Nanosize effects in metal oxide-supported gold catalysts

CO oxidation by O₂ in the air is a very important reaction from both scientific and practical aspects. The discovery that Au particles with a diameter of less than 5 nm supported on metal oxides (Au/MOs) exhibit extremely high catalytic activity for CO oxidation even at 203 K has led to the current remarkable development of Au nanocatalysts. Numerous experimental and theoretical studies have deepened our understanding of the reaction mechanism. The clue to clarifying the origin of this surprising catalytic reaction is to understand the nanosize effects of Au particles on the physical properties and reactivity of Au/MO catalysts. This perspective highlights various nanosize effects in Au/MO thermal catalysts. The basic part discusses, in order, the size control of Au nanoparticles (NPs) on MOs, the dimensional effect of Au particles, the quantum size effect of Au particles, the electronic Au NP-MO support interaction, and O₂ activation on Au/MO catalysts. By integrating these considerations with the important findings reported so far on low-temperature CO oxidation over Au/MO catalysts, we present a Langmuir-Hinshelwood (LH) type mechanism involving reductive O₂ activation via water-assisted electron transfer from the MO support to O₂ through Au NPs (SAO-ET) as the key step. This LH mechanism rationally explains the remarkable experimental results of the Au/MO catalysts in low-temperature CO oxidation, including the effects of Au particle size, MO support, and water addition. Subsequently, the thermal stability of Au nanocatalysts is described, which is the most important issue for practical applications, focusing on the atomic-level interfacial bonding with the MO support. Finally, we summarize the conclusions and important future challenges.