Catalysis Under Transient Conditions - American Chemical Society

9 Characterization of the Adsorbed Layer of a Silver Catalyst in the Oxidation of Ethylene from

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Its Transient Adsorption Behavior M. KOBAYASHI Kitami Institute of Technology, Department of Industrial Chemistry, 090 Kitami, Hokkaido, Japan

The adsorbed layer on a s i l v e r catalyst surface, during the course of ethylene oxidation at 9 1 ° C , was characterized by examining the transient adsorption behavior of CO2. The analysis of the results obtained by transient response, thermal desorption and pulse techniques suggested the existence of three different adsorbed oxygen species: isolated monoatomic oxygen (Oi s ), adjacent monoatomic oxygen (Oas) and diatomic oxygen species (Ods), d i f f e ring in their r e a c t i v i t i e s . Carbon dioxide and ethylene are competitively adsorbed on these three oxygen species and the strength of CO2 adsorption on Ois is weaker than C 2 H 4 . CO2 and C2H4 are not adsorbed on the surface reduced by H 2 but adsorbed on the surface reduced by C 2 H 4 on which there is Ois. The C 2 H 4 adsorbed on Ois produces no C2H4O and CO2, the C 2 H 4 on Oas produces an intermediate (In) in the complete oxidation and C2H4 on Ods produces C 2 H 4 O. Desorption of r e v e r s i b l y and i r r e v e r s i b l y adsorbed CO2 is accelerated by the removal of adsorbed oxygen with H2. The adsorption isotherm for CO2 obtained on an oxidized surface reaches saturation for Pco2=0.15 atm. The addition of Pco2= 0.15 atm into a reaction gas stream retarded the rates of C2H4O and CO2 formation to zero in spite of a presence of adsorbed C 2 H 4 , i n d i c a t i n g the complete blocking of CO2 to the adsorbed oxygen and no reaction of the adsorbed C 2 H 4 with gaseous O2. The adsorption of CO2 during reaction obeyed a Langmuir adsorption isotherm. The catalyst surface under steady state reaction is characterized as follows: appreciable parts of the s i l v e r surface larger than 0.4 is blocked by (In), i r r e v e r s i b l y adsorbed CO2 and adsorbed C2H4. It is sugg0097-6156/82/0178-0209$07.25/0 © 1982 American Chemical Society

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

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e s t e d t h a t a v e r y s m a l l f r a c t i o n a l p a r t of s u r f a c e oxygen i s a v a i l a b l e f o r the r e a c t i o n at h i gher c o n c e n t r a t i o n of e t h y l e n e . A l a r g e number o f i n v e s t i g a t o r s h a v e s t u d i e d t h e a d s o r p t i o n b e h a v i o r o f o x y g e n , c a r b o n d i o x i d e and e t h y l e n e on s i l v e r c a t a l y s t s . The chemisorρtion d a t a u s i n g a v a r i e t y of t e c h n i q u e s have s u g g e s t e d the f o l l o w i n g (_1) : o x y g e n i s c h e m i s o r b e d on s i l v e r s u r f a c e s to f o r m b o t h m o n o a t o m i c and d i a t o m i c o x y g e n s p e c i e s b a s e d on a r o u g h c l a s s i f i c a t i o n , e t h y l e n e i s c h e m i s o r b e d on s i l v e r i o n s (2^) w h e r e a s no i t s a d s o r p t i o n o c c u r s on r e d u c e d s i l v e r (_3) and c a r b o n d i o x i d e i s c h e m i s o r b e d on p r e v i o u s l y o x i d i z e d s u r f a c e s (4.,.5) . B a s e d on t h e s e e x p e r i m e n t a l f i n d i n g s i t i s d r a w n t h a t a d s o r b e d o x y g e n s p e c i e s s h o u l d p l a y an i m p o r t a n t role to f o r m a d s o r b e d i n t e r m e d i a t e s and a d s o r p t i o n s i t e s f o r r e a c t i o n gas c o m p o n e n t s . T h e r e f o r e , t o know t h e d e t a i l e d r e a c t i o n mechanism of e t h y l e n e o x i d a t i o n , i t i s n e c e s s a r y to c l a r i f y a s i t u a t i o n of the adsorbed l a y e r f o r m e d d u r i n g t h e r e a c t i o n , e s p e c i a l l y on t h e a d s o r b e d oxygen s p e c i e s a v a i l a b l e f o r the p r o g r e s s of react ion. C a r b o n d i o x i d e a d s o r b s on a d s o r b e d o x y g e n s p e c i e s , not bare s i l v e r s u r f a c e s , i n c o m p e t i t i o n w i t h o t h e r r e a c t i o n gas c o m p o n e n t s and r e t a r d s t h e o v e r a l l r e a c tion. One c a n c o n v e n i e n t l y u s e t h i s n a t u r e t o r e v e a l the s i t u a t i o n of the a d s o r b e d l a y e r d u r i n g the r e a c t i o n . F o r e x a m p l e , t h e a d s o r p t i o n i s o t h e r m o f CO^ o b t a i n e d on t h e s u r f a c e u s e d f o r r e a c t i o n w i l l p r e s e n t an e f f i c i e n t i n f o r m a t i o n f o r the amounts of the a d s o r b e d oxygen s p e c i e s f r e e f r o m a d s o r b e d i n t e r m e d i a t e s , when t h e i n t e r m e d i a t e s a r e i r r e v e r s i b l y a d s o r b e d and s t a b l y r e t a i n e d on t h e s u r f a c e . And an e x a m i n a t i o n f o r t h e c o m p e t i t i v e a d s o r p t i o n o f CO and C^H^ on i t s s u r f a c e w i l l p r o p o s e an i n f o r m a t i o n t o r t h e i r a d s o r p t i o n s t r e n gth and/or f o r a r e a c t i v i t y of the adsorbed e t h y l e n e to r e a c t w i t h the adsorbed oxygen or w i t h gaseous oxygen. In the p r e s e n t s t u d y , the t r a n s i e n t r e s p o n s e method (j6, 7_,J3) has b e e n e f f i c i e n t l y u s e d t o f o l l o w t h e c o m p e t i t i v e a d s o r p t i o n b e h a v i o r b e t w e e n CO ^ and o t h e r r e a c t i o n gas c o m p o n e n t s i n e t h y l e n e o x i d a t i o n , and t h e r e b y t h e a d s o r b e d l a y e r o f a s i l v e r c a t a l y s t was c h a r a c t e r i z e d . A l o w r e a c t i o n t e m p e r a t u r e as 90°C and a l a r g e amount o f c a t a l y s t w e r e u s e d t o f o l l o w more c l e a r l y t h e t r a n s i e n t s t a t e s o f a d s o r p t i o n and d e s o r p t i o n o f r e a c t i o n compo nents .


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The s i l v e r c a t a l y s t was p r e p a r e d by r e d u c i n g s i l v e r o x i d e . The s i l v e r o x i d e u s e d was p r e p a r e d by a d d i n g a s o l u t i o n o f p o t a s i u m h y d r o x i d e t o an a q u e o u s s o l u t i o n of s i l v e r n i t r a t e . A s m a l l amount o f 0.3% K^SO^ s o l u t i o n was a d d e d t o t h e s i l v e r o x i d e p o w d e r as a p r o m o t e r a n d , a f t e r m i x i n g , was d r i e d a t 105°C f o r 2 4 h r i n a d a r k room. T h i s was c o a t e d on a - A l ^ O ^ o f 20-42 mesh i n t h e p r e s e n c e o f a s m a l l amount o f e t h a n o l u n t i l t h e s a m p l e r e a c h e d a s i z e o f 12-14 mesh. After t h e e t h a n o l i n t h e s i l v e r o x i d e p o w d e r had b e e n comp l e t e l y v a p o r i z e d i n a i r a t room t e m p e r a t u r e , t h e s a m p l e was r e d u c e d i n a r e a c t o r w i t h a f l o w o f H f o r 12 h r a t 50°C and s u c c e s s i v e l y f o r 12 h r a t 100°C. The c o m p o s i t i o n o f t h e c a t a l y s t so p r e p a r e d was 206.0 g-Ag, 1.132 g - K S 0 / 5 3 . 5 g - A l 0 . The BET s u r f a c e a r e a was 0.3 m / g-A.g. The c o n s t a n t a c t i v i t y o f t h i s c a t a l y s t was o b t a i n e d by f l o w i n g t h e m i x t u r e o f 5% C«H , 20% 0 and 75% He a t 91°C f o r 48 h r . The r e a c t o r u s e d c o n s i s t s o f a P y r e x g l a s s t u b e c o n t a i n i n g 260.6 g c a t a l y s t and was i m m e r s e d i n an o i l bath. The t e m p e r a t u r e o f t h e c a t a l y s t bed r e m a i n e d c o n s t a n t w i t h i n + 0.1°C of the d e s i r e d temperature. O x y g e n (0 9 9 . 9 % ) , n i t r o g e n (N 99.9%), n i t r o u s oxide (N„,0 99 . 9 9 % ) , c a r b o n d i o x i d e ( 9 9 . 9 % ) and h e l i u m (He 997999%) from c o m m e r c i a l c y l i n d e r s were p u r i f i e d t h r ough a d r y i c e - m e t h a n o l t r a p t o r e m o v e w a t e r v a p o r . F o r He t h e f u r t h e r p u r i f i c a t i o n t o r e m o v e o x y g e n was c o n d u c t e d by p a s s i n g t h r o u g h m o l e c u l a r s i e v e s 5A w h i c h was c o o l e d a t t h e l i q u i d n i t r o g e n t e m p e r a t u r e . Ethy l e n e ( ^ 2 ^ 9 9 . 9 % ) f r o m a c o m m e r c i a l c y l i n d e r was u s e d without treatment. The t o t a l f l o w r a t e o f t h e gas was k e p t c o n s t a n t a t 160(± 2) ml ( N T P ) / m i n and t h e c o m p o s i t i o n o f r e a c t i o n m i x t u r e was v a r i e d by c h a n g i n g t h e c o n c e n t r a t i o n o f h e l i u m as a d i l u e n t . The t r a n s i e n t r e s p o n s e t o a c h a n g e i n t h e c o m p o s i t i o n o f He-N^ m i x t u r e was comp l e t e d w i t h i n 15 s e c . The i n t r a p a r t i c l e d i f f u s i o n r e s i s t a n c e was c o n f i r m e d t o be n e g l i g i b l e by e x a m i n i n g t h e r a t e d a t a f o r c a t a l y s t s o f d i f f e r e n t s i z e s , 12-14 mesh and 20-42 mesh a t 91°C. The e x t e r n a l mass t r a n s p o r t e f f e c t was a l s o f o u n d t o be n e g l i g i b l e a t 123°C by e x a m i n i n g t h e r a t e d a t a a t c o n s t a n t W/F w i t h v a r i o u s f l o w r a t e s and c a t a l y s t a m o u n t s . The r e a c t i o n c o n d i t i o n s w e r e c h o s e n i n s u c h a way t h a t t h e t o t a l c o n v e r s i o n o f e t h y l e n e d i d n o t e x c e e d 0.08 i n a l l e x p e r i m e n t s . I n t h i s c o n d i t i o n , i t was c o n f i r m e d , by a n a l y s i n g t h e gas a t f i v e p o s i t i o n s a l o n g t h e r e a c t o r l e n g t h d u r i n g t h e r e a c t i o n , t h a t t h e c o n c e n t r a t i o n o f p r o d u c t s (C^HÔ and CO^) i n c r e a s e d l i n e a r l y f r o m t h e e n t r a n c e o f t h e c a t a l y s t bed t o t h e e x i t . 2

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Three gas c h r o r n a t o g r a p h e s , each k e p t under d i f f e r e n t c o n d i t i o n s , were s i m u l t a n e o u s l y used t o a n a l y s e a l l r e a c t i o n g a s c o m p o n e n t s as c o n t i n u o u s l y a s p o s s i ble. In the s p e c i a l case of a very r a p i d response w i t h t h e p e r i o d s h o r t e r t h a n a f e w m i n u t e s , t h e same r e s p o n s e was r e p e a t e d a f e w t i m e s a n d t h e r e s u l t s o b t a i n e d were s u p e r i m p o s e d i n order t o draw a r e s p o n s e c u r v e as c o n t i n u o u s l y as p o s s i b l e . T h r e e f l o w s y s t e m s w i t h d i f f e r e n t gas c o m p o s i t i o n w e r e p r e p a r e d so t h a t t h e t r a n s i e n t r e s p o n s e e x p e r i m e n t s c o u l d be c o m p l e t e d f o r t h r e e d i f f e r e n t g a s m i x t u r e s w i t h i n a few m i n u t e s . A more d e t a i l e d d e s c r i p t i o n o f t r a n s i e n t r e s p o n s e method used i n t h i s s t u d y c a n be f o u n d e l s e w h e r e (6^, 7^,8^) . E x p e r i m e n t a l R e s u l t s and D i s c u s s i o n The r e s p o n s e o f t h e c o m p o n e n t Y i n t h e o u t l e t g a s m i x t u r e t o a s t e p change i n t h e c o n c e n t r a t i o n of X i n the i n l e t g a s s t r e a m i s d e s i g n a t e d a s X-Y r e s p o n s e . The f o l l o w i n g s y m b o l s w i l l be u s e d i w h e n X i s i n c r e a s e d , X ( i n c . , ) - Y ; when X i s d e c r e a s e d , X ( d e c . , ) - Y ; when X i s i n c r e a s e d f r o m n i l , X ( i n c . , 0 ) - Y ; when d e c r e a s e d t o n i l X ( d e c . , 0 ) - Y ; when X i s p u l s e d i n t h e i n l e t g a s s t r e a m , X ( p u l s e ) - Y r e s p o n s e ; when t h e t e m p e r a t u r e o f t h e c a t a l y s t bed i s r a i s e d l i n e a r l y w i t h r e s p e c t t o t h e e l a p s e d time, T ( l i n e a r ) - Y response. Transient

B e h a v i o r of Oxygen

In o r d e r t o a n a l y z e t h e a d s o r p t i o n b e h a v i o r o f c a r b o n d i o x i d e on s i l v e r i t was n e c e s s a r y t o u n d e r s t a n d the a d s o r p t i o n b e h a v i o r o f 0^ a n d i t s r e a c t i v i t y , b e c a u s e t h e a d s o r p t i o n o f CO strongly r e l a t e d t o the ads o r b e d o x y g e n s p e c i e s a s w i l l be d e s c r i b e d l a t e r . For t h i s r e a s o n , t h e f o l l o w i n g t r a n s i e n t e x p e r i m e n t s were p u r f ormed. T o t a l Amount o f A d s o r b e d O x y g e n . The amount o f a d s o r b e d o x y g e n c a n e a s i l y be e s t i m a t e d f r o m 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 °2~°2 P curve. After the c a t a l y s t s u r f a c e h a d c o m p l e t e l y b e e n r e d u c e d by a p u r e h y d r o g e n s t r e a m f o r a b o u t 5 h r s , t h e s t r e a m was s w i t c h e d o v e r t o a p u r e h e l i u m s t r e a m a n d t h e n t h e 0^ ( i n c . , 0 ) - 0 , the 0 (dec.,0)-0 and a g a i n t h e 0 ( i n c . , 0 ) -0 r e s p o n s e s were f o l l o w e d s u c c e s s i v e l y . T h i s p r o c e d u r e was r e p e a t e d a f t e r c h a n g i n g t h e c o n c e n t r a t i o n o f 0 · F i g . l c l e a r l y shows a d e l a y f o r t h e 0 ( i n c . , 0 ) - 0 r e s p o n s e ( R u n l ) a t f i r s t t i m e a n d shows an i n s t a n t a n e ous r e s p o n s e f o r b o t h t h e s u c c e s s i v e 0 ( d e c . , 0 ) - 0 r

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(Run 2) and 0 ( i n c . , 0 ) - 0 (Run 3) r e s p o n s e s a t s e c o n d time. T h e s e r e s u l t s i n d i c a t e d an i r r e v e r s i b l e a d s o r p t i o n o f 0^ and no a p p r e c i a b l e a d s o r p t i o n o f o x y g e n on the o x i d i z e d s u r f a c e . The i n t e g r a t e d amount o f t h e ^ o x y g e n w i t h Run 1 was e s t i m a t e d t o ^ b e 2 . 6 ( + 0 . 1 ) x l 0 mol/g-Ag (1.04(±0.04)xlO atoms/m ) no d e p e n d e n c e on t h e c o n c e n t r a t i o n o f 0^. The a d s o r p t i o n s t r e n g t h o f t h e a d s o r b e d o x y g e n c a n be d e t e r m i n e d by a t h e r m a l d e s o r p t i o n t e c h n i q u e . A f t e r t h e c a t a l y s t s u r f a c e had c o m p l e t e l y b e e n o x i d i z e d i n t h e 0 ( 2 0 % ) - H e m i x t u r e a t 91°C, t h e r e a c t o r was f l u s h e d w i t h a p u r e He s t r e a m and t e m p e r a t u r e o f c a t a l y s t bed was t h e n r a i s e d l i n e a r l y a t a r a t e o f 4.3°C/min f r o m 50 t o 280°C. Thus t h e T ( l i n e a r ) - 0 r e s p o n s e was f o l l o w ed. F i g . 2 shows t h a t t h e d e s o r p t i o n s p e c t r u m o f o x y g e n h a s a p e a k a t 250°C. The i n t e g r a t e d amount o f d e s o r b e d o x y g e n was e s t i m a t e d t o be 2 . 5 ( + 0 . 1 ) x l 0 mol/g-Ag (1.00(±0.04) x l O atoms/m ) , w h i c h i s v e r y c l o s e t o t h e v a l u e e s t i m a t e d from the 0 ^ ( i n c . , 0 ) - 0 ^ r e s p o n s e . From t h e s e r e s u l t s , i t was a p p a r e n t t h a t a l l t h e o x y g e n s p e c i e s a d s o r b e d a t 91°C w e r e d e s o r b e d a t a r o u n d 250°C. S c h o l t e n , K o n v a l i n k a and Beekman (9_) c a l c u l a t e d t h a t t h e t o t a l number o f s u r f a c e s i l v e r a t o m s was 1 . 3 1 x l 0 a t o m s p e r m2 b a s e d on S a n d q u i s t ' s d a t a ( 10), and t h e i r e x p e r i m e n t s on o x y g e n a d s o r p t i o n g a v e t h e number o f a d s o r b e d o x y g e n a t o m s t o be a b o u t 0 . 7 x l 0 a t 50°C and about 1 . 2 x l 0 atoms/m2 a t 100°C. Thus t h e t o t a l amount o f a d s o r b e d o x y g e n o b t a i n e d i n t h i s s t u d y f e l l w i t h i n S c h o l t e n e t a l s r a n g e and c o r r e s p o n d s t o 0.8 o f the s u r f a c e coverage e x p e c t e d f o r a monoatomic form of adsorbed oxygen. The s u r f a c e r e d u c e d by was a c t i v e f o r t h e d e c o m p o s i t i o n o f ^ 0 a t 91°C. A f t e r t h e c a t a l y s t had b e e n r e d u c e d by t h e H^ s t r e a m f o r 5 h r s a t 91°C, the r e a c t o r was f l u s h e d by t h e He s t r e a m and t h e n t h e NÔ (inc.,0)-N r e s p o n s e (Run 1) was f o l l o w e d . Run 1 i n F i g . 3 showed a t y p i c a l o v e r s h o o t t y p e r e s p o n s e o f w i t h an i n s t a n t a n e o u s maximum. T h i s i n s t a n t a n e o u s mode i n d i c a t e d the d i r e c t d e c o m p o s i t i o n of gaseous N 0 on a c t i v e s i t e s : t h i s i s a l s o s u g g e s t e d by t h e N o f d e c . , 0 ) -N 0 r e s p o n s e (Run 2) i n F i g . 3 w h i c h e x h i b i t e d no d e l a y i n d i c a t i n g no a d s o r p t i o n o f N 0 . The g r a p h i c a l i n t e g r a t i o n of the N 0 ( i n c . , 0 ) - N response curve c o r r e s p o n ded t o t h e amount o f a d s o r b e d o x y g e n a t o m s and was e s t i m a t e d t o be 2.2x10 mol/g-Ag (0.90(±0.08)xlO atoms/m2). T h i s i s c l o s e to the v a l u e 0 . 8 0 x l 0 which i s roughly e s t i m a t e d f r o m t h e f i g u r e g i v e n by S c h o l t e n e t a l . The d i f f e r e n c e b e t w e e n t h e amount o f a d s o r b e d o x y g e n f o u n d o x y g e n a d s o r p t i o n and as c o m p a r e d t o N 0 d e c o m p o s i t i o n may be a t t r i b u t e d t o t h e p r e s e n c e o f 2


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KOBAYASHi

Oxidation

of

Ethylene


9.


215


216

C A T A L Y S I S

(X10") 2U

U N D E R

C O N D I T I O N S

3

— Β (Run 1 )

z

T R A N S I E N T

P > m )P°(atm) N

5.1>

H

-

A

e

1.0 0

Β 0.2 1 4 0.7 8 6

0.8

ο 0.6 û- 0.4 z

Ό

C Ο

q° = 2.2X10' mol/g-Ag dS

6

.0.2 -o—o—o-o _i

ι

10 Figure 3.

ι

20

N 0(inc. 2

ι

0)-N

30 AO Ti me (min ) 2

L

50

and N 0(dec. 0)-N 0 2

2

o-

270 0

10

responses at 91 ° C .


20

Oxidation

KOBAYASHi

9.

of

217

Ethylene

adsorbed oxygen i n a d i a t o m i c form. The amount o f d i a t o m i c o x g e n i n t h e c a s e o f t h e 0^ a d s o r p t i o n was e s t i m a t e d t o be 0 . 4 x l 0 molecules/m , disregarding the r e c o m b i n a t i o n o f atomic oxygen formed d u r i n g t h e NÔ d e c o m p o s i t i o n .


1 9

2

R e a c t i v i t y o f Adsorbed Oxygen. The e x i s t e n c e o f two t y p e s o f a d s o r b e d o x y g e n on s i l v e r , d i a t o m i c a n d m o n o a t o m i c o x y g e n , h a s f r e q u e n t l y . b e e n claimed (1.) · I n t h e p r e s e n t s t u d y t h e c a t a l y t i c a c t i v i t y o f t h e two t y p e s o f a d s o r b e d o x y g e n was a l s o i n v e s t i g a t e d a g a i n using the pulse technique. Before use t h e pulse t e c h n i q u e one s h o u l d examine t h e d e s o r p t i o n b e h a v i o r o f e t h y l e n e oxide because the pulse spectrum of the ethyl e n e o x i d e formed by a r e a c t i o n o f e t h y l e n e w i t h a d s o r bed o x y g e n s p e c i e s i s s t r o n g l y a f f e c t e d b y i t . A f t e r t h e c a t a l y s t h a d b e e n o x i d i z e d i n a n O^-He s t r e a m f o r 5 h r s , t h e s t r e a m was s w i t c h e d o v e r t o e i t h e r a C H -0 -He m i x t u r e ( R u n 1) o r a C H,-He m i x t u r e ( R u n 2 ) a n d t h e C^H^, 0 ( i n c . , 0 )-C and - C 0 and t h e C H ^ ( i n c . , 0 ) - C H . 0 a n d - C 0 r e s p o n s e s w e r e f o l l o w e d s e p a r a t e l y . A l l t h e r e s p o n s e s i n t h e two Runs w e r e o f t h e o v e r s h o o t t y p e a s shown i n F i g . 4 . D u r i n g t h e i n i t i a l s t a g e o f t h e r e s p o n s e s , C H^0 and C0„, i n two Runs r e a c h e d t h e same maximum p o i n t s i r r e s p e c t i v e of whether oxygen e x i s t e d i n t h e gas phase. This r e s u l t s t r o n g l y i n d i c a t e d t h a t adsorbed oxygen n o t g a s e o u s o x y g e n was r e s p o n s i b l e f o r t h e p r o d u c t i o n o f C H ^ 0 a n d C 0 . F u r t h e r m o r e , t h e r e s p o n s e o f C H ^ 0 was o f t h e o v e r s h o o t t y p e w i t h a n i n s t a n t a n e o u s maximum. Such b e h a v i o r p r o b a b l y r e s u l t e d from t h e r a p i d d e s o r p t i o n o f t h e C H^0 p r o d u c e d . T h i s i d e a c a n a l s o be s u p p o r t e d by t h e f o l l o w i n g r e s u l t s . The h e i g h t a t t h e maximum p o i n t o f C ^ H ^ , 0 ( i n c . , 0 ) -C H^0 r e s p o n s e i n F i g . 4 c o r r e s p o n d s t o t h e r e a c t i o n r a t e (r^nô) adsorbed oxygen w i t h C o 4 * The r ^ H ^ . 0 was p l o t t e d a s a f u n c t i o n o f P C H A a n d a good s t r a i g h t l i n e o b t a i n e d a t t e m p e r a t u r e s r a n g i n g f r o m 80 t o 91°C as shown i n F i g . 5 . The A r r h e n i u s p l o t o f t h e a p p a r e n t r a t e c o n s t a n t e s t i m a t e d f r o m t h e s e s t r a i g h t l i n e s was 58.6 k J a s shown i n F i g . 6 . These r e s u l t s s t r o n g l y suggest t h a t C H , 0 is f o r m e d b y t h e r e a c t i o n b e t w e e n gaseous C H ^ and 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 mechanism, and t h a t t h e C H^0 formed d e s o r b s r a p i d l y . I n s u c h a c a s e , p u l s e t e c h n i q u e s c a n be c o n v e n i e n t l y u s e d t o examine t h e r e a c t i v i t y o f t h e a d s o r b e d spec i e s as f o l l o w s . A f t e r t h e c a t a l y s t had been r e d u c e d by a p u r e H s t r e a m a t 160°C f o r 2 h r s , t h e c a t a l y s t bed was f l u s h e d by He a n d t h e n t h e s r e a m c h a n g e d t o a n C H.(12.8%)-He stream. 0 a n d N 0 g a s e s w e r e sépara1

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218

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

Figure 4. Comparison of Run 1, the C H (inc. 0)-C H O and -C0 responses and Run 2, the C H (inc. O)- and 0 (dec. 0)-C H O and -C0 responses at 91 °C. 2

2

h

2

k

2

2

k

k

2

2



KOBAYASHi

Oxidation

of

Ethylene

P H 2 4 c

C

Figure 5.

The rate of C H O 2

h

formation at the maximum point of the C H (inc. C H O response. 2

2

h

h

1 /Τ Figure 6.

(atm)

H

(K)

Arrhenius plot for k H o C2

ll

nMX

at Ρ ° θ2

= 0.2 - 0.5 atm.


O)-

220

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 e l y p u l s e d i n t o the i n l e t stream of t h i s m i x t u r e i n v a r i o u s p u l s e s i z e s . The 0 ( p u l s e ) - C H

Ν> Ν)

to


9.

KOBAYASHi

Oxidation

of Ethylene

223

ϋ ο

Q 05

Catalysis Under Transient Conditions - American Chemical Society

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