Study On Ceria Based NH₃-SCO Catalyst For Diesel Exhaust Gases

With the sustained and rapid economic development,the per capita ownership of motor vehicles is increasing,and the problem of air pollution caused by mobile sources is becoming more and more serious.Currently,NH₃-SCR technology is mostly used to reduce the NO_x emitted by diesel vehicle exhaust.Excessive urea was injected to achieve high NO conversion rate,causing NH₃ slip in practical application.At present,ammonia selective catalytic oxidation technology（NH₃-SCO）is the most effective technology to achieve NH₃ removal.It lies in the development of NH₃-SCO catalysts with good low-temperature activity,high N₂ selectivity and excellent stability at high temperature.Transition metal oxides catalysts have attracted extensive attention due to their low price and easy availability.CeO₂ shows advantages in catalytic oxidation due to its unique physical,chemical properties and excellent oxygen storage and release capacity.However,its low-temperature activity and high-temperature N₂ selectivity need to be further improved.Therefore,based on CeO₂,this thesis focuses on the design of noble metal/CeO₂ supported catalysts to achieve high activity at low temperatures.Transition metal is used to modify Ce based catalyst to improve N₂ selectivity,and noble metal and modified Ce based bimetallic catalyst are combined to synchronously enhance catalytic performance.The relationship between catalyst structure and NH₃-SCO performance has been deeply explored and the NH₃-SCO mechanisms of these catalysts have been studied with In situ DRIFTS and DFT.The major work and conclusions are as follows:（1）In view of the high activity temperature window of CeO₂ catalyst,the catalytic performance of noble metal/CeO₂ supported catalysts with different preparation methods,noble metal types and loading amounts had been evaluated.It was found that Ag/CeO₂-IM catalyst prepared by incipient-wetness impregnation method showed better low-temperature NH₃ catalytic oxidation efficiency with T₉₀ at 200°C.The main reason was that Ag/CeO₂-IM catalyst possessed stronger redox performance and lower NH₃ catalytic oxidation temperature.Its high-temperature N₂ selectivity still needs to be improved（just 40%）,which may be related to the poor surface acidity of the catalysts at high temperatures.The NH₃-SCO reaction on the surface of these two catalysts followed the i-SCR（inter-SCR）mechanism.（2）In order to effectively solve the problem of poor N₂ selectivity of CeO₂ catalyst at high temperatures,CeSn bimetallic oxides catalysts were prepared by co-precipitation method.It was found that the high-temperature N₂ selectivity of all CeSn catalysts were more than 80%.The catalytic activity of CeSn catalysts with different molar ratio were different and Ce4Sn6 catalyst could achieve 90%NH₃ conversion at300°C.It was found that solid solution structure was formed in CeSn catalysts,and the increase of Ce³⁺content,surface adsorbed oxygen,surface acid sites and more active lattice oxygen were benefited to improve NH₃-SCO activity.NH₃-SCO reaction on the surface of Ce4sn6 catalyst followed the i-SCR mechanism.DFT calculation results showed that Sn doping not only enhanced the adsorption and transformation of NH₃ on Ce,but also Sn-O-NH₄⁺species formed at Sn-OH could further reduce NO₂ to N₂.The synergistic effect between Ce and Sn significantly was conducive to the formation of N₂.（3）In order to achieve high catalytic oxidation efficiency at low temperatures and good N₂ selectivity at high temperatures at the same time,combined with research results of the first two parts,Ag/CeSn catalysts with different Ag content were prepared by incipient-wetness impregnation method.It was found that Ag loading improved the low-temperature NH₃ oxidation capacity of Ce4Sn6 support.NH₃ conversion could reach 90%on 1.75Ag/CeSn catalyst at 200°C,T₉₀ could be reduced by 100°C,and the N₂ selectivity at high temperatures could still be maintained at more than 70%.After 5cycles and hydrothermal aging at 700°C,1.75Ag/CeSn catalyst still showed stable catalytic performance.The NH₃-SCO reaction on the surface of 1.75Ag/CeSn catalyst follows the i-SCR mechanism.The metal Ag species uniformly dispersed on the surface promoted the adsorption and dissociation of O₂.The precipitated Sn O₂ on the surface of catalyst enhanced the adsorption and storage capacity of NH₃.The formed Sn-O-NH₄⁺species could further react with the adsorbed oxygen atoms on metal Ag to form N₂.Compared with Ag/CeO₂ catalyst,the surface acidity of Ag/CeSn catalyst was significantly improved by Ce4Sn6 support due to the relatively enhancement of NH₃adsorption capacity,further improved its N₂ selectivity at high temperatures.The multi synergistic effect of Ag and support,Ag and Sn O₂ as well as Ce and Sn enhanced the adsorption and dissociation of NH₃ and O₂,further realizing the catalytic oxidation of NH₃ at low temperatures.