JPH0810574A - Nitrogen oxides purifying method - Google Patents

Abstract

PURPOSE:To purify NOX in an exhaust gas in a low temp. range at a high removing rate under an oxygen excess atmosphere by the action of hydrogen coexisting in the exhaust gas. CONSTITUTION:This nitrogen oxides purifying method is a nitrogen oxides reduction method for reducing nitrogen oxides contained in the exhaust gas in which the oxygen concn. is equal to or above the stoichiometric ratio required for oxidizing a component to be oxidized in the exhaust gas, and the exhaust gas containing excess oxygen coexisting with hydrogen is brought into contact with a nitrogen oxides reducing catalyst constituted so as to contain a noble metal element and molybdenum carried on a porous carrier.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has an oxygen concentration in exhaust gas discharged from a boiler, a diesel engine, etc., which is higher than a stoichiometric ratio required to oxidize components to be oxidized contained in the exhaust gas. Exhaust gas with excess oxygen (excess oxygen atmosphere)
The present invention relates to a nitrogen oxide purification method capable of purifying nitrogen oxides (NO _x ) contained in.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of protecting the global environment, nitrogen oxides in exhaust gas discharged from boilers, diesel engines, etc. have become a problem, and various solutions have been studied. However, the nitrogen oxides in the exhaust gas contain nitrogen (N ₂ ) in the oxygen excess atmosphere because the exhaust gas contains excess oxygen.
It is usually extremely difficult to reduce and purify the material. Under such circumstances, research and development of a catalyst that does not react with excess oxygen (O ₂ ) but selectively reacts with nitrogen oxides has been actively conducted.

In Japanese Patent Laid-Open No. 63-100919 (A), a carrier composed of a metal oxide such as alumina and a catalyst composed of a catalytic metal such as copper supported on the carrier is used as a hydrocarbon (HC) as a reducing agent. ) Is used to bring the exhaust gas containing the hydrocarbon into contact with the catalyst to reduce and purify nitrogen oxides in an oxygen excess atmosphere.

Further, for the purification of nitrogen oxides containing excess oxygen generated from a large-scale plant such as a factory, ammonia (NH ₃ ) is used as a reducing agent in a catalyst composed of vanadium / titania. A technique for reducing nitrogen oxides by contacting the catalyst with the exhaust gas containing ammonia is usually used (B).

Further, the present inventors have previously introduced hydrogen produced by electrolysis of a solid electrolyte or water vapor into exhaust gas containing nitrogen oxides and oxygen gas to obtain alumina, silica,
A technique for reducing and purifying nitrogen oxides by a catalyst in which platinum is supported on a carrier such as zeolite has been disclosed (Japanese Patent Laid-Open No. 5-168).
856 (C).

[0006]

However, in the method (A), since the hydrocarbon is used as the reducing agent, the chemical reaction rate at the time of nitrogen oxide reduction / purification is slow, and therefore, the large capacity is required. The above catalyst is required, and it is not suitable for mounting and using it for purification of nitrogen oxides generated from a moving body such as an automobile.

In the method (B), the reducing substance is harmful ammonia, and it is difficult to obtain and store the ammonia in a small-scale facility.

In the method (C), hydrogen is used as the reducing agent, and the catalyst is the alumina, silica,
A catalyst in which platinum is supported on a carrier such as zeolite has been proposed. However, this catalyst has a problem that the temperature range in which nitrogen oxides can be efficiently purified is as narrow as 60 to 130 ° C.

The inventors of the present invention have earnestly studied a suitable catalyst for utilizing hydrogen as a reducing agent, and as a result of various systematic experiments, the present invention has been accomplished.

An object of the present invention is to provide a method for purifying nitrogen oxides which exhibits high purifying activity in a wide temperature range of 90 to 250 ° C. by causing hydrogen to act as a reducing agent.

[0011]

[Means for Solving the Problems]

(First invention) In order to achieve the above object, the nitrogen oxide purification method of the first invention is such that the oxygen concentration in the exhaust gas is at least a stoichiometric ratio necessary to oxidize the oxidizable component in the exhaust gas. A nitrogen oxide purification method for purifying nitrogen oxides contained in certain oxygen-excess exhaust gas, wherein exhaust gas in which hydrogen coexists in the oxygen-excess exhaust gas is supported on a porous carrier and the porous carrier. It is characterized in that the nitrogen oxide in the exhaust gas is purified by bringing it into contact with a nitrogen oxide purification catalyst containing a noble metal element and molybdenum.

(Second invention) The method for purifying nitrogen oxides according to the second invention is the method for purifying nitrogen oxides according to the first invention.
It is characterized in that the amount of molybdenum supported is 0.1 or more in a molar ratio with respect to the amount 1 of carried noble metal element.

(Third invention) The method for purifying nitrogen oxides according to the third invention is the method according to the first invention or the second invention, wherein the catalyst for purifying nitrogen oxides is selected from the group consisting of alkali metals and alkaline earth metals. It is characterized by further containing at least one metal selected.

(Fourth Invention) The method for purifying nitrogen oxides according to the fourth invention is the method according to any one of the first invention to the third invention, wherein hydrogen coexisting in the oxygen-excess exhaust gas is present in the exhaust gas. It is characterized in that it is 1.2 or more in molar ratio with respect to the nitrogen oxide.

(Fifth Invention) A fifth aspect of the present invention is the method for purifying nitrogen oxides according to any one of the first to fourth aspects of the invention, wherein the exhaust gas in excess of oxygen in which hydrogen coexists is used to purify the nitrogen oxides. The temperature of the nitrogen oxide-purifying catalyst and / or the exhaust gas in excess of oxygen (hereinafter, simply referred to as the operating temperature of the nitrogen oxide-purifying catalyst) when contacting the catalyst is 90 to 2
It is characterized by being 50 ° C.

[0016]

[Action]

(Operation of the First Aspect of the Invention) The effect of reducing and purifying nitrogen oxides in exhaust gas by the method for purifying nitrogen oxides of the first aspect of the present invention is not clear, but it is presumed to be as described below. It is presumed that the selective reduction and removal reaction of nitrogen oxides by hydrogen on the catalyst proceeds by the adsorption and activation of hydrogen and nitrogen oxides on the precious metal element, which is the catalyst metal, as an important step. Particularly, in the temperature range of less than 150 ° C. where the activation of oxygen is suppressed, the nitrogen oxides are efficiently reduced.
On the other hand, in the temperature range where the activation rate of oxygen is high, which is 150 ° C. or higher, hydrogen (H ₂ ) is oxidized to water (H ₂ O) and NO is oxidized to NO ₂ , so that nitrogen oxide (N 2
O _X) reduction reaction is less likely to occur. Here, it is presumed that since the activation of oxygen on the noble metal element is suppressed by the interaction of the noble metal element and molybdenum, reduction purification of nitrogen oxide with hydrogen is possible.

(Operation of Second Invention) In the second invention,
The amount of molybdenum supported is 0.1 with respect to the precious metal element.
In the above case, a particularly large amount of noble metal element-molybdenum interaction sites are present on the catalyst surface, so that the nitrogen oxide purifying activity is further excellent. When the supported amount of molybdenum is less than 0.1 in terms of molar ratio with respect to the noble metal element, the interaction between the noble metal element and molybdenum is weak, and the purification activity in the high temperature range of 90 ° C. or higher is reduced, which is not preferable. The amount of molybdenum supported is effective even if the molar ratio to the noble metal element is 50 or more, but the effect is hardly increased.

(Operation of Third Invention) In the third invention,
When the catalyst for purifying nitrogen oxides further contains at least one metal selected from the group consisting of alkali metals and alkaline earth metals, the alkali metal and / or alkaline earth metal, a noble metal element, and molybdenum The activation of oxygen on the noble metal element is further suppressed by the interaction of. Therefore, it is presumed that the purification rate of nitrogen oxides by hydrogen can be further increased particularly in the temperature range where the oxygen activation rate is high.

(Operation of Fourth Invention) In the fourth invention,
When the amount of hydrogen as a reducing agent is 1.2 or more in molar ratio with respect to nitrogen oxides, the nitrogen oxide purification activity by the hydrogen is particularly effectively performed.

(Operation of Fifth Invention) In the fifth invention,
When the operating temperature of the catalyst for purifying nitrogen oxides is lower than 90 ° C., water generated by the reaction is aggregated and accumulated on the catalyst, so that the efficiency of purifying nitrogen oxides decreases. Also,
When the temperature exceeds 250 ° C, hydrogen directly reacts with oxygen and nitrogen oxides cannot be reduced, which is not preferable.

The operating temperature of the nitrogen oxide purifying catalyst is 90 to 2
In the case of 50 ° C., a state is realized in which hydrogen reacts with nitrogen oxide at a sufficiently practical rate, and coexisting oxygen does not participate in the reaction. Therefore, in this state, the purification reaction of nitrogen oxides proceeds efficiently using hydrogen as a reducing agent.
Further, in this temperature range, the undesirable side reaction that nitrogen oxides become nitrous oxide other than nitrogen, ammonia, and nitrogen dioxide hardly progress, which is particularly preferable.

[0022]

【The invention's effect】

(Effects of the first invention to the fifth invention) According to the first invention to the fifth invention, a nitrogen oxide purification method having high purification activity in a wide temperature range is provided by causing hydrogen to act as a reducing agent. be able to.

[0023]

EXAMPLES Examples of the present invention will be described in detail below.
The technical idea of the present invention is not limited to this embodiment.

(Specific Example of the Invention) In the first aspect of the present invention, the exhaust gas in excess of oxygen having an oxygen concentration equal to or higher than the stoichiometric ratio means that carbon monoxide and unsaturated hydrocarbon are each less than about 100 ppm, and Exhaust gas containing 0.1% or more of oxygen in addition to nitrogen oxides.
Here, the exhaust gas may further contain water vapor, carbon dioxide gas, and the like.

According to the nitrogen oxide purification method of the first invention,
When purifying nitrogen oxides in exhaust gas, when the exhaust gas contains carbon monoxide (CO) and unsaturated hydrocarbon components (HC), the exhaust gas in excess of oxygen is brought into contact with the nitrogen oxide purification catalyst. It is preferable to suppress the amount of each of the above components to about 100 ppm or less in advance, before carrying out. The reason is that when the component is contained in the exhaust gas in an amount exceeding 100 ppm, the component reduces the purification ability of the nitrogen oxides.

In order to suppress carbon monoxide and unsaturated hydrocarbon components in the exhaust gas in advance, an ordinary oxidation catalyst, for example, a catalyst in which palladium is supported on a carrier made of alumina is provided in the preceding stage of the exhaust gas passage of the nitrogen oxide purification catalyst. And the catalyst is used to oxidize the above-mentioned components.

As hydrogen coexisting in the exhaust gas, hydrogen produced by reforming fuel by steam reforming or water gas shift reaction of carbon monoxide, or hydrogen obtained from electrolysis of water is used. Any hydrogen supply means may be provided so long as it can coexist by any method.

As a method of supplying hydrogen, for example,
Supplying hydrogen continuously to the nitrogen oxide reduction catalyst or the exhaust gas flow path, intermittently supplying it, or by changing the hydrogen concentration according to the operating time and operating conditions of the engine and the state of the exhaust gas. The method to do is illustrated,
These methods can be used.

The type of the porous carrier constituting the nitrogen oxide purification catalyst is not particularly limited, but a porous carrier containing silicon among metal oxides used as the carrier,
It is preferable to use silicon oxide (silica; SiO ₂ ), zeolite or the like.

As the noble metal element supported on the porous carrier, platinum (Pt), rhodium (Rh), palladium (Pd) and the like can be used alone or in combination. Of the above-mentioned noble metal elements, platinum is excellent in purifying nitrogen oxides, and platinum is preferably used.

The amount of the noble metal element supported on the porous carrier is not particularly limited, but in practical use, it is preferably in the range of 0.1 to 20% by weight based on the carrier.

In the third aspect of the present invention, the alkali metal carried is sodium (Na), lithium (L
i), potassium (K), cesium (Cs), rubidium (Rb) and the like can be used, and as the alkaline earth metal, barium (Ba), calcium (Ca), strontium (Sr), and Magnesium (Mg) or the like can be used.

The amount of the alkali metal or alkaline earth metal is 0.001 in a molar ratio with respect to the amount 1 of the noble metal element supported.
-15 are preferable. When the supported amount of the metal is in this range,
The precious metal element-molybdenum interaction is particularly effective. When the amount of alkali metal carried exceeds 15,
It is presumed that the structure of the carrier is destroyed by the alkali metal and the specific surface area decreases, which is not preferable.

In the fifth aspect of the invention, the operating temperature of the nitrogen oxide purifying catalyst is preferably 100 to 200 ° C., more preferably 120 to 180 ° C. The reason is,
This is because the purification rate of nitrogen oxides is particularly increased in the operating temperature range described above.

(Preparation of Catalyst of this Example) Alkali metal or alkaline earth metal (sodium acetate, lithium nitrate, potassium nitrate) having a predetermined concentration prepared so as to obtain the supported amount of alkali metal or alkaline earth metal shown in Table 1. ,
An aqueous solution of calcium nitrate and magnesium acetate was impregnated in the silicon oxide (SiO ₂ ) powder, and evaporated to dryness, then 50
It was baked at 0 ° C. for 5 hours, and No. 2 to No. A catalyst having the amounts of sodium (Na), lithium (Li), potassium (K), calcium (Ca), and magnesium (Mg) shown in 9 was obtained. Next, the catalyst was impregnated with an ammonium molybdate aqueous solution having a predetermined concentration prepared so as to obtain the molybdenum (Mo) loading amount shown in Table 1,
After evaporating to dryness, baking was performed at 700 ° C. for 3 hours.
o. 2 to No. A catalyst having a molybdenum loading shown in 9 was obtained. Further, in Table 1, No. The silicon oxide powder was impregnated with an aqueous solution of ammonium molybdate having a predetermined concentration prepared so as to obtain the amount of molybdenum supported as shown in Table 1, evaporated to dryness, and then baked at 700 ° C. for 3 hours. A catalyst having a molybdenum loading of 1 was obtained.

[0036]

[Table 1]

Next, the No. 1 to No. A catalyst having a molybdenum loading shown in 9 was impregnated with a dinitrodiaminoplatinum aqueous solution having a predetermined concentration prepared so that 1.7% by weight of platinum was obtained, evaporated to dryness, and then calcined at 500 ° C. for 5 hours. , The catalyst No. of this example. 1 to No. 9 was prepared.

(Preparation of Catalyst of Comparative Example) The catalyst of the present Example except that the carrier was changed to alumina (Al ₂ O ₃ ) and the alkali metal, alkaline earth metal and molybdenum were not supported on the alumina carrier. No. By the same operation as that of the catalyst No. 2 of the comparative example. Got 10.

(Performance Evaluation Test) No. 1 shown in Table 1 1 to N
o. Diameter of 300 to 700 by compacting 10 catalysts
The pellet-shaped catalyst was molded into a pellet of 0.1 μm.
5 g was packed in a quartz reaction tube. A model gas of lean-burn exhaust having the gas components shown in Table 2 was circulated in the reaction tube, and catalytic activity (NO conversion rate) was evaluated by an exhaust gas analyzer. At this time, the hydrogen concentration was kept constant at 0.2%.

[0040]

[Table 2]

Next, the catalyst No. of the above-mentioned embodiment was used. 2 (pellets, 0.5 g), the catalyst activity (NO conversion rate) when the amount of hydrogen in the model gas was changed to 0 to 0.2% (H ₂ / NO ratio: 0 to 10) Was measured by an exhaust gas analyzer.

(Results of Performance Evaluation Test) FIGS. 1 to 4 show the NO purification rates of the catalysts of the present example and comparative example. The abscissa shows the incoming gas temperature, and the ordinate shows the NO _x (NO) purification rate. FIG. 1 shows NO when the amount of Na carried is changed.
_X Indicates the purification rate. From this result, the amount of Na supported may be zero, and as the amount of Na supported increases,
The NO _x purification rate is gradually decreasing. However, it can be seen that when the supported amount of Na is in the range of 0 to 5.6 in terms of molar ratio with respect to platinum, a good NO _x purification rate is exhibited (FIGS. 1 and 2). In the figure, the catalyst No. of the comparative example. The NO _x purification characteristics of No. 8 are also shown. The catalyst of this example has a higher NO _x purification rate than the catalyst of the comparative example.

FIG. 2 shows the NO _x (NO) purification rate when the supported alkali metal is changed. The vertical axis and the horizontal axis are the same as in FIG. From this result, it is understood that a good NO _x purification rate is exhibited even when the type of alkali metal is changed.

FIG. 3 shows the NO _x purification rate when the catalyst carrier is changed. From this result, it is understood that the NO _x purification rate is maintained at a high value even when the catalyst carrier is changed.

FIG. 4 shows the NO _x (NO) purification rate when an alkaline earth metal was supported. From this result, it can be seen that a good NO _x purification rate is exhibited even when the alkaline earth metal is supported.

FIG. 5 shows the measurement results of changes in the NO purification rate when the amount of hydrogen in the model gas was changed. The catalyst used was the catalyst No. 1 of this example. It is 2. The horizontal axis represents the amount of hydrogen as a ratio to NO, and the vertical axis represents the NO _x (NO) purification rate. From this result, it is understood that when the H ₂ / NO ratio is 1.2 or more, a good NO purification rate is exhibited.

[Brief description of drawings]

In the catalyst according to an embodiment of the invention, FIG diagrams showing the _the NO _x purification rate when varying the loading of Na, the _the NO _x purification rate of the catalyst of Comparative Example.

FIG. 2 is a diagram showing the NO _x purification rate when the type of alkali metal to be carried is changed in the catalyst according to the example of the present invention.

FIG. 3 is a diagram showing the NO _x purification rate when the type of carrier is changed in the catalyst according to the example of the present invention.

FIG. 4 is a diagram showing a NO _x purification rate when an alkaline earth metal is supported in the catalyst according to the example of the present invention.

FIG. 5 is a diagram showing changes in the NO _x purification rate when the amount of hydrogen in the model gas is changed in the catalyst according to the example of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiyuki Tanaka, Nagakute-cho, Aichi-gun, Aichi Prefecture, Nagatoji 1 1st side street, Toyota Central Research Institute Co., Ltd. (72) Inventor, Masahiro Taki Nagakute-cho, Aichi-gun, Aichi Prefecture 41 Chuo-dori, 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Yujiro Oshima, Nagakute-cho, Aichi-gun, Aichi Pref. Shoji, 1-chome Yokota Central Research Institute (72) Inventor Shinichi Matsumoto Aichi 1 Toyota-cho, Toyota City Toyota Motor Corporation

Claims (5)

[Claims] 1. Nitrogen oxide purification for purifying nitrogen oxides contained in exhaust gas in excess of oxygen in which the oxygen concentration in the exhaust gas is equal to or higher than the stoichiometric ratio required to oxidize components to be oxidized in the exhaust gas. In the method, the exhaust gas in which hydrogen coexists in the oxygen-excess exhaust gas is contacted with a nitrogen oxide purification catalyst composed of a porous carrier and a noble metal element and molybdenum supported on the porous carrier. A method for purifying nitrogen oxides, characterized by purifying nitrogen oxides in the exhaust gas. 2. The method for purifying nitrogen oxides according to claim 1, wherein the nitrogen oxide purifying catalyst has a molybdenum loading of 0.1 or more in a molar ratio with respect to a loading of 1 of the noble metal element. 3. The nitrogen oxide purifying catalyst according to claim 1, further comprising at least one metal selected from the group consisting of alkali metals and alkaline earth metals. A method for purifying nitrogen oxides. 4. The hydrogen coexisting in the exhaust gas in excess of oxygen according to any one of claims 1 to 3, having a molar ratio of 1.2 or more with respect to nitrogen oxides in the exhaust gas. A method for purifying nitrogen oxides, which comprises: 5. The nitrogen oxide purifying catalyst according to claim 1, and / or the nitrogen oxide purifying catalyst, when the oxygen-excess exhaust gas in which hydrogen coexists is brought into contact with the nitrogen oxide purifying catalyst. The temperature of exhaust gas with excess oxygen is 90-
A method for purifying nitrogen oxides, characterized in that the temperature is 250 ° C.