Distinct reaction pathways of methane oxidation on different oxidation states over Pd-based three-way catalyst (TWC)

Abstract

The impact of Pd oxidation state on CH4 oxidation kinetics over a supported Pd-based TWC was studied using combination of experimental and kinetic modeling approaches. Two distinct oxidation states were generated using different pre-treatment protocols that developed in this paper: pre-oxidized surface that was dominated by PdO and pre-reduced surface that was dominated by metallic Pd. The CH4 kinetics was investigated on the catalysts with these two different Pd oxidation states. It was discovered that metallic Pd played a vital role in low temperature CH4 oxidation activity, which was indicated by a higher reaction rate on the pre-reduced catalyst as compared to the pre-oxidized catalyst. The apparent activation energies were estimated to be 94.0 and 82.2 KJ/mole on PdO dominated and metallic Pd dominated surfaces, respectively. At higher temperature, CH4 conversions on pre-reduced catalysts presented a “bend” shape, which could not be merely explained by the CH4 kinetics built upon the pre-reduced catalyst. This “unusual” change in CH4 kinetics was explained by the transition of metallic Pd to PdO with increasing temperature. CH4 oxidation kinetics followed two distinct pathways on metallic Pd and PdO dominated surfaces: the activation of CH4 on PdO dominated surfaces requires a pair of site including a PdO site and a vacant site while activation of CH4 on metallic Pd dominated surfaces involves a pair of site consisting of a metallic Pd and an oxygen lattice from support. Additionally, it was revealed that CH4 activation was much faster on metallic Pd as compared to PdO. A CH4 oxidation kinetic model was developed to simulate the dynamic change of Pd active site pairs so as to accurately predict the CH4 oxidation with different Pd oxidation states under reaction conditions.