Reaction mechanism and dynamics of methanol electrooxidation on platinum(111)

Surface reactions in methanal electrooxidation to carbon dioxide are influenced by the accumulation of adsorbed partial decomposition products. We developed a Langmuir-Hinshelwood kinetic model for the surface chemistry based on current understanding of methanol electrooxidation. The model, which contains four kinetic and four mechanistic parameters, accounts for CO2 production by both CO oxidation (serial pathway) and oxidation of a non-CO reactive intermediate (parallel pathway). Model parameters were estimated by fitting time-dependent kinetic measurements of the charge passed following a potentiostatic step to 0.6 V on Pt(111) in 0.1 M methanal bearing electrolyte and the charge passed during voltammetric stripping of accumulated residue in blank electrolyte. The model gave an excellent fit to the experimental data, which spanned four orders of magnitude in time (0.03-300 s). Results from this exercise show that the non-CO reactive intermediate has the stoichiometry H:C:O prior to reacting to either CO or CO2; oxidation of H:C:O accounts for most of the CO2 produced. The most abundant adsorbate varies from H:C:O at short times, when the total adsorbate coverage is low, to CO at long times, when the total adsorbate coverage is high. Accumulation of H:C:O and CO not only poisons the surface, but also transfers kinetic control between different surface reactions. The predictive capability of the model was tested by comparing model predictions with experimental data obtained in 0.5 M methanal bearing electrolyte. We discuss the influence of residue accumulation on the rates of individual surface reactions, the significance of adsorbate interactions, and limitations of the model. (C) 1999 Elsevier Science S.A. All rights reserved.