In-situ detection of NO chemisorbed on platinum using infrared-visible sum-frequency generation (SFG)

In-situ sum-frequency generation (SFG) surface vibrational spectroscopy for different polarisation arrangements has been employed along with low-energ y electron diffraction (LEED) measurements and kinetic modelling calculatio ns to study NO adsorption on Pt(111) as a function of NO gas-phase pressure (p(NO) = 10(-9)-10(-4) mbar) and substrate temperature (T-s = 300-400 K). The observation of a NO stretching vibrational band with a frequency of 172 4 cm(-1) with different signal intensities for ssp and ppp polarisation com binations indicated the presence of tilted NO species at high NO coverages (theta greater than or equal to 0.5). At lower coverages (0.5 > theta > 0.2 5) the adsorption geometry was found to change towards upright NO giving ri se to a vibrational band with slightly lower frequencies of 1716-1720 cm(-1 ). LEED studies demonstrated that under adsorption/desorption equilibrium c onditions up to saturation coverage NO adsorption leads to the formation of ordered adsorbate structures with 2 x 2 periodicity. The detailed analysis of the pressure and temperature dependence of the LEED and SFG surface vib rational data is consistent with a NO adsorption mechanism involving the su ccessive formation of Pt(111)+p(2 x 2)-NO (theta = 0.25), Pt(111)+(2 x 2)-2 NO (theta = 0.5) and Pt(111)+(2 x 2)-3 NO (theta = 0.75) adsorbate structu res as the NO pressure is increased from 10(-9) to 10(-4) mbar. A tentative binding site assignment for the two higher-coverage structures with theta = 0.5 and 0.75 is proposed and discussed in the light of the present and pr evious experimental data.