Study of the activation process and catalytic behaviour of a supported iron ammonia synthesis catalyst

Abstract

A 3 wt% iron-supported catalyst has been prepared by free adsorption of K 4[Fe(CN) 6] precursor on acid-modified γ-alumina. The catalyst activation was studied by in situ X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and mass-spectrometry techniques. Thermal treatments under vacuum of H 2 : He flow produce the removal of cyanide ligands mainly as HCN and the formation of well dispersed Fe 2+ species which easily interact with the support. However, an activation under H 2:N 23:1 flow up to 673 K produces the elimination of cyanide ligands and formation of ammonia; small, well dispersed iron particles are obtained simultaneously. This activation process raises the initial K/Fe atomic surface ratio from 4.37 to 6.1. Chemisorption of dinotrogen species on potassium-rich small iron particles is related with an infrared absorption found at 2043 cm -1, which appears under H 2:N 23:1 activation conditions above 573 K. Catalytic behaviour in the ammonia-synthesis reaction has been studied in a flow reactor under pressure. A catalytic activity at 30 bar and 723 K as 405 mmol NH 3 per iron gram per hour has been determined.