Catalyst for purification of nitrogen oxide

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to a catalyst for efficiently removing nitrogen oxides contained in exhaust gas emitted from an internal combustion engine such as a boiler or an automobile or a nitric acid manufacturing plant.

【0002】

[Conventional technology]

In recent years, environmental conservation issues have been questioned on a global scale, but nitrogen oxides (NO), which are said to be the cause of acid rain in particular._X) And sulfur oxides (SO)_X) There is significant interest in removal measures. And of these, nitrogen oxide NO_XThe main source of is exhaust gas from internal combustion engines such as boilers and automobiles or nitric acid manufacturing plants, and its main component is NO. Conventionally, for example, an ammonia reduction method is known as a method for removing such nitrogen oxides.

【0003】

This ammonia reduction method is V₂O₅-TiO₂Nitric oxide is selectively reduced by an ammonia reducing agent using a catalyst. An example of this is the method proposed by M. Inomata (J. Catal., 62 (1980) 140.), Which uses ammonia. Is V₂O₅-TiO₂V on the catalyst surface⁵⁺NH next to = O₄⁺Adsorbed as the NH₄⁺Reacts with NO, which is the gas phase₂, H₂O and V-OH are produced. And after that, V-OH is O₂Or V⁵⁺It is reoxidized by = O. However, since this method uses ammonia, which is dangerous and difficult to handle, a method of using a reducing agent without such inconvenience instead of ammonia has been expected. Therefore, as a method of using a catalyst that can use a hydrocarbon as a reducing agent, for example, a method of JP-A-63-100919 has been proposed, and according to this method, copper is made of porous materials such as alumina, silica, and zeolite. A catalyst supported on a sex carrier is used.

【0004】

[Problems to be Solved by the Invention]

However, in the conventional method using the catalyst described in the above publication, the ability to remove nitrogen oxides is not always sufficient, especially in terms of catalytic performance. Also, the Cu catalyst is SO_XThere was also a problem that it was easily poisoned by. In view of such circumstances, it is an object of the present invention to provide a nitrogen oxide purification catalyst which is excellent in nitrogen oxide removing ability and can remove nitrogen oxides safely, easily and efficiently. ..

【0005】

[Means for solving problems]

The nitrogen oxide purification catalyst according to the present invention contains silver oxide and can remove nitrogen oxides by contacting with a gas containing nitrogen oxides in an oxidizing atmosphere in which hydrocarbons are present. ..

【0006】

Further, the nitrogen oxide purification catalyst according to the present invention is composed of one selected from alumina, aluminum phosphate, and alumina-silica and silver oxide, and is composed so that the carrying amount ratio of silver is 0.1 to 30 wt%. ing.

【0007】

[Action]

According to the present invention, for example, NO is first oxidized by oxygen to NO.₂Is generated, and this NO₂Is reduced by hydrocarbons. In this case, a catalytic component that quickly oxidizes NO is required, but if the oxidizing power is too strong, NO₂Even the hydrocarbons used to reduce the amount of carbon are oxidized. Since the nitrogen oxide purification catalyst according to the present invention contains silver, hydrocarbons react preferentially with nitrogen oxides over oxygen in an oxidizing atmosphere, thereby removing nitrogen oxides with high efficiency. Can be done.

【0008】

Further, the nitrogen oxide purification catalyst according to the present invention is supported on a carrier so that its surface area is increased, whereby its catalytic action is effectively activated.

【0009】

[Example]

Hereinafter, the first embodiment of the nitrogen oxide purification catalyst according to the present invention will be described. In the preparation of the catalyst according to the present invention, 0.85 g of silver nitrate was dissolved in 100 ml of water to prepare an aqueous silver nitrate solution, and this silver nitrate aqueous solution was added to 15 g of Al (OH).₃Powder (Aluminum hydroxide Grade G made by Nippon Ketchen, BET surface area; 340m²In addition to / g), it is evaporated and dried with stirring, dried at a temperature of 110 ° C for 20 hours, and then calcined in air at a temperature of 500 ° C for 3 hours. In this case, aluminum phosphate, silica-alumina, or the like can be used as the carrier in addition to alumina, and one or more of these carriers may be used as the carrier. The catalyst consists of silver oxide supported on an alumina carrier, and the amount of silver supported on the carrier is 5 wt%. The ratio of the amount of silver supported is preferably in the range of 0.1 to 30 wt%, but if it is less than 0.1 wt%, silver cannot be activated, and silver is sintered (baked) at a high temperature. This is because it is easy to cause sintering), and if it exceeds 30 wt%, it is not possible to increase the effective activation corresponding to the amount of the carrier.

【0010】

Next, a nitrogen oxide purification test conducted using such a catalyst will be described. 1 g of the catalyst prepared as described above is filled and set in a stainless steel reaction tube (inner diameter 10 mm) and held at a predetermined reaction temperature (4 types of 200 ° C, 300 ° C, 400 ° C and 500 ° C). , A gas containing nitrogen oxides (hereinafter referred to as a reaction gas) is circulated in the stainless steel reaction tube at a flow rate of 30 ml / min. Here, the composition of the above reaction gas is nitric oxide NO; 1050 ppm, propylene C.₃H₆1000ppm, oxygen O₂It was diluted with helium gas to 5%. NO in the reaction gas is N as the reaction gas passes through the catalyst.₂However, such evaluation of catalytic performance was performed based on the conversion rate of NO given by the following equation 1.

【0011】

[Formula 1]

<img file="JP2801423B2_D0001.tif" />NO and NO at the outlet of the reaction tube in the above formula 1.₂Concentrations make them chemiluminescent NO_XIt was obtained by analyzing with a meter. Table 1 shows the conversion rate of NO calculated from the results of the nitrogen oxide purification test conducted at each of the above reaction temperatures.

【0012】

[table 1]

<img file="JP2801423B2_D0002.tif" />【0013】

Here, as Comparative Example 2, the results of a nitrogen oxide purification test using a catalyst prepared so as to have a composition equivalent to that in the case where silver is not supported on an alumina carrier in the catalyst according to the first embodiment are shown. It is also shown in Table 1. First, looking at the conversion rate in the case of Comparative Example 2 shown in Table 1, each reaction temperature shows a relatively high value. As described above, various types of carriers can be selected, but some of the alumina carriers of this comparative example themselves have an effective catalytic action for the removal of nitrogen oxides. When such an alumina carrier is used, a nitrogen oxide digestion action unique to the carrier can be obtained as in Comparative Example 2. The silica carrier of Comparative Example 1 does not have such a unique catalytic action. Since this example is an Ag carrier catalyst using such an alumina carrier, the catalytic action of silver contained in the catalyst and the catalytic action unique to the alumina carrier can be obtained at the same time, and extremely high NO conversion is achieved by their synergistic effect. The rate is achieved. That is, due to the synergistic effect of the silver and alumina carriers, the NO conversion rate shows a remarkably high value even at each reaction temperature. When the reaction temperature rises above 500 ° C, the hydrocarbons in the reaction gas are directly oxidized to H.₂O and CO₂NO to form₂Is no longer reduced, and therefore the conversion rate tends to decrease. Therefore, the appropriate reaction temperature range is preferably 200 to 400 ° C.

【0014】

By the way, in the basic process of removing nitrogen oxides by the catalyst of the present invention, NO is first oxidized by oxygen to NO.₂Is generated, and this generated NO₂Is reduced by the hydrocarbons contained in the reaction gas in the same manner as the above oxygen. In this case, repeating the cycle of the nitrogen oxide removal process in a shorter time is a necessary condition for activating the nitrogen oxide removal reaction. That is, a catalyst is used to quickly oxidize NO. The catalyst of the present invention is also used for this purpose, but on the other hand, if the oxidizing power of the catalyst is too strong, NO.₂Even the hydrocarbons for reducing nitrogen oxides are oxidized by such catalysts, and the intended purpose of removing nitrogen oxides cannot be achieved. Since the catalyst of the present invention contains silver as described above, hydrocarbons in the reaction gas preferentially NO over oxygen in an oxidizing atmosphere.₂As a result, nitrogen oxides can be removed with high efficiency. That is, as is clear from the fact that silver contained in the catalyst has an action of partially oxidizing ethylene, the oxidizing power for hydrocarbons is not too strong and has a quick oxidizing action for NO, that is, so-called. It has mild oxidizing power. As a result, a high nitrogen oxide conversion rate as shown in Table 1 can be obtained.

【0015】

In the above case, the reaction gas contains oxygen in a predetermined ratio, and the oxidizing atmosphere here means a state in which oxygen is present, and air can be practically used, but the above-mentioned nitrogen oxides can be used. In the removal process, NO is oxidized and NO if it is not an oxidizing atmosphere.₂Therefore, the reduction reaction is not carried out because it remains in the NO state even with hydrocarbons, and therefore NO cannot be purified. Therefore, the state of such an oxidizing atmosphere is indispensable for using the catalyst of the present invention. Further, as in the case of such an oxidizing atmosphere, if the reaction gas does not contain hydrocarbons, NO is changed to NO by the catalyst of the present invention even in an oxidizing atmosphere.₂Even if it is oxidized to₂Is not reduced, and therefore the presence of hydrocarbons is also essential for the use of the catalysts of the present invention.

【0016】

As the form of existence of the hydrocarbon contained in the oxidizing atmosphere in the reaction gas, for example, it may be present in the exhaust gas of an internal combustion engine or the like, and if the amount is insufficient, only the required amount is introduced from the outside. May be good. And when introducing, the amount introduced is the sum of the amount of hydrocarbons already present in the exhaust gas, at least stoichiometrically reacting with the nitrogen oxides contained in the exhaust gas. N₂, H₂O and CO₂Is needed in sufficient quantity to produce. As the type of the hydrocarbon, for example, olefin-based hydrocarbons such as ethylene and propylene, paraffin-based hydrocarbons such as propane, and aromatic hydrocarbons such as toluene and xylene can be used.

【0017】

In the second embodiment of the nitrogen oxide purification catalyst of the present invention, the same catalyst as the catalyst according to the first embodiment is used, but the hydrocarbon in the reaction gas from which the nitrogen oxides should be removed by this catalyst is the first. (Iv) Propane is used instead of propylene in the case of the embodiment.

【0018】

The NO conversion rate in the case of the second embodiment is as shown in Table 1. The Ag-supported catalyst itself is the same as that of the first embodiment, but it can be seen that it also exerts an extremely excellent nitrogen oxide purifying effect on propane.

【0019】

Further, in the third embodiment of the nitrogen oxide purification catalyst of the present invention, aluminum phosphate is used as the catalyst carrier. Al (NO₃)₃9H₂O to 75g and H₃PO₄Dissolve 23 g in 1000 ml of water, add ammonia water at 1 ml / min to the solution so that the pH finally reaches 7 to 8, stir for about 2 hours, and leave to stand for 24 hours. Next, this left-over product is sucked and filtered, washed with pure water, and then dried at a temperature of 110 ° C. for 24 hours. Further, aluminum phosphate is obtained by firing in air at a temperature of 500 ° C. for 3 hours. To 19 g of such aluminum phosphate, an aqueous silver nitrate solution prepared by dissolving 1.7 g of silver nitrate in 100 ml of water is added, evaporated and dried while stirring, and further dried at a temperature of 110 ° C for 20 hours, and then at a temperature of 500 ° C. The Ag-supported catalyst is prepared by firing in.

【0020】

The NO conversion rate in the case of the third example is as shown in Table 1. In Comparative Example 3 with respect to this third example, a nitrogen oxide purification test was conducted using only the above aluminum phosphate carrier. As shown in Table 1, the aluminum phosphate carrier has a nitrogen oxide purifying action unique to the carrier, and therefore, like the alumina carrier, itself exerts an effective catalytic action for the removal of nitrogen oxides. As a result, an extremely high NO conversion rate is achieved by the synergistic effect of the nitrogen oxide purifying action of silver and the catalytic action of the aluminum phosphate carrier.

【0021】

[Effect of the invention]

As described above, according to the catalyst of the present invention, nitrogen oxides contained in exhaust gas and the like can be removed with high efficiency and safely, and by using silver, SO_XSO without being poisoned by_XThere is an advantage that the nitrogen oxide purifying action can be effectively exerted on the exhaust gas containing the above.