(689f) Spectro-Kinetic Observations of Ligand Exchanges for Single-Site Supported Metal Catalysts through the Development of Qxas at Ssrl
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 â M(C2H4)(CO) â 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.
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