JPH0464735B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a catalyst for purifying exhaust gas from a diesel engine and a method for producing the same. Specifically, the present invention relates to a catalyst for purifying diesel engine exhaust gas that has excellent performance in burning and removing carbon-based particulates present in diesel engine exhaust gas, and a method for producing the catalyst. In recent years, particulate matter (mainly composed of solid carbon particles, sulfur particles such as sulfates, and liquid or solid high molecular weight hydrocarbon particles) in diesel engine exhaust gas has become a problem in terms of environmental health. This is because most of these fine particles have particle diameters of 1 micron or less and are easily suspended in the atmosphere and easily taken into the human body through breathing. Therefore, studies are underway to tighten regulations on the emission of these particulates from diesel engines. By the way, methods for removing these fine particles can be broadly classified into the following two methods. One is to use a heat-resistant gas filter (ceramic foam, wire mesh, metal foam, sealed ceramic honeycomb, etc.) to pass through the exhaust gas and capture fine particles, and if the pressure drop increases, they will accumulate in a burner, etc. There is a method of regenerating the filter by burning the fine particles, and another method is to carry a catalyst substance on a carrier having this heat-resistant gas filter structure and perform a combustion operation as well as an over-operation to reduce the frequency of the above-mentioned combustion regeneration. This is a method of increasing the combustion activity of the catalyst to such an extent that regeneration is not necessary. In the former case, the higher the particle removal effect, the faster the pressure drop will rise and the more frequently the regeneration will occur, which will be cumbersome and economically disadvantageous. In comparison, the latter method is considered to be a much better method if a catalytic material that can maintain catalytic activity under the exhaust conditions (gas composition and temperature) of diesel engine exhaust gas is employed. However, the exhaust gas temperature of a diesel engine is much lower than that of a gasoline engine, and since light oil is used as fuel, the amount of SO ₂ in the exhaust gas is also large. Therefore, it has almost no ability to generate sulfate (SO ₂ is further oxidized to SO ₃ or sulfuric acid mist), and even the accumulated particulates can be ignited and combusted well within the temperature obtained under normal engine running conditions. Although there is a demand for a catalyst with performance that satisfies this requirement, the current situation is that no catalyst has been proposed that satisfactorily meets this requirement. The object of the present invention is to provide a catalyst that satisfies this requirement. Specifically, we aim to purify diesel engine exhaust gas in a diesel engine exhaust gas temperature range that occurs during normal city driving, in which the fuel behavior of fine particles is good, the pressure drop rises slowly, the specified exhaust gas temperature is reached, and combustion regeneration occurs quickly. The purpose is to provide a catalyst for That is, the present invention is specified as follows. (1) On a porous inorganic substrate supported on a refractory three-dimensional structure having a gas filter function, or on a porous inorganic substrate formed into a pellet, (a) barium molybdate and (b) a compound of at least one metal selected from the group consisting of platinum, rhodium and palladium. Catalyst for engine exhaust gas purification. (2) The catalyst according to the above item 1, wherein the compound selected from groups (a) and (b) has a molar ratio of (a)/(b) = 5 to 90. (3) The fire-resistant three-dimensional structure is ceramic foam,
3. The catalyst according to 1 or 2 above, which is a wire mesh, a metal foam, or a plugged ceramic honeycomb. (4) (a) barium molybdate on a porous inorganic base supported on a refractory three-dimensional structure having a gas filter function, or on a porous inorganic base formed into a pellet shape; and (b) at least one metal compound selected from the group consisting of platinum, rhodium, and palladium, and (b) a compound of at least one metal selected from the group consisting of platinum, rhodium, and palladium. A method for producing a catalyst for purifying diesel engine exhaust gas, which is characterized by heat treatment at a temperature in the range of °C. The present inventors found that the exhaust gas temperature from a diesel engine is extremely low, and the exhaust gas temperature during city driving does not reach 450℃ even at the manhold exit, so it is 350℃.
The combustion behavior of carbon-based fine particles is good even below, and the applanation temperature (the temperature at which the pressure increase due to accumulation of fine particles is equal to the pressure drop due to combustion of fine particles) is 330~
We have found a catalyst system that has characteristics such that the temperature is as low as 350℃, the combustion of accumulated particulates starts below 400℃, and the pressure drop decreases rapidly, and the formation of sulfate is hardly observed even at 450℃. Normally, with catalysts that use only base metals, the combustion behavior of fine particles is such that the pressure drop increases quickly until a predetermined temperature is reached, and if the regeneration temperature is not reached under normal running conditions, external forced regeneration is required frequently. It needs to be done expensively and lacks practicality. On the other hand, catalysts containing precious metals have the ability to oxidize carbon monoxide (CO) and hydrocarbons (HC), but at the same time
Oxidation of SO ₂ also occurs, producing sulfate, which is undesirable. However, even in the low temperature range, the combustible components of the fine particles are partially combusted, so the increase in pressure drop is gradual, and the applanation temperature is also lower than when only base metals are used. The present invention provides a catalyst composition that compensates for the above-mentioned drawbacks and does not impair the advantages of each catalyst component. Furthermore, although it is generally believed that molybdenum has high scattering properties and is prone to activity deterioration, the present inventors believe that component (a) can suppress the scattering of molybdenum and have good combustion behavior for fine particulate matter. The present invention has been completed based on the discovery that the sulfate-producing ability of the sulfate-generating ability of the sulfate-producing compound is significantly suppressed. According to the findings of the present inventors, in the above catalyst component dispersedly supported on an inorganic substrate, barium molybdate or lanthanum molybdate of group (a) acts extremely closely on the noble metal of group (b), and It exhibits the effect of effectively suppressing the sulfate generation ability of precious metals. Especially the final firing is 700~
The effect is fully demonstrated in catalysts that are processed at a high temperature of 1000°C. Moreover, the ratio of their coexistence is 5 in the molar ratio of (a)/(b).
90, preferably 8 to 60, and the supported amount of barium molybdate or lanthanum molybdate of group (a) is 8 to 120 g/- carrier,
Preferably it is 10 to 100 g/-carrier, and when the amount of noble metal of group (b) supported is in the range of 0.1-4.0 g/-carrier, preferably 0.3-3.0 g/-carrier, the sulfate-forming ability is most suppressed, Moreover, it was discovered that the combustion behavior of particulate matter is good. In the present invention, the above-mentioned molybdate is specified, but other molybdate metal salts,
For example, it has been recognized that potassium molybdate, lithium molybdate, vanadium molybdate, etc. are undesirable due to the high scattering of molybdenum, and it has also been recognized that strontium molybdate, cobalt molybdate, etc. have poor combustion behavior of particulate matter. Ta. The inorganic bases used in the present invention include alumina, silica, titania, which are usually used as carrier bases,
Zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia and the like are preferably used, but are not limited thereto. The specific method for preparing the catalyst according to the present invention is as follows. As an example, the above-mentioned inorganic substrate is slurried into a three-dimensional structure having a gas filter structure (for example, ceramic foam, wire mesh, metal foam, sealed type ceramic honeycomb) and coated with wash to form a support layer. A compound containing at least one metal selected from the group consisting of platinum, rhodium, and palladium is supported in the form of a water-soluble or organic solvent (alcohol etc.) solution or dispersion by an impregnation or dipping method. Dry or after drying 300~
Fire at 500℃. Next, a water-soluble or organic solvent-soluble salt of molybdenum is impregnated and supported, and after drying,
Bake at 300-500℃. The fired product is impregnated with a water-soluble or organic solvent-soluble salt of barium or a water-soluble or organic solvent-soluble salt of lanthanum, dried, and then fired at 700 to 1000°C for 1 to 5 hours. The above compounds include inorganic compounds such as oxides, hydroxides, nitrates, carbonates, phosphates, sulfates, halides, and metal salts, and organic compounds such as carboxylates and complex compounds such as acetates and formates. It is preferable to use one that is easily soluble in water or an alcoholic organic solvent. Furthermore, the order in which the catalyst components are supported may be changed. Furthermore, it is also possible to adopt a method in which the inorganic base material and each catalyst component group are mixed in advance, and then washed, dried, and calcined to obtain a finished catalyst, and a compromise between these methods can also be adopted as appropriate. The form of the catalyst is not limited to the three-dimensional structure described above, but the catalyst component may be supported on a pellet-like structure shown as an inorganic base. The present invention will be described in more detail below with examples and comparative columns. Example 1 Commercially available cordierite foam (bulk density 0.35 g/
cm ³ , porosity 87.5%, volume 1.7), 1 kg of alumina powder is slurried and supported using a wet mill,
After shaking off the excess slurry and drying at 150°C for 3 hours, the product was fired at 500°C for 2 hours to obtain a cordierite foam having an ammina coat layer. Next, a nitric acid solution of dinitrodiammine platinum containing 12.86 g as platinum (Pt) and rhodium (Rh)
The foam was immersed in mixed solution 2 of an aqueous rhodium nitrate solution containing 1.286 g of rhodium, the excess solution was shaken off, and the foam was dried at 150°C for 3 hours and then fired at 500°C for 2 hours to form a foam containing platinum-rhodium. A cordierite foam with an alumina coat layer was obtained. Next, the foam was immersed in aqueous solution 2 containing 350.6 g of ammonium paramolybdate, the excess aqueous solution was shaken off, and the foam was dried at 150°C for 3 hours and then fired at 500°C for 2 hours to produce molybdenum (Mo)-Pt. A cordierite foam with an alumina coat layer containing -Rh was obtained. Next, aqueous solution 2 containing 507.2 g of barium acetate
The foam was immersed, the excess aqueous solution was shaken off, the foam was dried at 150°C for 3 hours, and then fired at 750°C for 2 hours to form barium molybdate (BaMoO ₄ ).
A cordierite foam with an alumina coat layer containing BaMoO ₄ , Pt, and Rh was obtained. The amount of Pt and Rh supported at this time was 0.90g/0.90g/
- carrier, 0.09 g/- carrier, and the amount of BaMoO ₄ supported was 41.3 g/- carrier. The composition of the finished coating layer was 62.3% by weight of alumina, 36.8% by weight of BaMoO ₄ , and Pt+Rh (Pt/Rt=10/
1) It was 0.89% by weight. Example 2 353 g of ammonium paramolybdate was dissolved in the ion-exchanged water from step 2, and added while stirring into an aqueous solution in which 41.65 g of barium chloride had been previously dissolved in the ion-exchanged water from step 2, and the precipitate formed was overwashed. Dry at 150℃ for 5 hours, then dry at 500℃ for 2 hours.
About 530g of BaMoO ₄ powder was obtained by firing for a time. 472g of this powder and 800g of alumina powder were thoroughly mixed in a ball mill, then slurried in a wet mill, supported on cordierite foam 1.7, excess slurry was shaken off, dried at 150℃ for 3 hours, and then dried at 500℃ for 3 hours.
After firing for a period of time, a cordierite foam with an alumina coat layer containing BaMoO ₄ was obtained. Next, Pt and Rh were supported by a method according to Example 1.
After drying at 150°C for 2 hours, it was fired at 750°C for 2 hours. The composition of the finished coat layer at this time is Example 1
The composition was almost the same. Example 3 A catalyst was prepared in the same manner as in Example 1 except that an aqueous solution of lantum nitrate La(NO ₃ ) _{3.6H 2} O was used instead of _{barium acetate} .
-Support, lanthanum molybdate [La ₂ (MoO ₄ ) ₃ ]
The supported amounts of Pt and Rh were 35.1 g/-carrier and 0.90 g/-carrier and 0.09 g/-carrier, respectively. The composition of the finished coating layer is Al ₂ O ₃ 66.0% by weight,
La ₂ (MoO ₄ ) ₃ 33.1% by weight, Pt + Rh (Pt/Rh=10/
1) It was 0.94% by weight. Example 4 A catalyst was prepared in the same manner as in Example 2 except that Pd was used instead of Pt. The composition of the finished coat layer is 62.3% by weight of Al ₂ O ₃ ;
BaMoO ₄ 36.8% by weight, Pd+Rh (Pd/Rh=10/1)
It was 0.89% by weight. Example 5 Exactly the same as in Example 1 except that the cordierite foam was replaced with a sealed type honeycomb structure in which the adjacent holes on both end faces were closed differently to allow gas to pass through only through the partitions. A catalyst was prepared by the method described in . Example 6 Commercially available alumina pellets (3 to 6 mmφ) 1.7
A catalyst was prepared by supporting Pt, Rt, and BaMoO ₄ so as to have the composition of the finished coat layer of Example 1. Comparative Example 1 A catalyst was prepared in the same manner as in Example 1 except that Pt and Rh were not used, and a cordierite foam catalyst was obtained in which 70 g/- of Al ₂ O ₃ and 41.3 g/- of BaMoO ₄ were supported. Comparative Example 2 A catalyst was prepared in the same manner as in Example 1 except that barium acetate was not used, and 70 g of Al ₂ O ₃ /
-Support, MoO ₃ 20g/-Support, Pt0.90g/
A cordierite foam was obtained in which each of the -carrier and Rh0.09g/-carrier was supported. Comparative Example 3 A catalyst was prepared in the same manner as in Example 1 except that potassium nitrate was used instead of barium acetate, and Al ₂ O ₃ 70 g/- support and K ₂ MoO ₄ 33.1 g/
-Support, Pt0.90g/-Support, Rh0.09g/-
Cordierite foams each supported on a carrier were obtained. Comparative Example 4 A catalyst was prepared in the same manner as in Example 1 except that potassium molybdate was used instead of ammonium paramolybdate and barium acetate was not used, and a catalyst having the same composition as Comparative Example 3 was obtained. Comparative Example 5 A catalyst was prepared in the same manner as in Example 1 except that the final calcination temperature was changed to 600°C. Example 7 The catalysts obtained in Examples 1 to 6 and Comparative Examples 1 to 5 were evaluated using a 4-cylinder diesel engine with a displacement of 2300 c.c. Particulate capture was carried out for approximately 2 hours under the conditions of engine rotation speed 2500 rpm and torque 4.0 kg・m, and then the torque was
The pressure drop in the catalyst layer was continuously recorded by increasing the pressure at 0.5 kg/m intervals every 5 minutes, and as the exhaust gas temperature rose on the catalyst, the pressure increase due to the accumulation of fine particles and the pressure drop due to combustion of the fine particles were measured. The temperature (Te) at which these are equal to the temperature (Ti) at which ignition and combustion occur and the pressure drop rapidly decreases was determined. Also 2500rpm, torque
The value of ΔP (mmHg/H) was determined by calculating the change in pressure drop per hour from a chart when capturing fine particles at 4.0 kg·m. In addition, the conversion rate of SO ₂ to SO ₃ was determined at an exhaust gas temperature of 450℃.
I asked for it. The conversion rate of SO ₂ is determined by the inlet gas and outlet gas.
The SO ₂ concentration was analyzed using a non-dispersive infrared analyzer (NDIR method), and the conversion rate (%) of SO ₂ was determined using the following formula. SO ₂ conversion rate (%) = Inlet SO ₂ concentration (ppm) − Outlet SO ₂ concentration (
ppm)/Inlet SO ₂ concentration (ppm) x 100 The results are shown in Table 1 below. Next, for each catalyst, 2500 rpm, torque 14 Kg・m
Similar to the above test, SO ₂
The conversion rate, Te, and Ti were determined, and the Mo residual rate of the tested catalyst was determined by fluorescent X-ray analysis. The results are shown in Table-2.

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Claims (1)

[Scope of Claims] 1. On a porous inorganic base supported on a refractory three-dimensional structure having a gas filter function, or on a porous inorganic base formed into a pellet, (a) (b) a compound of at least one metal selected from the group consisting of barium molybdate and lanthanum molybdate; and (b) at least one metal selected from the group consisting of platinum, rhodium, and palladium. A catalyst for purifying diesel engine exhaust gas. 2. The catalyst according to claim 1, wherein the compounds selected from groups (a) and (b) have a molar ratio of (a)/(b)=5 to 90. 3 The fire-resistant three-dimensional structure is ceramic foam,
The catalyst according to claim 1 or 2, which is a wire mesh, a metal foam, or a plugged ceramic honeycomb. 4. (a) Barium molybdate and molybdenum are placed on a porous inorganic base supported on a refractory three-dimensional structure having a gas filter function, or on a porous inorganic base formed into a pellet. A compound of at least one metal selected from the group consisting of lanthanum acid and (b) at least one metal selected from the group consisting of platinum, rhodium, and palladium is dispersed and supported in the air at 700 to 1000°C. A method for producing a catalyst for purifying diesel engine exhaust gas, which is characterized by heat treatment at a temperature within a range.