Preparation method of Pd deposition-precipitation method supported walnut shell doped carbon nitride catalyst and application of catalyst in catalytic formic acid decomposition hydrogen production reaction

Abstract

The invention discloses a preparation method of a Pd deposition precipitation method loaded walnut shell doped carbon nitride catalyst and application of the catalyst in a reaction of catalyzing formic acid decomposition to produce hydrogen, the preparation method comprises the following steps: firstly, successfully preparing a walnut shell doped g-C3N4 carrier material by adopting a thermal polymerization method and taking walnut shells and melamine as precursors; then, a Pd-loaded walnut shell-doped carbon nitride catalyst is prepared by adopting a deposition precipitation method, and the catalyst is used for catalyzing a formic acid decomposition hydrogen production reaction and has relatively high catalytic reaction activity and very high catalytic performance; and no CO is generated in the reaction process, so that the catalyst is high in hydrogen selectivity (nearly 100%), and has excellent popularization and application prospects.

Description

Technical Field

[0001] The present invention relates to the field of catalyst preparation, and particularly relates to a preparation method of a Pd deposition-precipitation method supported walnut shell doped carbon nitride catalyst and its application in the catalytic decomposition of formic acid to produce hydrogen. Background Art

[0002] As a zero-carbon energy source, hydrogen gas (H2) occupies a very important position in the construction of a sustainable green energy system due to its advantages such as being renewable, clean and pollution-free. However, storing and transporting H2 in a safe and economical way is still a challenge in the current development of hydrogen energy, and there is an urgent need to develop new sustainable in-situ hydrogen production technologies. Formic acid hydrogen production is considered to be a very promising in-situ hydrogen production technology. Since formic acid (FA, HCOOH) is the main product of biomass processing and CO reduction, it has excellent stability, high hydrogen content (up to 4.4 wt.%) and low toxicity. There are two paths for formic acid decomposition: dehydrogenation (HCOOH→CO2+H2) and dehydration (HCOOH→CO+H2O). However, the production of CO in the dehydration path will poison the hydrogen production catalyst, resulting in the degradation and even inactivation of the catalyst performance, which is not conducive to the dehydrogenation reaction. Therefore, developing high-performance catalysts with high selectivity and no CO impurity generation is of great significance for hydrogen storage in FA.

[0003] In the formic acid decomposition to produce hydrogen reaction, Pd-based catalysts have both high catalytic activity and hydrogen selectivity, becoming a very advantageous catalyst system in this reaction. Most scholars believe that the optimization of the formic acid decomposition reaction performance highly depends on the precise regulation of the Pd size and electronic structure. The support effect is an effective means to regulate the Pd size. By modifying and anchoring groups on the support surface, fine regulation of nanoparticles can be achieved, enabling them to be uniformly dispersed in small sizes, thereby significantly improving the utilization rate of active sites and optimizing the electronic structure of Pd to enhance its activity, selectivity and stability. Currently, common and typical support materials mainly include metal oxides, graphitic carbon nitride (g-C3N4), activated carbon materials, and metal-organic frameworks, etc. Among them, g-C3N4 is a good support for Pd nanoparticles due to its advantages such as easy preparation, thermal / chemical stability, rich active sites, and easy surface modification. Guo et.al prepared C atom self-doped g-C3N4. By adjusting the molar ratio of uracil to dicyandiamide, the C doping level or the concentration of defects can be controlled, and the electronic structure and energy band structure of g-C3N4 can be precisely regulated. After introducing Pd species, Pd single atoms (Pd1) and small-sized Pd nanoparticles (Pd NP ) are formed. Cao Yong et al. successfully prepared a series of CNx supports with different pyridine N contents by pyrolyzing different ratios of chitosan / melamine mixed precursors at different temperatures, and synthesized a series of Pd / CNx catalysts by sodium borohydride reduction method, confirming that there is a strong correlation between the pyridine N content on the support surface and the catalyst activity.

[0004] Most studies have mainly explored the role of Pd 0 in the decomposition of formic acid to produce hydrogen, while the contribution of Pd 2+ in this process has the potential for further research. It has been reported that Pd ions play a significant role in improving the performance of catalysts. For example, Haiqin Wan et al. prepared N-rich mesoporous carbon materials and used them as carriers to load Pd (Pd / NMC), obtaining Pd 0 and Pd 2+ co-loaded catalysts. The authors believe that the basic sites (pyridine nitrogen) on the carrier adsorb formic acid molecules, promoting their dehydrogenation to produce HCOO - , and Pd 2+ adsorbs HCOO - through electrostatic effects, and then transfers to the Pd 0 site, promoting the cleavage of the C-H bond to produce CO2. The cleavage of the C-H bond is the rate-determining step, that is, Pd 0 can cooperate with Pd 2+ to improve the decomposition performance of formic acid. This indicates that the presence of Pd 2+ plays an important role in improving the catalytic performance. However, there are few current studies on the catalytic role of Pd 2+ , and the specific catalytic path and mechanism are not clear. In addition, there is no reference technical method on how to obtain a high and stable Pd 2+ / Pd 0 ratio on the surface of the carrier. Therefore, this patent will obtain a catalyst with a super-high proportion of Pd 2+ groups by regulating the carrier, and explore its synergistic catalytic mechanism with Pd 0 .

[0005] Based on the above content, this patent uses g-C3N4 as the carrier, and uses waste walnut shells as the carbon source, which is mixed and calcined with melamine. By adjusting the dosage of walnut shells, the content of defective sites and pyridine nitrogen in g-C3N4 is optimized to obtain g-C3N4 carrier materials doped with different ratios of walnut shells. The abundant defective sites and pyridine nitrogen in this carrier material contribute to the adsorption and stability of Pd 2+ , enabling it to load a high proportion of Pd 2+ / Pd 0 active groups during the deposition precipitation process, and finally forming a series of novel Pd / CN-nutshell-based catalysts. This catalyst exhibits excellent catalytic performance in the reaction of formic acid decomposition to produce hydrogen, and there is no by-product CO. Among them, the TOF of Pd / CN-nutshell 5:1 is as high as 5758 h -1Its excellent catalytic performance stems from the abundant N defect sites in the support, which not only confine Pd species to obtain ultrafine Pd nanoparticles, optimize the electronic structure of Pd particles, but also co-adsorb a large amount of active Pd with pyridine nitrogen 2+ groups, and these Pd 2+ adsorption of intermediates realizes fast catalytic hydrogen production from formic acid by Pd 0 . SUMMARY OF THE INVENTION

[0006] The object of the present invention is to overcome the shortcomings of the prior art and provide a preparation method of a Pd deposition-precipitation method supported walnut shell doped carbon nitride catalyst and its application in the catalytic decomposition of formic acid to produce hydrogen. The preparation method is as follows: First, a thermal polymerization method is adopted, using walnut shell and melamine as precursors, and a walnut shell doped g-C3N4 support material is successfully prepared; then, a Pd deposition-precipitation method is used to prepare a Pd supported walnut shell doped carbon nitride catalyst, and the above catalyst is used for the catalytic decomposition of formic acid to produce hydrogen reaction, and its catalytic reaction activity is high, with very high catalytic performance; no CO is found during the reaction process, indicating that the catalyst has strong hydrogen selectivity, almost 100%, and has excellent promotion and application prospects.

[0007] To achieve the above technical effects, the following technical solutions are adopted:

[0008] A preparation method of a Pd deposition-precipitation method supported walnut shell doped carbon nitride catalyst, comprising the following steps:

[0009] Step S1: Prepare a walnut shell doped g-C3N4 support material

[0010] Adopt a thermal polymerization method, using walnut shell and melamine as precursors, to prepare a walnut shell doped g-C3N4 support material, specifically:

[0011] After the walnut shell is crushed by a crusher, it is put into a ball mill and milled into powder. Weigh the melamine and walnut shell powder, mix them evenly, transfer them to a small crucible, put the crucible into a muffle furnace, roast it, wait for it to cool to room temperature, take it out and grind it into powder, and finally obtain a black porous solid, which is the walnut shell doped g-C3N4 support material.

[0012] Step S2: Prepare a Pd deposition-precipitation method supported walnut shell doped carbon nitride catalyst

[0013] Dissolve PdCl2 in an NaCl solution to prepare a Na2PdCl4 solution; weigh the carrier prepared in step S1 and disperse it in a beaker containing deionized water. After ultrasonic treatment, obtain deionized water with the carrier dispersed, pour it into the prepared Na2PdCl4 solution and stir, then add sodium hydroxide solution to adjust the pH and stir; filter the resulting solution, rinse it with absolute ethanol, put it in an oven, and after drying, take out the sample and grind it carefully to ensure uniform powder is obtained, which is the Pd-loaded walnut shell-doped carbon nitride catalyst.

[0014] Further, in step S1, the walnut shell is ball-milled to 100 - 200 mesh.

[0015] Further, in step S1, the mass ratio of melamine to walnut shell powder is 5:1 - 4.

[0016] Further, in step S1, it is calcined at 650 °C for 4 h.

[0017] Further, in step S1, the finally obtained black porous solid has a mesh number of 100 - 200 mesh.

[0018] Further, in step S2, the concentration of the NaCl solution is 0.048 mol / L, and the concentration of the Na2PdCl4 solution is 0.024 mol / L.

[0019] Further, in step S2, the amount of the carrier is 0.5 g, and the volume of deionized water is 20 mL; the ultrasonic time is 1 - 2 h; the volume ratio of deionized water with the carrier dispersed to the Na2PdCl4 solution is 2:1, and the stirring time in step S2 is 3 - 5 h.

[0020] Further, in step S2, the pH is adjusted to 10.5 - 11.0 and stirred for 12 - 18 h; dried at 60 °C for 2 - 4 h; the Pd-loaded walnut shell-doped carbon nitride catalyst has a mesh number of 100 - 200 mesh; the Pd loading amount in the Pd-loaded walnut shell-doped carbon nitride catalyst is 5 wt%.

[0021] A Pd deposition precipitation method for loading a walnut shell-doped carbon nitride catalyst is prepared by using any of the above preparation methods.

[0022] An application of a Pd deposition precipitation method for loading a walnut shell-doped carbon nitride catalyst in the catalytic decomposition of formic acid to produce hydrogen reaction.

[0023] The beneficial effects of the present invention are:

[0024] The present invention uses g-C3N4 as a carrier, and utilizes waste walnut shells as a carbon source. The walnut shells are mixed with melamine and calcined. By adjusting the dosage of the walnut shells, the content of defective sites and pyridine nitrogen in g-C3N4 is optimized, and g-C3N4 carrier materials doped with different proportions of walnut shells are obtained. The abundant defective sites and pyridine nitrogen in the carrier material contribute to the adsorption and stabilization of Pd 2+ , enabling a high proportion of Pd to be loaded during the deposition-precipitation process 2+ / Pd 0 active groups, and finally a series of novel Pd / CN-nutshell-based catalysts are formed. The catalysts exhibit excellent catalytic performance in the formic acid decomposition to hydrogen reaction and no by-product CO. Among them, the TOF of Pd / CN-nutshell 5:1 is as high as 5758 h -1 . Its excellent catalytic performance stems from the abundant N defective sites in the carrier, which not only confine the Pd species, obtaining ultrafine Pd nanoparticles and optimizing the electronic structure of the Pd particles, but also co-adsorb a large number of active Pd 2+ groups with pyridine nitrogen. The adsorption of these Pd 2+ on the intermediates realizes the fast catalytic hydrogen production from formic acid by Pd 0 . BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. The drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts

[0026] Figure 1 is the preparation flow chart of the Pd / CN series catalysts doped with walnut shells in the embodiments of the present invention

[0027] Figure 2 is the structural characterization diagram of the Pd / CN series catalysts doped with walnut shells in the embodiments of the present invention. Among them Figure 2 a is the XRD spectrum of the catalyst Figure 2 b is the nitrogen adsorption-desorption isotherm of the catalyst

[0028] Figure 3 is the SEM diagram of the Pd / CN catalyst doped with walnut shells in the embodiments of the present invention. Among them Figure 3 a and Figure 3 b are the SEM diagrams of Pd / CN Figure 3 c and Figure 3 d are the SEM diagrams of the Pd / CN-nutshell 5:1 catalyst Figure 3 e and Figure 3f is the SEM image of the Pd / CN-nutshell 5:4 catalyst;

[0029] Figure 4 These are the TEM images and the particle size distribution diagrams of Pd particles of the Pd / CN series catalysts in the embodiments of the present invention; among them, Figure 4 a and Figure 4 b are the TEM images of Pd / CN-nutshell 5:1, Figure 4 c is the particle size distribution diagram of Pd particles of Pd / CN-nutshell 5:1; Figure 4 d and Figure 4 e are the TEM images of Pd / CN, Figure 4 f is the particle size distribution diagram of Pd particles of Pd / CN;

[0030] Figure 5 These are the TEM images and the particle size distribution diagrams of Pd particles of the Pd / CN-nutshell 5:4 catalyst doped with walnut shell in the embodiments of the present invention; among them, Figure 5 a and Figure 5 b are the TEM images; Figure 5 c is the particle size distribution diagram of Pd particles;

[0031] Figure 6 These are the electron microscope images of the Pd / CN-nutshell 5:1 catalyst doped with walnut shell in the embodiments of the present invention, among which, Figure 6 a- Figure 6 c are the spherical aberration electron microscope images of Pd / CN-nutshell 5:1, Figure 6 d is the HAADF-STEM image and the elemental mapping image of Pd / CN-nutshell 5:1;

[0032] Figure 7 These are the HAADF-STEM images and the elemental mapping images of the Pd / CN-nutshell 5:4 catalyst doped with walnut shell in the embodiments of the present invention, among which, Figure 7 a is the HAADF-STEM image; Figure 7 b is the C elemental mapping image; Figure 7 c is the N elemental mapping image; Figure 7 d is the O elemental mapping image; Figure 7 e is the Pd elemental mapping image;

[0033] Figure 8 These are the HAADF-STEM images and the elemental mapping images of the Pd / CN catalyst doped with walnut shell in the embodiments of the present invention, among which, Figure 8 a is the HAADF-STEM image; Figure 8 b is the C elemental mapping image; Figure 8 c is the N elemental mapping image;Figure 8 d is the mapped image of the O element; Figure 8 e is the mapped image of the Pd element;

[0034] Figure 9 This is the XPS spectrum of the Pd / CN series catalysts doped with walnut shell in the embodiments of the present invention. Among them, Figure 9 a is the full spectrum; Figure 9 b is C1s; Figure 9 c is N1s; Figure 9 d is Pd3d;

[0035] Figure 10 This is the X-ray absorption fine structure characterization of the Pd / CN-nutshell 5:1 catalyst doped with walnut shell in the embodiments of the present invention. Among them Figure 10 a is the Pd K-edge XANES spectra of Pd / CN-nutshell 5:1, Pd foil, and PdO, Figure 10 b is k 3 Fourier transform of the weighted EXAFS spectrum, Figure 10 c is the EXAFS oscillation of Pd / CN-nutshell 5:1, Pd foil, and PdO at the Pd K-edge, with the fitted scattering contributions superimposed, Figure 10 d is k 3 Wavelet transform of the weighted EXAFS spectrum;

[0036] Figure 11 This is the Fourier transform fitting diagram of the k 3 weighted EXAFS spectrum of the Pd / CN-nutshell 5:1 catalyst in the embodiments of the present invention;

[0037] Figure 12 This is the performance diagram of the Pd / CN series catalysts doped with walnut shell in the embodiments of the present invention for hydrogen production by formic acid decomposition. Among them, Figure 12 a is the activity diagram; Figure 12 b is the multi-cycle activity diagram of Pd / CN-nutshell 5:1; Figure 12 c is the TOF diagram;

[0038] Figure 13 This is the product composition diagram of the Pd / CN series catalysts doped with walnut shell in the embodiments of the present invention after catalyzing formic acid decomposition;

[0039] Figure 14 This is the structure characterization diagram of the Pd / CN series catalysts in the embodiments of the present invention after the reaction. Among them, Figure 14 a and Figure 14 b are the TEM diagrams of Pd / CN-nutshell 5:1 after the reaction, Figure 14c is the particle size distribution diagram of Pd particles after the reaction of Pd / CN-nutshell 5:1; Figure 14 d and Figure 14 e are the TEM images after the reaction of Pd / CN, Figure 14 f is the particle size distribution diagram of Pd particles after the reaction of Pd / CN;

[0040] Figure 15 is the XPS spectrum diagram of the Pd / CN-nutshell 5:1 catalyst after multiple cyclic reactions in the examples of the present invention; among them, Figure 15 a is Pd3d, Figure 15 b is N1s;

[0041] Figure 16 is the in-situ infrared spectrum diagram of the Pd / CN-nutshell 5:1 catalyst in the examples of the present invention under the atmosphere of 5% HCOOH / Ar at 333K. Detailed implementation manners

[0042] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0043] It should be noted that the following detailed descriptions are all exemplary and are intended to provide further descriptions of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0044] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary implementation manners according to the present invention. As used herein, unless otherwise clearly specified in the context, the singular form is also intended to include the plural form. In addition, it should also be understood that when the terms "include" and / or "comprise" are used in this specification, they indicate the presence of features, steps, operations and / or combinations thereof.

[0045] Example 1:

[0046] Melamine, palladium chloride, sodium chloride, formic acid, sodium hydroxide, absolute ethanol, and sodium formate were all purchased from Sinopharm Chemical Reagent Co., Ltd.

[0047] As Figure 1 shown:

[0048] Step S1: Using the thermal polymerization method, with walnut shell and melamine as precursors, the g-C3N4 support material was successfully prepared.

[0049] They are named CN, CN-nutshell 5:1, CN-nutshell 5:2, CN-nutshell 5:3, and CN-nutshell 5:4 according to different doping ratios. The specific preparation methods are as follows:

[0050] CN: Weigh 10 g of melamine in a small crucible. Place the crucible in a muffle furnace and calcine it at 650 °C for 4 hours. After the calcination is completed, let it cool naturally to room temperature. Then take it out and grind it carefully until a fine powdery substance is formed. Finally, a light yellow and porous solid material (150 mesh) is obtained and marked as CN.

[0051] CN-nutshell 5:1: After the walnut shell is crushed by a crusher, it is ball-milled in a ball mill until it becomes powdery (150 mesh). Weigh 10 g of melamine and 2 g of walnut shell powder, mix them evenly, and transfer them to a small crucible. Place the crucible in a muffle furnace and calcine it at 650 °C for 4 h. Wait for it to cool to room temperature, take it out and grind it into powder. Finally, a black porous solid (150 mesh) is obtained and denoted as CN-nutshell 5:1.

[0052] The preparation methods of CN-nutshell 5:2, CN-nutshell 5:3, and CN-nutshell 5:4 are similar to that of CN-nutshell 5:1. Just change the mass of the doped walnut shell to 4 g, 6 g, and 8 g respectively, and the rest of the operation steps are the same.

[0053] Step S2: Prepare a walnut shell-doped carbon nitride catalyst loaded by the Pd deposition-precipitation method

[0054] Supported Pd / CN series catalysts are prepared by the deposition-precipitation method. The specific operation steps are as follows:

[0055] Dissolve PdCl2 in an NaCl solution (0.048 mol / L) to prepare a Na2PdCl4 solution (0.024 mol / L). Weigh 0.5 g of the above-prepared support and disperse it in a beaker containing 20 mL of deionized water. After ultrasonic treatment for 1 h, pour it into the above-prepared Na2PdCl4 solution and stir for 3 h (volume ratio 2:1). Then, add sodium hydroxide solution to adjust the pH to 10.8 and stir overnight. Filter the resulting solution by suction, rinse it with absolute ethanol, place it in an oven, and dry it at 60 °C for 2 - 4 h. After drying is completed, take out the sample and grind it carefully to ensure a uniform powder (150 mesh) is obtained. A series of Pd / CN catalysts doped with walnut shells can be obtained, with a Pd loading of 5 wt%. They are denoted as Pd / CN, Pd / CN-nutshell 5:1, Pd / CN-nutshell 5:2, Pd / CN-nutshell 5:3, and Pd / CN-nutshell 5:4 respectively.

[0056] Example 2:

[0057] Evaluate the catalysts obtained in Example 1:

[0058] The X-ray diffraction (XRD) analysis uses an X-ray diffractometer from Rigaku Corporation, Japan. The equipment parameters are a voltage of 40 KV, a current passing value of 40 mA, a Cu Ka target (λ = 1.5406 Å), a wavelength of 0.15406 nm, a test angle range of 5 - 90°, and data collection is carried out at a scanning speed of 10° / min.

[0059] The N2-adsorption / desorption is carried out using an Autosorb-iQ2 adsorption instrument from Quantachrome Instruments, USA, with a degassing temperature of 150 °C.

[0060] The scanning electron microscope (SEM) uses the SU8600 from Hitachi.

[0061] The transmission electron microscope (TEM) and high-magnification TEM are carried out on a Tecnai G2 F20 S-TWIN instrument from FEI Company, USA, with an operating voltage of 200 kV.

[0062] The X-ray photoelectron spectroscopy (XPS) uses an ESCALAB 250Xi instrument from ThermoFischer Scientific, USA.

[0063] The Pd K-edge X-ray absorption fine structure spectroscopy (XAFS) of the catalyst is collected on the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF), which operates in "top-up" mode at an energy of 3.5 GeV with a constant current of 260 mA.

[0064] The images of the spherical aberration corrected transmission electron microscope were taken using the Themis Z 03040701 instrument from Thermo Fisher Scientific, USA.

[0065] The in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) experiments were carried out on a Bruker Vertex 70 Fourier transform infrared spectrometer. The DRIFTS was equipped with an MCT detector, and the sample cell was equipped with a CaF2 window. To reduce the dead volume when switching different gases, an electronically controlled rapid switching system was used. Formic acid decomposition reaction experiment: Take 50 mg of the sample, heat it to 60 °C under Ar gas, after purging, switch to 5% HCOOH / Ar, collect data, and continuously scan the spectrum for 20 min; switch to Ar purge, collect data, and continuously scan the spectrum for 20 min. The infrared spectrum scanning range is 4000 - 1000 cm -1 and the spectral resolution is 4 cm -1 , and the spectrum collected in Ar before the experiment was selected as the background.

[0066] Evaluation of the hydrogen production performance by formic acid decomposition:

[0067] Turn on the power of the water bath and heat it to 60 °C, prepare a 6 mol / L formic acid solution for standby. Weigh 0.1 g of the catalyst prepared above and 0.2 g of sodium formate into a two-necked flask, add 2 mL of deionized water, place it in the water bath, turn on the stirrer, and preheat it at a constant temperature for about 5 min. After ensuring good airtightness, zero the electronic balance, inject 0.5 mL of formic acid solution from the septum of the two-necked flask with a syringe, start timing at the same time and record the reading every 5 seconds until no more gas is discharged.

[0068] For the stability test, after the above steps are completed, continue to inject 0.5 mL of formic acid solution, start timing at the same time and record the reading every 5 seconds until no more gas is discharged; inject formic acid solution again and repeat the above steps 10 times.

[0069] The gas composition after the reaction was detected offline by gas chromatography. The TOF calculation formula is as follows:

[0070] TOF = (PV / RT) / (2n*t)

[0071] where P represents the standard atmospheric pressure, and its value is 101.325 kPa;

[0072] V represents the total volume of the gas generated by the formic acid decomposition reaction (CO2 + H2);

[0073] R represents the ideal gas constant, and its value is 8.3145 J / (mol·K);

[0074] T represents the thermodynamic temperature, and its value is 298 K;

[0075] n represents the total amount of substance of Pd in the catalyst;

[0076] t represents the reaction time, which is the reaction time at a conversion rate of 20% in this experiment.

[0077] Structural Characterization of Pd / CN Series Catalysts

[0078] Figure 2 a is the XRD pattern of the catalyst, which is used to characterize its crystal structure. As Figure 2 shown in a, the catalysts all exhibit typical diffraction peaks of g-C3N4 (2θ = 27.60°, 12.90°), indicating that the catalysts all have a typical graphite-phase carbon nitride structure. The diffraction peak centered at 27.60° corresponds to the (002) crystal plane of graphite-phase carbon nitride, and the corresponding crystal plane spacing d value is 0.324 nm, belonging to the layered stacking structure. The diffraction peak centered at 12.90° corresponds to the (100) crystal plane of graphite-phase carbon nitride, and the corresponding crystal plane spacing d value is 0.691 nm, belonging to the in-plane tri-s-triazine repeating structural unit. With the increase of the walnut shell doping amount, the intensity of the diffraction peak of the (002) crystal plane becomes significantly lower, indicating that the structure of g-C3N4 is distorted and the crystallinity is reduced after doping; and the peak position shifts from 27.60° to 25.42°, indicating an increase in the layer spacing, indicating the composite of walnut shell and graphite-phase carbon nitride, which destroys part of the layered structure of carbon nitride. There are no diffraction peaks of Pd in all the catalysts, indicating that the loaded Pd has a small size and a high dispersion.

[0079] The specific surface area and pore structure of the samples were characterized. As Figure 2 shown in b, all the samples conform to the type IV isotherm and exhibit an H1 hysteresis loop. The specific surface area data, pore volume and average pore size distribution data are listed in Table 1. The specific surface area of Pd / CN is 30.18 m 2 / g. With the increase of the walnut shell doping amount, the specific surface area increases, and Pd / CN-nutshell 5:1 reaches 43.35 m 2 / g.

[0080] Table 1 Organizational Structure Parameters of Pd / CN Series Catalysts Doped with Walnut Shell

[0081]

[0082] As Figure 3 shown, Figure 3 a and Figure 3 b are the SEM images of Pd / CN. It can be seen from the figure that the morphology of the Pd / CN catalyst is fish-scale-like, and Pd / CN-nutshell 5:1 ( Figure 3 c,Figure 3 d) and Pd / / CN-nutshell 5:4( Figure 3 e, Figure 3 f) is porous, indicating that the doped walnut shell changes the morphological structure of g-C3N4. The more doped walnut shell, the more pores and the looser the structure, which is consistent with the XRD and N2 adsorption-desorption results.

[0083] The TEM images further estimated the size and distribution of Pd nanoparticles. Figure 4 It is the TEM and particle size distribution diagram of Pd particles of the Pd / CN catalyst doped with walnut shell in the embodiment of the present invention; wherein, Figure 4 a and Figure 4 b are the TEM images of Pd / CN-nutshell 5:1, Figure 4 c is the particle size distribution diagram of Pd particles of Pd / CN-nutshell 5:1; Figure 4 d and Figure 4 e are the TEM images of Pd / CN, Figure 4 f is the particle size distribution diagram of Pd particles of Pd / CN;

[0084] Figure 5 It is the TEM image and particle size distribution diagram of Pd particles of the Pd / CN-nutshell 5:4 catalyst doped with walnut shell in the embodiment of the present invention; wherein, Figure 5 a and Figure 5 b are the TEM images; Figure 5 c and Figure 5 b are the particle size distribution diagrams of Pd particles;

[0085] As Figure 4 and Figure 5 shown, Pd is uniformly distributed on the three supports. Compared with Pd / CN-nutshell 5:4 (3.0 nm) and Pd / CN (2.9 nm), Pd / CN-nutshell 5:1 has small-sized Pd nanoparticles uniformly distributed on the support, with an average particle size of 1.8 nm. The results show that the synergistic effect between the doped appropriate amount of walnut shell and g-C3N4 helps to form relatively small-sized and highly dispersed Pd nanoparticles.

[0086] Figure 6 It is the electron microscope image of the Pd / CN-nutshell 5:1 catalyst doped with walnut shell in the embodiment of the present invention, wherein, Figure 6 a- Figure 6 c are the spherical aberration electron microscope images of Pd / CN-nutshell 5:1, Figure 6 d is the HAADF-STEM image and element mapping diagram of Pd / CN-nutshell 5:1;

[0087] Figure 7 HAADF-STEM images and elemental mapping images of the Pd / CN-nutshell 5:4 catalyst doped with walnut shell in the embodiment of the present invention, wherein, Figure 7 a is the HAADF-STEM image; Figure 7 b is the C elemental mapping image; Figure 7 c is the N elemental mapping image; Figure 7 d is the O elemental mapping image; Figure 7 e is the Pd elemental mapping image;

[0088] Figure 8 HAADF-STEM images and elemental mapping images of the Pd / CN catalyst doped with walnut shell in the embodiment of the present invention, wherein, Figure 8 a is the HAADF-STEM image; Figure 8 b is the C elemental mapping image; Figure 8 c is the N elemental mapping image; Figure 8 d is the O elemental mapping image; Figure 8 e is the Pd elemental mapping image;

[0089] As Figure 6 and Figure 7 and Figure 8 shown, the aberration correction and HAADF-STEM analysis results indicate that Pd nanoparticles in the Pd / CN series catalysts are uniformly dispersed on the support, and Pd, N, O, and C elements are evenly distributed.

[0090] Figure 9 are the XPS spectra of the Pd / CN series catalysts doped with walnut shell; wherein Figure 9 a is the full spectrum; Figure 9 b is C1s; Figure 9 c is N1s; Figure 9 d is Pd3d;

[0091] Through XPS testing, the surface elemental chemical states and electronic states of the synthesized samples are analyzed. The full spectrum and the surface contents of each element are listed in Figure 9 Table 1 and Table 2. There are C, N, O, Pd, and S elements in the catalyst. The content of S element is extremely low and can be regarded as an impurity; with the increase of the doping amount of walnut shell, the C content gradually increases and the N content gradually decreases.

[0092] As Figure 9 shown in b, the C1s XPS spectrum can be deconvoluted into three peaks: 284.8, 286.2, and 288.3 eV. The peak at 284.8 eV is attributed to the C-C species; the peak at 288.3 eV is attributed to sp in the main structure of g-C3N4 2Hybrid carbon (N-C=N); the peak at 286.2 eV belongs to the carbon species in C-N-C or C-NH2. Meanwhile, the contents of C-C, N-C=N and C-N-C or C-NH2 are listed in the table. With the increase of walnut shell doping amount, the content of N-C=N gradually decreases and the content of C-C gradually increases. As Figure 9 shown in Fig. 2C / 3C N 3C is at the most central position and connected to three C atoms, which is very stable. Therefore, with the increase of walnut shell doping amount, the ratio of 2C N 3C / 2C N decreases, which is attributed to the N deficiency at the

[0093] Table 2 Elemental distribution on the catalyst surface measured by XPS

[0094]

[0095] Table 3 Contents of (2C) N (3C) and N-H obtained from XPS N 1s spectra

[0096]

[0097]

[0098] Table 4 Contents of 2+ Pd 0 and 0 Pd 2+

[0099]

[0100] As can be seen from the Pd 3d spectrum ( Figure 9 Fig. 3 / 2 d), two strong peaks appear at 337.5 eV and 342.8 eV in the Pd / CN series catalysts, which are attributed to Pd(II)3d 5 / 2 and Pd(II)3d 3 / 2 respectively. The low-intensity peaks at 335.6 eV and 340.7 eV are attributed to Pd(0)3d 5 / 2. As can be seen from Table 4, the content of Pd(II) is about 90% in all cases. Combining with the literature, pyridine N enhances the metal-support interaction by forming Pd-N covalent bonds, resulting in the transfer of electrons from Pd to N. Therefore, a high proportion of Pd exists in all Pd / CN series catalysts. 2+ . It should be noted that the peak position in Pd / CN-nutshell 5:1 shifts by 0.2 eV towards higher binding energy, and as the doping amount continues to increase, the peak position shifts back to lower binding energy, indicating a stronger electronic interaction between Pd and the support in Pd / CN-nutshell 5:1. The N 1s XPS shows that there are more nitrogen defects in Pd / CN-nutshell 5:1 compared to Pd / CN, which is consistent with the fact that C self-doping in g-C3N4 can increase the defect concentration in the literature. The defect structure of CN-nutshell 5:1 improves the electronic environment of the anchored Pd sites, making the electronic interaction between Pd and the support stronger, with more electrons transferred from Pd to N, causing 2+ the charge density of Pd to further decrease, thus shifting towards higher wavenumbers. The confinement effect of the defect structure can also effectively prevent the aggregation of Pd sites. Combining with TEM, the Pd particle size of Pd / CN-nutshell 5:1 is the smallest, which is consistent with more defect sites in CN-nutshell 5:1.

[0101] Figure 10 a shows the Pd K-edge XANES spectra of Pd / CN-nutshell 5:1, Pd foil, and PdO, Figure 10 b shows the Fourier transform of the k 3 weighted EXAFS spectra, Figure 10 c shows the EXAFS oscillations of Pd / CN-nutshell 5:1, Pd foil, and PdO at the Pd K-edge, with the superimposed fitted scattering contributions, Figure 10 d shows the wavelet transform of the k 3 weighted EXAFS spectra.

[0102] Figure 11 This is the Fourier transform fitting diagram of the k 3 weighted EXAFS spectra of the Pd / CN-nutshell 5:1 catalyst in the embodiment of the present invention;

[0103] X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) also provide information about the Pd structure of the catalyst. Consistent with the high Pd 2+ / Pd 0 ratio detected by XPS, Figure 9The XANES spectrum of Pd / CN-nutshell-5:1 in a is very similar to that of PdO, rather than Pd foil. The EXAFS of Pd / CN-nutshell-5:1 ( Figure 10 b and Figure 11 ) shows that there is a significant Pd-O peak at , a very small Pd-Pd peak at , and a significant Pd-O-Pd peak at . These peaks are due to the presence of bulk PdO. The Pd species in Pd / CN-nutshell-5:1 exhibits oscillations very similar to those of PdO ( Figure 10 c), indicating that most of the Pd in Pd / CN-nutshell-5:1 is in the oxidized state. The data from wavelet transform analysis ( Figure 10 d) is consistent with the EXAFS fitting data. In addition, part of the peaks can be attributed to single Pd atoms interacting with oxygen or nitrogen functional groups (i.e., Pd-Ox or Pd-Nx). In this system, the presence of Pd-Nx is speculated, which confirms the formation of a covalent bond between Pd and N.

[0104] Catalytic performance characterization of Pd / CN series catalysts

[0105] Figure 12 a is the activity diagram of the Pd / CN series catalysts doped with walnut shell for hydrogen production by formic acid decomposition; Figure 12 b is the multi-cycle activity diagram of Pd / CN-nutshell 5:1; Figure 12 c is the TOF diagram;

[0106] Figure 13 This is the product composition diagram after the Pd / CN series catalysts doped with walnut shell in the examples of the present invention catalyze formic acid decomposition;

[0107] The catalyst was used for the hydrogen production reaction by catalyzing formic acid decomposition. There are two possible paths for the formic acid decomposition reaction: dehydrogenation (HCOOH→CO2+H2) and dehydration (HCOOH→CO+H2O). However, the dehydration path will produce CO, which will poison and inactivate the active sites. Therefore, the dehydration path is not desired in formic acid decomposition. In this experiment, the product composition was detected by off-line chromatography. As Figure 13 shows, only hydrogen and carbon dioxide are produced in this reaction, and no CO is found at the lowest detection limit of the chromatography, indicating that the Pd / CN series catalysts doped with walnut shell have a high hydrogen selectivity of 100%.

[0108] The catalytic activity diagram is as Figure 12As shown in a, the five catalysts prepared all have good catalytic activity for the formic acid decomposition to hydrogen reaction, and almost complete conversion is achieved within 5 minutes at the beginning of the reaction. Under the action of Pd / CN, Pd / CN-nutshell 5:1 and Pd / CN-nutshell 5:2 catalysts, 136 mL of gas (CO2 + H2) is released during the decomposition of formic acid, among which the Pd / CN-nutshell 5:1 catalyst has the best catalytic effect. Pd / CN-nutshell 5:3 and Pd / CN-nutshell 5:4 have poor activity, and both release 118 mL of gas (CO2 + H2). Pd / CN-nutshell 5:1 was subjected to multiple rounds of experiments, and the results are as Figure 12 shown in b. A high catalytic activity can be ensured in the first four times, and then it decreases slightly. In the tenth test, the gas volume of 120 mL can still be released in ten minutes, indicating that the catalyst has good stability. Calculate the TOF in the reaction according to the Pd loading of the catalyst, as Figure 12 shown in c. The TOF value of Pd / CN-nutshell 5:1 is as high as 5758 h -1 , which is 1.9 times that of the Pd / CN catalyst; as the doping amount of walnut shell increases, the activity gradually decreases, and the Pd / CN-nutshell 5:4 catalyst has the worst catalytic effect, and its TOF value is only 1355 h -1 . The comparison of the activities with other similar catalysts is shown in Table 5, indicating that the Pd / CN series catalysts doped with walnut shell have very excellent catalytic performance.

[0109] Table 5 Comparison of the catalytic activities of the reported Pd-based catalysts and the catalysts synthesized in this study in the formic acid dehydrogenation reaction.

[0110]

[0111]

[0112] Structure characterization of the Pd / CN nutshell-5:1 catalyst after reaction

[0113] The TEM images of the catalyst after reaction estimated the size and distribution of Pd nanoparticles. Figure 14 This is the structure characterization diagram of the Pd / CN series catalyst doped with walnut shell after reaction in the embodiment of the present invention. Among them, Figure 14 a and Figure 14 b are the TEM images of Pd / CN-nutshell5:1 after reaction, Figure 14 c is the particle size distribution diagram of Pd particles of Pd / CN-nutshell 5:1 after reaction; Figure 14 d and Figure 14 e are the TEM images of Pd / CN after reaction, Figure 14f is the particle size distribution diagram of Pd particles after the Pd / CN reaction;

[0114] As Figure 15 shown, Pd is evenly distributed on the carrier. Compared with before the reaction, the Pd particles have all grown slightly after the reaction. Pd / CN-nutshell 5:1 increases from 1.8 nm to 2.7 nm; Pd / CN increases from 2.9 nm to 3.5 nm. The Pd / CN-nutshell 5:1 after multiple rounds of experimental reactions was characterized by XPS.

[0115] Figure 15 This is the XPS spectrum of the Pd / CN-nutshell 5:1 catalyst after multiple cyclic reactions in the examples of the present invention; among them, Figure 15 a is Pd3d, Figure 15 b is N1s;

[0116] From Figure 15 and Table 6, it can be seen that compared with before the reaction, the content of Pd 0 increases from 13.14% to 57.09%, and the content of Pd 2+ decreases from 86.86% to 42.91%, indicating that during the reaction process, as the Pd particles gradually increase, the ratio of Pd 0 to Pd 2+ on its surface changes. Combining with multiple rounds of activity tests ( Figure 12 b), the activity decreases after four times, further indicating that when the Pd particle size is smaller and the proportion of Pd 2+ is high, the catalyst can obtain excellent catalytic activity. The content of N (2C) (pyridine nitrogen) decreases from 71.15% to 42.73%, and the content of N (3C) (pyrrole nitrogen) increases from 20.17% to 48.74%. The content of graphitic nitrogen has little difference, indicating that during the reaction process, the content of pyridine nitrogen decreases, and the decrease of pyridine nitrogen leads to the reduction of Pd-N, and the ratio of Pd 0 / Pd 2+ is unbalanced.

[0117] Table 6 Contents of various species in Pd / CN-nutshell 5:1 after the reaction obtained from the XPS spectrum

[0118]

[0119] Reaction mechanism of Pd / CN series catalysts

[0120] In-situ infrared was used to study the active intermediates of Pd / CN-nutshell 5:1 in the catalytic decomposition reaction of formic acid, so as to speculate the reaction path. As Figure 16 shown, 2363 and 2336 cm -1The peak at [[]] is gaseous CO2, and the peaks at 1587 and 1350 cm -1 are the absorption peaks of the asymmetric and symmetric vibrations of OCO in the adsorbed HCOO*, respectively, and the peak at 2850 cm -1 is the C-H vibration peak of the adsorbed HCOOH. Moreover, from the off-line chromatographic detection of the product components ( Figure 13 ), it can be seen that only hydrogen and carbon dioxide are produced in this reaction, and no carbon monoxide is generated. This indicates that in the catalytic decomposition of formic acid by the Pd / CN series catalysts doped with walnut shell, HCOO* is the reaction intermediate and no by-product CO is generated.

[0121] It has been reported in the literature that divalent Pd and zero-valent Pd have synergy, and divalent Pd electrostatically adsorbs HCOO - , and the adjacent Pd 0 sites promote its decomposition to produce hydrogen. In the Pd / CN system doped with walnut shell, combined with XPS, due to the strong interaction between pyridine N and Pd, Pd 2+ accounts for about 90% in the Pd / CN series catalysts, and only a small amount of Pb 0 , and Pd 2+ can electrostatically adsorb HCOO - , so we speculate that the high proportion of Pd 2+ is the main reason for the high catalytic activity and high selectivity of this system. After multiple cycle activity tests, the Pb 2+ in the catalyst decreases, the Pb 0 increases, and the activity of the catalyst decreases slightly, which also proves that in this system, when Pb 2+ is much higher than Pb 0 , that is, when the proportion of Pb 2+ is high and the amount of Pb 0 is small, excellent catalytic activity is exhibited. Compared with the Pd-based catalyst supported on pure CN, Pd / CN-nutshell 5:1 improves the structural characteristics of the support and increases the nitrogen defect sites through biomass doping, thereby realizing the anchoring of Pd species and effectively preventing the aggregation of Pd sites, keeping the Pd particle size below 2 nm; and the peak of Pd 2+ in XPS shifts to a higher binding energy, indicating that the nitrogen defect sites improve the electronic environment of the anchored Pd sites and optimize the electronic structure of Pd, so it has the highest formic acid decomposition activity. When the content of walnut shell is too high, the content of pyridine N on the catalyst surface decreases sharply, the nitrogen defects decrease accordingly, the catalyst particles grow, and the catalytic performance decreases.

[0122] In summary, this patent uses the method of biomass doping. With walnut shell and melamine as precursors, nitrogen-deficient carbon nitride is obtained, and Pd is further loaded on this carrier. The resulting catalyst does not require calcination and reduction, and shows extremely high catalytic activity and selectivity in the formic acid decomposition reaction. Moreover, Pd / CN-nutshell 5:1 exhibits the highest TOF. Due to the strong interaction between pyridine nitrogen and Pd, the proportion of Pd in the Pd / CN series catalysts 2+ is much higher than that of Pb 0 , and Pd 2+ can electrostatically adsorb HCOO - , promoting the progress of the formic acid decomposition reaction. Therefore, the high proportion of Pd 2+ is the main reason for the excellent catalytic activity of this system. At the same time, biomass doping can effectively increase the amount of nitrogen defect sites on the surface of carbon nitride. The defect sites can not only anchor active metals to obtain fine Pd particles, but also optimize the electronic structure of Pd, thereby enhancing the catalytic activity. This study provides a new idea for understanding the determination of formic acid decomposition active sites and the design of high-performance catalysts.

[0123] In summary, the present invention discloses a preparation method of a Pd deposition precipitation method for loading walnut shell-doped carbon nitride catalyst and its application in the catalytic formic acid decomposition to produce hydrogen reaction. The preparation method is as follows: First, the thermal polymerization method is adopted, and with walnut shell and melamine as precursors, a walnut shell-doped g-C3N4 carrier material is successfully prepared; then, the deposition precipitation method is used to prepare a Pd-loaded walnut shell-doped carbon nitride catalyst, and the above catalyst is used for the catalytic formic acid decomposition to produce hydrogen reaction. Its catalytic reaction activity is relatively high, and it has very high catalytic performance; no CO is found during the reaction process, indicating that this kind of catalyst has strong hydrogen selectivity, almost 100%, and has excellent promotion and application prospects.

[0124] At this point, those skilled in the art recognize that although the embodiments of the present invention have been shown and described in detail herein, many other variations or modifications that conform to the principles of the present invention can still be directly determined or deduced based on the content disclosed in the present invention without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be understood and determined to cover all these other variations or modifications.

Claims

1. A preparation method of a Pd deposition-precipitation method supported walnut shell doped carbon nitride catalyst, characterized in that, The preparation method includes the following steps: Step S1: Prepare a walnut shell-doped g-C3N4 support material Using the thermal polymerization method, with walnut shells and melamine as precursors, prepare a walnut shell-doped g-C3N4 support material, specifically: After the walnut shells are crushed by a crusher, they are put into a ball mill and milled into a powder. Weigh the melamine and the walnut shell powder, mix them evenly, and transfer them to a small crucible. Place the crucible in a muffle furnace, calcine it, wait for it to cool to room temperature, take it out and grind it into a powder. Finally, a black porous solid is obtained, which is the walnut shell-doped g-C3N4 support material. Step S2: Prepare a Pd-deposited precipitation method-supported walnut shell-doped carbon nitride catalyst Dissolve PdCl2 in an NaCl solution to prepare a Na2PdCl4 solution; weigh the support prepared in Step S1, disperse it in a beaker containing deionized water, and after ultrasonic treatment, obtain deionized water with the support dispersed. Pour it into the prepared Na2PdCl4 solution and stir, then add sodium hydroxide solution to adjust the pH and stir; filter the obtained solution, rinse it with absolute ethanol, put it in an oven, and after drying, take out the sample and grind it carefully to ensure that a uniform powder is obtained, which is the Pd-supported walnut shell-doped carbon nitride catalyst.

2. The preparation method of a Pd deposition precipitation method supported walnut shell doped carbon nitride catalyst as described in claim 1, characterized in that, In Step S1, the walnut shells are ball milled to 100-200 mesh in the ball mill.

3. The preparation method of a Pd deposition precipitation method supported walnut shell doped carbon nitride catalyst as described in claim 1, characterized in that, In Step S1, the mass ratio of melamine to walnut shell powder is 5:1-4.

4. The preparation method of a Pd deposition precipitation method supported walnut shell doped carbon nitride catalyst as described in claim 1, characterized in that, In Step S1, it is calcined at 650 °C for 4 h.

5. The preparation method of a Pd deposition precipitation method supported walnut shell doped carbon nitride catalyst as described in claim 1, characterized in that, In Step S1, the finally obtained black porous solid has a mesh number of 100-200 mesh.

6. The preparation method of the Pd deposition precipitation method supported walnut shell doped carbon nitride catalyst as described in claim 1, characterized in that, In Step S2, the concentration of the NaCl solution is 0.048 mol / L, and the concentration of the Na2PdCl4 solution is 0.024 mol / L.

7. The preparation method of a Pd deposition-precipitation method supported walnut shell doped carbon nitride catalyst as described in claim 1, characterized in that, In Step S2, 0.5 g of the support is used, and the volume of deionized water is 20 mL; the ultrasonic time is 1-2 h; the volume ratio of the deionized water with the support dispersed to the Na2PdCl4 solution is 2:1, and the stirring time in Step S2 is 3-5 h.

8. The preparation method of a Pd deposition precipitation method supported walnut shell doped carbon nitride catalyst as described in claim 1, characterized in that, In Step S2, the pH is adjusted to 10.5-11.0, and stirred for 12-18 h; dried at 60 °C for 2-4 h; the mesh number of the Pd-supported walnut shell-doped carbon nitride catalyst is 100-200 mesh; the Pd loading amount in the Pd-supported walnut shell-doped carbon nitride catalyst is 5 wt%.

9. A Pd deposition precipitation method for loading a walnut shell-doped carbon nitride catalyst, characterized in that, The catalyst is prepared by the preparation method according to any one of claims 1-8.

10. Application of a Pd deposition precipitation method for loading a walnut shell-doped carbon nitride catalyst, characterized in that, The application of the catalyst in the catalytic decomposition of formic acid to produce hydrogen reaction.

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