Bifunctional versus Defect-Mediated Effects in Electrocatalytic Methanol Oxidation

The most prominent and intensively studied anode catalyst material for direct methanol oxidation fuel cells consists of a combination of platinum (Pt) and ruthenium (Ru). Classically, their high performance is attributed to a bifunctional reaction mechanism where Ru sites provide oxygen species at lower overpotential than Pt. In turn, they oxidize the adsorbed carbonaceous reaction intermediates at lower overpotential; among these, the Pt site-blocking carbon monoxide. We demonstrate that well-defined Pt modified Ru(0001) single crystal electrodes, with varying Pt contents and different local PtRu configurations at the surface, are unexpectedly inactive for the methanol oxidation reaction. This observation stands in contradiction with theoretical predictions and the concept of bifunctional catalysis for this reaction. Instead, we suggest that pure Pt defect sites play a more critical role than bifunctional defect sites on the electrodes investigated in this work.