Effect of non-Faradaic electrochemical modification of catalytic activity

Solid State Ionics 136–137 (2000) 721–725 www.elsevier.com / locate / ssi

Effect of non-Faradaic electrochemical modification of catalytic activity V.D. Belyaev*, T.I. Politova, V.A. Sobyanin Boreskov Institute of Catalysis, 5 Lavrentieva ave, 630090 Novosibirsk, Russia

Abstract The present paper briefly reviews our researches on non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) and discusses CO oxidation in a solid oxide electrolyte cell under the NEMCA conditions based on the chain reaction mechanism involving electrochemically generated oxygen species.  2000 Elsevier Science B.V. All rights reserved. Keywords: Solid oxide electrolyte cell; Non-Faradaic catalysis; Chain reaction mechanism

1. Introduction Recently much attention has been focused [1–3] on the study of oxidative conversion of gases (CO, CH 4 , C 2 H 4 , CH 3 OH) over metal electrode-catalysts in a solid oxide electrolyte cell: gas 1 O 2 , metal catalyst u ZrO 2 (8–10 mol% Y 2 O 3 ) u metal, air The current passing through the cell (a flow of O 22 ions through the electrolyte) was found to change remarkably and reversibly the catalytic activity of the metal catalyst. The induced change in the catalytic rate exceeded by several orders of magnitude the rate of oxygen flow through the yttriastabilized zirconia (YSZ) electrolyte. This effect has been termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect). *Corresponding author. Fax: 17-383-234-3056. E-mail address: [email protected] (V.D. Belyaev).

The effect of current on the rate of catalytic reaction over electrode deposited on YSZ is described by two parameters: r 5 r /r 0 and L 5 (r 2 r 0 ) /(I / 2F ), where r 0 and r are the rates of catalytic reaction (g-atom O / s) under the open- and closedcircuit conditions, respectively; I is a current; F is Faraday’s constant. The rate enhancement ratio r shows the current-affected variation of the reaction rate with regard to rate upon the cell circuit opening. Enhancement factor L reflects the change of the reaction rate (r 2 r 0 ) relative to the oxygen flow rate through the electrolyte (I / 2F ). Systems exhibiting NEMCA effect are characterized by r ±0 and uLu4 1. The NEMCA effect is of interest for heterogeneous catalysis, since the current passing through the cell, or electrode potential, is an additional parameter that can be applied to control catalytic properties of electrode-catalyst. Presently, this effect has been described for more than 50 catalytic reactions not only with O 22 conductor, but with Na 1 , F 2 and H 1 conductors as well [3].

0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00501-4

722

V.D. Belyaev et al. / Solid State Ionics 136 – 137 (2000) 721 – 725

Here we briefly summarize our previous data and current researches on the NEMCA effect. In particular, the origin of the NEMCA effect at carbon monoxide oxidation over Pt [4], Au [5] and Ag–Pd alloy [6] electrodes is discussed on the basis of chain reaction mechanisms involving electrochemically generated oxygen species.

the data on CO oxidation under the NEMCA conditions over a metal electrode in a cell with YSZ electrolyte exactly in the context of the chain mechanism concept.

3. CO oxidation under NEMCA conditions 2. Search for the NEMCA systems Investigations of non-Faradaic catalysis were started in our laboratory 9 years ago. We studied 16 systems (reaction1electrode / electrolyte), the results obtained were published in Refs. [5–19]. Only a half of the systems were found to exhibit the NEMCA effect. This is not surprising, since researchers still lack theoretical concepts allowing to predict a priori whether a new system will show the NEMCA effect, and under what experimental conditions. Vayenas and co-workers were the first who dealt with NEMCA and made significant contributions to this area [1–3]. They suggest that the essence of the NEMCA effect is that the electrode work function changes during polarization of metal electrode deposited on the solid oxygen-conducting electrolyte, i.e. when current passes through the cell. This leads to changes of the strength of the chemisorptive bond of adsorbed oxygen, and, thus, of the electrode catalytic properties. In the frame of this idea the scientists show that the expected value of factor L can be estimated by equation L | 2Fr 0 /I0 , where r 0 is the rate of catalytic reaction under open-circuit conditions, I0 is the exchange current. This equation allows easy calculating of factor L through trivial measuring of r 0 and I0 and thus predicting whether the system will show the NEMCA effect. However, special studies we have carried out in this field [13,18] revealed such predictions to be sometimes invalid. Based on the results of our studies, we suppose that the NEMCA effect may be attributed to either formation of new catalytically active sites on the electrode surface during polarization [9,11–13], or catalytic reaction running through the chain mechanism initiated by electrochemically generated oxygen species [6,20]. In the following section we discuss

The NEMCA effect was observed at CO oxidation on Pd [3], Ag [3], Pt [3,4], Au [5] and Ag–Pd alloy [6] electrodes deposited on YSZ electrolyte. However, no current–potential plots were reported for Pd and Ag electrodes. Therefore we shall consider the data for Pt, Au and Ag–Pd electrodes only. In Ref. [6,20] we have discussed the simplest chain reaction mechanism:

where Z 0 is a catalytically active site on the gasexposed surface of the electrode; Z 0 O is atomic oxygen species on the catalytically active site; Z is an electrochemically active site on the three-phase boundary gas–electrode–YSZ; ZO 2 and ZO 22 are charged oxygen species on the electrochemically active site; O 22 is the oxygen anion in the bulk of V YSZ. It was found that the theory satisfactorily reproduced the experimental data on CO oxidation over Pt and Au electrodes [20]. However, more profound further analysis of the calculation results showed that the surface coverage with ZO 2 2 species was too large. Moreover, in the case of Ag–Pd alloy electrodes the theory provided a poor description of the experimental data. It means that the simplest chain reaction mechanism must be modified. We suggest the following chain reaction mechanism under the NEMCA conditions:

V.D. Belyaev et al. / Solid State Ionics 136 – 137 (2000) 721 – 725

723

current. Steps 6 and 7 are the chain propagation steps producing no current. Their rates exceed those of initiation / termination steps. That is, under the NEMCA conditions the enhancement factor L may be as large as the chain length and is determined by the values of the steps rate constants. We have carried out numerical simulation of the experimental results [4–6] assuming all intermediates to obey steady-state condition and adlayer on each interface to be ideal. We have also assumed that the potential dependence of the rate constants of

The scheme is similar to the previously discussed one, except that the steps of the chain processes occur not at the three-phase boundary (Z-sites) but at the gas-exposed surface of the electrode (Z 0 -sites). In general, the following scheme of the process is valid: k1

→ 2Z 0 O 2Z 0 1 O 2 ← 2 k1

k2

Z 0 O 1 CO →CO 2 1 Z 0

(1) (2)

k3

→ 2ZO 2 2Z 1 O 2 1 2e ← 2 k3

(3)

Fig. 1. Effect of potential on current and on the rate of CO 2 formation over Ag–Pd alloy electrode deposited on YSZ. T5 5008C, [CO]55 vol.%, [O 2 ]525 vol.%. Points: experiment [6]; lines: theory.

k4

→ Z0O2 Z0 1 O2 1 e ← 2 2 k4

(4)

k5

→ Z 0 O 22 1 Z ZO 2 1 Z 0 O ← 2 k5

k6

Z 0 O 22 1 CO →CO 2 1 Z 0 O 2

(5) (6)

k7

→ Z0O2 Z0O2 1 ZoO ← 2 1 Z0 2 k7

(7)

k8

→ Z 1 O v22 ZO 2 1 e ← 2 k8

(8)

Steps (1) and (2) represent ordinary catalytic reaction following the Eley–Rideal mechanism. Steps (3)–(5) and (8) are the chain initiation / termination steps. Besides, Steps (3), (4) and (8) are current related steps. Their rates are controlled by the

Fig. 2. Effect of potential on current and on the rate of CO 2 formation over Au electrode deposited on YSZ. T54608C, [CO]5 1.4 vol.%, [O 2 ]521 vol.%. Points: experiment [5]; lines: theory.

V.D. Belyaev et al. / Solid State Ionics 136 – 137 (2000) 721 – 725

724

electrochemical reactions (current-producing steps) are given by Tafel’s law: k i 5 k 0i exp(2bi Fw /RT ), 2 k2 i 5 k 0i exp((1 2 bi )Fw /RT ),

where bi is symmetry coefficient, w is electrode potential; F is Faraday’s constant. As assumed, the area of the three-phase boundary equalled 1–5?10 23 cm 2 , the length of the three-phase boundary was

Fig. 3. Effect of potential on current and on the rate of CO 2 formation over Pt electrode deposited on YSZ. T54128C, [CO]5 0.47 vol.%, [O 2 ]516 vol.%. Points: experiment [4]; lines: theory.

1–5?10 4 cm, the area of gas-exposed surface was 100 cm 2 , all the values being referred to 1 cm 2 of electrode geometrical area. Parameters of kinetic model were calculated by the Newton method using Microsoft Excel 97 software. Figs. 1–3 show the experimental data on CO oxidation over, respectively, Ag–Pd alloy, Au and Pt electrodes under NEMCA conditions and the results of calculations. It is seen that experimental data fit the theory satisfactorily. Table 1 shows the rate parameters fitting experimental data obtained over Ag–Pd alloy and Pt electrodes. They seem to be quite realistic. Note in this regard that non-Faradaic increase of CO oxidation rate over Ag–Pd alloy and Pt electrodes (Figs. 1, 3) correlates with calculated increase of the electrode surface coverage with Z 0 O 22 species depending on the electrode potential (Fig. 4). Besides, the suggested model adequately simulates not only the dependencies of the reaction rate and current from the potential of electrode-catalysts (Figs. 1–3) but the dependencies of the reaction rate and current from the temperature and CO and O 2 concentrations. So, the chain process involving electrochemically generated oxygen species, most probably, is the key factor for the appearance of NEMCA effect of carbon monoxide oxidation over metal electrodes in the cell with YSZ electrolyte.

Table 1 Rate parameters for CO oxidation over Ag–Pd and Pt electrodes a Rate constant

Dimension

Ag–Pd electrode k0

21

21

s ?atm. s 21 ?atm.21 s 21 ?atm.21 s 21 ?atm.21 atom?s 21 ?cm 21 s 21 ?atm.21 s 21 s 21 s 21 s 21 s 21 atom?s 21 ?cm 21 s 21 s 21

k1 k2 k3 k4 k5 k6 k7 k8 k2 1 k2 3 k2 4 k2 5 k2 7 k2 8 a

E [kcal / mol] 6

8?10 8?10 3 1?10 4 30 6?10 17 8?10 4 1?10 12 2?10 7 1?10 12 2?10 3 1?10 4 3?10 17 1?10 12 7?10 5

0 8 14 25 19 3 10 20 30 8 12 15 13 14

Pt electrode

b

k0

E [kcal / mol] 6

0.69 0.71

0.59 0.69 0.71

0.59

k 0 is pre-exponential factor; E is activation energy; b is symmetry coefficient.

8?10 8?10 3 1?10 5 10 6?10 17 8?10 4 1?10 12 2?10 7 1?10 12 2?10 3 5?10 3 3?10 18 1?10 12 7?10 6

0 4 8 21 19 3 25 11 60 12 12 15 20 7

b

0.17 0.71

0.58 0.17 0.71

0.58

V.D. Belyaev et al. / Solid State Ionics 136 – 137 (2000) 721 – 725

Fig. 4. Effect of potential on the calculated coverage of Ag–Pd 2 alloy and Pt electrodes with Z 0 O 2 species at CO oxidation under NEMCA conditions (see captions to Figs. 1 and 3).

4. Conclusion Several ideas on the origin of the NEMCA phenomenon have been proposed. However, none of them could predict unambiguously new systems exhibiting this effect. Further efforts should be made to develop the theory of the NEMCA phenomenon.

References [1] C.G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625. [2] C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303. [3] C.G. Vayenas, M.M. Jaksic, S.I. Bebelis, S.G. Neophytides, in: Modern Aspects of Electrochemistry, Vol. No. 29, Plenum Press, NY, 1996, pp. 57–202.

725

[4] I.V. Yentekakis, C.G. Vayenas, J. Catal. 111 (1988) 170. [5] O.A. Mar’ina, V.A. Sobyanin, Catal. Lett. 13 (1992) 61. [6] T.I. Politova, V.V. Gal’vita, V.D. Belyaev, V.A. Sobyanin, Catal. Lett. 44 (1997) 75. [7] T.I. Politova, V.A. Sobyanin, V.D. Belyaev, React. Kinet. Catal. Lett. 41 (1990) 321. [8] T.I. Politova, V.A. Sobyanin, V.D. Belyaev, Elektrokhimiya 28 (1992) 1466, in Russian. [9] V.D. Belyaev, V.V. Galvita, V.P. Gorelov, V.A. Sobyanin, Catal. Lett. 30 (1995) 151. [10] O.A. Mar’ina, V.A. Sobyanin, V.D. Belyaev, Materials Sci. Eng. B13 (1992) 153. [11] O.A. Mar’ina, V.A. Sobyanin, V.D. Belyaev, V.N. Parmon, in: T. Inui et al. (Eds.), New Aspects of Spillover Effect in Catalysis, Studies in Surface Science and Catalysis, Vol. 77, Elsevier Science Publishers B.V., 1993, p. 337. [12] V.D. Belyaev, V.A. Sobyanin, A.K. Demin, A.S. Lipilin, V.E. Zapesotskii, Mendeleev Commun. 2 (1991) 53. [13] V.D. Belyaev, T.I. Politova, V.A. Sobyanin, Catal. Lett. 57 (1999) 43. [14] V.V. Gal’vita, V.D. Belyaev, V.A. Sobyanin, React. Kinet. Catal. Lett. 59 (2) (1996) 407. [15] V.D. Belyaev, V.V. Gal’vita, V.A. Sobyanin, React. Kinet. Catal. Lett. 63 (1998) 341. [16] V.D. Belyaev, T.I. Politova, O.A. Marina, V.A. Sobyanin, Appl. Catal. A: General 113 (1995) 47. [17] O.A. Mar’ina, PhD thesis, Boreskov Institute of Catalysis, Novosibirsk, 1993, 154 p. [18] G.L. Semin, V.V. Gal’vita, V.D. Belyaev, V.A. Sobyanin, Elektrokhimiya 34 (1998) 962, in Russian. [19] V.A. Sobyanin, V.I. Sobolev, V.D. Belyaev, O.A. Mar’ina, A.K. Demin, A.S. Lipilin, Catal. Lett. 18 (1993) 153. [20] V.A. Sobyanin, V.D. Belyaev, React. Kinet. Catal. Lett. 51 (1993) 373.