Structure and reactivity of alumina-supported iron catalysts for ammonia synth

Alumina-supported iron catalysts, obtained either by impregnation of iron from a K/sub 4/(Fe(CN)/sub 6/) aqueous solution upon several acid-modified ..gamma..-Al/sub 2/O/sub 3/ samples or by the incipient wetness method, were characterized and their activities for ammonia synthesis at atmospheric pressure and 593 K were studied. Characterization was carried out by temperature-programmed reduction (TPR), kinetics of hydrogen reduction, CO chemisorption, X-ray photoelectron spectroscopy (XPS), IR spectroscopy, and Moessbauer spectroscopy, resulting in the degree of reduction and the dispersion of the metallic phase dependent on the previous acid modification of the ..gamma..-Al/sub 2/O/sub 3/. The XPS surface composition expressed as M 2p/Al 2p (M = Fe, K) gave K 2p/Al 2p > Fe 2p/Al 2p. IR experiments, with NO as probe molecules, exhibited bands for reduced catalysts at 1778 and 1712 cm/sup -1/ whose intensity and position depended on the protonation degree of ..gamma..-Al/sub 2/O/sub 3/ and promoter content, respectively. Moessbauer spectra of the reduced samples showed the presence of surface ..cap alpha..-Fe, superparamagnetic Fe/sup 0/, Fe/sup 2 +/, Fe/sup 3 +/, and ..cap alpha..-Fe/sub 2/O/sub 3/ depending on the catalyst. Finally, the catalytic activity for ammonia synthesis was found to be dependent on the surface structure of the catalyst and hencemore » on the method of preparation.« less