Application of Ceria in CO2 Conversion Catalysis

Bismuth single atom supported CeO2 nanosheets for oxidation resistant photothermal reverse water gas shift reaction

Bi single atoms supported on CeO2 nanosheets combined with a Ti2O3 based photothermal device showed oxidation resistance and outperforming weak solar driven RWGS with a CO production rate of 31.00 mmol g−1 h−1 under 3 sun units of irradiation.

Theoretical insights into CO2 reduction reaction on a CuPc/graphene single-atomic catalyst

A theoretical insight into the CO2 reduction reaction on a CuPc/graphene single atomic catalyst.

Renaissance of Homogeneous Cerium Catalysts with Unique Ce(IV/III) Couple: Redox-Mediated Organic Transformations Involving Homolysis of Ce(IV)-Ligand Covalent Bonds

Recent advances in the catalytic application of cerium complexes were achieved through controlling the Ce(IV/III) redox couple. Although Ce(IV) complexes have been extensively investigated as stoichiometric oxidants in organic synthesis on the basis of their highly positive redox potentials, these complexes can be used as catalysts, not only by introducing supporting ligands around the coordination sphere of cerium, but also by taking advantage of the photoresponsive properties of Ce(IV) and Ce(III) species. Cerium is highly abundant, comparable to that of some first-row transition metals such as copper, nickel, and zinc. Cerium complexes are new and promising homogeneous catalyst candidates for a variety of organic transformations under mild reaction conditions. They are typically used to activate dioxygen to oxidize organic compounds and applied for organic radical generation using the photoresponsive character of Ce(IV) carboxylates and alkoxides as well as electronic transition of Ce(III), in which homolysis of Ce(IV)-ligand covalent bonds is an important step for the overall catalytic cycle. In this Perspective, we first review the early discovery of Ce(OAc)₄-mediated oxidative transformations to emphasize the importance of Ce(IV)-OAc bond homolysis in various C-C bond-forming reactions and its relation to recent developments. We then focus on the fundamental importance of Ce(IV) reactivity involving thermal and photoassisted homolysis of the Ce(IV)-ligand covalent bond and the developments regarding Ce(IV/III) redox changes in catalytic reactions together with our recent findings on cerium-based catalysis.

Electrospinning Highly Dispersed Ru Nanoparticle-Embedded Carbon Nanofibers Boost CO2 Reduction in a H2/CO2 Fuel Cell

A H₂/CO₂ fuel cell is a promising device that can convert CO₂ into hydrocarbon fuel with electricity generation. Herein, a facile electrospinning method has been used to synthesize the embedded Ru-CNF catalyst in which Ru nanoparticles are dispersed homogeneously within N-doped carbon nanofibers. This catalyst exhibits a high CH₄ production rate of 308.46 μmol g_cat^-1 h^-1 at 170 °C, which is superior to that of the Ru/CNF (242.53 μmol g_cat^-1 h^-1) and Ru/CNT (194.24 μmol g_cat^-1 h^-1). The enhanced CO₂RR performance of Ru-CNF is ascribed to the well-distributed Ru nanoparticles within the CNF matrix and synergistic effect of Ru sites with N species, which results in forming the increased CO₂RR active sites, hence improving the catalytic activity. Simultaneously, it can achieve a peak power density of 1.8 W m^-2 on the strength of anodic (H₂ oxidation) and cathodic (CO₂ reduction and H₂ evolution) reactions with remarkable stability. Such findings give a theoretical basis of CO₂RR in the H₂/CO₂ fuel cell system, which could hold great value to further develop the high-efficiency catalysts for CO₂ reduction.

How the Morphology of NiOx-Decorated CeO2 Nanostructures Affects Catalytic Properties in CO2 Methanation

Effects of morphology and exposed crystal planes of NiO_x-decorated CeO₂ (NiCeO₂) nanostructured catalysts on activity during CO₂ methanation were examined, using nanorod (NR), nanocube (NC), and nanooctahedron (NO) structures. The NiCeO₂ nanorods (NiCeO₂-NR) showed superior activity to NiCeO₂-NC and NiCeO₂-NO along with excellent selectivity for CH₄. This material also demonstrated exceptional durability, with no significant loss of catalytic activity or structural change after use. Comprehensive physicochemical characterization as well as density functional theory calculations determined that the high performance of the NiCeO₂-NR was closely related to the large quantity of surface oxygen vacancies and the high degree of reversibility associated with the Ce⁴⁺ ↔ Ce³⁺ redox cycle of the support. These effects originate from the enhanced reactivity of oxygen atoms on the (110) surfaces of the oxide compared with the (100) and (111) surfaces. This information is expected to assist in the rational design of practical catalysts for the activation of CO₂ molecules and other important transformations.

Flame Synthesis of Cu/ZnO-CeO2 Catalysts: Synergistic Metal-Support Interactions Promote CH3OH Selectivity in CO2 Hydrogenation

The hydrogenation of CO₂ to CH₃OH is an important reaction for future renewable energy scenarios. Herein, we compare Cu/ZnO, Cu/CeO₂, and Cu/ZnO–CeO₂ catalysts prepared by flame spray pyrolysis. The Cu loading and support composition were varied to understand the role of Cu–ZnO and Cu–CeO₂ interactions. CeO₂ addition improves Cu dispersion with respect to ZnO, owing to stronger Cu–CeO₂ interactions. The ternary Cu/ZnO–CeO₂ catalysts displayed a substantially higher CH₃OH selectivity than binary Cu/CeO₂ and Cu/ZnO catalysts. The high CH₃OH selectivity in comparison with a commercial Cu–ZnO catalyst is also confirmed for Cu/ZnO–CeO₂ catalyst prepared with high Cu loading (∼40 wt %). In situ IR spectroscopy was used to probe metal–support interactions in the reduced catalysts and to gain insight into CO₂ hydrogenation over the Cu–Zn–Ce oxide catalysts. The higher CH₃OH selectivity can be explained by synergistic Cu–CeO₂ and Cu–ZnO interactions. Cu–ZnO interactions promote CO₂ hydrogenation to CH₃OH by Zn-decorated Cu active sites. Cu–CeO₂ interactions inhibit the reverse water–gas shift reaction due to a high formate coverage of Cu and a high rate of hydrogenation of the CO intermediate to CH₃OH. These insights emphasize the potential of fine-tuning metal–support interactions to develop improved Cu-based catalysts for CO₂ hydrogenation to CH₃OH.

Highly catalytically active CeO2-x-based heterojunction nanostructures with mixed micro/meso-porous architectures

The architectural design of nanocatalysts plays a critical role in the achievement of high densities of active sites but current technologies are hindered by process complexity and limited scaleability. The present work introduces a rapid, flexible, and template-free method to synthesize three-dimensional (3D), mesoporous, CeO_2-x nanostructures comprised of extremely thin holey two-dimensional (2D) nanosheets of centimetre-scale. The process leverages the controlled conversion of stacked nanosheets of a newly developed Ce-based coordination polymer into a range of stable oxide morphologies controllably differentiated by the oxidation kinetics. The resultant polycrystalline, hybrid, 2D-3D CeO_2-x exhibits high densities of defects and surface area as high as 251 m² g^-1, which yield an outstanding CO conversion performance (T_90% = 148 °C) for all oxides. Modification by the creation of heterojunction nanostructures using transition metal oxides (TMOs) results in further improvements in performance (T_90% = 88 °C), which are interpreted in terms of the active sites associated with the TMOs that are identified through structural analyses and density functional theory (DFT) simulations. This unparalleled catalytic performance for CO conversion is possible through the ultra-high surface areas, defect densities, and pore volumes. This technology offers the capacity to establish efficient pathways to engineer nanostructures of advanced functionalities for catalysis.

Efficient electroreduction of CO2 to C2+ products on CeO2 modified CuO

Electrocatalytic reduction of CO₂ into multicarbon (C₂₊) products powered by renewable electricity offers one promising method for CO₂ utilization and promotes the storage of renewable energy under an ambient environment. However, there is still a dilemma in the manufacture of valuable C₂₊ products between balancing selectivity and activity. In this work, cerium oxides were combined with CuO (CeO₂/CuO) and showed an outstanding catalytic performance for C₂₊ products. The faradaic efficiency of the C₂₊ products could reach 75.2% with a current density of 1.21 A cm⁻². In situ experiments and density functional theory (DFT) calculations demonstrated that the interface between CeO₂ and Cu and the subsurface Cu₂O coexisted in CeO₂/CuO during CO₂RR and two competing pathways for C–C coupling were promoted separately, of which hydrogenation of *CO to *CHO is energetically favoured. In addition, the introduction of CeO₂ also enhanced water activation, which could accelerate the formation rate of *CHO. Thus, the selectivity and activity for C₂₊ products over CeO₂/CuO can be improved simultaneously.

CO₂ can be efficiently converted into C₂₊ products on CeO₂ modified CuO catalysts and the faradaic efficiency could reach 75.2% with a current density of 1.21 A cm⁻².

Nanoparticle O-H Bond Dissociation Free Energies from Equilibrium Measurements of Cerium Oxide Colloids

A novel equilibrium strategy for measuring the hydrogen atom affinity of colloidal metal oxide nanoparticles is presented. Reactions between oleate-capped cerium oxide nanoparticle colloids (nanoceria) and organic proton-coupled electron transfer (PCET) reagents are used as a model system. Nanoceria redox changes, or hydrogen loadings, and overall reaction stoichiometries were followed by both ¹H NMR and X-ray absorption near-edge spectroscopies. These investigations revealed that, in many cases, reactions between nanoceria and PCET reagents reach equilibrium states with good mass balance. Each equilibrium state is a direct measure of the bond strength, or bond dissociation free energy (BDFE), between nanoceria and hydrogen. Further studies, including those with larger nanoceria, indicated that the relevant bond is a surface O-H. Thus, we have measured surface O-H BDFEs for nanoceria-the first experimental BDFEs for any nanoscale metal oxide. Remarkably, the measured CeO-H BDFEs span 13 kcal mol^-1 (0.56 eV) with changes in the average redox state of the nanoceria colloid. Possible chemical models for this strong dependence are discussed. We propose that the tunability of ceria BDFEs may be important in explaining its effectiveness in catalysis. More generally, metal oxide BDFEs have been used as predictors of catalyst efficacy that, traditionally, have only been accessible by computational methods. These results provide important experimental benchmarks for metal oxide BDFEs and demonstrate that the concepts of molecular bond strength thermochemistry can be applied to nanoscale materials.

Efficient Imine Formation by Oxidative Coupling at Low Temperature Catalyzed by High-Surface-Area Mesoporous CeO2 with Exceptional Redox Property

High-surface-area mesoporous CeO₂ (hsmCeO₂ ) was prepared by a facile organic-template-induced homogeneous precipitation process and showed excellent catalytic activity in imine synthesis in the absence of base from primary alcohols and amines in air atmosphere at low temperature. For comparison, ordinary CeO₂ and hsmCeO₂ after different thermal treatments were also investigated. XRD, N₂ physisorption, UV-Raman, H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, EPR spectroscopy, and X-ray photoelectron spectroscopy were used to unravel the structural and redox properties. The hsmCeO₂ calcined at 400 °C shows the highest specific surface area (158 m²  g^-1 ), the highest fraction of surface coordinatively unsaturated Ce³⁺ ions (18.2 %), and the highest concentration of reactive oxygen vacancies (2.4×10¹⁵  spins g^-1 ). In the model reaction of oxidative coupling of benzyl alcohol and aniline, such an exceptional redox property of the hsmCeO₂ catalyst can boost benzylideneaniline formation (2.75 and 5.55 mmol  g ceria - 1  h^-1 based on >99 % yield at 60 and 80 °C, respectively) in air with no base additives. It can also work effectively at a temperature of 30 °C and in gram-scale synthesis. These are among the best results for all benchmark ceria catalysts in the literature. Moreover, the hsmCeO₂ catalyst shows a wide scope towards primary alcohols and amines with good to excellent yield of imines. The influence of reaction parameters, the reusability of the catalyst, and the reaction mechanism were investigated.

Evaluation of CO2 Hydrogenation in a Modular Fixed-Bed Reactor Prototype

Low-cost iron-based CO2 hydrogenation catalysts have shown promise as a viable route to the production of value-added hydrocarbon building blocks. It is envisioned that these hydrocarbons will be used to augment industrial chemical processes and produce drop-in replacement operational fuel. To this end, the U.S. Naval Research Laboratory (NRL) has been designing, testing, modeling, and evaluating CO2 hydrogenation catalysts in a laboratory-scale fixed-bed environment. To transition from the laboratory to a commercial process, the catalyst viability and performance must be evaluated at scale. The performance of a Macrolite®-supported iron-based catalyst in a commercial-scale fixed-bed modular reactor prototype was evaluated under different reactor feed rates and product recycling conditions. CO2 conversion increased from 26% to as high as 69% by recycling a portion of the product stream and CO selectivity was greatly reduced from 45% to 9% in favor of hydrocarbon production. In addition, the catalyst was successfully regenerated for optimum performance. Catalyst characterization by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), along with modeling and kinetic analysis, highlighted the potential challenges and benefits associated with scaling-up catalyst materials and processes for industrial implementation.

Mechanistic insights into the acetate-accelerated synthesis of crystalline ceria nanoparticles

Lithium acetate was reported to accelerate the growth of crystalline ceria nanoparticles in ozone-mediated synthesis through promoting alcohol-like condensation reactions.