Catalysis Under Transient Conditions - American Chemical Society

8 Transient and Steady-State Vapor Phase Electrocatalytic Ethylene Epoxidation Voltage, Electrode Surface Area, and Temperature

Effects

M I C H A E L S T O U K I D E S and C O S T A S G . V A Y E N A S

Downloaded by UNIV OF ARIZONA on April 24, 2017 | http://pubs.acs.org Publication Date: May 1, 1982 | doi: 10.1021/bk-1982-0178.ch008

Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA 02139

The catalytic activity and selectivity of polycrystalline silver catalysts used for ethylene epoxidation can be affected significantly by electrochemical O2- pumping. This new phenomenon was studied in the solid electrolyte cell C 2 H 4 ,C 2 H 4 O,CO 2 ,O 2 ,Ag|ZrO 2 (Y 2 O3) |Ag,air at temperatures between 300° and 420° and atmospheric pressure. In the present communication we report on the effects of voltage, temperature and electrode surface area on the transient and steady -state behavior of the system. The change in the rate of C2H4O production can exceed the rate of O2- pumping by a factor of 400 and is proportional to the anodic overvoltage both at steady state and during transients. The catalytic epoxidation of ethylene on silver has been studied extensively over the last thirty years. The literature in this area is very broad and has been reviewed by several authors (1,2,3). In recent years considerable progress has been made to wards a satisfactory understanding of the mechanism of this impor tant and complex catalytic system. Force and Bell have studied the infrared spectra of species adsorbed on silver during ethylene oxidation (4) and examined the relationship of these species to the reaction mechanism (5). Sev eral authors have studied the effects of catalyst support and crystal size on activity and selectivity (6,7). Cant and Hall (8) used 14C to study oxygen exchange between ethylene and ethylene oxide and found that C2H4O oxidation is much slower than direct CO2 formation from ethylene oxidation in agreement with indepen dent kinetic studies (1,6,12). Recent UHV studies have shown con clusively that CO2 is the only product of C2H4 reaction with atom ic oxygen (9) in agreement with previous studies on the interac tion of N2O and ethylene on silver (10). Kinetic measurements in a well mixed reactor have been recently combined with simultaneous in situ measurement of the thermodynamic activity of oxygen on 0097-6156/82/0178-0181$07.00/0 © 1982 American Chemical Society

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


182

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

porous s i l v e r c a t a l y s t f i l m s (11,12) . These r e s u l t s showed that thermodynamic e q u i l i b r i u m i s not g e n e r a l l y e s t a b l i s h e d between gaseous and s u r f a c e oxygen during r e a c t i o n and provided some a d d i t i o n a l evidence f o r the e x i s t e n c e of at l e a s t two a c t i v e adsorbed oxy gen species (12) i n agreement w i t h previous work (2) . However even one type of adsorbed oxygen only s u f f i c e s t o e x p l a i n many of the e x i s t i n g r e s u l t s (5) and there i s c o n s i d e r a b l e u n c e r t a i n t y about the exact nature of s u r f a c e and subsurface oxygen on s i l v e r during r e a c t i o n (2^9). There i s some evidence f o r surface oxide forma t i o n from the work of Seo and Sato (13) who observed a continuous e x o - e l e c t r o n emission during r e a c t i o n which p a r a l l e l s the r a t e of ethylene oxide p r o d u c t i o n . Despite the e x i s t i n g u n c e r t a i n t y about the exact nature of the oxygen species present during r e a c t i o n s e v e r a l d i f f e r e n t ap proaches have been reported which lead t o an i n c r e a s e i n the s e l e c t i v i t y t o ethylene oxide. These i n c l u d e a l l o y i n g s i l v e r w i t h other metals (14) and u s i n g d i f f e r e n t c a t a l y s t supports and v a r ious promoters (6,15). I t i s a l s o known and i n d u s t r i a l l y proven t h a t the a d d i t i o n of few PPM of c h l o r i n a t e d hydrocarbon "moderat o r s " t o the gas feed improves the s e l e c t i v i t y to ethylene oxide but decreases the c a t a l y s t a c t i v i t y (15). I t has a l s o been found r e c e n t l y by Carberry et a l that s e l e c t i v i t y i n c r e a s e s w i t h γ-irr a d i a t i o n of the c a t a l y s t (16). I n a recent communication a new e l e c t r o c h e m i c a l technique was described f o r a l t e r i n g the i n t r i n s i c a c t i v i t y and s e l e c t i v i t y of s i l v e r f o r ethylene e p o x i d a t i o n (17) . The new technique u t i l i z e s a s t a b i l i z e d z i r c o n i a s o l i d e l e c t r o l y t e c e l l w i t h a porous s i l v e r f i l m e l e c t r o d e exposed t o ethylene/02 mixtures and f u n c t i o n i n g both as an e l e c t r o d e and as a c a t a l y s t f o r ethylene e p o x i d a t i o n ( F i g . 1 ) . When the c i r c u i t i s open the s i l v e r f i l m a c t s as a r e g u l a r c a t a l y s t f o r ethylene e p o x i d a t i o n . When the c i r c u i t i s c l o s e d and an e x t e r n a l v o l t a g e V i s a p p l i e d t o the oxygen concen tration c e l l , 0 i s pumped e l e c t r o c h e m i c a l l y t o the s i l v e r c a t a l y s t through the z i r c o n i a e l e c t r o l y t e . The a c t i v i t y of oxygen species on s i l v e r can thus be a l t e r e d d r a m a t i c a l l y . This was shown t o cause a s i g n i f i c a n t change i n c a t a l y s t a c t i v i t y and s e l e c t i v i t y (17). I t was found that both c a t a l y s t a c t i v i t y and s e l e c t i v i t y i n c r e a s e when 0 i s pumped t o the c a t a l y s t and de crease when 0 i s e l e c t r o c h e m i c a l l y pumped from the c a t a l y s t by r e v e r s i n g the a p p l i e d v o l t a g e . I t was a l s o found that the i n crease i n the r a t e of ethylene oxide p r o d u c t i o n can exceed the r a t e of 0 t r a n s p o r t through the e l e c t r o l y t e by a f a c t o r of 400. This i n c r e a s e v a r i e s l i n e a r l y w i t h a p p l i e d c u r r e n t i f o r low v a l ues of c u r r e n t d e n s i t y ( H >

ο

00

oo

S T O U K I D E S

A N D

V A Y E N A S

Electrο catalytic

Epoxidation


8.

0

2

4

time, h Figure 4. Transient cell voltage response when a constant current i = +700 μΑ is applied at t = 0, Ρ — 0.140 bar, and Ρ τ = 0.008 bar. Conditions: RC 8, i = 100 μΑ, and 400°C. θ2

Ε


190

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

Figure 5. Galvanostatic transients of the rate of ethylene deep oxidation r and of the cell overvoltage. The rate of epoxidation r parallels r . Conditions: i = 100 μΛ, Ρ = 0.14 bar at 400°C. Key: Φ, RC 8, P » 0.008 bar; •, RC 10, PET « 0.0068 bar. 2

t

θ2

2

ET



8.

S T O U K I D E S

A N D V A Y E N A S

Electrocatalytic

Epoxidation

191

CM Ο Ο

Ε I.

Ο

Ήο

200

AV,mV

Figure 6. Overvoltage effect on the increase in the rate of ethylene epoxidation at constant gas phase composition and i = 100 μΑ. Key: O, RC 7, POJPET « 21.0, 400°C; •, RC 8, P /PET « 12.1, 400°C; V, RC 9, PQJPET ~ 7.9, 420°C; and Δ, RC 10, P /PET « 6.8,400°C. 02

02


192

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

α Δν

ΔΓ.

ι

(2)

Equation (2) i s v a l i d both during t r a n s i e n t s as w e l l as a t steady s t a t e i n which case (ΔΓ.)

ι max

Δν

α

max


As noted e a r l i e r the i n c r e a s e i n the r a t e s A r . can exceed the r a t e of oxygen t r a n s p o r t through the e l e c t r o l y t e i/4F by more than two orders of magnitude ( F i g . 7). Gas phase composition e f f e c t s : As shown i n f i g u r e 6 the i n crease i n the r a t e s Δ Γ . depends not only on the overvoltage Δ ν but a l s o on gas phase composition. This has been reported a l s o i n a previous communication where A r . / r . as w e l l as τ were found to i n c r e a s e w i t h decreasing ?^(17^. A s e r i e s of runs performed at constant Ρ showed that Δτ^/τ. i s p r o p o r t i o n a l to P . It was then e s t a b l i s h e d that A r . / r . i s p r o p o r t i o n a l to p / p ^ and that a l l the data, t r a n s i e n t and steady s t a t e , obtained w i t h ten d i f f e r e n t r e a c t o r s , each having d i f f e r e n t e l e c t r o d e s u r f a c e area can be c o r r e l a t e d i n terms of one s u r p r i s i n g l y simple equa tion, 1 0

Q2

Q 2

Δ

Γ

Ϊ

P

—^r.

. Q . x

E T

~

Ρ

ΙΟ

(3)

θ2

where αϊ ,012 ( i = l , 2 ) are This i s shown i n f i g u r e case equation (2) which tion. I t should be noted f i r s t order i n ethylene r

= α.Δν ι

E

k

constants w i t h a r a t i o 011/012= 1.65 + .3. 8. Equation (3) c o n t a i n s as a l i m i t i n g i s v a l i d f o r constant gas phase composi that a t temperatures above 350°C r . and zero order i n oxygen (12), i . e .

P

is

( 4 )

io - io^ ET

I t thus f o l l o w s that equation (3) can be w r i t t e n as Δ Γ . = k. ·α.·Ρ · Δ ν 1

10

1

(5)

Û2

which contains as a l i m i t i n g case the o b s e r v a t i o n t h a t Δ Γ . van ishes when Ρ -Κ), i . e . when the r e a c t o r i s run as a f u e l c e l l (17). Equations ( 3 ; J ( 4 ) and (5) are a l s o v a l i d both d u r i n g t r a n s i e n t s and a t steady s t a t e . Most g a l v a n o s t a t i c t r a n s i e n t s f o l l o w e d a f i r s t order response w i t h reasonable accuracy, i n agreement w i t h (25): Δν = Δν

t/T

max

( j.-e" o)

(6)

Table I I shows the dependence of τ on gas phase composition. For constant s u r f a c e area and c u r r e n t tRe time constant τ i s roughly p r o p o r t i o n a l to Δ ν ·Ρ /Ρ . I t i s a l s o shown t h a t τ max 0 ET ο &

J

r

r

2



02

2

ET

θ2

10

20

Figure 7. Effect of current on the steady-state increase in the rates of epoxidation (rj) and deep oxidation (r ). Comparison with the rate of oxygen transport through the electrolyte G = i/4F. Intrinsic selectivity r /r = 0.49. Conditions: RC 2 at 400°C, P ~ 0.016 bar, Ρ ~ 0.1 bar.


C/3 Η

194


C A T A L Y S I S

U N D E R

ο

T R A N S I E N T

C O N D I T I O N S

/' /

/

oo

/

/

ο

ο/ /ο

L

100

ο/

/ ο

Ο

#

/

/

^

_L 200 AV,mV

300

Figure 8. Overvoltage effect on the rates of ethylene epoxidation (i = 1, O) and deep oxidation (i = 2, %). At constant overvoltage the relative rate increases are proportional to FQJFET at 400°C.


8.

Electwcatalytic

STOUKiDES A N D ν A Y E N A S

Epoxidation

195

i s p r o p o r t i o n a l to A r . / r . i n agreement w i t h equation ( 3 ) . The decrease i n τ %τ with increasing Ρ has been a l s o r e p o r t e d p r e v i o u s l y (l9).° Table I I Gas phase composition e f f e c t on the r e l a x a t i o n time constant τ f o r i=100 A and T=400°C ο Reactor #

Ρ

Q

Û2

/Ρ

r

ET


moles O2

r2/r

r

max mV

i/ io

2 0

τ ο min

RC 8

5.2-10"

7

17.5

300

.64

.35

150

RC 8

5.2-10"

7

12.1

250

.27

.17

75

RC 8

5.2-10"

7

7.9

180

.15

.09

35

RC

10

10·ιο"

6.9

60

.02

.015

20

RC

10

10·10"

20.5

160

.16

.085

130

7

7

C a t a l y s t - E l e c t r o d e Surface Area E f f e c t s : At constant temper a t u r e , gas phase composition and c u r r e n t , t h e r e f o r e constant overv o l t a g e , the r e l a t i v e i n c r e a s e i n the r e a c t i o n r a t e s A r . / r . i s i 10 i n v e r s e l y p r o p o r t i o n a l to the e l e c t r o d e s u r f a c e area. This i s a consequence of equation (3) and i s shown i n f i g u r e 9 f o r f i v e d i f f e r e n t r e a c t o r s . Thus i n order t o maximize the r e l a t i v e e f f e c t of oxygen pumping on r i and r2* i . e . i n order to maximize y i e l d and s e l e c t i v i t y of ethylene oxide one must minimize the e l e c t r o d e s u r face area, i . e . minimize the porous s i l v e r f i l m t h i c k n e s s without causing a major decrease i n i t s c o n d u c t i v i t y . A decrease i n the e l e c t r o d e s u r f a c e area a l s o causes a moderate decrease i n the r e l a x a t i o n time constant τ . This i s shown i n f i g u r e 10 which con t a i n s data from 5 r e a c t o r s obtained at P Q / P ^7 and Τ = 400°C and shows that l/τ i s a l i n e a r i n c r e a s i n g f u n c t i o n of i/4FQ. An ex p l a n a t i o n f o r these observations i s given i n the d i s c u s s i o n s e c t i o n . Figure 11 demonstrates a problem f r e q u e n t l y encountered dur i n g the oxygen pumping experiments. The g a l v a n o s t a t i c t r a n s i e n t was obtained w i t h r e a c t o r RC9 which had the lowest e l e c t r o d e sur face area Q(Table 1 ) , t h e r e f o r e e x h i b i t e d the l a r g e s t r e l a t i v e e f f e c t A r . / r . , i . e . a t h r e e f o l d i n c r e a s e i n ethylene oxide y i e l d and a 210% i n c r e a s e i n CO2 p r o d u c t i o n w i t h a corresponding 20% i n c r e a s i n g i n s e l e c t i v i t y . As shown on the f i g u r e the constant c u r r e n t i = 100 μA was a p p l i e d a t t=o. At t=40 min and long be f o r e the r e a c t o r would reach steady s t a t e the c e l l r e s i s t a n c e sud denly dropped from a few ΚΩ to 1-2 Ohms i n d i c a t i n g e l e c t r o l y s i s of the z i r c o n i a e l e c t r o l y t e and m e t a l l i c z i r c o n i u m formation. This i s p o s s i b l e s i n c e the t o t a l c e l l v o l t a g e (AV+iR ) at t=40 min had reached approximately 2.5 V. As shown on the f i g u r e subse2

c


196

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


10

J* 8

_

6

/

2

/

/

-

ο

JL 0

.2

.4

4FQ,

.6 h

i Figure 9. Effect of A g catalyst-electrode surface area Q on the relative steadystate increase in the rates of epoxidation r and deep oxidation r at constant im posed current i = 100 μΑ, constant gas composition, 400°C, PO /PET ~ 7. Key: t

2

2

O,

r /'ΔΓ,; 10

and

Φ,

r /'ΔΓ . 20

2



STOUKiDES A N D

Τ

c

.15

-

-10

-

Electfοcatalytic

νA Y E NAS

Epoxidation

—

ε Τ

μ

ο ^RC2

.05

RCIO ^ TI-RC2 RC6 > ^ RCII ^ Ç ^ R C 2 Ύ

N

^RC2

0

l

X

K

V

r

C

8

I .05

1 .10

i/4FQ, min" Figure 10.

1 .15

1 .20

1

Effect of A g catalyst-electrode surface area Q on the cell relaxation time constant. Conditions: 400° C, PQJPET — 7.



198

C A T A L Y S I S

0

40

80 time,

U N D E R

T R A N S I E N T

120

C O N D I T I O N S

160

min

Figure 11. Transient galvanostatic response of the rates of ethylene oxidation r (O) and deep oxidation r (%) and of the cell overvoltage Δ F for reactor RC 9. Electrolyte breakdown occurred at 40 min. Conditions: RC 9, 420°C, i — 100 μΑ, Ρ = 0.095 bar, P = 0.012 bar. t

2

θ2

ET


8.

S T O U K I D E S

A N D

V A Y E NAS

Electrο catalytic

Epoxidation

199

quently to the e l e c t r o l y t e breakdown both τχ and r s l o w l y r e t u r n towards t h e i r i n t r i n s i c values r and r o s i n c e e l e c t r o n i c r a t h e r than i o n i c conduction i s now o c c u r i n g across the c e l l . The c e l l cannot f u n c t i o n as an oxygen c o n c e n t r a t i o n c e l l anymore s i n c e the e l e c t r o l y t e has been destroyed; the open c i r c u i t emf Ε always r e mains equal to zero r e g a r d l e s s of the r e a c t o r gas composition. It was thus concluded that t o t a l v o l t a g e s i n excess of 2.5 V should not be a p p l i e d to the c e l l at these r e l a t i v e l y low temperatures. 2

x 0

2

Temperature e f f e c t s : At constant current and gas composition, temperature has an i n t e r e s t i n g e f f e c t on the steady s t a t e r a t e i n creases A r i and A r ( F i g . 12). At temperatures below 350°C both A r i and Δ Γ i n c r e a s e r a p i d l y w i t h an a c t i v a t i o n energy E*=33+3 Kcal/mole. However at temperatures above 350°C hx\ and Δ Γ de crease w i t h an apparent negative a c t i v a t i o n energy E** = 1 9 + 2 Kcal/mole. The overvoltage Δ ν decreased monotonically from 90 mV at 320°C to 3-5 mV at 400°C. I t thus appears that the behavior of Δ ν p a r a l l e l s the behavior of Δ Γ Ι and Δ Γ at h i g h Τ but not a t temp e r a t u r e s below 350°C. A p o s s i b l e e x p l a n a t i o n i s given i n the d i s cussion s e c t i o n . Figure 12 a l s o shows that w i t h i n the accuracy of the experimental data the r a t i o Δ Γ Ι / Δ Γ remains p r a c t i c a l l y constant at 1.75+·25 which corresponds to .64+·04 s e l e c t i v i t y on the oxide. 2


2

2

2

2

Discussion The nature of the s i l v e r c a t a l y s t i s c l e a r l y a l t e r e d s i g n i f i c a n t l y during e l e c t r o c h e m i c a l oxygen pumping. The i n c r e a s e or decrease i n the r a t e s of ethylene e p o x i d a t i o n and combustion i s more than two orders of magnitude higher than the r a t e of oxygen t r a n s p o r t through the e l e c t r o l y t e . The s e l e c t i v i t y a l s o changes considerably. When oxygen i s pumped to the c a t a l y s t the a c t i v i t y of oxygen on the s i l v e r c a t a l y s t - e l e c t r o d e i n c r e a s e s c o n s i d e r a b l y because of the a p p l i e d v o l t a g e . I t thus becomes p o s s i b l e to at l e a s t p a r t l y o x i d i z e the s i l v e r c a t a l y s t e l e c t r o d e . I n a previous communica t i o n i t has been shown that the phenomenon i n v o l v e s surface r a t h e r than bulk o x i d a t i o n of the s i l v e r c r y s t a l l i t e s (17). The present r e s u l t s e s t a b l i s h the d i r e c t dependence of the change i n the r a t e s of e p o x i d a t i o n and combustion Δ Γ Ι and Δ Γ on the c e l l overvoltage (Equations 2,3, and 5) which i s d i r e c t l y r e l a t e d to the s u r f a c e oxygen a c t i v i t y . Furthermore, the o b s e r v a t i o n that the t r a n s i e n t r a t e change behavior p a r a l l e l s the t r a n s i e n t overvoltage behavior (Figures 5, 6 and equations 2,3 and 5) proves undoubtedly that the i n c r e a s e or decrease i n the r a t e s i s caused by a corresponding i n c r e a s e or decrease i n the s u r f a c e c o n c e n t r a t i o n of an adsorbed oxygen spec ies or s u r f a c e s i l v e r oxide. I n a previous communication we have denoted t h i s unknown s u r f a c e oxygen species by AgO* w i t h a con c e n t r a t i o n c(moles/cm ). There i s previous evidence i n the l i t e r 2

2



2

ET

t

θ2

Figure 12. Temperature effect on the steady-state rate increase in Ar (O) and Ar (Φ) at constant current i = 50 μΑ and gas composition. Conditions: Ρ = 0.10 bar, P — 0.015 bar.


2

Ο

H

l-H

§

Ο Ο

H

w

w

Ο

c!

> >

to ο ο

8.

Electwcdtalytic

STOUKiDES A N D V A Y E N A S

201

Epoxidation

ature f o r the e x i s t e n c e of s u r f a c e s i l v e r oxides i n c l u d i n g the work of Seo and Sato (13) who observed a continuous e x o e l e c t r o n emission from s i l v e r c a t a l y s t s during ethylene e p o x i d a t i o n which was p r o p o r t i o n a l to the r a t e of ethylene oxide formation. They ex p l a i n e d t h e i r r e s u l t s i n terms of formation of s u r f a c e s i l v e r oxide Ag 0 w i t h molecular oxygen i o n 0 adsorbed on i t (13). This p i c t u r e seems to be i n e x c e l l e n t agreement w i t h the present r e s u l t s , i . e . w i t h equation (5) 2

2

Δ Γ . = k. ·α.·Δν·Ρ (5) ι ίο 1 o i f the change i n the amount of s u r f a c e s i l v e r oxide i s p r o p o r t i o n a l to Δ ν and the coverage of molecular oxygen adsorbed on the oxide i s p r o p o r t i o n a l to P , or at l e a s t i f the r a t e s of e p o x i d a t i o n and combustion on the s i l v e r oxide are f i r s t order i n oxygen. Equations ( 2 ) , (3) and (5) which d e s c r i b e the dependence of Δ Γ . on the overvoltage are extremely simple but deserve some speci a l a t t e n t i o n . They a l l i n d i c a t e t h a t Δ Γ . i s p r o p o r t i o n a l to Δ ν but independent of the s u r f a c e area of s i l v e r e l e c t r o d e . This i s why the r e l a t i v e i n c r e a s e i n the r a t e s Δ Γ . / Γ . i s highest f o r r e a c t o r s w i t h s m a l l s u r f a c e area ( F i g . 11) and Becomes i n s i g n i f i c a n t f o r r e a c t o r s w i t h l a r g e s u r f a c e area. I t thus appears that a l though Δ ν i s an i n t e n s i v e v a r i a b l e i t i s n e v e r t h e l e s s a measure of the chang S'hc i n the mole number of s i l v e r oxide and not of the c o n c e n t r a t i o n change Ac. This i s because a l l the r e a c t o r s used had the same e l e c t r o l y t e area A = 2cm as e x p l a i n e d i n d e t a i l be low: N e g l e c t i n g the double l a y e r capacitance of the e l e c t r o d e e l e c t r o l y t e i n t e r f a c e the constant c u r r e n t i imposed d u r i n g the t r a n s i e n t can be s p l i t i n two p a r t s (25):


2

Q

1

2

1

ad

dt

+

X

CTR

U

)

where 1 > i s the c u r r e n t corresponding to the charge t r a n s f e r r e actions, i . e . ητΓτ

2

0" 2

0~->

+ C H *Products + 2e~ 2

lr

1/2 0 ( g ) + 2e" 2

(8) (9)

and C , i s the a d s o r p t i o n pseudo capacitance of the oxide which by d e f i n i t i o n equals (25):

where q i s the charge s t o r e d i n the oxide and λ i s a constant. I f one assumes that Sc v a r i e s l i n e a r l y w i t h V, i . e . t h a t the oxide pseudo capacitance i s constant one o b t a i n s c

=x- A-

K 2

2

S

P

' ' ET'

C

v

( 1 6 )

( 1 7 )

i . e . the model assumed t h a t oxide can form only on s i t e s covered o r i g i n a l l y w i t h molecular O 2 . According to t h i s equation the r e l a x a t i o n time constant of the system τ = τ i s given by 1

4 F S C

M

K 2 P

1

ET °

°

~c ~~ ~ ^ ν * 7 ° c O2 O2 O2 O2 Equation (18) i s i n e x c e l l e n t agreement w i t h f i g u r e 10 which shows l/τ f o r 5 d i f f e r e n t r e a c t o r s to be a l i n e a r i n c r e a s i n g func t i o n of w i t h a p o s i t i v e i n t e r c e p t . One of the i m p l i c a t i o n s i s that the amount of s u r f a c e oxide at f u l l coverage Sc^ i s comp a r a b l e t o the r e a c t i v e oxygen uptake Q of the c a t a l y s t . Temperature e f f e c t : There are s e v e r a l ways to account f o r the observed maximum i n Ar. at constant gas composition and imposed current i ( F i g . 12). Although the behavior shown i n the f i g u r e i s q u i t e r e p r o d u c i b l e , f u r t h e r experimental work i s r e q u i r e d to examine the e f f e c t of gas phase composition and i on the temper ature dependence of Ar.. One p o s s i b l e e x p l a n a t i o n f o r the ob served maximum can be obtained by c l o s e r examination of equation (7). The charge t r a n s f e r c u r r e n t i can be w r i t t e n as i^r™ ^+ i"CTR ^ where the former term corresponds to the charge t r a n s f e r r e a c t i o n (8) and the l a t t e r t o the charge t r a n s f e r r e a c t i o n ( 9 ) . Using the low f i e l d law (25) one o b t a i n s (

1

+

^CTR,A W

)

V

= i

Ï

(

4

T

F

0 )

B'

/ R T

( 1 + K

2v


( 1 8 )

( 1 9 >

( 2 0 )

204

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

The exchange current d e n s i t i e s i ^ ^ and i ^ a r e temperature dependent and can be expressed as β

9

VA

•

VB

=

K

E

O , A

VB

X

P ( "

e x p (

9

=

1

(25)

Combining equations (14),(24) and (25) one obtains -E*+AG Φ ^>Λ -Α.Ρ >1> ^χρ(-^—) ο

θ 2

Κ Τ

Γ ±

" 1 ^ , and from the c a t a l y s t r e s p e c t i v e l y . The use of i n s i t u s u r f a c e s c i ence techniques should prove very u s e f u l f o r the e l u c i d a t i o n of the exact nature of t h i s s u r f a c e oxide. The present study was l i m i t e d to temperatures above 300°C be cause of the severe i n c r e a s e i n the e l e c t r o l y t e r e s i s t a n c e w i t h decreasing temperature. Thinner (

Catalysis Under Transient Conditions - American Chemical Society

Recommend Documents