JPS5847020B2 - gas sensing element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas-sensitive element, and more particularly to a gas-sensitive element in which a catalyst layer is provided on the surface of a gas-sensitive member to improve sensitivity, selectivity, aging characteristics, and the like.

Gas-sensitive elements are known that utilize the fact that the specific resistance of the surface of an oxide semiconductor changes when gas comes into contact with the surface of the oxide semiconductor.

For example, ZnO2SnO2, Fe2O, which exhibits N-type semiconductivity
When a reducing gas comes into contact with the 3rd grade, the resistance value decreases, and when an oxidizing gas comes into contact with the 3rd grade, the resistance value increases.

Further, in an oxide semiconductor that is a P-type semiconductor, the increase and decrease in resistance value exhibits an inverse relationship.

In the above-mentioned oxide semiconductors, the reactivity or selectivity with various gases is determined by the semiconductor surface temperature, surface electron level structure, porosity, pore size, etc.
Generally, an oxide semiconductor alone has low sensitivity as a gas-sensitive element and cannot be said to have sufficient selectivity.

Therefore, attempts have been made to increase the sensitivity by adding a catalyst such as Pt or Pd to the oxide semiconductor, but these have the following drawbacks.

In other words, the oxide semiconductor and catalyst, which are the main components, have different optimal firing temperatures, making it extremely difficult to select a firing temperature that fully brings out the characteristics of both.

Furthermore, when used as a gas-sensitive element under high temperature conditions (
In order to increase the sensitivity of the gas-sensitive element, it is preferable to provide a heating section and use the oxide semiconductor surface at 300°C.)
The catalyst was solidly dissolved in the oxide semiconductor, causing a decrease in sensitivity and an increase in changes over time.

The present invention improves the drawbacks of the conventional element described above, and includes a pair of electrodes and ZnO provided between the electrodes.
5 to 20 mol%, 0.1 to 50 mol% of MeO (at least one of Co, Ni, and Mn) and 0.05 to 30 mol% of Me'203 (however, Me' is at least one of Co, Ni, and Mn). Ga, B, In,, Fe,
A.I.

Contains a gas sensitive material containing at least one type of Cr) and a Pt compound containing 70 or less atoms (not including 0) of at least one of Re and Rh as an additive to silica and alumina by 0.01 to 10% by weight★ It is an object of the present invention to provide a gas-sensitive element comprising a catalyst layer, which has excellent sensitivity and gas selectivity, has a particularly low humidity coefficient of resistance value, and has little change over time due to long-term use.

Note that the composition range in the present invention was limited for the following reasons.

In other words, if ZnO exceeds 99.85 mol★, MeO
is less than 0.1 mol, and M6□03 is 0.0
When ZnO is less than 5 mol φ, the change in resistance due to gas adsorption is small, and when ZnO is less than 20 mol φ, Me
When O exceeds 50 molφ and M6□03 is 30
If the molar value exceeds ★, the change in resistance due to gas adsorption will be small, and the change in resistance with respect to temperature will be large, so the composition of the gas sensitive material was set within the above range.

P containing at least 70 atoms (but not including O) of at least one of Re and Rh to silica and alumina
The reason why the amount of the t compound added was 0.01 to 10% by weight was because
This is because if the weight is less than o, oi weight φ, the change in resistance value due to gas adsorption will be small, and if it exceeds 10 weight, no improvement in the aging characteristics can be expected.

The reason for setting Re and Rh in the Pt compound to be 70 atoms or less is because sensitivity decreases when exceeding 70 atoms.

The present invention will be explained in detail below using examples.

First, the gas-sensitive element according to the present invention has a pair of electrodes 2 on the outer peripheral surface of a cylindrical insulating base 1, as shown in cross section in FIG. A gas sensitive body 3 is provided.

Further, on the surface of the gas sensitive body 3, Pt-Re, Pt-R
A catalyst layer 4 made of silica/alumina containing a Pt compound such as h-based compound is provided.

Further, the gas-sensitive element constructed as described above is assembled on a pin leg, for example, as shown perspectively in FIG.

In the figure, 5 indicates a lead wire, 6 indicates an insulating plate, and 7 indicates a heater.

The heater 7 is provided to improve the sensitivity of the gas sensitive element, and can be provided as appropriate if necessary.

Note that the catalyst layer 4 does not necessarily need to completely cover the surface of the gas sensitive body 3.

The gas-sensitive element according to the present invention is manufactured, for example, as follows.

That is, ZnO, MeO (Me is at least one of Co, Ni, and Mn) and M6203 (Me is Ga)
, B, In, Fe, AI, and Cr) in a predetermined composition ratio, mixed, water or a binder was added to form a paste, and an insulating material with a pair of electrodes 2 as shown in FIG. 1 was prepared. After coating on substrate 1 and drying 6
It is fired at 00 to 1000'C to form a gas sensitive body.

On the other hand, silica alumina calcined at 500 to 1300°C is ground into fine powder using a grinder such as a planetary mill or a pot mill.

Next, Pt compounds such as chloroplatinic acid, Re compounds, Rh
Weigh out the compound etc. in a predetermined component ratio, and add water to make an aqueous solution.

Thereafter, it is mixed with the silica/alumina fine powder in a predetermined weight ratio, and then subjected to a drying process to obtain a catalyst.

The starting materials for silica and alumina may be crystalline or amorphous as long as they become oxides at high temperatures.

This catalyst is coated on the gas sensitive member 3 and dried, and then
A gas-sensitive element is obtained by firing at a temperature of 00 to 900°C.

Next, examples of characteristics of the gas-sensitive element according to the present invention are shown in FIGS. 3 to 10.

The values for each characteristic were taken at a heating temperature of 370°C.

First, FIGS. 3 to 5 show the gas sensitive member components MeO (where Me is at least one of Co, Ni, and Mn) and Me2O3 (Me is Ga and B.

Sensitivity (at least one of In, Fe, AI, Cr) is determined by the ratio of the resistance Ro in air to the resistance Rg in an inbutane gas concentration of over 0.2.
Ro/Rg).

The catalyst layer is Pt-0,05Re with a weight of 0.2%.
Using 3A1203.2SiO2 containing -0,05R,h, curve 1 in the figure is Ga2O3, B2O3, Fe2O3
, resistance value in case of composite addition of Cr2O3, curve 2 is B2
The resistance value in the case of combined addition of O3゜I n2032 Cr 203, and curve 3 is A l 203, Fe 2
Figure 3 shows the resistance values in the case of composite addition of 03. Figure 3 shows Co01 as Me2O1 Figure 4 shows Ni01
This represents the case where O, is used.

Curves 1', 2' and 3' show sensitivities corresponding to curves 1, 2 and 3, respectively.

Furthermore, in the curve 3 in Figure 3 above, the amount of MeO added was increased to 10
Pt-Re system, Pt
The sensitivity of the -Rh catalyst to the Re and Rh contents was measured and is shown in FIG.

As a result, in either case, sensitivity decreases when the 70-atom slope is exceeded.

Note that the reason why the sensitivity depends on the content differs depending on Re or R, h is thought to be due to the difference in the crystal structure of the two.

As a result, as is clear from FIGS. 3 to 6, excellent sensitivity was always obtained in the gas-sensitive element according to the present invention.

Furthermore, in FIGS. 7 to 9, the amount of MeO added is fixed at approximately 2 mol for curve 1 in FIGS. 3 to 5, and the catalyst layer is composed of Pt supported on 3A1203.2SiO2.
Re when the amount of compound is fixed at 0.2% by weight,
Figure 2 shows the temporal characteristics of Rh content.

The measurement shows the rate of change in resistance value after energization for io and ooo times, and as a comparative example, the case without the addition of Re or Rh is also shown. Figure 7 shows Cod as MeO, and Figure 8 shows Ni.
O. FIG. 9 shows the case where MnO is used.

As a result, as is clear from FIGS. 7 to 9, in the gas-sensitive element according to the present invention, when used for a long period of time, the
Only a decrease of about % was observed.

The reason why the rate of change over time of a gas-sensitive element using a catalyst layer made of silica-alumina containing a Pt compound containing Re and Rh is small is considered to be as follows.

First, due to the two-layer structure that separates the gas sensitive material and the catalyst layer, the catalyst's Pt compound containing at least one of Re and Rh does not dissolve in the gas sensitive material, so there is no deterioration of the catalyst's performance. It is thought that this is because of this.

In addition, Pi, Pd, etc. or their oxides, which have been conventionally used as catalysts, cause grain growth and reduce the surface area of the catalyst when used, but as in the present invention, they have excellent heat resistance.
By using silica/alumina, which is inactive at high temperatures, as a carrier, Pi, RetRh, etc. are maintained in a state with a large surface area.Furthermore, since it contains Re and Rh, which have a higher melting point than Pt, it is possible to prevent sintering of the catalyst at the gas sensor usage temperature. This is thought to be because reduction in surface area can be prevented.

When no carrier is used, a decrease of about 20% is observed after about 1000 hours, and the present invention is superior, as a decrease of about 10% at most was observed even after 10,000 hours.

FIG. 10 shows Co, using the gas-sensitive element according to the present invention.
It shows the rate of change in resistance value with respect to the gas concentration of H2, C2H6, C3H8t C4H10, and as a result, it is clear that it has excellent selectivity.

As described above, the gas-sensitive element according to the present invention has excellent sensitivity selectivity and temporal change characteristics, and has excellent features never seen before.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a configuration example of the present invention, FIG. 2 is a perspective view showing an example of a device using a gas-sensitive element according to the present invention, and FIGS. 3 to 5 are resistance values and sensitivity with respect to the amount of MeO added. Figure 6 is a curve diagram showing the sensitivity to the Re and Rh contents in the catalyst, Figures 7 to 9 are the Re and Rh contents when the amount of MeO added is fixed at well over 2 moles. FIG. 10 is a curve diagram showing the selectivity of the gas-sensitive element according to the present invention. 2... Electrode, 3... Gas sensitive body, 4...
...Catalyst layer.

Claims (1)

[Claims] 1 A pair of electrodes and 9 ZnO provided between the electrodes.
9.85-20 mol%, MeO 0.1-50 mol% (
Tazashi Me' is at least one of Co, Ni, Mn) and M'e203 at 0.05 to 30 mol% (Tazashi Me' is Ga, B, In, Fe, AI, C
a gas sensitive body containing at least one of R) and a Pt compound containing 0.01 to 10% by weight of at least one of Re and Rh provided on the surface of the gas sensitive body, containing 70 atoms or less (however, excluding O); What is claimed is: 1. A gas-sensitive element comprising a catalyst layer made of silica and alumina containing