The influence of the Hubbard U parameter in simulating the catalytic behaviour of cerium oxide

Adsorption and Oxidation of CO on Ceria Nanoparticles Exposing Single-Atom Pd and Ag: A DFT Modelling

Various COx species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCOx moieties are found to be stabilised in the order MCO < MCO2 < MCO3, similar to the trend for COx species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO2 transformation are found for O4Pd site of the Pd/Ce21O42 model featuring a square-planar coordination of the Pd2+ cation. The removal of one O atom and formation of the O3Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO2-species are strongly bound in palladium carboxylate complexes, whereas the CO2 molecule easily desorbs from oxide-supported AgCO2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO3 nor AgCO3 carbonate intermediates before CO2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface COx moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO2 and CO32- species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction.

γ-Al2O3:Ce3+Cu2+ as a phosphor material; DFT+U and experimental approach

The γ-Al2O3 and Ce3+Cu2+-doped γ-Al2O3 powders have been synthesized by sol-gel method. Phases of the synthesized powders were characterized with X-ray diffraction. Morphological analysis and elemental composition of the samples were determined by scanning electron microscopy, high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Luminescence characterizations have been used to study the synthesized samples. Ab initio calculations by the use of local density approximation with the Hubbard U correlation were used to compute the structural, electronic and optical properties of γ-Al2O3 and Al2O3:Ce3+Cu2+. The results indicate that the particle size and morphology of the samples depend on the concentration of the dopants. In comparison with undoped γ-Al2O3 sample, the intensities of emission peaks at 430 and 458 nm of Ce3+Cu2+-doped γ-Al2O3 powders have been enhanced. This shows that, increasing Ce3+ and Cu2+ concentration causes an increase in the number of emitting ions which is expected in order to increase the number of applications of γ-Al2O3:Ce3+Cu2+ composite powders. The photoluminescence spectrum detected at $\lambda$ex = 253 nm shows a new peak located at 549 nm due to Cu2+ ions. This was confirmed computationally when the Ce_4f and Ce_5d states are found in the conduction band while the Cu_4p state was found at conduction band minimum and Cu_3d state at valence band maximum. This location of states showed there is no possible luminescence from the Ce3+ ions. The only possible luminescence was due to transition from Cu_4p to Cu_3d states.

Atomic-scale computational design of hydrophobic RE surface-doped Al2O3 and TiO2

Intrinsically hydrophobic rare-earth oxides (REOs) have emerged as a robust class of ceramics for a variety of applications. Recently, the hydrophobicity of REOs has been observed experimentally and subsequently scrutinized using electronic structure density functional theory (DFT) calculations. In this work, we applied the DFT method to analyze the possibility of tuning the wettability of commonly used hydrophilic Al₂O₃ and TiO₂ by surface doping with Ce. The calculations indicate that Ce can preferentially segregate to the surface of Al₂O₃ and TiO₂ and form a Ce-rich oxide layer, which is stable under a wide range of oxygen chemical potentials. A remarkable increase in the water contact angle is predicted for Ce-doped Al₂O₃(0001), whereas the water contact angle calculated for Ce-doped TiO₂(110) remains unchanged, regardless of the Ce concentration. The wetting properties of Ce-doped Al₂O₃ are governed by two factors: (1) the unique electronic structure of the rare-earth metal promotes hydrogen bond formation between H₂O and surface oxygen; (2) significant relaxation of the surface Ce and O atoms hampers direct interaction between H₂O and Al cations, preventing dissociative water adsorption. These results provide a valuable opportunity for Al₂O₃ surface modification, in terms of achieving hydrophobicity.

Adsorption of C2 gases over CeO2-based catalysts: synergism of cationic sites and anionic vacancies

Combined in situ DRIFTS and in silico DFT studies show the potential of Pd-substituted CeO₂ for the hydrogenation of C₂-gases.

Structural, elastic and thermo-electronic properties of paramagnetic perovskite PbTaO3

Self-consistent ab initio calculations with highly precise spin-polarised, density functional theory (DFT) have been performed for the first time, to study the structural stability, mechanical and magneto-electronic properties of cubic perovskite PbTaO₃.