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Issue Oil & Gas Science and Technology - Rev. IFP
Volume 64, Number 1, January-February 2009
Dossier: The Fischer-Tropsch Process
Page(s) 49 - 62
DOI 10.2516/ogst:2008039
Published online 28 October 2008

Oil & Gas Science and Technology - Rev. IFP, Vol. 64 (2009), No. 1, pp. 49-62
DOI: 10.2516/ogst:2008039

Microstructure of Supported Cobalt Fischer-Tropsch Catalysts

Microstructure de catalyseurs Fischer-Tropsch à base de cobalt supporté


O. Ducreux1, B. Rebours2, J. Lynch1, M. Roy-Auberger2 and D. Bazin3

1  Institut français du pétrole, IFP-Rueil, 1-4 avenue de Bois-Préau, 92852 Rueil-Malmaison - France
2  Institut français du pétrole, IFP-Lyon, BP 3, 69390 Vernaison - France
3  Laboratoire de Physique des Solides, Bât. 510, Université Paris XI, 91405 Orsay Cedex - France

olivier.ducreux@axens.net
bernadette.rebours@ifp.fr
john.lynch@ifp.fr
magalie.roy@ifp.fr
bazin@lps.u-psud.fr

Published online: 28 October 2008

Abstract
The structures of metallic cobalt particles supported on silica (with and without Ru promoter) and on alumina have been investigated in-situ by conventional and anomalous X-ray diffraction. Comparison with simulated XRD patterns shows the structure to consist of a defect phase characterised by the presence of stacking faults, the density of which is directly related to the temperature at which the cobalt oxide is reduced. Controlled modification of the cobalt microstructure, while maintaining the same support and promoter has been achieved with limited variation of the particle size, notably via CO – H2 treatment leading to the formation and decomposition of cobalt carbide. Independently of the support or the presence of a promoter, low reduction temperatures increase the tendency to form hexagonal close packed (hcp) structure whilst at high reduction temperatures face centred cubic (fcc) stacking is preferentially formed. The catalytic activity for CO conversion in Fischer-Tropsch conditions is greatly increased for catalysts with a majority of hcp stacking compared to those containing predominantly fcc phase particles.


Résumé
La structure de particules métalliques de cobalt supporté sur silice ou sur alumine, et en présence ou non d'un promoteur (Ru), a été étudiée in situ par diffraction des rayons X conventionnelle et anomale. Par comparaison avec des diagrammes simulés, il est montré que cette structure est caractérisée par la présence de défauts d'empilement dont la densité est directement reliée à la température de réduction du cobalt. Des traitements de carburation/décarburation sous CO et H2 nous ont permis de modifier de manière contrôlée la microstructure du cobalt métallique pour un même support tout en limitant la variation de la taille des particules. Quel que soit le support, et en présence ou non d'un promoteur, les basses températures de réduction favorisent la formation de la forme hexagonale compacte du cobalt, alors qu'une réduction à haute température favorisera la forme cubique faces centrées. Dans les conditions de la synthèse Fischer-Tropsch, les catalyseurs contenant du cobalt à forte densité d'empilements de type hexagonal compact montrent une activité en conversion du CO nettement supérieure à celle des catalyseurs majoritairement cubiques.



© IFP 2008


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