Current challenges in photocatalysis: Improved photocatalysts and appropriate photoreactor engineering

The photocatalytic activity of different commercially available titanium dioxide materials is compared employing dichloroacetate (DCA(-)) as the model pollutant. A mechanism is presented evincing that one photon is sufficient to initiate the complete mineralization of one DCA(-) molecule. The observed non-linear dependence of the photonic efficiency zeta of the DCA(-) degradation upon the incident photon rate is explained by a simple mathematical model considering only one-electron charge transfer and recombination reactions on the semiconductor particle. Since photonic efficiencies below 1% are observed when aromatic compounds are used as model pollutants, an electron-shuttle mechanism is proposed involving the benzoquinone/hydroquinone redox couple and resulting in an overall enhancement of the electon/hole recombination. Newly synthesized colloidal Ti/Fe mixed oxide particles exhibit higher activity for the degradation of dichloroacetate than pure TiO(2) colloids, however, they still suffer from cathodic corrosion problems. Finally, a self-contained thin film fixed bed reactor (TFFBR) is presented which can be operated as a stand-alone system gaining the energy for the pump operation from an appropriate photovoltaic module and regulating the water flow as a function of the solar flux.