Oxidative power of nitrogen-doped TiO2 photocatalysts under visible illumination

Nitrogen doping was recently shown to extend the absorptivity of TiO2 photocatalysts into the visible. We find that N-doped TiO2 materials fail, however, to catalyze the oxidation of HCOO- into CO2.- or of NH3- OH+ into NO3-, under visible illumination. By N-doping anatase at ambient or high temperature according to the literature we obtained yellow powders A and H, respectively, that absorb up to similar to520 nm. Aqueous H suspensions (pH similar to 6, 1 atm O-2) photocatalyze the oxidation of HCOO- into CO2.- radicals at lambda similar to 330 nm, but the quanturn yield of CO2.- formation at lambda > 400 nm remains below similar to2 x 10(-5) and is probably zero. A is similarly inert toward HCOO- in the visible region and, moreover, unstable in the UV range. Thus, the holes generated on N-doped TiO2 by visible photons are unable to oxidize HCOO- either by direct means or via intermediate species produced in the oxidation of water or the catalyst. Reports of the bleaching of methylene blue (MB) oil N-doped TiO2 which may proceed by direct oxidative or reductive photocatalytic pathways and also by indirect photocatalysis (i.e., induced by light absorbed by NIB rather than by the catalyst) even under aerobic conditions are, therefore, rather uninformative about the title issue.