Methane (CH4) cracking is emerging as a bridge technology for hydrogen (H2) production fostering the transition from current carbon-intensive production processes (e.g. steam reforming) to the fixation of the carbon content of the feedstock in the form of valuable carbon materials. With the valorization of the carbon product being a key factor to lower H2 prices by improving the overall process economics, the understanding and the description of the complex network of homogeneous and heterogeneous reactions involved in CH4 thermal cracking and, in particular, in the formation of valuable carbon and of undesired byproducts (carbon nanoparticles) is key to technology design and operations. In this work we present a systematic experimental study of pure methane cracking in a tubular quartz reactor in the temperature range T = 875-975 degrees C at initial flow rates QCH4,0 = 30, 45 and 60 ml/min. CH4 conversion and H2 yields have been measured quantitatively and complemented with the identification of key aromatic precursors supporting the interpretation of the mechanistic phenomena aided by an existing chemical kinetic model and chemical kinetic analyses. A careful analysis of the carbon products allowed to explore the different morphologies of deposited carbon and carbon nanoparticles in the gas phase, as well as their strong dependence from homogeneous gas-phase reactivity, motivating further model developments aimed to describe currently missing complex heterogeneous reaction pathways towards a quantitative description of the observed features.

Methane pyrolysis is now considered a promising process for producing clean hydrogen and high-value carbon materials. However, it requires very high temperatures (above 1000 degrees C) due to the kinetic barriers posed by the stable C-H bond, and the production of carbon presents a significant challenge. While solid catalysts can lower the operational temperatures to some extent, they are hindered by carbon accumulation, which deactivates the catalysts and clogs reactors, thus limiting process scalability. Recently, molten media have emerged as potential catalysts for methane pyrolysis. These media offer numerous advantages, including high thermal conductivity and resistance to deactivation via sintering or coking. Despite these advantages, a comprehensive understanding of how the physical properties and intrinsic catalytic activities of molten media influence methane pyrolysis is lacking. This review addresses this gap by examining the roles of physical properties, mainly surface tension, and catalytic activity in methane conversion and carbon morphology. The analysis of apparent activation energies across various molten media indicates that their physical properties significantly impact methane reactivity, challenging the conventional notion of catalytic activity. In summary, this review explores the synergistic effects of molten media's physical and catalytic properties on methane pyrolysis, highlighting the potential for these systems to revolutionize the process by enhancing efficiency and reducing operational challenges. Understanding these interactions is key to advancing the scalability and applicability of methane pyrolysis technologies for sustainable hydrogen production.

The Gibbs free energies of the formation of several polyynes (C6H2, C10H2 and C16H2) and aromatic species (C6H6, C10H8 and C16H10) from methane and acetylene at temperatures of 1000-2600 K and atmospheric pressure were obtained by quantum chemical calculations using the RI-MP2 method in the ORCA open source software. At lower temperatures, aromatic species form more readily than polyynes, while at temperatures >2200 K the trend reverses and polyyne formation becomes predominant.

The results of shock-tube measurements of acetylene concentration and characteristics of the ensemble of soot particles during the pyrolysis of benzene, ethylene, ethylene-methane and ethylene-propane diluted with argon are compared with the predictions of the unified kinetic model of soot formation implemented in the MACRON program based on the Galerkin method. The simulation results demonstrate good agreement with experimental data for the pyrolysis of ethylene alone and with the specified additives, thereby confirming the two- route mechanism of soot nuclei formation. The agreement for benzene pyrolysis is less satisfactory.

The kinetic patterns of the attainment of the equilibrium product composition in non-catalytic processes of partial oxidation and of steam and carbon dioxide reforming of hydrocarbons in the temperature range 1400–1800 K, characteristic of these processes, were analyzed. The need for such analysis is caused by the rapidly increasing consumption of natural gas as a chemical feedstock and by growing attention to environmental problems, in particular, to a decrease in СО 2 emissions or to partial CO 2 utilization. The forward and reverse water gas shift reactions (WGSRs) play an important role in approach to the equilibrium product composition in these processes. Analysis has shown that the elementary reactions characteristic of forward and reverse WGSRs start to play a significant role long before the equilibrium in the system is attained. Already in the intermediate steps of the process, the distribution of the major reaction products, Н 2 , СО, Н 2 О, and СО 2 , almost corresponds to the equilibrium value of K t = ([H 2 ][CO 2 ])/([CO][H 2 O]), close to the WGSR equilibrium constant K eq , and further conversion of the products occurs at K t values close to K eq .

Kinetic modeling of pyrolysis of acetylene diluted with argon showed a strong influence of small additives of oxygen on the routes of formation of soot nuclei. The influence of oxygen on various channels of formation and consumption of propargyl radicals C3H3, which are important precursors of soot formation, as well as the fundamental possibility of controlling the process of soot formation and its properties are considered.

A detailed kinetic modeling of the noncatalytic processes of thermal pyrolysis and steam and carbon dioxide reforming of methane revealed almost completely identical kinetics of the methane conversion in these processes. This suggests that, in the temperature range 1400–1800 K, the initial stage of conversion of methane in all these processes is its thermal pyrolysis. The modeling results agree well with the experimental data on methane pyrolysis. For the temperature range examined, the Arrhenius expressions (pre-exponential factors and activation energy) were obtained in the first-order kinetics approximation for the rate of methane conversion in the processes studied. The expressions derived may be useful for making preliminary estimates and carrying out engineering calculations.