The Study Of Preparation, Characterization And Catalytic Activity On Soot Combustion And Solid Electrolytes For CeO₂-based Mixed Oxides

Due to the high special oxygen-storage, oxygen-removal capacity and the fastoxygen vacancy transfer ability, CeO₂ and CeO₂-based mixed oxides are widely usedin catalytic field. In this work, the CeO₂-based mixed oxides were prepared by aSol-Gel method, and the CuO/CeO₂-Al₂O₃ catalysts were prepared by animpregnation method. In-situ XRD, Raman, UV-Visible, H₂-TPR and SEMtechniques were used to characterize the phase structure of the samples, meanwhile,the soot combustion activity of CeO₂-based mixed oxides, the conductivity ofCe_xGd_1-xO_2-δ solid electrolytes and CO oxidation activity of CuO/CeO₂-Al₂O₃catalysts were also studied.1. Soot combustion activity study of CeO₂-based mixed oxidesThe CeO₂-based mixed oxides have higher catalytic activity compared withsingle CeO₂, as the formation of CeO₂-based solid solutions can increase the quantityof oxygen vacancy, which hasten the transfer velocity of oxygen, and enhance thereducibility of the catalysts, resulting in improvement of catalytic ability. With theincreasing of calcination temperature, the crystallite sizes of catalysts become larger,therefore, The catalysts calcined at 900℃have lower activity compared with thesecalcined at 600℃. Combustion activity of Ce_0.7Zr_0.3O_2-δ catalyst calined at 600℃decreases slowly after 7 cycled runs, deducting from the effect of the using weight,indicating that the Ce_0.7Zr_0.3O_2-δ catalyst shows good catalytic stability.For CeO₂-based solid solutions, especially for Ce_0.7Zr_0.3O_2-δ and Ce_0.7Pr_0.3O)(2-δ),the catalytic activities are nearly the same as the 30% KNO₃ doped typical supports.As the doped KNO₃ could be easily dissolved and leached from the catalysts, causingthe environmental pollution and dramatic decline in oxidation activity, it can't beextended in the practical application. Therefore, according to the good catalytic abilityand stability, the Ce_0.7Zr_0.3O_2-δ catalyst could be a potential candidate for catalytic sootcombustion. A reaction mechanism was proposed according to the good reoxidizing-reducing ability of Ce_0.7Zr_0.3O_2-δ, namely, the lattice oxygen is the active oxygen species.2. Phase structure and conductivity study of Ce_xGd_1-xO_2-δ electrolytesBy the improved citric acid Sol-Gel method, solid solutions with smallercrystallite sizes could be prepared compared with the traditional method. Meanwhile,the improved method is benefical to stabilize the CeO₂-besed solid solutions. Withincreasing calcination temperature, the sample in forms of CeO₂-besed andGd₂O₃-based solid solution will transfer to the Gd₂O₃-based solid solution, while thesample in forms of Gd₂O₃-based solid solution and cubic Gd₂O₃ will transfer to cubicGd₂O₃. Raman spectroscopy has higher sensitivity than XRD, which can detect thetrace amount of CeO₂-besed solid solutions. Raman results indicate that the samplesprepared by two methods, no matter CeO₂-besed or Gd₂O₃-based solid solutions, canboth enhance the formation of oxygen vacancies.The Ce_xGd_1-xO_2-δ solid solution electrolytes have better conductivities than singleCeO₂ and Gd₂O₃, as well as the electrolytes prepared by the improved method havebetter conductivities than these prepared by the traditional method. As the crystallitesizes of samples prepared by the improved method are smaller and have more oxygenvacancies, which can enhance the transfer velocity of oxygen, then improve theconductivity of solid electrolytes. Both Ce_0.6Gd_0.4O_2-δ solid electrolytes prepared bytwo different methods show the best conductivity, so for Ce_xGd_1-xO_2-δ electrolytes, thebest Gd³⁺ doping scale is 40%.3. CO oxidation activity study of CuO/Al₂O₃ and CuO/CeO₂-Al₂O₃ catalystsCuO species and solid-solid interaction of CuO/Al₂O₃ catalysts werecharacterized by in-situ XRD, Raman spectroscopy and H₂-TPR techniques. CuAl₂O₄is formed as a result of solid-solid interaction between CuO and Al₂O₃ at hightemperature. The formation of CuAl₂O₄ inhibits CuO diffusing into Al₂O₃ support,which indicates that CuO can be stabilized on the surface region of the catalysts,resulting in improving CO oxidation activity of the catalysts. There are three H₂-TPRpeaks, which are attributed to the highly dispersed CuO species, bulk CuO species,and spinel CuAl₂O₄, respectively. Combined with H₂-TPR and CO oxidation activity, it is supposed that the catalytic activity is related to both the highly dispersed CuO andbulk CuO, with the former as dominant active species.For CuO/CeO₂-Al₂O₃ catalysts calcined at 300℃, two CuO species exist on thesurface, i.e. highly dispersed and bulk CuO crystalline phase. With increasingcalcination temperature, part of CuO on the surface has transferred into the internallayer of CeO₂, some to form CuAl₂O₄, and the rest exist as bulk CuO. The hightemperature enhances the transfer process and improves the formation of CuAl₂O₄.After calcined at 900℃, all CuO has transferred to form CuAl₂l₄. TheCuO(33.3%)/CeO₂-Al₂O₃ catalyst show better activity than CuO(33.3%)/Al₂O₃, due togreatly improved CO oxidation activity resulting from highly dispersed CuO on thesurface of CeO₂,...