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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Density functional theory (DFT) and ab initio thermodynamics are used to calculate the free energies of H2S adsorption and dissociation on CeO2(111) and CeO2(111) doped with La or Tb. Experimental sulfur capacities are reported for La- and Tb-doped CeO2 adsorbents for comparison with computed energetics. The DFT-based free energies of H2S adsorption, dissociation, and oxygen vacancy formation are evaluated at the operating conditions for the high temperature desulfurization of biomass gasifier effluents. DFT results indicate that the sulfur adsorption process occurs via H2S adsorption and dissociation over substoichiometric oxygen vacancies, and is rate limited by a strongly endergonic molecular adsorption of H2S. Sulfur incorporation is only favorable if multiple adjacent oxygen vacancies are present to provide the flexibility required to accommodate the larger coordination shell of sulfur atoms. The thermodynamics of the overall H2S adsorption process do not correlate with oxygen vacancy formation energy, implying that the optimization of ceria-based sulfur sorbents cannot be achieved by tailoring composition to maximize the exergonicity of vacancy formation. The rate-limiting step for H2S adsorption and dissociation involves reaching the transition state for dissociation of the first S–H bond to form SH* + H* (>2.23 eV over each surface), and this rate is highest over ceria-lanthana (>ceria-terbia > ceria). This is the same order as the experimental sulfur capacities, if these capacities are compared on a similar molar (Ce/La vs Ce/Tb) basis. Agreement with the experimental capacities suggests that the actual sulfur capacities of ceria-based mixed oxides are largely determined by the kinetics of the H2S dissociation.

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The surface properties of YSZ (111) have been investigated by X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM), temperature programmed desorption (TPD) of adsorbed formate, and computational studies using the ReaxFF reactive force field approach. XPS and computer simulations showed enrichment of the surface with yttria. STM studies indicated that a high density of step edges are readily formed with ∼35% of the surface sites located at steps. Step edges are identified as the primary adsorption sites for formate. The formate oxidizes in a dehydration reaction producing carbon monoxide and water at ∼600 K. This is contrasted to the reaction of formate on pure zirconia where formate reacts by both dehydration and dehydrogenation reactions. This shift in the selectivity between pure zirconia and yttria-doped zirconia is attributed to the modification of the active step edge sites by yttria segregation. Therefore, the modification of active sites by minority species in a mixed oxide can control the chemical surface functionality.