Synthesis of Thermally Stable and Highly Active Bimetallic Au−Ag Nanoparticles on Inert Supports

Abstract

A general two-step approach has been developed for the synthesis of very small and sintering-resistant bimetallic gold−silver nanoparticles on inert supports including commercial silica and alumina. In this approach, gold particles were formed in the first step on amino-functionalized silica or alumina support. Our density functional theory (DFT) calculations on selected model clusters show that the surface atoms of the gold particles formed in the first step carry slightly negative charges, which facilitates the subsequent Ag+ adsorption on the gold particle surface. Upon Ag+ adsorption and reduction by NaBH4 in the second step, specific nanoparticles with gold−silver alloy core and a silver nanoshell have been formed, as confirmed by our ultraviolet−visible spectroscopy (UV−vis), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) characterizations. Such particles have been found to be highly thermally stable, and their sizes remain substantially unchanged (∼3 nm) even upon calcination in air at 500 °C. After the final reduction treatment in H2, a randomly distributed alloy composed of gold and silver is formed, and the resultant Au−Ag alloy particles are highly catalytically active for CO oxidation, even superior to Au/TiO2. The role of Ag in stabilizing the particles has been discussed.