Nitrite Reduction to Nitrous Oxide and Ammonia by TiO2 Electrons in a Colloid Solution via Consecutive One-Electron Transfer Reactions

The mechanism of nitrite reduction by excess electrons on TiO2 nanoparticles (eTiO2–) was studied under anaerobic conditions. TiO2 was loaded with up to 75 electrons per particle, induced by γ-irradiation of acidic TiO2 colloid solutions containing 2-propanol. Time-resolved kinetics and material analysis were performed, mostly at 1.66 g L–1 TiO2. At relatively low nitrite concentrations (R = [eTiO2–]o/[nitrite]o > 1.5), eTiO2– decays via two consecutive processes; at higher concentrations, only one decay step is observed. The stoichiometric ratio Δ[eTiO2–]/[nitrite]o of the faster process is about 2. This process involves the one-electron reduction of nitrite, forming the nitrite radical (k1 = (2.0 ± 0.2) × 106 M–1 s–1), which further reacts with eTiO2– (k2) in competition with its dehydration to nitric oxide (NO) (k3). The ratios k2/k3 = (3.0 ± 0.5) × 103 M–1 and k2 > 1 × 106 M–1 s–1 were derived from kinetic simulations and product analysis. The major product of this process is NO. The slower stage of the kinetics involves the reduction of NO by eTiO2–, and the detailed mechanism of this process has been discussed in our earlier publication. The results reported in this study suggest that several intermediates, including NO and NH2OH, are adsorbed on the titanium nanoparticles and give rise to inverse dependency of the respective reaction rates on the TiO2 concentration. It is demonstrated that the reduction of nitrite by eTiO2– yields mainly N2O and NH3 via consecutive one-electron transfer reactions.