No abstract

The adsorption and photoreactions of CH 2 Cl 2 on powdered TiO 2 have been investigated by Fourier transform infrared spectroscopy. CH 2 Cl 2 is adsorbed molecularly or dissociatively to form chloromethoxy (CClH 2 O (a) ) at 35 °C. CClH 2 O (a) decomposes into CH 3 O (a) and HCOO (a) in a vacuum at temperatures higher than ∼100 °C. As TiO 2 in contact with gaseous CH 2 Cl 2 is heated in a closed cell, HCl from Cl (a) and OH (a) recombination, CH 2 O from CH 3 O (a) decomposition, and CO and CO 2 from HCOO (a) decomposition are detected. In the CH 2 Cl 2(a) photodecomposition in the absence of O 2 , CO (a) , CO 3(a) , and HCOO (a) are generated. The TiO 2mediated CH 2 Cl 2(a) photodecomposition is likely initiated by surface-active oxygen species instead of direct hole transfer. In the presence of O 2 , CH 2 Cl 2(a) photodecomposition is accelerated. In addition to CO (a) , CO 3(a) , and HCOO (a) , H 2 O (a) and CO 2(a) are generated as well. In the case of CO (a) formation, O 2 is also involved in addition to TiO 2 lattice oxygen. The O 2 participation may be via oxygen anion species.

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 °C in a vacuum. Under UV irradiation, CH4 decomposes to form CO(a), CO2(g), H20(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), H2 18O(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.

Photochemical reactions of methanol and formic acid on L64 copolymer-templated mesoporous SiO2 have been investigated by Fourier transform infrared spectroscopy. Although SiO2 has been generally recognized to be photochemically inert, the reaction pathways HCOOH(a), HCOO(a)/SiO2 --> CO(2(g)) and CH3OH(a), CH3O(a)/SiO2 --> CH2O(a) --> HCOO(a) --> CO(2(g)) are demonstrated. The photons used in this study are unable to promote the SiO2 valance band electrons to the conduction band. Therefore photoholes take no part in these reactions. Mechanisms involving charge transfer and the surface species possible to initiate the photooxidation on mesoporous SiO2 under irradiation are discussed. For comparison purposes, parallel studies are also conducted for nonporous SiO2 to show the effect of porosity of SiO2 on the reactivity.

The adsorption and reactions of CHCl3 on three commercially available TiO2 powders have been investigated by Fourier transform infrared spectroscopy in a gas‐solid reaction system. Probably, owing to the difference in surface morphology, CHCl3 is weakly adsorbed on two of the three TiO2 samples at 35 °C. But, as the more reactive TiO2 is exposed to CHCl3 at 35 °C, the surface is found to be covered with CHCl3, HCOO, H2O, and CO. In addition to these surface species, gaseous HCl and CO are generated at higher temperatures. Photoirradiation of CHCl3 on TiO2 in the absence or presence of O2 causes the decomposition of CHCl3. This photoprocess is enhanced in O2.