JPH0474535A - Exhaust gas purifying catalyst and preparation thereof - Google Patents

Abstract

PURPOSE:To obtain a catalyst suppressing the formation of H2S without lowering ternary activity by supporting a noble metal on an alumina carrier and subsequently bonding silica to the hydroxyl group bonded to the aluminum atom in alumina on the surface of the carrier through a covalent bond. CONSTITUTION:A noble metal such as platinum or rhodium is first supported on an alumina carrier composed of gamma-alumina or theta-alumina. Next, alkoxysilane or alkoxylalkylsilane is vapor-deposited on the surface of the alumina carrier to be heated. Whereupon, silica is bonded to the hydroxyl group bonded to the aluminum atom in alumina present on the surface of the carrier through a covalent bond. The catalyst thus obtained suppresses the formation of H2S without lowering ternary activity.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a catalyst for purifying exhaust gas, and more specifically, the present invention relates to a catalyst for purifying exhaust gas. This invention relates to an exhaust gas purification catalyst that can remove sulfur oxides and prevent sulfur oxides adsorbed on the catalyst from being reduced by hydrogen and hydrocarbons and being emitted as hydrogen sulfide, and a method for producing the same. It is.

[Prior art and problems]

Conventionally, three-way catalysts for exhaust gas purification generally consist of a carrier base material,
There is known a carrier consisting of an alumina coating formed on the surface of a carrier base material, and a catalyst metal supported on the carrier.

This exhaust gas purification catalyst is used to reduce hydrocarbons (HC) and -carbon oxides (CO) contained in gases exhausted by internal combustion engines.
is oxidized and nitrogen oxides (No.) are reduced by reduction. However, the sulfur in the fuel is either burned and emitted as sulfur dioxide (SO2), or is oxidized when exposed to exhaust gas or an oxidizing atmosphere while driving, becomes sulfur trioxide (SO3), which is adsorbed on the alumina carrier, and converts sulfates into sulfur. formed and accumulated. However, when the catalyst bed reaches a high temperature of approximately 600°C or higher and the exhaust gas contains a large amount of unburned hydrocarbons and carbon monoxide, reducing hydrogen is formed by the action of the noble metal catalyst, and the adsorbed hydrogen is The sulfate present in the fuel is reduced to form hydrogen sulfide (H2S), and when the amount of sulfur in the fuel is large, the amount of Has generated increases, leading to the emission of foul-smelling exhaust gas. In order to prevent the discharge and storage of H2S, Japanese Utility Model Publication No. 54-31210 discloses an example in which, in addition to the three-way catalyst, a catalyst for H2S removal is provided downstream of the three-way catalyst. However, in addition to the three-way catalyst, H
Installing a dedicated catalyst for removing 2S is undesirable because it increases cost and weight.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exhaust gas purifying catalyst that suppresses the production of H2S without reducing the three-way activity.

The inventor is an Environmental
ce & Technology Vol 13 (
1979) P., 321, based on the statement that sulfate accumulation occurs due to interaction with acid/base sites originating from hydroxyl groups on alumina.
The present invention was completed after extensive research into the possibility of utilizing silica (SiO□), which has no base sites.

(Description of the first invention) The first invention provides a) i. a Mina carrier, a noble metal supported on the surface of the carrier, and a hydroxyl group covalently bonded to an aluminum atom in alumina present on the surface of the carrier; The present invention relates to an exhaust gas purifying catalyst characterized by comprising SiO□ combined with SiO□.

The catalyst according to the present invention has noble metal and Si on the surface of the alumina carrier.
Since O2 coexists, noble metals can oxidize CO and HC and reduce NOx to remove them.
Since iC)+ is bonded to the hydroxyl group on the surface of the alumina support, it is possible to prevent S03, which causes the generation of H and S, from bonding with the hydroxyl group, and as a result, it is possible to prevent the generation of H2S. Become. Therefore, by using this catalyst, the generation of H2S can be suppressed without reducing the ternary activity. Further, the hydroxyl group to which 5i02 is bonded is covalently bonded to the aluminum atom in alumina. Therefore, under normal conditions of use, 5i02 cannot be removed from the carrier.

[Other inventions of the first invention] As the carrier supporting the catalyst, γ alumina or θ-alumina having a large specific surface area is used. Although the shape may be in the form of pellets, it is preferable to mix powdered alumina with water or the like to form a slurry and apply it to a base material to form a layered coating. The base material used in this case may be the same as conventional ones, such as a honeycomb-shaped monolithic carrier base material or a pellet-shaped carrier base material. Further, as the material of the carrier base material, known materials such as ceramics such as cordierite, mullite, alumina, magnesia, and spinel, or heat-resistant metals such as ferritic steel can be used.

In addition, noble metals include platinum (Pt) and rhodium (Rh).
), palladium (Pd), iridium (Ir), ruthenium (Ru), osmium (O8), and the like. The supported amount may be the amount normally used as a three-way catalyst.

Furthermore, SiO2 is bonded to hydroxyl groups on the surface of the alumina carrier. As shown in Figure 1, the amount of H2S generated is SiO□
decreases with increasing amount of 4.3X10-'mo
A/V-Cat can almost prevent the generation of H2S. S
As shown in FIG. 2, even if the amount of iO2 is increased, the 50% NO purification temperature hardly changes, indicating good activity.

(Description of the second invention) The second invention includes a step of supporting a noble metal on an alumina carrier,
An exhaust gas purifying catalyst comprising the steps of: depositing one or more of alkoxylsilanes, alkoxylalkylsilanes, and alkoxysilicone on the surface of the alumina carrier; and heating the carrier on which the alkoxylsilanes, etc. have been deposited. This relates to a manufacturing method.

According to this manufacturing method, noble metal and 3iC are added to the alumina carrier.
A catalyst in which L coexists can be easily produced. Furthermore, in this manufacturing method, noble metals can be supported before forming SiO□ by vapor deposition, and there is no reduction in ternary activity.

Although the reaction during the formation of 5i02 is not clear, it is thought to be as follows.

(Explanation of other inventions of the second invention) The catalyst of the present invention can be manufactured as follows.

First, a carrier base material is prepared, and an alumina carrier layer is formed on the surface of the carrier base material. The alumina carrier layer is formed by wash coating the surface of the carrier base material with an alumina slurry containing alumina powder. Thereafter, the coat layer is fired at a predetermined temperature to form a carrier made of an alumina layer.

Next, a noble metal as a catalyst metal is supported on the alumina carrier. The carrier base material on which the alumina carrier is formed is immersed in an aqueous solution containing the catalytic metal, the solution is impregnated into the carrier, and then the catalytic metal is supported by firing at a predetermined temperature.

Next, the base material carrying the catalytic metal is inserted into a chemical vapor deposition (CVD) device to form SiO□ on the surface of the alumina support. CVD may be performed using commonly used equipment.

The vapor of alkoxysilane or the like uses nitrogen as a carrier gas, and its amount is controlled by a feeder to supply a predetermined amount into the apparatus. The deposition temperature is 200 to 400°C and the processing time is 1 to 2 hours.

The heating process involves heating the silicon in an electric furnace or in the air, or heating it in the car and using the heat of the exhaust gas.
Including the case where O□ is formed.

Heating may be performed at 400 to 600° C. for a short time, for example, about 15 minutes.

Further, cerium oxide may be supported on the carrier. Cerium oxide can act as a cocatalyst and increase the three-way catalyst activity. Cerium oxide is prepared by a conventional method, that is, an alumina support is impregnated with an aqueous solution containing a water-soluble cerium salt, and then heated in air to a temperature of 600° C. or more to decompose and support the salt.

(Description of the Third Invention) The third invention includes a step of supporting a noble metal on an alumina carrier, a step of vapor depositing one or more of alkoxylsilane, alkoxylalkylsilane, and alkoxyl silicone on the surface of the alumina carrier, and a step of depositing alkoxylsilane, alkoxylalkylsilane, and alkoxyl silicone on the surface of the alumina carrier. A step of hydrolyzing silane, etc. and supporting a noble metal on an alumina carrier, a step of vapor depositing one or more of alkoxyl silane, alkoxylalkyl silane, and alkoxyl silicone on the surface of the alumina carrier, and hydrolyzing the vapor-deposited alkoxyl silane, etc. The present invention relates to a method for producing an exhaust gas purifying catalyst, which comprises a φ step of vapor depositing an alkoxyl silane or the like, and a step of heating a carrier on which the alkoxyl silane or the like is vapor deposited.

According to the manufacturing method of the third invention, hydroxyl groups are formed by the step of hydrolyzing the vapor-deposited alkoxylsilane, etc.
In the next vapor deposition step, tetramethoxysilane or the like is bonded to the hydroxyl group. By repeating these hydrolysis steps and vapor deposition steps, the amount of S io 2 increases after the heating step, and as shown in FIG. 1, the generation of H 2 S can be significantly reduced.

Moreover, this manufacturing method has the same effect as the manufacturing method of the second invention.

The reaction when SiO□ is formed by this manufacturing method is as follows:
It may not be clear, or it may be as follows.

For example, when tetramethoxysilane is used, tetramethoxysilane is vapor-deposited and bonded to the hydroxyl groups on the surface of the alumina carrier by the production method of the second invention, and then the tetramethoxysilane is hydrolyzed.

In the hydrolysis step, the functional group (-OMe) around silicon constituting the tetramethoxysilane is replaced with a hydroxyl group (
Reaction formula (2)), tetramethoxysilane is bonded onto the alumina support again to bond the tetramethoxysilane to the hydroxyl group (reaction formula (3)).

The tetramethoxysilane thus deposited on the alumina support is bonded to form SiO□. That is, AA O3i produced by the reaction of formula (3)
(O3i (OMe) 3 )- in methoxy group (-O
Me) is combined with oxygen in the heating step after the vapor deposition step,
Generate SiO□.

AAO3i (OMe)3 +3H20→AffO3i
(OH)3+3MeOHAAO8i (OH)3
+3Si (OMe)4→AA'O3i (O3i
(OMe) 2) 313 M e OH (3) (Description of other inventions of the third invention) The step of supporting the noble metal before the hydrolysis step and the step of vaporizing alkoxylsilane etc. are the same as in the case of the second invention. be.

For hydrolysis, after stopping the supply of alkoxylsilane, etc.
Water is bubbled with nitrogen gas, water vapor is introduced into the device using nitrogen as a carrier gas, and 200 to 400
This is done by heating at ℃ for 20 to 40 minutes.

The vapor deposition step and heating step after the main hydrolysis step are the same as in the case of the second invention.

(Example) This will be explained in detail below using examples.

[Preparation of Example Catalysts Nα1 to Nα4] 100 parts by weight of γ-alumina powder (average particle size 5 μm), boehmite powder (A A 203 ・H2O) (particle size 5
μm) was added to 114 parts of distilled water and mixed to prepare a slurry. A cordierite honeycomb carrier base material having a capacity of 35 cc was immersed in the slurry prepared as described above, and after taking it out, the excess slurry was wiped off. 20
After drying at 0℃ for 2 hours, baking at 650℃ for 1 hour,
A carrier consisting of an alumina coating layer was formed on the base material.

Next, the base material on which the carrier was formed as described above was mixed with cerium nitrate (
Immerse in an aqueous solution of Ce (No3)= ) and heat at 200°C.
After drying for 3 hours at 600° C. for 5 hours, cerium oxide was formed.

Next, after being sequentially immersed in a dinitrodiammine platinum aqueous solution and a rhodium chloride aqueous solution for 1 hour, it was dried, and Pt and Rh were each supported on a carrier made of alumina.

The supported amount is Pt 1.2 g/(!, Rh 0.4 g/j7.

The formation of SiO□ was carried out in the CVD apparatus shown in FIG.

As described above, a base material in which Pt and Rh were supported on an alumina carrier was placed in the furnace 3. After heating the furnace 3 to 300°C, nitrogen was pumped in at 150 ml/by mass flow regulator l.
min, and also tetramethoxysilane (S i
(OMe) 4) was fed from the microfeeder 2 at a rate of 0.88 ml/min.

A vapor deposition process was performed for 75 minutes to obtain a catalyst NcLl. Next, form an alumina carrier layer by the same operation as catalyst Nα1,
After supporting and vapor-depositing cerium oxide, Pt, and Rh, the cerium oxide, Pt, and Rh were kept in the CVD apparatus, water was bubbled with nitrogen gas, and water vapor was supplied into the furnace using nitrogen as a carrier gas. After hydrolysis treatment for 1 minute, the same vapor deposition treatment as in the case of catalyst N (11) was performed to obtain catalyst Nα2. The same hydrolysis treatment and vapor deposition treatment as for catalyst Nα2 were performed to obtain catalyst Nα3.In addition, the same hydrolysis treatment and vapor deposition treatment as for catalyst Nα3 were performed on a sample that had been subjected to vapor deposition treatment in the same manner as for catalyst Nα3. Nα4 was obtained.

(Preparation of Comparative Example Catalyst NCLCI) Comparative Example Catalyst NCLCI was obtained by carrying Pt and Rh in the same manner as the catalyst Nα1 without performing the vapor deposition treatment.
And so.

[Preparation of Comparative Example Catalyst N11C2] In the preparation method of Examples Nα1 to Nα4, commercially available amorphous SiO□ (Aerosil TT200) was used instead of γ-alumina powder and silica sol was used instead of boehmite powder to coat the carrier base material. A certain silica coating layer was formed.

Next, the base material on which the carrier was formed was made of cerium nitrate (Ce).
Example N without immersion in an aqueous solution of (NO3)-)
Pt and Rh were supported by water absorption using the same raw materials as α1 to Nα4. The catalyst obtained in this manner was used as comparative example catalyst N.
(It was set as 1C2.

[Preparation of Comparative Example Catalyst No., C3] In the preparation method of Example Catalysts Nα1 to Nα4, after forming a carrier consisting of an activated alumina coating layer, Pt and Rh were supported without supporting cerium oxide. The catalyst was a comparative example catalyst NαC3.

[Preparation of Example Catalyst Nα5] Comparative Example Catalyst No. C3 was subjected to vapor deposition treatment four times in the same manner as Example Catalyst Nα4 to obtain Example Catalyst Nα5.

[Evaluation of catalyst activity]

The present example catalysts Nα1 to Nα4, the comparative example catalyst NαC1
-N(lc2) was evaluated for hydrogen sulfide and ternary activity. The amount of hydrogen sulfide generated was measured at the temperature, time, and gas composition shown in Table 1. That is, 450°
After increasing the temperature, the temperature was lowered to 300°C, and the composition of a model gas imitating automobile exhaust gas was changed during the temperature increase step and the temperature decrease step. Hydrogen sulfide generation was evaluated by the amount of hydrogen sulfide generated in a peak shape in step 3.

In addition, to evaluate the three-way activity, a model gas with the same composition as the gas at the time of heating when measuring the amount of hydrogen sulfide generated was passed through the catalyst and heated at 15°C/min, and each of N01C0 and HC was The 50% purification temperature was measured.

The results are shown in Table 2 along with the amount of SiO2 deposited using the gravimetric method. Comparative Example Catalyst NcLC1 is a conventional alumina supported catalyst, so although it has excellent three-way activity,
A large amount of hydrogen sulfide is generated.

Table 2 Comparative Example Catalyst N (lC2 is a SiO2 supported catalyst, so hydrogen sulfide is not generated or the ternary activity is low.

In the example catalysts Nα1 to Nα4, the amount of hydrogen sulfide generated could be reduced without reducing the three-way activity.

In particular, with the example catalyst Nα4, no hydrogen sulfide was generated. This is thought to be because the accumulation of sulfate was suppressed by the vapor deposition of S i O2.

In addition, when the amount of hydrogen sulfide generated for Comparative Example Catalysts NO, C3 and Example Catalyst Nα5 was measured using the same evaluation method,
Hydrogen sulfide was generated in No. and C3 samples which were not subjected to vapor deposition treatment, whereas no hydrogen sulfide was generated in Nα5 sample which was subjected to vapor deposition treatment.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the amount of silica vapor deposited and the amount of H2S produced, and FIG. 2 is a diagram showing the relationship between the amount of silica vapor deposit and the 50% NO purification temperature. FIG. 3 is a diagram showing a chemical vapor deposition apparatus for manufacturing the exhaust gas purifying catalyst according to the present invention. Mass flow regulator Micro feeder furnace Sample thermocouple temperature regulator

Claims (3)

[Claims] (1) Exhaust gas characterized by being composed of an alumina carrier, a noble metal supported on the carrier surface, and silica bonded to a hydroxyl group covalently bonded to an aluminum atom in the alumina present on the carrier surface. Purification catalyst. (2) a step of supporting a noble metal on an alumina carrier; a step of vapor-depositing one or more of alkoxylsilanes, alkoxylalkylsilanes, and alkoxyl silicones on the surface of the alumina carrier; and a step of heating the carrier on which the alkoxylsilanes, etc. have been vapor-deposited. A method for producing an exhaust gas purifying catalyst, comprising: (3) A step of supporting a noble metal on an alumina carrier, a step of vapor depositing one or more of alkoxysilane, alkoxylalkylsilane, and alkoxyl silicone on the surface of the alumina carrier, and hydrolyzing the vapor-deposited alkoxylsilane, etc. 1. A method for producing an exhaust gas purifying catalyst, comprising a step of vapor-depositing alkoxylsilane, etc., and a step of heating a carrier on which the alkoxylsilane or the like is vapor-deposited.