Research On Reliability Of Cu/CHA NH₃-SCR Catalysts

The NO_x emission control of diesel vehicles is the core content of the emission abatement in China.It is the key to mitigate environmental pollution and improve the life quality of people.China VI emission standard proposes a big challenge on the catalysts applied in NOx abatement.In the field of NH₃-SCR technology,Cu/CHA catalysts have been paid much attention by the researchers due to its excellent NOx decomposition,N₂ selectivity,hydrothermal stability,the resistance of carbon accumulation and the hydrocarbon tolerance.However,to better cope with the China6 requirements of long-term stable catalysts,this paper focuses on the mechanism of alkali metal poisoning and sulfuring poisoning of Cu/CHA catalysts,so as to provide fundamental and theoretical support for commercial application of Cu/CHA catalysts.Firstly,the effect of SO₃ poisoning on the structure and catalytic performance of Cu/SSZ-13 catalysts was investigated,and the mechanism of SO₃ poisoning on Cu/SSZ-13 and Cu/SAPO-34 was compared.XRD,BET and IR results indicate that the framework structure is not affected during the sulfuration process.The copper sulfate is formed on the Cu/SSZ-13 catalysts and its formation increases with increasing SO₃ content.The Cu²⁺in eight-member-ring（8MR）is susceptible to poisoning and as a result,Cu/SSZ-13 loses a large amount of active sites,which changes the rate-limiting step of NH₃-SCR reaction.Secondly,the effects of different sulfates on the performance of Cu/SSZ-13catalysts were investigated,and the effects of different sulfate species on Cu/SSZ-13and Cu/SAPO-34 catalysts were compared.It suggests that different temperatures and ammonium sulfuration conditions have no effect on the structure of the catalysts.Under different sulfuration conditions,copper sulfate would be formed on Cu/CHA.By combining the SCR activity test results,we found that the influence mechanism of the different sulfate species on Cu/CHA NH₃-SCR reaction is that the numbers of the active centers decrease during sulfuration.However,unlike the case of Cu/SAPO-34catalyst for the formation of ammonium sulfate,due to the presence of[Cu（OH）]⁺in the Cu/SSZ-13 catalyst,the Cu/SSZ-13 catalyst produces a less stable ammonium hydrogen sulfate species;Also due to the presence of[Cu（OH）]⁺,active site loss is more pronounced on Cu/SSZ-13 than on Cu/SAPO-34,which leads to variation of reaction mechanism after formation of different sulfate species on Cu/SSZ-13catalyst.In addition,the effect mechanism of potassium on Cu/SSZ-13 catalyst under high-temperature hydrothermal conditions was investigated,and the mechanism of alkali metal poisoning of Cu/SSZ-13 and Cu/SAPO-34 catalysts high-temperature was compared.The results show that K ions enter the pores of Cu/CHA catalysts and ion exchange with acidic sites,thereby reducing the acidity of the catalyst and the content of copper ions.This allows the conversion of copper ions to Cu O species.The high content of K causes damage to the structure of Cu/CHA catalysts.Combining the results of activity evaluation,we found that the deactivation caused by K ions,the formation of copper oxide and the destruction of the CHA framework are all reasons for reducing the activity of Cu/CHA.Finally,based on the reliability mechanism of Cu/CHA catalyst,we take advantage of the improvement of the low-temperature hydrothermal stability of Cu/SAPO-34 catalyst modified by alkali metals.The dual catalyst system of Cu/SAPO-34 and Fe/Beta was studied.The results show that although the acid content of Cu/SAPO-34 catalyst decreases with the increasing exchanged potassium content and thus leads to the decrease of SCR activity,the introduction of K ions improve low temperature hydrothermal stability of the Cu/SAPO-34 catalysts.The improvement,in turn,promotes the protection of the Cu/SAPO-34 catalyst activity.At the same time,this study made up for the low-temperature catalytic activity of KCu/SAPO-34 catalyst by the excellent high-temperature SCR activity of Fe/Beta catalyst.It was found that the arrangement of Fe/Beta in the front stage and KCu/SAPO-34 in the back end would better benefit to excellent activity.