Shape matters too

Previous attempts to account for particle shapes employed the Wulff methodology to estimate the impact of specific particle facets. However, this method only considers nanoparticles’ equilibrium shapes and does not reflect their evolution under reaction conditions. Therefore, using a combination of density functional theory and Boltzmann statistics, the team modelled a large ensemble of nanoparticles ranging between 0.5 and 10 nm, including all possible shapes that may effectively accommodate the active sites under reaction conditions. In this way, they were able to reproduce the common experimental trend of activity — expressed as turnover frequency (TOF) — as a function of particle size, featuring a maximum at around 2.3 nm (pictured, panel a). Eventually, particles in this range are associated with a high prevalence of fourfold sites, which are identified as the most reactive sites. However, for a specific particle size range, different shapes exist, characterized by a different fraction of fourfold sites (pictured in panels b–d for particles of size ~2 nm). As a result, different particle shapes contribute differently to the observed experimental TOF.

This work offers a fresh perspective to rationalize the well-known structure sensitivity of CO₂ methanation on nickel catalysts. More generally, the study suggests how modelling could inform catalyst synthesis for other structure-sensitive reactions to obtained shape-tuned catalysts with high activity.