COADSORPTION OF WATER AND HYDROGEN ON PT(100) - FORMATION OF ADSORBED HYDRONIUM IONS

Coadsorption of water and hydrogen on Pt(100) was studied under conditions of ultrahigh vacuum with thermal desorption spectroscopy and high resolution electron energy loss spectroscopy. The experiments were conducted over a temperature range of 100-700 K and coverages ranging from submonolayer to several multilayers. The surface reaction H2O + H --> H3O+ + e- occurs in water/hydrogen coadsorbate; H3O+ was identified by a new vibrational peak at 1150 cm-1, assigned to the umbrella mode, and isotope exchange between coadsorbed H2O and D. Isotope exchange in the coadsorbate involves three reaction equilibria: intralayer exchange in the aqueous layer (primarily water), intralayer exchange in the chemisorbed deuterium layer, and interlayer exchange between the aqueous and chemisorbed deuterium layers. Both intralayer exchanges are essentially completely equilibrated for the conditions of the experiment, whereas interlayer exchange is kinetically limited, ranging from zero to 55% approach to equilibrium depending on the coverages of both species. Irreversible hydration of chemisorbed hydrogen, as probed by thermal desorption, is essentially negligible. This behavior is consistent with the production of hydronium ions, since the above surface reaction is approximately energetically neutral. These results are also consistent with recent attempts to unify the electrochemical and vacuum potential scales, which predict hydronium ion formation on high work function surfaces, like platinum, but not on low work function surfaces, like copper.