Selective Catalytic Reduction of Nitrogen Oxides by Ammonia over Fe3+-Exchanged TiO2-Pillared Clay Catalysts

Abstract

Fe-exchanged TiO2-pillared clay (PILC) catalysts were prepared and used for selective catalytic reduction (SCR) of NOx by ammonia. They were also characterized for surface area, pore size distribution, and by XRD, H2-TPR, and FT-IR methods. The Fe–TiO2–PILC catalysts showed high activities in the reduction of NOx by NH3 in the presence of excess oxygen. SO2 further increased the catalytic activities at above 350°C, whereas H2O decreased the activity slightly. The catalysts were about twice as active as commercial-type V2O5–WO3/TiO2 catalyst in the presence of H2O and SO2. Moreover, compared to the commercial catalyst, the Fe–TiO2–PILC catalysts had higher N2/N2O product selectivities (e.g., 0–1% vs 9% N2O at 400°C) and substantially lower activities (by 74–88%) for SO2 oxidation to SO3 under the same reaction conditions. The activity was further increased to over three times that of the vanadia-based catalyst when Ce was added. The high activity and low N2O selectivity for the Fe–TiO2–PILC catalysts were attributed to their low activity in the oxidation of ammonia, as compared with vanadia catalysts. XRD patterns of Fe–TiO2–PILC were similar to those of TiO2–PILC, showing no peaks due to iron oxide, even when the iron content reached 20.1%. The TPR results indicated that iron in the Fe–TiO2–PILC catalysts with lower iron contents existed in the form of isolated Fe3+ ions. The activities of Fe–TiO2–PILC catalysts were consistent with their surface acidities, which were identified by FT-IR of the NH3-adsorbed samples. The enhancement of activities by H2O+SO2 was attributed to the increase of surface acidity resulting from the formation of surface sulfate species of iron.