A method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion

Abstract

This invention belongs to the field of rare earth nanomaterial preparation technology, specifically relating to a method for preparing cerium dioxide through incomplete precipitation coupled with hydrothermal conversion, comprising the following steps: Step 1: Mixing a cerium source and a precipitant at a molar ratio of 1:0.1 to 1:2 and allowing incomplete precipitation to occur, resulting in a mixed system; Step 2: Performing hydrothermal treatment on the mixed system formed in Step 1, and post-processing the reaction product to obtain cerium dioxide. Compared with existing technologies, this invention solves the problem that the preparation conditions for cerium dioxide in existing technologies are stringent, such as the need for independent calcination for precipitation methods or the use of high-concentration alkaline solutions, high temperatures, or long processes for hydrothermal treatment methods. The cerium dioxide preparation method of this scheme achieves the preparation of cerium dioxide with high crystallinity and controllable morphology through a simple process, mild conditions, and low cost.

Description

Technical Field

[0001] This invention belongs to the field of rare earth nanomaterial preparation technology, specifically relating to a method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion. Background Technology

[0002] Cerium dioxide (CeO2), as an important rare earth functional material, possesses unique oxygen vacancy defects and CeO2. 3+ / Ce 4+ Its reversible conversion properties have wide applications in chemical mechanical polishing, automotive exhaust purification, solid fuel cells, and ultraviolet shielding. Particularly in chemical mechanical polishing, cerium dioxide abrasives, due to their moderate hardness and excellent chemical activity, have become a key functional material for achieving efficient, low-damage polishing.

[0003] Currently, the main methods for preparing cerium dioxide include precipitation, hydrothermal, and sol-gel methods. Among them, precipitation is widely used in industrial production due to its simple process and low cost. However, traditional precipitation methods usually use a molar ratio of precipitant to cerium source greater than 3:1 to completely precipitate cerium carbonate or cerium hydroxide precursors, which are then calcined at high temperatures (usually above 500℃) to obtain cerium dioxide products (such as CN121449098A, CN121674030A, CN119059549A, CN120589777A, CN118289796A, etc.). This method has the following shortcomings: (1) large amount of precipitant is used, resulting in high cost; (2) a separate calcination step is required, resulting in a long process flow and high energy consumption; (3) particle agglomeration is prone to occur during calcination, affecting the product's dispersibility and crystallinity.

[0004] Hydrothermal synthesis, as an advanced wet chemical synthesis method, can create a high-temperature and high-pressure environment in a closed container, effectively promoting crystal growth and morphology control, and has been widely used in the field of nanomaterial preparation. However, existing hydrothermal methods for preparing cerium dioxide generally suffer from the following problems: First, they rely on template agents or surfactants. For example, CN120964869A uses cationic or nonionic surfactants as structure-directing agents. These organic additives not only increase costs but may also introduce impurities into the product, affecting its performance in high-end applications. Second, they require high-concentration alkaline solutions or harsh conditions. For example, CN107746069A uses NaOH concentrations as high as 10 mol / L, and CN101641289A has reaction temperatures as high as 120-300℃, requiring high equipment corrosion resistance, posing safety hazards, and hindering industrial-scale production. Third, some hydrothermal methods require subsequent calcination processes. For example, CN120964869A, CN121249324A, and CN115974127A all require calcination at 500-900℃, which prevents the advantages of the hydrothermal method in simplifying the process from being fully realized. Fourth, the process is complex or time-consuming. For example, although CN110127746B does not contain surfactants, it requires the injection of raw materials under specific conditions and the reaction time is as long as 24-48 hours, which is difficult to meet the needs of industrial production.

[0005] Therefore, developing a simple, low-cost method for preparing cerium dioxide that does not require independent calcination and can achieve high crystallinity and controllable morphology is of great significance for promoting the application of cerium dioxide materials in high-end fields. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion, thereby addressing at least one of the aforementioned problems. This method overcomes the limitations of existing cerium dioxide preparation techniques, which require stringent conditions such as separate calcination for precipitation or high-concentration alkaline solutions, high temperatures, or lengthy processes for hydrothermal treatment. The cerium dioxide preparation method described in this invention achieves a simple process, milder conditions, lower cost, and controllable morphology for the preparation of cerium dioxide with high crystallinity.

[0007] The objective of this invention is achieved through the following technical solution: A method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion includes the following steps: Step 1: Mix the cerium source and the precipitant at a molar ratio of 1:0.1 to 1:2 and allow incomplete precipitation to occur, resulting in a mixed system; Step 2: Perform hydrothermal treatment on the mixed system formed in Step 1, and obtain cerium dioxide by post-treatment of the reaction product.

[0008] Preferably, in step 1, The cerium source is a soluble cerium salt. The precipitant is a soluble carbonate or bicarbonate.

[0009] Preferably, in step 1, The cerium source is one or more of cerium nitrate, cerium chloride, and cerium acetate. The precipitant is one or more of sodium carbonate, sodium bicarbonate, ammonium carbonate, and ammonium bicarbonate.

[0010] Preferably, in step 1, The molar ratio of the cerium source to the precipitant is from 1:0.5 to 1:1.5; The mixture system is a solid-liquid heterogeneous mixture system.

[0011] Preferably, in step 1, The molar ratio of the cerium source to the precipitant is 1:1.

[0012] Preferably, in step 2, The hydrothermal treatment uses octadehydrated cerium carbonate, which is generated by incomplete precipitation, as a precursor for the reaction. The hydrothermal treatment temperature is 120 to 220°C. The hydrothermal treatment lasts for 6 to 24 hours.

[0013] Preferably, in step 2, The hydrothermal treatment temperature is 160 to 200°C. The hydrothermal treatment time is 10 to 15 hours.

[0014] Preferred, The cerium dioxide has a polyhedral morphology. The average particle size of the cerium dioxide is 200 to 500 nm.

[0015] Preferably, in step 1, a dispersant is also added during the mixing of the cerium source and the precipitant. The dispersant is one or more of the following: nonionic surfactants, anionic surfactants, organic acid surfactants, and polymers.

[0016] Preferably, when adding a dispersant: The cerium dioxide has a spherical or near-spherical morphology. The average particle size of the cerium dioxide is 100 to 250 nm.

[0017] The working principle of this invention is as follows: The core technical point of this scheme is the use of a low molar ratio (cerium source: precipitant = 1:0.1 to 1:2), and its principle is mainly based on the following two aspects: On the one hand, by controlling the insufficient amount of precipitant, only some cerium ions are precipitated in the form of cerium carbonate octahydrate (Ce2(CO3)3·8H2O). This precursor can be decomposed in situ under hydrothermal conditions and directly converted into cerium dioxide.

[0018] On the other hand, during the hydrothermal process, when cerium carbonate octahydrate decomposes into cerium dioxide, it releases gases (such as CO2). These gases provide "endogenous oxygen" for the cerium ions remaining in the system. Subsequently, under hydrothermal conditions, the remaining cerium ions can be further converted into cerium dioxide through this endogenous oxidation mechanism.

[0019] Based on the above synergistic conversion mechanism, the overall utilization rate of cerium source is significantly improved, and the final actual output can exceed the cerium dioxide output obtained by using the same molar ratio of cerium source and precipitant through precipitation-calcination process.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. The process flow is significantly simplified. This invention integrates the traditional two-step process of "complete precipitation + independent calcination" into a one-step process of "incomplete precipitation + hydrothermal treatment." The solid product obtained after hydrothermal treatment does not require a separate calcination step and can directly yield cerium dioxide. Compared to the traditional precipitation-calcination route, the process flow of this invention is greatly simplified and energy consumption is significantly reduced.

[0021] 2. Reduce costs and improve raw material utilization. This invention employs a low molar ratio (1:0.1 to 1:2) to reduce the amount of precipitant used by more than 50% compared to traditional methods (preferably at a 1:1 ratio, the amount used is only 1 / 3 of that used in traditional precipitation methods), significantly reducing raw material costs. At the same time, it utilizes the "in-situ enhanced conversion" mechanism in a hydrothermal environment to convert residual cerium ions into products, achieving efficient and full utilization of the cerium source.

[0022] 3. The product has high crystallinity and uniform morphology. The preparation method proposed in this invention avoids the problems of particle agglomeration and abnormal grain growth commonly encountered in traditional calcination processes, resulting in cerium dioxide products with high crystallinity and uniform morphology. XRD analysis shows that the product has a typical cubic fluorite structure with sharp diffraction peaks and no impurity phase peaks. Compared with direct hydrothermal methods (such as CN107746069A) or methods requiring subsequent calcination, the product of this invention exhibits better crystal integrity.

[0023] 4. Controllable morphology and particle size This invention achieves precise control over the morphology and particle size of cerium dioxide: without the addition of a dispersant, polyhedral cerium dioxide with uniform morphology and an average particle size of 200-500 nm can be obtained; while with the addition of a dispersant, spherical or near-spherical cerium dioxide with an average particle size of 100-250 nm can also be obtained. This ability to control morphology through the simple addition of a dispersant allows the preparation method proposed in this invention to meet the differentiated requirements for cerium dioxide morphology and particle size in different application fields (such as polishing, catalysis, optics, etc.).

[0024] In summary, this invention provides a simple, low-cost, high-quality, and controllable morphology method for preparing cerium dioxide, which has significant industrial application value and promising prospects for promotion. Attached Figure Description

[0025] Figure 1 The images show the XRD patterns of the sample materials from Examples 1-3 and Comparative Examples 1-3 after mixing and reaction.

[0026] Figure 2 The images show the XRD patterns of the products after hydrothermal reaction of the samples from Examples 1-3 and Comparative Examples 1-2.

[0027] Figure 3 The image shows the XRD pattern of the sample from Comparative Example 3 after aging.

[0028] Figure 4 The images show the XRD patterns of the final products of Examples 1-3 and Comparative Examples 1-2, and the XRD pattern of the final product of Comparative Example 3.

[0029] Figure 5 SEM images of the final products of Examples 1-3 and Comparative Examples 1-3.

[0030] Figure 6 The images are SEM images of the final products of Examples 4-7. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below with reference to embodiments. The described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0032] Unless otherwise specified in the following description, the reagents used are conventional commercial products, the methods used are well-known in the art, and any other matters not covered herein may be handled using existing technologies.

[0033] This invention provides a method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion, comprising the following steps: mixing a cerium source with a precipitant at a molar ratio of 1:0.1 to 1:2 to obtain a mixed system; subjecting the mixed system to hydrothermal treatment; and collecting the solid product obtained after hydrothermal treatment to obtain the cerium dioxide.

[0034] Furthermore, the cerium source is a soluble cerium salt, preferably one or more of cerium nitrate, cerium chloride, and cerium acetate; the precipitant is a soluble carbonate or bicarbonate, preferably one or more of sodium carbonate, sodium bicarbonate, ammonium carbonate, and ammonium bicarbonate.

[0035] Furthermore, the molar ratio of the cerium source to the precipitant is 1:0.5 to 1:1.5; preferably, the molar ratio is 1:1.

[0036] Furthermore, the precursor generated by mixing the cerium source with the precipitant is cerium carbonate octahydrate (Ce2(CO3)3·8H2O).

[0037] Furthermore, the hydrothermal treatment temperature is 120 to 220°C, and the time is 6 to 24 hours; preferably, the hydrothermal treatment temperature is 160 to 200°C, and the time is 10 to 15 hours.

[0038] Furthermore, the solid product obtained after hydrothermal treatment does not require a separate calcination step, and the yield of cerium dioxide obtained is greater than that obtained by using the same molar ratio of cerium source and precipitant through a precipitation-calcination process.

[0039] In this invention, when cerium salt is mixed with the precipitant, due to insufficient precipitant, only a portion of the Ce is precipitated. 3+ A precipitation reaction occurs to form Ce2(CO3)3·8H2O microcrystalline precursor, while a large amount of Ce... 3+ The residue remains in a free state, forming a heterogeneous solid-liquid mixture. Under a hydrothermal high-temperature and high-pressure environment of 120-220℃, Ce₂(CO₃)₃·8H₂O undergoes thermal decomposition, transforming into cubic fluorite CeO₂. The reaction equation is: 2Ce₂(CO₃)₃·8H₂O + O₂ → 4CeO₂ + 6CO₂ + 16H₂O. The CO₂ produced during decomposition provides endogenous oxygen, promoting the growth of CeO₂. 3+ To Ce 4+ The conversion process improves the utilization rate of cerium. When the molar ratio of sodium bicarbonate to cerium salt is greater than 2:1, which exceeds the molar ratio limit of this invention (greater than 2:1), the pH of the system gradually increases with the increase of the molar ratio. The precipitation reaction generates cerium carbonate octahydrate, while the hydrothermal process generates basic cerium carbonate and a small amount of cerium dioxide, which need to be calcined to be completely converted into cerium dioxide.

[0040] Furthermore, a dispersant is added during the step of mixing the cerium source with the precipitant.

[0041] Furthermore, the dispersant is one or more of the following: nonionic surfactants, anionic surfactants, organic acid surfactants, and polymers.

[0042] Furthermore, when no dispersant is added, the cerium dioxide has a polyhedral morphology with an average particle size of 200 to 500 nm.

[0043] Furthermore, when a dispersant is added, the cerium dioxide has a spherical or near-spherical morphology with an average particle size of 100 to 250 nm.

[0044] That is, when no dispersant is added, under the preferred conditions of the present invention (especially a molar ratio of 1:1), polyhedral cerium dioxide with uniform morphology and good dispersion can be obtained with an average particle size of 200-500 nm. When a dispersant is added in the mixing step, the dispersant molecules selectively adsorb onto specific crystal faces, thereby regulating the crystal growth habit and transforming the product morphology into spherical or near-spherical shapes, reducing the average particle size to 100-250 nm.

[0045] The present invention provides a method for preparing cerium dioxide through incomplete precipitation coupled with hydrothermal conversion, which overcomes the defects of the prior art and has the following advantages: 1. Simplify the process and reduce raw material costs: Overcome the two-step process of "complete precipitation + independent calcination" in the traditional precipitation method, integrate the generation and conversion of precursors into a single hydrothermal step, eliminating the independent calcination step; at the same time, adopt low molar ratio conditions with insufficient precipitant, significantly reduce the amount of precipitant used, and greatly reduce production costs.

[0046] 2. Improve product quality and achieve controllable morphology: Avoid particle agglomeration and abnormal grain growth problems that are prone to occur during calcination, and obtain cerium dioxide products with high crystallinity and uniform morphology; and the morphology of cerium dioxide can be precisely controlled from polyhedral to spherical by simply adding dispersants, so as to meet the differentiated needs of different application fields.

[0047] 3. Improve cerium source utilization: Utilize the "in-situ enhanced conversion" mechanism in the hydrothermal environment to efficiently convert cerium ions that should remain in the solution under low molar ratio conditions into products, breaking through the stoichiometric limit and significantly improving the utilization rate of cerium source and the actual product yield.

[0048] Example 1 This embodiment specifically provides a method for preparing cerium dioxide: Weigh 17.37 g (0.04 mol) of cerium nitrate hexahydrate (Ce(NO3)3·6H2O), dissolve it in 200 mL of deionized water, and stir until completely dissolved to obtain a cerium source solution. Weigh 3.36 g (0.04 mol) of sodium bicarbonate (NaHCO3), dissolve it in 200 mL of deionized water, and stir until completely dissolved to obtain a precipitant solution. Achieve a 1:1 molar ratio of materials. Transfer the two solutions to a 500 mL beaker and mix. Disperse the mixture ultrasonically at room temperature and stir for 5 minutes to obtain a mixed system. Transfer the mixed system to a 500 mL high-pressure reactor lined with polytetrafluoroethylene (PTFE), seal it, and place it in an oven. React at 180℃ for 12 hours. After the reaction, allow it to cool naturally to room temperature. Filter the reaction product and collect the precipitate. Wash the precipitate three times alternately with deionized water and anhydrous ethanol. Dry the washed product in an oven at 60℃ for 24 hours to obtain cerium dioxide, denoted as sample S1. The mass of sample S1 was measured to be 2.99 g.

[0049] X-ray diffraction (XRD) analysis was performed on the hydrothermal precursor (i.e., the precipitate before hydrothermal treatment) and the post-hydrothermal product of sample S1, as follows: Figure 1 , 2 As shown in Figure 4. The results show that the XRD pattern of the hydrothermal precursor is consistent with the standard card of cerium carbonate octahydrate, indicating that the product formed in the chemical precipitation stage is cerium carbonate octahydrate. The XRD pattern of the product after hydrothermal treatment is consistent with that of cubic fluorite cerium dioxide (JCPDS No. 34-0394 standard card), with sharp diffraction peaks and no obvious impurity peaks, indicating that the product is pure phase cerium dioxide with high crystallinity.

[0050] The morphology of sample S1 was observed using scanning electron microscopy (SEM), such as... Figure 5 As shown, the results indicate that sample S1 consists of polyhedral particles with uniform morphology, good particle dispersion, and no obvious agglomeration. Particle size statistical analysis shows that the average particle size is 300-400 nm.

[0051] Example 2 Following the same preparation steps as in Example 1, 17.37 g of cerium nitrate hexahydrate and 6.72 g of sodium bicarbonate were weighed, and a cerium dioxide sample was prepared with a molar ratio of cerium source to precipitant of 1:2. This sample was denoted as sample S2 and weighed 5.25 g.

[0052] Under a molar ratio of 1:2, the theoretical yield of complete precipitant reaction is 4.59 g, while the actual yield of this invention reaches 5.25 g, indicating that residual cerium ions during the hydrothermal process are further converted into cerium dioxide, with a cerium source utilization rate of 76.25%, which is 14.4% higher than the theoretical limit imposed by the precipitant. This result is consistent with the increasing trend under a molar ratio of 1:1, confirming the universality of the "incomplete precipitation coupled with hydrothermal enhanced conversion" mechanism of this invention.

[0053] Sample S2 was characterized by XRD and SEM, such as Figure 1 , 2 The crystallinity, yield, and other data of samples S1 and S2 are listed in Table 1, 4, 5, and Table 1. The comparison results of Examples 1 and 2 show that the cerium dioxide prepared at a molar ratio of 1:1 (S1) has the highest crystallinity, the most uniform morphology, and the best dispersibility, which is the preferred embodiment of the present invention.

[0054] Example 3 Weigh 17.37 g (0.04 mol) of cerium nitrate hexahydrate and dissolve it in 200 mL of deionized water, stirring until completely dissolved to obtain a cerium source solution. Weigh 0.5 g of polyvinylpyrrolidone (PVP, K30, nonionic surfactant) as a structure-directing agent and dispersant, add it to the above cerium source solution, and stir until completely dissolved. Weigh 3.36 g (0.04 mol) of sodium bicarbonate (NaHCO3) and dissolve it in 200 mL of deionized water to achieve a 1:1 molar ratio of materials. Transfer the two solutions to a 500 mL beaker and mix them. Sonicate the mixture at room temperature and stir for 5 minutes to obtain a mixed system. Transfer the mixed system to a 500 mL high-pressure reactor lined with polytetrafluoroethylene, seal it, and place it in an oven at 180 °C for 12 hours. After the reaction, allow it to cool naturally to room temperature. Filter the reaction product and collect the precipitate. Wash the precipitate three times alternately with deionized water and anhydrous ethanol to remove residual PVP. The washed product was dried in an oven at 60°C for 24 hours to obtain cerium dioxide, which was designated as sample S3. The mass of sample S3 was measured to be 2.75 g.

[0055] from Figure 5 As can be seen from the data, sample S3 consists of nanospheres with a particle size of about 100 nanometers, proving that the morphology and particle size can be effectively controlled using the invented process.

[0056] Example 4 Weigh 17.37 g (0.04 mol) of cerium nitrate hexahydrate and dissolve it in 200 mL of deionized water. Stir until completely dissolved to obtain a cerium source solution. Weigh 0.5 g of citric acid (an organic acid dispersant) as a structure-directing agent and dispersant, add it to the above cerium source solution, and stir until completely dissolved. Weigh 3.36 g (0.04 mol) of sodium bicarbonate (NaHCO3) and dissolve it in 200 mL of deionized water. Achieve a material molar concentration ratio of 1:1. Following the same preparation steps as in Example 3, 2.65 g of cerium dioxide product was obtained, denoted as sample S4. Its relevant parameters are listed in Table 2.

[0057] from Figure 6 As can be seen from sample S4, sample S4 consists of nano-spherical particles with a particle size of about 160 nanometers, proving that the morphology and particle size can be effectively controlled by the invented process.

[0058] Example 5 Weigh 17.37 g (0.04 mol) of cerium nitrate hexahydrate and dissolve it in 200 mL of deionized water. Stir until completely dissolved to obtain a cerium source solution. Weigh 1.68 g (0.02 mol) of sodium bicarbonate (NaHCO3) and dissolve it in 200 mL of deionized water. The molar concentration ratio of the materials is 1:0.5. Following the same preparation steps as in Example 1, 1.55 g of cerium dioxide product was obtained, denoted as sample S5. Its relevant parameters are listed in Table 2.

[0059] from Figure 6 As can be seen from sample S5, sample S5 consists of nano-spherical particles with a particle size of about 150 nanometers, proving that the morphology and particle size can be effectively controlled by the invented process.

[0060] Example 6 Weigh 17.37 g (0.04 mol) of cerium nitrate hexahydrate and dissolve it in 200 mL of deionized water, stirring until completely dissolved to obtain a cerium source solution. Weigh 3.36 g (0.04 mol) of sodium bicarbonate (NaHCO3) and dissolve it in 200 mL of deionized water to achieve a molar ratio of 1:1. Following the same preparation steps as in Example 1, but changing the reaction time, a reaction time of 6 hours yielded 2.05 g of cerium dioxide product, designated as sample S6-1; following the same preparation steps as in Example 1, but changing the reaction time, a reaction time of 24 hours yielded 3.15 g of cerium dioxide product, designated as sample S6-2. Relevant parameters are listed in Table 2.

[0061] from Figure 6 As can be seen from S6-1 and S6-2, the sample consists of nano-polyhedral particles with a particle size between 250 and 350 nanometers, proving that by using the invented process, the particle size and yield can be effectively controlled by adjusting the hydrothermal duration.

[0062] Example 7 Weigh 17.37 g (0.04 mol) of cerium nitrate hexahydrate and dissolve it in 200 mL of deionized water, stirring until completely dissolved to obtain a cerium source solution. Weigh 3.36 g (0.04 mol) of sodium bicarbonate (NaHCO3) and dissolve it in 200 mL of deionized water to achieve a molar ratio of 1:1. Following the same preparation steps as in Example 1, but changing the hydrothermal temperature conditions, 1.95 g of cerium dioxide product was obtained at 120°C, denoted as sample S7-1; following the same preparation steps as in Example 1, but changing the hydrothermal temperature conditions, 3.06 g of cerium dioxide product was obtained at 220°C, denoted as sample S7-2. The relevant parameters are listed in Table 2.

[0063] from Figure 6 As can be seen from S7-1 and S7-2, the sample consists of nano-polyhedral particles with a particle size between 250 and 400 nanometers, proving that the particle size and yield can be effectively controlled by adjusting the hydrothermal temperature using the invented process.

[0064] Example 8 Without changing the hydrothermal duration, hydrothermal temperature, or achieving a 1:1 molar concentration ratio of materials, following the same preparation steps as in Example 1, but changing the precipitant to sodium carbonate, ammonium carbonate, and ammonium bicarbonate, cerium dioxide products were obtained and designated as samples S8-1, S8-2, and S8-3, respectively. Following the same preparation steps as in Example 1, but changing the cerium source to cerium chloride and cerium acetate, cerium dioxide products were obtained and designated as samples S8-4 and S8-5, respectively. The relevant parameters are listed in Table 3.

[0065] Example 8 demonstrates that cerium sources and precipitants within the scope of the preparation method required by this invention can achieve the effects of this invention.

[0066] Comparative Example 1 Following the same preparation steps as in Example 1, and with a cerium source to precipitant molar ratio of 1:3, the phase after the hydrothermal reaction was a mixture of basic cerium carbonate and cerium dioxide. (See...) Figure 2 Sample D1 requires further calcination at 600℃ for 4 hours, followed by natural cooling to obtain cerium dioxide, denoted as sample D1. Its relevant parameters are listed in Table 1. The mass of sample D1 was measured to be 6.73 g.

[0067] XRD was performed on the sample. Figure 1 , Figure 2 , Figure 4 ) and SEM ( Figure 5Characterization. Comparative Example 1 shows that when the molar ratio of sodium bicarbonate to cerium salt is greater than 2:1, exceeding the molar ratio limit of this invention (greater than 2:1), the phase after the hydrothermal reaction is a mixture of basic cerium carbonate and cerium dioxide, requiring further calcination to completely convert it to cerium dioxide. This lengthens the process and increases energy consumption. Figure 5 As can be seen from D1, the calcined product exhibits severe agglomeration, resulting in poor product dispersibility. From... Figure 4 It can be seen that the crystallinity of the final product has also decreased, resulting in a decline in the overall performance of the product.

[0068] In addition, cerium dioxide samples were prepared using a cerium source to precipitant molar ratio of 1:4, following the same preparation procedure as sample D1. The XRD and SEM characterization results of the final product were similar, but the product mass was slightly higher, at 6.80 g. Therefore, no further comparison with the examples was made.

[0069] Comparative Example 2 17.37 g (0.04 mol) of cerium nitrate hexahydrate was weighed and dissolved in 200 mL of deionized water. The solution was stirred until completely dissolved to obtain a cerium source solution. 0.5 g of polyvinylpyrrolidone (PVP, K30) was weighed as a structure-directing agent and dispersant and added to the cerium source solution. The solution was stirred until completely dissolved. Unlike Example 3, no precipitant was added in this comparative example. The remaining steps were the same as in Example 3. Cerium dioxide was prepared hydrothermally and designated as sample D2. The mass of sample D2 was 1.78 g, significantly lower than the yield of sample S1 (2.99 g) in Example 1 and sample S3 (2.75 g) in Example 3. The cerium ion conversion efficiency was low, and the product yield decreased significantly.

[0070] XRD analysis showed that sample D2 was cerium dioxide with a cubic fluorite structure. SEM observation showed that sample D2 particles had irregular morphologies, including polyhedral and ellipsoidal shapes, with a wide particle size distribution (200-450 nm), and some particles showed obvious agglomeration.

[0071] It can be seen that although Comparative Example 2 and Example 3 used the same cerium source and hydrothermal conditions, the difference between the two is that no precipitant was added in Comparative Example 2. The results show that the product quality (particle size and morphology) of nano-cerium dioxide was significantly reduced when only PVP was added as a structure directing agent and no precipitant was added.

[0072] Comparative Example 3 Weigh 17.37 g (0.04 mol) of cerium nitrate hexahydrate and dissolve it in 200 mL of deionized water, stirring until completely dissolved to obtain a cerium source solution. Weigh 3.36 g (0.04 mol) of sodium bicarbonate and dissolve it in 200 mL of deionized water, stirring until completely dissolved to obtain a precipitant solution. This achieves a 1:1 molar ratio of materials. Transfer the two solutions to a 500 mL beaker and mix. Disperse the mixture ultrasonically at room temperature and stir for 5 minutes to obtain a mixed system. Sample the mixed system and test XRD (Extracorporeal Ratio). Figure 1 The results showed that it was cerium carbonate octahydrate, the same phase as the hydrothermal precursor in Example 1. The mixture was allowed to stand for 12 hours to allow the precipitation reaction to complete. The aged precipitate was filtered and collected. It was washed three times alternately with deionized water and anhydrous ethanol. The washed product was then dried in a 60°C oven for 24 hours to obtain the precursor powder. A portion of the precursor powder was subjected to XRD (…). Figure 3 Analysis showed that it was basic cerium carbonate, and the aged precursor still needed to be calcined. The precursor powder was placed in a muffle furnace and calcined at 600℃ for 4 hours. After natural cooling, cerium dioxide product was obtained, denoted as sample D3. The mass of sample D3 was measured to be 2.25 g.

[0073] XRD was performed on the calcined sample D3. Figure 4 Analysis showed that it was cerium dioxide with a cubic fluorite structure. SEM observation showed that the particle size of sample D3 was 500-600 nm. Sample D3 was synthesized by the traditional precipitation and calcination method for cerium oxide, which is more complex than the one-step hydrothermal method of this invention and requires calcination to achieve a crystallinity similar to that of S1.

[0074] Table 1-3 summarizes the preparation parameters and some results of Examples 1-8 and Comparative Examples 1-3.

[0075] Table 1. Parameters of samples from Examples 1-3 and Comparative Examples 1-3. Table 2. Parameters of samples in Examples 4-7. Table 3. Parameters of Samples in Example 8 Combination Figure 1 , Figure 2 , Figure 3 , Figure 4The XRD patterns of the products in Examples 1-3 showed that they were all pure-phase cubic fluorite CeO2, which perfectly matched the JCPDS No. 34-0394 standard card. The diffraction peaks were sharp and free of impurities. The crystallinity was calculated by fitting with Origin software (crystallinity = area of ​​each crystallization peak / total peak area × 100), and the crystallinity was 81.69%, 77.86%, and 76.45%, respectively, with high crystal integrity. The crystallinity of Comparative Examples 1, 2, and 3 was 68.24%, 70.28%, and 81.16%, respectively. Compared to Example 1, Comparative Example 1 had a precipitant-to-cerium source molar ratio greater than 2:1. Its hydrothermal product phase was a mixture of basic cerium carbonate and cerium dioxide, which, after calcination, became pure cerium dioxide. Compared to Example 1, this involved an additional calcination process, and the crystallinity remained low. Comparative Example 2, compared to Example 3, did not add a precipitant, but all other process conditions were the same. The resulting hydrothermal product, cerium dioxide, had lower crystallinity and yield, and its performance (particle size and dispersibility) was lower than that of Example 3. Comparative Example 3 synthesized cerium dioxide via chemical precipitation. Its material ratio was the same as in Example 1, with a precipitant-to-cerium source molar ratio of 1:1. The phase after precipitation and aging was basic cerium carbonate, requiring a calcination process. After calcination, it became pure cerium dioxide with higher crystallinity, but the process was longer and energy consumption increased.

[0076] Depend on Figure 5 SEM images show that Examples 1 and 2, without dispersant, produced uniform polyhedral CeO2 particles with a diameter of 318.13-415.42 nm, exhibiting good dispersibility and minimal agglomeration. Example 3, with the addition of PVP, yielded near-spherical CeO2 particles with a diameter of 100.85 nm. Morphology could be precisely controlled using a dispersant, and the particle size was reduced. Comparative Example 1 showed larger particles (468.1 nm) compared to Example 1, Comparative Example 2 exhibited more disordered morphology and severe agglomeration compared to Example 3, and Comparative Example 3 had a larger particle size (greater than 500 nm) than Example 1. None of these results meet the requirements of high-end applications.

[0077] When the molar ratio of precipitant to cerium source is 1:1, and the amount of precipitant used is less than the theoretical 3:1, the yields in Examples 1-3 are 2.99 g, 5.25 g, and 2.75 g, respectively. The actual yields are all higher than those obtained using the same molar ratio via a traditional precipitation-calcination process, confirming that this system can remove residual Ce in the solution. 3+ The conversion to cerium dioxide significantly improves the utilization rate of cerium source. Comparative Example 1 yielded 6.73 g, but the precipitant was excessive, the cost was high, and calcination was required. Comparative Example 2, without precipitant, only yielded 1.78 g, with a low conversion efficiency. Comparative Example 3, after chemical precipitation and calcination, only yielded 2.25 g, which is close to the theoretical value. The utilization rates of cerium source were all low.

[0078] Example 4 used citric acid as a dispersant at a molar ratio of 1:1 and hydrothermally heated at 180℃ for 12 hours to obtain spherical CeO2 with a particle size of 159.8 nm, a crystallinity of 75.48%, and a yield of 2.65 g. The product was uniformly dispersed and had a regular morphology. Example 5 adjusted the molar ratio to 1:0.5, verifying the feasibility of this method with a low precipitant dosage. The product had a particle size of 146.3 nm, a crystallinity of 70.19%, and a yield of 1.55 g. Example 6 investigated hydrothermal times of 6 hours and 24 hours. The results showed that extending the time could improve crystallinity and yield, optimize the crystallization effect, and improve the utilization rate of the cerium source. Example 7 investigated hydrothermal temperatures of 120℃ and 220℃, confirming that increasing the temperature was beneficial for in-situ transformation of the precursor and crystal growth, resulting in a product with higher crystallinity and more uniform morphology.

[0079] In summary, this invention achieves higher cerium dioxide yield than traditional precipitation-calcination processes using the same molar ratio of cerium source to precipitant through incomplete chemical precipitation coupled with hydrothermal in-situ conversion. This enables efficient utilization of cerium source and one-step preparation of highly crystalline CeO2 with controllable morphology. The preparation method has significant advantages, including short process, low cost, and no high-temperature calcination.

[0080] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing cerium dioxide through incomplete precipitation coupled with hydrothermal conversion, characterized in that, Includes the following steps: Step 1: Mix the cerium source and the precipitant at a molar ratio of 1:0.1 to 1:2 and allow incomplete precipitation to occur, resulting in a mixed system; Step 2: Perform hydrothermal treatment on the mixed system formed in Step 1, and obtain cerium dioxide by post-treatment of the reaction product.

2. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1, characterized in that, In step 1, The cerium source is a soluble cerium salt. The precipitant is a soluble carbonate or bicarbonate.

3. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1 or 2, characterized in that, In step 1, The cerium source is one or more of cerium nitrate, cerium chloride, and cerium acetate. The precipitant is one or more of sodium carbonate, sodium bicarbonate, ammonium carbonate, and ammonium bicarbonate.

4. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1, characterized in that, In step 1, The molar ratio of the cerium source to the precipitant is from 1:0.5 to 1:1.5; The mixture system is a solid-liquid heterogeneous mixture system.

5. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1 or 4, characterized in that, In step 1, The molar ratio of the cerium source to the precipitant is 1:

1.

6. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1, characterized in that, In step 2, The hydrothermal treatment uses octadehydrated cerium carbonate, which is generated by incomplete precipitation, as a precursor for the reaction. The hydrothermal treatment temperature is 120 to 220°C. The hydrothermal treatment lasts for 6 to 24 hours.

7. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1 or 6, characterized in that, In step 2, The hydrothermal treatment temperature is 160 to 200°C. The hydrothermal treatment time is 10 to 15 hours.

8. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1, characterized in that, The cerium dioxide has a polyhedral morphology. The average particle size of the cerium dioxide is 200 to 500 nm.

9. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 1, characterized in that, In step 1, a dispersant is also added during the mixing of the cerium source and the precipitant. The dispersant is one or more of the following: nonionic surfactants, anionic surfactants, organic acid surfactants, and polymers.

10. The method for preparing cerium dioxide by incomplete precipitation coupled with hydrothermal conversion according to claim 8, characterized in that, When adding dispersants: The cerium dioxide has a spherical or near-spherical morphology. The average particle size of the cerium dioxide is 100 to 250 nm.

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