JP2010051862A - Catalyst, and exhaust gas cleaning device with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst which allows NOx in exhaust gas to be reduced/removed by using CO in the exhaust gas even when HC is not incorporated in the exhaust gas and allows the cost of an apparatus for cleaning the exhaust gas and the running cost thereof to be kept down and to provide the catalyst-incorporated apparatus for cleaning the exhaust gas. <P>SOLUTION: The catalyst for removing NOx contains: at least one component selected from cobalt and manganese; gold; and cerium dioxide for using CO as a reductant. In order to contact the catalyst efficiently with the exhaust gas, a corrugated steel sheet, which is used as a honeycomb base material in order to increase a contact area and used in a denitrification apparatus of a boiler, is coated with the powder of the catalyst. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a catalyst and an exhaust gas purification apparatus including the catalyst.

For the purpose of environmental improvement, reduction of carbon monoxide (CO), nitrogen oxides (NOx) and hydrocarbons (HC), which are harmful substances in the exhaust gas of boilers and internal combustion engines, is required. In addition, reduction of carbon dioxide (CO ₂ ), which is a greenhouse gas, is required to prevent global warming.

In order to continuously purify NOx in the exhaust gas of boilers and internal combustion engines, a method using a selective reduction catalyst with ammonia (hereinafter referred to as NH ₃ ), a method using urea which generates NH ₃ by thermal decomposition, a hydrocarbon (hereinafter referred to as “NO _3” ). , HC) and the like using a selective reduction catalyst. In the case of an internal combustion engine, there is a method using a lean NOx catalyst. This is because NOx adsorbed or occluded on the lean NOx catalyst in lean fuel flue gas (lean) and NOx adsorbed or occluded on the lean NOx catalyst in excess fuel flue gas is reduced by CO or HC in the exhaust gas. Purifying. In the case of a diesel engine, it is usually operated by lean fuel combustion, and fuel excessive combustion exhaust gas is produced by appropriately injecting light oil of fuel.

These methods require a system for supplying the reducing agents NH ₃ , urea, HC, and light oil (components are HC), which increases the cost of introducing the apparatus and at the same time adds the cost of the reducing agent to the operating cost. The

In order to reduce and purify NOx at a low cost, a method using CO present in the exhaust gas as a reducing agent can be considered. However, in the exhaust gas of boilers and diesel engines, since oxygen is present in a few percent, CO reacts with O ₂ in the exhaust gas to be consumed as CO ₂ , and NOx reduction is difficult to proceed. It was.

  Patent Document 1 discloses that when NOx discharged from an internal combustion engine is removed by a catalyst, particularly in an oxygen-excess atmosphere, deterioration of catalyst performance due to SOx in exhaust gas is prevented, and discharged CO is also removed at the same time. An exhaust gas purification device that removes NOx discharged from an internal combustion engine with a catalyst, particularly in an oxygen-excess atmosphere, for reducing NOx using HC as a reducing agent upstream of the NOx flow path And a catalyst for reducing and removing NOx by CO is disposed downstream of NOx.

  In Patent Document 2, for the purpose of further improving the purification performance of a gas purification catalyst using Au and efficiently purifying NOx and CO in exhaust gas, a composite oxide of cerium and zirconium, and gold are used. A catalyst containing the catalyst or a porous carrier containing cerium oxide, zirconium, and gold, and a gas purification method and a gas purification device using the catalyst are disclosed.

Japanese Patent Laid-Open No. 10-263368 JP 2008-73654 A

  An object of the present invention is to reduce and purify NOx with CO in exhaust gas even if HC is not contained in the exhaust gas, and to keep the apparatus cost and operation cost low.

  The catalyst of the present invention is a catalyst for purifying nitrogen oxides, comprising at least one component selected from cobalt and manganese, gold, and cerium dioxide for utilizing carbon monoxide as a reducing agent. It is characterized by including.

  According to the present invention, even if HC is not contained in the exhaust gas, NOx can be reduced and purified by CO in the exhaust gas, and the apparatus cost and the operating cost can be kept low.

  The present invention relates to a catalyst (also referred to as a NOx reduction catalyst) and an exhaust gas purification device including the catalyst, and more particularly to a catalyst for purifying nitrogen oxides contained in exhaust gas and an exhaust gas purification device including the catalyst.

The present invention is a combination of catalyst components effective for the reduction of nitrogen oxides (NOx) in oxygen-excess exhaust gas. Cerium dioxide (CeO ₂ ) has a redox function. In order to improve the properties of the CeO ₂ carrier, cobalt (Co) or manganese (Mn) is added to carry the active ingredient gold (Au).

  The mechanism of NOx purification on this catalyst is considered as follows.

(1) NO in the exhaust gas is adsorbed on the catalyst and dissociates into atomic N and O. (2) The dissociated O is taken into CeO _2-x in the reduced state and changed to CeO _2. (3) Dissociation generates N mutually bonded to _{N 2} was, (4) CO contained in the exhaust gas is changed to _{CeO 2-x} by reducing CeO _2. Said (1)-(4) advances repeatedly.

  For this reason, in this catalyst, the rapid change in the reduction and oxidation states of cerium oxide (2) and (4) described above is a factor that affects NOx purification.

  The inventor has found that when Co or Mn is added to cerium oxide, the reduction and the change of the oxidation state easily proceed.

  Although Au is known as a CO oxidation catalyst, it is also effective in reducing NO, and it is considered that the reactions (1) and (3) are likely to proceed on the Au catalyst.

Since the catalyst of the present invention reduces NOx using CO in the exhaust gas as a reducing agent, there is no need for a means for separately supplying HC, NH _3, etc., which are other reducing agents of NOx. That is, NOx can be reduced even if the amount of HC of the reducing agent contained in the exhaust gas is insufficient for reducing NOx.

  Hereinafter, embodiments of the present invention will be described in detail.

The composition of the catalyst of the present invention essentially comprises gold (Au), cobalt (Co) or manganese (Mn), and cerium dioxide (CeO ₂ ).

Au is included as a metal, and Co or Mn is included as an oxide. CeO ₂ preferably contains no element other than Co or Mn, but is applicable if it has an oxidation / reduction function. Therefore, a CeO ₂ —ZrO ₂ compound often used in a three-way catalyst, or a compound to which a catalyst function improving element such as lanthanum (La) or praseodymium (Pr) is added may be used.

  The raw material of Co or Mn used in the present invention is preferably a nitrate that is easily decomposed during catalyst preparation. However, if it is a compound that forms Co oxide or Mn oxide by firing in air, carbonate, Chlorides can also be used.

  There is no restriction | limiting in particular in the usage form of the catalyst of this invention. Since it is desirable to be able to contact the exhaust gas efficiently, it is desirable to coat the base material with the catalyst powder so that the contact area becomes high. For example, when the catalyst powder is coated on a corrugated steel plate used for a honeycomb base material used in automobile catalysts or boiler denitration equipment, the contact efficiency between the gas and the catalyst increases.

  In a conventional diesel engine, an oxidation catalyst, a diesel particulate filter (hereinafter referred to as DPF), and a NOx selective reduction catalyst are installed in the exhaust gas passage.

  Regarding the arrangement of the catalyst of the present invention, it is desirable that CO is not consumed by other devices or catalysts in the exhaust gas flow path, and it is most desirable to install the catalyst on the most upstream side of the engine exhaust gas flow path. However, the catalyst of the present invention may be installed in place of the NOx selective reduction catalyst of the diesel engine. In a boiler, when a plurality of devices and catalysts are installed in the exhaust gas flow path, it is not preferable that CO is consumed upstream of the catalyst of the present invention. Therefore, after removing dust from the boiler exhaust gas or downstream of the electric dust collector It is desirable to be installed downstream of equipment that does not involve oxidation reaction.

  Examples of the present invention are shown below.

Example 1 is a NOx reduction catalyst in which Au is supported after Ce is supported on CeO ₂ .

25.0 g of cerium oxide powder is impregnated with a solution obtained by dissolving 4.23 g of cobalt nitrate hydrate in 13.6 g of water and kneaded so that the whole becomes uniform. The amount of Co supported by element conversion is 10 mol% with respect to CeO ₂ . Then, it dried at 120 degreeC for 3 hours, and baked at 400 degreeC in air | atmosphere for 3 hours.

  An aqueous solution obtained by mixing chloroauric acid containing 0.15 g of Au and 200 g of water is kept at 60 ° C. While stirring the chloroauric acid aqueous solution maintaining the temperature, 0.1 M NaOH aqueous solution is added dropwise to adjust the pH to 10-12.

In this aqueous chloroauric acid solution, 15.0 g of CeO ₂ powder supporting Co is added and stirred for 1 hour.

After standing overnight, the solution was suction filtered, dried at 90 ° C. for 3 hours, and calcined in the atmosphere at 400 ° C. for 3 hours to obtain Au-supported Co / CeO ₂ powder.

The supported amount of Au is 1 wt% with respect to Co / CeO ₂ .

A slurry obtained by mixing 14.4 g of Au-supported Co / CeO ₂ powder, 7.2 g of silica sol (Nissan Chemical Co., Ltd., colloidal silica O), and 7.74 g of water for 15 minutes with a cracker is used as a cordierite honeycomb (400 cells, 7.2). It was poured into a mill, 17 × 17 × 75 mm) and applied. Then, the excess slurry was blown with air, dried at 150 ° C. for 15 minutes, and fired at 400 ° C. for 1 hour. The coating amount of the Au-supporting Co / CeO ₂ powder is 190 g / L per honeycomb volume. Thus, a NOx reduction catalyst of Au-supported Co / CeO ₂ was obtained.

In Example 2, Co in Example 1 was changed to Mn, and a NOx reduction catalyst was produced. Instead of the cobalt nitrate hydrate of Example 1, manganese nitrate hydrate was used. The amount of Mn supported is 10 mol% of CeO _{2, and} the amount of Au supported is 1 wt% with respect to Mn / CeO ₂ . The amount of honeycomb coating is 190 g / L.
(Comparative Example 1)
Comparative Example 1 is only CeO _2. In the same manner as in the coating method of Example 1, CeO ₂ was coated on the honeycomb. The coating amount is 190 g / L.
(Comparative Example 2)
Comparative Example 2 is an example of carrying Au to CeO _2. It was produced in the same manner as the Au carrying process of Example 1, and after Au carrying CeO ₂ was produced, the honeycomb was coated. The amount of Au supported is 1 wt% with respect to CeO ₂ . The amount of honeycomb coating is 190 g / L.
(Comparative Example 3)
In Comparative Example 3, Co in Example 1 was changed to copper (Cu) to produce a NOx reduction catalyst. Instead of the cobalt nitrate hydrate of Example 1, copper nitrate hydrate was used. The amount of Cu supported is 10 mol% of CeO _{2, and} the amount of Au supported is 1 wt% with respect to Cu / CeO ₂ . The amount of honeycomb coating is 190 g / L.
(Comparative Example 4)
In Comparative Example 4, Co in Example 1 was changed to tungsten (W) to produce a NOx reduction catalyst. Instead of the cobalt nitrate hydrate of Example 1, ammonium metatungstate ((NH ₄ ) ₆ (H ₂ W ₁₂ O ₄₀ )) was used. The W loading is 10 mol% of CeO ₂ and the Au loading is 1 wt% with respect to W / CeO ₂ . The amount of honeycomb coating is 190 g / L.
(Comparative Example 5)
In Comparative Example 5, Co in Example 1 was changed to lanthanum (La) to produce a NOx reduction catalyst. Instead of the cobalt nitrate hydrate of Example 1, lanthanum nitrate hydrate was used. The La loading amount is 10 mol% of CeO ₂ , and the Au loading amount is 1 wt% with respect to La / CeO ₂ . The amount of honeycomb coating is 190 g / L.
(Oxygen mobility evaluation)
When various elements were added to cerium oxide, changes in reduction and oxidation states were evaluated by H ₂ consumption.

An evaluation method for H ₂ consumption will be described. First, the temperature of the catalyst is raised to 300 ° C. in a He stream, and the O ₂ gas is switched to store oxygen in CeO ₂ . Thereafter, the inside of the reaction system was replaced with He, and finally 3% H ₂ —Ar was circulated. The reaction of the following formula occurs on the catalyst.

O ₂ atmosphere CeO _2−x + O ₂ → CeO ₂ (Formula 1)
H ₂ —Ar atmosphere H ₂ + CeO ₂ → CeO _2−x + H ₂ O (Formula 2)
As shown in (Equation 2), oxygen and _{H 2} of CeO ₂ reacts, _{H 2} O is produced. The thermal conductivity at 0 ° C. is 0.1684 W / m · K for H _{2 and} 0.0015 W / m · K for Ar, and the thermal conductivity of the catalyst outlet gas decreases due to consumption of H ₂ . The difference in thermal conductivity between the catalyst outlet gas and the reference gas (3% H ₂ —Ar) was measured with a thermal conductivity detector of gas chromatography, and H ₂ consumption was calculated. The oxygen mobility was evaluated based on the amount of H ₂ consumed when flowing 3% H ₂ —Ar for 1 hour.
(NOx purification performance test)
The NOx purification performance of the produced catalyst was evaluated. FIG. 1 shows an outline of a NOx purification performance evaluation apparatus.

  In this figure, the reaction tube 1a is installed in the electric furnace 1b, and the catalyst 1c is installed in the reaction tube 1a. Water and lean model gas are supplied to the reaction tube 1a from a water pump 1d and a lean model gas supply source 1e, respectively.

A catalyst 1c was set in a fixed bed flow type reaction tube (reaction tube 1a) in an electric furnace, a lean model gas shown in Table 1 was circulated, and water was dropped. The catalyst inlet temperature was adjusted to a predetermined temperature, and the catalyst outlet gas was measured with a NOx analyzer. The gas amount was 1.5 L / min, and was adjusted to SV15000h- ¹ . The NOx purification rate was defined by (Formula 3). The NOx meter used could not measure N ₂ O, but N ₂ O concentration was measured separately. As a result, since almost no N ₂ O was detected, it was interpreted that the NOx purification rate obtained by (Equation 3) was equal to the conversion rate from NO to N ₂ .

  NOx purification rate = (catalyst inlet NOx concentration−catalyst outlet NOx concentration) / catalyst inlet NOx concentration × 100% (formula 3)

Table 2 shows the H ₂ consumption and the NOx purification performance of the index indicating oxygen mobility. The standard for NOx purification performance is Au—CeO ₂ of Comparative Example 2.

The NOx purification rate in Examples 1 and 2 exceeded that of Au—CeO ₂ in Comparative Example 2. H ₂ consumption is similar, Examples 1 and 2 exceeds the comparative examples 1-5. The NOx purification rate tends to increase as the H ₂ consumption amount increases. From this, it is considered that when Co or Mn is added to CeO ₂ , the oxygen mobility is increased, the incorporation of O dissociated from NO by CeO ₂ is facilitated, and the NOx purification rate is increased.

Also, compared with Comparative Example 2 and Comparative Example 1, and H ₂ consumption reduced by carrying the Au. This is presumed to be because O ₂ occlusion / release characteristics changed with the temperature of CeO ₂ by supporting Au. In Examples 1 and 2, even when Au is supported, the H ₂ consumption is higher than that of Comparative Example 2. Therefore, it is presumed that the amount of H ₂ consumed is larger when Co or Mn is supported on CeO _2. The

However, as can be seen in comparison with Comparative Example 2 and Comparative Example 1, _{H 2} consumption is large Comparative Example 1 of CeO ₂ of the NOx purification rate is lower than the Au-CeO ₂ of Comparative Example 2 is the active ingredient Au loading is necessary for NOx purification.

Therefore, it can be seen that by adding Co or Mn to CeO ₂ , the oxygen mobility can be increased and NOx purification proceeds by supporting Au.

Table 3 shows the temperature dependence of the NOx purification performance of the catalyst using Au—Co / CeO ₂ of Example 1. From this table, it can be seen that when the catalyst inlet temperature is in the range of 200 to 400 ° C., the NOx purification rate becomes the highest when the catalyst inlet temperature is 300 ° C.

  Therefore, as a temperature condition for using the catalyst of the present invention, it is desirable to set the catalyst inlet temperature of the exhaust gas to about 300 ° C.

  When the catalyst inlet temperature is 300 ° C., strontium (Sr), lanthanum (La), copper (Cu), nickel (Ni), vanadium (V) instead of Co and Mn used in the examples according to the present invention. , Yttrium (Y), Molybdenum (Mo), Chromium (Cr), Tungsten (W), Bismuth (Bi), or Chlorine (Cl) are all added in comparison with the case where Co or Mn is added. The purification rate was low. Further, when sodium (Na) or cesium (Cs) is added instead of Co and Mn, the initial NOx purification rate is increased, but NOx is accumulated in the catalyst and the purification performance is gradually lowered. I understood.

  From the above, it was found that when Co or Mn was added, the NOx purification performance at a catalyst inlet temperature of 300 ° C. was particularly high.

  The present invention can be used for exhaust gas purification of internal combustion engines including boilers and diesel engines.

It is a schematic diagram of a NOx purification performance evaluation apparatus.

Explanation of symbols

  1a: reaction tube, 1b: electric furnace, 1c: catalyst, 1d: water pump, 1e: lean model gas supply source.

Claims (3)

  A catalyst for purifying nitrogen oxides, comprising one or more components selected from cobalt and manganese, gold, and cerium dioxide for using carbon monoxide as a reducing agent. catalyst.   The catalyst according to claim 1, wherein at least one component selected from cobalt and manganese is contained as an oxide.   An exhaust gas purification apparatus comprising the catalyst according to claim 1 or 2 in an exhaust gas flow path.

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