Dry phase titanium dioxide-mediated photocatalysis: Basis for in situ surface destruction of hazardous chemicals

The photocatalytic oxidation of 2,4,6-trichlorophenol (TCP) on the surface of titanium dioxide (TiO2) at varying water contents was investigated to provide fundamental data for incorporating photocatalysts into the surface of pavements to promote the destruction of spilled organic chemicals. 2,4,6-Trichlorophenol, spiked onto a thin layer of TiO2, was degraded to 20% of its original concentration over 24 h; the release of chloride confirmed the degradation of the parent compound on the surface of the dry TiO2. Addition of water (greater than or equal to 25% by weight) to the TiO2 increased the rate of photocatalysis resulting in degradation of TCP to below detectable levels after 20 h. Based on competition studies using the hydroxyl radical scavengers, bicarbonate and 1-octanol, the proposed mechanisms for the dry phase photocatalytic degradation of TCP was oxidation by the valence band hole on the surface of the TiO2 particle or dehalogenation by superoxide radical anions. Competition studies also confirmed that the more rapid TCP oxidation on wet TiO2 was primarily the result of generation of hydroxyl radicals through oxidation of water by the valence band hole. The results show that dry phase TiO2-mediated photocatalysis may be a potential system for the in situ surface destruction of chemicals that can be oxidized by nonhydroxyl radical mechanisms, such as valence band electron holes and dehalogenation processes.