CeO2 Catalysts Research Articles

Photothermal Synergistic Effect Induces Bimetallic Cooperation to Modulate Product Selectivity of CO2 Reduction on Different CeO2 Crystal Facets.

Product selectivity of solar-driven CO2 reduction and H2O oxidation reactions has been successfully controlled by tuning the spatial distance between Pt/Au bimetallic active sites on different crystal facets of CeO2 catalysts. The replacement depth of Ce atoms by monatomic Pt determines the distance between bimetallic sites, while Au clusters are deposited on the surface. This space configuration creates a favourable microenvironment for the migration of active hydrogen species (*H). The *H is generated via the activation of H2O on monatomic Pt sites and migrate towards Au clusters with a strong capacity for CO2 adsorption. Under concentrated solar irradiation, selectivity of the (100) facet towards CO is 100 %, and the selectivity of the (110) and (111) facets towards CH4 is 33.5 % and 97.6 %, respectively. Notably, the CH4 yield on the (111) facet is as high as 369.4 μmol/g/h, and the solar-to-chemical energy efficiency of 0.23 % is 33.8 times higher than that under non-concentrated solar irradiation. The impacts of high-density flux photon and thermal effects on carriers and *H migration at the microscale are comprehensively discussed. This study provides a new avenue for tuning the spatial distance between active sites to achieve optimal product selectivity.

Bimetal (CuNi and CuCo) substituted CeO2: An approach for low temperature dry reforming of methane

The solution combustion synthesis method was used to make CuNi substituted CeO2 (7.5 at.% of Cu+7.5 at.% of Ni), and CuCo substituted CeO2 (7.5 at.% of Cu+7.5 at.% of Co) catalysts. The catalysts were characterized by x-ray diffraction (XRD),XPS, BET surface area measurements, scanning electron microscopy (SEM), and H2-temperature-programmed reduction(H2-TPR). Further, these catalysts were tested for endothermic dry reforming of methane (DRM) reaction. The CuNi substituted CeO2 catalyst started dry reforming of methane (DRM) at around 350 °C, while the CuCo substituted CeO2 catalyst started DRM at around 450 °C. This indicates that the CuNi catalyst has a lower activation temperature for the DRM reaction compared to the CuCo catalyst. The CuNi substituted CeO2 catalyst converted CH4 better than the CuCo substituted CeO2 at all temperatures reaching 98 % conversion at 800 °C The CuNi substituted CeO2 showed higher H2/CO ratio in comparison to the CuCo substituted CeO2 but the long-term stability followed the opposite order. Thermal gravimetric analysis (TGA), SEM and O2-TPO were used to quantify as well as to understand the nature of carbon that had built up on spent catalysts and it is mostly amorphous. Transient studies have shown that along with the lattice oxygen participation, controlled methane decomposition step is necessary for stability of catalysts. Transient studies also recommended additional reaction steps during DRM where an exothermic methane partial oxidation step reduces the endothermicity of DRM. Also, CO2 activation also goes via the defect dissociation route, an additional step to the conventional carbon oxidation (CO2+C→CO) step. So, novelty of this work is in elucidating the exact role of lattice oxygen in imparting the activity and stability to the DRM catalyst.

Low Temperature NO and CO Conversion with a Mechanistic Approach on Ru-Composed Cerium Oxide.

Catalytic reduction of NO with CO at a lower temperature is an extremely challenging task, thus requiring conceivable surfaces to overcome such issues. Ru-substituted CeO2 catalysts prepared via the solution combustion method were employed in CO oxidation and NO-CO conversion studies. The characterization for material formation and surface structure was carried out through XRD, SEM, TEM, and BET surface area. The catalytic study revealed the promising behavior of 5% Ru in CeO2 for the 100% conversion of NO-CO at 150 °C, proving it to be an excellent exhaust material. These observed results are also supported by temperature-programmed studies, i.e. TPD of NO and CO in addition to NH3-TPD and H2-TPR for their convincible surface interaction that is inclined toward a significant change in the conversion path. Additionally, the proposed mechanism, based on the experimental evidence, sheds light on the NO-CO redox reaction, directing the reaction pathway toward the Langmuir-Hinshelwood and Mars-Van Krevelen-type route. Moreover, the exceptional performance can be attributed to the strategic incorporation of Ru in CeO2, where the strong interaction of Ru-Ce is able to gain a high synergy for NO and CO conversion.

Exploring the roles of atomic radius on methane oxidation in different M(Y, Sm, and Ni)-doped CeO2 catalysts using operando DRIFTS-MS and DFT computation

Ce-based oxides doped with different metal ions can greatly improve CH4 catalytic activity, but the relationship between the surface microstructure and its activity is not clear. In this article, M(Y, Sm, and Ni)-CeO2 catalyst prepared by aerosol method is used as the research object. CH4 is used as a molecular probe for the study. Operando DRIFTS-MS and DFT computation are used to verify the structure-activity relationship between the surface structure and activity of M-CeO2. Ni-CeO2 with the smallest atomic radius ratio has the best CH4 oxidation performance (550 °C) and excellent redox characteristics. Operando DRIFTS-MS results show that the NiO is more likely to adsorb CH4 and form the intermediates CH3*, gas-phase CO2, CO, and H2O at low temperatures. At high temperatures, Ni-OH, Ni-O-Ce, and CeO further react with CH4. Among them, the formation of M-Ov-Ce has a positive role in promoting the decomposition of O₂. Importantly, carbonate is the rate-determining step that affects the activation of M-Ov-Ce. Monodentate carbonates decompose more easily at low temperatures. These findings reveal a novel doping effect and provide a new idea for the design and development of Ce based catalysts with high CH4 removal ability

Revealing the effects of asymmetric M-O-Ce structure on CO oxidation over M(Cu, Mn, Fe, La and Zr)-doped CeO2 catalysts: Atomic radius and oxygen vacancies

Although CeO2 catalysts doped with different metal ions can significantly enhance CO activity, the intrinsic relationship between these activities and the catalyst structure is still not clearly understood. In this paper, M(Cu, Mn, Fe, La and Zr)-doped CeO2 catalysts prepared by aerosol method are used as research targets for CO oxidation. Cu-CeO2 with the smallest atomic radius ratio has the best redox capacity and more oxygen vacancy content. DFT results show that promoting the interface effect in the presence of Cu-Ov-Ce is conducive to enhancing CO adsorption capacity. Operando TPR-DRIFTS-MS results reveal that different types of oxygen species (including M=O, CeO, M-O-Ce, and M-OH) exhibit different reactivity on the surface of Ce-based catalysts. M(Cu, Mn, Fe, La and Zr)=O is more likely to react with CO above 100°C. CeO, M-O-Ce, and M-OH also react with CO to form oxygen vacancies (such as M-Ov-Ce and CeOv) and CO2 after reaching a certain temperature. Among them, the formation of M-Ov-Ce has a positive role in promoting the decomposition of O₂. Importantly, carbonate is the rate-determining step that affects the activation of M-Ov-Ce. Monodentate carbonates decompose more easily at low temperatures. These findings reveal a novel doping effect and provide a new idea for the design and development of Ce based catalysts with high CO removal ability.

Comparative study on CeO2 catalysts with different morphologies and exposed facets for catalytic ozonation: performance, key factor and mechanism insight

Morphology and facet effects of metal oxides in heterogeneous catalytic ozonation (HCO) are attracting increasing interests. In this paper, the different HCO performances for degradation and mineralization of phenol of seven ceria (CeO2) catalysts, including four with different morphologies (nanorod, nanocube, nanooctahedron and nanopolyhedron) and three with the same nanorod morphology but different exposed facets, are comparatively studied. CeO2 nanorods with (110) and (100) facets exposed show the best performance, much better than that of single ozonation, while CeO2 nanocubes and nanooctahedra show performances close to single ozonation. The underlying reason for their different HCO performances is revealed using various experimental and density functional theory (DFT) calculation results and the possible catalytic reaction mechanism is proposed. The oxygen vacancy (OV) is found to be pivotal for the HCO performance of the different CeO2 catalysts regardless of their morphology or exposed facet. A linear correlation is discerned between the rate of catalytic decomposition of dissolved ozone (O3) and the density of Frenkel-type OV. DFT calculations and in-situ spectroscopic studies ascertain that the existence of OV can boost O3 activation on both the hydroxyl (OH) and Ce sites of CeO2. Conversely, various facets without OV exhibit similar O3 adsorption energies. The OH group plays an important role in activating O3 to produce hydroxyl radical (∙OH) for improved mineralization. This work may offer valuable insights for designing Facet- and OV-regulated catalysts in HCO for the abatement of refractory organic pollutants.

Phosphorus-modulated surface active center reconstruction in cerium oxide for polyols conversion into carbonates

Ionic liquids (ILs) can effectively regulate the surface morphology and acid-base sites of metal oxides, which is of great significance in the preparation of metal oxide nanomaterials. Here, cerium oxide (CeO2) was prepared in phosphonium-based ILs and P-doped CeO2 materials were obtained by doping phosphorous (P) on the surface of CeO2. Moreover, the yield and selectivity for ethylene carbonate (EC) synthesis catalyzed by CeO2 catalyst prepared were 77.26 % and 95.99 % respectively. The results indicated that an appropriate content of P could generate both Lewis acidic and basic sites related to P on the catalyst surface, and P-mediated frustrated Lewis acid-base pairs (FLPs) composed of P-OH as the basic center and Ce3+ as the acidic center was constructed. This work provides a new strategy for preparing novel CeO2 catalysts and regulating acid-base sites on the surface of that.

Highly efficient dual-phase hydrogen-transporting membranes for NH3 decomposition coupling with CO2 reduction

Hydrogen-transporting membrane reactors play active roles in ammonia (NH3) decomposition and carbon dioxide (CO2) reduction by combining reaction and separation in one unit, simplifying the processes and reactor design and therefore saving the cost. These important reactions normally occur at high temperatures and in reducing and acidic atmosphere, thus posing great challenges on the membrane stability. Previous effort was devoted to thin Pd membranes, which however, suffer the shortcomings of hydrogen embrittlement, CO poisoning and high costs. Herein, we develop a H2-COx-tolerant and non-noble metal-ceramic dual-phase hydrogen permeable membrane reactors with a nominal composition of 60 vol% Ni-40 vol% La5.5WO11.25-δ (Ni-LWO). NH3 decomposition and hydrogen permeation was observed via the developed mixed protonic and electronic conducting membrane. A considerable production of CO in the sweep (CO2) side could also be achieved from CO2 reduction when the membrane was coupled by NH3 decomposition where the permeated hydrogen can consume the produced oxygen from favorable CO2 decomposition. Furthermore, we systematically investigated the performance of NiO–CeO2 catalyst coated Ni-LWO membrane reactor for CO2 reduction. Noteworthy that the CO generation rate obtained from Ni-LWO membrane reactor was increased in the presence of NiO–CeO2 catalysts. Hydrogen separation process coupled with surface reactions is jointly controlled by hydrogen surface exchange and bulk diffusion kinetics. These findings reveal a vital step towards the development of efficient hydrogen-transporting membrane reactor for the integration of chemical reactions and separation processes.

Concentrated solar CO2 reduction in H2O vapour with >1% energy conversion efficiency

H2O dissociation plays a crucial role in solar-driven catalytic CO2 methanation, demanding high temperature even for solar-to-chemical conversion efficiencies <1% with modest product selectivity. Herein, we report an oxygen-vacancy (Vo) rich CeO2 catalyst with single-atom Ni anchored around its surface Vo sites by replacing Ce atoms to promote H2O dissociation and achieve effective photothermal CO2 reduction under concentrated light irradiation. The high photon flux reduces the apparent activation energy for CH4 production and prevents Vo from depletion. The defects coordinated with single-atom Ni, significantly promote the capture of charges and local phonons at the Ni d-impurity orbitals, thereby inducing more effective H2O activation. The catalyst presents a CH4 yield of 192.75 µmol/cm2/h, with a solar-to-chemical efficiency of 1.14% and a selectivity ~100%. The mechanistic insights uncovered in this study should help further the development of H2O-activating catalysts for CO2 reduction and thereby expedite the practical utilization of solar-to-chemical technologies.

C2H2 selective hydrogenation over metal-doped CeO2 catalysts: Unraveling the role of metal and surface frustrated Lewis acid-base pair in tuning catalytic performance

Doping the second metal into CeO2 is used as a generally regarded strategy to promote C2H2 selective hydrogenation. This work employs DFT calculations and microkinetic modeling to explore the performance of CeO2 doped with different Cu, Ni, Pd, Pt, or Co atom towards C2H2 selective hydrogenation. The results show that the doped metal not only promotes the formation of surface oxygen-vacancy, but also generate different types of FLPs with surface O atom. FLPs promote H2 heterolytic dissociation to produce Ce-H/M-H and O-H species. Different forms of surface H species affect the performance of C2H2 selective hydrogenation. C2H4 selectivity follows a U-shaped dependence on surface Ce atom charge transfer amount. The screened catalyst Co/CeO2 presents excellent C2H4 production activity and selectivity, which could well prevent the generation of green oil. This work provides new insights for designing highly efficient metal-doped CeO2 catalysts by tuning the doped metal type.

Investigation into the photothermal catalytic CO2 decomposition over CeO2 with different morphologies: Behaviors of oxygen vacancies

CO2 is a viable renewable energy source, as global CO2 emissions are perennially increasing. The decomposition of CO2 to CO and O2 through high-temperature cracking of metal oxides via a two-step thermochemical cycle has been an effective strategy to reduce CO2 concentrations in the atmosphere. However, this thermochemical cycle requires a high reaction temperature (1273 K). Hence, in this study, we investigated photothermal CO2 decomposition over three CeO2 catalysts with different morphologies: porous CeO2 nanosheets (2D-CeO2), cubic-shaped CeO2 (C-CeO2), and octahedral CeO2 (O-CeO2). The photothermal synergistic effect eliminated the necessity of high temperatures, where light illumination increased the generation of oxygen vacancies and electron transfer to promote CO2 decomposition. The maximum rate of CO production by 2D-CeO2 at 250 °C was 69 μmol g-1h−1, which is significantly higher than those of O-CeO2 (47 μmol g-1h−1) and C-CeO2 (19 μmol g-1h−1). However, no activity was observed in dark conditions, even at 250 °C. In-situ DRIFTS, quasi in-situ EPR, XPS, and Ar-TPD experiments revealed that light irradiation promotes the production of oxygen vacancies on the catalyst surface and generates the intermediate CO2•− species, which cleave the C=O bonds and CO2 conversion, while temperature promotes the adsorption of CO2 on the catalyst surface and facilitates electron transfer. DFT calculations revealed the differences in oxygen vacancy formation and CO2 adsorption among various exposed crystal facets of CeO2. These findings provide new avenues for CO2 decomposition based on the adsorption and activation of CO2 on metal oxide surfaces and the low-temperature synergistic photothermal effect.

Machine-Learning-Assisted Descriptors Identification for Indoor Formaldehyde Oxidation Catalysts.

The development of highly efficient catalysts for formaldehyde (HCHO) oxidation is of significant interest for the improvement of indoor air quality. Up to 400 works relating to the catalytic oxidation of HCHO have been published to date; however, their analysis for collective inference through conventional literature search is still a challenging task. A machine learning (ML) framework was presented to predict catalyst performance from experimental descriptors based on an HCHO oxidation catalysts database. MnOx, CeO2, Co3O4, TiO2, FeOx, ZrO2, Al2O3, SiO2, and carbon-based catalysts with different promoters were compiled from the literature. Notably, 20 descriptors including reaction catalyst composition, reaction conditions, and catalyst physical properties were collected for data mining (2263 data points). Furthermore, the eXtreme Gradient Boosting algorithm was employed, which successfully predicted the conversion efficiency of HCHO with an R-square value of 0.81. Shapley additive analysis suggested Pt/MnO2 and Ag/Ce-Co3O4 exhibited excellent catalytic performance of HCHO oxidation based on the analysis of the entire database. Validated by experimental tests and theoretical simulations, the key descriptor identified by ML, i.e., the first promoter, was further described as metal-support interactions. This study highlights ML as a useful tool for database establishment and the catalyst rational design strategy based on the importance of analysis between experimental descriptors and the performance of complex catalytic systems.

Importance of Metal-Support Interactions for CO2 Hydrogenation: An Operando Near-Ambient Pressure X-ray Photoelectron Spectroscopy Study on Gold-Loaded In2O3 and CeO2 Catalysts.

Metal-support interactions, which are essential for the design of supported metal catalysts, used, e.g., for CO2 activation, are still only partially understood. In this study of gold-loaded In2O3 and CeO2 catalysts during CO2 hydrogenation using near-ambient pressure X-ray photoelectron spectroscopy, supported by near edge X-ray absorption fine structure, we demonstrate that the role of the noble metal strongly depends upon the choice of the support material. Temperature-dependent analyses of X-ray photoelectron spectra under reaction conditions reveal that gold is reduced on CeO2, enabling direct H2 activation, but oxidized on In2O3, leading to decreased activity of Au/In2O3 compared to bare In2O3. At elevated temperatures, the catalytic activity of the In2O3 catalysts strongly increases as a result of facilitated CO2 and (In2O3-based) H2 activation, while the catalytic activity of Au/CeO2 is limited by reoxidation by CO2. Our results underline the importance of operando studies for understanding metal-support interactions to enable a rational support selection in the future.

Preparation of CeO2 Supported on Graphite Catalyst and Its Catalytic Performance for Diethyl Phthalate Degradation during Ozonation

Catalysts for the efficient catalytic decomposition of ozone to generate reactive free radicals to oxidize pollutants are needed. The graphite-supported CeO2 catalyst was optimally prepared, and its activity in ozonation was evaluated using the degradation of diethyl phthalate (DEP) as an index. The stability of CeO2/graphite catalyst and the influence of operating conditions on its catalytic activity were investigated, and the mechanism of CeO2/graphite catalytic ozonation was analyzed. CeO2/graphite had the highest catalytic activity at a Ce load of 3.5% and a pyrolysis temperature of 400 °C with the DEP degradation efficiency of 75.0% and the total organic carbon (TOC) removal efficiency of 48.3%. No dissolution of active components was found during the repeated use of CeO2/graphite catalyst. The ozone dosage, catalyst dosage, initial pH, and reaction temperature have positive effects on the DEP degradation by CeO2/graphite catalytic ozonation. The presence of tert-butanol significantly inhibits the degradation of DEP at an initial pH of 3.0, 5.8, or 9.0, and the experimental results of the •OH probe compound pCBA indicate that the CeO2/graphite catalyst can efficiently convert ozone into •OH in solution. The DEP degradation in the CeO2/graphite catalytic ozonation mainly depends on the •OH in the bulk solution formed by ozone decomposition.

Fabrication of porous and highly defective CeO2-based catalysts for soot combustion with the strategy of soft-template addition and matrix doping by multiple rare earth elements

In this study, aiming to optimize the reactivity of CeO2 for soot combustion, rare earth elements (La, Pr, Sm and Y) co-doped CeO2 catalysts were synthesized via polyvinylpyrrolidone (PVP) and glycine (GNC) combustion methods. The PVP-CeO2-Re catalyst maintains a high specific surface area (82.9 m2 g−1) and abundant porous structure, thus being able to enhance the soot contact efficiency. The co-doping of multiple rare earth ions improves also the redox properties of the catalyst and increases the surface defects/vacancies, which is crucial to form the active surface O2- and O22- species to participate in soot combustion. The oxygen exchange rate over the catalyst with gaseous O2 can be accelerated by raising the temperature, and the exchange mechanism changes from a mixture of the R1 and R2 mechanisms to a domination by the R2 mechanism. Notably, PVP-CeO2-Re catalyst shows not only excellent activity (T50=373 °C) but also good stability. Our work could provide some reference for the design of promising catalysts applied in diesel particulate filters.

In situ/operando spectroscopic evidence on associative redox mechanism for periodic unsteady-state water–gas shift reaction on Au/CeO2 catalyst

Cyclic and repeatable CO2/H2 formation under periodic CO ↔ H2O feeds, that is unsteady-state water–gas shift reaction (uss-WGS), was found to be catalyzed by a gold nanoparticles-loaded CeO2 (Au/CeO2) catalyst. Kinetics of the CO-reduction of Ce4+ to Ce3+ and Ce3+ reoxidation by H2O in combination with transient CO2/H2 formation over Au/CeO2 were studied by operando ultraviolet–visible (UV–vis) and infrared (IR) spectroscopies at 175 °C. The Ce4+–OH species were reduced by CO to give Ce3+-□-Ce3+ (□: oxygen vacancy) and gas phase products (H2, CO2). The Ce3+-□-Ce3+ species were oxidized by H2O to give H2 and Ce4+–OH species. The reduction and reoxidation rates of Ce4+/Ce3+ redox couple were close to the rates of transient CO2/H2 formation. The X-ray absorption spectroscopy results showed that the oxidation states of Au remained unchanged during the redox reactions. These results indicate that the uss-WGS is primary driven by the Ce4+/Ce3+ redox couple. Combined with the observation of adsorbed carboxylate intermediates as a precursor of H2 and CO2, associative redox mechanism is proposed as the main pathway for the unsteady-state WGS reaction on Au/CeO2 at 175 °C.

Dimethyl Carbonate Synthesis from CO2 over CeO2 with Electron-Enriched Lattice Oxygen Species.

Direct synthesis of dimethyl carbonate (DMC) from CO2 plays an important role in carbon neutrality, but its efficiency is still far from the practical application, due to the limited understanding of the reaction mechanism and rational design of efficient catalyst. Herein, abundant electron-enriched lattice oxygen species were introduced into CeO2 catalyst by constructing the point defects and crystal-terminated phases in the crystal reconstruction process. Benefitting from the acid-base properties modulated by the electron-enriched lattice oxygen, the optimized CeO2 catalyst exhibited a much higher DMC yield of 22.2 mmol g-1 than the reported metal-oxide-based catalysts at the similar conditions. Mechanistic investigations illustrated that the electron-enriched lattice oxygen can provide abundant sites for CO2 adsorption and activation, and was advantageous of the formation of the weakly adsorbed active methoxy species. These were facilitating to the coupling of methoxy and CO2 for the key *CH3OCOO intermediate formation. More importantly, the weakened adsorption of *CH3OCOO on the electron-enriched lattice oxygen can switch the rate-determining-step (RDS) of DMC synthesis from *CH3OCOO formation to *CH3OCOO dissociation, and lower the corresponding activation barriers, thus giving rise to a high performance. This work provides insights into the underlying reaction mechanism for DMC synthesis from CO2 and methanol and the design of highly efficient catalysts.

Dimethyl Carbonate Synthesis from CO2 over CeO2 with Electron‐Enriched Lattice Oxygen Species

AbstractDirect synthesis of dimethyl carbonate (DMC) from CO2 plays an important role in carbon neutrality, but its efficiency is still far from the practical application, due to the limited understanding of the reaction mechanism and rational design of efficient catalyst. Herein, abundant electron‐enriched lattice oxygen species were introduced into CeO2 catalyst by constructing the point defects and crystal‐terminated phases in the crystal reconstruction process. Benefitting from the acid‐base properties modulated by the electron‐enriched lattice oxygen, the optimized CeO2 catalyst exhibited a much higher DMC yield of 22.2 mmol g‐1 than the reported metal‐oxide‐based catalysts at the similar conditions. Mechanistic investigations illustrated that the electron‐enriched lattice oxygen can provide abundant sites for CO2 adsorption and activation, and was advantageous of the formation of the weakly adsorbed active methoxy species. These were facilitating to the coupling of methoxy and CO2 for the key *CH3OCOO intermediate formation. More importantly, the weakened adsorption of *CH3OCOO on the electron‐enriched lattice oxygen can switch the rate‐determining‐step (RDS) of DMC synthesis from *CH3OCOO formation to *CH3OCOO dissociation, and lower the corresponding activation barriers, thus giving rise to a high performance. This work provides insights into the underlying reaction mechanism for DMC synthesis from CO2 and methanol and the design of highly efficient catalysts.

Effect of Ce2O3 phase transition on the catalytic oxidation for toluene over CeO2 catalysts

Cerium oxide was a potential replacement for precious metals as a catalyst for VOCs. Here, we prepared CeOx-350, CeOx-450, CeOx-550 and CeOx-650 catalysts by controlling the calcination temperature, the occurrence of Ce2O3 localized phase transitions at CeOx-350 and CeOx-550 was confirmed by XRD, HRTEM and TGA. This localized phase transition caused the change in specific surface area and crystallinity, and prolonged the Ce-O-Ce bond lengths at the heterogeneous interfaces, enhancing the redox capacity and activity of oxygen species. CeOx-550 exhibited the optimal low-temperature activity towards toluene due to the highest Ce3+ content, oxygen vacancy content and the best activity of reactive oxygen species. The oxidation of toluene experienced intermediates of benzyl alcohol, benzaldehyde, benzoate, maleic acid and carbonates. Among them, benzoic acid was the decisive step in the toluene oxidation pathway. Combined with the reaction pathway of toluene, the reaction stages of different oxygen species were determined. The active oxygen species located at the heterogeneous interface would serve as an adsorption site for toluene. The Ce2O3 phase would participate in the deep oxidation of toluene as an active site, which was equivalent to the oxygen vacancies. This study was conducted to explore the effect of local phase transition on catalytic activity, which provided a new strategy for the synthesis of highly active cerium oxide catalysts.

CO2 methanation over low-loaded Ni-M, Ru-M (M = Co, Mn) catalysts supported on CeO2 and SiC

The concentration of the major greenhouse gas CO2 is rapidly increasing in the atmosphere, leading to global warming and a range of environmental issues. An efficient circulation and utilization of CO2 is critical in the current environmental context. Methanation, an exothermic process, emerges as a critical strategy for effective CO2 utilization. On this front, there is a significant demand for rational design of catalysts that maintain high activity and methane selectivity over a wide temperature range (250–550 °C). The catalyst that can promise a consistent reaction even at 500 °C under an atmospheric pressure is thus obliged. The present study investigated bimetallic catalysts with SiC, which is known for its exceptional thermal conductivity, and CeO2, which is characterized by its CO₂ affinity, as base materials. We incorporated Ni-M and Ru-M (M = Co and Mn) as the active metals, each loaded at 2 %. Impressively, with merely 20 mg, the Ni-Co/SiC catalyst achieved a CO2 conversion rate of 77 % and CH₄ selectivity of 88 % at 500 °C, in a fixed-bed tubular reactor system with conditions of H2/CO2 = 4, a total flow rate of 70 ml min−1, and a steady GHSV of 12,000 h−1. Moreover, 2Ni-2Co/CeO2 catalyst demonstrated exceptional performance with a 76 % conversion of CO2 and a 83 % selectivity for CH4, all under identical conditions. The catalyst's durability was confirmed by a subsequent 40-hour stability test, which showed only a 3–5 % degradation. The developed catalysts were comprehensively characterized by BET/BJH, CO pulse chemisorption, H2-TPR, HAADF-STEM-EDS, SEM-EDS and XRD etc. to unveil their physicochemical and surface traits. It was found that Co and Mn, when integrated, effectively restrained the agglomeration of Ni and Ru particles, ensuring optimal metal dispersion on the support. In conclusion, our synthesized bimetallic catalysts shown a sustained catalytic capability, even in the high-temperature environment.