A large π-conjugated ligand in metal-organic framework as opticalswitch to regulate the electron transfer pathway for highly selective reduction of CO2 to CH4

Here, we report a Cu-based metal-organic framework (Cu-DBC), constructed by the large π-conjugated ligand dibenzo-[g,p]chrysene-2,3,6,7,10,11,14,15-octaol and the square-pyramidal CuO5 nodes, as the photo-coupled electrocatalysts for CO2 reduction to CH4. Under visible light, it exhibits high performance for photocatalytic reduction of CO2 to CH4 with selectivity of 100% and rate of 1.04 μmol g-1 h-1, without additional photosensitizer. Importantly, at -1.4 V vs. RHE, it exhibits high performance for photo-coupled electroreduction of CO2 to CH4 with a Faradaic eciency (CH4) of 93% and current density of 10.4 A g-1. Theoretical calculations, in-situ infrared spectroscopy investigation and Mott–Schottky measurements showed that the large conjugated ligand in Cu-DBC has the suitable lowest unoccupied molecular orbital (LUMO) to match well with the reduction potential of CO2/CH4 and serves as a photoswitch to regulate electron transfer pathway to the metal center, resulting highly selective photocatalytic reduction or photo-coupled electroreduction of CO2 to CH4.


Introduction
The increase of atmospheric CO 2 has a severe impact on global climate and ecological environment. [1][2][3][4][5][6] Scientists have carried out extensive researches on catalytic transformations of CO 2 molecules into high value-added chemicals, such as methane 7-8 and ethylene. 9 However, the CO 2 reduction to hydrocarbons usually involves multi-electron transfer processes (e.g. 8 electrons for CH 4 and 12 electrons for C 2 H 4 ), leading to slow kinetics and high overpotential. 10 Compared with traditional methods, such as thermal catalysis, 11 photocatalysis [12][13] and electrocatalysis, 14-15 the photo-coupled electrocatalytic CO 2 reduction (PECR) is a competitive method. [16][17][18] It has been suggested that light can induce the band bending, 19 thus reducing overpotential for CO 2 reduction and changing the catalytic pathways. In addition, the electrons and holes can be directionally separated in the external eld, facilitating charge separation. 20 However, the studies in this eld are very limited and challenging. Therefore, the development of new photo-coupled electrocatalyst system for CO 2 reduction is urgently necessary. CO 2 reduction can generate a variety of products such as HCOOH, CO, CH 4 and C 2 H 4 , etc. To date, most of reported PECR catalysts were shown to convert CO 2 to CO or HCOOH, rather than high-value-added chemicals (e.g., CH 4 and C 2 products). 17, 21-23 From the point of thermodynamics, it is possible to achieve high selectivity of CH 4 by rationally designing an electrocatalyst with the appropriate lowest unoccupied molecular orbital (LUMO) matching with the reduction potential of CO 2 /CH 4 (-0.24 V) (Scheme 1). Among all the metal catalysts, Cu has been demonstrated to be capable of converting CO 2 into CH 4 and C 2 species, which is related to the appropriate interaction of *CO intermediate with Cu catalytic sites, and making it possible for further hydrogenation rather than desorption to produce CO or HCOOH. [8][9] Based on the above analysis, both photosensitive units with suitable LUMO energy levels and copper-based active sites are required for a PECR catalyst to have high activity and high selectivity in CO 2 reduction into hydrocarbons. Metal-organic frameworks (MOFs) are constructed by self-assembly of metal ions or metal clusters with organic linkers. 24-26 Through altering ligands or catalytically active sites can expediently lead to various functional MOFs, making them the promising catalysts for PECR. Some organic materials bearing large π-conjugated structures usually possess narrow band gaps, 27-31 thus making the LUMO energy levels may match well with the reduction potential of CO 2 /CH 4 . Therefore, a MOF constructed by the copper ions and large π-conjugated ligands may be anticipated to serve as an ideal model for studying and realizing light-induced improvement of the electrocatalytic e ciency for reducing CO 2 to CH 4 .
(H 2 NC 2 H 6 ) 6 [Cu 2 O(dbc)] (Cu-DBC, H 8 dbc = dibenzo-[g,p]chrysene -2,3,6,7,10,11,14,15-octaol) possesses a 4fold interpenetration structure, 32 in which each dbc 8linker coordinates with four copper atoms, each copper ion is coordinated by four oxygen atoms from two catecholate ligands in the same layer and a single oxygen bridge in a distorted square-pyramidal fashion to form a CuO 5 node, and a pair of copper atoms from two adjacent layers are connected by the single oxygen bridges (Figure 1a-b). Such a 3D framework structure based on large π-conjugated organic ligands and square-pyramidal CuO 5 nodes could be a potential PECR catalyst. In this work, Cu-DBC was employed to investigate the structure and performance for selective conversion of CO 2 . Interestingly, with the assistance of light, the Faradaic e ciency (FE) of CH 4 and the current density were signi cantly enhanced over a wide potential range compared to those under the dark environment for Cu-DBC, indicating that the coupling of light energy plays an important role.
Cu-DBC was prepared by hydrothermal method according to the literature. 32 The purity of the as-prepared sample was demonstrated by the scanning electron microscopy (SEM) images and powder X-ray diffraction (PXRD) measurement ( Figure S2-3). The electron paramagnetic resonance (EPR) spectrum was consistent with X-ray photoelectron spectroscopy (XPS) ( Figure S4), indicating the existence of Cu(II) in the synthesized samples. To study the potential semiconductive property of Cu-DBC, band gap energy (E g ) was determined to be 1.68 eV by Tauc plot ( Figure S5). The Mott-Schottky measurements revealed that Cu-DBC was a typical n-type semiconductor and its LUMO position located in -0.27 V vs. RHE, which is very near to the reduction potential of CO 2 /CH 4 (-0.24 V) ( Figure 1c). Additionally, we conducted a photocurrent response test to investigate the light absorption capabilities of Cu-DBC, as shown in Figure   1d, revealing that the current density increases obviously under the trigger of light excitation, indicating that effective carrier generation and transfer in  In order to investigate the photocatalytic activity of Cu-DBC, we conducted a photocatalytic CO 2 RR in a quartz cell with saturated CO 2 , triethanolamine (TEOA) as sacri cial electron donor, and without photosensitizer. As shown in Figure S6, the amount of CH 4 was 0.025 μmol (i.e. 1.04 μmol g -1 •h -1 ) after 24 hrs and only trace CO was detected, suggesting that Cu-DBC is capable of converting CO 2 into CH 4 with nearly 100% selectivity under visible-light irradiation. However, the reaction rate is poor, which might be ascribed to the limited kinetic process. 34 It is worth mentioning that the applied bias voltage can promote the separation of photogenerated electrons and holes and enhance the photocatalytic e ciency. [35][36] Therefore, we further evaluated the photoelectrochemical reduction of CO 2 for Cu-DBC.

Results
The photoelectrochemical reduction of CO 2 was tested in 0.1 M KHCO 3 aqueous solution. All the potentials mentioned in this work are based on the reversible hydrogen electrode (RHE). The linear sweep voltammetry (LSV) (Figure 2a) revealed that compared with dark condition, the current density of Cu-DBC increases evidently under visible light irradiation (λ > 400 nm). 1 H nuclear magnetic resonance (NMR) spectrum ( Figure S7) and gas chromatography results (Figures 2b-c) revealed that no liquid-phase products were generated during CO 2 reduction over Cu-DBC. The FEs of different reduced products were tested (Figure 2b), showing the total FEs of gaseous products at all potentials are ~100%. In particular, the FE of CH 4 can reach up to 93.2% with the current density of 10.4 A g -1 at -1.4 V, which is the best one among all known MOFs for CH 4 production in CO 2 RR (Table S1). Actually, to our best knowledge, only a FN-CTF-400 compound exhibits a higher FE (FE CH4 = 99.3%) than Cu-DBC, but with a current density of only 0.2 A g -1 . 37 In order to explore the effect of light irradiation, the electrocatalytic CO 2 RR was tested in the same potential range without light irradiation (Figure 2c). By comparison, the maximal FE and current density of CH 4 signi cantly decrease from ~93% to ~54%, and from 10.4 to 3.9 A g -1 , respectively, at -1.4 V. This phenomenon indicates that the large π-conjugated ligand could serve as a photoswitch to regulate electron transfer pathway to the metal center, and coupling light with electric eld can indeed improve the catalytic performance of Cu-DBC for CO 2 RR.
In addition to high activity and selectivity, long-term stability is an important factor for PECR catalysts.
The i-t test was conducted at -1.4 V for 4 hours (Figure 2d), and it showed the current density was changed into 97% of the previous level. Meanwhile, the PXRD patterns, transmission electron microscopy (TEM) images and XPS analysis of Cu-DBC showed negligible change and no generation of Cu 2 O or Cu cluster after the CO 2 RR test for 4 hours ( Figure S8-S9), suggesting that Cu-DBC is stable and the catalytical performance can be ascribed to the MOF rather than Cu 2 O or Cu clusters. In the literature, 14 the square-planar CuO 4 nodes of MOF was unstable during the electrocatalytic process, highlighting the high stability of the square-pyramidal CuO 5 nodes in Cu-DBC.
A variety of products can be produced in CO 2 reduction reaction due to the different reaction intermediates formed over the catalysts. In order to investigate the reaction paths occurred on the Cu-DBC, in-situ attenuated total re ection infrared (ATR-FTIR) spectral measurements were performed with a home-made cell. Three bands located at 1288, 1366~1383, and 1616 cm -1 are associated with the OHdeformation, C−O stretching vibration and O−C=O asymmetric stretching vibration of *COOH intermediate, respectively, while a weaker band at 1020 cm -1 belongs to *CHO intermediate (Figures   3a,b). [38][39][40] Especially, both the *COOH and *CHO groups are crucial intermediates for the reduction of CO 2 to CH 4 . Based on the ATR-FTIR spectral analysis, a possible reaction mechanism of the reduction of CO 2 is proposed in Figure 3c. In words, a CO 2 molecule can be physically adsorbed on the square-pyramidal CuO 5 node of Cu-DBC and undergoes a proton-coupled electron transfer process to generate a *COOH intermediate, and then a *CO intermediate, which is successively hydrogenated into *CHO, *OCH 2 and *OCH 3 intermediates, and ultimately achieve the 8-electron transfer process to generate CH 4 .
In order to con rm the reaction mechanism, we used periodic density functional theory (PDFT) to simulate the reaction path and evaluate the corresponding energy barriers. As shown in the Figure 3d, the hydrogenation of *CO to *CHO involves a minimum barrier of -0.39 eV, while the production of CO involves a higher barrier of 0.18 eV. The low energy barrier allows the hydrogenation of *CO to be the main reaction. In fact, the energy barriers of hydrogenations of other intermediates are all lower than that of desorption (Figure 3d), which nally results in CH 4 as the main product rather than formic acid, formaldehyde or methanol. The structures of all intermediates obtained by PDFT calculations are illustrated in Figure S10. Comparing with the high energy barrier of hydrogen evolution reaction ( Figure   S11), the catalytic reduction of CO 2 to CH 4 involves lower energy, which is consistent with the experimental results. These facts suggest that the square-pyramidal CuO 5 nodes of Cu-DBC indeed can serve as the active sites for catalytic reduction of CO 2 to CH 4 .
Finally, the signi cant increase of the FE for CH 4 when coupling light with electrocatalytic CO 2 RR can be rationalized to that the formation of methane is thermodynamically feasible, but it must undergo a slow 8-electron transfer process. Through analyzing the frontier orbitals of Cu-DBC, it can be seen that the highest occupied molecular orbital (HOMO) is mainly located on the large π-conjugated ligand, and the lowest unoccupied molecular orbital (LUMO) is mainly located on the metal center. Under dark condition, electrons can enter the LUMO orbital but need a high overpotential to cross the HOMO-LUMO energy gap, resulting in a poor methane yield under dark condition. When Cu-DBC is light irradiated, the electrons in HOMO orbital can harvest photons and jump into the LUMO orbitals that match with the reduction potential of CH 4 , then can activate and reduce CO 2 molecule into CH 4 with a high selectivity.
To clarify the mechanism of photo-electrocatalysis and the effect of visible light, the electronic properties of S1 and T1 excited states of Cu-BDC were further calculated with PDFT. Both S1 and T1 exhibit clearly the ligand-to-metal charge-transfer (LMCT) excitation characteristic, which obviously facilitates the electron-ow and CO 2 activation occurring on the square-pyramidal CuO 5 node. When Cu-DBC is light irradiated, the large π-conjugated ligand is rst excited from the S 0 ground state to the S 1 state, and then spontaneously transformed to T 1 triplet state through the process of intersystem crossing. The energy gap between S 0 and S 1 is 1.71 eV, which is consistent with the band gap (1.68 eV) calculated from the UV-vis absorption spectrum, con rming the accuracy of the calculations. Since all the photophysical processes occur much faster than the electrocatalysis process, it can be proposed that the photo-coupled electrocatalysis of CO 2 proceeds mainly from the T 1 state rather than the S 0 state. The energy of T 1 is 1.13 eV higher than the S 0 state, suggesting that under light conditions, Cu-DBC will be excited to a higher energy state. Such high energy can enhance the electron transfer from the ligand to the active center, resulting in the improvement of current density (Figure 2a). These explain the inherent reason why Cu-DBC exhibits high performance for electrocatalytic reduction of CO 2 to CH 4 .

Discussion
In summary, a Cu-catecholate based MOF with active CuO 5 sites exhibits excellent activity and selectivity for CH 4 in PECR measurements. Compared with the condition of dark, the FE(CH 4 ) and current density increase very signi cantly upon light irradiation. Experimental and theoretical studies reveal that the high performance for PECR is ascribed to the highly catalytical activity of the CuO 5 node, the suitable conduction band edge position and the external light-eld can enhance the electron transfer to the adsorbed CO 2 . The results provide a new strategy to design and utilize photo-electrocatalysts with light harvesting and catalytic active sites for converting CO 2 into high-valued chemicals.

Methods
Materials and general methods. The ligand dibenzo-[g,p]chrysene -2,3,6,7,10,11,14,15-octaol was synthesized according to the literature .41 Other reagents were commercially available and without further puri cation. Power X-ray diffraction (PXRD) patterns were collected on a Bruker D8 Advance diffractometer (Cu Kα). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 spectrometer. A SU8010 scanning electron microscope was utilized to investigate the morphology of Cu-DBC. High resolution images of microcrystalline powder of Cu-DBC before and after CO 2 reduction reaction were obtained by a transmission electron microscope (TEM). 1 H Nuclear magnetic resonance ( 1 H NMR) measurements were performed on a Bruker advance III. Attenuated total re ection infrared (ATR-FTIR) spectra were recorded on a Nicolet 6700 (Thermo Fisher) to study the reaction intimidates formed over Cu-DBC in the electrochemical measurements.
Synthesis of Cu-DBC. dibenzo-[g,p]chrysene -2,3,6,7,10,11,14,15-octaol (8.6 mg) and Cu(OAc) 2 (6 mg) were dissolved in 500 μL DMF and 2 mL deionized water. After ultrasonic treatment for 30 minutes, the vessel was placed in an oven with the temperature of 85 °C for 72 hours. Then the MOF was separated from the reaction mixtures by washing with water and dried overnight in 60 °C to obtain the black product.

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