Ce-based Materials Research Articles

Cerium-Based Electrocatalysts for Oxygen Evolution/Reduction Reactions: Progress and Perspectives.

Ce-based materials have been widely used in photocatalysis and other fields because of their rich redox pairs and oxygen vacancies, despite research on the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) remaining scare. However, most pristine cerium-based materials, such as CeO2, are non-conductive materials. Therefore, how to obtain highly conductive and stable OER/ORR electrocatalysts is currently a hot research topic. To overcome these limitations, researchers have proposed a variety of strategies to promote the development of Ce-based electrocatalysts in recent years. This progress report focuses on reviewing new strategies concerning three categories of Ce-based electrocatalysts: metal-organic framework (MOF) derivatives, structure tuning, and polymetallic doping. It also puts forward the main existing problems and future prospects. The content of cerium in the crust is about 0.0046%, which is the highest among the rare earth elements. As a low-cost rare earth material, Ce-based materials have a bright future in the field of electrocatalysis due to replacing precious metal and some transition metals.

MOF-Derived CeO2 and CeZrOx Solid Solutions: Exploring Ce Reduction through FTIR and NEXAFS Spectroscopy.

The development of Ce-based materials is directly dependent on the catalyst surface defects, which is caused by the calcination steps required to increase structural stability. At the same time, the evaluation of cerium's redox properties under reaction conditions is of increasing relevant importance. The synthesis of Ce-UiO-66 and CeZr-UiO-66 and their subsequent calcination are presented here as a simple and inexpensive approach for achieving homogeneous and stable CeO2 and CeZrOx nanocrystals. The resulting materials constitute an ideal case study to thoroughly understand cerium redox properties. The Ce3+/Ce4+ redox properties are investigated by H2-TPR experiments exploited by in situ FT-IR and Ce M5-edge AP-NEXAFS spectroscopy. In the latter case, Ce3+ formation is quantified using the MCR-ALS protocol. FT-IR is then presented as a high potential/easily accessible technique for extracting valuable information about the cerium oxidation state under operating conditions. The dependence of the OH stretching vibration frequency on temperature and Ce reduction is described, providing a novel tool for qualitative monitoring of surface oxygen vacancy formation. Based on the reported results, the molecular absorption coefficient of the Ce3+ characteristic IR transition is tentatively evaluated, thus providing a basis for future Ce3+ quantification through FT-IR spectroscopy. Finally, the FT-IR limitations for Ce3+ quantification are discussed.

Violation of Pauli-Clogston limit in the heavy-fermion superconductor CeRh2As2 : Duality of itinerant and localized 4f electrons

We theoretically propose a mechanism to understand the violation of the Pauli-Clogston limit for the upper critical field H$_{\rm c2}$ observed in the Ce bearing heavy Fermion material CeRh$_2$As$_2$ from the view point of spin singlet pairing. It is based on a duality concept, the dual simultaneous aspects of an electron: the itinerant part and localized part of quasi-particles (QPs) originated from the 4f electrons of the Ce atoms. While the itinerant QPs directly participate in forming the Cooper pairs, the localized QPs exert the internal field so as to oppose the applied field through the antiferromagnetic exchange interaction between them. This is inherent in the dense Kondo lattice system in general. We argue that this mechanism can be applied not only to the locally noncentrosymmetric material CeRh$_2$As$_2$, but also to globally inversion symmetry broken Ce-based materials such as CePt$_3$Si. Moreover, we point out that it also works for strongly Pauli limit violated spin triplet pairing systems, such as UTe$_2$.

Mini Review on Active Sites in Ce-Based Electrocatalysts for Alkaline Water Splitting

Hydrogen energy has become one of the most attractive candidates to replace traditional fossil fuels because of its lack of pollution and its high energy density. Electrocatalytic water splitting is a “green” and sustainable way to produce hydrogen but is still not sufficiently efficient at this stage. In recent years, Ce-based materials have become very popular as the electrocatalysts for water splitting primarily because of the multivalence state of Ce and easily formed oxygen vacancies readily formed in CeO2. However, until now, this interesting subject has seldom been reviewed, especially for electrocatalysts for alkaline water splitting. Herein, we outline and discuss recent progress on the active sites of Ce-based electrocatalysts for hydrogen evolution and oxygen evolution. Oxygen vacancies and interfaces between CeO2 and mixed metal components could provide optimized binding of hydrogen evolution reaction (HER) intermediates, thus promoting HER performance. For the oxygen evolution reaction (OER), Ce3+/Ce4+ redox, oxygen vacancies, and exogenous transition metals could optimize the binding of OER intermediates toward top catalytic activities. The aim of this review is to seek an overall understanding about the reaction sites in Ce-based electrocatalysts for water splitting, which may provide a guide for the future development of HER and OER Ce-based electrocatalysts toward industrial applications.

Isotopic study of the influence of oxygen interaction and surface species over different catalysts on the soot removal mechanism

In order to improve the catalytic formulations for soot removal in after-treatment emission control technologies for gasoline and diesel engine vehicle, an isotopic study was approached using transitory labeled oxygen response method over model catalysts that allows the unraveling of soot oxidation mechanism. Ce-based materials promote oxygen exchange associated with the high population of lattice oxygen species (O2-) denoted as OI type. The incorporation of praseodymium produces a Pr3+ enrichment that decrease the energy for oxygen release and increase oxygen mobility through surface and subsurface oxygen centers (OII type) depending on the synthesis procedure. For PtBaK catalyst, OIII species are responsible for oxygen exchange. Gas-solid reaction between soot and gas phase molecular oxygen is responsible for direct uncatalyzed soot oxidation. For ceria containing catalysts, low-temperature soot removal takes place through the intervention of lattice atomic species and superoxide species. For DPNR model catalyst, PtBaK/Al2O3, the soot elimination occurs with the intervention of OIII type centers. In the presence NO, the assisted and cooperative mechanism due to NO2 and the intervention of the adsorbed nitrate species on the trimetallic catalyst enhances soot removal capacity.

Influences of Temperature on Band Energetics and Electrochemical Performance of Cerium Oxynitride in a Symmetric Aqua-Based Supercapacitor

The fast deterioration of fossil fuels motivates the development of eco-friendly, sustainable, and renewable energy resources. Concurrently enhancement of present energy storage technologies is crucial. Supercapacitors (SCs) are known for their quick energy delivery (1-10 kW kg-1) capability and long cycle life (~105 cycles). The advantages of SCs enable high-power applications in several sectors such as industrial, transportation, etc. The increasing demand and specific advantages of SCs have helped to cause a bloom in the market in the last couple of decades. Thus, research on novel materials to improve specific capacitance is increasing with time. Hence, understanding the band energetics of the electrode material and studying the electrochemical performances are pivotal in SC research.Among several electrode materials, Ce-based materials (oxides and related composites) are often studied in this aspect due to (i) interchangeable oxidation states of cerium (3+/4+), (ii) relative abundance of cerium on the earth’s crust (nearly as rich as copper sources, and indeed most abundant amongst all rare earth metals), and (iii) high-temperature stability. Research on metal oxynitrides is still in its infancy for supercapacitor application. Metal oxynitrides are compositionally in between pure oxides and nitrides. The existence of an oxide layer alongside the nitride phase ensures rapid redox reactions. Rapid redox reactions enhance rate capabilities and enable longer cycle life. Hence, cerium oxynitride is used here as the electrode material for a symmetric supercapacitor device based on an aqueous electrolyte. The influences of temperature on the band energetics of the material and electrochemical performances of the device are studied extensively.In this work, the lattice constant of CeOxNy is optimized concerning total energy calculations for various temperatures. The configuration of the material is found to be stable when varied against several temperatures (-5 to 60 oC). The electronic structure of CeOxNy for the temperature range from -5 to 60 oC is successfully predicted. The lattice parameter of CeOxNy increases monotonically with increasing temperature. The bandgap variance contributed by lattice thermal expansion is determined using the corresponding lattice constants. As with numerous different semiconductors, fundamental computational estimations in this work on CeOxNy show similar patterns of bandgap narrowing with expanding temperature. The bandgap is reduced by 1.18 eV at 60 oC compared to 0 oC.The total and partial density of states (DOS) was computed to analyze the electronic properties of CeOxNy at different temperatures. CeOxNy is found to be a wide bandgap semiconductor (Eg= 2.82 eV) at 0 oC. The DOS of CeOxNy shows a significant change in the conduction state because of the expansion in temperature. The Fermi level of CeOxNy also decreases and shifts downwards from 0 oC. As a result of the increased temperature, the width of the conduction state in CeOxNy is reduced, and the height of the peak is increased. The decrease in the bandgap is primarily due to a change in the conduction state and a decrease in the Fermi level value, which predominantly comes from the relatively minor hybridization between Ce-4f and O-2p state electrons.The observations due to the effect of temperature on the band energetics are reflected in the material’s electrochemical performance. The steady increase in areal capacitance with increasing temperature can be attributed to the narrowing bandgap. The areal capacitance exhibits an upsurge of ~13.4 times (74.73 mF cm-2 from 5.54 mF cm-2 0.1 mA current) at 60 oC than the room temperature (25 oC). However, the time constant increased in both freezing and high temperature. The optimum time constant is 12.12 millisecond at room temperature, which increased to 14.68 millisecond at -5 and 0 oC and 21.54 millisecond at 60 oC. Overall, high aerial capacitance and low time constant indicate CeOxNy as a promising supercapacitor electrode material even at high temperatures. Figure 1

Single band of red upconverison emission in Ce-based glass ceramics for light manipulation

Herein, single band of red upconversion (UC) emission in CeF3 nanostructured glass ceramics (GCs) is reported. CeF3 nanocrystals (NCs) were in-situ precipitated in aluminosilicate glass matrix, which was fabricated via quench-melting method and subsequent glass crystallization. Structural and spectrographic characterizations were also performed to verify the precipitation of CeF3 NCs and the partition of dopants. Ho3+ and Er3+ were elaborately selected to examine the cross relaxation (CR) process in Ce-based materials due to their ladder-like energy levels and similar energy gaps with Ce3+. Ultimately, single band of red UC emission was overtly observed independent of Ho3+ content, while a bit of green emission still stubbornly remained together with predominate red emission in Er3+ doped CeF3 NCs owing to the relative mismatch of energy gaps. Particularly, these results suggest that Ce-based materials are of importance in tailoring UC emissions, which could be a vital step toward realization of light manipulation for desirable emission.

Detrimental effect of Ce4+ ion on the Pt/C catalyst in polymer electrolyte membrane fuel cells

Ce-based materials have been widely used as a radical scavenger to improve the chemical durability of the membrane in polymer electrolyte fuel cells; however, any negative effects from Ce ions on power performance have not been reported as an issue before except for ionomer contamination by Ce3+. Herein, we propose and experimentally demonstrate the detrimental effect of Ce4+ on the Pt/C catalyst. Due to the strong oxidation power of Ce4+, Pt and carbon are subjected to oxidative decomposition under contact with Ce4+. Based on the electrochemical and spectroscopic analyses of the Pt/C stored in Ce3+ and Ce4+ electrolyte, Pt2+ dissolution and carbon corrosion by Ce4+ are verified. As a result, the active surface area and oxygen reduction reaction activity of Pt/C are considerably reduced after storage in the Ce4+ electrolyte. Additionally, at the membrane electrode assembly level, it is demonstrated that the Ce4+ leached out from the CeO2 causes a significant power performance decay along with the loss of the active surface area and an increase in the charge transfer resistance. The newly-found detrimental effect of Ce4+ on the Pt/C catalyst emphasizes the importance of preventing Ce4+ dissolution to improve the membrane durability by Ce-containing radical scavengers.

Effects of anaerobic SO2 treatment on nano-CeO2 of different morphologies for selective catalytic reduction of NOx with NH3

Tuning surface acidity is a common strategy to improve the reactivity of Ce-based materials for selective catalytic reduction (SCR) of NOx with NH3. Traditionally, it is achieved by wet impregnation of sulfate species or treatment under an atmosphere of sulfur dioxide and oxygen. However, the two methods do not bring any change to the electronic state of CeO2, and Ce4+ ions which cause undesired side reactions are largely retained on the surface. Herein, we report a sulfation strategy that can tune the surface acid sites and regulate the electronic state of nano-CeO2. Using SO2 as reducible gas for surface sulfation, we prepared sulfated CeO2 (S-CeO2) catalysts of different morphologies and tested them for the NH3-SCR reaction. The characterization results revealed that the interaction between SO2 and nano-CeO2 is obviously affected by ceria morphology. Among the sulfated catalysts, that with rod morphology (S-CeO2-Rod) shows the highest NOx conversion, and exhibits superior sulfur and water tolerance. The excellent SCR activity of S-CeO2-Rod is attributed to the enrichment of oxygen vacancies and surface Ce3+ ions as a result of the abundant presence of sulfate species.

Ce(III) nanocomposites by partial thermal decomposition of Ce-MOF for effective phosphate adsorption in a wide pH range

A series of hierarchical micro/nano Ce-based composites were derived from Ce-MOF via thermal treatment in N2 atmosphere. Different from conventional complete decomposed materials, forming cerium oxide in air, Ce-MOF that calcinated in N2 at lower temperatures (400 °C or 500 °C) showed a partial thermal decomposition with high percent content of Ce(III). Even though the complete decomposed products held higher surface areas, the partial decomposed samples exhibited extremely higher phosphate uptake, with working capacity 2–4 times higher than that of ceria. The results implied a predominant effect of different valence states on phosphate removal by Ce-based materials, in which Ce(III) species were demonstrated playing the major role to form binding with phosphate. The maximum adsorption capacity (189.4 mg/g) was achieved by Ce-MOF-500(S) with wide applicable scope of pH ranging from 2 to 12 and great selectivity for phosphate in the presence of competing anions. Remarkably, Ce-MOF-500(S) described obvious enhanced phosphate adsorption ability under alkaline condition. This was due to the fact that the hydrolyzed Ce(III) species brought more active sites in the form of hydroxyl groups for ligand exchange with phosphate. Furthermore, based on the analysis of FTIR, XPS, XRD and zeta potential, electrostatic attraction, ligand exchange and surface precipitation were confirmed as the main adsorption mechanisms for partial decomposed samples, while electrostatic attraction was the main mechanism for complete decomposed samples.

Electronic structure and 4f -electron character in Ce2PdIn8 studied by angle-resolved photoemission spectroscopy

The localized-to-itinerant transition of f electrons lies at the heart of heavy-fermion physics, but has only been directly observed in single-layer Ce-based materials. Here, we report a comprehensive study on the electronic structure and nature of the Ce 4f electrons in the heavy-fermion superconductor Ce2PdIn8, a typical n=2 CenMmIn3n+2m compound, using high-resolution and 4d-4f resonance photoemission spectroscopies. The electronic structure of this material has been studied over a wide temperature range, and hybridization between f and conduction electrons can be clearly observed to form a Kondo resonance near the Fermi level at low temperatures. The characteristic temperature of the localized-to-itinerant transition is around 120K, which is much higher than its coherence temperature Tcoh~30K.

Facile one-step in-situ synthesis of type-II CeO2/CeF3 composite with tunable morphology and photocatalytic activity

Ce-based materials have gained considerable prominence in catalysis as a type of the most valuable material for various applications ranging from automotive emission control to CO2 and water splitting. We here report a novel type-II CeO2/CeF3 composite, composed of two paradigms of Ce-based binary compounds, prepared by a facile one-step in-situ hydrothermal method, and explore its potential as a photocatalyst for solar energy conversion. Primarily, the morphologies, particle size and microstructures of CeO2/CeF3 composite can be modulated by the volume ratio of ethanol to water in the precursor solution. The photocatalytic activities firstly increase to a maximum and then decrease with the increased amount of ethanol. The hybrid CeO2/CeF3 particles with large size and polyhedron morphology exhibit superior photocatalytic activity, having a high rate constant k of MB decomposition about 7.6 times faster than that of the lowest one. A multiple synergetic mechanism in CeO2/CeF3 composite, which is featured by obvious promoted separation of photogenerated electrons and holes at the type-II heterojunction interfaces and in particular between the different facets of microcrystals, and the highly reactive facets exposed, has been proposed to account for its distinguished photocatalytic activity. This work provides some new insights for the synthesis and optimization of novel rare earth Ce-based composite photocatalyst with high photocatalytic performance.

Thermochemical Redox Cycles over Ce-based Oxides

Abstract Ce-based materials, that have been extensively investigated for catalytic applications, such as in automotive emission control and more specifically in the oxidation of diesel soot particles, were evaluated with respect to their performance on CO2 splitting. DFT calculations assisted towards the synthesis of two Zr-doped Ce-oxide formulations (Zr content: 20% and 40% respectively) via the LPSHS method. The materials were evaluated with respect to their CO2 splitting activity and compared with other Ce-Zr-based reference oxides. The most active materials were further evaluated under different CO2 splitting temperatures in the range of 800-1480oC in the course of successive redox cycles. The effect of thermal reduction temperature (T: 1480oC and 1600oC) on the most active material was also assessed. The Ce-based oxide formulation with the best performance (Ce0.8Zr0.2O2) was further investigated with respect to its activity towards H2O splitting. In addition, a porous flow-through structure consisting entirely of Ce0.8Zr0.2O2 was manufactured and the resulted body was also evaluated and compared to the respective powder in terms of its CO2 and H2O splitting ability.

Preparation of FexCe1-xOy solid solution and its application in Pd-only three-way catalysts

AbstractFeOx-CeO2 mixed oxides with increasing Fe/(Ce+Fe) atomic ratio (1–20 mol%) were prepared by sol-gel method and characterized by X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) and Hydrogen temperature-programmed reduction (H2-TPR) techniques. The dynamic oxygen storage capacity (DOSC) was investigated by mass spectrometry with CO/O2 transient pulses. The powder XRD data following Rietveld refinement revealed that the solubility limit of iron oxides in the CeO2 was 5 mol% based on Fe/(Ce+Fe). The lattice parameters experienced a decrease followed by an increase due to the influence of the maximum solubility limit of iron oxides in the CeO2. TPR analysis revealed that Fe introduction into ceria strongly modified the textual and structural properties, which influenced the oxygen handling properties. DOSC results revealed that Ce-based materials containing Fe oxides with multiple valences contribute to the majority of DOSC. The kinetic analysis indicated that the calculated apparent kinetic parameters obey the compensation effect. The three-way catalytic performance for Pd-only catalysts based on the Fe doping support exhibited the redundant iron species separated out of the CeO2 and interacted with the ceria and Pd species on the surface, which seriously influenced the catalytic properties, especially after hydrothermal aging treatment.

Comparative study on the preparation, characterization and catalytic performances of 3DOM Ce-based materials for the combustion of diesel soot

Three-dimensionally ordered macroporous (3DOM) praseodymium (Pr)-modified Ce-Zr solid solutions were successfully prepared by colloidal crystal templating method. They were characterized by the techniques of SEM, XRD, UV-Raman, N2 adsorption, and H2-TPR. 3DOM samples showed higher catalytic activities for soot combustion than corresponding disordered macroporous (DM) ones, which benefits from the enhanced contact efficiency between soot and ordered macroporous structure. In addition, the effects of Ce/Zr ratio on the phase composition and catalytic performances of Ce-Zr-O and Pr-Ce-Zr-O samples for soot oxidation were investigated. The results indicate that the phase compositions of Pr-modified ceria-zirconia solid solutions are similar to those of corresponding ceria-zirconia samples, and cubic–tetragonal phases coexist in the samples with moderate zirconia content, which is favorable for soot combustion. Furthermore, the effects of thermal and hydrothermal treatments on the catalytic properties for soot oxidation of 3DOM CeO2, Ce0.7Zr0.3O2 and Ce0.6Zr0.3Pr0.1O2 were comparatively studied. Compared with 3DOM bare CeO2, 3DOM Ce-Zr-based solid solutions, especially Pr-modified Ce-Zr solid solution Ce0.6Zr0.3Pr0.1O2 exhibited the highest thermal and hydrothermal stability because it possesses the lowest mobility of lattice oxygen among the three samples.

CeO 2–CoO x mixed oxides: Structural characteristics and dynamic storage/release capacity

CoO x –CeO 2 mixed oxides with increasing Co/(Ce + Co) atomic ratio (1–20 mol%) are prepared by sol–gel method and characterized by means of by X-ray powder diffraction (XRD), Brunauer–Emmett–Teller (BET) and Hydrogen temperature-programmed reduction (H 2-TPR) techniques. The dynamic oxygen storage capacity (DOSC) is investigated by mass spectrometry with CO/O 2 transient pulses. The powder XRD data following Rietveld refinement reveal that the solubility limit of cobalt oxides in the CeO 2 was 5 mol% based on Co/(Ce + Co). The lattice parameters have been observed to experience a decrease followed by an increase due to the influence of the maximum solubility limit of cobalt oxides in the CeO 2. TPR analysis reveals that Co introduction into ceria strongly modifies the textual and structural properties, which influenced the oxygen handling properties. DOSC results reveal that Ce-based materials containing Co oxides with multiple valences contribute to the majority of DOSC. The kinetic analysis indicates that the calculated apparent kinetic parameters obey the compensation effect.

Effects of Ni-Doping of Ceria-Based Materials on Their Micro-Structures and Dynamic Oxygen Storage and Release Behaviors

The structural, textural, and dynamic oxygen storage/release behaviors of Ni0.1Ce0.9Ox and Ni0.1Ce0.6Zr0.3Ox (prepared by sol–gel method) are investigated by X-ray diffraction (XRD), Raman, H2-TPR, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and CO/O2 pulses with mass spectrometry studies. The dynamic oxygen storage capacity and rate are largely promoted by Ni doing, and the thermal stability of the sample is also enhanced. XRD and Raman results indicate that Ni0.1Ce0.9Ox and Ni0.1Ce0.6Zr0.3Ox have the fluorite-type cubic structure similar to CeO2. TPR, EPR, and XPS analyses reveal that the introduction Ni into ceria and ceria–zirconia mixed oxides strongly modifies the structural and textural properties, which influence the kinetics of bulk oxygen diffusion. The dynamic oxygen storage capacity (DOSC) of Ni doped samples (Ni0.1Ce0.9Ox and Ni0.1Ce0.6Zr0.3Ox) and reference samples (CeO2 and Ce0.67Zr0.33O2) were investigated with CO/O2 transient pulses. The results show that Ce-based materials containing Ni oxides with variable valences contribute to the majority of DOSC and DOSR performances, and the stability of DOSC is obviously enhanced by Ni and Zr codoping.

Pressure studies of 4f and 5f intermetallics with strong electronic correlations

The pressure response of strongly correlated 4f and 5f intermetallic compounds is reviewed briefly. Special attention is given to the similarities and differences among isostructural U-, Ce- and Yb-based analogues. In general, pressure favors reduced electronic correlations in U- and Ce-based materials, whereas the opposite holds for Yb-based systems. A simple model is discussed that qualitatively accounts for these observations.