Acetonitrile (CH 3 CN), methanol (CH 3 OH), trichloroethylene (C 2 HCl 3 , TCE), and methylene chloride (CH 2 -Cl 2 ) were degraded in stirred batch reactors in the presence of O 2 at room temperature over films of pristine P-25 TiO 2 under irradiation centered at 360 nm. Molarities of organics, activities of O 2 , irradiation flux, deployment of catalyst, and reactor configuration were matched for vaporized and aqueous organics. The dependence of initial rates and the photoefficiencies of removal on concentrations of reactants and photon flux were measured. Identities and rates of formation and decay of some intermediates were determined. Langmuir coefficients were determined for dark adsorption of the organic vapors on films of P-25 in the presence of dry and water-saturated air. Surface concentrations in equilibrium with aqueous solutions in the dark were calculated with the aid of Henry's law constants from the Langmuir coefficients of the water-saturated vapors. Initial rates of removal of all four compounds from the vapor phase were much higher than from aqueous solution. Initial photoefficiencies of g1 molecule removed per incident photon were observed with vaporized CH 3 OH and TCE in dry air. Water vapor strongly inhibited the conversion of CH 3 OH and CH 2 Cl 2 . CH 3 CN, TCE, and CH 2 Cl 2 vapors appeared to react with both electrons and holes, and homolytic dissociation of the C-C and C-Cl bonds of CH 3 CN and CH 2 Cl 2 appeared to occur.

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