CO₂ Activation by Small Copper, Cobalt, and Yttrium Oxide Clusters Probed via Infrared Multiple-Photon Dissociation...

The interaction of CO₂ with Cu, Co, and Y oxide clusters of different size, composition, and charge is investigated via infrared multiple-photon dissociation (IR-MPD) spectroscopy. The IR-MPD spectra of M_xO_y(CO₂)^+/- (M = Cu, Co, Y) complexes reveal the non-activated binding of CO₂ to all cationic clusters. In contrast, CO₂ is activated by all anions via formation of carbonate-like CO₃ units. Density functional theory(DFT) calculations on M₂O₂ ^+/- model systems suggest that for CO₂ activation in general, the energy difference between the cluster highest occupied molecular orbital (HOMO) and the CO₂ lowest unoccupied molecular orbital (LUMO) is crucial. The binding motif of the activated CO₂ molecule, however, depends on a fine interplay between the nature of the orbitals close to the HOMO and the oxidation state of the metal atoms. CO₃ formation is favored when the cluster HOMO or any energetically close lying molecular orbital (MO) is at least partially localized on one of the cluster oxygen atoms. In contrast, if such MOs are energetically less favorable, η²-(C,O) and η²-(O,O) binding to one of the metal atoms can be favored, provided the already positively charged metal atom can transfer electron density, i.e., adopt an even higher oxidation state.