Fischer Tropsch synthesis : an in-situ TPR-EXAFS/XANES investigation of the influence of Group I alkali promoters on the local atomic and electronic structure of carburized iron/silica catalysts.
- Chemical Sciences and Engineering Division
The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomic structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hagg carbide and Fe{sub 3}O{sub 4}. After 10 h of CO/He treatment at 290 C, the main component observed was Hagg carbide. A model containing Hagg and {var_epsilon}-carbides, and Fe{sub 3}O{sub 4}, was also investigated. However, the r-factor was not significantly impacted by including {var_epsilon}-carbide in the fitting, and the resulting contribution of {var_epsilon}-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- FE; NETL; MRCAT; Kentucky Governor's Office of Energy Policy; Commonwealth of Kentucky
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 1035753
- Report Number(s):
- ANL/CSE/JA-64800; TRN: US201205%%368
- Journal Information:
- Journal of Physical Chemistry C, Vol. 114, Issue 17; ISSN 1932-7447
- Country of Publication:
- United States
- Language:
- ENGLISH
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