ADSORPTION OF CO ON NI-PROMOTED TIO2(110)

The valence-band structure and the vibrational modes of CO adsorbed on nickel-promoted TiO2(110) surfaces as a function of CO exposure have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy-loss spectroscopy (HREELS). It is found that CO exists in molecular form at room temperature on the nickel-promoted TiO2(110) surfaces and most likely binds to the Ni atoms or nickel-affected sites rather than to the substrate atoms. At saturation coverage, CO molecules adsorb simultaneously on the 2-fold bridge sites and terminal sites on the (111)-oriented Ni islands deposited upon TiO2(110). The occupation of the edge sites of Ni islands gives rise to an anomalously low frequency of the C-O stretching vibration. This frequency, indicative of a weakened C-O bond, suggests existence of a precursor to the dissociated state.