Application of a nickel-based catalyst in hydrogen production by ammonia decomposition

Abstract

This invention relates to the application of a nickel-based catalyst in the ammonia decomposition and hydrogen production reaction. In the presence of the nickel-based catalyst, ammonia undergoes a decomposition reaction to produce hydrogen. The nickel-based catalyst comprises elemental nickel, silicon dioxide, and nickel silicate, with the mass percentage of nickel in the nickel-based catalyst, based on the total amount of nickel in the nickel silicate and elemental nickel, being 10%–30%. The elemental nickel is in the form of nickel nanoparticles with a particle size of 2–5 nm. The nickel-based catalyst of this application exhibits excellent catalytic activity for the ammonia decomposition reaction, achieving near-complete conversion of ammonia (approximately 96%) at 450°C. It also demonstrates excellent high-temperature stability; after a stability test at 450°C for 50 hours, the catalyst activity did not decrease.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of hydrogen production by ammonia decomposition, and particularly relates to application of a nickel-based catalyst in hydrogen production by ammonia decomposition. BACKGROUND

[0002] Ammonia is a hydrogen carrier that has attracted wide attention due to its low price, high hydrogen density (about 17.7 wt%) and the absence of carbon and nitrogen elements (which can avoid the introduction of carbon oxides and nitrogen oxides). In addition, ammonia can be liquefied at room temperature and low pressure (about 10 bar), which makes it easy to transport, store and separate, and can well solve the storage and transportation problems of hydrogen. At present, the ammonia decomposition reaction (NH3 = 1 / 2N2 + 3 / 2H2, ΔH = +46 kJ mol -1 ) has been applied in small-scale industrial processes where hydrogen supply is inconvenient, and a mature ammonia decomposition hydrogen production process is expected to further promote the development and utilization of hydrogen energy.

[0003] The catalysts currently applied in the ammonia decomposition reaction process are mainly catalytic systems dominated by the noble metal ruthenium (Ru), and the high cost hinders its large-scale industrial application. The activity of the nickel-based catalyst in the ammonia decomposition reaction is only second to that of the ruthenium-based catalyst (the activity order is Ru > Ni > Rh > Co > Ir), and it is considered to be one of the non-noble metal-based catalysts that is most likely to replace Ru and be applied in the ammonia decomposition reaction. At present, although some nickel-based catalysts have been tried to be applied in the catalytic ammonia decomposition reaction, complete conversion of ammonia usually needs to be achieved at a temperature higher than 600 DEG C. The high reaction temperature not only causes high energy consumption, but also brings challenges to the safety of the reaction equipment, in addition, the catalytic stability of the catalyst is also difficult to maintain due to high-temperature sintering. Therefore, in order to faster promote the industrialization process of the ammonia decomposition reaction, how to develop a non-noble metal-based catalytic system with excellent ammonia decomposition reaction performance at a lower temperature (not higher than 450 DEG C) so as to make the ammonia decomposition reaction better and better produce hydrogen is a technical problem to be solved at present. SUMMARY

[0004] The application provides application of a nickel-based catalyst in hydrogen production by ammonia decomposition, and aims to make ammonia well decompose to produce hydrogen at a lower temperature by using a non-noble metal-based catalyst, so that the ammonia decomposition reaction is better and hydrogen is produced.

[0005] The application relates to application of a nickel-based catalyst in hydrogen production by ammonia decomposition, and ammonia is decomposed to produce hydrogen in the presence of the nickel-based catalyst; the nickel-based catalyst comprises elemental nickel, silicon dioxide and nickel silicate, the mass percentage of nickel in the nickel-based catalyst is 10% to 30% based on the total amount of nickel in the nickel silicate and the elemental nickel; the elemental nickel is nickel nanoparticles, and the particle size of the nickel nanoparticles is 2 to 5 nm.

[0006] Optionally, the reaction temperature for the decomposition reaction of ammonia to produce hydrogen is 300-600℃, preferably 450-600℃.

[0007] Optionally, the preparation method of the nickel-based catalyst comprises the following steps: (1) dissolving a nickel precursor in water to obtain a nickel precursor solution; adding an aqueous ammonia solution to the nickel precursor solution to adjust the pH value to 8-12 to obtain a nickel precursor solution after pH adjustment; (2) dispersing silica sol in the nickel precursor solution after pH adjustment, and then heating to generate nickel silicate, and separating out the solid phase to obtain a nickel-based catalyst precursor; (3) calcining and reducing the nickel-based catalyst precursor in a reducing atmosphere to obtain the nickel-based catalyst.

[0008] Optionally, in step (1), the nickel precursor is selected from one or more of the group consisting of nickel chloride hexahydrate, nickel nitrate hexahydrate and nickel sulfate hexahydrate; and / or, the mass ratio of the nickel precursor to water is (1-5):10.

[0009] Optionally, in step (1), the mass concentration of the aqueous ammonia solution is 20%-40%; and / or, the aqueous ammonia solution is added to the nickel precursor solution to adjust the pH value to 9-10.

[0010] Optionally, in step (2), the solid content of the silica sol is 30%-50%, and the particle size of the silica particles in the silica sol is 10-30 nm.

[0011] Optionally, in step (2), the molar ratio of the silica contained in the silica sol to the nickel precursor is (1-5):1.

[0012] Optionally, in step (2), the heating temperature is 50-90℃, preferably 60-80℃.

[0013] Optionally, in step (3), the reducing atmosphere comprises hydrogen and an inert gas, and the proportion of the hydrogen in the total volume of the reducing atmosphere is 5%-40%; the inert gas comprises argon and / or nitrogen.

[0014] Optionally, in step (3), the calcination and reduction temperature is 300-600℃, preferably 400-500℃, and the time is 1-4h, preferably 2-3h.

[0015] Beneficial effects:

[0016] (1) The nickel-based catalyst of the present application has excellent ammonia decomposition reaction catalytic activity in the ammonia decomposition reaction, and can basically achieve complete conversion of ammonia (about 96%) at 450℃, which can achieve the catalytic effect of a noble metal ruthenium-based catalyst, and can be scaled up, having great industrialization potential.

[0017] (2) The nickel-based catalyst of the present application has excellent high-temperature stability. After the catalyst has been subjected to a stability test at 450°C for 50 hours, the activity of the catalyst does not decrease;

[0018] (3) The nickel-based catalyst of the present application has excellent high-temperature recyclability. After the catalyst has been subjected to a 5-cycle test at low temperature (100°C) to high temperature (700°C), no significant deactivation is observed. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is an X-ray diffraction (XRD) pattern of the catalyst precursor and the reduced catalyst prepared in Example 1;

[0020] Figure 2 is a transmission electron microscope (TEM) photograph of the catalyst precursor prepared in Example 1;

[0021] Figure 3 is a TEM photograph of the reduced catalyst prepared in Example 1;

[0022] Figure 4 is a particle size statistical graph of the nickel nanoparticles in the reduced catalyst prepared in Example 1;

[0023] Figure 5 is a cyclic voltammogram (C-V graph) of the reduced catalyst prepared in Example 1 before and after immersion in a NaOH solution;

[0024] Figure 6 is an ammonia conversion rate-temperature graph obtained when the reduced catalyst prepared in Example 1 and the control samples ① to ④ prepared in Comparative Examples 1 to 4 are subjected to an evaluation test of ammonia decomposition activity;

[0025] Figure 7 is an ammonia conversion rate-time graph obtained when the reduced catalyst prepared in Example 1 is subjected to a catalyst stability evaluation test at a reaction temperature of 450°C for 50 hours;

[0026] Figure 8 is a cycle number-performance graph obtained when the reduced catalyst prepared in Example 1 is subjected to a recyclability test. DETAILED DESCRIPTION

[0027] The present application will be further explained in detail by the accompanying drawings and examples. Through these explanations, the features and advantages of the present application will become more apparent.

[0028] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any implementation described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other implementations. The various aspects of the embodiments are illustrated by way of example in the drawings and will be described in detail below.

[0029] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as there is no conflict.

[0030] In one aspect, the present application relates to an application of a nickel-based catalyst in an ammonia decomposition reaction for hydrogen production, wherein ammonia is decomposed to produce hydrogen in the presence of the nickel-based catalyst;

[0031] The nickel-based catalyst comprises elemental nickel, silicon dioxide and nickel silicate, and the mass fraction of nickel in the nickel-based catalyst is 10% to 30% based on the total amount of nickel in the nickel silicate and the elemental nickel.

[0032] The elemental nickel is nickel nanoparticles, and the particle size of the nickel nanoparticles is 2 to 5 nm.

[0033] It should be noted that the above-mentioned nickel-based catalyst of the present application is a core-shell structure, the elemental nickel is nanoparticles to form the core, and the silicon dioxide and the nickel silicate are wrapped outside the nanoparticle elemental nickel to form the shell. The nickel-based catalyst of the present application is obtained by reduction of the precursor nickel silicate, and the precursor nickel silicate (Ni3Si2O5(OH)4) is a layered structure composed of Si-O tetrahedron and Ni-O octahedron. Ni-O is easily reduced to form nickel nanoparticles, and Si-O is not easily reduced to form a wrapping layer wrapped on the surface of the nickel nanoparticles, thereby forming a core-shell structure.

[0034] It should be noted that although some nickel-based catalyst systems are also applied to ammonia decomposition reactions, there are still some significant shortcomings that hinder the large-scale application of nickel-based catalyst systems in ammonia decomposition reactions. Specifically, first, the existing nickel-based catalysts usually require a high reaction temperature (higher than 600℃) when catalyzing the ammonia decomposition reaction; second, the existing nickel-based catalysts are prone to sintering of active metals under long-time and high-temperature reaction conditions when used for ammonia decomposition reaction, which leads to a decrease in the activity of the catalyst and difficulty in maintaining the stability of the catalyst; third, the nickel-based catalysts are prone to oxidation, forming a dense oxide layer, which leads to a decrease or disappearance of the activity of the catalyst; therefore, the ammonia decomposition reaction catalyzed by the existing nickel-based catalysts cannot proceed better due to the activity problem of the catalyst.

[0035] The nickel-based catalyst contains three components: nickel, nickel silicate and silicon dioxide, and the mass percentage of nickel in the catalyst is 10% to 30%. The nickel-based catalyst has the core-shell structure, and the nickel nanoparticles in the nickel-based catalyst have a small particle size. The nickel-based catalyst has excellent ammonia decomposition reaction catalytic performance when used for catalyzing the ammonia decomposition reaction, can achieve the catalytic effect of the noble metal ruthenium-based catalyst, and has the potential to replace the ruthenium-based catalyst for industrial application.

[0036] According to one specific embodiment of the application, the reaction temperature for the decomposition reaction of ammonia to produce hydrogen is 300-600 DEG C, preferably 450-600 DEG C.

[0037] It should be noted that when other catalysts are used for catalyzing the ammonia decomposition reaction, the low-temperature (not higher than 450 DEG C) activity is low, and in order to achieve complete conversion of ammonia, a reaction temperature higher than 600 DEG C is usually required. However, when the nickel-based catalyst is used for catalyzing the reaction, complete conversion of ammonia can be basically achieved at a relatively low reaction temperature (about 450 DEG C), and the stability of the catalyst can be maintained for a long time, so that ammonia can be better decomposed to produce hydrogen.

[0038] According to another specific embodiment of the application, the preparation method of the nickel-based catalyst comprises the following steps:

[0039] (1) dissolving a nickel precursor in water to obtain a nickel precursor solution; adding an ammonia solution to the nickel precursor solution to adjust the pH value to 8-12 to obtain a nickel precursor solution after pH value adjustment;

[0040] (2) dispersing silica sol in the nickel precursor solution after pH value adjustment, and then heating to generate nickel silicate, and separating out a solid phase to obtain a nickel-based catalyst precursor;

[0041] (3) calcining and reducing the nickel-based catalyst precursor in a reducing atmosphere to obtain the nickel-based catalyst.

[0042] It is to be noted that the present application adopts a new nano-synthetic method to construct a new type of nickel-based catalyst system, which exhibits excellent low-temperature catalytic ammonia decomposition reaction performance. The nickel-based catalyst prepared based on the above preparation method can exhibit good catalytic activity at a lower temperature, such as less than 450°C, and can make ammonia well decomposed to produce hydrogen. In the above preparation method, in step (1), specifically, 20 mL of ultrapure water can be placed in a 50 mL round-bottom flask, then a certain amount, such as 1-5 g, preferably 2-4 g, of nickel precursor can be fully dissolved therein, and while stirring, a suitable amount of concentrated ammonia water can be slowly added to adjust the pH value, and the addition can be stopped until the solution is slightly clear, to obtain the adjusted pH value nickel precursor solution. In step (2), the silica sol can be added to the adjusted pH value nickel precursor solution obtained from step (1), and then transferred to a water bath at a certain temperature for continued stirring until the solution appears bright green (1-5 mL of silica sol Ludox HS-40, preferably 2-3 mL), indicating the formation of nickel silicate. The solid phase can be separated by conventional methods in the art, then washed with ultrapure water, and can be centrifuged 5 times, placed in a drying box for about 12 h, taken out, ground, and the precursor of the target catalyst is obtained. In step (3), the catalyst precursor is placed in a reducing atmosphere and reduced at a certain temperature to obtain the target catalyst, which can be denoted as Ni@SiO2(H450).

[0043] It is to be noted that, first, the nickel-based catalyst system prepared according to the above method has smaller and uniform metal nickel nanoparticles, and has excellent sintering resistance under high temperature reaction conditions, thus having excellent catalytic stability; second, the prepared nickel-based catalyst system has very high ammonia decomposition reaction catalytic activity, and can basically achieve complete conversion of ammonia gas under 450°C reaction conditions, reaching the catalytic effect of a noble metal ruthenium-based catalyst; third, using the above preparation method, the catalyst yield is high, and the catalyst can be easily scaled up, having great industrialization potential.

[0044] According to a specific embodiment of the application, in step (1), the nickel precursor is selected from one or more combinations of nickel chloride hexahydrate, nickel nitrate hexahydrate, and nickel sulfate hexahydrate; and / or,

[0045] The mass ratio of the nickel precursor to water is (1-5):10.

[0046] It should be noted that in the preparation of the above-mentioned nickel-based catalyst for ammonia decomposition reaction, the above-mentioned nickel precursor is selected, the mass ratio of the nickel precursor and water is controlled in step (1), and the prepared nickel-based catalyst can better catalyze the decomposition reaction of ammonia to prepare hydrogen, and can exhibit better catalytic activity and stability in the catalytic ammonia decomposition reaction.

[0047] According to one specific embodiment of the application, in step (1), the mass concentration of the ammonia solution is 20% to 40%, preferably 25% to 28%; and / or,

[0048] The pH value of the nickel precursor solution is adjusted to 9 to 10 by adding an ammonia solution.

[0049] It should be noted that as a preferred embodiment, the pH value of the nickel precursor solution is adjusted using the above-mentioned concentration of ammonia solution, and the pH value of the nickel precursor solution is adjusted to 9 to 10, so that the nickel precursor solution after adjusting the pH value is obtained, and the nickel-based catalyst prepared by subsequent steps (2) and (3) has better stability, recyclability, and exhibits more excellent catalytic activity, so that ammonia can be better decomposed to produce hydrogen.

[0050] According to one specific embodiment of the application, in step (2), the solid content of the silica sol is 30% to 50%, and the particle size of the silica particles in the silica sol is 10 to 30 nm.

[0051] It should be noted that using the above-mentioned solid content and particle size of the silica sol, a nickel-based catalyst with a core-shell structure having better catalytic activity can be prepared, which exhibits better stability when used for catalytic decomposition reaction, so that ammonia can be better decomposed to produce hydrogen.

[0052] According to one specific embodiment of the application, in step (2), the molar ratio of the silica contained in the silica sol to the nickel precursor is (1 to 5): 1.

[0053] It should be noted that by controlling the molar ratio of the silica contained in the silica sol to the nickel precursor as above, the nickel precursor and the silica sol can better react to form the above-mentioned nickel-based catalyst with a core-shell structure, and the prepared nickel-based catalyst with a core-shell structure exhibits better catalytic activity when used for catalytic ammonia decomposition reaction, so that ammonia can be better decomposed to produce hydrogen.

[0054] According to one specific embodiment of the application, in step (2), the heating temperature is 50 to 90°C, preferably 60 to 80°C.

[0055] It should be noted that the temperature of the heating in step (2) is controlled as above, so that the silica sol and the nickel precursor can be better reacted into the nickel-based catalyst precursor, and then the nickel-based catalyst with better catalytic activity can be prepared through step (3), and when used for catalyzing the ammonia decomposition reaction, ammonia can be better decomposed into hydrogen. The heating method can be water bath heating or the like.

[0056] According to one specific embodiment of the application, in step (3), the reducing atmosphere comprises hydrogen and inert gas, and the proportion of the hydrogen in the total volume of the reducing atmosphere is 5% to 40%; the inert gas comprises argon and / or nitrogen.

[0057] It should be noted that by controlling the proportion of the hydrogen in the total volume of the reducing atmosphere as above, the nickel-based catalyst precursor can be better calcined and reduced into the nickel-based catalyst in the reducing atmosphere, and the proportion of nickel and nickel silicate in the obtained target catalyst is well controlled within a certain range, so that the nickel-based catalyst has better catalytic activity for ammonia decomposition to hydrogen.

[0058] According to one specific embodiment of the application, in step (3), the calcination and reduction temperature is 300 to 600℃, preferably 400 to 500℃, and the time is 1 to 4h, preferably 2 to 3h.

[0059] It should be noted that by controlling the calcination and reduction temperature and time as above, the nickel silicate in the nickel-based catalyst precursor prepared in step (2) can be partially converted into nickel, and the nickel-based catalyst with the proportion of nickel and nickel silicate within a certain range can be prepared, so that the catalyst has good catalytic activity for ammonia decomposition to hydrogen.

[0060] In summary, the nickel-based catalyst is prepared according to the above steps (1) to (3). In step (1), the mass ratio of the nickel precursor and water is controlled to prepare the nickel precursor solution by mixing and dissolving, ammonia water solution is added dropwise to the solution, and the pH value of the solution is adjusted to a certain range to obtain the nickel precursor solution after pH adjustment; in step (2), the silica sol is added and heated to generate nickel silicate or the solution shows bright green, and the solid phase in the solution is separated to obtain the nickel-based catalyst precursor; in step (3), the nickel-based catalyst precursor is calcined and reduced in a reducing atmosphere with a certain composition at a certain temperature and for a certain time, and the nickel-based catalyst comprising nickel, nickel silicate and silicon dioxide with a certain proportion is obtained. When the prepared catalyst is used for catalyzing the ammonia decomposition reaction, the catalyst can exhibit more excellent catalytic activity, stability and cycle performance, and ammonia can be better decomposed into hydrogen.

[0061] In addition, the application can also relate to an ammonia decomposition reaction for hydrogen production, which comprises the following steps:

[0062] The NH3 is contacted with a nickel-based catalyst to produce hydrogen by decomposition reaction of NH3 at 300-600 ℃; the nickel-based catalyst comprises elemental nickel, silicon dioxide and nickel silicate, the mass ratio of nickel element in the nickel-based catalyst to the total amount of nickel in the nickel silicate and elemental nickel is 10%-30%; the elemental nickel is nickel nanoparticles, and the particle size of the nickel nanoparticles is 2-5 nm. The above nickel-based catalyst is a core-shell structure, the elemental nickel is nanoparticles to form a core, and the silicon dioxide and nickel silicate are wrapped outside the nanoparticle elemental nickel to form a shell.

[0063] In the ammonia decomposition hydrogen production reaction, (1) NH3 molecules are adsorbed on the catalyst surface; (2) NH3 molecules are decomposed on the catalyst surface to produce N2 and H2; (3) N2 and H2 are formed and desorbed on the catalyst surface. The reaction involves the decomposition of NH3 molecules and the formation of N2 and H2 molecules, and generally requires a highly active catalyst and a relatively high reaction temperature (above 600 ℃). The ammonia decomposition hydrogen production reaction based on the above nickel-based catalyst with a core-shell structure can basically achieve complete conversion of ammonia at 450-500 ℃.

[0064] The application will be further described in detail through the following examples, but the application is not limited by the examples. In the following examples, the experimental instruments and raw materials involved are commercially available products, unless otherwise specified. The silicon sol Ludox HS-40 used in the following examples is purchased from Shanghai Shuanglun Industrial Co., Ltd., the solid content of the silicon sol Ludox HS-40 is 40%, the density is 1.3 g / mL, and the particle size of the silicon dioxide particles contained is 12 nm; the concentrated ammonia water used in the following examples is 25%-28% of the concentrated ammonia water of China Pharmaceutical Group.

[0065] Example 1

[0066] (I) Preparation of a nickel-based catalytic system

[0067] (1) 20 mL of ultrapure water is taken in a 50 mL round-bottom flask;

[0068] (2) 3 g of Ni(NO3)2·6H2O is fully dissolved therein, and while stirring, a proper amount of 25%-28% concentrated ammonia water is slowly added dropwise to adjust the pH to 9-10 (until the solution is slightly clear);

[0069] (3) 4 mL of silicon sol Ludox HS-40 is further added dropwise, and the solution is transferred to a 60 ℃ water bath for continuous stirring until the solution appears bright green (nickel silicate is generated);

[0070] (4) The solution is washed with ultrapure water, centrifuged for 5 times, and then placed in a drying box for drying for about 12 h, taken out, ground, and then the precursor of the target catalyst is obtained;

[0071] (5) The catalyst precursor was placed in a H2 / Ar (v / v = 1 / 3) atmosphere for reduction (reduction conditions: 10°C min -1 , 450°C, 2h), i.e. the target catalyst Ni@SiO2(H450) was obtained.

[0072] (II) Evaluation experiment of ammonia decomposition reaction activity

[0073] 500mg of the target catalyst prepared in the above step was weighed, and an evaluation experiment of ammonia decomposition reaction activity was performed in the range of 200-600°C, and the obtained ammonia conversion rate-temperature graph is shown in Figure 6 .

[0074] The specific experimental process of the evaluation experiment of ammonia decomposition reaction activity: the catalyst was filled in a fixed bed microreactor for catalytic activity evaluation, the temperature was controlled by a temperature control device, the content of each component in the reaction gas was detected by online gas chromatography, and the conversion rate was calculated.

[0075] Comparative Example 1

[0076] (I) Preparation of Control Sample 1

[0077] (1) Ni / SiO2 control sample prepared by a traditional impregnation method. The specific preparation method is as follows: 10mL of ultrapure water was placed in a 25mL round-bottom flask. 1g of Ni(NO3)2·6H2O was dissolved therein, and then 1g of commercially available silica carrier was added, and the mixture was placed in a 80°C water bath for continuous stirring until the aqueous solution was evaporated to dryness. It was placed in a drying oven for drying for about 12h, taken out, ground, and then the catalyst precursor was obtained. Finally, the catalyst precursor was placed in a H2 / Ar (v / v = 1 / 3) atmosphere for reduction (reduction conditions: 10°C min -1 , 450°C, 2h), i.e. the control sample 1 was obtained.

[0078] (II) Evaluation experiment of ammonia decomposition reaction activity

[0079] The evaluation experiment was performed according to the method in Example 1, and the obtained ammonia conversion rate-temperature graph is shown in Figure 6 .

[0080] Comparative Example 2

[0081] (I) Preparation of Control Sample 2

[0082] (1) Ni / Al2O3 control sample prepared by traditional impregnation method. The specific preparation method is as follows: Take 10 mL of ultrapure water and place it in a 25 mL round-bottom flask. Dissolve 1 g of Ni(NO3)2·6H2O completely in it, and then add 1 g of commercially available alumina support. Place it in an 80℃ water bath and continue stirring until the aqueous solution evaporates to dryness. Place it in a drying oven and dry for about 12 h. After taking it out and grinding it, the catalyst precursor is obtained. Finally, the catalyst precursor is placed in an H2 / Ar (v / v=1 / 3) atmosphere for reduction (reduction conditions: 10℃ min). -1 (450℃, 2h) to obtain control sample ②.

[0083] (II) Evaluation Experiment of Ammonia Decomposition Reaction Activity

[0084] The evaluation experiment was conducted according to the method in Example 1, and the resulting ammonia conversion rate-temperature graph is shown below. Figure 6 As shown.

[0085] Comparative Example 3

[0086] (I) Preparation of control sample ③

[0087] (1) Replace the concentrated ammonia in Example 1 with an aqueous sodium hydroxide solution, and keep everything else the same.

[0088] (II) Evaluation Experiment of Ammonia Decomposition Reaction Activity

[0089] The evaluation experiment was conducted according to the method in Example 1, and the resulting ammonia conversion rate-temperature graph is shown below. Figure 6 As shown.

[0090] Comparative Example 4

[0091] (I) Preparation of control sample ④

[0092] (1) Only the processing atmosphere of the catalytic precursor in Example 1 was changed from H2 / Ar (v / v = 1 / 3) to air, while the others remained unchanged.

[0093] (II) Evaluation Experiment of Ammonia Decomposition Reaction Activity

[0094] The evaluation experiment was conducted according to the method in Example 1, and the resulting ammonia conversion rate-temperature graph is shown below. Figure 6 As shown.

[0095] The activity of catalyst sample Ni@SiO2(H450) and control samples ①②③④ was evaluated and compared. Figure 6It is evident that the catalyst sample prepared in Example 1 exhibits significantly better activity than the control samples ①②③④. This indicates that the catalyst prepared in Example 1 has a clear advantage over catalysts prepared by the traditional impregnation method, and the catalyst prepared in Example 1 can achieve near-complete conversion of ammonia (approximately 96%) at 450°C, achieving the catalytic effect of a noble metal ruthenium-based catalyst.

[0096] Test Example 1: Structural Characterization of Nickel-Based Catalytic System

[0097] (1) Approximately 50 mg of the catalyst precursor prepared in step (4) of Example 1 and the reduced Ni@SiO2(H450) catalyst prepared in step (5) of Example 1 were taken and subjected to X-ray diffraction (XRD) analysis. The obtained X-ray diffraction (XRD) spectra are shown below. Figure 1 As shown.

[0098] pass Figure 1 The X-ray diffraction (XRD) pattern shows that the prepared catalyst precursor is a layered nickel silicate crystal phase (Nickel silicate: Ni3Si2O5(OH)4). After hydrogen reduction treatment, the catalyst crystal phase formed is a mixed phase of nickel, silicon dioxide and nickel silicate. It was later confirmed that it is actually a silicon oxide-encapsulated nickel-based catalyst, abbreviated as Ni@SiO2(H450), in which the silicon dioxide is amorphous silicon dioxide.

[0099] In addition, according to inductively coupled plasma optical emission spectroscopy (ICP-OES) testing, the nickel-based catalyst finally prepared in Example 1 has a nickel mass ratio of 24% (the total amount of nickel in elemental nickel and nickel silicate), and the specific mass ratio of nickel silicate and silicon dioxide is difficult to determine.

[0100] Test Example 2

[0101] Approximately 1 mg of the catalyst precursor prepared in step (4) of Example 1 and the catalyst sample Ni@SiO2(H450) prepared in step (5) of Example 1 were dispersed in an ethanol solution and ultrasonically dispersed for approximately 30 min. Finally, a small amount of the ethanol solution of the sample was taken out and dropped onto the surface of an ultrathin carbon film for TEM experiment. The TEM image of the obtained catalyst precursor is shown below. Figure 2 As shown, the TEM image of the catalyst sample Ni@SiO2(H450) and the particle size distribution of the nickel nanoparticles are respectively as follows: Figure 3 As shown and Figure 4 As shown in the figure, the catalyst precursor is a sheet-like layered structure. After treatment in a reducing atmosphere, a large number of small (approximately 3.35 nm) and uniformly sized nickel nanoparticles are generated and embedded in the catalyst substrate, forming a Ni@SiO2(H450) core-shell catalyst.

[0102] Test Example 3

[0103] The target catalyst Ni@SiO2(H450) sample prepared in Example 1 and the Ni@SiO2(H450) sample after being immersed in NaOH solution (NaOH solution concentration, immersion time, 30 min) (Ni@SiO2(H450)-NaOH) were subjected to cyclic voltammetry tests in 0.5 M KHCO3 electrolyte under Ar atmosphere at a scan rate of 100 mV / s. -1 The resulting CV curve is shown below. Figure 5 As shown. Figure 5 As shown, after the NaOH solution washes away the silicon oxide coating on the surface of the Ni nanoparticles, a large amount of the Ni surface is exposed, and Ni is exposed at approximately 0.41V. 0 / Ni 2+ The significantly increased oxidation signal confirms that the coating layer on the surface of the Ni nanoparticles is silica and nickel silicate. This indicates that the final nickel-based catalyst has a core-shell structure, where nickel forms the core of the nanoparticles, and silicon oxides (nickel silicate and silica) encapsulate the nickel to form the shell.

[0104] Test Example 4

[0105] 100 mg of the catalyst sample Ni@SiO2(H450) prepared in Example 1 was weighed and subjected to a catalyst stability evaluation experiment at 450 °C for 50 h. The resulting ammonia conversion-time graph is shown below. Figure 7 As shown. By Figure 7 As can be seen, after a stability test at 450℃ for 50 hours, the activity of the catalyst did not decrease significantly, indicating that this encapsulated catalyst structure gives the catalyst excellent stability.

[0106] Test Example 5

[0107] 100 mg of the catalyst sample Ni@SiO2(H450) prepared in Example 1 was weighed and subjected to a catalyst cycleability evaluation experiment. During each cycle, the catalyst activity was tested within the temperature range of 100–700 °C, and then the temperature was lowered to 100 °C for a second test. The resulting catalyst cycle count-performance graph is shown below. Figure 8 As shown. (Through) Figure 8 It can be seen that after five cycles of low temperature (100℃) to high temperature (700℃), the activity of the catalyst did not show a significant decrease, indicating that the catalyst has excellent recyclability.

[0108] In the description of the present application, it needs to be explained that the terms "upper", "lower", "inner", "outer", "front", "back", "left", "right" and the like indicate the orientation or positional relationship based on the working state of the present application, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0109] In the description of the present application, it needs to be explained that the terms "mounting", "connecting", "connection" should be understood broadly, unless otherwise explicitly specified and limited. For those skilled in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0110] The above describes the present application in combination with the preferred embodiments, but these embodiments are only exemplary and serve only to illustrate. On this basis, various substitutions and improvements can be made to the present application, which all fall within the protection scope of the present application.

Claims

1. Use of a nickel-based catalyst in the reaction of ammonia decomposition for hydrogen production, characterized in that, Decomposing ammonia to produce hydrogen gas in the presence of the nickel-based catalyst; The nickel-based catalyst comprises elemental nickel, silicon dioxide and nickel silicate, and the mass percentage of nickel in the nickel-based catalyst is 10-30% based on the total amount of nickel in the nickel silicate and elemental nickel; The elemental nickel is in the form of nickel nanoparticles, and the particle size of the nickel nanoparticles is 2-5 nm. The nickel-based catalyst has a core-shell structure, wherein the elemental nickel forms the core, and the silicon dioxide and nickel silicate form the shell.

2. Use according to claim 1, characterized in that, The reaction temperature for decomposing ammonia to produce hydrogen gas is 300-600°C.

3. Use according to claim 1, characterized in that, The preparation method of the nickel-based catalyst comprises the following steps: (1) dissolving a nickel precursor in water to obtain a nickel precursor solution; adding an aqueous ammonia solution to the nickel precursor solution to adjust the pH value to 8-12 to obtain a nickel precursor solution after pH adjustment; (2) dispersing a silica sol in the nickel precursor solution after pH adjustment, and then heating to generate nickel silicate, and separating out the solid phase to obtain a nickel-based catalyst precursor; (3) performing calcination reduction of the nickel-based catalyst precursor in a reducing atmosphere to obtain the nickel-based catalyst.

4. Use according to claim 3, characterized in that, In step (1), the nickel precursor is selected from one or more combinations of nickel chloride hexahydrate, nickel nitrate hexahydrate and nickel sulfate hexahydrate; And / or, The mass ratio of the nickel precursor to water is (1-5):

10.

5. Use according to claim 3, characterized in that, In step (1), the mass concentration of the aqueous ammonia solution is 20-40%; and / or, The aqueous ammonia solution is added to the nickel precursor solution to adjust the pH value to 9-10.

6. Use according to claim 3, characterized in that, In step (2), the solid content of the silica sol is 30-50%, and the particle size of the silicon dioxide particles in the silica sol is 10-30 nm.

7. Use according to claim 3, characterized in that, In step (2), the molar ratio of the silicon dioxide contained in the silica sol to the nickel precursor is (1-5):

1.

8. Use according to claim 3, characterized in that, In step (2), the heating temperature is 50-90°C.

9. Use according to claim 3, characterized in that, In step (3), the reducing atmosphere comprises hydrogen and an inert gas, and the hydrogen accounts for 5-40% of the total volume of the reducing atmosphere; the inert gas comprises argon and / or nitrogen.

10. Use according to claim 3, characterized in that, In step (3), the calcination reduction temperature is 300-600°C, and the time is 1-4 h.

11. Use according to claim 1, characterized in that, The reaction temperature for decomposing ammonia to produce hydrogen gas is 450-600°C.

12. Use according to claim 3, characterized in that, In step (2), the heating temperature is 60-80°C.

13. Use according to claim 3, characterized in that, In step (3), the calcination reduction temperature is 400-500°C, and the time is 2-3 h.

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