Preparation of Ni-based catalyst and method for electrooxidation of 5-hydroxymethylfurfural (HMF) at low temperature and high alkali concentration

Abstract

The invention discloses preparation of a Ni-based catalyst and a method for electrooxidation of 5-hydroxymethylfurfural (HMF) at low temperature and high alkali concentration. The electrocatalyst with a novel structure is prepared, corresponding HMF electrooxidation performance characterization is carried out, and meanwhile conversion of Ni (OH) 2 to a main active species NiO (OH) under the electrochemical working condition is disclosed. The preparation method comprises the following steps: using hydrophobic carbon paper (CP) as a substrate; the precursor solution is a 100 mM nickel nitrate aqueous solution; and in a three-electrode system, under the condition that the constant potential is-0.8 V vs.RHE, continuous electro-deposition is carried out for 600 s. And finally, thoroughly cleaning the catalyst with deionized water, and drying the catalyst in a vacuum oven at 60 DEG C overnight to obtain the Ni / CP catalyst. The electrolysis method comprises the following steps: reducing the temperature to maintain the stability of the HMF while increasing the alkali concentration and accelerating the reaction kinetics, and electrolyzing the HMF under high substrate concentration.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrocatalysts, and particularly relates to a preparation method of a Ni-based catalyst and an optimization method of electrolysis conditions for 5-hydroxymethylfurfural (HMF) electro-oxidation. BACKGROUND

[0002] The current human society uses large-scale energy and chemicals mainly from the utilization and conversion of fossil fuels, which leads to serious energy crisis and environmental pollution problems. The value-added conversion of biomass and its derivatives has become a key research direction in the field of energy. As a rich and sustainable carbon resource, biomass is of great significance for replacing non-renewable fossil fuels in the production of chemicals and fuels, and helps to achieve the decarbonization and de-petroleumization goals of the industrial sector. Electro-catalytic oxidation is a new technology for efficient conversion of HMF to FDCA, which has attracted widespread attention due to its low energy consumption and mild green operating conditions. The electro-oxidation of HMF can be coupled with the process of hydrogen production by water electrolysis or carbon dioxide reduction, replacing the oxygen evolution reaction (OER), to simultaneously synthesize two high-value-added products in a single electrolytic cell. So far, a variety of catalytic systems including noble metals, transition metals and non-metals have been widely studied, and high HMF conversion rate and FDCA selectivity can be achieved at a relatively low substrate concentration. In an alkaline environment, HMF will decompose and self-polymerize during the oxidation to FDCA, forming humin. These degradation reactions not only reduce the selectivity and yield of the target product, but also increase the operating cost and process complexity. In order to solve these problems, researchers try to inhibit degradation by stabilizing the reactive functional groups in biomass derivatives, but this method increases additional steps and costs.

[0003] The technical bottleneck is that increasing the alkali concentration is beneficial to the kinetics of HMF electro-oxidation reaction, but it will also accelerate the degradation of HMF, thereby reducing the yield of FDCA. Therefore, it is crucial to design and invent a method that can inhibit the spontaneous degradation of HMF while accelerating the reaction kinetics. SUMMARY

[0004] The purpose of the present application is to provide a simple-to-prepare and structurally detailed transition metal Ni-based catalyst, which discards complex preparation methods and simplifies the use of additional reagents, and only uses constant potential electrodeposition method to prepare the catalyst. The catalytic material has a high specific surface area, providing a considerable number of catalytically active sites for the catalytic reaction. The preparation method involves low-cost raw materials and simple process steps. The obtained Ni / CP catalyst material mainly contains Ni and C elements, and the synthesized catalyst particle size ranges from 100-500nm.

[0005] The second object of the present application is to obtain a greater current density and to inhibit the spontaneous degradation of HMF to obtain a higher yield of FDCA by optimizing the electrolysis conditions, by increasing the base concentration to accelerate the electro-oxidation reaction kinetics, while reducing the temperature to maintain the stability of HMF. And can be electrolyzed under a larger substrate concentration.

[0006] To achieve the above results, the following technical solutions are mainly used:

[0007] (1) A series of cleaning steps are performed on the carbon paper, which is ultrasonically treated in 3M HCl solution for 30 minutes to remove surface oxides; then ultrasonically treated in acetone for 30 minutes to remove organic contaminants; finally ultrasonically treated in ethanol for 30 minutes to further clean the surface, and finally ultrasonically washed with deionized water for 30 minutes to remove residual chemicals, and through air drying treatment, to prepare for the next step of electrodeposition.

[0008] (2) Electrodeposition is carried out in a three-electrode system, in which the carbon paper is used as the working electrode, the carbon rod is used as the counter electrode, and the Ag / AgCl electrode is used as the reference electrode. The precursor solution is a 100mM aqueous solution prepared from nickel nitrate. Under the condition of constant potential -0.8V vs. RHE, electrodeposition is carried out for 600s.

[0009] (3) After electrodeposition is completed, the catalyst is thoroughly washed with deionized water, and finally the catalyst is placed in a vacuum oven at 60℃ overnight to dry, to ensure that it is completely dry and ready for subsequent experiments or applications.

[0010] (4) The catalyst material obtained in step (3) is subjected to HMF electrolysis experiment, in a higher base concentration (such as 2M KOH, 3M KOH or higher), while reducing the temperature to about 4℃ for electrolysis experiment.

[0011] (5) In step (4), a greater current density is obtained, which can realize high substrate concentration electrolysis,

[0012] Preferably, in the (1) step, a hydrophobic carbon paper is selected

[0013] Preferably, in the (2) step, the volume of the precursor solution is 30mL

[0014] Preferably, in the (3) step, after washing with deionized water, complete drying in a vacuum oven.

[0015] Preferably, in the (4) step, the base concentration is 2M KOH or higher, and the low temperature of 4℃ is achieved with an ice water bath.

[0016] Preferably, the substrate concentration is increased to 50 mM HMF, 100 mM HMF or higher in the (5) step.

[0017] Preparation of a Ni-based catalyst and an optimized method for its application in the electrolysis of 5-hydroxymethylfurfural (HMF).

[0018] Compared with the prior art, the present application has the following obvious substantial features and advantages:

[0019] 1. The material prepared by the present application has a large specific surface area and excellent HMF electrocatalytic performance, solving the problem of scarce resources of noble metal catalysts. The Ni-based material prepared by the method of the present application can be applied to various electrode catalytic reactions.

[0020] 2. The present application accelerates the HMF electro-oxidation reaction kinetics by increasing the alkali concentration, reduces the temperature to maintain the stability of HMF, and improves the yield of FDCA. High selective conversion of HMF is achieved under high substrate concentration. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is the thermogravimetric analysis diagram of the prepared catalyst (Ni / CP).

[0022] Figure 2 is the XRD diagram of the prepared catalyst (Ni / CP).

[0023] Figure 3 is the SEM diagram of carbon paper (a) and the SEM pictures of the Ni / CP catalyst at different magnifications (b-d).

[0024] Figure 4 is the Raman spectrum diagram of the prepared catalyst (Ni / CP) in 2M KOH+10mM HMF with potential change (a) and the Raman spectrum diagram before and after the phase transition of the electrode (b).

[0025] Figure 5 is the XPS full spectrum diagram of the prepared catalyst (Ni / CP) (a) and the XPS spectrum diagram of Ni2p (b).

[0026] Figure 6 is the LSV test in different alkali concentrations and temperatures (a-c) and the current density reached at 1.5V vs. RHE (d).

[0027] Figure 7 is the electrolysis curve of 2M KOH (4℃) and 0.1M KOH (25℃) electrolysis of 10mM HMF (a); HMF consumption rate and FDCA generation rate under different electrolysis conditions (b); FDCA generation rate (c). DETAILED DESCRIPTION

[0028] The above scheme is further illustrated in conjunction with specific examples. Preferred embodiments of the present application are described in detail as follows:

[0029] Example: Preparation method of Ni / CP catalyst with carbon paper as substrate and pre-test preparation, according to the following steps:

[0030] (1) First, cut the carbon paper (CP) into a size of 1 cm x 2 cm to facilitate subsequent processing and experimental operation. Then, the carbon paper is subjected to a series of cleaning steps. This step includes ultrasonic treatment in 3M HCl solution for 30 minutes to remove surface oxides; followed by ultrasonic treatment in acetone for 30 minutes to remove organic contaminants; and finally ultrasonic treatment in ethanol for 30 minutes to further clean the surface. After each step, ultrasonic cleaning with deionized water for 30 minutes is used to remove residual chemicals, and through air drying treatment, preparation for the next step of electrodeposition is made.

[0031] (2) In the electrodeposition stage, a three-electrode system is used, in which the carbon paper serves as the working electrode, the carbon rod as the counter electrode, and the Ag / AgCl electrode as the reference electrode. The precursor solution is a 100 mM aqueous solution prepared from nickel nitrate. The electrodeposition is carried out at a constant potential of -1.40 V vs. Ag / AgCl for 600 seconds. After electrodeposition is completed, the catalyst is thoroughly washed with deionized water, and finally the catalyst is placed in a vacuum oven at 60°C overnight to dry, to ensure that it is completely dry and ready for subsequent experiments or applications.

[0032] (3) When performing electrochemical tests, a three-electrode system is used, in which the Ni / CP catalyst serves as the working electrode, a platinum sheet as the counter electrode, and Hg / HgO as the reference electrode. The electrolyte used for testing is 0.1M KOH, 1M KOH, 2M KOH with or without 10mM HMF. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) tests are both carried out in a single electrolytic cell, with a scan rate set at 10mV s -1 to ensure consistency of test conditions.

[0033] (4) Constant potential electrolysis tests are carried out in an H-type electrolytic cell, using Nafion 117 membrane as a separator, which can effectively isolate the two electrolytic cells while allowing the transfer of electrons. During the entire test process, the electrolyte is kept at a uniform speed. The system temperature is controlled by an ice water bath, and this low temperature state is maintained by continuously adding ice. To ensure the accuracy of temperature control, a thermometer is used to calibrate the thermocouple in the experiment.

[0034] The ZDC-n catalyst prepared is characterized for morphology and electrochemical performance:

[0035] Figure 1 The TG analysis of Ni / CP shows that the two stages of weight loss in the range of 50-300 °C correspond to the removal of physically adsorbed water molecules and crystallization water, respectively. The carbon paper substrate ignites at about 745 °C, corresponding to a weight loss of about 67%, and the final residue (NiO) has a mass of about 21%.

[0036] Figure 2 The XRD pattern of Ni / CP shows several characteristic diffraction peaks, which correspond to different crystal phase structures in the material. Specifically, the diffraction peaks at 26.6° and 54.8° correspond to the characteristic peaks of the carbon paper substrate. In addition, the weak diffraction peak at 44.5° can be attributed to the Ni(111) crystal plane. The appearance of this peak confirms the successful deposition of the nickel catalyst and indicates that the main form of nickel is the face-centered cubic (FCC) structure.

[0037] Figure 3 SEM characterization results of carbon paper, pre-catalyst material. Figure 3 a is the SEM image of the original carbon paper substrate, which is composed of densely arranged carbon fibers, showing an interlaced network structure. Figure 3 b is the SEM image after electrodeposition treatment, showing a uniform layer of sheet structure covering the surface of the carbon fibers. Figure 3 c is the SEM image of the cross-section of the catalyst, showing that the sheet thickness is about 10-12 μm. Figure 3 d shows that these sheets are composed of nanoparticles with a size of about 100-500 nm, and the surface of the particles shows fine texture.

[0038] Figure 4 Raman spectra of Ni / CP in 2M KOH + 10mM HMF as a function of potential (a) and before and after phase transition of the electrode material (b). At the open circuit potential, characteristic peaks of Ni(OH)2 can be observed, which indicates that the catalyst surface exists in the form of Ni(OH)2 without applying potential. As the potential gradually increases, the catalyst begins to transition rapidly, and the characteristic peak of NiO(OH) begins to appear at 1.38 V vs. RHE, and at 1.40 V vs. RHE, the peak completely transforms into the characteristic peak of NiO(OH), indicating that the catalyst has completely transformed into NiO(OH) at this potential. This proves that Ni(OH)2 can be transformed into NiO(OH) under the driving of a certain external voltage. -1 The characteristic peaks of Ni(OH)2 are observed, which indicates that the catalyst surface exists in the form of Ni(OH)2 without applying potential. As the potential gradually increases, the catalyst begins to transition rapidly, and the characteristic peak of NiO(OH) begins to appear at 1.38 V vs. RHE, and at 1.40 V vs. RHE, the peak completely transforms into the characteristic peak of NiO(OH), indicating that the catalyst has completely transformed into NiO(OH) at this potential. This proves that Ni(OH)2 can be transformed into NiO(OH) under the driving of a certain external voltage.

[0039] Figure 5 XPS full spectrum of Ni / CP (a) and XPS spectrum of Ni2p (b). From the full spectrum, it can be seen that the catalyst is mainly composed of carbon, oxygen and nickel. The peak of carbon is the strongest, indicating that the catalyst is mainly composed of carbon paper. The peak of oxygen is relatively strong, indicating that the catalyst contains a certain amount of oxygen. The peak of nickel is relatively weak, indicating that the catalyst contains a small amount of nickel. Figure 5The XPS full spectrum results show that the catalyst surface contains Ni, N, C and O. Figure 5 The Ni 2p spectrum shows two characteristic peaks, the peak at 855.8 eV corresponds to Ni 2+ 2p3 / 2, and the peak at 856.6 eV corresponds to Ni 3+ 2p3 / 2. These peak positions are obtained by correcting the C1s signal at 284.8 eV. In the XPS spectrum, a satellite peak at 861.3 eV can also be observed, which is caused by the vibrational excitation of high-spin nickel ions.

[0040] Figure 6 The LSV tests in different base concentrations and temperatures (a-c) and the current density reached at 1.5 V vs. RHE (d) show that the overpotential can be reduced after adding HMF, which indicates that the Ni / CP catalyst exhibits significant catalytic activity in promoting the electro-oxidation of HMF. The current density obtained under high base concentration (2M KOH) and low temperature (4℃) is significantly higher than that under low base concentration (0.1M KOH) and room temperature (25℃). This finding indicates that the reaction kinetics can be effectively improved under low temperature and high base concentration, thereby achieving higher electrocatalytic activity while ensuring the stability of HMF.

[0041] Figure 7 The electrolysis curves of 10mM HMF in 2M KOH (4℃) and 0.1M KOH (25℃) (a); HMF consumption rate and FDCA generation rate under different electrolysis conditions (b); FDCA generation rate (c). The reaction time in 2M KOH (4℃) is more than twice shorter than that in 0.1M KOH (25℃) when the electrolysis is completed. As can be seen from the b graph, the HMF consumption rate and FDCA generation rate are both significantly faster in 2M KOH (4℃). As can be seen from the c graph, the FDCA generation rate is 2.08mM / h, which is twice that in 0.1M KOH (25℃). This proves that while reducing the temperature to inhibit the spontaneous degradation of HMF, a faster reaction rate can be obtained by increasing the concentration of the base.

[0042] In summary, the carbon paper is used as the substrate to prepare the Ni / CP electrocatalyst by constant potential electrodeposition, and systematic structural characterization is carried out, which reveals the morphology, crystal structure and surface electronic structure characteristics of the catalyst. Through in-situ electrochemical Raman spectroscopy, it is found that Ni(OH)2 undergoes phase transformation under electrochemical working conditions. The prepared catalyst also has excellent HMF electro-oxidation performance. It is proposed that by reducing the temperature of the reaction system and increasing the concentration of the base, the spontaneous degradation of HMF can be inhibited and the electro-oxidation reaction kinetics can be accelerated.

[0043] The preparation method of the above-mentioned embodiments of the present application is simple, the process equipment is simple, the product performance is stable and controllable, the raw materials are green and environmentally friendly, and the repeatability is high.

Claims

1. A process for the preparation of a Ni-based catalyst and its use for the electro-oxidation of 5-hydroxymethylfurfural (HMF) at low temperature and high alkali concentration, characterized in that, Ni / CP electrocatalyst was prepared by potentiostatic electrodeposition method. In-situ electrochemical Raman spectroscopy was used to investigate the phase transformation of Ni(OH)2 to NiOOH under electrochemical conditions. The chemical stability, spontaneous degradation kinetics of HMF at different alkali concentrations and temperatures, and the influence of electro-oxidation performance were explored. The method can inhibit the spontaneous degradation of HMF and accelerate the electro-oxidation reaction kinetics by reducing the temperature of the reaction system while increasing the alkali concentration. The method can be used for electrolysis at high substrate concentration.

2. The CP of claim 1, wherein, Hydrophobic carbon paper.

3. The Ni / CP electrocatalyst of claim 1, wherein, The deposition potential is -0.8V vs. RHE, and the deposition time is 600s.

4. The Ni / CP electrocatalyst of claim 1, wherein, The precursor solution is 100mM Ni(NO3)·6H2O.

5. The method of claim 1, wherein the catalyst is prepared by the steps of: The method comprises the following steps: 1) A series of cleaning steps were performed on the carbon paper. The surface oxides were removed by ultrasonic treatment in 3M HCl solution for 30 minutes. Then, the organic contaminants were removed by ultrasonic treatment in acetone for 30 minutes. Finally, the surface was further cleaned by ultrasonic treatment in ethanol for 30 minutes. Finally, the residual chemicals were removed by ultrasonic cleaning in deionized water for 30 minutes, and the sample was prepared for the next electrodeposition by air drying. 2) The electrodeposition was carried out in a three-electrode system, in which the carbon paper was used as the working electrode, the carbon rod was used as the counter electrode, and the Ag / AgCl electrode was used as the reference electrode. The precursor solution was a 100mM aqueous solution prepared from nickel nitrate. The electrodeposition was carried out at a constant potential of -0.8V vs. RHE for 600s. 3) After the electrodeposition was completed, the catalyst was thoroughly washed with deionized water, and finally the catalyst was placed in a vacuum oven at 60℃ overnight to dry, to ensure that it was completely dry and ready for subsequent experiments or applications.

6. A process for the preparation of a Ni-based catalyst and its use in the electrolysis of HMF at low temperature and high alkali concentration, characterized in that, The electrolysis method is to selectively electro-oxidize HMF to 2,5-furan dicarboxylic acid (FDCA) at low temperature and high alkali concentration.

7. An electrolysis method as claimed in claim 6, characterized in that The low temperature is about 4℃ (which can effectively inhibit the spontaneous degradation of HMF).

8. An electrolysis process as in claim 6, characterized in that The high alkali concentration is 2M KOH, 3M KOH or higher.

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