Role of Oxygen Vacancies and Water in Hydrogen Peroxide Production from Oxygen and Formic Acid Catalyzed by Zirconia-Supported Gold Nanoparticle

Hydrogen peroxide (H2O2) is not only a clean oxidant widely used in industry but also a promising energy carrier. There is a strong demand to develop an environmentally friendly and sustainable method for producing H2O2 as an alternative to the current energy-consuming anthraquinone autoxidation process. We recently found that Au nanoparticle-loaded ZrO2 (Au/ZrO2) rich in oxygen vacancy (Ov) or surface OH-rich exhibits much higher catalytic activity than Au/ZrO2 poor in Ov or surface OH for the production of H2O2 from O2 and HCOOH at ambient temperature and pressure. To clarify the reason, possible reaction pathways were evaluated by density functional theory calculations. The results indicated that a Langmuir–Hinshelwood-type reaction proceeds near the Au NP-ZrO2(Ov)-liquid three-phase interfaces to yield H2O2 via the two-electron oxygen reduction reaction (2e–-ORR) due to the cooperation of Ov and water. On the other hand, H2O is preferentially formed via 4e–-ORR when either Ov or water is lacking. The Ov generated on the ZrO2 surface near the Au NPs induces the efficient reductive activation of O2 by electron transfer from the defect-derived midgap levels to O2 through Au NPs. On the other hand, the preferential adsorption of H2O on the Ov sites inhibits the O–O bond cleavage of O2 leading to 4e–-ORR. This study provided important insight into the mechanism of the Au/metal oxide-catalyzed ORR in an aqueous medium, which was previously poorly understood.