(379a) Development and Validation of ReaxFF Forcefields for Mo – Ni – O – S – H Interactions Under High Pressure Conditions

Currently, the development of novel materials for advanced fossil energy systems remains slow because it is driven by a trial-and-error experimental approach. Atomistic Molecular Dynamic (MD) simulations have the potential to accelerate this development through the prediction of mechanical properties and corrosion resistance of new materials, reducing the time-to-market for new economically-viable materials to be used in fossil fuel systems. The success of MD simulations depends critically on the fidelity of interatomic potentials. Existing potentials typically are not able to account for reactions, or are not applicable for high-temperature simulations, or can be used only for modeling tiny nano-scale clusters.

These deficiencies were successfully addressed in this study by (1) developing QM-based Mo, Mo-O, Mo-Ni-O, Mo-H₂S and Mo-O-H₂S ReaxFF reactive potentials, (2) validating developed potentials against the Mo equation of state and temperature-dependent density and Young’s modulus in the temperature range of 300 K to 1500 K, (3) reproducing experimentally established trends of Ni-Mo alloy resistance to oxidation, and (4) sulfur agglomeration in molybdenum and molybdenum trioxide. These newly developed ReaxFF potentials offer a compromise between high-level QM description and computational speed.

Mechanisms of initial oxidation in Mo, Ni, and Mo-Ni were compared. Specifically, it was found that Mo oxidation involves oxygen transport inside the metal slab and large heat release melting the surface. Oxidation of Mo slab proceeded at a significantly higher rate than the oxidation of Ni slab. Interactions of H₂S with Mo and MoO slabs were also modeled to test Mo-H₂S and Mo-O-H₂S ReaxFF force fields. Some key trends were identified based on the test results: (1) H₂S consistently diffused into Mo and MoO slabs and decomposed there to form S and H, (2) Mo-Mo metal bonds were replaced by S-Mo bonds leading to S agglomeration in the slab, and (3) H preferably desorbed from Mo slab at temperatures above 1400 K and formed gaseous H₂.

Major applications of the ReaxFF database include (1) Design of refractory alloys for new turbine blades, (2) Design of 'smart' materials with unprecedented levels of aerodynamic efficiencies for aviation and astronautics, (3) Design of materials for new generation of electronic devices where the development time is of crucial importance, and (4) Chemical process design where computer-aided development could potentially revolutionize the current technology by, for example, making possible catalytic “dream” reactions. Currently, the materials used in the above-listed applications constitute a market worth of tens of billions dollars.