Keto-anthraquinone covalent organic framework for H2O2 photosynthesis with oxygen and alkaline water

Hydrogen peroxide photosynthesis suffers from insufficient catalytic activity due to the high energy barrier of hydrogen extraction from H2O. Herein, we report that mechanochemically synthesized keto-form anthraquinone covalent organic framework which is able to directly synthesize H2O2 (4784 μmol h−1 g−1 at λ > 400 nm) from oxygen and alkaline water (pH = 13) in the absence of any sacrificial reagents. The strong alkalinity resulted in the formation of OH-(H2O)n clusters in water, which were adsorbed on keto moieties within the framework and then dissociated into O2 and active hydrogen, because the energy barrier of hydrogen extraction was largely lowered. The produced hydrogen reacted with anthraquinone to generate anthrahydroquinone, which was subsequently oxidized by O2 to produce H2O2. This study ultimately sheds light on the importance of hydrogen extraction from H2O for H2O2 photosynthesis and demonstrates that H2O2 synthesis is achievable under alkaline conditions.

2200/PC diffractometer with a Cu-Kα radiation (λ=0.154nm).Solid-state nuclear magnetic resonances (NMR) spectra of C 13 were carried out on a Bruker BioSpin Avance III HD 400 high performance digital NMR spectrometer.X-ray photoelectron spectra (XPS) and Ultraviolet photoelectron spectroscopy (UPS) were implemented on an Axis Ultra DLD system (Shimadzu/Kratos) that was calibrated by C 1s at 284.6 eV.
Raman spectra were recorded on a Bruker Senterra R200-L dispersive Raman microscope at 532 nm.Fourier transform infrared (FTIR) spectra were obtained from a Nicolet 6700 spectrometer (Thermo Electron).Transmission electron microscopy (TEM) images were obtained from a TECNA1 G2F20 microscope at an accelerating voltage of 200 kV.Scanning electron microscope (SEM) images were obtained on a Hitachi S-4800 microscope.The UV-vis diffuse reflectance spectrophotometer (DRS, Lambda 950, PerkinElmer, USA) was used to investigate the optical properties of catalysts.The steady-state and transient-state photoluminescence (PL) spectra were obtained on an Edinburgh-250 instrument with an excitation wavelength at 250 nm.
Nitrogen adsorption-desorption isotherms were analyzed on a Micromeritics ASAP 2020 analyzer.

Kinetics analysis of H2O2
Kinetic analysis was carried out to assess the formation and decomposition rate constants of H2O2 during photocatalysis.The kinetic constants were calculated by fitting the H2O2 production curves via the following Box-Lucas model.
Here, kf is the zero-order H2O2 formation rate constant; kd is the first-order H2O2 decomposition rate constant; t is the reaction time.

Apparent quantum yield (AQY)
The apparent quantum yield (AQY) was calculated by measuring H2O2 photosynthesis of Kf-AQ under different monochromatic light irradiation.The light intensities of 400, 450, 550, 600 and 650 nm were measured by an optical power meter (FZ-A, China).The AQY (%) was calculated by the equation S3. 1

(S3)
The amount of H2O2 is presented in Table S3.The total photon number entering into the reactor can be calculated by the product of S, t, P, and λ, wherein, S is the irradiation area (m 2 ); t is the reaction time (3600 s); P is the incident monochromatic light intensity (W m -2 ); λ is the wavelength of the incident monochromatic light (m), respectively.

Solar-to-chemical conversion (SCC) efficiency
For the investigation of SCC efficiency, 5 mg Kf-AQ was dispersed into 30 mL ultrapure water with different pH.After 1.0 h visible light (λ > 400 nm) illumination, the total incident power and amount of generated H2O2 were well calculated (Table S4).
The SCC efficiency of Kf-AQ in different pH were determined by the equation S4, 2 wherein, △G (117 kJ mol -1 ) is represent the free energy of H2O2 generation;   (3600 s) is represent the irradiation time;   (3.74×10 -3 m 2 ) is represent the irradiation area;  400 (984 W m -2 ) is represent the incident light intensity, respectively.

Electron transfer number (n)
The electron transfer number for oxygen reduction reaction (ORR) was measured on a rotating disk electrode (RDE) in an O2-saturated Na2SO4 (0.1 mol L -1 ) system with different rotating speeds.The average numbers of electrons (n) were calculated by the Koutecky-Levich equation: 3 Here, i and ik are the current density (μA cm -2 ) and kinetic current density (μA cm -2 ), respectively; n is the number of electron transfer; F is the Faraday constant (96485 C mol -1 ); A is the working electrode area (0.196 cm 2 ); D is the oxygen diffusion coefficient (1.93×10 -5 cm 2 s -1 ); v is the kinematic viscosity of the electrolyte (0.0109 cm 2 s -1 ); C is the saturated oxygen concentration in water (1.26×10 -3 M); ω is the rotating speed.
Figure S2.FTIR characterization.(a) FTIR spectra of Tp, AQ and Kf-AQ, (b) the amplified regions of FTIR spectra in the range from 600 cm -1 to 2000 cm -1 .

Figure S5 .
Figure S5.BET characterization of Kf-AQ.(a) N2 adsorption-desorption isotherm and (b) pore size distribution of Kf-AQ.The specific surface area of Kf-AQ is calculated to be 141.7 m 2 g -1 , and the pore size is centered at 2.2 nm.The mesoporous distribution indicates the stacking structure of Kf-AQ.

Figure S17 .
Figure S17.Analysis of photogenerated charge separation.The steady-state (a) and transient-state (b) fluorescence spectra of TpAQ and Kf-AQ.

Figure S24 .Figure S27 .
Figure S22.Synthesis process and structural characterization of TpDA.(a) Synthesis of the TpDA.(b) PXRD patterns, experimentally observed (dark), simulated using eclipsed AA-stacking (red) model.(c) FIRT spectra of Tp, DA and TpDA.FTIR spectra revealed the formation of a new C-N stretching band at 1245 cm -1 , ascribing to the Tp and DA conjugation.(d) 13 C NMR spectra of TpDA.The chemical shift at 184 ppm, 175 ppm and 146 ppm are ascribing to C=O, C-OH and C-NH-, respectively.The keto-form structure is dominated in the TpDA.

Table S2 .
Kinetic constants of kf and kd.The kinetic constants of kf and kd at different

Table S4 .
SCC calculation parameter.Details of the SCC calculation for Kf-AQ.