JPH0578384B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to an exhaust gas purifying catalyst. More specifically, the present invention is a three-way catalyst for exhaust gas purification that simultaneously removes hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), which are harmful components contained in exhaust gas from internal combustion engines such as automobiles. It is related to. <Prior Art and Problems Theretofore, many catalysts for purifying exhaust gas emitted from internal combustion engines of automobiles and the like have been proposed and put into practical use. Initially, oxidation catalysts that removed HC and CO were put into practical use, but due to stricter regulations and the performance of the generation source, three-way catalysts that remove NOx as well as CO and HC are now mainstream. There is. This three-way catalyst simultaneously performs the oxidation reaction of CO and HC and the reduction reaction of NOx. However, gasoline, which is a fuel, contains trace amounts of sulfur compounds, which are emitted as sulfur oxides (SOx) into exhaust gas. When this exhaust gas is purified using a three-way catalyst, if the gas emitted from the internal combustion engine is in the reduction region, the SOx in the exhaust gas is reduced to hydrogen sulfide (H ₂ S) by the reduction action of the three-way catalyst and is emitted. . _H2S
is harmful not only to the odor but also to the human body,
To date, no effective catalyst has been proposed to suppress its generation. <Means for solving the problem> The present inventors have conducted intensive research to solve the problem of H ₂ S generation in order to suppress the emission of H ₂ S without impairing the performance of conventional catalysts. They discovered that this problem could be solved by using activated alumina loaded with a specific amount of noble metal and by adding nickel oxide. <Structure of the invention> The present invention contains 5 to 30% by weight of at least one noble metal among platinum palladium and 1% by weight of rhodium.
Activated alumina supported in the range of ~20% by weight
(a), cerium oxide (b), activated alumina (c), and nickel oxide (d) are coated and supported on a carrier having a honeycomb structure _. The present invention provides a catalyst that suppresses S generation. Exhaust gas from internal combustion engines such as automobiles contains SOx, which is produced when trace amounts of sulfur compounds in fuel are oxidized. The currently widely used three-way catalyst for purifying exhaust gas is such that when the conditions of the internal combustion engine deviate from the stoichiometric air-fuel ratio (A/F) and there is excess fuel, the exhaust gas switches to the reduction side and converts SOx in the exhaust gas into H ₂ S. It is reduced and discharged. In the case of automobiles, the A/F is normally well controlled around the theoretical value, but it shifts to the side of excess fuel when the accelerator is pressed temporarily or when the load is high. Emission of H ₂ S occurs at times like this. In order to suppress the generation of H ₂ S, the present inventors have conducted intensive research and found that by specifying the amount of noble metals used on activated alumina, the generation of H ₂ S can be suppressed, and in addition, nickel oxide can be added. By adding
It has been found that H ₂ S is hardly emitted. Conventionally, methods for supporting noble metals include pre-supporting activated alumina and then supporting the noble metal in the form of a solution by chemical adsorption or physical adsorption, or impregnating activated alumina in advance, dispersing the support, and then applying the activated alumina to the support. The most common method was to support it on In these methods, the supported noble metal exists on activated alumina in a highly dispersed state. However, this highly dispersed noble metal is in a state where the reduction reaction of SOx to H ₂ S is very likely to occur. Therefore, the present inventors investigated the degree of dispersion of noble metals and H ₂ S
As a result of examining the amount of H 2 S generated, it was found that H ₂ S generation could be suppressed by specifying the method of supporting H 2 S on activated alumina. That is, activated alumina supports at least one noble metal of platinum or palladium in a range of 5 to 30% by weight and rhodium in a range of 1 to 20% by weight, and cerium oxide and activated alumina that does not support anything are combined. It has been found that a catalyst obtained by supporting an aqueous slurry on a honeycomb structure suppresses H ₂ S generation compared to conventional catalysts, and does not impair catalyst performance at all. If the loading rate of noble metals on activated alumina is too high, the catalyst performance will be impaired, and if it is too low, the effect of suppressing H ₂ S generation cannot be obtained. The amount of catalyst composition supported on the honeycomb structure is 1 to 20% of activated alumina (a) per 1 carrier.
g, activated alumina (c) in the range of 50 to 200 g, and cerium oxide (b) in the range of 10 to 150 g as _CeO2 . Furthermore, in order to suppress even the slightest amount of H ₂ S emitted from the H ₂ S generation suppressing catalyst obtained in this way,
As a result of repeated research, it was discovered that nickel oxide is effective as an additive to capture the H ₂ S that is generated. Proposals have been made to add nickel or nickel oxide to improve catalyst performance. However, when nickel oxide is contained on the catalyst in a highly dispersed state, it has no effect of trapping H ₂ S, and on the contrary acts to promote H ₂ S generation. However, when nickel oxide (d) and at least one noble metal of platinum or palladium are mixed
By combining activated alumina (a), cerium oxide (b) and activated alumina (c) on which rhodium is supported in a range of 1 to 20% by weight, reacts with the generated H ₂ S, Almost all _H2S
It was discovered that it could collect . The amount of nickel oxide added depends on the amount of H ₂ S generated by the catalyst before addition, but is preferably 1 to 30 g per carrier. If it is too small, there is no effect, and if it is too large, it will adversely affect the catalyst activity. The method for adding nickel oxide depends on the properties of the nickel oxide, but it can be added during or after slurrying the catalyst composition. Furthermore, after supporting the catalyst material, it is also possible to additionally support the nickel oxide. <Examples> The present invention will be explained in more detail with reference to Examples below, but it goes without saying that the present invention is not limited only to these Examples. Example 1 A catalyst was prepared using a commercially available cordierite monolith carrier (manufactured by Nippon Glass). The monolith carrier used had a cylindrical shape with a cross section of 33 mmφ and a length of 76 mm L, with a cross section of about 400 gas flow cells per square inch, and a volume of about 65 mL. A mixture of a nitric acid aqueous solution of dinitroammineplatinum containing 1.5 platinum (Pt) and a rhodium nitrate aqueous solution containing 0.3 g of rhodium (Rh) was mixed with 7.5 g of activated alumina with a specific surface area of 100 m ² /g, and After drying, bake in air at 400℃ for 2 hours.
An alumina powder containing 16.1% by weight Pt and 3.2% by weight Rh was prepared. This Pt-Rh-containing alumina and activated alumina with a specific surface area of 100 m ² /g and no noble metal support
127 g, 15 g of nickel oxide (NiO, reagent), and 75 g of commercially available cerium oxide powder (manufactured by Nissan Kisensu)
An aqueous slurry for coating was prepared by wet-milling the mixture in a ball mill for 20 hours. After the monolithic carrier was immersed in this coating slurry and taken out, the excess slurry in the cells was blown with air to remove clogging in all the cells. Then, it was dried at 130°C for 3 hours to obtain catalyst (a). This catalyst contains 90g of alumina, 50g of cerium oxide, 1.0g of Pt, 0.2g of Rh, and 10g of nickel oxide.
It was being carried. Comparative Example 1 A mixture of a nitric acid aqueous solution of dinitroamine platinum containing 1.5 g of Pt and a rhodium nitrate aqueous solution containing 0.3 g of Rh was mixed with 150 g of the activated alumina used in Example 1, dried, and heated at 400°C for 2 hours. By firing for a period of time, an alumina powder carrying 1.0% by weight of Pt and 0.2% by weight of Rh was obtained. This powder and 75 g of cerium oxide similar to that used in Example 1 were ground in a ball mill for 20 hours to prepare an aqueous slurry for coating. Catalyst (b) was obtained using this slurry. Each catalyst supported 100 g of alumina, 50 g of cerium oxide, 1.0 g of Pt, and 0.2 g of Rh. Comparative Example 2 In Comparative Example 1, 135 g of activated alumina supporting Pt and Rh was used, and 1.11% by weight of Pt and
A catalyst (c) was prepared in the same manner as below by dispersing and supporting 0.22% by weight of Rh and adding 15 g of the nickel oxide used in Example 1. This catalyst contains 100g of alumina, 50g of cerium oxide, 1.0g of Pt, 0.2g of Rh, and 10g of nickel oxide.
was carried. Comparative Example 3 A nitric acid aqueous solution of dinitrodiamine platinum containing 1.5 g of Pt and an aqueous rhodium nitrate solution containing 0.3 g of Rh were mixed, and 136 g of the activated alumina used in Example 1 was added and mixed. Dry, bake,
Alumina powder containing 1.1% by weight of Pt and 0.22% by weight of Rh was prepared. Next, this powder and nickel nitrate (Ni
(NO ₃ ) ₂ 6H ₂ O) was mixed with an aqueous solution of 57.8 g dissolved in 150 ml of pure water, thoroughly dried, and then calcined in air at 500° C. to prepare alumina containing Pt, Rh, and Ni. Alumina powder prepared by the above procedure,
75.5 g of cerium oxide was wet-pulverized in a ball mill to prepare an aqueous slurry, and then the monolithic support used in Example 1 was immersed in the slurry, followed by drying in the same manner as in Example 1, and the catalyst
I got (d). Each catalyst carried 90 g of alumina, 50 g of cerium oxide, 1.0 g of Pt, 0.2 g of Rh, and 10 g of nickel oxide. <Effect of the invention) Four types of catalysts prepared in Example 1 and Comparative Examples 1 to 3
The amount of H ₂ S generated and catalyst performance were tested. The amount of H ₂ S generated is determined by using a commercially available electronically controlled engine (4 cylinders, 1800 c.c.), adding approximately 0.1 wt% of thiophene (reagent) as sulfur (S) to gasoline, and reducing the exhaust gas. This was carried out by introducing H 2 S into the catalyst and measuring the H ₂ S concentration in the catalyst outlet gas. The SOx concentration in engine exhaust gas was measured using the JIS-K0108 methylene blue absorbance method. The test method for H ₂ S generation is to first warm up the engine, set the catalyst inlet temperature to 500°C, set the A/F to 15.5, continue for 15 minutes, then set the A/F to 13.0 and at the same time sample the gas for H ₂ S measurement. starting with 1 every minute
, by doing it for 5 minutes. Table 1 shows the measurement results for catalysts (a) to (d). The catalyst exhaust gas purification performance is based on a commercially available electronically controlled engine (8 cylinders, 4400
cc), each catalyst was filled into a multi-converter and a durability test was conducted. The engine is in steady operation 60
It was operated for 50 hours under the condition that the catalyst inlet temperature was 800℃ at a steady state by operating in the mode of 2 seconds and 6 seconds of deceleration (fuel cut during deceleration). Performance evaluation of the catalyst after durability was performed using the same engine used for H ₂ S measurement at a space velocity of 90000 Hr ^-1 . The ternary characteristics are catalyst inlet temperature 400℃, A/
CO when the average A/F was changed from 15.1 to 14.1 while oscillating F at ±0.5, 1Hz,
Evaluation was made by measuring the purification rate of HC and NO.
In addition, for performance evaluation at low temperatures, the A/F was fixed at 14.6 and the catalyst inlet temperature was adjusted under the same operating conditions as above.
The temperature was varied from 200℃ to 450℃, the purification rate was determined, and the performance was evaluated. The results of the performance evaluation are shown in Table 1.

[Table] *1 Three-way characteristics at crossover point *2 Temperature at 50% purification rate of each component in exhaust gas

Claims (1)

[Claims] 1. Activated alumina (a), cerium oxide (b), on which 5 to 30% by weight of at least one noble metal selected from platinum and palladium and 1 to 20% by weight of rhodium are supported; 1. A catalyst for exhaust gas purification that suppresses hydrogen sulfide generation, characterized in that a catalyst composition comprising activated alumina (c) and nickel oxide (d) is coated and supported on a carrier having a honeycomb structure. 2 1 of the activated alumina (a) per 1 of the carrier
~20g, 50~200g of the activated alumina (c), 10~150g of the cerium oxide (b) as _CeO2 , and 1~30g of the nickel oxide (d). Catalysts as described.