Co_3-xCr_xO₄ spinel oxides for propane total oxidation: Essential roles of metal cations in octahedral coordination

The essential roles of octahedrally coordinated cations (B-site in AB(2)O(4) formula) in Co3-xCrxO4 spinel oxides in the total oxidation of propane were demonstrated, using a series of Co3-xCrxO4 spinel oxides with different Co/Cr molar ratios derived from layered double hydroxides. The catalytic activity was strongly related to the compositions of B-site cations. The Co2CrO4 catalyst with B-sites equally occupied with Co and Cr cations was more active than the Co3O4 (Co cations only in B-sites) and CoCr2O4 (Cr cations only in B-sites), giving turnover frequencies of 0.86 x 10(-3), 0.72 x 10(-3) and 0.11 x 10(-3) s(-1) at 250 degrees C (under kinetically-controlled reaction), respectively. Kinetics and density functional theory (DFT) calculations on the Co2CrO4 and CoCr2O4 catalysts indicated that the reaction followed a Langmuir-Hinshelwood mechanism, and the adsorption of oxygen on the former was easier. Moreover, the cleavage of the first C-H bond in the propane as the rate-determining step was more favored on the Co2CrO4, which accounted for its higher activity. These findings thus provide valuable insights on the mechanistic roles of B-site cations of spinel oxides in total oxidation of hydrocarbon.