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Abstract

The combination of dry reforming of methane (DRM) and methane pyrolysis allows methane (CH₄) and carbon dioxide (CO₂) to be converted into a mixture of 2:1 H₂:CO (syngas) while producing solid carbon as a co-product. However, traditional solid catalysts rapidly deactivate due to carbon deposition. This study explores molten metal catalysts, which can be used in bubble column reactors with continuous carbon separation for long-term stability. A total of 22 molten metals and alloys were experimentally compared, and Sn-In showed the best performance. It activated both CH₄ and CO₂ effectively. The initial activity for CO₂ activation was related to the Gibbs free energy of oxide formation. Sn-In remained active for over 22 hours without signs of deactivation in a bubble column reactor. Surface tension measurements were used to quantify the surface composition, showing the surface and bulk compositions of Sn-In were nearly identical. However, in Ni-In alloys, the surface is primarily In up to at least 50% Ni. For both Ni-In and Sn-In, increasing In content led to higher CH₄ and CO₂ conversions compared to pure In. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations showed that In donated electrons to Sn, causing In to become more positively charged. Short-pulsed reaction experiments were developed to measure reaction intermediates, leading to the proposal of a melt-based reaction pathway. Kinetic studies and DFT calculations confirmed that solvated [C] and [O] species play a key role, with mobile [O] reacting with [C] to produce CO. When carbon was present on the melt surface, the activation energy for CO₂ activation dropped to 75 kJ/mol, significantly lower than for a clean surface (153 kJ/mol) or CO₂ reacting with carbon in the absence of Sn-In (141 kJ/mol). Cu-In was also found to produce carbon nanofibers (CNFs), a first for molten catalysts in bubble columns. The CNFs had a mix of tubular and bamboo-like structures. Metal droplets assisted CNF growth, and a droplet-based mechanism was proposed. A heat treatment at 1096 °C removed residual metal impurities, yielding >99.9% pure carbon.