KR20020058179A - Catalyst for selective catalytic reduction of nitrogen oxides including sulfur dioxide at low temperature - Google Patents

Abstract

PURPOSE: A novel vanadium-rhodium/TiO2 catalyst for selective catalytic reduction of nitrogen oxides including sulfur dioxide at low temperature of less than 250deg.C is provided, which is characterized in that tungsten is impregnated on TiO2 to avoid catalyst poisoning due to ammonium sulfate. CONSTITUTION: The present invention is characterized in that based on the weight of TiO2, Rd and Vd are impregnated on W-TiO2 complex support material in an amount of 0.1-2 wt.% and 0.1-2 wt.%, respectively wherein tungsten is impregnated on TiO2 support material in an amount of 1 to 10 wt.%, based on the weight of TiO2.

Description

Catalyst for Selective Catalytic Reduction of Nitrogen Oxides Including Sulfur Dioxide at Low Temperature

The present invention relates to a selective reduction catalyst for removing nitrogen oxides in exhaust gas containing sulfur dioxide. More specifically, the present invention is to effectively remove nitrogen oxides contained in the exhaust gas generated from sulfur oil combustion internal combustion generator, waste incineration plant, marine engine, large-scale boiler, etc. at the low temperature while effectively preventing poisoning of the catalyst by sulfur dioxide. It relates to a selective reduction catalyst.

There are two types of techniques for removing nitrogen oxides from fixed sources, one of which is clean technology that controls the nitrogen content of the fuel or changes the burner and combustion process, and the other is combustion. End of pipe technology or flue-gas treatment technology to remove nitrogen oxides in the exhaust gas generated later.

When clean technology is used to remove nitrogen oxides, the nitrogen oxides that can be removed by reducing the nitrogen content of fuel are up to 60%, and are removed by process changes such as low NO _x burner (LNB) and two-stage combustion. The efficiency that can be achieved is 30-50%, and it is not commercialized because it cannot satisfy the environmental regulations such as NO _x regulation and total amount regulation.

Therefore, the NO _x removal technology by the excellent after-treatment technology in terms of nitrogen oxide removal efficiency can be applied to the actual commercialization process. Among them, the wet method has the advantage of being able to remove NO _x and SO _x at the same time, and has been applied to a process in which a small amount of NO _x is generated. However, this technique causes two serious problems, one of which is that NO is poorly soluble in water and must be oxidized to NO ₂ before absorbing NO in an aqueous solution, which is expensive. to be. Another problem is that NO ₃ and N ₂ O ₄ are formed as by-products in the process of oxidizing to NO ₂ , which causes the problem of treating these by-products again.

Due to this problem, the dry method is actively studied among post-treatment technologies, and the dry method selectively reduces NO _x to N ₂ and H ₂ O by NH ₃ injection only at a high temperature of 850 to 1050 ° C. without using a catalyst. Non-Catalytic Reduction) and SCR (Selective Catalytic Reduction) method is used to reduce NO _x to N ₂ and H ₂ O using a catalyst in a relatively low temperature range of 150 to 450 ° C. SNCR has the advantage of being able to remove more than 50% of NO _x at a low cost, but the unreacted ammonia released forms ammonium salts, etc., which can lead to plugging or corrosion in the device behind the reactor. It may be difficult to operate due to the narrow operating temperature.

Due to the above-mentioned problem, the technology that removes NO _x from the fixed source is the most popular technology in terms of economic and technical aspects SCR technology. The selective catalytic reduction method reduces the catalyst to nitrogen and water vapor using ammonia as the reducing agent, such as nitrogen monoxide (NO) or nitrogen dioxide (NO ₂ ), as shown in Schemes 1, 2, 3 and 4. Since the actual exhaust gas contains oxygen, nitrogen oxides are actually removed according to the equations (3) and (4).

6NO + 4NH ₃ → 5N ₂ + 6H ₂ O

6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O

4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O

2NO ₂ + 4NH ₃ + O ₂ → 3N ₂ + 6H ₂ O

However, ammonia, which should be used as a reducing agent, reacts with oxygen to produce nitrogen and nitrogen oxides, which side reactions occur as shown in Schemes 5, 6, 7 and 8.

4NH ₃ + 3O ₂ → 2N ₂ + 6H ₂ O

4NH ₃ + 4O ₂ → 2N ₂ O + 6H ₂ O

4NH ₃ + 5O ₂ → 4NO + 6H ₂ O

4NH ₃ + 7O ₂ → 4NO ₂ + 6H ₂ O

In general, the NH ₃ oxidation reaction occurs actively as the reaction temperature rises to a high temperature, it occurs competitively with the reduction reaction of NO _x , it shows a phenomenon that the NO conversion rate with the maximum temperature. When moisture is not included, Scheme 6 hardly occurs, and in particular, since NO _x is generated by Reactions 7 and 8, reactions to be suppressed, but these reaction rates increase as the reaction temperature increases.

In addition, moisture and sulfur oxides are present in the flue-gases, which form salts on the catalyst to deactivate the catalyst. As described above, the reaction which is the main cause of poisoning of the catalyst is carried out according to Schemes 9, 10, 11 and 12.

2NH ₃ + H ₂ O + ₂ NO ₂ → NH ₄ NO ₃ + NH ₄ NO ₂

2SO ₂ + O ₂ → 2SO ₃

NH ₃ + SO ₃ + H ₂ O → NH ₄ HSO ₄

SO ₃ + H ₂ O → H ₂ SO ₄

Scheme 9 is a reaction in which NO and NH ₃ react to form nitrate, and such nitrate is known to decompose at 150 ° C. or higher to not form catalyst poisoning. Eventually poisoning of the catalyst is caused by SO ₃ produced by Scheme 10 remaining sulfate-free on the surface of the catalyst, forming sulfates according to Scheme 11. In addition, sulfuric acid is generated by the reaction scheme 12 to provide a cause of corrosion of the catalyst layer and the equipment of the latter stage.

Therefore, there is an urgent need to develop a catalyst that minimizes SO ₃ generation by Scheme 10 and lowers the decomposition temperature of sulfate generated by Scheme 11 to a low temperature region.

Generally, catalysts used in the SCR technology have been proposed in various ways from metal oxide catalysts to zeolite catalysts, and many studies have focused on the effect of reaction conditions on catalysts. Precious metal catalysts are used to remove NO in automobile exhaust gases. When NO removal experiments are carried out on platinum catalysts using ammonia and hydrogen as reducing agents, NO removal activity disappears within 4 hours after the reaction in the presence of sulfur dioxide. can do. This is because the platinum surface is poisoned by the sulfur component. Therefore, the noble metal catalysts are severely poisoned even at 10 to 60 ppm sulfur dioxide, which is usually present in the exhaust gas, and thus are not commercialized as SCR catalysts. As zeolite catalysts, Y-type zeolite, mordenite and ZSM-5 are mainly studied, and the NO removal activity is increased by exchanging transition metals with cations. However, due to the structural disadvantages of the zeolite catalyst, it is difficult to use it as a commercialization catalyst because activity decreases in the presence of water and aluminum in the structure elutes so that durability decreases significantly.

Therefore, most of the recent SCR catalysts have been studied mainly on vanadium, and V ₂ O ₅ is used to precipitate or support V ₂ O ₅ in TiO ₂ , Al ₂ O ₃ , or SiO ₂ to give the best effect on SCR reaction. It is known. Among the above carriers, TiO ₂ is mainly used in consideration of the durability of the sulfur component. In addition, development of a catalyst in which tungsten, molybdenum or the like is added as a method for reducing SO ₃ generated by Scheme 10 has been actively conducted. Flue gas denitrification catalysts are currently commercialized in the form of tungsten, molybdenum, etc. added to the V ₂ O ₅ / TiO ₂ series to enhance the catalytic activity and endothelial toxicity to sulfur dioxide. Such catalysts exhibit high denitrification efficiency at temperatures around 350 ° C., but have a low denitrification efficiency at temperatures below 200 ° C. and are poisoned by ammonium sulfate formed on the surface of the catalyst and thus cannot be used at low temperatures. In most denitrification processes, a V ₂ O ₅ / TiO ₂ -based catalyst is installed at the rear end of the desulfurization plant, in which case additional energy is required to increase the temperature of the flue gas lowered below 100 ° C. In fact, it is known that it increases the temperature of exhaust gas by about 100 ℃ and consumes a huge amount of power of about 5 to 10% of the power generation capacity of the entire power plant. As mentioned above, the denitrification equipment of the actual process is a high energy consumption process installed due to the high temperature activity of the catalyst, and the catalytic activity of the V ₂ O ₅ / TiO ₂ -based catalyst is required to apply the denitrification process to the existing equipment. Since the installation space in the temperature range of 350 ° C or less is narrow, the catalyst has to be installed in a relatively low temperature space and an additional heat source is provided. In addition, when operating at a high temperature to accelerate the thermal fatigue of the catalyst layer to shorten the life.

On the other hand, the techniques related to the selective reduction catalyst studied in order to increase the nitrogen oxide removal efficiency in the flue gas containing sulfur dioxide and to prevent catalyst poisoning by ammonium sulfate are as follows.

U.S. Patent No. 5,409,681, assigned to Babcock-Hitachi Kabushiki Kaisha of Japan, discloses a precious metal selected from the group consisting of Pt, Pd and Rh as the main catalyst, V- (W and / or Mo) / TiO ₂ catalyst. It is disclosed that the catalyst may be physically mixed with the promoter supported on the carrier, but the main catalyst may be used to reduce and remove nitrogen oxides, and the subcatalyst may be used as a catalyst for oxidative removal of unreacted ammonia and carbon monoxide. At this time, it is described that the ratio of the two catalyst groups can be adjusted to 9: 1 to be used at 350 ℃ in the presence of 2 ppm of sulfur dioxide. However, the mixed catalyst was found to have a large amount of unreacted ammonia is discharged in the low temperature region of 200 ℃ or less.

U.S. Patent No. 4,152,296, assigned to Seitetsu Kagaku, Japan, carries 0.05 to 100 mol% at 300 to 520 DEG C by supporting vanadium sulfate (VSO ₄ ), vanadil sulfate (VOSO ₄ ), or a mixture thereof on a TiO ₂ carrier. A process for removing nitrogen oxides is provided by supplying a mixed gas consisting of ammonia and 99.95 to 0 mol% of an inert gas.

U.S. Patent No. 4,182,745, assigned to JGC Corporation of Japan, uses a heteropoly acid (silicotungstic acid, silicomolybdic acid, phosphotungstic acid, phosphomolybdic acid) as a carrier such as Cu, Ti, V, Cr, Mn, Fe, Co, Ni, etc. A process for removing nitrogen oxides at 250 to 550 ° C using a transition metal supported catalyst is proposed.

In addition, U. S. Patent No. 5,582, 809, assigned to Sakai Chemical Industry of Japan, supports 12-21 wt% of tungsten and / or molybten on a TiO ₂ carrier, and 0.15-0.5 wt. Of niobium in such supported oxide. A method of removing nitrogen oxides at 270 to 530 ° C using a supported catalyst is provided.

However, the above patents are mostly used to support the precious metal and transition metal in the TiO ₂ carrier or solid acid carrier, and requires the operation of the reactor temperature in the range of 250 ℃ to 550 ℃ at the time of sulfur dioxide input. Therefore, studies on catalysts capable of removing nitrogen oxides without poisoning of catalysts in the presence of sulfur dioxide in a low temperature region of less than 250 ° C have not been studied.

In contrast, the present inventors cannot operate at 300 ° C or lower in the presence of sulfur dioxide because the decomposition temperature of ammonium sulfate formed on the surface of the conventionally commercialized V ₂ O ₅ -WO ₃ / TiO ₂ catalyst is 300 ° C or higher. As a result of research to overcome the problem, it is possible to minimize the production of SO ₃ by oxidation of SO ₂ and to produce ammonium sulfate formed on the surface of the catalyst as a compound having a weak bond with the surface of the catalyst to a low temperature region of 250 ℃ It is to develop a catalyst that can decompose in.

Accordingly, it is an object of the present invention to provide a catalyst for removing nitrogen oxides that exhibits excellent denitrification efficiency in a low temperature region of 250 ° C or lower.

Another object of the present invention is to provide a catalyst for removing nitrogen oxides that can minimize catalyst poisoning by ammonium sulfate formed by sulfur dioxide in flue gas, suppress the emission of unreacted ammonia used as reducing agent, and extend the life of the catalyst. will be.

Still another object of the present invention is to provide a method for removing nitrogen oxide or simultaneously removing nitrogen oxide and dioxin using the catalyst.

One aspect of the present invention for achieving the above and other objects is a selective reduction catalyst for removing nitrogen oxides in the flue gas containing sulfur dioxide at low temperature, 0.1 to 2% by weight based on the TiO ₂ weight to TiO ₂ carrier Vanadium and 0.05 to 2% by weight of rhodium is characterized in that it is supported by the coprecipitation process was prepared.

Another aspect of the present invention for achieving the above and other objects is a selective reduction catalyst for removing nitrogen oxides in the flue gas containing sulfur dioxide at low temperature, it carries 1 to 10% by weight of tungsten, based on the weight of TiO ₂ 0.1-2% by weight of vanadium and 0.05-2% by weight of rhodium are supported on the TiO ₂ mixed carrier by carrying out the coprecipitation process.

1 is a diagram showing the removal rate of nitrogen oxide using the catalyst of the present invention prepared according to Examples 1 to 2 as a nitrogen oxide removal catalyst and changing the molar ratio of NH ₃ / NO _x .

FIG. 2 (a) is a diagram showing the adsorption state of ammonium sulfate formed on the surface of the catalyst in which rhodium and vanadium are co-precipitated.

Fig. 2 (b) is a diagram showing the state of adsorption of ammonium sulfate formed on the surface of a conventional vanadium catalyst.

Fig. 2 (c) is a diagram showing an adsorption state of ammonium sulfate formed on the surface of a conventional rhodium catalyst.

3 is a diagram showing a process in which ammonium sulfate formed by stopping the addition of sulfur dioxide after the formation of ammonium sulfate on the catalyst surface according to Example 2 is completely decomposed even at a low temperature below 250 ° C., thereby restoring nitrogen oxide removal activity.

Looking at the present invention in more detail as follows.

Selective reduction catalyst for removing nitrogen oxides in exhaust gas containing the sulfur dioxide of the present invention at a low temperature of less than 250 ℃ is using TiO ₂ as the carrier and, as active metals are one by co-precipitation of rhodium (rhodium), and vanadium (vanadium) The metal having two characteristics is supported in the carrier of. In addition, in order to minimize the rate at which SO ₂ is oxidized and converted to SO ₃ on the catalyst, it may be prepared by first supporting tungsten on the TiO ₂ carrier and then co-precipitating rhodium and vanadium. In the above, vanadium and rhodium, which are used as the active metal, are supported by 0.1 to 2% by weight and 0.05 to 2% by weight, respectively, based on the weight of the carrier TiO ₂ , and are primarily supported on the TiO ₂ carrier before supporting the rhodium and vanadium. The content of tungsten that can be supported by is 1 to 10% by weight based on the weight of TiO ₂ . In the case where the content of vanadium and rhodium is more than 2% by weight, there is a problem in that the manufacturing cost is increased in terms of economics, and when tungsten is more than 10% by weight, the secondary supported metals, vanadium and rhodium, are reduced due to the reduction of specific surface area. The problem of uneven distribution occurs.

The method for preparing the catalyst of the present invention is as follows.

First, the vanadium precursor and the rhodium precursor are dissolved in an aqueous solution. Ammonium metavanadate (NH ₄ VO ₃ ) is used as the vanadium precursor, and rhodium (III) chloride (RhCl ₃ ), rhodium sulfate (Rh ₂ (SO ₄ ) ₃ ), and rhodium nitrate (Rh ( NO ₃ ) ₃ ) and the like can be used. At this time, the same molar number of the organic acid as the vanadium precursor is added to an aqueous solution in which the two metal precursors are dissolved, and then dissolved together. Then, a certain amount of TiO ₂ to be used as a carrier is added to the aqueous solution. Since the two precursors are dissolved in the same aqueous solution, when TiO ₂ is mixed therewith, vanadium and rhodium are co-precipitation to the TiO ₂ carrier. . The slurry mixed as described above is adjusted to a viscosity of 30 to 300 cps and pH 2 to 3 and stirred while heating to 50 to 70 ° C. to evaporate moisture. Preferably, a vacuum evaporator is used. At this time, the viscosity and pH of the slurry is controlled to a certain range to improve the dispersion and durability of the catalyst, and evenly spreading the precursor metal aqueous solution evenly into the micropores of the carrier, the metal oxide uniform composition and It is intended to be distributed and formed by content. Therefore, when the viscosity of the slurry is less than 30 cps, there is a problem of disproportionation in which vanadium and rhodium are separated and agglomerated into one part when mixing in a vacuum evaporator. When the viscosity of the slurry exceeds 300 cps, the precursor aqueous solution is fine pores of titanium dioxide. It is not diffused until and aggregates only on the surface. In addition, when the pH is less than 2, the metal cations in the precursor are rapidly adsorbed on the surface of the titanium dioxide and are not evenly dispersed and aggregated. When the pH is above 3, the cations in the precursor are difficult to adsorb to the surface of the titanium dioxide. Then, it is dried for 20 to 30 hours at a temperature of 80 to 120 ℃, preferably about 100 ℃ and then reduced to a hydrogen atmosphere for 3 hours in a kiln of 230 to 270 ℃. By the above process, the chlorine component contained in the rhodium (III) chloride used as the precursor of rhodium can be removed. After the reduction by hydrogen as described above is calcined for 4 hours in an air atmosphere of 350 ~ 450 ℃ to prepare an oxide catalyst.

On the other hand, when using a mixed carrier in the form of tungsten supported on TiO ₂ , as described above, the step of supporting tungsten on TiO ₂ before co-precipitation of vanadium and rhodium is further included. That is, while tungsten precursors such as ammonium tungstate ((NH ₄ ) ₂ WO ₄ ) are dissolved in water, the same molar number of organic acids is added to the aqueous solution and dissolved together. In the process, the entire aqueous solution is heated to 50 to 60 ℃ completely dissolved, and then the aqueous solution is cooled to room temperature again. A certain amount of TiO ₂ is added to the solution to mix the viscosity of the entire slurry to 30 to 300 cps, and the pH of the slurry is adjusted to be 2-3. The slurry is then heated to 50-70 ° C. under stirring to evaporate the moisture, preferably using a vacuum evaporator. After the moisture evaporation step is completed, after drying for 24 hours or more at a temperature of 80 ~ 120 ℃ and fired for 4 to 6 hours in an air atmosphere of 450 ~ 550 ℃ to prepare a tungsten-supported TiO ₂ mixed carrier. The next step is the same as the process of supporting vanadium and rhodium on the above-described TiO ₂ single carrier.

The catalyst of the present invention can be used by coating on a metal plate, ceramic filter, honeycomb. At this time, the catalyst is uniformly ground to a particle size of 1 ~ 10 ㎛ coating. However, when grinding to less than 1 μm, there is a problem in the economics of the fine grinding step, and when it exceeds 10 μm, the uniformity and adhesion of the coating are reduced. In addition, the catalyst may be used in the form of extrusion or granular (monolith). Such coating process and extrusion process may be carried out according to methods well known in the art.

In the process of removing nitrogen oxides using the catalyst of the present invention, nitrogen oxides containing sulfur oxides and ammonia are supplied at a space velocity of 1000 to 50000 ^{hr −1} with ammonia molar ratio of nitrogen oxides of 0.8 to 1.2, and 170 The selective catalytic reduction reaction is carried out in the temperature range of ˜250 ° C. At this time. When the molar ratio is less than 0.8, there is a problem in that the reduction effect of ammonia is weak and the nitrogen oxide removal efficiency is low. When the molar ratio is higher than 1.2, unreacted ammonia is discharged or the consumption rate of ammonia is not economically preferable.

When using the catalyst of the present invention as described above, by using a selective reduction method has a merit that can be applied to a variety of process conditions by extending the temperature range for treating nitrogen oxides in the exhaust gas to a low temperature. In addition, since the vanadium and rhodium co-precipitated in the catalyst of the present invention have different adsorption properties, there is an advantage that the concentration ratio of ammonia to nitrogen oxides can be increased to 1.2 so that the ammonia concentration is excessive when using a conventional vanadium catalyst. Can significantly reduce the proportion of unreacted ammonia produced.

On the other hand, as the reaction temperature increases, the amount of SO ₂ contained in the exhaust gas is oxidized to SO ₃ increases. However, in the case of using the catalyst of the present invention, since it is possible to operate in a low temperature region, it is possible to minimize the production of ammonium sulfate and sulfuric acid by reducing the oxidation reaction.

The main difference between the catalyst of the present invention and the conventional vanadium catalyst lies in the denitrification reaction of nitrogen oxide at low temperature by co-precipitation of vanadium and rhodium and the decomposition temperature of ammonium sulfate formed on the catalyst surface (Zhenping Zhu et al. ., "Decomposition and Reactivity of NH ₄ HSO ₄ on V ₂ O ₅ / AC Catalysts Used for NO Reduction with Ammonia", Journal of Catalysis, 195, 268-278 (2000)). As shown in Figure 2, ammonium sulfate formed on a conventional vanadium catalyst is shown in Figure 2 (b), the decomposition temperature is 300 ℃ or more. Therefore, in the case of using a vanadium catalyst as the nitrogen oxide removal catalyst in the exhaust gas in which sulfur dioxide is present, it is necessary to operate at a temperature of 300 ° C. or higher to prevent catalyst poisoning. On the other hand, as shown in Fig. 2 (c), ammonium sulfate on the rhodium-supported catalyst has a higher decomposition temperature and thus cannot be decomposed even at 400 ° C or higher, and catalyst poisoning cannot be avoided. However, in the case of the vanadium-rhodium co-supported catalyst of the present invention, ammonium sulfate formed on the surface of the catalyst is adsorbed as shown in FIG. The poisoning can be eliminated or prevented.

In particular, using the catalyst of the present invention introduced a mixed gas of nitrogen oxides, ammonia and oxygen in a fixed bed reactor having an inner diameter of 8 mm and a height of 50 mm in a low temperature region of 190 ° C and experimented with the temperature, even in the low temperature region of 190 ° C Unlike the vanadium catalyst, a high conversion rate of 80% or more was obtained, and even when 150 ppm of sulfur dioxide was introduced, it can be seen that there was no decrease in conversion rate. However, even when vanadium and rhodium are physically mixed without coprecipitation or when one metal is first supported and then another metal is supported, such low temperature denitrification is not obtained because of the decomposition temperature of ammonium sulfate formed on the surface. Because is high.

The catalyst of the present invention can be used not only for the removal of nitrogen oxides but also for the dioxins removal process known as a highly toxic substance that causes various cancers and birth defects. Dioxin, commonly referred to as dioxin, is composed of two benzene rings substituted with chlorine atoms. Dioxins are known to be the most problematic chemicals that cause environmental pollution because dioxins are fairly stable, water-insoluble substances and their toxicity continues to exist. The process of removing the dioxins contained in the flue gas using the catalyst of the present invention is carried out at a temperature range of 130 to 400 ° C. while supplying the flue gas at a space velocity of 1000 to 50000 hr ⁻¹ . At this time, the molar ratio of nitrogen oxide and dioxin is preferably in the range of 1: 0.5 to 1: 2.0.

The present invention can be more clearly understood by the following examples, which are only intended to illustrate the invention and are not intended to limit the scope of the invention.

Preparation Example 1

Preparation of Rh-V / TiO ₂ Catalyst

0.459 g of ammonium metavanadate (NH ₄ VO ₃ ; 20555-9 of Aldrich Chemical Co.) and 0.513 g of rhodium (III) chloride (RhCl ₃ ; product number 808062 of KOJIMA Chemical Co. Ltd.) It was dissolved in ml of distilled water. In the aqueous solution in which the two metal precursors were dissolved, the same mole number of organic acid as the vanadium precursor was added to dissolve together. 10 g of TiO ₂ (P-25 from Daegu) was added to the aqueous solution to prepare a slurry, and then adjusted to a viscosity of 50 cps and a pH of 2.5. The slurry was heated to 60 ° C. under agitation using a vacuum evaporator, then dried at a temperature of 100 ° C. for 24 hours and reduced under hydrogen atmosphere for 3 hours in a 250 ° C. kiln. Then, it was calcined for 4 hours at a temperature of 400 ℃ and an air atmosphere. The prepared catalyst was analyzed by an element analyzer as described above, it was confirmed that the TiO ₂ vanadium 2.0 wt% and rhodium 2.0 wt% based on the weight on the TiO ₂ is supported.

Preparation Example 2

Preparation of Rh-V / W / TiO ₂ Catalyst

1.544.g of ammonium tungstate ((NH ₄ ) ₂ WO ₄ ; 32238-5 from Aldrich Chemical Co.) was dissolved in 20 ml of distilled water while the same mole of organic acid was added to the aqueous solution to dissolve together. After the aqueous solution was completely dissolved by heating to 60 ° C., the aqueous solution was cooled again to room temperature. Then, 10 g of TiO ₂ (P-25 from Daegu) was added to prepare a slurry, followed by viscosity of 50 cps and pH 2.5. Adjusted to. Then, the slurry was heated to 60 ° C. under stirring in a vacuum evaporator, dried at a temperature of 100 ° C. for 24 hours, and then calcined for 5 hours at a temperature of 500 ° C. and an air atmosphere to prepare a tungsten-supported TiO ₂ mixed carrier. . The mixture of the catalyst prepared using the carrier by co-precipitation of vanadium and rhodium according to the method of Production Example 1 was analyzed by using an element analyzer, 10.0% by weight tungsten as TiO ₂ by weight, based on TiO _2, vanadium 2.0 wt% And it was confirmed that rhodium 2.0% by weight is supported.

Example 1

The removal rate of NO _x according to the NH ₃ / NO _x mole ratio using the catalyst according to Preparation Examples 1 to 2 is shown in FIG. 1. At this time, the temperature of the reactor was maintained at 190 ℃, the concentration of the supplied NO _x 400ppm, SO ₂ 150ppm was added, the hourly space velocity was maintained at 10,000 hr ^-1 . In order to exclude the effects of water and oxidation value adsorbed on the catalyst before the reaction, the mixture was cooled to the reaction temperature after being maintained at 400 ° C. for 1 hour in an air atmosphere.

As a result of the experiment, as shown in FIG. 1, the NO _x conversion for each catalyst increased with increasing NH ₃ concentration, and both catalysts showed more than 90% NO _x conversion at NH ₃ / NO _x molar ratio of 1.0 or more. It can be seen. On the other hand, as a result of the experiment using each catalyst, the concentration of ammonia and nitrogen dioxide in the exhaust gas according to the NH ₃ / NO _x molar ratio is shown in Table 1 below.

NH ₃ / NO _x Rh-V / TiO ₂ Rh-V / W / TiO ₂ NH ₃ NO ₂ NH ₃ NO ₂ 0.8 0 0 0 0 0.9 0 0 0 0 1.0 0 0 0 0 1.1 0 0 3 0 1.2 0 0 8 0

The unit is ppm.

As can be seen from the table, unreacted ammonia was not generated in the Rh-V / TiO ₂ catalyst even at an NH ₃ / NO _x molar ratio of 1.2, and about 8 ppm of ammonia in the Rh-V / W / TiO ₂ catalyst containing tungsten. Was discharged. This means that in the coprecipitation of rhodium and vanadium, the excess ammonia is removed by the ammonia adsorbed on the vanadium oxide after being oxidized on the rhodium oxide surface and converted to NO _x . In addition, the amount of SO ₂ discharged according to the NH ₃ / NO _x molar ratio in the two catalysts is shown in Table 2 below.

NH ₃ / NO _x Rh-V / TiO ₂ Rh-V / W / TiO ₂ 0.8 135 150 0.9 135 150 1.0 130 150 1.1 140 140 1.2 140 145

The unit is ppm.

As can be seen from the above table, the Rh-V / TiO ₂ catalyst showed some SO ₂ reduction, but this does not mean that it is converted to SO ₃ , but it seems to be reduced to sulfur element. It can be seen that in the Rh-V / W / TiO ₂ catalyst added with tungsten, almost no conversion was achieved.

Example 2

Due to the ammonium sulfate formed on the surface of the catalyst by the reaction of sulfur dioxide with ammonia and water, the nitrogen oxide removal efficiency is greatly reduced under the condition that the molar ratio of NH ₃ / NO _x is low. Therefore, the removal efficiency of ammonium sulfate formed on the surface of the catalyst when 150 ppm of SO ₂ is added to the catalyst according to Preparation Example 1 and ammonium sulfate is decomposed in the low temperature region of 190 ° C when the sulfur dioxide is stopped. The process of restoring efficiency is shown in FIG. 3. At this time, the temperature of the reactor was maintained at 190 ° C, the concentration of supplied NO _x was 400 ppm, the concentration of SO ₂ was 150 ppm. In addition, the molar ratio of NH ₃ / NO _x was maintained at 0.7 and the hourly space velocity was 10,000 hr ⁻¹ . As a result, ammonium sulfate formed on the catalyst in which rhodium (Rh) and vanadium (V) were co-precipitated is adsorbed to the two oxide surfaces of rhodium and vanadium on the catalyst as shown in FIG. It can be seen that it is more unstable than ammonium sulfate adsorbed on the surface of a single oxide as in 2 (c) and decomposes even at a low temperature of 190 ° C.

Example 3

Oxidation removal characteristics for dioxins were measured using Rh-V / TiO ₂ , a catalyst according to Preparation Example 1, and Rh-V / W / TiO ₂ , according to Preparation Example 2, respectively. As a dioxin component reacting on the catalyst, 1,2-DCB, a precursor commonly used in dioxin removal experiments, was used. At the beginning of the reaction, 314 ppm of 1,2-DCB was added under oxygen 10%, reaction temperature 200 ° C., space velocity 10,000 hr ⁻¹ , and moisture 1.2% to measure the removal efficiency. In the condition of containing 50 ppm sulfur dioxide is the conversion of 1,2-DCB For a V-Rh / TiO ₂ there was the case of 62%, Rh-V / W / TiO 2 , the level of 58%, in the catalyst 250 ℃ All were 99.9%. Therefore, the catalyst of the present invention is expected to serve as a removal catalyst of dioxins simultaneously with removing nitrogen oxides.

Example 4 and Comparative Example 1

VW / TiO ₂ and 0.05% by weight of rhodium supported by a catalyst according to Preparation Examples 1 to 2 and 4% by weight of vanadium and 5% by weight of tungsten on a titanium dioxide carrier as disclosed in US Pat. No. 5,409,681 Physically mixed catalyst of Rh / Al ₂ O ₃ supported at a weight ratio of 9: 1 with NH ₂ / NO _x molar ratio of 1.0 in the absence of sulfur dioxide The measurement is shown in Table 3 below.

Reaction temperature (℃) Example 4 U.S. Patent 5,409,681 Rh-V / TiO ₂ Rh-V / W / TiO ₂ 160 120 72 300 170 70 0 224 180 24 0 128 190 0 0 75 200 0 0 14

The unit is ppm.

As shown in the above table, it can be seen that the catalyst in which vanadium and rhodium are simply physically mixed has a large amount of unreacted ammonia is discharged in a low temperature region of 200 ° C. or less as compared to the catalyst of the present invention.

The catalyst in which the vanadium and rhodium are co-precipitated in the TiO ₂ carrier or the W-TiO ₂ mixed carrier of the present invention has excellent denitrification efficiency at low temperature and reduces the adsorption power of ammonium sulfate formed on the surface and at the same time, releases unreacted ammonia. By effectively suppressing the catalyst poisoning phenomenon in the low temperature region by ammonium sulfate caused when using a conventional commercialized vanadium tungsten / titanium dioxide catalyst can be prevented. Therefore, there is an advantage that can be applied to various process conditions. In addition, the catalyst of the present invention can be usefully used for dioxin removal as well as nitrogen oxides.

Simple modifications and variations of the present invention can be readily used by those skilled in the art, and all such variations or modifications can be considered to be included within the scope of the present invention.

Claims (8)

In the selective reduction catalyst which removes nitrogen oxides in the flue gas containing sulfur dioxide at low temperature, TiO ₂ is used as a carrier and 0.1 to 2 wt% of vanadium and 0.1 to 2 wt% of rhodium are based on the TiO ₂ weight. Nitrogen oxide removal catalyst, characterized in that supported on the carrier through. The method of claim 1, wherein the carrier is based on the weight of TiO ₂ of 1 to 10% by weight of tungsten bearing the W-TiO ₂ mixed carrier of the NOx removal catalyst according to claim. The method of claim 1, wherein the vanadium and rhodium are co-precipitated on the carrier in the form of a slurry adjusted to have a viscosity of 30 to 300 cps and a pH of 2-3 to improve the dispersion and durability of the catalyst Nitrogen oxide removal catalyst. The catalyst for removing nitrogen oxides of claim 2, wherein the tungsten is supported on the TiO ₂ carrier in a slurry form adjusted to have a viscosity of 30 to 300 cps and a pH of 2 to 3. 3. A catalyst for removing nitrogen oxides, which is prepared by coating the catalyst of any one of claims 1 to 4 on a support selected from the group consisting of a metal plate, a ceramic filter and honeycomb. A catalyst for removing nitrogen oxides, which is prepared by extruding the catalyst of any one of claims 1 to 4 into a particulate or monolith. Nitrogen oxides and ammonia containing sulfur dioxide in the presence of the catalyst of claim 1 or 2 are supplied at a space velocity of 1,000 to 50,000 ^{hr -1} with a molar ratio of ammonia to nitrogen oxides of 0.8 to 1.2, and 170 to 250 캜. To remove nitrogen oxides at temperatures. In the presence of the catalyst of claim 1 or 2, nitrogen oxides and dioxins are supplied at a space velocity of 1,000 to 50,000 hr ^-1 at a molar ratio of nitrogen oxides and dioxins of 1: 0.5 to 1: 2.0 at 130 to 400 캜. Simultaneous removal of nitrogen oxides and dioxins at