Preparation method and application of doped hierarchical porous manganese dioxide

Abstract

The invention discloses a preparation method and application of doped hierarchical porous manganese dioxide, and belongs to the field of catalyst preparation and air purification. According to the invention, potassium permanganate is used as a manganese source and an oxidizing agent, a modified reducing agent is added, mixing and stirring reaction are carried out at room temperature, and the doped hierarchical porous manganese dioxide catalyst is synthesized after washing, drying and high-temperature treatment. The preparation method is simple in preparation process and high in repeatability; the prepared manganese dioxide realizes doping of potassium ions and formation of oxygen vacancies, and the catalytic activity of the manganese dioxide is improved; the doped manganese dioxide catalyst has a multi-stage structure of micropores, mesopores and macropores, the multi-stage pore structure endows the doped manganese dioxide catalyst with a relatively large specific surface area, the physical adsorption capacity of the doped manganese dioxide catalyst can be improved, and meanwhile, more active sites are exposed, so that the doped manganese dioxide catalyst shows relatively high activity and stability in a reaction for catalytically oxidizing gaseous pollutant formaldehyde at room temperature.

Description

Technical Field

[0001] The present invention relates to a catalyst material, belonging to the fields of catalyst preparation and air purification, and specifically to a preparation method of doped multi-level porous manganese dioxide and application thereof. Background Art

[0002] Formaldehyde, a gaseous pollutant that poses a threat to public health, is widely present in everyday items such as home decoration materials, furniture, and textiles. It is highly reactive, volatile, and releases formaldehyde over time. Long-term exposure to excessive formaldehyde levels increases the risk of respiratory, immune, nervous, and blood system diseases. It has been classified as a Group 1 carcinogen by the World Health Organization's International Agency for Research on Cancer. Controlling indoor formaldehyde pollution has become a key component of indoor air quality management and public health.

[0003] Indoor formaldehyde pollution control methods fall into two categories: source control and end-of-pipe treatment. Source control is the most effective measure for controlling formaldehyde pollution, but it cannot completely prevent the release of formaldehyde pollutants. End-of-pipe treatment remains a necessary method for formaldehyde pollution control. Currently, end-of-pipe treatment methods for formaldehyde can be divided into ventilation, physical adsorption, and chemical decomposition. Ventilation is the most convenient and economical method, but its operation is restricted by factors such as time, space, and human activities. Physical adsorption has a limited adsorption capacity and is prone to secondary pollution when adsorption reaches saturation. Chemical decomposition methods, especially catalytic decomposition, are gaining increasing attention as a method for completely decomposing formaldehyde.

[0004] Compared to titanium dioxide catalysts, transition metal oxide manganese dioxide (MnO2) has become a hot topic in research and application due to its unrestricted light source and high catalytic efficiency. However, the efficiency of MnO2 catalysts in formaldehyde degradation is heavily dependent on their active sites. These active sites on the catalyst surface are easily covered by the support in the reaction environment, causing pore clogging and deactivation of the catalytic sites, affecting their catalytic efficiency and stability. Furthermore, the properties of MnO2 are largely dependent on its morphology and crystal structure. Summary of the Invention

[0005] In response to the problems of the prior art, the present invention aims to overcome the problems of low catalytic activity and easy deactivation of active sites in the catalytic decomposition of formaldehyde in the manganese dioxide catalyst in the prior art, and provides a doped multi-level porous manganese dioxide catalyst. The manganese dioxide of the present invention has a hierarchical pore structure with a macropore-mesopore-micropore distribution, and is doped with a relatively high concentration of potassium ions, so that the prepared manganese dioxide has the advantages of high catalytic activity, stable performance and long life.

[0006] The second object of the present invention is to provide a method for preparing doped multi-level porous manganese dioxide, which is low in cost, simple to operate, and suitable for industrial-scale production.

[0007] To achieve the above object, the present invention provides the following technical solution: a method for preparing doped multi-level porous manganese dioxide, comprising the following steps:

[0008] Step a. Dissolve potassium permanganate in deionized water to prepare a manganese source solution.

[0009] Step b. dissolving potassium bicarbonate in deionized water, adding a reducing agent after complete dissolution, and continuing to stir until uniformly mixed to form a modified reducing agent solution; the reducing agent is at least one of decyl glucoside, octyl glucoside, nonyl glucoside, dodecyl glucoside, and tetradecyl glucoside.

[0010] Step c. While the modified reducing agent solution is being stirred, the manganese source solution is added to the modified reducing agent solution via a peristaltic pump at a predetermined flow rate. After the modified reducing agent solution is added, the stirring reaction is continued for a predetermined time. After the reaction is completed, a brown dispersion is obtained. The filter cake is filtered, washed, and dried. Preferably, the drying temperature is between 60° C. and 100° C.

[0011] Step d. Crushing and grinding the obtained dried filter cake, and then placing it in a vacuum furnace for vacuum heat treatment to obtain doped multi-level porous manganese dioxide. In some preferred embodiments, the vacuum heat treatment temperature is 240-300° C., the vacuum heat treatment time is 6-12 hours, and the vacuum degree is 0.1-0.01 Pa.

[0012] Furthermore, the molar ratio of the manganese source potassium permanganate to the reducing agent decyl glucoside is 1:(0.5-2). More preferably, the molar ratio of the manganese source to the reducing agent is 1:(0.5-1).

[0013] In some preferred embodiments, the concentration of the manganese source solution in step a is 0.1-0.25 mol / L.

[0014] In some preferred embodiments, the concentration of potassium bicarbonate or potassium carbonate used in the modified reducing agent solution in step b is 0-1.25 mol / L, and the concentration of the reducing agent is 0.02-0.2 mol / L.

[0015] The manganese dioxide prepared by the present invention has a birnessite crystal form and comprises a micropore-mesopore-macroporous structure, wherein the macropore diameter is between 50 nm and 100 nm, the mesopore diameter is between 2 nm and 50 nm, and the micropore diameter is between 0.7 nm and 2 nm. Potassium ions are doped in the pores, and the potassium ion doping concentration is 0.9% to 5.3%. The specific surface area of ​​the manganese dioxide is 150 to 220 m2 / g, pore capacity is 0.2~0.5cm 3 / g.

[0016] The catalytic reaction conditions and results of the present invention are as follows: Referring to the total attenuation test method in Appendix E of GB / T 18801-2022, the formaldehyde removal rate of Examples 1-3 and the comparative example was tested. The test time was 1 hour and the test chamber was 3m 3 The test temperature was 25 ± 2 °C and the humidity was 50 ± 5%. The manganese dioxide catalyst was loaded on the filter cotton with a loading of 0.02 g cm -2 , the total load is 2g, the filter cotton loaded with manganese dioxide catalyst is placed in a commercially available car purifier, replacing the purification filter element that comes with the machine, the fan power is 2.5W; the initial value of formaldehyde gaseous pollutants is C0: 1.02ppm, and the concentration value is recorded every 5min until it reaches 1h, and the recorded concentration value is C 60 The calculation formula for formaldehyde removal rate is (C0-C 60 ) / C0, each sample was tested three times. The present invention has the following beneficial effects:

[0017] (1) The present invention introduces a high-concentration potassium ion environment to prepare a manganese dioxide catalyst doped with a higher content of potassium ions. The higher concentration of potassium ions doping introduces more oxygen vacancies, providing more active sites for the catalyst.

[0018] (2) The present invention uses decyl glucoside, a nonionic surfactant with a reducing effect, as a surfactant template and a reducing agent to prepare a manganese dioxide catalyst with a microporous-mesoporous-macroporous multi-level pore structure.

[0019] (3) The raw materials used in the present invention are low in cost, the synthesis process is simple, and it is suitable for industrial-scale production.

[0020] (4) The doped manganese dioxide catalyst with a multi-level pore structure prepared by the present invention exhibits a high removal efficiency in the application of removing formaldehyde at room temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the pore size distribution diagram of Examples 1-3.

[0022] Figure 2 is the XRD spectrum of Example 1-3.

[0023] Figure 3 The XPS spectra of Examples 1-3 are shown.

[0024] Figure 4 The XPS K2p fine spectra of Examples 1-3.

[0025] Figure 5 This is the XPS O1s fine spectrum of Examples 1-3.

[0026] Figure 6 is the XRD spectrum of Comparative Example 1.

[0027] Figure 7 This is the pore size distribution diagram of Comparative Example 1.

[0028] Figure 8 This is a graph showing the 1h formaldehyde removal rates of Examples 1-3 and Comparative Example 1. DETAILED DESCRIPTION

[0029] To better understand the technical solutions of the present invention, embodiments of the present invention are described in detail below with reference to the accompanying drawings. It should be understood that the embodiments described are only a portion of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0030] Unless otherwise specified, the test materials and equipment used in the examples are common commercial products and can be purchased in the market.

[0031] In the synthesis of manganese dioxide catalysts, potassium permanganate is a commonly used raw material. Its concentration and the choice of reducing agent, as well as the type and concentration of cations in the solution, reaction conditions and drying conditions can all affect the crystal form, morphology and specific surface area of ​​the catalyst.

[0032] In an embodiment of the present invention, 0.1-0.25 mol / L potassium permanganate is used as a manganese source solution; potassium bicarbonate containing 0-1.25 mol / L and 0.02-0.2 mol / L decyl glucoside are used as a modified reducing agent solution. While keeping the modified reducing agent solution under stirring, the manganese source solution is added to the modified reducing agent solution through a peristaltic pump at a certain flow rate. After the addition of the modified reducing agent solution, the stirring reaction is continued for a certain time, and the reaction is carried out at room temperature (reaction temperature 25 ± 5 ° C). After the reaction is completed, a brown dispersion is obtained, and the filter cake is washed and dried after filtration. After drying, the filter cake is crushed and ground, placed in a vacuum furnace for vacuum heat treatment, and doped multi-level porous manganese dioxide is obtained. In some preferred embodiments, the vacuum heat treatment temperature is 240-300 ° C, the vacuum heat treatment time is 6-12 h, and the vacuum degree is 0.1-0.01 Pa. Furthermore, the molar ratio of potassium permanganate to decyl glucoside is 1: (0.5-2). More preferably, the molar ratio of the manganese source to the reducing agent is 1:(0.5-1).

[0033] During the development of the present invention, through extensive experiments, it was found that under the premise of determining the type of reducing agent, the crystal form of manganese dioxide is basically determined, and the raw material ratio has little effect on the crystal form. The influence on the pore size distribution of manganese dioxide is mainly related to the concentration of the reducing agent used. In addition to decyl glucoside, the reducing agent can also be one or more of octyl glucoside, nonyl glucoside, dodecyl glucoside, and tetradecyl glucoside. The reducing agent used is itself a nonionic surfactant whose functional group has a reducing effect. In the experiment, it acts as both a reducing agent and a surfactant.

[0034] The potassium bicarbonate in the modified reducing agent solution can also be replaced by potassium carbonate, which mainly serves to provide a potassium ion environment and an alkaline environment.

[0035] During the research and development process, it was also found that if the manganese source solution exceeds 0.25 mol / L, the particles will agglomerate and the dispersion will deteriorate.

[0036] Example 1

[0037] Step 1: Add 10 L of deionized water and 200 g of potassium permanganate into a stirred reactor, stir and dissolve to prepare a 0.13 mol / L manganese source solution.

[0038] Step 2: In another stirred reactor, add 10 L of deionized water and 1 kg of potassium bicarbonate. After stirring to dissolve, add 352 g of decyl glucoside and continue stirring to obtain a modified reducing agent solvent solution. The potassium bicarbonate concentration is approximately 1.00 mol / L, and the decyl glucoside concentration is approximately 0.11 mol / L.

[0039] Step 3: At room temperature, while stirring, gradually add the manganese source solution prepared in Step 1 to the modified reducing agent solution in Step 2 via a peristaltic pump at a rate of 0.5 L / min. Continue stirring and react for 6 hours. After the reaction is complete, filter the resulting dispersion and wash it three times with deionized water. Dry the resulting filter cake in a blast dryer at 80°C for 12 hours to obtain a brown-black cake. Because the reducing agent used has a weak reducing property, the reaction is relatively mild, and the flow rate has no significant effect on the reaction.

[0040] Step 4: The brown-black block filter cake is crushed by a crusher to obtain a powder sample, and the powder sample is placed in a vacuum furnace for vacuum heat treatment at a heat treatment temperature of 260°C, maintaining a vacuum degree of 0.05 Pa, and a vacuum treatment time of 8 hours to obtain doped multi-level porous manganese dioxide.

[0041] Example 2

[0042] The difference between Example 2 and Example 1 is that the amount of potassium bicarbonate added in step 2 is 100 g, the amount of reducing agent decyl glucoside is 203 g, and the rest remains unchanged. At this time, the potassium bicarbonate is about 0.1 mol / L, and the decyl glucoside is about 0.06 mol / L.

[0043] Example 3

[0044] The difference between Example 3 and Example 1 is that potassium bicarbonate is not added in step 2, and the rest remains unchanged.

[0045] Comparative Example 1

[0046] Referring to CN 113797925 B, the preparation method of Example 5 with the largest FCADR value was selected and implemented by the following steps:

[0047] Step 1: Add 2.0 L of deionized water and 220.5 g of potassium permanganate to a stirred reactor and stir for 30 minutes to dissolve to prepare a manganese source solution;

[0048] Step 2: In another stirred reactor, add 0.5 L of deionized water and 31.89 g of glucose powder and stir evenly to obtain an organic reducing agent solution;

[0049] Step 3: While maintaining stirring, the organic reducing agent solution prepared in step 2 is gradually added to the manganese source solution in step 1 at a rate of 66.3 g / min through a peristaltic pump. After the addition is completed, the stirring reaction is continued for 0.5 h. After the reaction is completed, the obtained dispersion is filtered and washed three times with deionized water. It is placed in a blower dryer and dried at 110°C for 12 h to obtain a brown powder.

[0050] In comparative example 2, commercially available nano manganese dioxide was used to test the formaldehyde removal rate, with an average particle size of 50 nm and a mesoporous pore size.

[0051] The samples obtained from Examples 1-3 and Comparative Example 1 were subjected to nitrogen adsorption and desorption tests. Figure 1 is the pore size distribution diagram of Examples 1-3, Figure 1 As shown, the pore size distribution of Examples 1-3 all runs through the micropore range of less than 2 nm (more specifically 0.7 nm-2 nm), the mesopore range of 2-50 nm, and the macropore range of more than 50 nm (50 nm-100 nm), showing a multi-level pore structure distribution. The multi-level pore structure increases the specific surface area of ​​the sample, exposes more active sites, and promotes the efficient catalytic oxidation of formaldehyde by the catalyst; Figure 7The pore size distribution diagram for Comparative Example 1 clearly shows that the pores are distributed only in the mesopore and macropore regions, with no distribution in the micropore region. Micropores, as the primary pore structure for adsorbing formaldehyde molecules, directly affect the efficiency of the catalyst in degrading formaldehyde. Table 1 shows the specific surface area (BET) and pore volume data calculated from nitrogen adsorption and desorption tests. Since the micropore area contributes most to the specific surface area, Examples 1-3 all have distribution in the micropore region. As can be seen from Table 1, the specific surface areas of Examples 1-3 are all greater than those of Comparative Example 1. The relatively large mesopores and macropores provide transmission channels for the adsorbed formaldehyde molecules in the micropores.

[0052] Table 1. Specific surface area (BET) and pore volume data of Examples 1-3

[0053] <![CDATA[Specific surface area (BET) (m 2 / g)]]> <![CDATA[Pore volume (m 3 / g)]]> Example 1 214.34 0.36 Example 2 178.82 0.41 Example 3 151.58 0.24 Comparative Example 1 100.31 0.28

[0054] The doped multi-level porous manganese dioxide obtained in Examples 1-3 was tested by X-ray photoelectron spectroscopy (XPS). Figure 3 As shown, Examples 1-3 all contain signal peaks of Mn, O, and K elements, indicating that each of the prepared samples is manganese dioxide doped with K ions.

[0055] The samples obtained in Examples 1-3 were subjected to X-ray diffraction tests. The diffraction peaks matched JCPDS No. 80-8018, indicating that the obtained samples corresponded to birnessite-type manganese dioxide. Figure 2 As shown, with the introduction of potassium bicarbonate, its diffraction peaks become diffuse and some diffraction peaks disappear, indicating that the crystallinity has deteriorated. The surface atomic arrangement of the manganese dioxide catalyst with poor crystallinity is disordered, and the surface defect sites increase, resulting in a higher surface free energy, which can better adsorb reactants, thereby improving the catalytic efficiency. Birnessite-type manganese dioxide has abundant unsaturated electron holes, which can activate oxygen and water in the air to generate active oxygen and active hydroxyl groups, and decompose formaldehyde into water and carbon dioxide. The multi-level pore structure of the present invention not only gives it a larger specific surface area, which can enhance its physical adsorption capacity, but also exposes more active sites, so that it exhibits higher activity and stability in the reaction of catalytic oxidation of gaseous pollutant formaldehyde at room temperature. Figure 6 This is the XRD pattern of Comparative Example 1, which has no obvious diffraction peak, indicating that Comparative Example 1 is amorphous manganese dioxide.

[0056] Figure 4The K2p XPS fine spectra of Examples 1-3 show that the intensity signals of K2p of Examples 1-3 gradually weaken, indicating that the amount of potassium ion doping gradually decreases. The test results corresponding to the potassium ion content are 5.3%, 2.8% and 0.9%, respectively. The amount of potassium ion doping has an important influence on the formation of defects and active sites in manganese dioxide. As the amount of potassium ion doping increases, the defect concentration and active sites will gradually increase, which has a promoting effect on the subsequent improvement of formaldehyde catalytic oxidation performance.

[0057] Figure 5 This is the O1s XPS fine spectrum of Example 1-3. It can be seen from the figure that the lattice oxygen energy level in the manganese dioxide crystal is at 529.5 eV; the energy level corresponding to the oxygen defect / vacancy is at 531.5 eV. The signal peak of Example 1-3 at 531.5 eV gradually becomes blurred and weakened, indicating that the concentration of oxygen defects / vacancies in Example 1-3 gradually decreases; oxygen defects / vacancies affect the local geometric structure and electronic structure of the catalyst, generating unsaturated coordination sites, thereby forming a large number of catalytic reaction active sites; oxygen defects / vacancies can also promote the adsorption of reactants and desorption of products, inhibit competitive reactions, and thus improve the overall catalytic performance.

[0058] Formaldehyde removal rate test: refer to the total attenuation test method in Appendix E of GB / T 18801-2022, and conduct formaldehyde removal rate test on Examples 1-3 and Comparative Examples 1-2. The test time is 1 hour and the test chamber is 3m 3 The test temperature was 25±2℃ and the humidity was 50±5%. The catalyst was loaded on the filter cotton with a loading of 0.02gcm -2 The total load is 2g. The filter cotton loaded with catalyst is placed in a commercially available car purifier, replacing the purification filter element that comes with the machine. The fan power is 2.5W. The initial value of formaldehyde gaseous pollutants is C0: 1.02ppm. The concentration value is recorded every 5 minutes until it reaches 1 hour. The recorded concentration value is C60. The formaldehyde removal rate is calculated as (C0-C 60 ) / C0, each sample was tested three times, and the results are shown in Table 2 and Figure 8 (Single test).

[0059] From Table 2 and Figure 8 It can be seen from the test data that the formaldehyde removal rates of Examples 1-3 are all higher than that of Comparative Example 1, and the formaldehyde removal rates in one hour are all above 80%, showing good catalytic efficiency, which is due to the high potassium ion doping and abundant active sites provided by the multi-level pore structure.

[0060] Table 2. 1h formaldehyde removal rate of Examples and Comparative Examples

[0061]

[0062] The above description is merely a preferred embodiment of the present invention and does not limit the technical scope of the present invention. Therefore, any minor modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention still fall within the scope of protection of the present invention.

Claims

1. A doped multi-level porous manganese dioxide, characterized in that: The manganese dioxide has a birnessite crystal form, comprising a micropore-mesopore-macroporous structure, wherein the macropore diameter is between 50 nm and 100 nm, the mesopore diameter is between 2 nm and 50 nm, and the micropore diameter is between 0.7 nm and 2 nm. Potassium ions are doped in the pores, and the potassium ion doping concentration is 0.9% to 5.3%.

2. The doped multi-level porous manganese dioxide according to claim 1, characterized in that: The specific surface area of ​​the manganese dioxide is 150 to 220 m 2 / g, pore capacity is 0.2~0.5cm 3 / g.

3. A method for preparing doped hierarchical manganese dioxide, characterized by: The following steps are involved: Step a. dissolving potassium permanganate in deionized water to prepare a manganese source solution; Step b. Dissolve potassium bicarbonate or potassium carbonate in deionized water. After complete dissolution, add a reducing agent and continue stirring until uniformly mixed to form a modified reducing agent solution; the reducing agent is decyl glucoside, octyl glucoside, nonyl glucoside, dodecyl glucoside, tetradecyl glucoside, at least one thereof; Step c. While maintaining the modified reducing agent solution under stirring, the manganese source solution is added to the modified reducing agent solution by a peristaltic pump at a certain flow rate. After the modified reducing agent solution is added, the stirring reaction is continued for a certain time. After the reaction is completed, a brown dispersion is obtained, and the filter cake is washed and dried after filtration; Step d. crushing the dried filter cake and placing it in a vacuum furnace for vacuum heat treatment to obtain doped multi-level porous manganese dioxide.

4. The method for preparing doped multi-level porous manganese dioxide according to claim 3, characterized in that: The molar ratio of the manganese source to the reducing agent is 1:(0.5-2).

5. The method for preparing doped multi-level porous manganese dioxide according to claim 3, characterized in that: The molar ratio of the manganese source to the reducing agent is 1:(0.5-1).

6. The method for preparing doped multi-level porous manganese dioxide according to claim 3, characterized in that: The concentration of the reducing agent is 0.02-0.2 mol / L; the concentration of the manganese source is 0.1-0.25 mol / L.

7. The method for preparing doped multi-level porous manganese dioxide according to claim 3, characterized in that: The concentration of potassium bicarbonate or potassium carbonate in step b is 0-1.25 mol / L.

8. The method for preparing doped multi-level porous manganese dioxide according to claim 3, characterized in that: The reaction temperature in step c is 25±5°C, and the drying temperature is 60-100°C.

9. The method for preparing doped multi-level porous manganese dioxide according to claim 3, characterized in that: In the step d, the vacuum heat treatment temperature is 240-300° C., the vacuum heat treatment time is 6-12 hours, and the vacuum degree is 0.1-0.01 Pa.

10. Use of the doped multi-level porous manganese dioxide according to any one of claims 1 to 2 or the doped multi-level porous manganese dioxide obtained by the preparation method according to any one of claims 3 to 9 in catalytic decomposition of formaldehyde gaseous pollutants.

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