III International Conference

CARBON INDUCED FCC-COBALT SURFACE RECONSTRUCTIONPP-I-60Ciobîcă I.M., van Santen R.A. 1 , van de Loosdrecht J. 2 , van BergeP.J. 2Sasol Technology Netherlands B. V., Eindhoven University of TechnologyP. O. Box 513, 5600 MB Eindhoven, The Netherlands1 Schuit Institute of Catalysis, Eindhoven University of Technology,P. O. Box 513, 5600 MB Eindhoven, The Netherlands2 Sasol Technology (Pty) Ltd, 1 Klasie Havenga Street, 1947 Sasolburg, South AfricaE-mail address: Ionel.Ciobica@Sasol.comThe interaction of carbon with cobalt surfaces is extremely important due to the technologicalrelevance of the system, particularly in the field of heterogeneous Fischer Tropsch synthesis(FTS) catalysis (integral part of the Gas-To-Liquids process), and the fundamental interest inexplaining the nature of the carbon induced surface reconstructions. Several types of carbon maybe present on the surface, for example, active carbon is formed at lower temperatures andpolymeric or graphitic carbon is formed at higher temperatures. This paper focuses on the role ofcarbon and other potential Fischer-Tropsch synthesis species which are present on the cobaltsurface and can induce surface reconstruction of specific cobalt surfaces.DFT calculations were performed with the VASP code (Vienna ab initio SimulationPackage). The model for the unreconstructed cobalt surfaces consists in 4, 5 and 7 layers forFCC-Co(111), FCC-Co(100) and respectively FCC-Co(110) and 5 layers of vacuum. For thereconstructed cobalt surfaces we used 5 layers from which 3 internal layers of FCC-Co(111)structure, plus 2 reconstructed layers. Full optimisation of the systems was carried out and nosymmetry constraints were applied. The comparison of the (111), (100) and (110) slabs is notmade based on the adsorption energies but rather on the total energy of the system.Due to the fact that small cobalt crystallites in Fischer Tropsch synthesis catalysts are inthe FCC phase [1] , the focus of this paper will be on this FCC phase, rather than on the HCPphase. O, CO, C, CH and CH 2 were selected to preliminary investigate as possible candidatesto effect (111) to (100) and (111) to (110) cobalt surface reconstructions, as they are typicalFTS intermediates.The difference in absolute energy of the Co(111) and Co(100) bare surfaces is 2.2 eV,which corresponds to a 2×2 unit cell (4 metal atoms on each layer, so 4 metal atoms at thesurface as well). Adsorbing C atoms on the cobalt surfaces results in a decrease of thedifference to 0.1 eV at 25% coverage and most important, the order of stability of the two113