Cobalt catalysts used in Fischer Tropsch synthesis deactivates over time for a variety of reasons. Promoters have been suggested as a way to retard the deactivation of catalysts caused by oxidation or carbidization. These promoters reduce the deactivation of catalyst by making the surface less susceptible to carbon deposition and poisoning caused by trace impurities in the feed. Saeys et al1, in a combined experimental and DFT study on B promoted Co showed that B and C have similar binding preferences and hence B can block C deposition. The deactivation rate was reduced 6 times without affecting selectivity and activity. Swart et al2, investigated the possible intermediates in the formation of carbon deposits in cobalt-based FTS and showed that graphene was formed via stable, smaller carbon clusters on the flat Co surface. Ru promoter enhanced Co catalyst stability by preventing filamentous carbon formation3.
In this study, we investigate if Ru added as a promoter to Co catalysts aids in preventing the deactivation of Co catalyst due to carbidization. A surface alloy model, where the promoter metal is dispersed on the top surface of the catalyst, is studied. The formation energies of dimer, trimer, linear, ring and graphitic carbon structures are calculated on both Ru promoted and unpromoted surfaces. The activation barrier for the diffusion of C on both the surfaces is also calculated to check the mobility of C atoms on the surfaces. If the formation energies of these C structures are reduced on the promoted surface, Ru prevents the deactivation of Co catalyst due to C formation. For the calculations, VASP (Vienna Ab Initio Simulation package) code4-6 with Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA)7 functional is utilized for the exchange and correlation functional. The electron-ion interactions are modeled by the projector-augmented wave (PAW)8 method. The activation barrier is calculated using the Climbing Image Nudged Elastic Band (CI-NEB) method 9-11.
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(2) Swart, J. C. W.; van Steen, E.; Ciobica, I. M.; van Santenc, R. A. Physical Chemistry Chemical Physics 2008, 11, 803.
(3) Bae, J. W.; Kim, S.-M.; Park, S.-J.; Prasad, P. S. S.; Lee, Y.-J.; Jun, K.-W. Industrial & Engineering Chemistry Research 2009, 48.
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