Method for producing conductive mayenite compound powder having large specific surface area

Abstract

If a conductive mayenite compound having a large specific surface area is obtained, the usefulness thereof in respective applications is remarkably increased. A conductive mayenite compound powder having a conduction electron density of 10.sup.15 cm.sup.?3 or more and a specific surface area of 5 m.sup.2g.sup.?1 or more is produced by: the following steps: (1) forming a precursor powder by subjecting a mixture of a starting material powder and water to a hydrothermal treatment; (2) forming a mayenite compound powder by heating and dehydrating the precursor powder; (3) forming an activated mayenite compound powder by heating the compound powder in an inert gas atmosphere or in a vacuum; and (4) injecting electrons into the mayenite compound through a reduction treatment by mixing the activated mayenite compound powder with a reducing agent.

Claims

1. A mayenite compound having a conduction electron concentration of 10.sup.15 cm.sup.?3 or more and a specific surface area of 5 m.sup.2g.sup.?1 or more.

2. The mayenite compound according to claim 1, wherein the mayenite compound is used as a support of a transition metal catalyst.

3. The mayenite compound according to claim 1, wherein the mayenite compound is in powder form.

4. The mayenite compound according to claim 1, wherein the mayenite compound is in a cage skeleton form.

5. The mayenite compound according to claim 1, wherein the mayenite compound is activated.

6. A supported metal catalyst comprising: a support comprising a conductive mayenite compound of claim 1; and a transition metal catalyst supported on the conductive mayenite compound support.

7. A method of synthesizing ammonia, the method comprising: providing a transition metal catalyst supported on the mayenite compound according to claim 1; supplying nitrogen gas (N.sub.2) and hydrogen gas (H.sub.2) to contact with the transition metal catalyst so as to react with each other to produce ammonia gas (NH.sub.3).

8. The mayenite compound according to claim 1, wherein the mayenite compound is a conductive mayenite compound.

9. The mayenite compound according to claim 1, wherein the mayenite compound has a specific surface area of 6 m.sup.2g.sup.?1 or more.

10. The mayenite compound according to claim 1, wherein the mayenite compound has a specific surface area of 10 m.sup.2g.sup.?1 or more.

Description

Description of Embodiments

(1) A production method of the present invention will now be described in detail.

(2) Crystals of a mayenite compound are formed by three-dimensionally connecting cage-shaped structures (cages) each having an inner diameter of about 0.4 nm while sharing wall surfaces thereof. In general, anions such as O.sup.2? are included inside the cages of a mayenite compound. However, these anions can be substituted with conduction electrons by providing a chemical treatment. The conduction electron concentration in the mayenite compound is increased by increasing the annealing time.

(3) In mayenite compounds, electrons that substitute oxide ions (O.sup.2?) included in the structures thereof function as conduction electrons. In the case of C12A7, the mayenite compound is represented by a composition formula ([Ca.sub.24Al.sub.28O.sub.64].sup.4+(O.sup.2?).sub.2-x(e.sup.?).sub.2x) (where 0<x<2). Furthermore, the conduction electron concentration is made to 1?10.sup.15 cm.sup.?3 or more by substituting the oxide ions with electrons. Accordingly, mayenite compounds including conduction electrons can be referred to as conductive mayenite compounds. In the case of C12A7:e.sup.?, a theoretical maximum concentration of conduction electrons is 2.3?10.sup.21 cm.sup.?3. A mayenite compound having a conduction electron concentration equal to the theoretical value can be obtained by the method described above.

(4) Conductive mayenite compounds generate light absorption at 2.8 eV and 0.4 eV. An electron density is determined by measuring an optical absorption coefficient of the light absorption. In the case of a powder sample, the electron density is easily determined by using a diffuse reflectance method. Alternatively, the electron density in cages can be measured by using electron spin resonance (ESR) because electrons in the cages are spin-active. Furthermore, when a mayenite compound including conduction electrons is dissolved in a solution containing iodine, the mayenite compound reduces iodine. By using this action, the electron density in cages can be measured by a redox titration.

(5) In the present invention, the term specific surface area refers to a value measured on the basis of an adsorption isotherm of nitrogen molecules at a liquid nitrogen temperature (?196? C.). The specific surface areas of synthesized conductive mayenite compounds were estimated by applying the BET (Brunauer, Emmett, and Teller) formula in the range of 0.05 to 0.3 of an equilibrium pressure (P/P.sub.0; where P represents a partial pressure (Pa) of an adsorption gas that is in an equilibrium state with a sample surface at ?196? C., and P.sub.0 represents a vapor pressure (Pa) of the adsorption gas) of the adsorption isotherm.

(6) <Synthesis of Mayenite Compound>

(7) In the method of the present invention, a mayenite compound used as a staring material of a target compound is more preferably in the form of a fine powder (primary particle size: 100 nm or less) or a bulk porous body having a porous structure. When the mayenite compound is in the form of fine particles, the surface area per gram increases and the gap between the particles is in the mesopore range (2 nm or more and 100 nm or less). A hydroxide serving as a precursor of the mayenite compound can be obtained by a hydrothermal treatment method.

(8) <Method for Synthesizing Mayenite Compound by Using Hydrothermal Treatment>

(9) A hydrothermal synthesis method has been studied for a long time as a method for synthesizing inorganic oxide fine particles having a good crystal quality. A precursor compound can be prepared by charging a solvent such as water or an alcohol and a raw material in a pressure-resistant container, and heating the resulting mixture at a temperature equal to or higher than a boiling point of the solvent for several hours to several days.

(10) Ca.sub.3Al.sub.2 (OH).sub.12, which is a hydroxide serving as a precursor of a mayenite compound C12A7, can be prepared by mixing water, calcium hydroxide, and aluminum hydroxide in a stoichiometric composition, and heating the resulting mixture, for example, at 150? C. for about six hours. The prepared precursor is dehydrated by heating in air in a range of about 400? C. to 1,000? C. Thus, a mayenite compound powder C12A7 having a large specific surface area (about 20 to 60 m.sup.2g.sup.?1) is obtained.

(11) <Pretreatment of Mayenite Compound>

(12) The mayenite compound powder having a large specific surface area and synthesized by way of the hydrothermal treatment method retains hydroxy groups that are strongly bonded on a surface of the powder or in a cage skeleton. In a step of allowing conduction electrons to be included, a reducing agent is consumed by reacting with the hydroxy groups (2CaH.sub.2+2OH.fwdarw.2CaO+3H.sub.2). Therefore, it is necessary that the surface of the powder or the inside of the cage skeleton be activated by removing the hydroxy groups as much as possible in a pretreatment step before the electron injection step. The specific surface area after the pretreatment is decreased with an increase in the temperature of the pretreatment. In a temperature range of 400? C. to 1,000? C., for example, the specific surface area is changed from 60 m.sup.2g.sup.?1 to 6 m.sup.2g.sup.?1.

(13) Regarding the pretreatment method, heating is preferably performed at a temperature in the range of 400? C. to 1,100? C. in an inert gas atmosphere or in a vacuum. The heating temperature is preferably in the range of 700? C. to 1,000? C. and more preferably 800? C. to 900? C. When the heating temperature is lower than 400? C., although a powder having a large specific surface area is obtained, a high conduction electron concentration cannot be obtained because, in the reduction treatment step, a reducing agent is consumed by a hydroxy group retained by the powder. On the other hand, when the heating temperature exceeds 1,100? C., although a high conduction electron concentration is obtained, a mayenite compound powder having a large specific surface area cannot be obtained because sintering of the powder proceeds. In order to sufficiently perform the activation, the heating is preferably conducted for three hours or more.

(14) <Step of Allowing Conduction Electron to be Included in Mayenite Compound by Reduction Treatment>

(15) In the case where a mayenite compound powder including conduction electrons is prepared, a raw material powder of the mayenite compound having a chemical equivalent composition is heated in a reducing atmosphere in the range of 400? C. to 1,100? C. The heating temperature is preferably in the range of 600? C. to 900? C., and more preferably in the range of 700? C. to 800? C. When the heating temperature is lower than 400? C., a reaction between an oxygen ion and a reducing agent in a cage does not sufficiently proceed, and a high conduction electron concentration cannot be obtained. On the other hand, at a heating temperature exceeding 1,100? C., although a high conduction electron concentration can be obtained, the specific surface area is decreased by sintering. The treatment time is preferably three hours or more in order to sufficiently diffuse oxygen ions and exchange the oxygen ions with conduction electrons.

(16) Any reducing agent may be used as long as the reducing agent reacts with an oxygen ion in a cage in the above heating temperature range. Examples of the reducing agent that can be used include alkali metals such as Na and Li; alkaline earth metals such as Mg, Ca, CaH.sub.2; and hydrides thereof. Calcium hydride (CaH.sub.2) becomes CaO after reduction and remains as an impurity, and thus the effective surface area of the conductive mayenite compound may be decreased. The higher the treatment temperature during the step of allowing conduction electrons to be included, the smaller the specific surface area of the mayenite compound powder subjected to the step becomes. For example, in the case where a mayenite compound powder sample is prepared by conducting a pretreatment at 800? C., and a reduction treatment of the sample is performed in a temperature range of 600? C. to 800? C., the specific surface area of the sample is changed, for example, from about 30 m.sup.2g.sup.?1 to about 20 m.sup.2g.sup.?1.

(17) <RTA Process>

(18) A part of a surface of the mayenite compound powder that has reacted with the reducing agent may be insulated as a result of being covered with, for example, calcium oxide. An RTA process can be used as a process for reducing the powder surface that has been insulated. The RTA process is an abbreviation for a rapid thermal annealing process, and is known as a method for improving crystal qualities of semiconductors. In an existing method for heating the surface of a powder, the temperature-increasing rate is as low as about 5 to 10? C. min.sup.?1, and a decrease in the surface area due to sintering of particles cannot be prevented. In contrast, by using the RTA process, the crystal quality of the surface of an electride can be improved without decreasing the surface area, and an electrical conduction property can be provided to the mayenite compound powder including the surface thereof. In the case where crystallization is performed by the RTA process, in an inert atmosphere, in a reducing atmosphere, or in a vacuum, the temperature is increased at a temperature-increasing rate of 30 to 60? C. min.sup.?1, and the temperature is held as a heating temperature at 900? C. to 1,100? C. for 5 to 15 seconds. The step of increasing the temperature and the step of holding the temperature under heating are repeated two to five times. The holding temperature is preferably in the range of 950? C. to 1,100? C.

(19) <Process for Producing Catalyst Including Conductive Mayenite Compound as Support>

(20) A catalyst can be produced, using an impregnation method, a physical mixing method, a thermal decomposition method, a liquid-phase method, a sputtering method, or a vapor deposition method, by allowing a transition metal catalyst such as Ru to be supported on the conductive mayenite compound powder produced by the method described above. In the physical mixing method, a conductive mayenite compound powder and a transition metal compound powder are mixed in a solid phase by a physical mixing method, and the transition metal compound is then reduced by heating the resulting mixture in a reducing atmosphere such as a hydrogen atmosphere in a temperature range of 50? C. to 600? C. Thus, a supported metal catalyst is obtained. From the viewpoint of suppressing sintering of supported metal particles, before the reduction by heating, increasing a temperature and holding a temperature are preferably repeated several times in a vacuum.

(21) The impregnation method includes a step of dispersing a conductive mayenite compound powder in a solvent solution of a transition metal compound, a step of forming a catalyst precursor composed of the transition metal compound that is dried by evaporating a solvent of the solvent solution, and a step of forming the metal catalyst by reducing the transition metal compound by heating in a reducing atmosphere.

(22) The support powder supporting a transition metal thereon clathrates electrons to the same degree as that in the initial state, even after the supporting step, and has a small work function in terms of support. Accordingly, the support powder has a high ability to donate electrons to a transition metal. In addition, since the support has a large specific surface area, the support powder significantly accelerates activation of nitrogen and hydrogen on a transition metal. As a result, the support powder functions as an ammonia synthesis catalyst with a high performance as compared with the case where a conductive mayenite powder having a small specific surface area is used. By using a transition metal catalyst supported on the conductive mayenite compound powder by using any of these methods, ammonia can be synthesized by allowing nitrogen and hydrogen, which are raw materials, to react with each other on the catalyst in a reactor at a reaction temperature of 100? C. or higher and 600? C. or lower and at a reaction pressure of 10 kPa to 30 MPa.

EXAMPLE 1

(23) <Synthesis of Mayenite Compound Powder>

(24) Calcium hydroxide (Ca(OH).sub.2) and aluminum hydroxide (Al(OH).sub.3) were weighed so as to satisfy Ca:Al=12:14, and mixed. Distilled water was weighed so that the concentration of the resulting mixed powder became 10% by weight, and a total of 160 g was stirred and mixed in a planetary ball mill for four hours. The resulting mixed solution was charged in a pressure-resistant hermetically sealed container, and a heat treatment (hydrothermal treatment) was performed at 150? C. for six hours while stirring. The resulting precipitate was separated by filtration, dried, and then pulverized. Thus, about 20 g of a precursor powder of a mayenite compound Ca.sub.3Al.sub.2(OH).sub.12 was obtained. This precursor powder was dehydrated by heating in air at 600? C. for five hours to prepare a mayenite compound powder as a raw material with a large specific surface area. This raw material had a specific surface area of 60 m.sup.2g.sup.?1.

(25) <Pretreatment>As a pretreatment, the powder was put in a silica glass tube, heated in a vacuum of 1?10.sup.?4 Pa at 900? C. for five hours under evacuation, and taken out from the glass tube. The powder obtained at this stage had a specific surface area of about 30 m.sup.2g.sup.?1.
<Electron Injection by Reduction Treatment>

(26) Subsequently, 0.4 g of CaH.sub.2 serving as a reducing agent was added relative to 3 g of the powder after the pretreatment, and sufficiently mixed to prepare a mixture. A tantalum (Ta) tube was then filled with the mixture. The Ta tube filled with the mixture was placed in a silica glass tube, and heated in a vacuum of 1?10.sup.?4 Pa at 700? C. for 15 hours. A conductive mayenite compound powder having a conduction electron concentration of 1.0?10.sup.21 cm.sup.?3 and a specific surface area of 17 m.sup.2g.sup.?1 was obtained.

COMPARATIVE EXAMPLE 1

(27) <Synthesis of Mayenite Compound Powder>

(28) A CaCO.sub.3 powder and an Al.sub.2O.sub.3 powder were mixed so that a ratio of Ca to Al became 11:14. A total of 30 g was heated in an alumina crucible at 1,300? C. for six hours. The resulting powder was inserted in a silica glass tube, and heated in a vacuum of 1?10.sup.?4 Pa at 1,100? C. for 15 hours to prepare a mayenite compound powder as a raw material. The powder obtained at this stage had a specific surface area of 1 m.sup.2g.sup.?1 or less.

(29) <Electron Injection by Reduction Treatment>

(30) Subsequently, 3 g of the powder prepared by the synthesis method described above was inserted into a silica glass tube together with 0.18 g of a metallic Ca powder, and heated at 700? C. for 15 hours, thereby generating a metallic Ca vapor atmosphere in the tube to allow the metallic Ca to react with the powder. The sample sealed in the tube in a vacuum state was taken out, and ground with a mortar. Subsequently, a silica glass tube is filled with the ground sample again, and sealed under evacuation. This silica glass tube was heated at 1,100? C. for two hours. Thus, a conductive mayenite compound powder C12A7:e.sup.? (denoted by C12A7e.sup.21) having a conduction electron concentration of 2?10.sup.21 cm.sup.?3 and a specific surface area of 1 m.sup.2g.sup.?1 was obtained.

EXAMPLE 2

(31) A mayenite compound powder having a large specific surface area was synthesized under the same conditions as in Example 1 except that the pretreatment was conducted at 800? C. instead of the pretreatment temperature of the raw material of 900? C. in Example 1. The powder obtained at this stage had a specific surface area of 40 m.sup.2g.sup.?1.

(32) <Electron Injection by Reduction Treatment>

(33) A conductive mayenite compound powder was synthesized under the same conditions as in Example 1 except that the reduction treatment was conducted at 600? C. instead of the reduction treatment temperature of 700? C. in Example 1. The conduction electron concentration was 1.0?10.sup.21 cm.sup.?3, and the specific surface area was 31 m.sup.2g.sup.?1.

EXAMPLE 3

(34) <Electron Injection by Reduction Treatment>

(35) A conductive mayenite compound powder was synthesized under the same conditions as in Example 1 except that the reduction treatment was conducted at 600? C. instead of the reduction treatment temperature of 700? C. in Example 1. The conduction electron concentration was 0.8?10.sup.21 cm.sup.?3, and the specific surface area was 20 m.sup.2g.sup.?1.

EXAMPLE 4

(36) <Pretreatment>

(37) A mayenite compound powder having a large specific surface area was synthesized under the same conditions as in Example 1 except that the pretreatment was conducted at 800? C. instead of the pretreatment temperature of the raw material of 900? C. in Example 1. The specific surface area at this stage was 40 m.sup.2g.sup.?1.

(38) <Electron Injection by Reduction Treatment>

(39) A conductive mayenite compound powder was synthesized by conducting a reduction treatment under the same conditions as in Example 1. The conduction electron concentration was 1.0?10.sup.21 cm.sup.?3, and the specific surface area was 23 m.sup.2g.sup.?1.

EXAMPLE 5

(40) <Pretreatment>

(41) A mayenite compound powder having a large specific surface area was synthesized under the same conditions as in Example 1 except that the pretreatment was conducted at 800? C. instead of the pretreatment temperature of the raw material of 900? C. in Example 1. The specific surface area at this stage was 40 m.sup.2g.sup.?1.

(42) <Electron Injection by Reduction Treatment>

(43) A conductive mayenite compound powder was synthesized under the same conditions as in Example 1 except that the reduction treatment was conducted at 800? C. instead of the reduction treatment temperature of 700? C. in Example 1. The conduction electron concentration was 0.4?10.sup.21 cm.sup.?3, and the specific surface area was 10 m.sup.2g.sup.?1.

COMPARATIVE EXAMPLE 2

(44) A mayenite compound powder was synthesized by the same method as in Example 1. However, the pretreatment of Example 1 was not performed, and the electron injection by the reduction treatment was also not performed. The conduction electron concentration was zero, and the specific surface area was 60 m.sup.2g.sup.?1.

EXAMPLE 6

(45) <Pretreatment>

(46) An electride was synthesized under the same conditions as in Example 1 except that the pretreatment was conducted at 1,000? C. instead of the pretreatment temperature of the raw material of 900? C. in Example 1. A conductive mayenite compound powder having a conduction electron concentration of 1.4?10.sup.21 cm.sup.?1 and a specific surface area of 6 m.sup.2g.sup.?1 was obtained.

EXAMPLE 7

(47) <Synthesis of Mayenite Compound>

(48) The Ca.sub.3Al.sub.2(OH).sub.12 prepared in Example 1 was dehydrated by heating at 800? C. for two hours in an oxygen stream. Thus, a mayenite compound powder was prepared as a raw material.

(49) <Pretreatment>

(50) As a pretreatment of this raw material, the powder was put in a silica glass tube, and heated in a vacuum of 1?10.sup.?4 Pa at 800? C. for 20 hours under evacuation.

(51) <Electron Injection by Reduction Treatment>

(52) Calcium (Ca) metal was used as a reducing agent instead of CaH.sub.2 in Example 1. To 2 g of the powder after the pretreatment, 0.12 g of Ca metal serving as a reducing agent was added. The resulting mixture was put in a silica glass tube, and was heated in a vacuum of 1?10.sup.?4 Pa at 700? C. for 15 hours.

(53) <RTA Process>

(54) Furthermore, in order to activate the surface of the powder, a Tammann tube was filled with the powder and vacuum-sealed. Subsequently, an RTA process was conducted by repeating twice a step of increasing the temperature at a temperature-increasing rate of 45 ? C. min.sup.?1 and then holding the temperature at 950? C. for five seconds under heating. A conductive mayenite compound powder having a conduction electron concentration of 0.5?10.sup.21 cm.sup.?1 and a specific surface area of 19 m.sup.2g.sup.?1 was obtained.

EXAMPLE 8

(55) <RTA Process>

(56) A conductive mayenite compound powder was synthesized under the same conditions as in Example 7 except that the RTA process was conducted at a process temperature of 1,000? C. instead of the RTA process temperature of 950? C. in Example 7. A conductive mayenite compound powder having a conduction electron concentration of 1.5?10.sup.21 cm.sup.?1 and a specific surface area of 14 m.sup.2g.sup.?1 was obtained.

(57) The synthesis and treatment conditions of Examples 1 to 8 and Comparative Examples 1 and 2 are summarized in Table 1.

(58) TABLE-US-00001 TABLE 1 Method for Pretreatment synthesizing of raw Reducing Reducing raw material material agent condition RTA process Example 1 Hydrothermal Evacuation, CaH.sub.2 700? C. Not synthesis 900? C. performed Example 2 Hydrothermal Evacuation, CaH.sub.2 600? C. Not synthesis 800? C. performed Example 3 Hydrothermal Evacuation, CaH.sub.2 600? C. Not synthesis 900? C. performed Example 4 Hydrothermal Evacuation, CaH.sub.2 700? C. Not synthesis 800? C. performed Example 5 Hydrothermal Evacuation, CaH.sub.2 800? C. Not synthesis 800? C. performed Example 6 Hydrothermal Evacuation, CaH.sub.2 700? C. Not synthesis 1,000? C. performed Example 7 Hydrothermal Evacuation, Ca 700? C. 950? C. synthesis 800? C. Example 8 Hydrothermal Evacuation, Ca 700? C. 1,000? C. synthesis 800? C. Comparative Solid-phase Evacuation, Ca 1,100? C. Not Example 1 method 1,100? C. performed Comparative Hydrothermal Not None None Not Example 2 synthesis performed performed

EXAMPLE 9

(59) <Supporting of Ru on Conductive Mayenite Compound Powder>

(60) In a Pyrex (registered trademark) glass tube, 1 g of the C12A7e.sup.? powder prepared in Example 1, the powder having an amount of electron injection of 1.0?10.sup.21 cm.sup.?3 and a specific surface area of 17 m.sup.2g.sup.?1, and 0.042 g of Ru.sub.3(CO).sub.12 were put, and the glass tube was vacuum-sealed. The vacuum-sealed tube was subjected to a heat treatment while rotating in an electric furnace using the following program.

(61) [40? C., increasing temperature for 20 min..fwdarw.40? C., holding for 60 min..fwdarw.70? C., increasing temperature for 120 min..fwdarw.70? C., holding for 60 min..fwdarw.120? C., increasing temperature for 120 min..fwdarw.120? C., holding for 60 min..fwdarw.250? C., increasing temperature for 150 min..fwdarw.250? C., holding for 120 min.]

(62) Subsequently, the vacuum-sealed tube was broken, and the resulting powder was heat-treated in a hydrogen gas (26.7 kPa) atmosphere by increasing the temperature to 300? C. over a period of 5 hours and holding the temperature for two hours. Thus, a conductive mayenite compound powder supporting 2% by weight of Ru was obtained.

(63) <Ammonia Synthesis Reaction>

(64) A reaction in which nitrogen gas (N.sub.2) and hydrogen gas (H.sub.2) were allowed to react with each other to produce ammonia gas (NH.sub.3) was conducted. The reaction was conducted in a fixed-bed flow-type reactor to which a quartz glass tube filled with 0.2 g of the prepared catalyst was attached. Regarding the flow rates of the gases, the flow rate of N.sub.2 was set to 15 mLmin.sup.?1, the flow rate of H.sub.2 was set to 45 mLmin.sup.?1, and the total flow rate was set to 60 mLmin.sup.?1. The reaction was conducted at a pressure of atmospheric pressure and at a reaction temperature in the range of 320? C. to 400? C. A gas discharged from the flow-type reactor was bubbled in a 0.005 M aqueous sulfuric acid solution so that the produced ammonia was dissolved in the solution. The produced ammonium ions were quantitatively determined by ion chromatography. The production rate of ammonia at 340? C. was 2,388 ?molg.sup.?1h.sup.?1.

COMPARATIVE EXAMPLE 3

(65) A 2wt % Ru-supported catalyst was prepared by the same method as in Example 9 except that the C12A7e.sup.21 powder prepared in Comparative Example 1, the powder having an amount of electron injection of 2.0?10.sup.21 cm.sup.?3 and a specific surface area of 1 m.sup.2g.sup.?1, was used. The ammonia synthesis reaction was conducted as in Example 9. The production rate of ammonia at 340? C. was 1,229 ?molg.sup.?1h.sup.?1.

EXAMPLE 10

(66) A 2wt % Ru-supported catalyst was prepared by the same method as in Example 9 except that the conductive mayenite compound powder prepared in Example 2, the powder having a specific surface area of 31 m.sup.2g.sup.?1, was used. The ammonia synthesis reaction was conducted as in Example 9. The production rate of ammonia at 340? C. was 1,575 ?molg.sup.?1h.sup.?1.

EXAMPLE 11

(67) A 2wt % Ru-supported catalyst was prepared by the same method as in Example 9 except that the conductive mayenite compound powder prepared in Example 3, the powder having a specific surface area of 20 m.sup.2g.sup.?1, was used. The ammonia synthesis reaction was conducted as in Example 9. The production rate of ammonia at 340? C. was 1,831 ?molg.sup.?1h.sup.?1.

EXAMPLE 12

(68) A 2wt % Ru-supported catalyst was prepared by the same method as in Example 9 except that the conductive mayenite compound powder prepared in Example 4, the powder having a specific surface area of 23 m.sup.2g.sup.?1, was used. The ammonia synthesis reaction was conducted as in Example 9. The production rate of ammonia at 340? C. was 1,696 ?molg.sup.?1h.sup.?1.

EXAMPLE 13

(69) A 2wt % Ru-supported catalyst was prepared by the same method as in Example 9 except that the conductive mayenite compound powder prepared in Example 5, the powder having a specific surface area of 10 m.sup.2g.sup.?1, was used. The ammonia synthesis reaction was conducted as in Example 9. The production rate of ammonia at 340? C. was 1,793 ?molg.sup.?1h.sup.?1.

COMPARATIVE EXAMPLE 4

(70) A 2wt % Ru-supported catalyst was prepared by the same method as in Example 9 except that the mayenite compound powder prepared in Comparative Example 2, the powder having a specific surface area of 60 m.sup.2g.sup.?1, was used. The ammonia synthesis experiment was conducted as in Example 9. The production rate of ammonia at 340? C. was 895 ?molg.sup.?1h.sup.?1.

(71) The results of Examples 9 to 13 and Comparative Examples 3 and 4 are summarized in Table 2.

(72) TABLE-US-00002 TABLE 2 Production rate of NH.sub.3 Support used (?molg.sup.?1h.sup.?1) Example 9 Example 1 2,388 Example 10 Example 2 1,575 Example 11 Example 3 1,831 Example 12 Example 4 1,696 Example 13 Example 5 1,793 Comparative Example 3 Comparative Example 1 1,229 Comparative Example 4 Comparative Example 2 895

INDUSTRIAL APPLICABILITY

(73) The conductive mayenite compound having a large specific surface area and produced by the method of the present invention can be used as electronic materials such as a transparent electrode and a cold emitter that have good electronic properties. Furthermore, the conductive mayenite compound of the present invention can be used as high-performance reducing agents, catalyst materials, etc.