ReversibleH2O Adsorption on Pt(111) +K: Work-Function Changes and Molecular Orientation

Ab initio calculations on a ${\mathrm{Pt}}_{10}$-K-${\mathrm{H}}_2$O cluster show that the ${\mathrm{H}}_2$O molecule coadsorbed with K is tilted 160° relative to the surface normal, with the hydrogen atoms pointing towards the surface. This unusual bonding geometry is supported experimentally by studies of ${\mathrm{H}}_2$O-K coadsorption on Pt(111). For K coverages ${\theta }_{\mathrm{K}}<0.10\mathrm{}$ the adsorption of ${\mathrm{H}}_2$O is fully reversible and characterized by a large work-function increase. The ${\mathrm{H}}_2$O-K interaction is short ranged and increases the ${\mathrm{H}}_2$O adsorption energy. The calculation suggests that the orientational change of the molecule is driven by the electrostatic interaction with the K ion.