Atomic-Scale Valence State Distribution inside Ultrafine CeO2 Nanocubes and Its Size Dependence

in situ observation of reversible phase transitions in Gd-doped ceria driven by electron beam irradiation

Ceria-based oxides are widely utilized in diverse energy-related applications, with attractive functionalities arising from a defective structure due to the formation of mobile oxygen vacancies (V O ⋅ ⋅ ). Notwithstanding its significance, behaviors of the defective structure andV O ⋅ ⋅ in response to external stimuli remain incompletely explored. Taking the Gd-doped ceria (Ce0.88Gd0.12O2-δ) as a model system and leveraging state-of-the-art transmission electron microscopy techniques, reversible phase transitions associated with massiveV O ⋅ ⋅ rearrangement are stimulated and visualized in situ with sub-Å resolution. Electron dose rate is identified as a pivotal factor in modulating the phase transition, and both theV O ⋅ ⋅ concentration and the orientation of the newly formed phase can be altered via electron beam. Our results provide indispensable insights for understanding and refining the microscopic pathways of phase transition as well as defect engineering, and could be applied to other similar functional oxides.

Field-Driven Solid-State Defect Control of Bilayer Switching Devices

We develop a framework for controlling and investigating reversible ionic transfer between two solid metal oxides layers by examining field-driven changes in electrical properties of the thin film bilayer oxide system Pr0.1Ce0.9O2/La1.85Ce0.15CuO4 (PCO/LCCO). We show that we can reversibly redistribute oxygen ions by applied voltage in a highly controlled and reversible fashion near ambient temperatures over large oxygen ion activity limits, which, for the first time, is directly interpretable by defect chemical models. This allowed us to determine how defect concentrations in each layer systematically varied with voltage and the subsequent impact on each film's conductance. These results showcase the relevance and applicability of defect chemical models, traditionally considered only at elevated temperatures, to the development of bilayer devices of importance to neuromorphic memory applications. This allows for a more systematic approach for studying and understanding the solid-solid exchange process in electrochemically controlled microelectronic devices. Moreover, our work sets the foundation for the development of large-area field-driven defect-controlled bilayer switching devices with potential application to a broad array of functionally modulated devices.

Composition-dependent morphologies of CeO2 nanoparticles in the presence of Co-adsorbed H2O and CO2: a density functional theory study

Catalytic activity is affected by surface morphology, and specific surfaces display greater activity than others. A key challenge is to define synthetic strategies to enhance the expression of more active surfaces and to maintain their stability during the lifespan of the catalyst. In this work, we outline an ab initio approach, based on density functional theory, to predict surface composition and particle morphology as a function of environmental conditions, and we apply this to CeO2 nanoparticles in the presence of co-adsorbed H2O and CO2 as an industrially relevant test case. We find that dissociative adsorption of both molecules is generally the most favourable, and that the presence of H2O can stabilise co-adsorbed CO2. We show that changes in adsorption strength with temperature and adsorbate partial pressure lead to significant changes in surface stability, and in particular that co-adsorption of H2O and CO2 stabilizes the {100} and {110} surfaces over the {111} surface. Based on the changes in surface free energy induced by the adsorbed species, we predict that cuboidal nanoparticles are favoured in the presence of co-adsorbed H2O and CO2, suggesting that cuboidal particles should experience a lower thermodynamic driving force to reconstruct and thus be more stable as catalysts for processes involving these species.

Fusion Growth and Extraordinary Distortion of Ultrasmall Metal Oxide Nanoparticles

Ultrasmall metal oxide nanoparticles (<5 nm) potentially have new properties, different from conventional nanoparticles. The precise size control of ultrasmall nanoparticles remains difficult for metal oxide. In this study, the size of CeO2 nanoparticles was precisely controlled (1.3-9.4 nm) using a continuous-flow hydrothermal reactor, and the atomic distortion that occurs in ultrasmall metal oxides was explored for CeO2. The crystalline nanoparticles grow rapidly like droplets via coalescence, although they reach a critical particle size (∼3 to 4 nm), beyond which they grow slowly and change shape through ripening. In the initial growth stage, the ultrasmall nanoparticles exhibit disordered atomic configurations, including stacking faults. In ultrasmall CeO2 nanoparticles (<3 to 4 nm), unusual electron localization occurs on Ce 4f orbitals (Ce3+) as a result of O disordering, regardless of O vacancy concentration. This behavior differs from ordinary electron localization caused by the presence of O vacancies. The ultrasmall metal oxides have extraordinary distortion states, making them promising for use in nanotechnology applications. Furthermore, the proposed synthesis method can be applied to various other metal oxides and allows exploration of their properties.

Reduction of (100)-Faceted CeO2 for Effective Pt Loading

Although the function and stability of catalysts are known to significantly depend on their dispersion state and support interactions, the mechanism of catalyst loading has not yet been elucidated. To address this gap in knowledge, this study elucidates the mechanism of Pt loading based on a detailed investigation of the interaction between Pt species and localized polarons (Ce3+) associated with oxygen vacancies on CeO2(100) facets. Furthermore, an effective Pt loading method was proposed for achieving high catalytic activity while maintaining the stability. Enhanced dispersibility and stability of Pt were achieved by controlling the ionic interactions between dissolved Pt species and CeO2 surface charges via pH adjustment and reduction pretreatment of the CeO2 support surface. This process resulted in strong interactions between Pt and the CeO2 support. Consequently, the oxygen-carrier performance was improved for CH4 chemical looping reforming reactions. This simple interaction-based loading process enhanced the catalytic performance, allowing the efficient use of noble metals with high performance and small loading amounts.

CeO2-Based Frustrated Lewis Pairs via Defective Engineering: Formation Theory, Site Characterization, and Small Molecule Activation

Activation of small molecules is considered to be a central concern in the theoretical investigation of environment- and energy-related catalytic conversions. Sub-nanostructured frustrated Lewis pairs (FLPs) have been an emerging research hotspot in recent years due to their advantages in small molecule activation. Although the progress of catalytic applications of FLPs is increasingly reported, the fundamental theories related to the structural formation, site regulation, and catalytic mechanism of FLPs have not yet been fully developed. Given this, it is attempted to demonstrate the underlying theory of FLPs formation, corresponding regulation methods, and its activation mechanism on small molecules using CeO2 as the representative metal oxide. Specifically, this paper presents three fundamental principles for constructing FLPs on CeO2 surfaces, and feasible engineering methods for the regulation of FLPs sites are presented. Furthermore, cases where typical small molecules (e.g., hydrogen, carbon dioxide, methane oxygen, etc.) are activated over FLPs are analyzed. Meanwhile, corresponding future challenges for the development of FLPs-centered theory are presented. The insights presented in this paper may contribute to the theories of FLPs, which can potentially provide inspiration for the development of broader environment- and energy-related catalysis involving small molecule activation.

Low-temperature synthesis of porous high-entropy (CoCrFeMnNi)3O4 spheres and their application to the reverse water-gas shift reaction as catalysts

A high-entropy porous spinel oxide [(Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4] was synthesized via a solvothermal method and calcination. Solvothermal conditions yielding homogeneous precursor composites with five metals were optimized. Low-temperature calcination of the amorphous composites at 500 °C for 60 min yielded porous spheres formed by small primary particles, with crystal structures attributed to single-phase spinels. The homogeneity of the five elements in the spheres was verified via scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy analysis. The high-entropy (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4 spheres exhibited superior catalytic activity and long-term stability for the reverse water-gas shift reaction at 700 °C for at least 15 h. The importance of the Cr component in stabilizing the spinel structure was demonstrated. Mn, Fe, Co, and Ni served as active sites in the reaction. The advantage of solvothermal synthesis for porous high-entropy materials was discussed.

Reversion of ACP Nanoparticles into Prenucleation Clusters via Surfactant for Promoting Biomimetic Mineralization: A Physicochemical Understanding of Biosurfactant Role in Biomineralization Process

Amphiphilic biomolecules are abundant in mineralization front of biological hard tissues, which play a vital role in osteogenesis and dental hard tissue formation. Amphiphilic biomolecules function as biosurfactants, however, their biosurfactant role in biomineralization process has never been investigated. This study, for the first time, demonstrates that aggregated amorphous calcium phosphate (ACP) nanoparticles can be reversed into dispersed ultrasmall prenucleation clusters (PNCs) via breakdown and dispersion of the ACP nanoparticles by a surfactant. The reduced surface energy of ACP@TPGS and the electrostatic interaction between calcium ions and the pair electrons on oxygen atoms of C-O-C of D-α-tocopheryl polyethylene glycol succinate (TPGS) provide driving force for breakdown and dispersion of ACP nanoparticles into ultrasmall PNCs which promote in vitro and in vivo biomimetic mineralization. The ACP@TPGS possesses excellent biocompatibility without any irritations to oral mucosa and dental pulp. This study not only introduces surfactant into biomimetic mineralization field, but also excites attention to the neglected biosurfactant role during biomineralization process.

Eutectoid decompositions in Ce-containing ABO3 perovskites: Part I, the case of cooperative growth in CeAlO3

Oxidation of perovskite CeAlO3 results in the eutectoid transformation to CeO2 and Al2O3. This phase transformation was recorded using thermal gravimetric analysis, X-ray diffraction, and scanning transmission electron microscopy. Lamellar features in the resultant microstructure indicates cooperative growth. Processing conditions dictate the lamellae sizes, which can be as small as a few nanometers, and coarsen into large domains with additional high temperature annealing.

Cerium Oxide Catalyzed Disproportionation of Hydrogen Peroxide: A Closer Look at the Reaction Intermediate

Cerium oxide nanoparticles (CNPs) have recently gained increasing interest as redox enzyme-mimetics to scavenge the intracellular excess of reactive oxygen species, including hydrogen peroxide (H2 O2 ). Despite the extensive exploration, there remains a notable discrepancy regarding the interpretation of observed redshift of UV-Visible spectroscopy due to H2 O2 addition and the catalase-mimicking mechanism of CNPs. To address this question, we investigated the reaction mechanism by taking a closer look at the reaction intermediate during the catalase mimicking reaction. In this study, we present evidence demonstrating that in aqueous solutions, H2 O2 adsorption at CNP surface triggers the formation of stable intermediates known as cerium-peroxo (Ce-O2 2- ) and/or cerium-hydroperoxo (Ce-OOH- ) complexes as resolved by Raman scattering and UV-Visible spectroscopy. Polymer coating presents steric hinderance for H2 O2 accessibility to the solid-liquid interface limiting further intermediate formation. We demonstrate in depth that the catalytic reactivity of CNPs in the H2 O2 disproportionation reaction increases with the Ce(III)-fraction and decreases in the presence of polymer coatings. The developed approach using UV-Visible spectroscopy for the characterization of the surface peroxide species can potentially serve as a foundation for determining the catalytic reactivity of CNPs in the disproportionation of H2 O2 .

Insight into modified CeMn based catalysts for efficient degradation of toluene by in situ infrared

Trace activated carbon (AC) and diatomaceous earth (DE) were used as structural promoters to be incorporated into Ce-Mn-based solid-solution catalysts by the redox precipitation method. The modified catalysts exhibit superior reducibility, with abundant Ce3+, Mn3+and reactive oxygen species, which are facilitated to the migration of oxygen and the generation of oxygen vacancies. In particular, the catalytic combustion temperatures of 90 % toluene (3000 ppm) on Ce1Mn3Ox-AC/DE were 84 °C (dry) and 123 °C (10 vol% H2O), respectively. The role of lattice oxygen and adsorbed oxygen was revealed by in situ DRIFTS. Additionally, in situ DRIFTS was employed to verify that the degradation of toluene by Ce1Mn3Ox-AC/DE satisfied the Langmuir-Hinshelwood (L-H) mechanism and the Mars-Van Krevelen (MvK) mechanism. The possible reaction pathway was elucidated (toluene → benzyl alcohol → benzoic acid → maleic anhydride → CO2 + H2O). Furthermore, final products attributed to toluene oxidation were detected by in situ DRIFTS at 50 °C in the absence of oxygen, confirming that the catalyst possessed outstanding performance at low temperatures beyond mere adsorption.

Lattice-Plane-Dependent Distribution of Ce3+ at Pt and CeO2 Interfaces for Pt/CeO2 Catalysts

The interaction between a metal and a support, which is known as the metal-support interaction, often plays a determining role in the catalytic properties of supported metal catalysts. Herein, we have developed model Pt/CeO2 catalysts, which enabled us to investigate the interface atomic and electronic structures between Pt and the {001}, {011}, and {111} planes of CeO2 using scanning transmission electron microscopy and electron energy-loss spectroscopy. We found that the number of Ce3+ ions around the Pt nanoparticles followed the order {001} > {011} > {111}, which was the opposite order of the generally accepted stability of low index surfaces of CeO2. Systematic first-principles calculations revealed that the presence of Pt nanoparticles facilitated the formation of oxygen vacancies and that the appearance of the Ptδ+ state was preferred when Pt nanoparticles were in contact with CeO2 {001} planes due to direct charge transfer from Pt to CeO2. These results provide important insights into the nature of the metal-support interaction for a comprehensive understanding of the properties of supported metal catalysts.

Hydroxyls on CeO2 Support Promoting CuO/CeO2 Catalyst for Efficient CO Oxidation and NO Reduction by CO

Transition metal catalysts, such as copper oxide, are more attractive alternatives to noble metal catalysts for emission control due to their higher abundance, lower cost, and excellent catalytic activity. In this study, we report the preparation and application of a novel CuO/CeO2 catalyst using a hydroxyl-rich Ce(OH)x support for CO oxidation and NO reduction by CO. Compared to the catalyst prepared from a regular CeO2 support, the new CuO/CeO2 catalyst prepared from the OH-rich Ce(OH)x (CuO/CeO2-OH) showed significantly higher catalytic activity under different testing conditions. The effect of OH species in the CeO2 support on the catalytic performance and physicochemical properties of the CuO/CeO2 catalyst was characterized in detail. It is demonstrated that the abundant OH species enhanced the CuOx dispersion on CeO2, increased the CuOx-CeO2 interfaces and surface defects, promoted the oxygen activation and mobility, and boosted the NO adsorption and dissociation on CuO/CeO2-OH, thus contributing to its superior catalytic activity for both CO oxidation and NO reduction by CO. These results suggest that the OH-rich Ce(OH)x is a superior support for the preparation of highly efficient metal catalysts for different applications.

Unravelling the interplay of local structure and valence transitions in Ce-doped CaYAlO4 luminescent materials

Cerium has been widely used as a dopant in luminescent materials due to its unique electronic configurations. It is generally anticipated that the luminescence properties of rare-earth-doped materials are closely related to the local environment of activators, especially for Ce3+ . In addition, it is convenient to modulate its emission wavelength by adjusting the composition and structure. In this study, we systematically analyzed the microstructure of the Ce-doped CaYAlO4 system at atomic resolution. The quantitive results indicated that the structure distortion greatly influenced the valence state of the Ce dopant, which is critical to its luminescence efficiency. In addition, valence variations also exist from surface to inner structure due to the big distortion area around the surface. Our results unravel the interplay of local structure and valence transitions in Ce-doped aluminate phosphors, which has the potential to be applied in other luminescent materials.

Photocatalytic Co-Reduction of N2 and CO2 with CeO2 Catalyst for Urea Synthesis

The effective conversion of carbon dioxide (CO2 ) and nitrogen (N2 ) into urea by photocatalytic reaction under mild conditions is considered to be a more environmentally friendly and promising alternative strategies. However, the weak adsorption and activation ability of inert gas on photocatalysts has become the main challenge that hinder the advancement of this technique. Herein, we have successfully established mesoporous CeO2-x nanorods with adjustable oxygen vacancy concentration by heat treatment in Ar/H2 (90 % : 10 %) atmosphere, enhancing the targeted adsorption and activation of N2 and CO2 by introducing oxygen vacancies. Particularly, CeO2 -500 (CeO2 nanorods heated treatment at 500 °C) revealed high photocatalytic activity toward the C-N coupling reaction for urea synthesis with a remarkable urea yield rate of 15.5 μg/h. Besides, both aberration corrected transmission electron microscopy (AC-TEM) and Fourier transform infrared (FT-IR) spectroscopy were used to research the atomic surface structure of CeO2 -500 at high resolution and to monitor the key intermediate precursors generated. The reaction mechanism of photocatalytic C-N coupling was studied in detail by combining Density Functional Theory (DFT) with specific experiments. We hope this work provides important inspiration and guiding significance towards highly efficient photocatalytic synthesis of urea.

Atomic-scale insights into the interfacial charge transfer in a NiO/CeO2 heterostructure for electrocatalytic hydrogen evolution

To understand the underlying mechanism of the interfacial charge transfer and local chemical state variation in the nonprecious-based hydrogen evolution reaction (HER) electrocatalysts, a model system of the NiO/CeO₂ heterostructure was chosen for investigation using a combination of the advanced electron microscopic characterization and first-principles calculations. The results directly proved that interfacial charge transfer occurs from Ni to Ce, leading to reduction in the valence state of Ce and increased formation of V_O. This would optimize ΔG_H* and facilitate the hydrogen evolution process, resulting in outstanding HER performance in 1 M KOH with a low overpotential of 99 mV at the current density of 10 mA•cm^-2 and a modest Tafel slope of 78.4 mV•dec^-1 for the NiO/CeO₂ heterostructure sample. Therefore, the improved HER performance could be attributed to the synergistic coupling interactions and electron redistribution at the interface of NiO and CeO₂. These results concretely demonstrate the direct determination of the interfacial structure of the heterostructure and provide atomistic insights to unravel the underlying mechanism of interfacial charge transfer induced HER performance improvement.

Synthesis of Stable Cerium Oxide Nanoparticles Coated with Phosphonic Acid-Based Functional Polymers

Functional polymers, such as poly(ethylene glycol) (PEG), terminated with a single phosphonic acid, hereafter PEG_ik-Ph are often applied to coat metal oxide surfaces during post-synthesis steps but are not sufficient to stabilize sub-10 nm particles in protein-rich biofluids. The instability is attributed to the weak binding affinity of post-grafted phosphonic acid groups, resulting in a gradual detachment of the polymers from the surface. Here, we assess these polymers as coating agents using an alternative route, namely, the one-step wet-chemical synthesis, where PEG_ik-Ph is introduced with cerium precursors during the synthesis. Characterization of the coated cerium oxide nanoparticles (CNPs) indicates a core-shell structure, where the cores are 3 nm cerium oxide and the shell consists of functionalized PEG polymers in a brush configuration. Results show that CNPs coated with PEG_1k-Ph and PEG_2k-Ph are of potential interest for applications as nanomedicines due to their high Ce(III) content and increased colloidal stability in cell culture media. We further demonstrate that the CNPs in the presence of hydrogen peroxide show an additional absorbance band in the UV-vis spectrum, which is attributed to Ce-O₂^2- peroxo-complexes and could be used in the evaluation of their catalytic activity for scavenging reactive oxygen species.

Probing and Leveraging the Structural Heterogeneity of Nanomaterials for Enhanced Catalysis

The marriage between nanoscience and heterogeneous catalysis has introduced transformative opportunities for accessing better nanocatalysts. However, the structural heterogeneity of nanoscale solids stemming from distinct atomic configurations makes it challenging to realize atomic-level engineering of nanocatalysts in the way that is attained for homogeneous catalysis. Here, we discuss recent efforts in unveiling and exploiting the structural heterogeneity of nanomaterials for enhanced catalysis. Size and facet control of nanoscale domains produce well-defined nanostructures that facilitate mechanistic studies. Differentiation of surface and bulk characteristics for ceria-based nanocatalysts guides new thoughts toward lattice oxygen activation. Manipulating the compositional and species heterogeneity between local and average structures allows regulation of catalytically active sites via the ensemble effect. Studies on catalyst restructurings further highlight the necessity to assess the reactivity and stability of nanocatalysts under reaction conditions. These advances promote the development of novel nanocatalysts with expanded functionalities and bring atomistic insights into heterogeneous catalysis.

Facet-Controlled Synthesis of CeO2 Nanoparticles for High-Performance CeO2 Nanoparticle/SnO2 Nanosheet Hybrid Gas Sensors

CeO2 nanocubes with metastable {100} facets and CeO2 nanooctahedrons with the most stable {111} facets are herein fabricated by controlling the morphology and facets of CeO2 nanoparticles. SnO2 nanosheet-based assembled films coated with these CeO2 nanocubes or CeO2 nanooctahedrons yield {100} CeO2 nanocubes/SnO2 nanosheets and {111} CeO2 nanooctahedron/SnO2 nanosheet hybrid gas sensors, respectively. The hybrid sensors with CeO2 nanoparticles exhibited enhanced sensing responses to numerous chemical species relative to a pristine SnO2 nanosheet gas sensor, including acetone, hydrogen, ethanol, ammonia, acetaldehyde, and allyl mercaptan. In particular, the responses of {100} CeO2 nanocubes/SnO2 nanosheets and {111} CeO2 nanooctahedron/SnO2 nanosheet gas sensors to acetone or allyl mercaptan were 6.8 and 10.3 times higher, respectively, than that of the pristine SnO2 nanosheet gas sensor. Furthermore, the sensor response to ammonia was 2.5 times higher than that of a commercial volatile organic compound (VOC) gas sensor (TGS2602, Figaro Engineering Inc.). The CeO2 nanocube-based sensor with exposed metastable {100} facets promotes the adsorption and oxidation of VOCs owing to the higher surface energy of the metastable {100} facets and therefore exhibits a higher sensing performance than the CeO2 nanooctahedron-based sensor with an exposed {111} facet. The developed sensors show excellent potential for the detection of gas markers in human breath and perspiration for disease diagnosis.

Discovering the key role of MnO2 and CeO2 particles in the Fe2O3 catalysts for enhancing the catalytic oxidation of VOC: Synergistic effect of the lattice oxygen species and surface-adsorbed oxygen

Highly active mesoporous Fe-Mn-Ce catalysts with high specific surface area (S_BET) were synthesized by a modified precipitation process for catalyzing toluene oxidation. The Fe_0.85Mn_0.1Ce_0.05 catalyst presents richer surface oxygen species (OS), a higher proportion of Mn⁴⁺ and Ce⁴⁺, a higher concentration of lattice defects and oxygen vacancies, the highest O_ads/O_latt ratio, and a superior low-temperature redox property compared with the Fe-Mn binary oxide and Fe₂O₃ and MnO₂ catalysts. The properties contribute to a high catalytic activity to achieve T₉₀% of toluene conversion at 264 °C and 185 °C with a gas hourly space velocity (GHSV) at 180,000 and 20,000 mL/(g∙h), respectively. The introduction of a slight quantity of Ce and Mn onto the Fe₂O₃ catalyst is the key to enhancing the synergistic effect of the lattice OS and surface-adsorbed oxygen, contributing to the activation oxidation procedure of toluene. In-situ DRIFTS analysis reveals that the rich oxygen vacancy concentration of catalysts accelerates the key steps for the generation and activation of oxidized products. These catalysts with rich oxygen vacancies can efficiently diminish the accumulation of a small number of the intermediary species (phenolate, C₆H₅-OH) produced during the catalytic oxidation of toluene.

Design of Ordered Mesoporous CeO2-YSZ Nanocomposite Thin Films with Mixed Ionic/Electronic Conductivity via Surface Engineering

Mixed ionic and electronic conductors represent a technologically relevant materials system for electrochemical device applications in the field of energy storage and conversion. Here, we report about the design of mixed-conducting nanocomposites by facile surface modification using atomic layer deposition (ALD). ALD is the method of choice, as it allows coating of even complex surfaces. Thermally stable mesoporous thin films of 8 mol-% yttria-stabilized zirconia (YSZ) with different pore sizes of 17, 24, and 40 nm were prepared through an evaporation-induced self-assembly process. The free surface of the YSZ films was uniformly coated via ALD with a ceria layer of either 3 or 7 nm thickness. Electrochemical impedance spectroscopy was utilized to probe the influence of the coating on the charge-transport properties. Interestingly, the porosity is found to have no effect at all. In contrast, the thickness of the ceria surface layer plays an important role. While the nanocomposites with a 7 nm coating only show ionic conductivity, those with a 3 nm coating exhibit mixed conductivity. The results highlight the possibility of tailoring the electrical transport properties by varying the coating thickness, thereby providing innovative design principles for the next-generation electrochemical devices.

Increasing Oxygen Vacancy of CeO2 Nanocrystals by Ni Doping and reduced Graphene Oxides Decoration towards the Electrocatalytic Hydrogen Evolution

The oxygen vacancy (VO) engineering is proved to be an effective approach for improving the hydrogen evolution reaction (HER) performance of low-cost metal oxides electrocatalysts. Cerium dioxide (CeO2), one of...

Roles of Oxygen Vacancies of CeO2 and Mn-Doped CeO2 with the Same Morphology in Benzene Catalytic Oxidation

Mn-doped CeO₂ and CeO₂ with the same morphology (nanofiber and nanocube) have been synthesized through hydrothermal method. When applied to benzene oxidation, the catalytic performance of Mn-doped CeO₂ is better than that of CeO₂, due to the difference of the concentration of O vacancy. Compared to CeO₂ with the same morphology, more oxygen vacancies were generated on the surface of Mn-doped CeO₂, due to the replacement of Ce ion with Mn ion. The lattice replacement has been analyzed through XRD, Raman, electron energy loss spectroscopy and electron paramagnetic resonance technology. The formation energies of oxygen vacancy on the different exposed crystal planes such as (110) and (100) for Mn-doped CeO₂ were calculated by the density functional theory (DFT). The results show that the oxygen vacancy is easier to be formed on the (110) plane. Other factors influencing catalytic behavior have also been investigated, indicating that the surface oxygen vacancy plays a crucial role in catalytic reaction.

Boosting the Catalytic Performance of CeO2 in Toluene Combustion via the Ce-Ce Homogeneous Interface

Catalytic combustion is an advanced technology to eliminate industrial volatile organic compounds such as toluene. In order to replace the expensive noble metal catalysts and avoid the aggregation phenomenon occurring in traditional heterogeneous interfaces, designing homogeneous interfaces can become an emerging methodology to enhance the catalytic combustion performance of metal oxide catalysts. A mesocrystalline CeO₂ catalyst with abundant Ce-Ce homogeneous interfaces is synthesized via a self-flaming method which exhibits boosted catalytic performance for toluene combustion compared with traditional CeO₂, leading to a ∼40 °C lower T₉₀. The abundant Ce-Ce homogeneous interfaces formed by both highly ordered stacking and small grain size endow the CeO₂ mesocrystal with superior redox property and oxygen storage capacity via forming various oxygen vacancies. Surface and bulk oxygen vacancies generate and activate crucial oxygen species, while interfacial oxygen vacancies further promote the reaction behavior of oxygen species (i.e., activation, regeneration, and migration), causing the splitting of redox property toward lower temperature. These properties facilitate aromatic ring decomposition, the important rate-determining step, thus contributing to toluene catalytic degradation to CO₂. This work may shed insights into the catalytic effects of homogeneous interfaces in pollutant removal and provide a strategy of interfacial defect engineering for catalyst development.

Surfactant-mediated morphology evolution and self-assembly of cerium oxide nanocrystals for catalytic and supercapacitor applications

Surfactant plays a remarkable role in determining the growth process (facet exposition) of colloidal nanocrystals (NCs) and the formation of self-assembled NC superstructures, the underlying mechanism of which, however, still requires elucidation. In this work, the mechanism of surfactant-mediated morphology evolution and self-assembly of CeO2 nanocrystals is elucidated by exploring the effect that surfactant modification has on the shape, size, exposed facets, and arrangement of the CeO2 NCs. It is directly proved that surfactant molecules determine the morphologies of the CeO2 NCs by preferentially bonding onto Ce-terminated {100} facets, changing from large truncated octahedra (mostly {111} and {100} exposed), to cubes (mostly {100} exposed) and small cuboctahedra (mostly {100} and {111} exposed) by increasing the amount of surfactant. The exposure degree of the {100} facets largely affects the concentration of Ce3+ in the CeO2 NCs, thus the cubic CeO2 NCs exhibit superior oxygen storage capacity and excellent supercapacitor performance due to a high fraction of exposed active {100} facets with great superstructure stability.

Sulfur-Resistant Ceria-Based Low-Temperature SCR Catalysts with the Non-bulk Electronic States of Ceria

Although ceria-based catalysts serve as an appealing alternative to traditional V₂O₅-based catalysts for selective catalytic reduction (SCR) of NO_x with NH₃, the inevitable deactivation caused by SO₂ at low temperatures severely hampers the ceria-based catalysts to efficiently control NO_x emissions from SO₂-containing stack gases. Here, we rationally design a strong sulfur-resistant ceria-based catalyst by tuning the electronic structures of ceria highly dispersed on acidic MoO₃ surfaces. By using Ce L₃-edge X-ray absorption near edge structure spectra in conjunction with various surface and bulk structural characterizations, we report that the sulfur resistance of the catalysts is closely associated with the electronic states of ceria, particularly expressed by the Ce³⁺/Ce⁴⁺ ratio related to the size of the ceria particles. As the Ce³⁺/Ce⁴⁺ ratio increases up to or over 50%, corresponding to CeO₂/MoO₃(x %, x ≤ 2.1) with the particle size of approximately 4 nm or less, the non-bulk electronic states of ceria appear, where the catalysts start to show strong sulfur resistance. This work could provide a new strategy for designing sulfur-resistant ceria-based SCR catalysts for controlling NO_x emissions at low temperatures.

Ordered mesoporous metal oxides for electrochemical applications: correlation between structure, electrical properties and device performance

Ordered mesoporous metal oxides with a high specific surface area, tailored porosity and engineered interfaces are promising materials for electrochemical applications. In particular, the method of evaporation-induced self-assembly allows the formation of nanocrystalline films of controlled thickness on polar substrates. In general, mesoporous materials have the advantage of benefiting from a unique combination of structural, chemical and physical properties. This Perspective article addresses the structural characteristics and the electrical (charge-transport) properties of mesoporous metal oxides and how these affect their application in energy storage, catalysis and gas sensing.

Ce3+-enriched spherical porous ceria with an enhanced oxygen storage capacity

Porous ceria was obtained using a unique solvothermal reaction in acetonitrile, applying high temperature and pressure. The resulting material comprised homogeneous and monodisperse spheres and exhibited an extremely large surface area of 152 m2 g-1. From catalytic performance evaluation by vapor- and liquid-phase reactions, the synthesized porous ceria showed superior and different reaction activity compared with commercial CeO2. To examine the origin of the reaction activity of the present porous ceria, synchrotron hard X-ray photoelectron spectroscopy (HAXPES) measurements were carried out. The systematic study of HAXPES measurements revealed that the obtained porous ceria with the present solvothermal method contained a very high concentration of Ce3+. Moreover, O2-pulse adsorption analyses demonstrated a significant oxygen adsorption capacity exceeding 268 μmol-O g-1 at 400 °C owing to its high contents of Ce3+ species.

Roles of Oxygen Vacancies in the Bulk and Surface of CeO2 for Toluene Catalytic Combustion

Catalytic combustion technology is one of the effective methods to remove VOCs such as toluene from industrial emissions. The decomposition of an aromatic ring via catalyst oxygen vacancies is usually the rate-determining step of toluene oxidation into CO₂. Series of CeO₂ probe models were synthesized with different ratios of surface-to-bulk oxygen vacancies. Besides the devotion of the surface vacancies, a part of the bulk vacancies promotes the redox property of CeO₂ in toluene catalytic combustion: surface vacancies tend to adsorb and activate gaseous O₂ to form adsorbed oxygen species, whereas bulk vacancies improve the mobility and activity of lattice oxygen species via their transmission effect. Adsorbed oxygen mainly participates in the chemical adsorption and partial oxidation of toluene (mostly to phenolate). With the elevated temperatures, lattice oxygen of the catalysts facilitates the decomposition of aromatic rings and further improves the oxidation of toluene to CO₂.

Novel Core-Shell (ε-MnO2/CeO2)@CeO2 Composite Catalyst with a Synergistic Effect for Efficient Formaldehyde Oxidation

A novel core-shell (ε-MnO₂/CeO₂)@CeO₂ composite catalyst with a synergistic effect was prepared by hydrothermal reaction and thermal decomposition and its application to high-efficiency oxidation removal of formaldehyde (HCHO) was systemically investigated. The (MnCO₃/CeO₂)@CeO₂ precursor was prepared first by the one-pot hydrothermal reaction of Mn²⁺ and Ce³⁺ solutions with a CO₂-storage material (CO₂SM) without any external templates or surfactants required. The thermal decomposition of the precursor afforded the core-shell (ε-MnO₂/CeO₂)@CeO₂ composite catalyst with excellent catalytic performance. HCHO in the feed gas (180 ppm HCHO, 21% O₂, N₂ balanced) at a gas hourly space velocity of 100 L/(g_cat h) is 100% converted over the catalyst at 80 °C. The conversion rate remains above 95% in 72 h and above 73.8% in 140 h, suggesting the strong stability of the catalyst at high gas flow rates and relatively low temperatures. The synergistic mechanism of the catalyst was explored by X-ray diffraction, Raman, Brunauer-Emmett-Teller, transmission electron microscopy, and X-ray photoelectron spectroscopy. The number of defects in the catalyst and the strength of the Mn-O bond in ε-MnO₂ can be tuned by adjusting the synthesis conditions. More oxygen vacancies on the surface of CeO₂ can make the synergistic effect of the catalyst stronger, which significantly improves the lattice oxygen (O_latt) activity on the surface of ε-MnO₂. Our work has provided new insights into the preparation of the desired composite catalysts with excellent performances.

Spiky-shaped niobium pentoxide nano-architecture: highly stable and recoverable Lewis acid catalyst

Niobium pentoxide particles with a complex three-dimensional (3D) nanostructure consisting of a spiky structure have been developed as recyclable and recoverable Lewis acid catalysts. The morphology of the niobium pentoxide was successfully controlled from 1D to 3D via a bridging-ligand-assisted hydrothermal treatment, without changing the crystal structure. Compared with dispersed one-dimensional (1D) niobium pentoxide nanorods with a major-axis length and minor-axis length of 20 nm and 5-8 nm, respectively, the spiky-shaped niobium pentoxide composed of 300 nm spherical cores and nanorods with a minor-axis length of 5 nm maintained its surface nanostructure even after calcination at 400 °C in air. The 400 °C-calcined spiky particles exhibited the highest production rate of 2-((4-methoxyphenyl)amino)-2-phenylacetonitrile (0.115 mmol m^-2) in a Strecker reaction, resulting in a nanoscale and ordered surface structure of spiky particles that simultaneously exhibit high specific reactivity and high structural stability. Acid site analysis and Raman spectroscopy revealed that stable nanorods that grew in the (001) orientation functioned as Lewis acid catalysts and that the origin of the acidity was a flexible Nb-O polyhedral structure in the single-nanoscale (<10 nm) niobium oxide rods. This study proposes that the spiky-shaped niobium pentoxide exhibits sintering resistivity and high activity and has potential applications as a recoverable and recyclable solid acid catalyst.

Unravelling the Role of Structural Geometry and Chemical State of Well-Defined Oxygen Vacancies on Pristine CeO2 for H2O2 Activation

Although H₂O₂ has been often employed as a green oxidant for many CeO₂-catalyzed reactions, the underlying principle of its activation by surface oxygen vacancy (V_o) is still elusive due to the irreversible removal of postgenerated V_o by water (or H₂O₂). The metastable V_o (ms-V_o) naturally preserved on pristine CeO₂ surfaces was adopted herein for an in-depth study of their interplay with H₂O₂. Their well-defined local structures and chemical states were found facet-dependent affecting both the adsorption and subsequent activation of H₂O₂. It is concluded that a strong adsorption of H₂O₂ on ms-V_o may not guarantee its subsequent activation. The ms-V_o can be only free for the next catalytic cycle when the electron density of surface Ce is high enough to reduce/break the O-O bond of adsorbed H₂O₂. This explains the 211.8 and 35.8 times enhancement in H₂O₂ reactivity when the CeO₂ surface is changed from (111) and (110) to (100).

Engineering well-defined rare earth oxide-based nanostructures for catalyzing C1 chemical reactions

In this review, we summarize the nanostructural engineering and applications of rare earth oxide-based nanomaterials with well-defined compositions, crystal phases and shapes for efficiently catalyzing C1 chemical reactions.

Synthesis of low- and high-index faceted metal (Pt, Pd, Ru, Ir, Rh) nanoparticles for improved activity and stability in electrocatalysis

Driven by the quest for future energy solution, faceted metal nanoparticles are being pursued as the next generation electrocatalysts for renewable energy applications. Thanks to recent advancement in solution phase synthesis, different low- and high-index facets on metal nanocrystals become accessible and are tested for specific electrocatalytic reactions. This minireview summarises the key approaches to prepare nanocrystals containing the most catalytically active platinum group metals (Pt, Pd, Ru, Ir and Rh) exposed with low- and high-index facets using solution phase synthesis. Electrocatalytic studies related to the different facets are highlighted to emphasise the importance of exposing facets for catalysing these reactions, namely oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), alcohol oxidation including methanol (MOR) and ethanol oxidation reactions (EOR), formic acid oxidation reaction (FAOR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The future outlook discusses the challenges and opportunities for making electrocatalysts that are even more active and stable.