Carbon Monoxide Adsorption Coverage Study on Platinum and Ruthenium Surfaces

Periodic density functional theory calculations elucidate carbon monoxide coverage effects on platinum and ruthenium surfaces. As expected the CO stretching frequencies increase with coverage. Unexpectedly, overlap population calculations show that increased stretching frequencies may not always correspond to stronger bonds. A theoretical framework is established based on a modified π-attraction σ-repulsion scheme. This phenomenological model directly relates the internal adsorbate bond strength to the net change of the carbon 2s and 2pxy contributions to the π- and σ-components, respectively. The variation of the metal–surface bond is examined by using the charges, polarizations, and electron densities of the adsorbate CO orbitals. For the systems studied here, the traditional frontier orbital model of the 5σ-donation/2π*-back-donation with the metal substrate bands is not always sufficient to explain the relative C–O and C–Metal bonds strengths.