Infrared Spectroelectrochemical Study of Dissociation and Oxidation of Methanol at a Palladium Electrode in Alkaline Solution

The dissociative adsorption and electrooxidation of CH3OH at a Pd electrode in alkaline solution are investigated by using in situ infrared spectroscopy with both internal and external reflection modes. The former (ATR-SEIRAS) has a higher sensitivity of detecting surface species, and the latter (IRAS) can easily detect dissolved species trapped in a thin-layer-structured electrolyte. Real-time ATR-SEIRAS measurement indicates that CH3OH dissociates to COad species at a Pd electrode accompanied by a dip at open circuit potential, whereas deuterium-replaced CH3OH doesn't, suggesting that the breaking of the C H bond is the rate-limiting step for the dissociative adsorption of CH3OH. Potential-dependent ATR-SEIRAS and IRAS measurements indicate that CH3OH is electrooxidized to formate and/or (bi)carbonate, the relative concentrations of which depend on the potential applied. Specifically, at potentials negative of ca. -0.15 V (vs Ag/AgCl), formate is the predominant product and (bi)carbonate (or CO2 in the thin-layer structure of IRAS) is more favorable at potentials from -0.15 to 0.10 V. Further oxidation of the COad intermediate species arising from CH3OH dissociation is involved in forming (bi)carbonate at potentials above -0.15 V. Although the partial transformation from interfacial formate to (bi)carbonate may be justified, no bridge-bonded formate species can be detected over the potential range under investigation.