Photocatalytic Reduction of NO by CO on Titanium Dioxide under Visible Light Irradiation

Abstract

It is shown for the first time that the photoassisted reduction of NO by CO into N2 and N2O can occur on TiO2 catalysts upon visible light irradiation (λ > 380 nm) at room temperature. The selectivity of photoreduction of NO into N2 reaches 90−95%. The CO2 formed which predominantly remains on the surface can be quantitatively desorbed after completion of the photoreaction by heating TiO2 to ∼500 K. The rates of NO consumption and product accumulation remain virtually constant upon successive admissions of the CO−NO mixture thus indicating a high stability of catalyst activity. It is found that the quantum yield of NO photoreduction by CO is considerably greater for visible light irradiation (λ = 405 + 436 nm) than for UV irradiation (λ = 365 nm). Experiments with 18O-enriched NO revealed that, under visible light irradiation, an intense oxygen isotopic exchange between NO and TiO2 develops. The photocatalytic reaction requires the presence in nonstoichiometric TiO2-x of electron-donor centers (Ti3+ ions, F and F+ centers) able to absorb visible light. A reaction mechanism is proposed which includes the following stages: (a) NO photoadsorption along two parallel routes, by e- capture from the electron-donor center by NO to yield adsorbed NO- species and by NO interaction with the hole O- to give adsorbed nitrite NO2-; (b) reduction of NO- into N2O and further into N2; and (c) photoreduction of adsorbed nitrite NO2- into N2O and further into N2 by CO which regenerates the donor centers. It is assumed that the photoinduced oxygen heteroexchange proceeds via adsorbed nitrite complexes.