Thermal Stability of Alumina-Overcoated Au₂₅ Clusters for Catalysis

One of the prevalent difficulties in using supported gold (Au) cluster or nanoparticle materials for catalytic applications is the ease at which such systems sinter at even moderate temperatures and conditions. Herein we demonstrate a stabilization strategy involving atomic-layer-deposition (ALD) of alumina overlayers onto supported Au clusters, which greatly alleviates high-temperature sintering. Control over both the number of cycles (5, 10, and 20) of alumina deposition over Au25 clusters and the surface chemistry of the clusters themselves was found to be important for optimal stabilization. Transmission electron microscopy and extended X-ray absorption fine structure analyses showed that Au25 clusters using 11-mercaptoundecanoic acid stabilizers with 20 cycles of alumina overcoating via ALD showed remarkable thermal stability, with particle sizes growing only slightly to ca. 2.5 nm after calcination at 650 °C. However, such stabilization does come with a cost; at moderate temperatures, 10-cycle alumina-coated catalysts showed better catalytic activity for a 4-nitrophenol reduction reaction than 20-cycle alumina-coated catalysts, which is likely because of increased mass-transfer resistance associated with the protective shell.