Oxygen Vacancy-Rich TiO2 as an Efficient Non-noble Metal Catalyst toward Mild Oxidation of Methane Using Hydrogen Peroxide as the Oxidant

Direct functionalization of methane remains a key challenge, especially for using non-noble metal catalysts. We demonstrated that TiO2 nanorods with abundant oxygen vacancies enabled mild oxidation of methane by H2O2 into formaldehyde (HCHO) without light irradiation. The activity of TiO2 nanorods with the concentration gradient of oxygen vacancies (VO) increased with the VO concentration. In H2O2 aqueous solution under 30 bar of CH4 at 70 °C for 1 h, the TiO2 nanorods with the most abundant VO exhibited a total oxygenate yield of 40.80 μmol, among which the selectivity for HCHO was 64.1%. On the basis of the catalytic and spectroscopic data, we identified the reaction intermediates and accordingly mapped the reaction scheme. Specifically, H2O2 is activated on Ti atoms near VO to form surface peroxo intermediates, followed by the activation of CH4 to produce methoxy groups. The methoxy group can react either with water to form methanol or with hydroxyl radicals to form CH3OOH. Methanol is attacked by hydroxyl radicals and dehydrated to form •CH2OH that further reacts with hydroxyl radicals and is dehydrated to HCHO. CH3OOH directly undergoes dehydration to engender HCHO.