It is well known that Co catalysts are efficient for ethanol steam reforming (ESR) reactions, but whether the active site is Co-0, Co2+, or Co-0-Co2+ is still debated. In the present work, density functional theory calculations are performed to study the reaction mechanisms of ESR over Co-0, Co2+, and Co-0-Co2+ sites. The mechanism of ESR on Co-0 sites is CH3CH2OH -> CH3CH2O -> CH3CHO -> CH3CO -> CH3 + CO, and H-2 is formed by the combination of adsorbed H species with a relatively high barrier (1.37 eV). On the CoO (1 0 0) surface, the main product CH3CHO is produced through the consecutive dehydrogenation of ethanol, but the Coo surface lacks the C-C bond activation that ESR requires. On Co catalysts with a combination of Co-0 and Co2+ (Co-10/CoO(1 0 0) model), a strong synergetic effect was found: On interface, it occurs through the path CH3CH2OH -> CH3CH2O -> CH3CHO -> CH3CO, the resulting CH3CO on interface will spread to Co-10 cluster and bind to OH on the interface which results from H2O dissociation on the CoO surface: CH3CO -> CH3COOH (interface), and the resulting acetic acid (CH3COOH) will spread to the Co-10 cluster and go on C-C scission with the path CH3COOH -> CH3 + trans-COOH -> CH3 + CO2 + H to form CO2. On CoO parts (Co2+ sites), H2O dissociation is more facile than that on Co-0 sites, and the formed OH (or O) migrates to the interface easily and reacts with CH, species to release carbon deposition. On the interface between Co-0 and Co2+, some key coupling reactions related to ESR are favored, such as H-2 formation, the formation of CH3COOH, and the oxidation of CHx species to release coke formation, thus leading to high activity for ESR. Possible reasons for the result that C-C bond breaking can take place on Co-0 sites instead of Co2+ sites have been analyzed, and it was found that Co-0 sites with a large ensemble size of Co favor dissociation reactions such as C-C scission. Based on the present work, it is expected that a proper catalyst for ESR reactions should have metallic sites with large ensemble size to break C-C bonds and oxidized metal sites with small ensemble size to favor water dissociation as well as acetate species formation. (C) 2018 Elsevier Inc. All rights reserved.