Module and device for detecting NOX gas

An element for detection of NO.sub.x gas in which at least one titanium-containing oxide which is selected from the group of (a) titanium oxide, (b) a solid solution wherein at least one element from the group of aluminum (Al), gallium (Ga), indium (In), scandium (Sc), magnesium (Mg), yttrium (Y), neodymium (Nd), tantalum (Ta), antimony (Sb) and arsenic (As) is contained in the solid-solution state in the titanium oxide, and (c) an oxide which contains titanium and other metal element(s) and has a perovskite structure, and has such an oxygen deficiency that its nonstoichiometric parameter (.delta.) is in the range of 0.03 to 0.3. Thereby, NO.sub.x of an exhaust gas can be measured easily.

The present invention relates to an element for detection of NO.sub.x gas 
consisting of NO and NO.sub.2 in a mixed gas, a module for detection of 
NO.sub.x, a device for detection of NO.sub.x and a process for detection 
of NO.sub.x. More specifically, the invention relates to an element for 
detection of NO.sub.x gas which has a selectively high sensitivity to 
NO.sub.x in a mixed gas and an excellent sensitivity even in a high 
NO.sub.x concentration of several thousands ppm, and a series of 
techniques utilizing it. 
Heretofore, nitrogen oxides NO and NO.sub.2 in a mixed gas, especially 
exhaust gas, have been referred to as "NO.sub.x ", and measurement of the 
concentration of NO.sub.x has been carried out using a method such as a 
chemiluminescence method, infrared absorption method or electrolysis 
method. However, these methods have problems, for example, that they need 
large-sized devices and thus are costly and require maintenance. 
For solution of these problems there have been developed elements for 
detection of NO.sub.x gas, as described below, using a certain 
semiconductor consisting of a metal oxide or an organic semiconductor such 
as a phthalocyanine complex, and devices for detection of NO.sub.x gas 
have been proposed which use the element for detection of NO.sub.x gas, 
and are small-sized and maintenance-free: 
(a) U.S. Pat. No. 4,169,369 and SAE Technical Paper Series 800537 
This U.S. patent discloses a method of detection or measurement of NO.sub.x 
gas in a mixed gas using a thin film comprising polycrystalline tin oxide 
having an oxygen to tin atomic ratio between 1.5 and 1.95, 
(b) Japanese Utility Model Publication No. 32360/1986 
This Japanese Utility Model Publication discloses a semiconductor device 
for detection of nitrogen oxides which uses a (NO+NO.sub.2) gas sensor 
consisting of a V.sub.2 O.sub.5 +Sm.sub.2 O.sub.5 +Ag semiconductor thin 
film layer and a NO.sub.2 gas sensor consisting of a V.sub.2 O.sub.5 +Ag 
semiconductor thin film layer. 
(c) Sensor and Actuators, 16, (1989), 379-392 
A paper titled "Kinetics factors in the response of organometallic 
semiconductor gas sensor" is carried in this literature, and results 
obtained when various phthalocyanine complexes are used as NO.sub.2 
sensors are reported therein. 
On the other hand, it is disclosed in SAE Technical Paper Series 750224 
published by Society of Automotive Engineers Inc. to use titania 
(TiO.sub.2) as an air to fuel ratio sensor for automobile exhaust. 
This literature discloses utilization of titania (TiO.sub.2) as a sensor to 
detect oxygen partial pressure in an exhaust gas, and therein a titania 
ceramic is maintained at a temperature of 700.degree. to 1,000.degree. C. 
However, this literature is utterly silent about titania having oxygen 
deficiency and its utilization for detection of NO.sub.x gas. 
The above NO.sub.x gas-detecting device using the NO.sub.x gas-detecting 
element or sensor is a device wherein a couple of electrodes are connected 
to the NO.sub.x gas-detecting element and wherewith change of electric 
resistance taking place when the NO.sub.x gas-detecting element absorbs 
NO.sub.x is measured to measure NO.sub.x amount. The device has drawn 
attention as a small-sized and maintenance-free NO.sub.x gas-detecting 
device. 
However, the above usual NO.sub.x gas-detecting devices have the following 
problems due to characteristics of the NO.sub.x gas-detecting element: 
(1) Due to their low sensitivity to NO, it is hard to accurately determine 
the NO.sub.x concentration of a gas wherein NO.sub.x occupies 90% or more 
of the contained NO.sub.x such as an exhaust gas from an internal 
combustion engine or the like. 
(2) As the upper limit of quantitatively determinable NO.sub.x gas 
concentration is several hundreds ppm and thus low, they cannot be used 
for detection of a gas whose NO.sub.x concentration reaches even several 
thousands ppm such as an exhaust gas from an internal combustion engine or 
the like. 
(3) As they have relatively high sensitivity also to other gases such as 
O.sub.2, CO and hydrocarbon (HC) than NO.sub.x gas, it is hard to 
accurately determine NO.sub.x concentration of a gas containing both 
NO.sub.x gas and these gases. 
(4) Concerning an organic semiconductor, it cannot be set inside the flue 
of an internal combustion engine or the like for lack of thermal 
stability. 
Thus, the first object of the present invention is to provide an NO.sub.x 
gas-detecting element which exhibits high sensitivities to both NO gas and 
NO.sub.2 gas in a mixed gas, namely a high sensitivity to the whole 
NO.sub.x gas. 
The second object of the invention is to provide an NO.sub.x gas-detecting 
element which has a selectively high sensitivity to NO.sub.x gas whether 
the concentration of NO.sub.x gas contained is low or high. 
Another object of the invention is to provide an NO.sub.x gas-detecting 
element having a selectively high sensitivity to NO.sub.x gas even when 
gases other than NO.sub.x gas such as O.sub.2, CO and hydrocarbons are 
contained in a mixed gas. 
Still another object of the invention is to provide an NO.sub.x 
gas-detecting element suitable for measurement of the concentration of 
NO.sub.x gas contained in an exhaust gas from an internal combustion 
engine. 
Still another object of the invention is to provide an NO.sub.x 
gas-detecting module and an NO.sub.x gas-detecting device wherein the 
above NO.sub.x gas-detecting elements are used, respectively. 
Still another object of the invention is to provide a process for 
measurement of NO.sub.x gas concentration in a mixed gas using the above 
NO.sub.x gas-detecting element. 
Still other objects of the invention will be apparent from the following 
descriptions. 
According to research by the present inventors it has been clarified that 
the above objects and benefits of the present invention can be 
accomplished by an element for detection of NO.sub.x gas substantially 
consisting of at least one titanium-containing oxide which is selected 
from the group consisting of 
(a) titanium oxide, 
(b) a solid solution wherein at least one element selected from the group 
consisting of aluminum (Al), gallium (Ga), indium (In), scandium (Sc), 
magnesium (Mg), yttrium (Y), neodymium (Nd), tantalum (Ta), antimony (Sb) 
and arsenic (As) is contained in the solid-solution state in the titanium 
oxide, and 
(c) an oxide which contains titanium and other metal element(s) and has a 
perovskite structure, 
and has such an oxygen deficiency that its nonstoichiometric parameter 
(.delta.) is 0.03 to 0.3. 
Further, there are provided as a result of research by the present 
inventors a module for detection of NO.sub.x gas which is composed of an 
electric insulating support; the above NO.sub.x gas-detecting element 
which is integrated thereinto and exposed on the surface; and a couple of 
electrodes connected to the detecting element at intervals of distance, as 
well as a device for detection of NO.sub.x gas which is composed of the 
above NO.sub.x gas-detecting module; a power source; and a measurement 
equipment for measurement of the electric voltage or electric current or 
electric resistance in the NO.sub.x gas-detecting module and wherein the 
NO.sub.x gas-detecting module, the electric power source and the 
measurement equipment are electrically connected. 
Further, there is provided as a result of research of the present inventors 
a process for measurement of the concentration of NO.sub.x gas in a mixed 
gas using the above NO.sub.x gas-detecting device, as later described. 
The present invention is detailedly and specifically described below. 
The NO.sub.x gas-detecting element of the present invention substantially 
consists, as above-mentioned, of at least one titanium-containing oxide 
which is selected from the group consisting of (a) titanium oxide, (b) the 
solid solution and (c) the oxide and has such an oxygen deficiency that 
its nonstoichiometric parameter (.delta.) is in the range of 0.03 to 0.3. 
The term "nonstoichiometric parameter (.delta.)" means a rate in which the 
oxygen content is short compared to the stoichiometrically represented 
chemical formula, and can specifically be represented by the following 
chemical formula: 
______________________________________ 
Stoichiometric titanium- 
Titanium-containing 
containing-oxide oxide of the invention 
______________________________________ 
(a) TiO.sub.2 (a) TiO.sub.2-.delta. 
(b) A.sub.z Ti.sub.1-z O.sub.2 
(b) A.sub.z Ti.sub.1-z O.sub.2-.delta. 
(c) BTiO.sub.3 (c) BTiO.sub.3-.delta. 
______________________________________ 
wherein A, B and z have later-described definitions, respectively. 
Thus, it can be said that the titanium-containing oxide in the invention is 
a titanium-containing compound in such a state that its oxygen is deleted 
from a stoichiometric titanium-containing compound such as the above (a), 
(b) or (c) in the proportion of (.delta.). 
The value of the nonstoichiometric parameter (.delta.) in the invention 
means a value calculated from the amount of each element measured using an 
ESCA (X-ray photoelectron spectral apparatus). That is, the 
nonstoichiometric parameter (.delta.) of the NO.sub.x gas-detecting 
element is a value calculated from the amount of each element of the 
NO.sub.x gas-detecting element measured using an ESCA. 
The titanium-containing oxide composing the NO.sub.x gas-detecting element 
of the invention has such an oxygen deficiency that the nonstoichiometric 
parameter (.delta.) is in the range of 0.03 to 0.3, preferably 0.04 to 
0.2, as above-described. Titanium-containing oxides having a 
nonstoichiometric parameter (.delta.) smaller than the above range have a 
higher sensitivity to oxygen (O.sub.2) and a much lower sensitivity to 
NO.sub.x. On the other hand, when the above (.delta.) goes beyond the 
above range, sensitivity to all gases is lowered to make utilization 
thereof as a detecting element impossible. 
Thus, typical examples of the titanium-containing oxides composing the 
NO.sub.x gas-detecting elements of the invention are specifically 
indicated below: 
(a-1) TiO.sub.2-.delta., 
(b-1) A.sub.z Ti.sub.l-z O.sub.2-.delta. or 
(c-1) BTiO.sub.3-.delta.. 
Each (.delta.) in the above (a-1), (b-1) and (c-1) represents a 
nonstoichiometric parameter. These oxides are more detailedly described 
below. 
TiO.sub.2-.delta. of (a-1) is a titanium oxide having an oxygen deficiency 
to titanium dioxide. 
A.sub.z T.sub.l-z O.sub.2-.delta. of (b-1) is a composite oxide containing 
the element A in the solid-solution state in titanium oxide, and the 
element A is Al, Ga, In, Sc, Mg, Y, Nd, Ta, Sb or As, or a mixture of two 
or more of them. The z in (b-1) represents the rate in which the element A 
is contained in the solid-solution state and is in the range satisfying 
0&lt;z&lt;0.1 when the element A is Al, Ga, In, Sc, Mg or Y and in the range 
satisfying 0&lt;z&lt;0.05 when the element A is Nd, Ta, Sb or As. Specific 
examples of composite oxides of (b-1) include the following compounds: 
Al.sub.z Ti.sub.l-z O.sub.2-.delta., Ga.sub.z Ti.sub.l-z O.sub.2-.delta., 
In.sub.z Ti.sub.l-z O.sub.2-.delta., Sc.sub.z Ti.sub.l-z O.sub.2-.delta., 
Mg.sub.z Ti.sub.l-z O.sub.2-.delta., Y.sub.z Ti.sub.l-z O.sub.2-.delta., 
Nd.sub.z Ti.sub.l-z O.sub.2-.delta., Ta.sub.z Ti.sub.l-z O.sub.2-.delta., 
Sb.sub.z Ti.sub.l-z O.sub.2-.delta., As.sub.z Ti.sub.l-z O.sub.2-.delta., 
Al.sub.zl Nb.sub.z2 Ti.sub.(1-z1-z2) O.sub.2-.delta., 
Ta.sub.zl Sb.sub.z2 Ti.sub.(1-z1-z2) O.sub.2-.delta., 
Ga.sub.zl As.sub.z2 Ti.sub.(1-z1-z2) O.sub.2-.delta., 
Y.sub.zl Nb.sub.z2 Ti.sub.(1-z1-z2) O.sub.2-.delta., 
wherein z is as defined above, and z1 and z2 represent positive numbers 
provided that they satisfy z1+z2=z. 
Preferred ones among composite oxides of (b-1) are composite oxides wherein 
the above element A is Al, Ga, In, Sc, Y or Mg. 
Further, the oxide of (c) in the invention which contains titanium and 
other metal element(s) and has a perovskite structure is represented by 
BTiO.sub.3-8 as shown in the above formula (c-1). In the above formula, B 
is lead (Pb), calcium (Ca), strontium (Sr), cadmium (Cd), lanthanum (La) 
or barium (Ba) or a mixture of two or more of these metals. 
Specific examples of the above oxides having a perovskite structure 
include, for example, PbTiO.sub.3-.delta., CaTiO.sub.3-.delta., 
SrTiO.sub.3-.delta., CdTiO.sub.3-.delta., LaTiO.sub.3-.delta., 
BaTiO.sub.3-.delta., Sr.sub.y Ba.sub.l-y TiO.sub.3-.delta., Ba.sub.y 
La.sub.l-y TiO.sub.3-.delta. and Ca.sub.y Sr.sub.l-y TiO.sub.3-.delta. 
wherein y is a positive number satisfying 0&lt;y&lt;1. 
Among the above oxides of a perovskite structure are preferred oxides 
wherein B in the above formula (c-1) is Ba, (Ba+La), Cd, or Sr. 
It is needed that the NO.sub.x gas-detecting element of the invention is 
the aforementioned titanium-containing oxide, but not particularly limited 
about its shape. The shape may appropriately be determined in accordance 
with the structure of the NO.sub.x gas-detecting module or NO.sub.x 
gas-detecting device. Usually, the NO.sub.x gas-detecting element may, for 
example, have a chip-like or thin film-like structure. Such a chip-like 
element can specifically be, for example, a circular, rectangular or 
elliptic flake, and it is advantageous that the thickness of these flakes 
is 0.05 to 5 mm, preferably 0.1 to 3 mm and the area of one side of the 
flakes is in the range of 0.1 to 150 mm.sup.2, preferably 0.3 to 100 
mm.sup.2. On the other hand, such a thin film-like element can be one 
which has a thickness in the range of 1.times.10.sup.-5 to 0.3 mm, 
preferably 1.times.10.sup.-4 to 0.2 mm and a film area of one side in the 
range of 0.001 to 100 mm.sup.2, preferably 0.003 to 100 mm.sup.2. 
The preparation method of the titanium-containing oxide as the NO.sub.x 
gas-detecting element in the invention is not particularly limited, and 
may typically be a sintering method, sputtering method, vacuum evaporation 
method, thermal decomposition method or the like. Among these preparation 
methods the sintering method is suitable for molding of chip-like and thin 
film-like NO.sub.x gas-detecting elements, whereas the sputtering vacuum 
evaporation and thermal decomposition methods are suitable for obtaining 
film-like elements. These methods are described below: 
(I) Sintering Method 
Usually preferred as this sintering method is a method wherein powder of a 
titanium-containing oxide having the above-mentioned composition is packed 
into a cavity having a predetermined shape, and heated after, or 
simultaneously with, compression molding. Proper pressure in the 
compression molding is 200 kg/cm.sup.2 to 1 t/cm.sup.2, (t=metric ton), 
generally 300 to 700 kg/cm.sup.2. In this method sintering temperature and 
sintering atmosphere have a particular important influence on 
determination of the nonstoichiometric parameter (.delta.). In a 
nonreducing atmosphere (N.sub.2, Ar or the like) the sintering temperature 
(T) may be a temperature in the range of 900.degree. C.&lt;T&lt;the melting 
point of the titanium-containing oxide. The sintering temperature for case 
of using an atmosphere of a reducing gas such as CO or H.sub.2 changes 
depending on the kind and concentration of the gas. However, it is 
desirable that the sintering temperature is 700.degree. C.&lt;T&lt;1,000.degree. 
C. in N.sub.2 containing 5% CO and 600.degree. C.&lt;T&lt;900.degree. C. in 
N.sub.2 containing 5% H.sub.2. Further, it is of course possible to use a 
combination of the above sintering conditions, for example, a method 
wherein the titanium-containing oxide is first sintered in a nonreducing 
atmosphere and then treated in a reducing atmosphere. 
There may be used as another sintering method a method wherein powder of 
the titanium-containing oxide is mixed with a dispersion medium to make 
paste, screen printing is made onto an insulating substrate using the 
paste to form a film, and the film is sintered at the above temperature 
and condition of sintering. 
In the above sintering method, it is also possible to use as the starting 
material another titanium-containing compound such as a hydroxide or 
alkoxide of titanium in place of the titanium-containing oxide and oxidize 
and sinter the compound at the same time. 
(II) Sputtering Method 
There may, for example, be used as the sputtering method a method wherein 
sputtering is made onto an insulating substrate in the presence of oxygen 
using metal titanium or the like as a target material to form a thin film 
and the thin film is burned in the air at 500.degree. to 800.degree. C. to 
obtain a thin film of TiO.sub.2. 
(III) Vacuum Evaporation Method 
There may, for example, be used as the vacuum evaporation method a method 
wherein metal titanium is evaporated under an oxygen pressure of 0.5 to 3 
Torr and this vapor is deposited on an insulating substrate such as 
alumina to form a TiO.sub.2 thin film. 
(IV) Thermal Decomposition Method 
There may be used as the thermal decomposition method a method wherein a 
mixed solution of an organo-metal compound such as an alkoxide of metal 
composing the desired titanium-containing oxide is applied onto a 
substrate such as alumina and thermally decomposed either in a nonreducing 
atmosphere, e.g. in the air or in a reducing atmosphere at 500.degree. C. 
to a temperature equal to, or less than, their melting point to form a 
TiO.sub.2 thin film. 
Although the sputtering, vacuum evaporation and thermal decomposition 
methods were described about a process of preparation of a thin film of 
TiO.sub.2, films of other titanium-containing oxides can be prepared in 
the same manner as above. 
The above-described titanium-containing oxides of the invention have a 
fully satisfactory sensitivity to NO.sub.x gas in a mixed gas. 
However, it has been found as a result of research by the present inventors 
that the concentration of NO.sub.x gas in a mixed gas containing besides 
NO.sub.x gas the afore-mentioned other gases in a low concentration to a 
high concentration can be measured in higher sensitivity and more 
accurately by making an oxidation catalyst exist at least on the surface 
of the element substantially consisting of the above titanium-containing 
oxide. 
There is no particular limitation about the oxidation catalyst which is 
made to exist at least on the surface of the element of the invention so 
long as it displays performances of oxidation catalyst at a temperature at 
which the NO.sub.x gas-detecting element is used. Typical examples of such 
oxidation catalysts include noble metal type oxidation catalysts such as 
Pt, Rh and Pd, metal type oxidation catalysts such as Ni and Fe, metal 
oxide type oxidation catalysts such as LaCO.sub.3, LaNiO.sub.3 and 
LaSr.sub.0.3 Co.sub.0.7 O.sub.3, etc. Particularly preferred among them 
are noble metal type oxidation catalysts such as Pt, Rh and Pd. It is 
quite enough that the above oxidation catalyst exists at least on the 
surface of the element consisting of the afore-mentioned 
titanium-containing oxide as a layer containing the oxidation catalyst. 
That is, there can be mentioned as the embodiments to make the layer 
containing the oxidation catalyst exist at least on the surface of the 
element comprising the titanium-containing oxide 1 an embodiment wherein 
the oxidation catalyst is made to exist in a state dispersed throughout 
the element, 2 an embodiment wherein the oxidation catalyst is made to 
exist in a state partially dispersed in the outer layer of the element, 3 
an embodiment wherein an oxidation catalyst-containing layer is formed on 
the surface of the element; etc. Particularly preferred among them are the 
embodiments of 1 and 2. In the embodiment of 3 the oxidation 
catalyst-containing layer may be formed by carrier particles carrying the 
oxidation catalyst or in some case by the oxidation catalyst alone. 
The embodiments of 1 and 2 are suitable for the above noble metal type 
oxidation catalysts and metal type oxidation catalysts. The concentration 
of the noble metal type oxidation catalyst in the matrix of the 
titanium-containing oxide containing the oxidation catalyst is generally 
10 to 1,000 ppm, preferably 100 to 600 ppm, whereas that of the metal type 
oxidation catalyst therein is generally 0.1 to 1 wt %, preferably 0.5 to 1 
wt %. On the other hand, the embodiment of 3 is applicable to all type of 
the oxidation catalysts. Particularly for the noble metal type oxidation 
catalysts and metal type oxidation catalysts is suitable the method of 
forming a layer by carrying them on the surface of carrier particles, and 
the suitable concentration of the oxidation catalyst on the surface of the 
carrier particles in this case accords with the concentration range 
thereof in the above oxidation catalyst-containing matrix. Further, it is 
preferred that the metal oxide type metal catalyst forms a layer by the 
catalyst alone. In the embodiments of 2 and 3 it is generally preferred 
that the thickness of the oxidation catalyst-containing layer is one 
micrometer or more. 
There is no particular limitation in the invention about the method for 
formation of an oxidation catalyst-containing layer in or on the element 
consisting of the titanium-containing oxide. For example, for the 
embodiment of the above 1 is suitable a method wherein in preparation of a 
titanium-containing oxide by the sintering method, thermal decomposition 
method or the like the oxidation catalyst or a compound capable of forming 
the oxidation catalyst by heating during sintering or thermal 
decomposition is compounded in the raw materials. Suitable for the 
embodiment of 2 is a method wherein the titanium-containing oxide is 
impregnated with a solution of a compound capable of forming the oxidation 
catalyst by heating and then heated. Examples of compounds capable of 
forming oxidation catalysts include soluble salts of the above-described 
noble metals or metals such as chlorides, nitrates or organic acid salts. 
Further, for the embodiment of 3 is mentioned a method wherein the 
oxidation catalyst is attached onto the surface of the element consisting 
of the titanium-containing oxide by a method such as a sputtering, vaccum 
evaporation, sintering of thermal decomposition method; or a method 
wherein the oxidation catalyst is carried on carrier particles of 
TiO.sub.2, Al.sub.2 O.sub.3, MgO.multidot.Al.sub.2 O.sub.3, SiO.sub.2 or 
the like and then the carrier particles are attached onto the surface of 
the element consisting of the titanium-containing oxide by a means such as 
sintering. 
Although the NO.sub.x gas-detecting element of the invention substantially 
consists, as hereinbefore described, of the titanium-containing oxide 
having an oxygen deficiency optionally together with the oxidation 
catalyst, there is no hindrance about that a small amount of another oxide 
is contained therein within such a range that the nonstoichiometric 
parameter (.delta.) of the NO.sub.x gas-detecting element of the invention 
is not much influenced. For example, the NO.sub.x gas-detecting element of 
the invention may contain in a small rate a titanium-containing oxide 
having such an oxygen deficiency that the nonstoichiometric parameter 
(.delta.) is out of the range defined in the invention or a carrier to 
carry the oxidation catalyst such as TiO.sub.2, Al.sub.2 O.sub.3, MgO 
Al.sub.2 O.sub.3 or SiO.sub.2. The amount of the oxide to be incorporated 
in the titanium-containing oxide depends on its kind, etc., but is on the 
order of 10 wt.% at most, preferably on the order of 5 wt.% at most. 
The present invention further provides a module for detection of NO.sub.x 
gas in a mixed gas which comprises 
(i) an electric insulating support, 
(ii) the above element for detection of NO.sub.x gas of the invention 
integrated into the surface of the support, and 
(iii) a couple of electrodes connected to the element for detection of 
NO.sub.x gas at intervals of distance; 
and a module for detection of NO.sub.x gas in a mixed gas which comprises 
the above module to which a means to maintain the temperature of the 
NO.sub.x gas-detecting element at about 200.degree. to about 700.degree. 
C., preferably at about 400.degree. to about 600.degree. C., is added as 
an constitutive element. 
The present invention still further provides a device for measurement of 
NO.sub.x gas concentration in a mixed gas which comprises 
(i) an electric insulating support, 
(ii) the above element for detection of NO.sub.x gas of the invention 
integrated into the surface of the support, 
(iii) a couple of electrodes connected to the element for detection of 
NO.sub.x gas at intervals of distance; 
(iv) a heating means to maintain the element for detection of NO.sub.x gas 
at a temperature of about 200.degree. to about 700.degree. C., preferably 
about 400.degree. to about 600.degree. C., and 
(v) a means to measure the electric resistance of the element for detection 
of NO.sub.x gas which resistance depends on the NO.sub.x gas concentration 
in the mixed gas to be measured. 
The present invention still further provides a process for measurement of 
NO.sub.x gas concentration in a mixed gas, particularly in an exhaust gas, 
in an internal combustion engine, which comprises 
(a) maintaining with heating the above element for detection of NO.sub.x 
gas of the invention at temperatures of about 200.degree. to about 
700.degree. C., preferably about 400.degree. to 600.degree. C., 
(b) contacting the specimen mixed gas with the heated surface of the 
element for detection of NO.sub.x gas, and 
(c) measuring the electric resistance of the element for detection of 
NO.sub.x gas. 
The above module for detection of NO.sub.x gas, device for measurement of 
NO.sub.x gas concentration and method for measurement of NO.sub.x gas 
concentration are described in more detail below in accordance with the 
attached FIGS. 1 and 2.

In measurement of NO.sub.x gas concentration using the above device for 
measurement of the concentration of NO.sub.x gas, heater 4 is made to 
operate, the NO.sub.x gas-detecting element 1 is placed in a mixed gas as 
a specimen while the NO.sub.x gas-detecting element 1 is heated to a 
predetermined temperature, for example, to about 200.degree. to about 
700.degree. C., preferably about 400.degree. to about 600.degree. C., and 
the voltage at that time is measured by volt meter 7. 
That is, there is the following relation among the voltage V.sub.c of power 
source 6 for circuit, the resistance R.sub.L of load resistance 8, output 
voltage V.sub.out measured by volt meter 7 and the resistance R.sub.s of 
the NO.sub.x gas-detecting element, and R.sub.s can easily be calculated 
according to the following equation (1) by measuring V.sub.out : 
EQU R.sub.s =R.sub.L (V.sub.c -V.sub.out)/V.sub.out (1) 
The NO.sub.x gas-detecting element of the invention exhibits, when the 
temperature of the element is constant, a definite resistance in 
accordance with the concentration of NO or NO.sub.2 in the atmosphere and 
moreover almost equal resistances in respect of NO and NO.sub.2. Thus, it 
is possible to know the NO.sub.x gas concentration in the specimen mixed 
gas by determining NO.sub.x gas concentration from a previously prepared 
calibration curve using the thus obtained R.sub.s value. 
EFFECT 
The NO.sub.x gas-detecting element of the invention has functions, for 
example, that 
(1) it has high and almost equal sensitivities to both NO and NO.sub.2, 
(2) it has an adequate sensitivity even to NO.sub.x gas in high 
concentration, 
(3) it hardly undergo influences of other coexisting gases. 
Therefore, it is possible to accurately measure NO.sub.x concentration in a 
mixed gas containing NO.sub.x over a wide range using the NO.sub.x 
gas-detecting element of the invention. 
Above all, (b-1) and (c-1) of the NO.sub.x gas-detecting elements in this 
invention have excellent characteristics to detect NO.sub.x gas in a high 
concentration. 
It is possible to detect NO.sub.x concentration in a general mixed gas by 
use of the NO.sub.x gas-detecting element of the invention. However, as 
the detecting element has characteristics that it has an adequately large 
sensitivity to NO.sub.x gas in higher concentration and does not undergo 
the influence of other coexisting gases, the detecting element is 
particularly suitable for continuously monitoring NO.sub.x concentration 
by directly setting the element inside the flue of internal combustion 
engines, etc. and changing the resistance of the element. Further, it is 
also possible to develop the element for a feedback system wherein 
resistance change of the element is obtained and the operation conditions 
for internal combustion engines, etc. are altered once an abnormal state 
takes place. 
EXAMPLE 
The present invention is specifically described below by examples, but not 
limited thereto. 
The sensitivities of NO, NO.sub.2, O.sub.2 and CO and the value of the 
nonstoichiometric parameter (.delta.) in the following Examples 1 to 40 
were determined by the following methods, respectively: 
(1) NO Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 5% O.sub.2 and 1,000 ppm NO to the 
resistance R.sub.1 in an N.sub.2 gas atmosphere containing 5% O.sub.2 and 
100 ppm NO. 
(2) NO.sub.2 Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 5% O.sub.2 and 1,000 ppm NO.sub.2 
to the resistance R.sub.1 in an N.sub.2 gas atmosphere containing 5% 
O.sub.2 and 100 ppm NO.sub.2. 
(3) O.sub.2 Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 900 ppm NO, 100 ppm NO.sub.2 and 
10% O.sub.2 to the resistance R.sub.1 in an N.sub.2 gas atmosphere 
containing 900 ppm NO, 100 ppm NO.sub.2 and 1% O.sub.2. 
(4) CO Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 5% O.sub.2, 900 ppm NO, 100 ppm 
NO.sub.2 and 1,000 ppm CO to the resistance R.sub.1 in an N.sub.2 gas 
atmosphere containing 5% O.sub.2, 900 ppm NO, 100 ppm NO.sub.2 and 10 ppm 
CO. 
(5) The value of the nonstoichiometric parameter (.delta.) 
The value of the nonstoichiometric parameter (.delta.) means a value 
calculated from the amount of each element measured using an ESCA (X-ray 
photoelectron spectral apparatus; using JPS-80 of JEOL. 
It can be said that the larger the value represented by the above log 
(R.sub.2 /R.sub.1) is, the higher the sensitivity to that gas is. 
EXAMPLE 1 
An aqueous ammonium sulfate solution and ammonia were added to an aqueous 
TiCl.sub.4 solution, and the formed precipitate was filtered, washed and 
burned in the air at 900.degree. C. for one hour. The obtained burned 
powder (having a .delta.-value of 0.01) was placed in a cavity, Pt 
electrodes were buried in the both sides, and then the powder was 
compression-molded to obtain a chip-like molding having a shape shown in 
FIG. 1. This chip-like molding was burned in the air at 1,200.degree. C. 
for 4 hours to obtain a sintered body of TiO.sub.2-.delta.. The value of 
.delta. in the obtained sintered body TiO.sub.2-.delta. was 0.05. The 
above chip-like molding had a thickness of 1 mm and longitudinal and 
transverse lengths of 2 mm and 2 mm, respectively. 
An NO.sub.x gas-detecting module having the structure shown in FIG. 1 was 
assembled using the above TiO.sub.2-.delta. chip. In this NO.sub.x 
gas-detecting module Al.sub.2 O.sub.3 and platinum were used as the 
insulating substrate and the heater, respectively. 
Sensitivity to various gases was measured using the thus obtained NO.sub.x 
gas-detecting device. The measurement was made under the condition that 
the NO.sub.x gas-detecting element was placed in the predetermined gases 
while being heated to 500.degree. C. 
The results were exhibited in Table 1. 
COMATIVE EXAMPLE 1 
TiO.sub.2 powder was molded into a chip-like form in the same manner as in 
Example 1 and then burned in oxygen at 900.degree. C. for 4 hours. The 
.delta. value of TiO.sub.2-.delta. of the obtained sintered body was 0. 
Sensitivities of the obtained chip-like sintered body to various gases 
were measured in the same manner as in Example 1. As the result were 
obtained NO sensitivity of 0.08, NO.sub.2 sensitivity of 0.09, O.sub.2 
sensitivity of 0.41 and CO sensitivity of 0.32. 
EXAMPLES 2 to 16 
Solid solutions shown in Examples 2 to 16 in the following Table 1 were 
prepared. Preparation of each solid solution was conducted by mixing an 
oxide of the element which is indicated in Table 1 and is to be contained 
in TiO.sub.2 in the solid-solution state with TiO.sub.2 in a predetermined 
molar ratio and then burning the mixture in the air at 1,000.degree. C. 
for one hour. As a result of ascertainment by X-ray diffraction, all of 
the obtained solid solutions exhibited diffraction peak of rutile type 
TiO.sub.2 alone. It was ascertained by this that in each of the 
titanium-containing oxides of Examples 2 to 16 the added element is 
contained in TiO.sub.2 in the solid-solution state. 
The obtained solid solution was molded into a chip-like form in the same 
manner as in Example 1 and then burned in the air at 1,200.degree. C. for 
4 hours to obtain a chip-like sintered body shown in FIG. 1. The 
nonstoichiometric parameters of the obtained chip-like sintered bodies are 
shown in Table 1. NO.sub.x sensitivity, O.sub.2 sensitivity and CO 
sensitivity were measured about these chip-like sintered bodies in the 
same manner as in Example 1. The results are shown in Table 1. 
It is understood from these results that the NO.sub.x gas-detecting 
elements of the invention have a high sensitivity to NO.sub.x in high 
concentrations and do not undergo influence of the other components in the 
gas. 
EXAMPLES 17 to 28 
Titanium-containing oxides having a perovskite structure shown in Examples 
17 to 28 in the following Table 1 were prepared. Preparation of these 
titanium-containing oxides was carried out by mixing a carbonate of metal 
other than titanium indicated in Table 1 with TiO.sub.2 in a predetermined 
molar ratio and burning the mixture in the air at 1,200.degree. C. for one 
hour. 
As a result of ascertainment by X-ray diffraction it was ascertained from 
the fact that each of the obtained titanium-containing oxides exhibited 
only peaks peculiar to an oxide of a perovskite structure that it is an 
oxide having a perovskite structure. 
Each of the thus obtained oxide having a perovskite structure, and a 
mixture of each of the thus obtained oxides having a perovskite structure 
with the titanium oxide powder obtained in Example 1 in a molar ratio of 
1:1 was molded into a chip-like form in the same manner as in Example 1 
and then burned in the air at 1,200.degree. C. for 4 hours to obtain a 
chip-like sintered body shown in FIG. 1. Nonstoichiometric parameters of 
the obtained chip-like sintered bodies are shown in Table 1. These 
chip-like sintered bodies were measured for NO.sub.x sensitivity, O.sub.2 
sensitivity and CO sensitivity in the same manner as in Example 1. The 
results are shown in Table 1. 
It is seen from these results that the NO.sub.x gas-detecting elements of 
the invention have a high sensitivity to NO.sub.x in high concentration 
and moreover does not undergo influence of the coexisting matters. 
COMATIVE EXAMPLE 2 
SnO.sub.2 powder was molded into a chip-like form in the same manner as in 
Example 1 and burned in the air at 1,200.degree. C. for 4 hours to obtain 
a chip-like sintered body having a nonstoichiometric parameter (.delta.) 
indicated in Table 1. The obtained chip-like sintered bodies were measured 
for sensitivity to various gases in the same manner as in Example 1. The 
results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Sensitivity to various gases 
NO.sub.x gas-detecting element 
NO NO.sub.2 
O.sub.2 
CO 
Kind of oxide 
(.delta.) 
sensitivity 
sensitivity 
sensitivity 
sensitivity 
__________________________________________________________________________ 
Example 
1 TiO.sub.2 0.05 
0.67 0.61 0.03 0.03 
2 Al.sub.0.01 Ti.sub.0.99 O.sub.2 
0.08 
0.68 0.60 0.08 0.05 
3 Al.sub.0.08 Ti.sub.0.92 O.sub.2 
0.05 
0.60 0.52 0.10 0.09 
4 Nb.sub.0.05 Ti.sub.0.95 O.sub.2 
0.20 
0.49 0.58 0.09 0.08 
5 Ta.sub.0.03 Ti.sub.0.97 O.sub.2 
0.18 
0.50 0.55 0.08 0.09 
6 Sb.sub.0.02 Ti.sub.0.98 O.sub.2 
0.15 
0.55 0.60 0.09 0.09 
7 As.sub.0.01 Ti.sub.0.99 O.sub.2 
0.13 
0.52 0.60 0.08 0.07 
8 Ga.sub.0.003 Ti.sub.0.997 O.sub.2 
0.09 
0.65 0.60 0.05 0.06 
9 In.sub.0.005 Ti.sub.0.995 O.sub.2 
0.10 
0.63 0.61 0.06 0.05 
10 Sc.sub.0.01 Ti.sub.0.99 O.sub.2 
0.11 
0.68 0.63 0.05 0.04 
11 Sc.sub.0.08 Ti.sub.0.92 O.sub.2 
0.08 
0.58 0.53 0.11 0.11 
12 Mg.sub.0.05 Ti.sub.0.95 O.sub. 2 
0.15 
0.62 0.59 0.06 0.05 
13 Y.sub.0.01 Ti.sub.0.99 O.sub.2 
0.13 
0.65 0.60 0.06 0.06 
14 Al.sub.0.01 Nb.sub.0.01 Ti.sub.0.98 O.sub.2 
0.13 
0.53 0.58 0.08 0.08 
15 Sc.sub.0.01 Sb.sub.0.01 Ti.sub.0.98 O.sub.2 
0.15 
0.55 0.58 0.07 0.07 
16 In.sub.0.01 Ta.sub.0.01 Ti.sub.0.98 O.sub.2 
0.12 
0.55 0.55 0.07 0.09 
17 BaTiO.sub.3 
0.10 
0.71 0.63 0.02 0.01 
18 PbTiO.sub.3 
0.06 
0.67 0.60 0.03 0.03 
19 CdTiO.sub.3 
0.05 
0.66 0.59 0.03 0.04 
20 CaTiO.sub.3 
0.09 
0.65 0.57 0.01 0.01 
21 SrTiO.sub.3 
0.08 
0.71 0.63 0.02 0.01 
22 LaTiO.sub.3 
0.08 
0.68 0.62 0.05 0.04 
23 BaTiO.sub.3 + TiO.sub.2 
0.12 
0.71 0.63 0.07 0.02 
24 CaTiO.sub.3 + TiO.sub.2 
0.10 
0.64 0.60 0.08 0.04 
25 SrTiO.sub.3 + TiO.sub.2 
0.11 
0.65 0.60 0.06 0.03 
26 Sr.sub.0.9 Ca.sub.0.1 TiO.sub.3 
0.09 
0.69 0.63 0.02 0.01 
27 Ba.sub.0.9 La.sub.0.1 TiO.sub.3 
0.13 
0.68 0.62 0.04 0.02 
28 Ca.sub.0.9 La.sub.0.1 TiO.sub.3 
0.14 
0.63 0.60 0.04 0.03 
Comparative 
SnO.sub.2 0.02 
0.08 0.19 0.20 0.35 
Example 2 
__________________________________________________________________________ 
EXAMPLES 29 to 34 
Titanium-containing oxides containing Ga in various proportions indicated 
in the following Table 2 and in the solid-solution state were prepared. 
That is, Ga.sub.2 O.sub.3 and TiO.sub.2 were mixed in various proportions 
so that the atomic percentages of Ga to Ti became the values indicated in 
Table 2, and each mixture was burned in the air at 1,000.degree. C. for 
one hour to prepare a composite oxide. As a result of measurement by X-ray 
diffraction it was ascertained that each of the obtained composite oxides 
exhibited only the diffraction peak of rutile type TiO.sub.2 and thus the 
entire gallium is contained in TiO.sub.2 in the solid-solution state. 
Each of the composite oxides was molded into a chip-like form in the same 
manner as in Example 1 and then burned in the air at 1,300.degree. C. for 
10 hours to obtain a chip-like sintered body indicated in Table 2. 
Nonstoichiometric parameters of the obtained chip-like sintered bodies are 
indicated in Table 2. These chip-like sintered bodies were measured for 
NO.sub.x sensitivity, O.sub.2 sensitivity and CO sensitivity in the same 
manner as in Example 1. The results are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
NO.sub.x gas-detecting element 
Sensitivity to various gases 
Atomic percentage 
NO NO.sub.2 
O.sub.2 
CO 
Example 
of Ga (.delta.) 
sensitivity 
sensitivity 
sensitivity 
sensitivity 
__________________________________________________________________________ 
Example 
29 0.01 0.10 
0.48 0.49 0.10 0.07 
30 0.05 0.13 
0.69 0.63 0.08 0.05 
31 0.1 0.13 
0.65 0.61 0.07 0.04 
32 0.5 0.15 
0.59 0.58 0.08 0.04 
33 1 0.15 
0.52 0.50 0.07 0.05 
34 5 0.14 
0.45 0.45 0.10 0.09 
__________________________________________________________________________ 
EXAMPLES 35 to 39 
The procedure of Example 31 was repeated except that as the burning 
conditions of the chip-like molding was employed the conditions that the 
chip-like molding was burned at 1,000.degree. C. for 10 hours in N.sub.2 
containing H.sub.2 in various concentrations indicated in the following 
Table 3, whereby chip-like sintered bodies were obtained. 
The nonstoichiometric parameters (.delta.) of the resulting chip-like 
sintered bodies are indicated in Table 3. These chip-like sintered bodies 
were measured for NO.sub.x sensitivity, O.sub.2 sensitivity and CO 
sensitivity in the same manner as in Example 1. The results are shown in 
Table 3. 
COMATIVE EXAMPLES 3 and 4 
The procedure of Examples 35 to 39 was repeated except that different 
H.sub.2 concentrations in the burning atmosphere were adopted to obtain 
chip-like sintered bodies. H.sub.2 concentrations, nonstoichiometric 
parameters (.delta.) and the measured NO.sub.x sensitivities, O.sub.2 
sensitivities and CO sensitivities are shown in Table 3. 
It is seen from these results that the NO.sub.x gas-detecting elements of 
the invention have a high sensitivity to NO.sub.x in high concentration 
and moreover do not undergo influence of the coexisting matters. 
TABLE 3 
__________________________________________________________________________ 
NO.sub.x gas-detecting element 
Sensitivity to various gases 
H.sub.2 concentration 
NO NO.sub.2 
O.sub.2 
CO 
Example 
(%) (.delta.) 
sensitivity 
sensitivity 
sensitivity 
sensitivity 
__________________________________________________________________________ 
Example 
35 0.05 0.04 
0.48 0.50 0.10 0.05 
36 0.1 0.07 
0.60 0.55 0.08 0.04 
37 0.5 0.10 
0.68 0.63 0.08 0.02 
38 1 0.14 
0.63 0.58 0.07 0.03 
39 4 0.18 
0.48 0.45 0.08 0.03 
Comparative 
Example 
3 0.01 0.005 
0.10 0.23 0.20 0.25 
4 20 0.61 
0.12 0.08 0.08 0.04 
__________________________________________________________________________ 
EXAMPLE 40 
An exhaust gas from automobile was analyzed for NO.sub.x concentration 
using the NO.sub.x gas-detecting devices of Examples 1 to 39 and 
Comparative Example 2 and a chemiluminescence type NO.sub.x -detecting 
device. The adopted operation conditions of the engine was such that the 
engine speed was 1,500 ppm and A/F was in the range of 13 to 20. As a 
result of comparison of the analytical value by each detecting device it 
was found that the analytical value by the chemiluminescence type NO.sub.x 
-detecting device and the analytical values by the detecting-devices using 
the titanium-containing oxides of Examples 1 to 39 respectively well 
accorded with one another and thus by the detecting devices of the 
invention NO.sub.x gas can accurately be detected up to high concentration 
without undergoing influences due to change of the concentration of the 
coexisting gas such as O.sub.2, CO or HC in the exhaust gas. On the other 
hand, the analytical value by the detecting device of Comparative Example 
2 using SnO.sub.2 was largely different from the analytical value on 
NO.sub.x concentration by the chemiluminescence type NO.sub.x -detecting 
device. 
In the following Examples 41 to 81, NO, CO and HC sensitivities were 
determined according to the following methods, respectively. 
(1) NO Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 5% O.sub.2 and 1,000 ppm NO to the 
resistance R.sub.1 in an N.sub.2 gas atmosphere containing 5% O.sub.2. No 
NO was listed as present in the atmosphere for measure R.sub.1. 
(2) CO Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 5% O.sub.2, 500 ppm NO and 5,000 
ppm CO to the resistance R.sub.1 in an N.sub.2 gas atmosphere containing 
5% O.sub.2, 500 ppm NO and 50 ppm CO. 
(3) H.sub.2 Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 5% O.sub.2, 500 ppm NO and 5,000 
ppm H.sub.2 to the resistance R.sub.1 in an N.sub.2 gas atmosphere 
containing 5% O.sub.2, 500 ppm NO and 50 ppm H.sub.2. 
(4) HC Sensitivity 
Represented by the ratio log (R.sub.2 /R.sub.1) of the resistance R.sub.2 
in an N.sub.2 gas atmosphere containing 5% O.sub.2, 500 ppm NO and 5,000 
ppm C.sub.3 H.sub.6 to the resistance R.sub.1 in an N.sub.2 gas atmosphere 
containing 5% O.sub.2, 500 ppm NO and 50 ppm C.sub.3 H.sub.6. 
(5) O.sub.2 Sensitivity 
EQU R.sub.1 ; NO900 ppm+NO.sub.2 100 ppm+N.sub.2 +O.sub.2 0.1% 
EQU R.sub.2 ; NO900 ppm+NO.sub.2 100 ppm+N.sub.2 +O.sub.2 10% 
It can be said that the larger the value represented by the above log 
(R.sub.2 /R.sub.1) is, the higher the the sensitivity to the gas is. 
EXAMPLE 41 
An aqueous ammonium sulfate solution and ammonia were added to an aqueous 
TiCl.sub.4 solution, and the formed precipitate was filtered, washed and 
burned in the air at 900.degree. C. for one hour. The resulting burned 
powder was placed in a cavity, and after burial of Pt electrodes into both 
ends compression molded into a chip-like molding shown in FIG. 1. This 
chip-like molding was burned in the air at 1,200.degree. C. for 4 hours to 
obtain a sintered body of TiO.sub.2-.delta.. The value of .delta. in the 
TiO.sub.2-.delta. sintered body was 0.05. The size of the chip-like 
molding was such that the thickness was 1 mm and the longitudinal and 
transverse lengths were 2 mm and 2 mm, respectively. 
An NO.sub.x gas-detecting module having the structure shown in FIG. 1 was 
produced using the above chip of TiO.sub.2-.delta.. In this NO.sub.x 
gas-detecting module Al.sub.2 O.sub.3 and platinum were used as the 
insulating substrate and heater, respectively. 
Sensitivity to various gases was measured using the thus obtained NO.sub.x 
gas-detecting device. The measurement was carried out by placing the 
NO.sub.x gas-detecting element in the predetermined gases while heating it 
to 500.degree. C. by means of the heater. 
The results are shown in Table 4. 
EXAMPLES 42 to 44 
An aqueous ammonium sulfate solution and ammonia were added to an aqueous 
TiCl.sub.4 solution, and the formed precipitate was filtered, washed and 
burned in the air at 900.degree. C. for one hour. Then, aqueous H.sub.2 
PtCl.sub.6, H.sub.2 PdCl.sub.4 and RhCl.sub.3 .multidot.4H.sub.2 O 
solution were added to TiO.sub.2 respectively so that Pt, Pd and Rh 
contents became 300 ppm respectively, and after drying the mixtures were 
burned at 500.degree. C. for one hour. The resulting powders were molded 
into a chip-like form in the same manner as in Example 41 and burned in 
the air at 1,200.degree. C. for 4 hours, respectively. These chip-like 
sintered bodies were measured for sensitivity to the various gases in the 
same manner as in Example 41. 
The results are shown in Table 4. 
EXAMPLES 45 to 47 
The Pt/TiO.sub.2 powder (Pt=300 or 800 ppm) or Pd/TiO.sub.2 powder (Pd=800 
ppm) obtained in the same manner as in Examples 42 to 44 respectively was 
applied onto the surface of the chip of TiO.sub.2-.delta. obtained in 
Example 41 and burned in the air at 1,200.degree. C. for one hour. This 
chip-like sintered body was measured for sensitivity to the various gases 
in the same manner as in Example 41. 
The results are shown in Table 4. 
EXAMPLE 48 
NiO was compounded into TiO.sub.2 so that the content became one wt.%, and 
the mixture was burned in the air at 1,200.degree. C. for one hour. The 
resulting powder was applied onto the surface of the chip of 
TiO.sub.2-.delta. obtained in Example 41 and burned in the air at 
1,200.degree. C. for one hour. This chip-like sintered body was measured 
for the various gases in the same manner as in Example 41. 
The results are shown in Table 4. 
EXAMPLE 49 
LaCO.sub.3 and Co(CH.sub.3 COO).sub.2 were mixed so that the molar ratio 
was 1:1, and burned in the air at 1,200.degree. C. for one hour. The 
resulting powder was applied onto the surface of the chip of 
TiO.sub.2-.delta. obtained in Example 41 and burned in the air at 
1,000.degree. C. for one hour. This chip-like sintered body was measured 
for sensitivity to the various gases in the same manner as in Example 41. 
The results are shown in Table 4. 
EXAMPLES 50 to 62 
Solid solutions indicated in Examples 50 to 62 in the following Table 4 
were prepared. Each solid solution was prepared by mixing the oxide of 
element indicated in Table 4 with TiO.sub.2 in a predetermined molar ratio 
and burning the mixture in the air at 1,000.degree. C. for one hour. 
As a result of ascertainment by X-ray diffraction each of the resulting 
solid solutions exhibited only diffraction peaks of rutile type TiO.sub.2. 
It was ascertained from this fact that in all of the titanium-containing 
oxides of Examples 50 to 62 the added element was contained in TiO.sub.2 
in the solid-solution state. The thus obtained solid solutions were molded 
respectively into a chip-like form in the same manner as in Example 41 and 
burned in the air at 1,200.degree. C. for 4 hours to obtain chip-like 
sintered bodies indicated in Examples 50 to 62 in the following Table 4. 
The Pt/TiO.sub.2 powder obtained in Example 42 was applied onto the 
surface of each of the obtained chip-like sintered bodies and burned in 
the air at 1,200.degree. C. for one hour. The resulting chip-like sintered 
bodies were measured for sensitivity to the various gases in the same 
manner as in Example 41. The results are shown in Table 4. 
EXAMPLES 63 to 74 
Titanium-containing oxides having a perovskite structure indicated in 
Examples 63 to 74 in the following Table 4 were prepared. Preparation of 
these titanium-containing oxides was carried out by mixing the carbonate 
of metals other than titanium indicated in Table 4 with TiO.sub.2 in a 
predetermined molar ratio and burning the mixture in the air at 
1,200.degree. C. for one hour. As a result of ascertainment by X-ray 
diffraction all of the resulting titanium-containing oxides exhibited only 
peaks peculiar to oxides of a perovskite structure, whereby it was 
ascertained that they are oxides of a perovskite structure. 
Each of the thus obtained oxide having a perovskite structure, and a 
mixture of one of the thus obtained oxides of a perovskite structure with 
the titanium oxide powder obtained in Example 41 in a molar ratio of 1:1 
was molded into a chip-like form in the same manner as in Example 41, and 
burned in the air at 1,200.degree. C. for 4 hours to obtain a chip-like 
sintered body indicated in Examples 63 to 74 in the following Table 4. The 
Pt/TiO.sub.2 powder obtained in Example 42 was applied onto the surface of 
each chip-like sintered body and burned in the air at 1,200.degree. C. for 
one hour. The resulting chip-like sintered bodies were measured for 
sensitivity to the various gases in the same manner as in Example 41. The 
results are shown in Table 4. 
EXAMPLES 75 to 81 
Titanium-containing oxides were prepared which contain Ga in the various 
rates indicated in Examples 75 to 81 in the following Table 4 in the 
solid-solution state. That is, Ga.sub.2 O.sub.3 and TiO.sub.2 were mixed 
in the predetermined rates and burned in the air at 1,000.degree. C. for 1 
hour to obtain titanium-containing oxides. It was found as a result of 
ascertainment by X-ray diffraction that the oxides exhibited only the 
diffraction peaks of rutile type TiO.sub.2 and all the Ga was contained in 
TiO.sub.2 in the solid-solution state. The oxides were respectively molded 
into a chip-like form in the same manner as in Example 41 and burned in 
the air at 1,300.degree. C. for 10 hours to obtain chip-like sintered 
bodies. On the other hand, H.sub.2 PtCl.sub.2 was added to Al.sub.2 
O.sub.3 so that the content became 300 ppm, and after drying the mixture 
was burned at 500.degree. C. for one hour to give oxidation catalyst 
powder. This powder was applied onto the surface of each of the chip-like 
sintered bodies and burned in the air at 1,200.degree. C. for one hour. 
The resulting chip-like sintered bodies were measured for sensitivity to 
the various gases in the same manner as in Example 41. 
The results are shown in Table 4. 
COMATIVE EXAMPLES 5 and 6 
Chip-like sintered bodies were obtained as in Examples 75 to 81 except that 
titanium oxide having a .delta. value of 0.01 or 0 was used and the 
surface contained 0.5% of Pt or 5% of Pt. The resulting chip-like sintered 
bodies were measured for sensitivity to various gases in the same manner 
as in Example 41. 
The results are shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Sensitivity to various gases 
NO.sub.x gas-detecting element 
NO CO H.sub.2 
HC O.sub.2 
Kind of oxide 
(.delta.) 
Oxidation catalyst 
sensitivity 
sensitivity 
sensitivity 
sensitivity 
sensitivity 
__________________________________________________________________________ 
Example 
41 TiO.sub.2 0.05 
-- 1.20 0.08 0.06 0.15 0.15 
42 " 0.03 
Pt 300 ppm 1.16 0.02 0.02 0.05 0.12 
43 " 0.04 
Pd 300 ppm 1.17 0.03 0.02 0.05 0.10 
44 " 0.03 
Rh 300 ppm 1.17 0.01 0.01 0.04 0.09 
45 " 0.05 
Pt 300 ppm (/TiO.sub.2) 
1.16 0.03 0.03 0.06 0.14 
46 " 0.05 
Pt 800 ppm (/TiO.sub.2) 
1.05 0.02 0.06 0.05 0.12 
47 " 0.05 
Pd 800 ppm (/TiO.sub.2) 
1.03 0.01 0.08 0.06 0.10 
48 " 0.05 
Ni 1% 1.18 0.07 0.06 0.08 0.15 
49 " 0.05 
LaCoO.sub.3 1% 
1.08 0.05 0.05 0.08 0.15 
50 Al.sub.0.01 Ti.sub.0.99 O.sub.2 
0.07 
Pt 300 ppm (/TiO.sub.2) 
1.25 0.01 0.09 0.02 0.10 
51 Nb.sub. 0.05 Ti.sub.0.95 O.sub.2 
0.16 
Pt 300 ppm (/TiO.sub.2) 
1.08 0.04 0.06 0.08 0.19 
52 Ta.sub.0.03 Ti.sub.0.97 O.sub.2 
0.15 
" 1.08 0.03 0.06 0.07 0.18 
53 Sb.sub.0.02 Ti.sub.0.98 O.sub.2 
0.12 
" 1.10 0.03 0.04 0.05 0.20 
54 As.sub.0.01 Ti.sub.0.99 O.sub.2 
0.10 
" 1.15 0.02 0.04 0.05 0.19 
55 Ga.sub.0.003 Ti.sub.0.997 O.sub.2 
0.09 
" 1.30 0.01 0.02 0.03 0.11 
56 Ta.sub.0.005 Ti.sub.0.995 O.sub.2 
0.09 
" 1.29 0.01 0.03 0.02 0.15 
57 Sc.sub.0.01 Ti.sub.0.99 O.sub.2 
0.10 
" 1.25 0.02 0.04 0.03 0.12 
58 Mg.sub.0.05 Ti.sub.0.95 O.sub.2 
0.13 
" 1.25 0.03 0.02 0.03 0.10 
59 Y.sub.0.01 Ti.sub.0.99 O.sub.2 
0.12 
" 1.28 0.02 0.02 0.02 0.13 
60 Al.sub.0.01 Nb.sub.0.01 Ti.sub.0.98 O.sub.2 
0.10 
" 1.08 0.03 0.04 0.06 0.18 
61 Sc.sub.0.01 Sb.sub.0.01 Ti.sub.0.98 O.sub.2 
0.13 
Pt 300 ppm (/TiO.sub.2) 
1.10 0.04 0.05 0.06 0.19 
62 In.sub.0.01 Ta.sub.0.01 Ti.sub.0.98 O.sub.2 
0.11 
" 1.12 0.04 0.04 0.05 0.17 
63 BaTiO.sub.3 
0.09 
" 1.29 0.03 0.03 0.06 0.20 
64 PbTiO.sub.3 
0.06 
" 1.14 0.02 0.02 0.05 0.18 
65 CdTiO.sub.3 
0.05 
" 1.20 0.01 0.02 0.03 0.13 
66 CaTiO.sub.3 
0.09 
" 1.10 0.02 0.02 0.03 0.03 
67 SrTiO.sub.3 
0.07 
" 1.28 0.01 0.03 0.05 0.05 
68 LaTiO.sub.3 
0.06 
" 1.18 0.03 0.04 0.07 0.15 
69 BaTiO.sub.3 + TiO.sub.2 
0.12 
" 1.20 0.01 0.02 0.05 0.12 
70 CaTiO.sub.3 + TiO.sub.2 
0.09 
" 1.18 0.02 0.03 0.04 0.04 
71 SrTiO.sub.3 + TiO.sub.2 
0.09 
Pt 300 ppm (/TO.sub.2) 
1.17 0.01 0.03 0.03 0.05 
72 Sr.sub.0.9 Ca.sub.0.1 TiO.sub.3 
0.09 
" 1.20 0.01 0.04 0.06 0.04 
73 Ba.sub.0.9 La.sub.0.1 TiO.sub.3 
0.10 
" 1.25 0.02 0.04 0.06 0.18 
74 Ca.sub.0.9 La.sub.0.1 TiO.sub.3 
0.12 
" 1.22 0.01 0.03 0.05 0.08 
75 Ga.sub.0.0001 Ti.sub.0.9999 O.sub.2 
0.09 
Pt 300 ppm (/Al.sub.2 O.sub.3) 
1.06 0.05 0.04 0.07 0.13 
76 Ga.sub.0.0005 Ti.sub.0.9995 O.sub.2 
0.12 
" 1.30 0.02 0.02 0.04 0.10 
77 Ga.sub.0.001 Ti.sub.0.999 O.sub.2 
0.13 
" 1.28 0.01 0.02 0.03 0.09 
78 Ga.sub.0.005 Ti.sub.0.995 O.sub.2 
0.13 
" 1.25 0.01 0.02 0.05 0.09 
79 Ga.sub.0.01 Ti.sub.0.99 O.sub.2 
0.15 
" 1.16 0.02 0.04 0.06 0.12 
80 Ga.sub.0.05 Ti.sub.0.95 O.sub.2 
0.14 
" 1.06 0.05 0.06 0.06 0.13 
81 Ga.sub.0.05 Ti.sub.0.95 O.sub.2 
0.14 
-- 1.08 0.13 0.16 0.22 0.18 
Comparative 
Example 
5 TiO.sub.2 0.01 
Pt 0.5% 0.32 0.19 0.58 0.39 0.45 
6 TiO.sub.2 0 Pt 5% 0.08 0.02 0.01 0.03 0.50 
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