ADSORPTION OF HYDROGEN AND OF OXYGEN ON AN OPEN METAL SURFACE-HREELS INVESTIGATION AT NI(311)

The adsorption behavior of hydrogen and oxygen on the stepped Ni(311) surface has been investigated by HREELS. A series of metastable phases was found for hydrogen adsorption at low temperatures with a successi on of different adsorption sites indicated by the following loss peaks : 55 and 149 meV for the threefold site, shirting with higher coverage to 65 and 155 meV, respectively; 40 and 90 meV for the fourfold site, shifting to 35 and 85 meV with coverage; and 110 and 124 meV for an a dditional site between close packed rows. Room temperature adsorption of hydrogen leads to the reconstruction of the surface with occupation of three- and fourfold sites, represented by loss peaks at 60 and 145 meV for the threefold site and 74 meV for the fourfold site. This pha se is the thermodynamically stable one. Oxygen is most likely initiall y adsorbed on a bridge site (loss peak at 66 meV). The stepped surface is already oxidized at very low exposures to oxygen, as seen by the c haracteristic vibration for oxide islands at 55 meV and later by the F uchs-Kliewer mode of NiO at 68 meV.