$Au@SiO_2$ nanoreactor frameworks for heterogeneous bifunctional catalysts and application to catalytic reaction

Abstract

Colloidal metallic nanoparticles have been interesting materials due to their use in catalysis. Despite their excellent catalytic activities, the use of ensemble catalysts has been limited to probe a catalytic property of a single particle in that those are averaged outcomes of significant amount of catalytic particles. The ideal nanoreactor is composed of an active metal core and silica hollow shell through which reactants pass and exhibit catalytic reaction on the surface of the active metal. Because the single active metal is isolated by solid shell, catalytic reaction occurs under homogeneous reaction condition in the shell like metal particles are evenly dispersed on the supports. The outer shell hinders the aggregation of neighboring particles and stabilizes the particle even under harsh reaction conditions such as high temperature or high pressure. The active metal of nanoreactor can be controlled to have various sizes and shapes and the silica shell can be prepared to have several diffusion coefficients. Moreover, selective ligand anchored onto metal surface can enhance the reaction rate. In this work, the new heterogeneous bifunctional catalyst, metal@silica nanoreactor framework with these advantages is represented as well as its catalytic application is demonstrated to prove the benefits. The fabricated gold nanoreactors which have different core size were applied to the catalytic reduction of p-nitrophenol. The turnover frequency of the gold nanoreactor showed a steady rise with decreasing the size of gold core, meaning that the reduction of p-nitrophenol is the size-dependent reaction. The functional group on the shell of gold nanoreactor could be modified for further application. Hydroxyl groups on the silica surface could be modified to trimethylsiloxyl groups by trimethylsilylation and the gold nanoreactor is anticipated to be used in different media with different property. It was also suggested that porous silica coating could realize t...