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

Binding of carbon monoxide, CO, and its activation on the surface of the Fe_nCu_mCO (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 Fe_nCu_m (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, d_CO, suffers a largest increase from 1.128 ± 0.014 Å for bare CO up to 1.21 Å. The CO stretching, ν_CO, 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 ν_CO of 2029 cm^–1. There is a correlation between the CO bond weakening and the increase of CO coordination in Fe_nCu_mCO, 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, Fe_nCu_m (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 Fe_nCu_mCO 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.