Nitric oxide reduction to ammonia by TiO₂ electrons in colloid solution via consecutive one-electron transfer steps

The reaction mechanism of nitric oxide (NO) reduction by excess electrons on TiO₂ nanoparticles (e_TiO2^-) has been studied under anaerobic conditions. TiO₂ was loaded with 10-130 electrons per particle using γ-irradiation of acidic TiO₂ colloid solutions containing 2-propanol. The study is based on time-resolved kinetics and reactants and products analysis. The reduction of NO by e_TiO2^- is interpreted in terms of competition between a reaction path leading to formation of NH₃ and a path leading to N₂O and N₂. The proposed mechanism involves consecutive one-electron transfers of NO, and its reduction intermediates HNO, NH₂O^•, and NH₂OH. The results show that e_TiO2^- does not reduce N₂O and N₂. Second-order rate constants of e_TiO2^- reactions with NO (740 ± 30 M^-1 s^-1) and NH₂OH (270 ± 30 M^-1 s^-1) have been determined employing the rapid-mixing stopped-flow technique and that with HNO (>1.3 × 10⁶ M^-1 s^-1) was derived from fitting the kinetic traces to the suggested reaction mechanism, which is discussed in detail.