JPH0676980B2 - NOx gas detection element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a NOx gas detection element for detecting the concentration of NOx gas contained in a mixed gas, and particularly to an NOx gas detection element contained in exhaust gas of an internal combustion engine or the like. The present invention relates to a NOx gas detection element effective for detecting high-concentration NOx gas.

[Conventional technology and problems]

Conventionally, the chemiluminescence method, the infrared absorption method, the electrolysis method, etc. have been used to measure the NOx gas concentration used.
The implementation of these methods requires a large-scale device and thus is expensive, and there is a problem that maintenance is difficult.

In order to solve these problems, metal oxide semiconductors such as SnO ₂ and AgO-V ₂ O ₅ and organic semiconductors such as phthalocyanine complexes are NO.
A large number of researchers have studied a compact and maintenance-free NOx gas detector used as an x gas detection element.

Such a NOx gas detector connects a pair of electrodes to the NOx gas detection element, and measures the change in resistance that occurs when the NOx gas detection element adsorbs NOx to measure the NOx amount.

However, the conventional NOx gas detector described above has the following problems due to the characteristics of the NOx gas detection element.

(1) Due to its low sensitivity to NO, it is difficult to accurately measure the NOx concentration of NOx that accounts for 90% or more of the contained NOx, such as exhaust gas from internal combustion engines. .

(2) Since the upper limit of the quantifiable NOx gas concentration is as low as several hundred ppm, the NOx concentration is several thousand ppm, like exhaust gas from internal combustion engines.
Cannot be used when reaching

(3) NOx gas other than O _2, CO, because of the relatively high sensitivity to hydrocarbon (HC) and the like of the gas, when the NOx gas and these gases contaminating the accurate measurement of NOx concentration Difficult to do.

[Means for solving problems]

The present inventor has conducted extensive studies in order to solve the above-mentioned problems, and as a result, has a layer containing an oxidation catalyst on at least the surface thereof, and has a metal oxide containing a titanium atom having a specific oxygen defect. By using it as a NOx gas detection element, it shows equivalent and high sensitivity to NO and NO ₂ , and also has a high upper limit of measurable NOx gas concentration, and has extremely low sensitivity to contaminant gases other than NOx, It has been found that it exhibits excellent characteristics that the effect of the contaminant gas is small,
The present invention has been completed.

The present invention has a non-stoichiometric parameter (δ) of 0.01 <δ
(A) TiO ₂ , (b) a composite oxide of titanium and a metal that forms a solid solution with TiO ₂ , and (c) a metal and titanium that form an oxide having a perovskite structure with titanium having an oxygen defect of <0.5. A NOx gas detection element comprising at least one metal oxide (hereinafter also referred to as titanium-containing oxide) selected from the group consisting of a complex oxide of and an oxide catalyst-containing layer on at least the surface. .

In the present invention, the non-stoichiometric parameter (δ)
The value of is measured using ESCA (X-ray photoelectron spectroscopy) and the degree of vacuum is 10
^-7 Pascal, X-ray output Under the conditions of 10 kV, 25 mA, the measured value after Ar sputtering was performed at 1 kV, 25 mA for 30 seconds.

The NOx gas detection element of the present invention has a non-stoichiometric parameter (δ) of 0.01 <δ <0.5, preferably 0.04 to 0.2, which has an oxygen defect, and (a) TiO ₂ (b) TiO ₂ forms a solid solution. And (c) at least one oxide selected from the group consisting of a complex oxide of a metal and titanium, and (c) a complex oxide of a titanium and a metal forming a perovskite structure oxide. It is necessary to selectively show high sensitivity to NOx. That is, in the present invention, when the non-stoichiometric parameter (δ) of the titanium-containing oxide is smaller than the above range, the sensitivity to oxygen is high and the sensitivity to NOx is remarkably lowered. Further, if the non-stoichiometric parameter (δ) is larger than the above range, the sensitivity to all gases decreases.

As a typical example of such titanium-containing oxide, as the titania of (a), TiO _{2 −} δ and (b)
Of the general formula AxTi _1- xO _2- δ (where A is A
l, Nb, Ta, Sb, As, Ga, In, Sc, Mg, and at least one element selected from the group consisting of Y, and 0 <x <1)
Further, the oxide represented by the formula (C) is used as a composite oxide of the general formula BTiO _3- δ (where B is Pb, Ca, Sr, Cd, Zn, La and Ba).
Indicating at least one element selected from the group consisting of)
And the like.

If Specific examples of the titanium-containing oxide represented by the general formula _{AxTi 1- xO 2- δ, AxTi 1-} xO 2- δ, NbxTi 1- xO 2- δ, Tax
_{Ti 1- xO 2- δ, SbxTi 1-} xO 2- δ, AsxTi 1- xO 2- δ, GaxTi 1- xO 2-
δ, InxTi _1- xO _2- δ, ScxTi _1- xO _2- δ, YxTi _1- xO _2- δ, MgxTi
_1- xO _2- δ, Alx ₁ Nbx ₂ Ti _(1- x _1- x ₂₎ O _2- δ, Inx ₁ Sbx ₂ Ti _(1- x _1-
x ₂₎ O _2- δ, Gax ₁ Asx ₂ Ti _(1- x _1- x ₂₎ O _2- δ, Yx ₁ Nbx ₂ Ti _(1- x _1- x
₂₎ O _2- δ and the like. In the above equation, x is
0 <x <1, preferably 0 <x ≦ 0.5, where x1 and x2 are
0 <(x1 + x2) <1, preferably 0 <(x1 + x2) ≦ 0.5
Is.

Further, the titanium-containing oxide represented by the general formula BTiO _{3 −} δ is an oxide having a perovskite type structure, and specific examples thereof include CaTiO _{3 −} δ, SrTiO _{3 −} δ, BaTiO _{3 −} δ, Pb
TiO _3- δ, Cd TiO _3- δ, ZnTiO _{3 -} δ, LaTiO _3- δ, BaTiO _3- δ, Nd
TiO _3- δ, SryBa _- yTiO _3- δ, BayLa _1- yTiO _3- δ, CaySr _1- yTiO
_3- δ, etc.

In the above formula, y is 0 <y <1, and these titanium-containing oxides are used alone or in combination with other titanium-containing oxides to form a NOx gas detection element.

The titanium-containing oxide which is the NOx gas detecting element of the present invention is represented by the above-mentioned chemical formula, but in order to increase the reactivity with NOx, it is generally used at 200 to 700 ° C., preferably 400 to 600 ° C. It is preferable to heat.

The shape of the NOx gas detection element of the present invention is not particularly limited, and the N
It may be appropriately determined according to the structure of the NOx gas detector formed using the Ox gas detection element. For example, chips, films, etc. are generally used. The production method of the titanium-containing oxide having such a shape is not particularly limited, but as a typical production method,
Examples thereof include a sintering method, a sputtering method, a vapor deposition method, a thermal decomposition method and the like. Among the above methods, the sintering method is suitable for forming a chip-shaped or film-shaped titanium-containing oxide, and the sputtering method, the vapor deposition method, or the thermal decomposition method is suitable for forming a film-shaped titanium-containing oxide. A specific manufacturing method is illustrated below.
First, as a manufacturing method by a sintering method, a method in which a titanium-containing oxide powder is filled in a cavity having a predetermined shape and compression-formed, or after compression-molding and simultaneously heating and sintering is preferable. is there. The pressure in the compression molding is
^{200kg / cm 2 ~1t / cm 2} , generally is suitable 300~700kg / cm ^2. Further, the sintering temperature and the sintering atmosphere determine the non-stoichiometric parameter (δ) and are particularly important. In a non-reducing atmosphere (N ₂ , Ar, etc.), the sintering temperature (T) may be 900 ° C. <T <melting point. Further, when using an atmosphere of a reducing gas such as CO, H ₂ or the like, a suitable sintering temperature varies depending on the kind and concentration of the gas, but in N ₂ containing 5% of CO, for example, 700 ° C <
T <1000 ° C. is desirable, 600 ° C. in a N ₂ containing H ₂ 5%
<T <900 ° C is desirable. Of course, a combination of the above-mentioned sintering conditions, for example, a method of sintering in a non-reducing atmosphere and then processing in a reducing atmosphere may be used.

Further, as another method of the sintering method, titanium-containing oxide powder is mixed with a dispersion medium to form a paste, which is screen-printed on an insulating substrate and then sintered as described above. There is a method of sintering at a temperature.

In the above-mentioned sintering method, a compound such as a hydroxide containing titanium or alkoxide may be used in place of the starting material containing titanium oxide, and the compound may be oxidized and sintered.

Further, as the sputtering method, for example, metal titanium is used as a target material, and a thin film is formed by performing sputtering on an insulating substrate such as alumina in the presence of oxygen,
Next, a method of obtaining a TiO ₂ thin film by firing the thin film at 500 to 800 ° C. in air can be mentioned.

Further, as a vapor deposition method, for example, metal titanium is oxygen pressure 0.
There is a method of evaporating under 5 to 3 Torr and vapor-depositing this vapor on an insulating substrate such as alumina to form a TiO ₂ thin film.

Furthermore, as the thermal decomposition method, a solution of an organic metal compound such as alkoxide of a metal constituting a target titanium-containing oxide is applied to a substrate such as alumina, and then a non-reducing atmosphere such as air or a reducing atmosphere is applied. A method of forming a TiO ₂ thin film by thermally decomposing at a temperature of 500 ° C. to a melting point or lower is mentioned.

Above, Ti by sputtering method, vapor deposition method, thermal decomposition method, etc.
Although the method of manufacturing a thin film of O ₂ has been described, other titanium-containing oxide thin films can be manufactured according to the above method.

The NOx gas detection element consisting of only the above titanium-containing oxide has extremely low sensitivity to contaminant gases,
When the concentration of the contaminant gas in the NOx gas to be measured is extremely high, 1000 ppm or more, the phenomenon that the measured value falls below a certain level occurs.

In the present invention, the presence of a layer containing an oxidation catalyst on at least the surface of the element comprising the above-mentioned titanium-containing oxide, even in the presence of a low concentration to high concentration of contaminant gas, always accurately and accurately Necessary for making measurements.

In the present invention, the oxidation catalyst is not particularly limited as long as it can exhibit the performance as an oxidation catalyst at the operating temperature of the NOx gas detecting element. For example, Pt, Ph, Pd and other noble metal-based oxidation catalysts, Ni, Fe and other metal-based oxidation catalysts, LaCO ₃ , LaNiO ₃ , La
Typical examples are metal oxide-based oxidation catalysts such as Sr _0.3 CO _0.7 O ₃ . Of these, noble metal-based oxidation catalysts are particularly preferable.

The oxidation catalyst may be present as a layer containing the oxidation catalyst on at least the surface of the element made of the titanium-containing oxide. That is, as an aspect in which a layer containing an oxidation catalyst is present on at least the surface of an element comprising a titanium-containing oxide, an aspect in which an oxidation catalyst is dispersed throughout the element, and an oxidation catalyst is added to the surface layer portion of the element Examples thereof include a mode in which the element is contained and partially dispersed, and a mode in which a layer containing an oxidation catalyst is provided on the surface of the element. Of these, the aspects (1) and (2) are particularly preferable. In the above aspect, the layer containing the oxidation catalyst may be formed by carrier particles carrying the oxidation catalyst, or may be formed by the oxidation catalyst alone in some cases.

The above and aspects are suitable for the above-mentioned noble metal-based oxidation catalyst and metal-based oxidation catalyst. In this case, the concentration of the noble metal-based oxidation catalyst in the titanium-containing oxide-containing catalyst matrix is 10 to 100 ppm, preferably 100 to 600 pp.
The concentration of m and the concentration of the metal-based oxidation catalyst are generally 0.1 to 5% by weight, preferably 0.5 to 3% by weight. In addition, the embodiment of can be applied to all oxidation catalysts. Particularly, the noble metal-based oxidation catalyst and the metal-based oxidation catalyst are
A method of supporting the carrier particles to form a layer is preferable,
In this case, the concentration of the oxidation catalyst on the surface of the carrier particles is preferably the concentration in the above-mentioned oxidation-containing catalyst matrix.
Further, the metal oxide-based metal catalyst preferably forms a layer by the catalyst alone. In the above and aspects, the thickness of the layer containing the oxidation catalyst is generally preferably 1 μm or more.

In the present invention, the method for forming the layer containing the oxidation catalyst on the element made of titanium-containing oxide is not particularly limited.
For example, in the above embodiment, when a titanium-containing oxide is produced by a sintering method, a thermal decomposition method, or the like, an oxidation catalyst is generated in the raw material, or an oxidation catalyst is generated by the sintering method or heating during thermal decomposition A method of compounding the compound is preferable. Also,
In this mode, it is preferable to heat the titanium-containing oxide after impregnating it with a solution of a compound that forms an oxidation catalyst by heating. Examples of the compound that produces an oxidation catalyst by such heating include soluble salts such as the above-mentioned noble metals or metal chlorides, nitrates, and organic acid salts. Further, in the embodiment, a method of attaching an oxidation catalyst to the surface of the element made of titanium-containing oxide by a method such as a sputtering method, a vapor deposition method, a sintering method, or a thermal decomposition method, TiO ₂ , Al ₂ O ₃ , MgO ・ Al ₂ O ₃ , Si
Examples include a method of supporting an oxidation catalyst on carrier particles such as O ₂ and then adhering the particles to the surface of an element made of a titanium-containing oxide by means such as sintering.

As the NOx gas detector using the NOx gas detecting element of the present invention, a known structure is adopted without particular limitation.

FIG. 1 is a perspective view showing a typical mode of a NOx gas detector using a square chip NOx gas detecting element. That is, the above
The NOx gas detector is provided with at least a part of the NOx gas detection element 1 exposed on a support base made of an insulating substrate 3,
A structure in which a pair of electrodes 2 are connected to the NOx gas detecting element 1 with a space therebetween and a heater 4 (a heater electrode is not shown) is provided so as to be located in the vicinity of the NOx gas detecting element 1. Have. In the above NOx gas detector, the insulating substrate 3 is for supporting the NOx gas detecting element 1, the heater 4, and the electrode 2 and has an insulating property.
A material having heat resistance to the heating temperature is used without particular limitation. Such materials include alumina, MgO / Al ₂ O
₃ , AlN and the like are preferable. Further, the heater 4 is for heating the NOx gas detection element 1 to enhance the reactivity with NOx. The heater 4 is preferably provided in the vicinity of the NOx gas detecting element 1 so that the NOx gas detecting element 1 can be heated via the insulating substrate 3. Specifically, as shown in FIG. 1, it is preferable that the NOx gas detecting element 1 be embedded in an insulating substrate in the vicinity thereof, or that the NOx gas detecting element 1 be attached to a surface other than the exposed surface. Further, the material of the heater 4 is not particularly limited as long as it can raise the temperature to a desired temperature by energization. Examples of suitable materials include platinum, tungsten, ruthenium oxide, and silicon carbide.

FIG. 2 shows a typical circuit diagram of a NOx gas measuring device incorporating a NOx gas detector. That is, the NOx gas detector 5 is connected in series with the circuit power supply 6 and the voltmeter 7 via the electrodes. Further, the load resistance 8 is connected in parallel with the voltmeter 7. On the other hand, the heater 4 is connected to the heater power source 9.

The NOx gas concentration is measured by the NOx gas measuring device by operating the heater 4 and setting the NOx gas detecting element 1 at a predetermined temperature,
For example, the NOx gas detecting element 1 is placed in the gas for measurement of damage while being heated to 400 to 600 ° C., and the voltage at that time is measured by the voltmeter 7.

That is, the voltage Vc of the circuit power supply 6 and the resistance R _L of the load resistance 8, the output voltage Vout measured by the voltmeter 7 and the resistance R of the NOx detection element.
There is the following relationship with _S, and by measuring Vout, R _S can be easily calculated from the following equation (1).

When _{R S = R L (Vc-} Vout) / Vout (1) NOx gas detecting element of the present invention the element temperature is constant, indicating the decided in accordance with the NOx concentration in the atmosphere resistance. Therefore,
The more R _S, by obtaining from the previously prepared calibration curve of the NOx gas concentration for this, it is possible to know the NOx gas concentration in the measurement gas.

[Effect]

The NOx gas detection element of the present invention is (1) both highly sensitive to NO and NO ₂ and equivalent, (2) sufficiently sensitive to high-concentration NOx gas, and (3) contaminated. It is characterized by being extremely insensitive to gas.

Therefore, it is possible to accurately measure the NOx concentration in the mixed gas containing NOx over a wide range.

The NOx gas detection element of the present invention can also detect the NOx concentration in a general mixed gas, but has a property that the sensitivity to a NOx gas having a higher concentration is sufficiently high, and that it is not affected by contaminant gas. Therefore, install the element directly on the flue of the internal combustion engine, etc., and track the resistance change of the element
Suitable for monitoring NOx concentration. Furthermore, it is possible to develop a feedback system that tracks the resistance change of the element and changes the operating conditions of the internal combustion engine or the like when an abnormality occurs.

〔Example〕

Examples will be shown below for specifically explaining the present invention, but the present invention is not limited to these examples.

In the examples, the NO, CO, H ₂ and HC sensitivities were obtained by the following methods.

(1) NO sensitivity; resistance R ₁ and O ₂ in an atmosphere containing O ₂ 5%
Ratio with resistance R ₂ in an atmosphere containing 5% and NO 1000ppm log
It is represented by (R ₂ / R ₁ ).

(2) CO sensitivity; O ₂ 5%, the ratio of the resistance R ₁ in an atmosphere containing NO500ppm and CO50ppm, O ₂ 5%, and the resistance R ₂ in the atmosphere containing NO500ppm and CO500ppm, log (R ₂ / R ₁ ).

The ratio of O ₂ 5%, and the resistance R ₁ in an atmosphere containing NO500ppm and _{H 2 50ppm, O 2 5%} , and the resistance R ₂ in the atmosphere containing NO500ppm and _{H 2 5000ppm; (3) H} 2 sensitivity , Log (R ₂ / R ₁ ).

(4) HC sensitivity; O ₂ 5%, and the resistance R ₁ in an atmosphere containing NO500ppm and _{C 3 H 6 50ppm, O 2} 5%, NO500ppm and C ₃ H ₆ 5000p
The ratio to the resistance R ₂ in the atmosphere containing pm is expressed by log (R ₂ / R ₁ ).

It can be said that the larger the value represented by log (R ₂ / R ₁ ) is, the higher the sensitivity to the gas is.

Comparative Example 1 An aqueous solution of ammonium sulfate and ammonia were added to an aqueous solution of TiCl ₄ , the precipitate formed was filtered, washed, and then calcined in air at 900 ° C. for 1 hr. The obtained fired powder is put into the cavity, Pt electrodes are embedded in both ends of the cavity, and compression molding is performed.
It was made into a chip shape as shown in the figure. Then, the chip-shaped molded body was fired in air at 1200 ° C. for 4 hours to obtain a TiO ₂ —δ sintered body. The value of δ of TiO ₂ −δ of the obtained sintered body was 0.05.

NOx with the structure shown in Fig. 1 using the TiO ₂ -δ chip
A gas detector was constructed. In this NOx gas detector, Al ₂ O ₃ was used as the insulating substrate and platinum was used as the heater.

Using the NOx gas detector thus obtained, the sensitivity to various gases was measured. The measurement is by the heater
The NOx detection element was placed in a predetermined gas while being heated to 500 ° C.

The results are shown in Table 1.

Examples 1 to 3 Ammonium sulfate aqueous solution and ammonia were added to TiCl ₄ aqueous solution, and the formed precipitate was filtered and washed, and then heated in air at 900 ° C. for 1 hr. _{Then, H 2 PtCl 6, H 2} PdCl 4, RhCl 3 -4
An aqueous solution of H ₂ O was added to Pt, Pd, and Rh to TiO _{2 at} 300 pp each.
It was added so as to have a size of m, dried and then baked at 500 ° C. for 1 hr. The obtained powder was formed into chips in the same manner as in Comparative Example 1, and then fired in air at 1200 ° C. for 4 hours. The chip-shaped sintered body was measured for sensitivity to various gases in the same manner as in Comparative Example 1.

The results are shown in Table 1.

Example 4 The Pt / TiO ₂ powder obtained in Example 1 was applied to the surface of the TiO ₂ −δ chip obtained in Comparative Example 1, and then 1200 in air.
Calcination was performed for 1 hr. The chip-shaped sintered body was measured for sensitivity to various gases in the same manner as in Comparative Example 1.

The results are shown in Table 1.

Examples 5 to 11 General formula AxTi ₁ -xO ₂ (A is Al, Nb, Ta, Sb, As, G
a, an In, SC, represents at least one element selected from the group consisting of Mg and Y, the production of O <X <1 and is) from consisting titanium-containing oxide, an oxide and TiO ₂ of A Was mixed at a predetermined molar ratio and baked in air at 1000 ° C. for 1 hr. Further, a titanium-containing oxide composed of BTiO ₃ (B represents at least one element selected from the group consisting of Ca, Sr, Ba, Pb, Cd, Zn and Ln) can be produced by using a carbonate of B and Ti.
It was performed by mixing O ₂ with a predetermined molar ratio and firing in air at 1200 ° C. for 1 hour. The obtained oxide and a mixture thereof were molded into chips in the same manner as in Comparative Example 1 and then calcined in air at 1200 ° C. for 4 hours to obtain a chip-shaped sintered body shown in Table 1. . The Pt / TiO ₂ powder obtained in Example 1 was applied to the surface of the obtained chip-shaped sintered body, followed by firing in air at 1200 ° C. for 1 hr. This chip-shaped sintered body was measured for sensitivity to various gases in the same manner as in Comparative Example 1.

The results are shown in Table 1.

Example 12 NiO was mixed with TiO _{2 in} an amount of 1%, and the mixture was mixed with 1% in air.
It was baked at 200 ° C for 1 hr. The obtained powder was applied to the surface of the TiO ₂ —δ chip obtained in Comparative Example 1 and then the powder was applied in air 12
It was baked at 00 ° C for 1 hr. The sensitivity to various gases was measured for this chip-shaped sintered material in the same manner as in Comparative Example 1.

The results are shown in Table 1.

Example 13 LaCO ₃ and Co (CH ₂ COO) ₂ were mixed at a molar ratio of 1: 1 and calcined in air at 1200 ° C. for 1 hr. The obtained powder was applied to the surface of the TiO ₂ —δ chip obtained in Comparative Example 1 and then fired in air at 1000 ° C. for 1 hr. The chip-shaped sintered body was measured for sensitivity to various gases in the same manner as in Comparative Example 1.

The results are shown in Table 1.

[Brief description of drawings]

FIG. 1 is a perspective view showing a typical embodiment in which the NOx gas sensing element of the present invention is used in a NOx gas detector, and FIG. 2 is NO.
The typical circuit diagram of the NOx gas measuring device incorporating the x gas detector is shown. In the figure, 1 is a NOx gas detection element, 2 is an electrode, 3 is an insulating substrate, 4 is a heater, 5 is a NOx gas detector, 6 is a circuit power supply, 7 is a voltmeter, 8 is a load resistance, and 9 is for a heater. Power supplies are shown respectively.

Claims (1)

[Claims] 1. A non-stoichiometric parameter (δ) is 0.01 <
(a) TiO ₂ and (b) a composite oxide of titanium and a metal that forms a solid solution with TiO ₂ , and (c) a metal that forms an oxide having a perovskite structure with titanium and that has an oxygen defect of δ <0.5. An NOx gas detection element, which comprises at least one metal oxide selected from the group consisting of complex oxides with titanium and has a layer containing an oxidation catalyst on at least the surface.