CN106876728B

CN106876728B - High-density transition metal monoatomic load graphene-based catalyst and preparation method thereof

Info

Publication number: CN106876728B
Application number: CN201710078259.1A
Authority: CN
Inventors: 宋礼; 刘道彬; 武传强; 谢垚峰; 甘伟
Original assignee: University of Science and Technology of China USTC
Current assignee: University of Science and Technology of China USTC
Priority date: 2017-02-14
Filing date: 2017-02-14
Publication date: 2020-01-03
Anticipated expiration: 2037-02-14
Also published as: CN106876728A

Abstract

The invention provides a preparation method of a high-density transition metal monoatomic load graphene-based catalyst, which comprises the following steps: dissolving a nitrogen-containing precursor, a carbon source and soluble transition metal salt in water, and then evaporating, drying, grinding and calcining to obtain the high-density transition metal monatomic supported graphene-based catalyst. The invention only uses soluble metal salt, glucose and the like as carbon sources, dicyanodiamine as a nitrogen doping source, and only uses a one-step calcination method to successfully prepare the high-efficiency catalyst with high active site density and high monatomic loading.

Description

High-density transition metal monoatomic load graphene-based catalyst and preparation method thereof

Technical Field

The invention relates to the technical field of catalysts, in particular to a high-density transition metal monatomic supported graphene-based catalyst and a preparation method thereof.

Background

The research on the catalytic active sites has important significance for developing novel high-efficiency catalysts, and a large number of experimental facts prove that the metal and alloy catalysts thereof only have the catalytic action of atoms on the surfaces of particles, so that the utilization rate of the atoms is greatly reduced. With the development and prosperity of nanotechnology, people find that the size of active part particles of the catalyst is reduced, the number and the dispersity of active sites are improved, and the utilization rate and the catalytic capacity of the catalyst are improved.

With the recent proposal of the monatomic concept, the synthesized monatomic active site catalyst shows the characteristics of high activity, high selectivity, high stability and the like, simultaneously realizes the maximum utilization of catalytic site atoms, and is an effective way for solving the problems of high price and resource shortage of the current noble metal catalyst.

At present, most of the synthesized monatomic catalysts have the problems of too low load capacity, complex preparation method and the like, and the large-scale commercial application of the monatomic catalysts is severely restricted.

Disclosure of Invention

In view of this, the technical problem to be solved by the present invention is to provide a high-density transition metal monatomic supported graphene-based catalyst and a preparation method thereof, wherein the prepared catalyst has high loading capacity and catalytic activity.

In order to solve the technical problems, the invention provides a preparation method of a high-density transition metal monoatomic load graphene-based catalyst, which comprises the following steps:

dissolving a nitrogen-containing precursor, a carbon source and soluble transition metal salt in water, and then evaporating, drying, grinding and calcining to obtain the high-density transition metal monatomic supported graphene-based catalyst.

The method specifically comprises the following steps:

first, a nitrogen-containing precursor and a carbon source are dissolved in water.

The nitrogen-containing precursor is dicyanodiamine, which has stronger complexing ability with metal ions and is easy to form a monatomic catalyst with higher dispersity, while other raw materials such as urea and the like are easy to form a plurality of metal carbides and nitride nanoparticles loaded graphene after high-temperature treatment.

The carbon source is an organic carbohydrate, preferably glucose or sucrose.

The mass ratio of the carbon source to the nitrogen-containing precursor is preferably greater than 1: 20. In the case of Fe single atom, the oxygen reduction catalyst obtained preferably has the best performance when the mass ratio of the two is 1: 20. In the optimization process, the larger the proportion of dicyanodiamine is, the larger the specific surface area of the finally obtained catalyst is, but the oxygen reduction performance is not further improved, because the high specific surface area has an excessively fluffy structure to prevent the oxygen diffusion process, thereby reducing the catalytic performance.

The amount of water used in the present invention is not particularly limited, and the nitrogen-containing precursor and the carbon source may be dissolved.

When the nitrogen-containing precursor is dicyanodiamide, the solubility in water is about 2.26% (13 ℃), and the amount of water added can be adjusted according to the actual amount added until the dicyanodiamide is completely dissolved.

The transition metal atom in the soluble transition metal salt is not particularly limited in the present invention, and may be a transition metal atom well known to those skilled in the art, and in some embodiments of the present invention, the transition metal atom is one or more of Fe, Co, Ni, Cu, Pt, Pd, and Ru.

The anion of the compound is not particularly limited, and the compound can be dissolved in water, and metal salts such as hydrochloride, sulfate, nitrate and the like are preferably used in the invention.

And then mixing the soluble transition metal salt aqueous solution with an aqueous solution containing a nitrogen precursor and a carbon source to obtain a precursor-containing mixed solution.

When the nitrogen-containing precursor is dicyandiamide and the carbon source is glucose, taking 5g of dicyandiamide and 0.25g of glucose as examples, the addition amount of the soluble metal salt is preferably 0-0.0375 mmol, and in some embodiments of the present invention, the addition amount is 0.025 mmol. During the optimization process, it was found that if the metal addition is greater than 0.0375mmol, some carbide and nitride nanoparticles are formed, mainly due to agglomeration that occurs when the metal atom loading reaches a maximum. Similar conclusions can be drawn based on such scaling up or down.

And then evaporating and drying the mixed solution containing the precursor to obtain a recrystallized solid.

The method of evaporation drying is not particularly limited in the present invention, and may be a general method of evaporation drying known to those skilled in the art, and in some embodiments of the present invention, the mixed solution may be evaporated to dryness on a heating stage.

The temperature of the evaporation drying is preferably < 80 ℃, more preferably 60-80 ℃ to avoid excessive temperature, decomposition of dicyanodiamide or carbonization of glucose.

The recrystallized solid was then ground to a powder.

And finally calcining the powder in an inert atmosphere to obtain the high-density transition metal monoatomic load graphene-based catalyst.

In the prepared high-density transition metal monoatomic load graphene-based catalyst, the molar ratio of C atoms to N atoms to metal atoms is 100: (9-20): (0-2.2).

In certain embodiments of the invention, the powder is placed into a quartz or corundum boat of suitable size and capped; the sample was then placed in a tube furnace and calcined under an inert atmosphere.

The inert atmosphere may be one conventional in the art, and in certain embodiments of the invention, the inert atmosphere is nitrogen or argon.

The calcination temperature is preferably more than 750 ℃, more preferably 800-1000 ℃ to ensure that the carbon nitride generated in the low-temperature process is completely decomposed; in certain embodiments of the invention, the calcination temperature is 900 ℃; the calcination time is preferably 1-3 h.

In the optimization process, the carbon nitride generated by dicyanodiamine is completely decomposed at the temperature of more than 750 ℃, and partial nitrogen atoms enter carbon lattices generated by glucose and play a role in fixing metal atoms. Taking the catalyst material prepared by using Fe single atoms as an example, when the calcination temperature is 900 ℃, the obtained oxygen reduction catalyst has the best performance, mainly because the electron conduction property of the graphene substrate is gradually improved along with the improvement of the temperature, which promotes the improvement of the catalyst performance; further increase in temperature reduces the nitrogen content and also causes some metal atoms to agglomerate, thereby reducing the oxygen reduction performance.

The preparation method has the advantages of simple process, common and easily-obtained raw material reagents, low cost, simplicity, easiness and good stability, and is suitable for large-scale production. Particularly, the Fe-N-C structure constructed by taking Fe single atom as the center shows excellent oxygen reduction performance, and is expected to replace Pt noble metal catalyst in the field of fuel cells to realize large-scale application.

The invention also provides a high-density transition metal single-atom loaded graphene-based catalyst prepared by the preparation method, wherein the catalyst comprises carbon, nitrogen and transition metal elements, wherein the atomic ratio of the transition metal to the carbon is 0-2.2 at.%, and the atomic ratio of the nitrogen to the carbon is 9-20 at.%. The catalyst is of a graphene lamellar structure, has a high specific surface area, and can be regulated and controlled by changing the proportion of dicyanodiamine.

The catalyst has high activity, dispersity, loading capacity and specific surface area, has a folded flake structure similar to graphene oxide, and has an anchoring effect on metal atoms when nitrogen atoms are doped into crystal lattices of a graphite-like carbon material, so that single metal atoms are anchored in graphene flake layers uniformly through coordination with nitrogen to form the catalyst with multiple active sites. The test results also show that the catalyst structure is a transition metal in the form of a single atom dispersed on the wrinkled thin graphene sheet.

The high-density transition metal monatomic supported graphene-based catalyst provided by the invention can be used for various catalytic reactions, and taking the Fe monatomic provided by the embodiment of the invention as an example, the high-density transition metal monatomic supported graphene-based catalyst can be used as a cathode oxygen reduction catalyst in the field of proton exchange membrane fuel cells or metal air cells.

The catalysts prepared by the above method were also tested for oxygen reduction performance by taking Fe single atom as an example, and compared with commercial platinum carbon catalysts, the catalysts prepared by the present application have oxygen reduction catalytic activity under alkaline (0.1mol/L KOH) condition and acidic (0.1mol/L HClO)₄) Under the conditions, the performance was comparable to 20% of commercial platinum-carbon catalyst. The initial potential is 998mV, 876mV, the half-wave potential is 845mV, 658 mV.

The preparation method only needs a few cheap and easily-obtained raw materials, such as glucose, dicyanodiamine, metal inorganic salt and the like, adopts water as a solvent to dissolve and mix the precursor material uniformly in a treatment mode, and only utilizes a one-step calcination method to successfully prepare the high-efficiency catalyst with high active site density and high monatomic loading.

The inert atmosphere adopted by calcination is safer and easier to realize, and a small part of ammonia gas (from decomposition of dicyanodiamine) generated in the sintering process is easy to treat, so that the method is a safe and simple method capable of preparing the transition metal monatomic catalyst on a large scale.

Taking the Fe monatomic catalyst prepared by the method as an example, an oxygen reduction performance test shows that the initial potential, half-wave potential and limit current value of the Fe-N catalyst in the alkaline electrolyte are superior to those of the currently commercially applied platinum-carbon catalyst; in addition, the oxygen reduction performance in acid electrolytes is also close to that of commercial platinum carbon. And the Fe monatomic catalyst does not have the problem of poisoning by carbon monoxide or methanol, and has good circulating stability.

Drawings

FIG. 1 is an X-ray diffraction pattern (XRD) of example 1, comparative example 1 and comparative example 2;

FIG. 2 is a Transmission Electron Microscopy (TEM) topography (a) and a sub-angstrom resolution dark field scanning transmission electron micrograph (HAADF-STEM) (b) of a sample prepared in example 1;

FIG. 3 is a TEM image of a sample prepared in comparative example 1;

FIG. 4 is a TEM image of a sample prepared in comparative example 2;

FIG. 5 is a linear sweep voltammogram of example 2at 1600rpm with 5mV/s sweep in 0.1mol/L KOH solution saturated with oxygen;

FIG. 6 shows HClO at 0.1mol/L for example 3₄In the solution, under the condition of oxygen saturation, the rotating speed is 1600rpm, and the sweeping speed is 5 mV/s;

FIG. 7 is a linear sweep voltammogram of example 4 and example 5 at 1600rpm and 5mV/s in 0.1mol/L KOH solution when saturated with oxygen;

FIG. 8 is a Scanning Transmission Electron Microscope (STEM) topography (a) and a sub-angstrom resolution dark field scanning transmission electron micrograph (HAADF-STEM) (b) of the sample prepared in example 6.

Detailed Description

In order to further illustrate the present invention, the high-density transition metal monatomic supported graphene-based catalyst and the preparation method thereof provided by the present invention are described in detail below with reference to examples.

Example 1

a. Weighing 0.25g of glucose and 5g of dicyanodiamide and dissolving the two in 200mL of deionized water (note that because dicyanodiamide has low solubility in water, the amount of water needs to be increased when the mass of dicyanodiamide is increased until the reagents are completely dissolved);

b. 2mL of prepared ferric trichloride solution with the molar concentration of 0.0125mol/L is measured by a liquid-transfering gun and is dripped into the dissolved glucose and dicyanodiamide solution;

c. putting the prepared solution containing the precursor on a heating table at 80 ℃ for drying by distillation, taking out the solution, grinding the solution into uniform powder by using a mortar, putting the powder into a quartz boat or a corundum boat with proper size, and covering the quartz boat or the corundum boat with a cover;

d. then the whole boat is put into a tube furnace, calcined for 2 hours at 900 ℃ under inert atmosphere (nitrogen or argon), and naturally cooled.

The result of the XRD test on the prepared catalyst is shown in figure 1, and from the XRD result in figure 1, the obtained sample only has the diffraction peak of carbon, and does not form crystalline nano-particles under the condition of high-temperature calcination due to the addition of ferric trichloride, which indicates that the nano-particles mainly exist in an amorphous form. It was further shown by HAADF-STEM characterization means (see results in fig. 2) that iron was uniformly supported on graphene substrate in an atomically dispersed form.

Comparative example 1

a. Weighing 0.25g of glucose and 5g of urea and dissolving the glucose and the urea in 200mL of deionized water;

b. 2mL of prepared ferric trichloride solution with the molar concentration of 0.0125mol/L is measured by a liquid-transferring gun and is added into the dissolved glucose and urea solution;

The results of the detection of the prepared catalyst by XRD are shown in figure 1, and as can be seen from the XRD results in figure 1, the obtained sample is also classified into Fe except the diffraction peak of carbon₃The peak of C compound indicates that the added iron source is agglomerated in the case of high-temperature calcination. Further characterization by TEMThe approach shows (see fig. 3 for the results, where a is a low-power transmission electron micrograph and b is a high-resolution lattice diagram of nanoparticles), that there are indeed particles varying from tens to tens of nanometers present and supported on graphene substrates.

Comparative example 2

a. Weighing 0.25g of glucose and 5g of dicyanodiamide and dissolving the two in 200mL of deionized water;

b. putting the prepared solution containing the precursor on a heating table at 80 ℃ for drying by distillation, taking out the solution, grinding the solution into uniform powder by using a mortar, putting the powder into a quartz boat or a corundum boat with proper size, and covering the quartz boat or the corundum boat with a cover;

c. then the whole boat is put into a tube furnace, calcined for 2 hours at 900 ℃ under inert atmosphere (nitrogen or argon), and naturally cooled.

The result of the XRD detection of the prepared catalyst is shown in figure 1, and as can be seen from the XRD result in figure 1, the diffraction peak of carbon exists in the obtained sample, and does not belong to the peak of carbon nitride, which indicates that the carbon nitride generated by high-temperature calcination (900 ℃) and low-temperature calcination is completely decomposed and a small part of nitrogen enters into the crystal lattice of carbon. Further, it is shown by TEM characterization means (see fig. 4 for results), that it presents a graphene sheet with few layers and folds, mainly because the carbon nitride formed in the low temperature process is a typical two-dimensional layered material, which acts as a template in the synthesis process, and acts as a confinement for the carbonization of glucose, so that it does not sinter into a bulk.

Example 2

The oxygen reduction performance of example 1, comparative example 2, and 20 wt% commercial platinum carbon in 0.1mol/L KOH electrolyte was evaluated. The test procedure and results are as follows:

5mg of each of the commercial platinum-carbon samples of example 1, comparative example 2 and 20 wt% were added 950. mu.L of a mixed solvent of isopropyl alcohol and water (in a volume ratio of 1:1), 50. mu.L of Nafion as a binder at a mass concentration of 5%, and the mixture was ultrasonically dispersed for about 2 hours to obtain a uniformly dispersed sample solution. Then 10 mu L of the prepared solution is taken out and dripped on a rotating disk electrode with the diameter of 5mm, and performance characterization is carried out after the solution is dried at room temperature to form a film.

A saturated Ag/AgCl electrode is selected as a reference electrode, a graphite rod is selected as a counter electrode, a rotating disc electrode for dripping samples is used as a three-electrode system of a working electrode, and the samples are tested under the conditions of oxygen saturation, 0.1mol/L KOH of electrolyte, 5mV/s of scanning speed and 1600rpm of rotating speed. The results of the experiment are shown in FIG. 5, and it can be seen from FIG. 5 that the starting potential and half-wave potential of the highly dispersed, high active site density iron monatomic sample prepared in example 1 are both comparable to 20 wt% commercial platinum carbon, at about 998mV and 845mV, respectively. In contrast, the initial potential and the half-wave potential of the sample obtained in comparative example 1 are 941mV and 802mV, respectively; the sample of comparative example 2 has a far lower oxygen reduction performance at the starting point and half-wave potential of 928mV and 769mV, respectively, than the sample obtained in example 1.

Example 3

Test evaluation examples 1, comparative examples 2, and 20 wt% commercial platinum carbon at 0.1mol/L HClO₄Oxygen reduction performance in the electrolyte. The test procedure and results are as follows:

Selecting a saturated Ag/AgCl electrode as a reference electrode, a graphite rod as a counter electrode, and a rotating disc electrode for dripping a sample as a three-electrode system of a working electrode, wherein the three-electrode system is saturated with oxygen and has 0.1mol/L of HClO (hydrogen chloride oxide) as electrolyte₄The above samples were tested at a scanning speed of 5mV/s and a rotation speed of 1600 rpm. The experimental results are shown in FIG. 6, and it can be seen from FIG. 6 that the initial potential and half-wave potential of the highly dispersed, high active site density iron-nitrogen monatomic sample prepared in example 1 are about 876mV and 658mV, respectively, which are slightly inferior to 20 wt% commercial platinum carbon, but better than the samples prepared in comparative example 1 and comparative example 2Much more.

Meanwhile, the sample prepared by the method has better stability compared with commercial platinum carbon, and experiments show that after the sample is tested for 1000 times, the oxygen reduction property of the commercial platinum carbon can be attenuated to a position equivalent to that of the highly-dispersed iron-nitrogen-doped graphene catalyst prepared by the method.

Example 4

b. 2mL of prepared ferric trichloride solution with the molar concentration of 0.0125mol/L is measured by a pipette and is added into the dissolved glucose and dicyanodiamide solution;

d. then the whole boat is put into a tube furnace, calcined for 2 hours at 800 ℃ under inert atmosphere (nitrogen or argon), and naturally cooled.

The prepared sample is tested by the test method of example 2, and the result is shown in fig. 7, and it can be seen from fig. 7 that the initial potential and the half-wave potential are 961mV and 771mV respectively, which shows that the performance is slightly worse than that of the sample prepared in example 1, mainly because the electron transmission rate is too slow and the catalytic reaction process is limited due to the high nitrogen content of the sample and the poor conductivity of the graphene substrate.

Example 5

d. then the whole boat is put into a tube furnace, calcined for 2 hours at 1000 ℃ under inert atmosphere (nitrogen or argon), and naturally cooled.

The test method of example 2 was used to test the prepared sample, and the result is shown in fig. 7, and it can be seen from fig. 7 that the initial potential and half-wave potential are 998mV and 854mV respectively, which shows that the performance is equivalent to that of the sample prepared in example 1, even the half-wave potential value is larger than that of example 1, but the limiting current value is smaller, mainly because the nitrogen content of the sample is reduced due to high temperature and the electron transport capability of the graphene substrate is slightly better than that of the sample treated at 900 ℃.

Example 6

b. 2mL of prepared H with the molar concentration of 0.0125mol/L is measured by a pipette₂PtCl₆Dripping the solution into the dissolved glucose and dicyanodiamine solution;

The product morphology is shown by STEM and HAADF-STEM characterization means, the result is shown in Table 8, and as can be seen from the result in Table 8, Pt is uniformly loaded on a graphene substrate in an atomically dispersed manner.

As can be seen from the above examples and comparative examples, the transition metal monatomic supported graphene-based catalyst prepared by the present invention has high activity and stability.

Although the above examples were developed using glucose as the carbon source, dicyanodiamine as the nitrogen source and ferric chloride as the iron source, it is certainly not excluded that the carbon source could be replaced by a carbohydrate like sucrose or the like, or that the iron source could be replaced by a soluble iron salt like sucrose, but similar results could still be obtained. Example 6 further illustrates the general applicability of this process, and the present application has also succeeded in preparing other transition metal monatomic catalysts, and is not exhaustive of all embodiments, and obvious variations or modifications are contemplated which are within the scope of the inventive preparation process.

The above description of the embodiments is only intended to facilitate the understanding of the method of the invention and its core idea. It should be noted that, for those skilled in the art, it is possible to make various improvements and modifications to the present invention without departing from the principle of the present invention, and those improvements and modifications also fall within the scope of the claims of the present invention.

Claims

1. A preparation method of a high-density transition metal monatomic supported graphene-based catalyst is characterized by comprising the following steps:

A) dissolving a nitrogen-containing precursor and a carbon source in water, and then mixing the nitrogen-containing precursor and the carbon source with a soluble transition metal salt aqueous solution to obtain a precursor-containing mixed solution; the nitrogen-containing precursor is dicyanodiamine; the carbon source is glucose or sucrose;

B) evaporating and drying the mixed solution containing the precursor obtained in the step A) to obtain a recrystallized solid;

C) grinding the recrystallized solid to a powder;

D) calcining the powder in an inert atmosphere to obtain the high-density transition metal monatomic supported graphene-based catalyst; the calcining temperature is 900-1000 ℃.

2. The method according to claim 1, wherein the transition metal atom in the soluble transition metal salt is one or more of Fe, Co, Ni, Cu, Pt, Pd, and Ru.

3. The preparation method according to claim 1, wherein in the high-density transition metal monoatomic supported graphene-based catalyst, the molar ratio of the C atoms to the N atoms to the metal atoms is 100: (9-20): (0-2.2).

4. The method of claim 1, wherein the temperature of the evaporative drying is < 80 ℃.

5. The preparation method according to claim 1, wherein the calcination time is 1 to 3 hours.

6. The high-density transition metal monatomic supported graphene-based catalyst produced by the production method according to any one of claims 1 to 5.