Size-Specific, Dissociative Activation of Carbon Dioxide by Cobalt Cluster Anions

The reaction of cobalt cluster anions Co n – (3 ≤ n ≤ 17) with CO 2 was studied experimentally and theoretically to explore the size-specific activation mode of CO 2 by Co n – . Mass spectrometric measurements revealed that the reactivity depends strongly on cluster size: the reactivity emerges abruptly at n = 7, peaks at n = 8–10, and then gradually decreases with increasing n . Infrared multiple photon dissociation spectra of Co n CO 2 – exhibit a single peak at ∼1870 cm –1, similarly to the previously reported spectra of Co n CO – . Density functional theory calculations for Co 7 CO 2 – as an example revealed that the dissociative adsorption of CO 2 into CO and O is energetically more favorable than nondissociative adsorption. The infrared spectra calculated for dissociated isomers Co 7 (CO)O – reproduced the experimental results, whereas those for nondissociated isomers Co 7 CO 2 – did not. The photoelectron spectra of Co n CO 2 – were shifted dramatically toward higher energies relative to those of Co n –, suggesting electron transfer from Co n – to the CO and O ligands. These results indicate that the CO 2 molecule adsorbs dissociatively on Co n –, in sharp contrast to its nondissociative adsorption onto the Co monomer anion.