Pre-exponential factor and mechanism of anodic oxidation of chemisorbed methanol and formaldehyde on platinum

Abstract

Summary The effect of temperature on the anodic oxidation of chemisorbed methanol and formaldehyde layers on platinized platinum has been studied in 0.5 M H2SO4 and in 0.1 M HCl+0.45 M H2SO4. The activation energy of the anodic oxidation process for the two films is the same and is attributed to a close structural similarity of the activated complexes formed in the two cases. The activation energy is the same in acid chloride as in the acid sulfate medium, and obeys the same potential dependence. The entropy of activation is virtually independent of electrode potential in 0.5 M H2SO4; in the acid chloride medium, however, it decreases markedly with increase in electrode potential. The results are interpreted in terms of the effect of chloride coverage on the free energy of activation. It is assumed, in conformity with literature data, that chloride coverage is also influenced by electrode potential and pH of the solution. On this basis, the kinetic data including the observed Tafel slopes and pH effects are explained in terms of a nonelectrochemical rate-determining step involving the adsorbed organic species and the adsorbed OH radical. Probable reaction paths are discussed. The relative effects of chloride ions and irreversibly adsorbed iodide ions on the kinetic parameters are examined in terms of the charge carried by the anion in the adsorbed state. It is suggested that the irreversibly adsorbed iodide anion is virtually a neutral particle and, as such, does not influence the reaction rate except through a blocking of the platinum sites.