Carbon monoxide hydrogenation over iron oxide, subjected to shear deformation under high pressure: role of vacancies

Abstract

The formation of catalytically active structures m both “unloaded” polycrystalline α-Fe₂O₃ and “loaded”, i.e. subjected to shear deformation under high pressure (SD+HP), was studied by “in situ” X-ray diffraction in an atmosphere of carbon monoxide and hydrogen, by transmission electron microscopy and by Mössbauer spectroscopy. Two main stages were observed in the activation process which correspond to different degrees of iron oxide reduction. The first stage (270–290°C ) was the reduction to nonstoichiometric magnetite, accompanied by carbon monoxide hydrogenation to saturated hydrocarbons. The second stage ( > 300 °C ) was the deep reduction to nonstoichiometric magnetite and clusters of iron carbide, accompanied by the appearance of unsaturated hydrocarbons. The overall catalytic activity was shown to be a function of both total length of spinel-carbide phase boundaries and the topology of the interface. In the case of the “unloaded” sample the formation of intercalated particles of carbide phase with alternating spinel-carbide layers led to a higher activity than that of the “loaded” one. Loose aggregates of carbide particles formed on the “loaded” sample block up active centres on the spinel surface and reduce the total activity. The nature of active centres on the surface of nonstoichiometric magnetite is discussed.