Akademik Veri Yönetim Sistemi
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, cilt.139, sa.15, ss.5444-5457, 2017 (SCI-Expanded)
The nucleation process yielding Ir(0)(similar to 300) nanoparticles from (Bu4N)(5)Na-3[(1,5-COD)Ir center dot P2W15Nb3O62] (abbreviated hereafter as (COD)Ir center dot POM8-, where POM9- = the polyoxometalate, P2W15Nb3O629-) under H-2 is investigated to learn the true molecularity, and hence the associated kinetically effective nucleus (KEN), for nanoparticle formation for the first time. Recent work with this prototype transition metal nanoparticle formation system (J. Am. Chem. Soc. 2014, 136, 17601-17615) revealed tliat nucleation in this system is an apparent second-order in the precatalyst, A = (COD)Ir center dot POM8-, not the higher order implied by classic nucleation theory and its nA reversible arrow A(n), "critical nucleus", A(n) concept. Herein) the three most reasonable more intimate mechanisms of nucleation are tested: bimolecular nucleation, termolecular nucleation, and a mechanism termed "alternative:termolecular nueleation" in which 2(COD)Ir+ and 1(COD)Ir center dot POM8- yield the transition state of the rate-deterrnining step of nucleation. The results obtained definitively rule out a simple bimolecular nucleation mechanism and provide evidence for the alternative termolecular mechanism with a KEN of 3, Ir-3. All higher molecularity nucleation mechanisms were also ruled out. Further insights into the KEN and its more detailed composition involving hydrogen, {Ir3H2xPOM}(6-), are also obtained from the established role of H-2 in the Ir(0)(similar to 300) formation balanced reaction stoichiometry, from the p(H-2) dependence of the kinetics, and from a D-2/H-2 kinetic isotope effect of 1.2(+/- 0.3). Eight insights and conclusions are presented. A section covering caveats in the current work, and thus needed future studies, is also included.