The interaction between Pd/CeO2 crystal surface and electric field: Application to complete oxidation of methane

Abstract

Pd/CeO2 with different crystal faces can significantly affect the methane oxidation process, but the catalytic behavior in electric field is not clear yet. The supported Pd/CeO2 with rod, octahedral and cubic morphologies were synthesized by hydrothermal method, and their methane oxidation efficiency in electric field was measured. The structure of Pd/CeO2 and reaction mechanism in electric field were investigated by XRD, BET, XPS, H2-TPR, TEM and in situ DRIFTs. Studies show that the synergistic effect between electric field and (111) crystal plane in octahedral CeO2 is the strongest and (110) in rod CeO2 is the main active restraint crystal plane. The Pd atoms on the surface of Pd/CeO2-oct are exposed to the most amount of Pd atoms and the main Pd nanoparticles are present. Pd species on CeO2-cube and CeO2-rod surface with relatively low activity exist mainly in the form of Pd2+ and Pd4+, whereas Pd0 and Pd2+ on the Pd/CeO2-oct surface are more favorable for CH4 oxidation in the catalytic cycle. Pd4+ is inactive for CH4 oxidation. In addition, H migration from Pd active site to the support induced by hydrogen spillover can enhance the oxidation activity of CH4 in electric field. The conversion of carbonate/hydrogencarbonate to formate is an advantageous step in the methane oxidation process, with the active sequence being octahedral (111) > cubic (100) > rod (110) faces in/without electric field.