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

Abstract

Alumina-supported iron catalysts, obtained either by impregnation of iron from a K 4[Fe(CN) 6] aqueous solution upon several acid-modified γ-Al 2O 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 Mössbauer spectroscopy, resulting in the degree of reduction and the dispersion of the metallic phase dependent on the previous acid modification of the γ-Al 2O 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 molecule, exhibited bands for reduced catalysts at 1778 and 1712 cm −1 whose intensity and position depended on the protonation degree of γ-Al 2O 3 and promoter content, respectively. Mössbauer spectra of the reduced samples showed the presence of surface α-Fe, superparamagnetic Fe 0, Fe 2+, Fe 3+, and α-Fe 2O 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 hence on the method of preparation.