Vibrational studies of the surface phases of CO on Pt{110} at 300 K

Abstract

Vibrational studies of the chemisorption of CO on Pt{110}, both as a function of coverage during adsorption at 300 K and during desorption from the ordered (2 × 1)plgl phase, are presented. High resolution electron energy loss spectroscopy (HREELS) results are discussed in detail, and are compared to data collected in a separate UHV chamber using reflection absorption infrared spectroscopy (RAIRS). HREELS revealed absorption bands at 475 cm −1 (Pt-CO stretch) and at 2105 cm −1(C-O stretch) assigned to chemisorbed CO bonded end-on to a single Pt atom. A third band, observed at 420 cm −1, is ascribed to a frustrated rotation of tilted linearly bonded CO at high coverages in the (1 × 1) and the (2 × 1)p1g1 phases. For the latter phase the effective dynamic charge for both the Pt-CO and the C-O stretching vibrations are calculated using dipole scattering theory. A 37 cm −1 frequency shift as a function of coverage is observed using RAIRS; and by using a mixture of 13CO/ 12CO this frequency shift is attributed purely to dipole-dipole coupling, and a singleton frequency of 2079 cm −1 is found. A detailed model for the adsorption sequence is presented, which extends our previous work (using mainly angular resolved UPS (ARUPS), LEED and thermal desorption spectroscopy (TDS)) on the surface phase transitions in CO chemisorption on Pt{110} at 300 K. In the coverage range 0.5 < θ < 0.9 after partial desorption from the p1g1 phase we observe an additional loss peak in HREELS at 1915 cm −1, assigned to the C-O stretch of CO adsorbed on short bridge sites. A maximum relative coverage of 25% bridge bonded CO is found at a partial CO coverage of θ ~0.7. ARUPS results are presented indicating that the bridged molecules are adsorbed with the molecular axis normal to the surface. A structural model in the coverage range 0.5 < θ < 0.9 after desorption from the ordered phase is proposed which involves a disordered array of tilted linearly bonded and upright bridge bonded CO molecules. Previous TD spectra are reinterpreted in the light of the new results. A new reconstruction-desorption model is proposed, which attributes the two different states in the CO desorption spectra to differences in the stability between reconstructed and unreconstructed Pt{110} phases.