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

Abstract

Binding of carbon monoxide, CO, and its activation on the surface of the Fe n Cu m CO (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 n Cu m (n + m = 6) clusters is thermochemically favorable. Atop and bridge CO cluster coordinations appear for pure, Fe 6 and Cu 6 , and mixed, Fe 2 Cu 4 and Fe 4 Cu 2 , clusters. Threefold coordination takes place for Fe 3 Cu 3 −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 Fe 3 Cu 3 CO and 1678 cm −1 for Fe 6 CO. In addition, in Cu 6 CO, 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 n Cu m CO, 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 Fe 6 increases the adsorption energies and the activation of CO. Indeed, Fe n Cu m (n + m = 6) are promising clusters to catalyze CO dissociation, particularly Fe 3 Cu 3 , Fe 5 Cu, and Fe 6 , which have large CO bond lengths and CO adsorption energies. The Bader analysis of the electronic density indicates that Fe n Cu m CO 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.

4-Phenylcoumarin (4-PC) Glucoside from Exostema caribaeum as Corrosion Inhibitor in 3% NaCl Saturated with CO2 in AISI 1018 Steel: Experimental and Theoretical Study

“…The full method, hereinafter labeled BPW91-D2/6-311++G(2d,2p), was used as coded in the quantum chemistry package Gaussian 09 D.01. Similar methods were used before to study small iron clusters [ 36 , 37 ], organic species, and their compounds [ 38 , 39 , 40 , 41 ].…”