JPH0580258B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention is a method for removing harmful gases such as CO and HC contained in the combustion exhaust gas of various combustion equipment and internal combustion engines.
This invention relates to a catalyst that can convert CO ₂ and H ₂ O and also purify NO _x in a reducing atmosphere.

Conventional Technology 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 this type of catalyst. The former is used as a three-way catalyst in automobile exhaust gas purification catalysts, and can oxidize CO, HC, etc., and purify NO _x in a reducing atmosphere, but is expensive. The latter is used as an oxidation catalyst for CO, HC, etc. mainly in the presence of oxygen because it is easily reduced and its catalytic activity decreases at high temperatures and in a reducing atmosphere. In contrast, Ln _1-x
A _x Co _1-y M _y O _3- 〓-based oxide catalyst has high oxidation activity for CO, HC, etc., is difficult to reduce even at high temperatures and in a reducing atmosphere, and purifies NO _x in a reducing atmosphere. Moreover, it is an oxide catalyst that is cheaper than noble metals such as Pt and Pd.

Problems to be Solved by the Invention The oxide catalyst represented by the general formula Ln _{1-x A x} Co _1-y M _y O _3- Earth elements easily turn into nitrates, reducing catalytic ability.

Means for Solving the Problems The present invention solves the above problems by using the general formula Ln _1-x A _x Co _1-y M _y O _3- (Ln is La, Ce, Pr, Nd
At least one element selected from A is Ca, Sr,
At least one element selected from Ba, M is Cr,
At least one element selected from Mn, Fe, Ni, and V, 0<x<1, 0<y<1, δ is the number of oxygen vacancies in the oxide at room temperature and atmospheric pressure, 0<δ<
MgO or
It is a mixture or fired product of oxide whose main component is MgO.

Function In the catalyst body of the present invention, MgO acts as a basic site and absorbs NO _x in the atmosphere up to a temperature of nearly 400°C, and decomposes and returns to its original state at temperatures above 400°C.
Therefore, by mixing with MgO or an oxide containing MgO as a main component or firing it, the catalytic activity of the oxide catalyst is prevented from decreasing due to NO _x .

Example In this example, a case will be described in which an oxide consisting of La _0.5 Sr _0.5 CoO _3- 〓, S=0.05 is used. La _0.5
Sr _0.5 CoO _3- 〓 is precipitated as a mixture of metal oxalates and metal hydroxides using a solution containing the metal salts of each component and a mixture of oxalic acid, amino acid or caustic alkali, filtered, After cleaning and drying, in the air
It was created by firing at 800℃ for 10 hours.

MgO was mixed with the above oxide catalyst at a weight ratio of 7:3, and the mixture was supported on a carrier made of metallic Ni foam to prepare a measurement sample.

To evaluate the characteristics of the catalyst, the catalyst was placed inside a quartz glass tube passed through a tubular electric furnace, and the temperature was raised to 300°C.
The oxidation rate of CO was measured by setting 20ppm of CO, 50ppm _{of NOx} , and 7% H _{2 O} to send oil stove combustion exhaust gas. In addition, as a cycle life test, while sending kerosene stove exhaust gas with the same composition as above,
The temperature of the catalyst body was alternately maintained at 300° C. for 3 hours and at 600° C. for 1 hour, and this was repeated 500 times as one cycle, and the CO oxidation rate at 300° C. was measured for each cycle.

La _0.5 Sr _0.5 made in a similar way for comparison
Similar tests were conducted on a catalyst body in which CoO _3- 〓 was supported on a metal Ni foam of the same weight.

Figure 1 shows the measurement results of the CO oxidation rate when kerosene stove combustion exhaust gas containing 20 ppm of CO, 50 ppm of NO _x , 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 mixing MgO, the catalytic performance did not decrease even if the catalyst was left in an exhaust gas atmosphere containing water vapor and NO _x for a long time. In addition, the catalyst body mixed with MgO, whose catalytic ability had decreased, was left at room temperature for one day and the CO oxidation rate at 300°C was measured, and as a result, recovery of the catalytic ability was observed. this is
This is because NO ₂ ^- and NO ₃ ^- were extracted from the part damaged by NO _x due to the basicity of MgO.

Oil stove combustion exhaust gas containing 20ppm of CO, 50ppm _{of NO}
Hold alternately at 300℃ for 3 hours and at 600℃ for 1 hour.
The changes in CO oxidation rate at 300℃ during 500 cycle life tests are shown. From this result, by mixing MgO, MgO that has absorbed NO _x can be heated to 600°C.
of the oxide catalyst to completely return to the original MgO.
It was observed that the decrease in catalytic performance due to NO _x was extremely small.

Furthermore, in a NO _x reduction test under a reducing atmosphere, the catalyst body containing MgO also showed La _0.5 Sr _0.5
It was confirmed that the catalyst reduced NO _x in the same way as the catalyst containing only CoO _3- 〓.

A catalyst body in which a catalyst mixed with MgO was 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 there was almost no decrease in catalytic performance. Nakatsuta. This is because in the case of a catalyst containing only La _0.5 Sr _0.5 CoO _3- 〓, Co reacted with SiO ₂ and Al ₂ O ₃ and the catalytic performance decreased, but in a catalyst containing MgO, MgO interacted with the catalyst and SiO ₂ This is to prevent reactions with Al ₂ O 3 and Al 2 O ₃ .

As described above, the catalyst body for exhaust gas purification according to the present invention shows almost no decrease in catalytic performance even in an exhaust gas atmosphere containing a large amount of water vapor and _NOx , and
It is a catalyst that can be regenerated at a temperature of 600°C, which can be easily obtained using general combustion equipment.

Furthermore, in the embodiment, only the case where Ln is La and A is Sr, but La is Ce, Pr, Nd or
When containing two or more types of La, Ce, Pr, and Nd, A is Ca,
When containing Ba or two or more types of Sr, Ca, and Ba, M
Similar effects were obtained with oxidation catalysts containing Cr, Mn, Fe, Ni, V, or two or more thereof. In addition, although only the case where metallic Ni foam was used as the carrier was described, Fe, Ti or Ni, Fe,
A similar effect was obtained even when a metal support containing Ti as a main component was used.

When a Pt group noble metal catalyst is added to such an oxide catalyst, it was possible to obtain the same level of catalytic activity with the addition of an extremely small amount of the Pt group noble metal catalyst compared to a catalyst body using a conventional Pt group noble metal catalyst. .

Effects of the Invention According to the present invention, by mixing or calcining MgO or an oxide mainly composed of MgO with an oxide catalyst represented by Ln _1-x A _x Co _1-y M _y O _3- Even in an exhaust gas atmosphere containing a large amount of NO _x , the catalytic performance hardly decreases, and an inexpensive catalyst body can be obtained.

[Brief explanation of drawings]

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.
Figure 2 is a characteristic diagram showing the stability of the CO oxidation rate. Exhaust gas containing water vapor and _NO When carrying out a cycle life test
FIG. 2 is a characteristic diagram showing the stability of CO oxidation rate at 300°C.

Claims (1)

[Claims] 1 General formula Ln _1-x A _x Co _1-y M _y O _3- (Ln is La, Ce,
At least one element selected from Pr and Nd, A is
At least one element selected from Ca, Sr, and Ba, M
is at least one element selected from Fe, Mn, Cr, and V, 0<x<1, 0<y<1, δ is the number of oxygen vacancies in the oxide at room temperature and atmospheric pressure, and 0<δ<
0.5) and an oxidation catalyst represented by MgO or an oxide whose main component is MgO. 2. The exhaust gas 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 main components. Catalyst for purification. 3. The exhaust gas purification catalyst body according to claim 1, characterized in that an oxide catalyst is supported on a ceramic carrier.