Production of hydrogen peroxide as a sustainable solar fuel from water and dioxygen

Abstract

Hydrogen peroxide was produced as a solar fuel from water and dioxygen using solar energy by combination of a water oxidation catalyst and a photocatalyst for two-electron reduction of O₂ in acidic aqueous solutions. Photocatalytic production of H₂O₂ occurred under photoirradiation of [Ru^II(Me₂phen)₃]²⁺ (Me₂phen = 4,7-dimethyl-1,10-phenanthroline) used as a photocatalyst with visible light in the presence of Ir(OH)₃ acting as a water oxidation catalyst in an O₂-saturated H₂SO₄ aqueous solution. Photoinduced electron transfer from the excited state of [Ru^II(Me₂phen)₃]²⁺ to O₂ results in the formation of [Ru^III(Me₂phen)₃]³⁺ and a superoxide radical anion (O₂˙⁻) which is protonated to produce H₂O₂via disproportionation of HO₂˙ in competition with back electron transfer (BET) from O₂˙⁻ to [Ru^III(Me₂phen)₃]³⁺. [Ru^III(Me₂phen)₃]³⁺ oxidises water with the aid of catalysis of Ir(OH)₃ to produce O₂. The photocatalytic reactivity of H₂O₂ production was improved by replacing Ir(OH)₃ nanoparticles by [Co^III(Cp*)(bpy)(H₂O)]²⁺ in the presence of Sc(NO₃)₃ in water. The optimised quantum yield of the photocatalytic H₂O₂ production at λ = 450 nm was determined using a ferrioxalate actinometer to be 37%. The value of conversion efficiency from solar energy to chemical energy was also determined to be 0.25%.