Modeling the shape and stability of supported Co nanoparticles under Fischer–Tropsch conditions via DFT calculations and Monte Carlo simulations: insights into CO-driven surface reconstruction

Abstract

In this work, we have expanded our recently published combined density functional theory (DFT) – Monte Carlo (MC) approach to model supported Co nanoparticles (NPs) to include the effects of Co–CO interactions. We derived coordination number (CN) – specific energy corrections based on DFT-calculated ΔG_ads of CO on different Co facets and incorporated them in our energy model. This allowed us to simulate supported Co NPs with increasing size under Fischer–Tropsch (FT) operating conditions, both at high and low CO conversion (X_CO). Our results reveal drastic surface reconstruction induced by CO, that consists of a contraction of the close-packed surfaces in favor of B5-A step sites. This transformation drives the NPs from a highly faceted toward a rounder shape, and is accompanied by the appearance of triangular terraces previously reported experimentally. The increase of the concentration of B5-A sites, on which CO can dissociate, is speculated to enhance catalyst activity. Additionally, we demonstrate that CO significantly lowers the surface energy of Co NPs, profoundly influencing their redox behavior and stability. While CO reduces the thermodynamic driving force for sintering, its positive impact on sintering kinetics is likely dominant. Our study provides a comprehensive theoretical description of fcc Co NPs under operating conditions, accounting simultaneously for the effects of particle size, temperature and both metal–support and metal–adsorbate interactions.