Size and Ligand Dependent Reactivity of Ru-H NanoSystems – SIDERUS

Nanoparticles (NPs) of various metals have long been used in heterogeneous catalysis for a number of important catalytic processes (arene hydrogenation, Fischer-Tropsch, hydrocracking, dehydrogenation and aromatization of paraffins…). More recently, an increasing number of catalytic processes have also involved the use of Nps in solution, for exemple arene hydrogenation or C-C coupling reactions. In all cases, the knowledge of the precise nature of the active site remains challenging. Ruthenium hydrides, whether as molecular complexes or as surface sites in large Nps, are known to be very reactive and to participate in numerous catalytic reactions: hydrogenation, Fischer-Tropsch, hydrocracking… In these reactions, the reactivity and the selectivity of these catalysts are controlled by the size of the particles which influences the number and the type of surface sites such as those found on faces, edges and corners and by the presence of ligands. The purpose of this proposal is to study the influence of the size and the environment on the surface chemistry and reactivity of precisely characterized Ru hydride systems based on small surface clusters and NPs (supported or not). We will combine experimental and theoretical studies to achieve a molecular understanding of these complex systems. The particles will be generated on supports or in solution, and a range of Ru NPs, varying by the size and the nature of the stabilizing agent (solvents, organic or organometallic ligands vs. oxide surface), will be prepared through an organometallic approach (decomposition under mild conditions of well-defined organometallic complexes). These systems will be characterized by the state-of-art physical techniques to probe: i) the particle size through TEM; ii) the structure by HREM, WAXS and EXAFS; iii) the surface ligands by adsorption (hydrides, CO, NO, O2), IR and NMR (gas phase, solution, solid state MAS or solid state static; NMR is accessible here because of the absence of Knight shift observed on Ru as a result of its low magnetic susceptibility) in combination with theoretical modeling. Coordination of ligands and reaction with molecules such as H2, alkanes, silanes, stannanes, olefins, arenes, will be used in order to establish the fundamental reactivity patterns of surface Ru hydrides as a function of size and chemical environments. Two reactions of utmost catalytic relevance will be particularly investigated: I) arene hydrogenation for which dependency of the size, or nature of exposed faces (related to size) are well-known; ii) elementary steps of hydrocracking and homologation reactions through reactivity with alkanes, diazocarbenes, alkenes and silanes involving C-, Si-C, C-N bond activation on a ruthenium surface. The final objective of this research is to monitor the reactivity of metal surfaces in a way similar to the state of the art in reactivity studies on organometallic complexes. Computational studies will address the influence of the structure and the size of the Ru particles on the dynamic, the spectroscopy and the reactivity of surface hydrides and ligands. After calibration and validation on well-defined Ru-hydride complexes, DFT calculations with the techniques useful for intermediate systems (QM/MM, effective group potentials) will be carried out for systems up to 10-15 atoms. Periodic calculations will also be used for evaluating the influence of surface defects and coverage. Complexes stabilized by ligands (models of systems in solution) or by oxide surface will be studied to understand the role of the ligands or the surface on the geometries and dynamic of the hydrides. The spectroscopic signatures (e.g. NMR chemical shifts quadrupolar splitting) of H/D, which are powerful reporters on the local environment, will be calculated for all coordination sites and environment of H/D. The reactivity of the hydrides towards representative molecules will be studied by exploring the potential energy surfaces. The results (structures, NMR signatures, reactivities) on small systems will serve to construct knowledge for the study of large systems. In these systems, the number of possible structures increases drastically. Dynamical studies, coupled with global optimization methods (Genetic algorithm, replica exchange method…) are planned in order to obtain a representative sampling of energy landscapes and to find stable structures. This requires long simulations (around the nanosec scale) and thus development of appropriate model Hamiltonians is needed. The model Hamiltonian will be calibrated and validated by the DFT calculations performed on the smaller systems. Model Hamiltonians are adapted to account for atoms of different natures and in different environments and thus they can integrate the surface ligands, hydrides etc. Therefore, they should help describing a unified bonding model valid from small to large ruthenium hydrides clusters in solution or stabilized by surfaces.