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Articles Nature Catalysis a b 0.0 –0.2 –0.4 –0.6 c –0.4 CO2 on Cu *CO *OCCO *CHCOH C–C coupling *CHCHOH C2H5OH *CCH C2H4 0.0 –0.2 1/9 2/9 3/9 *CO coverage 4/9 1/9 2/9 3/9 4/9 *CO coverage Fig. 1 | DFT calculations. a, Key reaction pathways for CO2rr to ethanol and ethylene illustrated using two *CO for C–C coupling at the presence of one additional *CO. Blue, copper; grey, carbon; red, oxygen; light blue, hydrogen. b, reaction energies (ΔEreaction) of two *CO forming a *OCCO via the C–C coupling step. The *CO coverage indicates the surface ratio of additional *CO on a 3 × 3 Cu(111) surface. c, reaction energies of *CHCOH to *CHCHOH (orange) or *CCH (cyan) at different *CO coverage on a 3 × 3 Cu(111) surface. ensuing reduction to ethanol on Cu sites. As a result of this strategy, we report CO2-to-ethanol conversion with an FE of 41% at a partial current density of 124 mA cm–2. We build a full-cell system, demon- strating the electrosynthesis of ethanol via CO2 reduction coupled to the water oxidation reaction, with a full-cell energy efficiency (EE) of 13%. results DFT calculations. We applied DFT calculations to predict the effect of local CO on the C–C coupling step. We first ascertained which type of CO (surface bound versus unbound) accounts for this effect on a Cu(111) surface (3×3 unit cell), a facet suggested by micro kinetics studies30,31 to have low *CO coverages during electrocata- lytic turnovers, offering us greater freedom to tune the coverage. We found that the presence of unbound CO did not affect the reac- tion energy of the coupling of two adsorbed *CO on the Cu surface (Supplementary Note 1 and Supplementary Figs. 1 and 2). Instead, the concentration of the unbound CO tuned the coverage of *CO on the Cu surface via the Langmuir isotherm model and then changed the reaction energy of the C–C coupling step (Supplementary Note 1). We then introduced various additional *CO coverage onto the Cu(111) surface with two *CO already adsorbed and compared the reaction energy of the C–C coupling step (Fig. 1a and Supplementary Fig. 3). The reaction energy was lowered when additional *CO was introduced, and the reduction in energy increased with further increase in *CO coverage with a coverage of 3/9 showing the largest margin (Fig. 1b): the adsorbate–adsorbate interaction became more marked with the increase in surface coverage32. We further calculated reaction energies associated with the hydrogenation of the intermediate *CHCOH towards either *CHCHOH or *CCH (Fig. 1a and Supplementary Fig. 4), a step found by Goddard and co-workers25,33 critical to controlling the differential production of ethanol versus ethylene. We found that in the presence of *CO, the reaction energy decreased more for the formation of *CHCHOH (ethanol path) compared with that of *CCH (ethylene path). These reaction energies differentiated fur- ther with higher *CO coverage (Fig. 1c), indicating that high *CO coverage steers selectivity from ethylene to ethanol. We found that the coverage of *CO also tuned the energy pro- files of reaction intermediates on the Cu(100) surface, though the changes were not as marked as those on the Cu(111) surface (Supplementary Figs. 5–7). Noting that Cu(111) is typically the dominant facet of polycrystalline Cu substrates (an approach com- monly employed in the study of Cu-based catalysts) we based our calculations on Cu(111). CO2RR performance. We pursued the intermediate-enrichment strategy experimentally by introducing a molecular catalyst on the surface of Cu (Fig. 2a), one that we posited would increase local CO concentration via its own electrocatalytic turnovers. Molecular catalysts have achieved near-unity FE for CO, commonly in non- aqueous solutions34,35. Their immobilization on conductive carbon substrates or covalent organic frameworks has enabled the trans- lation of water-insoluble molecular catalysts to operate instead in aqueous solutions and has enabled marked increases in the turnover frequency for CO production36–38. The use of molecular complexes is expected to create CO-rich interfaces via conformal contact with Cu surfaces, without affecting the CO2-to-C2+-active electronics of Cu. We used 5,10,15,20-tetraphenyl-21H,23H-porphine iron(iii) chloride (FeTPP[Cl], Fig. 2b), which exhibits a high FE for CO at low overpotentials in both homogeneous and immobilized forms35,39, as the molecular complex. We first examined CO2-to-CO conversion of FeTPP[Cl] immobilized on carbon substrates via the π–π interaction37, studying it in a flow cell (Supplementary Fig. 8) with a gas diffusion electrode (GDE) and employing 1M aqueous KHCO3 as the electrolyte (used throughout this study). The immo- bilized FeTPP[Cl] showed a clear increase of the reductive current under CO2 atmosphere compared with N2 atmosphere (linear 76 NATurE CATALYSiS | VOL 3 | JanUarY 2020 | 75–82 | www.nature.com/natcatal ∆Ereaction (eV) ∆Ereaction (eV)PDF Image | Cooperative CO2-to-ethanol conversion via enriched intermediates
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