Molecular chemisorption at a metal surface is a key step for many processes, such as catalysis, electrochemistry, surface treatment, tribology and friction. Modeling with density functional theory is largely used on these systems. From a detailed comparison with accurate micro-calorimetric data on ten systems (involving ethylene, cyclohexene, benzene, naphthalene, CO, O2, H2, methane, ethane), we study the accuracy, for chemisorption on Pt(111), of five exchange-correlation functionals including one generalized gradient approximation functional (PBE) and four functionals that take into account van der Waals interactions (optPBE-vdW, optB86b-vdW, BEEF-vdW, PBE-dDsC). If the functionals used provide very similar geometries and electronic structures, as shown by projected density of states, they give strikingly different results for the adsorption energy of molecules on Pt(111). Among the set of chemisorption data, the lowest mean absolute deviations (MAD) are obtained with the optPBE-vdW and PBE-dDsC functionals (∼0.2 eV) while PBE and optB86b-vdW give twice larger MAD (∼0.45 eV). BEEF-vdW is intermediate with a MAD of 0.33 eV. For laterally π-bound unsaturated hydrocarbons (cyclohexene, benzene, naphthalene) the PBE and the BEEF-vdW functionals are severally under-bound, while optPBE-vdW and PBE-dDsC provide a good match with experiments. Hence both the incorporation of van der Waals dispersive forces and the choice of the exchange functional have a key influence on the chemisorption energy. Vertically bound ethylidyne and CO are in contrast over-bound with all functionals, the best agreement being obtained with BEEF-vdW. None of the selected functionals hence provides a universally accurate treatment of chemisorption energies.

Co-adsorption is a key initial step in heterogeneous catalysis, which by bringing the reactants together at high coverage on the surface of the catalyst has a clear implication on the catalytic reaction activity. We show that when using Density Functional Theory (DFT) calculations, the choice of the exchange correlation functional can have a qualitative influence on the nature of the obtained most stable coadsorption state. The coadsorption of butadiene and hydrogen, the precursor state for catalytic hydrogenation, is studied on Pt(111) and on the surface alloy Pt2Sn/Pt(111). At typical hydrogenation conditions, the PBE exchange correlation functional gives as most stable situation on both model catalysts a surface fully covered with hydrogen, butadiene remaining in gas phase. This behavior does not agree with available experimental data, and results from an incorrect balance between H and butadiene adsorption energy, mainly by a poor description of dispersion energy for butadiene adsorption. The non-local optPBE-vdW functional provides opposite and correct results, with a co-adsorption of butadiene and hydrogen as most stable situation. The butadiene adsorption energy is strengthened by the description of dispersive forces, hence modifying the nature of the energetic competition between the two adsorbates. The co-asorption energy difference between PBE and optPBE-vdW amounts to 1.04 (resp. 0.7) eV for Pt(111) (resp. Pt2Sn/Pt(111)) on the considered 3x3 unit cell. The computational study of co-adsorption systems from DFT is hence delicate. Errors do not only impact the quantitative adsorption energy of one adsorbate, but they might cumulate over several species, finally providing a qualitatively wrong picture of the optimal coadsorption situation. Going beyond the standard Generalized Gradient Approximation hence appears mandatory for a correct description of catalytic reactivity of unsaturated hydrocarbons.