(359g) First Principle Guided Design of Cobalt Fischer-Tropsch Catalysts with Enhanced Stability

Supported cobalt catalysts slowly deactivate during Fischer-Tropsch Synthesis (FTS). Various deactivation mechanisms have been reviewed recently [1], and include catalysts sintering, oxidation by water, and deposition of resilient carbon species. To explore the importance of deactivation by carbon deposition, we evaluated the thermodynamic stability of various forms of deposited carbon using Density Functional Theory (DFT) [2]. A surface carbide phase and graphene islands were calculated to be thermodynamically stable under FT conditions. Two forms of deposited carbon were also identified by X-ray Photoelectron Spectroscopy and by temperature programmed hydrogenation after testing supported Co catalysts for 200 hours in a fixed bed reactor [2]. Based on these insights into the deactivation mechanism, boron was proposed as a promoter to enhance the stability of Co catalysts [3]. Calculations indicate that boron and carbon display similar relative binding preferences, and boron is proposed to selectively block the nucleation and growth of resilient carbon species. To evaluate the predictions, supported 20 wt% Co catalysts were promoted with 0.5 wt% boron and tested in a fixed bed reactor. Under enhanced deactivation conditions of 240 ºC and 20 bar, boron promotion was found to reduce the deactivation rate six-fold, without affecting selectivity and activity. This finding was subsequently confirmed at more typical FTS conditions.

References

1. Saib, A. M.; Moodley, D. J.; Ciobîcặ, I. M.; Hauman, M. M.; Sigwebela, B. H.; Westrate, C. J.; Niemantsverdriet, J. W.; Van de Loosdrecht, J., Catal. Today 154, 271 (2010)

2. Tan K. F.; Xu, J.; Chang, J.; Borgna, A.; Saeys, M., J. Catal. 274, 121 (2010)

3. Tan, K.F.; Chang, J.; Borgna, A.; Saeys, M., J. Catal., 280, 50 (2011)