JPH0462778B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a catalyst for purifying exhaust gas from a diesel engine. Specifically, the present invention has a low ability to convert sulfur dioxide (hereinafter referred to as SO ₂ ) present in the exhaust gas into sulfur trioxide (hereinafter referred to as SO 3), and has a low ability to convert sulfur dioxide (hereinafter referred to as SO ₃ ) present in the exhaust gas, and The present invention relates to a catalyst for purifying diesel engine exhaust gas that has excellent performance in complete combustion of hydrocarbons (hereinafter referred to as HC) and also has excellent performance in burning and removing carbon-based particulates coexisting in the exhaust gas. In recent years, particulate matter (mainly composed of solid carbon particles, sulfur-based 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, consideration is being given to tightening 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, the particles may be accumulated in the burner, etc. There is a method of regenerating the filter by burning particulates, and another method is to support a catalyst substance on a carrier with this heat-resistant gas filter structure and perform a combustion operation as well as over-operation to reduce the frequency of the combustion regeneration. This method increases the combustion activity of the catalyst to such an extent that it is not necessary. In the former case, the higher the particle removal effect, the faster the pressure drop increases and the frequency of regeneration increases, which is cumbersome and economically disadvantageous. In comparison, the latter method is considered to be a much better method if a catalytic material is employed that can maintain catalytic activity under the exhaust conditions (gas composition and temperature) of diesel engine exhaust gas. However, the exhaust gas temperature of diesel engines is generally in the range of 200 to 450 degrees Celsius, which is much lower than that of gasoline engines, and since diesel engines use light oil as fuel, the gas contains SO ₂
Despite the need for a catalyst that has a large amount of CO and HC at low temperatures and has the ability to oxidize SO ₂ to SO ₃ as low as possible, it has so far met these requirements. At present, no catalyst has been proposed that does this. The object of the present invention is to provide a catalyst that can satisfy this requirement. Specifically, solid carbon,
We have developed a so-called highly selective diesel engine exhaust gas purification catalyst that has high performance in combustion of carbon-based fine particles, which are liquid or solid hydrocarbon fine particles, and complete oxidation of CO and HC, and low oxidation ability from SO ₂ to SO ₃ . The purpose is to provide. That is, the present invention is specified as follows. (1) A heat-resistant inorganic carrier, alumina, a complex oxide or mixed oxide consisting of titanium and phosphorus, and chromium, manganese, iron, cobalt, nickel, copper, zinc, tungsten, alkali metals,
Forming a mixed oxide film comprising at least one metal oxide selected from the group consisting of alkaline earth metals and rare earth elements, and dispersing and supporting at least one noble metal element in the film. A catalyst for purifying exhaust gas from diesel engines. (2) The catalyst according to (1) above, wherein the carrier has a gas filter structure. (3) The total amount of precious metal elements is 0.2 to 1 per carrier.
The above (1) characterized in that 3.0g is supported.
or the catalyst described in (2). The present inventors have already reported on a catalyst for purification of general industrial exhaust gas, a catalyst for internal combustion, and a method for producing the same, which have a low conversion ability from SO ₂ to SO ₃ even in the presence of oxygen, yet have an excellent ability to fully oxidize CO and HC, and a method for producing the same. −
In the specification of Japanese Patent No. 15256, a catalyst system was proposed in which noble metals such as platinum and palladium were combined with a carrier consisting of alumina, titanium, and phosphorus. However, this catalyst composition is supported on a heat-resistant filter structure.
It has been found that the combustion performance of particulates in diesel engine exhaust gas is extremely insufficient, and the catalyst composition contains chromium, manganese, iron, cobalt, nickel, copper, zinc, tungsten, alkali metals, especially sodium and potassium, etc. The above drawbacks can be improved by adding and mixing at least one metal oxide selected from the group of alkaline earth metals, especially calcium, barium, magnesium, etc., and rare earths, especially cerium, lanthanum, samarium, etc. This is what we discovered. Light oil, the fuel used in diesel engines, is approximately
It contains 0.3% by weight of sulfur, and its exhaust gas contains a much higher concentration of SO ₂ than gasoline-powered vehicles. Furthermore, the exhaust gas temperature is significantly lower than that of gasoline-powered vehicles, making the catalyst highly resistant to sulfur poisoning. is required. If activated alumina is simply used as a catalyst base material, SO ₃ generated on the catalyst will accumulate irreversibly, impairing the strength of the catalyst adhesion layer, causing peeling, and significantly deteriorating the catalyst activity. Using the catalyst base material of alumina-titanium-phosphorus proposed by et al., the amount of sulfur accumulated was small even under the exhaust gas conditions of diesel cars, and the base material remained stable with little change even after long driving. It was found that catalytic performance was obtained. The composition of the catalyst base material consisting of alumina-titanium-phosphorus includes aluminum and titanium oxide (TiO ₂ ) equivalent to 50 to 98% by weight as alumina (Al ₂ O ₃ ).
Preferably, the constituent components are titanium equivalent to 0.5 to 26% by weight as phosphorus pentoxide (P ₂ O ₅ ) and phosphorus equivalent to 0.5 to 2.4% by weight as phosphorus pentoxide (P 2 O 5 ). The ability to convert SO ₂ to SO ₃ cannot be sufficiently suppressed, and the above-mentioned resistance to sulfur poisoning decreases, making it impossible to withstand long-term use. If the composition ratio of titanium and phosphorus is higher than this range, problems such as impairing the supporting and dispersing properties of metal elements and noble metal elements, which have the ability to increase the combustion efficiency of fine particles, which are other constituent components, will result in insufficient activity performance. There is. If a metal element having the ability to improve the combustion efficiency of particulates as proposed in the present invention is not added, the particulate purification rate is poor and the pressure drop of the gas filter structure increases quickly. Moreover, regarding the purification rate of CO and HC, the catalyst composition consisting of activated alumina and a noble metal, or the combination of alumina-titanium-phosphorus and a noble metal, and the catalyst composition consisting of the constituent components proposed in the present invention have a different purification rate. It has been confirmed that there is no difference in performance. Specifically, a catalyst base material made of activated alumina modified with titanium and phosphorus has a surface area of 20 to 300 m ² /g, preferably 40 m 2 /g.
Using activated alumina with ~200m ² /g, 40-98% by weight as alumina (AI ₂ O ₃ ), preferably
Aluminum equivalent to 50-95% by weight, 0.5-26% by weight as titanium oxide (TiO ₂ ), preferably 1-26% by weight
A metal containing titanium equivalent to 22% by weight and phosphorus equivalent to 0.5 to 24% by weight, preferably 1 to 22% by weight as phosphorus pentoxide (P ₂ O ₅ ) as constituent components, and having the performance of increasing the combustion efficiency of particulates. At least one metal element selected from chromium, manganese, iron, cobalt, nickel, copper, zinc, tungsten, alkali metals, alkaline earth metals, and rare earth elements as an oxide, Activated alumina is contained in a catalyst base modified with titanium and phosphorus in an amount of 0.5 to 50% by weight, preferably 1 to 25% by weight, and the amount of at least one noble metal element used is 0.2 to 3.0% by weight per filter structure.
The catalyst according to the invention is supported in an amount of 0.5 to 2.0 g, preferably in the range of 0.5 to 2.0 g. As the heat-resistant inorganic carrier used in the present invention, pellet carriers such as spherical, cylindrical, ring-shaped, and irregularly shaped carriers are also used, but in particular, carriers having a heat-resistant filter structure, such as ceramic foam,
Wire mesh, metal foam, plugged ceramic honeycomb carriers, and the like are preferably used. These carriers are made of alumina, zirconia, silica, titanium, aluminum, etc., but are not particularly limited as long as they are stable at temperatures of 500° C. or higher. The method for preparing the catalyst of the present invention involves preparing a catalyst base powder containing alumina, titanium, and phosphorus in advance, making the powder into a slurry and depositing it on a filter structure, and burning fine particles on the coating layer. It can be prepared by soaking a soluble salt of a metal element that has the ability to increase the efficiency, drying and firing, and then immersing and supporting a solution of a platinum metal, or by pre-preparing a metal element that has the ability to increase the combustion efficiency of fine particles. It may be contained in a catalyst base powder consisting of alumina-titanium-phosphorus, or a titanium compound and a phosphorus compound may be immersed and supported on a coating layer formed by adhering only alumina to the filter structure. A titanium-phosphorous composite oxide or a mixed oxide may be formed, or any other method may be used to sufficiently exhibit performance. Activated alumina that can be used in the present invention can be produced by conventionally known production methods.
The crystal forms include χ, ρ, κ, γ, δ, η, θ, etc. or amorphous alumina gel. The titanium phosphate used in the present invention includes water-soluble titanium compounds such as titanium tetrachloride and titanium sulfate, titanium hydroxide, anatase or rutile titanium oxide, or a mixture thereof, as a titanium source.
Titanium metal powder and a water-soluble phosphorus compound such as phosphoric acid or ammonium phosphate or an organic phosphorus compound as a phosphorus source in the form of an aqueous or acidic solution.
Mix the molar ratio of TiO ₂ and P ₂ O ₅ to the desired amount with stirring, age at room temperature to 100℃ to precipitate titanium phosphate crystals, and separate and dry at 400 to 1100℃.
It can be obtained by calcining it with a suitable pulverizing process. In addition, when using a water-insoluble titanium compound, it can be used in slurry form or in a state where a phosphorus compound is mixed with titanium compound powder.
It can also be used after being fired and pulverized at 400-1100°C. Although the molar ratio of TiO ₂ and P ₂ O ₅ can be adjusted arbitrarily,
Preferably TiO ₂ /P ₂ O ₅ = 5 to 0.5, particularly preferably TiO ₂ /P ₂ O ₅ = 2.5 to 1, and the surface area is 1 to 1.
400 m ² /g is preferred. Titanium phosphate is not limited to that obtained by the method described above, and TiO ₂ /
Titanium phosphate obtained by a manufacturing method other than the above can also be used satisfactorily in the present invention as long as the P ₂ O ₅ molar ratio is 5 to 0.5 and the surface area is 1 to 400 m ² /g. As a method other than the above-mentioned carrier manufacturing method, a predetermined amount of the above-mentioned alumina powder is added to activated alumina powder with a titanium compound such as titanium tetrachloride, titanium sulfate, titanium oxalate, or organic titanate, or in the form of a solution diluted with an aqueous solution or an organic solvent. The carrier is impregnated, then dried at 100 to 200°C, and then dried at 200°C.
Calcinate in air at ~600°C to decompose the titanium compound inside the pores of the alumina support. Next, to the alumina powder containing the titanium compound,
After impregnating with an aqueous solution of water-soluble phosphorus compounds such as phosphoric acid and ammonium phosphate and drying at 100 to 200℃,
A finished powder can also be obtained by firing at 800-1100°C for 2-6 hours. In some cases, the powder may be impregnated with a mixed solution or slurry of a titanium compound and a phosphorus compound at once, and then dried at 100 to 200°C and heated to 800 to 1100°C.
It is also possible to obtain a finished powder by baking for 2 to 6 hours at °C. Chromium, manganese, iron, cobalt, as metal elements that have the ability to increase the firing efficiency of fine particles.
Nickel, copper, zinc, tungsten, alkali metals, alkaline earth metals, and rare earth elements are preferably water-soluble salts such as nitrates, sulfates, carbonates, organic acid salts, and hydroxides, and in the case of non-water-soluble salts. Alternatively, the powder may be mixed with alumina-titanium-phosphorus powder to form a slurry, and the slurry may be adhered and supported. As the noble metal catalyst substance, for example, in the case of platinum, a water or organic solvent solution or an acidic or alkaline solution of chloroplatinic acid, platinum nitrate, dinitrodiamine platinum, etc. is used, and as a palladium compound, palladium nitrate, palladium chloride, etc. or palladium acetate, Water or an organic solvent solution or an acidic solution of a palladium compound such as organic acid palladium such as palladium oxalate is used, and as a rhodium compound, a water or organic solvent solution or an acidic or alkaline solution of rhodium chloride, rhodium nitrate, rhodium acetate, etc. is used. used. The evaluation method for each catalyst is to test it using actual diesel engine gas, and measure the conversion rate of SO ₂ to SO ₃ using a non-dispersive infrared analyzer (NDIR) at the inlet and outlet of SO ₂ . Find the rate and from this value
The conversion rate of SO ₂ to SO ₃ was determined. Regarding CO.
NDIR and HC were determined by the flame ionization current method (FID method). The particulate purification rate was determined by collecting particulates with a fluorocarbon gas filter and determining the weight increase of the filter. Furthermore, when we attempted to analyze the absorbance of the filters that had collected the particles at 530 nm using barium chlorate, we detected a considerable amount _{of SO 2-4} ⁱⁿ some samples. The pressure drop (mmHg) of the catalyst structure was determined from the difference between before and after the structure, and its change over time was compared for each catalyst. The diesel engine used had a displacement of 2300c.c.
It is a 4-cylinder engine, and its exhaust gas is used as the catalyst inlet temperature, which varies from 225°C to 350°C. The pressure drop in the catalyst layer was measured during 60km/hr running, and the rise process was compared and studied. EXAMPLES The present invention will be explained in more detail with reference to Examples below. Example 1 A 30% by weight aqueous phosphoric acid-ammonium solution containing 356 g as P ₂ O ₅ was added to a 50% by weight titanium tetrachloride aqueous solution containing 200 g as TiO ₂ with stirring at room temperature, and a white precipitate was obtained. (The molar ratio of TiO ₂ and P ₂ O ₅ was 1:1) was dried as it was at 150°C and fired at 500°C. The surface area of the fired product (referred to as titanium phosphate) was 53 m ² /g. Commercially available boehmite-like alumina monohydrate 1.2Kg
The fired titanium phosphate product prepared as above
Add 437g of water, add 1.9% of water, mix well using a kneader, dry at 170°C, sinter at 1000°C for 2 hours, and grind to produce 70% by weight of Al ₂ O ₃ , 10.8% by weight of TiO ₂ , P ₂ A powder consisting of alumina-titanium-phosphorus having a composition of 19.2% by weight of O ₅ was obtained. The surface area of the powder obtained is
It was 75m ² /g. Commercially available cordierite foam (bulk density 0.35g/
cm ³ , porosity 87.5%, apparent volume 1.5), the powder is slurried and supported using a wet mill to form a coating layer consisting of alumina-titanium-phosphorus,
After drying at 170°C for 2 hours, it was fired at 500°C for 2 hours. The amount supported was 70 g per foam. Next, 506.2 g of iron nitrate Fe(NO ₃ ) ₃ 9H ₂ O was dissolved in water to make 1.5, and the foam with the coated layer was immersed in this aqueous solution, and after sufficiently shaking off the solution. , dried at 170°C for 2 hours, and fired at 500°C for 2 hours. The amount of Fe ₂ O ₃ supported was 10 g per foam. The foam obtained as described above was immersed in a mixed aqueous solution of dinitrodiammine platinum in nitric acid and an aqueous solution of rhodium nitrate.
After drying at 500°C for 2 hours, it was fired at 500°C for 2 hours. Pt and Rh were supported in the obtained catalyst at a ratio of 0.9 g/-carrier and 0.1 g/-carrier, respectively. The finished composition of the coated layer of the catalyst is 60.5% by weight of Al ₂ O ₃ ,
The content was 9.3% by weight of TiO ₂ , 16.6% by weight of P ₂ O ₅ , 12.3% by weight of Fe ₂ O ₃ , and 1.23% by weight of Pt+Rh (Pt/Rh=9/1 atomic ratio). Example 2 10% by weight of titanium tetrachloride containing 40 g as TiO ₂
An aqueous solution of and 135 c.c. of 28% ammonia water are reacted with stirring, the resulting titanium hydroxide is washed with water, and 600 c.c. of water is added to the total amount of titanium hydroxide after washing.
The mixture was heated to ~60°C, and 95 g of oxalic acid was added and dissolved to obtain an aqueous solution of titanium oxalate. Commercially available activated alumina powder (γ-alumina) 615
The entire amount of the titanium oxalate aqueous solution was added to g, thoroughly mixed in a kneader, dried at 180°C for 2 hours, and then P ₂ O ₅
A phosphoric acid aqueous solution containing 28.4 g of TiO 2 and P 2 O 5 was mixed therein (the molar ratio of TiO ₂ and P ₂ O ₅ was 2.5:1), dried, and then fired at 800° C. for 2 hours. The composition of the fired product was 90% by weight of Al ₂ O ₃ , 5.8% by weight of TiO ₂ , and 4.2% by weight of P ₂ O ₅ , and the surface area was 147 m ² /g. It was prepared in the same manner as in Example 1 except that this powder was used. The composition of the coated layer of the finished catalyst is:
Al ₂ O ₃ 77.8% by weight, TiO ₂ 5.1% by weight, P ₂ O ₅ 3.6% by weight, Fe ₂ O ₃ 12.3% by weight, Pt + Rh (Pt/Rh = 9/1
(atomic ratio) was 1.23% by weight. Example 3 Catalysts having the following compositions were prepared in exactly the same manner as in Example 1 except that iron nitrate was not used. Nitrates were used in all cases except for ammonium dichromate for chromium and ammonium tungstate for tungsten. Note that the composition is a ratio in weight %. Also, the supported amount of Pt+Rh (Pt/Rh=9/1 atomic ratio) is 1.0
g/single substance.

[Table] Example 4 Al ₂ O ₃ 60.5, TiO ₂ 9.3,
P ₂ O ₅ 16.6, Fe ₂ O ₃ 12.3, Pt + Pd (Pt/Pd = 9/1
A catalyst having a coating layer having a composition (atomic ratio) of 1.23 (each ratio by weight %) was obtained. Example 5 The same method as in Example 1 was used except that the total amount of Pt and Rh was 0.5 g per carrier.
Al ₂ O ₃ 60.9, TiO ₂ 9.4, P ₂ O ₅ 16.7, Fe ₂ O ₃ 12.4, Pt
A catalyst having a coating layer having a composition of +Rh (Pt/Rh=9/1 atomic ratio) 0.62 (ratio of each weight %) was obtained. Example 6 In the same manner as in Example 1 except for changing the amount of iron, Al ₂ O ₃ 66.2, TiO ₂ 10.2, P ₂ O ₅ 18.2, Fe ₂ O ₃ 4.1,
A catalyst having a coating layer having a composition of Pt+Rh (Pt/Rh=9/1 atomic ratio) of 1.35 (ratio of each weight %) was obtained. The amount of Pt+Rh supported was 1.0 g/-carrier. Example 7 In Example 1, the cordierite foam was
A catalyst was prepared in exactly the same manner except that the honeycomb structure was replaced with a sealed type honeycomb structure in which adjacent holes on both end faces were alternately blocked to allow gas to pass only through the partition walls. Comparative Example 1 A catalyst was prepared in the same manner as in Example 1 except that Fe ₂ O ₃ was not added, and the composition of the coating layer was
Al ₂ O ₃ 69, TiO ₂ 10.65, P ₂ O ₅ 18.93, Pt + Rh (Pt/
Rh = 9/1 atomic ratio) 1.41 composition (each % by weight)
(ratio) of the catalyst was obtained. Comparative Example 2 The same activated alumina powder (γ/alumina) used in Example 2 was made into a slurry in cordieride foam, and a coating layer of 70 g per carrier was formed, and iron, Pt, and Rh were used in Example 2. The composition of the coating layer was as follows:
Al ₂ O ₃ 86.4, Fe ₂ O ₃ 12.3, Pt+Rh (Pt/Rh=9/
A catalyst having a composition (each % by weight) of 1.23 (1 atomic ratio) was obtained. Comparative Example 3 The same activated alumina powder (γ/alumina) used in Example 2 was made into a slurry on cordierite foam, and a coating layer of 70 g per carrier was formed, and Pt and Rh were applied to this in Example 3. As in 1, 1 g was supported per carrier, and the composition of the coating layer was Al ₂ O ₃ 98.6, Pt + Rh (Pt/Rh = 9/1 atomic ratio).
A catalyst with a composition of 1.41 (wt% each) was obtained. Example 8 A catalyst evaluation test was conducted on the catalysts obtained in Examples 1 to 7 and Comparative Examples 1 to 3 by operating a 4-cylinder diesel engine with a displacement of 2300 c.c. at 2000 rpm and a torque of 5 to 13 Kg-m. I did it. The composition of the inlet gas is
CO150~300ppm, HC100~300ppm, _SO2 80~
It was 150ppm, _O2 10-15%. The SO ₂ conversion rate (%) was determined by analyzing the SO ₂ concentration of the inlet gas and outlet gas using a non-dispersive infrared analyzer (NDIR method) using the following _calculation formula. The results are shown in Table-1. SO ₂ conversion rate (%) = Inlet SO ₂ concentration (ppm) - Outlet SO ₂ concentration (ppm) / Inlet SO ₂ concentration (ppm) x 100 The CO conversion rate (%) was determined using the following formula. The results are shown in Table-2. CO conversion rate (%) = Inlet CO concentration - Outlet CO concentration (ppm)
/Inlet CO concentration (ppm) x 100 To calculate the HC conversion rate, analyze the HC concentration of the inlet gas and outlet gas using the flame ionization current method (FID method), and calculate the HC conversion rate (%) using the following calculation formula. I asked for it. The results are shown in Table-2. HC conversion rate (%) = Inlet HC concentration (methane equivalent ppm) -
Outlet HC concentration (methane equivalent ppm) / inlet HC concentration (methane equivalent ppm) x 100 The particulate purification rate is 100 with a gas line that keeps the inlet gas and outlet gas warm, 47 mmφ, and particulates are removed with a fluorocarbon gas filter manufactured by PALLFLEX. The particulate purification rate (%) was determined from the weight increase of the filter using the following calculation formula. The results are shown in Table-3. Particulate purification rate (%) = Inlet filter - weight increase (
mg) - Outlet filter weight increase (mg) / Inlet filter weight increase (mg) x 100 In addition, the change in pressure drop (mmHg) of the catalyst over time is calculated by 60
Measurements were made during driving in Km/Hr to compare each catalyst.
The results are shown in Table 4.

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Claims (1)

[Claims] 1. A composite oxide or mixed oxide consisting of alumina (A), titanium, and phosphorus on a heat-resistant inorganic carrier.
(B) and at least one metal oxide (C) selected from the group consisting of chromium, manganese, iron, cobalt, nickel, copper, zinc, tungsten, alkali metals, alkaline earth metals and rare earth elements. 1. A catalyst for purifying exhaust gas from a diesel engine, characterized in that a mixed oxide film is formed, and at least one noble metal element is dispersed and supported in the film. 2. The catalyst according to claim 1, wherein the carrier has a gas filter structure. 3 The total amount of precious metal elements per carrier is 0.2~
3. The catalyst according to claim 1 or 2, wherein 3.0g is supported.