Single-site catalysts is 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. Some ligands have been shown to be part of the catalytic cycle while others are “spectators” that influence the activity of the metal center. 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, and thus provides information on reaction mechanism and catalyst activation. X-ray absorption spectroscopy (XAS) allows direct characterization of the metal site offering the ability to probe ligation without interference from the gas phase. 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, quick-scanning XAS (QXAS) methodology with scan rates up to the 10s of Hz, allows 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. We have observed the ligand exchange M(C2H4)2 to M(CO)2, where M = Ir or Rh, and the subsequent cycling of ligands, M(CO)2 → M(C2H4)(CO) → M(CO)2. 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. 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.

Spectro-kinetic observations of ligand exchanges for single-site supported metal catalysts through the development of QXAS at SSRL

2019 Fall National Meeting

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