The Study Of Preparation And Catalytic Activity For High-surface Area Nano-CeO₂ And Au/CeO₂

CeO₂ with fluorite-type crystal structure is widely used as a catalyst or a support because of its excellent oxygen storage and release property based on the potential redox chemistry through the shift between Ce(â…¢) and Ce(â…£) and thermal stability.Therefore, CeO₂ has attracted much attention in recent years.In this master thesis,a series of high-surface area nano-sized CeO₂ and Ce-based composite oxides were prepared using a surfactant-templated method.Meanwhile,Au/CeO₂ were also prepared using the CeO₂ as support.Their catalytic performances were tested for the CO oxidation.The CeO₂ and Ce-based composite oxides prepared with this method have high surface area,small crystallite size and good thermal stability,so as to improve their catalytic performance.The catalysts were characterized by XRD,Raman,TPR,N₂-sorption,TEM,SEM,XPS and DRIFT techniques.In this thesis,the main contents are as follows:1.The high-surface area nano-CeO₂ was prepared using Ce salt as precursor by precipitation method,taking surfactants as templating agent.The effects of the surfactants, Ce salts,addition rate of Ce salt and NaOH,aging temperature,aging time,amount of surfactant and calcination temperature on the surface area as well as on the crystallite size of nano-CeO₂ were studied.It was found that the optimal conditions were,namely, C₁₂H₂₅SO₄Na(SDS) as templating agent,CeCl₃ as precursor,CeCl₃:SDS:NaOH:H₂O=1: 2.7:4.3:1100(molar ratio),CeCl₃ addition rate = 2.5 mL·min^-1,NaOH addition rate=3 mL·min^-1,aging time = 24 h and aging temperature = 90℃.When calcination temperature was 200,400,600℃,respectively,the surface area of CeO₂ was 457,303,200 m²Â·g^-1 and the crystallite size was 2,2,3.2 nm,indicating that the good thermal stability of the resulting material.The concentration of oxygen vacancies in the CeO₂ was strongly related to the surface area of CeO₂ and the high-surface area was propitious to the formation of oxygen vacancies,which was responsible for the enhanced reactivity of CO oxidation.2.High-surface area nano-sized Ce_xZr_1-xO₂ with different ZrO₂ contents were prepared using C₁₂H₂₅SO₄Na(SDS) as templating agent,CeCl₃ and ZrOCl₂ as precursor, (CeCl₃+ZrOCl₂):SDS:NaOH:H₂O = 1:2.7:4.3:1100(molar ratio),CeCl₃+ZrOCl₂ addition rate = 2.5 mL·min^-1,NaOH addition rate = 3 mL·min^-1,aging time = 24 h and aging temperature = 90℃.The catalytic activities for CO oxidation were tested.The resulting materials were in form of Ce_xZr_1-xO₂ solid solutions,and the surface area increased as increasing ZrO₂ content and the highest surface area 433 m²Â·g^-1 was obtained on a Ce_0.2Zr_0.8O₂ calcined at 400℃.For Ce_0.5Zr_0.5O₂,when calcination temperature was 400,600,800℃,respectively,the surface area was 327,277,44 m²Â·g^-1,indicating that Zr improved the thermal stability of the conducted material.Besides,the introduction of Zr has some effect on the activity of CO oxidation.3.Au/CeO₂ catalysts used CeO₂ with different surface area as support were prepared by a deposition-precipitation method(DP) and tested for low-temperature CO oxidation. The surface property of CeO₂ played an important role in the reaction and the Au species provide sites for CO chemisorption.The reactivity was improved along with increasing surface area of CeO₂.The enhanced reactivity obtained on the Au/CeO₂ catalyst supported on high-surface area CeO₂ could be attributed to two facts:one was that Au particles formed on the high surface area CeO₂ may have a stronger interaction with the support, which could result in a higher degree of CO adsorption on Au particles.The other was the high surface area CeO₂ provided more oxygen vacancies by the formation of Au_xCe_1-xO₂ solid solution,on which oxygen molecules could be transformed to active species(such as superoxide species) for the reaction.