JPH0575457B2 - - 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-based particles such as sulfates, and high molecular weight hydrocarbon particles in liquid or solid form) in the exhaust gas of diesel engines has become a problem in terms of environmental health. It is in. 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, the methods for removing these fine particles are as follows:
There are two main methods as follows. 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. There is a method of regenerating the filter by burning the particulates, and another method is to carry a catalyst material on a carrier with 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 is, the higher the pressure drop will be and the faster the frequency of regeneration will be, which will be troublesome 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 it can effectively ignite and burn accumulated particulates within the temperature that can be 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, diesel engine exhaust gas purification has good combustion behavior of particulates within the diesel engine exhaust gas temperature range found during normal city driving, with a gradual rise in pressure drop, and where combustion regeneration occurs promptly when a predetermined exhaust gas temperature is reached. The purpose is to provide a catalyst for That is, the present invention is specified as follows. (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) at least one kind of tungstate, (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium and palladium, and (c) barium molybdate are dispersed and supported. A diesel engine exhaust gas purification catalyst featuring: (2) Compounds selected from groups (a) and (b) in molar ratio
The catalyst according to claim 1, wherein (a)/(b)=4 to 120. (3) The fire-resistant three-dimensional structure is ceramic foam,
3. The catalyst according to claim 1, which is a wire mesh, a metal foam, or a plugged ceramic honeycomb. (4) At least one of (a) A tungstate salt, (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium, and palladium, and (c) barium molybdate are dispersed and supported. A method for producing a catalyst for purifying diesel engine exhaust gas, which is characterized by heat treatment in air at a temperature in the range of 700 to 1000°C. The effects achieved by the catalyst according to the present invention will be explained in more detail below. As a result of the above, the present inventors found that the exhaust gas temperature from the diesel engine is extremely low, and the exhaust gas temperature during city driving does not reach 450°C even at the manifold outlet, so the combustion behavior of carbon-based particulates is good even below 350°C. The pressure equilibrium temperature (the temperature at which the pressure rise due to the accumulation of fine particles is equal to the pressure drop due to combustion of fine particles) is low at 330 to 350℃, and the accumulated fine particles are at 400℃.
We have discovered a catalyst system that has excellent properties, such as a catalyst that starts combustion at a temperature of 600 degrees Celsius, and has very low sulfate production of 5% or less even at 600 degrees Celsius. 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, in the case of a catalyst containing a platinum group element, although it has the ability to oxidize carbon monoxide (CO) and hydrocarbons (HC), it also oxidizes SO ₂ and produces sulfate, which is not desirable. However, even in the low temperature range, some of the combustible components in the fine particles burn, so the pressure drop increases slowly, and the pressure equilibrium 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. According to the findings of the present inventors, in the above catalyst component dispersedly supported on an inorganic substrate, tungstate of group (a) and barium molybdate of group (c) are combined with platinum group elements of group (b). It acts extremely closely against the metal, and exhibits the effect of effectively suppressing the sulfate-generating ability that the metal originally has. In particular, the effect is fully exhibited in catalysts whose final calcination is carried out at a high temperature of 700 to 1000°C. Moreover, the proportion of their coexistence is 4 in the molar ratio of (a)/(b).
When in the range of ~120, preferably in the range of 6 to 90,
Moreover, the supported amount of tungstate of group (a) is 8 to 8.
120g/l-carrier, preferably in the range of 10-100g/l-carrier, and the amount of supported platinum metal element of group (b) is
0.1-4.0g/- carrier, preferably 0.3-3.0g/
- It was found that the sulfate generation ability was suppressed the most when the carrier was in the range, and the combustion behavior of particulate matter was good. And again (c)
It has been found that the combustion performance of particulate matter is further improved by combining barium molybdate of the group in the range of 8 to 120 g/-carrier, preferably 10-100 g/-carrier. In the present invention, tungstate refers to tungstate of metals such as lithium, sodium, potassium, cesium, magnesium, calcium, strontium, barium, lanthanum, cerium, iron, cobalt, nickel, copper, silver, bismuth, tin, and lead. means acid salt. If the molar ratio of (a)/(b) is less than 4, the effect of suppressing sulfate formation will be poor, and under the condition of diesel exhaust gas at 600°C, more than 10% of SO ₂ will be produced.
Shows the conversion rate to SO ₃ . If the molar ratio of (a)/(b) is greater than 120, the combustion performance of fine particles at low temperatures (below 300°C) will deteriorate, and the pressure drop per unit time will increase, which is a characteristic of platinum group elements. This is undesirable because it interferes with the combustion performance of SOF (soluble organic fraction) at low temperatures. In addition, the firing temperature after supporting components (a) and (b) was set to 700℃.
When firing at temperatures below 1,000°C, there is a tendency for the conversion rate of SO ₂ to SO ₃ to increase, and when firing at temperatures above 1000°C, the self-regeneration temperature (pressure drop in the catalyst layer due to combustion of particulates decreases). temperature) becomes high, which is undesirable. 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 inorganic base may be formed into a three-dimensional structure having a gas filter structure (for example, ceramic foam, wire mesh, etc.).
A support layer is formed by slurrying a metal foam (metal foam, plugged ceramic honeycomb) and washing-coating it, and contains at least one metal selected from the group consisting of platinum group elements such as platinum, rhodium, and palladium. The compound is supported in the form of an aqueous or organic solvent (for example, alcohol) solution or dispersion by an impregnation or soaking method, and dried or fired at 300 to 500°C after drying. Next, a water-soluble or organic solvent-soluble salt of tungsten is impregnated and supported, dried and then fired at 300 to 500°C. Next, the metals (Li, Na, K, Cs, Mg, Ca, Sr, Ba, La,
A water-soluble or organic solvent-soluble salt of Ce, Fe, Co, Ni, Cu, Ag, Bi, Sn, Pb) is impregnated and supported, and after drying, it is fired at 700 to 1000°C for 1 to 5 hours. Then, each compound is impregnated and supported in the form of a water-soluble compound to form barium molybdate, and these molybdates are dispersed and formed on the catalyst.
It may be fired at 700-1000°C. The above raw material compounds include oxides, hydroxides, nitrates, carbonates, phosphates, sulfates, halides,
It is appropriately selected from inorganic compounds such as metal salts, carboxylic acid salts such as acetate and formate, and organic compounds such as complex compounds, and it is preferable to use compounds that are easily soluble in water or alcoholic organic solvents. Furthermore, the order in which the catalyst components are supported may be changed. Furthermore, the tungstate prepared and ground in advance and the powder containing the platinum group metal in the inorganic base material are mixed into a slurry using a wet mill, wash coated, and dried.
The catalyst may be completed by firing at 700 to 1000°C. A compromise between these methods may 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. EXAMPLES The present invention will be explained in more detail below with reference to Examples and Comparative Examples. Example 1 353 g of ammonium paramolybdate was dissolved in water from step 2, and 416.5 g of barium chloride was added in advance.
was poured into an aqueous solution of step 2 with stirring, the resulting precipitate was overwashed, dried at 150℃ for 5 hours, and calcined at 500℃ for 2 hours to obtain about 530g of
I got BaMoO ₄ powder. Add alumina powder to 800ml of a mixed solution of dinitrodiamine platinum nitric acid solution containing 9.0g as Pt and rhodium nitrate aqueous solution containing 0.9g as Rh.
700 g was added, mixed well, dried at 150°C for 5 hours, and then calcined at 500°C for 2 hours to obtain alumina powder containing Pt and Rh. 853 g of a commercially available ammonium metatungstate aqueous solution (solution containing 50% by weight as WO ₃ , manufactured by Japan Inorganic Chemical Industry Co., Ltd.) was taken and diluted to 1 with water. Separately, 470 g of barium acetate Ba(C ₂ H ₃ O ₂ ) ₂ was taken and dissolved in water to obtain 1. Mix both solutions with stirring and add barium tungstate BaWO ₄
A white precipitate was obtained. The precipitate is overwashed,
It was dried at 150°C for 5 hours and fired at 500°C for 2 hours. 472.6 g of the obtained BaWO ₄ powder and the BoMoO ₄
208.1 g of powder and 710 g of alumina powder containing Pt and Rh are thoroughly mixed in a ball mill, then slurried in a wet mill, supported on cordierite foam 1.7, and after shaking off excess slurry, heated at 150°C. After drying for 3 hours at 750° C., it was fired for 2 hours to obtain a cordierite foam having an alumina coat layer containing Pt, Rh, BaMoO ₄ and BaWO ₄ . The supported amounts of Pt and Rh in the obtained catalyst are respectively
0.9g/-carrier, 0.09g/-carrier,
The amount of BaMoO ₄ supported is 20.8g/− carrier, BaWO ₄
The amount supported was 47.3 g/-carrier. The composition of the finished coating layer is 50.3% by weight of Al ₂ O ₃ and BaMoO ₄
14.9% by weight, BaWO ₄ 34.0% by weight, Pt+Rh (Pt/
Rh=10/1) was 0.72% by weight. Comparative Example 1 Alumina powder was added to 800 ml of a mixed solution of dinitrodiamine platinum nitric acid solution containing 9.0 g as Pt and rhodium nitrate aqueous solution containing 0.9 g as Rh.
700 g was added, mixed well, dried at 150°C for 5 hours, and then calcined at 500°C for 2 hours to obtain alumina powder containing Pt and Rh. 853 g of a commercially available ammonium metatungstate aqueous solution (solution containing 50% by weight as WO ₃ , manufactured by Japan Inorganic Chemical Industry Co., Ltd.) was taken and diluted to 1 with water. Separately, 470 g of barium acetate Ba(C ₂ H ₃ O ₂ ) ₂ was taken and dissolved in water to obtain 1. Mix both solutions with stirring and add barium tungstate BaWO ₄
A white precipitate was obtained. The precipitate is overwashed,
It was dried at 150°C for 5 hours and fired at 500°C for 2 hours. 472.6 g of the BaWO ₄ powder and 710 g of the above alumina powder containing Pt and Rh were thoroughly mixed in a ball mill, then slurried in a wet mill, supported on cordierite foam 1.7, and the excess slurry was shaken off. After drying at 150°C for 3 hours, it was fired at 750°C for 2 hours to obtain a cordierite foam having an alumina cord layer containing Pt, Rh, and _BaWO4 . The amount of supported Pt and Rh in the obtained catalyst was 0.9 each.
g/- carrier, 0.09 g/- carrier,
The amount of BaWO ₄ supported was 47.3 g/-carrier.
The composition of the finished coating layer is Al ₂ O ₃ 59.2% by weight,
BaWO ₄ 40% by weight, Pt+Rh (Pt/Rh=10/1)
It was 0.84% by weight. Comparative Example 2 Commercially available cordierite foam (bulk density 0.35g/
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, it was fired at 500°C for 2 hours to obtain a cordierite foam having an alumina coat layer. The amount of alumina supported was 70.1 g/-carrier. Next, potassium tungstate (K ₂ WO ₄ ) 571.4
immersing the foam in an aqueous solution of 2 containing g;
After shaking off the excess aqueous solution, it was dried at 150°C for 3 hours, and then baked at 750°C for 2 hours. The amount of K ₂ WO ₄ supported on the obtained catalyst was 40 g/-
It was a carrier. The composition of the finished coat layer was 63.7% by weight of alumina and 36.3% by weight of K ₂ WO ₄ . Example 2 A catalyst evaluation test was conducted on the catalysts obtained in Example 1 and Comparative Examples 1 and 2 using a 4-cylinder diesel engine with a displacement of 2300 c.c. Particulate capture was carried out for approximately 2 hours at an engine speed of 2500 rpm and a torque of 4.0 kg・m, and then the torque was reduced to 0.5 kg/m.
The change in pressure drop in the catalyst layer is continuously recorded by raising the pressure at Kg/m intervals every 5 minutes, and as the exhaust gas temperature rises on the catalyst, the pressure rises due to the accumulation of particles and the pressure drops due to combustion of the particles. The temperature at which Te becomes equal to the temperature at which ignition and combustion occur and the pressure drop rapidly decreases
I asked for Ti. In addition, when capturing particulates at 2500 rpm and a torque of 4.0 kg・m, the change in pressure drop over time is calculated from the chart by calculating the change in pressure drop per hour by ΔP.
The value of (mmHg/H) was determined. In addition, the conversion rate of SO ₂ to SO ₃ is determined by the exhaust gas temperature 600
Calculated in °C. The conversion rate of SO ₂ is measured using the inlet gas, and the SO ₂ concentration of the outlet gas is measured using a non-dispersive infrared analyzer (NDIR method).
The conversion rate (%) of SO ₂ was determined using the following calculation formula. _SO2 conversion rate (%) = Inlet _SO2 concentration (ppm) ^- Outlet _SO2
Concentration (ppm)/Inlet SO ₂ concentration (ppm) x 100 The results are shown in Table 1. 【table】

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) Distributed support of at least one tungstate, (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium, and palladium, and (c) barium molybdate. A catalyst for purifying diesel engine exhaust gas that is characterized by the following characteristics: 2 (a)/(b)= molar ratio selected from groups (a) and (b)
The catalyst according to claim 1, having a molecular weight ranging from 4 to 120. 3 The fire-resistant three-dimensional structure is ceramic foam,
3. The catalyst according to claim 1, which is a wire mesh, a metal foam, or a plugged ceramic honeycomb. 4. 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) at least one type of tungstic acid. A salt, (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium, and palladium, and (c) barium molybdate are dispersed and supported in the air. 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 700°C to 1000°C.