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 NH3OH+ 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 ∼520 nm. Aqueous H suspensions (pH ∼ 6, 1 atm O2) photocatalyze the oxidation of HCOO- into CO2•- radicals at λ ∼ 330 nm, but the quantum yield of CO2•- formation at λ > 400 nm remains below ∼2 × 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) on N-doped TiO2, which may proceed by direct oxidative or re...