Role of Oxygen\nVacancies and Water in Hydrogen Peroxide\nProduction from Oxygen and Formic Acid Catalyzed by Zirconia-Supported\nGold Nanoparticle

Hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) is not\nonly a clean\noxidant widely used in industry but also a promising energy carrier.\nThere is a strong demand to develop an environmentally friendly and\nsustainable method for producing H<sub>2</sub>O<sub>2</sub> as an\nalternative to the current energy-consuming anthraquinone autoxidation\nprocess. We recently found that Au nanoparticle-loaded ZrO<sub>2</sub> (Au/ZrO<sub>2</sub>) rich in oxygen vacancy (O<sub>v</sub>) or surface\nOH-rich exhibits much higher catalytic activity than Au/ZrO<sub>2</sub> poor in O<sub>v</sub> or surface OH for the production of H<sub>2</sub>O<sub>2</sub> from O<sub>2</sub> and HCOOH at ambient temperature\nand pressure. To clarify the reason, possible reaction pathways were\nevaluated by density functional theory calculations. The results indicated\nthat a Langmuir–Hinshelwood-type reaction proceeds near the\nAu NP-ZrO<sub>2</sub>(O<sub>v</sub>)-liquid three-phase interfaces\nto yield H<sub>2</sub>O<sub>2</sub> via the two-electron oxygen reduction\nreaction (2e<sup>–</sup>-ORR) due to the cooperation of O<sub>v</sub> and water. On the other hand, H<sub>2</sub>O is preferentially\nformed via 4e<sup>–</sup>-ORR when either O<sub>v</sub> or\nwater is lacking. The O<sub>v</sub> generated on the ZrO<sub>2</sub> surface near the Au NPs induces the efficient reductive activation\nof O<sub>2</sub> by electron transfer from the defect-derived midgap\nlevels to O<sub>2</sub> through Au NPs. On the other hand, the preferential\nadsorption of H<sub>2</sub>O on the O<sub>v</sub> sites inhibits the\nO–O bond cleavage of O<sub>2</sub> leading to 4e<sup>–</sup>-ORR. This study provided important insight into the mechanism of\nthe Au/metal oxide-catalyzed ORR in an aqueous medium, which was previously\npoorly understood.