Carbon supported Rh-based nanoparticles with an average size of 1.0 nm have been tested for the hydrogenation of butadiene in two different forms: either metal Rh or hydride RhHx nanoparticles. Carbon supported Rh-based catalysts have been synthesized by a simple liquid impregnation method followed by reduction under hydrogen at 175 °C (1). Laboratory tests demonstrated that the Rh hydride nanocatalyst is more active than the metal counterpart, irrespective of the gas feed composition and temperature. Moreover, this difference is significantly more important at the initial stage of the reaction as compared to quasi-stationary conditions. However, the reaction mechanisms appear similar for metal and hydride nanocatalysts, as suggested by the similar selectivities to butenes, apparent reaction energies, and reaction orders. The local structures of both Rh and RhHx nanocatalysts were studied by operando X-ray Absorption Spectroscopy under stationary conditions (ROCK beam-line at SOLEIL synchrotron). The EXAFS analyses confirm that RhHx and Rh catalysts preserve their structure during the reaction as either hydride or metal phase, respectively. We suggest that the Mars-van Krevelen mechanism might occur at the initial stage but becomes progressively less predominant. Under quasi-stationary conditions only electronic effects might be invoked to explain the activity difference between the hydride and the metal phases. We hypothesize that the stabilization of Rh-H bonds at the surface in the presence of subsurface hydrogen, consistently with previous theoretical findings (2), explains the higher activity of the hydride catalyst. References: 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).