Effect of Fischer–Tropsch synthesis on the microstructure of Fe–Co-based metal/spinel composite materials Original Research Article

Fe–Co-based metal/oxide composite materials (Co0.730Fe0.270)0.82 [Co0.35Fe2.65O4], (Co0.950Fe0.050)0.62 [Co0.31Fe2.69O4] and (Co0.900Fe0.100) [Co0.10Fe2.90O4] were prepared to be used in Fischer–Tropsch (F–T) synthesis in view of producing light alkenes. The metal is an iron–cobalt alloy of bcc structure in the former and of fcc structure in the two latter and the oxide, a cobalt containing magnetite. The metal, obtained directly by preparation method, is embedded in the oxide phase. Then, catalysts were not subjected to a reducing pre-treatment. Although, catalysts were not reduced, the two former materials are efficient as catalysts in F–T reaction to produce C2–C4 (50 wt.%) with high alkene:alkane ratio and a low selectivity of CO2 (25% molar at CO conversion of 5%); the third catalyst presents a typical distribution of hydrocarbons obtained with cobalt catalyst in F–T reaction. The possible modifications of the catalysts during the catalytic reaction are studied by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). High resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) show the surface composition under test is identical for the first two catalysts, although their initial bulk composition is different. The characterisation results, obtained by combined methods (XRD, electron-microscopy, surface analysis) provide explanations to individual catalytic behaviour of the three catalysts studied.