JPH08323199A - Novel oxidation catalyst - Google Patents

Abstract

PURPOSE: To obtain an oxidation catalyst having high catalytic activity over the wide temp. range from a low temp. to a high temp. by imparting a pyrochlore structure to a metal oxide having a specified compsn. CONSTITUTION: This oxidation catalyst is an oxide having a compsn. represented by the formula (M1-x Cex )2 Sn2 O7 (where M is Ca or Sr and x≠0) and a pyrochlore structure. For example, CeO2 is wet or dry-mixed with SnO2 and MCO3 and this mixture is fired at about 1,000-1,600 deg.C in air to form the pyrochlore structure by a solid phase reaction. The resultant catalyst has high oxidation activity even at a low temp. and can be utilized for oxidation of CO or hydrocarbon such as methane or complete oxidation of an org. chlorine compd., etc.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a novel oxidation catalyst having a high catalytic activity in a wide range from low temperature to high temperature.

[0002]

2. Description of the Related Art Oxidation catalysts are used as catalysts for the complete oxidation reaction of hydrocarbons such as carbon monoxide and methane, and the complete removal reaction of harmful substances such as organic chlorine compounds. Conventionally, a catalyst made of a noble metal such as platinum or palladium has been known as an oxidation catalyst.
When used at the above high temperature, the catalytic activity is remarkably decreased due to aggregation and evaporation of the catalyst components platinum and palladium, and it is not always suitable as a catalyst for such a high temperature reaction.

As a high temperature oxidation reaction catalyst, a hexaaluminate type catalyst containing a transition metal such as manganese has been proposed, but the hexaaluminate type catalyst is not so high in oxidation activity itself, and is particularly comparatively high. When used at low temperatures, improvement of the oxidative activity of this product is a problem.

Under these circumstances, there is a demand for a catalyst which can be used under severe reaction conditions such as high temperature and subcritical atmosphere in a relatively low temperature range.

[0005]

The object of the present invention is to have a high oxidation catalytic activity not only at high temperature but also at low temperature.
Another object is to provide a catalyst having durability at high temperature.

[0006]

Means for Solving the Problems As a result of examining the catalytic activity of metal compounds having various structures, the present inventors have found that
The present invention has been completed by finding that a metal compound having a certain composition and structure solves the above problems.

That is, the present invention is represented by the following general formula (1) (M _1-x Ce _x ) ₂ Sn ₂ O ₇ (1) (wherein M is Ca or Sr and x is not 0). It is an oxidation catalyst characterized by having a pyrochlore structure.

The present invention will be described in more detail below.

The oxidation catalyst of the present invention has the above general formula (1)
And it is essential that the compound has a pyrochlore structure.

A compound having a pyrochlore structure has the general formula A ₂
It is a compound represented by B ₂ O ₇ , and its structure is determined by an X-ray diffraction experiment.

In the above general formula (1), the value of x is not limited to 0 and is not particularly limited, but x is preferably 0.4 or more and 0.7 or less for improving the catalytic activity. When x is less than 0.4, the MSnO ₃ phase precipitates, and when x exceeds 0.7, the CeO ₂ phase precipitates, so that it becomes difficult to obtain a remarkably high catalytic activity.

The oxidation catalyst of the present invention can be synthesized by various methods, that is, a solid phase reaction method, a precipitation reaction method and a hydrolysis method.

For example, in the solid phase reaction method, Ce
O ₂ , SnO ₂ , MCO ₃ (M is Ca or Sr) are wet- or dry-mixed, and the mixture is, for example, 1000 to 1 in air.
A compound having a pyrochlore structure can be obtained by firing at 600 ° C. In the precipitation reaction method,
The M salt (M is the same as above), the Ce salt and the Sn salt are dissolved in a solvent such as ethanol, a precipitating agent such as oxalic acid is added to this to form a precipitate, and the product is dried. 1000-16
It is obtained by firing at 00 ° C.

In the hydrolysis method, M (M is the same as above),
Dissolving Ce and Sn alkoxides in an organic solvent,
A pyrochlore compound can be obtained by dropping a solution containing water thereto to hydrolyze it to obtain a precipitate, which is then dried and baked at 1000 to 1600 ° C. The raw material components used in the above-mentioned various methods are used in stoichiometric amounts such that the composition represented by the above general formula is obtained.

The pyrochlore compound thus obtained is generally obtained in the form of powder, but it may be pulverized by a suitable pulverization method depending on the conditions of use.

The catalyst of the present invention can be used as a molded body by mixing the above pyrochlore compound with a binder such as clay and molding the mixture. The proportion of the pyrochlore compound at this time is not particularly limited, but is 0.1 to 50 wt% of the whole.
%, And it is also possible to use a pyrochlore compound supported on a honeycomb substrate made of cordierite or metal.

The present invention has a high oxidizing activity even at a low temperature, and can be used for the oxidation of hydrocarbons such as carbon monoxide and methane and the complete oxidation reaction of organic chlorine compounds.

[0018]

EXAMPLES The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 Commercially available calcium nitrate, cerium nitrate (trivalent) and tin chloride (divalent) were used in a molar ratio of Ca, Ce and Sn of 2: 3: 5.
And the mixture was dissolved in an ethanol solvent (15 wt% solution of the mixture). This solution was added dropwise as an oxalate salt to a mixed solution of oxalic acid-ethanol at a 10% excess over the amount for obtaining a precipitate to obtain a precipitate.
The obtained precipitate was dried and calcined in air at 1400 ° C. for 2 hours to obtain (Ca _0.4 Ce _0.6 ) ₂ Sn ₂ O ₇ (catalyst 1). The result of the X-ray diffraction measurement (CuKα ray) of the catalyst 1 is shown in FIG. From FIG. 1, a diffraction peak due to the pyrochlore structure was confirmed.

In order to evaluate the performance of the oxidation reaction of catalyst 1, the CO oxidation reaction known as the most general oxidation reaction was used as a model reaction. That is, the catalyst 1 was crushed after press molding and sized to 12 to 20 mesh, and 0.5 g of the crushed powder was charged into a fixed pressure fixed bed reaction tube. As a reaction gas, a mixed gas of CO (2400 ppm), O ₂ (4%), and He (balance) was flowed through the reaction tube at 5000 ml / hr, heated to 700 ° C., and held for 0.5 hour to pretreat And Then, the temperature was lowered and the CO conversion at a temperature of 650 to 150 ° C. was measured by TCD gas chromatography. Table 1 shows the results.

The CO conversion rate was calculated by the following equation.

CO conversion (%) = (CO _in −CO _out )
/ CO _in × 100 (CO _in and CO _out are CO concentrations at the fixed bed reaction tube inlet and outlet respectively)

[0023]

[Table 1]

Example 2 (Ca _0.6 Ce _0.4 ) ₂ Sn prepared in the same manner as in Example 1 except that a mixture of Ca, Ce and Sn was used in a molar ratio of 3: 2: 5. ₂ O ₇ was obtained (Catalyst 2). The result of the X-ray diffraction measurement of the catalyst 2 is shown in FIG. From FIG. 2, a diffraction peak due to the pyrochlore structure was confirmed. A CO oxidation reaction was performed in the same manner as in Example 1 using the catalyst 2.
The results are also shown in Table 1.

Comparative Example 1 For comparison with the catalyst of the present invention, Mn-substituted hexaaluminate, which is the most popular as a high temperature oxidation catalyst, was synthesized and used. 39 g of aluminum isopropoxide, 2.8 g of barium methoxide, 0.35 g of potassium nitrate, 1.79 g of manganese nitrate, and 2-ethoxy-ethanol-20.
It was dissolved in a mixed solvent of 0 ml and 80 ml of ethanol, and the solution was heated at 80 ° C. in nitrogen using a vessel equipped with a reflux condenser. Next, distilled water 10 ml, ethanol 80 ml, 2-
A mixed solution of 50 ml of ethoxy-ethanol was added dropwise to this alkoxide solution, and hydrolysis was performed to obtain a precipitate. The obtained precipitate was dried and calcined in air at 1300 ° C. for 2 hours to obtain Ba _0.8 K _0.2 MnAl ₁₁ O ₁₉ (catalyst 3).

The result of the X-ray diffraction measurement of the catalyst 3 is shown in FIG. The catalyst 3 was confirmed to have a diffraction peak due to the magnetoplumbite structure. Further, a CO oxidation reaction was performed using the catalyst 3 in the same manner as in Example 1. The results are also shown in Table 1.

[0027]

INDUSTRIAL APPLICABILITY The oxidation catalyst of the present invention has been treated at a relatively high temperature, and therefore is stable even at a high temperature,
Further, it is a catalyst having a high activity from a low temperature portion to a high temperature portion as compared with a conventional oxidation catalyst. Due to the high oxidizing power of this oxidation catalyst, it can be used as a catalyst for the oxidation reaction of hydrocarbons such as carbon monoxide and methane and the complete oxidation reaction of organic chlorine compounds.

[Brief description of drawings]

FIG. 1 is a diagram showing an X-ray diffraction result of catalyst 1.

FIG. 2 is a diagram showing an X-ray diffraction result of catalyst 2.

FIG. 3 is a diagram showing an X-ray diffraction result of catalyst 3.

Claims (2)

[Claims] 1. A pyrochlore represented by the following general formula (1) (M _1-x Ce _x ) ₂ Sn ₂ O ₇ (1) (wherein M is Ca or Sr and x is not 0). An oxidation catalyst characterized by having a structure. 2. The oxidation catalyst according to claim 1, wherein x is in the range of 0.4 ≦ x ≦ 0.7.