Ceria Defects Research Articles

Comprehensive characterization of irradiation induced defects in ceria: Impact of point defects on vibrational and optical properties

Validation of multiscale microstructure evolution models can be improved when standard microstructure characterization tools are coupled with methods sensitive to individual point defects. We demonstrate how electronic and vibrational properties of defects revealed by optical absorption and Raman spectroscopies can be used to compliment transmission electron microscopy (TEM) and x-ray diffraction (XRD) in the characterization of microstructure evolution in ceria under non-equilibrium conditions. Experimental manifestation of non-equilibrium conditions was realized by exposing cerium dioxide (CeO2) to energetic protons at elevated temperature. Two sintered polycrystalline CeO2 samples were bombarded with protons accelerated to a few MeVs. These irradiation conditions produced a microstructure with resolvable extended defects and a significant concentration of point defects. A rate theory (RT) model was parametrized using the results of TEM, XRD, and thermal conductivity measurements to infer point defect concentrations. An abundance of cerium sublattice defects suggested by the RT model is supported by Raman spectroscopy measurements, which show peak shift and broadening of the intrinsic T2g peak and emergence of new defect peaks. Additionally, spectroscopic ellipsometry measurements performed in lieu of optical absorption reveals the presence of Ce3+ ions associated with oxygen vacancies. This work lays the foundation for a coupled approach that considers a multimodal characterization of microstructures to guide and validate complex defect evolution models.

A novel double modification to enhance electrochemical performance of LiNi0.5Co0.2Mn0.3O2 by substituting Ce for Co site

Ce substitution can not only make Ce3+ doped into the crystal lattice, but stabilize the bulk structure by means of robust Ce-O bond, and form ceria coating layer at outer surface to protect matrix material. The influence of different Ce content on atomic occupation, lattice parameters and Li+/Ni2+ mixing extent were studied at length by Rietveld refinement. Profiting from the synergistic effect of lithium defects in ceria and enlarged crystal plane spacing, Ce 0.15%-NCM has superior electrochemical performance. Cycled at 5 C, Ce 0.15%-NCM has superior initial discharge capacity of 138.7 mAhg−1 and capacity retention of 83.99% after 200 cycles. So far as to the full-cells state, Ce 0.15%-NCM delivers the initial energy density of 406 Wh kg−1 and 72.94% retention after 200 cycles. Combined with the results of HRTEM and EDS spectrum, the cycled electrodes were investigated, and the mechanism of Ce modification at high potential cycling was further studied.

Strain‐Engineered Formation, Migration, and Electronic Properties of Polaronic Defects in CeO2

First‐principles investigations on the strain‐engineered formation, electronic structures, and migration properties of polaronic defects in ceria (CeO2), including single polarons, oxygen vacancies (Vo2+), and polaron–vacancy complexes [(Vo2+–1polaron)1+, (Vo2+–2polaron)0], are reported. Results show that the formation energy of oxygen vacancy increases with both tensile and compressive biaxial strain, whereas the formation energies of polarons and polaron–vacancy complexes reduce (increase) with tensile (compressive) strain, so that their defect concentrations behave drastically different with strain and temperature. Interestingly, due to the distinct deformation potentials, the polaronic defect states can shift toward band edges with strain, beneficial for enhanced electrical conductivity. The migration of polarons in CeO2 is further explored, and a tendency of strain‐induced directional migration is revealed. These findings not only shed light on the fundamental properties of polaronic defects in CeO2 but also provide an attractive approach of combining defect engineering with strain engineering for oxide‐based functional microelectronics, batteries, and catalysts.

CO Oxidation on Ceria Supported Gold Catalysts Studied by Combined Operando Raman/UV–Vis and IR Spectroscopy

We present a novel spectroscopic approach using a combination of operando Raman/UV–Vis spectroscopy and operando IR spectroscopy. It is demonstrated that a ceria supported gold catalyst exhibits the same catalytic activity profile in both setups when using room temperature CO oxidation as model reaction. Detailed analysis of the UV–Vis and visible Raman spectra during reaction reveals changes in the oxidation state of gold and the number of oxygen-vacancy defects in ceria. DRIFT spectra provide complementary information on adsorbates, in particular carbonates and hydrogen carbonates, as well as hydroxyl groups. Time-resolved operando spectra allow for a direct correlation with catalytic activity thus providing detailed information on activation and deactivation processes of ceria supported gold particles.

Disinfection effects of undoped and silver-doped ceria powders of nanometer crystallite size.

Being endowed with an ability of capturing and releasing oxygen, the ceria surface conventionally assumes the role of catalyzing redox reactions in chemistry. This catalytic effect also makes possible its cytotoxicity toward microorganisms at room temperature. To study this cytotoxicity, we synthesized the doped and undoped ceria particles of 8–9 nm in size using an inexpensive precipitation method and evaluated their disinfecting aptitudes with the turbidimetric and plate count methods. Among the samples being analyzed, the silver-doped ceria exhibits the highest sterilization ability, yet the undoped ceria is the most intriguing. The disinfection effect of undoped ceria is moderate in magnitude, demanding a physical contact between the ceria surface and bacteria cell wall, or the redox catalysis that can damage the cell wall and result in the cell killing. Evidently, this effect is short-range and depends strongly on dispersion of the nanoparticles. In contrast, the disinfection effects of silver-doped ceria reach out several millimeters since it releases silver ions to poison the surrounding microorganisms. Additionally, the aliovalent silver substitution creates more ceria defects. The synergetic combination, silver poisoning and heterogeneous redox catalysis, lifts and extends the disinfecting capability of silver-doped ceria to a superior level.

Hard-template preparation of Au/CeO2 mesostructured catalysts and their activity for the selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

Ordered mesoporous CeO2 has been prepared via a hard-template method using SBA-15 as a structure-directing agent. Leaching with NaOH and thermal treatment at 500 °C enabled the removal of the inorganic template, thus resulting in the formation of long-range ordered CeO2. Nevertheless, small amounts of silica were present in the final oxides. The resulting CeO2 samples were used as supports for Au nanoparticles and the prepared catalysts were tested in the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). Fine tuning of the silica template removal process was necessary to prepare active materials, by maximizing the close contact between the metal and the support, and thus confirming the important role of ceria defects in 5-hydroxymethylfurfural oxidation to 2,5-furandicarboxylic acid.

Effect of chemical redox on Gd-doped ceria mass diffusion

Cations’ valence and size influence mass diffusion and oxygen defects in ceria. Mass diffusion is limited by Gd3+ under oxidative conditions, where Ce4+ is the host cation, while it is unleashed under reducing conditions, where Ce3+ and Gd3+ have the same charge and similar size. Solid solutions with enhanced electrochemical properties are formed under controlled thermo-chemical conditions.

Conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over Au-based catalysts: Optimization of active phase and metal–support interaction

In the present work, a series of Au- and Au–Cu-containing catalysts based on different carriers such as TiO2 and CeO2 were studied with a view to increasing the activity and selectivity in 5-hydroxymethylfurfural oxidation by optimizing the interaction both between the metals and with the support. The results obtained demonstrated the high activity in HMF oxidation of gold supported on ceria and titania. Nevertheless, although the particle size of gold on both supports was comparable, Au/CeO2 showed significantly higher activity than Au/TiO2, thus corroborating the theory that not only the gold particle size, but the support also, plays a key role in HMF oxidation in the aqueous phase. Indeed, pre-made uniform nanoparticles, used for catalyst preparation, were surface-bound by poly(N-vinyl-2-pyrrolidone) – the stabilizer used during nanoparticle synthesis – whose presence proved to prevent the interaction of active phases with CeO2, while worsening the catalytic activity of both monometallic and bimetallic systems. The pre-treatment of the prepared catalysts was necessary to activate the materials, by maximizing the contact between the metal and the support and thus suggesting an important role of the ceria defects on 5-hydroxymethylfurfural oxidation to 2,5-furandicarboxylic acid.

The Influence of Platinum Addition on Nano-Crystalline Ceria Catalysts for the Total Oxidation of Naphthalene a Model Polycyclic Aromatic Hydrocarbon

The effect of adding Pt to a highly active ceria polycyclic aromatic hydrocarbon total oxidation catalyst has been investigated for the oxidation of naphthalene. The addition of Pt to ceria suppressed the performance of the catalyst for total oxidation. The addition of Pt reduced catalyst surface area, decreased the ceria crystallite size, decreased the concentration of ceria defects and increased the reducibility of the catalyst. The suppression of activity has been attributed to strong metal-support interaction between Pt and ceria, which limits the availability of lattice oxygen for the oxidation process, which follows the Mars-Van Krevelen redox mechanism in the absence of Pt. It is postulated that the presence of dispersed Pt, alters the mechanism of naphthalene oxidation over the ceria catalyst. The addition of platinum to a highly active naphthalene oxidation ceria catalyst modifies activity.

Enhanced Oxygen Buffering by Substitutional and Interstitial Ni Point Defects in Ceria: A First-Principles DFT+U Study

The defect chemistry and electronic structure of NiO/CeO2 solid solutions are studied by means of DFT+U calculations in the limit of low Ni doping. We consider four representative solid solutions in which the Ni atoms are present as substitutional and interstitial point defects in bulk crystalline CeO2, both in its stoichiometric form and in the presence of O vacancies. In all cases, Ni-doping significantly enhances the O buffering effect of ceria, controlled by O vacancy formation, but the actual microscopic mechanisms are different depending on the specific type and charge state of the point defects. The oxidation state of the Ni dopant is shown to univocally characterize the type of defect, whether interstitial (Ni+) or substitutional (Ni2+). Interstitial Ni+ defects result from a charge redistribution between the Ni and Ce cations that leads to the formation of characteristic Ni+−Ce3+ defect complexes. O release via vacancy formation in these interstitial solid solutions proceeds similarly as in pure ...

CO Adsorption on Gold Clusters Stabilized on Ceria−Titania Mixed Oxides: Comparison with Reference Catalysts

Fourier transform infrared spectra of CO adsorption from 120 K up to room temperature on two gold catalysts supported on different mixed ceria-titania oxides are discussed in comparison with those obtained on Au/TiO(2) and Au/Fe(2)O(3) reference catalysts provided by the World Gold Council. The spectra of adsorbed CO, run on the different samples before preliminary treatment, are shown and compared with those of the untreated catalysts and of the samples reduced either in CO or in hydrogen. Big differences have been found between the ceria-titania supported samples and the reference ones: unusual absorption bands, irreversible to outgassing, have been detected after CO interaction on the untreated and oxidized ceria containing samples. These absorptions are assigned to CO on Au(n)(+) small clusters stabilized at the ceria defects. By reduction in hydrogen, negatively charged Au(n)(-) species are produced on the same sample. Oxidized small particles are present on the reference catalysts, but only on the untreated samples; after treatment, only metallic step sites are evident.

The electronic structure of oxygen vacancy defects at the low index surfaces of ceria

Ceria, CeO 2, plays an important role in catalysis, participating directly in the conversion of environmentally sensitive molecules. This arises from the ability of ceria to store and release oxygen depending upon the conditions present in the exhaust gas. Obtaining a basic understanding of oxygen vacancy defects in ceria and the interaction of defective structures with such molecules is central to our understanding of the role of ceria in catalysis. In this work we examine using first principles density functional theory (DFT), with the inclusion of on site electronic correlations (DFT + U), the geometry and electronic structure of (1 1 1), (1 1 0) and (1 0 0) ceria surfaces that include oxygen vacancies. We find for all surfaces that the surface (atomistic) structure is strongly perturbed and the extraction of an oxygen vacancy is associated with a reduction of two neighbouring Ce(IV) species to Ce(III) rather than partial reduction of all Ce ions in the simulation cell. In the electronic density of states a new gap state appears between the top of the valence band and the bottom of the unoccupied Ce 4f states. Localisation of charge due to the gap state and excess spin density on Ce 3+ sites neighbouring the vacancy is observed for all three surfaces. These DFT + U results are validated by recent experimental results regarding the electronic structure of reduced ceria surfaces, in contrast to previous DFT results. We observe an interesting result that the vacancy formation energies do not follow the same order as the stabilities of the pure surfaces, as measured by the surface energy; thus, the (1 1 0) surface has the lowest vacancy formation energy. The impact of this for the study of catalytic reactions on ceria surfaces is discussed.

Effect of synthesis procedure on the low-temperature WGS activity of Au/ceria catalysts

Gold/ceria catalysts were prepared by two different methods (deposition-precipitation and modified deposition-precipitation). Considerable differences in the low-temperature water-gas shift (WGS) activity of both catalysts were observed. The preparation technique strongly influenced catalytic activity due to the large differences in gold particles size and to the availability of active gold sites in close contact with ceria defects on the surface. HRTEM and EDS have shown presence of very highly dispersed gold clusters (d about 1nm) on the surface of the catalyst prepared by deposition-precipitation. Large gold particles (average d≈15nm) have been found in gold/ceria catalyst prepared by modified deposition-precipitation. FTIR spectroscopy of adsorbed CO and X-ray photoelectron spectroscopy were employed to obtain information about the surface structure of both catalysts and to explain the differences in catalytic activity.

Thermodynamic Correlation Among Defects in Ceria—Zirconia Solid Solutions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Cadmium and indium defects in ceria and their interaction with oxygen vacancies and small polarons.

Atomistic simulation calculations have been used to investigate the energetics of defect clustering and migration in ceria. The defects considered are ${\mathrm{In}}^{3+}$, ${\mathrm{Cd}}^{2+}$, their associated oxygen vacancies, and small polarons modeled as ${\mathrm{Ce}}^{3+}$ ions. Thus a range of complex defect clusters is considered. The overall aim of the study is to generate a better understanding of these defects as they relate to recent experimental results obtained using perturbed-angular-correlation spectroscopy. The calculations are successful in this regard, correctly predicting both binding energies and an oxygen migration activation energy. More importantly, the calculations provide an atomistic explanation for certain of the experimental observations. As such, the synergy between calculations and experiment is an important feature of this paper.