Effect of 3.7 at% F doping on the atomic structure and reducibility of CeO2(111) surface: A first principles calculation

Density functional theory calculations was conducted to clarify effect of fluorination on the reducibility of CeO₂ polishing powder. The atomic structure and O vacancy formation energy of F-doped CeO₂(111) surface were systematically calculated, and electronic structure was also analyzed to understand the calculation results. It showed that the CeO_1.963(111) surface needs absorb 7.853 eV to be reduced further, so it is difficult to generate the second O vacancy (V₂). After fluorination, the first O vacancy (V₁) in CeO_1.926F_0.037(111) surface caused adjacent atoms to move significantly, and O vacancy formation energy decreased to 2.913 eV, which promoted the formation of V₂. Moreover, electronic structure calculation also showed that the interaction between O2p and Ce4f5d orbits was enhanced in reduced CeO_1.963(111) surface, which hindered the formation of V₂. The F2p orbit in CeO_1.926F_0.037(111) surface moved towards the low energy level, V₁ made the reduced surface stable and promoted the formation of V₂.     

Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型，用第一性原理计算方法，计算并分析了Pd在CeO₂(111)面上不同覆盖度时的吸附能，价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明：1)在单层吸附时，Pd的最佳吸附位置是O的顶位偏向Ce的桥位；在1/4单层吸附时，Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时，吸附原子Pd之间的作用较强；1/4单 关键词： 三元催化剂 Pd 2')" href="#">CeO₂ 吸附 密度泛函理论     

Effects of oxygen vacancies on the room-temperature ferromagnetism of Co-doped polycrystalline CeO2

Pure and Co-doped single-phase CeO₂ crystals were synthesized by a solid-state reaction method. Samples of different oxygen vacancy concentration were studied, including (1) as-sintered crystals, (2) powders ground from the same crystal, and (3) a cold-pressed pellet from the ground powder that was unannealed and annealed at 800 °C. By analyzing the magnetic behaviors, surface/volume ratio and O vacancy concentration, the effects of oxygen vacancies on the room-temperature ferromagnetism (RT-FM) of Co-doped CeO₂ were systematically investigated. The results confirm that the RT-FM observed in Co-doped CeO₂ has a direct relationship with the oxygen vacancy concentration, and support the oxygen vacancy mediated FM mechanism.     

Selective CO oxidation in hydrogen-rich gas over CuO/CeO2 catalysts

A series of CuO/CeO₂ catalysts with different Cu-Ce compositions were synthesized by co-precipitation method and characterized by X-ray diffraction, H₂-TPR, CO-TPD, SEM and X-ray photoelectron spectroscopy (XPS) techniques. The effects of Cu-Ce composition and water vapor on the catalytic properties for the selective CO oxidation in the hydrogen-rich gas were investigated. The results indicated that CuO (10%)/CeO₂ catalyst remained the maximum CO conversion and selectivity at 140 and 160 °C, while the performance of CuO/CeO₂ catalysts deteriorated with the CuO molar ratio further increased. The interfacial CuO and CeO₂ interaction and synergistic effect enhanced the redox properties of CuO/CeO₂ catalyst and the highly dispersed copper species were proposed as the active sites for the selective CO oxidation. The blockage of catalytic active sites by absorbed water and the formation of CO-H₂O surface complexes reduced the activity of CuO (10%)/CeO₂ catalyst. The decreasing of surface lattice oxygen and absorbed oxygen species and the agglomeration of copper particles were the plausible interpretations for the deactivation of CuO (10%)/CeO₂ catalyst.     

Lattice distortion and corresponding changes in optical properties of CeO2 nanoparticles on Nd doping

Doping of Nd distorts the lattice structure of CeO₂, increases the lattice strain and expands the lattice. Oxygen vacancies and other ceria related defects contribute to the lattice strain. Shifting and broadening of the F_2g Raman peak of doped sample, compared to pure CeO₂, is indicative of local structure distortion on doping. Dopant induced enhancement of oxygen vacancies, in the CeO₂ lattice, is further confirmed by the generation of a new Raman peak at 543 cm^?1 that is otherwise absent in the pure one. UV–vis spectroscopy gives an understanding of the different types of f–f electronic transition of Nd in the crystalline environment of CeO₂. Effective band gap of CeO₂ reduces upto Nd concentration of 2.5%. The band gap, however, increases at 4% of Nd due to Burstein–Moss shift. Photoluminescence intensity of pure CeO₂ decreases with Nd concentration owing to the increase in the number of non radiative oxygen vacancies. These vacancies act as luminescence quencher and reduce the emission intensity. Photoluminescence excitation spectra confirm the presence of these oxygen vacancies in the CeO₂ nanocrystallites.     

Vibrational spectroscopy of oxygen on the (100) and (111) surfaces of lanthanum hexaboride

Reflection absorption infrared spectroscopy (RAIRS) and high resolution electron energy loss spectroscopy (HREELS) have been used to study the adsorption of oxygen on the (100) and (111) surfaces of lanthanum hexaboride. Exposure of the surface at temperatures of 95 K and above to O₂ produces atomic oxygen on the surface and yields vibrational peaks in good agreement with those observed in previous HREELS studies. On the La-terminated (100) surface, RAIRS peaks correspond to vibrations of the boron lattice that gain intensity due to a decrease in screening of surface dipoles that accompanies oxygen adsorption. A sharp peak at ～ 734 cm^?1 in the HREEL spectrum shows isotopic splitting with RAIRS into two components at 717 and 740 cm^?1 with full widths at half maxima of only 12 cm^?1. The sharpness of this mode is consistent with its interpretation as a surface phonon that is well separated from both the bulk phonons and other surface phonons of LaB₆. On the boron-terminated LaB₆(111) surface, broad and weak features are assigned to both vibrations of the boron lattice and of boron oxide. On the (100) surface, oxygen blocks the adsorption sites for CO, and adsorbed CO prevents the dissociative adsorption of O₂.     

阳离子空位和吸附氧的协同作用对Na0.5Bi0.5TiO3(100)表面磁性影响的第一性原理研究

The combination effect of cation vacancies and O₂ adsorption on ferromagnetism of Na_0.5Bi_0.5TiO₃(100) surface is studied by using density functional theory.An ideal Na_0.5Bi_0.5TiO₃(100) surface is non-magnetic and the cation vacancy could induce the magnetism.By comparing the formation energies for Na,Bi and Ti vacancy,the Na vacancy is more stable than the others.Therefore,we focus on the configuration and electric structure for the system of O₂ molecule adsorption on the Na_0.5Bi_0.5TiO₃(100) surface with a Na vacancy.Among the five physisorption configurations we considered,the most likely adsorption position is Na vacancy.The O₂ adsorption enhances the magnetism of the system.The contribution of spin polarization is mainly from the O 2p orbitals.The characteristics of exchange coupling are also calculated,which show that the ferromagnetic coupling is favorable.Compared with the previous calculation results,our calculations could explain the room-temperature ferromagnetism of Na_0.5Bi_0.5TiO₃ nanocrytalline powders more reasonably,because of taking into account adsorbed oxygen and cation vacancies.Moreover,our results also show that adsorption of O₂ molecule as well as introduction of cation vacancies may be a promising approach to improve multiferroic materials.     

Oxygen reduction reactions in the SOFC cathode of Ag/CeO2

The interactions between oxygen molecules and a silver surface or a CeO₂(111) supported atomic layer of silver are predicted using first-principles calculations based on spin polarized DFT with PAW method. The juncture between the CeO₂(111), the atomic layer of silver, and O₂ represents a triple-phase boundary (TPB) whereas the interface between silver surfaces and O₂ corresponds to a 2-phase boundary (2PB) in a solid oxide fuel cell (SOFC). Results suggest that the O₂ dissociation process on a monolayer of silver supported by CeO₂(111) surfaces (or TPB) with oxygen vacancies has lower reaction barrier than on silver surfaces (or 2PB), and the dissociated oxygen ions can quickly bond with subsurface Ce atom via a barrierless and highly exothermic reaction. The oxygen vacancies at TPB are found to be responsible for the lower energy barrier and high exothermicity because of the strong interaction between subsurface Ce and adspecies, implying that oxygen molecules prefer being reduced at TPB than on silver surfaces (2PB). The results suggest that, for a silver-based cathode in a SOFC, the adsorption and dissociation of oxygen occur rapidly and the most stable surface oxygen species would be the dissociated oxygen ion with − 0.78|e| Bader charges; the rate of oxygen reduction is most likely limited by subsequent processes such as diffusion or incorporation of the oxygen ions into the electrolyte.     

Spectroscopic studies of interfacial structures of CeO2-TiO2 mixed oxides

We have investigated the geometric and electronic structures of the cerium oxide (CeO₂)-titanium dioxide (TiO₂) mixed oxides with various Ce/TiO₂ weight ratios prepared by the sol-gel method in detail by means of X-ray diffraction (XRD), high-resolution X-ray photoelectron spectroscopy (XPS), Raman spectroscopy excited by 325 and 514.5 nm lasers, and scanning electron microscope (SEM). Existence of cerium effectively inhibits the phase transition of TiO₂ from the anatase phase to the rutile phase. XRD peaks of TiO₂ anatase attenuate continuously with the increasing amount of CeO₂ in the mixed oxide, but the XRD peaks of cubic CeO₂ appear only after the weight ratio of Ce/TiO₂ reaches 0.50. The average crystalline sizes of TiO₂ anatase and cubic CeO₂ in CeO₂-TiO₂ mixed oxides are smaller than those in the corresponding individual TiO₂ anatase and cubic CeO₂. Raman spectroscopy excited by the 514.5 nm laser detects CeO₂ after the weight ratio of Ce/TiO₂ reaches 0.70 whereas Raman spectroscopy excited by the 325 nm laser detects CeO₂ after the weight ratio of Ce/TiO₂ reaches 0.90. XPS results demonstrate that Ti exists in the form of Ti⁴⁺ in the CeO₂-TiO₂ mixed oxide. Ce is completely in the form of Ce³⁺ in the mixed oxides with a 0.05 weight ratio of Ce/TiO₂. With the increasing weight ratio of Ce/TiO₂, Ce⁴⁺ dominates. On basis of these results, we proposed that CeO₂ initially nucleates at the defects (oxygen vacancies) within TiO₂ anatase, forming an interface bridged with oxygen between CeO₂ and TiO₂ anatase. At the interface, Ce species cannot substitute Ti⁴⁺ in the lattice of TiO₂ anatase whereas Ti⁴⁺ can substitute Ce⁴⁺ in the lattice of cubic CeO₂. The decreasing concentration of oxygen vacancies, the Ti-O-Ce interface, and the decreasing average crystalline size of TiO₂ anatase act to inhibit the phase transformation of TiO₂ anatase. With the increasing amounts of CeO₂, the CeO₂ clusters continuously grow and form cubic CeO₂ nanocrystals. Spectroscopic results strongly demonstrate that the surface region of CeO₂-TiO₂ mixed oxide is enriched with TiO₂.     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.