JP2011212626A - Catalyst for cleaning exhaust gas and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an epoch-making method which enables an adverse effect of poisoning to be avoided by suppressing accumulation of sulfur while maintaining a high activity of catalyst even under a high-temperature environment.SOLUTION: This catalyst carrier includes a rare earth element serving as the core, zirconia surrounding the rare-earth element, and titania surrounding the zirconia. A part of the rare earth element is solid-dissolved into the zirconia.

Description

  The present invention relates to a zirconia-titania powder and a method for producing the same, and is mainly suitable as a catalyst carrier or promoter, particularly as a catalyst carrier or promoter for automobile exhaust gas purification, and in a high-temperature gas containing sulfur. The present invention relates to a novel zirconia-titania powder suitable for use as a catalyst carrier and a method for producing the same.

  2. Description of the Related Art Conventionally, as a catalyst for exhaust gas purification of automobiles, a three-way catalyst that purifies by performing CO and HC oxidation and NOx reduction in exhaust gas simultaneously at a stoichiometric air-fuel ratio (stoichiometric) is used. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), Those carrying a catalytic noble metal such as palladium (Pd) are widely known.

By the way, the exhaust gas contains SO _x produced by burning sulfur (S) contained in the fuel. This SO _x is oxidized by a catalytic metal in the lean side and also joined react with water vapor, sulfite and sulfate ions are generated. When these react with the NO _x storage material to produce sulfite or sulfate, sulfur poisoning occurs in which the NO _x storage function of the NO _x storage material is impaired and the purification performance is reduced.

  In order to continuously reflect the automobile industry, it is necessary to meet the exhaust gas regulations that are being strengthened in each country and to minimize the use of precious metals with low reserves and high resource risks.

JP-A-8-192051 JP-A-8-281116 JP-A-8-224479

  A gasoline engine catalyst is equipped with a three-way catalyst having Pt, Pd, and Rh as main active species among noble metals. This three-way catalyst reduces hydrocarbons (HC) and carbon monoxide (CO), which are harmful gases, as reducing species, and reduces nitrogen oxide (NOx). The three-way catalyst has an oxygen occlusion mainly composed of cerium oxide that keeps the catalyst close to the theoretical air-fuel ratio (stoichiometric) so that the three gases can be purified uniformly so that the above-mentioned noble metals work with the highest activity. Material (OSC material) is included.

  When using a catalyst, catalyst poisoning is an unavoidable problem. The type of poisoning is mainly due to sulfur contained in gasoline, zinc dialkyldithiophosphate (ZDDP) contained in engine oil, or methylcyclopentanedi added to gasoline as an octane improver. The thing by an enyl manganese tricarbonyl (MMT) etc. are mentioned.

In order to recover from poisoning due to sulfur, it is necessary to vaporize and desorb as SO ₂ or H ₂ S by air-fuel ratio control or temperature control of the engine. On the other hand, when such control is performed, reduction in purification ability and catalyst deterioration are promoted, and therefore using a catalyst that hardly accumulates sulfur leads to avoiding poisoning.

  Titania has almost no base point on the surface, and therefore, acidic substances such as sulfur are difficult to adsorb and easily desorb. Therefore, it has the effect of preventing poisoning by acidic substances such as sulfur, and is used as a catalyst carrier or promoter. Can be used. However, titania has lower heat resistance than alumina or the like, and when employed as a catalyst, sintering may occur up to the metal on the support. That is, when titania is employed as a catalyst, there is a problem that the heat resistance of the catalyst itself deteriorates, so that it is difficult to use titania for a three-way catalyst that is exposed to a high-temperature gas.

  In order to improve the properties of titania used as a catalyst carrier or co-catalyst, zirconia has often been used as an additive along with alkaline earth metals, transition elements, and rare earth elements.

  Patent Document 1 (Japanese Patent Laid-Open No. 8-192051) proposes to use a Ti—Zr composite carrier. Patent Document 2 (Japanese Patent Laid-Open No. 8-281116) proposes to use zirconium phosphate. Patent Document 3 (Japanese Patent Laid-Open No. 8-224479) proposes improvements such as supporting a transition metal on a porous body.

  However, although any of the improved catalysts can suppress the accumulation of sulfur, there remains a problem that the catalytic activity is greatly reduced when used in a high-temperature environment of a gasoline engine.

  The present invention has been made in view of the above problems, and its object is to suppress the accumulation of sulfur while maintaining the high activity of the catalyst even in a high temperature environment, and to influence the poisoning. It is to provide an epoch-making method that can avoid the problem.

The following (1) to (6) are provided by the present invention.
(1) A catalyst carrier comprising a rare earth element as a core, zirconia surrounding the rare earth element, and titania surrounding the zirconia, wherein a part of the rare earth element is dissolved in the zirconia. A catalyst carrier.
(2) The catalyst carrier according to (1), wherein the rare earth element is yttrium, lanthanum, praseodymium, or neodymium.
(3) The catalyst carrier according to (1) or (2), wherein the rare earth element is 3% by mass to 5% by mass based on the total mass of the catalyst carrier.
(4) The catalyst carrier according to any one of (1) to (3), wherein titania is 5% by mass to 10% by mass based on the total mass of the catalyst carrier.
(5) in air, 900 ° C., a specific surface area after calcination for 5 hours is ^{90m 2 / g~120m 2 / g,} (1) a catalyst support according to any one of to (4).
(6) preparing an aqueous solution containing a rare earth element;
Adding aqueous ammonia to the aqueous solution to precipitate a rare earth element as a core;
Adding a salt containing zirconium and aqueous ammonia to the solution to surround the rare earth element core with zirconia;
Adding a titanium-containing salt and aqueous ammonia to the solution to surround the zirconia surrounding the rare earth core with titania; and the rare earth core, the zirconia surrounding the rare earth core, and Aging titania surrounding the zirconia surrounding the rare earth element core at 80 ° C. or higher and 100 ° C. or lower;
A process for producing a catalyst carrier comprising

FIG. 1 is a conceptual diagram of an example catalyst and a comparative catalyst. FIG. 2 is a graph showing the NOx purification rate (%) of the example catalyst and the comparative example catalyst. FIG. 3 is a diagram showing the pattern temperature and time of the high temperature durability treatment / sulfur (S) poisoning treatment. FIG. 4 is a diagram showing the pattern temperature and time for evaluation of catalyst activity after high temperature durability and S poisoning. FIG. 5 is a diagram showing a schematic image of stoichiometric activity evaluation in the model gas apparatus.

  The present invention provides a catalyst support comprising a rare earth element as a core, zirconia surrounding the rare earth element, and titania surrounding the zirconia, wherein a part of the rare earth element is dissolved in the zirconia. Will be described. FIG. 1 shows a conceptual diagram of the catalyst carrier of the present invention.

Examples of the rare earth element as the core include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium ( Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and the like. Particularly desirable is yttrium oxide (yttria, Y ₂ O ₃ ). The rare earth element acts as a NOx storage material. The ratio of the rare earth to the entire catalyst carrier is not particularly limited, but is preferably 1 to 10% by mass, and more preferably 3 to 5% by mass. With this mass ratio, effects such as a high NOx purification rate and a high specific surface area can be obtained. Mass ratio is likely to have less and _the NO _x storage capacity than 1 wt% NOx conversion performance is degraded less. When the mass ratio is more than 10% by mass, there is a possibility that problems such as saturation of NOx storage capacity and increase of HC emission may occur.

A core made of rare earth elements is surrounded by zirconia. Zirconia plays a role of improving heat resistance, and since zirconia is a basic oxide, NO _x tends to be close to each other, and the NO _x storage capacity of the catalyst is also increased. The ratio of zirconia to the entire catalyst support is not particularly limited, but is preferably 75 to 98% by mass, and more preferably 85 to 92% by mass.

  The zirconia that surrounds the core is surrounded by titania. That is, titania is thinly coated as a surface layer of the catalyst carrier. Titania acts as a catalyst support surface or cocatalyst. In the present invention, the ratio of titania relative to the entire catalyst support is not particularly limited, but is preferably 1 to 15% by mass, and more preferably 5 to 10% by mass. When the mass ratio of titania is within this range, effects such as a high NOx purification rate and a small sulfur (S) accumulation product can be obtained. With this configuration of the present invention (the core made of rare earth element is surrounded by zirconia and further surrounded by titania), sulfur poisoning can be suppressed with a small amount of titania compared to conventional titania-containing catalysts.

In the catalyst carrier of the present invention, a part of the core rare earth element is dissolved in zirconia. This stabilizes the crystal structure of zirconia. Unstabilized zirconia causes a phase transition at high temperatures. In order to stabilize the zirconia in cubic or tetragonal form, a rare earth element, preferably yttrium oxide, is added as a stabilizer. When the rare earth element is dissolved in zirconia, the effect of composite stabilization is obtained, and the heat resistance is improved.
In addition, since zirconia is a basic oxide, NO _x tends to be close thereto, and stabilized zirconia improves NO _x storage capacity.

In the catalyst carrier of the present invention, the method for producing the rare earth element core is not particularly limited, but a precipitation method may be used. The procedure when using the precipitation method is as follows.
(1) First, a salt containing a rare earth element is dissolved in ion-exchanged water, and then ammonia water is added to prepare a solution in which the salt containing the rare earth element is precipitated. This precipitate becomes a core made of rare earth elements.

In the catalyst carrier of the present invention, the core made of rare earth elements is surrounded by zirconia. The method of surrounding the core with zirconia is not particularly limited, but a precipitation method may be used. The procedure when using the precipitation method is as follows.
(2) While stirring the solution prepared in (1), a salt containing zirconium and ammonia water are added in small amounts at the same time, so that zirconia is deposited on the core made of rare earth elements, and the core is surrounded by zirconia. Precipitate is obtained.

The method of surrounding zirconia with titania is not particularly limited, but a precipitation method may be used (similar to the surrounding of the core with zirconia). The procedure when using the precipitation method is as follows.
(3) By adding a small amount of titanium-containing salt and aqueous ammonia to the solution after the treatment in (2), titania is further deposited on the precipitate surrounded by zirconia, and the precipitate surrounded by titania. A thing is obtained.

The method for dissolving a part of the rare earth element in zirconia is not particularly limited, but the precipitate obtained by the treatment (3) may be aged, dried and fired.
In this aging, the dispersion containing the precipitate obtained by the treatment of (3) is preferably at least 50 hours and preferably 120 hours at atmospheric pressure, preferably at a temperature range of 80 ° C. or higher and preferably 100 ° C. or lower. It is preferable to hold for not more than time, more preferably not less than 60 hours and more preferably not more than 100 hours. Here, the reason why the temperature range is set to 80 ° C. or more and 100 ° C. or less under atmospheric pressure is that if it is less than 80 ° C., an effective reaction rate cannot be obtained, whereas if it exceeds 100 ° C. This is because, since the solvent of the dispersion liquid containing the precipitate is water, the dispersion liquid may not be uniformly aged due to bumping or the like.
Next, drying is carried out with a dryer from the aging temperature to preferably 150 ° C., more preferably to 120 ° C., and further in the air, preferably 400 to 1100 ° C., more preferably 500 to 1000 ° C., more preferably 600 to 900. The catalyst carrier is produced by calcining at a temperature of preferably 30 minutes to 12 hours, more preferably 1 to 6 hours.
As a means for aging, drying, and baking, it is preferable to set the rate of temperature rise to 100 ° C./h or less. This is because it is effective for stably producing the catalyst carrier of the present invention. That is, when these precipitates are heated, aged, dried, and calcined, a considerable amount of a salt such as ammonium nitrate coexists as a by-product. In the heating process, these by-product salts are often oxidized by the titanium ions during the heating process, and the precipitate during drying often ignites or self-heats, and the temperature rises more than necessary, reducing the specific surface area. The phenomenon that occurs. In order to suppress this rapid self-heating, it is preferable to set the temperature rising rate to 100 ° C./h or less. More preferably, it is 50 ° C./h or less.

The catalyst carrier of the present invention can maintain a high specific surface area even after the above-described calcination treatment. The catalyst carrier of the present invention, in the air, 1000 ° C., after calcination for 3 hours, the specific surface ^area, shows the physical properties of 100m ² / g~130m 2 / g. The 900 ° C., after calcination for 5 hours, the specific surface area of ^{90m 2} / ^g~120m 2 / g. That is, the specific surface area is large even after being exposed to a high temperature.

  In addition, the catalyst carrier of the present invention can provide a catalyst carrier with greatly improved heat resistance without changing the characteristic that the number of basic sites on the titania surface is small. That is, even after being exposed to a high temperature, the feature that the number of basic sites on the titania surface is small is maintained, so that sulfur (S) poisoning is suppressed, and as a result, catalyst deterioration hardly occurs.

  Prior to the aging, drying, and calcination steps, a precipitation by filtration or decantation and / or a by-product removal step such as a water washing treatment can be used in combination. By using these steps in combination, the limitation of the heating temperature rise rate becomes relatively small.

  A catalyst noble metal is supported on the catalyst carrier of the present invention. As the catalyst noble metal, for example, one or more of Pt, Rh, Au, Ag and Pd can be used, and Pd is particularly desirable. The supported amount of any precious metal is preferably 0.1 to 20 g, particularly preferably 0.5 to 10 g, per 100 g of the carrier (corresponding to a volume of 1 liter of the whole catalyst). That is, based on the mass of the support, the amount of the catalyst noble metal supported is preferably 0.1 to 20 wt%, particularly preferably 0.5 to 10 wt%. Even if the amount of the catalyst noble metal supported is increased further, the activity is not improved, and the effective utilization cannot be achieved. On the other hand, if the amount of the catalyst noble metal supported is less than this, practically sufficient activity cannot be obtained.

Example Catalyst Production (A) A predetermined amount of a nitrate-based rare earth salt (see Table 3) was dissolved in ion-exchanged water. Next, 25 wt% aqueous ammonia was added so that the pH of the solution was 9 to 10, and a salt containing a rare earth element was precipitated in the solution. This precipitate was used as a core made of a rare earth element.
(B) While stirring the solution of (A) (stirring speed: 300 rpm), zirconium oxyate and 25 wt% aqueous ammonia were added little by little until the pH reached 9-10, so that zirconia was deposited on the core made of rare earth elements. A precipitate was obtained in which the core was surrounded by zirconia.
(C) While stirring the solution of (B) (stirring speed: 300 to 450 rpm), by adding titanyl sulfate (see Table 4) and 25 wt% aqueous ammonia at a time in small portions until pH 7-8, surrounded by zirconia Further, titania was deposited on the deposited precipitate to obtain a precipitate surrounded by titania.
(D) The solution containing the precipitate after the treatment of (C) was filtered and washed. Next, the aqueous solution containing the precipitate after filtration and washing was aged at 80 ° C. for 72 hours. Next, the precipitate after aging was dried at 120 ° C. for 12 hours. Next, the dried precipitate was fired at 900 ° C. for 5 hours to obtain a powder. This powder was used as a catalyst carrier.
(E) A predetermined amount of the catalyst carrier powder obtained by the treatment of (D) is immersed in an aqueous palladium nitrate solution (concentration: 8.2 wt%), stirred for 2 hours, evaporated to dryness, and 0. 5 wt% Pd was supported. Next, the supported catalyst carrier was dried at 120 ° C. for 12 hours. Finally, the dried catalyst carrier was calcined at 600 ° C. for 2 hours and pelletized to obtain an exhaust gas purifying catalyst.
An example catalyst as shown in FIG. 1 was obtained. The catalyst of the example has a structure in which a rare earth element is used as a core, the core is surrounded by zirconia, and zirconia is surrounded by titania.

Production of Comparative Catalyst (a) A predetermined amount of titanyl sulfate, zirconium oxynitrate, and nitrate-based rare earth salts (see Tables 3 and 4) were dissolved in ion-exchanged water to prepare a raw material aqueous solution.
(B) While stirring the raw material aqueous solution of (a) (stirring speed: 300 to 450 rpm), 25 wt% ammonia water is added little by little until the pH becomes 9 to 10, whereby titanium, zirconium and rare earth elements are uniformly mixed. A precipitate was obtained.
(C) The solution containing the precipitate after the treatment of (b) was filtered and washed. Next, the aqueous solution containing the precipitate after filtration and washing was dried at 120 ° C. for 12 hours. Next, the dried precipitate was fired at 900 ° C. for 5 hours to obtain a powder. This powder was used as a catalyst carrier.
(D) A predetermined amount of the catalyst carrier powder obtained by the treatment of (c) is immersed in an aqueous palladium nitrate solution (concentration: 8.2 wt%), stirred for 2 hours, evaporated to dryness, and the catalyst carrier powder is added to the catalyst carrier with a concentration of 0.00. 5 wt% Pd was supported. Next, the supported catalyst carrier was dried at 120 ° C. for 12 hours. Finally, the dried catalyst carrier was calcined at 600 ° C. for 2 hours and pelletized to obtain an exhaust gas purifying catalyst.
In FIG. 1, the conceptual diagram of a comparative example catalyst is shown. The comparative example catalyst has a structure in which rare earth elements, zirconia, and titania are uniformly mixed and dissolved.

Example Catalyst and Comparative Example Catalyst Evaluation Method Sample catalyst pellets were subjected to [1] high temperature durability treatment / sulfur (S) poisoning treatment, [2] S accumulation measurement, and [3] high temperature durability. -The catalytic activity after S poisoning was evaluated. [1] to [3] will be described below.

[1] High-temperature endurance treatment / sulfur (S) poisoning treatment For each of the exhaust gas purification catalysts (Example catalyst and Comparative example catalyst), 3 g of each pellet was placed in an endurance test apparatus. Next, the sulfur-free gas (1) and the sulfur-containing gas (2) shown in Table 1 were flown from the room temperature to 1000 ° C. in 90 minutes while flowing at 5 L / min to the durability test device in the pattern shown in FIG. The temperature was kept at 1000 ° C. for 4 hours and 30 minutes, the temperature was naturally lowered to room temperature, the temperature was raised from room temperature to 400 ° C. in 55 minutes, the temperature was kept at 400 ° C. for 90 minutes, and the temperature was naturally lowered to room temperature.

[2] S accumulation amount measurement [1] The S accumulation amount in the catalyst pellet subjected to the high temperature durability treatment / S poisoning treatment was measured with a CS meter (HORIBA EMIA-820W series carbon / sulfur analyzer). .
The measurement results of the S accumulation amount are shown in Table 4.

[3] Evaluation of catalyst activity after high temperature durability / S poisoning The evaluation of stoichiometric activity of catalyst pellets subjected to high temperature durability treatment / S poisoning treatment was performed with a model gas apparatus. FIG. 5 shows a schematic image of stoichiometric activity evaluation in the model gas apparatus.
About the exhaust gas purifying catalyst (Example catalyst and Comparative example catalyst) subjected to the above high temperature durability treatment / S poisoning treatment, 3 g of each pellet was placed in a model gas apparatus. Next, the model gas shown in Table 2 is heated from room temperature to 150 ° C. at a rate of 20 ° C./min, and stabilized at 150 ° C. for 5 minutes, while flowing the model gas in the pattern shown in FIG. 4 at a rate of 15 L / min. The temperature was raised from 150 ° C. to 500 ° C. at a rate of 20 ° C./minute, held at 500 ° C. for 3 minutes, and naturally cooled to room temperature.
The NOx concentration in the catalyst entering gas and the NOx concentration in the catalyst exit gas were measured with a gas analyzer (HORIBA MEXA-7100H automobile exhaust gas measuring device). The NOx purification rate (%) was determined as (NOx concentration in catalyst input gas−NOx concentration in catalyst output gas) / (NOx concentration in catalyst input gas) × 100.
The calculation results of the NOx purification rate (%) are shown in Tables 3 and 4. Further, FIG. 2 shows a graph comparing the NOx purification rates (%) of Example Catalyst 3 and Comparative Example Catalyst 4.

Moreover, about the produced catalyst, the specific surface area was measured (by a specific surface area measuring apparatus) before performing [1] high temperature durability treatment and sulfur (S) poisoning treatment (by a catalyst carrier model gas durability apparatus).
The measured specific surface area is shown in Table 3.

Table 3 shows the effect of the difference in the mixing method of the rare earth elements contained in the catalyst. The catalyst of the example had a structure in which a rare earth element was used as a core, the core was surrounded by zirconia, and zirconia was surrounded by titania. The catalyst of the comparative example had a structure in which rare earth elements, zirconia, and titania were uniformly mixed and dissolved. The example catalyst of the present invention showed a higher NOx purification rate than the comparative example catalyst. In particular, an extremely high NOx purification rate (87% or more) was exhibited at a mass ratio of 3% by mass to 5% by mass based on the entire rare earth element catalyst support.
Moreover, also about the specific surface area, when the same amount of rare earth elements was included, the Example had a larger surface area than the comparative example.

Table 4 shows the effects of differences in the amount of titania added to the catalyst and the mixing method.
First, the S accumulation amount will be considered. The amount of sulfur accumulated in the comparative catalyst was reduced by setting the titania addition amount to 20% by mass or more. In the catalyst of Example, the amount of sulfur accumulation was reduced by adjusting the titania addition amount to 5 mass% to 10 mass%. That is, the effect of reducing the amount of accumulated sulfur can be obtained with the Example catalyst with a smaller amount of titania added than the comparative example catalyst. Since the amount of titania added can be reduced, the heat resistance of the catalyst can be kept high. This means that the catalytic activity can be kept high even at high temperatures. As a result, the example catalyst had a higher NOx purification rate than the comparative example catalyst.

Claims (6)

  A catalyst carrier comprising a rare earth element as a core, zirconia surrounding the rare earth element, and titania surrounding the zirconia, wherein a part of the rare earth element is dissolved in the zirconia. Carrier.   The catalyst carrier according to claim 1, wherein the rare earth element is yttrium, lanthanum, praseodymium, or neodymium.   The catalyst carrier according to claim 1 or 2, wherein the rare earth element is 3% by mass to 5% by mass based on the total mass of the catalyst carrier.   The catalyst support according to any one of claims 1 to 3, wherein the titania is 5% by mass to 10% by mass based on the total mass of the catalyst support. In air, 900 ° C., a specific surface area after calcination for 5 hours is ^{90m 2} / ^g~120m 2 / g, the catalyst carrier according to any one of claims 1 to 4. Providing an aqueous solution containing a rare earth element;
Adding aqueous ammonia to the aqueous solution to precipitate a rare earth element as a core;
Adding a salt containing zirconium and aqueous ammonia to the solution to surround the rare earth element core with zirconia;
Adding a titanium-containing salt and aqueous ammonia to the solution to surround the zirconia surrounding the rare earth core with titania; and the rare earth core, the zirconia surrounding the rare earth core, and Aging titania surrounding the zirconia surrounding the rare earth element core at 80 ° C. or higher and 100 ° C. or lower;
A process for producing a catalyst carrier comprising

Images