Preparation of Fe₂O₃-Al₂O₃ Catalysts for Reduction of Nitrogen Oxide with Ammonia

Abstract

The preparation of a-Fe₂O₃-Al₂O₃ catalysts for the reduction of NO with NH₃ was studied with spherical γ-Al₂O₃ particles and iron(III) chloride, iron(III) nitrate or iron(III) sulfate solutions. The structure and the surface properties of the catalysts were related to their catalytic activity by means of X-ray powder diffraction, scanning electron microscope and ion microanalyzer.
A highly dispersed iron catalyst was made in an impregnation of the Al₂O₃ particles with a iron(III) chloride solution, while iron(III) nitrate and iron(III) sulfate solutions did not give highly dispersed iron catalysts. The FeCl₃-Al₂O₃ catalyst had a high activity for NO reduction because of iron ions highly dispersed (Fig. 3) and probably because of effects of ions (Fig. 2) upon the reaction rate of NO with NH₃. The FeCl₃-Al₂O₃ catalyst was, however, very volatile at the reaction temperature for practical use. In order to stabilize the iron ions on the carrier, we calcined the catalyst at high temperatures under various conditions. The activity of the calcined catalyst changed depending on its calcination conditions ; that is, the dispersion states of iron ions were modified by the calcination. A highly dispersed iron catalyst showed a high activity, and a less dispersed one a low activity (Table 2). In the case of a poorly dispersed iron catalyst, iron ions presented in a surface layer of the carrier forming large crystals (Fig. 5).
An attempt to find an optimum condition to calcinate the catalyst was made and some effective techniques were developed to get a highly dispersed iron catalyst. FeCl₃-Al₂O₃ catalyst should be dried completely before calcination. Then, it will be calcined in an oxygen rich atmosphere in order to prevent the crystallization of α-Fe₂O₃ particles in the catalyst.