(689f) Spectro-Kinetic Observations of Ligand Exchanges for Single-Site Supported Metal Catalysts through the Development of Qxas at Ssrl

Single-site catalysts, a single metal atom surrounded by ligands and the support, are a topic of growing interest in catalysis science. The ability to tune the ligand sphere around the metal site has led to the understanding and control of activity and/or selectivity of the metal center.[1] Some ligands have been shown to be part of the catalytic cycle while others are “spectators” that influence the activity of the metal center.[2] Recent studies using FTIR have demonstrated that ligands can be systematically exchanged, and the process is cyclically repeatable.[3] However, FTIR poses challenges in measuring the rates at which these ligands are exchanged as the signal is often dominated by the gas phase molecules with the species of interest being a minority. FTIR sample cells also are designed with samples as a pressed wafer or a large dead volume around a packed bed adding challenge to ascribing rates due to reaction or transport limitations. Being able to understand the rate at which ligands exchange allows for determining whether the exchange is limited by diffusion of molecules through the sample or if the binding energy of either ligand drives the association/disassociation process, provides information on reaction mechanism and catalyst activation.

X-ray absorption spectroscopy (XAS) allows direct characterization of the metal site, with X-ray absorption near edge structure (XANES) being sensitive to the electronic and ligand environment around the absorbing atom [3,4] offering the ability to probe ligation without interference from the gas phase. Extended X-ray absorption fine structure (EXAFS) also allows for the average local atomic structure to be determined aiding in ascribing the ligation of the metal center.[3,4] Conventional step-scan data collection methods for XAS are slow, ~10-20 minutes per scan, and thus prohibit the measurements of transient states that occur on the ~minute time scale. However, development of quick-scanning monochromators and quick-scanning XAS (QXAS) methodology have increased scans rates up to the 10s of Hz, allowing the study of these faster processes. Using the new development of QXAS at SSRL we probed the kinetics of ligand exchanges, CO for C₂H₄ and the reverse, for supported single-site metal catalysts, and thus able to determine the processes that control the exchange.

In-situ QXAS spectra were collected on beamline 2-2 at SSRL. Zeolite HY supported Ir(C₂H₄)₂ and Rh(C₂H₄)₂ catalysts, prepared in an X-ray transparent plug flow reactor, were exposed to a cycle of flowing: (1) helium, (2) 10% CO in helium, and (3) 10% C₂H₄ in helium at several temperatures. QXAS spectra were collected at 1 Hz, allowing the observation of changes in real time in the XAS spectra as the CO and C₂H₄ ligands were exchanged on the Ir or Rh sites: M(CO)₂ → M(C₂H₄)(CO) → M(CO)₂. Linear combination fitting (LCF) of the XANES was used to determine the ligation of the metal developing spectro-kinetic data. This analysis shows that the rate of addition of CO ligand and removal of C₂H₄ ligand is faster than the reverse process. By varying the temperature of the reaction, we are able to measure differences in the rate of the exchange process. Modeling of the data will allow us to discern the activation energy associated with the exchange process and/or determine if there are diffusion limitations associated with the exchange. EXAFS modeling was used to determine the average quantity of CO or C₂H₄ ligands after throughout the process.

This work highlights the development of QXAS at SSRL through studying ligand exchanges occurring over supported single-site metal catalysts. This work addresses the mechanisms associated with ligand association and dissociation. The technique development opens up the potential to study the kinetics of transient states at SSRL.

References

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