Exposure-Consistent Hydrogen Boundary Conditions Derived from Reactive Molecular Dynamics of Methane Decomposition on Nickel Surfaces

Hydrogen-assisted degradation of metals is governed by the local hydrogen population available at near-surface lattice sites and defects. However, atomistic mechanical simulations commonly employ hydrogen concentrations that are not representative of realistic exposure conditions. In this study, ReaxFF-based reactive molecular dynamics is used to translate sustained CH4 and CH4 + H2O exposure on a Ni(211) surface under an accelerated closed-cell protocol at 1000–1350 K into exposure-conditioned hydrogen availability descriptors. Three independent trajectories are performed for each exposure-temperature condition to quantify the normalized Ni-associated H inventory, hydrogen partitioning between adsorbed and subsurface regions, and localization across CN-defined step and terrace regions. A key finding is a dry–wet inversion in hydrogen accessibility. Although CH4 + H2O exposure produces a larger normalized Ni-associated H inventory (θH = 0.67–0.77), 71.6–86.1% of that inventory is primarily O-bound, while less than 3.5% of the geometrically defined near-surface H population lies within the subsurface window. In contrast, CH4-only exposure yields a smaller normalized Ni-associated H inventory (θH = 0.088–0.447) but substantially greater subsurface partitioning, reaching 12.3% at 1350 K. These results show that the surface with the largest normalized Ni-associated H inventory does not necessarily have the largest geometric subsurface fraction. Furthermore, CH4 + H2O suppresses carbon accumulation, sustains an oxygen-containing surface state, and maintains higher per-site hydrogen localization around CN-defined step regions. Additional Ni(111) and laterally doubled Ni(211) checks retain the wet-over-dry H areal-density ordering and the complementary dry-over-wet subsurface-fraction ordering at 1250 K. The resulting protocol-specific descriptors provide candidate inputs for controlled downstream simulations of hydrogen transport, trapping, and degradation.