Chemisorption of carbon monoxide on tungsten

Abstract

Abstract Results obtained by a variety of techniques for the interaction of CO with polycrystalline W ribbons and with the (100) face of a W single crystal are presented and discussed. Desorption spectra from a W ribbon possess three peaks, designated α, β1 an, β2 which are kinetically characterized. From electron stimulated desorption (e.s.d.) studies on both W ribbons and W(100) it is concluded that the α desorption peak is a composite from two states, α1and α2 which yield, respectively, CO+ and O+; the ionic e.s.d. cross-section for the β states is negligible by comparison with that for the α states. An interconversion from α1 to α2, occurring during both thermal and electronic depopulation of the α1 state, is quantitatively characterized. Reflexion-absorption infrared spectroscopy reveals a band at ca. 211000 m-1 for CO on polycrystalline W, and this is shown to be associated with the carbon-oxygen vibration in the α states. Oxygen is rapidly adsorbed by a CO-saturated W surface, with a preferential depletion of α1-CO. The β-CO states are not depleted by O2, but undergo (a) complete isotopic mixing with coadsorbed oxygen, and (b) convergence into a single monotonic desorption peak appearing at the β1 peak position. The results are consistent with dissociative adsorption in the β states.