Multistep Reduction of Oxygen on Polycrystalline Silver in Alkaline Solution

Transcription

1 2 Chinese Journal of Catalysis Vol. 31 No. Article ID: (2)-41-6 DI:.16/S (9)669-3 Article: ultistep Reduction of xygen on Polycrystalline Silver in Alkaline Solution. JAFARIAN 1,*, F. GBAL 2,. G. AHJANI 1,. HSSEINI ALIABADI 1 1 Department of Chemistry, K. N. Toosi University of Technology, , Tehran, Iran 2 Department of Chemistry, Sharif University of Technology, , Tehran, Iran Abstract: xygen reduction on a polycrystalline silver electrode was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The reaction occurred by a two-electron pathway. The steps in the mechanism were observed in the cyclic voltammograms recorded with different scan rates. The intermediates formed in the steps were detected by electrochemical impedance spectroscopy. Key words: silver; oxygen reduction; cyclic voltammetry; electrochemical impedance spectroscopy CLC number: 643 Document code: A Electrochemical systems such as fuel cells and brine electrolyzers [1 3] need electrocatalysts with high activity, corrosion resistance [4,], and cost effectiveness [6,7]. The oxygen reduction reaction (RR) determines the efficiency of these electrochemical systems because of its high over-voltage on different substrates, including Pt. xygen reduction on different metals in alkaline solution has been investigated by a number of authors [8 1]. xygen is reduced by two different pathways: a single step four-electron pathway 2 + 2H 2 + 4e 4H E =.41 V vs NHE (1) and by consecutive steps that each involves two electrons 2 + H 2 + 2e H + H 2 E =.146 V vs NHE (2) H 2 + H 2 + 2e 3H (3) The mechanism and kinetics of oxygen reduction on different metals have been investigated. Van Velzen et al. [9,] showed that there was no ph dependence of the kinetics of oxygen reduction to hydrogen peroxide on mercury in neutral and alkaline media. Wroblowa et al. [11] showed that the mechanism of oxygen reduction on low carbon steel surfaces in alkaline solution is a four-electron reduction process. A study of zinc surfaces revealed that the mechanism of RR can be affected by the potential dependence of the composition of the Zn/alkaline medium interface, and that the reaction occurred by a direct fourelectron reduction to hydroxyl ions on clean zinc surfaces [12]. The rate and mechanism of RR on corroded zinc depended on the thickness of the oxide or hydroxide film [13]. An investigation of an oxide-derived Pd electrode in alkaline medium showed that the RR was catalyzed by Pd (I) [14]. The mechanism of RR on Pd/C in alkaline medium followed the four-electron pathway because of the high catalytic activity of Pd for hydrogen peroxide reduction [1]. n silver electrodes, the activation energy for the surface dissociation of oxygen was high [16] (~1 kj/mol) and this had a negative effect on RR activity. In addition, silver is catalytically active for the chemical decomposition of peroxide (Eq. 4), which has motivated researchers to study RR on silver electrodes [17 26]. 2H H (4) It has been shown that oxygen reduction to a hydroxide ion occurred through a four-electron reaction that is accompanied by the catalytic decomposition of peroxide that was formed. There is, however, still a debate over the mechanism of RR on. A detailed mechanism has been proposed by Adanuvor et al. [23]. This mechanism consisted of four main steps and a number of sub-steps. Experimental results obtained in rotating disk electrode (RDE) studies were in good agreement with this mechanism. However, these results were only for the main steps. Savinova et al. [24] obtained valuable results from a combination of cyclic voltammetry and Raman spectroscopy. According to their work, H was converted to adsorbed atomic oxygen that then diffused into the bulk of the electrode [24]: H H ads + e () H + H ads ads + H 2 + e (6) ads sub (7) Four types of adsorbed oxygen, namely molecular, atomic, subsurface, and a species dissolved in the bulk, were observed [24]. Blizanac et al. [26] showed that on a single crystal in.1 mol/l KH, the RR proceeded through the four-electron reaction pathway with only a very Received date: 19 ctober 29. *Corresponding author. Tel: ; Fax: mjafarian@kntu.ac.ir English edition available online at ScienceDirect (

2 42 催化学报 Chin. J. Catal., 2, 31: small amount of peroxide formation, while in.1 mol/l HCl 4 at a low over-potential, the RR proceeded entirely as a two-electron process. Despite numerous investigations of the RR on there is still little information about the sub-steps of the RR mechanism. The purpose of this work was the detailed investigation of the multistep reduction of oxygen on silver in alkaline solution. 1 Experimental A polycrystalline rod of 99.99% purity was used as the working electrode. The geometric surface area of the electrode was.3 cm 2. A graphite rod and homemade /Cl (KCl saturated) (.222 V vs NHE) electrode were used as counter and reference electrodes, respectively. All potentials are reported in volts versus /Cl. NaH was obtained from erck and was analytical grade. The water was doubly distillated. The concentration of NaH was 1 mol/l in all experiments. All experiments used the soluble oxygen in NaH, 1. mol/l (164 4 ml/(ml of solvent) [27]) at 2 o C. Cyclic voltammograms were recorded using a EG&G model 273A potentiostat/galvanostat controlled by a PC with a GPIB interface and 27 commercial software. Impedance studies were carried out using a Solartron SI 12 frequency response analyzer with a 273A potentiostat/galvanostat run by a PC and 398 software. 2 Results and discussion Figure 1 presents the changes in the shape of the cyclic voltammogram upon increasing the cathodic limit. Three cathodic (A, B, and C) and anodic (A, B, and C ) peaks appeared in the voltammograms. The intensity of some of the peaks, e.g. A, increased with increasing cathodic limit A 4. C B Fig. 1. Cyclic voltammograms with different cathodic limit potentials. Working electrode: ; reference electrode: /Cl; scan rate: mv/s. C' B' A' The effects of an oxygen saturated and nitrogen saturated electrolyte on the response of the electrode in alkaline medium are shown in Fig. 2. Peaks A and B (Fig. 1) became larger with the presence of oxygen. This suggested that these peaks were due to the reduction and oxidation of oxygenated species. The minor peaks in the nitrogen saturated solution may be due to adsorbed oxygen on the surface of the electrode. I/μA (2) (1) E/mV Fig. 2. The effects of oxygen (1) and nitrogen (2) on the response of electrode. Working electrode: ; reference electrode: /Cl; scan rate: 2 mv/s. Fig. 3 presents the cyclic voltammograms recorded at various potential sweep rates in the range of 2 to 8 mv/s. Seven reduction and four oxidation peaks were observed. The peaks A and B had given rise to six peaks (1, 2, and 3) and (4,, and 6) mv/s mv/s mv/s 2 mv/s 4 mv/s 8 mv/s Fig. 3. Influence of changing scan rate on cathodic and anodic peaks. Working electrode: ; reference electrode: /Cl. To discuss this complex behavior, we need to know all the probable reactions that can take place in this region of the potential. The details of the mechanism of these reactions are presented in Scheme 1 (part (a) and (b)). This scheme is extracted from the reaction scheme in the appendix. Because at more negative potentials water can be re

3 JAFARIAN et al.: ultistep Reduction of xygen on Polycrystalline Silver in Alkaline Solution 43 [2] (a) 2 H [23] H2 H22 o + H2 - o H2 H o H H - o H o Na 2 H - H2 3 7b H2 + H - H2-2Na Electrochemical desorption 4 Without desorption + H - [1] H22 b H2 H2 - Chemical decomposition H2 a H22 +H - 6 H2 - Chemical adsorption - + H H +H - H H - Electrochemical desorption H - [23] 9 H - -H - Chemical adsorption [24] (b) 2 H2 H2 - - H H +2H- 2H - [23] (c) H2 H +H - H2 H H2 + H - [ 26] H H H2 Scheme 1. The possible reactions in the mechanism. duced to hydrogen, its reaction is also presented in the scheme (part (c)). The peaks were assigned from Adanuvor et al. [23] with peak 1 assigned to steps 2 4 (Scheme 1). The peroxide produced can decompose by step b, or undergo electrochemical reaction, step a, to yield hydroxide if the potential was negative enough. Electrochemical impedance spectroscopy is a useful tool for determining the actual path. The high frequency resistances obtained from the impedance results changed dramatically with direct current (DC) potential (Fig. 4). The high frequency resistance was due to the electrolyte and electrode resistances, and the changes were due to different adsorbed intermediates being formed at different potentials. At the beginning of the measurement, the high frequency (a) R/ohm 3 (b) Fig. 4. (a) Nyquist plots with different DC potentials. Working electrode: ; reference electrode: /Cl. (b) The changes in high frequency resistance versus DC potential.

4 44 催化学报 Chin. J. Catal., 2, 31: resistance, R Ω, was constant. When the potential was set at. V, this resistance was doubled. According to the mechanism presented in Scheme 1, this increase was due to adsorbed oxygen on the surface (step 1), which created an insulating surface that increased R Ω. When the potential became more negative (.1 V) and adsorbed oxygen was converted to 2, this resistance decreased because of the change from an insulator substrate to a charged surface. The next step of the spectrum was taken at.2 V and we observed an increase in R Ω. This was due to the conversion of 2 to adsorbed H 2 (step 3). We measured the decrease and increase in ohmic resistance in the next steps. These would be due to steps 4 6. So, an electrochemical desorption at.3 V has occurred (the 2nd peak). The following increase in resistance was due to peroxide adsorbing again on the surface, which reacted with water and produced adsorbed hydrogen peroxide. This product accepted two electrons in two steps and was converted to hydroxide and desorbed from the surface electrochemically (steps 6 8 that gave the third and fourth peaks). The sharp reduction in resistance confirmed this. n the other hand, peak was in the region of the potential that was characterized by the formation of H UPD [28 3]. The sixth peak was due to H PD [28 3]. The measured high frequency resistance increased in these potential regions because of the covering of the surface with adsorbed atomic hydrogen. When the evolution of hydrogen at more negative potential occurred (peak 7), the surface became empty and the resistance decreased. The Bode phase results showed phase angles bigger than 4º, which showed strong adsorption in most of the steps of the mechanism [31] (results not shown here). The first anodic peak (peak 8) was due to the oxidation of H PD. To understand the other anodic peaks, cathodic conditioning was applied for specific lengths of time prior to the anodic cycles. Increasing the conditioning time at.7 V generated and increased the current density of an anodic peak at.4 V and shifted it to.3 V (Fig. ). Hydroxide can be adsorbed on the surface at.6 V [24], so, this peak was due to the oxidation of hydroxide ion according to Eq. (). The current density of another anodic peak located at.1 V increased strongly. There was a shoulder between the two anodic peaks that was more towards the more anodic peak. According to Savinova et al. [24], this shoulder was explained by Eq. (6) when its product, ads, diffused into the subsurface and formed sub, Eq. (7), step 14, when the potential was more positive and scan rate slow enough for the penetration of ads into the subsurface layers [24]. The last anodic peak was due to the oxidation of H UPD. This suggestion was in good agreement with the higher stability of H UPD in comparison with H PD [28 3]. The effect of consecutive cycles was also investigated. The first and third cycles are presented in Fig. 6. In the first cycle, the broad peak located at.7 to.9 V consisted of peaks 4 and in Fig. 3. In the third cycle, the current due to peak was dramatically decreased and that of peak 4 increased. According to the mechanism, the product of peak 3 (adsorbed hydroxide) was reduced in peak 4. n the other hand, the current intensity of peak 4 was higher than that of peak 3 (Fig. 3). These intensities were accounted for as follows. During cycling, adsorbed oxygen was produced and reduced to adsorbed hydroxide ion at.6 V [24]. Two species that lead to electrochemical hydroxide desorption existed (step 8), ads and H 2 2ads s s s 2 s First cycle Fig.. Cyclic voltammograms with different initial conditioning time. Third cycle Fig. 6. Cyclic voltammograms with cycling. No. of cycles: 3; working electrode: ; reference electrode: /Cl; scan rate: mv/s. Silver is catalytically active for peroxide decomposition, so peroxide produced was simultaneously consumed by chemical reaction and electrochemically. At a low scan rate, the catalytic decomposition of peroxide was faster than the electrochemical reaction, and so the third peak (Fig. 3) did not appear at the low scan rate. According to Goszner et al. [2], adsorbed oxygen on the surface decreased the catalytic activity of silver, which was another reason for the en-

5 JAFARIAN et al.: ultistep Reduction of xygen on Polycrystalline Silver in Alkaline Solution 4 Scheme 2. Proposed mechanism for RR on a silver polycrystalline surface in alkaline solution. hancement of the electrochemical reaction over the catalytic reaction in the next experiments with a higher potential scan rate. The decrease in the current intensity of peak (Fig. 6) in the third cycle has been previously reported [28 3]. In the first cycle, H UPD was formed on the surface, while in the second potential scan, due to the covering of the surface with H ads and the remaining H UPD, there was not enough surface sites for the reduction of H UPD, and so the current intensity of peak was reduced. n the other hand, Giles et al. [32] reported that an alkaline metal ion (e + ) can be reduced on sites on silver in an alkaline medium to form a e phase: e + + e e (8) Then, water can be reduced on it: H 2 + e H. (e) + H (9) Reaction (8) takes place at potentials that are more positive than the hydrogen reduction potential and can change the silver surface. This reaction is in competition with the reduction of peroxide to hydroxide and even can delay oxygen reduction reaction [31]. According to this hypothesis, the constant or lower ohmic resistance in such a DC potential region is expected because of the covering of the surface with metallic sodium. However, the impedance diagram showed increased resistance. This phenomenon was due to the accumulation of H UPD and then H PD, and insulating agents on the surface. The proposed mechanism for RR on a silver polycrystalline surface in alkaline solution is shown in Scheme 2. It can be seen that the reduction of oxygen occurred by a twoelectron pathway. 3 Conclusions The oxygen reduction reaction on polycrystalline silver in an alkaline medium was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The reduction of oxygen occurred by a two-electron pathway. With different scan rates, all steps of the reaction appeared in the cyclic voltammograms. At low scan rates, the catalytic decomposition of peroxide was the main reaction but at high scan rates, the electrochemical reduction of peroxide dominated. The change of high frequency resistance with changing DC potential showed that the intermediates which were on the electrode at different potentials were different. These intermediates can be inferred from the mechanism by referring to the corresponding potentials. According to the change of resistance, the reduction of sodium ion at the silver surface at a more positive potential than the hydrogen reduction potential was rejected. Appendix Probable reactions [23,24] in the mechanism in our investigation: 2 + 2H 2 + 4e 4H (1) 2 + 2H 2 + 2e H + H 2 (2) step 1 (2.1). 2 + e 2 step 2 (2.2) 2 + H 2 H 2 + H step 3 (2.3) H 2 + e H 2 + step 4 (2.4) H 2 + H 2 + 2e 3H (3) H 2 + H 2 step 6 (3.1) H 2 + H 2 H H step a (3.2) H e H + H step 7 (3.3) H + e H + step 8 (3.4) 2H H (4) H 2 + H 2 step 6 (4.1) H 2 + H step b (4.2) + H H + step 7b (4.3) H H ads + e () H +. H step (.1)

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