Insight into the mechanism and possibility of ethanol formation from syngas on Cu(1 0 0) surface

• CH3 is the most favored monomer among all CHx species on Cu(1 0 0) surface.
• C2 oxygenates of ethanol precursor is mainly formed by CHO insertion into CH3.
• CHO insertion into CH3 is the most favorable among all reactions related to CH3.
• CH3OH formation is much easier than CH3 formation.
• Suppressing CH3OH and/or boosting CH3 sources can improve the yield of ethanol.

Low cost Cu-based catalysts have became an attractive option in catalyzing ethanol formation from syngas (2CO + 4H2 = C2H5OH + H2O). In this study, the complex mechanism of ethanol formation from syngas on Cu(1 0 0) surface has been investigated using density functional theory (DFT) calculations together with periodic slab models, in which all possible formation pathways of CHx(x = 1–3) and C2 oxygenates of ethanol precursor, as well as the hydrogenation of C2 oxygenates to ethanol have been considered. Our results suggest that ethanol formation starts with CO hydrogenation to CHO, subsequently, CHO is further hydrogenated to CH3O via CH2O intermediate, CH3O is eventually hydrogenated to CH3OH; alternatively, CH3O dissociation with hydrogen-assisted can also produce CH3; then, CHO insertion into CH3 can lead to the key intermediate CH3CHO, which is successively hydrogenated to ethanol via CH3CH2O intermediates; moreover, among all reactions related to CH3 species, CHO insertion into CH3 to CH3CHO is the most favorable reaction, which is responsible for C2 oxygenates formation. In the overall process of ethanol synthesis, CH3OH is more easily formed than CH3, and ethanol formation is controlled by CH3 formation, which means that the preferable CH3OH formation rather than CH3 is responsible for the low productivity and selectivity of ethanol. Therefore, aiming to obtain high productivity and selectivity to ethanol from syngas on Cu catalyst, we need to selectively modify Cu catalyst by using the promoters and/or supports, which should be able to boost CH3 formation and/or suppress CH3OH formation, this result can be applied to selectively modify and develop the novel Cu-based catalyst towards ethanol formation from syngas.

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