Effects of thermal activation on the oxidation pathways of methanol at bulk Pt–Ru alloy electrodes

The competition between pathways that lead to adsorbed CO and CO2 during the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO4 on two bulk Pt–Ru alloys (10 at.% Ru (XRu≈0.1) and 90 at.% Ru (XRu≈0.9)) was investigated for temperatures in the range of 25–80°C. On the high Ru content alloy studied (XRu≈0.9), the dissociative chemisorption of methanol was inhibited below 70°C; the faradaic current for methanol oxidation was low, and only small quantities of adsorbed CO and CO2 were detected with infrared spectroscopy between 0.2–0.8 V (vs. RHE). At 80°C, strong infrared bands from CO2 and adsorbed, atop coordinated CO were observed over the potential ranges of 0.4–0.8 V and 0.2–0.8 V, respectively. The infrared measurements are consistent with the observation that bulk, high Ru content alloy electrodes appear passivated toward methanol oxidation below 70°C. On the low Ru content alloy studied (XRu≈0.1), the methanol surface chemistry was similar to that of pure, polycrystalline Pt, but the electrode was more poison resistant than Pt. For both alloys, the persistence of strong adsorbed CO bands and rapid CO2 production between 0.4–0.8 V suggests CO functions as a reactive species with high steady-state coverages at these potentials.