CN116809070B

CN116809070B - Monoatomic catalyst for low-temperature reverse steam transformation and preparation method thereof

Info

Publication number: CN116809070B
Application number: CN202310859964.0A
Authority: CN
Inventors: 肖周荣; 王德松; 张长轩; 李鹏; 王继东; 谷建民
Original assignee: Yanshan University
Current assignee: Yanshan University
Priority date: 2023-07-13
Filing date: 2023-07-13
Publication date: 2025-05-13
Anticipated expiration: 2043-07-13
Also published as: CN116809070A

Abstract

The invention discloses a low-temperature inverse steam-shift monoatomic catalyst and a preparation method thereof, wherein the oxygen vacancy concentration and the specific surface area of the surface of the catalyst are improved by regulating the morphology of a carrier and doping rare earth elements, the rich oxygen vacancy concentration of the carrier is favorable for the adsorption, activation and dissociation of carbon dioxide of raw material gas, the conversion rate of the carbon dioxide is further increased, meanwhile, transition metal monoatomic (Co, fe and Mo) catalyst with specific load capacity is prepared for cooperatively improving the selectivity of the catalyst to carbon monoxide under the low-temperature condition. The invention is suitable for preparing carbon monoxide by catalytic hydrogenation of carbon dioxide.

Description

Monoatomic catalyst for low-temperature reverse steam transformation and preparation method thereof

Technical Field

The invention belongs to the field of carbon monoxide preparation by reverse steam conversion, namely carbon dioxide hydrogenation, and particularly relates to a low-temperature reverse steam conversion monoatomic catalyst and a preparation method thereof.

Background

The large-scale use of fossil fuels leads to the emission of a large amount of carbon dioxide, aggravates the greenhouse effect and causes global climate problems, and the concentration of carbon dioxide in the global atmosphere is continuously rising until now, so that the capture and conversion of greenhouse gases such as carbon dioxide are highly valued by various national governments and researchers. The conversion of carbon dioxide to fuel or chemicals not only effectively reduces the concentration of carbon dioxide in the atmosphere, but also artificially enhances carbon circulation and produces considerable economic benefits. The use of reverse steam reforming to convert carbon dioxide to carbon monoxide is an important chemical conversion process. However, thermodynamic studies have shown that the reaction barrier is high, it is required to be carried out at high temperatures (600-800 ℃) and the selectivity of the product CO does not reach 100%. Therefore, development of a catalyst with high performance at low temperature can effectively reduce the energy consumption of the reaction, and improvement of the conversion efficiency of carbon dioxide and the selectivity of carbon monoxide is particularly important.

The catalyst used in the process of preparing carbon monoxide by hydrogenating carbon dioxide is generally a low-cost and high-activity Cu-based catalyst, however, the Cu-based catalyst is particularly easy to sinter under the high-temperature condition, so that the stability of the catalyst is greatly reduced. Nickel-based catalysts have excellent carbon dioxide hydrogenation properties at low temperature, however, the target product is primarily methane rather than carbon monoxide. Thus, achieving selective hydrogenation of carbon dioxide to carbon monoxide at low temperature conditions using nickel-based catalysts is a significant challenge. The carrier used in the process of preparing carbon monoxide by hydrogenating carbon dioxide is generally alumina, silica, zirconia, lanthanum oxide or a mixture thereof, etc. The carrier plays an extremely important role in preparing the high-performance catalyst, namely, the existence of the carrier can greatly improve the dispersibility of the active components of the catalyst, and the existence of the carrier can promote the adsorption and activation of reactant molecules, so that the reaction rate is improved. At present, most nickel-based catalysts have the biggest defects that products are mainly methane at a low temperature, and how to adopt the nickel-based catalysts to realize the selective hydrogenation of carbon dioxide to prepare carbon monoxide at the low temperature and improve the overall reaction stability and efficiency is a current great challenge.

The present invention has been made to solve the above problems.

Disclosure of Invention

The invention aims to provide a low-temperature inverse steam-shift monoatomic catalyst and a preparation method thereof, wherein the oxygen vacancy concentration and the specific surface area of the surface of the catalyst are improved by regulating the morphology of a carrier and doping rare earth elements, the rich oxygen vacancy concentration of the carrier is favorable for the adsorption, activation and dissociation of carbon dioxide of raw material gas, so that the conversion rate of the carbon dioxide is increased, meanwhile, transition metal monoatomic (Co, fe and Mo) catalyst with specific load capacity is prepared for cooperatively improving the selectivity of the catalyst to carbon monoxide under the low-temperature condition.

The technical scheme of the invention is as follows:

The transition metal monoatomic catalyst for low-temperature high-selectivity reverse water gas shift comprises a carrier and an active component which grows on the carrier in situ, wherein the carrier is praseodymium cerium oxide in a nano sheet morphology, the active component comprises transition metal positioned on the carrier, the transition metal is one of Ni, co, fe, mo, the loading amount of the transition metal is 0.1-10% and the doping amount of Pr is 0-50% based on the total mass of the catalyst.

As a limitation of the invention, the thickness of the carrier is 4-40nm, and the active component is in a single atomic scale.

The invention also provides a preparation method of the catalyst, which is sequentially carried out according to the following step sequence:

(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 into a water bath kettle, and carrying out water bath for 0.5-8 hours at 40-80 ℃ to obtain CePr-MOFs nano sheets;

(2) After the water bath is finished, filtering and collecting solids, washing the solids with deionized water for 3-5 times, and drying the solids at 60-120 ℃ for 12-48 hours to obtain CePr-MOFs solids;

(3) Grinding the dried Ce-MOFs solid to obtain solid powder, dissolving the solid powder in an ethanol water solution, carrying out ultrasonic treatment and stirring uniformly to form a carrier solution, dripping the transition metal nitrate water solution into the carrier solution, stirring for 4 hours, washing with ethanol and water for 5-10 times respectively, and drying in an oven for 24 hours to obtain a precursor loaded with active components;

(4) And drying the precursor, grinding the dried precursor into powder, placing the powder in a muffle furnace, roasting the powder for 2-8 hours at 400-700 ℃ to obtain a catalyst precursor, and then reducing the catalyst precursor to prepare the final catalyst.

As one limitation of the preparation method of the invention, in the 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.

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

As a third limitation of the preparation method, in the step (4), the drying temperature is 60-120 ℃ and the drying time is 6-24 hours.

The preparation method of the invention is also limited, and in the step (4), the reduction preparation process is to reduce the catalyst precursor in 5% Vol H ₂/Ar mixed gas for 1H at 600 ℃.

According to the preparation method, the steps are closely related and can not be split, the praseodymium-doped Ce-MOFs nano-sheet is prepared by a water bath method, after the transition metal element is adsorbed and loaded, the praseodymium element is uniformly doped into the crystal lattice of cerium oxide during roasting, and more oxygen vacancies are formed due to the difference of atomic radiuses and valence states of praseodymium and cerium. The oxygen vacancy can promote the adsorption of carbon dioxide in the raw material gas and stabilize transition metal monoatoms, transition metal in the prepared catalyst exists in the form of monoatoms, the form can not only improve the atom utilization efficiency and inhibit the transition hydrogenation of carbon monoxide so as to improve the selectivity of carbon monoxide, but also be doped with rare earth element praseodymium so as to improve the adsorption, activation and stabilization of the corresponding transition metal monoatoms of the raw material carbon dioxide, and the synergistic effect of the transition metal monoatoms and the rare earth element promotes the stabilization of the structure of the transition metal monoatoms, strengthens the adsorption of CO ₂ in the raw material gas and weakens the adsorption strength of reaction intermediates, thereby greatly improving the performance of low-temperature reverse water-gas conversion.

After the technical scheme of the invention is adopted, the obtained technical effects are as follows:

1. The supported transition metal (Ni, co, fe, mo) monoatomic catalyst prepared by the invention has a certain degree of electropositivity relative to nano particles, not only maximizes the utilization efficiency of atoms, but also provides unique electronic properties, and can weaken dissociation of hydrogen and adsorption of intermediate product CO, thereby leading the catalyst to have excellent CO selectivity.

2. According to the invention, the concentration of oxygen vacancies on the surface of the carrier is increased by doping rare earth praseodymium, so that the abundant oxygen vacancies are beneficial to the stability of the transition metal monoatomic catalyst, and the adsorption, activation and dissociation of carbon dioxide in the raw material gas are enhanced, thereby increasing the conversion rate of carbon dioxide.

3. The preparation method is simple, the process is easy to control, the catalytic activity of the prepared catalyst is high at low temperature, and the selectivity of CO is as high as 99.5% at the temperature of less than 500 ℃.

The invention is suitable for preparing the catalyst for low-temperature high-selectivity reverse water gas shift.

The following detailed description of the invention refers to the accompanying drawings.

Drawings

FIG. 1 is SEM and TEM images of the catalyst Ni/CePr _0.2 of example 3, wherein a is an SEM image and b is a TEM image;

FIG. 2 is XRD patterns of Ni-NPs/CePr _0.2 (Ni nanoparticle catalysts) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, after calcination;

FIG. 3 is XRD patterns of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, after hydrogen reduction;

FIG. 4 is a synchrotron radiation diagram of catalyst Ni/CePr _0.2 of example 3;

FIG. 5 is a Raman spectrum of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, after hydrogen reduction;

FIG. 6 is an EPR diagram of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, after hydrogen reduction;

FIG. 7 shows the conversion of CO ₂ with temperature for the Ni-NPs/CePr _0.2 (Ni nanoparticle catalysts) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, at a gas space velocity of 24000mL/g _cat·h、CO₂:H₂ =1:4,

FIG. 8 is a graph showing the CO selectivity of the reaction products of comparative example 1 and examples 1-4, respectively, prepared Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalyst, at a gas space velocity of 24000mL/g _cat·h、CO₂:H₂ =1:4, as a function of temperature;

FIG. 9 is a graph showing the CO yields of the reaction products of comparative example 1 and examples 1 to 4, respectively, as a function of temperature for Ni-NPs/CePr _0.2 (Ni nanoparticle catalysts) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts at a gas space velocity of 24000mL/g _cat·h、CO₂:H₂ =1:4;

FIG. 10 is a graph showing the CO ₂ conversion and CO selectivity over time for the Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) catalyst prepared in comparative example 1 at a reaction temperature of 400℃and a gas space velocity of 24000mL/g _cat·h、CO₂:H₂ =1:4;

FIG. 11 is a graph showing CO ₂ conversion and CO selectivity over time for the Ni/CePr _0.2 catalyst prepared in example 3 at a reaction temperature of 400℃and a gas space velocity of 24000mL/g _cat·h、CO₂:H₂ =1:4.

Detailed Description

In the following examples, the reagents described were all commercially available unless otherwise specified, and the following experimental methods and detection methods were all employed according to the conventional experimental methods and detection methods unless otherwise specified.

Examples

Preparation of (one) catalyst

The following detailed description prepares a catalyst of transition metal single atoms supported by a cerium oxide and rare earth Pr doped cerium oxide carrier. Wherein the cerium oxide or Pr doped cerium oxide carrier is in the form of a nano-sheet. The prepared catalyst is dried, roasted, pressed, sieved and filled into a fixed bed. The purity of the chemical reagents described in the examples below were all analytically pure.

Comparative example 1 preparation of Ni-NPs/CePr _0.2 (Ni nanoparticles) catalyst

(1) 3.06 G of pyromellitic acid is weighed and dissolved in 225mL of ethanol solution, the solution A is obtained by stirring uniformly, then 4.17 g of cerium nitrate hexahydrate and 1.04 g of praseodymium nitrate hexahydrate are weighed and dissolved in 225mL of deionized water solution to obtain solution B, then the solution A and the solution B are uniformly mixed and placed in a water bath at 80 ℃ for 2 hours, the water bath is cooled to room temperature after the completion of the water bath, the Ce-Pr-MOFs are obtained by centrifugation, washing with ethanol and water for multiple times and drying, and finally the dried Ce-Pr-MOFs are placed in a muffle furnace for roasting at 600 ℃ for 4 hours, so that the praseodymium-doped CeO ₂ nano-sheets are obtained and recorded as CePr _0.2.

(2) 1.483 G of nickel nitrate hexahydrate is weighed to prepare 1.5mL of solution, 2.7 g of CePr _0.2 carrier is weighed, the prepared nickel nitrate hexahydrate solution is dripped into CePr _0.2 carrier by a straw until the solution is uniformly mixed, the solution is placed at normal temperature for 6 hours, then the solution is transferred into a 120 ℃ oven to be dried for 24 hours, then the solution is roasted in a 600 ℃ muffle furnace for 4 hours to prepare CePr _0.2 -loaded NiO (NiO/CePr _0.2), the oxide is subjected to 600 ℃ and 5% Vol H ₂/Ar mixed gas reduction for 1 hour to obtain a reduced catalyst, which is denoted as Ni-NPs/CePr _0.2, wherein the final loading amount of Ni is 10wt% based on the total mass of the catalyst.

EXAMPLE 1 preparation of Ni/CeO ₂ catalyst

(1) 3.06 G of pyromellitic acid is weighed and dissolved in 225mL of ethanol solution, the solution A is obtained after uniform stirring, then 5.22 g of cerium nitrate hexahydrate is weighed and dissolved in 225mL of deionized water solution to obtain solution B, the solution A and the solution B are uniformly mixed and placed in a water bath at 80 ℃ for 2 hours, the water bath is cooled to room temperature after the water bath is finished, and Ce-MOFs are obtained after centrifugation, ethanol and water washing for multiple times and drying.

(2) Weighing 1.483 g of nickel nitrate hexahydrate to prepare 20mL of solution, weighing Ce-MOFs synthesized in the step (1), dissolving the solution in 20mL of ethanol, dripping the prepared nickel nitrate hexahydrate solution into the ethanol solution of the Ce-MOFs, stirring the solution for 4H at normal temperature, centrifuging, washing the solution for multiple times by ethanol and water, transferring the solution into a 120 ℃ oven, drying the solution for 24H, roasting the solution in a 600 ℃ muffle furnace for 4H to prepare CeO ₂ -loaded NiO (NiO/CeO ₂), and reducing the oxide by 600 ℃ and 5% Vol H ₂/Ar mixed gas for 1H to obtain a reduced catalyst which is denoted as Ni/CeO ₂, wherein the final loading amount of Ni is 0.98wt% based on the total mass of the catalyst through atomic absorption spectrum detection.

EXAMPLE 2 preparation of Ni/CePr _0.1 catalyst

(1) 3.06 G of pyromellitic acid is weighed and dissolved in 225mL of ethanol solution, the solution A is obtained after uniform stirring, then 4.69 g of cerium nitrate hexahydrate and 0.52 g of praseodymium nitrate hexahydrate are weighed and dissolved in 225mL of deionized water solution to obtain solution B, then the solution A and the solution B are uniformly mixed and placed in a water bath at 80 ℃ for 2 hours, cooled to room temperature after the water bath is finished, and the solution A and the solution B are subjected to centrifugation, ethanol and water washing for multiple times and drying to obtain Ce-Pr _0.1 -MOFs.

(2) 1.483 G of nickel nitrate hexahydrate is weighed to prepare 20mL of solution, ce-Pr _0.1 -MOFs synthesized in the step (1) is weighed to be dissolved in 20mL of ethanol, the prepared nickel nitrate hexahydrate solution is dripped into the ethanol solution of Ce-Pr _0.1 -MOFs, stirring is carried out for 4H at normal temperature, then the solution is subjected to centrifugation, ethanol and water washing for multiple times, the solution is transferred to a 120 ℃ oven for drying for 24H, then the solution is baked for 4H in a 600 ℃ muffle furnace, cePr _0.1 -loaded NiO (NiO/CePr _0.1) is prepared, the oxide is reduced for 1H through 600 ℃ and 5% Vol H ₂/Ar mixed gas to obtain a reduced catalyst, which is denoted as Ni/CePr _0.1, wherein the final loading amount of Ni is 0.98wt% based on the total mass of the catalyst through atomic absorption spectrum detection.

EXAMPLE 3 preparation of Ni/CePr _0.2 catalyst

(1) 3.06 G of pyromellitic acid is weighed and dissolved in 225mL of ethanol solution, the solution A is obtained after uniform stirring, then 4.17 g of cerium nitrate hexahydrate and 1.04g of praseodymium nitrate hexahydrate are weighed and dissolved in 225mL of deionized water solution to obtain solution B, the solution A and the solution B are evenly mixed and placed in a water bath at 80 ℃ for 2 hours, the water bath is cooled to room temperature after completion, and the Ce-Pr _0.2 -MOFs are obtained after centrifugation, ethanol and water washing for multiple times and drying.

(2) 1.483 G of nickel nitrate hexahydrate is weighed to prepare 20mL of solution, ce-Pr _0.2 -MOFs synthesized in the step (1) is weighed to be dissolved in 20mL of ethanol, the prepared nickel nitrate hexahydrate solution is dripped into the ethanol solution of Ce-Pr _0.2 -MOFs, stirring is carried out for 4H at normal temperature, then the solution is subjected to centrifugation, ethanol and water washing for multiple times, the solution is transferred to a 120 ℃ oven for drying for 24H, then the solution is baked for 4H in a 600 ℃ muffle furnace, cePr _0.2 -loaded NiO (NiO/CePr _0.2) is prepared, the oxide is reduced for 1H through 600 ℃ and 5% Vol H ₂/Ar mixed gas to obtain a reduced catalyst, which is denoted as Ni/CePr _0.2, wherein the final loading amount of Ni is 0.98wt% based on the total mass of the catalyst through atomic absorption spectrum detection.

EXAMPLE 4 preparation of Ni/CePr _0.5 catalyst

(1) 3.06 G of pyromellitic acid is weighed and dissolved in 225mL of ethanol solution, the solution A is obtained after uniform stirring, then 2.61 g of cerium nitrate hexahydrate and 2.61 g of praseodymium nitrate hexahydrate are weighed and dissolved in 225mL of deionized water solution to obtain solution B, then the solution A and the solution B are uniformly mixed and placed in a water bath at 80 ℃ for 2 hours, cooled to room temperature after the water bath is finished, and the solution A and the solution B are subjected to centrifugation, ethanol and water washing for multiple times and drying to obtain Ce-Pr _0.5 -MOFs.

(2) 1.483 G of nickel nitrate hexahydrate is weighed to prepare 20mL of solution, ce-Pr _0.5 -MOFs synthesized in the step (1) is weighed to be dissolved in 20mL of ethanol, the prepared nickel nitrate hexahydrate solution is dripped into the ethanol solution of Ce-Pr _0.5 -MOFs, stirring is carried out for 4H at normal temperature, then the solution is subjected to centrifugation, ethanol and water washing for multiple times, the solution is transferred to a 120 ℃ oven for drying for 24H, then the solution is baked for 4H in a 600 ℃ muffle furnace, cePr _0.5 -loaded NiO (NiO/CePr _0.5) is prepared, the oxide is reduced for 1H through 600 ℃ and 5% Vol H ₂/Ar mixed gas to obtain a reduced catalyst, which is denoted as Ni/CePr _0.5, wherein the final loading amount of Ni is 0.98wt% based on the total mass of the catalyst through atomic absorption spectrum detection.

(II) characterization of the catalyst

As can be seen from the graph a in FIG. 1, the thickness of the carrier prepared in example 3 is 4-40nm, and the ultrathin structure is more beneficial to the uniform dispersion of active sites and the diffusion of reaction raw materials.

FIG. 2 shows XRD patterns of Ni-NPs/CePr _0.2 (Ni nanoparticle catalysts) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1 to 4, respectively, after calcination. As can be seen from FIG. 2, the catalyst after calcination has a distinct characteristic diffraction peak of cerium oxide, wherein the comparative catalyst nickel oxide characteristic diffraction peak appears at 43.5. Alpha. Indicating that the nickel oxide particle size is large and can be detected by XRD. The absence of characteristic peaks for nickel oxide in examples 1-4 suggests that nickel oxide is highly dispersed, possibly in a monoatomic state.

FIG. 3 shows XRD patterns of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalysts) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1 to 4, respectively, after hydrogen reduction. As can be seen from fig. 3, the reduced catalyst of comparative example 1 exhibited a significant metallic nickel diffraction peak, indicating that the nickel in the comparative example was larger in size, possibly in the form of nanoparticles, while the example did not detect nickel in the metallic state, indicating that the nickel was highly dispersed, possibly in the form of monoatoms. In addition, there is no independent characteristic peak of PrO ₂ after Pr doping in the examples, and the cerium oxide characteristic diffraction peak shifts to low angles, indicating that Pr is successfully doped into the cerium oxide lattice.

Fig. 4 is a synchrotron radiation plot of the catalyst Ni/CePr _0.2 prepared in example 3. From the synchrotron radiation results, it is clear that the nickel in example 3 is present in the form of a single atom, which advantageously supports the characteristic peaks of the XRD patterns of fig. 2 and 3, which are free of nickel oxide or metallic nickel nanoparticles.

FIG. 5 is a Raman spectrum of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, after hydrogen reduction, showing that as Pr doping increases, the peak of cerium oxide at 450cm ^-1 shifts to a low wavenumber, indicating that Pr doping is successful into the cerium oxide lattice, and especially when Pr is 0.5, the peak at 450cm ^-1 almost disappears, indicating that Pr doping damages the bulk structure of cerium oxide. In addition, the peak intensity of 550-610cm ^-1 increases with increasing Pr doping, indicating that rich oxygen vacancies are created after Pr doping.

FIG. 6 is an EPR graph of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, after hydrogen reduction, showing that the peak intensity increases after Pr doping, indicating an increase in oxygen defect concentration, wherein the oxygen defect concentration of the Ni/CePr _0.2 catalyst is maximized and the oxygen defect concentration of the Ni/CePr _0.5 catalyst is reduced, probably due to the destruction of the bulk structure of the cerium oxide caused by excessive Pr doping.

(III) evaluation of catalyst

The powder catalysts prepared in comparative example 1 and examples 1 to 4 are pressed and sieved to prepare catalyst particles with 20 to 40 meshes, the catalyst particles are uniformly mixed with quartz sand with 20 to 40 meshes, and the particles are filled into a quartz tube type fixed bed reactor, wherein the inner diameter of the reactor is phi 6mm, and the length of the reactor is 40cm. Then, the mixture was pretreated in H ₂/N₂ at 600℃for 1 hour, and the temperature was lowered to the reaction temperature under a nitrogen atmosphere. Introducing carbon dioxide, hydrogen and nitrogen (the gas volume ratio is 1:4:2) into a reactor at a flow rate of 40mL/min for reaction, taking the nitrogen as carrier gas and internal standard gas, condensing, separating and drying a gas phase product after the reaction, and then entering a gas chromatography GC-8860 for analysis.

The reaction data is shown in FIG. 7, where comparative example 1 and examples 1-4 are prepared Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalyst, respectively, and the conversion of CO ₂ varies with temperature, and the gas space velocity is 24000mL/g _cat·h,CO₂:H₂ =1:4. As can be seen from fig. 7, the catalyst prepared in comparative example 1 has a higher CO ₂ conversion, probably because it has more nickel nanoparticles with higher CO ₂ hydrogenation activity. The catalysts prepared in examples 1-4 have a lower CO ₂ conversion than the comparative examples, probably because nickel exists in the form of a single atom, which is difficult to dissociate from hydrogen, resulting in a low CO ₂ conversion. However, the conversion of CO ₂ in comparative example 1 decreases with increasing temperature, whereas the conversion of CO ₂ catalyzed by the catalysts prepared in examples 1-4 of the present invention increases with increasing temperature, because the reaction path of the catalyst of comparative example 1 is methanation, which is suitable for low temperature conditions, while the reaction path of the catalysts of examples 1-4 is reverse water shift.

FIG. 8 shows the CO selectivity of the reaction products of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalysts) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, as a function of temperature, provided that the gas space velocity was 24000mL/g _cat·h,CO₂:H₂ =1:4. It is evident from fig. 8 that the selectivity of CO is very low at low temperature (less than 500 ℃) for comparative example 1, whereas the selectivity of CO catalyzed by the catalysts prepared in examples 1-4 is significantly higher than comparative example 1 at low temperature for the same temperature, and the selectivity of CO is significantly improved as Pr doping increases, which may be that Pr doping promotes the formation of oxygen defects and passes through the reverse water shift reaction path on the monoatomic active site. Wherein the CO ₂ conversion of the Ni/CePr _0.2 catalyst of example 3 was the largest in examples 1-4 and the CO selectivity was the highest, reaching above 99.5%.

FIG. 9 shows the CO yields of the reaction products of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalysts) and Ni/CeO ₂、Ni/CePr_0.1、Ni/CePr_0.2、Ni/CePr_0.5 catalysts prepared in comparative example 1 and examples 1-4, respectively, as a function of temperature, provided that the gas space velocity was 24000mL/g _cat·h,CO₂:H₂ =1:4. As can be seen from fig. 9, the main product of comparative example 1 is methane, and thus, the CO yield thereof is the lowest. Pr doping in examples 1-4 increases the CO yield, whereas overdoping Pr leads to structural failure of the cerium oxide, and CO yield is reduced, with the CO yield of example 3 being highest.

FIG. 10 shows the CO ₂ conversion of the Ni-NPs/CePr _0.2 (Ni nanoparticle catalyst) catalyst prepared in comparative example 1, CO selectivity as a function of reaction time, catalytic conditions were a reaction temperature of 400℃and a gas space velocity of 24000mL/g _cat·h,CO₂:H₂ =1:4. As can be seen from FIG. 10, the Ni-NPs/CePr _0.2 catalyst of comparative example 1 has high CO ₂ hydrogenation performance under the condition of 400 ℃, the initial conversion rate can reach 83%, and the methane selectivity is close to 99.9%, which indicates that the Ni/CeO ₂ catalyst of comparative example 1 mainly undergoes methanation reaction. FIG. 11 shows the CO ₂ conversion, CO selectivity as a function of reaction time for the Ni/CePr _0.2 catalyst prepared in example 3, at a reaction temperature of 400℃and a gas space velocity of 24000mL/g _cat·h,CO₂:H₂ =1:4. As can be seen from FIG. 11, the Ni/CePr _0.2 catalyst prepared in example 3 has very high CO selectivity, which can reach more than 99.5%, indicating that the reverse water-gas shift reaction mainly occurs, and the initial conversion rate of CO ₂ can reach more than 23%. In addition, the Ni/CeIn _0.15 catalyst of example 3 has very high low temperature stability, the CO ₂ conversion rate is not obviously reduced after 72 hours of reaction, and the CO selectivity is maintained to be more than 99.5%, which indicates that the catalyst has low temperature and high reverse water-gas shift performance. It is known that in the process of catalyzing the hydrogenation of carbon dioxide, the catalytic route has various forms, such as reverse water gas shift and methanation reaction, the two reactions have different mechanisms, the reverse water gas shift mainly passes through formate, the formate can be rapidly decomposed to prepare CO, the methanation reaction can pass through a carbonate route and then gradually and deeply hydrogenate to form methane, in the process of catalyzing the reverse water gas shift, single atoms are more beneficial to pass through the formate route, and the single atoms can weaken the adsorption of intermediates, so that CO is rapidly desorbed, and the selectivity of CO is improved.

The above results demonstrate that CO ₂ hydrogenation is methanation path over nickel nanoparticle catalysts, whereas nickel monoatomic catalysts are favorable for reverse water gas shift reactions, and proper Pr doping increases the concentration of oxygen vacancies to stabilize the nickel monoatomic catalysts and promote reverse water gas shift reactions. The invention can effectively promote the reverse water-gas conversion performance under the low temperature condition by preparing the stable nickel monoatomic catalyst.

EXAMPLES 5-7 preparation of different catalysts

Examples 5-7 are different methods for preparing the monoatomic catalysts for low temperature high selectivity reverse water gas shift, respectively, the preparation process is similar to example 1, and the only difference is that the corresponding technical parameters in the preparation process are different, and the specific table is shown below.

It should be noted that the above-mentioned embodiments are merely preferred embodiments of the present invention, and the present invention is not limited thereto, but may be modified or substituted for some of the technical features thereof by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A low-temperature reverse water gas shift single-atom catalyst, characterized in that the catalyst consists of a carrier and an active component grown in situ on the carrier, the carrier is a praseodymium-cerium oxide in 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-atom 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 was dried and ground into powder, and calcined in a muffle furnace at 400-700°C for 2-8h to obtain a catalyst precursor, which was then reduced at 600°C in a 5% Vol H ₂ /Ar mixed gas for 1h to obtain the final catalyst.

2. A single-atom catalyst for low-temperature reverse water vapor shift 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 vapor shift single atom catalyst according to claim 1, characterized in that the steps are carried out in the following order:

4. The method for preparing a low-temperature reverse water gas 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 vapor 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.