J. Phys. Chem. B 2006, 110, 8657-8664
8657
In Situ Surface Oxidation Study of a Planar Co/SiO2/Si(100) Model Catalyst with Nanosized Cobalt Crystallites under Model Fischer-Tropsch Synthesis Conditions A. M. Saib,*,†,‡ A. Borgna,†,§ J. van de Loosdrecht,‡ P. J. van Berge,‡ and J. W. Niemantsverdriet† Schuit Institut of Catalysis, EindhoVen UniVersity of Technology, P. O. Box 513, EindhoVen 5600 MB, The Netherlands, and Sasol Technology (Pty) Ltd, P. O. Box 1, Sasolburg 1947, South Africa ReceiVed: December 18, 2005; In Final Form: March 8, 2006
The oxidation of nanosized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study a planar Co/SiO2/Si(100) model catalyst with well-defined cobalt crystallites, close to the threshold value reported for oxidation in the literature (410 nm), was prepared by the spin coating method. The planar Co/SiO2/Si(100) model catalyst was characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering. The surface oxidation behavior of the nanosized metallic cobalt crystallites of 4-5 nm was studied using in situ nearedge X-ray absorption fine structure under model FTS conditions, i.e., H2/H2O ) 1, PTotal ) 0.4 mbar, and 150-450 °C. No surface oxidation of metallic cobalt was observed under these model FTS conditions over a wide temperature range, i.e., 150-400 °C.
Introduction The production of liquid fuel from natural gas (GTL) via the Fischer-Tropsch synthesis (FTS) process is an attractive option for monetizing stranded natural gas and has potential to revolutionize the fuel industry.1 The major factors supporting this drive toward GTL are the historically high price of crude oil and global growth in the demand for clean diesel fuel.1 Supported cobalt catalysts are the system of choice for the FTS due to the high per pass conversion, low water gas shift activity, and paraffinic nature of the resulting synthetic crude oil.2-5 However, cobalt catalysts as used in the FTS are relatively expensive as compared to iron and need to have a high metal dispersion and long life to remain economically attractive.6 Hence, the development of economically attractive cobalt based FTS catalysts with a high stability requires a detailed fundamental understanding of the deactivation mechanisms at play for supported nanosized cobalt crystallites. Oxidation of metallic cobalt to cobalt oxide by means of water has long been postulated as a major deactivation mechanism during FTS which is thought to be related to the cobalt crystallite size distribution.2,7-12 It is known from thermodynamic calculations that bulk metallic cobalt does not oxidize to CoO or Co3O4 during realistic FTS conditions, e.g., 230 °C and PH2O/PH2 ) 1-1.5, whereas the formation of support compounds such as cobalt aluminate is thermodynamically favorable.3 It has been argued that the latter is kinetically inhibited and does not take place to a significant extent.3 Although bulk oxidation of cobalt metal is thermodynamically not favorable, a recent study showed that oxidation of nanosized cobalt crystallites seems to be thermodynamically possible under Fischer-Tropsch synthesis condi* To whom correspondence should be addressed: Telephone: +27 16 960 4263. Fax: +27 11 522 4488. E-mail:
[email protected]. † Eindhoven University of Technology. ‡ Sasol Technology (Pty) Ltd. § Present address: Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833.
tions.11 This is due to the contribution of the surface energy of nanosized cobalt crystallites to the overall oxidation process. From this study it was concluded that cobalt crystallites
