Ammonia synthesis catalyst, method for producing the same, and method for synthesizing ammonia using the same

Abstract

To provide an ammonia synthesis catalyst excellent in ammonia synthesis activity, and capable of more efficiently synthesizing ammonia.SOLUTION: An ammonia synthesis catalyst comprises: a CeR-containing oxide support containing cerium (Ce) and at least one element selected from the group consisting of rare earth elements (R) excluding Ce; and ruthenium (Ru) supported on the CeR-containing oxide support, and after the CeR-containing oxide support is fired at 700°C for 5 hours, when a pore volume x (cm3 g-1) of pores with a pore diameter of 2 nm or more and less than 16 nm, and the pore volume y (cm3 g-1) of pores with a pore diameter of 16 nm or more and 200 nm or less are identified by a BJH method, the values of x and y satisfy the condition (A): x≥0.005, the condition (B): y≥0.050, and condition (C):5x+y≥0.290.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to an ammonia synthesis catalyst, a method for producing the same, and a method for synthesizing ammonia using the same. [Background technology]

[0002] In recent years, ammonia has attracted attention as a component that can be applied to applications such as hydrogen energy carriers. While the Haber-Bosch process, using iron-based catalysts, has traditionally been used industrially to synthesize ammonia, recent research has focused on developing various types of ammonia synthesis catalysts with the aim of synthesizing ammonia under milder conditions than the Haber-Bosch process.

[0003] For example, Yuta Ogura et al., “Efficient ammonia synthesis over a Ru / La 0.5 Ce 0.5 O 1.75 "Catalyst pre-reduced at high temperature", Chemical Science, 2018, Vol. 9, pp. 2230-2237 (Non-patent Literature 1) describes an ammonia synthesis catalyst (Ru / La) in which ruthenium is supported on a ceria-lanthanum oxide complex oxide. 0.5 Ce 0.5 O 1.75 ) has been disclosed. In addition, Wenfeng Han et al., “La2Ce2O7 supported ruthenium as a robust catalyst for ammonia synthesis”, Journal of Rare Earths, 2019, Vol. 37, pp. 492-499 (Non-Patent Literature 2) discloses an ammonia synthesis catalyst (Ru / La2Ce2O7) in which ruthenium is supported on La2Ce2O7.

[0004] However, ammonia synthesis catalysts described in Non-Patent Documents 1 and 2 did not exhibit sufficient ammonia synthesis activity. Therefore, in the field of ammonia synthesis catalysts, there is a need for catalysts with higher ammonia synthesis activity. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Yuta Ogura et al., “Efficient ammonia synthesis over a Ru / La0.5Ce0.5O1.75 catalyst pre-reduced at high temperature”, Chemical Science, 2018, Vol. 9, pp. 2230-2237. [Non-Patent Document 2] Wenfeng Han et al., “La2Ce2O7 supported ruthenium as a robust catalyst for ammonia synthesis”, Journal of Rare Earths, 2019, Vol. 37, pp. 492-499 [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention has been made in view of the problems of the prior art, and aims to provide an ammonia synthesis catalyst that has excellent ammonia synthesis activity and enables more efficient ammonia synthesis, a method for producing the ammonia synthesis catalyst that can efficiently produce the ammonia synthesis catalyst, and a method for synthesizing ammonia using the ammonia synthesis catalyst. [Means for solving the problem]

[0007] As a result of intensive studies to achieve the above object, the inventors of the present invention have found that an ammonia synthesis catalyst contains a CeR-containing oxide carrier containing cerium (Ce) and at least one element selected from the group consisting of rare earth elements (R) excluding Ce; and ruthenium (Ru) supported on the CeR-containing oxide carrier. After the CeR-containing oxide carrier is calcined in air at 700 °C for 5 hours, when the pore volume x (cm 3 g -1 ) of pores having a pore diameter of 2 nm or more and less than 16 nm and the pore volume y (cm 3 g -1 ) of pores having a pore diameter of 16 nm or more and 200 nm or less are identified by the BJH method, the values of x and y satisfy the following conditions (A) to (C), and the catalyst has excellent ammonia synthesis activity and can synthesize ammonia more efficiently. Based on this finding, the present invention has been completed.

[0008] That is, the ammonia synthesis catalyst of the present invention is composed of cerium (Ce) and at least one element selected from the group consisting of rare earth elements (R) excluding Ce , and at least one additive metal element selected from the group consisting of Group 14 elements. and a CeR-containing oxide carrier, ruthenium (Ru) supported on the CeR-containing oxide carrier, and contains when the CeR-containing oxide carrier is calcined in air at 700 °C for 5 hours, and the pore volume x (cm 3 g -1 ) of pores having a pore diameter of 2 nm or more and less than 16 nm and the pore volume y (cm 3 g -1 ) of pores having a pore diameter of 16 nm or more and 200 nm or less are identified by the BJH method, the values of x and y satisfy the following conditions (A) to (C): [Condition (A)] x ≧ 0.005 [Condition (B)] y ≧ 0.050 [Condition (C)] 5x + y ≧ 0.290 It is characterized by satisfying the above conditions.

[0010] Furthermore, in the ammonia synthesis catalyst of the present invention, it is preferable that the rare earth element (R) other than Ce is at least one element selected from the group consisting of lanthanum (La) and praseodymium (Pr).

[0011] The method for producing the ammonia synthesis catalyst of the present invention is as follows: A step of obtaining a precursor powder containing Ce and at least one element selected from the group consisting of Ce and rare earth elements (R) other than Ce by complex polymerization using cerium nitrate as a raw material, A step of obtaining a CeR-containing oxide support by calcining the aforementioned precursor powder, Ruthenium (Ru) is supported on the CeR-containing oxide support. te A The process of obtaining a monia synthesis catalyst, Includes fruit, The aforementioned complex polymerization method utilizes a salt of at least one additive metal element selected from the group consisting of Group 14 elements, The ammonia synthesis catalyst of the present invention is obtained as the ammonia synthesis catalyst of the aforementioned ammonia synthesis catalyst. This method is characterized by the following:

[0012] Furthermore, in the method for producing the ammonia synthesis catalyst of the present invention, it is preferable to further add ethylene glycol to the complex solution in the complex polymerization method.

[0013] The present invention relates to a method for synthesizing ammonia, characterized by contacting the ammonia synthesis catalyst of the present invention with a gas containing hydrogen and nitrogen to synthesize ammonia.

[0014] Although the reason why the above objective is achieved by the ammonia synthesis catalyst of the present invention is not entirely clear, the inventors surmise the following. First, the ammonia synthesis catalyst of the present invention utilizes a CeR-containing oxide support containing cerium (Ce) and at least one element selected from the group consisting of rare earth elements (R) other than Ce. In the present invention, such a CeR-containing oxide support is used that satisfies the above conditions (A) to (C). It is thought that a support that satisfies such conditions (A) to (C) has a good balance of "pore volume of pores with a pore diameter of 2 nm or more and less than 16 nm" and "pore volume of pores with a pore diameter of 16 nm or more and 200 nm or less" in amounts above a certain level, and the inventors surmise that this allows for more efficient synthesis of ammonia. To explain this point more specifically, it is presumed that pores with a pore diameter of 2 nm or more and less than 16 nm serve as the reaction site for ammonia synthesis. Furthermore, pores with a diameter of 16 nm to 200 nm are presumed to primarily function as gas flow pathways capable of promoting the diffusion of reaction gases (raw material gases) during ammonia synthesis. Therefore, by creating a support that has a balanced ratio of "pore volume of pores with a diameter of 2 nm to less than 16 nm" and "pore volume of pores with a diameter of 16 nm to 200 nm," the proportion of pores with a diameter of 2 nm to less than 16 nm, which serve as the reaction site for ammonia synthesis, will be sufficient, as will the proportion of pores with a diameter of 16 nm to 200 nm, which promote the diffusion of reaction gases (raw material gases). Therefore, the inventors presum that when such a support is used, a sufficient amount of reaction site exists in the catalyst, and the reaction gases (raw material gases) can be more efficiently circulated to that reaction site, thereby improving ammonia synthesis activity (production activity). Furthermore, since the CeR-containing oxide support is composed of a composite oxide containing Ce and R (rare earth elements excluding Ce), the influence of R makes Ce more easily reduced (Ce 4+ →Ce 3+) Therefore, it becomes possible to more efficiently donate electrons from CeO2 in the support to the active species Ru, and with a CeR-containing oxide support, the active species Ru (Ru supported on the support) can be made more efficiently active. Thus, in the present invention, a CeR-containing oxide support is used in which the "pore volume of pores with a pore diameter of 2 nm or more and less than 16 nm" and the "pore volume of pores with a pore diameter of 16 nm or more and 200 nm or less" are each set to a certain amount or more. The inventors speculate that this allows for efficient use of the reaction field, and that the active species Ru can be made more efficiently active based on the type of element in the CeR-containing oxide support, resulting in higher ammonia synthesis activity. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide an ammonia synthesis catalyst that exhibits excellent ammonia synthesis activity and enables more efficient ammonia synthesis, a method for producing the ammonia synthesis catalyst that can efficiently produce the ammonia synthesis catalyst, and a method for synthesizing ammonia using the ammonia synthesis catalyst. [Brief explanation of the drawing]

[0016] [Figure 1] This graph shows the relationship between the pore volume x of pores with a diameter of 2 nm or more and less than 16 nm, and the pore volume y of pores with a diameter of 16 nm or more and less than 200 nm. [Figure 2] This graph shows the ammonia synthesis rate (unit: mmol / g·h) per gram of catalyst obtained in Example 1, Reference Examples 1-4, and Comparative Examples 1-4. [Modes for carrying out the invention]

[0017] The present invention will be described in detail below with reference to its preferred embodiments.

[0018] [Ammonia synthesis catalyst] The ammonia synthesis catalyst of the present invention is A CeR-containing oxide support comprising cerium (Ce) and at least one element selected from the group consisting of rare earth elements (R) other than Ce, Ruthenium (Ru) supported on the aforementioned CeR-containing oxide support, It contains, After firing the CeR-containing oxide support in air at 700°C for 5 hours, the pore volume x (cm³) of pores with a diameter of 2 nm or more and less than 16 nm is obtained. 3 g -1 ) and the pore volume y (cm³) of pores with a diameter of 16 nm to 200 nm. 3 g -1 When identified by the BJH method, the values ​​of x and y are given by the following conditions (A) to (C): [Condition (A)] x≧0.005 [Condition (B)] y≧0.050 [Condition (C)] 5x+y≧0.290 It is characterized by satisfying the following conditions.

[0019] The CeR-containing oxide support according to the present invention contains Ce and at least one element selected from the group consisting of rare earth elements (R) other than Ce. Examples of such rare earth elements R other than Ce include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and the like. The rare earth element R other than Ce may be used alone or in combination of two or more elements. Furthermore, from the viewpoint of further improving the ammonia synthesis activity in the resulting ammonia synthesis catalyst, La and Pr are preferred as the rare earth element R other than Ce, and La is more preferred.

[0020] The molar ratio of Ce to R in such a CeR-containing oxide support ([moles of Ce]:[total molars of R]) is preferably 1:9 to 9:1 (more preferably 3:7 to 7:3). When the molar ratio is within this range, based on that ratio, Ce is more easily reduced (Ce 4+ →Ce 3+ ) and Ce 3+ The electron-donating ability from to Ru is improved, and this tends to lead to improved ammonia synthesis activity.

[0021] Furthermore, from the viewpoint of increasing the proportion of components that contribute to ammonia synthesis activity, the total amount of Ce and R in the CeR-containing oxide support is preferably 70 mol% or more (more preferably 80-100 mol%, and even more preferably 90-100 mol%) relative to the total amount of all metal elements in the support.

[0022] Furthermore, the CeR-containing oxide support preferably further contains at least one additive metal element selected from the group consisting of Group 4 and Group 14 elements. Thus, the CeR-containing oxide support preferably contains Group 4 elements of the periodic table such as Ti, Zr, and Hf, and Group 14 elements of the periodic table such as Si, Ge, and Sn as additive metal elements. These additive metal elements may be used individually or in combination of two or more. Among these additive metal elements, Ti, Zr, Al, and Si are more preferred from the viewpoint that the cations are stable in a reducing atmosphere, and Ti and Si are even more preferred. Furthermore, as such additive metal elements, Group 14 elements are more preferred from the viewpoint that it is possible to maintain a larger pore volume for pores with a diameter of 2 nm or more and less than 16 nm during the preparation of the support, and that it is easier to obtain a support with a larger pore volume for pores with a diameter of 2 nm or more and less than 16 nm. Among these, Si is particularly preferred. Furthermore, if the carrier contains the added metal element, the added metal element makes the Ce in the carrier more easily reduced (Ce 4+ →Ce 3+ This tends to result in higher ammonia synthesis activity.

[0023] When the CeR-containing oxide support contains the added metal element, the total amount of Ce and the rare earth element R other than Ce is preferably 99 to 70 mol%, and more preferably 95 to 80 mol%, relative to the total molar amount of Ce, the rare earth element R other than Ce, and the added metal element. Furthermore, when the CeR-containing oxide support contains the added metal element, the content of the added metal element is preferably 1 to 30 mol% (more preferably 5 to 20 mol%) relative to the total molar amount of Ce, the rare earth element R other than Ce, and the added metal element. When the content of such an added metal element is above the lower limit, the Ce in the support is more easily reduced compared to when it is below the lower limit, and ammonia synthesis activity tends to improve further. On the other hand, when it is below the upper limit, the basic components (Ce, R) in the support increase compared to when it exceeds the upper limit, and ammonia synthesis activity tends to improve further.

[0024] Furthermore, the CeR-containing oxide support is defined by the following formula (I): Ce a R b A 1-a-b O c (I) (In formula (I) above, R represents a rare earth element other than Ce, A represents the added metal element, a represents the mole fraction of Ce with respect to the total amount of Ce, the rare earth element other than Ce, and the added metal element, b represents the mole fraction of the rare earth element other than Ce with respect to the total amount of Ce, the lanthanoid other than Ce, and the added metal element, 1-ab represents the mole fraction of the added metal element with respect to the total amount of Ce, the lanthanoid other than Ce, and the added metal element (if the carrier does not contain the added metal element, 1-ab is 0), and the value of c is a value uniquely determined from the composition and valency of the cations (Ce, the lanthanoid other than Ce, and the added metal element), and is between 1.5 and 2.0.) It is preferable that the composition has the form represented by .

[0025] Furthermore, in a CeR-containing oxide support having a composition represented by formula (I), from the viewpoint of obtaining higher ammonia synthesis activity, the value of a (mole fraction of Ce) is preferably 0.2 to 0.8 (more preferably 0.3 to 0.7), and the value of b (mole fraction of R) is preferably 0.2 to 0.8 (more preferably 0.3 to 0.7). Also, in formula (I), if 1-ab is not 0 (when the support contains the aforementioned additive metal element), the value of 1-ab (mole fraction of the aforementioned additive metal element) is preferably 0.01 to 0.3 (more preferably 0.05 to 0.2) (however, 1-ab may be 0). When the values ​​of a and b are within the above ranges, the ammonia synthesis activity tends to improve further.

[0026] Furthermore, the CeR-containing oxide support was fired in air at 700°C for 5 hours, and the pore volume x (cm³) of pores with a diameter of 2 nm or more and less than 16 nm was obtained. 3 g -1 ) and the pore volume y (cm³) of pores with a diameter of 16 nm to 200 nm. 3 g -1 When identified by the BJH method, the values ​​of x and y are given by the following conditions (A) to (C): [Condition (A)] x≧0.005 [Condition (B)] y≧0.050 [Condition (C)] 5x+y≧0.290 It must satisfy the following conditions.

[0027] In this invention, the condition "fired in air at 700°C for 5 hours" for the support used as a measurement sample when identifying (measuring) pore volumes x and y is considered to be satisfied if the support is subjected to such firing conditions at any stage during or after its manufacture (any stage before measurement). In other words, if the support is fired in air at 700°C for 5 hours at any stage before measurement (including during the manufacturing of the support), it can be used as a measurement sample when identifying (measuring) pore volumes x and y, as a support "fired in air at 700°C for 5 hours". For example, if the carrier (or precursor powder) is obtained by first firing it at a temperature below 700°C during the manufacturing process, and then the carrier is further fired continuously at 700°C in air for 5 hours or more to finally produce the carrier, then at the stage where the final carrier is obtained, it has been fired at 700°C in air for at least 5 hours. Therefore, the carrier can be considered as having been "fired at 700°C in air for 5 hours" and can be used as is as a sample (object of measurement) for measuring pore volume x and y. Furthermore, if firing at 700°C in air for 5 hours or more is not performed at any stage between the manufacturing stage of the carrier and the measurement of pore volume x and y, then a separate treatment (pre-measurement treatment) of firing at 700°C in air for 5 hours or more should be performed before measuring pore volume x and y, and the carrier after pre-measurement treatment should be used as a measurement sample to measure pore volume x and y. Thus, the pore volumes x and y can be identified by the BJH method using a carrier that has been calcined in air at 700°C for 5 hours continuously as the measurement sample (target of measurement) at any stage before measurement, including during the carrier's manufacture.

[0028] Furthermore, when identifying pore volumes x and y using the BJH (Barrett Joyner Hallenda) method, the support material (the sample to be measured, calcined in air at 700°C for 5 hours) is pretreated by standing it in a vacuum at a temperature of 300°C for 1 hour. After this pretreated support material is used as the measurement sample, nitrogen adsorption and desorption isotherms are determined under adsorption temperature conditions of -196°C, and the amount of nitrogen adsorbed under different relative pressures is measured and converted to pore volume. That is, the pore volume x (cm³) of pores with a diameter of 2 nm or more and less than 16 nm is determined.3 g -1 ) and the pore volume y (cm³) of pores with a diameter of 16 nm to 200 nm. 3 g -1 The pore volume can be determined by employing the BJH method, converting the relative pressure (P / P0) to pore diameter based on the nitrogen adsorption / desorption isotherm obtained as described above, determining the amount of nitrogen adsorbed within the range of pore diameters (ranges of different relative pressures), and then converting that to pore volume. Here, "pore diameter" refers to the diameter of the pore. The conditions (A) to (C) concerning pore volumes x and y will be explained in more detail below.

[0029] The CeR-containing oxide support is calcined in air at 700°C for 5 hours, and then the pore volume x (unit: cm) of pores with a pore diameter of 2 nm or more and less than 16 nm is identified by the BJH method. 3 g -1 The condition (A) that x is 0.005 or greater (x ≥ 0.005) must be met. In this way, by satisfying condition (A) that x is 0.005 or greater, the proportion of "pores with a diameter of 2 nm or more and less than 16 nm" that are considered to function as a reaction field becomes sufficient compared to the case where x is less than 0.005, making it possible to carry out the ammonia synthesis reaction efficiently. Such a value of x (unit: cm) 3 g -1 As for the same reason, a value of 0.010 or higher is more preferable, and a value of 0.020 or higher is particularly preferable, as it yields a higher effect from a similar perspective. There is no particular upper limit to the value of x, but it is preferable to be around 0.1.

[0030] Furthermore, the CeR-containing oxide support is calcined in air at 700°C for 5 hours, and then identified by the BJH method as having a pore volume y (unit: cm) of pores with a diameter of 16 nm to 200 nm. 3 g -1The condition (B) that y is 0.050 or greater (y≧0.050) must be met. By satisfying condition (B) that y is 0.050 or greater, the diffusion of the reaction gas can be sufficiently achieved compared to the case where y is less than 0.050, and the reaction gas can be efficiently circulated in the reaction field, thus enabling the ammonia synthesis reaction to proceed efficiently. 3 g -1 As for the same reason, a value of 0.070 or higher is more preferable, and a value of 0.090 or higher is particularly preferable, as it yields a higher effect from a similar perspective. There is no particular upper limit to the value of y, but it is preferably around 0.3.

[0031] Furthermore, the CeR-containing oxide support is calcined in air at 700°C for 5 hours, and the pore volume x (cm³) is identified by the BJH method. 3 g -1 ) and the pore volume y(cm 3 g -1 Regarding ), the sum of 5x and y (unit: cm) 3 g -1 The sum of 5x and y (5x+y≧0.290) must satisfy condition (C). By satisfying this condition that the sum of 5x and y (5x+y) is 0.290 or greater, the ratio of pores with a diameter of 2 nm or more and less than 16 nm to pores with a diameter of 16 nm or more and less than 200 nm becomes more balanced, and when the reaction gas is passed through, there is a sufficient amount of reaction sites and the diffusion rate of the reaction gas is sufficiently high, making it possible to carry out the ammonia synthesis reaction efficiently. 3 g -1As for the same reason, a value of 0.300 or higher is more preferable, and a value of 0.350 or higher is particularly preferable, as a higher effect can be obtained from the same viewpoint. There is no particular upper limit to the value of the sum of 5x and y (5x+y), but it is preferably around 0.8. Thus, in the present invention, by using a CeR-containing oxide support that satisfies the above conditions (A) to (C) as a support, it is possible to carry out the ammonia synthesis reaction more efficiently.

[0032] Furthermore, the CeR-containing oxide support may contain Ce and at least one element selected from the rare earth elements R other than Ce, and satisfy all of the above conditions (A) to (C). Within a range that does not impair the effects of the present invention, it may further contain Ce, R, and other metal elements other than the aforementioned additive metal elements, either individually or in combination of two or more. Such other metal elements are not particularly limited as long as they are metal elements used in ammonia synthesis catalysts, and examples include manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), etc. When the CeR-containing oxide support contains such other metal elements, the content is preferably 5 mol% or less, more preferably 1 mol% or less, and particularly preferably 0.1 mol% or less, relative to the total amount of all metal elements in the support. If the content of the other metal elements exceeds the above upper limit, the dissociation of N2 on Ru is inhibited, and the ammonia synthesis activity tends to decrease.

[0033] Furthermore, there are no particular limitations on the shape of the CeR-containing oxide support according to the present invention, and examples include ring-shaped, spherical, cylindrical, particulate, and pellet-shaped. Of these shapes, particulate is preferred from the viewpoint of being able to support a larger amount of Ru in a highly dispersible state. In addition, when the CeR-containing oxide support is in particulate form, the average particle size of the support is preferably 0.1 to 100 μm.

[0034] Furthermore, the ammonia synthesis catalyst of the present invention has Ru supported on the CeR-containing oxide support. There are no particular restrictions on the amount of Ru supported, but it is more preferable to have 0.5 to 10 parts by mass (more preferably 1 to 5 parts by mass) per 100 parts by mass of the CeR-containing oxide support. If the amount of Ru supported falls below the lower limit, the ammonia synthesis activity tends to decrease, while if it exceeds the upper limit, depending on the usage environment, sintering of Ru is likely to occur, which reduces the dispersion of Ru, which is the active site, making it difficult to obtain an effect commensurate with the amount of Ru supported, and thus tends to be disadvantageous in terms of cost, etc.

[0035] Furthermore, there are no particular restrictions on the average particle size of Ru supported on the CeR-containing oxide support, but it is preferably 0.5 to 100 nm, and more preferably 1 to 50 nm. If the average particle size of Ru falls below the lower limit, it tends to become difficult to utilize Ru in a metallic state, while if it exceeds the upper limit, the amount of active sites as a catalyst tends to decrease significantly.

[0036] The ammonia synthesis catalyst of the present invention is not particularly limited in form, and examples include a honeycomb-shaped monolithic catalyst and a pellet-shaped pellet catalyst. Alternatively, the powdered ammonia synthesis catalyst may be used by simply placing it at the desired location.

[0037] Furthermore, as a method for producing such an ammonia synthesis catalyst of the present invention, the method for producing the ammonia synthesis catalyst of the present invention described below can be suitably employed.

[0038] [Method for producing ammonia synthesis catalyst] The method for producing the ammonia synthesis catalyst of the present invention is as follows: A step (hereinafter referred to as the "first step" for convenience) is to obtain a precursor powder containing Ce and at least one element selected from the group consisting of Ce and rare earth elements (R) other than Ce by complex polymerization using cerium nitrate as a raw material. The process involves obtaining a CeR-containing oxide support by calcining the aforementioned precursor powder (hereinafter, for convenience, this will sometimes be simply referred to as the "second step"), The process of supporting ruthenium (Ru) on the CeR-containing oxide support to obtain the ammonia synthesis catalyst of the present invention (hereinafter, for convenience, may be simply referred to as the "third step"), This method is characterized by including [the specified element]. Herein, the "CeR-containing oxide support" obtained in the second step is the same as the "CeR-containing oxide support" described in the ammonia synthesis catalyst of the present invention (and the preferred one is also the same). The first to third steps will be described separately below.

[0039] <First step: Step to obtain precursor powder> In the present invention, in the first step, cerium nitrate is used as a cerium (Ce) raw material, and a precursor powder containing Ce and at least one element selected from the group consisting of Ce and rare earth elements (R) other than Ce is obtained by complex polymerization.

[0040] Such a complex polymerization method can be any method that uses cerium nitrate as a cerium (Ce) raw material to obtain a precursor powder containing Ce and at least one element selected from the group consisting of Ce and rare earth elements (R) other than Ce. It is not particularly limited, and known complex polymerization methods (and their manufacturing conditions, etc.) can be appropriately adopted, except for the use of cerium nitrate as a cerium (Ce) raw material. In this invention, a complex polymerization method using cerium nitrate as a cerium (Ce) raw material is adopted. By using cerium nitrate as a cerium (Ce) raw material, the formation of the desired complex becomes easier in the complex polymerization method compared to when other Ce raw materials are used, and as a result, it becomes possible to produce a CeR-containing oxide support that satisfies the above conditions (A) to (C).

[0041] Furthermore, a preferred method for the "first step" employing such a complex polymerization method is, for example, a method (X) in which cerium nitrate is used as a cerium (Ce) raw material to obtain a complex solution containing cerium nitrate, a salt of at least one element selected from the group consisting of rare earth elements (R) other than Ce, and an oxycarboxylic acid or phosphoric acid, and then the obtained composite metal complex containing Ce and R is heated and calcined in an oxidizing atmosphere (for example, in air) to obtain a precursor powder consisting of a composite oxide containing Ce and R. The above method (X) will be described below as a preferred method for the first step.

[0042] In such a method (X), first, a complex solution is prepared containing cerium nitrate, a salt of at least one element selected from the group consisting of rare earth elements (R) excluding Ce, and an oxycarboxylic acid or phosphoric acid. The method for preparing such a complex solution is not particularly limited, but it is preferable to obtain a metal salt solution containing cerium nitrate, a salt of at least one element selected from the group consisting of rare earth elements (R) excluding Ce, and a solvent, and then obtain the complex solution by adding and dissolving the oxycarboxylic acid or phosphoric acid in the metal salt solution.

[0043] Examples of salts of at least one element selected from the group consisting of rare earth elements (R) excluding Ce include sulfates, nitrates, chlorides, acetates, and various complexes of R. These salts of R may be used individually or in combination of two or more. Furthermore, from the viewpoint of improving the activity of the resulting ammonia synthesis catalyst, La and Pr are preferred as such R elements, with La being more preferred.

[0044] Furthermore, the solvent can be any solvent capable of dissolving a salt of at least one element selected from the group consisting of cerium nitrate and rare earth elements (R) excluding Ce, thereby generating Ce ions and R ions in the solution. There are no particular restrictions on the solvent, and examples include water, alcohol, and mixed solvents thereof. Of these solvents, water is preferred from the viewpoint of cost and safety. A metal salt solution can be obtained by dissolving a salt of at least one element selected from the group consisting of cerium nitrate and rare earth elements (R) excluding Ce in such a solvent.

[0045] The molar ratio of Ce to R ([moles of Ce]:[total molars of R]) in the final complex solution is preferably 1:9 to 9:1 (more preferably 3:7 to 7:3). When such a molar ratio is within the above range, a CeR-containing oxide support can be obtained containing Ce and R in a similar ratio, and the ammonia synthesis activity of the resulting ammonia synthesis catalyst tends to be further improved.

[0046] Examples of the oxycarboxylic acid include citric acid, malic acid, and lactic acid. Such oxycarboxylic acids or phosphoric acids are more preferable from the viewpoint that, in the calcination process in the second step, it is easier to remove organic matter derived from their components, and the remaining organic matter in the support after calcination can be further suppressed, thereby further improving the ammonia synthesis activity of the obtained catalyst. Among these, citric acid is particularly preferable. Furthermore, the amount of such oxycarboxylic acid or phosphoric acid added is preferably 0.1 to 50 equivalents, more preferably 1 to 20 equivalents, and particularly preferably 2 to 20 equivalents, relative to the total amount of all metal elements contained in the complex solution (total amount of cations in the solution). If the amount of oxycarboxylic acid or phosphoric acid added is less than the lower limit or more than the upper limit, a composite metal complex containing Ce and R will not be sufficiently formed, and a uniform CeR-containing oxide support will not be obtained.

[0047] Furthermore, if the structure (design) of the CeR-containing oxide support obtained in the second step includes at least one additive metal element selected from the group consisting of Group 4 and Group 14 elements, or other metal elements, it is preferable to add salts of those metal elements to the metal salt solution to prepare the complex solution.

[0048] Such salts of additive metal elements are not particularly limited as long as they are soluble in the solvent, and examples include sulfates, nitrates, chlorides, acetates, and various complexes of the additive metal elements. Such additive metal elements may be used individually or in combination of two or more. Furthermore, from the viewpoint of the stability of the cations in a reducing atmosphere, Ti, Zr, Al, and Si are more preferred, with Ti and Si being more preferred.

[0049] Furthermore, when using a salt of the aforementioned additive metal element, it is preferable that the content (amount used) of the additive metal element is 1 to 30 mol% (more preferably 5 to 20 mol%) relative to the total amount of all metal elements contained in the complex solution (total amount of cations in the solution).

[0050] Furthermore, the salt of the other metal element can be any salt that dissolves in the solvent and is not particularly limited. Examples include sulfates, nitrates, chlorides, acetates, and various complexes of the other metal element. The other metal element can be any metal element used in ammonia synthesis catalysts, such as manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu).

[0051] When using salts of other metal elements, the content of the other metal elements is preferably 5 mol% or less, more preferably 1 mol% or less, and particularly preferably 0.1 mol% or less, relative to the total amount of all metal elements contained in the complex solution (total amount of cations in the solution). If the content of the other metal elements exceeds the above upper limit, the dissociation of N2 on Ru is inhibited, and the ammonia synthesis activity tends to decrease.

[0052] Furthermore, after adding oxycarboxylic acid or phosphoric acid to the metal salt solution, other components may be added to the complex solution. When other components are added in this way, it is preferable to use glycol or glycerin (more preferably ethylene glycol) as the other component. That is, in the present invention, it is preferable to further add glycol or glycerin to the complex solution used in the complex polymerization method. When glycol or glycerin is added to the complex solution in this way, heating the solution causes a chain reaction of dehydration esterification between the carboxyl groups of the oxycarboxylic acid present in the solution and the hydroxyl groups of the glycol, making it possible to form a polyester polymer gel. When a polyester polymer gel is formed in this way, a gel is obtained in which Ce and R are more uniformly dispersed inside. When such a polyester polymer gel is heated and calcined, a precursor powder consisting of a composite oxide in which Ce and R are more uniformly dispersed tends to be obtained. Incidentally, by recovering the polyester polymer gel, the composite metal complex present in the gel is recovered as a result.

[0053] Examples of such glycols include ethylene glycol, propylene glycol, and diethylene glycol. Among glycols or glycerins, ethylene glycol is more preferred. Thus, it is preferable that the complex solution further contains ethylene glycol. The amount of glycol or glycerin (preferably ethylene glycol) added is preferably 0.01 to 50 equivalents, more preferably 0.05 to 20 equivalents, and particularly preferably 0.1 to 10 equivalents, relative to the total amount of all metal elements contained in the complex solution (total amount of cations in the solution). If the amount of glycol or glycerin (preferably ethylene glycol) added falls below the lower limit or exceeds the upper limit, a composite metal complex containing Ce and R (and, if the complex solution contains metal elements other than Ce and R, such metal elements) is not sufficiently formed, and a uniform CeR-containing oxide support tends not to be obtained.

[0054] Furthermore, after adding and dissolving oxycarboxylic acid or phosphoric acid in the metal salt solution, it is preferable to heat and stir the solution at a temperature of 50 to 380°C (more preferably 60 to 350°C) for 0.5 hours or more (more preferably 1 hour or more) to more reliably form the complex (a heating and stirring step for complex formation). It is preferable to perform such heating and stirring for complex formation after adding glycol or glycerin to the complex solution.

[0055] Furthermore, when glycol or glycerin is added to the complex solution, it is preferable to heat the complex solution at a temperature of 50 to 380°C (more preferably 60 to 350°C, even more preferably 80 to 300°C) for 0.5 hours or more (more preferably 1 hour or more, even more preferably 2 hours or more) after the heating and stirring step for complex formation. By performing such heating, the dehydration ester reaction can be carried out efficiently, and it tends to be possible to obtain a polyester polymer gel in which the metal elements contained in the complex solution are uniformly dispersed inside. When such a polyester polymer gel is heated and calcined, it is possible to obtain a precursor powder in which Ce and R are more uniformly dispersed. There is no particular upper limit to the heating time, although the process time will be longer.

[0056] Next, in method (X), the obtained composite metal complex containing Ce and R (or the obtained polyester polymer gel if a polyester polymer gel is formed) is heated and calcined in an oxidizing atmosphere (for example, in air). Such heating and calcination makes it possible to carbonize the organic matter, and a precursor powder consisting of a composite oxide containing Ce and R can be obtained.

[0057] The firing temperature of the composite metal complex (or polyester polymer gel if a polyester polymer gel is formed) during heating and firing is preferably 400 to 600°C, more preferably 425 to 575°C, and particularly preferably 450 to 550°C. When the firing temperature of the composite metal complex is within this range, the carbonization of organic matter (ligands of the complex and polyester) can be carried out more efficiently, and a precursor powder consisting of a more uniform composite oxide tends to be obtained. Furthermore, the firing time of the polyester polymer gel during heating and firing is preferably 30 minutes or more, more preferably 60 minutes or more, and particularly preferably 120 minutes or more. If the firing time of the polyester polymer gel is less than the lower limit, the carbonization of organic matter does not proceed sufficiently, and a uniform composite oxide tends not to be obtained. There is no particular upper limit to the firing time of the polyester polymer gel, although the process time will be longer. In this way, by heating and firing the composite metal complex in an oxidizing atmosphere (for example, in air), a precursor powder consisting of a composite oxide containing Ce and R can be obtained.

[0058] <Second step: Step to obtain a CeR-containing oxide support> The second step is to obtain a CeR-containing oxide support by calcining the precursor powder.

[0059] Such calcination may be carried out under either an oxidizing atmosphere (e.g., air) or an inert gas atmosphere (e.g., nitrogen or argon atmosphere), but it is more preferable to carry it out in air because it does not require adjustment of the gas atmosphere and simplifies the process. Furthermore, such calcination makes it possible to efficiently remove any carbon components that remain in the precursor powder.

[0060] The calcination temperature of such precursor powders is preferably 610 to 900°C (more preferably 625 to 800°C, and particularly preferably 650 to 750°C). If the calcination temperature of the precursor powder falls below the lower limit, it becomes difficult to sufficiently remove the carbon component, and the ammonia synthesis activity of the resulting ammonia synthesis catalyst tends to decrease. On the other hand, if the temperature exceeds the upper limit, the specific surface area of ​​the CeR-containing oxide support decreases, and the ammonia synthesis activity of the resulting ammonia synthesis catalyst also tends to decrease.

[0061] Furthermore, the calcination time of the precursor powder is preferably 1 to 48 hours (more preferably 2 to 36 hours, and particularly preferably 3 to 24 hours). If the calcination time of the precursor powder is less than the lower limit, it becomes difficult to sufficiently remove the carbon component, and the ammonia synthesis activity of the resulting ammonia synthesis catalyst tends to decrease. On the other hand, if it exceeds the upper limit, the specific surface area of ​​the CeR-containing oxide support decreases, and the ammonia synthesis activity of the resulting ammonia synthesis catalyst tends to decrease.

[0062] <Third step: Step to obtain the ammonia synthesis catalyst> The third step is to support Ru on the CeR-containing oxide support to obtain the ammonia synthesis catalyst of the present invention.

[0063] There are no particular limitations on the method for supporting Ru, but it is preferable to employ a method in which a Ru precursor is attached to the CeR-containing oxide support using a solution containing a Ru salt, the support with the Ru precursor attached is dried, and then calcined under a reducing gas atmosphere or an inert gas atmosphere to support Ru on the CeR-containing oxide support.

[0064] There are no particular restrictions on the salts of Ru mentioned above, and examples include acetates, carbonates, nitrates, ammonium salts, citrates, dinitrodiammine salts, chlorides, and various complexes (e.g., tetraammine complexes, carbonyl complexes) of Ru. Among these salts of Ru, dodecacarbonyltriruthenium[Ru3(CO) 12], ruthenium acetylacetonate, ruthenium nitrosylnitrate, and ruthenium nitrate are preferred. Furthermore, there are no particular restrictions on the solvent used in the solution containing the Ru salt, as long as the Ru salt dissolves and Ru ions are generated, for example, tetrahydrofuran (THF), water, alcohol, etc. The concentration of the Ru salt in the solution containing the Ru salt can be appropriately set according to the amount of Ru supported.

[0065] There are no particular limitations on the method for attaching the Ru precursor to the CeR-containing oxide support. Examples include immersing the CeR-containing oxide support in a solution containing the Ru salt to impregnate the CeR-containing oxide support with the Ru salt (impregnation method), or adsorbing the solution containing the Ru salt onto the CeR-containing oxide support (adsorption method). Among these, the impregnation method is preferred.

[0066] Furthermore, when supporting ruthenium, it is desirable to attach the Ru precursor to the CeR-containing oxide carrier such that the amount of Ru supported per 100 parts by mass of the CeR-containing oxide carrier is preferably 0.5 to 10 parts by mass, more preferably 1 to 5 parts by mass. If an amount of Ru precursor is attached that is less than the lower limit, the ammonia synthesis activity of the resulting ammonia synthesis catalyst tends to decrease. On the other hand, if it exceeds the upper limit, depending on the usage environment of the resulting ammonia synthesis catalyst, Ru sintering is likely to occur, which reduces the dispersion of Ru, which is the active site, making it difficult to obtain an effect commensurate with the amount of Ru supported, and thus tends to be disadvantageous in terms of cost, etc.

[0067] Furthermore, after attaching the Ru precursor to the CeR-containing oxide support, it is preferable to dry the support to which the Ru precursor is attached and calcine it under a reducing gas atmosphere or an inert gas atmosphere. By calcining in this manner, the ammonia synthesis catalyst of the present invention, in which Ru is supported on the CeR-containing oxide support, can be obtained. In addition, by calcining under a reducing gas atmosphere or an inert gas atmosphere (preferably under a reducing gas atmosphere), the Ru is supported on the CeR-containing oxide support in a metallic state, and the ammonia synthesis activity of the resulting catalyst tends to be further improved.

[0068] The drying temperature for drying the support to which the Ru precursor is attached is preferably 50 to 150°C, and more preferably 75 to 125°C. The drying time is preferably 3 hours or more, and more preferably 12 hours or more.

[0069] The reducing gas atmosphere may be any atmosphere containing a reducing gas such as hydrogen gas, carbon monoxide gas, or hydrocarbon gas. For example, a mixed gas atmosphere of the reducing gas and an inert gas (such as nitrogen gas or argon gas) is an example. The concentration of the reducing gas in such a mixed gas atmosphere is preferably 1 to 30% by volume, and more preferably 5 to 20% by volume. Examples of the inert gas atmosphere include a nitrogen gas atmosphere, an argon gas atmosphere, and a helium gas atmosphere.

[0070] The calcination temperature of the support after drying is preferably 200 to 500°C, and more preferably 300 to 500°C. The calcination time is preferably 0.5 to 10 hours, and more preferably 1 to 5 hours. If the calcination temperature or calcination time falls below the lower limit, even if calcined under a reducing gas atmosphere or an inert gas atmosphere, not all of the Ru can be sufficiently reduced to the metallic state, and Ru tends to remain in a precursor state. On the other hand, if it exceeds the upper limit, the Ru sintersects, making it difficult to adequately disperse the metallic Ru on the support, and the activity of the resulting ammonia synthesis catalyst tends to decrease.

[0071] In the method for producing an ammonia synthesis catalyst of the present invention, the ammonia synthesis catalyst produced in this manner may be molded into various forms by known methods. For example, it may be molded into pellets, or it may be coated onto various substrates such as monolithic substrates, pelletized substrates, or plate-shaped substrates.

[0072] [Method of ammonia synthesis] Next, the ammonia synthesis method of the present invention will be described. The ammonia synthesis method of the present invention is a method of synthesizing ammonia by contacting the ammonia synthesis catalyst of the present invention with a gas (mixed gas) containing hydrogen and nitrogen. There are no particular restrictions on the method of contacting the ammonia synthesis catalyst with a gas containing hydrogen and nitrogen, and known methods for synthesizing ammonia can be used as is.

[0073] In the ammonia synthesis method of the present invention, there are no particular restrictions on the synthesis conditions, and the conditions used in known ammonia synthesis methods can be adopted as they are. For example, the molar ratio of hydrogen to nitrogen (H2 / N2) is preferably 0.1 / 1 to 5 / 1, and more preferably 0.5 / 1 to 3 / 1. The gas containing hydrogen and nitrogen may contain an inert gas (such as argon gas) as a carrier gas, but from the viewpoint of ammonia production efficiency, it is preferable that the gas containing hydrogen and nitrogen consists only of hydrogen and nitrogen.

[0074] Furthermore, the reaction temperature is preferably 300 to 500°C, and more preferably 350 to 450°C. The reaction pressure is preferably 0.1 to 10 MPa, and more preferably 1 to 8 MPa. [Examples]

[0075] The present invention will be described more specifically below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0076] ( Reference example 1 ) <Preparation process for oxide support> Ce 0.5 La 0.5 O 1.75 An oxide support consisting of the following was synthesized. Specifically, first, cerium nitrate hexahydrate [Ce(NO3)3·6H2O] and lanthanum nitrate hexahydrate [La(NO3)3·6H2O] were dissolved in the minimum amount of deionized water required to dissolve them at room temperature (approximately 25°C) to obtain a metal salt solution (here, the "minimum amount" of deionized water is the minimum amount of deionized water that can completely dissolve a predetermined amount of cerium nitrate hexahydrate and lanthanum nitrate hexahydrate (see below) Reference example, The same applies to the examples and comparative examples.) In the process of obtaining such a metal salt solution, Ce(NO3)3·6H2O and La(NO3)3·6H2O were used so that the molar ratio of Ce to La cations in the metal salt solution ([Ce cation]:[La cation]) was 0.5:0.5. Next, citric acid (6 equivalents relative to the total amount of cations in the solution) was dissolved in the obtained metal salt solution and stirred at a temperature of 80°C for 5 hours to obtain a complex solution. Next, the obtained complex solution was allowed to stand at a temperature of 80°C for 12 hours to remove water from the solution and dry it to obtain a residue. Subsequently, the obtained residue was calcined in air at a temperature of 500°C for 5 hours to obtain a precursor powder. Next, the obtained precursor powder was calcined in air at a temperature of 700°C for 10 hours to obtain Ce 0.5 La 0.5 O 1.75 An oxide support consisting of the following was obtained.

[0077] <Ru-supporting process> The Ce obtained as described above was obtained by evaporation to dryness using tetrahydrofuran (THF) as the solvent, as follows. 0.5 La 0.5 O 1.75 An ammonia synthesis catalyst was obtained by supporting Ru on an oxide support consisting of the following: First, Ru3(CO) 12 Prepare a solution by dissolving in THF, and add the oxide support (Ce 0.5 La 0.5 O 1.75) was added and stirred for 5 hours. Note that Ru3(CO) 12 The amount used was adjusted so that in the final catalyst, the amount of Ru supported per 100 parts by mass of the oxide support was 3 parts by mass. Next, the solution, after being stirred for 5 hours, was subjected to a process of evaporating and removing THF for 2 hours to obtain a powder, and then the powder was dried at a temperature of 80°C for 18 hours. Next, the dried powder was calcined at a temperature of 500°C for 3 hours under a nitrogen atmosphere with N2 gas (100 vol%) flowing through it, thereby obtaining the oxide support (Ce 0.5 La 0.5 O 1.75 Ammonia synthesis catalyst (Ru / Ce) in which Ru is supported on ) 0.5 La 0.5 O 1.75 A catalyst was obtained in which the amount of Ru supported per 100 parts by mass of oxide support was 3 parts by mass.

[0078] [ Reference example 1 [Evaluation of the properties of catalysts, etc. obtained] <Measurement of pore volume of oxide support> Ce obtained in the oxide support preparation process 0.5 La 0.5 O 1.75The pore volume of the oxide support was identified using the Barrett-Joyner-Halenda (BJH) method. Specifically, to identify the pore volume, first, the oxide support (approximately 0.2 g) was packed into a reaction tube and pre-treated by standing it in a vacuum (under conditions of 1000 Pa or less) at a temperature of 300°C for 1 hour. Then, nitrogen adsorption / desorption isotherms were determined under conditions of adsorption temperature: -196°C. Next, using the obtained nitrogen adsorption / desorption isotherms, the relative pressure (P / P0) was converted to pore diameter to determine the amount of nitrogen adsorbed under relative pressure corresponding to pores with a pore diameter of 2 nm or more and less than 16 nm, and the amount of nitrogen adsorbed under relative pressure corresponding to pores with a pore diameter of 16 nm or more and less than 200 nm. By converting these to pore volume, the pore volume x of pores with a pore diameter of 2 nm or more and less than 16 nm, and the pore volume y of pores with a pore diameter of 16 nm or more and less than 200 nm were identified. The results obtained are shown in Table 1. Table 1 also shows the "5x+y" values ​​for pore volume x and pore volume y. Furthermore, Figure 1 shows a graph illustrating the relationship between pore volume x for pores with a diameter of 2 nm or more and less than 16 nm, and pore volume y for pores with a diameter of 16 nm or more and 200 nm or less.

[0079] <Evaluation test of ammonia synthesis activity> Reference example 1 The ammonia synthesis rate of the ammonia synthesis catalyst obtained was measured using a fixed-bed flow reactor. Specifically, first, 0.200 g of the ammonia synthesis catalyst was fixed in the reaction section of the fixed-bed flow reactor, and then a pretreatment was performed on the ammonia synthesis catalyst by supplying a mixed gas of H2 (75 vol%) / N2 (25 vol%) under the conditions of flow rate: 80 mL / min, pressure: 0.1 MPa, temperature: 600 °C, and time: 30 minutes. Next, while continuing to supply the mixed gas, the temperature was lowered to 350 °C, and the ammonia synthesis reaction was carried out at a temperature of 350 °C. After 1 hour, the ammonia concentration in the exhaust gas (gas after contact with the catalyst) discharged from the outlet was measured using an infrared spectrometer (IR spectrometer) connected to the outlet of the fixed-bed flow reactor, and the ammonia synthesis rate per 1 g of catalyst (unit: mmol / g·h) was determined. The obtained results are shown in Figure 2 and Table 1.

[0080] ( Reference example 2 ) In the preparation process for the oxide support, the step of obtaining the complex solution was changed to a step of dissolving citric acid (6 equivalents relative to the total amount of cations in the solution) in the metal salt solution, then adding ethylene glycol (12 equivalents relative to the total amount of cations in the solution) and stirring at a temperature of 80°C for 5 hours to obtain a complex solution containing ethylene glycol. Reference example 1 Similarly, the ammonia synthesis catalyst (Ru / Ce 0.5 La 0.5 O 1.75 A catalyst was prepared in which Ru was supported at a rate of 3 parts by mass per 100 parts by mass of oxide support. Reference example 1 Instead of the carrier and catalyst obtained in Reference example 2 Other than using the carrier and catalyst obtained from, Reference example 1 Similarly, "measurement of pore volume of oxide support" and "evaluation test of ammonia synthesis activity" were performed. The results obtained are shown in Table 1 and figure These are shown in 1 and 2 respectively.

[0081] ( Reference example 3 ) In the oxide support preparation process, the composition of the oxide support is Ce 0.5 La 0.4 Ti 0.1 O 1.8 To achieve this, the process of obtaining the metal salt solution was changed to a process in which Ce(NO3)3·6H2O and La(NO3)3·6H2O in amounts such that Ce:La:Ti = 0.5:0.4:0.1, and a 7M titanium oxynitrate [TiO(NO3)2] aqueous solution is used, Ce(NO3)3·6H2O and La(NO3)3·6H2O are dissolved in deionized water, and then a 7M titanium oxynitrate [TiO(NO3)2] aqueous solution is added to obtain the metal salt solution. Reference example 1 Similarly, the ammonia synthesis catalyst (Ru / Ce 0.5 La 0.4 Ti 0.1 O 1.8:In the catalyst, the amount of Ru supported on 100 parts by mass of the oxide carrier was 3 parts by mass). As the 7M titanium oxynitrate [TiO(NO3)2] aqueous solution, titanium tetraisopropoxide [Ti[(CH3)2CHO]4] was hydrolyzed, and the obtained precipitate was washed three times with distilled water to obtain titanium oxyhydroxide [TiO(OH)2]. Then, the titanium oxyhydroxide thus obtained was dissolved in the minimum amount of nitric acid aqueous solution (here, the "minimum amount" of the nitric acid aqueous solution is the minimum amount of the nitric acid aqueous solution capable of completely dissolving a predetermined amount of titanium oxyhydroxide). Also, Reference example 1 Instead of the carrier and catalyst obtained in Reference example 3 Except for using the carrier and catalyst obtained in Reference example 1 In the same manner as figure "Measurement of the pore volume of the oxide carrier" and "Evaluation test of ammonia synthesis activity" were carried out. The obtained results are shown in Table 1 and

[0082] ( Reference example 4 ) In the process of preparing the oxide carrier, in the step of obtaining the metal salt solution so that the composition of the oxide carrier becomes Ce 0.5 La 0.4 Ti 0.1 O 1.8 Using Ce(NO3)3·6H2O, La(NO3)3·6H2O, and a 7M titanium oxynitrate [TiO(NO3)2] aqueous solution in amounts such that Ce:La:Ti = 0.5:0.4:0.1, after dissolving Ce(NO3)3·6H2O and La(NO3)3·6H2O in ion-exchanged water, and then adding a 7M titanium oxynitrate [TiO(NO3)2] aqueous solution ( Reference example 3 The same as that used in Reference example 1 In the same manner as 0.5 La 0.4 Ti 0.1 O1.8 :In the catalyst, the amount of Ru supported on 100 parts by mass of the oxide carrier was 3 parts by mass) was prepared. Also, Reference example 1 Instead of the carrier and catalyst obtained in Reference example 4 Except for using the carrier and catalyst obtained in Reference example 1 In the same manner as figure "Measurement of the pore volume of the oxide carrier" and "Evaluation test of ammonia synthesis activity" were carried out. The obtained results are shown in Table 1 and figure 1 to 2, respectively.

[0083] ( Example 1 ) In the process of preparing the oxide carrier, in the process of obtaining the metal salt solution so that the composition of the oxide carrier becomes Ce 0.5 La 0.4 Si 0.1 O 1.8 The process was changed to a process of using a solution of Ce(NO3)3·6H2O, La(NO3)3·6H2O, and tetraethoxysilane in amounts such that Ce:La:Si = 0.5:0.4:0.1. After dissolving Ce(NO3)3·6H2O and La(NO3)3·6H2O in ion-exchanged water, a solution of tetraethoxysilane was further added to obtain a metal salt solution. The process of obtaining the complex solution was changed to a process of dissolving citric acid (6 equivalents with respect to the total cation amount in the solution) in the metal salt solution, then adding ethylene glycol (12 equivalents with respect to the total cation amount in the solution) and stirring at a temperature of 80 °C for 5 hours to obtain a complex solution containing ethylene glycol. And, except that the process of obtaining the precursor powder was changed to a process of heating the complex solution at 300 °C for 60 minutes to gel it, and then firing the obtained polyester polymer gel in the air at 500 °C for 5 hours to obtain the precursor powder, Reference example 1 In the same manner as 0.5 La 0.4 Si 0.1 O 1.8 :In the catalyst, the amount of Ru supported on 100 parts by mass of the oxide carrier was 3 parts by mass) was prepared. The solution of tetraethoxysilane was prepared by dropping nitric acid into a mixed solution of tetraethoxysilane (TEOS) and ethylene glycol and mixing at 80 °C. Also, Reference example 1 Instead of the carrier and catalyst obtained in Example 1Other than using the carrier and catalyst obtained from, Reference example 1 Similarly, "measurement of pore volume of oxide support" and "evaluation test of ammonia synthesis activity" were performed. The results obtained are shown in Table 1 and figure These are shown in 1 and 2 respectively.

[0084] (Comparative Example 1) The oxide support preparation process is carried out by the following "coprecipitation method by Ce 0.5 La 0.5 O 1.75 Except for the change to "the process of preparing an oxide support consisting of the following," Reference example 1 Similar to the process used in [the previous example], a comparative ammonia synthesis catalyst (Ru / Ce) was used. 0.5 La 0.5 O 1.8 A catalyst was prepared in which Ru was supported at a rate of 3 parts by mass per 100 parts by mass of oxide support. Reference example 1 Except for using the carrier and catalyst obtained in Comparative Example 1 instead of the carrier and catalyst obtained in [the previous example], Reference example 1 Similarly, "measurement of pore volume of oxide support" and "evaluation test of ammonia synthesis activity" were performed. The results obtained are shown in Table 1 and figure These are shown in 1 and 2 respectively. Furthermore, the following method, "coprecipitation by Ce 0.5 La 0.5 O 1.75 The "step of preparing an oxide support consisting of Ce" is adopted in the aforementioned Non-Patent Document 1. 0.5 La 0.5 O 1.75 The carrier preparation process mimics the carrier preparation method.

[0085] <Ce by coprecipitation method 0.5 La 0.5 O 1.75 Steps for preparing an oxide support consisting of the following: First, Ce(NO3)3·6H2O and La(NO3)3·6H2O were dissolved in deionized water at room temperature (approximately 25°C), and the resulting solution was added dropwise to aqueous ammonia to obtain a precipitate. In the process of obtaining such a solution, Ce(NO3)3·6H2O and La(NO3)3·6H2O were used so that the molar ratio of Ce to La cations in the metal salt solution ([Ce cation]:[La cation]) was 0.5:0.5. Next, the obtained precipitate was dried by standing at 80°C for 12 hours, and then calcined in air at 700°C for 5 hours to obtain Ce 0.5 La 0.5 O 1.75 An oxide support consisting of the following was prepared.

[0086] (Comparative Example 2) Except for using cerium ammonium nitrate ((NH4)2Ce(NO3)6) instead of cerium nitrate (Ce(NO3)3·6H2O) in the oxide support preparation process, Reference example 1 Similarly, the ammonia synthesis catalyst (Ru / Ce 0.5 La 0.5 O 1.75 A catalyst was prepared in which Ru was supported at a rate of 3 parts by mass per 100 parts by mass of oxide support. Reference example 1 Except for using the support and catalyst obtained in Comparative Example 2 instead of the support and catalyst obtained in [the first example], Reference example 1 Similarly, "measurement of pore volume of oxide support" and "evaluation test of ammonia synthesis activity" were performed. The results obtained are shown in Table 1 and figure These are shown in 1 and 2, respectively. Note that the "oxide support preparation process" used in Comparative Example 2 is a support preparation process that mimics the La2Ce2O7 preparation method used in Non-Patent Document 2 mentioned above.

[0087] (Comparative Example 3) In the oxide support preparation process, cerium ammonium nitrate ((NH4)2Ce(NO3)6) was used instead of cerium nitrate (Ce(NO3)3·6H2O), and the amount of ethylene glycol used was changed to 6 equivalents relative to the total amount of cations in the solution. Reference example 2 Similarly, the ammonia synthesis catalyst (Ru / Ce0.5 La 0.5 O 1.75 A catalyst was prepared in which Ru was supported at a rate of 3 parts by mass per 100 parts by mass of oxide support. Reference example 1 Except for using the carrier and catalyst obtained in Comparative Example 3 instead of the carrier and catalyst obtained in [the previous example], Reference example 1 Similarly, "measurement of pore volume of oxide support" and "evaluation test of ammonia synthesis activity" were performed. The results obtained are shown in Table 1 and figure These are shown in 1 and 2 respectively.

[0088] (Comparative Example 4) Except for using cerium ammonium nitrate ((NH4)2Ce(NO3)6) instead of cerium nitrate (Ce(NO3)3·6H2O) in the oxide support preparation process, Reference example 2 Similarly, the ammonia synthesis catalyst (Ru / Ce 0.5 La 0.5 O 1.75 A catalyst was prepared in which Ru was supported at a rate of 3 parts by mass per 100 parts by mass of oxide support. Reference example 1 Except for using the support and catalyst obtained in Comparative Example 4 instead of the support and catalyst obtained in [the previous example], Reference example 1 Similarly, "measurement of pore volume of oxide support" and "evaluation test of ammonia synthesis activity" were performed. The results obtained are shown in Table 1 and figure These are shown in 1 and 2 respectively.

[0089] Note: Example 1 , Reference examples 1-4 The carriers prepared in Example 1 and Comparative Examples 1-4 were all calcined at 700°C for 5 hours or more during the preparation of the carrier. Therefore, Example 1 , Reference examples 1-4 The carriers prepared in Comparative Examples 1-4 can all be considered to have been calcined in air at 700°C for 5 hours. From this viewpoint, Example 1 , Reference examples 1-4 And for the carriers prepared in Comparative Examples 1 to 4, the pore volume x (cm³) of pores with a pore diameter of 2 nm or more and less than 16 nm, as shown in Table 1 and Figure 1, is... 3 g -1 ), and the pore volume y (cm³) of pores with a diameter of 16 nm or more and 200 nm or less. 3 g -1The value of ) can be said to be the value identified by the BJH method after firing in air at 700°C for 5 hours.

[0090] [Table 1]

[0091] As is clear from the results shown in Table 1 and Figure 1, Example 1 and Reference Examples 1-4 The carrier prepared in this manner has pore volume x and pore volume y values ​​that meet the following conditions (A) to (C): [Condition (A)] x≧0.005 [Condition (B)] y≧0.050 [Condition (C)] 5x+y≧0.290 It satisfied all of the above conditions. And an ammonia synthesis catalyst (Example 1) was prepared using a support that satisfies all of the above conditions (A) to (C). and Reference Examples 1-4 The results shown in Table 1 and Figure 2 confirm that the ammonia synthesis rate is sufficiently fast, indicating that it has higher ammonia synthesis activity.

[0092] In contrast, following the aforementioned Non-Patent Document 1, Ce was prepared by coprecipitation. 0.5 La 0.5 O 1.75 When synthesized (Comparative Example 1), the pore volume x of the carrier was 0.041 cm³. 3 g -1 The volume increases, but the pore volume y is 0.015 cm³. 3 g -1 The size became smaller and did not satisfy the above conditions (B) and (C). Also, following the aforementioned Non-Patent Document 2, a complex polymerization method using cerium ammonium nitrate as the Ce raw material was adopted and Ce 0.5 La 0.5 O 1.75 When synthesized (Comparative Example 2), the pore volume y of the carrier was 0.254 cm³. 3 g -1 Although large, the pore volume x is 0.003 cm³. 3 g -1The size became smaller, and the above conditions (A) and (C) were not met. Furthermore, the carrier obtained in Comparative Example 3 had a pore volume x of 0.011 cm³. 3 g -1 It is also large, and the pore volume y is 0.065 cm³. 3 g -1 The result was large, satisfying conditions (A) and (B) above, but the value of 5x+y was small, failing to satisfy condition (C) above. In addition, the carrier obtained in Comparative Example 4 had a pore volume x of 0.015 cm³. 3 g -1 Although large, the pore volume y is 0.026 cm³. 3 g -1 The size was small and did not satisfy the above conditions (B) and (C). Furthermore, when catalysts were prepared using supports that did not satisfy one or more of the conditions (A) to (C) (Comparative Examples 1 to 4), it was found that a sufficient ammonia synthesis rate could not be obtained, as shown in Table 1 and Figure 2.

[0093] These results (Table 1 and Figures 1-2) confirm that the ammonia synthesis catalyst of the present invention exhibits excellent ammonia synthesis activity, enabling more efficient ammonia synthesis.

[0094] ( Reference example, (Discussion of Examples and Comparative Examples) [Regarding pore volumes x and y] First, Reference example 1 Comparing the catalyst obtained in [method 1] with the catalyst obtained in Comparative Example 2, Reference example 1 The catalyst obtained in [method 1] and the catalyst obtained in Comparative Example 2 had approximately the same pore volume y of the support (in Comparative Example 2, Reference example 1 While the size is 0.94 times that of the carrier, the pore volume x of the carrier is Reference example 1 The catalyst obtained was more than twice as effective (approximately 2.7 times more effective) than the catalyst obtained in Comparative Example 2. Reference example 1 is 0.008cm 3 g -1 Comparative Example 2 was 0.003 cm 3 g -1) is the size of the pores. Here, it is thought that larger pores contribute more to gas diffusion, Reference example 1 The difference in ammonia synthesis rates between Comparative Example 1 and Comparative Example 2 is thought to be due to the fact that pores with a diameter of 2 nm or more and less than 16 nm primarily function as the reaction field necessary for ammonia synthesis. Based on these considerations, the catalyst obtained in Comparative Example 2 is Reference example 1 In comparison to the results obtained, it was not possible to secure a sufficient reaction field necessary for ammonia synthesis, and as a result, Reference example 1 The inventors have investigated whether the ammonia synthesis rate was lower compared to the catalyst obtained using the other method.

[0095] next, Reference example 1 Comparing the catalyst obtained in [method 1] with the catalyst obtained in Comparative Example 1, Comparative Example 1 is: Reference example 1 In contrast, it can be seen that despite the large pore volume x of the support, the ammonia synthesis rate is slow. This is because, in the catalyst obtained in Comparative Example 1, the pore volume y of the support is 0.015 cm³. 3 g -1 It is as follows: Reference example 1 Since the size is 1 / 18th of the pore capacity y of the support used in the catalyst obtained, Reference example 1 In contrast to the support material, it is not possible to efficiently diffuse the reaction gases (H2, N2) necessary for ammonia synthesis into the catalyst, and as a result, Reference example 1 The inventors have investigated whether the ammonia synthesis rate was lower compared to the catalyst obtained using the other method.

[0096] [Regarding Ce raw materials] Example 1 and Reference Examples 1-4 In Example 1, cerium nitrate (more specifically, Ce(NO3)3·6H2O) was used as the Ce raw material in the complex polymerization method, whereas in Comparative Examples 2-4, cerium ammonium nitrate ((NH4)2Ce(NO3)6) was used as the Ce raw material in the complex polymerization method (Note that Comparative Example 1 utilized the coprecipitation method, and therefore the manufacturing method is different). Therefore, Example 1 and Reference Examples 1-4 We will examine the Ce raw material by comparing it with Comparative Examples 2-4.

[0097] First, in the case of using cerium nitrate (Example 1) and Reference Examples 1-4 ) For pores with a diameter of 2 nm or more and less than 16 nm, the pore volume x is 0.005 cm³. 3 g -1 In addition, the pore volume y of pores with a diameter of 16 nm to 200 nm is 0.050 cm³. 3 g -1 As shown above, it can be seen that both pore volumes x and y are at a sufficiently high level. Furthermore, when cerium nitrate is used (Example 1) and Reference Examples 1-4 In this case, the value of 5x+y is 0.310 or higher, indicating that a support material with a sufficiently high balance between pore volume x and y has been obtained.

[0098] In contrast, when cerium ammonium nitrate was used (Comparative Examples 2-4), the pore volume x was 0.005 cm³. 3 g -1 In cases where the value is less than (Comparative Example 2), or when the pore volume y is 0.050 cm³, 3 g -1 In some cases (Comparative Example 4), the result was less than 0.005 cm³, indicating that it is not possible to stably maintain both pore volume x and y at a sufficiently high level. In Comparative Example 3, an appropriate amount of ethylene glycol was used along with cerium ammonium nitrate to achieve a pore volume x of 0.005 cm³. 3 g -1 With the above in mind, the pore volume y is set to 0.050 cm³. 3 g -1 This makes it possible to achieve the above, and both the pore volume x and y are at a sufficiently high level, however, the value of 5x+y is 0.290 cm³. 3 g -1 The values ​​were less than the given range, indicating that the balance between pore volume x and y was not sufficient.

[0099] These results indicate that, in order to obtain a support that satisfies the above conditions (A) to (C), it is necessary to use cerium nitrate as a Ce raw material in the complex polymerization method. While the reason why using cerium nitrate as a Ce raw material in the complex polymerization method yields a support that satisfies the above conditions (A) to (C) is not entirely clear, the inventors speculate that this is because cerium nitrate is a less stable salt than cerium ammonium nitrate, and complex formation with compounds such as citric acid is easier. Example 1 shows a support obtained by using cerium nitrate as a Ce raw material in the complex polymerization method. and Reference Examples 1-4 As shown in Table 1 and Figure 2, all of the catalysts obtained exhibited a sufficiently fast ammonia synthesis rate and high ammonia synthesis activity.

[0100] Furthermore, considering the results of Comparative Examples 3 and 4, it is considered that, even if an appropriate amount of ethylene glycol is used, it is not possible to obtain a support that satisfies all of the above conditions (A) to (C) when cerium ammonium nitrate is used. In other words, even if the amount of ethylene glycol used, which is also used in conventional complex polymerization techniques, is appropriately adjusted, when cerium ammonium nitrate is used as the Ce raw material, the pore volume x and pore volume y of the resulting support have a trade-off relationship where improving one decreases the other. It is necessary to keep both pore volume x and pore volume y at a sufficiently high level, and furthermore, to set the value of 5x+y to 0.290 cm⁻¹. 3 g -1 It was found that no support material could be obtained in which pore volume x and pore volume y were sufficiently balanced to achieve the above. Furthermore, the catalysts obtained in Comparative Examples 2 to 4 were obtained using a support material that did not satisfy any of the above conditions (A) to (C), and therefore, Example 1 and Reference Examples 1-4 The inventors have investigated whether the ammonia synthesis rate was lower compared to the catalyst obtained using the other method.

[0101] [Regarding added metal elements] Based on the results shown in Table 1, the examples are as follows: and reference examplesIn cases where a manufacturing method that does not utilize ethylene glycol is employed ( Reference example 1 and Reference example 3 In comparison to the case where Ti is introduced into the composition of the carrier ( Reference example 3 ) If no additional metal elements are introduced into the composition of the carrier ( Reference example 1 It can be seen that the pore volume x of the support tends to be larger compared to the other material. The inventors speculate that this is because an increase in the number of constituent elements increases the energy required for crystal formation, suppressing the heat generation that leads to crystal aggregation during firing, and thus allowing for a more sufficient maintenance of the pore volume of finer pores, such as those with a pore diameter of 2 nm or more and less than 16 nm.

[0102] Furthermore, examples and reference examples In this case, when the same proportion of ethylene glycol is used during the production of the carrier, while each additional metal element is introduced into the composition of the carrier ( Reference example 4 and Example 1 In comparison to the case where Si is introduced into the composition of the support ( Example 1 ) When Ti is introduced into the composition of the support ( Reference example 4 It can be seen that the pore volume x of the support is larger than that of the support. From these results, it can be seen that adding a group 14 element (Si) is more effective than adding a group 4 element (Ti) in that it is possible to maintain the pore volume of fine pores more sufficiently during firing, etc.

[0103] Also, Reference example 1 and Reference Examples 3-4 and Example 1 Based on this comparison, the inventors surmise that by using at least one of the Group 4 and Group 14 elements as an additive metal element, it is possible to increase the pore volume x of pores with a diameter of 2 nm or more and less than 16 nm.

[0104] [About ethylene glycol] Example 1 , Reference Examples 1-4 andIn Comparative Examples 2 to 4, the carrier was manufactured using a complex polymerization method. Based on these examples, we will consider the effects obtained by using ethylene glycol. As is clear from the results shown in Table 1, when the complex solution used in the complex polymerization method further contains ethylene glycol ( Reference example 2, 4, Example 1, In Comparative Examples 3 and 4), the complex solution used in the complex polymerization method does not utilize ethylene glycol. Reference example Compared to 1, 3, and Comparative Example 2), the pore volume x of pores with a diameter of 2 nm or more and less than 16 nm tends to increase, while the pore volume y of pores with a diameter of 16 nm or more and less than 200 nm tends to decrease. From these results, it can be seen that when ethylene glycol is used, a support with a larger pore volume x of pores with a diameter of 2 nm or more and less than 16 nm, which serve as the reaction site, can be obtained. The reason why a support with a relatively large pore volume x and a relatively small pore volume y can be obtained is not entirely clear, but the inventors speculate that this is due to the fact that ethylene glycol allows for the polymerization of cation complexes through ester polymerization during synthesis, enabling the production of fine pores during calcination, and that ethylene glycol, being an organic substance, causes crystal aggregation during calcination, making it difficult for large pores to form.

[0105] As discussed above, Example 1 , Reference examples 1-4 From the results of Comparative Examples 1-4, it was found that a support satisfying the above conditions (A)-(C) can be obtained by a complex polymerization method using cerium nitrate, and that by using this as a catalyst, a catalyst with excellent ammonia synthesis activity can be obtained. Furthermore, it was found that by using ethylene glycol or added metal elements in the complex polymerization method using cerium nitrate, it is possible to more efficiently produce a support with a desired design (for example, a support with a larger pore volume x) while satisfying the above conditions (A)-(C). [Industrial applicability]

[0106] As described above, the present invention provides an ammonia synthesis catalyst that exhibits excellent ammonia synthesis activity and enables more efficient ammonia synthesis, a method for producing the ammonia synthesis catalyst that can efficiently produce the ammonia synthesis catalyst, and a method for synthesizing ammonia using the ammonia synthesis catalyst. The ammonia synthesis catalyst of the present invention exhibits excellent ammonia synthesis activity, and by utilizing it, ammonia can be synthesized efficiently. Therefore, it is particularly useful as a catalyst applied to the production of ammonia used as an energy carrier for hydrogen energy, for example.

Claims

1. A CeR-containing oxide support comprising cerium (Ce), at least one element selected from the group consisting of rare earth elements (R) other than Ce, and at least one additive metal element selected from the group consisting of Group 14 elements, Ruthenium (Ru) supported on the CeR-containing oxide support, It contains, After firing the CeR-containing oxide support in air at 700°C for 5 hours, the pore volume x (cm³) of pores with a diameter of 2 nm or more and less than 16 nm is obtained. ３ g －１ ) and the pore volume y (cm³) of pores with a diameter of 16 nm to 200 nm. ３ g －１ When identified by the BJH method, the values ​​of x and y are as follows: (A) to (C): [Condition (A)] x ≥ 0.005 [Condition (B)] y≧0.050 [Condition (C)] 5x+y≧0.290 An ammonia synthesis catalyst characterized by satisfying the following conditions.

2. The ammonia synthesis catalyst according to claim 1, characterized in that the rare earth element (R) other than Ce is at least one element selected from the group consisting of lanthanum (La) and praseodymium (Pr).

3. A step of obtaining a precursor powder containing Ce and at least one element selected from the group consisting of Ce and rare earth elements (R) other than Ce by complex polymerization using cerium nitrate as a raw material, A step of obtaining a CeR-containing oxide support by calcining the precursor powder, A step of obtaining an ammonia synthesis catalyst by supporting ruthenium (Ru) on the CeR-containing oxide support, Includes, The aforementioned complex polymerization method utilizes a salt of at least one additive metal element selected from the group consisting of Group 14 elements, A method for producing an ammonia synthesis catalyst, characterized in that the ammonia synthesis catalyst described in claim 1 or 2 is obtained as the ammonia synthesis catalyst.

4. The method for producing an ammonia synthesis catalyst according to claim 3, characterized in that ethylene glycol is further added to the complex solution in the complex polymerization method.

5. A method for synthesizing ammonia, characterized by contacting the ammonia synthesis catalyst described in Claim 1 or 2 with a gas containing hydrogen and nitrogen to synthesize ammonia.

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