Chemical Bonding of Unique CO on Fe(100)

Abstract

At low coverage, CO molecules are known to preferentially occupy the hollow sites of Fe(100) with considerably inclined molecular orientations. This CO configuration serves as the precursor state of CO dissociation, which is particularly important in several important catalytic reactions. In this study, we present a unique bonding picture of the precursor state from the spin, charge, and orbital perspectives. From the spin and orbital views, we show the antiferromagenetic nature of the adsorbate−metal coupling, where 2π magnetism prevails with a dominant spin-down channel. However, contrasting tendencies are found for the two 1π orbitals in two orthogonal directions: the 1π orbital in the vertical plane loses its symmetry, whereas the other 1π orbital remains intact with a preserved symmetry. The 1π symmetry in the vertical plane favors the 1π → 2π* excitation mechanism owing to the partial opening up of the 1π symmetry. In the charge perspective, we have identified a charge transfer mechanism involving the local structural I Fe C −C−O motif, in which the surface slightly charges the adsorbate with additional partial electrons located at the surface Fe atoms bonded to the carbon end, whereas the charges of the metallic atoms beneath the I Fe C −C−O motif are found to be depleted. In both the adsorbate and metal sides, the depletion of s electrons serves as a good measure of orbital repulsion and delocalization. Interestingly, the carbon and oxygen ends exhibit contrasting electron affinity with the metal surface: the carbon end is attractive, whereas the oxygen end is repulsive in terms of the contrasting charge rearrangement in the bonded metallic atoms.

Molecular Bonding of Predissociative CO on Fe(100): Molecular Orbital Perspective

“…The 2p x and 2p y states are found to be spin-dependent, although they have different spin-dependent behaviors. These observations are well collaborated with geometric and energetic data as well as Lowdin charges published in 2017 (ref ) and 2018 (ref ). In particular, the data of charges gives us an idea of how substantial these orbital processes and proposed hybridizations are, as it has demonstrated that these orbital processes lead to the attractive carbon end and the repulsive oxygen end.…”

“…However, transition states are hard to capture because of short lifetimes. Nevertheless, CO adsorption at Fe(100) displays an example that transition-state-like structures of CO bond cleavage are captured by the perfect metal surface, as the elementary reaction of CO dissociation fits into the category of an early barrier reaction where its transition state structurally resembles the unique reactant or adsorbate by simply elongating the C–O bond along the molecular axis. − Interestingly, both experiments and density functional theory (DFT) calculations consistently revealed that the unique CO species preferentially have the respective carbon and oxygen atoms simultaneously occupied at the hollow and bridge sites, − with considerably inclined molecular orientations. − The consensus means the generalized gradient approximation (GGA) level of theory not only provides a qualitatively good picture but a quantitatively correct scenario. Thus, Fe(100) is quite unique because only upstanding CO molecules can be found on other transition metal surfaces.…”