BIFUNCTIONAL YFEO3-BASED CATALYSTS USED IN THE SELECTIVE CATALYTIC REDUCTION OF NITROGEN MONOXIDE IN THE PRESENCE OF EXCESS OXYGEN

The catalytic activity of YFeO3-based catalysts in the lean-NOx proces s was studied for the sulfated perovskite YFeO3, composites between YF eO3 and the solid acid ZrO2, as well as sulfated composites YFeO3/ZrO2 . Moreover, a mechanical mixture between YFeO3 and sulfated tetragonal zirconia (t-ZrO2/SO4) was studied. It is shown that the lean-NOx acti vity of YFeO3 is improved by forming the composites YFeO3/ZrO2 as well as by sulfating samples, with the highest conversion rates of NOx int o N-2 being about 30% at about 400 degrees C. The disadvantage of thes e materials is that the activity decreases with increasing the oxygen content of the feed gas. However, the mechanical mixture between YFeO3 and t-ZrO2/SO4 shows not only an improved high-temperature activity a t about 400 degrees C but also a pronounced low temperature (<300-350 degrees C) activity. The latter clearly increases with increasing oxyg en content, whereas the former decreases. The opposing effect of oxyge n is explained by different reaction mechanisms. (C) 1998 Elsevier Sci ence Ltd.