Probing the Thermal Stability of (3-Mercaptopropyl)-trimethoxysilane-Protected Au25 Clusters by In Situ Transmission Electron Microscopy.

High-surface-area gold catalysts are promising catalysts for a number of selective oxidation and reduction reactions but typically suffer catalyst deactivation at higher temperatures. The major reason for catalyst deactivation is sintering, which can be triggered via two mechanisms: particle migration and coalescence, and Ostwald ripening. Herein, a direct method to synthesize Au25 clusters stabilized with 3-mercaptopropyltrimethoxysilane (MPTS) ligands is discussed. The sintering of Au25 (MPTS)18 clusters on mesoporous silica (SBA-15) is monitored by using an environmental in situ transmission electron microscopy (TEM) technique. Results show that agglomeration of smaller particles is accelerated by increased mobility of particles during heat treatment, while growth of immobile particles occurs via diffusion of atomic species from smaller particles. The mobility of the Au clusters can be alleviated by fabricating overlayers of silica around the clusters. The resulting materials show tremendous sinter-resistance at temperatures up to 650 °C as shown by in situ TEM and extended X-ray absorption fine structure analysis.