A hexanuclear cobalt metal–organic framework for efficient CO2 reduction under visible light

Increasing global challenges including climate warming and energy shortage have stimulated worldwide explorations for efficient materials for applications in the capture of CO2 and its conversion to chemicals. In this study, a novel pillared-layer porous metal–organic framework (Co6–MOF) with high nuclearity CoII clusters has been synthesized. This material exhibited a CO2 adsorption capacity of up to 55.24 cm3 g−1 and 38.17 cm3 g−1 at 273 K and 298 K, respectively. In a heterogeneous photocatalytic system of CO2 reduction, this material, co-operated with a ruthenium-based photosensitizer, can efficiently realize CO2 to CO conversion. Under visible-light irradiation for 3 hours, 39.36 μmol CO and 28.13 μmol H2 were obtained. This result is higher than those of most of the reported MOF materials under similar conditions and to the best of our knowledge, this is the first example of a high nuclear MOF used in CO2 reduction. The rooted reasons behind the high reactivity were studied through theoretical calculation studies. The results showed that electrons on reduced [Ru(bpy)3]Cl2·6H2O (bpy = 4,4′-bipyridine) could transfer to the Co6–MOF and the adsorbed CO2 molecule on the charged Co6–MOF could be activated more facilely. This work not only clarifies the reasons for high efficiency of the CO2 photoreduction system but also points out to us the direction for designing more effective MOF materials as photocatalysts for artificial CO2 photoreduction.


Introduction
In recent years, climate warming has become a global environmental problem which primarily results from excessive emissions of CO 2 from fossil fuel combustion. [1][2][3] To solve this issue, tremendous efforts have been devoted to reducing CO 2 concentration in air. CO 2 capture and sequestration (CCS) technologies using porous materials as sorbent materials are an accepted and working approach. [4][5][6] One kind of the outstanding porous material in this eld is metal-organic frameworks (MOFs) which are constructed from organic ligands and metal ions or metal clusters. Beneting from large surface areas and tailorable structures, these hybrid porous materials show intriguing applications in various elds including chemical separation, 7 catalysis, 8 drug delivery, 9 optical sensing detection, 10 and gas storage, 11,12 particularly in CO 2 capture. 1 The CO 2 adsorption of MOF materials is related to their pore size and shape, surface characteristics and porous functionalities. One promising strategy for increasing CO 2 adsorption is to introduce N-rich aromatic ligands. 13 Beyond the CCS route, the catalytic transformation of the captured CO 2 into value-added chemicals could be a more suitable alternative and sustainable method as CO 2 represents an abundant and inexpensive carbon source. [11][12][13] Owing to the chemical inertness of the CO 2 molecule, reducing it into chemical feedstocks needs high energy and appropriate catalysts. [14][15][16][17] Utilizing clean and renewable solar energy as the power source to accomplish this conversion could be the most economical and environmentally friendly choice. 18 Therefore, the exploration of high-performance photocatalytic systems for CO 2 reduction has been the fundamental goal of experts. In photocatalytic systems, there are three essential steps to achieve the transition from solar energy to chemical energy, [19][20][21][22] including light harvesting, generation of electron-hole pairs, and transferring redox equivalents to reactive centers for redox reactions. Noble metal complexes, such as Ru and Re complexes, are widely utilized as light harvesters in various photocatalytic systems, including water splitting [23][24][25] and CO 2 reduction. [26][27][28][29] Transition metals with multiple redox states and organic ligands are essential to form electron-transport chains together with light harvesters accelerating CO 2 reduction. For instance, cobalt complexes and oxides have been reported to serve as co-catalysts enhancing CO 2 photoconversion through boosting charge separation and surface reaction. 30,31 In addition to the three aspects above, for the gaseous reactions, adsorption and activation of CO 2 molecules are especially crucial in practical applications since effective charge transfer between catalytic centers and CO 2 molecules lies in their intimate and stable binding. 31 Therefore, in the process of CO 2 reduction, it is important to combine the adsorption and activation of CO 2 with transition-metal catalysis. Transition-metal MOFs with excellent capability for CO 2 adsorption might be materials with incomparable advantage in CO 2 reduction. Employing a classical cobalt-containing MOF material, Co-ZIF-9 (ZIF ¼ zeolitic imidazolate framework), as a co-catalyst 27 and [Ru(bpy) 3 ] Cl 2 $6H 2 O as a photosensitizer, Wang et al. reported an efficient CO 2 reduction system under visible light. In spite of this, the design and synthesis of a kind of MOF material with excellent capability for CO 2 adsorption meanwhile maintaining the highly matched energy band with a photosensitizer is a signicant challenge. [32][33][34][35] What's more, the mechanism in such CO 2 reduction systems needs to be revealed.
Herein, we report the synthesis of a hexanuclear cobaltcluster MOF [Co 3 (OH) 3 (NTB)(4,4 0 -bpy) 1.5 ] DEF (Co 6 -MOF, NTB ¼ 4,4 0 ,4 00 -nitrilotribenzoic acid ligands, 4,4 0 -bpy ¼ 4,4 0 -bipyridine, and DEF ¼ N,N-diethylformamide), in which the 2D layer is constituted by Co 6 (m 3 -OH) 6 clusters and NTB and the pillar is the 4,4 0 -bpy ligand. This novel porous material acting as a heterogeneous co-catalyst exhibits efficient CO 2 photochemical reduction by cooperating with a ruthenium-based photosensitizer. Under visible-light irradiation for 3 hours, 39.36 mmol CO and 28.13 mmol H 2 were obtained. Based on the theoretical calculation, the reaction mechanism was studied which indicated that the excited electrons can be efficiently transferred from the lowest unoccupied molecular orbital (LUMO) of the ruthenium-based photosensitizer to the LUMO of the hexanuclear cobalt-cluster MOF. And aer accepting electrons, the activation of the CO 2 molecule is facilitated by the charged Co 6 -MOF.

Experimental section
Materials Co(CH 3 CHO) 2 $6H 2 O, NTB and 4,4 0 -bpy for the synthesis were purchased from Alfa, and they were used as received. The purity of all reagents was of analytical grade.

Instrumentation
The C, H and N elemental analyses were performed on a Perkin-Elmer 2400 CHN elemental analyzer. The FT-IR spectra were examined in the range of 4000-400 cm À1 on a Mattson Alpha-Centauri spectrometer using KBr pellets. The UV-Vis absorption spectra were acquired on a Shimadzu UV-2550 spectrophotometer in the wavelength range of 200-800 nm. Powder Xray diffraction (PXRD) patterns were collected with an X-ray diffractometer of Rigaku, Rint 2000. Thermogravimetric analyses (TGA) were carried out on a Perkin-Elmer TG-7 analyzer heated from room temperature to 1000 C under a nitrogen gas atmosphere at a heating rate of 5 C min À1 .

X-ray crystallography study
Single-crystal X-ray diffraction data for the Co 6 -MOF were recorded by using a Bruker Apex CCD diffractometer with graphite-monochromated Mo Ka radiation (l ¼ 0.71069Å) at 293 K. Absorption corrections were applied using a multi-scan technique. All the structures were solved by the direct method of SHELXS-2014 and rened by full-matrix least-squares techniques using the SHELXL-2014 program within WinGX. Nonhydrogen atoms were rened with anisotropic temperature parameters. CCDC 1533801 for the Co 6 -MOF contains the supplementary crystallographic data for the work. †

Gas sorption experiments
The N 2 and CO 2 sorption measurements were performed on automatic volumetric adsorption equipment (Belsorp mini II). The solvent-removal assay has been employed before the gas adsorption measurements. The fresh as-synthesized samples were immersed in CH 2 Cl 2 for 48 h, and then transferred to CH 3 OH for 48 h to remove the DEF. Subsequently, the samples were gathered by decanting CH 2 Cl 2 similarly to remove CH 3 OH. When the CH 2 Cl 2 was removed by decanting, these samples were activated by heating under a dynamic vacuum at 100 C for 24 h to form the activated samples. Before the gas adsorption test, the samples needed to be dried again by using the 'outgas' function of the surface area analyzer for 12 h at 150 C. Aer activation, the samples were tested for N 2 and CO 2 sorption measurements.

Photocatalytic test
Photocatalytic reactions were carried out in a quartz tube with a cap at 1 atm CO 2 partial pressure. The reaction system contained [Ru(bpy) 3 ]Cl 2 $6H 2 O (0.01 mmol), Co 6 -MOF (0.005 mmol), acetonitrile (4 mL), H 2 O (1 mL), and triethanolamine (TEOA 1 mL). Aer the solution was puried by CO 2 for 10 min, it was beamed by a 150 W xenon lamp at 420 # l # 780 nm in a carousel irradiation apparatus. The reaction temperature was maintained at 35 C by using a water bath. The result of PXRD shows that at the end of the reaction, the crystal retains the original structure (Fig. S11 †). Aer the catalytic reaction completed, the gases (CO and H 2 ) were tested and analyzed using a GC instrument. To detect the formation of carbon monoxide from the reaction mixture, 500 mL from the middle of the test tube was taken out with a syringe and injected into a GC with a FID detector, using argon as the carrier gas and reference gas. The volume of the carbon monoxide produced was calculated by comparing the integrated area of the signals of carbon monoxide with a calibration curve. The injector and detector temperatures were set to 60 C. The retention time of hydrogen was 1.9 min. To detect the formation of hydrogen from the reaction mixture, 1000 mL of the headspace of the test tube was taken out with a syringe and injected into a GC with a TCD detector, using a 5Å molecular sieve column and argon as the carrier gas and reference gas. The volume of the hydrogen produced was calculated by comparing the integrated area of the signals of hydrogen with a calibration curve. The injector and detector temperatures were set to 60 C. The retention time of hydrogen was 0.7 min. All the photocatalytic reactions were repeated ve times to conrm the reliability of the data.

Crystal structures
The Co 6 -MOF was prepared through admixing 4,4 0 -bpy, NTB ligands and Co(CH 3 CHO) 2 $6H 2 O in DEF heated at 120 C for 2 days. The reaction produced violet crystals with a polyhedral morphology. The structures of the Co 6 -MOF were conrmed by single X-ray crystallography. Single-crystal X-ray diffraction analysis reveals that the Co 6 -MOF crystallized in the hexagonal P6 3 /mcm space group (Table S2 †). In the asymmetric unit, there are a half Co ion, one-sixth NTB ligands and one-fourth 4,4 0 -bpy ligands. The solvent DEF molecules in the channels are not crystallographically well dened. Six Co cations and six m 3 -OH groups construct Co 6 (m 3 -OH) 6 clusters serving as secondary building units (SBUs) in the structure. Each Co ion in the Co 6 (m 3 -OH) 6 clusters adopts six coordination connecting with one nitrogen atom of the 4,4 0 -bpy ligand and ve oxygen atoms provided by two carboxylates of different NTB ligands and three m 3 -OH groups ( Fig. 1(a)). Besides, each 4,4 0 -bpy ligand binds to two Co atoms in two Co 6 (m 3 -OH) 6 SBUs through N atoms in the 4,4 0 -position (Fig. S1 †). Each NTB ligand act as a 3-connecting node linked with three Co 6 (m 3 -OH) 6 SBUs forming twodimensional layers (Fig. S2 †). The adjacent Co 6 (m 3 -OH) 6 -NTB layers are further pillared by the bpy triangular prism ( Fig. 1(a)), constituting an overall 3D columnar supporting structure (Fig. S5 †). It is worth noting that such a triangular prism pillared-layer structure enhanced the stability of the architecture. The pillared layer structure contains three different types of channel continuity along a, b and c axes ( Fig. 1(b), (c) and S3 †) with the dimensions of 11.31 Â 12.56Å (a Â c or b Â c) (Fig.-S4(a) †) and 8.89 Â 8.89Å (a Â b) (Fig. S4(b) †). Topological analysis shows that the Co 6 (m 3 -OH) 6 SBUs can be regarded as 8connected nodes, and the NTB ligand was dened as a 3-connected node, so the architecture of the Co 6 -MOF can be reduced to a 3,8-connected tfz-d (a-UO 3 ) type topology with the point symbol of (4 3 ) 2 (4 6 .6 18 .8 4 ) (Fig. 1(d)). Many MOFs with the tfz-d type topology have been reported. [36][37][38] In this type of topology, 3-connected and 8-connected nodes exist simultaneously, and they will form a double connected 3D structure. PLATON calculation shows that the effective pore volume of the Co 6 -MOF is about 63.9% (4682.8Å 3 ) per unit cell (7324.9Å 3 ), which is occupied by DEF molecules.

Chemical and thermal stability
The purity of the synthesized Co 6 -MOF was veried by high similarities between simulated and experimental powder X-ray diffraction (PXRD) patterns. In order to determine the possibility of the practical application of the Co 6 -MOF, we investigated its stability toward moisture. The PXRD patterns still exhibit high similarities compared with simulated PXRD patterns aer soaking in water for 24 h. In addition, the acid resistance and alkali resistance of the Co 6 -MOF were also investigated. Surprisingly, aer placing it in a hydrochloric acid solution (pH ¼ 2) and sodium hydroxide solution (pH ¼ 12) for 24 h, it exhibits excellent acid and alkali resistance revealed by the similar shape and intensity of the PXRD (Fig. S6 †). Such high acid resistance and alkali resistance are quite rare in reported MOFs. 39 The thermal stability of the Co 6 -MOF is revealed through the TG curve. As shown in Fig. S7, † an evident weight loss of 11.2% was observed in the step from 270 to 285 C, which may be ascribed to the release of solvent molecules. Aer 285 C, the framework of the Co 6 -MOF starts to decompose.

Gas adsorption properties
To study the porosity and CO 2 adsorption capability of the Co 6 -MOF, we performed the N 2 and CO 2 adsorption experiments  (Fig. 2). The samples were adequately activated by the method represented in the Experimental section. The PXRD of the activated sample showed similar peaks to the as-synthesized sample, indicating the maintenance of the framework. N 2 adsorption of the activated Co 6 -MOF was carried out at 77 K. The adsorption isotherm of N 2 unfolds a characteristic type I behavior with good reversibility for microporous solids. The Brunauer-Emmett-Teller (BET) and Langmuir surface areas are 1957.5 m 2 g À1 and 2257.1 m 2 g À1 , respectively, calculated using the N 2 adsorption isotherms, and the total pore volume of the Co 6 -MOF is 0.87 cm 3 g À1 . The N 2 adsorption amount of the Co 6 -MOF at 1 bar is approximately 568.1 cm 3 g À1 , which is equivalent to 42.1 N 2 per unit cell, lower than the theoretical pore volume calculated using PLATON based on the single crystal X-ray diffraction data. In order to prove that the Co 6 -MOF structure remains intact in water, we performed N 2 sorption of the Co 6 -MOF which was soaked in water for 24 h. The corresponding adsorption isotherm of this sample is presented in Fig. S9 † and the result shows that its BET surface area is similar to that of the activated sample indicating that the structure of the Co 6 -MOF is maintained well aer being soaked in water.
The CO 2 sorption behaviors of the Co 6 -MOF were measured at 273 K and 298 K, respectively, and the sorption isotherms are shown in Fig. 2(b). At 1 atm and 273 K, the Co 6 -MOF has CO 2 uptakes at a saturation of 55.24 cm 3 g À1 , based on the total weight. And at 298 K, the CO 2 uptakes decreased slightly, up to 38.17 cm 3 g À1 (Fig. 2(b)). This capacity is comparable to classical MOFs Uio-66 and NH 2 -Uio-66 which were employed as catalysts for CO 2 reduction. 40 Photochemical CO 2 reduction properties The photocatalytic CO 2 reduction system consists of Co 6 -MOF, [Ru(bpy) 3 ]Cl 2 $6H 2 O, and TEOA (triethanolamine). The solvents of the photocatalytic system lled with atmospheric CO 2 are MeCN and H 2 O. Aer irradiation with visible light (l $ 420 nm) for 30 min, the reduction system generated CO (3.73 mmol) and H 2 (2.81 mmol) gases. In this reduction process, CO 2 was split into CO at a reaction rate of 0.13 mmol min À1 , along with the evolution of H 2 at a rate of 0.10 mmol min À1 (Fig. 3). Trace CH 3 OH was detected in the solution. With increasing reaction time, the evolution amount of CO and H 2 enhances consistently (Fig. 3). Aer 3 hours' irradiation, the total amount of CO and H 2 reaches 39.35 and 28.13 mmol (entry 1, Table 1). This activity is much higher than those of many classical MOF materials for converting CO 2 to CO listed in Table S1. † 20,27 The photochemical quantum yield of CO (F CO ) of the Co 6 -MOF under irradiation of 420 nm light was calculated as 0.758% (the detailed calculation is given in the ESI †).
In order to determine the importance of the components in the photocatalytic CO 2 reduction system, we provided a series of reference experiments and the results are summed up in Table  1. As we have seen, in total darkness, CO or H 2 was undetectable in the photocatalytic system (entry 2, Table 1). When the system was short of [Ru(bpy) 3 ]Cl 2 $6H 2 O, no gases were detected in the reaction system (entry 3, Table 1). Replacing [Ru(bpy) 3 ] Cl 2 $6H 2 O with tris(2-phenylpyridinato)iridium(III), CO gas wasn't detected in the reaction while only a trace amount of H 2 was produced (entry 13, Table 1). These results mentioned above indicate the necessity of the energy band matching of the photosensitizer and MOFs in the photocatalytic CO 2 reduction reaction. This was further ascertained by the fact that no CO or   In the experimental study, we also explored the inuence of the amount of Co 6 -MOF on the photocatalytic properties in the system (Fig. 4). In this gure, we can see that the yield of CO and H 2 changed obviously when the added amount of Co 6 -MOF changed a little. The optimum amount of Co 6 -MOF is 3 mg with maximum values of both CO and H 2 . Overall, these results have proved the major role of the Co 6 -MOF by fastening CO 2 acting as a CO 2 redox promoter and an absorber in the photocatalytic reduction system.
In the absence of the Co 6 -MOF, the turnover number decreases to 3.37, which is signicantly lower than that of the catalytic system containing [Ru(bpy) 3 ]Cl 2 $6H 2 O and Co 6 -MOF (entry 4, Table 1). To further conrm the role of the formation of MOFs as a co-catalyst in the photocatalytic reaction system, we destroyed the Co 6 -MOF by calcination at 900 C in N 2 gas, and then the residues were applied in the reaction system instead of the Co 6 -MOF (entry 5, Table 1). The results showed that CO and H 2 production decreased dramatically. This indicates the necessity of the complete framework structure of the Co 6 -MOF in the CO 2 splitting, possibly by the promotion of substrate concentration and carrier transfer. To further conrm the role of the formation of MOFs as a co-catalyst in the photocatalytic reaction system, we measured the photocatalytic efficiency of physical mixtures including Co 2+ and the ligand in a homogeneous system. We use 4,4 0 -bpy, NTB, and Co(CH 3 CHO) 2 $6H 2 O to replace the Co 6 -MOF, and the result reected that their physical mixture also plays a catalytic role in the photocatalytic reaction system, but with a lower TON than those with the Co 6 -MOF (entry 6, Table 1). These results noted above conrmed that as a heterogeneous co-catalyst, the Co 6 -MOF effectively promotes the efficiency of the photochemical reaction.
In the experiment of photocatalytic reaction, the participation of CO 2 was investigated by replacing CO 2 with N 2 (entry 7, Table 1). Once CO 2 was replaced by N 2 , the evolution of CO was not observed, and only 2.37 mmol H 2 was detected under the same reaction conditions. It is noticeable that when there was no CO 2 in the system, light-induced electrons could just promote H 2 generation.
In the course of the experiment, we found that the reaction medium has a great inuence on the catalytic effect of the Co 6 -MOF on the CO 2 photocatalytic reduction reaction. No reaction occurred when only H 2 O was used as the reaction medium which might be due to the weak chemical affinity towards CO 2 molecules (entry 10, Table 1). Meanwhile, MeCN and DMF were favorable reaction media for the CO 2 reduction (entry 11 and entry 12, Table 1). They possessed nitrogen or oxygen atoms that can interact with and solubilize CO 2 via Lewis acid-base interactions. 41,42 In addition, we replaced the Co 6 -MOF with Co 3 O 4 -Uio-66 and the turnover number (3.87) decreased, becoming signicantly lower than that of the original catalytic system (entry 14, Table 1 and Fig. S13 †).
In order to unambiguously determine the source of the C atom of CO, we conducted an isotopic tracing experiment by replacing CO 2 by 13 CO 2 under the same photocatalytic reaction conditions, and we used gas chromatography mass spectrometry to analyse the evolution of CO. Aer irradiation with visible light for 30 min, the peak at 1.99 min and m/z 29 was assigned to 13 CO (Fig. S12 †) and no sign at m/z 28 was detected. It turns out to be the case that the Co 6 -MOF can indeed stimulate the deoxygenative reduction of CO 2 to CO.
The results above clearly demonstrate the effective role of the Co 6 -MOF in the photocatalytic system. The next important target is to understand the root reason in such a system. Inspired by Tang's work, 43 the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the [Ru(bpy) 3 ]Cl 2 photosensitizer and  Fig. 4 The effect of the amount of Co 6 -MOF on the evolution of CO and H 2 from the CO 2 photoreduction system. Co 6 -MOF are studied rst. According to ref. 43 the HOMO and LUMO of [Ru(bpy) 3 ]Cl 2 are À5.68 and À3.19 eV, respectively. Theoretical calculation is introduced to reveal the HOMO and LUMO of the Co 6 -MOF. The calculation result shows the HOMO and LUMO of the Co 6 -MOF to be À6. 22 and À4.56 eV (Fig. S14 †). Since the LUMO of the Co 6 -MOF is lower than that of [Ru(bpy) 3 ]Cl 2 , the electrons in the LUMO of [Ru(bpy) 3 ]Cl 2 can be transferred to the LUMO of the Co 6 -MOF (Fig. 5).
As described above, the Co 6 -MOF has the capability of CO 2 adsorption, and the adsorption energy was investigated as well. As the density functional theory (DFT) calculation shows, the adsorption energy of the Co 6 -MOF in CO 2 adsorption is 0.275 eV since the activation of the CO 2 molecule is usually triggered by two-electron charges. 44 From rst-principles simulations, we calculated the adsorption energy changes aer the Co 6 -MOF accepted one or two electrons. The result shows that when receiving one electron, the energy increases from 0.275 to 0.361 eV. When obtaining two electrons, the energy further enhances to 0.907 eV. What's more, we simulated the potential energy surfaces with the variation of the O-C bond of CO 2 adsorbed on the Co 6 -MOF. From the simulation results, we can see that one-electron charge can hardly change the activation energy barrier (E B ) of CO 2 while two-electron charge can effectively reduce the E B from 5.66 to 4.98 eV (Fig. 5(c) and S15 †). Therefore, upon receiving electrons from the [Ru(bpy) 3 ]Cl 2 photosensitizer, the Co 6 -MOF can turn into an active photocatalyst material for the CO 2 activation further facilitating CO 2 reduction.
On the basis of the above results, a possible rooted reason for CO 2 reduction in the Co 6 -MOF participated system was proposed. Under irradiation, the photosensitizer [Ru(bpy) 3 ]Cl 2 is excited to the excited state. This excited state was then quenched reductively by the sacricial electron donor, TEOA 40 ( Fig. 5(a)), and formed a reduced photosensitizer. Then the electrons in the reduced photosensitizer transferred to the Co 6 -MOF. The adsorbed CO 2 molecule on the Co 6 -MOF was activated aer the Co 6 -MOF obtained the electrons. In the end, CO 2 is reduced to CO and released from the Co 6 -MOF.

Conclusions
In conclusion, based on highly symmetrical Co 6 (m 3 -OH) 6 cluster SBUs and organic ligands of NTB and 4,4 0 -bpy, a pillared-layer hexanuclear cobalt MOF, Co 6 -MOF, was obtained possessing a nanoporous structure. With this merit, the Co 6 -MOF displays a satisfactory amount of CO 2 uptake, up to 55.24 cm 3 g À1 . Under irradiation with visible light, the Co 6 -MOF can be used as a stable co-catalyst coupled to an appropriate photosensitizer ([Ru(bpy) 3 ]Cl 2 $6H 2 O) realizing the photocatalytic reduction of CO 2 to CO. Around 39.36 mmol CO and 28.13 mmol H 2 were produced aer 3 hours of irradiation. This result is higher than those of most of the reported classic MOF materials under similar conditions. To the best of our knowledge, this is the rst example of the use of a high nuclear MOF in CO 2 reduction. A possible mechanism was proposed through theoretical calculation studies. The results showed that electrons on reduced [Ru(bpy) 3 ]Cl 2 $6H 2 O could transfer to the Co 6 -MOF and the adsorbed CO 2 molecule on the charged Co 6 -MOF could be activated more facilely. The rooted reasons behind the high reactivity were revealed through theoretical calculation studies which showed that electrons on reduced [Ru(bpy) 3 ]Cl 2 $6H 2 O could transfer to the Co 6 -MOF and the adsorbed CO 2 molecule on the charged Co 6 -MOF could be activated more facilely. This work not only claries the reasons for the high efficiency of the CO 2 photoreduction system but also points out to us the direction for designing more effective MOF materials as photocatalysts for articial photochemical CO 2 reduction.