Infrared spectroscopic investigation of gas-solid heterogeneous photocatalytic oxidation of trichloroethylene

Transmission infrared spectra of untreated titania reveal that the surface is highly hydrated and contains both hydroxyl groups and chemisorbed water, and that in this state the surface does not chemisorb trichloroethylene (TCE). Near-ultraviolet (UV) illumination in the presence of TCE vapor leads to desorption of molecular water and subsequent formation of several adsorbed hydrocarbon intermediates and carbon dioxide. Formation of bands at 1609 and 1302 cm^-1 suggests the presence of an adsorbed dichlorinated olefin. An intense band at 1743 cm^- is assigned to the symmetric CO stretch of dichloroacetaldehyde. Comparison of the OH stretching regions of the spectra collected during illumination in the absence of TCE to those collected during illumination shows that hydroxyls are consumed in the TCE oxidation reaction. These observations are consistent with a mechanism in which water desorption is a prerequisite "trigger" step for oxygen adsorption and subsequent reactive hydroxyl radical and hydroperoxide radical formation, as well as for TCE adsorption. The evidence suggests that attack of adsorbed olefinic derivatives of TCE by hydroxyl radicals or by hydroperoxide radicals leads to production of the aldehydic intermediate. Ultraviolet illumination stimulates desorption of product CO₂ molecules formed by further attack of the aldehyde by reactive radicals and surface decomposition of the resulting intermediates.