Sinter-resistant metal nanoparticles encapsulated by zeolite nanoshell

摘要

Nanocatalysts have distinct activity and selectivity, which can be achieved by controlling their size, shape[1], surface composition, and electronic structure. In particular, gold nanoparticle possesses unique catalytic properties when they are supported on metal oxides/carbon and are utilized for various reactions such as CO oxidation[2], acetylene hydrochlorination[3], hydrogenation[4], selective oxidation[5] and epoxidation[6]. However, they may suffer from sintering and subsequent loss in activity due to harsh reactions conditions (e.g. high temperature, pressure, solvents etc.). This can be circumvented by encapsulating the gold nanoparticles with different shell materials such as silica, mesoporous carbon, and polymer shell. Although such approaches decrease sintering, the protective shell does not augment the product selectivity of the composite “nanoparticle core”-“protective shell” catalysts. Towards this goal, the present work explores and demonstrates the feasibility of structured encapsulants with well-defined pores to suppress sintering of encapsulated nanoparticles. We propose a general hydrothermal based synthetic strategy to assemble bifunctional catalysts in which a single gold nanoparticle is encapulated by a variety of zeolites-nanoshell (e.g. MCM-22 and ZSM-5). Gold nanoparticles were synthesized by reduction of chloroauric acid by sodium citrate[7]. These nanoparticles were coated with silica (denoted as Au@SiO2) followed by transformation into Au@MCM-22 and Au@ZSM-5 by modified hydrothermal synthesis, by varying amount of Si/Al. The zeolite nanoshells are of uniform shell thickness, morphology, possess high thermal stability and play the dual role of stabilizing the nanoparticles against sintering and their distinct pore structure can facilitate shape selective catalysis.