A DFT + U computational study on stoichiometric and oxygen deficient M–CeO2 systems (M = Pd1, Rh1, Rh10, Pd10 and Rh4Pd6)

Abstract

Molecular and dissociative adsorption processes of ethanol on stoichiometric and O-defected CeO₂(111) surfaces alone as well as in the presence of one metal atom (Pd or Rh) are studied using spin-polarized density functional theory (DFT) with the GGA + U method (U_eff = 5.0 eV). Dissociative adsorption (onto ethoxides) is slightly more stable than molecular adsorption onto stoichiometric CeO₂(111). The creation of surface oxygen vacancies further stabilizes both modes. In the case of ethoxide adsorbed onto a Ce³⁺ cation, adjacent to the oxygen vacancy, charge transfer to a nearest Ce⁴⁺ cation occurs. In addition, the interactions of Pd₁ (or Rh₁), Pd₁₀ (or Rh₁₀) as well as of a bimetal cluster (Rh₄Pd₆) with perfect and O-defected CeO₂(111) surfaces have been studied. From spin density calculations, it was found that the addition of metal changes the oxidation state of Ce⁴⁺ cations. The magnetic moment at the neighboring Rh or Pd induces a charge transfer to Ce⁴⁺ cations (i.e. Ce⁴⁺ (4f⁰) that becomes Ce³⁺ (4f¹)) and consequently M is oxidized to Pd^δ+ (or Rh^δ+). Similar to the atomic metal adsorption, Rh₁₀ has a stronger adsorption energy on the perfect surface than Pd₁₀ (E_ads = −6.49 and −5.75 eV, respectively), while that of Rh₄Pd₆ was in between (E_ads = −6.00 eV). The effect of one metal atom on the adsorption of ethanol was also studied. The presence of the metal further stabilized the adsorption energy of ethanol/ethoxide in its bridging configuration. The creation of an oxygen vacancy nearest the metal resulted in considerable stabilization of ethoxides (E_ads = −1.67 eV in the case of Pd) compared to those found on the O-defected CeO₂(111) surface alone (E_ads = −0.85 eV).