Metal nanoparticles are known for their catalytic and medicinal applications. These properties are connected to the particle size and size distributions, which can be tuned through surface ligands and reaction conditions. To control the final particle size and size distribution, one needs to fully understand the particle-formation processes. Restated, one needs to understand the nucleation, growth, and possibly agglomeration steps. We have used an iridium-based model system to better understand these steps, where we investigated the kinetics and mechanism of a second-generation bimetallic {[(1,5-COD)IrI●HPO4]2}2– nanoparticle precursor system that produces Ir(0)~150●(HPO4)x nanoparticles. We used six total techniques to monitor the kinetics of Ir(0)n nanoparticle formation. First, synchrotron X-ray absorption near-edge structure (XANES) was used to more directly monitor the reduction of IrI to Ir0. Next, synchrotron small-angle X-ray scattering (SAXS) was employed to monitor the formation of Ir(0)n vs time. Indirect, in-house cyclohexene catalytic reporter reaction (CHCRR) monitoring method was used to follow the formation of the catalytically active Ir(0)n. Next, gas-liquid chromatography (GLC) was used to quantify the amount of cyclooctane byproduct formed vs time as a second way to monitor the loss of the {[(1,5-COD)IrI●HPO4]2}2– precatalyst. Transmission Electron Microscopy (TEM) on reaction aliquots was used to determine the particle-size distribution (PSD) vs time. Then, the experimental kinetics data were fit and analyzed using a minimal, 2-step mechanism of nucleation and autocatalytic growth. Finally, the recently developed Mechanism-Enabled Population Balance Modeling (ME-PBM) was used to analyze the TEM PSD data vs time. Specifically, ME-PBM was used to answer the question of whether or not the minimum 2-step mechanism, consistent with all the kinetics data from the five physical methods, could explain the observed PSD? We found it could not! In fact, a more complex, 4-step mechanism was required to fit the PSD.

Nanoparticle formation kinetics and mechanism: Examination of a second-generation Ir(0)n particle formation system

ACS Spring 2022

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