| Gasoline engine exhaust has been a major cause of air pollution since pollutant emission from gasoline engine powered vehicles is harmful for both human beings and the environment, and the catalytic conversion is regarded as one of the most efficient and popular solution for gasoline engine pollution control. Three-way catalyst (TWC) has been proven to be a suitable alternative to reduce emissions from gasoline engine powered vehicles by converting basic pollutants like CO, HC and NOx to their inactive products CO2, N2and H2O simultaneously. The typical TWC formulation usually contains noble metals, such as platinum (Pt), palladium (Pd) and rhodium (Rh) as the active components. Pd exhibits excellent capability for low-temperature oxidation of carbon monoxide and hydrocarbons compared with Pt and Rh-supported catalysts. On the other hand, Pd is relatively economical and abundant than Pt and Rh, especially than Rh. Therefore, Pd-only TWC or low loading Rh TWC has received considerable attention. Ceria-zirconia mixed oxide is one of the key components in TWC’s coating material due to its outstanding oxygen storage/release capacity. Its major role is to minimize the influence of air-to-fuel (A/F) ratio fluctuation at around14.6on the catalytic activity of TWC during engine operation, thus ensuring that TWC works effectively within a narrow operating window near the stoichiometric A/F ratio.Legislations relating to automobile exhaust are continuously being tightened, requiring the emission control in cold-start stage since80～90%of the hydrocarbon emissions from automobiles equipped with modern TWC occurs during the cold-start. Among the several potential solutions to the cold-start problem have been proposed, close-coupled catalyst is the most direct and efficient. Close-coupled catalyst is placed in position near the engine valve in order to increase the catalyst temperature rapidly, and then most of hydrocarbons can be converted to CO2and H2O catalytically. However, the temperature in a close-coupled catalyst converter can rise even above1000℃in the practical process of three-way catalytic conversion, leading to the sintering of ceria-zirconia material and the losing of oxygen storage capacity. Therefore, the generation of thermal stability ceria-zirconia based material is the key factor to develop a novel TWC that has both high-temperature stability and low-temperature activity.In this dissertation, a multi-technique approach involving powder X-ray diffraction (XRD). small-angle XRD, N2adsorption/desorption, transmission electron microscopy (TEM).scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman, oxygen storage complete capacity (OSCC), dynamic oxygen storage capacity (DOSC), hydrogen temperature programmed reduction (H2-TPR), Fourier transform infrared reflectance (FTIR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were operated aiming at the research and development of novel high thermal stability ceria-zirconia based mixed oxide for TWC. In the present work, the effects of ceria/zirconia molar ratio on the physical and chemical properties of ceria-zirconia and ceria-zirconia-lanthana as well as their supported Pd-only TWC were studied. Then, the influence of rare earth addition (La, Nd, Pr, Sm and Y), loading content and preparation condition on the structural/textural properties of Zr-rich Ceo.2Zro.802(CZ) and its supported Pd-only TWC were investigated systemically. Some specific conclusions can be drawn as follows:1. The structural/textural properties of ceria-zirconia mixed oxide, such as crystal phase, specific surface area, oxygen storage/release capacity, redox behavior and thermal stability are strongly affected by the ceria/zirconia content. Ceria-zirconia mixed oxide with ceria/zirconia molar ratio1:1～1:4has the structure of tetragonal ceria-zirconia solid solution and shows the relatively bigger BET surface area (>105.9m2/g) and smaller crystallite size as well as better migration of bulk oxygen to the surface of ceria-based oxide, redox performance and thermal stability (the BET surface area of CZ14-a is24.3m2/g even after aging at1100℃for4h). Ceria-zirconia mixed oxide with ceria/zirconia molar ratio2:1～1:2exhibits good oxygen storage/release capacity. And therefore, its supported Pd-only TWC shows enlarged air-to-fuel operation window and excellent catalytic conversion activity to CO and NO2. What should be mentioned is that the Zr-rich ceria-zirconia solid solution supported Pd-only TWC has outstanding catalytic conversion activity to HC and NO, especially Pd/CZ14.2. The addition of La into ceria-zirconia solid solution results in the formation of ceria-zirconia-lanthana ternary solid solution, which shows better textural/structural properties than ceria-zirconia. Ceria-zirconia-lanthana mixed oxide shows better thermal stability than ceria-zirconia mixed oxide, and the BET surface area of CZL14and CZL14-a is as high as144.4and39.6m2/g, respectively. Ceria-zirconia-lanthana mixed oxide exhibits higher reducibility and oxygen storage/release capacity than ceria-zirconia mixed oxide. Typically, the oxygen storage/release capacity of Zr-rich ceria-zirconia mixed oxide is enhanced significantly due to the addition of La. Moreover, the incorporation of La into ceria-zirconia mixed oxide also leads to the improved interaction between the support and PdOx active species, which is beneficial to enhance the dispersion and stability of the active species. Therefore, the catalytic activity of ceria-zirconia-lanthana mixed oxide supported Pd-only catalyst is higher than the corresponding ceria-zirconia mixed oxide supported Pd-only catalyst, especially the catalytic conversion of HC. The introduction of La also leads to the increased thermal stability of the support and its supported catalyst, as well as the improved air-to-fuel operation window. 3. The CeO2-ZrO2-Re2O3(Re=La, Nd, Pr, Sm and Y) ternary solid solution can be obtained by introducing Re into the lattice structure of Ce0.2Zr0.8O2(CZ), and Zr is replaced by Re during the formation of ceria-zirconia-rare earth. All the rare earth modified CZ prepared in our work has the structure of tetragonal ceria-zirconia solid solution. Ceria-zirconia-rare earth mixed oxide with appropriate rare earth loading content shows improved themal stability than ceria-zirconia mixed oxide. The addition of La, Nd and Pr is propitious to inhibit the sintering of ceria-zirconia solid solution and enhance its themal stability, reducibility and oxygen storage/release capacity. In addition, the La, Nd and Pr modification leads to the stronger metal-support interaction, which is advantageous to the dispersion and stability of PdOx active species as well as the reducibility of catalyst. Therefore, the corresponding supported Pd-only TWC exhibit superior three-way catalytic activity and wide air-to-fuel operation window. CZL5and CZN5with5wt.%La and Nd doping content exhibits relatively better textural/structural properties due to the formation of more homogenous solid solution, and the corresponding supported Pd-only catalyst shows relatively higher three-way catalytic activity. The optimum doping concentration for praseodymia is8-10wt.%.4. The lattice structure, crystallite size, BET surface area, topography feature and thermal stability of ceria-zirconia based mixed oxide as well as the catalytic activity of its supported Pd-only TWC is affected by the drying method noticeably. Supercritical drying is applied as a new method contrast with the conventional drying techniques for the development of advanced ceria-zirconia-lanthana solid solution. During the process of supercritical drying, the vapor-liquid interface disappears, and the considerable structural shrinkage and hole collapse are restrained. Therefore, CZL-3prepared by supercritical drying technique shows larger specific surface area and pore volume as well as remarkable porosity and wide pore size distribution. The remarkable porosity and wide pore size distribution are beneficial to the adsorption/desorption of pollutants in the process of catalytic conversion and to the dispersion and reduction of PdOx active species, leading to the enhanced catalytic activity. Also. CZL-3exhibts higher redox properties and oxygen storage/release capacity due to the more homogenous structure. The excellent structural/textural properties of the forenamed CZL-3results in the outstanding catalytic activity, wide air-to-fuel operation window of the corresponding three-way catalyst, indicating its tremendous potential possibilities. |
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