First-principles Study On Stability And Oxygen Storage Capacity Of Cerium-zirconium Oxide Solid Solution

With the development of automobile industry,exhaust emission has become an important part of air pollution in China since 2009.The three-way catalyst as the core of the automobile exhaust converter is currently widely used in the automobile industry.Cerium oxides play an important role in the automobile exhaust purification due to its quick REDOX cycle and good oxygen storage capacity.However,pure cerium dioxide has poor oxygen storage capacity at low temperature and unideal thermal stability at working temperature.Thus,zirconia doping is usually adopted to improve ceria-based materials thermal stability and oxygen storage capacity.The addition of zirconium greatly improved the oxygen storage performance of pure cerium oxide at low temperature but also caused new problems.In experiments,it has been observed that the specific surface area and the oxygen storage capacity of the cerium zirconium catalytic material significantly decrease in the temperature range of 600～900℃.Therefore,it is urgent to improve the heterogeneity and stability of ceria-zirconia solid solution while maintaining its excellent catalytic performance.The new method to retard Zr segregation by co-doping,the description and quantification of ceria-zirconia thermal stability and segregation degree and the mechanism of cerium valence changes are studied by first-principles calculation and first-principles molecular dynamic simulations.The main conclusions are as follows:(1)A new way to retard the zirconium segregation in ceria-zirconia solid solutions is proposed.Adding a third metal(M)into the system is likely to retard this behavior if M and Zr attract each other and form atomic pairs to make the diffusion process harder.The formation energy of single-doping and co-doping systems,the association energy of M-Zr couples and the formation energy of oxygen vacancy are calculated by first-principles calculation.By comparing the formation energy of single-doping and co-doping system,it is found that the formation energies of all co-doping systems are lower than that of single doping systems.The lower the formation energy is,the easier the element is to be doped into the system,and the higher the doping concentration can be realized in actual experiments.According to the calculation results of the association energies of M-Zr couples it is found that the doped elements M(M=Cr,Al,Mn,Co,Mo,Pd,Tc,Rh and Cd)and Zr present a state of mutual attraction.The more negative association energy is,the stronger attraction between M and Zr atoms,which can help retard the diffusion and segregation of Zr atoms.Co-doping systems of M(M=Al,Cr,Mn,Co,Tc,Ru,Rh,Pd,Cd and Gd)can significantly lower the formation energy of oxygen vacancy.This means the co-doping systems can improve the oxygen storage capacity.Combined with the association energy and the oxygen vacancy formation energy,we conclude that M(M=Cr,Al,Mn,Co,Pd,Tc and Rh)can be selected as third doping metals to promote the homogeneity and OSC of the Zr-doped CeO2 materials.(2)In order to directly and quantitatively describe the dynamic process of Zr segregation,the vacuum slab structures on different surfaces were established and a series of first-principles molecular dynamics simulations were carried out at different temperatures.Two parameters,i.e.thermal displacement rate and phase segregation degree,were proposed to characterize the surface structure changes of ceria-zirconia solid solution.On the basis of molecular dynamics simulations,the thermal displacement rate of atoms in all slab structures was calculated.It was found that the closer the atom was to the vacuum layer,the greater the atomic thermal displacement rate was.The results meant that the surface layer structure would collapse at high temperature,resulting in the reduction of specific surface area.The thermal displacement rate and phase segregation degree increase sharply at 1 100 K.According to the thermal displacement rate and phase segregation degree results,we predict the temperature at which the solid solution specific surface area dramatically decreases to be～1 100 K.The theoretical temperature is in good agreement with previous experimental observations.This also indicates that the two parameters we defined can be reasonably used to describe the stability of ceria-zirconia solid solution systems and the surfaces.(3)A mixed valence state structure model of cerium atom was established.Taking the formation energy of neighbor oxygen vacancy as a probe,the mechanism of cerium valence changes was investigated.The calculation results show that the formation of oxygen vacancy will indeed cause the changes of the valence state of cerium atoms.But the formation of one oxygen vacancy cannot produce two trivalent cerium ions.The valence change of cerium atom is realized by adjusting the ratio of ionic bond to covalent bond.