CN112981454B - Manganese dioxide ultra-long nanowire catalyst with oxygen vacancy as well as preparation method and application thereof - Google Patents

Manganese dioxide ultra-long nanowire catalyst with oxygen vacancy as well as preparation method and application thereof Download PDF

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CN112981454B
CN112981454B CN202110148406.4A CN202110148406A CN112981454B CN 112981454 B CN112981454 B CN 112981454B CN 202110148406 A CN202110148406 A CN 202110148406A CN 112981454 B CN112981454 B CN 112981454B
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黄柏标
王光彬
郑昭科
王泽岩
王朋
程合锋
张晓阳
张倩倩
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Shandong University
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Abstract

The invention relates to a manganese dioxide ultra-long nanowire catalyst with oxygen vacancies and a preparation method and application thereof. MnO2Catalyst with ultra-long nano-wire, consisting of MnO of ultra-long length2The nano wires are of a 3D network structure, the diameter of each nano wire is about 10nm, the interplanar spacing is 0.69nm, and the nano wires have lattice distortion and oxygen vacancies. The preparation method comprises the steps of taking carbon paper as a conductive substrate, placing the carbon paper in a potassium permanganate solution for hydrothermal reaction, and obtaining MnO on the carbon paper2An ultra-long nanowire catalyst. Can spontaneously form oxygen vacancy in the process of hydrothermal reaction, improve MnO2The electronic structure of (1). And growing on the surface of the carbon paper to form a 3D structure, so that active sites with higher density can be exposed. The electro-catalytic nitrogen reduction performance can be effectively improved. Electrocatalytic nitrogen reduction can be performed at room temperature.

Description

Manganese dioxide ultra-long nanowire catalyst with oxygen vacancy as well as preparation method and application thereof
Technical Field
The invention belongs to the technical field of manganese dioxide nanowires, and particularly relates to a manganese dioxide ultra-long nanowire catalyst with oxygen vacancies as well as a preparation method and application thereof.
Background
The information in this background section is only for enhancement of understanding of the general background of the invention and is not necessarily to be construed as an admission or any form of suggestion that this information forms the prior art that is already known to a person of ordinary skill in the art.
Ammonia is the second largest chemical product in the world and is widely used in the manufacture of fertilizers, pharmaceuticals, dyes, explosives, and resins. It is also considered an energy carrier with no carbon footprint, high energy density and no carbon dioxide emissions. However, the major route to ammonia production on an industrial scale is currently the Haber-Bosch process, which requires high energy input, high reaction temperatures (400-. The electric energy is used as clean energy, can be provided by solar power generation, and belongs to inexhaustible energy. In recent years, the use of electrical energy to produce chemicals required by humans has been favored by researchers. The technology of converting nitrogen into ammonia by utilizing the electrochemical nitrogen fixation technology is considered to be a green, economic and sustainable synthetic ammonia process. However, this process is limited by the large energy barrier for activating the N ≡ N bond, requiring efficient and stable materials for cleaving the nonpolar N ≡ N bond of nitrogen. Therefore, the development of cheap and efficient electrocatalytic nitrogen reduction catalysts is of great importance for the industrial application of the electrochemical synthesis of ammonia.
MnO2As a transition metal oxide with abundant resources, the transition metal oxide has the advantages of low cost, good chemical stability, friendly interface with carbon materials and the like. The characteristics of various crystal structures and multivalence of the composite material are utilized, and the composite material is widely applied to the field of electrochemistry. However, the improvement of the reductive property of the electrocatalytic nitrogen is greatly limited due to its poor nitrogen adsorption capacity and insufficient activity of the active sites.
Disclosure of Invention
Aiming at the problems in the prior art, the invention aims to provide a manganese dioxide ultra-long nanowire catalyst with oxygen vacancies, and a preparation method and application thereof. In MnO2Oxygen vacancy is introduced to effectively improve the reduction performance of the electro-catalytic nitrogen. To make MnO2The long nanowire structure exposes active sites with higher density, provides more adsorption sites for charge transfer reaction, and utilizes oxygen vacancy engineering to control an electronic structure, thereby solving the problems of poor nitrogen adsorption capacity and insufficient activity of the active sites.
In order to solve the technical problems, the technical scheme of the invention is as follows:
in a first aspect, a manganese dioxide ultra-long nanowire catalyst with oxygen vacancies is prepared by using manganese dioxide in the form of ultra-long MnO2The nano wires are of a 3D network structure, the diameter of each nano wire is about 10nm, the interplanar spacing is 0.69nm, and the nano wires have lattice distortion and oxygen vacancies.
MnO2The nanowires are characterized as ultra-long nanowires. Has a 3D network structure with oxygen vacancies that can efficiently manipulate the electronic structure of the metal oxide and provide coordinated unsaturated sites for molecular chemisorption. MnO2Oxygen vacancies are introduced into the nanowires, so that the electro-catalytic nitrogen reduction performance can be effectively improved.
The super-long nanowire with the 3D network structure can enlarge the contact area between an electrode and an electrolyte and expose active sites with higher density.
In a second aspect, the MnO having oxygen vacancy as described above2The preparation method of the ultra-long nanowire catalyst comprises the following steps: carbon paper is used as a conductive substrate, the carbon paper is placed in potassium permanganate solution for hydrothermal reaction, and MnO is obtained on the carbon paper2An ultra-long nanowire catalyst.
Can spontaneously form oxygen vacancy in the process of hydrothermal reaction, improve MnO2The electronic structure of (1). Growing on the surface of the carbon paper to form a 3D structure, and exposing active sites with higher density.
In some embodiments of the invention, the concentration of the potassium permanganate solution is between 0.5 and 1 mol/L; preferably 0.6-0.8 mol/L; further preferably 0.75 mol/L. The concentration of potassium permanganate affects the yield of the ultra-long nanowires.
In some embodiments of the invention, the temperature of the hydrothermal reaction is 200-230 ℃, and the time of the hydrothermal reaction is 30-40 h; preferably, the temperature of the hydrothermal reaction is 200-220 ℃, and the time of the hydrothermal reaction is 32-38 h; further preferably, the temperature of the hydrothermal reaction is 220 ℃ and the time of the hydrothermal reaction is 36 hours. The temperature and time of the hydrothermal reaction affect the size of the nanowires and the formation of oxygen vacancies.
In some embodiments of the invention, the carbon paper is pretreated before being put into the potassium permanganate solution, and the pretreatment method comprises the following steps: cleaning with dilute hydrochloric acid, acetone and ethanol in sequence.
In some embodiments of the invention, the product obtained after the hydrothermal reaction is dried at 60-70 ℃ for 8-12 h; preferably at 60 ℃ for 12 h.
In a third aspect, the MnO having oxygen vacancy as described above2The application of the ultra-long nanowire catalyst in electrocatalytic nitrogen reduction.
In a fourth aspect, the MnO having oxygen vacancy as described above is used2The method for producing ammonia by carrying out electro-catalysis nitrogen reduction on the ultra-long nanowire catalyst comprises the following steps: using MnO2The method comprises the following steps of using an ultra-long nanowire catalyst as a working electrode, forming a three-electrode system with a counter electrode and a reference electrode, activating the electrode by using a cyclic voltammetry method, introducing nitrogen into an electrolyte, activating for the second time after the nitrogen is saturated, and then performing electrocatalysis at room temperature.
In some embodiments of the invention, the electrocatalytic potential is between-0.9V and-0.4V.
In some embodiments of the invention, the electrocatalysis time is 7000-7500 s.
In some embodiments of the invention, the voltage of the cyclic voltammetric activation is 0 to-1.0V, the scan rate is 45-55mV/s, the sampling interval is 0.001V, the resting time is 1.5-2.5s, and the number of scan segments is 450-.
In some embodiments of the invention, an inert gas is introduced before the introduction of nitrogen gas to remove dissolved nitrogen gas from the electrolyte.
Using MnO2The ultra-long nanowire catalyst is used for carrying out the electrocatalytic nitrogen reduction reaction, so that the electrocatalytic nitrogen reduction difficulty is reduced, and the electrocatalytic nitrogen reduction can be carried out at room temperature.
One or more technical schemes of the invention have the following beneficial effects:
MnO with oxygen vacancy prepared by the invention2The super-long nanowire is of a 3D network structure, can expose active sites with higher density, enlarge the contact area between an electrode and an electrolyte, and provide more adsorption sites for charge transfer reaction. In addition, the presence of spontaneously formed oxygen vacancies enables manipulation of the electronic structure of the catalyst, thereby effectively enhancing the adsorption and activation of nitrogen molecules. Can easily achieve these advantages, MnO2The ultra-long nanowires exhibit excellent electrocatalytic nitrogen reduction activity and stability. Expressed as MnO2Ultra-long nanowires are 0.1M Na2SO4In solution, a Faraday Efficiency (FE) of 8.8% and 1.13X 10 at-0.7V was achieved compared to a standard hydrogen electrode-10mol cm-2s-1The ammonia yield of (a). In addition, the material was tested for NH after 5 consecutive cycles at-0.7V vs. standard hydrogen electrode 3The yield and faraday efficiency changes were negligible; the current density was stable with no significant change over 24 hours of continuous electrolysis. This shows that the catalyst has good electrochemical stability and has wide prospect in the aspect of practical application.
MnO having oxygen vacancy in the invention2The ultra-long nanowire catalyst has excellent selectivity for reducing nitrogen gas into ammonia. By-product hydrazine was not detected at various potentials from-0.4 to-0.9V relative to a standard hydrogen electrode. In addition, the catalyst has the advantages of simple and adjustable preparation method, easily obtained reaction conditions, low cost, easy industrialization, environmental friendliness, no pollution and the like. Thus, MnO having oxygen vacancies prepared by the process of the present invention2The ultra-long nanowire catalyst is an electrocatalytic nitrogen reduction catalyst with potential commercial application value.
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The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the invention and not to limit the invention.
FIG. 1 shows MnO having oxygen vacancy prepared in example 1 of the present invention2Carrying out ultra-long nanowire XRD and fitting an XRD spectrum;
FIG. 2 shows MnO having oxygen vacancy prepared in example 1 of the present invention2An SEM image, a TEM image and a Mapping image of the ultra-long nanowire;
FIG. 3 shows MnO having oxygen vacancy prepared in comparative example 1 and example 2 of the present invention2An ultra-long nanowire SEM image;
FIG. 4 shows MnO with oxygen vacancy in example 3 of the present invention2Electrochemical testing of the ultra-long nanowire;
FIG. 5 is an ultraviolet-visible spectroscopy spectrum and calibration curve for ammonia production in example 3 of the present invention;
FIG. 6 shows MnO with oxygen vacancy in example 3 of the present invention2Ultraviolet-visible spectroscopy of the ultralong nanowires;
FIG. 7 shows MnO with oxygen vacancy in example 3 of the present invention2Ammonia yield and faraday efficiency of the ultralong nanowires;
FIG. 8 shows MnO with oxygen vacancy in example 3 of the present invention2And (4) testing the electrochemical stability of the ultralong nanowire.
Detailed Description
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise. The invention will be further illustrated by the following examples
Example 1
MnO with oxygen vacancy2The preparation method of the ultra-long nanowire catalyst comprises the following steps:
(1) ultrasonically cleaning a carbon paper conductive substrate:
cutting with scissors to obtain a carbon paper conductive substrate with the size of 2cm multiplied by 3cm, then respectively ultrasonically cleaning with dilute hydrochloric acid, acetone and ethanol for 20 minutes, and finally storing in ethanol solvent.
(2) One-step hydrothermal synthesis of MnO with oxygen vacancy2Ultra-long nanowires:
at room temperature, 3mmol of KMnO4Dissolved in 40mL of deionized water and magnetically stirred for 20 minutes to obtain a purple solution. Then transferring the solution into a reaction kettle with a 50 ml polytetrafluoroethylene lining, placing the clean carbon paper obtained in the step (1) in the kettle, carrying out hydrothermal reaction at 220 ℃ for 36 hours, and naturally cooling to obtain MnO uniformly growing on the carbon paper2An ultra-long nanowire.
The MnO2The XRD spectrum of the ultra-long nanowires was consistent with the results of the simulation, demonstrating the phase purity of the samples (as shown in fig. 1). As can be seen in FIG. 2(a), the carbon paper is completely entangled MnO2And covering the ultra-long nanowire to form a 3D network structure. HRTEM image showed clear lattice fringes, nanowires with a diameter of about 10nm and interplanar spacing of 0.69nm, corresponding to MnO 2The (110) crystal face of the crystal. At the same time, many lattice distortions can be observed, indicating the presence of oxygen vacancies (as shown in fig. 2 c). Mapping images also demonstrate MnO2The ultra-long nanowires exist in a form, and both elements of Mn and O are uniformly distributed (as shown in b in FIG. 2).
Comparative example 1
MnO with oxygen vacancy2The preparation method of the ultra-long nanowire catalyst comprises the following steps:
(1) ultrasonically cleaning a carbon paper conductive substrate:
cutting with scissors to obtain a carbon paper conductive substrate with the size of 2cm multiplied by 3cm, then respectively ultrasonically cleaning with dilute hydrochloric acid, acetone and ethanol for 20 minutes, and finally storing in ethanol solvent.
(2) One-step hydrothermal synthesis of MnO with oxygen vacancy2Ultra-long nanowires:
at room temperature, 3mmol of KMnO4Dissolved in 40mL of deionized water and magnetically stirred for 20 minutes to obtain a purple solution. Then transferring the solution into a reaction kettle with a 50 ml polytetrafluoroethylene lining, placing the clean carbon paper obtained in the step (1) in the kettle, carrying out hydrothermal reaction at 200 ℃ for 30 hours, and naturally cooling to obtain MnO uniformly growing on the carbon paper2An ultra-long nanowire. As can be seen from the SEM image, MnO was2The nanowires are short in size and difficult to form into a 3D network structure (as shown in a in fig. 3).
Example 2
MnO with oxygen vacancy2The preparation method of the ultra-long nanowire catalyst comprises the following steps:
(1) ultrasonically cleaning a carbon paper conductive substrate:
cutting with scissors to obtain a carbon paper conductive substrate with the size of 2cm multiplied by 3cm, then respectively ultrasonically cleaning with dilute hydrochloric acid, acetone and ethanol for 20 minutes, and finally storing in ethanol solvent.
(2) One-step hydrothermal synthesis of MnO with oxygen vacancy2Ultra-long nanowires:
at room temperature, 2mmol of KMnO4Dissolved in 40mL of deionized water and magnetically stirred for 20 minutes to obtain a purple solution. Then transferring the solution into a reaction kettle with 50 ml of polytetrafluoroethylene lining, placing the clean carbon paper obtained in the step (1) in the kettle, carrying out hydrothermal reaction at 220 ℃ for 36 hours, and naturally cooling to obtain MnO uniformly growing on the carbon paper2An ultra-long nanowire. As can be seen from the SEM image, MnO was2The ultra-long nanowires are sparse and yield is low (as shown in b in fig. 3).
Example 3 electrocatalytic nitrogen reduction experiment
1. The test method comprises the following steps:
the electrocatalytic nitrogen reduction ammonia production test was recorded by the electrochemical workstation (CHI 750E) using a three-electrode H-cell unit. The electrolytic cell separates the anode compartment and the cathode compartment by a Nafion membrane. MnO with oxygen vacancy to be grown on carbon paper 2The ultra-long nanowires (product prepared in example 1) were cut to 1cm × 1cm as working electrodes, carbon rods as counter electrodes, and Ag/AgCl electrodes as reference electrodes. The test was carried out at room temperature using a 0.1mol/L sodium sulfate solution as an electrolyte.
2. Electrocatalytic nitrogen reduction activity test:
MnO with oxygen vacancy grown on carbon paper2The ultra-long nanowire is used as a working electrode, and cyclic voltammetry test is performed in a three-electrode system to activate a sample. The voltage range of the cyclic voltammetry test is 0-1.0V (relative to an Ag/AgCl electrode), the highest potential is 0V, the lowest potential is-1.0V, the starting potential is 0V, and the stopping potential is-1.0V. The scanning speed was 50 mV/s. The sampling interval is 0.001V, the standing time is 2s, and the number of scanning sections is 500. Firstly, introducing argon into the electrolyte for 30min to remove dissolved nitrogen in the electrolyte, and performing a first linear voltage scanning test after the argon is saturated; and then introducing nitrogen into the electrolyte for 30min, and carrying out a second linear voltage scanning test after the nitrogen is saturated.
MnO with oxygen vacancy grown on carbon paper after activation by cyclic voltammetry2The ultra-long nanowire is used as a working electrode, a catalyst is subjected to a long-time electro-catalytic nitrogen reduction ammonia production test, the potentials are respectively set to be-0.4V, -0.5V, -0.6V, -0.7V, -0.8V and-0.9V (relative to an Ag/AgCl electrode), the running time is 7200s, and the test is carried out under the condition that nitrogen continuous bubbling is kept. The test results are shown in fig. 4.
3. Ammonia production test
(1) Drawing a calibration curve:
a plurality of standard solutions within 0-8 mu g/mL are respectively prepared in 0.1mol/L sodium sulfate solution by using ammonium chloride as a standard reagent, and the absorbance is tested by performing chromogenic reaction on the standard solutions, wherein the concentrations of the ammonium chloride in the standard solutions are (0, 0.382, 0.764, 1.528, 3.055, 4.583, 6.111 and 7.638) mu g/mL in sequence as shown in FIG. 4. 2mL of the standard solution was added to 2mL of a 1mol/L sodium hydroxide solution (solution A) containing 5 wt% of salicylic acid and 5 wt% of sodium citrate, followed by 1mL of a 0.05mol/L sodium hypochlorite solution (solution B), and preferably 0.2mL of a 1mol/L sodium nitroprusside dihydrate solution (solution C). Standing and developing for 2h at room temperature in dark place, performing spectral scanning with an ultraviolet-visible spectrophotometer at wavelength range of 550-800 nm, recording the value of absorbance at 655nm, and plotting with concentration to obtain calibration curve (as shown in FIG. 5).
(2) And (3) yield testing:
4mL of the electrolyte solution after 7200s of operation at each potential was taken, and then 2mL of solution A, 1mL of solution A and 0.2mLC were added in this order. Standing and developing for 2h at room temperature in a dark place, performing spectral scanning at wavelength of 550-800 nm by using an ultraviolet-visible spectrophotometer, recording the value of absorbance at 655nm (as shown in figure 6), and contrasting a calibration curve to obtain the concentration of ammonia. MnO with oxygen vacancy after data processing and calculation 2The ultra-long nano-wire has excellent performance of preparing ammonia by electro-catalysis nitrogen reduction, and the ammonia yield reaches 1.13 multiplied by 10 under the condition of-0.7V (relative to a standard hydrogen electrode)-10mol cm-2s-1The Faraday efficiency reached 8.8% (as shown in FIG. 7).
3. Stability test
MnO with oxygen vacancy grown on carbon paper2The ultra-long nm was used as the working electrode, and the ammonia yield and faraday efficiency changed negligibly after 5 consecutive cycles of testing (as shown in fig. 8 a). Within 24 hours of continuous electrolysis, the current density was stable with no significant change (as shown in fig. 8 b). This indicates that the catalyst has excellent electrochemical stability, again evidencing MnO with oxygen vacancies2The ultra-long nanowire catalyst has huge industrial application prospect.
Comparative example 2
The temperature of the hydrothermal reaction was 160 ℃ as compared to example 1. Other operating conditions were unchanged. MnO2The carbon paper is difficult to nucleate and grow on the surface, and an ultra-long nanowire structure cannot be formed. The active site number is less exposed and it is difficult to provide more adsorption sites for charge transfer reactions. In addition, the hydrothermal environment with lower temperature is not favorableFormation of oxygen vacancies, difficulty in changing MnO2The electronic structure of (2) is not favorable for the adsorption of nitrogen.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. A preparation method of a manganese dioxide ultra-long nanowire catalyst with oxygen vacancies is characterized by comprising the following steps: the method comprises the following steps: carbon paper is used as a conductive substrate, the carbon paper is placed in potassium permanganate solution for hydrothermal reaction, and MnO is obtained on the carbon paper2The concentration of the potassium permanganate solution is 0.075 mol/L, the temperature of the hydrothermal reaction is 220-.
2. The method of preparing manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 1, wherein: the temperature of the hydrothermal reaction is 220 ℃, and the time of the hydrothermal reaction is 36 h.
3. The method of preparing manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 1, wherein: the carbon paper is pretreated before being put into a potassium permanganate solution, and the pretreatment method comprises the following steps: and sequentially cleaning with dilute hydrochloric acid, acetone and ethanol.
4. The method of preparing manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 1, wherein: drying the product obtained after the hydrothermal reaction at the temperature of 60-70 ℃ for 8-12 h.
5. The method of preparing manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 4, wherein: the drying temperature is 60 ℃ for 12 h.
6. A manganese dioxide ultra-long nanowire catalyst having oxygen vacancies, prepared by the preparation method of any one of claims 1 to 5, wherein the preparation method comprises the following steps: the catalyst takes carbon paper as a conductive substrate and has ultra-long MnO2The nanowire grows on the surface of the carbon paper to form a 3D network structure, the diameter of the nanowire is 10nm, the interplanar spacing is 0.69nm, and the nanowire has lattice distortion and oxygen vacancies.
7. Use of the manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 6 in electrocatalytic nitrogen reduction.
8. The method for producing ammonia by electrocatalytic nitrogen reduction by using the manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 6, wherein the method comprises the following steps: the method comprises the following steps: using MnO2The method comprises the following steps of using an ultra-long nanowire catalyst as a working electrode, forming a three-electrode system with a counter electrode and a reference electrode, activating the electrode by using a cyclic voltammetry method, introducing nitrogen into an electrolyte, activating for the second time after the nitrogen is saturated, and then performing electrocatalysis at room temperature.
9. The method for electrocatalytic nitrogen reduction of manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 8 to produce ammonia, wherein: the electrocatalytic potential is-0.9V to-0.4V.
10. The method for electrocatalytic nitrogen reduction of manganese dioxide ultra-long nanowire catalyst with oxygen vacancies of claim 8 to produce ammonia, wherein: the electrocatalysis time is 7000-7500 s; or, the voltage of the cyclic voltammetry activation is 0 to-1.0V, the scanning speed is 45-55mV/s, the sampling interval is 0.001V, the standing time is 1.5-2.5s, and the number of scanning sections is 450-550; or before the nitrogen is introduced, introducing inert gas to remove the nitrogen dissolved in the electrolyte.
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