Laves phase intermetallic compound, catalyst using intermetallic compound, and method for producing ammonia

Abstract

The invention provides a Laves phase intermetallic compound having a composition represented by general formula ARu.sub.2 (A is Y, Sc, or at least one element selected from lanthanoid elements excluding Ce), the crystallite size thereof being 1 nm to 100 nm; a catalyst including the intermetallic compound as an active ingredient; and a method for producing ammonia using the same.

Claims

1. A Laves phase intermetallic compound having a composition represented by formula ARu.sub.2, wherein A is Y, Sc, or at least one lanthanoid element excluding Ce; and a crystallite size of the Laves phase intermetallic compound is 1 nm to 100 nm.

2. A Laves phase intermetallic compound complex, comprising the Laves phase intermetallic compound according to claim 1, and a catalyst support for supporting the intermetallic compound.

3. A catalyst, comprising a Laves phase intermetallic compound having a composition represented by formula ARu.sub.2, as a catalyst active component, wherein A is Y, Sc, or at least one lanthanoid element excluding Ce; and a crystallite size of the Laves phase intermetallic compound is 1 nm to 100 nm.

4. The catalyst according to claim 3, wherein the catalyst active component is supported on a catalyst support.

5. The catalyst according to claim 3, wherein the catalyst is a catalyst for producing ammonia.

6. A method for producing ammonia, the method comprising: passing a gas containing hydrogen and nitrogen into contact with a catalyst, wherein the catalyst comprises a Laves phase intermetallic compound having a composition represented by formula ARu.sub.2 as a catalyst active component; A is Y, Sc, or at least one lanthanoid element excluding Ce; and a crystallite size of the Laves phase intermetallic compound is 1 nm to 100 nm.

7. The method according to claim 6, wherein a reaction temperature of passing the gas into contact with the catalyst is 200 C. or more.

8. The method according to claim 6, wherein a reaction pressure of passing the gas into contact with the catalyst is 10 kPa to 15 MPa.

9. The method according to claim 6, wherein a volume ratio of hydrogen to nitrogen in the gas is 0.4 or more.

10. A powder comprising a Laves phase intermetallic compound having a composition represented by formula ARu.sub.2, wherein A is Y, Sc, or at least one lanthanoid element excluding Ce; and a crystallite size of the Laves phase intermetallic compound is 1 nm to 100 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram showing ammonia synthesis rates in Example 5 and Comparative Example 4.

(2) FIG. 2 is a diagram showing measurement results of XRI) before and after hydrogen adsorption-desorption treatment in Example 1.

(3) FIG. 3A is a diagram showing a measurement result of SEM before hydrogen adsorption-desorption treatment in Example 1.

(4) FIG. 3B is a diagram showing the measurement results of SEM after hydrogen adsorption-desorption treatment in Example 1.

(5) FIG. 4 is a graph showing changes with time of the ammonia synthesis rate in Example 6.

(6) FIG. 5 is a view showing XRD measurement results of the catalyst used in Example 6 at 24 hours and 49 hours.

DETAILED DESCRIPTION OF THE INVENTION

(7) The present invention will be described in detail below.

(8) The term powder is defined as an aggregate of particles having a maximum dimension of 1 mm or less (JIS Z 2500: 2,000 powder metallurgical term), and which is used in the same meaning in this specification. Also, although not strictly defined, it may be a powder having the particle size of several hundreds m or more is called as a coarse powder, several hundreds to several tens of m as ordinary powder, several tens to 1 m as fine powder, 1 to 0.1 m as submicron powder, and less than 0.1 m as ultrafine powder (Industrial Research Committee, item POWDER in Dictionary of Industrial Materials, first edition of 1997), and the term is used in the same meaning in this specification.

(9) <Intermetallic Compound>

(10) Intermetallic compounds are usually compounds composed of two or more kinds of metals.

(11) Intermetallic compounds are generally known to have different crystal structures from the constituent metallic elements constituting the intermetallic compounds and have specific properties not found in the component metallic elements.

(12) The atomic ratio of the metal elements constituting the intermetallic compounds is not particularly limited, but it may be an integer ratio. In the case of the intermetallic compounds represented by the composition formula, for example, in the case of an intermetallic compound composed of two kinds of metallic elements, when the constituent metallic elements are A and B, there is no particular limitation, but there is an intermetallic compound represented by the general formula AB.sub.5, and those represented by the general formula AB.sub.2.

(13) The compound represented by the general formula AB.sub.2 is known as a Laves phase intermetallic compound. The Laves phase is one of the phases of the intermetallic compound and may be composed of two kinds of metal elements A and B having different sizes, wherein the phase is formed when the ratio of the atomic radius Ra of atom A having a large atomic radius to the atomic radius Rb of a small atom B, Ra/Rb, is about 1.05 to 1.67. The intermetallic compound having such a phase is called a Laves phase intermetallic compound.

(14) The Laves phase intermetallic compound of the present invention (hereinafter sometimes simply referred to as intermetallic compound of the present invention) is a compound represented by the general formula ARu.sub.2 (A is Y, Sc, or one element selected from lanthanoid elements except Ce). Specifically, the lanthanoid elements excluding Ce are one or more lanthanoid elements selected from the group consisting of La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

(15) That is, the Laves phase intermetallic compound of the present invention is a Laves phase intermetallic compound having Ru as an essential constituent, and the metal element as a counterpart thereof including yttrium (Y), scandium (Sc) or a lanthanoid element except for Ce. The metal element as a counterpart of Ru is preferably Y which is relatively easy to obtain and inexpensive.

(16) The intermetallic compound of the present invention has a crystallite size of 1 to 100 nm. Preferably 2 to 90 nm, more preferably 5 to 75 nm, further preferably 5 to 50 nm. Due to this size, the intermetallic compound of the present invention can improve the catalytic activity while maintaining the properties of the intermetallic compound of the present invention, for example, when it is used as a catalyst.

(17) In addition, for example, when it is used as a catalyst, a catalyst having a large surface area may be obtained, so that it is possible to improve the catalytic activity.

(18) Here, the crystallite of the intermetallic compound of the present invention can be regarded as a single crystal in each particle. A single crystal is one in which the direction of atomic arrangement is the same in any part in a crystal. Also, one particle may be composed of a plurality of crystallites.

(19) When the size of the crystallite is in the range of about 1 to about 100 nm, the crystallite size can be obtained based on the Scherrer formula from the half width of the peak of X-ray diffraction. As a measurement method, a method based on H 7805:2005 Method for measuring crystallite diameter of metal catalyst by X-ray diffraction method can be used. In addition, Rietveld analysis can also be obtained from the full width at half maximum of the X-ray diffraction peak in the range of about 1 to about 100 nm. The size of the crystallite larger than the above range can be obtained by using any one of the measurement methods.

(20) The intermetallic compound of the present invention is not particularly limited, but may has cracks on its surface.

(21) A crack is a partial split or a fissure appearing on the material surface and can be confirmed by a surface analyzer such as an electron microscope.

(22) The amount of cracks is not particularly limited, but it is preferable that there are cracks in 20% or more of the total crystal.

(23) Although the specific surface area of the intermetallic compound of the present invention is not particularly limited, and the specific surface area may be small to the extent that it is difficult to measure by the BET method, the surface area of the catalyst is preferably 0.5 m.sup.2/g or more, more preferably 1.0 m.sup.2/g or more, in view that the catalytic activity is proportional to the magnitude of the surface area, and the catalytic activity per mass is higher as the surface area is larger.

(24) The shape of the intermetallic compound of the present invention are not particularly limited and may be any of lumps, powders, coatings, and the like, but it may be in the form of powder and regarding size, any one of rough powder, normal powder, fine powder, submicron powder and ultrafine powder may be used, but a fine powder having a size of about 50 m to 1 m is preferable.

(25) The method of producing the intermetallic compound of the present invention is not particularly limited as long as the effect is obtained, hut it may be produced according to the following method.

(26) First, a first step includes a step of producing the intermetallic compound ARu.sub.2 of the present invention from metal atom sources which are a components of the intermetallic compound of the present invention.

(27) A subsequent second step includes a step of passing the crystallite size of the intermetallic compound ARu.sub.2 into a predetermined range.

(28) <Production of ARu.sub.2>

(29) The method for producing the intermetallic compound of the present invention is not particularly limited, but it may be produced by melting and mixing the metal A and the metal Ru which are the constituent components.

(30) An raw materials ratio at the time of production is not limited as long as the intermetallic compound of the present invention can be obtained, but it may be adjusted so as to be 1:2 which is the composition molar ratio thereof.

(31) There are no particular restrictions on the method of melting and mixing the raw materials of the metals, for example, any one of the following methods of melting and mixing the raw materials of the metals can be selected appropriately: melting and mixing raw material metals by the are melting method; calcination in an electric furnace such as a high frequency furnace of raw material metal; microwave fusion synthesis of raw material metals by microwave oven etc.; mechanical alloying of metal A particles and metal Ru particles; spraying raw material metal melt; a reduction diffusion method in which an oxide powder of metal A and a metal Ru powder are mixed and the rare earth element oxide is reduced by heating it in the vapor of a reducing metal such as calcium to diffuse into the metal Ru; combine hydrogenated metal A powder and metal Ru powder by mixing, sputtering or the like, decompress to a vacuum state at high temperature, and extract hydrogen from metal A.

(32) In addition, the intermetallic compound of the present invention can be produced as a film by sputtering, vapor deposition, or a method of applying a solution of a metal compound as a raw material for an intermetallic compound to a carrier and reducing it by heating.

(33) Among them, the are melt method is preferably used because of its simplicity of operation.

(34) The are melt method is a method in which the metal is placed in a vacuum and then the metal is dissolved and mixed by are discharge.

(35) Although the conditions at this time are not particularly limited, in general, in order to prevent oxidation of the metal as a raw material, the treatment is carried out in an oxygen-free atmosphere, preferably under an Ar atmosphere in which discharge is likely to occur.

(36) The pressure at the time of melting is not particularly limited, but it may be carried out under a low pressure so that discharge easily occurs, preferably 0.1 or less, and more preferably 0.05 MPa or less.

(37) The voltage and the like at the time of are discharge are not particularly limited as long as melting and mixing occur, and can be appropriately selected according to the kind of the raw material and the amount thereof. Then, the molten metal is cooled to room temperature. The resulting ARu.sub.2 may be lumpy. By pulverizing it, ARu.sub.2 may be made to have a preferable particle diameter. Both mechanical pulverization and physical pulverization can be used for pulverization.

(38) <Step of making the Crystallite Size of ARu.sub.2 into a Predetermined Range>

(39) A second step of the method for producing the intermetallic compound of the present invention includes a step of making the crystallite of ARu.sub.2 obtained in the first step to a size of 1 to 100 nm. Here, ARu.sub.2 obtained in the first step may be reinafter be referred to as raw material ARu.sub.2 in some cases.

(40) The method is not particularly limited, and a method of increasing the so-called crystallite surface area is used.

(41) It may be difficult to make the raw material ARu.sub.2 into a fine particle form by a usual physical method. Therefore, it may be preferable to perform a treatment by hydrogen adsorption-desorption (hereinafter referred to as hydrogen adsorption-desorption treatment) to the raw material ARu.sub.2.

(42) Hereinafter, the hydrogen adsorption-desorption treatment will be described in detail.

(43) First, the raw material ARu.sub.2 is placed in an atmosphere having a high hydrogen partial pressure to adsorb hydrogen. The hydrogen partial pressure during the adsorption is not particularly limited, but it is preferably in the range of normal pressure to 10 MPa. Although the temperature at the time of adsorption is not particularly limited, it may be room temperature to 500 C., preferably 300 C. to 500 C. from the viewpoint that ARu.sub.2 does not decompose and sufficiently adsorb hydrogen. Further, although the temperature during adsorption may be constant or may be changed, by raising the temperature to a certain holding temperature with a rapid temperature change from room temperature and holding for a certain period of time, cracks are likely to occur on the surface of ARu.sub.2. It is preferable that it can increase the BET specific surface area. Although the rate of temperature rise at this time is not particularly limited, it may be raised at 5 C./min or more, preferably 10 C./min or more. The holding temperature is not particularly limited, but may be 300 C. to 500 C. The holding time after raising the temperature is not particularly limited, but it may be 10 minutes to 5 hours.

(44) Next, after ARu.sub.2 in which hydrogen has been absorbed is cooled to room temperature and is placed in a vacuum state, the adsorbed hydrogen is desorbed. The pressure in the vacuum state is not particularly limited, but it may be a hydrogen partial pressure of 10 kPa or less. The temperature at which desorption is carried out is not particularly limited, but it may be high temperature, specifically, it may be room temperature or more, preferably 300 C. to 500 C.

(45) The temperature at which hydrogen is desorbed may be constant or may be changed, but by raising the temperature to a certain holding temperature with a rapid temperature change from room temperature and holding for a certain period of time, cracks are formed on the surface of ARu.sub.2. As a result, the BET specific surface area can be increased, which is preferable.

(46) Although the rate of temperature rise at this time is not particularly limited, it may be raised at 5 C./min or more, preferably 30 C./min or more. The holding temperature is not particularly limited, but may be 300 C. to 500 C. The holding time after raising the temperature is not particularly limited, hut it may be 10 minutes to 5 hours.

(47) Since ARu.sub.2 used in the present invention may have a property of adsorbing hydrogen, it is considered that hydrogen embrittlement and miniaturization will occur by going through the aforementioned hydrogen adsorption-desorption treatment.

(48) As a result, the surface area of the intermetallic compound of the present invention increases.

(49) Although the shape of the intermetallic compound of the present invention obtained through the hydrogen adsorption-desorption treatment is not particularly limited, it may be fine powder, preferably One powder having particles of 10 m or more in 90 wt % or more.

(50) The intermetallic compound of the present invention can be used as a catalyst for various chemical reactions. In the case where the intermetallic compound of the present invention is used as a catalyst, the intermetallic compound of the present invention can be used as it is, or can be used by being supported on various commonly used catalyst supports in a complex.

(51) In the case where the intermetallic compound of the present invention is used as it is, the intermetallic compound can be used by using a commonly used method.

(52) The crystallite size and the specific surface area of the catalyst at this time are not particularly limited, but may have the same values as the crystallite size and the specific surface area of the above-mentioned intermetallic compound.

(53) The intermetallic compound of the present invention can be supported on various catalyst supports and used as a complex. The material of the catalyst supports used in the complex is not particularly limited as long as it can support the intermetallic compound of the present invention and does not inhibit the performance of the intermetallic compound of the present invention in the chemical reaction to be used. Various known catalyst supports may be used. For example, catalyst supports of inorganic oxides, for example, oxides of two or more kinds of metals such as silicon oxide (silica), zinc oxide, aluminum oxide (alumina), magnesium oxide (magnesia), indium oxide, calcium oxide, zirconium oxide (zirconia), titanium oxide (titanic), hafnium oxide, barium oxide, cerium oxide (Ceria), perovskite type compounds and mayenite type compounds may be used. In addition, nitride catalyst supports such as Ta.sub.3N.sub.5, BN, and Si.sub.3N.sub.4; or carbon catalyst supports such as activated carbon, silicon carbide, and the like may be used. And a catalyst support containing two or more kinds of catalyst supports selected from the catalyst supports of the above-mentioned examples can be used.

(54) Also, as a catalyst support, at least one or more types of the group consisting of conductive mayenite type compound described in the specification of WO2012/077658, two-dimensional electride compound or precursor compound thereof described in the specification of WO2015/129471, and complexes described in the specification of WO2016/088896 can be used as the catalyst support.

(55) The complex of the present invention can be used as a catalyst.

(56) The particle size of the catalyst support is not particularly limited, hut the catalyst support may have a large particle size and has a particle diameter similar to that of the catalyst support used.

(57) The specific surface area of the catalyst support is not particularly limited, but the specific surface area of the catalyst support may be dominant with respect to the value of the specific surface area of the complex, so it has a specific surface area according to the specific surface area of the catalyst support used.

(58) A complex is obtained by supporting the intermetallic compound of the present invention on a catalyst support. The complex can be used as a catalyst by physically mixing the raw material ARu.sub.2 with the catalyst support. Further, the complex can be used as a catalyst by coating the material ARu.sub.2 on a support such as the catalyst support or metal.

(59) Further, a complex can be obtained by supporting a compound of metal Ru which is a component of ARu.sub.2 and a compound of metal A which is a component of ARu.sub.2 in the present invention, on the catalyst support by using the impregnation method, chemical vapor deposition, sputtering or the like, and then by performing hydrogen reduction. The complex can be used as a supported metal catalyst supporting ARu.sub.2 particles.

(60) The compound of metal Ru is not particularly limited, for example, an inorganic metal compound which can easily thermally decompose or an organometallic complex such as tri-ruthenium dodecacarbonyl ([Ru.sub.3(CO).sub.12]), dishloro tetrakis (triphenylphosphine) ruthenium (II) ([RuCl.sub.2(PPh.sub.3).sub.4]), dichloro tris (triphenylphosphine) ruthenium (II) ([RuCl.sub.2(PPh.sub.3).sub.3]), tris (acetylacetonato) ruthenium (III) ([Ru(acac).sub.3]), ruthenocene ([Ru(C.sub.5H.sub.5)]), ruthenium chloride ([RuCl.sub.3]) can be used.

(61) As the impregnation method, the following step can be used. For example, a support powder is dispersed in a solution containing the compound of metal Ru and the compound of metal A, and the mixture is stirred. In this case, the total amount of the metal compound is not particularly limited, it may be 0.01% by mass or more with respect to the weight of the support used, preferably 0.02 wt % or more, more preferably 0.05 wt % or more, and it may be 40 wt % or less, preferably 30 wt % or less, more preferably 20 wt % or less. Thereafter, under an inert gas stream, such as argon, helium, or in a vacuum, the catalyst precursor is obtained by heating the mixture to dryness to evaporate the solvent. Temperature at this time is not particularly limited, it may be 50 C. to 200 C. The time required in the step is not particularly limited, it may be 30 minutes to 5 hours. Then, by reducing the catalyst precursor containing the dry compound of metal Ru and the compound of the metal A, a catalyst support supporting a coating of intermetallic compound ARu.sub.2 can be obtained.

(62) The intermetallic compound supported on the catalyst support obtained by the above-mentioned method, preferably after being treated by the hydrogen adsorption-desorption treatment, can be used as a catalyst in which the intermetallic compound of the present invention is supported on a catalyst support.

(63) Since the catalyst of the present invention in which the intermetallic compound is supported is suitable for ammonia synthesis, it can be used as an ammonia synthesis catalyst.

(64) Hereinafter, a method of producing ammonia of the present invention is described in detail.

(65) <Synthesis of Ammonia>

(66) In the method of producing ammonia of the present invention, the intermetallic compound of the present invention as a catalyst is used, and hydrogen and nitrogen are reacted on the catalyst to produce ammonia.

(67) In the method of producing ammonia of present invention, when a gas containing hydrogen and nitrogen are passed into contact with the catalyst of the present invention, it is not particularly limited, normally, the catalyst is heated.

(68) The reaction temperature is not particularly limited, it may be 200 C. or more, preferably 250 C. or more, more preferably 300 C. or more and it may be 600 C. or less, preferably 500 C. or less, more preferably 450 C. or less. Although a low-temperature region is advantageous for ammonia generation in terms of chemical equilibrium theory because ammonia synthesis is an exothermic reaction, temperature range described above is preferred in order to obtain a sufficient ammonia formation rate.

(69) The molar ratio of nitrogen and hydrogen passed into contact with the catalyst is not particularly limited, the ratio of hydrogen to nitrogen (H.sub.2/N.sub.2 (volume/volume)) may be 0.4 or more, preferably 0.5 or more, more preferably 1 or more and it may be 10 or less, preferably 5 or less.

(70) The reaction pressure of the ammonia production method of the present invention is not particularly limited, a pressure of the gas mixture containing nitrogen and hydrogen may be 0.01 MPa or more, preferably 0.1 MPa or more and it may be 20 MPa or less, preferably 15 MPa or less, more preferably 10 MPa or less. Since the activity of the catalyst of the present invention has high pressure dependence, in view of practical use, a pressure condition equal to or more than atmospheric pressure is preferred.

(71) As a type of the synthetic reaction, any one of a hatch reaction type, a closed circulation system reaction type, and a flow system reaction type may be use, however, the flow system reaction type is most preferred from the practical point of view. A high pressure and low temperature condition is advantageous in ammonia synthesis reaction in terms of equilibrium. Moreover, due to the exothermic reaction, it is advantageous if the reaction is performed while removing the reaction heat. Therefore, in order to increase the yield, it is possible to use a technique industrially usually performed in the ammonia production. For example, when using a flow system reactor, a method of obtaining a high ammonia yield by lowering the inlet temperature of each reactor may include connecting several reactors filled with the catalyst in series, and installing intercoolers to the outlet of each reactor to remove heat. For another example, the method may include precisely controlling the outlet temperature of each reaction layer by using a reactor having several catalyst layers therein.

(72) In the ammonia production method of the present invention, it is possible to perform the ammonia synthesis, in the same manner as in the conventional synthesis method, by using one type of reactor filled with catalyst, or by using several types of reactors. The fine powder catalyst of the Ru-based intermetallic compound alone or in combination of two or more catalysts selected from the fine power catalysts of Ru-based intermetallic compound, or in combination of the fine power catalysts of the Ru-based intermetallic compound and a conventional catalyst may be used. In addition, a method of connecting a plurality of reactors, and a method of providing a plurality of reaction layers in the same reactor may be used.

(73) In the equilibrium reaction conditions of industrial ammonia synthesis, after cooling removal of the ammonia produced in the reaction gas, an unreacted raw material is purged outside by separating the reaction gas and a portion of the impurities contained in the reaction gas by using membrane separation or the like. The unreacted raw material can be recycled as a raw material.

(74) As a hydrogen raw material used for the method of producing ammonia, it is possible to use hydrogen raw materials which can be produced through various production methods, such as a production method in which coal, petroleum or natural gas is used as a raw material; an autothermal reforming method which is obtained by combining the steam reforming and the partial oxidation; a method by combining these methods with a shift reaction; and 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.

(75) As the raw material for the ammonia synthesis, material derived from the natural gas material may be used. Specifically, the hydrogen gas and the nitrogen gas may be produced through a steam reforming step of the natural gas, a partial oxidation reforming step, a CO shift reacting step, a CO.sub.2 removing step, and a subsequent CO removing step performed by continuous CO methanation.

(76) On the other hand, as a method of obtaining a raw material having higher H/N ratio in order to carry out a synthesis reaction under conditions of excess hydrogen than the stoichiometric ratio N.sub.2:H.sub.2=1:3, 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 the air, oxygen is used to produce hydrogen through an auto-thermal method and nitrogen is used for the reaction gas or process nitrogen is preferable from the viewpoint of the energy saving. Either method can be used as the method of producing the raw material for the present invention.

(77) After the ammonia synthesis reaction is carried out in the above-mentioned reaction conditions by using the catalyst of the present invention, the reaction temperature is lowered to room temperature. In that case, it is possible to repeat the ammonia synthesis reaction since activity of the catalyst is recovered when the catalyst is placed on the reaction conditions again, which is preferred. This is because, in the course of lowering the reaction temperature to room temperature and then raising it again to the reaction temperature, the hydrogen adsorption amount of the intermetallic compound of the present invention changes by absorbing-de-sorbing hydrogen, and as a result, the activity of the catalyst is recovered by further miniaturization of the particles of the catalyst.

EXAMPLE

(78) Hereafter, the present invention will be described in more detail based on examples. An evaluation of the ammonia synthesis activity of the catalyst was carried out by determining the ammonia synthesis rate by quantified the production of NH.sub.3 by using a gas chromatograph or quantified the solution in which NH.sub.3 was dissolved in the sulfuric acid aqueous solution by using ion chromatography.

(79) (Method of Measuring BET Specific Surface Area)

(80) BET specific surface area was measured by adsorbing nitrogen gas at liquid nitrogen temperature on the surface of the object and calculating BET specific surface area from the adsorption-desorption isothermal curve based on the adsorption-desorption of nitrogen gas at 196 C. Measurement conditions are shown below.

(81) [Measurement Conditions]

(82) Measurement device: high-speed and specific surface area/pore distribution measuring apparatus NOVA 4200e (Quantachrome Instruments Co., Ltd.)

(83) Adsorbed gas: nitrogen 99.99995% by volume

(84) Adsorption temperature: liquid nitrogen temperature 196 C.

(85) Pretreatment: the measurement object was placed under the vacuum or reduced pressure, 130 C., and allowed to stand for 2 hours.

(86) (Ion Chromatogram Analysis)

(87) The ammonia gas discharged from the reaction vessel was dissolved in a 5 mM aqueous solution of sulfuric acid, and captured ammonium ion (NH.sub.4.sup.+) was analyzed by ion chromatography.

(88) Measurement conditions are shown below.

(89) [Measurement Conditions]

(90) Apparatus: JASCO Corporation LC-2000 plus Detector: electric conductivity detector CD-5 (Shodex Co., Ltd.) Column: Ion chromatogram column (manufactured by Shimadzu Corporation) Fluent: 3.0 mM oxalate+2.0 mM 18-crown-6-ether solution Flow rate: 1.0 ml/min. Column temperature: 40 C.

(91) (Method of Measuring the Reaction Order)

(92) Analysis of the reaction order of the ammonia synthesis reaction was carried out according to the conditions described in Applied Catalysis A: General 209(2001) pp.317 to 325. Specifically, the reaction order was determined by applying equations (1) to (13) described in item 2.3 Kinetic analysis of the same document at page 319.

(93) Analysis of the reaction order was carried out by measuring the value of the catalytic activity obtained by varying the partial pressure of the reactive gas under certain temperature and flow rate, and obtaining the reaction order represented by the following general formula (1).
r=k.Math.P.sub.N2.sup..Math.P.sub.H2.sup..Math.P.sub.NH3.sup.(1)

(94) Here, r is the reaction rate, k is the rate constant, P.sub.N2 is the partial pressure of nitrogen, P.sub.H2 is the hydrogen partial pressure, P.sub.NH3 parital pressure of ammonia, is reaction order of nitrogen, is reaction order of hydrogen, is the reaction order of ammonia.

(95) Reaction orders of nitrogen and hydrogen were obtained by the equation (1). Reaction order of ammonia was determined by heuristics from a value obtained by subtracting one from the value of the inverse of the slope of the resulting straight line plot of ammonia abundance vs. the reciprocal of total flow.

(96) In this measurement, in order to keep the gas flow rate constant, total flow was controlled by using Ar gas as an inert gas.

(97) [Measurement Conditions for Nitrogen Reaction Order]

(98) The nitrogen reaction order was determined by measuring ammonia synthesis rates by the following four flow conditions, taking the each natural logarithm plots of the nitrogen partial pressure and the ammonia synthesis rate, and calculating the nitrogen reaction order from the slope thereof.

(99) Flow rates (ml/min) of each gas were shown as follows.

(100) N.sub.2:H.sub.2:Ar=6:30:24

(101) N.sub.2:H.sub.2:Ar=10:30:20

(102) N.sub.2:H.sub.2:Ar=15:30:15

(103) N.sub.2:H.sub.2:Ar=20:30:10

(104) [Conditions of Measuring Hydgrogen Reaction Order]

(105) The hydrogen reaction order was determined by measuring ammonia synthesis rates by the following four flow conditions, taking the each natural logarithm plots of the hydrogen partial pressure and the ammonia synthesis rate, and calculating the hydrogen reaction order from the slope thereof.

(106) Flow rate (ml/min) is as follows for each gas.

(107) N.sub.2:H.sub.2:Ar=10:20:30

(108) N.sub.2:H.sub.2:Ar=10:25:35

(109) N.sub.2:H.sub.2:Ar=10:30:20

(110) N.sub.2:H.sub.2:Ar=10:40:10

(111) [Conditions of Measuring Ammonia Reaction Order]

(112) The ammonia reaction order was determined by measuring ammonia synthesis rates at four flow conditions as shown below, taking the natural logarithm plot of the reciprocal of the total gas flow rate and each outlet ammonia concentration, and calculating the ammonia reaction order () from the slope m by using the following equation, =11/m.

(113) Each flow rate (ml/min) for each gas were shown as follows.

(114) N.sub.2:H.sub.2:Ar=5:15:0

(115) N.sub.2:H.sub.2:Ar=10:30:0

(116) N.sub.2:H.sub.2:Ar=15:45:0

(117) N.sub.2:H.sub.2:Ar=20:60:0

(118) (SEM, XRD)

(119) Crystal structure of the catalytic material was measured using an X-rays diffractometcr (XRD, BRUKER Co., D8 ADVANCE).

(120) X-ray source: Cu K ray (=0.15418 nm, 40 kV, 50 mA)

(121) Surface structure of the catalyst material was measured using a scanning electron microscopy (SEM, JEOL Ltd., JSM-7600F).

Example 1

(122) <Synthesis of YRu.sub.2 Fine Powder>

(123) Commercial yttrium (Y) was weighed, and then in accordance with the weight of Y, commercial Ru metal was weighted so as that a molar ratio of Y and Ru was 1:2. Then a melt of YRu.sub.2 was obtained by melting the mixture of Y and Ru by using an are melting method using an are melting apparatus. Then, an YRu.sub.2 fine powder was obtained by cooling the melt to ambient temperature and crushing the resulting YRu.sub.2 mass in a mortar.

(124) <Hydrogen Adsorption-Desorption Treatment of the YRu.sub.2 Fine Powder>

(125) The YRu.sub.2 fine powder absorbed hydrogen by raising the temperature at 5 C./min from room temperature to 500 C. under hydrogen partial pressure of 10 atm or less, and maintaining the temperature for 1.5 hours, and then cooling it to normal temperature. Then, the resulting YRu.sub.2 fine powder absorbing hydrogen desorbed hydrogen by raising the temperature to 300 C. at 30 C./min from room temperature under vacuum, and then cooling it to normal temperature. As a result, a product of YRu.sub.2 fine powder after hydrogen adsorption-desorption treatment was obtained.

(126) As shown in FIGS. 3A and B, in which measurement results of SEM before and after hydrogen adsorption-desorption treatment were shown respectively, by analyzing the SEM photographs, change in crystal structure was not observed and it was confirmed that fine power having a size of several 10 m contained the crack as shown by the arrow in FIG. 3B. That is, the surface area of YRu.sub.2 fine power was increased.

(127) Further, as shown in FIG. 2, by evaluating change of line width of the diffraction line of powdered XRD, it was observed that by the hydrogen adsorption-desorption treatment, crystallite size was miniaturized from 250 nm, before the hydrogen adsorption-desorption treatment, to 20 nm, after the treatment.

Example 2

(128) <Ammonia Synthesis Reaction using the YRu.sub.2 Fine Power as a Catalyst after Hydrogen Adsorption-Desorption Treatment>

(129) The reaction was carried out to produce ammonia gas (NH.sub.3) by passing nitrogen gas (N.sub.2) and hydrogen gas (H.sub.2) into contact with the catalyst. An YRu.sub.2 fine powder was produced according to the method described in Example 1. After the ball milling, YRu.sub.2 fine powder was treated by hydrogen adsorption-desorption treatment according to the method described in Example 1. The resulting YRu.sub.2 fine powder 0.03 g was packed into a glass tube and was used as the catalyst in a fixed bed flow type system. The ammonia synthesis reaction was carried out by pass nitrogen gas and hydrogen gas into contact with the resulting YRu.sub.2 fine powder provided in the reactor.

(130) The ammonia synthesis reaction was carried out under a reaction temperature of 400 C.; at a flow rate of N.sub.2:15 ml/min, H.sub.2:45 nil/min, and a total of 60 ml/min; and at reaction pressure of atmospheric pressure. The gas coming out from the reactor was bubbled in 0.005M aqueous solution of sulfuric acid, and the resulting ammonia was dissolved in an aqueous sulfuric solution. The generated ammonium ions was determined by ion chromatography. The reaction rate of the ammonia synthesis reaction at 400 C. is 871 molg.sup.1h.sup.1. A specific surface area of the YRu.sub.2 fine power catalyst after reaction was 1.8 m.sup.2/g by BET specific surface area measurement. The results are shown in Table 1.

Example 3

(131) <High Activity of the YRu.sub.2 Catalyst after Hydrogen Adsorption-Desorption Treatment by Repeating the Ammonia Synthesis Reaction and Cooling to Room Temperature>

(132) After an ammonia synthesis reaction was carried out under the same reaction conditions as in Example 2 by using YRu.sub.2 fine power after hydrogen adsorption-desorption treatment described in Example 1 as a catalyst, the reaction temperature was lowered to room temperature. It was subsequently performed second ammonia synthesis reaction under the same reaction conditions as in Example 2 again. The reaction rate of the first ammonia synthesis reaction was 893 molg.sup.1h.sup.1, whereas the reaction rate of the second ammonia synthesis reaction increased to 1187 molg.sup.1h.sup.1. That is, the activity of catalyst per mass was increased.

Example 4

(133) <Measurement of Reaction Orders of H.sub.2, N.sub.2, and NH.sub.3 when using the YRu.sub.2 Fine Powder after Hydrogen Adsorption-Desorption Treatment as Catalyst>

(134) Using the same catalyst as that used in Example 2, an ammonia synthesis reaction was carried out in the same manner as Example 2 except that gas flow rates (ml/min) of N.sub.2 and H.sub.2 were varied in 5 ml/min to 60 ml/min.

(135) By using the calculation formula, a reaction order of nitrogen of 0.94, a reaction order of hydrogen of 0.81, and a reaction order of ammonia of 0.73 were obtained. The results are shown in Table 2.

Example 5

(136) <Pressure Dependence in the Ammonia Synthesis Reaction using YRu.sub.2 Fine Powder after Hydrogen Adsorption-Desorption as a Catalyst>

(137) At the reaction temperature of 380 C., ammonia synthesis reactions were carried out in the same manner as Example 2 except that the reaction pressures were 0.1 MPa, 0.4 MPa, 0.6 MPa, and 0.8 MPa.

(138) The reaction rates of the ammonia synthesis at 380 C. and at the pressure: 0.1 MPa, 0.4 MPa, 0.6 MPa and 0.8 MPa were 453 molg.sup.1h.sup.1, 713 molg.sup.1h.sup.1, 789 molg.sup.1h.sup.1, and 914 molg.sup.1h.sup.1, respectively. The results are shown in FIG. 1. The pressure dependence is observed in the ammonia synthesis reaction using the catalyst of the present invention, the reaction rate obtained at 0.8 MPa was twice the reaction rate obtained at 0.1 MPa.

Comparative Example 1

(139) <Ammonia Synthesis Reaction using YRu.sub.2 Fine Powder Catalyst without Hydrogen Adsorption-Desorption Treatment >

(140) Ammonia synthesis reaction was carried out in the same manner as in Example 2, by using YRu.sub.2 fine powder catalyst without hydrogen adsorption-desorption treatment described in Example 1 as a catalyst. Reaction rate of ammonia synthesis at 400 C. was 233 molg.sup.1h.sup.1, the catalyst activity was about of Example 2. The results are shown in Table 1. Specific surface area of YRu.sub.2 fine powder after reaction could not be measured due to very small.

Comparative Example 2

(141) <Ammonia Synthesis Reaction using Ru Metal Catalyst>

(142) Ammonia synthesis reaction was carried out in the same manner as in Example 2, except that Ru metal (Kojundo Chemical Laboratory Co., 3N, using powdered) was used as a catalyst and the amount of the catalyst was 0.5 g. The reaction rate of the ammonia synthesis reaction at 400 C. was 12 molg.sup.1h.sup.1. The results are shown in Table 1. The specific surface area of the Ru metal after reaction by the BET specific surface area measurement was 3.0 m.sub.2/g.

Comaprative Example 3

(143) <Reaction Orders of H.sub.2, N.sub.2, NH.sub.3 using Ru Metal Catalyst>

(144) The reaction order of the ammonia synthesis reaction was determined in the same manner as in Example 4, except that the metal Ru 0.5 g was used as a catalyst and the reaction temperature was 450 C.

(145) Reaction order of nitrogen was 1.01, reaction order of hydrogen was 0.49, and reaction order of ammonia was 0.09. The results are shown in Table 2.

Comparative Example 4

(146) <Ammonia Synthesis using Ru Metal Catalyst in Pressurized Conditions>

(147) Ammonia synthesis reaction was carried out in the same manner as in Example 5, except that the Ru metal 0.5 g was used as a catalyst and the reaction temperature was 450 C. The reaction rate of the ammonia synthesis at 450 C., under pressure: 0.1 MPa, 0.4 MPa, 0.6 MPa, and 0.8 MPa were 46 molg.sup.1h.sup.1, 54 molg.sup.1h.sup.1, 61 molg.sup.1h.sup.1, 64 molg.sup.1h.sup.1, respectively. The reaction rates obtained at 0.8 MPa was 1.4 times higher than the reaction rate obtained at 0.1 MPa. The results are shown in FIG. 1.

(148) [Comparison of Reaction Rates of Ammonia Synthesis]

(149) From the measurement results of reaction rates and the catalyst surface area after the ammonia synthesis shown in Table 1, although YRu.sub.2 had a specific surface area smaller than the Ru metal, the catalytic activity of YRu.sub.2 is approximately 73 times per mass higher than Ru metal, and catalytic activity of YRu.sub.2 per catalyst surface area after the reaction represented by the reaction rate of the ammonia synthesis per catalyst surface area after the reaction is about 130 times higher than that of Ru metal.

(150) TABLE-US-00001 TABLE 1 (i) Surface Area (ii) Reaction rate (ii)/(i) after reaction of ammonia (m.sup.2/g)/ Catalyst (m.sup.2/g) (molg.sup.1h.sup.1) (molg.sup.1h.sup.1) Example 2 YRu.sub.2 (after hydrogen 1.8 871 495 adsorption-desorption treatment) Comparative YRu.sub.2 (without Cannot be 233 Example 1 treatment) measured Comparative Metal Ru 3.0 12 4 Example 2 **Reaction Temperature: 400 C., Reaction Pressure: 0.1 MPa, Flow Rate: 60 mL/min, N.sub.2:H.sub.2 = 1:3(v/v)

(151) [Comparison of the Reaction Orders of H.sub.2, N.sub.2, NH.sub.3]

(152) As shown in Table 2, while the Ru metal catalyst had a negative value of the reaction orders of 0.49 hydrogen, YRu.sub.2 catalyst had a positive value of 0.81. As the results shown, no hydrogen poisoning occurred.

(153) TABLE-US-00002 TABLE 2 Reaction temperature Reaction Reaction Reaction Catalyst ( C.) order of N.sub.2 order of H.sub.2 order of NH.sub.3 Example 4 YRu.sub.2 (after 400 0.94 0.81 0.73 absorbtion- desorbtion treatment) Comparative Metal Ru 450 1.01 0.49 0.09 Example 2 **Reaction Pressure: 0.1 MPa,

(154) [Comparison of Reaction Rate of Ammonia Synthesis in High Pressure Conditions]

(155) As shown in FIG. 1, while reaction rate of ammonia synthesis in high pressure condition of 0.8 MPa was twice larger that that of 0.1 MPa when YRu.sub.2 catalyst was used, the reaction rate of 0.8 MPa was 1.4 times larger than that of 0.1 MPa when Ru metal catalyst was used. YRu.sub.2 catalyst has a high pressure dependence of the activity compared to Ru metal catalyst. The results show that the value of the reaction order of the hydrogen was positive when YRu.sub.2 catalyst was used.

Example 6

(156) <Durability of YRu.sub.2 Fine Powder Catalyst>

(157) Ammonia synthesis reaction was carried out by using YRu.sub.2 fine powder catalyst in the same manner as in Example 5, except that a reaction pressure was 0.8 MPa. As shown in FIG. 4, although up to about the first 20 hours decreased activity is observed, thereafter almost the same value of about 80% of the initial activity was shown. For example, catalytic activity measured after 23 hours is 771 molg.sup.1h.sup.1, and catalytic activity measured after 48 hours is 750 molg.sup.1h.sup.1. XRD was measured after 24 hours and after 49 hours, as shown in FIG. 5, since peak intensity almost did not change, YRu.sub.2 fine powder maintained a crystal structure without separation between the Y and Ru.

(158) The intermetallic compound of the present invention was found to have a hydrogen poisoning-resistant when used as an ammonia synthesis catalyst according to the comparison between Example 4 and Comparative Example 3. Therefore, it is possible to perform efficient ammonia synthesis at a high hydrogen partial pressure.

(159) Since the intermetallic compound of the present invention can absorb and desorb hydrogen easily, hydrogen poisoning can be prevented in the synthesis reaction. By changing Ru metal to a Laves phase intermetallie compound of ARu.sub.2 having hydrogen storage characteristics, ammonia synthesis activity using the intermetallic compound catalysts of the present invention are greatly improved compared to that using Ru single metal catalysts. Further, the kinetic analysis of the reaction rates of the ammonia synthesis, the catalyst used in the ammonia synthesis method of the present invention is different from the Ru metal catalyst. Since the reaction order of hydrogen is positive, it is possible to improve activity and have a higher reaction rate of ammonia synthesis reaction in the conditions of higher hydrogen partial pressure. Further, since the intermetallic compound has the property of absorbing hydrogen, the hydrogen atoms present excessively on the surface can be moved to the inside of the crystal. As a result, hydrogen poisoning can be suppressed, and activity becomes higher.

INDUSTRIAL APPLICABILITY

(160) When the intermetallic compound of the present invention is used as a catalyst, it is possible to obtain a high yield without causing hydrogen poisoning, particularly in the ammonia synthesis reaction. In order not to cause hydrogen poisoning even in the high hydrogen pressure conditions, it is possible to promote the further reaction in the high-pressure conditions, an ammonia reaction synthesis in the high-pressure conditions can be performed with higher efficiency.

(161) Further, it is possible to reduce the use ratio of a noble metal such as expensive Ru, and it is possible to inexpensively produce ammonia.