Preparation And Catalytic Properties Of CeO₂-based Nanocomposite Materials

With the acceleration of modernization and economic globalization,the environmental pollution such as air and water pollution has become a major obstacle to the sustainable development of modern society.In recent years,development of nanotechnology makes nanomaterials become more and more important in reduction of environment pollution,such as in desulfurization and denitration of fossil fuels,in degradation of organic dyes in wastewater and in exploration of new clean energy.One kind of nanomaterial,nano-metal oxides,is becoming popular in many modern industries because of its wide application in catalysts,new energy batteries,sensors,semiconductors and so on.China has become the biggest producer of rare eraths with more than 90%of the global production over a decade.Cerium（Ce）is one of the most abundant rare earth elements in the earth’s crust and its content is as high as 0.0046 wt%.As the most stable oxide of Ce,Ceria（CeO₂）possesses many unique physical and chemical properties,such as high oxygen hole concentration,fast oxygen migration rate and stable metal support.CeO₂ not only plays an important role in three ways catalyst and fuel cells,it has also been used as a new catalyst in some emerging reactions,such as photo-catalysis and water decomposition,which indicates CeO₂ will bring more breakthroughs to the development of catalyst market in the future.Based on the above analysis,this paper is focus on preparation of several Ce02-based nanocomposites and characterization of their catalytic properties in desulfurization of diesel,electrochemical catalyzed hydrogen evolution and decolorization of methyl blue（MB）.The main contents and results are as follows:In the third chapter,the process conditions for electrodeposition of CeO₂ onto AAO templates have been acquired by analyzing cyclic voltammetry（CV）curves and surface morphology of the deposition.The final electrodeposition parameters are as follows:the pretreatment of AAO is being vacuumed for 4 h,the current density for deposition is 0.1 mA/cm2,the electrolyte for deposition is 0.1 M of Ce（NO3）3·6H2O and 0.2 M of CH3COONH4 and the time for deposition is 4 h.The coaxial nanotubes（NTs）arrays of AAO-CeO₂ NTs have been successfully galvanostatically anodic deposited under the above conditions and the characterization results show that the fabrication process of AAO-CeO₂ NTs is accompanied with the formation of slight Ce2O3 and a new phase namely Al3Ce.In the fourth chapter,the new prepared AAO-CeO₂ NTs are adopted as the catalytic anode to remove the heterocyclic sulfurs（BT,DBT and DMDBT）in diesels,and the influences of the main operation conditions on the desulfurization efficiency are also investigated.The results show that electrochemical oxidation desulfurization can be an effective way to remove sulfur from diesel and the main oxidation products of diesel on the prepared AAO-CeO₂ NTs arrays are the soluble sulfates（SO42-）,especially Ce2（SO4）3.Moreover,the sulfur in BT is the easiest to be removed among the three typical sulfurs in diesels and which removal efficiency reaches 87.2%,and a possible electrochemical oxidation desulfurization pathway for diesel has also been proposed based on oxidation products and phase changes of catalyst AAO-CeO₂ NTs.Then,AAO-CeO₂ NTs are further used as anode catalyst to remove sulfur from the model diesel composed of 1000 ppm BT,600 ppm DBT and 300 ppm DMDBT.The results show that BT can be removed by two main routes,ie.,be oxidized to BTO₂ and to SO42-,while for DBT and DMDBT the main removal way is only to be oxidized to SO42-.The desulfurization efficiency of 1000 ppm BT,600 ppm DBT and 300 ppm DMDBT as individuals are 98.07,96.82 and 92.65%which are much higher than their respective desulfurization efficiency in model diesel.At last,density functional theory（DFT）calculations and CV test are used to study the electrochemical oxidation desulfurization behavior of the three sulfides respectively,it is found that there exist a critical point for sulfur concentration,when the concentration is lower than the point,the desulfurization efficiency increases with increasing the sulfur concentration,when the concentration is higher than the point,the desulfurization efficiency shows an opposite trend.In the fifth chapter,Ni NTs and CeO₂-Ni NTs have been prepared by galvanostatic electrodeposition in AAO templates.Their morphologies and catalytic properties as cathode for hydrogen evolution reaction（HER）have been tested.The results show that the preparation process of CeO₂-Ni NTs was accompanied by the formation of many new phases CeNix（x=1,2,3.5 or 5）and preferential orientation crystal face of Ni in CeO₂-Ni NTs is<111>,which is different from that Ni<200>in Ni NTs.Linear scan voltammetry（LSV）test of the two electrodes found that both of the two electrodes in 1 M NaCl aqueous solution（aq.）exhibited better catalytic property for HER than that of Ni films in 1 M NaOH（aq.）.Based on the kinetics analyses of HER process,it was deduced that Tafel slop b was mainly dependent on phase composition and electric conductivity of the electrode,while jo was mainly dependent on its real specific surface area.In the sixth chapter,CeO₂ nanorods（NRs）with different amounts（wt%）of reduced graphene oxide（RGO）have been prepared by a simple hydrothermal process in alkaline environment.The decolorization tests show that decolorization of MB is mostly attributed to its adsorption over the catalyst CeO₂ NRs-RGO,but the visible light irradiation also plays a vital role in increasing the decolorization efficiency.Doping of RGO into CeO₂ NRs not only changes the surface morphology and area,the particle size,the band gap,the crystal structure and the oxygen vacancy content of the latter,but also causes the mutual charge transfer between RGO and CeO₂ NRs,which therefore increases the decolorization efficiency of CeO₂ NRs-RGO（9.2 wt%）to be as high as 95.4%.Based on the influence of the electron scavenger H2O₂ and holes trapping agent CH3CH2OH on MB decolorization efficiency,a possible complete decolorization pathway of MB over Ce02 NRs-RGO（9.2 wt%）catalyst has been proposed.