Method for preparing supported iron-based nanocatalyst by impregnation annealing and application thereof

Abstract

The application relates to the field of metal nanocatalysts, and discloses a method for preparing a supported iron-based nanocatalyst by an impregnation annealing method and application, which comprises the following steps: S1, 2.0g of ketchen black is added into 200ml of 7.0M nitric acid solution, then heated and refluxed at 90 DEG C for 4h, after natural cooling, the black dispersion is centrifuged, then washed by ultrapure water until the pH is neutral, and vacuum dried at 60 DEG C for 10-12h to obtain activated carbon black powder, the application prepares a supported iron-based nanocatalyst with uniform dispersion and consistent morphology, from single atom, cluster to particle with different sizes, meanwhile, the huge specific surface area of the nitrogen-doped porous carbon substrate provides rich attachment sites for the iron element, and the rich -N functional groups on the surface and the supported metal iron adsorption form stable Fe-N bonds, the unique geometric and electronic structure makes the obtained iron nanocluster catalyst show excellent activity and selectivity for electrocatalytic nitrate reduction reaction.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of metal nanocatalysts, in particular to a method for preparing a supported iron-based nanocatalyst by impregnation and annealing and application thereof. BACKGROUND

[0002] Ammonia (NH3) is a key raw material for artificial fertilizers and various chemicals, and is one of the most promising carbon-free energy carriers. Currently, industrial synthesis of ammonia relies heavily on the energy- and carbon-intensive Haber-Bosch (H-B) process. Electrochemical synthesis of NH3 is carried out at room temperature and atmospheric pressure, using water as a proton source and renewable electricity as a driving force, and is a promising alternative process for synthesizing NH3. However, due to the extremely stable N≡N triple bond (945 kJ mol -1 ), the activity and selectivity of NRR are low. Therefore, finding an efficient and green method for producing ammonia is an important issue in modern chemical production.

[0003] Nitrate (NO3 - ), as a substance abundant in nature, is widely present in industrial and agricultural wastewater as an environmental pollutant. Therefore, in aqueous solution under environmental conditions, electrocatalytic conversion of NO3- to NH3 not only provides a green and sustainable ammonia synthesis technology, but also alleviates global energy and pollution problems. For example, patent application number "CN201811613389.1" proposes a high-efficiency self-supported iron-based nanocomposite material used as a catalyst for synthesizing ammonia and a method thereof, which has high activity and thermal stability.

[0004] However, the existing catalytic system still has many problems. For example, the ammonia synthesis catalyst proposed in patent application number "CN201811613389.1" has problems such as complex catalyst preparation, poor stability, insufficient selectivity, and the like. The reduction path from NO3- to NH3 involves a complex eight-electron transfer and multiple intermediates, and there is a hydrogen evolution side reaction (HER). Therefore, the main challenge at present is to achieve high selectivity and high yield of ammonia at low potential. SUMMARY

[0005] The present application relates to the field of metal nanocatalysts, in particular to a method for preparing a supported iron-based nanocatalyst by impregnation and annealing and application thereof.

[0006] The purpose of the present application can be achieved by the following technical solution: a method for preparing a supported iron-based nanocatalyst by impregnation and annealing, comprising the following steps:

[0007] S1, 2.0 g of Ketjen black was added into 200 mL of 7.0 M nitric acid solution, then heated under reflux at 90℃ for 4 h, after natural cooling, the black dispersion was centrifuged, then washed with ultrapure water until the pH was neutral, and dried at 60℃ under vacuum for 10-12 h to obtain activated carbon black powder;

[0008] S2, the activated carbon black powder in step S1 was mixed with melamine in a mortar at a mass ratio of 1:10, and the mixture was placed in a ceramic boat and then pyrolyzed and annealed under Ar atmosphere. The annealing process was as follows: first, the temperature was raised from 20℃ to 400℃ at a rate of 10℃ / min, and kept for 1 h; second, the temperature was raised to 800℃ and kept for 1 h. After the temperature dropped to room temperature, the nitrogen-doped porous carbon was obtained.

[0009] S3, 100 mg of the nitrogen-doped porous carbon prepared in step S2 was uniformly dispersed in 80 mL of anhydrous ethanol, and then an ethanol solution of iron nitrate nonahydrate or tririron dodecacarbonyl precursor was slowly added dropwise into the mixture. The mixture was stirred at room temperature for 12 h, then rotary evaporated, dried at 60℃ under vacuum, and finally annealed under Ar atmosphere to obtain a metal nanocatalyst.

[0010] Preferably, the Ketjen black in step S1 is activated by heating under reflux in a nitric acid solution to increase the number of hydrophilic functional groups such as hydroxyl groups on the surface.

[0011] Preferably, the separation and washing of the activated carbon black in step S1 is performed by a centrifuge at a speed of 8000 r / min for 5 min each time, and repeated for 7 times until the pH is neutral.

[0012] Preferably, the carbon black powder and melamine in step S2 are ground in a mortar for 30 min to mix them uniformly.

[0013] Preferably, the annealing process in step S2 is performed in a tube furnace under Ar atmosphere at a rate of 10℃ / min.

[0014] Preferably, in step S3, the anhydrous ethanol dispersion of the nitrogen-doped porous carbon is prepared by ultrasonic treatment for 1 h.

[0015] Preferably, the annealing process in step S3 is performed in a tube furnace at a rate of 5℃ / min.

[0016] Preferably, in step S3, the annealing temperature is 300℃-800℃, and the annealing time is 1 h-2 h.

[0017] A method for preparing a supported iron-based nanocatalyst by impregnation and annealing is applied in the electrocatalytic reduction of nitrate.

[0018] The beneficial effects of the present application are as follows:

[0019] The present application prepares a supported iron-based metal nanocatalyst with uniform dispersion and consistent morphology from single atoms, clusters to particles of different sizes. The large specific surface area of the nitrogen-doped porous carbon substrate provides abundant attachment sites for iron elements. The rich -N functional groups on the surface form stable Fe-N bonds with the loaded metal iron, thereby producing strong metal-support interactions. The unique geometric and electronic structure of the iron nanocluster catalyst effectively prevents the occurrence of nitrite byproducts and hydrogen production side reactions during the reduction of nitrate. The obtained iron nanocluster catalyst exhibits excellent activity and selectivity for electrocatalytic nitrate reduction.

[0020] The method of the present application can be universally used to prepare supported iron-based metal nanocatalysts of different sizes. The size effect of the iron nanocluster catalyst significantly improves the activity of the catalyst, with high atomic utilization. The bonding of the iron cluster catalyst with the -N functional groups on the support forms strong metal-support interactions, which to some extent endow the metal cluster with unique geometric and electronic properties, making the adsorption and conversion of reactant molecules more favorable in catalysis, with higher selectivity, activity and stability. The metal cluster is a potential alternative catalyst for the electrocatalytic reduction of nitrate to ammonia. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments or prior art description will be briefly introduced as follows. Obviously, for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings;

[0022] Figure 1 is the X-ray diffraction pattern of examples 1, 2 and 3;

[0023] Figure 2 is the high-angle annular dark-field scanning transmission electron microscope image of examples 1 and 2;

[0024] Figure 3 is the X-ray near-edge absorption structure spectrum of examples 1, 2 and 3;

[0025] Figure 4 is the performance comparison chart of examples 1, 2 and 3;

[0026] Figure 5 is the structure chart of the product prepared in examples 1, 2 and 3 under transmission electron microscope. DETAILED DESCRIPTION

[0027] With reference to the accompanying drawings, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0028] Embodiment 1

[0029] 2.0 g of Ketjen black (Ketjen black EC-600JD) was added into 200 mL of 7.0 M nitric acid solution, and then heated to reflux at 90 ℃ for 4 h. After natural cooling, the black dispersion was centrifuged, and then washed with ultrapure water until the pH was neutral. The activated carbon black powder was obtained by vacuum drying at 60 ℃ for 10-12 h, and was stored for later use.

[0030] The activated carbon black powder was mixed with melamine in a mortar at a mass ratio of 1:10, and the mixture was placed in a ceramic boat, and then pyrolysis annealing was performed under Ar atmosphere: the temperature was raised from 20 ℃ to 400 ℃ at a rate of 10 ℃min-1, and then kept for 1 h; and then the temperature was raised to 800 ℃ for 1 h. After the temperature dropped to room temperature, the obtained nitrogen-doped porous carbon was stored for later use.

[0031] 100 mg of the aforementioned nitrogen-doped porous carbon was uniformly dispersed in 80 mL of anhydrous ethanol, and then 14.43 mg of Fe(NO3)3·9H2O (iron nitrate nonahydrate) was dissolved in 10 mL of ethanol. The solution was slowly added dropwise into the dispersion of the nitrogen-doped porous carbon, and stirred at room temperature for 12 h. After stirring, centrifugation was performed, and the obtained sample was vacuum dried at 60 ℃. The sample was annealed at 300 ℃ for 2 h under Ar atmosphere, and then the iron metal nano-catalyst was obtained.

[0032] Embodiment 2

[0033] 2.0 g of Ketjen black was added into 200 mL of 7.0 M nitric acid solution, and then heated to reflux at 90 ℃ for 4 h. After natural cooling, the black dispersion was centrifuged, and then washed with ultrapure water until the pH was neutral. The activated carbon black powder was obtained by vacuum drying at 60 ℃ for 10-12 h, and was stored for later use.

[0034] The aforementioned activated carbon black powder was mixed with melamine in a mortar at a mass ratio of 1:10, and the mixture was placed in a ceramic boat, and then pyrolysis annealing was performed under Ar atmosphere: the temperature was raised from 20 ℃ to 400 ℃ at a rate of 10 ℃min-1, and then kept for 1 h; and then the temperature was raised to 800 ℃ for 1 h. After the temperature dropped to room temperature, the obtained nitrogen-doped porous carbon was stored for later use.

[0035] The 100 mg of the above nitrogen-doped porous carbon was uniformly dispersed in 80 mL of absolute ethanol, and then 14.99 mg of Fe3(CO) 12 (dodecacarbonyl iron precursor) was dissolved in 10 mL of ethanol and slowly added dropwise into the dispersion of the nitrogen-doped porous carbon, which was stirred at room temperature for 12 h. After stirring, rotary evaporation was performed, and vacuum drying was performed at 60 °C. The obtained sample was annealed at 400 °C for 2 h in a 10% H2 / Ar atmosphere to obtain a Fe metal nanocatalyst.

[0036] Example 3

[0037] The 2.0 g of Ketjen black was added to 200 mL of a 7.0 M nitric acid solution, which was then heated to reflux at 90 °C for 4 h. After natural cooling, the black dispersion was centrifuged, and then washed with ultrapure water until the pH was neutral. Vacuum drying was performed at 60 °C for 10-12 h to obtain an activated carbon black powder, which was stored for later use.

[0038] The above activated carbon black powder was uniformly mixed with melamine in a mortar at a mass ratio of 1:10. The mixture was placed in a ceramic boat, and then pyrolysis annealing was performed under an Ar atmosphere: the first step was to increase the temperature from 20 °C to 400 °C at a rate of 10 °C min-1, and the temperature was maintained for 1 h; the second step was to increase the temperature to 800 °C for 1 h. After the temperature decreased to room temperature, the obtained nitrogen-doped porous carbon was stored for later use.

[0039] The 100 mg of the above nitrogen-doped porous carbon was uniformly dispersed in 80 mL of absolute ethanol, and then 36.07 mg of Fe(NO3)3·9H2O was dissolved in 10 mL of ethanol and slowly added dropwise into the dispersion of the nitrogen-doped porous carbon, which was stirred at room temperature for 12 h. After stirring, rotary evaporation was performed, and vacuum drying was performed at 60 °C. The obtained sample was annealed at 800 °C for 2 h in an Ar atmosphere to obtain a Fe metal nanocatalyst.

[0040] Structure detection

[0041] From the XRD results of Figure 1 , it can be seen that the samples of Examples 1 and 2 do not have related diffraction peaks of Fe metal, indicating that the Fe atoms do not form a crystal structure of nanoparticles. The sample of Example 3 exists in the form of iron nanoparticles, and the HAADF-STEM image Figure 2 (a) of the sample of Example 2 shows that the Fe element is highly atomically dispersed, i.e., a single atom site, which indicates that by adjusting the type of precursor and the annealing temperature, controllable preparation of different size supported Fe-based metal nanocatalysts from single atoms, clusters to nanoparticles can be achieved. Figure 2

[0042] ​To further confirm the existence form of Fe on the surface of nitrogen-doped porous carbon in Examples 1, 2, 3, we characterized the samples by X-ray near-edge absorption structure spectroscopy (XANES), Figure 3 ) The valence state of iron elements in Examples 1-3 gradually decreases from the position of the absorption edge of the Fe K-edge of the sample, which is consistent with the structural change trend from single atom, cluster to nanoparticle. Then, the XANES spectrum of the sample is compared with that of the iron standard sample. From the peak shape characteristics, it can be analyzed that the sample of Example 3 is basically consistent with the iron standard sample, which confirms that the existence form of the sample is Fe nanoparticles. The pre-edge peak at the position of 7114 eV in Example 1 reflects the D4h symmetry distribution of the charge, which to some extent indicates the hybrid structure of Fe-N4, indicating that the single atom of Fe in Example 1 is dispersed; the valence state of the sample of Example 2 is just between Examples 1 and 3, and at the same time, it has the XANES peak shape characteristics of both, which is similar to the peak shape of the iron standard sample, and a smaller pre-edge peak appears at the position of 7114 eV compared with Example 1, indicating the hybridization between Fe and N, which confirms that the Fe in the sample of Example 2 is distributed in the form of clusters. Finally, the above analysis results correspond to the TEM results, which indicates that the nitrogen-doped porous carbon supported iron-based metal nanocatalysts with different sizes from single atom, cluster to nanoparticle are successfully synthesized.

[0043] Performance detection

[0044] To evaluate the electrocatalytic nitrate reduction activity and selectivity of the Fe metal nanocatalysts in Examples 1, 2, 3, we performed performance evaluation under the same test conditions, and the specific test steps and results are as follows:

[0045] (1) Preparation of ink: 5 mg of catalyst was dispersed in 1 mL of anhydrous ethanol and 30 μL of Nafion reagent, and after ultrasonic treatment for 2 h, a catalyst uniformly dispersed ink was obtained.

[0046] (2) Preparation of working electrode: 50 μL of ink was added dropwise on a carbon paper with a size of 0.4 x 0.5 cm 2 , and a working electrode was obtained after natural air drying.

[0047] (3) Assembly of three-electrode test system: the test selects an H-type electrolytic cell, and a 1M KOH + 0.5M KNO3 electrolyte is prepared. Then, the Hg / HgO reference electrode, platinum mesh counter electrode and prepared working electrode are assembled according to the three-electrode system to complete the test.

[0048] (4) Electro-catalytic test: After the electrolyte is bubbled with Ar for 30 min, CV is activated for 20 cycles, and then i-t test is performed for 30 min at different voltages of -0.2 V, -0.3 V, -0.4 V, -0.5 V, -0.6 V (vs. RHE).

[0049] (5) Product detection:

[0050] a. Ammonia (NH3) detection:

[0051] NH3 generated in the cathode electrolytic cell was detected by indigo phenol blue colorimetry. Due to the large concentration of generated NH3, the solution after electrolysis needs to be diluted before color development to ensure that the NH3 concentration in the detection solution is within the linear range of the indigo phenol blue colorimetry. After the electrolysis is completed, 2 mL of the diluted cathode electrolyte is taken and 2 mL of NaOH (1 M, containing 5 wt% salicylic acid and 5 wt% sodium citrate), 1 mL of 0.05 M NaClO and 0.2 mL of 1.0 wt% sodium nitrosoferricyanide are added respectively, and the mixture is mixed uniformly. After standing at room temperature for 1 h in the dark, the absorbance of the solution is detected at 655 nm by UV-visible spectrophotometer. The concentration of NH3 is calculated according to the standard ammonium chloride solution.

[0052] b. Nitrite (NO2 - ) detection:

[0053] 0.5 g of sulfanilamide is added to 50 mL of concentrated HCl and diluted with water to 100 mL. After mixing uniformly, it is color developing solution one. 100 mg of naphthalene ethylenediamine hydrochloride is added to 50 mL of water and diluted with water to 100 mL. After mixing uniformly, it is color developing solution two. 4 mL of the electrolyte after electrolysis is taken and 1 mL of color developing solution one is added. After shaking, it is allowed to stand for 10 min. Then 100 μL of color developing solution two is added to the above mixed solution. After shaking, it is allowed to stand for 20 min. The absorbance of the solution is detected at 540 nm by UV-visible spectrophotometer. The concentration of NO2 is calculated according to the standard NaNO2 solution.

[0054] (6) Faraday efficiency and yield:

[0055] The Faraday efficiency of electrocatalytic reduction of NO3 to synthesize NH3 is the ratio of the amount of charge required for the synthesis of NH3 to the total amount of charge in the electrolysis process. The calculation formula is as follows:

[0056]

[0057] where F represents the Faraday constant, represents the measured NH3 concentration (mg mL -1 ), V represents the volume of the electrolyte (mL), and Q is the total charge passing through the electrode during electrolysis.

[0058] The Faraday efficiency calculation formula for the reduction of NO3 to synthesize NO2 is as follows:

[0059]

[0060] By Figure 4 It can be seen that, compared with iron single atom and nanoparticles, the iron metal nanocluster sample has the best electrocatalytic nitrate reduction activity and selectivity, which can reach nearly 100% faradic efficiency and 21723·μg h -1 ·cm -1 of ammonia yield at-0.4V(vs.RHE), which further illustrates the important influence of size effect on catalytic performance, and in addition, the iron metal cluster catalyst successfully prepared by the present application is also proved to have excellent electrocatalytic nitrate reduction activity and ammonia selectivity.

[0061] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application, and various changes and improvements can be made without departing from the spirit and scope of the present application, and these changes and improvements all fall within the scope of the claimed present application.

Claims

1. An impregnation-annealing method for preparing supported iron-based nanocatalysts for use in electrocatalytic nitrate reduction reactions, characterized in that, The method for preparing supported iron-based nanocatalysts by impregnation annealing includes the following steps: S1. Add 2.0 g of Ketjen black to 200 mL of 7.0 M nitric acid solution, then heat under reflux at 90 °C for 4 h. After natural cooling, centrifuge the black dispersion, wash with ultrapure water until the pH is neutral, and vacuum dry at 60 °C for 10-12 h to obtain activated carbon black powder. S2. Grind and mix the activated carbon black powder and melamine in a mortar at a mass ratio of 1:10 until uniform. Place the mixture in a ceramic boat and then anneal it under an Ar atmosphere. The annealing steps are as follows: First, raise the temperature from 20 ℃ to 400 ℃ at a heating rate of 10 ℃ / min and hold for 1 h. Second, raise the temperature to 800 ℃ and anneal for 1 h. After the temperature drops to room temperature, nitrogen-doped porous carbon is obtained. S3. Take 100 mg of nitrogen-doped porous carbon prepared in step S2 and disperse it evenly in 80 mL of anhydrous ethanol. Then, slowly add the ethanol solution of ferric nitrate nonahydrate or dodecyltriferric precursor dropwise to the mixture. Impregnate and stir at room temperature for 12 h. After stirring, evaporate by rotary evaporation, dry in a vacuum environment at 60 °C, and finally anneal in an Ar atmosphere to obtain the supported iron-based nanocatalyst.

2. The application of the supported iron-based nanocatalyst prepared by the impregnation annealing method according to claim 1 in the electrocatalytic nitrate reduction reaction, characterized in that, In step S1, Ketjen Black is activated by heating and refluxing in nitric acid solution to increase surface hydroxyl groups.

3. The application of the supported iron-based nanocatalyst prepared by the impregnation annealing method according to claim 2 in the electrocatalytic nitrate reduction reaction, characterized in that, In step S1, the separation and washing of activated carbon black are carried out by centrifugation at a speed of 8000 r / min, centrifuged 7 times until the pH is neutral, and each centrifugation is 5 min.

4. The application of the supported iron-based nanocatalyst prepared by the impregnation annealing method according to claim 1 in the electrocatalytic nitrate reduction reaction, characterized in that, In step S2, the carbon black powder and melamine need to be ground in a mortar for 30 minutes to mix the carbon black powder and melamine evenly.

5. The application of the supported iron-based nanocatalyst prepared by the impregnation annealing method according to claim 4 in the electrocatalytic nitrate reduction reaction, characterized in that, In step S2, the annealing heating process is carried out in an Ar atmosphere in a tube furnace, with a heating rate of 10 °C / min.

6. The application of the supported iron-based nanocatalyst prepared by the impregnation annealing method according to claim 1 in the electrocatalytic nitrate reduction reaction, characterized in that, In step S3, an anhydrous ethanol dispersion of nitrogen-doped porous carbon is prepared by ultrasonication for 1 h.

7. The application of the supported iron-based nanocatalyst prepared by the impregnation annealing method according to claim 6 in the electrocatalytic nitrate reduction reaction, characterized in that, In step S3, the annealing heating is carried out in a tube furnace at a heating rate of 5 °C / min.

8. The application of the supported iron-based nanocatalyst prepared by the impregnation annealing method according to claim 7 in the electrocatalytic nitrate reduction reaction, characterized in that, In step S3, the annealing temperature is 300℃-800℃, and the annealing time is 1 h-2 h.

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