Efficient electrocatalytic reduction of CO2 to ethane over nitrogen-doped Fe2O3

doi:10.21203/rs.3.rs-1540591/v1

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Efficient electrocatalytic reduction of CO2 to ethane over nitrogen-doped Fe2O3

https://doi.org/10.21203/rs.3.rs-1540591/v1

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Non-copper catalysts are seldom reported to generate C2+ products, and the efficient over these catalysts were low. In this work, we reported an iron-based catalyst centered in nitrogen doped γ-Fe2O3 (xFe2O3-N), which yielded C2H6 as major product in H-cell. At –2.0 V vs Ag/Ag+, the Faradaic efficiency (FE) for ethane reaches 42% with current densities of 32 mA cm−2. This is the first report about selective CO2 reduction to ethane (C2H6) over iron-based catalyst. Results showed that catalyst possessing FeO1.5-nNn sites enriched with oxygen vacancies was beneficial for the stabilization of *COOH intermediate. The exposure of two adjacent surfaces of Fe atoms was conducive to lower the energy barrier for C−C coupling over FeO1.5-nNn sites, facilitating the generation of C2H6. This work provides a strategy for design of novel iron-based catalyst with tunable local coordination and electronic structures for converting CO2 into C2 products in CO2RR.

Electrocatalytic reduction of CO₂ under ambient conditions using renewable energy has emerged as an attractive way to maintain carbon balance, which is regarded as one of the cleanest and efficient approaches^1–3. Recently, numerous electrocatalysts have been developed for CO₂ reduction reaction (CO₂RR)^4–7. However, the majority of these catalyst systems have low efficient for multicarbon (C₂₊) products due to the multiple pathways involved in the reaction process⁸. Cu-based catalysts are mostly reported to convert CO₂ into C₂₊ products such as ethylene, alcohols and acetic acid^9–11. Up to now, non-copper catalysts are rarely reported to generate C₂₊ products^12,13.

Iron-based catalysts have been studied extensively for CO₂RR^14–17. The reported Fe-based catalysts, such as atomically dispersed Fe − N−C, single-crystalline Fe₃N, Fe-porphyrin carbon materials (FeN₄-C), mainly produce CO with high FE up to 90%^18–20. The element dopants together with the local coordination environment of Fe center play remarkable roles in affecting the electronic structures and catalytic performances²¹. Currently, there have been few reports about selective formation of multicarbon products over Fe based catalyst. A FeP nanoarray was reported to be capable of producing C₂H₅OH with FE of 14.1% and a low current density (2 mA cm^− 2)¹³. In addition, Fe₂P₂S₆ nanosheet was also reported with a maximum FE for C₂H₅OH of 23.1% and very low current density (< 0.5 mA cm^− 2)²². It is highly attractive but still remains a big challenge for electrocatalytic CO₂RR to multicarbon (C₂₊) products efficiently over non-copper catalysts.

In this work, we designed and prepared Fe-based moieties behaving core-shell architecture inlaid with nitrogen (N) doped γ-Fe₂O₃ and wrapped with carbonitride (CN) as shell. The as-prepared catalysts containing Fe-O-N sites with N coordination and oxygen defects exhibited outstanding performance toward selectivity for electrochemical conversion of CO₂ to C₂H₆.

Preparation and characterization of the iron-based catalysts.

We designed a pre-assemble strategy to fabricate iron-based catalysts through the in situ polymerization of dopamine (DA) in a solution with Fe source (Fe³⁺) and Tris (Tris(hydroxymethyl)aminomethane) buffer, using Pluronic F127 as template. After calcination of the as-prepared nanoparticles under N₂ atmosphere, the N doped Fe₂O₃ with CN shell were obtained, as illustrated in Fig. 1a. The iron-based moieties were denoted as _xFe₂O₃-N@CN, where the subscript x corresponds to the mole fraction of Fe in all the elements after carbonization, according to the results obtained by the inductively coupled plasma optical emission spectroscopy (ICP-OES). A series of catalysts with different Fe content, _0.05Fe₂O₃-N@CN, _0.11Fe₂O₃-N@CN, _0.17Fe₂O₃-N@CN, _0.2Fe₂O₃-N@CN and _0.3Fe₂O₃-N@CN were prepared by this method via controlling the amounts of Fe source (Fe₂(SO₄)₃) used.

The scanning electron microscopy (SEM) images showed that the _xFe₂O₃-N@CN exhibited core-shell structure with size distribution of 300 ~ 400 nm (Supplementary Fig. 1). As showed in Supplementary Fig. 2, the X-ray diffraction (XRD) patterns of the as-prepared _xFe₂O₃-N@CN were consistent with the characteristic peaks of γ-Fe₂O₃ (PDF # 39-1346). The relative diffraction intensity of Fe₂O₃ peaks in the XRD patterns was enhanced by the increase of iron content. A fine study on _0.3Fe₂O₃-N@CN with the highest content of Fe atom was carried out by using the transmission electron microscopy (TEM) and SEM, and the images indicated the core-shell architecture of the catalyst with a shell thickness of about 30 nm (Fig. 1b and Supplementary Fig. 3). The catalyst of _0.3Fe₂O₃-N@CN showed an intermediate mode between type I and type IV, indicative of the mesoporous and microporous features²³. Brunnauer–Emmett–Teller (BET) surface area (SA_BET: 248 m² g ) analysis and pore distribution were presented in Supplementary Fig. 4. In the high-resolution TEM (HRTEM) images (Fig. 1c), the emerged fringe spacings of 0.208 nm and 0.295 nm matched well with the (400) and (220) lattice planes of Fe₂O₃. The scanning transmission electron microscopy (STEM) image indicated that the Fe₂O₃ nanoparticles were coated by CN shell with rough surface. The associated energy dispersive X-ray spectroscopy (STEM-EDX) elemental mapping analysis (Fig. 1d) demonstrated that the existence of Fe, C, O and N elements in the core and shell, with Fe element as the primary component, which results were approximately equal to that of ICP-OES (Fe 65.7 wt%). In addition, the EDX analysis also displayed the distribution of each components (Supplementary Fig. 5), which showed an obvious separation interface between the N-doped Fe₂O₃ in the core and C, N elements in the shell section.

To explore the electronic states and local coordination environments of Fe atoms in the _xFe₂O₃-N@CN, the X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) measurements were conducted. The Fe K-edge XANES curves of the as-prepared _xFe₂O₃-N@CN demonstrated near-edge absorption energy between those of Fe foil and Fe₂O₃, implying that the oxidation state of Fe was Fe³⁺ (Fig. 1e)^24,25. The Fourier transform (FT) curves at the Fe K-edge of extended X-ray adsorption fine structure (EXAFS) spectra were further fitted to investigate the coordination information of Fe (Supplementary Fig. 6). The FT-EXAFS curves displayed a broad peak at about 1.4 ~ 1.7 Å which could be attributed to Fe–O/N coordination, and the peak around 2–3 Å in the FT-EXAFS was assigned to the Fe–Fe distance in Fe oxides^26,27, as illustrated in Fig. 1f. It is ambiguity to distinguish the Fe-O and Fe-N coordination in the EXAFS data²⁸. Moreover, the peak around 2.22 Å was observed in both samples of _0.05Fe₂O₃-N@CN and _0.11Fe₂O₃-N@CN, which verified the coexistence of zero-valent iron crystalline phase (Fe⁰)²⁹, but no Fe–Fe bond derived from Fe⁰ was detected in _xFe₂O₃-N@CN with higher content of Fe (x ˃ 0.11). Quantitative EXAFS fitting analysis (Supplementary Table 1) demonstrated that all the coordination numbers of Fe-O/N in _xFe₂O₃-N@CN were lower than that of Fe₂O₃ reference, suggesting the unsaturated coordination of Fe-O/N. The XPS spectra for _xFe₂O₃-N@CN also showed the existence of Fe, N, O and C elements (Supplementary Fig. 7a). The Fe 2p peaks (Supplementary Fig. 7b) were attributed to the status of Fe³⁺ (712.7 and 724.8 eV), which was identical with the results of XANES. The high-resolution N 1s spectra of _xFe₂O₃-N@CN (Fig. 1g) were mainly assigned to four components corresponding to pyridinic N (398.1 eV), pyrrolic N (400.8 eV), Fe-N coordination with pyridinic (399.6 eV) and pyrrolic (398.6 eV)³⁰. The graphitic N (402.1 eV) existed only in _xFe₂O₃-N@CN with much lower mole fraction of Fe, and no peak of graphitic N was observed in samples of _0.2Fe₂O₃-N@CN and _0.3Fe₂O₃-N@CN. The O 1s spectra of all catalysts (Supplementary Fig. 7c) exhibited typical peaks at ~ 529.8 eV, 531.5 eV and 533.5 eV, which were assigned to lattice oxygen, oxygen vacancy, and surface-adsorbed oxygen species, respectively³¹. Notably, the oxygen vacancy (531.5 eV) occupied noticeably larger proportion in the O 1s components, which showed an increasing trend with the increase of Fe content. In brief, no obvious difference was observed in the XRD patterns, XANES and XPS spectra of _xFe₂O₃-N@CN with high content of Fe, demonstrating virtually identical iron oxynitrides in _0.17Fe₂O₃-N@CN, _0.2Fe₂O₃-N@CN and _0.3Fe₂O₃-N@CN. Based on the above analysis, the as-prepared catalysts possessed N-doped γ-Fe₂O₃ centers with different coordination of Fe-O-N structures and rich oxygen defects.

Atomic structure analysis of _xFe₂O₃-N@CN by using ⁵⁷Fe Mössbauer spectroscopy.

⁵⁷Fe Mössbauer spectroscopy is sensitive to iron species of similar coordination environments but with different electronic states^32,33. To better identify the local coordination of Fe species in the catalysts, ⁵⁷Fe Mössbauer spectroscopy measurements were conducted to distinguish the detail structures of Fe-based catalysts. The Mössbauer spectra of _0.05Fe₂O₃-N@CN, _0.17Fe₂O₃-N@CN, _0.2Fe₂O₃-N@CN and _0.3Fe₂O₃-N@CN were collected and analyzed. The spectrum of _0.05Fe₂O₃-N@CN was fitted with one doublet and three types of sextets (Fig. 2a). According to the isomer shift (IS) and quadrupole splitting (QS) values (Supplementary Table 3), the doublet (doublet 1, D1) was assigned as characteristic for the presence of square-planar FeN₄^34,35. It is well known that N atoms are very mobile to diffuse into deeper layers forming a magnetic oxynitride, in which the N content is close to that in the initial iron nitrides³⁶. Sextet 1 could be identified as iron oxynitride phase, FeO_mN_4−m (0.2 ≤ m ≤ 0.58) (Supplementary Text) centers enriched with N, which formation might be attributed to the diffusion of N atoms from the surface region of FeN₄ into deeper layers³⁷. In addition, the hyperfine field (H) parameters of FeO_mN_4−m agreed well with the values of iron nitrides given in the literatures^36–39. Signals of sextet 2 (IS -0.11 mm/s, QS 0.03 mm/s) and sextet 3 were assigned to γ-Fe₂O₃ and ɑ-Fe, respectively⁴⁰, which was in agreement well with the results of XRD and EXAFS.

The increase of Fe content resulted in significant variations in the Mössbauer spectra. As illustrated in Fig. 2b and Supplementary Figs. 8a and 8b, a second doublet (D2) and another different sextet (sextet 3) signals emerged in the Mössbauer spectra of _xFe₂O₃-N@CN with Fe mole fraction higher than 0.17. The D1 and D2 in the Mössbauer spectrum of _0.3Fe₂O₃-N@CN were assigned to square-planar FeN₄ coordination with Fe in low spin states with different iron position⁴¹. The extra sextet 3 was assigned to a O-rich composition FeO_1.5−nN_n (0.34 ≤ n ≤ 0.55) (Supplementary Text) with small amount of N incorporated into Fe₂O₃. The discrimination derived from the combination of isomer shifts and hyperfine field (H) was significantly different from that of FeO_mN_4−m with N-rich (Supplementary Table 3)^34,42. Nitrogen atoms are more mobile at some elevated temperature, and part of them gradually diffuse into deeper layers, which resulted in the formation of O-rich FeO_1.5−nN_n phase, leading to O vacancies simultaneously³⁶. A comparison between these catalysts with low and high content of Fe displayed totally different signals in the Mössbauer spectra (Fig. 2c). The spectra of _0.17Fe₂O₃-N@CN, _0.2Fe₂O₃-N@CN and _0.3Fe₂O₃-N@CN exhibited similar signals signifying the presence of identical iron oxy-nitride structures. The signals attributed to γ-Fe₂O₃ (sextet 2), FeN₄ (D1, D2), FeO_mN_4−m (sextet 1) and FeO_1.5−nN_n (sextet 3) were all visible by the identical features in Mössbauer spectra of _xFe₂O₃-N@CN with high loading of Fe element. We focus on the variation of resonance area to estimate the different iron oxy-nitride centers in depth (Supplementary Table 3). The γ-Fe₂O₃ composition exhibited slight increase ranging from 20.6–30.5% with increasing iron content. Interestingly, it could be observed that both of the contents of FeN₄ and FeO_mN_4−m decreased obviously with the increase of iron content in _xFe₂O₃-N@CN. In contrast, the proportion of FeO_1.5−nN_n sites displayed a remarkable tendency of increase (Fig. 2d), along with the generation of more oxygen defects (Supplementary Fig. 7c). In brief, the coordination environment varied with iron content, and there were more O-rich FeO_1.5−nN_n sites and oxygen defects in the catalysts with higher Fe content.

Electrocatalytic CO₂ RR over the various catalysts.

The catalytic CO₂RR performance of _0.3Fe₂O₃-N@CN with the highest Fe content was evaluated in a three-electrode system in an ionic liquid (IL) based electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF₆/acetonitrile (MeCN)/H₂O (W/W/W = 30/65/5)). The linear sweep voltammetry (LSV) curves of _0.3Fe₂O₃-N@CN in N₂- and CO₂- saturated [Bmim]PF₆/MeCN/H₂O electrolyte were presented in Fig. 3a. In the CO₂- saturated electrolyte, a much higher current density was generated over _0.3Fe₂O₃-N@CN, indicating the efficient activity of CO₂RR. Controlled potential electrolysis of CO₂ was conducted to study the reduction products at potentials ranging from − 1.9 to -2.6 V vs Ag/Ag⁺. Gas-phase and liquid products were analyzed by gas chromatography (GC) and ¹H NMR spectroscopy, respectively. Only gaseous products were detected in this study, and no liquid product was generated. The products were quantified after electrolysis for 2 hours. C₂H₆ was the sole product from CO₂ at − 2.0 V vs Ag/Ag⁺, and hydrogen was the only by-product (Fig. 3b). The FE of C₂H₆ was 42% at − 2.0 V vs Ag/Ag⁺ with a current density of 32 mA cm^− 2. As the potential became more negative, the FE of C₂H₆ decreased gradually, accompanied with the production of CH₄ and CO. Control experiments were performed at the same conditions to confirm that the products were indeed generated by electrochemical reduction of CO₂ over _0.3Fe₂O₃-N@CN, and there was no carbonous product at any potential in N₂-saturated electrolyte. Subsequently, the catalytic performances of the catalysts with various Fe content were further evaluated systematically. All the as-prepared _xFe₂O₃-N@CN displayed much higher current density in CO₂-saturated electrolyte (Supplementary Fig. 9). Catalysts of _0.05Fe₂O₃-N@CN and _0.11Fe₂O₃-N@CN with relatively low content of Fe element produced CO and CH₄ at all applied potentials, as illustrated in Supplementary Figs. 10a and 10b. Notably, FE_C2H6 and the current density increased as a function of Fe content in the _xFe₂O₃-N@CN (Figs. 3c and 3d). At -2.0 V vs Ag/Ag⁺, the FE_C2H6 over _0.17Fe₂O₃-N@CN, _0.2Fe₂O₃-N@CN, and _0.3Fe₂O₃-N@CN were 22%, 27%, and 42%, respectively. The catalysts with low loading of Fe element produced C₁ products such as CO and CH₄. However, the high content of Fe³⁺ in _xFe₂O₃-N@CN led to high FE of C₂H₆ and current density. In other words, Fe content and the resulting Fe-O-N coordination were the primary differences in the as-prepared _xFe₂O₃-N@CN, which determined the selectivity for generating C₁ or C₂ products in CO₂RR. The porous CN shells of catalysts exhibited unconspicuous role in the selectivity of products, which could prevent the aggregation of the magnetic iron-based materials. A combination of the atomic structures obtained by Mössbauer analysis and catalytic performances of the various _xFe₂O₃-N@CN demonstrated that FeO_1.5−nN_n sites with small amount of incorporated N favored the generation of C₂H₆ in CO₂RR. Consequently, we can deduce that the active sites are related to the structure of FeO_1.5−nN_n.

The Nyquist plots under the open circuit potential in CO₂-saturated electrolyte were measured to explore the properties of the electrode/electrolyte interface. The experimental impedance data were fitted by using an equivalent circuit R(C(R(Q(RW)))) (Supplementary Fig. 11 and Supplementary Table 2)⁴³. Catalyst of _0.3Fe₂O₃-N@CN showed the lowest interfacial charge transfer resistance (R_ct) (Fig. 3e), indicating the facile charge transfer on the catalyst. The FE_C2H6 and current density showed no obvious change after 10 hours of electrolysis, illustrating the excellent stability of _0.3Fe₂O₃-N@CN (Fig. 3f). Meanwhile, the quasi-operando XPS spectra acquired after reaction over _0.3Fe₂O₃-N@CN displayed that Fe³⁺ in the as-prepared materials maintained the oxidation state at all potentials (Supplementary Fig. 12), verifying the stability of the catalyst.

To further test the above hypothesis, we also prepared a sample by pyrolysis of _0.3Fe₂O₃-N@CN under air at 600°C to change the N coordination (denoted as Fe₂O₃-CNF). The XRD pattern for the material showed characteristic peaks of ɑ-Fe₂O₃, indicating a transformation of configuration for ferric oxide centers (Supplementary Fig. 13a). No core-shell structure was observed in the morphology of Fe₂O₃-CNF (Supplementary Figs. 13b and 13c). Trace amounts of N (< 0.03 at %) was observed in the XPS spectra (Supplementary Fig. 14). Notably, the lattice oxygen escalated dramatically and oxygen vacancy decreased after the calcination in air (Supplementary Fig. 14b). Whereas only two types of sextet assigned to Fe₂O₃ (Supplementary Fig. 15 and Supplementary Table 4) were observed in the Mössbauer spectrum of Fe-CNF, and no signal about the coordination of Fe-O-N was observed. The electrocatalytic performance of Fe-CNF without FeO_1.5−nN_n sites exhibited only C₁ productions (CO, CH₄) with a maximum FE of 68%, but C₂ products were not detected (Supplementary Fig. 16). The results further indicated that the FeO_1.5−nN_n sites with oxygen vacancy in the catalysts of _xFe₂O₃-N@CN were crucial for the generation of C₂H₆. Meanwhile, the polydopamine during preparation in this method provided CN shell and N coordination with iron species to obtain the particular Fe-O-N sites.

DFT studies on the iron oxy-nitrides (FeO_1.5−n N_n) for CO₂ reduction.

Density functional theory (DFT) calculations were carried out to unveiling the electronic properties of Fe species and the intrinsic property of active species in iron oxy-nitrides (FeO_1.5−nN_n) for CO₂ reduction to ethane^44,45. According to the XAFS results and the different N, O coordination with iron species analyzed by ⁵⁷Fe Mössbauer spectroscopy, two catalyst models of FeO_1.5−nN_n and pristine Fe₂O₃ were constructed for the calculations. Fe₂O₃ (400) surface was considered based on the HRTEM and XRD results. The energy profiles of the most possible route for the activity of CO₂ reduction over N-doped Fe₂O₃ were presented, as shown in Fig. 4. All the corresponding structures of the two models are also given in Supplementary Fig. 17. The existence of oxygen vacancy in FeO_1.5−nN_n sites leaded to the emergence of two adjacent surface of Fe atom, of which configuration existed in the lowest free energy (Supplementary Fig. 17c). The first hydrogenation of *CO₂ preferred to take place on the emerged Fe atom (*COOH) with ΔG = 0.20 eV. Further, the *COOH intermediate was adsorbed and stabilized at the same surface. Subsequently, the energy barrier (1.37 eV) in the rate-determining step of the C − C coupling process (*CO*CO + H⁺+e⁻→*COCOH) over FeO_1.5−nN_n sites was obviously lower than that on catalyst of pristine Fe₂O₃ (1.99 eV). Then the hydrogenation process of *COHCOH to *CCOH and *CHCOH to *CCH were downhill pathway with ΔG = 0.07 and 0.05 eV respectively. Thereafter, the rest hydrogenation steps involving exothermic or endothermic process were facile with decreased energy barrier (Fig. 4). However, the hydrogenation of *COHCOH to *CCOH was difficult with ΔG = 1.33 eV over pristine Fe₂O₃, also implying that the C − C coupling process was not favorable. The electronic characteristics of Fe species were regulated by the different coordination environments, which inevitably affected the adsorption properties of intermediates⁴⁶. As illustrated in Supplementary Fig. 18, The density of states (DOS) of FeO_1.5−nN_n were distinctly different from that of pristine Fe₂O₃. The N-doped Fe₂O₃ centers exhibited a shifting of the 3d orbitals of central Fe indicative of the manipulation of electronic characteristics of Fe atom due to the N doping. The varied electronic properties in the Fe-O-N sites benefited the donation and back-donation of electrons between the catalyst and the reactant, which satisfied the adsorption and stabilization of *CO intermediate (1.09 eV) produced in the hydrogenation process of CO₂ (Supplementary Fig. 19). Additionally, with regards to Fe₂O₃, the desorption of *CO is facile with lower energy (-0.07 ev), demonstrating high selectivity for C₁ products. However, the difficult desorption of *CO is conducive to the proceeding of C-C coupling reaction.

In conclusion, a pre-assembly approach for construction of iron-based catalysts enriched with oxygen vacancy has been proposed. The well-constructed _xFe₂O₃-N@CN catalysts consisted of different components such as Fe-O-N sites enriched with N coordination (FeO_mN_4−m) or O-rich coordination (FeO_1.5−nN_n), γ-Fe₂O₃ and FeN₄ structures. The electrochemical investigation showed that catalysts possessing FeO_1.5−nN_n sites exhibited excellent catalytic performance for conversion of CO₂ to C₂H₆. The FE for ethane reached 42% with current densities of 32 mA cm^− 2 at − 2.0 V vs Ag/Ag⁺ in a H-cell, and HER was the only side reaction. Through the precise coordination of Fe with O and N, series of _xFe₂O₃-N@CN catalysts were obtained. The catalysts with high content of FeN₄ facilitated to produce C₁ products, whereas catalysts enriched with FeO_1.5−nN_n sites were conducive to selective conversion of CO₂ into C₂ product. The high activity of _xFe₂O₃-N@CN for producing C₂H₆ was due to the adsorption and stabilization of *COOH intermediate on FeO_1.5−nN_n sites with exposure of the adjacent Fe surface, resulting in the decrease of energy barrier for C − C coupling process. This work provides a successful example for electrocatalytic conversion of CO₂ into C₂ product over iron-based catalysts. The results pave a way for the design of efficient catalysts with tunable local coordination environment to promote the conversion of CO₂ into multicarbon products in CO₂RR.

Materials. CO₂ (Beijing Beiwen Gas Chemical Industry Co., Ltd., research grade) had a purity of 99.999% and used as received. Iron(III) sulfate hydrate (A. R. grade), Tris(hydroxymethyl)aminomethane (99.5%) and dopamine hydrochloride were purchased from Innochem, Pluronic F127 (Mn ~ 8400) was supplied by Alddin. Acetonitrile (AcN, A. R. grade), acetone (A. R. grade) were provided by Sinopharm Chemical Reagent Co., Ltd, China. Toray Carbon Paper (CP, TGP-H-60, 19×19 cm) and Nafion N-117 membrane (0.180 mm thick, ≥ 0.90 meg/g exchange capacity) were purchased from Alfa Aesar China Co., Ltd. Polytetrafluoroethylene (PTFE, 60 wt% aqueous solution) was purchased from Sigma-Aldrich Co. LLC. 1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF₆, purity > 99%) was obtained from the Centre of Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.

Preparation of Fe-based catalysts _xFe₂O₃-N@CN

The polymerization of dopamine (DA) was carried out according to the previous literature⁴⁷. To prepare _xFe₂O₃-N@CN, 0.4 g of dopamine hydrochloride (DA), 0.16 g of F127, desired amount of Fe₂(SO₄)₃, 16 mL of deionized water and 8 mL of EtOH were added into a glass bottle and stirred to form a clear solution at 30°C. One hour later, 8 mL of Tris(hydroxymethyl)aminomethane (Tris) solution (0.16 g of Tris in 8 mL of deionized water) was added under stirring. The reaction solution was stirred overnight. The product was collected by centrifugation, washed with water for twice and ethanol three times to remove inorganic salts and surfactants, and dried at 60°C under vacuum to obtain microsphere powder. The obtained nanoparticles were heated in a tubular furnace at 300°C for 3 h and 600°C for 2 h under N₂ atmosphere. The heating rate was 1°C min^-1. The corresponding products were named as _xFe₂O₃-N@CN, where the subscript x corresponds to the mole fraction of Fe in the catalysts after carbonization.

The corresponding amount of Fe₂(SO₄)₃ added for preparation _xFe₂O₃-N@CN with different Fe content are as follows: _0.05Fe₂O₃-N@CN: 1.05 g Fe₂(SO₄)₃, _0.11Fe₂O₃-N@CN: 1.58 g Fe₂(SO₄)₃, _0.17Fe₂O₃-N@CN: 2.1 g Fe₂(SO₄)₃, _0.2Fe₂O₃-N@CN: 2.63 g Fe₂(SO₄)₃, _0.3Fe₂O₃-N@CN: 3.68 g Fe₂(SO₄)₃.

When the amount of Fe₂(SO₄)₃ in the system was higher than 3.68g (the molar ratio of Fe³⁺ and DA is 8.4:1), the loading of Fe in the catalysts was almost unchanged due to the saturated coordination. Therefore, the mole fraction of 0.3 for Fe atom was the highest content of Fe atom in the as-prepared _xFe₂O₃-N@CN nanoparticles.

Preparation of the control sample without CN shell Fe₂O₃-CNF

The obtained _0.3Fe₂O₃-N@CN was heated in a muffle furnace in the air at 600°C for 6 h to obtain Fe₂O₃-CNF. The heating rate was 1°C min^− 1.

Material characterisations. PXRD data were collected on the X-ray diffractometer (Model D/MAX2500, Rigaka) with Cu-Kα radiation at a scan speed of 5 °/min. The morphologies of catalysts were characterized by scanning electron microscope (SEM) (TECNAI 20PHILIPS electron microscope) and transmission electron microscope (TEM) (JEOL-2100F). The energy dispersive X-ray spectroscopy (STEM-EDX) elemental mapping analysis was obtained by JEOL-2100F. The BET surface area and porosity properties of the materials were determined by N₂ adsorption-desorption isotherms using a Micromeritics ASAP 2020M system. The Fe loadings in the catalysts were determined by ICP-AES method (VISTA-MPX). X-ray photoelectron spectroscopy (XPS) analysis was performed on the Thermo Scientific ESCALab 250Xi using a 200 W monochromated Al Kα radiation.

⁵⁷Fe Mössbauer spectroscopy. The ⁵⁷Fe Mössbauer spectra were recorded on an SEE Co W304 Mössbauer spectrometer at room temperature, using a ⁵⁷Co/Rh source in transmission geometry which was equipped with a helium cryostat (Advanced Research Systems, Inc., 4 K). The data were fitted by using the MossWinn 4.0 software.

Electrocatalytic CO₂ reduction. The electrolysis experiments were performed at ambient temperature in a typical H-type cell. The Ag/Ag⁺ (0.01 M AgNO₃ in 0.1 M TBAP-MeCN) was used as the reference electrode and the Pt gauze was used as counter electrode. The cathode and anode compartments were separated through a Nafion 117 proton exchange membrane. [Bmim]PF₆/MeCN/H₂O (W/W/W = 30/65/5) solution was used as cathode electrolyte. 0.5 M of H₂SO₄ aqueous solution was used as anodic electrolyte. CO₂ was bubbled into the catholyte (25 mL/min) with continuous stirring for 30 min before electrolysis. After that, potentiostatic electrochemical reduction of CO₂ was carried out with CO₂ bubbling.

DFT calculations. The present spin-polarized first principle DFT calculations were performed by Vienna Ab initio Simulation Package(VASP)⁴⁸ with the projector augmented wave (PAW) method⁴⁹. The exchange-functional was treated using the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)⁵⁰ functional. The energy cutoff for the plane wave basis expansion was set to 450 eV and the force on each atom less than 0.02 eV/Å was set for convergence criterion of geometry relaxation. Four-layer Fe₂O₃ (400) was employed with a 15 Å vacuum along z direction in order to avoid the interaction between periodic structures. The Brillouin zone integration is performed using 2×2×1 k-point sampling through all the computational process. The self-consistent calculations apply a convergence energy threshold of 10^− 5 eV. The free energies of the CO₂ reduction steps (CRR) were calculated by the Eq. 5¹: ΔG = Δ𝐸_𝐷𝐹𝑇+Δ𝐸_𝑍𝑃𝐸−TΔS, where ΔE_DFT is the DFT electronic energy difference of each step, ΔE_ZPE and ΔS are the correction of zero-point energy and the variation of entropy, respectively, which are obtained by vibration analysis, T is the temperature (T = 298 K).

Acknowledgments

The authors thank the National Natural Science Foundation of China (22073104, 21890761, 21733011, 22121002), National Key Research and Development Program of China (2017YFA0403101, 2017YFA0403003, and 2017YFA0403102), Beijing Municipal Science & Technology Commission (Z191100007219009), and Chinese Academy of Sciences (QYZDY-SSW-SLH013). The XAS (1W1B) and SAXS (1W2A) measurements were performed at Beijing Synchrotron Radiation Facility, China.

Author contributions

Project design: PC, PZ

Material synthesis and characterizations: PC, PZ, XK

XAS test and data analysis: PC, PZ, LZ

Supervision: PZ, BH

Writing – original draft: PC, PZ

Writing – review & editing: PC, PZ, XK and BH

Competing interests

Authors declare that they have no competing interests.

Author Information

Correspondence and requests for materials should be addressed to P.Z. ([email protected]) or B.H. ([email protected]).

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Efficient electrocatalytic reduction of CO2 to ethane over nitrogen-doped Fe2O3

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