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A reactive force field molecular dynamics simulation of nickel oxidation in supercritical water

Summary

Reactive molecular dynamics with a reactive force field (ReaxFF-class) is used to study nickel surface oxidation in supercritical water (SCW) over a range of temperatures (300–800 °C) and densities (26–164 kg m⁻³) stated in the abstract. The motivation is to probe how extreme water fluid states—between dense liquid and low-density steam—change nickel passivation chemistry relative to near-ambient interfaces. The work follows water adsorption, dissociation, and deprotonation, together with nickel hydroxylation/oxidation, to interpret how SCW conditions alter reaction mechanism compared with ambient liquid water.

Methods

MD (LAMMPS, ReaxFF). §2 documents Ni–O–H ReaxFF (Assowe et al.) simulations in LAMMPS (papers/ReaxFF_others/2018_LiqiangAi_A reactive force field molecular dynamic simulation of nickel oxidation in supercritical water.pdf). A fcc Ni slab plus supercritical water (SCW) occupies 28.896 × 28.896 × 51.896 ų with 968–1078 atoms depending on (110)/(100)/(111) termination (Table 1). PBC span x and y; the z axis uses a fixed boundary with a reflecting wall so only one Ni–water interface reacts. Production runs use NVT integration, Berendsen thermostat, and Δt = 0.25 fs across 4,000,000 steps (~1 ns at that timestep); thermostat damping constants are not quoted in the §2 excerpt checked here. The Ni–water slab leg is constant-volume (no barostat on that segment). Auxiliary bulk SCW NPT trajectories (~0.25 ns per state point) map temperature–density grids against the FEQ Helmholtz EOS (Fig. 4). State points follow the abstract grids (300–800 °C at selected ρ, and 600 °C across 26.1–164 kg m⁻³). Electric fields and replica / enhanced sampling are not applied.

Force-field training: N/A — applies the published Assowe parameter set.

Static QM (validation). CASTEP spin-polarized PBE with ultrasoft pseudopotentials and a 571 eV cutoff benchmarks gas-phase water, water/Ni, and OH/Ni(110) energetics versus ReaxFF (§2, Figs. 1–2).

Findings

Time-resolved bond metrics (r_cut,NiO = 2.1 Å, r_cut,OH = 1.2 Å, §3) show hydration, water dissociation, hydroxylation, and oxide thickening with Ni/O rearrangement. SCW is argued to be a stronger oxidizer than ambient water for the sampled states, with rates rising with temperature and fluid density. Deprotonation statistics motivate homolytic-leaning water cleavage at high T and lower ρ, framed as more radical-like than ambient ionic chemistry. Ni(110)/(100)/(111) comparisons use Table 1 trajectories. Limitations: accuracy stays within the Assowe training manifold; DFT checks are spot validations, not exhaustive oxide coverage.

Limitations

This repo’s PDF filename suggests a 2018 volume imprint; the abstract header lists J. Supercrit. Fluids 133, 421–428 with ©2017 — reconcile against your PDF’s final bibliographic block. Supercritical water chemistry is sensitive to cluster statistics and surface facet choice; the abstract’s mechanistic classification should be read alongside the article’s full surface model and sampling duration rather than as a universal Ni oxidation rule across all crystallographic terminations.

Relevance to group

Application of ReaxFF-style reactive MD to corrosion/oxidation of Ni in extreme water environments (energy-systems context).

Citations and evidence anchors

  • DOI: 10.1016/j.supflu.2017.10.025.

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