The concept of Mechanism-Enabled Population Balance Modeling (ME-PBM) is reported, illustrated by its application to a prototype Ir(0) n nanoparticle formation reaction. ME-PBM is defined herein as the use of now available, experimentally established, disproof-based, deliberately minimalistic mechanisms of particle formation as the required input for more rigorous Population Balance models, critically including an experimentally established nucleation mechanism. ME-PBM achieves the long-sought goal of connecting such now available experimental minimum mechanisms to the understanding and rational control of particles size and size distributions. Twelve pseudoelementary step, particle-formation mechanisms are considered so that the approach to the ME-PBM is also extensively disproof-based. Resurrection of Smoluchowski's 1918 full Ordinary Differential Equation (ODE) approach to the PBM is another, critical aspect of our approach which, in turn, allows unbiased f itting of the information-laden particle-size distribution (PSD) including its shape. The results provide one solution to the "inverse problem" in which the PSD informs one as to the correct particle formation mechanism: A new, deliberately minimalistic 3-step particle-formation mechanism has been uncovered that is a single-additional-step expansion of the now broadly used Finke−Watzky (FW) 2-step mechanism, the new 3-step mechanism being: A → B (rate constant k 1 ), A + B → C (rate constant k 2 ), and A + C → 1.5C (rate constant k 3 ), where A represents the monomeric nanoparticle precursor, B represents "small" nanoparticles, and C represents "larger" nanoparticles. The results strongly support three paradigm shifts for nucleation and growth of particles, the most critical paradigm shift being that the "burst" nucleation assumption in LaMer's 1950s model of particle formation is not required to produce narrow, near-monodisperse PSDs. Instead, narrow PSDs can be and are achieved despite continuous nucleation because smaller particles grow faster than larger ones, k 2 > k 3 , thereby allowing the smaller particles to catch up in size to the more slowly growing larger particles.

Monodisperse nickel nanoparticles are prepared from the reduction of Ni(acac)(2) with borane tributylamine in the presence of oleylamine and oleic acid. Without any special treatment to remove the surfactants, the as-synthesized Ni nanoparticles supported on the Ketjen carbon support exhibit high catalytic activity in hydrogen generation from the hydrolysis of the ammonia-borane (H(3)NBH(3)) complex with a total turnover frequency value of 8.8 mol of H(2) x (mol of Ni)(-1) x min(-1). Such catalysis based on Ni nanoparticles represents a promising step toward the practical development of the H(3)NBH(3) complex as a feasible hydrogen storage medium for fuel cell applications.

To start, a brief introduction is provided on the importance of transition-metal nanoclusters, on the need to develop and then apply methods to rank the nanocluster formation and then stabilizing abilities of commonly employed anions, solvents, cations, and polymers, and on the somewhat confused literature of nanocluster stabilization. The fundamental importance of surface-adsorbed anions in transition-metal nanocluster stabilization is noted, the reason the present studies begin with a study of nanocluster-stabilizing anions. Next, five criteria, as well as the associated experimental methods, are developed to evaluate the efficacy of nanocluster stabilizing agents. The criteria are of fundamental significance in that they allow the separation of stabilizing agent effects on nanocluster formation from those on nanocluster stabilization. The results from applying the five criteria to four commonly employed anions lead to the first "anion series" of relative nanocluster-formation and stabilizing abilities, at least for the Ir(0) nanoclusters examined and by the following five criteria: [(P(2)W(15)Nb(3)O(61))(2)O](16-) (a Brphinsted-basic polyoxoanion) > C(6)H(5)O(7)(3-) (citrate trianion) > [-CH(2)-CH(CO(2))-](n)(n-) (polyacrylate) approximately Cl(-). In addition to the needed methods and the first anion series, six other (8 total) conclusions are reached, important insights in an area previously lacking hard information about which anions are the better choices for nanocluster formation and stabilization. The results are also of significance in establishing polyoxoanions, notably highly charged and basic polyoxoanions such as [(P(2)W(15)Nb(3)O(61))(2)O](16)(-), as the present "Gold Standards" among currently known nanocluster stabilizing anions, and according to the above five criteria. Such standards provide a reference point for future work aspiring to develop even better nanocluster stabilizing anions, solvents, cations, and polymers or their combinations.