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.

In conclusion, a pre-assembly approach for construction of iron-based catalysts enriched with oxygen vacancy has been proposed. The well-constructed _xFe₂O₃[email protected] 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[email protected] 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[email protected] 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[email protected]

The polymerization of dopamine (DA) was carried out according to the previous literature⁴⁷. To prepare _xFe₂O₃[email protected], 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[email protected], 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[email protected] with different Fe content are as follows: _0.05Fe₂O₃[email protected]: 1.05 g Fe₂(SO₄)₃, _0.11Fe₂O₃[email protected]: 1.58 g Fe₂(SO₄)₃, _0.17Fe₂O₃[email protected]: 2.1 g Fe₂(SO₄)₃, _0.2Fe₂O₃[email protected]: 2.63 g Fe₂(SO₄)₃, _0.3Fe₂O₃[email protected]: 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[email protected] nanoparticles.

Preparation of the control sample without CN shell Fe₂O₃-CNF

The obtained _0.3Fe₂O₃[email protected] 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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