Catalysis Today, Vol.334, 231-242, 2019
Generalized approach for the synthesis of silica supported Pd-Zn, Cu-Zn and Ni-Zn gamma brass phase nanoparticles
The existing literature suggests it is particularly difficult to access the catalytically relevant, and relatively complex, intermetallic gamma-brass crystal structure through traditional nanoparticle (NP) synthesis techniques. We introduce a simple and rational approach to access this phase in M-Zn (M=Pd, Cu, Ni) systems as silica-supported single-phase nanocrystals. This hybrid approach involves the initial synthesis of supported M/SiO2 through traditional approaches (dry impregnation and strong electrostatic adsorption) followed by heating to high temperatures in the presence of a stoichiometric amount of metallic Zn in an evacuated closed system for several hours. We demonstrate the generality of this method with three different catalytically important bimetallic systems: Pd-Zn, Ni-Zn and Cu-Zn. Of these three, Pd-Zn is by the far the most popular in terms of catalytic applications and yields the smallest particle size (similar to 8 nm). We tested the influence of various synthesis parameters on phase purity and particle size distribution in case of the synthesized gamma-brass Pd-Zn/SiO2 supported catalysts and provide general guidelines towards optimization of synthesis. Upon transformation of Pd/SiO2 to gamma-brass Pd-Zn/SiO2, a precipitous drop in CO adsorption and a 25 kJ/mol increase in the ethylene hydrogenation barrier is observed, indicating the catalytic active sites are significantly modified as a result of alloying. We anticipate these catalysts may find applications in various Pd-catalyzed chemistries.