Catalysis Under Transient Conditions - American Chemical Society

4 Dynamics of High-Temperature Carbon Monoxide Chemisorption on Platinum-Alumina by FastResponse IR Spectroscopy S. H. OH and L. L. HEGEDUS

1

Downloaded by AUBURN UNIV on March 1, 2016 | http://pubs.acs.org Publication Date: May 1, 1982 | doi: 10.1021/bk-1982-0178.ch004

General Motors Research Laboratories, Warren,MI48090 Since the early work of Langmuir (1), the chemisorption of carbon monoxide on platinum surfaces has been the subject of numerous investigations. Besides its s c i e n t i f i c interest, an understanding of CO chemisorption on Pt i s of considerable p r a c t i c a l importance; for example, the c a t a l y t i c reaction of CO over noble metals (such as Pt) i s an essential part of automobile emission control. There is a wealth of information available on CO chemisorption over s i n g l e - c r y s t a l and polycrystalline platinum surfaces under ultrahigh-vacuum conditions; research efforts i n this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) i s less well understood, due primarily to the i n a p p l i c a b i l i t y of most surface-sensitive techniques and to the difficulties involved i n characterizing supported metal surfaces. In part i c u l a r , the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. Transmission i n f r a r e d spectroscopy i s one o f the few t e c h niques a p p l i c a b l e to the i n s i t u study of supported c a t a l y s t systems (e.g., 12). Much of the evidence concerning the nature of the adsorbed s t a t e s of CO over supported P t c a t a l y s t s has been obtained from i n f r a r e d s p e c t r o s c o p i c s t u d i e s . F o r example, i t has been shown (13, 14) t h a t CO can chemisorb on supported P t c a t a l y s t s e i t h e r i n a l i n e a r o r i n a bridged form. A t h i g h temperatures, however, the a b s o r p t i o n band due t o the b r i d g e bonded CO disappears (e.g., 15), and thus the l i n e a r form o f adsorbed CO r e p r e s e n t s the dominant s t a t e , g i v i n g r i s e t o a strong,_Yell-defined i n f r a r e d band a t a frequency o f about 2070 cm . T h i s provides a convenient means o f m o n i t o r i n g t h e s u r f a c e c o n c e n t r a t i o n of CO under r e a l i s t i c o p e r a t i n g c o n d i t i o n s by observing the i n t e n s i t y o f the a s s o c i a t e d i n f r a r e d band. 1

Current address: W. R. Grace & Company, Washington Research Center, Columbia, MD 21044. 0097-6156/82/0178-0079$06.25/0 © 1982 A m e r i c a n Chemical Society

In Catalysis Under Transient Conditions; Bell, Alexis T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


80

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

Recently there has been a growing emphasis on the use of t r a n s i e n t methods to study the mechanism and k i n e t i c s of c a t a l y t i c r e a c t i o n s (16, 17, 18). These t r a n s i e n t s t u d i e s gained new impetus w i t h the i n t r o d u c t i o n of computer-controlled c a t a l y t i c converters f o r automobile emission c o n t r o l (19); i n t h i s l a r g e - s c a l e c a t a l y t i c process the composition of the feedstream i s o s c i l l a t e d as a r e s u l t of a feedback c o n t r o l scheme, and the frequency response c h a r a c t e r i s t i c s of the c a t a l y s t appear to p l a y an important r o l e (20). P r e l i m i n a r y s t u d i e s (e.g., 15) i n d i c a t e t h a t the t r a n s i e n t response of these c a t a l y s t s i s dominated by the r e l a x a t i o n of s u r f a c e events, and thus i t i s necessary to use f a s t - r e s p o n s e , s u r f a c e - s e n s i t i v e techniques i n order to understand the c a t a l y s t ' s behavior under t r a n s i e n t conditions. In t h i s paper we w i l l f i r s t d e s c r i b e a fast-response i n f r a r e d r e a c t o r system which i s capable of o p e r a t i n g a t h i g h temperatures and pressures. We w i l l d i s c u s s the r e a c t o r c e l l , the feed system which a l l o w s c o n c e n t r a t i o n step changes or c y c l i n g , and the m o d i f i c a t i o n s necessary f o r c o n v e r t i n g a commercial i n f r a r e d spectrophotometer to a high-speed i n s t r u ment. This m o d i f i e d i n f r a r e d s p e c t r o s c o p i c r e a c t o r system was then used to study the dynamics of CO a d s o r p t i o n and d e s o r p t i o n over a Pt-alumina c a t a l y s t a t 723 Κ (450°C). The measured step responses were analyzed using a t r a n s i e n t model which accounts f o r the k i n e t i c s of CO a d s o r p t i o n and d e s o r p t i o n , e x t r a - and i n t r a p e l l e t d i f f u s i o n r e s i s t a n c e s , s u r f a c e accumulation of CO, and the dynamics of the i n f r a r e d c e l l . F i n a l l y , we w i l l b r i e f l y d i s c u s s some of the t r a n s i e n t response ( i . e . , step and cycled) c h a r a c t e r i s t i c s of the c a t a l y s t under r e a c t i o n c o n d i t i o n s ( i . e . , CO + o ) . 2

Fast-Response I n f r a r e d Reactor System a. Reactor c e l l . The o b j e c t i v e was to c o n s t r u c t a rugged i n f r a r e d c e l l which can be operated a t h i g h temperatures (about 500°C), which has a s m a l l enough volume t o a l l o w f a s t response c h a r a c t e r i s t i c s , and which behaves as a well-mixed r e a c t o r (CSTR). The r e a c t o r body was manufactured of a p a i r of V a r i a n n o n r o t a t a b l e blank Confiât f l a n g e s , according to a suggestion of B e l l (21). These f l a n g e s were machined such that a s m a l l c a v i t y was created f o r placement of a t h i n c a t a l y s t d i s c . A t h i n thermocouple was placed i n d i r e c t contact w i t h the d i s c . The d i s c holder was machined to f i t i n t o the s t a i n l e s s s t e e l f l a n g e i n such a way t h a t i t d i r e c t s the gas to consecu t i v e l y sweep both faces of the c a t a l y s t d i s c , w i t h an expansion volume in-between. T h i s c o n f i g u r a t i o n provided good gas-phase mixing i n the c e l l , thus a l l o w i n g the r e a c t o r to be c h a r a c t e r i z e d as a CSTR. T h i s mode o f i n t e r n a l mixing e l i m i n a t e s the need f o r i n t e r n a l moving p a r t s or e x t e r n a l r e c y c l e loops and pumps.



4.

O H

A N D

H E G E D U S

Carbon

Monoxide

Chemisorption

81

The r e a c t o r assembly was heated by e l e c t r i c h e a t e r s . The maximum o p e r a t i n g temperature i s determined by the window con s t r u c t i o n . Sapphire windows (from EIMAC), brazed i n t o Kovar s l e e v e s , were used; the sleeves were then welded d i r e c t l y i n t o the s t a i n l e s s s t e e l r e a c t o r housing. We found t h a t the c e l l so constructed was capable of t r o u b l e - f r e e , continuous o p e r a t i o n a t 450°C; operations a t somewhat higher temperatures are probably s t i l l p o s s i b l e but were not explored. Sapphire was chosen as a window m a t e r i a l because i t i s i n s e n s i t i v e to water vapor and i s transparent i n tljie wave number range of our i n t e r e s t (about 2400 cm to 2000 cm i n these experiments). Moreover, the thermal expansion c h a r a c t e r i s t i c s of the r e a c t o r were found to match w e l l w i t h those of the window f i x t u r e . The c a t a l y s t was prepared by impregnating γ-alumina (Alon) to i n c i p i e n t wetness u s i n g an aqueous s o l u t i o n of H^CPtCl^). A f t e r impregnation, the powder was d r i e d , and c a l c i n e d i n a i r a t 773 Κ (500°C) f o r 2 h. The i n f r a r e d d i s c was prepared by com p r e s s i n g 0.08 g of the c a t a l y s t powder a t 58 840 N. The prop e r t i e s of the c a t a l y s t d i s c are l i s t e d i n Table I . b. Feed system. F i g u r e 1 shows a f l o w diagram of the feed system. Two feedstreams of d i f f e r i n g compositions were employed; these were a l t e r n a t i v e l y interchanged by f o u r , e l e c t r i c a l l y operated high-speed s o l e n o i d s w i t c h i n g v a l v e s (Automatic Switch Co.). I n c y c l e d experiments, the amplitude i s determined by the c o n c e n t r a t i o n d i f f e r e n c e between the two feedstreams, and the frequency by the v a l v e s w i t c h i n g frequency. An e l e c t r o n i c c l o c k was used to v a r y the s w i t c h i n g frequency i n the range of i n t e r e s t (ranging from s i n g l e steps to about 10 Hz) . The r e a c t o r was preceded by a preheater which had a s u f f i c i e n t l y s m a l l mixing volume so that i t d i d not c o n t r i b u t e s i g n i f i c a n t l y to the d i s p e r s i o n of the feedstream p u l s e s . Steady streams of ^ 0 vapor or SO2 could be introduced before the preheater to simulate automobile exhaust. When ^ 0 was added to the feedstream (we used a l i q u i d chromatographic pump), i t was condensed out before the gas entered the vent l i n e . However, no water- or SO^-containing feedstreams were used i n the examples shown i n t h i s paper. The gas-phase composition before or a f t e r the r e a c t o r was monitored by mass spectroscopy; although no such data a r e shown here, experience i n d i c a t e d the n e c e s s i t y of a f a s t - r e s p o n s e i n l e t system. c. Fast-response IR spectrophotometer. A Perkin-Elmer Model 180 i n f r a r e d spectrophotometer was m o d i f i e d f o r these experiments t o provide the necessary time response. The modi f i c a t i o n s i n c l u d e d the replacement of the o r i g i n a l chopper by a high-speed v a r i e t y (Laser P r e c i s i o n C o r p o r a t i o n , 15-1000 H z ) , and r e d i r e c t i n g the i n f r a r e d beam so t h a t i t was focused onto a



0.017

Thickness (cm)

130

2

(cm /s)**

^Determined by a CO f l o w - c h e m i s o r p t i o n technique.

. (A) micro

err

3

(g/cm )

0.586

0.0246

1.52

**Computed from the random pore model of Wakao and Smith (22) and g i v e n here a t 101.3 kPa and 723 Κ.

r

2.0

(A)

r

3

211 150

ε Ρ Diameter (cm)

0.346

oo, D - » oo. m

2

eff

2

m

m

eff

eff

1.0 r 0.8 -

,l

ι

ι

»

0

5

10

15

Time(s) Figure 10. Effects of internal and external transport resistances on the computed step-response of CO desorption. Curve A corresponds to our experimental condi tions. Key: A,k — 60 cm/s, D — 0.0246 cm /s; B, k - » oo, D = 0.0246 cm /s; and C, k -> oo, D -> oo. m

2

eff

2

m

m

eff

eff


96

C A T A L Y S I S

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T R A N S I E N T

C O N D I T I O N S

where (36/3 ) represents the slope of the a d s o r p t i o n isotherm. The q u a n t i t y l N a (30/3 )] i n Equation (15) accounts f o r the c a p a c i t y of the c a t a l y t i c surface f o r the chemisorption of CO. For porous c a t a l y s t p e l l e t s w i t h p r a c t i c a l l o a d i n g s , t h i s quan t i t y i s t y p i c a l l y much l a r g e r than the p e l l e t v o i d f r a c t i o n , i n d i c a t i n g t h a t the dynamic behavior of supported c a t a l y s t s i l dominated by the r e l a x a t i o n of s u r f a c e phenomena (e.g., 36). T h i s i m p l i e s that a q u a s i - s t a t i c approximation f o r Equation (1) (i.e., ΟΥ = 0) can o f t e n be s a f e l y invoked i n the t r a n s i e n t modeling of porous c a t a l y s t p e l l e t s . The c a l c u l a t i o n s showed t h a t the q u a s i - s t a t i c approximation i s indeed v a l i d i n our case; the model p r e d i c t e d v i r t u a l l y the same step responses, even when the v a l u e of was reduced by a f a c t o r of 10. I t f o l l o w s d i r e c t l y from Equation (15) that the c h a r a c t e r i s t i c response time f o r the i n t r a p e l l e t gas-phase c o n c e n t r a t i o n during the a d s o r p t i o n - d e s o r p t i o n process i s approximated by [ ( L / D ) N a (30/3c)]. (Note that t h i s c h a r a c t e r i s t i c r e s ponse time represents the product of the c h a r a c t e r i s t i c time f o r i n t r a p e l l e t d i f f u s i o n , L / D f f , and the s u r f a c e c a p a c i t y f o r CO chemisorption, Ν a (30/3c).) The e s t i m a t i o n of the response time i s complicated by the f a c t that the slope of the a d s o r p t i o n isotherm (30/3c) v a r i e s w i t h gas-phase CO c o n c e n t r a t i o n ; how ever, when i t s average v a l u e over the CO c o n c e n t r a t i o n range of 0 to 1 v o l % was taken to be 5x10° cm /mol (= 1 ( v o l see F i g u r e 4a), the c h a r a c t e r i s t i c response time was c a l c u l a t e d to be 0.75 s. I n s p e c t i o n of the time v a r i a t i o n of the i n t r a p e l l e t gasphase concentrations shown i n F i g u r e s 7 and 8 ( s o l i d l i n e s ) lends support to our estimated response time; that i s , the i n t r a p e l l e t gas-phase c o n c e n t r a t i o n r e l a x e s on the time s c a l e of 1 s during both the a d s o r p t i o n and d e s o r p t i o n processes. In c o n t r a s t to the i n t r a p e l l e t gas-phase c o n c e n t r a t i o n , the s u r f a c e c o n c e n t r a t i o n of CO i n the p e l l e t r e l a x e s at s i g n i f i c a n t l y d i f f e r e n t r a t e s , depending on the d i r e c t i o n of the step changes. As shown e a r l i e r , the surface c o n c e n t r a t i o n of CO decays much more s l o w l y during CO d e s o r p t i o n than i t i n creases during CO a d s o r p t i o n (compare F i g u r e s 5 and 6 ) . T h i s i s , however, merely due to the f a c t that c l o s e to a d s o r p t i o n e q u i l i b r i u m , the s u r f a c e coverage of CO can be high even at low gas-phase CO c o n c e n t r a t i o n s , as shown i n F i g u r e 4a. c

g

c


ε

2

e f f

g

e

b. R e a c t i v e experiments. In a d d i t i o n to the chemisorp t i o n s t u d i e s described above, we a l s o conducted t r a n s i e n t i n f r a r e d experiments under r e a c t i o n c o n d i t i o n s ( i . e . , CO + 0^ over Ρt-alumina). The r e s u l t s of these r e a c t i v e experiments, though not yet analyzed i n q u a n t i t a t i v e d e t a i l , w i l l be never t h e l e s s shown here ( i n the transmittance mode) because they i l l u s t r a t e some i n t e r e s t i n g f e a t u r e s of the t r a n s i e n t response of the C0/0 /Pt system. 9


4.

O H

A N D

Carbon

H E G E D U S

Monoxide

Chemisorption

97

F i g u r e 11 shows how 0 pretreatment of the c a t a l y s t i n f l u ences the step response during CO a d s o r p t i o n . P r i o r to t h i s experiment, the c a t a l y s t was f i r s t p r e t r e a t e d i n 0.7 v o l % 0 ( i n N ) f o r 5 min and then f l u s h e d w i t h N f o r 5 min ( a l l at 723 K j . A f t e r t h i s N f l u s h , the c a t a l y s t was suddenly exposed to a step flow of 1 v o l % CO ( i n N ) . As shown i n F i g u r e 11, the growth of the 2070 cm Pt-CO band i s s i g n i f i c a n t l y delayed a f t e r 0 pretreatment (Curve Β ) , i n d i c a t i n g t h a t a t the e a r l y stages of the step change, the CO on the P t surface i s r a p i d l y consumed by the s t r o n g l y chemisorbed oxygen. The r a t e of CO removal from the Pt surface i s a l s o a f f e c t e d by the presence of 0 i n the gas phase, as demonstrated i n F i g u r e 12. In t h i s experiment the c a t a l y s t , i n i t i a l l y i n e q u i l i b r i u m w i t h 1 v o l % CO ( i n N~), was suddenly exposed to a feedstream of 0.7 v o l % 0 ( i n N ;. I t can be seen from F i g ure 12 that the Pt-CO band decays much f a s t e r i n 0 (Curve B) than i n N (Curve A f o r r e f e r e n c e ) . This i n d i c a t e s that the surface r e a c t i o n between CO and oxygen i s f a s t e r than the r a t e of CO d e s o r p t i o n . F i g u r e 13 shows the time v a r i a t i o n of the s u r f a c e concen t r a t i o n of CO (on the transmittance s c a l e ) during feedstream composition c y c l i n g . C y c l i n g between two feedstreams, one o x i d i z i n g (1 v o l % CO, 1.03 v o l % 0 ) and the other reducing (1 v o l % CO, 0.23 v o l % 0 ) , was accomplished by f o u r f a s t - a c t i n g s o l e n o i d v a l v e s at a s w i t c h i n g frequency of 1 Hz. In the o x i d i z i n g feedstream, the P t s u r f a c e was found to be e s s e n t i a l l y f r e e of CO, w h i l e a s i g n i f i c a n t l y higher CO coverage was ob served over Pt i n e q u i l i b r i u m w i t h the reducing feedstream. I t i s i n t e r e s t i n g to note t h a t , as F i g u r e 13 shows, the s u r f a c e c o n c e n t r a t i o n of CO assumed the same s t a t i o n a r y p a t t e r n a f t e r s e v e r a l t r a n s i t o r y c y c l e s , r e g a r d l e s s of whether the c a t a l y s t was i n i t i a l l y s t a b i l i z e d i n an o x i d i z i n g or i n a reducing feedstream. 2

2

2

2

2

2

-1

2


2

2

2

2

2

2

2

Concluding Remarks A fast-response i n f r a r e d s p e c t r o s c o p i c r e a c t o r system has been described which i s capable of o p e r a t i n g a t high temperatures (e.g., 450-500°C). The i n f r a r e d r e a c t o r system was s u c c e s s f u l l y used to monitor the response of the surface c o n c e n t r a t i o n of CO to step changes or o s c i l l a t i o n s i n the feedstream composition, under both r e a c t i v e and nonreactive c o n d i t i o n s . The dynamics of high-temperature CO a d s o r p t i o n and desorp t i o n over Pt-alumina was analyzed i n d e t a i l using a t r a n s i e n t mathematical model. The model combined the mechanism of CO a d s o r p t i o n and d e s o r p t i o n ( e s t a b l i s h e d from ultrahigh-vacuum s t u d i e s over s i n g l e - c r y s t a l or p o l y c r y s t a l l i n e Pt s u r f a c e s ) w i t h e x t r a - and i n t r a p e l l e t t r a n s p o r t r e s i s t a n c e s . The numerical v a l u e s of the parameters which c h a r a c t e r i z e the s u r f a c e pro cesses were taken from the l i t e r a t u r e of c l e a n s u r f a c e s t u d i e s ;


98

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

CLEAN


Time Figure 11. Effect of 0 pretreatment on the rate of CO adsorption (transmittance mode) at 723 K, 257 cm /s, and 2,070 cm' . Key: A, prereduced Pt; and B, Pt preexposed to 0 and flushed with N . 2

3

1

2

2

CLEAN [Pt-CO]

EQUILIBRIUM

Time

^

Figure 12. Effect of O on the rate of CO removal (transmittance mode) at 723 K, 257 cm Is, and 2,070 cm . Key: A, CO removal by N ; and B, CO removal by 1% 0 . z

3

1

2

2

Time Figure 13. Surface transients over Pt-alumina, starting with an oxidizing feedstream (A) or with a reducing feedstream (B) (transmittance mode) at 723 K, 257 cm /s, 2,070 cm' , and 1 Hz switching frequency. 3

1



4.

O H

A N D

H E G E D U S

Carbon

Monoxide

Chemisorption

99

we found good agreement between the measured and c a l c u l a t e d step responses, i n d i c a t i n g t h a t , a t l e a s t i n t h i s case, i t i s p o s s i ble to apply s u r f a c e chemistry i n f o r m a t i o n to supported c a t a l y s t s o p e r a t i n g a t high temperature and pressure ( 4 5 0 ° C , s l i g h t l y above atmospheric p r e s s u r e ) . Parametric s e n s i t i v i t y a n a l y s i s showed that f o r nonreactive systems, the a d s o r p t i o n e q u i l i b r i u m assumption can be s a f e l y i n voked f o r t r a n s i e n t C O a d s o r p t i o n and d e s o r p t i o n , and that i n t r a p e l l e t d i f f u s i o n r e s i s t a n c e s have a strong i n f l u e n c e on the time s c a l e of the t r a n s i e n t s (they tend to slow down the responses). The l a t t e r o b s e r v a t i o n has important i m p l i c a t i o n s i n the a n a l y s i s of t r a n s i e n t a d s o r p t i o n and d e s o r p t i o n over supported c a t a l y s t s ; that i s , the r e s u l t s of t r a n s i e n t chemisorption s t u d i e s should be viewed w i t h c a u t i o n , i f the e f f e c t s o f i n t r a p e l l e t d i f f u s i o n r e s i s t a n c e s are not p r o p e r l y accounted f o r . Nomenclature 2

3

a

=

l o c a l P t s u r f a c e area, cm

Pt/cm

pellet

A

=

t o t a l e x t e r n a l s u r f a c e area of the c a t a l y s t p e l l e t , cm 2

A Q C

A

G

A

T

c

=

absorbance f o r adsorbed C O

=

absorbance corresponding a complete monolayer coverage of C O

=

C O c o n c e n t r a t i o n i n the i n t r a p e l l e t gas phase, mol/cm^ o r v o l %

C

Q

=

i n i t i a l C O c o n c e n t r a t i o n i n the i n t r a p e l l e t gas phase, mol/cm

c

g

=

s u r f a c e c o n c e n t r a t i o n of C O , mol/cm

c

œ

=

C O c o n c e n t r a t i o n i n the b u l k gas phase of the r e a c t o r c e l l , mol/cm^

c

=

i n i t i a l C O c o n c e n t r a t i o n i n the r e a c t o r c e l l , mol/cm

2

c

C O c o n c e n t r a t i o n a t the i n l e t of the r e a c t o r . 5 c e l l , mol/cm

œ,xn

t D ££ e

Pt

-

f r a c t i o n a l conversion of C O

=

e f f e c t i v e d i f f u s i v i t y of C O i n the c a t a l y s t p e l l e t , cm^/s


100

C A T A L Y S I S

T R A N S I E N T

C O N D I T I O N S

=

a c t i v a t i o n energy f o r d e s o r p t i o n , kJ/mol

=

a c t i v a t i o n energy f o r d e s o r p t i o n e x t r a p o l a t e d to zero s u r f a c e coverage of CO, kJ/mol

F

=

f l u x of CO molecules s t r i k i n g the s u r f a c e , mol/ (s-an P t )

km

=

e x t e r n a l mass t r a n s f e r c o e f f i c i e n t of CO,

L

=

h a l f - t h i c k n e s s of the c a t a l y s t p e l l e t , cm

M

=

molecular weight of CO, g/mol

=

s a t u r a t i o n CO c o n c e n t r a t i o n over the a c t i v e s i t e s , mol/cm P t

Q

=

gas v o l u m e t r i c f l o w r a t e ,

R

=

E^


U N D E R

N

o

g

cm/s

3

cm/s

7 2 2 gas constant, 8.3144x10 g cm / ( s -mol-K) or 8.32x10 kJ/(mol.K) 2 r a t e of CO a d s o r p t i o n , mol/(s*cm P t ) 2 r a t e of CO d e s o r p t i o n , mol/(s*cm P t ) J

R

=

R^

=

ο

r macro or = micro

i n t e g r a l - a v e r aog e rd pore r a d i i>, A

S

=

s t i c k i n g p r o b a b i l i t y f o r CO

=

i n i t i a l s t i c k i n g p r o b a b i l i t y f o r CO

Τ

=

temperature, Κ or °C

TÇQ

=

transmittance f o r adsorbed CO

t

=

time, s

V

=

volume of the r e a c t o r c e l l ,

=

p e l l e t macro- or micropore volumes, cm /g

=

d i s t a n c e from the p e l l e t c e n t e r , cm

S

Q

0

3

V

macro or micro

χ

cm 3

Greek L e t t e r s α

=

parameter d e s c r i b i n g the v a r i a t i o n of the desorp t i o n energy w i t h f r a c t i o n a l s u r f a c e coverage (see Equation 11), kJ/mol


4.

O H

Carbon

Monoxide

Chemisorption

ε Ρ

=

pellet void

Θ

=

f r a c t i o n a l s u r f a c e c o v e r a g e o f CO

Θ

=

integral-averaged CO i n t h e p e l l e t

Q

=

i n i t i a l CO c o v e r a g e

ν

=

pre-exponential t i o n , s""^-

pp

=

3 p e l l e t d e n s i t y , g/cm p e l l e t

p

=

3 p e l l e t s o l i d d e n s i t y , g/cm s o l i d

Q


A N D H E G E D U S

g

fraction

f r a c t i o n a l surface coverage o f

factor

f o r the r a t e o f desorp-

Acknowledgments E. J . S h i n o u s k i s b u i l t t h e i n f r a r e d r e a c t o r c e l l a n d c o n d u c t e d t h e i n f r a r e d r e a c t o r e x p e r i m e n t s . The a u t h o r s a r e i n d e b t e d t o J . A. S e l l f o r t h e m o d i f i c a t i o n s o f t h e i n f r a r e d s p e c t r o p h o t o m e t e r , a n d t o P r o f e s s o r A. T. B e l l f o r s h a r i n g h i s e x p e r i e n c e w i t h i n f r a r e d r e a c t o r c o n s t r u c t i o n . The CO c o n v e r s i o n d a t a o f F i g u r e 3 w e r e o b t a i n e d b y J . E. C a r p e n t e r .

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2.

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3.

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4.

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5.

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8.

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9.

101



102

CATALYSIS

U N D E R TRANSIENT

CONDITIONS

10.

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11.

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12.

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13.

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15.

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19.

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20.

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4.

O H

A N D

H E G E D U S

Carbon

Monoxide

Chemisorption

103

27.

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28.

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29.

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30.

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31.

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32. 33.

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34.

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35.

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Received July 28, 1981.


Catalysis Under Transient Conditions - American Chemical Society

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