The necessary swap from petroleum feedstock to biomass feedstock triggers the development of new catalysts, in particular in the conversion of oxygenated products. As an abundant biodiesel by-product, glycerol is a potential renewable building block for sustainable chemistry.[1] It is a C3 platform molecule, leading to several chemicals such as propanediols (PDO), lactic acid (LA), acrylic acid, etc. It can also be seen as a prototypical polyalcohol: results achieved could be extended to higher polyols such as glucose, starch, etc. The hydrogenolysis reaction catalyzed by a transition metal solid is a potential way to transform glycerol into important chemicals.[2] Surprising results have been obtained for the glycerol transformation by Rh catalysts on a carbon support in basic medium under a pressure of hydrogen. In addition to the expected propanediol, we obtained an oxidation product, namely LA, while operating under reductive conditions. Reversely, we still observe the hydrogenolysis product (PDO) in absence of H2. This clearly raises questions on the underlying mechanism. Moreover, the first step of glycerol hydrogenolysis is still under debate: it could be either a dehydration step, or a dehydrogenation step. Combining theoretical modeling of the elementary steps and detailed experimental study of our system, we will provide a better insight into the fundamental processes underlying those unexpected results. We will focus mainly here on the first step of glycerol transformation by DFT calculations. Materials and methods 100 mL 5wt% glycerol in NaOH 1M were introduced in a 200 mL stainless steel autoclave equipped with a graphite-stabilized Teflon® container, then 500 mg Rh/C catalyst was added and the reactor was flushed with helium, heated at 453 K then the reactor was purged with H2 and the pressure was adjusted to 50 bar. Periodic DFT calculations were carried out with the VASP package[3] using the PW91[4] exchange-correlation functional and a plane wave basis set. Reactions paths have been studied combining the NEB procedure[5] together with our local reaction path generator, CARTE[6]. Transition state are characterized a single imaginary frequency. The Rh(111) surface is modeled by a 3x3 slab of 4 layers, the two bottom layers are kept frozen in the bulk positions. Results/Discussion Experimentally, the glycerol transformation is performed in basic media (NaOH 1M) with Rh/C catalyst yielding concomitantly LA and PDO under inert (He) or reductive atmosphere (H2). The conversion is higher under He atmosphere and LA is the major product, while PDO is the major product under H2 atmosphere. We observe a dihydrogen production when operating under He atmosphere and hydrogen pressure is also initially increased by the reaction under H2 atmosphere. The total organic carbon analysis of the solution is stable all over the reaction, demonstrating that we do not have aqueous phase reforming. Thus, experiments suggest that dehydrogenation is the first step. To have a better insight into the molecular process, we have fully examined by the mean of DFT calculations the glycerol dehydrogenation mechanism on Rh(111). It can yield dihydroxyacetone or glyceraldehyde. Thermodynamics favours the formation of dihydroxyacetone but the relative stability is modified on the surface where the most favoured one is glyceraldehyde. It is also kinetically preferred, as indicated by the low calculated activation barriers. Moreover, the OH bond rupture is easier than the CH one in glycerol conversely to ethanol.[7] Thus, our DFT results sustain that the dehydrogenation step is feasible and leads to glyceraldehyde through OH scission followed by CH scission. Hence the combination of experiment and theory clearly demonstrates that, surprisingly, dehydrogenation is the first step for the catalysed hydrogenolysis reaction. 1. A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green Chem., 10, 13 (2008) 2. J. Chaminand, L. Djakovitch, P. Gallezot, P. Marion, C. Pinel, C. Rosier, Green Chem., 6, 359 (2004) 3. G. Kresse, J. Hafner, Phys. Rev. B, 47, 558, (1993) 4. J. Perdew, Y. Wang, Phys. Rev. B, 45, 13244 (1992) 5. D. Sheppard, R. Terrell, and G. Henkelman, J. Chem. Phys. 128, 134106, (2008) 6. P. Dayal, P. Fleurat-Lessard, to be published. 7. H.F. Wang, Z.P. Liu , J. Am. Chem. Soc. 130, 10996 (2008)