JPH0567337B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention converts harmful gases such as CO and HC contained in the exhaust gas of various combustion devices and internal combustion engines into harmless CO ₂
The present invention relates to a catalyst that can convert NO x into NO x and H ₂ O, and also purify NO _x in a reducing atmosphere.

Prior Art Conventionally, noble metal catalysts such as Pt and Pd, and oxide catalysts such as CuO, MnO ₂ and Co ₃ O ₄ have been used as catalysts of this type. The former is used as a three-way catalyst in automobile exhaust gas purification catalysts and is expensive because it can purify NO _x in an oxidizing or reducing atmosphere such as CO or HC. The latter is easily reduced and its catalytic activity decreases at high temperatures and in a reducing atmosphere, so it is mainly used as an oxidation catalyst for CO, HC, etc. in the presence of oxygen. In contrast, La _1-x A _x Co _1-y M _y
O _3- type oxide catalysts have high oxidation catalytic activity for CO, HC, etc., and are difficult to reduce even at high temperatures and in reducing atmospheres.
In addition, it is an oxide catalyst that can purify NO _x in a reducing atmosphere and is cheaper than noble metal catalysts such as Pt and Pd.

Problems to be Solved by the Invention An oxide of the general formula La _1-x A _x Co _1-y M _y O _3- is an alkaline earth element A in an exhaust gas atmosphere containing a large amount of water vapor and NO _x . easily converts into nitrate and covers the catalyst, reducing its catalytic ability. Furthermore, when regenerating the catalyst, when nitrate is denitrified by heating, it is converted to carbonate by CO ₂ gas in the atmosphere, so a long-term high-temperature treatment similar to the catalyst synthesis conditions is required.

Means for Solving the Problems In order to solve the above problems, the present invention has the following features:
La _1-x A _x Co _1-y M _y O _3- 〓 Made of MgO by replacing element A of the oxide with Ni and then mixing or firing with MgO or an oxide whose main component is MgO. Added a layer of protection.

Function In the catalyst body according to the present invention, MgO acts as a basic site and absorbs NO _x in the atmosphere up to a temperature of nearly 400°C to produce nitrate or basic nitrate, but it decomposes at temperatures above 400°C and returns to its original form. return. For this
An oxide catalyst with MgO or a protective layer mainly composed of MgO has less reaction with NO _x in the atmosphere and is less likely to produce nitrates. Furthermore, even if nitrate is generated in the oxide catalyst, the nitrate ion is extracted by the strong alkaline action of MgO. This allows catalyst regeneration at low temperatures. but,
La _1-x A _x Co _1-y M _y O _3- 〓If there is a large amount of alkaline earth element A in the oxide catalyst, denitrification ions by MgO will be difficult to occur, so alkaline earth element
By replacing with Ni, the denitrification ion effect by MgO was enhanced.

Example In this example, a case will be described in which an oxide consisting of La _0.35 Sr _0.65 Co _0.7 Fe _0.3 O _3- is used.

La _0.35 Sr _0.65 Co _0.7 Fe _0.3 O _3- 〓 is precipitated as oxalate salts and hydroxides using oxalic acid, amines and sodium hydroxide in an aqueous solution containing each metal salt, filtered and washed. After drying, it was baked in air at 800°C for 10 hours. Ni containing Ni corresponding to 5% by weight of Sr for a predetermined amount of the above oxide catalyst
(No. ₃ ) ₂ was dispersed in an aqueous solution and heated while stirring to replace 5% by weight of Sr with Ni. Since the substituted Ni is a hydroxide, after the substitution, it was filtered, washed, dried, and heat treated in the air at 400°C for 1 hour to convert the substituted Ni to nickel oxide.

MgO was mixed in a ratio of 8:2 to an oxide catalyst in which 5% by weight of Sr in the oxide was replaced with Ni, and the metal Ni
It was supported on a carrier made of foam and used as a measurement sample.

To evaluate the catalyst, the catalyst was placed in a quartz glass tube passed through a tubular electric furnace, the temperature was set at 300℃, and kerosene stove exhaust gas containing 20ppm of CO, 60ppm _{of NOx} , and ₇ % of H2Q was sent to the furnace. The oxidation rate was measured.
In addition, the temperature of the catalyst body was maintained at 300℃ for 3 hours, and 600℃
A cycle test was conducted for 1 hour at 300°C, and the CO oxidation rate was measured at 300°C.

For comparison, a similar test was also carried out on an oxide catalyst in which only La _0.35 Sr _0.65 Co _0.7 Fe _0.3 O _3- 〓 prepared in the same manner was supported in the same weight on a support made of metallic Ni foam. went.

Figure 1 shows the measurement results of the oxidation rate of CO when kerosene stove exhaust gas containing 20 ppm of CO, 50 ppm of _NOx , and 7% of H _{2 O} was continuously sent to a catalyst body set at 300°C. From this result, it was confirmed that by providing a protective layer made of MgO, the catalytic performance did not deteriorate even if the catalyst was left in an exhaust gas atmosphere containing NO _x and water vapor for a long time. The catalyst body with the MgO protective layer with reduced catalytic ability was left at room temperature overnight, and then heated to 300°C.
As a result of measuring the CO oxidation rate, recovery of the catalytic ability was observed.

Figure 2 shows a cycle test in which kerosene stove exhaust gas containing 20 ppm of CO, 5 ppm of NO _x , and 7% of H _{2 O} was sent continuously, and the catalyst body was held alternately at 300°C for 3 hours ← → 600°C for 1 hour. The change in CO oxidation rate at 300℃ is shown.

It was found that by providing a protective layer made of MgO, most of the catalyst was regenerated at a temperature of 600°C, resulting in very little loss of catalytic performance.

Furthermore, regarding NO _x reduction, a catalyst body with a protective layer made of MgO can also reduce NO _x in a CO reducing atmosphere in the same way as an oxide catalyst body consisting only of La _0.35 Sr _0.65 Co _0.7 Fe _0.3 O _3- 〓. It was approved to do so.

A catalyst with an MgO protective layer supported on a ceramic support such as cordierite or mullite was heat treated in air at 600℃ for 100 hours, and the CO oxidation rate was measured at 300℃, but no decrease in catalytic performance was observed. I couldn't help it. This is because in a catalyst body consisting only of La _0.35 Sr _0.65 Co _0.7 Fe _0.3 O _3- 〓, Co reacted with Si and Al, reducing the catalytic ability, but when solid acids such as SiO ₂ and Al ₂ O ₃ On the other hand, MeO, a solid base, protects the catalyst.

As described above, the exhaust gas purification catalyst of the present invention shows little decrease in catalytic performance even in an exhaust gas atmosphere containing a large amount of water vapor and _NOx , and can be easily obtained using general combustion equipment at 600°C. It is a catalyst that can be regenerated at temperature.

Furthermore, in the embodiment, the case where A is Sr and M is Fe is described, but the present invention is not limited to this; A is Ca, Ba, M is Mn,
Similar effects were obtained in the case of Cr and V. Also,
Although a metallic Ni foam was used as the carrier, similar effects were obtained with Fe, Ti, or metallic carriers mainly composed of Ni, Fe, and Ti.

When Pt group noble metals are added to such an oxide catalyst, a very small amount of Pt group noble metal is added compared to conventional Pt group catalysts.
The same level of catalytic performance could be obtained by adding Pt group elements.

Effects of the Invention The present invention provides a protective layer of MgO or an oxide mainly composed of MgO on an oxide catalyst in which alkaline earth elements are replaced with Ni, so that MgO acts as a strong basic point and oxidizes other metals. In addition to preventing substances from turning into nitrates, the denitrification effect from nitrates is stronger than NiO, so the catalytic performance against exhaust gas containing a large amount of water vapor and _NOx hardly decreases, and an inexpensive catalyst body can be obtained. I can do it.

[Brief explanation of the drawing]

Figure 1 shows how the catalyst of one embodiment of the present invention reacts to exhaust gas containing water vapor and NO _x at 300°C.
A characteristic diagram showing the stability of the CO oxidation rate. Figure 2 shows the exhaust gas containing water vapor and _NOx being continuously fed, and the temperature of the catalyst body kept at 300℃ for 3 hours←→〓Kept at 600℃ for 1 hour FIG. 2 is a characteristic diagram showing the stability of the CO oxidation rate at 300°C in a cycle test.

Claims (1)

[Claims] 1 General formula La _1-x A _x Co _1-y Me _y O _3- δ (A is Ca, Sr,
At least one element selected from Ba, M is Mn,
At least one element selected from Cr, V, Fe, 0
<x<1,0<y<1,0<δ<0.5), in which an oxide catalyst in which element A is replaced with Ni is mixed with MgO or an oxide whose main component is MgO. Or an exhaust gas purifying catalyst body characterized by being fired. 2. Exhaust gas purification according to claim 1, characterized in that the oxide catalyst is supported on a carrier made of a metal made of at least one element selected from Ni, Fe, and Ti, or an alloy made of these as a main component. Catalyst for use. 3. The catalyst body for exhaust gas purification according to claim 1, characterized in that an oxide catalyst is supported on a ceramic carrier.