Methane Activation and Oxygen Vacancy Formation over CeO2 and Zr, Pd Substituted CeO2 Surfaces

The catalytic properties of CeO2 for hydrocarbon oxidation reactions have implications for a variety of applications. Surface reduction and methane activation are key processes in the overall oxidation reaction, and are examined herein for pure CeO2 (111), (110) and (100) surfaces and surfaces with Zr or Pd substituted in Ce lattice positions. Density functional theory, with the inclusion of an on-site Coulombic interaction (DFT+U), was used to calculate the energetics of oxygen vacancy formation and methane activation. Oxygen vacancy formation is more exothermic for Zr and Pd substituted ceria surfaces than for pure ceria, with Pd-substituted surfaces being significantly more reducible. Methane adsorbs dissociatively as *H and *CH3, and the thermodynamics of dissociative methane adsorption correlate with those of oxygen vacancy formation over the pure and substituted surfaces. The lowest energy pathway for dissociative adsorption on the (111) surface proceeds through H abstraction and the formation of a methyl radical, and subsequent chemisorption of the radical species. Sensitivity of the reported results to the choice of U value within the DFT+U method is discussed.