H(2) scattering and dissociative adsorption on the W(100)-c(2 × 2)Cu surface alloy is studied based on DFT calculations. A strongly site dependent reactivity is observed in line with results obtained for the density of states projected onto the W and Cu atoms of the topmost layer. H(2) dissociation on a defect free terrace of W(100)-c(2 × 2)Cu is found to be a non-activated process like on W(100), despite the reduction of the number of energetically accessible dissociation pathways at low impact energies due to the presence of Cu atoms. A prominence of dynamic trapping and a reduction of the efficacy of trapping to promote dissociation is also verified, leading to a decrease of the initial sticking probability as a function of the molecular impact energy, in qualitative agreement with experimental findings. The heterogeneous reactivity is also evidenced by two different kinds of reflection events at low energies. Its combination gives rise to a broad specular peak superimposed on a cosine-like angular distribution of scattered molecules which is in good agreement with available experimental data.
We present a Molecular Dynamics (MD) study based on Density Functional Theory (DFT) calculations for H(2) interacting with a Pd-Cu(111) surface alloy for low Pd coverages, Θ(Pd). Our results show, in line with recent experimental data, that single isolated Pd atoms evaporated on Cu(111) significantly increase the reactivity of the otherwise inert pure Cu surface. On top of substitutional Pd atoms in the Pd-Cu(111) surface alloy, the activation energy barrier for H(2) dissociation is smaller than the lowest one found on Cu(111) by a factor of two: 0.25 eV vs. 0.46 eV. Also in agreement with experiments, our DFT-MD calculations show that a large fraction of the dissociating H atoms efficiently spillover from Pd (i.e. the active sites), thanks to their extra kinetic energy due to the ~0.50 eV chemisorption exothermicity. Still, our DFT-MD calculations predict a dissociative sticking probability for low energy H(2) molecules that is much smaller than the estimated value from scanning tunneling microscopy experiments. Thus, further theoretical and experimental investigations are required for a complete understanding of H(2) dissociation on low-Θ(Pd) Pd-Cu(111) surface alloys.
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