First-principles Studies On The Interaction Between Water Molecule And Ceria Based Catalysts

The facile valence exchange ( Ce 4 + ? Ce³⁺) of Cerium makes ceria a good catalyst with high oxygen storage/release capacity and good redox properities. Therefore, ceria based catalysts have been extensively used for fuel reforming (e.g., water gas shift reaction, CO preferential oxidation, the synthesis and decomposition of methane, methanol, etc.), automotive exhaust cleaning, fuel cell reactions and glass polishing. In these processes, water is present as a reactant, or product/spectator, therefore, it is very important to know the role that water molecule plays in the reactions. For rational design better catalysts, it is essential to investigate the interaction between water molecule and ceria or ceria based material at atomic and electronic level.The interaction of water molecule and a ceria(111) surface was systematically investigated using the DFT+U method (density functional theory with the inclusion of on site Coulomb interaction by introducing Hubbard U parameter) and supercell models. The results show that water molecules do not decompose and are adsorbed on the oxidized ceria(111) surface through a single H bond configuration. On a reduced ceria(111) surface, the water molecules may be adsorbed through a non-H-bond configuration. Furthermore, water molecules may also dissociate and form a more stable hydroxyl surface. The hydroxyl surface is much more stable than the physisorption state of H2 on the oxidized ceria(111) surface. In other words, reoxidation of the reduced ceria(111) surface through the dissociation of the hydroxyl surface and the generation of H2 molecules is an endothermic process. Therefore, there are mainly two adsorption states for water molecules on the reduced ceria(111) surface: i) chemisorption through a non-H-bond configuration and ii) dissociative adsorption with a hydroxyl surface. The hydroxyl surface may dissociate under certain conditions and reoxidize the reduced ceria(111) surface.Using the ab initio atomistic thermodynamic method which combines the DFT+U result and thermodynamics data, phase diagrams for oxidized and reduced ceria(111) surfaces in equilibrium with water vapor in the complete range of experimentally accessible gas phase condition are calculated. Our phase diagrams agree well with experiment and recent theortical work. Especially, it is shown that the reaction is preferable at lower temperature using water molecule to reoxidize the light reduced ceria(111). Zr doping can improve the thermal stability and oxygen activity. However, as a good oxidation catalyst, a doped oxide should attain a balance between two ambivalent requirements: It must make surface oxygen reactive but not so much that it will deter the healing of the oxygen vacancies created by the oxidation reaction. Therefore, the interaction of water molecule and Zr doped ceria(111) was investigated by using the DFT+U method. For the oxidized Zr doped ceria(111) surface, due to Zr doping improves the oxygen activity, water molecule can be adsorbed not only through one-H-bond configuration with its O* bonding to a surface Ce ion, but also through hydroxyls configuration with its O* bonding to a surface Zr ion, at variance with that on pure ceria(111), where only one-H-bond configuration was found. On the first kind of Zr doped reduced ceria(111), water molecule prefer to be dissociated at the Ce site with the forming of O*-Ce bond, while it tends to be adsorbed through one-H-bond configuration at the Zr site with the forming of O*-Zr bonds. On the second kind of reduced Zr doped ceria(111), water molecules have intricate behaviors, which depends on the initial structures and adsorption sites. However, it was noteworthy that Oas may involve in the adsorption reactions, which is different from that on the pure reduced ceria(111) surface.Additionally, because the affinity of Ce ion is stronger than that of Zr ions, the electronic interaction of water and Zr doped ceria(111) surface is mainly from O*-Ce and Hb-O. When water molecule dissociate, although O* fill the vacancy site, the surface haven't been reoxidized.