Dry Reforming of Methane on a Highly-Active Ni-CeO2 Catalyst: Effects of Metal-Support Interactions on C-H Bond Breaking

Insight into the Coordination Environment of Ru Single-Atom for Dry Reforming of Methane

Dry reforming of methane (DRM), a pivotal process for converting greenhouse gases into syngas is demanding rationally designed catalysts with high stability and ideal catalytic performance for industrial applications due to its stability of reactant molecules and characteristic of carbon deposition. However, the mechanistic understanding of how the coordination environment of the metal in a single-atom catalytic system may influence the catalytic performance remains limited. In this work, high- and low-coordinating Ru-based (RuHC and RuLC) catalysts with distinct Ru-O coordination numbers are prepared using one-pot and two-step methods. The difference in the stability (12.3% and negligible deactivation during 20 h test for RuLC and RuHC catalysts respectively) and selectivity (0.57 and 0.37 of H2/CO ratio) brought by the coordination environment signified the structure-function relationship of single-atom catalysts in DRM. The impact of the structure on the properties is systematically investigated by thorough structural and operando characterization as well as density functional theory (DFT) calculation. The findings contribute to the optimal design of single-atom catalysts for DRM, offering a theoretical basis for industrial catalyst development and the potential to improve the process's environmental impact.

Unveiling Direct Electrochemical Oxidation of Methane at the Ceria/Gas Interface

Solid oxide fuel cells (SOFCs) stand out in sustainable energy systems for their unique ability to efficiently utilize hydrocarbon fuels, particularly those from carbon-neutral sources. CeO2-δ (ceria) based oxides embedded in SOFCs are recognized for their critical role in managing hydrocarbon activation and carbon coking. However, even for the simplest hydrocarbon molecule, CH4, the mechanism of electrochemical oxidation at the ceria/gas interface is not well understood and the capability of ceria to electrochemically oxidize methane remains a topic of debate. This lack of clarity stems from the intricate design of standard metal/oxide composite electrodes and the complex nature of electrode reactions involving multiple chemical and electrochemical steps. This study presents a Sm-doped ceria thin-film model cell that selectively monitors CH4 direct-electro-oxidation on the ceria surface. Using impedance spectroscopy, operando X-ray photoelectron spectroscopy, and density functional theory, it is unveiled that ceria surfaces facilitate C─H bond cleavage and that H2O formation is key in determining the overall reaction rate at the electrode. These insights effectively address the longstanding debate regarding the direct utilization of CH4 in SOFCs. Moreover, these findings pave the way for an optimized electrode design strategy, essential for developing high-performance, environmentally sustainable fuel cells.

Structure Engineering of Ni/SiO2 Vegetable Oil Hydrogenation Catalyst via CeO2

Inspired by our finding that metallic Ni particles could be uniformly distributed on a reduced CeO2 surface and stabilized on Ce3+ sites, we suppose a possible improvement in the activity and selectivity of the MgNi/SiO2 vegetable oil hydrogenation catalyst by increasing the surface metal Ni availability via modification by ceria. The proposed approach involved the addition of a CeO2 modifier to the SiO2 carrier and as a catalyst component. Evaluation of the structure, reducibility, and surface and electronic states of the CeO2-doped MgNi/SiO2 catalyst was performed by means of the Powder X-ray diffraction (PXRD), Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) combined with High-resolution transmission electron microscopy (HRTEM), Temperature-programmed reduction with hydrogen (H2-TPR), and H2-chemisortion techniques. So far, no studies related to this approach of designing Ni/SiO2 catalysts for the partial hydrogenation of vegetable oil have been reported. The added ceria impact was elucidated by comparing fatty acid compositions obtained by the catalysts at an iodine value of 80. In summary, tuning the hydrogenation performance of Ni-based catalysts can be achieved by structural reconstruction using 1 wt.% CeO2. The introduction mode changed the selectivity towards C18:1-cis and C18:0 fatty acids by applying ceria as a carrier modifier, while hydrogenation activity was improved upon ceria operation as the catalyst dopant.

Atomistic insights from DFT calculations into the catalytic properties on ceria-lanthanum clusters for methane activation

Improving the catalytic performance of materials based on cerium oxide (CeO2) for the activation of methane (CH4) can be achieved through the following strategies: mixture of CeO2 with different oxides (e.g., CeO2-La2O3) and the use of particles with different sizes. In this study, we present a theoretical investigation of the initial CH4 dehydrogenation on (La2Ce2O7)n clusters, where n = 2, 4, and 6. Our framework relies on density functional theory calculations combined with the unity bond index-quadratic exponential potential approximation. Our results indicate that chemical species arising from the first dehydrogenation of CH4, that is, CH3 and H, bind through the formation of C-O and H-O bonds with the clusters, respectively. The coordination of the adsorption site and the chemical environment plays a crucial role in the magnitude of the adsorption energy; for example, species adsorb more strongly in the low-coordinated topO sites located close to the La atoms. Thus, it affects the activation energy barrier, which tends to be lower in configurations where the adsorption of the chemical species is stronger. During CH4 dehydrogenation, the CH3 radical can be present in a planar or tetrahedral configuration. Its conformation changes as a function of the charge transference between the molecule and the cluster, which depends on the CH3-cluster distance. Finally, we analyze the effects of the Hubbard effective parameter (Ueff) on adsorption properties, as the magnitude of localization of Ce f-states affects the hybridization of the interaction between the molecule and the clusters and hence the magnitude of the adsorption energies. We obtained a linear decrease in the adsorption energies by increasing the Ueff parameter; however, the activation energy is only slightly affected.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Comprehensive Density Functional and Kinetic Monte Carlo Study of CO2 Hydrogenation on a Well-Defined Ni/CeO2 Model Catalyst: Role of Eley-Rideal Reactions

A detailed multiscale study of the mechanism of CO2 hydrogenation on a well-defined Ni/CeO2 model catalyst is reported that couples periodic density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations. The study includes an analysis of the role of Eley-Rideal elementary steps for the water formation step, which are usually neglected on the overall picture of the mechanism, catalytic activity, and selectivity. The DFT calculations for the chosen model consisting of a Ni4 cluster supported on CeO2 (111) show large enough adsorption energies along with low energy barriers that suggest this catalyst to be a good option for high selective CO2 methanation. The kMC simulations results show a synergic effect between the two 3-fold hollow sites of the supported Ni4 cluster with some elementary reactions dominant in one site, while other reactions prefer the another, nearly equivalent site. This effect is even more evident for the simulations explicitly including Eley-Rideal steps. The kMC simulations reveal that CO is formed via the dissociative pathway of the reverse water-gas shift reaction, while methane is formed via a CO2 → CO → HCO → CH → CH2 → CH3 → CH4 mechanism. Overall, our results show the importance of including the Eley-Rideal reactions and point to small Ni clusters supported on the CeO2 (111) surface as potential good catalysts for high selective CO2 methanation under mild conditions, while very active and selective toward CO formation at higher temperatures.

Highly Efficient and Stable Methane Dry Reforming Enabled by a Single-Site Cationic Ni Catalyst

Zeolite-supported nickel (Ni) catalysts have been extensively studied for the dry reforming of methane (DRM). It is generally believed that prior to or during the reaction, Ni is reduced to a metallic state to act as the catalytic site. Here, we employed a ligand-protected synthesis method to achieve a high degree of Ni incorporation into the framework of the MFI zeolite. The incorporated Ni species retained their cationic nature during the DRM reaction carried out at 600 °C, exhibiting higher apparent catalytic activity and significantly greater catalytic stability in comparison to supported metallic Ni particles at the same loading. From theoretical and experimental evidence, we conclude that the incorporation of Ni into the zeolite framework leads to the formation of metal-oxygen (Niδ+-O(2-ξ)-) pairs, which serve as catalytic active sites, promoting the dissociation of C-H bonds in CH4 through a mechanism distinct from that of metallic Ni. The conversion of CH4 on cationic Ni single sites follows the CHx oxidation pathway, which is characterized by the rapid transformation of partial cracking intermediates CHx*, effectively inhibiting coke formation. The presence of the CHx oxidation pathway was experimentally validated by identifying the reaction intermediates. These new mechanistic insights elucidate the exceptional performance of the developed Ni-MFI catalyst and offer guidance for designing more efficient and stable Ni-based DRM catalysts.

Light-Reinforced Key Intermediate for Anticoking To Boost Highly Durable Methane Dry Reforming over Single Atom Ni Active Sites on CeO2

Dry reforming of methane (DRM) has been investigated for more than a century; the paramount stumbling block in its industrial application is the inevitable sintering of catalysts and excessive carbon emissions at high temperatures. However, the low-temperature DRM process still suffered from poor reactivity and severe catalyst deactivation from coking. Herein, we proposed a concept that highly durable DRM could be achieved at low temperatures via fabricating the active site integration with light irradiation. The active sites with Ni-O coordination (NiSA/CeO2) and Ni-Ni coordination (NiNP/CeO2) on CeO2, respectively, were successfully constructed to obtain two targeted reaction paths that produced the key intermediate (CH3O*) for anticoking during DRM. In particular, the operando diffuse reflectance infrared Fourier transform spectroscopy coupling with steady-state isotopic transient kinetic analysis (operando DRIFTS-SSITKA) was utilized and successfully tracked the anticoking paths during the DRM process. It was found that the path from CH3* to CH3O* over NiSA/CeO2 was the key path for anticoking. Furthermore, the targeted reaction path from CH3* to CH3O* was reinforced by light irradiation during the DRM process. Hence, the NiSA/CeO2 catalyst exhibits excellent stability with negligible carbon deposition for 230 h under thermo-photo catalytic DRM at a low temperature of 472 °C, while NiNP/CeO2 shows apparent coke deposition behavior after 0.5 h in solely thermal-driven DRM. The findings are vital as they provide critical insights into the simultaneous achievement of low-temperature and anticoking DRM process through distinguishing and directionally regulating the key intermediate species.

Systematic Characterization of Electronic Metal-Support Interactions in Ceria-Supported Pt Particles

Electronic metal-support interactions affect the chemical and catalytic properties of metal particles supported on reducible metal oxides, but their characterization is challenging due to the complexity of the electronic structure of these systems. These interactions often involve different states with varying numbers and positions of strongly correlated d or f electrons and the corresponding polarons. In this work, we present an approach to characterize electronic metal-support interactions by means of computationally efficient density functional calculations within the projector augmented wave method. We describe Ce3+ cations with potentials that include a Ce4f electron in the frozen core, overcoming prevalent convergence and 4f electron localization issues. We systematically explore the stability and chemical properties of different electronic states for a Pt8/CeO2(111) model system, revealing the predominant effect of electronic metal-support interactions on Pt atoms located directly at the metal-oxide interface. Adsorption energies and the reactivity of these interface Pt atoms vary significantly upon donation of electrons to the oxide support, pointing to a strategy to selectively activate interfacial sites of metal particles supported on reducible metal oxides.

Highly active Ru/TiO2 nanostructures for total catalytic oxidation of propane

Ruthenium is a robust catalyst for a variety of applications in environmental heterogeneous catalysis. The catalytic performance of Ru/TiO2 materials, synthesized by using the deposition precipitation with urea method, was assessed in the catalytic oxidation of C3H8, varying the ruthenium loading. The highest catalytic reactivity was obtained for a Ru loading of 2 wt. % in comparison with the 1, 1.5, 3, and 4 wt. % Ru catalysts. The physicochemical properties of the synthesized materials were investigated by XRD, N2 adsorption, TEM, FT-IR pyridine, H2-TPR, and XPS. The size of ruthenium particles was found to be greatly dependent on the pretreatment gas (air or hydrogen) and the catalytic activity was enhanced by the small-size ruthenium metal nanoparticles, leading to changes in the reduction degree of ruthenium, which also increased the Brönsted and Lewis acidity. Metal to support charge transfer enhanced the reactant adsorption sites while oxygen vacancies on the interface enabled the dissociation of O2 molecules as revealed through DFT calculations. The outstanding catalytic activity of the 2Ru/TiO2 catalysts allowed to convert C3H8 into CO2 at reaction temperatures of about 100 °C. This high activity may be attributed to the metal/support interaction between Ru and TiO2, which promoted the reducibility of Ti4+/Ti3+ and Ru4+/Ru0 species, and to the fast migration of TiO2 lattice oxygen in the catalyst. Furthermore, the Ru/TiO2 catalyst exhibited high stability and reusability for 30 h under reaction conditions, using a GHSV of 45,000 h-1. The underlying alkane-metal interactions were explored theoretically in order to explain the C-H bond activation in propane by the catalyst.

Imaging Interface and Particle Size Effects by In Situ Correlative Microscopy of a Catalytic Reaction

The catalytic behavior of Rh particles supported by three different materials (Rh, Au, and ZrO₂) in H₂ oxidation has been studied in situ by correlative photoemission electron microscopy (PEEM) and scanning photoemission electron microscopy (SPEM). Kinetic transitions between the inactive and active steady states were monitored, and self-sustaining oscillations on supported Rh particles were observed. Catalytic performance differed depending on the support and Rh particle size. Oscillations varied from particle size-independent (Rh/Rh) via size-dependent (Rh/ZrO₂) to fully inhibited (Rh/Au). For Rh/Au, the formation of a surface alloy induced such effects, whereas for Rh/ZrO₂, the formation of substoichiometric Zr oxides on the Rh surface, enhanced oxygen bonding, Rh-oxidation, and hydrogen spillover onto the ZrO₂ support were held responsible. The experimental observations were complemented by micro-kinetic simulations, based on variations of hydrogen adsorption and oxygen binding. The results demonstrate how correlative in situ surface microscopy enables linking of the local structure, composition, and catalytic performance.

Identification of Highly Selective Surface Pathways for Methane Dry Reforming Using Mechanochemical Synthesis of Pd-CeO2

The methane dry reforming (DRM) reaction mechanism was explored via mechanochemically prepared Pd/CeO2 catalysts (PdAcCeO2M), which yield unique Pd-Ce interfaces, where PdAcCeO2M has a distinct reaction mechanism and higher reactivity for DRM relative to traditionally synthesized impregnated Pd/CeO2 (PdCeO2IW). In situ characterization and density functional theory calculations revealed that the enhanced chemistry of PdAcCeO2M can be attributed to the presence of a carbon-modified Pd0 and Ce4+/3+ surface arrangement, where distinct Pd-CO intermediate species and strong Pd-CeO2 interactions are activated and sustained exclusively under reaction conditions. This unique arrangement leads to highly selective and distinct surface reaction pathways that prefer the direct oxidation of CH x to CO, identified on PdAcCeO2M using isotope labeled diffuse reflectance infrared Fourier transform spectroscopy and highlighting linear Pd-CO species bound on metallic and C-modified Pd, leading to adsorbed HCOO [1595 cm-1] species as key DRM intermediates, stemming from associative CO2 reduction. The milled materials contrast strikingly with surface processes observed on IW samples (PdCeO2IW) where the competing reverse water gas shift reaction predominates.

Ni-Cu Alloy Nanoparticles Confined by Physical Encapsulation with SiO2 and Chemical Metal-Support Interaction with CeO2 for Methane Dry Reforming

Fabrication of sintering- and carbon-free Ni catalysts for methane dry reforming (MDR), which is attractive to upgrade greenhouse gases CH₄ and CO₂, is challenging. In this work, we innovatively synthesized Ni-Cu alloy nanoparticles confined by physical encapsulation and chemical metal-support interaction (MSI); the synergism of alloy effect, size effect, MSI, and confinement effect in the catalysts gave high rates of CH₄ and CO₂ of 6.98 and 7.16 mmol/(g_Nis), respectively, at 1023 K for 50 h. The rates were 2-3 times enhanced compared to those in the literature. XRD, TEM, H₂-TPR, and so forth revealed that the alloy effect, size effect, and MSI of Ni-Cu and CeO₂ enhanced the MDR activity; MSI promoted the ceria surface lattice oxygen mobility and generated more oxygen vacancies, almost completely gasifying carbon deposits; chemical confinement from MSI and physical confinement from SiO₂ nanospheres realized sintering-free alloys and CeO₂ nanoparticles. The synergistic approach provides a universal strategy for sintering- and carbon-free Ni catalyst design for MDR reaction.

Tuning Selectivity in the Direct Conversion of Methane to Methanol: Bimetallic Synergistic Effects on the Cleavage of C-H and O-H Bonds over NiCu/CeO2 Catalysts

The efficient activation of methane and the simultaneous water dissociation are crucial in many catalytic reactions on oxide-supported transition metal catalysts. On very low-loaded Ni/CeO₂ surfaces, methane easily fully decomposes, CH₄ → C + 4H, and water dissociates, H₂O→ OH + H. However, in important reactions such as the direct oxidation of methane to methanol (MTM), where complex interplay exists between reactants (CH₄, O₂), it is desirable to avoid the complete dehydrogenation of methane to carbon. Remarkably, the barrier for the activation of C-H bonds in CH_x (x = 1-3) species on Ni/CeO₂ surfaces can be manipulated by adding Cu, forming bimetallic NiCu clusters, whereas the ease for cleavage of O-H bonds in water is not affected by ensemble effects, as obtained from density functional theory-based calculations. CH₄ activation occurs only on Ni sites, and H₂O activation occurs on both Ni and Cu sites. The MTM reaction pathway for the example of the Ni₃Cu₁/CeO₂ model catalyst predicts a higher selectivity and a lower activation barrier for methanol production, compared with that for Ni₄/CeO₂. These findings point toward a possible strategy to design active and stable catalysts which can be employed for methane activation and conversions.

Ni/CeO2 Catalyst Prepared via Microimpinging Stream Reactor with High Catalytic Performance for CO2 Dry Reforming Methane

Methane reforming with carbon dioxide (DRM) is one promising way to achieve carbon neutrality and convert methane to syngas for high-value chemical production. Catalyst development with better performance is the key to its potential large-scale industrial application due to its deactivation caused by carbon deposition and metal sintering. Hence, a Ni/CeO2 catalyst (Ni/CeO2-M) with higher CO2 conversion and better stability is prepared, supported on CeO2 precipitated via a novel microimpinging stream reactor. A series of ex-situ or in-situ characterizations, such as CO titration measurements, two-step transient surface reaction (two-step TSR), CO2 and CH4 temperature-programmed surface reaction (CO2-TPSR and CH4-TPSR), X-ray absorption fine structure (XAFS), and in-situ Raman spectroscopy study, were used to investigate its structure and mechanism. In contrast to Ni supported on commercial CeO2 (Ni/CeO2-C), the Ni/CeO2-M catalyst with stronger lattice oxygen mobility and higher oxygen storage capacity enhances its CO2 activation ability and carbon deposition. The Ni particle size of the Ni/CeO2-M catalyst decreased, and a higher oxidation state was obtained due to the strong metal–support interaction. Besides the reaction performance improvement of the Ni/CeO2-M catalyst, the novel microimpinging stream reactor could achieve catalyst continuous production with a high preparation efficiency. This work provides a novel method for the high-performance catalyst preparation for DRM reaction and its mechanism study gives a deep insight into high-performance catalyst development via bottom-up study.

Ni-CeO2/SBA-15 Catalyst Prepared by Glycine-Assisted Impregnation Method for Low-Temperature Dry Reforming of Methane

Developing low-temperature nickel-based catalysts with good resistance to coking and sintering for dry reforming of methane (DRM) is of great significance. In this work, Ni (5 wt%) and CeO2 (5 wt%) were supported on SBA-15 porous material by glycine-assisted impregnation method to obtain Ni-CeO2/SBA-15-G catalyst. XRD and TEM results showed that the addition of glycine can effectively promote the dispersion of NiO and CeO2 in the pores of SBA-15. H2-TPR and XPS results confirmed the formation of stronger metal-support interaction. In addition, after the addition of glycine, the NixCe1−xOy solid solution content was increased significantly, meanwhile, the Ce3+ concentration was increased from 31% to 49%, accompanied by more oxygen vacancies and generation of active oxygen species. For the above reasons, Ni-CeO2/SBA-15-G had better catalytic performance in the low-temperature DRM test (20 h, 600 °C) with high GHSV (600,000 mL/gcat/h), its CH4 conversion after reaction of 20 h was 2 times that of Ni-CeO2/SBA-15-C catalyst prepared by a conventional impregnation method. TGA-DTA test also proved that Ni-CeO2/SBA-15-G almost completely eliminated carbon deposition. The above advantages of the Ni-CeO2/SBA-15-G catalyst may have originated from the complexation of glycine with metal cations and can prevent them from gathering.

Structure and reactivity of model CeO2surfaces

As a key component in many industrial heterogeneous catalysts, the surface structure and reactivity of ceria, CeO₂, has attracted a lot of attention. In this topical review we discuss some of the approaches taken to form a deeper understanding of the surface physics and chemistry of this important and interesting material. In particular, we focus on the preparation of ultrathin ceria films, nanostructures and supported metal nanoparticles. Cutting-edge microscopic and spectroscopic experimental techniques are highlighted which can probe the behaviour of oxygen species and atomic defects on these model surfaces.

Sustainable Synthesis of a Highly Stable and Coke-Free Ni@CeO2 Catalyst for the Efficient Carbon Dioxide Reforming of Methane

A facile and green synthetic strategy is developed in this paper for the construction of an efficient catalyst for the industrially important carbon dioxide reforming of methane, which is also named the dry reforming of methane (DRM). Through controlling the synthetic strategy and Ni content, a high-performance Ni@CeO2 catalyst was successfully fabricated. The catalyst showed superb efficiency for producing the syngas with high and stable conversions at prolonged operating conditions. Incorporating Ni during the ceria (CeO2) crystallization resulted in a more stable structure and smaller nanoparticle (NP) size with a more robust interaction with the support than loading Ni on CeO2 supports by the conventional impregnation method. The H2/CO ratio was almost 1.0, indicating the promising applicability of utilizing the obtained syngas for the Fischer–Tropsch process to produce worthy chemicals. No carbon deposits were observed over the as-synthesized catalyst after operating the DRM reaction for 50.0 h, even at a more coke-favoring temperature (700 ∘C). Owing to the superb resistance to coke and sintering, control of the size of the Ni-NPs, uniform dispersion of the active phase, and potent metal interaction with the support, the synthesized catalyst achieved a magnificent catalytic activity and durability during serving for the DRM reaction for extended operating periods.

Effect of preparation methods of CeO2 on the properties and performance of Ni/CeO2 in CO2 reforming of CH4

CO₂ reforming of CH₄ (CRM) is not only beneficial to environmental protection, but also valuable for industrial application. Different CeO₂ supports were prepared to investigate the matching between Ni and CeO₂ over Ni/CeO₂ and its effect on CRM. The physicochemical properties of Ni/CeO₂-C (commercial CeO₂), Ni/CeO₂-H (hydrothermal method) as well as Ni/CeO₂-P (precipitation method) were characterized by XRD, N₂ adsorption at − 196 °C, TEM, SEM–EDS, H₂-TPR, NH₃-TPD and XPS. Ni⁰ with good dispersion and CeO₂ with more oxygen vacancies were obtained on Ni/CeO₂-H, proving the influence on Ni/CeO₂ catalysts caused by the preparation methods of CeO₂. The initial conversion of both CO₂ and CH₄ of Ni/CeO₂-H was more than five times that of Ni/CeO₂-P and Ni/CeO₂-C. The better matching between Ni and CeO₂ on Ni/CeO₂-H was the reason for its best catalytic performance in comparison with the Ni/CeO₂-C and Ni/CeO₂-P samples.

Promoting dry reforming of methane catalysed by atomically-dispersed Ni over ceria-upgraded boron nitride

Dry reforming of methane (DRM) is a very promising protocol to mitigate the greenhouse gases by making use of CO 2 and CH 4 to produce valuable syngas. Ni-Based catalysts exhibit high activity and low cost for DRM, but suffer from inferior stability because of serious carbon deposition. Herein, we proposed atomically dispersed Ni supported by ceria-upgraded boron nitride whose specific activity exceeds that of boron nitride supported Ni by 3 times. The results of temperature programmed surface reaction show ceria enhanced the adsorption of CO 2 and its surface-active oxygen species would contribute to the activation of CH 4 . Moreover, Ni exhibited a strong metal-support interaction which suppressed the metal sintering during DRM reaction while the incorporation of BN could suppress carbon deposition. The incorporation of active metal oxides into inert support provides a route to adjust the interaction between metal and support and achieve the synergistic promoting in catalytic performance.

Kinetic Monte Carlo simulations of the dry reforming of methane catalyzed by the Ru (0001) surface based on density functional theory calculations

This study shows the main pathways for the DRM reaction and the competitive RWGS reaction upon changing reaction conditions, displaying the importance of including lateral–lateral interactions to describe the reaction in agreement with the experiment.

Activation and catalytic transformation of methane under mild conditions

In the last few decades, worldwide scientists have been motivated by the promising production of chemicals from the widely existing methane (CH₄) under mild conditions for both chemical synthesis with low energy consumption and climate remediation. To achieve this goal, a whole library of catalytic chemistries of transforming CH₄ to various products under mild conditions is required to be developed. Worldwide scientists have made significant efforts to reach this goal. These significant efforts have demonstrated the feasibility of oxidation of CH₄ to value-added intermediate compounds including but not limited to CH₃OH, HCHO, HCOOH, and CH₃COOH under mild conditions. The fundamental understanding of these chemical and catalytic transformations of CH₄ under mild conditions have been achieved to some extent, although currently neither a catalyst nor a catalytic process can be used for chemical production under mild conditions at a large scale. In the academic community, over ten different reactions have been developed for converting CH₄ to different types of oxygenates under mild conditions in terms of a relatively low activation or catalysis temperature. However, there is still a lack of a molecular-level understanding of the activation and catalysis processes performed in extremely complex reaction environments under mild conditions. This article reviewed the fundamental understanding of these activation and catalysis achieved so far. Different oxidative activations of CH₄ or catalytic transformations toward chemical production under mild conditions were reviewed in parallel, by which the trend of developing catalysts for a specific reaction was identified and insights into the design of these catalysts were gained. As a whole, this review focused on discussing profound insights gained through endeavors of scientists in this field. It aimed to present a relatively complete picture for the activation and catalytic transformations of CH₄ to chemicals under mild conditions. Finally, suggestions of potential explorations for the production of chemicals from CH₄ under mild conditions were made. The facing challenges to achieve high yield of ideal products were highlighted and possible solutions to tackle them were briefly proposed.

Co3 O4 -CeO2 Nanocomposites for Low-Temperature CO Oxidation

In an effort to combine the favorable catalytic properties of Co₃ O₄ and CeO₂ , nanocomposites with different phase distribution and Co₃ O₄ loading were prepared and employed for CO oxidation. Synthesizing Co₃ O₄ -modified CeO₂ via three different sol-gel based routes, each with 10.4 wt % Co₃ O₄ loading, yielded three different nanocomposite morphologies: CeO₂ -supported Co₃ O₄ layers, intermixed oxides, and homogeneously dispersed Co. The reactivity of the resulting surface oxygen species towards CO were examined by temperature programmed reduction (CO-TPR) and flow reactor kinetic tests. The first morphology exhibited the best performance due to its active Co₃ O₄ surface layer, reducing the light-off temperature of CeO₂ by about 200 °C. In contrast, intermixed oxides and Co-doped CeO₂ suffered from lower dispersion and organic residues, respectively. The performance of Co₃ O₄ -CeO₂ nanocomposites was optimized by varying the Co₃ O₄ loading, characterized by X-ray diffraction (XRD) and N₂ sorption (BET). The 16-65 wt % Co₃ O₄ -CeO₂ catalysts approached the conversion of 1 wt % Pt/CeO₂ , rendering them interesting candidates for low-temperature CO oxidation.

Simple mechanisms of CH4 reforming with CO2 and H2O on a supported Ni/ZrO2 catalyst

To understand the metal-support interaction of oxide supported transition metal catalysts, we computed the reaction mechanisms of dry and steam reforming of methane on a tetragonal ZrO₂(101) supported Ni catalyst. Based on the limited number of active sites on the surface, an irregular and non-ideal Ni₁₃ cluster on ZrO₂(101) is identified as a catalyst. A simple reaction mechanism is proposed, and the first direct dissociation step of CO₂, CH₄ and H₂O is the most difficult based on the computed Gibbs free energies and no surface CH_XO and CH_XOH intermediates are involved, different from that on the flat Ni(111) surface. Analysis of other supported nickel catalysts shows that not only the support but also the size and shape of the metal clusters play an important role in the reaction mechanisms and kinetics.

New Insights on the Nickel State Deposited by Hydrazine Wet-Chemical Synthesis Route in the Ni/BCY15 Proton-Conducting SOFC Anode

Yttrium-doped barium cerate (BCY15) was used as an anode ceramic matrix for synthesis of the Ni-based cermet anode with application in proton-conducting solid oxide fuel cells (pSOFC). The hydrazine wet-chemical synthesis was developed as an alternative low-cost energy-efficient route that promotes ‘in situ’ introduction of metallic Ni particles in the BCY15 matrix. The focus of this study is a detailed comparative characterization of the nickel state in the Ni/BCY15 cermets obtained in two types of medium, aqueous and anhydrous ethylene glycol environment, performed by a combination of XRD, N₂ physisorption, SEM, EPR, XPS, and electrochemical impedance spectroscopy. Obtained results on the effect of the working medium show that ethylene glycol ensures active Ni cermet preparation with well-dispersed nanoscale metal Ni particles and provides a strong interaction between hydrazine-originating metallic Ni and cerium from the BCY15 matrix. The metallic Ni phase in the pSOFC anode is more stable during reoxidation compared to the Ni cermet prepared by the commercial mechanical mixing procedure. These factors contribute toward improvement of the anode’s electrochemical performance in pSOFC, enhanced stability, and a lower degradation rate during operation.

Selective Methane Oxidation to Methanol on ZnO/Cu2O/Cu(111) Catalysts: Multiple Site-Dependent Behaviors

Because of the abundance of natural gas in our planet, a major goal is to achieve a direct methane-to-methanol conversion at medium to low temperatures using mixtures of methane and oxygen. Here, we report an efficient catalyst, ZnO/Cu₂O/Cu(111), for this process investigated using a combination of reactor testing, scanning tunneling microscopy, ambient-pressure X-ray photoemission spectroscopy, density functional calculations, and kinetic Monte Carlo simulations. The catalyst is capable of methane activation at room temperature and transforms mixtures of methane and oxygen to methanol at 450 K with a selectivity of ∼30%. This performance is not seen for other heterogeneous catalysts which usually require the addition of water to enable a significant conversion of methane to methanol. The unique coarse structure of the ZnO islands supported on a Cu₂O/Cu(111) substrate provides a collection of multiple centers that display different catalytic activity during the reaction. ZnO-Cu₂O step sites are active centers for methanol synthesis when exposed to CH₄ and O₂ due to an effective O-O bond dissociation, which enables a methane-to-methanol conversion with a reasonable selectivity. Upon addition of water, the defected O-rich ZnO sites, introduced by Zn vacancies, show superior behavior toward methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH₄ → CH₃OH conversion are different from those engaged in methanol formation without water. The identification of the site-dependent behavior of ZnO/Cu₂O/Cu(111) opens a design strategy for guiding efficient methane reformation with high methanol selectivity.

Active Exsolved Metal-Oxide Interfaces in Porous Single-Crystalline Ceria Monoliths for Efficient and Durable CH4 /CO2 Reforming

Dry reforming of CH₄ /CO₂ provides an attractive route to convert greenhouse gas into syngas; however, the resistance to sintering and coking of catalyst remains a fundamental challenge at high operation temperatures. Here we create active and durable metal-oxide interfaces in porous single-crystalline (PSC) CeO₂ monoliths with in situ exsolved single-crystalline (SC) Ni particles and show efficient dry reforming of CH₄ /CO₂ at temperatures as low as 450 °C. We show the excellent and durable performance with ≈20 % of CH₄ conversion and ≈30 % of CO₂ conversion even in a continuous operation of 240 hours. The well-defined active metal-oxide interfaces, created by exsolving SC Ni nanoparticles from PSC Ni_x Ce_1-x O₂ to anchor them on PSC CeO₂ scaffolds, prevent nanoparticle sintering and enhance the coking resistance due to the stronger metal-support interactions. Our work would enable an industrially and economically viable path for carbon reclamation, and the technique of creating active and durable metal-oxide interfaces in PSC monoliths could lead to stable catalyst designs for many challenging reactions.

Methane Dry Reforming by Ni-Cu Nanoalloys Anchored on Periclase-Phase MgAlOx Nanosheets for Enhanced Syngas Production

Stable and efficient syngas production via methane dry reforming is highly desirable as it utilizes two greenhouse gases simultaneously. In this work, active Ni-Cu nanoalloys stably anchored on periclase-phase MgAlO_x nanosheets were successfully synthesized by a hydrothermal method. These highly dispersed small Ni-Cu alloys strongly interacted with the periclase-phase MgAlO_x nanosheets, on which abundant base sites were accessible. On the optimal catalyst (6Ni6CuMgAl-S), methane and carbon dioxide conversion always reached 85 and 90% at 700 °C under a gas hour speed velocity of 40,000 mL/g_cat h for more than 70 h. The hydrogen production rate was maintained at 1.8 mmol/min, and the ratio of H₂/CO was kept at approximately 0.96 under a CH₄ and CO₂ flow rate of 25 mL/min. Coke deposition and Ni sintering were effectively suppressed by the formation of a Ni-Cu alloy, the laminar structure, and the periclase phase of the MgAlO_x support. Moreover, the alloy nanoparticles were reconstructed into a segregated Ni-Cu alloy structure in response to the reaction environment, and this structure was more stable and still active. Density functional theory calculations showed that carbon adsorption was inhibited on the segregated Ni-Cu alloy. Furthermore, the experimental thermogravimetric and O₂-TPO results confirmed the significant decrease in carbon deposition on the Ni-Cu alloy catalysts.

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2surfaces

To study the dependence of the relative stability of surface (V^A) and subsurface (V^B) oxygen vacancies with the crystal facet of CeO₂, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce³⁺polarons, are facet- and concentration-dependent. At the higher vacancy concentration, theV^Ais more stable than theV^Bat the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both theV^Aand theV^Bhave similar stability. The stability of theV^Avacancies, compared to that of theV^B, is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce³⁺-Ce³⁺interactions.