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 gamma-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 N-2. 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 N-2. 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 x 106 M-(1) s-(1)) was derived from fitting the kinetic traces to the suggested reaction mechanism, which is discussed in detail.