JPH0260374B2 - - 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 consisting of solid carbon particles, sulfur-based particles such as sulfates, and liquid or solid high-molecular-weight hydrocarbon particles) in the exhaust gas of diesel engines has tended to become an environmental health problem. be. 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 particulates, and if the pressure drop increases, the particles may be accumulated in a burner, etc. One method is to regenerate the filter by connecting fine particles, and the other is to carry a catalyst substance on a carrier with this heat-resistant gas filter structure and perform over-operation as well as combustion operation to reduce the frequency of the 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 trivial 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 little ability to generate sulfate (SO ₂ is further oxidized to SO ₃ or sulfuric acid mist), and it is effective at igniting accumulated particulates within the temperatures found in normal engine running conditions. Although there is a demand for a catalyst with good performance for burning, the current situation is that no catalyst has been proposed that satisfactorily meets this condition. 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 Diesel engine exhaust gas purification characterized by dispersing and supporting tungstate and (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium and palladium. Catalyst for use. (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,
The catalyst according to claim 1 or 2, which is a wire mesh, a metal foam, or a plugged ceramic honeycomb. (4) At least one of (a) A tungstate salt and (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium, and palladium are dispersed and supported in the air at 700 to 1000
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 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 a catalyst that uses 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, forced regeneration from an external source is required frequently. It needs to be done expensively and lacks practicality. On the other hand, catalysts containing platinum metal elements 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 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 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, the tungstate of group (a) acts extremely closely on the platinum group elements of group (b), and It exhibits the effect of effectively suppressing the ability of the metal to generate sulfate. 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 60-90,
Moreover, the carrier number of group (a) tungstate is 8 to 8.
120g/one carrier, preferably in the range of 10 to 100g/one carrier, and the supported amount of group (b) platinum metal element is 0.1
It has been found that when the amount is in the range of ~4.0 g/carrier, preferably 0.3 to 3.0 g/carrier, the sulfate generation ability is suppressed the most and the combustion behavior of particulate matter is good. 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 oranic 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 to these. 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 compound containing at least one metal selected from the group of platinum group elements consisting of platinum, rhodium, and palladium, which is formed into a slurry and wash-coated on a metal foam (metal foam, plugged ceramic honeycomb) to form a support layer. is supported by an impregnating or dipping method in the form of a water-soluble or organic solvent (eg, alcohol) solution or dispersion, 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. The above raw material compounds include oxides, cedar oxides, 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 Commercially available cordierite foam (bulk density 0.35 g/
cm ³ , porosity 87.5%, voluminous 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. Next, the foam was immersed in mixed solution 2 of a dinitrodiammine platinum nitric acid solution containing 12.86 g of platinum (Pt) and a rhodium nitrate aqueous solution containing 1.286 g of rhodium (Rh), and the excess solution was shaken off. Cut and dry at 150℃ for 3 hours, then dry at 500℃ for 2 hours.
After firing for a period of time, a cordierite foam having an alumina coat layer containing platinum-rhodium was obtained. Then potassium tungstate (K ₂ WO ₄ )
The foam was immersed in an aqueous solution of 2 containing 571.4 g, and after shaking off the excess aqueous solution, it was heated at 150°C for 3
After drying for an hour, it was fired at 750°C for 2 hours. The supported amounts of Pt and Ph in the obtained catalyst are respectively
0.90g/one carrier, 0.09g/one carrier,
The amount of K ₂ WO ₄ supported was 40 g/carrier. The composition of the finished coating layer was 63.1% by weight of alumina, 36.0% by weight of K ₂ WO ₄ , and Pt+Rh (Pt/Rh=10/
1) It was 0.90% by weight. Example 2 700 g of alumina powder was added to 800 ml of a mixed solution of a dinitrodiammine platinum nitric acid solvent containing 9.0 g of Pt and an aqueous rhodium nitrate solution containing 0.9 g of Rh, mixed well, and dried at 150°C for 5 hours. After that, it was fired at 500°C for 2 hours to obtain alumina powder containing Pt and Rh. Dissolve 400g of potassium tungstate in water.
The entire amount of the Pt and Rh-containing alumina powder was added to the 800 ml solution and mixed well. After drying at 150℃ for 5 hours, baking at 500℃ for 2 hours to form Pt, Rh, K ₂ WO ₄
An alumina powder containing . The powder was made into a slurry using a wet mill, supported on a cordierite foam 1.7, and after shaking off the excess slurry, it was dried at 150°C for 3 hours and then fired at 750°C for 2 hours. The composition of the finished coating layer was 63.6% by weight of alumina, 35.5% by weight of K ₂ WO ₄ , and Pt+Rh (Pt/Rh).
Rh=10/1) 0.90% by weight, and had almost the same composition as Example 1. Example 3 700 g of alumina powder was added to 800 ml of a mixed solution of dinitrodiammine platinum nitric acid solution containing 9.0 g of Pt and rhodium nitrate aqueous solution containing 0.9 g of Rh, mixed well, and dried at 150°C for 5 hours. After that, it was fired 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 to obtain 1. Both solutions were mixed with stirring to obtain a white precipitate of barium tungstate BaWO ₄ . The precipitate was superwashed and heated at 150°C.
The mixture was dried for 5 hours at 500°C and fired for 2 hours at 500°C. 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 coat layer containing Pt, Rh, and _BaWO4 . The amount of Pt and Rh supported in the obtained catalyst was 0.9 each.
g/one carrier, 0.09g/one 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. Example 4 In the same manner as in Example 3 except that BaWo ₄ was replaced with the following tungstate, or the inorganic base Al ₂ O ₃ was replaced with another inorganic base, or Pt + Rh was replaced with another platinum group metal. Example 4-1
The catalysts from Example 4-23 were prepared. The supported amount (g/carrier) of each component of the obtained catalyst was
It was as shown in the table. Example 5 Potassium tungstate in Example 1
A catalyst was prepared in exactly the same manner, except that instead of using 571.4 g, a mixed solution of 285.7 g of potassium tungstate and 571.4 g of an aqueous ammonium metatungstate solution (a solution containing 50% by weight as Wo ₃ ) was used. The supported amounts of Pt and Rh in the obtained catalyst were 0.90g/one carrier, 0.09g/one carrier, respectively.
K ₂ WO ₄ was at 21 g/carrier and WO ₃ at 20 g/carrier. The composition of the finished coating layer is 63.3% by weight of alumina, 18.2% by weight of K ₂ Wo ₄ , and 17.6% by weight of WO _3.
% by weight, Pt+Rh 0.90% by weight. Example 6 Exactly the same as in Example 1, except that the cordierite foam was replaced with a so-called plugged type honeycomb structure in which the adjacent holes on both end faces were alternately closed with a honeycomb structure to allow gas to pass only through the partition walls. The catalyst was prepared by the method. The supported amounts of Pt and Rh in the obtained catalyst were 0.90 g/carrier and 0.09 g/carrier, respectively, and K ₂ WO ₄ was 40 g/carrier.
It was a carrier. The composition of the finished coat layer is 63.0% by weight of alumina, 36.1% by weight of K ₂ WO ₄ , and Pt + Rh.
It was 0.90% by weight. Example 7 Commercially available alumina pellets (3 to 6 mmφ) 1.7
A catalyst was prepared by supporting Pt, Rh, and K ₂ WO ₄ in the order of Example 1 so as to have the composition of the finished coating 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 70 g of Al ₂ O ₃ /
A cordierite foam catalyst supporting one support and 40 g of K ₂ WO ₄ per support was obtained. Comparative Example 2 A catalyst was prepared in the same manner as in Example 1 except that K ₂ WO ₄ was not used, and 70 g of Al ₂ O ₃ /
One carrier, Pt+Rh (Pt/Rh=10/1) 1.0g/
Cordierite foam catalysts each carrying one support were obtained. Comparative Example 3 A catalyst was prepared in the same manner as in Example 1 except that the final calcination temperature was changed to 600°C. Example 8 The catalysts obtained in Examples 1 to 7 and Comparative Examples 1 to 3 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 change 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 increased due to the accumulation of fine particles and the pressure due to the combustion of fine particles. The temperature at which the drop is equal (Te) and the temperature at which ignition and combustion occur and the pressure drop rapidly decreases (Ti) were determined. In addition, the value of ΔP (mmHg/H) was determined by calculating the change in pressure drop over time when capturing fine particles at 2500 rpm and a torque of 4.0 Kg·m from a chart of the amount of change in pressure drop per hour. In addition, the conversion rate of SO ₂ to SO ₃ is determined by the exhaust gas temperature 600
Calculated in °C. The SO ₂ conversion rate was determined by analyzing the SO ₂ concentrations of the inlet gas and outlet gas using non-dispersive infrared analysis (NDIR method), and determining the SO ₂ conversion rate (%) using the following calculation formula. SO ₂ conversion rate (%) = Inlet concentration SO ₂ concentration (ppm) - Outlet S
O ₂ concentration (ppm)/inlet SO ₂ concentration (ppm) x 100 The results are shown in Table 2 below.

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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) It is characterized by dispersively supporting at least one tungstate and (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium, and palladium. Catalyst for diesel engine exhaust gas purification. 2 Compounds selected from groups (a) and (b) in molar ratio
Claim 1 in which (a)/(b)=4 to 120
Catalysts as described. 3. The catalyst according to claim 1 or 2, wherein the refractory three-dimensional structure is a ceramic foam, wire mesh, metal foam, or 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 and (b) at least one metal and/or oxide selected from the group of platinum group elements consisting of platinum, rhodium, and palladium are dispersed and supported in air at a temperature in the range of 700 to 1000°C. A method for producing a catalyst for purifying diesel engine exhaust gas, which is characterized by heat treatment.