Selective catalytic reduction of nitrogen oxides by ammonia over Fe³⁺-exchanged TiO₂-pillared clay catalysts

Fe-exchanged TiO₂-pillared clay (PILC) catalysts were prepared and used for selective catalytic reduction (SCR) of NO_xby 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 NO_x 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.