JP2007125505A - Inhibitor of hydrogen sulfide generation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inhibitor of hydrogen sulfide H<SB>2</SB>S generation that can suppress the generation of H<SB>2</SB>S without using an environmentally hazardous substances such as nickel and can prevent a degradation in the activity of a three way catalyst to which the inhibitor is added. <P>SOLUTION: The inhibitor of H<SB>2</SB>S generation is comprised of a plurality of nuclei 1 comprised of an oxide of at least one element selected from alkali earth metal elements and rare earth elements, a porous oxide layer 2 whose average pore diameter is 0.5 nm to 10 nm formed on the surface of each of the nuclei 1, and noble metals 3, 4 deposited on the porous oxide layer 2. Sulfur dioxide in an exhaust gas capable of easily passing through the porous oxide layer 2 whose average pore diameter is 0.5 nm to 10 nm is oxidized by the noble metals 3, 4 while passing therethrough and reaches the nuclei 1 where it is converted to a sulfite or a sulfate which is adsorbed by the surface of the nuclei 1. The resulting sulfite or sulfate is a solid and grows in a form of bulk of particles incapable of passing through the porous oxide layer 2 whose average pore diameter is 0.5 nm to 10 nm and is fixed to the nuclei 1. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an H ₂ S generation suppressing material that suppresses the generation of hydrogen sulfide (hereinafter, H ₂ S) in exhaust gas from an automobile or the like. The H ₂ S production suppressing material of the present invention can be used alone or added to an exhaust gas purifying catalyst.

HC in exhaust gas of an automobile, as a catalyst for purifying CO and NO _x, the three-way catalyst is widely used. This three-way catalyst is formed by supporting a platinum group metal such as Pt and Rh on a porous oxide carrier such as alumina, ceria, zirconia, and ceria-zirconia, and oxidizes and purifies HC and CO. , to purify by reduction of NO _x. Since these reactions proceed most efficiently in an atmosphere in which an oxidizing component and a reducing component are present in approximately equivalent amounts, in a vehicle equipped with a three-way catalyst, the vicinity of the stoichiometric air-fuel ratio (Stoichi) (A / F = 14.6 ± The air-fuel ratio is controlled so that it is burned at about 0.2).

However, the three-way catalyst has a problem that when the exhaust gas atmosphere moves toward the reduction side, sulfur oxides in the exhaust gas are reduced and discharged as H ₂ S. For example, ceria with oxygen absorption / release capacity is an essential component of a three-way catalyst, but in a car equipped with a three-way catalyst using ceria, the exhaust gas atmosphere is on the rich side (reducing atmosphere) such as during acceleration. There was a problem that H ₂ S was generated. The mechanism that H ₂ S generates when ceria is used is explained as follows. That is, SO ₂ in the exhaust gas is oxidized by the catalytic metal to become SO ₃ or SO ₄ . Since ceria is an oxide having a relatively high basicity, it is easy to adsorb SO ₃ or SO ₄ . Then, it is considered that the adsorbed SO ₃ or SO ₄ is gradually concentrated on the support and is reduced in a reducing atmosphere to generate H ₂ S. Since H ₂ S is perceived by human olfaction even in trace amounts, it causes discomfort, so it is necessary to suppress the discharge.

Therefore, it is conceivable to further use an oxide of Ni or Cu as a component of the three-way catalyst. Ni or Cu oxide suppresses the generation of H ₂ S because SO _{2 is changed} to SO ₃ or SO ₄ in an oxidizing atmosphere, and sulfur components are stored as sulfides such as Ni ₂ S ₃ in a reducing atmosphere. Can do.

For example, Japanese Examined Patent Publication No. 08-015554 describes an exhaust gas purifying catalyst in which a noble metal is supported on a carrier made of nickel-barium composite oxide, alumina, and ceria. In this carrier, alumina and ceria capture sulfur oxides as sulfates in a lean atmosphere, and H ₂ S is captured by nickel barium complex oxide in a reducing atmosphere. Therefore, generation of H ₂ S can be suppressed.

Japanese Patent Publication No. 2000-515419 or Japanese Patent No. 02598817 describes that the production of H ₂ S is suppressed by using a carrier in which ceria is mixed with NiO ₂ , Fe ₂ O _3, or the like. Japanese Patent Application Laid-Open No. 07-194978 describes that the formation of H ₂ S is suppressed by using a carrier in which Ni and Ca are supported on ceria.

  However, since Ni or Cu is an environmentally hazardous substance, there is a current situation that its use is being restricted for automobile exhaust gas purification catalysts. Further, when barium or the like is added to the three-way catalyst, the original purification performance may be deteriorated.

Further, Japanese Patent Publication No. 02-020561 discloses a catalyst that contains a bismuth component and can oxidize and remove H ₂ S. However, since these catalysts oxidize H ₂ S in an oxidizing atmosphere, H ₂ S may be discharged in a stoichiometric or reducing atmosphere.
No. 08-015554 Special table 2000-515419 Patent No. 02598817 JP 07-194978 Japanese Patent Publication No. 02-020561

The present invention has been made in view of the above circumstances, can suppress the generation of H ₂ S without using environmentally hazardous substances such as nickel, and can prevent deterioration of catalyst performance when added to a three-way catalyst or the like. The problem to be solved is to use H ₂ S formation inhibitor.

The feature of the H ₂ S formation suppressing material of the present invention that solves the above-mentioned problems is formed on the surface of a nucleus comprising an oxide of at least one element selected from alkaline earth metals and rare earth elements, and the surface of the nucleus. In other words, it consists of a porous oxide layer having an average pore diameter of 0.5 to 10 nm and a noble metal supported on the porous oxide layer.

  The nucleus is preferably an oxide of at least one element selected from Ba, La, Pr and Nd.

According to the H ₂ S production inhibitor of the present invention, SO _{2 in} the exhaust gas easily passes through the porous oxide layer having an average pore diameter of 0.5 nm to 10 nm. SO ₂ is oxidized by the supported noble metal while passing through the porous oxide layer, and reaches SO2 as SO ₃ ions or SO ₄ ions. Since the nucleus is made of an oxide of at least one element selected from alkaline earth metals and rare earth elements, SO ₃ ions or SO ₄ ions are fixed to the surface of the nucleus as sulfites or sulfates. . Since sulfite or sulfate is solid and easily aggregates and grows in bulk, it is difficult to pass through a porous oxide layer with an average pore size of 0.5 nm to 10 nm. It is fixed in a state where it is captured.

That is, sulfur oxide does not return to the exhaust gas again from the sulfite or sulfate immobilized on the nucleus. Therefore, since the amount of sulfur oxides in the exhaust gas is greatly reduced, the generation of H ₂ S in the reducing atmosphere can be greatly suppressed.

  Further, since a porous oxide layer is formed on the surface of the nucleus, Ba or the like does not appear. Therefore, when added to a three-way catalyst or the like and used, there is no possibility of reducing the catalyst performance.

The H ₂ S generation suppressing material of the present invention is composed of a nucleus, a porous oxide layer formed on the surface of the nucleus, and a noble metal supported on the porous oxide layer.

The nucleus is made of an oxide of at least one element selected from alkaline earth metals and rare earth elements. The element species is not particularly limited as long as it is stable as an oxide. However, since oxides containing ceria have oxygen absorption / release capability, SO ₂ oxidation activity may be reduced and SO ₃ ions or SO ₄ ions may be reduced. It is desirable not to. Particularly desirable is at least one element selected from Ba, La, Pr and Nd, which easily reacts with SO ₃ ions or SO ₄ ions. Note that at least the inside of the nucleus may be an oxide, and the surface of the nucleus may be temporarily or partially carbonate or nitrate.

The nucleus preferably has a secondary particle size of 10 μm or less. When the secondary particle diameter exceeds 10 μm, the specific surface area of the H ₂ S formation inhibitor becomes small and the effect of inhibiting the formation of H ₂ S becomes insufficient.

As the oxide constituting the porous oxide layer, one kind or plural kinds such as alumina, zirconia, titania, ceria, silica, or plural kinds of complex oxides selected from these can be used. However, since the oxide containing ceria has the ability to absorb and release oxygen, the oxidation activity of SO ₂ may be reduced, and SO ₃ ions or SO ₄ ions may be reduced. It is desirable not to use it.

This porous oxide layer has an average pore diameter of 0.5 nm to 10 nm. If the average pore diameter is smaller than 0.5 nm, it is difficult to pass SO ₂ and sulfur oxide cannot be immobilized, and it is difficult to suppress the generation of H ₂ S. In addition, when the average pore diameter is larger than 10 nm, sulfite or sulfate fixed to the nucleus may be released through the porous oxide layer, which becomes H ₂ S in a reducing atmosphere. It is not preferable.

  In order to control the average pore diameter of the porous oxide layer as described above, for example, the oxide precursor is precipitated from an aqueous solution of a water-soluble oxide precursor, and the drying conditions and / or the firing conditions are controlled. Can be done. For example, if the calcination temperature is increased or the calcination time is lengthened, the average pore diameter is reduced by becoming dense. Conversely, if the firing temperature is lowered or the firing time is shortened, the average pore diameter can be increased.

A noble metal is supported on the porous oxide. As the noble metal, SO ₂ can be oxidized to SO ₃ or SO ₄ , and for example, at least one selected from Pt, Pd, Rh, and Ir can be used. Among these, it is desirable to use at least Pt having a high oxidation activity. Other precious metals or base metals may be co-supported.

The H ₂ S production suppressing material of the present invention can be used as it is disposed in the exhaust gas, but it is also preferable to use it by adding it to an exhaust gas purification catalyst such as an oxidation catalyst, a three-way catalyst or a NO _x storage reduction catalyst. . For example, a three-way catalyst is often used in which a coat layer made of a carrier powder such as alumina powder or ceria-zirconia solid solution powder is formed on a honeycomb substrate and Pt and Rh are supported on the coat layer. Therefore, it is preferable to mix the H ₂ S formation inhibitor powder of the present invention in the coat layer.

In such a three-way catalyst, when the exhaust gas fluctuates toward the lean side centering on the stoichiometric, HC and CO are oxidized and purified, and sulfur oxides are captured by the H ₂ S generation suppressing material. Moreover, HC and CO are also oxidized and purified by the noble metal supported on the H ₂ S formation suppressing material. In a rich atmosphere, NO _x is reduced and purified, and at the same time there is no release of sulfur oxides from the H ₂ S production inhibitor, so there is less sulfur oxide in the exhaust gas, and H ₂ S production is reduced. Is suppressed.

For example, when producing a three-way catalyst to which an H ₂ S production inhibitor is added, a H ₂ S production inhibitor material powder previously supporting a noble metal may be mixed in the coating layer, A noble metal powder composed of a porous oxide layer is mixed with a carrier powder to form a coat layer, and then a powder composed of a core and a porous oxide layer when carrying a Pt or the like to form a three-way catalyst. In addition, Pt can be supported as a H ₂ S formation suppressing material.

According to the H ₂ S production-suppressing member of the present invention, SO ₃ or SO _4, which is captured to maintain that state, trapping amount is either saturated. However, in the actual exhaust gas purification catalyst, H ₂ S emission is perceived only in the case of a new catalyst, and almost no H ₂ S emission occurs from a deteriorated catalyst. Therefore, the saturation of the amount of H ₂ S trapped in the H ₂ S generation suppressing material of the present invention has no problem in practical use.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

Example 1
FIG. 1 shows a schematic cross-sectional view of the H ₂ S formation suppressing material of this example. This H ₂ S production suppressing material is composed of a core body 1 made of BaO and having a secondary particle diameter of 3 μm, and an alumina layer 2 formed on the surface of the core body 1. The alumina layer 2 has an average pore diameter of 3.9 nm and a thickness of about 1.5 μm. The alumina layer 2 carries Pt3 and Rh4 with high dispersion. Hereinafter, the manufacturing method of this H ₂ S production | generation suppression material is demonstrated and it replaces with the detailed description of a structure.

  100 parts by weight of barium oxide powder was mixed with a solution obtained by mixing 788.9 parts by weight of alumina sol and pure water, and stirred at 60 ° C. for 1 hour or longer. 258.7 parts by weight of aqueous ammonia was added to the obtained dispersion and stirred for 30 minutes. Next, it was dehydrated with a rotary evaporator, dried by high frequency heating for 1 hour, fired at 700 ° C. for 5 hours in an electric furnace, and then pulverized by a ball mill to prepare a noble metal-free powder.

  This noble metal-free powder includes a core 1 and an alumina layer 2 formed on the surface of the core 1. When the pore diameter of the alumina layer 2 was measured with a mercury porosimeter, it was confirmed that the pore diameters of all the pores were in the range of 3 nm to 10 nm and the average pore diameter was 3.9 nm.

87 parts by weight of the noble metal powder obtained, 90 parts by weight of ceria-zirconia solid solution (molar ratio CeO ₂ : ZrO ₂ = 60: 40), 3 parts by weight of alumina hydrate, 44 parts by weight of 40% aluminum nitrate aqueous solution Each was weighed and mixed with a predetermined amount of pure water and milled to prepare a slurry.

  On the other hand, a cordierite honeycomb substrate (1.1L, diameter 103mm, length 130mm, cell density 400cpsu, wall thickness 100μm) was prepared, washed with the slurry, dried at 120 ° C, and dried at 650 ° C for 3 hours. Firing was performed to form a coat layer. The coating layer was formed in an amount of 200 g per liter of honeycomb substrate. Next, it was immersed in an aqueous rhodium nitrate solution having a predetermined concentration to adsorb and hold Rh, dried at 120 ° C. and then fired at 500 ° C. for 1 hour. Further, a predetermined amount of a dinitrodiammine platinum solution having a predetermined concentration was impregnated to absorb Pt with water, dried at 120 ° C., and then calcined at 500 ° C. for 1 hour. The loadings of Rh and Pt are 0.2 g of Rh and 1.0 g of Pt per liter of honeycomb substrate.

Rh and Pt were also carried on the alumina layer 2 formed on the surface of the noble metal-free powder core 1, and the H ₂ S formation suppressing material of this example was manufactured. The amount of Rh and Pt supported in the H ₂ S formation suppressing material is 70% by weight of noble metal in the alumina layer 1 in calculation. This H ₂ S generation inhibitor powder is uniformly mixed with the three-way catalyst powder, and together constitutes a catalyst coat layer.

(Example 2)
In the same manner as in Example 1, except that the noble metal powder was dried for 1 hour by high-frequency heating and calcined at 1000 ° C. for 5 hours in an electric furnace, a three-way catalyst added with H ₂ S formation inhibitor powder was prepared. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 1 nm to 3 nm, and the average pore diameter was 1.2 nm.

(Example 3)
In the same manner as in Example 1, except that the noble metal powder was dried for 1 hour by high-frequency heating and calcined for 5 hours at 1200 ° C. in an electric furnace, a three-way catalyst to which H ₂ S formation inhibitor powder was added was prepared. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 0.5 nm to 1 nm, and the average pore diameter was 0.8 nm.

(Comparative Example 1)
Inhibiting H ₂ S formation in the same manner as in Example 1 except that when precious metal-free powder was prepared, instead of high-frequency heating, it was dried at 100 ° C. for 24 hours in a dryer and baked at 700 ° C. for 5 hours in an electric furnace. A three-way catalyst with added powder was prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 50 nm or more, and the average pore diameter was 72 nm.

(Comparative Example 2)
When the noble metal-free powder preparation, except that calcined 5 hours at 1000 ° C. for 24 hours dry electric furnace at 100 ° C. in a drying machine instead of the high-frequency heating in the same manner as in Example 1, H ₂ S production-suppressing member A three-way catalyst with added powder was prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 10 nm to 20 nm, and the average pore diameter was 14 nm.

Example 4
A noble metal-free powder was prepared in the same manner as in Example 1 except that lanthanum oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 3 nm to 10 nm, and the average pore diameter was 6.8 nm.

(Example 5)
A noble metal-free powder was prepared in the same manner as in Example 2 except that lanthanum oxide powder was used in place of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. The alumina layer 2 had a pore diameter in the range of 1 nm to 3 nm and an average pore diameter of 2.5 nm.

(Example 6)
A noble metal-free powder was prepared in the same manner as in Example 3 except that lanthanum oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 0.5 nm to 1 nm, and the average pore diameter was 0.9 nm.

(Comparative Example 3)
A noble metal-free powder was prepared in the same manner as in Comparative Example 1 except that lanthanum oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 50 nm or more, and the average pore diameter was 83 nm.

(Comparative Example 4)
A noble metal-free powder was prepared in the same manner as in Comparative Example 2 except that lanthanum oxide powder was used in place of the barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 10 nm to 20 nm, and the average pore diameter was 19 nm.

(Example 7)
A noble metal-free powder was prepared in the same manner as in Example 1 except that praseodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 3 nm to 10 nm, and the average pore diameter was 7.1 nm.

(Example 8)
A noble metal-free powder was prepared in the same manner as in Example 2 except that praseodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 1 nm to 3 nm, and the average pore diameter was 2.2 nm.

Example 9
A noble metal-free powder was prepared in the same manner as in Example 3 except that praseodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 0.5 nm to 2 nm, and the average pore diameter was 1.2 nm.

(Comparative Example 5)
A noble metal-free powder was prepared in the same manner as in Comparative Example 1 except that praseodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added was prepared in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 50 nm or more, and the average pore diameter was 75 nm.

(Comparative Example 6)
A noble metal-free powder was prepared in the same manner as in Comparative Example 2 except that praseodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst added with H ₂ S formation inhibitor powder in the same manner as in Example 1 was prepared. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 10 nm to 30 nm, and the average pore diameter was 21 nm.

(Example 10)
A noble metal-free powder was prepared in the same manner as in Example 1 except that neodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 3 nm to 10 nm, and the average pore diameter was 4.5 nm.

(Example 11)
A noble metal-free powder was prepared in the same manner as in Example 2 except that neodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 1 nm to 3 nm, and the average pore diameter was 1.8 nm.

(Example 12)
A noble metal-free powder was prepared in the same manner as in Example 3 except that neodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 0.5 nm to 2 nm, and the average pore diameter was 1.0 nm.

(Comparative Example 7)
A noble metal-free powder was prepared in the same manner as in Comparative Example 1 except that neodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 50 nm or more, and the average pore diameter was 78 nm.

(Comparative Example 8)
A noble metal-free powder was prepared in the same manner as in Comparative Example 2 except that neodymium oxide powder was used instead of barium oxide powder, and a three-way catalyst to which H ₂ S formation inhibitor powder was added in the same manner as in Example 1. Prepared. In the alumina layer 2, the pore diameters of all the pores were in the range of 10 nm to 20 nm, and the average pore diameter was 15 nm.

(blank)
A three-way catalyst was prepared in the same manner as in Example 1 except that the same amount of alumina powder was used instead of the noble metal-free powder.

<Test and evaluation>
Each of the above-described catalysts was mounted as an underfloor catalyst for a vehicle having a 2.4 L gasoline engine with stoichiometric control. First, the vehicle was run for 1 hour at 40 km / hr so that the catalyst bed temperature would be 400 ° C. During this time, sulfur oxides are captured by the H ₂ S formation suppressing material. Next, the throttle is fully opened and accelerated to 80 km / hr in 10 seconds, maintained at that speed for 10 seconds, then decelerated and stopped in 20 seconds, and the amount of H ₂ S in the exhaust gas in the idle state 10 seconds after the stop It measured and evaluated by the peak value. The results are shown in Table 1 and FIG.

It can be seen that the catalyst of each Example and each Comparative Example has a lower H ₂ S peak concentration than the blank catalyst, and this is clearly the effect of adding an H ₂ S formation inhibitor. FIG. 2 clearly shows that the average pore diameter of the alumina layer 2 is preferably 10 nm or less, and that the core 1 is particularly preferably barium oxide.

The H ₂ S peak concentration in Example 3 is very low, 2 ppm, because the average pore diameter of the alumina layer 2 is as extremely small as 0.8 nm. However, if the average pore size of the alumina layer 2 is further reduced, the amount of sulfur oxides that reach the core 1 becomes very small and it becomes difficult to express the effect of the H ₂ S formation inhibitor. It is reasonable to set the lower limit to 0.5 nm.

The H ₂ S production suppressing material of the present invention can be used in an exhaust gas as it is, but it can also be used by being added to an exhaust gas purification catalyst such as an oxidation catalyst, a three-way catalyst or a NO _x storage reduction catalyst. .

It is a schematic sectional view showing a H ₂ S production-suppressing member according to an embodiment of the present invention. It is a graph showing the relationship between the average pore diameter and the H ₂ S peak concentration.

Explanation of symbols

  1: Nucleus 2: Alumina layer (porous oxide layer) 3: Pt 4: Rh

Claims (2)

A nucleus composed of an oxide of at least one element selected from alkaline earth metals and rare earth elements;
A porous oxide layer having an average pore diameter of 0.5 nm to 10 nm formed on the surface of the nucleus;
A noble metal supported on the porous oxide layer;
A hydrogen sulfide production-suppressing material comprising: The hydrogen sulfide production suppressing material according to claim 1, wherein the nucleus is an oxide of at least one element selected from Ba, La, Pr, and Nd.

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