Methanol Electrooxidation over Pt/C Fuel Cell Catalysts: Dependence of Product Yields on Catalyst Loading

We have investigated the influence of the catalyst loading (7−35 μg Pt ·cm - 2 ) on methanol oxidation product yields over Vulcan XC72 supported Pt catalyst by differential electrochemical mass spectrometry (DEMS), both under potentiodynamic and potentiostatic (0.6 V RHE ) conditions. Experiments were performed at room temperature and under continuous flow conditions (0.1 M CH 3 OH in 0.5 M H 2 SO 4 solution). Absolute calibration of the DEMS signal allowed determination of current efficiencies for the final reaction product CO 2 and for the reaction intermediates formic acid and formaldehyde. These could be converted into turnover frequencies (TOFs) on the basis of the physical (transmission electron microscopy) and electrochemical (CO stripping, H UPD ) determination of the absolute particle surface area. The good agreement between these values indicates 100% utilization of the catalyst. The electrochemical efficiencies, product distribution, and the turnover frequencies of partial reactions (methanol oxidation to formaldehyde, formic acid, and CO 2 ), during the methanol oxidation reaction (MOR) show a pronounced dependence on the catalyst loading. With increasing Pt loading the current efficiency for methanol oxidation to formaldehyde decays significantly (40% to almost 0%), while that for complete oxidation to CO 2 increases from 50 to 80%. The variation in current efficiency for methanol oxidation to formic acid is small (ca. 10%). The product distribution varies accordingly; the absolute numbers, however, are different: Low (5−20%) formic acid yields are accompanied by a decaying formaldehyde yield (60% to zero) and increasing CO 2 yield (30−80%) with increasing catalyst loading. The total TOF values calculated from the values for the individual reaction pathways and products are significantly higher than the Faradaic TOF numbers at low catalyst loadings, while at high loadings, where complete oxidation of methanol to CO 2 is preferred, they agree satisfactorily. The pronounced variations in product distribution/TOF values are attributed to increasing readsorption and subsequent complete oxidation of desorbed reaction intermediates (formaldehyde and formic acid) at higher catalyst loadings, while at low loadings or, even more pronounced, on smooth electrodes these intermediates are more likely to survive. This has to be considered for predictions of MOR characteristics under fuel cell conditions on the basis of studies on smooth electrodes.