Hydroxyl groups attached to Co2+ on the surface of Co3O4: a promising structure for propane catalytic oxidation†
Co3O4 catalysts with three specific morphologies (nanocubes, nanosheets, and nanooctahedra) were prepared using simple preparation methods and tested for catalytic combustion of propane under the same reaction conditions. Co3O4 nanosheets (Co3O4-S) with the highest Co2+ content presented the best catalytic activity among the three catalysts and achieved complete oxidation of propane at 210 °C compared with 320 °C for nanocubes and 400 °C for nanooctahedra in our work. It is confirmed by XPS characterization that there were a large number of surface hydroxyl groups on the surface of Co3O4-S, and they were more inclined to attach to Co2+ species, whereas the amount of hydroxyl groups on the surface of the other two catalysts (nanocubes and nanooctahedra) was negligible. DFT calculations indicate that the bond energy between a hydroxyl group and surface Co2+ is higher than that between a hydroxyl group and surface Co3+, which proves that hydroxyl groups are more likely to be attached to Co2+ on the surface of Co3O4-S. The hydroxyl groups on the surface of Co3O4-S altered the intermediates and the reaction pathway in the process of propane oxidation, which greatly enhanced the catalytic activity of Co3O4-S. This research can open up a facile and reliable strategy for the design and construction of efficient propane oxidation catalysts by modifying the functional groups on the surface of the catalysts.