Substantial pretreatment effect on CO oxidation over controllably synthesized Au/FeO_x hollow nanostructures via hybrid Au/β-FeOOH@SiO₂

A novel synthetic strategy has been adopted to deposit Au nanoparticles (NPs) (ca. 5 nm) on a hollow FeO_x substrate using Au/β-FeOOH hybrid nanocrystals as the precursor. Through the encapsulation of Au/β-FeOOH by SiO₂ shells and the calcination step, the Au/β-FeOOH can be transformed into Au/FeO_x with the hollow structural feature. Because of the protective SiO₂ shells, aggregation of the Au NPs is effectively prohibited, and the hemispherical morphology of the Au particles is essentially retained. The Au/FeO_x hollow composite is obtained by removing the SiO₂ shells, and the Au NPs in the final Au/FeO_x hollow composite are small-sized yet stable enough because of the calcination history. The derived Au/FeO_x hollow composite shows a substantial pretreatment effect on CO oxidation: with a pretreatment in the reaction feed at 180 C for 0.5 h, the hollow Au/FeO _x shows the T₁₀₀ of CO oxidation decreasing from 180 to 88 C. O₂ temperature-programmed desorption and X-ray photoelectron spectroscopy characterizations revealed that the pretreatment may result in (i) the creation of electron holes in the p-type FeO_x substrate and electron deficiency of Au nanoparticles as well as a strong Au-FeO_x interaction; (ii) appropriate coverage of oxygen adspecies on the Au NPs; and (iii) increased surface oxygen density, especially at the Au-FeO_x boundary region, as a result of the spillover effect of oxygen adspecies from Au NPs. All of these features are responsible for an overall enhanced activity of CO oxidation and better durability of the Au/FeO_x hollow composite.