How Carbon Monoxide Bonds to Metal Surfaces

An analysis is presented of the similarities and differences in the bonding of CO to Ni(100), Ni(l 11), Co(0001),Fe(110), Cr(110), and Ti(0001). The primary interactions in every case are the expected forward and back donation, the mixing of CO 5σ with surface d_z²and s states, and the mixing of CO 2π with metal d_xzyz. The latter interaction is dominant. Projections of the density of states and crystal orbital overlap population curves show these interactions clearly. As the metal changes from Ni to Ti, the effects of the rise of the Fermi level and the diffuseness of the d orbitals combine to put more and more electron density into 2π of CO. This is why CO dissociates on the earlier transition-metal surfaces. Our studies of surface coverage show little effect on the diatomic dissociation. There is some indication in the calculations that higher index surfaces should dissociate CO more readily than the lower index ones, but the effect is smaller than that of changing the metal. An analysis is presented of the similarities and differences in the bonding of CO to Ni(100), Ni(111), Co(0001).