Rival Kinetic Models in the Oxidation of Carbon Monoxide over a

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Rival Kinetic Models in the Oxidation of Carbon Monoxide over a Silver Catalyst by the Transient Response Method MASAYOSHI KOBAYASHI Kitami Institute of Technology, Department of Industrial Chemistry, 090 Kitami, Hokkaido, Japan

The oxidation of carbon monoxide by nitrous oxide and oxygen over a s i l v e r c a t a l y s t at 20°C was analysed by both the Hougen -Watson procedure and the transient response method. The rival models derived from both procedures were c l e a r l y distinguished by the mode of the t r a n s i e n t response curves of CO or Ν caused by the concentration jump of C O , O or N O. 2

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2

One f r e q u e n t l y e x p e r i e n c e s d i f f i c u l t y in discrimi n a t i n g between r i v a l k i n e t i c models d e r i v e d from t h e Hougen-Watson p r o c e d u r e , b e c a u s e o f t h e c o m p a r a b l e degree of f i t t i n g to steady s t a t e k i n e t i c data · I t i s v e r y c o n v e n i e n t and h e l p f u l i n t h e s e l e c t i o n o f a sound k i n e t i c model t o have a s i m p l e e x p e r i m e n t a l t e c h n i q u e which c a n s e r i o u s l y d i s t i n g u i s h between t h e r i v a l k i n e t i c models. To meet t h i s n e c e s s i t y , o u r r e c e n t works have p r o p o s e d t h e t r a n s i e n t réponse method [7J. In the present study, the t r a n s i e n t response method i s t y p i c a l l y a p p l i e d t o d i s t i n g u i s h between t h e r i v a l k i n e t i c m o d e l s i n CO o x i d a t i o n o v e r a s i l v e r c a t a l y s t d e r i v e d from t h e Hougen-Watson p r o c e d u r e . It i s a l s o shown how t h e b e s t k i n e t i c p a r a m e t e r - s e t c a n be d e t e r m i n e d among t h e r i v a l p a r a m e t e r - s e t s b y u s i n g t h e t r a n s i e n t r e s p o n s e method. Experimental

Procedure

The s i l v e r c a t a l y s t u s e d was p r e p a r e d f r o m silver o x i d e b y t h e same p r o c e d u r e a s t h e c a t a l y s t u s e d f o r e t h y l e n e o x i d a t i o n [8], w h i c h c o n t a i n e d a s m a l l amount o f K S 0 i » a s a p r o m o t e r a n d s u p p o r t e d on c i - A l 0 o f 2040 m e s h . T h e c o m p o s i t i o n o f t h i s s a m p l e was 1 5 4 g - A g , 0.827g-K S0 /40g-d-Al 0 . The d e t a i l e d e x p l a n a t i o n f o r t h e t r a n s i e n t r e s p o n s e m e t h o d c a n be f o u n d e l s e w h e r e [9^] . 2

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3

0097-6156/82/0196-0213$06.00/0 © 1982 American Chemical Society Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

214

CHEMICAL REACTION ENGINEERING

Steady

State Analysis

I n b o t h t h e N 0 - C 0 and t h e 0 - C 0 r e a c t i o n s , t h e r a t e of C0 f o r m a t i o n at steady s t a t e i s not first o r d e r w i t h r e s p e c t t o t h e c o n c e n t r a t i o n o f CO, N 0 or 0 . F o r i l l u s t r a t i n g t h e s e r a t e d a t a a l a r g e number o f p o s s i b l e m o d e l s w i l l be p r o p o s e d . Of t h e s e m o d e l s , t h r e e m o d e l s f o r t h e N 0 - C O r e a c t i o n and two m o d e l s f o r t h e 0 - C 0 r e a c t i o n , b a s e d on a L a n g m u i r - H i n s h e l w o o d t y p e and an E l e y - R i d e a l t y p e , a r e p r o p o s e d ( T a b l e I ) . Equa t i o n s 1-5 ( e a c h o f w h i c h c o r r e s p o n d s t o a m o d e l number i n T a b l e I) are l i n e a r i z e d a n d the s t e a d y s t a t e r a t e s at v a r i o u s p a r t i a l p r e s s u r e s o f CO, N 0 and 0 were compared w i t h the f i v e models. A l l of t h e s e m o d e l s were f o u n d t o g i v e t h e same d e g r e e o f f i t t i n g t o t h e s t e a d y s t a t e r a t e data, i n d i c a t i n g the d i f f i c u l t y i n d i s c r i m i n a t i n g b e t w e e n them. 2

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2

2

2

2

2

2

Table

I.

K i n e t i c Models N 0 - C O and t h e 2

2

f o r the Best F i t s 0 -C0 r e a c t i o n s . Κ

10'xk - , , mol/g.min.

Models

k K

K

P

P

co N 0 co N20 (l+K Ρ +κ Ρ „

0.539

2

u

;

Μ

v

co

co

N

λ

N 0

to

the

2

" -ι atm

K._ *° . - i atm Λ

N

24.3

Κ

Λ

°· . atm

1

13.9

) :

Ν ϋ

2

2

uv

ρ Ρ " »° co co N 0 N 0 XrV Ρ Ρ N*0 c o N 0 l+K Ρ +K ρ„ _ co c o N 0 N 0 N

(

2

)

1 + K

P

+ K

P

2

'

a

2

0.952 -»

210

HO

210

HO

a t -

1

>

8

2

Λ

2

kK (

4

)

co

(1«

Κ Ρ

co

o

Ρ 2

l+K

2-2 Ρ

co

Ρ

co

Γ

0 ' )• 0

·

1-36

££-24 -j-?+ Κ * P i / co 0 0

4.46

+

co ρ

(5)

atm

2

0

2

1

14.8

-

6.12

186.3

-

520.9

2

pi/2 /

2

2

2

2

atm

-ι

As h a s b e e n d e s c r i b e d i n o u r p r e v i o u s p a p e r s [21 » t h e mode o f t r a n s i e n t r e s p o n s e c u r v e s o f p r o d u c t s c a u s e d by t h e c o n c e n t r a t i o n jump o f r e a c t a n t s a t t h e i n l e t

Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

18.

KOBAYASHI

Kinetic Models in Carbon Monoxide Oxidation

215

of the r e a c t o r i s a f u n c t i o n of the r e a c t i o n mechanism. T h i s c a n c o n v e n i e n t l y be u s e d t o d i s t i n g u i s h b e t w e e n the p r e s e n t e d models. On t h e N 0 - C 0 r e a c t i o n , F i g u r e 1 r e p r e s e n t s the simulated t r a n s i e n t response curves of C0 or N c a u s e d by t h e s i m u l t a n e o u s c o n c e n t r a t i o n jump o f CO and N 0 ( d e s i g n a t e d a s t h e CO, N 0(inc.,0)-C0 and - N r e s p o n s e s ) , u s i n g the k i n e t i c parameters e s t i m a t e d f r o m t h e l i n e a r p l o t s o f M o d e l s 1, 2, and 3. H e r e , t h e f o r w a r d r a t e c o n s t a n t s o t h e r t h a n one r a t e d e t e r m i n i n g s t e p have a v a l u e f i v e hundred t i m e s l a r g e r than f o r the r a t e d e t e r m i n i n g s t e p . The comparison b e t w e e n t h e modes o f t h e t h r e e s i m u l a t e d r e s p o n s e c u r v e s ( C u r v e s 1, 2 and 3) a n d o f t h e e x p e r i m e n t a l c u r v e s , r e g a r d l e s s of the d e v i a t i o n of the v a l u e s a t t h e s t e a d y s t a t e s , c l e a r l y showed a d i s a g r e e m e n t . The simulated curves reached a r e a c t i o n steady s t a t e w i t h i n two m i n u t e s w i t h no d i f f e r e n c e b e t w e e n N and C 0 , and M o d e l s 1 and 2 i n d i c a t e d a s l i g h t o v e r s h o o t mode. I n c o n t r a s t t h e e x p e r i m e n t a l c u r v e s n e e d e d more t h a n s i x minutes to complete the r e a c t i o n steady s t a t e , e v e n though the response of N showed a s t e e p o v e r s h o o t mode d i f f e r i n g from the response of C0 w h i c h showed a Ss h a p e mode. In the 0 -C0 r e a c t i o n , the 0 - C 0 and C 0 - C 0 r e s p o n s e s w e r e s i m u l a t e d by u s i n g M o d e l s 4 and 5 i n T a b l e I and t h e r e s u l t s w e r e shown i n F i g u r e 2 by Runs 1 and 2. The s i m u l a t e d c u r v e s f r o m t h e two m o d e l s e x h i b i t e d an . o v e r s h o o t t y p e mode f o r M o d e l 4 i n Run2 and f o r M o d e l 5 i n Run 1, t h u s d i f f e r i n g f r o m t h e e x p e r i m e n t a l c u r v e s w h i c h were a m o n o t o n i e t y p e . C o n s e q u e n t l y , one c a n r e c o g n i z e t h a t t h e r e a r e no sound models i n the s i x models p r e s e n t e d . 2

2

2

2

2

2

2

2

2

2

2

2

Transient

2

2

2

State Analysis

P r e s e n t a t i o n of a R e a c t i o n Mechanism. In the N 0 -CO r e a c t i o n s , a f t e r t h e r e a c t i o n had a c h i e v e d a s t e a d y s t a t e , t h e r e a c t i o n g a s m i x t u r e was s w i t c h e d i n t o e i t h e r a pure helium stream or a 0 -He stream. The responses of C0 and CO w e r e t h e n f o l l o w e d . The C O ( d e c . , 0 ) - C O response obtained i n s t a n t a n e o u s l y responded zero with no d e l a y , i n d i c a t i n g t h a t t h e r e was no r e v e r s i b l y a d s o r b e d CO. Furthermore, the CO(dec.,0)-C0 response o b t a i n e d was n o t a f f e c t e d by t h e p r e s e n c e o f 0 i n the stream, s u g g e s t i n g the n o n e x i s t e n c e of i r r e v e r s i b l e a d s o r p t i o n o f CO w h i c h c o u l d r e a c t w i t h o x y g e n . Thus, a m o d e l o f t h e d i r e c t r e a c t i o n o f g a s e o u s CO w i t h a d s o r b e d o x y g e n , an E l e y - R i d e a l t y p e m e c h a n i s m , may be proposed. I t i s g e n e r a l l y a c c e p t e d t h a t t h e r e a r e two types o f a d s o r b e d o x y g e n on s i l v e r , monaatomic and d i a t o m i c . 2

2

2

2

2


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SJ A 0.055

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CM Ο

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0 Ρ·9./

5

10 15 Time ( m i n ) N 0(inc.,0)-C0 and-N responses on the reduced surface. 2

2

2

Figure 1. Simulated transient response curves at 2(PC of N O (including O), -CO , and -N, on a reduced surface. Model 1 ( ), Model 2 ( ), and Model 3 ( ; are calculated by the H-W procedure. Key: · , N ; O, CO ; and · · ·, calculated by the transient response method. t

t

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p

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D(Run2) ·

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A 0.09 9 0.0 5

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D 0.1 3 3 0.20

(atm) 8 Ti me

T=20'C PW 0.77 7 0.66 7

(atm) 0 (min)

2

10

Figure 2. Test for the discrimination of rival kinetic models by the transient re sponse method. Key: , Model 1 in Table I; , Model 2 in Table I; and - · ·, Set 8 in Table II.


18.

KOBAYASHI


217

Our p r e v i o u s s t u d y p r o p o s e d t h e a c t i v e o x y g e n s p e c i e s c o n t r i b u t i n g t o t h e o x i d a t i o n o f CO o n t h i s c a t a l y s t t o be a d i a t o m i c o x y g e n s p e c i e s , u s i n g t h e p u l s e technique of N 0 or 0 tljQ] · S i n c e o x y g e n was i r r e v e r s i b l y adsorbed onto a s u r f a c e reduced with H , t h e g r a p h i c a l integration of the 0 ( i n c . , 0 ) - 0 r e s p o n s e on t h e s u r f a c e g a v e a s a t u r a t e d amount o f t h e a d s o r b e d o x y g e n , 1.6x10 mol/g-Ag. On t h e o t h e r h a n d , t h e N 0 ( i n c . , 0 ) - N r e s p o n s e on the reduced s u r f a c e i n d i c a t e d a t y p i c a l o v e r s h o o t mode w i t h a n i n s t a n t a n e o u s maximum b e f o r e r e a c h i n g z e r o . T h i s c h a r a c t e r i s t i c mode s t r o n g l y s u g g e s t s t h a t N 0 i s d i r e c t l y decomposed on a c t i v e s i t e s , and t h a t t h e a c t i v e s i t e s are not regenerated r e s u l t i n g from the i r r e v e r s i b l e a d s o r p t i o n o f t h e formed oxygen atoms. This i s e a s i l y p r e s u m e d f r o m t h e r e l a t i o n b e t w e e n t h e mode o f t r a n s i e n t r e s p o n s e c u r v e a n d r e a c t i o n m e c h a n i s m [_Z1 · I n f a c t no o x y g e n was d e t e c t e d i n t h e e f f l u e n t g a s stream. The i n t e g r a t e d _ a m o u n t o f t h e a d s o r b e d oxygen i s e s t i m a t e d t o be 0 . 4 9 x l 0 ~ mol/g-Ag. T h i s i s about one t h i r d o f t h e t o t a l number o f a c t i v e s i t e s f o r o x y g e n a d s o r p t i o n , a s s u m i n g a monoatomic f o r m o f a d s o r p t i o n . For the e x p l a n a t i o n o f t h i s d i f f e r e n c e t h e b l o c k i n g e f f e c t o f oxygen (which i s c o n t a i n e d i n a N 0-He stream as an i m p u r i t y ) on t h e a c t i v e s i t e s f o r N 0 d e c o m p o s i t ion may be c o n s i d e r e d . However, i t c a n be e s t i m a t e d t o be a t m o s t 1 . 8 x 1 0 " mol/g-Ag. T h e number o f a c t i v e s i t e s f o r N 0 d e c o m p o s i t i o n s h o u l d t h e r e f o r e be s m a l l e r t h a n t h e number o f t h e a c t i v e s i t e s f o r o x y g e n a d s o r p t ion. S i n c e N 0 c a n n o t be decomposed on t h e o x i d i z e d s u r f a c e , i t may be c o n c l u d e d that the reduced surface is heterogeneous f o r the decomposition of N 0 : a cert a i n p a r t o f t h e a c t i v e s i t e s on t h e s u r f a c e i s n o t available f o r N 0 decomposition. T h i s h e t e r o g e n i t y may cause t h e d i f f e r e n c e i n r e a c t i o n mechanism between t h e N 0 - C 0 and t h e 0 - C 0 r e a c t i o n s and w i l l be d i s c u s s e d i n a later section. A f t e r t h e steady s t a t e o f t h e N 0-C0 r e a c t i o n had b e e n a c h i e v e d , o x y g e n g a s was i n t r o d u c e d i n t o t h e r e a c t o r w i t h no c h a n g e i n t h e c o n c e n t r a t i o n o f N 0 a n d CO ( R u n 1 ) . T h e r e a c t i o n g a s s t r e a m c o n t a i n i n g o x y g e n was then s w i t c h e d back t o t h e p r e v i o u s N 0-C0-He stream (Run 2 ) . T h e 0 ( i n c . , 0 ) - C 0 r e s p o n s e o b t a i n e d i n Run 1 showed a n o v e r s h o o t mode w i t h a n i n s t a n t a n e o u s maximum, a t t r i b u t i n g to the slow r e g e n e r a t i o n of a c t i v e s p e c i e s ( w h i c h m i g h t be d i a t o m i c o x y g e n ) a n d t o t h e r a p i d d e s o r ption of C0 . The 0 ( i n c . , 0 ) - N r e s p o n s e i n Run 1, o n the o t h e r hand, i n s t a n t a n e o u s l y responded z e r o . This i s due t o t h e b l o c k i n g e f f e c t o f i r r e v e r s i b l y a d s o r b e d o x y g e n on t h e a c t i v e s i t e s . I n Run 2, t h e r e s p o n s e s 2

2

2

2

2

2

2

2

6

2

2

2

2

2

2

2

2

2

2

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2

2

2

2

2


218


of C 0 and N w e r e v e r y s l o w f o r a b o u t 40 m i n . T h i s r e s u l t e d from the slow r e g e n e r a t i o n of the a c t i v e s i t e s f o r N 0 decomposition, p o s s i b l y because the r a t e of the r e c o m b i n a t i o n o f monoatomic o x y g e n t o f o r m b o t h one d i a t o m i c o x y g e n and one v a c a n t a c t i v e s i t e i s s l o w . The maximum v a l u e o f t h e N 0 ( i n c . , 0 ) - N response on t h e r e d u c e d s u r f a c e w i t h no CO c o r r e s p o n d s t o t h e decomposition rate of N 0 since N i s not adsorbed. T h i s r a t e i s measured a t d i f f e r e n t p r e s s u r e s o f N 0 . The p l o t o f t h i s r a t e v s . t h e p a r t i a l p r e s s u r e o f N 0 g i v e s a s t r a i g h t l i n e and f r o m i t s s l o p e an a p p a r e n t r a t e c o n s t a n t f o r N 0 d e c o m p o s i t i o n i s e s t i m a t e d t o be 6.7x10 mol/g-Ag.min.atm. The r e d u c e d s u r f a c e h a d b e e n e x p o s e d i n a CO-He s t r e a m a n d t h e n o x y g e n was p u l s e d i n t o t h e i n l e t o f t h e reactor using various sizes. The 0 ( p u l s e ) - C 0 response o b t a i n e d showed a s h a r p s p e c t r u m w i t h a n i n s t a n t a n e o u s maximum, f o l l w e d by a s t e e p d e c a y t o z e r o . This maximum p o i n t c o r r e s p o n d s t o a n a p p a r e n t r a t e o f C 0 f o r m a t i o n a t a g i v e n amount o f a d s o r b e d o x y g e n , e s t i mated from t h e g r a p h i c a l i n t e g r a t i o n o f t h e p u l s e spectrum. T h e p l o t s o f t h e r a t e v s . t h e amount o f a d s o r b e d o x y g e n o r v s . t h e p a r t i a l p r e s s u r e o f CO i n t h e CO-He s t r e a m r e p r e s e n t e d a l i n e a r r e l a t i o n . The s l o p e o f t h e two l i n e s _ § a v e a l m o s t t h e same a p p a r e n t r a t e c o n s t a n t , 9.6x10 mol/g-Ag.min.atm, f o r t h e r e a c t i o n o f a d s o r b e d o x y g e n a n d g a s e o u s CO. B a s e d on t h e e x p e r i m e n t a l f i n d i n g s d e s c r i b e d s o f a r , a s u i t a b l e r e a c t i o n m e c h a n i s m may be e x p r e s s e d : 2

2

2

2

2

2

2

2

2

2

2

2

2

N 0 2

(g) + S

^ k

2 0»S 0

(g) + S

2

C0(g)

*

^

+ 0 «S

C0 «0»S 2

**

>

0»S

(6)

0 »S + S

(7)

2

0 *S

(8)

C0 »0»S

(9)

C 0 ( g ) + 0»S

(10)

2

^

2

>

— kl

2

2

E q u a t i o n s ( 6 ) , ( 7 ) , (9) and (10) a r e f o r t h e N 0-C0 r e a c t i o n and E q u a t i o n s ( 7 ) , ( 8 ) , (9) and (10) a r e f o r the 0 -C0 r e a c t i o n . 2

2

K i n e t i c Parameter E s t i m a t i o n . Since the values o f k i a n d k* w e r e a l r e a d y e s t i m a t e d t o be 6.7x10 and 9.6xl0" mol/g-Ag.min.atm. r e s p e c t i v e l y , f i v e parameters, k , k , k k a n d k j » s h o u l d b e e s t i m a t e d by a p a r a meter o p t i m i z a t i o n t e c h n i q u e , u s i n g a d i g i t a l computer. 5

2

2

3f

5


18.

KOBAYASHI

219


I n t h i s e s t i m a t i o n t h e r e i s a p r o b l e m i n t h a t many r a t e c o n s t a n t - s e t s g i v i n g t h e same d e g r e e o f f i t t i n g t o a p a r t i c u l a r t r a n s i e n t d a t a m i g h t be c o n s i d e r e d . To minimize t h i s p o s s i b i l i t y , a cross checking of the e s t i m a t e d r a t e c o n s t a n t - s e t s i s n e c e s s a r y by e m p l o y i n g many t r a n s i e n t d a t a . On t h e N 0 - C 0 r e a c t i o n , k , k , k and k a r e unknown. Using the N 0,CO(inc.,0)-C0 response curve i n F i g u r e 1 t h e b e s t s i x p a r a m e t e r - s e t s were e v a l u a t e d w i t h t h e same d e g r e e o f f i t t i n g t o t h e c u r v e a n d t h e y w e r e p r e s e n t e d i n T a b l e I I b y s e t n u m b e r s 1-6. C o m p a r i s o n o f t h e d e g r e e o f f i t t i n g t o many o t h e r t r a n s i e n t and s t e a d y s t a t e r a t e d a t a was i m p o r t a n t i n s e l e c t i n g t h e b e s t one o f t h e s i x S e t s . One c o u l d r e c o g n i z e p a r a m e t e r - s e t number 6 t o b e b e s t . This Set c o n s i s t e n t l y e x p l a i n e d a l l o f t h e o b t a i n e d t r a n s i e n t d a t a , as c a n t y p i c a l l y b e s e e n i n F i g u r e s 3 a n d 4. In Fig.4, although the simulated curves of N and C 0 respectively showed a s i m i l a r mode a s t h e e x p e r i m e n t a l c u r v e s , t h e s t e a d y s t a t e v a l u e was i n a l e s s a g r e e m e n t . S u f f i c i e n t explanation f o r t h i s disagreement c o u l d not b e o f f e r e d a t the p r e s e n t time except t h e change i n t h e c a t a l y t i c activity. T h e a c t i v i t y o f t h e c a t a l y s t was i n f l u e n c e d by t h e d e g r e e o f t h e r e d u c t i o n w i t h H or of the o x i dation with 0 before s t a r t i n g the r e a c t i o n . 2

2

2

5

2

5

2

2

2

2

2

Table

Set No. 1 2 3 4 5 6 7 8

II.

K i n e t i c Parameters Response Curves. 10 xk

0.67 0.67 0.67 0.67 0.67 0.67

1.26 0.667 0.019 0.102 0.204 0.148 0.148 0.148

5

-

4

6

10 xk!

2

10 xk 10.0 5.0 1.0 0.5 1.0 0.5 0.5 50.0

Best

5

2

10 xk

0.64 50.0

3

Fitting

the Transient

10*xk«

10 xk

0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96

0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

2

2

5

10 xk

5

0.889 0.925 1.11 1.31 0.864 1.17 1.17 1.17

On t h e 0 - C 0 r e a c t i o n , o n l y k i s unknown when t h e k i n e t i c parameters e s t i m a t e d i n the N 0-C0 r e a c t i o n can d i r e c t l y be u s e d . T h e b e s t v a l u e o f ka was e v a l u a t e d so a s t o a g r e e w i t h t h e s t e a d y s t a t e r a t e d a t a r e g a r d l e s s o f t h e mode o f t h e i r t r a n s i e n t c u r v e s , a n d t h e v a l u e o b t a i n e d was p r e s e n t e d i n S e t 7 i n T a b l e I I . S e t 7, h o w e v e r , g a v e a s l i g h t o v e r s h o o t mode f o r t h e C 0 - C 0 r e s p o n s e c u r v e s i n c o n t r a s t w i t h t h e m o n o t o n i e mode o f the e x p e r i m e n t a l C0-C0 response curves (see F i g . 2 ) . This disagreement s t r o n g l y suggests the p o s s i b l i t y that some k i n e t i c p a r a m e t e r s f r o m t h e N 0 - C 0 r e a c t i o n m i g h t 2

3

2

2

2

2


220


(XIO- ^ 3

2.0 h

/

ο

Ε «β

\A

Ο* Ο

.i.oh

P* 0.0 5

0203 0.74 7

θ 0.20

0.203 0.547 (atm) T = 20 C

«8

#

A

10

6 8 Time (min)

Figure 3. CO-CO» and -N responses. Key: Φ, CO , and Ο, -Ν, experimentally observed responses; , CO calculated response; and —,N calculated response. g

t

g

t


KOBAYASHI


(x1Ô ) 3

0

10

20

30

Time

Figure 4.

N O-CO t

40

50

60

(min)

t

and -N responses. t


222


n o t be a v a i l a b l e f o r t h e 0 - C 0 r e a c t i o n . Remembering the h e t e r o g e n i t y of the a c t i v e s i t e s f o r oxygen adsorpt i o n on t h e r e d u c e d s u r f a c e , i t s h o u l d be c o n s i d e r e d a s t h e most p r o b a b l e p o s s i b i l i t y t h a t k and/or k a r e d i f f e r e n t between both r e a c t i o n s . F i n a l l y , t h e p a r a m e t e r o p t i m i z a t i o n t e c h n i q u e was a g a i n a p p l i e d f o r e v a l u a t i n g k , k , and k , u s i n g a p a r t i c u l a r CO(inc, 0)-C0 response curve. The c o n c l u s i v e v a l u e s o b t a i n e d a r e p r e s e n t e d i n T a b l e I I by p a r a m e t e r - s e t number 8. This Set 8 c o n s i s t e n t l y explained a l l other t r a n s i e n t r e s p o n s e c u r v e s , as t y p i c a l l y p r e s e n t e d i n F i g u r e 2 by a dotted curve. C o m p a r i n g S e t s 6 a n d 8 i n T a b l e 2, t h e value of k i n the 0 -C0 r e a c t i o n i s a hundred times l a r g e r than t h a t i n t h e N 0-C0 r e a c t i o n . T h i s means t h a t adsorbed oxygen i s v e r y q u i c k l y d i s s o c i a t e d into monoatomic oxygen, i n c o n t r a s t t o t h e N 0-C0 r e a c t i o n . T h i s i s c o n s i s t e n t w i t h C l a r k s o n and C i r i l l o ' s estimat i o n [ 1 1 ] i n w h i c h t h e y e v a l u a t e d o n l y 0.02% o f a l l a d s o r b e d o x y g e n t o be d i a t o m i c o x y g e n on o x i d i z e d silver. 2

2

2

2

2

3

2

2

2

2

2

The r a t e o f t h e 0 - C 0 r e a c t i o n i s m a i n l y controlled by two s t e p s , t h e r e a c t i o n o f a d s o r b e d d i a t o m i c o x y g e n w i t h g a s e o u s CO a n d t h e a d s o r p t i o n o f o x y g e n . The N 0 -CO r e a c t i o n , o n t h e o t h e r h a n d , i s c o n t r o l l e d b y two s t e p s , N 0 d e c o m p o s i t i o n a n d t h e r e c o m b i n a t i o n o f monoatomic oxygen. These a r e r e s p o n s i b l e i n g i v i n g t h r e e e x p l a n a t i o n s f o r t h e c h a r a c t e r i s t i c modes o f t h e e x p e r i m e n t a l t r a n s i e n t c u r v e s : ( 1 ) t h e S - s h a p e mode o f C 0 i n F i g u r e 1 a n d the o v e r s h o o t mode o f C 0 i n F i g u r e 3,which a r e c a u s e d by t h e s l o w f o r m a t i o n o f d i a t o m i c o x y g e n ; (2) t h e o v e r s h o o t mode o f N i n F i g u r e s 1 and 4 and t h e s l i g h t S - s h a p e mode o f N i n F i g u r e 3, w h i c h a r e d u e t o the slow r e g e n e r a t i o n o f the a c t i v e s i t e s f o r N 0 d e c o m p o s i t i o n ; a n d ( 3 ) t h e m o n o t o n i e mode o f t h e C 0 - C 0 response i n F i g u r e 2 which r e s u l t s from t h e c o m b i n a t i o n of t h e a d s o r p t i o n o f oxygen and t h e s u r f a c e r e a c t i o n . The a c t i v a t i o n e n e r g i e s a t t h e r e a c t i o n s t e a d y s t a t e a r e e s t i m a t e d t o be 14 K c a l / m o l f o r t h e 0 - C 0 r e a c t i o n and 10 K c a l / m o l f o r t h e N 0 - C 0 r e a c t i o n , g i v i n g s t r o n g support f o r the view that the r a t e - c o n t r o l l i n g step i s d i f f r e n t i n t h e two r e a c t i o n s . 2

2

2

2

2

2

2

2

2

2

2

The v a l i d i t y o f p a r a m e t e r - s e t s 6 a n d 8 s h o u l d a d d i t i o n a l l y be c o n f i r m e d by u s i n g t h e d a t a f r o m n o n isothermal experiments to reject a further p o s s i b i l i t y i n t h e e x i s t e n c e o f o t h e r p a r a m e t e r - s e t s w i t h a good degree of f i t t i n g . The n o n i s o t h e r m a l t r a n s i e n t e x p e r i ments, u n f o r t u n a t e l y , have n o t been conducted w i t h i n the p e r i o d g i v i n g a c o n s t a n t a c t i v i t y o f the c a t a l y s t . The mode o f t h e t r a n s i e n t r e s p o n s e c u r v e o f N i s s i g n i f i c a n t l y a f f e c t e d by t h e v a l u e o f k j , r a t h e r t h a n 2


18.

KOBAYASHI


223

that of C0 . Therefore, the experimental N 0 ( i n c . , 0 ) N r e s p o n s e c u r v e i n F i g u r e 1 c o u l d be u t i l i z e d f o r t h e parameter o p t i m i z a t i o n technique, i n stead of the use o f t h e n o n i s o t h e r m a l d a t a . T h e mode o f t h e r e s p o n s e curve of N is,however, g r e a t l y steep making a d i f f i c u l t y of the a p p l i c a t i o n of the o p t i m i z a t i o n technique, a s c a n b e s e e n i n F i g u r e 1. In the present study, a l a r g e number o f t r a n s i e n t d a t a b a s e d o n t h e i s o t h e r m a l experiment f o r t h e 0 - C 0 and t h e N 0-CO r e a c t i o n s have been employed t o d i s t i n g u i s h t h e p a r a m e t e r - s e t s e s t i m a t e d , e s p e c i a l l y c o m p a r i n g t h e mode o f t h e r e s p o n s e curve of N . 2

2

2

2

2

2

2

Literature Cited

1.

Kitrell,J.R.; Hunter, W.G.; Watson, C . C . AIChE Journal 1966, 12, 369. 2. H a n c i l , V . ; Mitschka, P; B e r a n i k , L . J . C a t a l . 1969, 13, 435. 3. Ford, F. E.; Perlmuter, F. E . Chem.Eng.Sci. 1964, 19, 371. 4. C u t l i p , M. P . ; Peters, M. S. Chem.Eng.Progr.Symp. Ser. 1968, No.89, 64, 1. 5. Knozinger, H . ; Hochel,K. ; Meye,W. J.Catal. 1973, 28, 69. 6. Lumpkin, R.E.; Smith, W.D. Douglas, J . M . IEC Fund. 1969, 8, 407. 7. Kobayashi, M. P r e p r i n t s , 5th Canadian Symposium on C a t a l y s i s , 1977 p202; Chem.Eng.Sci. 1982, 37, 393; ibd. 1982, 37, 403. 8. Kobayashi, M. Yamamoto, M. Kobayashi, H. Proc. 6th Int.Congr. C a t a l . 1976, 1, 336. 9. Kobayashi, H . ; Kobayashi, M. Catal.Rev. 1974, 10, 139. 10. Kobayashi, M . ; Takegami, H . ; Kobayashi, H. JCS Chem.Comm. 1977, 37. 11. Clarkson, R . B . ; Cirillo J r . A . C . J . C a t a l . 1974, 33, 392. RECEIVED April 2 7 , 1 9 8 2 .


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