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

du Plessis, Hester Esna (author)
de Villiers, J. P. R. (author)
Kruger, G. J. (author)
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Steuwer, Axel (author)
Lund University,Lunds universitet,European Spallation Source ESS AB,Stiftelser och övriga anknutna verksamheter,Other Institutions and Utilities
Brunelli, M. (author)
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 (creator_code:org_t)
2011
2011
English.
In: Journal of Synchrotron Radiation. - 1600-5775. ; 18, s. 266-271
  • Journal article (peer-reviewed)
Abstract Subject headings
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  • 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.

Subject headings

NATURVETENSKAP  -- Fysik -- Acceleratorfysik och instrumentering (hsv//swe)
NATURAL SCIENCES  -- Physical Sciences -- Accelerator Physics and Instrumentation (hsv//eng)

Keyword

Hagg carbide
Fischer-Tropsch synthesis
crystal structure
PDF

Publication and Content Type

art (subject category)
ref (subject category)

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