CH₄ suppresses bilayer-like H₂ interfacial enrichment on Fe (110): molecular dynamics evidence and trace-species modulation under hydrogen-blended natural gas conditions

Abstract

Methane (CH₄) can dominate the gas–steel interface and regulate the near-surface accessibility of hydrogen (H₂) under hydrogen-blended natural gas (HBNG) conditions. Here, classical molecular dynamics simulations of a “gas mixture–Fe (110)” interface representative of X80 pipeline steel quantify CH₄ suppression of bilayer-like H₂ interfacial enrichment and its modulation by trace species (H₂O, H₂S, CO, and CO₂). As CH₄ loading increases, H₂ near-surface enrichment is strongly suppressed: the time-averaged H₂ occupancy within 0–5 Å of the outermost Fe plane (sum of the first two interfacial layers) decreases from (NT=139.92) in the CH₄-free case to 25.00 at the highest CH₄ loading (−82.1%). Correspondingly, the near-surface H₂ enrichment peaks observed without CH₄ progressively attenuate and become indistinguishable at higher CH₄ loadings. In contrast, CH₄ retains a robust bimodal near-surface distribution, with characteristic peaks at 3.2 and 4.3 Å from the outermost Fe plane, consistent with a persistent CH₄-dominated bilayer-like occupation that limits H₂ approach and residence through preferential interfacial occupation and spatial exclusion. Within the examined trace fraction (0–4%), impurities reshape the interfacial composition but do not restore pronounced H₂ bilayer enrichment; instead, they induce species- and concentration-dependent changes in apparent H₂ mobility, including a marked suppression at high CO₂ fraction (DH2=117.63 Ų/ps at 4% CO₂, versus 142.04 Ų/ps in the impurity-free baseline). Overall, the results support a mechanism of “CH₄-dominated interfacial occupation → reduced H₂ accessibility → trace-species-dependent kinetic modulation”, informing composition-control strategies for HBNG.

Keywords: Hydrogen-blended natural gas (HBNG), Methane, Interfacial bilayer enrichment, Molecular dynamics, Fe (110)