Adsorption sites of an iron-aluminum catalyst for ammonia oxidation as studied by the IR spectroscopy of the adsorbed NO probe molecule

Abstract

The state of surface adsorption sites in the IK-42-1 oxide catalyst for ammonia oxidation depending on catalyst preparation conditions (the nature of raw materials and the temperature of calcination) was studied in this work with the use of the diffuse reflectance IR spectroscopy of the adsorbed NO probe molecule. Hematite, which was prepared by a sulfate or chloride technology, was used as the starting raw material; Al2O3 binding agents were prepared by the reprecipitation or hydration of thermally activated gibbsite; and acetic or nitric acid was used as an electrolyte. The samples were calcined at 900–1000°C. It was found that mono- and dinitrosyl complexes with reduced coordinatively unsaturated Fe2+ cations and nitrite-nitrate complexes were formed upon the adsorption of NO on the catalyst surface (regardless of the catalyst preparation conditions). The samples differed in the amount and degree of coordinative unsaturation of adsorption sites depending on the preparation conditions. It was concluded that the most coordinatively unsaturated Fe2+ adsorption sites observed were formed on the surface of a solid solution of iron cations in aluminum oxide, which was formed in the course of catalyst preparation. It was found that an increase in the catalyst calcination temperature resulted in a decrease in the number of coordinatively unsaturated adsorption sites, which correlated with the observed decrease in the yield of NO. This correlation had the shape of a saturation curve, which can reflect the occurrence of a reaction in the diffusion mode at high degrees of conversion for the majority of catalysts.