First-Principles Studies On CeO₂ -Based Catalysts

CeO₂ has the superior ability for storing/releasing oxygen (OSC) and excellent catalytic properties, which have been extensively investigated. In the present work, the catalytic activity and the related mechanisms of the CeO₂ based catalysts are studied using the DFT based method with slab super-cell models. The DFT+U method is used to describe the strong correlation of the Ce 4f electrons. Here, two major sytems studied in this thesis will be discussed.1. The study on the possible active oxygen species for the oxidation reactions on ceria, with C, C-dimer (C₂) and CO served as the probe molecules. It is found that there exist three types of active oxygen species for the oxidation reactions involving (i) the lattice oxygen; (ii) peroxide or superoxide species derived from the adsorption of O₂ on pre-reduced CeO₂; (iii) the activated lattice oxygen induced by the chemisorption of O₂ at the oxygen vacancy. Among of these, peroxide or superoxide is the most active species because the chemisorbed O₂ on vacancy carries two extra electrons which weaken the O-O bond of the adsorbed O₂ and result in the surface adsorbed oxygen atom ready to participate in the oxidation reactions. The probe molecules can be oxidized completely (C atom and CO) or partially (C₂) by these active oxygen species.2. The study on the deposition of the Cu clusters (Cux, x=1-4) on ceria and the H₂O dissociation on the Cu4/CeO₂(111) system. It is found that the copper clusters tend to bind with the surface oxygen atoms on the CeO₂(111) surface. The transition from the 2D planar structure to the 3D particles occurs with the barrier of 1.05 eV for the adsorbed Cu4 cluster, and the route with one Cu atom hopping from an interface site directly onto the top of the Cu triangle site is favorable. There exist two factors determining the morphology of Cu clusters on CeO₂: (1) the opportunity for the formation of Cu-O bonds at the interface and (2) the number and strength of Cu-Cu bonds in the Cu clusters. Charge transfer from Cu-3d and 4s to Ce-4f results in positively charged Cu_x clusters, which has distinct catalytic performance as compared to the unsupported Cu₄ clusters for the H₂O dissociation.The physisorbed, chemisorbed and nearly dissociated H₂O on the Cu4/CeO₂ are obtained, and the two most likely dissociation routes for H₂O molecule are considered, where the barriers are only 0.19 and 0.31 eV on the Cu_4-p/CeO₂(111) and Cu_4-t/CeO₂(111) surfaces, respectively. The highly catalytic activity of the Cu4/CeO₂ catalyst originates from the enhanced electrostatic interaction between the positively charged Cu sites and the polar H₂O molecule. In comparison with the traditional unsupported Cu-based catalyst, we conclude that the CeO₂ support not only activates the Cu sites as a buffer to accept/release electrons but also participates in the H₂O dissociation reaction at the Cu/O interface. The Cu/O interface sites on the Cu4-t/CeO₂(111) surface are identified to be the active centers for the H₂O dissociation.These results may lead to a better understanding for CeO₂ based catalysts and give clues for seeking the highly active catalysts.