Influence of the nature of the conduction band of transition metal oxides on catalytic activity for oxygen reduction

J. Electroanal. Chem., 83 (1977) 2 3 7 - - 2 4 3

237

© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

INFLUENCE OF THE NATURE OF THE CONDUCTION BAND OF TRANSITION METAL OXIDES ON CATALYTIC ACTIVITY FOR OXYGEN REDUCTION

Y. M A T S U M O T O , H. Y O N E Y A M A and H. T A M U R A

Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadakami, Suita, Osaka (Japan) (Received 26th O c t o b e r 1976)

ABSTRACT Catalytic activities for o x y g e n r e d u c t i o n of some transition m e t a l oxides with metallic conductivity such as LaTiO3, SrFeO3, SrVO3, SrRuO3, V0.2Til.sO 3 and Lal_~xSrxMnO 3 were investigated, and t h e y as well as the activities of o t h e r oxides r e p o r t e d already were c o m p a r e d with the nature of their c o n d u c t i o n bands. It was f o u n d that the catalytic activity of oxides having a o* c o n d u c t i o n band was high. The conclusion is drawn that in order for a transition metal o x i d e to have a high catalytic activity, (1) it must have a o* band and (2) the band must contain electrons. This conclusion will be useful to predict the catalytic activity for o x y g e n r e d u c t i o n of transition metal oxides. (I) INTRODUCTION

In a previous paper [1], it was shown that the feasibility of a* band formation of an oxide, which is dependent on the magnitude of the overlap integral between an eg orbital of a metal ion, M, and an Spo orbital of an oxygen ion, O, of the oxide, is a major factor in determining the electrocatalytic activity of the oxide for oxygen reduction. This conclusion suggests not only t h a t d orbitals of M must n o t be localized but form a band in order for the oxide to have a high catalytic activity, but also that the nature of the band influences the catalytic activity. According to Goodenough [2,3], there are three kinds of d bands in transition metal oxides; a 7r* band formed by interaction between t2g orbitals in M--M, a ~* band by interaction of a t2g orbital of M with a p~ orbital of O in M--O--M, and the o* band by an eg orbital of M with a spo orbital of O in M--O--M. Considering that an oxygen molecule makes an end-on type adsorption [1] in the first step of oxygen reduction, the oxygen molecule must interact with a transition metal ion on the oxide surface. If one takes into consideration the geometric arrangement of the respective orbital, it is quite reasonable to think that in the end-on type adsorption, the eg orbital rather than the t2g orbital of the transition metal ion will interact more easily with an oxygen molecular orbital to give an adsorbed intermediate. This means that oxides having the o* band are more powerful for oxygen reduction than those having lr* bands. If this view is acceptable, then one can predict the catalytic activity o f transition metal oxides from knowledge of the nature of the band of the oxides.

238 The main purpose of this paper is to demonstrate the validity of this view. For this purpose, the catalytic activities of some oxides with metallic conductivity such as LaTiO3, SrFeO3, SrVO3, SrRuO 3, Vo.2Til.803 and Lal_xSrxMnO3 were investigated, and they as well as the activities of other oxides which were already reported, were compared with the nature of their bands (2) EXPERIMENTAL LaTiO3, SrVO3, SrRuO3, Lal_xSrxMnO a and V0.2Til.803 were synthesized by similar methods to those described by Kestigian and Ward [4], Chamberland and Danielson [5], Callaghan et al. [6], Jonker [7] and Kawakubo et al. [8], respectively. SrFeO 3 was synthesized at 1200°C for 4--5 h in the air by using SrCO 3 and Fe203 as the starting materials [9]. SrFeO 3 presumably contained a small a m o u n t of oxygen ion vacancy as reported by MacChesney et al. [9]. In all the syntheses, starting materials were pressed into a tablet form with 100 kg cm -2. The specific resistivities were measured by the four probe method. LaTiO3, SrVO3, SrRuO 3 and V0.2Til.sO 3 could be synthesized to give firm sintered discs. Then they were used as the electrode merely by being waterproofed with polystyrene, as described previously [ 10]. In the case of SrFeO3 and Lal_~SrxMnO3, however, the oxides were not so firmly sintered when they were prepared. After being crushed with an agate mortar, the oxides were pressed into a tablet form with a binder to give electrodes, as described previously [10]. The electrolytes of 1 M NaOH and 0.5 M H2SO 4 were pre-electrolyzed for five days. The potential sweep m e t h o d was employed to study basic polarization behavior of the electrode itself with a sweep rate of 10 mV s-1. Besides this, potentiostatic polarization was conducted to evaluate the catalytic activity of the electrode for oxygen r e d u c t i o n . / R - f r e e current/potential curves were obtained by using a current interrupter. All the measurements were conducted at 25°C. (3) RESULTS Specific resistivities of sintered discs of LaTiOa, SrFeOa, SrVO 3, SrRuO3 and V0.2Til.sO 3 were 1.2, 5.0 × 10 -1, 6.0 × 10 -3, 9.4 × 10 -4 and 4.0 × 10 -4 cm, respectively. Resistivities of Lal_xSrxMnO 3 are shown in Fig. 1 as a function of the degree of substitution with strontium, x. When the resistivity was obtained for a sintered disc, it was fairly close to the value reported by Jonker [7]. The oxide disc with Afron binder, which was used as the electrode, had very large resistivity. As shown in this Figure, the tendency of the change of the resistivities as a function of x, however, was almost the same in both cases as expected. When the degree of substitution with strontium, x, becomes large, a localized eg orbital of the transition metal ion in the oxide becomes available to form the a* band with an oxygen ion [2]. Simultaneously the width of the a* band becomes great. The decrease of the resistivity with the increase of x in the range o f x = 0 to 0.3, therefore, seems to be due to the decrease of the effective mass of electrons, m*, bringing an increase of the mobility of electrons in the o* band, though the physical significance of the increase of the resistivity at x = 0.4 is n o t clear.

239 10 4

10 3

10 2

,01 Eu a~ lo o n,.

-1 10

10--2 I0

~ l I

I

0.1

0.2

I

0,3

0.4

X

Fig. 1. Resistivities of Lal__xSrxMnO 3 as a function of the degree of substitution, x. (©) Sample bonded by a binder (12.5 wt.%), (*) sintered disc, ($) values reported by Jonker [7].

Voltammograms of various oxides obtained in 0.5 M H2SO 4 and 1 M NaOH, are shown in Figs. 2 and 3. In the case of 0.5 M H2SO 4, a distinct wave of the electrochemical reaction of the oxide itself was observable only for SrVO 3 and

2.0

10

A

f! it

"/.5

I

I E Y E

5

~2.5

-1.0

I

-2.5

-1.0

-0.5

I

0

I

0.5

I

1.0

I

-1.0

ElY (v.s. HglHg2SO4)

Fig. 2. Potential sweep voltammograms in 0.5 M H2SO 4. ( Fig. 3. Potential sweep voltammograms in 1 M NaOH. ( ~ ) (. . . . . ) Lao.8Sro.2Mn03, (. . . . ) LaTi03.

[

I

I

-0.5 0 0.5 E/V (v.$. Hg/HgO

I

'1.0

) SrRuO3, ( - - - - - - ) SrVO 3. SrRuO3, ( - - - - --) SrVO 3,

~.5

240

SrRuO3. By electrolysis of these oxide electrodes for several tens of minutes at the potential giving the anodic peak current, the electrolyte turned light brown in the case of SrRuO3, while in the case of SrVO3, V ~+ ion was detected in the electrolyte by a color identificationusing a-benzoin oxim. These results show that the anodic dissolution t o o k place at the potential of the anodic peak current. In the case of 1 M NaOH, it was found that SrRuO3, LaTiO3 and La0.sSr0.2MnO 3 were electrochemically active as shown in Fig. 3. On the other hand, SrFeO3 and Vo.2Til.803 were stable. Similar electrochemical reactions to that of Lao.8Sro.2MnO 3 were observed for Lal_xSrxMnO 3 having x = 0, 0.1, 0.3 and 0.4, though the magnitude of the currents was different depending on x. Figures 4, 5 and 6 show the polarization curves of oxygen reduction. The polarization curve of oxygen reduction was obtained by subtracting background current measured in nitrogen atmosphere from the current measured in oxygen atmosphere at I atm. An appreciable cathodic current of oxygen reduction was observed only for the oxides presented in Figs. 4, 5 and 6. By comparing potential sweep voltammograms in Fig. 3 with steady-state polarization curves of oxygen reduction in Figs. 4, 5 and 6, it is noticed that the electrode surface might be slightly reduced during the measurements of the polarization curves of oxygen reduction, for SrVO 3 and Lal_xSrxMnO 3. However, the reduction, if any, is believed to bring no detrimental composition change in the electrode surface. In the case of SrRuO3, the polarization behaviour of the electrode was found to be quite analogous to that of LaNiO3. Therefore, the two distinct current peaks seem to be connected to a couple of the reaction of Ru3+/Ru 4+ and Ru4+/Ru 6+ to give SrRuO3_ 8 and SrRuO3+~, respectively [11,12]. It follows from this identification of the peaks that the oxygen reduction proceeded in potentials to bring no distinct change in the surface composition of the electrode. By comparing the polarization curves of oxygen reduction

10 -3

lO .3

,o-~

,o-5 lO-6

,04 I -1.0 -0.9

I I -~8-0.7

I I -0.6-0~

I -0A

i i i --0-3 - 0 ~ - 0 . 1

E1 V ( v . s . H g l H o O

)

I

I

-08

-03

i

i

i

-0.6 -0.5 -0.4 E/V

(v,s.

i

l

I

-0.3 -0.2 -~1

i

o

H g l H g 2 S O ~ ,)

Fig. 4. Current-potential curves of oxygen reduction in 1 M NaOH. (e) SrRuO3, (~) SrVO3, (©) V0.2Til.803. Fig. 5. Current-potential curves of oxygen reduction in 0.5 M H2SO 4. (o) SrRuO3, (©) V0.2Ti1.803 •

_

241

,d ¢ io.4

,d'

.uE 10-5

7/

1

)

..J

10-6

I

I

I

I

I

I

-200

-150

-700

-50

0

50

E/mV

(v.s. HglHgO

)

I00

0

I

I

I

i

0.1

0.2

0.3

0.4

X

Fig. 6. Current-potential curves o f o x y g e n r e d u c t i o n on L a l _ x S r x M n O 3 in 1 M NaOH. x = (0) 0.4, (e) 0.3, (e) 0.2, (5) 0.1, (A) O. Fig. 7. Exchange current densities of o x y g e n r e d u c t i o n o n Lal__xSrxMnO 3 as a f u n c t i o n of the degree o f substitution, x.

with each other, it is found that the catalytic activity was higher for 1 M NaOH than 0.5 M H2SO 4 and that the activity of Lal_xSrxMnO3 is the highest of all the oxides used in the present study. Figure 7 gives the exchange current density of oxygen reduction on Lal_xSrxMnO 3 as a function of the degree of substitution with St. The catalytic activity increased with an increase of the degree of substitution with strontium, x. The increasing tendency of the exchange current density with an increase of x can not be attributed to difference in the surface area of the electrodes, because Lal_~SrxMnO3 in the powder form had the surface area of 1.0--1.2 m2g -1 independent of the degree of substitution. From this result, it is concluded that the increase of the overlap integral, which is connected to promotion of the o* band formation, brings the increase of the catalytic activity of the oxide i n just the same manner as discussed on LaNil_~M~O3 [1]. All the Tafel slopes were the same and 47 mV per decade. Therefore, the rate determining step of the oxygen reduction on Lal_xSr~MnO3 seems to be the same as t h a t on LaNil_~M~Oa. The 7r* band formed by the M--O--M interaction is the conduction band in the cases o f LaTiOa, SrFeO3, S r R u O 3 a n d S r V O 3 , though the e m p t y o* band is also formed in these oxides [2]. On the other hand, the conduction band is the ~* band formed by M--M interaction in the case of Vo.2Til.sO s [9], and the o* band in the case of Lal_~Sr~MnOa [2]. By comparing the catalytic activities with the nature of the band, it is concluded that the catalytic activity of oxides having a a* conduction band is high, while t h a t of those having a ~* conduction band is low or negligible. These results give support to the idea t h a t the catalytic activities of oxides can be predicted roughly if the nature of the band is known.

242 (4) DISCUSSION

From the above results, it is clear that the following two conditions must be fulfilled in order for a transition metal oxide to have a high catalytic activity: (1) the 0* band must be formed; (2) it must contain electrons. The importance of the first condition is demonstrated in the fact that the catalytic activity changed with the degree of substitution, x, in Lal_xSrxMnO 3 and LaNil_xM~O 3 [1]. The importance of the second condition is observable in the fact t h a t LaTiOa, SrVO3, SrFeO 3 and SrRuO3, which have an e m p t y o* band, showed low catalytic activities. Qualitative relations between the catalytic activities and the nature of the conduction band are summarized in Table 1 for a variety of transition metal oxides. In this Table, the results obtained in this study as well as in other literature are collected. From this Table, it is noticed that the conduction band of

TABLE 1 d-Electron configuration of oxide and catalytic activity for oxygen reduction Oxide

d-Electron configuration [ 3 ]

Conductivity a

Activity

LaTiO 3

7r* 1 o* 0

Metallic

Negligible

SrVO 3

lr* 10, 0

Metallic

Low

SrFeO 3

t.3T~ 1o* 0

Metallic

Negligible

SrRuO 3

7r'40 *0

Metallic

Low

LaCrO 3 [ 19 ]

t* 30*0

Semi

Low

Lal_xSrxCoO 3 [13]

t*60 *° or t*40 . 2 (x = 0) 7t'*no*n(x > O) b

Semi-metallic

High

Lal__xSrxMnO 3

t*3eg *1 (x = 0)

Semimetallic

Low (x = 0)

t*30 *l-'x (x > 0) LaNil_xMxO 3

t , 6 0 , 1 (x = 0)

(M: Fe, Co)

t*6eg *n (x > 0)

NaxWO 3 [15]

High (x > 0)

Semimetallic?

High (x = 0)

7r'no *0

Metallic

Low

Vo.2Til.803

?T,n c

Metallic

Low

Li dopedNiO [18]

eg* n (< TN)

Semi

Relatively high (
O*n (> TN)

Low (x > 0)

(>TN) a "Metallic" and " s e m i " denote metallic conductivity and semiconductivity, respectively. b Overlap of ~* band with O* band. c This o* band is formed by the interaction of M--M.

243

transition metal oxides must satisfy the above mentioned two conditions in order for the oxides to have a high catalytic activity for oxygen reduction. Tseung and Bevan [13] reported that Lal-xSrxCoO3 gave a sufficiently high catalytic activity as to give the equilibrium potential of oxygen at the potential expected by thermodynamics. According to Goodenough [3], the conduction band of this oxide is formed by overlapping the o* band with the ~* band. Hence, this oxide satisfies the above two conditions. Two different bands have been proposed for the conduction band of NaxWO3. Mackintosch [14] proposed that the band was formed by interaction between sodium orbitals, while Goodenough considered the u* band to be formed by the M--O--M interaction [3]. The theory of the ~* band seems to be more appropriate to understand the physical properties of NaxWO 3. If the ~* band is the conduction band of Na~WO3, the catalytic activity for oxygen reduction must be low. It was reported by Bockris and McHardy [15] that the catalytic activity of Na~WO3 is low if the oxide is not doped with platinum. Although NiO is a well k n o w n p-type semiconductor [16], the eg orbital of Ni 2+ is localized in the oxide [3]. However, the magnitude of the overlap integral of the eg orbital of Ni 2+ with the spo orbital of oxygen ion in the oxide is equal to that of the critical overlap integral, that is, Acaac~ Ae, in the temperature range above the Neel point [17]. Therefore, formation of a partially filled o* band seems to be possible at temperatures above the Neel point. Then, NiO satisfies the t w o necessary conditions mentioned above. It was already reported that the catalytic activity of Li-doped I~iO became suddenly high above the Neel point [18]. If the end-on type adsorption takes place in this case, then the overlapping of the ~* orbital of an oxygen molecule, whose orbital is half occupied by electrons and energetically highest [20], with the t2g orbital of a transition metal ion of the oxide will be very weak on the adsorption. Therefore, electrons in the o* band rather than in the u* band seem to be more easily transferred to the ~* orbital of an oxygen molecule. This is one possible reason w h y a transition metal oxide having the o* conduction band shows a high catalytic activity for oxygen reduction. REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Y. M a t s u m o t o , H. Y o n e y a m a a n d H. T a m u r a , J. E l e c t r o a n a l . C h e m . , 7 9 ( 1 9 7 7 ) 3 1 9 . J.B. G o o d e n o u g h , J. A p p l . P h y s . , 37 ( 1 9 6 6 ) 1 4 1 5 . J . B . G o o d e n o u g h , P r o g r e s s in S o l i d - s t a t e C h e m i s t r y , V o l . 5, P e r g a m o n , O x f o r d , 1 9 7 1 , p p . 1 4 5 - - 3 9 9 . M. K e s t i g i a n a n d R . W a r d , J. A m e r . C h e m . S o c . , 76 ( 1 9 5 4 ) 6 0 2 7 . B.L. C h a m b e r l a n d a n d P.S. Damielson, J. S o l i d S t a t e C h e m . , 3 ( 1 9 7 1 ) 2 4 3 . A. C a l l a g h a n , C.W. M o e l l e r a n d R , W a r d , I n o r g . C h e m . , 2 7 ( 1 9 6 6 ) 1 5 7 2 . G.H. Jonker, Physica (Utrecht), 20 (1954) 1118. T. K a w a k u b o , T. Y a n a g i a n d S. N o m u r a , J. P h y s . S o c . J a p . , 1 5 ( 1 9 6 0 ) 2 1 0 2 . J . B . M a c C h e s n e y , R . C . S h e r w o o d a n d J . F . P o t t e r , J. C h e m . P h y s . , 4 3 ( 1 9 6 5 ) 1 9 0 ~ . Y. M a t s u m o t o , H. Y o n e y a m a a n d H. T a m u r a , C h e m . L e t t . , ( 1 9 7 5 ) 6 6 1 . Y. M a t s u m o t o , H. Y o n e m a y a a n d H. T a m u r a , J. E l e c t r o a n a l . C h e m . , 8 0 ( 1 9 7 7 ) 1 1 5 . T. K u d o , H. O b a y a s h i a n d T. G e n j o , J. E l e c t r o c h e m . S o c . , 1 2 2 ( 1 9 7 5 ) 1 5 9 . A . C . C . T s e u n g a n d H . L . B e v a n , J. E l e c t r o a n a l . C h e m . , 4 5 ( 1 9 7 3 ) 4 2 9 . A . R . M a c k i n t o s h , J. C h e m . P h y s . , 3 8 ( 1 9 6 3 ) 1 9 9 1 . J . O ' M . B o c k r i s a n d J. M c H a r d y , J. E l e c t r o c h e m . S o c . , 1 2 0 ( 1 9 7 3 ) 6 1 . S. v a n H o u t e n , J. P h y s . C h e m . Solids, 1 7 ( 1 9 6 0 ) 7. J.B. G o o d e n o u g h , M a t . Res. Bull., 2 ( 1 9 6 7 ) 1 6 5 . H.L. Bevan and A.C.C. Tseung, Electrochim. Acta, 9 (1974) 201. D.B. M e a d o w k r o f t , N a t u r e , 2 2 6 ( 1 9 7 0 ) 8 4 7 . G . H . Olive a n d S. Olvie, A n g e w . C h e m . I n t . Ed. Engl., 1 3 ( 1 9 7 4 ) 2 9 .