Ammonia synthesis over a supported iron catalyst prepared from an amorphous iron-zirconium precursor: III. Mechanism of nitrogen adsorption and ammonia synthesis kinetics

Abstract

Photoemission studies (UPS and XPS) of nitrogen adsorption on amorphous Fe 91Zr 9 at 79 K indicate the presence of both molecular and atomic nitrogen on the surface. Atomic nitrogen is found to be the reactive intermediate in ammonia synthesis catalyzed by this material. Upon addition of small amounts of hydrogen to the surface covered with atomic nitrogen, this species is desorbed, presumably as NH 3, leaving nondissociated dinitrogen behind. The results of the UPS and XPS studies, including binding energies and line profiles, agree well with the results of similar investigations carried out on single crystal surfaces of iron. This supports the view that the active component of the amorphous iron-zirconium alloy is elemental iron and that the local electronic structure of the adsorption sites in elemental iron and in the amorphous alloy surface are similar. The question as to whether an amorphous surface exhibits different adsorption characteristics than a crystalline surface is discussed in the light of the nitrogen adsorption results. The findings of the photoemission studies are supported by a kinetic study of ammonia synthesis over the catalyst prepared from the amorphous alloy. Kinetic experiments were carried out in a continuous fixed-bed tubular reactor in the range 623–723 K and at 4 bar total pressure. The experimental results could be well described with a kinetic model based on the assumption that ∗N is the most abundant reaction intermediate. The activation energy of nitrogen adsorption derived from the kinetic data amounts to 85 kJ/mol. This value agrees well with activation energies reported in the literature for ammonia synthesis over unpromoted iron and supports the conclusion that elemental iron is the active component of the catalyst prepared by in situ activation of amorphous Fe 91Zr 9.