English

• INTRODUCTION

  The rapid economic development has increased the global demand for fossil fuels and produced excessive CO₂, which has led to problems such as energy crisis, global warming, etc.^[1] To solve these problems, many methods have been explored to capture and utilize CO₂.^[2-4] The electrocatalytic reduction of CO₂ (CO₂RR) has a potential application prospect due to its simple device and mild conditions.^[5] A variety of electrocatalysts, including metal/metal oxide, molecular-based catalysts, carbon-based materials and so on for CO₂RR have been developed.^[6-8] Among them, metal-doped nitrogen-carbon (M-N-C) materials are currently one of the most promising catalysts which were usually obtained through the pyrolysis process.^{[9, 10]} The combination of nitrogen atoms and carbon structures can form different chemical functional groups, including pyrrole N, graphite N, pyridine N, etc., and metal atoms can coordinate with them, creating potential M-N_x sites for CO₂RR.^[11] However, their chemical structure needs to be characterized by X-ray absorption fine structure (XAFS), and the influence of other types of functional N and inorganic metal impurities cannot be excluded, which is not conducive to the study of the reaction mechanism.^[12]

  Metalloporphyrins, as molecular catalysts with definite M-N₄ structure, are extensively used in CO₂RR because their macrocyclic conjugated electron system is beneficial to electron migration.^[13-15] It is revealed that M-N₄ is the most likely active site. However, this kind of catalyst is usually used in a homogeneous system, and its catalytic activity is often limited by its solubility in the solution.^[16] Since the heterogeneous system can better control the chemical environment of the catalyst's active site, it is a meaningful subject to explore the application of porphyrin-based molecular catalysts in the heterogeneous system.^[17-19]

  Metal-organic framework (MOF) is a kind of porous material assembled by metal nodes and organic ligands, with the advantages of adjustable pore size and functionality, high specific surface, which may provide a suitable catalytic environment for the CO₂RR.^[20] Porphyrin-based MOFs formed by connecting metalloporphyrin-based carboxylate with Zr₆/Zr₈ clusters in a high oxidation state have become an ideal choice for discussing CO₂RR activity with M-N₄ center, which also brings out a high hydro-stability,^[21-28] such as PCN-222,^[22] PCN-221,^[23] PCN-224,^[24] and so forth. In our previous work, we have proved that PCN-222(Fe) has a high selectivity of 91% for transforming CO₂ to CO.^[29] In addition, we also discussed the influence of topological structure on the CO₂RR activity with PCN-222 (Cu) and PCN-224 (Cu) as models.^[30] We found that PCN-222(Cu) with a tubular structure and a diameter of 3.2 nm showed a higher mass transfer rate and higher current density, achieving electron transfer more quickly.

  As established by Hod et al., the redox jump on these periodically arranged and chemically equivalent sites—metalloporphyrins is the way of electron conduction in porphyrin-based MOFs.^[31] Recently, they have investigated iron porphyrin-based MOFs formed by the same Zr₆ node and free TCPP(Fe^Ⅲ) ligand (tetrakis (4-carboxyphenyl) porphyrinato iron(Ⅲ)) with different topological structures, that is, PCN-222(Fe) (csq), MOF-525(Fe) (ftw), NU-902(Fe) (scu), and PCN-225(Fe) (sqc), on the redox jump rate (k_hop).^[32] The results showed that in PCN-222(Fe), the redox jump path is in the direction of the hexagonal tubular channel along the c-axis. When the metal center of Fe coordinates with 1-methylimidazole (MIM) to form a six-coordinated low spin TCPP(Fe)/MIM complex, the k_hop in the redox jump process and the current density increase significantly, indicating that the charge transport is improved. Given this, in the same topological structure, changing the active metal center would have an impact on the redox jump rate and even the catalytic activity of the extended system.

  Nevertheless, the conductivity and catalytic activity of porphyrin-based MOFs are limited by the electrically insulated Zr clusters. Herein, the highly conductive material of carbon nanotube (CNT) was employed to combine with PCN-222(M) (M = Mn, Co, Ni, Zn) and PCN-222 (without metal center). With the aid of density functional theory (DFT) calculation and electrocatalytic measurement, we will reveal the CO₂RR mechanism and clarify the relationship between structure and catalytic activity.

  RESULTS AND DISCUSSION

  Characterizations of PCN-222(M). The series of porphyrin-based MOFs were structurally characterized by IR, PXRD, and XPS (Figure S1-S4, ESI). The PXRD shows that the diffraction peak of the as-synthesized sample is consistent with the simulated one on the basis of the reported single-crystal diffractions, indicating that the prepared material is the target product and has good crystallinity. XPS results indicate that the corresponding metal center's oxidation states are Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺, respectively (Figure S3, ESI). The fitted high-resolution N1s spectra further prove the existence of N-H (397.7 eV) and M-N group (397.9-398.8 eV, Figure S4, ESI).^{[11, 33, 34]} The porosity and specific surface area were evaluated by N₂ adsorption-desorption isotherms (Figure S5, Table S1, ESI). Triangular micropore and hexagonal mesopore are exhibited, consistent with the reported ones in the literature.^[22] The existence of mesopores is verified to facilitate the transfer of reactant and product during the electrocatalysis process.

  The UV spectra of the free ligands (H₂-TCPP and M-TCPP) and the corresponding MOFs were investigated to clarify the electronic structure of each catalyst (Figure S6, ESI). As for the strong absorption of the Soret band (320-480 nm) in each free ligand, the ones in the extended frameworks are all blue-shifted, except that of PCN-222(Mn), which is red-shifted (Figure S6a-e). In general, the metal center accepts lone-pair electrons from the porphyrin ring (forming σ bonding M←N). At the same time, orbital overlapping happens between the metal center (3d) and the porphyrin ring (forming backbonding π). Therefore, electron flows easily within the delocalized π system.^{[35, 36]} If the σ bonding exerts a more potent influence than that of the backbonding π, the electronic transition on the porphyrin ring will require higher energy, which leads to a blue-shift of the Soret band and vice versa.

  The formation of the extended MOF structure facilitates the enhancement of symmetry. The Soret bands in PCN-222(M) and PCN-222 exhibit blue-shift (λ = 387, 366, 347, and 353 nm, respectively for Co, Ni, Zn, and metal-free) compared with the free ligand except for PCN-222(Mn) (λ = 463 nm), which exhibits a little red-shift. When compared with the metal-free PCN-222, they are all red-shifted in PCN-222(Mn), PCN-222(Co), and PCN-222(Ni), among which it shifts the most for Mn-center. This fact is probably due to the big radius of Mn³⁺ (0.58 Å) and the ligand axial coordination role of Cl, resulting in a weaker ability of binding with electrons and a high tendency of electron transport from Mn 3d to the porphyrin ring. As a result, increasing electron-cloudy in the latter would reduce the required energy for electronic transition (Figure S6f, ESI). The Zn metal insertion into the core of PCN-222 (λ = 353 nm) displays a blue-shifted Soret band (λ = 347 nm). These results hint that the metal center has a specific effect on the electronic transition in each MOF and, therefore, would cause diffe-rent CO₂RR catalytic activities.

  Characterizations of PCN-222/CNT and PCN-222(M)/CNT. According to our previous investigation, the composite catalysts were prepared at a ratio of PCN-222/PCN-222(M): CNT = 1:30. The PXRD for the composite was examined, as shown in Figure 1a, indicating the main contribution of CNT. A faint diffraction peak belonging to MOFs at 7.1° can be detected. The pristine MOFs show rod-like morphology, and the composite displays the fibrous character of CNT (taking the PCN-222(Co) and PCN-222(Co)/CNT as examples, Figure 1b-1c). The element mapping result of PCN-222(Co)/CNT (Figure 1d) shows that C, N, and O elements are evenly distributed on the surface of CNT. The Co element is almost unobservable (Figure S7, ESI), indicating that the low content of PCN-222(Co) is loaded on the surface of CNT. The successful loading of extended molecular catalyst on CNT is also supported by the IR spectra characterization, on which both of the characteristic peaks of -OH (from CNT) and carboxylic groups (from MOF) are exhibited (Figure S8, ESI).

  Figure 1

  

  Figure 1.  (a) PXRD patterns of CNT, PCN-222/CNT, and PCN-222(M)/CNT; SEM images of PCN-222(Co) (b) and PCN-222(Co)/CNT (c); (d) HAADF-STEM and EDS elemental mappings of PCN-222(Co)/CNT.

  DownLoad: Full-Size Img PowerPoint

  Electrochemical Performance Test. The PCN-222/CNT and PCN-222(M)/CNT modified electrodes were tested by linear sweep voltammetry (LSV) in the N₂- or CO₂-saturated 0.5 M KHCO₃ solution, respectively (Figure S9, ESI). The reduction potential appears earlier in the CO₂-saturated solution than in the N₂-saturated one for all catalysts. Taking PCN-222/CNT as an example, a higher current density of 37.7 mA cm^-2 is shown at the same potential (-1.80 V vs. Ag/AgCl) in the former, and 27.5 mA cm^-2 is observed in the latter, indicating that CO₂ is catalyzed.^[37] Moreover, the combination with CNT can significantly improve the conductivity of the catalyst and generate considerable catalytic effects even with low catalyst loading, which is proved by comparing CV curves to the electrodes made by mixing with active carbon in 1:2 (Figure S10, ESI). The composite catalysts (PCN-222(M)/CNT and PCN-222/CNT) also show a more positive reduction potential and higher current density than CNT alone (Figure S11, ESI).^[38]

  The electrolysis experiments were performed at various potentials for 2 h from -0.50 to -0.75 V vs. RHE (Figure 2), showing a noticeable difference in the current density at different metal-porphyrin centers. The average density versus potential is plotted in Figure 2f. The PCN-222(Co)/CNT composite shows the highest current density, while PCN-222(Mn)/CNT and PCN-222(Zn)/CNT are slightly inferior but considerably higher than that of PCN-222/CNT at higher potential. The PCN-222(Ni)/CNT composite exhibits a much lower current density than the others in the same potential range of -0.50 to -0.75 V vs. RHE. The Faraday efficiencies (FE) of the gas products for each catalyst electrolyzed at different potentials are shown in Figure 3. Hydrogen and carbon monoxide are the main products with all the catalysts, but CO₂RR activity is dependent on the metal center in each one (Table S2-S6, ESI). As for PCN-222/CNT composite, it exhibits moderate CO₂RR activity that FE_CO maintains at 60%-80%, and FE_H2 lies in 20%-35% (FE_COmax = 80.7%, j = 0.9 mA cm^-2, -0.65 V vs. RHE). PCN-222(M)/CNT (M = Mn, Co, Zn) displays much higher catalytic activity (FE_CO > 80%) than PCN-222/CNT from -0.60 to -0.70 V vs. RHE. Especially for PCN-222(Mn)/CNT (FE_COmax = 88.5%, j = 1.1 mA cm^-2, -0.60 V vs. RHE, η = 494 mV) and PCN-222(Co)/CNT (FE_COmax = 89.3%, j = 3.8 mA cm^-2, -0.65 V vs. RHE, η = 544 mV), they are featured with low overpotential and high current density, respectively, for reaching a high CO₂→CO conversion efficiency. PCN-222(Zn)/CNT presents a higher selectivity to CO, with 92.5% of FE_COmax (j = 1.8 mA cm^-2, -0.70 V vs. RHE) with a bigger overpotential 594 mV. To our surprise, PCN-222(Ni)/CNT exhibits better activity towards the hydrogen evolution reaction (HER), while the FE_COmax (52.0%) is even less than that of PCN-222/CNT at -0.80 V vs. RHE (j = 0.8 mA cm^-2). Moreover, under low overpotential, it displays almost 100% FE_H2 but much slight current density (j < 0.1 mA cm^-2). The blank experiment for pure CNT was also performed, exhibiting the entire H₂ product (-0.60 V vs. RHE, FE_H2 = 96.2%, Table S7, ESI).

  Figure 2

  

  Figure 2.  Chronoamperograms recorded for 2 h at various potentials in 0.5 M KHCO₃ aqueous solution: (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Ni)/CNT; (e) PCN-222(Zn)/CNT; (f) Plotting of the average current density versus potential for each catalyst.

  DownLoad: Full-Size Img PowerPoint

  Figure 3

  

  Figure 3.  Faradaic efficiencies of (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Ni)/CNT; (e) PCN-222(Zn)/CNT; (f) Faradaic efficiencies of each catalyst at the optimal potential.

  DownLoad: Full-Size Img PowerPoint

  The CO₂RR activity-metal center correlation hints that the current density plays an essential role in the highly efficient transformation of CO₂ to CO. Therefore, we investigated the electrochemically active area (ECSA, Figure S12, ESI) and electrochemical impedance spectroscopy (EIS) of each catalyst (Figure 4a). PCN-222(M)/CNT (M = Mn, Co, Zn) all behave with higher ECSA (763.5, 740.0, and 775.5 cm², respectively, Table S8, ESI), and lower charge transfer resistance (R_ct = 37.2, 10.4 and 39.6 Ω, Table S9, ESI), indicating their effective electrochemical area for electrocatalysis^{[27, 38]} and faster charge transfer rate,^{[37, 39]} which are superior to that of PCN-222/CNT (628.5 cm², 44.0 Ω). In contrast, PCN-222(Ni)/CNT presents a flawed character (647.0 cm², 61.3 Ω).

  Figure 4

  

  Figure 4.  (a) Nyquist plot at -0.6 V vs. RHE and (b) Tafel plot for CO of each catalyst.

  DownLoad: Full-Size Img PowerPoint

  The reaction kinetics for the CO formation is elucidated by the Tafel slopes based on j-η data (Figure 5b). Remarkably, the slopes for PCN-222(Mn)/CNT, PCN-222(Co)/CNT, and PCN-222(Zn)/CNT are 159.85, 174.93, and 184.54 mV decade^-1, respectively, suggesting the faster kinetics for the CO formation,^{[27, 40]} which are significantly lower than that of PCN-222/CNT (336.20 mV decade^-1) and PCN-222(Ni)/CNT (386.68 mV decade^-1).

  Figure 5

  

  Figure 5.  Long-term chronoamperometry test (20 h) and Faradaic efficiency for CO as a function of time for each catalyst: (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Zn)/CNT.

  DownLoad: Full-Size Img PowerPoint

  We also performed the long-time durability tests (20 h) for each catalyst at its optimal potential to evaluate the stability (Figure 5, Figure S13, ESI). The corresponding FE_CO can be retained at > 70% within 10 h for PCN-222(Mn)/CNT, PCN-222(Co)/CNT, and PCN-222(Zn)/CNT (Table S10, ESI). In contrast, the FE_CO of PCN-222/CNT drops to 59.5%. The substantial stability of PCN-222(M)/CNT (M = Mn, Co, Zn) guarantees their efficient electrocatalytic transformation of CO₂ to CO. In comparison to some heterogeneous catalysts with M-N_x centers, the composites of PCN-222(M)/CNT (M = Mn, Co, Zn) behave remarkable advantages in both CO₂→CO efficiency and overpotential (Table S11, ESI).

  DFT Calculation of PCN-222 and PCN-222(M). To clarify the role of metal centers in PCN-222(M) for CO₂RR, we chose the free ligands of H₂-TCPP and M-TCPP (M = Mn^Ⅲ, Co^Ⅱ, Ni^Ⅱ, Zn^Ⅱ), as the models to perform density functional theoretical (DFT) calculations (the detailed calculation process is listed in ESI). In general, three steps are involved during the CO₂→CO process: the formation of ^*COOH and ^*CO, and the final CO desorption.^[41] Based on the calculated free energy result, for CO₂RR in each model, the initial proton-coupled electron transfer of CO₂ to form the ^*COOH intermediate is sluggish, indicating the rate-determining step (RDS) is the formation of ^*COOH (Figure 6). Mn-TCPP and Co-TCPP possess a much smaller free energy of ΔG_^*COOH (0.87 and 0.58 eV, respectively), being consistent with the high catalytic activity and selectivity of PCN-222(Mn)/CNT and PCN-222(Co)/CNT. H₂-TCPP and Zn-TCPP possess similar ΔG_^*COOH of 2.35 and 2.34 eV, respectively, slightly higher than Ni-TCPP (2.15 eV).

  Figure 6

  

  Figure 6.  The free energy diagrams of CO₂ reduction to CO for (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Ni)/CNT; (e) PCN-222(Zn)/CNT; (f) Comparisons of free energy for each catalyst.

  DownLoad: Full-Size Img PowerPoint

  Since the CO₂RR process involves the electron transfer and the central metal ions would affect the distribution of frontier molecular orbital, we further investigated each catalyst's HOMO and LUMO orbital energies and its 1e^- reduced form (Table S12-13, ESI). As reported, the frontier molecular orbital is closely related to the redox potential of each catalyst, and the HOMO-LUMO gap (ΔE_L-H) should account for the electrocatalytic reduction ability. Mn-TCPP and Co-TCPP display lower ΔE_L-H of 0.036 and 0.057 eV, respectively, further decreasing to 0.023 and 0.024 eV, respectively, after another electron acquisition. They also exhibit good conductivity and efficient internal electron transfer, confirmed by the partial density of states (PDOS) result (Figure 7c, d).^[25] The PDOS near the Fermi level of their reduced form is dominated by Mn/Co 3d and N1-2p states, with gaps of 0.01 and 0.09 eV, respectively, indicating the high density of carriers near the Fermi level.^{[42, 43]} However, although Ni-TCPP exhibits a small band peak gap of 0.02 eV, the contribution of the Ni-3d state is much less than that of Mn-TCPP/Co-TCPP, hinting at the low density of carriers near the Fermi level and low conductivity (Figure 7e). Such electronic structural feature, together with the higher ΔE_L-H value of 0.060 eV (0.055 eV for 1e^- reduction), should account for the insufficient CO₂RR activity of PCN-222(Ni)/CNT.

  Figure 7

  

  Figure 7.  (a) Illustration of the configuration of A-TCPP (A = H₂, Mn, Co, Ni, Zn) with different N, O environments; the PDOS of (b) H₂-TCPP; (c) Mn-TCPP; (d) Co-TCPP; (e) Ni-TCPP; (f) Zn-TCPP.

  DownLoad: Full-Size Img PowerPoint

  As for H₂-TCPP and Zn-TCPP, they display similar ΔE_L-H gaps of 0.062 and 0.066 eV (0.060 and 0.063 eV for 1e^- reduction), but the Zn insertion contributes to decreasing the band peak gap. Different from those of Mn-TCPP/Co-TCPP/Ni-TCPP, the PDOS near the Fermi level of H₂-TCPP and Zn-TCPP is determined by the contribution of two types of N-2p states (N1, N2) in the porphyrins, with the gaps of 0.05 eV and close to zero, respectively (Figure 7b, f). This fact indicates that the active sites are concentrated on the N atoms in the porphyrin ring in each model, while the Zn center is essential for increasing electron mobility in the porphyrin ring.^[44-46] Therefore, the maximum FE_CO for PCN-222(Zn)/CNT is significantly higher than PCN-222/CNT.

  Based on the experimental results and theoretical calculations, we seem to get the correlation between structure and performance. The strength of the backbonding π of the transition metal and its influence on the electron mobility in the porphyrin ring can affect its CO₂RR activity. The high electron-cloud density and good electron mobility of the porphyrin ring would help improve its activity (CO₂→CO), such as Mn-TCPP, Co-TCPP, and Zn-TCPP. It should be stressed that the coordination environment around the metal center (Mn^ⅢN₄Cl) would also have an influence on the CO₂RR activity.^[47]

  CONCLUSION

  In summary, this work employed PCN-222(M)/CNT (M = Mn, Co, Ni, Zn) with a clear M-N₄ structure as model catalysts and explored the structure-activity relationship between the metal center and CO₂RR through the combination of theoretical calculation and experimental results. We found that PCN-222(Mn)/CNT, PCN-222(Co)/CNT, and PCN-222(Zn)/CNT composite materials exhibit high catalytic activity for CO₂RR, with the FE_COmax of 88.5%, 89.3%, and 92.5%, respectively. They display fast kinetics for the CO formation, with the Tafel slopes of 159.85, 174.93, and 184.54 mV decade^-1, respectively. The metal center in M-N₄ with stronger backbonding π strength and higher electron-cloud density is more conducive to improving the CO₂→CO activity. This work provides new ideas for exploring catalysts with high catalytic activity, which would be suggestive for further research.

  EXPERIMENTAL

  Synthesis of PCN-222 and PCN-222(M). PCN-222 and PCN-222(M) were synthesized according to a process described in the literature.^[22]

  Synthesis of PCN-222/CNT and PCN-222(M)/CNT. Take the synthesis of PCN-222/CNT as an example. ZrCl₄ (70.0 mg), tetrakis(4-carboxyphenyl)-porphyrin (H₂-TCPP, 50.0 mg), benzoic acid (2.7 g), and hydroxylated CNTs (1.5 g) were dissolved in 8.0 mL DMF and were placed in a polytetrafluoroethylene autoclave. The mixture was maintained at 120 ℃ for 48 h. Then, the dark sample was harvested by filtration, followed by acid treatment for 12 h (8.0 M HCl). The obtained material was washed with DMF and acetone, respectively. According to the above-mentioned method, PCN-222(M)/CNT composite materials were synthesized when replacing H₂-TCPP with M-TCPP in each process.

  
  ACKNOWLEDGEMENTS: This work was financially supported by the NNSFC (No. 21671169), and the Foundation from the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors declare no competing interests.
  COMPETING INTERESTS
  For submission: https://mc03.manuscriptcentral.com/cjsc
  ADDITIONAL INFORMATION
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0057
  
• 1. [1]

     Porosoff, M. D.; Yan, B.; Chen, J. G. Catalytic reduction of CO₂ by H₂ for synthesis of CO, methanol and hydrocarbons: challenges and opportunities. Energy Environ. Sci. 2016, 9, 62-73. doi: 10.1039/C5EE02657A

  2. [2]

     Jiang, J.; Wu, Q.; Huang, Y. B.; Cao, R. Reticular frameworks and their derived materials for CO₂ conversion by thermo-catalysis. EnergyChem 2021, 3, 100064. doi: 10.1016/j.enchem.2021.100064

  3. [3]

     Bhatia, S. K.; Bhatia, R. K.; Jeon, J. M.; Kumar, G.; Yang, Y. H. Carbon dioxide capture and bioenergy production using biological system – a review. Renew. Sust. Energy Rev. 2019, 110, 143-158. doi: 10.1016/j.rser.2019.04.070

  4. [4]

     Hussain, I.; Jalil, A. A.; Hassan, N. S.; Hamid, M. Y. S. Recent advances in catalytic systems for CO₂ conversion to substitute natural gas (SNG): perspective and challenges. J. Energy Chem. 2021, 62, 377-407. doi: 10.1016/j.jechem.2021.03.040

  5. [5]

     Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and homogeneous approaches to conversion of CO₂ to liquid fuels. Chem. Soc. Rev. 2009, 38, 89-99. doi: 10.1039/B804323J

  6. [6]

     Qiao, J. L.; Liu, Y. Y.; Hong, F.; Zhang, J. J. A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels. Chem. Soc. Rev. 2014, 43, 631-675. doi: 10.1039/C3CS60323G

  7. [7]

     Zhang, W. J.; Hu, Y.; Ma, L. B.; Zhu, G. Y.; Wang, Y. R.; Xue, X. L.; Chen, R. P.; Yang, S. Y.; Jin, Z. Progress and perspective of electrocatalytic CO₂ reduction for renewable carbonaceous fuels and chemicals. Adv. Sci. 2018, 5, 1700275(1-24).

  8. [8]

     Zhang, M. D.; Dong, Y. J.; Huang, Y. B.; Cao, R. Covalent triazine frameworks-derived N, P dual-doped porous carbons for highly efficient electrochemical reduction of CO₂. Chin. J. Struct. Chem. 2021, 40, 1213-1222.

  9. [9]

     Paul, S.; Kao, Y. L.; Ni, L. M.; Ehnert, R.; Geppert, I. H.; Krol, R.; Stark, R. W.; Jaegermann, W.; Kramm, U. I.; Bogdanoff, P. Influence of the metal center in M-N-C catalysts on the CO₂ reduction reaction on gas diffusion electrodes. ACS Catal. 2021, 11, 5850-5864. doi: 10.1021/acscatal.0c05596

  10. [10]

      Wang, F. Y.; Liu, Y.; Song, Z. L.; Miao, Z. C.; Zhao, J. P. Ni-N-doped carbon-modified reduced graphene oxide catalysts for electrochemical CO₂ reduction reaction. Catal. 2021, 11, 561. doi: 10.3390/catal11050561

  11. [11]

      Varela, A. S.; Ju, W.; Bagger, A.; Franco, P.; Rossmeisl, J.; Strasser, P. Electrochemical reduction of CO₂ on metal-nitrogen-doped carbon catalysts. ACS Catal. 2019, 9, 7270-7284. doi: 10.1021/acscatal.9b01405

  12. [12]

      Ju, W.; Bagger, A.; Hao, G. P.; Varela, A. S.; Sinev, I.; Bon, V.; Roldan, C. B.; Kaaskel, S.; Rossmeisl, J.; Strasser, P. Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO₂. Nat. Commun. 2017, 8, 944-952. doi: 10.1038/s41467-017-01035-z

  13. [13]

      Ren, S. X.; Joulié, D.; Salvatore, D.; Torbensen, K.; Wang, M.; Robert, M.; Berlinguette, C. P. Molecular electrocatalysts can mediate fast, selective CO₂ reduction in a flow cell. Science 2019, 365, 367-369. doi: 10.1126/science.aax4608

  14. [14]

      Meng, D. L.; Zhang, M. D.; Si, D. H.; Mao, M. J.; Hou, Y.; Huang, Y. B.; Cao, R. Highly selective tandem electroreduction of CO₂ to ethylene over atomically isolated nickel-nitrogen site/copper nanoparticle catalysts. Angew. Chem. Int. Ed. 2021, 60, 25485-25492. doi: 10.1002/anie.202111136

  15. [15]

      Hou, Y.; Huang, Y. B.; Liang, Y. L.; Chai, G. L.; Yi, J. D.; Zhang, T.; Zang, K. T.; Luo, J.; Xu, R.; Lin, H.; Zhang, S. Y.; Wang, H. M.; Cao, R. Unraveling the reactivity and selectivity of atomically isolated metal-nitrogen sites anchored on porphyrinic triazine frameworks for electroreduction of CO₂. CCS Chem. 2019, 1, 384-395. doi: 10.31635/ccschem.019.20190011

  16. [16]

      Hu, X. M.; Rønne, M. H.; Pedersen, S. U.; Skrydstrup, T.; Daasbjerg, K. Enhanced catalytic activity of cobalt porphyrin in CO₂ electroreduction upon immobilization on carbon materials. Angew. Chem. Int. Ed. 2017, 56, 6468-6472. doi: 10.1002/anie.201701104

  17. [17]

      Weng, Z.; Jiang, J. B.; Wu, Y. S.; Wu, Z. S.; Guo, X. T.; Materna, K. L.; Liu, W.; Batista, V. S.; Brudvig, G. W.; Wang, H. L. Electrochemical CO₂ reduction to hydrocarbons on a heterogeneous molecular Cu catalyst in aqueous solution. J. Am. Chem. Soc. 2016, 138, 8076-8079. doi: 10.1021/jacs.6b04746

  18. [18]

      Yan, T. T.; Guo, J. H.; Liu, Z. Q.; Sun, W. Y. Metalloporphyrin encapsulation for enhanced conversion of CO₂ to C₂H₄. ACS Appl. Mater. Interfaces 2021, 13, 25937-25945. doi: 10.1021/acsami.1c03557

  19. [19]

      Watpathomsub, S.; Luangchaiyaporn, J.; Sariciftci, N. S.; Thamyongkit, P. Efficient heterogeneous catalysis by pendant metalloporphyrin-functionalized polythiophenes for the electrochemical reduction of carbon dioxide. New J. Chem. 2020, 44, 12486-12495. doi: 10.1039/D0NJ01381A

  20. [20]

      Zhou, H. C.; Kitagawa, S. Metal-organic frameworks (MOFs). Chem. Soc. Rev. 2014, 43, 5415-5418. doi: 10.1039/C4CS90059F

  21. [21]

      Bai, Y.; Dou, Y. B.; Xie, L. H.; Rutledge, W.; Li, J. R.; Zhou, H. C. Zr-based metal-organic frameworks: design, synthesis, structure, and applications. Chem. Soc. Rev. 2016, 45, 2327-2367. doi: 10.1039/C5CS00837A

  22. [22]

      Feng, D. W.; Gu, Z. Y.; Li, J. R.; Jiang, H. L.; Wei, Z. W.; Zhou, H. C. Zirconium-metalloporphyrin PCN-222: mesoporous metal-organic frameworks with ultrahigh stability as biomimetic catalysts. Angew. Chem. Int. Ed. 2012, 51, 10307-10310. doi: 10.1002/anie.201204475

  23. [23]

      Feng, D. W.; Jiang, H. L.; Chen, Y. P.; Gu, Z. Y.; Wei, Z. W.; Zhou, H. C. Metal-organic frameworks based on previously unknown Zr₈/Hf₈ cubic clusters. Inorg. Chem. 2013, 52, 12661-12667. doi: 10.1021/ic4018536

  24. [24]

      Feng, D. W.; Chung, W. C.; Wei, Z. W.; Gu, Z. Y.; Jiang, H. L.; Chen, Y. P.; Darensbourg, D. J.; Zhou, H. C. Construction of ultrastable porphyrin Zr metal-organic frameworks through linker elimination. J. Am. Chem. Soc. 2013, 135, 17105-17110. doi: 10.1021/ja408084j

  25. [25]

      Wang, Y. R.; Huang, Q.; He, C. T.; Chen, Y. F.; Liu, J.; Shen, F. C.; Lan, Y. Q. Oriented electron transmission in polyoxometalate-metalloporphyrin organic framework for highly selective electroreduction of CO₂. Nat. Commun. 2018, 9, 4466-4473. doi: 10.1038/s41467-018-06938-z

  26. [26]

      Ahrenholtz, S. R.; Epley, C. C.; Morris, A. J. Solvothermal preparation of an electrocatalytic metalloporphyrin MOF thin film and its redox hopping charge-transfer mechanism. J. Am. Chem. Soc. 2014, 136, 2464-2472. doi: 10.1021/ja410684q

  27. [27]

      Hod, I.; Sampson, M. D.; Deria, P.; Kubiak, C. P.; Farha, O. K.; Hupp, J. T. Fe-porphyrin-based metal-organic framework films as high-surface concentration, heterogeneous catalysts for electrochemical reduction of CO₂. ACS Catal. 2015, 5, 6302-6309. doi: 10.1021/acscatal.5b01767

  28. [28]

      Xin, Z. F.; Wang, Y. R.; Chen, Y. F.; Li, W. L.; Dong, L. Z.; Lan, Y. Q. Metallocene implanted metalloporphyrin organic framework for highly selective CO₂ electroreduction. Nano Energy 2020, 67, 104233. doi: 10.1016/j.nanoen.2019.104233

  29. [29]

      Dong, B. X.; Qian, S. L.; Bu, F. Y.; Wu, Y. C.; Feng, L. G.; Teng, Y. L.; Liu, W. L.; Li, Z. W. Electrochemical reduction of CO₂ to CO by a heterogeneous catalyst of Fe-porphyrin-based metal-organic framework. ACS Appl. Energy Mater. 2018, 1, 4662-4669. doi: 10.1021/acsaem.8b00797

  30. [30]

      Liu, M. J.; Cao, S. M.; Feng, B. Q.; Dong, B. X.; Ding, Y. X.; Zheng, Q. H.; Teng, Y. L.; Li, Z. W.; Liu, W. L.; Feng, L. G. Revealing the structure-activity relationship of two Cu-porphyrin-based metal-organic frameworks for the electrochemical CO₂-to-HCOOH transformation. Dalton Trans. 2020, 49, 14995-15001. doi: 10.1039/D0DT02683B

  31. [31]

      Kung, C. W.; Goswami, S.; Hod, I.; Wang, T. C.; Duan, J. X.; Farha, O. K.; Hupp, J. T. Charge transport in zirconium-based metal-organic frameworks. Acc. Chem. Res. 2020, 53, 1187-1195. doi: 10.1021/acs.accounts.0c00106

  32. [32]

      Maindan, K.; Li, X. L.; Yu, J. R.; Deria, P. Controlling charge-transport in metal-organic frameworks: contribution of topological and spin-state variation on the iron-porphyrin centered redox hopping rate. J. Phys. Chem. B 2019, 123, 8814-8822. doi: 10.1021/acs.jpcb.9b07506

  33. [33]

      Wang, F. Y.; Liu, Y.; Song, Z. L.; Miao, Z. C.; Zhao, J. P. Ni-N-doped carbon-modified reduced graphene oxide catalysts for electrochemical CO₂ reduction reaction. Catal. 2021, 11, 561. doi: 10.3390/catal11050561

  34. [34]

      Matanovic, I.; Babanova, S.; Perry Ⅲ, A.; Serov, A.; Artyushkova, K.; Atanassov, P. Bio-inspired design of electrocatalyst for oxalate oxidation: a combined experimental and computational study of Mn-N-C catalysts. Phys. Chem. Chem. Phys. 2015, 17, 13235-13244. doi: 10.1039/C5CP00676G

  35. [35]

      Wang, L. Z.; She, Y. B. Spectroscopic analysis of substituted tetraphenylporphyrin iron, manganese, cobalt, copper and zinc complexes. Spectrosc. Spect. Anal. 2008, 28, 2312-2317.

  36. [36]

      Soldatova, A. V.; Ibrahim, M.; Spiro, T. G. Electronic structure and ligand vibrations in FeNO, CoNO, and FeOO porphyrin adducts. Inorg. Chem. 2013, 52, 7478-7486. doi: 10.1021/ic400364x

  37. [37]

      Cao, C. S.; Ma, D. D.; Gu, J. F.; Xie, X. Y.; Zeng, G.; Li, X. F.; Han, S. G.; Zhu, Q. L.; Wu, X. T.; Xu, Q. Metal-organic layers leading to atomically thin bismuthene for efficient carbon dioxide electroreduction to liquid fuel. Angew. Chem. Int. Ed. 2020, 59, 15014-15020. doi: 10.1002/anie.202005577

  38. [38]

      Chen, E. X.; Yang, J.; Qiu, M.; Wang, X. Y.; Zhang, Y. F.; Guo, Y. J.; Huang, S. L.; Sun, Y. Y.; Zhang, J.; Hou, Y.; Lin, Q. P. Understanding the efficiency and selectivity of two-electron production of metalloporphyrin-embedded zirconium-pyrogallol scaffolds in electrochemical CO₂ reduction. ACS Appl. Mater. Interfaces 2020, 12, 52588-52594. doi: 10.1021/acsami.0c14135

  39. [39]

      Jadhav, H. S.; Roy, A.; Thorat, G. M.; Chung, W. J.; Seo, J. G. Hierarchical free-standing networks of MnCo₂S₄ as efficient electrocatalyst for oxygen evolution reaction. J. Ind. Eng. Chem. 2019, 71, 452-459. doi: 10.1016/j.jiec.2018.12.002

  40. [40]

      Han, N.; Wang, Y.; Ma, L.; Wen, J. G.; Li, J.; Zheng, H. C.; Nie, K. Q.; Wang, X. X.; Zhao, F. P.; Li, Y. F.; Fan, J.; Zhong, J.; Wu, T. Q.; Miller, D. J.; Lu, J.; Lee, S. T.; Li, Y. G. Supported cobalt polyphthalocyanine for high-performance electrocatalytic CO₂ reduction. Chem 2017, 3, 652-664. doi: 10.1016/j.chempr.2017.08.002

  41. [41]

      Cheng, Q. Q.; Mao, K.; Ma, L. S.; Yang, L. J.; Zou, L. L.; Zou, Z. Q.; Hu, Z.; Yang, H. Encapsulation of iron nitride by Fe-N-C shell enabling highly efficient electroreduction of CO₂ to CO. ACS Energy Lett. 2018, 3, 1205-1211. doi: 10.1021/acsenergylett.8b00474

  42. [42]

      Arriagada, D. C.; Rojas, S. M.; Mora, F. C.; Labbé, A. T. First-principles study of hybrid nanostructures formed by deposited phthalocyanine/porphyrin metal complexes on phosphorene. J. Mol. Liq. 2021, 333, 115948-11557. doi: 10.1016/j.molliq.2021.115948

  43. [43]

      Wang, C.; Zhu, C. Y.; Zhang, M.; Geng, Y.; Li, Y. G.; Su, Z. M. An intriguing window opened by a metallic two-dimensional lindqvist-cobaltporphyrin organic framework as an electrochemical catalyst for the CO₂ reduction reaction. J. Mater. Chem. A 2020, 8, 14807-14814. doi: 10.1039/D0TA04993J

  44. [44]

      Wu, Y. S.; Jiang, J. B.; Weng, Z.; Wang, M. Y.; Broere, D. L. J.; Zhong, Y. R.; Brudvig, G. W.; Feng, Z. X.; Wang, H. L. Electroreduction of CO₂ catalyzed by a heterogenized Zn-porphyrin complex with a redox-innocent metal center. ACS Cent. Sci. 2017, 3, 847-852. doi: 10.1021/acscentsci.7b00160

  45. [45]

      Cao, M. J.; Yu, Y. M.; Fu, H. Y.; She, Y. B. Effect of substituents and central metal ions on electronic structure and catalytic activity of porphyrins. CIESC J. 2013, 64, 88-97.

  46. [46]

      Wu, H.; Fan, S. H.; Zhang, H.; Li, H. Q.; Yang, M. Q.; Zhang, C. Y. Porphyrins with different electron groups: spectral and DFT study. Spectrosc. Spect. Anal. 2014, 34, 1060-1063.

  47. [47]

      Yan, T. T.; Wang, P.; Xu, Z. H.; Sun, W. Y. Copper(Ⅱ) frameworks with varied active site distribution for modulating selectivity of carbon dioxide electroreduction. ACS Appl. Matter. Interfaces 2022, 14, 13645-13652. doi: 10.1021/acsami.2c00487

• 

  Figure 1  (a) PXRD patterns of CNT, PCN-222/CNT, and PCN-222(M)/CNT; SEM images of PCN-222(Co) (b) and PCN-222(Co)/CNT (c); (d) HAADF-STEM and EDS elemental mappings of PCN-222(Co)/CNT.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Chronoamperograms recorded for 2 h at various potentials in 0.5 M KHCO₃ aqueous solution: (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Ni)/CNT; (e) PCN-222(Zn)/CNT; (f) Plotting of the average current density versus potential for each catalyst.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Faradaic efficiencies of (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Ni)/CNT; (e) PCN-222(Zn)/CNT; (f) Faradaic efficiencies of each catalyst at the optimal potential.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Nyquist plot at -0.6 V vs. RHE and (b) Tafel plot for CO of each catalyst.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Long-term chronoamperometry test (20 h) and Faradaic efficiency for CO as a function of time for each catalyst: (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Zn)/CNT.

  下载: 全尺寸图片 幻灯片
  

  Figure 6  The free energy diagrams of CO₂ reduction to CO for (a) PCN-222/CNT; (b) PCN-222(Mn)/CNT; (c) PCN-222(Co)/CNT; (d) PCN-222(Ni)/CNT; (e) PCN-222(Zn)/CNT; (f) Comparisons of free energy for each catalyst.

  下载: 全尺寸图片 幻灯片
  

  Figure 7  (a) Illustration of the configuration of A-TCPP (A = H₂, Mn, Co, Ni, Zn) with different N, O environments; the PDOS of (b) H₂-TCPP; (c) Mn-TCPP; (d) Co-TCPP; (e) Ni-TCPP; (f) Zn-TCPP.

  下载: 全尺寸图片 幻灯片
•