Porous transition metal-nitrogen-carbon catalyst as well as preparation method and application thereof

Abstract

The invention discloses a porous transition metal-nitrogen-carbon catalyst and a preparation method and application thereof, and relates to the technical field of catalysts.The preparation method comprises the steps that wood is subjected to cyanoethylation and amidoximation reactions under the alkaline condition, lignin in the wood is dissolved, meanwhile, part of cellulose on the wall of a pore channel is peeled off, and therefore the wood can be obtained; further, a cross-linked pore structure is generated, so that the wood can obtain an integral carbon material with interconnected pore structures after pyrolysis; according to the method provided by the invention, during preparation, cellulose in wood is taken as a grafting site, an amidoxime group is grafted to a wood precursor, and abundant surface anchoring sites are provided for metal species, so that high-efficiency loading of different types of metal species on wood derived carbon is realized. According to the method, in the pyrolysis process, zinc ions can gradually form zinc oxide or zinc nanoparticles and then evaporate, large-scale aggregation of target metal species is inhibited in the process, and the high dispersion state of the target metal species is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and particularly relates to a porous transition metal-nitrogen-carbon catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Metal-air batteries are an important type of electrochemical energy conversion device, and their performance is mainly limited by the kinetically slow oxygen reduction reaction (ORR) at the cathode. Currently, platinum-based materials are considered ideal catalysts for efficiently catalyzing the ORR process. However, since platinum is a noble metal, its high cost limits its large-scale application. Therefore, the development of low-cost non-noble metal catalysts has become a research hotspot in the field of metal-air batteries. Transition metal-nitrogen-carbon materials (M-N-C) have become non-noble metal electrode catalysts that have received much attention due to their large specific surface area, excellent electrical conductivity, and good stability. Due to their high ORR catalytic activity, M-N-C is expected to replace platinum-based catalysts as the electrode material for the cathode of metal-air batteries. The main preparation method of M-N-C is high-temperature pyrolysis, that is, high-temperature treatment of a mixture containing carbon, nitrogen, and a transition metal precursor in an inert gas atmosphere. In this material, highly dispersed metal species are considered to be highly efficient catalytic active sites. Therefore, the key to preparing highly active M-N-C lies in how to avoid the aggregation of transition metal species and achieve their high dispersion.

[0003] Traditional M-N-C catalysts are mostly powder-like materials. In practical applications, they need to go through a complex electrode preparation process, including steps such as powder dispersion, coating, and drying. In addition, an ion-conductive binder needs to be introduced additionally, and a suitable electrode substrate needs to be selected. The activity of the electrocatalytic reaction not only depends on the activity of the catalyst itself but is also significantly affected by the state of the electrode and the catalyst on the electrode surface. An inappropriate electrode preparation process may lead to a reduction in the active surface area of the catalyst, an excessive dead volume, and a poor catalytic interface, thereby hindering electron and mass transfer and increasing the contact resistance. Summary of the Invention

[0004] In view of the above deficiencies of the prior art, the present invention provides a porous transition metal-nitrogen-carbon catalyst rich in mesopores and micropores, which can be directly used as an electrode without additional binders, conductive agents, and current collectors in actual electrocatalytic reactions, as well as a preparation method and an application thereof.

[0005] To achieve the above invention object, the technical solution adopted by the present invention is as follows:

[0006] Provide a preparation method of a porous transition metal-nitrogen-carbon catalyst, which includes the following steps:

[0007] S1: immersing the wood block in a 10 wt.% sodium hydroxide aqueous solution and leaving it at room temperature for 0.5 to 5 h to obtain a wood block from which lignin has been removed;

[0008] S2: immersing the wood block obtained in step S1 in acrylonitrile, and dropping 10 wt.% sodium hydroxide aqueous solution to adjust the pH value of the reaction system to alkaline, stirring and reacting at room temperature for 3 to 10 hours, neutralizing the reaction solution with 1 wt.% acetic acid solution, taking out the wood block, rinsing it, and freeze-drying it to obtain a cyanoethylated wood block;

[0009] S3: placing the cyanoethylated wood block in a mixed aqueous solution of hydroxylamine hydrochloride and sodium hydroxide, heating and stirring to react for 5 to 12 hours, and after the reaction is completed, taking out the wood block, rinsing it, and freeze-drying it to obtain an amidoximated wood block;

[0010] S4: soaking the amidoximated wood block in a mixed aqueous solution containing a transition metal salt and a zinc salt for 6 to 72 hours, allowing the mixture to react, taking out the wood block, rinsing it, and freeze-drying it to obtain an amidoximated wood block chelated with both transition metal ions and zinc ions;

[0011] S5: placing the amidoximated wood block chelated with transition metal ions and zinc ions in an inert atmosphere for thermal stabilization treatment, and then pyrolyzing it to obtain a porous transition metal-nitrogen-carbon catalyst.

[0012] Furthermore, in step S2, a sodium hydroxide aqueous solution is added dropwise to make the pH value of the reaction system 8-12.

[0013] Furthermore, in step S2, in the mixed aqueous solution of hydroxylamine hydrochloride and sodium hydroxide, the concentration of hydroxylamine hydrochloride is 0.5-5 mol / L, and the concentration of sodium hydroxide is 0.5-5 mol / L; and the heating temperature during the heating and stirring reaction is 50-90°C.

[0014] Furthermore, the transition metal salt is one or a combination of manganese nitrate, manganese chloride, manganese sulfate, cobalt nitrate, cobalt chloride, cobalt sulfate, nickel nitrate, nickel chloride, nickel sulfate, copper nitrate, copper chloride and copper sulfate, and the zinc salt is zinc nitrate, zinc chloride or zinc sulfate.

[0015] Furthermore, the total concentration of transition metal ions and zinc ions is 0.02-2 mol / L, and the ratio of transition metal ions to the total metal ion concentration is 0.005-0.5.

[0016] Furthermore, in step S5, the pyrolysis temperature is 900-1200°C.

[0017] Furthermore, the wood block is balsa wood, pine wood, basswood, fir wood or birch wood.

[0018] The present invention also provides a porous transition metal-nitrogen-carbon catalyst prepared by the above preparation method.

[0019] The present invention also provides an application of the above-mentioned porous transition metal-nitrogen-carbon catalyst in the preparation of a metal-air battery electrode.

[0020] The beneficial effects of the present invention are as follows:

[0021] Through the cyanoethylation and amidoximation reactions of wood under alkaline conditions, the lignin in the wood is dissolved, and at the same time, part of the cellulose on the pore walls is peeled off, thereby generating a cross-linked pore structure, so that an integral carbon material with interconnected pore structures can be obtained after pyrolysis of the wood; and when the method proposed by the present invention is prepared, the amidoxime group is grafted onto the wood precursor with the cellulose in the wood as the grafting site, providing rich surface anchoring sites for metal species, thereby realizing the efficient loading of different types of metal species on the wood-derived carbon.

[0022] During the pyrolysis process of the present invention, zinc ions will gradually form zinc oxide or zinc nanoparticles and then evaporate. This process inhibits the large-scale aggregation of target metal species, ensures the highly dispersed state of target metal species, and at the same time is conducive to the generation of rich mesopores and micropores in the integral carbon material. Thereby, the catalytic active sites can be increased, so that the prepared metal-air battery electrode has higher catalytic activity for oxygen reduction reaction, and the battery prepared has higher voltage and peak power density. Description of the Drawings

[0023] Figure 1 Scanning electron micrograph of the catalyst prepared in Example 1;

[0024] Figure 2 Nitrogen adsorption-desorption isotherm diagram of the catalyst prepared in Example 1;

[0025] Figure 3 Schematic diagram of the pore size distribution of the catalyst prepared in Example 1;

[0026] Figure 4 Schematic diagram of the comparison of X-ray diffraction patterns of the catalysts prepared in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3;

[0027] Figure 5 Schematic diagram of the comparison of ORR polarization curves of the catalysts prepared in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3;

[0028] Figure 6 Schematic diagram of the comparison of ORR polarization curves of the catalysts prepared in Example 2, Example 3 and Example 4;

[0029] Figure 7 Polarization curve diagram of the aluminum-air battery prepared with the catalyst prepared in Example 1. Detailed Embodiments

[0030] The following describes the detailed embodiments of the present invention to facilitate those skilled in the art of this technology to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the detailed embodiments. For those of ordinary skill in the art of this technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are within the scope of protection.

[0031] Example 1

[0032] The manganese-nitrogen-carbon catalyst was prepared in the following steps in this example:

[0033] S1: Immerse the balsa wood block with dimensions of 30×10×2 mm 3 in a 10 wt.% sodium hydroxide aqueous solution, and let it stand at room temperature for 1 h to remove the lignin of the balsa wood block. In specific implementation, it is impossible to completely remove the lignin, and the residual part of the lignin does not affect the subsequent preparation.

[0034] S2: Immerse the balsa wood block with partially removed lignin in step S1 in 30 mL of acrylonitrile, add 1 mL of 10 wt.% sodium hydroxide aqueous solution to adjust the pH value of the reaction system to 10, stir at room temperature for 7 h, then neutralize the reaction solution with 1 wt.% acetic acid solution, take out the wood block in the neutralized reaction solution, rinse it with ultrapure water, and then perform freeze-drying to obtain a cyanoethylated wood block.

[0035] S3: Place the cyanoethylated wood block in a mixed aqueous solution containing 3.45 mol / L hydroxylamine hydrochloride and 3.45 mol / L sodium hydroxide, heat and stir for 6 h. Rinse the reacted wood block with ultrapure water, and then perform freeze-drying to obtain a amidoximated wood block.

[0036] S4: Immerse the amidoximated wood block in a mixed aqueous solution containing 40 mmol / L manganese nitrate and 160 mmol / L zinc nitrate for 48 h, rinse it with deionized water, and then perform freeze-drying to obtain an amidoximated wood block chelated with both manganese ions and zinc ions.

[0037] S5: Place the amidoximated wood block chelated with manganese ions and zinc ions in a nitrogen atmosphere for heat stabilization treatment at 250 °C for 1 h, and then perform pyrolysis at 1000 °C for 2 h in a nitrogen atmosphere to obtain a manganese-nitrogen-carbon catalyst, labeled as m-Mn 0.2 NC, and the subscript 0.2 represents the proportion of manganese ions in the total of manganese ions and zinc ions during the chelation of ions.

[0038] Example 2

[0039] The difference between this example and Example 1 is as follows: The amidoximated wood blocks were immersed in an aqueous solution containing 40 mmol / L cobalt nitrate and 160 mmol / L zinc nitrate for 48 h, rinsed with deionized water, and then freeze-dried to obtain amidoximated wood blocks chelated with cobalt ions and zinc ions simultaneously. After the same heat stabilization and pyrolysis, a cobalt-nitrogen-carbon catalyst was obtained, labeled as m-Co 0.2 NC.

[0040] Example 3

[0041] The difference between this example and Example 1 is as follows: The amidoximated wood blocks were immersed in an aqueous solution containing 40 mmol / L nickel nitrate and 160 mmol / L zinc nitrate for 48 h, rinsed with deionized water, and then freeze-dried to obtain amidoximated wood blocks chelated with nickel ions and zinc ions simultaneously. After the same heat stabilization and pyrolysis, a nickel-nitrogen-carbon catalyst was obtained, labeled as m-Ni 0.2 NC.

[0042] Example 4

[0043] The difference between this example and Example 1 is as follows: The amidoximated wood blocks were immersed in an aqueous solution containing 40 mmol / L copper nitrate and 160 mmol / L zinc nitrate for 48 h, rinsed with deionized water, and then freeze-dried to obtain amidoximated wood blocks chelated with copper ions and zinc ions simultaneously. After the same heat stabilization and pyrolysis, a copper-nitrogen-carbon catalyst was obtained, labeled as m-Cu 0.2 NC.

[0044] Comparative Example 1

[0045] The difference between this example and Example 1 is as follows: The amidoximated wood blocks were immersed in an aqueous solution containing 200 mmol / L manganese nitrate for 48 h, rinsed with deionized water, and then freeze-dried to obtain amidoximated wood blocks chelated with manganese ions. After the same heat stabilization and pyrolysis, a manganese-nitrogen-carbon catalyst was obtained, labeled as m-Mn 1 NC.

[0046] Comparative Example 2

[0047] The difference between this example and Example 1 is as follows: The balsa wood blocks with partial lignin removed were directly immersed in an aqueous solution containing 40 mmol / L manganese nitrate and 160 mmol / L zinc nitrate for 48 h, rinsed with deionized water, and then freeze-dried to obtain balsa wood blocks impregnated with manganese ions and zinc ions simultaneously. After the same heat stabilization and pyrolysis, a manganese-nitrogen-carbon catalyst was obtained, labeled as m-Mn 0.2 NC-i.

[0048] Comparative Example 3

[0049] The difference between this example and Example 1 is as follows: The amidoximated wood blocks were immersed in an aqueous solution containing 200 mmol / L zinc nitrate for 48 h, rinsed with deionized water, and then freeze-dried to obtain amidoximated wood blocks chelated with zinc ions. After the same heat stabilization and pyrolysis, a nitrogen-carbon catalyst was obtained, labeled as m-NC.

[0050] Morphology and X-ray diffraction detection of the catalyst in Example 5

[0051] For the catalyst m-Mn 0.2 NC prepared in Example 1, scanning electron microscopy was performed, and the results are as Figure 1 shown. Among them, Figure 1 the left side is the cross-sectional scanning electron micrograph of the catalyst m-Mn 0.2 NC, Figure 1 and the right side is the longitudinal-sectional scanning electron micrograph of the catalyst m-Mn 0.2 NC; It can be seen from Figure 1 this that m-Mn 0.2 NC retains those micron-sized pores arranged horizontally and extending longitudinally in the wood, and at the same time generates some cross-linked pore structures.

[0052] For the catalyst m-Mn 0.2 NC prepared in Example 1, nitrogen adsorption-desorption tests were carried out to obtain the nitrogen adsorption-desorption isotherm as shown in Figure 2 the figure; The pore size distribution of the catalyst m-Mn 0.2 NC was detected and statistically analyzed to obtain the pore size distribution diagram as shown in Figure 3 the figure; Based on the BET method, the specific surface area of the catalyst m-Mn 0.2 NC was 130 m 2 / g. It can be seen from this that the catalyst m-Mn 0.2 NC has a very large number of active sites and can improve the catalytic efficiency.

[0053] For m-Mn 0.2 NC, m-Mn 1 NC, m-Mn 0.2 NC-i and m-NC prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3, X-ray diffraction detection was carried out, and the results are as shown in Figure 4 the figure. It can be seen from Figure 4 this that there are no obvious diffraction peaks of metals or metal oxides in the XRD pattern of m-Mn0 .2 NC, only showing the diffraction peaks of graphite. In contrast, significant diffraction peaks of metals or metal oxides appear in the XRD pattern of m-Mn 1 NC. This indicates that when using amidoximated wood as a precursor, it is possible to simultaneously chelate the target metal ions and zinc ions, which is beneficial to the high dispersion of the target metal ions. At the same time, m-Mn 0.2In the XRD pattern of NC-i, obvious diffraction peaks of metals or metal oxides also appeared. This result indicates that when introducing metals into wood precursors by the conventional impregnation method, it is difficult for metal species to remain highly dispersed during pyrolysis. In contrast, amidoximated wood can efficiently anchor metal ions, thereby significantly restricting their agglomeration during pyrolysis and achieving a high degree of dispersion of metal species in the electrode.

[0054] Example 6 Catalyst Performance Test

[0055] Using the rotating disk test technique, the ORR polarization curves of the catalysts prepared in Examples 1-4 and Comparative Examples 1-3 were collected in a saturated 0.1 mol / L potassium hydroxide solution. The electrode rotation speed was 1600 rpm, the potential range was 0.05 - 1.1 V vs RHE, and the scanning speed was 10 mV·s 2 -1. -1 .

[0056] The m-Mn 0.2 NC, m-Mn 1 NC, m-Mn 0.2 NC-i, m-NC, and commercial platinum-carbon (Pt / C) catalysts' oxygen reduction reaction polarization curves are as Figure 5 shown. It can be seen from Figure 5 that the half-wave potential of the oxygen reduction reaction of m-Mn 0.2 NC is significantly higher than that of m-NC, indicating that introducing metal species can efficiently catalyze the oxygen reduction reaction process. Although both m-Mn 1 NC and m-Mn 0.2 NC-i contain Mn species, their half-wave potentials are much lower than that of m-Mn 0.2 NC. This is because a large amount of Mn species in m-Mn 1 NC and m-Mn 0.2 NC-i agglomerate, resulting in fewer actual catalytic active sites; while the Mn species in m-Mn 0.2 NC are highly dispersed, with a large number of actual catalytic sites.

[0057] The catalyst m-Mn 0.2 NC prepared in Example 1 was compared with common powdered Mn-N-C catalysts in terms of performance. Regarding the half-wave potential, which is an important indicator for evaluating the oxygen reduction reaction activity of catalysts, m-Mn 0.2The half-wave potential of Mn-NC reaches 0.89 V, which is significantly higher than that of the powdered Mn-N-C catalyst and commercial Pt / C catalyst in the existing literature "H.-Y. Kim, Y.-W. Ju, Fabrication of Mn-NC catalyst for oxygen reduction reactions using Mn-embedded carbon nanofiber, Energies, 13(2020)2561.". It can be seen that the invention provides a method for preparing an integrated Mn-N-C electrode with high metal dispersion for oxygen reduction reaction by pyrolyzing amidoximated wood blocks while chelating manganese ions and zinc ions, and this electrode can efficiently catalyze the oxygen reduction reaction.

[0058] m-Co prepared in Example 2, Example 3 and Example 4 0.2 NC, m-Ni 0.2 NC and m-Cu 0.2 The ORR polarization curves of Mn-NC are as Figure 6 shown. It can be Figure 6 seen that the integrated electrodes containing different metal species can be prepared by the method proposed in the present invention, and these electrodes all exhibit high ORR activity.

[0059] The m-Mn 0.2 NC prepared in Example 1 and an aluminum sheet are respectively placed on both sides of a potassium hydroxide gel electrolyte to form an aluminum-air battery, and the polarization curve is as Figure 7 shown. It can be Figure 7 seen that the open-circuit voltage of the battery can reach 2.3 V, and the peak power density reaches 11.25 mW / cm 2 . This shows that the integrated electrode prepared in the present invention can be directly used as a cathode electrode catalyst for constructing a metal-air battery.

Claims

1. A method for preparing a porous transition metal-nitrogen-carbon catalyst, characterized in that: The following steps are involved: S1: immersing the wood block in a 10 wt.% sodium hydroxide aqueous solution and leaving it at room temperature for 0.5 to 5 h to obtain a wood block from which lignin has been removed; S2: immersing the wood block obtained in step S1 in acrylonitrile, and dropping 10 wt.% sodium hydroxide aqueous solution to adjust the pH value of the reaction system to alkaline, stirring and reacting at room temperature for 3 to 10 hours, neutralizing the reaction solution with 1 wt.% acetic acid solution, taking out the wood block, rinsing it, and freeze-drying it to obtain a cyanoethylated wood block; S3: placing the cyanoethylated wood block in a mixed aqueous solution of hydroxylamine hydrochloride and sodium hydroxide, heating and stirring to react for 5 to 12 hours, and after the reaction is completed, taking out the wood block, rinsing it, and freeze-drying it to obtain an amidoximated wood block; S4: soaking the amidoximated wood block in a mixed aqueous solution containing a transition metal salt and a zinc salt for 6 to 72 hours, allowing the mixture to react, taking out the wood block, rinsing it, and freeze-drying it to obtain an amidoximated wood block chelated with both transition metal ions and zinc ions; S5: placing the amidoximated wood block chelated with transition metal ions and zinc ions in an inert atmosphere for thermal stabilization treatment, and then pyrolyzing it to obtain a porous transition metal-nitrogen-carbon catalyst.

2. The preparation method according to claim 1, characterized in that: In step S2, a sodium hydroxide aqueous solution is added dropwise to make the pH value of the reaction system 8-12.

3. The preparation method according to claim 2, characterized in that: In step S2, in the mixed aqueous solution of hydroxylamine hydrochloride and sodium hydroxide, the concentration of hydroxylamine hydrochloride is 0.5-5 mol / L, and the concentration of sodium hydroxide is 0.5-5 mol / L; and the heating temperature during the heating and stirring reaction is 50-90°C.

4. The preparation method according to claim 3, characterized in that: The transition metal salt is one or a combination of manganese nitrate, manganese chloride, manganese sulfate, cobalt nitrate, cobalt chloride, cobalt sulfate, nickel nitrate, nickel chloride, nickel sulfate, copper nitrate, copper chloride and copper sulfate, and the zinc salt is zinc nitrate, zinc chloride or zinc sulfate.

5. The preparation method according to claim 4, characterized in that: The total concentration of transition metal ions and zinc ions is 0.02-2 mol / L, and the ratio of transition metal ions to the total metal ion concentration is 0.005-0.

5.

6. The preparation method according to claim 5, characterized in that: In step S5, the pyrolysis temperature is 900-1200°C.

7. The preparation method according to claim 6, characterized in that: The wood blocks are balsa, pine, basswood, fir or birch.

8. A porous transition metal-nitrogen-carbon catalyst prepared by the preparation method according to claim 7.

9. Use of the porous transition metal-nitrogen-carbon catalyst according to claim 8 in the preparation of metal-air battery electrodes.

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