An iron-based catalyst, its preparation method and use

Abstract

The application relates to the technical field of hydrogen production by ammonia decomposition, and discloses an iron-based catalyst and a preparation method and application thereof. x Al y Fe z , wherein x:y:z=2:(2-3.5):(2-0.5), and y+z=4. The lithium-aluminum-iron hydrotalcite-like compound prepared by adopting the constant-pH coprecipitation method is used as a precursor of the iron-based catalyst, and then the iron-based catalyst can be prepared through calcination treatment and reduction treatment. The iron-based catalyst prepared by the application has excellent ammonia decomposition catalytic activity and catalytic stability. The ammonia conversion rate can reach 73% at 30000 mL.g ‑1 ·h ‑1 , 600 DEG C; the ammonia conversion rate can be kept above 99% after continuous reaction for 100 h at 30000 mL.g ‑1 ·h ‑1 , 700 DEG C.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogen production by decomposing ammonia, and in particular to an iron-based catalyst and a preparation method and application thereof. Background Art

[0002] In recent years, with the continued rapid growth of population, energy shortages and resource pressures faced by the world have become increasingly apparent. The current energy situation forces fossil energy to play a major role in meeting the ever-increasing energy demand. However, the environmental pollution problems caused by fossil energy are becoming increasingly serious, which has had a serious impact on people's daily lives. Therefore, people urgently need to develop clean energy. Among the new energy sources that have been developed and utilized, hydrogen energy has the advantages of being pollution-free, having a high calorific value, and having a wide range of resources. Therefore, hydrogen energy is regarded as a clean energy with great application prospects.

[0003] However, the low volume density of hydrogen limits its storage and transportation. To solve this problem, people store hydrogen in molecules such as methanol (CH3OH), methane (CH4), and ammonia (NH3) to achieve hydrogen storage and transportation. Among the many hydrogen storage materials, carbon-based hydrogen storage materials such as methanol and methane will inevitably produce carbon oxides (CO x ), these carbon oxides will cause environmental pollution; in contrast, ammonia does not contain carbon elements, and its hydrogen production products are only nitrogen and hydrogen, which will not produce carbon oxides that pollute the environment. Moreover, the hydrogen content in ammonia is relatively high (about 17.6%), so ammonia is considered to be an excellent hydrogen storage material.

[0004] The ammonia decomposition method for producing hydrogen has the advantages of high purity, small footprint, simple operation, low cost and no harmful by-products, making it a good method for producing hydrogen. Currently, the industry mainly uses thermal catalytic decomposition of ammonia to produce hydrogen. The reaction equation is This reaction is an endothermic reaction with volume expansion, so high temperature and low pressure are conducive to the progress of this reaction; relevant experimental data show that the ammonia conversion rate of this reaction is almost 0 at 700℃ and 101.325kPa. It can be seen that ammonia decomposition is difficult to carry out under mild conditions. Therefore, when decomposing ammonia to produce hydrogen, it is often necessary to add an ammonia decomposition catalyst to increase the ammonia conversion rate.

[0005] At present, ammonia decomposition catalysts mainly include precious metal catalysts represented by Ru, Ir, Pd, and Pt, and transition metal catalysts represented by Fe, Co, Ni, and Mo. It has been reported in the literature that Ru-based catalysts have the highest catalytic activity among precious metal catalysts. For example, K-5% Ru / CNTs (K / Ru=1) has the highest catalytic activity at 60000 mL·g -1 cat ·h -1, the ammonia conversion rate at 450°C is 97.3%; however, precious metals such as Ru are expensive and have high usage costs, which seriously limits the large-scale commercial application of precious metal catalysts. In contrast, transition metal catalysts show greater advantages due to their low cost and abundant resources.

[0006] Many studies have been conducted on Fe-based ammonia decomposition catalysts, including the structure and morphology control of catalysts (mesoporous confinement, core-shell structure, etc.), carrier types (carbon nanomaterials, metal oxides, silicon oxide, molecular sieves, etc.), doping with transition metals (Co, Ni, etc.), and modification with additives (alkali metals, rare earth oxides, etc.).

[0007] There is a literature that uses a combination of hydrothermal and heat treatment to prepare Fe@GC series graphitized carbon-embedded iron nanoparticle catalysts; the catalyst has a high specific surface area and porosity, good dispersibility and graphitized carbon structure. Due to space limitations, the Fe nanoparticles in the catalyst are almost completely embedded in the graphitized carbon matrix. This isolated structure enhances the stability of the Fe nanoparticles, so the catalyst shows good stability. However, the catalyst has a high stability at 600℃ and 6000h. -1 Under the conditions of , the ammonia decomposition conversion rate is only 80%, and after 20 hours of reaction, the ammonia decomposition conversion rate is only maintained at about 65%, which shows that the activity of the catalyst in catalyzing ammonia decomposition is poor.

[0008] There are literatures that use two-dimensional mica nanosheets (MS) as carriers and use uniform precipitation method (HP), impregnation method (IM) and deposition precipitation method (DP) to prepare Fe / MS catalysts; at 700℃, 30000h -1 Under the conditions of 40 ℃ and 80 ℃, the ammonia conversion rates of the 5% Fe / MS-HP catalyst, the 5% Fe / MS-DP catalyst and the 5% Fe / MS-IM catalyst were 97.1%, 86.0% and 77.4% respectively. The difference in the activity of the catalysts for catalytic ammonia decomposition was due to the production of different Fe species by different preparation methods. The excellent catalytic performance of the Fe / MS-HP catalyst was mainly attributed to the high dispersion of Fe species, the layered structure of mica and the strong metal-support interaction between Fe species and mica. However, the Fe / MS-DP and Fe / MS-IM catalysts had poor activity for catalytic ammonia decomposition due to the uneven dispersion of Fe particles and the wide particle size distribution.

[0009] There is a literature that uses mesoporous carbon material CMK-5 as a carrier to prepare a highly dispersed α-Fe2O3 nanoparticle catalyst. Due to the porous structure and high specific surface area of ​​CMK-5, the active component α-Fe2O3 nanoparticles are well dispersed. -1Under the conditions of , the ammonia conversion rate reaches 100%, but the high-temperature stability of the catalyst is poor, and some Fe2O3 nanoparticles are easily sintered and migrated, resulting in a decrease in catalyst activity.

[0010] Therefore, preparing an ammonia decomposition catalyst with good catalytic activity and stability has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the Invention

[0011] The purpose of the present invention is to provide an iron-based catalyst for decomposing ammonia with good catalytic activity and stability, as well as a preparation method and application thereof.

[0012] To achieve the above object, the present invention provides the following technical solutions:

[0013] One of the technical solutions of the present invention:

[0014] An iron-based catalyst with the chemical formula Li x Al y Fe z , where x:y:z=2:(2~3.5): (2~0.5), y+z=4.

[0015] The second technical solution of the present invention:

[0016] The preparation method of the above-mentioned iron-based catalyst comprises the following steps:

[0017] Step 1: dissolve LiNO3, Al(NO3)3·9H2O and Fe(NO3)3·9H2O in water to form a lithium aluminum iron mixed salt solution, and dissolve Na2CO3 in water to form a Na2CO3 bottom solution;

[0018] Step 2: slowly dropwise adding the lithium aluminum iron mixed salt solution obtained in step 1 to the Na2CO3 bottom solution obtained in step 1 to form a suspension, and then aging, filtering, and drying to obtain a lithium aluminum iron hydrotalcite compound;

[0019] Step 3: calcining the lithium aluminum iron hydrotalcite compound obtained in step 2 to obtain a mixed metal oxide;

[0020] Step 4: performing a reduction treatment on the mixed metal oxide obtained in step 3 to obtain the iron-based catalyst.

[0021] Furthermore, in step one, the molar concentration of lithium ions in the lithium-aluminum-iron mixed salt solution is 0.2058-0.2417 mol / L, the molar concentration of aluminum ions is 0.2058-0.4230 mol / L, and the molar concentration of iron ions is 0.0604-0.2058 mol / L.

[0022] Furthermore, in step 1, the molar concentration of the Na2CO3 base solution is 0.1029 to 0.1209 mol / L.

[0023] Furthermore, step 2 is carried out at room temperature under magnetic stirring, wherein the rotation speed of the magnetic stirring is 180 r / min.

[0024] Furthermore, in step 2, the dripping rate of the slow addition is 40 d / min.

[0025] Furthermore, in step 2, when slowly adding the lithium aluminum iron mixed salt solution, it is also necessary to add a NaOH solution with a molar concentration of 2.0 mol / L to maintain the pH of the Na2CO3 base solution at 10±0.5.

[0026] Furthermore, in step 2, the aging is static aging for 24 hours.

[0027] Furthermore, in step 2, the drying is performed at 100° C. for 24 hours.

[0028] Furthermore, in step 3, the calcination treatment is calcining at 500° C. for 5 h in an air atmosphere;

[0029] Furthermore, the calcination process was carried out from room temperature to 500° C. at a heating rate of 3° C. / min.

[0030] Furthermore, after the calcination treatment in step 3, the product needs to be ground, tableted, and passed through a 60-mesh sieve.

[0031] Furthermore, in step 4, the reduction treatment is carried out at 700° C. for 30 min in a hydrogen atmosphere;

[0032] Furthermore, the reduction treatment was performed by heating the temperature from room temperature to 700° C. at a heating rate of 10° C. / min.

[0033] The present invention adopts a constant pH co-precipitation method to prepare a lithium aluminum iron hydrotalcite compound (Li-Al-Fe layered compound) as an iron-based catalyst precursor, and calcines the lithium aluminum iron hydrotalcite compound, which can effectively improve the catalytic activity and catalytic stability of the iron-based catalyst in catalyzing ammonia decomposition.

[0034] Hydrotalcite-like compounds are a type of two-dimensional anionic clay composed of positively charged hydrotalcite layers, charge-compensating anions, and crystal water. The chemical formula is [M 2+ 1-x M 3+ x (OH)2][A n- ] x / n ·zH2O, where M 2+ is a divalent metal cation, M 3+ A is a trivalent metal cation, n- It is an inorganic or organic anion used for charge compensation, and x usually varies between 0.2 and 0.33. After calcination to remove the anions and crystal water of the hydrotalcite-like compound, it has a suitable pH value, has a dispersing effect on the active components, and can form a uniform mesoporous structure with a high specific surface area. If the active metal component is inserted into the calcined hydrotalcite-like compound, the active metal component can be evenly dispersed, thereby improving the catalytic activity and catalytic stability of the iron-based catalyst for ammonia decomposition. The lithium aluminum iron hydrotalcite compound prepared by the present invention has a single crystal phase, a good layered structure and a flaky morphology, and lithium ions, aluminum ions and iron ions are evenly dispersed in the positively charged hydrotalcite-like layer, and Fe 3+ Partial replacement of Al 3+ , which effectively increases Fe 3+ The content of Fe 3+ The dispersion of the iron-based catalyst is improved, thereby improving the catalytic activity and catalytic stability of the iron-based catalyst in catalytic ammonia decomposition.

[0035] The present invention performs calcination on the lithium aluminum iron hydrotalcite compound, and can also increase the pore size of the lithium aluminum iron hydrotalcite compound, so that its BET specific surface area reaches 109-200m 2 / g, which is beneficial to the dispersion of Fe metal particles and the diffusion of catalytic reactants (NH3), thereby being beneficial to the catalytic activity and catalytic stability of the iron-based catalyst.

[0036] The third technical solution of the present invention:

[0037] Application of the above iron-based catalyst in catalytic decomposition of ammonia.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1) The present invention adopts a constant pH co-precipitation method to prepare a lithium aluminum iron hydrotalcite compound as an iron-based catalyst precursor, and the iron-based catalyst can be obtained by calcination and reduction treatment. The preparation process is simple, the raw materials are cheap and readily available, and the reproducibility is good;

[0040] 2) The iron-based catalyst prepared by the present invention has excellent catalytic activity and catalytic stability for ammonia decomposition, wherein Li2Al 2.5 Fe 1.5 At a space velocity of 30000 mL·g -1 ·h -1 , the ammonia conversion rate can reach 73% at 600℃; at 30000mL·g -1 ·h -1 , the reaction was continued at 700℃ for 100h, and the ammonia conversion rate was maintained above 99%. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present invention. The same reference symbols are used throughout the drawings to represent the same components. In the drawings:

[0042] Figure 1 This is the X-ray powder diffraction pattern of the lithium aluminum iron hydrotalcite compound prepared in Example 1;

[0043] Figure 2 is the X-ray powder diffraction pattern of the mixed metal oxide prepared in Example 1;

[0044] Figure 3 Li2Al prepared in Example 1 3.5 Fe 0.5 X-ray powder diffraction pattern of;

[0045] Figure 4 This is the X-ray powder diffraction pattern of the lithium aluminum iron hydrotalcite compound prepared in Example 2;

[0046] Figure 5 is the X-ray powder diffraction pattern of the mixed metal oxide prepared in Example 2;

[0047] Figure 6 This is the X-ray powder diffraction pattern of Li2Al3Fe1 prepared in Example 2;

[0048] Figure 7 This is the X-ray powder diffraction pattern of the lithium aluminum iron hydrotalcite compound prepared in Example 3;

[0049] Figure 8 is the X-ray powder diffraction pattern of the mixed metal oxide prepared in Example 3;

[0050] Figure 9 Li2Al prepared in Example 3 2.5 Fe 1.5 X-ray powder diffraction pattern of;

[0051] Figure 10 This is a scanning electron microscope image of the lithium aluminum iron hydrotalcite compound prepared in Example 3;

[0052] Figure 11 The Li2Al prepared in Example 3 2.5 Fe 1.5 HAADF-STEM scan image;

[0053] Figure 12The Li2Al prepared in Example 3 2.5 Fe 1.5 High-resolution transmission electron microscopy images;

[0054] Figure 13 This is the X-ray powder diffraction pattern of the lithium aluminum iron hydrotalcite compound prepared in Example 4;

[0055] Figure 14 is the X-ray powder diffraction pattern of the mixed metal oxide prepared in Example 4;

[0056] Figure 15 This is the X-ray powder diffraction pattern of Li2Al2Fe2 prepared in Example 4;

[0057] Figure 16 is the X-ray powder diffraction pattern of the mixed metal oxide prepared in Comparative Example 1;

[0058] Figure 17 Fe prepared in Comparative Example 1 1.5 / Li2A l 2.5 X-ray powder diffraction pattern of;

[0059] Figure 18 The X-ray powder diffraction patterns of different iron-based catalysts after catalytic ammonia decomposition reaction, where a is the Li2Al prepared in Example 1. 3.5 Fe 0.5 , b is Li2Al3Fe1 prepared in Example 2, c is Li2Al3Fe1 prepared in Example 3 2.5 Fe 1.5 d is Li2Al2Fe2 prepared in Example 4, e is Fe2 prepared in Comparative Example 1 1.5 / Li2A l 2.5 ;

[0060] Figure 19 The Li2Al prepared in Example 3 after catalytic ammonia decomposition reaction 2.5 Fe 1.5 HAADF-STEM scan image;

[0061] Figure 20 The Li2Al prepared in Example 3 after catalytic ammonia decomposition reaction 2.5 Fe 1.5 High-resolution transmission electron microscopy scan of

[0062] Figure 21 This is a graph showing the test results of the catalytic activity of the iron-based catalysts prepared in Examples 1 to 4 for ammonia decomposition;

[0063] Figure 22Graph showing the test results of the catalytic activity for ammonia decomposition of the iron-based catalysts prepared in Example 3 and Comparative Example 1;

[0064] Figure 23 Li2Al prepared in Example 3 at different space velocities 2.5 Fe 1.5 Ammonia decomposition catalytic activity test results diagram;

[0065] Figure 24 Li2Al prepared in Example 3 2.5 Fe 1.5 Ammonia decomposition catalytic stability test results. DETAILED DESCRIPTION

[0066] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0067] In the following examples, all drugs used were commercially available and of analytical grade unless otherwise specified.

[0068] In the following examples, the room temperature is 25±2°C.

[0069] Example 1

[0070] An iron-based catalyst

[0071] 1) According to the molar ratio of Li:Al:Fe=2:3.5:0.5, 1.6663 g of LiNO3, 15.8674 g of Al(NO3)3·9H2O, and 2.4412 g of Fe(NO3)3·9H2O were weighed and dissolved in 100 mL of deionized water to obtain a lithium aluminum iron mixed salt solution;

[0072] 2) Weigh 1.2809 g of Na2CO3 and dissolve it in 100 mL of deionized water to obtain a Na2CO3 base solution;

[0073] 3) Weigh 8.0000 g of NaOH and dissolve it in 100 mL of deionized water to obtain a NaOH solution;

[0074] 4) Slowly adding the lithium aluminum iron mixed salt solution obtained in step 1) to the Na2CO3 base solution obtained in step 2) at room temperature under magnetic stirring to form a suspension, during which the NaOH solution obtained in step 3) was added to maintain the pH of the Na2CO3 base solution at 10±0.5;

[0075] The speed of magnetic stirring was 180 r / min; the dripping speed of slow addition was 60 d / min;

[0076] 5) The suspension obtained in step 4) was allowed to stand for aging for 24 hours, filtered, and the precipitate was washed with 1.5 L of deionized water and dried in an oven at 100° C. for 24 hours to obtain a lithium aluminum iron hydrotalcite compound;

[0077] 6) placing the lithium aluminum iron hydrotalcite compound obtained in step 5) in a muffle furnace, calcining it at 500° C. for 5 h in an air atmosphere, and then grinding, tableting, and passing through a 60-mesh sieve to obtain a mixed metal oxide;

[0078] The temperature was raised from room temperature to 500°C at a heating rate of 3°C / min;

[0079] 7) The mixed metal oxide obtained in step 6) was reduced in a hydrogen atmosphere at 700° C. for 30 min to obtain the iron-based catalyst, which was recorded as Li2Al 3.5 Fe 0.5 ;

[0080] The temperature was increased from room temperature to 700°C at a heating rate of 10°C / min.

[0081] The lithium aluminum iron hydrotalcite compound, mixed metal oxide and Li2Al prepared in Example 1 were 3.5 Fe 0.5 The X-ray powder diffraction method was used for characterization. The X-ray powder diffraction pattern is shown in Figures 1 to 3 As shown;

[0082] exist Figure 1 In the figure, characteristic diffraction peaks of hydrotalcite-like compounds can be observed, with 2θ=13.5°, 23.5°, 27.4°, 41.2°, 41.6°, 47.7°, 56.3°, 74.9° and 76.5° corresponding to (002), (101), (004), (006), (112), (202), (008), (303) and (110) crystal planes, respectively, indicating that a good layered structure is formed, i.e., a lithium aluminum iron hydrotalcite-like compound is formed;

[0083] exist Figure 2 In the sample, characteristic diffraction peaks of Fe2O3 and Al2O3 can be observed, and no other miscellaneous peaks are observed, which indicates that mixed metal oxides are formed;

[0084] exist Figure 3 Characteristic diffraction peaks of Al2O3 and Fe can be observed, which indicates that the iron metal is fully reduced. According to the Scherrer formula, the average grain size of the Fe metal particles is calculated to be 20 nm.

[0085] Example 2

[0086] An iron-based catalyst

[0087] 1) According to the molar ratio of Li:Al:Fe=2:3:1, 1.5747 g of LiNO3, 12.8532 g of Al(NO3)3·9H2O, and 4.6141 g of Fe(NO3)3·9H2O were weighed and dissolved in 100 mL of deionized water to obtain a lithium aluminum iron mixed salt solution;

[0088] 2) Weigh 1.2105 g of Na2CO3 and dissolve it in 100 mL of deionized water to obtain a Na2CO3 base solution;

[0089] 3) Weigh 8.0000 g of NaOH and dissolve it in 100 mL of deionized water to obtain a NaOH solution;

[0090] 4) Slowly adding the lithium aluminum iron mixed salt solution obtained in step 1) to the Na2CO3 base solution obtained in step 2) at room temperature under magnetic stirring to form a suspension, during which the NaOH solution obtained in step 3) was added to maintain the pH of the Na2CO3 base solution at 10±0.5;

[0091] The speed of magnetic stirring was 180 r / min; the dripping speed of slow addition was 60 d / min;

[0092] 5) The suspension obtained in step 4) was allowed to stand for aging for 24 hours, filtered, and the precipitate was washed with deionized water and dried in an oven at 100° C. for 24 hours to obtain a lithium aluminum iron hydrotalcite compound;

[0093] 6) placing the lithium aluminum iron hydrotalcite compound obtained in step 5) in a muffle furnace, calcining it at 500° C. for 5 h in an air atmosphere, and then grinding, tableting, and passing through a 60-mesh sieve to obtain a mixed metal oxide;

[0094] The temperature was raised from room temperature to 500°C at a heating rate of 3°C / min;

[0095] 7) reducing the mixed metal oxide obtained in step 6) at 700° C. for 30 min in a hydrogen atmosphere to obtain the iron-based catalyst, which is designated as Li2Al3Fe1;

[0096] The temperature was increased from room temperature to 700°C at a heating rate of 10°C / min.

[0097] The lithium aluminum iron hydrotalcite compound, mixed metal oxide and Li2Al3Fe1 prepared in Example 2 were characterized by X-ray powder diffraction. The X-ray powder diffraction pattern is shown in FIG. Figures 4-6 As shown;

[0098] exist Figure 4 In the figure, characteristic diffraction peaks of hydrotalcite-like compounds can be observed, with 2θ=13.5°, 23.5°, 27.4°, 41.2°, 41.6°, 47.7°, 56.3°, 74.9° and 76.5° corresponding to (002), (101), (004), (006), (112), (202), (008), (303) and (110) crystal planes, respectively, indicating that a good layered structure is formed, i.e., a lithium aluminum iron hydrotalcite-like compound is formed;

[0099] exist Figure 5 In the sample, characteristic diffraction peaks of Fe2O3 and Al2O3 can be observed, and no other miscellaneous peaks are observed, which indicates that mixed metal oxides are formed;

[0100] exist Figure 6 Characteristic diffraction peaks of Al2O3 and Fe can be observed, which indicates that the iron metal is fully reduced. According to the Scherrer formula, the average grain size of the Fe metal particles is calculated to be 34.1 nm.

[0101] Example 3

[0102] An iron-based catalyst

[0103] 1) According to the molar ratio of Li:Al:Fe=2:2.5:1.5, 1.4927 g of LiNO3, 10.1530 g of Al(NO3)3·9H2O, and 6.5606 g of Fe(NO3)3·9H2O were weighed and dissolved in 100 mL of deionized water to obtain a lithium aluminum iron mixed salt solution;

[0104] 2) Weigh 1.1475 g of Na2CO3 and dissolve it in 100 mL of deionized water to obtain a Na2CO3 base solution;

[0105] 3) Weigh 8.0000 g of NaOH and dissolve it in 100 mL of deionized water to obtain a NaOH solution;

[0106] 4) Slowly adding the lithium aluminum iron mixed salt solution obtained in step 1) to the Na2CO3 base solution obtained in step 2) at room temperature under magnetic stirring to form a suspension, during which the NaOH solution obtained in step 3) was added to maintain the pH of the Na2CO3 base solution at 10±0.5;

[0107] The speed of magnetic stirring was 180 r / min; the dripping speed of slow addition was 60 d / min;

[0108] 5) The suspension obtained in step 4) was allowed to stand for aging for 24 hours, filtered, and the precipitate was washed with deionized water and dried in an oven at 100° C. for 24 hours to obtain a lithium aluminum iron hydrotalcite compound;

[0109] 6) placing the lithium aluminum iron hydrotalcite compound obtained in step 5) in a muffle furnace, calcining it at 500° C. for 5 h in an air atmosphere, and then grinding, tableting, and passing through a 60-mesh sieve to obtain a mixed metal oxide;

[0110] The temperature was raised from room temperature to 500°C at a heating rate of 3°C / min;

[0111] 7) The mixed metal oxide obtained in step 6) was reduced in a hydrogen atmosphere at 700° C. for 30 min to obtain the iron-based catalyst, which was recorded as Li2Al 2.5 Fe 1.5 ;

[0112] The temperature was increased from room temperature to 700°C at a heating rate of 10°C / min.

[0113] The lithium aluminum iron hydrotalcite compound, mixed metal oxide and Li2Al prepared in Example 3 were 2.5 Fe 1.5 The X-ray powder diffraction method was used for characterization. The X-ray powder diffraction pattern is shown in Figures 7-9 As shown;

[0114] exist Figure 7 In the figure, characteristic diffraction peaks of hydrotalcite-like compounds can be observed, with 2θ=13.5°, 23.5°, 27.4°, 41.2°, 41.6°, 47.7°, 56.3°, 74.9° and 76.5° corresponding to (002), (101), (004), (006), (112), (202), (008), (303) and (110) crystal planes, respectively, indicating that a good layered structure is formed, i.e., a lithium aluminum iron hydrotalcite-like compound is formed;

[0115] exist Figure 8 In the sample, characteristic diffraction peaks of Fe2O3 and Al2O3 can be observed, and no other miscellaneous peaks are observed, which indicates that mixed metal oxides are formed;

[0116] exist Figure 9 Characteristic diffraction peaks of Al2O3 and Fe can be observed, which indicates that the iron metal is fully reduced. According to the Scherrer formula, the average grain size of the Fe metal particles is 39 nm.

[0117] The lithium aluminum iron hydrotalcite compound prepared in Example 3 was subjected to electron microscope scanning. The electron microscope scanning image is as follows: Figure 10 As shown;

[0118] exist Figure 10 In the experiment, it can be observed that the lithium aluminum iron hydrotalcite compound is in the form of flakes and has a typical hydrotalcite-like morphology.

[0119] The Li2Al prepared in Example 3 2.5 Fe 1.5 Perform HAADF-STEM scanning, the HAADF-STEM scanning diagram is as follows Figure 11 As shown;

[0120] exist Figure 11 In the MgO sample, it can be observed that the average grain size of Fe metal particles is 39 nm.

[0121] The Li2Al prepared in Example 3 2.5 Fe 1.5 Perform high-resolution transmission electron microscopy scanning, and the high-resolution transmission electron microscopy scanning image is as follows Figure 12 As shown;

[0122] exist Figure 12 In the figure, lattice fringes with a spacing of 0.198 nm can be observed, which corresponds to the (110) crystal plane in Fe. Figure 12 In the lattice, lattice fringes with a lattice fringe spacing of 0.237 nm can also be observed, which corresponds to the (311) crystal plane in Al2O3.

[0123] Example 4

[0124] An iron-based catalyst

[0125] 1) According to the molar ratio of Li:Al:Fe=2:2:2, 1.4188 g of LiNO3, 7.7203 g of Al(NO3)3·9H2O, and 8.3144 g of Fe(NO3)3·9H2O were weighed and dissolved in 100 mL of deionized water to obtain a lithium aluminum iron mixed salt solution;

[0126] 2) Weigh 1.0907 g of Na2CO3 and dissolve it in 100 mL of deionized water to obtain a Na2CO3 base solution;

[0127] 3) Weigh 8.0000 g of NaOH and dissolve it in 100 mL of deionized water to obtain a NaOH solution;

[0128] 4) Slowly adding the lithium aluminum iron mixed salt solution obtained in step 1) to the Na2CO3 base solution obtained in step 2) at room temperature under magnetic stirring to form a suspension, during which the NaOH solution obtained in step 3) was added to maintain the pH of the Na2CO3 base solution at 10±0.5;

[0129] The speed of magnetic stirring was 180 r / min; the dripping speed of slow addition was 60 d / min;

[0130] 5) The suspension obtained in step 4) was allowed to stand for aging for 24 hours, filtered, and the precipitate was washed with deionized water and dried in an oven at 100° C. for 24 hours to obtain a lithium aluminum iron hydrotalcite compound;

[0131] 6) placing the lithium aluminum iron hydrotalcite compound obtained in step 5) in a muffle furnace, calcining it at 500° C. for 5 h in an air atmosphere, and then grinding, tableting, and passing through a 60-mesh sieve to obtain a mixed metal oxide;

[0132] The temperature was raised from room temperature to 500°C at a heating rate of 3°C / min;

[0133] 7) reducing the mixed metal oxide obtained in step 6) at 700° C. for 30 min in a hydrogen atmosphere to obtain the iron-based catalyst, which is designated as Li2Al2Fe2;

[0134] The temperature was increased from room temperature to 700°C at a heating rate of 10°C / min.

[0135] The lithium aluminum iron hydrotalcite compound, mixed metal oxide and Li2Al2Fe2 prepared in Example 4 were characterized by X-ray powder diffraction. The X-ray powder diffraction pattern is as follows: Figures 13-15 As shown;

[0136] exist Figure 13 In the figure, characteristic diffraction peaks of hydrotalcite-like compounds can be observed, with 2θ=13.5°, 23.5°, 27.4°, 41.2°, 41.6°, 47.7°, 56.3°, 74.9° and 76.5° corresponding to (002), (101), (004), (006), (112), (202), (008), (303) and (110) crystal planes, respectively, indicating that a good layered structure is formed, i.e., a lithium aluminum iron hydrotalcite-like compound is formed;

[0137] exist Figure 14 In the sample, characteristic diffraction peaks of Fe2O3 and Al2O3 can be observed, and no other miscellaneous peaks are observed, which indicates that mixed metal oxides are formed;

[0138] exist Figure 15 Characteristic diffraction peaks of Al2O3 and Fe can be observed, which indicates that the iron metal is fully reduced. According to the Scherrer formula, the average grain size of the Fe metal particles is calculated to be 43.6 nm.

[0139] Comparative Example 1

[0140] An iron-based catalyst

[0141] 1) According to the molar ratio of Li:Al = 2:2.5, 2.6291 g of LiNO3 and 17.8824 g of Al(NO3)3·9H2O were weighed and dissolved in 100 mL of deionized water to obtain a lithium aluminum mixed salt solution;

[0142] 2) Weigh 2.0210 g of Na2CO3 and dissolve it in 100 mL of deionized water to obtain a Na2CO3 base solution;

[0143] 3) Weigh 8.0000 g of NaOH and dissolve it in 100 mL of deionized water to obtain a NaOH solution;

[0144] 4) Slowly adding the lithium aluminum mixed salt solution obtained in step 1) to the Na2CO3 base solution obtained in step 2) at room temperature under magnetic stirring to form a suspension, during which the NaOH solution obtained in step 3) was added to maintain the pH of the Na2CO3 base solution at 10±0.5;

[0145] The speed of magnetic stirring was 180 r / min; the dripping speed of slow addition was 60 d / min;

[0146] 5) The suspension obtained in step 4) was allowed to stand for aging for 24 hours, filtered, and the precipitate was washed with deionized water and dried in an oven at 100° C. for 24 hours to obtain a lithium aluminum hydrotalcite compound;

[0147] 6) The lithium aluminum hydrotalcite compound obtained in step 5) was placed in a muffle furnace and calcined at 500° C. for 5 h in an air atmosphere, and then ground, pressed into tablets, and passed through a 60-mesh sieve to obtain Li2Al 2.5 -MMO;

[0148] The temperature was raised from room temperature to 500°C at a heating rate of 3°C / min;

[0149] 7) Weigh 2.1869 g of Fe(NO₃)₃·9H₂O and dissolve it in 10 mL of deionized water to obtain a ferric nitrate solution.

[0150] 8) 0.5678g of Li2Al obtained in step 6) 2.5 -MMO was placed in a polytetrafluoroethylene tank, and the ferric nitrate solution obtained in step 7) was added dropwise to the Li2Al 2.5 -MMO surface makes Li2Al 2.5 -MMO moistened, stirred evenly and dried in an oven at 100°C for 10 min, repeating the process until all the ferric nitrate solution was added;

[0151] 9) Add the Li2Al solution in step 8) 2.5-MMO was placed in a muffle furnace and calcined at 500 °C for 5 h in an air atmosphere, and then ground, pressed into tablets, and passed through a 60-mesh sieve to obtain a mixed metal oxide;

[0152] The temperature was raised from room temperature to 500°C at a heating rate of 3°C / min;

[0153] 10) The mixed metal oxide obtained in step 9) was reduced in a hydrogen atmosphere at 700° C. for 30 min to obtain the iron-based catalyst, which was recorded as Fe 1.5 / Li2A l 2.5 ;

[0154] The temperature was increased from room temperature to 700°C at a heating rate of 10°C / min.

[0155] The mixed metal oxides prepared in Comparative Example 1 and Fe 1.5 / Li2A l 2.5 The X-ray powder diffraction method was used for characterization. The X-ray powder diffraction pattern is shown in Figures 16-17 As shown;

[0156] exist Figure 16 In the sample, characteristic diffraction peaks of Fe2O3 and Al2O3 can be observed, and no other miscellaneous peaks are observed, which indicates that mixed metal oxides are formed;

[0157] exist Figure 17 Characteristic diffraction peaks of Al2O3 and Fe can be observed, which indicates that the iron metal is fully reduced. According to the Scherrer formula, the average grain size of the Fe metal particles is calculated to be 38.5 nm.

[0158] Effect verification

[0159] 1. Catalytic activity test

[0160] 50 mg of the iron-based catalysts prepared in Examples 1 to 4 and Comparative Example 1 were placed in different fixed-bed quartz tube reactors, respectively, under normal pressure, pure ammonia, 30000 mL·g -1 ·h -1 Catalyzes the decomposition of ammonia under the reaction conditions;

[0161] Among them, the flow rate of pure ammonia is 25mL / min;

[0162] The X-ray powder diffraction method was used to characterize the different iron-based catalysts after the ammonia decomposition reaction at 750 ° C. The X-ray powder diffraction patterns are shown in Figure 2. Figure 18 As shown, where a is the Li2Al prepared in Example 1 3.5 Fe 0.5b is the X-ray powder diffraction pattern of Li2Al3Fe1 prepared in Example 2, and c is the X-ray powder diffraction pattern of Li2Al prepared in Example 3. 2.5 Fe 1.5 d is the X-ray powder diffraction pattern of Li2Al2Fe2 prepared in Example 4, and e is the X-ray powder diffraction pattern of Fe2 prepared in Comparative Example 1. 1.5 / Li2A l 2.5 X-ray powder diffraction pattern of;

[0163] In Figure a, it can be observed that the characteristic peak of Fe disappears, and Fe is completely transformed into Fe4N. According to the Scherrer formula, the average grain size of Fe4N is calculated to be 18nm;

[0164] In Figure b, it can be observed that the characteristic peak of Fe disappears, and Fe is completely transformed into Fe4N. According to the Scherrer formula, the average grain size of Fe4N is calculated to be 22nm;

[0165] In Figure c, it can be observed that the characteristic peak of Fe disappears, and Fe is completely transformed into Fe4N. According to the Scherrer formula, the average grain size of Fe4N is calculated to be 26nm;

[0166] In Figure d, characteristic diffraction peaks of Fe and Fe4N can be observed. This is because the average particle size of the Fe metal particles after reduction is too large and cannot be completely transformed into Fe4N. According to the Scherrer formula, the average grain size of the Fe metal particles is 32.9 nm, and the average grain size of Fe4N is 27.3 nm.

[0167] In Figure e, characteristic diffraction peaks of Fe and Fe4N can be observed. This is because the iron-based catalyst prepared by the impregnation method is unevenly distributed inside the carrier and has a small specific surface area. After reduction, the average particle size of the Fe metal particles is too large and cannot be completely transformed into Fe4N. According to the Scherrer formula, the average grain size of the Fe metal particles is calculated to be 24.1nm, and the average grain size of Fe4N is 22.4nm.

[0168] The iron-based catalyst after the ammonia decomposition reaction in Example 3 was subjected to HAADF-STEM scanning. Figure 19 As shown;

[0169] exist Figure 19 In the MgO sample, it can be observed that the average grain size of Fe metal particles is 31.8 nm.

[0170] The iron-based catalyst after the ammonia decomposition reaction in Example 3 was scanned by high-resolution transmission electron microscopy. Figure 20 As shown;

[0171] exist Figure 20 In the figure, lattice fringe spacing of 0.214 nm can be observed, which corresponds to the (111) crystal plane in Fe4N. Figure 20 In the lattice, lattice fringes with a lattice fringe spacing of 0.198 nm can also be observed, which corresponds to the (400) crystal plane in Al2O3.

[0172] The iron-based catalysts prepared in Examples 1 to 4 and Comparative Example 1 were subjected to N2 physical adsorption and desorption experiments, and the BET (specific surface area), pore volume, and average pore diameter were measured, where BET was calculated based on the BET specific surface area method, the pore volume was calculated based on the total volume of single-point adsorption, and the average pore diameter was calculated based on the Barrett-Joyner-Ha l enda (BJH) desorption average pore diameter.

[0173] The comparison of the Fe average grain size, Fe4N average grain size, BET (specific surface area), pore volume and average pore diameter in the iron-based catalysts prepared in Examples 1 to 4 and Comparative Example 1 is shown in Table 1;

[0174] Table 1 Grain size, BET, pore volume, average pore diameter

[0175]

[0176] The test results of the catalytic activity of the iron-based catalysts prepared in Examples 1 to 4 for ammonia decomposition are as follows: Figure 21 As shown;

[0177] Depend on Figure 21 It can be seen that with the increase of Fe content, the ammonia decomposition activity first increases. When the Fe content is 1.5, the ammonia decomposition catalytic activity of the iron-based catalyst is the highest. When the Fe content increases further, the ammonia decomposition catalytic activity of the iron-based catalyst decreases. This is because the Fe metal particles are too large to be completely converted into Fe4N, resulting in the presence of large Fe metal particles in the iron-based catalyst, which reduces the ammonia decomposition activity.

[0178] The test results of the catalytic activity of the iron-based catalysts prepared in Example 3 and Comparative Example 1 for ammonia decomposition are as follows: Figure 22 As shown;

[0179] Depend on Figure 22 It can be seen that the catalytic activity of ammonia decomposition of the iron-based catalyst prepared in Example 3 is higher than that of the iron-based catalyst prepared in Comparative Example 1. This is because the iron-based catalyst prepared by the impregnation method has lower specific surface area, pore volume and average pore diameter, and the Fe metal particles are unevenly distributed inside the carrier, which leads to a decrease in the catalytic effect of the iron-based catalyst.

[0180] 50 mg of Li2Al prepared in Example 3 was added 2.5 Fe1.5 Placed in different fixed-bed quartz tube reactors, catalytic ammonia decomposition was carried out under the reaction conditions of normal pressure, pure ammonia, and different space velocities;

[0181] Among them, the flow rate of pure ammonia is 25mL / min;

[0182] Li2Al prepared in Example 3 at different space velocities 2.5 Fe 1.5 The test results of ammonia decomposition catalytic activity are as follows Figure 23 As shown;

[0183] Depend on Figure 23 It can be seen that as the space velocity increases, the conversion rate of NH3 decreases; Li2A l 2.5 Fe 1.5 At a space velocity of 5000 mL·g -1 ·h -1 Under the reaction conditions of 500℃ and 600℃, the ammonia conversion rate reaches 98%.

[0184] 2. Catalytic stability test

[0185] 50 mg of the Li2Al prepared in Example 3 was added 2.5 Fe 1.5 Placed in a fixed bed quartz tube reactor, at a space velocity of 30000 mL g -1 ·h -1 , catalytic ammonia decomposition stability test under reaction conditions of 700℃;

[0186] Among them, the flow rate of pure ammonia is 25mL / min;

[0187] The results of the ammonia decomposition catalytic stability test are as follows: Figure 24 As shown;

[0188] Depend on Figure 24 It can be seen that the Li2Al prepared in Example 3 2.5 Fe 1.5 At a space velocity of 30000 mL·g -1 ·h -1 During the reaction process at 700℃ and 100h, the ammonia conversion rate remained unchanged at 99%, showing good stability.

[0189] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. An iron-based catalyst, characterized in that The chemical formula of the iron-based catalyst is Li x Al y Fe z , where x:y:z=2:(2~3.5): (2~0.5), y+z=4; The preparation method of the iron-based catalyst comprises the following steps: Step 1: dissolve LiNO3, Al(NO3)3·9H2O and Fe(NO3)3·9H2O in water to form a lithium aluminum iron mixed salt solution, and dissolve Na2CO3 in water to form a Na2CO3 bottom solution; Step 2: slowly dropwise adding the lithium aluminum iron mixed salt solution obtained in step 1 to the Na2CO3 bottom solution obtained in step 1 to form a suspension, and then aging, filtering, and drying to obtain a lithium aluminum iron hydrotalcite compound; Step 3: calcining the lithium aluminum iron hydrotalcite compound obtained in step 2 to obtain a mixed metal oxide; Step 4: performing a reduction treatment on the mixed metal oxide obtained in step 3 to obtain the iron-based catalyst.

2. The iron-based catalyst according to claim 1, characterized in that In step 1, the molar concentration of lithium ions in the lithium-aluminum-iron mixed salt solution is 0.2058-0.2417 mol / L, the molar concentration of aluminum ions is 0.2058-0.4230 mol / L, and the molar concentration of iron ions is 0.0604-0.2058 mol / L; the molar concentration of the Na2CO3 base solution is 0.1029-0.1209 mol / L.

3. The iron-based catalyst according to claim 1, characterized in that In step 2, the dripping rate of the slow addition is 60d / min.

4. The iron-based catalyst according to claim 1, characterized in that In step 2, when slowly adding the lithium aluminum iron mixed salt solution, it is also necessary to add a NaOH solution with a molar concentration of 2.0 mol / L to maintain the pH of the Na2CO3 base solution at 10±0.

5.

5. The iron-based catalyst according to claim 1, characterized in that In step 2, the aging is static aging for 24 hours, and the drying is drying at 100° C. for 24 hours.

6. The iron-based catalyst according to claim 1, characterized in that In step 3, the calcination treatment is carried out at 500° C. for 5 hours in an air atmosphere.

7. The iron-based catalyst according to claim 1, characterized in that In step 4, the reduction treatment is carried out at 700° C. for 30 minutes in a hydrogen atmosphere.

8. Use of the iron-based catalyst according to claim 1 in catalytic decomposition of ammonia.

Images