Binding and orientations of CO on Fe(110), (100), and (111): A surface structure effect from molecular orbital theory

Abstract

Calculations based on the atom superposition and electron delocalization molecular orbital method show that at low coverage CO binds on the high-coordinate site in the perpendicular orientation on Fe(110), on the high-coordinate site in the lying-down orientation on Fe(100), and on the di-σ bridging site in the lying-down orientation on Fe(111). These findings confirm the earlier suggestions regarding CO binding site and orientation on the (110) and (100) surfaces. However, they do not support the previously proposed deep-hollow binding site for CO on the (111) surface. The differences for thee favored CO binding site and orientation on Fe(110), (100), and (111) surfaces are explained on the basis of surface-atom coordinations and atom-atom spacings. In the favored lying-down CO orientations on the (100) and (111) surfaces, 4σ and π donation interactions coupled with the familiar 5σ donation to the surfaces, and back-donation to the CO π ∗ orbitals are responsible for binding to the surfaces.