Coordination Chemistry Reviews 412 (2020) 213262

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Coordination Chemistry Reviews

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Review

Photocatalytic CO2 reduction over metal-organic framework-basedmaterials

https://doi.org/10.1016/j.ccr.2020.2132620010-8545/� 2020 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: jianglab@ustc.edu.cn (H.-L. Jiang).

Dandan Li a,b, Meruyert Kassymova b, Xuechao Cai b,c, Shuang-Quan Zang c, Hai-Long Jiang b,d,⇑a Institutes of Physics Science and Information Technology, Anhui University, Hefei 230601, PR ChinabHefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Soft Matter Chemistry, Department of Chemistry, University of Science and Technologyof China, Hefei, Anhui 230026, PR ChinacCollege of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, PR Chinad State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Fujian Institute of Innovation, Chinese Academy of Sciences, Fuzhou,Fujian 350002, PR China

a r t i c l e i n f o

Article history:Received 16 January 2020Accepted 21 February 2020

Keywords:Metal-organic frameworksMOF-based materialsPhotocatalysisCO2 reduction

a b s t r a c t

Photocatalytic reduction of carbon dioxide (CO2) into high value-added chemicals using clean and renew-able solar energy is a very promising pathway to address energy and environmental issues. Recently,metal-organic frameworks (MOFs) have been intensively exploited in photocatalytic CO2 reduction owingto their promising CO2 capture capability, photochemical and textural properties. In this review, we pro-vide an overview of recent progress achieved in MOF-based photocatalysts for CO2 reduction on the basisof the reduced products, including photocatalytic conversion of CO2 into CO and the other organic chem-icals (formic acid, methanol and methane). Diverse modification techniques for improving relevant pho-tocatalytic performance and the corresponding structure-activity relationships are highlighted. Particularemphasis is placed on the role of CO2 capture capacity for the photocatalytic CO2 reduction performanceover MOF-based materials. Furthermore, the opportunities, challenges and future prospects of the appli-cation of MOF-based materials for photocatalytic CO2 conversion are given, aiming at rational design ofmore creativeMOF-based photocatalytic systems for CO2 utilizationwith a green and sustainable strategy.

� 2020 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Fundamentals of MOF photocatalysts for CO2 reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23. Photocatalytic CO2 reduction by MOF-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.1. Light-driven reduction of CO2 to CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.1.1. MOFs as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.1.2. MOF composites as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.1.3. MOF derivatives as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2. Light-driven reduction of CO2 to organic chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.1. MOFs as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2.2. MOF composites as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.3. MOF derivatives as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4. The role of CO2 capture capacities of MOF-based materials in photocatalytic CO2 reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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1. Introduction

Excessive emission of carbon dioxide (CO2) from the rapid con-sumption of the fossil fuels leads to ever increasing atomosphericCO2 concentration (over 400 ppm) [1,2], which is the major con-tributor to climatic warming [3,4]. To mitigate the above environ-mental issue, much effort has been devoted to CO2 capture, storageand utilization [5–7]. Thereinto, direct CO2 utilization by convert-ing it into high value-added chemicals presents a promising routeto remove excessive CO2 with chemical feedstocks/fuels output [8–15]. However, in the case of CO2 conversion, the intractable chal-lenge is that the highly stable structure of CO2 requires a signifi-cant energy input for C@O bond cleavage (bond enthalpy+ 805 kJ mol�1) [16]. In this context, as an alternative sustainableapproach, the utilization of free and abundant solar energy forthe photocatalytic reduction of CO2 is extremely appealing [17–27]. Indeed, this field presents a continuously growing researchinterest in exploitation of efficient photocatalysts for simple C1/C2 fuels production (such as carbon monoxide, methane, formicacid, methanol and ethanol) [28–32].

Typically, the light-driven CO2 reduction process mainlyincludes the following three principal steps: (1) Light-harvesting.To achieve high solar energy utilization, amounts of visible light-responsive photocatalysts have been developed due to the fact thatvisible light accounts for about 53% of the whole solar energy whilethe ultraviolet light occupies only about 4% [33]. (2) Separation andtransfer of photogenerated charge carrier. Generally, the photogen-erated electron and hole (e-h) pairs are formed within catalystsupon light irradiation and then migrate to the interface of photo-catalysts, subsequently participating in reduction and oxidationprocesses, respectively [23]. (3) The adsorbed and activated CO2

molecules initiated the reaction. In this regards, the increasedabsorbed CO2 concentration and accessible active sites of photocat-alysts can expedite the CO2 reduction process [34,35]. Bearing theabove aspects in mind, significant progress has been achieved inexpanding the optical absorption range and promoting energy-conversion efficiency during the past decades [24,28,36–43]. How-ever, the efficiency of photocatalytic CO2 reduction is still low.Moreover, the photocatalytic selectivity control of high value-added chemicals remains challenging by now. Thus, it is crucialto develop efficient photocatalytic systems, which possessextended light-harvesting ability, efficient photogenerated chargeseparation, abundant active sites and excellent CO2 adsorptioncapacity, to address the above issues.

Scheme 1. Schematic showing the photocatalytic CO2 reduction into CO andorganic chemicals over MOFs.

Metal-organic frameworks (MOFs) are well-known crystallinematerials constructed by metal ions/clusters interconnected withmulti-dentate organic linkers [44–50]. Benefiting from their largesurface areas, tunable structures and high porosity, MOFs havebeen employed for various advanced applications, such as gassorption and separation, sensors, drug delivery, heterogeneouscatalysis, etc [51–76]. In particular, MOFs recommended them-selves as very promising catalysts in the area of photocatalysisdue to their unique properties and advantages [77–94], such asthe adjustable light-harvesting ability over broad range, theimproved electron-hole (e-h) separation, the uniformly distributedcatalytic active sites and the ease of their accessibility for catalysis,the promising platform for mechanistic understanding ofstructure-activity relationships, and so on. Of note, the promisingCO2 capture capability of MOFs [6,58,95–101] further endows theirparticular merits toward photocatalytic conversion of CO2 intohigh-value chemicals by concentrating/enriching CO2 moleculesaround the active sites [80,102–104]. In this sense, MOF materialshold their particular advantages in light-driven CO2 reduction pro-cess by optimizing the above-mentioned three principal steps.

In pursuit of rational design and development of more creativeMOF-based systems for light induced CO2 reduction, it is necessaryto provide a comprehensive and timely overview of the researchadvances in this area. Although several reviews have been pub-lished previously [26,80,82], an updated review which offers a dif-ferent perspective for readers to understand the past, the presentand what yet to solve in this hot topic should be provided. In thisreview, the particular advantages and recent progress of MOFs inphotocatalytic CO2 reduction are presented at first. Different fromprevious reviews, on the basis of the category of generated prod-ucts, including photocatalytic conversion of CO2 into CO and otherorganic chemicals (HCOOH, CH3OH and CH4) (Scheme 1), an over-view on the up-to-date main progress achieved in MOF-based pho-tocatalysts for CO2 reduction will be summarized. Moreover, thephotocatalytic activity related to the CO2 capture capabilities ofMOF-based materials will also be discussed, especially the out-standing CO2 reduction performance even under low CO2 concen-trations. Finally, the limitations, remaining challenges andopportunities of MOFs for CO2 photoreduction will be criticallypointed out.

2. Fundamentals of MOF photocatalysts for CO2 reduction

Photocatalytic CO2 reduction is an attractive approach that canreduce CO2 into valuable fuels and chemicals by using renewablesolar energy over photocatalysts. For MOF-based photocatalysts,typically, the CO2 reduction half reaction starts from the formationof photoexcited electrons and holes from the highest occupiedmolecular orbitals (HOMO) to the lowest unoccupied molecularorbitals (LUMO) upon light irradiation. With a charge-separationstate obtained, the photogenerated electrons on the HOMO diffuseto the catalytic centers to react with adsorbed CO2 initiating thephotocatalytic reaction (Scheme 2). Moreover, for a MOF photocat-alyst to be active towards CO2 reduction, the LUMO must be abovethe redox potential for the CO2 reduction half reaction, whichdepends on the formed product. For example, the redox potentialfor the reduction of CO2 into CO is �3.50 eV, while those for formicacid, methanol and methane are �3.42, �3.65 and �3.79 eV,respectively [81,105,106].

In general, the HOMO and LUMO of MOFs are mainly con-tributed by the organic linker and the metal cluster [107,108],respectively. The structural controllability of MOFs (tunableorganic linkers and/or metal clusters) offers possibility in tuningthe optical and electronic responses (HOMO and LUMO energylevels) [109]. Therefore, the photocatalytic CO2 reduction

Scheme 2. Schematic showing the process of photocatalytic CO2 reduction over MOFs and the redox potentials for the reduction of CO2 into CO, HCOOH, CH3OH and CH4.

D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262 3

performance can be optimized through rational design of theorganic linkers and coordination metal ions/clusters [81,110,111].In addition, MOFs are promising photocatalysts for CO2 reductionbecause of the following features [26,112–115]: 1) both organiclinkers and/or unsaturated metal ions can act as catalytically activesites; 2) high CO2 adsorption capacity leads to high CO2 concentra-tion level around the active sites, facilitating the photocatalyticreactions; 3) the porous structures enable MOFs to incorporateguest photoactive species, offering synergetic effect toward effi-cient photocatalytic CO2 reduction.

3. Photocatalytic CO2 reduction by MOF-based catalysts

It was reported that as opposed to typical photocatalysts whichlead to a mixture of products, most MOF catalysts were able to givehigh product selectivity [116–120]. In this section, centered on thephotocatalytic products, including (1) the reduction of CO2 to COand (2) conversion of CO2 into organic chemicals (HCOOH, CH3OHand CH4), the recent key progress achieved by using MOF-basedcatalysts will be summarized and discussed according to the cate-gory of photocatalysts (MOFs, MOF composites and MOF deriva-tives). For MOFs, the selection/modification of the constituents(organic ligands and metal clusters) has found to be a criticaldeterminant for their photocatalytic performance. Those MOFswith no (or weak) photoactivity can be integrated with other com-ponents (such as semiconductors, photosensitizers or metalnanoparticles) to obtain MOF composites for photocatalysis. Insome cases, to achieve better activity and particularly efficientactive sites, MOFs can be converted to related derivatives, whichwould not only inherit the merits of MOFs to some extent but alsoprovide some additional advantages that parent MOFs might notpossess.

3.1. Light-driven reduction of CO2 to CO

As a common pathway to consume CO2, the photoreduction ofCO2 to CO is a two-electron process which presents the relativelylow kinetic barrier with respect to the other processes in the for-mation of complicated organic chemicals.

3.1.1. MOFs as photocatalystsQuite a few studies unveil that the modification of organic

ligands and metal clusters of MOFs can reduce the HOMO-LUMO

energy gaps, giving rise to extended light-harvesting ability. In thispart, several typical strategies, including the introduction ofphotosensitizer-functionalized linkers, electron-rich conjugatedlinkers, and metalloligands, have been employed for photocatalyticCO2-to-CO reduction [121–128].

As a very early stage work, the light-driven reduction of CO2 toCO was explored by the UiO-67 type MOF involvingphotosensitizer-functionalized linker [ReI(CO)3(dcbpy)Cl] [121].This MOF served as an active catalyst for photocatalytic reductionof CO2 to CO with a total turnover number (TON) of 10.9, which ismore than 3-fold to that of the photoactive homogeneous complex[ReI(CO)3(dcbpy)Cl]. Later, another MOF with a similar structurebased on (bpy)Re(CO)3Cl-containing elongated dicarboxylateligands was fabricated as a single-site catalyst to photochemicallyreduce CO2 to CO [122]. The above two reports proposed that theCO2 reduction in the MOF system undergoes a unimolecular path-way as a result of the immobilization of the Re catalyst in the MOFframework. Similarly, a multifunctional MOF comprising cuprousphotosensitizers and Re catalysts has been reported for CO2-to-CO conversion very recently [128]. In addition to the introductionof photosensitive Re complex linked to organic linkers, a series ofzirconium polyphenolate-decorated-(metallo)porphyrin MOFs,ZrPP-n, were constructed from simulation to synthesis [124]. Theintroduction of electron-rich conjugated porphyrin linker centeredwith Co-metallation gave rise to ZrPP-1-Co with high CO2 uptakecapability (�90 cm3 g�1 at 1 atm, 273 K, among the highest in Zr-MOFs) and high photocatalytic activity for reduction of CO2 intoCO (�14 lmol g-1h�1) without cocatalyst under visible-light irradi-ation (Fig. 1). With the aid of electron spin resonance (ESR) testingand theoretical calculations, the electron traps and reaction kinet-ics have been clarified. It was proposed that, for ZrPP-1-Co, theeclipsed metalloporphyrin array plays a crucial role in CO2-specific capture and CO2-adduct stabilization for enhanced photo-catalysis. This work provides a robust platform for light-drivenCO2-to-CO conversion by uniformly isolating active sites in MOFs.

The ultrathin two-dimensional (2D) MOF nanosheets have cap-tured increasing interest in recent years for their unique properties[65,129]. Recently, a photosensitizing metal-organic layer (MOL),Hf12-Ru, was built by Hf12 cluster and [Ru(bpy)3]2+-based dicar-boxylate linker [125]. Upon capping of Hf12-Ru with M(bpy)(CO)3X (M = Re and X = Cl or M = Mn and X = Br)-based monocar-boxylic acids through carboxylate exchange reaction, the obtainedmultifunctional Hf12-Ru-M (M = Re and Mn) MOLs exhibited highactivity for photocatalytic CO2 reduction to CO (Fig. 2). The efficient

Fig. 1. Structure of ZrPP-1-Co and time courses of CO obtained from CO2 photoreduction with catalysts of ZrPP-1-M under visible-light irradiation. Adapted from Ref. [124]with permission from Wiley-VCH, copyright 2018.

Fig. 2. Schematic showing the synthesis of Ru-Hf12-M (M = Re or Mn) and the mechanism of photocatalytic CO2 reduction. Reprinted from Ref. [125] with permission fromthe American Chemical Society, copyright 2018.

4 D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262

photocatalytic activity of Hf12-Ru-Mmay be attributed to the prox-imity of the MOF skeleton to the capping ligands (1–2 nm), whichfacilitated electron transfer from the [Ru(bpy)3]2+ photosensitiveunit to M(bpy)(CO)3X catalytic unit. The capping of Hf12-Ru withM(bpy)(CO)3X afforded multifunctional MOLs for efficient photo-catalytic CO2 reduction. This work provides a versatile strategyfor multifunctional catalyst fabrication to study artificialphotosynthesis.

3.1.2. MOF composites as photocatalystsThe integration MOFs with semiconductors, photosensitizers or

metal nanoparticles (MNPs), have been investigated for photocat-alytic CO2 reduction. The obtained MOF composites not only can

inherit the advantages of both materials, especially the CO2 captureability of MOFs, but also can accelerate e-h separation and improvethe availability of photogenerated electrons for enhanced photore-duction process due to the formation of heterojunctions orelectron-trapping sites [130–132].

Firstly, the fabrication of composites by coupling MOFs withsemiconductors is an effective strategy for enhanced e-h separa-tion. Wang and co-workers employed Co-ZIF-9 as a co-catalystcoupling with several semiconductor photocatalysts as light har-vesters, including g-C3N4 [133] and CdS [134]. Thanks to the con-siderable visible light-harvesting ability of CdS, the CdS/Co-ZIF-9photoreduction system attained a high apparent quantum yield(AQY) of 1.93% for CO2-to-CO conversion at 420 nm monochro-

Fig. 3. (a) Schematic illustration of the preparation of the Zn2GeO4/Mg-MOF-74 composite. (b) Time-production plot of CO over irradiated Zn2GeO4/Mg-MOF-74, Mg-MOF-74and Zn2GeO4. Adapted from Ref. [136] with permission from Royal Society of Chemistry, copyright 2018.

Fig. 4. (a) Schematic showing the structure of Co-ZIF-9. (b) The dependence of the wavelength of incident light on the gas evolution of CO and H2 over the Co-ZIF-9/[Ru(bpy)3]Cl2�6H2O composite. Adapted from Ref. [144] with permission from Wiley-VCH, copyright 2014. (c-e) The calculated CO2 binding capacity and free energy of CO2 reductionand H2 evolution for MOFs (MOF-Ni, MOF-Co and MOF-Cu). Reproduced from Ref. [147] with permission from the American Chemical Society, copyright 2019.

D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262 5

matic irradiation [134]. When the Co-ZIF-9 cocatalyst wasremoved from the reaction system, the catalytic activity decreaseddramatically, demonstrating the crucial role of Co-ZIF-9 on pro-moting the reduction of CO2. UiO-66 was coupled with carbonnitride nanosheets (CNNS) via a facile electrostatic self-assembly

synthesis for visible light driven reduction of CO2 to CO by Yeand coworkers [135]. This work manifested that the conductionband (CB) edge of CNNS matched well with the LUMO of UiO-66,favoring photogenerated electron transfer from CNNS to UiO-66.The photogenerated long-lived electrons finally go to CO2 adsorbed

6 D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262

onto UiO-66 and substantially suppress e-h recombination, unveil-ing the co-catalytic role of UiO-66 in the composites. Cao, Gao andcoworkers prepared Zn2GeO4/Mg-MOF-74 composite by thehydrothermal method for light-driven CO production (Fig. 3a)[136]. The results showed that Zn2GeO4/Mg-MOF-74 sampleexhibited a higher photocatalytic activity than bare Zn2GeO4

nanorods or the physical mixture of Zn2GeO4 and Mg-MOF-74. Itwas suggested that the dynamic interaction between Zn2GeO4

and Mg-MOF-74 might act as a transportation path to accelerateelectron transfer from Zn2GeO4 to Mg-MOF-74. Therefore, withthe assistance of excellent CO2 capture capacity of Mg-MOF-74,the Zn2GeO4/MgMOF-74 composite exhibited enhanced CO2-to-CO photoreduction efficiency (Fig. 3b). Similar strategies were alsoemployed in other photocatalytic systems comprising MOFs andsemiconductors for photocatalytic conversion of CO2 to CO [137–142].

Secondly, coupling the photosensitizers with MOFs is one of themost common methods to kinetically promote CO2 photoreductionas well [143]. The use of Co-ZIF-9 (Fig. 4a) as a co-catalyst coupledwith photosensitizers for photoreduction of CO2 to CO was firstreported byWang and coworkers in 2014 [144]. The photocatalyticreaction was conducted in the mixture of acetonitrile and waterwith [Ru(bpy)3]Cl2�6H2O as a photosensitizer (Fig. 4b), Co-ZIF-9as a co-catalyst and triethanolamine (TEOA) as a sacrificial electrondonor under visible light irradiation. The reactant CO2 was identi-fied as the source of the produced CO by 13C-labelled isotropicexperiments. A series of controlled experiments disclosed the vitalrole of Co-ZIF-9 in CO2 photoreduction catalysis by promotingcharge transfer and enriching CO2 molecules. The similar approachwas also applied in ZIF-67 system to promote the photocatalyticreduction of CO2 to CO under visible light irradiation [145,146].Very recently, Lan, Li and coworkers fabricated three stable andisostructural MOFs (MOF-Ni, –Co and -Cu) [147]. The results

Fig. 5. Structures of Ren-MOF and Ag � Ren-MOF for plasmon-enhanced photocatalytic COSociety, copyright 2017.

showed that the MOF-Ni exhibited very high selectivity of 97.7%for photoreduction of CO2 to CO with triisopropanolamine as anelectron donor and [Ru(bpy)3]Cl2�6H2O as an auxiliary photosensi-tizer. The authors demonstrated that the electrons were allowed tomigrate from the [Ru(bpy)3]Cl2�6H2O photosensitizer to the MOFsfor visible light driven CO2 reduction due to their matched LUMOpositions. Additionally, the specific effects of different metal ionspecies on photoreduction of CO2 were studied through densityfunctional theory (DFT) calculations. The MOF-Ni presented thestrongest CO2-to-CO conversion activity with the highest selectiv-ity among the three MOFs because of its strong CO2 binding capac-ity (Fig. 4c), low CO2 reduction free energy (Fig. 4d) and high sidereaction (H2 evolution) free energy (Fig. 4e).

Compared with bulk MOFs, 2D MOF nanosheets possess higherphotocatalytic efficiency due to the better exposed active sites[129]. Zhu et al. successfully demonstrated exfoliated nanosheetsfrom a conductive 2D-MOF Ni3(HITP)2 as an efficient co-catalystfor CO2-to-CO conversion in a visible-light photocatalytic systemusing [Ru(bpy)3]2+ as the photosensitizer and TEOA as the electrondonor [148]. Besides acting as active sites in the composites ofMOFs and photosensitizers, 2D MOFs can be applied as light-harvesting agents as well. For instance, a valid approach has beendeveloped by Gao and coworkers, in which a visible-light-responsive 2D porphyrin-based MOF was coupled with a photoac-tive dinuclear cobalt complex as co-catalysts for photocatalytic COevolution [149]. Importantly, the abundant coordinatively unsatu-rated metal sites of 2D MOFs in these studies provide the high den-sity catalytic sites for efficient CO2 photoreduction.

Thirdly, MOFs are elegant host matrices for confining diverseguest species due to their permanent porosity [64,77,150], partic-ularly metal nanoparticles (MNPs), for their synergistic effect inlight-driven CO2 reduction. The typical example of MNPs/MOFcomposites for photoreduction of CO2 to CO was presented by

2 reduction. Reprinted from Ref. [118] with permission from the American Chemical

Fig. 6. (a) Schematic illustration of the synthesis of In2S3-CdIn2S4 heterostructured nanocube and (b) its band structure and recyclability for photocatalytic CO2 reduction.Reproduced from Ref. [155] with permission from the American Chemical Society, copyright 2017.

D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262 7

Yaghi, Yang and coworkers [118]. In this report, Re(CO)3(bpydc)Clcomplex, as the catalytic center, was attached to the linkers toobtain photoactive Re-UiO-67 (Fig. 5a). Moreover, Re-UiO-67 wasfurther coated on Ag nanocubic to give Ag � Re-UiO-67 (Fig. 5b),which resulted in a 7-fold enhancement of CO2-to-CO photoreduc-tion activity compared with Re-UiO-67 under visible light withlong-term stability maintained up to 48 h. Later, Ag@Co-ZIF-9was fabricated for photocatalytic reduction of CO2 to CO under vis-ible light irradiation with the assistance of a photosensitizer, againdemonstrating that Ag NPs doping may provide a possible way topromote the efficiency and selectivity of MOF materials in CO2

photoreduction [151].

3.1.3. MOF derivatives as photocatalystsAs an alternative, MOFs can be converted to related materials in

a controlled manner. The resultant materials usually provide well-dispersed active sites that are not found in MOFs, enhanced stabil-ity and high surface area [152,153].

Ye and coworkers realized the CO2-to-CO photoreduction usinga MOF-derived Fe@C photocatalyst [154]. Later, starting with an In-based MOF (MIL-68) hexagonal prism as the precursor, Lou andcoworkers prepared hierarchical In2S3-CdIn2S4 heterostructurednanotubes through a liquid phase sulfidation process followed byheat treatment and cation exchange reaction (Fig. 6) [155]. Accord-ingly, these hierarchically heterostructured nanotubes facilitatethe separation and mobility of photogenerated electrons and holes,promote the adsorption of CO2 molecules, and offer rich active sitesfor surface photochemical reactions. Consequently, the optimizedIn2S3-CdIn2S4 nanotubes manifest remarkable performance forCO2 reduction with high CO production rate (825 lmol h�1 g�1)and outstanding stability under visible light irradiation. In anotherwork, they presented sandwich-like ZnIn2S4-In2O3 hierarchicaltubular heterostructures by using the In-MIL-58 as the precursorfor photocatalytic CO2-to-CO conversion with outstanding perfor-mance and high stability [156]. Recently, a controllable approachto prepare ultrathin two-dimensional porous Co3O4 catalysts byair calcining of the ultrathin MOFs nanosheet templates has beendeveloped for CO2-to-CO photoreduction as well [157]. The abovestudies proved the particular advantages of MOF derivatives forphotocatalytic CO2 reduction.

3.2. Light-driven reduction of CO2 to organic chemicals

CO2 was also successfully reduced to a few other useful organicchemicals by MOF-based photocatalysts, mainly including HCOOH,CH3OH and CH4, which requires the transfer of 2, 6 and 8 electrons

during the photocatalytic process, respectively, suggesting theincreased kinetic barrier should be overcame in the correspondingreactions. Apart from this, the further use of the obtained organicchemicals is also quite different. The produced gas product (CH4)can directly supply the energy consumption, while the liquidchemicals (HCOOH and CH3OH) are important feedstocks foramounts of chemical reactions. To meet the practical require-ments, the clarification of the mechanism of these processes andthe regulation of HOMO and LUMO energy levels of the MOF-based photocatalysts are of great importance in the control of theproduct selectivity.

3.2.1. MOFs as photocatalystsGiven the well-defined structures and structural controllability,

MOFs offer great advantages to understand the reaction mecha-nism and tune the electronic responses (HOMO and LUMO energylevels) for photocatalytic selectivity control. Extensive experimen-tal results have shown that CO2 can be reduced to organic chemi-cals over MOF photocatalysts via the modification of organiclinkers (e.g. amine-modified linkers, photosensitizer-functionalized linkers, electron-rich conjugated linkers, metal-coordinated linkers, etc.) and the functionalization of metalclusters.

As early as 2012, Li and co-workers reported a photoactive cat-alyst NH2-MIL-125(Ti) using amine-modified terephthalic acid(ATA) as the linker to reduce CO2 to formate anion HCOO� withTEOA as the sacrificial agent under visible light irradiation [117].Benefiting from the amino functionalization, the optical absorptionof NH2-MIL-125(Ti) has been expanded, its absorption edge wasshifted from 350 nm (that of MIL-125(Ti)) to 550 nm (Fig. 7a).Besides enhanced light harvesting, another positive change inNH2-MIL-125(Ti) induced by the amino functionality is its higheradsorption capability toward CO2 (132.2 cm3 g�1) than that ofMIL-125(Ti) (98.6 cm3 g�1). Accordingly, the HCOO� formation rateof NH2-MIL-125(Ti) (16.28 mmol h�1 g�1) was much higher thanthe inactive MIL-125(Ti). Moreover, the authors demonstrated thatthe photogenerated Ti3+ moiety via photoexcited organic linkers toTi4+ ions is responsible for the enhanced photocatalytic perfor-mance of NH2-MIL-125(Ti) (Fig. 7b). Later on, they presentedNH2-UiO-66(Zr) involving amine-modified linker ATA for photo-catalytic HCOOH production [158]. The photoinduced electrontransfer from the excited ATA to Zr-oxo clusters in NH2-UiO-66(Zr) to generate ZrIII was confirmed by ESR and photoluminscencestudies. These reports presented a successful visible light respon-sive MOF fabrication strategy by amino functionality for photocat-alytic reduction of CO2 to HCOOH, and then a series of MOFs were

Fig. 7. Schematic showing the visible light capturing ability of NH2-MIL-125(Ti) and proposed mechanism for the photocatalytic CO2 reduction. Reproduced from Ref. [117]with permission from Wiley-VCH, copyright 2012.

8 D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262

further designed along this line. In 2014, a series of earth-abundantFe-containing MOFs were functionalized with amine group (NH2-MIL-101(Fe), NH2-MIL-53(Fe) and NH2-MIL-88B(Fe)) and per-formed for CO2 reduction under the same experimental conditionsas above [159]. This work analyzed the mechanism for theenhanced photocatalytic HCOOH production, unveiling that notonly the Fe-O clusters but also the NH2-functionalied linkers canharvest light and induce electron transfer to generate Fe2+ forCO2 reduction. Very recently, Lan and coworkers reported two bio-mimetic MOFs containing auxochromic –NH2 groups for efficientCO2-to-HCOOH conversion [119]. The reaction could be performedin aqueous solution due to the high structural stability of bothMOFs involving hydrophobic alkyl groups on the organic linkers.Their free-energy calculations for CO2 reduction indicated thatthe CO2-to-HCOOH conversion could readily occur at the bare aro-matic N atom of adenine with the assistance from o-amino groupthrough strong binding with the activated CO2 intermediate.

As efficient photocatalysts and photosensitizers for CO2 reduc-tion, the metalloligands based on Ru are preferred for photoactiveMOF fabrication [160–166]. Recently, Kong and coworkers pre-sented a Ru-metalloligand-based MOF featuring dinuclear Eu(III)2clusters as connecting nodes to drive CO2 reduction under visiblelight irradiation [167]. The results showed that the obtainedphotosensitizer-functionalized MOF can reduce CO2 to HCOOHwith high selectivity. The transient absorption measurementsand theoretical calculations unveiled that the electron transferfrom Ru metalloligands to Eu(III)2 catalytic centers initiated thephotoreduction reaction. Similarly, a photoactive Zr-MOF incorpo-rating bis(40-(4-carboxyphenyl)-terpyridine)Ru(II) complex (Ru(cptpy)2) in its backbone was prepared [168]. The obtained MOFexhibited highly efficient CO2-to-HCOOH reduction activity, whichcan be attributed to the long emission lifetime of Ru(cptpy)2 linker,the high CO2 adsorption capability of obtained MOF and the possi-bility of CO2 coordination to the zirconium center. In addition,based on the experimental analysis and the DFT calculations, theauthors proposed that the fast ruthenium-to-cptpy charge transferinitiated the CO2 photoreduction process.

In general, porphyrin-like pigments array as light-harvestingantennas in natural photosynthesis. To this end, Jiang, Zhang andcoworkers used porphyrin-based Zr-MOF (PCN-222) for effectiveCO2 capture and reduction of CO2 to HCOOH by mean of solarenergy [116]. Given the more negative potential of LUMO forPCN-222 than the reduction potential of CO2 to formate anions, itwas found that 30 lmol of HCOO– were generated after 10 h of vis-ible light irradiation, which was significantly higher than that bythe homogeneous porphyrin ligand (2.4 lmol of HCOO–) undersimilar conditions. The improved photocatalytic performance canbe ascribed to the existence of an extremely long-lived electrontrap state in PCN-222 uncovering by ultrafast transient absorption

and photoluminescence spectroscopy measurements (Fig. 8a).Very recently, two mixed-ligand MOFs (PCN-137 and PCN-138)involving electron-rich conjugated porphyrin linkers werereported [169]. Thereinto, the large cages within PCN-138 skeletonendow its uptake capability toward CO2. Accordingly, PCN-138 forCO2-to-HCOOH conversion gave high photocatalytic activity bene-fitting by the coexistence of photosensitizing porphyrin fragmentsand Zr-oxo centers. In another work, two isomorphic Zr-MOFs(pbz-MOF-1 and PCN-136) via a single-crystal-to-single-crystalprocess were fabricated (Fig. 8b) [120]. The authors demonstratedthat the HCOOH evolution rate over PCN-136 was about 3 timeshigher than that over pbz-MOF-1 due to the large p-conjugatedfragments of PCN-136, unveiling the key role of electron-rich con-jugated linkers within MOF photocatalysts for catalytic reactions.

In addition to the common organic linker modification[170–172], the photocatalytic performance of MOFs can be improvedvia metal cluster functionalization. In 2015, Ti-substituted NH2-UiO-66(Zr/Ti) was prepared via post-synthetic metal exchange methodfor photocatalytic CO2 conversion to HCOOH [173]. DFT calculationsand ESR results revealed that the enhanced photocatalytic perfor-mance was induced by a Ti-mediated electron transfer mechanism.Meanwhile, Cohen, Kang and coworkers reported a mixed-ligandand mixed-metal UiO-66-type MOF via postsynthetic exchange(PSE) for light-driven HCOOH production (Fig. 9a) [174]. Lately, dif-ferent from the above mixed-metal MOFs via PSE method, Lan, Liuand coworkers utilized stable Fe2M(l3-O)(OAc)6(H2O)3 clusters todirectly assemble a series of heterometallic Fe2M cluster-based MOFs(NNU-31-M, M = Co, Ni, Zn), which could be employed for overallreaction (coupling CO2 reduction with H2O oxidation) without theassistance of additional sacrificial agent and photosensitizer(Fig. 9b) [175]. Among these MOF photocatalysts, the highest CO2-to-HCOOH conversion performance was obtained from NNU-31-Zndue to the fact that CO2 reduction reaction was more likely to occuron metal Zn and H2O oxidation reaction occured on metal Fe. Thiswork provides a promising platform for designing MOF photocata-lysts to realize artificial photosynthetic overall reaction.

In comparison with the photocatalytic reduction of CO2 toHCOOH, relatively few works were conducted for photoreductionof CO2 to the chemicals of CH3OH and CH4 by MOF photocatalysts.In 2013, Zhang, Huang and coworkers mimicked the photosynthe-sis using a Cu porphyrin-based MOF (SCu) for CO2-to-CH3OH con-version and its activity was compared with that of an analogousMOF without Cu2+ (SP) [176]. The CH3OH evolution rate achievedby SCu is seven times that of SP due to the more favorable CO2

adsorption on SCu compared to SP as evidenced by FT-IR spec-troscopy. As reported, the porphyrin-based MOF can be employedto convert CO2 to CH4 as well [177]. Sharifnia and coworkersdemonstrated that the Zn porphyrinic MOF can be applied as aphotocatalyst for hydrocarbon fuel (CH4) evolution under UV/visi-

Fig. 8. (a) View of the 3D network of PCN-222 and mechanisms underlying the photoexcited dynamics involved in PCN-222. Reproduce from Ref. [116] with permission fromthe American Chemical Society, copyright 2015. (b) Schematic showing the preparation of PCN-136 for CO2-to-HCOOH photoreduction. Reprinted from Ref. [120] withpermission from the American Chemical Society, copyright 2019.

D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262 9

ble light irradiation in the presence of water vapor as electrondonor. In another work, Ye and coworkers implanted singlet Coatom into porphyrin-based MOF (Fig. 10a), called MOF-525(Zr)–Co, for enhanced CO2-to-CH4 reduction (36.67 mmol g-1h�1) dueto Co-existed as the atomically dispersed catalytic centers [178].The results of CO2 uptake measurements and simulated potentialenergy surfaces clearly indicated that the photoexcited electronsof porphyrin units were released to the coordinately unsaturatedCo centers under the light irradiation, after which Co centersbecame active for CO2 reduction (Fig. 10b). Very recently, twostable polyoxometalate-grafted metalloporphyrin coordinationframeworks were presented for light-driven CO2-to-CH4 reductionwith high selectivity (>96%). Lan and coworkers demonstrated thatthe introduction of polyoxometalate building blocks can theoreti-cally donate adequate electrons to fulfill the eight-electron reduc-tion process of CO2 to CH4 transformation [179].

3.2.2. MOF composites as photocatalystsFor improved activity of photoreduction of CO2 into organic

chemicals, the fabrication of composites by coupling MOFs withlight-harvesting molecules, semiconductor materials or metalnanoparticles is an effective way.

Firstly, the hybrid structures involving semiconducting materi-als with MOFs are usually employed for converting CO2 to CH3OHand CH4 [180–183]. In 2013, Wang and coworkers integrated theco-catalyst ZIF-8(Zn) with Zn2GeO4 nanorods for photocatalyticreduction of CO2 to CH3OH in aqueous solution [184]. The ZIF-8(Zn) acted as a platform that is capable of enhancing the enrich-

ment of CO2 to promote the conversion. In similar way, Cu3(btc)2was conjugated with TiO2 for CO2-to-CH4 conversion by Xiongand coworkers due to the good CO2 uptake capability of Cu3(btc)2[185]. The fabricated hybrid catalyst showed a five-fold enhance-ment in photocatalytic activity and significantly promoted selec-tivity of CH4 in comparison with pristine TiO2 photocatalyst. Thefirst principle simulation unveiled that the one-electron chargewould alter the activation-energy barrier for CO2 to facilitate theimproved performance.

Later, Su et al. presented a series of hybrid MOFs by integrat-ing different amount of NH2-UiO-66(Zr) with Cd0.2Zn0.8S to pro-duce CH3OH through photocatalytic conversion of CO2 [186]. Thehighest performance was obtained when the content of NH2-UiO-66(Zr) was 20 wt%, giving the CH3OH production rate of6.8 lmol h-1g�1. In another work, halide perovskite@MOF(CsPbBr3@ZIFs) composites were prepared for photoreduction ofCO2 to CO and CH4 with reinforcing activity (Fig. 11) [187].The ZIF coating largely improved the stability of CsPbBr3 towardwater molecules, CO2 capturing ability and charge separationefficiency. Moreover, the catalytically active Co centers withinZIF-67 could further facilitate the charge separation processand activate the adsorbed CO2 molecules for efficient CO2 con-version. Very recently, the low-cost CH3NH3PbI3 (MAPbI3) per-ovskite quantum dots (QDs) were encapsulated by Fe-porphyrin based MOF PCN-221(Fex) for CH4 production usingwater as a sacrificial reductant [188]. With the aid of steady-state and time-resolved photoluminescence experiments, theauthors revealed that the photogenerated electrons in the encap-

Fig. 9. (a) Synthesis of mixed-ligand and mixed-metal MOF. Reprinted from Ref. [174] with permission from Royal Society of Chemistry, copyright 2015. (b) Schematicshowing the functions of the heterometallic cluster-based organic framework (NNU-31-M, M = Co, Ni, Zn) as photocatalyst. Adapted from Ref. [175] with permission fromWiley-VCH, copyright 2020.

10 D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262

sulated QDs could rapidly transfer to the Fe catalytic sites inMOF, which led to the much higher photocatalytic activity ofresultant photocatalyst MAPbI3@PCN-221(Fe0.2), 38-foldenhancement, than that of corresponding PCN-221(Fe0.2).

Secondly, the homogenous catalysts or photosensitizer mole-cules can be incorporated into MOFs for enhanced photocatalyticCO2 reduction efficiency as well [189–192]. Cohen, Kubiak andcoworkers used Mn(bpydc)-(CO)3Br to incorporate into UiO-67for CO2 photocatalytic reduction with [Ru(dmb)3](PF6)2 as a photo-sensitizer (Fig. 12) [193]. The TON of approximately 110 wasachieved for this composite in 18 h for CO2 to formate conversionunder visible light irradiation, extensively exceeding to the bareUiO-67. The enhanced photocatalytic performance was ascribedto the isolated active sites within the designed framework. Lateron, a series of UiO-67 type MOFs (RuCl@UiO, RuOH2@UiO,RhCl@UiO and RhOH2@UiO) linked by the Ru- and Rh-half-sandwich complexes were presented for CO2-to-HCOOH conver-sion using Ru(bpy)3Cl2 as photosensitizer [191]. The resultsrevealed that Ru-based MOF catalysts showed better performancethan Rh-MOFs for HCOOH production. In addition, the [M-OH2]

(M = Rh or Ru) group gave higher photocatalytic activity than thosewith [M-Cl] due to the easier accommodation and dissociation ofphotocatalytic intermediates on the coordinated H2O sites.

Thirdly, MNPs/MOF composites, where MNPs act as electronacceptors or cocatalysts, are also effective to boost e-h separationfor enhanced photocatalytic performance [150]. In the early stage,Li and co-workers reported Pt and Au-doped NH2-MIL-125(Ti) pho-tocatalysts for the photocatalytic CO2-to-HCOOH conversion withTEOA as the sacrificial agent under visible-light irradiation [194].The results demonstrated that the Pt/NH2-MIL-125(Ti) and Au/NH2-MIL-125(Ti) presented positive and negative effect for the for-mate formation, respectively. Thanks to the Pt doping, an addi-tional pathway to form Ti3+ within the Pt/NH2-MIL-125(Ti) wasachieved and an enhanced catalytic performance was realized. Inanother work, Duan, Chen and coworkers reported a simple strat-egy for engineering heterostructures of Au NPs loaded MOFnanosheets (PPF-3) to achieve CO2-to-HCOOH conversion [195].In this work, the enhanced photocatalytic performance over Au/PPF-3 via a plasmon resonance energy transfer process from AuNPsto PPF-3 was observed.

Fig. 10. (a) Construction of MOF-525-Co. (b) The optimized structure for CO2 adsorption on a porphyrin-Co unit and O-C bond length-dependent CO2 activation energybarrier. Adapted from Ref. [178] with permission from Wiley-VCH, copyright 2016.

Fig. 11. Schematic illustrations of the CO2 photoreduction process of CsPbBr3@ZIFs.Reprinted from Ref. [187] with permission from the American Chemical Society,copyright 2018.

D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262 11

3.2.3. MOF derivatives as photocatalystsMOFs are attracting considerable attention for their use as both

the precursor and templates to prepare metal oxides or carbon-based composites for photocatalytic CO2 conversion. For instance,gold nanoparticles (GNPs)/NH2-MIL-125 was pyrolyzed to attainGNPs/TiO2 photocatalyst for CH4 production (Fig. 13a) [196]. Asreported, the obtained TiO2 displayed the similar morphologyand size as the parent MOF crystals, which enriched the conven-tional techniques for the GNP/TiO2 preparation in a controlledmanner. Later, porous ZnO@Co3O4 composite was prepared byusing the core–shell ZIF-8@ZIF-67 as the precursor (Fig. 13b)[197]. The results unveiled that the CH4 generation rate overZnO@Co3O4 composite was much higher than that of commercial

ZnO and TiO2 (P25) due to its advantageous porous structure andthe co-catalytic properties of its constituents (ZnO and Co3O4).Thus, these MOF derivatives have enriched the selection of promis-ing photocatalysts for CO2 reduction into organic chemicals.

4. The role of CO2 capture capacities of MOF-based materials inphotocatalytic CO2 reduction

Different from other catalytic materials, MOFs are particularand very promising in CO2 capture as their structures can be pre-cisely tailored to create interactions with CO2 molecules. Thesound CO2 capture capability of MOFs plays a significant role inits diverse catalytic conversions [115], including CO2 photoreduc-tion. To provide the guidelines for rational design of efficient pho-tocatalysts for CO2 reduction, in this part, the relationship betweenCO2 capture and photocatalytic CO2 reduction performance withMOF-based materials will be discussed. Particular emphasis isplaced on the outstanding of sound CO2 reduction performanceeven under low CO2 concentrations.

To improve CO2 uptake of MOFs, the introduction of aminogroup onto the organic linker is a well-established strategy. Theincreased CO2 capture of MOFs will then lead to enhanced photo-catalytic CO2 reduction activity. For instance, the amount of CO2

uptake by amine-functionalized Ti-MOF [NH2-MIL-125(Ti)]increased from 98.6 to 132.2 cm3 g�1 [117]. Similarly, amino-functionalized NH2-UiO-66(Zr) shows a higher adsorption capabil-ity toward CO2 than its parent UiO-66(Zr) [158]. Accordingly, thebetter CO2 uptake properties bring about more HCOO� productgenerated under visible light irradiation. As illustrated above, theintroduction of metalloligands will increase the number of activesites and extend the light response of the catalytic system. Also,this strategy can strengthen the interaction between the frame-work and CO2 molecules and thus boost CO2 adsorption. As copper

Fig. 12. Schematic showing the structure of UiO-67-Mn(bpy)(CO)3Br and Proposed mechanism for the formation of HCOOH. Reproduce from Ref. [193] with permission fromthe American Chemical Society, copyright 2015.

Fig. 13. (a) Schematic showing the preparation of GNPs/NH2-MIL-125-derived GNPs/TiO2. Reproduce from Ref. [196] with permission from the American Chemical Society,copyright 2015. (b) The fabrication of ZIF-8-derived ZnO and the ZIF-8@ZIF-67-derived ZnO@Co3O4. Adapted from Ref. [197] with permission from Royal Society of Chemistry,copyright 2016.

12 D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262

D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262 13

organic complexes are well-known for their assistance in CO2 cap-ture and activation [198], the amount of CO2 adsorption capacityover Cu porphyrin-based MOF (SCu) reached 277.4 mg/g (1 atm,298 K), which is much higher than that of an analogous MOF with-out Cu2+ (SP) (153.1 mg/g) [176]. Moreover, the chemical adsorp-tion of CO2 with SCu gives rise to lower reaction barrier andimproved photocatalytic efficiency. In addition, the enhanced CO2

uptake of MOFs can also be realized by incorporating coordina-tively unsaturated single sites in MOFs. As presented by Ye andcoworkers, the CO2 adsorption behavior of MOF-525-Co is higherthan that of MOF-525-Zn and the parent MOF-525 [178]. It isdemonstrated that the introduction of unsaturated single Co sites

Fig. 14. (a) Photochemical reduction of CO2 with RuII-CO (1) and UiO-67/RuCO (3): leftReprinted from Ref. [102] with permission from Wiley-VCH, copyright 2016. (b) Comparand 0.1 atm. (c) Comparison of the CO2 and H2O binding energies of reduced MAF-X27-Chemical Society, copyright 2018. (d) CO2 photoreduction performance over Ni MOLs anMOLs and Co MOLs. Adapted from Ref. [103] with permission from Wiley-VCH, copyrightand diluted CO2. Adapted from Ref. [200] with permission from Elsevier, copyright 2020

enhanced CO2 adsorption based on the open sites of Co porphyrins,thus realizing the activation of molecular CO2.

Not limited to the above typical strategy of linker functionaliza-tion, the increase of the CO2 capture capacities of MOF-based mate-rials for enhanced CO2 photoreduction has also been achieved by avariety of approaches, including the functionalization of metalclusters of MOFs [175], the fabrication of MOF composites [185]and MOF derivatives [155]. As revealed in the above reports, thehigh CO2 capture capacities of MOF-based photocatalysts lead tohigh CO2 concentration level around the active sites, thus facilitat-ing the photocatalytic reactions, which can be further proved bythe outstanding CO2 conversion performance even under low CO2

y-axis, catalytic activity (bar graph); right y-axis, product selectivity (line graph).ison of the TOF values of MAF-X27-Cl, MOF-74-Co and MAF-X27-OH under 1.0 atmCl and MAF-X27-OH. Adapted from Ref. [104] with permission from the Americand Co MOLs in pure CO2 and diluted CO2. (e) CO2 and H2O adsorption energies of Ni2018. (f) Comparison of CO2 photoreduction performance over NiCo2O4 in pure CO2

.

14 D. Li et al. / Coordination Chemistry Reviews 412 (2020) 213262

concentrations. Several typical examples are discussed as followsto illustrate this issue.

The photoactive RuII-CO complex ([RuII(5,50-dcbpy)(tpy)(CO)](PF6)2) modified UiO-67 (UiO-67/RuCO) uptakes higher CO2 thanunmodified UiO-67 due to the decreased pore size and increasedaffinity sites for adsorbates [102]. In stark contrast to the drasti-cally reduced photocatalytic performance of the homogeneous cat-alyst RuII-CO with the decreased partial CO2 pressure, UiO-67/RuCO affords almost unchanged catalytic activity even under lowCO2 concentrations (5% CO2) (Fig. 14a). It clearly demonstrates thatUiO-67/RuCO is able to concentrate CO2 molecules around theactive sites for subsequent reaction. Similarly, the catalystPt/Au@Pd@MOF-4 photocatalyzes the conversion CO2 to CH4 atlow CO2 concentration (800 ppm) [199]. Besides catalytic activity,product selectivity is another point of interest in the evaluationfor CO2 photoreduction. As presented by Zhang, Liao and cowork-ers, a series of isostructural/isoreticular honeycomb-like MOFs(MAF-X27-Cl, MAF-X27-OH and MOF-74-Co) were synthesizedfor photocatalytic CO2-to-CO conversion [104]. Notably, theydemonstrate that the turnover frequencies (TOFs) of MAF-X27-Cland MOF-74-Co decrease significantly by more than 90% whenthe CO2 partial pressure was reduced to 0.1 atm. In sharp contrast,under similar CO2 pressure, MAF-X27-OH gives a comparable TOF(23 � 10-3 s�1) with that of 1 atm CO2 (28 � 10-3 s�1) (Fig. 14b).Further DFT calculations reveal that the strong CO2 binding affinityof l-OH- ligands neighboring the cobalt center within MAF-X27-OH can stabilize the initial Co-CO2 adduct to promote the CO2

reduction (Fig. 14c), endowing an unchanged CO selectivity of97.2% under low CO2 concentration. In another work, a Ni metal–organic framework monolayers (Ni MOLs) was reported for pho-toreduction of diluted CO2 with a CO selectivity of 96.8%(Fig. 14d) [103]. Experimental and theoretical investigations showthat the strong CO2 while weak H2O adsorption energy of catalystNi MOLs are responsible (Fig. 14e), which stabilizes the Ni-CO2

adducts and suppresses the Ni-H2O adducts, facilitating CO2-to-CO conversion with high selectivity. More recently, hierarchicalNiCo2O4 hollow nanocages have been constructed based on theassembly of NiCo Layered Double Hydroxide (LDHs), which areobtained by using ZIF-67 as the precursor, for diluted CO2 photore-duction [200]. The reported NiCo2O4 features abundant coordina-tively unsaturated Ni active sites, which facilitate CO2 uptake andactivation, resulting in light-driven CO2-to-CO conversion in pureand diluted CO2 (Fig. 14f). All these results further provide a high-lighted window for getting insight into the relationship betweenCO2 capture capacity of MOF-based materials and their photocat-alytic performance for CO2 reduction.

5. Conclusions and perspectives

This review summarizes the recent advances of MOF-basedphotocatalysts in CO2 reduction on the basis of the classificationof obtained products. Benefiting from the structural tunability ofMOFs, various strategies including metal substitution, the intro-duction of visible-light responsive units and ligand replacementhave been explored to improve light harvesting and CO2 conver-sion. Moreover, the photocatalytic activity can be enhanced byintegrating homogenous catalysts, photosensitizer molecules orsemiconductors with MOFs. Encapsulation of diverse active unitswithin MOFs is also a promising method to boost the photocat-alytic performance [150]. In some cases, the chemical or thermalconversion of MOFs is an effective strategy to produce promisingphotocatalysts for CO2 reduction. In particular, MOF-based materi-als hold great promise for photocatalytic CO2 reduction especiallydue to their outstanding CO2 capture capacities. Indeed, MOF-based materials hold their particular advantages in light-driven

CO2 reduction and all of these effective modification strategies willprovide the guidelines to the rational design of MOF-based photo-catalysts with enhanced catalytic performance.

Despite the fact that the great progress has been made in thedevelopment of MOF-based photocatalysts for CO2 reduction, somechallenges and more endeavors need to be further made. First,proper MOF-based materials for the photocatalytic CO2 conversionare still very limited. Single-metal atom or single-site photocata-lysts confined in MOFs is recognized as a promising way toimprove photocatalytic performance [178,201,202]. Besides, thefabrication of 2D MOFs and hierarchically porous MOFs with moreexposed active sites is also demonstrated as a valid platform toimprove the catalytic performance. These strategies open the doorsto explore novel MOF photocatalysts with stable and high photo-catalytic activity for CO2 conversion. Second, to date, photocat-alytic CO2 reduction over MOF-based materials requires certainconditions, e.g. organic solvents with the presence of sacrificialagents, which is not environmental friendly and not the final tar-get. Exploration of MOF photocatalysts for CO2 reduction coupledwith the oxidation of water [175] or organics to generate valuablechemicals [203] is recognized to be a sustainable strategy. Third,the mechanism for photocatalytic CO2 reduction over MOFs isnot well clear. Advanced characterization (e.g. transient absorptiontechniques and in-situ characterization) and density functionaltheory calculations should be conducted for deep understandingof the structure–activity relationships, which are of great signifi-cance for the rational design of MOF photocatalysts with optimumstructures to realize CO2 reduction with superior performance. Lastbut not the least, the main products of MOF-based photocatalystsfor CO2 conversion are C1 chemicals (including CO, CH4, HCOOHand CH3OH) instead of the much more valuable C2+ products (hy-drocarbons with more than two carbons). Photoelectrocatalytic isappearing with a great potential for reduction of CO2 to C2 productin aqueous solution [204]. In addition, the photothermal effect isan alternative approach for the efficient CO2 reduction by localizedheating to reduce reaction temperature for CO2 hydrogenation[77,205]. The electro/thermal-assisted photocatalysis over MOF-based materials would be an important solution to allow the reac-tions to proceed for C2+ chemicals production.

In summary, the current state-of-art has shown that MOFs arepromising materials for photocatalytic CO2 reduction. Since chal-lenges and opportunities coexist, we do believe that with the rapiddevelopment of MOF materials, it will stimulate new perspectivesin MOF-based photocatalysts for CO2 reduction and fulfill therequirements for practical applications in the not-too-distantfuture.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

Acknowledgments

This work was supported by the NSFC (21725101, 21673213,21701160, 21871244, and 21521001), DNL Cooperation Fund,CAS (DNL201911) and the Hefei National Laboratory for PhysicalSciences at the Microscale (KF2019002).

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