Visible light decomposition of ammonia to N2 with Ru(bpy)32+ sensitizer

Abstract

Visible light decomposition of aqueous NH₃ to N₂ was investigated using a photocatalyst aqueous solution based on molecular photoelectron relay systems of either sensitizer (tris(2,2′-bipyridine)ruthenium(II), (Ru(bpy)₃²⁺)/potassium peroxodisulfate(K₂S₂O₈) or Ru(bpy)₃²⁺/methylviologen dichloride(MV²⁺)/O₂, capable of using visible light instead of UV-driven semiconductors such as TiO₂. It was confirmed by using an in situ visible absorption spectral change under irradiation that the Ru(II) complex is oxidized to the Ru(III) complex by K₂S₂O₈, and that the Ru(III) complex formed is stable without NH₃, while the added NH₃ was oxidized by the Ru(III) complex to produce the Ru(II) complex. In the presence of 1 mM NH₃ aqueous solution, the Ru(III) complex was the predominant species under the photostationary state, but in the presence of 100 mM NH₃, Ru(II) predominated. Gas-chromatographic analysis of the gaseous phase in the presence of 8.1 M NH₃ showed that the photochemical oxidation of ammonia yielded N₂. It was also demonstrated by using the in situ visible absorption spectrum under irradiation of the NH₃ (1 M)/Ru(bpy)₃²⁺ (0.1 mM)/MV²⁺ (10 mM) system under Ar that MV⁺˙ is accumulated, showing that NH₃ works as an electron donor for MV⁺˙ accumulation with simultaneous formation of the oxidized product of ammonia ((NH₃)ox) without producing N₂. It was suggested that the reduced product (MV⁺˙) and the oxidized product ((NH₃)ox) are in a kind of dynamic equilibrium prohibiting further oxidation of (NH₃)ox by Ru(bpy)₃³⁺ to N₂. In the O₂ atmosphere, the oxidation of MV⁺˙ to MV²⁺ takes place to accumulate Ru(III) complex, so that (NH₃)ox was further oxidized to N₂. The high activity of IrO₂ as a cocatalyst in this system was demonstrated.