JPH04284825A - Purification of exhaust gas - Google Patents

Abstract

PURPOSE:To purify exhaust gas by catalytically decomposing nitrogen oxide in the exhaust gas into N2 and O2 within a relatively low temp. region under an oxygen rich atmosphere by a coexisting reducing agent. CONSTITUTION:An exhaust gas purifying method consists of a reducing agent introducing process introducing hydrocarbon into exhaust gas as a reducing agent and a catalytic decomposition process bringing the exhaust gas into which hydrocarbon is introduced into contact with a catalyst composed of metal- containing zeolite containing at least one kind of an element selected from copper, manganese, cobalt, iron, nickel, chromium and vanadium capable of relatively easily changing in valence as oxide to decompose nitrogen oxide. The aforementioned hydrocarbon is characterized in that the number of carbon atoms is set to 2-7.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] This invention relates to an exhaust gas purification method for purifying nitrogen oxides in exhaust gas by catalytically decomposing them into N2 and O2 using a coexisting reducing agent in an oxygen-rich atmosphere. .

0002

[Prior Art] Conventionally, the technologies for removing nitrogen oxides, which are harmful components in exhaust gas from automobile engines, include purification methods using three-way catalysts such as Pt-Rh, and purification methods using ammonia, urea, etc. Purification methods that apply selective reduction methods, purification methods that use copper ion exchange zeolite catalysts, etc.
Various types are known.

However, in a purification method using a three-way catalyst such as a Pt-Rh system, nitrogen oxides can be removed on the rich side of the stoichiometric air-fuel ratio, but on the lean side (that is, in an oxygen-rich atmosphere), nitrogen oxides can be removed. , there is a fatal problem that cannot be removed. In addition, purification methods that apply selective reduction using ammonia, urea, etc. have a high purification rate of nitrogen oxides, but the equipment becomes large and new pollution occurs due to the secondary emission of ammonia. There is a problem with this. Furthermore, in purification methods using copper ion exchange zeolite catalysts, nitrogen oxides can be removed even on the lean side, unlike the purification methods using three-way catalysts such as the Pt-Rh system mentioned above. It has been pointed out that the purification efficiency is low and that the purification performance deteriorates at relatively low temperatures.

[0004] In recent years, in such conventional exhaust gas purification methods, even when oxygen coexists at a high concentration, such as in exhaust gas from a diesel engine or exhaust gas from a lean fuel gasoline engine, nitrogen oxidation As an exhaust gas purification method and catalyst that can stably decompose and remove substances,
A technique disclosed in Japanese Unexamined Patent Publication No. 100919/1983 is known. This conventional publication discloses that nitrogen oxides in the exhaust gas are removed by preparing a catalyst containing copper and bringing the exhaust gas containing nitrogen oxides into contact with the catalyst in the presence of hydrocarbons in an oxidizing atmosphere. and a catalyst for removing nitrogen oxides in exhaust gas in an oxidizing atmosphere, characterized in that copper is supported on a porous carrier such as alumina, silica, zeolite, etc. An exhaust gas purification catalyst is disclosed. As a result, by implementing the technical content disclosed in this prior art publication, a method and catalyst for efficiently removing nitrogen oxides in an oxidizing atmosphere will be provided.

[0005]

[Problems to be Solved by the Invention] However, even when such conventional technology is used, nitrogen oxides are decomposed by utilizing the high-temperature activity of the catalyst. In a low-temperature region where the exhaust gas temperature has not risen sufficiently, the catalyst cannot reach a sufficiently high temperature and cannot utilize its high-temperature activity, resulting in extremely poor purification performance. As a result, even when using the technology disclosed in the above-mentioned conventional publications, nitrogen oxides are released into the atmosphere without being decomposed until the catalyst temperature rises to a predetermined high temperature range. There remains a major issue.

Furthermore, as mentioned above, since the high-temperature activity of the catalyst is utilized, the catalyst itself is subject to severe high-temperature deterioration, and there remains a problem in terms of durability. This invention was made in view of the above-mentioned problems, and an object of the invention is to provide an exhaust gas purification method that can obtain good nitrogen oxide purification performance from a relatively low temperature side.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the exhaust gas purification method according to the present invention includes a reducing agent introduction step of introducing hydrocarbons as a reducing agent into the exhaust gas, and a step of introducing hydrocarbons as a reducing agent into the exhaust gas. The exhaust gas is brought into contact with a catalyst made of a metal-containing zeolite containing at least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium whose valence as an oxide can be changed relatively easily, The method includes a catalytic cracking step for decomposing nitrogen oxides, and the hydrocarbon is characterized in that the number of carbon atoms is set to 2 to 7.

[0008]

[Operation] The operation of the exhaust gas purification method according to the present invention will be explained in detail below. First, the oxidizing atmosphere defined in the present invention is a stable atmosphere that completely oxidizes reducing substances such as carbon monoxide, hydrogen, and hydrocarbons contained in exhaust gas and hydrocarbons introduced in the present invention. H2 O and C
This refers to a state in which oxygen is contained in excess of the amount of oxygen required to convert it into O2. That is, it refers to the atmosphere of exhaust gas from a diesel engine or the atmosphere of exhaust gas from a gasoline engine that burns a lean mixture with a large air-fuel ratio (that is, lean).

The exhaust gas purification method according to the present invention involves introducing a hydrocarbon as a reducing agent into the exhaust gas in the above-mentioned oxidizing atmosphere, and then bringing it into contact with a catalyst made of metal-containing zeolite. Based on the discovery that in decomposing nitrogen oxides, by limiting the number of carbon atoms in hydrocarbons to a range of 2 to 7, good purification performance of nitrogen oxides can be achieved from a relatively low temperature range. ing.

That is, when oxygen generated in an oxidizing atmosphere and by the decomposition of nitrogen oxides is adsorbed to the active sites of metal-containing zeolite, it is difficult for oxygen to be desorbed and nitrogen oxides lose adsorption sites, resulting in nitrogen oxidation. Purification of substances is limited, and it becomes impossible to maintain high nitrogen oxide purification performance continuously. Here, if specific hydrocarbons are allowed to coexist in the exhaust gas to be purified, the purification efficiency of nitrogen oxides will be dramatically improved, and in particular, purification performance can be demonstrated from a relatively low temperature range. Become. In particular, the use of hydrocarbons ranging from ethane with 2 carbon atoms to heptane with 7 carbon atoms is thought to extend the purification performance to a relatively low temperature range for the following reasons.

That is, the conversion rate of each hydrocarbon to combustion is
In all cases, the conversion rate is higher than that of nitrogen oxides, but the temperature range of combustion roughly corresponds to the conversion range of nitrogen oxides, and the combustion of hydrocarbons depletes the oxygen on the surface of the metal contained in the zeolite. This is because it is believed that this prevents excessive oxidation and helps the progress of the decomposition reaction of nitrogen oxides. Here, the catalyst used in this invention is a metal-containing zeolite, and is not a metal ion-exchanged zeolite. Further, the metal contained in the zeolite is at least one type of transition metal such as copper, manganese, cobalt, iron, nickel, chromium, and panadium, and the combination thereof is not limited at all.

As described above, by configuring the means for solving the problem, nitrogen oxides in exhaust gas can exhibit good purification performance from a relatively low temperature range even in an oxidizing atmosphere. For example, even if the engine has not been sufficiently warmed up immediately after starting, it will begin to purify nitrogen oxides from an early stage, making it possible to suppress nitrogen oxides from being released into the atmosphere as much as possible. . In addition, since this exhaust gas purification method exhibits good purification performance from a relatively low temperature range, for example, the catalyst may be placed in the exhaust pipe far away from the engine to minimize the heat received from the engine and exhaust gas. By reducing the size and maintaining the temperature of the catalyst itself in a relatively low temperature range, the life of the catalyst can be extended.

[0013]

Effects of the Invention As detailed above, the exhaust gas purification method according to the present invention includes a reducing agent introduction step of introducing hydrocarbons as a reducing agent into exhaust gas, and a reducing agent introduction step of introducing hydrocarbons as a reducing agent into exhaust gas, and Nitrogen oxides are decomposed by contacting with a catalyst made of metal-containing zeolite containing at least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium, whose valence as an oxide can be changed relatively easily. The hydrocarbon is characterized in that the number of carbon atoms is set to 2 to 7.

Therefore, according to the present invention, there is provided an exhaust gas purification method that can obtain good nitrogen oxide purification performance from a relatively low temperature side. In addition, compared to the method of purifying exhaust gas using a three-way catalyst, it can purify nitrogen oxides with high efficiency even in an oxidizing atmosphere, making the exhaust gas of diesel engines and gasoline engines pollution-free. In addition, compared to selective reduction methods using ammonia, etc., the equipment is extremely small and inexpensive, and there is no problem of secondary pollution such as the discharge of excess ammonia. In comparison, it has excellent thermal stability and can maintain stable purification performance for a long time even when used under conditions where the exhaust gas temperature is high.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of an embodiment of the exhaust gas purification method according to the present invention will be explained in detail below with reference to FIGS. 1 and 4. First, copper-containing type A zeolite constituting the catalyst used in this example was prepared in the following manner.

That is, predetermined amounts of copper nitrate, sodium aluminate, water glass, and sodium hydroxide are mixed in an ice bath,
Al/Si=1.0, Cu/Si=0.5 obtained by stirring
A copper-containing type A zeolite was prepared by hydrothermally synthesizing a gel having the following composition at a constant temperature of 85° C. for 6 hours. This copper-containing type A zeolite was washed with water, dried, and then formed into tablets.
It was crushed into a mesh, and 0.5 g was subjected to standardization treatment in a He gas stream and then used for the reaction. When performing hydrogen reduction treatment, 10
In a 0% H2 gas flow, the temperature was raised from room temperature to 400°C/1.5 h and maintained for 0.5 h.

[Example 1] For the reaction, a normal pressure flow reactor was used, the temperature was raised and lowered at a constant rate of 2.5°C/min, and the GHSV
= 2500h-1, nitrogen monoxide as nitrogen oxide (
NO), methane (
A mixed gas of CH4) and He was passed through and brought into contact with the copper-containing type A zeolite prepared as described above.

The gas that came into contact with this copper-containing type A zeolite was analyzed using gas chromatography and an infrared gas analyzer, and the nitrogen (
The amount of N2) produced was measured, and the conversion rate of nitrogen monoxide was calculated from the measured amount of nitrogen produced. This result is shown in Figure 1.
Indicated with a mark. Here, the methane concentration is 4.8%, the oxygen concentration is 3.1%, the nitrogen monoxide concentration (NO/He) to helium is 2.0%, and the combustion stoichiometric ratio of oxygen (O2) is , 0.4, respectively.

On the other hand, the combustion stoichiometric ratio of oxygen (O2) is set to 0.
The temperature dependence of methane combustion was measured when the temperature was set to 4, and the results are shown in FIG. 2 by circles. Here, the methane concentration is 2.1%, and the oxygen concentration (O2
/He) is set at 1.7%. As a result,
As shown in FIG. 1, no results were obtained suggesting that the conventional catalyst temperature could be extended to a relatively low temperature range by introducing methane in nitrogen monoxide decomposition in the coexistence of O2. That is, methane was found to be inappropriate as the hydrocarbon of the present invention.

[Example 2] Under the same conditions as in Example 1, the hydrocarbon introduced into the mixed gas was changed to ethane having 2 carbon atoms, and the mixed gas was circulated. And in a similar way,
The conversion rate of nitrogen monoxide was calculated.

[0021] The results are shown in Fig. 1 with a mark ◇. here,
The ethane concentration is 2.4%, the oxygen concentration is 2.8%, the nitric oxide concentration relative to helium (NO/He) is 2.3%, and the combustion stoichiometric ratio of oxygen (O2) is 0. 4 respectively. On the other hand, the temperature dependence of ethane combustion was measured when the combustion stoichiometric ratio of oxygen (O2) was set to 0.4, and the results are shown in FIG. 2 with a mark ◇. Here, the ethane concentration is 2.0%, and the oxygen concentration with respect to helium (
O2/He) was set at 2.9%.

As a result, as shown by the mark ◇ in FIG. 1, temperature dependence of the conversion rate of nitrogen monoxide when ethane is introduced appears, and the temperature dependence is approximately 4
It was found that purification performance was exhibited from 00°C. The combustion stoichiometric ratio of oxygen (O2) in this case is 0.4, and the temperature dependence of ethane combustion in this case is shown in FIG. From these figures 1 and 2, the conversion rate of ethane to combustion is higher than the conversion rate of nitrogen monoxide, but the temperature range of combustion is
It is understood that the area roughly corresponds to the nitrogen monoxide conversion region, and therefore, the combustion of ethane depletes the oxygen on the surface of the copper contained in zeolite, prevents its excessive oxidation, and causes the decomposition reaction of nitrogen monoxide. It is thought that the nitrogen monoxide purification performance can be exhibited even down to the relatively low temperature range, which is the lower limit of combustion of ethane.

In this Example 2, the influence of the combustion stoichiometric ratio of coexisting oxygen (O2) to ethane was investigated, and the results are shown in FIG. Note that in FIG. 3, each mark is set as shown in Table 1 below.

[0024]

[Table 1] As is clear from FIG. 3, in the absence of oxygen (O2), the conversion rate of carbon monoxide sharply increased around 500°C. This temperature range approximately corresponds to the maximum ethane conversion temperature shown in FIG. Furthermore, even when the oxygen (O2) coexistence ratio was increased, the conversion rate of nitrogen monoxide did not decrease much. From this, it was confirmed that consistency between the decomposition activation temperature of nitrogen monoxide and the combustion temperature of coexisting ethane is important.

[Example 3] Under the same conditions as in Example 1, the hydrocarbon introduced into the mixed gas was changed to propane having 3 carbon atoms, and the mixed gas was circulated. Then, the conversion rate of nitrogen monoxide was calculated in the same manner.

[0026] This result is shown in FIG. 2 with a black mark. here,
The propane concentration is 1.8%, the oxygen concentration is 2.9%, and the nitric oxide concentration relative to helium (NO/He)
is set to 2.1%, and the combustion stoichiometric ratio of oxygen (O2) is set to 0.4. On the other hand, the temperature dependence of propane combustion was measured when the combustion stoichiometric ratio of oxygen (O2) was set to 0.4, and the results are shown in FIG. 2 with black circles. here,
The propane concentration is 2.1%, and the oxygen concentration relative to helium (O2/He) is set to 4.0%.

[0027] As a result, it was found that the conversion rate of nitrogen monoxide at the time of propane introduction was temperature dependent, as shown by the black mark in Fig. 1, and that the purification performance was exhibited from about 400°C. The combustion stoichiometric ratio of oxygen (O2) in this case is 0.4, and the temperature dependence of propane combustion in this case is shown in FIG. From these Figures 1 and 2, it is understood that the conversion rate of propane to combustion is higher than the nitrogen monoxide conversion rate, but the combustion temperature range almost corresponds to the nitrogen monoxide conversion area, and therefore, The combustion of propane depletes the oxygen on the surface of the copper contained in zeolite, prevents its excessive oxidation, and helps the progress of the decomposition reaction of nitrogen monoxide, so that the temperature reaches a relatively low temperature range, which is the lower combustion limit of propane. It is thought that the purification performance of nitric oxide can be exhibited.

In this Example 3, the influence of the combustion stoichiometric ratio of coexisting oxygen (O2) to propane was investigated, and the results are shown in FIG. In this FIG. 4, each mark is set as shown in Table 2 below.

[0029]

[Table 2] As is clear from FIG. 4, in the absence of oxygen (O2), the temperature dependence of the conversion of carbon monoxide was gentler than in the case of introducing ethane. This is because the combustion of propane is easier than the combustion of ethane, so it burns at a lower temperature than the decomposition of nitric oxide, which contributes to maintaining the catalyst surface in a reductive state during the decomposition of nitric oxide. This is probably because there were few.

[Example 4] Under the same conditions as in Example 1, the hydrocarbon introduced into the mixed gas was changed to heptane, which is a normal paraffin having 7 carbon atoms, and the mixed gas was circulated. Then, the conversion rate of nitrogen monoxide was calculated in the same manner.

[0031] The results are shown in FIG. 2 by ◆. here,
Heptane concentration is 0.85% and oxygen concentration is 3.0%
, and the concentration of nitric oxide relative to helium (NO/He
) is 1.6%, and the combustion stoichiometric ratio of oxygen (O2) is 0.4
are set respectively. On the other hand, the temperature dependence of heptane combustion was measured when the combustion stoichiometric ratio of oxygen (O2) was set to 0.4, and the results are shown in FIG. 2 with a mark ◇. Here, the heptane concentration is 1.2%, and the oxygen concentration relative to helium (O2/He) is set to 5.2%.

[0032] As a result, it was found that the conversion rate of nitrogen monoxide when heptane was introduced showed temperature dependence, as shown by the ◆ mark in Fig. 1, and that the purification performance was exhibited from about 300°C. The combustion stoichiometric ratio of oxygen (O2) in this case is 0.4, and the temperature dependence of heptane combustion in this case is shown in FIG. From these Figures 1 and 2, it is understood that the conversion rate of heptane to combustion is higher than the nitrogen monoxide conversion rate, but the combustion temperature range almost corresponds to the nitrogen monoxide conversion area, and therefore, Combustion of heptane depletes oxygen on the surface of the copper contained in zeolite, prevents its excessive oxidation, and helps the progress of the decomposition reaction of nitrogen monoxide, so that the temperature reaches a relatively low temperature range, which is the lower limit of combustion of heptane. It is thought that the purification performance of nitric oxide can be exhibited.

In particular, when heptane is introduced, although heptane is more combustible than propane, the conversion of nitrogen monoxide proceeds at a lower temperature, reducing the stoichiometric ratio of coexisting oxygen combustion to 0.8.
Even when the temperature was increased, there was little decrease in the conversion rate of nitrogen monoxide. This is because the complete combustion of heptane, which requires a large amount of surface oxygen at the same time, is not achieved, and some intermediate oxides are formed, which in turn accumulate closer to the surface until complete combustion occurs. This is thought to be due to the expansion of the temperature range in which the reduced surface is maintained.

As is clear from the first to fourth embodiments detailed above, hydrocarbons are introduced into the exhaust gas as a reducing agent with the number of carbon atoms set to 2 to 7, and the hydrocarbons are The introduced exhaust gas is brought into contact with a catalyst made of metal-containing zeolite containing at least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium whose valence as an oxide can be changed relatively easily. This makes it possible to decompose nitrogen oxides in the exhaust gases of diesel engines and lean-burn gasoline engines, which contain an excess of oxygen, in a relatively low temperature range.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of coexisting hydrocarbons on nitrogen monoxide decomposition in the coexistence of O2 when the exhaust gas purification method according to the present invention is applied.

FIG. 2 is a diagram showing the temperature dependence of hydrocarbon fuel when the O2 coexistence ratio is 0.4 of the fuel stoichiometric ratio.

FIG. 3 is a diagram showing the influence of the oxygen coexistence ratio on nitrogen monoxide decomposition when ethane is introduced.

FIG. 4 is a diagram showing the influence of hydrogen reduction and oxygen coexistence ratio on nitrogen monoxide decomposition when propane is introduced.

Claims (1)

[Claims] 1. A reducing agent introduction step of introducing a hydrocarbon as a reducing agent into the exhaust gas, and the exhaust gas into which the hydrocarbon has been introduced is made of copper, whose valence as an oxide can be changed relatively easily. manganese, cobalt, iron, nickel, chromium,
a catalytic cracking step in which the nitrogen oxides are decomposed by bringing them into contact with a catalyst made of metal-containing zeolite containing at least one type of panadium; Characteristic exhaust gas purification method.