Small Transition-Metal Mixed Clusters as Activators of the C-O Bond. FenCum-CO (n + m = 6): A Theoretical Approach.

Binding of carbon monoxide, CO, and its activation on the surface of the FenCumCO (n + m = 6) clusters are studied in this work. Using the BPW91/6-311 + G(2d) method, we have found that adsorption of the CO molecule on the surface of FenCum (n + m = 6) clusters is thermochemically favorable. Atop and bridge CO cluster coordinations appear for pure, Fe6 and Cu6, and mixed, Fe2Cu4 and Fe4Cu2, clusters. Threefold coordination takes place for Fe3Cu3-CO where the CO bond length, dCO, suffers a largest increase from 1.128 ± 0.014 Å for bare CO up to 1.21 Å. The CO stretching, νCO, as an indicator for the CO bond weakening is redshifted, from 2099 ± 4 cm-1 for isolated CO up to 1690 cm-1 for Fe3Cu3CO and 1678 cm-1 for Fe6CO. In addition, in Cu6CO, the strongest CO bond is slightly weakened as it has a bond length of 1.15 Å and a νCO of 2029 cm-1. There is a correlation between the CO bond weakening and the increase of CO coordination in FenCumCO, which in turns promotes the transference of charges from the metal core into the antibonding orbitals of CO. Substitution of up to three Cu atoms in Fe6 increases the adsorption energies and the activation of CO. Indeed, FenCum (n + m = 6) are promising clusters to catalyze CO dissociation, particularly Fe3Cu3, Fe5Cu, and Fe6, which have large CO bond lengths and CO adsorption energies. The Bader analysis of the electronic density indicates that FenCumCO species with threefold coordination show a rise in the C-O covalent character due to the less electronic polarization. They also show important M → CO charge transfer, which favors the weakening of the CO bond.