Methane Activation Research Articles

From Methane to Methanol: Pd-iC-CeO2 Catalysts Engineered for High Selectivity via Mechanochemical Synthesis.

In the pursuit of selective conversion of methane directly to methanol in the liquid-phase, a common challenge is the concurrent formation of undesirable liquid oxygenates or combustion byproducts. However, we demonstrate that monometallic Pd-CeO2 catalysts, modified by carbon, created by a simple mechanochemical synthesis method exhibit 100% selectivity toward methanol at 75 °C, using hydrogen peroxide as oxidizing agent. The solvent free synthesis yields a distinctive Pd-iC-CeO2 interface, where interfacial carbon (iC) modulates metal-oxide interactions and facilitates tandem methane activation and peroxide decomposition, thus resulting in an exclusive methanol selectivity of 100% with a yield of 117 μmol/gcat at 75 °C. Notably, solvent interactions of H2O2 (aq) were found to be critical for methanol selectivity through a density functional theory (DFT)-simulated Eley-Rideal-like mechanism. This mechanism uniquely enables the direct conversion of methane into methanol via a solid-liquid-gas process.

A and B Site Doped Barium Niobates with Improved Electrical Properties of Electrochemical Oxidative Coupling of Methane

Electrochemical oxidative coupling of methane (E-OCM) to produce ethylene is gaining renewed interest due the ability to control the conversion rate and product selectivity though temperature and applied potential. We previously demonstrate Mg, Fe and Ca, Fe codoped barium niobates have remarkable methane activation properties coupled with good chemical stability. Nevertheless, their electrical conductivity remained less than 50 mScm-1 that is low for electrode application. Hence, we attempted both A and B site doping on barium niobates with various dopants such as Sr, La (A site), and Ca, Mg, Fe, Co, Ni, Y (B site) to improve their electrical properties. These doped niobates showed enhanced thermochemical stability in SOFC relevant conditions and catalytic activity towards methane activation. The redox behavior of the Nb4+/5+ couple seem to be at a key reason behind this redox stability while the size and electronegativity of the dopants affect the electrical properties. The chemical stability was analyzed by TGA measurements followed by analysis of the perovskite powders using PXRD measurements. Impedance measurements under difference gas environments were utilized to analyze their electrical conductivity. Chronoamperometric measurements were utilized to evaluate the E-OCM properties of these catalysts. Our results demonstrate doped barium niobates as a promising candidate for stable operation in high temperature electrochemical applications. References H. Denoyer, A. Benavidez, A. Brearley, K. P. Ramaiyan*, F. H. Garzon*, “Electrochemical oxidative coupling of methane: deciphering the exceptional properties of BaMg0.33Nb0.67-xFexO3-δ for enhanced electrocatalysis and durable operation” Under revision J. Electrochem. Soc, (December 2023).H. Denoyer, A. Benavidez, F. H. Garzon*, K. P. Ramaiyan*, “Highly Stable Doped Barium Niobate Based Electrocatalysts for Effective Electrochemical Coupling of Methane to Ethylene” Adv. Mater. Interfaces, 2022, 2200796 (1-9). Ramaiyan, L. H. Denoyer, A. Benavidez, F. H. Garzon*, “Role of Dopants in Fine-Tuning the Electrical and Catalytic Properties of Barium Niobate Perovskites Towards High-Temperature Electrolyzer Application” ECS Transactions 2023, 111, 1139.H. Denoyer, A. Benavidez, A. Brearley, K. Ramaiyan, F. H. Garzon*, “Chemical Stability of BaMg0.33Nb0.67-XFexO3-δ in High Temperature Methane Conversion Environments” ECS Transactions, 2023, 111, 587.

Atomically Dispersed Iron‐Copper Dual‐Metal Sites Synergistically Boost Carbonylation of Methane

AbstractThe direct liquid‐phase oxidative carbonylation of methane, utilizing abundant natural gas, offers a mild and straightforward alternative. However, most catalysts proposed for this process suffer from low acetic acid yields due to few active sites and rapid C1 oxygenate generation, impeding their industrial feasibility. Herein, we report a highly efficient 0.1Cu/Fe‐HZSM‐5‐TF (TF denotes template‐free synthesis) catalyst featuring exclusively mononuclear Fe and Cu anchored in the ZSM‐5 channels. Under optimized conditions, the catalyst achieved an unprecedented acetic acid yield of 40.5 mmol gcat−1 h−1 at 50 °C, tripling the previous records of 12.0 mmol gcat−1 h−1. Comprehensive characterization, isotope‐labeled experiments and density functional theory (DFT) calculations reveal that the homogeneous mononuclear Fe sites are responsible for the activation and oxidation of methane, while the neighboring Cu sites play a key role in retarding the oxidation process, promoting C−C coupling for effective acetic acid synthesis. Furthermore, the methyl‐group carbon in acetic acid originates solely from methane, while its carbonyl‐group carbon is derived exclusively from CO, rather than the conversion of other C1 oxygenates. The proposed bimetallic catalyst design not only overcomes the limitations of current catalysts but also generalizes the oxidative carbonylation of other alkanes, demonstrating promising advancements in sustainable chemical synthesis.

Effect of Li-Na sorbent phase in Ru/Al2O3 dual function materials for the Integrated CO2 capture and methanation

The design of highly performing dual functional materials (DFMs) is the key to the successful deployment of Integrated Carbon Capture and Methanation (ICCM) process that combines both steps into a single unit operation with high energy efficiency. Intending to enhance the CO2 working capacity while preserving high catalytic methanation activity, herein we set out to investigate the effect of combining Li and Na as the CO2 sorbent phase in Ru/Al2O3 DFMs. ICCM tests in a fixed bed reactor operated with alternate feeds indicated that combining Na with Li (or partially substituting it for Li) effectively increases the CO2 capture capacity and methane production of the DFM. However, this generally requires higher temperatures to properly activate the catalytic methanation and to regenerate the DFM compared to a pure Li-based formulation. Transient operando DRIFTS and catalytic kinetic experiments provided mechanistic insights into the activation/inhibition of different reaction paths on those DFMs containing only Li and/or Na.

The effect of calcination gas atmosphere on the structural organization of Ru/Ce0.75Zr0.25O2 catalysts for CO2 methanation

In this study, supported 5 wt% Ru/Ce0.75Zr0.25O2 catalysts were prepared by sorption-hydrolytic deposition technique, followed by calcination under different conditions, namely in reductive H2/N2 (Ru/CeZr_red) and oxidative air (Ru/CeZr_ox) atmospheres. A thorough characterization of the catalysts was carried out by XRD, CO chemisorption, HRTEM, Raman spectroscopy, XPS, and in situ DRIFTS. The findings showed that the choice of the calcination atmosphere has a great impact on the structural organization of the catalyst. Calcination in air results in the formation of the larger crystalline RuO2 particles, which tend to enlarge upon reduction under CO2 methanation conditions. The reductive treatment results in a much higher dispersion of supported Ru species and a more pronounced metal-support interaction (MSI). In the Ru/CeZr_red catalyst, a large amount of atomic scale Ru species was detected along with metallic Ru nanoparticles and clusters. However, it was found that the superiority of the Ru/CeZr_red catalyst in the Ru dispersion did not result in the significant advantage in the CO2 methanation activity. In situ DRIFTS results suggested that the MSI affects the ability of Ru species to adsorb and activate reagent molecules, in particular, enhances the adsorption strength of the intermediate CO species.

Integrated Study on Methane Activation: Exploring Main Group Frustrated Lewis Pairs through Density Functional Theory, Machine Learning, and Machine-Learned Force Fields.

Frustrated Lewis Pairs (FLP) are an important advance in metal-free catalysis due to their ability to activate a variety of small molecules. Many studies have focused on a very limited sample of Lewis acids and bases. Herein, we disclose an automated exploration algorithm using density functional methods, artificial neural networks (ANNs), and a molecule builder that incentivizes the exploration of favorable FLP space for the activation of methane via two mechanisms: deprotonation and hydride abstraction. The exploration algorithm creates FLPs with different Lewis acids (LA), Lewis bases (LB), and their substituents (LA/LB), which proved successful in quickly converging in the favorable chemical space, suggesting chemically sound structures, and generating thousands of potential candidates for methane activating FLPs. By modeling thousands of reactions, an FLP database of methane activation was created, allowing one to data mine properties, e.g., adduct bond length, highest occupied molecular orbital-lowest-unoccupied molecular orbital (HOMO-LUMO) gap, global electrophilicity index, favored Lewis acids/bases/substituents, and substituent steric volume. These properties not only successfully narrow the FLP chemical space but also provide meaningful insight into the chemical nature of competent methane activators. The machine learning discovery strategy disclosed here is general enough to be applicable to many chemical optimization tasks. This study also investigates the efficacy of a Machine-Learned Force Field (MLFF) in predicting the formation energies of Frustrated Lewis Pairs (FLPs). Our model, exhibiting a test error of ±10 kcal/mol, highlighted impressive computational efficiency by enabling the calculation of all possible FLP permutations within our chemical space. The MLFF demonstrated proficiency in predicting energies, providing a significant acceleration compared to quantum mechanics methods. However, challenges emerged in accurately capturing forces, necessitating recourse to classical force fields for reliable structure relaxation. The present study sheds light on the MLFF's potential as a tool for rapid energy predictions, emphasizing the need for further refinement to enhance its accuracy, particularly in force predictions, to expand its utility in chemical simulations.

Bimetallic Single Atom/Nanoparticle Ensemble for Efficient Photochemical Cascade Synthesis of Ethylene from Methane

AbstractLight‐driven photoredox catalysis presents a promising approach for the activation and conversion of methane (CH4) into high value‐added chemicals under ambient conditions. However, the high C−H bond dissociation energy of CH4 and the absence of well‐defined C−H activation sites on catalysts significantly limit the highly efficient conversion of CH4 toward multicarbon (C2+) hydrocarbons, particularly ethylene (C2H4). Herein, we demonstrate a bimetallic design of Ag nanoparticles (NPs) and Pd single atoms (SAs) on ZnO for the cascade conversion of CH4 into C2H4 with the highest production rate compared with previous works. Mechanistic studies reveal that the synergistic effect of Ag NPs and Pd SAs, upon effecting key bond‐breaking and ‐forming events, lowers the overall energy barrier of the activation process of both CH4 and the resulting C2H6, constituting a truly synergistic catalytic system to facilitate the C2H4 generation. This work offers a novel perspective on the advancement of photocatalytic directional CH4 conversion toward high value‐added C2+ hydrocarbons through the subtle design of bimetallic cascade catalyst strategy.

Bimetallic Single Atom/Nanoparticle Ensemble for Efficient Photochemical Cascade Synthesis of Ethylene from Methane.

Light-driven photoredox catalysis presents a promising approach for the activation and conversion of methane (CH4) into high value-added chemicals under ambient conditions. However, the high C-H bond dissociation energy of CH4 and the absence of well-defined C-H activation sites on catalysts significantly limit the highly efficient conversion of CH4 toward multicarbon (C2+) hydrocarbons, particularly ethylene (C2H4). Herein, we demonstrate a bimetallic design of Ag nanoparticles (NPs) and Pd single atoms (SAs) on ZnO for the cascade conversion of CH4 into C2H4 with the highest production rate compared with previous works. Mechanistic studies reveal that the synergistic effect of Ag NPs and Pd SAs, upon effecting key bond-breaking and -forming events, lowers the overall energy barrier of the activation process of both CH4 and the resulting C2H6, constituting a truly synergistic catalytic system to facilitate the C2H4 generation. This work offers a novel perspective on the advancement of photocatalytic directional CH4 conversion toward high value-added C2+ hydrocarbons through the subtle design of bimetallic cascade catalyst strategy.

Regenerable Ni-Au/La2O3 catalysts for dry reforming of methane

Dry reforming of methane (DRM) has been proposed as an effective way of converting CO2 and CH4, two major greenhouse gases, into synthesis gas (H2 + CO), a ubiquitous feedstock for various chemical industries. Ni-based bimetallic catalysts have been widely explored as an attractive route to improve DRM activity, selectivity, and stability. In this study, we synthesized LaNiO3 (LNO) and 1% Au-doped LaNiO3 (Au/LaNiO3 – A/LNO) using the modified sol-gel method. The material properties were studied using various characterization techniques, including XRD, BET, H2-TPR, TPO, STEM-HAADF with EDX mapping, Raman spectroscopy, and XPS. Ni nanoparticles with sizes ranging from 10 to 70 nm were successfully exsolved from the perovskite material after reduction at 800 °C. DRM activity of the reduced catalysts was probed from 400 to 800 °C. The Au-doped LNO exhibits significant enhancement in DRM activity at a lower temperature range (600–700 °C) compared to LNO. The A/LNO was also shown to retard coke formation and improve the selectivity for DRM. A computational study using density functional theory (DFT) reveals that the Ni3Au sites have a lower methane activation barrier due to the formation of NiAu alloy and the synergetic effect between Ni and Au.

Tandem dual-heterojunctions in Au/ZnGa2O4/ZnO for promoted photocatalytic nonoxidative coupling of methane

Rational design of photocatalysts to enhance the photocatalytic non-oxidative coupling of methane (NOCM) activity remains a great challenge. Herein, a composite photocatalyst Au/ZnGa2O4/ZnO (Au/ZGO/ZnO) with tandem heterojunctions was prepared by a hydrothermal method and impregnation reduction method for the photocatalytic NOCM reaction. The tandem heterojunctions effectively promote the migration of photogenerated electrons of ZGO and ZnO through the ZGO/ZnO S-scheme heterojunction and the Au/ZnO Schottky junction to Au active sites, significantly enhancing the photocatalytic NOCM activity. The C2H6 and H2 yields of the optimized photocatalyst Au0.75/ZGO/ZnO-15 are 12.2 and 10.3 μmol·g−1, with a ratio close to 1:1, within 4 hours under simulated solar light irradiation. The C2H6 production of Au0.75/ZGO/ZnO-15 is 4.5 and 21.4 times higher than that of Au0.75/ZnO and Au0.75/ZGO, respectively. This work provides a strategy for enhancing the photocatalytic NOCM of semiconductor photocatalysts and marks an important step toward the design and synthesis of dual-heterostructures composite photocatalysts.

In-situ exploration of divergent methane coupling pathways in dry and aqueous environments on silver and palladium heteropolyacid-titania photocatalysts

Methane, abundant but inert, contributes significantly to greenhouse gases, primarily through combustion, which emits vast amounts of CO2. Photocatalytic methane coupling at ambient temperature offers a method to convert it into ethane, a more versatile hydrocarbon. This study examines selective methane-to-ethane coupling using heterogeneous photocatalysts comprising silver and palladium salts dispersed on titania. Through a combination of ex-situ and in-situ techniques along with DFT simulations, distinct mechanisms and active phases in these catalysts are revealed. In silver catalysts, dispersed cationic Ag+ species are crucial for methane activation, while in palladium catalysts, palladium primarily exists in metallic form during coupling. Water strongly enhances coupling rates with both catalysts. DFT modeling identified methane adsorption sites, suggesting methane activation via •OH radicals, experimentally supported by EPR under in-situ conditions.

Experimental Reactivity of (MoO3)NO- (N = 1-21) Cluster Anions with C1-C4 Alkanes: A Simple Model to Predict the Reactivity with Methane.

Metal oxide clusters with atomic oxygen radical anions are important model systems to study the mechanisms of activating and transforming very stable alkane molecules under ambient conditions. It is extremely challenging to characterize the activation and conversion of methane, the most stable alkane molecule, by metal oxide cluster anions due to the low reactivity of the anionic species. In this study, using a ship-lock type reactor that could be run at relatively high pressure conditions to provide a high number of collisions in ion-molecule reactions, the rate constants of the reactions between (MoO3)NO- (N = 1-21) cluster anions and the light alkanes (C1-C4) were measured under thermal collision conditions. The relationships among the reaction rates of different alkanes were obtained to establish a model to predict the low rate constants with methane from the high rate constants with C2-C4 alkanes. The model was tested by using available experimental results in literature. This study provides a new method to estimate the relatively low reactivity of atomic oxygen radical anions with methane on metal oxide clusters.

NiAlFe catalysts based on hydrotalcite-like precursors for low temperature CO2 methanation: Electronic effects among components and intrinsic activity of Ni site

NiAlFe catalysts based on the hydrotalcite-like precursor were prepared and the catalysts exhibit excellent low temperature catalytic activity for CO2 methanation. CO2 conversion over the NiAlFe0.3 catalyst is nearly 80 % under the condition of T = 225 °C, P = 1 atm, and WHSV = 24000 mL/(g∙h). With the increase in the substitution of Fe for Al, the hydrotalcite-like structure of the catalyst precursor is gradually destroyed; the reducibility of NiO component is promoted; the amount of surface basic site decreases. There is a strong electronic push–pull effect between NiO and Al2O3 in the calcined catalyst, and the electronic effect is weakened with the introduction of Fe because Fe3+ shares the responsibility of Ni2+ for donating electron to Al3+. In the reduced catalysts, the Ni3Fe alloy was formed and the electrons are pushed from Ni0 to Fe0. With the increase in the Fe amount, the number of surface Ni atom decreases, while the intrinsic activity of Ni active site enhances. The number and intrinsic activity of Ni site are the main two factors determining the CO2 methanation activity at low temperature. Moreover, the substitution of Al3+ by Fe3+ is beneficial for the stability of the catalysts by inhibiting the agglomeration of catalyst particle.

Enzymatic Activation and Continuous Electrochemical Production of Methane from Dilute CO2 Sources with a Self-Healing Capsule.

Converting dilute CO2 source into value-added chemicals and fuels is a promising route to reduce fossil fuel consumption and greenhouse gas emission, but integrating electrocatalysis with CO2 capture still faced marked challenges. Herein, we show that a self-healing metal-organic macrocycle functionalized as an electrochemical catalyst to selectively produce methane from flue gas and air with the lowest applied potential so far (0.06 V vs reversible hydrogen electrode, RHE) through an enzymatic activation fashion. The capsule emulates the enzyme' pocket to abstract one in situ-formed CO2-adduct molecule with the commercial amino alcohols, forming an easy-to-reduce substrate-involving clathrate to combine the CO2 capture with electroreduction for a thorough CO2 reduction. We find that the self-healing system exhibited enzymatic kinetics for the first time with the Michaelis-Menten mechanism in the electrochemical reduction of CO2 and maintained a methane Faraday efficiency (FE) of 74.24% with a selectivity of over 99% for continuous operation over 200 h. A consecutive working lab at 50 mA·cm-2, in an eleven-for-one (10 h working and 1 h healing) electrolysis manner, gives a methane turnover number (TON) of more than 10,000 within 100 h. The integrated electrolysis with CO2 capture facilitates the thorough reduction of flue gas (ca. 13.0% of CO2) and first time of air (ca. 400 ppm of CO2 to 42.7 mL CH4 from 1.0 m3 air). The new self-healing strategy of molecular electrocatalyst with an enzymatic activation manner and anodic shifting of the applied potentials provided a departure from the existing electrochemical catalytic techniques.

Enhanced low-temperature activity of CO2 methanation over Ni/CeO2 catalyst: Influence of preparation methods

Enhanced low-temperature activity of CO2 methanation over Ni/CeO2 catalyst: Influence of preparation methods

Directed Evolution of Protein-Based Sensors for Anaerobic Biological Activation of Methane.

Microbial alkane degradation pathways provide biological routes for converting these hydrocarbons into higher-value products. We recently reported the functional expression of a methyl-alkylsuccinate synthase (Mas) system in Escherichia coli, allowing for the heterologous anaerobic activation of short-chain alkanes. However, the enzymatic activation of methane via natural or engineered alkylsuccinate synthases has yet to be reported. To address this, we employed high-throughput screening to engineer the itaconate (IA)-responsive regulatory protein ItcR (WT-ItcR) from Yersinia pseudotuberculosis to instead respond to methylsuccinate (MS, the product of methane addition to fumarate), resulting in genetically encoded biosensors for MS. Here, we describe ItcR variants that, when regulating fluorescent protein expression in E. coli, show increased sensitivity, improved overall response, and enhanced specificity toward exogenously added MS relative to the wild-type repressor. Structural modeling and analysis of the ItcR ligand binding pocket provide insights into the altered molecular recognition. In addition to serving as biosensors for screening alkylsuccinate synthases capable of methane activation, MS-responsive ItcR variants also establish a framework for the directed evolution of other molecular reporters, targeting longer-chain alkylsuccinate products or other succinate derivatives.

Atomically Dispersed Iron-Copper Dual-Metal Sites Synergistically Boost Carbonylation of Methane.

The direct liquid-phase oxidative carbonylation of methane, utilizing abundant natural gas, offers a mild and straightforward alternative. However, most catalysts proposed for this process suffer from low acetic acid yields due to few active sites and rapid C1 oxygenate generation, impeding their industrial feasibility. Herein, we report a highly efficient 0.1Cu/Fe-HZSM-5-TF (TF denotes template-free synthesis) catalyst featuring exclusively mononuclear Fe and Cu anchored in the ZSM-5 channels. Under optimized conditions, the catalyst achieved an unprecedented acetic acid yield of 40.5 mmol gcat -1 h-1 at 50 °C, tripling the previous records of 12.0 mmol gcat -1 h-1. Comprehensive characterization, isotope-labeled experiments and density functional theory (DFT) calculations reveal that the homogeneous mononuclear Fe sites are responsible for the activation and oxidation of methane, while the neighboring Cu sites play a key role in retarding the oxidation process, promoting C-C coupling for effective acetic acid synthesis. Furthermore, the methyl-group carbon in acetic acid originates solely from methane, while its carbonyl-group carbon is derived exclusively from CO, rather than the conversion of other C1 oxygenates. The proposed bimetallic catalyst design not only overcomes the limitations of current catalysts but also generalizes the oxidative carbonylation of other alkanes, demonstrating promising advancements in sustainable chemical synthesis.

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.

Single-atom catalysts on ceria substrates: Exploring cluster and surface effects on methane activation

The activation of methane (CH4) is a critical step in its conversion to high value products, which require the use of catalysts due to the higher stability of the C-H bond. In this study, we combined density functional theory calculations and the unit bond index-quadratic exponential potential approximation to investigate the first CH4 dehydrogenation in the TM/(CeO2)10 system, where TM represents a single transition-metal specie (Fe, Co, Ni, Cu) supported in the compact (CeO2)10 cluster. In addition, substrate size effects will be addressed by comparing our results with previous surface science studies by our group. We found a direct correlation between the magnitude of the TM adsorption energy on the CeO2 substrates (hollow and bridge sites) and the magnitude of charge transfer from the TM adatoms to the substrates, while CH4 has a weaker physisorption binding energy to the ceria cluster and the surface substrates. Molecular fragments derived from CH4 (CH3 and H) bind through chemisorption interactions to the TM and O sites, respectively. The estimated energy barrier for the cleavage of the C-H bond is lower when the reaction occurs in the cluster rather than on the surface. The molecular fragments exhibited better stabilization on cluster substrates, resulting in an exergonic dehydrogenation reaction; whereas, for surfaces, the reaction was endergonic. Among the selected TM species, Fe and Co adatoms are promising candidates for the first CH4 dehydrogenation on CeO2 substrates, which can be explained by the magnitude of the metal-support interaction and stabilization of the CH3 specie, which results in the lowest barriers among the evaluated transition metals.

Boosting CO2 methanation activity by tuning Ni crystal plane and oxygen vacancy in Ni/CeO2 catalyst

It is challenging to boost CO2 methanation activity over Ni-based catalysts, and new findings that decipher the relationship between the essential laws of catalysts and their catalytic performance will be transformative. Herein, three kinds of Ni/CeO2 catalysts with different Ni crystal facets and oxygen vacancy are developed by varied preparation methods, which show significant differences in catalytic performance for CO2 methanation, and these catalysts are taken as a case study for the investigation of the crystal plane and oxygen vacancy effects in CO2 methanation as well. It is observed that the activity of Ni/CeO2-SG catalyst with abundant oxygen vacancies and principally exposed Ni (111) crystal facet is highly active for CO2 methanation reaction, which is 1–3 times than that of Ni/CeO2 catalysts with poor oxygen vacancy and/or Ni (111) crystal face. In addition, the former one cannot suffer from deactivation even in operation 130 h at 300 °C. Both structural investigations and catalytic evaluations indicate that the adsorption and activating ability of H2 and CO2 can be controlled by adjusting the Ni-exposed crystal face and oxygen vacancy concentration, respectively. It is also verified that the synergistic contribution of the Ni crystal facets and the oxygen vacancy play a crucial role in boosting the catalytic activity of the Ni/CeO2 catalysts by affecting the adsorption and activation of reactants. The Ni/CeO2 catalysts exposed Ni (111) crystal plane facilitate the catalytic activity by strengthing the H2 adsorption/dissociation capacity, while the abundantly available oxygen vacancies maximize the activity via acting as CO2 activation sites. Furthermore, the in situ DRIFT experiments reveal that the direct formate hydrogenation pathway is involved in the CO2 methanation process over the Ni/CeO2-SG catalyst.