Selectivity of H₂O₂ and O₂ by water oxidation on metal oxide surfaces

Water oxidation is an important chemical reaction that yields electrons for downstream reduction reactions such as hydrogen generation or CO₂ and/or N₂ reduction. When producing O₂, the reaction involves 4 electrons and 4 protons and tends to be kinetically unfavored. A competing pathway leading to the formation of H₂O₂ would only involve 2 electrons and 2 protons and may serve as a favorable alternative to O₂ formation while meeting the needs for electron production by water oxidation. Although H₂O₂ as a product of water oxidation has been observed experimentally, the bifurcating point that determines whether O₂ or H₂O₂ is the favored product has not been identified by experiments previously. Here, we report a detailed experimental study aimed at correcting this deficiency. We propose that the ease or difficulty of protonation or deprotonation of -OOH intermediates is a key to the selectivity between H₂O₂ and O₂. That is, we hypothesize that the (de)protonation of M-OOH, where M represents an active metal center, is the bifurcating point of the water oxidation catalytic cycle. Ready deprotonation of this intermediate leads to the eventual formation and release of O₂, whereas the protonation of this intermediate enables the formation of H₂O₂. The dependence of product selectivity on pH as observed by quantitative H₂O₂ detection supports this hypothesis. Additional experimental evidence based on isotope effects is also obtained. The results will likely find broad implications in catalyst design for high-performance water oxidation reactions.