Fischer–Tropsch synthesis: Iron catalysts supported on N-doped carbon spheres prepared by chemical vapor deposition and hydrothermal approaches

Conventional carbon supports are generally treated in acid or base solutions to produce functionalized groups on the carbon surface in order to anchor active metal or metal oxide species. The use of nitrogen-doped carbon materials as supports can avoid this environmentally unfriendly step because the N atoms in the carbon materials can act as the anchoring sites. In this study, we have synthesized three different nitrogen-doped carbon sphere (NCS) supports by two different strategies: chemical vapor deposition (CVD), in either a horizontal or a vertical furnace, or using a hydrothermal approach. The three different NCSs were found to have different physical and chemical properties as revealed by TGA, XPS, Raman spectroscopy, and electron spectroscopy studies. NCSs prepared by CVD in a vertical furnace (NCSver) gave the smallest diameter and highest surface area materials. The NCSver material presented the most defected sites and the highest N content. The NCSs prepared by the hydrothermal method (NCShyd) gave the largest diameter, the lowest surface area and the least amount of defect sites and the materials with the lowest N content. The NCShor prepared by CVD in the horizontal furnace exhibited defect sites and an N content between the amounts measured for the NCSver and NCShyd materials. The surface N types, determined by XPS, varied with the synthesis strategy used. Iron species were successfully deposited on the three NCSs without a functionalization step. The Fischer–Tropsch synthesis (FTS) performances of the three Fe/NCS catalysts were found to correlate with the Fe particle size which was influenced by the different N contents, the N type, and the defect sites. It is suggested that pyrollic and pyridinic N atoms play a key role in binding the Fe atoms and that quaternary N atoms play a minor role. The Fe/NCSver contained well-dispersed Fe oxide particles on CSs that had the highest N content (made of pyrollic/pyridinic N atoms) and this led to the highest FT activity. The Fe/NCShor catalyst showed the lowest FT activity due to the presence of the largest Fe oxide particle sizes (50% quaternary N atoms).