Bulk Oxygen Research Articles

Unraveling the importance between electronic intensity and oxygen vacancy on photothermocatalytic toluene oxidation over CeO2

• CeO 2 with different electron density is obtained via regulation on facet and oxygen vacancy. • High electron density plays more important role than oxygen vacancy in photothermocatalytic toluene oxidation. • Toluene conversion rate can achieve 93% at 145 °C under GHSV of 60,000 mL·g −1 ·h −1 . • Decrement of electron density and increase of oxygen vacancy are responsible for the decline of activity. Developing low-temperature, high-efficiency and anti-poisoning catalysts is crucial for VOCs removal. CeO 2 materials with different electron density are prepared via regulation on facets and oxygen vacancies. It reveals that high electron density of materials plays a decisive role in the production of reactive oxygen species, reducibility of surface lattice oxygen and improvement of photothermocatalytic activity for toluene oxidation compared with highly concentrated oxygen vacancies and large special surface area. Photothermocatalytic conversion rate for 1500 ppm of toluene over the optimized defective CeO 2 could reach 93 % at 145 °C under the gas hourly space velocity of 60,000 mL·g −1 ·h −1 . H 2 -TPR result shows that excessive oxygen vacancies are detrimental to the appearance of mobile reactive oxygen species due to the strong chemical interaction and the escape probability of surface lattice oxygen of CeO 2 . It is also found that the decrement of electron density of the recovered sample can account for the slight catalyst deactivation, accompanied by the increasement of bulk oxygen vacancies in the recovered sample, exclusive of disappearance of surface oxygen vacancies, aggregations and deposition of organic carbon. This work provides an insight into the design of high-efficiency non-noble catalysts applied to VOCs removal.

An optimized numerical method on corrosion in the non–isothermal lead–bismuth eutectic loops with solid–phase oxygen control system

In the present work, a new calculation method for oxygen and iron concentration parameters that can be used to calculate corrosion was proposed for a non–isothermal liquid lead–bismuth eutectic (LBE) loop with solid–phase oxygen control system. We aimed to optimize the prediction of one–dimensional distribution of the bulk oxygen and iron concentrations, as well as the dissolution/precipitation rates of corrosion products along the entire LBE loop by incorporating the dissolution process of PbO particles and the oxygen mass transfer process. In numerical terms, three mass transfer models were simultaneously applied. The simulation results indicated that the iron concentration dissolved in the liquid LBE was negatively related to oxygen concentration, and correlation between dissolution/precipitation rate and temperature clearly revealed that dissolution and precipitation areas corresponded respectively to hot region and cold region in the LBE loop. Dissolved corrosion products were transported by the flowing liquid to the cold region of the loop and re–precipitates there. These regulars were consistent with previous reports. Comparisons of calculated and experimental results from CRAFT and DELTA loops indicated that the optimized calculation method and corrosion model were applicable to other LBE loops.

Perovskite Oxides in Catalytic Combustion of Volatile Organic Compounds: Recent Advances and Future Prospects

Volatile organic compounds are a kind of important indoor and outdoor air pollutants. In recent years, more and more attention has been paid to the ways of volatile organic compound elimination because of its potential long‐term effects on human health. Among the various available methods for volatile organic compound elimination, the catalytic combustion is the most attractive method due to its high efficiency, low cost, simple operation, and easy scale‐up. Perovskite oxides, as a large family of metal oxides with their A‐site mainly of lanthanide element and/or alkaline earth metal element and B‐site of transition metal element, have been extensively investigated as active and stable catalysts for volatile organic compound removal reactions due to their abundant compositional elements, high thermal/chemical stability, and compositional/structural flexibility. The catalytic performance of perovskite oxides is strongly depended on its material composition, morphology, and surface/bulk properties, while the doping, tailored synthesis route, and composite construction may have a significant effect on the bulk (oxygen vacancy concentration, lattice structure), surface (oxygen species, defect) properties, and particulate morphology, consequently the catalytic activity and stability for volatile organic compound removal. Herein, a comprehensive review about the recent advances in perovskite oxides for volatile organic compound elimination reactions based on catalytic combustion is presented from different aspects with a special emphasis on the material design strategies, such as compositional tuning, morphology control, nanostructure building, hybrid construction, and surface modification. At last, some perspectives are presented on the development and design of perovskite oxide‐based catalysts for volatile organic compound removal applications by highlighgting the critical issues and challenges.

Oxygen vacancy concentration in nanoobjects of CeO2−δ: Effects of characteristic size, morphology, and temperature

A theoretical model has been developed that adequately describes the formation of oxygen vacancies in ceria at the nanoscale. The intrinsic regime of vacancy formation under equilibrium conditions is considered. An analytical relation describing the dependence of the enthalpy of oxygen vacancy formation on the characteristic size and morphology of nanoobjects is derived. This made it possible to perform quantitative calculations of the concentration of oxygen vacancies in cerium oxide nanoobjects for a wide range of their characteristic sizes, morphology and temperature. It is shown that the size effect becomes noticeable for the concentration of surface and bulk oxygen vacancies, approximately, at the characteristic sizes of nanoobjects less than 20 nm and 30 nm, respectively. The effect of morphology on the concentration of oxygen vacancies increases with a decrease in the characteristic size of nanoobjects. This effect is more significant for the concentration of bulk oxygen vacancies than for the concentration of surface oxygen vacancies. In addition, the morphological effect and the size-effect decrease with increasing temperature. In thin films of ceria, the concentration of bulk oxygen vacancies at 300 K is 1011 and 103 times lower than that of spherical ceria nanoparticles, with their characteristic size of 4 nm and 40 nm, respectively. At 1000 K, this ratio is 3∙103 and 7, respectively. Thus, the oxygen storage capacity is higher for spherical nanoparticles and increases with a decrease in their characteristic size.

The effects of bulk composition on planetesimal core sulfur content and size

This study explores the compositions and sizes of metallic cores that result from planetesimals forming from a range of chondritic bulk compositions. Our models examine the influence of starting bulk composition on core size and composition, how oxygen fugacity (fO2), temperature, pressure, and bulk composition affect sulfur partitioning between the core and silicate mantle of planetesimals, and the formation and fate of immiscible sulfur-rich liquid during core solidification. We apply experimentally-derived equations for the sulfur distribution coefficient to the bulk compositions of ordinary chondrites (H,L,LL) and carbonaceous chondrites (CM, CI, CO, CK, CV) under conditions appropriate for melting planetesimals.The sulfur content of all modeled cores is above 6 wt% S, which is greater than the amount of sulfur needed to form an immiscible sulfide liquid in the presence of other light elements (e.g., C, Si, and/or P). We concluded that early planetesimal cores likely formed either an immiscible sulfide liquid, a eutectic sulfide liquid, or most surprisingly, were composed of mostly monosulfide solid solution, [(Fe, Ni)1-xS].

A Baltic Perspective on the Early to Early Late Ordovician δ13C and δ18O Records and Its Paleoenvironmental Significance

AbstractThe current study presents new bed‐by‐bed brachiopod δ13C and δ18O records from Öland, Sweden, which together with previously published data from the East Baltic region, constitutes a high‐resolution paired brachiopod and bulk rock carbon and oxygen isotope archive through the Lower to Upper Ordovician successions of Baltoscandia. This new data set refines the temporal control on the global Ordovician δ18O‐trend considerably, improving paleoenvironmental reconstructions through the main phase of the Great Ordovician Biodiversification Event (GOBE). The new brachiopod carbon and oxygen isotope records from Öland display strong similarity with the East Baltic records, elucidating the regional consistency as well as global correlation utility of the ensuing composite Baltoscandian Lower to Middle Ordovician carbon and oxygen isotope record. The carbon isotope record from Öland indicates that the widely reported Middle Ordovician carbon cycle perturbation—MDICE (Mid‐Darriwilian Carbon Isotope Excursion)—is recorded in both brachiopods and bulk carbonates. The oxygen isotope record reveals a long‐term Lower to Upper Ordovician trend of increasingly heavier brachiopod δ18O values, with a pronounced increase during the Middle Ordovician Darriwilian Stage. We interpret this trend as dominantly reflecting a paleotemperature signal indicating progressively cooler Early to Middle Ordovician climate with glacio‐eustasy. Our Baltic δ18O values are therefore consistent with postulations that the biotic radiations during the GOBE and climatic cooling during the Darriwilian were strongly linked.

A Strategy to Enhance the Catalytic Activity of Electrode Materials by Doping Bismuth for Symmetrical Solid Oxide Electrolysis Cells

Lanthanum chromate-based perovskites are potential ceramic electrodes for symmetrical solid oxide electrolysis cells (Sym-SOECs). In order to improve its electrochemical activity for CO2 reduction reaction (CO2RR), bismuth is introduced into the A-site. While all the components are stable in the anode atmosphere, the cathode application limits the doping content to 0.2, below which bismuth has the suboxidation state between 0 and +3. As revealed with experimental investigations and density functional theory calculations, doping bismuth could effectively increase the physicochemical properties for CO2RR, including oxygen vacancy concentration, CO2 adsorption capacity, conductivities in CO–CO2 atmospheres, oxygen bulk diffusion coefficient, as well as surface exchange coefficient. Consequently, Rp for CO2RR is reduced by 82% to 1.07 Ω cm2 at 800 °C. And the Sym-SOEC performance for CO2 electrolysis is increased by 46% to 0.79 A cm–2 at 800 °C, 1.5 V, indicating that bismuth-doped material is an excellent ceramic electrode material for Sym-SOECs.

Flexible TiO2 nanograss array film decorated with oxygen vacancies introduced by facile chemical reduction and their photocatalytic activity

Flexible TiO 2 nanograss array film was prepared by simple chemical oxidation of Ti wire mesh at a low temperature of 353 K. To improve its visible-light-driven photocatalytic activity, we further carried out a low-temperature chemical reduction to introduce oxygen vacancies in the solution of ethylene glycol (EG) and ammonium fluoride (NH4F). The results of XPS and EPR confirmed the formation of surface and bulk oxygen vacancies was confirmed, and their concentrations increased as the reduction time increased. Compared with the as-annealed TiO 2 film, the as-reduced specimens exhibited higher visible-light-driven photocatalytic activity. When reduction time was 48h, the photocatalytic activity was increased by 15.8 times compared to the as-annealed sample. The remarkable improvement in photocatalytic activity should be ascribed to the oxygen vacancies’ introduction. The most excellent photocatalytic performance was obtained when a suitable concentration ratio of surface oxygen vacancies to bulk oxygen vacancies was reached. Low-temperature chemical reduction in the solution of EG + NH4F is very efficient and convenient for introducing defects such as oxygen vacancies and Ti3+.

Boosting CO2 directly electrolysis by electron doping in Sr2Fe1.5Mo0.5O6-δ double perovskite cathode

Commercialization of solid oxide electrolysis cell (SOEC) depends on the development of cathode materials with outstanding catalytic activity for CO2 electroreduction. Herein, Pure La-doped Sr2-xLaxFe1.5Mo0.5O6-δ double perovskite oxide is prepared via the citric acid-glycine method and used as a cathode to evaluate its electrochemical performance for CO2 electrolysis. La substitution at Sr-site promotes the oxygen surface exchange and bulk diffusion in Sr2-xLaxFe1.5Mo0.5O6-δ oxide, leading to higher CO2 electrolysis performance compared to Sr2Fe1.5Mo0.5O6-δ parent oxide. At 850 °C and 1.5 V, a current density of −2760 mA cm−2 is obtained on an LSGM-supported single cell with Sr1.9La0.1Fe1.5Mo0.5O6-δ cathode. In addition, the electrolysis cell displays excellent stability under a constant voltage of 1.2 V, suggesting that Sr1.9La0.1Fe1.5Mo0.5O6-δ oxide is a promising SOEC cathode material for pure CO2 electrolysis.

Fast and pervasive diagenetic isotope exchange in foraminifera tests is species-dependent

Oxygen isotope compositions of fossil foraminifera tests are commonly used proxies for ocean paleotemperatures, with reconstructions spanning the last 112 million years. However, the isotopic composition of these calcitic tests can be substantially altered during diagenesis without discernible textural changes. Here, we investigate fluid-mediated isotopic exchange in pristine tests of three modern benthic foraminifera species (Ammonia sp., Haynesina germanica, and Amphistegina lessonii) following immersion into an 18O-enriched artificial seawater at 90 °C for hours to days. Reacted tests remain texturally pristine but their bulk oxygen isotope compositions reveal rapid and species-dependent isotopic exchange with the water. NanoSIMS imaging reveals the 3-dimensional intra-test distributions of 18O-enrichment that correlates with test ultra-structure and associated organic matter. Image analysis is used to quantify species level differences in test ultrastructure, which explains the observed species-dependent rates of isotopic exchange. Consequently, even tests considered texturally pristine for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange; critical paleo-temperature record re-examination is warranted.

Multiscale Understanding of Surface Structural Effects on High-Temperature Operational Resiliency of Layered Oxide Cathodes.

The worldwide energy demand in electric vehicles and the increasing global temperature have called for development of high-energy and long-life lithium-ion batteries (LIBs) with improved high-temperature operational resiliency. However, current attention has been mostly focused on cycling aging at elevated temperature, leaving considerable gaps of knowledge in the failure mechanism, and practical control of abusive calendar aging and thermal runaway that are highly related to the eventual operational lifetime and safety performance of LIBs. Herein, using a combination of various in situ synchrotron X-ray and electron microscopy techniques, a multiscale understanding of surface structure effects involved in regulating the high-temperature operational tolerance of polycrystalline Ni-rich layered cathodes is reported. The results collectively show that an ultraconformal poly(3,4-ethylenedioxythiophene) coating can effectively prevent a LiNi0.8 Co0.1 Mn0.1 O2 cathode from undergoing undesired phase transformation and transition metal dissolution on the surface, atomic displacement, and dislocations within primary particles, intergranular cracking along the grain boundaries within secondary particles, and intensive bulk oxygen release during high state-of-charge and high-temperature aging. The present work highlights the essential role of surface structure controls in overcoming the multiscale degradation pathways of high-energy battery materials at extreme temperature.

An ab initio study of the oxygen defect formation and oxide ion migration in (Sr1-xPrx)2FeO4±δ

The Sr n+1 Fe n O 3n+1 Ruddlesden-Popper (RP) perovskite family displays promising oxygen permeability and serves as a host stoichiometry for the design of solid oxide electrode materials. A strategy to tune electronic and ionic properties is the introduction of substitutional dopants like Pr 3+ to the A-site. In this study, we investigate the bulk structural, electronic, and oxygen migration properties for the n = 1 RP perovskite (Sr 1-x Pr x ) 2 FeO 4± δ (x = 0, 0.125, 0.25, 0.375, and 0.5). The oxygen partial pressure is adjusted to elucidate how anodic and cathodic operating conditions influence the formation of oxygen defects. Under anodic conditions, the oxygen vacancy is the dominant oxide defect for all dopant-configurations. Under cathodic conditions, oxygen vacancy defects dominate for configurations from x = 0 to x = 0.375 while the oxygen peroxide interstitial defect becomes the primary defect for x = 0.5. Next, we examine the relationship between Pr 3+ concentration, iron oxidation state, and charge compensation with defect formation to explain the trends in vacancy and peroxide interstitial formation energies. Results clarify the role of lanthanide A-site substitutional dopants on the electronic conductivity and oxide defect formation and migration in Sr-based RP perovskites. These atomic-scale insights suggest design directions for RP perovskites in SOFCs. • Doping strategy of Sr 2 FeO 4 with Pr 3+ for targeted electronic and ionic properties. • Mechanism of vacancy defect formation outlined in terms of charge delocalization. • Mechanism of interstitial defect formation outlined in terms of A-site identity. • Studied all non-symmetrical vacancy-mediated migration paths for given Pr 3+ value.

Spatter oxidation during laser powder bed fusion of Alloy 718: Dependence on oxygen content in the process atmosphere

In laser powder bed fusion ( L -PBF), powder degradation is mainly driven by the accumulation of highly oxidized spatter particles in the powder bed. Although the amount of spattering can be controlled by the melt pool stability, spatter formation is an unavoidable characteristic of PBF processes. Oxidized spatter risks defect formation in the printed components. However, the factors influencing the level of spatter oxidation during L -PBF processing are not yet fully understood. Herein, the residual oxygen in the process atmosphere was reduced from the traditionally applied 1000–20 ppm using an oxygen partial pressure control system to process Alloy 718 powder. Spatter particles accumulated on the gas inlet were further analyzed to reveal the effect of the oxygen content in the process atmosphere on the spatter oxidation by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Increasing the residual oxygen in the process atmosphere increased surface coverage by oxide phases rich in Al and Cr. The XPS analysis confirmed that the surface of Alloy 718 spatter particles were covered with Al- and Cr-based oxides, whose thickness increased with the oxygen content in the process atmosphere. The bulk oxygen content in the spatter powder showed the same trend with approximately thrice the oxygen content in spatters generated at 1000 ppm O 2 (608 ppm O in the sample) compared to spatters generated with oxygen at 20 ppm (206 ppm O in the sample). Thermodynamic simulations demonstrate a transition from thick Al- and Cr-based mixed corundum and spinel-type oxides to Al-based corundum oxide with decreasing oxygen partial pressure, consistent with the XPS findings. • The spatter oxidation decreased with the residual oxygen content in the L -PBF atmosphere. • Significant changes in the surface oxide chemistry of spatter particles occurred as the residual oxygen varied. • The thickness of oxide phases decreased with the residual oxygen content. • The surface oxide was significantly enriched with Al and Cr as the oxygen level in the L -PBF atmosphere increased. • Thermodynamic calculations confirm preferential Al oxidation at lower oxygen levels compared to Cr.

Carbon and oxygen stable isotope record of upper Kimmeridgian shallow-marine ramp carbonates (Iberian Basin, NE Spain): the imprint of different burial and tectonic histories

Bulk carbon and oxygen stable isotopes of ancient shallow-marine carbonates can record the effects of multiple palaeoenvironmental factors, but also the imprint of several post-depositional processes, which may alter the original marine isotopic composition. In this study, carbon and oxygen stable isotope analyses were performed on bulk carbonate, bivalve calcitic-shell (Trichites) and calcite vein samples from two stratigraphic sections (Tosos and Fuendetodos, present-day distance 15km), representing proximal inner- and distal mid-ramp environments, respectively, of the uppermost Kimmeridgian ramp facies deposited in the northern Iberian Basin (NE Spain). These successions underwent different diagenetic pathways that altered the primary marine isotopic composition in each section in different ways. Different burial histories, tectonic uplift and a variable exposure to meteoric diagenesis from the end of the Kimmeridgian to the Cenozoic (following Alpine tectonic uplift) are reflected in the different alteration patterns of the carbon and oxygen stable isotope signatures. A significant deviation to lower values in both δ13O and δ18O is recorded in those carbonates mostly exposed to meteoric diagenesis (distal mid-ramp Fuendetodos section), because of post-depositional tectonic uplift (telogenesis). On the other hand, the deposits mainly affected by burial diagenesis (proximal inner-ramp Tosos section) only record low δ18O with respect to expected values for pristine Kimmeridgian marine carbonates. The different burial and tectonic uplift histories of these deposits in each sector, due to their different tectonic evolution in this part of the basin, resulted in a variable degree of diagenetic resetting. However, in spite of the different diagenetic resetting reported of the carbon and oxygen stable isotope signatures in each section, these carbonates show similar cement types in termsof fabrics and cathodoluminescence properties. The diagenetic resetting reported for these carbonates prevents the use of the δ13O and δ18O records for addressing palaeoenvironmental interpretations, but instead highlights useful features regarding the variable diagenetic overprint of the studied shallow-marine carbonate successions concerning their specific post-depositional history.

Oxygen vacancies mediated Bi12O17Cl2 ultrathin nanobelts: Boosting molecular oxygen activation for efficient organic pollutants degradation

Photocatalysis technology has been considered as a sustainable and promising strategy for pollutant degradation. However, the photocatalytic activity is limited by the unsatisfactory carrier separation efficiency of photocatalysts and insufficient reactive oxygen species. Herein, the oxygen vacancies (OVs) mediated Bi12O17Cl2 ultra-thin nanobelt (ROV Bi12O17Cl2) was fabricated via solvothermal method. The surface oxygen vacancies can act as the ‘electron sink’ and boost charge separation. Thus, the ROV Bi12O17Cl2 shows superior photocatalytic performance, which is 2.72 and 4.52 times compared to deficient oxygen vacancies Bi12O17Cl2 (DOV Bi12O17Cl2) and Bulk Bi12O17Cl2 for colored organic pollutants degradation, respectively. Besides, the ROV Bi12O17Cl2 also displays excellent removal efficiency for refractory antibiotics, roughly 4.00 and 7.45 times compared to that of DOV Bi12O17Cl2 and Bulk Bi12O17Cl2, respectively. Furthermore, the intermediates for photocatalytic degradation were determined through HPLC-MS and the possible degradation paths of the target molecules were inferred. Capture experiment and ESR spectra confirmed that the •O2− played a vital role for the organic pollutant degradation. This work provides a new perspective for the design of advanced semiconductors for organic pollutants degradation.

Unlocking bulk and surface oxygen transport properties of mixed oxide-ion and electron conducting membranes with combined oxygen permeation cell and oxygen probe method

Surface exchange kinetics and bulk diffusion of oxygen are of paramount importance to the activity of oxygen electrocatalysis and performance of electrochemical devices such as fuel cell, metal-air batteries, and oxygen separation membranes. Conventional approaches to obtaining these transport properties are often limited to single property under a specific non-operation related condition. Here we use a combined oxygen permeation cell and oxygen probe methodology to simultaneously attain rates of oxygen surface exchange and bulk conductivity/chemical diffusivity of three representative mixed oxide-ion and electron conductors, namely SrCo0.9Ta0.1O3-δ (SCT), La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4FeO3-δ (LSF), operated under a steady-state oxygen flux. The results explicitly show that SCT exhibit the highest oxide-ion conductivity/chemical diffusivity, fastest rates of surface oxygen exchange kinetics, thus promising to be the best oxygen electrocatalyst. We have also mapped out the distribution of oxygen chemical potential gradient across the membranes and applied B-transport number concept to illustrate the rate-limiting steps in the overall oxygen permeation process.

Synthesis and characterization of the flower-like LaxCe1−xO1.5+δ catalyst for low-temperature oxidative coupling of methane

Lanthanum-based oxides are promising candidates for low-temperature oxidative coupling of methane (OCM). To further lower the OCM reaction temperature, the Ce doped flower-like La2O2CO3 microsphere catalysts were synthesized, achieving a significantly low reaction temperature (375 °C) while maintaining high C2+ hydrocarbon selectivity (43.0%). Doping Ce into the lattice of La2O2CO3 created more surface oxygen vacancies and bulk lattice defects, which was in favor of the transformation and migration of oxygen species at 350–400 °C. The designed H2 temperature-programmed reduction (H2-TPR) experiments provided strong evidence that the low reaction temperature of LaxCe1−xO1.5+δ can be attributed to the transformation and migration of oxygen species, which dynamically generated surface oxygen vacancies for continuous oxygen activation to selectively convert methane. Moreover, designed temperature-programmed surface reaction (TPSR) clarified that two kinds of surface oxygen species in LaxCe1−xO1.5+δ catalysts were concerned with catalytic performance, that is, the surface chemisorbed oxygen species for the activation of CH4 and the formation of CH3• intermediates, surface La-Ce-O lattice oxygen species that caused the excessive oxidation of CH3• intermediates. Finally, the factors affecting the transformation and migration of oxygen species were explored.

Oxygen Kinetic Isotope Effects in the Thermal Decomposition and Reduction of Ammonium Diuranate.

Oxygen stable isotopes in uranium oxides processed through the nuclear fuel cycle may have the potential to provide information about a material’s origin and processing history. However, a more thorough understanding of the fractionating processes governing the formation of signatures in real-world samples is still needed. In this study, laboratory synthesis of uranium oxides modeled after industrial nuclear fuel fabrication was performed to follow the isotope fractionation during thermal decomposition and reduction of ammonium diuranate (ADU). Synthesis of ADU occurred using a gaseous NH3 route, followed by thermal decomposition in a dry nitrogen atmosphere at 400, 600, and 800 °C. The kinetic impact of heating ramp rates on isotope effects was explored by ramping to each decomposition temperature at 2, 20, and 200 °C min–1. In addition, ADU was reduced using direct (ramped to 600 °C in a hydrogen atmosphere) and indirect (thermally decomposed to U3O8 at 600 °C, then exposed to a hydrogen atmosphere) routes. The bulk oxygen isotope composition of ADU (δ18O = −16 ± 1‰) was very closely related to precipitation water (δ18O = −15.6‰). The solid products of thermal decomposition using ramp rates of 2 and 20 °C min–1 had statistically indistinguishable oxygen isotope compositions at each decomposition temperature, with increasing δ18O values in the transition from ADU to UO3 at 400 °C (δ18OUO3 – δ18OADU = 12.3‰) and the transition from UO3 to U3O8 at 600 °C (δ18OU3O8 – δ18OUO3 = 2.8‰). An enrichment of 18O attributable to water volatilization was observed in the low temperature (400 °C) product of thermal decomposition using a 200 °C min–1 ramp rate (δ18OUO3 – δ18OADU = 9.2‰). Above 400 °C, no additional fractionation was observed as UO3 decomposed to U3O8 with the rapid heating rate. Indirect reduction of ADU produced UO2 with a δ18O value 19.1‰ greater than the precipitate and 4.0‰ greater than the intermediate U3O8. Direct reduction of ADU at 600 °C in a hydrogen atmosphere resulted in the production of U4O9 with a δ18O value 17.1‰ greater than the precipitate. Except when a 200 °C min–1 ramp rate is employed, the results of both thermal decomposition and reduction show a consistent preferential enrichment of 18O as oxygen is removed from the original precipitate. Hence, the calcination and reduction reactions leading to the production of UO2 will yield unique oxygen isotope fractionations based on process parameters including heating rate and decomposition temperature.

Electrode properties of CuBi2O4 spinel oxide as a new and potential cathode material for solid oxide fuel cells

Herein, the spinel oxide CuBi 2 O 4 is prepared by solid-state reaction method and characterized for the first time as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). CuBi 2 O 4 (CBO) crystallizes in tetragonal structure and is chemically compatible with Ce 0.9 Gd 0.1 O 1.9 (GDC) electrolyte at 800 °C. Electrical conductivity relaxation is conducted to assess the oxygen surface exchange coefficient k chem and oxygen bulk diffusion coefficient D chem of CBO material. It is found that CBO exhibits better oxygen transport properties than the traditional La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathodes. The electrochemical activity of CBO-based cathode is investigated by impedance spectroscopy on the symmetric cell with GDC electrolyte. The CBO cathode forms good contact with the GDC electrolyte and displays low polarization resistance ( R P ) values, e.g., 0.58 Ωcm 2 at 700 °C in air. The introducing GDC to form a composite cathode with CBO further reduces the R P to 0.39 Ωcm 2 . An anode-supported single cell with the CBO-GDC cathode exhibits a peak power density of 507 mWcm −2 at 750 °C, suggesting the viability of CBO oxide as a cathode material for IT-SOFC. • CuBi2O4 exhibits fast surface oxygen exchange and bulk diffusion properties. • CuBi2O4 forms good contact with GDC electrolyte at low sintering temperatures. • CuBi2O4 cathode shows improved electrochemical performance below 700 °C.

Tendencies in ABO3 Perovskite and SrF2, BaF2 and CaF2 Bulk and Surface F-Center Ab Initio Computations at High Symmetry Cubic Structure

We computed the atomic shift sizes of the closest adjacent atoms adjoining the (001) surface F-center at ABO3 perovskites. They are significantly larger than the atomic shift sizes of the closest adjacent atoms adjoining the bulk F-center. In the ABO3 perovskite matrixes, the electron charge is significantly stronger confined in the interior of the bulk oxygen vacancy than in the interior of the (001) surface oxygen vacancy. The formation energy of the oxygen vacancy on the (001) surface is smaller than in the bulk. This microscopic energy distinction stimulates the oxygen vacancy segregation from the perovskite bulk to their (001) surfaces. The (001) surface F-center created defect level is nearer to the (001) surface conduction band (CB) bottom as the bulk F-center created defect level. On the contrary, the SrF2, BaF2 and CaF2 bulk and surface F-center charge is almost perfectly confined to the interior of the fluorine vacancy. The shift sizes of atoms adjoining the bulk and surface F-centers in SrF2, CaF2 and BaF2 matrixes are microscopic as compared to the case of ABO3 perovskites.