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Supercritical water gasification of naphthalene over iron oxide catalyst: A ReaxFF molecular dynamics study

Summary

ReaxFF MD (ADF/SCM) explores iron-oxide-catalyzed supercritical water gasification (SCWG) of naphthalene as a PAH surrogate. SCW supplies H for H\(_2\) and O for CO while Fe\(_2\)O\(_3\) clusters provide labile oxygen and catalytic C–C activation. Simulations sweep composition/temperature to map H\(_2\)/CO evolution, catalyst deactivation (carbon, O vacancy, Fe loss), and O\(_2\) regeneration pathways. International Journal of Hydrogen Energy framing connects SCWG to renewable waste streams where PAHs model tar chemistry from biomass pyrolysis condensates.

Methods

  • Catalyst: α-Fe\(_2\)O\(_3\)-derived cluster (~15 Å diameter) after heat/cool protocol + minimization (details in SI).
  • Cell: Periodic 60×60×60 Å\(^3\) boxes with random reactant placement (ADF library).
  • Potential: ReaxFF parametrization from Shin et al. (Fe–O–C–H).
  • MD: Energy minimization → heat 0–300 K (30 K/ps) → hold 300 K for 20 ps → ramp to target (100 K/ps) → 800 ps production at 3000 K (main text); NVT, Berendsen thermostat 100 fs damping, 0.25 fs timestep. ADF ReaxFF module for integration and analysis (Table 1 cases).
  • Rationale: Elevated T accelerates chemistry to fit ~ns windows (discussion cites Voter et al.).

Engine, cell, and pressure targets. Reactive molecular dynamics uses the ADF/SCM ReaxFF module (not LAMMPS for the main production runs tabulated) in periodic 60 Å cubic cells with α-Fe₂O₃ clusters and random reactant placement as above. Atoms: total counts follow from stoichiometries in Table 1 of the article. NVT at 3000 K with Berendsen thermostat (100 fs damping) and 0.25 fs timestep for 800 ps production after heating ramps. Barostat / hydrostatic pressure: N/A — constant-volume NVT without GPa servocontrol in the summarized cases. External electric field: N/A. Enhanced sampling: N/A — brute-force MD at elevated temperature to accelerate chemistry.

Findings

  • Synergy between SCW and Fe oxide promotes NAP conversion and H\(_2\)/CO formation.
  • Water acts as both H and O source; oxide supplies lattice oxygen, activates SCW, and weakens C–C bonds.
  • Parameter trends: More H\(_2\)O raises H\(_2\)/CO yields and lattice-O replenishment but can slow CO generation rate; high NAP loading increases H\(_2\) recovery yet lowers carbon gasification efficiency.
  • Deactivation: Carbon deposition, lattice oxygen depletion, Fe loss; SCW mitigates Fe loss; calcination in O\(_2\) clears C and restores O. Parameter sweeps over naphthalene loading and water fraction illustrate trade-offs between H\(_2\) yield and carbon gasification efficiency quoted in the abstract-level discussion.

Limitations

3000 K, sub-ns trajectories are accelerated conditions; quantitative agreement with pilot SCWG requires extrapolation.

Continuous flow reactors, sulfur/alkali poisons, and supported catalyst morphologies differ from isolated Fe\(_2\)O\(_3\) clusters in periodic boxes; scale Fe/O stoichiometry trends cautiously to fixed-bed pilot units.

Char gasification kinetics for lignin/cellulose oligomers beyond naphthalene may require additional ReaxFF training if oxygenated PAH intermediates dominate tar under real feedstocks.

Pressure vessels for SCW operation introduce mechanical constraints and concentration gradients absent from uniform periodic supercells in the MD study.

Relevance to group

Uses van Duin-lineage ReaxFF Fe parameters extended by Shin and collaborators for heterogeneous SCW chemistry.

Citations and evidence anchors

  • papers/ReaxFF_others/Han_Fe_supercritical_IJHE_2019.pdf