A mild and environmentally benign strategy towards hierarchical CeO2/Au nanoparticle assemblies with crystal facet-enhanced catalytic effects for benzyl alcohol aerobic oxidation

Abstract

The exposure of the crystallographic facets of CeO₂ nanocrystals may alter their surface structure and composition, leading to significant discrepancies in their reactivity with respect to catalyzing different reactions. In this paper, a facile strategy of etching hollow CeO₂ spheres (@CeO₂) with a fluorine-containing ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]), under hydrothermal conditions was developed to achieve surface-fluorinated @CeO₂ nanoparticles with exposed active high-energy facets, which were further assembled in situ into @CeO₂ nanoparticle assemblies. The IL, [Bmim][BF₄], is believed to play an important role in determining the formation of CeO₂ nanoparticles, and the mild release of fluorine species from [Bmim][BF₄] may account for the surface-fluorination and the exposure of active high-energy facets of CeO₂. The CeO₂ nanoparticle assemblies are ideal model materials for studying crystal facet-dependent catalytic behavior. By loading well-dispersed Au nanoparticles via a sol-impregnation method, @CeO₂/Au nanoparticle assemblies were achieved, which showed an enhanced catalytic performance for benzyl alcohol aerobic oxidation. In comparison with @CeO₂/Au nanoparticle assemblies obtained by etching @CeO₂ spheres with other fluorine-containing agents, NH₄BF₄, NaBF₄, and NH₄F, under identical reaction conditions, @CeO₂/Au nanoparticle assemblies achieved by etching with [Bmim][BF₄] exhibited the highest catalytic activity for benzyl alcohol aerobic oxidation. The surface-fluorination and the exposure of active high-energy facets of CeO₂ nanoparticles are believed to be responsible for the enhancement of their catalytic activity. This methodology provides a robust strategy to create CeO₂ materials with the active high-energy facets exposed and afford model materials for investigating crystal facet-dependent catalytic behavior, which currently represents an exciting issue in the nanocatalysis community.