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

Abstract

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.

Pt–MoOx-carbon nanotube redox couple based electrocatalyst as a potential partner with polybenzimidazole membrane for high temperature Polymer Electrolyte Membrane Fuel Cell applications

“…This can help the system to retain steady performance during oxygen rich-lean fluctuations and transitions [14][15][16][17]. Such types of redox couple based catalysts have been successfully used in cleaning automotive exhaust and in eliminating coke deposition during reforming of hydrocarbons [18][19][20]. In both these cases, the redox couple assisted oxygen transfer helps the systems to perform better in their respective reaction environments.…”

Oxygen Defects at Reducible Oxide Surfaces: The Example of Ceria and Vanadia

“…Ce 2 O 3 þ 1 2 O 2 ) energies [43,57,[69][70][71][72]. Also, the excess charge localization and the defect formation energy as a function of U in bulk CeO 2-x [71,72], and reduced surfaces [58,60], have been addressed. Suggested U values lie in the range of 2.0-8.0 eV, depending on underlying exchange-correlation functional (LDA or GGA), the method used to compute the U [57,67] or the experimental properties being matched; a recent study even recommended 0.2 eV with GGA for the energy of CeO 2 to Ce 2 O 3 reduction [73].…”

“…Several studies have applied the (periodic) DFT+U methodology to oxygen defects at CeO 2 (111) [9,45,47,57,[59][60][61][62][63][64][66][67][68], while a minority applied hybrid DFT approaches [9,45,46]. All of these calculations satisfactorily described the localization of two electrons per O vacancy driving the Ce 4+ →Ce 3+ reduction, and the appearence of defect states lying inside the O 2p -Ce 5d ceria band gap.…”

“…Isolated near-surface oxygen defects at the CeO 2 (111) surface have been modeled using either the periodic [9,45,47,57,[59][60][61][62][63][64][66][67][68] or cluster approach [49] (cf. Sect.…”

“…Experimental values of the heat of reduction per oxygen vacancy in bulk CeO 2 determined by termogravimetry, electrical conductivity and calorimetry, are in the 4.12-4.67 eV range [118], and those derived by coulometric titration between 3.89-4.15 eV [4]. A number of theoretical works have considered the formation of oxygen defects in the bulk using DFT (GGA)+U [9,57,60,65,67,71,72,74,75,113,114,119,120] and HSE06 [9]. There is agreement that the bulk defect is more difficult to create than the surface one [9,79].…”