JP4512691B2 - Catalyst for selective reduction of nitrogen oxides by carbon monoxide and its preparation - Google Patents

Description

  The present invention relates to a catalyst for removing nitrogen oxides in diesel exhaust gas.

Diesel exhaust gas contains nitrogen oxides (NOx), which is an oxidative harmful component, and when it is reduced and rendered harmless, it reacts with carbon monoxide (CO) contained in the exhaust gas. This is a desirable method because there is no need to greatly change the operating conditions of the engine or to install a reducing agent in the diesel vehicle. This detoxification reaction is a reaction in which both the reactions are performed in an oxidizing atmosphere in which oxygen remains, and a catalyst containing a platinum group element has been used. For this reaction, an iridium catalyst (Patent Document 1) and a rhodium catalyst (Patent Document 2) that exhibit good oxidation performance and are durable in an oxygen-excess atmosphere have been developed.
As the iridium catalyst, iridium fine particles supported on a carrier made of a metal oxide such as silica (Non-patent Document 1), tungsten oxide (Patent Document 2), and zinc oxide (Patent Document 2) are used. The iridium catalyst supported on silica is effective when a relatively large amount of sulfur dioxide is contained in the exhaust gas.

In recent years, NOx emissions regulations for diesel vehicles have been increasingly tightened. On the other hand, the sulfur concentration in diesel fuel has been regulated to an increasingly lower level. When such a low-sulfur fuel is used, the exhaust gas has an extremely low SO ₂ concentration generated by combustion. Therefore, in order to deal with such an exhaust gas having an extremely low SO ₂ concentration, NOx contained in the exhaust gas is reduced. There is a need to develop catalysts that can be detoxified by CO.
As a catalyst having a space velocity of tens of thousands of h ⁻¹ or more under a condition of SO _{2 of} 1 ppm or less and a NOx conversion rate exceeding 80%, a catalyst using Ir / silicalite is known (Non-patent Document 3). In this case, in the case of actual industrialization, a support that is a thermally stable oxide, inexpensive and capable of large-scale synthesis is required, and the development of an Ir-supported catalyst that can solve this point is required. It has been demanded.
Environmental problems due to air pollution is increasingly serious, with fuel sulfur concentration is low as a diesel fuel, for very low exhaust gas SO ₂ concentration of SO ₂ 1 ppm or less for generating, NOx contained therein Therefore, there is an urgent need to develop a catalyst that can be detoxified using CO.
JP 2001-286759 A JP 2003-33654 A “Promotional effect of SO2 on the activity of Ir / SiO2 for NO reduction with CO under oxygen-richconditions”, M. Haneda, Pursparatu, Y. Kintaichi, I. Nakamura, M. Sasaki, T. Fujitani and H. Hamada, J. Catal., 229, 197-205 (2005) “Selective Catalytic Reduction of NO by COover Supported Iridium and Rhodium Catalysts”, Masahide Shimokawabe, Noriyoshi Umeda, Chem. Lett., 33, 534-535 (2004) “Catalytic Activity of Ir for NO-CO Reactionin the Presence of SO2 and Excess Oxygen”, Masaru Ogura, AyaKawamura, Masahiko Matsukata, Eiichi Kikuchi, Chem. Lett., 146-147 (2000)

An object of the present invention is to use a fuel having a low sulfur concentration as a diesel fuel, correspond to an extremely low exhaust gas in which the generated SO ₂ concentration is 1 ppm or less of SO ₂ , NOx contained therein is converted to CO It is another object of the present invention to provide a catalyst that can be detoxified by reduction and a method for producing the same.

The inventors have used a composite obtained by mixing tungsten oxide (WO ₃ ) and silica (SiO ₂ ) under simulated exhaust gas conditions in the absence of SO ₂ as a carrier, and a catalyst having iridium supported thereon. And found that NOx can be selectively reduced (hereinafter referred to as CO-SCR) by CO in the exhaust gas, and the present invention has been completed.

According to the present invention, the following inventions are provided.
(1) Iridium Ri to Do since supported on composite comprising tungsten oxide and silica, and characterized in that it is obtained by reduction treatment in hydrogen, optionally nitrogen oxides by carbon monoxide Reduction catalyst for reduction.
(2) a peroxide polytungstic acid solution, the addition of silica sol and dried to obtain a complex consisting of Risan tungsten and silica by the firing, impregnated with iridium compound in the resulting complex , iridium supported on said complex, followed characterized by reduction treatment in hydrogen production method of reducing catalyst which selectively reduces nitrogen oxides by carbon monoxide.

According to the catalyst obtained by the present invention, when a fuel having a low sulfur concentration is used, a NOx conversion rate exceeding 80% can be obtained in the selective reduction reaction of nitrogen oxides in the exhaust gas, and also about 90%. Useful catalysts are obtained that achieve the N ₂ selectivity that is reached.

The catalyst of the present invention is a catalyst obtained by supporting iridium on a composite composed of tungsten oxide and silica.
In this catalyst, the ratio of the composite composed of tungsten oxide and silica to iridium is in the range of 1: 1 to 1:50, preferably in the range of 1: 2 to 1:30. If the composite of tungsten oxide and silica is used in excess of the ratio of 1:50, the effect of iridium will not be obtained sufficiently, and the content of iridium will exceed the ratio of 1: 1. Even if it makes it, the special effect by having used a large amount of iridium cannot be acquired.

The composite consisting of tungsten oxide and silica is mixed with a precursor of tungsten oxide such as tungstic acid and a precursor of silica such as colloidal silica or silica sol, and the resulting product is dried and then subjected to a firing treatment. To be obtained as an oxide. The baking treatment is performed in the range of about 400 ° C to 600 ° C. These are considered to form a complex in the form of a mixture of the two. The catalyst exists in a chemically stable state as a complex under conditions of about 200 to 400 ° C., which is the temperature range in which the catalyst performs its function. In addition, the mechanical strength is maintained at a high temperature in this range.
The ratio of tungsten oxide and silica can exert the maximum CO-SCR activity when the content of tungsten oxide in the composite is 1 to 40% by weight, most preferably 5 to 30% by weight.
In order to keep this range, it can be performed by adjusting the mixing ratio of tungsten oxide and silica as raw materials.
The composite by the sintering process is porous.
The form of the composite can be appropriately determined according to the form of the catalyst used. By using a cylindrical shape, a granular shape, or a shape supported on a honeycomb support, it can be used for normal use. The capacity | capacitance of the composite_body | complex of the said form can be arbitrarily determined according to the state to be used.
As a catalyst carrier, it is clear by comparing Examples and Comparative Examples described below that the effects are obtained as compared with the case where tungsten oxide and silica are used as the carrier, respectively.

A composite composed of tungsten oxide and silica as a carrier is produced as follows.
The carrier in which WO ₃ and SiO ₂ are combined is prepared by adding ammonia (NH ₃ ) water to a peroxide polytungstic acid solution (pH = about 1.0) prepared by reacting hydrogen peroxide water with metal tungsten (W). Add excessive amount to tungstic acid to make it weakly alkaline (pH = 8.5), add a desired amount of colloidal silica or jelly-like silica sol, dry at 110 ° C, and then baked in air at 500 ° C for 4 hours. Can be obtained. Alternatively, a composite carrier obtained by performing the same treatment without adding NH ₃ in this step may be used.
By the above treatment, the composite carrier is obtained in an oxide state.

The method for supporting Ir on the composite is as follows.
The WO ₃ —SiO ₂ composite support prepared according to the above procedure is immersed in an aqueous solution composed of an Ir compound such as Ir (NH ₃ ) ₆ (OH) ₃ , H ₂ IrCl ₆ , Ir (NO ₃ ) ₃ , An impregnation method in which the composite is impregnated with Ir is employed.
The carrier is impregnated with any of the above Ir compounds, dried and fired.
Alternatively, Ir (NH ₃ ) ₆ (OH) ₃ , H ₂ IrCl in a mixed state of the polytungstic acid peroxide solution and a silica precursor such as colloidal silica or jelly-like silica sol before obtaining the WO ₃ —SiO ₂ composite support _{6. A} method of further mixing a compound such as Ir (NO ₃ ) ₃ and then drying and firing may be used.

The obtained Ir / WO ₃ —SiO ₂ catalyst exhibits its activity by being reduced at 400 ° C. or higher in a hydrogen (H ₂ ) stream regardless of the preparation conditions. The upper limit is about 700 ° C. or less.

Exhaust gas treatment is performed by bringing the catalyst obtained by supporting iridium thus obtained on a composite made of tungsten oxide and silica into contact with exhaust gas.
The composition of the exhaust gas has an SO ₂ concentration of 1 ppm or less of SO ₂ , and the ratio of NO and CO is approximately 1:10.
The temperature of the exhaust gas is 200 to 400 ° C.
Examples of the present invention will be described below.

By adding 70 mL of 15% H ₂ O ₂ to a ₂ L beaker containing 11 g of W powder in a metallic state, W and H ₂ O ₂ react vigorously, W is oxidized and dissolved, and polytungstic peroxide A solution was obtained. Next, 1/10 volume of concentrated NH ₃ water was added to this solution to obtain an ammonium peroxide polytungstate solution. Furthermore, colloidal silica (catalyst conversion, Catalloid S-20L) was mixed with this solution so that the weight ratio of WO ₃ and SiO ₂ was 1: 9. Thereafter, the mixed solution was evaporated to dryness at 100 ° C. or lower, and further calcined in air at 500 ° C. for 4 hours. The obtained WO ₃ —SiO ₂ composite was impregnated with an Ir (NH ₃ ) ₆ (OH) ₃ aqueous solution whose amount was adjusted so that the Ir weight was 0.5% by weight, and in air, at 110 ° C., After drying for 12 hours and firing at 500 ° C. for 4 hours, Ir / WO ₃ —SiO ₂ having various WO ₃ —SiO ₂ composition ratios were obtained.
For comparison, 0.5 wt% Ir / SiO ₂ and 0.5 wt% Ir / WO ₃ prepared from similar raw materials were prepared.

The obtained Ir / WO ₃ —SiO ₂ was pulverized to a particle size of 0.25 mm or less, and 0.1 g was mixed with silicon carbide particles sized to a particle size of 0.37 to 0.6 ml. The catalyst bed was packed so that its height was 8 mm. A thermocouple for measuring the catalyst bed temperature was placed near the center of the catalyst bed. Before the reaction, the catalyst was reduced in a 10% H ₂ / He stream at 600 ° C. for 1 hour. 1000 ppm NO, 5000 ppm CO, 10% O ₂ ,
The He diluent gas mixture containing 1% H ₂ O with a reaction gas, flows into the catalyst bed at a flow rate of 225 ml / min, and the product gas, FT-IR, which established the gas cell (NO, NO ₂ analysis) and micro gas chromatograph (CO, CO ₂ , N ₂ analysis). The activity was evaluated based on the NOx (= NO + NO ₂ ) conversion rate and N ₂ selectivity shown in the following formula.
NOx conversion rate = [(NOx inlet concentration−NOx outlet concentration) / NOx inlet concentration] × 100 (%),
N ₂ selectivity = [N ₂ outlet concentration / (N ₂ + N ₂ O outlet concentration)] x 100 (%)

NOx conversion of 0.5wt% Ir / WO ₃ -SiO ₂ as compared to 0.5wt% Ir / SiO ₂ and 0.5wt% Ir / WO ₃ showed significantly higher values. Both NOx conversion and N ₂ selectivity of 0.5 wt% Ir / WO ₃ —SiO ₂ reached a maximum at a reaction temperature of 260 ° C., reaching 86% and 89%, respectively.

In Example 1, an activity test was performed on 0.5 wt% Ir / WO ₃ —SiO ₂ and 0.5 wt% Ir / SiO ₂ under the condition that 0.5 ppm SO ₂ was added to the raw material gas, and in the presence and absence of SO ₂ . Table 1 shows the maximum NOx conversion and N ₂ selectivity.

It was found that 0.5 wt% Ir / WO ₃ —SiO ₂ maintains high activity even when 0.5 ppm SO ₂ is added. Ir / SiO _2, which has been reported to promote activity by the addition of SO _2, did not show a significant promotion effect due to its low SO ₂ concentration.

Effect of SO ₂ coexisting in reaction gas (Example 2)

In Example 1, colloidal silica (catalyst conversion, Catalloid) was prepared by adjusting the amount of the ammonium peroxide polytungstate solution to a desired WO ₃ / (WO ₃ + SiO ₂ ) weight ratio (from 0 to 1). S-20L) was mixed, and the same activity test was performed on the catalyst which was subjected to the same treatment. FIG. 2 shows the relationship between the WO ₃ / (WO ₃ + SiO ₂ ) weight ratio, the maximum NOx conversion rate, and the N ₂ selectivity. The NOx conversion showed a value of 80% or more when the WO ₃ content was 5 to 30%, and the N ₂ selectivity was also about 90%.

In the preparation of Example 1, after mixing the colloidal silica with the amount adjusted to be WO ₃ : SiO ₂ = 3: 7 with respect to the polytungstic acid solution or ammonium polytungstate solution, A similar activity test was conducted on the catalyst that had been treated in the same manner. Table 2 shows the maximum NOx conversion rate and N ₂ selectivity at that time. Regardless of whether or not NH _{3 was} added to the peroxide polytungstic acid, the maximum NOx conversion reached 50% or more, and the N ₂ selectivity reached 80% or more.

Effect of the presence or absence of NH ₃ addition to peroxide polytungstic acid on activity (Example 4)

In the preparation of Example 4, colloidal silica was mixed with adjusting the amount of WO ₃ : SiO ₂ = 3: 7 with respect to the polyperoxytungstic acid solution or the ammonium peroxytungstate solution, and Ir. After mixing Ir (NH ₃ ) ₆ (OH) ₃ aqueous solution whose amount was adjusted so that the weight was 0.5% by weight, this mixed solution was evaporated to dryness at 100 ° C. or lower, and further, 500 ° C. for 4 hours. A similar activity test was performed on the catalyst obtained by calcination in air. Table 3 shows the maximum NOx conversion rate and N ₂ selectivity at that time. Since these results were lower than those of Ir impregnation supported in Table 2, it was found that it is preferable that the Ir precursor is impregnated and supported after producing WO ₃ —SiO ₂ .

Effect of the presence or absence of NH ₃ addition on the activity when Ir precursor is mixed during the preparation of WO ₃ —SiO ₂ (Example 5)

Table 4 shows the maximum NOx conversion rate and N ₂ selectivity at that time for Ir / WO ₃ —SiO ₂ catalyst prepared in Example 1 with or without H ₂ treatment before reaction. The difference in maximum NOx conversion with and without H ₂ pretreatment was about 50%, indicating that H ₂ pretreatment is essential for this catalyst.

Effect of presence or absence of H ₂ pretreatment (Example 6)

The catalyst of the present invention exhibits an effective activity for reducing NOx in exhaust gas from diesel engines, and diesel vehicles in which exhaust gas regulations are being tightened, or O ₂ remains in exhaust gas as in the case of diesel vehicles. It is expected to be used as an exhaust gas treatment technology for lean burn gasoline vehicles that are difficult to reduce and detoxify.

Temperature dependence of activity of 0.5 wt% Ir / WO ₃ —SiO ₂ (WO ₃ : SiO ₂ = 1: 9) (Example 1) Dependence of WO ₃ / (WO ₃ + SiO ₂ ) weight ratio on the activity of a catalyst having 0.5 wt% Ir supported on a WO ₃ —SiO ₂ composite support (Example 3)

Claims (2)

Iridium Ri Do since was carried on a complex consisting of tungsten oxide and silica, and characterized in that it is obtained by reduction treatment in hydrogen, selectively reducing nitrogen oxides by carbon monoxide reduction Catalyst. Peroxide polytungstic acid solution, the addition of silica sol and dried to obtain a complex consisting of O Risan tungsten and silica by firing, impregnated with iridium compound in the resulting complex, iridium the supported on complex subsequently characterized by reduction treatment in hydrogen production method of reducing catalyst which selectively reduces nitrogen oxides by carbon monoxide.

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