Low Temperature Methane Activation Research Articles

Low-Temperature Methane Activation Reaction Pathways over Mechanochemically-Generated Ce4+/Cu+ Interfacial Sites.

Methane is a valuable resource and its valorization is an important challenge in heterogeneous catalysis. Here it is shown that CeO2/CuO composite prepared by ball milling activates methane at a temperature as low as 250°C. In contrast to conventionally prepared catalysts, the formation of partial oxidation products such as methanol and formaldehyde is also observed. Through an in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and operando Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS) approach, it can be established that this unusual reactivity can be attributed to the presence of Ce4+/Cu+ interfaces generated through a redox exchange between Ce3+ and Cu2+ atoms facilitated by the mechanical energy supplied during milling. DFT modeling of the electronic properties confirms the existence of a charge transfer mechanism. These results demonstrate the effectiveness and distinctiveness of the mechanical approach in creating unique and resilient interfaces thereby enabling the optimization and refining of CeO2/CuO catalysts in methane activation reactions.

Low temperature activation of methane to hydrogen depending on tailored electron transfer over Ni–Cr composite oxide

Current industrial use of methane depends mainly on high-temperature conversion, resulting in high energy consumption and catalyst sintering inactivation. In this paper, metal oxide catalysts with different Ni/Cr ratios were obtained by the sol-gel method for the methane activation reaction. The structure-property relationships of the catalysts were investigated by XRD, XPS, H2-TPR, TG, N2 adsorption-desorption, and CV, focusing on the interaction between Ni–Cr in the composite oxide catalysts. The results showed that Ni0–NiCrO3/Cr2O3 structure was formed after the Ni–Cr oxides underwent the reduction and calcination process, which provided a structural support for electron transfer between the two metals. The least prone to sintering nickel metal particles and the highest content of high-valent chromium ions were present in Ni1Cr2. XPS confirmed that this counterpart acts as the donor and acceptor for electron transfer in the catalysts, accelerating oxidation-reduction reaction over the catalyst. The catalyst exhibited the most excellent conversion and product selectivity in the methane activation reaction.

Small iridium clusters supported on TiO2 as catalysts for intensifying low-temperature methane activation and reforming

Hydrogen production via low-temperature (<550 °C) steam reforming of methane (SRM) has attracted increasing research attention due to its importance as an alternative, efficient, and environmentally benign energy carrier. The thermodynamic limitation of low-temperature SRM can be eased by process design such as using a membrane reactor or an adsorption-enhanced reforming process, and an active low-temperature SRM catalyst with high stability would be indispensable. Herein, we report a 2 wt% Ir/TiO2 catalyst that exhibits high methane conversion activity in low-temperature SRM with a relatively high turnover frequency (TOF) and a low activation energy in comparison with the literature. Long-term catalyst stability test demonstrates a good resistance to coking and sintering during low-temperature SRM reactions. In situ spectroscopy analyses and density functional theory (DFT) calculations reveal that the small Ir clusters and Ir single atoms (SAs) are both active for methane conversion, while the clusters exhibit higher activity than SAs for low-temperature SRM reaction. Microkinetic simulation based on DFT calculations shows consistent product formation temperatures and the simulation nicely reproduces the product distribution observed in experimental analysis, which confirms the superior SRM activity of the TiO2-supported Ir clusters in this work.

Non-Functionalized Subnanometer Copper Nanoparticles for Low-Temperature Methane Activation

Non-Functionalized Subnanometer Copper Nanoparticles for Low-Temperature Methane Activation

Topology-Determined Structural Genes Enable Data-Driven Discovery and Intelligent Design of Potential Metal Oxides for Inert C-H Bond Activation.

The identification of appropriate structural genes that influence the active-site configuration for a given reaction is critical for discovering potential catalysts with reduced reaction barriers. In this study, we introduce bulk-phase topology-derived tetrahedral descriptors as a means of expressing a catalyst's "material structural genes". We combine this approach with an interpretable machine learning model to accurately and efficiently predict the effective barrier associated with methane C-H bond cleavage across a wide range of metal oxides (MOs). These structural genes enable high-throughput catalyst screening for low-temperature methane activation and ultimately identify 13 candidate catalysts from a pool of 9095 MOs that are recommended for experimental synthesis. The topology-based method that we describe can also be extended to facilitate high-throughput catalyst screening and design for other dehydrogenation reactions.

Co3+–O Bond Elongation Unlocks Co3O4 for Methane Activation under Ambient Conditions

The pursuit of low-temperature methane activation is a paramount challenge for heterogeneous catalysis because of the high stability of the C–H bonds (435 kJ mol–1). Very few catalysts fulfill this through either constructing complex interfaces or using special noble metal oxides. Bond length, which is highly temperature-sensitive, has long been ignored in catalyst studies. Here, we exploit a bond-length elongation strategy to unlock a nonprecious metal oxide for dissociating methane at normal temperature and pressure. Mesoporous bubblelike Co3O4 with elongated Co3+–O bonds, synthesized through a unique “melting–foaming–solidifying” route, is found to activate methane at room temperature to form COx species on the very surface revealed by temperature-programmed reaction spectroscopy experiments. A combination of experimental and theoretical methods suggests that the elongated Co3+–O bonds contribute to the greatly enhanced methane adsorption, in contrast to the weak adsorption of methane on conventional nonprecious metal oxides. Accordingly, the activation energy for the C–H bond cleavage is smaller than that for methane desorption, which finally leads to the low-temperature methane activation. The simultaneous enhancement in intrinsic activity and mass transfer endows mesoporous bubblelike Co3O4 with excellent catalytic activity for methane combustion, where the complete conversion temperature is much below that over the conventional Co3O4 nanocatalysts.

Potential of intrinsic reactivity toward value added products from methane oxidation on RhO2(1 1 0) surface

Catalytic studies focusing on the direct transformation from methane (CH4) to value-added products have attracted considerable attention due to the dramatic increase in natural gas production. This study investigated computationally and experimentally the potentials of intrinsic reactivity for RhO2(110) surface toward value-added products (C2H4 and CH2O) from methane oxidation. The simulation predicted that RhO2(110) is capable of low-temperature methane activation, but it is less active than IrO2(110), which has superior reactivity toward methane, leading to complete oxidation. In addition, the complete oxidation of methane was computationally predicted to be facile, and the results were validated by In-situ DRIFT experiments showing multiple intermediates of methane oxidation. Finally, the potential mechanism for C–C coupling was evaluated to produce the value-added product of C2H4 from CH4 oxidation on RhO2(110) surface. These results showed that the reaction kinetics of C2H4 formation is competitive with the further oxidation, which is the opposite kinetic behavior of IrO2(110). The results suggest that the RhO2(110) surface has a high potential to effectively produce the value-added product because of the low-temperature activation preventing the acceleration of further oxidation and the competitive reactions (production of value-added product vs. further oxidation). With the proper strategy, hindering further oxidation (deactivation of surface oxygens), and the moderate reactivity of RhO2(110), the additional enhancement in the selectivity toward value-added products would be achieved.

Low-Temperature Activation of Methane with Nitric Oxide and Formation of Hydrogen Cyanide over an Alumina-Supported Platinum Catalyst

This study demonstrated methane activation with subsequent conversion to hydrogen cyanide (HCN) at low temperatures using nitric oxide (NO) as the sole oxidant together with an alumina-supported platinum catalyst (Pt/Al2O3). This process afforded HCN even at 300 °C, indicating that C–H bond cleavage, NO dissociation, and simultaneous C–N coupling all occurred at this reduced reaction temperature. The HCN yield increased with increasing temperature, resulting in a 3.2% yield with a selectivity of 49% at 425 °C. This yield was much greater than that obtained from the reaction of CH4 with NH3 and O2, which suggests a reaction mechanism different from the Andrussow process. The HCN production rate was 11.4 mmol g–1 h–1 and the corresponding turnover frequency was 253 h–1, both of which were far superior to those obtained in previous studies at similar reaction temperatures. The Pt catalyst was found to be stable and could continuously produce HCN for at least 100 h. In situ X-ray absorption fine structure analyses suggested that the high resistance of this material to deep oxidation facilitated HCN formation. The difference between X-ray absorption near-edge structure spectra before and during the reaction indicated that the specific adsorbates on the catalyst were dependent on the reaction temperature and that the extent of adsorbed Pt-CN species was correlated with the reactivity of the material.

Temporal evolution of electron energy distribution function and its correlation with hydrogen radical generation in atmospheric-pressure methane needle–plane discharge plasmas

Non-thermal plasmas show great potential in low-temperature activation of methane (CH4) owing to the abundant energetic active species. Motivated by the fact that the chemical reactions in plasma-based CH4 conversion are dominated and regulated by the energetic electrons and various radicals, the temporal evolution of the electron energy distribution function (EEDF) and its relation to hydrogen (H) radical generation in an atmospheric-pressure CH4 needle–plane discharge plasma have been investigated numerically. The simulations are carried out using one-dimensional particle-in-cell Monte-Carlo collision and fluid dynamic models. It can be shown that during the formation and development of the streamer, a characteristic time exists, before and after which the evolution characteristic of the EEDF is reversed. This is mainly attributed to the competition between the energies continuously obtained from the electric field and the increasingly strong inelastic collisions and fast-growing low-energy electron population. When the amplitude of the applied voltage is increased, the fraction of electrons with high enough energy to participate in dissociation or ionization reactions of CH4 increases, leading to an increased H density. Besides, the characteristic time decreases exponentially, and the energy efficiency of the activation of CH4 molecules is decreased. An appropriate electron energy distribution and H radical density should be chosen to ensure acceptable product selectivity and conversion rate without excessive energy consumption; this will depend on the required products. The results presented in this work provide a partial theoretical basis for effectively optimizing the content of high-energy electrons and H radicals.

Low Temperature Activation of Methane on Metal-Oxides and Complex Interfaces: Insights from Surface Science.

ConspectusThe abundance of cheap, natural gas has transformed the energy landscape, whereby revealing new possibilities for sustainable chemical technologies or impacting those that have relied on traditional fossil fuels. The primary component, methane, is underutilized and wastefully exhausted, leading to anthropogenic global warming. Historically, the manipulation of methane remained "clavis aurea," an insurmountable yet rewarding challenge and thus the focus of intense research. This is primarily due to an inability to dissociate C-H bonds in methane selectively, which requires a high energy penalty and is an essential prerequisite for the direct conversion of methane into a large set of value-added products. The discovery of such processes would promise an energy gainful use of natural gas benefiting several essential chemical processes associated with C1 chemistry. This first C-H bond dissociation step of the methane molecule appears in numerous catalytic mechanisms as the rate-determining step or most essential barrier sequence for all subsequent steps that follow in the production of C-C, C-O, or Cx-Hy-Oz bonds found in value added products. A main goal is to catalytically reduce the energy barrier for the first C-H bond dissociation to be able to achieve the activation of methane at low or moderate temperatures. As such there is great value in understanding the fundamental nature of the active sites responsible for bond breaking or formation and thus be able to facilitate better control of this chemistry, leading to the development of new technologies for fuel production and chemical conversion. Surface science studies offer enhanced perspectives for a careful manipulation of bonds over the last layer atoms of catalyst surfaces, an essential factor for the design of atomically precise catalysts and unravelling of the reaction mechanism. With the advent of new surface imaging, spectroscopy, and in situ tools, it has been possible to decipher the surface chemistry of complex materials systems and further our understanding of atomic active sites on the surfaces of metals, oxides, and carbides or metal-oxide and metal-carbide interfaces. The once considered near impossible step of C-H bond activation is now observed at low temperatures with high propensity over a collection of oxide, metal-oxide, and metal-carbide systems in a conventional or inverse configuration (oxide or carbide on metal). The enabling of C-H activation at low temperature has opened interesting possibilities for the specific production of chemicals such as methanol directly from methane, a step toward facile synthesis of liquid fuels. We highlight the most recent of these results and present the key aspects of active site configurations engineered from surface science studies which enable such a simple reactive event through careful manipulation of the last surface layer of atoms found in the catalyst structure. New concepts which help in the activation and conversion of methane are discussed.

Electronic band contraction induced low temperature methane activation on metal alloys

The catalytic conversion of methane under mild conditions is an appealing approach to selectively produce value-added products from natural gas.

Low-temperature activation of methane on doped single atoms: descriptor and prediction.

Catalytic transformation of methane under mild conditions remains a grand challenge. Fundamental understanding of C-H activation of methane is crucial for designing a catalyst for the utilization of methane at low temperature. Recent experiments show that strong methane chemisorption on oxides of precious metals leads to facile C-H activation. However, only a very few such oxides are capable (for example, IrO2 and PdO). Here we show for the first time that strong methane chemisorption and facile C-H activation can be accomplished by single transition-metal atoms on TiO2, some of which are even better than IrO2. Using methane adsorption energy as a descriptor, we screened over 30 transition-metal single atoms doped on TiO2 for the chemisorption of methane by replacing a surface Ti atom with a single atom of another transition metal. It is found that the adsorption energies of methane on a single atom of Pd, Rh, Os, Ir, and Pt doped on rutile TiO2(110) are greater than or similar to those on rutile IrO2(110), a benchmark for the chemisorption of methane on transition oxides. Electronic structure analysis uncovered orbital overlap and mixing between methane and the single atom, as well as significant localization of the charge between the molecule and the surface, demonstrating chemical bonding of CH4 to doped single atoms. Facile C-H dissociation has been found on the single-atom sites with the transition state energies lower than desorption energies. Our computational studies predict that Pd, Rh, Os, Ir, and Pt single atoms on rutile TiO2(110) can activate C-H of methane at a temperature lower than 25 °C.

Low-temperature biological activation of methane: structure, function and molecular interactions of soluble and particulate methane monooxygenases

Mechanistic aspects of oxidation of methane to methanol by methanotrophic bacteria via methane monooxygenase (MMO) is still not well understood. Elucidating how various molecules pertinent to methane oxidation interact with each other at the MMO active site offers critical insights on low-temperature activation of methane, which is one of the greatest technical challenges in hydrocarbon chemistry. In this review, most recent contributions to the area are analyzed comparing soluble (sMMO) and particulate (pMMO) forms. Initially, the taxonomical, morphological and physiological differences of different methanotrophs are discussed. Then, the structural and functional differences of sMMO and pMMO are analyzed while considering substrate/product-cofactor-active site interactions. A docking analysis was performed using Autodock Vina to uncover interactions between cofactors and corresponding enzymes. With natural gas becoming a significant contributor to the energy continuum, this literature analysis and molecular simulations conducted brings new insights to low-temperature activation of methane.

Ni/TiO2 for low temperature steam reforming of methane

Titanium dioxide-supported nickel (Ni/TiO2) catalyst was studied for the low temperature steam reforming of methane (SRM). While studies have frequently reported deactivation/inactivity of TiO2-supported metal catalysts under conventional high temperature methane reforming conditions, it is shown here that by applying activation treatments at a milder temperature of 400°C, Ni/TiO2 is able to maintain stable and effective hydrogen production at 500°C even at a low steam to methane input ratio of 1:1. Temperature programmed reduction studies showed that the presence of nickel species with stronger support interaction is crucial for the low temperature activation of methane while more weakly interacting nickel species contributed to hydrogen production via the water gas shift reaction. This observation was further confirmed when an identical reaction was performed with nickel supported on an inert oxide (silica) whereby the nickel species predominantly possessed a weaker metal–support interaction. When the amount of steam fed into the SRM system was increased, an enhancement in methane conversion by Ni/TiO2 was observed with up to 45% methane conversion achieved at 500°C for a H2O/CH4 feed ratio of 3. The Ni/TiO2 catalyst was shown to be capable of maintaining stable conversions and product selectivities for at least 96h, depending on the H2O/CH4 feed ratio.

Low temperature activation of methane over a zinc-exchanged heteropolyacid as an entry to its selective oxidation to methanol and acetic acid.

A Zn-exchanged heteropolyacid supported onto silica (Zn-HPW/SiO2) activates methane at 25 °C into Zn-methyl. At higher temperatures and with CH4/O2 or CH4/CO2, it gives methanol and acetic acid respectively.

Low-temperature activation of methane over rare earth metals promoted Zn/HZSM-5 zeolite catalysts in the presence of ethylene

Abstract At low temperature of 723 K, methane can be easily activated in the presence of ethylene in the feed, and converted to higher hydrocarbons (C 2-C 4) and aromatics (C 6-C 10), through its reaction over rare metals modified Zn/HZSM-5 zeolite catalysts without undesirable carbon oxides formation. Methane can get 37.3% conversion over the above catalysts under low temperature, and the catalysts show a longer lifetime than usual metal supported HZSM-5 zeolite catalysts without adding any rare earth metals. The effects of methane activation over various rare earth metal promoted Zn/HZSM-5 catalysts on the products and influences of several reaction conditions such as temperature, catalyst lifetime and molar ratio of CH 4/C 2H 4 have been discussed.

Structure, Bonding, and Stability of a Catalytica Platinum(II) Catalyst: A Computational Study

Periana et al. [Science 1998, 280, 560] previously reported two catalysts for low-temperature methane activation to methanol: PtCl2(NH3)2 and PtCl2(bpym). It was shown that the ammine catalyst is much more active, but it decomposes rapidly in sulfuric acid to form a PtCl2 precipitate, while the bpym system is long-lived. To have a basis for developing new catalysts that would not decompose, we undertook a study of the structure, bonding, and stability of the PtCl2(NH3)2 and PtCl2(bpym) catalysts, using quantum mechanics (QM) [density functional theory (DFT) at the B3LYP/LACVP**(+) level] including solvation in sulfuric acid via the Poisson−Boltzmann continuum approximation. Critical results include the following: (1) The influence of a trans ligand Y on the Pt−X bond follows the order Cl- > NH3 (bpym) > OSO3H- > □ (empty site). Thus the Pt−N bond length is longer (up to 0.04 Å) and the Pt−N bond is weaker (up to 18 kcal/mol) when trans to a Cl- as compared to trans to OSO3H-. (2) The bpym ligand acts as both a σ-donor and a π-acceptor. As bpym is protonated, the Pt−N bond strength decreases (by up to 51 kcal/mol). Thus, ΔH(soln, 453 K) for Pt(OSO3H)2(bpym) (69.6) > [Pt(OSO3H)2(bpymH)]+ (49.4) > [Pt(OSO3H)2(bpymH2)]2+ (18.7). (3) In sulfuric acid replacing the ammine ligands with bisulfate ligands is thermodynamically favorable [by ΔG(soln, 453 K) = −23 kcal/mol], whereas replacement of bpym with OSO3H- is unfavorable [by ΔG(soln, 453 K) = +16 kcal/mol]. (4) Replacement of chloride ligands with bisulfate ligands is thermodynamically unfavorable [by ΔG(soln, 453 K) = ∼7 kcal/mol for ammine and ∼12 kcal/mol for bpym]. (5) Protonation of PtCl2(bpym) is thermodynamically favorable, leading to [PtCl2(bpymH)]+ as the stable species in sulfuric acid (by 8 kcal/mol). Thus we conclude that in hot concentrated sulfuric acid it is quite favorable for PtCl2(NH3)2 to lose its ammine ligands to form PtCl2, which in turn will dimerize and oligomerize, leading eventually to a (PtCl2)n precipitate and catalyst death. We find that PtCl2(bpym) is resistant to solvent attack, favoring retention of the bpym ligand in hot concentrated sulfuric acid. These results agree with experimental findings. The insights from these findings should help screen for stable new ligands in the design of new catalysts.

CO hydrogenation and methane activation over Pd–Co/SiO 2 catalysts prepared by sol/gel method

Silica supported cobalt, palladium and cobalt–palladium bimetallic catalysts were prepared by sol/gel technique and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES). The CO hydrogenation and low temperature methane activation under non-oxidative condition were used as test reactions. It was established that, in contrast to other Pd–Co systems, bimetallic particles were not formed, only a part of cobalt was reduced in the presence of palladium and the cobalt was segregated to the catalyst surface. In the CO hydrogenation over a catalyst with the ratio of Co/Pd=2, a synergism was observed, while over cobalt and palladium catalysts alone lower activities were measured. Over pure cobalt sample, which has a limited reducibility, only short chain hydrocarbons, mainly alkenes, are formed, whereas in the presence of palladium (sites for enhanced hydrogen activation), the amount of alkanes increased. The limited reducibility of the cobalt was explained by the small particle size due to the preparation technique. Palladium acts in the bimetallic system not only as a component, facilitating cobalt reduction, but as a source of hydrogen participating in the reaction.

Low-Temperature Methane Activation under Nonoxidative Conditions over Supported Ruthenium–Cobalt Bimetallic Catalysts

Dissociative chemisorption of methane over ruthenium, cobalt, and ruthenium–cobalt bimetallic catalysts supported by alumina, silica, and NaY was investigated under a wide range of temperatures. The extent of hydrogen loss from methane was monitored by deuterium uptake of the surface carbonaceous species (CHx) formed from methane and/or by the amount of hydrogen evolved during the course of methane chemisorption. The presence of a high average number of deuteriums in the desorbing methane suggested a wide spread dissociation of methane. The initial distribution of the deuterated products generally decreased in the sequence CD4>CHD3>CH2D2. The amount of chemisorbed methane and the evolution of hydrogen during methane chemisorption increase with temperature and follow the sequence of reducibility of the supported metals and the particle size which, in turn, depends on the support and the alloy formed. CH species prevailed on alumina- and silica-supported catalysts, while on NaY-supported metals, CH2species are dominant when small metal particles are stabilized inside the supercage.