Catalytic oxidation of methane to formaldehyde and methanol has attracted attention because of its advantage in energy efficiency over the conventional multistep reaction process involving endothermic steam reforming of CH4; however, it is challenging to selectively obtain the partial oxidation products in the direct oxidation of CH4. In the present study, Co/SiO2 with various Co loadings was tested for the CH4/O2/H2O gas flow reaction. As a result, Co/SiO2 with a low loading of ≤0.1 wt % showed high selectivity for the partial oxidation reaction and mainly produced HCHO, while Co/SiO2 with high Co loadings proceeded to complete oxidation. Structural analysis using X-ray absorption fine-structure spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and scanning transmission electron microscopy suggested that single Co atoms generated at low Co loadings were effective in the selective oxidation of CH4 to HCHO, while Co3O4 nanoparticles generated at high Co loadings promoted the complete oxidation.
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