Study On Low-temperature NH₃-SCR Catalytic Activity And Bench Test Of Cu-SSZ-13 Molecular Sieve

Nitrogen oxides（NO_x）in diesel vehicle exhaust is a main source of air pollution in China.At present,ammonia selective catalytic reduction（NH₃-SCR）is the most efficient and mature technology for NO_x conversion.Cu-SSZ-13 molecular sieve has great potential in NO_x removal from diesel exhaust due to its adjustable acid content,high specific surface area and considerable cation loading.In this paper,the adaptability of Cu-SSZ-13 catalyst in the stage of China’s emission standards was discussed from the preparation of Cu-SSZ-13 catalyst formula and engine bench test.A series of Cu-SSZ-13 zeolites with different Si/Al ratios（Si/Al=47,30,20,15）were prepared by hydrothermal method and liquid ion exchange technology,which were used for NH₃-SCR reaction.The physicochemical properties of the samples were characterized by a number of analytical techniques.It is shown that the NH₃-SCR performances of different Si/Al ratios are significantly different,especially in the low-temperature range.The light off temperature T₅₀（the temperature when the conversion efficiency is≥50%）fellows the order of Si/Al=20<Si/Al=15<Si/Al=30<Si/Al=47.Characterization results show that different Si/Al ratios could affect the surface morphology and crystal structure of the zeolite catalysts,and further exert an influence on distribution and quantity of Cu species as well as the acidity.Among the four zeolite catalysts,Si/Al-20 possesses the largest number of isolated Cu²⁺and acid sites,which are conducive to the low-temperature NH₃-SCR activity.The influence of Cu concentration on the selective catalytic reduction（SCR）of NO with ammonia（NH₃-SCR）for copper exchanged aluminosilicate zeolite catalysts（Cu-SSZ-13）was investigated.The results demonstrate that the SCR performance of the catalysts increases initially and decreases subsequently with a rise in Cu concentration in low-temperature region（100～300℃）.The highest low-temperature SCR activity（light-off temperature is 149℃）is obtained in Cu_0.100-SSZ-13 catalyst,i.e.the concentration of Cu salt solution reaches up to 0.100 mol/L.BET and XRD results illustrate that the concentration of the Cu salt solution does not have a significant effect on the structure of the samples.The NH₃-TPD and H₂-TPR outcomes demonstrate that the amount of Lewis acidity and isolated Cu²⁺on Cu-SSZ-13 catalysts rises as the Cu Cl₂ concentration increases from 0 to 0.125 mol/L.However,most of H in the Br（?）nsted acid sites（Si-OH-Al）are replaced by isolated Cu²⁺,which limits the storage and transfer capacity of ammonia.As a result,the catalytic efficiency of the Cu-SSZ-13 is reduced with further increasing Cu Cl₂ concentration.Cu-SSZ-13 catalysts with the nearly same content of copper species were prepared by ion exchange technology with copper acetate,copper chloride,copper nitrate and copper sulfate as precursors,respectively.The effects of copper source on NH3selective catalytic reduction（NH₃-SCR）were probed detailly.The physicochemical properties of the catalysts were investigated by XRD,ICP,N₂ absorption-desorption,SEM,H₂-TPR,XPS and NH₃-TPD.The results of activity test display that the performance of Cu-SSZ-13 zeolite for ammonia selective catalytic reduction at low temperature is significantly different.The light off temperature T₅₀ follows the order of Cu-SSZ-13（A）<Cu-SSZ-13（Cl）<Cu-SSZ-13（S）<Cu-SSZ-13（N）.The characterization results manifest that the distribution and quantity of copper species and the acidity of zeolite catalysts are affected by copper source.The Cu-SSZ-13（A）catalyst possesses the largest number of isolated Cu²⁺and strong L acid sites,which are conducive to the low-temperature NH₃-SCR reaction.The results show that the NO_x emission of Cu-SSZ-13 catalyst in WHSC and WHTC cycle is 0.954 and 0.846 g/k Wh,respectively.The resistance of the catalyst at the calibration point is 16.8 k Pa,which meets the requirements of post-treatment calibration（the resistance at the calibration point is less than 25 k Pa）.The conversion efficiency of DOC to NO is lower than the target value below 400℃,and the conversion efficiency of DOC to pollutants reaches the limit value of the Sixth National emission standard for diesel vehicles.