Ultra-violet and X-ray photoelectron spectroscopy (UPS and XPS) of CO, CO2, O2 and H2O on molybdenum and gold films

Abstract

Electron spectroscopic (XPS and UPS) studies of the interaction of CO, CO₂, O₂ and H₂O with Mo, and H₂O and CO₂ with Au are reported. The object was first to explore how far one could distinguish and between different states of chemisorption; whether there was any relation between bond strength and electron binding energy and whether electron spectroscopy, particularly UPS, could provide unambiguous information on the molecular nature of surface species. Physical adsorption data for CO₂ and H₂O on Au at 77 K provide reference binding-energy data.

Arguments are given for believing that, with molybdenum, CO can exist in both molecular and dissociated forms, that CO₂ dissociates into C and O and that water exists in both molecular and dissociated forms. The role of the substrate temperature in determining the relative stability of the different states of the molecules is emphasized. Implicit is the assumption that identical electron-binding data by XPS reflect identical surface-bonding situations. In this context the role of complementary UPS data is important. Where information was available on the relative strengths of surface bonding then for any given adsorbate the electron binding energy was greatest when bonding to the surface was weakest.