THE INTERACTION OF H2O AND CL-2 ON FE POLYCRYSTALLINE SURFACES

Temperature-programmed desorption and Anger spectroscopy were used to study the interaction of H2O and Cl-2 on an Fe polycrystalline surface under UHV conditions over the temperature range of 90-900 K. The TPD spectra of H2O on a clean Fe(poly) surface show the desorption of H2O from two states at 175 and 205 K. There is no recombinant H2O desorption from the Fe(poly), in contrast to the Fe(100) surface. The adsorption and dissociation of H2O is significantly affected by the presence of Cl at the Fe(poly) surface. Pre-exposure of Cl-2 results in dissociatively adsorbed Cl (Cl-a) at the surface. Cl-a acts as a site blocker, preventing H2O dissociation. Cl-2 adsorption on an Fe(poly) surface pre-exposed to H2O at 90 K leads to a Cl-2-H2O reaction which results in the formation of an OH-Cl complex. This complex is stabilized to temperatures above 400 K in UHV and results in recombinant H2O desorption over a broad temperature range. Desorption of an FeOCl complex is observed at 240 K regardless of dosing sequence, but only at low adsorbate pre-coverages. These data demonstrate that Cl-a inhibits hydroxylation of the Fe(poly) surface. The low temperature volatilization of the Fe surface via Fe oxychloride formation does not directly involve H2O(a), but can only occur after H2O dissociation.