First principles prediction of CH₄ reactivities with Co₃O₄ nanocatalysts of different morphologies

Co₃O₄ nanocatalysts have been experimentally shown to have excellent performance in catalyzing CH₄ combustion. These nanocatalysts of different morphologies, such as nanoparticle/nanocube, nanorod/nanobelt, and nanoplate/nanosheet, were previously synthesized and characterized to mainly expose the (001), (011), and (112) surfaces, respectively, with distinct reactivities. In this study, rigorous first principles calculations were performed to investigate CH₄ reactivities of the above Co₃O₄ surfaces of different terminations. CH₄ dissociation was predicted to occur at the Co-O pair site on these surfaces. For each surface, the most reactive Co-O pair site was identified based on calculated energy barriers of the different active sites, which should contribute most significantly to the reactivity of that surface. The lowest energy barriers for the (001), (011), and (112) surfaces were predicted to be 0.96, 0.90, and 0.79 eV, respectively, suggesting CH₄ reactivity to increase in that order for the different Co₃O₄ surfaces, consistent with the trend found experimentally for Co₃O₄ nanocatalysts of different morphologies. Direct comparison between the estimated and experimental CH₄ reaction rates per gram of the nanocatalysts at 325 °C further indicate that their relative ratios were well reproduced by considering three main factors: the effective energy barrier for CH₄ dissociation, the surface area of the nanocatalyst, and the number of independent active sites per unit surface area. The important influence of surface area on CH₄ reactivity is also demonstrated by the significant difference in the reactivities of the nanocatalysts when exposing the same facet but with distinct surface areas.