Recently we discovered that supported-nanoparticle formation from Ir(1,5-COD)Cl/?-Al2O3 under H2,in contact with acetone solution and in the presence of cyclohexene, produces Ir(0)~170/?-Al2O3 supported-nanoparticle heterogeneous catalysts through significantly different nanoparticle formation kinetics than the 2-step mechanism kinetics we observed previously and which led to larger, Ir(0)~900/?-Al2O3 supported-nanoparticles (Mondloch, J. E.; Wang, Q.; Frenkel, A. I.; Finke, R. G. J. Am. Chem. Soc. 2010,  132, 9701; see also Mondloch, J.E.; Finke, R.G. J. Am. Chem. Soc. 2011  , 133, 7744). Specifically, we have obtained kinetic, product stoichiometry, and particle-size defocusing evidence (a �30% nanoparticle size distribution) consistent with and supportive of a previously undocumented 4-step mechanism of supported transition-metal nanoparticle formation in contact with solution. That 4-step mechanism consists of: slow continuous nucleation, A ? B (rate constant k1), autocatalytic surface growth, A + B ? 2B (rate constant k2), bimolecular aggregation, B + B ? C (rate constant k3), and secondary autocatalytic surface growth, A + C ? 1.5C (rate constant k4), where A is nominally the Ir(1,5-COD)Cl/?-Al2O3 precursor, B the growing Ir(0) particles, and C the larger, catalytically active nanoparticles. The findings are significant in that they comprise the first documentation of a 4-step mechanism for supported-nanoparticle formation under any conditions (18 alternative mechanisms were disproven en route to the proposed 4-step mechanism). Moreover, when the present study is combined with our 2011 study, op cit., one now has precedent for two specific, balanced pseudo-elementary kinetic steps and their precise chemical descriptors�bimolecular particle aggregation, B + B ? C, and autocatalytic agglomeration, B + C ? 1.5C�steps involved in, for example, other significant processes in heterogeneous catalysis such as catalyst sintering.

Four-step mechanism for the formation of supported-nanoparticle heterogenous catalysts in contact with solution: The conversion of Ir(1,5-COD)Cl/?-Al2O3 to Ir(0)~170/?-Al2O3

247th National Meeting (2014)

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