Photooxidation of Methane over TiO₂

Abstract

Abstract Fourier‐transform infrared spectroscopy has been employed to investigate the adsorption and photo‐oxidation of CH 4 over powdered TiO 2 . The interaction between the CH 4 and TiO 2 surface is weak. It is found that no CH 4 molecules are adsorbed on the surface at 35 °C in a vacuum. Under UV irradiation, CH 4 decomposes to form CO (a) , CO 2(g) , H20 (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 18 O 2 is used. The major 18 O‐containing products are C 18 O (a) , C 18 O 2(g) , H 2 18 O (a) , HC 16 O 18 O (a) , and HC 18 O 18 O (a) after 180 min UV irradiation. However, the extent of 18 O incorporating into the adsorbed formate is dependent on UV irradiation time. In the early stage of UV irradiation HC 16 O 16 O (a) is the major formate form indicating the involvement of TiO 2 lattice oxygens for its formation, but HC 18 O 18 O (a) becomes the major one after 180 min indicating the involvement of 18 O 2 . Formate on TiO 2 further photodecomposes to CO 2(g) , but not to CO (a) . CO (a) formation is directly from CH 4 photodecomposition with the participation of TiO 2 lattice oxygens and O 2 .