Adsorption of unsaturated hydrocarbons on Pd(111) and Pt(111): A DFT study

The adsorption of unsaturated hydrocarbons on metallic surfaces is of fundamental and applied interest, in view of its implications as a model system for catalysis in the refining industry. In particular, the relation between the type of hydrocarbon and its adsorption strength on metallic surfaces remains an open question. Thus, we propose a comparative DFT study of the adsorption of several relevant hydrocarbons on the catalytically relevant Pd(111) and Pt(111) surfaces. The key adsorption features (structural, electronic, energetic, and spectroscopic) are analyzed in a consistent way for two olefins (ethylene, 1-butene), one aromatic (benzene), one alkyne (acetylene), and one diolefin (1,3-butadiene). Whereas ethylene and 1-butene are adsorbed in a di-sigma mode, the flat adsorption of benzene takes place in a bridge position. For acetylene, the adsorption occurs in a 3-fold hollow site, whereas 1,3-butadiene adsorbs in a tetra-a configuration whatever the coverage. In the latter case, a vibrational analysis of the adsorption modes is proposed and compared to available HREELS data. A thermodynamic model, giving the variation of the Gibbs free energy of adsorption, enables a ranking of the relative stability of the adsorbates as a function of pressure and temperature. Finally, we suggest that the more selective hydrogenation of diolefins observed for Pd may be attributed to the lower adsorption strength of 1-butene on Pd, preventing its further hydrogenation into butane.