On the distribution of aluminium and magnesium oxides in wustite catalysts for ammonia synthesis

The distribution of aluminium and magnesium oxides in wusite catalysts for ammonia synthesis was studied using the selective etching method, energy dispersive spectroscopy (EDS) and nitrogen chemisorption. Aluminium oxide was found to occur in three forms in the wustite catalyst's precursor: one in the solid solution with wustite, the other two in the spaces between wustite grains, being distinguished by solubility in hydrochloric acid. The rising content of CaO in the precursor causes an increase of the HCl-soluble form of Al2O3 on behalf of the non-HCl-soluble form. The amount of Al2O3 dissolved in the wustite phase was the same in all investigated wustite samples, and amounted to 0.3 wt.%. The magnesium oxide is well dispersed in the wustite phase, and it has no influence on the forms of occurrence of Al2O3. Considering the surface area of the reduced catalyst, the exclusive promotion with Al2O3 is twice as effective as exclusive promotion with MgO, while catalysts promoted with both oxides possess much larger surface areas, an indication of a synergistic effect. (C) 2003 Elsevier Science B.V. All rights reserved.