JPH0722705B2 - Method for producing oxidation catalyst - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing an oxidation catalyst that completely burns unburned hydrocarbons and carbon monoxide discharged from various combustors into carbon dioxide gas and water. .

2. Description of the Related Art Generally, platinum, such as platinum and palladium, has the highest activity as an oxidation catalyst for completely oxidizing unburned hydrocarbons into carbon dioxide and water vapor in the presence of air. Therefore, platinum group catalysts supported on various carriers such as alumina and silica are widely used as oxidation catalysts in various combustors. On the other hand, with respect to so-called base metals such as cobalt, nickel and iron, various composite oxides have recently been studied rather than as individual metal oxides. In particular, those having a perovskite type crystal structure have attracted attention because they have high activity. (For example, Nakamura, Misono, et al., Nikka, 1980, 1679) Problems to be solved by the invention Platinum, such as platinum, palladium, and rhodium, have high oxidation activity per se, and are problematic when used at a temperature of 500 ° C or lower. However, when it is used at a temperature of 500 ° C. or higher, particularly 700 to 800 ° C. or higher, there is a problem of thermal deterioration in that the particle size of the supporting metal called Siritung increases and the activity decreases. Therefore, various methods for improving heat resistance have been proposed, but sufficient results have not been obtained. Furthermore, the platinum group has a high cost and a wide range of price fluctuations, and there is a problem in terms of stable supply. On the other hand, oxides of so-called base metals such as nickel, cobalt, iron, etc. have not been actually used because they have low oxidation activity and low heat resistance. Recently, it has been reported that a composite oxide having a perovskite structure represented by the crystal structural formula ABO ₃ has high oxidation activity and high heat resistance. Particularly, various studies have been made on the ones in which the A site is composed of a rare earth element such as lanthanum and neodymium and the B site is composed of a transition metal such as cobalt, nickel and iron, because they have high oxidation activity. In each of these, a catalyst powder having a perovskato structure is produced by evaporating or precipitating a solution obtained by mixing various salts such as acetate and nitrate in a stoichiometric ratio and firing the mixture. For practical use, a mixture of this catalyst powder with alumina sol, silica sol, etc. was carried on a honeycomb etc. molded of a heat resistant material such as cordierite, alumina, silica, mullite to serve as a catalyst. Therefore, when it is used as a catalyst, there is always a problem of peeling, and also from the viewpoint of catalytic activity, the problem that alumina sol and silica sol added as a binder shield the catalyst powder and the activity decreases, or the surface area of the catalyst carrier is used in the powder method. However, there was a problem that only those having a small specific surface area could be obtained.
For this reason, in order to solve these problems, as a result of conducting an investigation to directly prepare a perovskite type crystal structure of fine particles having a high specific surface area from a solution on a catalyst carrier, the following problems were clarified. That is, it is necessary to bake at a temperature of at least 700 ° C. or more to form a perovskite structure,
Under such temperature conditions, there is a problem that the components of the carrier and the supported metal react rather, and the desired perovskite structure cannot be obtained. For example, when cobalt or nickel is used as the transition metal of the B site, and when alumina is used as the carrier, cobalt aluminate (CoAl ₂ O ₄ ) and nickel aluminate (NiAI ₂ O ₄ ) are produced, and There is a problem that the perovskite that does not generate.

Therefore, the effect of so-called precoat, in which various metal oxides are preliminarily supported in order to eliminate the interaction with the carrier as described above, was examined. As a result, 1 to 15% by weight of rare earth oxides such as lanthanum and cerium or alkaline earth metal oxides such as strontium and magnesium relative to the carrier, preferably 5 to 10% by weight, is higher than the temperature for pre-synthesizing the perovskite. 50
It has been found that the interaction with the carrier can be suppressed by sintering at a temperature as high as ~ 100 ° C. However, in order to pre-coat the catalyst carrier with a rare earth element oxide as described above, it is necessary to use 7 to 10% based on the weight of the carrier from ordinary nitrates and oxides. If the loading amount is less than this, the effect of interaction with the carrier appears and the activity decreases. However, conversely, when the precoat is carried by about 10%, the carrier is covered and the specific surface area becomes small, so that the original specific surface area of the carrier cannot be effectively utilized. The present invention is intended to solve the problems in producing a highly active perovskite type catalyst by the direct loading method as described above.

Means for Solving the Problems In order to solve the problems related to the direct loading as described above, lanthanum produced by the alkoxide method on a catalyst carrier whose base material is an inorganic heat-resistant material such as alumina, silica, cordierite, cerium, etc. A structure in which a thin film of an oxide of a rare earth element or a thin film of an oxide of an alkaline earth metal such as strontium or magnesium is prepared and then a composite oxide having a perovskite structure is supported is adopted.

Action A thin film of a rare earth element such as lanthanum or cerium or an oxide of an alkaline earth metal such as strontium or magnesium is supported by an alkoxide method on a catalyst carrier whose base material is an inorganic heat-resistant material such as alumina, silica, or cordierite. Adhesion between the catalyst carrier and the precoat material is very good, and the role of precoat can be fulfilled with a small loading amount, and it is possible to produce a highly active perovskite type complex oxide on the catalyst carrier without interaction with the carrier. It was

Examples Examples of catalysts prepared by using the present invention will be described below.

(Example 1) Isopropoxy lanthanum (La (i-
opr) 3) isopropanol solution (0.1g / 100ml) 25m
The solution was injected, left standing for 24 hours, and dried by nitrogen bubbling. After drying, it was baked at 900 ° C. for 5 hours. The amount of lanthanum oxide supported at this time was 3% by weight with respect to the carrier. After that, impregnation into a mixed acetate solution with a molar ratio of La: Sr: Co = 0.8: 0.2: 1.0 and drying were repeated.
_0.8 Sr _0.2 CoO _{3 As a} whole, 5.8 wt% was supported, and then the mixture was calcined in air at 850 ° C. for 5 hours to obtain a catalyst.

(Example 2) As in Example 1, after supporting 5% by weight of cerium oxide by the alkoxide method and baking at 900 ° C for 1 hour, the molar ratio was set to La: Ce: Co = 0.9: 0.1: 1.0. Repeated impregnation in a nitrate solution and drying, La _0.9 Ce _0.1 CoO ₃ 5.8 as a whole
After supporting by weight percent, it was calcined in air at 800 ° C. for 1 hour to obtain a catalyst. When the oxidation activity of propane was examined using the catalysts of Examples 1 and 2 above, 10% of lanthanum oxide was loaded from nitrate as a precoat, and then La _0.8 Sr _0.2
As compared with the catalyst supporting 7 weight percent of C ₀ O ₃ perovskite type complex oxide, the catalyst of any of the examples obtained about twice the activity at the reaction rate per weight. In the above examples, rare earth oxides such as lanthanum and cerium were used for precoating, but similar results were obtained when precoating with alkaline earth oxides such as strontium and magnesium. Further, La _0.8 Sr _0.2 CoO ₃ and La _0.9 Ce _0.1 CoO ₃ were used as examples of the perovskite composite oxide used in the examples, but other perovskite composite oxides (lanthanum, strontium, cerium at the A site, and neodymium in addition to cerium) were used. , Praseodymium, etc., and B site using other than cobalt, such as iron, nickel, manganese, etc.), similar results were obtained.

As described above, according to the present invention, cordierite, alumina,
A perovskite-type composite oxide is synthesized after supporting a rare earth metal such as lanthanum or cerium or an oxide of an alkaline earth metal such as strontium or magnesium as a thin film by the alkoxide method as a precoat on a catalyst carrier whose base material is an inorganic heat-resistant material such as silica. The stoichiometric ratio was adjusted so that
As a result of preparing an oxidation catalyst by impregnating with a solution of acetate, nitrate or the like and firing, the following effects were obtained.

Since the precoating is performed by the alkoxide method, the adhesion between the carrier and the precoat material such as rare earth or alkaline earth metal and the shielding property are improved, the amount of the precoat carried is small, and the cost can be reduced. Furthermore, since the amount of the precoat carried is small, the surface area of the carrier is reduced, and as a result, the surface area of the carrier can be utilized when synthesizing the perovskite, and it is possible to synthesize the fine particle perovskite composite oxide. As a result, the oxidation activity was significantly improved as compared with the catalyst synthesized from powder.

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Claims (1)

[Claims] 1. A catalyst carrier comprising an inorganic heat-resistant material in the form of a honeycomb, a foamed ceramic, or a cloth, and an alkoxide method comprising an oxide of one or more of lanthanum, cerium, strontium, and magnesium on the carrier. After supporting in a thin layer with a total of 1 to 15% by weight of the oxide as a carrier, the A site of the perovskite-type composite oxide represented by the crystal structural formula ABO ₃ is lanthanum, It is composed of at least one element from rare earth and alkaline earth metals such as neodymium and praseodymium, and B site is selected from at least one element from iron, cobalt, nickel and manganese, and the amount supported as perovskite type complex oxide is used as a carrier. On the other hand, a method for producing an oxidation catalyst, which is configured to be supported in the range of 1 to 15 weight percent.