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

Abstract

Pd x Au100–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 x Au100–x NPs 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.

Rapid Synthesis of Rhodium–Palladium Alloy Nanocatalysts

“…The H binding energy of Rh is closer to the top of the volcano than that of Pd, as shown in Figure c . Whereas Pd has a more suitable binding energy for CHE, the overall predicted hydrogenation activity is lower than that for Rh.…”

“…Previously, we reported that H and CHE binding energies could be used as descriptors to evaluate the catalytic performance of hydrogenation catalysts . A volcano‐shaped activity plot shows that the H binding energies on both Rh(1 1 1) and Pd(1 1 1) are too strong to reach the highest TOF activity . To understand the experimental results of this study, DFT calculations were performed to calculate the H and CHE binding energies on Rh(1 1 1), Pd(1 1 1), and RhPd(1 1 1) random alloy surfaces.…”

“…It was shown previously in the microwave-assisted synthesis of PdAu alloy NPs that the most easily reducible metal (i.e.,P d) underwent near-instantaneous nucleation to form small (2 nm) NPs that subsequently facilitated random incorporation of less easily reducible Au atoms, which were autocatalytically reduced at the growing NP surfaces. [27] In this instance, after the metal-ion precursorsw ere consumed, prolonged heating resulted in ripening to give fewer,l arger alloy NPs. In the case of Rh III and Pd II ,b oth metal ions tend to undergo facile reduction in hot ethylene glycol (EG) at approximately the same temperature ( % 90 8C).…”

“…[15] Av olcano-shaped activity plot shows that the Hb inding energies on both Rh(111) and Pd(111)a re too strong to reacht he highest TOF activi-ty. [15,27] To understand the experimental resultso ft his study, DFT calculations were performed to calculate the Ha nd CHE binding energies on Rh(111), Pd(111), and RhPd(111)r andom alloy surfaces. Details of the calculations are provided in the Supporting Information.…”

Wet‐Chemical Synthesis of Pd‐Au Core‐Shell Nanoparticles (8 nm): From Nanostructure to Biological Properties

“…Kunal et al . have reported a microwave‐based synthesis of alloyed Pd−Au nanoparticles (polyol process) with about 2.5 nm diameter with variable composition and tested them in the catalytic hydrogenation of alkenes . Hahn et al .…”