Preparation method of MXene aggregate with high specific surface area and application of MXene aggregate in CO2 hydrogenation reaction

Abstract

The invention belongs to the technical field of material science and chemical engineering, and particularly relates to a preparation method of a high-specific-surface-area MXene aggregate and application of the high-specific-surface-area MXene aggregate in catalysis of CO2 hydrogenation reaction. Comprising the following steps: (1) mixing MXene, a stripping agent and a solvent, and carrying out ultrasonic treatment to obtain a single-layer MXene suspension and a few-layer MXene suspension; and (2) adding the salt / acid / alkali-containing electrolyte solution into the single-layer and few-layer MXene suspension liquid, enabling MXene to spontaneously aggregate and precipitate, filtering and washing the obtained MXene aggregate precipitate, and drying the filter cake in vacuum to obtain the MXene aggregate with high specific surface area. The prepared MXene has the maximum specific surface area exceeding 90 m < 2 > / g and reaches 10 times or above that of traditional single-layer MXene powder, the specific surface area is remarkably increased, and wide application prospects are shown in the fields of catalytic materials, rechargeable batteries, supercapacitors, sensors and the like.

Description

Technical Field

[0001] The present invention belongs to the field of material science and chemical engineering technology, and specifically relates to a method for preparing a high specific surface area MXene aggregate and its application in catalytic CO2 hydrogenation reaction. Background Art

[0002] In the field of materials science, two-dimensional materials have attracted widespread attention due to their unique physical and chemical properties and broad application prospects. MXene, as an emerging class of two-dimensional inorganic compounds, has attracted much attention since it was first reported in 2011 due to its excellent electrical conductivity, hydrophilicity and potential applications in energy storage, electromagnetic shielding, sensors and other fields. The general formula of MXene is M n+1 X n T x , where M represents a transition metal (such as Ti, Mo, etc.), X represents carbon or nitrogen, n is usually 1 to 3, and T x Represents the surface terminal groups (such as -OH, -F, -O, etc.).

[0003] However, due to the van der Waals force, MXene materials tend to stack layers, and their specific surface area is very small, usually less than 10m 2 / g. To solve the problem of MXene interlayer stacking, the main method currently adopted is to fill other substances between MXene layers. Professor Xu Bing of Beijing University of Chemical Technology proposed a method of using polyvinylidene fluoride (PVDF) as an intercalation material to hinder the stacking of MXene layers. The specific surface area of ​​the composite reached 168m 2 / g (Adv. Funct. Mater. 2019, 1906282). However, PVDF has poor thermal stability and cannot be used in high-temperature scenarios. Weizhai Bao et al. from the University of Technology Sydney also obtained high-specific-surface-area MXene using a freeze-drying process. The principle is to freeze the MXene before stacking to prevent it from stacking, and use freeze-drying to remove the solvent (Joule 2, 2018, 778–787). Although these methods can effectively synthesize high-specific-surface-area MXene, other substances need to be added to the MXene or complex synthesis processes need to be used. Therefore, the development of a process for synthesizing high-specific-surface-area MXene is still an important research direction at present. Summary of the invention

[0004] In view of the fact that MXene is easy to stack, resulting in too small interlayer spacing and limiting the influence of material diffusion, the present invention provides a method for synthesizing high specific surface area MXene aggregates using salting-out effect. The method not only increases the specific surface area of ​​MXene, but also simplifies the production process and reduces production costs. At the same time, the impact on the environment is also significantly reduced. When applied to the catalytic CO2 hydrogenation reaction, the CO2 hydrogenation conversion rate can reach 20%, and the selectivity for CO is close to 100%, which significantly improves the catalytic activity of MXene aggregates for CO2 hydrogenation.

[0005] In order to achieve the above object, the technical solution of the present invention is as follows:

[0006] In one aspect, the present invention provides a method for preparing a high specific surface area MXene aggregate, the method comprising the following steps:

[0007] (1) Preparation of monolayer and few-layer MXene suspensions: MXene, exfoliant, and solvent are mixed and then ultrasonically treated to obtain monolayer and few-layer MXene suspensions;

[0008] (2) Preparation of high specific surface area MXene aggregates by salting out method:

[0009] Add a salt / acid / base electrolyte solution to the monolayer and few-layer MXene suspension obtained in step (1) to allow the MXene to spontaneously aggregate and precipitate. Filter the obtained MXene aggregate precipitate, wash, and then vacuum dry the filter cake to obtain a high specific surface area MXene aggregate.

[0010] Step (1) The MXene has M n X n-1 T 2n The general chemical formula is as follows, wherein M is a transition metal, X is C or N, and T is a surface end group, including but not limited to -O, -OH, -F, and -Cl. The MXene is purchased from outside or prepared by the following preparation method, which includes: using phosphoric acid, hydrofluoric acid, and hydrochloric acid aqueous solution to etch MXene under hydrothermal conditions; using HF etching to etch MXene at room temperature, wherein the HF etching method also includes a method of using hydrochloric acid and a fluoride salt to mix to prepare hydrofluoric acid in situ.

[0011] In the above technical solution, further, the stripping agent includes one or both of tetramethylammonium hydroxide and tetrapropylammonium hydroxide.

[0012] In the above technical solution, further, the solvent includes one or more of water, ethanol, acetone, ether, and acetonitrile, preferably water or ethanol aqueous solution.

[0013] In the above technical solution, further, the salt / acid / base electrolyte includes one or more of ammonium carbonate, sodium carbonate, sodium sulfate, sodium hydroxide, potassium hydroxide, hydrochloric acid, sulfuric acid, and formic acid; the salt / acid / base electrolyte is dissolved in the solution to ionize H + OH - NH4 + , metal ions, non-metal ions and ion clusters, and use the salting-out effect to change the surface charge state of MXene, destroy the surface ionization balance of MXene, and cause it to spontaneously aggregate and precipitate.

[0014] Another aspect of the present invention provides a MXene aggregate prepared by the above method, wherein the specific surface area of ​​the MXene aggregate is 15-100m 2 / g.

[0015] In the above technical solution, further, the specific surface area of ​​the MXene aggregate is 50-100m 2 / g.

[0016] The present invention also provides an application of the above MXene aggregate in CO2 hydrogenation to produce CO.

[0017] The beneficial effects of the present invention are:

[0018] The method of the present invention utilizes the salting-out effect to precipitate single-layer and few-layer MXene to prepare high specific surface area MXene aggregates, and the prepared MXene aggregates have a maximum surface area of ​​more than 90m 2 / g specific surface area, which is more than 10 times that of traditional single-layer MXene powder, and the specific surface area has been significantly increased. Under SEM, MXene presents a curled stacking structure, avoiding the close stacking between layers. At the same time, the MXene aggregates have micropores and mesoporous structures with pore sizes of 10 to 100 nm. These unique pore characteristics give the material application prospects in the field of catalytic materials. In addition to its application in the field of catalysts, this high specific surface area MXene aggregate can also be used as a catalyst carrier, and has shown broad application prospects in many fields such as rechargeable batteries, supercapacitors, and sensors.

[0019] Compared with methods such as compounding MXene with nano-microspheres and polymer materials, the use of salting-out effect to precipitate MXene does not require the addition of any substances that affect the physical and chemical properties of MXene. In addition, MXene aggregates have good mechanical strength and are suitable for catalyst materials.

[0020] In terms of synthesis process, the present invention provides a method for rapidly precipitating single-layer and few-layer MXene using the salting-out effect. The precipitated MXene aggregates have low affinity for water, ethanol, etc. and high density. When washing MXene, filtration or precipitation can be used to separate the solution and MXene. The MXene suspension will not precipitate in the solution and is difficult to filter, which effectively simplifies the production process. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 sa-Mo2CT of Example 1 x SEM images of MXene-(NH4)2CO3, a is magnified 8000 times, b is magnified 30000 times;

[0022] Figure 2 Mo2CT in Example 1 x MXene, d-Mo2CT x MXene and sa-Mo2CT x XRD pattern of MXene-(NH4)2CO3;

[0023] Figure 3 Mo2CT in Examples 1-4 x MXene, d-Mo2CT x MXene and sa-Mo2CT x N2 adsorption-desorption isotherms and pore size distribution diagrams of MXene-X (X = (NH4)2CO3, Na2CO3, NaOH, HCl), a is the N2 adsorption-desorption isotherm diagram, b is the pore size distribution diagram;

[0024] Figure 4 Mo2CT in Example 1-2 x MXene and sa-Mo2CT x The test results of CO2 conversion rate and CO selectivity during the CO2 hydrogenation to CO reaction catalyzed by MXene-X (X = (NH4)2CO3 and Na2CO3), where a is the CO2 conversion rate and b is the CO selectivity. DETAILED DESCRIPTION

[0025] The following examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0026] Unless otherwise specified, the materials used in the embodiments of the present invention can be obtained through commercial channels or prepared according to conventional methods well known to those skilled in the art.

[0027] Example 1

[0028] This embodiment uses (NH4)2CO3 salting out single layer and few layer Mo2CT x MXene is used to prepare MXene aggregates with high specific surface area, and the method comprises the following steps:

[0029] S1: Preparation of MXene materials

[0030] 1.0 g Mo2Ga2C and 50 mL 10% H3PO4 were placed in a 100 ml polytetrafluoroethylene-lined stainless steel hydrothermal reactor and hydrothermally reacted at 180 °C for 24 h. After the hydrothermal reactor was cooled, the mixture was filtered and washed with deionized water for 3 times to obtain Mo2CT x MXene, T is the surface end group including but not limited to -O, -OH;

[0031] S2: Preparation of monolayer and few-layer MXene suspensions

[0032] Mo2CT prepared by S1 x MXene was placed in a 100 mL round-bottom flask, and 20 mL of deionized water, 10 mL of anhydrous ethanol, and 1 mL of a 25% tetrapropylammonium hydroxide aqueous solution were added. The mixture was ultrasonically treated in an ultrasonic cleaner for 1 h with an ultrasonic power of 200 W and a frequency of 40 kHz to obtain a single-layer and few-layer MXene suspension, which was recorded as d-Mo2CT x ;

[0033] S3: Preparation of high specific surface area MXene aggregates by (NH4)2CO3 salting out

[0034] To the monolayer and few-layer MXene suspension prepared by 5 mL S2, 5 mL of 20% (NH4)2CO3 aqueous solution was added, and the MXene was shaken until the MXene was completely aggregated and the supernatant was transparent or translucent. The mixed solution was filtered, and the filter cake was washed three times with deionized water. The filter cake was dried in a vacuum drying oven at 60 ° C overnight to obtain high specific surface area Mo2CT x MXene aggregates, the sample is denoted as sa-Mo2CT x -(NH4)2CO3.

[0035] Example 2

[0036] This embodiment uses Na2CO3 to salt out single-layer and few-layer Mo2CT x MXene is used to prepare MXene aggregates with high specific surface area. Steps S1 and S2 of the method are exactly the same as those in Example 1, except that step S3 is as follows:

[0037] Add 5 mL of 20% Na2CO3 aqueous solution to the monolayer and few-layer MXene suspensions prepared by 5 mL of S2, shake until the MXene is completely aggregated and the supernatant is transparent or translucent, filter the mixture, wash the filter cake three times with deionized water, and dry the filter cake in a vacuum drying oven at 60 ° C overnight to obtain high specific surface area Mo2CT x MXene aggregates, the sample is denoted as sa-Mo2CT x -Na2CO3.

[0038] Example 3

[0039] This example uses NaOH to salt out single-layer and few-layer Mo2CT x MXene is used to prepare MXene aggregates with high specific surface area. Steps S1 and S2 of the method are exactly the same as those in Example 1, except that step S3 is as follows:

[0040] S3: Add 5 mL of 20% NaOH aqueous solution to 5 mL of the monolayer and few-layer MXene suspensions prepared in S2, shake until the MXene is completely aggregated and the supernatant is transparent or translucent, filter the mixture, wash the filter cake three times with deionized water, and dry the filter cake in a vacuum drying oven at 60 ° C overnight to obtain high specific surface area Mo2CT x MXene aggregates, the sample is denoted as sa-Mo2CT x -NaOH.

[0041] Example 4

[0042] This example uses HCl to salt out single-layer and few-layer Mo2CT x MXene is used to prepare MXene aggregates with high specific surface area. Steps S1 and S2 of the method are exactly the same as those in Example 1, except that step S3 is as follows:

[0043] Add 5 mL of 20% HCl aqueous solution to the monolayer and few-layer MXene suspensions prepared by 5 mL of S2, shake until the MXene is completely aggregated and the supernatant is transparent or translucent, filter the mixture, wash the filter cake three times with deionized water, and dry the filter cake in a vacuum drying oven at 60 ° C overnight to obtain high specific surface area Mo2CT x MXene aggregates, the sample is denoted as sa-Mo2CT x -HCl.

[0044] Example 5

[0045] In this embodiment, (NH4)2CO3 is used to salt out single-layer and few-layer Ti3C2T x MXene preparation of Ti3C2Tx aggregates, the steps are as follows:

[0046] S1: Ti3C2T x MXene (Foshan Xinxi Technology Co., Ltd.) was placed in a 100 mL round-bottom flask, and 20 mL of deionized water, 10 mL of anhydrous ethanol, and 1 mL of a 25% tetrapropylammonium hydroxide aqueous solution were added. The mixture was ultrasonically treated in an ultrasonic cleaning machine for 1 h with an ultrasonic power of 200 W and a frequency of 40 kHz to obtain Ti3C2T x MXene suspension;

[0047] S2: 5mL Ti3C2T x Add 5 mL of 20% (NH4)2CO3 aqueous solution to the MXene suspension and shake until the Ti3C2T x The mixture was completely aggregated, and the supernatant was transparent or translucent. The mixture was filtered, and the filter cake was washed three times with deionized water. The filter cake was dried in a vacuum drying oven at 60 ° C overnight to obtain a high specific surface area Ti3C2T x MXene aggregates, the sample is denoted as sa-Ti3C2T x -(NH4)2CO3.

[0048] Figure 1 sa-Mo2CT of Example 1 of the present invention x -(NH4)2CO3 SEM photo. It can be seen from the figure that after salting out treatment, the edges and surfaces of the MXene sheets are stacked alternately, and no obvious interlayer stacking structure is formed; it can also be observed from the figure that the disordered overlapping between the sheets forms a large number of mesoporous and macroporous structures.

[0049] Figure 2 Mo2CT in Example 1 of the present invention x 、d-Mo2CT x and sa-Mo2CT x -XRD diagram of (NH4)2CO3. It can be seen from the figure that salting out will not affect the crystal structure of MXene and will not destroy the original crystal structure of MXene.

[0050] Figure 3 Mo2CT in Examples 1-4 of the present invention x d-Mo2CT x and sa-Mo2CT x-X (X = (NH4)2CO3, Na2CO3, NaOH, HCl) N2 adsorption-desorption isotherms and pore size distribution diagrams indicate that the use of (NH4)2CO3, Na2CO3, etc. can significantly increase the specific surface area of ​​MXene by more than 10 times. The use of NaOH salting out has a slightly weaker effect on the improvement of the specific surface area, with the specific surface area increased by 7 times. The use of HCl salting out has a poor effect, but the specific surface area is also increased by two times.

[0051] Table 1 is the Mo2CT of Examples 1-4 of the present invention x d-Mo2CT x and sa-Mo2CT x -X (X = (NH4)2CO3, Na2CO3, NaOH, HCl) specific surface area, pore volume and average pore size test results.

[0052] Table 1 Specific surface area, pore volume and average pore diameter of samples in Examples 1-5

[0053]

[0054]

[0055] Application Example 1

[0056] The CO2 hydrogenation performance of the sample was tested using a micro fixed bed reactor. 20 mg of sa-Mo2CT in Example 1 was taken. x -(NH4)2CO3 and sa-Mo2CT in Example 2 x The MXene-Na2CO3 samples were mixed with 1.00g of quartz sand, loaded into a quartz tube with an outer diameter of 10mm and an inner diameter of 8mm, and then installed on a CO2 hydrogenation reaction device; the samples were treated with H2 at 450°C for 2h and then cooled to 200°C; 24mL / min CO2+72mL / min H2+4mL / min N2 were introduced into the reactor as reaction gases to test the CO2 hydrogenation activity of the catalyst, and the gas after the reaction was detected by gas chromatography.

[0057] Figure 4 Mo2CT in Example 1-2 x 、sa-Mo2CT x -(NH4)2CO3 and sa-Mo2CT x-Na2CO3 sample catalyzes the CO2 hydrogenation to CO reaction. As can be seen from the figure, at 400°C, the CO2 hydrogenation conversion rate increased from 9.5% to 20%, and the CO selectivity increased from 98% to nearly 100%. This is because the MXene after salting out has a larger specific surface area, which significantly improves the catalytic activity and CO selectivity of MXene aggregates for CO2 hydrogenation.

[0058] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing a high specific surface area MXene aggregate, characterized in that: The method comprises the following steps: (1) Preparation of monolayer and few-layer MXene suspensions: MXene, exfoliant, and solvent are mixed and then ultrasonically treated to obtain monolayer and few-layer MXene suspensions; (2) Preparation of high specific surface area MXene aggregates by salting out method: Add a salt / acid / base electrolyte solution to the monolayer and few-layer MXene suspension obtained in step (1) to allow the MXene to spontaneously aggregate and precipitate. Filter the obtained MXene aggregate precipitate, wash, and then vacuum dry the filter cake to obtain a high specific surface area MXene aggregate.

2. The preparation method according to claim 1, characterized in that: The stripping agent includes one or both of tetramethylammonium hydroxide and tetrapropylammonium hydroxide.

3. The preparation method according to claim 1, characterized in that: The solvent includes one or more of water, ethanol, acetone, ether and acetonitrile.

4. The preparation method according to claim 1, characterized in that: The salt / acid / base electrolyte in the salt / acid / base electrolyte solution includes one or more of ammonium carbonate, sodium carbonate, sodium sulfate, sodium hydroxide, potassium hydroxide, hydrochloric acid, sulfuric acid, and formic acid.

5. A MXene aggregate prepared by the method according to any one of claims 1 to 4, characterized in that: The specific surface area of ​​the MXene aggregate is 15-100m 2 / g.

6. The MXene aggregate according to claim 5, characterized in that: The specific surface area of ​​the MXene aggregate is 50-100 m 2 / g.

7. Use of the MXene aggregates described in claim 5 or 6 in CO2 hydrogenation to produce CO.

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