1 Highly Active Au/δ-MoC and Cu/δ-MoC Catalysts for the Conversion of CO 2 : The Metal/C Ratio as a Key Factor Defining Activity, Selectivity, and Stability Sergio Posada-Pérez, a Pedro J. Ramírez, b Jaime Evans, b Francesc Viñes, a Ping Liu, c Francesc Illas a,* and José A. Rodriguez c,* a Departament de Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/ Martí i Franquès 1, 08028 Barcelona, Spain b Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela c Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA *Corresponding authors: Francesc Illas (francesc.illas@ub.edu) and Jose. A Rodríguez (rodrigez@bnl.gov) Abstract The ever growing increase of CO 2 concentration in the atmosphere is one of the main causes of global warming. Thus, CO 2 activation and conversion towards valuable added compounds is a major scientific challenge. A new set of Au/δ-MoC and Cu/δ-MoC catalysts exhibits high activity, selectivity, and stability for the reduction of CO 2 to CO with some subsequent selective hydrogenation towards methanol. Sophisticated experiments under controlled conditions and calculations based on density functional theory have been used to study the unique behavior of these systems. A detailed comparison of the behavior of Au/β-Mo 2 C and Au/δ-MoC catalysts provides evidence of the impact of the metal/carbon ratio in the carbide on the performance of the catalysts. The present results show that this ratio governs the chemical behavior of the carbide and the properties of the admetal, up to the point of being able to switch the rate and mechanism of the process for CO 2 conversion. A control of the metal/carbon ratio paves the road for an efficient reutilization of this environmental harmful greenhouse gas. Keywords: CO 2 hydrogenation ● CO 2 reduction ● methanol ● reverse water-gas shift reaction ●metal carbides ● copper