Methanol Electro-Oxidation on Bimetallic PtMo/C Catalysts and Pt/C - Mo/C Mechanical Mixtures

The role of molybdenum in carbon-supported Pt-Mo electrocatalysts was studied with the aim of obtaining active materials to be used as anodes in the methanol electro-oxidation reaction. The catalysts were synthesized by the thermolysis of Mo and Pt carbonyls. Two series were obtained: bimetallic PtMo/C, and Pt/C plus Mo/C mechanical mixtures. The Mo content was changed, with the atomic ratio ranging from 0.0 to 1.0. XRD and XPS analyses of the fresh materials indicated the presence of Pt0 and molybdenum oxides (MoOx). XRD of Mo/C treated electrochemically at constant potential in H2SO4 confirmed the formation of molybdenum bronzes with different protonation degrees (HxMoO3 0.3 < x < 2). Cyclic voltammetry indicated that Mo/C does not present the capacity to oxidize methanol nor the intermediate species, but it did show promoter behavior. In the PtMo/C series, low molybdenum contents present the maximum promoting effect on methanol electro-oxidation, as evidenced by lower onset potentials for the methanol oxidation reaction with respect to the Pt/C catalyst. Similar results were registered by the mechanical mixtures, where the Pt sites are separated from the molybdenum bronzes, suggesting that Pt poisoned sites, caused by methanol electro-adsorption, are cleaned by the interaction with molybdenum bronzes (HxMoO3) through a process of surface diffusion of oxygen-containing species.