Effects of coadsorbates in dissociative chemisorption of hydrogen on metallic surfaces

The promotion and poisoning of hydrogen dissociation on metallic surfaces by atomic adsorbates is discussed in the context of calculations on the system H-2/Cu(111) + O. First-principles simulations, based on density functional theory and using a slab geometry, show an increase in the H-2 dissociation barrier (poisoning) for sites close to adsorbed oxygen atoms, and a longer-ranged decrease in the barrier height (promotion). Three mechanisms which have been proposed to explain the effects of promoters and poisons are analysed in detail. It is shown that the short-ranged poisoning can be understood in terms of an interaction between the hydrogen molecular orbitals and the metal d-bands. Electrostatic effects lead to a longer-ranged promotion but the strength is insufficient to explain the first-principles results. The calculated promotion may be due to an observed decrease in the surface electron density caused by oxygen adsorption, but a quantitative model of this is not yet available. The significance of these results for the effects of coadsorbates on a range of hydrogen dissociation reactions are discussed.