Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

Abstract In this study, new Fe 2 O 3 based materials are developed for the selective catalytic reduction (SCR) of NO x by NH 3 in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO 2 is considered to be the most effective carrier for Fe 2 O 3 . The modification of the Fe 2 O 3 /ZrO 2 system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO 2 that is coated with 1.4 mol% Fe 2 O 3 and 7.0 mol% WO 3 (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO x is obtained between 285 and 430 °C with selective formation of N 2 . Here, the turnover frequency of NO x per Fe atom is found to be 35 × 10 −5  s −1 that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H 2 O and CO 2 , whereas it is increased by NO 2 . Temperature programmed reduction by H 2 (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe 2 O 3 , whereby relatively small oxide entities are also present. The strongly aggregated Fe 2 O 3 species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH 3 that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N 2 and H 2 O. In the final step, the reduced Fe sites be regenerated by oxidation with O 2 . As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO 2 or O 2 into NO.