| The number of gasoline automobile is greatly increased with the development of economy and transportation. And the automotive exhaust has been one of the main sources of air pollution, which is harmful to both human beings and the environment. Three-way catalyst (TWC) can diminish CO, HC and NOx simultaneously and is regarded as the most efficient way to control automotive exhaust. The coating materials of typical TWCs are usually composed of ceria-zirconia based mixed oxide and y-alumina, and the active components are usually included of precious metals such as Rh, Pt and Pd. Ceria-zirconia based mixed oxides have excellent oxygen storage ability to broaden the operation window for the pollutants during rich/lean perturbations. And y-alumina has high specific surface area and thermal stability, which may be beneficial to the fine dispersion of precious metal with good thermal stability. In order to meet the requirements of higher emission legislation, besides improving catalysts low temperature catalytic performance, usually the mounting of TWCs closer to the engine is explored to remove pollution gases during cold start period. However, with the mount close to the engine, the catalyst would be exposed to wide temperature fluctuations, resulting in a high requirement for thermal stability of the catalyst.In spite of improving the oxygen storage capacity and thermal stability of ceria-zirconia based mixed oxides to improve the three-way catalytic performance, as is known to all, the catalytic performance of supported noble metal catalyst is not only depended on coating materials, but also the nature of precious metal compounds and the load methods, which would have direct effects on the dispersion, chemical state, stability and redox ability of precious metal. Thus, it is very important to reveal these structure-activity relationships to further improve the catalytic performance of the catalysts.In this paper, based on the real application of TWC, the dispersion state, chemical state and thermal stability of PdOx species on Ce0.67Zr0.3302-Al2O3 support and the interaction between PdOx and different support are systematic studied. In addition, the influence of Pd precursors and pretreatment gas on the nature of PdOx species and the three-way catalytic performance is studied in detail. All the catalysts were characterized and studied by XRD, N2 adsorption/desorption, CO chemisorption, XPS, HRTEM, H2-TPR, OSCC, UV-Raman, in-situ DRIFTS and so on. Some specific conclusions from this work are drawn as follows:1. Two kinds of (Ceo.67Zro.3302)-Al203 mixed oxides were prepared using mechanically mixed and co-precipitation methods to study the influence of heat treatment condition on the texture/structure properties and three-way catalytic performance of the catalysts prepared with H2PdCl4 as Pd precursor. Compared with fresh Pd/CZA-c(Cl) catalyst, Pd/CZA(Cl) showed a better catalytic activity for HC, CO, NO and NO2 conversion. High temperature treatment resulted in the increase of interaction between CZ and Al2O3, which was beneficial to improve the thermal stability of the catalyst. The interaction between CZ and Al2O3 in Pd/CZA-c(Cl) catalyst was much stronger, and the catalyst showed a better thermal stability under extreme high-temperature (1100℃). PdOx species were mainly dispersed on Al2O3-rich grains surface for the fresh Pd/CZA(C1) catalyst, but easily migrated onto CZ-rich grains surface under high temperature treatment, which was helpful to keeping high oxidation state of PdOx species during reaction process. Small PdOx partiles dispersed on CZ would promote the oxidation of HC and CO, while the PdOx particles dispersed on Al2O3 would promote the reduction of NO in the light-off process. Moreover, more PdOx species which had strong interaction with CZ-rich grains formed during high temperature treatment, and this would significantly promote the catalytic performance of aged Pd/CZA catalysts.2. The influence of palladium precursor on the nature and configuration of noble metal particles in Pd/CZ catalysts and their Three-way catalytic performance was systematic studied. Pd/CZ(NO) catalyst prepared with Pd(NO3)2 as the metal precursor exhibited the best catalytic performance for HC and CO elimination because of abundant small Pdn clusters and strong Pd-support interaction that facilitated electron transfer from PdOx particles to the CZ support. Pd/CZ(NH) catalyst prepared with Pd(NH3)4(NO3)2 as the metal precursor exhibited good performance for NO and NO2 elimination due to abundant bigger Pdn clusters and oxidized/metallic Pd coexistence. Po/CZ(Cl) catalyst prepared with H2PdCl4 as the metal precursor exhibited low catalytic activity as a result of low Pd dispersion, weak Pd-support interaction, and the trace amount of CeOCl which inhibited oxygen vacancy creation. Then, the influence of palladium precursor on the nature of noble metal particles supported on CZA mixed oxide, metal-support interaction and their Three-way catalytic performance were further studied. For Pd/CZA(NO) and Pd/CZA(NH) catalysts, PdOx species were mainly dispersed on CZ-rich grains surface with smaller Pd particle size and strong Pd-support interaction. And Pd/CZA(NO) catalyst exhibited a good catalytic performance for HC, CO and NO2 conversion. While for Pd/CZA(Cl) catalyst, PdOx species were mainly dispersed on Al2O3-rich grains surface with larger PdOx particle size and higher electron density, showing a different PdOx nature compared with that in Pd/CZ(Cl) catalyst, and Pd/CZA(Cl) catalyst exhibited good catalytic performance for NO conversion.3. The influence of pretreatment gas on the nature of PdOx species and their Three-way catalytic performance were systematic studied in Pd/CZ and Pd/CZA catalysts. The results showed that pretreatment gas mainly influenced PdOx particle size and chemical state in the fresh catalysts. Oxidizing gas pretreatment gave oxidized state and less growth of PdOx particle size in the catalysts, which exhibited good catalytic performance for HC oxidation.In contrast, PdOx species were mainly in the metallic state and exhibited more growth in reducing gas. In addition, reducing gas pretreatment also increased the concentration of oxygen vacancies, resulting in not only promoting the conversion of NO, but also broadening the operation window. For the catalysts pretreated with reacting gas, some active sites in the catalyst surface were easily blocked by strongly adsorbed HC species, hindering HC and NOx conversion.4. High temperature treatment influenced both the nature of PdOx species and their Three-way catalytic performance in Pd/CZ and Pd/CZA catalysts. After aging treatment, the PdOx partice size was greatly increased, which weakened the interaction between Pd and support. And the catalytic performance of the aged catalysts was decreased. While for the catalysts prepared with H2PdCl4 as Pd precursor, due to the remove of residual chlorine species which promoted the interaction between PdOx and the support, Pd/CZ(Cl)-a and Pd/CZA(Cl)-a catalysts still exhibited good catalytic performance for HC and NO2 conversion. And for the CZA supported Pd catalysts, new oxygen vacancies were formed during high temperature treatment, which improved the migration ability of surface oxygen. And it was helpful to promote the catalytic performance of aged Pd/CZA catalysts, especially broadening the NOx operation window, showing different features with Pd/CZ catalysts. |
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