Co-decorated Cu alloy catalyst for C2 oxygenate and ethanol formation from syngas on Cu-based catalyst: insight into the role of Co and Cu as well as the improved selectivity

Abstract

Co-decorated Cu alloy, a potential material for use in syngas conversion based on less expensive Cu metal, can efficiently promote the formation of C₂ oxygenates and ethanol. Herein, the mechanisms of C₂ oxygenate and ethanol formation from syngas on Co-decorated Cu alloy catalyst have been investigated to probe into the role of Co and Cu and identify the catalytic selectivity of Cu catalysts toward C₂ oxygenates and ethanol. DFT calculations with microkinetic modeling have been performed, and the Co-decorated Cu alloy catalyst is modeled using Co-doped Cu(211) surface. Our results suggest that CO initial adsorption and activation occur at the Cu sites of Co-doped Cu(211). CO prefers to be hydrogenated to CHO at the Cu site, and subsequently, CHO species at the Cu site easily migrate to the most stable Co–Cu mixed site. Starting from CHO species adsorbed at the Co–Cu mixed site, CH₂ and CH₃ species are the most favorable CH_x monomers formed via H-assisted CH_xO (x = 2, 3) dissociation, which are more favorable than CH₃OH formation both thermodynamically and dynamically, suggesting that the CoCu(211) surface can provide more CH_x resources for C–C chain formation. Further, starting from CH₂ and CH₃ species, ethanol is formed by CO insertion into CH₂ and CH₃ to CH₂CO and CH₃CO, respectively; subsequently, CH₂CO and CH₃CO are successively hydrogenated to ethanol via CH₂CHO, CH₃CHO and CH₃CH₂O intermediates. Based on microkinetic modeling, the CoCu(211) surface is highly selective for ethanol rather than methanol and methane. Moreover, the function of Cu is to provide the undissociated CO/CHO at the Cu sites; Co promotes CH_x formation by accelerating the C–O bond cleavage of CH_xO species, and the synergetic effect of Co and Cu facilitates C–C chain formation, which is typical of a “dual-site” mechanism. Thus, the synergetic effects between the active Co and Cu sites promote the formation of C₂ oxygenates and ethanol; the productivity and selectivity of ethanol over the Co-decorated Cu-based catalyst can be improved compared to that of the pure Cu catalyst. In addition, we believe that the insight derived from this study can be valuable for the design of other types of Cu-based catalysts involved in C₂ oxygenate synthesis from syngas.