Ceria-Based Materials for Thermocatalytic and Photocatalytic Organic Synthesis

Unveiling Synergetic Photocatalytic Activity from Heterometallic Ti/Ce Clusters

Titanium-oxo clusters, with their robust structure and suitable optical and electronic properties, have been widely investigated as photocatalysts. Heterometallic Ti/M-oxo clusters provide additional tunability and functionality, which enable systematic structure-activity investigations to elucidate the reaction mechanisms and improve the catalyst design. Incorporating cerium into Ti-oxo clusters can provide additional redox (CeIV/CeIII) and oxygen harvesting ability, but to date, only a limited number of structurally defined titanium-cerium (Ti/Ce) clusters have been reported due to their synthetic challenges. Herein, we report the synthesis and photocatalytic properties of two structurally defined Ti/Ce-oxo clusters, Ti8Ce2(BA)16 and Ti9Ce4(BA)20, as well as a TiCe-BA cluster with a calculated formula of Ti20Ce9O36(BA)42. Photocatalytic study of these clusters demonstrates that the amount of Ce3+ species greatly impacts its photocatalytic oxidation performance, and their superior photocatalytic reactivity toward aerobic alcohol oxidation can be contributed to the synergistic effects of the multiple radical species generated upon light absorption. This work represents a significant milestone in the construction of stable Ti/Ce-oxo clusters, enriching the current library of known heterometallic Ti/M-oxo clusters, and providing a series of crystalline materials with great promise of photoluminescence and photovoltaic chemistry.

Promoted oxygen adsorption on porous CeO2 cubes with abundant oxygen vacancies for efficient gaseous formaldehyde removal

Photocatalytic degradation stands as a promising method for eliminating gas-phase pollutants, with the efficiency largely hinging on the capture of photogenerated electrons by oxygen. In this work, we synthesized a porous CeO2 single crystal cube with abundant oxygen vacancies as photocatalyst, employing urea as a pore-forming agent and for gas-phase formaldehyde degradation. Compared with the CeO2 cubes without pores, the porous ones were superior in specific surface area, akin to conventional CeO2 nanoparticles. The photocatalytic degradation for gas-phase formaldehyde on porous CeO2 cubes was significantly accelerated, of which degradation rate is 3.3 times and 2.1 times that of CeO2 cubes without pores and CeO2 nanoparticles, respectively. Photoelectric tests and DFT calculations revealed that this enhancement stemmed from facilitated oxygen adsorption due to pronounced oxygen vacancies. Consequently, the capture of photoelectrons by oxygen was promoted and its recombination with holes was suppressed, along with an accelerated generation of curial free radicals such as ·OH. This work reveals the pivotal role of surface oxygen vacancies in promoting adsorbed oxygen, proposing a viable strategy to enhance the photocatalytic degradation efficiency for gas-phase pollutants.

Unveiling the Structure of Oxygen Vacancies in Bulk Ceria and the Physical Mechanisms behind Their Formation

Understanding the structures of oxygen vacancies in bulk ceria is crucial as they significantly impact the material's catalytic and electronic properties. The complex interaction between oxygen vacancies and Ce3+ ions presents challenges in characterizing ceria's defect chemistry. We introduced a machine learning-assisted cluster-expansion model to predict the energetics of defective configurations accurately within bulk ceria. This model effectively samples configurational spaces, detailing oxygen vacancy structures across different temperatures and concentrations. At lower temperatures, vacancies tend to cluster, mediated by Ce3+ ions and electrostatic repulsion, while at higher temperatures, they distribute uniformly due to configurational entropy. Our analysis also reveals a correlation between thermodynamic stability and the band gap between occupied O 2p and unoccupied Ce 4f orbitals, with wider band gaps indicating higher stability. This work enhances our understanding of defect chemistry in oxide materials and lays the groundwork for further research into how these structural properties affect ceria's performance.

Generic heterostructure interfaces bound to Co9S8 for efficient overall water splitting supported by photothermal

The increase of reaction temperature of electrocatalysts and the construction of heterogeneous structures is regarded as an efficient method to improve the electrocatalytic water splitting activity. Here, we report an approach to enhance the local heat and active sites of the catalyst by building a heterostructure with Co9S8 to significantly improve its electrocatalytic performance. The as-fabricated Co9S8@Ce-NiCo LDH/NF electrode possesses a notable photothermal ability, as it effectively converts near-infrared (NIR) light into the local heat, owing to its significant optical absorption. Leveraging these favorable qualities, the prepared Co9S8@Ce-NiCo LDH/NF electrode showed impressive performance in both hydrogen evolution reaction (HER) (η100 = 144 mV) and oxygen evolution reaction (OER) (η100 = 229 mV) under NIR light. Compared to the absence of the NIR light, the presence of NIR irradiation leads to a 24.6 % increase in catalytic efficiency for HER and a 15.8 % increase for OER. Additionally, other dual-functional electrocatalysts like NiCo-P, NiFeMo, and NiFe(OH)x also demonstrated significantly enhanced photothermal effects and improved catalytic performance owing to the augmented photothermal conversion when combined with Co9S8. This work offers novel pathways for the development of photothermal-electrocatalytic systems that facilitate economically efficient and energy-conserving overall water splitting processes.

Strengthening the Metal-Acid Interactions by Using CeO2 as Regulators of Precisely Placing Pt Species in ZSM-5 for Furfural Hydrogenation

Understanding the synergism between the metal site and acid site is of great significance in boosting the efficiency of bi-functional catalysts in many heterogeneous reactions, particularly in biomass upgrading. Herein, a "confined auto-redox" strategy is reported to fix CeO2-anchored Pt atoms on the inner wall of a ZSM-5 cage, achieving the target of finely controlling the placements of the two active sites. Compared with the conventional surface-supported counterpart, the encapsulated Pt/CeO2@ZSM-5 catalyst possesses remarkably-improved activity and selectivity, which can convert >99% furfural into cyclopentanone with 97.2% selectivity in 6 h at 160 °C. Besides the excellent catalytic performance, the ordered metal-acid distribution also makes such kind of catalyst an ideal research subject for metal-acid interactions. The following mechanization investigation reveals that the enhancement is strongly related to the unique encapsulation structure, which promotes the migration of the reactants over different active sites, thereby contributing to the tandem reaction.

Low-temperature synthesis of porous organic polymers with donor-acceptor structure and β-ketoenamine for photocatalytic oxidative coupling of amines

In light of the widespread use of fossil fuels and the resulting environmental pollution, it is crucial to develop efficient photocatalysts for renewable energy applications that utilize visible light. Organic photocatalysts based on β-ketoenamine offer several advantages, including facile preparation, high stability, structural controllability, and excellent photovoltaic properties. However, in previous studies, the synthesis of porous organic polymers (POPs) often involved long, high-temperature processes. In this study, POPs with donor (D)-acceptor (A) structure were constructed by utilizing various branched bridging groups and 2,4,6-triformylphloroglucinol, across multiple temperature gradients. Through adjustments in hydrothermal temperature, we successfully synthesized a series of POPs with varying enol-keto structure ratios. Among these POPs, the dimethoxybenzidine-POPs (DMDPOPs) with methoxy electron-rich branched chains exhibited superior photovoltaic performance, electron transfer rate, and photocatalytic activity compared to the dihydroxybenzidine-POPs (DHDPOPs) with electron-deficient hydroxyl branched chains. Notably, DMDPOP-30 demonstrated outstanding performance, achieving a conversion rate of 98% within 3 h. Additionally, other POPs exhibited favorable conversions (90%), further confirming the feasibility of this synthetic approach. Moreover, the synthesis of DMDPOP-30 was achieved under mild conditions at room temperature, highlighting its significant potential for practical applications.

Facet impact of CeO2@C 2D core-shell structure on electrochemical reaction kinetic factor and efficient detection of nitrite

Fine-tuning the surface structure of transition metal oxides at the atomic level is a promising way to improve the catalytic properties of materials. However, the influence of crystal surface structure on electrode reaction kinetics is still limited. In this study, we propose an in-situ synthesis strategy to obtain two-dimensional carbon/cerium oxide core-shell nanosheets by thermal decomposition of Ce-MOF nanosheets grown on the surface of carbon nanostructures, and fine-tuning the surface structure by introducing oxygen vacancies through defect engineering during the oxide nucleation process is conducted to obtain controllable exposed {111} and {110} surface CeO2@C composites. Both experiments and theoretical calculations show that the {110} -dominated nanocomplex (CeO2@C-350S) has better kinetic behavior and catalytic activity due to its abundant surface defects, which is manifested in higher active surface area, richer carrier concentration, and better promotion of diffusion and adsorption. In addition, CeO2@C-350S electrode has an extremely wide linear range and good stability in the electrochemical detection of nitrite. After 1000 times of the accelerated cycle experiments, CeO2@C-350S electrode still maintains 79.3 % of its initial current response, and recovers to 87.3 % after 10 min of stopping the test. The electrode stability is excellent, which is attributed to the clever carbon shell structure of the material. This synthesis strategy can be extended to other carbon-based oxide composite catalysts to improve the electrocatalytic performance and overall stability by adjusting the surface structure.

Rare-earth Element-based Electrocatalysts Designed for CO2 Electro-reduction

Electrochemical CO2 reduction presents a promising approach for synthesizing fuels and chemical feedstocks using renewable energy sources. Although significant advancements have been made in the design of catalysts for CO2 reduction reaction (CO2RR) in recent years, the linear scaling relationship of key intermediates, selectivity, stability, and economical efficiency are still required to be improved. Rare earth (RE) elements, recognized as pivotal components in various industrial applications, have been widely used in catalysis due to their unique properties such as redox characteristics, orbital structure, oxygen affinity, large ion radius, and electronic configuration. Furthermore, RE elements could effectively modulate the adsorption strength of intermediates and provide abundant metal active sites for CO2RR. Despite their potential, there is still a shortage of comprehensive and systematic analysis of RE elements employed in the design of electrocatalysts of CO2RR. Therefore, the current approaches for the design of RE element-based electrocatalysts and their applications in CO2RR are thoroughly summarized in this review. The review starts by outlining the characteristics of CO2RR and RE elements, followed by a summary of design strategies and synthetic methods for RE element-based electrocatalysts. Finally, an overview of current limitations in research and an outline of the prospects for future investigations are proposed.

Kinetic pathways of sub-bandgap induced electron transfer in Ag/TiO2 and the effect on isopropanol dehydrogenation under gaseous conditions

Electron transfer and its kinetics play a major role in the photocatalysis of metal/semiconductor systems. Using in situ photoconductances, in situ photoabsorption, and photoinduced spectroscopic techniques, the present research aimed to gain a deep insight into electron transfer pathways and their kinetics for Ag/TiO2 systems under sub-bandgap light illumination and gaseous conditions. The results revealed that electrons generated in TiO2 can transfer to Ag nanoparticles at fast rates, and plasmon-generated electrons in Ag nanoparticles can also transfer to TiO2. However, it was found that plasmon-assisted hot electron transfer efficiency is much lower than the electron transition from the valence band to the conduction band of TiO2. Rather than plasmonic active spots, the results showed that Ag nanoparticles acted as co-catalyst sites bridging electron transfer to recombination in a methanol-containing N2 atmosphere. As a result, photocatalytic isopropanol dehydrogenation was decreased. Independent of Ag decorations, it was also indicated that isopropanol dehydrogenation mainly occurred over TiO2 surfaces; therefore, Ag nanoparticles did not increase photocatalytic activities. Our results may provide a different viewpoint on sub-bandgap light-induced Ag/TiO2 photocatalysis under gaseous conditions; this may also facilitate the understanding of the photocatalytic mechanism of metal/semiconductor systems.

Emerging two dimensional metastable-phase oxides: insights and prospects in synthesis and catalysis

Since the discovery of graphene, the development of new two-dimensional (2D) materials has received considerable interest. Recently, as a newly emerging member of the 2D family, 2D metastable-phase oxides that combine the unique advantages of metal oxides, 2D structures, and metastable-phase materials have shown enormous potential in various catalytic reactions. In this review, the potential of various 2D materials to form a metastable-phase is predicted. The advantages of 2D metastable-phase oxides for advanced applications, reliable methods of synthesizing 2D metastable-phase oxides, and the application of these oxides in different catalytic reactions are presented. Finally, the challenges associated with 2D metastable-phase oxides and future perspectives are discussed.

Superoxide dismutase-like cerium dioxide hollow sphere-based highly specific photoelectrochemical biosensing for ascorbic acid

Conventional N-type semiconductor-based photoelectrochemical (PEC) sensors are difficult to achieve high selectivity for ascorbic acid (AA) detection in real samples because co-existing reducing agents act as hole sacrificial agents like AA to promote the increase of photocurrent. Cerium dioxide (CeO2) is a superoxide dismutase-like nanozyme with the reversible Ce3+/Ce4+ redox pair as well as one of alternative N-type semiconductors. To address the problem of PEC detection selectivity of AA, bifunctional CeO2 is a good choice. Herein, a novel and rational PEC biosensor for AA is constructed based on CeO2 hollow spheres as both AA superoxide dismutase-like nanozyme and the photoelectric beacon, which enable the PEC approach with high selectivity. In this protocol, AA can selectively induce a decrease in the CeO2-based photoanode current, which is significantly different from the conventional N-type semiconductor-based PEC sensor, this unique working mechanism is also proposed. The results show that the CeO2-based photocurrent response decreases linearly with AA concentrations in the ranges of 1 μM-600 μM and 600 μM-3000 μM, with a limit of detection of 0.33 μM. Moreover, the fabricated PEC biosensor has advantages of cost-effectiveness, replicability, and stability. Additionally, the sensor is competent for AA determination in practical settings and has achieved satisfactory results.

Constructing Built-In Electric Field in NiCo2 O4 -CeO2 Heterostructures to Regulate Li2 O2 Formation Routes at High Current Densities

Developing catalysts with suitable adsorption energy for oxygen-containing intermediates and elucidating their internal structure-performance relationships are essential for the commercialization of Li-O2 batteries (LOBs), especially under high current densities. Herein, NiCo2 O4 -CeO2 heterostructure with a spontaneous built-in electric field (BIEF) is designed and utilized as a cathode catalyst for LOBs at high current density. The driving mechanism of electron pumping/accumulation at heterointerface is studied via experiments and density functional theory (DFT) calculations, elucidating the growth mechanism of discharge products. The results show that BIEF induced by work function difference optimizes the affinity for LiO2 and promotes the formation of nano-flocculent Li2 O2 , thus improving LOBs performance at high current density. Specifically, NiCo2 O4 -CeO2 cathode exhibits a large discharge capacity (9546 mAh g-1 at 4000 mA g-1 ) and high stability (>430 cycles at 4000 mA g-1 ), which are better than the majority of previously reported metal-based catalysts. This work provides a new method for tuning the nucleation and decomposition of Li2 O2 and inspires the design of ideal catalysts for LOBs to operate at high current density.

Bio-based hyperbranched epoxy resins: synthesis and recycling

Epoxy resins (EPs), accounting for about 70% of the thermosetting resin market, have been recognized as the most widely used thermosetting resins in the world. Nowadays, 90% of the world's EPs are obtained from the bisphenol A (BPA)-based epoxide prepolymer. However, certain limitations severely impede further applications of this advanced material, such as limited fossil-based resources, skyrocketing oil prices, nondegradability, and a "seesaw" between toughness and strength. In recent years, more and more research has been devoted to the preparation of novel epoxy materials to overcome the compromise between toughness and strength and solve plastic waste problems. Among them, the development of bio-based hyperbranched epoxy resins (HERs) is unique and attractive. Bio-based HERs synthesized from bio-derived monomers can be used as a matrix resin or a toughener resulting in partially or fully bio-based epoxy thermosets. The introduction of a hyperbranched structure can balance the strength and toughness of epoxy thermosets. Here, we especially focused on the recent progress in the development of bio-based HERs, including the monomer design, synthesis approaches, mechanical properties, degradation, and recycling strategies. In addition, we advance the challenges and perspectives to engineering application of bio-based HERs in the future. Overall, this review presents an up-to-date overview of bio-based HERs and guidance for emerging research on the sustainable development of EPs in versatile high-tech fields.

Cerium oxide boosted CoFe-N codoped carbon nanotubes with abundant oxygen-vacancies toward efficient oxygen reduction and methanol oxidation reaction

In this study, we designed a novel strategy that utilizes N-doped carbon nanotubes as the chemical bond supporter to stabilize ultrafine CoFe alloy and introduces secondary CeO2 active sites into the hybrid, resulting in the formation of CeO2/CoFe-NCNTs heterostructures with exceptional bifunctional electrocatalytic capabilities. To be specific, solution dispersion and high-temperature calcination methods were employed to create the CoFe-NCNTs active sites through the introduction of ethylenediamine into the network interstitials of Co-EDTA and Fe-EDTA. The CeO2/CoFe-NCNTs hybrid not only promotes oxygen absorption and conversion of intermediates, but also accelerates charge transfer capability, thus enhancing oxygen reduction reaction (ORR) performance, while simultaneously inducing boosted the methanol oxidation reaction (MOR) activity. Moreover, the well-dispersed CoFe nanoparticles within the hybrid hold significant potential for establishing metal-nitrogen bonds with the N-doped carbon nanotube network, resulting in efficient catalytic behavior driven by synergistic effects with CeO2 nanoparticles, which contributes to reactant activation. As expected, the resultant CeO2/CoFe-NCNTs-2 exhibits remarkable electrocatalytic performance, with a current density of 281.40 mA cm-2 at a scan rate of 200 mV s-1 and a low Tafel slope (71.3 mV dec-1) for MOR, as well as achieving excellent half-wave potential and onset potential values of 0.834 and 0.90 V (vs. RHE) for ORR. Additionally, it exhibits durable cycle stability for both MOR and ORR, retaining 92.8% and 96.4% of its initial current density during the I-t test, respectively. This work establishes a highly efficient bifunctional earth-abundant electrocatalysts for both anode and cathode reactions in methanol fuel cells.

Modifying charge transfer between rhodium and ceria for boosted hydrogen oxidation reaction in alkaline electrolyte

Sluggish kinetics of hydrogen oxidation reaction (HOR) in alkaline solution has restricted the rapid development of hydrogen economy. Constructing catalyst with metal-oxide heterostructures can enhance HOR performance; however, little studies concentrate on charge transfer between them, and the corresponding effects on reactions remain unclear. Herein, we report charge-transfer-adjustable CeO2/Rh interfaces uniformly dispersed on multiwalled carbon nanotube (CNT), which exhibit excellent alkaline HOR performance. Results confirm that the charge transfer from Rh to CeO2 could be conveniently tuned via thermal treatment. Consequently, the adsorption free energies of H* in Rh sites and OH* adsorption strength in CeO2 could be adjusted, as corroborated by density functional theory study. The optimized CeO2/Rh interfaces exhibit an exchange current density and a mass-specific kinetic current of 0.53 mA cmPGM-2 and 830 A gPGM-1 at an overpotential of 50 mV, respectively, which surpasses most of the advanced noble-metal-based electrocatalysts. This work provides a new insight of harnessing charge transfer of heterostructure to enhance catalytic activities.

Ce4+/Ce3+ Redox Effect-Promoted CdS/CeO2 Heterojunction Photocatalyst for the Atom Economic Synthesis of Imines under Visible Light

The employment of stoichiometric alcohols and amines for imine synthesis under mild and green reaction conditions is still a challenge in the field. In this work, based on our research foundation in the thermocatalytic synthesis of imines over ceria, a CdS/CeO2 heterojunction photocatalyst was constructed and successfully realized the atom-economic synthesis of imines under visible light without additives at room temperature. Mechanistic experiments and corresponding characterizations indicated that the CdS/CeO2 heterojunction can improve the separation efficiency of photogenerated carriers, which can be further enhanced by the Ce4+/Ce3+ redox pair by rapidly combining photogenerated e-. The in situ-reduced Ce3+ can better activate O2 to form Ce-O-O·, which, together with h+, efficiently accelerates alcohol oxidation, which is the rate-determined step for the synthesis of imines via oxidative coupling reaction of alcohol and amine. In addition, our photocatalyst exhibited fairly decent reusability and substrate universality. This work solves problems of using base additives and excess amine or alcohol in the reported photocatalytic systems and provides new insight for designing CeO2-based photocatalytic oxidation catalysts.

Metal-Organic Frameworks as a New Platform to Construct Ordered Mesoporous Ce-Based Oxides for Efficient CO2 Fixation under Ambient Conditions

Metal-organic frameworks (MOFs) are proved to be good precursors to derive various nanomaterials with desirable functions, but so far the controllable synthesis of ordered mesoporous derivatives from MOFs has not been achieved. Herein, this work reports, for the first time, the construction of MOF-derived ordered mesoporous (OM) derivatives by developing a facile mesopore-inherited pyrolysis-oxidation strategy. This work demonstrates a particularly elegant example of this strategy, which involves the mesopore-inherited pyrolysis of OM-CeMOF into a OM-CeO2 @C composite, followed by the oxidation removal of its residual carbon, affording the corresponding OM-CeO2 . Furthermore, the good tunability of MOFs helps to allodially introduce zirconium into OM-CeO2 to regulate its acid-base property, thus boosting its catalytic activity for CO2 fixation. Impressively, the optimized Zr-doped OM-CeO2 can achieve above 16 times higher catalytic activity than its solid CeO2 counterpart, representing the first metal oxide-based catalyst to realize the complete cycloaddition of epichlorohydrin with CO2 under ambient temperature and pressure. This study not only develops a new MOF-based platform for enriching the family of ordered mesoporous nanomaterials, but also demonstrates an ambient catalytic system for CO2 fixation.

Nanomaterials for Advanced Photocatalytic Plastic Conversion

As the disposal of waste plastic emerges as a societal problem, photocatalytic waste plastic conversion is attracting significant attention. Ultimately, for a sustainable future, the development of an eco-friendly plastic conversion technology is essential for breaking away from the current plastic use environment. Compared to conventional methods, photocatalysis can be a more environmentally friendly option for waste plastic reprocessing because it uses sunlight as an energy source under ambient temperature and pressure. In addition to this, waste plastics can be upcycled (i.e., converted into useful chemicals or fuels) to enhance their original value via photocatalytic methods. Among various strategies for improving the efficiency of the photocatalytic method, nanomaterials have played a pivotal role in suppressing charge recombination. Hence, in recent years, attempts have been made to introduce nanomaterials/nanostructures into photocatalytic plastic conversion on the basis of advances in material-based studies using simple photocatalysts. In line with this trend, the present review examines the nanomaterials/nanostructures that have been recently developed for photocatalytic plastic conversion and discusses the direction of future development.

Photo-induced heterogeneous regeneration of Fe(Ⅱ) in Fenton reaction for efficient polycyclic antibiotics removal and in-depth charge transfer mechanism

Fenton reaction is regarded as a potential treatment for antibiotics removal, but challenges remain due to the sluggish reaction kinetics of Fe(III) reduction and incomplete degradation from insufficient active substance. Distinguished from traditional Fe(Ⅱ) regeneration techniques, this work focuses on utilizing the aliovalent redox pairs and built-in electric field to induce photo-excited electrons to cross the material interface and achieve Fe(III) reduction (heterogeneous regeneration). Herein, oxygen-deficient CeO₂ particles are anchored on metal-organic frameworks (MIL-88A) and thus constitute the heterojunction with enhanced photoelectric properties, accelerating the directional charge transfer. Consequently, the synthesized MIL-88A/CeO₂(OV) composite can degrade 95.76% of oxytetracycline within 60 min in photo-Fenton reaction and maintain a high mineralization rate (75.33%) after 4 cyclic tests. Furthermore, the charge transfer mechanisms of Fe cycle and antibiotics mineralization are both unveiled via experiment results and theorical calculation. This work proposes a new paradigm for constructing self-sufficient photo-Fenton catalytic system for efficient and sustainable removal of polycyclic antibiotics.

Three-dimensional porous CuO-modified CeO2-Al2O3 catalysts with chlorine resistance for simultaneous catalytic oxidation of chlorobenzene and mercury: Cu-Ce interaction and structure

Chlorine poisoning effects are still challenging to develop efficient catalysts for applications in chlorobenzene (CB) and mercury (Hg⁰) oxidation. Herein, three-dimensional porous CuO-modified CeO₂-Al₂O₃ catalysts with macroporous framework and mesoporous walls prepared via a dual template method were employed to study simultaneous oxidation of CB and Hg⁰. CuO-modified CeO₂-Al₂O₃ catalysts with three-dimensional porous structure exhibited outstanding activity and stability for simultaneous catalytic oxidation of CB and Hg⁰. The results demonstrated that the addition of CuO into CeO₂-Al₂O₃ can simultaneously enhance the acid sites and redox properties through the electronic inductive effect between CuO and CeO₂ (Cu²⁺+Ce³⁺↔Cu⁺+Ce⁴⁺). Importantly, the synergistic effect between Cu and Ce species can induce abundant oxygen vacancies formation, produce more reactive oxygen species and facilitate oxygen migration, which is beneficial for the deep oxidation of chlorinated intermediates. Moreover, macroporous framework and mesoporous nanostructure dramatically improved the specific surface area for enhancing the contact efficiency between reactants and active sites, leading to a remarkable decrease of byproducts deposition. CB and Hg⁰ had function of mutual promotion in this reaction system. In tune with the experimental results, the possible mechanistic pathways for simultaneous catalytic oxidation of CB and Hg⁰ were proposed.

Synthesis of mesoporous SiO2-CeO2 hybrid nanostructures with high catalytic activity for transamidation reaction

Transamidation reactions catalyzed by boronic acid derivatives and metal catalysts are well known nevertheless their requirement for elevated temperatures and long reaction times were considered major obstacles in converting amides to N-alkyl amides with the coupling of primary amides and amines. The acidic-basic co-existence of ceria nanoparticles is considered a perfect choice for different catalytic activities. Mesoporous silica on the other hand is well known for its use as a supporting material for catalysts owing to its excellent characteristics like large surface area, good absorption capacity, and high-temperature stability. The SiO₂-CeO₂ hybrid nanocomposite was prepared by solvothermal route followed by annealing and the formation of the catalyst was confirmed by XRD, EDX, FTIR, and TEM characterization techniques. The hybrid catalyst shows high catalytic activity towards transamidation reaction at very low temperatures and in solvent-free conditions compared to pure ceria nanoparticles. The SiO₂-CeO₂ catalyst showed more than 99% selectivity and a remarkable catalytic activity of above 90% for the conversion of N-heptyl amine with acetamide to N-heptyl acetamide at a very low temperature of 120 °C for 3 hours. Furthermore, the catalyst remains stable and active for repeated catalytic cycles. It established 80% catalytic activity even after 4 repeated cycles making it suitable for multiple-time usages.

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.

CeO2-CDs clusters decorated Co(OH)2 nanosheets for improved photocatalytic ammonia synthesis

The green synthesis process of photocatalytic ammonia production has received more and more attentions. Herein, a Z-scheme heterojunction with all-solid-state structures is constructed, in which carbon dots can act as electron transferring mediators. The photocatalytic measurement shows that the modified photocatalysts exhibit much higher activities, in which the ammonia production rates can reach above 232 µmol·g_cal^-1·h^-1 under the light irradiation. The improved catalytic properties can be credited to the significantly increased number of photoinduced oxygen vacancies, the excellent visible-light adsorption abilities and photogenerated electron-hole separation efficiencies for the carbon dots bridged heterostructures. More hydroxyl and superoxide radicals can be simultaneously produced in the composites. This work provides reasonable guidance for applications in photocatalytic ammonia synthesis and a promising construction strategy of efficient Z-scheme photocatalysts.

Salt-templated porous melamine-based conjugated polymers for selective oxidation of amines into imines under visible light

Conjugated polymers have a broad application foreground in the field of photocatalytic organic synthesis to produce value-added chemicals due to their functional diversity, broad light responsive ability, high thermal and chemical stability, and tunable band structure. Herein, using mixed chloride salts (i.e., NaCl/LiCl) as building template, a series of porous conjugated polymers constructed by melamine and terephthalaldehyde monomers were obtained through a Schiff-base reaction in the absence of any external solvent. Melamine-terephthalaldehyde polymer (i.e., PMTPA-x, x represents the mass ratio of salt-mixture to mixed precursors of PMTPA) materials displayed porous morphologies and possessed different energy band structures via regulating the mass ratio of mixed-salt to monomers. Specifically, PMTPA-20 has a larger specific surface area and more suitable redox potential towards the photocatalytic oxidative coupling of amines to imines. Under visible light, with molecular oxygen as oxidant, PMTPA-20 achieves 97% conversion of benzylamine in 8 h which is 3.9 times higher than that of pristine PMTPA (25% conversion in 8 h). In addition, PMTPA-20 catalyst has good structure stability and reusability performance for photocatalytic reactions.

Recent advances on catalysts for photocatalytic selective hydrogenation of nitrobenzene to aniline

Selective hydrogenation of nitrobenzene (SHN) is an important approach to synthesize aniline, an essential intermediate with extremely high research significance and value in the fields of textiles, pharmaceuticals and dyes. SHN reaction requires high temperature and high hydrogen pressure via the conventional thermal-driven catalytic process. On the contrary, photocatalysis provides an avenue to achieve high nitrobenzene conversion and high selectivity towards aniline at room temperature and low hydrogen pressure, which is in line with the sustainable development strategies. Designing efficient photocatalysts is a crucial step in SHN. Up to now, several photocatalysts have been explored for photocatalytic SHN, such as TiO2, CdS, Cu/graphene and Eosin Y. In this review, we divide the photocatalysts into three categories based on the characteristics of the light harvesting units, including semiconductors, plasmonic metal-based catalysts and dyes. The recent progress of the three categories of photocatalysts is summarized, the challenges and opportunities are pointed out and the future development prospects are described. It aims to give a clear picture to the catalysis community and stimulate more efforts in this research area.

Synthesis of Hierarchical Layered Quasi-Triangular Ce(OH)CO3 and Its Thermal Conversion to Ceria with High Polishing Performance

Layered quasi-triangular Ce(OH)CO₃ assembled from primary nanoparticles was synthesized via a solvothermal method and converted into CeO₂ abrasive particles by calcination at 800-1000 °C. With the increase of calcination temperature, the primary particle size increased and the microstructure, mechanical hardness, and chemical activity of the CeO₂ particles changed, thus affecting the polishing performance. The calcined products obtained at 800, 850, and 900 °C maintained the layered edge structure of the Ce(OH)CO₃ precursor and had a relatively high specific surface area and surface Ce³⁺ concentration. The samples calcined at 950 and 1000 °C lost the layered structure due to the large-scale melting of the primary particles, and their surface chemical activity decreased. The polishing experiments on K9 glass showed that, with the calcination temperature rising from 800 to 1000 °C, the material removal rate (MRR) first increased and then decreased sharply. The initial increase of MRR was attributed to the increase of mechanical hardness of the layered quasi-triangular CeO₂, and the subsequent decrease of MRR was related to the decrease in surface chemical activity and disappearance of the layered edge structure. The product calcined at 900 °C had the highest MRR and best surface quality after polishing due to the layered edge structure and optimal match of chemical activity and mechanical hardness.

Controlled gold-palladium cores in ceria hollow spheres as nanoreactor for plasmon-enhanced catalysis under visible light irradiation

Visible-light-driven organic transformations boosting by localized surface plasmon resonance (LSPR) have been attracting considerable interests. Gold-palladium (Au-Pd) bimetallic nanoparticles (NPs) are considered as ideal plasmonic catalysts realizing efficient light-driven catalysis. Nevertheless, stability and adjustability of plasmonic Au-Pd NPs remain to be a challenging task. Herein, we designed the controlled Au-Pd cores in ceria (CeO₂) hollow spheres (Au-Pd@h-CeO₂) as nanoreactor for Suzuki cross-coupling reactions. Under visible light irradiation, the Au-Pd@h-CeO₂ exhibited remarkable photocatalytic performance with a turnover frequency (TOF) value as high as 797 h^-1. More impressively, the coupling reactions of aryl chlorides bearing electron-withdrawing groups proceeded better and afforded the corresponding desired products in good yields. Detailed structural, optical and photoelectrochemical characterizations unraveled that the enhanced photocatalytic efficiency of Au-Pd@h-CeO₂ was attributed to the LSPR effect of controllable Au-Pd cores and their synergetic effect of hollow CeO₂ shells. The merits of this hollow sphere architecture lied on as followed: (I) Incident light could be reflected and refracted between the inner cores and outer shells, which extended the trapping of incident light, and then enhanced the light harvesting efficiency; (II) the mesoporous architecture of CeO₂ hollow spheres provided a huge specific surface area and numerous mesoporous channels, which could enhance the absorption of reactants and provided more active sites; (III) LSPR excitation of Au-Pd NPs and band-gap excitation of CeO₂ simultaneously occurred under visible light illumination, inducing a more efficient separation and transfer of charge carriers. Furthermore, due to the confinment effect of CeO₂ shells, the Au-Pd@h-CeO₂ exhibited an excellent reusability after six cycles without significant deactivation of yield. Our findings provided a facile way to design highly efficient plasmonic-enhanced photocatalysts utilized for catalytic organic reactions.

Interfacial Reaction-Directed Green Synthesis of CeO2-MnO2 Catalysts for Imine Production through Oxidative Coupling of Alcohols and Amines

Direct oxidative coupling of alcohols with amines over cheap but efficient catalysts is a promising choice for imine formation. In this study, porous CeO₂-MnO₂ binary oxides were prepared via an interfacial reaction between Ce₂(SO₄)₃ and KMnO₄ at room temperature without any additives. The as-prepared porous CeO₂-MnO₂ catalyst has a higher fraction of Ce³⁺, Mn³⁺, and Mn⁴⁺ and contains larger surface area and more oxygen vacancies. During the oxidative coupling reaction of alcohol with amine to imine, the as-obtained CeO₂-MnO₂ catalyst is motivated by the above encouraging characteristics and exhibits superior catalytic activity (98% conversion and 97% selectivity) and can also work effectively under a wide scope of temperatures and substrates. The in-depth in situ DRIFTS and density functional theory (DFT) results demonstrate that there is a strong interaction between CeO₂ and MnO₂ in the CeO₂-MnO₂ catalyst, exhibiting especially a positive synergistic effect in the direct coupling of alcohol and amine reaction.

Simultaneous electrical impedance and optical absorption spectroscopy for rapid characterization of oxygen vacancies and small polarons in doped ceria

Mixed ionic-electronic conductors (MIECs) play a central role in emerging energy conversion and energy efficient computational technologies. However, it is both challenging and resource demanding to characterize MIECs over the broad range of experimental conditions of interest, thereby significantly limiting their study and applications. Here, a novel method of a simultaneous measurement of electrical conductivity and optical absorption of thin films in out-of-equilibrium state, i.e. during a reduction process, is employed for a comprehensive study of a MIEC oxide, Pr_xCe_1-xO_2-δ (PCO). It enables, orders of magnitude faster than by established techniques, characterization of the oxygen vacancy and small polaron formation and transport as a function of temperature (demonstrated here down to 200 °C), in a wide range of deviation from stoichiometry, δ. For instance, at 600 °C the PCO properties were obtained during a ten minute reduction process, in the pO₂ range from 1 to 10^-13 bar. The experimental results show that the oxygen vacancy mobility is constant while the small polaron mobility is linear in δ, in the whole pO₂ range, which yields the total conductivity quadratic in δ. Furthermore, the method was applied to study the modification of PCO's transport properties with composition change. It was shown that increasing x from 0.1 to 0.2 suppresses the ionic mobility and, at the same time, enhances the small polaron mobility. Finally, the optically determined δ was used to define an instantaneous oxygen activity in PCO that can be accessed in the out-of-equilibrium experiments. This work opens up new possibilities to study the effects of microstructure, strain and other applied external stimuli on the transport and thermodynamic properties of PCO and similar types of MIEC materials.

Facial One-Pot Synthesis, Characterization, and Photocatalytic Performance of Porous Ceria

A facial one-step synthesis procedure was applied to prepare porous sponge-like ceria (CeO2). The synthesis was performed by mixing cerium nitrate with citric acid, followed by thermal treatment. The produced solid material was characterized by several techniques, such as XRD, SEM, N2 sorption measurement, DR-UV-vis, and Raman spectroscopy. The characterization data showed that the nanoparticles of the porous ceria were formed with a three-dimensional pore system. Moreover, the measured surface area of the porous sample was eight times higher than the commercially available ceria. The photocatalytic performance of the porous ceria was investigated in two different applications under visible light illumination. The first was the decolorization of a methyl green aqueous solution, while the second was the photocatalytic elimination of a gaseous mixture consisting of five short-chain hydrocarbons (C1–C3). The obtained results showed that the photocatalytic activity of porous ceria was higher than that of the commercial sample. Finally, the recycling of porous ceria showed low deactivation (less than 9%) after four consecutive runs.

MIL-47(V)-derived carbon-doped vanadium oxide for selective oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran

The development and transformation of biomass-derived platform compounds is a sustainable way to deal with the fossil fuel crisis. 5-Hydroxymethylfurfural (HMF) can be reduced or oxidized to produce many high-value compounds; however, it is challenging to effectively produce 2,5-diformylfuran (DFF) due to overoxidation. In this work, a carbon-doped V₂O₅ (C-V₂O₅) material was obtained through pyrolysis of MIL-47(V) nanorods, a typical metal-organic framework material. The X-ray diffraction patterns and X-ray photoelectron spectra showed that the graphitized carbon species were incorporated in C-V₂O₅. High-efficiency HMF oxidation, high specific selectivity for DFF and excellent recycling could be achieved with the C-V₂O₅ catalyst. Fourier-transform infrared spectroscopy combined with density functional theory (DFT) calculation revealed that graphitized carbon weakens the VO bond and promotes the formation of oxygen vacancies in C-V₂O₅, thus improving the catalytic activity in the oxidation of furfuryl alcohols. The V⁴⁺ induced by oxygen vacancies will be oxidized by O₂ to form V⁵⁺, so that the cycle can be realized. It exhibits remarkable selectivity in the oxidation of different alcohols produced from biomass based on the relatively constant active sites in C-V₂O₅.

Mechanistic Investigation of Enhanced Catalytic Selectivity toward Alcohol Oxidation with Ce Oxysulfate Clusters

Ceria-based materials have been highly desired in photocatalytic reactions due to their redox properties and strong oxygen storage and transfer ability. Herein, we report the structures of one CeCe₇₀ oxysulfate cluster and four MCe₇₀ clusters (M = Cu, Ni, Co, and Fe) with the same Ce₇₀ core. As noted, single-crystal X-ray diffraction confirmed the structures of CeCe₇₀ and the MCe₇₀ series, while Raman spectroscopy indicated an increase in oxygen defects upon the introduction of Cu and Fe ions. The clusters catalyzed the oxidation of 4-methoxybenzyl alcohol under ultraviolet light. CuCe₇₀ and FeCe₇₀ exhibited enhanced reactivity compared to CeCe₇₀ and improved aldehyde selectivity compared to control experiments. In comparison with their homogeneous congeners, the CeCe₇₀/MCe₇₀ clusters altered the location of radical generation from the bulk solution to the clusters' surfaces. Mechanistic studies highlight the role of oxygen defects and specific transition metal introduction for efficient photocatalysis. The mechanistic pathway in this study provides insight into how to select or design a highly selective catalyst for photocatalysis.

Acetylene hydrogenation catalyzed by bare and Ni doped CeO2(110): the role of frustrated Lewis pairs

Ceria (CeO₂) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H₂ dissociation and acetylene hydrogenation on bare and Ni doped CeO₂(110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO₂(111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (O_vs) on the oxide surfaces. On bare CeO₂(110) with a single O_v (CeO₂(110)-O_v), two surface Ce cations with one non-adjacent O anion are shown to form (Ce³⁺-Ce⁴⁺)/O quasi-FLPs, while for the Ni doped CeO₂(110) surface with one (Ni-CeO₂(110)-O_v) or two (Ni-CeO₂(110)-2O_v) O_vs, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H₂ dissociation via a heterolytic mechanism, while the resulting surface O-H and Ce-H species catalyze the subsequent acetylene hydrogenation. With CeO₂(110)-O_v and Ni-CeO₂(110)-2O_v, our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni-CeO₂(110)-O_v, the reaction is shown to be controlled by the H₂ dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO₂(111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs - the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H₂ but also lead to a higher barrier of acetylene hydrogenation.