Introduction The necessary swap from petroleum feedstock to biomass feedstock triggers the development of new catalysts, in particular in the conversion of oxygenated products.[1] In this context, the dehydrogenation of alcohols is one of the central issues. In alkanes, the C-H bond scission is mainly catalyzed by supported metallic particles, the metal and the support being tailored depending on the desired products. For the C-H and O-H bond dissociation in alcohols, supported metallic particles are also used, but they have to be adapted to the presence of the aqueous phase, the alcohols being generally solubilized in water.[2-3] Consequently, a better understanding of such processes is required. Using periodic calculations in the DFT framework, we will focus on the role of H-bonded neighbors on the C-H and O-H bond scissions in water, ethanol and glycerol. Computational details The metallic surfaces have been modeled by a slab made of four layers, separated by 5 layers-equivalent of vacuum. The two bottom layers are frozen in the ab initio bulk position. The supercell is a 3x3 cell. The calculations have been performed at the DFT level using the VASP code. Various XC energy functionals have been used. A tight convergence of the plane-wave expansion was obtained with a cutoff of 400 eV. The electron-ion interactions were described by the projector augmented wave method (PAW). A Monkhorst-Pack mesh of 3x3x1 K-points was used for the 2D Brillouin zone integration. Geometries have been converged until forces were less than 0.01eV. Transitions states have been characterized by the presence of a unique imaginary frequency. Results and discussion OH bond scission in water -- We have considered close-packed surfaces of Co, Rh, Ir and Ni, Pd, Pt. At a 2/9 ML coverage, water preferentially adsorbs as a dimer rather than as two monomers: a first water molecule is adsorbed at atop site, the second one is H-bonded as the H-bond acceptor, farther from the surface. This process is strongly synergetic: both the H-bond and the metal-O bond are shortened, and the energy gain is greater than the formation of a H-bond plus the weak interaction between the surface and the second water molecule. The synergy can be quantified around 0.25 eV per dimer. In this water dimer, the OH bond scission is facilitated compared with the isolated water for most of the considered metals (up to 30% decrease of the activation barrier) expect Pt. Those calculations have been performed using a GGA functional (PW91). For sake of comparison, the same analysis has been also performed using PBE but also PBE+D to take better into account the dispersion[4-5] and optB86, a functional recently proposed by Klimes et al.[6] based on the non-local Van der Walls density functional initially proposed by Dion et al.[7] OH and CH bond scission in alcohols -- Then, we have focused on the effect of an adsorbed water molecule on O-H and C-H scission in ethanol on Rh(111) and Pt(111). The situation is analogous to the water dimer, the ethanol replacing the physisorbed H-bonded water molecule. Here again, the OH scission is assisted on Rh,[8] not on Pt. In addition, the CH scission is inhibited (up to 30% increase of the activation barrier). Thus, the presence of the adsorbed water modifies strongly the reactivity of ethanol at a metallic surface: the OH scission becomes easier than the CH scission and rhodium becomes more active than platinum. At a lesser extent, the same effect can be pinpointed in polyols such as glycerol.[9] Conclusions According to DFT calculations using various functionals, H-bonded neighbors are key players in the reactivity of oxygenated compounds at metallic surfaces. Their influence depends on the metal in consideration. Acknowledgements Calculations were performed using the local HPC resources of PSMN and of GENCI (CINES/IDRIS), project x2010075609. The French ANR supports this project (GALAC). References [1] Corma, A., Iborra, S., Velty, A., Chemical Reviews, 107 (2007) 2411-2502 [2] Chaminand, J., Djakovitch, L., Gallezot, P., Marion, P., Pinel, C., Rosier, C., Green Chemistry, 6 (2004) 359-361 [3] Chheda, J. N., Huber, G. W. , Dumesic, J. A. Angewandte Chemie International Edition, 46 (2007) 7164-7183 [4] Grimme, S., Journal of Computational Chemistry, 27 (2006) 1787-1799 [5] Hujo, W., Grimme, S., Physical Chemistry Chemical Physics, 13 (2011) 13942-13950 [6] Klimes, J., Bowler, D. R., Michaelides, A., Journal of Physics-condensed Matter, 22 (2010) 022201 [7] Dion, M., Rydberg, H., Schröder, E., Langreth, D. C., Lundqvist, B. I. Physical Review Letters, 92, (2004) 246401 [8] C. Michel, F. Auneau, F. Delbecq, Ph. Sautet, ACS Catalysis, 1, (2011) 1430 [9] F. Auneau, C. Michel, F. Delbecq, C. Pinel, Ph. Sautet, Chemistry: A European Journal, on line (doi:10.1002/chem201101318)