Palladium nanocatalysts protected by polyacids

Palladium(0) Nanoparticle Formation, Stabilization, and Mechanistic Studies: Pd(acac)₂ as a Preferred Precursor, [Bu₄N]₂HPO₄ Stabilizer, plus the Stoichiometry, Kinetics, and Minimal, Four-Step Mechanism of the Palladium Nanoparticle Formation and Subsequent Agglomeration Reactions

Palladium(0) nanoparticles continue to be important in the field of catalysis. However, and despite the many prior reports of Pd(0)n nanoparticles, missing is a study that reports the kinetically controlled formation of Pd(0)n nanoparticles with the simple stabilizer [Bu4N]2HPO4 in an established, balanced formation reaction where the kinetics and mechanism of the nanoparticle-formation reaction are also provided. It is just such studies that are the focus of the present work. Specifically, the present studies reveal that Pd(acac)2, in the presence of 1 equiv of [Bu4N]2HPO4 as stabilizer in propylene carbonate, serves as a preferred precatalyst for the kinetically controlled nucleation following reduction under 40 ± 1 psig initial H2 pressure at 22.0 ± 0.1 °C to yield 7 ± 2 nm palladium(0) nanoparticles. Studies of the balanced stoichiometry of the Pd(0)n nanoparticle-formation reaction shows that 1.0 Pd(acac)2 consumes 1.0 equiv of H2 and produces 1.0 equiv of Pd(0)n while also releasing 2.0 ± 0.2 equiv of acetylacetone. The inexpensive, readily available HPO4(2-) also proved to be as effective a Pd(0)n nanoparticle stabilizer as the more anionic, sterically larger, "Gold Standard" stabilizer P2W15Nb3O62(9-). The kinetics and associated minimal mechanism of formation of the [Bu4N]2HPO4-stabilized Pd(0)n nanoparticles are also provided, arguably the most novel part of the present studies, specifically the four-step mechanism of nucleation (A → B, rate constant k1), autocatalytic surface growth (A + B → 2B, rate constant k2), bimolecular agglomeration (B + B → C, rate constant k3), and secondary autocatalytic surface growth (A + C → 1.5C, rate constant k4), where A is Pd(acac)2, B represents the growing, smaller Pd(0)n nanoparticles, and C represents the larger, most catalytically active Pd(0)n nanoparticles. Additional details on the mechanism and catalytic properties of the resultant Pd(0)n·HPO4(2-) nanoparticles are provided in this work.

Synthesis and self-organization of soluble monodisperse palladium nanoclusters

Control over size and size distribution of nanoparticles has been the subject of much interest during the past decade. In the present study, a simple strategy is described for obtaining monodisperse Pd nanoparticles with size less than 5 nm and size distribution around 10%. Without size-selective precipitation but with a simple reducing agent, hypophosphite, the synthesized Pd nanoparticles can form 2D well-ordered arrays on the TEM grids. When reducing agents were changed, no obvious size change of Pd nanoparticles was observed within experimental errors but the size distribution varied dramatically. In addition, stability, self-organized pattern, and solubility can be controlled by changing the capping agent. The present route is very simple and reproducible, and further study on the properties of the Pd nanoparticles is underway.