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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

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.