Electrochemical promotion (NEMCA) of CH4 and C2H4 oxidation on Pd/YSZ and investigation of the origin of NEMCA via AC impedance spectroscopy

The catalytic activity of Pd for the oxidation of methane to CO, can be mar kedly and reversibly affected using the effect of non-Faradaic electrochemi cal modification of catalytic activity (NEMCA effect) or electrochemical pr omotion (EP), i.e. by interfacing polycrystalline Pd films with Y2O3-stabil ized ZrO2 (YSZ) and varying Pd catalyst-electrode potential in galvanic cel ls of the type: CH4, O-3 CO2, Pd/YSZ/Au, CH4, O-2, CO2. It was found that b y applying positive overpotentials or currents and thus, supplying O2- onto the catalyst surface, up to 70-fold increase in the catalytic rate of CH, oxidation can be obtained, compared to the open circuit (unpromoted) cataly tic rate. Electrochemical oxygen removal from the Pd catalyst-electrode sur face, following negative overpotential or current application, also enhance s the catalytic rate by up to a factor of 10. The induced changes in cataly tic rate were: typically two orders of magnitude higher than the correspond ing rate of ion transfer to the catalyst-electrode surface, i.e. Faradaic e fficiency Lambda values of the order of 150 were attained. The results can be rationalized on the basis of the theoretical considerations invoked to e xplain NEMCA behavior, i.e. the effect of changing work function on chemiso rptive bond strengths of catalytically active electron donor or acceptor ad sorbates. The existence of charged back-spillover species giving rise to ca talyst work function change for positive overpotential application is manif est in this work for the first time using AC impedance spectroscopy. (C) 20 00 Elsevier Science B.V. All rights reserved.