The Physical And Chemical Properties Of Ceria Surfaces:A First-principle Theory Study

Ceria and ceria based systems are widely used in the areas of solid oxide cell,automobile exhaust gas treatment,bio-sensors,and purification of hydrogen energy,with the ability of ceria to store,release oxygen being crucial to its functionality in such applications.Thus,the detailed knowledge of the location and dynamic behavior of ceria surface oxygen vacancy is critical to further improve its functionalities.However,during the past decade,the near-surface vacancy structures at Ce O₂?111?have been questioned due to the contradictory results from experiments and theoretical simulations.Whether surface vacancies agglomerate,and which is the most stable vacancy structure are being heatedly debated.The lack knowledge of the fundamental behaviors of oxygen vacancies at ceria surface hindered us to further modify the performance of ceria based materials.Here,we systematically investigated the locations and types of the near-surface oxygen vacancies at CeO₂?111?surface using density functional theory calculations and Monte Carlo simulations.We found at 600 K and 15%vacancy concentration,the numbers of single surface oxygen vacancy and single subsurface oxygen vacancy are almost the same and the single surface oxygen vacancies are the main surface oxygen vacancy type.As the increase of oxygen vacancy concentration,the surface oxygen vacancies have the tendency to form linear surface oxygen vacancy clusters and after the linear surface vacancy clusters are the triangular vacancy trimer clusters.However,at 80 K and 10%oxygen vacancy concentration,the near-surface oxygen vacancies are almost locate at the subsurface oxygen layer and form ordered structures.These results are well consistent with the experimental results and solve the decade debate about the structures of the near-surface oxygen vacancies.Based on the density functional theory calculations and ab initio moleculer dynamics simulations,we thouroughly studied the kinetic behaviors of the oxygen vacancies at CeO₂?111?surface.When the creation of one oxygen vacancy at the near-surface of ceria two electrons are left behind which are located at the 4f orbitals of cerium atoms.Through the calculations,we find at 300 K both the 4f electrons and oxygen vacancy at the near-surface of ceria are immobile.At 500 K,the 4f electrons are still immobile,however,the oxygen vacancy can exchange between the surface and subsurface sites at located area.At 700 K,both the 4f electrons and oxygen vacancy can diffuse at the near-surface of ceria.Further,we notice that the location of the 4f electrons can drastically change the energy barriers of the oxygen vacancy diffusion.The diffusion of oxygen vacancy is entanglement with the 4f electron hopping.Finally,at 900 K,the frequency of 4f electrons hopping are sharply increased while that of oxygen vacancy diffusion does not increase.These phenomena are unexpected.The ab initio molecular dynamics simulations indicate that the oxygen atoms,having more mass than the electrons,move slower than the lighter electrons.Further,we investigate the dynamic interplay between water and the reduced CeO₂?111?surface.We find water can induce the migration of oxygen vacancy from subsurface to surface site which results in the dissociation of water and the formation of surface hydroxyl.The diffusion of surface hydroxyl species at ceria surface plays crucial role on ceria supported Au and Pt nanoparticles in CO oxidation and water-gas shift reactions.We find the hopping of the 4f electron can simulate the short-range or long-range diffusion of surface hydroxyl species mediated by water molecules.These results show the critical roles of near-surface oxygen vacancy,4f electrons,and water molecules on the ceria supported transition metal nanopartical catalysts.Meanwhile,we find the structures and dynamic behavior of near-surface oxygen vacancy at CeO₂?111?surface can also influence the catalytic performance of ceria supported transition metal adatoms in water dissociation and CO oxidation reactions.In general,we can either get high stability or low activity of the catalyst or inversely.However,through our calculation we find that the dynamic exchange of oxygen vacancy between the surface and subsurface site of ceria in the reaction process can achieve both high stability and activity of the supported gold adatoms toward CO oxidation reactions.Our results provide the foundations for understanding the locations and dynamic behaviors of near-surface oxygen vancancies at ceria surface,and therefore,considering that such vacancies strongly affect the catalytic activity of ceria system,they should be useful in the design of more efficient ceria-based catalysts.