Solvent-Free Photoreduction of CO2 to CO Catalyzed by Fe

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Article Cite This: Inorg. Chem. 2019, 58, 8517−8524

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Solvent-Free Photoreduction of CO2 to CO Catalyzed by Fe-MOFs with Superior Selectivity Xiao-Yao Dao, Jin-Han Guo, Yuan-Ping Wei, Fan Guo, Yi Liu, and Wei-Yin Sun* Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China

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ABSTRACT: It is deemed as a desired approach to utilize solar energy for the conversion of CO2 into valuable products, and the majority of the MOFs-based photocatalytic reductions of CO2 have focused on formic acid (HCOOH) production with an organic solvent as the reaction medium. Herein, we report a solvent-free reaction route for the photoreduction of CO2 catalyzed by Fe-MOFs, namely, NH2-MIL-53(Fe) [(Fe(OH)(NH 2−BDC)]•G, NH2-MIL-88B(Fe) [Fe3O(H2O)3(NH2−BDC)3]Cl•G, and NH2-MIL-101(Fe) [Fe3O(H2O)3(NH2−BDC)3]Cl•G (NH2−BDC = 2-aminoterephthalic acid; G = guest and/or solvent molecules). Compared with the orthodox reaction route, the present out-of-the-way photocatalytic reduction of CO2 with superior selectivity to CO occurs at the gas−solid interface. The reaction procedure is environmentally friendly and provides a possibility to address the CO2 emission problem. Importantly, NH2-MIL-101(Fe) shows the highest photocatalytic activity among these Fe-MOFs due to its efficient charge separation and electron transfer.

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tunable functionality.15,16 Due to their inherently large surface areas, controllable pores, and confined catalytic active sites, MOFs materials possess unparalleled advantages as well as application scope.17−20 Interestingly, photoredox catalysis has shown great opportunities for CO2 transformation.12,21−23 In 2011 it was reported that for the photoreduction of CO2 in MOF, the photoreduction of CO2 to CO was realized by incorporating molecular catalyst ReI(CO)3(5,5′-dcbpy)Cl into the framework of UiO-67 using acetonitrile (MeCN) as the solvent and triethanolamine (TEOA) as the sacrificial agent.24 Subsequently, Li and co-workers presented the photocatalytic reduction of CO2 to HCOO− under visible light irradiation by using amino-functionalized MOFs, including NH2-MIL125(Ti), NH2−UiO-66(Zr), and Fe-based MOFs of aminofunctionalized NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe).25−27 What is more, Jiang et al. employed a mesoporous Zr-porphyrin MOF (PCN-222) for effective integration of CO2 capture and CO2 reduction to HCOO− under visible-light irradiation.28 Recently, Wang and coworkers developed a Co-containing ZIF-9 and coupled it with different materials such as Ru complex, CdS, and C3N4 for reduction of CO2 into CO with organic solvent as the reaction medium, and all of these photoreduction reactions give H2 as a byproduct.29−31 In spite of these excellent works, the reaction medium is organic solvent; furthermore, the photocatalytic

INTRODUCTION Carbon dioxide (CO2), a product of fuel combustion, is one of the main greenhouse gases and is rapidly increasing in the atmosphere, which may cause catastrophic consequences such as ice melting at the north and south poles, rising sea level, and increasing global temperature.1−4 Thus, CO2 capture and conversion technologies are considered as the most potential and effective strategies to deal with the problem.5−8 Among the varied technologies, artificial photosynthesis is an up-andcoming method to obtain solar-to-chemical energy conversion with new chemical bonds formation.9−12 Importantly, the photocatalytic reduction of CO2 can not only reduce the CO2 emission but also solve the energy problems. To date, traditional photocatalysts, such as inorganic catalysts and molecular complexes, have been developed for photocatalytic reduction of CO2. However, these materials endure some inevitable defects even though they have respective preponderances. Although most of inorganic catalysts exhibit high stability during the reaction, these materials are relatively inert in chemical reactivity and photogenerated charge carriers.13 As for the molecular complexes, the active sites can have high activity; however, the catalysts are usually difficult to recollect and recycle for reuse.14 Hence, it is essential to develop new effective materials combining the advantages of both inorganic and molecular complexes to overcome their drawbacks. Metal−organic frameworks (MOFs) have been demonstrated to be a promising class of hybrid materials constructed from metal centers and polydentate organic linkers with © 2019 American Chemical Society

Received: March 21, 2019 Published: June 11, 2019 8517

DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524

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Scheme 1. Schematic Illustration for the Photocatalytic CO2 Reduction in Solvent-Free System at the CO2/Fe-MOF Interfacea

(a) CO2 steel cylinder, (b) mass flow controller, (c) vacuum pump, (d) circulating pump, (e) photoreactor, (f) heating plate, (g) automatic gas sampler, (h) gas chromatograph, (i) pressure meter, and (j) light source. a

Figure 1. SEM images and XRD patterns of NH2-MIL-53(Fe) (A, a), NH2-MIL-88B(Fe) (B, b), and NH2-MIL-101(Fe) (C, c).

guest and/or solvent molecules) were employed as photocatalysts for photocatalytic reduction of CO2. Amino functional group was introduced into the framework to improve light absorption and enhance the affinity toward CO2. In this reaction system, MOF was uniformly dispersed into a glass fiber film with assistance of TEOA as a sacrificial electron donor for the photocatalytic CO2 conversion. Simultaneously, gas was circulated in the pipeline by a pump to increase the touch area between the catalyst and reaction gas. Hence, the solvent-free route generates a gas−solid interface at CO2 and MOF. Compared with a conventional gas−liquid−solid reaction route, the solvent-free, gas−solid interfacial route has advantages, such as being environmentally friendly and creating a large contact area, which benefits in situ CO2

CO2 reduction still remains at a relatively low efficiency, and the explorations were mainly concentrated on the HCOOH production. To improve the photocatalytic efficiency and enhance the selectivity of the products, great effects have been adopted, including the predesign of bridging ligand and modification of photocatalysts. However, it is still a challenge to update the reaction route of the photocatalysis, which is significant and attractively promising for enhancing CO2 conversion. Herein, we employ a solvent-free route to photoredox catalytic CO2 conversion (Scheme 1). In this strategy, three classical Fe-based MOFs of NH2-MIL-53(Fe) [(Fe(OH)(NH2−BDC)]•G, NH2-MIL-88B(Fe) [Fe3O(H2O)3(NH2− BDC)3]Cl•G, and NH2-MIL-101(Fe) [Fe3O(H2O)3(NH2− BDC)3]Cl•G (NH2−BDC = 2-aminoterephthalic acid; G = 8518

DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524

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Figure 2. XPS spectra of NH2-MIL-53(Fe) (a), NH2-MIL-88B(Fe) (b), and NH2-MIL-101(Fe) (c): survey (a), and Fe 2p (b) spectra.

transform infrared (FT-IR) spectra were recorded (Figure S2). Characteristic bands of NH2−BDC at 3510 and 3390 cm−1 were assigned to the asymmetric and symmetric stretching vibrations of the N−H bond, and blue-shifts were observed for the characteristic peaks of the amino group in the Fe-MOFs. In addition, the peaks around 1251 cm−1 correspond to the C−N stretching vibrations.34 Furthermore, shifts were also found for the asymmetric and symmetric stretching vibration bands of the coordinated carboxylate groups in Fe-MOFs compared with the ones in NH2−BDC.35 These results prove that the expected amino-functionalized Fe-MOFs have been successfully achieved. In addition to the EDX analysis, X-ray photoelectron spectroscopy (XPS) was further employed to examine the composition of the Fe-based MOFs (Figure 2a). It can be seen that the results of XPS match well with those of EDX. The high resolution spectra of Fe 2p (Figure 2b) show two distinct peaks corresponding to the Fe 2p1/2 and Fe 2p3/2,36−38 and the separation of 13.7 eV between them ensures the presence of Fe(III). In order to elucidate the semiconductor behavior of FeMOFs, the optical response of NH2-MIL-53(Fe), NH2-MIL88B(Fe), and NH2-MIL-101(Fe) was examined by UV−vis diffuse-reflectance spectroscopy (DRS). As shown in Figure 3, all samples have absorption in the visible region suggesting the feasibility of photocatalytic ability. Mott−Schottky measurements were further conducted. As displayed in Figure S3, the positive slopes of the samples imply that these MOFs possess an n-type semiconductor character.39 The flat-band potentials of NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL101(Fe) are determined to be −1.1, −0.73, and −0.78 V vs NHE, respectively (Figure S3), which are corresponding to the conduction band (CB) for n-type semiconductors. Given the more negative CB potentials of these MOFs than the redox potential of CO2 to CO (−0.53 V vs NHE), these Fe-MOFs may have potential for the photocatalytic reduction of CO2. Catalytic Performance. The photoreduction of CO2 catalyzed by Fe-MOFs was investigated in a solvent-free reaction system with dispersed catalyst in filter film by employing TEOA as a sacrificial electron donor. Figure 4a and 4b show the yield of photoreduction of CO2 within 5 h under visible light irradiation (400−780 nm). The nonlinear CO production curves in Figure 4a are attributed to the high

adsorption as well as providing a possibility for addressing the reduction of the CO2 emission problem.

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EXPERIMENTAL SECTION

Syntheses of Fe-MOFs: NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe). NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe) were synthesized according to the literature with some modifications.27,32 NH2-MIL-101(Fe) was prepared by reaction of NH2−BDC (0.1 mmol) with FeCl3·6H2O (0.2 mmol) in DMF (10 mL) at 110 °C for 24 h. NH2-MIL-88B(Fe) was prepared similarly to NH2-MIL-101(Fe) except for addition of methanol (2 mL) to DMF (8 mL). As for NH2-MIL-53(Fe), NH2−BDC and FeCl3·6H2O (1:1, 0.5 mmol) were dissolved in deionized (DI) water (10 mL) and stirred for 30 min, and then the mixture was transferred to a Teflonlined autoclave for the hydrothermal treatment at 150 °C for 24 h. After being cooled to room temperature, the resultant precipitates were separated by centrifugation and washed thoroughly with DMF and ethanol. The obtained solids were dried in a vacuum oven at 80 °C for 12 h for further characterization and investigation.

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RESULTS AND DISCUSSION Characterization of Catalysts. Fe-MOFs of NH2-MIL53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe) were synthesized through hydro-solvothermal reactions and characterized by varied methods. The morphologies of these assynthesized Fe-MOFs crystals were observed via scanning electron microscopy (SEM). As illustrated in Figure 1(A−C), NH2-MIL-53(Fe) exhibits favorable dispersibility but irregular shapes with micronano sizes and NH2-MIL-88B(Fe) presents a spindle-shaped morphology with an average size of 1 μm in length and 400 nm in diameter, while NH2-MIL-101(Fe) embodies uniform feature and representative octahedral morphology with an average edge length of around 1 μm. The crystalline structures of these as-prepared products were confirmed by powder X-ray diffraction (PXRD). The PXRD patterns of these Fe-based MOFs are shown in Figure 1(a−c), and the as-obtained samples display well-defined diffraction peaks that matched well with the simulated ones,32,33 manifesting the pure phase of the synthesized Fe-MOFs. Furthermore, scanning electron microscopy analysis associated with energy dispersive X-ray spectroscopy (EDX) indicates the existence of Fe, C, O, and N in Fe-MOFs (Figure S1). To further ensure the chemical composition and demonstrate the presence of amino functional groups in the Fe-MOFs, Fourier 8519

DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524

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inhibition activity was expressed in the replicated tests, implying that the catalyst is stable during the photocatalytic reactions. A slight decrease in photocatalytic activity may be partly caused by loss of photocatalyst during each evacuation. Subsequently, the used catalysis was collected by centrifugation and filtration, and the PXRD patterns of the fresh and reused NH2-MIL-101(Fe) samples match well (Figure 5b), further emphasizing the considerable durability of the photocatalyst. The employed Fe-MOFs consist of Fe(III) and NH2−BDC linkers but different framework structures (Figure S6).45 NH2MIL-53(Fe) contains μ2−OH and corner-sharing Fe(III) octahedra without weak coordinated water molecules, and there are μ3-O-bridged Fe3O clusters in both NH2-MIL88B(Fe) and NH2-MIL-101(Fe) with terminal coordinated H2O molecules, which can be removed upon activation to generate coordination unsaturated metal sites that can act as catalytic active sites.46−50 In addition, the results of CO2 adsorption of Fe-MOFs show that the CO2 uptakes are 8.6, 9.2, and 61.6 cm3/g for NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe), respectively (Figure S7). It is noteworthy that NH2-MIL-101(Fe) and NH2-MIL-88B(Fe) exhibit better photocatalytic capability than NH2-MIL-53(Fe), even though NH2-MIL-88B(Fe) has poor CO2 adsorption. It implies that the coordination unsaturated metal sites in NH2MIL-101(Fe) and NH2-MIL-88B(Fe) are dominant in determining the catalytic activity. The μ−O-Fe clusters in the amino-functionalized Fe-MOFs can be directly excited by visible light,51,52 and the semiconductor behaviors of the Fe-MOFs have been identified by UV−vis DRS and Mott−Schottky measurements (vide ante). Thus, the band gap can be estimated by intercept of the tangents of (Ahυ)2 vs photon energy, where A is a constant and hυ is the incident photon energy (Figure 6a).40 The band gap values of the Fe-MOFs are determined to be about 1.98, 1.72, and 1.77 eV for NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe), respectively. It can be seen that NH2-MIL88B(Fe) and NH2-MIL-101(Fe) with similar metal clusters have a close band gap as well as the flat-band potentials (Figure S3). To further understand the photocatalytic reactions, photoluminescence (PL) spectra for NH2-MIL-53(Fe), NH2MIL-88B(Fe), and NH2-MIL-101(Fe) were measured upon excitation at 350 nm (Figure 6b). It is notable that NH2−BDC ligand shows a robust emission at 485 nm,52,53 while the Fe-

Figure 3. Diffuse-reflectance UV/vis spectra of NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe).

boiling point of TEOA, resulting in little CO generation at the initial stage due to the slow volatilization of TEOA as observed in the reported gas−solid photocatalytic reaction system.40,41 It is noteworthy that no H2 and CH4 evolution was detected during the photocatalytic reaction (Figure S4). In addition, no liquid product such as HCOOH was observed as ensured by 1 H NMR spectral measurements (Figure S5), which is ascribed to the solvent-free reaction system.40−44 The results indicate that all the Fe-MOFs possess a superior selectivity for conversion of CO2 to CO. Under the uniform reaction conditions, NH2-MIL-101(Fe) shows the best catalytic performance for photocatalytic reduction of CO2 to CO among the three Fe-MOFs, with a CO formation rate of 87.6 μmol g−1, which is about 5.6 times higher than that of NH2MIL-53(Fe) (15.7 μmol g−1). To understand the behavior of photocatalytic reaction, a range of control experiments were carried out (Table S1). In the absence of catalyst, light, CO2, or TEOA, the target product is undetectable. Obviously, all of these in one can make the photocatalysis effective. Stability is one of the main factor in evaluation the performance of a photocatalyst. As shown in Figure 5a, NH2-MIL-101(Fe) was consecutively utilized as the deoxygenative CO2 conversion catalyst for four cycles. After multiple cycles, no overt

Figure 4. Photocatalytic CO production (a) and photocatalytic CO production rates (b) of NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL101(Fe). 8520

DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524

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Figure 5. (a) Recycling performance and (b) XRD patterns of NH2-MIL-101(Fe) before and after photocatalysis.

Figure 6. (a) Plots of (Ahv)2 vs photon energy, (b) PL emission spectra (λex= 350 nm), (c) photocurrent responses, and (d) EIS Nyquist plots for NH2-MIL-53(Fe), NH2-MIL-88B(Fe) and NH2-MIL-101(Fe).

MOFs display remarkably decreased fluorescence. The evident luminescence quenching implies the effect of ligand-to-metal charge transfer (LMCT) upon the incorporation of the organic groups into the framework of Fe-MOFs. To further validate the photoresponsive properties, the photocurrent measurements were also employed to reflect the efficiency of photogenerated charge separation, in which a higher photocurrent often leads to a better photocatalytic property.54 The I-

t curves (Figure 6c) with visible-light illumination show that NH2-MIL-101(Fe) possesses the highest photocurrent among the Fe-MOFs, suggesting that NH2-MIL-101(Fe) may have higher active catalytic capacity. In addition, electrochemical impedance spectroscopy (EIS) can be used to characterize the carrier mobility of these samples (Figure 6d).55 Obviously, the results demonstrate that NH2-MIL-101(Fe) exhibits the smallest resistance, reflecting that more efficient charge transfer 8521

DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524

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support of this work. This work was also supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

occurred. The conductivities of the samples were calculated from the Nyquist plots by using equation, σ = L/(S × Ret), where L (cm) and S (cm2) are the thickness and area of the sample, respectively, and the electron-transfer resistance (Ret) is equivalent to the diameter of semicircle in the Nyquist diagram.56 The results exhibit that NH2-MIL-101(Fe) has a higher conductivity of 4.1 × 10−6 S cm−1 than those of NH2MIL-53(Fe) and NH2-MIL-88B(Fe) with conductivities of 1.1 × 10−6 and 2.2 × 10−6 S cm−1, respectively. The difference of the conductivities can be explained by their distinct framework structures. Closely arranged four μ3-O-bridged Fe3O clusters as a tetrahedron in NH2-MIL-101(Fe) (Figure S6c) is more efficient for electron mobility than the triangular bipyramidal arrangement of five Fe3O clusters in NH2-MIL-88B(Fe) (Figure S6b), while NH2-MIL-53(Fe) with μ2−OH bridges (Figure S6a) has poorer conductivity than NH2-MIL-88B(Fe) and NH2-MIL-101(Fe) with μ3-O-bridged Fe3O clusters. To sum up, NH2-MIL-101(Fe) shows brilliant photocatalytic activity, which may be ascribed to the unique structure, matched band gap, efficiently charge separation, as well as electron transfer.

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CONCLUSIONS In summary, we employed a solvent-free route to photoredox catalytic CO2 conversion, which is environmentally benign. Furthermore, the solvent-free route can generate a gas−solid interfacial reaction that can increase the touch probability between the catalyst and the reaction gas. Thus, three classical Fe-based MOFs of NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe) with amino-functionalized organic linkers were adopted as catalysts for the photocatalytic reduction of CO2, and the results demonstrate that they exhibit photocatalytic activity with superior selectivity for conversion of CO2 to CO. The distinct photocatalytic activities of the Fe-MOFs catalysts were discussed. NH2-MIL-101(Fe) performs with the highest photocatalytic activity, which may be ascribed to the unique structure with unsaturated coordination metal sites as well as effective electron transfer. It is anticipated that the solvent-free route can be applied to a variety of catalytic reactions, particularly for gas-involved reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00824. Additional characterization data and photocatalytic results (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 25 89682309. ORCID

Wei-Yin Sun: 0000-0001-8966-9728 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Basic Research Program of China (2017YFA0303504) and the National Natural Science Foundation of China (21573106) for financial 8522

DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524

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Inorganic Chemistry (55) Bag, P. P.; Wang, X. S.; Sahoo, P.; Xiong, J.; Cao, R. Efficient photocatalytic hydrogen evolution under visible light by ternary composite CdS@NU-1000/RGO. Catal. Sci. Technol. 2017, 7, 5113− 5119. (56) Chen, E. X.; Xu, G.; Lin, Q. P. Robust Porphyrin-Spaced Zirconium-Pyrogallate Frameworks with High Proton Conduction. Inorg. Chem. 2019, 58, 3569−3573.

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DOI: 10.1021/acs.inorgchem.9b00824 Inorg. Chem. 2019, 58, 8517−8524