Investigating the Role of Intermediate Carbon Species on Methane Activation over Molten In–Sn and In–Ni

Abstract Methane pyrolysis over molten metal catalysts has gained attention as a promising route for CO2-free hydrogen production owing to the intrinsic resistance of liquid catalysts to carbon-induced deactivation. In this work, we investigate the role of intermediate carbon species on the surface composition and methane activation over molten In–Sn and In–Ni alloys using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. Free-energy calculations indicate that methane dissociation proceeds through the formation of carbon monomer and dimer intermediates, which are likely to serve as the initial building blocks for carbon nucleation and subsequent growth of larger poly-aromatic carbon structures. Carbon exhibits distinct segregation behavior in the two molten alloys, preferentially occupying the surface and subsurface regions in In–Sn, while migrating toward the subsurface and bulk in In–Ni. These contrasting distributions are largely independent of the carbon concentration and arise from the preferential interaction of carbon with Sn in In–Sn and Ni in In–Ni. Although carbon segregation modifies the local surface environment, methane activation barriers remain qualitatively unchanged, while the dissociated intermediates are preferentially stabilized on surface carbon species rather than on the molten alloy surface, highlighting the sustained catalytic activity of the molten catalysts.