Pt/Ba/Sn_xCe_1-xO₂ Catalysts For High-Temperature Lean NO_x Removal With High H₂O And CO₂ Tolerance

NO_x storage reduction（NSR）technology is recognized as an effective NO_xelimination technology under lean-burn conditions.Researches on the performance of NSR catalysts usually concentrate in the temperature range from 250 to 400℃.Nevertheless,the wide application of gasoline direct injection engine enables the exhaust temperature above 400℃.Previous researches on high-temperature（HT）NSR catalysts were carried out in the absence of H₂O or CO₂,which are inevitable in realistic condition,leading to catalysts deactivation.Therefore,it is of great significance to develop the catalysts with high tolerance to H₂O and CO₂ at high temperatures.A series of Pt/Ba/Sn_xCe_1-xO₂ catalysts were prepared by coprecipitation impregnation method.The NO_x removal percentage（NRP）of Pt/Ba/Sn_xCe_1-xO₂ were tested in the absence of H₂O and CO₂ at the temperature range from 250 to 550℃,and compared with Pt/Ba/Al₂O₃.The NO_x removal percentage（NRP）of all the catalysts varies in a volcanic curve with the increase of temperature,while the temperature of the optimal De-NO_x activity is different.The De-NO_x activity of the catalysts at high temperatures is directly linked to the NO_x storage capacity.The NO_xstorage capacity is related to the amount of highly dispersed Ba CO₃ species on the catalysts.Pt/Ba/Sn_0.8Ce_0.2O₂ has the most highly dispersed Ba CO₃,promoting the NO_x storage capacity,thus improving its De-NO_x activity.NO_x mainly exists in two forms on the catalysts,stable monodentate nitrate species and unstable ionic nitrate species.On Pt/Ba/Sn_0.8Ce_0.2O₂,NO_xmainly exists in the form of the stable monodentate nitrate.The effect of H₂O and CO₂ on the NO_x removal performance of the catalysts was investigated.Both H₂O and CO₂ have negative effects on the De-NO_x activity of the catalysts.And the negative effect of CO₂ is stronger than that of H₂O.In the presence of H₂O or/and CO₂,the NRP of Sn-rich catalysts is higher than that of Pt/Ba/Al₂O₃and Ce-rich catalysts.Pt/Ba/Sn_0.8Ce_0.2O₂ shows the best De-NO_x activity with NRP over 86%even in the presence of H₂O and CO₂ at 500℃.H₂O and CO₂ mainly compete with free ionic nitrate with low thermal stability for adsorption,inhibiting the storage of NO_x on the catalysts,and thus weaken the NO_x removal performance of the catalysts.NO_xmainly exists in the form of stable monodentate nitrate on Pt/Ba/Sn_0.8Ce_0.2O₂,as a result,H₂O and CO₂ have less effect on it.