Prussian blue analogue derived Ce doped Co-based catalyst and preparation method thereof

Abstract

The invention provides a Prussian blue analogue derivative Ce-doped Co-based catalyst and a preparation method thereof.The preparation method comprises the steps that cerium salt and cobalt salt are dissolved and mixed to prepare a mixed solution, cobalt potassium cyanide solution is prepared, the cobalt potassium cyanide solution is dropwise added into the mixed solution, and the Prussian blue analogue derivative Ce-doped Co-based catalyst is obtained after the cobalt potassium cyanide solution is dropwise added into the mixed solution; the Ce-doped Co-based catalyst is prepared by dropwise adding a mixed solution or a cobalt cyanide potassium solution, adding polyvinylpyrrolidone into the mixed solution or the cobalt cyanide potassium solution before dropwise adding to further regulate and control the reaction rate, carrying out stirring reaction and aging to obtain a metal organic framework precursor with an irregular morphology, and finally calcining to obtain the Ce-doped Co-based catalyst. Based on earlier-stage scientific achievements, when cobalt salt is additionally added to regulate and control the proportion of Ce and Co elements, it is accidentally found that when other cobalt salt and cerium salt coexist, Co-Co coordinated Ce-doped irregular-morphology Prussian blue analogues can be formed, and under the specific Ce and Co element proportion condition, the Prussian blue analogues can be used for preparing the Prussian blue analogues with the Co-Co coordinated Ce-doped irregular-morphology Prussian blue analogues with the Co-Co coordinated Ce-doped irregular-morphology Prussian blue analogues. And after being calcined, the catalyst has obviously higher ammonia decomposition conversion rate under a relatively low-temperature condition.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hydrogen production catalysts by ammonia decomposition, and relates to a Prussian blue analogue derivative Ce-doped Co-based catalyst and a preparation method thereof. BACKGROUND

[0002] With the promotion of the global "double carbon" goal, hydrogen energy, as a kind of secondary energy with abundant resources, green and low carbon, and wide application, is regarded as a key carrier to build a clean energy system in the future. Under the background of the transformation of global energy structure to low carbonization, hydrogen energy, as a kind of efficient and clean secondary energy carrier, is regarded as a key path to solve the crisis of fossil energy and environmental problems. However, the low volumetric energy density of hydrogen and the serious safety problems of storage and transportation restrict its large-scale application, which seriously hinders the development of hydrogen energy economy.

[0003] Ammonia (NH3) as an ideal hydrogen storage medium has attracted widespread attention due to its high hydrogen storage density, and its synthesis, storage and transportation industrial infrastructure are relatively mature. Ammonia has high hydrogen storage density, with hydrogen content of 17.6 wt%, and volumetric hydrogen storage density of 121 kg H2 / m 3 (10 bar), which is significantly better than liquid hydrogen; the liquefaction condition is mild, which can be liquefied at room temperature, greatly reducing the energy consumption of storage and transportation; the safety is good, the explosion limit range is narrow (16%~25%), and the leakage risk can be controlled; and ammonia as a carbon-free hydrogen carrier will not produce carbon-containing gas in the decomposition process, and is considered as a hydrogen energy carrier with great application potential.

[0004] However, the ammonia decomposition reaction has a significant kinetic barrier, so it needs to rely on high-efficiency catalysts to reduce the activation energy, and therefore the design and development of related catalysts have become the focus of attention of the academic and industrial circles.

[0005] For example, the inventors of the present application previously applied for a patent "Long-acting and low-cost Ru-based catalyst for ammonia decomposition to produce hydrogen and a preparation method thereof" (CN120305963A), which disclosed a long-acting and low-cost Ru-based catalyst for ammonia decomposition to produce hydrogen and a preparation method thereof. The preparation method is to first prepare a mixed solution by adding calcium salt or strontium salt and cerium nitrate hexahydrate into deionized water and adjusting the pH value, then performing closed heating treatment to obtain a precipitate, and then performing calcination and grinding in sequence to prepare a carrier powder, adding an appropriate amount of ruthenium salt to form a Ru-carrier dispersion liquid by stirring uniformly, and finally performing calcination and grinding in sequence to obtain the Ru-based catalyst for ammonia decomposition to produce hydrogen. The catalyst has a lower conventional Ru loading amount, and can exhibit good ammonia decomposition hydrogen production performance at a 10% NH3 / Ar mass space velocity (GHSV) of 30000 ml / (g cat •h) and a low temperature of 450℃, and it is unexpectedly found that the catalyst can maintain long-term stability for at least 100 hours or more.

[0006] However, in the actual industrial system, we noticed that although Ru-based catalysts exhibit excellent activity at low temperature, the scarcity and high cost of noble metal Ru restrict its large-scale industrial application. Therefore, the development of non-noble metal catalysts becomes the key to promote the practical application of ammonia decomposition hydrogen production technology, among which Co-based catalysts have become a research hotspot in recent years due to their activity and cost balance advantage. However, Co-based catalysts still have the problem of insufficient low-temperature activity, mainly because the Co-N binding energy is weak, which leads to insufficient ability of Co-based catalysts to activate NH3 and break N-H bonds. SUMMARY

[0007] To solve the above problems in the prior art, a Prussian blue analogue derived Ce doped Co-based catalyst and a preparation method thereof are provided. Based on the previous research results of the inventors' team: hexagonal nanosheet structure metal organic framework based on Prussian blue analogue Ce and Co isomerism is used as a precursor to prepare low-temperature and high-efficiency ammonia decomposition hydrogen production Co-based catalyst, when additional cobalt salt is added to regulate the element ratio of Ce and Co, it is accidentally found that when other cobalt salt and cerium salt coexist, Co-Co coordination Ce doped irregular morphology Prussian blue analogue will be formed, and under the condition of a specific element ratio of Ce and Co, the ammonia decomposition conversion rate after calcination is obviously higher (up to 95%) at a relatively low temperature (550℃).

[0008] To achieve the above purpose, the present application is realized by the following technical measures.

[0009] In one aspect, the present application provides a preparation method of a Prussian blue analogue derived Ce doped Co-based catalyst, mainly comprising the following steps:

[0010] (1) Dissolve and mix cerium salt and cobalt salt to prepare a mixed solution, and dissolve K3[Co(CN)6] to prepare a potassium cobalt cyanide solution;

[0011] Among them, the molar ratio of Ce element to Co element in the mixed solution is (0.5-2):(3-4.5), and the total molar amount of Ce element and Co element in the mixed solution is equal to the molar amount of Co element in the potassium cobalt cyanide solution;

[0012] (2) Drop the potassium cobalt cyanide solution into the mixed solution, and further control the reaction rate by adding polyvinylpyrrolidone (PVP) in the mixed solution or the potassium cobalt cyanide solution before dropping, continue to stir for 2.5-3.5 h after dropping, and then age for at least 12 h, separate the reaction product, and obtain the Prussian blue analogue Ce doped Co-Co coordination irregular morphology metal organic framework precursor after washing and drying;

[0013] (3) calcining the irregular morphology metal-organic framework precursor obtained in step (2) under a temperature condition of 550-650 °C to prepare a Prussian blue analogue Ce-doped Co-Co coordination derived Co / CeO2 catalyst, i.e. a Prussian blue analogue derived Ce-doped Co-based catalyst.

[0014] In this context, the cerium salt in step (1) is a selection of a conventional metal salt raw material used in a cerium ion solution, and the cobalt salt is a selection of a conventional metal salt raw material used in a cobalt ion solution, and those skilled in the art can make a specific selection according to the conventional metal salts known to be added to a metal ion solution in the prior art and in combination with actual needs, the cerium salt includes but is not limited to cerium nitrate, cerium chloride, etc., and the cobalt salt includes but is not limited to cobalt nitrate, cobalt chloride, cobalt acetate, etc.; in order to simplify the experimental operation and design a control experiment, the cerium salt selected in the following specific embodiments is cerium nitrate, and the cobalt salt is cerium nitrate. Those skilled in the art should clearly recognize that the cerium salt and the cobalt salt in step (1) mainly provide cerium ions and cobalt ions, which are different from K3[Co(CN)6] in Prussian blue analogue, and the selection of the specific anion radical in the cerium salt and the cobalt salt generally does not affect the technical effects achieved by the present application. However, it should be further pointed out that the selected cerium salt and cobalt salt should have solubility in the selected solvent to form a mixed solution of cerium ions and cobalt ions.

[0015] Generally, in step (1), the cerium salt and the cobalt salt are dissolved and mixed to prepare a mixed solution, and K3[Co(CN)6] is dissolved to prepare a potassium cobalt cyanide solution. Since it involves a metal coordination reaction, the solvent of the mixed solution and the potassium cobalt cyanide solution should be an aqueous solvent, for example, deionized water as the solvent.

[0016] In order to better illustrate the present application and provide a technical solution for reference, the solvent of the mixed solution and the potassium cobalt cyanide solution is deionized water, the molar concentration of cerium ions in the mixed solution is 0.02-0.08 mmol / mL, the molar concentration of cobalt ions in the mixed solution is 0.12-0.18 mmol / mL, and the molar concentration of K3[Co(CN)6] in the potassium cobalt cyanide solution is 0.1-0.3 mmol / mL.

[0017] It should be noted that although the Ce element still has a certain proportion in the mixed solution in step (1), during the formation of the Prussian blue analogue, due to the closer charge and ionic radius, potassium cobalt cyanide is more easily coordinated with cobalt nitrate, and Ce is finally doped in the Prussian blue analogue in the form of adsorption. In order to fully guarantee the accuracy of the ratio of Ce and Co elements in the precursor, the sum of the molar amounts of Ce and Co elements in the mixed solution is equal to the molar amount of Co element in the potassium cobalt cyanide solution.

[0018] It should be emphasized that through subsequent comparative tests, it is found that when other cobalt salts exist, the obtained Prussian blue analogue Ce-doped Co-Co-coordinated metal organic framework has an irregular morphology under a microscope, which is significantly different from the Prussian blue analogue Ce, Co-equal coordination hexagonal nanosheet structure metal organic framework, proving that the two are different reaction products.

[0019] In this paper, the further regulation of the reaction rate by adding polyvinylpyrrolidone (PVP) to the mixed solution or potassium cobalt cyanide solution before dropping in step (2) means that a proper amount of polyvinylpyrrolidone (PVP) is added to the mixed solution or potassium cobalt cyanide solution obtained in step (1) and mixed uniformly to regulate the reaction rate in the subsequent reaction process, guide the crystal to grow towards a specific shape that is more stable in thermodynamics, and limit the disorder and rapid growth of the crystal in various directions. It should be emphasized that if the reaction is carried out without adding polyvinylpyrrolidone (PVP), the synthesized Prussian blue analogue precursor will have serious problems such as agglomeration, morphology out of control, size unevenness, and poor crystallinity. It should be noted that the regulation of the reaction rate by adding polyvinylpyrrolidone (PVP) is a process step known in the art for regulating the reaction rate, so those skilled in the art can know the specific amount of polyvinylpyrrolidone (PVP) added by referring to existing technical literature and combining with specific experimental phenomena.

[0020] In order to better illustrate the present application, and provide a preferred technical solution for reference, the further regulation of the reaction rate by adding polyvinylpyrrolidone (PVP) to the mixed solution or potassium cobalt cyanide solution before dropping in step (2) means that a proper amount of polyvinylpyrrolidone (PVP) is added to the mixed solution or potassium cobalt cyanide solution obtained in step (1) and mixed uniformly, and the addition amount of polyvinylpyrrolidone (PVP) is 0.15-0.25 g per 1 mmol K3[Co(CN)6].

[0021] In this paper, the reaction product is separated in step (2), washed and dried. The washing and drying of the separated reaction product are conventional processes in the chemical industry, and those skilled in the art can choose appropriate washing and drying methods according to common knowledge or conventional processes.

[0022] In order to better illustrate the present application, and provide a technical solution for reference, the washing and drying in step (2) are as follows: the washing is alternating washing with deionized water and anhydrous ethanol for at least 3 times to better remove impurity ions in the reaction product; the drying is preferably drying treatment of the washed reaction product at a temperature of 60-80 DEG C for at least 24 h, and keeping dry until calcination.

[0023] In this paper, the irregular morphology metal organic framework precursor in step (3) is calcined at a temperature of 550-650 DEG C, wherein the calcination is to convert the metal components in the precursor into oxides, and the specific calcination mode / parameters in the conventional chemical process can be directly referred to by those skilled in the art for processing.

[0024] In order to better illustrate the present application, and provide a technical solution for reference, the calcination at a temperature of 550-650 DEG C in step (3) includes the following specific process parameters: heating at a heating rate of 4.5-5.5 DEG C / min to 550-650 DEG C, keeping for 2.5-3.5 h, and finally naturally cooling to room temperature.

[0025] In this paper, the irregular morphology metal organic framework precursor in step (3) is calcined at a temperature of 550-650 DEG C, wherein the calcination is to convert the metal components in the precursor into oxides, and the specific calcination mode / parameters in the conventional chemical process can be directly referred to by those skilled in the art for processing.

[0026] The team of the present inventors found in previous research that the Ce, Co-based Prussian blue analogue hexagonal nanosheet structure metal organic framework as a precursor, unlike the traditional deposition and precipitation method of doping metal ions, because Ce and Co present an atomic level alternating distribution form, the precursor forms Co nanoparticles with uniform distribution in the calcination process, and there is a close interface contact between the Co nanoparticles and the CeO2 carrier rich in oxygen vacancies, which enhances the strong metal-support interaction between CeO2 and the surface Co nanoparticles. The strong metal-support interaction in the Co-based catalyst obtained after calcination promotes the transfer of electrons from CeO2 to Co nanoparticles, thereby increasing the electron density of the active sites, enhancing the ability of Co sites to adsorb and activate NH3, and weakening the energy barrier of N-H bond breaking. And the CeO2 surface rich in oxygen vacancies promotes the transfer of H species produced by NH3 decomposition from Co to the carrier, thereby timely releasing the active sites and avoiding the problem of poisoning of the active sites.

[0027] The Co-based catalyst for low-temperature and high-efficiency ammonia decomposition prepared in the above prior study can reach 90% ammonia decomposition conversion rate at 550 DEG C under the condition that the pure NH3 mass space velocity (GHSV) is 30000 ml / (g cat •h).

[0028] To further improve the ammonia decomposition conversion rate of the catalyst, in the subsequent research and development process, based on the specific coordination structure of the Prussian blue analogue, when the cobalt salt is additionally added to regulate the element ratio of Ce and Co, it is accidentally found that when other cobalt salts coexist with cerium salts, a Co-Co coordination Ce-doped irregular morphology Prussian blue analogue is formed, and under the condition of a specific element ratio of Ce and Co, the catalyst prepared after calcination can reach 74% ammonia decomposition conversion rate at 500 DEG C and 95% ammonia conversion rate at 550 DEG C under the condition that the pure NH3 mass space velocity (GHSV) is 30000 ml / (g cat •h).

[0029] In this paper, the mixing, washing and drying all follow the conventional principles in the chemical process, and those skilled in the art can perform specific operations according to common knowledge.

[0030] The present application has the following beneficial effects:

[0031] 1. The present application provides a Prussian blue analogue derived Ce-doped Co-based catalyst and a preparation method thereof, the Ce-doped Co-based catalyst is prepared by calcining a Co-Co coordination Ce-doped irregular morphology Prussian blue analogue as a precursor, and it is found that under the condition of a specific element ratio of Ce and Co, the catalyst has a significantly higher ammonia decomposition conversion rate (up to 95%) at a relatively low temperature (550 DEG C), which further improves the ammonia conversion rate compared with the prior study.

[0032] 2. The present application provides a Prussian blue analogue derived Ce-doped Co-based catalyst and a preparation method thereof, the catalyst can reach 95% ammonia decomposition conversion rate at 550 DEG C and 74% ammonia decomposition conversion rate at 500 DEG C under the condition that the pure NH3 mass space velocity (GHSV) is 30000 ml / (g cat •h). BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The scanning electron microscope (SEM) images of the precursor before calcination and the sample obtained after calcination in Example 1 of the present application are shown. The left image is the scanning electron microscope (SEM) image of the precursor before calcination in Example 1, and the right image is the scanning electron microscope (SEM) image of the sample obtained after calcination in Example 1.

[0034] Figure 2 The ammonia decomposition conversion rate of the catalyst prepared in Examples 1-3 and Comparative Examples 1-3 of the present application under various temperature conditions is shown in the following figures. Figure (a) is a line graph showing the ammonia decomposition conversion rate of the catalyst prepared in Examples 1-3 and Comparative Examples 1-3 under various temperature conditions, and Figure (b) is a column graph showing the ammonia decomposition conversion rate of the catalyst prepared in Examples 1-3 and Comparative Examples 1-3 at 500°C.

[0035] Figure 3 The hydrogen production rate of the catalyst prepared in Examples 1-3 and Comparative Examples 1-3 of the present application under various temperature conditions is shown in the following figures.

[0036] Figure 4 The ammonia decomposition conversion rate of the catalyst prepared in Example 1 of the present application under various temperature conditions is shown in the following figures. Figure (a) is a line graph showing the ammonia decomposition conversion rate of the catalyst prepared in Example 1 under various temperature conditions, and Figure (b) is a line graph showing the ammonia decomposition conversion rate of the catalyst prepared in Example 1 under various mass space velocities.

[0037] Figure 5 The XRD diffractogram of the precursor before calcination in Examples 1-3 and Comparative Examples 1-3 of the present application is shown in the following figure.

[0038] Figure 6 The XRD diffractogram of the catalyst prepared in Examples 1-3 and Comparative Examples 1-3 of the present application after calcination is shown in the following figure.

[0039] Figure 7 The XRD diffractogram of the catalyst prepared in Examples 1-3 and Comparative Examples 1-3 of the present application after reduction with 20% H2 / Ar during testing is shown in the following figure.

[0040] Figure 8 The XPS full spectrum, K 2p XPS spectrum and Co 2p XPS spectrum of the catalyst prepared in Examples 1-3 and Comparative Examples 1-3 of the present application are shown in the following figures. DETAILED DESCRIPTION

[0041] For a further understanding of the present application, preferred embodiments thereof will be described in conjunction with examples, it being understood, however, that this description is made only by way of further illustration and is not intended to limit the scope of the application. Those skilled in the art can adapt the process parameters to the specific situations with the content herein. It is specifically pointed out that all such similar alterations and modifications of the devices and methods described herein are contemplated as being within the scope of the present application. The methods and applications of the present application have been described by way of preferred embodiments, and it will be obvious to those skilled in the art that the methods and applications described herein can be modified or adapted or combined appropriately without departing from the content, spirit and scope of the present application to realize and apply the present application technology. Although the following terms are believed to be well understood by those skilled in the art, the following definitions are set forth to facilitate the understanding of the presently disclosed subject matter.

[0042] In one aspect, the present application provides a method for preparing a Prussian blue analogue derived Ce-doped Co-based catalyst, mainly comprising the following steps:

[0043] (1) Dissolving and mixing cerium salt and cobalt salt to prepare a mixed solution, and dissolving K3[Co(CN)6] to prepare a potassium cobalt cyanide solution;

[0044] wherein the molar ratio of Ce element to Co element in the mixed solution is (0.5-2):(3-4.5), and the sum of the molar amounts of Ce element and Co element in the mixed solution is equal to the molar amount of Co element in the potassium cobalt cyanide solution;

[0045] (2) Adding the potassium cobalt cyanide solution dropwise into the mixed solution, and further controlling the reaction rate by adding polyvinylpyrrolidone (PVP) into the mixed solution or the potassium cobalt cyanide solution before dropping, continuing to stir for 2.5-3.5 h after dropping is completed, and then aging for at least 12 h, separating the reaction product, and washing and drying to obtain a Prussian blue analogue Ce-doped Co-Co coordinated irregular morphology metal organic framework precursor;

[0046] (3) Calcining the irregular morphology metal organic framework precursor obtained in step (2) at a temperature of 550-650 °C to prepare a Co / CeO2 catalyst derived from a Prussian blue analogue Ce-doped Co-Co coordination, i.e. a Prussian blue analogue derived Ce-doped Co-based catalyst.

[0047] In the present disclosure, the cerium salt in step (1) is selected from the metal salt raw materials commonly used in cerium ion solution, and the cobalt salt is selected from the metal salt raw materials commonly used in cobalt ion solution. Those skilled in the art can select the conventional metal salt added in metal ion solution according to the prior art and in combination with the actual needs. In one embodiment, the cerium salt includes but is not limited to cerium nitrate, cerium chloride, etc., and the cobalt salt includes but is not limited to cobalt nitrate, cobalt chloride, cobalt acetate, etc. In order to simplify the experimental operation and facilitate the design of control experiments, the cerium salt selected in the following specific embodiments is cerium nitrate, and the cobalt salt is cerium nitrate. Those skilled in the art should clearly understand that the cerium salt and the cobalt salt in step (1) mainly provide cerium ions and cobalt ions, which are different from K3[Co(CN)6] in Prussian blue analogues. The selection of the specific anion radical in the cerium salt and the cobalt salt generally does not affect the technical effects achieved by the present disclosure. However, it should be further pointed out that the selected cerium salt and cobalt salt should have solubility in the selected solvent to form a mixed solution of cerium ions and cobalt ions.

[0048] Generally, the cerium salt and the cobalt salt are dissolved and mixed to prepare a mixed solution in step (1), and K3[Co(CN)6] is dissolved and prepared to obtain a potassium cobalt cyanide solution. Since metal coordination reactions are involved, the solvents of the mixed solution and the potassium cobalt cyanide solution are preferably aqueous solvents. In one embodiment, for example, deionized water is used as the solvent.

[0049] In order to better illustrate the present disclosure and provide an embodiment for reference, the solvent of the mixed solution and the potassium cobalt cyanide solution is deionized water, the molar concentration of cerium ions in the mixed solution is 0.02-0.08 mmol / mL, for example, 0.02 mmol / mL, 0.03 mmol / mL, 0.04 mmol / mL, 0.05 mmol / mL, 0.06 mmol / mL, 0.07 mmol / mL, 0.08 mmol / mL, or any range or point value therebetween; the molar concentration of cobalt ions in the mixed solution is 0.12-0.18 mmol / mL, for example, 0.12 mmol / mL, 0.13 mmol / mL, 0.14 mmol / mL, 0.15 mmol / mL, 0.16 mmol / mL, 0.17 mmol / mL, 0.18 mmol / mL, or any range or point value therebetween; and the molar concentration of K3[Co(CN)6] in the potassium cobalt cyanide solution is 0.1-0.3 mmol / mL, for example, 0.1 mmol / mL, 0.15 mmol / mL, 0.2 mmol / mL, 0.25 mmol / mL, 0.3 mmol / mL, or any range or point value therebetween.

[0050] It should be noted that although the Ce element still has a certain proportion in the mixed solution in step (1), during the formation of the Prussian blue analogue, due to the closer charge and ionic radius, potassium cobalt cyanide is more easily coordinated with cobalt nitrate, and Ce is finally doped in the Prussian blue analogue in the form of adsorption. In order to fully guarantee the accuracy of the ratio of Ce and Co elements in the precursor, the total amount of the molar quantity of the Ce element and the Co element in the mixed solution is equal to the molar quantity of the Co element in the potassium cobalt cyanide solution.

[0051] It should be emphasized that through subsequent comparative experiments, it was found that when other cobalt salts are present, the obtained Prussian blue analogue Ce-doped Co-Co-coordinated metal organic framework has an irregular morphology under a microscope, which is significantly different from the Prussian blue analogue Ce, Co-equivalent-coordinated hexagonal nanosheet structure metal organic framework, proving that the two are different reaction products.

[0052] In this paper, the further regulation of the reaction rate by adding polyvinylpyrrolidone (PVP) to the mixed solution or potassium cobalt cyanide solution before dropping in step (2) means that a suitable amount of polyvinylpyrrolidone (PVP) is added to the mixed solution or potassium cobalt cyanide solution obtained in step (1) and mixed uniformly to regulate the reaction rate in the subsequent reaction process, guide the crystal to grow towards a specific shape that is more stable thermodynamically, and limit the disorder and rapid growth of the crystal in all directions. It should be emphasized that if the reaction is carried out without the addition of polyvinylpyrrolidone (PVP), the synthesized Prussian blue analogue precursor will have serious problems such as agglomeration, morphology loss of control, size unevenness, and poor crystallinity. It should be noted that the process step of regulating the reaction rate by adding polyvinylpyrrolidone (PVP) is known in the art, so the skilled person can know the specific amount of polyvinylpyrrolidone (PVP) added by referring to the existing technical literature and combining with the specific experimental phenomena.

[0053] In order to better illustrate the present application, and to provide a preferred embodiment for reference, the further regulation of the reaction rate by adding polyvinylpyrrolidone (PVP) to the mixed solution or potassium cobalt cyanide solution before dropping in step (2) means that a suitable amount of polyvinylpyrrolidone (PVP) is added to the mixed solution or potassium cobalt cyanide solution obtained in step (1) and mixed uniformly, and the amount of polyvinylpyrrolidone (PVP) added is 0.15-0.25 g per 1 mmol of K3[Co(CN)6], for example, 0.15 g, 0.16 g, 0.17 g, 0.18 g, 0.19 g, 0.2 g, 0.21 g, 0.22 g, 0.23 g, 0.24 g, 0.25 g, or any range or point value therebetween.

[0054] In one embodiment, the stirring is continued for 2.5-3.5 h, for example 2.5 h, 2.6 h, 2.7 h, 2.8 h, 2.9 h, 3 h, 3.1 h, 3.2 h, 3.3 h, 3.4 h, 3.5 h or any range or point value between them after the dropwise addition in step (2) is completed.

[0055] In this context, the separation of the reaction product in step (2), washing and drying are conventional processes in chemical industry, and the person skilled in the art can select appropriate washing and drying methods according to common knowledge or conventional processes.

[0056] In order to better illustrate the present application and provide a reference embodiment, the washing and drying in step (2) are as follows: the washing is alternating washing with deionized water and anhydrous ethanol for at least 3 times to better remove impurity ions in the reaction product; the drying is preferably drying treatment of the washed reaction product at a temperature of 60-80 ℃ for at least 24 h, and keeping dry until calcination.

[0057] In this context, the calcination of the irregular morphology metal organic framework precursor in step (3) is carried out at a temperature of 550-650 ℃, in which the metal components in the precursor are converted into oxides, and the person skilled in the art can directly refer to the conventional method of preparing cerium dioxide by calcination for processing.

[0058] In order to better illustrate the present application and provide a reference embodiment, the calcination in step (3) is carried out at a temperature of 550-650 ℃, and the specific process parameters of the calcination include: heating at a heating rate of 4.5-5.5 ℃ / min to 550-650 ℃, keeping for 2.5-3.5 h, and finally naturally cooling to room temperature.

[0059] In this context, the calcination of the irregular morphology metal organic framework precursor in step (3) is carried out at a temperature of 550-650 ℃, in which the metal components in the precursor are converted into oxides, and the person skilled in the art can directly refer to the conventional method of preparing cerium dioxide by calcination for processing.

[0060] The inventors of the present application found in previous studies that based on the Ce, Co analog of Prussian blue, the hexagonal nanosheet structure metal organic framework with equal coordination as a precursor, unlike the traditional deposition precipitation method for doping metal ions, because Ce and Co present an atomic level of alternating distribution form, therefore the precursor forms Co nanoparticles with uniform distribution during calcination, and there is a close interface contact between the Co nanoparticles and the CeO2 carrier rich in oxygen vacancies, which enhances the strong metal-support interaction between CeO2 and the surface Co nanoparticles. The strong metal-support interaction in the Co-based catalyst obtained after calcination promotes the transfer of electrons from CeO2 to Co nanoparticles, thereby increasing the electron density of the active sites, enhancing the ability of Co sites to adsorb and activate NH3, and weakening the energy barrier of N-H bond breaking. And the CeO2 surface rich in oxygen vacancies promotes the transfer of H species produced by NH3 decomposition from Co to the carrier, thereby timely releasing the active sites and avoiding the problem of poisoning of the active sites.

[0061] In the above-mentioned previous studies, the low-temperature and high-efficiency ammonia decomposition hydrogen production Co-based catalyst prepared can achieve an ammonia decomposition conversion rate of 90% at 550 ℃ under the condition of a pure NH3 mass space velocity (GHSV) of 30000 ml / (g cat •h).

[0062] In order to further improve the ammonia decomposition conversion rate of the catalyst, in the subsequent research and development process, based on the specific coordination structure of the Prussian blue analog, when additional cobalt salt is added to regulate the element ratio of Ce and Co, it is accidentally found that when other cobalt salt coexists with cerium salt, Co-Co coordinated Ce-doped irregular morphology Prussian blue analogs will be formed, and under the condition of a specific element ratio of Ce and Co, the catalyst prepared after calcination can achieve an ammonia decomposition conversion rate of 74% at 500 ℃ and 95% at 550 ℃ under the condition of a pure NH3 mass space velocity (GHSV) of 30000 ml / (g cat •h), further improving the ammonia decomposition conversion rate of the catalyst.

[0063] In this paper, the mixing, washing, drying all follow the conventional principles in chemical process, and those skilled in the art can perform specific operations according to common knowledge.

[0064] The present application will be further explained in detail with reference to the following examples. However, those skilled in the art should understand that these examples are provided only for illustrative purposes, and are not intended to limit the present application.

[0065] Example

[0066] The embodiments of the present application will be described in detail below with examples, but those skilled in the art will understand that the following examples are only for illustration of the present application and should not be regarded as limiting the scope of the present application. The specific conditions not noted in the examples are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not noted by the manufacturer, which are all conventional products that can be obtained by market purchase. The present application should not be interpreted as being limited to the specific examples described.

[0067] 1. Raw materials

[0068]

[0069] 2. Preparation method

[0070] (1) 0.5-2 mmol of cerium nitrate hexahydrate and 3-4.5 mmol of cobalt nitrate hexahydrate were added to 25 mL of deionized water to dissolve and prepare a mixed solution;

[0071] 5 mmol of potassium cobalt cyanide and 1 g of polyvinylpyrrolidone (PVP) were added to 25 mL of deionized water to dissolve and prepare a potassium cobalt cyanide solution;

[0072] (2) The potassium cobalt cyanide solution was added dropwise to the mixed solution, and after the dropwise addition was completed, the reaction was continued to be stirred for 3.5 h, and then aged for 12 h. The solid reaction product was separated, washed, and dried to obtain a Prussian blue analogue Ce-doped Co-Co coordinated irregular morphology metal organic framework precursor;

[0073] The washing and drying were specifically that the reaction product was washed with deionized water and anhydrous ethanol alternately for 3 times to better remove impurity ions in the reaction product; and the drying was that the washed reaction product was dried at 60 °C for 24 h, and kept dry until calcination;

[0074] (3) The irregular morphology metal organic framework precursor obtained in step (2) was heated to 600 °C at a heating rate of 5 °C / min in air, and then calcined at 600 °C for 3 h. Finally, after natural cooling to room temperature, the Co / CeO2 catalyst derived from the Prussian blue analogue Ce-doped Co-Co coordinated was prepared by grinding as a sample.

[0075] 3. Test method

[0076] The test method is as follows: 0.1 g of catalyst (20-40 mesh) is placed in a stainless steel reactor with an inner diameter of 8 mm. The temperature is raised to 400°C at a rate of 8°C / min under an Ar atmosphere, and the gas is changed to 20% H2 / Ar (30 ml / min) for 1 h of reduction; then the gas is changed to Ar, and the temperature is lowered to 350°C for 30 min of purging; then pure NH3 (50 ml / min) is introduced into the reactor, the reactor is heated to the required temperature, and the product is analyzed by a gas chromatograph (GC9790 II). The detection temperature range is 350-600°C, and an activity test point is taken every 50°C.

[0077] X NH3 :

[0078]

[0079] X NH3 represents the conversion rate of ammonia,

[0080] represents the peak area of ammonia before the reaction,

[0081] represents the peak area of ammonia after the reaction.

[0082] R H2 :

[0083]

[0084] R H2 represents the H2 production rate,

[0085] represents the NH3 flow rate, in ml / min,

[0086] m cat represents the catalyst mass.

[0087] Example 1

[0088] This example is prepared according to the above-mentioned “2, preparation method” steps. In step (1), 1 mmol of cerium nitrate hexahydrate and 4 mmol of cobalt nitrate hexahydrate are added to 25 mL of deionized water to prepare a mixed solution. The Prussian blue analogue Ce-doped Co-Co coordinated irregular morphology metal organic framework precursor prepared in step (2) is used as a sample, denoted as Ce1Co4 / CoPBA, and the Prussian blue analogue Ce-doped Co-Co coordinated derivative Co / CeO2 catalyst is finally prepared as a sample, denoted as KCe1Co9O δ .

[0089] Example 2

[0090] This example is in reference to the above-mentioned "2, preparation method" steps, in step (1) is to add 0.5 mmol of cerium nitrate hexahydrate and 4.5 mmol of cobalt nitrate hexahydrate into 25 mL of deionized water, dissolve and prepare a mixed solution; the Prussian blue analogue Ce-doped Co-Co coordination irregular morphology metal organic framework precursor prepared in step (2) is taken as a sample, denoted as Ce 0.5 Co 4.5 / CoPBA, and finally a Prussian blue analogue Ce-doped Co-Co coordination derived Co / CeO2 catalyst is prepared as a sample, denoted as KCe 0.5 Co 9.5 O δ .

[0091] Example 3

[0092] This example is in reference to the above-mentioned "2, preparation method" steps, in step (1) is to add 2 mmol of cerium nitrate hexahydrate and 3 mmol of cobalt nitrate hexahydrate into 25 mL of deionized water, dissolve and prepare a mixed solution; the Prussian blue analogue Ce-doped Co-Co coordination irregular morphology metal organic framework precursor prepared in step (2) is taken as a sample, denoted as Ce2Co3 / CoPBA, and finally a Prussian blue analogue Ce-doped Co-Co coordination derived Co / CeO2 catalyst is prepared as a sample, denoted as KCe2Co8O δ .

[0093] Comparative Example 1

[0094] This comparative example is in reference to the above-mentioned "2, preparation method" steps, but in step (1) no cerium nitrate hexahydrate is added, only 5 mmol of cobalt nitrate hexahydrate is added into 25 mL of deionized water, dissolved and prepared a mixed solution; the Prussian blue analogue Co-Co coordination metal organic framework precursor prepared in step (2) is taken as a sample, denoted as Co5 / CoPBA, and finally a Prussian blue analogue Co-Co coordination derived Co-based catalyst is prepared as a sample, denoted as KCo3O4.

[0095] Comparative Example 2

[0096] The comparative example is referred to the above-mentioned “2, preparation method” steps, but in step (1), 4 mmol cerium nitrate hexahydrate and 1 mmol cobalt nitrate hexahydrate are added to 25 mL deionized water to dissolve and prepare a mixed solution; the prussian blue analogue Ce-doped Co-Co coordinated irregular morphology metal organic framework precursor prepared in step (2) is taken as a sample, recorded as Ce4Co1 / CoPBA, and finally the prussian blue analogue Ce-doped Co-Co coordinated derived Co / CeO2 catalyst is prepared as a sample, recorded as KCe4Co6O according to the ratio calculation. δ .

[0097] Comparative example 3

[0098] The comparative example is referred to the above-mentioned “2, preparation method” steps, but in step (1), 4 mmol cerium nitrate hexahydrate and 1 mmol cobalt nitrate hexahydrate are added to 25 mL deionized water to dissolve and prepare a mixed solution; the prussian blue analogue Ce-doped Co-Co coordinated irregular morphology metal organic framework precursor prepared in step (2) is taken as a sample, recorded as Ce4Co1 / CoPBA, and finally the prussian blue analogue Ce-doped Co-Co coordinated derived Co / CeO2 catalyst is prepared as a sample, recorded as KCe4Co6O according to the ratio calculation. δ .

[0099] The catalysts prepared in example 1 and comparative example 1 are taken as samples, and the results are shown in Figures 1-8 . As shown in Figure 1 , the precursor obtained in step (2) of example 1 shows irregular morphology under microscope.

[0100] As can be seen from Figure 2 , the ammonia decomposition conversion rate of all catalysts increases with the increase of reaction temperature, which is consistent with the exothermic nature of ammonia decomposition to hydrogen. With the increase of Ce molar ratio, the NH3 decomposition conversion rate shows a trend of first increasing and then decreasing. Under the condition of pure ammonia with GHSV of 30000 ml / g•h, KCe1Co9O δ catalyst can reach 74% at 500 ℃ and 95% at 550 ℃, far exceeding the KCo3O4 catalyst.

[0101] As shown in Figure 3 , at a relatively low temperature condition (≤500 ℃), KCe1Co9O δ catalyst shows a significantly better hydrogen production rate.

[0102] As shown in Figure 4 , under different GHSV conditions, KCe1Co9O δThe catalysts were tested for NH3 decomposition performance. With the continuous increase of GHSV, the ammonia decomposition conversion rate at 600℃ can basically remain unchanged, and ammonia can be almost completely decomposed. At lower temperatures, the ammonia decomposition conversion rate will show a downward trend. The reason is that the increase of GHSV leads to insufficient contact time between part of NH3 molecules and the catalyst for reaction, so with the increase of GHSV, the ammonia decomposition conversion rate will decrease to a certain extent.

[0103] As Figure 5 shown, according to the XRD results of the precursors, it is proved that Ce x Co 5-x / CoPBA is successfully synthesized; the precursors of examples 1~3 mainly show the XRD spectrum of Co-CoPBA. The reason may be that the ion radius difference between Co 3+ in Co(NO3)•6H2O and Co 2+ in K2[Co(CN)6] is small, so it is easier to coordinate, thereby forming Co-CoPBA with good crystallinity. The ion radius difference between Ce in Ce(NO3)•6H2O precursor and Co 3+ in K2[Co(CN)6] is large, when Co(NO3)•6H2O exists, competitive coordination will be formed, and Co-CoPBA will be obtained. Ce may enter the crystal lattice of Co-CoPBA in the form of doping. With the increase of Ce content, the crystallinity of Co-CoPBA becomes poor, because the entry of Ce into PBA will cause the distortion of its crystal lattice.

[0104] As Figure 6 shown, the XRD after calcination shows that, compared with the KCo3O4 catalyst obtained by calcining Co / CoPBA of comparative example 1, after calcination of the samples doped with Ce (examples 1~3 and comparative example 2), with the increase of Ce content, the peak intensity of the (111) crystal plane of CeO2 gradually increases, and the crystal phase of Co gradually weakens; and the Co3O4 phase will move to low wave number, indicating that the doping of Ce will cause the lattice expansion of Co3O4, because the radius of Ce ion is larger than that of Co. The Ce5Co5O δ catalyst obtained by calcining Ce / CoPBA has obvious CeO2 crystal phase, and the characteristic peak of Co3O4 nanoparticles is weak. As Figure 7 shown, after H2 reduction, the peak of Co3O4 will be partially converted into metallic Co, which is the active site for ammonia decomposition to produce hydrogen.

[0105] According to Figure 8XPS full spectrum in the middle can know that there is K element in the Prussian blue analogue Co-Co coordination derived catalyst; from the XPS spectrum of K 2p can know that with the increase of Ce content, the binding energy of K increases first and then decreases, and the KCe1Co9O δ The charge density of Co in the catalyst reaches the maximum, indicating that the charge density is the lowest. K element is a typical alkali metal and has strong electron-donating ability, so the charge of K may be transferred to other components in the catalyst. Combined with the results of Co 2p XPS, the charge density of metal Co is Ce5Co5O δ >KCe1Co9O δ >KCe2Co8O δ >KCe4Co6O δ >KCe 0.5 Co 9.5 O δ >KCo3O4, in the Prussian blue analogue Co-Co coordination derived K-containing catalyst, KCe1Co9O δ The charge density of Co in the catalyst is the highest, indicating that the electron of K in the catalyst is transferred to the Co site. The higher the charge density of Co, the more conducive to the adsorption and activation of NH3 on the Co site, thereby having excellent ammonia decomposition conversion rate; although Ce5Co5O δ has the highest metal Co charge density, but Ce5Co5O δ The Co content in the catalyst is much lower than that in KCe1Co9O δ The catalyst, which reduces the active site for NH3 decomposition, has lower ammonia decomposition performance than KCe1Co9O δ The catalyst. Therefore, KCe1Co9O δ The catalyst balances the relationship between the charge density and the geometry of Co, and finally realizes the most excellent ammonia decomposition performance.

[0106] In order to better illustrate that the Prussian blue analogue derived Ce-doped Co-based catalyst provided by the present application has excellent catalytic performance, the following Table 2 is compared with the Co-based catalysts in the currently published literature:

[0107] Table 2: Comparison of catalysts obtained in Example 1 with Co-based catalysts in published literature

[0108]

[0109] The literature involved in Table 2 is as follows:

[0110]

[0111] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above embodiments, and any changes, modifications, substitutions, combinations, simplifications, etc. made without departing from the spirit and principles of the present application should be equivalent replacement manners and should be included in the protection scope of the present application.

Claims

1. A method for preparing a Prussian blue analogue-derived Ce-doped Co-based catalyst, characterized in that... The main steps include: (1) Dissolve and mix cerium salt and cobalt salt to prepare a mixed solution, and dissolve K3[Co(CN)6] to prepare a potassium cobalt cyanide solution; The molar ratio of Ce to Co in the mixed solution is (0.5~2):(3~4.5), and the total molar amount of Ce and Co in the mixed solution is equal to the molar amount of Co in the potassium cobalt cyanide solution. (2) Potassium cobalt cyanide solution was added dropwise to the mixed solution, and the reaction rate was further controlled by adding polyvinylpyrrolidone to the mixed solution or potassium cobalt cyanide solution before the addition. After the addition was completed, the reaction was stirred for 2.5 to 3.5 h, and then aged for at least 12 h. The reaction product was separated, and after washing and drying, an irregular morphology metal-organic framework precursor with Ce-doped Co-Co coordination, a Prussian blue analog, was obtained. (3) The irregular morphology metal-organic framework precursor obtained in step (2) is calcined at a temperature of 550~650 ℃ to prepare a Co / CeO2 catalyst derived from Prussian blue analog Ce doping Co-Co coordination, namely, a Prussian blue analog Ce doping Co-based catalyst.

2. The preparation method according to claim 1, characterized in that: The cerium salt mentioned in step (1) includes any one of cerium nitrate and cerium chloride; the cobalt salt includes any one of cobalt nitrate, cobalt chloride, and cobalt acetate.

3. The preparation method according to claim 1, characterized in that: The molar concentration of cerium ions in the mixed solution in step (1) is 0.02~0.08 mmol / mL, the molar concentration of cobalt ions in the mixed solution is 0.12~0.18 mmol / mL, and the molar concentration of K3[Co(CN)6] in the potassium cobalt cyanide solution is 0.1~0.3 mmol / mL.

4. The preparation method according to claim 1, characterized in that: The reaction rate is further controlled by adding polyvinylpyrrolidone to the mixed solution or potassium cobalt cyanide solution before the addition in step (2). Specifically, polyvinylpyrrolidone is added to the mixed solution or potassium cobalt cyanide solution obtained in step (1) and mixed evenly. The amount of polyvinylpyrrolidone added is 0.15~0.25 g per 1 mmol K3[Co(CN)6].

5. The preparation method according to claim 1, characterized in that: The washing and drying in step (2) are as follows: the washing is performed by alternating washing with deionized water and anhydrous ethanol at least 3 times; the drying is performed by drying the washed reaction product at a temperature of 60~80 ℃ for at least 24 h and keeping it dry until calcination.

6. The preparation method according to claim 1, characterized in that: The calcination process described in step (3) involves heating the temperature to 550-650 ℃ at a rate of 4.5-5.5 ℃ / min, maintaining the temperature for 2.5-3.5 h, and finally allowing the temperature to drop naturally to room temperature.

7. The Prussian blue analogue-derived Ce-doped Co-based catalyst prepared by the method described in claim 1.

8. The application of the Prussian blue analogue-derived Ce-doped Co-based catalyst as described in claim 7 in the field of ammonia decomposition for hydrogen production.

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