Iron-based Fischer–Tropsch catalysts, which are applied in the conversion of CO and H2 into longer hydrocarbon chains, are historically amongst the most intensively studied systems in heterogeneous catalysis. Despite this, fundamental understanding of the complex and dynamic chemistry of the iron-carbide system is still a developing field. One prominent question is what active sites are exposed under typical process conditions. There are many iron-carbide phases and each phase can expose a variety of surface facets. In turn, these surface facets harbor a variety of active sites. To construct an atomic/mechanistic kinetic model of Fisher-Tropsch synthesis, it is critical that the stability and presence of these active sites is known.
In previous research projects, a vast dataset of atomic structures was developed using density functional theory calculations. From such a dataset, it is possible to generate a reactive force field (ReaxFF) that describes the multi-body interactions between the atoms. Such force fields descriptions are typically 103-105 times faster to evaluate than the DFT calculations, which makes performing larger scale simulations using such force fields computationally feasible.
Once a ReaxFF force fields has been developed, we aim to utilize it in larger scale simulated annealing computations. These computations are essentially molecular dynamics simulations wherein the temperature is increased and decreased. At higher temperatures in the simulation, there is sufficient thermal energy to overcome the energetic barriers allowing the system to transverse the phase space. At the lower temperature in the simulation, we aim to seek the local minima.
From a large set of these simulated annealing computations, we can explore which kind of active sites are exposed. In this step, an in-house written surface atom pattern recognition script is used. This analysis would reveal which active sites are expected to be present and which are the prime candidates for future detailed density functional theory calculations.
Figure 1: Conceptual picture of a growing hydrocarbon chain on a iron-carbide surface.