`Selective Catalytic Reduction of Nitrogen Oxides by Ammonia over Fe~(3+)-Exchanged TiO_2-Pillared Clay Catalysts

摘要

Fe-exchanged TiO_2pillared clay (PILC) catalysts were prepared and used for selective catalytic reduction (SCR) of NO_x by ammonia.They were also characterized for surface area,pore size distribution,and XRD,H_2-TPR,and FT-IR methods.The Fe-TiO_2-PILC catalysts showed high activities in the reduction of NO_x by NH_3 in the presence of excess oxygen.SO_2 further increased the cata-lytic activities at above 350deg C,whereas H_2O decreased the activity slightly.The catalysts were about twice as active as commercial-type V_2O_5-WO_3/TiO_2 catalyst in the presence of H_2O and SO_2.Moreover,compared to the commercial catalyst,the Fe-TiO_2-PILC catalysts had higher N_2/N_2O product selectivities (e.g.,0-1% vs 9% N_2O at 400deg C) and substantially lower activities (by 74-88%) for SO_2 ox-idation to SO_3 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 N_2O selec-tivity for the Fe-TiO_2-PILC catalysts were attributed to their low activity in the oxidation of ammonia,as compared with vanadia catalysts.XRD patterns of Fe-TiO_2-PILC were similar to those of TiO_2-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-TiO_2-PILC catalysts with lower iron contents existed in the form of isolated Fe~(3+) ions.The activities of Fe-TiO_2-PILC catalysts were consisitent with their surface acidities,which were identified by FT-IR of the NH_3-adsorbed samples.The enhancement of activities by H_2O+SO_2 was attributed to the increase of surface acidity re-sulting from the formation of surface sulfate species of iron.