Mechanism of the Catalytic Oxidation of Glycerol on Polycrystalline Gold and Platinum Electrodes

Abstract

This paper addresses the oxidation mechanism of glycerol on Au and Pt electrodes under different pH conditions. Intermediates and/or reaction products were detected by using an online high-performance liquid chromatography technique (for soluble products) and online electrochemical mass spectrometry (for CO2). In alkaline media, the main product of glycerol oxidation on the Pt electrode is glyceric acid produced via glyceraldehyde. Glyceric acid is the primary oxidation product on the Au electrode, which is further oxidized to glycolic acid and formic acid at high potentials (>= 0.8 V), yielding high current densities. As the pH of the solution is lowered, the glycerol oxidation becomes significantly more sluggish on both Au and Pt electrodes, which results in glyceraldehyde being the main oxidation product under neutral conditions, especially on gold. In acidic solutions, only the Pt electrode shows catalytic activity with a relatively low conversion rate, mainly to glyceraldehyde. At positive potentials corresponding to the formation of a Pt surface oxide, the PtOx surface oxide catalyzes the conversion of glyceraldehyde finally to formic acid and CO2, but only under acidic conditions. Gold catalyzes glycerol oxidation only under alkaline conditions, in contrast to a "real catalyst," that is, platinum, which catalyzes glycerol oxidation over the entire pH range.