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Rietveld and pair distribution function study of Hagg carbide using synchrotron X-ray diffraction

du Plessis, Hester Esna; de Villiers, J. P. R.; Kruger, G. J.; Steuwer, Axel LU and Brunelli, M. (2011) In Journal of Synchrotron Radiation 18. p.266-271
Abstract
Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe5C2 Hagg carbide (chi-HC) during FT synthesis. Arguably, chi-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of chi-HC to iron oxide, a step-wise decarburization to cementite (theta-Fe3C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of chi-HC for the understanding of the synthesis process and for... (More)
Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe5C2 Hagg carbide (chi-HC) during FT synthesis. Arguably, chi-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of chi-HC to iron oxide, a step-wise decarburization to cementite (theta-Fe3C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of chi-HC for the understanding of the synthesis process and for comparison with the first-principles ab initio modelling. Here the results of high-resolution synchrotron X-ray powder diffraction data are reported. The atomic arrangement of chi-HC was confirmed by Rietveld refinement and subsequent real-space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of chi-HC correspond well with that of a pseudo-monoclinic phase of space group Pi [a = 11.5661 (6) A, b = 4.5709 (1) A, c = 5.0611 (2) A, alpha = 89.990 (5)degrees, beta = 97.753 (4)degrees, gamma = 90.195 (4)degrees], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Hagg carbide, Fischer-Tropsch synthesis, crystal structure, PDF
in
Journal of Synchrotron Radiation
volume
18
pages
266 - 271
publisher
Wiley-Blackwell
external identifiers
  • wos:000287530900022
  • scopus:79951918605
ISSN
1600-5775
DOI
10.1107/S0909049510048958
language
English
LU publication?
yes
id
a3c1fdae-ba27-45b7-9f51-ca33d3353342 (old id 1872716)
date added to LUP
2011-04-18 13:02:12
date last changed
2017-08-13 03:54:52
@article{a3c1fdae-ba27-45b7-9f51-ca33d3353342,
  abstract     = {Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe5C2 Hagg carbide (chi-HC) during FT synthesis. Arguably, chi-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of chi-HC to iron oxide, a step-wise decarburization to cementite (theta-Fe3C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of chi-HC for the understanding of the synthesis process and for comparison with the first-principles ab initio modelling. Here the results of high-resolution synchrotron X-ray powder diffraction data are reported. The atomic arrangement of chi-HC was confirmed by Rietveld refinement and subsequent real-space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of chi-HC correspond well with that of a pseudo-monoclinic phase of space group Pi [a = 11.5661 (6) A, b = 4.5709 (1) A, c = 5.0611 (2) A, alpha = 89.990 (5)degrees, beta = 97.753 (4)degrees, gamma = 90.195 (4)degrees], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure.},
  author       = {du Plessis, Hester Esna and de Villiers, J. P. R. and Kruger, G. J. and Steuwer, Axel and Brunelli, M.},
  issn         = {1600-5775},
  keyword      = {Hagg carbide,Fischer-Tropsch synthesis,crystal structure,PDF},
  language     = {eng},
  pages        = {266--271},
  publisher    = {Wiley-Blackwell},
  series       = {Journal of Synchrotron Radiation},
  title        = {Rietveld and pair distribution function study of Hagg carbide using synchrotron X-ray diffraction},
  url          = {http://dx.doi.org/10.1107/S0909049510048958},
  volume       = {18},
  year         = {2011},
}