Study On High-temperature Pd-based Close Coupled Catalysts Used For Exhaust Abatement During Cold Start Of The Vhicles

As the economics is much more developed today, the air pollution is getting worse with the increase of industrial level. The automobile exhaust is a main source of pollution which can not be ignored nowadays and grips lots of attentions. For exhaust pollution abatement, the more and more stringent regulations are employed, such as ultra low emission vehicle (SLEV) and zero emission vehicle (ZEV). As for the traditional three way catalyst (TWC), the unburned hydrocarbons and CO released during cold start can not be completely removed. Considering this, the close coupled catalyst (CCC) is proposed. At present study, the Pd-based catalysts are systematically investigated. The effects of Ce, Zr doping on catalytic performance is studied, including the ratio of Ce to Zr. Then the third promoter such as Y, Ca and Ba is comparatively studied concerning the sulfur resistance ability. The Y doping shows positive effects. In this context, the Y content is optimized and water resistance is studied. At last, the Pd precursor is selectively studied and the appropriate precursor is obtained.A series of Pd/Ce-Zr/Al₂O₃ catalyst with different ratio of Ce to Zr were prepared by ultrasonic-assisted successive impregnation. The catalysts were examined by a series of characterization measurements. The results of activity evaluation show that the doping of Ce and Zr can form into solid solution, which shows positive effects on catalytic activity, which is confirmed by TPR data. According to the results of XRD, it is deduced that the Ce-Zr solid solution can stabilize the Pd species and suppress the agglomeration of Al₂O₃ support.A series of high-temperature close coupled catalysts Pd/Ce-Zr-M/Al₂O₃ (M=Y, Ca or Ba) were prepared by ultrasonic-assisted successive impregnation. The catalysts were subjected to a series of characterization measurements. The results of activity evaluation show that Y is the best promoter for propane total oxidation, especially at the calcination temperature of 1100℃. It is interesting that although the BET specific surface areas and the dispersion of Pd species decrease, the Y promoted catalyst calcined at 1100℃shows higher catalytic activity than the corresponding one calcined at 900℃and better sulfur-resisting performance. The results of TEM, TPHD and CO chemisorption indicate that Y can remarkably increase the dispersion of Pd species. However, the dispersion is hard to be connected with the activity increase as the calcination temperature is elevated from 900℃to 1100℃. The change of active phases and the interaction between Pd species and the supports may account for the activity enhancement. Combined with XRD, H2-TPR and O₂-TPD results, it is deduced that the coexistence of metallic Pd and PdO species in the catalysts calcined at 1100℃may be also favorable to C₃H₈ oxidation. In a word, Pd/Ce-Zr-Y/Al₂O₃ is indeed a promising high-temperature close coupled catalyst applicable to high temperature.High-temperature close coupled catalysts Pd/Ce-Zr/Al₂O₃ and Pd/Ce-Zr-Y/Al₂O₃ with different Y content were prepared by ultrasonic-assisted successive impregnation, and subjected to C₃H₈ total oxidation. The results show that the catalyst doped with 5% Y exhibits the best performance with 99% of propane oxidized at 365℃, which is about 70℃lower than that for Pd/Ce-Zr/Al₂O₃. The coexistence of Pd and PdO species is confirmed by XRD, which seems favorable to propane oxidation. The characterization results of TEM, TPHD and CO chemisorption conformably indicate that the dispersion of Pd species is remarkably increased by Y addition, especially at 5% Y loading. Although the PdO species in Pd/Ce-Zr/Al₂O₃ possess high thermal stability, as reflected by O₂-TPD, they exhibit very low dispersion, resulting in very low activity; while the PdO species in the Y-promoted catalyst containing 5% Y simultaneously possess high dispersion and high thermal stability, showing novel catalytic performance. The results of activity for propane total oxidation with the presence of water vapor show that with 5 % Y loading, the water resistance of the catalyst is greatly improved. Combining the activity results and Pd dispersion data, four possible schemes are proposed to expatiate on the mechanism of dehydration. Last, the selection of Pd precursor is preliminarily studied. According to the results of activity evaluation, it can be obtained that the catalyst prepared with palladium nitrate and calcined at 1100℃shows the best activity, which is confirmed by TPR results, showing that the reduction temperature of PdO is the lowest. In the context of above-mentioned theories and present experiment data such as XRD, TPHD, TEM and CO chemisorption, it can be deduced that the Pd dispersion of corresponding catalyst is the highest. From O₂-TPO profiles, it can be seen that the catalyst shows the best thermal stability when the temperature ascending. During the cooling run, the metallic Pd is first oxidized to PdO which indicates the active Pd species are preserved.After several series of comparative experiments and discussions, the most appropriate close coupled catalyst system has been sorted out among those prepared catalysts, which is of great application potential for exhaust abatement during cold start period.