Methane Activation Research Articles

Photocatalytic methane conversion to high-value chemicals

Methane has a substantial and widespread reserve on Earth. As a very abundant carbon and hydrogen source, as well as an energy vector, upgrading methane to higher-value fuels and chemicals (carbon oxygenates, C₂₊ hydrocarbons, et al.) is a promising technology in the supply of energy and chemicals by a low-carbon process. Due to the stable and inert nature of methane, activating and converting the molecule is extremely challenging. Currently, commercial methane conversion technology operates at high temperatures and/or high pressure, suffering from intense energy consumption and high capital investment. Photocatalysis, using photons as the only energy input, can operate at mild temperatures and under atmospheric pressure, which is a promising and green technology for methane conversion to highvalue products. In this review, fundamental understandings of photocatalytic methane activation and product yield involving different oxidants are discussed. Then recent advances in photocatalysts for methane conversion to hydrocarbons and oxygenates are detailed, including the relevant reaction mechanism and reaction pathways. Finally, the challenges and perspectives for photocatalytic methane conversion will be discussed based on the current progress and fundamental understanding.

Structural design and particle size examination on NiO-CeO2 catalysts supported on 3D-printed carbon monoliths for CO2 methanation

3D-printed high-surface carbon monoliths have been fabricated and tested as catalyst supports of CO2 methanation active phases (NiO-CeO2, 12 wt% Ni). The carbon carriers show a developed microporosity and good adherence to the catalytic phases of NiO-CeO2, showing great stability and cyclability. Two monolith designs were used: a conventional parallel-channeled structure (honeycomb) and a complex 3D network of non-linear channels built upon interconnected circular sections (circles), where flow turbulences along the reactant gas path are spurred. The effect of the active phases particle size on the catalyst distribution and the overall performance has been assessed by comparing NiO-CeO2 nanoparticles of 7 nm average (Np), with a reference counterpart of uncontrolled structure (Ref). The improved radial gases diffusion in the circles monolith design is confirmed, and nanoparticles show enhanced CO2 methanation activity than the uncontrolled-size active phase at low temperatures (< 300 ºC). On the contrary, the Ref catalysts achieve higher CH4 production at higher temperatures, where the reaction kinetics is controlled by mass transfer limitations (T > 300 ºC). SEM and Hg porosimetry evidence that nanoparticles are deposited at deeper penetration through the narrow micropores of the carbon matrix of the monolithic supports, which tend to accumulate on the channels surface remaining more accessible to the reactant molecules. Altogether, this study examines the impact of the channel tortuosity and the active phase sizing on the CO2 methanation activity, serving as ground knowledge for the further rational and scalable fabrication of carbon monolith for catalytic applications.

Uncovering the active sites of single atom-doped rutile oxides during methane activation by data-driven approach

Uncovering the active sites of single atom-doped rutile oxides during methane activation by data-driven approach

Magnetic Exchange Coupling in Zeolite Copper Dimers and Its Contribution to Methane Activation.

The highly reactive binuclear [Cu2O]2+ active site in copper zeolites activates the inert C-H bond of methane at low temperatures, offering a potential solution to reduce methane flaring and mitigate atmospheric methane levels. While substantial progress has been made in understanding the activation of methane by this core, one critical aspect, the active site's spin, has remained undetermined. In this study, we use variable-temperature, variable-field magnetic circular dichroism spectroscopy to define the ground state spin of the [Cu2O]2+ active sites in Cu-CHA and Cu-MFI. This novel approach allows for site-selective determination of the magnetic exchange coupling between the two copper centers of specific [Cu2O]2+ cores in a heterogeneous mixture, circumventing the drawbacks of bulk magnetic techniques. These experimental findings are coupled to density functional theory calculations to elucidate magnetostructural correlations in copper zeolites that are different from those of homogeneous binuclear Cu(II) complexes. The different spin states for the [Cu2O]2+ cores have different reactivities governed by how methane approaches the active site. This introduces a new understanding of zeolite topological control on active site reactivity.

Activation of Methane in Vapor and Molten Sodium Catalyst

Activation of Methane in Vapor and Molten Sodium Catalyst

Thermal Reactions of NiAl3O6+ and Al4O6+ with Methane: Reactivity Enhancement by Doping.

Investigation of the reactivity of heteronuclear metal oxide clusters is an important way to uncover the molecular-level mechanisms of the doping effect. Herein, we performed a comparative study on the reactions of CH4 with NiAl3O6+ and Al4O6+ cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl3O6+ and Al4O6+ could bring about hydrogen atom abstraction reaction to generate CH3• radical; however, only NiAl3O6+ has the potential to stabilize [CH3] moiety and then transform [CH3] to CH2O. Density functional theory calculations demonstrate that the terminal oxygen radicals (Ot-•) bound to Al act as the reactive sites for the two clusters to activate the first C-H bond. Although the Ni atom cannot directly participate in methane activation, it can manipulate the electronic environment of the surrounding bridging oxygen atoms (Ob) and enable such Ob to function as an electron reservoir to help Ot-• oxidize CH4 to [H-O-CH3]. The facile reduction of Ni3+ to Ni+ also facilitates the subsequent step of activating the second C-H bond by the bridging "lattice oxygen" (Ob2-), finally enabling the oxidation of methane into formaldehyde. The important role of the dopant Ni played in improving the product selectivity of CH2O for methane conversion discovered in this study allows us to have a possible molecule-level understanding of the excellent performance of the catalysts doping with nickel.

Mapping the Catalytic-Space for the Reactivity of Metal-free Boron Nitride with O2 for H2O-Mediated Conversion of Methane to HCHO and CO.

Transition-metal based catalysts have been widely employed to catalyze partial oxidation of light alkanes. Recently, metal-free hexagonal-boron nitride (h-BN) has emerged as a promising catalyst for the oxidation of CH4 to HCHO and CO; however, the intricate catalytic surface of h-BN at molecular and electronic levels remains inadequately understood. Key questions include how electron-deficient boron atoms in h-BN reduce O2, and whether the partial oxidation of methane over h-BN exhibits similarities to traditional transition-metal catalysts. In our study, we computationally-mapped in-detail the surface catalytic-space of h-BN for methane oxidation. We considered different structures of h-BN and show that these structures contain numerous sites for O2 binding and therefore various routes for methane oxidation are possible. The activation barriers for methane oxidation via various paths varies from ~83 to ~123 kcal mol-1. To comprehend the differences in activation barriers, we employed geometrical, orbital and distortion/interaction analysis (DIA). Orbital analysis reveals that methane activation over h-BN in presence of dioxygen follows a standard hydrogen atom transfer mechanism. It is also shown that water plays an intriguing role in reducing the barrier for HCHO and CO formation by acting as a bridge.

Charge transfer boosts up methane adsorption and activation on three-coordinated metal sites

The activation and conversion of CH4 has attracted great interest due to its mitigation impact on the greenhouse gas and contribution to the high value-added chemicals. However, the activation of CH4 is the rate-limiting step due to the high symmetry of the molecule and its chemical inertness. In this study, we constructed active sites with specific geometry for methane activation based on the Nickel and other post-transition metals, where the interactions were significantly enhanced by 0.2∼0.6 eV varying with different VIII group metal types. With advanced electronic structure analysis methods, the origin of this interaction was unraveled, that is the Coulomb attraction induced by charge transfer from hydrogen of methane to three-coordinated metal Ni sites, rather than the covalent bonding or dispersion interaction. This enhancement not only increases the collision possibility by several orders of magnitude, but also lowers the barrier of CH bond breaking. These findings provide deep insight on the methane activation on metal catalysts and shed the light to design more efficient methane conversion catalysts.

Photocatalytic oxidation of methane to methanol over zinc titanate supported silver catalysts

The direct activation of methane under mild condition to achieve highly selective of oxygenates is a challenging project. In this study, a well dispersed silver supported ZnTiO3 catalyst was prepared to achieve selective preparation of methanol from methane and water under mild condition. X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy characterizations demonstrate that silver species are uniformly dispersed on ZnTiO3 surface in the form of metallic silver nanoparticles. The photoelectric characterizations reveal that the addition of silver species enhances light absorption and promotes charge separation of the catalysts. Under the reaction conditions of 50 °C and 3 MPa, the methanol is obtained as the only liquid product over the designed Ag/ZnTiO3 catalyst under light irradiation. In this photocatalytic process, the holes generated by ZnTiO3 activate water to produce intermediate ·OH, which further reacts with methane to synthesize methanol. The silver species as co-catalysts extend the light absorption range of ZnTiO3 as well as promote charge separation.

Effects of Ru particle size over TiO2 on the catalytic performance of CO2 hydrogenation

The influence of metal particle size over heterogeneous catalysts on the catalytic performance has always been an interesting subject due to its significance from both fundamental and practical perspectives. In this work, we regulated the synthesis of TiO2 by the hydrolysis of different titanium precursors, and as-prepared TiO2 supports were employed to load Ru with impregnation method. Interestingly, we synthesized three Ru-TiO2 catalysts with different Ru particle sizes of 1.1, 2.9 and 5.2 nm. The Ru-TiO2 catalysts were next tested for CO2 hydrogenation reaction, the results show that the medium Ru particles on TiO2 exhibited the best CO2 methanation activity. Based on the characterizations, we observed that the medium Ru particles exhibited the best redox properties compared with small and large Ru particles, which was beneficial to the H2 dissociation; in addition, the medium Ru particles exhibited a moderate ability to activate CO2, resulting in the most favorable formation and decomposition of bidentate bicarbonate. Consequently, the Ru-TiO2 catalyst with medium Ru particles showed the highest activity for CO2 methanation. This study provides a new method to prepare Ru-TiO2 catalysts with different Ru sizes and also presents an understanding of Ru particle size effects on CO2 hydrogenation.

Research Progress and Outlook on Photocatalytic Conversion of Methane to Methanol

AbstractMethane, a main component of natural gas, shale gas, and combustible ice, is widely distributed in nature. The direct conversion and efficient utilization of methane have become crucial topics within the scientific community. However, two main challenges persist in this field: methane activation and overoxidation inhibition. Recently, research on this subject has reached its pinnacle stage. Herein, we present some works addressing these concerns, particularly employing photocatalytic approaches, and with a specific focus on strategies aimed at enhancing the selectivity towards methanol as the desired product. Finally, we discuss the existing challenges and propose potential solutions for advancing direct methane‐to‐methanol conversion technology.

DFT study of electrostatic effects upon methane activation by early, high-valent multiply bonded complexes

The research presented here focuses on the electrostatic effect of varying alkali metal cations (Li+ through Cs+) in the outer coordination sphere of model methane activating complexes with metal-nitrogen multiple bonds. Lighter metal alkali metal cations yield reduced methane activation barriers (ΔG‡), with the lowest barrier computed for the Li+ salt; moreover, all modeled salts have lower ΔG‡ than the corresponding anion in polar solvents. Via variation of the polarity of continuum solvents, this research also indicates that electrostatic effects are likely to be manifested most significantly in the low polarity, hydrophobic solvents typically used for inorganic and organometallic complexes capable of methane activation. Taken together, the results of these DFT simulations suggest that electrostatic effects have the potential to significantly decrease methane activation barriers.

Screening Cu-Zeolites for Methane Activation Using Curriculum-Based Training.

Machine learning (ML), when used synergistically with atomistic simulations, has recently emerged as a powerful tool for accelerated catalyst discovery. However, the application of these techniques has been limited by the lack of interpretable and transferable ML models. In this work, we propose a curriculum-based training (CBT) philosophy to systematically develop reactive machine learning potentials (rMLPs) for high-throughput screening of zeolite catalysts. Our CBT approach combines several different types of calculations to gradually teach the ML model about the relevant regions of the reactive potential energy surface. The resulting rMLPs are accurate, transferable, and interpretable. We further demonstrate the effectiveness of this approach by exhaustively screening thousands of [CuOCu]2+ sites across hundreds of Cu-zeolites for the industrially relevant methane activation reaction. Specifically, this large-scale analysis of the entire International Zeolite Association (IZA) database identifies a set of previously unexplored zeolites (i.e., MEI, ATN, EWO, and CAS) that show the highest ensemble-averaged rates for [CuOCu]2+-catalyzed methane activation. We believe that this CBT philosophy can be generally applied to other zeolite-catalyzed reactions and, subsequently, to other types of heterogeneous catalysts. Thus, this represents an important step toward overcoming the long-standing barriers within the computational heterogeneous catalysis community.

Alloying and oxidation of PdAu thin films

The relation between catalytic activity and the presence of oxides on the catalyst’s surface has proven very complex, especially in the case of methane oxidation over Pd. While the metallic Pd surface and a thin, but at least two atomic layers thick, oxide film has been found catalytically active, a single-layer surface oxide or a too thick oxide film are both low-active. Unfortunately, under reaction conditions, the oxide tends to grow thick and deactivate. The reason for this deactivation is believed to be exposure of the PdO(100) surface, which does not have any suitable active sites for the methane adsorption and activation, in contrast to PdO(101), which is exposed for thin films. In an attempt to limit the thickness of the oxide film, and hence stabilise the active PdO orientation, we have investigated the oxidation of thin PdAu films. The effects of different mixtures (25%, 50%, and 75% Au, respectively) and treatments on the oxidation and orientation of the alloy and oxide films were investigated. As intended, PdAu turned out to be significantly more difficult to oxidise compared to pure Pd. This effect was even stronger than expected. Depending on the sample temperature and the amount of Au present in the alloy, the orientation of the oxide is affected. At lower temperatures and Au concentration, the desired [301] oxide orientation (corresponding to the (101) planes being parallel to the sample surface) is favoured, while higher Au concentration favours the low-active [100] oxide orientation. Thus, PdAu might be a good candidate for methane oxidation if the Au concentration is low, probably below 25%. The larger lattice constant of Au compared to Pd might also affect the oxide orientation, so another choice of alloying material, e.g. PdPt, may also work to stabilise PdO[301]. Independent of the alloy composition, the sample temperature during oxide growth should be optimised in order to achieve an active oxide film.

Activation and Transformation of Methane on Boron-Doped Cobalt Oxide Cluster Cations CoBO2.

The cleavage of inert C-H bonds in methane at room temperature and the subsequent conversion into value-added products are quite challenging. Herein, the reactivity of boron-doped cobalt oxide cluster cations CoBO2+ toward methane under thermal collision conditions was studied by mass spectrometry experiments and quantum-chemical calculations. In this reaction, one H atom and the CH3 unit of methane were transformed separately to generate the product metaboric acid (HBO2) and one CoCH3+ ion, respectively. Theoretical calculations strongly suggest that a catalytic cycle can be completed by the recovery of CoBO2+ through the reaction of CoCH3+ with sodium perborate (NaBO3), and this reaction generates sodium methoxide (CH3ONa) as the other value-added product. This study shows that boron-doped cobalt oxide species are highly reactive to facilitate thermal methane transformation and may open a way to develop more effective approaches for methane (CH4) activation and conversion under mild conditions.

Lattice capacity-dependent activity for CO2 methanation: crafting Ni/CeO2 catalysts with outstanding performance at low temperatures.

In the pursuit of understanding lattice capacity threshold effects of oxide solid solutions for their supported Ni catalysts, a series of Ca2+-doped CeO2 solid solutions with 10 wt% Ni loading (named Ni/CaxCe1-xOy) was prepared using a sol-gel method and used for CO2 methanation. The lattice capacity of Ca2+ in the lattice of CeO2 was firstly determined by the XRD extrapolation method, corresponding to a Ca/(Ca + Ce) molar ratio of 11%. When the amount of Ca2+ in the CaxCe1-xOy supports was close to the CeO2 lattice capacity for Ca2+ incorporation, the obtained Ni/Ca0.1Ce0.9Oy catalyst possessed the optimal intrinsic activity for CO2 methanation. XPS, Raman spectroscopy, EPR and CO2-TPD analyses revealed the largest amount of highly active moderate-strength alkaline centers generated by oxygen vacancies. The catalytic reaction mechanisms were revealed using in situ IR analysis. The results clearly demonstrated that the structure and reactivity of the Ni/CaxCe1-xOy catalyst exhibited the lattice capacity threshold effect. The findings offer a new venue for developing highly efficient oxide-supported Ni catalysts for low-temperature CO2 methanation reaction and enabling efficient catalyst screening.

Unraveling the effect of particle size of active metals in Ni/MgO on methane activation and carbon growth mechanism.

For dry reforming of methane, the active metal particle size of the catalyst has a significant effect on both the reaction activity and the resistance to carbon deposition. In this study, nickel particles of different sizes (Ni13, Ni25, and Ni37) supported on the MgO(100) slab are used to study the mechanism of CH4 activation and carbon growth based on DFT theoretical calculations. According to the results, the energy of adsorption for reaction intermediates changes depending on the size of the active metal. The adsorption process of CH3, CH2, CH and C on Ni25/MgO has a maximum exothermic value. Furthermore, the energy barriers of CH4 four-step dehydrogenation are lowest on Ni25/MgO during the CH4 activation process. The growth process of carbon deposition on the catalysts is also investigated in this work. The results indicate that the growth of carbon from C5 to C6 is difficult to proceed on Ni13/MgO due to size and active site limitation. Additionally, with an increase in particle size of the active metal, the absolute value of growth energy and average carbon binding energy of Cn increase on both Ni25/MgO and Ni37/MgO. It is proved that smaller particle size presents better resistance to carbon deposition. In the studied size range, Ni25/MgO is demonstrated to have greater catalytic activity and better resistance to carbon deposition.

Effect of metal loading sequences in CO2 methanation activity on samarium-doped ceria supported bimetallic catalysts

The La/Ni/SDC catalyst from one pot microwave-heating gives higher methanation activity than La–Ni/SDC and Ni–La/SDC from stepwise heating.

Methane C(sp3)-H bond activation by water microbubbles.

Microbubble-induced oxidation offers an effective approach for activating the C(sp3)-H bond of methane under mild conditions, achieving a methane activation rate of up to 6.7% per hour under optimized parameters. In this study, microbubbles provided an extensive gas-liquid interface that promoted the formation of hydroxyl (OH˙) and hydrogen radicals (H˙), which facilitated the activation of methane, leading to the generation of methyl radicals (CH3˙). These species further participated in free-radical reactions at the interface, resulting in the production of ethane and formic acid. The microbubble system was optimized by adjusting gas-liquid interaction time, water temperature, and bubble size, with the optimal conditions (150 s of water-gas interaction, 15 °C, 50 μm bubble size) yielding a methane conversion rate of 171.5 ppm h-1, an ethane production rate of 23.5 ppm h-1, and a formic acid production rate of 2.3 nM h-1 during 8 h of continuous operation. The stability and efficiency of this process, confirmed through electron spin resonance, high-resolution mass spectrometry, and gas chromatography, suggest that microbubble-based methane activation offers a scalable and energy-efficient pathway for methane utilization.

Ligand effect on Ru-centered species toward methane activation.

Ligands have been known to profoundly affect the chemical transformations of methane, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we demonstrate that the conversion of methane can be regulated by Ru centered cations with a series of ligands (C, CH, CNH, CHCNH). Gas-phase experiments complemented by theoretical dynamic analysis were performed to explore the essences and principles governing the ligand effect. In contrast to the inert Ru+, [RuC]+, and [RuCNH]+ toward CH4, the dehydrogenation dominates the reaction of ligand-regulated systems [RuCH]+/CH4 and [RuCHCNH]+/CH4. In active cases, CH acts as active sites, and regulates the activation of CH4 assisted by the "seemingly inert" CNH ligand.