The Effect Of Preparation Method And Transition Metal On The Three-way Catalytic Performance Of Pd/CZA Catalysts

The problem of pollutants emitted by vehicles have received the widespread attention in the society. The application of catalytic invert technique is the most effective method to realize automotive eahaust purification (over Euro-â…£standard). High performance thulium modified alumina material (ACZO) is the pivotal component in the catalysts of automotive exhaust purification, and it has been applied in practice. As the redox reaction of three way catalyst mainly take place in the catalysts' surface, the high surface and high thermal stability of high performance ACZO material is the precondition of high oxygen-storage capacity. Recently to satisfy the catalyst's cold-starting performance , catalytic invert instrument is near to the vent of the engine during pratical application. This measure needs to have better performance of high thermal stability and large surface area.γ-Al₂O₃ is one of supports of the catalysts which are the most far-ranging application. But the problem of thermal stability ofγ-Al₂O₃ is a long-term one, which people is puzzled, especially in the atmosphere of high temperature and vapour being present at. Al₂O₃ is easy to take place phase transition and sinter, and it evolves toα-phase being steady in thermodynamics and large particle causing a drastic decrease in the surface area. This is one of important reasons which contribute to the thermal deactivation of the catalysts. Therefore, investigation and exploitation of new thulium modified alumina material with high thermal stability and high surface area has importantly academic and practical significance for improving durability and cold-starting performance of three-way catalysts for automotive exhaust purification.In this work , the effects of prepared method , transition metals (Cr,Mn,Fe,Co and Ni) , preparation condition on CZA material and the effect of catalysts are discussed in detail. The CZA supports are synthesized by five different methods, which are Sol-gel, coprecipitation, Citric acid , Microemulsion and Impregnation methods. The Pd/CZA catalysts are tested for exhaust pollution purification. Effects of transition metals (Cr,Mn,Fe,Co and Ni) and preparation condition on exhaust pollution destruction over Pd/CZA and Pd/CZA-Ni catalysts have been studied. At the same time, bulk composition and redox behavior over these catalysts are characterized by some experiments. Some specific conclusions are drawn from this work as follows:1 The effects of the preparation methods on the CZA mixed oxides and the three-way catalytic behaviors over Pd/CZA catalysts were investigated. The results show that the CZA prepared by Sol-gel exhibits the highest thermal stability. The BET surface area of CZA-SG is 93m²/g after calcinations at 1100℃for 4h. Theα-alumina phase isn't discovered. Automotive exhaust purification over Pd/CZA catalysts has been studied. The Pd/CZA-SG catalyst prepared by Sol-gel exhibits the highest catalytic activity, thermal stability and oxygen-storage capacity. The catalytic activity is related with the reducibility and quantity ofα-PdO dispersed on Ce-Zr-Al-rich grains, the high thermal stability of PdO and the capacity and quantity of the surface oxygen desorption under lower temperature.2 Effects of transition metals (Cr, Mn, Fe, Co and Ni) on the exhaust pollution purification over Pd/CZA-SG catalysts have been studied. The results show that the CZA-Ni exhibits the highest thermal stability. The BET surface area of CZA-Ni is higher than 101 m²/g. The addition of Ni and Co obviously improves the activity, thermal stability and oxygen-storage capacity of Pd/CZA catalyst for the exhaust pollution purification, especially the Pd/CZA-Ni catalyst. The addition of Ni and Co improves the reducibility of PdO on the surface. The catalytic activity is related with the reducibility and quantity ofα-PdO dispersed on the surface of catalysts. The addition of Fe Co and Ni improve the reducibility ofβ-PdO on the surface,which will improves the activity of HC under the learn conditions. In addition,the addition of Fe,Co,Mn and Ni improve the reducibility capacity and quantity of the surface oxygen in the Ce-Zr solid solution. The Ni and Co are help to dispersion and stability of PdOx , so the Pd/CZA-Ni and Pd/CZA-Co exhibit the highest thermal stability3 Effects of quantity of Ni on exhaust pollution purification over Pd/CZA catalysts have been studied. The results show that the best condition is that the ratio of Ni and Ce is 1/10(mol ratio). The Pd/CZA-Ni (1/10) catalyst exhibits the highest catalytic activity and oxygen-storage capacity for transform of CO, HC and NO_x. The best quantity of Ni improves the the reducibility and stability of PdO. and the ability of the surface oxygen desorption in the Ce-Zr solid solution.4 Effects of treatment temperature of supports on exhaust pollution purification over Pd/CZA-Ni catalysts have been studied. The high treatment temperature of supports will make the catalytic activity of the catalysts decline sharply. The T_20% and T_98% of Pd/CZA-Ni-800 catalyst increases 40℃-50℃after calcined at 1100℃.As treatment temperature increases, the ability of desportion and reducibility on surface and crystal oxygen and the stability,reduction-reoxidation properties of the active PdO species decline. Because of the reasons above, the catalytic activity of CO, HC and NO_x decreases.