Associative vs. dissociative binding of CO₂ on M₄ transition metal clusters

Density functional theory calculations were performed to determine reaction paths for the reaction of CO₂ with M₄ transition metal clusters (M = Nb, Mo, Ru, Rh, Pd, Ag, Pt). Geometries incorporating associatively bound (CO₂), partly dissociated (O + CO) and fully dissociated (O + C + O) carbon dioxide were identified for all clusters except Ag₄. Nb₄ and Mo₄ are likely to dissociate CO₂ fully. For Ru₄, both partly and fully dissociative geometries were competitive, while Rh₄, Pd₄ and Pt₄ activate CO₂ without breaking either CO bond. Ag₄ was found to interact only minimally with CO₂. The change in ν_bend, the energy of the CO₂ π_u orbital in the physisorbed M₄CO₂ capture species and the charge transfer to the CO₂ molecule, q(CO₂), in the first transition state were found to correlate with the eventual fate of the CO₂ molecule.