Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with “in situ” controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H2, CO, NO and C2H4 are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some “key studies” and trying to evidence similarities and differences among surfaces and probes (572 references).

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Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

Cobalt particle size effects in Fischer–Tropsch synthesis: structural and in situ spectroscopic characterisation on reverse micelle-synthesised Co/ITQ-2 model catalysts

In order to generate sulfate radicals (SRs) as oxidizing species for the degradation of 2,4-dichlorophenol (2,4-DCP) in water, we explored heterogeneous activation of peroxymonosulfate (PMS) by supported cobalt catalysts. More attention was given to the effect of support materials (Al2O3, SiO2, TiO2) and cobalt precursors (Co(NO3)2, CoCl2, CoSO4) on cobalt–support interaction, cobalt leaching, and reactivity of the catalysts. Especially, the feasibility of simultaneous generation of SRs and hydroxyl radicals (HRs) in PMS-Co/TiO2 systems was first studied under ultraviolet (UV) radiation. Much lower cobalt leaching was observed in Co/Al2O3 and Co/TiO2 systems than that of Co/SiO2 most probably due to their relatively strong cobalt-support interaction. Co/TiO2 catalyst prepared with Co(NO3)2, compared to CoCl2 or CoSO4 (where Cl− and SO42−, respectively, were not completely removed upon heat treatment at 500 °C), showed strong cobalt–support interaction, and thereby exhibited negligible cobalt leaching. Under UV radiation, Co/TiO2 at Co/Ti molar ratio of 0.001 showed significant improvement in the degradation of 2,4-DCP due to HRs. The effective generation of HRs in the system can be explained with Co(III)-mediated charge transfer from the photoinduced electrons to PMS, inducing facilitation of photoinduced electron-hole separation. However, high cobalt loading (i.e., Co/Ti molar ratio of 0.1) on TiO2 surface exhibited negligible enhancement of 2,4-DCP transformation under UV radiation since the penetration of UV light to TiO2 was prohibited by the cobalt.

Heterogeneous activation of peroxymonosulfate by supported cobalt catalysts for the degradation of 2,4-dichlorophenol in water: The effect of support, cobalt precursor, and UV radiation

Fischer–Tropsch synthesis (FTS) process aims at converting synthesis gas to liquid fuels. Due to high activity and long catalyst life, cobalt-based catalyst is currently the catalyst of choice for gas to liquid (GTL) technology. Water is most undesirable byproduct of FTS process. Due to low water–gas-shift (WGS) activity of cobalt-based catalyst, the water concentration rises with time-on-stream (TOS) in FTS. This paper reviews the effects of water on the performances of various cobalt catalysts for FTS. The effects of water on FTS is quite complex and depends on the support and its nature, Co metal loading, its promotion with noble metals, and preparation procedure. Added water up to certain concentrations has positive effects (in terms of higher CO conversion, C 5+ selectivity, olefin selectivity and lower methane and CO 2 selectivity) on unsupported cobalt oxide catalysts. If the effects of support are taken into account, water has positive effect for silica-supported catalysts. The effects are negative for alumina where as for titania support, water has little positive effect. However in general, oxidation of cobalt active site depending on the cluster size and water partial pressure, the removal of transport restrictions via the formation of water-rich intra-pellet liquids, and kinetic effects have been considered as the main responsible factors. The effects are strongly influenced by the cobalt cluster size as well as on pore size of the support. Addition of noble metals at low cobalt loading increases the dispersion of cobalt on the support and hence improves its activity. Higher cobalt dispersion enhances the negative impact of water especially at higher water partial pressures under FTS conditions.

Rafene A. Makhothe

Papers

TPR Study on the Preparation of Impregnated Co/SiO2Catalysts