Abstract
The kinetics and mechanism of a second-generation iridium, bimetallic {[(1,5-COD)IrI·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)IrI·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)IrI·HPO4]2}2-precatalyst. Synchrotron X-ray absorption near-edge structure is used next to more directly monitor the reduction of IrIto 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.
| Original language | English |
|---|---|
| Pages (from-to) | 13449-13476 |
| Number of pages | 28 |
| Journal | Journal of Physical Chemistry C |
| Volume | 125 |
| Issue number | 24 |
| DOIs | |
| State | Published - Jun 24 2021 |
| Externally published | Yes |
Funding
We greatly appreciate and acknowledge Patrick Kent for his contributions beginning before 2014. We further thank Patrick for providing us with his XANES work as cited herein, so this long-term effort could finally be published without further delay. We offer our heartfelt condolences to Patrick on the tragic passing of his wife due to cancer in the otherwise prime of her and his lives. Professor Murielle Watzky’s always careful and thoughtful comments on the manuscript and proofreading help are a pleasure to acknowledge as are the thoughtful, helpful comments from two referees. The Stanford Synchrotron Radiation Lightsource and Australian Synchrotron are thanked and acknowledged for providing access to conduct synchrotron XAFS and SAXS, respectively. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Sciences under Contract no. DE-AC-02-76SF00515. Operations at the beamlines BL2-2 of the National Synchrotron Light Source-I were supported in part by the Synchrotron Catalysis Consortium (U.S. DOE, Office of Basic Energy Sciences, grant no. DE-SC0012335). XAS analysis work of A.I.F. was supported in part by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award no. DESC0001004. The SAXS portion of this research was undertaken on the SAXS/WAXS beamline at the Australian Synchrotron, part of ANSTO. The work at Colorado State University was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Catalysis Science Program, via Award SE-FG402-02ER15453.