How to synthesize ammonia

Abstract

To provide an ammonia synthesis method capable of synthesizing ammonia at a high reaction rate under atmospheric pressure.SOLUTION: The method includes a step (A) of reacting lithium hydride (LiH) with nitrogen (N2) to form lithium imide (Li2 NH) and a step (B) of reacting lithium imide (Li2 NH) generated in the step (A) with hydrogen (H2) to form ammonia (NH3). The step (A) is performed by mixing lithium hydride (LiH) with a substance that does not react with lithium hydride (LiH) at a temperature where the lithium hydride (LiH) and nitrogen (N2) react with each other.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a method for synthesizing ammonia under atmospheric pressure. [Background technology]

[0002] In recent years, ammonia (NH3), which has attracted attention as a hydrogen carrier, has been synthesized widely using the Haber-Bosch process. This method uses iron as a catalyst to synthesize ammonia from hydrogen (H2) and nitrogen (N2), but because the nitrogen molecule is very stable, it must be carried out under high temperatures and pressures of 400-600°C and 200-400 atmospheres.

[0003] Therefore, various methods capable of synthesizing ammonia under milder conditions (low temperature, low pressure) have been investigated. For example, Non-Patent Document 1 discloses a method in which lithium hydride (LiH) is reacted with N2 to produce lithium imide (Li2NH), and the produced Li2NH is reacted with H2 to produce NH3.

[0004] In this method, LiH is regenerated in the process of generating NH3 by reacting Li2NH with H2, so it is possible to generate NH3 continuously by repeating the two reaction steps. It has also been suggested that nickel (Ni), cobalt (Co), iron (Fe), etc. function as a catalyst to dissociate nitrogen in the reaction step of LiH with N2, accelerating the reaction rate. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Wenbo Gao et al. Production of ammonia via chemical looping process based on metal imides as nitrogen carriers. Nature Energy. 2018, 3, 1067-1075 Summary of the Invention [Problem to be solved by the invention]

[0006] In the method of synthesizing ammonia described in Non-Patent Document 1, Ni and the like function as a catalyst that dissociates nitrogen, so that the reaction initiation temperature can be lowered. However, there is a problem in that, in the process of repeating the reaction steps, LiH and the catalyst aggregate, inhibiting the reaction between Li2NH and H2, and as a result, the reaction rate decreases.

[0007] The present invention has been made in consideration of the above problems, and a main object of the present invention is to provide an ammonia synthesis method capable of synthesizing ammonia at a high reaction rate under atmospheric pressure. [Means for solving the problem]

[0008] The method for synthesizing ammonia according to the present invention includes a step of reacting lithium hydride (LiH) with nitrogen (N2) to generate lithium imide (Li2NH), and a step of reacting the generated lithium imide (Li2NH) with hydrogen (H2) to generate ammonia (NH3). The step of generating lithium imide (Li2NH) is characterized in that it is performed by mixing lithium hydride (LiH) with a substance that does not react with lithium hydride (LiH) at a temperature at which lithium hydride (LiH) reacts with nitrogen (N2). Effect of the Invention

[0009] According to the present invention, there is provided an ammonia synthesis method capable of synthesizing ammonia at a high reaction rate under atmospheric pressure. [Brief description of the drawings]

[0010] [Figure 1] 1 is a graph showing the weight change and reaction yield in the reaction of LiH with N2, and the MS spectrum of H2 (m / z=2). [Diagram 2]FIG. 2 shows an X-ray diffraction pattern of the reaction product produced by the reaction of LiH with N2. [Diagram 3] 1 is a graph showing the reaction yield in the reaction of Li2NH and H2 and the MS spectrum of HH3 (m / z=17). [Figure 4] 1 is a graph showing the reaction yield in the reaction of LiH with N2. [Diagram 5] (a)-(c) are SEM images of the reaction products after reacting with N2. [Figure 6] 1 is a conceptual diagram illustrating the reaction between LiH and N2. [Figure 7] 1 is a graph showing the weight change and reaction yield in the reaction of a mixture of LiH and Li2O with N2, and the MS spectrum of H2 (m / z=2). [Figure 8] FIG. 1 shows an X-ray diffraction pattern of the reaction product produced by reacting a mixture of LiH and Li2O with N2. [Figure 9] (a) and (b) are SEM images of the reaction product after reacting with N2, and (c) and (d) show EDS mapping. [Figure 10] 1 is a conceptual diagram illustrating the reaction of a mixture of LiH and Li2O with N2. [Figure 11] 1 is a graph showing the reaction yield in the reaction of Li2NH and H2 and the MS spectrum of HH3 (m / z=17). [Figure 12] 1 is a graph showing the weight change in the reaction of LiH with N2 and the MS spectrum of H2 (m / z=2). [Figure 13] 1 is a graph showing the weight change in the reaction of Li2NH with H2 and the MS spectrum of HH3 (m / z=17). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the following embodiment. In addition, appropriate modifications are possible within the scope of the effects of the present invention.

[0012] The method for synthesizing ammonia in this embodiment is based on the two steps shown in the following reaction formulas (1) and (2).

[0013]

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[0014]

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[0015] First, as shown in reaction formula (1), lithium hydride (LiH) is reacted with nitrogen (N2) to generate lithium imide (Li2NH). Next, as shown in reaction formula (2), the generated lithium imide (Li2NH) is reacted with hydrogen (H2) to generate ammonia (NH3). Here, as shown in reaction formula (2), lithium hydride (LiH) is regenerated in the process of generating ammonia (NH3), so ammonia (NH3) can be continuously generated by repeating the two steps shown in reaction formulas (1) and (2).

[0016] The chemical progress of the above reactions (1) and (2) was verified by the method described below.

[0017] [Reaction of LiH with N2] The thermal reaction characteristics of LiH and N2 were measured using a thermogravimetry (TG) device (TG8120, manufactured by Rigaku Corporation) installed in a glove box, and the generated gas was measured using a mass spectrometry (MS) device (M-QA200TS, manufactured by Canon Anelva Corporation) equipped with the TG device. The measurement was performed by subjecting 0.6 mg of LiH (manufactured by Alfa Aesar, purity 99.4%) to 300 cm2 of N2 (G1 grade) in a glove box at 1 atm.3 While flowing at a flow rate of 1 / min, the sample was heated from room temperature to 500°C at a rate of 5°C / min for a period of time sufficient for the weight change of the sample to become saturated.

[0018] Figure 1 shows the weight change and reaction yield (χ N2 ) and the MS spectrum of H2 (m / z=2) measured by an MS device. Here, the reaction yield (χ N2 ) is defined by the following formula (3). In formula (3), W N2 is the weight gain experimentally obtained by TG measurement, W theo is the theoretical weight gain from the initial sample weight.

[0019]

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[0020] As shown in Figure 1, a gradual increase in weight was observed from approximately 400°C, and H2 generation was confirmed in sync with this weight change.

[0021] Next, the reaction products generated by the thermal reaction of LiH with N2 were identified using an X-ray diffraction (XRD) device (Rigaku Corporation, RINT 2500V).

[0022] Figure 2 shows the diffraction patterns of the X-ray diffraction measurements, where (a) shows the measurement results before the reaction, and (b) shows the measurement results after the reaction. (c) shows the measurement results after the reaction of the reaction product with H2, which will be described later. Here, because the diffraction intensity of LiH is weak, an enlarged view is shown on the right side to show the change in LiH. (d) and (e) show the diffraction peaks specific to LiH and Li2NH, respectively.

[0023] As shown in Figures 2(a) and (b), the diffraction pattern corresponding to LiH before the reaction changes to the diffraction pattern corresponding to Li2NH. This result verifies that the reaction between LiH and N2 produces Li2NH and H2 according to reaction formula (1).

[0024] [Reaction of Li2NH with H2] The thermal reaction characteristics of Li2NH generated by the reaction of LiH with N2 and H2 were measured using a TG device in the same manner as above, and the generated gas was measured using an MS device. The measurement was performed by subjecting the generated Li2NH to 300 cm3 of H2 (G1 grade) in a glove box at 1 atm. 3 The temperature was increased from room temperature to 500°C at a rate of 5°C / min while flowing at a flow rate of 100 / min.

[0025] Figure 3 shows the reaction yield (χ NH3 ) and the MS spectrum of NH3 (m / z=17) measured by an MS device. Here, the absolute values ​​of the weight changes in the above reaction formulas (1) and (2) are the same, so the reaction yield of NH3 (χ NH3 ) is defined by the following equation (4): N2 is the experimental weight gain from the reaction of LiH with N2, and W NH3 is the experimentally obtained weight loss from the reaction of Li2NH with H2.

[0026]

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[0027] As shown in FIG. 3, a gradual weight loss was observed from about 260°C, and the generation of NH3 was confirmed in synchronization with this weight loss. In addition, the reaction yield (χ NH3 ) reached nearly 100%, and NH3 production was complete.

[0028] Next, the reaction products produced by the thermal reaction of Li2NH with H2 were identified using X-ray diffraction in the same manner as above.

[0029] Figure 2(c) shown above is a diffraction pattern showing the measurement result of X-ray diffraction. As shown in Figures 2(b) and (c), the diffraction pattern corresponding to Li2NH before the reaction changes to the diffraction pattern corresponding to LiH. From this result, it was verified that NH3 and LiH are produced by the reaction of Li2NH and H2.

[0030] As shown in Figure 1, the reaction yield (χ N2 ) was about 60% after 150 minutes of reaction. After that, the reaction was continued at 500°C for about 1200 minutes, as shown in Figure 4. After about 200 minutes, the reaction began to slow down, and the reaction yield saturated at about 80%. This result is consistent with the result that a diffraction peak corresponding to LiH was faintly observed after the reaction, as shown in the enlarged view of Figure 2(b).

[0031] To investigate this phenomenon, the product after the reaction with N2 was observed using a scanning electron microscope (SEM) (JEOL, JSM-6380A) equipped with an energy dispersive X-ray spectrometer (EDS).

[0032] Figures 5(a)-(c) are SEM images taken at different magnifications. As shown in Figure 5(a), it can be seen that the product (Li2NH) has coarsened into large particles of over 200 μm. As shown in Figure 5(b), the surface has a smooth net-like shape, which suggests that the product has melted and is connected to adjacent particles. As shown in Figure 5(c), particles with sharp edges are observed in the high-magnification SEM image, suggesting that the product crystallized during the cooling process to room temperature.

[0033] From the above results, as shown in the conceptual diagrams of Figures 6(a)-(c), it is inferred that the reaction between LiH and N2 proceeds on the surface of LiH, the product Li2NH aggregates, and LiH is covered by the aggregated Li2NH. As a result, as the reaction proceeds, contact between the inside of LiH and N2 is hindered, so the reaction rate gradually decreases and the reaction becomes saturated.

[0034] The inventors of the present invention have investigated various methods for suppressing the aggregation of Li2NH and improving the reaction rate in the reaction of LiH and N2, and have found that mixing a substance that acts as a scaffold for the reaction with LiH is effective, leading to the invention. Note that in the present invention, a substance that does not react with LiH is used as the substance to be mixed with LiH, which is essentially different from the method using a catalyst that involves a chemical reaction.

[0035] The effectiveness of the present invention will be explained below by taking as an example the case where lithium oxide (Li2O) is used as the substance to be mixed with LiH.

[0036] [Reaction of a mixture of LiH and Li2O with N2] First, a mixture of LiH (Alfa Aesar, purity 99.4%) and Li2O (Kojundo Chemical Laboratory, purity 99% or more) was prepared in a glove box filled with Ar (G1 grade) using an agate mortar at a molar ratio of 1:2 over 10 minutes. The amount of LiH was fixed at 0.6 mg to compare with the case where Li2O was not mixed. In addition, if Li2O contains impurities (e.g., LiOH, etc.) that react with LiH, the reaction between LiH and the impurities will proceed, which may reduce the reaction yield between LiH and N2. Therefore, it is preferable to heat-treat Li2O in advance to remove the impurities before mixing.

[0037] The thermal reaction characteristics of the mixture of LiH and Li2O with N2 were measured using a TG device as described above, and the generated gas was measured using an MS device. Also, as described above, the mixture of LiH and Li2O was measured in a glove box at 1 atmosphere with 300 cm2 of N2. 3 While flowing at a flow rate of 1 / min, the sample was heated from room temperature to 500°C at a rate of 5°C / min for a period of time sufficient for the weight change of the sample to become saturated.

[0038] The graph in FIG. 7 shows the weight change and reaction yield (χ N27) and the MS spectrum of H2 (m / z=2) measured by an MS device. Here, graph A shows the case where Li2O was mixed, and graph B shows the case where Li2O was not mixed (same as the graph in Figure 1). As shown in Figure 7, a gradual weight increase can be seen from around 390°C, which is slightly lower than when Li2O was not mixed, and the generation of H2 can be confirmed in sync with this weight change.

[0039] What is noteworthy here is that the reaction yield (χ N2 ) reaches 100%, and it can be seen that the reaction rate is dramatically improved by mixing LiH with Li2O.

[0040] Next, the reaction products produced by the thermal reaction of the mixture of LiH and Li2O with N2 were identified using an X-ray diffraction apparatus in the same manner as above.

[0041] Figure 8 shows the diffraction patterns of the X-ray diffraction measurements, where (a) shows the measurement results before the reaction, and (b) shows the measurement results after the reaction. Note that (c) shows the measurement results after the reaction of the reaction product with H2, which will be described later. Also, (d), (e), and (f) show the diffraction peaks specific to LiH, Li2NH, and Li2O, respectively.

[0042] As shown in FIG. 8(a), before the reaction, the diffraction patterns corresponding to LiH and Li2O are observed, but the peak intensity of LiH is lower than that of Li2O due to the small number of electrons. After the reaction with N2, as shown in FIG. 8(b), the diffraction pattern corresponding to LiH changes to that corresponding to Li2NH. This result is consistent with the high reaction yield (χ N2 )

[0043] In order to investigate the mechanism by which the reaction rate with N2 was dramatically improved by mixing LiH with Li2O, the product (Li2NH) after reaction with N2 was observed using an SEM in the same manner as above.

[0044] 9(a) and (b) are SEM images taken at different magnifications, and FIGS. 9(c) and (d) show EDS mappings of FIGS. 9(a) and (b), respectively.

[0045] As shown in Fig. 9(a), the size of the reaction product is less than 50 μm, which is smaller than the size of the reaction product (200 μm) observed when Li2O was not mixed in. This is thought to be because the reaction proceeded without the aggregation of Li2NH during the reaction process.

[0046] As shown in the EDS mapping in Figure 9(c) and (d), the particles observed mainly contain oxygen (O), and therefore these particles are considered to be Li2O. In addition, since nitrogen (N) is distributed in small amounts around the Li2O, it is considered that Li2NH was generated by dispersing around the Li2O.

[0047] In addition, the high-magnification SEM image shown in Figure 9(b) shows that small particles are attached to the surface of Li2O, suggesting that the reaction product Li2NH has crystallized into small particles around the Li2O.

[0048] From the above results, it is considered that when a mixture of LiH and Li2O is reacted with N2, as shown in the conceptual diagrams in Figures 10(a) and (b), the Li2O dispersed among the LiH functions as an effective scaffold, suppressing the aggregation of Li2NH, and as a result, the reaction rate is dramatically improved.

[0049] In the above description, Li2O is used as an example of the substance to be mixed with LiH, but in view of the mechanism for suppressing the aggregation of Li2NH described above, the material is not particularly limited as long as it is a stable substance that does not react with LiH at the temperature at which LiH reacts with N2. For example, in addition to lithium hydride (Li2O), examples of substances to be mixed with LiH include lithium carbonate (Li2CO3), lithium nitrate (LiNO3), aluminum oxide (Al2O3), zirconium oxide (ZrO2), silicon oxide (SiO2), crystalline graphite, and crystalline boron nitride (BN).

[0050] [Reaction of Li2NH with H2] The thermal reaction characteristics of Li2NH generated by the reaction of a mixture of LiH and Li2O with N2 and H2 were measured using a TG device as described above, and the generated gas was measured using an MS device. Also, the measurement was performed in the same manner as above, by subjecting the generated Li2NH to 300 cm2 of H2 in a glove box at 1 atm. 3 The temperature was increased from room temperature to 500°C at a rate of 5°C / min while flowing at a flow rate of 100 / min.

[0051] FIG. 11 shows the reaction yield (χ NH3 ) and the MS spectrum of NH3 (m / z=17) measured by an MS device. Here, graph A shows the case where Li2O was mixed, and graph B shows the case where Li2O was not mixed (same as the graph in Figure 3). As shown in Figure 11, a gradual weight loss was observed from about 260°C, and NH3 was generated in sync with this weight loss. In addition, the reaction yield (χ NH3 ) reached nearly 100%, and NH3 production was complete.

[0052] Next, the reaction products produced by the thermal reaction of Li2NH with H2 were identified using X-ray diffraction in the same manner as above.

[0053] Figure 8(c) shown above is a diffraction pattern showing the measurement result of X-ray diffraction. As shown in Figures 8(b) and (c), the diffraction pattern corresponding to Li2NH before the reaction has changed to a diffraction pattern corresponding to LiH. From this result, it is considered that NH3 and LiH are produced by the reaction of Li2NH and H2.

[0054] As described above, the method for synthesizing ammonia in this embodiment includes a step (A) of reacting LiH with N2 to generate Li2NH, and a step (B) of reacting the Li2NH generated in the step (A) with H2 to generate NH3, and the step (A) is performed by mixing LiH with a substance that does not react with LiH at a temperature at which LiH and N2 react. This makes it possible to synthesize ammonia at a high reaction rate under atmospheric pressure.

[0055] In step (B), LiH is regenerated, and NH3 produced in step (B) can be continuously produced by repeating steps (A) and (B). Specifically, steps (A) and (B) can be repeatedly performed while switching between the introduction of N2 and H2. The reaction between LiH and N2 starts at about 400°C, and since the reactions in steps (A) and (B) are both exothermic reactions, it is necessary to input thermal energy until the first reaction starts, but the subsequent continuous reactions can proceed while maintaining a high temperature state by self-heating.

[0056] In this embodiment, the amount of material such as Li2O to be mixed with LiH may be appropriately determined depending on the particle size of LiH and the material to be mixed, the reaction temperature, the reaction pressure, and the like.

[0057] Fig. 12 is a graph showing the weight change and the MS spectrum of H2 (m / z=2) when the amount of Li2O mixed is changed in the reaction of a mixture of LiH and Li2O with N2. Fig. 13 is a graph showing the weight change and the MS spectrum of HH3 (m / z=17) in the reaction of the produced Li2NH with H2. Here, the graph shown by A is the case where the ratio (molar ratio) of LiH to Li2O is 1:1, and the graph shown by B is the case where the ratio (molar ratio) of LiH to Li2O is 1:2.

[0058] As shown in Figures 12 and 13, in both reactions, the reaction start time and the slope of the weight change after that were almost the same regardless of the amount of Li2O mixed. The amount of Li2O or other material mixed with LiH is not particularly limited, but in order for the material such as Li2O dispersed among the LiH to function as an effective scaffold, it is preferable to mix the material at a molar ratio of 1 or more to LiH.

[0059] Although the present invention has been described above with reference to the preferred embodiments, such description is not limiting and, of course, various modifications are possible.

Claims

1. Lithium hydride (LiH) and nitrogen (N ２ ) to form lithium imide (Li ２ NH) (A); The lithium imide (Li ２ NH) to hydrogen (H ２ ) to produce ammonia (NH ３ (B) producing Including, The step (A) is a step of reacting the lithium hydride (LiH) with nitrogen (N ２ ) at a temperature at which lithium hydride (LiH) reacts with a substance that does not react with said lithium hydride (LiH).

2. In the step (B), lithium hydride (LiH) is regenerated, By repeating the steps (A) and (B), ammonia (NH ３ 2. The method for synthesizing ammonia according to claim 1, wherein

3. In the step (A), the substance is lithium oxide (Li ２ O), lithium carbonate (Li ２ CO ３ ), lithium nitrate (LiNO ３ ), aluminum oxide (Al ２ O ３ ), zirconium oxide (ZrO ２ ), silicon oxide (SiO ２ 2. The method for synthesizing ammonia according to claim 1, wherein the catalyst material comprises at least one selected from the group consisting of crystalline graphite and crystalline boron nitride (BN).

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