A supercritical carbon dioxide / cosolvent modified g-C3N4, its preparation method and application

Abstract

The application discloses supercritical carbon dioxide / cosolvent modified g-C3N4, a preparation method and application thereof, g-C3N4 is added into a reactor, a solvent is added, high-pressure CO2 is introduced, temperature control is 40-65 DEG C, pressure control is 5-10 MPa, then heating to 250-350 DEG C, then cooling, and supercritical carbon dioxide / cosolvent modified g-C3N4 is obtained. The ScCO2 / cosolvent system not only has excellent transmission of ScCO2, but also has reactivity of the cosolvent, can significantly reduce the required amount of organic solvent, and has a higher heating rate under the same wall temperature heat transfer conditions. The whole treatment process can be completed in 5 min, and the yield reaches 92.3%. The application can simultaneously realize the bulk-surface structure regulation of g-C3N4, and cooperatively improve the visible light hydrogen production and CO2 reduction activity of the modified g-C3N4.

Description

Technical Field

[0001] This invention belongs to the field of photocatalyst preparation technology, specifically relating to a supercritical carbon dioxide / cosolvent modified g-C3N4, its preparation method, and its application. Background Technology

[0002] Graphite carbonitrides (g-C3N4) are commonly prepared by thermal polymerization in air using nitrogen-rich precursors (such as dicyandiamide, melamine, and urea). The raw materials are abundant, the preparation method is simple, and it poses virtually no environmental harm. Furthermore, g-C3N4 can absorb visible light in the 450–600 nm range, thermodynamically satisfying the simultaneous needs for hydrogen and oxygen production and complete water splitting. g-C3N4 also exhibits excellent thermal and chemical stability, and its structural composition allows for easy control at the molecular level.

[0003] The bulk phase of g-C3N4 forms the core of the photocatalyst, determining its structural composition, microstructure, and band structure. However, the surface composition of g-C3N4 often differs significantly from the bulk phase. The surface can contact other components and is the primary site for photocatalytic reactions, influencing the contact, diffusion, and adsorption processes of reactants and products in heterogeneous catalysis. Common post-treatment modification methods for g-C3N4 include hydrothermal / solvothermal methods, thermal treatment, plasma treatment, ionothermal treatment, and surface deposition, all affecting both the bulk and surface phases of the catalyst. However, existing methods suffer from drawbacks such as long processing times, high energy consumption, low yields, and difficulty in large-scale application. Finding an effective, time-saving, high-yield, and scalable post-treatment strategy remains a significant challenge.

[0004] As mentioned above, the thermal polymerization method for preparing g-C3N4 has the advantages of being simple, environmentally friendly, and inexpensive. If combined with efficient post-processing modification methods, it can achieve large-scale production and development of highly efficient g-C3N4-based photocatalysts. Therefore, developing green, efficient, high-efficiency, and scalable post-processing modification methods for g-C3N4-based photocatalysts has significant research value and practical demand. Summary of the Invention

[0005] To overcome the problems of long processing time, high energy consumption, low yield, and difficulty in large-scale utilization in the existing g-C3N4 modification process, the purpose of this invention is to provide a supercritical carbon dioxide / cosolvent modified g-C3N4, its preparation method, and its application. This method uses greenhouse gas CO2 as a supercritical fluid, which is a strategy of turning waste into treasure and greatly reduces the amount of organic solvent used in the process. It is a green, time-saving, energy-efficient, high-yield method that can be scaled up.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing supercritical carbon dioxide / cosolvent modified g-C3N4 includes the following steps:

[0008] g-C3N4 was added to a reactor, along with a solvent, and high-pressure CO2 was introduced. The temperature was controlled at 40–65 °C and the pressure at 5–10 MPa. The mixture was then heated to 250–350 °C and cooled to obtain supercritical carbon dioxide / cosolvent modified g-C3N4.

[0009] Furthermore, the ratio of g-C3N4 to solvent is 100–500 mg: 100–1000 μL.

[0010] Furthermore, the ratio of g-C3N4 to solvent is 100–500 mg: 500 μL.

[0011] Furthermore, the solvent is one or more of methanol, acetonitrile, water, and acetone.

[0012] Furthermore, the solvent is methanol and / or acetonitrile.

[0013] Furthermore, before introducing high-pressure CO2, CO2 is injected to replace the gas inside the reactor.

[0014] Furthermore, water cooling is used for cooling.

[0015] A supercritical carbon dioxide / cosolvent modified g-C3N4.

[0016] Application of supercritical carbon dioxide / cosolvent modified g-C3N4 in photocatalytic CO2 reduction to CO.

[0017] Application of supercritical carbon dioxide / cosolvent modified g-C3N4 in photocatalytic water splitting for hydrogen production.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] This invention utilizes a ScCO2 / cosolvent system, composed of supercritical carbon dioxide (ScCO2) and an organic solvent as cosolvents, to treat pristine g-C3N4. This system possesses not only the excellent transport properties of ScCO2 but also the reactivity of the cosolvent. Compared to treatment with pure supercritical organic fluids, the ScCO2 / cosolvent system significantly reduces the required amount of organic solvent and exhibits a higher heating rate under the same wall temperature heat transfer conditions. The entire preparation process of this invention can be completed within 5 minutes, with a yield of up to 92.3%. During the ScCO2 / cosolvent treatment, CO2, the cosolvent, and the reaction intermediates can form a homogeneous phase with excellent heat and mass transfer properties, allowing g-C3N4 particles to fluidize in the reactor and fully contact other substances. There is a synergistic effect between ScCO2 and the cosolvent on the structural regulation of g-C3N4, and the ScCO2 / cosolvent system can achieve structural regulation effects close to those of pure supercritical organic fluids.

[0020] Furthermore, since different organic solvents have different chemical properties, specific co-solvents can be selected to achieve specific structural regulation of g-C3N4. When methanol (MeOH), acetonitrile (MeCN), and a mixture of methanol / acetonitrile (MeOH / MeCN) are used as co-solvents to post-treat pristine g-C3N4, bulk modification, surface modification, and simultaneous bulk-surface modification of g-C3N4 are achieved, respectively. MeOH, with the assistance of ScCO2, can partially and orderly transform g-C3N4 into a Q-PHI structure rich in methyl and hydroxyl groups. Meanwhile, MeCN can polymerize into triazine derivatives under high temperature and pressure, and then be covalently functionalized and grafted onto the surface of modified g-C3N4. Using MeOH and MeCN as co-solvents can respectively achieve bulk and surface structural regulation of g-C3N4.

[0021] Furthermore, co-treatment with ScCO2, MeOH, and MeCN can simultaneously achieve bulk-surface structure regulation of g-C3N4, which has a synergistic effect on improving the photocatalytic hydrogen production and CO2 reduction activity of modified g-C3N4. Simultaneous bulk-surface structure regulation can comprehensively enhance light absorption capacity, promote the separation and migration of photogenerated carriers, increase the number of photocatalytic reaction sites, and enhance the interfacial interaction with the sacrificial agent (triethanolamine) and photosensitizer (cobalt(II)2,2'-bipyridine complex) in the photocatalytic reaction system, thereby achieving a breakthrough in photocatalytic activity. Attached Figure Description

[0022] Figure 1 Photocatalytic hydrogen production activity diagrams of pristine g-C3N4, modified g-C3N4 under Examples 1, 2, and 3;

[0023] Figure 2Photocatalytic CO2 reduction to CO activity diagrams for original g-C3N4, modified g-C3N4 under Examples 1, 2, and 3;

[0024] Figure 3 Schematic diagrams of the molecular structure changes of g-C3N4 induced by ScCO2 / MeOH treatment and induced by ScCO2 / MeCN treatment; wherein, a is a schematic diagram of the molecular structure changes of g-C3N4 induced by ScCO2 / MeOH treatment and b is a schematic diagram of the molecular structure changes of g-C3N4 induced by ScCO2 / MeCN treatment.

[0025] Figure 4 The XRD patterns are of the original g-C3N4 and the modified g-C3N4 from Examples 1, 2, and 3.

[0026] Figure 5 The graphs show the heating curves of the modified g-C3N4 process in Examples 1, 2, and 3.

[0027] Figure 6 Thermogravimetric curves of the original g-C3N4 and the modified g-C3N4 under Examples 1, 2 and 3 are shown.

[0028] Figure 7 The diagram shows the mass content of elements and the atomic ratio of C / N in the original g-C3N4, and the modified g-C3N4 in Examples 1, 2 and 3.

[0029] Figure 8 UV-Vis spectra of the original g-C3N4 and the ScCO2 / MeCN-350 prepared in Example 3, and the ScCO2 / MeCN-350 sample after ultrasonic treatment obtained in Example 3;

[0030] Figure 9 The photocatalytic hydrogen production activity of g-C3N4 modified by ScCO2, atCO2 / cosolvent and ScCO2 / cosolvent systems at different temperatures in Examples 1-3, Comparative Examples 1 and 2 is shown in the figure.

[0031] Figure 10 The photocatalytic water splitting hydrogen production activity curves are shown for the original g-C3N4, the ScCO2 / AC-310 prepared in Example 6, and the ScCO2 / H2O-250 obtained in Example 7 after ultrasonic treatment. Detailed Implementation

[0032] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0033] Thanks to the unique reactivity, excellent mass transfer performance, and good solubility of supercritical fluids (SCFs), SCF-based post-treatment strategies are an effective way to control the structure of photocatalysts. ScCO2 (supercritical carbon dioxide) operates under relatively mild supercritical conditions (31.1℃, 7.4MPa) and is a gas at room temperature and pressure, easily separated from other substances without residue. ScCO2's excellent heat and mass transfer properties and solvation behavior make it a suitable "carrier" for carrying metal precursors or other substances into porous or layered materials. A ScCO2 / co-solvent system, consisting of ScCO2 and a small amount of organic solvent as a co-solvent, was used to treat pristine g-C3N4. This system not only possesses the excellent transport properties of ScCO2 but also the reactivity of the co-solvent. Compared to pure supercritical organic fluid treatment, the ScCO2 / co-solvent system can significantly reduce the amount of organic solvent required and has a higher heating rate under the same wall temperature heat transfer conditions. It offers advantages such as being green, efficient, and scalable. Based on this, the present invention uses a supercritical carbon dioxide / cosolvent system to post-process and modify g-C3N4.

[0034] A method for preparing supercritical carbon dioxide / cosolvent modified g-C3N4 according to the present invention includes the following steps:

[0035] First, pristine g-C3N4 was prepared by thermal polymerization. The obtained pristine g-C3N4 was placed in a high-temperature, high-pressure reactor, and a certain amount of organic solvent was added. Then, high-pressure CO2 was pumped in, and the reactor was rapidly heated. Once the target temperature was reached, the reactor was immediately placed in water for cooling. The resulting solid product was collected, washed, and dried to obtain ScCO2 / co-solvent modified g-C3N4. The specific steps are as follows:

[0036] 1) Add 100-500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 100-1000 μL of methanol (or solvents such as acetonitrile, water, acetone, etc. and mixed solvents), and then seal the reactor.

[0037] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. To prevent CO2 from liquefying, the initial temperature can be controlled at 40-65℃ and the initial pressure can be controlled at 5-10MPa.

[0038] 3) Place the reactor in an electric furnace and heat it to 250-350°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0039] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / cosolvent modified g-C3N4.

[0040] This invention also provides an application of supercritical carbon dioxide / cosolvent modified g-C3N4 in photocatalytic water splitting for hydrogen production and CO2 reduction for CO production.

[0041] On the one hand, bulk structure modulation using MeOH as a cosolvent enabled the construction of a closely contacted Type II heterojunction of g-C3N4 / Q-PHI with abundant methyl and hydroxyl groups. The interleaved band positions of the two components resulted in the enrichment of photogenerated electrons and holes in the Q-PHI and g-C3N4 phases, respectively. Simultaneously, the introduced methyl and hydroxyl groups enhanced light absorption and broke the asymmetry of the in-layer planar arrangement. Both the Type II heterojunction and the structural asymmetry effectively promoted the separation and migration of photogenerated carriers. Furthermore, the breaking of the C–N bonds between the heptaazine rings led to a decrease in catalyst particle size and an increase in specific surface area, increasing photocatalytic reaction sites and promoting mass transfer in the photocatalytic reaction. On the other hand, surface structure modulation using MeCN as a cosolvent allowed for the covalent grafting of the acetonitrile polymer C6H9N3 triazine compound onto the photocatalyst surface. Triazine functional groups can serve as photocatalytic reaction sites, expand the 2D conjugated electron system to achieve spatial carrier separation, improve the adsorption capacity of substances in photocatalytic reactions, and enhance light absorption. Based on these two aspects, simultaneous bulk-surface structural modulation can comprehensively enhance light absorption, promote the separation and migration of photogenerated carriers, increase the number of photocatalytic reaction sites, and enhance interfacial interactions with sacrificial agents and photosensitizers in the photocatalytic reaction system, thereby achieving a breakthrough improvement in photocatalytic activity.

[0042] The technical solution of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings. These embodiments do not constitute a limitation on the present invention.

[0043] In this invention, the original g-C3N4 is obtained by thermal polymerization and is denoted as CN.

[0044] Example 1

[0045] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 500 μL of MeOH / MeCN mixture (MeOH to MeCN volume ratio of 1:1), and then seal the reactor.

[0046] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 65℃ and the initial pressure is controlled at 8.0MPa.

[0047] 3) Place the reactor in an electric furnace and heat it to 330°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0048] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / MeOH / MeCN modified g-C3N4. The obtained sample is labeled as ScCO2 / MeOH / MeCN-330.

[0049] Example 2

[0050] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 500 μL of MeOH. Then seal the reactor.

[0051] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 65℃ and the initial pressure is controlled at 8.0MPa.

[0052] 3) Place the reactor in an electric furnace and heat it to 320°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0053] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / MeOH modified g-C3N4. The obtained sample is labeled as ScCO2 / MeOH-320.

[0054] Example 3

[0055] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 500 μL of MeCN. Then seal the reactor.

[0056] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 65℃ and the initial pressure is controlled at 8.0MPa.

[0057] 3) Place the reactor in an electric furnace and heat it to 350°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0058] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / MeCN modified g-C3N4. The obtained sample is labeled as ScCO2 / MeCN-350.

[0059] Figure 1 The image shows the photocatalytic hydrogen production activity of pristine g-C3N4 and modified g-C3N4 under Examples 1, 2, and 3. The photocatalytic hydrogen production activity was tested as follows, using Pt as a cocatalyst and triethanolamine as a sacrificial agent: a side-illuminated bottle reactor was used, with the reactor body made of Pyrex glass and an outer jacket for circulating water. The effective volume of the entire reactor was 100 mL. 50 mg of pristine g-C3N4 and 50 mg of modified g-C3N4 were thoroughly dispersed in 80 mL of 10 vol% triethanolamine aqueous solution, respectively, followed by the addition of H2PtCl6 aqueous solution corresponding to 0.5 mg of Pt (c...). Pt =0.765 mg / mL). Subsequently, the photocatalytic reaction solution was purged with high-purity Ar for 15 minutes using a purge tube (outer diameter 4 mm) inserted into the bottle mouth to remove air. After purging, the bottle mouth was sealed with a silicone stopper. A xenon lamp equipped with a cutoff filter (λ≥400 nm) was used as a simulated visible light source. Before the test, it was preheated for 15 minutes to stabilize the light intensity. In addition, the circulating water system was turned on in advance to keep the photocatalytic reaction solution at 35°C. The light-receiving surface of the photocatalytic reactor was aligned with the center of the xenon lamp, and the photocatalytic reaction solution was stirred with a magnetic stirrer at 500 rpm. Every 1 hour, 200 μL of gas was manually injected into the gas chromatograph using a high-precision microsyringe inserted into the silicone stopper of the bottle mouth to test the amount of hydrogen produced.

[0060] Depend on Figure 1 It can be seen that the photocatalytic hydrogen production rate of CN is 109.2 μmol·h⁻¹. -1 ·g -1 The photocatalytic hydrogen production activities of modified g-C3N4 obtained by ScCO2 / cosolvent treatment were significantly higher than those of CN. The photocatalytic hydrogen production rates of ScCO2 / MeOH-320 and ScCO2 / MeCN-350 were 1238.1 μmol·h⁻¹, respectively. -1 ·g -1 and 891.7 μmol·h -1 ·g -1 The photocatalytic hydrogen production rate of ScCO2 / MeOH / MeCN-330 is 2126.1 μmol·h⁻¹. -1 ·g -1This value is approximately equal to the sum of the photocatalytic hydrogen production rates of ScCO2 / MeOH-320 and ScCO2 / MeCN-350. This result indicates that bulk and surface modifications of g-C3N4 have a synergistic effect on improving photocatalytic hydrogen production activity.

[0061] Figure 2 The image shows the photocatalytic CO2 reduction to CO activity of the original g-C3N4 and the modified g-C3N4 under Examples 1, 2, and 3. The photocatalytic CO2 reduction to CO activity test method is as follows: In the test, cobalt(II) 2,2'-bipyridine complex was used as the photosensitizer, triethanolamine as the sacrificial agent, and acetonitrile as the CO2 co-solvent. A quartz-top sealed reactor was used, with one 19mm inner diameter bottle opening on each side of the upper part of the reactor, and a circulating water jacket on the lower half of the reactor. The effective volume of the entire reactor was 130mL. First, the photocatalytic reaction solution was prepared by fully dispersing 10mg of original g-C3N4 and 10mg of modified g-C3N4 in 4mL of 1μM CoCl2 aqueous solution, and adding 6mL of acetonitrile, 2mL of triethanolamine, and 25mg of 2,2'-bipyridine. The reaction solution was then sonicated in an ultrasonic machine for 5min. Subsequently, a fluororubber sealing ring and high-transparency quartz glass were installed on the top of the reactor. After sealing the top of the reactor with a flange, a purge tube (4mm outer diameter) was inserted into the bottle opening to purge the photocatalytic reaction solution with high-purity CO2 for 30 minutes to remove air. After purging, the bottle opening was sealed with a silicone stopper. A xenon lamp (PerfectLight, 300W) equipped with a cutoff filter (λ≥400nm) was used as a simulated visible light source. Before the test, it was preheated for 15 minutes to stabilize the light intensity. In addition, the cryogenic bath was turned on in advance to maintain the photocatalytic reaction solution at 10℃. Then, the top of the photocatalytic reactor was aligned with the center of the xenon lamp, and the magnetic stirrer was turned on to stir the photocatalytic reaction solution at 500rpm. Every 1 hour, a high-precision micro-syringe was inserted into the silicone stopper on one side of the bottle opening, and 500μL of gas was manually injected into the gas chromatograph to test the amount of hydrogen and carbon monoxide produced.

[0062] Depend on Figure 2 It can be seen that the photocatalytic CO2 reduction to CO rate of ScCO2 / MeOH / MeCN-330 is 874 μmol·h. -1 ·g -1 It is much higher than ScCO2 / MeOH-320 (246 μmol·h⁻¹). -1 ·g -1 ) and ScCO2 / MeCN-350 (486 μmol·h -1 ·g -1 CN, on the other hand, has only weak photocatalytic CO2 reduction activity.

[0063] Figure 3 Schematic diagrams illustrating the molecular structural changes in g-C3N4 induced by ScCO2 / MeOH treatment and ScCO2 / MeCN treatment. (See also...) Figure 3 In methods a and b, MeOH, with the assistance of ScCO2, can orderly transform g-C3N4 into a homologous compound (Q-PHI) with a poly(heptaazine imide) structure rich in methyl and hydroxyl groups. Meanwhile, MeCN can polymerize into a triazine derivative under high temperature and pressure, and then be covalently functionalized and grafted onto the surface of modified g-C3N4.

[0064] Figure 4 The images show the XRD patterns of the original g-C3N4 and the modified g-C3N4 from Examples 1, 2, and 3. Figure 4 It can be seen that the XRD diffraction peak of ScCO2 / MeOH-320 at 12.8° splits into two parts, which is caused by the structural distortion of the heptaazine ring within the g-C3N4 layer. Furthermore, two distinct XRD characteristic peaks appear at 7.1° and 7.5° in ScCO2 / MeOH-320, belonging to the (100) crystal plane of the newly formed Q-PHI phase. Due to the interactions of hydrogen bonds, methyl groups, and hydroxyl groups within the Q-PHI layer, the intralayer arrangement of Q-PHI is asymmetrical, thus the XRD characteristic peak of the Q-PHI (100) crystal plane in ScCO2 / MeOH-320 splits into two independent peaks. The XRD pattern of ScCO2 / MeCN-350 is similar to that of CN, indicating that the bulk structure of g-C3N4 was not changed by ScCO2 / MeCN treatment. Diffraction peak information corresponding to the Q-PHI phase can also be found in the XRD pattern of ScCO2 / MeOH / MeCN-330.

[0065] Figure 5 The figures show the temperature rise curves for the modified g-C3N4 process in Examples 1, 2, and 3. Figure 5 It can be seen that the temperature of the system increases linearly (approximately 1.8℃·s). -1 The entire solvent heat treatment process takes only 5 minutes.

[0066] Table 1 shows the maximum pressure, heating time, and yield of the modified g-C3N4 process and the blank group under Examples 1, 2, and 3. The yields of ScCO2 / MeOH-320, ScCO2 / MeCN-350, and ScCO2 / MeOH / MeCN-330 were 88.8%, 95.5%, and 92.3%, respectively. The ScCO2 / co-solvent system strategy for treating g-C3N4 has the advantages of being time-saving and yielding high yields.

[0067] Table 1

[0068]

[0069] Figure 6 Thermogravimetric curves of the original g-C3N4, and the modified g-C3N4 under Examples 1, 2, and 3 are shown. Figure 6 It can be seen that for CN, the g-C3N4 phase begins to decompose at approximately 520℃ and decomposes completely at 750℃. Compared to CN, the TG curve of ScCO2 / MeCN-350 shifts towards the lower temperature range, with more weight loss in the 520–750℃ temperature range attributed to the thermal decomposition of the surface-grafted C6H9N3 functional groups. The TG curves of ScCO2 / MeOH-320 and ScCO2 / MeOH / MeCN-330 show significant changes in shape, indicating two distinct weight loss processes. In addition to the latter thermal decomposition of the g-C3N4 phase, a weight loss process with an initial decomposition temperature of 310℃ also occurs, attributed to the thermal decomposition of the Q-PHI phase.

[0070] Figure 7 The mass content of elements and the C / N atomic ratio of the original g-C3N4, and the modified g-C3N4 under Examples 1, 2, and 3 are given. Figure 7 It can be seen that, compared with CN, the C / N atomic ratio of ScCO2 / MeOH-320 increases from 0.715 to 0.744, while the H content increases from 2.0% to 2.6%. In ScCO2 / MeCN-350, due to the surface grafting of C6H9N3 functional groups, the C / N atomic ratio (0.718) and H content (2.1%) are also slightly increased compared to CN. Since bulk phase regulation targets the overall composition of the catalyst, while surface regulation targets the surface portion, the effects of bulk and surface regulation on the overall elemental composition of g-C3N4 differ, with bulk phase regulation having a more significant effect on elemental regulation. As expected, the C / N atomic ratio (0.738) and H content (2.4%) of ScCO2 / MeOH / MeCN-330 are between those of ScCO2 / MeOH-320 and ScCO2 / MeCN-350.

[0071] Figure 8 UV-Vis spectra of the original g-C3N4 and the ScCO2 / MeCN-350 prepared in Example 3, and the ScCO2 / MeCN-350 sample after ultrasonic treatment obtained in Example 3. Figure 8It is evident that ScCO2 / MeCN treatment anchors C6H9N3 molecules to the surface of g-C3N4, significantly altering the UV-Vis spectrum of g-C3N4. Furthermore, conventional ultrasonic treatment cannot remove the surface-anchored C6H9N3 molecules from ScCO2 / MeCN-350, suggesting that the C6H9N3 molecules are anchored to the C6H9N3 surface through covalent functionalization rather than weak physical adsorption.

[0072] Comparative Example 1

[0073] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 500 μL of MeOH / MeCN mixture (MeOH to MeCN volume ratio of 1:1), and then seal the reactor.

[0074] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and control the initial pressure at room temperature to 0.1 MPa.

[0075] 3) Place the reactor in an electric furnace and heat it to 330°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0076] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain g-C3N4 modified by atmospheric CO2 atmosphere and MeOH / MeCN, which is denoted as atCO2 / cosolvent system modified g-C3N4.

[0077] Comparative Example 2

[0078] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and then seal the reactor.

[0079] 2) Inject CO2 into the reactor to fully displace the internal atmosphere, then pump in high-pressure CO2 using a gas compressor. The initial temperature is controlled at 65℃, and the initial pressure is controlled at 8.0 MPa.

[0080] 3) Place the reactor in an electric furnace and heat it to 330°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0081] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 modified g-C3N4, which is denoted as ScCO2 system modified g-C3N4.

[0082] Figure 9The image shows the photocatalytic hydrogen production activity of g-C3N4 modified with ScCO2, atCO2 / cosolvent, and ScCO2 / cosolvent systems at different temperatures in Examples 1-3, Comparative Examples 1 and 2. Figure 9 It can be seen that, under different treatment temperatures, the visible light photocatalytic hydrogen production activity of ScCO2 / cosolvent modified g-C3N4 is significantly higher than that of ScCO2 modified g-C3N4 and g-C3N4 modified with a small amount of cosolvent at ambient pressure CO2, further confirming the synergistic effect between ScCO2 and cosolvent on the structural regulation of g-C3N4.

[0083] Example 4

[0084] 1) Add 100 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 500 μL of MeOH / MeCN mixture (volume ratio 1:1), then seal the reactor.

[0085] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 65℃ and the initial pressure is controlled at 8.0MPa.

[0086] 3) Place the reactor in an electric furnace and heat it to 330°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0087] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / MeOH / MeCN modified g-C3N4.

[0088] Example 5

[0089] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 500 μL of MeOH / MeCN mixture (MeOH to MeCN volume ratio of 1:1), and then seal the reactor.

[0090] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 65℃ and the initial pressure is controlled at 6.0MPa.

[0091] 3) Place the reactor in an electric furnace and heat it to 300°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0092] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / MeOH / MeCN modified g-C3N4.

[0093] Example 6

[0094] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor and add 500 μL of acetone, then seal the reactor.

[0095] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 65℃ and the initial pressure is controlled at 8.0MPa.

[0096] 3) Place the reactor in an electric furnace and heat it to 310°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0097] 4) Collect the solid product, wash it three times with water, then once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / acetone modified g-C3N4. The obtained sample is designated as ScCO2 / AC-310.

[0098] Example 7

[0099] 1) Add 500 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 500 μL of water. Then seal the reactor.

[0100] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 65℃ and the initial pressure is controlled at 8.0MPa.

[0101] 3) Place the reactor in an electric furnace and heat it to 310°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0102] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain ScCO2 / water modified g-C3N4. The obtained sample is recorded as ScCO2 / H2O-250.

[0103] Figure 10 The photocatalytic activity curves of water splitting for hydrogen production are shown for the original g-C3N4, the ScCO2 / AC-310 prepared in Example 6, and the ScCO2 / H2O-250 obtained in Example 7 after ultrasonic treatment. Figure 10 It is evident that in the ScCO2 / cosolvent system, besides MeCN and MeOH as typical cosolvents, more cosolvent combinations can be explored to achieve surface-to-bulk structure modulation of g-C3N4. For example, the typical solvents water and acetone result in modified g-C3N4 exhibiting enhanced photocatalytic hydrogen production activity.

[0104] Example 8

[0105] 1) Add 200 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 100 μL of MeOH / MeCN mixture (MeOH to MeCN volume ratio of 2:1), and then seal the reactor.

[0106] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 40℃ and the initial pressure is controlled at 5.0MPa.

[0107] 3) Place the reactor in an electric furnace and heat it to 250°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0108] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain supercritical carbon dioxide / cosolvent modified g-C3N4.

[0109] Example 9

[0110] 1) Add 300 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 300 μL of MeOH / water mixture (MeOH to water volume ratio of 3:1), and then seal the reactor.

[0111] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 45℃ and the initial pressure is controlled at 10.0MPa.

[0112] 3) Place the reactor in an electric furnace and heat it to 270°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0113] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain supercritical carbon dioxide / cosolvent modified g-C3N4.

[0114] Example 10

[0115] 1) Add 400 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 1000 μL of MeOH. Then seal the reactor.

[0116] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 50℃ and the initial pressure is controlled at 6.0MPa.

[0117] 3) Place the reactor in an electric furnace and heat it to 300°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0118] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain supercritical carbon dioxide / cosolvent modified g-C3N4.

[0119] Example 11

[0120] 1) Add 250 mg of raw g-C3N4 to a high-temperature and high-pressure reactor, and add 800 μL of MeOH. Then seal the reactor.

[0121] 2) Inject CO2 into the reactor to fully replace the internal atmosphere, and then pump in high-pressure CO2 through a gas compressor. The initial temperature is controlled at 60℃ and the initial pressure is controlled at 7.0MPa.

[0122] 3) Place the reactor in an electric furnace and heat it to 340°C. Once the thermocouple in the reactor detects that the temperature has reached the specified temperature, immediately remove the reactor from the electric furnace and immerse it in a room temperature water bath to cool it down.

[0123] 4) Collect the solid product, wash it three times with water, then wash it once with ethanol, and dry it at 60℃ for 6 hours to obtain supercritical carbon dioxide / cosolvent modified g-C3N4.

[0124] The above description is only of the preferred embodiment of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All variations made within the scope of the independent claims of the present invention are also within the scope of protection of the present invention.

[0125] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

Claims

1. A method for preparing supercritical carbon dioxide / cosolvent modified g-C3N4, characterized in that, Includes the following steps: Add g-C3N4 to the reactor, along with methanol and / or acetonitrile as a co-solvent, and purge with high-pressure CO2 while maintaining the temperature at 40-65°C. o C, the pressure is controlled at 5~10 MPa, then heated to 250~350℃. o C, then cooled, to obtain supercritical carbon dioxide / cosolvent modified g-C3N4; the ratio of g-C3N4 to cosolvent was 100~500 mg: 500 μL; methanol, acetonitrile and a mixture of methanol / acetonitrile were used as cosolvents to post-treat g-C3N4, to achieve bulk phase modification, surface modification and simultaneous bulk-surface modification of g-C3N4 respectively.

2. The method for preparing supercritical carbon dioxide / co-solvent modified g-C3N4 according to claim 1, characterized in that, Before introducing high-pressure CO2, CO2 is injected to replace the gas inside the reactor.

3. The method for preparing supercritical carbon dioxide / co-solvent modified g-C3N4 according to claim 1, characterized in that, Water cooling is used for cooling.

4. A supercritical carbon dioxide / cosolvent modified g-C3N4 prepared by the method according to any one of claims 1-3.

5. The application of supercritical carbon dioxide / cosolvent modified g-C3N4 prepared according to any one of claims 1-3 in photocatalytic CO2 reduction to CO.

6. The application of supercritical carbon dioxide / cosolvent modified g-C3N4 prepared by any one of claims 1-3 in photocatalytic water splitting for hydrogen production.

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