An inelastic neutron scattering investigation of the temporal behaviour of the hydrocarbonaceous overlayer of a prototype Fischer-Tropsch to olefins catalyst

Abstract

A dual sodium and sulfur promoted haematite, representative of a candidate Fischer-Tropsch to olefins (FTO) catalyst, is prepared and contrasted with the performance of an unpromoted hematite sample in the ambient pressure CO hydrogenation reaction at 623 K as a function of time-on-stream (0–24 h). In-situ post-reaction temperature-programmed oxidation measurements show the carbon evolutionary phase of the catalyst conditioning process to be retarded for the FTO catalyst. Ex-situ inelastic neutron scattering measurements show the promoters perturb the formation of a previously described hydrocarbonaceous overlayer. Specifically, whilst the sp3 hybridised C–H modes of the hydrocarbonaceous overlayer are almost unaffected by the additives, the formation of the overlayer’s sp2 hybridised C–H modes are noticeably impeded. The results are discussed in terms of the Na/S promoters disturbing the formation of an ordered hydrocarbonaceous overlayer that is thought to constrain the supply of adsorbed hydrogen atoms, which favours the formation of unsaturated hydrocarbons associated with the FTO process.