In situ solid-state NMR studies of trichloroethylene photocatalysis:: Formation and characterization of surface-bound intermediates

In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation of trichloroethylene (TCE) over two TiO(2)-based catalysts, Degussa P-25 powder and a monolayer TiO(2) catalyst dispersed on porous Vycor glass. (13)C magic angle spinning (MAS) experiments reveal that similar reaction intermediates form on the surfaces of both catalysts. Long-lived intermediates, including dichloroacetyl chloride (Cl(2)HCCOCl, DCAC), carbon monoxide, and pentachloroethane and final products CO(2), phosgene (Cl(2)CO), and HCl were observed under dry conditions. The presence of molecular oxygen was found to be essential for TCE photooxidation to proceed. Adsorbed water was found to greatly reduce the formation of phosgene. The formation of surface-bound dichloroacetate and trichloroacetate species was observed and identified via (13)C cross polarization MAS experiments. Dichloroacetate, which forms from mobile DCAC, appears to be bound to the nonirradiated surfaces of the powdered TiO(2) catalyst and further degradation was not possible. Formation of di-and trichloroacetate also takes place on the TiO(2)/PVG catalyst in the absence of light; however, their concentrations are low. Degradation studies of these surface-bound species indicate that the photooxidation of dichloroacetate is slow and results in the formation of phosgene and CO(2), while trichloroacetate remains resistive to degradation on the TiO(2)/PVG catalyst. Our results also indicate that the formation of DCAC and phosgene seems to be a general result of TCE degradation which is not limited to TiO(2) photocatalysis but instead may be more characteristic of the types of initiating species which are formed by UV irradiation. However, the TiO(2) surface is the most effective in terms of the observed initial rates of degradation.