The effect of ZnO addition on the oxidation behavior of Co along with catalytic performance in vapor and aqueous phase reforming of ethanol were investigated on Co supported on carbon black (XC-72R). Carbon was selected to minimize the support interactions. Effect of ZnO addition during both vapor and aqueous phase reforming were compared at 250 degrees C. ZnO addition inhibited the reduction of cobalt oxides by H-2 and created surface sites for H2O activation. During vapor phase reforming at 450 degrees C the redox of cobalt, driven by steam oxidation and H-2 reduction, trended to an equilibrium of Co-0/Co2+. ZnO showed no significant effect on cobalt oxidation, inferred from the minor changes of C1 product yield. Surface sites created by ZnO addition enhanced water activation and oxidation of surface carbon species, increasing CO2 selectivity. At 250 degrees C cobalt reduction was minimal, in situ XANES demonstrated that ZnO addition significantly facilitated oxidation of Co-0 under vapor phase reforming conditions, demonstrated by lower C-1 product yield. Sites introduced by ZnO addition improved the COx selectivity at 250 degrees C. Both Co/C and Co-ZnO/C rapidly oxidized under aqueous phase reaction conditions at 250 degrees C, showing negligible activity in aqueous phase reforming. This work suggests that ZnO affects the activation of H2O for Co catalysts in ethanol reforming. (C) 2014 Elsevier B.V. All rights reserved.