THE ADSORPTION OF CO ON NI(110) AND ITS INTERACTION WITH HYDROGEN - A RAIRS STUDY

The adsorption of CO on clean Ni{110} and on Ni{110} with preadsorbed hydrogen has been investigated in the temperature range 110-320 K using high resolution infrared spectroscopy together with LEED and TDS. Below room temperature the infrared spectra are very sensitive to the presence of traces of adsorbed H, which favours occupancy of bridged sites. In the absence of hydrogen atop and bridged sites are occupied, with CO bands in the frequency ranges 2000-2060 cm-1 and 1880-1940 cm-1, respectively, for coverages below 0.8 monolayers, the atop band being dominant; a streaky missing-beam c(4 X 2) structure observed at this coverage suggests the formation of a distributed bridge-bonded/atop structure. At full monolayer coverage a single band at 1998 cm-1 signifies the conversion to the tilted-molecule (2 X 1)p2mg overlayer, with molecules occupying bridge sites. In the presence of H the atop site is destabilised in favour of a bridged species, for CO coverages below 0.5 monolayer. However, for higher CO coverages this species is itself destabilised, as the CO adlayer switches to atop sites with a band at 2070 cm-1, not previously reported. This effect is independent of initial hydrogen coverage. Structural models are suggested, and analogies are drawn between the properties of the hydrogen-atop-CO mixed adlayer and the SIGMA-states of H-2 and CO on Ni{100}, where SIGMA-H-2 is molecularly chemisorbed.