Controllable Preparation Of Cu-SAPO-34 With Enhanced Water And Sulfur Resistance And Study On DeNO_x Of Diesel Engine Exhaust

"China Mobile Source Environmental Management Annual Report(2022)"shows that mobile source pollution has become the main source of air pollution in China,among which diesel vehicle NOx emissions more than 80%of the total automobile emissions.The emission of nitrogen oxides into the atmosphere will cause a series of serious environmental problems,so the elimination of nitrogen oxides is urgent.In order to control the motor vehicle exhaust emission to the direction of low emission,ultra-low emission and even zero emission,countries in the world have formulated increasingly strict emission standards and various regulations,which put forward higher requirements for diesel exhaust denitrification technology.NH3-SCR denitrification technology is the most effective denitrification technology for diesel exhaust gas at present,and catalyst is the core of this technology.Although Cu-SSZ-13 catalyst has been stably applied commercially at present,Cu-SAPO-34 has better high-temperature hydrothermal stability,low cost and a wide selection of template agents,so it has become a more potential catalyst for denitrification of diesel exhaust NH3-SCR.However,in the practical application process of diesel motor vehicle,SO2 and H2O inevitably exist in exhaust gas.On the one hand,under low temperature hydrothermal conditions,water molecules attack the molecular framework of Cu-SAPO-34,resulting in hydrolysis of Si-O(H)-Al bond,skeleton collapse and Si island,resulting in rapid deactivation of catalyst.In addition,the hydrolyzed aluminum reacts with the active copper species to form stable copper species,resulting in irreversible deactivation of the catalyst.On the other hand,in the process of NH3-SCR reaction,SO2/SO3 will react with reducing agent NH3 to form ammonium sulfate salt,causing pore blockage.It also reacts with active copper species to form stable copper sulfate,resulting in irreversible deactivation of the catalyst.Therefore,this paper aims to modify Cu-SAPO-34 to improve its water and sulfur resistance,and to study the mechanism of its H2O/SO2 deactivation.In the first part,Cu-S34-C18 molecular sieve was synthesized by using octadecyl trimethoxysilane(OTMOS)as the common silicon source of tetraethyl orthosilicate(TEOS)and MOR as the template.Compared with traditional Cu-SAPO-34 molecular sieve,Cu-S34-C18 molecular sieve showed better denitrification activity and low temperature hydrothermal stability.In addition,the structural morphology,hydrophobic properties,acidic sites and redox sites of the catalyst indicated that the long carbon chain of OTMOS affected the pore size distribution and copper species distribution of the Cu-S34-C18 molecular sieve during the catalyst synthesis process,and reduced the hydroxyl group on the surface of the Cu-S34-C18 molecular sieve,and enhanced the hydrophobicity.In situ infrared results showed that the long carbon chain of OTMOS did not affect the catalytic reaction mechanism of Cu-S34-C18 molecular sieve,but helped to accelerate the reaction speed of NH3-SCR.The first part of the results showed that the introduction of OTMOS can promote the NH3-SCR reaction of Cu-SAPO-C18 molecular sieve,improve the denitrification activity and low temperature hydrothermal stability of the molecular sieve.The reason may be that during the crystallization of Cu-SAPO-34 molecular sieve,OTMOS long carbon chains interspersed inside or outside the molecular sieve,which helped to optimize the distribution of copper species.When the molecular sieve was calcined at high temperature,the decomposition of long carbon chains was helpful to produce more mesoporous structures,reduce the surface hydroxyl group and enhance the hydrophobicity.In order to further study the anti-sulfur poisoning performance of Cu-SAPO-34 molecular sieve,the second part uses the shielding effect of core-shell catalyst to improve and explore the sulfur poisoning problem of Cu-SAPO-34 catalyst.Therefore,based on the mature core-shell molecular sieve synthesis technology,Cu-SAPO-34 was used as the nuclear layer,and SAPO-34 molecular sieve was wrapped around it.A Cu-S34@S34 molecular sieve with high sulfur poisoning resistance was synthesized by secondary crystallization.The results of denitrification activity test show that Cu-S34@S34 molecular sieve has better NH3-SCR catalytic activity and sulfur toxicity resistance.By SEM characterization,it was found that a layer of SAPO-34 nanosheets and nanoparticles grew on the surface of Cu-S34@S34 catalyst.The characterization results of H2-TPR,XPS and EPR indicate that the distribution of Cu species in Cu-S34@S34 core-shell molecular sieve is optimized.In situ infrared results show that Cu-S34@S34 catalyst follows L-H mechanism.In addition,the characterization of the Cu-S34@S34 acid sites showed that the core-shell molecular sieve had fewer hydroxyl groups on its surface.In summary,on the one hand,there are more active sites in Cu-S34@S34 core-shell molecular sieve,and the distribution of copper species has been optimized,which may be the main reason for its excellent denitrification activity.On the other hand,the core-shell structure of Cu-S34@S34 molecular sieve is beneficial to protect the active site of catalyst from the influence of SO2,and its special nanosheet structure and pore structure are conducive to the decomposition of ammonium sulfate.These two aspects may be the main reason for the high sulfur resistance of Cu-S34@S34 molecular sieve.