Selective Photo-oxidation of Methane to Methanol with Oxygen over Dual-Cocatalyst-Modified Titanium Dioxide

Abstract

Direct and selective oxidation of CH4 with dioxygen to methanol is a "dream reaction" in modern catalysis yet remains a great challenge. Here, we report that TiO2 loaded with dual cocatalysts, that is, nanometals and cobalt oxide (CoOx) nanoclusters, is capable of selectively catalyzing CH4 to CH3OH at room temperature under photoexcitation using abundant and inexpensive O-2 as an oxidant. The best activity for the formation of the primary products, CH3OOH and CH3OH, is up to 50.8 pmol for 2 h with 95% selectivity. Mechanistic studies elucidate that the superior activity and selectivity result from the synergistic effect of nanometals and CoOx. Nanometals enhance CH4 conversion by promoting the separation of the photoexcited electron and the reduction of O-2. The CoOx mediates a mild CH4 oxidation process by suppressing the formation of highly oxidative (OH)-O-center dot radicals that can further oxidize CH3OH to HCHO and CO2, thereby preserving a high selectivity toward oxygenated products. This work provides a prototype in designing efficient photocatalysts for selective oxidation of CH4 to CH3OH with O-2 under mild conditions.