Preparation method of high oxygen storage cerium-zirconium solid solution

Abstract

The present invention belongs to the technical field of metal oxides, and specifically relates to a method for preparing a high oxygen storage cerium-zirconium solid solution. The preparation method is to add zirconium salt, cerium salt and doped metal salt into water, stir and keep warm to obtain a salt solution; add a dispersant and a precipitant to the salt solution in sequence and mix, and obtain a suspension through precipitation; the suspension is aged and solid-liquid separated in sequence, and the obtained precipitate is washed and roasted to obtain a high oxygen storage cerium-zirconium solid solution. The present invention improves the problem of insufficient oxygen storage capacity of the cerium-zirconium solid solution in the current preparation method of the cerium-zirconium solid solution by selecting new precipitants and dispersants, and the obtained high oxygen storage cerium-zirconium solid solution has good high temperature sintering resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of metal oxides, and in particular relates to a method for preparing a high oxygen storage cerium-zirconium solid solution. Background Art

[0002] Three-way catalyst, referred to as TWC, its main active ingredients are precious metals such as platinum (Pt), rhodium (Rh) and palladium (Pd). Traditionally, three-way catalysts are made by coating precious metals on carriers such as porous ceramics. They can simultaneously treat multiple pollutants such as carbon monoxide, hydrocarbons and nitrogen oxides, and are often used for automobile exhaust purification. The relationship curves of the treatment efficiency-air-fuel ratio of three-way catalysts for carbon monoxide, hydrocarbons and nitrogen oxides are not completely overlapped, but there is a narrow air-fuel ratio range, called the working window. Only within this working window can the treatment of carbon monoxide, hydrocarbons and nitrogen oxides be taken into account; however, due to the complexity of the engine operating conditions, the exhaust gas discharged from the vehicle will fluctuate irregularly under oxygen-rich and oxygen-poor conditions, resulting in the engine air-fuel ratio generally being difficult to stabilize within the working window. At present, it is generally necessary to load zirconium-cerium solid solution on the three-way catalyst. Ce exists in the cerium-zirconium solid solution. 3+ / Ce 4+ Ion pair, through Ce 3+ The oxidation process of Ce 4+ The reduction process releases oxygen ions when oxygen is scarce, forming a buffer against changes in oxygen concentration and improving the treatment efficiency of the three-way catalyst for a variety of pollutants; and the oxygen storage capacity (OSC) of the cerium-zirconium solid solution directly affects the treatment capacity of automobile exhaust.

[0003] At present, the coprecipitation method is commonly used in the industry to prepare cerium-zirconium solid solution. The coprecipitation method is to make cerium and zirconium ions coprecipitate through the action of a precipitant to obtain a precursor precipitate, and then wash, dry and roast to obtain a cerium-zirconium solid solution. It is simple to operate, low cost, easy to promote on a large scale, and therefore favored by the industry. In addition, the sol-gel method and the surfactant template method are also commonly used methods for preparing cerium-zirconium solid solution.

[0004] Chinese patent CN117878265A discloses a method for preparing a cerium-zirconium-based solid solution precursor, which comprises dispersing a cerium source and a zirconium source in an anhydrous organic solvent, stirring and fully dissolving; then adding ethylene glycol and a complexing agent citric acid, heating and reacting in an oil bath, and obtaining a cerium-zirconium-based solid solution precursor by a sol-gel method. Chinese patent CN103111280A discloses a method for preparing a transition metal element-doped cerium-zirconium solid solution high-space-velocity denitration catalyst, which comprises uniformly mixing a soluble salt of a transition metal element (vanadium, chromium, manganese, iron, cobalt, copper), a soluble salt of cerium, a soluble salt of zirconium, citric acid, ethanol, and deionized water, and then refluxing at 80°C, forming a gel at 110°C, foaming, pre-calcining at 200°C, grinding after cooling, and roasting at 400-800°C for 4-8 hours to obtain the high-space-velocity denitration catalyst.

[0005] The above two patents use citric acid as a complexing agent; as a commonly used complexing agent, citric acid is easily affected by pH environment, temperature, and solvent environment, resulting in insufficient complexing; in addition, citric acid will decompose and produce gas during the high-temperature roasting stage, and these gases will cause pores, cracks and other damage to the surface structure of the cerium-zirconium solid solution when they overflow, aggravating the high-temperature sintering phenomenon and affecting the oxygen storage capacity of the zirconium-cerium solid solution. With increasingly stringent environmental protection standards, this cerium-zirconium solid solution made with citric acid complexing agent is difficult to meet market needs.

[0006] Chinese patent CN101940921A discloses a method for preparing a double-layer oxygen storage material, comprising the following steps: (1) taking a lanthanum nitrate solution, a cerium nitrate solution, a yttrium nitrate solution and a zirconium nitrate solution, placing them in a container, adding ammonia water for precipitation to obtain a precipitate; (2) adding industrial hydrogen peroxide of cerium oxide quality to the precipitate in step (1) to obtain a cerium-zirconium precursor compound Re x Ce y Zr 1-x-y (OH)2; (3) Re x Ce y Zr 1-x-y (OH)2 and γ-Al2O3 are mixed in a mass ratio of 1:1 and stirred to obtain a mixture; (4) the mixture in step (3) is burned to obtain a cerium-zirconium precursor compound Re x Ce y Zr 1-x-y (OH)2 decomposes into Re x Ce y Zr 1-x-y O2, and air flow pulverization is performed to obtain a double-layer structured powder oxygen storage material. Chinese patent CN118416881A discloses a method for preparing a cerium-zirconium solid solution powder, which comprises mixing cerium nitrate and zirconium nitrate and adding them to an ammonia solution for reaction, taking the precipitate after the reaction and dispersing it in a solvent containing a pore-forming agent, centrifuging and drying, and ball milling to obtain a cerium-zirconium solid solution powder.

[0007] The above two patents use ammonia as a precipitant; since ammonia can naturally ionize into OH in aqueous solution, - , while OH - Can be used with Ce 3+ 、Zr 4+ The metal ions react quickly to form precipitation, and it is difficult to ensure uniformity when adding them, which will make the local concentration too high and make it difficult to control the particle size of the cerium-zirconium solid solution; the nanoparticles precipitated earlier will serve as crystal nuclei, and due to the energy potential difference, the nanoparticles precipitated later will agglomerate around the crystal nuclei, affecting the dispersibility of the cerium-zirconium solid solution. Large particle size and poor dispersibility will affect the specific surface area of ​​the cerium-zirconium solid solution, thereby affecting the oxygen storage capacity of the zirconium-cerium solid solution; with increasingly stringent environmental protection standards, this cerium-zirconium solid solution prepared using ammonia precipitant is difficult to meet market needs.

[0008] The deficiencies in the prior art will affect the specific surface area of ​​the cerium-zirconium solid solution. Since the specific surface area is one of the most important factors affecting the oxygen storage capacity, with the increasing requirements for the catalytic efficiency of three-way catalysts, how to improve the problems existing in the complexing agent and precipitant in the prior art, so as to effectively control the particle size of the cerium-zirconium solid solution, obtain a cerium-zirconium solid solution with a narrow distribution particle size, and give the cerium-zirconium solid solution better high-temperature sintering resistance, thereby making the cerium-zirconium solid solution have a larger specific surface area, is the key to further improving the oxygen storage capacity of the cerium-zirconium solid solution. Summary of the invention

[0009] The purpose of the present invention is to provide a method for preparing a cerium-zirconium solid solution with high oxygen storage capacity, and to improve the problem of insufficient oxygen storage capacity of the cerium-zirconium solid solution in the current preparation method of the cerium-zirconium solid solution by selecting a new precipitant and a dispersant.

[0010] To achieve the above object, the technical solution adopted by the present invention is:

[0011] The preparation method of the high oxygen storage cerium-zirconium solid solution of the present invention comprises the following steps: adding zirconium salt, cerium salt and doped metal salt into water, stirring and keeping warm to obtain a salt solution; sequentially adding a dispersant and a precipitant into the salt solution and mixing evenly, and obtaining a suspension through precipitation; sequentially subjecting the suspension to aging and solid-liquid separation, washing and roasting the obtained precipitate to obtain a high oxygen storage cerium-zirconium solid solution; wherein the suspension contains a solid solution precursor of a solid phase.

[0012] in:

[0013] The zirconium salt is zirconium nitrate or zirconium oxynitrate, the cerium salt is cerium nitrate or ammonium cerium nitrate, and the doped metal salt is one or more of yttrium nitrate, praseodymium nitrate, lanthanum nitrate, neodymium nitrate, iron nitrate, strontium nitrate, aluminum nitrate or barium nitrate.

[0014] The zirconium accounts for 20-70%, the cerium accounts for 20-70%, and the doped metal accounts for 5-15%, with the total mass of zirconium, cerium and doped metal being 100%.

[0015] The ratio of the total amount of the zirconium salt, cerium salt and doped metal salt to the amount of water added is 10: (4.2-5.0), wherein the total amount of the zirconium salt, cerium salt and doped metal salt is measured in mol, and the unit of water is L; the insulation temperature is 55-60°C, and the insulation time is 40-60min.

[0016] The dispersant is sodium polyepoxysuccinate and lauric acid, and the mass ratio of sodium polyepoxysuccinate to lauric acid is 1: (1-2); the precipitant is thiourea; the ratio of the total amount of zirconium, cerium and doped metal and the added amount of the dispersant and the precipitant is 1: (46-58): (4.3-4.6), the total amount of zirconium, cerium and doped metal and the precipitant are measured in mol, and the dispersant is measured in g.

[0017] The precipitation temperature is 60-70°C, and the precipitation time is 90-120 minutes.

[0018] The aging temperature is 50-70° C., the aging time is 1-2 hours, and the pH value during the aging process is 9.0-10.0. Washing is performed with water.

[0019] The conductivity of the precipitate after washing is ≤40mS / cm.

[0020] The calcination temperature is 650-1000°C, and the calcination time is 5-7h.

[0021] The beneficial effects of the present invention are as follows:

[0022] The present invention adds sodium polyepoxysuccinate, lauric acid and precipitant thiourea to utilize the synergistic effect of the three:

[0023] (1) On the one hand, part of the -COONa in sodium polyepoxysuccinate undergoes hydrolysis reaction in water to generate carboxyl and hydroxyl groups, which provide an alkaline environment for thiourea hydrolysis and reduce the thiourea hydrolysis temperature. When the reaction is carried out at a lower temperature, the thiourea can be prevented from further decomposing and producing by-products, which affects the precipitation efficiency. On the other hand, experiments have found that when the alkaline hydrolysis temperature of thiourea is reached (60-70°C), the chelating force and dispersing force of sodium polyepoxysuccinate also reach a relatively balanced state. At this temperature, the molecular chain of sodium polyepoxysuccinate moves smoothly and is relatively stretched. On the one hand, it is mainly through single-point coordination with Ce 4+ (or Ce 3+ ), Zr 4+Plasma forms a complex; single-point coordination avoids molecular chain agglomeration and shielding of chelated metal ions on the side chain, so that the precipitant can fully contact with the metal ions; on the other hand, the more stretched molecular chain acts as a template agent, through the complex Ce 4+ (or Ce 3 + ), Zr 4+ Plasma, guided Ce 4+ (or Ce 3+ ), Zr 4+ After the plasma is precipitated and calcined, a cerium-zirconium solid solution with uniform morphology is finally generated. At the same time, the molecular chain of sodium polyepoxysuccinate forms O...H...H hydrogen bonds with lauric acid, and they are adsorbed on the surface of the solid solution precursor through the carboxyl group, forming a steric effect, reducing the agglomeration of the solid solution precursor during high-temperature calcination. In addition, the hydrophobic alkyl chain at the other end of lauric acid makes the surface of the solid solution precursor hydrophobic, which can isolate water and reduce the surface tension of water in the pores, reducing the aggregation of water in the pores. Since the solid solution precursor has many pores and a large structure, It is relatively fragile. When calcining to prepare high oxygen storage cerium-zirconium solid solution, as the temperature rises, the evaporation of water will generate stress. The collapse of pores at high temperatures is one of the reasons for high-temperature sintering. Therefore, by reducing the residual water in the pores and thus reducing the collapse of the pores during the calcination process, the pore network structure of the material is improved, and the problem of rapid decrease in specific surface area due to high-temperature sintering is improved. Lauric acid and polyepoxysuccinic acid work together to reduce the high-temperature sintering phenomenon during the calcination of the solid solution precursor, and ultimately the prepared cerium-zirconium solid solution obtains a more ideal specific surface area.

[0024] (2) The precipitant is a urea analogue. In the present invention, thiourea (SC(NH2)2) is used. When thiourea is in an alkaline aqueous solution environment at a temperature above 60°C, it is hydrolyzed to generate ammonia water, which then reacts with Ce 4+ (or Ce 3+ ), Zr 4+ Plasma reacts to generate Ce(OH)4, Zr(OH)4 and other precipitates. Therefore, sodium polyepoxysuccinate can be added to the salt solution to make the salt solution alkaline. Thiourea can be dissolved in the alkaline salt solution in advance to make it fully dispersed. Then, the thiourea is hydrolyzed by heating it to above 60°C to react with Ce(OH)4. 4+ (or Ce 3+ ), Zr 4+ Plasma generates precipitation, thus avoiding the problem in the prior art that when a precipitant such as ammonia is added, the particle size is large and the dispersion is poor, resulting in a reduction in specific surface area due to excessive local concentration.

[0025] (3) Due to the differences in concentration and precipitation rate of each component in the salt solution, some ions such as Ce 4+ (or Ce 3+) will precipitate in advance, making it difficult to ensure the uniformity of Ce / Zr composition distribution at different positions in the cerium-zirconium solid solution. As the precipitation process proceeds, the Zr content increases, causing the precipitated solid solution precursor phase to transform from a cubic fluorite structure to a cubic phase and a monoclinic phase structure, and the lattice distortion between the phase interfaces will increase the internal stress, prompting the solid solution precursor to rearrange and combine at high temperatures to reduce the system energy, thereby causing the cerium-zirconium solid solution to sinter in a high-temperature environment. The present invention adds sodium polyepoxysuccinate, and sodium polyepoxysuccinate and Ce 4+ (or Ce 3+ ), Zr 4+ The stable complex can control the migration and aggregation of metal ions at high temperature and inhibit the 4+ (or Ce 3+ ) precipitation activity, thereby preventing grain growth and phase separation, increasing the specific surface area of ​​cerium-zirconium solid solution, and reducing Ce 4+ (or Ce 3+ ) to prevent premature precipitation, thereby reducing sintering in high temperature environments. DETAILED DESCRIPTION

[0026] The present invention is specifically described and illustrated below in conjunction with embodiments.

[0027] Example 1

[0028] Prepare raw materials: use ammonium cerium nitrate, zirconium oxynitrate, lanthanum nitrate, neodymium nitrate and yttrium nitrate as raw materials, and add 1000g (1000g is the total mass of cerium, zirconium and doped metal) in a ratio of 20:68:12 among cerium, zirconium and doped metal, and 0.22mol, 0.2mol and 0.68mol of lanthanum, neodymium and yttrium respectively, 179.5g of sodium polyepoxysuccinate, 359g of lauric acid and 42.87mol of thiourea.

[0029] Add ammonium cerium nitrate, zirconium oxynitrate, lanthanum nitrate, neodymium nitrate and yttrium nitrate to 5.0 L of water, start stirring, and keep warm at 55°C for 60 minutes to obtain a salt solution; add sodium polyepoxysuccinate and lauric acid to the salt solution, and stir at 500 r / min for 10 minutes; add thiourea, continue stirring for 5 minutes to mix, pass into a reactor, reflux at 65°C for 100 minutes for precipitation, and obtain a suspension.

[0030] The suspension was aged at 70°C for 1 hour, and a small amount of ammonia water was added as needed to maintain the pH at 9.0-10.0; the aged suspension was placed in a centrifuge for centrifugal separation to obtain a precipitate; the precipitate was washed with water until the conductivity of the precipitate was ≤40mS / cm, and then the precipitate was placed in a muffle furnace and calcined at 1000°C for 5 hours to obtain a high oxygen storage cerium-zirconium solid solution.

[0031] Example 2

[0032] Prepare raw materials: Use ammonium cerium nitrate, zirconium nitrate, lanthanum nitrate, praseodymium nitrate, strontium nitrate and ferric nitrate as raw materials, and add 1000g (1000g is the total mass of cerium, zirconium and doped metal) in a ratio of 65:20:15 between cerium, zirconium and doped metal, and 0.2mol, 0.2mol, 0.6mol and 0.75mol of lanthanum, praseodymium, strontium and iron respectively, 247.95g of sodium polyepoxysuccinate, 247.95g of lauric acid and 39.33mol of thiourea.

[0033] Add ammonium cerium nitrate, zirconium nitrate, lanthanum nitrate, praseodymium nitrate, strontium nitrate and ferric nitrate into 3.6L of water, start stirring, and keep warm at 60°C for 40 minutes to obtain a salt solution; add sodium polyepoxysuccinate and lauric acid to the salt solution, and stir at 500r / min for 13 minutes; add thiourea, continue stirring for 5 minutes to mix, pass into a reactor, reflux at 60°C for 120 minutes for precipitation, and obtain a suspension.

[0034] At 60°C, the suspension was aged for 1.3 hours, at which time a small amount of ammonia water was added as needed to maintain the pH at 9.0-10.0; the aged suspension was placed in a centrifuge for centrifugal dispersion to obtain a precipitate; the precipitate was washed with water until the conductivity of the precipitate was ≤40mS / cm, and then the precipitate was placed in a muffle furnace and calcined at 650°C for 7.5 hours to obtain a high oxygen storage cerium-zirconium solid solution.

[0035] Example 3

[0036] Prepare raw materials: Use cerium nitrate, zirconium nitrate, yttrium nitrate, neodymium nitrate, aluminum nitrate and barium nitrate as raw materials, and add 1000g (1000g is the total mass of cerium, zirconium and doped metal) in the ratio of 50:45:5 of cerium, zirconium and doped metal, and 0.1mol, 0.1mol, 0.5mol and 0.1mol of yttrium, neodymium, aluminum and barium respectively, 170.6g of sodium polyepoxysuccinate, 255.8g of lauric acid and 41.7mol of thiourea.

[0037] Add cerium nitrate, zirconium nitrate, yttrium nitrate, neodymium nitrate, aluminum nitrate and barium nitrate into 4.2L of water, start stirring, and keep warm at 58°C for 55 minutes to obtain a salt solution; add sodium polyepoxysuccinate and lauric acid to the salt solution, and stir at 500r / min for 18 minutes; add thiourea, continue stirring for 5 minutes to mix, pass into a reactor, reflux at 70°C for 90 minutes for precipitation, and obtain a suspension.

[0038] The suspension was aged at 50°C for 2 hours, at which time a small amount of ammonia water was added as needed to maintain the pH at 9.0-10.0; the aged suspension was placed in a centrifuge for centrifugal dispersion to obtain a precipitate; the precipitate was washed with water until the conductivity of the precipitate was ≤40mS / cm, and then the precipitate was placed in a muffle furnace and calcined at 850°C for 5.8 hours to obtain a high oxygen storage cerium-zirconium solid solution.

[0039] Comparative Example 1

[0040] Without adding lauric acid, the remaining steps were the same as those in Example 1.

[0041] Comparative Example 2

[0042] Without adding sodium polyepoxysuccinate, the remaining steps were the same as those in Example 1.

[0043] Comparative Example 3

[0044] The thiourea was replaced by ammonia water, and the remaining steps were the same as those in Example 1.

[0045] Comparative Example 4

[0046] The precipitation was carried out by reflux at 45°C, and the remaining steps were the same as those in Example 1.

[0047] Comparative Example 5

[0048] The precipitation was carried out by reflux at 95°C, and the remaining steps were the same as those in Example 1.

[0049] Implementation effect evaluation

[0050] Oxygen storage capacity OSC test: The oxygen storage capacity (OSC) of the high oxygen storage cerium zirconium solid solution prepared in Examples 1 to 3 and Comparative Examples 1 to 5 was tested using the H2-TPR method; 100 mg of high oxygen storage cerium zirconium solid solution was added to a conical quartz tube, and the conical quartz tube was connected to a thermal conductivity cell detector TCD and a chemical adsorption instrument, the TPR mode of the chemical adsorption instrument was turned on, N2 was introduced at a rate of 22 mL / min, and the purification treatment was carried out at 280°C for 35 min; then the temperature was lowered To 52°C, introduce Ar / H2 mixed gas (H2 concentration is 10vol%) to purge until the chromatographic recording baseline is stable, and then perform programmed temperature reduction; maintain the introduction of Ar / H2 mixed gas, and increase the temperature from 52°C to 1000°C at a rate of 10°C / min, and keep warm for 30min to obtain the H2-TPR curve. Take 400°C, 450°C, 500°C, 600°C, 800°C and 1000°C respectively to calculate the hydrogen consumption, which is obtained by the peak area method. The above oxygen storage OSC test is repeated using the standard substance CuO with known oxygen storage capacity, and the oxygen storage capacity of the high oxygen storage cerium zirconium solid solution is calculated by comparing the peak area. The results are shown in Table 1.

[0051]

[0052] The high oxygen storage cerium-zirconium solid solution obtained in Examples 1 to 3 of the present invention has the characteristic of having a cerium oxide ratio of no more than 70 wt%. The high oxygen storage cerium-zirconium solid solution obtained in Examples 1 to 3 was subjected to a high temperature sintering test. After calcination at 1100° C. for 12 h, the air specific surface area of ​​the high oxygen storage cerium-zirconium solid solution obtained in Examples 1 to 3 was ≥63 m 2 / g, and the hydrothermal specific surface area ≥56m 2 / g, oxygen storage capacity ≥1062μmol / g.

Claims

1. A method for preparing a high oxygen storage cerium-zirconium solid solution, characterized in that: The zirconium salt, the cerium salt and the doped metal salt are added into water, stirred and kept warm to obtain a salt solution; a dispersant and a precipitant are sequentially added into the salt solution and mixed evenly, and a suspension is obtained through precipitation; the suspension is sequentially aged and solid-liquid separated, and the obtained precipitate is washed and roasted to obtain a high oxygen storage cerium-zirconium solid solution; The dispersant is sodium polyepoxysuccinate and lauric acid, and the mass ratio of sodium polyepoxysuccinate to lauric acid is 1:(1-2); the precipitant is thiourea; the precipitation temperature is 60-70°C; the zirconium salt is zirconium nitrate or zirconium oxynitrate, the cerium salt is cerium nitrate or ammonium cerium nitrate, and the doped metal salt is one or more of yttrium nitrate, praseodymium nitrate, lanthanum nitrate, neodymium nitrate, iron nitrate, strontium nitrate, aluminum nitrate or barium nitrate; Zirconium accounts for 20-70%, cerium accounts for 20-70%, and doped metals account for 5-15%, with the total mass of zirconium, cerium and doped metals being 100%.

2. The method for preparing a high oxygen storage cerium-zirconium solid solution according to claim 1, characterized in that: The ratio of the total amount of zirconium, cerium and doped metal to the amount of water added is 10:(4.2~5.0), where the measurement unit of zirconium, cerium and doped metal is mol, and the measurement unit of water is L; the insulation temperature is 55~60℃, and the insulation time is 40~60min.

3. The method for preparing a high oxygen storage cerium-zirconium solid solution according to claim 1, characterized in that: The ratio of the total amount of zirconium, cerium and doped metals and the added amount of dispersant and precipitant is 1:(46~58):(4.3~4.6), wherein the total amount of zirconium, cerium and doped metals and the precipitant are measured in mol, and the dispersant is measured in g.

4. The method for preparing a high oxygen storage cerium-zirconium solid solution according to claim 1, characterized in that: The precipitation time is 90~120min.

5. The method for preparing a high oxygen storage cerium-zirconium solid solution according to claim 1, characterized in that: The aging temperature is 50~70°C, the aging time is 1~2h, and the pH during the aging process is 9.0~10.

0.

6. The method for preparing a high oxygen storage cerium-zirconium solid solution according to claim 1, characterized in that: Wash with water.

7. The method for preparing a high oxygen storage cerium-zirconium solid solution according to claim 1, characterized in that: The conductivity of the precipitate after washing is ≤40mS / cm.

8. The method for preparing a high oxygen storage cerium-zirconium solid solution according to claim 1, characterized in that: The calcination temperature is 650~1000℃ and the calcination time is 5~7h.

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