Iron catalyst supported on carbon nanotubes for Fischer-Tropsch synthesis: Effects of Mo promotion

Our studies on the application of carbon nanotubes (CNTs) as support have shown that iron catalysts supported on CNTs are active and selective catalysts for Fischer-Tropsch synthesis (FTS). However, these catalysts experienced deactivation as a result of active site agglomerations. In order to control the agglomeration of active site, which is an important step in developing a novel catalyst supported on carbon-based supports, the effects of Mo promotion on deactivation behavior of iron catalysts supported on CNTs were studied. In this work the properties and catalytic performance of unpromoted iron catalysts were compared with a promoted catalyst with different Mo contents (0.5, 1, 5, and 12 wt%). Based on TEM and XRD analyses, promotion of the catalysts with Mo resulted in production of smaller metal particles compared to the unpromoted iron catalyst. According to XRD analysis, Mo species were deposited in their amorphous structure. TPR analyses showed that addition of Mo increased reduction temperature significantly. Based on TEM and XRD analyses, the particle size of the iron oxides in the unpromoted catalyst increased from 16 to 25 nm under FT operating conditions, while the particle size of the iron oxide in the Mo promoted catalysts (∼12-14 nm) did not change noticeably under the same operating conditions. Activity, selectivity and stability of the unpromoted and Mo promoted catalysts showed that addition of 0.5-1 wt% Mo resulted in a more stable catalyst. Higher contents of Mo (5 and 12 wt%) decreased the activity of the catalysts due to catalytic site coverage and lower extent of reduction. Mo promotion (0.5-12 wt%) increased the selectivity of the catalysts toward lighter hydrocarbons. The promotion of the iron catalyst with 0.5 wt% of Mo stabilized the activity of the catalyst with minimal increase (2%) in methane selectivity.