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Title: Fischer–Tropsch Synthesis: XANES Spectra of Potassium in Promoted Precipitated Iron Catalysts as a Function of Time On-stream

Abstract

XANES K-edge spectra of potassium promoter in precipitated Fe catalysts were acquired following activation by carburization in CO and as a function of time on-stream during the course of a Fischer–Tropsch synthesis run for a 100Fe:2K catalyst by withdrawing catalysts, sealed in wax product, for analysis. CO-activated and end-of-run spectra of the catalyst were also obtained for a 100Fe:5K catalyst. Peaks representing electronic transitions and multiple scattering were observed and resembled reference spectra for potassium carbonate or potassium formate. The shift in the multiple scattering peak to higher energy was consistent with sintering of potassium promoter during the course of the reaction test. The catalyst, however, retained its carbidic state, as demonstrated by XANES and EXAFS spectra at the iron K-edge, suggesting that sintering of potassium did not adversely affect the carburization rate, which is important for preventing iron carbides from oxidizing. This method serves as a starting point for developing better understanding of the chemical state and changes in structure occurring with alkali promoter.

Authors:
 [1];  [2];  [2];  [2];  [2];  [3];  [4];  [4];  [2]
  1. Univ. of Kentucky, Lexington, KY (United States). Center for Applied Energy Research; Univ. of Texas, San Antonio, TX (United States). Dept. of Biomedical and Chemical Engineering
  2. Univ. of Kentucky, Lexington, KY (United States). Center for Applied Energy Research
  3. Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
  4. Canadian Light Sources, Inc., Saskatoon, SK (Canada)
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL), Upton, NY (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Synchrotron Radiation Lightsource (SSRL)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); Commonwealth of Kentucky; Canadian Light Sources, Inc., Saskatoon, SK (Canada)
OSTI Identifier:
1433948
Report Number(s):
BNL-203462-2018-JAAM
Journal ID: ISSN 1011-372X
Grant/Contract Number:
SC0012704; AC02-76SF00515
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Catalysis Letters
Additional Journal Information:
Journal Volume: 147; Journal Issue: 8; Journal ID: ISSN 1011-372X
Publisher:
Springer
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 43 PARTICLE ACCELERATORS; Fischer-Tropsch synthesis; iron-based catalyst; potassium promoter; olefin selectivity; XANES/EXAFS

Citation Formats

Jacobs, Gary, Pendyala, Venkat Ramana Rao, Martinelli, Michela, Shafer, Wilson D., Gnanamani, Muthu Kumaran, Khalid, Syed, MacLennan, Aimee, Hu, Yongfeng, and Davis, Burtron H.. Fischer–Tropsch Synthesis: XANES Spectra of Potassium in Promoted Precipitated Iron Catalysts as a Function of Time On-stream. United States: N. p., 2017. Web. doi:10.1007/s10562-017-2090-9.
Jacobs, Gary, Pendyala, Venkat Ramana Rao, Martinelli, Michela, Shafer, Wilson D., Gnanamani, Muthu Kumaran, Khalid, Syed, MacLennan, Aimee, Hu, Yongfeng, & Davis, Burtron H.. Fischer–Tropsch Synthesis: XANES Spectra of Potassium in Promoted Precipitated Iron Catalysts as a Function of Time On-stream. United States. doi:10.1007/s10562-017-2090-9.
Jacobs, Gary, Pendyala, Venkat Ramana Rao, Martinelli, Michela, Shafer, Wilson D., Gnanamani, Muthu Kumaran, Khalid, Syed, MacLennan, Aimee, Hu, Yongfeng, and Davis, Burtron H.. Tue . "Fischer–Tropsch Synthesis: XANES Spectra of Potassium in Promoted Precipitated Iron Catalysts as a Function of Time On-stream". United States. doi:10.1007/s10562-017-2090-9.
@article{osti_1433948,
title = {Fischer–Tropsch Synthesis: XANES Spectra of Potassium in Promoted Precipitated Iron Catalysts as a Function of Time On-stream},
author = {Jacobs, Gary and Pendyala, Venkat Ramana Rao and Martinelli, Michela and Shafer, Wilson D. and Gnanamani, Muthu Kumaran and Khalid, Syed and MacLennan, Aimee and Hu, Yongfeng and Davis, Burtron H.},
abstractNote = {XANES K-edge spectra of potassium promoter in precipitated Fe catalysts were acquired following activation by carburization in CO and as a function of time on-stream during the course of a Fischer–Tropsch synthesis run for a 100Fe:2K catalyst by withdrawing catalysts, sealed in wax product, for analysis. CO-activated and end-of-run spectra of the catalyst were also obtained for a 100Fe:5K catalyst. Peaks representing electronic transitions and multiple scattering were observed and resembled reference spectra for potassium carbonate or potassium formate. The shift in the multiple scattering peak to higher energy was consistent with sintering of potassium promoter during the course of the reaction test. The catalyst, however, retained its carbidic state, as demonstrated by XANES and EXAFS spectra at the iron K-edge, suggesting that sintering of potassium did not adversely affect the carburization rate, which is important for preventing iron carbides from oxidizing. This method serves as a starting point for developing better understanding of the chemical state and changes in structure occurring with alkali promoter.},
doi = {10.1007/s10562-017-2090-9},
journal = {Catalysis Letters},
number = 8,
volume = 147,
place = {United States},
year = {Tue Jun 06 00:00:00 EDT 2017},
month = {Tue Jun 06 00:00:00 EDT 2017}
}

Journal Article:
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  • Coal-to-liquids. Iron-based Fischer-Tropsch synthesis catalysts are not only used commercially for high temperature Fischer-Tropsch synthesis, but are increasingly becoming the focus for converting low H{sub 2}/CO ratio synthesis gas at lower temperature. Such low temperature processing yields hydrocarbon distributions with higher {alpha} values, and as a consequence, much less light hydrocarbon gas production (and especially, less methane). Another benefit to the use of iron-based catalysts is that the product slate is richer in {alpha}-olefinic products, which are more valuable than the paraffinic products produced using cobalt-based catalysts. Iron-based catalysts are often used to convert low H{sub 2}/CO ratio syngas, becausemore » the catalysts can intrinsically adjust the syngas ratio upward by converting a fraction of CO by reaction with H{sub 2}O to produce H{sub 2} and CO{sub 2} via the water-gas shift reaction.« less
  • The kinetics of the gas-solid Fischer-Tropsch (FT) synthesis over a commercial Fe-Cu-K-SiO{sub 2} catalyst was studied in a continuous spinning basket reactor. Experimental conditions were varied as follows: reactor pressure of 0.8--3.2 MPa, H{sub 2}/CO feed ratio = 0.5--2.0, and a space velocity of 0.5--2.0 {times} 10{sup {minus}3} Nm{sup 3}/kg{sub cat} s at a constant temperature of 523 K. A new product distribution model for linear hydrocarbons is proposed. Deviations from conventional Anderson-Schulz-Flory distribution can be quantitatively described with an {alpha}-olefin readsorption product distribution model. The experimentally observed relatively high yield of methane, relatively low yield of ethene, and bothmore » the exponential decrease of the olefin-to-paraffin ratio and the change of the chain growth parameter with chain length can all be predicted from this new model. It combines a mechanistic model of olefin readsorption with kinetics of chain growth and termination on the same catalytic sites. The hydrocarbon formation is based on the surface carbide mechanism by CH{sub 2} insertion. The olefin readsorption rate depends on the chain length because of increasing physisorption strength on the catalyst surface and increasing solubility in FT wax with increasing chain length. Interfacial concentrations of reactive olefins near the gas-wax and wax-catalyst surface are used in the kinetic model. With optimization of three parameters per experimental product distribution, the olefin readsorption product distribution model proved to predict product selectivities accurately over the entire range of experimental conditions. The relative deviations are 10.1% and 9.1% for the selectivity to paraffins and olefins with n < 11, respectively.« less
  • The effects of potassium and copper promotion on the activity and selectivity of precipitated iron catalysts for Fischer--Tropsch synthesis (FTS) were studied in a fixed bed reactor at 1.48 MPa and 235--265{degrees} C using synthesis gas with a H{sub 2}/CO = 1 molar feed ratio. It was found that both potassium and copper increase the catalyst activity for FTS and the water gas shift reaction. Potassium promotion ({approximately} 0.2--1 wt %) results in an increase in the average molecular weight of hydrocarbon products and suppression of secondary reactions (olefin hydrogenation and isomerization of 1-alkenes to 2-alkenes). Copper promotion ({approximately} 3more » wt %) has a similar effect on the hydrocarbon distribution, but it enhances slightly the secondary reactions. The activity of doubly promoted (100 Fe/3 Cu/{ital x}K, {ital x} = 0.2 or 0.5) catalysts was higher than that of singly promoted catalysts and was independent of potassium loading, whereas their selectivity behavior was strongly influenced by their potassium loading. Product selectivities on the 100 Fe/3 Cu/0.2 K catalyst were similar to those of the 100 Fe/3 Cu catalyst, whereas selectivities of the 100 Fe/3 Cu/0.5 K catalyst were similar to those obtained in tests with the 100 Fe/0.5 K catalyst.« less
  • 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 atomicmore » 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.« less