Nitrogen doping was recently shown to extend the absorptivity of TiO 2 photocatalysts into the visible. We find that N-doped TiO 2 materials fail, however, to catalyze the oxidation of HCOOinto CO 2 •-, or of NH 3 -OH + into NO 3 -, 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 O 2 ) photocatalyze the oxidation of HCOOinto CO 2 •radicals at λ ∼ 330 nm, but the quantum yield of CO 2 •formation at λ > 400 nm remains below ∼2 × 10 -5 and is probably zero. A is similarly inert toward HCOOin the visible region and, moreover, unstable in the UV range. Thus, the holes generated on N-doped TiO 2 by visible photons are unable to oxidize HCOOeither 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 TiO 2 , which may proceed by direct oxidative or reductive photocatalytic pathways and also by indirect photocatalysis (i.e., induced by light absorbed by MB rather than by the catalyst) even under aerobic conditions are, therefore, rather uninformative about the title issue.

Significance The Fenton reaction, Fe 2+ + H 2 O 2 , plays fundamental roles in vivo and in advanced oxidation processes. Its mechanism and the identity of the intermediates involved, however, remain controversial. Here we present direct, mass-specific evidence of the prompt formation of mono- and poly-iron Fe IV =O (ferryl) species on the surface of aqueous FeCl 2 microjets exposed to gaseous H 2 O 2 or O 3 beams. Remarkably, Fe 2+ ions at the aqueous surface react with H 2 O 2 and O 3 >10 3 times faster than Fe(H 2 O) 6 2+ in bulk water. Our results suggest that interfacial Fenton and Fenton-like chemistries could play a more significant role than hitherto envisioned.