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-energy electron diffraction (LEED) measurements and kinetic modelling calculations to study NO adsorption on Pt(111) as a function of NO gas-phase pressure (pNO = 10-9-10-4 mbar) and substrate temperature (Ts = 300-400 K). The observation of a NO stretching vibrational band with a frequency of 1724 cm-1 with different signal intensities for ssp and ppp polarisation combinations indicated the presence of tilted NO species at high NO coverages (θ ≥ 0.5). At lower coverages (0.5 > θ > 0.25) the adsorption geometry was found to change towards upright NO giving rise to a vibrational band with slightly lower frequencies of 1716-1720 cm-1. LEED studies demonstrated that under adsorption/desorption equilibrium conditions up to saturation coverage NO adsorption leads to the formation of ordered adsorbate structures with 2× 2 periodicity. The detailed analysis of the pressure and temperature dependence of the LEED and SFG surface vibrational data is consistent with a NO adsorption mechanism involving the successive formation of Pt(111)+p(2×2)-NO (θ = 0.25), Pt(111)+(2 ×2)-2 NO (θ = 0.5) and Pt(111)+(2×2)-3 NO (θ = 0.75) adsorbate structures 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 θ = 0.5 and 0.75 is proposed and discussed in the light of the present and previous experimental data.