Catalytic and electrochemical evaluation of the role of metal oxides on Pd nano-catalysts for complete methane oxidation

Catalytic complete methane oxidation over Pd-MOx (MO_x = SnO₂, FeO_x, and ZnO) nanoparticles deposited on YSZ solid electrolyte was evaluated for and compared to a monometallic Pd catalyst. To this end, the nanoparticles were synthesized via the polyol method and tested for methane oxidation in a temperature range from 200 to 475 °C under reducing, stoichiometric, and oxidizing reaction conditions in open-circuit conditions. The light-off experiments revealed that the presence of a second phase in the form of metal oxide (SnO₂ and ZnO) increased the catalytic rate of the reaction compared to monometallic palladium in all gas compositions. However, the addition of iron oxide to Pd showed a different behaviour, i.e., a strong inhibition of the reaction rate in the oxidizing and stoichiometric conditions and significant promotion in the reducing conditions. To gain an insight into the role of MO_x in various conditions, the detailed electrochemical measurements were carried out at selected temperatures. The exchange current density (i_o) of the electrochemical process at the three phase bpoundary was found to depend on the oxidation state of the catalyst, which in turn influences the catalytic rate of Pd-MO_x. Overall, the addition of the cheaper oxide to Pd significantly promotes the catalytic reaction, and the inverse relation between the catalytic rate and i_o was found in agreement with the electrochemical promotion of catalysis (EPOC) mechanism, where the lower exchange current density values correspond to the higher catalytic reaction rate of complete methane oxidaiton.