Influence of pH on noble metal catalysed alcohol oxidation: reaction kinetics and modelling

The Pt-catalysed oxidation of the alcohol methyl α-D-glucopyranoside to 1-O-methyl α-D-glucuronic acid has been studied under varying pH conditions. Two types of catalysts with different metal dispersion are used, that is, Pt on active carbon support and Pt on graphite support. The pH of the reaction medium is varied between 6 and 10. The initial reaction rate increases with an increase in pH, and so does the rate of catalyst deactivation, because of overoxidation, for both catalysts. The carbon-supported Pt catalyst gives higher initial reaction rates compared with the graphite-supported Pt catalyst. A dynamic, pH-dependent, electrochemical kinetic model is presented based on a detailed investigation of the mechanism of alcohol oxidation. The model adequately describes the observed pH effect by considering the alcohol dehydrogenation occurring in two parallel reaction steps, one playing a role under acidic conditions, which is independent of the hydroxyl concentration, and a second one playing a role under alkaline conditions, which is linearly dependent on the hydroxyl concentration.