Direct CO₂ photoreduction from flue gas by synergistic catalysis of a nickel metal–organic framework and a ruthenium polypyridyl complex

Abstract

Direct photoreduction of CO₂ from flue gas is an energy-saving avenue to realize the carbon-neutral cycle but it is still in its infancy. Herein, we constructed a new anionic metal–organic framework based on trinuclear Ni clusters and thiophenecarboxylic acid for CO₂ photoreduction of exhaust gas from a power plant. Under visible-light irradiation, the yield of CO was 17.4–26.3 mmol g⁻¹ in diluted CO₂ with a concentration of 5–20%. The apparent quantum yield (A.Q.Y.) under a 10% CO₂ atmosphere was determined to be 2.1%, which ranks among the highest values of the reported photocatalysts under similar conditions. Importantly, in real flue gas containing 10% CO₂, a selectivity of 90.4% was achieved and CO generation reached 18.2 mmol g⁻¹. In situ transient photovoltage (TPV) and density functional theory (DFT) calculations showed that the formation of a CO₂ bridged photocatalytic interface between Ni-MOF1 and a Ru complex ([Ru(bpy)₃]Cl₂/[Ru(bpy)₂]Cl₂) was a crucial factor for the efficient CO₂-to-CO conversion in diluted CO₂.

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1. Introduction

With the gradual increase in the consumption of fossil fuels, the energy crisis and global warming resulting from heavy emission of CO₂ are thorny issues facing human society.^1–4 Reduction of CO₂ into usable carbon-based fuel using solar light has been considered a promising avenue to solve these problems.^5–9 Tremendous efforts have been made in this field and high reaction activity has been achieved but mainly focusing on the utilization of pure CO₂.^10–17 The purification and condensation of low concentration CO₂ ²⁰ utilizing dropwise condensation or supersonic separation technologies²¹ is an energy-intensive and complicated process;^22–24 however, the direct usage of CO₂ from the exhaust gas of heavy industries, which contains 3–20% CO₂,^18,19 as the carbon source is an energy-saving, low-cost and sustainable way. However, direct photoreduction of CO₂ from real flue gas is still in its initial stages.

Metal–organic frameworks (MOFs) have attracted tremendous attention in low-concentration CO₂ photoreduction^25–29 due to their advantages of high-density metal nodes and excellent CO₂ adsorption capability.^30–32 Besides, their well-defined crystalline structures can assist in understanding the structure–activity relationship.^33,34 For example, Han et al.³⁵ have demonstrated CO₂ reduction activity of metal–organic monolayers (Ni MOLs) under 10% CO₂ using [Ru(bpy)₃]Cl₂ as a photosensitizer. Similarly, Wang and co-workers³⁶ have reported that Co-based MAF-X27-OH can convert CO₂ into CO with [Ru(bpy)₃]Cl₂ as a photosensitizer and the yield from 10% CO₂ is comparable to that from pure CO₂. Although remarkable progress has been achieved towards the practical application of MOFs,^37,38 still the catalytic efficiency at low CO₂ concentration needs to be further improved. Since [Ru(bpy)₃]Cl₂ is a commonly used reagent in MOF systems, the development of efficient interface interaction between a MOF, Ru complex, and CO₂ may be significant to enhance the catalytic efficiency, particularly in low-concentration CO₂ reduction.³⁹ Until now, there has been a lack of profound understanding of the MOF, Ru complex, and CO₂ catalytic interface kinetics and it is still a big challenge to explore such systems.

Herein, a new MOF (Ni-MOF1) with an anionic framework based on thiophenecarboxylic acid and a trinuclear Ni cluster is synthesized for direct CO₂ photoreduction from the flue gas of a power plant. The anionic framework could help to facilitate the collaboration between the MOF and [Ru(bpy)₃]²⁺/[Ru(bpy)₂]²⁺. Under visible-light irradiation, the yield of CO is 17.4–26.3 mmol g⁻¹ in diluted CO₂ with a concentration of 5–20%. And the apparent quantum yield (A.Q.Y.) is up to 2.1% which is better than that of most reported MOFs under similar conditions.^11,18 Meanwhile, a selectivity of 90.4% is achieved and the production of CO is up to 18.2 mmol g⁻¹ on Ni-MOF1 in flue gas containing 10% CO₂. Mechanistic studies based on DFT calculations and TPV experiments reveal that CO₂ bridged the photocatalytic interface between Ni-MOF1 and the Ru complex in the reduction process, which may account for the relatively high catalytic activity at low concentrations of CO₂.

2. Experimental methods

2.1. Preparation of (C₂H₆NH₂)⁺[Ni₃(μ₃-OH)(H₂O)₃(DMTDC)₃](C₂H₅OH)(DMA)₂ (Ni-MOF1)

Ni(NO₃)₂·6H₂O (0.058 g, 0.2 mmol) was added to a mixture of DMA (8 mL) and absolute ethanol (2 mL) containing H₂DMTDC (Shanghai Tensus Bio-tech Co., Ltd) (0.05 g, 0.02 mmol) and KOH (0.0056 g, 0.1 mmol). Then the mixture was placed in a Teflon reactor and heated at 150 °C for 72 h. Brown and long strip crystals were obtained after the reaction system was cooled to room temperature slowly. The product was filtered and washed with DMA. The yield was 82% based on H₂DMTDC. The final empirical formula was determined by combining crystallographic data, and elemental and thermogravimetric analysis data. Elemental analysis calcd: C, 39.33; H, 4.95; N, 3.27. Found: C, 39.21; H, 4.98; N, 3.28. IR data (KBr pellet cm⁻¹): 3286 (w), 3007 (w), 2934 (w), 2858 (w), 2464 (w), 1663 (w), 1614 (m), 1534 (w), 1497 (m), 1420 (w), 1391 (m), 1362 (s), 1334 (m), 1249 (w), 1183 (w), 1012 (m), 969 (w), 895 (w), 797 (w), 768 (m), 678 (w), 652 (w), 598 (w). (C₂H₆NH₂)⁺[Ni₃(μ₃-OH)(H₂O)₃(DMTDC)₃](C₂H₅OH)(DMA)₂ (Ni-MOF1, DMTDC²⁻ = 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylate).

2.2. CO₂ photoreduction

The procedure for photoreduction of CO₂ was carried out in a sealed quartz reactor (50 mL). A circulating cooling water system was used to maintain the reaction temperature at around 20 °C. In a typical reduction reaction, Ni-MOF1 (1 mg), [Ru(bpy)₃]Cl₂ (Shanghai Bidepharmatech Co., Ltd) (bpy = 2′,2-bipyridine, 7 mg), water (1 mL), acetonitrile (MeCN, 4 mL) and triethanolamine (TEOA, 1 mL) were placed in a quartz reactor. Then the quartz reactor was filled with a low concentration of CO₂ (CO₂/Ar v/v). The system was irradiated using a 300 W Xe lamp with a cut-off filter (λ > 420 nm). To detect the composition of gas products, 0.5 mL of gas products were sampled and injected into an FID. The yield of CO was calibrated with standard gas mixtures and their identity was determined using their retention time (1.9 min for CO). The injector and detector temperatures were set to 60 °C. To detect the content of H₂, 1 mL of the gas products were injected into a TCD. Subsequently, standard gas mixtures and retention time (0.7 min for H₂) were used to determine the identity and calibrate the yield of H₂.

To explore the source of CO produced by photocatalytic reduction of CO₂, gas chromatography-mass spectrometry (GC-MS) was used to detect the products after an isotopic experiment. In this isotopic experiment, the reaction conditions were the same as described above except that ¹²CO₂ was replaced by ¹³CO₂.

3. Results and discussion

3.1. Characterization of the structure

Single-crystal X-ray structural analysis revealed that Ni-MOF1 crystallizes in the P6₃/mmc space group. The asymmetric unit consists of one Ni(II) ion, one coordinated water molecule, one third μ₃-OH⁻, and one DMTDC²⁻ ligand. Every Ni(II) ion is six-coordinated with four oxygen atoms from four different DMTDC²⁻ ligands, one coordinated water molecule, and one μ₃-OH⁻. The structure of the crystal consists of Ni₃(μ₃-OH) cores and DMTDC²⁻ ligands, which is similar to the structure of MIL-88B (Fig. 1a).⁴⁰ Five Ni₃(μ₃-OH) cores and six DMTDC²⁻ ligands form a trigonal bipyramid-type cage (Fig. 1b). From a topological perspective, the whole structure of Ni-MOF1 can be described as a 6-connected acs network (Fig. 1c). Running along the c axis, there are hexagonal honeycomb porous channels with a pore size of about 10.1 Å (Fig. 1d). The infrared (IR) spectrum was used to elucidate the connection between the metal and the ligand in Ni-MOF1 (Fig. 2a). It displays saturated hydrocarbon stretching vibrations at 2934 cm⁻¹ and 2858 cm⁻¹, and carbonyl vibration at 1663 cm⁻¹. Meanwhile, the peaks at 1497 cm⁻¹ and 1420 cm⁻¹ are attributed to the skeletal vibrations of the aromatic ring. The flexural vibrational peak of saturated hydrocarbons is observed at 1391 cm⁻¹. No vibrational peak of the oxygen–hydrogen (hydroxyl) bond in the carboxyl group was observed, indicating that the Ni(II) atom of Ni-MOF1 is coordinated with the carbonyl oxygen of the carboxyl group in the ligand. Powder X-ray diffraction (PXRD) was used to determine the phase purity of Ni-MOF1. A good agreement between peak positions of the simulated prototype and the synthesized material proved the crystallinity and high purity of Ni-MOF1 (Fig. 2b). The difference between them may be due to the various crystal orientations of the powder samples.^41–43

Fig. 1 The structure of Ni-MOF1. (a) The coordination environment of inorganic SBUs. (b) The cage structure. (c) The 6-connected topology network. (d) View of the 3D structures along the c axis (blue balls: Ni; red balls: O; yellow balls: S; and grey balls: C).

Fig. 2 Basic characterization of Ni-MOF1. (a) Infrared (IR) spectra. (b) Powder X-ray diffraction (PXRD) (black curve: single-crystal simulation and red curve: experimental synthesis). (c) TGA curves. (d) UV-vis diffuse reflectance spectra.

To investigate the thermal stability of Ni-MOF1, thermogravimetric analysis (TGA) was conducted from 35 °C to 800 °C in N₂ (Fig. 2c). The framework remains intact up to 294 °C, and above that temperature the structure starts to decompose. To understand the light-absorption ability of Ni-MOF1, UV-vis diffuse reflectance was tested. It showed that Ni-MOF1 has broad absorption in the visible light region (380–700 nm) (Fig. 2d).

3.2. CO₂ photoreduction

Considering that the CO₂ concentration in the exhaust emission of the heavy industry sector is between 3 and 20%, the reactivity of Ni-MOF1 in low CO₂ concentration was first investigated by performing reduction reactions in nitrogen diluted CO₂ with 5%, 10%, and 20% CO₂ concentrations under the model conditions of acetonitrile, H₂O, [Ru(bpy)₃]Cl₂ and TEOA (Fig. 3a). When 5% CO₂ was used, the CO yield could reach 17.4 mmol g⁻¹ while the selectivity was only 72.8% after reacting for three hours, which may be attributed to inadequate CO₂ and the unused reductive sites in the production of H₂. When 10% CO₂ and 20% CO₂ were used, the yields of CO could be increased to 23.1 mmol g⁻¹ and 26.3 mmol g⁻¹ with the selectivities of 84.3% and 89.2%, respectively. Meanwhile, an apparent quantum yield (A.Q.Y.) of approximately 2.1% at 420 nm and 10% CO₂ concentration was identified, which is higher than the A.Q.Y. of most reported MOFs at the same concentration of CO₂ (Table S2†).^35,36,44,45 To trace the source of the generated CO, an isotopic experiment was performed with ¹³CO₂ as the substrate under similar conditions of CO₂ photoreduction and chromatography-mass spectrometry (GC-MS) was used to examine the products. The strong m/z peak signal at 29 was assigned to ¹³CO, which clearly indicated that the produced CO originates from photoreduction of CO₂ instead of other species in the reaction system (Fig. S1, ESI†).

Fig. 3 CO₂ photoreduction performance and recycling. (a) Ni-MOF1 under different CO₂ concentrations. (b) Ni-MOF1 under various reaction conditions. (c) Cycling tests under 3 h of irradiation in a 10% CO₂ atmosphere over Ni-MOF1. (d) The yield of CO and H₂ on Ni-MOF1 in real flue gas.

To elucidate the effect of each component of the system on CO₂ photoreduction, four sets of control experiments were performed (Fig. 3b). No CO and H₂ production was observed in the presence of Ar, indicating that the products did not come from the degradation of organic matter in the reaction system. In the absence of Ni-MOF1 or [Ru(bpy)₃]Cl₂, no CO and H₂ were produced, demonstrating that Ni-MOF1 and [Ru(bpy)₃]Cl₂ are indispensable. And no products were detected in the absence of TEOA, indicating that TEOA as a sacrificial reagent is crucial for CO₂ photoreduction. Cycling experiments revealed that the stability of Ni-MOF1 at 10% CO₂ concentration was relatively good although a slight decline of CO production in the third reaction run was observed. The slight decline was probably owing to a little weight loss of Ni-MOF1 when cleaning and transferring (Fig. 3c).

Due to the high catalytic activity of Ni-MOF1 at low CO₂ concentrations, the performance of this system towards photoreduction of CO₂ directly from flue gas (5% and 10% CO₂ concentrations) was investigated. Of note, yields of CO of 13.4 and 18.2 mmol g⁻¹ could still be obtained in the flue gas with 5% and 10% CO₂ concentration, respectively (Fig. 3d). Peculiarly, a high selectivity of 90.4% was observed for the proposed catalyst in the flue gas with 10% CO₂ concentration. Comparing the yields of CO in 5% CO₂ (17.4 mmol g⁻¹) and 10% CO₂ (23.1 mmol g⁻¹), the slight decrease in yield may be mainly attributed to the competitive adsorption and negative effect of other components such as H₂S, SO_x and NO_x.⁴⁶ These results indicated that Ni-MOF1 showed pretty good catalytic performance even under harsh reaction conditions, which would provide a valuable guideline to design efficient photocatalysts for practical applications of CO₂ photoreduction.

3.3. Possible catalytic mechanism

To reveal the root reason behind the high reactivity of the system, an in situ TPV test was carried out in CO₂ and N₂-saturated ACN solutions. As shown in Fig. 4a, c, and d, the curves of CO₂ and N₂ obtained from Ni-MOF1 are almost the same while the intensity of the curves of CO₂ obtained from the Ru complex and Ru complex/Ni-MOF1 is lower than that of N₂. Compared with that in the Ru complex, a higher intensity curve of N₂ (Fig. 4b) was identified in Ru complex/Ni-MOF1. The above results show that the Ni-MOF1/Ru complex is preponderant in the generation of photoelectrons and is more favorable as an active center for CO₂ photoreduction. For the Ru complex and Ru complex/Ni-MOF1, the equalization (Q₀ − Q₁)/Q₁ (the ratio of used number of electrons in CO₂ reduction to the total number of photogenerated electrons) was calculated to be 66.83% and 74.37% (Fig. 4c and d), respectively. A slightly lower value for Ru complex/Ni-MOF1 (66.83%) than that for the Ru complex reveals that the bare Ru complex has a faster reaction rate for CO₂ reduction.

Fig. 4 The in situ TPV patterns of different materials in N₂-saturated ACN solution (black curve) and CO₂-saturated ACN solution (red curve). (a) Ni-MOF1. (b) Ru complex/Ni-MOF1 (black curve) and Ru complex (red curve) in N₂-saturated ACN solution. (c) Ru complex. (d) Ru complex/Ni-MOF1. Charge annihilation process of (e) the Ru complex and (f) Ru complex/Ni-MOF1.

Furthermore, a detailed analysis of the relaxation process using the TPV curves of Ru complex/Ni-MOF1 and Ru complex was conducted. According to the normalized curves of N₂ and CO₂ (Fig. 4e and f), the τ value, representing the time for electron and hole recombination, is also calculated. The τ of the Ru complex under N₂ conditions (0.07511 ms) is larger than that under CO₂ conditions (0.06191 ms) (Fig. 4e). This suggests that the Ru complex can transfer electrons directly to CO₂ for the next reaction step. However, the τ of Ru complex/Ni-MOF1 under N₂ conditions (0.07328 ms) is smaller than that under CO₂ conditions (0.11277 ms) (Fig. 4f). These results indicate that a new photoelectric interface is generated in the CO₂/Ru complex/Ni-MOF1 system. In other words, there is a new CO₂ bridged photocatalytic interface between Ni-MOF1 and the Ru complex. Notably, this kind of CO₂ bridged photocatalytic interface shows a less conductive characteristic than the Ru complex/Ni-MOF1 catalytic interface, but the CO₂ bridged nature is beneficial for the photoreduction of low-concentration CO₂. According to the reported study,⁴⁷ the interface has some storage capacity for charge, and therefore it slows down the rate of electron-charge transport. A slower electron transfer process will ensure that the photoexcited charge on the sample surface is maintained for a longer period of time. The extended lifetime of the electrons is an important factor in facilitating the photocatalytic process.

Density functional theory (DFT) calculations were performed to further understand the reaction mechanism of CO₂ reduction to CO. In combination with the results of TPV, the free energy changes (ΔG) on the sites of bare Ni-MOF1, the Ru complex and the CO₂ bridged interface of Ru complex/Ni-MOF1 for CO₂ reduction was calculated. A key step in the reported mechanism is the activation of the classically inert CO₂. In our calculations, ΔG values of COOH* formation on the Ru complex, bare Ni-MOF1 and the Ru complex/Ni-MOF1 interface were 2.15, 0.57 and 0.44 eV, respectively, indicating that the COOH* formed on the Ru complex/Ni-MOF1 interface is more stable (Fig. 5a). In the subsequent step, COOH* will be hydrogenated to CO* by reacting with another proton–electron pair. The ΔG for COOH* hydrogenation to form CO* + H₂O is −0.46 and −0.25 eV, respectively. Besides, due to the weak adsorption of CO on the Ru complex, CO can directly desorb from the substrate with a ΔG of −1.52 eV. The final step is the CO desorption from Ni-MOF1 and the Ru complex/Ni-MOF1 interface after overcoming the larger thermodynamic barrier. Compared with bare Ni-MOF1 as catalyst sites, the synergistic catalytic process of the Ru complex and Ni-MOF1 results in lower energy (ΔG_max = 0.44 eV) and more stable intermediates. Thus the CO₂ photoreduction is prone to occur on the CO₂ bridged Ru complex/Ni-MOF1 interface (Fig. 5b). The high catalytic yield at low CO₂ concentrations may be ascribed to this synergistic catalytic process.

Fig. 5 The DFT results and possible mechanism. (a) Free energy profile for the CO₂RR on Ni-MOF1 and Ru complex/Ni-MOF1 (the free energy zero is set as that of a CO₂ molecule in the gas phase and the catalyst). (b) Synergistic effect of a CO₂ bridged photocatalytic interface of the Ru complex and Ni-MOF1.

4. Conclusions

In summary, a new kind of anionic MOF (Ni-MOF1) based on a trinuclear nickel cluster and DMTDC²⁻ ligand was constructed for direct CO₂ photoreduction from flue gas. Under visible light, a CO yield of 23.1 mmol g⁻¹ and an A.Q.Y. of 2.1% were achieved with the assistance of the Ru complex in diluted CO₂ with a concentration of 10%. The A.Q.Y. obtained in this work is comparable to the highest values of all reported photocatalysts under similar conditions. More importantly, in the flue gas containing 10% CO₂ concentration, CO generation was still 18.2 mmol g⁻¹ with a selectivity of 90.4%. TPV experiments and DFT calculations indicated that the synergistic catalysis of a CO₂ bridged photocatalytic interface of the Ru complex and Ni-MOF1 played an important role in CO₂ photoreduction, which might be the reason for the high reactivity in low concentration of CO₂. This work provides new insights not only for the construction of a catalytic interface in CO₂ photoreduction but also for the development of efficient photocatalysts for CO₂ reduction directly from flue gas.

Author contributions

M. D. and Y. T. contributed equally to this paper. M. D. and C. Y. S. conceived the ideas. M. D. and X. H. W. designed the experiments, synthesized the sample, measured the photocatalytic performance and wrote the paper. L. X. W. assisted in performing photocatalysis experiments. Y. T. was responsible for the theoretical calculation part. B. S. H. was responsible for crystal analysis. J. W. was responsible for TPV analysis. A. Y. was responsible for article polishing. C. Y. S. and Z. H. K. supervised the project, analyzed the data and revised the paper. J. X. G. helped analyze data and modified the article. X. L. W. and Z. M. S. supervised the project and analyzed the data.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was made possible as a result of a generous grant from the National Natural Science Foundation of China (21971032 and 22175033) and the Education Department of Jilin Province (JJKH20221153KJ).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2061508. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02366k

‡ These authors contributed equally.

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