Journal
TOPICS IN CATALYSIS
Volume 57, Issue 6-9, Pages 479-490Publisher
SPRINGER/PLENUM PUBLISHERS
DOI: 10.1007/s11244-013-0204-1
Keywords
Fischer-Tropsch synthesis (FTS); Gas-to-liquids (GTL); Cobalt crystallites; Size-dependent reoxidation
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Funding
- NASA [NNX11AI75A]
- commonwealth of Kentucky
- US DOE, Division of Materials Science and Chemical Science
- Fulbright-Thailand Research Fund scholarship program
- U.S. DOE, Office of Fossil Energy, NETL
- U.S. DOE, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
- NASA [NNX11AI75A, 144170] Funding Source: Federal RePORTER
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Freshly H-2-reduced catalyst samples and FTS catalyst samples (i.e., freshly reduced and immediately exposed to the onset of FTS conditions corresponding to 50 % CO conversion) were prepared. Each sample was coated in situ using molten polywax and solidified so that an air-protected sample was obtained, which was stored in inert gas. XAS was utilized to investigate the oxidation state of cobalt. A fraction of cobalt crystallites in the freshly reduced research catalysts having lower-than-commercial loading and smaller crystallites undergoes a degree of oxidation to CoO at the onset of FTS conditions simulating 50 % CO conversion (i.e., the H2O partial pressure is high enough to induce some oxidation). Therefore, by decreasing Co content with the aim of improving the dispersion of cobalt and Co efficiency, very small Co crystallites are obtained. Their reoxidation at the onset of FTS is an unintended consequence. Thus, catalysts should be designed to have an optimum narrow cluster size range-small enough to increase Co surface site densities, but large enough to avoid reoxidation, and the stability problems that arise from having unreduced Co in the working catalyst (e.g., a complex coalescence and reduction mechanism).
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