(130d) Fabrication and Characterization of Ultrasmall Au–Cu Nanoalloy Clusters Encapsulated By Silica for High Temperature Catalysis

Significant characteristics (such as optical, electronics, and catalytic) of gold nanoparticles (NPs) have attracted the attention of many scientists since gold NPs are capable of being utilized for a wide range of engineering and biomedical purposes. It has been showed that small gold nanoparticles with a diameter less than 5nm are highly active for selective oxidation reactions such as carbon monoxide oxidation. The tendency of the gold clusters to sinter at elevated temperatures is a key limitation of gold catalysts to be utilized in practical applications. Obviously, the aggregation of gold clusters quickly leads to catalyst deactivation. A supported gold catalyst with a strong affinity between gold and the support which has high thermal stability is one of the solutions which has been proposed to solve this sintering issue in relevant reactions conducted at high temperatures. The other alternative solution is the fabrication of core-shell structure nanocatalysts in which gold and a thermally stable metal oxide serve as a core and shell in a catalyst, respectively. The motivation of this study is the fabrication and characterization of gold-copper nanoalloy clusters encapsulated by silica which can be a good candidate for CO oxidation at elevated temperatures. Considering the ligand and ensemble effect of copper on gold we are expecting to see a higher activity for Au-Cu nanoalloys compared with Au catalyst. In addition, silica shell can protect the gold-copper cluster from aggregation since silica has a high thermal and mechanical stability. In our research, inverse microemulsion route was used to fabricate Cu-Au@SiO₂ nanoparticles. Cyclohexane and Brij C10 was used as the solvent and surfactant, respectively. Copper chloride and gold(III) chloride trihydrate were used as precursors. Different volume ratios (%) of CuCl₂ and HAuCl₄ solutions 0:100, 20:80, 50:50, 80:20, and 100:0 were applied to produce different copper-gold alloys. A silica shell formed by tetraethyl orthosilicate when (3-Aminopropyl) triethoxysilane was used as a bridge between copper-gold cluster surface and silica. The obtained catalysts were characterized by X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). XRD patterns revealed that Au₃Cu, AuCu, and AuCu₃ were successfully synthesized. EDS plots justified the formation of Au-Cu@SiO₂ nanoparticles. TEM images showed that the produced gold-copper alloy clusters had average diameters around 3 nm. Coated silica thickness were around 10 nm. Nanoparticles were very well dispersed and with mainly a single core/shell structure. However, some multi core/shell nanoparticles were observed as well.