Water adsorption and reactions on small sodium chloride clusters

Energetics, structures, and reactions of water with small sodium chloride clusters, Na4Cln, 2 less than or equal to n less than or equal to 4, and of OH-doped nonstoichiometric, Na4Cl3(OH), Na4Cl2(OH), and Na4Cl2(OH)(2), clusters, are investigated employing electronic structure calculations using the local spin-density functional method with exchange-correlation gradient corrections in conjunction with norm-conserving nonlocal pseudopotentials, structural optimizations, and Born-Oppenheimer molecular dynamics simulations. in OH-doped clusters an OH- is formed, substituting for the missing halide anion, and the hydroxyl anion is oriented approximately along the body diagonal of the cuboid structure of the duster. Energetically and structurally, clusters in the sequence Na4Cl4-m(OH)(l) with m = 1, 2 and 1 less than or equal to l less than or equal to m, are similar to the corresponding undoped clusters, Na4Cl4-m+l. On the stoichiometric cuboid Na4Cl4 cluster, a water molecule adsorbs with the molecular plane tilted by similar to 13 degrees toward the chlorine anions with respect to the ''(100) facet'' of the cluster, with a binding energy of similar to 0.49 eV. For nonstoichiometric clusters Na4Cln, n = 2, 3, H2O adsorption at the halide-vacant, F-center site, with a binding energy of similar to 0.32 eV, is accompanied by partial expulsion of the excess electron from the F-center site. This effect is correlated with a decrease in the vertical and adiabatic ionization potentials upon H2O adsorption. The thermochemistry of several reactions of water with alkali chloride clusters is evaluated. The hydrogen generation reaction, Na4Cl2(H2O)(2) --> Na4Cl2(OH)(2) + H-2, is catalyzed by the surface, with an exothermicity of 2.63 eV, and involving a barrier of 0.33 eV, compared to an exothermicity of 1.22 eV and a barrier of 1.56 eV for the reaction Na-2(H2O)(2) --> (NaOH)(2) + H-2. The mechanism of the cluster-catalyzed reaction is discussed.