Crystal facet engineering and defect engineering are both critical strategies to improve the catalytic hydrogenation performance of catalyst. Herein, ethylene hydrogenation on the perfect and oxygen defective Co3O4(110) surfaces has been studied by using periodic density functional theory calculations. The results are compared with that on Co3O4(111) surface to clarify the problem of which facet for Co3O4 is more reactive, and to illuminate the role of oxygen vacancy. The low oxygen vacancy formation energy suggests that Co3O4(110) surface with defective site is easily formed. The whole mechanism of H-2 dissociation and stepwise hydrogenation of ethylene to ethane is examined, and the most favorable pathway is heterolytic dissociation of H-2 follows two stepwise hydrogenation of ethylene process. The results show that ethyl hydrogenation to ethane on perfect Co3O4(110) surface is the rate limiting step with an activation energy of 1.19 eV, and the presence of oxygen vacancy strongly reduces the activation energies of main elementary steps, and the activation energy of rate limiting step is only 0.47 eV. Compared with that on Co3O4(111), ethylene hydrogenation is preferred on Co3O4(110) surface. Therefore, Co3O4 with exposed (110) facet is predicted as an excellent catalyst for ethylene hydrogenation. (C) 2017 Elsevier B.V. All rights reserved.