A melt-based reaction pathway for CO2 and CH4 conversion to syngas and carbon using liquid In–Sn

Abstract

Molten In–Sn has been recently reported as an effective catalyst for directly producing 2:1 H₂:CO syngas and solid carbon from CO₂ and CH₄ in a single reaction, equivalent to the combined reactions of methane pyrolysis and dry reforming. In the present work, when reactants are alternately fed, mass balances indicate that CO₂ reacts to form an [O] species that oxidizes CH₄. Also, CH₄ is converted to a [C] species that reduces CO₂. The presence of accumulated [O] did not significantly affect the experimentally determined activation energies of 229 kJ/mol vs. 222 kJ/mol. We performed ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations, to probe the dynamic structural evolution and to obtain atomistic insights into the behavior of the molten In–Sn alloy at the molecular level. This integrated approach also facilitated the evaluation of activation energy barriers associated with key reaction pathways. Simulations indicate that [O] are solvated; rapidly switching neighbors between Sn and In, which is unique to a molten catalyst. The presence of accumulated [C] significantly decreased the experimentally observed apparent CO₂ activation energy from 154 kJ/mol to 75 kJ/mol, which is lower than the direct reaction between CO₂ and solid graphite. This finding supports the simulations that indicate [C] formed in the melt is solvated, chemically distinct from solid carbon, and can play a critical role in enhancing catalytic performance. Large fluctuations in adsorbate binding energies were theoretically observed over time, suggesting the creation of transient sites that are fleetingly active, and facilitate the formation of solvated [O] and [C] intermediates unique to a molten catalyst surface.