Synthesis of gold nanoclusters embedded in mesoporous silica for catalytic application

Abstract

Nanoclusters are so small that the quantum confinement effect gives rise to molecular-like electronic orbital, which exhibits unique electronic, optical, and catalytic properties. Also, the high surface to volume ratio of nanoclusters and their distinctive atomic structure provide potentials for high catalytic activity and high selectivity. However, the protecting ligands of the nanoclusters inhibit the approach of the reactant to the active sites. Thus, fabricating the catalyst by utilizing ligand-off nanoclusters with minimizing any size and structural modifications is of great interest. In this thesis paper, we have studied the synthesis of gold nanoclusters embedded in two types of mesoporous silica (MCM-41 and MCM-48) for catalytic application by adsorption of gold nanoclusters to the ionic surfactant, which is the template of mesoporous silica. During the formation of the mesoporous silicas, the silicate precursor was condensed near the gold nanoclusters interacted with the surfactants by an electrostatic Coulomb force. This methodology can prevent the sintering of the gold nanoclusters and secure the high surface area and pore accessibility due to the mesoporous silica. As expected, the average size of the gold nanoclusters embedded in mesoporous silica less than 3.0nm through HAADF-STEM image after calcination at a high temperature. Also, the ordered pore structures of mesoporous silica after the addition of the gold nanoclusters were preserved by the Small-angle X-ray scattering (SAXS). 4-nitrophenol reduction and CO oxidation reaction were utilized to evaluate the catalytic activity of two synthesized materials (Au@MCM-41, Au@MCM-48). They showed high catalytic activity for both reactions in spite of the low gold amount, even 100% CO conversion rate at room temperature. This unprecedented result gives the possibility for new catalytic application of nanoclusters supported on silica.