T. Shirman
Harvard University,
United States
Keywords: bimetallic AuAg nanoparticles, selective oxidation, raspberry colloids
Summary:
Despite the well-known inertness of bulk gold, metal oxide supported gold nanoparticles (AuNPs) have been shown to be highly efficient heterogeneous catalyst in many industrially relevant reactions including low-temperature CO oxidation, catalytic combustion of hydrocarbons, ozone decomposition, water gas shift reactions, and hydrogenation of unsaturated hydrocarbons. In particular, gold-based bimetallic nanoparticles have emerged as promising catalysts for selective oxidation of alcohols, esterification, and amidization reactions. Control over the composition and morphology, as well as an efficient incorporation of metallic NPs are critical factors for obtaining efficient catalysts. Herein we describe a material platform based on hybrid colloid templated porous (CTP) materials featuring a high degree of control over the composition of the various components and placement of the functional nanoparticles. Our approach is based on the co-assembly of metal NPs-decorated polymer colloids (raspberry colloids) as sacrificial templates and a matrix precursor material. Removal of the polymer via calcination produces a fully interconnected porous network with metal NPs present exclusively at the pore-matrix interface, which in turn results in superior mass transport within the catalyst and improved accessibility of the metal NPs. Importantly, the composition, morphology and size of the metal NPs can be pre-designed for specific catalytic reactions, preserved during the fabrication processes and maintained by matrix stabilization during operation. The versatility of these types of materials was demonstrated for a number of selective chemical transformations using diluted Ag-in Au bimetallic NPs supported on CTP silica matrix (CTP AuAg/SiO2). These systems were characterized using various techniques including UV-vis, TEM, STEM-EDS, XPS and ICP-MS in order to elucidate structural, morphological, and compositional properties of the system throughout their fabrication and catalytic experiments. The results confirm that this approach yields high control over the morphology, composition, homogeneity of incorporation, size, and size distribution of NPs. The CTP AuAg/SiO2 structures were tested in oxidative coupling of methanol to methyl formate and exhibited high conversion efficiency, selectivity, and long-term stability as well as absence of mass transport limitations under applied reaction condition. The raspberry template method provides a platform for making and testing a plethora of rationally designed catalysts with controlled bimetallic NP size and composition and a well-defined structural and compositional hierarchy which are sustained under catalytic conditions. The synthesis provides the ability to systematically vary the nature of the metal oxide support, the pore size and structure, and the composition, size, loading, and placement of the single- or multi-metallic nanoparticles. Hence, the presented approach creates immense possibilities in terms of the catalyst composition towards efficient and selective chemical transformations in a sustainable and material-efficient way.