Carbon dioxide low-temperature methanation catalyst, its preparation method and application

Abstract

This invention relates to the field of carbon dioxide methanation catalysis technology, and discloses a low-temperature carbon dioxide methanation catalyst, its preparation method, and its applications. The catalyst comprises an active component, a support, and an auxiliary agent; the active component is at least one selected from nickel, ruthenium, and rhodium; the support is nitrogen-doped carbon nanotubes; and the auxiliary agent includes at least one selected from transition metal elements and / or rare earth metal elements. The methanation catalyst of this invention exhibits excellent methanation catalytic activity at low temperatures and good stability at high temperatures, producing highly selective methane. The catalyst preparation method is simple and has promising prospects for industrial application.

Description

Technical Field

[0001] This invention relates to the field of carbon dioxide methanation catalysis technology, and particularly to a low-temperature carbon dioxide methanation catalyst, its preparation method, and its application. Background Technology

[0002] The massive combustion of fossil fuels has led to a sharp increase in carbon dioxide emissions into the atmosphere, exacerbating the greenhouse effect and having a significant impact on the global climate. Reducing carbon emissions and developing efficient carbon dioxide capture, utilization, and storage (CCUS) technologies are of great importance. Among these technologies, converting carbon dioxide into high-value-added clean fuel methane can both realize the resource utilization of carbon dioxide and help reduce a country's dependence on imported natural gas.

[0003] The methanation of carbon dioxide is a strongly exothermic reaction (CO2 + H2 → CH4 + 2H2O, ΔH). 298K From a thermodynamic perspective, a lower reaction temperature (-165 kJ / mol) favors the forward reaction; however, from a kinetic perspective, CO2 molecules have high chemical stability, and the kinetic barrier for their conversion to CH4 is high. Therefore, improving the low-temperature activity of the catalyst is a crucial consideration in the design of methanation catalysts. Currently, catalysts for CO2 methanation reactions are typically supported catalysts, generally composed of an active component, a support, and promoters. Commonly used active components mainly include noble metals (Ru, Rh, Pd, Pt, etc.) and non-noble metals (Fe, Co, Ni, etc.). It is well known that Ru-based catalysts exhibit the highest activity in methanation reactions, but the scarcity and high cost of Ru resources limit its industrial application. Ni-based catalysts, with their advantages of low cost, high catalytic activity, and high methane selectivity, have become a research focus in methanation catalysts in recent years. However, Ni-based catalysts are prone to sintering during the reaction, and carbon deposits easily form at high temperatures, resulting in unstable long-term operation, low low-temperature catalytic activity, and poor high-temperature stability. The support has a significant impact on the performance of Ni-based catalysts. A suitable support helps to disperse the active components and form good metal-support interactions, thereby improving the performance of the catalyst in the methanation reaction. Commonly used supports include Al2O3, TiO2, and ZrO2. However, many supports will rapidly accumulate heat during the methanation process, leading to rapid sintering and deactivation of the catalyst.

[0004] Given the complexity and difficulty in controlling the methanation reaction process, existing catalysts still face problems such as low activity at low temperatures, poor stability at high temperatures, low carbon dioxide conversion rate, and low methane selectivity, making it difficult to meet the needs of actual production. Summary of the Invention

[0005] This invention addresses the technical problems of existing methanation catalysts, such as easy sintering, easy carbon deposition at high temperatures, low low-temperature activity, poor high-temperature stability, low carbon dioxide conversion rate, and low methane selectivity. This invention provides a low-temperature carbon dioxide methanation catalyst, its preparation method, and its applications. The methanation catalyst of this invention has advantages such as high low-temperature activity, good high-temperature stability, high carbon dioxide conversion rate, and high methane selectivity.

[0006] One objective of this invention is to provide a low-temperature methanation catalyst for carbon dioxide, the catalyst comprising an active component, a support, and an auxiliary agent; the active component is at least one of nickel, ruthenium, and rhodium, the support is nitrogen-doped carbon nanotubes, and the auxiliary agent comprises at least one of transition metal elements and / or rare earth metal elements. The metal selected for the active component is different from the metal selected for the auxiliary agent.

[0007] This invention uses only nitrogen-doped carbon nanotubes as a support, avoiding the use of supports such as Al2O3, TiO2, and ZrO2. Nitrogen-doped carbon nanotubes effectively prevent rapid heat accumulation during methanation, which can lead to rapid catalyst sintering and deactivation. Nitrogen-doped carbon nanotubes possess extremely high thermal conductivity and superior mechanical properties, facilitating the good dispersion and removal of reaction heat during the reaction. The catalyst prepared by this invention is less prone to sintering and deactivation due to localized high temperatures, thus exhibiting excellent stability. Furthermore, the alkaline surface of the nitrogen-doped carbon nanotube support promotes the adsorption of acidic CO2 gas, fostering a local CO2 concentration gradient near the active sites, thereby facilitating the methanation reaction and achieving high methane selectivity.

[0008] The catalyst of this invention uses nitrogen-doped carbon nanotubes as a support. Carbon nanotubes are one-dimensional tubular carbon materials formed by rolling up single or multiple layers of graphene. They have a high specific surface area. After nitrogen doping, the surface of the support provides abundant binding sites for the metal active components, which helps to load and highly disperse the catalyst particles, effectively inhibits catalyst sintering and agglomeration, and improves the catalytic activity of the catalyst.

[0009] According to some embodiments of the present invention, the nitrogen-doped carbon nanotubes are prepared by chemical vapor deposition; and / or, the additive is at least one selected from iron, cobalt, copper, manganese, lanthanum and cerium.

[0010] This invention uses nitrogen-doped carbon nanotubes, which are different from untreated carbon nanotubes. Nitrogen-containing gas is introduced through chemical vapor deposition to achieve in-situ nitrogen doping during the growth of carbon nanotubes. The nitrogen is then used to load active components, which can anchor active metals and enhance the adsorption of CO2 gas. There is no need to introduce other ligands in the preparation of methanation catalysts, and there is no need to artificially introduce oxygen elements on the surface.

[0011] According to some embodiments of the present invention, the nitrogen content of the nitrogen-doped carbon nanotubes is 3-15%, preferably 3-8%. Specifically, the nitrogen content of the nitrogen-doped carbon nanotubes is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%.

[0012] According to some embodiments of the present invention, the content of the active component is 20-50% based on the total weight of the catalyst, the content of the nitrogen-doped carbon nanotubes is 40-79%, and the content of the auxiliary agent is 1-10%.

[0013] Preferably, based on the total weight of the catalyst, the content of the active component is 25-40%, the content of the nitrogen-doped carbon nanotubes is 60-70%, and the content of the auxiliary agent is 2.5-8%.

[0014] More preferably, based on the total weight of the catalyst, the content of the active component is 25-37%, the content of the nitrogen-doped carbon nanotubes is 60-70%, and the content of the auxiliary agent is 2.5-5%.

[0015] In one embodiment, the content of the active component is 20%, 21%, 23%, 25%, 27%, 29%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, or 50%. The content of nitrogen-doped carbon nanotubes is 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, or 70%. The content of the auxiliary agent is 2.5%, 3%, 4%, 5%, 6%, 7%, or 8%.

[0016] According to some embodiments of the present invention, the additive is at least two of iron, cobalt, copper, manganese, lanthanum and cerium, preferably cerium and lanthanum, and more preferably the molar ratio of cerium and lanthanum is 1:5-3:1.

[0017] In one embodiment, the molar ratio of cerium to lanthanum is 1:5, 3:5, 1:1, 2:1, or 3:1.

[0018] The second objective of this invention is to provide a method for preparing a low-temperature carbon dioxide methanation catalyst. The method includes: preparing nitrogen-doped carbon nanotubes by chemical vapor deposition; then loading active components and additives onto the nitrogen-doped carbon nanotubes by an impregnation method. Preferably, the nitrogen-doped carbon nanotubes are added to an impregnation solution of the active component precursor and the additive precursor for impregnation treatment, followed by drying and calcination to obtain the methanation catalyst.

[0019] Preferably, the preparation of nitrogen-doped carbon nanotubes by chemical vapor deposition is carried out under nitrogen-containing conditions, and the catalyst catalyzes the carbon nanotubes to undergo a catalytic reaction at a set temperature; the reactants are cleaned to remove impurities, and nitrogen-doped carbon nanotubes are obtained.

[0020] According to some embodiments of the present invention, the nitrogen-containing conditions are acetylene / argon / ammonia-containing conditions; and / or,

[0021] The catalyst is an iron / alumina catalyst; and / or,

[0022] The catalytic reaction is carried out at a temperature of 700-800℃ for a time of 1.5-2.5 h; and / or,

[0023] The cleaning process comprises alkaline cleaning, acid cleaning, and optionally water cleaning; preferably, the alkaline solution is a sodium hydroxide solution; and / or, the acid solution is a hydrochloric acid solution; and / or,

[0024] The cleaning process is followed by a drying process;

[0025] The impregnation solution is prepared by dissolving the active component precursor and the auxiliary agent precursor in their respective solvents to obtain an active component precursor solution and an auxiliary agent precursor solution, and then mixing the active component precursor solution and the auxiliary agent precursor solution to obtain an impregnation solution containing the active component precursor and the auxiliary agent precursor.

[0026] In one embodiment, the method for preparing the nitrogen-doped carbon nanotubes includes using acetylene / argon / ammonia as the reactant gas and iron / alumina as the catalyst, reacting the carbon nanotubes at 700-800℃ for 1.5-2.5 h; treating the prepared powder with sodium hydroxide solution and hydrochloric acid solution respectively to remove alumina and iron, then washing with deionized water and drying to obtain nitrogen-doped carbon nanotubes; and / or,

[0027] The method for preparing the methanation catalyst includes dissolving the active component precursor and the auxiliary precursor in a solvent, and after complete dissolution, thoroughly mixing the active component precursor solution and the auxiliary precursor solution to obtain an impregnation solution, and then adding nitrogen-doped carbon nanotubes into the impregnation solution for impregnation; removing the impregnated nitrogen-doped carbon nanotubes, and subjecting them to drying and calcination treatment to obtain the methanation catalyst.

[0028] According to some embodiments of the present invention, the carbon nanotubes are at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes; and / or, the morphology of the carbon nanotubes is bamboo-like; and / or, the diameter of the carbon nanotubes is 1–40 nm; preferably 10–30 nm; and / or,

[0029] The active component precursor is selected from at least one water-soluble salt of nickel, ruthenium, and rhodium; preferably, it is selected from at least one nitrate, sulfate, acetate, and chloride salt of nickel, ruthenium, and rhodium; and / or,

[0030] The auxiliary agent precursor is selected from at least one water-soluble salt of iron, cobalt, copper, manganese, lanthanum, and cerium; preferably at least one nitrate of iron, cobalt, copper, manganese, lanthanum, and cerium; and / or,

[0031] The solvent used for the active component precursor and the solvent used for the auxiliary agent precursor are respectively selected from at least one of water, ethanol, methanol, isopropanol, acetone, benzene and toluene; preferably a mixture of water and ethanol, wherein the volume of ethanol accounts for 20-40% of the total volume of the mixture.

[0032] According to some embodiments of the present invention, the impregnation temperature is 30–80°C, and the impregnation time is 1–10 h; preferably, the impregnation temperature is 40–60°C, and the impregnation time is 2–4 h, and / or,

[0033] The drying temperature is 60–150°C, and the drying time is 5–25 h; preferably, the drying temperature is 80–110°C, and the drying time is 10–14 h, and / or,

[0034] The roasting temperature is 200–500°C, and the roasting time is 1–10 h; preferably, under an inert atmosphere, the roasting temperature is 300–400°C, and the roasting time is 4–6 h.

[0035] In one embodiment, the calcination temperature is 300°C, 320°C, 350°C, 380°C, 400°C, 420°C, 450°C, 480°C, or 500°C.

[0036] The third objective of this invention is to provide the application of a low-temperature carbon dioxide methanation catalyst in the carbon dioxide methanation reaction.

[0037] The beneficial effects of this invention are as follows: the methanation catalyst of this invention has excellent methanation catalytic activity at low temperatures and good stability at high temperatures, and the generated methane has high selectivity. The catalyst is simple to prepare and has prospects for industrial application. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments, but this does not constitute any limitation on the present invention.

[0039] In the following embodiments, the nitrogen-doped carbon nanotubes were prepared by chemical vapor deposition: acetylene / argon / ammonia was used as the reaction gas, iron / alumina was used as the catalyst, and the bamboo-shaped carbon nanotubes were reacted at 750°C for 2 hours; the prepared powder was treated with sodium hydroxide and hydrochloric acid solutions to remove alumina and iron, then washed with deionized water and dried to obtain nitrogen-doped carbon nanotubes.

[0040] The specific reaction conditions are as follows: the feed gas composition is 15% CO2, 60% H2 and 25% N2; the pressure is 2.0 MPa; and the space velocity is 6000 h⁻¹. -1 The reaction temperatures were 250, 300, and 350℃, respectively.

[0041] Unless otherwise specified, the nitrogen content of the nitrogen-doped carbon nanotubes used in the examples and comparative examples is 5%.

[0042] Unless otherwise specified, the nitrogen-doped carbon nanotube powder used in the examples and comparative examples has an average diameter of 30 nm.

[0043] In the following examples, due to different degrees of impregnation, the final mass fractions of active components, additives, and supports in the catalyst are based on the characterization results (see Table 1 for details).

[0044]

Example 1

[0045] 1) Dissolve 58.2 g of nickel nitrate hexahydrate powder in 100 mL of 20% ethanol-water solution and stir until completely dissolved to obtain solution A1 with a Ni concentration of 2 mol / L; 2) Dissolve 10.8 g of lanthanum nitrate hexahydrate powder in 50 mL of 20% ethanol-water mixture to obtain solution B1 with a La element concentration of 0.5 mol / L; 3) Dissolve 10.8 g of cerium nitrate hexahydrate powder in 50 mL of 20% ethanol-water mixture to obtain solution B1 with a Ce element concentration of 0. 4) Mix solutions A1, B1 and C1 thoroughly to obtain solution D1; 4) Weigh 11.5g of nitrogen-doped carbon nanotube powder and add it to solution D1 at 40℃ with magnetic stirring. Continue stirring for 80min to obtain mixture E1; 5) Place mixture E1 in a 70℃ oven to dry for 12h, and then place it in a muffle furnace and calcine at 350℃ in a nitrogen atmosphere for 5h to obtain catalyst S1 (composition shown in Table 1 below).

[0046]

Example 2

[0047] 1) Dissolve 58.2g of nickel nitrate hexahydrate powder in 100mL of 20% ethanol-water solution and stir until completely dissolved to obtain solution A1 with a Ni concentration of 2mol / L; 2) Dissolve 21.6g of lanthanum nitrate hexahydrate powder in 50mL of 20% ethanol-water mixture to obtain solution B2 with a La element concentration of 0.5mol / L; 4) Mix solutions A1 and B2 thoroughly to obtain solution C2; 5) Weigh 11.5g of nitrogen-doped carbon nanotube powder and add it to solution C2 under magnetic stirring at 40℃. Continue stirring for 80min to obtain mixture D2; 6) Place mixture D2 in a 70℃ oven to dry for 12h, and then place it in a muffle furnace and calcine at 350℃ in a nitrogen atmosphere for 5h to obtain catalyst S2 (composition shown in Table 1 below).

[0048]

Example 3

[0049] The only difference from Example 2 is that "21.6g of lanthanum nitrate hexahydrate powder" was replaced with "21.6g of cerium nitrate hexahydrate powder", resulting in catalyst S3 (composition shown in Table 1 below).

[0050]

Example 4

[0051] The only difference from Example 2 is that "21.6g of lanthanum nitrate hexahydrate powder" was replaced with "12.1g of ferric nitrate powder" to obtain catalyst S4 (the composition is shown in Table 1 below).

[0052]

Example 5

[0053] The only difference from Example 2 is that "21.6g of lanthanum nitrate hexahydrate powder" was replaced with "14.6g of cobalt nitrate hexahydrate powder" to obtain catalyst S5 (the composition is shown in Table 1 below).

[0054]

Example 6

[0055] The only difference from Example 2 is that "21.6g of lanthanum nitrate hexahydrate powder" was replaced with "12.08g of copper nitrate trihydrate powder" to obtain catalyst S6 (the composition is shown in Table 1 below).

[0056]

Example 7

[0057] 1) Dissolve 20.74 g of ruthenium trichloride powder in 100 mL of 20% ethanol-water solution and stir until completely dissolved to obtain solution A2 with a Ru concentration of 1 mol / L; 2) Dissolve 2.16 g of lanthanum nitrate hexahydrate powder in 50 mL of 20% ethanol-water mixture to obtain solution B3 with a La element concentration of 0.1 mol / L; 3) Dissolve 2.17 g of cerium nitrate hexahydrate powder in 50 mL of 20% ethanol-water mixture to obtain solution B3 with a Ce element concentration of 0.1 mol / L. 4) Mix solutions A2, B3 and C3 thoroughly to obtain solution D3; 5) Weigh 13.2g of nitrogen-doped carbon nanotube powder and add it to solution D3 at 40℃ with magnetic stirring. Continue stirring for 80min to obtain mixture E3; 6) Place mixture E5 in an 80℃ oven to dry for 12h, and then place it in a muffle furnace and calcine at 350℃ in a nitrogen atmosphere for 5h to obtain catalyst S7 (composition shown in Table 1 below).

[0058]

Example 8

[0059] 1) Dissolve 20.74 g of ruthenium trichloride powder in 100 mL of 20% ethanol-water solution and stir until completely dissolved to obtain solution A2 with a Ru concentration of 1 mol / L; 2) Dissolve 2.16 g of lanthanum nitrate hexahydrate powder in 50 mL of 20% ethanol-water mixture to obtain solution B4 with a La element concentration of 0.1 mol / L; 4) Mix solutions A2 and B4 thoroughly to obtain solution C4; 5) Weigh 13.2 g of nitrogen-doped carbon nanotube powder and add it to solution C4 under magnetic stirring at 40 °C. Continue stirring for 80 min to obtain mixture D4; 6) Place mixture D4 in an oven at 80 °C for 12 h and then place it in a muffle furnace and calcine at 350 °C in a nitrogen atmosphere for 5 h to obtain catalyst S8 (composition shown in Table 1 below).

[0060]

Example 9

[0061] The only difference from Example 8 is that "2.16g of lanthanum nitrate hexahydrate powder" was replaced with "2.17g of cerium nitrate hexahydrate powder" to obtain catalyst S9 (the composition is shown in Table 1 below).

[0062]

Example 10

[0063] The only difference from Example 1 is that "nickel nitrate hexahydrate powder" is replaced with an equal weight of "nickel chloride", and the catalyst S10 is finally obtained (the composition is shown in Table 1 below).

[0064]

Example 11

[0065] The only difference from Example 1 is that the calcination temperature during the catalyst preparation process is 600°C, and the final catalyst S11 is obtained (the composition is shown in Table 1 below).

[0066]

Example 12

[0067] The only difference from Example 1 is that, based on the total weight of the catalyst, the content of the active component is 38%, the content of the promoter is 4%, and the content of nitrogen-doped carbon nanotubes is 58%. Finally, catalyst S12 was obtained (the composition is shown in Table 1 below).

[0068]

Example 13

[0069] The only difference from Example 1 is that the nitrogen content of the nitrogen-doped carbon nanotubes is 15%. Finally, catalyst S13 was obtained (its composition is shown in Table 1 below).

[0070]

Example 14

[0071] The only difference from Example 7 is that the average diameter of the nitrogen-doped carbon nanotubes is 3 nm, and the catalyst S14 is finally obtained.

[0072]

Example 15

[0073] The only difference from Example 7 is that "2.16g of lanthanum nitrate hexahydrate powder" is replaced with "1.21g of copper nitrate hexahydrate powder" to obtain catalyst S15 (the composition is shown in Table 1 below).

[0074] Comparative Example 1

[0075] The only difference from Example 1 is that no additives were added during the preparation of the catalyst (that is, the support only loaded the active component), and the final catalyst SD1 was obtained (the composition is shown in Table 1 below).

[0076] Comparative Example 2

[0077] The only difference from Example 1 is that in Example 1, nitrogen-doped carbon nanotubes were used, while in Comparative Example 3, undoped carbon nanotubes were used, resulting in catalyst SD2 (the composition of which is shown in Table 1 below).

[0078] Comparative Example 3

[0079] The only difference from Example 1 is that 50 wt% of the support was replaced with alumina (that is, the support is N-doped carbon nanotubes and alumina in a mass ratio of 1:1), resulting in catalyst SD3 (the composition is shown in Table 1 below).

[0080] Comparative Example 4

[0081] The only difference from Example 1 is that the support was replaced with alkali-modified carbon nanotubes. The modification method was as follows: 10.5 g of commercial carbon nanotubes were dispersed in 100 mL of deionized water and ultrasonically dispersed. A 1 mol / L sodium hydroxide solution was slowly added to the carbon nanotube dispersion, and the mixture was stirred at 50°C until homogeneous. After drying, the mixture was kept warm in an inert atmosphere and then dispersed in water until neutral. After drying, alkali-modified carbon nanotubes were obtained. Catalyst SD4 was obtained (composition shown in Table 1 below).

[0082] Comparative Example 5

[0083] The only difference from Example 1 is that the nitrogen-doped carbon nanotubes were prepared by high-temperature carbonization. The preparation method was to carbonize self-assembled polyaniline nanotubes at 800°C to obtain nitrogen-doped carbon nanotubes with a nitrogen content of 9% and an electrical conductivity of 0.75 S / cm. Finally, the catalyst SD5 was obtained (the composition is shown in Table 1 below).

[0084] Comparative Example 6

[0085] The composite structure additive of Example 6 of patent "CN114471512A" was used. The composite structure additive contains 7 wt% carbon nanotubes, 13 wt% diatomaceous earth and 20 wt% alumina, and 6 wt% additives. Finally, catalyst SD6 was obtained (the composition is shown in Table 1 below).

[0086] Comparative Example 7

[0087] The only difference from Example 3 is that "cerium nitrate hexahydrate powder" was replaced with an equal mass of "potassium chloride", and the catalyst SD7 was finally obtained (the composition is shown in Table 1 below).

[0088] The catalysts S1-S14 prepared in Examples 1-14 and catalysts SD1-SD7 prepared in Comparative Examples 1-7 were subjected to BET surface area testing and XRD characterization. The size of the active components was calculated according to the Scherrer Equation. The composition and characterization results of each catalyst are shown in Table 1. The solubility of the active components, promoters, or supports obtained by different processing steps will change. For example, as shown in Examples 13 and 14, the solubility of the catalysts changed after different processing steps, and the mass of the support contained in the final catalysts was different.

[0089] The catalysts were placed in a tube furnace and reduced at 400°C for 3 hours under a hydrogen atmosphere, followed by low-temperature activity evaluation. Feed gas composition: CO2: 15%, H2: 60%, N2: 25%. Reaction conditions: pressure 2.0 MPa, space velocity 6000 h⁻¹. -1 The reaction temperatures were 250, 300, and 350℃. The content of each component in the product gas was analyzed by gas chromatography, and the results are shown in Table 2.

[0090] To further verify the high-temperature stability of the catalyst, the above catalyst was subjected to the above feed gas composition, pressure of 2.0 MPa, and space velocity of 6000 h⁻¹. -1 The test was conducted at 500℃ for 24 hours, and then the temperature was lowered to 250℃ for low-temperature activity testing. The results are shown in Table 3.

[0091] Table 1 Catalyst composition and characterization

[0092]

[0093]

[0094] Table 2 Results of catalyst low-temperature activity determination

[0095]

[0096]

[0097] Table 3. Results of low-temperature activity determination of catalysts after high-temperature testing.

[0098]

[0099] As shown in Table 1, the catalysts S1 to S10 prepared in Examples 1 to 10 have small active component sizes and high specific surface areas. The abundant active component binding sites on the carbon nanotube support surface and the introduction of promoters facilitate the loading and dispersion of catalyst particles, preventing catalyst agglomeration.

[0100] The two-component additives used in Examples 1 and 7 help to obtain catalyst particles with smaller sizes.

[0101] In Example 11, due to the high calcination temperature (600℃), the carbon nanotube support structure was destroyed, resulting in a lower specific surface area of ​​the prepared catalyst. At the same time, the dispersion effect of the support was weakened, and the catalyst size became larger, which was not conducive to the catalytic effect.

[0102] In Examples 12 and 13, the catalyst performance and CO2 adsorption capacity did not reach the optimal level because the catalyst preparation conditions were not optimal.

[0103] In Example 14, the carbon nanotubes have a small diameter and fewer nitrogen doping sites, resulting in a relatively low specific surface area of ​​the prepared support.

[0104] Comparative Example 1, lacking an additive, suffered from poor catalyst dispersion, resulting in larger catalyst size. Comparative Example 2, with its undoped carbon nanotubes, exhibited poor catalyst dispersion. Comparative Examples 3 and 6, with the addition of alumina and other supports, did not show a synergistic promoting effect; compared to a single support, they were less conducive to catalyst particle dispersion. Comparative Examples 4 and 5, using carbon nanotube supports modified or prepared by methods other than nitrogen doping, showed poorer structural integrity compared to those prepared by chemical vapor deposition, thus affecting catalyst dispersion and methanation reaction efficiency. Comparative Example 7, using potassium chloride as an additive, suffered from poor inhibition of active component aggregation due to potassium being a non-transition metal or rare earth metal additive.

[0105] As shown in Table 2, the catalysts prepared in the examples all exhibited good CO2 conversion and methane selectivity at lower temperatures (250–350 °C). This is due to the excellent structure and properties of nitrogen-doped carbon nanotubes, which act as a support to facilitate the methanation reaction. With the addition of promoters and optimization of conditions, the low-temperature methanation activity of the catalysts can be further improved.

[0106] The catalysts prepared by this invention all exhibit high-temperature resistance and retain high catalytic activity even after high-temperature treatment. Taking Examples 1 and 7 as examples, as shown in Table 3, the catalysts prepared in Examples 1 and 7 maintain high catalytic activity at low temperatures after high-temperature testing. The catalysts of this invention have good stability, with the ruthenium-based catalyst prepared in Example 7 exhibiting the highest activity and stability.

[0107] In summary, the preparation process of this invention is simple, and the resulting methanation catalyst has good low-temperature reactivity and high-temperature stability, which is beneficial for achieving efficient conversion of carbon dioxide and highly selective preparation of methane, and has promising industrial application prospects.

[0108] Any numerical value mentioned in this invention, if there is only a two-unit interval between any minimum and any maximum value, includes all values ​​that increase by one unit each time from the minimum to the maximum value. For example, if the amount of a component, or a process variable such as temperature or time, is stated as 700-800, in this specification it means specifically listing values ​​such as 701-799, 702-798... and 748-752 and 749-751. For non-integer values, it may be appropriate to consider a unit of 0.1, 0.01, 0.001, or 0.0001. These are merely some specifically specified examples. In this application, in a similar manner, all possible combinations of numerical values ​​between the listed minimum and maximum values ​​are considered to have been disclosed.

[0109] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A low-temperature methanation catalyst for carbon dioxide, characterized in that, The catalyst comprises an active component, a support, and an additive; the active component is at least one of nickel, ruthenium, and rhodium; the support is nitrogen-doped carbon nanotubes; and the additive comprises at least one of transition metal elements and / or rare earth metal elements.

2. The carbon dioxide low-temperature methanation catalyst according to claim 1, characterized in that, The nitrogen-doped carbon nanotubes were prepared by chemical vapor deposition.

3. The low-temperature methanation catalyst for carbon dioxide according to claim 1 or 2, characterized in that, The nitrogen-doped carbon nanotubes have an N content of 3-15%, preferably 3-8%.

4. The carbon dioxide low-temperature methanation catalyst according to any one of claims 1-3, characterized in that, Based on the total weight of the catalyst, the content of the active component is 20-50%, the content of the nitrogen-doped carbon nanotubes is 40-79%, and the content of the auxiliary agent is 1-10%. Preferably, based on the total weight of the catalyst, the content of the active component is 25-40%, the content of the nitrogen-doped carbon nanotubes is 60-70%, and the content of the auxiliary agent is 2.5-8%. More preferably, based on the total weight of the catalyst, the content of the active component is 25-37%, the content of the nitrogen-doped carbon nanotubes is 60-70%, and the content of the auxiliary agent is 2.5-5%.

5. The carbon dioxide low-temperature methanation catalyst according to any one of claims 1-4, characterized in that, The additive is at least one of iron, cobalt, copper, manganese, lanthanum and cerium, preferably cerium and lanthanum, and more preferably the molar ratio of cerium and lanthanum is 1:5-3:

1.

6. The method for preparing the carbon dioxide low-temperature methanation catalyst according to any one of claims 1-5, characterized in that, The method includes: preparing nitrogen-doped carbon nanotubes by chemical vapor deposition; then loading active components and additives onto nitrogen-doped carbon nanotubes by impregnation method, preferably, adding nitrogen-doped carbon nanotubes into an impregnation solution of active component precursor and additive precursor for impregnation treatment, and then drying and calcining to obtain a methanation catalyst. Preferably, the preparation of nitrogen-doped carbon nanotubes by chemical vapor deposition is carried out under nitrogen-containing conditions, and the catalyst catalyzes the carbon nanotubes to undergo a catalytic reaction at a set temperature; the reactants are cleaned to remove impurities, and nitrogen-doped carbon nanotubes are obtained.

7. The preparation method according to claim 6, characterized in that, The nitrogen-containing conditions are those containing acetylene / argon / ammonia; and / or, The catalyst is an iron / alumina catalyst; and / or, The catalytic reaction is carried out at a temperature of 700-800℃ for a time of 1.5-2.5 h; and / or, The cleaning process includes alkaline cleaning, acid cleaning, and optional water cleaning; preferably, the alkaline solution is a sodium hydroxide solution; and / or, the acid solution is a hydrochloric acid solution. And / or, The cleaning process is followed by a drying process; The impregnation solution is prepared by dissolving the active component precursor and the auxiliary agent precursor in their respective solvents to obtain an active component precursor solution and an auxiliary agent precursor solution, and then mixing the active component precursor solution and the auxiliary agent precursor solution to obtain an impregnation solution containing the active component precursor and the auxiliary agent precursor.

8. The preparation method according to claim 6 or 7, characterized in that, The carbon nanotubes are at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes; and / or, the morphology of the carbon nanotubes is bamboo-like; and / or, the diameter of the carbon nanotubes is 1–40 nm; preferably 10–30 nm; and / or, The active component precursor is selected from at least one of nitrates, sulfates, acetates, and chlorides; and / or, The auxiliary precursor is selected from at least one water-soluble salt of iron, cobalt, copper, manganese, lanthanum, and cerium; and / or, The solvent used for the active component precursor and the solvent used for the auxiliary agent precursor are respectively selected from at least one of water, ethanol, methanol, isopropanol, acetone, benzene and toluene.

9. The preparation method according to any one of claims 6-8, characterized in that, The impregnation treatment is performed at an impregnation temperature of 30–80°C for 1–10 hours; and / or, The drying temperature is 60–150°C; and / or, The roasting temperature is 200–500℃, and the roasting time is 1–10 h.

10. The application of the low-temperature carbon dioxide methanation catalyst according to any one of claims 1-5 or the low-temperature carbon dioxide methanation catalyst prepared by the preparation method according to any one of claims 6-9 in the carbon dioxide methanation reaction.

Images