JPH0671176A - Catalyst for purifying nitrogen oxide - Google Patents

Abstract

PURPOSE:To display catalytic activity over a wide temp. range in the catalytic reduction of NOx as regards a catalyst having Pt and Sn supported on the carrier having a high specific surface area by a method wherein Pt and Sn are supported thereon within a specific range of ratio and the atomic ratio of Sn to Pt is given within a predetermined range. CONSTITUTION:The catalyst having Pt and Sn supported simultaneously on the carrier with a high specific surface area accelerates a selective reduction of hydrocarbons by NOx in an atmosphere of excessice oxygen. In particular, if the atomic ratio of Pt to Sn is changed, the active temp. range of Pt and Sn catalyst is shifted toward high temp. side. More specifically, 0.01-5wt.% Pt and 0.01-10wt.% Sn are supported on the carrier and the atomic ratio of Sn to Pt is given within the range of 0.1-2, whereby the active temp. of the Pt-Sn catalyst can be set within a desired range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to exhaust gas, and more particularly to a catalyst capable of purifying NO _{x in a} wide temperature range even in an oxygen excess atmosphere where the air-fuel ratio is lean.

[0002]

2. Description of the Related Art Nitrogen oxides (NO _x ) are contained in various exhaust gases such as exhaust gases from internal combustion engines such as boilers and automobiles or exhaust gases from nitric acid manufacturing plants, etc., but are harmful to humans. It is a major air pollutant and one of the causes of acid rain, which is regarded as a problem from the viewpoint of global environmental protection. Therefore, NO _{x can be} efficiently emitted from these various exhaust gases.
It is desired to develop a method for removing the above. Ideal NO _x
The removal technique is direct decomposition of NO as shown in the equation (1). 2NO → N ₂ + O ₂ (1) The equation (1) is overwhelmingly advantageous to the right-hand side generation system in terms of equilibrium, and for example, in JP-A-60-125250, C
The catalyst in which u is supported on zeolite by ion exchange is N
It is disclosed that the direct decomposition reaction of O is promoted.
However, in the reaction of the formula (1), the generated oxygen is preferentially adsorbed on the catalytic active site, so that the removal efficiency is gradually reduced, and further, the condition that excess oxygen exists in the reaction system (oxygen excess atmosphere) Then the reaction is completely hindered.

Many methods have been proposed for purifying NO _x using a reducing agent such as carbon monoxide, hydrogen, hydrocarbons, ammonia and hydrazine in the presence of a catalyst. The platinum group elements also _the NO _x purification catalyst to be used for the purification of these NO _x including the catalyst supported on a carrier,
Various other catalysts have been proposed. However, the conventional catalyst has a good N 2 content when used in an atmosphere containing less oxygen than the theoretical amount of oxygen required to completely oxidize a reducing agent such as carbon monoxide, hydrocarbons and hydrogen, that is, in a reducing atmosphere.
Although it has a purifying effect on O _x , it has a drawback that the purifying ratio of NO _x sharply decreases in an oxygen-excess atmosphere in which the amount of oxygen is large. That is, in the catalytic reduction of NO _x in an oxygen-excess atmosphere, there is a drawback that the selectivity of the reduction reaction is remarkably lowered because the combustion reactions of coexisting reducing agents such as carbon monoxide, hydrogen and hydrocarbon occur simultaneously. The catalyst effective for the NO _x purification reaction in the oxygen excess atmosphere has not been completed yet.

The present applicants have already devised and proposed a denitration process in an oxygen excess atmosphere using a catalyst in which Pt is supported on a carrier and using hydrocarbon as a reducing agent. In this process, the reduction reaction of NO _x to N ₂ proceeds at a high rate in the temperature range of 100 to 250 ° C., but the N ₂ production rate decreases in the temperature range of 250 ° C. or higher. . When this process is applied to an actual engine exhaust gas purification system, the exhaust gas temperature rises to a temperature of about 400 ° C depending on the position where the catalytic converter is placed. Cooling the exhaust gas by installing a heat exchanger in a mobile source such as an automobile is not realistic due to space limitations, and it becomes necessary to treat the water vapor in the exhaust gas as a drain, so the exhaust gas is high in temperature. It is preferable to process as it is. Therefore, a catalyst and a catalytic process that exhibit NO _x reduction activity in a wide temperature range even in an oxygen-excess atmosphere have been earnestly desired.

[0005]

The present invention has been made in view of the above circumstances, and NO in an oxygen excess atmosphere is used.
It is an object of the present invention to provide a catalyst that exhibits activity in a wide temperature range for the catalytic reduction reaction of _x .

[0006]

Means for Solving the Problems As a result of intensive studies by the present inventors, NOx hydrocarbon selective reduction of NO _x in an oxygen excess atmosphere by a catalyst in which Pt and Sn are simultaneously supported on a carrier having a high specific surface area. The reaction is promoted, and especially when the atomic ratio of Pt and Sn to be supported is changed, Pt—S is reduced in the selective reduction reaction of hydrocarbons of NO _x in an oxygen excess atmosphere.
The inventors have found that the active temperature range of the n-catalyst shifts to the high temperature side and completed the present invention. That is, the present invention is a catalyst in which Pt and Sn are supported on a high specific surface area carrier, and the Pt loading rate is 0.01 to 5 wt% (wt% is expressed as a metal) and the Sn loading rate is 0.01 to 10 wt%. %
The above problem can be solved by a nitrogen oxide purifying catalyst having an Sn / Pt atomic ratio in the range of 0.1 to 2.

[0007]

The heat-resistant inorganic oxide that can be used as the carrier raw material in the present invention is silica, alumina, titania, zirconia, silica-alumina, silica-titania, etc., which is crystalline or amorphous. It doesn't matter. Preference is given to using silica, alumina or silica-alumina having a high specific surface area. The shape of these carriers is
It is not particularly limited to spherical, cylindrical, honeycomb, rod, spiral, granular, etc., and the shape, size, etc. can be arbitrarily selected according to the use conditions. The Pt raw material used in the present invention is platinum (IV) chloride hexahydrate, platinum (II) chloride chloride, platinum (IV) chloride chloride, platinum (II) chloride sodium hexahydrate, ammonium chloride platinum (I).
V) and other platinum salts or platinum tetraammine complexes 2
Platinum complex salts such as chloride and platinum tetraammine complex nitrate can be used. The Sn raw material used in the present invention is tin (II) acetate, tin (II) bromide, tin (IV) bromide, stannous chloride hydrate, stannous chloride hydrate or tin (II) sulfate. Can be used.

The active metal impregnating solution is prepared by dissolving the above-mentioned platinic acid, platinate or platinum complex salt and tin salt in distilled water or ion-exchanged water. Pt and Sn in solution
The concentration of Pt and Sn in all solutions used is
It suffices that the amount of Pt and Sn that can be effectively supported by the finished catalyst be within a range that can be supported, and it can be appropriately adjusted as necessary. The amount of the active metal salt aqueous solution used in the ion exchange method or the immersing method is not particularly limited as long as the active metal salt can be sufficiently impregnated into the inorganic oxide as the carrier, but usually the amount of the carrier About 2 to 20 times is appropriate. The amount of the active metal salt aqueous solution used in the incipient wetness method is the amount of water that the porous inorganic oxide as a carrier can occlude in its pore structure, and is used as a carrier before the impregnation treatment. It is determined by measuring the water absorption of the inorganic oxide.

In the present invention, the supported amount of Pt can be variously changed, but 0.01 to 5 wt% with respect to the carrier.
%, Preferably 0.1 to 1 wt% is suitable. Considering that Pt is expensive, the catalytic activity is not sufficiently exhibited when the amount of Pt supported is less than the above range, and there is no improvement in the catalytic activity particularly with an increase in the amount supported even when the amount is greater than the above range. The above range is suitable. In the present invention, the supported amount of Sn can be variously changed, but 0.01 to 10 wt%, preferably 0.1 to 5 w, relative to the carrier.
t% is suitable. As will be shown later in Comparative Examples, Sn alone has extremely small NO _x reduction activity in an oxygen excess atmosphere, and it is presumed that Sn plays a role of a promoter for Pt in the NO _x reduction reaction. Therefore S
The loading rate of n will be determined according to the loading rate of Pt and in which range the active temperature range of the finished catalyst is set.

The atomic ratio of Sn to Pt is in the range of 0.1 to 2 and is determined according to the required activation temperature range of the finished catalyst. As shown in FIG. 1, the Sn loading is low and Sn / Pt <
When it is 0.1, the active temperature range of the finished catalyst is almost the same as that of Pt alone. Sn / Pt = 0.1
In No. 2, the active temperature range of the finished catalyst can be shifted to the high temperature side of 100 to 200 ° C. as compared with the case of Pt alone. In addition, by further increasing the Sn loading rate, Sn / P
When t> 2, the active temperature range of the finished catalyst is Sn / Pt.
Similar to the case of <0.1, there is almost no difference from the case of Pt alone. Therefore, the active temperature range of the catalyst can be selected by the atomic ratio of Sn and Pt. Predetermined amount of P by the above method
The carrier containing t and Sn is dried and calcined by a conventional method to complete the catalyst used in the present invention. It is preferable to carry out the firing step in an air atmosphere at a temperature of 400 to 600 ° C. economically, from the viewpoint of catalytic activity and durability of the catalyst.

[0011]

The present invention will be described in more detail with reference to the following examples, but it goes without saying that the present invention is not limited to these examples. Example 1 200 g of alumina hydrate having a pseudo-boehmite structure (trade name G base) manufactured by Nippon Ketjen was calcined at a temperature of 500 ° C. for 3 hours, and 170 g of alumina powder containing gamma alumina as a main component. Was manufactured. The specific surface area of this alumina powder determined by the nitrogen adsorption method was 295 m ² / g, and the pore volume by the mercury porosimetry was 2.8 ml / g. 100 ml of an aqueous solution of chloroplatinic acid containing 1 g of Pt metal in 100 ml of the solution,
100 ml of the solution is mixed with 10 ml of an aqueous solution of stannous chloride containing 1 g of Sn metal and further diluted with ion-exchanged water to prepare a total aqueous solution of chloroplatinic acid-tin chloride of 280 ml. Mechanical stirring is continued while gradually adding 100 g of the chloroplatinic acid-tin chloride mixed aqueous solution to the alumina carrier to impregnate alumina with the chloroplatinic acid-tin chloride mixed aqueous solution. Dry it in air at 120 ° C,
Further, it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst 1.
ICP (Inductive Coupled Plas)
ma) The platinum loading rate of the catalyst 1 obtained from the emission analysis is 0.
91 wt%, tin loading rate is 0.11 wt%, Sn
The / Pt atomic ratio was 0.2.

1 g of the above catalyst was filled in a stainless steel reaction tube having an inner diameter of 10 mm, and a reaction gas (gas composition NO:
1,000 ppm, C ₃ H ₅ : 1,000 ppm,
O ₂ : 5 vol%, He: remaining amount) were passed at a flow rate (SV = 1,000 / h) of 30 ml / min. The reaction tube exit gas composition was measured in NO _x analyzer chemiluminescence is the concentration of NO and NO _2, N ₂ O concentration is a gas chromatograph equipped with a silica gel column - was measured using a thermal conductivity detector . The temperature of the catalyst layer was set to a predetermined temperature in the range of 100 to 400 ° C., and the value at the time when the reaction tube outlet gas composition became steady was adopted as the measured value. The results are shown in Table 1.

When the reaction gas passes through the catalyst, NO in the reaction gas is converted into NO ₂ and N ₂ O and N _{2. The} NO conversion rate, N ₂ O production rate and N ₂ production rate are as follows. Defined as N ₂ production rate (%) = NO conversion rate−N ₂ O production rate Conventionally, the NO conversion rate is evaluated only by the NO conversion rate referred to in the present invention. The chemiluminescence NO _x analyzer does not detect N ₂ O in principle, and one is that N ₂ O is less toxic than NO and NO ₂ and is specified as NO _x in the current emission regulations. In some cases, N ₂ O is one of the causative substances that cause ozone depletion, although it may not exist.

Example 2 100 ml of an aqueous solution of chloroplatinic acid containing 1 g of Pt metal in 100 ml of a solution and 50 ml of an aqueous solution of stannous chloride containing 1 g of Sn metal in 100 ml of the solution were mixed, and ion-exchanged water was further added. 280m diluted with
1 of chloroplatinic acid-tin chloride mixed aqueous solution was prepared, and mechanical stirring was continued while gradually adding to 100 g of an alumina carrier prepared by the same method as in Example 1 to prepare the chloroplatinic acid-tin chloride mixed aqueous solution as alumina. Impregnated. This in the air 12
Catalyst 2 was prepared by drying at 0 ° C. and further calcining in air at 500 ° C. for 3 hours. Table 1 shows the results of activity evaluation conducted in the same manner as in Example 1. The catalyst 2 obtained by ICP emission analysis had a platinum loading of 0.85 wt% and a tin loading of 0.
It was 48 wt% and the Sn / Pt atomic ratio was 1.

Example 3 100 ml of an aqueous solution of chloroplatinic acid containing 1 g of Pt metal in 100 ml of a solution, and 100 ml of an aqueous solution of stannous chloride containing 1 g of Sn metal in 100 ml of the solution.
, And then diluted with deionized water for a total of 280
ml of a chloroplatinic acid-tin chloride mixed aqueous solution was prepared, and mechanical stirring was continued while gradually adding it to 100 g of an alumina carrier prepared by the same method as in Example 1, and the chloroplatinic acid-tin chloride mixed aqueous solution was added to alumina. Impregnated. This in the air 1
A catalyst 3 was prepared by drying at 20 ° C. and further calcining in air at 500 ° C. for 3 hours. Table 1 shows the results of activity evaluation conducted in the same manner as in Example 1. The platinum loading rate of catalyst 3 obtained from the ICP emission analysis was 0.88 wt%, the tin loading rate was 0.88 wt%, and the Sn / Pt atomic ratio was 1.7.

Comparative Example 1 100 ml of a chloroplatinic acid aqueous solution containing 1 g of Pt metal in 100 ml of a solution was diluted with ion-exchanged water to prepare a total amount of 280 ml of a chloroplatinic acid aqueous solution, which was prepared in the same manner as in Example 1. Mechanical stirring was continued while gradually adding to 100 g of the alumina carrier prepared above, and the aqueous solution of chloroplatinic acid was impregnated into the alumina. This was dried in air at 120 ° C. and further calcined in air at 500 ° C. for 3 hours to prepare a catalyst A. Table 1 shows the results of activity evaluation conducted in the same manner as in Example 1. The platinum loading rate of catalyst A obtained from ICP emission analysis was 0.91 wt%.

Comparative Example 2 100 ml of an aqueous solution of stannous chloride containing 1 g of Sn metal in 100 ml of the solution was diluted with ion-exchanged water to prepare an aqueous solution of tin chloride in a total amount of 280 ml, and prepared in the same manner as in Example 1. Mechanical stirring was continued while gradually adding to 100 g of the alumina carrier prepared above, and the tin chloride mixed aqueous solution was impregnated into the alumina. This was dried in air at 120 ° C. and further calcined in air at 500 ° C. for 3 hours to prepare a catalyst B.
Table 1 shows the results of activity evaluation conducted in the same manner as in Example 1. The tin support rate of the catalyst B obtained from ICP emission analysis was 0.87 wt%.

[0018]

[Table 1]

It can be seen from Table 1 that the relationship between the reaction temperature and the NO conversion rate, N ₂ O production rate, N ₂ production rate changes by changing the atomic ratio of Sn and Pt. Of these, Sn and Pt
FIG. 1 shows the relationship between the reaction temperature and the N ₂ production rate when the atomic ratio of is changed. From FIG. 1, when the Sn / Pt atomic ratio is 1, the N ₂ production rate is about 50% at the reaction temperature of about 250 to 300 ° C., and when the Sn / Pt atomic ratio is 0.2 or 1.7, the reaction temperature is about 200. to 250 DEG ° C. in N catalysts by changing the Sn / Pt atomic ratio because it shows the value N ₂ production rate is higher than that of Pt alone at ₂ generation rate of 50% becomes around further 200 ° C. or more high temperature side It can be seen that the active temperature region can be shifted to the high temperature side of the desired range.

[0020]

As has been described in the foregoing, according to the present invention, the catalyst simultaneously carrying Pt and Sn on a carrier having a high specific surface area, the hydrocarbon selective reduction reaction of the NO _x in an oxygen-rich atmosphere is promoted In particular, when the atomic ratio of Pt and Sn to be supported is changed, N in an oxygen excess atmosphere is changed.
In the hydrocarbon selective reduction reaction of O _x active temperature region of the Pt-Sn catalyst can be set to a desired range.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the reaction temperature and the N ₂ production rate when the atomic ratio of Sn and Pt is changed.

Claims (1)

[Claims] 1. A catalyst in which Pt and Sn are supported on a high specific surface area carrier, and the loading ratio of Pt is 0.01 to 5 wt.
%), The loading ratio of Sn is 0.01 to 10 wt%,
A nitrogen oxide purifying catalyst having an n / Pt atomic ratio in the range of 0.1 to 2.