Evidence for and mitigation of the encapsulation of gold nanoparticles within silica supports upon high-temperature treatment of Au/SiO₂ catalysts: Implication to catalyst deactivation

Silica is one of the most widely used catalyst supports for metal nanocatalysts. Although the sintering of metal nanoparticles on various silica supports has been extensively studied, the restructuring of silica supports and its effect on supported metal nanoparticles have been seldom investigated. In this paper, silica-supported gold catalysts were used as a model system to probe the interplay of silica supports and metal nanoparticles under high-temperature treatment conditions. Gold was loaded onto mesoporous SiO₂ (SBA-15) using Au(en)₂Cl₃ as the precursor in the presence of aqueous NaOH (pH ∼ 10). The influence of high-temperature treatment on the textural and structural changes of SBA-15 and Au/SBA-15 was studied by X-ray diffraction (XRD), N₂ adsorption-desorption, and transmission electron microscopy (TEM). Control experiments were conducted using an amorphous SiO₂ (Cab-O-Sil) as the support. It was found that SBA-15 undergoes significant phase transformation to crystalline cristobalite upon high-temperature treatment, resulting in the dramatic decrease in surface area. More interestingly, the crystallization of SiO₂ leads to the encapsulation of gold nanoparticles inside the SiO₂ matrix. This conclusion was proven by aqua regia leaching, EDX, and SEM/TEM experiments. Gold nanoparticles can also be encapsulated into the SiO₂ matrix when using Cab-O-Sil as the support, but the process takes place under much higher temperatures. The encapsulation of gold nanoparticles can be mitigated by coating Au/SBA-15 with amorphous Al₂O₃ or by coating SBA-15 with Al₂O₃ before loading gold. Our findings shed new light on the deactivation of supported gold catalysts under high-temperature conditions.