Towards H2O catalyzed N2-fixation over TiO2 doped Run clusters (n = 5, 6): a mechanistic and kinetic approach

Abstract

H₂O driven N₂ fixation is known as the best alternative pathway to synthesise NH₃ under ambient conditions. The thermodynamic non-spontaneous reaction can be accomplished by a photocatalytic water splitting reaction over a TiO₂ supported surface with oxygen vacancies. Previous experiments have also shown N₂ activation over a neutral Ru cluster whose catalytic activity was remarkably enhanced by TiO₂ doping. In this article, we have investigated the detailed mechanism and kinetics of the H₂O catalyzed nitrogen reduction reaction (NRR) over bare and TiO₂ doped Ru₅ clusters in conjunction with DFT and TST calculations. The lack of photochemical activity of the small model cluster provoked us to explore an alternative route of NH₃ formation via H₂O catalysis. For this, we have considered H₂ as co-reactant. The partial reduction of N₂ into NH₃ or N₂H₄ could be achieved by a H₂O oxidation reaction, however, catalytic regeneration requires additional H₂ which effectively makes the overall reaction catalyzed by H₂O. Above all, the present investigation suggests that NH₃ is most favorably produced through the distal mechanism. Analysis of the rate constants demonstrates that the doping with TiO₂ accelerates the kinetics of NRR by a few orders of magnitude. Furthermore, an increase of the size of the metal cluster would not significantly enhance the overall performance of NRR.