Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases, Two effects can change the CO stretching frequency: the adsorbate-adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2 pi(.) CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that GO-CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2 pi(.) CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the (CO)-C-12-(CO)-C-13 dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the (CO)-C-12-(CO)-C-13 dipole coupling is indeed weak compared with the (CO)-C-12-(CO)-C-12. dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm(-1) to higher frequency. However, we should point out that when one (CO)-C-12 molecule is surrounded by a (CO)-C-13 environment, the (CO)-C-12 stretching band shifts 10 cm(-1) upwards. We have also computed the heat of adsorption of CO on Pt(100)-(1 x 1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol-(l1)and then drops to 180 kJ mol(-1) at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the GO-CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol(-1). (C) 2000 Elsevier Science B.V. All rights reserved.