Rhodium-iron bimetallic catalyst, its preparation method and application

Abstract

The application provides a rhodium-iron bimetallic catalyst and a preparation method and application thereof, and relates to the technical field of carbon dioxide hydrogenation catalyst preparation. The rhodium-iron bimetallic catalyst is prepared by using a sequential reduction method, rhodium atoms are pre-reduced under the condition of a dispersing agent, agglomeration of the rhodium atoms is avoided, the rhodium atoms are uniformly dispersed on the surface of a titanium dioxide carrier, iron is further reduced, high-dispersed rhodium-iron bimetallic catalytic active sites are formed, the atomic utilization efficiency of active components in the catalyst is improved, and the catalyst has the characteristics of high carbon dioxide conversion rate and high methane selectivity in a carbon dioxide methanation reaction.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon dioxide hydrogenation catalyst preparation, in particular to a rhodium-iron bimetallic catalyst and a preparation method and application thereof. BACKGROUND

[0002] With the large-scale burning of global fossil fuels, the content of carbon dioxide emitted into the atmosphere has increased sharply, accounting for more than 80% of the total amount of greenhouse gas emissions, which has intensified the impact of the greenhouse effect on the global ecosystem. By coupling carbon dioxide with "green hydrogen" to convert into commercially valuable products such as carbon monoxide, methane, methanol and hydrocarbons, etc. industrial products with added value, achieving economic and environmental benefits, is an important means of carbon dioxide resource utilization. Among them, the carbon dioxide methanation reaction converts carbon dioxide into important fuel methane with high heat value and clean product, which is considered an effective way to utilize carbon dioxide.

[0003] The carbon dioxide methanation reaction is an exothermic reaction. According to thermodynamic research, the reaction is favorable in the low temperature range of 25-400℃, but it is difficult to activate carbon dioxide at a lower temperature, and the kinetics limits the industrial application of the reaction. Therefore, developing a carbon dioxide methanation catalyst with high efficiency at low temperature is the main research direction at present. Compared with non-noble metal-based catalysts, noble metal-based catalysts (Ru, Rh, etc.) have better low-temperature activity and carbon deposition resistance, and among them, noble metals Ru and Rh both exhibit good activity and selectivity in the carbon dioxide methanation reaction. Chinese patent CN 111036199 A prepared a highly dispersed catalyst supported by rutile titanium dioxide through a simple impregnation method, and showed high methane selectivity, but the carbon dioxide conversion rate of the catalyst was low, and the catalytic activity needed to be further optimized. Chinese patent CN104148065A used noble metal Ru or Rh as the main active component, and prepared catalysts with different supports and additives through a coating method, which showed good activity and selectivity in the carbon dioxide methanation reaction, but the reaction conditions were harsh and the preparation process was complex.

[0004] In summary, in the current carbon dioxide methanation reaction, the low-temperature carbon dioxide methanation catalyst still faces problems such as low carbon dioxide conversion rate and low methane selectivity. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a catalyst for preparing methane by carbon dioxide hydrogenation and a preparation method thereof. The highly dispersed rhodium-iron bimetallic catalyst prepared by the method has the characteristics of high carbon dioxide conversion rate and high methane selectivity in the carbon dioxide methanation reaction.

[0006] One of the purposes of the present application is to provide a preparation method of a rhodium-iron bimetallic catalyst.

[0007] The second object of the present application is to provide a rhodium-iron bimetallic catalyst prepared by the preparation method.

[0008] The third object of the present application is to provide an application of the rhodium-iron bimetallic catalyst.

[0009] In order to achieve the above objects of the present application, the following technical solutions are adopted:

[0010] In a first aspect, the present application provides a preparation method of a rhodium-iron bimetallic catalyst, comprising the following steps:

[0011] S1, uniformly stirring nano-TiO2, a rhodium salt and a dispersant in water to obtain a solution A, wherein the content of rhodium ions is 1wt%-10wt% based on the mass of nano-TiO2; dissolving a reducing agent in water, and obtaining a uniformly dispersed solution B after ultrasonic treatment; adding the solution B dropwise into the solution A and stirring to obtain a solution C;

[0012] S2, uniformly stirring an iron salt into the solution C to obtain a mixed solution, wherein the molar ratio of rhodium to iron is 1:0.2-5;

[0013] S3, washing and drying the obtained mixed solution to obtain a rhodium-iron bimetallic catalyst.

[0014] The high-dispersion rhodium and iron bimetallic catalyst is prepared by a sequential reduction method in the presence of a dispersant, first reducing rhodium and then reducing iron, and no further calcination treatment is required after reduction. In the presence of a dispersant, a high-dispersion rhodium catalyst is first obtained, and after further reduction of iron, due to the electronic interaction between the two, a rhodium-iron bimetallic catalyst is formed, which not only changes the electronic structure of rhodium and weakens its excessive hydrogenation ability, but also promotes the non-dissociative activation of carbon monoxide intermediates, thereby achieving excellent catalytic performance.

[0015] Step S1:

[0016] In step S1, the nano-titania is a commercial nano-titania, preferably with an average particle size of 20-30 nm and a rutile phase structure.

[0017] In some embodiments, in step S1, the rhodium salt is one or more of rhodium chloride, rhodium nitrate, rhodium phosphate, and rhodium acetylacetone;

[0018] In some embodiments, in step S1, the dispersant is polyvinylpyrrolidone (PVP), and the type number K of PVP is 16-32;

[0019] In some embodiments, in step S1, the ratio of the molar mass of PVP to the total molar mass of rhodium and iron is 2-10:1;

[0020] In some embodiments, in step S1, the concentration of the rhodium salt in solution A is 0.0006-0.05 g / mL.

[0021] In some embodiments, in step S1, the stirring speed is 200 rpm-1000 rpm, and the stirring time is 0.5 h-5 h.

[0022] In some embodiments, in step S1, the reducing agent is one or more of sodium borohydride, sodium borohydride, ammonia borane, and hydrazine hydrate.

[0023] The concentration of the reducing agent in solution B is 10-100 g / mL.

[0024] In some embodiments, in step S1, the molar ratio of the reducing agent to the total molar amount of rhodium and iron ions is 5-20:1.

[0025] In some embodiments, in step S1, the ultrasonic time is 0.5 h-1 h.

[0026] In some embodiments, in step S1, the dropping speed is 0.1-0.5 mL / min, the stirring speed is 200-1000 r / min, and the stirring time is 0.5-2 h.

[0027] Step S2:

[0028] In some embodiments, in step S2, the iron salt is one or more of ferric nitrate, ferric sulfate, ferric chloride, and ferric tribromide.

[0029] In some embodiments, in step S2, the stirring speed is 200 rpm-1000 rpm, and the stirring time is 0.5 h-2 h.

[0030] Step S3:

[0031] In some embodiments, in step S3, the mixed solution of acetone and water is used for centrifugal washing, wherein the volume ratio of acetone to water is 1-5:1.

[0032] In some embodiments, in step S3, the drying operation process is vacuum drying, the drying temperature is 30-100℃, and the drying time is 3-24 h.

[0033] In a second aspect, the present application provides a rhodium-iron bimetallic catalyst prepared by the above preparation method, wherein the rhodium-iron bimetallic catalyst comprises a nano-TiO2 carrier and high-dispersed rhodium-iron bimetallic nanoparticles loaded on the nano-TiO2 carrier, and the average particle size of the rhodium-iron bimetallic nanoparticles is 5-20 nm.

[0034] The loading of rhodium is 1wt%-10wt% based on the mass of the TiO2 carrier, and the molar ratio of rhodium to iron is 1:1-5.

[0035] In some embodiments, the average particle size of the nano-TiO2 carrier is 20-30 nm, and the phase is a rutile phase structure.

[0036] In a third aspect, the application provides a use of the above rhodium-iron bimetallic catalyst in a catalytic reaction of carbon dioxide hydrogenation to prepare methane.

[0037] Preferably, the catalytic reaction is carried out in a fixed bed reactor;

[0038] The catalyst is loaded into a fixed bed reactor for carbon dioxide hydrogenation after being pressed into a tablet, crushed, and sieved, and the particle size of the sieving is 10-80 mesh;

[0039] The reaction conditions are as follows: the reaction temperature is 200-400℃, the reaction pressure is 2-4 MPa, the gas space velocity is 3000-8000 mL / g / h, and the gas used is a mixture of CO2 and H2, wherein the volume ratio of CO2 to H2 is 1:1-5.

[0040] Preferably, the rhodium-iron bimetallic catalyst is subjected to hydrogen pre-reduction treatment before use, and the reduction conditions are as follows: the hydrogen space velocity is 3000 mL / g / h-10000 mL / g / h, the temperature is 200℃-400℃, and the reduction time is 0.5h-5h.

[0041] Technical effects:

[0042] (1) The preparation method of the application is simple, the catalyst does not need to be calcined, and the operation process is convenient;

[0043] (2) The rhodium-iron bimetallic catalyst prepared by the application avoids the agglomeration of rhodium atoms by pre-reducing the rhodium atoms under the condition of a dispersing agent, so that the rhodium atoms are uniformly dispersed on the surface of the titanium dioxide carrier, and further reduction of iron forms highly dispersed rhodium-iron bimetallic catalytic active sites, thereby improving the atomic utilization efficiency of the active components in the catalyst;

[0044] (3) The rhodium-iron bimetallic catalyst can significantly enhance the activation ability of carbon dioxide and the hydrogenation ability of intermediate products, which helps to improve the activity of the catalyst and avoid the production of by-products such as carbon monoxide, thereby improving the selectivity of methane, and further regulating the dispersion of rhodium-iron bimetallic catalyst by changing the molar ratio of rhodium to iron, etc., to regulate the catalytic performance;

[0045] (4) The catalyst prepared by the application exhibits excellent catalytic performance at a relatively low temperature (270℃), and has high methane selectivity while maintaining high carbon dioxide conversion rate.

[0046] The application has been described in detail above, but the above embodiments are merely illustrative in nature and are not intended to limit the application. Furthermore, the present application is not limited by any theory of prior art or the following examples or the following examples described in the summary. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 X-ray diffraction (XRD) spectra of catalysts prepared for Example 1 and Example 2.

[0048] Figure 2 Test results of catalysts prepared for Examples and Comparative Examples. DETAILED DESCRIPTION

[0049] The application will be further described in conjunction with the following examples, it should be noted that the following examples are provided for illustrative purposes only and do not constitute a limitation on the scope of the application.

[0050] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the examples are conventional raw materials, reagents, methods in the art.

[0051] Example 1

[0052] (1) 25.3 mg of rhodium chloride hydrate (0.01 mmol, 9.9 mg of rhodium), 500 mg of nano-titania (P25), and 66.6 mg of PVP (0.6 mmol, molecular weight calculated as monomer, PVP monomer molar mass is 111) were dissolved in 30 mL of deionized water, stirred at 500 rpm for 1 h, and recorded as solution A, the loading amount of rhodium was 2 wt.%, the K value of PVP was 29-32, and the ratio of the molar amount of PVP to the sum of the molar amounts of rhodium and iron was 2:1;

[0053] (2) 56.7 mg of sodium borohydride was dissolved in 1 mL of deionized water, ultrasonic for 1 h to obtain a uniformly dispersed solution B, the ratio of the molar amount of sodium borohydride to the sum of the molar amounts of rhodium and iron was 5:1;

[0054] (3) Solution B was slowly added to solution A, and the addition was completed in about 5 min, and stirred at 500 rpm for 1 h;

[0055] (4) 80 mg of iron nitrate nonahydrate (0.2 mmol, 11 mg of iron) was added to the above solution, and stirred at 500 rpm for 1 h, the molar ratio of rhodium to iron was 1:2, and the loading amount of iron was 2.2 wt.%;

[0056] (5) The solution was centrifugally washed with acetone and water in a volume ratio of 3:1, and the washing was repeated 3 times;

[0057] (6) The centrifuged sample was placed in a vacuum drying oven and dried at 80°C for 12h, and the catalyst was collected for testing;

[0058] The catalyst was subjected to hydrogen pre-reduction treatment before reaction test, and the pretreatment conditions were as follows: hydrogen space velocity 8000 mL / g / h, reduction temperature 300°C, and reduction time 2h. After the temperature dropped to room temperature, the reaction gas was introduced into the fixed bed reaction device for reaction performance evaluation, and the reaction temperature was 270°C, the reaction pressure was 3MPa, the space velocity was 6000 mL / g / h, and the volume ratio of CO2 / H2 used was 1:4. The reaction performance test results are shown in Table 1. Figure 2 .

[0059] Example 2

[0060] (1) 25.3 mg of rhodium chloride hydrate (0.1 mmol, 9.9 mg of rhodium), 500 mg of nano-titanium dioxide (P25), and 44.4 mg of PVP (0.4 mmol, the molecular weight is calculated according to the monomer, and the PVP monomer molar mass is 111) were dissolved in 30 mL of deionized water, stirred at 500 rpm for 1h, and recorded as solution A, the rhodium loading was 2wt.%, the K value of PVP was 29-32, and the ratio of the molar amount of PVP to the sum of the molar amounts of rhodium and iron was 2:1;

[0061] (2) 37.8 mg of sodium borohydride was dissolved in 1 mL of deionized water, and ultrasonic treatment was performed for 1h to obtain a uniformly dispersed solution B, and the ratio of the molar amount of sodium borohydride to the sum of the molar amounts of rhodium and iron was 5:1;

[0062] (3) Solution B was slowly added to solution A, and the addition was completed in about 5 min, and the stirring was continued at 500 rpm for 1h;

[0063] (4) 40 mg of iron nitrate nonahydrate (0.1 mmol, 5.5 mg of iron) was added to the above solution, and the stirring was continued at 500 rpm for 1h, and the molar ratio of rhodium to iron was 1:1, and the iron loading was 1.1wt.%;

[0064] (5) The solution was washed by centrifugation with acetone and water in a volume ratio of 3:1, and the washing was repeated for 3 times;

[0065] (6) The centrifuged sample was placed in a vacuum drying oven and dried at 80°C for 12h, and the catalyst was collected for testing;

[0066] The catalyst underwent hydrogen pre-reduction treatment before reaction testing. The pre-treatment conditions were: hydrogen space velocity (HHSV) 8000 mL / g / h, reduction temperature 300℃, and reduction time 2 h. After the temperature cooled to room temperature, the reaction performance was evaluated in a fixed-bed reactor using reaction gases at a temperature of 270℃, a reaction pressure of 3 MPa, a HHSV of 6000 mL / g / h, and a CO2 / H2 volume ratio of 1:4. The reaction performance test results are shown below. Figure 2 .

[0067] The X-ray diffraction (XRD) spectra of the catalysts in Examples 1 and 2 are as follows: Figure 1 As shown, from Figure 1 As can be seen from the spectrum, when the molar ratio of rhodium to iron is 1:2, diffraction peaks of iron oxide appear in the catalyst, indicating that some iron agglomerates. When the molar ratio of rhodium to iron is 1:1, only diffraction peaks of the titanium dioxide support are present in the spectrum, and no diffraction peaks of rhodium and iron are observed, indicating that rhodium and iron are uniformly dispersed on the titanium dioxide support.

[0068] Example 3

[0069] (1) Dissolve 50.6 mg of hydrated rhodium chloride (0.2 mmol, 19.8 mg rhodium), 500 mg of nano titanium dioxide (P25), and 66.6 mg of PVP (0.6 mmol, molecular weight calculated based on monomers, molar mass of PVP monomer is 111) in 30 mL of deionized water and stir at 500 rpm for 1 h. This solution is denoted as solution A. The rhodium loading is 4 wt.%, the K value of PVP is 29-32, and the ratio of the molar amount of PVP to the sum of the molar amounts of rhodium and iron is 2:1.

[0070] (2) Dissolve 56.7 mg of sodium borohydride in 1 mL of deionized water and sonicate for 1 h to obtain a uniformly dispersed solution B. The ratio of the molar amount of sodium borohydride to the sum of the molar amounts of rhodium and iron is 5:1.

[0071] (3) Slowly add solution B to solution A, and finish adding it in about 5 minutes. Stir at 500 rpm for 1 hour.

[0072] (4) Add 40 mg of ferric nitrate nonahydrate (0.1 mmol, 5.5 mg iron) to the above solution and stir at 500 rpm for 1 h. The molar ratio of rhodium to iron is 1:0.5 and the iron loading is 1.1 wt.%.

[0073] (5) Wash the solution by centrifugation with acetone and water at a volume ratio of 3:1, and repeat the process 3 times.

[0074] (6) Place the centrifuged sample in a vacuum drying oven and dry at 80°C for 12 hours. Collect the catalyst for testing and use.

[0075] The catalyst was pre-reduced by hydrogen before reaction test. The pre-reduction conditions were as follows: hydrogen space velocity 8000 mL / g / h, reduction temperature 300 °C, and reduction time 2 h. After the temperature dropped to room temperature, the reaction gas was introduced into the fixed bed reaction device for reaction performance evaluation. The reaction temperature was 270 °C, the reaction pressure was 3 MPa, the space velocity was 6000 mL / g / h, and the volume ratio of the gas CO2 / H2 used was 1:4. The reaction performance test results are shown in Table 1. Figure 2 .

[0076] Example 4

[0077] (1) 25.3 mg of rhodium chloride hydrate (0.1 mmol, 9.9 mg of rhodium), 500 mg of nano-titania (P25), and 111 mg of PVP (1 mmol, the molecular weight was calculated according to the monomer, and the PVP monomer molar mass was 111) were dissolved in 30 mL of deionized water, stirred at 500 rpm for 1 h, and recorded as solution A. The loading amount of rhodium was 2 wt.%, the K value of PVP was 29-32, and the ratio of the molar amount of PVP to the sum of the molar amounts of rhodium and iron was 5:1;

[0078] (2) 37.8 mg of sodium borohydride was dissolved in 1 mL of deionized water, and a uniform dispersion solution B was obtained by ultrasonic treatment for 1 h. The ratio of the molar amount of sodium borohydride to the sum of the molar amounts of rhodium and iron was 5:1;

[0079] (3) Solution B was slowly added to solution A, and the addition was completed in about 5 min. The solution was stirred at 500 rpm for 1 h;

[0080] (4) 40 mg of iron nitrate nonahydrate (0.1 mmol, 5.5 mg of iron) was added to the above solution, and the solution was stirred at 500 rpm for 1 h. The molar ratio of rhodium to iron was 1:1, and the loading amount of iron was 1.1 wt.%;

[0081] (5) The solution was washed by centrifugation with acetone and water in a volume ratio of 3:1, and the washing was repeated 3 times;

[0082] (6) The centrifuged sample was placed in a vacuum drying oven and dried at 80 °C for 12 h. The catalyst was collected for testing;

[0083] The catalyst was pre-reduced by hydrogen before reaction test. The pre-reduction conditions were as follows: hydrogen space velocity 8000 mL / g / h, reduction temperature 300 °C, and reduction time 2 h. After the temperature dropped to room temperature, the reaction gas was introduced into the fixed bed reaction device for reaction performance evaluation. The reaction temperature was 270 °C, the reaction pressure was 3 MPa, the space velocity was 6000 mL / g / h, and the volume ratio of the gas CO2 / H2 used was 1:4. The reaction performance test results are shown in Table 1. Figure 2 .

[0084] The catalyst prepared in Comparative Example 1 without introducing iron

[0085] (1) 39.7 mg of rhodium chloride hydrate (0.15 mmol, 15.4 mg of rhodium), 500 mg of nano-titania (P25), 83.3 mg of PVP (0.75 mmol, molecular weight calculated as monomer, PVP monomer molar mass is 111) were dissolved in 30 mL of deionized water, stirred at 500 rpm for 1 h, recorded as solution A, the loading of rhodium is 3.1 wt.%, the K value of PVP is 29-32, the molar ratio of PVP to rhodium is 5:1;

[0086] (2) 28.4 mg of sodium borohydride was dissolved in 1 mL of deionized water, ultrasonic for 1 h to obtain a uniformly dispersed solution B, the molar ratio of sodium borohydride to rhodium is 5:1;

[0087] (3) Solution B was slowly added to solution A, about 5 min for dropwise addition, stirred at 500 rpm for 1 h;

[0088] (4) The solution was centrifuged and washed with acetone and water in a volume ratio of 3 to 1, repeated 3 times;

[0089] (5) The centrifuged sample was placed in a vacuum drying oven and dried at 80°C for 12 h, and the catalyst was collected for testing;

[0090] The catalyst was pretreated with hydrogen before reaction test, the pretreatment conditions were: hydrogen space velocity 8000 mL / g / h, reduction temperature 300°C, reduction time 2 h. After the temperature dropped to room temperature, the reaction gas was introduced into the fixed bed reaction device for reaction performance evaluation, the reaction temperature was 270°C, the reaction pressure was 3 MPa, the space velocity was 6000 mL / g / h, and the volume ratio of CO2 / H2 used was 1:4. The reaction performance test results are shown in Figure 2 .

[0091] Comparative Example 2 without introducing rhodium element to prepare the catalyst

[0092] (1) 111.8 mg of iron nitrate nonahydrate (0.28 mmol, 15.5 mg), 500 mg of nano-titania (P25), 155.4 mg of PVP (1.4 mmol, molecular weight calculated as monomer, PVP monomer molar mass is 111) were dissolved in 30 mL of deionized water, stirred at 500 rpm for 1 h, recorded as solution A, the loading of iron is 3.1 wt.%, the K value of PVP is 29-32, the molar ratio of PVP to iron is 5:1;

[0093] (2) 53 mg of sodium borohydride was dissolved in 1 mL of deionized water, ultrasonic for 1 h to obtain a uniformly dispersed solution B, the molar ratio of sodium borohydride to iron is 5:1;

[0094] (3) slowly drop solution B into solution A, about 5 min drop completion, 500 rpm stirring for 1 h;

[0095] (4) centrifugal wash the solution with acetone and water in the ratio of 3:1 by volume, repeat 3 times;

[0096] (5) place the centrifuged sample in a vacuum drying oven, dry at 80°C for 12 h, collect the catalyst for testing standby;

[0097] The catalyst is pretreated with hydrogen before reaction test, the pretreatment conditions are: hydrogen space velocity 8000 mL / g / h, reduction temperature 300°C, reduction time 2 h. After the temperature drops to room temperature, the reaction gas is introduced into the fixed bed reaction device for reaction performance evaluation, the reaction temperature is 270°C, the reaction pressure is 3 MPa, the space velocity is 6000 mL / g / h, and the volume ratio of the used gas CO2 / H2 is 1:4. The reaction performance test results are shown in Figure 2 .

[0098] Comparative example 3, catalyst prepared without introducing a dispersant

[0099] (1) dissolve 25.3 mg of hydrated rhodium chloride (0.1 mmol, 9.9 mg of rhodium), 500 mg of nano-titania (P25) in 30 mL of deionized water, 500 rpm stirring for 1 h, mark as solution A, the loading of rhodium is 2 wt.%;

[0100] (2) dissolve 37.8 mg of sodium borohydride in 1 mL of deionized water, ultrasonic for 1 h, get a uniform dispersed solution B, the molar ratio of sodium borohydride to the sum of the molar amount of rhodium and iron is 5:1;

[0101] (3) slowly drop solution B into solution A, about 5 min drop completion, 500 rpm stirring for 1 h;

[0102] (4) add 40 mg of iron nitrate nonahydrate (0.1 mmol, 5.5 mg of iron) to the above solution, 500 rpm stirring for 1 h;

[0103] (5) centrifugal wash the solution with acetone and water in the ratio of 3:1 by volume, repeat 3 times;

[0104] (6) place the centrifuged sample in a vacuum drying oven, dry at 80°C for 12 h, collect the catalyst for testing standby;

[0105] The catalyst was subjected to hydrogen pre-reduction treatment before reaction test, the pretreatment conditions were: hydrogen space velocity 8000 mL / g / h, reduction temperature 300°C, reduction time 2h. After the temperature dropped to room temperature, the reaction gas was introduced into the fixed bed reaction device for reaction performance evaluation, the reaction temperature was 270°C, the reaction pressure was 3 MPa, the space velocity was 6000 mL / g / h, and the volume ratio of the gas CO2 / H2 used was 1:4. The reaction performance test results are shown in Table 1. Figure 2 .

[0106] Catalyst prepared by changing the reduction order in Comparative Example 4

[0107] (1) 40 mg of iron nitrate nonahydrate (0.1 mmol, 5.5 mg of iron), 500 mg of nano-titania (P25), and 111 mg of PVP (1 mmol, the molecular weight is calculated according to the monomer, and the PVP monomer molar mass is 111) were dissolved in 30 mL of deionized water, stirred at 500 rpm for 1 h, and recorded as solution A, the loading amount of iron was 1.1 wt.%, the K value of PVP was 29-32, and the ratio of the molar amount of PVP to the sum of the molar amounts of rhodium and iron was 5:1;

[0108] (2) 37.8 mg of sodium borohydride was dissolved in 1 mL of deionized water, and ultrasonic treatment was performed for 1 h to obtain a uniformly dispersed solution B, and the ratio of the molar amount of sodium borohydride to the sum of the molar amounts of rhodium and iron was 5:1;

[0109] (3) Solution B was slowly added to solution A, and the addition was completed in about 5 min, and the stirring was continued at 500 rpm for 1 h;

[0110] (4) 25.3 mg of rhodium chloride hydrate (0.1 mmol, 9.9 mg of rhodium) was added to the above solution, and the stirring was continued at 500 rpm for 1 h, and the loading amount of rhodium was 2 wt.%;

[0111] (5) The solution was centrifugally washed with acetone and water at a volume ratio of 3:1, and the washing was repeated for 3 times;

[0112] (6) The centrifuged sample was placed in a vacuum drying box and dried at 80°C for 12 h, and the catalyst was collected for testing;

[0113] The catalyst was subjected to hydrogen pre-reduction treatment before reaction test, the pretreatment conditions were: hydrogen space velocity 8000 mL / g / h, reduction temperature 300°C, reduction time 2h. After the temperature dropped to room temperature, the reaction gas was introduced into the fixed bed reaction device for reaction performance evaluation, the reaction temperature was 270°C, the reaction pressure was 3 MPa, the space velocity was 6000 mL / g / h, and the volume ratio of the gas CO2 / H2 used was 1:4. The reaction performance test results are shown in Table 1. Figure 2 .

[0114] The conversion rate of CO2 and the selectivity of were calculated as follows:

[0115]

[0116] wherein [CO2] inlet and [CO2] outlet respectively represent the molar concentration of CO2 in the raw gas and tail gas.

[0117]

[0118] wherein [CO], [C i H x ], [CH4] respectively represent the molar concentration of CO, hydrocarbon and methane products.

[0119] From Figure 2 It can be seen that the catalyst prepared by the present application exhibits high catalytic activity and methane selectivity; compared with the catalytic performance of the comparative catalyst, the rhodium-iron bimetallic catalyst prepared by the sequential reduction method can significantly enhance the activation ability of carbon dioxide, has strong hydrogenation ability, avoids the desorption of carbon monoxide byproduct, and thus has excellent catalytic performance under low temperature conditions. Taking Example 4 as an example, under the condition of 270℃, the conversion rate of carbon dioxide is 98%, and the selectivity of methane can reach 99%.

[0120] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it. Although the present application has been described in detail with reference to the foregoing examples, it should be understood by those skilled in the art that the technical solutions recorded in the foregoing examples can be modified, or some or all of the technical features can be replaced equivalently without departing from the spirit and essence defined in the claims of the present application; and these modifications or replacements are still within the scope defined in the claims of the present application.

Claims

1. A method for preparing a rhodium-iron bimetallic catalyst, characterized by, The method comprises the following steps: S1, uniformly stirring nano-TiO2, rhodium salt and dispersant in water to obtain solution A, the content of rhodium ions is 1wt%-10wt% based on the mass of nano-TiO2; dissolving a reducing agent in water, and obtaining uniformly dispersed solution B after ultrasonic treatment; adding solution B dropwise into solution A, and stirring to obtain solution C; The reducing agent is one or more of sodium borohydride, sodium borohydride, ammonia borane and hydrazine hydrate; The concentration of the reducing agent in solution B is 10-100g / mL; S2, adding an iron salt into solution C and continuously stirring to obtain a mixed solution, the molar ratio of rhodium to iron is 1:0.2-5, and the molar ratio of the molar amount of the reducing agent to the total molar amount of rhodium and iron ions is 5-20:1; S3, washing and drying the obtained mixed solution to obtain a rhodium-iron bimetallic catalyst.

2. The production method according to claim 1, characterized by, In step S1, the rhodium salt is one or more of rhodium chloride, rhodium nitrate, rhodium phosphate and rhodium acetylacetone.

3. The preparation method according to claim 1, characterized in that, In step S1, the dispersant is polyvinylpyrrolidone, and the type number K of the polyvinylpyrrolidone is 16-32. In step S1, the molar ratio of the molar amount of polyvinylpyrrolidone to the total molar amount of rhodium and iron is 2-10:

1. In step S1, the concentration of the rhodium salt in solution A is 0.0006-0.05g / mL.

4. The method of claim 1, wherein, In step S1, the dropping speed is 0.1-0.5mL / min, the stirring speed is 200-1000r / min, and the stirring time is 0.5-2h.

5. The preparation method according to claim 1, characterized in that, In step S2, the iron salt is one or more of iron nitrate, iron sulfate, iron chloride and iron tribromide.

6. The method of claim 1, wherein, In step S2, the stirring speed is 200rpm-1000rpm, and the stirring time is 0.5h-2h.

7. A rhodium-iron bimetallic catalyst characterized in that, The rhodium-iron bimetallic catalyst is prepared by the preparation method of any one of claims 1-6, and comprises a nano-TiO2 carrier and high-dispersed rhodium-iron bimetallic nanoparticles loaded on the nano-TiO2 carrier, the average particle size of the rhodium-iron bimetallic nanoparticles is 5-20nm; The loading amount of rhodium is 1wt%-10wt% based on the mass of the TiO2 carrier, and the molar ratio of rhodium to iron is 1:0.2-5.

8. Application of the rhodium-iron bimetallic catalyst of claim 7 in a carbon dioxide hydrogenation catalytic reaction for preparing methane.

9. Use according to claim 8, characterized in that, The catalytic reaction is carried out on a fixed bed reactor, and the reaction conditions are as follows: the reaction temperature is 200-400℃, the reaction pressure is 2-4MPa, the gas space velocity is 3000-8000mL / g / h, and the gas used is a mixed gas of CO2 and H2, wherein the volume ratio of CO2 to H2 is 1:1-5.

10. Use according to claim 9, characterized in that, The rhodium-iron bimetallic catalyst is subjected to hydrogen pre-reduction treatment before use, and the reduction conditions are as follows: the hydrogen space velocity is 3000mL / g / h-10000mL / g / h, the temperature is 200℃-400℃, and the reduction time is 0.5h-5h.

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