Reaction rates of catalytic cycles over supported metal catalysts are normalized by the exposed metal atoms on the catalyst surface, reported as site time yields which provide a rigorous standard to compare distinct metal surfaces. Defined as the fraction of exposed metal surface atoms to the total number of metal atoms, it is important to measure the dispersion of supported metal catalysts to report standardized rates for kinetic investigations. Multiple characterization techniques such as electron microscopy, spectroscopy and chemisorption are exploited for catalyst dispersion measurements. While effective, electron microscopy and spectroscopy are not readily accessible due to cost and maintenance requirements. Commercial instruments therefore typically rely on chemisorption measurements, but can be cost prohibitive nonetheless, hindering the ability of catalysis research to report rigorous measures of activity. Thus, a dispersion measurement technique based on gas chromatograph (GC) ubiquitous in catalysis research is proposed, based on the principle of dynamic carbon monoxide (CO) chemisorption, where number of exposed metal surface atoms are estimated based on the amount of adsorbed CO.
In this technique, the supported metal catalyst is packed into a liner, and inserted in the temperature-controlled inlet of the GC. The catalyst is pre-treated, purged with inert gas, and pulses of known amount of CO are passed through it via an automated sequence. The CO chemically adsorbs on the supported metal catalyst and the unadsorbed CO is detected by the flame ionization detector/methanizer on the GC. The amount of CO adsorbed is estimated by the difference between the amount of CO pulsed and detected, translated to estimate the number of exposed metal surface atoms using a stoichiometry factor. Dispersion measurements for several group VIII metal catalysts were conducted using this technique to demonstrate its applicability across a range of weight loadings and support identities. An agreement between catalyst dispersion measured using this technique and commercially available instruments indicated the reliability of this technique. The amount of dispersed metal as low as 0.02 mg could be estimated by this technique.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:masters_theses_2-2234 |
| Date | 28 June 2022 |
| Creators | Thakkar, Shreya |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Masters Theses |
| Rights | http://creativecommons.org/licenses/by/4.0/ |
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