Activity and reduction behavior of fused iron catalysts containing cobalt for ammonia synthesis: a structure study

Abstract

Mössbauer effect spectroscopy (MES) studies of cobalt-containing fused iron catalysts show that different cobalt distributions occur in irregularly shaped catalysts (formed by natural cooling of the fused mass in air) and in ball-shape catalysts (formed in water by sudden cooling). The Co 2+ ions replace Fe 2+ ions in the B sites of the inverse spinel structure of Fe 2O 4 for the former catalyst, while a relatively even distribution over A and B sites occurs for the latter. The above results are supported by X-ray diffraction (XRD). For the irregular catalysts, the activity and specific surface area increase to some extent with increasing cobalt content and decreasing atomic iron ratios (i.e. Fe 2+/Fe 3+ ). For the ball-shape catalyst, in spite of the somewhat smaller specific surface area, a higher specific activity results in an overall activity that is incomparable to that of the irregular catalyst; the pore surface distribution is shifted to larger pores. The reaction rate constant decreases with increasing total pressure. The ball-shaped catalyst follows the relation k T.P∼exp(jp) , while the irregular catalyst is fits the Nielsen equation, k T.P∼p i , where i and j are small negative numbers. The intrinsic reduction kinetics of the ball-shape catalyst are best in line with the contracting sphere model, and reduction of industrial size catalysts also shows that cobalt promotes the reduction of the catalyst. On the other hand, the uneven distribution of Co 2+ ions in the irregular catalyst causes its intrinsic reduction kinetics to deviate from the contracting sphere model. We consider that the easier reduction of the cobalt containing catalyst may be caused by the much easier reduction of the CoFe 2O 4 present.