Electron Localization On CeO₂Surfaces And Their Effect On CO Catalytic Oxidation: A First-principle Investigation

Using first-principle calculations based on density functional theory, we have investigatedthat effect of oxygen vacancy formation, metal atoms M（M=Cu, Ag, Au） adsorption and COadsorption on the atomic and electronic structures of CeO₂surfaces.Atomic and electronic structures of CeO₂（111）,（110） and （100） upon the oxygen vacancyformation have been studied in detail and compared with the available experimental results.（1）Stability of oxygen vacancy formation depends on the different surface: for the CeO₂（110） and（100） surfaces, the surface oxygen vacancy is energetically more favorable than the subsurfaceone; The opposite relationship is true for the CeO₂（110） surface; CeO₂（110） exhibits the lowestoxygen vacancy formation energy, which is followed by CeO₂（100） and CeO₂（111） surfaces.（2）Redistribution of two electrons left due to the formation of oxygen vacancy depends on thedifferent surface: For the CeO₂（111） and （110） surfaces, two excess electrons prefer to localize attwo Ce ions which are the next nearest-neighbor to the oxygen vacancy; For the CeO₂（100）surface, the most stable structure is the one with two excess electrons localized at the first andsecond nearest Ce ions from the oxygen vacancy.We have investigated the adsorption behavior of metal atoms M（M=Cu, Ag, Au） on theCeO₂（100） surface. The adsorption at the oxygen bridge （Ou-Ou） site exhibits the largestadsorption energy for each species and the sequence among them is: Cu>Au>Ag. The adsorptioncauses M atom to be oxidized to M+ions while the left electron prefers to localize at the Ce ionwhich is the nearest-neighbor to the M atom. The adsorption of M on the CeO₂（100） isenergetically more favorable than those on the CeO₂（111） and CeO₂（111） surfaces.We have studied the interaction mechanism between CO and the CeO₂surface. Our majorfinding is that the interaction is strongly surface dependent, consistent with experiment. Uponinteraction of CO with the （111） surface, weak bonding is found with little perturbation to thesurface or the molecule. For the （110） and （100） surfaces, the most stable adsorbate is that in which the CO molecule bridges two oxygen atoms and pulls these atoms out of their lattice sites,with formation of a CO₃^2-species. The interaction between CO molecule and CeO_2-x（111）p（2×2）surface with an oxygen vacancy is weak and the adsorption energy is smaller than0.5eV. WhenCO molecule is adsorbed on CeO_2-x（111）p（2×2） surface with two oxygen vacancies, C atomprefers to interact with the surface. CO bond may break in the role of oxygen vacancies, therebystrengthening the interaction between CO molecules and CeO_2-x（111）.