(768g) The Role of Added Promoters in Reducing the Deactivation of Co Catalyst Used in Fischer Tropsch Synthesis

 Cobalt catalysts used in Fischer Tropsch synthesis deactivate 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. In an experimental study, Ru promoter was shown to enhance Co catalyst stability by preventing filamentous carbon formation^1,2. Park et al.³ observed different deactivation behavior of a Co catalysts with respect to the type of noble metal used for promotion. They found that Pt promoted catalyst had higher catalytic stability than Ru promoted catalyst where large amounts of polymeric carbons were deposited on the surface.

In this study, we investigate if Ru or Pt added as promoters to Co catalysts aid 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 used in this study. The adsorption energies of dimer, trimer, linear, ring and graphitic carbon structures were calculated on both Ru promoted and unpromoted surfaces. The activation barrier for the diffusion of C on both the surfaces was calculated to check the mobility of C atoms on the surfaces. The activation barriers for C-H, C-C, C-C-H and C-C-C coupling were also calculated to determine if the promoters inhibit or enhance C-C coupling thereby preventing or facilitating the deactivation of the catalyst surface by the formation of various C compounds. The adsorption energies were found to follow the order Pt promoted<Ru promoted<unpromoted catalyst. In addition, the barriers for C-C and C-C-C coupling were higher on the promoted catalyst suggesting that Pt and Ru promoters help prevent C deposition on Co catalyst.

References:

                (1)          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.

                (2)          Bae, J. W.; Kim, S.-M.; Park, S.-J.; Prasad, P. S. S.; Lee, Y.-J.; Jun, K.-W. Ind. Eng. Chem. Res. 2009, 48.

                (3)          Park, S.-J.; Bae, J. W.; Lee, Y.-J.; Ha, K.-S.; Jun, K.-W.; Karandikar, P. Catal. Commun. 2011, 12, 539.