Electrocatalytic oxidation and adsorption rate of methanol at Pt stepped single-crystal electrodes and effect of Ru step decoration: a DEMS study.

Abstract

The adsorption and oxidation of methanol at Pt(331) and Ru-step-decorated Pt(331) electrodes are studied recording currents and ion currents by online differential electrochemical mass spectrometry. The CO(2) current efficiencies and the degree of surface poisoning with CO(ad) formed during methanol oxidation are independent of the flow rate, confirming the parallel pathway mechanism. The CO(2) current efficiencies decrease with increasing methanol concentration and increase with increasing potential, whereas those of methyl formate show a reverse trend. At potentials higher than 0.6 V, neither the CO(2) current efficiencies nor the methanol oxidation currents increase with increasing Ru coverage. Instead, methanol oxidation is inhibited due to blocking of the most active platinum step sites. At potentials lower than 0.6 V, however, not only the onset of methanol oxidation shifts negatively, by about 0.1 V, but also the methanol oxidation current and the CO(2) current efficiencies increase. Crucial for the use in fuel cells is the complete oxidation to CO(2), which can be achieved if the reactants first adsorb at the electrode surface along the reaction path with adsorbed CO as an intermediate. Therefore, we directly determine the methanol adsorption rates at Pt(331) as well as at Ru-step-decorated Pt(331), Pt(332), Pt(100), and Pt(11,1,1) electrodes. The methanol adsorption rate is doubled by a double step density in the case of the Pt(331) and Pt(332) electrodes, higher at higher Ru coverages, and increases by a factor of three upon increasing the potential by 0.1 V (corresponding to a Tafel slope of approximately 200 mV dec(-1)). At Pt(331) electrodes with partial step decoration, stripping of adsorbed CO (from CO gas) reveals two adsorbate states, which are also discernable when the adsorbate formed from methanol dehydrogenation is stripped.