When ammonia contained in various exhaust gases is discharged in the environment, it is necessary to detoxifying ammonia because ammonia has an odor, and for example, a method of oxidatively decomposing ammonia by contact with oxygen, and a method of directly decomposing ammonia to hydrogen, and the like have been proposed. Ammonia decomposition reaction is industrially used for producing atmospheric gas composed of nitrogen and hydrogen used for bright annealing of stainless steel, nickel steel, and the like.
Also, use of hydrogen as a clean energy source has recently attracted attention from the viewpoint of environmental protection and, for example, a method of recovering hydrogen from ammonia produced from organic waste and fuel cell vehicles using hydrogen as fuel have been actively developed. Hydrogen is preferred as clean energy, but particularly when hydrogen is used as a raw material of fuel cells for automobile cars, a method of supplying hydrogen to fuel cells has been a problem because of the necessity for a large volume of storage. As a method for resolving the problem, attention has recently been paid to a method of storing hydrogen as liquid ammonia and producing hydrogen by contact decomposition of vaporized ammonia.
Decomposition reaction of ammonia is volume expansion-type endothermic reaction represented by 2NH3→3H2+N2, and thus the reaction at lower pressure and higher temperature is advantageous for reaction equilibrium. Although a high temperature of 800° C. or more, preferably 1000° C. or more, is required for ammonia thermal decomposition, contact decomposition using a catalyst can be performed at a reaction temperature of 300° C. to 700° C.
A catalyst for ammonia synthesis can be considered to be used for ammonia decomposition reaction, but development of a catalyst for ammonia decomposition is required because ammonia decomposition is basically different from ammonia synthesis for two reasons below. The first reasons is that ammonia synthesis reaction is preferably performed under the conditions of a low temperature and high pressure such as 300° C. to 500° C. and 30 MPa due to equilibrium, while ammonia decomposition reaction which is reverse reaction is preferably performed under the conditions of a low pressure and high temperature. The second reason is that a rate-controlling step of ammonia synthesis reaction is activation of nitrogen molecules, while a rate-controlling step of ammonia decomposition reaction is desorption of nitrogen species adsorbed on a catalyst surface and produced by ammonia decomposition.
An optimum catalytic metal for ammonia contact decomposition is ruthenium (Ru) and, for example, there have been proposed a method (Patent Literature 1) in which a catalyst including a basic compound which is added to Ru supported on alumina is used as a catalyst suitable for decomposing ammonia recovered from a coke oven into hydrogen and nitrogen at a middle temperature of 400° C. to 500° C. under the atmospheric pressure, a method (Patent Literature 2) in which a catalyst composed of Ru supported on α-alumina and having a specific surface area of 8.5 to 100 m2/g is used at a reaction temperature of 300° C. to 800° C., a method (Patent Literature 3) in which a catalyst prepared by substituting, with a catalytically active metal such as Ru, a portion of A-sites or B-sites of a perovskite-type composite oxide represented by general formula ABO3 and formed by firing a raw material mixture at a high temperature of 1000° C. or more is used at a reaction temperature of 400° C. to 900° C., and the like.
Further, there have been proposed a method (Patent Literature 4) in which a catalyst containing an iron-group metal as an active metal and prepared by supporting an iron-group metal compound on at least one metal oxide selected from the group consisting of ceria, zirconia, yttria, lanthanum oxide, alumina, magnesia, tungsten oxide, and titania and then reducing the compound is used at a reaction temperature of 180° C. to 950° C., a method (Patent Literature 5) in which a catalyst prepared by supporting at least one metal element belonging to the group VIII to group X in the long-period periodic table on a support composed of a composite oxide containing ceria, alumina, and if required, zirconia, is used at a reaction temperature of 150° C. to 650° C., a method (Non-Patent Literature 1) in which a catalyst prepared by compounding a metallic component of Ni, Cu, or Zn with alumina cement composed of calcia and alumina, and the like. However, in the catalyst of this method, Ni easily reacts with alumina to produce a NiO—Al2O3 solid solution, thereby failing to produce a mayenite-type structure.
A hydrogen generating apparatus in which hydrogen produced by decomposing liquid ammonia is supplied to a fuel cell preferably uses a hydrogen producing catalyst capable of producing high-purity hydrogen with a high conversion rate at as a low reaction temperature as possible. Patent Literature 6 discloses that a noble metal catalyst such as Pt, Rh, Pd, Ru, or the like is preferred as a hydrogen producing catalyst exhibiting stable performance for ammonia decomposition reaction for fuel cell vehicles in which initiation or termination of reaction is repeated.
Also, Patent Literature 7 discloses that a Ni-based hydrogen producing catalyst is a preferred catalyst but requires a longer contact time for achieving the same conversion efficiency as a Ru-based catalyst, and the contact time of the Ru-based catalyst is 1/10 of that of the Ni-based catalyst. It is also disclosed that other preferred ammonia decomposition catalysts include Fe, Rh, Ir, Pd, Pt, and Re catalysts, and compounds containing such an element.
Patent Literature 8 discloses that an ammonia decomposition catalyst including a Na metal, K metal, Na compound, or K compound present on the surfaces of composite oxide particles containing La, Ni, Co, and Fe is suitable as a catalyst for efficiently producing hydrogen and nitrogen from ammonia with a high conversion rate.
On the other hand, there is a substance called the mineral name “mayenite” as calcium aluminosilicate containing CaO, Al2O3, and SiO2 as constituent components, and compounds having the same type crystal structure as the crystal of mayenite is referred to as “mayenite-type compounds”. It is reported that the mayenite-type compounds have a typical composition of 12CaO.7Al2O3 (referred to as “C12A7” hereinafter), and C12A7 crystals have a peculiar crystal structure (space group I4-3d) in which among the 66 oxygen ions present in a unit cell containing 2 molecules, 2 oxygen ions are included as “free oxygen” in the space of a cage formed by a crystal skeleton (chemical formula [Ca24Al28O64]4+(O2−)2 (referred to as “C12A7:O” hereinafter) (Non-Patent Literature 2).
In the mayenite-type compounds, Ca constituting the formula of the typical composition may be partially or entirely substituted by at least one typical metal element or transition metal element selected from the group consisting of Li, Na, K, Mg, Sr, Ba, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ir, Ru, Rh, and Pt. Also, Al constituting the formula of the typical composition may be partially or entirely substituted by at least one typical metal element or transition metal element selected from the group consisting of B, Ga, C, Si, Fe, and Ge. Further, O constituting the formula of the typical composition may be partially or entirely substituted by at least one typical element or metal element selected from the group consisting of H, F, Cl, Br, and Au.
In and after 2003, the inventors of the present invention have made it clear that the free oxygen ions can be substituted by various anions. In particular, when C12A7 is held in a strong reducing atmosphere, all free oxygen ions can be substituted by electrons. C12A7 in which free oxygen ions are substituted by electrons can be represented by the chemical formula [Ca24Al28O64]4+(e−)4 (may be referred to as “C12A7:e−” hereinafter).
Therefore, a substance in which anions are substituted by electrons is called “electride” and the electride has the characteristic of exhibiting good electron conductive properties (Non-Patent Literature 3 and Patent Literature 9). Also, electrons in the cage have the property of readily reacting with hydrogen in a gas phase and being taken as hydrogen anions (hydride) in C12A7 (Non-Patent Literature 4). When C12A7 is reduced with a reducing agent such as Ca, Ca(OH)2, CaH2, or the like, C12A7 including hydrogen anions can be easily synthesized (Non-Patent Literature 5). The C12A7 including hydrogen anions releases hydrogen and returns to electride by light irradiation, heating, or the like (Non-Patent Literature 4).
Although mayenite-type compounds containing hydrogen anions (H−, H2−) at a concentration of 1×1018 cm−3 or more and a method for producing the compounds are reported (Patent Literatures 10 to 12 and Non-Patent Literature 5), examples of application of hydrogen anion-including C12A7 are little known.
The inventors filed applications for patent for an invention (Patent Literature 13) relating to a catalyst for ammonia synthesis reaction, the catalyst containing a conductive mayenite-type compound and a metal, such as Ru, Fe, or the like, supported on the compound, and a method for synthesizing ammonia using the catalyst under the conditions of a reaction temperature of room temperature to 600° C. or less and a reaction pressure of 10 kPa to 20 MPa, and an invention (Patent Literature 14) relating to a method for reducing carbon dioxide to carbon monoxide by using a conductive mayenite-type compound.
Even when C12A7 has no conductivity, it has application as a catalyst and a catalyst support and, for example, a catalyst produced by spray-drying a solution of a raw material complex and then calcining at 1300° C. to 1400° C. for 2 hours or more is known to be used as a catalyst for steam decomposition reaction for producing soft olefins (Patent Literature 15). In recent years, a method for producing a C12A7 powder with a high specific surface area has been proposed, in which a precursor is synthesized by a hydrothermal method or a sol-gel method and is then fired (Non-Patent Literatures 6 and 7).