Adsorption of naphthalene and quinoline on Pt, Pd and Rh: a DFT study

The adsorption of naphthalene and quinoline on Pt(111), Pd(111) and Rh(111) surfaces is studied using density functional theory. The metal surfaces are simulated by means of large confined clusters and for Pt by means of a slab with periodic boundary conditions (PBC). Calculation parameters such as basis set convergence, basis set superposition error and effects of cluster relaxation and size are analyzed in order to assess the aptness of the cluster model. For all the metals, the preferred sites of adsorption are analyzed, thus revealing their different behaviors concerning structure and stability of adsorption modes. On Pt, the molecules have the richest theoretical configurational variety. Naphthalene and quinoline are found to adsorb preferentially on di-bridge[7] sites on the three metals, and Rh exhibits higher adsorption energies than Pt and Pd. Structural features of the adsorbed molecules are correlated to the calculated adsorption energies. The di-bridge[7] adsorption modes are studied in deeper detail decomposing the adsorption energies in two terms arising from molecular distortion and binding interaction to the metal. Molecular distortion is correlated to the HOMO-LUMO energy gap. The larger adsorption energies found for interactions with Rh result from the lower contribution of the distortion term. Binding interactions are described by analyzing the wave functions of naphthalene and quinoline adsorbed on a subunit of the large clusters in order to reduce the complexity of the analysis. Molecular orbitals are studied using concepts of Frontier Molecular Orbitals theory. This approach reveals that in the adsorption of naphthalene and quinoline on Pt and Pd, an antibonding state lies below the Fermi energy, while on Rh all antibonding states are empty, in agreement with the larger interaction energies. In addition, further insight is gained by projecting the density of states on the d band of the clean surfaces and of the adsorbed systems. This results in the rationalization of the structural features in terms of the concepts of electronic structure theory. The distributions of electronic density are described by means of Hirshfeld charges and isosurfaces of differential electron density. The net electron transfer from the metals to the molecules for most of the sites correlates with the trends of the adsorption energies.