Reduced Cerium Oxide Research Articles

Mechanisms of photoluminescence in cerium oxide pellets with different types of defects

We systematically investigated the photoluminescence (PL) spectra of sintered pellets of cerium oxide, CeO2. The crystal defects were controlled by commonly used techniques in the field of solid-state ionics. We observed the transitions between the O-2p and Ce-4f energy levels in intrinsic CeO2. By doping with trivalent lanthanide ions, which results in oxygen vacancies by charge compensation, a new transition band appears. It involves electronic levels that are similar to those of Ce2O3 clusters, rather than to those of oxygen vacancies. In reduced cerium oxide CeO2-δ, where the number of Ce3+ ions is larger, electrons partially fill the energy levels of defects similar to Ce2O3 clusters, and thus an optical response derived from the Ce-4f–Ce-5d transition appears.

Bulk hydroxylation and effective water splitting by highly reduced cerium oxide: The role of O vacancy coordination

Bulk hydroxylation and effective water splitting by highly reduced cerium oxide: The role of O vacancy coordination

Chemical transformation of additive phase in MgH2/CeO2 hydrogen storage system and its effect on catalytic performance

This study provides important information for understanding the hydrogen storage mechanism of ceria additive incorporated magnesium hydride (MgH2). It is observed that dehydrogenation activation energy of MgH2 can be lowered by at least 50 kJ/mol.H2 upon incorporating a small amount of CeO2 with MgH2. The ceria additive partially transforms to CeO1.96 and hcp Ce2O3 phases when we mechanically mix ceria with MgH2. Subsequently, upon further dehydrogenation at 300 °C, the remaining CeO2, CeO1.96 and hcp Ce2O3 phases transform to cubic Ce2O3. A substantial quantity of ceria undergoes crystalline to amorphous transformation upon intense mechanical milling but the crystallinity gets restored during hydrogenation/dehydrogenation reactions. With appropriate evidence, the current study also explains, “how reduced cerium oxide phase(s) may be mistaken as cerium hydride(s)” and “why the beliefs existing in the literature regarding CeH2 as the active in situ catalyst in MgH2/CeO2 hydrogen storage system needs to be further scrutinized”.

Structure of Reduced Cerium Oxide Ultrathin Films on Pt(111): Local Atomic Environment and Long‐Range Order

AbstractTo optimize the catalytic functionality of cerium oxide it is important to understand the structural modifications associated with reduction and the role of the proximity of metals, which are often coupled with the oxide in the applications. For this purpose, the evolution of the short‐ and long‐range structure of cerium oxide ultrathin epitaxial films and nanostructures supported on Pt(111) is investigated using X‐ray absorption spectroscopy at the Ce L3 edge and surface X‐ray diffraction, during reduction by thermal treatments in vacuum. In epitaxial nanoislands reduction is associated with a contraction of the Ce–O distance and with the appearance of CePt bonds. The formation of a phase with a (2 × 2) periodicity after a thermal treatment at 1023 K is ascribed to the formation of a Pt5Ce alloy. Films of 3 nm thickness do not show, on average, significant structural modifications with the same thermal treatment, consistent with the hypothesis that the reduction involves only the topmost surface layers and it does not influence significantly the bulk structure of the material. This study demonstrates a strong interaction between cerium oxide and platinum, which has implications for the reactivity and stability of catalysts based on metals combined with reducible oxides.

Toward the design of monolithic 23.1% efficient hysteresis and moisture free perovskite/c-Si HJ tandem solar cell: a numerical simulation study

In recent years, research and development of high-efficiency perovskite solar cells (PSCs) have gained momentum. However, the inherent issues of high-temperature stability and hysteresis have constrained the device from commercial feasibility. Researchers have proposed different electron transport layer (ETL) based PSCs to minimize the aforesaid issues. Recently, reduced cerium oxide/[6,6]-phenyl-C61-butyric acid methyl ester (CeOx/PCBM ETL) based PSC device is developed with power conversion efficiency (PCE) of 16.85% and improved stability. In the present work, CeOx/PCBM ETL based PSC device is simulated and calibrated to provide the scope for further improvement in terms of the overall conversion efficiency of the device. The device is further optimized by parametric variation such as doping and thickness of CeOx/PCBM ETL layers. The optimized device with added carbon nanotubes CNTs (to enhance moisture stability) is employed in the monolithic tandem solar cell, and the efficiency potential of a monolithic, hysteresis and moisture free perovskite/crystalline silicon heterojunction (c-Si HJ) tandem solar cell is investigated. Silicon-based, i.e. hydrogenated p-type microcrystalline silicon oxide (µc-Si1−xOx:H) and hydrogenated n-type amorphous silicon tunnel junction (TJ) is used to model the TJ between two diodes. Comprehensive analysis and optimization of the tandem device are done in terms of optical and electrical performance with different thicknesses of perovskite and c-Si. The tandem device proposed in this work yielded a maximum PCE of 23.08%.

In Situ Study of Particle Precipitation in Metal-Doped CeO2 during Thermal Treatment and Ion Irradiation for Emulation of Irradiating Fuels

Metallic particles formed in oxide fuels (e.g., UO2) during neutron irradiation have an adverse impact on fuel performance. A fundamental investigation of particle precipitation is needed to predict the fuel performance and potentially improve fuel designs and operations. This study reports on the precipitation of Mo-dominant β-phase particles in polycrystalline CeO2 (surrogate for UO2) films doped with Mo, Pd, Rh, Ru, and Re (surrogate for Tc). In situ heating scanning transmission electron microscopy indicates that particle precipitation starts at ∼1073 K with a limited particle growth to ∼10 nm. While particle concentration increases with increasing temperature, particle size remains largely unchanged up to 1273 K. There is a dramatic change in the microstructure following vacuum annealing at 1373 K, probably due to phase transition of reduced cerium oxide. At the high temperature, particles grow up to 75 nm or larger with distinctive facets. The particles are predominantly composed of Mo with a body-c...

On the structural stability of crystalline ceria phases in undoped and acceptor-doped ceria materials under in situ reduction conditions.

The reduction of pure and Sm-doped ceria in hydrogen has been studied by synchrotron-based in situ X-ray diffraction to eventually prove or disprove the presence of crystalline cerium hydride (CeH x ) phases and the succession of potential structural phase (trans)formations of reduced cerium oxide phases during heating-cooling cycles up to 1273 K. Despite a recent report on the existence of bulk and surface CeH x phases during reductive treatment of pure CeO2 in H2, structural analysis by Rietveld refinement as well as additional 1H-NMR spectroscopy did not reveal the presence of any crystalline CeH x phase. Rather, a sequence of phase transformations during the re-cooling process in H2 has been observed. In both samples, the reduced/defective fluorite lattice undergoes at first a transformation into a bixbyite-type lattice with a formal stoichiometry Ce0.58 3+Ce0.42 4+O1.71 and Sm0.15 3+Ce0.39 3+Ce0.46 4+O1.73, before a transformation into rhombohedral Ce7O12 takes place in pure CeO2. This phase is clearly absent for the Sm-doped material. Finally, a triclinic Ce11O20 phase appears for both materials, which can be recovered to room temperature, and on which a phase mixture of bixbyite Ce0.66 3+Ce0.34 4+O1.67, rh-Ce0.60 3+Ce0.40 4+O1.70 and tri-Ce0.48 3+Ce0.52 4+O1.76 (for pure CeO2) or bixbyite Sm0.15 3+Ce0.47 3+Ce0.38 4+O1.69 and tri-Sm0.15 3+Ce0.31 3+Ce0.54 4+O1.77 (for Sm-doped CeO2) prevails. The absence of the rhombohedral phase indicates that Sm doping leads to the stabilization of the bixbyite phase over the rhombohedral one at this particular oxygen vacancy concentration. It is worth noting that recent work proves that hydrogen is indeed incorporated within the structures during the heat treatments, but under the chosen experimental conditions it has apparently no effect on the salient structural principles during reduction.

Numerical simulations: Toward the design of 18.6% efficient and stable perovskite solar cell using reduced cerium oxide based ETL

This work presents an extensive study of one of the potential alternatives in electron transport material (ETM) for perovskite solar cell (PSC). Reduced cerium oxide (CeOx) as an electron transport layer (ETL) is studied for different composition of oxygen and analysis with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) interface layer thickness if stacked in between CeOx ETL and perovskite layer has also been done. A detailed investigation has been carried out, incorporating the mobility of CeOx due to small polaron hopping mechanism at the different composition of x, the effect of variation in doping concentration for CeOx and PCBM have also been studied and the optimum efficiency 18.2% is obtained for PSC. Furthermore, to improve the shortcomings in terms of hysteresis and moisture stability, a device is proposed incorporating carbon nanotube (CNT) layer stacked between perovskite and hole transport layer (HTL) which shows 18.6% conversion efficiency.

Nucleation, morphology, and structure of sub‐nm thin ceria islands on Rh(111)

The early stages of ceria growth on Rh(111) at high temperature have been investigated by low‐energy electron microscopy and photoemission electron microscopy. Ceria was deposited by reactive Ce deposition at substrate temperatures between 700°C and 900°C in an oxygen ambient of 5 × 10−7 Torr. At 700°C, we observe a high nucleation density of 100‐nm‐sized islands. With elevated temperature, the average island size increases, and the nucleation density decreases. Triangularly shaped islands nucleate preferentially at step edges, with seemingly abrupt interfaces between Ce and Rh. At 900°C, the island edges are still straight, but during growth the islands lose their triangular form. Instead, growth along the substrate step edges becomes favorable, leading to a maze‐like morphology. Atomic force microscopy reveals islands of 0.3 to 0.6‐nm height, consistent with ceria islands formed by one or two trilayers (O―Ce―O) of ceria. Moreover, the second layer of the islands is also triangularly shaped, with lateral dimensions of 50 nm and similar step heights. IV‐LEEM analysis leads to the conclusion that the rhodium surface is covered by a layer of reduced cerium oxide, which is partially overgrown by smaller islands of CeO2.

Oxychlorination of methane over FeOx/CeO2 catalysts

Methane activation through oxychlorination is in the spotlight due to the relatively mild reaction conditions at atmospheric pressure and in the temperature range of 450–550 °C. Although CO2 is known to exhibit good activity for methane oxychlorination, significant amounts of by-products such as CO2, CO and carbon deposits are produced during the reaction over CO2. We investigated the effect of iron in FeOx/CO2 catalysts on methane oxychlorination. FeOx/CO2 with 3 wt% iron shows the maximum yield at 510 °C with 23% conversion of methane and 65% selectivity of chloromethane. XRD and H2 TPR results indicate that iron-cerium solid solution was formed, resulting in the production of more easily reduced cerium oxide and the suppression of catalysts sintering during the reaction. Furthermore, the selectivity of by-products decreased more significantly over FeOx/CO2 than cerium oxide, which can be attributed to the facilitation of HCl oxidation arising from the enhanced reducibility of the former sample.

Bulk Hydroxylation and Effective Water Splitting by Highly Reduced Cerium Oxide: The Role of O Vacancy Coordination

Reactions of reduced cerium oxide CeOx with water are fundamental processes omnipresent in ceria-based catalysis. Using thin epitaxial films of ordered CeOx, we investigate the influence of oxygen vacancy concentration and coordination on the oxidation of CeOx by water. Upon changing the CeOx stoichiometry from CeO2 to Ce2O3, we observe a transition from a slow surface reaction to a productive H2-evolving CeOx oxidation with reaction yields exceeding the surface capacity and indicating the participation of bulk OH species. Both the experiments and the ab initio calculations associate the effective oxidation of highly reduced CeOx by water to the next-nearest-neighbor oxygen vacancies present in the bixbyite c-Ce2O3 phase. Next-nearest-neighbor oxygen vacancies allow for the effective incorporation of water in the bulk via formation of OH– groups. Our study illustrates that the coordination of oxygen vacancies in CeOx represents an important parameter to be considered in understanding and improving the reactivity of ceria-based catalysts.

In Situ Characterization of Mesoporous Co/CeO2 Catalysts for the High-Temperature Water-Gas Shift

Mesoporous Co/CeO2 catalysts were found to exhibit significant activity for the high-temperature water-gas shift (WGS) reaction with cobalt loadings as low as 1 wt %. The catalysts feature a uniform dispersion of cobalt within the CeO2 fluorite type lattice with no evidence of discrete cobalt phase segregation. In situ XANES and ambient pressure XPS experiments were used to elucidate the active state of the catalysts as partially reduced cerium oxide doped with oxidized cobalt atoms. In situ XRD and DRIFTS experiments suggest facile cerium reduction and oxygen vacancy formation, particularly with lower cobalt loadings. In situ DRIFTS analysis also revealed the presence of surface carbonate and bidentate formate species under reaction conditions, which may be associated with additional mechanistic pathways for the WGS reaction. Deactivation behavior was observed with higher cobalt loadings. XANES data suggest the formation of small metallic cobalt clusters at temperatures above 400 °C may be responsible. N...

Lead-Free Sn-Ce-O Composite Coating on Cu Produced by Pulse Electrodeposition from an Aqueous Acidic Sulfate Electrolyte

Pulse-electrodeposited Sn-Ce-O composite solder coatings were synthesized on a Cu substrate from an aqueous acidic solution containing stannous sulfate (SnSO4·3H2O), sulfuric acid (H2SO4), and Triton X-100 as an additive. The codeposition was achieved by adding nano-cerium oxide powder in varying concentrations from 5 g/L to 20 g/L into the electrolytic bath. Microstructural characterization was carried out using x-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy. The XRD analysis showed that the deposits consist mainly of tetragonal β (Sn) with reduced cerium oxide species. The composite coatings thus obtained exhibit a smaller grain size, possess higher microhardness, and a lower melting point than the monolithic Sn coating. The electrical resistivity of the developed composites increases, however, but lies within the permissible limits for current lead-free solder applications. Also, an optimum balance of properties in terms of microhardness, adhesion, melting point and resistivity can be obtained with 0.9 wt.% cerium oxide in the Sn matrix, which enables potential applications in solder joints and packaging.

System efficiency for two-step metal oxide solar thermochemical hydrogen production – Part 1: Thermodynamic model and impact of oxidation kinetics

A comprehensive solar-to-hydrogen (STH) efficiency model, which includes the effect of oxidation kinetics, is developed for two-step solar thermochemical redox water splitting processing. Active materials flow through separate reduction and oxidation reactors. Two active redox materials are considered and compared in order to assess the impact of the rate of redox and the hydrogen productivity per cycle on STH efficiency. Reported oxidation rates for reduced cerium oxide (fast kinetics/lower H2 productivity/cycle) and a ferrite/zirconia composite (slow kinetics/higher H2 productivity/cycle) are used in the model in order to make a realistic comparison. Generally, the efficiency at thermodynamic equilibrium is higher for the ferrite/zirconia composite than ceria. Interactions between material specific parameters are compared, such as the combination of heat capacity and flow rate on sensible heating loads. Additionally, the sensitivity of oxidation kinetics on the overall cycle efficiency is illustrated. Model results show that kinetics can have a drastic effect on STH efficiency. Near-isothermal redox processing is more optimal for materials with slower kinetics, especially when moderate to high gas heat recuperation is possible. The kinetic effects are negligible for active materials having fast oxidation rates, i.e. ceria, which benefit from a larger temperature difference (thermodynamic driving force) between the reduction and oxidation steps. This leads to different optimal operating conditions when oxidation kinetics are included in the analysis as compared to prior models when only thermodynamic equilibrium is considered.

Effect of the surface state on the catalytic performance of a Co/CeO2 ethanol steam-reforming catalyst

This work examines the impact of the Co/CeO2 catalysts’ surface oxidation state and composition on the ethanol steam reforming (ESR) reaction performance. To this purpose, in situ and ex situ X-ray photoelectron spectroscopy (XPS) combined with on-line mass spectrometry were applied in a wide pressure range (0.2–20mbar). When the reaction was performed at 0.2mbar, metallic cobalt and partly reduced cerium oxide were found regardless of the catalysts’ pretreatment conditions. This surface state favors CO and H2 production, indicating that CC bond cleavage is the most important pathway in this pressure regime. A higher reduction degree of ceria gave rise to a larger population of adsorbed hydroxyl groups, which, contrary to the expected behavior, suppressed the activity and the CC bond cleavage yield. Under higher pressure (4–20mbar), gradual oxidation of cobalt and ceria was noted. The presence of ionic cobalt species appears to enhance CO2 and acetaldehyde yields. On the basis of the present results and the available literature a plausible pressure-dependent reaction mechanism is proposed.

Observation of surface reduction in porous ceria thin film grown on graphite foil substrate

In this study, we report transmission electron microscopy and electron energy loss spectroscopy study of cerium oxide thin layer grown on graphite foil substrate by direct liquid injection chemical vapour deposition. Transmission electron microscopy experiments have revealed the porous morphology of the deposited layer. Energy electron loss spectroscopy measurements were also performed in scanning mode to study the evolution of the cerium valence. In addition to Ce4+ inside the layer, the presence of reduced cerium oxide with Ce3+ valence is pointed out at the layer surface and at the surface of the porosity developed by the layer. Besides, studies of high resolution images coupled to electron energy loss spectroscopy have indicated the presence of crystallized ceria nanoparticles, and the absence of cerium carbide.

Impact of Rh–CeOx interaction on CO oxidation mechanisms

We have investigated the impact of electronic metal–support interaction on catalytic properties of Rh/CeOx system, i.e. its capacity for the reverse and direct oxygen spillover, in the context of CO oxidation. The studies were carried out on two types of samples consisting of Rh particles supported on polycrystalline nearly stoichiometric and partially reduced cerium oxide films using temperature programmed reaction, temperature programmed desorption, and conventional X-ray and resonant photoelectron spectroscopies. We have found that the electronic Rh–CeOx interaction leads to a buildup of a net positive charge on Rh particles supported on stoichiometric cerium oxide, and a net negative charge on Rh particles on reduced cerium oxide. The effect of the Rh–CeOx interaction is manifested in suppression of the reverse and direct oxygen spillover on Rh particles supported on partially reduced ceria.

Structural and morphological modifications of thermally reduced cerium oxide ultrathin epitaxial films on Pt(111).

The modifications of the stoichiometry, morphology and surface structure of cerium oxide ultrathin films induced by thermal treatments under vacuum and oxygen partial pressure were studied using in situ X-ray photoemission spectroscopy, scanning tunnelling microscopy and low energy electron diffraction. The effect of the film nominal thickness, heating temperature and heating time on the degree of reduction of the film was investigated. The reduction is more relevant on the film surface, where different ordered surface structures were observed at different degrees of reduction for very thin films. The obtained results are discussed taking into account the dimensionality of the oxide and the effects of the proximity of the Pt substrate. After reduction it was always possible to re-oxidize the films back to their original oxidation state by thermal treatment under oxygen-rich conditions.

Origin of chemical contrast in low-energy electron reflectivity of correlated multivalent oxides: The case of ceria

A combined experimental and theoretical study of the local chemistry of cerium oxide films and islands on Ru(0001) is presented. Based on intensity-voltage low-energy electron microscopy [$I(V)$-LEEM] and resonant x-ray photoemission spectroscopy, we establish a one-to-one correspondence between the local oxidation state of ${\mathrm{Ce}}^{3+}$ [cubic Ce${}_{2}$O${}_{3}$(111)] and ${\mathrm{Ce}}^{4+}$ [cubic CeO${}_{2}$(111)] and their respective spatially resolved $I(V)$ curves. Ab initio scattering theory explains the difference between the $I(V)$ curves in the low-energy range in terms of the ${k}_{\ensuremath{\parallel}}=0$ projected band structure arising from the different structure of the Ce 5d states in fully oxidized and reduced ceria. The theoretical analysis unambiguously attributes the LEEM contrast observed for chemically reduced cerium oxide to a variation in oxidation state on the nanometer scale, which is not present for the as-grown islands.

Catalytic conversion of carbon dioxide into dimethyl carbonate using reduced copper-cerium oxide catalysts as low as 353 K and 1.3 MPa and the reaction mechanism.

Synthesis of dimethyl carbonate (DMC) from CO2 and methanol under milder reaction conditions was performed using reduced cerium oxide catalysts and reduced copper-promoted Ce oxide catalysts. Although the conversion of methanol was low (0.005–0.11%) for 2 h of reaction, DMC was synthesized as low as 353 K and at total pressure of as low as 1.3 MPa using reduced Cu–CeO2 catalyst (0.5 wt% of Cu). The apparent activation energy was 120 kJ mol−1 and the DMC synthesis rates were proportional to the partial pressure of CO2. An optimum amount of Cu addition to CeO2 was 0.1 wt% for DMC synthesis under the conditions at 393 K and total pressure of 1.3 MPa for 2 h (conversion of methanol: 0.15%) due to the compromise of two effects of Cu: the activation of H2 during reduction prior to the kinetic tests and the block (cover) of the surface active site. The reduction effects in H2 were monitored through the reduction of Ce4+ sites to Ce3+ based on the shoulder peak intensity at 5727 eV in the Ce L3-edge X-ray absorption near-edge structure (XANES). The Ce3+ content was 10% for reduced CeO2 catalyst whereas it increased to 15% for reduced Cu–CeO2 catalyst (0.5 wt% of Cu). Moreover, the content of reduced Ce3+ sites (10%) associated with the surface O vacancy (defect sites) decreased to 5% under CO2 at 290 K for reduced Cu–CeO2 catalyst (0.1 wt% of Cu). The adsorption step of CO2 on the defect sites might be the key step in DMC synthesis and thus the DMC synthesis rate dependence on the partial pressure of CO2 was proportional. Subsequent H atom subtraction steps from methanol at the neighboring surface Lewis base sites should combine two methoxy species to the adsorbed CO2 to form DMC, water, and restore the surface O vacancy.