Defects and photocatalytic activity of pure TiO2 and TiO2 doped with chromium or vanadium ions

The development of new processes for the transformation of toxic compounds in polluted air and waste waters is an important goal for chemists pursuing environmentally "benign" or "green" chemistry. Promising candidates as environmentally harmonious catalysts are TiO2 -based photo-catalysts which are, in addition to their cleanness and stability, comparatively cheap. Extending the wavelength response from the ultraviolet to the visible, thereby using a larger fraction of the solar energy belongs to the major efforts in the field of TiO2 photochemistry. Recent findings show that Cr- and V- implanted TiO2 can effectively operate under visible light irradiation However, the energies of the dopant levels with respect to the conduction band and the effect of these extrinsic defects on electron-hole recombination or photochemistry are not known. The present project aims at a better understanding of the effect of doping of TiO2 with Cr and V as well as the role of intrinsic surface defects (i.e. vacancies) on the surface reactivity of TiO2 in photocatalytic processes. The photooxidation of mimics for environmentally harmful organic compounds on doped and undoped material will be studied by comparative mechanistic investigations. They comprise kinetic studies on single crystal surfaces providing values for quantum efficiency and cross section of the photooxidation step and, on the other hand complementing spectroscopic studies on high surface area TiO2 that should evidence molecular species as surface intermediates. Systematic measurements using both UV and visible radiation should answer the question wether there are differences in the photooxidative mechanism caused by doping and, thus, extension of the catalyst response into the visible wavelength range.