JPH0722709B2 - Method for producing denitration catalyst using ceramics paper - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a NOx removal catalyst for selective reduction of nitrogen oxides (NOx) in exhaust gas by using ammonia (NH ₃ ), and more specifically, so-called ceramic paper. The present invention relates to a method for producing a NOx removal catalyst.

Conventional technology and its problems Conventionally, as an NH ₃ reduction selective denitration catalyst, a catalyst in which a metal oxide such as V ₂ O ₅ is supported on anatase type titania (TiO ₂ ) is known to show excellent performance. ing. This catalyst is prepared by adding a suitable inorganic binder and water to a powdery catalyst component, kneading the mixture well, and granulating the kneaded product into a granular catalyst having an appropriate particle size, or a monolysis catalyst having an appropriate size (first It is manufactured by extrusion molding (see FIG. 2). In the selective reduction denitrification method using NH _3, a monolysis type catalyst is considered to be more advantageous than a granular catalyst in consideration of gas flow resistance in the catalyst bed and clogging of the catalyst bed due to dust.

However, anatase-type TiO ₂ lacks thermal stability, and even at a relatively low temperature of, for example, 500 to 600 ° C., a specific surface area decreases due to sintering, and a transition to catalytically inactive rutile type crystals occurs. Therefore, the sintering after extrusion cannot be sufficiently performed, and the mechanical strength of the monolysis catalyst is very low. Therefore, in the monolysis catalyst shown in Fig. 2, for a catalyst with a large opening (1) used for high flow rate gas, the wall thickness (2) generally needs to be 1 mm or more, and for a low flow rate with a small opening (1). Even with a catalyst, it is said that it is difficult to reduce the wall thickness (2) to 0.5 mm or less. Further, this catalyst has a problem that it requires careful handling and hinders the efficiency of the catalyst filling work.

In order to overcome the above drawbacks, a plate catalyst reinforced with a metal wire or metal lath-like metal has been proposed. However, since this has a large plate thickness of about 1 mm and is expensive to manufacture, its advantage is doubtful as compared with an extrusion-molded monolysis catalyst. Furthermore, in this catalyst, since it is necessary to form the catalyst as a dense oxide as much as possible in order to improve the mechanical strength, the gas diffusibility of NOx, NH _3, etc. in the catalyst pores is impaired, and as a result, V ₂
There is a tendency that the original performance of the O ₅ / TiO ₂ based catalyst is not exhibited.

The present inventors have previously proposed, for the purpose of solving the above-mentioned problems, a denitration catalyst characterized in that TiO ₂ fine powder is dispersed and held in a glass fiber aggregate (preform body) (Japanese Patent Laid-Open Publication No. Sho.
55-155744 and Japanese Patent Publication No. 58-1976).
However, although high activity is obtained with this catalyst,
The following new problems arose. That is, (1) it is necessary to use a metal supporting plate to impart rigidity to the glass fiber aggregate, (2) the plate thickness of the catalyst becomes as thick as 1 mm to 2 mm, and (3) the catalyst is subjected to a severe mechanical impact. The powder peels off little by little and scatters.

Therefore, the present inventors also proposed a denitration catalyst having a novel structure using ceramic paper as a catalyst capable of overcoming the above-mentioned problems of the prior art, at the same time as the present case. In this catalyst, ceramic paper is impregnated with titania sol, dried and calcined, and the resulting titania holding paper carries vanadium oxide. This catalyst is thin and light, has high mechanical strength, and the binder effect of TiO ₂ cures the paper, giving rigidity to the catalyst and increasing bending strength, and as a result, powder falling due to mechanical impact is reduced. Further, the inter-fiber voids ensure the diffusibility of the reaction gas inside the catalyst and exhibit excellent activity. In this catalyst, the ceramic paper is heated in a low oxygen atmosphere as a pretreatment during the manufacturing process, and is further baked after impregnation, so that all the organic binder is burned and decomposed. Therefore, the strength of the catalyst after burning out of the binder relies exclusively on the binder effect of TiO ₂ . However, TiO ₂ obtained by drying the sol is relatively inferior in strength and has low adhesiveness to fibers, so that it does not show such a strong binder effect. If calcination is performed at a temperature of 500 to 600 ° C, the strength of TiO ₂ increases and the binder effect also improves, but anatase type TiO ₂
Lacks thermal stability, and even when heated at a relatively low temperature as described above, the specific surface area decreases and the catalyst becomes inactive due to transfer to rutile type crystals.

An object of the present invention is to provide a method for producing a thinner and lighter denitration catalyst while maintaining the excellent characteristics of the above-mentioned catalyst using ceramics paper as it is, and further increasing the strength of the catalyst.

Means for Solving the Problems In order to achieve the above-mentioned object, the method for producing a denitration catalyst according to the present invention impregnates a ceramic paper produced by paper-making of silica-alumina-based or alumina-based ceramic fibers with a mixture of titania sol and silica sol. It is characterized in that it is dried and calcined, and the vanadium oxide is carried on the obtained solid holding paper.

(1) In the present invention, a mixture of titania sol and silica sol is used as a sol for impregnating ceramic paper.

A catalyst composed of ceramic paper impregnated with a mixture of titania sol and silica sol, dried and fired, and vanadium oxide supported on the obtained solid holding paper is a catalyst obtained by using only titania sol as an impregnating sol. On the other hand, as shown in the vibration resistance test of the examples described later, it is superior in each step in terms of vibration resistance, and as shown in the bending strength test of the examples described below, it is comparable in bending strength. This is because when the sol is dried and solidified in the ceramics paper, the mixed silica sol causes the solid matter to form relatively uniformly on the surface of the paper, and the solid matter having an excessive particle size is localized on the surface. It is considered that this is due to the effect of preventing

The denitration activity of the catalyst tends to decrease due to the incorporation of SiO ₂ , as is clear from the activity test results of the examples described later, but the SiO ₄ incorporation rate = [SiO ₂ / (TiO ₂ + SiO ₂ )] × 100 Is 80
If the amount is less than or equal to the weight%, there is almost no effect of mixing SiO ₂ . The mixing ratio of the titania sol and the silica sol is set so that the mixing ratio of SiO ₂ is usually 5 to 16% by weight, preferably 7 to 13% by weight.

(2) The catalyst powder is dispersed and held in the fiber preform.

Since it is theoretically impossible to improve the strength of the V ₂ O ₅ / TiO ₂ type plate-like catalyst and / or monolysis catalyst without losing the catalytic activity, consider reinforcement with some fibrous substance. There is a need. In this case, a method of kneading the catalyst powder and the fiber and producing a monolysis catalyst by extrusion molding (kneading method) and a method of dispersing and holding the catalyst powder in the fiber aggregate, that is, the fiber preform (fiber preform method) are considered. To be

For plate catalysts, the kneading method is excellent for increasing the amount of catalyst per plate area, but in the kneading method, the diffusibility of the reaction gas inside the catalyst is low and the original performance of the catalyst cannot be exhibited. Further, it is difficult to reduce the plate thickness to 0.5 mm or less due to the extrusion moldability of the paste in which the fibers are kneaded.

In the case of the fiber preform method, for example, if the reaction temperature is 200 ° C. or higher, the catalyst powder may be 100 g / m ² or less, and the plate thickness is determined by the thickness of the fiber preform body and is 0.15 mm or more. Can be set freely. Therefore, the catalyst by the fiber preforming method is advantageous.

(3) Ceramic paper is used as the fiber preform.

Glass fiber paper and ceramic paper are examples of fiber preforms in which fibers are accumulated as thinly and densely as possible and the rigidity of the fibers themselves can be expected to some extent.

Since glass fiber paper generally has a low binder effect of an inorganic binder against the glass fiber paper and has a lower strength as paper than the strength of ceramic paper, it is preferable to use ceramic paper.

To form the inorganic fiber as paper, it is necessary to add a binder to the fiber. As the binder, since an organic binder decomposes and / or burns under the use condition of the catalyst and loses its effect, it is in principle preferable to use an inorganic binder.

(4) A sol impregnation method is applied to hold the TiO ₂ powder or the catalyst powder containing the TiO ₂ powder on the inorganic fiber paper.

As a method of holding the catalyst powder on the inorganic fiber paper, a method of impregnating TiO ₂ colloid sol (sol impregnation method)
And a method of filtering TiO ₂ powder or catalyst powder supporting V ₂ O ₅ on it at the papermaking stage (catalyst powder filtering method). If allowed to hold the TiO ₂ sol impregnation method, ceramic fibers, contact between the TiO ₂ is good compared with the adhesion between the glass fibers and TiO _2, dusting of the catalyst compared to the case of the glass fiber paper using Shows the feature of small quantity.

When TiO ₂ is retained by the sol impregnation method when using ceramics paper, the retained TiO ₂ exhibits the effect as a binder and improves the rigidity as a plate catalyst, especially the bending strength. Is more advantageous than

The catalyst produced in this manner has a high diffusivity of the reaction components inside the catalyst, and the catalyst powder can exhibit its original performance. Therefore, the catalyst has higher activity than the conventional monolysis catalyst prepared by the extrusion molding method. Moreover, the catalyst thickness can be reduced to about 0.1 mm. Therefore, when the catalyst surface area per volume is constant, this catalyst exhibits a higher porosity and therefore a lower gas flow resistance than the conventional V ₂ O ₅ / TiO ₂ monolithic catalyst.

(5) It is preferable to preheat the ceramic paper in a low oxygen atmosphere.

In order to increase the rigidity of the catalyst, it is necessary to use high density ceramic paper. High-density ceramic paper is generally made by using an organic binder and an inorganic binder in combination. When such a paper is subjected to the sol impregnation method, the paper is heated in a low oxygen atmosphere before impregnation with the sol to improve the adhesion between TiO ₂ and SiO ₂ and the fiber, and the organic binder on the fiber surface is heated. It is preferable to remove the carbonization. In this case, if oxygen is present in an amount of 5% or more, the organic binder is completely removed, and the strength of the paper is remarkably reduced. Further, if oxygen is not present at all, a large amount of carbon remains on the fiber surface and the adhesion cannot be improved. Therefore, it is desirable to heat the ceramic paper with an oxygen concentration of 1 to 5%. Heating conditions are 300-500 ℃ for 0.5-4 hours, preferably 350-400 ℃
It takes 2-3 hours.

As TiO ₂ , anatase-type TiO ₂ formed by hydrolysis of titanium salt such as titanium sulfate, titanium tetrachloride, tetrapropyl titanate is preferable. The TiO ₂ thus formed is
Since it has many large pores of 1000Å or more, the diffusion rate of the reaction gas in the powder is high.

The titanium content is ₂₀ to 300 g, preferably 30 to 200 g as TiO ² per 1 m ^{2 of the} surface of the ceramic paper.

Commercially available silica sol can be used. The vanadium oxide is supported on the ceramics paper, for example, by impregnating the ceramics paper with an ammonium metavanadate solution, drying and firing. The vanadium oxide precursors are not limited to those described above.

EFFECTS OF THE INVENTION The method for producing a denitration catalyst according to the present invention is to impregnate a ceramic paper with a mixture of titania sol and silica sol so that TiO ₂ and SiO ₂ are retained in the paper, and vanadium oxide is supported on the obtained solid-retaining paper. By using ceramic paper, the thickness of the catalyst can be remarkably reduced and the mechanical strength can be greatly increased. Further, by using a mixture of titania sol and silica sol, TiO ₂ can be firmly fixed to the fibers of the ceramic paper. That is, in the present invention, the titania sol acts not only as a catalyst, but also as a binder by getting between the fibers. Therefore, it is possible to harden the ceramic paper more strongly, give rigidity to the catalyst, and increase bending strength. As a result, powder falling due to mechanical shock can be minimized. Furthermore, the inter-fiber voids of the fibers that make up the ceramic paper ensure the diffusivity of the reaction gas inside the catalyst, and the conventional V ₂
It can exhibit higher activity than the O ₅ / TiO ₂ based denitration catalyst.

Examples Next, examples of the present invention will be shown together with comparative examples.

a. Preparation of catalyst Three types of ceramic paper (A) with the specifications shown in Table 1
(B) and (C) are mixed with 3% oxygen and the residual N ₂ in a mixed gas of 400
It was baked at ℃ for 2 hours. Then, ceramic paper was added to a mixture of titania sol (TiO ₂ content = 33% by weight, pH = 3 to 4) and silica sol (SiO ₂ content = 20% by weight, pH = 6 to 7) having various mixing ratios shown in Table 2. Each was dipped, dried at 120 ° C., and baked at 300 ° C. for 3 hours. In this way, the solid material of TiO ₂ and SiO ₂ was held on the ceramic paper. The amount of solids retained was adjusted by diluting the mixed solution with pure water.

Next, the ceramic papers holding TiO ₂ and SiO ₂ were respectively immersed in a room temperature saturated solution of ammonium metavanadate for 8 hours, dried at 120 ° C., and calcined at 300 ° C. for 3 hours. Thus, 9 kinds of V ₂ O ₅ / TiO ₂ based catalysts shown in Table 2 were obtained.

b. Vibration resistance For each of the catalysts prepared as described above, the amount of powder falling of the catalyst due to vibration was measured using a vibration resistance tester shown in FIG. The tester includes a box (3), a 0.8 mm thick stainless steel diaphragm (4) horizontally arranged on the box, and a 60 Hz vibration exciter (5) provided on the upper surface of the box. The test catalyst (6) is fixed to the lower surface of the diaphragm (4) with a double-sided adhesive tape.

Table 2 shows the measurement results of the accumulated powder drop amount. As is clear from the table, the catalyst obtained by using a mixture of titania sol and silica sol as the impregnating sol has a significantly smaller amount of powder fallout even under violent vibration, as compared with the catalyst obtained by using only the titania sol. , And is extremely superior in terms of vibration resistance.

c. Bending strength Using the ceramic papers (A) and (B) shown in Table 1 above and according to the method described above, various kinds of catalysts having different TiO ₂ retentions and different solid retentions at a SiO ₂ content of 10% by weight. Multiple catalysts were prepared respectively. A three-point bending test was performed on these catalysts at a fulcrum distance of 20 mm, and bending strength was measured according to the following formula.

Bending strength (kgf / mm) = [3 x breaking load (kgf) x distance between fulcrums (mm)] / [2 x test piece width (mm) x test piece thickness (mm)] Solid retention and bending strength The relationship is shown in the graph of FIG. As is clear from the graph, the catalyst obtained by using the mixture of titania sol and silica sol as the impregnating sol is comparable to the catalyst obtained by using only the titania sol in terms of bending strength.

d. Mixing Ratio of SiO ₂ and Catalytic Activity Using the ceramic paper (B) shown in Table 1 above and according to the method described above, various kinds of catalysts having different mixing ratios of SiO ₂ and solid content retention amounts were prepared. Each of these catalysts was subjected to an activity test using a stainless steel flow type reaction tube having an inner diameter of 1 inch. That is, the catalyst was filled in the above reaction tube and fixed, and then the reaction temperature was controlled to a predetermined value, and the reaction of the prepared exhaust gas for test having a composition of inlet NOx = 50 ppm, oxygen = 15%, steam = 10% by volume. Pour into a tube. Area velocity (A · V), that is, the flow rate of gas (Nm ³ / hour) per geometric surface area (m ² ) of the catalyst is 43 m / hour, and the NH ₃ ratio (inlet NH ₃ concentration ppm / outlet NH ₃ concentration ppm) Was 1.2, and the NH ₃ concentration at the inlet was 60 ppm.

For each catalyst, the denitration rate at temperatures of 150 ° C., 200 ° C. and 300 ° C. = [(Inlet NOx concentration ppm−outlet NOx concentration ppm) / inlet NOx concentration ppm] × 100 was determined. Table 3 shows the obtained NOx removal rates.
Shown in. Further, the relationship between the mixing ratio of SiO ₂ and the catalytic activity is shown in the graph of FIG. 4, and the relationship between the solid content retention amount and the catalytic activity is shown in the fifth graph.
Shown in the graph in the figure.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a vibration resistance tester, FIG. 2 is a partial perspective view of a conventional monolysis type catalyst, FIG. 3 is a graph showing the relationship between the amount of solids retained and bending strength, and FIG. 4 is SiO ₂ FIG. 5 is a graph showing the relationship between the mixing ratio and the denitration ratio, and FIG. 5 is a graph showing the relationship between the solid content retention amount and the denitration ratio.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01J 23/22 ZAB A 8017-4G

Claims (1)

[Claims] 1. A vanadium oxide is added to a solid-holding paper obtained by impregnating a ceramics paper produced by papermaking of silica-alumina-based or alumina-based ceramics fiber with a mixture of titania sol and silica sol, drying and firing. A method for producing a denitration catalyst using ceramics paper, which comprises supporting