Direct Observation of Surface-Bound Intermediates During Methanol Oxidation on Platinum Under Alkaline Conditions

Direct methanol fuel cells (DMFCs) using alkaline electrolytes are of interest due to the applicability of nonprecious metal-based materials for electrocatalysts. However, the lack of understanding of the methanol oxidation reaction (MOR) mechanism in alkaline media hinders the development of active catalysts for the MOR. In this work, ambient-pressure XPS and in situ surface-enhanced infrared spectroscopy were performed on the Pt surface in order to gain experimental insights into the reaction pathway for the MOR. We present a comprehensive reaction mechanism for the MOR in alkaline media and show that the MOR proceeds via two different pathways depending on the electrode potential. We confirmed the formation of partially hydrogenated CO adsorbates [H_xCO_ad···(OH) (1 < x < 3)] via water and/or hydroxide ion-mediated dissociation of methanol. The H_xCO_ad···(OH) species were further dehydrogenated to CO_ad in the potential range of 0.40–0.60 V_RHE and subsequently oxidized to CO₂ by reactive OH_ad on the Pt surface at 0.65 V_RHE (pathway I). Furthermore, H₃C–O_ad intermediates were observed at potentials higher than 0.9 V_RHE, at which the MOR proceeds mainly via H₃C–O_ad instead of CO_ad intermediates (pathway II). The oxidation current related to this conversion from H₃C–O_ad to CO₂ (pathway II) dominates the overall MOR current, suggesting that the H₃C–O_ad pathway could be one of the keys to enhancing the MOR activity in an alkaline environment. Our findings pave the way toward a design strategy for MOR electrocatalysts with improved activity based on the experimental reaction mechanisms that have been identified.