A subcritical oxidation catalyst for sludge reduction and a preparation method thereof

Abstract

This invention belongs to the field of environmental engineering and catalytic materials, and discloses a subcritical oxidation catalyst for sludge reduction and its preparation method. The catalyst uses functionalized mesoporous hollow silica spheres as a support, grafts phosphate and amino groups onto the surface, loads ternary metal oxides with synergistic active sites, and is prepared using a combined microwave and hydrothermal process to obtain a highly dispersed sub-nano metal cluster structure. The prepared catalyst can efficiently generate free radicals in subcritical aqueous media, significantly promoting the deep oxidative decomposition of organic components in sludge, achieving the dual effects of volume reduction and pollutant removal; simultaneously, due to the functionalization of the support and the synergistic effect of multiple metals, it exhibits excellent anti-sintering and cycling stability.

Description

Technical Field

[0001] This invention belongs to the field of environmental engineering and catalytic materials, specifically relating to a subcritical oxidation catalyst for sludge reduction and its preparation method. Background Technology

[0002] In municipal and industrial wastewater treatment, residual sludge is characterized by high water content, high viscosity, and large volume. Traditional mechanical dewatering, thermal drying, and landfill treatment methods suffer from high energy consumption, expensive operating costs, and significant risks of secondary pollution. In recent years, subcritical water oxidation technology has attracted widespread attention due to its ability to rapidly generate strong oxidizing free radicals under subcritical conditions, efficiently decomposing organic matter into CO2, water, and harmless salts. However, commonly used subcritical water oxidation catalysts in practical applications often employ single-metal or bimetallic oxides, which suffer from uneven distribution of active sites, limited specific surface area, and susceptibility to sintering deactivation and poisoning carbon deposition, severely restricting the stability and economic efficiency of continuous, large-scale sludge reduction treatment. Furthermore, existing subcritical water oxidation reactors rely on external heating for temperature control, resulting in low thermal efficiency and difficulty in achieving local temperature compensation, further increasing energy consumption and reducing reaction efficiency. To address these shortcomings, existing research has attempted to improve activity by optimizing catalyst supports and refining synthesis processes, but these efforts largely remain at the level of single technological approaches, lacking innovative designs that integrate multiple levels of systems, including support functionalization, multi-metal synergy, and process coupling. Meanwhile, how to balance high catalyst activity, long lifespan, and low energy consumption is a key bottleneck in realizing the engineering application of subcritical water oxidation technology. Summary of the Invention

[0003] To address the shortcomings mentioned in the background art, the present invention aims to provide a subcritical oxidation catalyst for sludge reduction and its preparation method. The catalyst is a subcritical water oxidation catalyst with ternary metal synergistic oxidation active sites supported on a functionalized mesoporous hollow silica sphere support. It can achieve high free radical yield, support anti-sintering and cycle stability, and has the self-heating function of phase change material. It can be used for municipal and industrial sludge reduction treatment.

[0004] The objective of this invention can be achieved through the following technical solutions:

[0005] A subcritical oxidation catalyst for sludge reduction comprises the following raw materials in parts by weight: 80-100 parts of mesoporous hollow silica sphere support, 2-10 parts of (3-aminopropyl)triethoxysilane, 5-7 parts of ferric nitrate nonahydrate, 3-5 parts of copper nitrate trihydrate, and 0.5-1.5 parts of cerium nitrate hexahydrate.

[0006] The subcritical oxidation catalyst has the following structural features: a bifunctionalized layer is formed by grafting phosphate groups and amino groups onto the surface of a mesoporous hollow silica sphere support; and active sites composed of iron, copper, and cerium metal oxides are highly dispersed and loaded in the form of sub-nano clusters on the surface of the functionalized support.

[0007] More preferably, the preparation method of the mesoporous hollow silicon sphere carrier specifically includes the following steps:

[0008] S101. Add tetraethoxysilane, triethyl phosphate, ammonia, ethanol and deionized water to a high-pressure reactor and stir to mix all components thoroughly.

[0009] S102. After sealing the reactor, heat it to 180 ℃, maintain the temperature at this temperature for the reaction, and after the reaction is completed, allow it to cool naturally to room temperature and release the pressure in the reactor.

[0010] S103. Separate the solid phase from the reaction product by vacuum filtration, wash the solid phase three times each with ethanol and deionized water, and remove the surface residue by vacuum filtration.

[0011] S104. The filter cake is placed in an oven to dry, the dried product is spread on a boat, placed in a tube furnace for calcination, and then cooled to room temperature to obtain the mesoporous hollow silicon sphere carrier.

[0012] More preferably, the surface-grafted phosphate functionalized layer is prepared by immersing the dried mesoporous hollow silica sphere carrier in a phosphoric acid solution, causing the hydroxyl groups on the carrier surface to react chemically with the phosphate groups, followed by drying under mild conditions to remove residual solvent, and then calcining in air at high temperature to solidify and stabilize the phosphate groups.

[0013] More preferably, the amine group functionalized layer on the carrier surface is formed by dispersing the phosphate-functionalized mesoporous hollow silica sphere carrier in anhydrous ethanol, adding (3-aminopropyl)triethoxysilane and reacting under reflux conditions, and washing and drying after the reaction is completed, thereby forming a stable amine graft layer.

[0014] More preferably, the metal oxide synergistic active sites are dispersed on the surface of the functionalized support in the form of sub-nanometer metal clusters.

[0015] More preferably, the pore size of the mesoporous hollow silicon sphere carrier is 2–10 nm.

[0016] A method for preparing a subcritical oxidation catalyst for sludge reduction includes the following steps:

[0017] S1. Tetraethoxysilane, triethyl phosphate, ammonia, ethanol and water are mixed in a reaction vessel and subjected to hydrothermal reaction. After the reaction is completed, the mixture is washed and dried to obtain a mesoporous hollow silica sphere support.

[0018] S2. The mesoporous hollow silica sphere carrier is immersed in an aqueous solution of phosphoric acid, then dried and calcined in air to fix the phosphate graft layer;

[0019] S3. The support was then dispersed in anhydrous ethanol and (3-aminopropyl)triethoxysilane was added. The reaction was carried out under reflux, and the mixture was washed and dried to obtain the bifunctionalized support.

[0020] S4. Dissolve the precursors of ferric nitrate nonahydrate, copper nitrate trihydrate, and cerium nitrate hexahydrate in deionized water, add the bifunctionalized carrier to the metal solution, and stir until evenly dispersed;

[0021] S5. The mixture is placed in a microwave reactor for nucleation treatment, then transferred to a hydrothermal reactor. After the reaction is completed, the product is collected and preliminarily dried. The product is then placed in a drying oven to completely remove water, and finally calcined in a high-temperature furnace to obtain the subcritical oxidation catalyst.

[0022] More preferably, before step S5, the stearic acid-paraffin phase change material and the carrier are added to the metal solution mixture in a blending manner, so that the phase change material and the metal precursor are co-loaded in the subsequent microwave-hydrothermal and calcination processes.

[0023] More preferably, after step S5 and before the end of the calcination process, a carbon source is introduced in situ onto the catalyst surface to form a 1–3 wt% carbon shell layer through low-temperature (400–450 °C) thermal decomposition, thereby enhancing the catalyst's resistance to sintering and poisoning.

[0024] The beneficial effects of this invention are:

[0025] The subcritical oxidation catalyst provided by this invention significantly improves the free radical generation efficiency of the catalyst in subcritical water media by grafting phosphate and amine functional groups onto the surface of mesoporous hollow silica spheres and loading ternary metal oxides as synergistic active sites. This enables rapid and thorough decomposition of organic components in sludge, achieving efficient volume reduction and pollutant removal. Simultaneously, the mesoporous and hollow structure of the functionalized support not only improves the dispersion and specific surface area of ​​the metal oxides but also effectively inhibits sintering and particle agglomeration, enhancing the catalyst's thermal stability and poisoning resistance. Sub-nanometer-scale metal clusters can be obtained using a microwave and hydrothermal composite process, providing better surface exposure for active sites and reducing synthesis energy consumption. Furthermore, the introduction of phase change materials into the catalyst achieves local temperature compensation through a phase change endothermic and exothermic mechanism, reducing the external heating burden and thus lowering overall reaction energy consumption while maintaining temperature uniformity. This catalyst maintains high activity even after multiple cycles of use. Combined with low-temperature in-situ hydrogen peroxide regeneration technology, the regeneration process can be simplified and the service life extended. Attached Figure Description

[0026] The invention will now be further described with reference to the accompanying drawings.

[0027] Figure 1 The bar chart shows the comparison of sludge volume reduction rate and COD removal rate of the catalysts in Examples 1-3 and Comparative Examples 1-2.

[0028] Figure 2 The graphs show the anti-sintering cycle performance of the catalysts in Examples 1-3 and Comparative Examples 1-2.

[0029] Figure 3 The bar chart shows the activity retention rate of the catalysts in Examples 1-3 and Comparative Examples 1-2 during the third round of catalyst regeneration. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0031] Example 1

[0032] I. Preparation of Mesoporous Hollow Silica Spherical Carriers

[0033] Weigh 20.0 g of tetraethoxysilane, 10.0 g of triethyl phosphate, 10 mL of ammonia (25 wt%), 200 mL of ethanol, and 100 mL of deionized water. Add all the above components to a 500 mL stainless steel autoclave and stir at 600 rpm for 30 min at room temperature. Then, seal the autoclave and heat it to 180 °C, holding it at this temperature for 10 h for hydrothermal synthesis. After the reaction is complete, allow it to cool naturally to room temperature, slowly release the pressure inside the autoclave, and open the lid. Transfer the reaction product to a vacuum filtration funnel, wash it twice with ethanol, and then twice with deionized water to remove unreacted raw materials and impurities. Remove the washing liquid using vacuum filtration until there is no obvious residual liquid on the surface of the filter cake. Place the filter cake in an 80 °C oven to dry for 12 h until the surface of the solid powder is dry and free of moisture. The dried powder was spread evenly on a high-temperature resistant boat and placed in a tube furnace. Under an air atmosphere, the temperature was increased to 550 ℃ at a rate of 5 ℃ / min and held for 3 h. After calcination, the furnace was cooled to room temperature and the mesoporous hollow silicon sphere carrier was obtained.

[0034] II. Preparation of Subcritical Oxidation Catalysts

[0035] The subcritical oxidation catalyst comprises the following raw materials in parts by weight: 80 parts of mesoporous hollow silica sphere support, 2 parts of (3-aminopropyl)triethoxysilane, 5 parts of ferric nitrate nonahydrate, 3 parts of copper nitrate trihydrate, and 0.5 parts of cerium nitrate hexahydrate;

[0036] The preparation steps are as follows: 80 g of mesoporous hollow silica spheres were weighed and placed in 100 mL of 0.5 mol / L phosphoric acid aqueous solution. After shaking at room temperature for 6 h, the solid was filtered out and dried in an oven at 80 ℃ for 8 h. The dried product was placed in a tube furnace and calcined at 500 ℃ at a rate of 5 ℃ / min for 3 h in air atmosphere to obtain a phosphate-functionalized carrier. The carrier was then dispersed in 80 mL of anhydrous ethanol and sonicated for 30 min. 2 g of (3-aminopropyl)triethoxysilane was added to the dispersion and refluxed at 78 ℃ for 4 h. After cooling, the mixture was centrifuged, washed three times with ethanol, and dried at 80 ℃ for 12 h to obtain a bifunctionalized carrier. 5 g of ferric nitrate nonahydrate, 3 g of copper nitrate trihydrate, and 0.5 g of cerium nitrate hexahydrate were dissolved in 50 mL of deionized water. The bifunctionalized carrier was added, and the mixture was stirred at room temperature for 2 h. The solid was filtered out and preliminarily dried at 80 ℃ for 6 h. The obtained solid and mother liquor were transferred together to a microwave reactor and irradiated at 600 W for 4 min. The mixture was then transferred to a hydrothermal reactor and held at 180 °C for 10 h. After natural cooling, the solid was filtered off and dried at 80 °C for 6 h. The solid was then dried in a vacuum drying oven at 110 °C for 12 h, transferred to a tube furnace, and heated to 500 °C at a rate of 5 °C / min, held for 3 h, and then allowed to cool naturally to obtain the subcritical oxidation catalyst.

[0037] Example 2

[0038] The preparation steps of the mesoporous hollow silicon sphere carrier are the same as in Example 1.

[0039] The preparation method of subcritical oxidation catalyst is as follows:

[0040] The subcritical oxidation catalyst comprises the following raw materials in parts by weight: 100 parts of mesoporous hollow silica sphere support, 10 parts of (3-aminopropyl)triethoxysilane, 7 parts of ferric nitrate nonahydrate, 5 parts of copper nitrate trihydrate, and 1.5 parts of cerium nitrate hexahydrate.

[0041] The preparation steps of the subcritical oxidation catalyst are the same as in Example 1.

[0042] Example 3

[0043] The preparation steps of the mesoporous hollow silicon sphere carrier are the same as in Example 1.

[0044] The preparation method of subcritical oxidation catalyst is as follows:

[0045] The subcritical oxidation catalyst comprises the following raw materials in parts by weight: 90 parts of mesoporous hollow silica sphere support, 6 parts of (3-aminopropyl)triethoxysilane, 6 parts of ferric nitrate nonahydrate, 4 parts of copper nitrate trihydrate, and 1.0 part of cerium nitrate hexahydrate;

[0046] The preparation steps of the subcritical oxidation catalyst are the same as in Example 1.

[0047] Comparative Example 1 (Cerium nitrate without hexahydrate)

[0048] The preparation steps of the mesoporous hollow silicon sphere carrier are the same as in Example 1.

[0049] The preparation method of subcritical oxidation catalyst is as follows:

[0050] The subcritical oxidation catalyst comprises the following raw materials in parts by weight: 90 parts of mesoporous hollow silica sphere support, 6 parts of (3-aminopropyl)triethoxysilane, 6 parts of ferric nitrate nonahydrate, and 4 parts of copper nitrate trihydrate.

[0051] The preparation of the subcritical oxidation catalyst is the same as in Example 1, except that cerium nitrate hexahydrate is not added during the adsorption of the metal precursor.

[0052] Comparative Example 2 (Phosphate / amine functionalization process not performed)

[0053] The preparation steps of the mesoporous hollow silicon sphere carrier are the same as in Example 1.

[0054] The preparation method of subcritical oxidation catalyst is as follows:

[0055] The subcritical oxidation catalyst comprises the following raw materials in parts by weight: 90 parts of mesoporous hollow silica sphere support, 6 parts of (3-aminopropyl)triethoxysilane, 6 parts of ferric nitrate nonahydrate, 4 parts of copper nitrate trihydrate, and 1.0 part of cerium nitrate hexahydrate;

[0056] The preparation steps are as follows: 6.0 g of ferric nitrate nonahydrate, 4.0 g of copper nitrate trihydrate, and 1.0 g of cerium nitrate hexahydrate were dissolved sequentially in 50 mL of deionized water and ultrasonically dispersed for 10 min. 90 g of mesoporous hollow silica spheres were added to the metal solution, and the mixture was magnetically stirred at room temperature for 2 h to ensure uniform adsorption of metal ions. The solid was separated by vacuum filtration and pre-dried in an 80 ℃ oven for 6 h. The pre-dried solid and the remaining mother liquor were transferred to a microwave reactor and irradiated with microwaves at 600 W for 4 min for nucleation. The nucleation liquid was immediately transferred to a stainless steel hydrothermal reactor and kept at 180 ℃ for 10 h. After the reaction, the mixture was allowed to cool naturally to room temperature. The product was removed from the reactor, washed twice with deionized water, and dried at 80 ℃ for 6 h. The dried product was then dried in a 110 ℃ vacuum drying oven for 12 h. The dried solid was spread evenly on a high-temperature resistant boat and placed in a tube furnace, where the temperature was increased to 500 ℃ at a rate of 5 ℃ / min. The catalyst was heated to ℃ and kept at that temperature in air for 3 hours. After calcination, it was cooled to room temperature in the furnace to obtain the subcritical oxidation catalyst.

[0057] Performance testing

[0058] 1. Sludge reduction rate and COD removal rate test

[0059] 1.0 g of the catalyst samples from Examples 1–3 and Comparative Examples 1–2 were weighed and added to a high-pressure reactor containing 50 mL of deionized water. After stirring evenly, 50 g of moist sludge was added. The reactor was sealed and placed in an electric heating furnace, heated to 300 °C and pressurized to 15 MPa, and maintained for 60 min. After the reaction was completed, the mixture was allowed to cool naturally to room temperature, and the solid and liquid phase mixture was removed. The solid phase was separated by filter paper, and the volume of the filter residue was measured to calculate the sludge volume reduction rate. The liquid phase was filtered through a 0.45 μm filter membrane, and the COD concentration was determined using the potassium dichromate method. The COD was compared with that of the original sludge water sample to calculate the COD removal rate. The results are shown in Table 1 below.

[0060] Table 1. Sludge reduction rate and COD removal rate

[0061] sample Sludge volume reduction rate (%) COD removal rate (%) Example 1 58 92 Example 2 56 90 Example 3 60 95 Comparative Example 1 48 85 Comparative Example 2 45 80

[0062] As shown in Table 1, the catalysts in the examples significantly outperformed the comparative examples in both sludge volume reduction and COD removal rate, fully demonstrating the synergistic effect mechanism of this invention. In the examples, the mesoporous hollow silica spheres, after being bifunctionalized with phosphate and amine groups, not only enhanced the adsorption and dispersion of metal oxides but also constructed highly efficient free radical generation sites in the subcritical water medium. The ternary metal synergistic oxidation sites further improved the yield and selectivity of hydroxyl radicals, accelerating the oxidative cracking of organic components. In contrast, Comparative Example 1 lacked Ce synergistic co-catalytic effect, resulting in a decrease in free radical yield and catalytic efficiency; Comparative Example 2 lacked a functionalized layer, and the metal clusters were prone to sintering and agglomeration, resulting in insufficient exposure of active sites and a decrease in reaction rate.

[0063] 2. Free radical yield and reaction kinetics

[0064] The -OH radical generation capacity of each catalyst under subcritical conditions was determined using the pyridine-EDTA trapping method. 1.0 g of catalyst sample was added to 50 mL of pyridine-EDTA aqueous solution (0.1 mmol / L), and the reaction was carried out in an autoclave at 300 °C and 15 MPa for 30 min. After the reaction, the mixture was rapidly cooled, and samples were taken. The absorbance of the hydroxylation products in the reaction solution was measured by ultraviolet spectrophotometry at λ=325 nm, and converted to the radical generation rate (μmol·L⁻¹·min⁻¹). Simultaneously, the reaction rate constant of each catalyst was tested at 260–300 °C, and the rate constant k was calculated using the first-order kinetic fitting formula ln(C0 / C) = kt. The results are shown in the two tables below.

[0065] Table 2. Rate of OH formation and rate constant

[0066] sample -OH generation rate (μmol·L⁻¹·min⁻¹) Reaction rate constant k (min⁻¹) Example 1 8.5 0.062 Example 2 7.9 0.057 Example 3 9.2 0.068 Comparative Example 1 6.4 0.045 Comparative Example 2 5.2 0.038

[0067] Table 2 shows that the radical generation rate and reaction rate constant of Examples 1–3 are significantly higher than those of the comparative examples, indicating that the catalyst of this invention has stronger activity and reaction kinetic advantages in the subcritical oxidation process. The mesoporous hollow silica sphere support modified with phosphate and amine groups significantly improves the dispersibility and electron transfer efficiency of the metal oxide, enabling the ternary Fe–Cu–Ce synergistic system to form a high-density redox cycle center; Fe³⁺ / Fe²⁺ and Ce… 4 Electron coupling between ⁺ and Ce³⁺ promoted the continuous generation of -OH radicals. In contrast, Comparative Example 1 showed a significantly reduced radical yield due to the lack of Ce co-catalysis; Comparative Example 2, due to the lack of functionalized support, resulted in severe metal cluster aggregation and insufficient exposure of active sites, leading to a slower reaction rate.

[0068] 3. Thermal stability and anti-sintering cycle test

[0069] Each catalyst sample was calcined at 500 °C for 3 h in air to simulate sintering aging during long-term high-temperature operation. After calcination, the catalyst was cooled to room temperature, and 1.0 g was weighed for a standard sludge oxidation experiment (300 °C, 15 MPa, 60 min). The sludge volume reduction rate and COD removal rate were measured as the baseline values ​​for the first cycle. Subsequently, the catalyst samples were recovered, washed with water, dried, and reused in the next round of reaction. The test was repeated continuously for a total of 20 cycles. Performance data were recorded every 5 cycles to evaluate the catalyst's resistance to sintering and its service life under high-temperature conditions. The results are shown in Table 3.

[0070] Table 3 Thermal stability and cyclic anti-sintering performance (COD removal rate %)

[0071] sample initial 5th 10th 15th 20th Example 1 92 91 90 89 88 Example 2 90 89 88 86 85 Example 3 95 94 93 92 91 Comparative Example 1 85 81 77 72 68 Comparative Example 2 80 75 69 61 55

[0072] As shown in Table 3, after calcination at 500℃ and 20 consecutive cycles, the COD removal rate of Examples 1–3 remained above 88%, especially Example 3, which only decreased by 4 percentage points, demonstrating excellent thermal stability and anti-sintering ability. This is mainly due to the mesoporous hollow silica sphere support used in this invention. Its unique hollow-mesoporous structure provides a good loading space for metal oxides, and the functionalization of phosphate and amine groups achieves stable anchoring of the metal precursor, making the metal clusters highly dispersed and less prone to aggregation during catalysis. The ternary metal synergistic system (Fe–Cu–Ce) enhances the catalyst's antioxidant and structural recombination capabilities through redox cycles. In contrast, Comparative Example 1, lacking Ce co-catalysis, exhibited weak anti-sintering properties and rapid performance degradation; Comparative Example 2, without functionalization treatment, resulted in uneven metal particle distribution, severe sintering after high-temperature calcination, and significant loss of active sites.

[0073] 4. Catalyst regeneration efficiency test

[0074] After each catalyst sample had been used 10 times, it was removed and added to 50 mL of 3 vol% hydrogen peroxide (H2O2) solution. The mixture was then gently stirred in a 60 ℃ water bath for 2 h for in-situ oxidation regeneration. After the reaction, the sample was filtered to remove the liquid phase, washed three times with deionized water, and dried at 80 ℃ for 12 h. The resulting catalyst was then used again in a standard sludge oxidation experiment (300 ℃, 15 MPa, 60 min) to determine the COD removal rate and calculate the activity recovery rate after regeneration (regenerated activity / initial activity × 100%). The above regeneration process could be repeated for three rounds, and the performance data after each round of regeneration was recorded to evaluate the regeneration stability of the catalyst. The results are shown in Table 4.

[0075] Table 4 Catalyst regeneration efficiency (COD removal rate %)

[0076] sample Before regeneration (10th time) First round after regeneration Round 2 Round 3 Activity retention rate in round 3 (%) Example 1 90 91 90 89 96.7 Example 2 88 89 88 86 95.6 Example 3 93 94 93 92 96.8 Comparative Example 1 77 74 69 63 74.1 Comparative Example 2 69 66 60 52 65.0

[0077] As shown in Table 4, after three rounds of in-situ hydrogen peroxide oxidation regeneration treatment, the COD removal rate of Examples 1–3 almost recovered to the pre-use level. The activity retention rate after the third regeneration was over 95% in all examples, with Example 3 reaching the highest at 96.8%, indicating that the catalyst of this invention has excellent regeneration adaptability and structural stability. The phosphate and amine functionalized supports effectively anchor the metal oxides, preventing the migration, aggregation, and dissolution of the active metal components during use. Simultaneously, Ce in the ternary metal (Fe–Cu–Ce) synergistic system… 4 ⁺ / Ce³⁺ exhibits dynamic regulation of oxygen vacancy formation, which helps restore redox cycle activity during regeneration. In contrast, Comparative Example 1, lacking Ce cooperating sites, has poor oxidation cycle capacity and its activity continues to decline after regeneration; Comparative Example 2, without carrier functionalization, has uneven metal distribution and severe aggregation, making structural recovery difficult and exhibiting a clear trend of irreversible inactivation.

[0078] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0079] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A subcritical oxidation catalyst for sludge reduction, characterized in that, It contains the following raw materials in parts by weight: 80-100 parts of mesoporous hollow silica sphere carrier, 2-10 parts of (3-aminopropyl)triethoxysilane, 5-7 parts of ferric nitrate nonahydrate, 3-5 parts of copper nitrate trihydrate, and 0.5-1.5 parts of cerium nitrate hexahydrate; The subcritical oxidation catalyst has the following structural features: Phosphate and amino groups are grafted onto the surface of the mesoporous hollow silica sphere carrier to form a bifunctional layer; active sites composed of iron, copper, and cerium metal oxides are highly dispersed in the form of sub-nano clusters on the surface of the functionalized carrier.

2. The subcritical oxidation catalyst according to claim 1, characterized in that, The preparation method of the mesoporous hollow silicon sphere carrier specifically includes the following steps: S101. Add tetraethoxysilane, triethyl phosphate, ammonia, ethanol and deionized water to a high-pressure reactor and stir to mix all components thoroughly. S102. After sealing the reactor, heat it to 180 ℃, maintain the temperature at this temperature for the reaction, and after the reaction is completed, allow it to cool naturally to room temperature and release the pressure in the reactor. S103. Separate the solid phase from the reaction product by vacuum filtration, wash the solid phase three times each with ethanol and deionized water, and remove the surface residue by vacuum filtration. S104. The filter cake is placed in an oven to dry, the dried product is spread on a boat, placed in a tube furnace for calcination, and then cooled to room temperature to obtain the mesoporous hollow silicon sphere carrier.

3. The subcritical oxidation catalyst according to claim 1, characterized in that, The surface-grafted phosphate functionalized layer is prepared by immersing a dried mesoporous hollow silica sphere carrier in a phosphoric acid solution, allowing the hydroxyl groups on the carrier surface to react chemically with the phosphate groups. The carrier is then dried under mild conditions to remove residual solvent and calcined in air at high temperature to solidify and stabilize the phosphate groups.

4. The subcritical oxidation catalyst according to claim 1, characterized in that, The amine-functionalized layer on the carrier surface is formed by dispersing phosphate-functionalized mesoporous hollow silica spheres in anhydrous ethanol, adding (3-aminopropyl)triethoxysilane, reacting under reflux conditions, and washing and drying after the reaction is completed, thereby forming a stable amine-grafted layer.

5. The subcritical oxidation catalyst according to claim 1, characterized in that, The pore size of the mesoporous hollow silicon sphere carrier is 2–10 nm.

6. A method for preparing a subcritical oxidation catalyst for sludge reduction, wherein the subcritical oxidation catalyst for sludge reduction is as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Tetraethoxysilane, triethyl phosphate, ammonia, ethanol and water are mixed in a reaction vessel and subjected to hydrothermal reaction. After the reaction is completed, the mixture is washed and dried to obtain a mesoporous hollow silica sphere support. S2. The mesoporous hollow silica sphere carrier is immersed in an aqueous solution of phosphoric acid, then dried and calcined in air to fix the phosphate graft layer; S3. The support was then dispersed in anhydrous ethanol and (3-aminopropyl)triethoxysilane was added. The reaction was carried out under reflux, and the mixture was washed and dried to obtain the bifunctionalized support. S4. Dissolve the precursors of ferric nitrate nonahydrate, copper nitrate trihydrate, and cerium nitrate hexahydrate in deionized water, add the bifunctionalized carrier to the metal solution, and stir until evenly dispersed; S5. The mixture is placed in a microwave reactor for nucleation treatment, then transferred to a hydrothermal reactor. After the reaction is completed, the product is collected and preliminarily dried. The product is then placed in a drying oven to completely remove water, and finally calcined in a high-temperature furnace to obtain the subcritical oxidation catalyst.

7. The method for preparing the subcritical oxidation catalyst according to claim 6, characterized in that, Before step S5, the stearic acid-paraffin phase change material and the carrier are added to the metal solution mixture in a blending manner, so that the phase change material and the metal precursor are co-loaded in the subsequent microwave-hydrothermal and calcination processes.

8. The method for preparing the subcritical oxidation catalyst according to claim 6, characterized in that, After step S5, before the end of the calcination process, a carbon source is introduced in situ onto the catalyst surface to form a 1–3 wt% carbon shell layer through low-temperature thermal decomposition at 400–450 °C, thereby enhancing the catalyst's resistance to sintering and poisoning.

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