CO adsorption on Pt(111) modified with sulfur

Abstract

CO adsorption on clean and S-covered Pt(111) was studied using temperature programmed desorption (TPD), electron stimulated desorption ion angular distribution (ESDIAD), LEED, and work function measurements. Special attention was paid to comparing the CO adsorption rate, binding energy, and soft bending modes on a clean surface and on p(2×2) S/Pt(111) with S coverage =0.25 S/Pt. It was found that on p(2×2) 0.25 S/Pt(111), the CO adsorption rate is decreased by a factor of 2 and only one CO adsorption state with maximum coverage, θCO ≂0.25 CO/Pt is detected. On the basis of the ESD data the CO adsorption state on p(2×2) 0.25 S/Pt(111) is assigned to terminal-CO residing on the next nearest Pt atom and separated from S by 3.72 Å. When compared with the same CO configuration on clean Pt(111) in the limit of low θCO, the adsorption binding energy of the terminal CO on sulfided Pt(111) is decreased by 8 kcal/mol. For this same overlayer, the cross sections for production of all ESD products (CO+ , O+ , and metastable CO*) is increased by 30%–50%. This result is interpreted considering the possible S-induced perturbations of the CO–5σ/metal and metal/CO–2π* coupling. An important result in the present study is the observation of a substantial decrease of the polar angle of the ESDIAD patterns of all CO ESD products from the sulfided surface which indicates a decrease of the amplitude of the CO bending modes. This appears to be direct experimental evidence for S-induced stiffening of the soft CO-bending vibrations. Approximate estimations (on the basis of the measured polar angles of the ESDIAD patterns) showed that the frequency of the CO frustrated translational modes increases by about a factor of 2—from 48 cm−1 for CO/Pt(111) to ≃100 cm−1 for CO/p(2×2) 0.25 S/Pt(111).