Photooxidation of methane over TiO2

Fourier-transform infrared spectroscopy has been employed to investigate the adsorption and photo-oxidation of CH4 over powdered TiO2. The interaction between the CH4 and TiO2 surface is weak. It is found that no CH4 molecules are adsorbed on the surface at 35 degreesC in a vacuum. Under UV irradiation, CH4 decomposes to form CO(a), CO2(g), H2O(a), and HCOO(a) in the presence of O-2. The photoreaction rate is retarded and only small amounts of CO(a) and HCOO(a) are formed in the absence of O-2. It is observed that the oxygen atoms of O-2 are incorporated into these photoproducts as O-18(2) is used. The major O-18-containing products are (CO(a))-O-18, (CO2(g))-O-18, (H2O(a))-O-18, (HCOO(a))-O-16-O-18, and (HCOO(a))-O-18-O-18 after 180 min UV irradiation. However, the extent of O-18 incorporating into the adsorbed formate is dependent on UV irradiation time. In the early stage of UV irradiation (HCOO(a))-O-16-O-16 is the major formate form indicating the involvement of TiO2 lattice oxygens for its formation, but (HCOO(a))-O-18-O-18 becomes the major one after 180 min indicating the involvement of O-18(2). Formate on TiO2 further photodecomposes to CO2(g), but not to CO(a). CO(a) formation is directly from CH4 photodecomposition with the participation of TiO2 lattice oxygens and O-2.