Preparation method of cobalt-manganese oxide synergistic monolithic catalyst and application of cobalt-manganese oxide synergistic monolithic catalyst in degradation of VOCs

Abstract

The invention discloses a preparation method of a cobalt-manganese oxide synergistic monolithic catalyst and application of the cobalt-manganese oxide synergistic monolithic catalyst in degradation of VOCs, and belongs to the technical field of air purification. The preparation method of the cobalt-manganese oxide synergistic monolithic catalyst comprises the following steps: mixing cobalt salt and urea to prepare a precursor solution, then putting a substrate into the precursor solution, carrying out hydrothermal reaction, and calcining to obtain a Co3O4 monolithic catalyst, and adding the Co3O4 monolithic catalyst into an organic manganese salt solution for in-situ electro-deposition, and then roasting to obtain the cobalt-manganese oxide synergistic monolithic catalyst. The cobalt-manganese oxide synergistic monolithic catalyst prepared by the invention does not contain noble metal components, and has the advantages of good catalytic activity, low cost and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of air purification, in particular to a preparation method of cobalt-manganese oxide synergistic monolithic catalyst and application of the catalyst in degradation of VOCs. BACKGROUND

[0002] Many volatile organic compounds (VOCs) have high toxicity and irritability, and after entering the human body through the skin and respiratory tract, they can cause throat discomfort, skin allergy and other phenomena; some VOCs also have strong mutagenicity and strong carcinogenicity, increasing the risk of cancer in the human body, which is harmful to human health. At the same time, VOCs have been proven to contribute to the formation of ozone, organic aerosols and photochemical smog, thereby leading to a general decline in the air quality of the earth, so it is of great significance to develop effective and applicable large-scale treatment methods to reduce the emission of gaseous pollutants.

[0003] At present, the methods for treating VOCs mainly include adsorption method, plasma decomposition method, thermal decomposition method, photocatalytic method and catalytic oxidation method. Among them, the catalytic oxidation method has the advantages of wide application range, high removal efficiency, small secondary pollution, efficient treatment process and low operation cost compared with other methods.

[0004] There are various types of catalysts, which can be divided into traditional granular catalysts and new monolithic catalysts according to the macroscopic physical form. The granular catalyst has small particle size, which leads to large gas resistance of the reaction gas through the bed, high pressure drop of the catalytic bed and increased energy consumption; at the same time, heat is also easy to accumulate between the interfaces of the granular catalyst, resulting in local flying temperature and sintering inactivation of the active component. In order to solve this problem, researchers designed monolithic catalysts to replace traditional granular catalysts, using regular carriers as the basic framework of monolithic catalysts, mainly for supporting and loading the auxiliary catalyst and active component, and improving the overall mechanical strength of the catalyst. Since the regular carriers selected for the monolithic catalyst usually have many parallel channels or macro-pore structures, the monolithic catalyst has the advantages of small reaction gas resistance, low bed pressure drop, strong anti-blocking performance, good heat transfer performance and fast heating rate.

[0005] The active components of monolithic catalysts are divided into two categories: precious metals and transition metal oxides. Monolithic catalysts containing precious metals (such as Pt, Pd, and Au) have good catalytic performance for the catalytic oxidation of VOCs, but they are expensive and easily sintered into agglomerates, making them difficult to apply on a large scale in actual industry. Transition metal oxide catalysts, such as manganese oxide catalysts, have the advantages of variable valence states, good catalytic activity, environmental friendliness, and low cost. However, their low-temperature activity is poor and their ignition temperature is high, which still lags behind the activity of precious metal catalysts. Therefore, how to prepare a catalyst with good catalytic activity and low cost has become a technical problem that those skilled in the art urgently need to solve. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a cobalt-manganese oxide synergistic monolithic catalyst and its application in degrading VOCs, so as to solve the problems existing in the above-mentioned prior art.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] One of the technical solutions of the present invention is a method for preparing a cobalt-manganese oxide synergistic monolithic catalyst, comprising the following steps:

[0009] Cobalt salt and urea are mixed to prepare a precursor solution, and then a substrate is placed in the precursor solution, subjected to a hydrothermal reaction, and then calcined to obtain a Co3O4 monolithic catalyst;

[0010] The Co3O4 monolithic catalyst is added to an organic manganese salt solution for in-situ electrodeposition, and then calcined to obtain the cobalt-manganese oxide synergistic monolithic catalyst.

[0011] Furthermore, the cobalt salt includes cobalt nitrate.

[0012] Furthermore, the concentration of cobalt salt in the precursor solution is 0.02-0.15 mol / L, and the concentration of urea is 0.15-0.90 mol / L.

[0013] Furthermore, the temperature of the hydrothermal reaction is 100-130° C., and the time is 6-12 hours.

[0014] Furthermore, the calcination temperature is 250-500° C., and the time is 3-6 hours; and the calcination atmosphere is air atmosphere.

[0015] Furthermore, the calcination temperature is 350° C. and the calcination time is 3 to 6 hours.

[0016] Furthermore, the organic manganese salt solution includes a manganese acetate solution with a concentration of 0.1 to 0.5 mol / L.

[0017] Furthermore, the in-situ electrodeposition conditions include: using a Co3O4 monolithic catalyst as a working electrode, a platinum sheet as a counter electrode, saturated calomel as a reference electrode, and a deposition potential of 0 to 1.5 V (cyclic voltammetry).

[0018] Furthermore, the calcination temperature is 200-400° C., and the calcination time is 3-6 hours; and the calcination atmosphere is air atmosphere.

[0019] Furthermore, the calcination temperature is 300-400° C. and the calcination time is 3-6 hours.

[0020] Furthermore, before placing the substrate in the precursor solution, the method further includes a step of pre-treating the substrate to remove oil stains and surface oxide layers;

[0021] The pretreatment comprises: firstly subjecting the substrate to ultrasonic treatment in a dilute hydrochloric acid solution, then sequentially adding deionized water and anhydrous ethanol for ultrasonic treatment, and finally rinsing with deionized water and vacuum drying for standby use.

[0022] Furthermore, the material of the substrate includes nickel foam; the concentration of the dilute hydrochloric acid solution is 1 to 3 mol / L; and the time of the ultrasonic treatment is 2 to 10 minutes.

[0023] Furthermore, after the hydrothermal reaction and before calcination, a drying step is also included, with the drying temperature being 80° C. and the drying time being 3 to 7 hours.

[0024] Furthermore, the in-situ electrodeposition and the calcination further include a drying step, wherein the drying temperature is 80° C. and the drying time is 3 to 7 hours.

[0025] The cobalt-manganese oxide synergistic monolithic catalyst prepared by the present invention (for example, when the substrate is nickel foam, the prepared catalyst is MnOx@Co3O4 / Ni foam, where x is 1.0-2.0) forms a layered distribution on the substrate, with a BET specific surface area of ​​up to 63.83 m 2 / g, which is much higher than the corresponding single-component monolithic catalyst Co3O4 / Ni foam (17.26m 2 / g) and MnOx / Ni foam (19.10m 2 Calculated by mass percentage of active metal elements, the content of Co in the cobalt-manganese oxide synergistic monolithic catalyst prepared by the present invention is 1-3 wt.%, and the loading amount of MnOx is about 40 wt.%.

[0026] The traditional immersion-coating process first requires the pre-preparation of active component powders, which are then loaded onto a support by immersion, coating, or spraying. While this allows the preparation of transition metal oxide monolithic catalysts, it is difficult to securely support the active components on the substrate. The preparation process of the present invention utilizes an in-situ growth method to load the active components (Co3O4 and MnOx) onto the metal substrate. This ensures that the active components are evenly distributed on the metal substrate surface, securely loaded, and more tightly bonded at the loading interface than in the traditional immersion-coating process, contributing to improved catalytic performance.

[0027] In the prior art, the catalyst is generally loaded on a carrier by impregnation to prepare a monolithic catalyst, but this method is difficult to load the catalyst on a smooth metal substrate, and the loaded active component easily falls off. The present invention first pre-treats the surface of the substrate (such as nickel foam) to remove grease and passivation film on the surface of the metal substrate, and increases the exposed surface of the substrate itself (increasing the exposed surface of the substrate (carrier) itself can increase its specific surface area and optimize the surface structure, thereby having more physical surface and pore inner surface, and increasing the space, allowing the active component precursor to be more evenly distributed, reducing local oversaturation and the risk of heterogeneous nucleation and agglomeration), which is conducive to the subsequent loading of active components. Then, a hydrothermal method is used to utilize the alkaline environment formed by the decomposition of urea to first generate basic cobalt carbonate 2CoCO3·3Co(OH)2·n in situ on the substrate. H2O, followed by calcination, forms Co3O4 nanoarrays on a gold substrate, resulting in a uniformly distributed grid-like active component layer. This significantly improves the substrate's pore structure, particularly increasing the mesopore volume, thereby significantly increasing the specific surface area and the number of exposed catalytically active sites. (When the support surface has a large number of evenly distributed active sites, Mn precursor ions tend to adsorb dispersedly on these sites rather than aggregate at a few sites or spontaneously nucleate in solution, allowing the MnOx active component to form uniformly and simultaneously or sequentially across the support surface.) This provides ample deposition sites for the subsequent uniform loading of the MnOx catalytically active component. Subsequently, MnOx components were in situ grown on the surface of the uniformly distributed Co3O4 nanoarray substrate by electrodeposition, resulting in the preparation of a cobalt-manganese oxide-synergistic monolithic catalyst, MnOx@Co3O4 / Ni foam, with a higher specific surface area.

[0028] MnOx is the main catalytically active component of the catalyst of the present invention. Therefore, a high-specific-surface-area grid-like Co3O4 nanoarray is first formed on the surface of a smooth, dense, and regular nickel foam substrate by a hydrothermal method. On the one hand, this is more conducive to the uniform and firm loading of the subsequent MnOx catalytically active component; on the other hand, MnOx will also undergo redox reaction with the Co3O4 nanoarray during the in-situ deposition process (Co 3+ +Mn 3+ =Co2+ +Mn 4+ ), this interaction between cobalt and manganese can adjust the ratio of the oxidized state to the reduced state of the cobalt and manganese components in the catalytic active layer, thereby affecting the activity and stability of the catalytic oxidation reaction of VOCs. As shown by the XPS results, in the catalytic oxidation degradation reaction of toluene, the ratio of the oxidized state to the reduced state of the MnOx active component in the catalyst of the present invention (Mn 4+ / Mn 3+ ) can be maintained at 1.08, which is closer to a ratio of 1:1. Because the reaction mechanism of MnOx catalyst oxidation degradation of VOCs generally follows the Mars-van Krevelen catalytic oxidation reaction mechanism, that is, the oxidation state (Mn 4+ ) The active site first adsorbs the reactant toluene and degrades it into CO2 and water, while reducing itself to Mn 3+ ; while the reduced state (Mn 3+ ) can react with oxygen molecules in the sample to regenerate Mn 4+ , to achieve regeneration; therefore Mn 4+ and Mn 3+ Maintaining a molar ratio close to 1:1 is more conducive to the sustained and stable operation of the catalytic cycle. Therefore, the catalyst developed by the present invention, by introducing a Co species, can maintain an ideal ratio between the oxidized and reduced states of the main active component, manganese oxide, forming a favorable catalytic cycle and ultimately enhancing the catalyst's low-temperature catalytic activity. Compared to precious metal catalysts, the MnOx@Co3O4 / Ni foam catalyst prepared by the present invention has significantly lower economic costs and significantly improved catalytic oxidation activity compared to single-component transition metal oxide catalysts.

[0029] The degradation of VOCs by transition metal oxides involves the Mars-van Krevelen mechanism, in which lattice oxygen (corresponding to Mn 4+ ) reacts with toluene molecules and converts into chemically adsorbed oxygen (corresponding to Mn 3+ ), toluene is converted into carbon dioxide and water. In order to maintain the catalytic cycle, the chemically adsorbed oxygen needs to react with the oxygen in the gas phase to regenerate into lattice oxygen, forming a catalytic cycle. The catalyst of the present invention can maintain Mn in the degradation reaction. 4+ / Mn 3+ The ratio is close to 1:1, which means that the catalyst forms an ideal cycle, the catalytic activity is high, and the temperature required for the reaction is low.

[0030] The second technical solution of the present invention: a cobalt-manganese oxide synergistic integral catalyst prepared by the above preparation method.

[0031] The third technical solution of the present invention: an application of the above-mentioned cobalt manganese oxide in synergistically combining the integral catalyst in the degradation of VOCs.

[0032] Furthermore, the VOCs include toluene.

[0033] The present invention discloses the following technical effects:

[0034] (1) The cobalt-manganese oxide synergistic integral catalyst prepared by the present invention does not contain precious metal components and has the advantages of good catalytic activity and low cost.

[0035] (2) The cobalt-manganese oxide synergistic monolithic catalyst prepared by the present invention has a synergistic effect of bimetallic oxides. Compared with single-component catalysts, it has advantages such as a high specific surface area and can significantly improve the low-temperature purification activity of toluene.

[0036] (3) The preparation method of the present invention is not only applicable to metal substrates of various shapes, but also can ensure that the active components are uniformly and firmly loaded on the metal substrate, and has excellent toluene catalytic oxidation performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0038] Figure 1 The test results of toluene catalytic oxidation performance of the catalysts prepared in Example 1 and Comparative Examples 1 to 5 at different temperatures are shown;

[0039] Figure 2 The test results of the catalytic oxidation performance of the catalyst prepared in Example 1 on toluene of different concentrations at different temperatures;

[0040] Figure 3 The stability test results of the catalyst prepared in Example 1 for the catalytic oxidation of toluene at 190°C;

[0041] Figure 4 The SEM images of the catalysts prepared in Example 1 and Comparative Examples 1 and 2, wherein (a) is the Co3O4 / Ni foam prepared in Comparative Example 1, (b) is the MnOx / Ni foam prepared in Comparative Example 2, and (c) is the MnOx@Co3O4 / Nifoam prepared in Example 1;

[0042] Figure 5 The nitrogen adsorption-desorption isotherms and BJH pore size distribution diagrams of the blank nickel foam substrate, Example 1, and the catalysts prepared in Comparative Examples 1 and 2, wherein (a) is the nitrogen adsorption-desorption isotherm and (b) is the pore size distribution. DETAILED DESCRIPTION

[0043] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0044] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0045] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0046] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0047] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0048] It should be pointed out that the matters not described in detail in the present invention are conventional operating means in this field and are not the focus of the present invention.

[0049] Example 1

[0050] A method for preparing a cobalt-manganese oxide synergistic monolithic catalyst:

[0051] (1) A 30 mm long and 20 mm wide nickel foam was first ultrasonically treated in a 2 mol / L HCl solution for 2 min to remove the oxide layer, and then ultrasonically treated in deionized water and anhydrous ethanol solution for 10 min in sequence. Finally, it was rinsed with deionized water and vacuum dried to obtain a pretreated substrate for later use.

[0052] (2) 3.75 mmol of cobalt nitrate hexahydrate and 22.5 mmol of urea were dissolved in 30 mL of deionized water to form a uniform solution to obtain a precursor solution; the precursor solution and the pretreated substrate were transferred to a 50 mL polytetrafluoroethylene-lined reactor, sealed, and placed in an oven for hydrothermal reaction at 120°C for 10 h. The sample was ultrasonically cleaned with deionized water and placed in a forced air drying oven at 80°C for 5 h. Finally, it was calcined at 350°C for 3 h in an air atmosphere to obtain a Co3O4 monolithic catalyst (i.e., Co3O4 / Nifoam).

[0053] (3) In situ electrodeposition was carried out on Co3O4 / Ni foam using a three-electrode system (Co3O4 / Ni foam as the working electrode, platinum sheet as the counter electrode, and saturated calomel as the reference electrode) with a 0.25 mol / L manganese acetate solution as the electrolyte. The deposition potential was 0-1.5 V (cyclic voltammetry) and the number of deposition cycles was 180. After the deposition was completed, the sample was taken out and rinsed with deionized water. The sample was dried at 80°C for 5 h and calcined at 300°C in air for 3 h to obtain a cobalt-manganese oxide synergistic monolithic catalyst (i.e., MnOx@Co3O4 / Ni foam, x is 1.0-2.0), with a Co content of 3 wt.% and a MnOx content of approximately 40 wt.%.

[0054] Comparative Example 1

[0055] Preparation method of Co3O4 / Ni foam:

[0056] (1) A 30 mm long and 20 mm wide nickel foam was first ultrasonically treated in a 2 mol / L HCl solution for 2 min to remove the oxide layer, and then ultrasonically treated in deionized water and anhydrous ethanol solution for 10 min in sequence. Finally, it was rinsed with deionized water and vacuum dried to obtain a pretreated substrate for later use.

[0057] (2) 3.75 mmol of cobalt nitrate hexahydrate and 22.5 mmol of urea were dissolved in 30 mL of deionized water to form a uniform solution to obtain a precursor solution; the precursor solution and the pretreated substrate were transferred together into a 50 mL polytetrafluoroethylene-lined reactor, sealed, and placed in an oven for hydrothermal reaction at 120°C for 10 h. The sample was ultrasonically cleaned with deionized water and placed in a forced air drying oven at 80°C for 5 h. Finally, it was calcined at 350°C for 3 h in an air atmosphere to obtain a Co3O4 monolithic catalyst (i.e., Co3O4 / Nifoam) with a Co content of 3%.

[0058] Comparative Example 2

[0059] Preparation method of MnOx / Ni foam:

[0060] (1) A 30 mm long and 20 mm wide nickel foam was first ultrasonically treated in a 2 mol / L HCl solution for 2 min to remove the oxide layer, and then ultrasonically treated in deionized water and anhydrous ethanol solution for 10 min in sequence. Finally, it was rinsed with deionized water and vacuum dried to obtain a pretreated substrate for later use.

[0061] (2) In situ electrodeposition was performed on the pretreated substrate using a three-electrode system (pretreated substrate as working electrode, platinum sheet as counter electrode, and saturated calomel as reference electrode) with a 0.25 mol / L manganese acetate solution as the electrolyte. The deposition potential was 0-1.5 V (cyclic voltammetry) and the number of deposition cycles was 180. After the deposition was completed, the sample was removed and rinsed with deionized water. The sample was dried at 80°C for 5 h and calcined at 300°C in air for 3 h to obtain a MnOx monolithic catalyst (i.e., MnOx / Nifoam, x was 1.0-2.0) with a MnOx content of approximately 40%.

[0062] Comparative Example 3

[0063] Preparation method of MnOx@Co3O4 / Ni foam:

[0064] (1) A 30 mm long and 20 mm wide nickel foam was first ultrasonically treated in a 2 mol / L HCl solution for 2 min to remove the oxide layer, and then ultrasonically treated in deionized water and anhydrous ethanol solution for 10 min in sequence. Finally, it was rinsed with deionized water and vacuum dried to obtain a pretreated substrate for later use.

[0065] (2) 3.75 mmol of cobalt nitrate hexahydrate and 22.5 mmol of sodium carbonate were dissolved in 30 mL of deionized water to form a uniform solution to obtain a precursor solution; the precursor solution and the pretreated substrate were transferred to a 50 mL polytetrafluoroethylene-lined reactor, sealed, and placed in an oven for hydrothermal reaction at 120°C for 10 h. The sample was ultrasonically cleaned with deionized water and placed in a forced air drying oven at 80°C for 5 h. Finally, it was calcined at 350°C for 3 h in an air atmosphere to obtain a Co3O4 monolithic catalyst (i.e., Co3O4 / Ni foam).

[0066] (3) In situ electrodeposition was carried out on Co3O4 / Ni foam using a three-electrode system (Co3O4 / Ni foam as the working electrode, platinum sheet as the counter electrode, and saturated calomel as the reference electrode) with a 0.25 mol / L manganese acetate solution as the electrolyte. The deposition potential was 0-1.5 V (cyclic voltammetry) and the number of deposition cycles was 180. After the deposition was completed, the sample was taken out and rinsed with deionized water. The sample was dried at 80°C for 5 h and calcined at 300°C in air for 3 h to obtain a cobalt-manganese oxide synergistic monolithic catalyst (i.e., MnOx@Co3O4 / Ni foam, x is 1.0-2.0), with a MnOx content of approximately 40.0 wt.%.

[0067] Comparative Example 4

[0068] (1) A 30 mm long and 20 mm wide nickel foam was first ultrasonically treated in a 2 mol / L HCl solution for 2 min to remove the oxide layer, and then ultrasonically treated in deionized water and anhydrous ethanol solution for 10 min in sequence. Finally, it was rinsed with deionized water and vacuum dried to obtain a pretreated substrate for later use.

[0069] (2) 3.75 mmol of cerium nitrate hexahydrate and 22.5 mmol of urea were dissolved in 30 mL of deionized water to form a uniform solution to obtain a precursor solution; the precursor solution and the pretreated substrate were transferred to a 50 mL polytetrafluoroethylene-lined reactor, sealed, and placed in an oven for hydrothermal reaction at 120°C for 10 h. The sample was ultrasonically cleaned with deionized water and placed in a forced air drying oven at 80°C for 5 h. Finally, it was calcined at 350°C for 3 h in an air atmosphere to obtain a CeO2 monolithic catalyst (i.e., CeO2 / Nifoam).

[0070] (3) In situ electrodeposition was carried out on CeO2 / Ni foam using a three-electrode system (CeO2 / Ni foam as the working electrode, platinum sheet as the counter electrode, and saturated calomel as the reference electrode) with a 0.25 mol / L manganese acetate solution as the electrolyte. The deposition potential was 0-1.5 V (cyclic voltammetry) and the number of deposition cycles was 180. After the deposition was completed, the sample was taken out and rinsed with deionized water. The sample was dried at 80°C for 5 h and calcined at 300°C in air for 3 h to obtain a cobalt-manganese oxide synergistic monolithic catalyst (i.e., MnOx@CeO2 / Ni foam, x is 1.0-2.0), with a MnOx content of approximately 28 wt.%.

[0071] Comparative Example 5

[0072] Preparation method of MnOx@Co3O4 / Ni foam:

[0073] (1) The metal foam nickel with length of 30 mm and width of 20 mm was first treated in 2 mol / L HCl solution for 2 min under ultrasonic to remove the oxide layer, then treated in deionized water and anhydrous ethanol solution for 10 min under ultrasonic, and finally washed with deionized water, vacuum dried to obtain a pretreated substrate, which was ready for use.

[0074] (2) 3.75 mmol of cobalt nitrate hexahydrate and 22.5 mmol of urea were dissolved in 30 mL of deionized water to form a homogeneous solution to obtain a precursor solution; the precursor solution and the pretreated substrate were transferred into a 50 mL polytetrafluoroethylene-lined reaction kettle, sealed, placed in an oven at 120°C for hydrothermal reaction for 10 h, washed with deionized water under ultrasonic and placed in a forced air drying oven at 80°C for drying for 5 h, and finally calcined at 350°C for 3 h under air atmosphere to obtain a Co3O4 monolithic catalyst (i.e. Co3O4 / Ni foam).

[0075] (3) A 0.25 mol / L manganese acetate solution was used as a precursor solution; 30 mL of the precursor solution and the Co3O4 monolithic catalyst were transferred into a 50 mL polytetrafluoroethylene-lined reaction kettle, sealed, placed in an oven at 120°C for hydrothermal reaction for 10 h, washed with deionized water under ultrasonic and placed in a forced air drying oven at 80°C for drying for 5 h, and finally calcined at 300°C for 3 h under air atmosphere to obtain a cobalt-manganese oxide synergistic monolithic catalyst (i.e. MnOx@Co3O4 / Ni foam, x is 1.0-2.0).

[0076] Effect Example 1

[0077] (1) During the experiment, the sample to be tested (catalyst prepared in the examples or comparative examples) was placed in a quartz tube with an inner diameter of 8 mm. A catalytic reaction fixed bed device was used, and toluene was selected as a probe molecule to evaluate the catalytic performance of the sample at different temperatures. The reactant was a gas mixture of toluene and air (the concentration of toluene in air was 1.0 g / m 3 , and the pressure of the reactant was 0.1 MPa). In order to control the inlet concentration of toluene, one gas stream generated bubbles through a saturator filled with liquid toluene and delivered toluene vapor, which was then mixed with another gas stream (air, 20 vol.% oxygen + 80 vol.% nitrogen) to adjust the toluene inlet concentration. The total flow rate in the reactor was set to 60 mL / min using a mass flow meter. An Agilent HP6820 gas chromatograph was used to measure the inlet and outlet stream concentrations online using a TCD detector and an FID detector, respectively. Under all test conditions, no other by-products were found except CO2 and H2O, and the data results are shown in Table 1 and Figure 1 .

[0078] Table 1 Toluene catalytic oxidation performance test of catalysts

[0079]

[0080] From Table 1 and Figure 1 It can be seen that the single-component monolithic catalysts Co3O4 / Ni foam and MnOx / Ni foam catalytically oxidize 90% of toluene at reaction temperatures of 230℃ and 215℃, respectively, while the cobalt-manganese oxide synergistic monolithic catalyst MnOx@Co3O4 / Ni foam can catalytically oxidize 90% of toluene at only 188℃, indicating that there is a synergistic effect between cobalt oxide and manganese oxide, which is beneficial to the catalytic oxidation of toluene.

[0081] (2) During the experiment, the sample to be tested (catalyst prepared in Example 1) was placed in a quartz tube with an inner diameter of 8 mm. A fixed-bed catalytic reaction apparatus was used, and toluene was selected as the probe molecule to evaluate the catalytic performance of the sample at different temperatures and different toluene concentrations. The reactants were a gas mixture of toluene and air (the toluene concentration in the air was 1.0 g / m 3 (i.e. 250ppm), 2.0g / m 3 (i.e. 500ppm) or 4.0g / m 3 (i.e. 1000ppm), the pressure of the reactants is 0.1MPa). In order to control the toluene concentration at the inlet, a stream of gas is passed through a saturator filled with liquid toluene to generate bubbles and transport toluene vapor, which is then mixed with another stream of gas (air, 20vol.% oxygen + 80vol. nitrogen) to adjust the toluene inlet concentration. The total flow rate in the reactor is set to 60mL / min using a mass flow meter. An Agilent HP6820 gas chromatograph is used to measure the inlet and outlet gas concentrations online using a TCD detector and an FID detector, respectively. Under all test conditions, no other by-products were found except CO2 and H2O. The data results are shown in the table. Figure 2 .

[0082] (3) During the experiment, the sample to be tested (catalyst prepared in Example 1) was placed in a quartz tube with an inner diameter of 8 mm. A fixed-bed catalytic reaction apparatus was used, and toluene was selected as the probe molecule to evaluate the stability of the catalytic performance of the sample at 190°C. The reactants were a gas mixture of toluene and air (the toluene concentration in the air was 1.0 g / m 3, the pressure of the reactants is 0.1 MPa). In order to control the toluene concentration at the inlet, a gas stream is passed through a saturator filled with liquid toluene to generate bubbles and transport toluene vapor, which is then mixed with another gas stream (air, 20 vol.% oxygen + 80 vol. nitrogen) to adjust the toluene inlet concentration. The total flow rate in the reactor is set to 60 mL / min using a mass flow meter. An Agilent HP6820 gas chromatograph is used to measure the inlet and outlet gas stream concentrations online using a TCD detector and an FID detector, respectively. Under all test conditions, no other by-products were found except CO2 and H2O. The data results are shown in Figure 2. Figure 3 .

[0083] Effect Example 2

[0084] X-ray photoelectron spectroscopy (XPS) was used to investigate the surface valence state changes of the catalyst samples prepared in Example 1 and Comparative Examples 1-2. The data results are shown in Table 2.

[0085] Table 2 Chemical state results of catalyst surface elements obtained by XPS test

[0086]

[0087]

[0088] As can be seen from Table 2, the oxidized state / reduced state of manganese ions in Comparative Example 2 decreased to a lower ratio after the reaction, while the oxidized state / reduced state of manganese ions in Example 1 was maintained at a ratio close to 1:1. The degradation of VOCs by transition metal oxides involves the Mars-van Krevelen mechanism, in which lattice oxygen (corresponding to Mn 4+ ) reacts with toluene molecules and converts into chemically adsorbed oxygen (corresponding to Mn 3+ ), toluene is converted into carbon dioxide and water. In order to maintain the catalytic cycle, the chemically adsorbed oxygen needs to react with the oxygen in the gas phase to regenerate into lattice oxygen, forming a catalytic cycle. The catalyst of the present invention can maintain Mn in the degradation reaction. 4+ / Mn 3+ The ratio is close to 1:1, which means that the catalyst forms an ideal cycle. Of course, Mn 4+ The higher proportion than that of the comparative example also means that the lattice oxygen proportion of the embodiment is high, the catalytic activity is higher, the performance is good, and the temperature required for the reaction can be reduced.

[0089] Effect Example 3

[0090] The catalyst samples prepared in Example 1 and Comparative Examples 1-2 were tested for catalyst loading firmness. The specific method is as follows:

[0091] Different catalyst samples were placed in 15 mL of deionized water and ultrasonicated in an ultrasonic water bath at 40 kHz and 100 W for 30 min. The samples were weighed before ultrasonication and after ultrasonication and drying, and the mass loss was calculated. The results are shown in Table 3.

[0092] Table 3 Ultrasonic vibration test on the mass changes of three different monolithic catalysts

[0093]

[0094] Effect Example 4

[0095] The SEM image of the catalyst prepared in Example 1 is shown in Figure 4 , wherein (a) is the Co3O4 / Nifoam prepared in Comparative Example 1, (b) is the MnOx / Ni foam prepared in Comparative Example 2, and (c) is the MnOx@Co3O4 / Ni foam prepared in Example 1.

[0096] from Figure 4 It can be seen that after the hydrothermal reaction, a dense nano-Co3O4 array grew on the metal foam nickel substrate; while the single-component monolithic catalyst MnOx / Ni foam, which only underwent a one-step electrodeposition reaction without undergoing a hydrothermal reaction, formed a layered MnOx active component; in contrast, the cobalt manganese oxide-coordinated monolithic catalyst MnOx@Co3O4 / Ni foam, which underwent a hydrothermal reaction followed by an electrodeposition reaction, formed a dense spherical MnOx active component with a larger exposed area.

[0097] Nitrogen adsorption-desorption isotherms and BJH pore size distribution diagrams of the nickel foam, Example 1 and the catalysts prepared in Comparative Examples 1 and 2 are shown in FIG. Figure 5 , where (a) is the nitrogen adsorption-desorption isotherm and (b) is the pore size distribution.

[0098] from Figure 5 It can be seen that the mesopores of the cobalt-manganese oxide-coordinated monolithic catalyst MnOx@Co3O4 / Ni foam are significantly increased, and the specific surface area (63.83m 2 / g) is much larger than that of the single-component monolithic catalyst Co3O4 / Ni foam (17.26m 2 / g) and MnOx / Ni foam (19.10m 2 / g), and Figure 4 The electron microscopy results are consistent.

[0099] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a cobalt-manganese oxide synergistic monolithic catalyst, characterized in that: The following steps are involved: Cobalt salt and urea are mixed to prepare a precursor solution, and then a substrate is placed in the precursor solution, subjected to a hydrothermal reaction, and then calcined to obtain a Co3O4 monolithic catalyst; The Co3O4 monolithic catalyst is added to an organic manganese salt solution for in-situ electrodeposition, and then calcined to obtain the cobalt-manganese oxide synergistic monolithic catalyst.

2. The preparation method according to claim 1, characterized in that The cobalt salt includes cobalt nitrate.

3. The preparation method according to claim 1, characterized in that The cobalt salt concentration in the precursor solution is 0.02-0.15 mol / L, and the urea concentration is 0.15-0.90 mol / L.

4. The preparation method according to claim 1, characterized in that The temperature of the hydrothermal reaction is 100-130° C., and the time is 6-12 hours.

5. The preparation method according to claim 1, characterized in that The calcination temperature is 250-500° C., and the calcination time is 3-6 hours.

6. The preparation method according to claim 1, characterized in that The organic manganese salt solution includes a manganese acetate solution with a concentration of 0.1 to 0.5 mol / L.

7. The preparation method according to claim 1, characterized in that The in-situ electrodeposition conditions include: using a Co3O4 monolithic catalyst as a working electrode, a platinum sheet as a counter electrode, saturated calomel as a reference electrode, and a deposition potential of 0 to 1.5V.

8. The preparation method according to claim 1, characterized in that The calcination temperature is 200-400° C., and the calcination time is 3-6 hours.

9. A cobalt-manganese oxide synergistic monolithic catalyst prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the cobalt-manganese oxide synergistic monolithic catalyst according to claim 9 in degrading VOCs.

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