Study On Selective Catalytic Reduction Of NO With NH₃ Over Ce-Fe/Cu-SSZ-13 Catalysts And The Reaction Mechanism

With legislation of NO_x emissions becoming more and more stringent,it is urgent to find an effective way to remove such pollutants from the diesel exhaust.Nowadays,one of the most promising technologies to eliminate the NO_x pollution is selective catalytic reduction（SCR）of NO_x with ammonia.Recently,Cu/zeolite catalysts with chabazite（CHA）crystal structure,such as Cu-SSZ-13,has received much attention.A series of CHA-type trimetallic composite zeolites of Ce_x-Fe_y/Cu-SSZ-13 catalysts were prepared by the one-pot synthesized Cu-SSZ-13 and subsequent ion-exchange with Fe³⁺ and Ce³⁺.The catalysts were characterized including XRD,SEM and BET.Their catalytic performances,sulfur-water tolerance,anti carbon deposition,resistance against high space velocity were investigated completely.In addition,In situ DRIFTS were used to investigate the reaction pathway on Ce_0.017-Fe_0.017/Cu-SSZ-13,Cu-SSZ-13^a（one-pot synthesis）)and Cu-SSZ-13^b（Cu ion-exchanged SSZ-13）.The results of XRD revealed that the crystal structure of zeolite Ce-Fe/Cu-SSZ-13 is same as Cu-SSZ-13^a and Cu-SSZ-13^b.Among the prepared Ce_x-Fe_y/Cu-SSZ-13 catalysts,Ce_0.017-Fe _0.017/Cu-SSZ-13 displayed the best SCR performance.The NO conversion was more than 90% between 200 and 500 oC.N₂ selectivity was above 98% within the wider temperature range of 150-550 oC.In addition,the catalyst demonstrated good sulfur-water tolerance,good anti-coke properties and effective resistance against high space velocity.The phenomena suggest that synergistic effects of Cu,Fe and Ce species improve the SCR performances and make Ce-Fe/Cu-SSZ-13 catalyst as a promising candidate for NH₃-SCR technology.From the DRIFT study,it was evident that the path way of the SCR reaction on Ce_0.017-Fe_0.017/Cu-SSZ-13 and Cu-SSZ-13^a is prosed that the ammonium nitrates from the reaction of surface ammonia and nitrates are further reduced by NO to form N₂ and H₂O.However,during the reaction on Cu-SSZ-13^b,NO₂ was involved as important intermediates in the SCR reaction besides the ammonium nitrates（similar to the reaction path way on Cu-SSZ-13^a）.