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.