Where does hydrogen adsorb on Ru nanoparticles? A powerful joint (2)H MAS-NMR/DFT approach.

Several studies have shown that H NMR, among many other techniques, is a versatile tool for the study of hydrogen coordination modes in transition metal complexes or clusters. Thanks to H gas-phase and H solid-state MAS-NMR spectroscopy, the coexistence of ancillary organic ligands, with mobile and reactive hydride ligands coordinated to ruthenium nanoparticles (NPs) has been observed. Similar results were also found in the case of Ru NPs embedded in the cavities of metal-organic frameworks (MOFs). However, secure assignment of the experimental information is hardly straightforward, as few reliable reference data are available for different bonding situations and chemical environments. As a matter of fact, solid-state NMR spectra of molecular compounds with nonequivalent deuterons generally consist of a complex superposition of subspectra with different shapes, which depend on the motion of the corresponding deuterons. Here, the need for systematic theoretical studies for better interpretation of NMR spectra is clear. Recently, some of the authors have proposed the foundation of this work, with molecular density functional theory (DFT) calculations of quadrupole coupling constants Qcc and the asymmetry parameter hQ, (see the Supporting Information for definitions) of deuteron in ruthenium complexes directly comparable with the experimental data. There is no doubt now that joint theoretical/experimental studies are a very powerful approach to complete the partial understanding of deuteron solid-state NMR spectra. Herein, we propose to explore the stability and to estimate quadrupole coupling parameters of deuterium atoms, on and below the surface of Ru NPs at low coverage, by means of DFT calculations on an infinite Ru(0001) slab surface model. Moreover, new quantitative insights into the kinetic behavior of chemically adsorbed hydrogen atoms are brought with the help of diffusion barrier estimations for surface hopping and subsurface paths. These investigations should form a first basis for the assignment of deuteron MAS-NMR spectra of ruthenium NPs. At a coverage value of 1/4 monolayer (ML), high-symmetry adsorption sites on the Ru surface, as depicted in Figure 1, were considered, yielding energetic and structural parameters