Synthesis, characterization, and catalytic evaluation of Co 3 O 4 /γ-Al 2 O 3 as methane combustion catalysts: Significance of Co species and the redox cycle

A series of Co3O4/γ-Al2O3 catalysts with different Co3O4 loadings were prepared by incipient wetness impregnation (IWI) and a combination of IWI and subsequent combustion synthesis (CS), and tested for catalytic combustion of CH4. The IWI/CS-X (X = 5, 10, 30 and 50, represented the Co3O4 loading as wt%) samples prepared via IWI/CS method exhibited higher catalytic activity than the IWI-X (X = 5, 10, 30 and 50) ones via IWI method. IWI/CS-30 showed the highest catalytic activity with T10 (10% CH4 conversion) at 300 °C and T100 (complete conversion) at 550 °C. The structure characterization indicated the IWI/CS-X catalysts had a greater surface area with relatively lower Co3O4 crystallization and better Co3O4 dispersion compared with IWI-X samples. X-ray photoelectron spectroscopy (XPS) and UV–vis results revealed that surface Co3+/(Co2+ + Co3+) ratios of the IWI/CS-X samples was relatively higher than those of IWI-X ones. The surface Co3+ of the IWI/CS-X catalysts was easily reduced to Co2+ at 280–330 °C, whereas, only one difficult reduction process of Co3O4 to Co0 was observed at 400–600 °C for all of the IWI-X samples according to hydrogen temperature-programmed reduction (H2-TPR) results. Hence, the abundance of surface Co3+, easy and fast redox cycle between Co2+ and Co3+ at low temperatures were the underlying mechanisms for the excellent catalytic performance of IWI/CS-X catalysts in CH4 combustion.