CN106881096A - Mesoporous LaFeO3The preparation method of perovskite type composite oxide catalyst material - Google Patents

Abstract

The invention belongs to inorganic functional material technical field, and in particular to a kind of mesoporous LaFeO₃The preparation method of perovskite type composite oxide catalyst material, comprises the following steps：(1) citric acid is dissolved in deionized water and is stirred, to addition La (NO in the citric acid solution after dissolving₃)₃·6H₂O、Fe(NO₃)₃·9H₂O dissolves, and adjusts pH value, adds FDU 12, heating stirring, into xerogel；(2) heat, grinding adds NaNO₂It is well mixed；(3) calcine, alkali cleaning, then obtained final product after suction filtration, washing, drying, grinding.Compared with prior art, the invention has the advantages that：(1) raw material are more cheap and easy to get, the more preferable advantage of heat endurance；(2) mesoporous material has specific surface area high, increase with the effective contact area of gas, so as to improve to CO gas catalysis conversion efficiencies；(3) the more low-carbon environment-friendly of condition needed for.

Description

Preparation method of mesoporous LaFeO3 perovskite composite oxide catalyst material

technical field

The invention belongs to the technical field of inorganic functional materials, and in particular relates to a preparation method of a mesoporous _LaFeO3 perovskite type composite oxide catalyst material.

Background technique

The structural characteristics of mesoporous materials are mainly reflected in the following aspects: first, the pore diameter of mesoporous materials is concentrated between 2 and 50 nm, and the pore size distribution is narrow and adjustable; second, mesoporous materials generally have larger specific surface area and high porosity; third, mesoporous materials can have better thermal stability and a certain degree of hydrothermal stability after optimized synthesis conditions or after treatment; fourth, The composition of mesoporous materials can be diversified and adjustable. Because of these excellent characteristics, mesoporous materials have been widely used in various fields, especially in catalysis.

FDU-12 is an ordered cage-shaped mesoporous silica material, which has very large and adjustable cage-shaped pores and inlets, especially its super-large pore size, which is generally unmatched by other mesoporous structure types of materials. It cannot be achieved, so it is favored by researchers. Moreover, compared with the cylindrical type mesoporous structure system, the large pores with multi-directional openings in this material are considered to be very conducive to the three-dimensional diffusion of substances in it.

Commonly used low-temperature catalytic CO oxidation catalysts include noble metal catalysts and perovskite catalysts. Noble metal catalysts are a class of catalysts that have high low-temperature catalytic activity and can resist corrosion and oxidation in humid environments. The noble metal catalysts used for low-temperature catalytic oxidation of CO mainly include gold-based, platinum-based, palladium-based, and silver-based. Reasons such as easy sintering limit its practical application to some extent. Compared with noble metal catalysts, perovskite catalysts have the following advantages: ①It has a certain oxygen storage capacity, which enables the catalyst to play a better catalytic role in a relatively wide range of air-fuel ratio; ②It has better Excellent durability, thermal stability and certain ability to resist lead poisoning; ③ The source of raw materials is abundant and the price is low. Compared with noble metals Pt, Pd and Rh, rare earth perovskite catalysts are low in price and abundant in reserves. Among the La-based perovskite-type composite oxide materials, LaFeO ₃ has better high-temperature thermal stability and hydrothermal stability than LaMnO ₃ and LaCoO ₃ , and its constituent elements La and Fe are also relatively cheap The constituent elements of La-based perovskite-type composite oxides reduce the cost of raw materials for the preparation of perovskite catalysts, so the present invention selects _LaFeO3 as the research object of perovskite catalyst materials.

However, due to the relatively dense structure of traditional perovskite, its specific surface area is relatively low (<5m ² /g). Practical use of automobile exhaust catalysis. There are many preparation methods for perovskite-type automobile exhaust catalysts. In the research of nearly half a century, pyrolysis method, co-precipitation method, sol-gel method, citric acid complexation method, sintering method and spray drying method have been used successively. , cooling and drying methods. The specific surface area of the perovskite catalyst powder prepared by these methods is basically low. In many early studies, the catalytic activity of the catalyst was improved by doping different metal ions to increase the crystal defects and oxygen vacancies of the catalyst. The research on the modification has been relatively thorough. To further improve its catalytic activity, some new methods must be considered. Synthetic process or system. Generally speaking, the larger the specific surface area, the better the activity of the catalyst, and the larger the specific surface area, the lower the light-off temperature, and the pore structure is also an important factor affecting the catalytic activity. Therefore, increasing the specific surface area of a catalyst is an effective way to improve its catalytic activity. Some researchers use mesoporous carbon materials to prepare perovskite materials to increase the specific surface area of perovskite materials, but perovskite materials usually require high temperature calcination synthesis, and mesoporous carbon materials are thermally stable at high temperatures. Therefore, in recent years, people have turned their attention to mesoporous silica materials with better thermal stability. Using mesoporous silica as the template precursor material, the prepared mesoporous perovskite material has the structural advantages of perovskite materials such as catalytic oxidation ability, good thermal stability and large specific surface area of mesoporous materials. Promising catalytic materials for automotive exhaust purification.

Contents of the invention

The object of the present invention is to provide a kind of preparation method of mesoporous _LaFeO3 perovskite type composite oxide catalyst material, prepare the _LaFeO3 material with mesoporous structure by sol-gel method in conjunction with molten salt method to improve its specific surface area, To increase its effective area for adsorption-desorption in the catalytic process, thereby reducing its catalytic initial conversion temperature for CO gas and improving the catalytic conversion efficiency of the material.

The technical scheme of the present invention to achieve the above object is as follows: the preparation method of mesoporous LaFeO _perovskite type composite oxide catalyst material is characterized in that it comprises the following steps:

(1) Dissolve citric acid in deionized water and stir evenly, then add La(NO ₃ ) ₃ ·6H ₂ O and Fe(NO ₃ ) ₃ ·9H ₂ O to the dissolved citric acid solution in turn and stir until completely dissolved, Slowly add ammonia water dropwise to the mixed solution to adjust the pH value of the solution, then add FDU-12, stir at room temperature, then heat up and stir, the solution is gradually evaporated to dryness into a xerogel;

( ₂ ) heat-treating the xerogel, then grinding the cooled solid particles after the treatment, adding NaNO and continuing to grind until uniformly mixed;

(3) Calcining the mixed solid particles, stirring the cooled product with alkali at constant temperature, and then suction filtering, washing, drying and grinding to obtain the mesoporous LaFeO ₃ perovskite composite oxide catalyst.

According to the above scheme, the preparation method of FDU-12 comprises the following steps:

(a) F127, TMB, and KCl are dissolved in HCl solution and stirred to obtain a mixed solution;

(b) adding TEOS to the above mixed solution and stirring at constant temperature, heating the mixed solution to carry out hydrothermal reaction, and drying the product after suction filtration and washing;

(c) Calcining and cooling the dried product at high temperature to obtain FDU-12.

According to the above scheme, the molar concentration of HCl solution in step (a) is 1～4mol/L, wherein the mass ratio of HCl and F127 is 3～5, the ratio of HCl and TMB mass is 3～4, the mass ratio of HCl and KCl The ratio is 2 to 1.2.

According to the above scheme, the mixed solution in step (a) was stirred at room temperature and kept for 24 h.

According to the above scheme, the mass ratio of TEOS and F127 contained in the mixed solution in step (b) is 4-6, and the constant temperature stirring is carried out at 40° C. for 24 hours.

According to the above scheme, the hydrothermal reaction temperature in step (b) is 80° C. to 120° C., and the holding time is 24 to 72 hours.

According to the above scheme, the product in step (c) is calcined in a muffle furnace at 450° C. to 650° C. for 2 hours to 8 hours, and the heating rate is controlled at 1° C./min to 5° C./min.

According to the above scheme, the ratio of the amount of deionized water to citric acid in step (1) is 3000-4500, the ratio of deionized water to La(NO ₃ ) ₃ 6H ₂ O is 8000-14000, La The ratio of (NO ₃ ) ₃ 6H ₂ O to Fe(NO ₃ ) ₃ 9H ₂ O is 1:1, the pH value of the solution is adjusted to 8.0 with ammonia water, and the molar ratio of FDU-12 and La ⁺ is 1～ 4. The stirring time is 2 hours at room temperature, the heating temperature is 40°C-80°C, and the stirring time is 8h-10h.

According to the above scheme, the heat treatment temperature in step (2) is controlled at 200°C to 300°C until sparks no longer appear, the grinding time after cooling is 20min, ^{and the ratio of the amount of NaNO2 to La+} _added is 10 to 40 , Grinding time 20min again.

According to the above scheme, the calcining temperature in step (3) ranges from 450°C to 650°C, and the heating rate is controlled at 1°C/min to 5°C/min; Stir at ℃ for 4h~10h to wash with alkali.

The invention uses a sol-gel method combined with a molten salt method to prepare a mesoporous _LaFeO3 perovskite catalyst material. The material uses FDU-12 as a hard template to synthesize a LaFeO ₃ perovskite catalyst material with a mesoporous structure. The effective contact area of the gas is reduced to reduce the initial conversion temperature of the catalytic oxidation of CO gas and improve the catalytic conversion efficiency.

Compared with the prior art, the present invention has the following beneficial effects: (1) Among the La-based perovskite catalysts, compared with LaMnO ₃ and LaCoO ₃ , LaFeO ₃ has cheaper raw materials and better thermal stability (2) The mesoporous material has a high specific surface area (92-300m ² /g), which increases the effective contact area with the gas, thereby improving the catalytic conversion efficiency of CO gas; (3) the traditional sol-gel The preparation of LaFeO ₃ perovskite-type composite oxide requires calcination at a high temperature of 750°C to synthesize the product, and the resulting product has large particles, compact structure, and small specific surface area. The present invention combines the sol-gel method on the basis of The advantage of the low-temperature sintering of the molten salt method reduces the calcination temperature to 500°C-600°C, making the conditions required for the preparation lower carbon and environmentally friendly; (4) The raw materials used in the preparation are very common materials that are cheap and easy to obtain, and the equipment is simple , the preparation process is simple and easy.

Description of drawings

Fig. 1 is the SAXS figure of the FDU-12 that the embodiment of the present invention 1 prepares;

Figure 2 is a TEM image of FDU-12 prepared in Example 1 of the present invention, wherein the scale bar of Figure 2-a is 100nm, and the scale bar of Figure 2-b is 50nm;

Fig. 3 is the XRD spectrum of the mesoporous _LaFeO3 perovskite type composite oxide prepared by the embodiment of the present invention 1-5;

Fig. 4 is the XRD spectrum of the mesoporous _LaFeO3 perovskite type composite oxide prepared by the embodiment of the present invention 3;

Fig. 5 is the SEM image of the mesoporous LaFeO ₃ perovskite composite oxide prepared in Example 2 of the present invention, wherein the scale bar of 5-a is 200nm, and the scale bar of 5-b is 100nm;

Fig. 6 is the SEM image of the mesoporous LaFeO ₃ perovskite composite oxide prepared in Example 3 of the present invention, wherein the scale bar of 6-a is 200nm, and the scale bar of 6-b is 100nm;

Figure 7 is an SEM image of the mesoporous LaFeO ₃ perovskite composite oxide prepared in Example 4 of the present invention, wherein the scale bar of 7-a is 200nm, and the scale bar of 7-b is 200nm;

Fig. 8 is the _N2 adsorption-desorption isotherm diagram obtained from the BET test of the mesoporous _LaFeO3 perovskite composite oxide prepared in Example 3 of the present invention;

Fig. 9 is a diagram of the catalytic oxidation conversion rate of mesoporous LaFeO ₃ perovskite composite oxides prepared in Examples 3, 6, 7 and 8 of the present invention to CO.

detailed description

In order to better understand the content of the present invention, the following will be further described in conjunction with specific examples. However, it should be pointed out that the implementation of the present invention is not limited to the following several implementation modes.

Example 1 investigated the effects of different calcination temperatures on mesoporous LaFeO ₃ perovskite composite oxides.

(1) First, dissolve 2.0g F127, 2.4gTMB, and 5g KCl in 120ml (2mol/L) of HCl solution, and stir for 24 hours at room temperature;

(2) Add 8.3g TEOS to the mixed solution, then stir at 40°C for 24h; transfer the mixed solution to the reaction kettle, heat to 100°C for 72h, heat preservation and hydrothermal reaction, after the reaction, suction filter and wash with deionized water until Dry the isolated product at 40°C until no white foam is produced;

(3) Put the dried product in an alumina crucible and bake it in a muffle furnace at 550°C for 6h, the heating rate is controlled at 2°C/min, and the FDU-12 mesoporous material is obtained after cooling;

(4) Dissolve 5.76g of citric acid in 200ml of deionized water, add 4.33g of La(NO ₃ ) ₃ ·6H ₂ O and 4.04g of Fe(NO ₃ ) ₃ ·9H ₂ O to the solution in turn, drop by drop after complete dissolution Add fresh ammonia water to adjust the pH value of the solution to 8.0 to be stable; weigh 1.2 g of the FDU-12 template prepared in step (3) and slowly add it to the solution under magnetic stirring at room temperature, and continue stirring for 2 hours; then place the beaker containing the solution in Stir in a constant temperature oil bath at 80°C for 8 to 10 hours to form dry gel, stop stirring;

(5) Transfer the dry gel to a clean crucible and place it on a resistance wire furnace with an asbestos net for heat treatment at about 260°C, and a fluffy porous solid is obtained in the crucible. Grind the obtained porous solid in a mortar for about 20 minutes, then add 13.80 g NaNO ₂ solid particles and continue grinding for 20 minutes until the mixture is uniform;

(6) Put the mixed powder into a clean alumina crucible and keep it in a muffle furnace at 450° C. for 2 hours, and control the heating rate at 2° C./min to obtain a template-containing sample. Prepare a 2mol/L NaOH solution, weigh a certain amount of template-containing sample and pour it into a certain amount of NaOH solution (2mol/L), stir magnetically in an oil bath at 80°C for 6 hours, repeatedly suction filter, wash until neutral, and dry to obtain Detemplated mesoporous LaFeO ₃ perovskite composite catalyst powder.

As shown in Figure 1 is the SAXS characterization diagram of the FDU-12 sample prepared in Example 1, there is a sharp peak with a very high peak intensity near 0.75° in the figure, indicating that the sample is a highly ordered mesoporous material; Figure 2 is the TEM characterization diagram of the FDU-12 sample prepared in Example 1. The scale bar in Figure 2-a is 100nm, and the scale bar in Figure 2-b is 50nm. From the figure, it can be clearly seen that the sample has a ordered mesoporous structure.

Example 2-5

Other steps and parameters of embodiment 2-4 are identical with embodiment 1, and difference is: in step (6), in muffle furnace, calcining temperature is 500 ℃ of insulation 2h (embodiment 2), 550 ℃ of insulation 2h (embodiment 3), heat preservation at 600°C for 2h (Example 4), and heat preservation at 650°C for 2h (Example 5).

Comparing Examples 1-4, according to the XRD diffraction analysis and characterization of the products prepared at different calcination temperatures in Figure 3, when the calcination temperature is 450 ° C, the XRD characteristic _LaFeO3 characteristic diffraction peak of the sample is not obvious enough ; and when the calcination temperature reaches 650°C, there are more miscellaneous peaks in the XRD characterization chart of the sample; when the calcination temperature is 500°C, 550°C, and 600°C, obvious perovskite characteristic peaks can be seen. Further analysis, when comparing the SEM images (Figures 5, 6, and 7), it can be seen that when the calcination temperature is 500°C, the morphology of the sample is mainly flake, and the internal structure is relatively dense; when the calcination temperature is 600°C, the sample The particle distribution of the sample is not uniform, and each size is different; when the calcination temperature is 550 ° C, the spherical large structure of the sample appears a small particle structure with uniform size distribution, and the morphology of this sample is the most ideal. Figure 4 is the XRD diffraction pattern of the sample in Example 3, which clearly shows that the sample has the characteristic peaks of the perovskite crystal form, the peak shape is sharp and the peak intensity is high without other impurity peaks. It shows that the synthesized samples have good crystallinity, typical perovskite structure, high purity and no impurities. The curve in Fig. 8 shows the _N2 adsorption-desorption-adsorption isotherm diagram of the sample in Example 3. The ASAP2020M automatic specific surface area pore distribution instrument produced by American Mike Instrument Company was used for the test characterization. In this _N2 adsorption-desorption-attachment isotherm diagram, the sample adsorption line and desorption line of embodiment 3 constitute a very obvious hysteresis loop between relative atmospheric pressure 0.4～1.0, show that this sample has mesoporous structure), Calculated according to the BET method, the surface area of the sample prepared in Example 3 was 92.43 m ² /g, the average pore diameter was 10.18 nm, and the average pore volume was 0.24 cm ³ /g.

In summary, the calcination temperature of 550°C, that is, the calcination preparation temperature in Example 3, produces the best mesoporous LaFeO ₃ perovskite phase.

Examples 6-8 investigated the effects of different dosages of FDU-12 on the catalytic performance of mesoporous LaFeO ₃ perovskite oxides.

The mesoporous LaFeO ₃ prepared by the template method uses the FDU-12 silica template as a precursor, injects metal nitrate into the mesoporous pores of FDU-12, and then prepares the composite oxide with perovskite structure through molten salt method calcination . Considering that the preparation of templates and metal nitrates in different proportions may have a certain impact on the crystallinity of perovskite, therefore, different proportions of FDU-12 templates were used for casting.

Other parameter steps of embodiment 6-8 are identical with embodiment 3, difference is: the consumption of FDU-12 is respectively 0.6g (embodiment 6), 1.8g (embodiment 7) and 2.4g ( Example 8). The experimental process for evaluating the CO catalytic activity of the mesoporous _LaFeO3 catalyst sample is roughly as follows: Mix the appropriate amount of catalyst powder particles and asbestos evenly and put them into a quartz tube with a length of 1.5 m and an inner diameter of 8 mm, and then put it into a fixed In the quartz bed reactor, argon (carrier gas), oxygen, and carbon monoxide are introduced, and the gas flow is controlled by an electronic flow valve. The temperature of the catalyst and the reaction gas is controlled by heating the quartz tube through a heating furnace to simulate the tail exhaust of a car. In the working environment of the tube, the catalyzed gas in the quartz tube is passed into the gas chromatograph, and the contents of CO and CO ₂ in the catalyzed "vehicle exhaust" at different temperatures are detected in FID mode to analyze and calculate the catalytic effect of the catalyst. Catalytic conversion efficiency of CO. The curves in FIG. 9 represent the CO catalytic conversion rate spectra of the samples prepared in Examples 3, 6, 7, and 8. The sample prepared in comparative example 3,6,7,8, when the ratio of the amount of substance: when n _(FDU-12) than n _(La+) from 1 to 4, the synthetic LaFeO _The change of the catalytic efficiency of sample The trend is to increase first and then decrease. When the ratio of n _(FDU-12) and n _(La+) is 2, the initial conversion temperature of the prepared mesoporous LaFeO ₃ perovskite catalyst sample for catalytic oxidation of CO is the lowest, and the catalytic conversion rate Highest.

The above-mentioned embodiment is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions and combinations made without departing from the spirit and principle of the present invention , simplification, are all equivalent replacement methods, and are all included in the protection scope of the present invention.

Claims (10)

1. mesoporous LaFeO₃The preparation method of perovskite type composite oxide catalyst material, it is characterised in that including following step Suddenly：

(1) citric acid is dissolved in deionized water and is stirred, to sequentially adding La (NO in the citric acid solution after dissolving₃)₃· 6H₂O、Fe(NO₃)₃·9H₂O is stirred to whole dissolvings, is slowly added dropwise the pH value of regulation solution in ammoniacal liquor to mixed solution, then FDU-12 is added, heat up heating stirring again after being stirred under normal temperature, solution is gradually evaporated into xerogel；

(2) xerogel is heated, is then ground solid particle cooled after treatment, add NaNO₂Continue to grind It is milled to well mixed；

(3) hybrid solid particle is calcined, the product constant temperature stirring alkali cleaning after cooling, then after suction filtration, washing, drying, grinding Just mesoporous LaFeO is obtained₃Perovskite type composite oxide catalyst.

2. mesoporous LaFeO according to claim 1₃The preparation method of perovskite type composite oxide catalyst material, it is special Levy and be：The preparation method of FDU-12 includes following steps：

A be dissolved in HCl solution for F127, TMB, KCl and stir by (), obtain mixed solution；

B TEOS is added constant temperature in above-mentioned mixed solution to stir by (), heating mixed solution carries out hydro-thermal reaction, product suction filtration, is washed Dried after washing；

C dried product high-temperature roasting cooling is obtained FDU-12 by ().

3. mesoporous LaFeO according to claim 2₃The preparation method of perovskite type composite oxide catalyst material, it is special Levy and be：HCl solution molar concentration is that the mass ratio of 1~4mol/L, wherein HCl and F127 is 3~5, HCl in step (a) It is 2~1.2 with the mass ratio that TMB mass ratios are 3~4, HCl and KCl.

4. mesoporous LaFeO according to claim 2₃The preparation method of perovskite type composite oxide catalyst material, it is special Levy and be：Mixed solution is stirred at normal temperatures in step (a), and keeps 24h.

5. mesoporous LaFeO according to claim 2₃The preparation method of perovskite type composite oxide catalyst material, it is special Levy and be：The mass ratio of TEOS, F127 contained by mixed solution is 4~6 in step (b), and constant temperature stirring is stirred at a temperature of 40 DEG C Mix 24h.

6. mesoporous LaFeO according to claim 2₃The preparation method of perovskite type composite oxide catalyst material, it is special Levy and be：Hydrothermal temperature is 80 DEG C~120 DEG C in step (b), and soaking time is 24~72h.

7. mesoporous LaFeO according to claim 2₃The preparation method of Perovskite Catalytic agent material, it is characterised in that：Step C product 450 DEG C~650 DEG C roasting 2h~8h in Muffle furnace in (), heating rate is controlled in 1 DEG C/min~5 DEG C/min.

8. mesoporous LaFeO according to claim 1₃The preparation method of perovskite type composite oxide catalyst material, it is special Levy and be：The ratio between amount of deionized water and lemon acid substance is 3000~4500, deionized water and La (NO in step (1)₃)₃· 6H₂The ratio between amount of O materials is 8000~14000, La (NO₃)₃·6H₂O and Fe (NO₃)₃·9H₂The ratio between amount of O materials is 1:1, Ammoniacal liquor adjusts solution ph to 8.0, FDU-12 and La⁺Mol ratio be 1~4, mixing time 2h under normal temperature, heating-up temperature is 40 DEG C~80 DEG C, stir 8h~10h.

9. mesoporous LaFeO according to claim 1₃The preparation method of perovskite type composite oxide catalyst material, it is special Levy and be：Heat treatment temperature control untill no longer there is Mars, is ground at 200 DEG C~300 DEG C after cooling in step (2) Time consuming 20min, the NaNO of addition₂With La⁺The ratio between the amount of material be 10~40, regrind time 20min.

10. mesoporous LaFeO according to claim 1₃The preparation method of perovskite type composite oxide catalyst material, its It is characterised by：Calcination temperature range is 450 DEG C~650 DEG C in step (3), and heating rate is controlled in 1 DEG C/min~5 DEG C/min； 4h~10h alkali cleanings are stirred under 80 DEG C of environment in the NaOH solution of 2mol/L~10mol/L after being cooled to room temperature.