Iron-based ammonia synthesis catalysts prepared via non-oxidic precursors

Abstract

Supported iron catalysts were prepared by adsorption of potassium hexacyanoferrate (II) complex onto acid-modified alumina surfaces. The impregnated preparations were reduced with hydrogen without a calcination step and tested in the ammonia synthesis reaction. At 583 K and a total pressure of 76 atm of a stoichiometric mixture (H 2-to-N 2 molar ratio = 3:1), the rate of formation of ammonia increased with increasing iron content of the catalyst, viz., decreasing iron dispersion (as measured by nitric oxide chemisorption). Several iron species were detected on reduced catalysts by Mössbauer and infrared spectroscopy. The Mössbauer spectra all indicated a slight contribution of a magnetic component and three overlapping doublets (two of Fe 2+ and one of Fe 3+). The infrared spectra of adsorbed nitric oxide revealed the presence of Fe 2+ and eventually Fe 0 sites. On Fe 0 centres, the nitric oxide is adsorbed as a mononitrosyl whereas Fe 2+ centers of two configurations lead either to a dinitrosyl (NO) 2 or a mononitrosyl. It was also shown by X-ray photoelectron spectroscopy that, during the air-drying step of the catalyst precursors, a fraction of the cyanide ligands are removed and iron is simultaneously oxidized to Fe 3+. This latter ion strongly interacts at the Al 3+ vacancies resulting from acid treatment of the carrier before the anchorage of the hexacyanoferrate (II) complex.