JPH04354536A - Catalyst for decomposition of nitrogen oxide and method for decomposing nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide a catalyst for decomposition of NOx enabling decomposition of NOx contained in exhaust gas from the diesel engine of an automobile, etc., or various burners for an industrial furnace, etc., in which combustion with excess oxygen is carried out into harmless compds. without using ammonia. CONSTITUTION:One or more kinds of alkaline earth metals and/or silver is supported on a carrier such as gamma-alumina by a soln. impregnation method to obtain a catalyst and NOx in exhaust gas is decomposed with the catalyst while feeding hydrocarbon such as C3H8.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is designed to reduce emissions from diesel engines mounted on trucks, buses, ships, etc., diesel engines used for power generation, various household combustors, and combustors of various industrial furnaces. This article relates to a NOx decomposition catalyst that decomposes nitrogen oxides (hereinafter abbreviated as NOx) contained in exhaust gas containing excessive O2 into substances that are harmless to the human body, and a method for decomposing NOx contained in these exhaust gases. It is.

0002

[Prior Art] Exhaust gas emitted from diesel engines and various combustors usually contains a problematic amount of harmful NOx.
It is said that these not only cause photochemical smog, but also have a negative impact on the human respiratory system. For this reason, there is a strong demand for measures to reduce and remove NOx, and policies are being put forward to strengthen legal regulations.

[0003] The amount of NOx generated can be reduced to some extent by improving the combustion method in the equipment that is the source of the NOx, but the degree of reduction is not sufficient, and currently it is necessary to separately treat the exhaust gas. There is.

On the other hand, if the amount of oxygen in the exhaust gas is small, it can be treated using the three-way catalyst technology used in gasoline engines, but if the amount of oxygen is The use of this three-way catalyst has no effect on lean-burn exhaust gas in which the exhaust gas is excessive, and even if attempts are made to reduce it with unburned hydrocarbons, carbon monoxide, etc. present in the exhaust gas, it cannot be purified.

[0005] As a method for removing NOx from lean combustion exhaust gas, NOx is used as a reducing agent using ammonia.
A catalytic decomposition process for x has been established industrially. However, adding a device to control the amount of ammonia injected according to the amount of NOx generated makes the denitrification device expensive and bulky, making it unsuitable as a catalyst device for small-scale combustion devices or mobile sources.

Furthermore, if excessive ammonia is injected, it is necessary to detoxify the ammonia remaining as a reducing agent downstream of the catalyst. For practical purposes, it is not possible to attach this detoxification processing device to the engine.

[0007]

[Problems to be Solved by the Invention] The present invention provides a catalyst that decomposes NOx in exhaust gas that contains excess oxygen without using ammonia, and a NOx catalyst that uses this catalyst.
The present invention aims to provide a method for removing Ox.

[0008]

[Means for Solving the Problems] The present invention has been made to solve the above problems, and the nitrogen oxide decomposition catalyst of the present invention decomposes nitrogen oxides in exhaust gas in the presence of hydrocarbon gas. The nitrogen oxide decomposition catalyst used for this purpose is characterized by having one or more alkaline earth metals and/or silver supported on a carrier.

The nitrogen oxide decomposition catalyst of the present invention is prepared by impregnating a carrier, preferably an alumina carrier, with one or more alkaline earth metals and/or silver in the form of an aqueous solution of a metal salt, drying, and then calcining the catalyst. can be obtained.

The nitrogen oxide decomposition catalyst of the present invention is particularly suitable for removing NOx present in diesel engine exhaust gas. In systems where O2 and SO2 coexist in the exhaust gas, the catalyst is oxidized by O2 and poisoned by SO2.In systems where poisonous components do not coexist, such as in gasoline engines, a three-way catalyst exhibits high NOx decomposition activity. Almost no activity is expressed.

With the nitrogen oxide decomposition catalyst of the present invention, even in systems where poisonous substances coexist, such as diesel engine exhaust gas, when hydrocarbons are injected and coexisted, reduction occurs due to the action of the acid sites of the catalyst. It is believed that NOx decomposition activity is developed by converting hydrocarbons that function as agents into active intermediates and improving reactivity with NOx.

[0012] Furthermore, the activated hydrocarbons suppress the oxidation of the catalyst surface by O2 and the poisoning by SO2, and the catalyst is maintained in a highly active state, so that the catalyst maintains high NOx decomposition activity and long life. becomes possible.

Silver or alkaline earth metal, which is the main component of the nitrogen oxide decomposition catalyst of the present invention, exhibits effective NOx decomposition activity even when supported alone on an alumina carrier, but when both are combined. It exhibits a synergistic effect and exhibits higher NOx decomposition activity.

The starting materials of silver and alkaline earth metals used in the preparation of the nitrogen oxide decomposition catalyst of the present invention may be in any compound form, such as nitrates, hydrochlorides, etc. of these metal elements. Carbonates, acetates, hydroxides, and complex salts can be used, and the solution is prepared by a preparation method that is compatible with the compound form of the raw materials, and has good solubility in solvents and good dispersibility of supported catalyst components. Since it is possible to secure
In particular, it is preferable to use nitrates, acetates, etc. as starting materials.

After the salts of these metals are dissolved in water or an organic solvent such as a solution in methanol, ethanol, acetone, etc., particularly preferably water, and impregnated into a carrier such as alumina. It is convenient to prepare by drying. In this case, two or more types of alkaline earth metal salts may be used in combination.

The amount of silver or alkaline earth metal supported on the carrier is generally about 0.1 to 10% by weight, and preferably 0.5 to 2% by weight from the viewpoint of economy and effectiveness.

The support used in the catalyst of the invention is preferably an alumina support, particularly preferably γ-alumina. Further, the specific surface area of the carrier is preferably 10 m2/g or more, more preferably 100 m2/g or more. To prepare the catalyst, a carrier is impregnated with a solution of a metal salt, dried and then calcined, with the calcining temperature ranging from 450°C to 700°C.
, preferably from 500°C to 650°C, with a holding time of 1
~10 hours, preferably 2~6 hours, but these conditions are not necessarily limited.

[0018] In the NOx decomposition catalyst of the present invention, NOx
Hydrocarbons that are thought to act as reducing agents for NOx have different reaction characteristics depending on their combustion characteristics and the presence of partially oxidized intermediates. It makes a difference.

[0019] Types of hydrocarbons include methane, LPG
Any hydrocarbon that can be vaporized, such as gasoline or the like, can be used. Among the above, hydrocarbons having a relatively large number of carbon atoms are preferred. In place of or in addition to the above hydrocarbons, alcohols, aldehydes, etc., which are partial oxides of these hydrocarbons, can also be used. It is believed that the above hydrocarbons or their partial oxides remain in an active form on the catalyst surface and maintain the catalyst surface in an active state, thereby exhibiting higher NOx decomposition activity.

[0020] The amount of hydrocarbon added is preferably 0.3 to 2 equivalents per equivalent of NOx in order to maintain a high NOx decomposition rate;
Even if a hydrocarbon is added at a concentration below the reaction stoichiometric ratio, NO
x Degrading activity is expressed.

[0021] The higher the concentration of hydrocarbons added, the lower the NO of the catalyst.
x The decomposition activity will be high, but if it is excessive, it is necessary to reduce the emission of unreacted hydrocarbons from downstream of the catalyst as much as possible, so it is recommended to add so that the reaction stoichiometric ratio with NOx is 1 or less. Particularly preferred.

[0022] The exhaust gas of a diesel engine from which NOx is preferably removed by the NOx decomposition catalyst of the present invention usually contains 10 to 300 ppm by volume of SO2 and O2.
2 2-20%, CO2 5-15%, H2O 5
~15%, NOx 200~3000ppm, and soot dust 0.05~0.6g/Nm3, but with the NOx decomposition catalyst of the present invention, even if SO2 is contained up to about 500ppm and O2 is contained up to about 20%. It is active and can handle all diesel engine exhaust gas compositions. However, it is not necessary to particularly limit the composition of the exhaust gas.

[0023] The reaction conditions for the catalyst include a temperature of 150°C.
The temperature range is preferably from 300°C to 600°C, particularly from 300°C to 600°C. Spatial velocity (SV) is 2000~1
00000h-1, especially 5000-50000h-
It is preferable to set it as the range of 1.

[0024]

[Operation] By injecting hydrocarbons into the exhaust gas from a diesel engine so that they coexist, catalyst deterioration due to poisoning components such as SO2 and O2 is eliminated, and the catalyst surface can always be kept in a highly active state. and higher NOx
Express decomposition activity.

In the preferred nitrogen oxide decomposition catalyst of the present invention, by supporting a combination of silver and alkaline earth metal on an alumina carrier, hydrocarbons are further activated and N of the catalyst is supported.
Ox decomposition activity is further enhanced.

[0026]

Examples Examples 1 to 6 Commercially available γ-alumina powder (specific surface area: 128 m2/g) was used as an alumina carrier, and was impregnated with aqueous solutions of nitrates and acetates of various alkaline earth metals. 120 of these
After drying at 600° C. for a day and night, the mixture was calcined at 600° C. for 4 hours to support an alkaline earth metal oxide on an alumina carrier, and the resulting product was used as a catalyst.

[0027] The amount of alkaline earth metal oxide supported in each catalyst was 0.5 to 2% by weight relative to the alumina support in terms of metal.

The amount of alkaline earth metal supported was adjusted by changing the concentration of the aqueous solution, and each catalyst was
After press-molding at g/cm2, the catalyst was crushed and classified with a sieve to have a particle size of 10 to 22 mesh and used as a test catalyst.

Each of these catalysts was placed in an ordinary atmospheric pressure fixed bed flow reactor and reacted under the following conditions.

[0030] Reaction gas composition; NOx 1200ppm,
C3H 8900ppm, O2 12%, (by volume) SO2 0 ~ 300ppm, He
Remaining reaction temperature: 400 to 600°C Space velocity: 30,000 h-1 to 52,000 h-1 Table 1 summarizes the preparation conditions and test results for each catalyst.

[0031]

[Table 1]

Examples 7 to 9 Commercially available γ-alumina powder (specific surface area: 135 m 2 /g) was used as an alumina carrier, and was impregnated with an aqueous solution of silver nitrate, an ethanol solution of silver acetate, and the like. 11 of these
After drying at 0°C for 3 hours, it was fired at 500°C for 2 hours.

In this case, the supported amount of silver is γ-
It is 1 to 1.5% by weight externally based on alumina,
The amount of silver supported was adjusted by changing the concentration of each solution. Each catalyst thus prepared was made into a particle size of 7 to 15 mesh by the same method as in Examples 1 to 6.

Each of the prepared catalysts was placed in a conventional atmospheric pressure fixed bed flow reactor and reacted under the following conditions. Reaction gas composition; NOx 1000ppm, C3H8 1
000ppm, O2 10%, (by volume) SO2
0 to 300ppm, remainder He reaction temperature
;400-600℃ Space velocity;36000
~51000h-1 The preparation conditions and experimental results for each catalyst are summarized in Table 2.

[0035]

[Table 2]

Examples 10 to 16 Commercially available γ-alumina powder (specific surface area: 110 m 2 /g) was used as an alumina carrier and impregnated with a mixed aqueous solution containing both nitrates and acetates of various alkaline earth metals and silver nitrate.

[0037] After drying these at 100°C for 10 hours, they were calcined at 550°C for 3 hours to obtain a catalyst supporting both alkaline earth metal and silver.

[0038] The supported amounts of alkaline earth metals and silver were 1 to 1.5 wt% based on γ-alumina in metal terms, and the supported amounts of alkaline earth metals and silver were adjusted by changing the concentration of the solution. .

These catalysts were made to have a particle size of 20 to 40 mesh in the same manner as in Examples 1 to 6, respectively. Each of these catalysts is placed in a normal pressure fixed bed flow reactor,
The reaction was carried out under the following conditions.

Reaction gas composition; NOx 900ppm, C
3H8 600ppm, O2 11%, (by volume) SO2 0 ~ 500ppm, balance
He reaction temperature: 300 to 600°C Space velocity: 35,000 to 50,000 h-1 The preparation conditions and test results for each catalyst are summarized in Table 3.

[0041]

[Table 3]

Example 17 (Comparative Example) Commercially available γ-alumina powder (specific surface area 138 m2/g)
was adjusted to a particle size of 10 to 20 mesh in the same manner as in Examples 1 to 6, and placed in a normal atmospheric pressure fixed bed flow reactor and reacted under the following conditions. Almost no NOx decomposition activity was expressed.

Reaction gas composition: NOx 1100ppm,
C3H8 1000ppm, O2 10%, (by volume) SO2 200ppm, balance He
Reaction temperature: 300-600℃ Space velocity
;35000 h-1

[0044]

[Effects of the invention] (1) The NOx decomposition catalyst of the present invention:
It is highly active even in the case of exhaust gas with a small concentration of NOx, where there is a large excess of oxygen (10% or more oxygen and ppm order of NOx), and it is effective in decomposing NOx into harmless N2 and O2. It is.

(2) The high activity of the NOx decomposition catalyst of the present invention is maintained even in the coexistence of poisoning components such as SO2 up to about 500 ppm.

(3) When using the NOx decomposition catalyst of the present invention, there is no need for any special pretreatment of the exhaust gas to be treated, and the reaction gas can be directly introduced into the catalyst layer.

(4) Since the NOx decomposition catalyst of the present invention uses hydrocarbons, NOx in exhaust gas from a diesel engine can be removed without using ammonia as in conventional NOx decomposition catalysts.

Claims (5)

[Claims] Claim 1: A catalyst used for decomposing nitrogen oxides in exhaust gas in the presence of hydrocarbon gas, the catalyst comprising one or more alkaline earth metals and/or silver supported on a carrier. A nitrogen oxide decomposition catalyst characterized by: 2. The nitrogen oxide decomposition catalyst according to claim 1, wherein the carrier is an alumina carrier. 3. The nitrogen oxide decomposition catalyst according to claim 1, wherein the exhaust gas is exhaust gas from a diesel engine. 4. A method for decomposing nitrogen oxides in which nitrogen oxides in exhaust gas are decomposed while injecting hydrocarbon gas, the method comprising using one or more alkaline earth metals and/or silver as a nitrogen oxide decomposition catalyst. A method for decomposing nitrogen oxides, characterized by using nitrogen oxides supported on an alumina carrier. 5. The method for decomposing nitrogen oxides according to claim 4, wherein the exhaust gas is exhaust gas from a diesel engine.