Bi-Pd / Mg x Cd 1-x S catalyst, preparation method and application thereof in catalytic hydrogenation of carbon dioxide to methanol

Abstract

The present invention discloses a Bi-Pd / Mg x Cd 1‑x S catalyst, a preparation method thereof, and an application thereof in the catalytic hydrogenation of carbon dioxide to methanol, belonging to the technical field of catalysis. The technical solution is as follows: A Bi-Pd / Mg x Cd 1‑x S catalyst is provided, which includes a Mg x Cd 1‑x S nanoflower support, supported PdO nanoparticles, and Bi2O3, where x = 0.2-0.8, the Pd loading is 1.5-15 wt%, and the Bi loading is 1.2-15 wt%. The preparation method includes the following steps: 1) Preparation of the Mg x Cd 1‑x S nanoflower support; 2) Loading of Pd; 3) Coprecipitation of Bi. The catalyst of the present invention has excellent catalytic activity, stability, and selectivity, provides a new approach for the catalytic hydrogenation of carbon dioxide to methanol, and has broad application prospects.

Description

Technical Field

[0001] The invention belongs to the field of catalytic technology and specifically relates to a Bi-Pd / Mg x Cd 1-x S catalyst, preparation method and application thereof in catalytic hydrogenation of carbon dioxide to methanol. Background Art

[0002] The extensive use of fossil fuels has produced a lot of carbon dioxide, leading to global warming and rising sea levels. Therefore, corresponding technologies are needed to reduce the concentration of carbon dioxide in the air and convert it into other fuel forms, such as alcohols and alkanes. Currently, the most researched are photocatalytic CO2 reduction conversion, CO2 hydrogenation thermal catalytic conversion, etc. Although photocatalytic technology is environmentally friendly and has low energy consumption, it is still a long way from true industrialization due to technical limitations. In the field of catalyst technology, especially in the development of high-efficiency catalysts for specific chemical reactions, the composition, structure and preparation method of the catalyst have a decisive influence on the catalytic performance.

[0003] Traditional catalysts often have problems such as uneven distribution of active components, insufficient carrier stability or limited catalytic efficiency, making it difficult to meet the needs of modern industry for efficient, highly selective and environmentally friendly catalysts. Chinese invention patent CN113842920A provides a carbon dioxide hydrogenation methanol catalyst and its molding method and use, the molding method comprising the following steps: 1) mixing the catalyst powder and the molding aid evenly; 2) pre-treating the particle size; 3) pressing the pre-treated powder into sheets and finally calcining. The catalyst obtained by this molding method has high activity, and the CO2 single-pass conversion rate can reach 35%, which effectively avoids the problem of reduced catalyst activity after molding, and solves the problem of uniformity of the catalyst side pressure strength after molding. Chinese invention patent CN103721719A discloses a catalyst for synthesizing methanol by hydrogenation of carbon dioxide, which contains Cu, Zn, Al, X, halogen and oxygen elements, and is composed of oxides and halides. The molar ratio of various elements is: [Cu+Zn+MA]: [Al+MB] = 2-18, Cu: Zn = 0.5-5, MA: [Cu+Zn] = 0-5, MB: Al = 0-9, halogen: Al = 0.05-5, and MA and MB cannot be 0 at the same time; wherein MA represents monovalent and / or divalent metal ions in X, MB represents trivalent and / or tetravalent metal ions in X, and X is one or a combination of several elements selected from Li, K, Mg, B, Ga, In, transition metal elements and rare earth metal elements. The catalyst has the advantages of high carbon dioxide conversion rate, good methanol selectivity and high methanol yield, but there may be a problem of catalyst recycling. Therefore, it is necessary to develop a new catalyst for catalytic hydrogenation of carbon dioxide to methanol with good catalytic activity, selectivity and stability. Summary of the invention

[0004] The present invention provides a Bi-Pd / Mg x Cd 1-x S catalyst, preparation method and application thereof in catalytic hydrogenation of carbon dioxide to methanol. The catalyst has excellent catalytic activity, stability and selectivity, provides a new way for catalytic hydrogenation of carbon dioxide to methanol and has broad application prospects.

[0005] The technical solution of the present invention is:

[0006] In the first aspect, a Bi-Pd / Mg x Cd 1-x S catalysts, including Mg x Cd 1-x S nanoflower carrier, loaded PdO nanoparticles and Bi2O3, wherein x=0.2-0.8, Pd loading amount 1.5-15wt%, Bi loading amount 1.2-15wt%.

[0007] In a second aspect, a method for preparing the catalyst is provided, comprising the following steps:

[0008] 1) Mg x Cd 1-x Preparation of S nanoflower carrier: Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O and thioacetamide are dissolved in water and mixed evenly; the obtained uniform solution is continuously stirred, and then NaOH solution is added to obtain a mixed solution; the mixed solution is transferred to a reactor and calcined at a temperature of 300-400°C for 3-6 hours. After the reaction is completed, the precipitated product is separated by centrifugation, which is Mg x Cd 1-x S nanoflower carrier, where x = 0.2-0.8;

[0009] 2) Pd loading: Mg x Cd 1-x The S nanoflower carrier was placed in a Pd(NO3)2·2H2O solution and allowed to stand; dried in a constant temperature oven, ground, and then baked in a muffle furnace at 350-450℃ for 1-3h, and then heated to 750-900℃ and baked for a second time for 3-4h to prepare Pd / Mg x Cd 1-x S catalyst, where x=0.2-0.8; the first baking is equivalent to preheating, which promotes the diffusion of active component atoms, and the second baking further promotes the diffusion of active components on the catalyst surface and removes impurities on the catalyst surface.

[0010] 3) Coprecipitation of Bi: Weigh Bi(NO3)3·5H2O and 25% NH3·H2O in water to make an aqueous solution, then add Pd / Mgx Cd 1-x S is added to the aqueous solution, and then thiourea as a precipitant is added, and the mixture is stirred in a water bath at 60-100°C for 6-12 hours, precipitated at room temperature for 12-24 hours, filtered, washed, and dried at 60-100°C for 8-16 hours. The obtained solid is ground to 100-500 μm, and calcined in a tube furnace at 450-650°C for 3-6 hours under argon or helium conditions. The obtained precipitate is filtered, washed, dried in a constant temperature oven at 80-90°C, ground, and calcined again in a muffle furnace at 500-600°C for 1-2 hours to obtain the target product Bi-Pd / Mg x Cd 1-x S catalyst. The first calcination is equivalent to preheating, which promotes the diffusion of active component atoms. The second calcination further promotes the diffusion of active components on the catalyst surface. At the same time, abundant oxygen vacancies and more active sites are generated on the catalyst surface, which plays an important role in improving catalytic performance.

[0011] Preferably, in step 1), the molar volume ratio of Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O, thioacetamide and water is (0.2-0.8 mol):(0.2-0.8 mol):(0.01-0.02 mol):(500-800 mL), and the continuous stirring time is 1-2 h.

[0012] Preferably, in step 1), NaOH solution is added to adjust the pH to 7-9.

[0013] Preferably, in step 2), the standing time is 10-12 hours, the oven drying temperature is 80-100° C., and the drying time is 5-8 hours.

[0014] Preferably, in step 2), Mg x Cd 1-x The mass ratio of S nanoflower carrier and Pd(NO3)2·2H2O is (16-26):(1-5).

[0015] Preferably, in step 3), Pd / Mg x Cd 1-x The mass volume ratio of S, Bi(NO3)3·5H2O and 25% NH3·H2O is (16-30g):(1-5g):(0.1-0.2mL).

[0016] Preferably, in step 3), the precipitant thiourea and Pd / Mg x Cd 1-x The mass ratio of S is (0.1-0.2):(15-30).

[0017] In a third aspect, the Bi-Pd / Mg x Cd1-x The application of S catalyst in catalytic hydrogenation of carbon dioxide to methanol comprises the following steps:

[0018] a. Combine Bi-Pd / Mg x Cd 1-x S catalyst was added to the fixed bed reactor, where x = 0.2-0.8, and nitrogen was introduced at a flow rate of 100 mL / min for pretreatment for 60 min. The temperature of the preheater and the fixed bed reactor was set to 200°C and 350°C, and the temperature was increased and the reaction pressure was set to 0.2 MPa.

[0019] b. Set the raw gas to H2, N2 and CO2, and the volume ratio of H2, N2 and CO2 to (60-78): (2-10): (20-30), and obtain the gas phase product methanol, and analyze the content of the product online by gas chromatography-mass spectrometry.

[0020] Mg x Cd 1-x The S nanoflower structure has a flexible framework and adjustable composition, which improves the catalytic performance by creating active sites. During the CO2 hydrogenation process, these active sites promote the cleavage and activation of CO2 bonds. The addition of Pd increases the concentration of strong base sites and oxygen vacancies in the catalyst. The abundant oxygen vacancies help the catalyst capture and activate CO2 to generate formic acid intermediates. The presence of Bi2O3 can accelerate the dissociation of H2, promote the further hydrogenation of formic acid to produce methanol, and improve the conversion rate of CO2. In addition, Bi and Pd promote the dissociative adsorption of CO bonds and the subsequent coupling between methanol molecular bonds, improving the catalytic efficiency of CO2 hydrogenation.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention has a bimetallic synergistic effect: Bi can promote H2 dissociation, Pd can enhance CO2 adsorption and oxygen vacancy formation, and synergistically improve the hydrogenation efficiency of formic acid intermediates;

[0023] 2. Mg x Cd 1-x The S nanoflower structure not only has a flexible framework and adjustable composition, but its unique crystal structure also provides abundant active sites for catalytic reactions. The Mg and Cd ions in the nanoflower structure can be combined in different proportions (x=0.2-0.8). This adjustability optimizes the surface properties, electronic structure and catalytic activity of the catalyst. In addition, the presence of the S element further enhances the stability and anti-toxicity of the catalyst.

[0024] 3. The staged calcination process during the catalyst preparation process makes the catalyst grains more uniform and dense, stabilizes the catalyst structure, and thus improves the mechanical properties and thermal stability of the catalyst. At the same time, staged calcination also helps to form more active sites, further improving the catalytic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Mg prepared in Example 1 of the present invention 0.2 Cd 0.8 Morphology of S nanoflower carrier.

[0026] Figure 2 This is a morphology diagram of the catalyst prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] In order to enable persons skilled in the art to better understand the technical solution in the present invention, the technical solution will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0028] Example 1

[0029] The Bi-Pd / Mg x Cd 1-x S catalysts, including Mg x Cd 1-x S nanoflower carrier, loaded PdO nanoparticles and Bi2O3, where x=0.2, Pd loading amount 10wt%, Bi loading amount 8wt%.

[0030] The method for preparing the catalyst comprises the following steps:

[0031] 1) Mg x Cd 1-x Preparation of S nanoflower carrier: Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O and thioacetamide are dissolved in water, wherein the molar volume ratio of Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O, thioacetamide and water is 0.8 mol: 0.2 mol: 0.015 mol: 800 mL, and the mixture is stirred and mixed evenly; the obtained uniform solution is stirred continuously for 1 h, and then NaOH solution is added to adjust the pH of the obtained mixed solution to 9; the mixed solution is transferred to a reactor and calcined at a temperature of 350°C for 5 h. After the reaction is completed, the precipitated product is separated by centrifugation, which is Mg 0.2 Cd 0.8 S nanoflower carrier, the morphology of the nanoflower carrier is as follows Figure 1 As shown;

[0032] 2) Pd loading: Mg 0.2 Cd 0.8The S nanoflower carrier was placed in a 0.05 mol / L Pd(NO3)2·2H2O solution, and Mg 0.2 Cd 0.8 The mass ratio of S nanoflower carrier to Pd(NO3)2·2H2O was 20:2, and the mixture was allowed to stand for 12 h. The mixture was dried in a constant temperature oven at 100 °C for 5 h, ground, and then baked in a muffle furnace at 450 °C for 1 h. The mixture was then heated to 900 °C and baked for a second time for 3 h to prepare Pd / Mg 0.2 Cd 0.8 S catalyst;

[0033] 3) Coprecipitation of Bi: Add Bi(NO3)3·5H2O and 25% NH3·H2O to 50 mL of water to prepare an aqueous solution, then add Pd / Mg 0.2 Cd 0.8 S is added to the aqueous solution, and then thiourea is added as a precipitant, wherein Pd / Mg 0.2 Cd 0.8 The mass volume ratio of S, Bi(NO3)3·5H2O and 25% NH3·H2O is 20g:2g:0.15mL, and the precipitant thiourea and Pd / Mg 0.2 Cd 0.8 The mass ratio of S is 0.15:20; the mixture is stirred in a water bath at 100°C for 6 hours, precipitated at room temperature for 24 hours, filtered, washed, and dried at 100°C for 8 hours. The solid obtained is ground to 500 μm and calcined at 650°C in a tube furnace for 3 hours under argon conditions. The precipitate obtained is filtered, washed, dried in a constant temperature oven at 90°C, ground, and calcined again in a muffle furnace at 600°C for 1 hour to obtain the target product Bi-Pd / Mg 0.2 Cd 0.8 S catalyst, the morphology of the catalyst is as follows Figure 2 shown.

[0034] The Bi-Pd / Mg 0.2 Cd 0.8 The application of S catalyst in catalytic hydrogenation of carbon dioxide to methanol comprises the following steps:

[0035] a. Add 50 mg of Bi-Pd / Mg 0.2 Cd 0.8 S catalyst was added to the fixed bed reactor, and nitrogen was introduced at a flow rate of 100 mL / min for pretreatment for 60 min. The preheater temperature was set at 200 °C, the fixed bed reactor reaction temperature was set at 350 °C, and the temperature was increased. The reaction pressure was set to 0.2 MPa.

[0036] b. Set the raw gas to H2, N2 and CO2, and the volume ratio of H2, N2 and CO2 to 60:8:25, to obtain gas phase product methanol, and analyze the content of the product online by gas chromatography-mass spectrometry.

[0037] Example 2

[0038] The Bi-Pd / Mg x Cd 1-x S catalysts, including Mg x Cd 1-x S nanoflower carrier, loaded PdO nanoparticles and Bi2O3, where x=0.8, Pd loading amount 15wt%, Bi loading amount 15wt%.

[0039] The method for preparing the catalyst comprises the following steps:

[0040] 1) Mg x Cd 1-x Preparation of S nanoflower carrier: Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O and thioacetamide are dissolved in water, wherein the molar volume ratio of Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O, thioacetamide and water is 0.2 mol: 0.8 mol: 0.02 mol: 500 mL, and the mixture is stirred and evenly mixed; the obtained uniform solution is continuously stirred for 1.5 h, and then NaOH solution is added to obtain a mixed solution and the pH is adjusted to 8; the mixed solution is transferred to a reactor and calcined at a temperature of 400°C for 3 h. After the reaction is completed, the precipitated product is separated by centrifugation, which is Mg 0.8 Cd 0.2 S nanoflower carrier;

[0041] 2) Pd loading: Mg 0.8 Cd 0.2 The S nanoflower carrier was placed in a 0.05 mol / L Pd(NO3)2·2H2O solution, and Mg 0.8 Cd 0.2 The mass ratio of S nanoflower carrier to Pd(NO3)2·2H2O was 16:5, and the mixture was allowed to stand for 11 hours; dried in a 90°C constant temperature oven for 6 hours, ground, and then baked in a 400°C muffle furnace for 2 hours, and then heated to 800°C for a second baking for 3.5 hours to prepare Pd / Mg 0.8 Cd 0.2 S catalyst;

[0042] 3) Coprecipitation of Bi: Weigh Bi(NO3)3·5H2O and 25% NH3·H2O in 50 mL of water to prepare an aqueous solution, then add Pd / Mg 0.8 Cd 0.2 S is added to the aqueous solution, and then thiourea is added as a precipitant, wherein Pd / Mg 0.8 Cd 0.2The mass volume ratio of S, Bi(NO3)3·5H2O and 25% NH3·H2O is 16g:5g:0.2mL, and the precipitant thiourea and Pd / Mg 0.8 Cd 0.2 The mass ratio of S is 0.1:16; the mixture is stirred in a water bath at 80°C for 9 hours, precipitated at room temperature for 15 hours, filtered, washed, and dried at 80°C for 12 hours. The obtained solid is ground to 300 μm and calcined at 500°C in a tube furnace for 4 hours under helium conditions. The obtained precipitate is filtered, washed, dried in a constant temperature oven at 85°C, ground, and calcined again in a muffle furnace at 550°C for 1.5 hours to obtain the target product Bi-Pd / Mg 0.8 Cd 0.2 S catalyst.

[0043] The Bi-Pd / Mg 0.8 Cd 0.2 The application of S catalyst in catalytic hydrogenation of carbon dioxide to methanol comprises the following steps:

[0044] a. Add 50 mg of Bi-Pd / Mg 0.8 Cd 0.2 S catalyst was added to the fixed bed reactor, and nitrogen was introduced at a flow rate of 100 mL / min for pretreatment for 60 min. The temperature of the preheater and the fixed bed reactor was set to 200 °C for the preheater and 350 °C for the fixed bed reactor. The temperature was increased and the reaction pressure was set to 0.2 MPa.

[0045] b. Set the raw gas to H2, N2 and CO2, and the volume ratio of H2, N2 and CO2 to 78:10:30, to obtain gas phase product methanol, and analyze the content of the product online by gas chromatography-mass spectrometry.

[0046] Example 3

[0047] The Bi-Pd / Mg x Cd 1-x S catalysts, including Mg x Cd 1-x S nanoflower carrier, loaded PdO nanoparticles and Bi2O3, where x=0.5, Pd loading amount 1.5wt%, Bi loading amount 1.2wt%.

[0048] The method for preparing the catalyst comprises the following steps:

[0049] 1) Mg x Cd 1-xPreparation of S nanoflower carrier: Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O and thioacetamide are dissolved in water, wherein the molar volume ratio of Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O, thioacetamide and water is 0.5 mol: 0.5 mol: 0.01 mol: 600 mL, and the mixture is stirred and evenly mixed; the obtained uniform solution is continuously stirred for 2 h, and then NaOH solution is added to obtain a mixed solution and the pH is adjusted to 7; the mixed solution is transferred to a reactor and calcined at a temperature of 300°C for 6 h. After the reaction is completed, the precipitated product is separated by centrifugation, which is Mg 0.5 Cd 0.5 S nanoflower carrier;

[0050] 2) Pd loading: Mg 0.5 Cd 0.5 The S nanoflower carrier was placed in a 0.05 mol / L Pd(NO3)2·2H2O solution and allowed to stand for 10 h; Mg 0.5 Cd 0.5 The mass ratio of S nanoflower carrier to Pd(NO3)2·2H2O was 26:1, dried in a constant temperature oven at 80℃ for 8h, ground, and then baked in a muffle furnace at 350℃ for 3h, and then heated to 750℃ for a second baking for 4h to prepare Pd / Mg 0.5 Cd 0.5 S catalyst;

[0051] 3) Coprecipitation of Bi: Weigh Bi(NO3)3·5H2O and 25% NH3·H2O in 50 mL of water to prepare an aqueous solution, then add Pd / Mg 0.5 Cd 0.5 S is added to the aqueous solution, and then thiourea is added as a precipitant, wherein Pd / Mg 0.5 Cd 0.5 The mass volume ratio of S, Bi(NO3)3·5H2O and 25% NH3·H2O is 30g:1g:0.1mL, and the precipitant thiourea and Pd / Mg 0.5 Cd 0.5 The mass ratio of S is 0.2:30; the mixture is stirred in a 60°C water bath for 12 hours, precipitated at room temperature for 12 hours, filtered, washed, and dried at 60°C for 16 hours. The obtained solid is ground to 100 μm and calcined at 450°C in a tube furnace for 6 hours under argon conditions. The obtained precipitate is filtered, washed, dried in a constant temperature oven at 80°C, ground, and calcined again in a muffle furnace at 500°C for 2 hours to obtain the target product Bi-Pd / Mg 0.5 Cd 0.5 S catalyst.

[0052] The Bi-Pd / Mg 0.5 Cd0.5 The application of S catalyst in catalytic hydrogenation of carbon dioxide to methanol comprises the following steps:

[0053] a. Add 50 mg of Bi-Pd / Mg 0.5 Cd 0.5 S catalyst was added to the fixed bed reactor, and nitrogen was introduced at a flow rate of 100 mL / min for pretreatment for 60 min. The temperature of the preheater and the fixed bed reactor was set to 200 °C for the preheater and 350 °C for the fixed bed reactor. The temperature was increased and the reaction pressure was set to 0.2 MPa.

[0054] b. Set the raw gas to H2, N2 and CO2, and the volume ratio of H2, N2 and CO2 to 65:2:20, to obtain gas phase product methanol, and analyze the content of the product online by gas chromatography-mass spectrometry.

[0055] Comparative Example 1

[0056] Different from Example 1, this comparative example does not include step 3) co-precipitation of Bi, and the remaining preparation methods and steps are the same as those of Example 1.

[0057] Comparative Example 2

[0058] Different from Example 1, this comparative example does not include step 2) Pd loading, and the remaining preparation methods and steps are the same as those of Example 1.

[0059] Comparative Example 3

[0060] Different from Example 1, the molar ratio of Cd(CH3COO)2·2H2O and Mg(CH3COO)2·2H2O added in step 1) of this comparative example is 0.9:0.1, and the rest of the preparation method and steps are the same as those of Example 1.

[0061] Comparative Example 4

[0062] The difference from Example 1 is that step 2) is: 2) Pd loading: Mg 0.2 Cd 0.8 The S nanoflower carrier was placed in a 0.05 mol / L Pd(NO3)2·2H2O solution, and Mg 0.2 Cd 0.8 The mass ratio of S nanoflower carrier to Pd(NO3)2·2H2O was 20:2, and the mixture was allowed to stand for 12 h. The mixture was dried in a 100°C constant temperature oven for 2 h, ground, and then baked in a 900°C muffle furnace for 4 h to prepare Pd / Mg 0.2 Cd 0.8 S catalyst; the rest of the preparation methods and steps are the same as those in Example 1.

[0063] Comparative Example 5

[0064] Different from Example 1, in step 3) of this comparative example, the obtained solid is not ground, and the other preparation methods and steps are the same as those of Example 1.

[0065] The catalysts prepared in the above examples and comparative examples were tested for performance, and the test results are shown in Table 1.

[0066] Table 1 Test results

[0067]

[0068] The evaluation performance of the above examples and comparative examples for catalytic production of methanol from carbon dioxide is shown in Table 2. The CO2 conversion rate ( ), the selectivity of methanol (S) is calculated according to the following formula:

[0069]

[0070] .

[0071] Table 2 Evaluation of catalyst performance in preparing methanol

[0072]

[0073] As can be seen from Table 2, Comparative Example 1 does not contain the co-precipitation of Bi, and Comparative Example 2 does not contain the loading of Pd. The conversion rate of CO2 and the selectivity of methanol are significantly reduced. This is mainly because the lack of the synergistic effect of the bimetallic makes it impossible to achieve high-performance catalysis. Bi2O3 accelerates the dissociation of H2, and Pd increases the concentration of strong base sites and oxygen vacancies, synergistically promoting the hydrogenation of formic acid intermediates to produce methanol. If Bi or Pd is missing alone, this synergistic hydrogenation mechanism is broken, resulting in low catalytic efficiency; in Comparative Example 3, the ratio of Mg and Cd is changed, and the conversion rate of CO2 and the selectivity of methanol are significantly reduced. This is mainly because the excessive Mg content may destroy the nanoflower structure and lead to a reduction in active sites, which not only directly reduces the catalytic activity, but also affects the loading and synergy of PdO nanoparticles and Bi2O3. Because of the reduction in active sites, PdO and Bi2O3 are difficult to play an effective role, which in turn affects the CO2 adsorption, activation and hydrogenation reaction process; in Comparative Example 4, Mg x Cd 1-xDuring the preparation of the S nanoflower carrier, no secondary baking is performed, and the conversion rate of CO2 and the selectivity of methanol are significantly reduced. The main reason is that the Pd particles are sintered by a high-temperature baking, which changes their dispersion state on the carrier surface and reduces the effective active sites. At the same time, it affects the electronic structure of the catalyst surface, weakens the synergistic effect with Bi2O3, and makes it impossible to efficiently carry out each step in the CO2 hydrogenation reaction (such as CO2 adsorption, H2 dissociation, formic acid hydrogenation, etc.), and finally reduces the catalytic performance; in comparative example 5, grinding is not performed, and the conversion rate of CO2 and the selectivity of methanol are significantly reduced, mainly because grinding does not affect the specific surface area of ​​the catalyst, resulting in a decrease in bulk density and pore volume, a decrease in specific surface area, and a decrease in the contact area between the reactant and the catalyst, which hinders the adsorption and reaction of CO2 and H2 on the catalyst surface. At the same time, the changes in bulk density and pore volume affect the diffusion of substances, making it difficult for the reaction products to escape from the catalyst pores, and inhibiting the forward reaction. This not only affects the action of a single active component, but also destroys the synergy between Bi, Pd and the carrier, resulting in a decrease in catalytic performance.

[0074] For the catalyst prepared in Example 1, after the reaction is completed, nitrogen is introduced to exhaust the gas in the reactor, and then oxygen is introduced into the fixed bed. The fixed bed reactor is set to a temperature of 600°C for 3 hours, and then cooled to room temperature for the next experiment. The catalyst prepared in Example 1 is cycled 10 times. The cycle performance of catalyzing carbon dioxide to prepare methanol is shown in Table 3.

[0075] Table 3 Cyclic performance results

[0076]

[0077] It can be seen from the table that the catalyst prepared by the present invention can be recycled, and the catalytic performance of the recycled catalyst is still good. With the increase of the number of cycles, the CO2 conversion rate and the selectivity of methanol are relatively stable.

Claims

1. Bi-Pd / Mg x Cd 1-x S catalyst, characterized in that Including Mg x Cd 1-x S nanoflower carrier, loaded PdO nanoparticles and Bi2O3, wherein x=0.2-0.8, Pd loading amount 1.5-15wt%, Bi loading amount 1.2-15wt%; The method for preparing the catalyst comprises the following steps: 1) Mg x Cd 1-x Preparation of S nanoflower carrier: Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O and thioacetamide are dissolved in water and mixed evenly; the obtained uniform solution is continuously stirred, and then NaOH solution is added to obtain a mixed solution; the mixed solution is transferred to a reactor and calcined at a temperature of 300-400°C for 3-6 hours. After the reaction is completed, the precipitated product is separated by centrifugation, which is Mg x Cd 1-x S nanoflower carrier, where x = 0.2-0.8; 2) Pd loading: Mg x Cd 1-x The S nanoflower carrier was placed in a Pd(NO3)2·2H2O solution and allowed to stand; dried in a constant temperature oven, ground, and then baked in a muffle furnace at 350-450℃ for 1-3h, and then heated to 750-900℃ and baked for a second time for 3-4h to prepare Pd / Mg x Cd 1-x S catalyst, where x = 0.2-0.8; 3) Coprecipitation of Bi: Weigh Bi(NO3)3·5H2O and NH3·H2O in water to make an aqueous solution, then add Pd / Mg x Cd 1-x S is added to the aqueous solution, and then thiourea as a precipitant is added, and the mixture is stirred in a water bath at 60-100°C for 6-12 hours, precipitated at room temperature for 12-24 hours, filtered, washed, and dried at 60-100°C for 8-16 hours. The obtained solid is ground to 100-500 μm, and calcined in a tube furnace at 450-650°C for 3-6 hours under argon or helium conditions. The obtained precipitate is filtered, washed, dried in a constant temperature oven at 80-90°C, ground, and calcined again in a muffle furnace at 500-600°C for 1-2 hours to obtain the target product Bi-Pd / Mg x Cd 1-x S catalyst, where x=0.2-0.

8.

2. A method for preparing the catalyst according to claim 1, characterized in that: The following steps are involved: 1) Mg x Cd 1-x Preparation of S nanoflower carrier: Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O and thioacetamide are dissolved in water and mixed evenly; the obtained uniform solution is continuously stirred, and then NaOH solution is added to obtain a mixed solution; the mixed solution is transferred to a reactor and calcined at a temperature of 300-400°C for 3-6 hours. After the reaction is completed, the precipitated product is separated by centrifugation, which is Mg x Cd 1-x S nanoflower carrier, where x = 0.2-0.8; 2) Pd loading: Mg x Cd 1-x The S nanoflower carrier was placed in a Pd(NO3)2·2H2O solution and allowed to stand; dried in a constant temperature oven, ground, and then baked in a muffle furnace at 350-450℃ for 1-3h, and then heated to 750-900℃ and baked for a second time for 3-4h to prepare Pd / Mg x Cd 1-x S catalyst, where x = 0.2-0.8; 3) Coprecipitation of Bi: Weigh Bi(NO3)3·5H2O and NH3·H2O in water to make an aqueous solution, then add Pd / Mg x Cd 1-x S is added to the aqueous solution, and then thiourea as a precipitant is added, and the mixture is stirred in a water bath at 60-100°C for 6-12 hours, precipitated at room temperature for 12-24 hours, filtered, washed, and dried at 60-100°C for 8-16 hours. The obtained solid is ground to 100-500 μm, and calcined in a tube furnace at 450-650°C for 3-6 hours under argon or helium conditions. The obtained precipitate is filtered, washed, dried in a constant temperature oven at 80-90°C, ground, and calcined again in a muffle furnace at 500-600°C for 1-2 hours to obtain the target product Bi-Pd / Mg x Cd 1-x S catalyst, where x=0.2-0.

8.

3. The method for preparing the catalyst according to claim 2, characterized in that: In step 1), the molar volume ratio of Cd(CH3COO)2·2H2O, Mg(CH3COO)2·2H2O, thioacetamide and water is (0.2-0.8 mol): (0.2-0.8 mol): (0.01-0.02 mol): (500-800 mL), and the continuous stirring time is 1-2 h.

4. The method for preparing the catalyst according to claim 2, characterized in that: In step 1), NaOH solution is added to adjust the pH to 7-9.

5. The method for preparing the catalyst according to claim 2, characterized in that: In step 2), the standing time is 10-12 hours, the oven drying temperature is 80-100°C, and the drying time is 5-8 hours.

6. The method for preparing the catalyst according to claim 2, characterized in that: Step 2) Mg x Cd 1-x The mass ratio of S nanoflower carrier and Pd(NO3)2·2H2O is (16-26):(1-5).

7. The method for preparing the catalyst according to claim 2, characterized in that: Step 3) Pd / Mg x Cd 1-x The mass volume ratio of S, Bi(NO3)3·5H2O and NH3·H2O is (16-30g):(1-5g):(0.1-0.2mL).

8. The method for preparing the catalyst according to claim 2, characterized in that: Step 3) Precipitating agent thiourea and Pd / Mg x Cd 1-x The mass ratio of S is (0.1-0.2):(15-30).

9. A Bi-Pd / Mg as claimed in claim 1 x Cd 1-x The application of S catalyst in catalytic hydrogenation of carbon dioxide to methanol is characterized in that: The following steps are involved: a. Combine Bi-Pd / Mg x Cd 1-x S catalyst was added to the fixed bed reactor, where x = 0.2-0.8, and nitrogen was introduced at a flow rate of 100 mL / min for pretreatment for 60 min. The temperature of the preheater and the fixed bed reactor was set to 200°C and 350°C, and the temperature was increased and the reaction pressure was set to 0.2 MPa. b. Set the raw gas to H2, N2 and CO2, and the volume ratio of H2, N2 and CO2 to (60-78): (2-10): (20-30), and obtain the gas phase product methanol, and analyze the content of the product online by gas chromatography-mass spectrometry.

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