Construction Of Ceria-Based Nanocomposites And Their Structure Correlated Automobile Exhaust Catalytic Properties

With the developing of society,vehicle has been widely used as the transportation,which could induce a big environment issue.Increasing attention has been paid to automotive emission conversion,such as nitrogen oxides(NOx),carbon monoxide(CO)and hydrocarbons(HC).Cerium oxide,on the basis of its excellent properties in oxygen storage capability and rich redox chemistry,has been widely used in emission control system.Comparing with pure CeO2,ceria-based materials have excellent oxygen vacancy on the basis of the interaction of different components,which could result superior catalytic property.Moreover,the catalytic property of catalyst is related with the structure,surface species and active sites.Therefore,the purpose of this work is to correlate the catalytic property with the structure,exposed facets and surface species.(1)On the basis of a prolonged stirring process,we have successfully performed a controllable conversion between 1D and 3D structures.Based on the previous report,templates and additives have been widely used to control ceria structure.Different with that,in our work,1D and 3D structures are successfully synthesized by prolonging the stirring process.When the stirring time increased from 5 min to 90 min,the final products underwent a morphology transformation form rods to irregular flowers,finally to regular 3D flowers.TEM results indicate that 3D flowers are consist of nanorods,which emanate from one center.Systematic study confirm that the prolonged stirring process is beneficial for the mass transport of solution and the generation of the big seeds from the small seeds.Furthermore,with long enough stirring time,intimate mixing is performed to produce regular 3D flowers.Moreover,we utilized the redox property of sample to deposit Au,which resulted a morphology transformation from solid to hollow structure based on the acid etching and oxidation-reduction process.With the introducing Au and enlarged surface area,Au/CeO2 show excellent catalytic properties toward CO oxidation.(2)The exposed facets of supports show a great influence on the construction and catalytic property of multi-components.In this work,the effect of ceria facets({110} and {100} facets)in nickel-ceria catalysts on the nickel species and NO + CO catalytic property were studied.Generally,two kinds of nickel species were formed:NiO strongly or weakly interacted with the surface and Ce-O-Ni solid solution.Results demonstrated that the ceria facets have great influences on the states of nickel species.Compared with {100} facets,the active sites in {110} facets were beneficial for formation of strong interaction between ceria and nickel species to generate NiO strongly interacted with the surface and Ce-O-Ni,which leaded the generation of stable construction.The catalysts with {100} facets shown the best catalytic performance(e.g.:with 5%nickel loading,the complete conversion temperature of{100} and {100} facets were 154 ℃ and 229 ℃,respectively).Correlation with the structure,NiO strongly interacted with the surface and Ce-O-Ni species were responsible for the superior catalytic activity.(3)For ceria-based three-way catalysts,introducing palladium species can dramatically enhance the catalytic activity.The catalytic property of noble metal system is related to the interaction mode between noble metal and supports.This work is mainly focus on the structure evolution of palladium species in palladium-ceria catalysts during calcination process and subsequent catalytic process.Generally,three types of palladium species were formed:PdO interacted with ceria supports(PdOx/Pd-O-Ce),PdO nanoparticles and Pd-O-Ce solid solution.Results demonstrate that the calcination rate in calcination process show great effect on the distribution of Pd species:rapid calcination rate contributes to the generation of PdO species(PdOx/Pd-O-Ce and PdO),whereas slow calcination rate promotes the formation of Pd-O-Ce.Moreover,the subsequent catalytic process results a further evolution of Pd species:decomposition of PdOx/Pd-O-Ce with forming PdO.Correlation of the catalytic performances and analysis of palladium structures,the best catalytic activities are shown in 0.5%Pd/CeO2 catalysts with fast calcination rate,which result from the synergy effect of PdO nanoparticles and Pd-O-Ce solid solution.(4)Palladium-ceria catalysts have been widely used in automotive exhaust control and indoor air purification due to their low-temperature activity in CO oxidation.In this work,we utilized ceria with {110} facets as substrates to synthesize the palladium-ceria catalysts by deposition-precipitation method.By comparing with the catalysts treated in different atmospheres(O2 and CO),we systematically investigated the structure evolution of palladium species during the CO oxidation process.In O2-pretreatment process,three types of palladium components were formed:PdOx/Pd-O-Ce interacted with ceria supports,PdO nanoparticles on the surface and Pd-O-Ce solid solution.O2-pretreated catalysts shown low-temperature catalytic performance(e.g.:the complete conversion temperature of 1%Pd/CeO2 was 103 ℃).The following CO-treated process would cause the decomposition of PdOx/Pd-O-Ce components with the formation of PdO nanoparticles,which resulted a high conversion temperature.On the basis of the catalysts structure,the low-temperature activity toward CO oxidation is due to PdOx/Pd-O-Ce.Notably,the catalytic process did not cause any differences in catalytic performance and catalysts structure.Therefore,based on the structure evolution in O2 and CO treated process,during the catalytic process CO firstly reacted with PdOx/Pd-O-Ce components;meanwhile the abundant O2 in catalytic system would promote the regeneration of PdOx/Pd-O-Ce,which is beneficial for catalysts to keep the amount of active sites;finally the low-temperature catalytic activity is remained.