Analysis of Intrinsic Defects in CeO2 Using a Koopmans-Like GGA+U Approach

Spin-density for a Ce vacancy, with U applied to O and Ti states, to give a defect state localised as four oxygen holes.We have investigated the formation of intrinsic defects in CeO2 using density functional theory with the generalized gradient approximation (GGA) corrected for on-site Coulombic interactions (GGA+U). We employed an ab initio fitting procedure to determine a U {O2p} value that satisfies a Koopmans-like condition and obtained a value of U {O2p} = 5.5 eV. We subsequently demonstrated that by applying GGA+U to the O2p states, in addition to the Ce 4f states, we were able to model localized holes in addition to localized electrons, thus improving the description of p-type defects in CeO2. Our results show that under oxygen-poor conditions the defects with the lowest formation energy are oxygen vacancies, while oxygen interstitials, which form peroxide ions, will be more favorable under oxygen-rich conditions. We carried out temperature and pressure dependence analyses to determine the relative abundance of intrinsic defects under real-world conditions and determined that oxygen vacancies will always be the dominant defect. Furthermore, we determined that at the dilute limit none of the defects studied can account for the intrinsic ferromagnetism that has been observed in nanosized CeO2.

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The Origin of the Enhanced Oxygen Storage Capacity of Ce1-x(Pd/Pt)xO2

Doping CeO2 with Pd or Pt increases the oxygen storage capacity (OSC) and catalytic activity of this environmentally important material. To date, however, an understanding of the mechanism underlying this improvement has been lacking. We present a density functional theory analysis of Pd- and Pt-doped CeO2, and demonstrate that the increased OSC is due to a large displacement of the dopant ions from the Ce lattice site. Pd(II)/Pt(II) (in a d8 configuration) moves by ~1.2 Å to adopt a square-planar coordination due to crystal field effects. This leaves three three-coordinate oxygen atoms that are easier to remove, and which are the source of the increased OSC. These results highlight the importance of rationalising the preferred coordination environments of both dopants and host cations when choosing suitable dopants for next generation catalysts.

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