Microwave-Assisted Synthesis of Pd_xAu_100-x Alloy Nanoparticles: A Combined Experimental and Theoretical Assessment of Synthetic and Compositional Effects upon Catalytic Reactivity

Pd_xAu_100-x nanoparticle (NP) catalysts with well-defined morphologies and compositions can be rapidly prepared using a simple microwave-assisted synthetic approach. Common Pd(II) and Au(III) precursors are coreduced in ethylene glycol to give small and nearly monodisperse (2.5 ± 0.6 nm) NPs with homogeneously alloyed structures in less than 300 s at 150 °C. A comparison of the nucleation and growth processes responsible for the formation of PdAuNPs by microwave and conventional methods revealed faster and more reproducible product formation under microwave-assisted heating. Pd-rich NPs were rapidly formed, into which Au atoms were subsequently incorporated to give the alloyed NPs. The value of x in the Pd_xAu_100-xNPs obtained can be finely controlled, allowing the surface electronic structure of the NPs to be broadly tuned. This permits model heterogeneous reaction studies, in which catalytic reactivity can be directly related to Pd:Au composition. Vapor-phase alkene hydrogenation studies using a series of PdAuNPs with varying compositions revealed that Pd₅₉Au₄₁NPs were catalytically the most active. Detailed theoretical studies of the entire hydrogenation reaction catalyzed at randomly alloyed PdAu surfaces were performed using a density functional theory (DFT) approach. Local ensemble effects and longer range electronic effects in the alloys were considered, leading to a prediction for optimal hydrogenation activity by Pd₅₇Au₄₃NPs. PdAuNPs obtained from microwave-assisted syntheses were also found to be more highly active than analogous NPs prepared conventionally. Quantitative solution-state ¹H NMR studies suggest that significantly less PVP was incorporated into PdAuNPs synthesized under microwave heating.