Hide/Show Apps

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

2016-04-19
Özkar, Saim
Finke, Richard G.
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](2)HPO4 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](2)HPO4 as stabilizer in propylene carbonate, serves as a preferred precatalyst for the kinetically controlled nucleation following reduction under 40 +/- 1 psig initial H-2 pressure at 22.0 +/- 0.1 degrees 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 H-2 and produces 1.0 equiv of Pd(0)(n) while also releasing 2.0 +/- 0.2 equiv of acetylacetone. The inexpensive, readily available HPO42- also proved, to be as effective a Pd(0)(n) nanoparticle stabilizer as the more anionic, sterically larger, "Gold Standard" stabilizer P2W15Nb3O629-. The kinetics and associated minimal mechanism of formation of the [Bu4N](2)HPO4-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 k(1)), autocatalytic surface growth (A + B -> 2B, rate constant k(2)), bimolecular agglomeration (B + B -> C, rate constant k(3)), and secondary autocatalytic surface growth (A + C -> 1.5C, rate constant k(4)), where A is Pd(acac)(2), B represents the growing, smaller Pd(0)(n) nanopartieles, 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)center dot HPO42- nanoparticles are provided in this work.