Titanium-doped biochar and preparation method and application thereof

Abstract

This invention belongs to the field of biomass materials technology, specifically relating to a titanium-doped biochar, its preparation method, and its applications. In this invention, supercritical fluid is used for composite processing. Its unique high diffusion and low surface tension characteristics enable uniform distribution of the titanium source within the protein fibers and induce condensation reactions between hydroxyl groups on the titanium surface and carboxyl groups in the protein fibers within the supercritical system, forming high-density Ti-O-C chemical bonds. Furthermore, low-temperature thermal induction is performed based on the supercritical pre-swollen matrix structure to complete the ordered reconstruction of the carbon skeleton structure and the regulation of active sites, achieving in-situ self-doping of N and S while avoiding the TiO2 phase transformation. This further improves the stability and photocatalytic performance of the titanium-doped biochar, enabling visible light response.

Description

Technical Field

[0001] This invention belongs to the field of biomass materials technology, specifically relating to a titanium-doped biochar, its preparation method, and its application. Background Technology

[0002] Carbon-titanium composites have attracted increasing interest as excellent photocatalysts because carbon materials can act as electron traps to collect photoinduced charge carriers from the photocatalyst surface. Meanwhile, carbon doping is an effective method for introducing heteroatoms into the TiO2 lattice, which can reduce the band gap and achieve visible light absorption. Biochar, due to its abundant natural resources, non-metallic properties, acid and alkali resistance, excellent ecological sustainability, and favorable physicochemical properties, including high specific surface area, porous framework, oxygen-containing groups, intrinsic minerals, and active binding sites, exhibits exceptional competitiveness among carbonaceous materials. These properties are beneficial for the binding and growth of photocatalyst nanoparticles, thereby improving their stability and reactivity.

[0003] Although titanium-doped biochar composites possess excellent photocatalytic activity and adsorption properties, the differences between the functional groups and porous structure of biochar surface and the chemical properties and crystal structure of TiO2 surface lead to uneven dispersion and insufficient interfacial chemical bonding, affecting the stability and service life of the material. Furthermore, enhancing the reduction reaction activity requires controlling the anatase phase of TiO2, while high-temperature pyrolysis easily triggers the rutile phase transformation of TiO2. Summary of the Invention

[0004] The purpose of this invention is to provide a titanium-doped biochar, its preparation method, and its application. The method provided by this invention can further improve the stability and photocatalytic performance of titanium-doped biochar, and achieve visible light response.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention provides a method for preparing titanium-doped biochar, comprising the following steps:

[0007] Protein fibers and titanium sources were combined in a supercritical carbon dioxide reaction system to obtain keratin biochar precursors.

[0008] The keratin biochar precursor was carbonized to obtain the titanium-doped biochar.

[0009] Preferably, the protein fiber includes at least one of rabbit hair, wool, cashmere, yak hair, and feathers.

[0010] Preferably, the titanium source includes at least one of titanium dioxide, titanate, titanium tetrachloride, titanium oxysulfate, organic titanium source, and titanium powder;

[0011] The organic titanium source includes at least one of tetrabutyl titanate and tetraisopropyl titanate.

[0012] Preferably, the mass ratio of the protein fiber to the titanium source is 10:1 to 10:10.

[0013] Preferably, the pressure of the supercritical carbon dioxide reaction system is 10~100MPa, the reaction temperature is 25~250℃, and the time is 10~180 min.

[0014] Preferably, the carbonization temperature is 250~1100℃, the holding time is 0.5~2 h, and the heating rate is 1~10℃ / min; the carbonization is carried out under a protective atmosphere.

[0015] The present invention also provides titanium-doped biochar prepared by the preparation method described in the above technical solution.

[0016] Preferably, the titanium-doped biochar comprises a carbon matrix and doping elements; the doping elements include N, S and Ti, wherein Ti exists in the anatase phase and forms Ti-OC bonds with the carbon matrix.

[0017] Preferably, the mass percentage of N doping is 0.4% to 16%;

[0018] The doping mass percentage of S is 0.2% to 5%;

[0019] The Ti doping mass percentage is 1-35%.

[0020] This invention also provides the application of titanium-doped biochar as a photocatalyst as described in the above technical solution.

[0021] This invention provides a method for preparing titanium-doped biochar, comprising the following steps: compounding protein fibers and a titanium source in a supercritical carbon dioxide reaction system to obtain a keratin biochar precursor; and carbonizing the keratin biochar precursor to obtain the titanium-doped biochar.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] (1) The titanium-doped biochar prepared by this invention is a precursor structure-guided biochar design. It directly utilizes the natural amino acid sequence and secondary structure of keratin to regulate the distribution of active sites in the carbon skeleton, realizes in-situ self-doping of N and S, and then achieves visible light response, breaking through the limitations of traditional physical doping.

[0024] (2) This invention proposes a supercritical fluid-gradient thermal induction synergistic process, which achieves uniform loading of titanium source at the nanoscale by utilizing the high diffusion and low surface tension characteristics of supercritical fluid. Combined with gradient pyrolysis to precisely control the evolution of N and S self-doped functional groups and the formation of Ti-OC covalent bonds, the problem of heterostructure interface stability is solved.

[0025] (3) Based on the supercritical pre-swollen matrix structure, the present invention performs low-temperature thermal induction to complete the orderly reconstruction of the carbon skeleton structure and the regulation of active sites, while avoiding the transformation of TiO2 phase structure (i.e. transformation to rutile phase), further improving the stability and photocatalytic performance of titanium-doped biochar. Attached Figure Description

[0026] Figure 1 An optical microscope image of the precursor obtained in the example;

[0027] Figure 2 The image shows the SEM-EDS diagram of the precursor obtained in the example.

[0028] Figure 3 FITR (a) and XRD (b) of the precursor obtained in the example;

[0029] Figure 4 The image shows the SEM-EDS image of the titanium-doped biochar obtained in the example.

[0030] Figure 5 The FITR image of titanium-doped biochar obtained in the example;

[0031] Figure 6 The XRD pattern of the titanium-doped biochar obtained in the example is shown below.

[0032] Figure 7 The UV-vis DRS (a) and band gap diagram (b) of the titanium-doped biochar obtained in the example are shown.

[0033] Figure 8 The transient photocurrent response (a) and electrochemical impedance (b) of the titanium-doped biochar obtained in the examples are shown.

[0034] Figure 9 The graph shows the removal of methylene blue by titanium-doped biochar obtained in the example.

[0035] Figure 10 The graph shows the removal of hexavalent chromium by titanium-doped biochar obtained in Example 1. Detailed Implementation

[0036] This invention provides a method for preparing titanium-doped biochar, comprising the following steps:

[0037] Protein fibers and titanium sources were combined in a supercritical carbon dioxide reaction system to obtain keratin biochar precursors.

[0038] The keratin biochar precursor was carbonized to obtain the titanium-doped biochar.

[0039] This invention mixes protein fibers and a titanium source and performs compounding in a supercritical carbon dioxide reaction system to obtain a keratin biochar precursor.

[0040] In this invention, the protein fiber preferably includes at least one of rabbit hair, wool, cashmere, yak hair, and feathers; the feathers are preferably chicken feathers. In this invention, before mixing, the protein fiber is preferably pretreated, the pretreatment preferably including: heating the protein fiber in a water bath, then washing and drying; the water bath heating temperature is preferably 100°C, and the time is preferably 1 hour; the washing is preferably done until the washing liquid is clear and free of impurities; the drying temperature is preferably 60°C.

[0041] In this invention, the titanium source preferably includes at least one of titanium dioxide, titanate, titanium tetrachloride, titanium oxysulfate, organic titanium source, and titanium powder; the organic titanium source preferably includes at least one of tetrabutyl titanate and tetraisopropyl titanate.

[0042] In this invention, the mass ratio of the protein fiber to the titanium source is preferably 10:1 to 10:10, specifically 10:1, 10:2, 10:3, 10:4, 10:5, 10:6, 10:7, 10:8, 10:9, or 10:10.

[0043] In this invention, the pressure of the supercritical carbon dioxide reaction system is preferably 10~100 MPa, specifically 10MPa, 20MPa, 30MPa, 40MPa, 50MPa, 60MPa, 70MPa, 80MPa, 90MPa, or 100MPa; the reaction temperature is preferably 25~250℃, specifically 25℃, 50℃, 100℃, 150℃, 200℃, or 250℃; and the reaction time is preferably 10~180 min, specifically 10 min, 30 min, 60 min, 90 min, 120 min, 150 min, or 180 min.

[0044] The keratin biochar precursor is obtained by carbonizing the keratin biochar precursor to obtain the titanium-doped biochar.

[0045] In this invention, the carbonization temperature is preferably 250~1100℃, specifically 250℃, 300℃, 400℃, 500℃, 600℃, 700℃, 800℃, 900℃, 1000℃, or 1100℃; the holding time is preferably 0.5~2 h, specifically 0.5h, 1.0h, 1.5h, or 2.0h; the heating rate is preferably 1~10℃ / min, specifically 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min. In this invention, the carbonization is preferably carried out under a protective atmosphere, which is argon.

[0046] The present invention also provides titanium-doped biochar prepared by the preparation method described above, wherein the titanium-doped biochar comprises a carbon matrix and doping elements; the doping elements include N, S, and Ti, wherein Ti exists in the anatase phase and forms Ti-OC bonds with the carbon matrix. In the present invention, the doping mass percentage of N is preferably 0.4-16%; the doping mass percentage of S is preferably 0.2-5%; and the doping mass percentage of Ti is preferably 1-35%.

[0047] This invention also provides the application of titanium-doped biochar as a photocatalyst as described in the above technical solution.

[0048] Unless otherwise specified, the materials and equipment used in this invention are all commercially available products in the field.

[0049] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0050] Example 1

[0051] The rabbit hair was placed in a beaker containing deionized water and bathed (100℃, 1h). Then it was washed with deionized water until the water was clear. The washed rabbit hair was then dried in an oven at 60℃ to obtain pre-treated rabbit hair.

[0052] Pretreated rabbit hair and TiO2 were placed in a supercritical CO2 reaction system at a mass ratio of 10:1 for compounding. The reaction pressure was controlled at 16 MPa, the reaction temperature at 130 °C, and the reaction time at 120 min to obtain a keratin biochar precursor, denoted as 16 MPa / RH@TiO2.

[0053] The keratin biochar precursor obtained above was placed in a high-temperature tube furnace and carbonized under an argon atmosphere at a carbonization temperature of 450°C, a heating rate of 5°C / min, and a carbonization time of 2h to obtain the titanium-doped biochar, denoted as 16MPa / 450°C RBC@TiO2.

[0054] Example 2

[0055] Keratin biochar precursor and titanium-doped biochar were prepared according to the method in Example 1;

[0056] The pressure of the supercritical CO2 reaction system is 25 MPa, and the keratin obtained is 25 MPa / RH@TiO2;

[0057] The obtained titanium-doped biochar is designated as 25MPa / 450℃ RBC@TiO2.

[0058] Example 3

[0059] Keratin biochar precursor and titanium-doped biochar were prepared according to Example 1;

[0060] The carbonization temperature was 800℃, and the resulting titanium-doped biochar was designated as 16MPa / 800℃ RBC@TiO2.

[0061] Example 4

[0062] Keratin biochar precursor and titanium-doped biochar were prepared according to the method in Example 2;

[0063] The carbonization temperature was 800℃, and the resulting titanium-doped biochar was designated as 25MPa / 800℃ RBC@TiO2.

[0064] Example 5

[0065] The wool was placed in a beaker containing deionized water and bathed (100℃, 1h), then rinsed with deionized water until the water was clear. The washed wool was then dried in an oven at 60℃ to obtain pretreated wool.

[0066] Pretreated wool and TiO2 were placed in a supercritical CO2 reaction system at a mass ratio of 10:5 for compounding. The reaction pressure was controlled at 50 MPa, the reaction temperature at 130 °C, and the reaction time at 120 min to obtain a keratin biochar precursor, denoted as 50 MPa / WF@TiO2.

[0067] The keratin biochar precursor obtained above was placed in a high-temperature tube furnace and carbonized under an argon atmosphere at a carbonization temperature of 550℃, a heating rate of 5℃ / min, and a carbonization time of 2h to obtain the titanium-doped biochar, denoted as 50MPa / 550℃ WBC@TiO2.

[0068] Example 6

[0069] The wool was placed in a beaker containing deionized water and bathed (100℃, 1h), then rinsed with deionized water until the water was clear. The washed wool was then dried in an oven at 60℃ to obtain pretreated wool.

[0070] Pretreated wool and TiO2 were placed in a supercritical CO2 reaction system at a mass ratio of 10:10 for compounding. The reaction pressure was controlled at 80 MPa, the reaction temperature at 100℃, and the reaction time at 180 min to obtain a keratin biochar precursor, denoted as 80 MPa / WF@TiO2.

[0071] The keratin biochar precursor obtained above was placed in a high-temperature tube furnace and carbonized under an argon atmosphere at a carbonization temperature of 650℃, a heating rate of 5℃ / min, and a carbonization time of 2h to obtain the titanium-doped biochar, denoted as 80MPa / 650℃ WBC@TiO2.

[0072] Example 7

[0073] Place chicken feathers in a beaker containing deionized water for a water bath (100℃, 1h), then rinse with deionized water until the water is clear. Place the rinsed chicken feathers in an oven at 60℃ to dry, and obtain pre-treated chicken feathers.

[0074] Pretreated chicken feathers and TiO2 were placed in a supercritical CO2 reaction system at a mass ratio of 10:2 for compounding. The reaction pressure was controlled at 10 MPa, the reaction temperature at 110℃, and the reaction time at 120 min to obtain a keratin biochar precursor, denoted as 10 MPa / FF@TiO2.

[0075] The keratin biochar precursor obtained above was placed in a high-temperature tube furnace and carbonized under an argon atmosphere at a carbonization temperature of 350°C, a heating rate of 5°C / min, and a carbonization time of 2h to obtain the titanium-doped biochar, denoted as 10MPa / 350°C FBC@TiO2.

[0076] Example 8

[0077] Yak hair was placed in a beaker containing deionized water and bathed (100℃, 1h). Then it was washed with deionized water until the water was clear. The washed yak hair was dried in an oven at 60℃ to obtain pre-treated yak hair.

[0078] Pretreated yak hair and TiO2 were placed in a supercritical CO2 reaction system at a mass ratio of 10:8 for compounding. The reaction pressure was controlled at 100 MPa, the reaction temperature at 130 °C, and the reaction time at 120 min to obtain a keratin biochar precursor, denoted as 100 MPa / YH@TiO2.

[0079] The keratin biochar precursor obtained above was placed in a high-temperature tube furnace and carbonized under an argon atmosphere at a carbonization temperature of 800℃, a heating rate of 5℃ / min, and a carbonization time of 2h to obtain the titanium-doped biochar, denoted as 100MPa / 800℃ YBC@TiO2.

[0080] Performance testing

[0081] Test methods and equipment:

[0082] (1) Morphological and compositional characteristics

[0083] The microstructure and composition of the sample surface were measured using SEM-EDS. The samples were sputtered with gold at 18 mA and scanned at an accelerating voltage of 20 kV.

[0084] (2) Crystal structure characterization

[0085] The phase composition and crystal structure of the samples were determined by XRD under the following conditions: a Cu Ka radiation source was used, with a test power of 40 kV × 40 mA, continuous scanning mode, a scanning speed of 5° / min, and a scanning angle 2θ ranging from 10° to 80°. The data were analyzed using Jade 9 software.

[0086] (3) Chemical structure characterization

[0087] FTIR was used to characterize the functional groups in the samples. The samples were ground with dried KBr powder in an agate mortar (mass ratio 1:100), and then compressed into transparent thin sheets before analysis. The scanning range was 400–4000 cm⁻¹. -1 100 scans, 4.0cm resolution -1 .

[0088] (4) Characterization of optical response range

[0089] The photoresponse range of the samples was determined using UV-vis DRS under the following conditions: BaSO4 substrate, wavelength range 200–800 nm. The band gap width of the samples was calculated using the Tauc polt method based on the test data, as shown in the following formula:

[0090]

[0091] In the formula, α is the absorption coefficient of the material, in cm. -1 ;

[0092] h — Planck's constant, J\cdotps;

[0093] ν—frequency of light, Hz;

[0094] B—constant, (cm) -1 ·eV )1 / n ·eV -1 ;

[0095] E g —The bandgap width of a semiconductor, in eV;

[0096] n—an index related to semiconductor type.

[0097] (5) Electrochemical performance analysis

[0098] Photocurrent and electrochemical impedance spectroscopy (EIS) tests were performed on the samples using an electrochemical workstation (CHI 760E). 10 mg of sample was added to 5 mL of a mixture (ethanol: Nafion dispersion = 9:1) and ultrasonically dispersed for 1 h. 0.1 mL of the mixture was then slowly added dropwise onto a 1 cm × 1.3 cm fluorine-doped SnO2 conductive glass (FTO) and dried in an oven at 60 °C for 2 h for later use.

[0099] Test conditions: A three-electrode system was used for testing, with an Ag / AgCl electrode as the reference electrode, a platinum (Pt) electrode as the auxiliary electrode, and FTO coated with the sample as the working electrode.

[0100] Photocurrent (it) test: A 300W xenon lamp (>420nm) was used for the test, with an illumination interval of 20s (20s on, 20s off), and the cycle was repeated 6 times. The base solution was a 0.5mol / L Na2SO4 solution.

[0101] EIS test: frequency range 0.1~100KHz, amplitude 10mV, electrolyte 2.5mmol / L potassium ferricyanide solution, and light exposure required.

[0102] Test 1

[0103] Morphological characteristics of precursors

[0104] Figure 1The microstructural evolution of rabbit hair and biochar precursors prepared using supercritical fluid CO2 is shown (optical microscope, 1000×). The results show that the original rabbit hair morphology (a and b) exhibits a typical scaly layer structure with regular scale arrangement and clear interlayer interfaces. The cross-section shows periodic bamboo-like medullary cavities. Images (c and d) after loading TiO2 at 16 MPa pressure show that TiO2 nanoparticles preferentially deposit in the open-angle regions of the scales, forming a uniform coating layer, with submicron-sized aggregates in local areas. Some TiO2 is also present in the cortex. The medullary cavity septa are thinned, and the periodic bamboo-like structures are partially disrupted, but the overall structure remains continuous. Images (e and f) of the rabbit hair sample loaded with TiO2 at 25 MPa pressure show that the scaly layer is deformed, the interlayer interfaces are blurred, the TiO2 loading on the scaly layer surface is significantly reduced, and the particles tend to embed in the cortex and the scaly basal layer. The cavity structure of the medulla showed enhanced stability, the septum thickness was further reduced, and high-density TiO2 deposition was observed within the medullary cavity. This indicates that the 25 MPa high pressure caused fiber swelling, promoting particle migration into the medullary cavity.

[0105] The microstructure of precursors prepared under supercritical conditions of 16 MPa and 25 MPa was characterized by SEM-EDS. Figure 2 (where a is 16 MPa and b is 25 MPa). The results showed that a small amount of Ti element was scanned on the surface of the precursor samples under pressure conditions of 16 MPa and 25 MPa, indicating that TiO2 was successfully loaded onto the surface of rabbit hair fibers. Comparison of the surface morphology of the two showed that the rabbit hair fibers under 16 MPa were less damaged and a relatively complete scale layer structure could be observed, with TiO2 mostly loaded on the fiber surface; the rabbit hair fiber structure under 25 MPa was severely damaged, the scale layer was opened, and there was a large warp angle, with TiO2 loaded into the fiber interior through the warp angle. Combining the elemental content chart, it was found that the contents of C, N, O, S, and Ti in the 16MPa / RH@TiO2 sample were 68.63%, 4.64%, 18.47%, 5.84%, and 2.42%, respectively, while the contents of C, N, O, S, and Ti in the 25MPa / RH@TiO2 sample were 72.64%, 7.62%, 9.93%, 8.54%, and 1.27%, respectively. This indicates that under 16MPa conditions, Ti is mainly enriched on the fiber surface, while under 25MPa conditions, Ti is mainly enriched in the fiber medullary cavity, which is consistent with the results observed by optical microscopy.

[0106] Test 2

[0107] Precursor Structure

[0108] Figure 3Figure a shows the FITR spectra of precursors prepared under different pressure conditions. Rabbit hair exhibits distinct protein characteristic peaks located at 3300–3500 cm⁻¹. -1 Stretching vibrations of NH and OH at 2920 cm⁻¹ (amino, hydroxyl, or water), 2920 cm⁻¹ -1 and 2850cm -1 Stretching vibrations of CH at 1600~1650 cm⁻¹ (aliphatic structure) -1 C=O stretching vibration at (amide I band), 1530~1550cm -1 The NH bending and CN stretching vibrations at (amide II band) and at 1230~1300 cm -1 Complex vibrations of CN and NH at the (amide III band). Rabbit hair loaded with TiO2 using supercritical CO2 technology also showed vibrations at 3300~3500 cm⁻¹. -1 OH characteristic peak, 1600 cm⁻¹ -1 The characteristic peaks nearby are the overlap of amide I from the original rabbit hair and water adsorbed on the TiO2 surface, located at 1300~1400 cm⁻¹. -1 The two narrow absorption peaks at the point are related to the Ti-O-Ti and Ti-OH bonds in TiO2, indicating that TiO2 was successfully introduced into the rabbit hair matrix through supercritical technology.

[0109] Figure 3Figure b shows the XRD patterns of precursors prepared under different pressure conditions, revealing the evolution characteristics of the material's crystal structure. The results show that, compared with the original rabbit hair, 16MPa / RH@TiO2 and 25MPa / RH@TiO2 exhibit characteristic peaks of the α-helix (100) and β-sheet (002) crystal planes unique to rabbit hair keratin at approximately 2θ=9° and 21°, respectively. The difference lies in the fact that rabbit hair is dominated by the α-helix structure, while the prepared precursors show a trend of transition from α-helix to β-sheet. Furthermore, 16MPa / RH@TiO2 is still dominated by the α-helix (Iβ / Iα=1.3) structure, while 25MPa / RH@TiO2 is dominated by the β-sheet (Iβ / Iα=2.1) structure. The study shows that under high pressure induction, the breakage of the hydrogen bond network in the fiber promotes the conformational transition from α-helix to β-sheet. Compared with pure TiO2, the prepared precursors all showed a weak diffraction peak at 25.3° (d=3.52Å), which confirmed the initial loading of the anatase phase TiO2 (101) crystal plane. However, due to the low loading (2.42 at.%) and the shielding effect of the amorphous fiber matrix, other characteristic peaks were not observed. Calculations of the lattice parameters of the samples showed that the crystallinity of the original rabbit hair was 25.63%, while the crystallinity of 16MPa / RH@TiO2 decreased to 4%, and the crystallinity of 25MPa / RH@TiO2 was 5.73%. This indicates that supercritical CO2 loading of TiO2 can significantly reduce the crystallinity of rabbit hair. Supercritical CO2 penetrates into the amorphous region of the fiber, weakening intermolecular hydrogen bonds or van der Waals forces, leading to enhanced polymer chain mobility and disordering of some crystalline regions. Theoretically, the higher the pressure of a supercritical fluid, the stronger its dissolving power and permeability, making it easier to enter the amorphous region of the fiber or even the edge of the crystalline region, leading to a decrease in crystallinity. However, crystallinity calculations show that the crystallinity of 25MPa / RH@TiO2 is slightly higher than that of 16MPa / RH@TiO2, which may be related to the fact that rabbit hair is loaded with more TiO2 at 25MPa.

[0110] Test 3

[0111] Morphology and composition of titanium-doped keratin biochar

[0112] SEM-EDS of titanium-doped keratin biochar prepared under different pressures, such as Figure 4As shown (where b is 16MPa / 450℃ RBC@TiO2, c is 16MPa / 800℃ RBC@TiO2, e is 25MPa / 450℃ RBC@TiO2, and f is 25MPa / 800℃ RBC@TiO2), the titanium-doped biochar prepared from rabbit hair loaded with TiO2 after carbonization is mainly granular, with enrichment of TiO2 loaded at 16MPa. After carbonization at 450℃, the C, N, and Ti content increased and the O and S content decreased in the samples prepared under both pressures. During the carbonization process, the organic matter in the rabbit hair undergoes pyrolysis, and the chemical bonds between carbon atoms break and recombine, forming more stable carbon structures with higher aromaticity, such as graphitic carbon, resulting in a relative increase in the carbon content in the biochar. Nitrogen forms new chemical bonds with titanium due to the surface TiO2 loading, and some nitrogen may combine with carbon to form stable nitrogen-containing aromatic ring structures, such as pyridine and pyrrole rings, which are largely retained. Oxygen mainly exists in rabbit hair in the form of oxygen-containing functional groups, such as hydroxyl and carboxyl groups. During carbonization, these oxygen-containing functional groups are unstable and easily decompose and volatilize in the form of gases such as carbon dioxide and carbon monoxide, thus reducing the oxygen content. The increase in titanium content is due to the release of the internally loaded TiO2 after fiber carbonization. Samples carbonized at 800℃ show a further increase in carbonization degree, further decomposition of organic matter, a significant increase in the relative content of C, a further decrease in O content, and some nitrogen loss in the form of small molecules such as ammonia, leading to a decrease in N content. At higher temperatures, some anatase TiO2 loaded on the sample surface detaches and is damaged to some extent, resulting in uneven and reduced Ti distribution during surface scanning, causing TiO2 aggregation.

[0113] Test 4

[0114] Structure of titanium-doped keratin biochar

[0115] (1) Chemical structure

[0116] The chemical structure of titanium-doped keratin biochar prepared under pressures of 16 MPa and 25 MPa was analyzed using infrared spectroscopy. The results are as follows: Figure 5 As shown. The results show that the spectra of the four samples are similar, with a wavelength of 3736 cm⁻¹. -1 The peak at 3427 cm⁻¹ corresponds to the stretching vibration of hydroxyl groups (-OH) on the TiO₂ surface; the peak at 3427 cm⁻¹ corresponds to the stretching vibration of hydroxyl groups (-OH) on the TiO₂ surface. -1Broad absorption peaks appeared at 3400 cm⁻¹, attributed to the stretching vibrations of -OH and -NH in rabbit hair. The peak intensities at this point were high for both 16 MPa / RH@TiO₂ and 25 MPa / RH@TiO₂ samples. After high-temperature carbonization, such as at 450℃, the peak intensity decreased, and the decrease became more pronounced at 800℃. This phenomenon is attributed to the potential dehydration of hydroxyl groups (-OH) in the biochar during high-temperature carbonization, leading to the formation of more aromatic ring structures. This results in a reduction in the hydroxyl content, thus causing the peak intensity to be lower at 3400 cm⁻¹ in the infrared spectrum. -1 The hydroxyl stretching vibration peaks around the left and right are weakened. (2816 cm⁻¹) -1 and 2716cm -1 The peak value at 2331 cm⁻¹ corresponds to the stretching vibration of aliphatic CH₄ in the biochar residue, indicating carbon desaturation and dehydrogenation. -1 and 2362cm -1 The peak value at 1604 cm⁻¹ is attributed to the antisymmetric stretching vibration of CO₂ in the TiO₂ adsorption environment. The peak value intensifies with increasing carbonization temperature, which is related to unsaturated bonds such as C≡C or C≡N. This is because nitrile groups are formed during carbonization, and the formation of these thermally stable nitrile groups fixes some of the nitrogen element in the biochar. -1 The peak values ​​near the 16MPa / RH@TiO2 and 25MPa / RH@TiO2 samples are relatively smooth, without other weak small peaks. This is due to the stretching and bending vibrations of adsorbed water on the TiO2 surface. After carbonization at 450℃, the peak value of the 16MPa / 450℃ RBC@TiO2 sample at this point weakens and decreases to around 1630 cm⁻¹. -1 The presence of numerous weak peaks at this point can be attributed to the disruption of N–H bonds and the formation of double bonds (N=N) in the biochar. The smaller peak variation observed in the 25 MPa / 450℃ RBC@TiO2 sample at this point is likely due to the higher TiO2 loading on the rabbit hair fibers, which affects the carbonization process. (1385 cm⁻¹) -1 The peak at this point is attributed to the -COO-Ti vibration. Compared with other samples, the peak at this point is weaker in the 16MPa / 450℃ RBC@TiO2 sample, possibly indicating that biochar interacted with TiO2, resulting in a new structure. (400~800cm) -1 The absorption band at 671 cm⁻¹ corresponds to the stretching vibration of the Ti-O bond. The 16 MPa / RH@TiO₂ and 25 MPa / RH@TiO₂ samples show this absorption band at 671 cm⁻¹. -1The peak value was very weak, possibly because the peaks of the rabbit hair fiber itself affected the manifestation of the TiO2 characteristic peaks. After carbonization, the influence of the rabbit hair disappeared, and the peak value increased slightly. Additionally, the 16MPa / RH@TiO2 sample showed a broad absorption band with a wide peak area due to the presence of disulfide bonds. With increasing load pressure and high-temperature carbonization, this peak value disappeared, possibly due to the breakage of disulfide bonds leading to the disappearance of the characteristic peak. In conclusion, TiO2 was successfully incorporated into rabbit hair keratin biochar.

[0117] (2) Crystal structure

[0118] The crystal structure of titanium-doped keratin biochar prepared under pressures of 16 MPa and 25 MPa was analyzed by XRD, and the results are as follows: Figure 6 As shown in the figure. The results show that after carbonization, the titanium-doped keratin biochar sample exhibited characteristic diffraction peaks of TiO2 at approximately 25.2°, 37.7°, 48.0°, 53.8°, 55.0°, 62.6°, 70.2°, and 75.0°, corresponding to the (101), (004), (200), (105), (211), (220), (204), and (215) crystal planes of the anatase phase, respectively. The characteristic peaks of the original α-helical and β-folded structures of rabbit hair disappeared, transforming into a more disordered structure, with a corresponding decrease in fiber crystallinity, crystal quality, and crystal volume. A weak, broad peak appeared near 25°, corresponding to the short-range ordered (002) crystal plane of graphite microcrystals. It should be noted that, in addition to graphite, many two-dimensional sheet-like structures containing C, N, and O elements, such as crystalline carbonitrides, can also show the (002) crystal plane near 25°, indicating that the ordering of the carbon matrix promotes TiO2 crystallization. In addition, the 16MPa / 800℃ RBC@TiO2 sample exhibits a new broad peak near 44°, corresponding to the (100) or (101) crystal plane of the graphite material, reflecting sp 2 The enhanced in-plane ordering of carbon, coupled with the stable presence of the anatase phase (absence of the 27.5° rutile phase), confirms that the carbon matrix effectively suppresses the TiO2 phase transition. This structural evolution indicates that the 450℃ system forms an anatase / amorphous carbon composite structure, suitable for photocatalytic surface reactions; while the 800℃ system constructs an anatase / graphite synergistic system, possessing both catalytic activity and electronic conductivity advantages.

[0119] The carbonization stage exhibited a significant pressure effect; the graphite (002) peak intensity of the 25 MPa / 450℃ RBC@TiO2 sample decreased compared to the 16 MPa sample. This is attributed to the increased carbon layer stacking and defect density resulting from the increased TiO2 loading. After carbonization at 800℃, the graphite crystallite size of the 25 MPa sample decreased, but its anatase phase stability was maintained. These results indicate that while the 25 MPa condition is favorable for deep TiO2 loading, it sacrifices the orderliness of the carbon matrix, while the 16 MPa system is more advantageous in constructing highly graphitized / highly dispersed composite materials. Combined with lattice parameter calculations (Table 1), the crystallinity of the heat-treated titanium-doped keratin biochar is significantly increased compared to the precursor, and the higher the pyrolysis dimension, the greater the crystallinity. This is because high-temperature pyrolysis promotes the rearrangement of carbon atoms into a graphite microcrystalline structure. At the same carbonization temperature, the greater the pressure, the greater the crystallinity of the prepared titanium-doped keratin biochar. High-pressure supercritical fluid promotes the uniform penetration of titanium precursor into the interior of keratin, forming Ti-O-C or Ti-C bonds during carbonization. These bonded structures can serve as "templates" to guide the orderly arrangement of carbon atoms.

[0120] Table 1. Lattice parameters of titanium-doped keratin biochar

[0121]

[0122] Test 5

[0123] Light absorption properties of titanium-doped keratin biochar

[0124] Figure 7Figure 'a' shows the UV-Vis diffuse reflectance absorption spectra of rabbit hair-loaded TiO2 samples and their carbonization products prepared under different supercritical pressures. The results show that all samples exhibit significant light absorption characteristics in the UV region below 360 nm. This phenomenon is mainly attributed to the inherent optical properties of TiO2: its 3.2 eV band gap enables strong absorption in the UV region, allowing for the generation of electron-hole pairs through photoexcitation. Compared to the absorbance of pure TiO2 in the UV region (λ < 360 nm) of 1.2, doped keratin biochar significantly alters the light absorption characteristics of the material. 16 MPa / RH@TiO2 and 25 MPa / RH@TiO2 exhibit strong absorption in the UV region (< 400 nm) (A ≈ 1.2), while absorption in the visible region (400–800 nm) is weak (A ≈ 0). After carbonization, the ultraviolet absorption decreased to varying degrees (ΔA≈0.3), while the visible light absorption significantly increased (from 450~800 nm to above 0.7), with a noticeable redshift at the absorption edge (Δλ≈40 nm). This phenomenon indicates that the construction of the TiO2-biochar composite system effectively expanded the optical response range of TiO2. The heteroatoms (such as N, S, etc.) doped in biochar may have introduced new impurity energy levels into the TiO2 band structure, thereby extending the material's optical response from the pure ultraviolet region (<387 nm) to the visible light region. These changes stem from the following synergistic mechanisms: First, the porous structure formed during the carbonization process provides more light-harvesting sites; second, the hierarchical porosity enhances the light scattering effect and extends the optical path length; third, the oxygen-containing functional groups (-COOH, -OH) on the biochar surface form Ti-OC interface bonds with TiO2, promoting charge transfer. It is worth noting that the RBC@TiO2 sample at 16MPa / 450℃ exhibited the best light absorption performance, which is closely related to its rich content of surface functional groups.

[0125] Optical band gap calculated by the Tauc plot method ( Figure 7 (b) Further confirmation shows that the 16MPa / 450℃ RBC@TiO2 has the narrowest band gap (2.76 eV), significantly smaller than that of the uncarbonized sample (3.72 eV), indicating that the composite material prepared under this condition has the best bonding effect and stronger light absorption performance, which is beneficial to improving photocatalytic performance. This is attributed to the strong electronic coupling between the carbon matrix and TiO2, forming an intermediate energy level. The band gap values ​​of other samples are as follows: 16MPa / 800℃ RBC@TiO2 (2.95 eV), 25MPa / 450℃ RBC@TiO2 (3.17 eV), 25MPa / 800℃ RBC@TiO2 (2.87 eV), and 25MPa / RH@TiO2 (3.58 eV). This band gap regulation shows a good correlation with the photocatalytic activity of the material.

[0126] Test 6

[0127] Electrochemical properties of titanium-doped keratin biochar

[0128] To investigate the transfer and separation efficiency of photogenerated charges in titanium-doped biochar composites, this chapter selected 16MPa / 450℃ RBC@TiO2 samples with good light absorption performance and pure TiO2 samples for photocurrent and electrochemical impedance (EIS) testing. Figure 8 In the transient photocurrent response, the photocurrent density of the RBC@TiO2 sample at 16 MPa / 450℃ was stronger than that of pure TiO2, being 1.22 times that of the TiO2 sample. During six photo-irradiation cycles, the photocurrent response of the sample showed some attenuation, but remained higher than that of TiO2 overall, indicating better photochemical stability. This suggests that titanium-doped biochar possesses higher charge transfer and separation capabilities, forming new active catalytic sites and reaction centers. This is mainly related to the increased light absorption range and the higher efficiency of photogenerated carrier separation. Biochar has unique structure and optical properties; its light absorption performance shows that titanium-doped biochar can absorb visible and ultraviolet light, broadening the photoresponse region and thus generating more photogenerated carriers, increasing the photocurrent density. Meanwhile, after TiO2 is photoexcited to generate photogenerated electrons and holes, these elements easily recombine, reducing photocatalytic efficiency and photocurrent density. Biochar has a high carbon content and good electrical conductivity, which can serve as an electron acceptor and transport channel. It can quickly receive and conduct photogenerated electrons generated by TiO2, effectively separating electron-hole pairs, reducing the recombination probability, and allowing more photogenerated charge carriers to participate in the electrode reaction, thereby increasing the photocurrent density.

[0129] The smaller the radius of the EIS, the lower the charge transfer resistance and the faster the electron migration rate. Compared with pure TiO2, Figure 8Electrochemical impedance spectroscopy (EIS) in section b shows that in the high-frequency region, the semi-circular diameter of the 16 MPa / 450℃ RBC@TiO2 sample is smaller than that of the pure TiO2 sample, indicating a decrease in interfacial charge transfer resistance (RCT). Biochar has relatively high electrical conductivity and contains some freely moving electrons. When combined with TiO2, biochar can act as a bridge for electron transport, providing a fast migration channel for photogenerated electrons from TiO2, enabling electrons to transport more efficiently within the material and reducing obstacles during electron transport, thereby lowering the impedance of the composite material. FITR and XRD results confirm that a Ti-OC bond is formed between biochar and TiO2, and this interfacial interaction is beneficial for electron transfer between different phases. Compared with pure TiO2, the interface within the composite material provides more electron transport paths, and the resistance of these paths is relatively low, resulting in a reduction in overall impedance. Meanwhile, due to the abundance of functional groups in keratin biochar, the combination of biochar and TiO2 increases the number of active sites in the material. On one hand, the surface functional groups of biochar itself can participate in electrochemical reactions as active sites; on the other hand, the composite of biochar and TiO2 alters the surface properties of the material, allowing more sites to interact with ions in the electrolyte, promoting charge transfer processes, reducing charge transfer resistance, and thus decreasing the impedance of the composite material. The results of photocurrent and electrochemical impedance spectroscopy are consistent, indicating that the combination of TiO2 and biochar is a pathway to promote effective charge separation and transfer, thereby enhancing the photocatalytic activity of the composite material.

[0130] Test 7

[0131] Using the titanium-doped keratin biochar prepared in Examples 1-5 and 8 as a photocatalyst, and methylene blue as a model pollutant, the removal effect of titanium-doped keratin biochar on organic pollutants was investigated. The specific test procedure was as follows: At room temperature, 10 mg of titanium-doped keratin biochar sample was dispersed in 50 mL of MB solution (20 mg / L) and treated in the dark for 30 min to achieve adsorption-desorption equilibrium of the dye on the biochar surface. Then, the photocatalytic cup containing the sample was placed under a 300 W xenon lamp (λ≥420 nm) for visible light irradiation. Every 30 min, 4 mL of the reaction solution was taken and filtered through a 0.22 μm aqueous filter membrane to obtain a clear solution. The absorbance was measured at the maximum absorption wavelength of 664 nm using an ultraviolet spectrophotometer.

[0132] The results are as follows Figure 9 As shown in Table 2;

[0133] Table 2 Degradation rates (%) of different biochar

[0134]

[0135] It can be seen that under visible light irradiation, the RBC@TiO2 sample at 16MPa / 450℃ exhibits the best synergistic degradation effect, with the C / C0 value dropping to about 0.417 after 2.5h, and the degradation rate reaching 58.32%.

[0136] Test 8

[0137] Using the titanium-doped keratin biochar prepared in Example 1 as a photocatalyst, and hexavalent chromium as a model pollutant, the removal effect of the titanium-doped keratin biochar on organic pollutants was investigated. The specific test procedure was as follows: 50 mL of a 20 mg·L⁻¹ solution was prepared. -1 A Cr(VI) solution was prepared, pH adjusted to 3, and 20 mg of titanium-doped keratin biochar was added. The Cr(VI) concentration in the solution was tested at regular intervals. The results are as follows: Figure 10 As shown in Table 3;

[0138] Table 3 Catalytic effect of biochar obtained in Example 1

[0139]

[0140] It can be seen that after 6 hours of visible light irradiation, the removal rate of hexavalent chromium by RBC@TiO2 at 16 MPa / 450℃ was 55.22%.

[0141] In summary, it can be seen that:

[0142] (1) Optical microscopy and scanning electron microscopy showed that the surface morphology of rabbit hair loaded with TiO2 changed significantly. At 16 MPa pressure, TiO2 particles were uniformly distributed on the surface of rabbit hair fibers, and the outward protrusion of the scale layer decreased. At 25 MPa pressure, the physical structure of rabbit hair changed further, the scale layer was severely damaged, and TiO2 particles were mainly loaded in the medullary cavity. With further carbonization treatment, the surface morphology of the material changed, and the increase in carbonization temperature led to more TiO2 particles being embedded in the biochar.

[0143] (2) Fourier transform infrared spectroscopy showed that TiO2 was successfully modified onto the surface of rabbit hair fibers and biochar, and the results were obtained at 671 cm⁻¹. -1 Stretching vibration peaks of Ti-O bonds appeared at all locations. During carbonization, the content of hydroxyl (-OH) groups decreased, the stretching vibrations of aliphatic CH groups weakened, and the aromatic ring structure increased. High-temperature carbonization promoted the formation of nitrile groups, fixed some nitrogen elements, and the characteristic peaks of Ti-O bonds became more pronounced after carbonization. XRD results showed that TiO2 was successfully loaded in samples prepared under pressures of 16 MPa and 25 MPa. After carbonization, the characteristic diffraction peaks of TiO2 became more pronounced, with characteristic diffraction peaks of anatase TiO2 appearing at 25.3°, 37.8°, 48°, 54.7°, 63°, 69.1°, and 75.4°.

[0144] (3) The light absorption capacity of RBC@TiO2 prepared by supercritical fluid-thermal induction is significantly improved in the visible light region, the absorption edge is red-shifted, and the band gap energy is reduced. In particular, the band gap energy level of the 16MPa / 450℃ RBC@TiO2 sample is reduced to 2.76eV, the photocurrent density is increased to 1.22 times that of the TiO2 sample, and the photocurrent response density of the sample is still higher than that of TiO2 during 6 light irradiation cycles, and the impedance is reduced.

[0145] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing titanium-doped biochar, characterized in that, Includes the following steps: Protein fibers and a titanium source are combined in a supercritical carbon dioxide reaction system to obtain a keratin biochar precursor. The supercritical carbon dioxide reaction system operates at a pressure of 10–100 MPa, a reaction temperature of 25–250 °C, and a reaction time of 10–180 min. The protein fibers include at least one of rabbit hair, wool, cashmere, yak hair, and feathers. The titanium source includes at least one of titanium dioxide, titanate, titanium tetrachloride, titanium oxysulfate, organic titanium source, and titanium powder. The organic titanium source includes at least one of tetrabutyl titanate and tetraisopropyl titanate. The keratin biochar precursor was carbonized to obtain the titanium-doped biochar. The carbonization temperature is 250~1100℃, the holding time is 0.5~2h, and the heating rate is 1~10℃ / min; the carbonization is carried out under a protective atmosphere. The titanium-doped biochar comprises a carbon matrix and doping elements; the doping elements include N, S and Ti, wherein Ti exists in the anatase phase and forms Ti-OC bonds with the carbon matrix.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the protein fiber to the titanium source is 10:1 to 10:

10.

3. Titanium-doped biochar prepared by the preparation method according to claim 1 or 2.

4. The titanium-doped biochar according to claim 3, characterized in that, The titanium-doped biochar comprises a carbon matrix and doping elements; the doping elements include N, S and Ti, wherein Ti exists in the anatase phase and forms Ti-OC bonds with the carbon matrix.

5. The titanium-doped biochar according to claim 4, characterized in that, The N doping mass percentage is 0.4% to 16%; The doping mass percentage of S is 0.2% to 5%; The Ti doping mass percentage is 1-35%.

6. The application of titanium-doped biochar according to any one of claims 3 to 5 as a photocatalyst.

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