A low-temperature reverse water vapor shift single atom catalyst and its preparation method

Abstract

The present invention discloses a low-temperature reverse water vapor shift single atom catalyst and a preparation method thereof, which increases the oxygen vacancy concentration and specific surface area of ​​the catalyst surface by regulating the carrier morphology and rare earth element doping, and the carrier-rich oxygen vacancy concentration is conducive to the adsorption, activation and dissociation of the raw gas carbon dioxide, thereby increasing the conversion rate of carbon dioxide, and at the same time, a transition metal with a specific loading amount is prepared to prepare a transition metal single (Co, Fe, Mo) atom catalyst to synergistically improve the catalyst's selectivity for carbon monoxide under low temperature conditions; the present invention adopts a transition metal nickel (Co, Fe, Mo) single atom catalyst, which not only maximizes the atomic utilization efficiency, but also provides unique electronic properties and unique geometric shapes, so that the catalyst has superior catalytic activity and carbon monoxide selectivity. The present invention is suitable for catalytic hydrogenation of carbon dioxide to produce carbon monoxide.

Description

Technical Field

[0001] The invention belongs to the field of reverse water vapor shift, i.e., preparing carbon monoxide by hydrogenating carbon dioxide, and specifically relates to a single-atom catalyst for low-temperature reverse water vapor shift and a preparation method thereof. Background Art

[0002] The large-scale use of fossil fuels has led to a large amount of carbon dioxide emissions, exacerbating the greenhouse effect and causing global climate problems. To this day, the concentration of carbon dioxide in the global atmosphere is still rising, so the capture and conversion of greenhouse gases such as carbon dioxide has received great attention from governments and researchers. Converting carbon dioxide into fuels or chemicals can not only effectively reduce the concentration of carbon dioxide in the atmosphere, but also artificially enhance the carbon cycle and generate considerable economic benefits. Using the reverse water vapor shift to convert carbon dioxide into carbon monoxide is an important chemical conversion method. However, thermodynamic studies have shown that the reaction barrier is high and needs to be carried out at high temperatures (600-800°C), and the selectivity of the product CO cannot reach 100%. Therefore, it is particularly important to develop high-performance catalysts at low temperatures to effectively reduce the energy consumption of the reaction, improve the conversion efficiency of carbon dioxide and the selectivity of carbon monoxide.

[0003] The catalysts used in the process of hydrogenating carbon dioxide to produce carbon monoxide are generally cheap and highly active Cu-based catalysts. However, Cu-based catalysts are particularly prone to sintering under high temperature conditions, resulting in a significant reduction in catalyst stability. Nickel-based catalysts have excellent carbon dioxide hydrogenation performance under low temperature conditions, but their target product is mainly methane rather than carbon monoxide. Therefore, it is a huge challenge to use nickel-based catalysts to achieve selective hydrogenation of carbon dioxide to produce carbon monoxide under low temperature conditions. The carriers used in the process of hydrogenating carbon dioxide to produce carbon monoxide are generally aluminum oxide, silicon oxide, zirconium oxide, lanthanum oxide or mixtures thereof. The carrier plays an extremely important role in the preparation of high-performance catalysts: the presence of the carrier can greatly improve the dispersibility of the active components of the catalyst; some carriers can promote the adsorption and activation of reactant molecules, thereby increasing the reaction rate. At present, the biggest drawback of most nickel-based catalysts is that the product is mainly methane under low temperature conditions. How to use nickel-based catalysts to achieve selective hydrogenation of carbon dioxide to produce carbon monoxide under low temperature conditions and improve the overall reaction stability and efficiency is a huge challenge at present.

[0004] In order to solve the above problems, the present invention is proposed. Summary of the invention

[0005] The present invention aims to provide a single-atom catalyst for low-temperature reverse water vapor shift and a preparation method thereof. The oxygen vacancy concentration and specific surface area on the catalyst surface are increased by regulating the carrier morphology and rare earth element doping. The oxygen vacancy concentration of the carrier is conducive to the adsorption, activation and dissociation of carbon dioxide in the raw gas, thereby increasing the conversion rate of carbon dioxide. At the same time, a transition metal single (Co, Fe, Mo) atom catalyst is prepared with a specific loading amount of transition metal to synergistically improve the catalyst's selectivity for carbon monoxide under low temperature conditions. The present invention adopts a transition metal nickel (Co, Fe, Mo) single-atom catalyst, which not only maximizes the atomic utilization efficiency, but also provides unique electronic properties and unique geometric shapes, so that the catalyst has excellent catalytic activity and carbon monoxide selectivity.

[0006] The technical solution of the present invention is as follows:

[0007] A low-temperature, high-selectivity transition metal single-atom catalyst for reverse water-gas shift, the catalyst consisting of a carrier and an active component grown in situ on the carrier, the carrier being a praseodymium-cerium oxide in nanosheet morphology, the active component comprising a transition metal on the carrier, the transition metal being one of Ni, Co, Fe, and Mo; based on the total mass of the catalyst, the loading amount of the transition metal is 0.1-10%, and the doping amount of Pr is 0-50%.

[0008] As a limitation of the present invention, the thickness of the carrier is 4-40 nm; and the active component is at a single-atom scale.

[0009] The present invention also provides a method for preparing the catalyst, which is carried out in the following steps in sequence:

[0010] (1) mixing a cerium nitrate hexahydrate aqueous solution, a praseodymium nitrate hexahydrate aqueous solution, and an ethanol solution of pyromellitic acid, stirring, transferring the mixed solution to a water bath, and bathing the mixture at 40 to 80° C. for 0.5 to 8 hours to obtain CePr-MOFs nanosheets;

[0011] (2) After the water bath is completed, the solid is collected by filtration, washed with deionized water for 3 to 5 times, and dried at 60 to 120° C. for 12 to 48 hours to obtain CePr-MOFs solid;

[0012] (3) Grinding the dried Ce-MOFs solid to obtain a solid powder, then dissolving it in an ethanol aqueous solution, ultrasonicating and stirring to form a carrier solution, dropping a transition metal nitrate aqueous solution into the carrier solution, stirring for 4 hours, then washing with ethanol and water for 5 to 10 times respectively, and drying in an oven for 24 hours to obtain a precursor loaded with active components;

[0013] (4) The precursor is dried and ground into powder, which is then calcined in a muffle furnace at 400-700° C. for 2-8 hours to obtain a catalyst precursor, which is then reduced to obtain the final catalyst.

[0014] As a limitation of the preparation method of the present invention, in step (1), the molar ratio of the cerium nitrate hexahydrate aqueous solution, the praseodymium nitrate hexahydrate aqueous solution and the ethanol solution of pyromellitic acid is 1:(0-1):1.

[0015] As a second limitation of the preparation method of the present invention, in step (3), the concentration of the transition metal nitrate aqueous solution is 1 to 6 mol / L.

[0016] As a third limitation of the preparation method of the present invention, in step (4), the drying temperature is 60 to 120° C., and the drying time is 6 to 24 hours.

[0017] The preparation method of the present invention has another limitation. In step (4), the reduction preparation process is to reduce the catalyst precursor in a 5% Vol H2 / Ar mixed gas at 600°C for 1 hour.

[0018] The above-mentioned preparation method of the present invention as a whole, each step is closely related and cannot be separated. The present invention prepares Ce-MOFs nanosheets doped with praseodymium by a water bath method. After adsorbing and loading transition metal elements, praseodymium elements are uniformly doped into the cerium oxide lattice during calcination. Due to the difference in atomic radius and valence state between praseodymium and cerium, more oxygen vacancies are formed. Oxygen vacancies can promote the adsorption of raw gas carbon dioxide and stabilize transition metal single atoms; the transition metal in the prepared catalyst exists in the form of single atoms, which can not only improve the atomic utilization efficiency and inhibit the transition hydrogenation of carbon monoxide, thereby improving the selectivity of carbon monoxide; at the same time, rare earth element praseodymium is doped in the catalyst to improve the adsorption, activation and stabilization of raw carbon dioxide. The synergistic effect of the transition metal unit and the rare earth element promotes not only stabilizing the transition metal single atom structure, strengthening the adsorption of raw gas CO2 and weakening the adsorption strength of the reaction intermediate, thereby greatly improving the performance of low-temperature reverse water gas shift.

[0019] After adopting the technical solution of the present invention, the technical effects achieved are as follows:

[0020] 1. The single-atom catalyst loaded with transition metals (Ni, Co, Fe, Mo) prepared by the present invention has a certain degree of positive charge relative to nanoparticles, which not only maximizes the atomic utilization efficiency, but also provides unique electronic properties, which can weaken the dissociation of hydrogen and the adsorption of the intermediate product CO, thereby making the catalyst have excellent CO selectivity.

[0021] 2. The present invention increases the surface oxygen vacancy concentration of the carrier by doping with the rare earth element praseodymium. The abundant oxygen vacancies are beneficial to the stability of the transition metal single-atom catalyst, while strengthening the adsorption, activation and dissociation of the raw gas carbon dioxide, thereby increasing the conversion rate of carbon dioxide.

[0022] 3. The preparation method is simple and the process is easy to control. The prepared catalyst has high catalytic activity at low temperatures, and the selectivity of CO at a temperature below 500°C is as high as 99.5%.

[0023] The invention is suitable for preparing a catalyst for low-temperature high-selectivity reverse water-gas shift.

[0024] The specific implementation of the present invention will be further described in detail below with reference to the accompanying drawings.

[0025] Instruction Manual

[0026] Figure 1 Example 3 catalyst Ni / CePr 0.2 SEM and TEM images, where: Figure a is the SEM image, and Figure b is the TEM image;

[0027] Figure 2 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 XRD pattern of the catalyst after calcination;

[0028] Figure 3 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 XRD pattern of the catalyst after hydrogen reduction;

[0029] Figure 4 Example 3 catalyst Ni / CePr 0.2 Synchrotron radiation pattern;

[0030] Figure 5 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5Raman spectrum of the catalyst after hydrogen reduction;

[0031] Figure 6 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 EPR diagram of the catalyst after hydrogen reduction;

[0032] Figure 7 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 The catalyst was heated at a gas phase space velocity of 24000 mL / g cat h, CO2:H2=1:4, the CO2 conversion rate changes with temperature,

[0033] Figure 8 Comparative Example 1 and Examples 1-4 are Ni-NPs / CePr 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 The catalyst was heated at a gas phase space velocity of 24000 mL / g cat ·h, CO2:H2=1:4 reaction product CO selectivity versus temperature;

[0034] Fig. 9 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 The catalyst was heated at a gas phase space velocity of 24000 mL / g cat ·H, CO2:H2=1:4 reaction product CO yield versus temperature;

[0035] Fig.10 Ni-NPs / CePr prepared in Comparative Example 1 0.2 (Ni nanoparticle catalyst) The catalyst was heated to 400°C and the gas phase space velocity was 24000 mL / g cat·H2, CO2 conversion rate and CO selectivity change with reaction time under the condition of CO2:H2=1:4;

[0036] Fig.11 Ni / CePr prepared in Example 3 0.2 The catalyst was heated to 400°C and the gas phase space velocity was 24000 mL / g. cat ·h, CO2:H2=1:4 conditions, CO2 conversion rate and CO selectivity versus reaction time. DETAILED DESCRIPTION

[0037] In the following examples, the reagents described are all commercially available unless otherwise specified, and the following experimental methods and detection methods are all based on existing experimental methods and detection methods unless otherwise specified.

[0038] Example

[0039] (I) Preparation of catalyst

[0040] The following specific implementation method prepares a catalyst of transition metal single atoms supported by cerium oxide and rare earth Pr-doped cerium oxide carriers. The cerium oxide or Pr-doped cerium oxide carrier is in the form of nanosheets. The prepared catalyst is dried, calcined, pressed and sieved, and loaded into a fixed bed. The purity of the chemical reagents described in the following examples is analytically pure.

[0041] Comparative Example 1: Ni-NPs / CePr 0.2 Preparation of (Ni nanoparticle) catalyst

[0042] (1) Weigh 3.06 g of pyromellitic acid, dissolve it in 225 mL of ethanol solution, stir it evenly to obtain solution A, then weigh 4.17 g of cerium nitrate hexahydrate and 1.04 g of praseodymium nitrate hexahydrate and dissolve them in 225 mL of deionized water solution to obtain solution B, then mix solution A and solution B evenly and place them in a water bath at 80°C for 2 h, cool them to room temperature after the water bath is completed, centrifuge, wash them with ethanol and water for multiple times, and dry them to obtain Ce-Pr-MOFs, and finally place the dried Ce-Pr-MOFs in a muffle furnace at 600°C for 4 h to obtain praseodymium-doped CeO2 nanosheets, denoted as CePr 0.2 .

[0043] (2) Weigh 1.483 g of nickel nitrate hexahydrate to make a 1.5 mL solution, weigh 2.7 g of CePr 0.2 The prepared nickel nitrate hexahydrate solution was added dropwise to the CePr 0.2 The mixture was mixed in the carrier until uniform, placed at room temperature for 6 hours, transferred to a 120°C oven for drying for 24 hours, and then calcined in a muffle furnace at 600°C for 4 hours to obtain CePr 0.2Loaded NiO (NiO / CePr 0.2 ), the oxide was reduced at 600℃ and 5% Vol H2 / Ar mixed gas for 1h to obtain the reduced catalyst, which was recorded as Ni-NPs / CePr 0.2 , wherein the final loading amount of Ni is 10 wt % based on the total mass of the catalyst.

[0044] Example 1: Preparation of Ni / CeO2 catalyst

[0045] (1) Weigh 3.06 g of pyromellitic acid, dissolve it in 225 mL of ethanol solution, and stir it to obtain solution A. Then weigh 5.22 g of cerium nitrate hexahydrate and dissolve it in 225 mL of deionized water solution to obtain solution B. Then, mix solution A and solution B evenly and place them in a water bath at 80° C. for 2 h. After the water bath is completed, cool them to room temperature, centrifuge, wash them with ethanol and water for multiple times, and dry them to obtain Ce-MOFs.

[0046] (2) Weigh 1.483 g of nickel nitrate hexahydrate to prepare a 20 mL solution, weigh the Ce-MOFs synthesized in step (1) and dissolve it in 20 mL of ethanol, add the prepared nickel nitrate hexahydrate solution dropwise to the ethanol solution of Ce-MOFs, stir at room temperature for 4 h, then centrifuge, wash with ethanol and water multiple times, transfer to a 120°C oven and dry for 24 h, then calcine in a muffle furnace at 600°C for 4 h to obtain CeO2-loaded NiO (NiO / CeO2), and reduce the oxide at 600°C and 5% VolH2 / Ar mixed gas for 1 h to obtain a reduced catalyst, denoted as Ni / CeO2, wherein the final Ni loading is 0.98 wt% based on the total mass of the catalyst as determined by atomic absorption spectroscopy.

[0047] Example 2: Ni / CePr 0.1 Catalyst preparation

[0048] (1) 3.06 g of pyromellitic acid was weighed and dissolved in 225 mL of ethanol solution, and stirred to obtain solution A. Then 4.69 g of cerium nitrate hexahydrate and 0.52 g of praseodymium nitrate hexahydrate were weighed and dissolved in 225 mL of deionized water solution to obtain solution B. Then, solution A and solution B were mixed and placed in a water bath at 80° C. for 2 h. After the water bath was completed, the mixture was cooled to room temperature, centrifuged, washed with ethanol and water for multiple times, and dried to obtain Ce-Pr 0.1 -MOFs.

[0049] (2) Weigh 1.483 g of nickel nitrate hexahydrate to make a 20 mL solution, weigh the Ce-Pr synthesized in step (1) 0.1-MOFs was dissolved in 20 mL of ethanol and the prepared nickel nitrate hexahydrate solution was added dropwise to the Ce-Pr 0.1 -MOFs ethanol solution was stirred at room temperature for 4 h, then centrifuged, washed with ethanol and water for multiple times, transferred to an oven at 120 °C for 24 h, and then calcined in a muffle furnace at 600 °C for 4 h to obtain CePr 0.1 Loaded NiO (NiO / CePr 0.1 ), the oxide was reduced at 600℃ and 5% Vol H2 / Ar mixed gas for 1h to obtain the reduced catalyst, which was recorded as Ni / CePr 0.1 , wherein based on the total mass of the catalyst, the final loading amount of Ni is 0.98wt% as detected by atomic absorption spectroscopy.

[0050] Example 3: Ni / CePr 0.2 Catalyst preparation

[0051] (1) 3.06 g of pyromellitic acid was weighed and dissolved in 225 mL of ethanol solution, and stirred to obtain solution A. Then 4.17 g of cerium nitrate hexahydrate and 1.04 g of praseodymium nitrate hexahydrate were weighed and dissolved in 225 mL of deionized water solution to obtain solution B. Then, solution A and solution B were mixed and placed in a water bath at 80° C. for 2 h. After the water bath was completed, the mixture was cooled to room temperature, centrifuged, washed with ethanol and water for multiple times, and dried to obtain Ce-Pr 0.2 -MOFs.

[0052] (2) Weigh 1.483 g of nickel nitrate hexahydrate to make a 20 mL solution, weigh the Ce-Pr synthesized in step (1) 0.2 -MOFs was dissolved in 20 mL of ethanol and the prepared nickel nitrate hexahydrate solution was added dropwise to the Ce-Pr 0.2 -MOFs ethanol solution was stirred at room temperature for 4 h, then centrifuged, washed with ethanol and water for multiple times, transferred to an oven at 120 °C for 24 h, and then calcined in a muffle furnace at 600 °C for 4 h to obtain CePr 0.2 Loaded NiO (NiO / CePr 0.2 ), the oxide was reduced at 600℃ and 5% Vol H2 / Ar mixed gas for 1h to obtain the reduced catalyst, which was recorded as Ni / CePr 0.2 , wherein based on the total mass of the catalyst, the final loading amount of Ni is 0.98wt% as detected by atomic absorption spectroscopy.

[0053] Example 4: Ni / CePr 0.5 Catalyst preparation

[0054] (1) 3.06 g of pyromellitic acid was weighed and dissolved in 225 mL of ethanol solution, and stirred to obtain solution A. Then 2.61 g of cerium nitrate hexahydrate and 2.61 g of praseodymium nitrate hexahydrate were weighed and dissolved in 225 mL of deionized water to obtain solution B. Then, solution A and solution B were mixed and placed in a water bath at 80° C. for 2 h. After the water bath was completed, the mixture was cooled to room temperature, centrifuged, washed with ethanol and water for multiple times, and dried to obtain Ce-Pr 0.5 -MOFs.

[0055] (2) Weigh 1.483 g of nickel nitrate hexahydrate to make a 20 mL solution, weigh the Ce-Pr synthesized in step (1) 0.5 -MOFs was dissolved in 20 mL of ethanol and the prepared nickel nitrate hexahydrate solution was added dropwise to the Ce-Pr 0.5 -MOFs ethanol solution was stirred at room temperature for 4 h, then centrifuged, washed with ethanol and water for multiple times, transferred to an oven at 120 °C for 24 h, and then calcined in a muffle furnace at 600 °C for 4 h to obtain CePr 0.5 Loaded NiO (NiO / CePr 0.5 ), the oxide was reduced at 600℃ and 5% Vol H2 / Ar mixed gas for 1h to obtain the reduced catalyst, which was recorded as Ni / CePr 0.5 , wherein based on the total mass of the catalyst, the final loading amount of Ni is 0.98wt% as detected by atomic absorption spectroscopy.

[0056] (II) Catalyst characterization

[0057] Figure 1 As can be seen from Figure a, the thickness of the carrier prepared in Example 3 is 4-40 nm. This ultra-thin structure is more conducive to the uniform dispersion of active sites and the diffusion of reaction raw materials.

[0058] Figure 2 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 XRD pattern of the catalyst after calcination. Figure 2 It can be seen that the catalyst after calcination has obvious characteristic diffraction peaks of cerium oxide, and the characteristic diffraction peak of nickel oxide in the comparative catalyst appears at 43.5°α, indicating that the nickel oxide particles are larger in size and can be detected by XRD. There is no characteristic peak of nickel oxide in Examples 1-4, indicating that the nickel oxide is highly dispersed and may be in a single atomic state.

[0059] Figure 3 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 XRD pattern of the catalyst after hydrogen reduction. Figure 3 It can be seen that the catalyst after reduction in Comparative Example 1 has obvious metallic nickel diffraction peaks, indicating that the nickel in the comparative example is larger in size and may exist in the form of nanoparticles, while no metallic nickel is detected in the example, indicating that nickel is highly dispersed and may be distributed in the form of single atoms. In addition, after Pr doping in the example, there is no independent characteristic peak of PrO2, and the characteristic diffraction peak of cerium oxide shifts to a low angle, indicating that Pr is successfully doped into the lattice of cerium oxide.

[0060] Figure 4 The catalyst Ni / CePr prepared in Example 3 0.2 The synchrotron radiation pattern shows that nickel in Example 3 exists in the form of a single atom, which supports the Figure 2 and Figure 3 The XRD spectrum of the embodiment does not have the characteristic peaks of nickel oxide or metallic nickel nanoparticles.

[0061] Figure 5 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 Raman spectrum of the catalyst after hydrogen reduction. It can be seen from the figure that as the Pr doping increases, the cerium oxide at 450cm -1 The peak of 450cm- -1 The peak of 550-610cm -1 The peak intensity increases with the increase of Pr doping, indicating that abundant oxygen vacancies are generated after Pr doping.

[0062] Figure 6 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5The EPR diagram of the catalyst after hydrogen reduction is Figure 6 It can be seen that the peak intensity increases after Pr doping, indicating that the oxygen defect concentration increases. 0.2 The oxygen defect concentration of the catalyst reaches the maximum, while Ni / CePr 0.5 The oxygen defect concentration of the catalyst is reduced, which may be due to the destruction of the bulk structure of cerium oxide caused by excessive Pr doping.

[0063] (III) Catalyst evaluation

[0064] The powder catalyst prepared in the above comparative example 1 and examples 1-4 was pressed and sieved to obtain catalyst particles of 20 to 40 mesh, and mixed evenly with quartz sand of 20 to 40 mesh, and the particles were loaded into a quartz tube fixed bed reactor with an inner diameter of Φ6mm and a length of 40cm. Then, it was pretreated at 600°C for 1h in a H2 / N2 mixed gas, and the temperature was lowered to the reaction temperature under a nitrogen atmosphere. After that, carbon dioxide, hydrogen, and nitrogen (gas volume ratio of 1:4:2) were introduced into the reactor at a flow rate of 40mL / min for reaction, and nitrogen was used as a carrier gas and an internal standard gas. The gas phase product after the reaction was separated by condensation and dried, and then entered the gas chromatograph GC-8860 for analysis.

[0065] The reaction data looks like this: Figure 7 Comparative Example 1 and Examples 1-4 are Ni-NPs / CePr 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 The CO2 conversion rate of the catalyst changes with temperature, and the gas phase space velocity is 24000mL / g cat ·h, CO2:H2=1:4. Figure 7 It can be seen that the catalyst prepared in Comparative Example 1 has a higher CO2 conversion rate, which may be because it has more nickel nanoparticles with higher CO2 hydrogenation activity. The CO2 conversion rate of the catalyst prepared in Examples 1-4 is lower than that of the comparative example, which may be because nickel exists in a single atom form and it is difficult to dissociate hydrogen, resulting in a low CO2 conversion rate. However, the CO2 conversion rate of Comparative Example 1 decreases with increasing temperature, while the CO2 conversion rate catalyzed by the catalyst prepared in Examples 1-4 of the present invention increases with increasing temperature. This is because the reaction path of the catalyst in Comparative Example 1 is methanation, which is suitable for low temperature conditions, while the reaction path of the catalyst in Examples 1-4 is reverse water gas shift.

[0066] Figure 8 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2(Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 The selectivity of the catalyst reaction product CO varies with temperature, under the following conditions: gas phase space velocity is 24000 mL / g cat ·h, CO2:H2=1:4. Figure 8 It can be clearly seen that the selectivity of CO in Comparative Example 1 is very low under low temperature (less than 500°C), while the selectivity of CO catalyzed by the catalysts prepared in Examples 1-4 under the same low temperature condition is significantly higher than that in Comparative Example 1, and the selectivity of CO is significantly improved with the increase of Pr doping. This may be because Pr doping promotes the formation of oxygen defects and the reverse water gas shift reaction path at the single atom active site. 0.2 The CO2 conversion rate of the catalyst is the largest in Examples 1-4, and the CO selectivity is the highest, reaching more than 99.5%.

[0067] Fig. 9 Ni-NPs / CePr prepared in Comparative Example 1 and Examples 1-4, respectively 0.2 (Ni nanoparticle catalyst), Ni / CeO2, Ni / CePr 0.1 、Ni / CePr 0.2 、Ni / CePr 0.5 The yield of the catalyst reaction product CO varies with temperature, under the following conditions: gas phase space velocity is 24000 mL / g cat ·h, CO2:H2=1:4. Fig. 9 It can be seen that the main product of Comparative Example 1 is methane, so its CO yield is the lowest. In Examples 1-4, the CO yield increases with the increase of Pr doping, while excessive Pr doping will cause the structure of cerium oxide to be destroyed, and the CO yield decreases. Among them, the CO yield of Example 3 is the highest.

[0068] Fig.10 Ni-NPs / CePr prepared in Comparative Example 1 0.2 (Ni nanoparticle catalyst) The CO2 conversion rate and CO selectivity of the catalyst vary with reaction time. The catalytic conditions are: reaction temperature 400℃, gas phase space velocity 24000mL / g cat ·h, CO2:H2=1:4. Fig.10 It can be seen that at 400°C, the Ni-NPs / CePr 0.2 The catalyst has high CO2 hydrogenation performance, with an initial conversion rate of 83% and a methane selectivity of nearly 99.9%, indicating that the Ni / CeO2 catalyst in Comparative Example 1 mainly undergoes methanation reaction. Fig.11Ni / CePr prepared in Example 3 0.2 The CO2 conversion rate and CO selectivity of the catalyst vary with reaction time. The catalytic conditions are: reaction temperature 400℃, gas phase space velocity 24000mL / g cat ·h, CO2:H2=1:4. Fig.11 It can be seen that the Ni / CePr prepared in Example 3 0.2 The catalyst has a very high CO selectivity, which can reach more than 99.5%, indicating that it mainly undergoes a reverse water-gas shift reaction, and the initial CO2 conversion rate can be as high as more than 23%. 0.15 The catalyst has very high low-temperature stability. After 72 hours of reaction, the CO2 conversion rate did not decrease significantly, and the CO selectivity remained above 99.5%, indicating that the catalyst has low-temperature and efficient reverse water-gas shift performance. As we all know, in the process of catalytic hydrogenation of carbon dioxide, there are many forms of catalytic pathways, such as reverse water-gas shift and methanation reaction. The mechanisms of these two reactions are different. The reverse water-gas shift mainly passes through formate, and formate can be quickly decomposed to produce CO, while the methanation reaction can pass through the carbonate pathway and then gradually hydrogenate to form methane. In the process of catalytic reverse water-gas shift, single atoms are more conducive to passing through the formate pathway, and single atoms can weaken the adsorption of intermediates, thereby allowing CO to desorb quickly and improve CO selectivity.

[0069] The above results show that CO2 hydrogenation on nickel nanoparticle catalysts goes through a methanogenic pathway, while nickel single-atom catalysts are beneficial to the reverse water-gas shift reaction. Appropriate Pr doping increases the concentration of oxygen vacancies, thereby stabilizing the nickel single-atom catalyst and promoting the reverse water-gas shift reaction. The present invention can effectively promote the reverse water-gas shift performance under low temperature conditions by preparing a stable nickel single-atom catalyst.

[0070] Example 5-7 Preparation of different catalysts

[0071] Examples 5-7 are respectively methods for preparing single atom catalysts of different low temperature and high selectivity for reverse water gas shift. The preparation process is similar to that of Example 1, and the only difference is that the corresponding technical parameters in the preparation process are different, as shown in the table below.

[0072]

[0073] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the scope of protection of the claims of the present invention.

Claims

1. A low-temperature reverse water vapor shift single atom catalyst, characterized in that: The catalyst is composed of a carrier and an active component grown in situ on the carrier, the carrier is a praseodymium-cerium oxide in a nanosheet morphology, the active component includes a transition metal located on the carrier, the transition metal is Ni, and the active component is at a single atomic scale; based on the total mass of the catalyst, the loading amount of the transition metal is 0.1-10%, and the doping amount of Pr is 0-50%, and is not 0; The method for preparing the low-temperature reverse water vapor shift single-atom catalyst is carried out in the following order: (1) Mixing a hexahydrate cerium nitrate aqueous solution, a hexahydrate praseodymium nitrate aqueous solution, and an ethanol solution of pyromellitic acid, stirring, transferring the mixed solution to a water bath, and bathing at 40 to 80° C. for 0.5 to 8 hours to obtain CePr-MOFs nanosheets; (2) After the water bath is finished, the solid is collected by filtration, washed with deionized water for 3 to 5 times, and dried at 60 to 120 °C for 12 to 48 h to obtain CePr-MOFs solid; (3) The dried CePr-MOFs solid is ground to obtain a solid powder, which is then dissolved in an ethanol aqueous solution, ultrasonicated and stirred to form a carrier solution, and a transition metal Ni nitrate aqueous solution is added dropwise to the carrier solution, stirred for 4 hours, and then washed with ethanol and water for 5 to 10 times respectively, and placed in an oven to dry for 24 hours to obtain a precursor loaded with active components; (4) The precursor is dried and ground into powder, which is then calcined in a muffle furnace at 400-700°C for 2-8 h to obtain a catalyst precursor, which is then reduced in a 5% Vol H2 / Ar mixed gas at 600°C for 1 h to obtain the final catalyst.

2. A low-temperature reverse water gas shift single atom catalyst according to claim 1, characterized in that: The thickness of the carrier is 4-40 nm.

3. The method for preparing a low-temperature reverse water gas shift single atom catalyst according to claim 1, characterized in that: Follow the steps below in order: (1) Mixing a hexahydrate cerium nitrate aqueous solution, a hexahydrate praseodymium nitrate aqueous solution, and an ethanol solution of pyromellitic acid, stirring, transferring the mixed solution to a water bath, and bathing at 40 to 80° C. for 0.5 to 8 hours to obtain CePr-MOFs nanosheets; (2) After the water bath is finished, the solid is collected by filtration, washed with deionized water for 3 to 5 times, and dried at 60 to 120 °C for 12 to 48 h to obtain CePr-MOFs solid; (3) The dried CePr-MOFs solid is ground to obtain a solid powder, which is then dissolved in an ethanol aqueous solution, ultrasonicated and stirred to form a carrier solution, and a transition metal Ni nitrate aqueous solution is added dropwise to the carrier solution, stirred for 4 hours, and then washed with ethanol and water for 5 to 10 times respectively, and placed in an oven to dry for 24 hours to obtain a precursor loaded with active components; (4) The precursor is dried and ground into powder, which is then calcined in a muffle furnace at 400-700°C for 2-8 h to obtain a catalyst precursor, which is then reduced in a 5% Vol H2 / Ar mixed gas at 600°C for 1 h to obtain the final catalyst.

4. The method for preparing a low-temperature reverse water vapor shift single atom catalyst according to claim 3, characterized in that: In step (1), the molar ratio of cerium nitrate hexahydrate, praseodymium nitrate hexahydrate and pyromellitic acid is 1:(0-1):1, and the molar amount of praseodymium nitrate hexahydrate is not 0.

5. The method for preparing a low-temperature reverse water gas shift single atom catalyst according to claim 3, characterized in that: In step (3), the concentration of the transition metal Ni nitrate aqueous solution is 1-6 mol / L.

6. The method for preparing a low-temperature reverse water gas shift single atom catalyst according to claim 3, characterized in that: In step (4), the drying temperature is 60-120° C. and the drying time is 6-24 hours.

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