Fourier-transform infrared spectroscopy has been employed to investigate the adsorption and photooxidation of CH4 over powdered TiO 2. The interaction between the CH4 and TiO2 surface is weak. It is found that no CH4 molecules are adsorbed on the surface at 35°C in a vacuum. Under UV irradiation, CH4 decomposes to form CO(a), CO2(g), H2O (a), and HCOO(a) in the presence of O2. The photoreaction rate is retarded and only small amounts of CO(a) and HCOO(a) are formed in the absence of O2. It is observed that the oxygen atoms of O2 are incorporated into these photoproducts as 18O2 is used. The major 18O-containing products are C18O(a), C18O2(g), H218O(a), HC16O18O (a), and HC18O18O(a) after 180 min UV irradiation. However, the extent of 18O incorporating into the adsorbed formate is dependent on UV irradiation time. In the early stage of UV irradiation HC16O16O(a) is the major formate form indicating the involvement of TiO2 lattice oxygens for its formation, but HC18O18O(a) becomes the major one after 180 min indicating the involvement of 18O2. 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 O2.
All Science Journal Classification (ASJC) codes