Selective oxidation of methyl α- d-glucoside on carbon supported platinum : III. Catalyst deactivation

Abstract

Platinum supported on activated carbon and platinum supported on graphite were characterized before, during and after oxidation of methyl α- d-glucoside in water. Ex situ information was obtained by X-ray photoelectron spectroscopy, cyclic voltammetry, scanning transmission electron microscopy and carbon monoxide chemisorption. In situ information was obtained by cyclic voltammetry. Furthermore, the open circuit potential of a platinized platinum foil was monitored during reaction. A distinction could be made between a reversible deactivation, occurring on a time scale of 10 ks and an irreversible deactivation, occurring on a time scale of 100 ks. The reversible deactivation is attributed to a slowly increasing oxygen surface coverage rather than to the formation of platinum oxide. Chemisorption takes place beyond the steady-state degree of oxygen coverage expected from the intrinsic initial reaction kinetics and causes a reversible decrease of the reaction rate. The irreversible deactivation is caused by the dissolution and subsequent redeposition of platinum. The preferential dissolution of small platinum particles and redeposition on larger ones leads to a decrease of the fraction of exposed platinum atoms from 0.7 to 0.5. The effect of the irreversible decrease of the platinum surface area on the specific reaction rate is strongly attenuated by an antipathetic structure sensitivity. An increase of the average platinum particle diameter results in an increase of the turnover frequency.