Photooxidative and Photoreductive Degradation of Chlorinated Hydrocarbons on Aqueous Titanium Dioxide Colloids

Citation

Choi, Wonyong (1996) Photooxidative and Photoreductive Degradation of Chlorinated Hydrocarbons on Aqueous Titanium Dioxide Colloids. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/D1KR-1M50. https://resolver.caltech.edu/CaltechETD:etd-05052006-084215

Abstract

Studies on photocatalytic degradation reactions of chlorinated hydrocarbons on TiO₂ colloids are presented in this thesis. Photoreactivities of metal-ion doped quantum-sized TiO₂ colloids and photochemical mechanisms of CHCl₃ and CCl₄ degradation are investigated in detail.

A systematic study of 21 metal-ion doped quantum-sized (2-4 nm) TiO₂ colloids is performed by measuring their photoreactivities and the transient charge-carrier recombination dynamics. Doping with Fe³⁺, Mo⁵⁺, Ru³⁺, Os³⁺, Re⁵⁺, V⁴⁺, and Rh³⁺ at 0.1-0.5 atom% significantly increases the photoreactivity for both CHCl₃ oxidation and CCl₄ reduction while Co³⁺ and Al³⁺ doping decreases the photoreactivity. The quantum yields obtained during CW photolyses are quantitatively correlated with the measured transient absorption signals of the charge-carriers.

The photoreductive degradation of CCl₄ in TiO₂ particulate suspensions in the presence of a variety of organic electron donors (alcohols, carboxylic acids, and benzene derivatives) has been examined. The rate of CCl₄ dechlorination can be enhanced significantly when alcohols and organic acids are used as electron donors. It is demonstrated that CCl₄ can be fully degraded under both oxic and anoxic conditions. A photodegradation mechanism of CCl₄ that includes both one-electron and two-electron transfer is proposed.

The mechanism of photoreduction of CCl₄ on illuminated TiO₂ surfaces is investigated by selectively trapping transient free radical intermediates. Dichlorocarbene and trichloromethyl radical are trapped with 2,3-dimethyl-2-butene during the photocatalytic degradation of CCl₄. The rate of formation of trapped :CCl₂ and •CCl₃ is found to be a function of [H₂O], pH, [CCl₄], the nature of the dissolved gas, and light intensity. A two-electron photoreductive pathway (via dichlorocarbene formation) is suggested to be the dominant mechanism leading to the full degradation of CCl₄.

The photocatalytic degradation reactions of CHCl₃, CHBr₃, CCl₄, and CCl₃CO₂^- are investigated in aqueous TiO₂ suspensions. CHCl₃ and CHBr₃ are degraded into carbon monoxide and halide ion in the absence of dissolved oxygen. The anoxic degradation proceeds through a dihalocarbene intermediate which is produced by sequential reactions of the haloform molecule with a valence band hole and a conduction band electron. Degradation of haloform is enhanced dramatically at pH >11. This enhancement is ascribed to "photoenhanced hydrolysis".

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