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

Abstract

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 (O_v) or surface OH-rich exhibits much higher catalytic activity than Au/ZrO₂ poor in O_v 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(O_v)-liquid three-phase interfaces to yield H₂O₂ via the two-electron oxygen reduction reaction (2e^–-ORR) due to the cooperation of O_v and water. On the other hand, H₂O is preferentially formed via 4e^–-ORR when either O_v or water is lacking. The O_v 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 O_v 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.