Supported metal catalyst and method of synthesizing ammonia using the same

Abstract

A catalyst is provided which is used for continuously synthesizing ammonia using a gas containing hydrogen and nitrogen as a raw material, wherein a transition metal which exhibits catalytic activity is supported by a support, and the support is a two-dimensional electride or a precursor thereof. The two-dimensional electride or the precursor thereof is a metal nitride represented by MxNyHz (M represents one or two or more of Group II metals selected from the group consisting of Mg, Ca, Sr and Ba, and x, y and z are in ranges of 1x11, 1y8, and 0z4 respectively, in which x is an integer, and y and z are not limited to an integer) or M.sub.3N.sub.2 (M is the same as above), or a metal carbide selected from the group consisting of Y.sub.2C, Sc.sub.2C, Gd.sub.2C, Tb.sub.2C, Dy.sub.2C, Ho.sub.2C and Er.sub.2C. These catalysts are used for continuously reacting nitrogen with hydrogen, which are raw materials, on the catalyst, wherein the reaction is performed in an ammonia synthesis reaction system under the preferable conditions of a reaction temperature which is equal to or higher than 100 C. and equal to or lower than 600 C., and a reaction pressure which is equal to or higher than 10 kPa and lower than 20 MPa.

Claims

1. A supported metal catalyst, comprising: a transition metal which exhibits catalytic activity, wherein the transition metal is supported by a support, and the support is a two-dimensional electride or a precursor thereof, wherein the two-dimensional electride or a precursor thereof is a metal nitride represented by MxNyHz or a hydride thereof, wherein M represents one or two or more of Group II metals selected from the group consisting of Mg, Ca, Sr and Ba, and x, y and z are in ranges of 1x11, 1y8, and 0z4 respectively, in which x is an integer, and y and z are not limited to an integer.

2. The supported metal catalyst according to claim 1, wherein the supported metal catalyst is a catalyst suitable for synthesizing ammonia.

3. The supported metal catalyst according to claim 1, wherein the supported metal catalyst is a catalyst suitable for synthesizing ammonia, wherein a gas comprising hydrogen and nitrogen is used as a raw material.

4. The supported metal catalyst according to claim 2, wherein the ammonia is synthesized continuously.

5. The supported metal catalyst according to claim 1, wherein the two-dimensional electride or a precursor thereof is a metal nitride represented by M.sub.3N.sub.2.

6. The supported metal catalyst according to claim 1, wherein the two-dimensional electride or a precursor thereof is at least one of Ca.sub.3N.sub.2 and Mg.sub.3N.sub.2.

7. The supported metal catalyst according to claim 1, wherein the two-dimensional electride or a precursor thereof is a CaNH-based compound.

8. The supported metal catalyst according to claim 1, wherein the transition metal is at least one selected from the group consisting of Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh and Ir.

9. The supported metal catalyst according to claim 1, wherein the transition metal is at least one selected from the group consisting of Ru, Co and Fe.

10. The supported metal catalyst according to claim 1, wherein an amount of the transition metal is in a range of 0.01 to 30 wt % with respect to that of the support.

11. A method of synthesizing ammonia, comprising: preparing (a) the supported metal catalyst of claim 1, and (b) gas comprising hydrogen and the nitrogen, which is used as a raw material; and reacting the nitrogen with the hydrogen on the catalyst in an ammonia synthesis reaction device, in which the supported metal catalyst is provided, to synthesize ammonia.

12. The method of synthesizing ammonia according to claim 11, wherein the reaction of the nitrogen and the hydrogen is performed at a reaction temperature which is equal to or higher than 100 C. and equal to or lower than 600 C.

13. The method of synthesizing ammonia according to claim 11, wherein the reaction of the nitrogen and the hydrogen is performed at a reaction pressure which is equal to or higher than 10 kPa and lower than 20 MPa.

14. A supported metal catalyst, comprising: a transition metal which exhibits catalytic activity, wherein the transition metal is supported by a support, and the support is a two-dimensional electride or a precursor thereof, wherein the two-dimensional electride or a precursor thereof is selected from (a) to (c): (a) at least one compound selected from the group consisting of Ca.sub.2N, Sr.sub.2N, Ba.sub.2N, Ca.sub.2NH, CaNH, Ca(NH.sub.2).sub.2, Y.sub.2C, Sc.sub.2C, Gd.sub.2C, Tb.sub.2C, Dy.sub.2C, Ho.sub.2C and Er.sub.2C; (b) at least one compound selected from the group consisting of Ca.sub.2N, Sr.sub.2N and Ba.sub.2N, wherein a part of Ca of Ca.sub.2N, Sr of Sr.sub.2N, and Ba of Ba.sub.2N is substituted with at least one or more of alkaline metal elements selected from the group consisting of Li, Na. K, Rb and Cs; and (c) a metal carbide selected from the group consisting of Y.sub.2C, Sc.sub.2C, Gd.sub.2C, Tb.sub.2C, Dy.sub.2C, Ho.sub.2C and Er.sub.2C, or is a hydride thereof.

15. A method of synthesizing ammonia, comprising: preparing (a) the supported metal catalyst of claim 14, and (b) gas comprising hydrogen and the nitrogen, which is used as a raw material; and reacting the nitrogen with the hydrogen on the catalyst in an ammonia synthesis reaction device, in which the supported metal catalyst is provided, to synthesize ammonia.

16. The method of synthesizing ammonia according to claim 15, wherein the reaction of the nitrogen and the hydrogen is performed at a reaction temperature which is equal to or higher than 100 C. and equal to or lower than 600 C.

17. The method of synthesizing ammonia according to claim 15, wherein the reaction of the nitrogen and the hydrogen is performed at a reaction pressure which is equal to or higher than 10 kPa and lower than 20 MPa.

18. The supported metal catalyst according to claim 14, wherein the transition metal is at least one selected from the group consisting of Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co,Rh and Ir.

19. The supported metal catalyst according to claim 14, wherein the transition metal is at least one selected from the group consisting of Ru, Co and Fe.

20. The supported metal catalyst according to claim 14, wherein an amount of the transition metal is in a range of 0.01 to 30 wt % with respect to that of the support.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating catalytic activity of an ammonia synthesis reaction performed by a catalyst in which Ru is supported on various types of supports.

(2) FIG. 2 is a graph illustrating an ammonia synthesis rate (vertical axis) when the ammonia synthesis reaction is performed by using Ru-supported Ca.sub.3N.sub.2 and Ru-supported C12A7:e.sup. at various reaction temperatures (horizontal axis; C.).

(3) FIG. 3 is a diagram illustrating a result of a Raman spectra of Ru-supported Ca.sub.3N.sub.2(a) and Ca.sub.2N(b) under the same conditions as those in the ammonia synthesis reaction.

(4) FIG. 4 is a graph illustrating a result of a stability evaluation test for a catalyst of Example 2 (horizontal axis; reaction time, and vertical axis; ammonia synthesis rate).

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention relates to a catalyst for continuously synthesizing ammonia wherein a gas containing hydrogen and nitrogen is used as a raw material in the synthesis. In the catalyst, a transition metal which exhibits catalytic activity is supported by a support, and the support is a two-dimensional electride or a precursor thereof.

(6) The two-dimensional electride or the precursor thereof is represented by MxNyHz (here, M represents one or two or more of Group II metals selected from the group consisting of Mg, Ca, Sr and Ba, and x, y, and z are in ranges of 1x11, 1y8, and 0z4, respectively, in which x is an integer, and y and z are not limited to an integer). In the aforementioned formula, when z=0, the formula represents a metal nitride, and when 0<z, the formula represents a hydride. N may cause a defect in some cases, and therefore y is not limited to an integer. In addition, hydrogen is incorporated into the metal nitride, and thus z is not limited to an integer.

(7) Representative examples of the metal nitride include the metal nitride represented by M.sub.3N.sub.2 (here, M represents one or two or more of Group II metals selected from the group consisting of Mg, Ca, Sr and Ba), when x=3, y=2 and Z=0. Specific examples of the metal nitride include the calcium nitride represented by Ca.sub.3N.sub.2 or Ca.sub.2N in a case where M is Ca. The two-dimensional electride or the precursor thereof is not limited to the metal nitride, but may a metal carbide selected from the group consisting of Y.sub.2C, Sc.sub.2C, Gd.sub.2C, Tb.sub.2C, Dy.sub.2C, Ho.sub.2C and Er.sub.2C, or may be a hydride thereof.

(8) Hereinafter, a catalyst of the present invention, a method of producing the catalyst, and a method of synthesizing ammonia using the catalyst (hereinafter, referred to as a method of the present invention) will be specifically described by exemplifying a calcium nitride as a specific example. Regarding other two-dimensional electride compounds, the description can be also used in the same way.

(9) As examples of a calcium nitride-based compound and hydride thereof, -Ca.sub.3N.sub.2, -Ca.sub.3N.sub.2, -Ca.sub.3N.sub.2, Ca.sub.11N.sub.8, Ca.sub.2N, Ca.sub.2NII, CaNII, and Ca(NII.sub.2).sub.2 are known, and Ca can be included up to 11, N can be included up to 8, and H can be included up to 4. Accordingly, the calcium nitride and the CaNH-based compound of the catalysts of the present invention are comprehensively represented by formula CaxNyHz (here, M represents one or two or more of Group II metals selected from the group consisting of Mg, Ca, Sr and Ba, and x, y, and z are in ranges of 1x11, 1y8, and 0z4, respectively, in which x is an integer, and y and z are not limited to an integer). When 0<z, the formula represents the hydride, and in the case of the hydride, z may be a value which is less than 1, and therefore z is not limited to an integer.

(10) Further, it is considered that the metal nitrides such as Mg.sub.3N.sub.2, Sr.sub.2N and Ba.sub.2N other than the calcium nitride also change the structure thereof similar to the structure of Ca.sub.3N.sub.2 during the ammonia synthesis reaction. Thus nitride or nitride containing hydrogen of the catalysts of the present invention can be represented by formula MxNyHz (here, M represents one or two or more of Group II metals selected from the group consisting of Mg, Ca, and Sr, and Ba, and x, y, and z are in ranges of 1x11, 1y8, and 0z4, respectively, in which x is an integer, and y and z are not limited to an integer).

(11) <Synthesizing of Calcium Nitride>

(12) The calcium nitride (Ca.sub.3N.sub.2) is commercially available, and can be obtained by using a method of dissolving metal calcium in liquid ammonia, and thermally decomposing the obtained calcium amide in a nitrogen gas stream. For example, the calcium nitride (Ca.sub.2N) is obtained by heating Ca.sub.3N.sub.2 with metal calcium at approximately 1000 C. in a vacuum condition.

(13) <Synthesizing of CaNH-Based Compound Catalyst>

(14) The hydride of the calcium nitride represented by formula CaxNyHz (x, y, and z are in ranges of 1x11, 1y8, and 0<z4, in which x is an integer, and y and z are not limited to an integer) is also used as a support and exhibits high catalytic activity similar to Ca.sub.2N. Specific examples of the CaNH-based compound include Ca.sub.2NH, CaNH, Ca(NH.sub.2).sub.2 and the like.

(15) When the ammonia synthesis is performed by supporting a transition metal element catalyst such as Ru on the calcium nitride (Ca.sub.3N.sub.2), Ca.sub.3N.sub.2 is reduced during the synthesis reaction, and is changed to Ru-supported Ca.sub.2N, a Ru-supported CaNH compound, or a mixed compound thereof. In addition, as another method, Ca.sub.3N.sub.2 or Ca.sub.2N is heated at approximately 500 C. in the hydrogen gas stream to obtain the CaNH compound, and the obtained CaNH compound functions as a catalyst by supporting the transition metal element such as Ru thereon.

(16) <Step of Supporting Transition Metal Such as Ru>

(17) The transition metal element is used for various types of synthesis reactions as a homogenous catalyst and a heterogeneous catalyst. It is known that particularly, a Group VI, Group VIII, or Group IX transition metal such as Fe, Ru, Os, Co and Mo is suitably used as the catalyst which is used to synthesize ammonia by direct reaction between hydrogen and nitrogen. In the present invention, as the transition metal element, each element of a Group IV metal selected from Cr, Mo and W, a Group VII metal selected from Mn, Tc and Re, a Group VIII metal selected from Fe, Ru and Os, and a Group IX metal selected from Co, Rh and Ir can be used alone or in combination thereof. In addition, the compounds of these elements such as Co.sub.3Mo.sub.3N, Fe.sub.3Mo.sub.3N, Ni.sub.2Mo.sub.3N and Mo.sub.2N can be used.

(18) In a case where the support such as the calcium nitride and the CaNH-based compound are used, a substance obtained in the step of synthesizing the calcium nitride is mixed with the transition metal compound such as Ru by using an impregnation method and a physical mixing method. Examples of the physical mixing method include a method performed in such a manner that the complex of metal such as Ru is mixed with the calcium nitride while being pulverized by using a mortar, a mixing method performed by putting them in a glass tube and shaking or rotating a glass tube after sealing the glass tube, and a pulverizing and mixing method performed by a ball mill.

(19) In a case where a solid sintered body, a thin film or a solid single crystal is used as a support, it is possible to use an impregnation method, similar to powder or a porous body, and a method wherein the transition metal compound such as Ru is deposited on the surface of the support by using a CVD method (chemical vapor deposition method), a sputtering method and the like, and the transition metal compound is thermally decomposed so as to deposit the transition metals. In a case of using the aforementioned compound of the transition metals, for example, it is possible to use a method wherein each of the metal raw materials are deposited on the support by using the CVD method or the like, are thermally decomposed, and then are nitrided by an ammonia gas.

(20) The transition metal compound is not particularly limited, and examples thereof include an inorganic metal compound or an organic metal complex such as tri-ruthenium dodecacarbonyl [Ru.sub.3(CO).sub.12], dichloro tetrakis (triphenyl phosphine) ruthenium (II) [RuCl.sub.2(PPh.sub.3).sub.4] dichloro-tris (triphenyl phosphine) ruthenium (II) [RuCl.sub.2(PPh.sub.3).sub.3], tris(acetyl acetonato) ruthenium (III) [Ru(acac).sub.3], ruthenocene [Ru(C.sub.5H.sub.5)], ruthenium chloride [RuCl.sub.3], iron pentacarbonyl [Fe(CO).sub.5], tetracarbonyl iron iodide [Fe(CO).sub.4I.sub.2)], iron chloride [FeCl.sub.3], ferrocene [Fe(C.sub.5H.sub.5).sub.2], tris(acetyl acetonato) iron (III) [Fe(acac).sub.3], triiron dodecacarbonyl [Fe.sub.3(CO).sub.12], cobalt chloride [CoCl.sub.3], tris(acetyl acetonato) cobalt (III) [Co(acac).sub.3], cobalt (II) acetyl acetonate [Co(acac).sub.2], cobalt octacarbonyl [Co.sub.2(CO).sub.8], cobaltocene [Co(C.sub.5H.sub.5).sub.2], triosmium dodecacarbonyl [Os.sub.3(CO).sub.12], and molybdenum hexacarbonyl [Mo(CO).sub.6], which are likely to be thermally decomposed.

(21) As the impregnation method, the following steps can be used. For example, the support powders, or the molded body is dispersed into a transition metal compound solution (for example, a hexane solution of a Ru carbonyl complex), and stirred. At this time, the content of the transition metal compound is 0.01 to 40 wt %, preferably in a range of 0.02 to 30 wt %, and further preferably in a range of approximately 0.05 to 20 wt % with respect to support powders. After that, in an inert gas streams such as nitrogen, argon and helium, or in a vacuum state, the transition metal compound is heated at 50 C. to 200 C. for 30 minutes to 5 hours so as to evaporate a solvent, and thereby the transition metal compound is dried and solidified. Then, a catalyst precursor which is the dried and solidified transition metal compound is reduced. With the above-described steps, it is possible to obtain the supported metal catalyst in which transition metals such as Ru as the fine particles having a particle diameter of several nm to several hundreds of nm are supported on the support powders.

(22) The specific surface area of the supported metal catalyst is in a range of approximately 0.1 to 200 m.sup.2/g, and the supporting amount of the transition metals such as Ru is in a range of 0.01 to 30 wt %, is preferably 0.02 to 20 wt %, and is more preferably 0.05 to 10 wt %, with respect to the support powders or the molded body.

(23) Further, instead of using the above-described method, it is possible to obtain the same supported metal catalyst by using a method, wherein, under the same conditions as those described in the above-described method, the calcium nitride or the CaNH-based compound and the transition metal compound powder such as Ru are solid-phase mixed through the physical mixing method, and then heated so as to reduce and decompose the transition metal compound such as Ru to the transition metals such as Ru.

(24) In addition, the supported metal catalyst of the present invention can be used as a molded body by using a typical molding technology. Specifically, examples of the shapes of the molded body include a granular shape, a spherical shape, a tablet shape, ring, a macaroni shape, a four-leaf clover shape, a dice shape, and a honeycomb shape. In addition, it is possible to use the supported metal catalyst of the present invention such that the catalyst is coated on an appropriate supporting body.

(25) <Synthesis of Ammonia>

(26) An ammonia synthesis method of the present invention is a method of synthesizing ammonia wherein nitrogen is reacted with hydrogen on a catalyst, and the above-described supported metal catalyst is used as the catalyst. A representative form of the reaction is a method performed in such a manner that a gas obtained by mixing nitrogen and hydrogen which are used as raw materials is directly reacted under heating and under pressure, and ammonia which is produced by the reaction of N.sub.2+3H.sub.2.fwdarw.2NH.sub.3 is cooled or absorbed by water so as to be separated, similar to the conventional Haber-Bosch method. The nitrogen and hydrogen gases are supplied to be in contact with the supported metal catalyst installed in the reactor. Unreacted nitrogen and hydrogen gases are recycled and circulated in the reactor after removing the produced ammonia. Before supplying the nitrogen and hydrogen gases, the surface of the supported metal catalyst is preferably subjected to a reduction treatment by using the hydrogen gas or the mixed gas of hydrogen and nitrogen to perform a pre-treatment of reducing and removing an oxide or the like which is formed on the supported transition metal such as Ru.

(27) When the calcium nitride or the CaNH-based compound is left in the atmosphere, water is preferentially absorbed and the compound is likely to be decomposed under an excessive amount of moisture. Therefore, it is preferable that the ammonia synthesis reaction is performed in an atmosphere which contains extremely low moisture. That is, it is preferable to use the nitrogen and hydrogen gas having a content of moisture which is equal to or lower than 100 ppm, and more preferably equal to or lower than 50 ppm, as raw materials.

(28) Next, the supported metal catalyst is heated under the atmosphere of the mixed gas of nitrogen and hydrogen which are the raw materials to synthesize ammonia. It is preferable to use a condition wherein the molar ratio of nitrogen to hydrogen is approximately 1/10 to 1/1, and preferably 1/5 to 1/1. A reaction temperature is preferably equal to or higher than 100 C. and lower than 600 C., preferably in a range of approximately 200 C. to 500 C., and more preferably in a range of approximately 250 C. to 500 C. As the reaction temperature is low, equilibrium is in favor of the ammonia production. The above-described range is preferable in order to realize a sufficient ammonia production rate and to make the equilibrium in favor of the ammonia production.

(29) The reaction pressure used for the mixed gas of nitrogen and hydrogen at the time of performing the synthesis reaction is not particularly limited. However, the reaction pressure is preferably equal to or higher than 10 kPa and lower than 20 MPa. In terms of practical use, the reaction is preferably performed under pressurized conditions, and thus, the reaction pressure is more preferably in a range of approximately 100 kPa to 20 MPa in terms of practical use, and even in a case of being less than 3 MPa, the synthesis reaction can be sufficiently performed.

(30) Examples of types of the synthesis reaction include a hatch type reaction, a closed circulation type reaction, and a flow type reaction system. The flow type reaction system is most preferably used from a practical point of view. In terms of the equilibrium, conditions of high pressure and low temperature are advantageous for the ammonia synthesis reaction. In addition, since the reaction is exothermic reaction, it is favorable that the reaction be performed while removing the reaction heat, and thus various improvements have been made to industrially increase the yield. For example, in a case of using a flow type reaction system, a method of obtaining a high yield of ammonia has been proposed, and the method has been proposed wherein a plurality of reactors with which the catalyst is filled are connected to each other in a row, an intercooler is installed at an outlet of each of the reactors so as to remove the heat, and thus an inlet temperature of each of the reactor is decreased, thereby increasing the yield of ammonia. Further, a method of accurately controlling an outlet temperature of each of the reaction layers has been also proposed, and the method is performed by using a reactor including a plurality of catalyst layers with which an Fe catalyst and a Ru-based catalyst are filled in the inside thereof.

(31) In the present invention, similar to the conventional method which has been performed, it is possible to perform the ammonia synthesis by using one reactor or a plurality of reactors with which the catalyst is filled. As a catalyst to be used, the catalyst of the present invention can be used alone, a combination of two or more types of the catalyst selected from the catalysts of the present invention can be used, or a combination of the catalyst of the present invention and the well-known catalyst can be used. In addition, either of the method of connecting the plurality of reactors to each other, or the method of providing the plurality of reaction layers in the same reactor can be used.

(32) In the present invention, in a case where the catalysts are used in combination, the activity of the catalyst of the present invention is high at the low temperature, and thus the catalyst is preferably used in the last reactor. That is, it is possible to obtain a high yield of ammonia by performing the last reaction at the low temperature which is in favor of the equilibrium.

(33) Under the equilibrium reaction conditions for industrial ammonia synthesis, due to the equilibrium limitation, the concentration of ammonia in the reaction gas at the outlet of the reactor is equal to or less than 20%. Accordingly, after the ammonia produced in the reaction gas is cooled and removed, unreacted raw material can be recycled and used as the raw material after a portion of the impurities included in the reaction gas or the unreacted raw material is separated by membrane separation or the like and purged from the system.

(34) As a hydrogen raw material used for the method of synthesizing ammonia, it is possible to use hydrogen raw materials which can be produced through various production methods shown below. Production methods, wherein coal, petroleum or natural gas is used as a raw material, such as a steam reforming method, a partial oxidation reforming method, an autothermal reforming method which is obtained by combining the steam reforming and the partial oxidation, and methods wherein these methods are combined with a shift reaction, and production methods such as a method of using biomass as a raw material, a method performed by water electrolysis, and a method of water decomposition performed by an optical catalyst can be used to generated the hydrogen.

(35) In a case where the natural gas raw material is used as the raw material of the method of synthesizing ammonia, the hydrogen gas and the nitrogen gas are produced through the steps which include a steam reforming step of the natural gas, a partial oxidation reforming step, a CO shift reaction step, a CO.sub.2 removing step, and a subsequent CO removing step performed by continuous CO methanation. Since the steam reforming reaction is endothermic, the reaction heat which is generated in the auto-thermal reaction is used, and the H/N ratio in a case where air is used as the nitrogen gas raw material is in a range of approximately 1.7 to 2.5 molar ratio. The unreacted gas of the steaming reforming method contains the hydrogen gas, and thus is preferably used as a recycle gas by circulating in the reforming step. Methods of efficiently performing a reaction by controlling the ratio of the fresh gas to the recycle gas have been developed, and in the present invention, the above-described method can be also used.

(36) On the other hand, as a method of obtaining a raw material having the high H/N ratio, a method of using oxygen-enriched air has been developed. When such a raw material is used, an amount of recycled gas is decreased, and thus the aforementioned method is energetically preferred. Furthermore, a method in which, after compressing and separating of the air is performed, oxygen is used to produce hydrogen through an auto-thermal method and nitrogen is used for the reaction gas or industrial nitrogen is a method which is preferable in terms of the energy saving, and can be used as the method of producing the raw material used in the present invention.

(37) Hereinafter, the present invention will be specifically described with reference to examples. The evaluation of the ammonia synthesis activity of the catalysts in the present invention and Comparative example was performed by acquiring a production amount of NH.sub.3 by using a gas chromatograph, or by dissolving the produced NH.sub.3 into a sulfuric acid aqueous solution, and quantifying the solution by using an ion chromatograph so as to acquire the ammonia synthesis rate.

EXAMPLE 1

(38) <Production of Ru-supported Ca.sub.3N.sub.2>

(39) 1 g of Ca.sub.3N.sub.2 powder (BET surface area: about 1 m.sup.2 g.sup.1) which is a commercially available reagent was mixed with 0.042 g of Ru.sub.3(CO).sub.12 powder in a glove box under an Ar atmosphere, and the mixture was enclosed in vacuum quartz glass. The glass-enclosed sample was heated at 250 C. for 1.5 hours while being rotated. With this, a Ca.sub.3N.sub.2 catalyst on which 2 wt % of Ru metal particles were supported was obtained. The BET surface area of the aforementioned catalyst was about 1 m.sup.2 g.sup.1, and the BET surface area was hardly changed after being supported. The dispersion degree (%) of Ru measured by using a CO adsorption method was 3.0.

(40) <Ammonia Synthesis Reaction>

(41) The synthesis reaction in which the nitrogen gas (N.sub.2) reacts with the hydrogen gas (H.sub.2) so as to produce an ammonia gas (NH.sub.3) was performed. 0.2 g of the catalyst obtained by using the above-described method was packed in a glass tube in a case where the reaction temperature was 400 C., and 0.1 g of the catalyst obtained by using the above-described method was packed in a glass tube in a case where the reaction temperature was 340 C., so as to perform the synthesis reaction by using a fixed bed type flow reaction apparatus. The reaction was performed by setting the flow rate of gas such that N.sub.2 was 15 mL/min and H.sub.2 was 45 mL/min, which was 60 mL/min in total, and setting the pressure to be an atmospheric pressure. The gas discharged from the flow system reactor was caused to be bubbled in 0.005 M sulfuric acid aqueous solution to dissolve the produced ammonia in the solution, and quantification of the produced ammonia ion was performed using the ion chromatograph. The production rate of ammonia at 400 C. was 2760 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 3164 mmol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 164.2.

EXAMPLE 2

(42) 2 wt % of Ru/Ca.sub.2N catalyst was prepared by using the same method as that in Example 1 and the ammonia production reaction was performed under the same conditions as those in Example 1, except that Ca.sub.2N (BET surface area of 1 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 400 C. was 2750 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 3386 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 171.2.

EXAMPLE 3

(43) 2 wt % of Ru/Ca(NH.sub.2).sub.2 catalyst was prepared by using the same method as that in Example 1 and the ammonia production reaction was performed under the same conditions as those in Example 1, except that Ca(NH.sub.2).sub.2 (BET surface area of 120 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 340 C. was 2118 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 3.2.

EXAMPLE 4

(44) 2 wt % of Ru/CaNH catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that CaNH (BET surface area of 1 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 340 C. was 1107 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 38.4.

EXAMPLE 5

(45) 2 wt % of Ru/Y.sub.2C catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that Y.sub.2C (BET surface area of 0.2 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 400 C. was 580 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 1101 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 59.4.

(46) Table 1 indicates the results of the ammonia synthesis reactions performed using the catalysts in which Ru was supported on various types of supports (reaction temperature: 340 C., flow rate: 60 mL min.sup.1, pressure: 0.1 MPa, and N.sub.2:H.sub.2=1:3).

(47) TABLE-US-00001 TABLE 1 Catalyst Ru NH.sub.3 Example No. surface disper- production TOF Comparative area sion rate (mol (10.sup.3 example No. Catalyst (m.sup.2 g.sup.1) (%) g.sup.1 h.sup.1) s.sup.1) Comparative Ru/-Al.sub.2O.sub.3 170 20 10 0.1 example 7 Comparative Ru/CaO 3 4.9 261 7.5 example 6 Comparative BaRu/ 310 25.2 29 0.2 example 5 Active carbon Comparative CsRu/MgO 12 25 2367 13.3 example 4 Comparative Ru/CaOAl2O3 1 4.5 22 0.7 example 3 Comparative Ru/C12A7:O.sup.2 1 4.6 88 2.7 example 2 Comparative Ru/C12A7:e.sup. 1 4.7 1571 52.1 example 1 Example 5 Ru/Y2C 0.2 2.6 1101 59.4 Example 4 Ru/CaNH 1 4.5 1107 38.4 Example 3 Ru/Ca(NH.sub.2).sub.2 120 93.6 2118 3.2 Example 2 Ru/Ca.sub.2N 1 3.1 3386 171.2 Example 1 Ru/Ca.sub.3N.sub.2 1 3.0 3164 164.2

EXAMPLE 6

(48) 0.5 wt % Ru/Ca.sub.3N.sub.2 catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that the amount of Ru supported on Ca.sub.3N.sub.2 in Example 1 was set to be 0.5 wt %. The synthesis rate of ammonia at 340 C. was 2009 mol g.sup.1 h.sup.1 as illustrated in Table 2.

EXAMPLE 7

(49) 1.0 wt % Ru/Ca.sub.3N.sub.2 catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that the amount of Ru supported on Ca.sub.3N.sub.2 in Example 1 was set to be 1.0 wt %. The production rate of ammonia at 340 C. was 2399 mol g.sup.1 h.sup.1 as illustrated in Table 2.

EXAMPLE 8

(50) 3.0 wt % Ru/Ca.sub.3N.sub.2 catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that the amount of Ru supported on Ca.sub.3N.sub.2 in Example 1 was set to be 3.0 wt %. The production rate of ammonia at 340 C. was 3446 mol g.sup.1 h.sup.1 as illustrated in Table 2.

EXAMPLE 9

(51) 5.0 wt % Ru/Ca.sub.3N.sub.2 catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that Ru loading supported on Ca.sub.3N.sub.2 in Example 1 was set to be 5.0 wt %. The production rate of ammonia at 340 C. was 3922 mol g.sup.1 h.sup.1 as illustrated in Table 2.

(52) Table 2 indicates the results of the ammonia synthesis reactions performed using Ca.sub.3N.sub.2 having different Ru-loading (reaction temperature: 340 C., flow rate: 60 mL min.sup.1, pressure: 0.1 MPa, and N.sub.2:H.sub.2=1:3).

(53) TABLE-US-00002 TABLE 2 Example No. Ru Surface Ammonia Comparative loading area production rate example No. (wt %) (m.sup.2 g.sup.1) (mol g.sup.1 h.sup.1) Example 6 0.5 1.0 2009 Example 7 1 1.0 2399 Example 1 2 1.0 3164 Example 8 3 1.0 3446 Example 9 5 1.0 3922

EXAMPLE 10

(54) <Synthesis of Co-supported Ca.sub.3N.sub.2>

(55) 1 g of Ca.sub.3N.sub.2 powder, which is a commercially available reagent, was mixed with 0.058 g of Co.sub.2(CO).sub.8 powder in a glove box under an Ar atmosphere, and the mixture was enclosed in vacuum quartz glass. The glass-enclosed sample was heated at 250 C. for 15 hours. With this, a Ca.sub.3N.sub.2 catalyst on which 2 wt % of Co metal was supported was obtained. The BET surface area of the aforementioned catalyst was about 1 m.sup.2 g.sup.1. The ammonia synthesis reaction was performed using the synthesized catalyst under the same conditions as those in Example 1. The production rate of ammonia at 340 C. was 439 mol g.sup.1 h.sup.1 as illustrated in Table 3.

EXAMPLE 11

(56) <Synthesis of Fe-supported Ca.sub.3N.sub.2>

(57) 1 g of Ca.sub.3N.sub.2 powder which is a commercially available reagent was mixed with 0.065 g of Fe.sub.2(CO).sub.8 powder in a glove box under an Ar atmosphere, and the mixture was enclosed in vacuum quartz glass. The glass-enclosed sample was heated at 250 C. for 15 hours. With this, a Ca.sub.3N.sub.2 catalyst on which 2 wt % of Fe metal was supported was obtained. The BET surface area of the aforementioned catalyst was about 1 m.sup.2 g.sup.1. The ammonia synthesis reaction was performed by using the synthesized catalyst under the same conditions as those in Example 1. The production rate of ammonia at 340 C. was 284 mol g.sup.1 h.sup.1 as illustrated in Table 3.

(58) Table 3 indicates the results of the ammonia synthesis reactions performed using Ca.sub.3N.sub.2 on which various types of transition metals were supported (reaction temperature: 340 C., flow rate: 60 mL min.sup.1, pressure: 0.1 MPa, and N.sub.2:H.sub.2=1:3).

(59) TABLE-US-00003 TABLE 3 Example No. Surface Ammonia Comparative area production rate example No. Catalyst (m.sup.2 g.sup.1) (mol g.sup.1 h.sup.1) Example 1 Ru/Ca.sub.3N.sub.2 1.0 3164 Example 10 Co/Ca.sub.3N.sub.2 1.0 439 Example 11 Fe/Ca.sub.3N.sub.2 1.0 284

EXAMPLE 12

(60) 2 wt % of Ru/Mg.sub.3N.sub.2 catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the conditions in Example 1, except that Mg.sub.3N.sub.2 (BET surface area of 1 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 340 C. was 365 mol g.sup.1 h.sup.1 as illustrated in Table 4. TOF (10.sup.3 s.sup.1) was 16.9.

EXAMPLE 13

(61) 2 wt % of Ru/Sr.sub.2N catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the conditions in Example 1, except that Sr.sub.2N (BET surface area of 1 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The productions rate of ammonia at 340 C. was 1520 mol g.sup.1 h.sup.1 as illustrated in Table 4. TOF (10.sup.3 s.sup.1) was 70.2.

EXAMPLE 14

(62) 2 wt % of Ru/Ba.sub.2N catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the conditions in Example 1, except that Ba.sub.2N (BET surface area of 1 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 340 C. was 566 mol g.sup.1 h.sup.1 as illustrated in Table 4. TOF (10.sup.3 s.sup.1) was 26.1.

(63) Table 4 indicates the results of the ammonia synthesis reactions performed using the catalyst in which Ru (2 wt %) was supported on the various types of metal nitride supports (reaction temperature: 340 C., flow rate: 60 mL min.sup.1, pressure: 0.1 MPa, and N.sub.2:H.sub.2=1:3).

(64) TABLE-US-00004 TABLE 4 Example No. Surface Ammonia Comparative area production rate TOF example No. Catalyst (m.sup.2g.sup.1) (mol g.sup.1 h.sup.1) (10.sup.3 s.sup.1) Example 1 Ru/Ca.sub.3N.sub.2 1.0 3164 164.0 Example 2 Ru/Ca.sub.2N 1.0 3368 171.0 Example 12 Ru/Mg.sub.3N.sub.2 1.0 365 16.9 Example 13 Ru/Sr.sub.2N 1.0 1520 70.2 Example 14 Ru/Ba.sub.2N 1.0 566 26.1

COMPARATIVE EXAMPLE 1

(65) <Synthesis of mayenite Type Compound Powder>

(66) CaCO.sub.3 and Al.sub.2O.sub.3 powders were mixed such that the ratio of Ca to Al was 11:14, and 30 g of the mixture in total was heated at 1300 C. for six hours in an alumina crucible. The obtained powders were put into a silica glass tube and heated at 1100 C. for 15 hours in a vacuum of 110.sup.4 Paso as to obtain mayenite type compound powders of the raw material. The specific surface area in this stage was equal to or less than 1 m.sup.2 g.sup.1.

(67) <Electron Injection by the Reduction Treatment>

(68) 3 g of powders obtained as described above were put into a silica glass tube together with 0.18 g of metal Ca powders, and heated at 700 C. for 15 hours so as to react with the powders by setting the inside of the silica glass tube to be in a state of the metal Ca vapor atmosphere. The enclosed sample in the vacuum state was then took out, crushed by using a mortar, and packed in the silica glass tube again and sealing was performed while being in the vacuum state. The sample was heated at 1100 C. for two hours to obtain conductive mayenite type compound powder C12A7:e.sup. having a concentration of conduction electron of 210.sup.21 cm.sup.3 and a specific surface area of 1 m.sup.2 g.sup.1. 2 wt % of Ru was supported and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that C12A7:e.sup. was used. The production rate of ammonia at 400 C. was 2684 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 1571 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 52.1.

COMPARATIVE EXAMPLE 2

(69) 2 wt % of Ru/C12A7 catalyst was prepared by using the same method as that in Example 1 and the ammonia production reaction was performed under the same conditions as those in Example 1, except that C12A7 (non-doping), which has a chemical equivalent composition wherein conduction electron was not included, was used instead of the conductive mayenite type compound used in Comparative example 1. The production rate of ammonia at 400 C. was 546 mmol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 88 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 2.7.

COMPARATIVE EXAMPLE 3

(70) 2 wt % of Ru/CA catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that CaO.Al.sub.2O.sub.3 (denoted by CA) (BET surface area of 1 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 400 C. was 467 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 22 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 0.7.

COMPARATIVE EXAMPLE 4

(71) 2 wt % of RuCs/MgO catalyst (Cs/Ru element ratio=1) was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that Cs-added MgO (denoted by Cs/MgO) (BET surface area of 12 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 in Example 1. The production rate of ammonia at 400 C. was 2264 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The synthesis rate of ammonia at 340 C. was 2367 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 13.3.

COMPARATIVE EXAMPLE 5

(72) 2 wt % of RuBa/C catalyst (Ba/Ru element ratio=1) was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that Ba-added active carbon (denoted by Ba/AC) (BET surface area of 310 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 used in Example 1. The production rate of ammonia at 400 C. was 148 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 29 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 0.2.

COMPARATIVE EXAMPLE 6

(73) 2 wt % of Ru/CaO catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that CaO (BET surface area of 3 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 in Example 1. The production rate of ammonia at 400 C. was 158 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 261 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 7.5.

COMPARATIVE EXAMPLE 7

(74) 2 wt % of Ru/Al.sub.2O.sub.3 catalyst was prepared by using the same method as that in Example 1 and the ammonia synthesis reaction was performed under the same conditions as those in Example 1, except that Al.sub.2O.sub.3 (BET surface area of 170 m.sup.2 g.sup.1) was used instead of Ca.sub.3N.sub.2 in Example 1. The production rate of ammonia at 400 C. was 8.5 mol g.sup.1 h.sup.1 as illustrated in FIG. 1. The production rate of ammonia at 340 C. was 10 mol g.sup.1 h.sup.1 as illustrated in Table 1. TOF (10.sup.3 s.sup.1) was 0.1.

(75) [Comparing of Ammonia Synthesis Rates]

(76) From the ammonia production rates shown in Table 1, it was found that, as compared with the catalysts in which Ru was supported on the known catalyst supports (Al.sub.2O.sub.3, CaO, Ba/AC, Cs/MgO, and CA), Ca.sub.3N.sub.2 and Ca.sub.2N on which Ru was supported were very excellent catalysts having high catalytic activity per the same weight, and exhibited a high TOF value even though having a small specific surface area. It was also found that the aforementioned catalytic activity is higher than that of C12A7:e.sup. on which Ru was supported.

(77) On the other hand, Y.sub.2C on which Ru was supported used in Example 5 had a inferior ammonia production rate per unit weight compared with Ru/C12A7:e.sup.. However, taking the BET surface area thereof which is 0.2 m.sup.2/g.sup.1 into consideration, it was confirmed that the catalyst had catalystic performance which was comparable with Ru/C12A7:e.sup. regarding the catalytic activity per surface area.

(78) [Evaluation Test 1]

(79) The reaction temperature dependency of the catalyst was evaluated by performing an ammonia synthesis reaction under the same conditions as those in Example 1 except for the reaction temperature. The reaction was performed by setting the amount of the catalyst to be 0.1 g, the flow rate of the gas such that N.sub.2 was 15 mL/min and H.sub.2 was 45 mL/min, which was 60 mL/min in total, and the pressure to be an atmospheric pressure. FIG. 2 illustrates the results obtained by performing the ammonia synthesis reactions by using 2 wt % of Ru/Ca.sub.3N.sub.2 and 2 wt % of Ru/C12A7:e.sup. as the catalyst at various reaction temperatures. The activity was almost the same at 400 C. However, in a low temperature area at equal to or lower than 340 C., it was clear that 2 wt % of Ru/Ca.sub.3N.sub.2 had approximately two times the catalytic activity compared with 2 wt % of Ru/C12A7:e.sup..

(80) [Evaluation Test 2]

(81) FIG. 3 illustrates the results obtained by examining the change of structures of 2 wt % of Ru/Ca.sub.3N.sub.2 catalyst (a) and 2 wt % of Ru/Ca.sub.2N catalyst (b) based on a Raman spectra while performing the ammonia synthesis reaction under the same conditions as those in Example 1 except for the reaction temperature. When the reaction temperature was increased, a peak derived from CaN stretching vibration which was shown at 258 cm.sup.1 was decreased in 2 wt % of Ru/Ca.sub.3N.sub.2, and when the reaction temperature became 350 C., two peaks were shown at 180 cm.sup.1 and 322 cm.sup.1.

(82) On the other hand, when 2 wt % of Ru/Ca.sub.2N was heated under the nitrogen and hydrogen gas stream, which was the ammonia synthesis condition, similarly, two peaks were shown at 180 cm.sup.1 and 322 cm.sup.1. From the above-described results, it was clear that when 2 wt % Ru/Ca.sub.3N.sub.2 was heated at 350 C. under the conditions of the ammonia synthesis reaction, the structure thereof was changed to a structure similar to Ca.sub.2N, and high catalytic activity was exhibited.

(83) [Evaluation Test 3]

(84) The stability of the catalyst was evaluated by continuously performing the synthesis reaction at the reaction temperature of 340 C. for 20 hours. FIG. 4 illustrates the results of the ammonia synthesis performed by using 2 wt % of Ru/Ca.sub.2N of Example 2 as the catalyst. It was found that ammonia was stably produced even with the reaction of about 20 hours, and the reaction activity was hardly deteriorated.

(85) The ammonia synthesis activity of the catalyst, in which Co or Fe other than Ru was supported on Ca.sub.3N.sub.2, evaluated at the reaction temperature of 340 C. is indicated in Table 2. The performance of the catalyst on which Co or Fe was supported (Example 6 and Example 7) was inferior compared with that of 2 wt % of Ru/Ca.sub.3N.sub.2 used in Example 1. However, the aforementioned catalyst showed catalyst performance which was comparable with the known Ru catalyst other than RuCs/MgO used in Comparative example 4, or showed even superior catalyst performance.

(86) The activity of ammonia synthesis of the catalyst in which 2 wt % of Ru was supported on a calcium nitride (Ca.sub.3N.sub.2 and Ca.sub.2N), a magnesium nitride, a strontium nitride and a barium nitride (Mg.sub.3N.sub.2, Sr.sub.2N, and Ba.sub.2N), evaluated at the reaction temperature of 340 C. was indicated in Table 3. The performance of the catalysts of 2 wt % of Ru/Mg.sub.3N.sub.2 in Example 8, 2 wt % of Ru/Sr.sub.2N in Example 9 and 2 wt % of Ru/Ba.sub.2N in Example 10 was inferior compared with 2 wt % of Ru/Ca.sub.3N.sub.2 or 2 wt % of Ru/Ca.sub.2N. However, the aforementioned catalysts exhibited higher catalyst performance than the known Ru catalyst other than RuCs/MgO used in Comparative example 4.

INDUSTRIAL APPLICABILITY

(87) Currently, the synthesizing method (the Haber-Bosch method) performed by using a doubly promoted iron catalyst containing several weight % of Al.sub.2O.sub.3 and K.sub.2O in Fe.sub.3O.sub.4, which is widely used in producing ammonia, requires high pressure of about equal to or higher than 20 MPa. On the other hand, in the method of the present invention, high pressure is not required, and the synthesis reaction can be sufficiently performed at a pressure which is lower than 20 MPa, or at a relatively low pressure of lower than 3 MPa. Further, it can be said that the method of the present invention is a preferable method in terms of simplification of the manufacturing process, and reduction of energy consumption. In addition, it is possible to produce ammonia inexpensively with remarkably high efficiency compared with the conventional Ru catalyst.