Mechanism and Energetics of Hydrogen Sulfide Thermolysis from Reactive Molecular Dynamics: Cutoff-Radius Effects, Thermochemically Validated Energy Costs, and the Elementary Reaction Network

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🔬研究者:Provides validated molecular-level data on H2S thermolysis mechanism and energy costs, useful for further simulation and experimental studies.

🏢実務担当者:Offers insights for engineering H2S valorization processes, potentially relevant for chemical and energy companies seeking hydrogen production routes.

🏛政策担当者:Highlights the potential of H2S as a hydrogen source, which could inform policies on hydrogen supply diversification and decarbonization.

Hydrogen sulfide (H₂S), a high-volume by-product of the hydrodesulfurization of fossil fuels, can be valorized by thermolysis to recover both molecular hydrogen and elemental sulfur, rather than being oxidized as in the conventional Claus process. The viability of this route depends on quantitative knowledge of the reaction mechanism and of the energy costs of dissociation, which are difficult to obtain experimentally at the temperatures involved. Here we study H₂S thermolysis by reactive molecular dynamics (RMD) with the ReaxFF potential for systems of 1000 H₂S molecules at 1 atm, addressing three coupled questions: the simulation parameters required for dilute gases, the energetics of dissociation, and the elementary reaction mechanism. The interaction cutoff radius proved critical: the original 10 Å value, parametrized for condensed systems, misses about 23 eV of attractive non-bonded interaction energy in the gaseous system and fails to capture dissociation at 3000 K within 20 ns, whereas radii of 30–40 Å converge. Using a 40 Å cutoff at 2500, 3000 and 3500 K, atom-resolved species-transition records reveal a free-radical chain mechanism with temperature-invariant elementary steps: S–H homolysis initiates the chain, hydrogen abstraction (H• + H₂S → H₂ + HS•) is essentially the exclusive source of H₂ (persistent H•+H• recombination contributed only 1, 13 and 17 events, below 0.5% of the abstraction count), and a slow sulfur-condensation stage (S₂ → S₃ → S₄) limits the net conversion, which reached 8.7%, 26.7% and 46.3%. The enthalpy of the system rises linearly with the number of H₂S molecules consumed (R² ≥ 0.99), defining energy costs of 2.41 ± 0.07, 3.03 ± 0.06 and 3.89 ± 0.18 eV per molecule that increase with temperature by ≈1.36 eV per 1000 K; at 3500 K the cost is statistically indistinguishable from the complete-dissociation limit of 3.90 eV obtained independently from Kirchhoff's law and the experimental H–SH bond energy. These results provide a thermochemically validated, molecular-level basis for engineering the valorization of residual H₂S as a source of green hydrogen.

• Research Square https://doi.org/10.20944/preprints202607.2258.v1first seen 2026-08-07 04:38:28