A Density Functional Theory Study Of The Structures And Raman Spectra Of Ceria-based Catalysts

CeO₂ with a fluorite structure has a high oxygen storage/release capacity and high ionic conductivity due to the conversion between Ce4+ and Ce³⁺,and has been widely used in heterogeneous catalysis.However,the catalytic performance of pure CeO₂ is relatively low and its thermodynamic stability is poor.It is reported that the doped and supported ceria with other metal atoms can effectively improve these properties.Therefore,it is of great significance to study the properties of the doped and supported ceria catalysts for the development of high performance ceria-based catalysts.In this thesis,we systematically investigate the structure and distribution of Ce and Zr atoms in model oxide of ZrxCei6-xO32,and the effects of different surface structures on metal-support interactions,using Ag/CeO₂ as a case,as well as the Raman spectra of ceria-based catalysts.By using the artificial intelligence program,site-occupancy disorder（SOD）,all possible configurations of different crystalline phases of ZrxCe-6-xO32 with the Ce/Zr molar ratios of 7/1,3/1,5/3,1/1,3/5,1/3 and 1/7 are generated.Then,all the configurations are geometrically optimized by using the VASP package based on the first principles.The theoretical results indicate that the distribution of Ce and Zr atoms in cubic and tetragonal phase of ZrxCe16-xO32 tend to be evenly distributed in the solid solution oxide.When the Ce/Zr ratio is larger than 1/1,ZrxCe-6-xO32 prefers to exist in cubic phase;while it might exist in the tetragonal phase when the ratio decreases to 3/5,and the two phase might coexist simultaneously if the ratio betweenthe two values above.When the Ce/Zr ratio decreases to 1/7,ZrxCe16-xO32 tends to exist in the monoclinic phase.The adsorption behaviors of Ag on CeO₂（111）,CeO₂（110）,CeO₂（100）and CeO₂（111）step surface are systematically investigated by density functional theory calculations.It is indicated that Ag atom prefers to locate at the hollow sites on CeO₂（111）and CeO₂（110）surfaces,and the corresponding adsorption energies are-1.44 eV and-1.63 eV,respectively.For CeO₂（100）surface,Ag prefers to adsorb on the O-bridge-X site with the adsorption energy of-2.46 eV.While Ag atom prefers to locate at the step site which the coordination numbers of oxygen atoms are two and three on CeO₂（111）step surface.Its adsorption energy is-2.37 eV.The reasons for the differences of adsorption structures and energies of Ag are the different geometric and electronic structures of different terminal surfaces of ceria,including the different coordination numbers of oxygen atoms,the type and arrangement of surface atoms,the partial charge on ceria surfaces near Fermi level and the bond angle of Ag-O-Ce.We propose a normalizing method of the bond number of Ag-O.Hence,the adsorption energy of Ag atom on ceria can be transformed into the interaction energy per Ag-O bond.Another important parameter,the difference between the Ag-O-Ce bond angle and the bulk Ce-O-Ce bond angle,is also firstly proposed as a measure of the Ag-O-Ce bond angle deviating from the sp3 orbital hybrization of O atom.Based on the analysis of this parameter,we find out the quantitative relationship between the interaction energy of a single Ag-O bond and the deviation degreeof the Ag-O-Ce bond angel from the the sp3 hybrid orbital angle of O atom.It is reported that oxygen vacancy plays the key role in ceria-based catalysts.Generally,the Raman peak at 570 cm-1 of the ceria-based catalyst with metal ions doped into ceria support,such as Pr,La,Ga,Y,Sm and Lu,etc.,is attributed to the contribution of oxygen vacancy.Further experimental results find that it can be divided into two peaks,550 and 600 cm-1.It is deduced that the peak at 550 cm-1 might be produced by the oxygen vacancy;while the latter one is contributed by the MO8-type complex,in which M is the doped metal ion.Since the Raman spectroscopy is derived from the spatial derivatives of the macroscopic polarization for the Raman active phonon modes of vibration and rotation,is the Raman band at 550 cm-1 really produced by O vacancy in ceria?Similar doubt exists for the Raman band at 600 cm-1.Due to the limitation of present experimental characterization techniques,these very important scientific issues are still suspended for a long time.In this thesis,the Raman spectroscopy of ceria with oxygen defects is calculated by the finite displacement algorithm using density functional theory（DFT）.Our results unambiguous prove that the peak of 560 cm-1 is attributed to the M³⁺-O-Ce4+ vibration mode due to the existence of oxygen vacancy;while the peak at about 610 cm-1 might be due to the vibration of Mn+-O-Ce4+ mode without oxygen vacancy.The peak position in Raman spectroscopy is dependent on the radius and charge of doped Mn+ ions.