Ab initio investigation of the role of the d-states on the adsorption and activation properties of CO 2 on 3d, 4d, and 5d transition-metal clusters.

We report a theoretical investigation of the adsorption and activation properties of CO ₂ on eight-atom 3d, 4d, and 5d transition-metal (TM) clusters based on density functional theory calculations. From our results and analyses, in the lowest energy configurations, CO ₂ binds via a chemisorption mechanism on Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt (adsorption energy from -0.49 eV on Pt up to -1.40 eV on Os), where CO ₂ breaks its linearity and adopts an angular configuration due to the charge transfer from the clusters toward the C atom in the adsorbed CO ₂ . In contrast, it binds via physisorption on Cu, Ag, and Au and maintains its linearity due to a negligible charge transfer toward CO ₂ and has a small adsorption energy (from -0.17 eV on Cu up to -0.18 eV on Ag). There is an energetic preference for twofold bridge TM sites, which favors binding of C with two TM atoms, which enhances the charge transfer ten times than on the top TM sites (onefold). We identified that the strength of the CO ₂ -TM ₈ interaction increases when the energy values of the highest occupied molecular orbital (HOMO) of the TM ₈ are closer to the energy values of the lowest unoccupied molecular orbital of CO ₂ , which contributes to maximize the charge transfer toward the molecule. Beyond the energy position of the HOMO states, the delocalization of 5d orbitals plays an important role in the adsorption strength in TM, especially for the iron group, e.g., the adsorption energies are -1.08 eV (Fe, 3d), -1.19 eV (Ru, 4d), and -1.40 eV (Os, 5d).