Formation of a mixed-valence Cu(i)/Cu(ii) metal–organic framework with the full light spectrum and high selectivity of CO2 photoreduction into CH4

Based upon the hetero-N,O ligand of pyrimidine-5-carboxylic acid (Hpmc), a new semiconductive Cu(i)/Cu(ii) mixed-valence MOF with the full light spectrum and a novel topology of {43·612·86}2{43·63}2{63}6{64·82}3, {(Cu4I4)2.5[Cu3(μ4-O) (μ3-I) (pmc)3(Dabco)3]·2.5DMF·2MeCN}∞ (NJU-Bai61, NJU-Bai for Nanjing University Bai group; Dabco = 1,4-diazabicyclo [2.2.2] octane), was synthesized stepwise. NJU-Bai61 exhibits good water/pH stabilities and a relatively large CO2 adsorption capacity (29.82 cm3 g−1 at 1 atm, 273 K) and could photocatalyze the reduction of CO2 into CH4 without additional photosensitizers and cocatalysts and with a high CH4 production rate (15.75 μmol g−1 h−1) and a CH4 selectivity of 72.8%. The CH4 selectivity is the highest among the reported MOFs in aqueous solution. Experimental data and theoretical calculations further revealed that the Cu4I4 cluster may adsorb light to generate photoelectrons and transfer them to its Cu3OI(CO2)3 cluster, and the Cu3OI(CO2)3 cluster could provide active sites to adsorb and reduce CO2 and deliver sufficient electrons for CO2 to produce CH4. This is the first time that the old Cu(i)xXyLz coordination polymers' application has been extended for the photoreduction of CO2 to CH4 and this opens up a new platform for the effective photoreduction of CO2 to CH4.


Section 1. Experimental sections
Materials and general methods. All chemical reagents were obtained from commercial sources and, unless otherwise noted, were used as received without further purification. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 240 analyzer. The IR spectra were obtained in the 4000∼400 cm -1 on a VECTOR TM 22 spectrometer using KBr pellets. The in situ FTIR experiments were performed in an IR cell made of quartz on the PerkinElmer Frontier FT-IR Spectrometer with the MCT detector. Thermal gravimetric (TG) analyses were performed under N 2 atmosphere (100 ml min -1 ) with a heating rate of 10 °C min -1 using a 2960 SDT thermogravimetric analyzer. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 ADVANCE X-ray diffractometer with Cu/Kα radiation. X-ray photoelectron spectroscopy (XPS) was used a ULVAC -PHI 5000 VersaProbe with an Al Kα microfocused X-ray source and the C1s peak at 284.8 eV as internal standard. UV-Vis-NIR absorption spectra were recorded in diffuse reflectance mode on a UV-3600 Shimadzu spectrometer.
The optical band gap (E g ) of samples calculation based on UV-Vis diffusion spectra could be estimated following the equation proposed by Tauc, Davis, and Mott: (αhν) 2 = hν -E g (Where α stands for absorption coefficient, h is Planck's constant and ν represents frequency of vibration) 1 .
Photoluminescence (PL) spectra were measured on a Hitachi F-4600 photoluminescence spectrophotometer. The decay lifetime was measured on an Edinburgh Instruments FLS 980 fluorescence spectrometer. The 1 H NMR spectra were recorded on a Bruker DRX-500 spectrometer with tetramethylsilane as an internal reference. The 13 C NMR spectra were measured using a chromatography-mass spectrometry (7890A and 5975C, Agilent).

Conductivity tests
The conductivity of the samples was obtained from Keithley 2400 source meter on CRX-4K High Performance Closed Cycle Refrigerator-based Probe Station at room temperature. The sample powders were pressed under pressure of 10 Mpa into pellets using the conductive carbon adhesive contacts with the "two-probe method" for the current-voltage (I-V) measurements in -10 to 10 V. The electrical conductivity σ can be expressed as, σ = G•L/A, where L and G are the length, electrical conductance of the pellet, respectively, and A is the area of the conductive carbon adhesive.

Electrochemical measurements
The Mott-Schottky measurements were performed on a three-electrode electrochemical workstation CHI 660E (CH Instruments, USA) at frequencies of 500, 1000 and 1500 Hz. Preparation of S2 the working electrode: 2 mg samples were dispersed in a mixed solution of 990 μL ethanol and 10 μL Nafion D-520 dispersion solutions to generate the homogeneous slurry. Subsequently, 200 μL of slurry was transferred and coated on fluoride-tin oxide (FTO) glass plates (1 cm × 2 cm) then dried at room temperature. The Ag/AgCl electrode was employed as the reference electrode and platinum plate was used as the counter electrode, respectively. A 0.2 M of Na 2 SO 4 solution was used as the electrolyte.
The transient photocurrent responses were carried out under light irradiation conditions (300 W xenon arc lamp, CEL-HXF300/CEL-HXUV300, 200 mW/cm 2 ). The preparation of working electrode was the same as above, but the electrolyte was instead of 0.5 M Na 2 SO 4 aqueous solution.

Gas Sorption Measurements.
Low-pressure adsorption isotherms of N 2 (99.999%) and CO 2 (99.999%) were performed on Quantachrome Autosorb IQ-2 surface area and pore size analyzer. Before analysis, about 100 mg samples were activated by using the "outgas" function of the surface area analyzer. Helium (99.999%) was used for the estimation of the free space (warm and cold), assuming that it was not adsorbed at any of the studied temperatures. The specific surface area was determined using the Brunauer-Emmett-Teller (BET) and the Langmuir equation from the N 2 sorption data at 77 K. When applying the BET theory, we made sure that our analysis satisfied the two consistency criteria as detailed by Walton and co-workers. 2

Calculations of isosteric heat of adsorption (Q st ).
A virial-type 3 expression comprising the temperature-independent parameters a i and b j was employed to calculate the enthalpies of adsorption for CO 2 (at 273 and 298 K) on NJU-Bai61. In each case, the data were fitted using the equation: Here, P is the pressure expressed in Torr, N is the amount adsorbed in mmol g -1 , T is the temperature in K, a i and b j are virial coefficients, and m, n represent the number of coefficients required to adequately describe the isotherms (m and n were gradually increased until the contribution of extra added a and b coefficients was deemed to be statistically insignificant towards the overall fit, and the average value of the squared deviations from the experimental values was minimized). The values of the virial coefficients a 0 through a m were then used to calculate the isosteric heat of adsorption using the following expression.
Q st is the coverage-dependent isosteric heat of adsorption and R is the universal gas constant. The heat of CO 2 adsorption for NJU-Bai61 in the manuscript are determined by using the adsorption data measured in the pressure range from 0 ~ 1 bar (273 and 298 K), which is fitted by the virial equation very well (R 2 > 0.9999).

Section 2. Computational details
All the calculations were performed with the Gaussian 16 package suite. 4 Geometry optimizations were carried out by using density functional theory (DFT) with the B3LYP functional. 5  was performed to test the optimized stationary points, and to calculate the free energies along reaction pathways of the photocatalytic CO 2 -to-CH 4 conversion at the Cu 3 OI(CO 2 ) 3 cluster in NJU-Bai61. The binding energy (E b ) was obtained by calculating the energy difference between the total energy of the complex system (E sys ) and the sum of individual energy of the MOF (E MOF ) and CO (E CO ), respectively, which was expressed as： To understand the fundamental electronic structures of NJU-Bai61, band structure and density of states (DOS) calculations were performed by density functional theory (DFT), as implemented in Cambridge Serial Total Energy Package (CASTEP) 7 module in the Materials Studio software package. 8 The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) 9 forms was employed, and Grimme method 10,11 was applied to take the van der Waals interaction into consideration.
In Figure X2, NJU-Bai61 shows a narrow band gap of 0.65 eV, which is agreement with the experimental results, indicating that it can absorb the light for the catalytic reaction from CO 2 to CH 4 .

Section 3. Single-crystal X-ray structure determination
Single-crystal X-ray diffraction data were measured on a Bruker Apex II CCD diffractometer at 296 K using graphite monochromated Mo/Kα radiation (λ = 0.71073 Å). Data reduction was made with the Bruker SAINT program. The structures were solved by direct methods and refined with full-matrix least squares technique using the SHELXTL package. 12 Non-hydrogen atoms were refined with S4 anisotropic displacement parameters during the final cycles. Organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to 1.2 × U eq of the attached atom. The unit cell includes a large region of disordered solvent molecules, which could not be modeled as discrete atomic sites. We employed PLATON/SQUEEZE 13 to calculate the diffraction contribution of the solvent molecules and thereby, to produce a set of solvent-free diffraction intensities; structures were then refined again using the data generated.
A summary of the crystallographic data are given in Table S1.           Figure S11. IR spectra of NJU-Bai61p and NJU-Bai61.

S10
The peak at 1710 cm -1 could be assigned to the characteristic peak of -COOH in Hpmc ligand and NJU-Bai61p. This peak was not observed in NJU-Bai61 indicating that all the -COOH groups in NJU-Bai61 were participated in the coordination with Cu(II) ions.  (b) for NJU-Bai61.            There was no CH 4 detected in the absence of catalyst, illumination, TEA, using ligands or the ligands, CuI and CuCl 2 mixture directly, which indicates that NJU-Bai61, light and sacrificial agent maintain the photocatalytic progress together. When Ar was used instead of CO 2 , CH 4 was not been detected, indicating that CH 4 was reduced from CO 2 rather than skeleton decomposition. S29 Section 9. The research of photocatalytic pathway of CO 2 -to-CH 4 reduction Figure S28. N 2 adsorption and desorption isotherms of NJU-Bai61 at 77 K.        The 470 nm (440 nm calculated by TDDFT) light launches an electronic transition from the ground state (S 0 ) to a certain excited state (S n ) in Cu 4 I 4 cluster, followed by quick deactivation processes to the first excited state (S 1 ). TDDFT calculations predicted the emission at 574 nm, which is in agreement with the experiment of 555 nm. The electron may be transferred to the Cu 3 OI(CO 2 ) 3 cluster, while the Cu 3 OI(CO 2 ) 3 clusters could supply electrons to the adsorbed CO 2 for CH 4 evolution. In the first step, the adsorbed CO 2 molecule is reduced to the COOH* species with releasing 0.7 eV of free energy. Further the COOH* combines with the electron-proton pair to generate CO* with the ∆G of -3.2 eV. Then CO* accepts two electrons and a proton to form CHO*, which is endothermic process with the ∆G of 1.2 eV.

Chemical stability tests
The Cu 4 I 4 cluster could serve as a photosensitizer and donate the energy of 2.16 eV to the process of CO* to CHO* at the Cu 3 OI(CO 2 ) 3 cluster. The CHO* protonates to form CH 2 O* (∆G = -5.7 eV).
Similarly, two electrons are transferred together with a proton to the CH 2 O* to form CH 3 O*, then combining with a proton to obtain CH 3 OH*. The ∆G values of these two steps are 0.5 and -5.7 eV, respectively. The CH 3 OH* is reduced with two electrons and one proton to generate CH 3 * and H 2 O* (∆G = -1.6 eV). A proton is subsequently transferred to the CH 3 * to generate CH 4 *, which the ∆G is -4.1 eV. The catalytic cycle is completed after release of CH 4 and H 2 O molecules calculated by -0.1 eV.