Density functional theory calculations have been employed to investigate the effect of promoter Co on C-2 oxygenate formation from syngas on pure Cu(100) and two kinds of Co-Cu bimetallic surfaces. Based on the results of previous studies that CH3O is a key intermediate in ethanol formation on Cu catalyst, five reactions starting from CH3O were taken into consideration. Different from the results on Cu(100) that CH3OH is the most favorable product on pure Co sites, CH3 formation is more favorable both kinetically and thermodynamically than CH3OH formation, which leads to more CH3 available for CHO insertion to form C-2 oxygenates. On the other hand, Co-Cu bimetallic sites can facilitate CHO insertion into CH3 energetically, which is favorable for carbon chain growth. And the addition of Co can make the barrier of CH3 hydrogenation and CH3 coupling to CH4 and CH3CH3 higher, making CH3CHO much more selective than hydrocarbons. In conclusion, by introducing Co into Cu catalyst, the productivity and selectivity of C-2 oxygenate precursor of ethanol can be effectively improved. The optimum Co-Cu catalyst should contain Cu ensembles and Co ensembles with proper sizes, and offer enough Co-Cu bimetallic sites at the same time. (C) 2017 Elsevier B.V. All rights reserved.