ

Ο co 6

d>

E i t h e r coverage dependent terms i n the r a t e c o e f f i c i e n t s or f r a c t i o n a l exponents i n the coverage terms m u s t be i n c o r p o r a t e d . Table I summarizes our a n a l y s i s of s e v e r a l cases u s i n g the e q u a t i o n R

=

co

2

A

LH

exp

* α

{ CO

θ

co

(E „ + α θ + LH 00 T

)/RT}

e

m

ο

θ

n

co

A l t h o u g h we c a n n o t d e m o n s t r a t e u n a m b i g u o u s l y w h i c h of t h e s e , i f any, i s p h y s i c a l l y most s a t i s f a c t o r y , we p r e f e r m o d e l I V b e c a u s e i t p r e s e r v e s t h e e x p o n e n t s o f u n i t y on t h e c o v e r a g e s , as an e l e m e n t a r y equation m u s t , b e c a u s e i t g i v e s a good f i t to t h e d e t a i l e d r e a c t i o n r a t e v e r s u s t i t r a t i o n t i m e a t 360K ( c a l c u l a -


CREiGHTON

A N D

W H I T E

Catalytic

Carbon

Monoxide

Oxidation


TIMEAsec.) 15

T/(IO · K ) Applications of Surface Science Figure 3. C0 pressure vs. time during heating a polycrystalline Rh sample cov ered with CO and O. These species were deposited at 360 Κ in varying amounts by predosing oxygen (15.9 L) and then titrating with CO(g) at 7 X 10 ton. The heating was then done in vacuo. Titration times are marked in minutes. The small high temperature peak is due to CO interactions with the walls (14). 2

9


C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S


Table I Kinetic R

C0

=

A

e X

2

[

P "

( E

LH

Case #

m = order in θ

I II III IV V

1 1.6 1 1 1

+

α

analysis,

θ

+ a

0 0

η = order in Θ Γ Π

1 1 0.66 1 1

9

C0 C0)

/ R T

ot ot 0 CO k c a l mole'^ k c a l mole"

^ molec. 41.1 48.5 41.0 47.6 42.1

Scaled

± ± ± ± ±

cm~^s"^ 1.02 0.56 0.40 0.40 0.29

according

LH k c a l mole""- 1

8.97 13.31 8.64 14.30 8.60

to Ε „ γ

± ± ± ± ±

0 0 0 0 2.1

0 0 0 -4.4 0

E

n

e

^0 C0

0.86 0.48 0.34 0.34 0.24

for direct

Standard deviation α 0.299 0.114 0.121 0.073 0.087 a

a

a

a

comparison with I .


2.

C R E I G H T O N

A N D

Catalytic

W H I T E

Carbon

Monoxide

Oxidation

41

ted from a p l o t of Q v e r s u s t i t r a t i o n t i m e ) , and because i t g i v e s the b e s t s t a t i s t i c a l f i t to the d a t a . P e r h a p s some c o m b i n a t i o n o f m o d e l s I V and V w o u l d be e v e n b e t t e r b u t o u r d a t a c e r t a i n l y do n o t r e q u i r e i t . I n any c a s e , m o d e l V a l o n e d o e s n o t g i v e a l a r g e e n o u g h CO^ p r o d u c t i o n r a t e when t h e o x y g e n c o v e r a g e i s h i g h , so i t c a n n o t be u s e d a l o n e . The i n t e r p r e t a t i o n o f t h e s e coverage-dependent e f f e c t s i n v o l v e s s u c h i d e a s as i s l a n d f o r m a t i o n , m i x e d d o m a i n s o f CO and 0, t h e m o b i l i t y o f CO and 0 and t h e a d s o r p t i o n o f CO on o x y g e n - c o v e r e d r e g i o n s ( 1 , 1 8 , 2 6 , 27). A deeper u n d e r s t a n d i n g of the r o l e s of these p r o c e s s e s comes f r o m i s o t h e r m a l e x p e r i m e n t s i n v o l v i n g pressure transients. A l l of the r e s u l t s d i s c u s s e d to t h i s p o i n t i n f e r s u r f a c e c o v e r a g e s f r o m gas p h a s e c o m p o s i t i o n m e a s u r e ments. Although these i n f e r e n c e s are soundly based i n t h e c a s e o f CO o x i d a t i o n , we w o u l d be much more c o m f o r t a b l e w i t h d i r e c t measurement of the s u r f a c e coverage.


q

Transients

Induced

by

Pressure

Changes

The m o s t s o p h i s t i c a t e d and i n c i s i v e t r a n s i e n t experiments are those d e r i v e d from modulated m o l e c u l a r beam r e a c t i v e s c a t t e r i n g e x p e r i m e n t s . R e s u l t s f r o m s u c h e x p e r i m e n t s w i l l be d i s c u s s e d below a f t e r p r e s e n t a t i o n of data from s i m p l e r e x p e r i m e n t s w h i c h n o t o n l y a r e c o n s i s t e n t w i t h t h e beam r e s u l t s but a l s o extend the range of t e m p e r a t u r e c o v e r a g e c o n d i t i o n s a n d , i n some c a s e s , d i r e c t l y assess the coverage. F i g u r e 4 shows t h e r e s u l t s o f a v e r y s i m p l e e x p e r i m e n t i n v o l v i n g t r a n s i e n t CO2 p r o d u c t i o n on a p o l y c r y s t a l l i n e P t f o i l a t 540K ( 1 5 ) . The t r a n s i e n t was s e t up by e s t a b l i s h i n g a s t e a d y - s t a t e r e a c t i o n w i t h 0 / C O - 1 0 and t h e n r a p i d l y c l o s i n g (-3 s e c ) t h e 0 l e a k v a l v e i n t o the c o n t i n u o u s l y pumped r e a c t i o n c h a m b e r . In the absence of r e a c t i o n , t h e CO l e a k r a t e g a v e a p a r t i a l p r e s s u r e o f 2 x 1 0 " t o r r . The e n s u i n g t r a n s i e n t CO2 comes f r o m t h e r e m o v a l o f c h e m i s o r b e d o x y g e n atoms p r e s e n t a t i t s o r i g i n . From F i g u r e 4 i t i s i m m e d i a t e l y c l e a r t h a t t h e CO2/CO r a t i o , and t h e r e f o r e t h e C 0 p r o d u c t i o n rate per u n i t CO p r e s s u r e , r e m a i n s n e a r l y c o n s t a n t o v e r t h e f i r s t 2 min of the t i t r a t i o n w h i l e the oxygen c o v e r a g e d r o p s o f f more t h a n a f a c t o r o f 2. This r e s u l t c l e a r l y e l i m i n a t e s the p o s s i b i l i t y of r e a c t i o n through a s i m p l e ER p r o c e s s o f t h e f o r m 2

2

2


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CATALYSIS

CO(g) for

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(3)

A c c o r d i n g t o t h i s e x p r e s s i o n t h e r a t e p e r u n i t CO p r e s s u r e s h o u l d be p r o p o r t i o n a l t o t h e c o v e r a g e o f o x y g e n w h i c h i t i s n o t . O r i g i n a l l y (15) t h i s d a t a was i n t e r p r e t e d i n t e r m s o f a p h y s i s o r b e d p r e c u r s o r s t a t e l a r g e l y b e c a u s e no s i g n i f i c a n t accumulation o f c h e m i s o r b e d CO i s p o s s i b l e a t t h i s temperature and i t was f e l t t h a t t h e CO r e s i d e n c e t i m e was p r o b a b l y v e r y s h o r t (), w h e r e φ i s t h e p h a s e l a g , as a f u n c t i o n o f Τ can be i n t e r p r e t e d i n t e r m s o f an a c t i v a t i o n e n e r g y d i f f e r e n c e ( E - E^) b e t w e e n r e a c t i o n and d e s o r p t i o n . The r e s u l t f o r P t ( l l l ) i s -10.8 k c a l m o l e " as shown i n F i g u r e 12 ( 1 6 b ) . F o r an E l e y - R i d e a l p a t h w a y i n w h i c h a gas p h a s e CO m o l e c u l e makes a d i r e c t o r i m p a c t a t t a c k on an o x y g e n a d a t o m , E^ = 0 and we r e q u i r e r

1


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2

2

2

2

2

r

1


52

U N D E R

T R A N S I E N T

C O N D I T I O N S


C A T A L Y S I S

Journal of Chemical Physics Figure 12. Logarithmic plot of the tangent of the phase lag as a function of tem perature for CO2 reactively scattered from Pt (111). The experiment was run with a background CO pressure of 10' ton and a modulated 0 beam (10 Hz) which had an equivalent pressure of 6.3 χ 10 ton. Key: Φ, data; and , model, E * = 24.1 kcal/mol, v = 0.11 cm /particles s (16b). 7

2

7

LH

h

2


2.

CREiGHTON

A N D

Catalytic

W H I T E

Carbon

Monoxide

53

Oxidation

E < 0 which i s unreasonable. I f a p h y s i s o r b e d CO i s i n v o l v e d , i t s b i n d i n g energy would have to exceed 10 k c a l m o l e i n order f o r E t o become p o s i t i v e and i t i s not r e a s o n a b l e to expect a b i n d i n g energy of t h i s magnitude. We a r e t h u s l e d t o t h e c o n c l u s i o n t h a t t h e r e a c t i o n o c c u r s t h r o u g h an LH p a t h w a y . The same c o n c l u s i o n s h o l d f o r PdÇlll)Çl6a) w h e r e , on t h e b a s i s o f p h a s e s h i f t m e a s u r e m e n t s , E - E^ = -9.5 k c a l mole"" . I n t h i s same v e r y i n t e r e s t i n g w o r k , t r a n s i e n t p r e s s u r e jump e x p e r i m e n t s w i t h e x c e l l e n t time r e s o l u t i o n were p e r f o r m e d . An e x a m p l e i s shown i n F i g u r e 13 i n w h i c h a CO beam, w i t h a f l u x e q u i v a l e n t to P = 6x10 t o r r i m p i n g e s on a P d ( H l ) s u b s t r a t e immersed i n a c o n s t a n t ambient P 1x10" t o r r . The i m p o r t a n t p o i n t s t o n o t e a r e t h e i n d u c t i o n t i m e a t 375K and t h e t r a n s i e n t s , w i t h c l e a r m a x i m a n o t e d a t 375 and 450K. S i n c e t h e beam c a n be t u r n e d on i n a b o u t 1 msec t h e r e s p o n s e shown i n F i g u r e 13 i s n o t an a r t i f a c t o f t h e p r o c e d u r e and we m u s t c o n c l u d e t h a t more t h a n ER k i n e t i c s i s i n v o l v e d e v e n u n d e r t h e s e c o n d i t i o n s o f h i g h o x y g e n c o v e r a g e and r e l a t i v e l y l o w t e m p e r a t u r e s ; an ER p a t h w o u l d show no i n d u c t i o n time. The maxima o b s e r v e d i n F i g u r e 13 a r e r e l a t e d t o t h e v a r i a t i o n of t h e C 0 p r o d u c t i o n r a t e w i t h c o v e r a g e s o f CO and 0. At s h o r t times 0 i s low w h i l e 0 i s h i g h and t h e r e a c t i o n r a t e g r o w s as CO a c c u m u l a t e s . W i t h t h e p a s s a g e of t i m e , 9 d e c l i n e s and 9 continues t o grow. The r e s u l t i n g p r o d u c t Q ^ c o i i when θ = θ = 0.5. A t l o n g e r t i m e s and l o w e r t e m p e r a t u r e s 6 i n h i b i t s d i s s o c i a t i v e o x y g e n a d s o r p t i o n and t h e r a t e d r o p s t o some s t e a d y - s t a t e v a l u e . A t h i g h temp eratures, 6 cannot r i s e to i n h i b i t 0 adsorption. C o n s e q u e n t l y , t h e r a t e r i s e s u p o n a d m i s s i o n o f CO b u t t h e r e i s no p r o n o u n c e d maximum. F i n a l l y , we n o t e a v e r y c l e v e r i s o t o p e e x p e r i m e n t d o n e by T a t s u o M a t s u s h i m a Ç24). The r e s u l t s a r e s u m m a r i z e d i n F i g u r e 14 and show t h a t when CO i s p r e a d s o r b e d on P t and t h e n a m i x t u r e o f l^CO j Q i s i n t r o d u c e d i n t o t h e gas p h a s e , t h e p r o d u c t C 0 m o l e c u l e s i n i t i a l l y formed a l l a r e l a b e l l e d w i t h ^C. I n f a c t t h e e x p e r i m e n t i s n o t q u i t e so s i m p l e . An i n i t i a l ^ C 0 / ^ C 0 r a t i o i s measured ( u n i t y i n F i g u r e 14) w h i c h i s e q u a l t o t h e C O / C O r a t i o i n t h e p r e a d s o r b e d CO. The c o n c l u s i o n i s t h a t C 0 i s p r o d u c e d by t h e r e a c t i o n o f a d s o r b e d , r a t h e r t h a n g a s e o u s , CO and a d s o r b e d 0 a t o m s - i . e . t h e L a n g m u i r H i n s h e l w o o d p r o c e s s . T h i s c o n c l u s i o n has d o m i n a t e d a l l o f t h e r e s u l t s p r e s e n t e d h e r e and a p p e a r s t o be u b i q u i t o u s f o r a l l t r a n s i t i o n m e t a l s under a l l c o n d i t i o n s t h a t have been s t u d i e d t o d a t e . r

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C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

T I M E (sec) Journal of Chemical Physics Figure 13. Transient C0 production rates as a function of time for the titration of preadsorbed oxygen on Pd (111) at 375, 450, and 500 K. The experiment was run in a background of 10' torr of 0 , and the CO pressure 6 X 10 torr was introduced at the point indicated (16a,). 2

7

2

8


2.

CREiGHTON

A N D

Catalytic Carbon

W H I T E

Monoxide

55

Oxidation

2.5 (α)

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Transient Low-Pressure Studies of Catalytic Carbon Monoxide Oxidation

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