Research On Application Of Manganese Oxide Octahedral Molecular Sieve And Cu-SAPO-18 Catalysts In NH₃-SCR Reaction

The emission of nitrogen oxide(NO_x)would cause great harm to the ecological environment and human health.Diesel vehicles have become an important target for pollution prevention and control due to their high NO_x emissions.NH₃ selective catalytic reduction of NO_x technology(NH₃-SCR)has become one of the most effective denitrification technologies.The core of this technology depends on the development of catalysts.The national VI emission regulations for diesel vehicles not only tighten the NO_xemission limits of vehicles under different operating conditions,but also improve the durability requirements of SCR catalysts.Manganese oxide catalysts exhibit high de NO_xperformance at low temperature,therefore,it could be used as low-temperature SCR catalyst to solve the problem of NO_x emissions during the cold start of diesel vehicle.Simultaneously,Cu-SAPO-18(Cu-18)zeolite has attracted some attention due to its good NH₃-SCR activity and hydrothermal stability,but its activity and hydrothermal stability need to be further improved to meet more stringent emission standards.Furthermore,the SCR catalyst would suffer sulfur poisoning and deactivation during long-term use,due to the sulfur contained in diesel fuel.Hence,this dissertation explored the sulfur poisoning mechanism of Cu-SAPO-18,it is expected to provide a theoretical basis for the design of SCR catalysts with better sulfur resistance in the future.The main content of the dissertation is as follows:Firstly,a one-pot reflux method was used to prepare manganese oxide octahedral molecular sieve(OMS-2)catalysts doped with different metal cations(Li⁺,Na⁺,Mg²⁺,Ca²⁺,Al³⁺,Fe³⁺,Co²⁺,Ni²⁺,Cu²⁺,Zn²⁺and Ag⁺).A series of characterizations were carried out to explore the intrinsic relationship between the physiochemical properties of M-OMS-2 and its low-temperature NH₃-SCR activity.The results showed that Na-doped OMS-2 had the best low-temperature de NO_x activity due to its higher specific surface area,better reducibility and the most Br(?)nsted acid sites.Further research on Na-OMS-2 with different Na content was conducted,it was found that when Na/(Na+Mn)was 0.030,Na-OMS-2possessed a high redox ability and the highest acid site content,thus leading to better low-temperature de NO_x performance.Then the Cu-SAPO-18 was post-treated with a mixed solution of Mn²⁺and NH₄⁺,after the post-treatment with optimal concentration mixed solution,the Mn/Cu-18-?was obtained,and its NH₃-SCR activity and hydrothermal stability were better than Cu-18.This is mainly because Mn/Cu-18-?contains more acid sites and Cu²⁺species,but does not contain Cu O species.Moreover,in order to further improve the activity and hydrothermal stability of Cu-18,La,Ce,Sm or Zr was introduced into the mixed solution used to prepare Mn/Cu-18-III.The results showed that Sm doping significantly enhanced the NH₃-SCR activity and hydrothermal stability of the Mn/Cu-18-?.The NH₃-SCR reaction mechanism of the fresh and aged Cu-18,Mn/Cu-18-?,La and Sm Mn/Cu-18-?were investigated,it was found that these catalysts mainly followed the Langmuir–Hinshelwood(L–H)reaction mechanism,with the L–H reaction route of fresh and aged Mn/Cu-18-?,La and Sm Mn/Cu-18-?faster than that of Cu-18.Finally,Cu-SAPO-18 was exposed to SO₂+O₂ to investigate the variations in the physicochemical properties and activity of the Cu-SAPO-18 before and after sulfur poisoning.It was found that sulfate species forming on the surface of sulfated Cu-SAPO-18 could interact with isolated Cu²⁺sites,causing the isolated Cu²⁺poisoning,and the Cu²⁺in the cage was more susceptible to sulfur poisoning.In the NH₃-SCR reaction,sulfur species formed on the sulfated Cu-SAPO-18 surface could react with NH₃ to form ammonium salts,which further covered the Cu²⁺sites.These ultimately led to a significant decrease in the low-temperature NH₃-SCR activity of sulfur poisoning Cu-SAPO-18.Furthermore,the reaction mechanism of Cu-SAPO-18 before and after sulfur poisoning was investigated,and it was found that the fresh and sulfated Cu-SAPO-18 mainly followed the L–H mechanism in the NH₃-SCR reaction,but the L–H mechanism of sulfur poisoning Cu-SAPO-18 was inhibited.In addition,the sulfur-poisoned Cu-SAPO-18 was regenerated under SCR atmosphere at 550?,and its low-temperature NH₃-SCR activity could be restored to a large extent.