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 (e_TiO2^-) 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 = [e_TiO2^-]_o/[nitrite]_o > 1.5), e_TiO2^- decays via two consecutive processes; at higher concentrations, only one decay step is observed. The stoichiometric ratio Δ[e_TiO2^-]/[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 e_TiO2^- (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 e_TiO2^-, 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 e_TiO2^- yields mainly N₂O and NH₃ via consecutive one-electron transfer reactions.