Identifying the surface species is critical in developing a realistic understanding of supported metal catalysts working in water. Regarding supported Ru catalysts, several hypotheses have been proposed to explain their enhanced reactivity to valorize biomass in aqueous conditions, notably, the presence of hydrogen-bonded water molecules[1] and the generation of active surface hydroxyls.[2] Since most of these transformations are performed under a pressure of H2, it is imperative to investigate the surface state of metallic Ru nanoparticles under varying conditions of temperature, pressure and aqueous environment. To identify the surface species exposed at the Ru/water interface, we randomly generated more than 800 configurations, along with well-known ordered structures, involving adsorption of H/O/OH/H2O species on 3 different Ru surfaces. We found that under a pressure of hydrogen the most stable phase is a monolayer of H on the three facets under investigation. However, if little or no H2 pressure is supplied, the surface tends to oxidize by O/OH species reaching a monolayer of oxygen at temperatures above 400 K. Notably, in presence of liquid water, full dissociation into O and H is favored over a partial dissociation into OH and H, as illustrated on Ru(0001) in Fig 1a vs. 1b. This is due to a solvation stabilization of -0.279 J.m-2 relative to the partially-dissociated water monolayer found on Ru(0001) in UHV conditions.[2] This contribution is evaluated using our QM/MM approach (MMSolv) [3] that has been designed to keep a highlevel description of the surface/adsorbate interactions. A careful analysis of the structuring of explicit water molecules reveals that the O and H species on the surface act in a synergistic fashion to draw MM water molecules closer to the surface in localized pockets leading to stronger solvation. In fact, some water molecules can approach the surface very closely above the H-covered zone to engage in hydrogen bonding with the surface O atoms (see Fig 1c). Those results [4] will be compared and contrasted with the one obtained on small Ru particles supported on TiO2. References [1] C. Michel, J. Zaffran, A.M. Ruppert, J. Matras-Michalska, M. Jedrzejczyk, J. Grams, P. Sautet. Role of water on metal catalyst performance for ketone hydrogenation. A join experimental and theoretical study on levulinic acid conversion into gamma-valerolactone. Chem. Comm., 50, 12450 (2014) [2] P.J. Feibelman, Partial dissociation of water on Ru(0001), Science, , 295, 99 (2002) [3] P. Clabaut, B. Schweitzer, A. W. Goetz, C. Michel, S. N. Steinmann, Ten Facets, One Force Field: The GAL19 Force Field for Water-Noble Metal Interfaces J. Chem. Theo. Comp. 2020, 16, 6539 (2020) [4] M.A. Ramzan, R. Wischert, S. N. Steinmann, C. Michel, Toward a Realistic Surface State of Ru in Aqueous and Gaseous Environments J. Phys. Chem. Lett., 14, 4241 (2023)