Irradiated aqueous suspensions of (Fe, Ru or Os)/TiO2 catalysts were found to reduce nitrogen to ammonia. The order of catalytic activity was found to be Ru > Fe > Os, while reverse order was found for the stability of the catalysts with respect to ammonia formation. A mechanism involving interaction between H-ads and chemisorbed nitrogen is proposed to explain the reactivity and deactivation of these catalysts with the aid of electron paramagnetic resonance results.