EFFECTS OF ELEVATED HYDROGEN PRESSURES ON CARBON-MONOXIDE CHEMISORBED ON NI(111)

The influence of hydrogen gas on CO chemisorbed on Ni(111) has been studied by infrared reflection absorption spectroscopy, temperature-programmed desorption, and low-energy electron diffraction. For CO coverages below 0.5 ML, both bridge-bonded and terminal-CO species coexist on Ni(111). Under the influence of elevated pressures of hydrogen, the terminal-CO species disappear, and CO is compressed into a disordered phase containing only bridge-bonded species. Simultaneously, a high-frequency CO species is produced by short-range interactions with electronegative coadsorbed hydrogen. These changes are all reversible, and only take place above a threshold temperature (approximately 240 K) and hydrogen pressure (approximately 10(-3) mbar). The experimental observations point to an activated motion of the CO molecules from their normal adsorption sites under the influence of a weakly-bound hydrogen species in equilibrium with H2(g).