JP4895090B2 - NOX selective reduction catalyst - Google Patents

Description

The present invention relates to a NO _X selective reduction catalyst for reducing NO _X in exhaust gas discharged from a lean combustion internal combustion engine or the like with NH ₃ .

A NO _X selective reduction catalyst that selectively reduces NO _X with NH ₃ in an oxygen-excess atmosphere has been put into practical use in fixed sources such as a thermal power generation boiler, a sintering furnace, a coke oven, and a gas turbine. As such a catalyst component, V as an active species and TiO ₂ as a carrier are mainly used. On the other hand, also in mobile sources such as automotive diesel engines, the use of _the NO _X selective reduction with NH ₃ or NH ₃ precursor such as urea have been demanded, it has led recently commercialized. As such a catalyst component, a V / TiO ₂ catalyst that is also used as a fixed source or an Fe / zeolite catalyst in which Fe is supported on zeolite has been the mainstream.

However, in the V / TiO ₂ catalyst and the Fe / zeolite catalyst, the exhaust gas whose temperature changes greatly in the range of 200 to 700 ° C. such as exhaust gas discharged from a moving generation source such as an automobile diesel engine exceeds a predetermined level. Could not be purified stably. Furthermore, the V / TiO ₂ catalyst has a problem that vanadium oxide melts and scatters when exposed to a high temperature atmosphere of 700 ° C. or higher, and the Fe / zeolite catalyst has a temperature of 700 ° C. or higher. aluminum in the the zeolite lattice exposed to a high temperature atmosphere of a problem such that NO _X purification activity decreased by dealumination phenomenon get out of the lattice.

  In order to solve the above problems, for example, Japanese Patent Laying-Open No. 2005-81189 (Patent Document 1) discloses a denitration catalyst for high-temperature exhaust gas containing nitrogen oxide, and has an acid strength of Ho ≦ −11. 35 supports, or at least one oxide selected from the group consisting of vanadium, tungsten, molybdenum, iron, chromium, copper, manganese and cobalt on a support having a solid acid amount of 0.2 mmol / g or more, or A denitration catalyst for high-temperature exhaust gas characterized in that these composite oxides are supported is disclosed. In the specification, a denitration catalyst for high-temperature exhaust gas in which a metal such as iron is supported on sulfate titania zirconia is disclosed. Are listed. Japanese Patent Application Laid-Open No. 2005-137984 (Patent Document 2) discloses a catalyst for reducing nitrogen oxides in the presence of ammonia, which is composed of iron-containing sulfate zirconia. Has been.

However, even in the denitration catalyst as described in literature, particularly NO _X purification activity in the temperature range of 200 to 350 ° C. it was not yet sufficient.
JP 2005-81189 A JP 2005-137984 A

The present invention has been made in view of the above-described problems of the prior art, has excellent heat resistance in a high temperature atmosphere of 700 ° C. or higher, and has a high level of NO _X purification in a temperature range of 200 to 700 ° C. An object is to provide an NO _X selective reduction catalyst having activity.

As a result of intensive studies to achieve the above-mentioned object, the present inventors have supported at least one selected from the group consisting of W and Mo on a zirconia support and a combination of Fe and 700 ° C. or higher. It has been found that a NO _X selective reduction catalyst having excellent heat resistance in a high temperature atmosphere and having a high level of NO _X purification activity in a temperature range of 200 to 700 ° C. is achieved, and the present invention has been completed. It was.

That, _the NO _X selective reducing catalyst of the present invention is a _the NO _X selective reducing catalyst for reducing NO _X discharged from the internal combustion engine by NH _3, and a zirconia support, W is supported on the carrier and / or features and Mo, said has a Fe carried on the carrier, the der Rukoto made by brought further carrying Fe after immobilizing by firing allowed carrying W and / or Mo in the zirconia support It is what.

Furthermore, in the NO _X selective reduction catalyst of the present invention, the supported amount of W and / or Mo is preferably in the range of 0.1 to 30% by mass with respect to the total mass of the NO _X selective reduction catalyst. .

In the NO _X selective reduction catalyst of the present invention, the amount of Fe supported is preferably in the range of 0.1 to 15% by mass with respect to the total mass of the NO _X selective reduction catalyst.

In the NO _X selective reduction catalyst of the present invention, the reason why excellent heat resistance in a high-temperature atmosphere of 700 ° C. or higher and high-level NO _X purification activity in the temperature range of 200 to 700 ° C. is not necessarily clear. However, the present inventors speculate as follows. That is, W and Mo itself exhibit NO _X selective reduction activity at a temperature of 400 ° C. or higher, but a strong acid point is expressed by fixing W or Mo on ZrO ₂ . When Fe exists on such a strong acid point and such a strong acid point and Fe coexist, a high level of NO _x purification activity in a temperature range of 200 to 700 ° C. is achieved. They guess. In addition, W, Mo, Fe, and ZrO ₂ do not have the problem of melting or scattering like vanadium oxide even at a temperature of 700 ° C. or higher, and the dealumination phenomenon does not occur like aluminum. Therefore, the present inventors speculate that the NO _X selective reduction catalyst of the present invention has excellent heat resistance in a high temperature atmosphere of 700 ° C. or higher.

According to the present invention, it is possible to provide a NO _X selective reduction catalyst having excellent heat resistance in a high temperature atmosphere of 700 ° C. or higher and having a high level of NO _X purification activity in a temperature range of 200 to 700 ° C. It becomes possible.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

_The NO _X selective reducing catalyst of the present invention is a _the NO _X selective reducing catalyst for reducing NO _X discharged from the internal combustion engine by NH _3, and a zirconia support, and W and / or Mo, which is supported on the support And Fe supported on the carrier.

Such a zirconia support is not particularly limited as long as it contains ZrO ₂ after firing. For example, zirconium oxide (ZrO ₂ ), zirconium hydroxide (Zr (OH) ₄ nH ₂ ). O).

In the NO _X selective reduction catalyst of the present invention, W and / or Mo described later in the zirconia support and the later described from the viewpoint of achieving a high level of NO _X purification activity in a temperature range of 200 to 700 ° C. It is necessary to carry Fe.

  As a method for supporting W and / or Mo on the zirconia support in this way, for example, after impregnating the zirconia support in an aqueous solution containing a W source and / or Mo source and supporting W and / or Mo. Examples thereof include a firing method, a W source and / or Mo source, and a method in which the zirconia support is physically mixed and fired. Among these methods, from the viewpoint of supporting W and Mo on the zirconia support in a highly dispersed state, the aqueous solution containing the W source and / or Mo source is impregnated with the zirconia support to support W and / or Mo. A method of firing after heating is preferable.

Such a W source and / or Mo source may be a salt of W and / or Mo, and is not particularly limited. From the viewpoint of the acid strength of the catalyst, for example, an ammonium salt (for example, (NH ₄ ) _{10 W 12 O 41, (NH 4} ) 6 Mo 7 O 24), heteropolyacid salt containing P and Si _{(e.g., H 3 PW 12 O 40,} H 3 PMo 12 O 40, H 4 SiW 12 O 40, H 4 SiMo ₁₂ O ₄₀ ) is preferred.

In addition, the amount of W and / or Mo supported is preferably in the range of 0.1 to 30% by mass, and 1 to 20% by mass with respect to the total mass of the NO _X selective reduction catalyst of the present invention. More preferably, it is the range. If the supported amount is less than the lower limit, the amount of strong acid points is not sufficient, and therefore the amount of Fe coexisting with the strong acid points tends to decrease. On the other hand, if the upper limit is exceeded, W and / or Mo are adsorbed in multiple layers. In order to cover the strong acid point, the amount of Fe coexisting with the strong acid point tends to decrease. Furthermore, as the conditions for firing after supporting W and / or Mo, firing temperature is preferably 600 to 800 ° C. and firing time is 1 to 5 hours.

Examples of a method for supporting Fe on the zirconia support as described above include, for example, a method in which an aqueous solution containing an Fe source is impregnated with the zirconia support to support Fe and then firing, and FeCl ₃ sublimation is utilized. And calcination after carrying the chemical vapor deposition on the zirconia support. Among these methods, from the viewpoint of supporting Fe only in the vicinity of a strong acid point, a method of firing after supporting Fe on the zirconia support by chemical vapor deposition using sublimation of FeCl ₃ is preferable. An example of such a Fe source is FeCl ₃ .

The amount of Fe supported is preferably in the range of 0.1 to 15% by mass and preferably in the range of 1 to 10% by mass with respect to the total mass of the NO _X selective reduction catalyst of the present invention. It is more preferable. The Fe loading amount is less than the lower limit of, located in the NO _X purification activity is low tendency to amount of Fe coexist with a strong acid point is small, whereas if it exceeds the upper limit, the amount of Fe coexisting with a strong acid point is saturated, Fe tends to NO _X purifying activity decreases to multilayer adsorption. Furthermore, as conditions for firing after supporting Fe, it is preferable that the firing temperature is 600 to 800 ° C. and the firing time is 1 to 5 hours.

In addition, the NO _X selective reduction catalyst of the present invention has W and / or Mo supported on the zirconia support and immobilized by calcination to express a strong acid point from the viewpoint of supporting Fe in the vicinity of the strong acid point. It is preferable that Fe is further supported.

  Examples of the method for supporting Fe on such a carrier include the same methods as those for supporting Fe on the zirconia carrier described above.

Furthermore, the shape of the NO _X selective reduction catalyst of the present invention is not particularly limited, and examples thereof include powder, granule, pellet, and honeycomb. Further, the NO _X selective reduction catalyst of the present invention may be non-porous or porous.

In addition, the method for reducing NO _X discharged from the internal combustion engine with NH ₃ using the NO _X selective reduction catalyst of the present invention is not particularly limited, and a known method can be appropriately selected. Further, in such a method, a known NO _X selective reduction catalyst can be used in addition to the NO _X selective reduction catalyst of the present invention.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
First, 300 ml of a 25% aqueous ammonia solution was added while stirring 350 ml of a 1.0 mol / L zirconium oxynitrate aqueous solution with a propeller stirrer to precipitate zirconium hydroxide. Next, the operation of filtering the obtained precipitate after washing with ion-exchanged water was repeated three times to remove remaining nitrate ions and ammonium ions. Then, it was made to dry at 100 degreeC for 12 hours, and white zirconium hydroxide was obtained.

Next, 10 g of zirconium hydroxide obtained was impregnated in a solution of 4 g of ammonium molybdate dissolved in 150 ml of ion-exchanged water, evaporated and dried, and then fired at 600 ° C. for 5 hours to obtain Mo-supported ZrO ₂ powder. Next, 5 g of the obtained Mo-supported ZrO ₂ powder and 0.83 g of FeCl ₃ powder were physically mixed in nitrogen and heat-treated at 600 ° C. for 1.5 hours under a nitrogen flow. After washing and drying, Fe is supported by firing in the atmosphere at 600 ° C. for 5 hours, and further compacted and sized after being compacted at about 1000 kgf / cm ² to 0.5. The NO _X selective reduction catalyst was obtained as a pellet of ˜1.0 mm.

(Examples 2 to 6)
Instead of 4 g of ammonium molybdate, 2 g of silicomolybdic acid (Example 2), 4 g of silicomolybdic acid (Example 3), 2 g of silicotungstic acid (Example 4), 4 g of silicotungstic acid (Example 5) or phosphotungstic acid A NO _X selective reduction catalyst was obtained in the same manner as in Example 1 except that 2 g (Example 6) was used.

(Comparative Example 1)
A comparative NO _X selective reduction catalyst was obtained in the same manner as in Example 1 except that ZrO ₂ powder not supporting Mo was used instead of the Mo-supporting ZrO ₂ powder.

(Comparative Example 2)
A comparative NO _X selective reduction catalyst was obtained in the same manner as in Example 1 except that Fe was not supported on the Mo-supported ZrO ₂ powder.

(Comparative Example 3)
A comparative NO _X selective reduction catalyst was obtained in the same manner as in Example 1 except that SiO ₂ (manufactured by Nippon Aerosil Co., Ltd., AEROSIL (registered trademark) 380PE) was used instead of zirconium hydroxide.

<Characteristic evaluation of NO _X selective reduction catalyst>
(I) Heat resistance test Each of the NO _X selective reduction catalysts obtained in Examples 1 to 6 and Comparative Examples 1 to 3 contains air containing 3% H ₂ O at 700 ° C. at a gas flow rate of 1 L / min. The heat supply test was conducted by supplying for hours.

(Ii) NO _X purification rate With respect to the NO _X selective reduction catalysts obtained in Examples 1 to 6 and Comparative Examples 1 to 3, measurement of the NO _X purification rate using each NO _X selective reduction catalyst before and after the heat resistance test as a catalyst sample. Went. That is, first, 1 g of each NO _X selective reduction catalyst was installed as a catalyst sample in an atmospheric pressure fixed bed flow type reactor (Cata-5000, manufactured by Best Sokki Co., Ltd.). Next, a model composed of NO (0.1%), NH ₃ (0.1%), O ₂ (8%), CO ₂ (10%), H ₂ O (8%), and N ₂ (balance). The gas was supplied at a gas flow rate of 3.5 L / min, and the gas temperature with catalyst was adjusted to 150 ° C. Thereafter, while maintaining the temperature of the gas containing the catalyst at 150 ° C. for 15 minutes, the NO _X concentration of the gas containing the catalyst and the gas emitted from the catalyst in the steady state was measured, and the NO _X purification rate was calculated from those measured values. Then, the temperature of the gas containing the catalyst was increased by 25 ° C., and the NO _X purification rate in the steady state was calculated every 150 ° C. from 150 ° C. to 350 ° C. in the same manner as described above.

(Iii) Evaluation results Table 1 shows the results obtained for the catalyst samples before the heat resistance test. The results obtained for the catalyst sample after the heat test are shown in Table 2. Furthermore, Figure 1 shows the graph showing the NO _X purification rate of each _the NO _X selective reducing catalyst in the catalyst-containing gas temperature 350 ° C..

As apparent from the results shown in Tables 1 and 2 and FIG. 1, the NO _X selective reduction catalysts (Examples 1 to 6) of the present invention exhibit a high level of NO _X purification activity, and further NO after the heat resistance test. _X purification activity was maintained. On the other hand, instead of a comparative NO _X selective reduction catalyst that does not contain Mo and W (Comparative Example 1), a comparative NO _X selective reduction catalyst that does not contain Fe (Comparative Example 2), and ZrO ₂ The comparative NO _X selective reduction catalyst using SiO ₂ (Comparative Example 3) did not show sufficient NO _X purification activity. Therefore, it is confirmed that the NO _X selective reduction catalyst of the present invention has excellent heat resistance in a high temperature atmosphere of 700 ° C. or higher, and has a high level of NO _X purification activity in a temperature range of 200 to 700 ° C. It was.

As described above, according to the present invention, NO _X selective reduction having excellent heat resistance in a high temperature atmosphere of 700 ° C. or higher and having a high level of NO _X purification activity in a temperature range of 200 to 700 ° C. A catalyst can be provided.

Therefore, the NO _X selective reduction catalyst of the present invention is useful as a NO _X selective reduction catalyst for reducing NO _X in exhaust gas discharged from a lean combustion type internal combustion engine or the like with NH ₃ .

Is a graph showing the NO _X purification rate of each _the NO _X selective reducing catalyst in the catalyst-containing gas temperature 350 ° C..

Claims (3)

A NO _X selective reduction catalyst for reducing NO _X discharged from an internal combustion engine with NH ₃ , comprising a zirconia support, W and / or Mo supported on the support, Fe supported on the support, and includes a, NO _X selective reducing catalyst, characterized in der Rukoto made by brought further carrying Fe after immobilizing by firing allowed carrying W and / or Mo in the zirconia support. Wherein W and / or the loading amount of Mo is, _the NO _X selective reducing catalyst according to claim 1, characterized in that in the range of 0.1 to 30% by weight relative to the total weight of the _the NO _X selective reducing catalyst . The supported amount of Fe, the _the NO _X selective reducing _the NO _X selective reducing catalyst according to claim 1 or 2, characterized in that in the range of 0.1 to 15% by weight relative to the total weight of the catalyst.

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