Investigating the Origins of the pH-Dependent Oxidation of Glycerol over Platinum Using Differential Electrochemical Mass Spectrometry

The glycerol oxidation reaction (GOR) to give CO₂ could be harnessed to produce energy in a direct glycerol fuel cell. However, state-of-the-art electrocatalysts for this reaction exhibit poor voltage and current efficiency. Thus, there is a need to develop superior electrocatalysts for this process. However, rational electrocatalyst design is difficult due to the complicated reaction mechanism, which is poorly understood. In the present study, differential electrochemical mass spectrometry is utilized to investigate the mechanism of glycerol oxidation over Pt. This is accomplished by investigating the products formed during the electrochemical oxidation of glycerol and various potential reaction intermediates during cyclic voltammetry in acidic and alkaline media. Mass spectrometric cyclic voltammograms showed that the m/z = 44 signal, assigned to CO₂, was detected during GOR over a Pt electrode in a 0.5 M H₂SO₄ solution. At electrode potentials greater than 1.0 V vs RHE, the m/z = 29 signal was also detected, suggesting the formation of formic acid (FA) by C-C bond cleavage reaction. The influence of electrolyte pH on the adsorption of glycerol and its various potential oxidation products on Pt(110) and Pt(100) active sites were investigated. Glycerol, glyceraldehyde, dihydroxyacetone, and FA were oxidized at an overpotential of ca. 0.7 V in 0.5 M H₂SO₄. However, the oxidation of most of the species potentially derived from glycerol required a higher overpotential (>1.0 V), providing an explanation for the low GOR kinetics in acidic media. Conversely, glyceric acid, hydroxypyruvic acid, mesoxalic acid, and glycolic acid were oxidized to CO₂ at an overpotential of 0.5 V in alkaline media. Thus, alkaline media facilitates GOR kinetics by enabling intermediate oxidation products to remain adsorbed to the electrode surface.