Iron-zirconium oxide catalysts for the hydrogenation of carbon monoxide: In situ studies by iron-57 Mössbauer spectroscopy

Abstract

Some unsupported iron-zirconium oxide catalysts have been prepared by the calcination in air of precipitates containing 15 mole % iron. The catalyst formed at 500°C was shown by powder X-ray diffraction to consist of a non-equilibriated solid solution of iron(III) in a tetragonal or cubic zirconium dioxide structure whereas the catalyst formed at 1000°C was found to contain a zirconium-doped α-iron(III) oxide, or a magnetically ordered iron-zirconium oxide, in combination with an iron-containing monoclinic polymorph of zirconium dioxide. The⁵⁷Fe Mössbauer spectra recorded in situ following the pretreatment of the solids in nitrogen, carbon monoxide and hydrogen showed that little change is induced in the catalysts under such conditions. The⁵⁷Fe Mössbauer spectra also showed that the pretreated catalysts were unchanged by exposure to a 1:1 mixture of carbon monoxide and hydrogen at 270°C and 1 atmosphere pressure but were partially converted to iron carbide when used for the hydrogenation of carbon monoxide at 330°C and at 20 atmospheres pressure. The hydrocarbon product distribution showed Schulz-Flory α-values of 0.73 to 0.76 which were higher than the α-values obtained from pure iron catalysts which had been prepared and pretreated in a similar fashion. The⁵⁷Fe Mössbauer spectra and the results of the catalytic evaluation may be associated with an interaction between zirconium(IV) and the electron-rich atoms of the reactant carbon monoxide.