Ni / porous carbon composite catalyst and preparation method and application thereof

Abstract

The invention discloses a Ni / porous carbon composite catalyst and a preparation method and application thereof.The preparation method comprises the steps that zinc acetate, a template agent and nickel nitrate are dissolved in methyl alcohol, and a solution A is obtained through ultrasonic treatment; adding a methanol solution of 2-methylimidazole into the solution A, and reacting to obtain Ni / ZIF-8; adding Ni / ZIF-8 into the organic ligand solution, carrying out a hydrothermal reaction, and calcining the product in a nitrogen atmosphere to prepare a product B; and soaking the product B in an acid solution to prepare the Ni / porous carbon composite catalyst. The Ni / porous carbon composite catalyst prepared by the preparation method has a relatively large specific surface area, good stability and a controllable pore diameter and gap structure, and the problem that the catalyst is easy to inactivate is effectively avoided. The Ni / porous carbon composite catalyst prepared by the invention is combined with a low-temperature plasma technology, shows excellent catalytic activity and stability in a tar reforming process, and has a good application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of tar reforming, and relates to a Ni / porous carbon composite catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Biomass gasification is an efficient biomass energy utilization technology. Biomass syngas is obtained through high-temperature reactions. However, the by-product tar generated during the gasification process is a bottleneck restricting the development and application of this technology. Tar refers to a complex mixture containing various condensable organic compounds such as monocyclic aromatic hydrocarbons, hydrocarbons, oxygen-containing hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs). Toluene is the main component of tar, accounting for up to 10%. Therefore, toluene is often selected as a tar model compound. Generally, tar is in a gaseous state at high temperatures (>300 °C); at low temperatures (<200 °C, the dew point temperature of tar vapor), tar is easily condensed and combines with other by-products in the gasification process, such as ash, water vapor, and carbon particles and other gas-solid impurities, forming more complex substances, which affects the subsequent application of syngas. Tar not only blocks downstream pipelines, causes environmental pollution, and endangers human health, but also reduces the energy efficiency of biomass gasification, resulting in energy losses. Due to the harm of biomass tar, how to efficiently remove tar and convert it into high-value-added products, thereby realizing the resource utilization of biomass tar, has become a key problem that urgently needs to be solved in the current development of biomass gasification technology.

[0003] Tar reforming technology is a technology that converts tar into syngas or other useful gases or liquid fuels, which can effectively utilize tar, reduce waste emissions, and improve resource utilization efficiency. Currently, the main tar reforming technologies include catalytic conversion, low-temperature plasma, etc. Among them, the low-temperature plasma method has advantages such as fast and efficient, low pollution, simple process, and mild reaction conditions. However, it is difficult to achieve the directional conversion of target products by reforming tar with plasma alone. Therefore, combining low-temperature plasma with a catalyst to form a low-temperature plasma catalytic reforming technology and constructing a low-temperature plasma catalytic reforming system can not only utilize the high-activated molecule ability of plasma but also utilize the high product selectivity of the catalyst, significantly improve the tar conversion efficiency, reduce the reaction energy consumption, avoid the formation of secondary by-products, and at the same time improve the selectivity and yield of target products. Nickel-based catalysts are widely used in the reforming of tar model compounds such as toluene due to their excellent C-H bond and C-C bond breaking abilities, low cost, and easy availability. Although the catalyst can react at a relatively low temperature and reduce the energy consumption during the tar conversion process, the catalyst generally has problems such as poor stability and easy deactivation, which limit its effective application. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention provides a Ni / porous carbon composite catalyst, a preparation method thereof and an application thereof, so as to solve the technical problems that in the process of toluene reforming by low-temperature plasma synergistic catalyst in the prior art, the catalyst is prone to deactivation and has poor stability.

[0005] The present invention is realized by the following technical solutions: A preparation method of a Ni / porous carbon composite catalyst, comprising the following steps: S1: Dissolve zinc acetate, a template agent and nickel nitrate in methanol, and ultrasonically treat to completely dissolve them to obtain solution A; then add a methanol solution of 2-methylimidazole to solution A, after ultrasonic oscillation reaction, carry out aging, centrifugation, and drying overnight to obtain Ni / ZIF-8; S2: Add the Ni / ZIF-8 to an organic ligand solution, mix evenly and carry out a hydrothermal reaction, then centrifuge and dry, dry the obtained solid product, and carry out a calcination treatment in a nitrogen atmosphere to obtain product B; S3: Immerse the product B in an acid solution for soaking treatment and then wash with deionized water to obtain the Ni / porous carbon composite catalyst.

[0006] Preferably, in step S1, the molar ratio of nickel atoms to zinc atoms is (1-5):10; the dosage of the template agent accounts for 5%-10% of the total mass of zinc acetate, the template agent, nickel nitrate and 2-methylimidazole.

[0007] Preferably, in step S1, the template agent is at least one of polyvinylpyrrolidone, polyvinyl alcohol, polyacrylamide and polystyrene.

[0008] Preferably, in step S1, the time of ultrasonic oscillation reaction is 20-30 min.

[0009] Preferably, in step S2, the temperature of the hydrothermal reaction is 100-120 °C and the time is 2-3 h.

[0010] Preferably, in step S2, during the calcination treatment, the heating rate is 3-6 °C / min, the calcination temperature is 800-900 °C, and the calcination time is 3-5 h.

[0011] Preferably, in step S3, the acid solution is an HCl solution or an HNO3 solution; the concentration of the acid solution is 0.1-1 mol / L.

[0012] Preferably, in step S3, the soaking time is 3-5 h.

[0013] A Ni / porous carbon composite catalyst is prepared by the above method.

[0014] Application of the above-mentioned Ni / porous carbon composite catalyst in the field of tar reforming. The Ni / porous carbon composite catalyst is placed in a dielectric barrier discharge plasma reactor, and the reforming reaction with toluene as a tar model compound is realized through plasma discharge. During the plasma discharge process, the discharge power is 30-90 W; in the reforming reaction, the toluene conversion rate is 87%-99%, the H2 selectivity is 23%-50%, and the CO selectivity is 21%-43%.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention discloses a preparation method of a Ni / porous carbon composite catalyst. First, zinc acetate and nickel nitrate are used as metal sources and dissolved in methanol on the premise of adding a template agent. These compounds can be fully dissolved under ultrasonic treatment to form a uniform solution A. Subsequently, a methanol solution of 2-methylimidazole is added to solution A. 2-Methylimidazole is a key ligand for forming ZIF-8. Through ultrasonic reaction, 2-methylimidazole coordinates with metal ions in solution A. The nitrogen atom in 2-methylimidazole provides a lone pair of electrons to form a coordination bond with the empty orbital of zinc ions. Multiple such coordination units are connected to each other to construct a three-dimensional ZIF-8 skeleton. In this process, nitrogen atoms are fixed in the ZIF-8 structure, realizing the transfer of nitrogen elements from 2-methylimidazole to ZIF-8, forming an N-doped ZIF-8 structure. By introducing Ni ions, active sites of nickel can be introduced into the ZIF-8 framework, providing an active center for subsequent catalytic reactions. The addition of the template agent helps to improve the morphology and stability of the material and promotes the uniform distribution of Ni. Then, Ni / ZIF-8 is added to the organic ligand solution. The organic ligand serves as a carbon source and interacts with Ni / ZIF-8 under hydrothermal reaction conditions. After the hydrothermal reaction, the obtained solid product is dried to remove residual moisture and solvents. Finally, calcination treatment is carried out under a nitrogen atmosphere. Calcination under nitrogen protection can promote the carbonization of the organic ligand. At the same time, the Zn element in the Ni / ZIF-8 structure evaporates at high temperature, leaving a porous carbon skeleton and Ni nanoparticles. In this step, through the carbonization of the organic ligand and the decomposition of Ni / ZIF-8, the simultaneous co-doping of nickel and nitrogen in the porous carbon is realized, a porous carbon skeleton is constructed, and Ni nanoparticles are fixed. The porous structure helps to increase the specific surface area of the catalyst and improve the catalytic efficiency. Calcination under a nitrogen atmosphere avoids the oxidation of the material and maintains the integrity of the carbon skeleton. Finally, product B is soaked in an acid solution. The purpose of this step of treatment is to remove residual ZnO or other impurities and further adjust the pore structure and surface properties of the catalyst. Through acid treatment, the purity of the catalyst can be improved, and the influence of unnecessary impurities on the catalytic reaction can be reduced. Acid treatment may also open or expand the pores of the catalyst, increasing the accessibility of active sites, thereby improving the catalytic performance. Therefore, the present invention optimizes the preparation process of the catalyst, loads the catalyst Ni by a one-step method, and prepares a composite catalyst with porous carbon as the carrier. The porous carbon material has a large specific surface area, good stability, controllable pore size and pore structure, effectively avoiding the problems of easy deactivation and poor stability of the catalyst. The easy formation of microdischarge inside the pores of the catalyst can also improve its discharge characteristics and enhance the synergy between the plasma and the catalyst. In addition, the high-porosity carrier material is conducive to the high dispersion of the active phase, thereby enhancing the catalytic activity and preventing the formation of carbon deposition during the reaction process.

[0016] Further, in step S1, the molar ratio of nickel atoms to zinc atoms is (1-5):10; the dosage of the template agent accounts for 5%-10% of the total mass of zinc acetate, the template agent, nickel nitrate, and 2-methylimidazole. By adjusting the molar ratio of nickel atoms to zinc atoms, the number of nickel active sites in Ni / ZIF-8 can be precisely controlled. A higher nickel content means more active sites, which may improve the catalytic activity of the catalyst. However, an excessively high nickel content may also lead to overcrowding of active sites and affect the catalytic efficiency. Therefore, choosing an appropriate molar ratio can find a balance between the number of active sites and the catalytic efficiency. The addition of the template agent helps to improve the morphology and stability of the material. An appropriate amount of the template agent can promote the formation of the Ni / ZIF-8 structure and endow it with a better pore structure and morphology, which is helpful for the construction of the porous carbon skeleton and the uniform distribution of Ni nanoparticles during the subsequent calcination process. The template agent may also promote the uniform distribution of Ni ions in the ZIF-8 framework through its unique chemical or physical properties, which helps to reduce the agglomeration of Ni nanoparticles and improve the catalytic activity and stability of the catalyst.

[0017] Further, in step S1, the template agent is at least one of polyvinylpyrrolidone, polyvinyl alcohol, polyacrylamide, and polystyrene. Polyvinylpyrrolidone (PVP) has good biocompatibility and low toxicity, which gives it significant advantages in the preparation of catalysts for high-purity products. PVP can stabilize nanoparticles and prevent them from agglomerating during the synthesis process, thus ensuring the uniformity and stability of the Ni / ZIF-8 structure. Polyvinyl alcohol (PVA) is a mild polymer with low toxicity and no harm to humans and the environment, making it suitable for the preparation process of catalysts that require high safety. Polyacrylamide (PAM) has the water solubility of a macromolecular compound and active acyl groups on the main chain, which enables it to provide good dispersibility and stability during the catalyst preparation process. Polystyrene (PS) has a stable chemical structure and can maintain its performance unchanged under harsh conditions such as high temperature, which helps to maintain the structural stability of the catalyst during the calcination process. Different types of template agents have different characteristics and advantages, and their combined use may produce a synergistic effect, thereby improving the catalytic performance, stability, and durability of the catalyst. In summary, choosing at least one of polyvinylpyrrolidone, polyvinyl alcohol, polyacrylamide, and polystyrene as the template agent in step S1 can effectively improve the catalyst performance, reduce costs, increase the flexibility and controllability of the preparation process, etc.

[0018] Further, in step S1, the time of the ultrasonic oscillation reaction is 20 - 30 min. Ultrasonic treatment can generate strong acoustic vibrations, which can form tiny bubbles in the solution and rapidly burst, producing the so-called "cavitation effect". This effect helps to break the intermolecular forces in the solution and promote the uniform mixing of reactant molecules. Within the ultrasonic reaction time of 20 - 30 minutes, sufficient time allows the reactant molecules to fully contact and mix, ensuring the uniformity of the Ni / ZIF-8 structure. Ultrasonic treatment not only promotes mixing but also accelerates the rate of chemical reactions through its physical effects (such as local high temperature and high pressure). In the synthesis process of Ni / ZIF-8, ultrasonic reaction helps the coordination reaction between 2-methylimidazole and metal ions (such as Ni²⁺ and Zn²⁺) to proceed more rapidly. A relatively short reaction time (such as 20 - 30 minutes) can achieve a relatively high coordination efficiency, reducing unnecessary reaction time and improving the preparation efficiency. Ultrasonic treatment can also affect the crystal growth process. An appropriate ultrasonic reaction time helps to control the size and morphology of Ni / ZIF-8 crystals, avoiding overgrowth or agglomeration, which helps to maintain the pore structure and specific surface area of the catalyst in subsequent steps, thereby improving its catalytic performance. A shorter ultrasonic reaction time helps to reduce the occurrence of side reactions and the generation of impurities. At the same time, ultrasonic treatment can also promote the dispersion and removal of impurities in the solution, thereby improving the purity of the final product. In the synthesis of Ni / ZIF-8, a high-purity product is beneficial to the formation of porous carbon skeletons and Ni nanoparticles during the subsequent calcination process. In summary, setting the ultrasonic reaction time in step S1 to 20 - 30 minutes can significantly promote the uniform mixing of reactants, accelerate the coordination reaction, control crystal growth, improve the product purity, etc. These effects act together on the synthesis process of Ni / ZIF-8, laying a solid foundation for the subsequent preparation of high-performance Ni / porous carbon composite catalysts.

[0019] Further, in step S2, the temperature of the hydrothermal reaction is 100 - 120 °C, and the time is 2 - 3 h. The hydrothermal reaction is carried out under high-temperature conditions, which can significantly increase the reaction rate. Within the temperature range of 100 - 120 °C, the rates of most chemical reactions will be accelerated, which is beneficial to the formation and stability of the Ni / ZIF-8 structure. Temperature is one of the important factors affecting the selectivity of chemical reactions. Conducting the hydrothermal reaction within an appropriate temperature range can optimize the product distribution and selectivity and reduce the formation of by-products, which helps to obtain a Ni / ZIF-8 precursor with high purity and high activity. Appropriate temperature contributes to the growth of Ni / ZIF-8 crystals and the improvement of crystallinity. At high temperatures, the thermal motion between molecules is more intense, which is conducive to the formation of a more complete and dense crystal structure. Although high temperature is beneficial to the reaction, too high a temperature may lead to overheating, affecting the quality and stability of the product. Setting the temperature in the range of 100 - 120 °C can avoid the occurrence of overheating and ensure that the reaction proceeds under controllable conditions. The hydrothermal reaction requires a certain amount of time to complete the conversion of reactants and the formation of products. Setting the reaction time to 2 - 3 hours can ensure that the reaction proceeds fully, improving the conversion rate and purity of the product. Appropriate reaction time helps to optimize the crystal structure of Ni / ZIF-8. During the reaction process, the crystals will go through stages such as nucleation, growth, and stabilization. By controlling the reaction time, Ni / ZIF-8 crystals with specific morphology and size can be obtained. By optimizing the temperature and time settings of the hydrothermal reaction, a Ni / ZIF-8 precursor with high purity, high crystallinity, and good morphology can be obtained, which will contribute to the formation and distribution of the porous carbon skeleton and Ni nanoparticles in the subsequent steps, thereby improving the catalytic performance and stability of the final catalyst. Reasonable temperature and time settings can simplify the preparation process, reduce unnecessary steps and condition adjustments, which will reduce production costs and operation difficulties and improve the controllability and repeatability of the preparation process.

[0020] Further, in step S2, during the calcination process, the heating rate is 3 - 6 °C / min, the calcination temperature is 800 - 900 °C, and the calcination time is 3 - 5 h. The slow heating rate (3 - 6 °C / min) helps to reduce the thermal stress generated by the rapid temperature change during the heating process of the material, thereby reducing the risk of material cracking or deformation. During the calcination process, the organic components in the precursor will decompose and release gases (such as CO2, H2O, etc.). The slow heating rate can provide sufficient time for the release of these gases, avoiding the accumulation of gases inside the material and causing structural damage. An appropriate heating rate helps the crystals to gradually grow and optimize the structure during the calcination process, improving the crystallinity and stability of the product. The calcination temperature is 800 - 900 °C. Within this temperature range, the organic components in the Ni / ZIF-8 precursor can be fully decomposed to form a porous carbon skeleton and Ni nanoparticles, which helps to obtain a catalyst precursor with high purity and high activity. The appropriate calcination temperature helps to control the size and distribution of Ni nanoparticles, as well as the pore structure of the porous carbon skeleton, which is crucial for improving the specific surface area and catalytic performance of the catalyst. Too high a temperature may lead to the agglomeration of Ni nanoparticles and the sintering of the porous carbon skeleton, thereby reducing the specific surface area and catalytic activity of the catalyst. Therefore, setting the calcination temperature within the range of 800 - 900 °C can avoid this problem. An appropriate calcination time can ensure that the organic components in the Ni / ZIF-8 precursor are completely decomposed and form a stable porous carbon skeleton and Ni nanoparticles. The length of the calcination time will affect the morphology of Ni nanoparticles and the structure of the porous carbon skeleton. Within an appropriate calcination time, Ni nanoparticles with uniform size and good dispersion, as well as a porous carbon skeleton with a rich pore structure, can be obtained. By optimizing the heating rate, calcination temperature, and calcination time settings during the calcination process, a Ni / porous carbon composite catalyst with high purity, high crystallinity, good morphology, and rich pore structure can be obtained, which will help to improve the catalytic performance, stability, and durability of the catalyst.

[0021] Further, in step S3, the acid solution is an HCl solution or an HNO3 solution, and the concentration of the acid solution is 0.1 - 1 mol / L. As strong acids, HCl and HNO3 can ensure that the reaction proceeds in a stable acidic environment, which is beneficial to the smooth progress of the reaction and the formation of products. By adjusting the concentration of the acid, the reaction rate can be controlled to a certain extent to meet the requirements of experiments or production. A lower acid concentration reduces the safety risk during the operation process and is beneficial to protecting the safety and health of operators.

[0022] Further, in step S3, the soaking treatment time is 3 to 5 h. During the soaking process, the acid solution can fully penetrate into the pores and surface of the material and react with the components therein. This time setting ensures that the acid solution has sufficient time to fully contact the material, thereby achieving more complete infiltration and reaction. The acid solution can dissolve or remove impurities such as oxides and dirt on the surface of the material, thereby improving the purity and surface quality of the material. By controlling the soaking time, it can be ensured that the material reaches the optimal treatment effect in the acid solution. Too short a soaking time cannot fully treat the material, while too long a soaking time will lead to a decline in the material performance. Therefore, a soaking time of 3 to 5 hours is a relatively reasonable choice. An appropriate soaking time also helps to improve the stability of the material.

[0023] In addition, the present invention also discloses a Ni / porous carbon composite catalyst prepared by the above method. The pore volume of the composite catalyst is 0.2 to 0.4 cm 3 / g, and the average pore diameter is 6 to 8 nm. It has high catalytic activity, selectivity and stability for toluene at low energy consumption, and effectively inhibits the deactivation problems of the catalyst caused by carbon deposition and sintering during the reforming process. In the present invention, the toluene conversion rate can reach 98.94%, and the H2 selectivity and CO selectivity can reach 50.3% and 43.3% respectively. It is a good toluene reforming method and is suitable for wide application. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as a limitation of the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0025] Figure 1 It is a flow chart of a method for preparing a Ni / porous carbon composite catalyst in the present invention; Figure 2 It is a scanning electron microscope image of the Ni / porous carbon composite catalyst prepared in Example 1 of the present invention; Figure 3 It is an X-ray photoelectron spectroscopy image of the Ni / porous carbon composite catalyst prepared in Example 1 of the present invention; Figure 4 It is an X-ray diffraction pattern of the Ni / porous carbon composite catalyst prepared in Example 1 of the present invention; Figure 5 It is a pore size distribution diagram of the Ni / porous carbon composite catalyst prepared in Example 1 of the present invention; Figure 6This is a stability test diagram of the Ni / porous carbon composite catalyst prepared in Example 1 of the present invention and the low-temperature plasma system. Detailed implementation mode

[0026] To enable those skilled in the art to understand the features and effects of the present invention, the following is a general description and definition of the terms and phrases mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein shall have the ordinary meaning understood by those skilled in the art with respect to the present invention. In case of conflict, the definition in this specification shall prevail.

[0027] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall in no way limit the scope of the present invention, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0028] In this article, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are only for the sake of brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values within the range (including integers and fractions).

[0029] In this article, unless otherwise specified, "comprising", "including", "containing", "having" or similar terms cover the meanings of "consisting of" and "consisting essentially of". For example, "A comprises a" covers the meanings of "A comprises a and others" and "A only comprises a".

[0030] In this article, for the sake of concise description, all possible combinations of all technical features in each embodiment or example are not described. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered as the scope described in this specification.

[0031] As Figure 1 shown, the present invention provides a preparation method of a Ni / porous carbon composite catalyst, comprising the following steps: S1: Dissolve zinc acetate, the template agent, and nickel nitrate in methanol, and ultrasonically treat to completely dissolve them to obtain solution A. Then, add the methanol solution of 2-methylimidazole to solution A, ultrasonically oscillate and react for 20 - 30 min, then place it at 4 °C to cool for 1 h. Then, age the reaction solution at room temperature for 24 h, centrifuge (7000 r / min, 3 min) and wash with methanol several times, and dry overnight at 60 - 120 °C to obtain the Ni / ZIF-8 solid product. Here, the cooling treatment helps slow down the crystal growth rate, thus allowing more time to form a regular and less defective crystal structure. Such a cooling process can reduce grain boundaries and other crystal defects, improving the crystallinity and porosity of the material. Aging treatment at room temperature further improves the crystal quality, optimizes the pore structure, and enhances the thermal stability and chemical stability of the material.

[0032] Among them, the molar ratio of nickel atoms to zinc atoms is (1 - 5):10; the dosage of the template agent accounts for 5% - 10% of the total mass of zinc acetate, the template agent, nickel nitrate, and 2-methylimidazole. The template agent is at least one of polyvinylpyrrolidone, polyvinyl alcohol, polyacrylamide, and polystyrene.

[0033] The zinc acetate here can be zinc acetate dihydrate, and the nickel nitrate is nickel nitrate hexahydrate.

[0034] S2: Add the Ni / ZIF-8 to the organic ligand solution, mix evenly, and carry out a hydrothermal reaction at 100 - 120 °C for 2 - 3 h. Subsequently, centrifuge, wash with methanol, and dry to obtain a light yellow solid product. After drying and grinding the obtained light yellow solid product, raise the temperature to 800 - 900 °C at a heating rate of 3 - 6 °C / min for calcination treatment, and the calcination treatment time is 3 - 4 h. After calcination, product B is prepared. The organic ligand can be glucose. As one of the sources of organic ligands, glucose participates in constructing the three-dimensional structure of ZIF-8 together with 2-methylimidazole. Glucose molecules contain multiple functional groups, and these functional groups can interact with Zn²⁺ ions, helping to guide the orderly arrangement of Zn ions and promoting the formation of ZIF-8 crystals. Substances replacing glucose should have functional groups capable of forming coordination bonds with zinc ions and have appropriate spatial dimensions to adapt to the crystal structure of ZIF-8. Some possible alternative substances include other organic ligands containing nitrogen heterocyclic structures, such as pyridine, benzimidazole, etc., or other organic molecules with appropriate functional groups, such as urea, thiourea, etc.

[0035] S3: Place product B in an acid solution with a concentration of 0.1 - 1 mol / L, soak and react at 60 - 100 °C for 3 - 5 h, and then wash with deionized water to obtain the Ni / porous carbon composite catalyst.

[0036] The present invention provides a Ni / porous carbon catalyst with a large specific surface area, a rich pore structure and easy adjustment, so as to improve the synergy between the plasma and the catalyst and develop a green and efficient method for catalytic reforming of toluene by low-temperature plasma. The present invention is a method for catalytic reforming of toluene by coupling low-temperature plasma with Ni / porous carbon. The Ni / porous carbon catalyst is prepared by the deposition-precipitation method combined with the organic ligand-assisted hydrothermal method, and is coupled with a DBD reactor to construct an in-situ plasma catalytic reforming method for toluene. The microstructure of the catalyst is optimized by regulating the catalyst synthesis method to improve its reaction performance.

[0037] The present invention also discloses the application of the above-mentioned Ni / porous carbon composite catalyst in the field of tar reforming; the application includes placing the Ni / porous carbon composite catalyst in a dielectric barrier discharge plasma reactor, and realizing the reforming reaction with toluene as the tar model compound through plasma discharge. During the plasma discharge process, the discharge power is 30-90 W; in the reforming reaction, the toluene conversion rate is 87%-99%, the H2 selectivity is 23%-50%, and the CO selectivity is 21%-43%. During the reforming reaction process, the initial concentration of toluene can be set to 500-1500 ppm.

[0038] The method for catalytic reforming of toluene by coupling low-temperature plasma with Ni / porous carbon provided by the present invention uses a nitrogen-doped porous carbon material as a carrier. The porous carbon material has a large specific surface area, good stability, controllable pore size and pore structure. Microdischarge is easily formed inside the pores, which can also improve its discharge characteristics and enhance the synergy between the plasma and the catalyst; in addition, the high-porosity carrier material is conducive to the high dispersion of the active phase, thereby enhancing the catalytic activity and preventing the formation of carbon deposition during the reaction process. In the present invention, low-cost nickel is selected as the active component, and the preparation method is simple, reducing the preparation cost of the catalyst. The method for catalytic reforming of toluene by coupling low-temperature plasma with Ni / porous carbon provided by the present invention has high catalytic activity, selectivity and stability for toluene at low energy consumption, and effectively inhibits the deactivation problem of the catalyst caused by carbon deposition and sintering during the reforming process. In the present invention, the highest toluene conversion rate reaches 98.94%, and the H2 selectivity and CO selectivity can reach 50.3% and 43.3% respectively. It is a good toluene reforming method and is suitable for wide application.

[0039] The following further elaborates the present invention in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0040] Conventional instruments and equipment in the art are used in the following examples. For the experimental methods without specific conditions noted in the following examples, they are generally carried out under conventional conditions or according to the conditions recommended by the manufacturer. Various raw materials are used in the following examples. Unless otherwise specified, commercially available products are used, and their specifications are conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" represents weight percentage, "parts" represents weight parts, and the ratio represents weight ratio.

[0041] Example 1 A preparation method of a Ni / porous carbon composite catalyst is as follows: (1) Weigh zinc acetate dihydrate, polyvinylpyrrolidone, and nickel nitrate hexahydrate in a methanol solution. The total volume ratio of zinc acetate dihydrate and polyvinylpyrrolidone to methanol is 1:50, and the volume ratio of zinc acetate dihydrate to polyvinylpyrrolidone is 1:1. Then add zinc acetate dihydrate, nickel nitrate hexahydrate, and polyvinylpyrrolidone to methanol to obtain a mixed solution A. Then, weigh 2-methylimidazole and methanol according to a volume ratio of 1:50, and add 2-methylimidazole to methanol to obtain a mixed solution B. The amount of methanol used in mixed solution A and mixed solution B is the same. After uniformly mixing mixed solution A and mixed solution B, ultrasonic oscillation is carried out for 30 min, and then it is cooled at 4 °C for 1 h. Among them, the molar ratio of nickel atoms to zinc atoms is 1.5:10 (i.e., the nickel loading is 15%).

[0042] (2) Age the above mixed solution at room temperature for 24 h.

[0043] (3) Finally, centrifuge (7000 r / min, 3 min) and wash with methanol several times, and dry at 60 °C overnight to obtain a Ni / ZIF-8 solid product.

[0044] (4) Take 0.5 g of Ni / ZIF-8 solid and disperse it in 40 mL of 0.5 M glucose solution, mix well, pour the mixed solution into a hydrothermal reaction kettle, react at 120 °C for 2 h, and then centrifuge, wash with methanol, and dry to obtain a light yellow solid product.

[0045] (5) Grind the above obtained light yellow solid, place it in a tubular furnace, and calcine it at 900 °C for 3 h with a heating rate of 5 °C / min in a N2 atmosphere. Then, pour the sample into an HCl solution and react at 60 °C for 4 h to remove impurities.

[0046] (6) Then, wash the obtained product repeatedly with deionized water until the pH value of the washing solution is neutral to obtain a black hollow Ni / porous carbon composite catalyst. The pore volume of this composite catalyst is 0.3 cm 3 / g, and the average pore diameter is 7 nm.

[0047] Example 2 The difference between this example and Example 1 is that nickel acetate hexahydrate is used as the precursor solution.

[0048] Example 3 The difference between this example and Example 1 is that nickel chloride hexahydrate is used as the precursor solution.

[0049] Furthermore, in order to verify the technical effects of the technical solution of the present invention, the catalytic materials prepared in Examples 1 to 3 were combined with plasma for the catalytic conversion of toluene. When the molar ratio of nickel atoms to zinc atoms was 1.5:10, the discharge power was 75 W, the total flow rate was 1 L / min, and the initial toluene concentration was set at 1000 ppm, the conversion efficiency and H2 and CO selectivities of the plasma catalytic system are shown in Table 1.

[0050] Table 1 Toluene conversion rates and H2 and CO selectivities corresponding to Ni / porous carbon catalysts prepared with different ZIF-8 nickel precursors

[0051] As can be seen from Table 1, the catalyst prepared with nickel nitrate as the precursor has better effects than those prepared with nickel acetate and nickel chloride. This is mainly because nickel nitrate has good solubility in methanol and is easy to mix with organic ligands, which is conducive to the formation of a uniform solution, thus facilitating the crystal growth of ZIF-8. As a ligand, nitrate can decompose to produce oxygen at high temperatures, which helps the calcination process of the material.

[0052] Example 4 The difference between this example and Example 1 is that the molar ratio of nickel atoms to zinc atoms is 1:10 (i.e., the nickel loading is 10%).

[0053] Example 5 The difference between this example and Example 1 is that the molar ratio of nickel atoms to zinc atoms is 2:10 (i.e., the nickel loading is 20%).

[0054] When the ratio of zinc acetate dihydrate to polyvinylpyrrolidone is 1:1, the ZIF-8 solvent is methanol, the nickel precursor is nickel nitrate, the discharge power is 75 W, the total flow rate is 1 L / min, and the initial toluene concentration is set at 1000 ppm, the degradation efficiency and CO selectivity of the plasma catalytic system are shown in Table 2.

[0055] Table 2 Toluene conversion rates and H2 and CO selectivities corresponding to Ni / porous carbon catalysts prepared with different Ni loadings

[0056] Figure 2SEM image of the Ni / porous carbon composite catalyst prepared in Example 1. After loading Ni, the 15% Ni / N-C particles retain the cubic structure of the rhombic dodecahedron of ZIF-8, and granular substances are formed on the surface, indicating the loading of Ni particles.

[0057] Figure 3 XPS characterization result diagram of the Ni / porous carbon composite catalyst prepared in Example 1, indicating that the configurations of N in the carbon material framework are divided into graphitic N, pyrrolic N, pyridinic N, and oxidized N, and Ni exists in the catalyst in the form of Ni 0 and Ni 2+ .

[0058] Figure 4 XRD results of the Ni / porous carbon composite catalyst prepared in Example 1. The Ni / porous carbon composite catalyst obtained after calcination treatment exhibits two broad diffraction peaks at about 2θ = 25° and 44°, corresponding to the (002) and (101) planes of graphitic carbon (JCPDS 99-0057), respectively, showing the main characteristics of amorphous carbon.

[0059] Figure 5 Pore characteristics of the Ni / porous carbon composite catalyst prepared in Example 1. As can be seen from the figure, the Ni / porous carbon catalyst is mainly composed of micropores and mesopores, and there is also a small amount of macroporous porous structure, which can provide more active sites for the reforming reaction.

[0060] As can be seen from Table 2, when the Ni loading is 15%, the catalyst has the best performance. This is because when the loading is 15%, the specific surface area and average pore diameter of the catalyst are the largest. The high specific surface area and rich pore structure are beneficial to the mass transfer during the reaction process and can provide more active sites for the reforming reaction.

[0061] Furthermore, to study the performance of the catalytic material prepared in the present invention during use, taking the Ni / porous carbon catalyst prepared with a Ni loading of 15% in Example 1 as an example, its plasma catalytic performance at different discharge powers was investigated, as shown in Comparative Examples 1-2.

[0062] Comparative Example 1 The catalyst was prepared according to the steps and conditions of Example 1. Different from the test process in Table 1, the discharge power was 60 W.

[0063] Comparative Example 2 The catalyst was prepared according to the steps and conditions of Example 1. Different from the test process in Table 1, the discharge power was 90 W.

[0064] When the ratio of zinc acetate dihydrate to polyvinylpyrrolidone is 1:1, the solvent for ZIF-8 is methanol, the nickel precursor is nickel nitrate, the Ni loading is 15%, that is, the molar ratio of nickel atoms to zinc atoms is 1.5:10, the total flow rate is 1 L / min, and the initial toluene concentration is set at 1000 ppm, the conversion efficiency and CO selectivity of the plasma catalytic system are shown in Table 3.

[0065] Table 3 Toluene conversion and H2 and CO selectivity corresponding to the Ni / porous carbon catalyst at different discharge powers

[0066] As can be seen from Table 3, the experimental results show that the toluene conversion, H2, and CO selectivity under the condition of 90 W power are slightly higher than those under 75 W. This is due to the synergistic effect of increased plasma density, improved activation degree, increased radical generation, enhanced thermal effect, and control of side reactions. However, the energy efficiency under the condition of 90 W is 2.66 g / kWh, and the energy efficiency under the condition of 75 W is 3.2 g / kWh. From the perspective of industrial comprehensive application, the condition of 75 W is more suitable.

[0067] Furthermore, taking the discharge power of 75 W as an example, the reaction performances of three systems, namely the single plasma system, the single catalytic system, and the plasma catalytic system, are listed, as shown in Comparative Examples 3 to 4.

[0068] Comparative Example 3 Compared with the test conditions in Table 1 of Example 1, in this comparative example, only the Ni / porous carbon composite catalyst prepared in Example 1 was used during the test, and no plasma was used for toluene reforming.

[0069] Comparative Example 4 Compared with the test conditions in Table 1 of Example 1, in this comparative example, only plasma was used during the test, and no catalyst was used for toluene reforming.

[0070] When the ratio of zinc acetate dihydrate to polyvinylpyrrolidone is 1:1, the solvent for ZIF-8 is methanol, the nickel precursor is nickel nitrate, the Ni loading is 15%, the discharge power is 75 W, the total flow rate is 1 L / min, and the initial toluene concentration is set at 1000 ppm, the conversion efficiency and CO selectivity of different test systems are shown in Table 4.

[0071] Table 4 Toluene conversion and H2 and CO selectivity corresponding to different systems

[0072] Figure 6Figure showing the reforming stability of the Ni / porous carbon composite catalyst prepared in Example 1 during the reaction time. It shows that as the reaction time extends, compared with the initial activity, there is a slight loss in the toluene conversion rate, H2 and CO selectivity. Among them, when the reaction proceeds for 30 h, the toluene conversion rate shows a downward trend, and the final toluene conversion rate is 93.44%, only 4.96% lower than the initial toluene conversion rate. For the main products H2 and CO, as the reforming reaction continues, their selectivities decrease from the initial 45.25% and 35.3% to 40.8% and 31.1% respectively. This is because as the reaction proceeds, carbon deposition gradually forms on the catalyst surface, resulting in a decrease in the catalyst activity. Nevertheless, the 15% Ni / porous carbon catalyst still maintains relatively high catalytic activity and stability, and has good reforming performance at around 30 h of testing.

[0073] As can be seen from Table 4, the experimental results show that the toluene conversion rate and CO selectivity in the plasma-catalytic system are much greater than the sum of the single catalyst and single plasma, demonstrating the good synergistic effect between the plasma and the catalyst.

[0074] In summary, in the present invention, the highest toluene conversion rate is 98.94%, the H2 selectivity is 50.3%, the CO selectivity is 43.3%, the nickel loading is 15%, and the reforming effect is the best when the power is 75 W, indicating that the low-temperature plasma combined with the Ni / porous carbon catalyst has great application prospects in toluene reforming.

[0075] Example 6 A preparation method of a Ni / porous carbon composite catalyst, comprising the following steps: S1: Dissolve zinc acetate, polyvinyl alcohol, and nickel nitrate in methanol, and ultrasonically treat to completely dissolve to obtain solution A; then add the methanol solution of 2-methylimidazole to the solution A, ultrasonically oscillate and react for 20 min, then place it at 4 °C and cool for 1 h, and then age the reaction solution at room temperature for 24 h, centrifuge (7000 r / min, 3 min) and wash several times with methanol, and dry overnight at 60 °C to obtain the Ni / ZIF-8 solid product; wherein, the molar ratio of nickel atoms to zinc atoms is 1:10; the dosage of the template agent accounts for 5% of the total mass of zinc acetate, the template agent, nickel nitrate, and 2-methylimidazole.

[0076] S2: Add the Ni / ZIF-8 to the pyridine solution, mix evenly, and carry out hydrothermal reaction at 100 °C for 2 h. Subsequently, centrifuge, wash with methanol, and dry to obtain a light yellow solid product. After drying and grinding the obtained light yellow solid product, raise the temperature to 800 °C at a heating rate of 5 °C / min for calcination treatment, and the calcination treatment time is 5 h. After calcination, product B is prepared; S3: Place the product B in a 0.1 mol / L acid solution, soak and react at 60 °C for 5 h. Finally, wash the obtained product repeatedly with deionized water and dry it to obtain the Ni / porous carbon composite catalyst. The pore volume of this composite catalyst is 0.4 cm 3 / g, and the average pore diameter is 6.3 nm.

[0077] Example 7 A preparation method of a Ni / porous carbon composite catalyst, comprising the following steps: S1: Dissolve zinc acetate, polyacrylamide, and nickel nitrate in methanol, and ultrasonically treat to completely dissolve them to obtain solution A; then add a methanol solution of 2-methylimidazole to solution A, ultrasonically oscillate and react for 30 min, then place it at 4 °C and cool for 1 h, and then age the reaction solution at room temperature for 24 h. Centrifuge (7000 r / min, 3 min) and wash with methanol several times, and dry overnight at 120 °C to obtain a Ni / ZIF-8 solid product; wherein, the molar ratio of nickel atoms to zinc atoms is 5:10; the dosage of the template agent accounts for 10% of the total mass of zinc acetate, the template agent, nickel nitrate, and 2-methylimidazole.

[0078] S2: Add the Ni / ZIF-8 to a benzimidazole solution, mix evenly, and carry out a hydrothermal reaction at 120 °C for 3 h. Subsequently, centrifuge, wash with methanol, and dry to obtain a light yellow solid product. After drying and grinding the obtained light yellow solid product, raise the temperature to 900 °C at a heating rate of 5 °C / min for calcination treatment. The calcination treatment time is 3 h. After calcination, product B is obtained; S3: Place the product B in a 1 mol / L acid solution, soak and react at 100 °C for 3 h. Finally, wash the obtained product repeatedly with deionized water and dry it to obtain the Ni / porous carbon composite catalyst. The pore volume of this composite catalyst is 0.25 cm 3 / g, and the average pore diameter is 7.04 nm.

[0079] Example 8 A preparation method of a Ni / porous carbon composite catalyst, comprising the following steps: S1: Dissolve zinc acetate, polystyrene and nickel nitrate in methanol, and ultrasonically treat to completely dissolve them to obtain solution A; then add the methanol solution of 2-methylimidazole to solution A, ultrasonically oscillate and react for 25 min, then place it at 4 °C and cool for 1 h, and then age the reaction solution at room temperature for 24 h. Centrifuge (7000 r / min, 3 min) and wash several times with methanol, and dry overnight at 100 °C to obtain the Ni / ZIF-8 solid product; among them, the molar ratio of nickel atoms to zinc atoms is 1.5:10; the dosage of the template agent accounts for 8% of the total mass of zinc acetate, the template agent, nickel nitrate and 2-methylimidazole.

[0080] S2: Add the Ni / ZIF-8 to the urea solution, mix evenly and carry out hydrothermal reaction at 110 °C for 2.5 h. Subsequently, centrifuge, wash with methanol and dry to obtain a light yellow solid product. After drying and grinding the obtained light yellow solid product, raise the temperature to 850 °C at a heating rate of 5 °C / min for calcination treatment. The calcination treatment time is 3.5 h. After calcination, product B is prepared; S3: Immerse the product B in a 0.5 mol / L acid solution and soak and react at 80 °C for 4 h. Finally, wash the obtained product repeatedly with deionized water and dry to prepare the Ni / porous carbon composite catalyst. The pore volume of this composite catalyst is 0.32 cm 3 / g, and the average pore diameter is 6.9 nm.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A preparation method of a Ni / porous carbon composite catalyst, characterized in that, It includes the following steps: S1: Dissolve zinc acetate, a templating agent, and nickel nitrate in methanol, and subject it to ultrasonic treatment to completely dissolve it to obtain solution A; then add a methanol solution of 2-methylimidazole to the solution A, perform ultrasonic oscillation reaction, followed by aging, centrifugation, and drying overnight to prepare Ni / ZIF-8; S2: Add the Ni / ZIF-8 to an organic ligand solution, mix evenly and then carry out a hydrothermal reaction. Subsequently, after centrifuging and drying the obtained solid product, perform a calcination treatment in a nitrogen atmosphere to obtain product B; S3: Immerse the product B in an acid solution for soaking treatment and then wash it with deionized water to prepare the Ni / porous carbon composite catalyst.

2. The preparation method of a Ni / porous carbon composite catalyst according to claim 1, characterized in that, In step S1, the molar ratio of nickel atoms to zinc atoms is (1~5):10; the dosage of the templating agent accounts for 5%~10% of the total mass of zinc acetate, the templating agent, nickel nitrate, and 2-methylimidazole.

3. The preparation method of a Ni / porous carbon composite catalyst according to claim 1, characterized in that, In step S1, the templating agent is at least one of polyvinylpyrrolidone, polyvinyl alcohol, polyacrylamide, and polystyrene.

4. The preparation method of a Ni / porous carbon composite catalyst according to claim 1, characterized in that, In step S1, the time of the ultrasonic oscillation reaction is 20~30 min.

5. The preparation method of a Ni / porous carbon composite catalyst according to claim 1, characterized in that, In step S2, the temperature of the hydrothermal reaction is 100~120 °C, and the time is 2~3 h.

6. The preparation method of a Ni / porous carbon composite catalyst according to claim 1, wherein In step S2, during the calcination treatment, the heating rate is 3-6 °C / min, the calcination temperature is 800~900 °C, and the calcination time is 3~5 h.

7. The preparation method of a Ni / porous carbon composite catalyst according to claim 1, characterized in that, In step S3, the acid solution is an HCl solution or an HNO3 solution; the concentration of the acid solution is 0.1~1 mol / L.

8. The preparation method of a Ni / porous carbon composite catalyst according to claim 1, characterized in that, In step S3, the time of the soaking treatment is 3~5 h.

9. A Ni / porous carbon composite catalyst, characterized in that, It is prepared by the method according to any one of claims 1~8.

10. Use of a Ni / porous carbon composite catalyst as described in claim 9 in the field of tar reforming, characterized in that, Place the Ni / porous carbon composite catalyst in a dielectric barrier discharge plasma reactor, and realize the reforming reaction with toluene as the tar model compound through plasma discharge. During the plasma discharge process, the discharge power is 30~90 W; in the reforming reaction, the toluene conversion rate is 87%~99%, the H2 selectivity is 23%~50%, and the CO selectivity is 21%~43%.

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