The Nottingham Pulse Rig Towards Determining The Mechanism Of The Fischer Tropsch Reaction
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This Chapter discusses the progress made using the Nottingham Pulse Rig towards determining the mechanism of the Fischer Tropsch reaction. A summary of the three main mechanisms proposed in the literature was outlined in Chapter 1 but is still the subject of much debate.1-6 Generally the carbide mechanism is accepted for Fe catalysts,7 but the mechanism over Co catalysts is less clear.1 Ge and Neurock,8 as well as Ojeda et al.,4 have suggested that the carbide mechanism is the pathway for FT synthesis on Co catalysts. However, they disagree over the route to CO dissociation; direct dissociation of the carbonyl bond has been proposed by Ge and Neurock, whilst Ojeda et al. have concluded that carbonyl dissociation proceeds via an H assisted…show more content… 3.1.1 On the Mechanism and Active Site of the FT Process Over Co Catalysts
Several studies have investigated the effect of syngas on the structure of metallic Co catalysts.9-12 For instance, Karaca et al. investigated the effect of syngas on a CoPt/Al2O3 catalyst using X-Ray Diffraction (XRD).9 After reducing the catalyst in H2 at 623 K, XRD peaks were observed for face centred cubic (fcc) and hexagonal close packed (hcp) structures of metallic Co. After exposure to 20 bar of syngas at 493 K, the fcc XRD peaks became narrower over 5 hrs, which was suggested to be due to an increase in the particle size due to sintering of the Co metal phase. Over the subsequent 5 hrs this peak decreased in intensity and a new peak assigned to a carbide phase grew in. Therefore, the authors concluded that deactivation of the catalyst was due initially to sintering and then, over longer periods of time, to carbidisation.
Wilson and de Groot have investigated the surface reorganisation of a model Co catalyst upon exposure to 4 bar of a H2/CO mixture (2:1 ratio) at 523 K.10-11 The surface was examined ex-situ in an Ultra High Vacuum (UHV) chamber before and after exposure to syngas using a Scanning Tunnelling Microscope (STM). Before exposure to CO the surface consisted of flat terraces, with a Co(0001) structure. The terrace surfaces were separated by cliff edges ranging in size from single atom high steps