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

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 RuII(Me2phen)32+ (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 RuII(Me2phen)32+ to O2 results in the formation of RuIII(Me2phen)33+ 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 RuIII(Me2phen)33+. RuIII(Me2phen)33+ 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 CoIII(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%.