Samarium manganese mullite catalyst rich in Sm vacancy as well as preparation method and application of samarium manganese mullite catalyst

Abstract

The invention discloses a samarium manganese mullite catalyst rich in Sm vacancy and a preparation method and application thereof, and belongs to the technical field of VOCs waste gas catalytic oxidation. According to the invention, samarium nitrate and manganous nitrate are used as precursors, citric acid monohydrate is used as a chelating agent, urea is used as an additive, and the samarium manganese mullite catalyst rich in Sm vacancies is synthesized through a sol-gel method. The Sm vacancy samarium manganese mullite catalyst prepared by the invention shows excellent toluene catalytic oxidation low-temperature activity due to selective removal of inert Sm atoms on the surface of samarium manganese mullite, more active Mn atoms are exposed, and the surface lattice oxygen migration performance is enhanced. The catalyst has excellent performance in a stability test and a water vapor resistance test, and has a wide industrial application prospect.

Description

Technical Field

[0001] The invention relates to a samarium manganese mullite catalyst rich in Sm vacancies and a preparation method and application thereof, belonging to the technical field of catalytic oxidation of VOCs waste gas. Background Art

[0002] VOCs are common atmospheric pollutants. They are not only toxic, mutagenic, carcinogenic, and teratogenic, but also serve as precursors to photochemical smog, ozone, and secondary aerosols, severely polluting the atmosphere. With the continued advancement of global industrialization and the continued use of fossil fuels, VOC emissions are increasing annually, posing a serious threat to human health and the natural environment. Thermal catalytic oxidation, with its high catalytic efficiency, lack of adsorption saturation, zero secondary pollution, and thorough degradation, has become the mainstream method for VOC end-of-pipe treatment. Precious metal catalysts offer advantages such as high low-temperature activity, high stability, corrosion resistance, excellent regeneration, and high selectivity. However, their relatively high cost and susceptibility to deactivation due to sintering or poisoning significantly limit their practical application. Transition metal catalysts, with their high activity, low cost, and excellent thermal stability, have attracted considerable attention and are expected to replace precious metal catalysts in playing a significant role in VOC treatment.

[0003] Samarium manganese mullite (SmMn2O5) is a unique manganese oxide with high thermal stability and a unique Mn-Mn dimer structure that can generate four different lattice oxygen species. This allows for a wide range of activity control and has attracted increasing attention in the fields of diesel exhaust and VOC purification. However, since the Sm element is more readily exposed on the surface of SmMn2O5 than Mn, its ability to oxidize toluene is suppressed. Current approaches to this problem primarily involve acid etching to remove Sm atoms. However, the depth of the acid etch is difficult to control, which can easily cause structural collapse and the formation of new phases, making it difficult to maintain structural properties. Therefore, new surface modification methods are needed to achieve the selective removal of Sm atoms while maintaining the structural stability of manganese-based mullite. Summary of the Invention

[0004] In order to overcome the problems in the background technology, the purpose of the present invention is to provide a samarium manganese mullite catalyst rich in Sm vacancies and its preparation method and application, so as to solve the problems of insufficient exposure of Mn atoms, low catalytic activity and poor water resistance of the original samarium manganese mullite catalyst.

[0005] In order to achieve the above object, the present invention is implemented through the following technical solutions:

[0006] A method for preparing a samarium-manganese mullite catalyst rich in Sm vacancies comprises the following steps:

[0007] (1) dissolving samarium nitrate hexahydrate and manganese nitrate in deionized water to obtain solution A;

[0008] (2) dissolving urea and citric acid monohydrate in solution A to obtain a mixed solution, then heating and stirring to obtain a gel product, and drying the gel product to obtain a precursor;

[0009] (3) The precursor is calcined in an air atmosphere to obtain a Sm vacancy-rich Sm manganese mullite catalyst.

[0010] Preferably, the molar ratio of the samarium nitrate hexahydrate to the manganese nitrate is 1:2.

[0011] Preferably, the ratio of the amount of urea to the sum of the amounts of samarium nitrate and manganese nitrate is 6:1-1:1, and the ratio of the amount of citric acid monohydrate to the sum of the amounts of samarium nitrate and manganese nitrate is 2:1-0.5:1.

[0012] Preferably, the heating temperature is 70-90° C. and the heating time is 6-10 h.

[0013] Preferably, the calcination temperature is increased at a rate of 1-3°C / min, the temperature is 700°C-900°C, and the calcination time is 4h-6h.

[0014] The present invention also claims protection of the Sm vacancy-rich Sm-manganese mullite catalyst prepared by the preparation method of the Sm vacancy-rich Sm-manganese mullite catalyst.

[0015] The present invention also claims to protect the use of the samarium manganese mullite catalyst rich in Sm vacancies in catalytic oxidation of VOCs.

[0016] Beneficial effects of the present invention:

[0017] (1) The present invention improves the preparation method of samarium manganese mullite by adding urea and adding a sol-gel process, so that the Sm atoms on the surface of the original samarium manganese mullite are selectively removed, forming Sm vacancies, resulting in a catalyst with a higher surface manganese content, strengthening the electron transfer between samarium vacancies, manganese atoms and lattice oxygen, enhancing the migration performance of surface lattice oxygen and the adsorption capacity for toluene, effectively improving the catalytic oxidation activity of toluene, and at the same time does not change the crystal structure of mullite, and still has excellent structural stability.

[0018] (2) The present invention promotes the dissociation of water vapor at the Sm vacancy by constructing Sm vacancies, forming surface active hydroxyl groups as active oxygen species to participate in the toluene oxidation reaction process, thereby exhibiting excellent water resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The XRD spectra of the samples obtained in Comparative Example 1 and Example 2 of the present invention are shown.

[0020] Figure 2 Figures 1 and 2 are SEM images and EDS elemental mapping images of Sm, Mn, and O of the samples obtained in Comparative Example 1 and Example 2 of the present invention; a is an SEM image of the sample obtained in Comparative Example 1; b is a high-magnification SEM image of the boxed area in Figure a; c is an EDS elemental mapping image of Sm, Mn, and O of the sample obtained in Comparative Example 1; d is an SEM image of the sample obtained in Example 2; e is a high-magnification SEM image of the boxed area in Figure d; f is an EDS elemental mapping image of Sm, Mn, and O of the sample obtained in Example 2.

[0021] Figure 3 These are the high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission images (HAADF STEM) of the sample of Comparative Example 1 of the present invention; a is the HRTEM image, b is the HAADF-STEM image, and c is the lattice spacing measurement result of Line 1 and Line 2 in Figure b.

[0022] Figure 4 These are high-resolution transmission electron microscopy (HRTEM) images and high-angle annular dark field scanning transmission images (HAADF STEM) of the sample of Example 2 of the present invention; a is the HRTEM image, b is the HAADF-STEM image, and c is the lattice spacing measurement result image of Line 1 and Line 2 in Figure b.

[0023] Figure 5 These are the H2-TPR and O2-TPD results of the samples obtained in Comparative Example 1 and Example 2 of the present invention, wherein a is the H2-TPR result diagram, and b is the O2-TPD result diagram.

[0024] Figure 6 This is a graph showing the toluene oxidation performance test results of the samples obtained from Comparative Examples 1-2 and Examples 1-3 of the present invention.

[0025] Figure 7 This is a diagram showing the water resistance test results of the sample obtained in Comparative Example 1 of the present invention.

[0026] Figure 8 This is a diagram showing the water resistance test results of the sample obtained in Example 2 of the present invention.

[0027] Figure 9 This is a diagram showing the water resistance test results of the sample obtained in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described in detail below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto.

[0029] Example 1

[0030] A method for preparing a samarium-manganese mullite catalyst rich in Sm vacancies comprises the following steps:

[0031] Step 1: Dissolve 2.225 g (5 mmol) of Sm(NO3)3·6H2O and 2.33 mL (10 mmol) of Mn(NO3)2 solution (50 wt.%) in 100 mL of deionized water and stir magnetically at room temperature for 3 h to obtain solution A.

[0032] Step 2: Dissolve 1.35 g of urea and 3.467 g of citric acid monohydrate in solution A and continue stirring for 1 h to obtain a mixed solution.

[0033] Step 3: The beaker containing the mixed solution was then transferred to an oil bath maintained at 70°C, stirred and evaporated for 10 h to obtain a gel-like product.

[0034] Step 4: Transfer the gel-like product to a constant temperature drying oven at 120°C and dry it for 12 hours, and then grind it to obtain a precursor.

[0035] Step 5: Transfer the powdered precursor to a tube furnace and heat it to 900°C at a heating rate of 1°C min -1 , and calcined in air atmosphere for 4 hours. After calcination, the powder obtained was samarium manganese mullite catalyst containing Sm vacancies (SMO-0.5U).

[0036] Example 2

[0037] A method for preparing a samarium-manganese mullite catalyst rich in Sm vacancies comprises the following steps:

[0038] Step 1: Dissolve 2.225 g of Sm(NO3)3·6H2O and 2.33 mL of Mn(NO3)2 solution (50 wt.%) in 100 mL of deionized water and stir magnetically at room temperature for 3 h to obtain solution A.

[0039] Step 2: Dissolve 2.7 g of urea and 3.467 g of citric acid monohydrate in solution A and continue stirring for 1 h to obtain a mixed solution.

[0040] Step 3: The beaker containing the mixed solution was then transferred to an oil bath maintained at 90°C, stirred and evaporated for 8 h to obtain a gel-like product.

[0041] Step 4: Transfer the gel-like product to a constant temperature drying oven at 120°C and dry it for 12 hours, and then grind it to obtain a precursor.

[0042] Step 5: Transfer the powdered precursor to a tube furnace and heat it to 800°C at a heating rate of 1°C min -1, and calcined in air atmosphere for 5 hours, and the calcined powder obtained the samarium manganese mullite catalyst containing Sm vacancies (SMO-U).

[0043] Example 3

[0044] A method for preparing a samarium-manganese mullite catalyst rich in Sm vacancies comprises the following steps:

[0045] Step 1: Dissolve 2.225 g of Sm(NO3)3·6H2O and 2.33 mL of Mn(NO3)2 solution (50 wt.%) in 100 mL of deionized water and stir magnetically at room temperature for 3 h to obtain solution A.

[0046] Step 2: Dissolve 5.4 g of urea and 1.575 g of citric acid monohydrate in solution A and continue stirring for 1 h to obtain a mixed solution.

[0047] Step 3: The beaker containing the mixed solution was then transferred to an oil bath maintained at 80°C, stirred and evaporated for 6 h to obtain a gel-like product.

[0048] Step 4: The gel-like product was transferred to a constant temperature drying oven at 100°C and dried for 15 hours, and then ground to obtain a precursor.

[0049] Step 5: Transfer the powdered precursor to a tube furnace and heat it to 700°C at a heating rate of 3°C min -1 , and calcined in air atmosphere for 4 hours, and the calcined powder obtained the samarium manganese mullite catalyst containing Sm vacancies (SMO-2U).

[0050] Example 4

[0051] A method for preparing a samarium-manganese mullite catalyst rich in Sm vacancies comprises the following steps:

[0052] Step 1: Dissolve 2.225 g (5 mmol) of Sm(NO3)3·6H2O and 2.33 mL (10 mmol) of Mn(NO3)2 solution (50 wt.%) in 100 mL of deionized water and stir magnetically at room temperature for 3 h to obtain solution A.

[0053] Step 2: Dissolve 0.9 g of urea and 6.3 g of citric acid monohydrate in solution A and continue stirring for 1 h to obtain a mixed solution.

[0054] Step 3: The beaker containing the mixed solution was then transferred to an oil bath maintained at 70°C, stirred and evaporated for 10 h to obtain a gel-like product.

[0055] Step 4: Transfer the gel-like product to a constant temperature drying oven at 120°C and dry it for 12 hours, and then grind it to obtain a precursor.

[0056] Step 5: Transfer the powdered precursor to a tube furnace and heat it to 900°C at a heating rate of 1°C min -1 , and calcined in air atmosphere for 4 hours. After calcination, the powder obtained was samarium manganese mullite catalyst (SMO-0.5U-1) containing Sm vacancies.

[0057] Comparative Example 1

[0058] A method for preparing a samarium manganese mullite catalyst (SMO) free of Sm vacancies comprises the following steps:

[0059] Step 1: Dissolve 2.225 g of Sm(NO3)3·6H2O and 2.33 mL of Mn(NO3)2 solution (50 wt.%) in 100 mL of deionized water and stir magnetically at room temperature for 3 h to obtain solution A.

[0060] Step 2: Dissolve 3.467 g of citric acid monohydrate in solution A and continue stirring for 1 h to obtain a mixed solution.

[0061] Step 3: The beaker containing the mixed solution was then transferred to an oil bath maintained at 90°C, stirred and evaporated for 8 h to obtain a gel-like product.

[0062] Step 4: Transfer the gel-like product to a constant temperature drying oven at 120°C and dry it for 12 hours, and then grind it to obtain a precursor.

[0063] Step 5: Transfer the powdered precursor to a tube furnace and heat it to 800°C at a heating rate of 1°C min -1 , and calcined in air atmosphere for 5 hours. After calcination, the powder obtained was samarium manganese mullite catalyst (SMO) without Sm vacancies.

[0064] Comparative Example 2

[0065] A method for preparing a samarium-manganese-mullite catalyst free of Sm vacancies comprises the following steps:

[0066] Step 1: 2.225 g of Sm(NO3)3·6H2O, 1.165 g of Mn(NO3)2, 1.35 g of urea and 3.467 g of citric acid monohydrate were mixed to obtain a powdery precursor material.

[0067] Step 2: Transfer the powdered precursor to a tube furnace and heat it to 800°C at a heating rate of 1°C min -1 , and calcined in air atmosphere for 5 h. After calcination, the powder obtained was a samarium manganese mullite catalyst (SMO-mix) without Sm vacancies.

[0068] Effect Examples

[0069] Figure 1The XRD spectra of SMO and SMO-U samples obtained in Comparative Example 1 and Example 2 are shown in FIG. Figure 1 It can be seen that SMO and SMO-U exhibit diffraction peaks that are almost consistent with those of the standard card PDF#52-1096, among which 2θ=15.5°, 23.9°, 25.9°, 28.6°, 30.5°, 33.5°, 35.3°, 41.2°, 51.3°, 53.4° and 58.9° correspond to the (001), (200), (021), (121), (211), (130), (112), (212), (331), (042) and (332) crystal planes of SmMn2O5, respectively. This indicates that the synthesized SMO and SMO-U have a single SmMn2O5 crystal feature. After adding urea, the diffraction peak intensity of SMO-U weakened, indicating that urea can reduce the crystallinity of SmMn2O5 to a certain extent, but does not change its crystal type.

[0070] Figure 2 The SEM images and EDS element mappings of Sm, Mn and O of the SMO and SMO-U samples obtained in Comparative Example 1 and Example 2 are shown in FIG. Figure 2 As shown in ab, the surface of the SMO block is dense and flat, while the surface of the SMO-U particle is fluffy and spongy. Under high magnification, its details are found to be lamellar structures ( Figure 2 de). The surface Sm, Mn, and O elements of the sample were scanned using EDS energy spectrum. The results showed that the surface Sm / Mn of SMO was 0.50 ( Figure 2 c), which is consistent with the atomic ratio of SmMn2O5, while the Sm / Mn ratio of the SMO-U surface is 0.48 ( Figure 2 f), which is lower than the theoretical value, indicating that urea modification allows more Mn to be exposed on the surface, which is beneficial to the improvement of catalytic activity.

[0071] The element contents of SMO and SMO-U samples were analyzed by ICP. The results are shown in Table 1. The Sm content in the SMO-U sample is lower than that in SMO. The calculated Sm / Mn atomic ratio of SMO is 0.50, which is consistent with the theoretical value of SmMn2O5. However, the Sm / Mn atomic ratio of SMO-U is 0.47, indicating that Sm atoms are indeed selectively removed during the urea modification pyrolysis process.

[0072] Table 1 Elemental analysis results of SMO and SMO-U samples

[0073]

[0074] Figure 3 and Figure 4The high-resolution TEM images and high-angle annular dark field scanning transmission images (HAADFSTEM) of the SMO and SMO-U samples obtained in Comparative Example 1 and Example 2 are shown. Figure 3 a and Figure 4 a It can be seen that the lattice spacing of SMO and SMO-U samples are 0.285nm and 0.288nm respectively, both corresponding to the (002) crystal plane of SmMn2O5. Figure 3 b It can be observed that the atomic arrangement of the SMO sample surface is relatively uniform in brightness, while in the STEM of SMO-U ( Figure 4 In b), we can see that there are obviously grid points with different color depths, and different types of metal atoms have different brightness. Scan the height of each atom on Line 1 and Line 2. Figure 3 c shows that the peak width and peak height of the SMO sample are uniformly distributed, indicating that the elements on the SMO surface are relatively uniformly distributed. Figure 4 The large difference in peak width and peak height distribution along the SMO-U scanning path in c is due to the uneven distribution of Sm and Mn, and the dark spots are Sm atomic vacancies.

[0075] Figure 5 The H2-TPR and O2-TPD results of the samples obtained in Comparative Example 1 and Example 2 of the present invention are shown in FIG. Figure 5 It can be seen from a that the center temperature of the H2 reduction peak in the low and high temperature ranges of SMO-U is reduced by 42°C and 33°C respectively compared with SMO, indicating that the formation of Sm vacancies significantly improves the reduction performance of SMO-U. The oxygen desorption curves of SMO and SMO-U are shown in Figure 2. Figure 5 As shown in Figure 2b, a desorption peak of chemically adsorbed oxygen is observed on SMO at around 207°C, while no obvious desorption peak of chemically adsorbed oxygen is observed on SMO-U, indicating that the adsorbed oxygen content on SMO-U is lower than that on SMO. However, a large and broad surface lattice oxygen desorption peak is detected on SMO-U at 359°C, while no peak is observed on SMO. This indicates that the surface lattice oxygen of SMO-U has better mobility and is easier to desorb from the material surface than SMO. This is the main reason for its improved catalytic oxidation performance of toluene.

[0076] The specific process of the toluene oxidation performance test of the present invention includes: using a fixed bed reactor to test the catalytic oxidation performance of toluene, 0.1g of powdered catalyst is mixed with 0.2g of quartz sand and filled in the middle of a quartz tube, and then fixed in the constant temperature zone of a tubular furnace. After checking the air tightness of the system, the reaction gas is first introduced from the bypass, and the initial concentration of toluene is measured after the system is stabilized. After that, the raw gas is switched to the reactor, and the temperature is raised after the catalyst is saturated with toluene. The reaction temperature is precisely controlled by the K-type digital temperature control system integrated in the tubular furnace. Data is measured every 15 minutes at each temperature point, and the average value is taken as the experimental result at the temperature three times in a row. Finally, the catalytic performance curve of the catalyst at different reaction temperatures can be obtained. Figure 6 The toluene oxidation performance test results of the samples obtained from Comparative Example 1 and Examples 1-3 of the present invention are shown. The toluene conversion rates of all samples showed an "S"-shaped upward trend with increasing temperature. The catalytic activities of SMO-0.5U, SMO-U, SMO-2U, and SMO-0.5U-1, which are rich in Sm vacancies, were superior to those of the SMO samples without Sm vacancies. The prepared SMO-U showed the best catalytic performance, T 90 is 236℃, compared with the T of SMO (259℃) 90 The T of Comparative Example 2 was reduced by 23°C. 90 It is 273℃.

[0077] Figure 7 and 8 The water resistance test results of the samples obtained in Comparative Example 1 and Example 2 of the present invention are shown in FIG. Figure 7 and 8 It can be seen that at similar initial toluene conversion, the responses of SMO and SMO-U to water vapor are significantly different. Figure 7 In the experiment, the toluene conversion rate on the SMO decreased with the addition of water vapor, and the degree of decrease was positively correlated with the water vapor content. When 10 vol.% water vapor was introduced, the toluene conversion rate dropped by as much as 29.4%, indicating that water vapor had a significant inhibitory effect on the conversion of toluene. Once the water vapor was stopped, the toluene conversion rate recovered rapidly, indicating that the effect of water vapor was reversible. Figure 7 After nearly 900 minutes of testing (the 900-minute test included a transient response experiment process in which the reaction conditions of 3 vol.% water (120 minutes), 5 vol.% water (120 minutes), and 10 vol.% water (160 minutes) were alternately switched with the water-free condition (500 minutes in total), during which the initial toluene concentration was maintained at 600 ppm and the reaction temperature was 250°C, so that the initial toluene conversion rate was maintained at 70-80%), the toluene conversion rate was still 3% lower than the initial conversion rate. Figure 7In the experiment, after the introduction of 3 vol.% water vapor, the toluene conversion rate of SMO-U decreased by only 1.4%, while after the introduction of 5 vol.% water vapor, the toluene conversion rate decreased by 4.3%, which was less than the decrease of SMO under the same reaction conditions. In addition, after the introduction of 10 vol.% water vapor, the toluene conversion rate did not decrease but increased, indicating that water vapor has a promoting effect on toluene conversion. Figure 8 After nearly 900 minutes of testing ((the 900-minute test included a transient response experiment process in which 3 vol.% water (120 minutes), 5 vol.% water (120 minutes), and 10 vol.% water (120 minutes) were alternately switched with water-free conditions (a cumulative 500 minutes), during which the initial toluene concentration was maintained at 600 ppm and the reaction temperature was 230°C, so that the initial toluene conversion rate was maintained at 70-80%), the toluene conversion rate was increased by 3.7% compared to the initial stage of the reaction. Based on the opposite results of the responses of SMO and SMO-U to water vapor and the differences in their own structures, it can be judged that this is due to the different results caused by the presence of Sm vacancies on the surface. The above results confirm that the SMO-U catalyst obtained by the preparation method of Example 2 has excellent catalytic activity stability and water resistance, and has prospects for industrial application.

[0078] Figure 9 This is the water resistance test result of Comparative Example 2. When 3% water vapor was introduced, the toluene conversion rate dropped rapidly by 23% and showed a continuous downward trend. After the water vapor was stopped, the catalytic efficiency could not return to the initial state.

[0079] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made in form and details without departing from the scope defined by the claims of the present invention.

Claims

1. A method for preparing a samarium-manganese mullite catalyst rich in Sm vacancies, characterized in that: The steps include: (1) dissolving samarium nitrate hexahydrate and manganese nitrate in deionized water to obtain solution A; (2) dissolving urea and citric acid monohydrate in solution A to obtain a mixed solution, then heating and stirring to obtain a gel product, and drying the gel product to obtain a precursor; (3) The precursor is calcined in an air atmosphere to obtain a Sm vacancy-rich Sm manganese mullite catalyst.

2. The method for preparing a samarium-manganese mullite catalyst rich in Sm vacancies according to claim 1, wherein: The molar ratio of the samarium nitrate hexahydrate to the manganese nitrate is 1:

2.

3. The method for preparing a Sm vacancy-rich samarium manganese mullite catalyst according to claim 1, wherein: The ratio of the amount of urea to the sum of the amounts of samarium nitrate and manganese nitrate is 6:1-1:1, and the ratio of the amount of citric acid monohydrate to the sum of the amounts of samarium nitrate and manganese nitrate is 2:1-0.5:

1.

4. The method for preparing a Sm vacancy-rich samarium manganese mullite catalyst according to claim 3, wherein: The heating temperature is 70-90° C. and the heating time is 6-10 h.

5. The method for preparing a Sm vacancy-rich samarium manganese mullite catalyst according to claim 1, wherein: The calcination temperature rise rate is 1-3°C / min, the temperature is 700°C-900°C, and the time is 4h-6h.

6. The Sm vacancy-rich Sm-manganese mullite catalyst prepared according to the preparation method of the Sm vacancy-rich Sm-manganese mullite catalyst according to any one of claims 1 to 5.

7. Use of the Sm vacancy-rich samarium manganese mullite catalyst according to claim 6 in catalytic oxidation of VOCs.

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