High Methane Conversion Research Articles

Biogas dry reforming over nickel-silica sandwiched core–shell catalysts with various shell thicknesses

A series of nickel-silica sandwiched catalysts were synthesized to investigate the effect of shell thicknesses on the reaction of dry reforming. The thicknesses of the silica shell were adjusted by TEOS content and coating time of the silica shell. The outcomes exhibited that encapsulating the Ni-SiO2 catalyst by mesoporous silica shell effectively decreased carbon formation. Higher carbon resistance property of the nickel-silica sandwiched catalyst was related to the confined nickel particles by silica shell, which immobilizes metal particles, hampers the agglomeration and prevents uprooting Ni particles by the growing filamentous carbon. The results revealed that the amount of carbon deposition gradually decreased with the augment of TEOS content. The sandwiched catalyst with 2 ml TEOS exhibited high methane and CO2 conversions of 71.5% and 91.5% under CH4/CO2 = 1.5 and T = 700 °C for 20 h, respectively. Also, negligible carbon was observed on the surface of the catalyst and the original structure of the catalyst was well preserved after the stability test. The results revealed that with an increase in coating time, the catalytic activity improved and carbon deposition decreased.

Efficient hole abstraction for highly selective oxidative coupling of methane by Au-sputtered TiO2 photocatalysts

AbstractPhotocatalytic oxidative coupling of methane (OCM) produces C2 molecules that can be used as building blocks for synthesis of fuels and chemicals. However, the yield rate and the selectivity of C2 products are still moderate due to the stable nature of methane molecules. Here we develop a Au nanocluster-loaded TiO2 photocatalyst by a sputtering approach, achieving a high methane conversion rate of 1.1 mmol h−1, C2 selectivity of ~90% and apparent quantum efficiency of 10.3 ± 0.6%. The high C2/C2+ yield rate is on the same order of magnitude as the benchmark thermal catalysts in OCM processes operated at high temperature (>680 °C). Au nanoparticles are shown to prolong TiO2 photoelectron lifetimes by a factor of 66 for O2 reduction, together with Au acting as a hole acceptor and catalytic centre to promote methane adsorption, C–H activation and C–C coupling. This work underscores the importance of multifunctional co-catalysts and mechanistic understanding to improve photocatalytic OCM.

Catalyst-enhanced autothermal chemical looping reforming: Intrinsic SMR kinetics and numerical simulation

In this work, we propose a catalyst-enhanced autothermal chemical looping reforming (CE-aCLR) to address the environmental concerns associated with toxic Ni-based oxygen carrier (OC) and the low fuel conversion related to the low reactivity of Fe-based OC. In this process, a non-toxic noble-metal-based steam methane reforming (SMR) catalyst is employed for methane conversion, while a Fe-based OC is employed for oxygen transfer between two reactors. Firstly, a commercial Rh-based SMR catalyst is studied in a micro-fixed bed reactor and its intrinsic reaction kinetics are determined. Then, a 1-D CE-aCLR model is developed based on a verified multi-scale aCLR model to facilitate analyzing commercial-scale feasibility. This model accounts for the essentials of the reactor hydrodynamics, mass and heat transfer, detailed reaction kinetics, and the influence of the solids residence time distribution in the fuel reactor. The main dimensions of a commercial CE-aCLR unit are determined and this unit is simulated using the developed model. The results demonstrate the feasibility of the novel, environmentally friendly process that enables high methane conversion using Fe-based OC within realistically sized reactors and an acceptable solid circulation rate. Moreover, sensitivity study shows that the CE-aCLR allows using OCs with low activity (potentially low cost) while maintaining high CH4 conversion.

Photocatalytic Conversion of Methane: Current State of the Art, Challenges, and Future Perspectives.

With 28-34 times the greenhouse effect of CO2 over a 100-year period, methane is regarded as the second largest contributor to global warming. Reducing methane emissions is a necessary measure to limit global warming to below 1.5 °C. Photocatalytic conversion of methane is a promising approach to alleviate the atmospheric methane concentrations due to its low energy consumption and environmentally friendly characteristics. Meanwhile, this conversion process can produce valuable chemicals and liquid fuels such as CH3OH, CH3CH2OH, C2H6, and C2H4, cutting down the dependence of chemical production on crude oil. However, the development of photocatalysts with a high methane conversion efficiency and product selectivity remains challenging. In this review, we overview recent advances in semiconductor-based photocatalysts for methane conversion and present catalyst design strategies, including morphology control, heteroatom doping, facet engineering, and cocatalysts modification. To gain a comprehensive understanding of photocatalytic methane conversion, the conversion pathways and mechanisms in these systems are analyzed in detail. Moreover, the role of electron scavengers in methane conversion performance is briefly discussed. Subsequently, we summarize the anthropogenic methane emission scenarios on earth and discuss the application potential of photocatalytic methane conversion. Finally, challenges and future directions for photocatalytic methane conversion are presented.

Partial Oxidation of Bio-methane over Nickel Supported on MgO–ZrO2 Solid Solutions

Syngas can be produced from biomethane via Partial Oxidation of Methane (POM), being an attractive route since it is ecofriendly and sustainable. In this work, catalysts of Ni supported on MgO–ZrO2 solid solutions, prepared by a one-step polymerization method, were characterized by HRTEM/EDX, XRD, XPS, H2-TPR, and in situ XRD. All catalysts, including Ni/ZrO2 and Ni/MgO as reference, were tested for POM (CH4:O2 molar ratio 2, 750 ºC, 1 atm). NiO/MgO/ZrO2 contained two solid-solutions, MgO–ZrO2 and NiO-MgO, as revealed by XRD and XPS. Ni (30 wt%) supported on MgO–ZrO2 solid solution exhibited high methane conversion and hydrogen selectivity. However, depending on the MgO amount (0, 4, 20, 40, 100 molar percent) major differences in NiO reducibility, growth of Ni0 crystallite size during H2 reduction and POM, and in carbon deposition rates were observed. Interestingly, catalysts with lower MgO content achieved the highest CH4 conversion (~ 95%), high selectivity to H2 (1.7) and CO (0.8), and low carbon deposition rates (0.024 g carbon.gcat−1 h−1) with Ni4MgZr (4 mol% MgO) turning out to be the best catalyst. In situ XRD during POM indicated metallic Ni nanoparticles (average crystallite size of 31 nm), supported by MgO–ZrO2 solid solution, with small amounts of NiO–MgO being present as well. The presence of MgO also influenced the morphology of the carbon deposits, leading to filaments instead of amorphous carbon. A combustion-reforming mechanism is suggested and using a MgO–ZrO2 solid solution support strongly improves catalytic performance, which is attributed to effective O2, CO2 and H2O activation at the Ni/MgO–ZrO2 interface.

Synthesis of COx-free hydrogen via natural gas decomposition over titanium dioxide-supported bimetallic catalysts

Nickel and cobalt-based titania-supported bimetallic catalysts were synthesized using the co-precipitation technique. The total metal loading varied between 20 and 60 wt% while keeping nickel to cobalt ratio of unity. In comparison with low-loading catalysts i.e., 10 and 15 wt% each of nickel and cobalt supported over titania (10NCT and 15NCT), high-loading catalysts (20NCT, 25NCT, and 30NCT) have shown higher activity and stability performances. Among the high-loading catalysts, 25NCT catalyst outperformed the rest of the catalysts with the highest methane conversion and carbon yield. The higher specific surface area (115.1 m2/g), evaluated from physisorption, facilitates better metal particle dispersion; suitable metal-support interaction, and number of reducible species determined by temperature-programmed reduction, have played a vital role in enhanced activity of 25NCT. The morphological analysis revealed the formation of carbon nanotubes via following both tip-growth and base-growth mechanisms.

Double adjustment of Ni and Co in CeO2/La2Ni2-xCoxO6 double perovskite type oxygen carriers for chemical looping steam methane reforming

Chemical looping steam methane reforming (CL-SMR) is a clean and efficient technology for the co-production of syngas and hydrogen. In this work, a series of Ni and Co doped CeO2/La2Ni2-xCoxO6 double perovskite composite oxygen carriers were developed. The introduction of Co increase a large number of surface active sites, which accelerates the surface activation of CH4, while the addition of Ni provides oxygen vacancies to promote the migration of lattice oxygen in the particles. The optimal substitution ratios of cobalt and nickel are 0.6 and 1.4, respectively. The CeO2/La2Ni1.4Co0.6O6 sample exhibits excellent syngas selectivity (95%) accompanied with high methane conversion (86%) in the methane reforming stage at 850℃, and close to 100% hydrogen concentrations in the steam oxidation stage. Density functional theory calculations demonstrated that the partial oxidation of methane is a strongly exothermic process, and the double perovskite loaded with CeO2 is more conducive to the activation of CH4 and prevents the generation of carbon deposition. The Ni-Co synergistic effect formed by the appropriate doping ratio of Ni and Co can also greatly promote the partial oxidation of methane. Subsequently, the deeply reduced metals combined with oxygen vacancies provide sufficient active stable points for water vapor cracking to generate high-purity hydrogen.

Doping Fe and Zn to modulate Ni nanoparticles on IM-5 for methane decomposition to form hydrogen and CNTs

Catalysts Ni/IM-5 doped with Fe and/or Zn were studied to promote the performance in the catalytic decomposition of methane (CDM) to simultaneously produce hydrogen and carbon nanotubes (CNTs). The catalyst Ni20Fe5Zn5/IM-5 showed high methane conversion (c.a. 65%) and 100% selectivity towards H2 in 300 min at 670 °C. In the Fe atoms inhibited the formation of larger Ni particles. Zn atoms enhanced the migration of C atoms over the surface of the catalyst. IM-5 was speculated to provide better diffusion of C atoms to avoid the excessive deposition of carbon. The synergistic effect between metal and IM-5 enabled the catalyst to selectively produce pure and high graphitization CNTs. A carbon migration model was proposed to explain the synergistic effect between metals and zeolite. The atom erosion as dominant reason of catalyst deactivation was confirmed by XRD and TEM. According to density functional theory (DFT), bimetallic Fe and Zn promoter catalyst was more favorable for the initial bond cleavage of CH4.

Methane Photooxidation with Nearly 100 % Selectivity Towards Oxygenates: Proton Rebound Ensures the Regeneration of Methanol.

Restrained by uncontrollable dehydrogenation process, the target products of methane direct conversion would suffer from an inevitable overoxidation, which is deemed as one of the most challenging issues in catalysis. Herein, based on the concept of a hydrogen bonding trap, we proposed a novel concept to modulate the methane conversion pathway to hinder the overoxidation of target products. Taking boron nitride as a proof-of-concept model, for the first time it is found that the designed N-H bonds can work as a hydrogen bonding trap to attract electrons. Benefitting from this property, the N-H bonds on the BN surface rather than C-H bonds in formaldehyde prefer to cleave, greatly suppressing the continuous dehydrogenation process. More importantly, formaldehyde will combine with the released protons, which leads to a proton rebound process to regenerate methanol. As a result, BN shows a high methane conversion rate (8.5 %) and nearly 100 % product selectivity to oxygenates under atmospheric pressure.

Methane Photooxidation with Nearly 100 % Selectivity Towards Oxygenates: Proton Rebound Ensures the Regeneration of Methanol

AbstractRestrained by uncontrollable dehydrogenation process, the target products of methane direct conversion would suffer from an inevitable overoxidation, which is deemed as one of the most challenging issues in catalysis. Herein, based on the concept of a hydrogen bonding trap, we proposed a novel concept to modulate the methane conversion pathway to hinder the overoxidation of target products. Taking boron nitride as a proof‐of‐concept model, for the first time it is found that the designed N−H bonds can work as a hydrogen bonding trap to attract electrons. Benefitting from this property, the N−H bonds on the BN surface rather than C−H bonds in formaldehyde prefer to cleave, greatly suppressing the continuous dehydrogenation process. More importantly, formaldehyde will combine with the released protons, which leads to a proton rebound process to regenerate methanol. As a result, BN shows a high methane conversion rate (8.5 %) and nearly 100 % product selectivity to oxygenates under atmospheric pressure.

The study of CO2 reforming of methane over Ce/Sm-promoted NiCaAl catalysts

In this work, we synthesized Ce and Sm-promoted NiCaAl layered catalysts via co-precipitation and freeze-drying strategies for alleviating some problems, such as coke deposition and agglomeration of active sites, in dry reforming of methane. Employing such promoters leads to rising O2 vacancies on the catalyst's surface, resulting from adsorbing CO2 gas under the reaction. X-ray diffraction (XRD) analysis confirmed the formation of a hydrotalcite network. High-resolution transmission electron microscopy (HR-TEM) and field emission scanning electron microscopy (FE-SEM) images also indicated a wealth of nanocatalysts, well distributed on the surface and within a porous scaffold structure. The temperature-programmed reduction (H2-TPR) proved that cerium (H2 consumption = 1.2 mmol.g-1) could weaken the interactions between nickel and Ca/Al, leading to better reducibility than samarium (H2 consumption = 1.35 mmol.g-1). Moreover, the temperature-programmed deposition (CO2-TPD) showed the Ce-promoted catalyst possesses more basic sites (0.41 mmol.g-1) on its surface than the Sm-enhanced one (0.25 mmol.g-1). Consequently, the Ce-enhanced catalyst with a smaller crystallite size (8.9 nm), higher dispersion of Ni (10.8%), better reducibility, and more basic sites, which caused the highest methane conversion (82%), and H2/CO ratio (0.9) at 700 °C without any coke deposition after 5 h of time on stream.

Distinguishing anaerobic digestion from electrochemical anaerobic digestion: Metabolic pathways and the role of the microbial community

In this study, we explored why electrochemical anaerobic digestion (EAD) results in higher methane conversion and lower CO2 emissions than anaerobic digestion (AD). Single-chamber AD and EAD reactors were used in this experiment, and the temperature was set as the disturbance factor. Current, pH, electrode potential, gas content, and microbial community were used as indicators for our analysis. Flux balance analysis (FBA) and high-pass next-generation sequencing (NGS) were used to explore the relationships between AD and EAD methane-producing metabolic fluxes and microorganisms. The results showed that the average methane fluxes were 22.27 (AD) and 29.65 (EAD). Compared with AD, EAD had improved hydrogen-dependent CO2 reduction pathway. Trichloromonas was the dominant electricity-producing microorganism on the EAD anode film, which was closely related to the H2 flux at the cathode. Oscillibacter and Syntrophomonas were the dominant bacteria in the fermentation broth, specific to EAD. The abundance of Oscillibacter was positively correlated with the H2 flux, and the presence of Oscillibacter enhanced CO2 reduction by hydrogen. Methanosaeta was the only dominant methanogenic bacterium in AD and EAD, and its abundance was higher in the experimental group with a greater methane flux.

Non‐Oxidative Coupling of Methane Driven by Non‐Thermal Plasma: Effect of Packing Materials on Micro‐Electric Field

AbstractNon‐oxidative coupling of methane (CH4) by plasma has received particular attention due to the achievement of high methane conversion and product selectivity at a low temperature. In this work, experimental results suggest that the packing materials directly affect the electric field, which in turn determines the plasma discharge and, subsequently, the conversion of CH4 and the products selectivity. High selectivity to ethylene was obtained by BaTiO3 packing, which can be ascribed to the moderate electric field generated with special physiochemical properties. Furthermore, in the cases of packing by ZSM‐5, γ‐Al2O3 or SiO2 granules, small size of granules contributed to generate a stronger electric field, which improves CH4 conversion. However, reverse trend has been found in the case of packing by BaTiO3. Finally, the specific indicator of micro‐electric field was developed to illustrate the relationship between reaction performance and packing materials.

Investigation of methane and ethane pyrolysis with highly active and durable iron-alumina catalyst to produce hydrogen and valuable nano carbons: Continuous fluidized bed tests and reaction rate analysis

The goal of this research work was to evaluate a novel iron-alumina catalyst for methane and ethane pyrolysis with high conversion rates to H2 with a long life time to address two major issues reported with previous catalysts. Fluidized bed tests demonstrated continuous hydrogen production while collecting carbon containing fines. During the 230 h fluid bed test the methane conversion rate to hydrogen at 700 °C remained above 80% for 160 h while creating valuable nano carbons. Similarly high catalyst lifetimes with high methane conversions have not been reported before. Ethane was more active than methane for pyrolysis with 100% conversion to H2. Transmission electron microscopy and Raman analysis of the carbon showed single and multiwall nanotubes and conglomerate nano-fibers. Ethane did not impact the allotrope of carbon formed. TGA was used to create a data array that was used for kinetic analysis and reaction model parametric regressions. Ethane had a higher rate of pyrolysis that was 2.3–2.8 times that of methane. Ethane pyrolysis was found to be more sensitive to temperature with a higher activation energy of 62 kJ/mol as compared to methane at 43 kJ/mol. The frequency factor, Ai, was significantly higher for ethane at 0.73 [[kmol/m3-sec]/[N/m2]n] compared to 0.03 for methane on the same basis. Both gases had a functional partial pressure order dependence of ∼0.6 based on the regression analysis.

Silica-encapsulated Fe2O3 oxygen carriers for selective chemical looping combustion of hydrogen

Methane dehydroaromatization (MDA) combined with chemical looping (CL) combustion of hydrogen can overcome the MDA thermodynamic constraints conventionally limiting conversion. Development of selective hydrogen-oxidation oxygen carriers in the presence of aromatics in the CL is key to improve MDA performance in the coupled reaction system. Here we show that a novel oxygen carrier (silica-encapsulated Fe2O3 with Fe/Si mole ratio of 1.2) yields high overall methane conversion (49%) and aromatics yield (up to 36%) during 4 h of recirculating a stream in the developed system. A silica layer encapsulating Fe2O3 favors hydrogen transport and oxidation, preventing aromatics diffusing to the Fe2O3 surface. A close physical contact between iron oxide particles and silica, and near complete encapsulation of the particles by the silica layer can facilitate hydrogen conversion without full depletion of iron oxides by forming iron silicates. The silica-encapsulated iron oxide particles can be reoxidized in air after depletion of its oxidative capacity.

Production of syngas by methane dry reforming over the catalyst ZNi1‐xCex: effects of catalyst calcination and reduction temperature

AbstractBACKGROUNDA nickel‐based catalyst is active in the dry reforming of methane. However, the nickel‐metal particles' sintering at high reaction temperatures and the rapid catalyst deactivation due to coke deposition are still significant issues.RESULTSA series of catalysts, Ni1‐xCex, was prepared by the sol–gel technique and the synthesized catalysts were used for the methane dry reforming reaction considering various parameters, such as the ceria to nickel ratio, total loading, catalyst calcination, and reduction temperature. The addition of ceria to nickel increased the CH4 and CO2 percent conversion. The catalyst 40Ni0.75Ce0.25/Al2O3 calcined at 700 °C possessed a high conversion of CO2 and CH4. The reduced catalyst (40Ni0.75Ce0.25/Al2O3) showed better catalytic activity than the calcined catalyst. However, the reduced catalyst's performance declined due to coke deposition. The calcined catalyst was more stable than the reduced catalyst. The time‐on‐stream study (up to 16 h) reflected that the percent conversion and yield dropped more sharply for the reduced catalyst (% CO2 Conversion dropped: 100% to 94%) than for the calcined catalyst (% CO2 conversion dropped: 94% to 93%). The accumulation of ceria and support (alumina) increased the catalyst surface area, which improved the overall activity and stability of the catalyst. Scanning Electron Microscopy (SEM) and Raman spectroscopy analyses detected the formation of Multi Walled ‐ Carbon Nanotubes (MW‐CNT) on the used catalyst. They also show the formation of a smaller diameter of MW‐CNT (60–70 nm) over the calcined catalyst than the reduced catalyst (80–139 nm).CONCLUSIONThe calcined catalyst, 40Ni0.75Ce0.25/Al2O3–700 °C, was very active with high methane (91%) and CO2 (94%) conversions; also, the reduced catalyst was active and possessed high methane (88%) and CO2 (100%) conversions at low reaction temperatures. Overall, the calcined catalyst was comparatively more stable than the reduced catalyst. © 2022 Society of Chemical Industry (SCI).

Recent Insights into Cu-Based Catalytic Sites for the Direct Conversion of Methane to Methanol.

Direct conversion of methane to methanol is an effective and practical process to improve the efficiency of natural gas utilization. Copper (Cu)-based catalysts have attracted great research attention, due to their unique ability to selectively catalyze the partial oxidation of methane to methanol at relatively low temperatures. In recent decades, many different catalysts have been studied to achieve a high conversion of methane to methanol, including the Cu-based enzymes, Cu-zeolites, Cu-MOFs (metal-organic frameworks) and Cu-oxides. In this mini review, we will detail the obtained evidence on the exact state of the active Cu sites on these various catalysts, which have arisen from the most recently developed techniques and the results of DFT calculations. We aim to establish the structure-performance relationship in terms of the properties of these materials and their catalytic functionalities, and also discuss the unresolved questions in the direct conversion of methane to methanol reactions. Finally, we hope to offer some suggestions and strategies for guiding the practical applications regarding the catalyst design and engineering for a high methanol yield in the methane oxidation reaction.

Producing the Methane Conversion by Steam Methane Reforming Over SiO2-NiO Catalyst

Methane is the main compound of natural gas and is today used in various industrial processes and also as energy source in gas turbines or natural gas fuelled vehicles. Methane is considered a substantially more potent greenhouse gas than CO2. This study presents an approach for the catalyst of SiO2-NiO foam in steam methane reforming. The 35 wt.% composition of Silica (SiO2) that produced from Rice Husk Ash (RHA) was used in this study by using slurry technique to produce of SiO2-NiO foam at 950°C and 1250°C of sintering temperatures. The morphologies of SiO2 -NiO foams were observed by using the Scanning Electron Microscopy (SEM) to identify the pore size as a catalyst filter. The pore size of SiO2-NiO foam were found in the range 13µm to 43µm. SiO2-NiO catalyst were characterized by temperature programmed reduction (TPR). The TPR result showed present of SiO2-NiO was successfully activated at the 320°C at 30 minutes. Meanwhile, in the temperature range of 500°C–600°C, the catalyst was shown to be particularly active and stable. The high methane conversion 42% was produced at the reaction temperature of 600°C.

Non-thermal plasma-assisted steam methane reforming for electrically-driven hydrogen production

Plasma-assisted steam methane reforming (SMR) has become a promising approach for low temperature and small-scale hydrogen production. To increase H2 yields, water-gas-shift reactions are needed to drive the formed CO to CO2 and H2. In this study, bulk gas temperature, plasma power and water feed rate strongly impacted the CO and CO2 product selectivity at high methane conversions of 60–80% in the presence of a Ni-based catalyst. CO2-enriched hydrogen could be formed directly with H2O/methane ratios > 4. To further increase the CO2/CO product selectivity, a “one-pot” cascade design with a Cu/ZnO/Al2O3/MgO catalyst bed placed downstream of the plasma zone achieved substantially higher CO2/CO selectivity (>15) in the effluent gas at 60% methane conversion and 300 °C. Comparably, placing the Cu-based catalyst in the plasma zone does not alter the CO2/CO selectivity. This study highlights the use of plasma reactor systems to directly tune the catalytic SMR performance and lead to an electrified route for hydrogen production.

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed.