Method for preparing spinel type catalyst rich in oxygen vacancies under assistance of magnetization treatment and application of spinel type catalyst

Abstract

The invention provides a method for preparing a spinel type catalyst rich in oxygen vacancies under the assistance of magnetization treatment and application of the spinel type catalyst. The synthesis method of the catalyst comprises the following steps: (1) preparing a precursor of the catalyst by adopting a hydrothermal method: putting a metal foam carrier into a mixed solution of metal salt ions such as ferric nitrate, manganous nitrate and cobalt nitrate, adding ammonium fluoride and urea, carrying out hydrothermal reaction, repeating the hydrothermal reaction for several times, and washing and drying to obtain the precursor; and (2) putting the catalyst precursor into a magnetization reactor, putting the magnetization reactor in the center of an alternating magnetic field coil, and carrying out magnetization treatment on the catalyst precursor in an air or oxygen atmosphere to obtain the spinel type catalyst rich in oxygen vacancies. The method is simple in process and low in cost, and the obtained product is uniform in morphology and good in repeatability and has high photo-thermal catalytic oxidation toluene performance. Through a clean, pollution-free and in-situ directional heating electromagnetic induction auxiliary technology, removal of oxygen atoms on the surface of the spinel catalyst is promoted, so that rich oxygen vacancies are introduced into cobalt-manganese spinel, the exchange effect between the catalyst and oxygen molecules is accelerated, and the reaction rate of catalytic oxidation of VOCs is promoted. When the intensity of simulated sunlight is 500 cm <-2 >, the concentration of toluene is 1000 ppm, the air speed is 45000 mL.gcat <-1 >. H <-1 >, the conversion rate of toluene reaches 86% or above, and high-efficiency oxidation of toluene can be realized under the driving of the simulated sunlight.

Description

Technical Field

[0001] The present invention relates to the preparation of a photothermal catalyst, and particularly to a method and application for magnetically assisted preparation of a spinel catalyst rich in oxygen vacancies.

[0002] Transition metal oxides, such as NiO, Fe 2 O 3 、CeO 2 、Co 3 O 4 etc., because the lowest empty orbitals of these materials are composed of cation d orbitals with lower energy, these oxides usually have semiconductor properties with a band gap < 3 eV. The removal of oxygen causes the redistribution of excess electrons on the cation empty energy levels to exchange oxygen in a relatively easy way. Therefore, oxygen vacancies can regulate the electronic structure of metal oxides. When oxygen vacancies are present, defect energy levels appear in the band gap, thereby reducing the band width, improving the light absorption performance, and promoting the separation of carriers. At the same time, oxygen vacancies promote the conversion of excitons into carriers, accelerate the surface reduction reaction, and promote the separation of carriers. In addition, oxygen vacancies act as active sites on the surface (edges, corners or planes) of the oxide, optimizing the adsorption energy of reactants on the catalyst surface, thereby reducing the reaction energy and promoting molecular activation. Oxygen vacancies and nearby active metal sites play a synergistic catalytic role in the catalyst.

[0003] The application of electromagnetic induction technology in catalytic reactions generally involves using the influence of an electric field or a magnetic field to change the reaction rate, selectivity or product distribution, which has been widely studied and has become one of the most promising methods for synthesizing oxygen vacancies. Using the vacuum calcination method, TiO 2 、ZrO 2 、MnO 2 、MoO 3 、WO 3A series of metal oxides generate an oxygen-deficient structure with high quality. However, this method can operate under high-temperature or high-pressure conditions, which is neither convenient nor time-saving. Chemical reductant treatment, as an active solution reduction and room-temperature process, is also a common and effective method, but it is limited by the relatively low content of oxygen vacancies. Through the active metal reduction method, oxygen-rich original vacancy metal oxides can be obtained, but its operation process is usually very complex. High-energy particle bombardment, calcination-vacuum activation, and ultrasonic treatment have limited application ranges, and only a small part of metal oxides with oxygen vacancies can be synthesized through these methods. In addition, in the hydrogen reduction method, the content of oxygen vacancies in metal compounds increases with the increase of temperature and hydrogen pressure. However, excessive reductants may lead to the emergence of new phases or even metal elemental phases. Therefore, as an auxiliary energy source for heating the reaction system, improving the reaction rate or reducing the reaction temperature, the electromagnetic field treatment method has begun to be studied. Due to the presence of a magnetic field, it affects the rotation, molecular orientation, or magnetic interaction of reactants to influence the catalytic reaction. The electromagnetic field regulates the catalyst structure, that is, the electric field or magnetic field can change its catalytic performance by regulating the catalyst structure. For example, the electric field can adjust the density of active sites on the catalyst surface or adjust the electronic structure of the catalyst, thereby affecting its catalytic activity and selectivity.

[0004] The monolithic catalyst with metal foam as the carrier has more active sites exposed and shorter reaction gas diffusion paths while the active components are evenly dispersed. The cobalt-manganese spinel catalyst has the characteristics of a high specific surface area and multivalent multi-metal ions. The abundant active sites are conducive to the adsorption and reaction of VOCs molecules, and the abundant defect structure can reduce electron-hole recombination and increase the formation of oxygen vacancies. In the oxidation reaction, oxygen molecules need to be adsorbed on the catalyst surface and active oxygen atoms need to be released from the oxygen molecules. This oxygen activation is very important for the catalytic reaction. Abundant oxygen vacancies can provide active adsorption and dissociation sites for oxygen molecules. Therefore, the preparation of spinel catalysts rich in oxygen vacancies is crucial for the photo-thermal catalytic oxidation of VOCs. According to specific catalysts and application requirements, methods such as redox treatment, high-temperature treatment, doping and modification, and ion exchange can be selected to introduce oxygen vacancies in the catalyst preparation stage. Different from the conventional oxygen vacancy introduction strategies, the present invention promotes the removal of oxygen atoms on the surface of the spinel catalyst through a clean and pollution-free, in-situ directional heating electromagnetic induction-assisted technology, thereby introducing abundant oxygen vacancies in the cobalt-manganese spinel, accelerating the exchange interaction between the catalyst and oxygen molecules, and promoting the reaction rate of VOCs catalytic oxidation. Summary of the Invention

[0005] The object of the present invention is to address the above problems existing in the prior art, and a method for magnetically treating a catalyst precursor with an alternating magnetic field to assist in the preparation of a spinel catalyst rich in oxygen vacancies is proposed, and it is applied to the photothermal catalytic oxidation of VOCs, and the obtained catalyst has high catalytic activity.

[0006] The object of the present invention is achieved by the following technical solutions.

[0007] A preparation method for magnetically treating to assist in the preparation of a spinel catalyst rich in oxygen vacancies, using a hydrothermal method to prepare a catalyst precursor: a clean metal foam is placed in a mixed metal salt solution, and a hydrothermal reaction is carried out, and the hydrothermal process is repeated 1 to 3 times to obtain a precursor Fe / MCO@NF-B; the Fe / MCO@NF-B precursor is placed in a magnetization reactor and simultaneously placed at the center of an alternating magnetic field coil, and magnetization treatment is carried out in an air or oxygen atmosphere to obtain Fe / MCO@NF-B-EMIH.

[0008] Specifically, the method and application for magnetically treating to assist in the preparation of a spinel-type catalyst rich in oxygen vacancies include the following steps:

[0009] Step (1), the metal foam carrier is ultrasonically cleaned with hydrochloric acid and ethanol and then dried, a mixed solution of metal salt ions such as iron nitrate, manganese nitrate, and cobalt nitrate is prepared, and the metal foam carrier is added to the mixed metal ion solution. Ammonium fluoride and urea are added to the above solution and stirred evenly, and then placed in an oven for hydrothermal reaction at 95 °C for 12 h, and the hydrothermal process is repeated 1 to 3 times. After washing, drying, and calcination, the precursor Fe / MCO@NF-B is obtained;

[0010] Step (2), the catalyst precursor is placed in a magnetization reactor and placed at the center of the alternating magnetic field coil. In an air or oxygen atmosphere, the catalyst precursor is magnetically treated to obtain a spinel-type catalyst Fe / MCO@NF-B-EMIH rich in oxygen vacancies;

[0011] In the step (2), the voltage of the magnetization treatment is 30 - 50 V, the time of the magnetization treatment is 0.5 - 1 hour, the frequency of the magnetization treatment is 50 - 200 kHz, and the magnetic field strength of the magnetization treatment is 5 - 10 mT.

[0012] The above method for magnetically treating to assist in the preparation of a spinel-type catalyst rich in oxygen vacancies is applied to the photothermal catalytic oxidation of VOCs reaction. First, a spinel-type catalyst rich in oxygen vacancies is prepared by magnetically treating to assist; simulated sunlight is introduced into the reaction system for catalytic oxidation of VOCs. Using air as the balance gas, the concentration of VOCs is 1000 ppm, and the space velocity is 45000 mL·g cat -1 ·h -1, the VOCs are preferably toluene; the intensity of the simulated sunlight is 500 - 900 mW cm -2 . The applicant independently designed a room-temperature and atmospheric-pressure continuous-flow gas-solid phase reaction device. The catalyst is placed in a quartz reactor with a heat preservation device. The simulated sunlight passes through the quartz window to reach the surface of the catalyst, and the surface temperature of the catalyst is measured in real time by a thermocouple.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] The present invention uses magnetization treatment to assist in the preparation of a spinel-type catalyst rich in oxygen vacancies. The process is simple and the cost is low. The obtained product has a uniform morphology and good reproducibility. In the photothermal catalytic oxidation reaction of VOCs, compared with the untreated Fe / MCO@NF-B catalyst, the Fe / MCO@NF-B-EHIM catalyst prepared by magnetization assistance has a significantly increased activity for the photothermal catalytic oxidation of toluene. The magnetization treatment changes the magnetic properties of the catalyst and significantly increases the oxygen vacancy concentration and the content of active oxygen species on the surface of the catalyst, thereby significantly improving the photothermal catalytic oxidation performance of the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 XRD spectra of the catalysts prepared in Examples 1 and 2.

[0016] Figure 2 SEM images of the catalyst prepared in Example 2.

[0017] Figure 3 EPR spectra of the catalyst prepared in Example 1.

[0018] Figure 4 XPS data graphs of the catalysts prepared in Examples 1 and 2.

[0019] Figure 5 Conversion rates of the catalysts prepared in Examples 1 and 2 for the photothermal catalytic oxidation of toluene. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described in detail below with reference to the drawings and examples.

[0021] Example 1

[0022] (1) After pickling and ultrasonic treatment with ethanol, the nickel foam is dried to obtain pure nickel foam; weigh Mn(NO 3 ) 2 ·4H 2 O (2 mmol), Co(NO 3 ) 2 ·6H 2 O (4 mmol), NH 4F (12 mmol), urea (24 mmol) were dissolved in 80 ml of deionized water and stirred at room temperature until a clear solution was obtained. It was transferred into a polytetrafluoroethylene liner, and nickel foam was added. Then it was placed in a reaction kettle and put into an oven at 95 °C for a 12-hour hydrothermal reaction. Then the hydrothermal step was repeated once. After being washed with ethanol, it was transferred into a vacuum drying oven and dried at 80 °C for 12 h. Finally, it was placed in a muffle furnace and calcined in an air atmosphere at 400 °C for 2 h with a heating rate of 5 °C / min to obtain a monolithic catalyst, denoted as MCO@NF-B. MCO@NF-B was placed in a reactor and simultaneously placed at the center of an alternating magnetic field coil for magnetization treatment. The alternating voltage was 40 V and the treatment time was 30 minutes. The catalyst prepared in this example was denoted as MCO@NF-B-EMIH.

[0023] Example 2

[0024] (1) Nickel foam was pickled and ultrasonically treated with ethanol and then dried to obtain pure nickel foam; Weigh Fe(NO 3 ) 2 ·9H 2 O (0.33 mmol), Mn(NO 3 ) 2 ·4H 2 O (2 mmol), Co(NO 3 ) 2 ·6H 2 O (4 mmol), NH 4 F (12 mmol), urea (24 mmol) were dissolved in 80 ml of deionized water and stirred at room temperature until a clear solution was obtained. It was transferred into a polytetrafluoroethylene liner, and nickel foam was added. Then it was placed in a reaction kettle and put into an oven at 95 °C for a 12-hour hydrothermal reaction. Then the hydrothermal step was repeated once. After being washed with ethanol, it was transferred into a vacuum drying oven and dried at 80 °C for 12 h. Finally, it was placed in a muffle furnace and calcined in an air atmosphere at 400 °C for 2 h with a heating rate of 5 °C / min to obtain a monolithic catalyst, denoted as Fe / MCO@NF-B. MCO@NF-B was placed in a reactor and simultaneously placed at the center of an alternating magnetic field coil for magnetization treatment. The alternating voltage was 40 V and the treatment time was 30 minutes. The catalyst prepared in this example was denoted as Fe / MCO@NF-B-EMIH.

[0025] The XRD pattern of the catalyst prepared in Example 1 is as Figure 1 shown. It can be seen from the figure that there is no obvious change in the diffraction peaks, and it conforms to the standard PDF card of cubic phase MnCo 2 O 4 spinel.

[0026] The SEM image of the catalyst prepared in Example 2 is as Figures 2 to 4 shown. FromFigure 2 It can be seen that the Fe / MCO@NF-B consists of a nano-flower structure composed of nanosheets with a thickness of 10-50 μm, and a nano-array on its surface. There are pores of different sizes on the nanosheets, which may be caused by the formation of gas during the formation of spinel. It is speculated that the change in surface area and surface defects is caused by iron doping. Comparing the SEM images before and after magnetization ( Figure 3 ), it shows that the morphology of Fe / MCO@NF-B-EHIM has not changed significantly after magnetization. The nanoporous hexagonal prism nanosheets still serve as the substrate to form the nano-flower array, indicating that magnetization does not change the catalyst morphology.

[0027] The Electron Paramagnetic Resonance (EPR) test of the catalyst prepared in Example 1 is as Figure 5 shown. In the test of the catalyst defect structure, it is analyzed that the g value of the oxygen vacancy is 2.003. From the EPR, it is found that the peak intensity of the MCO@NF-B-EMIH catalyst increases here, indicating an increase in the concentration of oxygen vacancies in the MCO@NF-B-EMIH catalyst.

[0028] The O1s of the XPS spectra of the catalysts prepared in Examples 1 and 2 can be divided into 4 peaks after fitting. There are mainly two types of oxygen species on the surface of these catalysts: the surface adsorbed oxygen species at 531.7 eV and the lattice oxygen species at 529.8 eV. After quantitative analysis, the order of the O ads . / O latt ratio is MCO@NF-B-EMIH (0.41) > MCO@NF-B (0.39) > Fe / MCO@NF-B-EMIH (0.29) > Fe / MCO@NF-B (0.27). It can be seen that the magnetized catalysts show stronger oxygen activation ability, which may be related to the fact that they have more oxygen vacancies on their surfaces. Therefore, the rich surface adsorbed oxygen species is one of the reasons for the excellent catalytic oxidation activity of the Fe / MCO@NF-B-EMIH catalyst for VOCs.

[0029] The catalytic performance of the catalysts prepared in Examples 1 and 2 for the photocatalytic oxidation of VOCs was evaluated. The reaction conditions were as follows: 500 mg of the catalyst (100 mg of the active substance and 400 mg of the nickel foam substrate) was placed in a cylindrical quartz reactor, and a xenon lamp was used as the source of simulated sunlight energy. The feed gas was introduced from one end of the reactor, and the total gas flow rate was 45000 mL·g cat -1 ·h -1 , the concentration of the VOCs feed gas was 1000 ppm, and the catalytic oxidation degradation performance of four catalysts for toluene was evaluated. For the convenience of comparison, all catalysts were irradiated at 500 mW cm-2 The toluene conversion rate and surface temperature under light intensity were plotted as a bar chart (as Figure 5 ). The photo-thermal catalytic oxidation activity and surface temperature of the Fe / MCO@NF-B-EMIH and MCO@NF-B-EMIH catalysts were both superior to those of the unmagnetized Fe / MCO@NF-B and MCO@NF-B. Therefore, it was concluded that in the photo-thermal catalytic reaction, magnetization treatment could increase the concentration of oxygen-rich vacancies on the spinel surface, and the spinel catalyst rich in oxygen vacancies prepared by magnetization-assisted method showed high photo-thermal catalytic oxidation activity for toluene.

[0030] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method and application of preparing a spinel catalyst rich in oxygen vacancies by magnetization treatment, characterized in that: The synthesis method of the catalyst comprises: (1) preparing a catalyst precursor by a hydrothermal method: placing a metal foam carrier in a mixed solution of metal salt ions such as iron nitrate, manganese nitrate, cobalt nitrate, etc., adding ammonium fluoride and urea for hydrothermal reaction, repeating the hydrothermal reaction several times, and obtaining a precursor after washing and drying; (2) placing the catalyst precursor in a magnetization reactor, placing it at the center of an alternating magnetic field coil, and magnetizing the catalyst precursor in an air or oxygen atmosphere to obtain a spinel catalyst rich in oxygen vacancies.

2. The method according to claim 1, characterized in that The step (1) is as follows: the metal foam carrier is ultrasonically cleaned with hydrochloric acid and ethanol and then dried, a mixed solution of metal salt ions such as iron nitrate, manganese nitrate, and cobalt nitrate is prepared, and the metal foam carrier is added to the mixed solution of metal ions. Ammonium fluoride and urea are added to the above solution and stirred evenly, and then placed in an oven for hydrothermal reaction at 95° C. for 12 hours, and the hydrothermal reaction is repeated 1 to 3 times, and then washed and dried to obtain a precursor.

3. The method and application according to claim 2, characterized in that: The metal foam carrier is a metal carrier with magnetocaloric effect such as foam nickel, foam iron, stainless steel, etc.

4. The method according to claim 1, characterized in that The step (2) is as follows: placing the catalyst precursor in a magnetization reactor, placing it at the exact center of the alternating magnetic field coil, and magnetizing the catalyst precursor in an air or oxygen atmosphere to obtain a spinel catalyst rich in oxygen vacancies.

5. The method and application according to claim 4, characterized in that: The voltage of the magnetization treatment is 30-50V.

6. The method and use according to claim 4, characterized in that: The magnetization treatment time is 0.5 to 1 hour.

7. The method and use according to claim 4, characterized in that: The frequency of the magnetization treatment is 50 to 200 kHz.

8. The method and use according to claim 4, characterized in that: The magnetic field intensity of the magnetization treatment is 5-10 mT.

9. The method and use according to claim 1, characterized in that: The catalyst is used for photothermal catalytic oxidation of volatile organic compounds.

10. The method and use according to claim 9, characterized in that: First, magnetization treatment is used to assist in the preparation of a spinel catalyst rich in oxygen vacancies; simulated sunlight is introduced into the reaction system of the catalyst for photothermal catalytic oxidation of toluene.

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