This greater free energy is required for the generation of hydroxyl radicals, as the free energy of the hydroxyl species on the catalyst surface must be endergonic relative to water. This establishes an upper threshold for the free energy of OH * on the surface during the formation of oxygen molecule or hydrogen peroxide. Therefore, in order to avoid the one-electron oxidation reaction (H 2 O → • OH(aq)+H + +e -) and facilitate the two/four-electron oxidation pathways, it is necessary that ∆G OH * should be less than or equal to 2.4 eV.