Double-rare earth monatomic catalyst and application thereof in photocatalytic reduction of CO2

Abstract

The invention discloses a double-rare earth monatomic catalyst and application thereof in photocatalytic reduction of CO2. The preparation method of the catalyst comprises the following steps: dissolving urea, Er (NO3) 3 and Nd (NO3) 3 in deionized water containing melamine sponge, stirring at room temperature for 2-4 hours, and performing liquid nitrogen freezing and vacuum drying; and then under the protection of nitrogen, raising the temperature to 500-600 DEG C and keeping the temperature for 1-4 hours. The prepared catalyst takes g-C3N4 as a support body, and Er < 3 + > and Nd < 3 + > are respectively coordinated with N atoms in two tri-s-triazine rings. The obtained catalyst shows excellent C2H5OH and CH3OH generation capacity in a photocatalytic reduction CO2 performance test, the generation rates are 1761.4 and 987.7 [mu] mol g <-1 > h <-1 > respectively, the ethanol selectivity is up to 64%, and 98.4% of catalytic performance and initial morphology and structure are still kept after three times of circulation.

Description

Technical Field

[0001] The invention belongs to the technical field of photocatalysis, and specifically relates to a double rare earth single atom catalyst and its application in photocatalytic reduction of CO2. Background Art

[0002] Artificial photocatalytic conversion of CO2 and H2O to fuels is a powerful and green approach to alleviate the energy crisis and environmental pollution. So far, the proton transfer involved by multiple electrons and the slow CC coupling process have led to problems such as low yield and poor selectivity. Researchers are increasingly aware that a suitable steric environment of the C1 intermediate is a prerequisite for achieving effective CC coupling. In principle, the binding strength of the key C1 intermediate on the catalyst should be stronger than that of the desorbed C1 product, while the binding strength of the steric C1 species needs to be weaker than that of the C1 intermediate. Among various C1 intermediates, CO * It is recognized as a key species in the synthesis of C2H5OH. * The study of the mechanism of conversion to C2H5OH has received extensive attention. In particular, * The vibrational signals of OCCOH and theoretical calculations suggest a precise mechanism for the CO * Synthesis by dimerization and hydrogenation * OCCOH, which is a key step in the precipitation process of C2H5OH. It can be seen that better CO * Adsorption kinetically accelerates CC coupling, and the charge polarization strategy can effectively regulate CO * Electronic interactions with the catalyst. Summary of the invention

[0003] The purpose of the present invention is to provide a novel method for synthesizing double rare earth single atom catalysts, namely, atomic confinement and coordination method, in view of the shortcomings of existing photocatalytic reduction of CO2 to synthesize fuels or chemicals, such as low yield and difficulty in CC coupling. This synthesis method is simple and easy to implement, low in cost, and can obtain rare earth catalysts with high single atom concentration, providing a feasible solution for the development of new rare earth catalytic CO2 reduction technology. The double rare earth single atom catalyst is based on g-C3N4 as a support, Er 3+ and Nd 3+ In the two tri-s-triazine rings, they are coordinated to the N atoms respectively.

[0004] The preparation method of the double rare earth single atom catalyst is as follows: urea, Er(NO3)3 and Nd(NO3)3 are dissolved in deionized water containing melamine sponge, stirred at room temperature for 2-4 hours and then freeze-dried in liquid nitrogen in a vacuum; then, under nitrogen protection, the temperature is raised to 500-600°C and maintained for 1-4 hours.

[0005] The molar ratio of Er(NO3)3 to Nd(NO3)3 is 0.3-3.

[0006] The double rare earth single atom catalyst prepared as above is applied to the photocatalytic reduction of CO2 reaction.

[0007] The specific operation of the photocatalytic reduction of CO2 reaction is: dispersing the double rare earth single atom catalyst in a triethanolamine aqueous solution, then charging CO2 to adsorption-desorption equilibrium, and photocatalytically reducing CO2 to obtain C2H5OH and CH3OH.

[0008] The present invention predicts and analyzes the electronic structure and electronic interaction between the 4f orbital of a single rare earth atom and the 2p orbital of a N atom in a carbon-nitrogen ligand unit based on the state density and differential charge density. According to the Gibbs free energy, it is revealed that CO * The spatial environment on the Er and Nd surfaces. Secondly, the double rare earth single atom catalyst with Er6N and Nd6N as the main structural units was successfully prepared by atomic confinement and coordination method. By integrating two adjacent rare earth single atom metals, the CO species produced on the surface of one single atom can be provided to another single atom, which is expected to improve the selectivity of high-order products. The mechanistic pathway of the synergistic photocatalytic reduction of CO2 to C2H5OH and CH3OH by double rare earth single atoms was further explored. The combination of theoretical calculations, X-ray absorption near-edge structure analysis and in situ FT-IR spectroscopy analysis showed that the Er-N bond is conducive to charge transfer and is the active center for the formation of CC bonds, while the Nd single atom provides more required CC coupling in the photocatalytic C2H5OH synthesis. * CO intermediate. Gibbs free energy shows that this catalytic strategy gives single-atom stability * CO species and promotion * The ability to generate CHO intermediates leads to the production of high yields of C2H5OH and CH3OH.

[0009] The beneficial effects of the present invention are:

[0010] 1. The preparation method of the double rare earth single atom catalyst obtained by the present invention is simple and low in cost, and a single atom rare earth catalyst with a high metal concentration can be obtained under relatively mild conditions. The contents of Er single atom and Nd single atom in the Er1Nd3 / CN sample after the synthesis conditions are optimized are 10.36wt% and 10.41wt%, respectively.

[0011] 2. The catalyst showed excellent C2H5OH and CH3OH generation capabilities in the photocatalytic reduction of CO2 performance test. Using a 300W xenon lamp equipped with a 420nm cut-off filter to simulate visible light, the double rare earth single atom catalyst set a new record for the formation rate of C2H5OH and CH3OH, with the generation rates of 1761.4 and 987.7μmol g, respectively. -1 h -1 , ethanol selectivity is as high as 64%. Among them, Er1Nd3 / CN showed excellent durability in the stability experiment, and still maintained 98.4% of the catalytic performance and the original morphology and structure after 3 cycles.

[0012] 3. CO on Nd / CN surface * It is easier to desorb and transfer to the Er single atom surface, thereby promoting the CC coupling process to produce ethanol. * Adsorption reveals CC coupling mechanism and provides a basis for CO2 to C n H 2n+1 It provides new ideas for the industrial-grade photocatalytic conversion of OH (n=1,2). BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is the theoretically calculated differential charge diagram.

[0014] Figure 2 This is the transmission electron microscope spectrum of the Er1Nd3 / CN single atom prepared in Example 1.

[0015] Figure 3 This is the XRD spectrum of the Er1Nd3 / CN single atom catalyst prepared in Example 1.

[0016] Figure 4 This is the synchrotron radiation test image of the Er1Nd3 / CN single atom catalyst prepared in Example 1.

[0017] Figure 5 This is the performance diagram of the photocatalytic reduction of CO2 of the Er1Nd3 / CN single atom catalyst prepared in Example 1.

[0018] Figure 6 This is the in-situ infrared image of the Er1Nd3 / CN single atom catalyst prepared in Example 1 under light conditions.

[0019] Figure 7 This is a diagram showing the mechanism of photocatalytic reduction of CO2 by the double rare earth single atom catalyst of the present invention. DETAILED DESCRIPTION

[0020] Example 1

[0021] 1) 5.40 g urea, 0.11 g Er(NO3)3·5H2O and 0.33 g Nd(NO3)3·6H2O were dissolved in 20 ml deionized water containing melamine sponge (1.2 cm×1.2 cm×2 cm) and stirred at room temperature for 120 min until the melamine sponge was uniformly filled with the mixed solution of urea, Er(NO3)3·5H2O and Nd(NO3)3·6H2O.

[0022] 2) The melamine sponge filled with the mixed solution was quickly frozen with liquid nitrogen and treated in a vacuum freeze dryer for about 3 days.

[0023] 3) Under nitrogen protection, the dried sponge was heated at 550°C for 10°C min -1 The sample was calcined at a rate of 2 h and labeled as Er1Nd3 / CN.

[0024] The double rare earth single atom catalyst Er1Nd3 / CN prepared by the above method was applied to the photocatalytic CO2 reduction reaction: 25 mg of Er1Nd3 / CN catalyst was uniformly dispersed in a water system containing 5% triethanolamine, and high-purity gas CO2 was purified at 40 ml min -1 After the CO2 has completed the adsorption-desorption equilibrium on the catalyst surface (about 30 min), the CO2 rate is adjusted to 10 mL min -1 , irradiated with 300W Xe lamp for 2h. The products were detected by gas chromatography and nuclear magnetic resonance to quantify the yield and selectivity of the catalyst. No products such as CO, CH4, HCOOH, HCOH, and CH3CHO were detected in the gas and liquid environments after the reaction.

[0025] The catalyst prepared in Example 1 was characterized:

[0026] Depend on Figure 1 It can be seen that electrons are transferred from rare earth atoms to the connected N atoms, creating positive charges on the Er and Nd surfaces. This more positive charge on the Nd surface will make it easier to achieve CO2 - The stability of the intermediate. At the same time, the remaining charge density is asymmetrically distributed on top of the Er atom, which will be beneficial for the activation of CO2 through electron donors.

[0027] Figure 2 The typical nanotube structure of Er1Nd3 / CN is shown, and the pore size distribution of the nanotube is 10-15nm. The white spots clearly observed on the surface of the nanotube are marked by the yellow dotted box, and there are no obvious rare earth metal particles, which may be due to the high dispersion of rare earth in the form of single atoms.

[0028] Figure 3The introduction of rare earth metals significantly weakens the intensity of the diffraction peak at 26.28°, indicating the destructive stacking of the carbon conjugated aromatic system. In addition, no Er or Nd species were found, which is attributed to the high dispersion of high concentrations of single atoms.

[0029] Figure 4 It shows that the coordination number of Er, Nd and N is 6, indicating that the double rare earth single atom was successfully synthesized.

[0030] Figure 5 The catalyst performance after optimized synthesis conditions was 1761.4 μmol g -1 h -1 The C2H5OH rate and 987.7 μmol g -1 h -1 The CH3OH rate and the C2H5OH generation capacity are Er / CN (77.6 μmol g -1 h -1 ) and Nd / CN (64.8 μmol g -1 h -1 ) are 22.7 times and 27.2 times.

[0031] Figure 6 It shows that as the illumination time increases to 30min, HCO3 - Strength (1720-1790cm -1 ) gradually decreased, indicating that CO2 was first converted into bicarbonate ions and then reduced in sequence. -1 and 1648cm -1 A new peak was observed at * COOH group, which is the key intermediate in the formation of CH3CH2OH and CH3OH. 1236cm -1 May belong to through a short * CO- * CO coupling * OCCOH, which is the key evidence of CC coupling.

[0032] Example 2 and Comparative Example

[0033] The catalysts used in the photocatalytic reduction of CO2 performance evaluation experiment mentioned in Example 1 were replaced with Er / CN, Nd / CN and Er-Nd / CN in turn. These three catalysts were synthesized by the same preparation steps as Er1Nd3 / CN, except that the mass ratios of Er(NO3)3·5H2O to Nd(NO3)3·6H2O were 4:0, 0:4 and 2:2, respectively.

Claims

1. A double rare earth single atom catalyst, characterized in that: The catalyst is supported by g-C3N4. 3+ and Nd 3+ In the two tri-s-triazine rings, they are coordinated to the N atoms respectively.

2. A method for preparing double rare earth single atom catalyst, characterized in that: The specific operation of the preparation method is: dissolving urea, Er(NO3)3 and Nd(NO3)3 in deionized water containing melamine sponge, stirring at room temperature for 2-4 hours and then freezing and vacuum drying with liquid nitrogen; then heating to 500-600°C and maintaining for 1-4 hours under nitrogen protection.

3. The preparation method according to claim 1, characterized in that: The molar ratio of Er(NO3)3 to Nd(NO3)3 is 0.3-3.

4. Use of the double rare earth single atom catalyst prepared according to the method of claim 2 or 3 in the photocatalytic reduction of CO2 reaction.

5. The use according to claim 4, characterized in that: The specific operation of the photocatalytic reduction of CO2 reaction is: dispersing the double rare earth single atom catalyst in a triethanolamine aqueous solution, then charging CO2 to adsorption-desorption equilibrium, and photocatalytically reducing CO2 to obtain C2H5OH and CH3OH.

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