Oxygen vacancies dependent Au nanoparticle deposition and CO oxidation

Abstract

Oxygen vacancies are critical for both the reactivity and the reaction mechanisms for Au-deposited oxides. The deposition behavior of Au on a CeO₂ support was studied by determining the content of oxygen vacancies induced by ascorbic acid (VC) treatment. Oxygen vacancies were introduced on CeO₂ nanorods (NRs) by VC reduction, and Au nanoparticles (NPs) were loaded by a deposition–precipitation (DP) method. The formation of oxygen vacancies and their effects on Au NP deposition were evaluated by catalytic CO oxidation. The reaction mechanism of Au anchoring was closely linked to the concentration of oxygen vacancies. On the CeO₂ nanorods with an optimal number of oxygen vacancies, the deposition of Au behaved more like a “lattice substitution mechanism”, where charge transfer occurs to form positively charged Au³⁺ species and reduced Ce³⁺, associated with the creation of oxygen vacancies, which are active for CO oxidation. VC treatment induced a large number of oxygen vacancies on CeO₂ NRs, resulting in highly increased reducibility of CeO₂ and strong interaction between Au and CeO₂. Consequently, Au³⁺ cations were reduced directly with a fast reduction rate, instead of undergoing hydrolysis into the hydroxychloro gold(III) complex, [Au(OH)_xCl_4−x]⁻, which was generally generated during the DP procedure. Such strong charge transfer interaction between the oxygen vacancies and Au³⁺ leads to sintering of the reduced Au species to form Au NPs with a larger size and an uneven distribution, and to a decreased Ce³⁺/Ce⁴⁺ ratio, with a decrement of surface oxygen atoms, as well as the reduction of Au³⁺ species to Au⁺; these events are together connected to the activity loss of the catalyst.