*2.2. Turquoise Hydrogen*

Thermal or thermocatalytic decomposition (cracking or pyrolysis) is one of the methods to produce hydrogen with a low carbon footprint. The reaction of splitting methane molecules into hydrogen and carbon is carried out in a pyrolysis reactor at temperature from 600 to 1200–1400 ◦C [23]. Many studies are devoted to the search of effective catalysts in pyrolysis. However, the problem of catalyst deactivation, highlighted in these studies, cannot be solved by burning carbon from the catalyst surface, since in this case CO2 emissions become comparable to the emissions in steam reforming of methane [24]. The problem of coking is less acute when using carbon catalysts—activated and black carbon, graphite, nanostructured carbon, etc. [25–27]. In this case, the reactor contains a fluidized bed with carbon catalyst particles [24]. However, all catalysts are subject to deactivation, and the formation of solid carbon as a product can lead to reactor fouling [28]. Another method of methane pyrolysis is the removal of these disadvantages thanks to the use of molten metal as a heat carrier and a bubble column for the extraction of solid carbon and gaseous hydrogen. In addition, the produced carbon, as well as the liquid metal, can act as a catalyst for the process [29]. As a rule, Sn [15,29] and Pb [17] are used as molten metal; however, there are studies aimed at finding new materials, e.g., Ni-Bi melt is proposed in [16]. Ni-Bi melt possesses catalytic properties capable of increasing methane conversion, in contrast to non-catalytic Sn and Pb melt [13,30–39]. The effect of temperature on the direct cracking of methane was studied in [30]. This research confirmed that the accumulation of carbon black in the reaction tube is the main technological obstacle in the implementation of the process of direct thermal cracking of methane. There were also studies on the types of catalysts used for methane cracking in order to reduce the amount of carbon dioxide generated in hydrogen production from methane [32,35]. The thermochemical model for the assessment of factors influencing the process of obtaining hydrogen by pyrolysis of methane was presented in [34]. It was discovered that the temperature and residence time of the gas have the greatest impact on the yield of methane conversion. It was also confirmed in [37,38]. In [37,39] there was analyzed the process of thermocatalytic

decomposition of methane in a fluidized bed reactor aimed at reducing the amount of harmful emissions.

According to preliminary modeling, hydrogen with a negative carbon footprint can be produced by pyrolysis of biogas, in which carbon has been captured naturally from the air during biomass formation. In addition, it is assumed that solid carbon produced by pyrolysis of methane can be used for the production of nanostructured materials such as carbon nanotubes and fullerenes [40]. At present, the high cost of such materials limits their broad use, e.g., in the construction of highways or cement production.
