Glycerol Oxidation Reaction Mechanism on Ag-Modified Pt Electrode in Alkaline Medium

Abstract

Introduction Recently, biodiesel fuel (BDF) attracts attention due to the progress of global warming. Glycerol as a byproduct is produced with BDF, and practically used for pharmaceuticals, cosmetics, food additives and so on. With the mass production of BDF, new applications of the glycerol byproduct need to be found. The application of glycerol as anode for direct alcohol fuel cells (DAFCs) can be an important solution because it has low toxicity and high specific energy (5.0 kWh kg-1). However, glycerol has two C-C bonds and three OH groups, so the mechanism for glycerol oxidation reaction (GOR) is complex,1 and the rate for GOR is so slow that the complete oxidation to CO2 is very difficult. Palladium is known to be one of active materials for GOR. We have found the alloy nanoparticles of Pd with Ag or Au improved the GOR activity and tolerance to the poisoning species on Pd nanoparticle in alkaline medium due to the electronic and bi-functional effects.2 Meanwhile, the Pt electrode had higher GOR activity than the Pd electrode in terms of the onset potential of GOR current, and Ag would improve the GOR activity of Pt. In this study, the effect of the modification with Ag on the GOR activity of a Pt substrate was evaluated by cyclic voltammetry (CV). Moreover, the mechanism for GOR on the Ag-modified Pt (Ag/Pt) electrode was discussed based on in situ infrared reflection absorption spectra at different potentials. Experimental The Ag/Pt electrode was prepared by underpotential deposition of Cu (Cu-upd) on a Pt substrate and the following galvanic replacement with Ag. The coverage of Ag (θ Ag) on the Pt substrate were calculated from the charge for H desorption before and after the Ag modification. The Ag/Pt electrodes with θ Ag = 0.8 and 0.5 were used in this study, which are denoted as Ag(0.8)/Pt and Ag(0.5)/Pt, respectively. The electrolytes were 1 M KOH or (1 M KOH + 0.5 M glycerol) solutions. Pt plate (1 cm × 1 cm) and a Hg/HgO electrode were used as counter and reference electrodes, respectively. In situ infrared reflection-absorption spectroscopy (IRAS) was used to qualitatively analyze the products of GOR on the Pt, Ag(0.8)/Pt and Ag(0.5)/Pt electrodes in alkaline solution. All electrochemical measurements were performed at room temperature. Results and Discussion The GOR activity for the Pt, Ag(0.8)/Pt and Ag(0.5)/Pt electrodes in alkaline medium was evaluated by CV (Fig. 1). Both Ag(0.8)/Pt and Ag(0.5)/Pt electrodes had twice higher current density and about 150 mV lower onset potential than the Pt substrate. This means that the enhancement of the GOR activity is caused by the modification of Ag atomic layers. Moreover, as θ Ag increased, a GOR wave was separated into two. The durability for GOR was evaluated by potentiostatic electrolysis for 60 min at -0.1 V and -0.3 V (vs. Hg/HgO). At -0.1 V, the durability for GOR was decreased in the order of Ag(0.5)/Pt > Pt > Ag(0.8)/Pt. On the other hand, at -0.3 V, the Ag(0.5)/Pt electrode still showed the best tolerance to the poisoning species, but the durability for GOR of Ag(0.5)/Pt was poorer than that of Pt. These results indicated that the modification of Ag on Pt improved the GOR activity and durability. IRAS gives us the information on the GOR mechanism. In the case of the Pt electrode, the mainly absorption band are observed at 1310 cm-1, 1335 cm-1, 1350 cm-1, 1385 cm-1 and 1575 cm-1 which can be attributed to glyceraldehyde or glycerate, dihydroxyacetone, hydroxypyruvate, symmetric and symmetric O-C-O stretching of carboxylate, respectively. At higher potentials, the absorption band at 1310 cm-1 was obviously observed. On the other hand, in the case of both Ag(0.8)/Pt and Ag(0.5)/Pt electrodes, the absorption band at 1335 cm-1 due to dihydroxyacetone was more obviously observed at -0.4 to -0.2 V, whereas at -0.2 to 0.1 V, the absorption band at 1310 and 1350 cm-1 was strengthen. This results indicate that the oxidation of primary and secondary OH groups depends on potential, and the secondary OH (-0.4 to -0.2 V) and the primary OH ( -0.1 V) are preferentially oxidized on the Ag modified Pt electrodes. Acknowledgement This work was partially supported by JSPS KAKENHI Grant Number 15H04162. References 1) M. Simoes et al., Appl. Catal. B: Environ., 93, 354 (2010).2) B. T. X. Lam et al., J. Power Sources, 297, 149 (2015). Figure 1