Photocatalytic Reduction of Carbon Dioxide with Iron Complexes

Chenliu Tang; Yunjie Zou; Mingkai Xu; Lan Ling

doi:10.7536/PC201231

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (1) : 142-154 DOI: 10.7536/PC201231

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Photocatalytic Reduction of Carbon Dioxide with Iron Complexes

Chenliu Tang ¹^,²
, Yunjie Zou ¹^,²
, Mingkai Xu
, Lan Ling ¹^,²^,^*

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Abstract

Photocatalytic CO₂ reduction for fuel production has attracted much attention due to its potential for simultaneously solving energy and global warming problems. As a molecular catalyst, earth-abundant and eco-friendly iron complexes take the advantages of adjustable structure, rich valence, and easy synthesis, exhibiting good CO₂ photocatalytic reduction performance, and hence have attracted much attention in the field of CO₂ photocatalytic reduction. This review focuses on the recent progress in photocatalytic reduction of CO₂ based on iron complexes. First, the homogeneous photocatalytic CO₂ reduction systems using iron complex as catalyst, including iron porphyrin, iron polypyridine and iron pentadentate complex, are summarized. Visible-light-driven CO₂ reduction system is generally composed of three basic components: photosensitizer for absorption of visible light, catalyst for catalytic reduction of CO₂, and sacrificial electron donors for providing electrons in reduction reaction. Beyond catalytic efficiency, CO₂ photoreduction is a multi-electron transfer process boosted by the catalysts and inevitable competition with hydrogen evolution is a general issue for molecular catalysis of the CO₂-to-CO conversion, therefore the selectivity of the products is an important indicator. The selectivity and efficiency could be tuned by changing the ligand of iron complex, photosensitizer and sacrificial electron donors. Moreover, the mechanisms for the homogeneous photocatalytic CO₂ reduction, including catalyst activation and reduction process, are deciphered in detail. Second, the recent works of heterogeneous catalytic systems, which combine semiconductor nanomaterials/quantum dots with metal iron complexes as catalysts, are introduced. Considering the superior stability and fairly strong light absorption capacity of inorganic materials to the organic counterparts, the solid nanomaterials can be used as the photosensitizers to incorporate with the molecular catalysts. At the end, the current issues and perspectives on photocatalytic reduction of CO₂based on iron complexes are discussed. For examples, porphyrin metal organic frameworks become a new research interest, and the design and construction of iron porphyrin metal organic frameworks is a promising way for getting new photocatalytic systems functioning in aqueous conditions. Besides, further efforts could be made on the mechanistic studies, especially the 8e^-/8H⁺ reduction to methane.

Contents

1 Introduction

2 Homogeneous photocatalytic CO₂reduction systems using iron complex as catalyst

2.1 Iron porphyrin as photocatalyst

2.2 Iron polypyridine as photocatalyst

2.3 Iron pentadentate complex as photocatalyst

3 Heterogeneous photocatalytic CO₂reduction systems using iron complex as catalyst

3.1 Semiconductor nanomaterials as photosensitizers

3.2 Quantum dots as photosensitizers

4 Conclusion and outlook

Graphical abstract

Keywords

CO₂ photoreduction / iron porphyrin / iron complex / homogeneous photocatalysis / artificial photosynthesis

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Chenliu Tang, Yunjie Zou, Mingkai Xu, Lan Ling. Photocatalytic Reduction of Carbon Dioxide with Iron Complexes. Progress in Chemistry, 2022, 34(1): 142-154 DOI:10.7536/PC201231

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1 Introduction

The continuous consumption of fossil energy has brought unprecedented energy crisis to human beings. Studies have shown that fossil energy accounts for 87.9% of global energy consumption, and because of its non-renewable nature, fossil energy will inevitably be exhausted^[1]. In addition, the use of fossil energy can cause environmental problems, such as the production of greenhouse gases and the aggravation of global warming. In particular, under the trend of global warming, in recent years, a large number of Arctic and Antarctic sea ice has melted and permafrost has thawed ahead of schedule, which has threatened the earth's ecological environment. Carbon dioxide (CO₂), as the main greenhouse gas, has the largest "contribution" to the greenhouse effect (among them, CO₂-55%, chlorofluorocarbons (CFCs) -24%, methane N₂O)-6%, and nitrous oxide (N₂O)-6%)^[2]. Therefore, how to control the concentration of CO₂ in the atmosphere has become the focus of the world. Transforming CO₂ into high value-added energy products and chemical raw materials through green ways can not only effectively reduce the content of CO₂ in the atmosphere, but also realize the sustainable development of resources, which is an ideal solution.

The molecular structure of CO₂(Δ

G f 0

= -394.39 kJ/mol) is very stable, so the conversion of CO₂ requires high energy input. For example, the potential of directly reducing CO₂ to C

O 2 · -

is -1.90 V (relative to standard hydrogen electrode NHE, 25 ℃, atmospheric pressure), so the possibility of this reaction is very small. However, under the condition of multiple proton-electron coupling, the potential of reducing the CO₂ can be effectively lowered. For example, the potential at which a CO₂ couples to two protons to form carbon monoxide (CO) can be reduced to -0.53 V (vs. standard hydrogen electrode NHE, 25 ° C, atmospheric pressure); the potential at which a CO₂ couples to eight protons to form CH₄ can be reduced to -0.24 V (vs NHE). Equations (1 ~ 8) list the possible reactions of CO₂ reduction and H₂O redox and the corresponding redox potentials (vs NHE).

(1)CO₂ + e^- → CO₂^·- E⁰ =-1.90 V

(2)CO₂ + 2H⁺ + 2e^- → CO + H₂O E⁰ =-0.53 V

(3)CO₂ + 2H⁺ + 2e^- → HCOOH E⁰ =-0.61 V

(4)CO₂ + 4H⁺ + 4e^- → HCHO + H₂O E⁰ =-0.48 V

(5)CO₂ + 6H⁺ + 6e^- → CH₃OH + H₂O E⁰ =-0.38 V

(6)CO₂ + 8H⁺ + 8e^- → CH₄ + 2H₂O E⁰ =-0.24 V

(7)2H⁺ + 2e^- → H₂ E⁰ =-0.41 V

(8)H₂O + 2h⁺ → 0.5O₂ + 2H⁺ E⁰ = 1.23 V

Conversion and utilization technologies of CO₂ mainly include electrocatalysis, photocatalysis, thermocatalysis, photoelectrocatalysis, bioconversion and so on^[3⇓⇓~6]^[2,7⇓~9]^[10⇓~12]^[13,14]^[15,16]. Among them, photocatalytic technology has attracted extensive attention of researchers because of its most direct use of solar energy, mild reaction conditions, environmental friendliness and other advantages. The catalysts used in photocatalytic reduction of CO₂ are mainly divided into two types: (1) semiconductor photocatalysts, such as titanium dioxide (TiO₂), nitrogen carbide (C₃N₄), etc., which belong to heterogeneous catalytic systems.Can realize the conversion from CO₂ to CH₄, CO, formic acid (HCOOH), methanol (CH₃OH) and other substances by taking water as a reducing agent, and has the characteristics of good stability in the environment, easy separation from reactants and products and the like^{[17⇓⇓~20]}^{[21⇓⇓⇓~25]}. However, this kind of system has the disadvantages of low reactivity and poor selectivity of products^[7]. (2) Molecular photocatalysts, mainly metal complexes. It has the advantages of high stability, controllable structure and good product selectivity, which can effectively reduce CO₂ by photocatalysis and has certain industrial application potential.

Based on the research of Lehn et al., the reduction of CO₂ by metal complexes as molecular catalysts has become a hot research topic in the field of photocatalysis^[26⇓~28]. Noble metal complexes (Ru, Ir, Rh, etc.) Are commonly used as molecular catalysts in the photocatalytic reduction of CO₂. Considering the practical application and promotion of the photocatalytic reduction of CO₂, people begin to pay attention to the first subgroup transition metal elements such as Co, Ni and Mn, which are abundant in nature, as the central atoms of molecular catalysts^[28⇓~30]^[31⇓~33]^[34⇓~36]^{[37⇓⇓⇓⇓⇓~43]}^{[44⇓⇓⇓⇓~49]}^{[50⇓⇓~53]}. Based on the early research of Neta et al., scholars have successfully applied the earth's environmentally friendly metal Fe as the central atom of molecular catalyst in the photocatalytic reduction of CO₂^{[54⇓⇓~57]}.

Based on this, this review will summarize the recent developments in the use of iron complexes as photocatalysts for the reduction of CO₂, mainly from the homogeneous and heterogeneous catalytic systems.The main types of iron complexes were summarized, and the mechanism of photocatalytic reduction of CO₂ by iron complexes as catalysts was reviewed, as well as the future research directions and problems of photocatalytic reduction of CO₂ by iron complexes.

2 Homogeneous Photocatalytic Reduction of CO₂ Based on Metal Iron Complex

2.1 Iron porphyrin as a photocatalyst

2.1.1 Activation of catalyst

The catalytically active state of the catalyst is Fe (0). Starting from Fe (Ⅲ) porphyrin, three consecutive bimolecular photochemical reduction steps are required. The first step, from Fe (III) to Fe (II), occurs by homolytic cleavage of the Fe-Cl bond (formula (9)) under irradiation of the porphyrin ligand to metal charge transfer (LMCT) band. This reaction can be carried out in the absence of a sacrificial electron donor (SD), while the yield and rate of the reaction can be greatly improved when a typical electron donor such as triethylamine (TEA) is present in the solution (formula (10)). In the next two steps, Fe (II) is converted to Fe (I) and finally to Fe (0) by a bimolecular reductive quenching reaction between the porphyrin excited state and the electron sacrificial agent (formula (11, 12)). It should be noted that the SD^·+ generated during the reaction will undergo an acid-base reaction with the parent molecule (formula (13)) to generate a radical with strong reducibility and high activity to further reduce Fe (III) (formula (14)). In addition, the formation of protonated strongly acidic SDH⁺ (formula (13)) may play a role in the catalytic mechanism. Finally, the disproportionation of Fe (I) may also be a secondary route to Fe (0) (formula (15)).

(9)Fe(Ⅲ)—Cl $\stackrel{hv}{\longrightarrow}$ Fe(Ⅱ) + Cl·

(10)Fe(Ⅲ) + SD $\stackrel{hv}{\longrightarrow}$ Fe(Ⅱ) + SD^·+

(11)Fe(Ⅱ) + SD $\stackrel{hv}{\longrightarrow}$ Fe(Ⅰ)^- + SD^·+

(12)Fe(Ⅰ)^- + SD $\stackrel{hv}{\longrightarrow}$ Fe(0)^2- + SD^·+

(13)SD^·+ + SD → SDH⁺ + SD^· - SD

(14)SD^· - SD + Fe(Ⅲ)⁺ → SD⁺ - SD + Fe(Ⅱ)

(15)2Fe(Ⅰ)^- ⇌Fe(0)^2- + Fe(Ⅱ)

2.1.2 Photosensitization

In the homogeneous photocatalytic reduction system of CO₂, solar energy is regarded as an inexhaustible clean energy source, which is the ideal energy input source of photocatalytic system, but CO₂ is chemically inert under ultraviolet and visible light (200 ~ 900 nm).Because iron porphyrin and other molecular catalysts are prone to photodegradation under long-term UV irradiation, photosensitizer (PS) is usually introduced into the system to achieve the following goals: (1) its band gap is appropriate and it is easily excited by visible light, thus reducing the light energy input in the system; The (2)PS^·- can be used as an electron donor to reduce the CO₂-CAT adduct, and can improve the efficiency of the photocatalytic process by promoting the formation of CO in the final step; (3) increase the stability of the iron catalyst. The use of photosensitizers is a common strategy to broaden and/or change the light absorption range of a system. Overall, its role is to efficiently collect light and then trigger the photocatalytic cycle of the system. The excited state PS^* is efficiently quenched by the electron sacrificial agent, thereby producing the radical anion PS^·-, a strong reducing agent capable of reducing the iron catalyst to its active state (CAT⁰) (see below). In addition, another important function of the photosensitizer is to separate the light absorption step from the catalytic process, thereby improving the catalytic efficiency.

Typical photosensitizers used in the photochemical CO₂ reduction system include noble metal complexes, such as Ru and Ir, etc. These noble metal photosensitizers can reduce the absorption energy in the visible region, thereby further improving the photoreduction efficiency^{[44,45,53,58]}^[31,59~60]; However, because the use of precious metals will greatly increase the production cost, it is difficult to popularize and apply. Robert and Bonin et al. reported the photochemical conversion of CO₂ to CO using the organic dye purpurin as a photosensitizer in an aqueous solution (pH = 6.7) environment under visible light irradiation (> 420 nm)^[61]. In addition, the pure organic compound 9-cyanoanthracene (9CNA) (V vs SCE = − 1.58 V vs SCE) has also been used as an inexpensive photosensitizer by researchers to participate in the photocatalytic reduction of CO₂^[62]^[63].

2.1.3 Electron sacrificial agent

Sacrificial agents are substances that donate electrons in the process of electron transfer. Therefore, the choice of electron sacrificial agents has also attracted the attention of researchers. As shown in Scheme 3, commonly used electron sacrificial agents include typical amines (TEA, triethanolamine (TEOA), N, N-Diisopropylethylamine (DiPEA)), as well as very efficient electron donors such as 1,3-dimethyl-2-phenylbenzimidazole (BIH) and the like. However, they have some disadvantages such as self-degradation and by-products in the catalytic process. For example, tertiary amines and ethylenediaminetetraacetic acid (EDTA) are oxidized to form amine radicals and imines, and secondary amines and short aldehydes are formed in the presence of water^[64,65].

2.1.4 Catalytic efficiency and selectivity

Due to the diversity of CO₂ reduction products and the inevitable generation of hydrogen in the system, there is also a problem of product selectivity in the CO₂ reaction system. The molecular photocatalytic system is generally evaluated by two indexes: Turnover Number and Selectivity.

(16)TON=

n 还 原 产 物 n 催 化 剂

(17)Selectivity=

n 特 定 还 原 产 物 n 总 产 物

Table 1 summarizes the main reduction products in different CO₂ photocatalytic reduction systems, the selectivity of the products, and the catalytic efficiency of the system. In all kinds of systems, iron porphyrins can show CO₂ reduction performance, but the type and selectivity of the products are limited by the reaction conditions of the system (such as photosensitizer and electron sacrificial agent), therefore, the products of photocatalytic reduction of iron porphyrin CO₂ are classified (mainly CO and CH₄) in order to summarize and discuss the reduction mechanism of various systems.

2.1.4.1 Photocatalytic reduction of CO₂ to CO

The process of reducing CO₂ to obtain high-value organic matter can not only effectively reduce carbon emissions, but also promote its resource utilization. The most common CO₂ reduction products are CO and CH₄. Compared with CH₄, CO can be used as a component of syngas to directly participate in industrial processes such as Fischer-Tropsch synthesis to produce other advanced organic compounds, so the reduction of CO₂ to CO has attracted wide attention from academia. CO is often the intermediate product of the reduction of CO₂ to CH₄, so how to control the selectivity of the catalyst to produce CO efficiently becomes an urgent problem to be solved.

In 1997, Neta et al. First reported the photocatalytic reduction of CO₂ by ferric porphyrin^[54]. In this catalytic system, TEA and N, N-Dimethylformamide (DMF) were used as electron sacrificial agent and solvent, respectively, and the photoexcitation of the ligand-to-metal charge transfer (LMCT) band at 360 nm led to the one-electron reduction of the central metal from Fe^III to Fe^II and the release of chloride ligands. Fe^II can be further reduced to Fe^I by TEA. Disproportionation of two Fe^I molecules produces catalytically active Fe⁰, which binds to CO₂. The main product of the CO₂ reduction is CO (TON = ~ 70) with the formation of the by-product H₂. However, the photocatalytic reduction system of iron porphyrins has been further studied and optimized due to the disadvantages of the photodegradation of iron porphyrins and the low photoreduction efficiency of CO₂.

In 2014, Bonin et al. Used Fe-o-OH as an iron-based homogeneous catalyst for photocatalytic reduction of CO₂ in the visible range (λ > 400 nm), and the main product was CO^[63]. When the reaction time is more than 50 H, the catalytic system still has high selectivity for CO formation (no or a small amount of H₂ is produced), and the amount of CO formation increases linearly, indicating that the catalytic system has good stability. When Ir(ppy)₃ and 9 CNA were used as photosensitizers, TON was 140 and 60, respectively, and the selectivity of the product was as high as 93% and 100%. The results show that iron porphyrins with phenolic groups have high catalytic activity for the photocatalytic reduction of CO₂. In the same year, they studied the catalytic mechanism of photocatalytic reduction of CO₂ by FeTPP, Fe-o-OH and Fe-o-OH-F^[66]. Compared with the unmodified TPPs, the introduction of the phenolic groups into the CO₂ adducts is responsible for the stability of the adducts, which is attributed to the hydrogen bonding between the phenolic groups, the CO₂ and the metal center.In addition, the addition of the phenolic group can also accelerate the cleavage of C — O bonds, which lays a foundation for designing more stable and durable photocatalytic systems.

In 2017, Rao et al. proposed a new photocatalytic reduction CO₂ system without photosensitizer, in which modified tetraphenyl iron porphyrin Fe-p-TMA (Figure 1) was used as photocatalyst and BIH (Figure 3) was used as electron sacrificial agent^[67]. In order to solve the problem of weak absorption of visible light in this system, Rao et al. Introduced trimethylamine on the para-position of the benzene ring of iron porphyrin, and expanded the delocalized π bond of the benzene ring by using the lone pair electrons on the nitrogen of trimethylamine, which made the complex active under visible light and photochemically reduced CO₂ to CO without adding photosensitizer. The results showed that: firstly, the introduction of trimethylamine group made the catalyst active under visible light and prevented photodegradation; Second, it hinders the formation of competing hydrides; Third, no decrease in catalyst activity was observed during the 60 H reaction, proving that the four positively charged groups played a strong stabilizing role. Furthermore, the researchers found that when a simple organic photosensitizer purpurin (Scheme 2) was added to the reaction system (visible light, aqueous solution (acetonitrile/water = 1:9 V/V)), Fe-p-TMA as a molecular catalyst could selectively reduce CO₂ to CO with a selectivity of up to 95%. This work opens up new perspectives for the development of efficient CO2 photocatalysis in water^[61].

As shown in Figure 1, based on the analysis and research on the catalytic reduction of CO₂ by iron porphyrin catalysts such as FeTPP, Fe-o-OH and Fe-o-OH-F, Bonin et al. proposed the reaction mechanism of photocatalytic reduction of CO₂ by iron porphyrin and emphasized several reaction pathways after the catalyst is activated^[68].

When FeTPP was used as the catalyst, the reduction product was mainly H₂, which was attributed to the in situ generation of a strong acid, namely protonated TEA. It is not only beneficial to the formation of porphyrin hydride and the gradual release of H₂, but also beneficial to the degradation of macrocycle, that is, the hydrogenation reaction under light. This degradation pathway leads to rapid inactivation of porphyrins, which is also slightly increased in the presence of external weak acids. The formation of CO is related to the stability of the CO₂- porphyrin adduct (formula (18)). For example, for CAT and FCAT, the — OH group located on the porphyrin ring can form an internal hydrogen bond, which greatly stabilizes the adduct. For FeTPP, however, the stability of the adduct is difficult to maintain, so the formation of CO is hardly observed. However, the improved stability of CO₂- porphyrin adducts implies that the formation of CO requires an additional electron (compared to FeTPP) to participate in the electron transfer coupled with proton transfer pathway to trigger C — O bond cleavage (formula (20)). This step is facilitated by protonation of the intermediate adduct (formula (19)) due to the ability of the phenol group in its internal structure to be reprotonated by an external acid. An intrinsic limitation of the reaction is that the extra electron could only come from another highly reducing species, the Fe (0) porphyrin, at a very low steady-state concentration. Therefore, it can limit the catalytic performance of CAT and FCAT. On the other hand, when CO is released, CAT and FCAT in the catalytic cycle return to the Fe (Ⅰ) state instead of the Fe (Ⅱ) state of FeTPP. Therefore, the cycle time of these two catalysts is shorter, which also explains why the catalytic performance of Fe-O-OH and Fe-O-OH-F is better than that of FeTPP. In general, the catalytic selectivity of Fe-O-OH and Fe-O-OH-F is better than that of FeTPP, because the binding between Fe (0) and CO₂ is faster, and correspondingly, the hydride formation is less.

(18)Fe(0)^2- + CO₂ → [Fe(Ⅱ) - CO₂]^2-

(19)[Fe(Ⅱ) -CO₂]^2-+ H⁺ → [Fe(Ⅱ) - CO₂H]^-

(20)[Fe(Ⅱ) -CO₂H]^- + e^- + H⁺ → Fe(Ⅰ)^- + CO + H₂O

From the above reaction mechanism analysis, it can be concluded that there are three limiting factors for the photochemical catalytic reduction of CO₂ to CO when iron porphyrin is used as a homogeneous catalyst: in the absence of photosensitizer, porphyrin requires the excitation of high-energy ultraviolet light (< 300 nm); The formation of strong acids (electron sacrificial agents after oxidation) leads to the formation of competing hydrides, which leads to the deactivation of the catalyst by hydrogenation; When the catalysts are Fe-o-OH and Fe-o-OH-F, the strong reducing substances are necessary to initiate the cleavage of the C — O bond in the CO₂ adduct.

2.1.4.2 Photocatalytic reduction of CO₂ to CH₄;

In the previous study, Robert et al. Used Fe-p-TMA catalyst to realize the two-electron catalytic reduction of CO₂-CO in nearly neutral aqueous solution (acetonitrile: water = 1:9 V/V) under visible light conditions, using purpurin and TEA as photosensitizer and electron sacrificial donor respectively, and the product selectivity of CO was as high as 95%^[61]. Furthermore, on the basis of previous experiments, the photosensitizer in the system was replaced by iridium complex Ir(ppy)₃ (Fig. 2), TEA was used as the electron sacrificial donor, and acetonitrile was used as the solvent for the photocatalytic reduction of CO₂. The main product in the system is still CO, and the selectivity is 78%; In particular, a part of CH₄ is produced in the system, the selectivity is about 17%, and the TON is 79; In addition, a small amount of by-product H₂ is generated. In order to explore the effect of CO on CH₄ production more directly, the photocatalytic reduction system was directly irradiated with visible light in CO-saturated acetonitrile solution, and the product was CH₄ with a selectivity of more than 80%, while H₂ was the only detected by-product. This indicates that in the whole photocatalytic reduction of CO₂ system, CO₂ is first converted to CO, and then CO is further converted to CH₄, which is one of the breakthroughs in the realization of secondary conversion of products so far^[69]. On this basis, in order to avoid the use of photosensitizers based on noble metals, they were replaced by organic phenazine groups (Phen1 and Phen2, Scheme 2). The values of the reduction potentials of the excited triplet States of Phen1 and Phen2 are E⁰(²Phen1^·+/³Phen1^*)=-2.09 V vs Fc+/Fc and E⁰(²Phen2^·+/³Phen2^*)=-2.20 V vs Fc⁺/Fc, respectively, which are similar to those of Ir(ppy)₃ (E⁰(Ir(IV)/³Ir(Ⅲ)^*)=-2.13 V vs Fc⁺/Fc)^[70]. Compared with the photosensitizer Ir(ppy)₃, the photocatalysis efficiency of the system with Phen as the photosensitizer is much higher, and the effect of Phen2 is better than that of Phen1. Under similar conditions, the yield of CH₄ is almost twice that of Phen1, and the quantum yield is about three times that of Phen1. The catalytic system continued to produce methane after a long period of illumination for 4 days. Using CO as raw material, the CH₄,TON of 80 can be obtained with a selectivity of 85% and a quantum yield of 0.47%. In addition, the redox properties of organic photosensitizers and the acidity of proton source play a key role in driving the 8e^-/8H⁺ process.

The 8e^-/8H⁺ reaction mechanism for the reduction of CO₂ to CO is shown in Figure 2. Fe (III) porphyrin is first reduced to Fe (0) with catalytic activity by three electrons, and then the CO₂ is complexed with Fe (0) (Fe^IICO₂ intermediate) to form Fe^IICO intermediate after protonation and dehydration, which has been confirmed by UV-visible absorption spectroscopy^[63,67,68]^[67]. Even if the experimental conditions are optimized, the reaction of electron-rich Fe (0) with 2H⁺ to form Fe (Ⅱ) and the by-product H₂ is still the secondary pathway of the reaction. The Fe^IICO intermediate can remove CO product and form Fe (Ⅱ), and further participate in the 6e^-/6H⁺ reduction reaction to obtain the final product CH₄.

2.2 Iron polypyridine as a photocatalyst

In 2016, Guo et al. Found that the iron complex catalyst [Fe(qpy)(OH₂)₂]²⁺(qpy=2,2':6',2″:6″,2‴-quaterpyridine) formed by tetrapyridine ligand and Fe can reduce CO₂ to CO with high efficiency and selectivity^[38]. In this system, the light source is light-emitting diode (LED) blue light (λ = 460 nm), the solvent is a mixture of acetonitrile (MeCN) and TEOA (MeCN ∶ TEOA = 4 ∶ 1 V/V), and the photosensitizer and electron sacrificial agent are Ru (bpy

) 3 2 +

and BIH, respectively. The product selectivity of CO is 85%, and the TON and quantum yield are as high as 3844 and 8.8%, respectively. The main by-products are H₂ and formate. To reduce the cost, they replaced Ru (bpy

) 3 2 +

with purpurin as a photosensitizer. In addition, due to the poor solubility of purpurin in MeCN, the solvent was also replaced by N, N-dimethylformamide. Under these conditions, the selectivity and TON of CO product were 92% and 1365, respectively, and no by-product H₂ was detected. Fig. 3 shows the mechanism of the reduction of CO₂ to CO in the Ru (bpy)

) 3 2 +

/BIH/TEOA system. Briefly, first, that two electron accepted by the Fe^II are reduce by the Ru (bpy

) 3 +

, which is generated by quenching of the Ru (bpy

) 3 2 + *

by BIH) to the active Fe⁰. The CO₂ then binds to the reducing metal via the carbon atoms to form the Fe-CO₂ adduct, which is finally protonated to produce CO and regenerate the catalyst. The mechanism of the catalytic reaction using purpurin as a photosensitizer is similar. This study is of great significance to the development of more practical catalysts for CO₂ reduction.

Qin et al recently reported the application effect of [Fe^II(qnpy)(H₂O)₂]²⁺(qnpy=2,2':6',2″:6″,2‴:6‴,2'‴-quinquepyridine), an antitumor agent, in the reduction of CO₂^[71]^[72]. Since qnpy is a pentadentate ligand and qpy is a tetradentate ligand, this iron complex is more stable than the previously described [Fe(qpy)(OH₂)₂]²⁺,[Fe^II(qnpy)(H₂O)₂]²⁺. The experimental results show that the addition of water can significantly improve the yield of CO. The catalyst has high activity even in the presence of 50% water content, with a maximum TON of 14.095 and a CO selectivity of 98%. In addition, the system maintains a high and stable CO selectivity throughout the whole catalytic process. The iron complex is a state-of-the-art technology for visible-light-driven catalytic CO₂.

2.3 Iron pentadentate complex

Chen et al. Used the same pentadentate ligand N5 (2,13-dimethyl-3,6,9,12,18-pentaazabicyclo [12.3.1] octa-eca-1 (18), 2,12,14,16-pentaene) to synthesize complexes with Fe^III and Co^II as metal centers, respectively, and explored their ability to reduce CO₂ under photochemical conditions^[39]. Photocatalytic experiments were carried out under visible light (λ > 460 nm) with MeCN as solvent, Ir(ppy)₃ as photosensitizer and TEA as sacrificial electron donor. They found that when the Co^II was used as the metal center, the reduction product was mainly CO; When Fe^III is used as the metal center, the reduction product is mainly HCOOH. They gave a reasonable explanation for this phenomenon, and the interaction between the :CO₂ and the metal was the key step in the reaction. As shown in Fig. 4, CO₂ may be bound to the metal through the carbon atom (η₁-CO₂ coordination), but the reduced and protonated adduct does not undergo C — O cleavage, but is converted to the η₁-OCOH coordination complex, which eventually releases the formic acid molecule. On the other hand, the quantum calculation shows that the intermediate of the reaction is Fe-CO₂H, in which the Fe^III is a poor π-donor, so that the C — O bond cleavage is relatively slow and the isomerization is easier, eventually promoting the formation of HCOOH.

3 Heterogeneous Photocatalytic Reduction of CO₂ Based on Metal Iron Complex

3.1 Semiconductor nanomaterials as photosensitizers

Considering that inorganic materials usually show higher stability than organic materials, inorganic semiconductor nanomaterials as light absorbers combined with molecular catalysts have become a hot research topic^[29,73,74]. The hybrid system not only has the high efficiency and selectivity of molecular catalysts, but also has the advantages of high stability and high visible light activity of inorganic nanomaterials. More importantly, the light-harvesting process is spatially and temporally separated from the catalytic process, enabling efficient separation and transfer of charge carriers.

Lin et al. prepared g-C₃N₄/FeTCPP heterogeneous catalyst by a simple self-assembly method with low-cost graphite-phase carbon nitride (g-C₃N₄) nanosheets as the photosensitive element and FeTCPP as the catalytic center (as shown in Figure 5)^[75]. The strong interaction between the g-C₃N₄ nanosheets and FeTCPP can be formed through the hydrogen bond between the edge amino group of the g-C₃N₄ nanosheets and the carboxyl group of FeTCPP and the π-π stacking interaction between the tri-s-triazine unit and porphyrin, thereby promoting the charge transfer between the two. The results show that the g-C₃N₄/FeTCPP catalyst has high activity for CO₂ reduction under visible light irradiation, and the photoinduced electrons can be effectively transferred from g-C₃N₄ nanosheets to FeTCPP, the CO yield within 6 H is 6.52 mmol·g^-1, and the selectivity of the product can reach 98%. Similar to the mechanism of homogeneous catalytic reduction of CO₂ by iron porphyrin mentioned in the previous section, photoinduced electrons in g-C₃N₄ nanosheets are first transferred to Fe^IIITCPP, prompting the formation of Fe^IITCPP. At the same time, the holes left in the g-C₃N₄ are trapped by the electron sacrificial agent TEOA. The Fe^IITCPP is combined with the CO₂ to obtain a Fe^IITCPP-CO₂ intermediate,After receiving the electrons generated from the g-C₃N₄, the Fe^ITCPP-CO₂ and the Fe⁰TCPP-CO₂ are generated in turn. Finally, the catalytically active Fe⁰TCPP reduces CO₂ to CO. In contrast to FeTPP, the carboxyl group on FeTCPP can stabilize the above CO₂ adducts through intramolecular hydrogen bonding, as previously reported for the role of the phenolic functional group. In 2020, Zhang et al. Further used carbon dots (CDs) doped g-C₃N₄ as a photosensitizer to combine with FeTPP^[76]. The g-C₃N₄-C_0.05/FeTCPP composite photocatalyst was highly active under visible light irradiation, and the CO yield was 23.1 mmol·g^-1,H₂ and 71.1 mmol·g^-1 within 6 H, which was about three times higher than that of the previous g-C₃N₄/FeTCPP system. The existence of trace carbon dots in the system provides another way for electron transfer, that is, electrons from g-C₃N₄ to FeTCPP through CDs, thus promoting interfacial charge separation and electron transfer.

Li et al. used a simple ion-exchange reaction to prepare CdS/Bi₂S₃ heterostructures, which were combined with FeTCPP to construct a semiconductor/molecular catalyst hybrid system (CdS/Bi₂S₃/FeTCPP) to photocatalytically reduce CO₂ to CO and H₂ under visible light^[77]. Likewise, in this heterogeneous photocatalytic system, the CdS/Bi₂S₃ heterostructure serves as a light collection unit, and FeTCPP serves as a catalytic center. As shown in fig. 6, if that CdS/Bi₂S₃ is not combine with FeTCPP, the photogenerated electrons in the CdS/Bi₂S₃ heterostructure will migrate from the conduction band of the CdS to the conduction band of the Bi₂S₃ from a thermodynamic point of view, while the electron can also be transferred from the Bi₂S₃ conduction band to the CdS valence band through the Z-type heterostructure. When the CdS/Bi₂S₃ is combined with FeTCPP, the photogenerated electrons in CdS can be effectively transferred to FeTCPP, and then the CO₂ reduction occurs on the FeTCPP molecular catalyst, and the electron transfer from CdS to Bi₂S₃ will be greatly inhibited. Therefore, the introduction of FeTCPP can change the migration direction of electrons in the CdS/Bi₂S₃ heterojunction. More importantly, due to the feasibility of the Z-type heterostructure in the hybrid catalyst, the strong reducibility of electrons in the conduction band of CdS is preserved, which is beneficial to the transfer to FeTCPP. At the same time, the separation and utilization efficiency of carriers will be greatly improved. In addition, the reduction of sulfur vacancies in the heterostructure can further promote the separation of carriers. Therefore, the CdS/Bi₂S₃/FeTCPP heterogeneous composite photocatalyst has high catalytic activity for CO₂, and the CO yield is 7.73 mmol·g^-1,H₂ and the yield is 24.30 mmol·g^-1 after 4 H of visible light irradiation. This work will contribute to a better understanding of the reduction of CO₂ in the semiconductor heterostructure/molecular catalyst composite system, providing a feasible route for the preparation of efficient heterogeneous photocatalysts for solar fuels.

3.2 Quantum dots as photosensitizer

The key to the sensitization process is the efficient transfer of redox equivalents from the photosensitizer to the catalyst. In the system of molecular photosensitizer, this transfer is usually very slow, because the electron transfer between the two is mainly mediated by bimolecular collision reaction, and its rate and catalytic efficiency are limited by diffusion and concentration. In addition, these commonly used noble metal sensitizers need to adjust their redox potentials or absorption spectra to suit specific applications or catalysts by changing the coordination environment of the metal or the size of the organic π system^[47,78]. Colloidal quantum dots (QDs) as photosensitizers for the reduction of CO₂ can provide a solution to these problems.

Lian et al. introduced the electrostatic assembly using negatively charged colloidal CuInS₂/ZnS quantum dot (QD) sensitizer and positively charged FeTMA catalyst (fig. 7)^[78,79]. In this study, the extinction coefficient ε of CuInS₂/ZnS core/shell QDs without heavy metals at 450 nm is about 70 000 M^-1·cm^-1, which is 3 ~ 25 times higher than that of Ir(ppy)₃ and Ru(bpy)₃. In addition, the surface chemistry of colloidal quantum dot sensitizers is easy to adjust, and their electronic characteristics are relatively insensitive to neighboring charges, which makes them easier to adjust than molecular photosensitizers (such as Ir or Ru-based dyes) (molecular photosensitizers are difficult to functionalize without changing their redox potentials and absorption spectra). In particular, a simple change in its diameter increases its reduction potential from − 1.7 V vs SCE for bulky V vs SCE to ~ − 2.4 V vs SCE for V vs SCE with a diameter of 2.5 nm, which is sufficient to drive the three-step reduction process of FeTPP to achieve the binding and reduction of V vs SCE. Under the illumination of 450 nm, this system can photocatalytically reduce CO₂ in water to CO with a high selectivity of 99%, and the sensitization efficiency of this system is 11 times higher than that of the previously recorded iron porphyrin catalyst, which is attributed to the ultrafast electron transfer between QD and FeTPP (the time constants of Fe^IIITPP→Fe^IITPP and Fe^IITPP→Fe^ITPP are both less than 200 fs). The ultrafast electron transfer capability between the quantum dot photosensitizer and the catalyst center is essential for reactions that require multi-step redox processes at low photon fluxes, opening up the possibility of developing artificial photocatalytic systems that work under natural light.

4 Conclusion and prospect

In summary, this paper reviews the recent progress in the application of low-cost iron-based complexes in the photocatalytic reduction of CO₂, including homogeneous and heterogeneous catalytic systems, mainly from the types and selection of iron complexes molecular catalysts, photosensitizers and electron sacrificial agents.It is emphasized that iron complexes as catalysts have great potential and application prospects in the reduction of CO₂ by visible light and selective conversion of them into high value-added products such as CO and CH₄ (that is, artificial photosynthesis). In recent years, the combination of molecular catalysts and solid materials has attracted much attention. In particular, porphyrin metal-organic frameworks have become a new research hotspot. The design and construction of iron porphyrin metal-organic frameworks may inject new vitality into the efficient capture and conversion of CO₂. In addition, there are still many difficulties to be overcome in the practical application of iron complex catalysts. For example, the research on the mechanism still needs to be further deepened, especially the understanding of the reduction of 8e^-/8H⁺ to methane, which may need to be analyzed by the combination of time-resolved infrared spectroscopy and quantum computing, which will provide a theoretical basis for improving the diversity of CO₂ conversion products (such as methanol and other fuels and multi-carbon products). Amines are still the main electron sacrificial agents in this reaction system, and it is also an important challenge to replace amines as sacrificial electron donors with water or develop more valuable oxidation processes. These studies will provide new ideas for solar-driven carbon dioxide conversion using water as a proton source.

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Funding

National Natural Science Foundation of China(21822607)

State Key Laboratory of Photocatalysis on Energy and Environment(SKLPEE-KF201701)

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1 Introduction

2 Homogeneous Photocatalytic Reduction of CO2 Based on Metal Iron Complex

2.1 Iron porphyrin as a photocatalyst

2.1.1 Activation of catalyst

2.1.2 Photosensitization

2.1.3 Electron sacrificial agent

2.1.4 Catalytic efficiency and selectivity

2.1.4.1 Photocatalytic reduction of CO2 to CO

2.1.4.2 Photocatalytic reduction of CO2 to CH4;

2.2 Iron polypyridine as a photocatalyst

2.3 Iron pentadentate complex

3 Heterogeneous Photocatalytic Reduction of CO2 Based on Metal Iron Complex

3.1 Semiconductor nanomaterials as photosensitizers

3.2 Quantum dots as photosensitizer

4 Conclusion and prospect

References

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2 Homogeneous Photocatalytic Reduction of CO₂ Based on Metal Iron Complex

2.1.4.1 Photocatalytic reduction of CO₂ to CO

2.1.4.2 Photocatalytic reduction of CO₂ to CH₄;

3 Heterogeneous Photocatalytic Reduction of CO₂ Based on Metal Iron Complex