Passivation and oxidation of an ammonia iron catalyst

Abstract

A process of ammonia iron catalyst passivation and oxidation with oxygen and water vapour was investigated by thermogravimetry, Mössbauer spectroscopy and X-ray diffraction. The optimal passivation conditions (T ≤ 100 °C, pO2≤0.0013bar) have been proposed. The thickness (0.9–1.1 nm) and the structure (paramagnetic Fe2O3) of a protective, passive layer have been determined. It was found that oxidation of a reduced catalyst with pure oxygen (pO2=1bar) and water vapour (pH2O=0.023bar) did not lead to its passivation. The oxidation with pure oxygen, at temperatures above 250 °C, led to a creation of magnetite layer with built-in aluminum oxide. The thickness of the magnetite layer decreased and the amount of the built-in aluminum oxide grew along with the increase of oxidation temperature. An adsorption model has also been introduced for the interpretation of oxidation with water vapour: the whole iron nanocrystallites were oxidized successively from the smallest to the largest ones, and the oxidation process was limited by dissociative adsorption of water vapour on the iron surface.