JPH02161345A - Catalyst combustion type gas detecting element - Google Patents

Abstract

PURPOSE:To form the catalyst combustion type gas detecting element which is free from erroneous alarming and failure in alarming and is excellent in reliability by providing a tin oxide carrier deposited with a noble metal catalyst and increasing the temp. rise when a combustible gas makes contact combustion in the carrier. CONSTITUTION:A palladium chloride soln. is impregnated in tin oxide powder having, for example, 3mum average grain size and 20m<2>/g specific surface area at such a ratio at which prescribed weight % is attained as palladium. The powder is gradually heated over a water bath to remove the moisture and is heated in air after drying to decompose the palladium chloride. A silica sol is added to this tin oxide powder with the palladium so as to attain prescribed weight % as silica to prepare paste. The tin oxide with the palladium is applied on a coil 1 so as to cover the coil 1. The coil 1 is then dried and heated to form the tin oxide carrier 2 with the palladium around the coil 1. Since the tin oxide carrier is deposited with the noble metal catalyst in this way, the temp. rise when the combustible gas burns in the carrier increases and the contact combustion type gas detecting element which is free from the erroneous alarming and the failure in alarming is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a catalytic combustion type gas sensing element, and particularly to the structure of a carrier for the sensing element.

[Conventional technology]

As shown in Fig. 3, the catalytic combustion type gas detection element consists of a hot wire 1 made of a coiled wire made of a material with a high temperature coefficient of resistance, such as platinum, and coated with a carrier 2 such as activated alumina. It is formed by supporting a catalyst that has excellent oxidation performance against toxic gases. On the other hand, the temperature compensating element is formed in the same manner as the gas sensing element except that it is inert to combustible gas. The pixel elements are connected together with a resistor 5 in an adjacent branch as shown in FIG.
The output of the gas detection element is detected by the output detection section 7 provided between the connection point of the gas detection element and the connection point of the gas detection element.

As a carrier for a conventional catalytic combustion type gas detection element, for example, platinum is used in porous activated alumina, as disclosed in US Pat. No. 3,092,799.

Those carrying noble metals such as palladium are generally used. Such activated alumina supported yellow metal catalysts are known to exhibit high activity in catalytic oxidation reactions of combustible gases, and are widely used in gas leak alarm sensors for LP gas, city gas, etc. .

In conventionally used gas detection elements made of the above materials, the temperature rise due to catalytic combustion in, for example, 0.2% isobutane gas is 10 to 20°C when the element size is approximately 1.5*mφ. As a result, a bridge output value of 10 to 20 mV was obtained.

[Problem to be solved by the invention]

However, even if the sensing element is used in combination with a temperature compensation element to form a bridge, when used for a long period of time, there will be differences in the heat dissipation state of the pixel element due to dust adhesion and differences in the degree of sintering. Fluctuations occur in the zero gas output (hereinafter referred to as the zero point), reaching several mV over several years, and there is a risk that the bridge output of the above-mentioned sensing element may cause problems such as malfunction or failure to provide an alarm.

This invention was made in view of the above points, and its purpose is to increase the temperature change of the gas detection element due to the catalytic combustion reaction and increase the detection element output, thereby eliminating the influence of zero point fluctuation due to long-term use. An object of the present invention is to provide a catalytic combustion type gas detection element with excellent reliability.

[Means to solve the problem]

According to the present invention, the above-mentioned object is to provide a gas detection element that catalytically burns a combustible gas on the internal surface of a carrier and detects the temperature rise due to the combustion reaction as a resistance change of a coil embedded inside the carrier. This is achieved by providing a tin oxide carrier 2 on which is supported.

As noble metal catalysts, both platinum and palladium exhibit similar high activity.

[Effect]

Activated alumina mainly has the function of increasing the dispersibility of precious metals, whereas tin oxide actively adsorbs oxygen on its particle surface, and this adsorbed oxygen and precious metal catalyst act synergistically to cause flammability. This is thought to be due to high oxidation activity against gases.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. Tin oxide powder with an average particle size of 3 μm and a specific surface area of 20 d/i,
Impregnated with a palladium chloride solution to a concentration of 0.5% by weight as palladium, gradually heated on a water bath to remove water, dried at 150°C for 2 hours, and then heated at 600°C for 3 hours.
Heat in air for an hour to decompose the palladium chloride. Silica sol is added to this palladium-coated tin oxide powder to give a concentration of 3 m - 11% as silica to form a paste, and the palladium-coated tin oxide is applied so as to cover the coil 1 shown in FIG. Then, after drying at room temperature for 2 hours,
Heating in the air at 0°C for 3 hours forms a tin oxide carrier with palladium around the coil (hereinafter, this element is referred to as P
d/5n02 Seiko).

Next, for comparison, the average particle diameter is 3 μm, the specific surface area is 150, ? 1
/g of r-alumina powder is impregnated with a palladium chloride solution to give a concentration of 0.5% by weight as palladium, and the subsequent operations are carried out in the same manner as for the Pd/Sn0g element, using a gas using palladium-attached r-1'J20s as a carrier. A sensing element is produced (hereinafter this element will be referred to as a Pa/A1320s t=child).

The above two elements were each set at the position of the gas detection element 3 shown in FIG. 4, and the output with respect to inbutane gas was measured. However, as the temperature compensation element 4, an element was used in which each gas detection element was covered with a hermetically sealed cap to prevent gas from coming into contact with it.

FIG. 1 compares the bridge output with respect to isobutane gas concentration between a Pd/5n02 element and a 20M2O3 element when the element temperature is maintained at 400 DEG C. in an atmosphere without isobutane gas. The solid line A in the figure is the Pd/5n of the present invention.
02 element output, solid line B is Pa/A13 as a comparative example
The output of 203 elements is shown. As shown in Figure 1, Pd/5
The n02 element is Pd, which is used in conventional gas detection elements.
/) It can be seen that the bridge output is about 4 times higher than that of the J203X.

Next, the Pd/SnO2 element prepared in the above example and the Pd/SnO2
In order to compare the temperature rise upon contact with d/to-e ZOs crystal 0 gas, the surface temperature of the element was measured using a thermograph. Figure 2 shows 0.2% when changing the element temperature in the atmosphere.
The temperature rise value when contacting with inbutane gas is calculated, solid line C is Pd/5nOz element, solid line C is Pd%203
Indicates the characteristics of the element. As shown in FIG. 2, it can be seen that at a device temperature of 400° C., the Pd/5nOz device exhibits a temperature increase approximately four times that of the 20M2O3 device.

〔Effect of the invention〕

According to this invention, in a gas detection element that catalytically burns a combustible gas on the internal surface of a carrier and detects the temperature rise due to the combustion reaction as a resistance change in a coil embedded inside the carrier, tin oxide on which a noble metal catalyst is supported is used. Since it is equipped with a carrier, the temperature rise when combustible gas catalytically burns on the carrier is greater than that of conventional elements, which increases the element output and eliminates the effects of aging of the zero point, eliminating false alarms and failure to alarm. It becomes possible to provide a catalytic combustion type gas detection element with excellent reliability. Furthermore, due to the high sensitivity change of the element, it is possible to measure with high precision even for low concentration gases, and it is also possible to expand the range of use, such as application as a gas concentration meter.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the bridge output and inbutane concentration of the gas detection element according to the embodiment of the present invention in comparison with the characteristics of the conventional element, and Figure 2 is the diagram according to the embodiment of the present invention. Diagram showing the relationship between the temperature rise value of the gas detection element and the element temperature in comparison with the characteristics of a conventional element, ii! FIG. 3 is a partially cutaway perspective view showing the catalytic combustion type gas sensing element, and FIG. 4 is a connection diagram showing a bridge circuit for taking out the output of the gas sensing element. Iftan is also concentrated 1L (%) Figure 1 Element temperature (-C) Dream 2 Figure

Claims (1)

[Claims] 1) A gas detection element that catalytically burns combustible gas on the internal surface of the carrier and detects the temperature rise due to the combustion reaction as a change in resistance of a coil embedded inside the carrier, is equipped with a tin oxide carrier on which a noble metal catalyst is supported. A catalytic combustion type gas detection element featuring: