Initial Sticking Coefficient of H2 on the Pd–Cu(111) Surface Alloy at very Low Coverages

We present an experimental study of the initial dissociative sticking probability of H2 on the Pd–Cu(111) surface alloy at Pd coverages between 1–10%. The measurements have been performed using a supersonic molecular beam with an incident energy range Ei = 75–163 meV. In agreement with a recent STM study, our results confirm that small amounts of Pd atoms significantly increase the reactivity of the inert Cu(111) surface, although the measured initial sticking probability values were found to be much lower than the ones estimated from the STM work. For a Pd coverage of 1% and Ei ∼ 0.10 eV, the H2 initial sticking probability was found to be 2 × 10−3, increasing to 4 × 10−3 for a Pd coverage of 10%. This agrees well with a recent molecular dynamics study, in which the initial sticking probability was estimated in 7 × 10−3 for a Pd coverage of 11% and Ei ∼ 0.15–0.20 eV. Essentially the same values were measured in the whole incident energy range investigated. Therefore, our results support the main conclusion of the molecular dynamics calculations about the mechanism leading to the increase of reactivity of the Pd–Cu(111) surface alloy, which combines a lower activation energy barrier for dissociation of ∼ 0.25 eV on top of substitutional Pd atoms with spillover onto the Cu(111) surface.