Light-induced electron-transfer reactions. Part 1. Kinetics of formation of hydrogen peroxide by irradiation with visible light of aqueous solutions containing tris(2,2′-bipyridine)ruthenium(II), oxalic acid, manganese(II), and oxygen

Abstract

Hydrogen peroxide is formed by the light-catalyzed reaction of [Ru(bipy)₃]²⁺(bipy = 2,2′-bipyridine) in an aqueous dilute sulphuric acid solution containing oxalic acid, manganese(II) sulphate, and oxygen. The amount of H₂O₂ formed increases greatly with increasing intensity of visible light, with increasing concentration of [Ru(bipy)₃]²⁺ or oxygen, and with increasing concentration of manganese(II), and increases linearly with increasing hydrogen-ion concentration up to about 0.3 mol dm^–3 but then decreases with further increase. On the other hand, the rate of formation of H₂O₂ is almost independent of the concentration of oxalic acid when [H₂C₂O₄] < [Mn^II] and slightly decreases when [H₂C₂O₄][Mn^II]. No formation of H₂O₂ is found in the dark or in the absence of any one of [Ru(bipy)₃]²⁺, manganese(II), and oxalic acid. The formation of H₂O₂ is greatly retarded by the presence of copper(II) sulphate or methylviologen. A chain mechanism of reaction is proposed to account for these facts.