JP2020093232A - No adsorbing-material - Google Patents

Abstract

To provide, since at low temperatures such as when the internal combustion engine is started or when the engine is cold, the oxidation reaction of NO is unlikely to occur and a NO in the exhaust gas is difficult to be purified, a NO adsorbing-material that exhibits high adsorption performance at low temperatures (for example, 100°C).SOLUTION: Provided are: a NO adsorbing-material characterized by containing Pd and a complex oxide containing Mo and V as metal atoms; a NO adsorbing-material characterized in that the complex oxide is Mo3VOx; and a NO adsorbing-material having a Pd content of 0.01 to 10 mass% for the entire NO adsorbing-material.SELECTED DRAWING: Figure 2

Description

The present invention relates to NO adsorbents.

Exhaust gas from an internal combustion engine such as an automobile engine or a diesel engine contains nitric oxide (NO). This NO is usually oxidized to NOx and then reduced and purified to N ₂ by a three-way catalyst. It However, at low temperatures such as when the internal combustion engine is started or when the engine is cold, the NO oxidation reaction is unlikely to occur, and thus NO in exhaust gas is difficult to purify. Therefore, in an exhaust gas purifying apparatus for purifying exhaust gas from an internal combustion engine, an NO adsorbent is usually arranged on the upstream side of the three-way catalyst, and when the internal combustion engine is cold or when it is cold such as cold. By this NO adsorbent, NO in the exhaust gas is adsorbed and removed, and NO emission is suppressed. The NO adsorbed on the NO adsorbent is released at a high temperature, is oxidized, and is reduced and purified to N ₂ by the three-way catalyst.

Examples of the adsorbent that adsorbs NO at such a low temperature and releases NO at a high temperature include, for example, a noble metal (for example, Pd) and a small pore molecular sieve (for example, aluminosilicate) having a maximum ring size of eight tetrahedral atoms. A passive NOx adsorber (Molecular Sieve) (Japanese Patent Publication No. 2017-501329 (Patent Document 1)), a noble metal (for example, Pd) and a molecular sieve having a MAZ skeleton type (for example, aluminosilicate zeolite). A passive NOx adsorbent (Japanese Patent Publication No. 2018-513773 (Patent Document 2)) is known. However, these passive NOx adsorbers and passive NOx adsorbents do not always have sufficient NO adsorption performance at low temperatures.

Japanese Patent Publication No. 2017-501329 Special table 2018-513773 gazette

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide an NO adsorbent that exhibits high adsorption performance at low temperatures (for example, 100°C).

As a result of intensive studies to achieve the above-mentioned object, the present inventors blended Pd into a complex oxide containing Mo and V as metal atoms, and thus exhibited high adsorption at low temperature (for example, 100° C.). The inventors have found that an NO adsorbent exhibiting performance can be obtained, and completed the present invention.

That is, the NO adsorbent of the present invention is characterized by containing Pd and a composite oxide containing Mo and V as metal atoms. In such an NO adsorbent, the composite oxide is preferably Mo ₃ VO _x , and the content of Pd is preferably 0.01 to 10 mass% with respect to the entire NO adsorbent. ..

The reason why the NO adsorbent of the present invention exhibits high adsorption performance at low temperatures (for example, 100°C) is not always clear, but the present inventors speculate as follows. That is, in the NO adsorbent of the present invention, Pd is mixed with the composite oxide containing Mo and V as metal atoms. Since this Pd acts as an NO adsorption site even at low temperatures, it is speculated that the NO adsorbent of the present invention exhibits high NO performance at low temperatures.

According to the present invention, it is possible to obtain an NO adsorbent that exhibits high adsorption performance at low temperatures (for example, 100°C).

3 is a graph showing an X-ray diffraction pattern of the MoV composite oxide obtained in Example 1. Is a schematic view showing the crystal structure of Mo ₃ VO _x. 3 is a graph showing the NO adsorption performance at 100° C. of the adsorbents obtained in Example 1 and Comparative Examples 1 and 2.

Hereinafter, the present invention will be described in detail with reference to its preferred embodiments.

The NO adsorbent of the present invention contains Pd and a complex oxide containing Mo and V as metal atoms.

The composite oxide used in the present invention contains Mo and V as metal atoms (hereinafter, also referred to as “MoV composite oxide”), and is a metal atom from the viewpoint of improving NO adsorption performance at low temperatures. It is preferable that it contains only Mo and V. By blending Pd with such a MoV composite oxide, an NO adsorbent having excellent NO adsorbing performance at low temperatures can be obtained.

The molar ratio (Mo/V) of Mo and V in such a MoV composite oxide is preferably 0.2/1 to 10/1, more preferably 0.25/1 to 7/1, 0.33/1 to 6/1 is particularly preferable. When Mo/V deviates from the above range, the number of sites where Pd is ion-exchanged (acid points of MoV composite oxide; hereinafter also referred to as “ion-exchange sites”) tends to decrease.

Specific examples of such a MoV composite oxide include Mo ₃ VO _x , V ₉ Mo ₆ O ₄₀ , and V ₂ MoO ₈ . Among them, Mo ₃ VO _x is preferable from the viewpoint of easy formation of ion exchange sites.

When the MoV composite oxide is in the form of particles, the average particle size thereof is preferably 1 nm to 20 μm, more preferably 2 nm to 15 μm, and particularly preferably 5 nm to 10 μm. If the average particle size of the MoV composite oxide is less than the lower limit, handling tends to be difficult, while if it exceeds the upper limit, the NO adsorption rate tends to decrease.

The NO adsorbent of the present invention contains such a MoV composite oxide and Pd, and is excellent in NO adsorbing performance at low temperatures. Such Pd is preferably present in the NO adsorbent in a state of being ion-exchanged on the acid sites of the MoV composite oxide (more preferably, supported on the MoV composite oxide). This improves the NO adsorption performance at low temperatures.

In the NO adsorbent of the present invention, the content of Pd is preferably 0.01 to 10% by mass, more preferably 0.05 to 5% by mass, and 0.1 to 4% by mass with respect to the entire NO adsorbent. % Is particularly preferred. When the content of Pd is less than the lower limit, NO adsorption sites tend to decrease, and the NO adsorption amount tends to decrease. On the other hand, when the Pd content exceeds the upper limit, Pd aggregates and NO adsorption performance tends not to appear. ..

The shape of the NO adsorbent of the present invention is not particularly limited, and may be, for example, powder, pellet-shaped, or fixed on a substrate. The shape of the base material is not particularly limited, and a known shape such as a pellet shape or a honeycomb shape can be appropriately adopted. Examples of such a substrate include a DPF substrate, a monolith substrate, a pellet substrate, and a plate substrate.

When the NO adsorbent of the present invention is in the form of powder, the average particle size thereof is preferably 1 nm to 20 μm, more preferably 2 nm to 15 μm, and particularly preferably 5 nm to 10 μm. If the average particle diameter of the powdery NO adsorbent is less than the lower limit, handling tends to be difficult, while if it exceeds the upper limit, the NO adsorption rate tends to decrease.

Moreover, when the NO adsorbent of the present invention is in the form of pellets, the average particle size thereof is preferably 1 μm to 100 mm, more preferably 2 μm to 80 mm, and particularly preferably 10 μm to 50 mm. If the average particle size of the pellet-shaped NO adsorbent is less than the lower limit, handling tends to be difficult, while if it exceeds the upper limit, the gas does not sufficiently diffuse inside the pellet-shaped NO adsorbent, and NO adsorption Performance tends to decrease.

Such an NO adsorbent of the present invention can be manufactured, for example, by the following method. That is, first, a Mo compound and a V compound are dissolved in a solvent as a precursor of the MoV composite oxide to prepare a precursor solution.

The Mo compound is not particularly limited as long as it is soluble in a solvent, and examples thereof include methylammonium isopolymolybdate, ammonium metamolybdate, molybdic acid, and ammonium paramolybdate. The V compound is not particularly limited as long as it can be dissolved in a solvent, and examples thereof include vanadium oxide (IV) sulfate, vanadium chloride, and vanadium oxyoxalate. The solvent is not particularly limited as long as it can dissolve the Mo compound and the V compound, and examples thereof include water, ethanol, and methanol.

The ratio of the Mo compound and the V compound in the precursor solution is preferably 0.2/1 to 10/1 in terms of a molar ratio of Mo ions and V ions (Mo ions/V ions), and 0. 0.25/1 to 7/1 is more preferable, and 0.33/1 to 6/1 is particularly preferable. If the Mo ions/V ions deviate from the above range, the amount of impurities other than the desired MoV composite oxide tends to increase.

Next, the obtained precursor solution is heated. Thereby, the MoV composite oxide according to the present invention can be synthesized.

The heating method of the precursor solution is not particularly limited, and examples thereof include microwave heating and electric furnace heating. The heating temperature is preferably 80 to 300°C, more preferably 90 to 260°C, and particularly preferably 100 to 250°C. If the heating temperature is lower than the lower limit, the synthetic reaction does not proceed sufficiently, Mo and V react independently of each other, and the target MoV composite oxide tends to be not obtained, while the upper limit is not exceeded. If it exceeds, the pressure in the reaction vessel tends to rise too much. The heating time is preferably 0.5 minutes to 100 hours, more preferably 1 minute to 90 hours, and particularly preferably 5 minutes to 80 hours. If the heating time is less than the lower limit, the synthesis reaction does not proceed sufficiently, Mo and V react independently of each other, and the desired MoV composite oxide tends to be not obtained, while the upper limit is not exceeded. If it exceeds, crystal growth is likely to proceed and the grain size tends to be too large.

Next, the MoV composite oxide thus obtained and the Pd precursor are mixed. At this time, it is preferable to ion-exchange Pd of the Pd precursor on the acid sites of the MoV composite oxide. This makes it possible to obtain an NO adsorbent having improved NO adsorbing performance at low temperatures.

The Pd precursor is not particularly limited as long as it is soluble in the solvent, and examples thereof include tetraammine palladium and palladium nitrate. It is preferable to use such a Pd precursor in a solution state (that is, as a Pd precursor solution).

The method of mixing the MoV composite oxide and the Pd precursor is not particularly limited, and examples thereof include a method of adding the MoV composite oxide to the Pd precursor solution. Moreover, as a mixing ratio of the MoV composite oxide and the Pd precursor, the ratio of the content of Pd in the obtained NO adsorbent is 0.01 to 10 mass% with respect to the entire NO adsorbent. Is preferable, a ratio of 0.05 to 5 mass% is more preferable, and a ratio of 0.1 to 4 mass% is particularly preferable. If the content of Pd is less than the lower limit, the NO adsorbent obtained will have fewer NO adsorption sites and the NO adsorption amount will tend to decrease. On the other hand, if the upper limit is exceeded, Pd will aggregate and NO adsorption Performance tends to not be expressed.

Next, the mixture of the MoV composite oxide thus obtained and the Pd precursor is fired in an oxidizing atmosphere (for example, in air). As a result, the NO adsorbent of the present invention containing MoV composite oxide and Pd can be obtained.

The firing temperature of the mixture is preferably 150 to 500°C, more preferably 200 to 400°C, and particularly preferably 200 to 350°C. If the firing temperature is less than the lower limit, the unnecessary Pd precursor tends not to be sufficiently removed, while if it exceeds the upper limit, Pd tends to aggregate and NO adsorption performance tends not to be exhibited. The firing time is preferably 1 minute to 10 hours, more preferably 2 minutes to 8 hours, and particularly preferably 5 minutes to 5 hours. If the firing time is less than the lower limit, the unnecessary Pd precursor tends not to be sufficiently removed, while if it exceeds the upper limit, the cost tends to be too high.

Hereinafter, the present invention will be described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following Examples. The methylammonium isopolymolybdate used in the examples was synthesized by the following method.

(Synthesis example 1)
100 ml of ion-exchanged water and 100 ml of a 70% ethylamine solution (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) were mixed, and 77.1 g of molybdenum oxide (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) was added to the resulting mixed solution. After stirring, the molybdenum oxide was completely dissolved. The obtained solution was dried using an evaporator to obtain methylammonium isomolybdate.

(Example 1)
2.632 g of vanadium (IV) oxide sulfate n-hydrate (VOSO ₄ .nH ₂ O, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) and 7.196 g of methylammonium isopolymolybdate obtained in Synthesis Example 1 are ion-exchanged. It was dissolved in 80 ml of water, and the obtained solution was subjected to microwave heating at 225° C. for 30 minutes using a microwave synthesizer (“monowave 450” manufactured by Anton paar) to carry out a synthesis reaction. The reaction product was washed with a 0.4 mol/L oxalic acid aqueous solution to obtain a MoV composite oxide. When the molar ratio of Mo to V in this MoV composite oxide was measured by using a fluorescent X-ray (XRF) analyzer, it was Mo/V=5.5/1.

4 g of this MoV composite oxide was added to 200 ml of a tetraamminepalladium solution to ion-exchange Pd on the acid sites of the MoV composite oxide, followed by firing in air at 300° C. for 15 minutes to obtain a MoV composite oxide. An adsorbent E1 containing Pd was obtained. When the amount of Pd contained in the adsorbent E1 was measured using a fluorescent X-ray (XRF) analyzer, the Pd content was 1% by mass.

<X-ray diffraction measurement>
The X-ray diffraction pattern of the MoV composite oxide obtained in Example 1 was measured using an X-ray diffractometer (“Ultima IV” manufactured by Rigaku Corporation). The result is shown in FIG. In addition, FIG. 1 also shows an X-ray diffraction pattern of Mo ₃ VO _x prepared based on a crystallographic information common data format (CIF: Crystallographic Information File). As shown in FIG. 1, the X-ray diffraction pattern of the MoV composite oxide obtained in Example 1 matches the X-ray diffraction pattern of Mo ₃ VO _x , and the MoV composite oxidation obtained in Example 1 was obtained. It was confirmed that the product had the same crystal structure (FIG. 2) as Mo ₃ VO _x .

(Comparative Example 1)
A MoV composite oxide was prepared in the same manner as in Example 1, and this was used as adsorbent C1.

(Comparative example 2)
Except that 4 g of zeolite (“HSZ-800 series 840HOA type” manufactured by Tosoh Corporation) was used instead of the MoV composite oxide, in the same manner as in Example 1, after ion exchange of Pd, firing was performed, An adsorbent C2 containing zeolite and Pd was obtained. When the amount of Pd contained in the adsorbent C2 was measured using a fluorescent X-ray (XRF) analyzer, the Pd content was 1% by mass.

<NO adsorption test>
Crushed pellets were prepared from each of the adsorbents obtained in Examples and Comparative Examples by using a sieve of 50 μm mesh and 150 μm mesh. Using a fixed fluidized bed catalyst evaluation device, the mixed gas (CO ₂ (10%)+H ₂ O(5%)+NO(1000ppm)+N ₂ (remainder)) was heated to 1 g with respect to 1 g of the adsorbent formed into a pellet. The mixture was allowed to flow for 15 minutes under the conditions of 100° C. and a flow rate of 5 L/min, and the NO adsorption amount during this period was measured. The result is shown in FIG. As is clear from this result, the adsorbent E1 containing Pd and MoV composite oxide having the same crystal structure as Mo ₃ VO _x contains the adsorbent C1 and the zeolite and Pd which are composed of only the MoV composite oxide. It was found that the amount of NO adsorbed was larger than that of the adsorbent C2 containing the adsorbent C2, and the NO adsorbent performance at a low temperature was excellent.

As described above, according to the present invention, it is possible to obtain an NO adsorbent that exhibits high adsorption performance at low temperatures (for example, 100° C.). The NO adsorbent of the present invention as described above is useful as an NO adsorbent in an exhaust gas purifying apparatus for purifying exhaust gas from an internal combustion engine such as an automobile engine or a diesel engine.

Claims (3)

An NO adsorbent, comprising Pd and a complex oxide containing Mo and V as metal atoms. The NO adsorbent according to claim 1, wherein the composite oxide is Mo ₃ VO _x . Content of Pd is 0.01-10 mass% with respect to the whole NO adsorbent, The NO adsorbent of Claim 1 or 2 characterized by the above-mentioned.