Relationship between the Conductivities of CeO2-ZrO2-MOx (M = Bi, Ca, Sn, Ni, Fe) Solid Solutions and Catalytic Activities during Methane Oxidation

We investigated the relationship between the conductivities of CeO2-ZrO2-MOx (M = Bi, Ca, Sn, Ni, Fe) solid solutions and the catalytic abilities of catalysts composed of PdO on CeO2-ZrO2-MOx/γ-Al2O3 supports. The oxide-ionic conductivities of the CeO2-ZrO2-MOx promoters increased with increasing oxygen vacancy concentrations and the electronic conductivities of these promoters were significantly enhanced by doping with Sn4+/2+, Ni3+/2+ and Fe3+/2+, all of which readily undergo valency changes. Studies of the oxygen release characteristics of CeO2-ZrO2-MOx/γ-Al2O3 samples found that the reduction temperatures obtained from hydrogen temperature programmed reduction data were primarily determined by the electronic conductivity of the promoter rather than by the oxide-ionic conductivity. However, the oxygen storage capacity of each CeO2-ZrO2-MOx/γ-Al2O3 sample was related to both the oxide-ionic and electronic conductivities. The catalytic methane oxidation activities of these materials were evidently affected by both the reduction temperature and the oxygen storage capacity. As a result, the activities of the PdO/CeO2-ZrO2-MOx/γ-Al2O3 catalysts were clearly dependent on the oxide-ionic and electronic conductivities of the CeO2-ZrO2-MOx promoters.