Elevating Photooxidation of Methane to Formaldehyde via TiO2 Crystal Phase Engineering

Abstract Photocatalytic conversion of methane to value-added products under mild conditions, which represents a long-sought-after goal for industrial sustainable production, remains extremely challenging to afford high production and selectivity using cheap catalysts. Herein, we present the crystal phase engineering of commercially available anatase TiO2 via simple thermal annealing to optimize the structure-property correlation. Biphase catalyst of anatase (90%) and rutile (10%) TiO2 exhibits the exceptional performance in the oxidation of methane to formaldehyde under the reaction condition of water solvent, oxygen atmosphere and full-spectrum light irradiation. An unprecedented production of 24.27 mmol gcat-1 with an excellent selectivity of 97.4% towards formaldehyde is acquired at room temperature after a 3 h reaction. Both experimental results and theoretical calculation disclose that the crystal phase engineering of TiO2 lengthens the lifetime of photogenerated carriers and favors the formation of intermediate methanol species, thus maximizing the efficiency and selectivity in aerobic oxidation of methane to formaldehyde. More importantly, the feasibility of scale-up production of formaldehyde is demonstrated by inventing the "pause-flowing" reactor. This work opens the avenue towards industrial methane transformation in a sustainable and economical way.