Electrochemical promotion (NEMCA) of CH 4 and C 2H 4 oxidation on Pd/YSZ and investigation of the origin of NEMCA via AC impedance spectroscopy

Abstract

The catalytic activity of Pd for the oxidation of methane to CO 2 can be markedly and reversibly affected using the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) or electrochemical promotion (EP), i.e. by interfacing polycrystalline Pd films with Y 2O 3-stabilized ZrO 2 (YSZ) and varying Pd catalyst-electrode potential in galvanic cells of the type: CH 4, O 2, CO 2, Pd|YSZ|Au, CH 4, O 2, CO 2. It was found that by applying positive overpotentials or currents and thus, supplying O 2− onto the catalyst surface, up to 70-fold increase in the catalytic rate of CH 4 oxidation can be obtained, compared to the open circuit (unpromoted) catalytic rate. Electrochemical oxygen removal from the Pd catalyst-electrode surface, following negative overpotential or current application, also enhances the catalytic rate by up to a factor of 10. The induced changes in catalytic rate were typically two orders of magnitude higher than the corresponding rate of ion transfer to the catalyst-electrode surface, i.e. Faradaic efficiency Λ values of the order of 150 were attained. The results can be rationalized on the basis of the theoretical considerations invoked to explain NEMCA behavior, i.e. the effect of changing work function on chemisorptive bond strengths of catalytically active electron donor or acceptor adsorbates. The existence of charged back-spillover species giving rise to catalyst work function change for positive overpotential application is manifest in this work for the first time using AC impedance spectroscopy.