Pyrrole nitrogen modified carbon oxide catalyst, preparation method and application of pyrrole nitrogen modified carbon oxide catalyst in production of hydrogen peroxide through electrocatalytic oxygen reduction

Abstract

The preparation method comprises the following steps: carrying out acid treatment on a carbon-based material, inducing a pyrrole monomer to form cationic free radicals by taking ammonium persulfate as an initiator, and disproportionating the cationic free radicals with a carbon matrix, so as to obtain the high-purity pyrrole nitrogen modified carbon-based catalyst. The high-purity pyrrole nitrogen modified carbon oxide catalyst prepared by the invention shows excellent productivity and excellent stability for producing hydrogen peroxide by electrocatalytic oxygen reduction. The method is simple in preparation process, low in equipment requirement and high in controllable degree, and the production cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of electrochemistry, and in particular relates to a method for preparing a high-purity pyrrole nitrogen-modified carbon oxide catalyst and an application of the catalyst in producing hydrogen peroxide by electrocatalytic oxygen reduction. Background Art

[0002] Hydrogen peroxide (H2O2) is one of the important chemicals and is widely used in various industrial and healthcare fields, such as disinfection, bleaching, and water treatment. Today, the industrial production of H2O2 is mainly carried out through a multi-step anthraquinone oxidation process, which has the disadvantages of high energy consumption and serious pollution. The electrocatalytic oxygen reduction reaction (ORR) that forms H2O2 via two protons and electrons provides a promising alternative for green H2O2 synthesis. Since O2 molecules can also be completely reduced to H2O via four concerted proton-electron transfer steps, which is the preferred pathway in fuel cell applications, the development of highly selective and active 2e − ORR catalysts are a prerequisite for this sustainable synthetic route.

[0003] From a molecular level, the two ORR pathways have a common intermediate *OOH, whose adsorption strength at the active site determines the final product distribution. Catalytic materials with moderate binding energy for *OOH tend to retain the O−O bond and are selective for H2O2. It is particularly noteworthy that non-metallic heteroatom-modified carbon-based catalysts have become an interesting platform for green H2O2 electrosynthesis due to their abundant sources and flexible structures. The binding strength of the intermediate *OOH can be fine-tuned through adjustable non-metal atoms and atomic configurations. Taking nitrogen-modified carbon-based catalysts as an example, studies on model catalysts based on pyridinic nitrogen have clearly shown that the carbon atom adjacent to the pyridinic nitrogen is 4e − The active site of ORR is located at 4e − Interestingly, recent studies have shown that carbon-based materials containing pyrrolic nitrogen configurations can change the coordination environment of carbon atoms, thereby weakening the binding strength of *OOH and effectively shifting the ORR to 2e −pathway. However, there is still controversy about the role of pyrrolic N in ORR selectivity and activity. In the work of Li et al. on nitrogen-rich few-layer graphene (N-FLG) for ORR, the H2O2 selectivity was increased from 65% to 95% by adjusting the nitrogen configuration, and the structure-activity relationship analysis found that the pyrrolic nitrogen content was positively correlated with the H2O2 selectivity. Similarly, Peng et al. reported that as the pyrrolic nitrogen content in graphene / mesoporous carbon composites (P-NMG) increased from 23 at% to 50 at%, the conversion of O2 to H2O2 increased from 70% to 91% at 0.5 V vs. RHE. It is worth noting that the reported nitrogen-modified carbon materials are usually obtained by high-temperature pyrolysis process, resulting in uncontrollable nitrogen configuration. For example, the N-FLG catalyst containing 24.9 at% pyrrolic nitrogen also has 30.6 at% pyridinic N and 34.8 at% graphitic N. Therefore, more controllable synthetic methods are needed to precisely regulate the nitrogen configuration to obtain high-purity pyrrolic nitrogen-modified carbon materials.

[0004] Based on this, the present invention develops a high-purity pyrrolic nitrogen-modified carbon oxide catalyst through a mild chemical oxidation strategy, which makes the relative content of pyrrolic N as high as 90%. This type of catalyst effectively overcomes the activity-selectivity "trade-off" effect and efficiently produces H2O2 (7.5 g L) in a flow reaction system. -1 h -1 ), and the corresponding Faraday efficiency is as high as 90%. Summary of the invention

[0005] The purpose of the present invention is to overcome the defects of the prior art, provide a method for preparing a high-purity pyrrole nitrogen-modified carbon oxide catalyst, and apply it to the electrocatalytic cathode oxygen reduction to produce hydrogen peroxide, thereby solving the problem of uncontrollable nitrogen configuration in the current nitrogen-modified catalyst for producing hydrogen peroxide.

[0006] The present invention also provides a method for preparing the high-purity pyrrole nitrogen-modified carbon oxide catalyst and its application in producing hydrogen peroxide by electrocatalytic oxygen reduction.

[0007] To achieve the above object, the present invention adopts the following technical solution: A method for preparing a high-purity pyrrole nitrogen-modified carbon oxide catalyst comprises the following steps: 1) Pretreatment of carbon materials: Mix nitric acid and carbon materials and heat and stir them in a reflux device at 80-100°C for 6-24 hours, and obtain oxidized carbon materials through solid-liquid separation, washing and drying; 2) Preparation of catalyst: The oxidized carbon material obtained in step 1) is uniformly dispersed in deionized water, and pyrrole monomer and initiator ammonium persulfate are added at room temperature and stirred for reaction for 10-36 h to induce disproportionation between the pyrrole monomer and the carbon material, and then the catalyst is obtained by rotary evaporation and drying.

[0008] Specifically, in step 1), the carbon material includes but is not limited to one or more of carbon nanotubes, carbon black and graphene, etc. The concentration of the nitric acid can be 10-18 M; nitric acid is calculated as HNO3, and 3-7 mol of HNO3 is preferably added to 1g of carbon material.

[0009] Specifically, in step 1), the drying temperature may be 80-100° C., and the drying time may be 8-12 h. When washing, deionized water is used for multiple washings until neutral.

[0010] Specifically, in step 2), the stirring reaction time is preferably 12-24 h, and more preferably, stirring is performed on a constant temperature magnetic stirrer for 12-14 h.

[0011] Furthermore, in step 2), 0.3-0.6 mmol of ammonium persulfate is added to 1 g of carbon oxide material. The ammonium persulfate can be added directly or in the form of an aqueous solution, and the concentration of the added ammonium persulfate aqueous solution can be 11-12 mM.

[0012] Specifically, in step 2), the mass ratio of nitrogen element in the pyrrole monomer to the carbon oxide material is 10-70:100, that is, the mass percentage of nitrogen to carbon is 10-70 wt%.

[0013] The invention provides a pyrrole nitrogen modified carbon oxide catalyst prepared by the preparation method.

[0014] The invention provides application of the pyrrole nitrogen modified carbon oxide catalyst in producing hydrogen peroxide by electrocatalytic cathode oxygen reduction.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1) The equipment used for the synthesis of the catalyst of the present invention is cheap and readily available, has low requirements on the equipment, is highly controllable, reduces production costs, and has a simple preparation process, which is expected to achieve large-scale industrial production.

[0016] 2) The pyrrole nitrogen content in the catalyst of the present invention is as high as 90 at%, which enables precise regulation of the nitrogen configuration in the material. The catalyst of the present invention can effectively reduce oxygen to hydrogen peroxide under electrocatalytic conditions and achieve high yield in the flow cell, thereby solving the problems of high price, non-mass production and low efficiency of existing catalysts used to produce hydrogen peroxide.

[0017] 3) The high-purity pyrrole nitrogen-modified carbon oxide catalyst prepared by the present invention exhibits excellent production capacity and excellent stability for electrocatalytic oxygen reduction to produce hydrogen peroxide. −2 Stable conversion of O2 to H2O2 under intermittent and continuous electrolysis, with H2O2 remaining at 7.8 g L-1 over an 8 h run time.-1 h -1 (1.4 mol g -1 h -1 ) with a corresponding Faradaic efficiency of 95%. Even after 24 test cycles in 48 h intermittent operation mode, the Faradaic efficiency remained at about 90%, while the H2O2 generation rate was stabilized at about 7.5 g L -1 h -1 , proving that 2e − Excellent stability of ORR. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 HR-TEM images of the catalyst CNT (a), O-CNT (b) and NO-CNT (c) prepared in Example 1 and the comparative example; Figure 2 The XPS spectra (a) and N 1s high-resolution XPS graph (b) and the relative atomic percentage graph of nitrogen configuration (c) of the catalyst CNT, O-CNT, N-CNT and NO-CNT prepared in Example 1 and the comparative example; Figure 3 Figure 3 shows the H2O2 production rate and Faradaic efficiency of CNTs running in the flow cell for 48 h. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further described in detail below in conjunction with the embodiments, but the protection scope of the present invention is not limited thereto.

[0020] In the following examples, unless otherwise specified, the raw materials or devices used are common commercial products that can be directly purchased, or can be prepared according to conventional methods in the art.

[0021] Room temperature refers to 25±5°C.

[0022] Example 1 A method for preparing a high-purity pyrrole nitrogen-modified carbon oxide catalyst comprises the following steps: 1) Pretreatment of carbon materials: 1 g of carbon nanotubes (CNT) was mixed with 300 mL of 17 M HNO3 and heated and stirred at 80°C for 24 h. After cooling naturally to room temperature, the mixture was filtered and washed with deionized water several times until the pH value reached neutral. The sample was then dried in an oven at 80°C for 12 h to obtain oxidized carbon nanotubes (O-CNT). 2) Preparation of catalyst: Add 0.6 g of O-CNT obtained in step 1) to 100 mL of deionized water, stir at room temperature for 10 min to disperse evenly, then add 0.894 mL of pyrrole monomer and continue stirring for 30 min (N:C=30 wt%). Then slowly add 22.330 mL of 11.6 mM (NH4)2S2O8 aqueous solution, and continue stirring and reacting for 12 h. After the reaction is completed, use a rotary evaporator to remove the aqueous solution. Vacuum dry for 24 h to obtain the pyrrole nitrogen modified carbon oxide catalyst NO-CNT.

[0023] Comparative Example 1 A certain amount of carbon nanotubes CNT without pretreatment was directly prepared by the same method as step 2) in Example 1 to obtain a product named N-CNT.

[0024] Figure 1 HR-TEM images of catalysts CNT, O-CNT and NO-CNT are given. In order to confirm the microscopic chemical structure of the catalyst, high-resolution transmission electron microscopy (HR-TEM) was performed. It can be seen from the figure that compared with CNT (0.357 nm) (Figure 1a), the interlayer spacing of O-CNT increased to 0.371nm (Figure 1b) due to the attachment of more oxygen groups to the carbon nanotube matrix during the oxidation process. The interlayer spacing of nitrogen-modified NO-CNT decreased to 0.363 nm (Figure 1c), which may be due to the removal of some oxygen functional groups and the incorporation of smaller nitrogen atoms during the chemical oxidation process, resulting in lattice contraction.

[0025] Figure 2 The XPS spectra of catalyst CNT, O-CNT, N-CNT and NO-CNT, as well as the N 1s high-resolution XPS graph and the relative atomic percentage graph of nitrogen configuration are given. After nitrogen modification of CNT and O-CNT, additional new N 1s peaks were observed in the XPS measurement spectra of N-CNT and NO-CNT ( Figure 2 a). Figure 2 As shown in Fig. 2a, after careful deconvolution, it is found that N-CNT has three different nitrogen configurations: pyrrolic N (399.5 eV), graphitic N (401.3 eV), and oxidized N (403.2 eV). It is worth noting that the oxidized N disappears in the NO-CNT obtained by nitrogen-modified oxidized carbon nanotubes, while the pyrrolic N content increases from 68 at% on N-CNT to 90 at% on NO-CNT ( Figure 2 c) The formation of high-purity pyrrolic nitrogen configuration in carbon-based materials was achieved.

[0026] Application of catalysts in electrocatalytic oxygen reduction to produce hydrogen peroxide: To simulate the actual device, a conventional flow cell was used, including a gas chamber, a cathode chamber, and an anode chamber. The gas chamber was continuously purged with high-purity O2 at a flow rate of ≈5 mL min −1 The cathode chamber and the anode chamber were circulated with 15 mL and 20 mL of mixed electrolyte containing 0.1 M KOH and 0.5 M Na2SO4, respectively, with a flow rate of ≈0.5 mL min −1 The anode chamber and cathode chamber are separated by Nafion 117 membrane. Pt wire is used as anode and gas diffusion electrode (GDE) is used as cathode.

[0027] The ability of the high-purity nitrogen-modified carbon oxide catalyst (NO-CNT) prepared in Example 1 of the present invention to produce H2O2 by oxygen reduction at the electrocatalyst cathode was evaluated using a flow cell. Figure 3 showed that at 40 mA cm −2 Stable conversion of O2 to H2O2 in batch and continuous electrolysis using Ce 4+ / Ce 3+ The actual amount of H2O2 produced was quantified by colorimetry. The H2O2 on the NO-CNTs remained at 7.8 g L-1 over an operating time of 8 h. -1 h -1 (1.4 mol g -1 h -1 ) with a corresponding Faradaic efficiency of 95%. Even after 24 test cycles in 48 h intermittent operation mode, the Faradaic efficiency remained at about 90%, while the H2O2 generation rate was stabilized at about 7.5 g L -1 h -1 , proving that 2e − Excellent stability of ORR.

[0028] In summary, the pyrrole nitrogen content in the catalyst of the present invention is as high as 90 at%, which achieves precise regulation of the nitrogen configuration in the material. The catalyst of the present invention can effectively reduce oxygen to hydrogen peroxide under electrocatalytic conditions, and achieves high yield in the flow cell, thereby solving the problems of high price, non-mass production and low efficiency of the existing catalysts used to produce hydrogen peroxide.

[0029] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a pyrrole nitrogen modified carbon oxide catalyst, characterized in that: The steps include: 1) Pretreatment of carbon materials: mixing nitric acid and carbon materials and heating them at 80-100°C for 6-24 hours, followed by solid-liquid separation, washing and drying to obtain oxidized carbon materials; 2) Preparation of catalyst: The carbon oxide material obtained in step 1) is uniformly dispersed in deionized water, pyrrole monomer and initiator ammonium persulfate are added at room temperature, stirred and reacted for 10-36 hours, and then rotary evaporated and dried to obtain the catalyst.

2. The method for preparing the pyrrole nitrogen modified carbon oxide catalyst according to claim 1, characterized in that: In step 1), the carbon material is one or more of carbon nanotubes, carbon black and graphene.

3. The method for preparing the pyrrole nitrogen modified carbon oxide catalyst according to claim 1, characterized in that: In step 1), the drying temperature is 80-100°C and the drying time is 8-12 h.

4. The method for preparing the pyrrole nitrogen modified carbon oxide catalyst according to claim 1, characterized in that: In step 1), the concentration of nitric acid is 10-18 M; nitric acid is calculated as HNO3, and 3-7 mol of HNO3 is added to 1 g of carbon material.

5. The method for preparing the pyrrole nitrogen modified carbon oxide catalyst according to claim 1, characterized in that: When washing in step 1), use deionized water to wash multiple times until it becomes neutral.

6. The method for preparing the pyrrole nitrogen modified carbon oxide catalyst according to claim 1, characterized in that: In step 2), the stirring reaction time is 12-24 h.

7. The method for preparing the pyrrole nitrogen modified carbon oxide catalyst according to claim 1, characterized in that: In step 2), 0.3-0.6 mmol of ammonium persulfate is added to 1 g of carbon oxide material.

8. The method for preparing the pyrrole nitrogen modified carbon oxide catalyst according to claim 1, characterized in that: In step 2), the mass ratio of nitrogen element in the pyrrole monomer to carbon oxide material is 10-70:

100.

9. A pyrrole nitrogen modified carbon oxide catalyst prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the pyrrole nitrogen modified carbon oxide catalyst according to claim 9 in producing hydrogen peroxide by electrocatalytic oxygen reduction.

Images