The photocatalytic degradation of chloroform has been investigated in aqueous suspensions of TiO2 over the wavelength range of 310-380 nm. A detailed reaction mechanism has been proposed in which the rate-determining step is the reaction of surface-bound .OH with adsorbed CHCl3. A pH-stat titration technique was developed for the measurement of the rates of degradation of chlorinated hydrocarbons. The quantum efficiency (PHI = 0.56 at lambda = 330 nm) of the degradation of CHCl3 was found to be inversely proportional to the square root of the incident light intensity. This relationship can be explained in terms of a direct competition between a second-order recombination of surface-bound .OH and the rate-determining reaction of surface-bound .OH with CHCl3. The rates of degradation of several electron donors have been correlated with their computed surface speciation. The results of this study show that the adsorption of electron donors and acceptors to the TiO2 surface plays a more important role in determining the rate of the photocatalytic reactions than the effect of pH-dependent Fermi-level shifts.
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