Why Methanol Electro-oxidation on Platinum in Water Takes Place Only in the Presence of Adsorbed OH

Abstract

Untangling the mechanism of the methanol electro-oxidation on platinum in water as a model reaction is essential to optimize fuel cells using alcohols as fuel. Recent experiments unexpectedly suggested that this electro-oxidation process would take place only in the presence of adsorbed OH. The here reported results, carefully obtained under low methanol concentrations on the three basal planes of platinum at different scan rates to discriminate between oxidation and adsorption processes, confirm such an unexpected preliminary observation. It is found that adsorbed CO from methanol is only formed when adsorbed OH is already present on the surface. This observation is a clear indication that adsorbed OH is involved in the mechanism beyond providing the oxygen group required to oxidize adsorbed CO, which has never been considered before. Supported by density functional theory calculations, the role played by adsorbed OH in the methanol electro-oxidation to CO on platinum in water and the reason why this reaction is not observed in the absence of adsorbed OH are also here both elucidated. A combination of kinetic and thermodynamic factors, such as the presence of multiple water molecules per methanol molecule, the high adsorbed OH mobility on the surface, the favorable coadsorption of methanol in the presence of adsorbed OH, and the favorable and virtually barrier-less hydrogen transference from the hydroxy group of methanol to adsorbed OH to yield water result in the immediate activation of methanol (as soon as the molecule approaches the surface) through the favorable substitution of adsorbed OH by adsorbed methoxy. This contribution represents a change of paradigm in the understanding of how alcohols are electro-oxidized in reference systems and have crucial implications in the search for better electrocatalysts.