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Currently, hydrogen is mainly generated by steam methane reforming, with significant CO2 emissions, thus exacerbating the greenhouse effect. This environmental concern promotes methane cracking, which represents one of the most promising alternatives for hydrogen production with theoretical zero CO/CO2 emissions. Methane cracking has been intensively investigated using metallic and carbonaceous catalysts. Recently, research has focused on methane pyrolysis in molten metals/salts to prevent both reactor coking and rapid catalyst deactivation frequently encountered in conventional pyrolysis. Another expected advantage is the heat transfer improvement due to the high heat capacity of molten media. Apart from the reaction itself that produces hydrogen and solid carbon, the energy source used in this endothermic process can also contribute to reducing environmental impacts. While most researchers used nonrenewable sources based on fossil fuel combustion or electrical heating, concentrated solar energy has not been thoroughly investigated, to date, for pyrolysis in molten media. However, it could be a promising innovative pathway to further improve hydrogen production sustainability from methane cracking. After recalling the basics of conventional catalytic methane cracking and the developed solar cracking reactors, this review delves into the most significant results of the state-of-the-art methane pyrolysis in melts (molten metals and salts) to show the advantages and the perspectives of this new path, as well as the carbon products’ characteristics and the main factors governing methane conversion.

The study experimentally investigated a novel approach for producing hydrogen from methane cracking in dielectric barrier discharge catalytic plasma reactor using a nanocatalyst. Plasma-catalytic methane (CH4) cracking was undertaken in a dielectric barrier discharge (DBD) catalytic plasma reactor using Ni/MgAl2O4. The Ni/MgAl2O4 was synthesised through co-precipitation followed customised hydrothermal method. The physicochemical properties of the catalyst were examined using X-ray diffraction (XRD), scanning electron microscopy—energy dispersive X-ray spectrometry (SEM-EDX) and thermogravimetric analysis (TGA). The Ni/MgAl2O4 shows a porous structure spinel MgAl2O4 and thermal stability. In the catalytic-plasma methane cracking, the Ni/MgAl2O4 shows 80% of the maximum conversion of CH4 with H2 selectivity 75%. Furthermore, the stability of the catalyst was encouraging 16 h with CH4 conversion above 75%, and the selectivity of H2 was above 70%. This is attributed to the synergistic effect of the catalyst and plasma. The plasma-catalytic CH4 cracking is a promising technology for the simultaneous H2 and carbon nanotubes (CNTs) production for energy storage applications.

The investigation of the course of the Boudouard reaction and methane cracking was performed over nickel catalysts based on oxides of calcium, aluminum, and magnesium. The catalytic samples were synthesized by the impregnation method. The physicochemical characteristics of the catalysts were determined using atomic adsorption spectroscopy (AAS), Brunauer–Emmett–Teller method analysis (BET), temperature-programmed desorption of ammonia and carbon dioxide (NH3- and CO2-TPD), and temperature-programmed reduction (TPR). Qualitative and quantitative identification of formed carbon deposits after the processes were carried out using total organic carbon analysis (TOC), temperature-programmed oxidation (TPO), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The selected temperatures for the Boudouard reaction and methane cracking (450 and 700 °C, respectively) were found to be optimal for the successful formation of graphite-like carbon species over these catalysts. It was revealed that the activity of catalytic systems during each reaction is directly related to the number of weakly interacted nickel particles with catalyst support. Results of the given research provide insight into the mechanism of carbon deposit formation and the role of the catalyst support in this process, as well as the mechanism of the Boudouard reaction.

When driven by sunlight, molten catalytic methane cracking can produce clean hydrogen fuel from natural gas without greenhouse emissions. To design solar methane crackers, a canonical plug flow reactor model was developed that spanned industrially relevant temperatures and pressures (1150–1350 Kelvin and 2–200 atmospheres). This model was then validated against published methane cracking data and used to screen power tower and beam-down reactor designs based on “Solar Two,” a renewables technology demonstrator from the 1990s. Overall, catalytic molten methane cracking is likely feasible in commercial beam-down solar reactors, but not power towers. The best beam-down reactor design was 9% efficient in the capture of sunlight as fungible hydrogen fuel, which approaches photovoltaic efficiencies. Conversely, the best discovered tower methane cracker was only 1.7% efficient. Thus, a beam-down reactor is likely tractable for solar methane cracking, whereas power tower configurations appear infeasible. However, the best simulated commercial reactors were heat transfer limited, not reaction limited. Efficiencies could be higher if heat bottlenecks are removed from solar methane cracker designs. This work sets benchmark conditions and performance for future solar reactor improvement via design innovation and multiphysics simulation.

A molten-metal catalyst exhibits strong resistance to carbon encapsulation and deactivation, due to its unique physical and chemical properties, demonstrating excellent catalytic activity and stability. This paper overviews recent developments in molten-metal catalysts for methane cracking and hydrogen production. It thoroughly examines the stability of reactors, carbon products, and catalysts for each molten-metal system. The kinetics and mechanism of the catalysts in each system have also been analyzed. Finally, for future development, several recommendations for hydrogen production via methane cracking have been proposed, addressing the following research challenges: (1) gaining a deeper understanding of the active sites and methane conversion process, which can provide crucial guidance for designing high-performance catalysts; (2) fostering the advancement of new reaction interfaces; and (3) attempting to develop a low-eutectic-point molten salt system for chemical vapor deposition reactions. The molten-metal catalyst exhibits strong resistance to carbon encapsulation and deactivation due to its unique physical and chemical properties, demonstrating excellent catalytic activity and stability.

Methane cracking is considered a bridge technology between gray and green hydrogen production processes. In this work an experimental study of methane cracking in molten tin is performed. The tests were conducted in a quartz reactor (i.d. = 1.5 cm, L = 20 cm) with capillary injection, varying temperature (950–1070 °C), inlet methane flow rate (30–60 mL·min –1) and tin height (0–20 cm). The influence of the residence time in the tin and in the headspace on methane conversion and on carbon morphology was investigated. The conversions obtained in tin and in the empty reactor were measured and compared with results of detailed kinetic simulations (CRECK). Furthermore, an expression of a global kinetic constant for methane conversion in tin was also derived. The highest conversion (65% at Q 0 = 30 mL·min –1 and t = 1070 °C) is obtained for homogeneous gas phase reaction due to the long residence time (70 s), the presence of tin leads to a sharp decrease of residence time (1 s), obtaining a conversion of 35% at 1070 °C, thus meaning that tin owns a role in the reaction. Carbon characterization (scanning electron microscopy, Raman) reported a change in carbon toward sheet-like structures and an increase of the carbon structural order in the presence of molten tin media.

High mass loading (ca. 30 mg/cm2) electrodes were prepared with carbon recovered from catalytic methane cracking (MC). As-fabricated supercapacitors displayed 74% of capacitance retention from 6 mA/cm2 to 60 mA/cm2 and a Ragone plot’s slope of −7 Wh/kW (compared to 42% and −31 Wh/kW, respectively, for high mass loading devices fabricated with commercial carbon). The high-rate capability of the MC-recovered carbon is attributed to the presence of carbon black and carbon nanotubes produced during the reaction, which likely increased the electronic and ionic conductivity within the electrode. These results suggest that the by-product of this hydrogen generation route might be a suitable active material for supercapacitors.

The catalytic behaviour of Co 3 Mo 3 C, Co 6 Mo 6 C, Co 3 Mo 3 N and Co 6 Mo 6 N for methane cracking has been studied to determine the relationship between the methane cracking activity and the chemical composition. The characterisation of post-reaction samples showed a complex phase composition with the presence of Co 3 Mo 3 C, α-Co and β-Mo 2 C as catalytic phases and the deposition of different forms of carbon during reaction. Graphical Abstract

Up to 80% of hydrogen production is currently carried out through CO2 emission-intensive natural gas reforming and coal gasification. Water-splitting electrolysis using renewable energy (green H2) is the only process that does not emit greenhouses gases, but it is a quite energy-demanding process. To significantly contribute to the clean energy transition, it is critical that low-carbon hydrogen production routes that can replace current production methods and can expand production capacity to meet new demands are developed. A new path, alternative to steam reforming coupled with CCS (blue H2) that is based on methane cracking, in which H2 production is associated with solid carbon instead of CO2 (turquoise H2), has received increasing attention recent years. The reaction takes place inside the liquid bath, a molten metal reactor. The aim of this article is to model the main kinetic mechanisms involved in the methane cracking reaction with molten metals. The model developed was validated using experimental data produced by the University of La Sapienza. Finally, such a model was used to scale up the reactor architecture.

Methane pyrolysis is a transitional technology for environmentally benign hydrogen production with zero greenhouse gas emissions, especially when concentrated solar energy is the heating source for supplying high-temperature process heat. This study is focused on solar methane pyrolysis as an attractive decarbonization process to produce both hydrogen gas and solid carbon with zero CO2 emissions. Direct normal irradiance (DNI) variations arising from inherent solar resource variability (clouds, fog, day-night cycle, etc.) generally hinder continuity and stability of the solar process. Therefore, a novel hybrid solar/electric reactor was designed at PROMES-CNRS laboratory to cope with DNI variations. Such a design features electric heating when the DNI is low and can potentially boost the thermochemical performance of the process when coupled solar/electric heating is applied thanks to an enlarged heated zone. Computational fluid dynamics (CFD) simulations through ANSYS Fluent were performed to investigate the performance of this reactor under different operating conditions. More particularly, the influence of various process parameters including temperature, gas residence time, methane dilution, and hybridization on the methane conversion was assessed. The model combined fluid flow hydrodynamics and heat and mass transfer coupled with gas-phase pyrolysis reactions. Increasing the heating temperature was found to boost methane conversion (91% at 1473 K against ~100% at 1573 K for a coupled solar-electric heating). The increase of inlet gas flow rate Q0 lowered methane conversion since it affected the gas space-time (91% at Q0 = 0.42 NL/min vs. 67% at Q0 = 0.84 NL/min). A coupled heating also resulted in significantly better performance than with only electric heating, because it broadened the hot zone (91% vs. 75% methane conversion for coupled heating and only electric heating, respectively). The model was further validated with experimental results of methane pyrolysis. This study demonstrates the potential of the hybrid reactor for solar-driven methane pyrolysis as a promising route toward clean hydrogen and carbon production and further highlights the role of key parameters to improve the process performance.