Preparation Of Ce-rich Ceria-Zirconia Composite Oxide And Its Application In Pd-only Three-way Catalysts

Environmental pollution has attracted more and more attention and especially, the exhaust from vehicle has posed significant abatement problems. In response to the challenges associated with treatment of these exhaust, three-way catalysts (TWCs) have been widely used to reduce pollutant emissions from gasoline engine powered vehicles. CexZr1-xO2 solid solutions are the key components in TWCs. Through the transition between Ce3+ and Ce4+, the oxygen storage material buffers the lean-rich swings to achieve a stoichiometric atmosphere. However, for practical performance, TWCs should be able to stand a temperature as high as 1000～1100℃. At such a high temperature, CeO2-ZrO2 will deactivate significantly due to the loss of surface area and oxygen storage capacity (OSC). Another problem of TWCs is its high conversion efficiency can only be achieved within a very narrow "operation window" of air to fuel ratio. In practice, the air to fuel ratio has to fluctuate around the theoretic value to some extent due to the change of the operation modes of engine, resulting in that TWCs can not eliminate all three kinds of pollutants at the same time. The high-performance of OSC of CeO2-ZrO2 solid solution can moderate the effect of oscillates between rich and lean conditions. As a result, the width of the operation window enlarges. In addition, the amount of precious metal can be reduced by using CeO2-ZrO2 solid solution and the cost of TWCs can be reduced. Recently, the preparation and adding other metals into CeO2-ZrO2 solid solutions have attracted increasing interests in order to improving the thermal resistance and OSC performance of CeO2-ZrO2 solid solutions.In the present dissertation, we used X-ray powder diffraction (XRD), N2 adsorption/desorption, UV-Raman spectra (UV-Raman), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), oxygen storage capacity complete (OSCC) and temperature-programmed reduction (H2-TPR) techniques to study systematically the effect of preparation methods of Ce-rich ceria-zirconia composite oxide (Ceo.67Zro.33O2), preparation condition and rare earths doping on the textural-structural property, surface morphology, redox property, oxygen storage-release capacity, thermal stability of support and the performance of its supported Pd-only TWCs. Some specific conclusions are drawn from this work as follows:1. The effect of preparation methods (microemulsion, homogeneous precipitation, hydrothermal and coprecipitation methods) on the textural-structural property, oxygen storage-release capacity, thermal stability of Ceo.67Zro.33O2 (CZ) and the performance of its supported Pd-only TWCs was investigated in detail. The results show that the OSCC can be strongly enhanced by increasing the homogeneity and defect of the CZ solid solution. The pore-size distribution and redox properties of support seem to be important for the three-way catalytic activity of Pd-only catalyst. Compared to other preparation methods, the fresh CZ solid solution prepared by coprecipitation combined with dried under supercritical condition (CZCP) obtains the largest diameter (20.87 nm) and mesopore volume (0.455 cm3/g) as well as wider mesopore-size distribution and better thermal stability. After aging at 1100℃, the CZCP exhibits higher surface area (10.2 m2/g), the smallest crystallite size (21.4 nm) and higher OSCC value. Furthermore, strong interaction between PdO species and CZCP exists, which favors the stability of PdO species. The Pd-only catalyst supported on CZCP exhibits good redox property, thermal stability and excellent three-way catalytic activity as well as the widest operation window.2. A series of CZ were prepared by coprecipitation route. The effects of pH, aging temperature and drying temperature on CZ properties were studied, as well as the catalytic performance of Pd-only TWC supported on the prepared CZ. The results show that with the increase of aging temperature during precipitation, the crystallite size of CZ increases significantly, while the OSCC drops and no notable effect on porosity of CZ is observed. The porosity is mainly determined by drying temperature and pH value during precipitation. Prepared under optimized conditions (precipitation at the pH of 9.5, aging at 25℃and then drying under supercritical condition in alcohol), the CZ solid solutions exhibit good properties. This preparation procedure leads to CZ with proper pore size distribution, larger average pore size (20.42 nm), higher pore volume (0.69 cm3/g) and enhanced OSCC value (383.0μmol O2/g). Furthermore, the Pd-only TWC supported on the CZ synthesized under the optimized conditions exhibits satisfactory catalytic activity and very low reduction temperature (53℃).3. The effect of rare earth doping on the textural-structural property, oxygen storage-release capacity, thermal stability of CZ and the performance of its supported Pd-only TWC was investigated. The results show that the amount of oxygen vacancy and the OSCC are improved at different degree by rare earth ion doping. Compared to the doping of Y, Sm or La, more homogeneous Ce-Zr-M-O ternary solid solutions are obtained and only cubic phase can be seen by the adding of Nd or Pr into CZ. The structural and textural thermal stability of CZ solid solution is improved remarkably by the rare earth doping. After aging at 1100℃, CZ support doped by the rare earth keeps larger surface area, pore volume, smaller crystallite size and more homogeneous structure. Furthermore, the interaction between PdO species and support is also improved by the rare earth doping, which promotes the stability of PdO species and the reduction of Ce4+ located at the interface of palladium species. For fresh catalysts, the introduction of rare earth has no obvious effect on the width of three-way catalytic window but promotes the conversion of NOx. The thermal stability of aged catalyst is improved to some extent by the adding of different rare earth metal. Moreover, the conversion of HC at lean-oxygen atmosphere and the reduction of NOx at rich-oxygen condition are promoted obviously by the presence of rare earth metal. For the aged catalysts, compared to the adding of Y, Sm or La, the catalyst supported on CZ modified with Nd or Pr exhibits favorable three-way catalytic activity and the widest operation window.4. The effect of praseodymium content on the textural-structural property, oxygen storage-release capacity, thermal stability of CZ and the performance of its supported Pd-only TWC was studied. The results show that the textural-structural stability and OSCC value can be improved obviously when the molar ratio of Pr:Ce is 1:8. The CZP(1/8) support remains ternary cubic solid solution even after aging at 1100℃for 16 h. Furthermore, the interaction between PdO species and support is strengthened, which promotes the reduction of PdO species and Ce4+ located at the interface of palladium species. The light-off temperature of CO, NO and NO2 is obviously decreased over Pd-only catalyst supported on CZ doped with proper amount of praseodymium and the Pd/CZP(1/8) catalyst represents good three-way catalytic performance.