Spherical carbons as model supports for Fe, Co and Fe-Co Fischer-Tropsch catalysts

Abstract

The production of liquid transportation fuels and chemicals by the Fischer-Tropsch (FT) synthesis continues to garner attention due to its economic and environmental benefits. This interest is also compounded by the flexibility to use readily available materials as feedstocks for synthesis gas production, with coal, natural gas, biomass and recently shale gas being used. Although this process is over 90 years old, challenges still remain. In this study, we have attempted to understand several FT synthesis challenges by exploring the use of carbon spheres as a model support for Co, Fe and Fe-Co FT catalysts. Thus the synthesis, characterization and application of carbon spheres with distinct architectures are described. The synthesis of solid carbon spheres using a sucrose precursor yielded materials that were mono-dispersed (600 nm) and adopted a necklace-like accreted conformation. Upon further investigation, it was demonstrated that annealing is useful for tuning the properties of the as-prepared materials to have high surface areas (> 500 m2/g), good thermal stability (>660 °C) and a mesoporous (> 2 nm) pore structure. Deposition of a Fe-Co bimetallic catalyst yielded oxides of the monometallic species with relatively small crystallites, with sizes in the range 7.9 – 14.4 nm. Reduction of the bimetallic samples was monitored by using in situ PXRD and TPR techniques, which revealed that a Co-Fe type-alloy is one of the phases formed on Co-rich samples at T > 450 °C. Interestingly, high relative abundances of this alloy did not correlate with high C5+ selectivities in Fischer-Tropsch synthesis; instead Co-rich/Fe-poor catalysts gave the best selectivity. The effect of the support morphology in heterogeneous catalysis was investigated by using high surface area solid and hollow carbon spheres (>560 m2/g) prepared from a resorcinol-formaldehyde precursor as support material. Loading the Co and Fe precursors on these two supports was shown by TEM and PXRD to result in smaller and well dispersed metal particles on the hollow support material. This corresponded with high activities and C5+ selectivities for the Co and Fe catalysts supported on the hollow carbon spheres. TEM studies revealed that the Co and Fe particles tended to sinter significantly when dispersed on a material with a solid architecture. iv Post-synthesis N-doping using a melamine precursor was shown by XPS to incorporate high quantities of nitrogen (up to 13%) on to the surface of the 30 nm thick shells of the hollow carbon spheres. On further investigation, N-doping by this method was shown to have minimal effects on the thermal stability and crystallinity of the materials. The N-doped HCSs were shown to be good anchors of Co particles as displayed by the good dispersion, activity and minimal sintering tendency of catalysts supported on N-doped HCSs. Studies conducted herein have demonstrated the versatility of carbon spheres as a model support, and how their properties can be tailored to suit the desired specifications by simply adjusting the synthesis parameters. We have also highlighted how the chemical inertness of these materials allows for studies on metal-metal interactions at elevated temperatures for bimetallic catalyst systems. The monodisperse, morphology-tunable aspects of carbon spheres were particularly useful in modelling the effect of the support morphology in Fischer-Tropsch synthesis. It is believed that the versatility of CSs demonstrated in this study can also be exploited in other heterogeneous catalytic systems.