Oxidation of Reduced Ceria by Incorporation of Hydrogen

The interaction of hydrogen with reduced ceria (CeO_2?x) powders and CeO_2?x(111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce³⁺ species to hydrogen, which results in the formation of Ce⁴⁺, and thus in oxidation of ceria.     

Growth and Electronic Properties of Ag Nanoparticles on Reduced CeO2-x(111) Films

Ag nanoparticles grown on reduced CeO_2-x thin films have been studied by X-ray photoelec-tron spectroscopy and resonant photoelectron spectroscopy of the valence band to understand the effect of oxygen vacancies in the CeO_2-x thin films on the growth and interfacial elec-tronic properties of Ag. Ag grows as three-dimensional particles on the CeO_2-x(111) surface at 300 K. Compared to the fully oxidized ceria substrate surface, Ag favors the growth of smaller particles with a larger particle density on the reduced ceria substrate surface, which can be attributed to the nucleation of Ag on oxygen vacancies. The binding energy of Ag3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. The interfacial interaction between Ag and CeO_2-x(111) is weak. The resonant enhancement of the 4f level of Ce³⁺ species in RPES indicates a partial Ce⁴⁺→Ce³⁺ re-duction after Ag deposited on reduced ceria surface. The sintering temperature of Ag on CeO_1.85(111) surface during annealing is a little higher than that of Ag on CeO₂(111) surface, indicating that Ag nanoparticles are more stable on the reduced ceria surface.     

Understanding the Role of Surface Oxygen in Hg Removal on Un-Doped and Mn/Fe-Doped CeO2(111)

Effects of surface-adsorbed O and lattice O for the CeO₂(111) surface on Hg removal has been researched. In this work, periodic calculations based on density functional theory (DFT) were performed with the on-site Coulomb interaction. Hg is oxidized to HgO via the surface-adsorbed O by overcoming a Gibbs free energy barrier of 114.1 kJ·mol⁻¹ on the CeO₂(111) surface. Mn and Fe doping reduce the activation Gibbs free energy for the Hg oxidation, and energies of 70.7 and 49.6 kJ·mol⁻¹ are needed on Ce_0.96Mn_0.04O₂(111) and Ce_0.96Fe_0.04O₂(111) surfaces. Additionally, lattice O also plays an important role in Hg removal. Hg cannot be oxidized leading to the formation of HgO on the un-doped CeO₂(111) surface owing to the inertness of lattice O, which can be easily oxidized to HgO on Ce_0.96Mn_0.04O₂(111) and Ce_0.96Fe_0.04O₂(111) surfaces. It can be seen that both surface-adsorbed O and lattice O play important roles in removing Hg. The present study will shed light on understanding and developing Hg removal technology on un-doped and Mn/Fe-doped CeO₂(111) catalysts. © 2019 Wiley Periodicals, Inc.     

In Situ Investigation of Methane Dry Reforming on Metal/Ceria(111) Surfaces: Metal–Support Interactions and C−H Bond Activation at Low Temperature

Studies with a series of metal/ceria(111) (metal=Co, Ni, Cu; ceria=CeO₂) surfaces indicate that metal–oxide interactions can play a very important role for the activation of methane and its reforming with CO₂ at relatively low temperatures (600–700 K). Among the systems examined, Co/CeO₂(111) exhibits the best performance and Cu/CeO₂(111) has negligible activity. Experiments using ambient pressure X‐ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO₂(111) at temperatures as low as 300 K—generating CH_x and CO_x species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co²⁺/CeO₂(111), and to only 0.05 eV on Co⁰/CeO_2−x (111). At 700 K, under methane dry reforming conditions, CO₂ dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CH_x formed on the Co⁰/CeO_2−x (111) catalyst recombines to yield ethane or ethylene.     

Tuning the concentration of surface/bulk oxygen vacancies in CeO2 nanorods to promote highly efficient photodegradation of organic dyes

To enhance the photodegradation ability of CeO₂ for organic dyes, an effective strategy is to introduce oxygen vacancies (V_o). In general, the introduced V_o are simultaneously present both on the surface and in the bulk of CeO₂. The surface oxygen vacancies (V_o-s) can decrease the band gap, thus enhancing light absorption to produce more photogenerated e⁻ for photodegradation. However, the bulk oxygen vacancies (V_o-b) will inhibit photocatalytic activity by increasing the recombination of photogenerated e⁻ and V_o-b. Therefore, regulating the concentrations of V_o-s to V_o-b is a breakthrough for achieving the best utilization of photogenerated e⁻ during photodegradation. We used an easy hydrothermal method to achieve tunable concentrations of V_o-s to V_o-b in CeO₂ nanorods. The optimized CeO₂ presents a 70.2% removal of rhodamine B after 120 min of ultraviolet−visible light irradiation, and a superior photodegradation performance of multiple organics. This tuning strategy for V_o also provides guidance for developing other advanced metal-oxide semiconductor photocatalysts for the photodegradation of organic dyes.     

Adsorption and migration growth of the Ce,O, and F adatoms on the CeO2 (111) surface: A density functional theory study

In order to investigate the microscopic behavior of the crystal surface growth of the fluorinated cerium dioxide polishing powder, the adsorption and migration of the Ce, O, and F atoms on the CeO₂ (111) surface were studied by using density functional theory with Hubbard correction +U. The adsorption energies of three single atoms at five high-symmetry sites and the migration activation energies along the migration pathway on the CeO₂ (111) surface were calculated. Results show that the most stable adsorption sites of the Ce, O, and F atoms were the O_h, Ce_bri, and Ce_t sites, respectively. The Ce atom migrated from the O_h to the O_t site. The O atom migrated from the Ce_bri to the O_bri site. The F atom migrated from the Ce_t to the O_h site. The migration activation energies of the Ce, O, and F atoms along the migration pathways were 1.526, 0.597, and 0.263 eV, respectively. The F adatom does not change the spatial configuration of the Ce and the O atoms. When the O vacancy occurs on the CeO₂ (111) surface, the F adatom can make up for the O vacancy defect.     

乙二醇溶剂热合成的CeO2的可逆氧化还原性及CO2捕获性能

利用乙二醇的还原性,采用乙二醇溶剂热法制备了表面具有丰富氧空穴的CeO₂-GST纳米晶,对其进行了X射线衍射、透射电镜、X射线光电子能谱、原位H₂还原-O₂氧化循环和CO₂原位红外漫反射表征,并研究了其可逆氧化还原性及CO₂捕获性能. 结果表明,与CeO₂-nanorod和柠檬酸溶胶法合成的CeO₂-CA样品相比,CeO₂-GST纳米晶具有最好的可逆氧化还原性能和循环稳定性,同时在50 ℃下具有最好的CO₂吸附性能（149 μmol/g）. 利用原位红外漫反射光谱研究了CO₂在还原CeO₂表面的吸附情况,发现CO₂主要以双齿碳酸盐和桥连碳酸盐两种形式吸附在CeO₂表面,其中桥连碳酸盐物种不稳定,He吹扫可脱附. 此外,CO₂在CeO₂-nanorod上还会生成稳定的甲酸盐和单齿碳酸盐物种.     

Structure and surface properties of ZrO<Subscript>2</Subscript>, CeO<Subscript>2</Subscript>, and Zr<Subscript>0.5</Subscript>Ce<Subscript>0.5</Subscript>O<Subscript>2</Subscript> prepared by a microemulsion method according to X-ray diffraction,TPR, and EPR data

It was established by X-ray diffraction, TPR, and EPR that microemulsion (m.e.) synthesis yields the binary oxides ZrO₂(m.e.) and CeO₂(m.e.) and the mixed oxide Zr_0.5Ce_0.5O₂(m.e.) in the form of a tetragonal, cubic, and pseudocubic phase, respectively, having crystallite sizes of 5–6 nm. The bond energy of surface oxygen in the (m.e.) samples is lower than in their analogues prepared by pyrolysis. Hydrogen oxidation on the oxides under study occurs at higher temperatures than CO oxidation. ZrO₂(m.e.) and CeO₂(m.e.) are active in O₂⁻ formation during NO + O₂ adsorption, while CeO₂ is active during CO + O₂ adsorption, too. However, its amount here is one-half to one-third its amount in the pyrolysis-prepared samples, signifying a reduced number of active sites, which are Zr⁴⁺ and Ce⁴⁺ coordinatively unsaturated cations and Me⁴⁺-O²⁻ pairs. O₂⁻ radical anions are stabilized in the coordination sphere of Zr⁴⁺ coordinatively unsaturated cations via ionic bonding, and in the sphere of Ce⁴⁺ cations, via covalent bonding. Ionic bonds are stronger than ionic-covalent bonds and do not depend on the ZrO₂ phase composition. Zr_0.5Ce_0.5O₂ is inactive in these reactions because of the strong interaction of Zr and Ce cations. It is suggested that Ce^{(4 + β)+} coordinatively unsaturated cations exist on its surface, and their acid strength is lower than that of Zr⁴⁺ and Ce⁴⁺ cations in ZrO₂ and CeO₂, according to the order ZrO₂ > CeO₂ ≥ Zr_0.5Ce_0.5O₂. Neither TPR nor adsorption of probe molecules revealed Zr cations on the surface of the mixed oxide.     

Structural and catalytic properties of lanthanide (La, Eu, Gd) doped ceria

Ce_0.9M_0.1O_2−δ mixed oxides (M=La, Eu and Gd) were synthesized by coprecipitation. Independent of the dopant cation, the obtained solids maintain the F-type crystalline structure, characteristic of CeO₂ (fluorite structure) without phase segregation. The ceria lattice expands depending on the ionic radii of the dopant cation, as indicated by X-ray diffraction studies. This effect also agrees with the observed shift of the F_2g Raman vibrational mode. The presence of the dopant cations in the ceria lattice increases the concentration of structural oxygen vacancies and the reducibility of the redox pair Ce⁴⁺/Ce³⁺. All synthesized materials show higher catalytic activity for the CO oxidation reaction than that of bare CeO₂, being Eu-doped solid the one with the best catalytic performances despite of its lower surface area.     

Influence of chemical equilibrium in introduced oxygen vacancies on resistive switching in epitaxial Pt-CeO<Subscript>2</Subscript> system

We investigate the introduction of oxygen vacancies by the interaction of Pt with CeO₂(111) (ceria) thin epitaxial film grown on Cu(111) and the influence of the vacancies on resistive switching behavior. For this purpose, we used X-ray photoelectron spectroscopy and conductive atomic force microscopy. We found out that after Pt deposition, the ceria film was partially reduced. By our estimation, the reduction occurs not only at the Pt/CeO₂ interface, but also on the surface of the ceria film which is not covered by Pt, after Pt deposition and annealing. A different distribution of oxygen vacancies in the film proves to have an influence on the resistance switching process of the film. Finally, the proper balance between the reduced and the unreduced species in order to obtain relatively stable repeatable resistance switch with clear resistance window is discussed.     

Analysis of Dopant Atom Distribution and Quantification of Oxygen Vacancies on Individual Gd‐Doped CeO2 Nanocrystals

This work reports the analysis of the distribution of Gd atoms and the quantification of O vacancies applied to individual CeO₂ and Gd‐doped CeO₂ nanocrystals by electron energy‐loss spectroscopy. The concentration of O vacancies measured on the undoped system (6.3±2.6 %) matches the expected value given the typical Ce³⁺ content previously reported for CeO₂ nanoparticles. The doped nanoparticles have an uneven distribution of dopant atoms and an atypical amount of O vacant sites (37.7±4.1 %). The measured decrease of the O content induced by Gd doping cannot be explained solely by the charge balance including Ce³⁺ and Gd³⁺ ions.     

Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: A DFT + U study

The interactions and reduction mechanisms of O₂ molecule on the fully oxidized and reduced CeO₂ surface were studied using periodic density functional theory calculations implementing on‐site Coulomb interactions (DFT + U) consideration. The adsorbed O₂ species on the oxidized CeO₂ surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within ?0.05 to 0.02 eV and 1530–1552 cm^?1, respectively. For the reduced CeO₂ surface, the adsorption of O₂ on Ce⁴⁺, one‐electron defects (Ce³⁺ on the CeO₂ surface) and two‐electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O₂, O₂^a? (0 < a < 1), superoxide (O₂^?), and peroxide (O₂^2?) species. Their corresponding adsorption energies are ?0.01 to ?0.09, ?0.20 to ?0.37, ?1.34 and ?1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O₂^a? (0 < a < 1) and physisorbed species are 897, 1234, 1323–1389, and 1462–1545 cm^?1, respectively, which are in good agreement with experimental data. Potential energy profiles for the O₂ reduction on the oxidized and reduced CeO₂ (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria‐based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT‐SOFC electrolyte materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Effect of preparation procedure on the properties of CeO2

The effect of preparation procedure on the physicochemical and catalytic properties of CeO₂ was studied. Differences in the electronic and structural characteristics of CeO₂ depending on preparation procedure and treatment temperature were found using X-ray diffraction analysis, transmission electron microscopy, UV-visible electronic spectroscopy, and X-ray photoelectron spectroscopy. With the use of the temperature-programmed reaction with CO, it was demonstrated that CeO₂ samples with a high concentration of point defects—oxygen vacancies caused by the presence of Ce³⁺—were characterized by an increased mobility of bulk oxygen. The samples of CeO₂ with a high concentration of structural defects—micropores of size 1–2 nm and stepwise vicinal faces in crystallites—exhibited a high catalytic activity in the reaction of CO oxidation.     

Oxygen vacancy ordering within anion-deficient Ceria

The structural properties of anion deficient ceria, CeO_2−δ, have been studied as a function of oxygen partial pressure, p(O₂), over the range 0≥log₁₀ p(O₂)≥−18.9 at 1273(2) K using the neutron powder diffraction technique. Rietveld refinement of the diffraction data collected on decreasing p(O₂) showed increases in the cubic lattice parameter, a, the oxygen nonstoichiometry, δ, and the isotropic thermal vibration parameters, u_Ce and u_O, starting at log₁₀ p(O₂)~−11. The increases are continuous, but show a distinct kink at log₁₀ p(O₂)~−14.5. Analysis of the total scattering (Bragg plus diffuse components) using reverse Monte Carlo (RMC) modelling indicates that the O²⁻ vacancies preferentially align as pairs in the 〈111〉 cubic directions as the degree of nonstoichiometry increases. This behaviour is discussed with reference to the chemical crystallography of the CeO₂-Ce₂O₃ system at ambient temperature and, in particular, to the nature of the long-range ordering of O²⁻ vacancies within the crystal structure of Ce₇O₁₂.     

Relationship between oxygen species and activity/stability in heteroatom (Zr,Y)-doped cerium-based catalysts for catalytic decomposition of CH3SH

CeO₂, Ce_1–xZr_xO₂, and Ce_1–xY_xO_2–δ (x = 0.25, 0.50, 0.75, and 1.00) have been rapidly synthesized to estimate their catalytic behavior in decomposing CH₃SH. The role of oxygen vacancies, and the relationship between the oxygen species and catalytic properties of CeO₂ and Zr-doped and Y-doped ceria-based materials are investigated in detail. Combining the observed catalytic performance with the characterization results, it can be deemed that surface lattice oxygen plays a critical role in methanethiol catalytic conversion over cerium oxides. Ce_0.75Zr_0.25O₂ shows higher catalytic activity for CH₃SH decomposition due to the large amount of surface lattice oxygen, readily available oxygen species, and excellent redox properties. Ce_0.75Y_0.25O_2–δ displays better catalytic stability owing to the greater number of oxygen vacancies that would promote bulk lattice oxygen migration to the surface of the catalyst in order to replenish surface lattice oxygen. In addition, the results show that the difference in chemical valence between Ce and the heteroatoms would strongly influence the amount of surface lattice oxygen as well as the mobility of bulk-phase oxygen in these catalysts, thus affecting their activity and stability.     

Role of step sites on water dissociation on stoichiometric ceria surfaces

The adsorption and dissociation of water on CeO₂(111), CeO₂(221), CeO₂(331), and CeO₂(110) has been studied by means of periodic density functional theory using slab models. The presence of step sites moderately affects the adsorption energy of the water molecule but in some cases as in CeO₂(331) is able to change the sign of the energy reaction from endo- to exothermic which has important consequences for the catalytic activity of this surface. Finally, no stable molecular state has been found for water on CeO₂(110) where the reaction products lead to a very stable hydroxylated surface which will rapidly become inactive.     

Interface-controlled synthesis of CeO2(111) and CeO2(100) and their structural transition on Pt(111)

Ceria-based catalytic materials are known for their crystal-face-dependent catalytic properties. To obtain a molecular-level understanding of their surface chemistry, controlled synthesis of ceria with well-defined surface structures is required. We have thus studied the growth of CeO_x nanostructures (NSs) and thin films on Pt(111). The strong metal-oxide interaction has often been invoked to explain catalytic processes over the Pt/CeO_x catalysts. However, the Pt-CeO_x interaction has not been understood at the atomic level. We show here that the interfacial interaction between Pt and ceria could indeed affect the surface structures of ceria, which could subsequently determine their catalytic chemistry. While ceria on Pt(111) typically exposes the CeO₂(111) surface, we found that the structures of ceria layers with a thickness of three layers or less are highly dynamic and dependent on the annealing temperatures, owing to the electronic interaction between Pt and CeO_x. A two-step kinetically limited growth procedure was used to prepare the ceria film that fully covers the Pt(111) substrate. For a ceria film of ~3–4 monolayer (ML) thickness on Pt(111), annealing in ultrahigh vacuum (UHV) at 1000 K results in a surface of CeO₂ (100), stabilized by a c-Ce₂O₃(100) buffer layer. Further oxidation at 900 K transforms the surface of the CeO₂(100) thin film into a hexagonal CeO₂(111) surface.     

Nanosized CeO2–SiO2, CeO2–TiO2, and CeO2–ZrO2 Mixed Oxides: Influence of Supporting Oxide on Thermal Stability and Oxygen Storage Properties of Ceria

The influence of SiO₂, TiO₂, and ZrO₂ on the structural and redox properties of CeO₂ were systematically investigated by various techniques namely, X-ray diffraction (XRD), Raman spectroscopy (RS), UV–Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HREM), BET surface area, and thermogravimetry methods. The effect of supporting oxides on the crystal modification of ceria was also mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahigh dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO₂–SiO₂ sample primarily consists of nanocrystalline CeO₂ on the amorphous SiO₂ surface. Both crystalline CeO₂ and TiO₂-anatase phases were noted in the case of CeO₂–TiO₂ sample. Formation of cubic Ce_0.75Zr_0.25O₂ and Ce_0.6Zr_0.4O₂ (at 1073 K) were observed in the case of CeO₂–ZrO₂ sample. The cell ‘a’ parameter estimations revealed an expansion of the ceria lattice in the case of CeO₂–TiO₂, while a contraction is noted in the case of CeO₂–ZrO₂. The DRS studies suggest that the supporting oxides significantly influence the band gap energy of CeO₂. Raman measurements disclose the presence of oxygen vacancies, lattice defects, and displacement of oxide ions from their normal lattice positions in the case of CeO₂–TiO₂ and CeO₂–ZrO₂ samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation states, Si(IV), Ti(IV), and Zr(IV) at the surface of the materials. Cerium is present in both Ce⁴⁺ and Ce³⁺ oxidation states. The HREM results reveal well-dispersed CeO₂ nanocrystals over the amorphous SiO₂ matrix in the case of CeO₂–SiO₂, isolated CeO₂ and TiO₂ (A) nanocrystals and some overlapping regions in the case of CeO₂–TiO₂, and nanosized CeO₂ and Ce–Zr oxides in the case of CeO₂–ZrO₂ sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO₂ is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of mixed oxides is more than that of pure CeO₂ and the CeO₂–ZrO₂ exhibits highest OSC.     

A DFT Study of the Structures of Aux Clusters on a CeO2(111) Surface

Studying the structures of metal clusters on oxide supports is challenging due to their various structural possibilities. In the present work, a simple rule in which the number of Au atoms in different layers of Au_x clusters is changed successively is used to systematically investigate the structures of Au_x (x=1–10) clusters on stoichiometric and partially reduced CeO₂(111) surface by DFT calculations. The calculations indicate that the adsorption energy of a single Au atom on the surface, the surface structure, as well as the Au? Au bond strength and arrangement play the key roles in determining Au_x structures on CeO₂(111). The most stable Au₂ and Au₃ clusters on CeO₂(111) are 2D vertical structures, while the most stable structures of Au_x clusters (x>3) are generally 3D structures, except for Au₇. The 3D structures of large Au_x clusters in which the Au number in the bottom layer does not exceed that in the top layer are not stable. The differences between Au_x on CeO₂(111) and Mg(100) were also studied. The stabilizing effect of surface oxygen vacancies on Au_x cluster structures depends on the size of Au_x cluster and the relative positions of Au_x cluster and oxygen vacancy. The present work will be helpful in improving the understanding of metal cluster structures on oxide supports.     

Thermal-, photo- and electron-induced reactivity of hydrogen species on rutile TiO2(110) surface: Role of oxygen vacancy

The formation and reactivity of various types of hydrogen species on rutile TiO₂(110), including surface hydroxyl group, surface hydride species and bulk hydrogen species sensitively depend on the oxygen vacancy concentration and structure.