Ab initio investigation of quantum size effects on the adsorption of CO2, CO, H2O, and H2 on transition-metal particles

Abstract

Adsorption is a crucial preliminary step for the conversion of CO₂ into higher-value chemicals, nonetheless, the atomistic understanding of how substrate particle size affects this step is still incomplete. In this study, we employed density functional theory to investigate the effects of particle size on the adsorption of model molecules involved in the CO₂ transformations (CO₂, CO, H₂O and H₂) on Co_n, Ni_n and Cu_n particles with different sizes (n = 13, 55, 147) and on the respective close-packed surfaces. We found significant size-dependence of the adsorption properties for physisorbed (linear) and chemisorbed (bent) CO₂ on the substrates and distinct (symmetric or asymmetric) stretching of the C–O bonds, which can play a crucial role to understand the CO₂ dissociation pathways. For CO and H₂, some properties showed small oscillations, due to size effects that induced alternation of the adsorption site preference for different particle sizes; for H₂O, the adsorption properties were almost independent of particle size. The presence of low-coordinated adsorption sites resulted in a trend for stronger adsorption and greater charge transfer for smaller clusters. Fixing the size-independent factors (e.g., type of metal), our results show that CO₂ adsorption on transition-metal clusters is significantly affected by particle size, suggesting that substrate particle size could be a key factor to understand and control the catalytic transformations of CO₂.