JPH0692938B2 - Gas detection method and apparatus used therefor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial field of application" The present invention relates to a gas capable of selectively and accurately measuring the concentration of a specific gas, and analytically measuring the composition and concentration of each component of a mixed gas. The present invention relates to a detection method and an apparatus used for this method.

"Prior Art" As a conventional gas detection method, the following is known.

(1) Catalytic combustion thermometer method Using the fact that the catalytic combustion rate of combustible gas on the surface of a solid catalyst is proportional to the concentration of gas phase combustible gas, the temperature rise ΔT of the catalyst due to combustion heat is measured. Is a method of obtaining the gas concentration.

(2) Method for measuring electric conductivity of gas-sensitive semiconductor This method utilizes the phenomenon that the electric conductivity of a semiconductor changes depending on the atmosphere.

(3) Diaphragm galvanic cell method This is a method of obtaining a gas concentration by measuring an electrolytic current due to a specific electrochemical reaction of gas molecules dissolved in an electrolytic solution through a diaphragm.

(4) Gas chromatography This is a method of selectively and analytically detecting a gas by utilizing the time difference between adsorption and desorption of a sampled gas.

(5) Light absorption method This is a method of detecting the gas selectively and analytically by measuring the light absorption spectrum specific to the gas.

(6) Photochemical reaction method This is a method of selectively detecting a gas by utilizing a unique photochemical reaction between specific gases.

"Problems to be Solved by the Invention" However, the above method has the following drawbacks.

Since the method (1) uses only the information of temperature regardless of the type of gas, it is essentially impossible to perform selective and analytical detection.

In (2), only the information on the electric conductivity is used regardless of the type of gas. Therefore, essentially selective, analytical detection is not possible. Further, although the sensitivity is high, the electric conductivity value σ in the reference atmosphere is unstable, and the sensitivity depends on σ, so that the temporal stability, the measurement accuracy, and the accuracy are poor.

The method (3) uses a specific reaction and thus has good selectivity, but since it measures only the amount of current, analytical detection is essentially impossible. Furthermore, it is necessary to replace the diaphragm contaminated with dust and to replace the electrolyte solution that has been consumed or changed due to the reaction, which is a troublesome maintenance.

(4) Since it is a batch sampling method, continuous measurement is not possible, and a thermostatic chamber for thermally desorbing gas is large and expensive.

In the method (5), since the light absorption coefficient of gas is small, a long optical path is required, and the difference in absorbance with the comparative gas must be measured precisely. Therefore, there is a drawback that the device becomes large and expensive.

The method (6) requires a large-scale device equipped with a gas cylinder or the like because a reaction gas that causes a unique reaction with the gas to be detected must be constantly supplied, and a device for detecting weak gas phase emission is required. Get expensive.

By the way, the ideal gas detection method is (1) capable of detecting low-concentration gas, that is, having high sensitivity, (2) capable of selective and analytical detection, and (3) stable measurement for a long period of time. The requirements must be satisfied, (4) continuous measurement is possible, (5) no consumables are required, and (6) the measuring device is small and inexpensive.

However, each of the conventional detection methods has merits and demerits, and none of them satisfy all the above requirements.

[Means for Solving the Problems] The present inventor has proposed a semiconductive or electrically insulating solid catalyst in a temperature range in which the surface catalytic oxidation reaction between adsorbed gases proceeds in a diffusion-controlled state (that is, the phenomenon of catalytic oxidation When the reaction rate is maintained in a temperature range where it is almost independent of the catalyst temperature), the luminescence of the light intensity corresponding to the gas-phase component gas concentration is emitted from the catalyst solid, and the emission of the luminescence. Is stable, and that its luminescence spectrum depends on the type of adsorbed gas.
discovered.

Based on this discovery, the present invention aims to provide an ideal gas detection method satisfying all the requirements described above and an apparatus used for the method, and has the following configurations.

The gas detection method uses a semiconductive or electrically insulating solid catalyst,
The surface of the gas is maintained at a temperature at which a surface oxidation reaction occurs in a diffusion-controlled state, the luminescence emitted from the catalyst is measured, and the type and composition of the gas is determined from the luminescence spectrum. The gas concentration is measured.

In the gas detection device used in this gas detection method, one or a plurality of spectroscopic devices and an optical sensor are sequentially arranged on a line having a surface of a semiconductive or electrically insulating solid catalyst as an optical field of view. Then, a means for measuring the output of the optical sensor and a heater for heating the catalyst are provided.

In the gas detection device, it is possible to provide an ultraviolet light source or a radiation source for irradiating the catalyst surface instead of the heater. Or, of the heater, UV light source, and radiation source, 2
It may be a combination of the above.

"Operation" When a semiconductive or electrically insulating solid catalyst is heated, a gas phase component is chemically adsorbed on the surface thereof to form a surface level. Generally, the reducing gas forms a surface donor level on the catalyst surface, and the oxidizing gas forms a surface acceptor level. Usually, the surface donor level has a higher electron potential energy than the surface acceptor level, so that the localized electrons at the surface donor level directly recombine with the localized holes at the surface acceptor,
Light is emitted according to the energy difference. Or, indirectly, recombination (for example, localized holes of the surface acceptor are thermally excited to form free holes in the valence band, and these free holes move to the vicinity of the surface donor, and electrons of the surface donor are Recombine with) and emit light according to the energy difference.

In each case, the spectrum of the emitted light reflects the energy level of the surface level formed by the chemisorption of the gas, and the surface level differs depending on the type of the adsorbed gas. The spectrum will contain information about the type of gas. Therefore, the type of gas can be determined from the spectrum.

On the other hand, when the catalyst is heated, a surface oxidation reaction occurs between the reducing gas adsorbed on the surface and the oxidizing gas. as a result,
The surface level that emitted light also disappears and a new reaction product is created, but this reaction product desorbs from the catalyst surface and returns to the gas phase.

The adsorption, luminescence generation, reaction, and desorption processes are continuously repeated, so that the luminescence continues. In other words, part of the energy released by the surface oxidation reaction is
Instead of reaction heat, it is emitted in the form of light energy. The emission intensity naturally depends on the surface reaction rate. When the temperature of the catalyst is maintained at a temperature at which the surface oxidation reaction rate is diffusion-controlled, the emission process of luminescence is stable and the emitted light intensity shows a one-to-one correspondence with the gas concentration. A constant light intensity is maintained as long as the gas is present. Therefore, the gas concentration can be measured from the light intensity.

In an apparatus used for such a gas detection method, when a semiconductive or electrically insulating solid catalyst is heated by a heater, luminescence is emitted as described above.

Since this luminescence has specific spectral characteristics depending on the type of gas to be detected, it is possible to determine the type of gas to be detected by spectrally measuring the spectrum using a spectroscopic device such as a prism, diffraction grating, or optical filter. it can.

Luminescence can also be converted to an electrical signal by a photosensor via a spectroscopic device. The intensity of the electric signal is measured by the means for measuring the output of the electric signal, and the concentration of the gas to be detected is measured.

"Embodiment" First, an embodiment of the gas detector shown in FIG. 1 will be described.

Reference numeral 1 denotes a base made of an electrically insulating material, and two metal columns 2 are inserted therethrough. A metal wire coil heater 3 is located above the pedestal 1 and is connected to the upper portions of the two columns 2 by a conductive wire.

Reference numeral 4 denotes a solid catalyst, which is sintered and molded by enclosing the heater 3.

Reference numeral 5 denotes two types of optical filters that are detachably fixed to the upper part of a mounting hole penetratingly provided in the center of the base 1, and respectively pass light of different specific wavelength regions. Below the filter 5, a pair of optical sensors 6 are similarly fixed in the mounting holes to convert the luminescence emitted by the catalyst 4 into an electric signal and output the electric signal.

Reference numeral 7 is a wire mesh for protecting the catalyst 4, and 8 is a power source for heating the heater 3, which is connected to the pillar 2.

Reference numeral 9 denotes an operational amplifier, which amplifies an output signal of an electric signal of the optical sensor 6 to perform an arithmetic process. Reference numeral 10 is a meter for displaying the output signal of the operational amplifier 9. The meter 10 is a means for measuring the output of the optical sensor.

Hereinafter, a specific example in which a sintered body of Al ₂ O ₃ powder is used as the catalyst 4 in this apparatus and ethanol is first selected as the gas to be detected and then a mixture of ethanol and acetone is selected will be described.

First, a case will be described in which one of the filters 5 is shielded and the other is a filter 5 that transmits light of 640 nm or less in order to cut light in a long wavelength range due to black body radiation.

An electric current is applied to the heater 3 to heat the catalyst 4 by Joule heat to maintain the temperature of the catalyst 4 at T ° C. When ethanol vapor is introduced into the air around the catalyst 4, reaction heat is generated by the catalytic oxidation reaction between ethanol and oxygen adsorbed on the surface of the catalyst 4, and the temperature of the catalyst 4 rises by ΔT.

At the same time, the recombination process between the electrons possessed by the adsorbed ethanol and the free holes thermally excited from the adsorbed oxygen proceeds in parallel, and luminescence occurs.

Here, in FIG. 2, the logarithm of the luminescence intensity I and Δ
The results of plotting the logarithm of T and the reciprocal of the temperature T of the catalyst 4 are shown as curves I and II, respectively.

When the temperature of the catalyst 4 is 300 ° C. or higher, the curve II hardly depends on T, and the catalytic oxidation reaction rate depends only on the diffusion rate of ethanol to the surface of the catalyst 4 and does not depend on the catalytic activity of the surface. It shows the characteristic characteristics in.

At 420 ° C or higher, the luminescence intensity does not depend on T either, but at 300 ° C or higher, luminescence shows a stable response to changes in ethanol concentration.

For example, Al ₂ O ₃ catalyst temperature T = 275 ° C., 300 ° C., 325 ° C., 350
The response waveform when 0.32% ethanol was introduced into the air at 300C for 300 to 600 seconds from the start of measurement,
It is shown by curves I, II, III and IV in FIG.

The curve I representing the luminescence intensity at 275 ° C., which is below the diffusion-controlled temperature region, gradually increases from the time when the gas is introduced, and shows a peak after the gas concentration becomes zero. Shows a response waveform. This is 30
In the reaction rate-limited state of 0 ° C. or lower, the adsorbed ethanol cannot be immediately removed by reaction, so that the adsorbed ethanol accumulates on the surface of the catalyst 4, so that the adsorbed oxygen decreases and recombines with the electrons of the adsorbed ethanol. It is thought that this is due to a shortage of holes in the origin of adsorbed oxygen.

On the other hand, curves III and IV representing the luminescence intensity at 325 ° C. and 350 ° C. in the diffusion-controlled temperature region show stable response waveforms almost corresponding to changes in gas concentration. That is, it was newly found that the response of luminescence changes qualitatively at the boundary between the diffusion-controlled and the reaction-controlled.

FIG. 4 shows the relationship between the ethanol concentration in air and the luminescence intensity at a catalyst temperature T = 350 ° C. at which the Al ₂ O ₃ catalyst shows a stable response.

Next, as a simple example, one method for analyzing the composition of a binary gas mixture leaking into the air will be described.

At the Al ₂ O ₃ catalyst temperature T = 350 ° C., the luminescence spectra of the case where only ethanol is mixed in the air and the case where only acetone is mixed in the air are shown in curves I and II of FIG. Are shown in each. This is a result of providing a diffraction grating spectroscope instead of the filter 5, changing the transmission wavelength thereof sequentially, and measuring the transmitted light intensity using a photomultiplier tube.

In this case, since the catalyst is not a single crystal but a sintered body of powder, the spectrum is broad, but the peak wavelengths are clearly different for each gas.

The peak wavelengths of luminescence when ethanol and acetone are mixed in air independently are λ ₁ and λ ₂ respectively.
And Further, luminescence intensities by ethanol at wavelengths of λ ₁ and λ ₂ at unit gas concentration are defined as a ₁ and a _2, and luminescence intensity by acetone at b ₁ and b ₂ .
Since the linear addition rule holds for the luminescence intensity,
Ethanol concentration is c _a, when acetone concentration of c _b, luminescence intensity of the wavelength _{λ 1 I 1 = a 1,} c a + b 1, c b ···· wavelength lambda ₂ of the luminescence intensity I ₂ = a ₂ , C _a + b ₂ , c _b ...

It is clear that if a ₁ , a ₂ , b ₁ , b ₂ are measured and determined in advance, then I ₁ , I ₂ can be measured and the above simultaneous equations can be solved to obtain c _a , c _b. It is well known that this can be realized by an analog or digital arithmetic processing device.

Especially, by adjusting the intensity of the luminescence incident on the optical sensor that measures the wavelengths of λ ₁ and λ ₂ by using a neutral density filter and a pinhole, I ₁ = c _a + b ₁ ′ c _b. .. 'I ₂ = c _a + b ₂ ' c _b ... 'It is easy to set.

At this time, by inputting the optical sensor outputs of I ₁ and I ₂ to the differential amplifier, I ₁ −I ₂ = (b ₁ ′ −b ₂ ′) c _b c _b = (I ₁ −I ₂ ) / ( Since b ₁ ′ −b ₂ ′) ..., 1 / (b ₁ ′ −b ₂ ′) of the output of the amplifier directly represents the acetone concentration.

Moreover, if the photosensor outputs of I ₁ and I ₂ are passed through the amplifiers of b ₂ ′ times and b ₁ ′ times respectively and then input to the differential amplifier, b ₂ ′ I ₁ −b ₁ ′ I ₂ = ( b ₂ ′ −b ₁ ′) c _a c _a = (b ₁ ′ I ₁ −b ₁ ′ I ₂ ) / (b ₂ ′ −b ₁ ′) ...
Since the can output of the amplifier 1 _{/ (b 2 '-b 1'} ) is to directly represent the ethanol concentration c _a.

In the embodiment shown in FIG. 1, the transmission wavelengths of the two filters 5 provided below the catalyst 4 are set to λ ₁ and λ ₂ , respectively, and the output of the optical sensor 6 is calculated by the equation. By inputting it to the operational amplifier 9, it is possible to switch and display either of the two gas concentrations on the meter 10.

Furthermore, in the detection of mixed gas of multi-component system, it is necessary to measure the luminescence intensity in more wavelength range and to perform complicated arithmetic processing, which can be easily realized by today's technology. Is.

When selectively detecting only a specific gas, it is only necessary to measure the luminescence in the spectral region peculiar to the gas. Especially as a catalyst, instead of powder, single crystal,
For example, when sapphire was used, the luminescence intensity was weakened, but the spectrum peculiar to each gas was emphasized and it was confirmed that the spectrum became close to the line spectrum. Of course, whiskers or oriented needle crystals can be used to sharpen the spectrum without significantly lowering the luminescence intensity.

In order to generate luminescence as described above, when the gas adsorbed on the catalyst surface is reducing, free holes are
Also, free electrons are required when oxidizable. It was experimentally confirmed that it is possible to generate luminescence at a lower temperature by irradiating ultraviolet rays or radiation to generate these free carriers by thermal excitation.

For example, when Al ₂ O ₃ was used as a catalyst, the luminescence due to the catalytic catalytic oxidation of ethanol generally did not appear unless the temperature was 200 ° C. or higher, but by irradiating with ²² Na or ⁹⁰ Sr radiation, at 50 ° C. Luminescence has occurred.

Further, the catalyst, SnO ₂ , a semiconductive material such as ZnO, Al ₂ O ₃ powder,
Sapphire, CaCO ₃ powder, Yongju stone, BaSO ₄ , CaSO ₄ , MgO,
An experiment was conducted using an electrically insulating solid such as BeO, but a stable luminescence response to gas was obtained in the temperature range in which the catalytic reaction occurs in the diffusion-controlled state, although the luminescence intensity of the sample was large or small. There was no example that violates the newly discovered rule of indicating.

Although it was possible to enhance the luminescence sensitivity by slightly mixing the above catalyst with a metal catalyst such as Pt or Rh, the luminescence disappeared when the surface was completely covered with metal.

Among the catalysts used in the experiment, Al ₂ O ₃ and CaCO ₃ emitted particularly strong luminescence. By doping the inside or the surface of the catalyst solid with an impurity serving as an emission center, the luminescence intensity was increased and the spectrum was made sharper to improve the gas selective detection characteristics.

For example, when BaSO ₄ was doped with Εu, emission from the Εu ³⁺ excited level appeared near 615 nm, and as a result, luminescence in the air containing ethanol was emphasized.

It is considered that this is because the emission center receives the energy due to the non-radiative transition from the surface level where the direct transition is not allowed, becomes the excited state, and emits luminescence when returning to the ground state again.

Although the catalyst was heated by the heating wire heater in the above example, the catalyst was heated by using an infrared heater that irradiates the catalyst with infrared rays through an optical fiber, and the generated luminescence was sent to an optical sensor through an optical fiber. It was also possible to guide and detect the gas at a point remote from the catalyst. This is effective as a gas detection means in an atmosphere that requires the prevention of ignition and explosion of gas by electric sparks.

[Effect] As described above in detail, the gas detection method of the present invention determines the type and concentration of gas by measuring the luminescence emitted from the catalyst solid when the detection gas undergoes a catalytic oxidation reaction on the catalyst surface. Since we know it, the only background of measured values is blackbody radiation. Blackbody radiation hardly changes over time, and its spectrum is completely different from luminescence, so it can be removed using a filter. Therefore, since the zero point of the optical measurement is practically zero, the sensitivity of the optical sensor for detecting luminescence can be increased, and the gas of minute concentration can be accurately detected.

In addition, since the luminescence that is stably generated is measured under the condition that the reaction rate is proportional to the essentially stable amount of the gas diffusion rate, the gas sensitivity does not change over time for a long period of time.

More importantly, the present invention is the only gas analysis method using a solid-state sensor and the apparatus used therefor, which can measure the luminescence spectrum that changes depending on the type of gas and analyze the composition of the mixed gas. That's what it means.

Moreover, the device has many advantages such as being smaller and cheaper than the conventional device.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a gas detection device for explaining an embodiment of the present invention. FIG. 2 is a diagram showing the temperature dependence of the luminescence intensity due to gas and the temperature change of the catalyst, FIG. 3 is a diagram showing the difference in the ethanol response waveform of luminescence with temperature, and FIG. 4 is a diagram showing the luminescence intensity. It is a figure showing ethanol concentration dependence. FIG. 5 is a diagram showing a difference in luminescence spectrum between ethanol and acetone.

Claims (3)

[Claims] 1. A semiconductive or electrically insulating solid catalyst is kept at a temperature at which a surface oxidation reaction occurs in a diffusion-controlled state of a gas on the surface thereof, and the luminescence emitted from the solid catalyst is measured to measure the luminescence. The method for detecting gas is characterized by determining the type of gas from the spectrum and measuring the concentration of gas from the intensity. 2. A means for sequentially arranging one or a plurality of spectroscopic devices and an optical sensor on a line having an optical field of view of the surface of a semiconductive or electrically insulating solid catalyst and measuring the output of the optical sensor. And a heater for heating the catalyst. 3. The gas detection device according to claim 2, wherein an ultraviolet light source or a radiation source for irradiating the catalyst surface is provided instead of the heater, or two or more of the heater, the ultraviolet light source and the radiation source are combined. A gas detection device characterized by being combined.