Effect of Silver Modification on Glycerol Oxidation Reaction Activity of Palladium Electrode in Alkaline Medium

Abstract

Introduction Biodiesel fuel (BDF) attracts attention as a carbon-neutral fuel as well as bioethanol. As the production of BDF increases, the production of the glycerol byproduct is also increasing. It is important to develop new uses for glycerol to cope with the mass production of BDF. The use of glycerol as a fuel for direct alcohol fuel cells is promising, because anodic oxidation of glycerol produced by bioprocesses occurs in carbon-neutral cycles and direct glycerol fuel cells are expected to produce electricity with a low environmental load and to be energy efficient. It is well-known that Pd is cheaper than Pt and Au, and Pd-based alloy electrodes are highly active for the oxidation of alcohols in alkaline media. We also have reported the PdAg alloys-loaded carbon exhibited higher glycerol oxidation reaction (GOR) activity and tolerance to poisoning species than Pd.1 The mechanism for GOR, however, is not so clear. In this study we prepared an Ag atomic layer-modified Pd model electrode, and investigated the potential dependence of GOR products by in situ infrared reflectance-absorption spectroscopy (IRAS). In addition, we discussed the GOR mechanism based on these data. Experimental The Ag atomic layer-loaded Pd (Ag/Pd) electrode was prepared by underpotential deposition of Cu (Cu-upd) on a Pt polycrystalline substrate and the following galvanic replacement with Ag. The coverage of Ag (θ Ag) on the Pd substrate was evaluated to be 0.5 from the charges for Cu-upd before and after the Ag modification. 1 M KOH and (1 M KOH + 0.5 M glycerol) solution were used for electrochemical measurements. In situ infrared reflection-absorption spectroscopy (IRAS) was used to qualitatively analyze the products of GOR on the Pd and Ag/Pd electrodes in alkaline solution. All electrochemical measurements were performed at room temperature. Results and Discussion The cyclic voltammograms of the Pd and Ag/Pd electrodes in a (1 M KOH + 0.5 M glycerol) solution is shown in Fig. 1. The Ag/Pd electrode had higher oxidation current and more negative onset potential of the oxidation current than the Pd electrode, clearly indicating that the GOR activity was enhanced by the modification of the Ag atomic layer. In addition, the peak potential of the oxidation current for the Ag/Pd electrode was more positive than that for the Pd electrode, suggesting that the former was superior in tolerance to poisoning species to the latter. IRAS spectra for the Pt electrode exhibited that the absorption peak at 1335 cm-1 assigned to the formation of dihydroxyacetone was mainly increased at lower potentials, but the absorption peak at 1310 cm-1 assigned to the formation of glycerate was increased at higher potentials. This suggests that the secondary OH group in a glycerol molecule is preferentially oxidized at smaller overpotentials, and the primary OH group is greatly oxidized at larger overpotentials. For the Ag/Pd electrode, the oxidation of the primary OH group occurred even at smaller overpotentials, and hydroxypyruvate was also formed at larger overpotentials. Acknowledgement This work was partially supported by JSPS KAKENHI Grant Number 15H04162. Reference 1 B. T. X. Lam, M. Chiku, E. Higuchi, H. Inoue, J. Power Sources, 297, 149 (2015). Figure 1