JP2017185424A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for purifying an exhaust gas capable of preventing decomposition of a surface later of a SCR catalyst even when exposed to high humidity atmosphere for overcoming current situation that a temperature of a catalyst is dramatically increased when the exhaust gas having 100 to 150°C inflows onto the catalyst absorbing moisture suddenly, humidity becomes extremely high locally as absorbed moisture is detached at a stretch and performance for decomposing nitrogen oxide is largely reduced temporarily in the case that the SCR catalyst is used as a catalyst for purifying nitrogen oxide of an automobile exhaust gas, also a structure of a surface layer of the SCR catalyst is broken and activity is reduced when exposed to such high humidity environment.SOLUTION: There is provided a catalyst for purifying exhaust gas consisting of zeolite covered by metal oxide having area of a surface part thereof of 63% or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to an exhaust gas purification catalyst, and more particularly to a NO _x selective reduction catalyst.

In recent years, exhaust gas regulations have been strengthened worldwide year by year from the viewpoint of protecting the global environment. As a countermeasure, an exhaust gas purifying catalyst is used in an internal combustion engine. In this exhaust gas purifying catalyst, hydrocarbon (hereinafter also abbreviated as HC), CO and nitrogen oxides (hereinafter also abbreviated as NO _x ) in the exhaust gas are efficiently removed. In addition, noble metals such as Pt, Pd, and Rh are used as catalyst components.

  Various systems are used to improve fuel efficiency as well as catalyst activity in automobiles using the exhaust gas-purifying catalyst, such as gasoline engine cars or diesel engine cars. For example, in order to improve fuel efficiency, combustion is performed under conditions where the air-fuel ratio (A / F) is lean (excess oxygen) during steady operation, and stoichiometric (theoretical air-fuel ratio, A / F = 14.7) Burning under rich (fuel excess) conditions.

This is known Pt, Pd, catalysts such as noble metals Rh, etc. have low _the NO _x purification performance in an oxidizing conditions, it requires a reducing atmosphere by the addition of HC or CO, etc. in order to improve the purification performance It is. Because of this influence on the catalyst activity, the air-fuel ratio (A / F) cannot be increased even during steady operation, and there is a limit to the improvement of fuel consumption with catalysts such as the noble metals.

As described above, conventionally known catalysts such as precious metals require a fuel for temporarily setting the purification catalyst in a reducing atmosphere and a low air-fuel ratio (A / F) in the engine. In order to improve the fuel efficiency of internal combustion engines including automobile engines, there has been a demand for new purification catalysts that can exhibit NO _x purification performance in a lean atmosphere, for example.

  Conventionally, a selective reduction catalyst (SCR) has been used as one of the NOx purification means in an oxygen-excess atmosphere. As this SCR catalyst, a zeolite catalyst carrying a catalyst has been proposed in recent years. In particular, it is known that a catalyst in which a metal is supported on silicoaluminophosphate (hereinafter referred to as SAPO), which is a zeolite containing silicon, aluminum, and phosphorus atoms, is highly active in purifying nitrogen oxides.

  When such an SCR catalyst is used as a catalyst for purifying nitrogen oxides in automobile exhaust gas, there are the following problems. That is, when the engine is started after the engine is stopped, exhaust gas at 100 to 150 ° C. passes over the catalyst. When the engine is stopped, the catalyst adsorbs moisture corresponding to 0.3 to 0.4 times its weight from the atmosphere. When exhaust gas of 100 to 150 ° C. suddenly flows onto the catalyst adsorbing moisture in this way, the temperature on the catalyst suddenly rises and the adsorbed moisture is desorbed all at once. The humidity will be high, and the ability to temporarily decompose nitrogen oxides will be greatly reduced. Further, exposure to such a high humidity atmosphere may cause the structure of the surface layer of the SCR catalyst to collapse, resulting in a decrease in activity.

  In order to solve such a problem of water supply deterioration of the SCR catalyst, Patent Document 1 describes that by adding an oxide particle binder to SAPO, the water adsorption amount of SAPO is reduced and the above water supply deterioration is suppressed. It has been proposed to do.

JP 2012-148272 A

  However, as a result of conducting a water resistance test on the catalyst obtained by adding the oxide fine particle binder to SAPO, it was found that the water absorption deterioration could not be sufficiently suppressed and the activity deterioration was caused by moisture. For these reasons, there is a need for SAPO that suppresses water absorption deterioration.

  As a result of verifying factors caused by water deterioration, the present inventors have found that the degree of deterioration is greatly influenced by the state of oxide coating on the SAPO surface, and have reached the present invention.

Aspects of the present invention are as follows.
An exhaust gas purifying catalyst comprising a zeolite in which 63% or more of the surface area is coated with a metal oxide.

  According to the embodiment of the present invention, the use of zeolite in which 63% or more of the surface area is coated with a metal oxide can improve the feedwater deterioration resistance.

2 is a model diagram of an oxide coat state of a sample manufactured in Example 1. FIG. 2 is a TEM image of a sample manufactured in Example 1. FIG. It is a model figure of the oxide coat state of the sample produced by the comparative example. It is a TEM image of the sample produced by the comparative example. It is a graph which shows the relationship of an oxide coverage and an activity maintenance factor about each sample of an Example and a comparative example.

  The catalyst according to the present invention is made of zeolite in which 63% or more of the surface area is coated with a metal oxide.

  The zeolite used in the present invention contains a zeolite containing at least a silicon atom, an aluminum atom and a phosphorus atom in the skeleton structure. In addition, the framework structure of the zeolite in the present invention may contain atoms other than aluminum, phosphorus and silicon atoms. Other atoms that may be included include lithium, magnesium, titanium, zirconium, vanadium, chromium, manganese, iron, cobalt, nickel, palladium, copper, zinc, gallium, germanium, arsenic, tin, calcium, boron, etc. 1 type or 2 types or more of these atoms are mentioned, Preferably, an iron atom, a copper atom, and a gallium atom are mentioned.

  The particle diameter of the zeolite in the present invention is not particularly limited, but is usually 1 μm or more, more preferably 2 μm or more, more preferably 3 μm or more, and usually 15 μm or less, preferably 10 μm or less.

  Zeolite in the present invention is a compound known per se and can be produced according to a commonly used method. The zeolite used in the present invention is usually obtained by hydrothermal synthesis after mixing an aluminum atom raw material, a phosphorus atom raw material, a silicon atom raw material, and a template as necessary. When a template is mixed, an operation for removing the normal template is performed after hydrothermal synthesis.

  The zeolite in the present invention may carry a metal. In the present invention, the metal supported on the zeolite is not particularly limited as long as it can be supported on the zeolite and exhibit catalytic activity, but preferably iron, cobalt, palladium, iridium, platinum, copper , Silver, gold, cerium, lanthanum, praseodymium, titanium, zirconia and the like. The metal supported on zeolite may be one of these, or a combination of two or more metals may be supported on zeolite. The metal supported on the zeolite is more preferably iron and / or copper.

  The amount of the metal supported on the zeolite in the catalyst of the present invention is not particularly limited, but is usually 0.1% or more, preferably 0.5% or more, more preferably 1% or more, by weight ratio of the metal to the zeolite. Usually, it is 10% or less, preferably 8% or less, more preferably 5% or less. If the amount of metal supported is less than the lower limit, the active point tends to decrease, and the catalyst performance may not be exhibited. If the amount of metal supported exceeds the upper limit, metal agglomeration tends to be remarkable, and the catalyst performance may be lowered.

  The method for supporting the metal on the zeolite when producing the catalyst of the present invention is not particularly limited, but generally used ion exchange method, impregnation support method, precipitation support method, solid phase ion exchange method, CVD method, etc. Is used. The ion exchange method and the impregnation support method are preferable.

  The metal source of the metal to be supported is not particularly limited, but metal salts, metal complexes, simple metals, metal oxides, etc. are used. Usually, supported metal salts are used, for example, nitrate, sulfuric acid, etc. An inorganic acid salt such as a salt or hydrochloride, or an organic acid salt such as acetate can be used.

As a method for coating the zeolite of the present invention with a metal oxide, a metal alkoxide liquid to be coated is used as a precursor, and after coating the surface of the zeolite with the alkoxide solution, it is oxidized by hydrolysis, A metal oxide is coated. As the oxide material to be coated, ZrO ₂ and TiO ₂ are preferable, and an effect of Nb or Si oxide is recognized to some extent. As for the amount of oxide to be coated, the effect is recognized even with a small amount of 5 wt%, but if it is coated too much, the activity may decrease due to a decrease in NOx and NH ₃ gas diffusibility inside the zeolite. Is preferably 50 wt%.

Example 1
In 200 g of ethanol, 30 g of silicoaluminophosphate (Cu / SAPO) powder supporting copper by an ion exchange method was added and dispersed uniformly. Zirconium propoxide was added dropwise thereto so that the ZrO ₂ ratio was 5 to 50 wt% with respect to Cu / SAPO, and the mixture was stirred. This was oxidized at 80 ° C. by slowly hydrolyzing with moisture in the atmosphere or moisture in the zeolite while evaporating to dryness. This was dried at 120 ° C. overnight and then fired at 600 ° C. for 5 hours to prepare ZrO ₂ -coated Cu / SAPO.

Example 2
A TiO ₂ -coated Cu / SAPO was produced in the same manner as in Example 1 except that titanium tetraisopropoxide was used in place of zirconium propoxide as the alkoxide to be added.

Comparative Example 30 g of Cu / SAPO powder was added to 200 g of water and dispersed uniformly. The Taki Chemical Ltd. ZrO ₂ sol (Bairaru Zr-C20) thereto, was added in an amount of 25 wt% relative to Cu / SAPO with ZrO ₂ ratio was stirred. This was evaporated to dryness at 80 ° C., then dried at 120 ° C. overnight, and then fired at 600 ° C. for 5 hours to prepare ZrO ₂ -coated Cu / SAPO.

Evaluation method (coverage measurement)
The element distribution of the prepared sample was measured by XPS, and the amount of Al in the zeolite component elements exposed on the surface was quantified. The oxide coverage was calculated from the following formula from the amount of Al in the uncoated oxide Cu / SAPO and the amount of Al in the sample.

(Water resistance test)
6 g of the prepared sample was immersed in ion-exchanged water at room temperature for 10 minutes, then taken out, placed in an electric furnace heated to 450 ° C., and dried for 30 minutes. Then, it is allowed to cool to room temperature. This series of operations was repeated 10 times.

(NH ₃ -SCR activity evaluation)
For each of the samples before the water resistance test and after the water resistance test, weigh 5 cc, and after the pretreatment at 600 ° C. for 5 minutes in the evaluation apparatus, at a temperature of 450 ° C., 410 ° C. and 330 ° C., a rich-lean transient Evaluation was performed 10 cycles. Data were averaged over the last 3 cycles. The gas conditions are NO = 150 ppm for rich, NH ₃ = 550 ppm, H ₂ O = 5%, CO ₂ = 10% for 10 seconds, and lean for NO = 50 ppm, H ₂ O = 5%, O ₂ = CO ₂ = A cycle of 60% at 10% was performed. The gas flow rate was 15 L / min and the space velocity was 180,000 h- ¹ .

  From the activity of the sample after the water resistance test and the activity of the sample before the water resistance test (initial activity), the activity maintenance rate was determined by the following formula.

  From the obtained results, the relationship between the oxide coverage and the activity maintenance ratio is shown in FIG. By setting the oxide coverage to 63% or more, the catalytic activity of the zeolite can be maintained.

  About each produced sample, the model figure and TEM image of an oxide coat state are shown in FIGS. As shown in FIGS. 1 and 2, in Example 1, by adding alkoxide, which is a liquid, as an oxide precursor, zirconium propoxide, which is an oxide precursor, is uniformly deposited on Cu / SAPO1, Thereafter, the oxide is converted into zirconium oxide by hydrolysis, whereby a uniform oxide coat 2 can be formed, and a high oxide coverage is achieved.

  On the other hand, in the comparative example in which an oxide is added to Cu / SAPO instead of an alkoxide, the oxide coating cannot be controlled as shown in FIGS. 3 and 4, and the film thickness 2 of the oxide is uneven. Therefore, it is considered that the exposed portion of Cu / SAPO1 becomes large, and deterioration starts from this exposure.

  As described above, according to the exhaust gas purifying catalyst according to the present invention, the use of zeolite in which 63% or more of the surface area is coated with a metal oxide can improve the feedwater deterioration resistance. .

Claims (1)

  An exhaust gas purifying catalyst comprising a zeolite in which 63% or more of the surface area is coated with a metal oxide.