Nanoparticle Formation Kinetics, Mechanisms, and Accurate Rate Constants: Examination of a Second-Generation Ir(0)n Particle 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-1 center dot HPO4](2)}(2-) nanoparticle precursor system that produces Ir(0)(similar to 150)center dot(HPO4)(x) nanoparticles 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-1 center dot 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-1 center dot HPO4](2)}(2-) precatalyst. Synchrotron X-ray absorption near-edge structure is used next to more directly monitor the reduction of Ir-I to Ir-0, and small-angle X-ray scattering is employed in separate experiments at a second synchrotron to monitor the formation of WO), versus 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 constant k(1)), and autocatalytic growth, A + B -> 2B (rate constant k(2)). 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)(n) nanoparticle formation kinetics and mechanisms.

Automated summary

This study investigates the kinetics and mechanism of a second-generation iridium, bimetallic nanoparticle precursor system. By using five experimental techniques and mechanism-enabled population balance modeling, the study addresses seven open questions and examines the consistency of rate constants across different methods, as well as the overall accuracy of the kinetics in monitoring particle formation. The analysis reveals that a minimal mechanism consistent with all the kinetic data from the physical methods cannot fully explain the observed particle size distribution.