Methane Conversion Research Articles

Development and verification of a multi-component model for steam methane reforming

RELEVANCE. Steam methane reforming is the dominant method of hydrogen production. Its significant share in global CO2 emissions highlights the importance of optimizing technological parameters to reduce environmental impact. The developed multi-component model of steam methane reforming in COMSOL Multiphysics is relevant not only due to its applicability for optimizing existing production facilities but also for its potential in developing new methods for utilizing associated petroleum gas. In the context of import substitution in the hydrogen energy sector, this model is also of interest, allowing for the calculation of technological parameters of industrial installations.THE PURPOSE. The aim of the work is to develop and verify a multi-component model of steam methane reforming.METHODS. The research methodology includes the use of experimental data from the literature and industrial indicators for integration into a multi-component model in COMSOL Multiphysics. This enables the modelling of complex chemical interactions under conditions characteristic of the industrial steam methane reforming process.RESULTS. The developed multi-component model allows calculating key parameters of the steam methane reforming process, including the concentration of components (methane, hydrogen, carbon monoxide, and carbon dioxide) and temperature along the reactor. The model successfully describes the chemical interactions between components and takes into account the influence of operating conditions, such as temperature, pressure, and steam/gas ratio, on process efficiency. The model verification was carried out by comparing the modelling results with experimental data and indicators of real industrial processes. Their correspondence confirms the high degree of reliability and suitability of the model for practical application in engineering calculations and optimization of steam methane reforming processes.CONCLUSION. The conclusions made based on the modelling can be used for further improvement of methane conversion technologies, contributing to their efficiency and environmental friendliness. There is also potential for using the model to calculate the stages of installations for the utilization of products from the processing of associated petroleum gas.

Exploring Dry Reforming of CH4 to Syngas Using High Entropy Materials: A Novel Emerging Approach

The high global warming potential of natural gas methane necessitates its conversion to valuable products, typically achieved through syngas production, with dry reforming of methane and integrating carbon capture followed by dry reforming of methane standing out among different technologies for methane valorization. However, persistent challenges such as the reverse water gas shift reaction, coke formation, and sintering associated with methane dry reforming have redirected scientific focus toward multi‐metallic catalysts with supports or promoters. High entropy materials have gained attention as promising catalysts because their flexible composition allows fine‐tuning of lattice oxygen reactivity and catalytic activity. Entropy plays a key role in catalysis, and recent research focuses on the enthalpy‐entropy relationship that influences reaction pathways. Alongside entropy, core effects like lattice distortion, sluggish diffusion, and cocktail effects improve catalytic performance by synergistic effects, prevent carbon buildup, and maintain stability at high temperatures, enabling efficient methane conversion. These advancements in high entropy materials drive interest in using entropy‐stabilized systems to address the challenges of methane dry reforming. This review summarizes the recent progress in dry reforming of methane and integrating carbon capture followed by dry reforming of methane using high entropy materials.

High selectivity syngas generation by double perovskite oxygen carriers La2Fe2-xCoxO6 for chemical looping steam methane reforming

The generation of high-quality syngas combined with high-purity hydrogen is an essential challenge in the chemical looping steam methane reforming (CL-SMR) process, in which the design of oxygen carriers (OCs) is crucial. In this study, La2FeBO6 (BCr, Ni，Co) and La2Fe2-xCoxO6 (x = 0.2, 0.4, 0.6, 0.8, 1.0) double perovskites have been investigated as OCs in the CL-SMR process. Various analytical techniques (BET, XPS, XRD, H2-TPR, Raman.) have been deployed to characterize the OCs' specific surface area, crystal structure, and surface oxygen species. The fixed-bed experiments revealed that the Cr-modified perovskite had the lowest reactivity, which was attributed to the creation of LaCrO3 limiting the oxygen release rate of the OC. Meanwhile, Ni doping exhibited the maximum methane conversion (99.15 %), but it resulted in more carbon deposition due to methane cracking, and its carbon deposition was four times than that of Co doping. Whereas, the Co doped double perovskite OCs showed excellent overall performance, in which La2Fe1.8Co0.2O6 (L2F1.8Co0.2) exhibited the optimum CO selectivity (90.72 %), H2 selectivity (96.91 %), and H2 purity (96.84 %), as well as the lowest carbon deposition. Moreover, the L2F1.8Co0.2 also exhibited outstanding stability in 10 cycles, and it could be a prospective material for CL-SMR, enabling the simultaneous generation of high-quality syngas and high-purity hydrogen.

Transition Metal-Promoted LDH-Derived CoCeMgAlO Mixed Oxides as Active Catalysts for Methane Total Oxidation

A series of M(x)CoCeMgAlO mixed oxides with different transition metals (M = Cu, Fe, Mn, and Ni) with an M content x = 3 at. %, and another series of Fe(x)CoCeMgAlO mixed oxides with Fe contents x ranging from 1 to 9 at. % with respect to cations, while keeping constant in both cases 40 at. % Co, 10 at. % Ce and Mg/Al atomic ratio of 3 were prepared via thermal decomposition at 750 °C in air of their corresponding layered double hydroxide (LDH) precursors obtained by coprecipitation. They were tested in a fixed bed reactor for complete methane oxidation with a gas feed of 1 vol.% methane in air to evaluate their catalytic performance. The physico-structural properties of the mixed oxide samples were investigated with several techniques, such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), elemental mappings, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction under hydrogen (H2-TPR) and nitrogen adsorption–desorption at −196 °C. XRD analysis revealed in all the samples the presence of Co3O4 crystallites together with periclase-like and CeO2 phases, with no separate M-based oxide phase. All the cations were distributed homogeneously, as suggested by EDX measurements and elemental mappings of the samples. The metal contents, determined by EDX and ICP-OES, were in accordance with the theoretical values set for the catalysts’ preparation. The redox properties studied by H2-TPR, along with the surface composition determined by XPS, provided information to elucidate the catalytic combustion properties of the studied mixed oxide materials. The methane combustion tests showed that all the M-promoted CoCeMgAlO mixed oxides were more active than the M-free counterpart, the highest promoting effect being observed for Fe as the doping transition metal. The Fe(x)CoCeMgAlO mixed oxide sample, with x = 3 at. % Fe displayed the highest catalytic activity for methane combustion with a temperature corresponding to 50% methane conversion, T50, of 489 °C, which is ca. 40 °C lower than that of the unpromoted catalyst. This was attributed to its superior redox properties and lowest activation energy among the studied catalysts, likely due to a Fe–Co–Ce synergistic interaction. In addition, long-term tests of Fe(3)CoCeMgAlO mixed oxide were performed, showing good stability over 60 h on-stream. On the other hand, the addition of water vapors in the feed led to textural and structural changes in the Fe(3)CoCeMgAlO system, affecting its catalytic performance in methane complete oxidation. At the same time, the catalyst showed relatively good recovery of its catalytic activity as soon as the water vapors were removed from the feed.

Hierarchically structured nanospherical fibrous silica-supported bimetallic catalysts: An enhanced performance in methane decomposition

Hydrogen production by methane catalytic decomposition is a promising method that also allows the formation of valuable carbon nanomaterials which can be utilized in transportation fuels, chemical synthesis, and fuel cell technology. In this study, bimetallic 3d transition metal (Ni, Fe, Co) catalysts supported on spherical mesoporous nanofibrous silica (KCC1) were successfully synthesized using a hydrothermal method followed by sono co-impregnation. The catalysts were evaluated for their performance in the catalytic decomposition of methane. Comprehensive characterization of the catalysts was conducted utilizing XRD, FTIR, SEM, TEM, STEM, XPS, H2-TPR, and BET techniques. XRD and TEM analyses confirmed the formation of Ni–Fe, Ni–Co, and Co–Fe bimetallic alloys on the nanofibrous silica without compromising its dendrimeric hierarchical structure. STEM HAADF imaging revealed a uniform dispersion of bimetallic nanoparticles within the spherical fibrous framework. Catalytic tests demonstrated that all bimetallic catalysts showed high stability and activity for methane cracking at 700 °C over 180 min of time on stream (TOS). Among them, Fe-based catalysts Ni–Fe/KCC1 and Co–Fe/KCC1 achieved the highest hydrogen yields of 80% and 60%, respectively. Methane conversion was 1.56 and 2.23 times higher in Ni–Fe/KCC1 compared to Co–Fe/KCC1 and Ni–Co/KCC1, respectively. Post-reaction characterization of the spent catalysts was performed using XRD, Raman spectroscopy, SEM, TEM, and TGA. XRD and Raman spectroscopy were used to evaluate the degree of graphitization and crystallinity in the carbon nanotubes (CNTs) produced on the catalysts, enabling a correlation with the catalyst properties. TEM examination was used to investigate the structural morphology and growth methods of the CNTs, including tip or base growth and chain-like or parallel-wall types. TGA results revealed that all bimetallic catalysts exhibited no amorphous carbon during methane cracking reaction.

Hybrid plasma catalysis-thermal system for non-oxidative coupling of methane to ethylene and hydrogen

We report a two-stage hybrid plasma catalysis-thermal system for non-oxidative coupling of methane (NOCM) to ethylene (C2H4) and hydrogen (H2), achieving 66 % C2H4 selectivity and 60 % H2 selectivity with 28 % CH4 conversion. This corresponds to a C2H4 yield of 18 %, which is one of the highest reported in literature. The system consists of the first plasma catalysis stage (stage 1) and the second thermal cracking stage (stage 2). Comprehensive analyses using in-situ mass spectrometry (MS) and gas chromatography (GC) reveal that the combination of plasma and Pt/ZrO2 catalyst in stage 1 predominantly facilitates the conversion of methane to ethane. Characterizations employing X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy-scanning transmission electron microscopy-energy dispersive X-ray (HRTEM-STEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR), indicate that the coordinatively unsaturated site of Pt could be the active sites for C–H bond cracking to generate abundant CH3 radicals (CH3·), enhancing the C2 selectivity by inhibiting the non-selective formation of coke and higher hydrocarbons. Subsequently, the products from stage 1 (especially C2H6) undergo pyrolysis reactions at 880 °C in stage 2, which further boosts the C2H4 selectivity and yield. Based on the reaction performance, catalyst characterization, optical emission spectroscopy (OES), and in-situ Fourier transform infrared spectroscopy (FTIR) results, we reveal the reaction mechanism within the hybrid two-stage plasma catalysis-thermal system for converting CH4 to C2H4 and H2.

An efficient Au/ZnO catalyst for the photocatalytic conversion of methane to formaldehyde

The photocatalytic oxidation of methane to value-added chemical products under mild conditions is crucial for resource utilization and environmental protection. However, inefficient generation of reactive oxygen species partially limits methane activation. To tackle this issue, we have developed highly dispersed Au-modified ZnO nanosheets through photo-deposition method for the photocatalytic oxidation of methane to formaldehyde. The optimized Au0.1/ZnO catalyst (0.1 wt% Au on ZnO) achieves a HCHO yield of 19.14 μmol h−1 with a selectivity of 87.94 %. Electron paramagnetic resonance, Raman, X-ray photoelectron spectroscopy and density functional theory show that Au enhances the concentration of oxygen vacancies (OVs) in ZnO. This enhancement is attributed to a strong interaction between Au and ZnO, which facilitates the transfer of electrons from the ZnO surface to Au. Under irradiation, Au and OVs function as hole and electron acceptors, respectively. This interaction promotes carrier separation, reduces charge recombination, enhances oxygen adsorption, and effectively generates reactive oxygen species, such as ·OH. Subsequently, CH4 is oxidized to ·CH3 by holes from Au. Concurrently, O2 is reduced by electron from OVs, following the pathways O2 → OOH→H2O2 → ·OH. The rapid oxidation of CH3OH by ·CH3 and ·OH results in the formation of HCHO.

Highly efficient Co-added Ni/CeO2 catalyst for co-production of hydrogen and carbon nanotubes by methane decomposition

The catalytic decomposition of methane (CDM) is a hydrogen and nanostructured carbon production process with minimal CO2 emission. Among the transition metal-based catalysts (e.g. Ni, Fe, Co, etc.), Ni-based catalysts are most widely studied due to the higher catalytic activity in decomposing methane. However, the limited lifespan of the catalyst makes it unsuitable for practical applications. Effective methane decomposition catalysts should be designed to optimize both reaction efficiency and catalyst lifetime. A Ni/CeO2 catalyst, developed in previous studies, Co was added to promote low-temperature (< 700 °C) activity manipulating the redox property of Co. Among the prepared catalysts with varying Ni:Co ratio, the methane conversion rate of the Ni8Co2/CeO2 catalyst was approximately twice that of the Ni10/CeO2 catalyst, confirming its excellent low-temperature activity. The reaction rate of Ni8Co2/CeO2 catalyst was 4.38 mmol/min∙gcat at 600 °C with WHSV of 36 L/gcat∙h. In terms of characteristics of carbon products, Raman spectroscopy analysis revealed that the carbon grown on the catalyst surface exhibited high crystallinity, with D-G band ratio (ID/IG) of 1.01. The fresh and used catalyst samples were characterized by TEM, XPS, XAS, and other methods to analyze the parameters affecting catalytic activity.

Non-Oxidative Coupling of Methane via Plasma-Catalysis Over M/γ-Al2O3 Catalysts (M = Ni, Fe, Rh, Pt and Pd): Impact of Active Metal and Noble Gas Co-Feeding

Plasma-catalysis has attracted significant interest in recent years as an alternative for the direct upgrading of methane into higher-value products. Plasma-catalysis systems can enable the electrification of chemical processes; however, they are highly complex with many previous studies even reporting negative impacts on methane conversion. The present work focuses on the non-oxidative plasma-catalysis of pure methane in a Dielectric Barrier Discharge (DBD) reactor at atmospheric pressure and with no external heating. A range of transition and noble metals (Ni, Fe, Rh, Pt, Pd) supported on γ-Al2O3 are studied, complemented by plasma-only and support-only experiments. All reactor packings are investigated either with pure methane or co-feeding of helium or argon to assess the role of noble gases in enhancing methane activation via energy transfer mechanisms. Electrical diagnostics and charge characteristics from Lissajous plots, and electron temperature and collision rates calculations via BOLSIG+ are used to support the findings with the aim of elucidating the impact of both active metal and noble gas on the reaction pathways and activity. The optimal combination of Pd catalyst and Ar co-feeding achieves a substantial improvement over non-catalytic pure methane results, with C2+ yield rising from 30% to almost 45% at a concurrent reduction of energy cost from 2.4 to 1.7 \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\ ext{M}\ ext{J}\\:{\ ext{m}\ ext{o}\ ext{l}}_{\ ext{C}{\ ext{H}}_{4}}^{-1}$$\\end{document} and from 9 to 4.7 \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\ ext{M}\ ext{J}\\:\ ext{m}\ ext{o}{\ ext{l}}_{{\ ext{C}}_{2+}}^{-1}$$\\end{document}. Pd, along with Pt, further displayed the lowest coke deposition rates among all packings with overall stable product composition during testing.

Enhanced performance of red mud oxygen carrier for chemical looping combustion via tandem reactions

Biomethane power generation, particularly chemical looping combustion technology, has garnered significant attention recently due to its ability to capture CO2 in situ. In this study, dual bed layers oxygen carriers (DBL) for chemical looping combustion of methane were designed. The LaFe0.93Ni0.07O3 oxygen carrier with excellent CH4 partial oxidation was applied in upper bed layer and low-cost red mud was located into lower bed layer. This tandem reaction showed much higher methane conversion (87.1 %) than that over the single oxygen carrier of red mud (33.9 %) and the mechanically mixed oxygen carrier of LaFe0.93Ni0.07O3 and red mud (77.7 %). In addition, the mechanically mixed sample showed a quick deactivation, with the methane conversion dropping 54.2 % after 40 redox cycles due to the encapsulation of the active LaFe0.93Ni0.07O3 by red mud, while the double bed system showed high stability in the long-term redox cycle. The limitedly increased crystallite size of LaFeO3, as well as completely replenished lattice oxygen, were responsible for superior activity and stability of DBL. This study provides a new strategy to improve the efficiency of chemical looping combustion of methane in fixed bed reactor by tandem reactions over different oxygen carriers with complementary functions in terms of activity and cost.

Enhanced conversion of methane to liquid-phase oxygenates via hollow ferrite nanotube@horseradish peroxidase based photoenzymatic catalysis

A novel photoenzymatic catalytic system employing hollow ferrite nanotubes and horseradish peroxidase (HRP) has been developed to significantly enhance the conversion of methane to liquid-phase oxygenates. The 1.5 % HRP-Fe3O4 HNTs demonstrated an exceptional liquid product yield of 177.87 μmol g−1 h−1 and exhibited reusability for methane photo-oxidation under ambient conditions. The physicochemical characterization revealed that the immobilization of HRP enhanced the visible light absorption and charge carrier separation efficiency. The Fe3O4 HNTs facilitated methane (CH4) molecule adsorption, while photogenerated electrons and H2O2 participated in the Fe2+-porphyrin and Fe3+-porphyrin cycle of HRP, enabling continuous generation of hydroxyl radicals, which activated CH4 to methyl radicals. In situ EPR and FTIR results indicated that CH3OH and CH3CH2OH were produced via the formation and consumption of •CH3O and •CH2 intermediates. This work presents an efficient strategy and provides new insights into the direct photo-oxidation of methane reaction through a photoenzyme catalytic system.

A highly active and stable palladium zeolite catalyst for wet methane combustion

Natural gas engines are the most viable alternative to diesel and gasoline ones. However, the current state-of-art catalyst, palladium (Pd) on alumina, for eliminating unburnt methane from engine exhaust suffers from high light-off temperature (> 400 °C) and poor water tolerance. Here we systematically investigated the effects of zeolite structure and Si/Al ratio, catalyst calcination temperature and extra-framework cation type on catalytic performance of zeolite-supported Pd catalysts for wet methane combustion to overcome these limitations. We found that a 3.0 wt% Pd catalyst supported on Na+-post-exchanged 500 °C-calcined IWV zeolite with a Si/Al ratio of 45 has a light-off temperature as low as 290 °C for methane combustion in the presence of 10 % water vapor while maintaining ca. 85 % methane conversion at 330 °C for 100 h. This study provides a new direction for bringing supported Pd catalysts close to real-world applications.

Optimization of methane partial oxidation for hydrogen production with local free space addition: An efficient combination of porous media reactor and thermoelectric conversion system

Thermoelectric technology plays a pivotal role in the efficient conversion and utilization of exhaust heat. Abundant waste heat resources are present in the tail gas of hydrogen production, significantly influenced by combustion characteristics. To investigate the impact of combustion performance on hydrogen yield and power output, an efficient porous media reactor was designed and integrated with a thermoelectric conversion system. The combustion characteristics of a thermoelectric system incorporating unsteady porous media were examined. Furthermore, the effects of the porous media’s structural parameters on temperature distribution, gas concentration, and power generation were analyzed under various operating conditions. The results revealed that radial free space structures exhibited higher hydrogen and methane conversion efficiencies compared to axial free space structures at verge and toroidal positions. However, the height of the radial free space exerted a more substantial influence on temperature distribution and gas production than the size of the free space itself. At a height of 39 mm, hydrogen concentration and yield reached their peak values of 14.5 % and 53 %, respectively. Combustion was found to significantly impact power generation, with a downward shift of the flame resulting in decreased power output. The inlet velocity was observed to affect the thermoelectric generator power, with a maximum steady-state power of 3.95 W achieved at a velocity of 16 cm/s. These findings provide practical guidance for the efficient utilization of industrial waste heat.

Highly Selective Synthesis of Acetic Acid from Hydroxyl‐Mediated Oxidation of Methane at Low Temperatures

Direct methane conversion and, in particular, the aerobic oxidation to acetic acid, remain an eminent challenge. Here, we reported a zeolite‐supported Au‐Fe catalyst (Au‐Fe/ZSM‐5) that converted methane to acetic acid with molecular oxygen as an oxidant in the presence of CO. Specifically, Au nanoparticles catalyzed the formation of hydroxyl species from the reaction of CO, O2, and H2O, meanwhile ZSM‐5‐supported atomically dispersed Fe species were responsible for the hydroxyl‐mediated coupling of CH4 and CO to generate acetic acid. The reaction over 50 mg of Au‐Fe/ZSM‐5 under 62 bar (CH4: CO: O2 = 14: 14: 3) at 120 °C for 3.0 h yielded 5.7 millimoles of acetic acid per gram of the catalyst (mmol gcat‐1) with the selectivity of 92%, outperformed most of reported catalysts. Significantly, the catalyst remained active even at 60 °C. We anticipate that this hydroxyl‐mediated route may guide the design of optimized catalysts for the direct methane functionalization at low temperatures.

Highly Selective Synthesis of Acetic Acid from Hydroxyl-Mediated Oxidation of Methane at Low Temperatures.

Direct methane conversion and, in particular, the aerobic oxidation to acetic acid, remain an eminent challenge. Here, we reportedazeolite-supported Au-Fe catalyst (Au-Fe/ZSM-5)that convertedmethane to acetic acid with molecular oxygen as an oxidant in the presence of CO. Specifically, Au nanoparticles catalyzedthe formation of hydroxyl species from the reactionofCO, O2, and H2O, meanwhile ZSM-5-supported atomically dispersed Fe species were responsible for the hydroxyl-mediated coupling of CH4and CO to generateacetic acid.The reaction over 50mg of Au-Fe/ZSM-5under 62 bar (CH4:CO:O2= 14:14:3)at 120 °C for 3.0 hyielded 5.7millimoles of acetic acid per gram of the catalyst (mmolgcat-1) with the selectivity of 92%, outperformed most of reported catalysts. Significantly, the catalyst remained active even at 60 °C. We anticipate that this hydroxyl-mediated route may guide the design of optimized catalysts for the direct methane functionalizationat low temperatures.

Role of MgO in Al2O3‐supported Fe catalysts for hydrogen and carbon nanotubes formation during catalytic methane decomposition

AbstractCatalytic methane decomposition is a promising technology for reducing the reliance on fossil fuels and mitigating the effects of climate change by producing clean hydrogen and value‐added carbon without the emission of greenhouse gases. The aim of the study was to investigate the use of Al2O3‐modified MgO doped iron‐based catalysts for the catalytic decomposition of methane. The catalysts were synthesized using the impregnation method and characterized using various analysis techniques, including Brunauer, Emmett, and Teller, temperature programmed reduction, temperature programmed oxidation, X‐ray diffraction, thermal gravimetric analysis, Raman, scanning electron microscopy, and transmission electron microscopy. The activity of the synthesized catalysts was tested in a packed‐bed reactor with a gas flow rate of 20 mL/min at a temperature of 800°C. The investigation focuses on the influence of incorporating magnesium into alumina catalysts with MgO concentration ranging from (20%–70%), where higher magnesium levels improve catalytic activity by creating more active sites, positively impacting methane decomposition. Enhanced catalyst reducibility and increased particle dispersion lead to improved catalytic properties despite the reduced surface area. The FA70M and FA63M catalysts exhibited almost the same catalytic characteristics and the highest stability and methane conversion among the catalysts investigated, reaching 87% and 85% at 800°C for 120 min. Moreover, both catalysts showed hydrogen yields of 86% and 85%, respectively. The introduction of MgO further increased the total carbon yield from 103% with FA and 39% for FM to 114% and 120% for the respective catalysts (FA70M and FA63M). During the methane decomposition reaction, carbon nanotubes of varying diameters were produced. Higher iron loading resulted in a positive trend.

Hydrophobic TaOx Species Overlayer Tuning Light-Driven Methane Chlorination with Inorganic Chlorine.

Halogenated methane serves as a universal platform molecule for building high-value chemicals. Utilizing sodium chloride solution for photocatalytic methane chlorination presents an environmentally friendly method for methane conversion. However, competing reactions in gas-solid-liquid systems leads to low efficiency and selectivity in photocatalytic methane chlorination. Here, an in situ method is employed to fabricate a hydrophobic layer of TaOx species on the surface of NaTaO3. Through in-situ XPS and XANES spectra analysis, it is determined that TaOx is a coordination unsaturated species. The TaOx species transforms the surface properties from the inherent hydrophilicity of NaTaO3 to the hydrophobicity of TaOx/NaTaO3, which enhances the accessibility of CH4 for adsorption and activation, and thus promotes the methane chlorination reaction within the gas-liquid-solid three-phase system. The optimized TaOx/NaTaO3 photocatalyst has a good durability for multiple cycles of methane chlorination reactions, yielding CH3Cl at a rate of 233 µmol g-1 h-1 with a selectivity of 83%. In contrast, pure NaTaO3 exhibits almost no activity toward CH3Cl formation, instead catalyzing the over-oxidation of CH4 into CO2. Notably, the activity of the optimized TaOx/NaTaO3 photocatalyst surpasses that of reported noble metal photocatalysts. This research offers an effective strategy for enhancing the selectivity of photocatalytic methane chlorination using inorganic chlorine ions.

Enhanced low-temperature catalytic activity and stability in methane combustion of Pd−CeO2 nanowires@SiO2 by Pt dispersion

The long-standing contradiction between low-temperature activity and high-temperature stability is one of the difficulties in catalytic combustion of low-concentration methane. The traditional Pd−CeO2 catalyst system has been applied to the oxidation of methane with low concentrations. However, the problem of sintering at high temperatures still exists. In this work, we prepared the Pt-modified Pd−CeO2 nanowires (NW) sample (in which the actual Pt, Pd, and Ce contents were 0.12, 0.86, and 9.8 wt%, respectively) using the one-pot reverse-micelle emulsion method. It was found that Pt-Pd−CeO2NW@SiO2 showed the highest low-temperature catalytic activity at a space velocity of 20,000 mL/(g h) and the best water resistance and high-temperature stability in the combustion of methane. The T50 % and T90 % (the temperatures for achieving methane conversions of 50 and 90 %) were 298 and 342 ℃, respectively, methane reaction rate at 270 ℃ was 0.49 μmol/(gcat s), and turnover frequency (TOF) at 270 °C was 0.198 s−1 over Pt-Pd−CeO2NW@SiO2; whereas over Pd−CeO2NW@SiO2 (in which the actual Pd and Ce contents were 0.82 and 10.6 wt%, respectively), the T50 % and T90 % were 360 and 420 ℃, respectively, methane reaction rate at 270 ℃ was 0.074 μmol/(gcat s), and TOF at 270 °C was 0.032 s−1. The introduction of the highly dispersed Pt to Pd−CeO2NW@SiO2 could effectively increase the PdOx sites of unsaturated coordination through the electron-donating interaction of the Pt with PdO, which played an important role in activating the C−H bonds in methane. In addition, the unique structure of encapsulation also rendered the Pt-Pd−CeO2NW@SiO2 sample to possess good water resistance and thermal stability in methane combustion. We are sure that the present work provides a possibility for developing the catalysts with stable catalytic and water-resistant performance at low and high temperatures in the combustion of methane.

Selective oxidation of methane to C2+ products over Au-CeO2 by photon-phonon co-driven catalysis

Direct methane conversion to high-value chemicals under mild conditions is attractive yet challenging due to the inertness of methane and the high reactivity of valuable products. This work presents an efficient and selective strategy to achieve direct methane conversion through the oxidative coupling of methane over a visible-responsive Au-loaded CeO2 by photon-phonon co-driven catalysis. A record-high ethane yield of 755 μmol h−1 (15,100 μmol g−1 h−1) and selectivity of 93% are achieved under optimised reaction conditions, corresponding to an apparent quantum efficiency of 12% at 365 nm. Moreover, the high activity of the photocatalyst can be maintained for at least 120 h without noticeable decay. The pre-treatment of the catalyst at relatively high temperatures introduces oxygen vacancies, which improves oxygen adsorption and activation. Furthermore, Au, serving as a hole acceptor, facilitates charge separation, inhibits overoxidation and promotes the C-C coupling reaction. All these enhance photon efficiency and product yield.

Synergistic effect of oxygen vacancies and functionalized ligands selectively enhanced the photocatalytic oxidation of methane to carbon monoxide

The titanium-based metal–organic framework MIL-125(Ti) has been widely investigated in photocatalytic carbon dioxide reduction and water splitting, but rarely studied in photocatalytic methane conversion by employing only water as an oxidant under mild conditions. Simultaneously controlling products with high yield and selectivity is highly challenging in methane conversion. Herein, we develop a series of functionalized titanium metal–organic frameworks via a facile organic ligand exchange process. The abundant Ti3+ active sites induced by oxygen vacancies and functionalized ligands synergistically facilitate the adsorption and activation of methane molecules. The carbon monoxide yield over NH2-MIL-125(Ti) increased to 198.6 μmol·gcat−1·h−1 with a high selectivity of 87.1 % under simulated solar illumination. Combined with in-situ characterizations and density functional theoretical calculations, the results confirmed that Ti3+ active sites induced by oxygen vacancies and amino functional groups synergistically enhanced the photocatalytic conversion of methane and elucidate CH4-to-CO conversion mechanism. This study does not only provide insights into the rational design of metal–organic frameworks with copious oxygen vacancies and functional groups, but also provides some significant cognition for photocatalytic methane conversion.