Coadsorption of carbon monoxide and hydrogen on the Ni(100) surface: A theoretical investigation of site preferences and surface bonding

The CO/H coadsorption on the Ni(100) surface is discussed in this study. Relative stabilities of various possible surface structures are compared for the initial state (lower temperature form), as well as the final state (higher temperature form) of the coadsorption system. The surface-adsorbate bonding in the Ni(100)/H(4)/CO(t) structure (H(4) stands for hydrogen atoms adsorbed in a 4-fold hollow site and CO(t) stands for carbon monoxide in the on-top position), the most favorable choice of the lower temperature state, resembles that of the singly adsorbed systems. The adsorbate-adsorbate interaction does not lead to any chemical bonds but does affect the surface-CO π bonding. Destabilization of the CO 2π orbitale due to the 2π-1s(H) interaction results in a depopulation of the 2π states and a strengthening of the C-O bond. For the observed higher temperature c(2√2×√2)R45° geometry of CO, possible H and H₂ (adsorbed hydrogen molecule) arrangements in the final surface state are compared. Energy and crystal overlap population analyses show that the 4-fold adsorption site for both H and H₂ gives good agreement with the experimental observations. A new adsorbate-adsorbate coupling between CO(t) and H₂(4) (hydrogen molecule adsorbed in a 4-fold hollow site) is seen, but this coupling does not significantly affect the surface-adsorbate bonding. Again, no C-H or O-H bonds are formed. Besides the favorable CO(t)/CO(4) (half of the CO's adsorbed terminally and the other half in the 4-fold hollow site) configuration, our calculations also reveal the possibility of a CO(t)/CO(b) combination (half of the CO's adsorbed terminally and the other half in a bridging manner) in the final state.