Temperature-dependent surface free energy and the Wulff shape of iron and iron carbide nanoparticles: A molecular dynamics study

The morphology change is crucial to the catalysis performance of catalyst nanoparticles in heterogeneous catalysis. Iron and iron carbide nanoparticles are used as high temperature heterogeneous catalyst, such as Fischer-Tropsch synthesis and carbon nanotube growth. Here we have investigated the effect of temperature and entropy on the surface free energy and morphology of the iron and iron carbide (θ-Fe₃C, χ-Fe₅C₂, and o-Fe₇C₃) nanoparticles using molecular dynamics. The free energies of all the bulk and most of the surface systems drop following a parabolic curve with elevating temperature due to entropic effect. The nanoparticles are covered by low index surfaces at low temperature. At low temperature, the surface free energies of all surfaces usually decrease with a similar slope with increasing temperature. However, a critical temperature exists at which the high-index surfaces starts to dominate the catalyst particles. Fe₇C₃ shows an unusual minimum surface free energy at 400 K in all the surfaces. This study provides fundamental insights into the modulation of iron-based nanocatalysts morphologies with desired catalytic performance.