Different Surface Chemistries of Water on Ru0001: From Monomer Adsorption to Partially Dissociated Bilayers

Density functional theory has been used to perform a comparative theoretical study of the adsorption and dissociation of H(2)O monomers and icelike bilayers on Ru0001. H(2)O monomers bind preferentially at atop sites with an adsorption energy of approximately 0.4 eV/H(2)O. The main bonding interaction is through the H(2)O 1b(1) molecular orbital which mixes with Ru d(z)2 states. The lower-lying set of H(2)O molecules in an intact H(2)O bilayer bond in a similar fashion; the high-lying H(2)O molecules, however, do not bond directly with the surface, rather they are held in place through H bonding. The H(2)O adsorption energy in intact bilayers is approximately 0.6 eV/H(2)O and we estimate that H bonding accounts for approximately 70% of this. In agreement with Feibelman (Science 2002, 295, 99) we find that a partially dissociated OH + H(2)O overlayer is energetically favored over pure intact H(2)O bilayers on the surface. The barrier for the dissociation of a chemisorbed H(2)O monomer is 0.8 eV, whereas the barrier to dissociate a H(2)O incorporated in a bilayer is just 0.5 eV.