Introduction

The CO2 reduction (CO2RR) to form valuable chemicals has been an active area of research in recent years1. In order to achieve the goal of converting CO2 into valuable items, photoelectrocatalytic Carbon Dioxide Reduction (CO2R) is an effective way to convert inorganic materials into high-value organic materials2,3,4,5.

Photosynthesis is a process used by plants to convert carbon dioxide into valuable carbohydrates6. Photosynthesis occurs primarily through chlorophyll, the porphyrin ring, which enables carbon dioxide reduction. Porphyrin rings have strong selective absorption of blue-violet and red light, and their large conjugated π-π system provides a transport platform for photoelectrons. Several studies have investigated the use of porphyrin materials for CO2 electrochemical reduction. For example, a study by Ping et al. reported the use of cobalt-porphyrin complexes as electrocatalysts for the reduction of CO2 to formic acid with high selectivity and efficiency7. Another study by Wu et al. investigated the use of zinc-porphyrin complexes as electrocatalysts for CO2 reduction and demonstrated improved performance compared to traditional catalysts8. Whereas Zheng et al. have reported Iron (III) tetraphenylporphyrin thiocyanate having a subtle electrostatic interaction between the SCN ligand and the metal center9. Electrostatic interaction has led to a non-negligible effect on its electrocatalytic activity. Tridecene is a trimethylene derivative with internal free volume (IFV) channel, which has specific absorption of CO2. COFs materials prepared based on porphyrin and trichothecene (TC) can be simultaneously photoelectrically active and efficiently adsorb CO2.

Urea in the production process, the need to use extremely high energy consumption and flammable high-purity hydrogen, such preparation conditions will further deepen the environmental pollution10,11. For example, the process generate large amounts of greenhouse gases (GHGs) and nitrogen oxides (NOx) and aggravates global energy and environmental problems12. The synthesis of NH2CONH2 by electrochemical reduction13 instead of the Bosch-Meiser method is an attractive alternative to reduce chemical energy consumption, reduce environmental pollution, and improve efficiency. The electrochemical reduction of N2 and CO2 to make NH2CONH2 has been widely used, but the inertness of the N-N bond and the higher dissociation energy were the difficulties of the reaction. The dissociation energy of the N-O bond was much lower than that of the dissociation energy of the N-N bond14, NO3RR and NO2RR instead of N2RR was developed, which made the photoelectrocatalytic synthesis of NH2CONH2 easier. High concentration of nitrate and nitrite (NO2−, NO3−) leads to microbial enrichment, bacterial growth, and when absorbed by the organism it reacts with amines and causes methemoglobinemia and gastrointestinal cancers in the organism, so NO2RR is simpler and has significance in protecting the environment for generating NH2CONH2.

Catalysts such as single atom catalysts (SACs) and metal-organic frameworks (MOFs) are commonly used in photoelectrocatalysis14,15,16,17,18,19,20,21. Single-atom catalysts require fine tuning to avoid agglomeration during preparation; metal-organic frameworks not only have high morphological requirements, but also tend to deactivate during catalysis22. In addition, most of these catalysts require ionic solvents and critical conditions, leading to cumbersome post-treatments and consuming extra energy23. In contrast, covalent organic frameworks (COFs) consist of strong covalent bonds and light elements with good water stability24, which reduces the abuse of metals and lowers the difficulty of recycling. Their \(\pi\text{-}\pi\) conjugated electronic structure allows COFs to have suitable energy levels and moderate visible light-accepting band gaps25,26. Based on these features, COFs are considered as a promising metal-free semiconductor photocatalyst13,27,28,29,30,31,32,33,34,35,36,37. The use of COFs materials for photoelectrocatalytic CO2RR avoids complicated post-processing as well as harsh reaction conditions.

In this paper, a COFs catalyst based on porphyrin parent core and triptycene skeleton was prepared for the reduction coupling of CO2 with NO2− to generate NH2CONH2, which is the first time that COF_PP−TAT, a non-metallic catalyst, was applied to CO2RR for the synthesis of NH2CONH2. Characterisation data illustrated the excellent optical property and electrocatalytic activity. Under the optimal conditions,

Fig. 1
figure 1

Synthesis of COF_PP−TAT.

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COF_PP−TAT achieved continuous photoelectrocatalytic CO2 and NO2− co-reduction to generate NH2CONH2 for 8 h with a yield of 0.32 µmol h− 1. And the synthesis mechanism was described by infrared data and mechanism diagram.

Experimental section

Materials

All chemicals and reagents were used without any further purification. 4-formylphenylboronic acid, 10,15,20-tetrakis(4-bro-mophenyl)-porphyrin (TBPP), trichothecenes (TC), tetrakis(tri-phenylphosphine) palladium (10% Pd(PPh3)4), palladium-carbon catalyst (Pd/C), and potassium carbonate (K2CO3) were purchased from Anergy Chemistry, Indophenol blue reagent was purchased from Shanghai Amperexperiment Technology. Deionized water was used in all experiments.

Synthesis

Synthesis of 5, 10, 15, 20-tetrakis(4-formyl-biphenyl)-porphyrin (TBPP)

TBPP (1.20 g, 1.29 mmol) and 4-formylphenylboronic acid (TC, 1.40 g, 9.34 mmol) were dissolved in 80 mL DMF, 160 mL toluene and 40 mL H2O in a three-necked flask, then added 10% Pd(PPh3)4 (0.18 g). The solution was heated to reflux and reacted for 72 h under N2. The product was suction filtered and collected as violet black cake. The crude product was washed with in Dichloromethane (DCM). The mass of synthesized pure TBPP products is 1.21 g.

Synthesis of 2,6-Dinitrotriptycene (TAT)

Triptycene (2.54 g, 10.0 mmol) was dissolved in DCM (50 mL). Cerium ammonium nitrate (11.0 g, 20.0 mmol) was added, followed by sulfuric acid (1.92 mL, 3.53 g, 36.0 mmol). The reaction was stirred at room temperature overnight under N2. The mixture was filtered to remove the cerium salts, and the filter-cake was washed with DCM until colorless. The comb-ined organic fractions were basified with K2CO3 solution (10%, 100 mL), the organic layer was taken, and the aqueous phase was extracted with further DCM (50 mL). The combined organic fractions were dried (MgSO4), taken todryness under reduced pressure, and purified by column chromatography (gradient: 2:1 to 1:1 petrol: DCM) to give pure 2,6-dinitrotriptycene. Ethanol (100 mL) was cooled in an ice-bath under N2. Palladium on carbon (10 wt %, 0.34 g) was added, followed by 2,6 - dinitrotriptycene (3.44 g, 10.0 mmol), and hydrazine hydrate (3.4 mL) dropwise. The reaction was heated to reflux under N2 overnight and then cooled to room temperature. The product was collected by washing with ethanol and the mass of pure TAT products is 2.08 g.

Fig. 2
figure 2

(A) UV-visible diffuse reflectance spectra of COF_PP−TAT, (B) SEM spectra of COF_PP−TAT, (C) CO2 adsorption curve of COF_PP−TAT, (D) COF_PP−TAT sunder different atmospheres, (E) CV curve of COF_PP−TAT under different wavelengths of light irradiation (F) PP, TAT and COF_PP−TAT under light and dark conditions.

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Synthesis of COF_PP-TAT

At first, TAT (0.31 g, 1105.60 µmol) was dissolved in 100 ml anhydrous DCM. A solution of TBPP (0.31 g, 280.00 µmol) was prepared in 100 ml DCM. 0.5 ml of 6 M acetic acid catalyst was added. This solution was added dropwise to the TAT solution at room temperature under Ar. Once the addition was completed the reaction mixture was stirred for 24 h. Finally, the reaction was refluxed at 50 ℃ for 48 h under O2. This precipitate was then collected by filtration and thoroughly washed with DCM. The mass of pure COF_PP−TAT products is 0.183 g. All of the synthesis routes are shown in Fig. 1.

Results and discussion

Catalyst characterization

UV spectroscopy, fluorescence spectroscopy, and photo-thermal experiments were performed to investigate the optical properties of the material. The absorption spectrum of COF_PP-TAT was obtained in DCM (Fig. 2A). The spectrum in solution is characteristic of a porphyrin, featuring an intense Soret band at 413 nm and four Q bands at 512, 548, 589, and 644 nm. The UV absorption capacity of the material is interconnected with the fluorescence maximum excitation wavelength, and the fluorescence spectra show (Fig. S4) that the material has high fluorescence intensity under fluorescence excitation at 300, 450, and 650 nm, which is correspondingly linked with its UV absorption peak. The absorption peak at 670 nm is close to the region of near-infrared light, and by irradiating the samples with different concentrations by 808 nm lamp source, the rate of COF_PP-TAT warming was positively correlated with its concentration (Table S1), and the above results indicate that the material can absorb and utilise photoelectrons at multiple wavelengths for photoelectrocatalysis and has the ability of photothermogenesis.

In order to investigate the surface morphology and adsorption of CO2 on COF_PP−TAT photocatalyst, scanning electron microscope (SEM) and BET surface area were used to characterize the photocatalysts. Materials showed that the large mesopores with large surface area (Fig. 2B), and the adsorption curve (Fig. 2C) indicates that the COF_PP−TAT has a better adsorption of CO2, which are favourable for performing the photocatalytic reduction.

Electrochemical reduction of CO2

To evaluate the CO2RR performance of as-prepared materials, a series of electrochemical tests were conducted in a standard H-cell. As shown in Fig. 2D, the COF_PP−TAT cathode exhibited a significantly higher current density in the CO2-saturated or N2-saturated electrolyte than that in the Ar-saturated solution, indicating that CO2RR or N2RR is more favorable than HER. Furthermore, The CV curves (Fig. 2E) to evaluate the light response performance catalyzed by materials. COF_PP−TAT at the 808 nm light illumination shows the highest catalytic activity of -8.4 µA cm− 2 compared to those of 460 nm ( -6.9 µA cm− 2) and Dark ( -5.7 µA cm− 2), indicating its excellent photocatalytic activity of CO2RR.

The photocatalytic activities of PP and TAT were also examined, and it can be seen from Fig. 2F that TAT has almost no photocatalytic activity. PP has a more powerful current response value under 808 nm illumination ( -35.2 µA cm− 2), which is better than that under dark conditions ( -21.6 µA cm− 2). COF_PP−TAT showed a 15% increase in photoelectron utilisation under 808 nm light compared to the dark condition. The porphyrin-based COFs has good electrical conductivity and photocatalytic.

A series of concentrations of NaHCO3 solutions were set to investigate the optimal CO2RR concentration of COF_PP−TAT. When the concentrations were at a low level (0.125 M to 0.5 M), the CO2RR ability of COF_PP−TAT increased with increasing concentration. The catalytic performance increased with the increase of c(NaHCO3), indicating that COF_PP−TAT has CO2RR performance, and the optimal catalytic reduction c(NaHCO3) is 0.5 M.

A series of concentrations of NaNO2 solutions were set to investigate the optimal NO2RR concentration of COF_PP−TAT. When the concentrations were at a low level (0.05 M to 0.4 M), the NO2RR ability of COF_PP−TAT increased with increasing concentration. In addition, the increasing oxidation peak at 0.2 V (vs. SCE) is caused by NO2− being catalytically oxidised. Catalytic performance increased with the increase of c(NaNO2), indicating that COF_PP−TAT has NO2RR performance, and the optimal catalytic reduction c(NaNO2) is 0.4 M.

The photoelectrocatalytic reduction reaction of COF_PPTAT was carried out in a reaction system of 0.5 m NaHCO3 and 0.4 M NaNO2 under the irradiation of a 300 W xenon lamp at an applied voltage of -0.78 V vs. SCE.

Online gas chromatography (GC),1H-NMR and indophenol blue spectrophotometric method were adopted to detect gaseous and water-soluble products of CO2RR. The electrocatalytic performance of COF_PP-TAT was tested in 0.5 M mixture of NaHCO3 and NaNO2 using an H-type cell equipped with a cation exchange membrane to separate each compartment. The GC spectrum of the products (Fig. S5) reveals the presence of CO2、O2、N2 and CO at retention time of 3.006 min, 9.126 min, 9.561 min, and 11.409 min, respectively. CO2 is the gas supply to the system, O2 and N2 are from air. CO is the gaseous reduction product obtained in photoelectrocatalytic reduction. Formic acid, and urea are detected by NMR spectroscopy (Fig. 3A) at the chemical shifts of 8.33 and 5.68 ppm severally. When the liquid sample was mixed with indophenol blue colour developer, the solution changed from yellow to blue-green (Fig. S6), indicating that NH3 was present in the system.

Urea quantification

Set up a series of concentrations of ammonia standard solution, after the reaction with indophenol blue colour developer, take the absorbance value at 665 nm to plot the standard curve calculation (Fig. S7). The concentration of NH2CONH2 in the sample was calculated to be 21.3 µmol L− 1, and the yield per unit time was 0.32 µmol h− 1.

Catalytic properties and mechanism

The functional group changes of the electrolyte before and after catalysis were detected to illustrate the mechanism of NH2CONH2 synthesis. The O-H bending vibrational peak located at 2000 cm− 1 at the electrolyte before photoelectrocatalysised (Figure S8). When the electrolyte was photoelectrocatalyzed, the peaks at 1161, 1370, 1458 and 1600 cm− 1 in the liquid product belonged to the -NH2 in-plane deformation vibration, stretching vibration peaks, C-N stretching vibration peak and C = O stretching vibration peak, which were the characteristic absorption peaks of NH3 and NH2CONH2, respectively. It indicated that the COF_PP−TAT photoelectro-catalytic CO2 reduction to generate NH2CONH2 is accompanied by the by-product NH3.The mechanism of CO2 and NO2− synthesis of NH2CONH2 is as follows (Fig. 3B), the NO2− in the system adsorbed to the surface of the catalyst to form *NO2, which was reduced to *NH by electron transfer and dehydration, and CO2 was adsorbed, reduced, and dehydrated to obtain *CO, and the two of them were coupled in situ on the surface of the catalyst by electron transfer, and reduction and hydrogenation, which ultimately led to the formation of NH2CONH2.

Fig. 3
figure 3

1H-NMR spectrum of reduction product (A), urea synthesis mechanism (B).

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Conclusions

In conclusion, we synthesized a non-metallic photocatalyst COF_PP−TAT. It was synthesised based on porphyrin and tridecene, which can effectively activate CO2 and NO2− and couple to synthesise urea. The synthesised COF_PP−TAT possesses the large conjugated \(\pi\text{-}\pi\) structure of PP and the IFV channel of TAT, which results in a rich pore structure and efficient adsorption of CO2 in the material. It provides a new green way for efficient synthesis of urea and even amino acids.