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 investigat e the photocatalytic oxidation of trichloroethylene (TCE) over two TiO 2-based catalysts, Degussa P-25 powder and a monolayer TiO2 catalyst d ispersed on porous Vycor glass. C-13 magic angle spinning (MAS) experi ments reveal that similar reaction intermediates form on the surfaces of both catalysts. Long-lived intermediates, including dichloroacetyl chloride (Cl2HCCOCl, DCAC), carbon monoxide, and pentachloroethane and final products CO2, phosgene (Cl2CO), and HCl were observed under dry conditions. The presence of molecular oxygen was found to be essentia l for TCE photooxidation to proceed. Adsorbed water was found to great ly reduce the formation of phosgene. The formation of surface-bound di chloroacetate and trichloroacetate species was observed and identified via C-13 cross polarization MAS experiments. Dichloroacetate, which f orms from mobile DCAC, appears to be bound to the nonirradiated surfac es of the powdered TiO2 catalyst and further degradation was not possi ble. Formation of di-and trichloroacetate also takes place on the TiO2 /PVG catalyst in the absence of light; however, their concentrations a re low. Degradation studies of these surface-bound species indicate th at the photooxidation of dichloroacetate is slow and results in the fo rmation of phosgene and CO2, while trichloroacetate remains resistive to degradation on the TiO2/PVG catalyst. Our results also indicate tha t the formation of DCAC and phosgene seems to be a general result of T CE degradation which is not limited to TiO2 photocatalysis but instead may be more characteristic of the types of initiating species which a re formed by UV irradiation. However, the TiO2 surface is the most eff ective in terms of the observed initial rates of degradation.