Abstract
Freshly H2-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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Acknowledgments
This work was sponsored by a NASA grant (Relating FTS catalyst properties to performance No.NNX11AI75A) and the commonwealth of Kentucky. This research was carried out, in part, at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the US DOE, Division of Materials Science and Chemical Science. We are also grateful to the Fulbright-Thailand Research Fund scholarship program for financial support for Mr. Thani Jermwongratanachai. Argonne’s research was supported in part by the U.S. DOE, Office of Fossil Energy, NETL. The use of the APS was supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. MRCAT operations are supported by the DOE and the MRCAT member institutions.
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Jermwongratanachai, T., Jacobs, G., Shafer, W.D. et al. Fischer–Tropsch Synthesis: Oxidation of a Fraction of Cobalt Crystallites in Research Catalysts at the Onset of FT at Partial Pressures Mimicking 50 % CO Conversion. Top Catal 57, 479–490 (2014). https://doi.org/10.1007/s11244-013-0204-1
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DOI: https://doi.org/10.1007/s11244-013-0204-1