Density functional cluster model study of bonding and coordination modes of CO2 on Pd(111)

The cluster model approach within the B3LYP, density functional theory base d, computational method has been used to investigate the coordination modes and adsorption sites involved in the interaction of CO2 on Pd(111). Three different surface species corresponding to two coordination modes only, eta (1)-C and eta(2)-side-on, are predicted. These are an almost undistorted CO 2 molecule and two highly bent adsorbed species. The undistorted adsorbate appears at a rather large distance from the surface and is tentatively attr ibuted to a precursor physisorbed state. The other two species closely rese mble CO2- and, in both cases, the equilibrium distance above the surface is much smaller than in the previous case. These two species correspond to CO 2 chemisorbed in the eta(1)-C and eta(2)-side-on coordination modes. The tw o chemisorbed species have very similar geometrical parameters and vibratio nal frequencies, and hence it is not possible to use these properties to di fferentiate both species. Nevertheless, the larger energetic stabilization of the eta(1)-C chemisorbed CO2 suggests that this is the one which is obse rved experimentally. (C) 1999 Elsevier Science B.V. All rights reserved.