Nanoparticle Nucleation Is Termolecular in Metal and Involves Hydrogen: Evidence for a Kinetically Effective Nucleus of Three {Ir 3 H 2 x ·P 2 W 15 Nb 3 O 62 } 6– in Ir(0) n Nanoparticle Formation From [(1,5-COD)Ir I ·P 2 W 15 Nb 3 O 62 ] 8– Plus Dihydrogen
The nucleation process yielding Ir(0)~300 nanoparticles from (Bu4N)5Na3[(1,5-COD)Ir·P2W15Nb3O62] (abbreviated hereafter as (COD)Ir·POM8-, where POM9- = the polyoxometalate, P2W15Nb3O629-) under H2 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 that nucleation in this system is an apparent second-order in the precatalyst, A = (COD)Ir·POM8-, not the higher order implied by classic nucleation theory and its nA ⇌ An, “critical nucleus”, An concept. Herein, the three most reasonable more intimate mechanisms of nucleation are tested: bimolecular nucleation, termolecular nucleation, and a mechanism termed “alternative termolecular nucleation” in which 2(COD)Ir+ and 1(COD)Ir·POM8- yield the transition state of the rate-determining 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, Ir3. 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 H2 in the Ir(0)~300 formation balanced reaction stoichiometry, from the p(H2) dependence of the kinetics, and from a D2/H2 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.
- Research Organization:
- Colorado State Univ., Fort Collins, CO (United States)
- Sponsoring Organization:
- USDOE
- Grant/Contract Number:
- FG02-03ER15453
- OSTI ID:
- 1415068
- Alternate ID(s):
- OSTI ID: 1473888
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Vol. 139 Journal Issue: 15; ISSN 0002-7863
- Publisher:
- American Chemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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