CN114870862A - Composite oxide catalyst for purifying automobile exhaust and preparation method thereof - Google Patents

Abstract

The invention discloses a composite oxide catalyst used for automobile exhaust gas purification, which coats cerium oxide on a MOF frame, and is characterized in that the nano-cube frame has an inner pore size, that is, the pores between the surface of the nano-cube frame and the pores and The inside has a macroporous structure, and the inner and outer surfaces of the cerium oxide-coated cobalt-iron nano-cube frame are also uniformly distributed with nano-Ag particles. The invention prepares a new type of catalyst with special structure, which is composed of a cobalt-iron nano-cube frame inside and wrapped with silver-loaded cerium oxide outside. The present invention has good catalytic activity, utilizes the nano-cube frame structure of Co-Fe material, enhances the contact between the catalyst and soot, and combines it with CeO ₂ and Ag to form a "Co-Fe→CeO ₂ →Ag secondary""Oxygen transfer channel" to increase the source of oxygen supply. In the presence of the catalyst, a reduction of 20-45°C in T ₅₀ (the temperature at which the soot conversion rate is 50%) can be achieved to simulate the atmosphere of vehicle exhaust. At the same time, the synthesis method is environmentally friendly and has great application value.

Description

A kind of composite oxide catalyst used for automobile exhaust purification and preparation method thereof

technical field

The invention belongs to the technical field of flue gas purification, and particularly relates to a composite oxide catalyst used for purification of automobile exhaust gas and a preparation method and application thereof.

Background technique

With the progress of the times and the development of science and technology, in 2020, the total emissions of four pollutants from motor vehicles in my country will reach 15.93 million tons, and the pollutants generated by vehicles account for 90% of all pollutants emitted by motor vehicles. Among them, particulate matter emissions have reached 64,000 tons. The particulate matter in the exhaust gas is mainly soot particles. The particle size of soot particles is generally less than 10 μm, or even less than 1 μm, which seriously endangers the atmospheric environment and human health. At present, it is mainly improved by installing a particulate matter filter device after the automobile engine, which reduces the ignition point of soot through the catalyst in the device, so that the soot can be converted into relatively harmless CO ₂ at the ambient temperature of the exhaust gas. With the increase in the price of traditional precious metals, it is particularly important to find new catalysts that are economical, efficient and green.

my country is rich in rare earth resources. In the research of various catalysts, researchers have turned their attention to the feasibility of rare earth element oxides in exhaust after-treatment devices. Bueno-López et al. studied a variety of non-precious metal catalysts and found that cerium-based catalysts have better performance, and cerium-based catalysts have gradually become the focus of research. The 5d orbital of cerium is favorable for electron transfer, so there will be a situation where Ce ³⁺ /Ce ⁴⁺ changes with each other. CeO ₂ still maintains the cubic fluorite structure after oxygen vacancies appear during the conversion process, which is conducive to absorption and deoxygenation, and can meet the needs of new engines to a certain extent without the help of "external force". However, studies have found that the light-off temperature of CeO ₂ catalyst for effectively oxidizing soot is above 400 °C, which needs to be continuously improved to further meet the needs of new engines.

Compared with granular nanocatalysts, the preparation of catalysts with special morphologies or the assembly of nanoparticles into ordered structures may make the catalysts better match the contact with soot, obtain more contact sites, and shorten the time of reactive oxygen species from " The distance from the "generation site" to the "reaction site", thereby reducing the possibility ^of _Ox- "deactivation in overflow" and realizing the "throttling" utilization of reactive oxygen species. For example, Makkee et al. prepared CeO2 crystallites (100 nm) by nitrate calcination, using _O2 as the oxidant T10 (the temperature at which the soot conversion reaches ₁₀ % ₎ in loose contact mode with soot above 530 °C , T ₅₀ is about 600 °C; after increasing the calcination temperature, using O ₂ as an oxidant in the mode of close contact with soot, T ₁₀ and T ₅₀ decreased by about 30 ° C [Krishna K, ABueno-López, Makkee M, et al. Potential rare earth modified CeO ₂ catalysts for soot oxidation[J]. Applied Catalysis B: Environmental, 2007, 75(3–4):189-200.]. Machida et al. found through research that among the cerium-based catalysts modified with various noble metals (platinum, gold, silver, rhodium, etc.), the silver _- loaded CeO2 catalyst showed the best catalytic oxidation performance, and the silver-loaded CeO catalyst exhibited the best catalytic oxidation performance. When not less than 5 wt.%, Ag/CeO ₂ catalyst exhibits the most excellent low-temperature soot catalytic oxidation activity [Machida M, Murata Y, Kishikawa K, et al. On the reasons for high activity of CeO ₂ catalyst for soot oxidation. Chemistry of Materials, 2008, 20(13): 4489-4494]. Zhang et al. synthesized Ag/CeO ₂ nanocubes (12~42 nm) and nanorods (12 × 77 nm) by hydrothermal method and impregnation method and explored their soot catalytic activity, in which the Ag/CeO ₂ nanocube T ₅₀ was 447 ℃[Meisheng, Zhang, Baofang, et al. Ozone activatedAg/CeO ₂ catalysts for soot combustion: The surface and structural influences- ScienceDirect[J]. The Chemical Engineering Journal, 375(C):121961-121961.]. In addition, another way to improve the performance of CeO ₂ catalyst is to make full use of the bulk oxygen of the catalyst. Due to its small oxygen release, common transition metal oxides such as Fe ₂ O ₃ and Co ₃ O ₄ are used. As an oxygen supply source can effectively solve this problem. Wang et al. coated Ag/CeO ₂ on the outer surface layer of Fe ₂ O ₃ nanocubes, and constructed a "Fe ₂ O ₃ → CeO ₂ → Ag secondary oxygen transport channel", which has a higher performance than Ag under simulated GPF conditions. The catalytic performance of /CeO ₂ is increased to 3 times [Wang H ,Jin B ,Wang H , et al. Study of Ag promoted Fe ₂ O ₃ @CeO ₂ as superior soot oxidationcatalysts: The role of Fe ₂ O ₃ crystal plane and tandem oxygen delivery[J].Applied Catalysis B: Environmental, 2018, 237:251-262.]. Wang et al. used Co ₃ O ₄ as the oxygen supply core to construct the Ag/Co@Ce material, which further reduced the soot rapid combustion temperature to 450 °C [Wang X, JinB, Feng R, et al. A robust core- shell silver soot oxidation catalyst driven by Co ₃ O ₄ : Effect of tandem oxygen delivery and Co ₃ O ₄ -CeO ₂ synergy[J]. AppliedCatalysis B: Environmental, 2019, 250:132-142.].

To sum up, based on the selection of low-cost, high-performance new catalytic materials, the catalyst structure can be reasonably designed to enhance its contact with soot and the design of oxygen supply source materials, which is expected to break through the existing The soot removal efficiency is limited, which contributes to the emission reduction of automobile exhaust particulate matter in my country.

SUMMARY OF THE INVENTION

In view of the above problems, an object of the present invention is to provide a soot removal catalyst with low price, good catalytic activity, improved contact between soot and catalyst, and efficient removal of soot particles.

Another object of the present invention is to provide a simple and efficient preparation method of the above-mentioned composite oxide catalyst.

In the present invention, the nano-cubic frame structure of Co-Fe material is combined with the "Co-Fe→CeO2 _→ Ag secondary oxygen transfer channel" structure composed of CeO ₂ and Ag, and the silver-loaded cerium oxide is evenly wrapped in the Ag/Co-Fe@CeO2 _- coated catalysts with nanocubic framework structure were fabricated on Co-Fe materials. The catalyst uses a CeO ₂ carrier with strong oxygen-conducting ability activated by silver components, which has strong intrinsic oxidation performance; the nano-cubic frame structure helps to enhance the contact between the catalyst and soot, and increases the active site of the catalyst for oxidizing soot. number of points. The above-mentioned material components work together with the special structure of the catalyst to achieve a reduction of T50 (temperature at ₅₀ % soot conversion) of 20-45°C. In addition, silver, cerium oxide, cobalt acetate and other components are not expensive materials, so the cost is greatly reduced compared to commercial platinum-based catalysts.

In order to achieve the above object, the concrete technical scheme adopted in the present invention is as follows:

A composite oxide catalyst used for automobile exhaust purification, comprising a ceria-coated Co-Fe nano-cube frame structure, characterized in that the ceria-coated Co-Fe nano-cube frame has an internal pore structure, that is, a nano-cube The pores between the frames and the surface and interior of the nano-cube frame have macroporous structures, and the inner and outer surfaces of the cerium oxide-coated Co-Fe nano-cube frame structure also distribute nano-Ag particles uniformly.

The average particle size of the cerium oxide-coated Co-Fe nano-cubic frame structure is 200 nm, and the inner pore size is 40-70 nm.

A kind of preparation method of the composite oxide catalyst used as automobile exhaust gas purification is characterized in that it comprises the following steps:

(1) Preparation of cobalt-iron mixed oxide: weigh an appropriate amount of cobalt salt and shape-directing agent and mix, add an appropriate amount of solvent, and stir at room temperature until completely dissolved to form solution A; weigh an appropriate amount of iron salt, add an appropriate amount of solvent, and stir at room temperature until completely Dissolved to form solution B. Add solution B to solution A, stir at room temperature until the mixture is uniform, and obtain a precipitate after aging. The precipitate was annealed in an air atmosphere to obtain a cobalt-iron mixed oxide catalyst.

(2) Coating of ceria: Weigh an appropriate amount of the cobalt-iron mixed oxide obtained in step (1), add an appropriate amount of solvent, sonicate at room temperature until uniform, add an appropriate amount of cerium salt and pH adjuster, stir at room temperature until fully mixed, Reflux and vigorous stirring under _N2 atmosphere, the precipitate was obtained after cooling the precipitate. After calcining the precipitate, a ceria-coated cobalt-iron mixed oxide catalyst is obtained.

(3) Loaded silver component: an appropriate amount of silver salt is prepared into a solution, and the cerium dioxide obtained in step (2) is immersed in the cobalt-iron mixed oxide catalyst, so that the inner and outer surfaces of the cubic frame structure are evenly loaded with silver salt, and the Mixing uniformly, then drying and calcining, the novel soot removal catalyst is finally prepared.

Further, in the above step (1), the cobalt salt is cobalt acetate tetrahydrate, the shape directing agent is sodium citrate dihydrate, and the mass ratio of cobalt acetate tetrahydrate and sodium citrate dihydrate is 50:49.

Further, in the above step (1), the solvent is deionized water.

Further, in above-mentioned step (1), aging temperature is 35 ℃, and aging time is 18h.

Further, in the above-mentioned step (1), the annealing temperature is 350°C, the holding time is 2h, and the heating rate is controlled at 2°C/min.

Further, in the above step (2), the cerium salt is cerium nitrate, the pH regulator is hexamethylenetetramine, and the adjusted pH value is 7.35.

Further, in the above step (2), the solvent is ethanol and deionized water.

Further, in above-mentioned step (2), reflux temperature is 70 ℃, and reflux time is 2h.

Further, in the above-mentioned step (2), the calcination temperature is 500°C, the calcination time is 2h, and the calcination heating rate is controlled at 2°C/min.

Further, in the above step (3), the silver salt is silver nitrate, and the mass ratio of silver to catalyst is 1:20.

Further, in the above-mentioned step (3), the calcination temperature is 500°C, the calcination time is 2h, and the calcination heating rate is controlled at 2°C/min.

The beneficial effects of the present invention are: silver, cerium oxide, cobalt acetate and other components are not expensive materials, so the cost is greatly reduced compared with commercial platinum-based catalysts; At the same time, this structure builds a "Co-Fe→CeO ₂ →Ag secondary oxygen transfer channel" and has macropores that can match the soot particles, thereby enhancing the contact between the catalyst and soot. In the presence of the catalyst, the temperature at which the soot conversion reaches 50% ( T ₅₀ ) can be reduced by 20-45°C to simulate the atmosphere of vehicle exhaust gas, which has great application value.

Description of drawings

1 is a scanning electron microscope image of the material prepared in Example 1 of the present invention.

2 is a scanning electron microscope image and a transmission electron microscope image of the material prepared in Example 2 of the present invention.

FIG. 3 is a comparison diagram of the soot conversion rates of Examples 1 to 3 of the present invention and Comparative Examples 1 to 3.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described below with reference to the comparative examples and accompanying drawings and through specific embodiments, but the protection scope of the present invention should not be limited accordingly.

Example 1:

Dissolve 0.15g of cobalt acetate tetrahydrate and 0.147g of sodium citrate dihydrate in 20ml of deionized water, stir at room temperature until completely dissolved to form solution A; dissolve 0.1g of potassium hexacyanoferrate in 30ml of deionized water to form solution B. Then under magnetic stirring, solution B was added to solution A in 15s, and the mixture was continuously stirred for 1min to make it evenly mixed, and the obtained mixed solution was aged for 18h at 35°C. After cooling, the precipitate was collected by centrifugation, washed with deionized water and ethanol for many times, dried at 70°C overnight, and annealed for 2h in a muffle furnace at 350°C with a heating rate of 2°C/min in air. , a Co-Fe mixed oxide (Co-Fe) catalyst with nanocube framework structure was obtained.

The prepared catalyst samples were evaluated by simulated gas distribution in the laboratory. During the evaluation, the samples were mixed with catalyst, soot and quartz sand in a weight ratio of 100 mg: 10 mg: 300 mg. Before mixing, the catalyst was mixed in a mortar. Grind with soot for 5 min to achieve "close contact". The mixed sample was placed in a quartz reaction tube with a diameter of 10 mm. The soot oxidation test reaction atmosphere was 500 ml/min of 1% O ₂ /N ₂ (space velocity 100,000 h ⁻¹ ). The temperature range of the activity test was 30 to 700 °C, and the heating rate was 5 °C/min. During the reaction, the temperature controller was used to control the temperature of the electric furnace, and the concentration of _CO generated during the reaction was measured by an infrared spectrometer. It can be seen that the catalytic performance of the Co-Fe catalyst is not good, with T10 of ₄₆₃ °C and _T50 of 493 °C.

And it was characterized. It can be seen from the scanning electron microscope in Figure 1 that the nanocube frame structure is obvious, the average particle size is 200nm, and the internal aperture is 40~70nm.

Example 2:

The method of this example is basically the same as that of Example 1, the difference is that: on the basis of Example 1, cerium oxide is supported by the method of calcination.

0.05g of Co-Fe was added to 20 ml of ethanol and 20 ml of H ₂ O, ultrasonicated for 15 min, 0.08 g of cerium nitrate and hexamethylenetetramine were added to adjust the pH of the solution to 7.35, fully stirred and transferred to a three-necked flask, the suspension was It was heated to 70 ° C in flowing N , and at this temperature, refluxed and vigorously stirred for ₂ h. After cooling, it was centrifuged, washed with deionized water and ethanol many times, and dried at 110 ° C. After grinding, it was ground into powder, and then with The Co-Fe@CeO catalyst was obtained after heating the muffle furnace to 500 °C at a rate of 2 °C/min for ₂ h of calcination.

It was characterized. From the scanning electron microscope image and transmission electron microscope image in Figure 2, it can be seen that the basic morphology of the Co-Fe@CeO ₂ catalyst sample after loading ceria, the nano ceria particles are uniformly coated on the Co-Fe nanocube frame , the internal pore size is 30~60nm, which is slightly reduced. Through the laboratory simulated gas evaluation system, the measured T10 is ₄₅₄ °C, and the _T50 is 490°C . Experiments show that the structure optimization of Co-Fe coated with ceria can utilize Co and Fe oxides as oxygen supply sources and enhance their contact with soot, thereby improving catalyst performance.

Example 3:

The method of this example is basically the same as that of Example 2, the difference is: on the basis of Example 2, Ag is supported on the catalyst by an equal volume impregnation method.

Weighing Ag quality is that the silver nitrate of catalyst quality 5% is configured into a solution, dropwise added Co-Fe@CeO ₂ in the catalyst sample and stirring, in 80 ℃ of oven drying samples, put into muffle furnace at 500 °C for 2 h of calcination to obtain the Ag/Co-Fe@CeO ₂ catalyst.

Through the laboratory simulated gas evaluation system, the measured T10 is 430°C, and the _T50 is ₄₇₂ °C . It is obvious that the soot light-off temperature and T ₅₀ of the Ag/Co-Fe@CeO ₂ catalyst after Ag loading are significantly reduced, which is attributed to the formation of the “Co-Fe→CeO ₂ →Ag secondary oxygen transfer channel” by the catalyst. ” structure and enhanced its contact with soot.

Comparative Example 1:

Dissolve 0.15g of cobalt acetate tetrahydrate and 0.147g of sodium citrate dihydrate in 20ml of deionized water, stir at room temperature until completely dissolved to form solution A; dissolve 0.1g of potassium hexacyanoferrate in 30ml of deionized water to form solution B. Then under magnetic stirring, solution B was added to solution A in 15s, and the mixture was continuously stirred for 1min to make it evenly mixed, and the obtained mixed solution was aged for 18h at 35°C. After cooling, the precipitate was collected by centrifugation, washed with deionized water and ethanol for many times, dried at 70°C overnight, and annealed for 2h in a muffle furnace at 350°C with a heating rate of 2°C/min in air. , a Co-Fe mixed oxide (Co-Fe) catalyst with nanocube framework structure was obtained.

The prepared catalyst samples were evaluated by simulated gas distribution in the laboratory. In the evaluation process, the samples were mixed with catalyst, soot and quartz sand in a weight ratio of 100 mg: 10 mg: 300 mg. Before mixing, the catalyst and carbon were mixed with a spoon. The smoke was stirred and mixed for 2 min to achieve "loose contact". The mixed sample was placed in a quartz reaction tube with a diameter of 10 mm. The soot oxidation test reaction atmosphere was 500 ml/min of 1% O ₂ /N ₂ (space velocity 100,000 h ⁻¹ ). The temperature range of the activity test was 30 to 700 °C, and the heating rate was 5 °C/min. During the reaction, the temperature controller was used to control the temperature of the electric furnace, and the concentration of _CO generated during the reaction was measured by an infrared spectrometer. It can be seen that the catalytic performance of the Co-Fe catalyst is poor, with a T10 of ₄₇₂ °C and a _T50 of 560 °C .

Comparative Example 2:

The method of this comparative example is basically the same as that of the comparative example 1, the difference is: on the basis of the comparative example 1, the cerium oxide is supported by the method of calcination.

0.05g of Co-Fe was added to 20 ml of ethanol and 20 ml of H ₂ O, ultrasonicated for 15 min, 0.08 g of cerium nitrate and hexamethylenetetramine were added to adjust the pH of the solution to 7.35, fully stirred and transferred to a three-necked flask, the suspension was It was heated to 70 ° C in flowing N , and at this temperature, refluxed and vigorously stirred for ₂ h. After cooling, it was centrifuged, washed with deionized water and ethanol many times, and dried at 110 ° C. After grinding, it was ground into powder, and then with The Co-Fe@CeO catalyst was obtained after heating the muffle furnace to 500 °C at a rate of 2 °C/min for ₂ h of calcination.

It was characterized. From the scanning electron microscope image and transmission electron microscope image in Figure 2, it can be seen that the basic morphology of the Co-Fe@CeO ₂ catalyst sample after loading ceria, the nano ceria particles are uniformly coated on the Co-Fe nanocube frame , the internal pore size is 30~60nm, which is slightly reduced. The catalyst and soot were stirred and mixed to achieve "loose contact", and the measured T50 was ₅₅₆ °C through the laboratory simulated gas evaluation system. Experiments show that the structure optimization of Co-Fe coated with ceria can utilize Co and Fe oxides as oxygen supply sources and enhance their contact with soot, thereby improving catalyst performance.

Comparative Example 3:

The method of the present comparative example is basically the same as that of the comparative example 2, the difference is that: on the basis of the comparative example 2, the same volume impregnation method is adopted to support the Ag on the catalyst.

Weighing Ag quality is that the silver nitrate of catalyst quality 5% is configured into a solution, dropwise added Co-Fe@CeO ₂ in the catalyst sample and stirring, in 80 ℃ of oven drying samples, put into muffle furnace at 500 °C for 2 h of calcination to obtain the Ag/Co-Fe@CeO ₂ catalyst.

The catalyst and soot were stirred and mixed to achieve "loose contact". Through the laboratory simulated gas evaluation system, the measured T ₁₀ was 468°C and T ₅₀ was 516°C. It is obvious that the soot light-off temperature and T ₅₀ of the Ag/Co-Fe@CeO ₂ catalyst after Ag loading are significantly reduced, which is attributed to the formation of the “Co-Fe→CeO ₂ →Ag secondary oxygen transfer channel” by the catalyst. ” structure and enhanced its contact with soot.

Claims (10)

1. a composite oxide catalyst used for automobile exhaust purification, comprising a ceria-coated cobalt-iron nano-cubic frame structure, characterized in that the ceria-coated cobalt-iron nano-cube frame has an internal aperture structure, that is, a nano-cube The pores between the frames and the surface and interior of the nano-cube frame have macroporous structures, and the inner and outer surfaces of the cerium oxide-coated cobalt-iron nano-cube frame structure also distribute nano-silver particles uniformly. 2 . The composite oxide catalyst according to claim 1 , wherein the cerium oxide-coated cobalt-iron nano-cubic frame structure has an average particle size of 200 nm and an internal pore size of 40 to 70 nm. 3 . 3. a preparation method of the described composite oxide catalyst of claim 1, is characterized in that it comprises the following steps: (1) Preparation of cobalt-iron mixed oxide: weigh an appropriate amount of cobalt salt and shape-directing agent and mix, add an appropriate amount of solvent, and stir at room temperature until completely dissolved to form solution A; weigh an appropriate amount of iron salt, add an appropriate amount of solvent, and stir at room temperature until completely dissolve to form solution B; Adding solution B to solution A, stirring at room temperature until the mixture is uniform, and aging to obtain a precipitate; annealing the precipitate in an air atmosphere to obtain a cobalt-iron mixed oxide catalyst; (2) Coating of ceria: Weigh an appropriate amount of the cobalt-iron mixed oxide obtained in step (1), add an appropriate amount of solvent, sonicate at room temperature until uniform, add an appropriate amount of cerium salt and pH adjuster, stir at room temperature until fully mixed, Under the _N2 atmosphere, reflux and vigorous stirring, cooling and precipitation to obtain a precipitate; calcining the precipitate to obtain a ceria-coated cobalt-iron mixed oxide catalyst; (3) Loaded silver component: an appropriate amount of silver salt is prepared into a solution, and the cerium dioxide obtained in step (2) is immersed in the cobalt-iron mixed oxide catalyst, so that the inner and outer surfaces of the cubic frame structure are evenly loaded with silver salt, and the Mixing uniformly, then drying and calcining, the novel soot removal catalyst is finally prepared. 4. The preparation method according to claim 3, characterized in that in step (1), the cobalt salt is cobalt acetate tetrahydrate, the shape-directing agent is sodium citrate dihydrate, the combination of cobalt acetate tetrahydrate and sodium citrate dihydrate. The mass ratio is 50:49. 5. The preparation method according to claim 3, wherein in step (1), the solvent is deionized water, the aging temperature is 35°C, and the aging time is 18h. 6. preparation method according to claim 3 is characterized in that in step (1), annealing temperature is 350 DEG C, holding time is 2h, and the temperature rise rate is controlled at 2 DEG C/min. 7 . The preparation method according to claim 3 , wherein in step (2), the cerium salt is cerium nitrate, the pH regulator is hexamethylenetetramine, and the adjusted pH value is 7.35. 8 . 8. preparation method according to claim 3 is characterized in that in step (2), solvent is ethanol, deionized water, reflux temperature is 70 ℃, and reflux time is 2h. 9. preparation method according to claim 3 is characterized in that in step (2), calcination temperature is 500 DEG C, calcination time is 2h, and calcination heating rate is controlled at 2 DEG C/min. 10. preparation method according to claim 3 is characterized in that in step (3), silver salt is silver nitrate, silver and catalyst mass ratio are 1:20, calcination temperature is 500 ℃, calcination time is 2h, calcination The heating rate was controlled at 2°C/min.

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