Methanol oxidation on a carbon-supported Pt fuel cell catalyst - A kineticand mechanistic study by differential electrochemical mass spectrometry

Methanol oxidation on a supported Pt fuel cell catalyst was investigated by on-line differential electrochemical mass spectrometry (DEMS) at continuou s electrolyte flow and defined catalyst utilization, employing a thin-film electrode setup and a thin-layer flow-through cell. The active surface of t he Pt/Vulcan (E-TEK) high surface area catalyst was characterized quantitat ively by H-upd and preadsorbed CO monolayer stripping. Methanol stripping D EMS experiments, oxidizing the adsorbed dehydrogenation products formed upo n methanol adsorption at potentials in the hydrogen adsorption region, show that the coverage of these products and hence the methanol uptake depend o n the electrode potential, in contrast to the potential-independent COad co verage. The dehydrogenation products cannot be displaced by H-upd. The numb er of close to two electrons used per oxidation of one adsorbed dehydrogena tion product identifies this as COad species. Further methanol dehydrogenat ion is hindered when the CO adlayer reaches a density of 1/3 monolayers. Si de reactions during bulk methanol oxidation were identified directly by DEM S, showing methylformate formation in addition to the main product, CO2. Th e extent of formaldehyde and formic acid formation was estimated from mass spectrometric and faradaic currents to be between 25% and 50% per dehydroge nation step. The exclusive formation of fully deuterated methylformate upon oxidation of deuterated methanol underlines the irreversibility of methano l dehydrogenation and rules out H/D exchange. A rather low kinetic H/D isot ope effect implies that the removal of poisoning COad intermediates rather than C-H bond dissociation determines the methanol oxidation rate, although there is a contribution from the latter step. Reduction of an anodically p reformed PtO monolayer by methanol under open-circuit conditions indicates that Pt oxy species are equally active for methanol oxidation.