Catalytic implications of local electronic interactions between carbon monoxide and coadsorbed promoters on nickel surfaces

Abstract

Recent experimental and theoretical work has highlighted the electronic factors that influence the interactions between CO and coadsorbed species on transition-metal surfaces. Although the Blyholder model and modifications thereof provide a useful framework for interpreting many of these results, the microscopic mechanisms by which coadsorbates either poison or promote catalytic reactions are still not well understood. The development of theoretical approaches that can account for established experimental data, while having the flexibility to explore possible reaction paths is therefore an important step in being able to design more efficient catalysts. We present multiple scattering-Xα and Green function density of states calculations for Ni(100) and Ni(111) with CO and coadsorbed poisons (e.g. sulphur) and promoters (e.g. lithium potassium) for the methanation reaction. For the promoted Ni(100) surface we find that the polarisation of the alkali-metal charge towards the surface induces a decrease in the work function and a large electrostatic shift of the CO levels to greater binding energies, which increases the occupancy of the 2π^*orbital. There is furthermore evidence of direct CO–promoter interaction in the 1π and 2π^* levels, although the 5σ appears to be largely unaffected by the alkali metal. The implications of these calculations for the promotion of the methanation reaction by nickel catalysts are discussed. For sulphur on Ni(111) we find an oscillatory interaction, with some sites within the range of poisoning showing an enhanced local density of states. This is not observed on the (100) surface and is attributed to the absence of a CO adsorption site at the appropriate distance from the sulphur atom.