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Articles Nature Catalysis agreed with the main role for the FeTPP[Cl] complex as a CO2- to-CO conversion enhancer. This view was further supported by the fact that a family of CO2-to-CO catalytically active porphyrin- based complexes with different ligand structures and cobalt centres also showed enhanced performance for CO2-to-ethanol (Fig. 4b and Supplementary Fig. 23). The range of molecular enhancers demon- strated here showcases the wider applicability of the homogeneous– heterogeneous cooperative strategy in reprogramming CO2RR. Taking these experimental findings together with the DFT cal- culations, we conclude that the functional complexes in this study generate a CO-rich environment at the molecule–metal interface, increasing the coverage of *CO on the Cu surface. This lowers the barrier to C–C coupling and steers selectivity towards ethanol, resulting in increased control over CO2RR. To gain molecular-level insight into the cooperative role of the heterogenized FeTPP[Cl] and Cu substrate, we interrogated the FeTPP[Cl]/Cu surface using in situ Raman spectroscopy (Supplementary Fig. 24). We acquired Raman spectra on both Cu and FeTPP[Cl]/Cu electrodes at a range of applied potentials from the open-circuit potential to −0.57V (Fig. 4c, d). Under negative applied potentials, the peaks associated with surface copper oxide (CuOx) disappeared, a finding we attributed to the reduction of the oxide to metallic Cu. On bare Cu electrodes, three bands were identified associated with surface-adsorbed *CO at 280, 365 and the range of 1,900– 2,130cm–1 (Fig. 4c). These bands emerged at −0.45V and corre- sponded to the Cu–CO frustrated rotation, the Cu–CO stretch and the C≡O stretch, respectively43,44. On the FeTPP[Cl]/Cu catalyst, the same bands emerged at an earlier onset potential of −0.41V (Fig. 4d); this was consistent with the notion that CO concentration was enhanced on the FeTPP[Cl]/Cu electrode. We also observed an emerging band at 535cm–1, which we attributed to the Fe–CO bending vibration owing to the interaction between CO and Fe in the iron porphyrin segment45–47. This finding supports the view that immobilized FeTPP[Cl] produced CO, which was then converted to C2+ products on a nearby Cu site, as evidenced by the Cu–CO feature and by the fact that the FE of CO on FeTPP[Cl]/Cu did not increase compared with that on bare Cu at similar applied poten- tials. This view agreed with the picture of cooperative electrocata- lytic CO2-to-ethanol conversion by the heterogenized FeTPP[Cl] on the Cu substrate in Fig. 2a. We confirmed, carrying out Ar and supported by its reversibility following electrolysis; the oxidation state of Fe recovered to the same value as that before the reaction and there was no Fe–Fe bond formation, findings we obtained examining the post-electrolysis catalyst using XAS (Supplementary Fig. 18). With the aid of both Cu K-edge XANES and EXAFS spec- troscopies, we confirmed that Cu remained in the metallic state and that the porphyrin did not coordinate with Cu under catalytically active conditions (Supplementary Fig. 27). Conclusions In summary, we present an intermediate-enrichment-enhanced electrocatalytic CO2-to-ethanol conversion strategy by coopera- tive catalysis at the molecule–metallic interface. Using this strat- egy, implemented using a family of porphyrin-based metallic complexes on a Cu catalyst, we reported a CO2-to-ethanol con- version with an ethanol FE of 41% at a partial current density of 124 mA cm–2 in neutral media. We achieved a full-cell EE of 13% for the electrosynthesis of ethanol from CO2 and water. The find- ings suggest a wider strategy for improving CO2 conversion into value-added liquid fuels using renewable electricity with the aid of cooperative effects between adsorbed molecular and heteroge- neous approaches. Methods DFT calculations. All the DFT calculations in this work were carried out with a periodic slab model using the Vienna ab initio simulation program51–54. The generalized gradient approximation was used with the Perdew–Burke– Ernzerhof exchange-correlation functional. The projector-augmented wave 56,57 method was utilized to describe the electron–ion interactions and the cut-off energy for the plane–wave basis set was 450 eV. To illustrate the long-range dispersion interactions between the adsorbates and catalysts, we employed the D3 correction method as described by Grimme et al.58. Brillouin zone integration was accomplished using a 3 × 3 × 1 Monkhorst–Pack k-point mesh. All the adsorption geometries were optimized using a force-based conjugate gradient algorithm. For the modelling of Cu(111) and Cu(100), the crystal structure was optimized; Cu(111) and Cu(100) were modelled with a periodic four-layer p(3 × 3) model with the two lower layers fixed and the two upper layers relaxed. FeTPP[Cl]/Cu(111) was modelled with a periodic three-layer p(9 × 9) model with the one lower layer fixed and two upper layers relaxed. The atomic coordinates of the optimized models are provided in Supplementary Data 1. Electrode preparation. All chemicals were purchased from Sigma Aldrich and were used without further purification. The PTFE electrode was prepared by sputtering 200 nm Cu onto a piece of PTFE membrane (pore size of 450 nm, with a polypropylene support on the backside) using a pure Cu target (99.99%) at a –1 sputtering rate of 0.67 Å s . The porphyrin-based complexes were dissolved in a mixture of tetrahydrofuran and acetonitrile (6:4 by volume) at a concentration of 0.1 mM. The complexes were then spray-coated on either GDE (Sigracet, Fuel Cell Store) or the PTFE electrode. The nominal concentration was estimated on the basis of the concentration of solution used for the spray coat and the true loading concentration was quantified by inductively coupled plasma mass spectrometry (ICP–MS; Supplementary Fig. 21). The optimized loading of the FeTPP[Cl] molecules for the GDE was approximately fivefold higher than that for the Cu/ PTFE electrode owing to the larger surface area of the GDE. CO2RR and product analysis. Most CO2RR measurements were conducted in an electrochemical flow cell setup (details in Supplementary Fig. 8). The applied potentials were converted to the RHE scale with iR correction through the following equation: ERHE 1⁄4E þ0:059 ́pHþ0:210þiR ð2Þ versus Ag=AgCl where i is the current at each applied potential and R is the equivalent series resistance measured via electrochemical impedance spectroscopy in the frequency range of 105–0.1 Hz with an amplitude of 10 mV. The MEA (see details in Supplementary Fig. 15) system was used to carry out the full electrosynthesis of ethanol. The full-cell voltage was gradually increased from 3 V to 3.7 V and kept constant over the course of electrolysis. The gaseous products were analysed by gas chromatography (PerkinElmer Clarus 600), equipped with a thermal conductivity detector and a flame ionization detector. Liquid products were quantified by 1H NMR spectroscopy (600 MHz Agilent DD2 NMR spectrometer) using dimethyl sulfoxide as the internal standard. Isotopic experiments were carried out using 1 M KCl and 1 M KHCO3 as 13 catholyte and anolyte, respectively, to avoid the exchange of carbon between CO2 and H12CO3–. 13CO controls (Supplementary Fig. 25), that the Raman peaks cor- 2 responding to Cu–CO and Fe–CO as discussed above were indeed from CO2RR. The band at 535cm–1 was ruled out as being from CuOx using X-ray absorption spectroscopy (XAS; vide infra). We investigated further the chemical structure and coordinat- ing environment of FeTPP[Cl] under operating conditions using XAS. The Fe K-edge X-ray absorption near edge structure (XANES) spectra of the FeTPP[Cl]/Cu showed that the central Fe ions of the FeTPP[Cl] molecules were slightly reduced under catalytic turn- over conditions (Fig. 4e). Using linear combination fitting, we calculated the average oxidation state of Fe to be ~1.7–1.8 under operating conditions (Supplementary Fig. 26), consistent with the reported mechanism for the electrochemical CO2-to-CO conver- sion by FeTPP[Cl], where the porphyrin-coordinated Fe2+, Fe1+ and Fe0 were supposed to be intermediate species48,49. We also observed a higher pre-edge feature compared with that for the dry sample, a finding suggesting a more distorted structure by coordination of the metal centre with CO2RR intermediates50. We examined the Fe K-edge extended X-ray absorption fine structure (EXAFS) spec- tra of the FeTPP[Cl]/Cu catalyst. Over the applied potential range relevant to CO2RR, we identified only the Fe–N bond, whereas the Fe–Fe bond was not observed (Fig. 4f). This finding suggests that FeTPP[Cl] maintained its original complex status instead of being reduced to iron nanoparticles or nanoclusters under operating conditions. The stability of the FeTPP[Cl] molecule was further 80 NATurE CATALYSiS | VOL 3 | JanUarY 2020 | 75–82 | www.nature.com/natcatal 55PDF Image | Cooperative CO2-to-ethanol conversion via enriched intermediates
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