H2 dissociation on individual Pd atoms deposited on Cu(111).

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.