Asymmetrical Interactions between Ni Single Atomic Sites and Ni Clusters in a 3D Porous Organic Framework for Enhanced CO2 Photoreduction

Abstract 3D porous organic frameworks, which possess the advantages of high surface area and abundant exposed active sites, are considered ideal platforms to accommodate single atoms (SAs) and metal nanoclusters (NCs) in high‐performance catalysts; however, very little research has been conducted in this field. In the present work, a 3D porous organic framework containing Ni1 SAs and Nin NCs is prepared through the metal‐assisted one‐pot polycondensation of tetraaldehyde and hexaaminotriptycene. The single metal sites and metal clusters confined in the 3D space created a favorable micro‐environment that facilitated the activation of chemically inert CO2 molecules, thus promoting the overall photoconversion efficiency and selectivity of CO2 reduction. The 3D‐NiSAs/NiNCs‐POPs, as a CO2 photoreduction catalyst, demonstrated an exceptional CO production rate of 6.24 mmol g−1 h−1, high selectivity of 98%, and excellent stability. The theoretical calculations uncovered that asymmetrical interaction between Ni1 SAs and Nin NCs not only favored the bending of CO2 molecules and reducing the CO2 reduction energy, but also regulated the electronic structure of the catalyst leading to the optimal binding strength of intermediates.

Fourier transform infrared (FT-IR) spectra were recorded on a Bruker ALPHA II FT-IR spectrometer.
The X-ray photoelectron spectroscopy (XPS) measurements were tested using Thermo Scientific K-Alpha.The C peak at 284.8 eV was used as a reference to correct for charging effects.In situ XPS (Thermo Fischer, ESCALAB 250Xi) was conducted by Thermo Fisher Scientific (ESCALAB 250Xi) equipped with light source device.
Solid-state UV-vis diffuse reflectance spectra were collected on UH4150 spectrophotometer using BaSO4 as the reference.
Nitrogen sorption isotherms were measured at liquid nitrogen temperature (77 K) by using automatic volumetric adsorption equipment (Belsorp Max) after a degassed process at 120 ℃ for 12 h.Specific surface areas were obtained by using the Brunauer-Emmet-Teller (BET) model, pore size distributions were simulated by the nonlocal density functional theory (NLDFT) model.CO2 adsorption isotherms were measured at 273 K and 298 K by using automatic volumetric adsorption equipment (Belsorp Max) after a degassed process at 120 ℃ for 12 h.The inductively coupled plasma mass spectrometry (ICP-MS) analysis was recorded on Agilent 5110 (OES).
Morphology of all samples were carried out using Zeiss Sigma 500 on a scanning electron microscopy (SEM) measurement.
Transmission electron microscopy (TEM) images were obtained in FEI TalosF200S.
Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were obtained in FEI Titan cubed Themis G2 300 STEM with aspherical aberration corrector.
Photoluminescence spectra and luminescence decay was measured on a HORIBA FluoroLog-3 fluorescence spectrometer.
Electrochemical test was carried out on electrochemical working station CHI 660E (Shanghai).
Isotopic labeling control experiments were obtained on gas chromatography-mass spectrometry (7890A and 5975C, Agilent Technologies).
Thermogravimetric analysis (TGA) was recorded by a TGA Q50 thermal analyzer from room temperature to 800 ℃ under N2 atmosphere using a heating rate of 10 ℃/min.The produced gas was monitored by Agilent GC7820 Gas Chromatograph (N2 as gas carrier, and the columns of GC are Porapak Q and MolSieve 5A).
The CO2RR reaction pathways of as-prepared catalysts were detected via in situ DRIFTS spectrometer (BRUKER INVENIO S).
Synthesis of 3D-POPs: HATT (16.9 mg, 0.03 mmol,1 equiv.),TFBD (13.4 mg, 0.045 mmol, 1.5 equiv.)were carefully placed into a vial with a mixed solution of DMF (0.2 mL) and n-BuOH (0.8 mL).The mixture was sonicated few minutes until it becomes homogeneous solution, then aqueous acetic acid (6.0 M, 0.2 mL) was added.After heating in an isothermal oven at 120℃ for 5 days, the mixture was cooled to room temperature.The resulting brown solid was soaked in DMF solvent for 3 days (the solvent was changed many times during this period) to remove all unreacted TFBD and HATT.After this process, the sampled was isolated and activated by Soxhlet extractor with THF for 2 days and dried at 80 ℃ under vacuum overnight to give 3D-POPs in a yield of 50%.
The precipitate was isolated after centrifuged, washed with MeOH for 12 h to ensure complete removal of residual Ni (OAc)2•4H2O, and then dried at 80 ℃ under vacuum overnight, to give 3D-NiSAs-POPs.

Photocatalytic Experiments:
The photocatalytic CO2 reduction experiments of all the synthesized catalyst was carried out in a 50 mL double-walled quartz reactor with a 300 W Xenon lamp equipped with UV cutoff filter (λ > 420 nm) as light source.The reaction was kept at ambient temperature for stirring, the detailed procedure is as follows: Primarily, 1 mg catalyst, 5 mg photosensitizer (Ru(bpy)3Cl2•6H2O) and 25 mg sacrificial agent (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole, BIH) were put into a mixed solvent system of 4 mL deionized water and 6 mL acetonitrile.After sonicating, the suspension become homogeneous.
Next, high-purity carbon dioxide was introduced into the reaction system for 25 min to remove the air and full of CO2 atmosphere.The straight-line distance from light source to the outer wall of the reactor was fixed to 3 cm.Agilent GC7820 Gas Chromatograph equipped with FID detector and TCD detector was used to monitor the gas product (300 μL of the headspace was injected into the gas chromatography every hour.).Three runs of consecutive photocatalytic reduction of CO2 to CO by adding equal amounts of Ru(bpy)3Cl2•6H2O and BIH at each run to the photocatalytic system after 3 h of reaction and reintroducing CO2.In the pre-phase of photocatalytic experiment, using a white LED lamp (400 nm ≤ λ ≤ 800 nm) as the light source to screen the best photocatalytic conditions.

CO2-TPD measurement:
The sample was dried and pre-treated at 10 ℃ min −1 from room temperature to 100 ℃ in a He stream (50 mL min −1 ).When it is cooled to 50℃, 10% CO2/He mixed gas was injected until adsorption saturation, and the He air flow is switched to 50mL min −1 to remove the physically adsorbed CO2 on the surface.Then the thermal desorption of chemisorbed CO2 was performed in flowing He at a ramp rate of 10 ℃ min −1 to a final temperature of 400 ℃.During the process, the outgoing gas was detected by TCD detector.

Apparent quantum efficiency:
The apparent quantum efficiency (AQE) of the catalysts was measured by different bandpass filters (including 380, 420, 425, 475, 500 and 550 nm,) under the same photocatalytic reaction conditions.The AQE values were calculated as follow: where  CO is the molar number of the CO,   is Avogadro's constant (6.022×10 23 mol −1 ), ℎ is the Planck's constant (6.63×10 -34 m 2 kg s -1 ),  is the speed of light (3×10 8 m s −1 ), S is the irradiation area (cm 2 ), P is irradiation intensity (W cm −2 ), T is irradiation time (s), and λ is the wavelength of the light source, respectively.

Computational methods:
All the calculations were performed within the framework of the density functional theory (DFT) as implemented in the Vienna Ab initio Software Package (VASP 5.4.4) code within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation and the projected augmented wave (PAW) method [1-4] .The cutoff energy for the plane-wave basis set was set to 450 eV.The Brillouin zone of the surface unit cell was sampled by Monkhorst-Pack (MP) grids, with k-point mesh density of 2π × 0.04 Å −1 for structures optimizations [5] .The convergence criterion for the electronic self-consistent iteration and force was set to 10 −5 eV and 0.01 eV/Å, respectively.The vacuum layer of 15 Å was introduced to avoid interactions between periodic images.

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Photoelectrochemical measurements: All photoelectrochemical measurements (Electrochemical impedance spectroscopy, Mott-Schottky spots and Photocurrent) were performed on an electrochemical workstation CHI 660E (Chenhua Instrument, Shanghai, China) via a standard three-electrode system in 0.5 M Na2SO4 solution, which contains a working electrode, a platinum plate as counter electrode, and a saturated Ag/AgCl (KCl saturated) electrode as a reference electrode at ambient temperature.The catalyst (1 mg) was dispersed into a solution of 20 μL 5 wt% Nafion and 1 mL ethanol.For Mott-Schottky plot and electrochemical impedance spectroscopy (EIS) measurements, the resulting mixture was deposited onto the surface of ITO and left in the air for drying to prepare the working electrode.Mott-Schottky plots were recorded at frequencies of 500, 1000 and 1500 respectively.For photocurrent, glassy carbon electrode coated by Ru(bpy)3Cl2•6H2O and catalyst with Nafion as the working electrode and 300 W Xenon lamp with UV cutoff filter (λ > 400 nm) as light source.

Figure S4 .
Figure S4.Differential charge density by representing the depletion (blue) and accumulation (yellow) of electron distributions.

Figure S5 .
Figure S5.TEM images of 3D-POPs and corresponding EDS element mapping images.

Figure S21 .
Figure S21.The apparent quantum yields of 3D-NiSAs/NiNCs-POPs and the light absorption spectrum of the Ru photosensitizer.

Figure S28 .
Figure S28.PL spectra of the CO2 photoreduction system with the addition of increasing amounts of BIH.

Figure S32 .
Figure S32.Proposed photocatalytic CO2 to CO reduction pathways on models of NiNCs.

Figure S33 .
Figure S33.Proposed photocatalytic CO2 to CO reduction pathways on models of NiSAs.

Table S2 .
EXAFS fitting parameters at the Ni K-edge for various samples a CN, coordination number; b R, distance between absorber and backscatter atoms; c σ 2 , Debye-Waller factor to account for both thermal and structural disorders; d ΔE0, inner potential correction; R factor indicates the goodness of the fit.

Table S3 .
CO2 geometric parameter of different models.

Table S4 .
Comparison of CO2 photoreduction performance catalyzed by various Ni-based porous photocatalysts.