Copper grafted on SBA-15 as efficient catalyst for the selective oxidation of methane by oxygen

CuOx/SBA-15 catalysts prepared by a grafting approach (denoted as CuOx/SBA-15-gra) have been characterized and studied for the selective oxidation of methane to formaldehyde by oxygen. Our characterizations using XRD, H2-TPR, UV–vis and ESR suggest that Cu2+ ions are almost isolated on the surface of SBA-15 at a Cu content ≤0.6 wt%. The increase in the Cu content to >0.6 wt% decreased the fraction of isolated Cu2+ ions and increased that of aggregated CuOx clusters. On the other hand, the fraction of isolated Cu2+ ions was much smaller in the CuOx/SBA-15 catalyst prepared by the impregnation method (denoted as CuOx/SBA-15-imp) even with a lower Cu content (0.5 wt%). The CuOx/SBA-15-gra provided significantly higher HCHO selectivity in the selective oxidation of CH4 than the CuOx/SBA-15-imp, and the CuOx/SBA-15-gra catalyst with a Cu content of 0.6 wt% showed the best performance for HCHO formation. In situ FT-IR spectroscopy of adsorbed CO and XPS measurements suggest the generation of Cu+ after the reaction, and the Cu+ site is likely responsible for the activation of oxygen, forming an active oxygen species for the selective oxidation of CH4 to HCHO.