A Density Functional Theory Study Of The Structures And Catalysis Of Metal Supported Ceria-Based Catalysts

Ceria has an excellent oxygen storage capacity(OSC)due to the reversible transformation between Ce4+ and Ce3+ ions,and has been widely applied into heterogeneous catalysts and solid oxide fuel cell.However,pure ceria has the low catalytic performance and poor thermodynamic stability.Experimental and theoretical results show that its catalytic performance is greatly improved by doping metal ions into ceria or metal nanoparticles supported on CeO2.To theoretically investigate of the structures and catalytic roles of metal and oxide nanoparticles supported on ceria is of great significance.It will provide the important theoretical information for developing the ceria-based catalysts with high efficiency at low temperature.In this thesis,the structures and catalytic roles of RhOx/CeO2(100)catalysts are systematically studied by density functional theory method.Meanwhile,the effects of Ti modification and temperature on the structures of ceria-based catalysts are also investigated.RhOx nanoparticles supported on ceria catalysts(RhOx/CeO2)have been widely used in the purification of automobile exhaust.It is very important to study their structures and catalytic roles.In this paper,the adsorption and reaction behaviors of CO on Rh/CeO2(100)and Rh2O3/CeO2(100)catalysts have been studied by using density functional theory method.It is indicated that the catalytic oxidation of CO by Rh/CeO2(100)catalyst follows the Eley-Rideal-mechanism.According to the adsorption and reaction behaviors of CO at the different active sites on Rh2O3/CeO2(100)catalyst,it is found that the CO oxidative activity of oxygen species at the interface between Rh2O3 and ceria is higher than those on the individual Rh2O3 and ceria;and the activity of the two coordinated oxygen species is higher than that of the three coordinated one.Based on the analysis of the reaction of surface oxygen species and O2 molecule,the reaction recycling on catalyst is realized.When the surface of Rh/CeO2 catalyst is oxidized,the CO oxidation activation energy decreases at first and then increases with the coverage of surface O.This indicates that the Rh/CeO2 catalyst with the moderate oxidation has the higher activity for CO oxidation,which is consistent with the experimental results.Using the theoretical calculation,the active site and reaction mechanism of the CO oxidation on Rh/CeO2 and Rh2O3/CeO2 catalysts are well explained,which provides the important information for the development of catalysts with high efficiency at low temperature.Agglomeration of metal nanoparticles is one of the most important reasons for the deactivation of ceria supported metal nanoparticles(M/CeO2)catalysts.How to improve the stability of metal nanoparticles is the hot topic in catalysis science.In order to design and develop M/CeO2 catalysts with high performance and sintering resistance,this paper propose two ways to enhance the interactions between metal nanoparticles and ceria support,as well as the stability of the catalysts.By introduce Ti atom on the surface of CeO2(1 11)carrier to form monatomically dispersed Ti,TiOx or TiO2-like species on ceria.Metal atoms interact strongly with TiOx and TiO2-like species to improve their stability on support.Density functional theory calculations show that the adsorption energy of Au on the surface of Ti modified CeO2(1 11)supports is between-2.64-3.42 eV,which is much higher than that on CeO2(1 11)surface(-1.04 eV).The charge density difference and spin density show that the strong interactions are attributed to the electron transfer from the Ti modified ceria substrate to Au.The surface charge density analysis shows that the charge protuberance of surface O atoms near Ti atom results in the strong interactions between metal and ceria due to the doping of Ti atoms.In view of carrier modification,the adsorption and electronic properties of Au on Ti modified ceria are systematically investigated.According to the effects of the interaction between metal-metal and metal-support on the catalyst structure,it provides new ideas and theoretical information for the development of catalyst with high activity and stability.The temperature under the treatment and reaction conditions plays an important role in the structure of catalyst.Investigating the qualitative and quantitative relationship of the temperature effects on the catalyst structures is of great significance in theory and experiment.In this paper,the molecular dynamics method based on the first principle is used to simulate the molecular dynamics behavior of the carrier and Agn nanoparticles on the surface of CeO2(111)carrier at 298-1173 K.The dynamic structures of Agn nanoparticles on the stoichiometric and reduced CeO2(1 11)are systematically investigated.The effects of temperature on the structures of Agn/CeO2(111)and Agn/CeO2-x(111)catalysts are obtained.It is indicated that the oxygen vacancy on CeO2-x(1 11)migrates between the surface and subsurface with the increase of temperature;and the Ag atoms migrate freely on Ce02(1 11).On the reduced and contain step surface CeO2(111),the Ag atoms nucleate on the surface oxygen vacancies and step edges,and the vacancies are no longer migrated with temperature.When two Ag atoms are dispersed on CeO2(111),the aggregation occurs at 873 K due to their surface migration;the migration rate of Ag atoms is relatively low at the reduced CeO2(111)surface and the agglomeration occurs at 1173 K,while the agglomeration of Ag nanoparticles did not occur on CeO2(1 11)with stepped surfaces.Therefore,Ag atoms first nucleate at the surface oxygen vacancies and step edges,its migration rate is low.That is,the Agn/CeO2-x(1 11)catalyst has a higher stability,which is consistent with the experimental results.It also provides the theoretical information for the design of ceria-based catalysts with high stability.