Investigation of Molten Metals and Alloys as Catalysts for Methane Pyrolysis

Hydrogen is a high-potential vector in the transition of the current fossil energy system to a sustainable one. In order to achieve the ambitious net-zero emission goal, not only the conventional, carbon-intensive fuels and reduction agents must be substituted, but the required replacements must also be generated in a climate-friendly yet economical manner. In this regard, methane pyrolysis emerges as a promising alternative to the conventional H2 production route, i.e., steam reforming of methane, as no carbon dioxide is emitted in the base reaction. In this process, methane is heated and decomposed in an oxygen-free atmosphere. The products are gaseous hydrogen and solid carbon. Several approaches for technical implementation exist, e.g., regarding varying reactor concepts or the utilization and the type of catalysts. This thesis focuses on methane pyrolysis in molten metals and alloys. The primary purpose of the investigated metallic melts is to transfer heat to the injected methane and catalyze its decomposition. Various metals, binary and ternary alloys, are compared in terms of the methane conversion achieved in an experimental reactor. The results prove that methane pyrolysis can be effectively catalyzed with an appropriate selection of alloying constituents, such as nickel, in a base metal, such as tin. However, the catalysts only have a significant effect at process conditions, where the reaction itself is rate-limiting, i.e., at lower temperatures. Reduced surface tension and viscosity are identified as beneficial features of the melts in all temperature ranges. This effect is attributed to an increased reaction rate, resulting, inter alia, from a decreased average size of the bubbles the injected methane forms in the liquid catalyst. Some specific metals, for example, tin and bismuth, are assumed to have those advantageous properties. Besides the quantity of the produced solid carbon, its quality, i.e., its purity and morphology, is influenced by the composition of the molten catalyst. The findings provide a profound base for effectively designing industrial reactors that facilitate the generation of marketable, high-quality products, hydrogen and carbon.