Preparation And Mechanism Of Mn-based Oxide Catalysts For Low-temperature No_x Reduction

Nitrogen oxides emitted by diesel vehicle exhaust have become one of the main sources of air pollution in our country.Currently,ammonia selective catalytic reduction（NH₃-SCR）is the most widely used nitrogen oxide purification（denitrification）technology.Cu-CHA molecular sieves are widely used as commercial catalysts for denitration of diesel vehicle exhaust,but their low-temperature activity needs to be further improved.However,V₂O₅/WO₃-TiO₂ commercial catalysts have problems such as poor low-temperature activity and easy volatilization of toxic vanadium.Therefore,the development of low-temperature denitration catalysts for green metal oxides is imminent.In this paper,a series of Mn-based oxide denitrification catalysts with excellent low-temperature activity and hydrothermal stability are designed and prepared.Through a series of experimental characterizations,the reaction mechanism of NH₃-SCR denitration,hydrothermal stability and N₂ selectivity improvement mechanism are systematically studied.The specific research contents are as follows:（1）The alumina supported manganese oxide catalyst modified by triethanolamine is prepared by the simple impregnation method.The results show that even after hydrothermal aging of 650 ℃ in an atmosphere of 10 vol.%H₂O for 24 h,the developed catalysts still show more than 90%NO_x conversion in the range of 200-360 ℃.Through UV-vis,XPS,in situ UV-vis and other analytical characterizations,the modification of triethanolamine can engineer highly dispersed low-coordination Mn species with excellent hydrothermal stability,and these low-coordination Mn species can promote the production of more acidic sites and redox sites,thereby significantly accelerating the adsorption and activation of NO_x and NH₃ species in the NH₃-SCR reaction,and these low-coordination Mn species remain stable even after hydrothermal aging treatment.This work provides a new exploration for the application of Mn-based oxide catalysts in the low-temperature denitration of diesel vehicle exhaust.（2）Although alumina supported manganese oxide catalyst modified by triethanolamine have excellent hydrothermal stability,their low-temperature performance needs to be further improved.Here,the cerium-modified alumina-supported manganese oxide catalyst is prepared by simply impregnating a cerium-modified alumina carrier with manganese oxide,and its low-temperature NH₃-SCR denitration performance is investigated.The results show that even after hydrothermal aging of 650 ℃ in an atmosphere of 10 vol.%H₂O for 50 h,the developed catalyst can still achieve 80%NO_x conversion around 180 ℃ under the evaluation condition of GHSV of 200000 ml·g^-1·h^-1.Through UV-vis,XPS,in situ DRIFTS and other analyses and characterization,it is found that the cerium oxide in the carrier can generate electron transfer between manganese oxide and manganese oxide,which can further promote the production of more highly dispersed low-coordination manganese species.It can promote the adsorption of NO_x on the catalyst surface and convert it into nitrite species that are more likely to participate in the reaction.Even after severe hydrothermal aging,this electron transfer effect can still exist stably.This work lays a scientific foundation for the application of Mn-based oxide catalysts in the low-temperature denitration of diesel vehicle exhaust.（3）The cerium-modified alumina-supported manganese oxide catalyst has excellent hydrothermal stability and low-temperature NO_x conversion at higher space velocity,but its N₂ selectivity still has room for further improvement.Therefore,in this work,through compounding NH₄⁺-ZSM-5 molecular sieve in the cerium-modified alumina carrier,and then simply impregnating manganese oxide,the molecular sieve and ceria-modified alumina composite carrier-supported manganese oxide catalyst are prepared.The results show that even after hydrothermal aging of 650 ^℃ in an atmosphere of 10 vol.%H₂O for 50 h,the developed catalyst can still achieve 80%NO_xconversion around 180 ℃ and less than 75 ppm N₂O production under the evaluation condition of GHSV of 200000 ml·g^-1·h^-1.Through the analyses and characterization of NH₃-TPD,H₂-TPR,in situ DRIFTS,etc.,the carrier compounded with NH₄⁺-ZSM-5molecular sieve can provide more acid sites,and the ammonia adsorbed on these acid sites will not be peroxidized,but the"Fast SCR"reaction is carried out with gaseous NO₂ according to the Eley-Rideal mechanism.These acidic sites are stable even after severe hydrothermal aging.Therefore,this work provides a new technology for the application of manganese-based oxide catalysts in the low-temperature denitration of diesel vehicle exhaust.