Nanoparticle Formation Kinetics, Mechanisms, and Accurate Rate Constants: Examination of a Second-Generation Ir(0)_nParticle Formation System by Five Monitoring Methods plus Initial Mechanism-Enabled Population Balance Modeling

The kinetics and mechanism of a second-generation iridium, bimetallic {[(1,5-COD)Ir^I·HPO4]₂}^2-nanoparticle precursor system that produces Ir(0)_∼150·(HPO₄)_xnanoparticles are investigated herein. Specifically, a list of seven open questions is addressed via a total of five experimental techniques used to monitor the kinetics of the {[(1,5-COD)Ir^I·HPO4]₂}^2-system plus mechanism-enabled population balance modeling (ME-PBM), hence six total methods. To start, an indirect but in-house cyclohexene catalytic reporter reaction monitoring method is used to follow the formation of the catalytically active Ir(0)_n. Next, gas-liquid chromatography is used to quantify the amount of cyclooctane product formed versus time as a second way to monitor the loss of the {[(1,5-COD)Ir^I·HPO4]₂}^2-precatalyst. Synchrotron X-ray absorption near-edge structure is used next to more directly monitor the reduction of Ir^Ito Ir⁰, and small-angle X-ray scattering is employed in separate experiments at a second synchrotron to monitor the formation of Ir(0)_nversus time. Transmission electron microscopy (TEM) on reaction aliquots is used to determine the particle size distribution (PSD) versus time. The experimental kinetics data are then fit and analyzed to start using a minimal, two-step mechanism of nucleation, A → B (rate constantk₁), and autocatalytic growth, A + B → 2B (rate constantk₂). How well the rate constants agree between the various methods is addressed as is the overall estimated accuracy of the kinetics in light of the multiple methods employed to monitor the particle formation kinetics. ME-PBM is then used to analyze the TEM PSD data versus time, specifically to answer the question of whether or not the minimum mechanism consistent with all the kinetic data from the five physical methods can explain the observed PSD? An important finding is that it cannot. The Discussion section returns to the seven primary questions posed in the Introduction and includes 16 recommendations for future studies. A Conclusions section is also provided in this final experimental study from our group of prototype Ir(0)_nnanoparticle formation kinetics and mechanisms.