Mechanism of transformation from Fe_3—xAl_xO₄ to porous Fe by exposure to H₂ gas

Abstract

We address the long-standing puzzle that the presence of a few per cent of aluminium in magnetite results in an improved catalyst for the synthesis of ammonia in the Haber—Bosch process. Using optical microscopy and transmission electron microscopy, we have observed significant changes in the microstructure of the porous iron catalyst as a function of the aluminium content. We discuss these observations with reference to local strains at the interface between iron and the magnetite phase. Based on epitaxial considerations and diffusion mechanisms we present a model of the reduction front, which attempts to describe how the microstructure of the porous iron catalyst is influenced by the local concentration of aluminium.