Ceria Materials Research Articles

Enhanced Hydrogen Production in Microwave‐Driven Water‐Splitting Redox Cycles by Engineering Ceria Properties

AbstractSustainable hydrogen, produced from renewable sources such as solar or wind, plays a decisive role in driving industrial decarbonization. Among hydrogen production technologies, steam electrolysis, and solar‐driven thermochemical cycles using reducible solid oxides show promise but face challenges due to high operation temperatures. Microwave‐driven redox chemical looping enables the direct, contactless electrification of the process, reducing the operation temperature and complexity. Previous works showed that microwaves can efficiently drive reduction/water‐splitting cycles using Gd‐doped ceria at low temperatures (<250 °C), but adjustment of material properties is needed. Here, the key properties of materials are explored that affect the redox mechanism by screening a series of doped ceria materials to enhance microwave‐driven hydrogen production. Evaluation of trivalent dopants (La3+, Gd3+, Y3+, Yb3+, Er3+, and Nd3+) reveals that reduction correlates with lattice and electronic properties. The composition Ce0.9La0.1O2‐δ achieves 1.41 mL g−1, the highest hydrogen production among the studied series. Its narrower bandgap allows for reaching higher conductivity upon microwave‐driven reduction at lower temperatures, while a larger ionic lattice size boosts solid‐state oxygen diffusion. Overall, this research remarks on the critical properties of ceria‐based materials that enhance hydrogen production in microwave‐driven water‐splitting cycles, supporting the design of more efficient materials for sustainable chemical production technology.

Operando Monitoring of Homolytic Cleavage of H2 into Surface Hydrides on Defective Cerium Dioxide Nanoparticles

AbstractThe use of ceria as catalyst in hydrogenation reactions is a recent trend, as they were uncovered to be active on a number of substrates, including alkynes, alkenes, and enones. While some studies focused on the use of well‐defined shapes, others emphasized the interest of defective structures. Overall, the meeting point of these studies is the need to characterize the surface reactivity vs. H2, by considering a number of interesting textures (films, nanopowders with various grain size or porosity), using monitoring techniques that preserve the original features of the materials. A key question is in particular to assess the possibility to form surface hydride through homolytic H2 splitting at cerium sites rather than at the oxygen one, a route recently highlighted as plausible. To address this question on small crystallites of CeO2, we designed a low‐temperature synthetic route for nanorods with an average grain size of 10 nm. Then, we employed near‐ambient‐pressure X‐ray photoelectron spectroscopy as an operando tool to monitor the cerium surface oxidation state during the initial annealing of the nanopowders, followed by exposure to a moderate pressure of H2 (0.52 mbar). We demonstrate that H2 homolytic splitting at cerium sites is the main activation process on this surface at 100 °C, leading to the oxidation of ca. 30 % of the surface cerium atoms from Ce3+ to Ce4+. The surface hydride species were however not stable at 250 °C, H2 was released, and cerium reduced back to Ce3+. A similar mechanism was observed on a defective ceria material obtained by calcination of CeO(OH)2, with comparable intensities. Overall, we report here CeO2 nanorods, exposing predominantly {100} and {110} facets, as showing an interesting surface reactivity for potential application in hydrogenation reactions, and we expose a straight operando methodology to delineate suitable temperatures to be used.

Application of ceria materials modified by oxygen vacancies: In photocatalytic process

Cerium-based materials are widely used in various applications such as photocatalytic environmental remediation, CO2 photoreduction and photocatalytic hydrogen production due to their unique optical properties and special oxygen vacancy formation mechanisms. The special external electronic structure of cerium (4f15d16s2f15d16s2), the highly electronic 4f orbital, promotes the formation of oxygen vacancies through Ce4+/Ce3+ conversion, thereby improving the optical properties of the catalyst. Consequently, the application of cerium-based materials in the field of photocatalysis has been of great interest to researchers. In this paper, we briefly review the synthesis of cerium-based photocatalysts and their applications in pollutant removal, CO2 reduction, and hydrogen production, as well as their promising applications.

Porous ceria materials for efficient direct conversion of carbon dioxide and methanol to dimethyl carbonate

A porous CeO2 catalyst produced via a simple and low temperature approach achieves a high dimethyl carbonate formation rate of 14.8 mmol g−1 h−1 at 5.0 MPa and 140 °C due to the superimposition of a number of properties of the catalyst.

Boosting the Stability and Performance of SOFCs: A New Approach to Oxygen Storage Capacity Using Lanthanum-Doped Ceria Interlayer Technology

Being carbon neutral is one of the world's most important objectives. More than 65 percent of the countries that emit harmful greenhouse gases have committed to achieving net-zero emissions by the middle of the century.[1] There are numerous essential ways to gather energy for worldwide missions, including fuel cells, solar power, wind power, etc. The growing marketplace of fuel cells is following the trend of green-energy demand. Solid oxide fuel cells (SOFCs) are gaining popularity in the next-generation energy industry due to their high energy conversion efficiency, fuel flexibility, and low emissions.[2]The main challenges for the commercialization of SOCs that need to address are high performance, long-term durability, and effective cost. Stabilized zirconia such as Yttria-stabilized zirconia (YSZ) and Scandia-stabilized zirconia (ScSZ) is commonly used as a self-standing electrolyte support. The inability of zirconia-based electrolytes to react with Sr-rich cathode materials like LSC, LSCF, BSCF, etc. Therefore, the electrodes and the zirconia-based electrolyte must have the best-interlayered buffer structure. Optimizing interlayer microstructures to make them more effective at high-performance ESCs is of great interest. Several Rare-earth doped ceria (RDC) materials, including (Gd, Ce)O2, (Sm, Ce)O2, and (La, Ce)O2, have been used as efficient interlayers to lessen reactivity and preserve ionic conductivity in the interface. However, the difference in the thermal expansion coefficient (TEC) between the GDC and the YSZ poses a significant problem for starting and halting SOCs. In the present work, we focus on the commercial lanthanum-doped ceria (Ce0.6La0.4O1.8 - LDC) as a buffer layer between the YSZ electrolyte and the LSCF cathode because of its higher oxygen storage capacitance. The reactivity of the YSZ/GDC composite was higher than that of the YSZ/LDC composite. And the total conductivity of the LDC/YSZ composite is higher than GDC/YSZ composite even though LDC has lower conductivity than GDC. Moreover, the mismatching of TEC was reduced by replacing the GDC with the LDC bilayer.Consequently, the maximum power density for the electrolyte-supported cell SOFC with LDC interlayer was ~0.55 W.cm−2 at 1073 K. A variety of additives (Co, Mn, and Cu) was added to the LDC buffer layer to examine the sintering properties. The addition of copper not only obtained the highest microstructural density but also improve mechanical stability under cost-effective processes.

Chemistry and electrochemistry of CeO<sub>2</sub>-based interlayers: Prolonging the lifetime of solid oxide fuel and electrolysis cells

Research and development of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) are currently of paramount importance in terms of realizing hydrogen energy and carbon emission reduction programs, which many countries have committed to. Although, there are many outstanding results in the fabrication and characterization of SOFCs and SOECs with promising oxygen-ionic and proton-conducting electrolytes, conventional zirconia electrolytes are still widely used not only in a lab-scale setup, but also in the form of enlarged cells and stacks, with the experimental operation of the latter during 10 000–100 000 h. To ensure good performance stability and microstructural integrity of such multilayered cells, a special attention should be paid to the chemical activity of functional materials toward their interaction with each other, especially in long-term focus. The literature analysis has shown that many undesirable processes occur in SOFCs and SOECs with the classical pairs of zirconia electrolytes and strontium-containing electrodes, including element segregation and interdiffusion, insulating phase formation, microscopic defect appearance, and delamination. Some of these processes can be efficiently eliminated by using so-called interlayers designed from doped ceria materials. Due to their numerous beneficial functions, such interlayers have several synonymous names: blocking, barrier, buffer, or protecting layers. Herein, we review the recent progress and achievements in the fundamental and applied researches dealing with the ceria interlayers and their impact on chemistry and electrochemistry of solid oxide cells based on classical zirconia electrolytes as well as promising oxygen-ionic and proton-conducting analogs. <br> The bibliography includes 405 references.

Recent Developments in Ceria-Driven Solar Thermochemical Water and Carbon Dioxide Splitting Redox Cycle

Metal oxide (MO) based solar thermochemical H2O (WS) and CO2 splitting (CDS) is one of the most promising and potential-containing processes that can be used to produce H2 and syngas (liquid fuel precursor). Several non-volatile and volatile MOs were considered redox materials for the solar-driven WS and CDS operation. Among all the examined redox materials, based on their high O2 storage capacity, faster oxidation kinetics, and good stability, ceria and doped ceria materials are deemed to be one of the best alternatives for the operation of the thermochemical redox reactions associated with the WS and CDS. Pure ceria was used for solar fuel production for the first time in 2006. A review paper highlighting the work done on the ceria-based solar thermochemical redox WS and CDS cycle from 2006 until 2016 is already published elsewhere by the author. This review paper presents all the significant findings reported in applying pure ceria and doped ceria materials for the WS and CDS by research teams worldwide.

Selective Discrimination between CO and H2 with Copper-Ceria-Resistive Gas Sensors.

The production of hydrogen and the utilization of biomass for sustainable concepts of energy conversion and storage require gas sensors that discriminate between hydrogen (H2) and carbon monoxide (CO). Mesoporous copper-ceria (Cu-CeO2) materials with large specific surface areas and uniform porosity are prepared by nanocasting, and their textural properties are characterized by N2 physisorption, powder XRD, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The oxidation states of copper (Cu+, Cu2+) and cerium (Ce3+, Ce4+) are investigated by XPS. The materials are used as resistive gas sensors for H2 and CO. The sensors show a stronger response to CO than to H2 and low cross-sensitivity to humidity. Copper turns out to be a necessary component; copper-free ceria materials prepared by the same method show only poor sensing performance. By measuring both gases (CO and H2) simultaneously, it is shown that this behavior can be utilized for selective sensing of CO in the presence of H2.

Stepwise photoassisted decomposition of carbohydrates to H2

Stepwise photoassisted decomposition of carbohydrates to H2

Fine control for the preparation of ceria nanorods (111).

The morphologies and exposed surfaces of ceria nanocrystals are important factors in determining their performance. In order to establish a structure-property relationship for ceria nanomaterials, it is essential to have materials with well-defined morphologies and specific exposed facets. This is also crucial for acquiring high resolution 17O solid-state NMR spectra. In this study, we explore the synthesis conditions for preparing CeO2 nanorods with exposed (111) facets. The effects of alkali concentration, hydrothermal temperature and time, cerium source and oxidation agent are investigated and optimal synthesis conditions are found. The resulting CeO2 nanorods show very narrow 17O NMR peaks for the oxygen ions in the first, second and third layers, providing a foundation for future research on mechanisms involving ceria materials using 17O solid-state NMR spectroscopy.

Ceria nanoflowers decorated Co3O4 nanosheets electrodes for highly efficient electrochemical supercapacitors

Overcoming a low energy density (1–50 Whkg−1) limitation is a challenge for supercapacitor electrode materials. To achieve higher performance (up to 100 Whkg−1), the transition metal-based pseudocapacitors (PCs) have been utilized owing to advantages viz. Faradaic reversible redox reaction significantly boosts the capacitance compared to that of the traditional electrical double layers capacitors (EDLCs). In this work, ceria nanoflowers decorated Co3O4 nanosheets structured active material deposited on nickel foam (denoted as CeO2@Co3O4/NF) by hydrothermal synthesis, demonstrates enhanced supercapacitor performance due to high conductivity, large surface area, and stable reversible oxidation states of Ce3+/Ce4+ and Co2+/Co3+ and greatly supported the Faradaic redox reaction of PCs. The CeO2@Co3O4/NF electrode showed a high areal-specific capacitance of 1598 mFcm−2 at 1 mAcm−2 for the 0–0.5 V potential windows and stability up to 96.6% after 6000 cycles. In an asymmetric supercapacitor (ASC) device, the CeO2@Co3O4/NF was used as a positive electrode which revealed an excellent energy density and power density viz. 92 Whkg−1 and 1580 Wkg−1, respectively. Overall, the present research findings signify the advantages of the novel combination of nanostructured ceria and cobalt oxide active materials for enhanced supercapacitor performance.

Deoxydehydration of glycerol to allyl alcohol catalysed by ceria-supported rhenium oxide

The Deoxydehydration (DODH) of glycerol to allyl alcohol was studied over ceria-supported rhenium oxide catalyst. Mesoporous ceria materials were synthetized via a nanocasting process using SiO2 and activated carbon as hard templates. The as-obtained ceria supports were impregnated with 2.5-10 wt.% ReOx. and applied in the DODH reaction of glycerol to allyl alcohol at 175°C in batch conditions using 2-hexanol as solvent and hydrogen donor. As the characterisations revealed that the template removal was a critical step in the synthesis of the mesostructured ceria via the nanocasting method, the influence of the presence of the hard template was studied in detail by comparison to commercial ceria supports. The catalyst based on the nanocasting ceria showed higher performance of up to 88% yield in allyl alcohol and was reusable for 3 cycles without reactivation step. No evidence of leaching was observed via hot filtration test. The characterisation of the catalyst by XPS revealed the presence of Re4+ species after test, which led us propose that two redox couples, namely Re7+/Re5+ and Re6+/Re4+, are involved during DODH of glycerol to allyl alcohol, which was further confirmed by DFT calculations.

Highly Active and Stable Ni/La-Doped Ceria Material for Catalytic CO2 Reduction by Reverse Water-Gas Shift Reaction

The design of an active, effective, and economically viable catalyst for CO2 conversion into value-added products is crucial in the fight against global warming and energy demand. We have developed very efficient catalysts for reverse water-gas shift (rWGS) reaction. Specific conditions of the synthesis by combustion allow the obtention of macroporous materials based on nanosized Ni particles supported on a mixed oxide of high purity and crystallinity. Here, we show that Ni/La-doped CeO2 catalysts─with the "right" Ni and La proportions─have an unprecedented catalytic performance per unit mass of catalyst for the rWGS reaction as the first step toward CO2 valorization. Correlations between physicochemical properties and catalytic activity, obtained using a combination of different techniques such as X-ray and neutron powder diffraction, Raman spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, electron microscopy, and catalytic testing, point out to optimum values for the Ni loading and the La proportion. Density functional theory calculations of elementary steps of the reaction on model Ni/ceria catalysts aid toward the microscopic understanding of the nature of the active sites. This finding offers a fundamental basis for developing economical catalysts that can be effectively used for CO2 reduction with hydrogen. A catalyst based on Ni0.07/(Ce0.9La0.1Ox)0.93 shows a CO production of 58 × 10-5 molCO·gcat-1·s-1 (700 °C, H2/CO2 = 2; selectivity to CO > 99.5), being stable for 100 h under continuous reaction.

Novel Ceria and Ceria-based Nanocomposites as Potential Catalysts for Methanol Decomposition and Total Oxidation of Ethyl Acetate

The focus of our study is the development of novel ceria-based materials that can be successfully used as catalysts in various environmentally friendly processes due to a significant improvement of ceria intrinsic characteristics. In particular, the combination of ceria with low-dimensional carbon materials such as graphene oxide (GO) and nanodiamond (ND) appears as a very promising approach. Actually, the decrease of ceria crystallite sizes deep in the nanoscale have been found to convert them into highly effective destructive adsorbent materials owing to their high surface area, strong adsorbability, and large number of highly reactive sites. Besides, the addition of a nanostructured carbon containing component (nanodiamond or graphene oxide) contributes to both mechanical and catalytic properties of the obtained nanocomposite hybrids. The results show that the novel ceria and ceria-based composite materials significantly outperform the hydrothermally obtained ceria catalyst used for comparison in both activity and selectivity to CO and hydrogen in the methanol decomposition reaction, and in the process of total oxidation of ethyl acetate due to their significantly improved redox behaviour and textural characteristics.

Synthesizing active and durable cubic ceria catalysts (

Dehydrogenation is considered as one of the most important industrial applications for renewable energy. Cubic ceria-based catalysts are known to display promising dehydrogenation performances in this area. Large particle size (>20 nm) and less surface defects, however, hinder further application of ceria materials. Herein, an alternative strategy involving lactic acid (LA) assisted hydrothermal method was developed to synthesize active, selective and durable cubic ceria of <6 nm for dehydrogenation reactions. Detailed studies of growth mechanism revealed that, the carboxyl and hydroxyl groups in LA molecule synergistically manipulate the morphological evolution of ceria precursors. Carboxyl groups determine the cubic shape and particle size, while hydroxyl groups promote compositional transformation of ceria precursors into CeO2 phases. Moreover, enhanced oxygen vacancies (Vö) on the surface of CeO2 were obtained owing to continuous removal of O species under reductive atmosphere. Cubic CeO2 catalysts synthesized by the LA-assisted method, immobilized with bimetallic PtCo clusters, exhibit a record high activity (TOF: 29,241 h−1) and Vö-dependent synergism for dehydrogenation of bio-derived polyols at 200 °C. We also found that quenching Vö defects at air atmosphere causes activity loss of PtCo/CeO2 catalysts. To regenerate Vö defects, a simple strategy was developed by irradiating deactivated catalysts using hernia lamp. The outcome of this work will provide new insights into manufacturing durable catalyst materials for aqueous phase dehydrogenation applications.

Evolution of photocatalytic activity of CeO2–Bi2O3 composite material for wastewater degradation under visible-light irradiation

The Ce-doped bismuth oxide have been fascinated significantly due to the potential applications in ecological decontamination by photodegradation of the dye pollutants and agrochemical wastewater in to ecofriendly byproducts. Ceria (CO), Bismuth oxide (BO) and Ce-modified bismuth oxide (CBO) nanocrystalline materials were synthesized and thoroughly characterized by XPS, powder XRD, SEM-EDS, FT-IR, Raman, UV–Visible diffuse reflectance (UV–vis-DRS), photocurrent measurements, electrochemical impedence spectroscopy and photoluminescence (PL) spectral analysis. The band gap energy of Bi2O3 was reduced from 3.35 to 2.64 eV, the transformation of the monoclinic (α-Bi2O3) phase to the tetragonal (β-Bi2O3) phase and increase in BET surface area was observed upon Ce4+ doping into BO. Low fluorescence intensity, lesser arc radius of the Nyquist plots, high photocurrent response of CBO specifies a reduction in the rate of recombination of photogenerated electron and hole pair. The photocatalytic activity of BO, CO and CBO were examined for photodegradation of methylene blue (MB), cotton blue (CB) and agrochemical industrial wastewater (ACIWW) under visible light treatment. CBO exhibited greater degradation of MB, CB and ACIWW whereas pristine BO was less active in the degradation of pollutants. The greater photocatalytic activity of CBO is ascribed to stronger absorption of visible-light energy because of the higher surface area and shrinking of band gap energy as well as the easy production of •OH radicals during the photocatalytic reaction evidencing from scavenger studies. Even after five cycles of photodegradation of MB, CB and ACIWW, the photocatalyst CBO is found to be consistent in photocatalytic activity and powder XRD pattern as compared to pure CBO material.

Additive manufacturing and two-step redox cycling of ordered porous ceria structures for solar-driven thermochemical fuel production

This study focuses on thermochemical H2O and CO2-splitting processes using non-stoichiometric metal oxides and concentrated solar energy to produce solar fuels. The redox process involves two distinct reactions: (i) a thermal reduction at high temperature of the oxide with creation of oxygen vacancies in its crystallographic structure, resulting in released O2; (ii) a re-oxidation of the metal oxide by H2O and/or CO2, yielding H2 and/or CO. Hierarchically-ordered porous ceria materials offer high potential for solar-driven thermochemical fuel production based on two-step redox cycles for H2O and CO2-splitting. The emergence of additive manufacturing processes allows to develop architected reactive materials with 3D-ordered geometry and hierarchical structure (porosity gradient), able to enhance the volumetric solar absorptivity and provide homogeneous heating of the oxygen carrier. The investigation of additive-manufactured ordered porous ceria monoliths made from 3D-printed polymer scaffolds was performed in a solar reactor.In comparison with reticulated ceria foams, an improvement of the oxygen yields was achieved with 3D-ordered porous structures. A low total pressure during the reduction step (inducing low pO2) favored the reduction extent and associated fuel yields. The fuel production rate during the exothermal oxidation step was enhanced by decreasing the temperature and by increasing the CO2 partial pressure. In addition, since the oxidation step is a surface-controlled reaction, a natural pore former (woody biomass) was used to create µm-size pores within the struts of ceria scaffolds. This enhanced the oxidation kinetics with a maximal CO production rate of 4.9 mL/min/g and fuel yield up to 333 µmol/g (with a reduction step at 1400 °C under ~0.10 bar of total pressure). Thus, using a 3D-ordered open-cell geometry with addition of micro-scale porosity in the struts enhanced the thermochemical performance.

Effect of Reduced Atmosphere Sintering on Blocking Grain Boundaries in Rare-Earth Doped Ceria

Rare-earth doped ceria materials are amongst the top choices for use in electrolytes and composite electrodes in intermediate temperature solid oxide fuel cells. Trivalent acceptor dopants such as gadolinium, which mediate the ionic conductivity in ceria by creating oxygen vacancies, have a tendency to segregate at grain boundaries and triple points. This leads to formation of ionically resistive blocking grain boundaries and necessitates high operating temperatures to overcome this barrier. In an effort to improve the grain boundary conductivity, we studied the effect of a modified sintering cycle, where 10 mol% gadolinia doped ceria was sintered under a reducing atmosphere and subsequently reoxidized. A detailed analysis of the complex impedance, conductivity, and activation energy values was performed. The analysis shows that for samples processed thus, the ionic conductivity improves when compared with conventionally processed samples sintered in air. Equivalent circuit fitting shows that this improvement in conductivity is mainly due to a drop in the grain boundary resistance. Based on comparison of activation energy values for the conventionally processed vs. reduced-reoxidized samples, this drop can be attributed to a diminished blocking effect of defect-associates at the grain boundaries.

Two-step thermochemical cycles using fibrous ceria pellets for H2 production and CO2 reduction in packed-bed solar reactors

The thermochemical redox activity and performance of commercial-grade fibrous ceria pellets were determined including fuel production rates and yields from H2O and CO2 dissociation. Two solar reactors integrating the ceria pellets undergoing two-step thermochemical cycling (with temperature-swing between alternating redox steps) were experimentally tested. They consisted of packed-bed tubular and cavity-type solar reactors with direct or indirect heating of the reacting materials. The obtained fuel production rates compared favorably with the performance of other previously considered microstructured ceria materials such as templated foams, sintered felts, bioinspired or ordered macroporous morphologies. H2 production rates reached 2.3 mL.g−1.min−1 (with H2/O2 ratios approaching 2) in the indirectly-heated tubular reactor after reduction at 1400 °C and oxidation upon free cooling below 950 °C in steam atmosphere (57% molar content). The highest fuel production rate (~9.5 mL.g−1.min−1) and peak solar-to-fuel energy efficiency (~9.4%) were reached in the directly-irradiated cavity-type reactor (with a reduction step at 1400 °C under reduced pressure, and an oxidation step in pure CO2 below 950 °C). The reduction extent was favored at low pO2 (δ up to 0.056) and the fuel yields were thus improved notably (up to 315 μmol/g) by decreasing the total pressure below 0.1 bar during the reduction step. The fuel production rate increased when the oxidation temperature decreased (because the reaction was thermodynamically more favorable). An increase in CO2 partial pressure further promoted the oxidation kinetics (and decreased the pCO/pCO2 ratio, thus shifting the thermodynamic equilibrium toward CO product). The fibrous ceria structures exhibited stable morphologies with high fuel production performance similar to less scalable porous structures, thus offering the potential for versatile applications in packed-bed solar reactors.

Effect of Cation Point Defects in Doped Ceria Materials on Surface Oxygen Vacancies and Exchange Reactions

Effect of Cation Point Defects in Doped Ceria Materials on Surface Oxygen Vacancies and Exchange Reactions