RSC Adv. 2022 Jul 4;12(30):19394-19401. doi: 10.1039/d2ra02753d. eCollection 2022 Jun 29.
Cu2O based electrocatalysts generally exhibit better selectivity for C2 products (ethylene or ethanol) in electrochemical carbon dioxide reduction. The surface characteristic of the mixed Cu+ and Cu0 chemical state is believed to play an essential role that is still unclear. In the present study, density functional theory (DFT) calculations have been performed to understand the role of copper chemical states in selective ethanol formation using a partially reduced Cu2O surface model consisting of adjacent Cu+/Cu0 sites. We mapped out the free energy diagram of the reaction pathway from CO intermediate to ethanol and discussed the relation between the formation of critical reduction intermediates and the configuration of Cu+/Cu0 sites. The results showed that Cu+ sites facilitate the adsorption and stabilization of *CO, as well as its further hydrogenation to *CHO. More importantly, as compared to the high reaction energy (1.23 eV) of the dimerization of two *CO on Cu+/Cu0 sites, the preferable formation of *CHO on the Cu+ site makes the C-C coupling reaction with *CO on the Cu0 site happen under a relatively lower energy barrier of 0.58 eV. Furthermore, the post C-C coupling steps leading to the formation of the key intermediate *OCHCH2 to C2 compound are all thermodynamically favoured. Noteworthily, it is found that *OCHCH2 inclines to the ethanol formation because the coordinatively unsaturated Cu+ site could maintain the C-O bond of *OCHCH2, and the weak binding between *O and Cu+/Cu0 sites helps inhibit the pathway toward ethylene. These findings may provide guidelines for the design of CO and CO2 reduction active sites with enhanced ethanol selectivity.