Identifying Influential Parameters of Octahedrally Coordinated Cations in Spinel ZnMn_xCo_{2- x}O₄ Oxides for the Oxidation Reaction

Transition metal oxides are potential alternatives to precious metal catalysts for oxidation reactions. Among these earth abundant oxide catalysts, cobalt- or manganese-based spinel oxides have attracted consistent interest because of their superior catalytic performances. It has been found that the octahedral sites in spinels are responsible for their catalytic activities. However, little is known about the parameters of the octahedrally coordinated cations that influence their activity. Herein, a series of ZnMn_xCo_2-xO₄ (x = 0-2.0) spinel oxides are investigated, employing CO oxidation as the model reaction, with particular attention being paid to the variation in activity caused by tuning the ratio of octahedrally occupied Mn to Co. Both Mn and Co contribute to the activity with Mn cations as the primary active species when they coexist; the intrinsic specific activity is found to be dependent on composition, and the highest activity is seen at a Mn/Co molar ratio of 0.11. The presence of Mn⁴⁺ and Mn³⁺ in a proper ratio is another key for achieving high oxidation activity and can be rationalized by the moderate oxygen adsorption during CO oxidation, which facilitates O vacancy refilling. This is also supported by the density function theory calculation, showing that the high activity of ZnMn_0.2Co_1.8O₄ originates from having the O p-band center neither too far from nor too close to the Fermi level. The e_g occupancy of Mn cations and the O p-band center relative to the Fermi level, which are the indices of how the electronic structure influences the oxygen addition- and removal-related processes, are proposed to serve as the activity descriptors. This work may provide a different insight into understanding the activity of transition metal spinel oxides for oxidation reactions.