Oxygen Storage Material Research Articles

Enhanced Low‐Temperature Oxidation Activity of Cu‐Doped Highly Active Pd‐Based Catalysts

AbstractPd‐based catalysts supported on cerium–zirconium (CZ) oxygen storage materials (OSM) have emerged as promising candidates for the catalytic oxidation of CH4 and CO. However, achieving efficient and stable treatment of CH4 and CO in exhaust gases with Pd‐based catalysts remains a long‐term challenge. To address this, we propose a straightforward strategy of doping the supports with Cu to enhance the catalytic performance of Pd/Ce0.4Zr0.5La0.1O1.95 catalysts. Performance tests revealed that the catalytic activity of the Cu‐doped catalysts significantly improved, with T90 (CH4) decreasing from 418–301 °C and T90 (CO) decreasing from 141–113 °C. Notably, the copper‐doped catalysts exhibited enhanced water and sulfur resistance while maintaining excellent thermal stability. The CO conversion decreased by less than 2 % after a thermal deactivation experiment lasting over 100 hours. The reduction in particle size and the increase in specific surface area due to copper doping are key factors contributing to the improved total oxygen storage capacity (OSC). The main reasons for the increase in total OSC are likely the reduced particle size and increased specific surface area.

Multifunctional surface modification to enhance the electrochemical performance of lithium-rich manganese-based cathode materials

The preparation of high-performance lithium-rich manganese-based cathode materials (LLOs) require attention to the redox of anions and the migration of transition metals (TM) during the cycling process. In this study, Ce ion-doped Li-rich cathode coated with La0.2Ce0.8O2, which has abundant oxygen vacancies on its surface, was prepared. The presence of the oxygen storage material La0.2Ce0.8O2 can effectively absorb the irreversible oxygen generated during cycling, preventing it from reacting with the electrolyte to form thick cathode electrolyte interface (CEI) coatings. These coatings hinder the relative activity of lattice oxygen in the bulk phase, reducing the specific capacity of the material. Additionally, the 5d metal ion Ce carries a large nuclear charge, causing a 'pinning effect' when it integrates into the LLOs structure. This effect reduces thermal oscillation in the lattice, impedes TM migration during battery charging and discharging, and effectively prevents voltage decay. Results indicate that after 100 cycles at 1C and 25 °C, the surface-coated materials maintain a discharge-specific capacity of 194 mAh/g, compared to 158 mAh/g for uncoated material. Voltage decay decreased from 4.4 mV/cycle to 1.8 mV/cycle, with an ICE of 91.5 % versus the original material's 82.8 %. The modified material also exhibits improved rate performance, retaining a specific capacity of 162 mAh/g at 3C, in contrast to the original material's 123 mAh/g.

The effect of the oxygen storage material on a three-way catalyst spatio-temporally resolved with Spaci-MS

A methodology that allows the estimation of the stored oxygen (oxygen storage capacity (OSC)) along the TWC catalyst length by spatio-temporally resolving the O2 profiles during rich to lean cycle conditions with the use of the Spaci-MS technique has been developed. As a case study, the effect of the presence of mixed cerium-zirconium oxide as oxygen storage material has been investigated. The oxygen storage capacity for the TWC containing Ce material was much higher than without. The study also explores the reaction pathways typical of TWC along the catalyst coordinates by spatio-temporally resolving O2, H2, CO, and NO, as well as the related products and intermediates providing important understanding on, for example, the water-gas shift reaction leading to H2 production and how they develop differently depending on the OSM (oxygen storage material) in the catalyst. H2 production occurs closer to the front of the TWC length in the case of the catalyst without Ce, and results in an earlier production of undesired NH3 from NO reaction with H2. However, more N2O has been observed for the catalyst with Ce, which suggests the catalyst composition could be optimised to minimize overall secondary emissions.

A revolutionary design concept: full-sealed lithium-oxygen batteries

At this moment, non-aqueous rechargeable lithium-oxygen batteries (LOBs) with extremely high energy density are regarded as the most viable energy storage devices to potentially replace petroleum. One of the most crucial impediments to their implementation has been ensuring facile oxygen availability. Moreover, as semi-sealed systems, LOBs have confronted challenges including oxygen impurities, product degradation, anode corrosion, frequent side reactions, and mediocre cycling performance. In this work, utilizing the physical adsorption of porous (micro-, meso- and macro-porous) solid carbon materials, we incorporate an oxygen storage layer (OSL) with reversible oxygen ad/desorption capabilities into a LOB to develop novel fully-sealed lithium-oxygen batteries (F-S-LOBs). The results demonstrate mesoporous carbons exhibit optimal oxygen adsorption/desorption kinetics, rendering them highly suitable for F-S-LOBs without developing complex oxygen-permeable membranes or carrying oxygen tanks. The OSL fabricated with mesoporous carbon can sustain battery charge/discharge at various current densities with exceptional cycling performance. Additionally, we provide approximate pore size guidelines for oxygen storage materials to aid future research. This study is anticipated to offer a new robust research direction for metal-air batteries and to forge a new path toward promoting the commercialization and development of this technology.

Perovskites as oxygen storage materials for chemical looping partial oxidation and reforming of methane.

The traditional partial oxidation, dry reforming and steam reforming of methane technologies are separated into two reactors for execution by chemical looping technology, which can avoid the defects exposed in the traditional process (avoiding carbon accumulation, reducing costs, etc.). The key to chemical looping technology is to find suitable oxygen carriers (OCs), which can store and release oxygen to form a closed loop in the chemical looping. The purpose of this review is to summarize the current status of perovskite oxides for partial oxidation and reforming of methane in chemical looping, describe the structure, oxygen capacity, oxygen migration rate and common synthesis methods of perovskites in chemical looping. In addition, the effects of impregnation loading, ion doping, and structural morphology on the catalytic conversion of CH4 by perovskite OCs and the reaction mechanism on the OCs are also discussed.

Impact of Oxygen Storage Components in Prototype Pd-Based Three-Way Catalysts under Exhaust Conditions Relevant to Propane Engines

With increasing concerns about global warming, the push for sustainable and eco-friendly fuels is accelerating. Propane, recognized as liquefied petroleum gas or LPG, has garnered research interest as an alternative fuel due to its notable advantages, including a high-octane rating, reduced greenhouse gas emissions, and potential cost-effectiveness. However, to realize its full potential as an alternative fuel it is essential to develop catalysts that efficiently handle emissions at low temperatures. In our research, we investigated three distinct palladium (Pd)-based three-way catalyst (TWC) formulations (PdRh, Pd-only, and Pd-OSC) to investigate the influence of typical TWC components rhodium (Rh) and oxygen storage components (OSC) in exhaust scenarios relevant to propane-fueled engines. Among these, the formulation containing oxygen storage components (Pd-OSC) showed the highest reactivity for both NO and C3H8 while minimizing performance degradation from hydrothermal aging (HTA). Notably, the temperature of 50% conversion (T50) for propane in the Pd-OSC fresh and HTA sample was lower by 30 °C and 13 °C, respectively, compared to the Pd-only sample, highlighting the role of oxygen storage materials in enhancing catalyst performance, even without dithering. Additionally, N2 physisorption showed that the Pd-OSC sample has a higher surface area and increased pore volume. This underscores the idea that OSC materials not only augment the catalyst’s porosity but also optimize reactant accessibility to active sites, thus elevating catalytic efficiency. In addition to evaluating performance, we further explored the performance and characteristics of the catalysts using catalytic probe reactions, such as water–gas shift and steam reforming reactions.

Synthesis of high-performance Pd/Ce0.4Zr0.5La0.05Y0.05O1.95 catalysts for low-temperature methane combustion with the ultrasound-assisted field

In this work, ultrasound-assisted sol–gel (UASG) and wet incipient impregnation (WIP) methods were employed to synthesize a series of Ce0.4Zr0.5La0.05Y0.05O1.95 oxygen storage materials (OSM) and Pd/Ce0.4Zr0.5La0.05Y0.05O1.95 catalysts under different frequency ultrasound external field. The effect of the ultrasonic external field on the low-temperature catalytic activity of the catalyst was investigated. The BET and TEM results demonstrated that ultrasound field can increase the catalyst's specific surface area by 25% and decrease the particle size from 10.1 nm to 7.3 nm. The XPS results showed that the concentrations of surface Pd2+, Oads, and Ce3+ species increased from 21%, 44%, and 43% to 42%, 65%, and 46%, respectively, enhancing the formation of Pd2+, Oads and Ce3+ species on the catalyst surface. The performance tests revealed that T90(CH4) of the prepared Pd/Ce0.4Zr0.5La0.05Y0.05O1.95 catalyst by UASG method decreased from 536 ℃ to 331 ℃, improving its low-temperature catalytic activity. Additionally, the catalysts synthesized using by UASG method exhibited the enhanced resistance to water and sulfur poisoning and maintained their catalytic performance for 20 h.

Oxidation Kinetics of Nanocrystalline Hexagonal RMn1-xTixO3 (R = Ho, Dy).

Hexagonal manganites, RMnO3 (R = Sc, Y, Ho-Lu), are potential oxygen storage materials for air separation due to their reversible oxygen storage and release properties. Their outstanding ability to absorb and release oxygen at relatively low temperatures of 250-400 °C holds promise of saving energy compared to current industrial methods. Unfortunately, the low temperature of operation also implies slow kinetics of oxygen exchange in these materials, which would make them inefficient in applications such as chemical looping air separation. Here, we show that the oxidation kinetics of RMnO3 can be improved through Ti4+-doping as well as by increasing the rare earth cation size. The rate of oxygen absorption of nanocrystalline RMn1-xTixO3 (R = Ho, Dy; x = 0, 0.15) was investigated by thermogravimetric analysis, X-ray absorption near-edge structure, and high-temperature X-ray diffraction (HT-XRD) with in situ switching of atmosphere from N2 to O2. The kinetics of oxidation increases for larger R and even more with Ti4+ donor doping, as both induce expansion of the ab-plane, which reduces the electrostatic repulsion between oxygen in the lattice upon oxygen ion migration. Surface exchange rates and activation energies of oxidation were determined from changes in lattice parameters observed through HT-XRD upon in situ switching of atmosphere.

Enhanced Oxygen Storage Capacity of Porous CeO2 by Rare Earth Doping.

CeO2 is an important rare earth (RE) oxide and has served as a typical oxygen storage material in practical applications. In the present study, the oxygen storage capacity (OSC) of CeO2 was enhanced by doping with other rare earth ions (RE, RE = Yb, Y, Sm and La). A series of Undoped and RE-doped CeO2 with different doping levels were synthesized using a solvothermal method following a subsequent calcination process, in which just Ce(NO3)3∙6H2O, RE(NO3)3∙nH2O, ethylene glycol and water were used as raw materials. Surprisingly, the Undoped CeO2 was proved to be a porous material with a multilayered special morphology without any additional templates in this work. The lattice parameters of CeO2 were refined by the least-squares method with highly pure NaCl as the internal standard for peak position calibrations, and the solubility limits of RE ions into CeO2 were determined; the amounts of reducible-reoxidizable Cen+ ions were estimated by fitting the Ce 3d core-levels XPS spectra; the non-stoichiometric oxygen vacancy (VO) defects of CeO2 were analyzed qualitatively and quantitatively by O 1s XPS fitting and Raman scattering; and the OSC was quantified by the amount of H2 consumption per gram of CeO2 based on hydrogen temperature programmed reduction (H2-TPR) measurements. The maximum [OSC] of CeO2 appeared at 5 mol.% Yb-, 4 mol.% Y-, 4 mol.% Sm- and 7 mol.% La-doping with the values of 0.444, 0.387, 0.352 and 0.380 mmol H2/g by an increase of 93.04, 68.26, 53.04 and 65.22%. Moreover, the dominant factor for promoting the OSC of RE-doped CeO2 was analyzed.

Enhanced performance of La1-xFeO3-δ oxygen carrier via A-site cation defect engineering for chemical looping dry reforming of methane

Chemical looping is a process-intensification tool that focuses on the fields of fossil fuel combustion, CO2 capture, hydrocarbon valorization, energy storage, and other energy-related and environmental-related areas. Perovskite-structured LaFeO3, a typical oxygen storage material, shows great potential for chemical looping partial oxidation and H2O/CO2 splitting. Here, we report a simple and efficient strategy to enhance catalytic activity by tuning A-site cation defects in perovskites via a sol-gel method. Systematically studies show that A-site cation defects affect both the physical and chemical properties. With the A-site cation defect engineering, the crystallite size of the La1-xFeO3-δ perovskites is reduced. The catalytic performance for CH4 oxidation and CO2 splitting is enhanced with increasing oxygen vacancy concentration. The La0.93FeO3-δ oxygen carrier exhibits the highest catalytic performance with 88% CO selectivity and 90% methane conversion at 750 °C. In addition, the La0.93FeO3-δ shows higher stability in the successive CH4 partial oxidation/CO2 splitting redox cycles, with almost unchanged CH4 conversion and CO selectivity. All these results demonstrate that the strategy of adjustment of different molar ratios of A/B site in perovskite metal oxides can effectively regulate the oxygen vacancy concentration.

Hydrogenation of CO2 over Mn-Substituted SrTiO3 Based on the Reverse Mars–van Krevelen Mechanism

Oxygen storage materials (OSMs) such as CeO2 have attracted great attention as catalysts for the reverse water gas shift (RWGS) reaction, replacing metal-supported catalysts from the perspectives of thermal stability and CO selectivity. Because perovskite-type oxides are also known to be powerful OSM candidates, here we employed perovskite-type Mn-substituted SrTiO3 as a catalyst for RWGS and investigated the effects of oxygen vacancies on the activity toward CO generation. Among the solid solutions with different Mn ratios, SrTiO3 with 20% Mn substitution exhibited the highest activity. It is suggested that oxygen vacancies generated by the substitution of Mn ions for Ti ions in SrTiO3 exhibit selective activity toward CO2 splitting, whereas the redox of bulk Mn oxides does not have the capacity for CO generation in the control experiments. Quantitative H2-temperature programmed reduction and CO2-temperature programmed oxidation experiments revealed that half of the oxygen vacancies in Mn-substituted SrTiO3, which were generated by reduction with H2, were used as active sites for CO2 splitting. We conclude that this generated active oxygen vacancy, which is attributed to the valence change of the Mn ions, contributes to efficient CO2 splitting to form CO through the reverse Mars–van Krevelen mechanism.

Synthesis of a nanosized homogeneous Al2O3–CeO2–ZrO2 composite as an oxygen-storage material for highly improved thermal durability

Al2O3–CeO2–ZrO2 (ACZ) is widely used in automotive catalysts as an oxygen-storage material. However, ACZ prepared by conventional synthesis methods has poor compositional homogeneity, resulting in thermal degradation of the oxygen storage capacity (OSC) because of the sintering of CeO2–ZrO2 (CZ) crystals. In this study, a nanosized homogeneous ACZ composite is synthesized by an improved co-precipitation method that uses a high-shear agitation reactor and a polyamine dispersant. The sol obtained using the abovementioned method is a monodisperse colloid with an average particle size of 3.2 nm. A quantitative analysis of transmission electron microscopy images reveals that the distribution of Al2O3 and CZ after calcination at 500 °C is 1.6 times more homogeneous in the resulting ACZ than that in the ACZ powder obtained by conventional co-precipitation. Such excellent homogeneity of Al2O3 and CZ distribution results in smaller CZ crystals (11.2 nm) than the conventional one (14.6 nm) obtained after calcination at 1100 °C. The smaller CZ crystals have 1.3 times higher OSC than that of the conventional one. Therefore, the developed method yields homogeneous and thermally stable ACZ that can be used as an efficient automotive catalyst. Furthermore, this method would also be useful for synthesizing homogeneous nano-composites with various elemental combinations.

CeO2-based oxygen storage capacity materials in environmental and energy catalysis for carbon neutrality: extended application and key catalytic properties

ABSTRACT Reducing carbon dioxide emissions is one of the largest energy and environmental challenges currently faced globally. To achieve carbon neutrality, the development of new technologies and simultaneous improvement of conventional technologies are required. Catalysis, which determines the success or failure and efficiency of the process, is at the center of addressing these problems. Innovative advancements in catalysis have been accomplished through the extensive research on catalytic materials. Oxygen storage materials with oxygen storage capacity (OSC) have been widely applied in supports and active cocatalysts for energy and environmental catalytic applications. This review provides knowledge on the extended applications (reforming, water-gas shift reaction, and partial oxidation of methane (POM) for high-value-added chemicals, including hydrogen production, deoxygenation of fatty acids for second-generation biofuel production, selective catalytic reduction of NO x with NH3, hydrocarbon (HCs) oxidation for emission control, CO oxidation for vehicle exhaust control and purification of gaseous product fuel, and soot oxidation for the removal of motor-type pollutants) of these materials, which are mainly used as three-way catalysts (TWC) in automotive catalysis, to the carbon-neutral field. In particular, this study focuses on the physico-chemical properties that are closely synergistic with OSC, such as reducibility, dispersion, surface acid/base properties, and physical properties, such as surface structure or morphology.

A proxy for oxygen storage capacity from high-throughput screening and automated data analysis.

Oxygen storage and release is a foundational part of many key pathways in heterogeneous catalysis, such as the Mars-van Krevelen mechanism. However, direct measurement of oxygen storage capacity (OSC) is time-consuming and difficult to parallelise. To accelerate the discovery of stable high OSC rare-earth doped ceria-zirconia oxygen storage catalysts, a high-throughput robotic-based co-precipitation synthesis route was coupled with sequentially automated powder X-ray diffraction (PXRD), Raman and thermogravimetric analysis (TGA) characterisation of the resulting materials libraries. Automated extraction of data enabled rapid trend identification and provided a data set for the development of an OSC prediction model, investigating the significance of each extracted quantity towards OSC. The optimal OSC prediction model produced incorporated variables from only fast-to-measure analytical techniques and gave predicted values of OSC that agreed with experimental observations across an independent validation set. Those measured quantities that feature in the model emerge as proxies for OSC performance. The ability to predict the OSC of the materials accelerates the discovery of high-capacity oxygen storage materials and motivates the development of similar high-throughput workflows to identify candidate catalysts for other heterogeneous transformations.

Effects of Hydrothermal Treatment on Mesopore Structure and Connectivity in Doped Ceria-Zirconia Mixed Oxides.

Pore size and pore connectivity control diffusion-based transport in mesopores, a crucial property governing the performance of heterogeneous catalysts. In many cases, transition-metal oxide catalyst materials are prepared from molecular precursors involving hydrothermal treatment followed by heat treatment. Here, we investigate the effects of such a hydrothermal aftertreatment step, using an aqueous ammonia solution, on the disordered mesopore network of CexZr1-x-y-zYyLazO2-δ mixed oxides. This procedure is a common synthesis step in the preparation of such ceria-based oxygen storage materials applied in three-way catalysis, employed to improve the materials' thermal stability. We perform state-of-the-art Ar-physisorption analysis, especially advanced hysteresis scanning, to paint a detailed picture of the alterations in mesopore space caused by the hydrothermal aftertreatment and subsequent aging at 1050 °C. Furthermore, we investigate the network characteristics by electron tomography in combination with suitable statistical analysis, enabling a consistent interpretation of the desorption scans (physisorption). The results indicate that the hydrothermal aftertreatment enhances the mesopore connectivity of the continuous 3D network by widening pores and especially necks, hence facilitating accessibility to the particles' internal surface area and the ability to better withstand high temperatures.

Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si-Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance.

Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (kchem ) is examined for Pr0.1 Ce0.9 O2-δ (PCO), a model mixed ionic electronic conductor, via electrical conductivity relaxation measurements, and the area-specific resistance (ASR) by electrochemical impedance spectroscopy. It is demonstrated that even low silica levels, introduced by infiltration, depress kchem by a factor 4000, while the ASR increases 40-fold and weattribute this to its acidity relative to that of PCO. The ability to fully regenerate the poisoned surface by the subsequent addition of basic Ca- or Li-species is further shown. This ability to not only recover Si-poisoned surfaces by tuning the relative surface acidity of an oxide surface, but subsequently outperform the pre-poisoned response, promises to extend the operating life of materials and devices for which the catalytic oxygen/solid interface reaction is central.

Recent Insights into the Production of Syngas and Hydrogen Using Chemical Looping-Steam Reforming (CL-SR)

Chemical looping-steam reforming (CL-SR) is a prospective technology for the simultaneous production of syngas and hydrogen (H2). The quality of syngas and hydrogen (H2) obtained from CL-SR process has garnered the interest of the scientific society. A recyclable oxygen storage material (oxygen carrier) is reacted through an alternating cycle of fuel (reduction step) and steam (oxidation step) at higher temperatures. Oxygen carrier (OC) plays a vital role in directing the path of the reaction in the CL-SR process. Hence, developing and selecting a proper OC is crucial. The current study intends to address recent breakthroughs in the development and performance of OCs subjected to different reacting fuel species such as methane (CL-SMR), CO2 (CL-HG), and liquid fuels. The performance of various classes of OCs such as metal oxide, mixed metal oxides, and perovskites toward conversion and lattice oxygen selectivity is investigated. The thermodynamic study and constraints related to the performance of OCs in CL-SR are discussed along with the economic evaluation of the CL-SR process and the conventional SMR process. Further, the theoretical approaches involved in determining the kinetics of the CL-SR process and kinetic models developed by the researchers are reviewed. Finally, the scientific barriers and the suggestions for optimized operation of the CL-SR process are listed.

A novel Ni–MoCxOy interfacial catalyst for syngas production via the chemical looping dry reforming of methane

<h2>Summary</h2> Modulating the oxygen-metal interface that enables efficient and selective C–H activation remains challenging. Herein, we present a novel MoC_xO_y oxygen carrier for syngas production via the chemical looping CH₄–CO₂ reforming (CL-DRM) reaction. Molybdenum carbide additive induces the re-dispersion of Ni particles from Al₂O₃ surface to α-MoC driven by the strong interaction between Ni and α-MoC. A dynamic structure transformation between MoC_x and MoC_xO_y occurs and new Ni–MoC_xO_y interfaces form during the redox cycling, which is discovered to be crucial for the low-temperature CH₄ activation using surface construction experiments and <i>in-situ</i> spectroscopic methods. As a result, Ni–(α-MoC)/Al₂O₃ exhibits near 100% selectivity to syngas with a H₂/CO ratio of 2:1 at 500°C, which is a significantly lower temperature than that of conventional systems. This study first employs molybdenum carbide as an oxygen storage material (OSM) in CL-DRM reaction, paving the way of utilizing transition metal carbides as OSMs in chemical looping processes.

Ground-state structure, orbital ordering and metal-insulator transition in double-perovskite PrBaMn2O6

In recent years, A-site ordered half-doped double-perovskite manganites $\rm RBaMn_2O_6$ (R=rare earth) have attracted much attention due to their remarkable physical properties and a prospect of application as magnetoresistance, multiferroic, and oxygen storage materials. The nature of the ground state in ${\rm RBaMn_2O_6}$ as well as sequence of phase transitions taking place at cooling are not yet well understood due to complexity in both experimental and theoretical studies. Here we address the origin of the ground-state structure in PrBaMn$_2$O$_6$ as well as its electronic and magnetic properties. Utilizing GGA+U approach and specially designed strategy to perform structural optimization, we show that the system has two competing AFM-A and AFM-CE magnetic structures with very close energies. The AFM-A structure is a metal, while AFM-CE is an insulator and the transition to the insulating state is accompanied by the charge Mn$^{3+}$/Mn$^{4+}$, and orbital $3x^2-r^2$/$3y^2-r^2$ orderings. This orbital ordering results in strong cooperative Jahn-Teller (JT) distortions, which lower the crystal symmetry. Our findings give a key to understanding contradictions in available experimental data on ${\rm PrBaMn_2O_6}$ and opens up the prospects to theoretical refinements of ground-state structures in other ${\rm RBaMn_2O_6}$ compounds.

Perovskite-type Co/Ti-substituted Ba0.15Sr0.85FeO3-δ as oxygen storage materials for oxygen enrichment application: structural, sintering, and temperature-swing oxygen sorption uptake properties at high temperatures

Perovskite-type Co/Ti-substituted Ba0.15Sr0.85FeO3-δ as oxygen storage materials for oxygen enrichment application: structural, sintering, and temperature-swing oxygen sorption uptake properties at high temperatures