Selective Oxidation of Sulfuric Acid to Peroxydisulfuric Acid on a Tungsten Trioxide Anode

The mechanisms involved in the oxidation of H2SO4 to peroxydisulfuric acid (H2S2O8) and H2O to O-2 on WO3, SnO2, and their interface models were investigated using density functional theory. Regardless of the metal oxide, the free energy changes for the two proton-coupled electron transfer (PCET) steps of the H2SO4 oxidation mechanism indicated that the first PCET is the rate-limiting step that requires a greater energy input than the second one. Based on the free energy change in the rate-limiting step, the WO3 anode with SnO2 was expected to produce electrochemically more H2S2O8 in an aqueous H2SO4 solution than SnO2 only without O-2 evolution from H2O. We also experimentally confirmed that the Faraday efficiency of H2S2O8 production using the WO3 anode deposited on a F-doped SnO2 conductive glass (FTO) with lower applied potential is superior to FTO only.