Ligand effects observed for the adsorption of CO on Co-Pt alloys

The adsorption of CO on well-defined Co-Pt alloy surfaces prepared by physical vapour deposition in ultra-high vacuum was studied by Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) and temperature-programmed desorption (TPD) spectroscopy. The adsorption behaviour was found to depend strongly on the composition of the surface and the first subsurface atomic layer. In the case of low Co concentrations at the surface CO adsorbs preferentially on Pt sites, however with an enthalpy of adsorption which is substantially lowered when compared to CO adsorption on Pt single crystals. TPD shifts of ∼60 K were found. When Co dominates the surface composition, CO adsorbs preferentially on Co sites, however with an enthalpy of adsorption leading to a TPD shift by ∼80 K to higher temperatures with respect to monometallic single crystals. This behaviour can be explained by ligand effects, i.e., by an electronic influence of the presence of a second metal in the chemical environment of a given adsorption center. The experimental findings were compared with theoretical calculations.