A method for preparing a walnut shell doped carbon nitride catalyst loaded by a Pd deposition precipitation method and application of the catalyst in a hydrogen production reaction by catalytic decomposition of formic acid

Abstract

The application discloses a preparation method of a Pd deposition precipitation method loaded walnut shell doped carbon nitride catalyst and application of the catalyst in a hydrogen production reaction of formic acid decomposition, and the preparation method is as follows: firstly, a thermal polymerization method is adopted, walnut shells and melamine are used as precursors, and a walnut shell doped g-C3N4 carrier material is successfully prepared; then, a deposition precipitation method is adopted to prepare a Pd loaded walnut shell doped carbon nitride catalyst, and the catalyst is used in the hydrogen production reaction of formic acid decomposition, and the catalyst has relatively high catalytic reaction activity and very high catalytic performance; no CO is found in the reaction process, indicating that the catalyst has strong hydrogen selectivity, is close to 100%, and has excellent popularization and application prospect.

Description

Technical Field

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

[0002] Hydrogen (H2), as a zero-carbon energy source, plays a crucial role in building a sustainable green energy system due to its renewable, clean, and pollution-free advantages. However, storing and transporting H2 safely and economically remains a challenge in hydrogen energy development, necessitating the development of new sustainable in-situ hydrogen production technologies. Formic acid hydrogen production is considered a highly promising in-situ hydrogen production technology because formic acid (FA, HCOOH), a major product of biomass processing and CO reduction, possesses excellent stability, high hydrogen content (up to 4.4 wt.%), and low toxicity. Formic acid decomposition occurs via two pathways: dehydrogenation (HCOOH → CO2 + H2) and dehydration (HCOOH → CO + H2O). However, the CO produced during the dehydration pathway poisons the hydrogen production catalyst, leading to catalyst performance degradation or even deactivation, which is detrimental to the dehydrogenation reaction. Therefore, developing high-performance catalysts with high selectivity and no CO impurity generation is of great significance for FA-based hydrogen storage.

[0003] In the formic acid decomposition reaction for hydrogen production, Pd-based catalysts possess both high catalytic activity and hydrogen selectivity, making them a highly advantageous catalyst system for this reaction. Most researchers believe that optimizing the performance of the formic acid decomposition reaction highly depends on the precise control of the Pd size and electronic structure. The support effect is an effective means of controlling the Pd size. By modifying the support surface with anchoring groups, the nanoparticles can be precisely controlled, allowing them to be uniformly dispersed at small sizes. This significantly improves the utilization rate of active sites and optimizes 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. Among them, g-C3N4 is a good support for Pd nanoparticles due to its advantages such as convenient preparation, thermal / chemical stability, abundant active sites, and easy surface modification. Guoet.al prepared C-doped g-C3N4. By adjusting the molar ratio of uracil to dicyandiamide, the C doping level or defect concentration could be controlled, precisely tuning the electronic and band structures of g-C3N4. The introduction of Pd species led to the formation of Pd single atoms (Pd1) and small-sized Pd nanoparticles (Pd2). NP Cao Yong et al. successfully prepared a series of CNx supports with different pyridine N contents by pyrolyzing chitosan / melamine mixed precursors with different ratios at different temperatures. They also synthesized a series of Pd / CNx catalysts by sodium borohydride reduction, confirming a strong correlation between the pyridine N content on the support surface and the catalyst activity.

[0004] Most studies primarily explored Pd 0 The role of Pd in ​​the decomposition of formic acid to produce hydrogen. 2+ The contribution of Pd ions in this process has the potential for further research. Previous reports have shown that Pd ions have a significant effect on improving catalyst performance; for example, Haiqin Wanet.al prepared N-rich mesoporous carbon materials and used them as supports to load Pd (Pd / NMC), obtaining Pd... 0 and Pd 2+ Co-supported catalyst. The authors believe that the adsorption of formic acid molecules at the basic sites (pyridine nitrogen) on the support promotes their dehydrogenation to HCOO. - Pd 2+ Adsorption of HCOO via electrostatic effect - Then transferred to Pd 0 The site triggers the breakage of the CH bond, generating CO2. The breakage of the CH bond is the rate-determining step, i.e., Pd. 0 Can be used with Pd 2+ Synergistically enhances the decomposition performance of formic acid. This indicates that Pd 2+ The presence of Pd plays a crucial role in enhancing catalytic performance. However, current understanding of Pd... 2+ Research on catalysis is still limited, and the specific pathways and mechanisms of catalysis remain unclear. Furthermore, how to obtain and maintain high and stable Pd values ​​on a support surface is a key challenge. 2+ / Pd 0 There is currently no comparable technical method available. Therefore, this patent aims to obtain a Pd ratio of extremely high by controlling the carrier. 2+ Catalysts with functional groups, and their interaction with Pd. 0 The synergistic catalytic mechanism.

[0005] Based on the above, this patent uses g-C3N4 as a carrier and waste walnut shells as a carbon source, mixing and calcining them with melamine. By adjusting the amount of walnut shells, the content of defect sites and pyridine nitrogen in g-C3N4 is optimized, resulting in g-C3N4 carrier materials with different proportions of walnut shell doping. The abundant defect sites and pyridine nitrogen in this carrier material contribute to Pd 2+ Its adsorption and stabilization properties allow it to be loaded with a high proportion of Pd during the deposition process. 2+ / Pd 0 The active groups ultimately formed a series of novel Pd / CN-nutshell-based catalysts. These catalysts exhibited excellent catalytic performance in the formic acid decomposition to hydrogen production reaction, with no CO as a byproduct. Among them, the Pd / CN-nutshell 5:1 catalyst achieved a TOF as high as 5758 h⁻¹. -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 and optimize the electronic structure of Pd particles, but also co-adsorb a large amount of active Pd with pyridine nitrogen. 2+ Groups, these Pd 2+ The adsorption of intermediates enabled Pd 0 Fast catalytic hydrogen production from formic acid. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing a Pd-supported walnut shell-doped carbon nitride catalyst by deposition and precipitation, and its application in the catalytic decomposition of formic acid to produce hydrogen. The preparation method is as follows: First, a walnut shell-doped g-C3N4 support material was successfully prepared using a thermal polymerization method with walnut shell and melamine as precursors. Then, a Pd-supported walnut shell-doped carbon nitride catalyst was prepared by deposition and precipitation. When this catalyst was used to catalyze the decomposition of formic acid to produce hydrogen, it exhibited high catalytic activity and very high catalytic performance. No CO generation was observed during the reaction, indicating that this catalyst has strong hydrogen selectivity, nearly 100%, and possesses excellent prospects for widespread application.

[0007] To achieve the above technical effects, the following technical solution is adopted:

[0008] A method for preparing a Pd deposition-precipitation method for supporting carbon nitride-doped walnut shell catalyst includes the following steps:

[0009] Step S1: Preparation of walnut shell-doped g-C3N4 support material

[0010] A walnut shell-doped g-C3N4 carrier material was prepared using a thermal polymerization method with walnut shells and melamine as precursors. Specifically:

[0011] After the walnut shells are crushed by a grinder, they are put into a ball mill and ground into powder. Melamine and walnut shell powder are weighed and mixed evenly, then transferred to a small crucible. The crucible is placed in a muffle furnace and calcined. After cooling to room temperature, it is taken out and ground into powder, finally obtaining a black porous solid, which is the walnut shell-doped g-C3N4 carrier material.

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

[0013] PdCl2 was dissolved in NaCl solution to prepare Na2PdCl4 solution; the support prepared in step S1 was weighed and dispersed in a beaker containing deionized water, and after sonication, the deionized water containing the support was obtained, which was poured into the prepared Na2PdCl4 solution and stirred, and then sodium hydroxide solution was added dropwise to adjust the pH and stirred; the obtained solution was filtered, rinsed with anhydrous ethanol, placed in an oven, and dried. After drying, the sample was taken out and carefully ground to ensure that a uniform powder was obtained, which is the Pd supported walnut shell doped carbon nitride catalyst.

[0014] Furthermore, in step S1, the walnut shells are ball-milled to 100-200 mesh.

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

[0016] Furthermore, in step S1, the food is calcined at 650°C for 4 hours.

[0017] Furthermore, in step S1, the final black porous solid obtained has a mesh size of 100-200.

[0018] Furthermore, 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] Furthermore, in step S2, the carrier is 0.5g, the volume of deionized water is 20mL, the ultrasonic time is 1-2h, the volume ratio of deionized water to Na2PdCl4 solution in which the carrier is dispersed is 2:1, and the stirring time in step S2 is 3-5h.

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

[0021] A Pd deposition-precipitation method for supporting carbon nitride-doped walnut shells is prepared using any of the above-mentioned preparation methods.

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

[0023] The beneficial effects of this invention are as follows:

[0024] This invention uses g-C3N4 as a carrier and waste walnut shells as a carbon source, mixing and calcining them with melamine. By adjusting the amount of walnut shells, the content of defect sites and pyridine nitrogen in g-C3N4 is optimized, resulting in g-C3N4 carrier materials with different proportions of walnut shell doping. The abundant defect sites and pyridine nitrogen in this carrier material contribute to Pd 2+ Its adsorption and stabilization properties allow it to be loaded with a high proportion of Pd during the deposition process. 2+ / Pd 0 The active groups ultimately formed a series of novel Pd / CN-nutshell-based catalysts. These catalysts exhibited excellent catalytic performance in the formic acid decomposition to hydrogen production reaction, with no CO as a byproduct. Among them, the Pd / CN-nutshell 5:1 catalyst achieved a TOF as high as 5758 h⁻¹. -1 Its excellent catalytic performance stems from the abundant N defect sites in the support, which not only confine Pd species to obtain ultrafine Pd nanoparticles and optimize the electronic structure of Pd particles, but also co-adsorb a large amount of active Pd with pyridine nitrogen. 2+ Groups, these Pd 2+ The adsorption of intermediates enabled Pd 0 Fast catalytic hydrogen production from formic acid. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0026] Figure 1 This is a flowchart illustrating the preparation process of the walnut shell-doped Pd / CN series catalysts according to an embodiment of the present invention.

[0027] Figure 2 The diagram shows the structural characterization of the Pd / CN series catalysts doped with walnut shells according to embodiments of the present invention. Figure 2 a is the XRD pattern of the catalyst; Figure 2 b is the nitrogen adsorption-desorption curve of the catalyst;

[0028] Figure 3 Here is a SEM image of the walnut shell-doped Pd / CN catalyst from an embodiment of the present invention; wherein, Figure 3 a and Figure 3 b is the SEM image of Pd / CN; Figure 3 c and Figure 3 d is a SEM image of the Pd / CN-nutshell 5:1 catalyst; Figure 3 e and Figure 3f is a SEM image of the Pd / CN-nutshell 5:4 catalyst;

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

[0030] Figure 5 These are TEM images and particle size distribution diagrams of the Pd / CN-nutshell 5:4 catalyst doped with walnut shells, as shown in this embodiment of the invention; wherein, Figure 5 a and Figure 5 b is the TEM image; Figure 5 c is the particle size distribution diagram of Pd particles;

[0031] Figure 6 This is an electron micrograph of the Pd / CN-nutshell 5:1 catalyst doped with walnut shells, according to an embodiment of the present invention. Figure 6 a- Figure 6 c is an aberration-corrected electron microscope image of Pd / CN-nutshell 5:1. Figure 6 d represents the HAADF-STEM diagram and elemental mapping diagram of Pd / CN-nutshell 5:1;

[0032] Figure 7 These are HAADF-STEM images and elemental mapping images of the walnut shell-doped Pd / CN-nutshell 5:4 catalyst from an embodiment of the present invention. Figure 7 a is a HAADF-STEM image; Figure 7 b is the C element mapping image; Figure 7 c is an N-element mapping image; Figure 7 d is the O-element mapping image; Figure 7 e is the image mapped to element Pd;

[0033] Figure 8 These are HAADF-STEM images and elemental mapping images of the walnut shell-doped Pd / CN catalyst from an embodiment of the present invention. Figure 8 a is a HAADF-STEM image; Figure 8 b is the C element mapping image; Figure 8 c is an N-element mapping image; Figure 8 d is the O-element mapping image; Figure 8 e is the image mapped to element Pd;

[0034] Figure 9 The above is an XPS spectrum of the Pd / CN series catalyst doped with walnut shells from an embodiment of the present invention. Figure 9 a represents the full spectrum; Figure 9 b is C1s; Figure 9 c is N1s; Figure 9 d is Pd3d;

[0035] Figure 10 X-ray absorption fine structure characterization of the walnut shell-doped Pd / CN-nutshell 5:1 catalyst of this invention, wherein... Figure 10 a represents 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 weighted EXAFS spectrum Figure 10 c represents the EXAFS oscillations of Pd / CN-nutshell 5:1, Pd foil, and PdO at the Pd K-edge, superimposed with the fitted scattering contribution. Figure 10 d is k 3 Wavelet transform of weighted EXAFS spectrum;

[0036] Figure 11 For the embodiment of this invention, Pd / CN-nutshell 5:1 catalyst k 3 Fourier transform fit plot of weighted EXAFS spectrum;

[0037] Figure 12 The diagram shows the performance of the Pd / CN series catalyst doped with walnut shells in the catalytic decomposition of formic acid to produce hydrogen, according to an embodiment of the present invention. Figure 12 a is the activity diagram; Figure 12 b is the activity profile of Pd / CN-nutshell 5:1 after multiple cycles; Figure 12 c is the TOF plot;

[0038] Figure 13 This is a composition diagram of the products after formic acid decomposition catalyzed by the Pd / CN series catalyst doped with walnut shells in an embodiment of the present invention.

[0039] Figure 14 The diagram shows the structural characterization of the Pd / CN series catalysts after reaction in this embodiment of the invention. Figure 14 a and Figure 14 b is a TEM image of Pd / CN-nutshell after the reaction at a ratio of 5:1. Figure 14c is the particle size distribution of Pd particles after the Pd / CN-nutshell 5:1 reaction; Figure 14 d and Figure 14 e is a TEM image after the Pd / CN reaction. Figure 14 f is the particle size distribution of Pd particles after the Pd / CN reaction;

[0040] Figure 15 The above are XPS spectra of the Pd / CN-nutshell 5:1 catalyst after multiple cycles of reaction in an embodiment of the present invention; wherein, Figure 15 a is Pd3d. Figure 15 b is N1s;

[0041] Figure 16 The image shows the in-situ infrared spectrum of the Pd / CN-nutshell 5:1 catalyst in an embodiment of the present invention at 333 K under a 5% HCOOH / Ar atmosphere. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0043] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0044] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" 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, anhydrous ethanol, and sodium formate were all purchased from Sinopharm Chemical Reagent Co., Ltd.

[0047] like Figure 1 As shown:

[0048] Step S1: Using thermal polymerization, g-C3N4 carrier material was successfully prepared using walnut shells and melamine as precursors.

[0049] Based on different doping ratios, they are named CN, CN-nutshell 5:1, CN-nutshell 5:2, CN-nutshell 5:3, and CN-nutshell 5:4, respectively. The specific preparation methods are as follows:

[0050] CN: Weigh 10g of melamine into a small crucible, place the crucible in a muffle furnace, and calcine at 650℃ for 4 hours. After calcination, allow it to cool naturally to room temperature, then remove it and grind it finely until a fine powder is formed. A light yellow, porous solid material (150 mesh) was finally obtained and labeled CN.

[0051] CN-nutshell 5:1: After crushing the walnut shells with a grinder, grind them into powder (150 mesh) in a ball mill. Weigh out 10g of melamine and 2g of walnut shell powder, mix them evenly, and transfer them to a small crucible. Place the crucible in a muffle furnace and bake at 650℃ for 4 hours. After cooling to room temperature, take it out and grind it into powder. Finally, a black porous solid (150 mesh) is obtained, which is recorded as CN-nutshell5:1.

[0052] The preparation methods for CN-nutshell 5:2, CN-nutshell 5:3, and CN-nutshell 5:4 are similar to those for CN-nutshell 5:1, except that the mass of the walnut shells added is changed to 4g, 6g, and 8g, respectively, and the other steps are the same.

[0053] Step S2: Preparation of Pd deposition-precipitation method supported walnut shell-doped carbon nitride catalyst

[0054] Supported Pd / CN series catalysts were prepared using a deposition-precipitation method. The specific operational steps are as follows:

[0055] PdCl2 was dissolved in NaCl solution (0.048 mol / L) to prepare Na2PdCl4 solution (0.024 mol / L). 0.5 g of the prepared support was weighed and dispersed in a beaker containing 20 mL of deionized water. After sonication for 1 h, the mixture was poured into the prepared Na2PdCl4 solution and stirred for 3 h (volume ratio 2:1). Sodium hydroxide solution was then added dropwise to adjust the pH to 10.8, and the mixture was stirred overnight. The resulting solution was filtered, rinsed with anhydrous ethanol, and dried in an oven at 60 °C for 2-4 h. After drying, the sample was removed and carefully ground to ensure a uniform powder (150 mesh). This yielded a series of walnut shell-doped Pd / CN catalysts with a Pd loading of 5 wt%. These were designated 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] The catalyst obtained in Example 1 was evaluated:

[0058] X-ray diffraction (XRD) analysis was performed using an X-ray diffractometer from Rigaku Corporation, Japan. The equipment parameters were: voltage 40 kV, current 40 mA, Cu Ka target (λ = 1.5406 A), wavelength 0.15406 nm, test angle range of 5-90°, and data acquisition at a scanning speed of 10° / min.

[0059] The N2 adsorption-desorption was performed using the Autosorb-iQ2 adsorption instrument from the American company, Quanta Computer, with a degassing temperature of 150℃.

[0060] The scanning electron microscope (SEM) used was a Hitachi SU8600.

[0061] Transmission electron microscopy (TEM) and high-magnification TEM were performed on a Tecnai G2 F20 S-TWIN instrument from FEI Corporation in the United States, with an operating voltage of 200 kV.

[0062] X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250Xi instrument from ThermoFischer, USA.

[0063] The Pd K-edge X-ray absorption fine structure spectrum (XAFS) of the catalyst was collected on the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) at 3.5 GeV in “top-up” mode with a constant current of 260 mA.

[0064] The aberration-corrected transmission electron microscope images were obtained using a Themis Z03040701 instrument from ThermoFisher, Inc., USA.

[0065] In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed on a Bruker Vertex 70 Fourier transform infrared spectrometer. The DRIFTS was equipped with an MCT detector and a CaF2 window in the sample cell. An electronically controlled rapid switching system was used to reduce dead volume when switching between different gases. Formic acid decomposition reaction experiment: 50 mg of sample was heated to 60°C under Ar gas, purged, switched to 5% HCOOH / Ar, and data were collected, with continuous spectral scanning for 20 min; then switched to Ar purging, data were collected, and continuous spectral scanning for 20 min was performed. The infrared spectral scanning range was 4000–1000 cm⁻¹. -1 Spectral resolution of 4 cm -1 The spectrum collected in Ar before the experiment was selected as the background.

[0066] Performance evaluation of hydrogen production from formic acid decomposition:

[0067] Turn on the power to the water bath and heat it to 60°C to prepare a 6 mol / L formic acid solution. 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 stir, and preheat at a constant temperature for about 5 minutes. After ensuring good airtightness, zero the electronic balance, and inject 0.5 mL of formic acid solution into the two-necked flask through the flap stopper using a syringe. Start timing and record the reading every 5 seconds until no more gas is released.

[0068] The stability test is performed by injecting 0.5 mL of formic acid solution after the above steps are completed, starting the timer and recording the reading every 5 seconds until no more gas is expelled; then inject formic acid solution again and repeat the above steps 10 times.

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

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

[0071] In the formula, P represents standard atmospheric pressure, which is 101.325 kPa;

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

[0073] R represents the ideal gas constant, with a value of 8.3145 J / (mol·K);

[0074] T represents the thermodynamic temperature, which is 298K.

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

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

[0077] Structural characterization of Pd / CN series catalysts

[0078] Figure 2 a is the XRD pattern of the catalyst, used to characterize its crystal structure. For example... Figure 2 As shown in Figure a, all catalysts exhibit typical g-C3N4 diffraction peaks (2θ = 27.60°, 12.90°), indicating that they all possess a typical graphitic carbon nitride structure. The diffraction peak centered at 27.60° corresponds to the (002) crystal plane of graphitic carbon nitride, with a corresponding interplanar spacing d of 0.324 nm, belonging to a layered stacked structure. The diffraction peak centered at 12.90° corresponds to the (100) crystal plane of graphitic carbon nitride, with a corresponding interplanar spacing d of 0.691 nm, belonging to an in-plane tri-s-triazine repeating structural unit. As the doping amount of walnut shell increased, the intensity of the (002) crystal plane diffraction peak decreased significantly, indicating that the g-C3N4 structure was distorted and the crystallinity decreased after doping. Furthermore, the peak position shifted from 27.60° to 25.42°, indicating an increase in interlayer spacing. This suggests that the composite of walnut shell and graphitic carbon nitride disrupted part of the layered structure of carbon nitride. No Pd diffraction peaks were observed in any of the catalysts, indicating that the supported Pd particles were small in size and highly dispersed.

[0079] The specific surface area and pore structure of the samples were characterized. For example... Figure 2 As shown in b, all samples conform to the Type IV isotherm and exhibit an H1 hysteresis loop. 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 m². 2 / g, with the increase of walnut shell impurity, the specific surface area increases, and the Pd / CN-nutshell 5:1 reaches 43.35m². 2 / g.

[0080] Table 1. Microstructural parameters of Pd / CN series catalysts doped with walnut shells

[0081]

[0082] like Figure 3 As shown, Figure 3 a and Figure 3 b is the SEM image of Pd / CN. The image shows that the Pd / CN catalyst has a fish-scale morphology, with a Pd / CN-nutshell ratio of 5:1. Figure 3 c, Figure 3 d) and Pd / / CN-nutshell 5:4( Figure 3 e, Figure 3 f) The loose porous structure indicates that the doping with walnut shells alters the morphology and structure of g-C3N4. The more walnut shells are added, the more pores there are and the looser the structure becomes, which is consistent with the XRD and N2 adsorption-desorption results.

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

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

[0085] like Figure 4 and Figure 5 As 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 indicate that the synergistic effect between appropriate doping of walnut shell and g-C3N4 contributes to the formation of relatively small-sized and highly dispersed Pd nanoparticles.

[0086] Figure 6 This is an electron micrograph of the Pd / CN-nutshell 5:1 catalyst doped with walnut shells, according to an embodiment of the present invention. Figure 6 a- Figure 6 c is an aberration-corrected electron microscope image of Pd / CN-nutshell 5:1. Figure 6 d represents the HAADF-STEM diagram and elemental mapping diagram of Pd / CN-nutshell 5:1;

[0087] Figure 7 These are HAADF-STEM images and elemental mapping images of the walnut shell-doped Pd / CN-nutshell 5:4 catalyst from an embodiment of the present invention. Figure 7 a is a HAADF-STEM image; Figure 7 b is the C element mapping image; Figure 7 c is an N-element mapping image; Figure 7 d is the O-element mapping image; Figure 7 e is the image mapped to element Pd;

[0088] Figure 8 These are HAADF-STEM images and elemental mapping images of the walnut shell-doped Pd / CN catalyst from an embodiment of the present invention. Figure 8 a is a HAADF-STEM image; Figure 8 b is the C element mapping image; Figure 8 c is an N-element mapping image; Figure 8 d is the O-element mapping image; Figure 8 e is the image mapped to element Pd;

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

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

[0091] XPS analysis was used to analyze the surface elemental chemical and electronic states of the synthesized sample. The full spectrum and surface content of each element are listed below. Figure 9 a and Table 2. The catalyst contains C, N, O, Pd and S elements. The S element content is extremely low and can be regarded as an impurity. As the amount of walnut shell doping increases, the C content gradually increases and the N content gradually decreases.

[0092] like Figure 9 As shown in b, the C1s XPS spectrum can be divided into three peaks: 284.8, 286.2, and 288.3 eV. The peak at 284.8 eV belongs to the CC species; the peak at 288.3 eV belongs to the sp species in the g-C3N4 main structure. 2Hybrid carbon (NC=N); the peak at 286.2 eV is attributed to carbon species in CNC or C-NH2. The contents of CC, NC=N, and CNC or C-NH2 are listed in the table. With increasing walnut shell doping concentration, the NC=N content gradually decreases, while the CC content gradually increases. Figure 9 As shown in Figure c, the N 1s XPS spectrum can be divided into three peaks: 398.8, 400.1, and 401.3 eV, which are assigned to pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen, respectively. With increasing walnut shell doping concentration, the peak intensity of N 1s gradually decreases. The contents of pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen, as well as the pyridine nitrogen / pyrrole nitrogen ratio, are listed in Table 3. 2C / N 3C The ratio can reflect the degree of N atom deficiency. 3C Located at the very center and connected to three C atoms, it is very stable. Therefore, as the amount of walnut shell impurity increases, N... 2C / N 3C The ratio decreased, attributed to N 2C The N deletion at the site forms an N defect site, and the defect site originates from pyridine nitrogen.

[0093] Table 2 shows the elemental distribution on the catalyst surface as measured by XPS.

[0094]

[0095] Table 3 shows the N1s spectrum obtained from XPS N1s spectra. (2C) N (3C) and NH content

[0096]

[0097]

[0098] Table 4 shows the Pd obtained from XPS Pd3d spectra. 2+ and Pd 0 The content of Pd 0 / Pd 2+

[0099]

[0100] From the Pd 3d spectrum ( Figure 9 d) It can be seen that two strong peaks appeared at 337.5 eV and 342.8 eV in the Pd / CN series catalysts, which can be attributed to Pd(II)3d, respectively. 3 / 2 and Pd(II)3d 5 / 2 The low-intensity peaks at 335.6 eV and 340.7 eV are attributed to Pd(0)3d, respectively. 3 / 2 and Pd(0)3d 5 / 2Table 4 shows that the Pd(II) content is around 90% in all cases. Based on literature, pyridine N enhances the interaction between the metal and the support by forming Pd-N covalent bonds, leading to electron transfer from Pd to N. Therefore, Pd / CN series catalysts all contain a high proportion of Pd. 2+ Notably, the peak position in Pd / CN-nutshell 5:1 shifts to a higher binding energy by 0.2 eV, but shifts back to a lower binding energy as the doping concentration continues to increase, indicating a stronger electronic interaction between Pd and the support in Pd / CN-nutshell 5:1. N1s XPS analysis shows that Pd / CN-nutshell 5:1 has more nitrogen defects compared to Pd / CN, consistent with the literature finding that C self-doping with g-C3N4 can increase the defect concentration. The defect structure of CN-nutshell 5:1 improves the electronic environment of the anchored Pd sites, resulting in stronger electronic interactions between Pd and the support, more electrons transferred from Pd to N, and enhanced Pd... 2+ The charge density further decreases, thus shifting towards higher wavenumbers. The confinement effect of the defect structure can also effectively prevent the aggregation of Pd sites. Combined with TEM, the Pd particles in Pd / CN-nutshell 5:1 are the smallest and have more defect sites that match those in CN-nutshell 5:1.

[0101] Figure 10 a represents 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 weighted EXAFS spectrum Figure 10 c represents the EXAFS oscillations of Pd / CN-nutshell 5:1, Pd foil, and PdO at the Pd K-edge, superimposed with the fitted scattering contribution. Figure 10 d is k 3 Wavelet transform of weighted EXAFS spectrum.

[0102] Figure 11 For the embodiment of this invention, Pd / CN-nutshell 5:1 catalyst k 3 Fourier transform fit plot of weighted EXAFS spectrum;

[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. This contrasts with the high Pd detected by XPS. 2+ / Pd 0 The ratios are consistent. Figure 9The XANES spectrum of Pd / CN-nutshell-5:1 in a medium is very similar to that of PdO, rather than Pd foil. The EXAFS spectrum of Pd / CN-nutshell-5:1 is also very similar to that of PdO, but not to Pd foil. Figure 10 b and Figure 11 ) shows that in There is a significant Pd-O peak at [location missing]. There is a very small Pd-Pd peak at that point, and at... There is a significant Pd-O-Pd peak, which originates from 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) indicates that most of the Pd in ​​Pd / CN-nutshell-5:1 is in the oxidized state. Data analyzed by wavelet transform ( Figure 10 d) Consistent with EXAFS fitted data. Furthermore, Some of the peaks can be attributed to individual Pd atoms interacting with oxygen or nitrogen functional groups (i.e., Pd-Ox or Pd-Nx). The presence of Pd-Nx in this system is presumed, confirming the formation of covalent bonds 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 shells catalyzing the decomposition of formic acid to produce hydrogen; Figure 12 b is the activity profile of Pd / CN-nutshell 5:1 after multiple cycles; Figure 12 c is the TOF plot;

[0106] Figure 13 This is a composition diagram of the products after formic acid decomposition catalyzed by the Pd / CN series catalyst doped with walnut shells in an embodiment of the present invention.

[0107] The catalyst was used to catalyze the decomposition of formic acid to produce hydrogen. The formic acid decomposition reaction has two possible pathways: dehydrogenation (HCOOH→CO2+H2) and dehydration (HCOOH→CO+H2O). However, the dehydration pathway produces CO, which poisons and inactivates the active sites; therefore, the dehydration pathway is undesirable in formic acid decomposition. In this experiment, offline chromatography was used to detect the product components. Figure 13 It can be seen that the reaction only produces hydrogen and carbon dioxide. No CO was detected at the lowest detection limit of chromatography, indicating that the Pd / CN series catalyst doped with walnut shells has high hydrogen selectivity, which is 100%.

[0108] Catalytic activity diagram as follows Figure 12As shown in Figure a, all five prepared catalysts exhibited good catalytic activity for the formic acid decomposition to hydrogen production, achieving almost complete conversion within the first 5 minutes of the reaction. Under the action of Pd / CN, Pd / CN-nutshell 5:1, and Pd / CN-nutshell 5:2 catalysts, the formic acid decomposition released 136 mL of gas (CO2+H2), with Pd / CN-nutshell 5:1 showing the best catalytic effect. Pd / CN-nutshell 5:3 and Pd / CN-nutshell 5:4 showed poorer activity, each releasing 118 mL of gas (CO2+H2). Multiple rounds of experiments were conducted on Pd / CN-nutshell 5:1, and the results are as follows... Figure 12 As shown in b, the catalyst maintained high catalytic activity in the first four tests, then decreased slightly. In the tenth test, it still released 120 mL of gas in ten minutes, indicating good catalyst stability. The TOF in the reaction was calculated based on the Pd loading of the catalyst, as follows: Figure 12 As shown in c, the TOF value of Pd / CN-nutshell 5:1 is as high as 5758h. -1 The activity was 1.9 times that of the Pd / CN catalyst; however, the activity gradually decreased with increasing walnut shell doping content, and the Pd / CN-nutshell 5:4 catalyst showed the worst catalytic effect, with a TOF value of only 1355 h⁻¹. -1 Table 5 shows the activity comparison with other similar catalysts, indicating that the walnut shell-doped Pd / CN series catalysts have excellent catalytic performance.

[0109] Table 5 compares the catalytic activity of reported Pd-based catalysts with that synthesized in this study in the formic acid dehydrogenation reaction.

[0110]

[0111]

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

[0113] TEM images of the catalyst after the reaction estimated the size and distribution of Pd nanoparticles. Figure 14 The image shows the structural characterization of the Pd / CN series catalyst doped with walnut shells after the reaction, as described in this embodiment of the invention. Figure 14 a and Figure 14 b is a TEM image of Pd / CN-nutshell 5:1 after the reaction. Figure 14 c is the particle size distribution of Pd particles after the Pd / CN-nutshell 5:1 reaction; Figure 14 d and Figure 14 e is a TEM image after the Pd / CN reaction. Figure 14f is the particle size distribution of Pd particles after the Pd / CN reaction;

[0114] like Figure 15 As shown, Pd is uniformly distributed on the support. Compared with before the reaction, the Pd particles all increased slightly after the reaction, with the Pd / CN-nutshell 5:1 increasing from 1.8 nm to 2.7 nm; and the Pd / CN increasing from 2.9 nm to 3.5 nm. The Pd / CN-nutshell 5:1 particles were characterized by XPS after multiple rounds of experimental reactions.

[0115] Figure 15 The above are XPS spectra of the Pd / CN-nutshell 5:1 catalyst after multiple cycles of reaction in an embodiment of the present invention; wherein, Figure 15 a is Pd3d. Figure 15 b is N1s;

[0116] Depend on Figure 15 As shown in Table 6, compared to before the reaction, Pd 0 The content increased from 13.14% to 57.09%, Pd 2+ The content decreased from 86.86% to 42.91%, indicating that as the Pd particles gradually increased in size during the reaction, the surface Pd content decreased. 0 With Pd 2+ The proportion changed, combined with multiple rounds of activity testing ( Figure 12 b) The activity decreased after four cycles, further indicating that the Pd particles were smaller and Pd... 2+ Only when the proportion of N is high can the catalyst achieve excellent catalytic activity. (2C) The pyridine nitrogen content decreased from 71.15% to 42.73%, N (3C) The pyrrolidone nitrogen content increased from 20.17% to 48.74%, while the graphitic nitrogen content remained relatively unchanged. This indicates that the pyridine nitrogen content decreased during the reaction, and this decrease in pyridine nitrogen led to a reduction in Pd-N. 0 / Pd 2+ The proportions are unbalanced.

[0117] Table 6 shows the content of each species in the post-reaction Pd / CN-nutshell 5:1 mixture obtained from XPS spectra.

[0118]

[0119] Reaction mechanism of Pd / CN series catalysts

[0120] In-situ infrared spectroscopy was used to study the active intermediate of Pd / CN-nutshell 5:1 in the catalytic decomposition of formic acid, thereby inferring the reaction pathway. Figure 16 As shown, 2363 and 2336cm -1The peaks at 1587 and 1350 cm⁻¹ represent gaseous CO₂. -1 The absorption peaks at 2850 cm⁻¹ represent the asymmetric and symmetric vibrations of OCO in the adsorbed HCOO*, respectively. -1 The peak represents the CH vibration in the adsorbed HCOOH. Furthermore, the product components were detected by offline chromatography. Figure 13 As can be seen, the reaction only produces hydrogen and carbon dioxide, with no carbon monoxide generated. This indicates that in the catalytic formic acid decomposition reaction using walnut shell-doped Pd / CN series catalysts, HCOO* is the reaction intermediate, and no CO byproduct is generated.

[0121] Some literature indicates a synergistic effect between divalent and zero-valent Pd, with divalent Pd electrostatically adsorbing HCOO. - Adjacent Pd 0 The site promotes its decomposition and hydrogen production. In the walnut shell-doped Pd / CN system, combined with XPS, due to the strong interaction between pyridine N and Pd, the Pd in ​​the Pd / CN series catalysts... 2+ It accounts for about 90%, with only a small amount of Pb. 0 And Pd 2+ It can electrostatically adsorb HCOO - Therefore, we speculate that a high proportion of Pd 2+ This is the main reason why this system exhibits high catalytic activity and high selectivity. The Pb content in the catalyst after multiple cyclic activity tests... 2+ Reduce Pb 0 The increase in Pb, coupled with a slight decrease in catalyst activity, also demonstrates that in this system, Pb... 2+ Much higher than Pb 0 At that time, i.e., a high proportion of Pb 2+ and a small amount of Pb 0 It exhibits excellent catalytic activity. Compared to pure CN-supported Pd-based catalysts, the Pd / CN-nutshell 5:1 catalyst, through biomass doping, improves the structural characteristics of the support, increases nitrogen defect sites, thereby anchoring Pd species and effectively preventing Pd site aggregation, maintaining the Pd particle size below 2 nm; and the Pd in ​​XPS... 2+ The peak shifts towards higher binding energies, indicating that nitrogen defect sites improve the electronic environment of anchored Pd sites and optimize the electronic structure of Pd, thus resulting in the highest formic acid decomposition activity. However, when the walnut shell content is too high, the pyridine N content on the catalyst surface decreases sharply, nitrogen defects decrease accordingly, catalyst particles grow, and catalytic performance deteriorates.

[0122] In summary, this patent utilizes a biomass doping method, employing walnut shells and melamine as precursors to obtain nitrogen-deficient carbon nitride, which is then used as a support for further loading Pd. The resulting catalyst requires no calcination or reduction and exhibits extremely high catalytic activity and selectivity in the formic acid decomposition reaction. Furthermore, the Pd / CN-nutshell 5:1 ratio demonstrates the highest TOF. Due to the strong interaction between pyridine nitrogen and Pd, the Pd in ​​the Pd / CN series catalysts... 2+ The proportion is much higher than that of Pb 0 Pd 2+ It can electrostatically adsorb HCOO - This promotes the decomposition reaction of formic acid, thus resulting in a high proportion of Pd. 2+ This is the main reason why this system exhibits excellent catalytic activity. Simultaneously, biomass doping can effectively increase the amount of nitrogen defect sites on the carbon nitride surface. These defect sites can not only anchor the active metal, resulting in fine Pd particles, but also optimize the electronic structure of Pd, thereby enhancing catalytic activity. This study provides a new approach for understanding the determination of active sites in formic acid decomposition and the design of high-performance catalysts.

[0123] In summary, this invention discloses a method for preparing a Pd-supported walnut shell-doped carbon nitride catalyst via deposition and precipitation, and its application in the catalytic decomposition of formic acid to produce hydrogen. The preparation method involves: firstly, using a thermal polymerization method with walnut shell and melamine as precursors, successfully preparing a walnut shell-doped g-C3N4 support material; then, using a deposition and precipitation method to prepare a Pd-supported walnut shell-doped carbon nitride catalyst. When this catalyst is used to catalyze the decomposition of formic acid to produce hydrogen, it exhibits high catalytic activity and very high catalytic performance. No CO generation was observed during the reaction, indicating that this catalyst has strong hydrogen selectivity, nearly 100%, and possesses excellent prospects for widespread application.

[0124] Therefore, those skilled in the art will recognize that although embodiments of the present invention have been shown and described in detail herein, many other variations or modifications conforming to the principles of the present invention can be directly determined or derived from the disclosure of the present invention without departing from the spirit and scope of the invention. Therefore, the scope of the present invention should be understood and recognized as covering all such other variations or modifications.

Claims

1. The application of a Pd deposition-precipitation method supported walnut shell-doped carbon nitride catalyst in the catalytic decomposition of formic acid to produce hydrogen, characterized in that, The method for preparing the catalyst includes the following steps: Step S1: Preparation of walnut shell-doped g-C3N4 support material A walnut shell-doped g-C3N4 carrier material was prepared using a thermal polymerization method with walnut shells and melamine as precursors. Specifically: After the walnut shells are crushed by a grinder, they are ball-milled into powder. Melamine and walnut shell powder are weighed and mixed evenly, and then transferred to a small crucible. The mass ratio of melamine to walnut shell powder is 5:1-4. The crucible is placed in a muffle furnace and calcined. After cooling to room temperature, it is taken out and ground into powder, finally obtaining a black porous solid, which is the walnut shell-doped g-C3N4 carrier material. Step S2: Preparation of Pd deposition-precipitation method supported walnut shell-doped carbon nitride catalyst PdCl2 was dissolved in NaCl solution to prepare Na2PdCl4 solution; the support prepared in step S1 was weighed and dispersed in a beaker containing deionized water, and after sonication, the deionized water containing the support was obtained, which was poured into the prepared Na2PdCl4 solution and stirred, and then sodium hydroxide solution was added dropwise to adjust the pH and stirred; the obtained solution was filtered, rinsed with anhydrous ethanol, placed in an oven, and dried. After drying, the sample was taken out and carefully ground to ensure that a uniform powder was obtained, which is the Pd supported walnut shell doped carbon nitride catalyst.

2. The application as described in claim 1, characterized in that, In step S1, the walnut shells are ball-milled to 100-200 mesh.

3. The application as described in claim 1, characterized in that, In step S1, the food is roasted at 650 °C for 4 h.

4. The application as described in claim 1, characterized in that, In step S1, the final black porous solid obtained has a mesh size of 100-200.

5. The application as described in claim 1, characterized in that, In step S2, the concentration of NaCl solution is 0.048 mol / L and the concentration of Na2PdCl4 solution is 0.024 mol / L.

6. The application as described in claim 1, characterized in that, In step S2, 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 to Na2PdCl4 solution in which the carrier is dispersed is 2:1; and the stirring time in step S2 is 3-5 h.

7. The application 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; then dried at 60 ℃ for 2-4 h; the Pd-supported walnut shell-doped carbon nitride catalyst has a mesh size of 100-200 mesh; and the Pd loading in the Pd-supported walnut shell-doped carbon nitride catalyst is 5 wt%.

Images