1377

THE NATURE OF ACTIVE OXYGEN SPECIES IN COPPER VANADATE CATALYST

Satohiro YOSHIDA, Akio UEDA and Kimio TARAMA

Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan

ABSTRACT: The reactivity of oxygen ions of copper orthovanadate has been investigated by studies of heterophase oxygen exchange reac-

tion, reduction by CO and CO oxidation over the vanadate. The re-

activity has been found to be intermediate between that of V205 and

CuO. A part of the lattice oxygen ions in a surface layer about

20 1 thick has been considered to be reactive in the reactions. Auger electron spectroscopy has revealed that copper ions in the

layer are appreciably mobile and the enrichment of the ions on the surface has been observed when the vanadate was treated by oxygen

after pre-reduction at 20OoC.

1. INTRODUCTION

In the studies for development of new catalysts for the reduc-

tion of NO by NH3 at low temperatures, copper orthovanadate

(Cu3V208) was found to be an active catalyst with high selectivity to N2 at 15OoC1). The activity was enhanced profoundly by an oxy-

gen treatment at 2OO0C after a slight pre-reduction of the vana-

date. By a study using isotope tracers, the interaction between

surface oxygen species and adsorbed NO and NH3 was clarified and it

was concluded that surface oxygen species play an important role in

the reduction of NO. The present work has been carried out to in-

vestigate the nature of the active oxygen species in the copper

orthovanadate by kinetic studies of heterophase oxygen exchange

reaction and by measurements of the rate of reduction by CO as well

as the activity in CO oxidation.

2. EXPERIMENTAL

Copper orthovanadate (CuV) was prepared from a vanadium oxide

solution and a copper sulfate solution according to the process by Strupler2).

stored. The catalyst thus obtained was evacuated fo r 1 hr and oxidized by oxygen f o r 1 hr at 39OoC, then evacuated at a desired

The vanadate was calcined at 39OoC in air for 1 hr and

1378 S. Yoshida, A . Ueda, K. Tarama

temperature before a reaction was started. This will be referred

to as the virgin CuV. The specific surface area as determined by

the standard BET method was 23 m’g-’.

98 ,84 % as atomic concentration of l80) supplied from Yeda R&I) was

used for the oxygen exchange reactions. For CO oxidation, the gas

diluted by natural O2 was used.

commercial bombs and purified by vacuum distillation. The oxygen

exchange reaction was carried out in a low pressure gas circulating

reactor system with a dead volume of ca. 280 ml. The reaction gas

was analyzed by a quadrupole mass spectrometer (Shimadzu Maspeq

with a secondary electron multiplier). Oxidation of CO was carried

out by the same apparatus. Reduction by CO was followed by meas- urement of the weight loss by means of an electromicro balance

(Cahn RG). For Auger electron spectroscopic analysis, a scanning

Auger electron spectrometer (JOELCO Jamp-3) equipped with an A r ion

bombardment device was used.

Heavy oxygen gas (purity,

Other gases were supplied from

3. RESULTS AND DISCUSSION

3.1. Oxygen exchange reaction

The exchange reaction between gaseous oxygen and lattice oxygen

in the virgin CuV was observed to progress at temperatures higher than 30OoC.

rate was appreciably fast. It has been reported that the exchange

reaction progresses with a measurable rate at temperatures below

30G°C over copper oxide (CUO)~) but scarcely at 37OoC over vanadium

oxide ( V 2 0 5 ) 4 ) . in the lattice is sufficiently fast compared with the rate of ex- change at temperatures, e.g. 5OO0C where the exchange proceeds with

a measurable rate; over CuO the reactivity of surface oxygen ions

is so large that only oxygen ions on the surface participate in the

reaction at temperatures below 3OOoC.

intermediate between that of V205 and C u O , the ions in a surface

layer would participate in the reaction. I n order to obtain about the diffusion rate in the bulk phase and the number of oxygen ions

which exchange rapidly compared with the bulk diffusion, kinetic

studies were carried out. The results were analyzed by the method

described by Teichner et They treated the exchange reaction

between C 0 2 and oxygen ions in V205 where the exchange of oxygen ions on the surface proceeded relatively fast.

The rate at 3OO0C was very slow, while at 38OoC the

In the case of V205, the djffusion of oxygen ions

As the rate over CuV was

Figure 1 shows the fitness of equation (1) to the reactions over

N a t u r e of A c t i v e Oxygen Species i n Copper V a n a d a t e C a t a l y s t 1379

w h e r e at is t h e concen- t r a t i o n o f l80 i n oxygen gas a t t i m e t a n d D is t h e d i f f u s i o n con- s t a n t . O t h e r s y m b o l s s h o u l d b e r e f e r r e d t o t h e r e f e r e n c e ( 5 ) . The e q u a t i o n was d e r i v e d on t h e a s s u m p t i o n t h a t t h e e q u i l i b r a t i o n be tween

4

h

c\1

0

4J3

v E: 3

2

g a s e o u s oxygen and t h e I n . . . . . . . . I s u r f a c e oxygen w a s a t - 0 2 4 6 8 10

t a i n e d i n s t a n t l y . ff (h*)

The d e v i a t i o n f rom t h e Fig. 1. The p l o t s of c o n c e n t r a t i o n o f "0 i n g a s p h a s e a c c o r d i n g t o e q . ( 1 ) f o r oxygen e x c h a n g e r e a c t i o n o v e r t h e v i r g i n straight line shows that

t h e e q u i l i b r a t i o n was cuv . = 5 T o r r ; 0 , 38OoC; , 3 6 O o C ; c) , 3 4 0 O C ;

po2 n o t a t t a i n e d i n t h e e a r l y s t a g e o f t h e re- a c t i o n . A t 38OoC, a s t r a i g h t l i n e was ob- t a i n e d i n t h e l a te r s t a g e t h a n 25 h r a f t e r t h e start o f t h e r e a c t i o n . From t h e s l o p e of t h e l i n e a d i f f u s i o n c o n s t a n t was c a l c u l a t e d a n d by e x t r a p o l a t i n g t h e l i n e t o t = O , t h e v a l u e o f a,,

was o b t a i n e d . The v a l u e /

o f a, a l lows u s t o ca l - c u l a t e t h e number of oxygen i o n s ( n s ) which e x c h a n g e r a p i d l y on t h e s u r f a c e . On t h e o t h e r h a n d , t h e c o n c e n t r a t i o n t h e c a l c u l a t e d c o n c e n t r a t i o n a t t h e of i n t h e s u r f a c e s u r f a c e (Bt) a t 3 8 O o C .

( B t ) was e s t i m a t e d a t a g i v e n t i m e t f rom t h e d a t a a t t h e e a r l y s t a g e . The c h a n g e of Bt is shown i n F i g . 2 a s w e l l a s t h a t o f u t . T h e n , w e c a n o b t a i n t h e

rate c o n s t a n t of t h e e x c h a n g e r e a c t i o n on t h e s u r f a c e .

8 , 3 0 0 ° C . t h e r u n a t 36OoC a n d 200 m g of t h e C u V was u s e d f o r o t h e r r u n s .

283 mg of t h e CuV w a s u s e d for

0 ' 1 1

0 10 20

Time ( h )

F i g . 2 . The t i m e c o u r s e s of t h e concen- t r a t i o n o f l80 i n g a s p h a s e (at) and

where n i s t h e t o t a l number of oxygen atoms in g a s p h a s e , S is t h e g

1.380 s. Yoshida, A . Ueda, K. Tarama

T a b l e 1. K i n e t i c p a r a m e t e r s o f oxygen e x c h a n g e r e a c t i o n s

a ) c a l c u l a t e d f rom t h e i n i t i a l r a te .

s u r f a c e area and k is t h e r a t e c o n s t a n t d e f i n e d as t h e number o f atoms w h i c h e x c h a n g e i n u n i t s u r f a c e area p e r u n i t t i m e .

T a b l e 1 shows t h e v a l u e s o f D , ns a n d k a t 38OoC t h u s o b t a i n e d . A t lower t e m p e r a t u r e s , v e r y l o n g e l a p s e o f t i m e w a s n e c e s s a r y f o r t h e d e t e r m i n a t i o n o f D a n d w e c o u l d n o t o b t a i n e d t h e a c c u r a t e v a l u e s of D , n a n d k f rom t h e e x p e r i m e n t a l d a t a o f 10 d a y s l o n g . However, a p p r o x i m a t e v a l u e s of k c a n b e o b t a i n e d f rom t h e i n i t i a l r a te o f d e c r e a s e i n at. o b t a i n e d f rom t h e i n i t i a l r a t e , w h e r e a s t h e v a l u e o f 3 . 0 ~ 1 0 ~ ~ atoms m-2 h r - l w a s o b t a i n e d f rom t h e a n a l y s i s m e n t i o n e d a b o v e . The a g r e e - ment is s a t i s f a c t o r y , s o t h e v a l u e s o f k a t t e m p e r a t u r e s b e l o w 38OoC were c a l c u l a t e d f rom t h e i n i t i a l ra tes a n d are a l so shown i n T a b l e 1. The t a b l e i n c l u d e s t h e r e p o r t e d v a l u e s o f D and k f o r V205 a n d k f o r CuO.

We m u s t n o t i c e t h a t t h e v a l u e o f D w a s o b t a i n e d f r o m t h e d a t a a t

A t 380°C, t h e v a l u e o f 2 . 8 ~ 1 0 ~ ~ atoms m-’hr-l w a s

l a t e r stage o f t h e r e a c t i o n , t h a t i s , t h e v a l u e c o r r e s p o n d s t o t h e d i f f u s i o n p r o c e s s i n t h e b u l k p h a s e o f CuV a n d t h e v a l u e shows t h a t t h e m o b i l i t y o f oxygen i o n s i n t h e b u l k p h a s e is almost t h e same as t h a t i n V 2 0 5 . Whil.e, t h e v a l u e o f ra te c o n s t a n t o f CuV a t 38OoC is c o m p a r a b l e t o t h a t o f V205 a t 5OO0C a n d t h a t of CuO a t 200°C. a c t i v a t i o n e n e r g y f o r t h e s u r f a c e r e a c t i o n w a s c a l c u l a t e d as 36 Kcal mo1-l- f o r CuV from t h e v a l u e s o f k i n t h e table . T h i s is i n t e r m e d i - a t e b e t w e e n t h a t f o r V205 ( 4 6 K c a l G m ~ l - ~ ) ~ ) a n d t h a t f o r CuO ( 3 0 K ~ a l . m o l - ~ ) ~ ) . r e a c t i v e t h a n t h a t of V205 b u t less r eac t ive t h a n t h a t of CuO.

i n CuV, w h e r e a s t h e number o f s u r f a c e oxygen c a n be e s t i m a t e d as 2 % of t h e t o t a l oxygen f r o m t h e knowledge o f t h e s u r f a c e area a n d

The

T h u s , t h e s u r f a c e oxygen o f t h e v i r g i n CuV w a s more

The v a l u e o f ns c o r r e s p o n d s t o a b o u t 6 .6 % o f t h e t o t a l oxygen

Nature of Active Oxygen Spec ie s i n Copper Vanadate Catalyst 1381

the crystal structure8).

surface but also a part of the ions under the surface participated

in the reaction with an equal probability. This suggests that a

rapid scramble of oxygen ions occurred in a surface layer at 380°C.

The mobility of the ions will be dicussed later.

Hence, not only the oxygen ions on the

3.2. Reduction by CO

temperatures of around 15OoC over CuV and the catalytic activity

was closely related to the nature of oxygen ions of CuV. In connec-

tion with the NO + NH3 reaction, it is desirable to investigate the nature of oxygen ions at temperatures of around 150OC.

found that the reduction of CuV by CO and the oxidation of CO by O2

over CuV progressed at temperatures of around 150OC.

to obtain informations on the nature of oxygen species through

analysis of these reactions.

In the previous work, NO + NH3 reaction was found to proceed at

It was

We attempted

The reduction was followed by measuring of the weight loss of

CuV under CO atmosphere (40 Torr). The time courses of the weignt loss normalized to the initial weight are shown in Fig. 3. In the

case of the virgin CuV, it is evident that the reduction proceeded

by two stages at 20OoC.

parabolic law and then the second stage was followed as shown in

Fig. 4 . At lower temperatures, the two stages were also observed

with a long period of the first stage which was beyond the time

scale of Fig. 4 . The amount of oxygen ions which was taken off by the first stage was about 4 % of the total oxygen ions in CuV and it corresponds to the amount of the ions contained in the layer

4 1 thick.

The rate of the first stage obeyed the

However, the fact that the reduction rates obeyed the

0

0 0 rl

X

p -1

5 --.

-2 0 10 20 30

Time (min)

Fig. 3. The time courses of the normalized weight l o s s of CuV in the reduction by CO. (a):virgin CuV,1500C;(b):virgin C~V,175~C;(c):virgin CuV,200°C; (d):the oxygen treated CuV after pre-reduced to C U ~ V ~ O ~ . ~ , ~ O O ~ C .

1382 S . Yoshida, A . Ueda, K. Tarama

parabolic law shows that

these ions should be

distributed in a thicker

layer and that the rate

of reduction was con-

trolled by the diffusion

through the layer. The

thickness of the layer

was estimated to be

about 20 A from the in-

formation of Auger

electron spectroscopy

as discussed later.

Using the semi-infinite

nlode~9), we can calcu-

late the diffusion con-

stant D'. The prime

2

0 0 rl

X

P 2 1 1

0 0 2 4 6 8

K (mins)

Fig. 4. The plots of normalized weight loss versus E. (a):virgin CuV,150°C;(b):virgin CuV,175OC; (c):virgin CuV,2000C;(d):the oxygen treated CuV,200°C.

denotes that the diffusion constant is of a different kind from D shown in Table 1. The flux of oxygen ions removing at the surface

is given as follows.

This is the same equation found in the reference (5). Co is the

concentration of the mobile ions in the layer at t=O. Let no and

d be the number oP the mobile oxygen ions and the thickness of the

layer, then C, is given as no/Sd, where S is the surface area. The

f l u x is equal to the rate of the reduction, (l/S)(dn/dt), where n is

the number of mobile oxygen ions in the layer at a given time t.

or its integrated form

is obtained.

Assuming no = 4.5~10~' atoms.g-CuV-' (4 % of the total oxygen ions) and d = 20 8 , the diffusion constants were calculated as 37x

9.3~10-'~ and 2 . 4 ~ 1 0 - l ~ cm2hr-l .at 200°, 175O and 15OoC,

respectively. The values are much larger than that of D in Table 1 .

As mentioned previously, the value of D should be applied to the

dilfusion process in the bulk phase. Hence, there should be a dif-

ferent kind of oxygen ions in the surface layer from the oxygen ions in the bulk phase. The activation energy f o r E ' was obl.ained as

Nature of Active Oxygen Spec ie s in Copper Vanadate Catalyst 1383

22 Kcal.mo1-l.

the activation energy to 38OoC, a value of 2.2~10-lo cm2hr-l is

predicted for D ' at 38OoC. The predicted value is extremely larger

than that of D at 38OoC. The extremely large value of D ' allows us to assume a rapid scrambling of oxygen ions in the surface layer at

38OoC and rationalize a fairly large value of n

Thus, it is reasonably concluded that the fairly mobile oxygen ions

at 2OO0C are the same kind ions as the ions which participated in

the oxygen exchange reactions on the surface at high temperatures.

If it is allowed to assume the constant value of

shown in Table 1.

In the previous work, very active catalysts for NO + NH3 reac-

tion were obtained when Cu3V208 was pre-reduced to the composition

of Cu3V207.75 - 7.8 and then treated by oxygen at 20OoC. treatment will referred to as "the oxygen treatment" hereafter.

X-ray analysis revealed no difference on the structure of the CuV

before and after the pre-reduction and it was found that the weight

of the sample was restored to that of the virgin CuV after the oxygen treatment. Curve d in Fig. 3 shows the time course of

reduction of the oxygen treated CuV at 20OoC. Evidently, the ini-

tial rate was much faster than that of the virgin CuV, showing the formation of very reactive species by the oxygen treatment. Figure

4 shows that the rate obeyed the parabolic law after 1 minute in

this case. The line has the same slope as that of curve c and also

broke at a point where the degree of reduction was the same as that

of the breakpoint of the curve c. This shows that a part of the

mobile oxygen ions was converted to very reactive species. When

the straight line was extrapolated to t=O (dotted line), the inter-

cept gave a value of 0.39 % which corresponds to 1.2 % of the total oxygen ions. The value can be regarded as the amount of the very

reactive species. Thus, by the oxygen treatment, about one-fourth

of the mobile oxygen ions was converted to the very reactive

species.

The

3.3. Oxidation of CO

The mobile oxygen ions mentionedin above section are expected to

play an important role in catalysis in which oxygen molecules parti-

cipate at relatively low temperatures. Oxidation of CO seems to be

suitable for the investigation of the reactivity of the mobile

oxygen ions. The oxidation progressed with a measurable rate at

temperatures above 13OoC over CuV.

of CO conversion at 150OC.

stoichiometric composition of CO and 02.

short induction period was observed, then a rapid reaction was

followed. The steady state reaction proceeded after 10 minutes.

Figure 5 shows the time courses

The gas mixture was comprised of the

Over the virgin CuV, a

1384 S . Yoshida, A . Ueda, K

During the rapid reaction

process, an increase in

the temperature of cata-

lyst bed by 27OC was

observed. This should

be resulted from the

rapid evolution of heat

of reaction. Over the

oxygen treated CuV, the

oxidation progressed

without the induction

period as shown in the

figure. In this case,

an increase in the reac-

tion temperature was

also observed at initial

stage. The initial rate

was almost the same as

Tarama

h

El00

c 0 Ti

m sl aJ 2 50 0

0 u

A " 0 10 20 30

Time (min)

Fig. 5 . The time courses of CO conversion in the oxidation over CuV at 150OC. Initial pressure:PCO=lOO Torr,Po2=50 Torr.

The formed C 0 2 was trapped by a cold trap during the reaction. o :virgin CuV; 0 :the oxygen treated CuV after pre-reduction to Cu V 0

3 2 7.83'

that of the rapid process over the virgin CuV. By the 50 min r u n , the color of catalysts changed from reddish brown to dark brown,

suggesting a slight reduction of the surface.

It seems that a slight reduction occurred during the induction

period in the reaction over the virgin CuV and then the very active

oxygen species were formed hy adsorption of 0 2 . The rapid process

would be the reaction between the very reactive oxygen and CO. On

the oxygen treated CuV, there would be the very reactive oxygen

species from the beginning. To confirm the above assumption, the oxidation of CO by oxygen gas containing heavy oxygen was carried

out over the virgin CuV.

gas was 2 9 . 1 '%. Figure 6 shows the relation of the amount of '*O atoms in the formed CO t o the amount of oxidized CO, The dotted

line is the expected one if no lattice oxygen ions of CuV partici- patein the reaction. After 0.1 mmol of CO was oxidized (CO conver-

sion; 13 $) , the line was linear but in the e a r l y stage of the oxi- dation the concentration of "0 in the formed C02 was less than that

at the steady state. The fact shows that the oxidation proceeded

by the redox mechanism and the intercept at abscissa could be re-

garded as the amount of oxygen ions which were initially removed by

CO from CuV.

about 2 % of the total oxygen ions in CuV. The value is in satis-

factory agreement with that of the mobile oxygen ions mentioned in

the previous section. From the straight line in the figure, a ratio

of (mol of C180160 + 2C1'0 )/(mol of the oxidized CO) in the gas

The concentration of l80 in the oxygen

2

The value was 2.2~10~' atoms-g-'. corresponding to

2

Nature of Active Oxygen Species in Copper Vanadate Catalyst 1385

0 1 2 3 4

Amount of oxidized CO (10-4m01)

Fig. 6. The relation of amount of l80 in C02 to the amount of oxidize 29.1 % of "0. Cat:virgin CuV 100 mg;initial press.:Po2f25 Torr,P~0=50 Torr. The dotted line is the expected amount if no lattice oxygen ions in the CuV participate in the reaction.

CO in the oxidation at 15OoC by 0 2 containing

phase was obtained as 28.2 %. The value is a little less than the

concentration of l80 in the gaseous oxygen. Thus, a part of ad-

sorbed oxygen should diffuse slowly into the inner phase of C u V .

3.4. Auger electron spectroscopic analysis

From the results mentioned above, it is concluded that the oxygen

ions in a surface layer of CuV are fairly mobile and they partici- pate in the CO oxidation. Furthermore, some of those are converted

to very reactive species by the oxygen treatment. The movement of

oxygen ions in the layer would be accompanied by the movement of

metal ions. In order to investigate the composition change in the

layer by the reduction or the oxygen treatment, Auger electron

spectroscopic analysis was carried out. For the analysis of the

composition change in the depth, some surface layers were sputtered

by argon ion bombardment successively.

It was found that there was no composition change to the depth

of 30 in the virgin CuV. By the treatments, the concentration of copper ions in the region was changed profoundly, while that of vanadium ions did not vary significantly. Figure 7 shows the change

of atomic ratio of Cu/V as a function of the depth from the surface

by the treatments. Evidently, copper ions moved into inner phase

by a slight reduction and interestingly, the concentration of copper

ions on the surface became quite large when the pre-reduced CuV was

treated by O2 at 2OOoC.

creased profoundly in the region 5 - 20 deep. This shows that

the copper ions moved to the surface from a thin layer underneath

On the other hand, the concentration de-

1386 S. Yoshida. A. Ueda, K. Tarama

t h e s u r f a c e by t h e oxy- gen t r e a t m e n t . I t is a r e a s o n a b l e assumpt ion t h a t t h e l a y e r i n which t h e coppe r i o n s are mobi le c o r r e s p o n d s t o t h e l a y e r i n which t h e mob i l e oxygen i o n s e x i s t

L- u 3 s

1.5

1 . 0

The t h i c k n e s s o f t h e l a y e r is e s t i m a t e d as

20 - 25 1. The e n r i c h - 0 10 20 30 0 . 5

ment of coppe r i o n s on Depth (1) t h e s u r f a c e bv t h e oxv-

F i g . 7 . The change o f compos i t ion i n a gen treatment be s u r f a c e l a v e r o f CuV bv v a r i o u s treat- a s s o c i a t e d w i t h t h e men t s . Abscissa d e n o t e s t h e t h i c k n e s s

s p u t t e r e d by A r i o n bombardment. 0 : v i r g i n CuV; 0 : r educed CuV(Cu3V207 8); * : r e d u c e d CuV(Cu3V207 .5 ) ;U: the oxygen t r e a t e d CuV a f t e r p r e - r e d u c t i o n t o

formation Of very reac- t i v e oxygen s p e c i e s .

cu3v207.8 .

In c o n c l u s i o n , t h e r e are mob i l e oxygen i o n s i n a s u r f a c e l a y e r of CuV. The oxygen i o n s can p a r t i c i p a t e i n oxygen exchange reaction and CO o x i d a t i o n . The t h i c k n e s s of t h e l a y e r is e s t i m a t e d as about 20 i. v e r y r e a c t i v e s p e c i e s by t h e oxygen t r e a t m e n t a f t e r p r e - r e d u c t i o n . The coppe r i o n s i n t h e s u r f a c e l a y e r are c o n s i d e r a b l y mob i l e and t h e en r i chmen t o f t h e i o n s is caused by t h e oxygen t r e a t m e n t . I n t h e r e d u c t i o n of NO by NH3 a t low t e m p e r a t u r e s , t h e e x i s t e n c e of t h e s e oxygen s p e c i e s would b e c l o s e l y c o r r e l a t e d t o t h e c a t a l y t i c a c t i v i t y o f copper o r t h o v a n a d a t e .

About one - fou r th o f t h e mob i l e oxygen can b e c o n v e r t e d t o

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Nature of A c t i v e Oxygen S p e c i e s in Copper Vanadate C a t a l y s t 1387

DISCUSSION

Y. Murakami (Nagoya Univ.)

enhanced by oxygen t r e a t m e n t at 2OO0C a f t e r p re - r educ t ion of coppe r o r thovanada te . I n F i g . 7 , you showed t h a t t h e Cu i o n s i n t h e s u r f a c e l a y e r are c o n s i d e r a b l y mob i l e and t h e en r i chmen t of t h e surface Cu i o n s is caused by t h e oxygen t r e a t m e n t . I imagine t h a t new a c t i v e sites c o n t a i n i n g Cu or Cu i o n s are pro- duced on t h e s u r f a c e by oxygen t r e a t m e n t . That p o i n t , I b e l i e v e , is impor t an t . Could you pe rhaps e l a b o r a t e on it?

S. Yoshida

The c a t a l y t i c a c t i v i t i e s f o r NO-NH3 and CO-O2 r e a c t i o n s w e r e

I a g r e e w i t h your p o i n t t h a t t h e a c t i v e s p e c i e s i n CuV c o n t a i n copper i o n s and t h e i o n s ' s h o u l d i n t e r a c t w i t h r e a c t a n t s d i r e c t l y . However, t h e coppe r i o n s shou ld have d i f f e r e n t n a t u r e from t h o s e i n CuO, because a c o n s i d e r a b l e amount o f N 2 0 was formed o v e r CuO i n NO + NH3 r e a c t i o n s a t 150°C, w h i l e o n l y a trace o f N20 w a s observed o v e r CuV (S. Yoshida e t a l . , IEC Prod. R e s . and Dev., - 18, 283 ( 1 9 7 9 ) ) . Thus, t h e n a t u r e of t h e copper i o n s shou ld be i n f l u e n c e d by vanadium i o n s . What is t h e role of t h e vanadium i o n s i n CuV? I t is r e p o r t e d t h a t copper i o n s i n C u ( I 1 ) s t a t e c a t a l y z e t h e NO + NH3 r e a c t i o n w i t h h i g h s e l e c t i v i t y t o N2 ( O t t o and S h e l e f , J . Phys. Chem., 76, 37 ( 1 9 7 2 ) ) . Thus, one p o s s i b l e role o f t h e vanadium i o n s i n CuV is t o keep copper i o n s i n Cu(I1) s t a t e d u r i n g t h e r e a c t i o n . In f a c t , w e observed an i n c r e a s e i n ESR i n t e n s i t y of CuV when CuV w a s reduced s l i g h t l y by CO (un- p u b l i s h e d r e s u l t s ) . A s Cu(1) i o n s are d iamagne t i c s p e c i e s , t h e r e s u l t s s u g g e s t t h e fo rma t ion of V(1V) i o n s by t h e r e d u c t i o n .

I.W. Geus (Univ. Utrecht) R e f e r r i n g t o V205 t h e vanadium i o n s are l i k e l y t o m i g r a t e i n t o

t h e o x i d e when t h e material i s reduced t o a lower v a l e n c e vanadium o x i d e . With t h e oxygen exchange, on t h e o t h e r hand, oxygen i o n s must m i g r a t e . D i f f e r e n t ra tes o f exchange and re- d u c t i o n may p o i n t t o m i g r a t i o n of oxygen and of metal i o n s , r e s p e c t i v e l y , de t e rmin ing t h e ra te of t h e p r o c e s s .

Did you g e t any ev idence f o r s e g r e g a t i o n of a copper ( t ) o x i d e or copper (0) phase i n t h e reduced c a t a l y s t ?

S . Yoshida

W e t r i e d t o g e t i n f o r m a t i o n s about t h e fo rma t ion of new phases i n t h e reduced c a t a l y s t s by X-ray d i f f r a c t i o n and I R s p e c t r o s c o p y .

1388 S . Yoshida, A . Ueda, K. Tarama

There i s no ev idence t o i n d i c a t e t h e fo rma t ion of new p h a s e s f o r a s l i g h t l y reduced CuV ( C U ~ V ~ O ~ . ~ ) which co r re sponds t o t h e end s ta te of f i r s t s t a g e i n F i g . 4. By s e v e r e r e d u c t i o n s , an u n a s s i g n a b l e phase w a s formed. The phase d i s a p p e a r e d by calci- n a t i o n o f t h e sample i n a i r a t 250°C. Tn o r d e r t o p r e p a r e copper vanada te from VaOg and CuO, a mixed powder must be c a l c i n e d a t a t e m p e r a t u r e h i g h e r t h a n 60OoC. Thus, w e b e l i e v e t h a t t h e new phase is n o t a s e g r e g a t e d coppe r ox ide .

-0 (Kyushu Univ. , Fukuoka)

c a t a l y t i c a c t i v i t y of t h e copper vanada te . However, t h e mob i l e oxygen is produced when you t rea t t h e sample w i t h oxygen a f t e r p re - r educ t ion . What do you t h i n k abou t t h e p a r t i c i p a t i o n o f t h i s oxygen i n t h e s t e a d y s t a t e c a t a l y t i c r e a c t i o n ?

Y. Yoshida

You mentioned t h a t mob i l e oxygen is r e s p o n s i b l e f o r t h e

The term "mobile oxygen" and "very r e a c t i v e oxygen" i n t h e t e x t shou ld b e d i s t i n g u i s h e d . A f t e r p r e - r e d u c t i o n , t h e v e r y r e a c t i v e oxygen is formed i f oxygen molecules e x i s t i n t h e r e a c t i o n s y s t e m . I n CO o x i d a t i o n , t h e v i r g i n CuV s h o u l d b e reduced s l i g h t l y a t f i r s t and t h e n t h e v e r y r e a c t i v e oxygen was formed as s t a t e d i n t h e t e x t i n connec t ion w i t h F ig . 5 and 6. We can expec t t h e r e g e n e r a t i o n of t h e ve ry reac t ive oxygen by t h e redox mechanism i n a s t e a d y s ta te c o n d i t i o n .

Ji-Yong Ryu (Exxon Res., A l l e n d a l e )

C02 on v i r g i n c a t a l y s t and s l i g h t l y reduced c a t a l y s t ? be adsorbed on t h e c a t a l y s t s u r f a c e , s u r f a c e oxygen atoms which is a s s o c i a t e d t o t h e C02 chemiso rp t ion c o u l d be less r e a c t i v e oxygen s p e c i e s f o r t h e CO o x i d a t i o n r e a c t i o n .

S. Yoshida

Do you have any ev idence of a d s o r p t i o n ( c h e m i s o r p t i o n ) of I f O2 c a n

We obse rved t h a t CO o x i d a t i o n was r e t a r d e d by C02 which w a s formed d u r i n g t h e r e a c t i o n . by a c o l d t r a p of l i q u i d n i t r o g e n , t h e r e t a r d a t i o n w a s no t obse rved . ve ry weak. reduced c a t a l y s t s w a s a l so confirmed by IR spec t roscopy .

However, when t h e C02 w a s f r e e z e d

Thus, C02 i s adsorbed on CuV b u t t h e a d s o r p t i o n is The weak a d s o r p t i o n of C02 on b o t h t h e v i r g i n and

N. Yamazoe (Kyushu Univ . , Fukuoka)

o b t a i n e d t h e dec reased c o n c e n t r a t i o n of Cu i o n i n t h e s u r f a c e In t h e AES dep th p r o f i l i n g o f reduced c a t a l y s t s ( F i g . 7), you

Nature of Active Oxygen Spec ie s i n Copper Vanadate Catalyst 1389 0

l a y e r ( l e s s t han 30 A ) . Where do you expec t t h e m i s s i n g Cu i o n s e x i s t ? Does t h e observed Ccu/Cv of t h e reduced s u r f a c e l a y e r sugges t t h e fo rma t ion of any p a r t i c u l a r compound? For t h e case o f oxygen t r e a t e d CuV, can you e x c l u d e a p o s s i b i l i t y t h a t CuO is somehow formed on t h e o u t e r most s u r f a c e d u r i n g t h e t r e a t m e n t and c o n t r i b u t e s t h e c a t a l y t i c a c t i v i t y for subsequen t r e a c t i o n .

S . Yoshida

We t h i n k t h a t copper i o n s mig ra t ed i n t o t h e i n n e r phase deeper 0

t h a n 30 A i n t h e reduced c a t a l y s t s . A s f o r t h e p o s s i b i l i t y of fo rma t ion of any p a r t i c u l a r compound, see answers t o P r o f . Murakami and Dr. Gues.