Thermodynamic properties of hydrogen dissociation reaction from the small system method and reactive force field ReaxFF

Thermodynamic attributes are important in chemical engineering and chemistry in general. These properties have been used to describe macroscopic systems, in laboratory chemical reactor scales. The descriptions are insufficient at the nanoscale owing to the fact that thermodynamic variables are not extensive when the particle number drops to a few particles.But the Small System Method can be used to connect properties of the system in the nanoscale with macroscopic limit. It has now been applied to compute thermodynamic correction factors, partial molar enthalpies, derivatives of activity coefficients, and reaction enthalpies of macroscopic systems. The results obtained in all these cases indicate agreement with data obtained from other approaches.

Thermodynamic factors enable us to compute Maxwell-Stefan diffusion coefficients from Fick diffusion coefficients for both binary and ternary systems. Most of the computations have been based on non-reactive systems. Therefore, obtaining thermodynamic data in reacting systems would be groundbreaking for this method.

Researchers led by Professor Signe Kjelstrup at the Norwegian University of Science and Technology demonstrated a combination of the small system method and the Reactive Force Field in a bid to obtain thermodynamic data of the hydrogen dissociation reaction. In their work, they provided a number of properties of the reaction such as heat of reaction, heat capacity, thermodynamic correction factors with the reactive force field. They compared their results with the already obtained values from the three-body interaction potential. Their work is published in peer-reviewed journal, Chemical Physics Letters.

The development of accurate thermodynamic methods and precise potentials for the dissociation of hydrogen gas is the starting point for a number of complex computations. The dissociation normally occur at high temperatures, therefore the need for computer simulations. The authors simulated a cubic box system with periodic boundary conditions in three dimensions.

The researchers varied the box size from 28.47 Å-100 Å and the number of hydrogen molecules from 37-1500. They then performed NVT simulations with 0.1 fs step size. They equilibrated the system with 1 ns run at 300 K.

In general, the authors applied the Small System Method to the hydrogen dissociation reaction, implementing Reactive Force Field to obtain thermodynamic data. The main aim of the study was to compare the results from the Reactive Force Field with the results obtained earlier from the accurate interaction potential. They reported heat capacity, thermodynamic correction factors, activation energy, and reaction enthalpy.

The results of their study indicated that the two approaches of computing thermodynamic data were in agreement. However, simplifying ideal mixture approximation was considered for enthalpies of reaction. Three body potential outcomes appeared more precise, but the Reactive Force Field gave the trends and magnitudes that were expected.

These results open the possibility of combining small system method and reactive force field for more complex reactions. The approach would be helpful in bringing important information about reactions away from equilibrium, and studying transport characteristics in mixtures where chemical reactions occur.