Ultra-small ceria sphere supported palladium catalyst, its continuous flow preparation method and application

Abstract

The application discloses a kind of ultra-small cerium oxide ball supported palladium catalyst and its high-pressure continuous flow preparation method and application, belong to nanometer catalysis technical field.The application uses high-pressure continuous flow reaction system, with wet basis cerium hydroxide precursor that is not dried calcination instead of finished product cerium oxide for loading, completely solve the industry pain point that cerium oxide is easy to settle and block, prepare about 5nm, the BET specific surface area of grape cluster-shaped cerium oxide carrier is not less than 130m² / g, realize 1~3nm palladium nanoparticles highly uniform dispersion.The obtained catalyst is as low as 400 DEG C in methane catalytic combustion T90, 1000h Continuous operation stability is excellent, process repeatability is good, easy to enlarge, and industrial application prospect is wide.

Description

Invention Field

[0001] This invention belongs to the field of nanocatalytic material preparation technology, specifically relating to a Pd / CeO2 nanocatalyst, and more particularly to a high-pressure continuous flow preparation method of a palladium catalyst supported on ultra-small cerium oxide spheres with uniform particle size, and the application of this catalyst in the catalytic combustion degradation of low-concentration methane. Background Technology

[0002] Methane is the second largest greenhouse gas after carbon dioxide, with a global warming potential approximately 28 to 36 times that of carbon dioxide on a centennial timescale. Human activities such as natural gas extraction leaks, coal mine gas emissions, and agricultural production release large amounts of methane into the atmosphere, further exacerbating global climate change. Catalytic oxidation of low-concentration methane, such as from ventilation gases, biogas, and unburned natural gas tail gas, into carbon dioxide and water is an important way to mitigate its greenhouse effect and achieve energy utilization. Currently, the treatment of low-concentration methane mainly employs two technologies: direct combustion and catalytic oxidation. Compared to direct combustion processes that require temperatures above 1000℃, catalytic oxidation can achieve complete methane conversion at 300–500℃, offering advantages such as low energy consumption, high efficiency, and good safety. Therefore, developing high-performance methane purification catalysts is key to advancing this technology.

[0003] Pd / CeO2 is a recognized high-efficiency catalyst system for the catalytic oxidation of methane. In existing technologies, Pd / CeO2 catalysts are mainly prepared using traditional batch processes, including impregnation, precipitation, and sol-gel methods. However, these traditional preparation methods suffer from the following insurmountable drawbacks: ① Difficulty in particle size control: Traditional batch reaction has low mass and heat transfer efficiency, uneven temperature and concentration distribution in the reaction system, and difficulty in accurately controlling the nanoscale size and distribution uniformity of the support and active components, resulting in poor batch stability of catalyst and low utilization of active sites. ② Poor carrier dispersibility: Cerium oxide (CeO2) has a high density and settles easily in aqueous phase. Maintaining long-term stable suspension is difficult, both during the stirring process in a batch reactor and during subsequent material transport. Extensive experiments have shown that conventional methods such as physical stirring, ultrasonic dispersion, and the addition of various polymeric suspending agents cannot fundamentally solve the sedimentation problem of cerium oxide. This ultimately leads to uneven metal loading, large fluctuations in product performance, and a high risk of clogging production equipment and pipelines.

[0004] The aforementioned defects severely restrict the large-scale production and industrial application of Pd / CeO2 catalysts. Therefore, it is urgent to develop a novel preparation method that can solve the above technical problems and achieve precise and controllable synthesis and stable batch production of Pd / CeO2 catalysts. Summary of the Invention

[0005] In view of the above-mentioned defects in the prior art, the object of the present invention is to achieve at least one of the following: To address the technical problems of easy sedimentation and uneven metal loading of cerium oxide supports, a catalyst preparation process is provided that can achieve stable suspension of the support without the addition of polymeric dispersants. A palladium catalyst supported on ultra-small cerium oxide spheres with uniform and controllable particle size and high specific surface area is provided, which gives it excellent low-temperature methane catalytic combustion activity and long-term stability. This invention provides a high-pressure continuous flow preparation method with good reproducibility and easy scale-up, enabling precise and controllable synthesis and industrial production of catalysts.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: One aspect of the present invention provides a palladium catalyst supported on ultrasmall cerium oxide spheres, comprising a cerium oxide support and an active component palladium supported on the surface of the cerium oxide support. The cerium oxide support is composed of ultrasmall cerium oxide nanospheres with a native particle size of approximately 5 nm and a particle size deviation of no more than ±1 nm; the ultrasmall cerium oxide nanospheres are prepared by a continuous flow reaction at 100-120 °C, and are characterized by a grape-like aggregate morphology by high-resolution transmission electron microscopy, with a BET specific surface area of ​​not less than 130 m² / g as measured by nitrogen adsorption; the active component palladium is highly uniformly dispersed on the surface of the cerium oxide support in the form of elemental nanoparticles with a particle size of 1-3 nm, and exhibits no characteristic diffraction peaks of metallic palladium by X-ray diffraction, indicating a strong interfacial interaction between palladium and the cerium oxide support; the catalyst achieves a T90 temperature of 400 °C when the methane conversion reaches 90% in the methane catalytic combustion reaction.

[0007] Furthermore, the loading of the active component palladium is 1% of the total mass of the catalyst.

[0008] Furthermore, in a 1000-hour continuous methane catalytic combustion reaction, the fluctuation range of the methane conversion T90 temperature of the catalyst does not exceed ±10℃.

[0009] Another aspect of the present invention provides a continuous flow preparation method for the above-mentioned ultra-small cerium oxide sphere-supported palladium catalyst, which is prepared using a high-pressure continuous flow reaction system. The high-pressure continuous flow reaction system includes a high-pressure injection pump, a stainless steel coil reactor immersed in an oil bath for precise temperature control, and a back pressure valve located at the outlet end of the stainless steel coil reactor for controlling the system pressure. The preparation method includes the following steps: S1 Continuous flow synthesis of cerium hydroxide precursor: A cerium source precursor solution, an alkaline solution, and a separating phase silicone oil are simultaneously injected into the stainless steel coil reactor using a high-pressure injection pump. The system pressure is controlled by the back pressure valve, and the reaction temperature is controlled at 100~120℃. The residence time of the material in the reactor is 8~13 min to complete the precipitation reaction. The reaction product is centrifuged and washed to obtain the cerium hydroxide precursor. S2 In-situ reduction of supported palladium without dispersant: The cerium hydroxide precursor obtained in step S1 is directly dispersed in water to prepare a stable suspension with a standing suspension time of not less than 1 h without the addition of any polymeric dispersant. The suspension and palladium source precursor solution are mixed evenly to obtain a mixed feed. The mixed feed and reducing agent solution are simultaneously injected into a continuous flow reactor using a high-pressure injection pump. The reaction temperature is controlled at 70~80℃, and the residence time of the material in the reactor is 8~15min to complete the liquid-phase reduction reaction, so that palladium nanoparticles are deposited in situ on the surface of the cerium hydroxide precursor to obtain a palladium-loaded cerium hydroxide precursor. S3 Calcination and conversion molding: The palladium-loaded cerium hydroxide precursor obtained in step S2 is centrifuged and washed, and then calcined in an air atmosphere to convert cerium hydroxide into cerium oxide, while fixing palladium nanoparticles, to obtain the ultra-small cerium oxide sphere-loaded palladium catalyst.

[0010] Further, in step S1, the cerium source precursor is cerium nitrate, and its aqueous solution concentration is 0.05M; the alkaline solution is sodium hydroxide aqueous solution with a concentration of 2M; the system pressure is controlled at 0.2MPa by a back pressure valve; the stainless steel coil reactor has an inner diameter of 1mm and a length of 5m.

[0011] Furthermore, the cerium hydroxide precursor used in step S2 to prepare the stable suspension is a wet-based cerium hydroxide solid that has not undergone drying or calcination after centrifugation and washing in step S1.

[0012] Further, in step S2, the palladium source precursor is palladium chloride, and its concentration in the mixed feed is 10 mM; the reducing agent is ascorbic acid aqueous solution, and its molar amount is 10 times the theoretical molar amount of palladium; the continuous flow reactor is a stainless steel coil reactor with an inner diameter of 1 mm and a length of 10 m, the reaction temperature is controlled at 75 °C, and the material residence time is 10 min.

[0013] Furthermore, in step S3, the calcination conditions are as follows: the temperature is increased to 500°C at a heating rate of 2°C / min, and then calcined for 3 hours.

[0014] In another aspect, the present invention provides the application of the above-mentioned ultra-small cerium oxide sphere supported palladium catalyst in the catalytic combustion degradation of low-concentration methane.

[0015] Furthermore, the volume fraction of the low-concentration methane is 0.5% to 5.0%, the space velocity of the catalytic combustion reaction is 10,000 to 50,000 mL・g⁻¹・h⁻¹, the reaction temperature is 250 to 400°C, and the reaction is carried out in an air atmosphere. Beneficial effects

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention effectively solves the technical problems of easy sedimentation of cerium oxide carrier and uneven metal loading. It creatively uses undried, uncalcined wet-based cerium hydroxide precursor to replace finished cerium oxide for loading. Cerium hydroxide has excellent aqueous suspension stability, showing no significant sedimentation after standing for more than 1 hour. The system remains stable without the addition of any polymeric dispersant, avoiding pipeline blockage and eliminating the risk of impurities introduced by the dispersant.

[0017] 2. High-pressure continuous flow technology was successfully applied to the large-scale preparation of Pd / CeO2 catalysts for the first time. Utilizing the excellent mass and heat transfer performance of the high-pressure continuous flow reaction system, grape-like CeO2 nanospheres with a particle size of approximately 5 nm and a particle size deviation of ≤±1 nm were successfully prepared through precise temperature control of 100~120℃, with a BET specific surface area ≥130 m² / g. At the same time, the Pd nanoparticle size was controlled within the range of 1~3 nm, with highly uniform distribution and no obvious agglomeration, significantly improving the active site density of the catalyst.

[0018] 3. The catalyst exhibits excellent catalytic performance and good stability. The in-situ reduction method enables a strong and close interfacial interaction between the Pd species and the CeO2 support. The resulting catalyst demonstrates excellent low-temperature activity in the catalytic degradation of methane, with a T90 temperature below 400℃. Furthermore, the T90 temperature fluctuation does not exceed ±10℃ during 1000h of continuous operation, indicating promising prospects for industrial applications.

[0019] 4. Good process repeatability and easy scale-up. The high-pressure continuous flow process parameters used in this invention have strong controllability and excellent batch stability, and can be directly scaled up for mass production, solving the problem of significant scale-up effects in traditional batch processes.

[0020] The above summary provides a simplified overview of some concepts, which will be further described in detail in the following specific embodiments. The above summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to help determine the scope of the claimed subject matter. The claimed subject matter is not limited to embodiments that address any or all the shortcomings pointed out in the background art.

[0021] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by means of embodiments thereof. Attached Figure Description

[0022] Figure 1 These are the X-ray diffraction patterns of the pure CeO2 support and Pd / CeO2 catalyst prepared in Example 1 of this invention; Figure 2 This is a high-resolution transmission electron microscope image of the Pd / CeO2 catalyst prepared in Example 1 of this invention; Figure 3 This is the methane catalytic combustion activity curve of the Pd / CeO2 catalyst prepared in Example 1 of this invention. Detailed Implementation

[0023] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0024] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. It should be further understood that, as used herein, the singular forms "a," "an," and "the" are intended to also include the plural forms, unless the context indicates otherwise. Furthermore, the terms "or," "and / or," "including at least one of the following," etc., as used herein, can be interpreted as inclusive, or mean any one or any combination thereof. Exceptions to this definition only arise when combinations of elements, functions, steps, or operations are inherently mutually exclusive in some manner.

[0025] It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit the scope of this application. Unless otherwise specified, the materials, reagents, etc., used in the following embodiments are commercially available. Experimental methods not specifically described in the embodiments are generally performed under conventional conditions or as recommended by the manufacturer.

[0026] The present invention will be further described below with reference to the embodiments and accompanying drawings. Example

[0027] The specific steps for preparing the palladium catalyst supported on ultrasmall cerium oxide spheres in this embodiment are as follows: A high-pressure continuous flow reaction system was constructed using a stainless steel coil reactor with an inner diameter of 1 mm and a length of 5 m. The reactor was immersed in an oil bath, and the reaction temperature was controlled at 110℃ by the oil bath. A back pressure valve was installed at the reactor outlet to control the system pressure at 0.2 MPa.

[0028] Three feed solutions were prepared: feed A was a 0.05M cerium nitrate (Ce(NO3)3) aqueous solution, feed B was a 2M sodium hydroxide (NaOH) aqueous solution, and feed C was silicone oil (as a separator phase to form a stable segmented flow pattern and enhance mixing). The three feed solutions were separately loaded into high-pressure injection pumps, and the flow rates of the three feeds were adjusted to ensure a residence time of 10 minutes for the materials in the reactor.

[0029] The high-pressure injection pump was turned on, allowing three streams of feed liquid to simultaneously enter the stainless steel coil reactor for precipitation reaction. The reactor effluent was collected, and the silicone oil phase was removed by centrifugation. The solid product was repeatedly washed with deionized water until neutral to obtain a wet-based cerium hydroxide precursor, which was neither dried nor calcined.

[0030] The above-mentioned wet-based cerium hydroxide precursor was directly dispersed in deionized water to prepare a suspension with a solid content of 5 g / L. After standing for 1 hour, no obvious sedimentation was observed. A 10 mM palladium chloride aqueous solution was added to the suspension, and the theoretical loading of Pd was controlled to be 1 wt% (based on the mass of the final CeO2 generated). The mixture was stirred evenly to obtain a mixed feed.

[0031] An aqueous solution of ascorbic acid was prepared as a reducing agent, with a molar amount 10 times that of the theoretical amount of Pd. The mixed feed and the reducing agent solution were separately loaded into high-pressure injection pumps and simultaneously injected into a stainless steel coil reactor with an inner diameter of 1 mm and a length of 10 m. The reaction temperature was controlled at 75 °C and the material residence time was 10 min to carry out the liquid-phase reduction reaction.

[0032] The reaction effluent was collected, centrifuged, and washed three times alternately with deionized water and ethanol. The resulting solid was placed in a muffle furnace and heated to 500°C at a rate of 2°C / min, held at that temperature for 3 hours, and then naturally cooled to room temperature to obtain the Pd / CeO2 nanocatalyst, denoted as sample S1. Example

[0033] The only difference between this embodiment and Example 1 is that in step 2, the reaction temperature of the stainless steel coil reactor is controlled at 100°C using an oil bath, and the feed flow rate is adjusted to ensure that the residence time of the material in the reactor is 13 minutes. All other steps and process parameters are the same as in Example 1, resulting in a Pd / CeO2 nanocatalyst, denoted as sample S2. Example

[0034] The only difference between this embodiment and Example 1 is that in step 2, the reaction temperature of the stainless steel coil reactor is controlled at 120°C using an oil bath, and the feed flow rate is adjusted to ensure that the residence time of the material in the reactor is 8 minutes. All other steps and process parameters are the same as in Example 1, resulting in a Pd / CeO2 nanocatalyst, denoted as sample S3.

[0035] Comparative Example 1 This comparative example uses a finished cerium oxide support for continuous flow loading. The specific steps are as follows: The wet-based cerium hydroxide precursor was prepared according to steps 1-3 of Example 1. The precursor was placed in a muffle furnace and calcined at 500°C for 2 hours at a heating rate of 2°C / min to obtain the finished CeO2 powder.

[0036] The CeO2 powder was dispersed in deionized water to prepare a suspension with a solid content of 5 g / L. The remaining steps were performed according to steps 4-6 of Example 1. Results: The CeO2 powder settled rapidly in the feed line and reactor. Significant blockage occurred in the pipeline after 10 minutes of reaction, making continuous flow experiments impossible. Characterization of the collected small amount of product showed that the Pd particles were extremely unevenly distributed and contained a large number of agglomerates.

[0037] Comparative Example 2 This comparative example uses the traditional impregnation method to prepare Pd / CeO2 catalysts. The specific steps are as follows: CeO2 powder was prepared according to step 1 of Comparative Example 1.

[0038] The CeO2 powder was soaked in a 10 mM palladium chloride solution and stirred for 12 h. Ascorbic acid was added for reduction. After washing and drying, the powder was placed in a muffle furnace and calcined at 500 °C for 3 h at a heating rate of 2 °C / min to obtain the Pd / CeO2 catalyst, which was designated as control sample D1.

[0039] Characterization and comparative analysis: This experimental example performs structural characterization and catalytic performance testing on the catalysts prepared in the above examples and comparative examples.

[0040] Structural characterization was performed using an X-ray diffractometer (Brook D8 Advance type) for phase analysis of the sample, with CuKα radiation, a scanning range of 2θ = 10°~80°, and a scanning rate of 5° / min.

[0041] The morphology and particle size of the samples were characterized using a high-resolution transmission electron microscope (JEOLJEM-2100F) with an accelerating voltage of 200 kV.

[0042] The specific surface area of ​​the samples was tested using a specific surface area and pore size analyzer (McASAP2460). The samples were degassed at 200℃ for 2 hours before the test.

[0043] The characterization results show that: Figure 1 As shown, the sample prepared in Example 1 exhibited characteristic diffraction peaks of cubic fluorite CeO2 at 2θ = 28.5°, 33.1°, 47.5°, and 56.3°, corresponding to the (111), (200), (220), and (311) crystal planes of cubic fluorite CeO2 (JCPDS card number: 34-0394), respectively. This indicates that the CeO2 prepared by this continuous flow process has good crystallinity and is a single phase. In the XRD pattern of the Pd / CeO2 sample, no characteristic diffraction peaks of palladium were observed. This indicates that the palladium species are highly dispersed on the CeO2 support surface, with a grain size smaller than the XRD detection limit (typically <5 nm), and no large-sized independent metallic phase has formed.

[0044] The Pd / CeO2 catalyst prepared in Example 1 was characterized by HRTEM, and the results are as follows: Figure 2 As shown in the HRTEM image, the CeO2 support consists of nanospheres with a diameter of approximately 5 nm, exhibiting a grape-like aggregate structure. This structure ensures both a small size effect and a certain degree of mesoporous structure, which is beneficial for mass transfer during the catalytic reaction. The Pd nanoparticles have a size of 1–3 nm and are uniformly distributed on the CeO2 surface.

[0045] The BET specific surface area test results are as follows: sample S1 is 136 m² / g, sample S2 is 142 m² / g, and sample S3 is 135 m² / g, all of which meet the requirement of not less than 130 m² / g.

[0046] The catalytic performance of the samples was evaluated using a fixed-bed reactor. The catalyst dosage was 0.1 g, the reaction gas composition was 0.5 vol% CH4 + 20 vol% O2 + 79.5 vol% N2, the space velocity (GHSV) was 50000 mL・g⁻¹・h⁻¹, and the reaction temperature range was 200–500 °C. The gas composition before and after the reaction was analyzed online using gas chromatography (Agilent 7890A). Simultaneously, a 1000-hour long-term stability test was conducted on the catalyst to examine the change in T90 temperature. The test results are shown in the table below: sheet:

[0047] Note: T10, T50, and T90 are the reaction temperatures at which the methane conversion rate reaches 10%, 50%, and 90%, respectively.

[0048] As shown in the table above, the catalyst prepared in the embodiments of the present invention exhibits significantly better low-temperature catalytic activity than the comparative sample prepared by the traditional impregnation method, and also demonstrates excellent long-term stability. The performance differences in Examples 1-3 are within an acceptable range, proving that the process parameter range of the present invention has good applicability. The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0049] In this application, the same or similar terms, concepts, technical solutions and / or application scenario descriptions are generally described in detail only when they appear for the first time. When they appear again, they are generally not repeated for the sake of brevity. When understanding the technical solutions and other contents of this application, the same or similar terms, concepts, technical solutions and / or application scenario descriptions that are not described in detail later can be referred to their previous relevant detailed descriptions.

[0050] In this application, the descriptions of the various embodiments have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0051] The technical features of the present application can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of the present application.

[0052] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A palladium catalyst supported on ultra-small cerium oxide spheres, characterized in that, The catalyst comprises a cerium oxide support and an active component palladium loaded on the surface of the cerium oxide support. The cerium oxide support is composed of ultrasmall cerium oxide nanospheres with a native particle size of approximately 5 nm and a particle size deviation of no more than ±1 nm. The ultrasmall cerium oxide nanospheres exhibit a grape-like aggregate morphology, and the BET specific surface area, as measured by nitrogen adsorption, is not less than 130 m² / g. The active component palladium is highly uniformly dispersed on the surface of the cerium oxide support in the form of elemental nanoparticles with a particle size of 1-3 nm. The catalyst exhibits no characteristic diffraction peaks of metallic palladium as characterized by X-ray diffraction, and a strong interfacial interaction is formed between the active component palladium and the cerium oxide support. In the catalytic combustion reaction of methane, the catalyst achieves a methane conversion rate of 90% at a T90 temperature of less than or equal to 400 °C.

2. The palladium catalyst supported on ultra-small cerium oxide spheres according to claim 1, characterized in that, The loading of the active component palladium is 1% of the total mass of the catalyst.

3. The palladium catalyst supported on ultra-small cerium oxide spheres according to claim 1, characterized in that, In a 1000-hour continuous methane catalytic combustion reaction, the T90 temperature fluctuation of the catalyst when the methane conversion rate reaches 90% does not exceed ±10℃.

4. A continuous flow preparation method for an ultra-small cerium oxide sphere-supported palladium catalyst according to any one of claims 1 to 3, characterized in that, The catalyst is prepared using a high-pressure continuous flow reaction system, which includes a high-pressure injection pump, a stainless steel coil reactor immersed in an oil bath for precise temperature control, and a back pressure valve located at the outlet of the stainless steel coil reactor to control the system pressure. The preparation method includes the following steps: S1 Continuous Flow Synthesis of Cerium Hydroxide Precursor: A cerium source precursor solution, an alkaline solution, and a separating phase silicone oil are simultaneously injected into the stainless steel coil reactor using a high-pressure injection pump. The back pressure valve controls the system pressure at 0.1~1MPa, the reaction temperature at 100~120℃, and the residence time of the material in the reactor at 8~13min to complete the precipitation reaction. The reaction product is centrifuged and washed to obtain the cerium hydroxide precursor. S2 In-situ Reduction of Palladium-Loaded Precursor without Dispersant: The cerium hydroxide precursor obtained in step S1 is directly dispersed in water to prepare a stable suspension with a standing suspension time of not less than 1 hour without the addition of any polymeric dispersant; the suspension is mixed evenly with the palladium source precursor solution to obtain a mixed feed; the mixed feed and the reducing agent solution are simultaneously injected into a continuous flow reactor using a high-pressure injection pump; the reaction temperature is controlled at 70~80℃ and the residence time of the material in the reactor is 8~15 minutes to complete the liquid phase reduction reaction, so that palladium nanoparticles are deposited in situ on the surface of the cerium hydroxide precursor to obtain a palladium-loaded cerium hydroxide precursor; S3 Calcination and Conversion: The palladium-loaded cerium hydroxide precursor obtained in step S2 is centrifuged, washed, and then calcined in air to convert cerium hydroxide into cerium oxide, while simultaneously fixing palladium nanoparticles, to obtain the ultra-small cerium oxide sphere-loaded palladium catalyst.

5. The preparation method according to claim 4, characterized in that, In step S1, the cerium source precursor is cerium nitrate, and its aqueous solution has a concentration of 0.05M; the alkaline solution is sodium hydroxide aqueous solution with a concentration of 2M; the system pressure is controlled at 0.2MPa by a back pressure valve; the stainless steel coil reactor has an inner diameter of 1mm and a length of 5m.

6. The preparation method according to claim 4, characterized in that, The cerium hydroxide precursor used in step S2 to prepare the stable suspension is a wet-based cerium hydroxide solid that has not been dried or calcined after centrifugation and washing in step S1.

7. The preparation method according to claim 4, characterized in that, In step S2, the palladium source precursor is palladium chloride, and its concentration in the mixed feed is 10 mM; the reducing agent is ascorbic acid aqueous solution, and its molar amount is 10 times the theoretical molar amount of palladium; the continuous flow reactor is a stainless steel coil reactor with an inner diameter of 1 mm and a length of 10 m, the reaction temperature is controlled at 75 °C, and the material residence time is 10 min.

8. The preparation method according to claim 4, characterized in that, In step S3, the calcination conditions are as follows: the temperature is increased to 500°C at a heating rate of 2°C / min, and then calcined for 3 hours.

9. The application of the palladium catalyst supported on ultra-small cerium oxide spheres according to any one of claims 1 to 3 in the catalytic combustion degradation of low-concentration methane.

10. The application according to claim 9, characterized in that, The low concentration of methane has a volume fraction of 0.5% to 5.0%, the space velocity of the catalytic combustion reaction is 10,000 to 50,000 mL・g⁻¹・h⁻¹, the reaction temperature is 250 to 400℃, and the reaction is carried out in an air atmosphere.

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