Carbon supported RhHx particles with an average size of 1.0 nm have been synthesized by a simple liquid impregnation method followed by reduction under hydrogen at 175 °C.1 Two Rh-based nanocatalysts were tested for the hydrogenation of butadiene : hydride RhHx (as-synthesized nanoparticles) or metal Rh (as-synthesized nanoparticles pre-treated at 300 °C under He). Laboratory catalytic tests were performed for the partial hydrogenation of butadiene for different gas feed compositions and temperatures. They proved that the Rh hydride nanocatalyst is more active than its metal counterpart, irrespective of the reaction conditions. However, the apparent activation energies and the selectivities to butenes are almost identical for both Rh and RhHx catalysts, suggesting similar reaction mechanisms. In order to probe in situ the local structure, XAS experiments were carried out in operando conditions on the ROCK beam line. The EXAFS analyses demonstrate the stability of the Rh or RhHx structure throughout the reaction at room temperature: neither hydrogen depletion from the hydride phase nor hydride formation from the metal phase were observed over a period of two hours. As previously suggested theoretically,2 hydrogen adsorption at the nanoparticle surface may be stabilized in the presence of subsurface hydrogen, which would explain the higher activity of the hydride phase relative to the metal catalyst. 1. C. Zlotea, Y. Oumellal, M. Msakni, J. Bourgon, S. Bastide, C. Cachet-Vivier and M. Latroche, Nano Lett.15, 4752–4757 (2015). 2. H. A. Aleksandrov, S. M. Kozlov, S. Schauermann, G. N. Vayssilov and K. M. Neyman, Angew. Chem. Int. Ed., 53, 13371–13375 (2014).