Straightforward Synthesis of Gold Nanoparticles Supported on Commercial Silica-Polyethyleneimine Beads

Stable silica-supported gold nanoparticles (Au-Nps) suitable for catalysis applications were conveniently obtained in a straightforward, one-step synthesis by simply adding an aqueous solution of HAuCl4 to commercial polyethyleneimine-functionalized silica beads (SiO2-PEI) as the only reactant without any external reducing agent and/or conventional stabilizing moieties. Six different types of Au-Nps/(SiO2-PEI) beads termed Au(x-y)h, where x is the initial HAuCl4 concentration (1, 5, or 10 mM) and y is the reaction time (1 or 24 h), were prepared and characterized by UV-vis diffuse reflectance spectroscopy, X-ray fluorescence, FE-SEM microscopy, and X-ray absorption spectroscopy. The SEM micrographs of Au(x-y)h samples showed that the particle size distribution decreases with the increase of the starting gold concentration, i.e., 70-100 nm for Au(1-x)h, 40-70 nm for Au(5-x)h, and Au(10-x)h, whereas on passing from 1 to 24 h the aggregation phenomena overcome the nucleation ones, promoting the formation of bigger aggregates at the expense of small Au-Nps. The XAS analysis as a combination of XANES and EXAFS studies provided detailed structural information regarding the coordination geometry and oxidation state of the gold atoms present on the beads. Moreover, the catalytic activity of the modified silica beads in the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 was investigated and in one case the XAS analysis was repeated after recovery of the catalyst, demonstrating further reduction of the Au site to Au(0)