Density functional calculation of the vibrational stretching mode of CO coadsorbed with ammonia on palladium clusters

Abstract

Vibrational analysis of the shift of the stretching mode of carbon monoxide coadsorbed with ammonia on model palladium clusters is presented. The CO molecule is adsorbed on the threefold site while NH 3 is adsorbed on an on-top site on the opposite side of the cluster. The electronic structure calculations are made by using a local density approximation. The vibrational analysis involves a two-dimensional potential depending on the CO and MeC distances. Several effects are analysed; firstly the addition of electrons to the cluster shifts the CO vibration to lower frequencies. This is explained as, primarily, a chemical effect; a small charge transfer to the adsorbate is observed and this is confirmed by the decrease of the binding energy of CO on the surface. Second, the effect of coadsorbed ammonia is to weaken the CO bond and to increase the CO chemisorption energy. The stretching mode frequency ω co is decreasing by about 20 cm −1 depending slightly on the cluster size while the MeCO beating mode has its frequency slightly increased. Changes in work function, σ-d and d-π ∗ backdonation, core electron energy of C, O and N atoms, as well as Mulliken population analyses are discussed and the link with the vibrational behavior of both studied vibrations is made. It is concluded that the CO is influenced indirectly by the ammonia which polarizes the cluster and hence increases the donation-backdonation between CO and the cluster although there is no charge transfer from NH 3 to the CO molecule through the palladium surface. It is shown in all cases that the anharmonicity contribution and the coupling of the stretching with the beating of the CO on the surface are small.