(205a) Atomically Precise Gold Clusters with Modulated Ligand and Support Environment for Oxidation Catalysis

Noble metal atom clusters have gained a lot of attention recently, because of their fascinating properties that emerge due to their unique electronic and geometric structures. These clusters find broad applications in fields ranging from catalysis to biomedicine. Different methodologies have been developed to synthesise and stabilise monodisperse atomic clusters. The most common method of preparation is colloidal preparation, which generally employs stabilizing ligands (mainly surfactants, ionic liquids, organic moieties) to prevent them from aggregating. In preparing these clusters, the metal-ligand interactions play a very important role to tune the electronic structure, which influences their final properties, especially in catalysis.

Herein, we report a facile, one-pot methodology to synthesize very stable, atomically precise gold clusters (< 2 nm core size) using triphenylphosphine as a stabilizing ligand. In addition to batch synthesis, we also employ a continuous microfluidic system. The latter has various advantages over the batch system, such as controlled mixing and a narrow residence time distribution. Gold (I) precursors of varying steric hindrance were used to tune the final size of the gold clusters and to understand the role of bound organic ligands. UV/Vis spectroscopy and electron microscopy (HAADF-STEM) are used to determine the particle size. A relationship has been discovered that correlates the steric hindrance around the metal to the final size of the nanoparticles. The synthesized gold clusters are successfully dispersed onto different metal oxide supports, such as TiO₂, ordered mesoporous silica SBA-15 and disordered SiO₂, without any alteration in the final size of the immobilized gold clusters. The catalytic activity of these supported gold clusters in the conversion of benzyl alcohol to benzaldehyde and propylene to propylene epoxide is presented. The effect of the different ligands and the support materials on the catalytic activity of these small gold clusters will be demonstrated.