Modeling the Shape and Stability of Co Nanoparticles as a Function of Size and Support Interactions through DFT Calculations and Monte Carlo Simulations

Abstract

In this work, we have employed a combined density functional theory (DFT)-Monte Carlo (MC) approach to produce structural models of Co nanoparticles (NPs), widely employed in the Fischer–Tropsch (FT) synthesis for the production of sustainable aviation fuels (SAFs), in the 2–10 nm size range including the effects of temperature and metal–support interactions (MSI). We make use of a lattice model where the energy of Co atoms is estimated based on their first-shell coordination number (CN), an approach that was validated via DFT calculations. We report a marked increase in step and kink sites at the expense of terraces with increasing particle size, which we linked to the experimentally observed increase in turnover frequency (TOF). Increasing MSI led to a flattening of the NPs on the support as well as to decreasing Co dispersion but hardly affected the site distribution, suggesting that they do not alter the NPs intrinsic activity. We additionally report the size-dependent surface energies and chemical potentials of Co NPs, which are both shown to decrease fast in the 2–6 nm size range and approach convergence afterward. Our models provide a description of these quantities accounting simultaneously for particle size, nonideality of surface morphologies, temperature, and MSI and thus overcome several approximations that previous studies had to rely on.