Ab Initio Investigation of CO₂ Adsorption on 13-Atom 4d Clusters

In this work, we report an ab initio investigation based on density functional theory calculations within van der Waals D3 corrections to investigate the adsorption properties and activation of CO₂ on transition-metal (TM) 13-atom clusters (TM = Ru, Rh, Pd, Ag), which is a key step for the development of subnano catalysts for the conversion of CO₂ to high-value products. From our analyses, which include calculations of several properties and the Spearman correlation analysis, we found that CO₂ adopts two distinct structures on the selected TM₁₃ clusters, namely, a bent CO₂ configuration in which the OCO angle is about 125 to 150° (chemisorption), which is the lowest energy CO₂/TM₁₃ configuration for TM = Ru, Rh, Pd. As in the gas phase, the linear CO₂ structure yields the lowest energy for CO₂/Ag₁₃ and several higher energy configurations for TM = Ru, Rh, Pd. The bent CO₂ (activated) is driven by a chemisorption CO₂-TM₁₃ interaction due to the charge transfer from the TM₁₃ clusters toward CO₂, while a weak physisorption interaction is obtained for the linear CO₂ on the TM₁₃ clusters. Thus, the CO₂ activation occurs only in the first case and it is driven by charge transfer from the TM₁₃ clusters to the CO₂ molecule (i.e., CO₂^-δ), which is confirmed by our Bader charge analysis and vibrational frequencies.