Kinetic Study of the Reaction of Adsorbed CO with Methanol and Ethanol on Pt Electrode

Abstract

Direct alcohol fuel cells (DAFCs), which mostly use low-molecular weight alcohols such as methanol and ethanol as fuel, have attracted considerable interest in recent years. Methanol is recognized as a promising fuel for DAFCs, and thus the anode reaction in direct methanol fuel cells (DMFCs), namely, the electrooxidation of methanol, has been extensively studied in order to improve performance of DMFCs and to achieve cost reduction. Among various electrocatalysts for the oxidation, Pt-based anodes show excellent electrocatalytic activities. Complete electrooxidation of a methanol molecule, which is necessary for efficient oxidation, gives six electrons: CH3OH + H2O → CO2 + 6H+ + 6e-. (1) It is well accepted that the oxidation reaction proceeds on Pt electrodes via a dual path mechanism consisting of indirect and direct paths. The indirect path involves adsorbed carbon monoxide (COad) as a poisoning intermediate, and the direct path involves a non-CO intermediate as a reactive one. Water produces oxygenated spices such as hydroxide and oxide, and it plays an important role in removal of COad. Thus, it seems that the methanol oxidation does not proceed without water. However, as we have reported previously [1, 2], the oxidation produces current without water (panel a1 of Figure 1), and the current is of the same order of magnitude as that observed in the presence of water, e.g., 0.1 and 1 M methanol aqueous solutions. Detailed studies using surface-enhanced infrared absorption spectroscopy and high-performance liquid chromatography have revealed that COad reacts with methanol to form methyl formate (HCOOCH3): COad + CH3OH → HCOOCH3, (2) and also that the current is produced in the absence of water partly by the formation of formaldehyde (HCHO) and mainly by that of methyl formate via non-CO pathway: CH3OH → HCHO + 2H+ + 2e- (3) and 2CH3OH → HCOOCH3 + 4H+ + 4e-. (4) Methanol is toxic for human and is mostly produced from natural gas, which is a major disadvantage of using methanol as fuel. On the other hand, ethanol is non-toxic and can be sustainably produced on a large scale from biomass. In these regards, direct ethanol fuel cells have a strong advantage over DMFCs. Moreover, ethanol has a higher energy density than methanol, that is, its complete electrooxidation with water gives twelve electrons: C2H5OH + 3H2O → 2CO2 + 12H+ + 12e-. (5) The cleavage of the C–C bond, which is involved in the complete oxidation, requires a high activation energy. Thus, even when Pt is used as the anode, most ethanol is incompletely oxidized to acetaldehyde (CH3CHO) which may be further oxizied to acetic acid (CH3COOH): C2H5OH → CH3CHO + 2H+ + 2e- (6) and CH3CHO + H2O →CH3COOH + 2H+ + 2e-. (7) We have recently found that ethanol is also oxidized without water (panel b1). From the analogy of reaction 2, it has been deduced that COad reacts with ethanol to form ethyl formate (HCOOC2H5): COad + C2H5OH → HCOOC2H5. (8) As the electrode potential (E) increases, the coverage of COad gradually decreases (panel b2) and the current increases in the potential region above 0.4 V vs. SHE (panel b1). This indicates that the number of vacant sites on Pt surface increases gradually due to reaction 8, and reaction 6 proceeds at vacant sites at E > 0.4 V. To verify the occurrence of reactions 2 and 8, this present work conducts a kinetic study on the reaction of COad with the alcohols: the time course of the coverage of COad can be well interpreted by Langmuir-Hinshelwood mechanism, and the rate constants for the reactions 2 and 8 at 0.8 V are estimated to be 0.0042 and 0.0051 s-1, respectively (panels a3 and b3). Furthermore, the dependence of the rate constants on the E is studied, which will be discussed in the presentation. REFERENCES [1] H. Okamoto, T. Gojuki, N. Okano, T. Kuge, M. Morita, A. Maruyama, Y. Mukouyama, Electrochim . Acta, 136 (2014) 385. [2] Y. Mukouyama, S. Yamaguchi, K. Iida, T. Kuge, M. Kikuchi, S. Nakanishi, ECS Trans., 80 (2017) 1471. FIGURE CAPTION Figure 1 (top) Cyclic voltammograms for the oxidation of (left) methanol and (right) ethanol at a sweep rate of 0.1 Vs-1. The concentration of methanol was 24 M, and that of ethanol was 17 M. The high-concentration alcohol solutions contained 0.5 M H2SO4 as an electrolyte. (middle) The dependence of the coverage of COad on E for (left) the 24 M metanol oxidation and (right) the 17 M ethanol oxidation. (bottom) Time courses of the coverage of COad at 0.8 V. Figure 1