Stepwise reduction of coordinated dinitrogen to ammonia via diazinido and hydrazido intermediates on a visible light irradiated Pt/CdS · Ag 2S/RuO 2 particulate system suspended in an aqueous solution of K[Ru(EDTA-H)Cl]2H 2O

Abstract

We report the photocatalytic fixation of coordinated dinitrogen to ammonia in aqueous solution under ambient conditions (30 °C and 1 atm N 2) in the presence of the modified semiconductor particulate system Pt/CdS · Ag 2S/RuO 2 catalysed by K[Ru(EDTA-H)Cl]2H 2O. Doping of the photon absorber CdS with Ag 2S (0.05 wt.% relative to the weight of CdS) leads to an enhanced rate of production of NH 3 at 505 nm. The reaction shows a first-order dependence on the catalyst concentration. The quantum yields of the product were obtained at five monochromatic absorption lines between 450 and 550 nm. Illumination of the system with 505–525 nm light gives the highest quantum yield of 0.35. A mechanism is proposed for the formation of NH 3 which involves the in situ formation of a dinitrogen complex of ruthenium(II). The —NN bond in this complex appears to undergo stepwise reduction to NH 3 via diazinido and hydrazido intermediates which were characterized by the nuclear magnetic resonance (NMR) spectra of the solution from the reaction cell. The estimated deuterium kinetic isotope effect is 1.77 indicating a hydrogen atom transfer in the rate-determining step. The source of the nitrogen atom in NH 3 is dinitrogen as confirmed by 15N 2 studies. Photo-oxidation of H 2O to oxygen is confirmed by performing studies in H 2 18O (10%) enriched medium.