STRUCTURE AND REACTIVITY OF THE SURFACE METHOXY SPECIES ON AG(111)

The structure and reactivity of the surface methoxy species on Ag{111} have been studied using a combination of reflection absorption infrared spectroscopy and temperature-programmed desorption. Methoxy can be generated by exposing a preoxidized Ag{111} surface to methanol at 180 K and adsorbs in an upright geometry possessing effective 3-fold symmetry, with its C-O bond axis oriented normal to the surface at all coverages. This conclusion is based on a detailed symmetry analysis coupled with rigorous application of the metal-surface selection rule and careful assignment of the infrared absorption bands with the aid of deuterium substitution. It is shown that strong Fermi resonance interactions are the cause behind unusual features observed in the C-H stretching region of the infrared spectra of methoxy on many metal surfaces. Significant interadsorbate dipole coupling and chemical/electrostatic effects are responsible for the coverage-dependent frequency shifts exhibited by the infrared active bands. Methoxy is stable on Ag{111} up to 290 K, above which it decomposes to yield formaldehyde as the major product, which desorbs in a reaction-limited step that is consistent with first-order kinetics. Formate has been identified as a minor surface species when excess surface oxygen is present and acts as the intermediate in the further oxidation of formaldehyde to CO2 and H-2.