Deciphering Size and Shape Effects on the Structure Sensitivity of the CO2 Methanation Reaction on Nickel

Abstract

This study advances the understanding of structure sensitivity in CO2 methanation over nickel-based catalysts by highlighting the combined influence of the metal nanoparticle (NP) size and shape on catalytic performance. Density functional theory (DFT) calculations of the metal nanoparticle structure and activity provide the theoretical underpinnings of the experimentally observed structure sensitivity of CO2 methanation over nickel-based catalysts. This is achieved by taking into account the diversity of shapes of metal nanoparticles (NPs) under the reaction conditions and the corresponding distribution of active sites at different metal NP sizes. We built a large ensemble of Ni metal NPs with different shapes and sizes in the range of 0.5-10 nm and quantified the distribution of the potential active sites for each NP. We then computed the reaction rate over each of these active sites on the metal surface to evaluate the activity as a function of the metal NP diameter. Our calculations reveal that the activity at the active sites located at the edge between the Ni(100) and Ni(111) facets largely dominates the overall observed activity. Furthermore, metal NPs can be categorized into families based on their shape, specifically the fraction of exposed Ni(100) facets. The observed maximum in turnover frequency (TOF) for 2-3 nm metal NPs is linked to the dominance of NP families with high Ni(100) fractions. Conversely, experimental conditions favoring NP families with higher Ni(111) fractions result in a hockey stick trend in the TOF. These findings resolve key debates on structure sensitivity in CO2 methanation and offer broader applicability to other structure-sensitive reactions, such as ammonia synthesis, decomposition, and Fischer-Tropsch synthesis, where similar sensitivities have been widely debated.