Ammonia decomposition catalysts

Abstract

An ammonia decomposition catalyst wherein a first catalyst having a crystalline silicate which is represented by the formula in terms of molar ratio of oxides as dehydrated:(1±0.8)R2O.[aM2O3.bM′O.cAl2O3].ySiO2, wherein R denotes an alkaline metal ion and / or hydrogen ion, M denotes a VIII Group element, rare earth element, titanium, vanadium, chromium, niobium, antimony or gallium, M′ denotes magnesium, calcium, strontium or barium, a≧0, 20>b≧0, a+c=1, 3000>y>11 or a specific porous material as a carrier and iridium or a noble metal as an active metal is present together with or covered with a second catalyst having at least one element selected from the group consisting of titanium, vanadium, tungsten and molybdenum, if necessary, as well as a method of using the same.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a catalyst for decomposing ammonia contained in waste gases to nitrogen and a method of using the same for ammonia decomposition.[0003]2. Description of the Related Art[0004]Ammonia is widely used as a starting material for fertilizers and nitric acid as well as a reducing agent for removal of nitrogen oxides (NOx) in coolants and exhaust gases. As a result, a large amount of ammonia is emitted from various chemical plants, industrial waste disposal facilities for such things as refrigerators, and combustion exhaust gas treatment facilities.[0005]Ammonia is a gas having a characteristic irritating odor, and should be emitted into the atmosphere as little as possible. In reality, however, the formation of ammonia by degradation of organisms, emission of ammonia from coolants in disposed machines, and emission of unreacted ammonia in the reduction process of nitrogen oxides in flue gases r...

AI Technical Summary

Benefits of technology

[0008]An object of the present invention is to provide an ammonia decomposition catalyst capable of decomposing and removing ammonia with a high efficiency without producing by-products nitrogen oxides which may cause air pollution, whereby solving the problems associated with the conventional method described above.

[0018]The catalysts according to the present invention exhibits stable ammonia decomposition performance without degradation even in the presence of SO2 in exhaust gases. In addition, no disadvantageous production of acid ammonium sulfate is observed because of a low ability of oxidizing SO2 to SO3.

[0020]The crystalline silicate of the catalyst employed in the first aspect of the present invention can be synthesized by hydrothermal synthesis from compounds containing elements constituting such silicate. Alternatively, as such crystalline silicate, a layered composite crystalline silicate may also be employed in which using a crystalline silicate previously synthesized as a mother crystal a crystalline silicate consisting of Si and O having the crystalline structure identical to that of the mother crystal is grown on the surface of the mother crystal. In such layered composite crystalline silicate, H2O is prevented from penetrating into the crystalline silicate as a result of the hydrophobic effect of the crystalline silicate consisting of Si and O grown on the external surface (called silicalite). Accordingly, the release of a metal, such as aluminum, from the lattice of the crystalline silicate due to H2O is suppressed, whereby suppressing deterioration of the catalyst.

[0022]Catalyst A of the second aspect of the present invention can be utilized as an ammonia decomposition catalyst as described above. When catalyst A is used in the presence of a standard denitration catalyst (catalyst B), formation of NOx as a by-product is further prevented, whereby promoting selective conversion from NH3 to N2. Thus, additional effect is provided in which NOx formed as a by-product by catalyst A is converted by catalyst B to N2 in the reaction according to the equation shown below.4NH3+4NO+O2→4N2+6H2O

[0029]As a method for carrying at least one noble metal selected from the group consisting of platinum, palladium, rhodium and ruthenium as an active metal on various carriers in catalyst C, an ion exchange method for incorporating such noble metal ions into the carrier material or an impregnation method in which the carrier is impregnated with an aqueous solution of a noble metal salt, such as a chloride, may be employed. The active metal (noble metal) to be carried can exhibit a satisfactory activity at a concentration of 0.002% by weight, and for a higher activity about 0.02% by weight is preferred.

Problems solved by technology

As a result, a large amount of ammonia is emitted from various chemical plants, industrial waste disposal facilities for such things as refrigerators, and combustion exhaust gas treatment facilities.

In reality, however, the formation of ammonia by degradation of organisms, emission of ammonia from coolants in disposed machines, and emission of unreacted ammonia in the reduction process of nitrogen oxides in flue gases result in the release of large amounts of ammonia into the atmosphere.

However, when using a conventional catalyst NO, NO2 and N2O are generated as a result of the side reactions shown below and cause the another problem of air pollution.

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