Photocatalytic Dechlorination of Aqueous Carbon Tetrachloride Solutions in TiO₂ Layer Systems: A Chain Reaction Mechanism

Quantum yields of chloride ions exceeding unity are obtained upon illumination of TiO₂ layers in aqueous solutions containing CCl₄ and methanol. The TiO₂ layers were prepared by spin coating using concentrated colloidal solution (prepared by hydrolysis of the propoxide). Absorbed light intensities were 7 × 10^-11 to 2 × 10^-8 ein cm^-2 s^-1. There is only a small, if any, effect of methanol concentration on the Cl^- yield in the range 0.2-5 M. Above 5 M, the yield remains nearly constant in the absence of air but decreases in aerated solutions. Only negligible Cl^- yield is obtained in the absence of methanol. In the presence of oxygen, the rate of photocatalytic Cl^- build up is proportional to the square root of the light intensity, as expected in a chain reaction. A similar study in oxygen-free solutions shows [Cl^-] leveling off at the higher light intensities. The quantum yield increases with pH in both the presence and absence of oxygen, reaching values φ ≈ 7 at pH 12.2, at the lowest light intensity. In the absence of oxygen, there is no observable effect of CCl₄ concentration on the Cl^- yield above 1 × 10^-3 M, while the oxygen-containing systems show nearly linear increase of the yield upon increasing [CCl₄]. Thermal catalyzed formation of chloride is observed in the absence of oxygen. In the absence of oxygen, removal of adsorbed hydroxyl radicals, OH^•_ads, by the methanol enables the electrons that have escaped recombination to react with CCl₄, producing chloride ions and CCb₃^• radicals. This is followed by electron injection to the TiO₂ conduction band and subsequent hydrolysis of the CCl₃⁺ intermediate to carbon dioxide and HCl. A chain reaction mechanism is proposed also for oxygen-containing systems. The CCl₄ - O₂^•- adduct is the chain carrier and the termination involves dismutation of O₂^•- radical ions.