Structure and Reactivity of AgCu/SiO2 Bimetallic Catalysts for Alkene Epoxidation

Abstract

AgCu bimetallic materials have attracted significant interest as catalysts for selective oxidation reactions. Near-surface alloys of Ag on Cu have been shown to activate O₂ efficiently at exposed reverse-segregated isolated Cu atoms within the surface Ag layers. This study focuses on the synthesis of nanoparticle analogs of these alloys and measurement of their performance for ethylene and propylene epoxidation. Supported Cu/SiO₂ catalysts were prepared by strong electrostatic adsorption and partial galvanic replacement of Cu atoms with Ag cations in aqueous media, which led to bimetallic nanoparticles of 3–10 nm average size and 0.25–1.6 Ag:Cu atomic ratios. Galvanic exchange stoichiometry, elemental maps and UV–vis spectroscopy reveal coexistence of Ag and Cu in the nanoparticles. Diffuse reflectance infrared spectra of bound CO indicate the formation of nanoparticles with a Cu core and Ag shell when catalysts with high Ag:Cu ratios are reduced to metallic form. The bimetallic catalysts show improvement over monometallic Ag and Cu and their physical mixtures, exhibiting higher rates, higher initial selectivity than Ag, and resistance to secondary reactions that decrease the selectivity of Cu catalysts at higher alkene conversion. These results demonstrate a simple synthesis method of more selective bimetallic AgCu nanoparticles with core-shell like structures, which may be of use in a variety of selective oxidation reactions.