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At least 253 records · Page 14

Electronic structure of reduced CeO 2 (111) surfaces interacting with hydrogen as revealed through electron energy loss spectroscopy in comparison with theoretical investigations

Based on both, ab-initio cluster calculations as well as periodic density functional based loss function calculations, we have assigned the origins of the valence electron excitation regime in electron energy loss spectra of well-ordered ceria films in (111) orientation at various states of reduction as well as after exposing the reduced films to hydrogen from the gas phase (Li et al, Angew. Chem. Int. Ed., 58 (2019) 14686–14693). The explicit calculation of intensity distributions using the dipole approximation allow us to draw conclusions about the nature of oxygen vacancies, which occur upon reduction and how those interact with hydrogen to form hydride species. Herein, we also reports a brief discussion of vibrational excitations of hydrogen loaded ceria and corroborates previous interpretations, based on inelastic neutron scattering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

A comparative study of SrO and BaO doping to CeO{sub 2}-ZrO{sub 2}: Characteristic and its catalytic performance for three-way catalysts

Graphical abstract: Display Omitted Highlights: ► The prepared materials have a face-centered cubic structure and nanosize particles. ►Comparing to CZB, aged CZS has 494 μmol/g of OSC and 30 m{sup 2}/g of surface area. ► CZS and CZB have similar NO sorption and reductive properties and different H{sub 2} uptake. ► T{sub 50} of Pt-Rh/CZS/LA is as low as 199 °C for CO, 228 °C for NO, and 252 °C for C{sub 3}H{sub 8}. ► Pt-Rh/CZS/LA has wider working-window at 320 °C under different λ value. -- Abstract: Ceria-zirconia-strontia (Ce{sub 0.35}Zr{sub 0.55}Sr{sub 0.10}O{sub 1.9}) and ceria-zirconia-baria (Ce{sub 0.35}Zr{sub 0.55}Ba{sub 0.10}O{sub 1.9}) were synthesized using an oxidation-co-precipitation method with hydrogen peroxide (H{sub 2}O{sub 2}) as oxidant. The physical and chemical properties of the prepared materials were investigated using Brunauer–Emmett–Teller surface area characterization, transmission electron microscopy, X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, and oxygen pulse reaction. The prepared materials were used in preparing three-way catalysts with low Pt and Rh content. Moreover, catalytic activities were evaluated at a fixed bed under a simulated gaseous mixture. The results are as follows: (1) the prepared materials have a face-centered cubic structure and are nano-sized; (2) aged Ce{sub 0.35}Zr{sub 0.55}Sr{sub 0.10}O{sub 1.9} has higher oxygen storage capacity (494 μmol/g), better thermal stability (30 m{sup 2}/g), good low-temperature reducibility, and high hydrogen uptake after TPR-redox cycles; (3) the light-off temperature (T{sub 50}) of Pt-Rh/CZS/LA can be as low as 199 °C for CO, 228 °C for NO, and 252 °C for C{sub 3}H{sub 8}; and (4) Pt-Rh/CZS/LA has a fairly wide working-window.

36 MATERIALS SCIENCE↗

Temperature-dependent void formation and growth at ion-irradiated nanocrystalline CeO 2 –Si interfaces

Ceria is a thermally stable ceramic that has numerous applications in the nuclear industry, including use in nuclear fuels and waste forms. Recently, interest has surged in nanostructured ceria due to its increased mechanical properties and electronic conductivity in comparison with bulk ceria and its ability to self-heal in response to energetic ion bombardment. Here, nanocrystalline ceria thin films grown over a silicon substrate are irradiated to fluences of up to ~4 × 10 16 ions/cm 2 under different irradiation conditions: with differing ion species (Si + and Ni + ), different ion energies (1.0–1.5 MeV), and at varying temperatures (160–600 K). Finally, while the nanocrystalline ceria is found to exhibit exceptional radiation resistance under all tested conditions, severe ion irradiation-induced mixing, void formation, and void growth are observed at the ceria/silicon interface, with the degree of damage proving to be temperature dependent.

Environmental Molecular Sciences Laboratory↗

Transient Studies of CO 2 Adsorption over CeO 2 Nanostructures with In Situ DRIFTS and Modulation Excitation

Experiments of in-situ DRIFTS combined with modulation excitation (ME) spectroscopy showed a rich surface chemistry associated with the adsorption of CO 2 on nanocubes and nanospheres of ceria. The nanocubes exposed faces with a (100) orientation, with the edges and corners displaying (110) and (111) orientations, respectively. Here, the nanospheres mainly contained ceria (111) and (110) planes. DFT calculations showed that CO 2 is a multidentate adsorbate on ceria that can undergo changes in its bonding configuration depending on the chemical environment. At 250 °C, a temperature typically used for the conversion of CO 2 into oxygenates, alkanes and olefins, CO 2 reacted with O centers or OH groups present on the nanocubes and nanospheres to yield bi- and tri-dentate carbonates, hydroxycarbonates, and formates. Both nanostructures were highly reactive and a dynamic equilibrium was established: carbonate species were rapidly generated upon the injection of CO 2 and they decomposed upon the removal of CO 2 from the gas phase. In the case of the ceria nanocubes, the adsorption/desorption processes were essentially reversible, opening the door to catalytic transformations. A larger concentration of defects in the ceria nanospheres led to strongly bound carbonates and formates that may be spectators, site blockers, or surface modifiers in catalytic processes. In the ME studies, additional intermediates were detected, and it was clear that the response of surface species to the presence/absence of CO 2 was highly dependent on the morphology of the ceria nanostructures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Nucleation of Cu n ( n = 1–5) Clusters and Equilibrium Morphology of Cu Particles Supported on CeO 2 Surface: A Density Functional Theory Study

In the present work, the interactions between Cun (n=1~4) clusters and three low-index structural CeO2 (111, 110, 100) surfaces were investigated using density functional theory calculations. The atomic Cu prefers to aggregate into large clusters on the CeO2(111) surface while it is unfavorable on both CeO2(110) and CeO2(100) surfaces. Once the planar Cu4-p cluster is formed, it would convert into the 3D tetrahedral Cu4-t cluster on CeO2(110) and CeO2(111) surfaces, i.e., 2D to 3D transition growth is thermodynamically favorable and kinetically feasible on CeO2(110) and CeO2(111) surfaces. The effects of ceria support structure on the morphology of the large Cu nanoparticle was examined using calculated adhesion and interfacial energies of periodic Cu(khl)/CeO2 model systems. The calculated macroscopic contact angles (? > 100?) of various model systems suggest that the supported Cu nanoparticle is in bad wetting condition. Finally, the morphologies of large Cu nanoparticles on different CeO2 surfaces were predicted using Wulff-Kaichew construction principle.

Cu/CeO2 catalyst, Adsorption and nucleation, DFT c↗

Aliovalent Doping of CeO 2 Improves the Stability of Atomically Dispersed Pt

Atomically dispersed supported catalysts hold considerable promise as catalytic materials. The ability to employ and stabilize them against aggregation in complex process environments remains a key challenge to the elusive goal of 100% atom utilization in catalysis. In this work, using a Gd-doped ceria support for atomically dispersed surface Pt atoms, we establish how the combined effects of aliovalent doping and oxygen vacancy generation provide dynamic mechanisms that serve to enhance the stability of supported single atom configurations. Using correlated, in situ X-ray absorption, photoelectron and vibrational spectroscopies for analysis of the samples on the two types of support (with and without Gd doping), we establish that the Pt atoms locate proximal to Gd dopants, forming a speciation that serves to enhance the thermal stability of Pt atoms against aggregation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗