Local Chemical Reactivity of a Metal Alloy Surface

The chemical reactivity of a metal alloy surface is studied by density functional theory investigating the interaction of ${\mathrm{H}}_{2}$ with NiAl(110). The energy barrier for ${\mathrm{H}}_{2}$ dissociation is largely different over the Al and Ni sites without, however, reflecting the barriers over the single component metal surfaces. This local chemical behavior is due to the covalent nature of the ( ${\mathrm{H}}_{2}$ ${\ensuremath{\sigma}}_{g}$)-(Ni ${3d}_{{z}^{2}}$) and ( ${\mathrm{H}}_{2}$ ${\ensuremath{\sigma}}_{u}^{*}$)-(Ni ${3d}_{\mathrm{xz}}$) interactions. Thus, it cannot be described in terms of the Harris-Andersson model (i.e., Pauli repulsion and its weakening by empty $d$ states).