Effects of Catalyst Physicochemical Properties on the Catalytic Activity of Cu Zeolite Selective Catalytic Reduction at Different Inlet Gas Conditions

Selective catalytic reduction (SCR) systems are important technical means of mitigating NOx emissions for diesel engines. The major challenge of this technique is to improve the low-temperature activity of the catalyst during engine cold start and the thermal stability on repeated exposure to high-temperature exhaust. A two-stage SCR system with a Cu-SSZ-13 catalyst was analyzed with SCR sample experiments and NH3 oxidation experiments. The effects of copper content, doping metals, structural arrangement, NO2/NOx ratio, and NH3/NOx ratio (ANR) on the NOx conversion reaction at different temperatures were investigated. Furthermore, the Cu-SSZ-13 catalyst activity was characterized by Fourier transform infrared spectroscopy, H-2 temperature-programmed reduction, and X-ray diffraction. The results showed that the low-temperature catalytic performance first increased with increasing copper content and decreased with a further increase. The optimal value of copper content was around 3.4%. The thermal stability of Cu(S)SZ-13 catalysts decreased with the increase in the copper content. The temperature window of the high NOx conversion rate was widened by doping Zr/Mn/Ce, and the temperature window was widened by 15.1-33.1%. The two-stage SCR system was compared with a single SCR, which showed that the NH3 oxidation rate increased by 46.9-88.7%, the NOx conversion rate increased by 67.7-85.5%, and T-90 was optimized by 3.3-13.5% during engine cold start. This made the total NOx conversion rate to maintain a high level above 80% at temperatures over 500 degrees C. An appropriate NO2/NOx ratio and ANR could improve the NOx conversion rate at a low-temperature range.