A method for preparing a reverse water gas shift catalyst and its application

Abstract

This invention discloses a method for preparing a reverse water-gas shift catalyst and its application, comprising the following steps: (1) dissolving tetrabutyl titanate, cerium nitrate, or gallium nitrate in water with ammonium molybdate, stirring until homogeneous, to prepare a mixed solution; (2) adding an organic acid to the mixed solution, stirring until homogeneous, and adjusting the pH of the solution; (3) heating the solution from step (2) until gelled, drying overnight, and then calcining to obtain the catalyst. This invention is the first to prepare a molybdenum-based bimetallic catalyst using the sol-gel method, which has the advantages of simple preparation steps, readily available raw materials, and safe operation. Furthermore, the catalyst obtained by this invention exhibits excellent CO2 conversion rate and nearly 100% CO selectivity.

Description

Technical Field

[0001] This invention belongs to the field of catalyst technology, and particularly relates to a reverse water-gas shift catalyst, its preparation method, and its application. Background Technology

[0002] With industrial development and increasingly intense human activities, the concentration of carbon dioxide (CO2) in the global atmosphere has been increasing year by year. As a major component of greenhouse gases, the rising concentration of CO2 in the atmosphere has triggered a series of severe environmental problems such as global warming, glacial melting, and ocean acidification, seriously threatening the human living environment. CO2 is an abundant, inexpensive, non-toxic, non-flammable, and renewable single-carbon structural unit, making it a good carbon source. Therefore, in addition to developing new energy sources, carbon capture and storage (CCS) and carbon capture and utilization (CCU) technologies have attracted great attention due to their high efficiency and ease of application in capturing large amounts of CO2.

[0003] Furthermore, from both environmental protection and CO2 utilization perspectives, catalytic hydrogenation of CO2 is a promising method for mitigating the greenhouse effect and providing energy resources. In industrial production, it is necessary to consider stable catalysts that can achieve high activity and high selectivity at relatively low temperatures; however, designing a catalyst that simultaneously meets these requirements is a challenge. This is because CO2 has a very stable C=O bond, making it thermodynamically difficult to activate. Therefore, preparing simple and easily synthesized, highly active, and highly selective catalysts to convert carbon dioxide into CO and subsequently produce high-value-added products is of great significance for improving the natural environment and addressing energy needs. Summary of the Invention

[0004] Purpose of the invention: The present invention aims to provide a method for preparing a reverse water gas shift catalyst. The preparation method is simple, the raw materials are readily available, and the operation is safe. The catalyst prepared can achieve high activity and high selectivity at relatively low temperatures.

[0005] This invention also provides the application of the reverse water gas shift catalyst in the reverse water gas shift reaction.

[0006] Technical solution: To achieve the above objectives, the method for preparing the reverse water gas shift catalyst of the present invention includes the following steps:

[0007] (1) Dissolve tetrabutyl titanate or cerium nitrate or gallium nitrate in water with ammonium molybdate, stir until uniform, and prepare a mixed solution;

[0008] (2) Add organic acid to the mixture, stir well, and then adjust the pH of the solution;

[0009] (3) Heat the solution from step (2) until it gels, dry it overnight, and then calcine it to obtain the catalyst.

[0010] Further, in step (1), the mass ratio of tetrabutyl titanate or cerium nitrate or gallium nitrate to ammonium molybdate is 1-4:10-15.

[0011] Further, the organic acid in step (2) is any one of citric acid, acetic acid, malic acid, tartaric acid, ascorbic acid, succinic acid, and oxalic acid.

[0012] Furthermore, the concentration of the organic acid in step (2) is 1.0 mol / L to 10 mol / L.

[0013] Furthermore, the pH value of the solution in step (2) is 0.4 to 1.

[0014] Furthermore, the solution heating temperature in step (3) is 50–70°C.

[0015] Furthermore, the drying temperature in step (3) is 110–140°C, and the drying time is 12–24 hours.

[0016] Furthermore, the calcination temperature in step (3) is 440–460°C, and the calcination time is 2–3 hours.

[0017] The application of the reverse water gas shift catalyst described in this invention in the reverse water gas shift reaction.

[0018] Further, the application process is as follows: (i) the catalyst is compressed into tablets, sieved and added to the reaction vessel, hydrogen is introduced and calcined at 500-600°C for 0.5-1 hour;

[0019] (ii) H2 and CO2 are introduced into the reaction vessel, and the reverse water-gas shift reaction is carried out at 300-550℃.

[0020] Preferably, the application process is as follows: (i) a fixed-bed reactor is used as the evaluation device for catalyst reactivity, wherein the reaction tube is made of quartz glass; (ii) the catalyst is pressed into tablets and sieved, and catalyst particles of 30-50 mesh are selected. 0.1 g of catalyst is weighed and mixed with 0.6 g of quartz sand, and placed into a quartz tube for testing; (iii) the corresponding gas cylinder and hydrogen and air generators are opened, the computer is started, the gas flow rate entering the reaction system is controlled by a mass flow meter, and then the reaction pipeline is purged with nitrogen to remove air and check for leaks; (i) v) Turn on the programmed temperature controller and heat to the specified reduction temperature at a rate of 5℃ / min. Then, reduce the calcined sample at 600℃ for 1 hour using hydrogen at a certain flow rate. (v) Turn on argon gas and start the gas chromatograph. After the column oven temperature and detector temperature reach 80℃ and 140℃ respectively, turn on the TCD detector and set the bridge current to 50mA. (vi) After the reduction is complete, purge with nitrogen gas to lower the reaction temperature. Then, turn on the reactant gas to start the reaction. The reaction products are analyzed online by the gas chromatograph.

[0021] Mechanism of invention: Adding promoters to molybdenum-based catalysts can improve the catalyst's reactivity. XRD, TEM, and SEM characterization revealed that MoTi has a smaller grain size, which may expose more active sites. Combined with H2-TPR results, it was found that it can consume more H2 and has a strong ability to adsorb and dissociate H2, which may form more oxygen vacancies. This is consistent with the results of Raman characterization analysis, indicating that oxygen vacancies, as active sites, promote the reverse water-gas shift reaction.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0023] This invention presents the first preparation of a molybdenum-based bimetallic catalyst using a sol-gel method, offering advantages such as simple preparation steps, readily available raw materials, and safe operation. When applied to the reverse water-gas shift reaction, this catalyst effectively avoids methanation and overcomes the problems of low carbon dioxide conversion rates and complex molybdenum carbide preparation methods inherent in molybdenum-based catalysts. Furthermore, the catalyst obtained in this invention exhibits excellent CO2 conversion and near-100% CO selectivity.

[0024] The catalyst obtained by this invention is compared with the existing technology in terms of carbon dioxide conversion rate at 500℃. The catalyst of this invention can achieve a maximum CO2 conversion rate of 52% at 500℃, and the CO selectivity can reach close to 100% at 300℃. Attached Figure Description

[0025] Figure 1 The conversion rates of CO2 using different catalysts are shown; MoTi, MoCe, and MoGa correspond to the catalysts prepared in Examples 1, 2, and 3, respectively.

[0026] Figure 2 The selectivity of different catalysts for CO was demonstrated; MoTi, MoCe, and MoGa correspond to the catalysts prepared in Examples 1, 2, and 3, respectively.

[0027] Figure 3 XRD patterns of MoGa, MoCe, and MoTi;

[0028] Figure 4 SEM images of MoGa, MoCe, and MoTi;

[0029] Figure 5 TEM images of MoCe, MoTi, and MoGa catalysts;

[0030] Figure 6 EDS elemental mapping of catalyst samples: (a) MoCe, (b) MoTi, (c) MoGa;

[0031] Figure 7 H2-TPR diagrams of MoCe, MoTi, and MoGa catalysts;

[0032] Figure 8 The Raman spectra of MoGa, MoCe and MoTi are shown. Detailed Implementation

[0033] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0034] Example 1

[0035] MoTi bimetallic oxide catalyst was prepared using the sol-gel method. 5 mL of deionized water was added to a beaker, and 0.1136 g of tetrabutyl titanate and 1.1918 g of ammonium molybdate were weighed and dissolved in the beaker, then stirred until homogeneous. Subsequently, 1.4931 g of citric acid was weighed and added to the beaker, and the mixture was stirred until homogeneous. The pH of the solution was then adjusted to 0.5 with a 1 mol / L nitric acid solution. The resulting mixture was then heated and stirred in a 60°C water bath until it reached a gel state, and then dried in a constant temperature oven at 120°C for 24 hours. After drying, the resulting sample was calcined in an air atmosphere at 450°C for 3 hours to obtain the reverse water-gas shift catalyst MoTi.

[0036] Example 2

[0037] MoCe bimetallic oxide catalyst was prepared using the sol-gel method. 5 mL of deionized water was added to a beaker, and 0.1532 g of cerium nitrate and 1.1541 g of ammonium molybdate were weighed and dissolved in the beaker, then stirred until homogeneous. Subsequently, 1.4483 g of citric acid was weighed and added to the beaker, and the mixture was stirred until homogeneous. The pH of the solution was then adjusted to 0.5 with a 1 mol / L nitric acid solution. The resulting mixture was then heated and stirred in a 60°C water bath until it reached a gel state, and then dried in a constant temperature oven at 120°C for 24 hours. The resulting sample was calcined in a muffle furnace at 450°C in air atmosphere for 3 hours to obtain the reverse water-gas shift catalyst MoCe.

[0038] Example 3

[0039] MoGa bimetallic oxide catalyst was prepared using the sol-gel method. 5 mL of deionized water was added to a beaker, and 0.0924 g of gallium nitrate and 1.1874 g of ammonium molybdate were weighed and dissolved in the beaker, then stirred until homogeneous. Subsequently, 1.4875 g of citric acid was weighed and added to the beaker, and stirred until homogeneous. The pH of the solution was adjusted to 0.5 with a 1 mol / L nitric acid solution. The resulting mixture was then heated and stirred in a 60°C water bath until it reached a gel state, and then dried in a constant temperature oven at 120°C for 24 hours. The resulting sample was calcined in a muffle furnace at 450°C in air atmosphere for 3 hours to obtain the reverse water-gas shift catalyst MoGa.

[0040] like Figure 3 As shown, the XRD patterns of the MoGa, MoCe, and MoTi catalysts prepared in Examples 1-3 range from 5° to 80°, and the XRD patterns of the MoCe, MoTi, and MoGa samples are recorded. Strong diffraction peaks were observed in MoO3, indicating its high crystallinity. No Ti or Ga-related species were observed in MoTi and MoGa, possibly due to their low crystallinity. The grain sizes of the three catalysts at the (020) crystal plane at 20 = 12.8° were calculated using the Sherrer formula, and were found to be 78.7 nm, 40.0 nm, and 62.0 nm, respectively. It was found that the grain sizes of the composite oxides were all larger than those of pure MoO3 (37.9 nm).

[0041] like Figure 4 The SEM images of the MoGa, MoCe, and MoTi catalysts prepared in Examples 1-3 are shown, illustrating that different metal doping results in different particle sizes. Notably, close-up observation reveals that MoTi exhibits relatively small layered nanosheets. MoCe nanosheets, on the other hand, have relatively large volumes. The layered nanosheets of MoGa fall somewhere in between. These characterization results are consistent with the grain size calculations obtained through XRD.

[0042] like Figure 5 As shown in the TEM images of the MoCe, MoTi, and MoGa catalysts prepared in Examples 1-3, the catalysts exhibit a layered structure, and the incorporation of different metals results in different layer sizes. MoCe has the largest layer size, while MoTi has the smallest. These results are consistent with XRD and SEM, indicating that Ti incorporation can form smaller layered nanosheets. The EDS mapping results of the catalysts are shown below. Figure 6 The elemental distribution shows that MoGa has the least amount of O on its surface, MoCe has more Ce and O on its surface, and MoTi has more Mo and O on its surface. It is possible that MoTi can reduce more Mo oxides in an H2 atmosphere.

[0043] like Figure 6 As shown, the EDS elemental mapping of the samples is: (a) MoCe, (b) MoTi, and (c) MoGa. MoGa has the least amount of O on its surface, while the Ga content on the catalyst surface is between that of MoCe and MoTi. MoCe has more Ce and O on its catalyst surface, while MoTi has more Mo and O on its surface. This may be because MoTi can reduce more Mo oxides in a hydrogen atmosphere. The above experiments effectively demonstrate the successful synthesis of the three materials in this invention.

[0044] like Figure 7 The figures show the H2-TPR spectra of the MoCe, MoTi, and MoGa catalysts prepared in Examples 1-3. H2-TPR characterization was used to study the reduction performance of the MoCe, MoTi, MoGa, and MoO3 catalysts. The complete reduction of MoO3 occurs in two steps: the first step reduces it to MoO2, and the second step, the reduction of MoO2 to elemental Mo, is relatively difficult, thus requiring temperatures above 900℃. All four catalysts exhibit a single Mo atom at temperatures above 900℃. 4+ Become Mo 0 The weak reduction peaks are observed. MoO3 exhibits a major reduction peak at 809℃, corresponding to the reduction of MoO3 to MoO2. MoCe, MoTi, and MoGa catalysts show a distinct major reduction peak in the temperature range of 500-800℃, appearing successively at 750℃, 746℃, and 730℃, corresponding to the weak reduction peaks observed in MoO3. 6+ Restored to Mo 4+Furthermore, a shoulder peak can be observed around 700℃ for both MoCe and MoGa. The shoulder peak for MoCe may be due to the reduction of Ce2(MoO4)3 to Ce2Mo3O9, while the shoulder peak for MoGa may also be due to a reduction similar to that of molybdenum-gallium compounds. The addition of Ce, Ti, and Ga shifts the main reduction peak to lower temperatures, indicating that the reduction of Mo oxides is promoted. Based on the H2 consumption data calculated by integrating the hydrogen consumption peak area of ​​H2-TPR, the H2 consumption of MoCe, MoTi, and MoGa catalysts, obtained by integrating the reduction peaks from 500-800℃, is 7.69 mmol / gcat, 9.17 mmol / gcat, and 8.84 mmol / gcat, respectively. Among the MoCe, MoTi, and MoGa catalysts, MoCe has the lowest H2 consumption, indicating weak H2 adsorption. MoTi has the highest H2 consumption, suggesting a relatively higher number of oxygen-containing functional groups or oxygen vacancies on the MoTi surface.

[0045] like Figure 8 As shown, the Raman spectra of MoGa, MoCe, and MoTi prepared in Examples 1-3 are 600-1100 cm⁻¹. -1 Three peaks appear, each corresponding to a different vibrational mode of MoO3. (666 cm⁻¹) -1 The peak value at this point belongs to the tensile vibration of oxygen shared at the edges of three octahedral MoO3 cells (vasMo-O3). 816 cm -1 This belongs to the stretching vibration of an oxygen atom shared at two corners of two MoO6 octahedrons (vas Mo-O-Mo). For 992 cm⁻¹ -1 The vibrations are non-shared terminal oxygen stretching vibrations (vas Mo=O). For Raman spectroscopy, only the spectral characteristics of MoO3 are shown because the Raman scattering efficiency of Mo and O vibrations is much higher than that of other metals with O vibrations. Although the peak positions are the same, the Raman intensities of these peaks change significantly, indicating that the vibrations of the MoO3 nanocrystals on the catalyst are affected by the doping metal. Among the three catalysts, MoTi has the weakest Raman diffraction peak intensity, indicating the presence of more oxygen vacancies. This is also why MoTi exhibits the highest activity among all catalysts.

[0046] Example 4

[0047] The detection process of this invention:

[0048] (i) A fixed-bed reactor was used as the evaluation device for the catalyst's reactivity, wherein the reaction tube was made of quartz glass; (ii) The MoTi bimetallic oxide catalyst prepared in Example 1 was pressed into tablets and sieved, and catalyst particles of 30-50 mesh were selected. 0.1 g of catalyst was weighed and mixed with 0.6 g of quartz sand, and placed into a quartz tube for testing; (iii) The corresponding gas cylinders and hydrogen and air generators were turned on, the computer was started, and the gas flow rate entering the reaction system was controlled by a mass flow meter (gas flow rate was 50 mL·min). -1 (iv) Then purge the reaction pipeline with 100% high-purity nitrogen to remove air and check for leaks; turn on the programmed temperature riser and heat to the specified reduction temperature of 600℃ at a heating rate of 5℃ / min, then pass gas through a gas flow rate of 40mL·min -1 (v) Reduce the calcined sample with 10% hydrogen by 1 hour; (vi) Turn on argon gas and gas chromatograph. After the column oven temperature and detector temperature reach 80℃ and 140℃ respectively, turn on the TCD detector and set the bridge current to 50mA; (vi) After the reduction is complete, purge with nitrogen to lower the reaction temperature to 300-550℃. Then turn on the reaction raw material gas (a mixture of hydrogen and carbon dioxide with a volume fraction of 4:1) to start the reaction. The reaction products are analyzed online by gas chromatograph.

[0049] The detection process for MoCe prepared in Example 2 and MoGa prepared in Example 3 is the same as the above detection process, except that the catalysts used in step (ii) are MoCe and MoGa bimetallic oxide catalysts, respectively. The gas content was detected online using gas chromatography to compare the effects of the three bimetallic oxide catalysts (MoTi, MoCe, and MoGa) on the catalytic activity of the reverse water-gas shift reaction.

[0050] like Figure 1 The figure shows the carbon dioxide conversion rate at a hydrogen:carbon dioxide ratio of 4:1. It can be seen from the graph that as the reaction temperature increases from 300℃ to 500℃ for the three catalysts, the carbon dioxide conversion rate increases accordingly, reaching a maximum of 52% at 500℃. This is because the reverse water-gas shift reaction is endothermic, and increasing the temperature promotes the forward reaction. The order of conversion rate for the three catalysts is molybdenum-titanium > molybdenum-cerium > molybdenum-gallium, indicating that the catalyst of this invention can achieve excellent conversion rates at 500℃.

[0051] like Figure 2 The figure shows the carbon monoxide selectivity for hydrogen:carbon dioxide = 4:1. As can be seen from the figure, the carbon monoxide selectivity approaches 100% for all three catalyst pairs as the reaction temperature increases from 300℃ to 550℃.

[0052] Example 5

[0053] The effects and structures of the bimetallic oxide catalysts prepared in Examples 1-3 and existing bimetallic oxide catalysts are shown in Table 1 below:

[0054] Table 1. RWGS reaction conditions with different catalysts and their CO2 conversion and CO selectivity

[0055]

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[0065]

[10] Ronda-lloret,M.Yang,LQQ;Hammerton,M.Marakatti,VS;Tromp,M.Sofer,Z.Sepulveda- Clerk,A.Ramos-Fernandez,EV:Delgado,JJRothenberg,G.Queen,TRShiju,NRMolybdenum oxide supported on Ti3AlC2 is an activereverse water gas shift catalyst.ACS Sustain.Chem.Eng.,2021,9,4957.

[0066] In summary, the CO2 conversion rate and other parameters proposed in the prior art are all optimal results under optimal conditions. However, the CO2 conversion rate and CO selectivity results of the present invention are significantly better than those of the prior art, and the bimetallic oxide catalyst MoTi is more effective than the other two.

Claims

1. The application of a counter-current gas shift catalyst in a counter-current gas shift reaction; the preparation method of the counter-current gas shift catalyst includes the following steps: (1) Dissolve tetrabutyl titanate, cerium nitrate, or gallium nitrate in water and ammonium molybdate, stir until homogeneous, and prepare a mixed solution; (2) Add organic acid to the mixture, stir well, and then adjust the pH of the solution; (3) Heat the solution from step (2) until it gels, dry it overnight, and then calcine it to obtain the catalyst; In step (1), the mass ratio of tetrabutyl titanate, cerium nitrate, or gallium nitrate to ammonium molybdate is 1~4:10~15; In step (3), the calcination temperature is 440~460℃ and the calcination time is 2~3 hours.

2. Use according to claim 1, characterized in that, In step (2), the organic acid is any one of citric acid, acetic acid, malic acid, tartaric acid, ascorbic acid, succinic acid, and oxalic acid.

3. Use according to claim 1, characterized in that, In step (2), the concentration of organic acid is 1.0 mol / L to 10 mol / L.

4. Use according to claim 1, characterized in that, In step (2), the pH value of the solution is 0.4~1.

5. The use according to claim 1, characterized in that, The solution is heated to a temperature of 50-70°C in step (3).

6. Use according to claim 1, characterized in that, In step (3), the drying temperature is 110~140℃ and the drying time is 12~24 hours.

7. The application according to claim 1, characterized in that, The application process is as follows: (i) The catalyst is compressed into tablets, sieved and added to the reaction vessel, hydrogen gas is introduced and calcined at 500~600℃ for 0.5~1 hour; (ii) H2 and CO2 are introduced into the reaction vessel and the reverse water-gas shift reaction is carried out at 300~550℃.

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