Predicting the surface topology of iron carbide nanoparticles.

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.

Research questions

• Which phases of iron carbide are observed as function of temperature and carbon potential?
• What kind of surface sites are dominant as function of temperature and carbon potential?

Figure 1: Conceptual picture of a growing hydrocarbon chain on a iron-carbide surface.

Project-specific Learning goals

• Build a Reactice Force field (ReaxFF) using density function theory calculations.
• Understand how atomic scale calculations can be used as input for mesoscale models.
• Perform simulated annealing experiments to explore the surface composition of nanoparticles.
• Utilize Python to handle large datasets and effectively produce and post-process results.

General learning goals

• Construct relevant research questions.
• Construct a working hypothesis.
• Develop a scientific methodology to validate or invalidate the working hypothesis.
• Build a research report based on the research topic.
• Present the most salient results in an oral contribution.