(165d) Deactivation of Molybdenum Carbide-Based Fischer-Tropsch Synthesis Catalysts

Early transition metal carbides have been demonstrated to be active and selective for the Fischer-Tropsch Synthesis reaction. A challenge with regard to the use of these materials is their deactivation.  This paper will describe results for Mo₂C-based catalysts.  While their bulk structures were not altered, these catalysts deactivated with time on stream.  The deactivation occurred in two stages.  During the first 10 h on stream, the reaction rate decreased rapidly, reaching a steady state rate that was ~35% of the initial rate. Based on X-ray photoelectron spectroscopy (XPS) and changes in selectivity, this initial deactivation was attributed to an increase in oxygen concentration on the Mo₂C surface. Recent theoretical studies suggest that increasing surface coverage of oxygen on Mo­_2­C alters the binding energies of carbon and hydrogen, which may help explain the decreased activity [1].  The second stage of deactivation was much more gradual and occurred over long time periods. Based on XPS and SEM results, this deactivation was attributed to the build-up of carbon, in the form of higher hydrocarbons (not coke), on the Mo­_­2­C surface. After treatment in 15% CH­₄/H­₂ at 590°C, the catalyst was able to regain all of its lost activity. CO uptakes for the fresh and regenerated catalysts were consistent with the activity trends as all of the CO site densities were regained.  These results suggest that surface oxygen strongly affects the activity and selectivity of the Mo₂C catalyst and that M­o₂C is moderately-resistant to coke formation.

[1] A.J. Medford, A. Vojvodic, F. Studt, F. Abild-Pedersen, J.K. Nørskov. J. Catalysis 290 (2012) 108–117.