Reduction of dinitrogen to ammonia in aqueous solution mediated by colloidal metals

Abstract

Reduction of N₂ to NH₃ has been investigated by radiolysis and photolysis in N₂-saturated aqueous solutions containing colloidal dispersions of Ru, Pt or Rh stabilized either by neutral [poly(vinyl alcohol), PVA] or charged (polybrene, PB, or polystyrene sulphonate, PSS) polymers. Reducing radicals formed from propan-2-ol, methyl viologen, or 4-sulphonatomethylbenzophenone (SBP) were produced radiolytically or photochemically and used to charge the colloidal particles. The yield of NH₃ in γ-radiolysis experiments was negligible with Ru/PSS, small with Rh/PB and Pt/PVA, and high with Ru/PB and Ru/PVA. The yield depended on the irradiation time, colloid concentration, type of reducing radical, and pH. Detailed studies with Ru/PVA gave radiation efficiencies close to unity at short irradiations (dose ⩽ 1 kGy). At longer irradiations the concentration of NH₃ increased at a much slower rate until the reaction stopped. The results suggest fast and efficient reduction of any N₂ that was preadsorbed at highly reactive sites on the surface of the colloid. As this N₂ was consumed the active sites were destroyed by hydrogen produced from water reduction. Upon photolysis, H₂ and NH₃ were produced in competition; longer irradiation times or lower pH favouring water reduction. In all cases, the overall yields of NH₃ remained very low. Experiments with colloidal oxides of iron or ruthenium gave very low yields of NH₃.