Nitrogen-doped porous carbon and preparation method and application thereof

Abstract

The application relates to the technical field of energy materials, and discloses nitrogen-doped porous carbon as well as a preparation method and application thereof, which is prepared by taking peanut shells as a carbon source and taking ammonia as a nitrogen source; the nitrogen-doped porous carbon has a pore size of 2-50 nm and a specific surface area of 4.187-5.711 m2.g-1 ‑1 The nitrogen-doped porous carbon has nitrogen in the form of pyrrole nitrogen, pyridine nitrogen and graphitized nitrogen, and the nitrogen content in the nitrogen-doped porous carbon is 0.51-7.5%. The preparation method comprises the following steps: uniformly mixing crushed peanut shells, water and ammonia, reacting at 160-240 DEG C for 2-10 h, purifying and drying to obtain peanut shell hydrothermal carbon, and preparing the nitrogen-doped porous carbon for adsorbing methylene blue. The one-step method is used, ammonia is added as a nitrogen source, nitrogen element doping of the carbon material is completed, peanut shell-based nitrogen-doped porous carbon with high adsorption performance is prepared, and the time consumption is short and the steps are few.

Description

Technical Field

[0001] This invention belongs to the field of energy materials technology, specifically relating to a nitrogen-doped porous carbon, its preparation method, and its application. Background Technology

[0002] Biomass energy refers to a form of energy in which all non-fossil organic matter absorbs carbon dioxide during its growth, converting solar energy into chemical energy. Biomass is a renewable carbon source found in nature, and fully utilizing biomass to prepare high-performance carbon materials is highly beneficial to carbon emission reduction. Hetero-element doping is also an important method for the functionalization of biomass-based carbon materials. Doping the surface of carbon with functional groups of different elements during pre-carbonization or modification will facilitate the complexation of adsorbents and adsorbates, thereby increasing the adsorption capacity of the adsorbent.

[0003] Adding different elements to carbon materials can effectively enhance their adsorption capacity. Nitrogen is highly stable and inexpensive, and can increase the nitrogen-containing functional groups on the surface of hydrothermal carbon, enhancing its electrostatic interactions and improving its adsorption performance. Existing technology for preparing biomass-based nitrogen-doped porous carbon (referencing Chinese patent CN 106629655 B) uses cellulose carbamate as a raw material and urea as a nitrogen source. The mixture is uniformly mixed with sodium hydroxide solution, dried to form a sol, and then calcined at high temperature to prepare biomass-based nitrogen-doped porous carbon for adsorption of methylene blue. However, this method requires forming a sol before high-temperature calcination to obtain biomass-based nitrogen-doped porous carbon, involving too many process steps. Summary of the Invention

[0004] This invention provides a nitrogen-doped porous carbon, its preparation method, and its application, solving the problem that existing technologies require the formation of a sol and high-temperature calcination before biomass-based nitrogen-doped porous carbon can be obtained, resulting in too many process steps.

[0005] A nitrogen-doped porous carbon is prepared using peanut shells as the carbon source and ammonia water as the nitrogen source. The nitrogen-doped porous carbon has a pore size of 2–50 nm and a specific surface area of ​​4.187–5.711 m²·g. -1 The nitrogen in nitrogen-doped porous carbon exists in the forms of pyrrole nitrogen, pyridine nitrogen and graphitized nitrogen, and the mass ratio of nitrogen in nitrogen-doped porous carbon is 0.51 to 7.5%.

[0006] The second objective of this invention is to protect the method for preparing nitrogen-doped porous carbon, the specific steps of which are: mixing crushed peanut shells, water, and ammonia water evenly, reacting at 160-240℃ for 2-10 hours, purifying and drying to obtain nitrogen-doped porous carbon; wherein the ratio of peanut shells:water:ammonia water is 30g:250-100mL:50-200mL.

[0007] Preferably, the peanut shell:water:ammonia water ratio is 30g:100mL:200mL.

[0008] Preferably, the concentration of the ammonia water is 5% to 20.83%.

[0009] Preferably, the mixing is performed by stirring, with a stirring speed set to 600–1000 r / min. -1 Stir for 30–60 minutes.

[0010] Preferably, the drying conditions are: drying at 80–105°C for 24–72 hours.

[0011] Preferably, the nitrogen-doped porous carbon is reacted with water vapor at 750–850°C for 0.5–1.5 h under an N2 atmosphere to obtain water vapor-activated modified nitrogen-doped porous carbon.

[0012] Preferably, the nitrogen-doped porous carbon is reacted with water vapor at 750°C for 1.5 h under an N2 atmosphere to obtain water vapor-activated modified nitrogen-doped porous carbon.

[0013] Preferably, when the nitrogen-doped porous carbon is (0.05–0.1) g, the water vapor flow rate is 0.06 mL·(g·min). -1 .

[0014] A third objective of this invention is to protect the application of the nitrogen-doped porous carbon in the adsorption of methylene blue.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] 1. Using a one-step method, ammonia water is added as a nitrogen source during the preparation of nitrogen-doped porous carbon to complete the nitrogen doping of the carbon material, thus preparing peanut shell-based nitrogen-doped porous carbon with high adsorption performance. The method involves fewer steps, simpler process, shorter time consumption, and no additional consumption required.

[0017] 2. A relatively environmentally friendly, low-requirement, and relatively fast reaction rate steam activation method was selected to further improve the adsorption performance of nitrogen-doped porous carbon. Orthogonal experiments were used to explore the simple, energy-efficient, and high-adsorption-performance carbon material preparation process parameters. Attached Figure Description

[0018] Figure 1 A diagram of the experimental apparatus for water vapor-activated modification of nitrogen-doped porous carbon provided by the present invention;

[0019] Figure 2 The adsorption results of methylene blue on hydrothermal carbon prepared under different conditions provided by the present invention;

[0020] Figure 3The effect of the amount of PSC200-6-100(a) and HAC133(b) added on the adsorption of methylene blue;

[0021] Figure 4 The effect of initial methylene blue concentration on the adsorption of methylene blue by PSC200-6-100(a) and HAC133(b);

[0022] Figure 5 The effect of adsorption reaction temperature on the adsorption of methylene blue by PSC200-6-100(a) and HAC133(b);

[0023] Figure 6 The effect of adsorption time on the adsorption of methylene blue by PSC200-6-100, PSC200-6-0(a) and HAC133, HC130(b);

[0024] Figure 7 Scanning electron microscope images of PSC200-6-0(a) and PSC200-6-100(b);

[0025] Figure 8 The N2 adsorption-desorption isotherms (a) and pore size distribution diagram (b) are for PSC200-6-0 and PSC200-6.

[0026] Figure 9 Raman spectra of PSC200-6-100 and PSC200-6-0;

[0027] Figure 10 XRD patterns of PSC200-6-100 and PSC200-6-0;

[0028] Figure 11 FT-IR spectra of PSC200-6-100 and PSC200-6-0;

[0029] Figure 12 XPS plots for PSC200-6-100 and PSC200-6-0;

[0030] Figure 13 The adsorption effect of activated modified carbon on methylene blue under different conditions;

[0031] Figure 14 Scanning electron microscope images of HAC133(a) and HC130(b);

[0032] Figure 15 The N2 adsorption-desorption isotherms (a) and pore size distribution diagram (b) are for HAC133 and HC130.

[0033] Figure 16Raman spectra of HAC133 and HC130;

[0034] Figure 17 XRD patterns of HAC133 and HC130;

[0035] Figure 18 FT-IR spectra of HAC133 and HC130;

[0036] Figure 19 XPS spectra of HAC133 and HC130.

[0037] Explanation of reference numerals in the attached figures:

[0038] 1-Fixed bed tube furnace; 2-High pressure constant flow pump; 3-Quartz crucible. Detailed Implementation

[0039] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Unless otherwise specified, the experimental methods described in the various embodiments of this invention are conventional methods. Unless otherwise specified, the materials and reagents used are commercially available.

[0041] 1. Experimental materials, reagents and instruments

[0042] (1) Experimental materials

[0043] Peanut shells (PS) originate from Dengzhou City, Henan Province. After being washed with deionized water, they are baked at 105℃ for 48 hours, pulverized through an 80-mesh sieve, dried, and stored for later use.

[0044] (2) Experimental reagents

[0045] The reagents used in the experiment include:

[0046] Methylene Blue (MB): Analytical grade, Tianjin Fengchuan Chemical Reagent Technology Co., Ltd.; Potassium dihydrogen phosphate: Analytical grade, Tianjin Kemeio Chemical Reagent Co., Ltd.; Sodium dihydrogen phosphate, dodecahydrate: Analytical grade, Chengdu Kelong Chemical Co., Ltd.; Ammonia water: Analytical grade, Tianjin Fuyu Chemical Co., Ltd.; Nitrogen: Purity >99.5%, Henan Yingzhong Chemical Products Co., Ltd.

[0047] (3) Experimental instruments

[0048] The instruments used in the experiment include:

[0049] High-speed universal pulverizer: FW-130, Beijing Zhongxing Weiye Instrument Co., Ltd.; Electric thermostatic drying oven: DHG-9070A, Shanghai Jinghong Experimental Equipment Co., Ltd.; Miniature magnetic stirring autoclave: PCK-500, Nanjing Zhengxin Instrument Co., Ltd.; Fixed bed tube furnace: CD-1200NT, Zhengzhou Brothers Kiln Co., Ltd.; Gas flow meter: FL-1002, Shenzhen Flömei Measurement and Control System Co., Ltd.; High-pressure constant flow pump: STI-501Plus, Saizhi Technology (Hangzhou) Co., Ltd.; Electric thermostatic oscillating water bath: DKZ-2, Shanghai Jinghong Experimental Equipment Co., Ltd.; Ultraviolet spectrophotometer: UV-3900, Hitachi, Ltd., Japan; Electronic balance: FA2004N, Shanghai Minqiao Precision Scientific Instrument Co., Ltd.

[0050] 2 Experimental Methods

[0051] 2.1 Preparation of the target product

[0052] (1) Preparation of nitrogen-doped porous carbon

[0053] Weigh 30g of peanut shell powder and place it into the ceramic liner of a micro magnetic stirring autoclave. Then add 300mL of a mixture of water and ammonia (peanut shell:water:ammonia = 30g:100mL:200mL). Seal the autoclave with sealing film and set the stirring speed to 600 rpm. -1 Stir for 30 minutes to ensure thorough mixing of the solid and liquid. Place the mixture in a reactor and set the hydrothermal reaction temperature to 160, 200, and 240℃ respectively, maintaining the temperature at the target for 2, 6, and 10 hours. After the reaction, remove the reactor and allow it to cool naturally in air. Rinse the mixture in the reactor first with anhydrous ethanol three times, then with deionized water three times, repeating this rinsing process. Filter and wash continuously under vacuum filtration until the filtered liquid is colorless and transparent. Dry the filter residue at 105℃ for 24 hours, then grind it through an 80-mesh sieve to obtain peanut shell-based nitrogen-doped hydrothermal carbon (PSC), i.e., nitrogen-doped porous carbon. When the amount of ammonia added is 100 mL, it is denoted as PSC xy (x represents the hydrothermal temperature, y represents the hydrothermal time) according to different temperature and time conditions. At a hydrothermal temperature of 200℃, a reaction time of 6 hours, and keeping the total volume of the liquid constant, it is named PSC xyz (z represents the amount of ammonia added) based on changes in the amount of ammonia added. Therefore, the names of the samples with different amounts of ammonia added and different reaction conditions according to the naming rules are shown in Table 1. Due to space limitations when drawing the graph, PSC200-6-100 is PSC200-6 in the figure.

[0054] Table 1 Experimental parameters and naming conventions

[0055]

[0056]

[0057] (2) Preparation of nitrogen-doped porous carbon modified by steam activation

[0058] Activation device for steam-activated modified nitrogen-doped porous carbon, such as Figure 1 As shown, 5g of nitrogen-doped porous carbon was placed in the middle of a fixed-bed tube furnace 1. Under a N2 atmosphere, after reaching the target temperature, deionized water was pumped into a quartz crucible 3 at a certain flow rate using a high-pressure constant-flow pump 2. The high temperature of the fixed-bed tube furnace 1 converted the deionized water into steam, which reacted with the material to achieve activation. After the reaction, the high-pressure constant-flow pump 2 was turned off, and the mixture was cooled to room temperature under N2 protection. This yielded steam-activated modified peanut shell-based nitrogen-doped hydrothermal carbon (HAC), i.e., steam-activated modified nitrogen-doped porous carbon.

[0059] The orthogonal experimental method selected activation reaction temperature, activation reaction time, and the flow rate of water vapor in the activation medium as reference factors, and the adsorption value of methylene blue as the evaluation index. Three experimental levels were selected for each experimental factor, using an L9(3)2 model. 3 The experiment was arranged using an orthogonal array, and the factor level table is shown in Table 2. Each prepared material was named HACxyz (x: level number of activation temperature; y: level number of activation time; z: level number of water vapor flow rate in the activation medium).

[0060] Table 2. Orthogonal experimental design for preparing HAC at three levels and three factors.

[0061]

[0062] 2.2 Characterization and analysis of the physicochemical properties of the products

[0063] (1) Yield analysis

[0064] The mass yield of the processed sample is calculated using the following formula:

[0065] Among them, Y hydro It is the mass yield of the processed sample, in %. M solid，d It is the solid mass of nitrogen-doped porous carbon collected after drying, in g, M free，d It is the total mass of biomass, in grams (g).

[0066] (2) Elemental Analysis

[0067] The carbon, hydrogen, nitrogen and sulfur of the sample were analyzed using an elemental analyzer (UNICUBE, ElementarAnalysensysteme, Germany), and the oxygen content was calculated using the difference method.

[0068] (3) Pore Specific Surface Area Analysis (BET)

[0069] The textural properties of the samples were investigated using a gas adsorption analyzer (BELSORPMINIⅡ, Japan) at liquid nitrogen temperature (77 K) using a nitrogen adsorption-desorption isotherm. Prior to the adsorption experiments, the samples were heated at 200 °C under vacuum (10–4 MPa) for 5 h. Specific surface area was analyzed using the BET (Brunauer-Emmett-Teller) method. Pore size distribution was analyzed using the BJH (Barrett-Joiner-Halenda) model.

[0070] (4) Raman analysis

[0071] Raman spectra of activated carbon were collected using a Raman spectrometer (LabRAM HR Evolution, HORIBA Jobin Yvon, France) and a 532 nm laser. Among them, I... D and I G These are approximately 1350 cm⁻¹ in the Raman spectral mode after curve fitting. -1 and 1580cm -1 The peak area at the Raman shift.

[0072] (5) Crystal structure analysis (XRD)

[0073] X-ray diffraction (XRD) information of the samples was obtained using an X-ray diffractometer (Smart Lab, Rigaku, Japan) with a CuKα source, 45 kV, 200 mA, 2θ range of 10° to 80°, Δθ = 0.01, and a scan rate of 10° / min. -1 .

[0074] (6) Fourier transform infrared spectroscopy analysis (FT-IR)

[0075] The surface functional groups of the samples were studied using the KBr particle method via Fourier transform infrared spectroscopy (FT-IR, Vertex 80, Bruker, Germany). -1 Resolution between 4000 and 400cm -1 The spectrum was collected by 16 scans within the range.

[0076] (7) X-ray electron spectroscopy (XPS)

[0077] To determine the chemical state of the selected elements and the surface composition of the samples, X-ray surface photoelectron spectroscopy (XPS) analysis was performed using a Thermo K-Alpha instrument manufactured in the United States, employing Al Kα rays (hv = 1486.6 eV) with 5 cycles of signal accumulation.

[0078] 2.3 Methylene Blue Adsorption Performance Test

[0079] 2.3.1 Plotting the standard curve for methylene blue solution

[0080] Prepare 2 mg·L -1 4 mg·L -1 6 mg·L -1 8 mg·L -1 and 10 mg·L -1 For methylene blue solutions, quartz cuvettes were used, and the absorbance of each solution was measured at 665 nm using a UV-3900 microscope. Based on the different concentrations of the methylene blue solutions and their corresponding absorbance measurements, a standard curve was plotted to fit the relationship between the methylene blue solution and the absorbance.

[0081] 2.3.2 Adsorption experiment and calculation of methylene blue

[0082] A certain mass of nitrogen-doped porous carbon was weighed and placed into a 50 mL centrifuge tube. Then, 40 mL of a methylene blue solution of a certain concentration was added. Under controlled temperature and 100 rpm conditions, after adsorption by closed shaking, the mixture was filtered through a 0.45 μm inorganic filter membrane and the remaining concentration of methylene blue was determined using a UV-3900 spectrophotometer (Hitachi, Japan). The maximum wavelength of methylene blue is 665 nm. The adsorption capacity Q was calculated using the following formula. t And adsorption rate R.

[0083]

[0084] In the formula, Q t The adsorption capacity (mg·g) of a unit mass sample for methylene blue at time t (min) -1 ), mg·g -1 C0 and C t The concentrations of methylene blue at time t (min) and t (min) represent the initial concentration and the concentration (mg·L) of methylene blue, respectively. -1 V represents the solution volume (L); W represents the amount of adsorbent added (g).

[0085] 2.3.3 Effects of Different Factors on the Adsorption of Methylene Blue (Experiment)

[0086] Effect of addition amount on the adsorption performance of methylene blue by the target product: A certain mass of nitrogen-doped porous carbon was weighed and added to a 50 mL centrifuge tube, and then 40 mL of 100 mg·L⁻¹ sodium chloride solution was added to the centrifuge tube. -1 Methylene blue solution. The mixture was subjected to a closed-loop shaking condition at 30°C and 100 vibrations per minute for 24 hours. The remaining concentration was measured, and the adsorption capacity and adsorption rate were calculated.

[0087] Effect of initial concentration on the adsorption performance of methylene blue by the target product: A certain mass of nitrogen-doped porous carbon sample was weighed and placed in a 50 mL centrifuge tube. 40 mL of the sample was then added with initial concentrations of 80, 100, 120, 140, 160, 180, and 200 mg·L⁻¹, respectively. -1 The methylene blue solution was placed in a constant temperature shaker and shaken in a closed container at 30°C and 100 shakes per minute for 24 hours. The remaining concentration was measured, and the adsorption capacity and adsorption rate were calculated.

[0088] Experiment on the effect of temperature on the adsorption performance of methylene blue by the target product: A certain mass of nitrogen-doped porous carbon sample was weighed and placed in a 50 mL centrifuge tube, and 40 mL of 100 mg·L⁻¹ of the solution was added to each tube. -1 Methylene blue solution was subjected to closed shaking in a water bath at 30, 40, and 50°C for 24 hours at 100 shakes per minute. The remaining concentration was measured, and the adsorption capacity and adsorption rate were calculated.

[0089] Effect of adsorption time on the adsorption performance of methylene blue by the target product: A certain mass of nitrogen-doped porous carbon was weighed and placed in a 50 mL centrifuge tube, and 40 mL of 100 mg·L⁻¹ sodium chloride solution was added to each tube. -1 Methylene blue solution was placed in a constant temperature shaker and shaken in a closed container at 30°C and 100 shakes per minute. One bottle was taken out at 5, 10, 15, 30, 60, 75, 90, 120, 150, 180, 240, 300, 360, 720, 1080, and 1440 min to determine the remaining concentration, and the adsorption capacity and adsorption rate were calculated.

[0090] 2.3.4 Effects of different hydrothermal conditions on the yield and specific surface area of ​​nitrogen-doped porous carbon

[0091] Table 3 shows that, under the same ammonia addition and reaction time, the yield of nitrogen-doped porous carbon generally decreases with increasing reaction temperature. Particularly at a reaction time of 10 hours, the yield decreases most significantly, by 33.702%, when the reaction temperature increases from 160℃ to 240℃. Even at a reaction time of 2 hours, the yield decreases by as much as 26.378% when the hydrothermal reaction temperature increases from 160℃ to 240℃. This is likely because at 160℃, the primary reaction is the decomposition of hemicellulose, while cellulose and lignin fail to dissolve, resulting in a lower degree of carbonization and a higher solids yield. As the temperature further increases, some cellulose and lignin continue to decompose into liquid and gas, intensifying the dehydration and decarboxylation reactions, thus leading to a decrease in solids yield.

[0092] Table 3 shows that, at the same temperature and ammonia addition, the yield of nitrogen-doped porous carbon decreased with increasing reaction time. Particularly at a hydrothermal reaction temperature of 200℃, the yield decreased most drastically, by 27.939%, when the reaction time increased from 2 hours to 10 hours. At a hydrothermal reaction temperature of 240℃, the yield decreased by only 6.521% when the reaction time increased from 2 hours to 10 hours. This may be because, under low temperature and short reaction time conditions, the components in the peanut shell powder had not yet undergone complete hydrothermal carbonization, resulting in a higher yield. The decrease in yield with increasing reaction temperature and time is related to the intensified deoxidation reaction of the raw materials during the reaction process.

[0093] Under the conditions of a reaction temperature of 200℃ and a reaction time of 6 hours, the amount of ammonia added was varied. It can be seen that the yield decreases with the increase of ammonia addition. This may be because as the amount of ammonia increases, the pH of the medium gradually rises, and the increased alkalinity affects the solubility of cellulose, resulting in a reduction in solid carbon conversion. Simultaneously, it can be seen that the effect of temperature on the yield is far greater than the effect of pH changes caused by the amount of ammonia added. Under the same conditions, changing only the temperature resulted in a yield range of 33.702%, while changing only the pH caused a yield range of only 4.893%. In the hydrothermal carbonization process, temperature has a more significant impact on the yield than the pH of the medium; the yield is not significantly affected by pH.

[0094] The specific surface area of ​​the target product changes significantly with the hydrothermal reaction conditions. Table 3 shows that, with the same amount of ammonia added, the nitrogen-doped porous carbon with the smallest specific surface area of ​​PSC160-2 is 0.753 m². 2 ·g -1 As the reaction temperature and reaction time increase, the maximum value of PSC200-10 is 5.711m. 2 ·g -1 The range is 4.958m. 2 ·g -1 As the reaction temperature and reaction time further increase, the specific surface area of ​​nitrogen-doped porous carbon tends to stabilize. This is likely mainly because the fibrous structure in biomass decomposes with increasing reaction temperature and time, resulting in a smoother surface. When the reaction temperature exceeds 200℃ and the reaction time exceeds 6 hours, the specific surface area of ​​various hydrothermal carbons under different conditions does not differ significantly, ranging from 4.187 to 5.711 m². 2 ·g -1 Between them, the range is 1.524m. 2 ·g -1 .

[0095] The effect of ammonia addition was investigated at a reaction temperature of 200℃ and a reaction time of 6 h. The results showed that the specific surface area of ​​the nitrogen-doped porous carbon with added ammonia was smaller than that of the nitrogen-doped porous carbon PSC200-6-0 without ammonia. This is likely because the addition of ammonia forms more functional groups, reducing the specific surface area. Furthermore, the change in specific surface area with increasing ammonia addition was not as pronounced (see Table 3).

[0096] Table 3. Yields and specific surface areas of nitrogen-doped porous carbon prepared under different reaction conditions

[0097] PSC160-2 74.082 0.753 PSC160-6 70.359 1.921 PSC160-10 78.259 0.853 PSC200-2 72.487 1.808 PSC200-6-0 68.023 7.487 PSC200-6-50 67.134 4.519 PSC200-6-100 64.337 5.653 PSC200-6-150 63.536 4.351 PSC200-6-200 63.130 5.456 PSC200-10 54.548 5.711 PSC240-2 51.078 2.565 PSC240-6 45.793 5.408 PSC240-10 44.557 5.352

[0098] 2.3.5 Effects of different hydrothermal conditions on the adsorption performance of methylene blue on peanut shell-based nitrogen-doped hydrothermal carbon

[0099] Weigh out 0.1 g of nitrogen-doped porous carbon prepared under different hydrothermal reaction conditions and add them to 50 mL centrifuge tubes. Then add 40 mL of 100 mg·L⁻¹ solution. -1 The effect of different hydrothermal reaction conditions on the adsorption of methylene blue was investigated by shaking the solution in a closed system at 30°C and 100 times per minute for 24 hours. The results are as follows: Figure 2 As shown, at an ammonia addition of 100 mL and a temperature of 160 °C, the adsorption capacity and adsorption rate increase with time. This indicates that the biomass hydrolysis reaction has occurred, with hemicellulose undergoing pyrolysis and gradually forming a porous structure. Furthermore, the reaction may deepen over time, forming more functional groups that contribute to the improved adsorption capacity of the biochar. At an ammonia addition of 100 mL and reaction temperatures of 200 °C and 240 °C, both adsorption capacity and adsorption rate initially increase and then decrease with increasing hydrothermal reaction time, reaching their maximum at 6 hours. This is likely because at 2 hours, the decomposition of hemicellulose, cellulose, and lignin, as well as the volatilization and dissolution of other soluble components of the biomass, result in insufficient carbonization and the formation of enough functional groups. At 10 hours, the increased carbonization and destruction of some functional groups may be due to the longer reaction time. Under the same reaction time and ammonia addition of 100 mL, the removal of methylene blue by nitrogen-doped porous carbon first increased and then decreased with increasing reaction temperature. The adsorption effect was best at 200℃. This may be because the hydrothermal reaction of peanut shells is more complete at 200℃, resulting in nitrogen-doped porous carbon with more porous structure and active sites.

[0100] When the reaction temperature was 200℃ and the reaction time was 6h, the adsorption capacity of the prepared nitrogen-doped porous carbon increased with the increase of ammonia water addition, and the specific surface area also showed the same trend. However, the adsorption capacity of PSC200-6-100 (ammonia water addition 100mL) for methylene blue was 39.403mg·g. -1 The adsorption rate was 98.861%, which was only 0.28 mg·g lower than that of PSC200-6-200 (with 200 mL of ammonia added) for methylene blue. -1 The adsorption rate decreased by 0.446%. However, compared to PSC200-6-0 (with 0 mL of ammonia added), the adsorption capacity increased by 25.45 mg·g. -1 The adsorption rate was 63.941% higher, indicating that PSC200-6-100 is 2.84 times that of the undoped nitrogen-treated hydrothermal carbon PSC200-6-0. Its adsorption capacity is 0.965 mg·g higher than that of PSC200-6-50 (with 50 mL of ammonia added). -1 The adsorption rate increased by 1.611%, thus PSC200-6 not only has a good adsorption effect, but also has the advantages of lower reaction temperature, shorter reaction time, lower energy consumption, and less ammonia addition. The optimal reaction conditions for preparing peanut shell-based nitrogen-doped hydrothermal carbon for adsorbing methylene blue were selected as follows: temperature 200℃, time 6h, and ammonia addition of 100mL.

[0101] 2.3.6 Effect of Addition Amount on the Adsorption Performance of Methylene Blue on Peanut Shell-Based Nitrogen-Doped Hydrothermal Carbon

[0102] 0.02, 0.05, 0.1, 0.2, and 0.5 g of PSC200-6-100 were weighed out respectively, and the effect of the dosage on carbon adsorption performance was investigated according to the experimental method in section 2.3.3. The results are as follows. Figure 3 As shown in (a), the adsorption capacity decreases with increasing dosage, while the adsorption rate increases. This may be because the adsorption active sites provided by PSC200-6-100 are directly proportional to the dosage; the number of active sites increases with increasing PSC200-6-100 dosage. Simultaneously, the mass transfer resistance between the methylene blue solution and PSC200-6-100 decreases with increasing adsorbent dosage, allowing the adsorbent to absorb more methylene blue molecules, thus increasing the adsorption rate. However, with a constant amount of methylene blue, the excess active sites cannot be fully utilized, resulting in a decrease in adsorption capacity. When the dosage is 0.02 g and 0.05 g, the adsorption capacity of peanut shell-based nitrogen-doped hydrothermal carbon PSC200-6-100 is very high, at 89.307 mg·g⁻¹, respectively. -1 and 65.390 mg·g -1However, the adsorption rates were low, at 44.653% and 82.555% respectively. This clearly indicates incomplete adsorption, with active sites being fully occupied and insufficient for binding with methylene blue molecules. Consequently, the methylene blue solution could not be fully adsorbed. When the addition amounts were 0.2g and 0.5g, the adsorption capacity of peanut shell-based nitrogen-doped hydrothermal carbon PSC200-6-100 was very low, at 19.88 mg·g⁻¹ respectively. -1 and 7.965 mg·g -1 However, the adsorption rates were very high, at 99.649% and 99.672% respectively. The adsorption was clearly very thorough, but this resulted in a waste of PSC200-6-100, as the active sites provided by the peanut shell-based nitrogen-doped hydrothermal carbon could not be fully utilized, and not enough methylene blue molecules were provided to bind with it. When the addition amount was 0.1 g, the adsorption capacity was 39.403 mg·g. -1 The adsorption rate was 98.861%. The methylene blue solution and the active sites provided by PSC200-6-100 were fully combined, achieving a reasonable ratio without wasting PSC200-6-100. Therefore, the optimal addition amount of PSC200-6-100 was determined to be 0.1g in subsequent studies.

[0103] 2.3.7 Effect of initial concentration on the adsorption performance of methylene blue on peanut shell-based nitrogen-doped hydrothermal carbon

[0104] Weigh out 0.1 g of PSC200-6-100 and follow the experimental method in section 2.3.3 regarding the effect of initial concentration on the adsorption performance of methylene blue on carbon. The results are as follows. Figure 4 As shown in (a), the figure reveals that when the amount of PSC200-6-100 added is 0.1 g, the adsorption capacity increases with increasing initial methylene blue concentration, while the adsorption rate decreases. This is likely because an increase in the initial methylene blue concentration leads to an increase in the driving force for adsorption, thus increasing the adsorption capacity. However, since the amount of PSC200-6-100 providing active sites is limited, as the initial concentration of the methylene blue solution increases, these active sites are consumed, resulting in too many methylene blue molecules failing to bind, thus causing a decrease in the adsorption rate. Therefore, to ensure sufficient adsorption of methylene blue, subsequent studies selected 100 mg·L⁻¹. -1 As the initial concentration of the methylene blue solution.

[0105] 2.3.8 Effect of temperature on the adsorption performance of methylene blue on peanut shell-based nitrogen-doped hydrothermal carbon

[0106] Weigh 0.1 g of PSC200-6-100 and follow the experimental method in section 2.3.3 regarding the effect of temperature on the adsorption performance of methylene blue on carbon. The results are as follows. Figure 5As shown in (a). The amount of PSC200-6-100 added was 0.1 g, and the initial concentration of methylene blue was 100 mg·L⁻¹. -1 Under these conditions, the adsorption capacity increases slightly with increasing temperature, while the adsorption rate increases relatively significantly with increasing temperature. When the temperature increases from 30℃ to 50℃, the adsorption capacity increases from approximately 39.403 mg·g⁻¹. -1 Increased to 39.602 mg·g -1 The adsorption rate increased from 98.861% to 99.237%. This is likely because the adsorption reaction is endothermic; as temperature increases, the molecules become more active, their mobility increases, and the porosity and active sites increase. While increasing temperature helps improve adsorption capacity, the amount of PSC200-6-100 added was 0.1 g, and the initial concentration of methylene blue was 100 mg·L⁻¹. -1 Under the given conditions, the input of high temperature is not well matched with the increase in adsorption capacity. Taking into account both energy consumption and adsorption effect, 30℃ was selected as the reaction temperature.

[0107] 2.3.9 Effect of adsorption time on the adsorption performance of methylene blue on peanut shell-based nitrogen-doped carbon

[0108] Sixteen groups of 0.1g each of PSC200-6-100 and PSC200-6-0 carbon materials were weighed out respectively. The experiment on the effect of adsorption time on the adsorption performance of carbon materials was investigated according to the method described in section 2.3.3. The results are as follows: Figure 6 As shown in (a), the adsorption capacity of peanut shell hydrothermal PSC200-6-100 and PSC200-6-0 for methylene blue initially increases with time, then becomes almost unchanged and tends to level off, reaching adsorption equilibrium. This reflects the maximum methylene blue adsorption capacity of the adsorbents under the operating conditions. At adsorption equilibrium, the adsorption capacity of PSC200-6-100 reaches 39.403 mg·g⁻¹. -1 The adsorption capacity is much higher than that of PSC200-6-0 (12.488 mg·g). -1This is likely because the ammonia-modified peanut shell-based hydrothermal char possesses a more and better pore structure and binding sites, which also increases the electrostatic interaction sites on the biochar surface, promoting ion exchange and electrostatic attraction of methylene blue molecules in aqueous solution. This results in better adsorption performance of PSC200-6-100 under the same adsorption conditions. Throughout the adsorption process, nitrogen-doped peanut shell-based hydrothermal char PSC200-6-100 significantly outperformed undoped peanut shell-based hydrothermal char PSC200-6-0. Adsorption is typically divided into three stages: rapid adsorption, slow adsorption, and adsorption equilibrium. In the rapid adsorption stage, compared to undoped peanut shell-based hydrothermal char PSC200-6-0, nitrogen-doped peanut shell-based hydrothermal char PSC200-6-100 showed a greater rate of increase and a longer duration of adsorption, reaching 200 min, significantly higher than the former's 60 min. In the slow adsorption stage, PSC200-6-100 also showed a significantly longer duration and greater rate of increase. At 1440 min, the PSC200-6-100 reaction was considered to have reached equilibrium. Prolonging the reaction time excessively did not effectively increase the adsorption of methylene blue by carbon, but instead increased power consumption and wasted energy. Therefore, subsequent studies selected 24 h as the adsorption time for PSC200-6-100.

[0109] 2.4 Characterization and analysis results of nitrogen-doped porous carbon

[0110] 2.4.1 Morphology analysis of nitrogen-doped porous carbon

[0111] Figure 7 The images show scanning electron microscope (SEM) images of PSC200-6(b) and PSC200-6-0(a). No fibrous tubular structures were observed in either PSC200-6-100 or PSC200-6-0. The resulting char was brown in color. The surface of the unmodified peanut shell hydrothermal char was relatively rough, with fewer and unevenly distributed pores. The surface also showed unevenly distributed microgrooves of varying shapes and sizes. This may be due to the dehydration of cellulose and other substances in the biomass raw material, or the decomposition and gasification of some substances. The particulate matter on the surface may be crystals formed from certain mineral elements. Figures b1 and b2 show that the surface of the peanut shell char modified with ammonia was smoother, with increased and more uniformly distributed pores. The pores became smoother and smaller, exhibiting a more developed pore structure. This may be because ammonia deepened the carbonization process. Spherical carbon particles were also observed on the surface, which may be single, dispersed carbon microspheres formed by the thermal condensation of cellulose, hemicellulose, and lignin. Comparing Figures b1 and a1 under high magnification, it is particularly evident that the modified peanut shell hydrothermal carbon has a more diverse pore size, which is more conducive to adsorption.

[0112] 2.4.2 Pore Size Analysis of Nitrogen-Doped Porous Carbon

[0113] Figure 8 (a) shows the N2 adsorption-desorption isotherms of PSC200-6-0 and PSC200-6-100. It can be seen that the N2 adsorption isotherms of PSC200-6-100 and PSC200-6-0 are similar to type V isotherms with an H3-type hysteresis loop. As the relative pressure increases in the range of 0–0.5, the adsorption capacity initially increases very slowly, possibly because the interaction between adsorbate molecules is strong while the interaction between the adsorbent and adsorbate is too weak, making it difficult for the adsorbate to be adsorbed initially. Simultaneously, the adsorption and desorption lines coincide, indicating the presence of a microporous structure in the hydrothermal carbon. As the relative pressure increases in the range of 0.5–0.9, the adsorption capacity increases rapidly, possibly due to capillary condensation within the mesopores, exhibiting a self-accelerating adsorption phenomenon and displaying an adsorption-desorption hysteresis loop corresponding to the relative pressure range. This suggests that the hydrothermal carbon may contain narrow plate-slit structures, cracks, and wedge-shaped structures. With increasing relative pressure within the range of 0.9–1, the adsorption capacity increased very rapidly, almost vertically, gradually forming multilayer adsorption. However, no saturation adsorption plateau was observed, indicating that the prepared peanut shell-based hydrothermal carbon had a highly irregular surface and consisted entirely of mesoporous structures. Figure 8 (b) The pore size distribution diagram shows that the hydrothermal carbon contains a rich pore structure. After treatment with ammonia, it is obvious that the pore size of the hydrothermal carbon is concentrated in the range of 2-50 nm, which is larger than the pore size of the untreated carbon.

[0114] Table 4 Hole structure parameters of PSC200-6-100 and PSC200-6-0

[0115] PSC200-6-100 5.653 0.0315 22.282 PSC200-6-0 7.487 0.0369 19.718

[0116] Table 4 shows that both PSC200-6-100 and undoped PSC200-6-0 hydrothermal carbon exhibit rich pore structures. The specific surface area and total pore volume of the PSC200-6-0 sample are slightly smaller than those of PSC200-6-100, possibly due to the addition of ammonia forming more functional groups, thus reducing the specific surface area and total pore volume. Tessmer et al.'s research also indicates that surface functional groups in carbon materials can block micropores to reduce the BET surface area. Simultaneously, the slightly larger average pore size of PSC200-6-100 compared to PSC200-6-0 suggests that the ammonia gas formed upon heating the ammonia water plays a role in pore expansion. Adsorption efficiency is influenced by factors such as the pore structure and surface functional groups of the adsorbent; therefore, specific surface area is not the sole determining factor for adsorption.

[0117] 2.4.3 Raman analysis of nitrogen-doped porous carbon

[0118] Figure 9These are the Raman spectra of PSC200-6-100 and PSC200-6-0, from which the D band (1360 cm⁻¹) can be observed. -1 ) and G-band (1582cm) -1 Two strong peaks. The D peak is generally considered to indicate a disordered structure or structural defects. The G band, on the other hand, corresponds to the carbon single bond vibration within the graphite plane, similar to the vibration of a complete honeycomb sp bond. 2 E of hybrid carbon networks 2g The vibrational modes are related, representing the ordered carbon structure. Therefore, the ratio of the fitted areas of the D and G bands can be used to analyze the degree of graphitization of carbon materials. For PSC200-6-100 and PSC200-6-0, their R-values ​​I D / I G The R values ​​were 3.796 and 2.03, respectively. The R value is inversely proportional to the degree of graphitization of the material; the higher the degree of graphitization, the higher the degree of carbonization. This indicates that under the same reaction temperature, reaction time, and solid-liquid ratio, adding an appropriate amount of ammonia effectively introduced heteroatoms (N). The added N atoms enhanced the defect effect, and the increased number of defects increased the intensity of the D peak while decreasing the intensity of the G peak and the degree of graphitization. This resulted in a lower proportion of graphitized structures but a higher proportion of disordered structures in PSC200-6-100, forming more defect sites that are beneficial for adsorption. Furthermore, at 2700 cm⁻¹... -1 2D bands representing graphene can be observed at the point, which proves that graphitized structures are generated during the preparation process.

[0119] 2.4.4 XRD Analysis of Nitrogen-Doped Porous Carbon

[0120] Figure 10 These are the XRD patterns of PSC200-6-100 and PSC200-6-0. In typical lignocellulosic biomass, cellulose, due to its high order, has only a small portion that is amorphous, thus enabling it to form a relatively stable crystalline structure. Meanwhile, hemicellulose and lignin are in a relatively disordered and amorphous state. Therefore, analyzing the crystal structure of biomass helps to further understand the microscopic mechanisms of hydrothermal carbon formation. (Observation) Figure 10It can be seen that PSC200-6-100 exhibits three 2θ peaks at approximately 15.0° (101), 22.5° (002), and 34.5° (040), which are similar to the characteristic peaks of cellulose. This is due to the lateral arrangement of the microcrystals and the longitudinal structure of the polymer. This indicates that hemicellulose has decomposed, resulting in more prominent diffraction peaks of the cellulose crystal structure on XRD. The 2θ peak at 22° shows a broad diffraction peak, indicating the presence of graphitic carbon. However, because its diffraction peak is not sharp, is widely diffuse, and has low intensity, it indicates that the cellulose skeleton was destroyed during formation, resulting in poor carbon crystallinity. This proves that PSC200-6-100 is an amorphous amorphous crystal. The relatively few diffraction peaks in the spectrum also prove that PSC200-6-100 has a low degree of graphitization and poor order, which is consistent with Raman's results. In summary, XRD analysis confirms that PSC200-6-100 is an amorphous carbon with some graphite crystal structure. This material does not exhibit the sharp peaks characteristic of crystalline materials, but rather displays an amorphous structure at 2θ = 26.54°, a typical feature of activated carbon.

[0121] 2.4.5 FT-IR characterization analysis of nitrogen-doped porous carbon

[0122] Figure 11 The images show the FT-IR spectra of PSC200-6-100 and PSC200-6-0. As can be seen from the figures, the spectrum from 3392 to 3335 cm⁻¹... -1 The stretching vibration of -OH groups formed by hydrogen bonds between hydrothermal biomass carbons; in the range of 2930–2918 cm⁻¹. -1 The absorption peaks between these peaks should be due to the stretching vibrations of aliphatic -CH groups; these peaks are located between 1626 and 1606 cm⁻¹. -1 The absorption peak at 1512 cm⁻¹ is likely due to the stretching vibration of the C=C aromatic ring in biochar, which plays a crucial role in the adsorption capacity of biochar; -1 The absorption peak at that point should be due to the absorption vibration of -NO2, 1463~1461cm. -1 The absorption peaks between these peaks are attributed to the vibrations of COOH or CH=O; 1429–1426 cm⁻¹ -1 The absorption peak between these peaks should be due to the shear vibration of -CH, 1160–1110 cm⁻¹ -1 The absorption peaks between 1099 and 1032 cm⁻¹ are attributed to CO vibrations. -1 The absorption peaks between these peaks are likely caused by the stretching vibrations of C=O, belonging to the pyridine and pyrrole groups of the heteroaromatic ring. At 990 cm⁻¹... -1 The absorption peak at 613–560 cm⁻¹ is the absorption peak of the bending vibration of -CH; -1 The absorption peaks between these peaks are caused by in-plane bending vibrations of the CC=O plane. Notably, the 1700 cm⁻¹ peak was not observed in PSC200-6. -1The vibration of unsaturated aliphatic aldehydes was also observed at 1269 cm⁻¹. -1 The absorption peak should be due to the COC stretching vibration in the alkyl aryl ether bond of peanut shell lignin. Compared with the weaker signal intensity of PSC200-6-0, the peak is at 1512 cm⁻¹. -1 The absorption peak intensity at that location is also relatively weak. This indicates that the surface of PSC200-6-100 is rich in nitrogen-containing functional groups, such as pyridine nitrogen and pyrrole nitrogen; hydroxyl functional groups and aromatic compounds can provide π electrons, which can undergo surface complexation with methylene blue. Therefore, the surface of the resulting hydrate contains a large number of nitrogen-containing and oxygen-containing functional groups, such as -COOH and -OH, and exhibits a certain degree of graphitization. These surface functional groups can act as reactive groups, adsorbing aromatic pollutants through hydrogen bonding or π-π interactions, and can also be used as substitution sites for the production of N-doped carbon materials.

[0123] 2.4.6 XPS Analysis of Nitrogen-Doped Porous Carbon

[0124] The surface composition and oxidation state changes of ammonia-modified PSC200-6-100 and unmodified PSC200-6-0 were investigated using X-ray photoelectron spectroscopy (XPS). Figure 12 The full spectrum clearly shows that the N peak value in PSC200-6-100 is significantly higher and the signal is more pronounced than that in PSC200-6-0, proving that the addition of ammonia effectively increases the N content of peanut shell-based hydrothermal carbon. Further analysis... Figure 12 (b) and (c) are fine spectra of PSC200-6-100 and PSC200-6-0. The results show that the carbon material after hydrothermal treatment with ammonia water is mainly composed of pyrrole nitrogen (N-5), pyridine nitrogen (N-6), and graphitized nitrogen (NQ), while the content of oxidized nitrogen (NO) is relatively low. In contrast, the hydrothermal carbon without ammonia water treatment is also mainly composed of pyrrole nitrogen (N-5), graphitized nitrogen (NQ), and oxidized nitrogen (NO), while the content of pyridine nitrogen (N-6) is relatively low. Ammonia water effectively increases the relative content of pyrrole nitrogen (N-5) and pyridine nitrogen (N-6), and reduces the relative content of graphitized nitrogen (NQ) and oxidized nitrogen (NO), demonstrating that the addition of ammonia water effectively changes the valence state of nitrogen in the carbon. Increased nitrogen content leads to more crystallization defects in the graphite structure. The lone pairs of electrons in pyridine nitrogen and pyrrole nitrogen can serve as active sites for adsorption, thus demonstrating that the addition of ammonia water helps improve the adsorption of methylene blue by hydrothermal carbon. Detailed XPS measurements of elemental percentages are shown in Table 5. Ammonia water treatment increased the nitrogen content in the hydrothermal carbon and altered the distribution of nitrogen-containing functional groups.

[0125] Table 5. Percentage of XPS-measured Elements in PSC200-6-100 and PSC200-6-0

[0126] PSC200-6-100 66.98 3.92 23.59 41 26.24 9.17 PSC200-6-0 70.03 2.32 6.23 37.82 37.77 18.19

[0127] 2.4.7 Determination of Adsorption Conditions for Methylene Blue by Steam-Activated Modified Carbon Materials

[0128] 2.4.7.1 Effect of different activation conditions on the adsorption performance of methylene blue by activated modified carbon materials

[0129] Previous research indicates that activation temperature, activation time, and the steam flow rate of the activation medium are crucial factors affecting the activation effect. To investigate the extent to which these three factors influence the adsorption of methylene blue on carbon materials, an orthogonal experimental method was employed. Using peanut shell-based nitrogen-doped hydrothermal carbon prepared by this invention and activated by steam, and referring to the experimental method for the influence of methylene blue adsorption performance in nitrogen-doped porous carbon, L9(3) was selected. 3 The results of the orthogonal array are shown in Table 6.

[0130] Table 6. Orthogonal experimental results of HAC preparation

[0131]

[0132] k i (i represents the level number of each factor) indicates the average of the same level under different factors. The range R reflects the degree of influence of the selected factor level changes on the adsorption of methylene blue by activated nitrogen-doped porous carbon; the larger the R, the stronger the influence. It can be seen that among the three factors selected in this experiment—activation temperature (A), activation reaction time (B), and water vapor flow rate (C)—the influence on the adsorption of methylene blue by activated modified peanut shell-based nitrogen-doped hydrothermal carbon is in the following order: activation reaction time (B) > water vapor flow rate (C) > activation temperature (A). In A1B3C3 (activation temperature 750℃, activation reaction time 1.5h, water vapor flow rate 0.06mL·(g·min)). -1 Under the experimental conditions, the activated carbon exhibited a maximum adsorption capacity of 131.693 mg·g for methylene blue. -1 Therefore, the optimal process conditions were obtained: activation temperature of 750℃, activation time of 1.5h, and water vapor flow rate of the activation medium of 0.06mL·(g·min). -1 The char prepared under these conditions was named HAC133, see [link to article]. Figure 13 .

[0133] 2.4.7.2 Effect of Addition Amount on the Adsorption of Methylene Blue by Water Vapor-Activated Modified Nitrogen-Doped Porous Carbon

[0134] 0.01, 0.02, 0.03, 0.04, and 0.05 g of steam-activated modified nitrogen-doped porous carbon were weighed out respectively, and experiments were conducted according to the experimental method in section 2.3.3 regarding the effect of addition amount on the adsorption performance of methylene blue by carbon. The experimental results are as follows. Figure 3 As shown in (b), when the amount added increased from 0.01 g to 0.05 g, the adsorption capacity decreased from 190.005 mg·g⁻¹. -1 Decreased to 75.559 mg·g -1 Therefore, the adsorption capacity decreases with increasing HAC133 addition. The adsorption rate, however, tends to plateau with increasing HAC133 addition. When the addition amount increases from 0.01g to 0.03g, the adsorption rate increases from 48.433% to 99.427%. But when the addition amount increases from 0.03g to 0.05g, the adsorption rate only increases from 99.427% to 99.747%, remaining almost flat. This may be because the surface functional groups, pores, and active sites on HAC133 are positively correlated with the addition amount. When the addition amount is low, there are fewer surface functional groups, pores, and active sites on HAC133, which are fully occupied by methylene blue molecules, resulting in a large adsorption capacity but a low adsorption rate. Conversely, as the addition amount increases, HAC133 provides more surface functional groups, pores, and active sites, offering more for methylene blue molecule binding, thus reducing the adsorption capacity. The adsorption rate initially increases and then plateaus. Therefore, when the addition amount is 0.03 g, it provides sufficient surface functional groups and pores, and active sites to bind with methylene blue molecules, maintaining a high adsorption capacity of 131.693 mg·g. -1 With a high adsorption rate of 99.427%, it does not cause waste of carbon materials. Therefore, subsequent studies determined that the optimal addition amount of HAC133 was 0.03g.

[0135] 2.4.7.3 Effect of initial concentration on the adsorption performance of water vapor-activated modified nitrogen-doped porous carbon

[0136] Seven groups of 0.03g HAC133 were weighed out, and the effect of initial concentration on adsorption performance of methylene blue was investigated according to the experimental method in section 2.3.3. The results are as follows. Figure 4 As shown in (b): when the amount of HAC133 added was 0.03 g, the adsorption capacity increased with increasing initial methylene blue concentration, while the adsorption rate decreased with increasing initial methylene blue concentration. The reason is the same as in 2.3.7. Because the surface functional groups, pores, and active sites provided by a quantitative amount of HAC133 are limited, they are consumed as the initial concentration of the methylene blue solution increases, resulting in too many methylene blue molecules failing to bind and be adsorbed, thus causing a decrease in the adsorption rate. Therefore, to avoid wasting methylene blue solution, subsequent studies selected 100 mg·L⁻¹.-1 The initial concentration of methylene blue was used to assess the adsorption performance of HAC133.

[0137] 2.4.7.4 Effect of temperature on the adsorption performance of water vapor-activated modified nitrogen-doped porous carbon

[0138] Three groups of 0.03 g HAC133 were weighed out, and the experiment was conducted according to the method described in section 2.3.3 regarding the effect of temperature on the adsorption performance of methylene blue on carbon. The HAC133 addition amount was 0.01 g, and the initial methylene blue concentration was 100 mg·L⁻¹. -1 Under the following conditions, such as Figure 5 As shown in (b), the adsorption capacity and adsorption rate change only slightly with increasing temperature, while the adsorption rate increases from 99.427% to 99.67% with increasing temperature. The reason is the same as in 2.3.8. Although increasing temperature helps to improve the adsorption capacity, under these experimental conditions, the effect of temperature on the adsorption capacity is very limited. In order to avoid wasting electrical energy, 30℃ was chosen as the reaction temperature.

[0139] 2.4.7.5 Effect of adsorption time on the adsorption performance of methylene blue on water vapor-activated modified nitrogen-doped porous carbon

[0140] Sixteen groups of 0.03g HAC133 and sixteen groups of 0.03g carbon HC130 reacted at the same temperature and time but without the introduction of water vapor were weighed out. A comparative experiment was conducted using the experimental method described in section 2.3.3 regarding the effect of adsorption time on the adsorption performance of carbon materials. The results are as follows: Figure 6 As shown in (b), the adsorption capacity of both HAC133 and HC130 for methylene blue increased with time. However, HAC133 reached adsorption equilibrium in the later stages of the adsorption reaction, and the growth slowed down, while HC130 still showed a relatively significant slow increase in the later stages of the adsorption reaction, indicating that the reaction had not reached equilibrium and thus did not reflect the maximum methylene blue adsorption capacity of the adsorbent under the operating conditions. The introduction of water vapor into HAC133 effectively improved the adsorption capacity of carbon for methylene blue, achieving an adsorption capacity of 131.693 mg·g⁻¹. -1 The adsorption capacity for methylene blue by HC130 without steam activation (88.487 mg·g) is significantly higher than that of carbon without steam activation under the same adsorption conditions. -1 This may be because the nitrogen-doped porous carbon modified by steam activation possesses more and better pore structures, surface functional groups, and binding sites, resulting in better adsorption performance under the same conditions. In the rapid adsorption phase, compared to HC130, HAC133, after steam modification, exhibits a greater rate of increase and a longer duration of adsorption, reaching 150 min (with an adsorption capacity of 122.64 mg·g⁻¹). -1The adsorption capacity was significantly higher than that of the former at 60 min (reaching 33.81 mg·g). -1 Even in the slow-speed phase, the steam-modified hot char showed a significantly longer adsorption duration and a greater adsorption rate. The HAC133 reaction was considered to have reached equilibrium at 1440 min. Excessively extending the reaction time did not effectively increase the adsorption of methylene blue by the char. Considering both energy consumption and adsorption efficiency, subsequent studies selected 24 h as the adsorption experiment duration for HAC133.

[0141] 2.5 Characterization and analysis results of water vapor-activated modified nitrogen-doped porous carbon

[0142] 2.5.1 Morphology analysis of water vapor-activated modified nitrogen-doped porous carbon

[0143] pass Figure 14 It can be seen that after high-temperature treatment, the carbon materials all exhibit a smoother surface with more pores and a deeper black color compared to hydrothermal carbon, and fibrous tubular structures can be observed in all cases. Nitrogen-doped porous carbon without water vapor has more grooves and fewer pores on its surface. After water vapor is introduced, nitrogen-doped porous carbon forms more cracks and fractures, resulting in larger and more numerous pore structures. These pores are due to the reaction between water vapor and carbon atoms during carbonization, and the removal of surface impurities and amorphous components, significantly altering the surface structure. Simultaneously, spherical carbon particles are observed on the surface of nitrogen-doped porous carbon without water vapor, which may be monodisperse carbon microspheres formed by the thermal condensation of cellulose, hemicellulose, and lignin.

[0144] 2.5.2 Pore Size Analysis of Water Vapor-Activated Modified Nitrogen-Doped Porous Carbon

[0145] Figure 15 (a) shows the N2 adsorption-desorption isotherms of HAC133 and HC130. It can be seen that the N2 adsorption isotherm of HAC133 is similar to a Type IV isotherm with an H3-type hysteresis loop, which is characteristic of mesoporous materials. HC130 carbon exhibits a Type IV isotherm similar to one with an H4-type hysteresis loop, indicating the presence of micropores and mesopores. Both HAC133 and HC130 show inflection points (P / P) in the low-pressure region. o<0.1), which is usually caused by the formation of monolayer adsorption points. With increasing relative pressure in the range of 0–0.4, the adsorption and desorption lines of HAC133 and HC130 coincide, indicating the presence of a microporous structure in the hydrothermal carbon. With increasing relative pressure in the range of 0.4–1, HAC133 exhibits an H3-type hysteresis loop, possibly indicating capillary condensation in the carbon material's pore structure, suggesting that steam-activated HAC133 contains narrow plate-slit structures, cracks, and wedge-shaped structures; while HC130 exhibits an H4-type hysteresis loop, indicating that HC130 treated at high temperature without steam introduction contains micropores and mesopores. It was also found that no adsorption saturation plateau appeared with increasing relative pressure, indicating that the surfaces of both HAC133 and HC130 are relatively disordered, also suggesting multilayer interactions between the adsorbate and the carbon material. Figure 15 (b) The pore size distribution diagram shows that HAC133 exhibits a narrower BJH pore size distribution between 3.0 and 4.8 nm. In terms of both total pore volume and average pore size, HAC133 is superior to HC130. The smaller specific surface area may be due to the enrichment of functional groups in HAC133 after steam oxidation. Park et al. showed similar adsorption and isotherm patterns in their study, indicating that the abundance of mesopores and micropores in carbon facilitates adsorption. Table 7 shows that the introduction of water vapor effectively expanded the pore size and total pore volume.

[0146] Table 7 Pore structure parameters of HAC133 and HC130

[0147] HAC133 516.0950 0.505154 3.9152 HC130 550.6575 0.343217 2.4931

[0148] 2.5.3 Raman analysis of water vapor-activated modified nitrogen-doped porous carbon

[0149] The structure of carbon was studied using Raman spectroscopy. For example... Figure 16 As shown, two characteristic peaks observed in carbonaceous materials were measured in both HAC133 and HC130. The D peak corresponding to disorder defects in HAC133 and HC130 appears at 1345 cm⁻¹. -1 The G peaks corresponding to the graphite lattices of HAC133 and HC130 appear at 1587 cm⁻¹. -1 and 1595cm -1 Compared to HC130, the G peak of HAC133 shifted to the right. Furthermore, compared to untreated carbon, the D and G peaks of HAC 133 and HC130, after high-temperature treatment, became significantly sharper. The sharpening of the D and G peaks and the rightward shift of the G peak may be due to the removal of functional groups such as epoxides on HAC 133, reducing defects and forming sp... 2Double bonds. The R values ​​for HAC 133 and HC130 are 2.65 and 2.446, respectively, indicating that HAC 133 has a significantly higher defect degree, consistent with SEM observations. This suggests that the introduced water vapor effectively enhances the defect effect; the increased number of defects increases the intensity of the D peak while decreasing the intensity of the G peak and the degree of graphitization.

[0150] 2.5.4 XRD Analysis of Water Vapor-Activated Modified Nitrogen-Doped Porous Carbon

[0151] The results of XRD analysis of the prepared carbon materials are as follows: Figure 17 As shown, in Figure 17 As can be seen, HAC133 and HC130, compared to the hydrothermal carbon without high-temperature treatment, also exhibit broad diffraction peaks at 2θ = 22°, indicating the presence of graphitic carbon. Similarly, the broad dispersion and weak intensity of the diffraction peaks suggest that the cellulose framework was further disrupted during high-temperature carbonization, resulting in poor carbon crystallinity and a typical amorphous structure, indicating that HAC133 is an amorphous amorphous crystal. The peak intensity decreases after water vapor activation; this effect may be due to structural changes caused by activation, indicating the formation of an inhomogeneous lattice strain in HAC133, indicating poor lattice order, consistent with Raman's results.

[0152] 2.5.5 FT-IR Analysis of Water Vapor-Activated Modified Nitrogen-Doped Porous Carbon

[0153] Fourier transform infrared spectral analysis was performed on HAC133 and HC130, and the results are as follows: Figure 18 As shown. Between 3423-3432cm -1 The absorption peaks between these two values ​​should be attributed to the stretching vibrations of -OH molecules formed by intermolecular hydrogen bonds during the high-temperature pyrolysis of hydrothermal carbon; 2940-2850 cm⁻¹ -1 The absorption peaks between these values ​​should be due to the -CH stretching vibration of aliphatic molecules; 1570-1550 cm⁻¹ -1 The vibrational peaks may be due to the stretching vibration of the -NO2 double bond in biochar; 1040-1020 cm⁻¹ -1 The absorption peaks are likely caused by the C=O stretching vibration. Meanwhile, the infrared Fourier transform results show that the absorption peak intensity of the activated carbon is increased compared to carbon treated at high temperature under the same conditions. This indicates that introducing water vapor promotes the formation of CO and -OH functional groups, enhancing hydrophilicity and thus improving the adsorption capacity of carbon for methylene blue.

[0154] 2.5.6 XPS Analysis of Water Vapor-Activated Modified Nitrogen-Doped Porous Carbon

[0155] Table 8. Elemental percentages measured by XPS for HAC133 and HC130

[0156] HAC133 79.13 2.81 26.86 29.52 29.09 14.52 HC130 81.66 1.84 27.21 34.65 22.84 15.66

[0157] XPS was used to study the changes in surface composition and oxidation state of HAC133 and HC130, and the results are as follows: Figure 19 As shown. In Figure 19 In the full spectrum of (a), it is clear that the N peak in HAC133 is slightly higher than that in HC130. Further analysis... Figure 19 The fine spectra of HAC133 and HC130 in (b) and (c) show that, similar to HC130, HAC133 is mainly composed of pyrrole nitrogen (N-5), pyridine nitrogen (N-6), and graphitized nitrogen (NQ), while the content of oxidized nitrogen (NO) is relatively low. Compared with the hydrothermal carbon without high-temperature treatment, the relative content of N decreased, and the carbon without steam introduction showed a greater reduction in nitrogen compared with the carbon with steam introduction. High temperature and steam may both affect the amount of nitrogen functional groups. XPS results confirm that HAC133 carbon is still a carbon material with a relatively high N content. Compared with pyrrole nitrogen, pyridine nitrogen is more difficult to decompose at high temperatures. In addition, the increase of nitrogen oxides (NO) in HAC133 is likely due to the oxidation of carbon exposed to a steam atmosphere. The content of nitrogen functional groups is an important factor affecting methylene blue adsorption, as they can serve as basic active sites for interaction with methylene blue, including dipole-dipole interactions, H bonds, and covalent bonds.

[0158] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A nitrogen-doped porous carbon, characterized in that, It is a nitrogen-doped porous carbon prepared using peanut shells as the carbon source and ammonia water as the nitrogen source; the pore size of the nitrogen-doped porous carbon is 2~50 nm, and the specific surface area is 4.187~5.711 m². 2 ·g -1 The nitrogen in the nitrogen-doped porous carbon exists in the forms of pyrrole nitrogen, pyridine nitrogen, and graphitized nitrogen, and the nitrogen content in the nitrogen-doped porous carbon is 0.51~7.5% by mass. The specific steps for preparing the nitrogen-doped porous carbon are as follows: crushed peanut shells, water, and ammonia are mixed evenly, reacted at 160~240 ℃ for 2~10 h, purified, and dried to obtain nitrogen-doped porous carbon; the ratio of peanut shell:water:ammonia is 30 g:250~100mL:50~200mL.

2. The method for preparing nitrogen-doped porous carbon according to claim 1, characterized in that, The specific steps are as follows: mix crushed peanut shells, water, and ammonia evenly, react at 160~240 ℃ for 2~10 h, purify and dry to obtain nitrogen-doped porous carbon; the ratio of peanut shell:water:ammonia is 30 g:250~100mL:50~200mL.

3. The method for preparing nitrogen-doped porous carbon according to claim 2, characterized in that, The ratio of peanut shells:water:ammonia water is 30 g:100 mL:200 mL.

4. The method for preparing nitrogen-doped porous carbon according to claim 3, characterized in that, The concentration of the ammonia water is 5-20.83%.

5. The method for preparing nitrogen-doped porous carbon according to claim 4, characterized in that, During the mixing process, stirring is employed, with a stirring speed set to 600~1000 r·min. -1 Stir for 30-60 minutes.

6. The method for preparing nitrogen-doped porous carbon according to claim 5, characterized in that, The drying conditions are: drying at 80~105 ℃ for 24~72 h.

7. The method for preparing nitrogen-doped porous carbon according to claim 2, characterized in that, Under a N2 atmosphere, the nitrogen-doped porous carbon is reacted with water vapor at 750~850 °C for 0.5~1.5 h to obtain water vapor activated modified nitrogen-doped porous carbon.

8. The method for preparing nitrogen-doped porous carbon according to claim 7, characterized in that, Under a N2 atmosphere, the nitrogen-doped porous carbon was reacted with water vapor at 750 °C for 1.5 h to obtain water vapor-activated modified nitrogen-doped porous carbon.

9. The method for preparing nitrogen-doped porous carbon according to claim 8, characterized in that, When the nitrogen-doped porous carbon is 0.05~0.1 g, the water vapor flow rate is 0.06 mL·(g·min). -1 .

10. An application of the nitrogen-doped porous carbon according to claim 1, characterized in that, Used to adsorb methylene blue.

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