Stepwise reduction of coordinated dinitrogen to ammonia via diazinido and hydrazido intermediates on a visible light irradiated Pt/CdS · Ag₂S/RuO₂ particulate system suspended in an aqueous solution of K[Ru(EDTA-H)Cl]2H₂O

Abstract

We report the photocatalytic fixation of coordinated dinitrogen to ammonia in aqueous solution under ambient conditions (30°C and 1 atm N₂) in the presence of the modified semiconductor particulate system Pt/CdS · Ag₂S/RuO₂ catalysed by K[Ru(EDTA-H)Cl]2H₂O. Doping of the photon absorber CdS with Ag₂S (0.05 wt.% relative to the weight of CdS) leads to an enhanced rate of production of NH₃ 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 NH₃ 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 NH₃ 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 NH₃ is dinitrogen as confirmed by ¹⁵N₂ studies. Photo-oxidation of H₂O to oxygen is confirmed by performing studies in H₂¹⁸O (10%) enriched medium.