Preparation method of radio frequency plasma modified NiMgAl catalyst for dry reforming of methane to syngas and application thereof

Abstract

This invention discloses a method for preparing a radio frequency plasma-modified NiMgAl catalyst for methane dry reforming to syngas and its application, belonging to the fields of energy chemical engineering and environmental catalysis. First, 0.523 g of Ni(NO3)2·6H2O, 3.23 g of Mg(NO3)2·6H2O, and 1.35 g of Al(NO3)3·9H2O are weighed and dissolved in a mixed solution of ethanol and deionized water. After thorough stirring, appropriate amounts of urea and citric acid are added. After stirring at room temperature for 1 hour, the resulting solution is transferred to a high-pressure hydrothermal reactor and subjected to hydrothermal treatment at 160°C for 48 hours. The resulting powder is washed three times with ethanol and three times with deionized water, and then dried in an oven for 12 hours. After drying, the powder sample is placed in a radio frequency discharge plasma surface treatment instrument under an Ar atmosphere at a power of 200W for 30 minutes to obtain the catalyst. The use of radio frequency plasma to construct surface oxygen vacancy defects in the catalyst offers advantages such as simple process, ease of operation, and low cost. It exhibits good catalytic stability and strong resistance to carbon deposition in methane dry reforming reactions. It can also be used in other reactions such as CO2 methanation, making it suitable for industrial production.

Description

Technical Field

[0001] This invention relates to a method for preparing radio frequency plasma modified NiMgAl catalyst for dry reforming of methane to syngas and its application. Specifically, it is a catalyst for converting two major greenhouse gases, CO2 and CH4, into syngas (CO and H2), belonging to the fields of energy chemical engineering and environmental catalysis. Background Technology

[0002] Implementing comprehensive dual control over total carbon emissions and intensity, and accelerating the transformation of the energy system towards "new and green" energy, are major national strategic requirements. Natural gas, as a clean and low-carbon fossil fuel, is not only the "main force" in replacing traditional high-carbon energy sources, but also a "key force" supporting the large-scale development of renewable energy. CH4, as the core component of natural gas, is being converted into high-value-added chemicals and liquid fuels. This is not only an important way to achieve the clean and efficient utilization of natural gas resources, but also a concrete practice of promoting the clean and efficient utilization of fossil energy and vigorously developing green and low-carbon technologies. How to efficiently utilize CO2 to produce high-value-added products has become a cutting-edge focus in the energy and environment fields.

[0003] CH4 dry reforming (DRM) is considered a preferred technology with the potential for large-scale CO2 conversion due to its combined value in resource utilization and emission reduction. This reaction converts both into syngas (H2 and CO) with a 1:1 molar ratio of hydrogen to carbon monoxide. This syngas can be used in Fischer-Tropsch synthesis to produce cleaner liquid fuels and other high-value-added chemical products.

[0004] Because CH4 and CO2 molecules have stable electronic structures, they are difficult to activate under mild conditions. Therefore, the DRM reaction requires a large amount of energy at high temperatures to drive the reaction, which inevitably leads to catalyst sintering and carbon deposition, resulting in catalyst deactivation. Numerous studies have confirmed that the aforementioned sintering and carbon deposition problems are not only a core bottleneck restricting the industrial application of DRM technology, but also the dominant cause of catalyst deactivation, ultimately leading to a significant decline in reactant conversion efficiency and target product selectivity. Therefore, developing catalytic materials with strong anti-carbon deposition capabilities, anti-metal sintering properties, and good thermal stability is currently a key challenge.

[0005] Nickel-containing hydrotalcite (chemical formula [M]) 2+ 1-x M 3+ x (OH)2][(A n x / nLDHs (Laminated Dihydrogen Hydroxide) are catalytic materials derived from layered double hydroxide (LDH) catalysts, which have shown promising application prospects in the field of dry methane reforming (DRM) in recent years. (Recent advances in layered double hydroxide (LDH)-based materials: fabrication, modification strategies, characterization, promising environmental catalytic applications, and prospective aspects. Energy Advances, 2024 3(9): 2136-2151. Bu et al. (Methane dry reforming over boron nitride interface-confined and LDHs-derived Ni catalysts. Applied Catalysis B: Environmental, 2019 252: ) (86-97) A boron nitride interface-constrained layered double hydroxide (LDH) derived Ni catalyst (NiMA-BN-MR) was designed for methane dry reforming. The interface constraint between the (Ni,Mg)Al2O4 sheets derived from h-BN and LDHs, and the strong metal support interaction, are the reasons for the good dispersion of Ni nanoparticles. However, the preparation of core-shell structured catalysts is often complex and costly.

[0006] The design concept of this patent is to prepare NiMgAl-LDHs catalysts with highly dispersed active components, small particle size, and more oxygen vacancies. Firstly, NiMgAl-LDHs catalysts were prepared using a hydrothermal method. According to literature reports, catalysts prepared based on NiMgAl-LDHs can effectively reduce the size of Ni particles after the reaction, improve the acidity and basicity of the catalyst surface, and enhance the interaction between the active metal and the support oxide. However, contrary to expectations, the calcined and reduced catalyst exhibited severe active metal sintering and significant carbon deposition during the reaction. Therefore, we conducted extensive research. The stability of the catalyst is closely related to the interaction force between the active component and the support; the stronger the interaction force, the higher the catalyst stability. Significantly improving the interaction force between the active component and the support, while exposing more active sites on the catalyst, especially more oxygen vacancies on the catalyst surface, would significantly improve the activity and stability of the catalyst. Therefore, we boldly proposed introducing low-temperature plasma to bombard the catalyst surface with high-energy electrons, constructing numerous defects on the catalyst surface, exposing more oxygen vacancies, forming more CO2 adsorption active sites, and accelerating the carbon removal reaction during the catalyst reaction. Therefore, we conducted research on the application of radio frequency cold plasma technology in the enhanced preparation of reforming catalysts. Extensive exploratory work was carried out, optimizing the plasma treatment atmosphere, time, and power to obtain the optimally plasma-modified NiMgAl-LDHs catalyst. Experimental results show that the PAr-200w-30min catalyst synthesized using the optimal process parameters exhibits excellent stability and anti-sintering properties, with minimal carbon deposition after the reaction, demonstrating promising application prospects. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing a radio frequency plasma-modified NiMgAl catalyst for dry reforming of methane to syngas and its application. This method has the advantages of simple process, ease of operation, and low cost. Using NiMgAl-LDHs as a precursor, abundant oxygen vacancy defects are constructed on the catalyst surface through plasma treatment. The catalyst prepared by this method significantly improves the long-term stability and anti-coking performance.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a catalyst for dry reforming methane to produce syngas includes the following steps: (1) Weigh out the active components Ni(NO3)2·6H2O, Mg(NO3)2·6H2O, and Al(NO3)3·9H2O in the specified proportions. The molar percentage of the active metal Ni is 10%, and n(Ni 2+ +Mg 2+ ):n(Al 3+Add 4 to 60 ml of a 1:1 mixture of ethanol and water, stir until completely dissolved, and obtain a green transparent solution. (2) Add the measured amounts of urea and citric acid sequentially to the green transparent solution obtained in step (1); (3) Stir the mixed slurry obtained in step (2) at room temperature for 1 hour; (4) Transfer the green transparent solution obtained in step (3) to a 100 ml polyethylene hydrothermal reactor, and heat at 160 °C. o C hydrothermal 48h; (5) Wash the powder obtained in step (4) three times with ethanol and deionized water respectively; (6) Place the powder obtained in step (5) in an oven and dry it for 12 hours; (7) The powder obtained in step (6) is either placed in a muffle furnace and roasted, then naturally cooled to room temperature, or placed in a radio frequency plasma device for processing, pressed into tablets, and ground into a catalyst of 20-40 mesh to obtain the methane dry reforming syngas catalyst.

[0009] According to the above scheme, the processing method is radio frequency (RF) plasma.

[0010] According to the above scheme, the molar percentage content of the active metal Ni is 10%, and n(Ni) 2+ +Mg 2+ ):n(Al 3+ =4.

[0011] According to the above scheme, the molar ratio of urea and citric acid to the active metal is 1:0.06:0.3.

[0012] According to the above scheme, the plasma discharge atmosphere is Ar gas, the discharge power is 200W, and the discharge time is 30min.

[0013] According to the above scheme, the hydrothermal temperature is 160°C. o C, hydrothermal time 48h.

[0014] According to the above scheme, the drying temperature is 110℃; the drying time is 12h.

[0015] The present invention also provides the application of the methane dry reforming syngas catalyst prepared by any of the above technical solutions in carbon dioxide conversion.

[0016] Compared with the prior art, the beneficial technical effects of the technical solution of this invention application are as follows: 1. The radio frequency plasma-modified NiMgAl-LDHs catalyst of this invention is used in the CH4 / CO2 reforming reaction. Its catalytic stability and anti-coking performance are significantly improved compared to the catalyst without cold plasma treatment. After 20 hours of reaction, the CH4 conversion rate decreased by only 1.6%, and the coking amount after the reaction was only 3.6 wt%, while the calcined catalyst showed a 5.8% decrease in CH4 conversion and a coking amount reaching 24.9 wt%.

[0017] 2. A method for constructing surface oxygen vacancy defects in catalysts using radio frequency plasma is employed. This method is simple, easy to operate, and low in cost. It exhibits good catalytic stability and strong resistance to carbon deposition in methane dry reforming reactions, and can also be used in reactions such as CO2 methanation, making it suitable for industrial production.

[0018] 3. Modification of NiMgAl-LDHs catalyst using radio frequency plasma Figure 1 The graphs show the catalytic performance test results of the catalysts prepared in Examples 1, 2, 3, 4 and 5.

[0019] Figure 2 The graphs show the catalytic performance test results of the catalysts prepared in Examples 1, 6, 7 and 8.

[0020] Figure 3 The graphs show the catalytic performance test results of the catalysts prepared in Examples 6, 9 and 10.

[0021] Figure 4 The graphs show the catalytic performance test results of the catalysts prepared in Examples 6 and 11.

[0022] Figure 5 The EPR spectra of the catalysts prepared in Examples 6 and 11 are shown.

[0023] Figure 6 XPS spectra of the catalysts prepared in Examples 6 and 11.

[0024] Figure 8 The XRD patterns are those of the catalysts prepared in Examples 6 and 11. Detailed Implementation

[0025] The present invention will be further illustrated by some implementation examples below, but these examples do not limit the scope of the invention.

[0026] Example 1 The steps for preparing the catalyst for dry reforming of methane are as follows: 0.523 g of Ni(NO3)2·6H2O, 3.23 g of Mg(NO3)2·6H2O, and 1.35 g of Al(NO3)3·9H2O were weighed and dissolved in a mixed solution of ethanol and deionized water. After thorough stirring, appropriate amounts of urea and citric acid were added. After stirring at room temperature for 1 h, the resulting solution was transferred to a high-pressure hydrothermal reactor and subjected to hydrothermal treatment at 160°C for 48 h. The resulting powder was washed three times each with ethanol and deionized water, and then dried in an oven for 12 h. The dried powder sample was then placed in an RF discharge plasma surface treatment instrument under an Ar atmosphere at a power of 200 W for 1 h to prepare the PAr-200 W-60 min catalyst.

[0027] Catalyst evaluation The stability of the catalyst was evaluated in a self-made continuous flow fixed-bed reactor. The reaction tube was a quartz tube with an inner diameter of 6 mm and a length of 33 cm. The reaction temperature was measured by a thermocouple placed in the middle of the reaction tube and controlled by a programmed temperature controller. The gas flow rate was controlled by a mass flow meter, and the space velocity was 36000 ml / (g). cat The reaction feed gases were 24.2 ml / min CH4 and 26.4 ml / min CO2. The catalyst dosage was 0.1 g, the reaction temperature was 700℃, and samples were taken every hour. The gas composition of the reactants and products was analyzed using a gas chromatograph (SC8000) to calculate the conversion rates of CH4 and CO2. The conversion curves of the catalyst to CH4 and CO2 at different time points are shown in the figure. Figure 1 As shown in Table 2, the inactivation rate of CH4 is as follows.

[0028] Examples 2-5 Compared with Example 1, only the discharge atmosphere during the preparation process was different; all other processes were the same as in Example 2, and the finished catalysts were obtained. The catalyst preparation conditions for Examples 2 to 5 are shown in Table 1.

[0029] Catalyst evaluation Following the evaluation method of Implementation Case 1, the conversion curves of the catalyst for CH4 and CO2 at different time points are shown below. Figure 1 As shown in Table 2, the inactivation rate of CH4 is as follows.

[0030] Example 6 Compared with Example 1, the difference lies in the plasma treatment time; the other processes are the same as in Example 1. After drying, the powder sample is placed in an RF discharge plasma surface treatment instrument under an Ar atmosphere, with a power of 200W, and discharged for 30 minutes to obtain the PAr-200W-30min catalyst. The catalyst composition of Example 5 is shown in Table 1.

[0031] Catalyst evaluation Following the evaluation method of Implementation Case 1, the conversion curves of the catalyst for CH4 and CO2 at different time points are shown below. Figure 2 As shown in Table 2, the inactivation rate of CH4 is as follows.

[0032] Examples 7-8 Compared with Example 6, the difference lies in the plasma treatment time; the other processes are the same as in Example 5, yielding the finished catalysts. The catalyst compositions of Examples 6 to 8 are shown in Table 1.

[0033] Catalyst evaluation Following the evaluation method of Implementation Case 1, the conversion curves of the catalyst for CH4 and CO2 at different time points are shown below. Figure 2 As shown in Table 2, the inactivation rate of CH4 is as follows.

[0034] Examples 9-10 Compared to Example 6, the difference lies in the plasma treatment power; the other processes are the same as in Example 1, yielding the finished catalysts. The catalyst compositions of Examples 9 to 10 are shown in Table 1.

[0035] Catalyst evaluation Following the evaluation method of Implementation Case 1, the conversion curves of the catalyst for CH4 and CO2 at different time points are shown below. Figure 3 As shown in Table 2, the inactivation rate of CH4 is as follows.

[0036] Example 11 Compared to Example 6, the difference is that air calcination was used; the other processes were the same as in Example 1, yielding the finished catalyst. The catalyst composition of Example 12 is shown in Table 1.

[0037] Catalyst evaluation Following the evaluation method of Implementation Case 1, the conversion curves of the catalyst for CH4 and CO2 at different time points are shown below. Figure 4 As shown in Table 2, the inactivation rate of CH4 is as follows.

[0038] Catalyst composition table for examples: Specific embodiments of radio frequency plasma activation preparation are shown in Table 1: The specific CH4 deactivation rate and carbon deposition after the catalyst reaction are shown in Table 2: EPR characterization To characterize the oxygen vacancy defects on the catalyst surface, such as Figure 5As shown, the signal observed at g=2.005 is generally attributed to unpaired electrons trapped by oxygen vacancies. Notably, the higher intensity peak of PAr-200w-30min indicates that O atoms escape from the unit cell after Ar-cooled plasma bombardment, resulting in a certain concentration of oxygen vacancies. In contrast, the calcined LDH-C catalyst exhibits a smaller intensity peak, suggesting a lower oxygen vacancy content on the catalyst surface. This demonstrates that we have successfully generated abundant oxygen vacancy defects on the catalyst surface using plasma treatment.

[0039] XPS characterization Figure 6 The O 1s spectrum is shown, with peaks at 533.1, 531.9, 531.1, and 529.6 eV, named Oads, Ov, and O. OH and O L These correspond to adsorbed oxygen, defect sites with low oxygen coordination, oxygen atoms bound to hydroxyl species, and metal-oxygen bonds on the surface. The Ov content of PAr-200w-30min is higher than that of LDH-C, indicating that the PAr-200w-30min catalyst has abundant oxygen vacancies. This further illustrates that we have successfully generated abundant oxygen vacancy defects on the catalyst surface using Ar gas plasma treatment, which can enhance the capture of oxygen-related intermediates (formates).

[0040] XRD characterization Further revealing the structural changes of the catalyst after stability testing, Figure 7 The XRD patterns of the catalysts after the reaction are shown. The calcined LDH-C catalyst exhibited a distinct graphite characteristic diffraction peak at 2θ = 26.6°, while no identifiable graphitic carbon diffraction peak was detected in the PAr-200w-30min catalyst, indicating that a large amount of carbon was deposited on the LDH-C surface during the reaction. In contrast, the Ni diffraction peak intensity of LDH-C was stronger than that of JCPDS No. 04-0850, indicating a significant increase in Ni particle size during the reaction, while PAr-200w-30min maintained a lower Ni diffraction peak intensity. This demonstrates that Ar plasma treatment can significantly improve the catalyst's resistance to carbon deposition and sintering.

[0041] TG characterization Figure 8The data shows the weight loss of the catalyst after the reaction. The mass loss in the 30-150 °C range is attributed to the removal of adsorbed water from the catalyst surface; the mass loss in the 150-450 °C range is due to the oxidative combustion of surface-bound active carbon and intermediate carbon species; and the mass loss in the 450-700 °C range is caused by the combustion of graphite carbon in the sample. Comparing the LDH-C and PAr-200w-30min samples, the PAr-200w-30min sample showed the lowest carbon deposition at only 3.9 wt%, while the LDH-C catalyst had a higher carbon deposition at 22.6 wt%. This indicates that Ar plasma treatment is beneficial for improving the catalyst's resistance to carbon deposition.

Claims

1. A method for preparing radio frequency plasma-modified NiMgAl catalyst for dry reforming of methane to syngas and its application, characterized in that, The method includes the following steps: (1) Weigh out the active components Ni(NO3)2·6H2O, the support Mg(NO3)2·6H2O, and Al(NO3)3·9H2O in the specified proportions. n(Ni 2+ +Mg 2+ ):n(Al 3+ Add 4 to 60 ml of a 1:1 mixture of ethanol and water, stir until completely dissolved, and obtain a green transparent solution. (2) Add the measured amounts of urea and citric acid sequentially to the green transparent solution obtained in step (1); (3) Stir the mixed slurry obtained in step (2) at room temperature for 1 hour; (4) Transfer the green transparent solution obtained in step (3) to a 100 ml polyethylene hydrothermal reactor, and heat at 160 °C. o C hydrothermal 48h; (5) Wash the powder obtained in step (4) three times with ethanol and deionized water respectively; (6) Place the powder obtained in step (5) in an oven and dry it for 12 hours; (7) The powder obtained in step (6) is either placed in a muffle furnace and roasted, then naturally cooled to room temperature, or placed in a radio frequency plasma device for processing, pressed into tablets, and ground into a catalyst of 20-40 mesh to obtain the methane dry reforming syngas catalyst.

2. The method for preparing radio frequency plasma-modified NiMgAl-LDHs for methane dry reforming to syngas according to claim 1, characterized in that, The process described is radio frequency (RF) plasma.

3. The radio frequency plasma-modified NiMgAl-LDHs for methane dry reforming to syngas according to claims 1-2, characterized in that, The molar percentage of active metal Ni is 10%, and n(Ni) 2+ +Mg 2+ ):n(Al 3+ =4.

4. The radio frequency plasma-modified NiMgAl-LDHs for methane dry reforming to syngas according to any one of claims 1-3, characterized in that, The molar ratio of urea and citric acid to the active metal is 1:0.06:0.

3.

5. The method for preparing radio frequency plasma-modified NiMgAl-LDHs for methane dry reforming to syngas according to any one of claims 1-4, characterized in that, The hydrothermal temperature is 160°C. o C, hydrothermal time 48h.

6. The method for preparing radio frequency plasma-modified NiMgAl-LDHs for methane dry reforming to syngas according to any one of claims 1-5, characterized in that; The drying temperature is 110℃; the drying time is 12h.

7. The method for preparing radio frequency plasma-modified NiMgAl-LDHs for methane dry reforming to syngas according to any one of claims 1-6, characterized in that, The plasma discharge atmosphere was Ar, the discharge power was 200W, and the discharge time was 30min.

8. The application of the catalyst for dry reforming methane to syngas according to any one of claims 1-7 in carbon dioxide conversion.

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