The meso-substituent electronic effect of Fe porphyrins on the electrocatalytic CO₂ reduction reaction

Abstract

We report Fe porphyrins bearing different meso-substituents for the electrocatalytic CO₂ reduction reaction (CO₂RR). By replacing two and four meso-phenyl groups of Fe tetraphenylporphyrin (FeTPP) with strong electron-withdrawing pentafluorophenyl groups, we synthesized FeF₁₀TPP and FeF₂₀TPP, respectively. We showed that FeTPP and FeF₁₀TPP are active and selective for CO₂-to-CO conversion in dimethylformamide with the former being more active, but FeF₂₀TPP catalyzes hydrogen evolution rather than the CO₂RR under the same conditions. Experimental and theoretical studies revealed that with more electron-withdrawing meso-substituents, the Fe center becomes electron-deficient and it becomes difficult for it to bind a CO₂ molecule in its formal Fe⁰ state. This work is significant to illustrate the electronic effects of catalysts on binding and activating CO₂ molecules and provide fundamental knowledge for the design of new CO₂RR catalysts.

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Developing active and selective electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) has attracted great interest in the past few decades.^1–4 First, the electrocatalytic CO₂RR provides an appealing strategy to convert green electrical energy to chemical energy,^5–14 which is stored in small molecules, such as CO, HCOOH, and CH₄. On the other hand, by using the greenhouse gas CO₂ as a C1 source, this process can produce value-added chemicals.^15–19 Recent efforts have led to the development of a large variety of molecular complexes as active CO₂RR electrocatalysts.^20–28 Importantly, by studying these molecular catalysts, the correlation between the structure of the catalysts and their catalytic performance can be established.^29–33 Among many studied structural factors, electronic effects were shown to play critical roles in determining catalytic activity and selectivity.^34,35 However, understanding the electronic effects of catalysts on the CO₂RR is still required from a fundamental aspect.

Fe porphyrins have been greatly studied as electrocatalysts for the CO₂RR.^36–44 Recent works from us and others showed that the activity of Fe porphyrins can be significantly boosted by modifying second-sphere environments of the Fe active site through the introduction of substituents with proton relay capability,^45,46 hydrogen-bonding,^47,48 and electrostatic features.^49,50 In addition to these studies, we considered that Fe porphyrins are appealing models to study the electronic effects on the CO₂RR. Porphyrin ligands are able to provide a rigid and stable Fe–N₄ coordination environment. With this benefit, we can introduce substituents of different electronic effects to the ligand backbone to fine tune the electronic structure of Fe porphyrins, while maintaining the same Fe–N₄ coordination environment. Note that keeping the same coordination environment of the metal center is essential to study the intrinsic electronic effect of the substituents on catalysis.

Herein, we report on the electronic effects of Fe porphyrins with different meso-substituents on the electrocatalytic CO₂RR. We designed and synthesized FeTPP and its structural analogues FeF₁₀TPP and FeF₂₀TPP by replacing two and four meso-phenyl groups of FeTPP with pentafluorophenyl groups, respectively (Fig. 1). We showed that FeTPP and FeF₁₀TPP are effective to catalyze the reduction of CO₂ to CO in dimethylformamide (DMF) with an activity order of FeTPP > FeF₁₀TPP, but FeF₂₀TPP catalyzes the reduction of protons rather than CO₂ under the same conditions. Importantly, results from both experimental and theoretical studies revealed that the introduction of strong electron-withdrawing pentafluorophenyl groups decreases the electron density of the Fe center, making the binding of a CO₂ molecule at the corresponding Fe⁰ state more difficult. This work is therefore significant to provide insights into understanding the electronic effect of catalysts on the CO₂RR, which will be valuable for the design and development of more efficient electrocatalysts.

Fig. 1 Molecular structures of FeTPP, FeF₁₀TPP, and FeF₂₀TPP.

Fe porphyrins FeTPP, FeF₁₀TPP, and FeF₂₀TPP were synthesized according to the methods described in the ESI.† High-resolution mass spectrometry analysis displayed an ion at a mass-to-charge ratio of 668.1646 for FeTPP (Fig. S4, ESI†), 848.0709 for FeF₁₀TPP (Fig. S5, ESI†), and 1027.9770 for FeF₂₀TPP (Fig. S6, ESI†), confirming the identity and purity of these Fe porphyrins. The UV-vis absorption spectra of FeTPP, FeF₁₀TPP, and FeF₂₀TPP each displayed characteristic Soret and Q bands at almost identical positions (Fig. 2a), indicating the similar coordination environment of the Fe ion in the three complexes.

Fig. 2 (a) UV-vis spectra of FeTPP, FeF₁₀TPP, and FeF₂₀TPP in DMF. (b) CVs of FeTPP, FeF₁₀TPP, and FeF₂₀TPP in DMF. Conditions: 0.1 M Bu₄NPF₆, 0.5 mM catalyst, 100 mV s⁻¹ scan rate.

Next, the electrochemical features of these Fe porphyrins were tested in DMF. The cyclic voltammogram (CV) of FeTPP under argon displayed three reversible redox waves at −0.59, −1.51, and −2.13 V versus ferrocene, which could be assigned to the formal Fe^III/II, Fe^II/I, and Fe^I/0 redox couple, respectively (Fig. 2b). Note that all potentials reported in this work are referenced to ferrocene. Similar results were observed for the other two Fe porphyrins, showing reversible redox waves at −0.49, −1.38, and −1.94 V for FeF₁₀TPP and at −0.42, −1.24, and −1.81 V for FeF₂₀TPP (Fig. 2b). These redox potentials are summarized in Table S1 (ESI†). As compared to FeTPP, the reduction waves of FeF₁₀TPP and FeF₂₀TPP were notably shifted in the anodic direction. This result is consistent with the replacement of meso-phenyl groups in FeTPP with meso-pentafluorophenyl groups in FeF₁₀TPP and FeF₂₀TPP. In addition, in CV measurements, the reduction peak currents of all three Fe porphyrins increased linearly with the square root of the scan rates (Fig. S7–S9, ESI†), indicating that these reduction processes are diffusion-controlled.

The electrocatalytic CO₂RR performance of these Fe porphyrins was subsequently investigated in DMF at room temperature. As shown in Fig. 3a, the CV of FeTPP in CO₂-saturated DMF with the addition of phenol as the proton source displayed a pronounced catalytic wave at the potential corresponding to the Fe^I/0 redox couple. The same measurement under argon in the presence of phenol showed no such catalytic wave. This result indicated that (1) this catalytic wave was due to the reduction of CO₂ instead of protons and (2) the Fe⁰ form of FeTPP was the catalytically active species for the CO₂RR. We also examined FeF₁₀TPP and FeF₂₀TPP for the electrocatalytic CO₂RR in DMF. Our results showed that FeF₁₀TPP was also active for the electrocatalytic CO₂RR and the catalytic wave showed a shift of 220 mV to the anodic direction as compared to FeTPP (Fig. 3c). However, the catalytic current with FeF₁₀TPP was much smaller than that with FeTPP. For FeF₂₀TPP, it catalyzed the reduction of protons instead of the CO₂RR (Fig. 3e) as its CVs in the presence of phenol under CO₂ and under argon were similar to each other. These results suggested that strong electron-withdrawing meso-substituents of Fe porphyrins, such as pentafluorophenyl groups, are not favored for the CO₂RR. Moreover, we determined the turnover frequency (TOF) of FeTPP and FeF₁₀TPP for the electrocatalytic CO₂RR using the method reported in the literature (Fig. S10, ESI†).^20,45 As shown in Table S2 (ESI†), the TOF value is 124 s⁻¹ for FeTPP and is 8.38 s⁻¹ for FeF₁₀TPP.

Fig. 3 CVs of FeTPP (a), FeF₁₀TPP (c), and FeF₂₀TPP (e) in DMF under argon and CO₂. CVs of FeTPP (b), FeF₁₀TPP (d), and FeF₂₀TPP (f) in acetonitrile under argon and CO₂. Conditions: 0.1 M Bu₄NPF₆, 0.5 mM catalyst, 100 mV s⁻¹ scan rate.

In addition to DMF, we also examined these Fe porphyrins for electrocatalytic CO₂RR in acetonitrile. As shown in Fig. 3b and d, in the CVs of FeTPP and FeF₁₀TPP, the Fe^I/0 wave under argon became a large catalytic wave under CO₂ in the presence of phenol. Note that the catalytic CO₂RR current obtained in acetonitrile is much higher than that in DMF, which is consistent with previous studies from us and others.^51,52 Interestingly, for FeF₂₀TPP, its CV with phenol showed little catalytic wave under argon but showed much larger catalytic current under CO₂ (Fig. 3f), indicating that FeF₂₀TPP became more selective for the CO₂RR over hydrogen evolution in acetonitrile. Because illustrating the solvent effect on CO₂RR selectivity is not the interest of the current work, we would like to not discuss the different electrocatalytic behaviors of FeF₂₀TPP for the CO₂RR in DMF and in acetonitrile herein.

Electrolysis under controlled potentials was then performed in DMF under CO₂ at room temperature. For FeTPP, the electrolysis at −2.10 V showed stable currents at 0.48 mA cm⁻² (Fig. S12, ESI†). The UV-vis spectra of the DMF solution of FeTPP before and after electrolysis were almost identical (Fig. S12, ESI†). The electrode after electrolysis displayed no electrocatalytic activity in a catalyst-free DMF solution under CO₂ (Fig. S13, ESI†). These results together confirmed that FeTPP is stable by functioning as a molecular electrocatalyst for the CO₂RR. Gas chromatographic analysis confirmed the generation of CO during the electrolysis and gave a 98% faradaic efficiency for the CO₂-to-CO conversion with FeTPP (Fig. S14, ESI†). For FeF₁₀TPP, it was also stable during the electrocatalytic CO₂RR (Fig. S15, ESI†) and gave a 94% faradaic efficiency for CO₂-to-CO conversion (Fig. S16, ESI†). However, for FeF₂₀TPP, the faradaic efficiency for the CO₂RR and for hydrogen evolution was 3% and 97%, respectively. These results are in an agreement with previous CV studies, showing that FeF₂₀TPP has a poor selectivity for the CO₂RR over hydrogen evolution.

To gain insights into the meso-substituent electronic effect on the CO₂RR, we determined the CO₂ binding feature of Fe porphyrins in DMF. By measuring the shift of the Fe^I/0 redox potential under CO₂ and argon, we determined the CO₂ binding constant (K_CO2) using the method reported in the literature.^53–55 No proton sources were added in the solution to prevent subsequent catalysis. With a high scan rate of 2.0 V s⁻¹, the electron transfer and CO₂ binding occurred quickly to achieve reversible kinetics (Fig. 4). Our results showed that the K_CO2 value is 2.07 M⁻¹ for FeTPP, 1.14 M⁻¹ for FeF₁₀TPP, and 0.26 M⁻¹ for FeF₂₀TPP (Table S2, ESI†). We considered that with four strong electron-withdrawing meso-pentafluorophenyl substituents, the Fe⁰ state of FeF₂₀TPP becomes highly electron-deficient to bind and activate a CO₂ molecule.

Fig. 4 CVs of FeTPP (a), FeF₁₀TPP (b), and FeF₂₀TPP (c) in DMF under argon and CO₂. Conditions: 0.1 M Bu₄NPF₆ DMF, 0.5 mM catalyst, 2.0 V s⁻¹ scan rate.

Density functional theory (DFT) calculations were then carried out to study the influence of meso-pentafluorophenyl substituents on the electronic structure and corresponding CO₂ binding ability of these Fe porphyrins. First, we studied the electronic structure of the formal Fe⁰ state. As shown in Fig. S17 and Table S3 (ESI†), the Fe⁰ species of all three Fe porphyrins exhibited triplet ground states with the spin localized on the central Fe atom, suggesting metal-centered reductions. Next, for CO₂ binding at the Fe⁰ center, the calculated reaction free energy change was −5.6 kcal mol⁻¹ for FeTPP, −4.3 kcal mol⁻¹ for FeF₁₀TPP, and −2.0 kcal mol⁻¹ for FeF₂₀TPP (Table S4, ESI†). This result revealed that the CO₂-binding ability of the Fe⁰ state of these Fe porphyrins has an order of FeTPP > FeF₁₀TPP > FeF₂₀TPP.

In addition, we illustrated in Fig. 5a the partial density of states (PDOS) distributions of the Fe⁰ center in the three Fe porphyrins, with the diagrams and the energy levels of the singly-occupied molecular orbital (SOMO). The results showed that the introduction of meso-pentafluorophenyl substituents leads to the reduction of the d orbital levels, which originated from their electron-withdrawing natures and can be verified by examining the electrostatic potential (ESP) maps (Fig. 5b). As a consequence, meso-pentafluorophenyl substituents cause an increased energy gap between the π* orbital of CO₂ and the d orbitals of Fe (e.g. d_z² and d_xz), leading to a reduced orbital interaction and thus a weakened intermolecular charge transfer efficiency between CO₂ and the Fe⁰ center.

Fig. 5 (a) PDOS distributions of Fe and the SOMO diagrams and energy levels of the Fe⁰ state of FeTPP, FeF₁₀TPP, and FeF₂₀TPP. (b) ESP maps of the Fe⁰ state of FeTPP, FeF₁₀TPP, and FeF₂₀TPP.

In conclusion, we report the meso-substituent electronic effect of Fe porphyrins on the electrocatalytic CO₂RR. By introducing more meso-pentafluorophenyl substituents, Fe porphyrins became less active and selective for the CO₂RR in DMF. We revealed that with strong electron-withdrawing meso-substituents, the central Fe atom of Fe porphyrins became electron-deficient, leading to decreased CO₂ binding affinity of the corresponding Fe⁰ state. Therefore, although the introduction of electron-withdrawing substituents will make the catalytic wave shift in the anodic direction, it will also notably decrease the electron density of the metal ions, leading to decreased CO₂ binding affinity.

We are grateful for support from the National Natural Science Foundation of China (22371177, 22171176 and 22325202), Key Research and Development Program of Shaanxi (2023-YBGY-296), Fok Ying-Tong Education Foundation for Outstanding Young Teachers in University, Fundamental Research Funds for the Central Universities (GK202306007 and GK202403004), and Research Funds of Shaanxi Normal University.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details; Fig. S1–S17; Tables S1–S4; Cartesian coordinates of calculated structures. See DOI: https://doi.org/10.1039/d4cc01630k

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