(289c) Ab Initio Studies of Ceria-Based Oxides for Direct Hydrocarbon Oxidation
Ceria (CeO2) offers unique properties as a heterogeneous catalyst or catalyst support for a number of applications, such as three-way automotive catalysts, preferential CO oxidation, and catalytic oxidation of hydrocarbons. For each of these applications, the use of ceria is motivated by its ability to store and release oxygen, or more generally to readily transition between oxidation states. Ceria exhibits activity for the oxidation of hydrocarbons, making it suitable for catalytic combustion applications and as anode electrocatalysts for direct hydrocarbon solid oxide fuel cells (SOFCs). The addition of low levels of rare-earth or noble metals alters the redox properties and hydrocarbon oxidation activity of ceria-based materials. The redox properties of ceria-based materials have been probed using experimental and theoretical methods, however the correlation between redox properties and hydrocarbon oxidation activity with changes in transition metal-ceria composition is unclear. Moreover, the surface structure of the metal-CeO2 catalyst under operating conditions, and the impact of the surface structure on hydrocarbon oxidation activity remains unresolved. We use density functional theory, with the inclusion of an on-site Coulombic interaction (DFT+U), to examine the energetics of dissociative methane adsorption and oxygen vacancy formation over ceria surfaces with the addition of Zr or Pd, and address the thermodynamic preference for Zr or Pd to substitute into the CeO2 lattice. The thermodynamic preference for Zr or Pd substitution into the ceria lattice versus segregation into separate phases (Zr or Pd supported on ceria), is evaluated by comparing the relative energies of substituted and segregated configurations. The results presented provide insight into the relationship between composition, surface reduction and catalytic activity of ceria-based materials.
The partial reduction of ceria may occur through the formation of oxygen vacancies, during which cerium atoms are formally reduced from Ce4+ to Ce3+. The formation of surface oxygen vacancies alters the coordination sites available for hydrocarbon oxidation and influences elementary reaction energetics occurring on the ceria surface. The energy of oxygen vacancy formation, and thereby reducibility, is lowered by the addition of rare-earth or noble metals to ceria. Oxygen vacancy formation energies are examined for single crystal surfaces of pure ceria and ceria with the addition of Zr or Pd. Addition of Zr or Pd to each surface is accomplished by substitution into surface Ce sites within the CeO2 lattice. For Zr-substituted surfaces, the oxygen vacancy formation energy is more exothermic with respect to pure ceria, and the formation of an oxygen vacancy reduces surface Ce atoms. For Pd-substituted surfaces, the vacancy formation energy is more exothermic than for pure ceria or Zr-substituted ceria and reduces surface Pd atoms.
The activation of C-H bonds is the rate limiting step for hydrocarbon oxidation over many metal oxides, and thus the energetics of this reaction step may provide useful insight into the relative activity of ceria surfaces. As an initial probe of the activity of ceria surfaces for hydrocarbon oxidation, the reaction energy and barrier of methane activation are determined for intact and defective single crystal ceria surfaces with the addition of Zr or Pd. We find that methane adsorption occurs by dissociative adsorption of H and CH3 fragments to available oxygen coordination sites on each ceria surface. The adsorption energy of methane is more exothermic over Zr-substituted ceria surfaces than over pure ceria, and even more exothermic over Pd-substituted surfaces. The lowest energy pathway for dissociative adsorption proceeds through H abstraction and the formation of a methyl radical, and subsequent chemisorption of the radical species. The barrier for dissociative adsorption is lower over Zr-substituted ceria than for pure ceria, and even lower for Pd-substituted ceria. Dissociative adsorption of H and CH3 reduces surface Ce atoms for both pure ceria and Zr-substituted ceria, and reduces surface Pd atoms for Pd-substituted ceria. Both oxygen vacancy formation and dissociative methane adsorption result in similar reduction of the ceria surface. We show that the energetics of methane adsorption and vacancy formation directly correlate, and thus the thermodynamics of the initial step in methane oxidation over ceria are dependent on surface reducibility.
These results provide insight into the mechanism by which metal addition alters the redox properties and catalytic activity of ceria. Under certain temperatures and oxygen partial pressures, it is thermodynamically favored for rare-earth or noble metals such as Zr or Pd to substitute into the CeO2 lattice at the surface. This substitution results in greater surface reducibility, more exothermic dissociative adsorption of CH4 over ceria, and a lower energy barrier for this reaction step. Vacancy formation and methane adsorption are both surface reducing processes on ceria, and the energetics of each directly correlate with each other. These results aid in both interpreting experimental behavior and guiding design of improved ceria-based catalysts for hydrocarbon oxidation.