A Density Functional Theory Study of Spectroscopic and Thermodynamic Properties of Surfacic Hydrides on Ru (0001) Model Surface: The Influence of the Coordination Modes and the Coverage

The characterization of hydrides on the surface of bulk metals and organometallic nanoparticles is of primary importance, especially in the context of catalysis, such as olefin hydrogenation. Although hydride titration as well as combination of nuclear magnetic resonance (NMR), and to a lesser extent infrared (IR) spectroscopies, confirm the presence and the potential reactivity of these simple species, the unambiguous characterization of their coordination on the surface and subsurface is still challenging owing to the number of degrees of freedom accessible at intermediate coverage rate. In this work, the influence of varying coordination modes (bridging, face capping, terminal, subsurfacic) and coverage upon the thermodynamic, kinetic, and spectroscopic properties have been investigated by means of density functional theory (DFT) calculations achieved on a ruthenium slab model. The predicted IR and (2)H NMR observables reinforce the previous spectral assignment. Interestingly, following our seminal work for a monolayer coverage value (Phi)) of 1/4 (Chem. Phys. Chem. 2009, 10, 2939), quadrupolar parameters obtained for higher Phi confirm that experimental (2)H NMR spectra measured on Ru nanoparticles result from the presence of deuterides adsorbed on terminal, 2-fold and 3-fold symmetry sites. The kinetic and thermodynamic parameters-namely, diffusion barriers for the former and adsorption energies and phase diagrams for the latter-show that the probability of finding a subsurfacic hydride increases with Phi, whereas the saturation threshold on pristine Ru surfaces should stay close to the unity-for very low and moderate values of H(2) pressure-in order to ensure favorable thermodynamic conditions. Finally, this study partially opens the route to DFT studies of multistep hydrogenation reactions at the surface of ruthenium nanoparticles monitored by spectroscopic techniques. Conversely, it also demonstrates that the use of finite size models will be mandatory in the future if one wants to reach a reliable description of the metallic nanoparticle properties.