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.

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Palladium supported on ceria is an effective catalytic material for three-way automotive catalysis, catalytic combustion, and solid-oxide fuel cell (SOFC) anodes. The morphology, oxidation state, and particle size of Pd on ceria affect catalytic activity and are a function of experimental conditions. This work utilizes ab initio thermodynamics using density functional theory (DFT) (DFT+U) methods to evaluate the stability of Pd atoms, PdO(x) species, and small Pd particles in varying configurations on CeO(2) (111), (110), and (100) single crystal surfaces. Over specific oxygen partial pressure and temperature ranges, palladium incorporation to form a mixed surface oxide is thermodynamically favorable versus other single Pd atom states on each ceria surface. For example, Pd atoms may incorporate into Ce fluorite lattice positions in a Pd(4+) oxidation state on the CeO(2) (111) surface. The ceria support shifts the transition between formal Pd oxidation states (Pd(0), Pd(2+), Pd(4+)) relative to bulk palladium and stabilizes certain oxidized palladium species on each surface. We show that temperature, oxygen pressure, and cell potential in a SOFC can influence the stable states of palladium supported on ceria surfaces, providing insight into structural stability during catalytic operation.

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