Associative vs. dissociative binding of CO₂ on M₅ transition metal clusters

Reaction paths were calculated using density functional theory for the reaction of carbon dioxide with a series of transition metal pentamers, M₅ + CO₂, (M = Nb, Mo, Ru, Rh, Pd, Ag, Pt). A stochastic search algorithm was used to identify geometries with intact CO₂, as well as geometries where the CO₂ molecule was partly (O + CO) and fully dissociated (O + C + O). Nb₅ and Mo₅ clusters were found to thermodynamically dissociate CO₂. Pd₅ and Ag₅ were found to leave the CO₂ molecule intact, Ru₅ could partly dissociate CO₂, while for Rh₅ and Pt₅, the fate of the adsorbed CO₂ was dependent on the cluster geometry. The change in the CO₂ π_u orbital energy in the capture species on initial reaction with the M₅ cluster was found to distinguish clusters where CO₂ fully dissociated, but could not distinguish clusters where CO₂ was found to partly dissociate. The charge transfer to the CO₂ molecule at the first transition state, did however, distinguish clusters that fully dissociate CO₂, those that partly dissociate CO₂ to O + CO, and those that leave CO₂ fully intact.