JPH0347144B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a catalyst that purifies harmful gas components in exhaust gas. Structure of conventional examples and their problems La _0.5 Sr _0.5 MnO ₃ and La _0.8 Sr _0.2 CoO ₃ , which are perovskite-type composite oxides with compositions similar to those of the present invention, have been studied using cheaper exhaust gas as a purification catalyst. Ta. These have the advantage of being highly heat resistant and having catalytic activity comparable to other precious metal catalysts, but their targets are CO and hydrocarbon HC, which are reducing gases in exhaust gas, and they cannot be used in the same exhaust gas. Almost no attempts have been made to treat the included oxidizing gas _NOx . In addition, the catalytic activity of these oxides is not very high, and furthermore, when exposed to a reducing gas atmosphere for a long time, structural changes occur, leading to a decrease in catalytic activity, loss of reproducibility, etc. It has the disadvantage of being lacking in OBJECTS OF THE INVENTION In view of the above-mentioned drawbacks, the present invention aims to improve catalyst activity and stabilize characteristics, and to provide a catalyst that can simultaneously oxidize reducing gases and decompose and reduce oxidizing gases. purpose. Structure of the invention The present invention is based on the general formula

[Formula] (Me is an element selected from Fe, Mn, Cr, V, and Ti, 0<x<1), and has the ability to simultaneously oxidize reducing gases and decompose and reduce oxidizing gases. This is a catalyst made of a perovskite-type complex oxide. It was discovered that the catalytic activity is highest when the ratio of each component element is determined as shown in the formula, and that by replacing Co at the B site with Me element, stable activity can be obtained that is resistant to reducing atmospheres. It is based on Description of Examples FIG. 1 shows an example of the activity of an oxide catalyst having the composition La _1-x Sr _x Co _1-y Fe _y O ₃ . The catalyst is a sintered composite oxide obtained by mixing oxides of each component element or salts such as carbonates and acetates so as to have a predetermined composition and baking the resulting composite oxide into cylindrical pellets weighing 1 g/piece. I used the one I made. Four of the prepared catalyst pellets were placed in a reaction container with a fixed volume, and a homogeneous mixed gas of 100 ppm CO, 5% O ₂ , and the balance N ₂ adjusted to atmospheric pressure was introduced into the container, and the container was exposed to the catalyst for 1 minute. The gas pressure inside was measured, and the rate of change was defined as the catalytic activity. The temperature was kept constant at 300°C. From this, when y=0, x=0.5, and when y=0.3, x=
When the element ratio becomes 0.65, y=0.6 and x=0.8, and y=1.0 and x=1.0, the gas pressure change rate is maximum, respectively.
That is, it can be seen that the activity is maximized. It is also clear that the activity is improved by increasing the amounts of Sr and Fe substitution at the A site and the B site to some extent. Figure 2 shows NO ₂ for a catalyst with the same composition as above.
This is an example of data obtained regarding the decomposition of . Forty-five catalyst pellets prepared in the same manner as above were filled into a quartz tube with an inner diameter of 20 mm, and the tube was fixed in an electric furnace and the temperature was kept constant at 300°C. A homogeneous mixed gas containing 40 ppm of NO ₂ and the remainder of N ₂ was introduced into the catalyst layer, and when a steady state was reached, the NO ₂ concentration on the outlet side was measured to determine the rate of decrease with respect to the inflow gas concentration. In this case, as in the case of CO oxidation,
The NO ₂ removal rate is maximized when each element ratio is x = 0.5 at y = 0, x = 0.65 at y = 0.3, x = 0.8 at y = 0.6, and x = 1.0 at y = 1.0. Obtained. In each curve of FIG. 1 and FIG. 2, the compositions showing the activity peak are La _0.5 Sr _0.5 CoO ₃ and La _0.35 Sr _0.65.
Co _0.7 Fe _0.3 O ₃ , La _0.2 Sr _0.8 Co _0.4 Fe _0.6 O ₃ , SrFeO ₃ ,
It is. In LaCoO ₃ , Co exists with a valence of +3, but when La at the A site is replaced with Sr ⁺²
Co ⁴⁺ is produced. Furthermore, replace Co at B site
Assuming that Fe enters with a valence of +4, in the oxides with the above four compositions, it becomes Co ⁺³ due to the electrically neutral condition.
Equal amounts of Co ⁴⁺ will be present. That is, Co ³⁺
A site and B site so that equal amounts of Co 4+ and Co ⁴⁺ are present.
It is thought that high activity can be imparted by determining the amount of site substitution. That is, B site substitution element
If the Me substitution amount is x, then the highly active oxide composition is

It can be expressed as [Formula]. Next, when x=0.3, that is, La _0.35 Sr _0.65 Co _0.7
The catalytic activity of an oxide expressed as Me _0.3 O ₃ will be shown. Figure 3 shows the CO reduction rate, and Figure 4 shows the NO ₂ reduction rate. Fill a quartz tube with an inner diameter of 20 mm with 40 catalyst pellets prepared in the same manner as in the previous case, fix it in an electric furnace, and uniformly mix 150 ppm of CO, 50 ppm of NO ₂ , and the balance N ₂ while maintaining the temperature at 300°C. Gas was introduced into the catalyst layer, and each concentration on the outlet gas side when a steady state was reached was measured to determine the rate of decrease with respect to the inflow gas concentration. This data also shows specific stability. That is, the diameter of 40 catalyst pellets is
A piece of 120mm heat-resistant and corrosion-resistant wire mesh sandwiched evenly between two pieces was placed on top of the combustion tube of a portable gas stove, and the cycle of 30 minutes of burning and 30 minutes of extinguishing was repeated 1000 times. The temperature of the catalyst layer during combustion is approximately 700°C. Then, perform the above activity measurement every 1000 times,
I did this a total of 5000 times. La _0.5 Sr _0.5 as comparison
Similar catalyst pellets of MnO ₃ and La _0.8 Sr _0.2 CoO ₃ were tested. Both CO reduction rate and NO ₂ removal rate
La _0.35 Sr _0.65 Co _0.7 Me _0.33 (Me=Fe, Mn, Cr, V,
Ti) has high activity, and there is almost no decrease in activity even after 5000 combustion tests. Compared to this La _0.5
Sr _0.5 MnO ₃ and La _0.8 Sr _0.2 CoO ₃ are inferior in both activity and stability. It is thought that the effects of the B-site substitution and the amount of substitution are reflected in the high activity and high property stability at the same time. Next, La _0.35 Sr _0.65 Co _0.7 Fe _0.33 , La _0.35 Sr _0.65 Co _0.7
The results of a combustion test in which 50 catalyst pellets each of Mn _0.3 O ₃ and La _0.5 Sr _0.5 MnO ₃ sandwiched between stainless steel SUS316 gold steel were incorporated into an actual gas combustion device are shown. FIG. 5 is a schematic cross-sectional view of this combustion section. 1 is a burner, and 2 is a catalyst pellet placed in the opening of a cylinder 3 surrounding the burner 1 and sandwiched between wire meshes 4. The exhaust gas temperature on the upstream side of the catalyst layer during combustion was approximately 600°C. The results are shown in the table below.

[Table] As is clear from the test results, the catalyst according to the present invention has a much higher purification ability than similar conventional examples. Although pellet-shaped catalysts have been described in the above embodiments, they can also be formed into a honeycomb-like structure having a large number of ventilation holes, or supported on various types of phase bodies. As a method for supporting the catalyst on a carrier, for example, a porous alumina carrier is impregnated with a mixed aqueous solution of acetate as a component of the catalyst, and the salt is thermally decomposed at about 400°C.
The catalyst can also be supported on the carrier by calcining in air at 1100°C for about 3 hours. Effects of the Invention As described above, the catalyst of the present invention has an activity of simultaneously oxidizing a reducing gas and decomposing and reducing an oxidizing gas with high efficiency, and has extremely stable characteristics. It has high activity and stability that cannot be obtained with conventional similar catalysts, and also has high heat resistance.
Furthermore, since it is relatively inexpensive and easy to manufacture, it is extremely useful as an exhaust gas purification catalyst.

[Brief explanation of drawings]

Figure 1 shows the CO oxidation activity of La _1-x Sr _x Co _1-y Fe _y O ₃ , Figure 2 also shows the NO ₂ decomposition reduction activity, and Figure 3 shows the CO oxidation activity of La 1-x Sr x Co _1- y Fe y _O 3. Co _0.7 Me _0.3 O ₃ (Me
= Fe, Mn, Cr, V, Ti) and the conventional example. Figure 4 is a diagram also showing the NO ₂ decomposition reduction activity. Figure 5 is the gas combustion equipment. FIG. 2 is a schematic vertical cross-sectional view showing a state in which a catalyst is attached to the holder.

Claims (1)

[Claims] 1 General formula [Formula] (Me is an element selected from Fe, Mn, Cr, V, Ti, 0<
An exhaust gas purifying catalyst comprising a perovskite composite oxide represented by x<1).