Adsorbate-Induced Reconstruction of p(2X2)X Adlayers on Nickel(100)

The interactions between CO and Ni(100)-p(2x2)X, X = C, N, O, S, Cl, have been examined experimentally with LEED, AES, temperature-programmed desorption, and reflection absorption infrared spectroscopy and modeled with a semiempirical tight-binding model. Adsorption at 170 K was reduced on all the surfaces with adlayers relative to clean Ni(100). The adsorption enthalpies, as estimated from TPD desorption energies, were in the order clean <0<C1<C<S<N. Infrared spectroscopy found that at 170 K both bridge and on-top CO species were found on the surfaces with adlayers of C, N, and Cl; only bridge-bonded CO was found on surfaces with O and S adlayers. Above 300 K only on-top bonded species were seen on surfaces with O, S, and Cl adlayers. No CO remained on the p(2x2)N surface above 300 K, and both bridgeand on-top-bonded CO persisted above 300 K on the carbided surface. The calculations indicated that CO adsorption in on-top sites is greatly inhibited by the presence of a p(2X2)X adlayer, with sulfur and chlorine being the most detrimental to CO adsorption. The effect of the adatoms on CO adsorption is dominated by the CO(5<r) adatom p overlap, such that CO should be preferentially adsorbed on bridge sites on the surfaces with p(2X2)X adlayers. The experimental results have been explained by considering the reconstruction of the p(2X2)X adlayer into islands of C(2x2)X and clean surface. It is shown that this reconstruction makes energetically more favorable on-top binding sites available for CO, and the thermodynamic driving force for this reconstruction is favorable for all the adatoms except nitrogen. This model is able to account for the experimentally observed CO binding-site transformations from bridge sites to on-top sites, and it also accounts for why nitrogen had the most deleterious effect on CO adsorption.