JP2009229369A - Hydrogen gas sensing element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable safe sensing of the presence of a hydrogen gas. <P>SOLUTION: A hydrogen gas sensing element 101 is configured so that an interdigital electrode 112, which is a part of a circuit element of an equivalent circuit having a predetermined impedance, is coated by tungsten oxide that supports a noble metal catalyst having hydrogen dissociation power, and the impedance in the equivalent circuit changes with changes in the conductivity of the tungsten oxide which is changed by the dissociation and absorption of hydrogen atoms. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a hydrogen gas detection element, and more particularly to a hydrogen gas detection element that can detect hydrogen at room temperature by including a metal oxide supporting a noble metal catalyst having hydrogen dissociation ability.

  In recent years, hydrogen has been in the limelight as a clean energy source that does not emit carbon dioxide and other greenhouse gases. Necessary.

  By the way, tungsten oxide is used as a hydrogen-sensitive material because tungsten bronze is formed by a reduction reaction and the electrical or optical characteristics change greatly.

As for tungsten oxide, for example, as shown in the formula (1), tungsten bronze is generated by implantation of protons and alkali metal ions accompanying electrochemical reduction. In formula (1), A represents an element such as hydrogen (H), lithium (Li), or sodium (Na), and x is a numerical value in the range of 0 <x <1.
WO ₃ + xA ⁺ + xe ⁻ ⇔ AxWO ₃ (1)

Tungsten oxide exhibits a light yellow-green color that is almost transparent, but tungsten bronzes exhibit a deep blue color. Absorption of light of tungsten oxide is performed by valence transfer by electrons that transition between pentavalent tungsten and hexavalent tungsten, as shown in Formula (2). That is, it is considered that W ⁶⁺ receives electrons to become W ⁵⁺ by receiving light, and the electrons localized there contribute to light absorption as a color sensor. In the formula (2), A and B indicate the positions of electrons.
W ⁵⁺ (A) + W ⁶⁺ (B) → W ⁶⁺ (A) + W ⁵⁺ (B) (2)

NaxWO ₃ is known to have a different color depending on the value of x, and is golden when x = 0.9, orange-red when x = 0.6, and dark blue when x = 0.3.

  For example, the reduction reaction of tungsten oxide is performed in this way, and these reactions are such that tungsten oxide is reduced at room temperature with the aid of an electrochemical reaction or a photochemical reaction.

  That is, when tungsten oxide is used as a hydrogen-sensitive material based on these reduction reactions, for example, an electrical or optical configuration is separately required. Hydrogen gas that requires a device for applying a voltage between the electrodes is disclosed in Patent Document 1.

  Thus, for example, it is conceivable to use tungsten oxide as a hydrogen-sensitive material based on a reduction reaction based on a chemical reduction power of hydrogen that does not require an electrical or optical configuration.

JP 2002-328108 A

  However, the reduction reaction of tungsten oxide by the chemical reducing power of hydrogen is usually confirmed only at high temperatures.

  That is, when tungsten oxide is used as a hydrogen-sensitive material based on a reduction reaction due to the chemical reducing power of hydrogen, it is necessary to heat the element, for example, it becomes a source of ignition of hydrogen gas, which is a flammable gas. there were.

  The present invention has been devised under such a background, and enables the reduction reaction of tungsten oxide by the chemical reduction power of hydrogen at room temperature, and it is possible to reduce the hydrogen content with a safe and simple configuration. The purpose is to be able to detect the presence.

  One aspect of the present invention is a hydrogen gas detection element comprising a circuit element having predetermined electrical characteristics and a metal oxide supporting a noble metal catalyst having hydrogen dissociation ability, wherein at least a part of the circuit element comprises: Coated with the metal oxide, the conductivity of the metal oxide is changed by dissociative adsorption of hydrogen atoms to the metal oxide, and the electrical characteristics of the circuit element are the conductivity of the metal oxide. It is a hydrogen gas detection element characterized by changing in accordance with the change of.

  The metal oxide is tungsten oxide supporting a noble metal catalyst having hydrogen dissociation ability, and tungsten bronze is generated by the reducing force of hydrogen atoms dissociated and adsorbed on the tungsten oxide, whereby the conductivity of the tungsten oxide is increased. Sex can also be changed.

  The circuit element may be represented by an equivalent circuit having a predetermined impedance, and the impedance or phase may be changed according to a change in conductivity of the metal oxide.

  The equivalent circuit may have a predetermined resonance frequency, and the resonance frequency of the equivalent circuit may be changed according to a change in the impedance according to a change in conductivity of the metal oxide.

  According to the present invention, the reduction reaction of tungsten oxide by the chemical reducing power of hydrogen can be performed at room temperature, and the presence of hydrogen can be detected with a simple configuration.

  FIG. 1 is a top view of an external configuration example of a hydrogen gas detection element 1 to which the present invention is applied, and FIG. 2 is a bottom view thereof. FIG. 3 is a cross-sectional view taken along the line A-A ′ of FIG. 1.

  The substrate 11 is a member obtained by forming a dielectric material such as polyethylene, polypropylene, or polyester into a plate shape.

  On the upper surface of the substrate 11 (FIG. 1), a thin film electrode 13 is provided in a substantially rectangular region 11A. The antenna coil 12 is arranged around the thin film electrode 13 in a shape in which a rectangle is wound five times. The outermost end of the antenna coil 12 is connected to a through hole 14 penetrating in the vertical direction, and the innermost end is electrically connected to the thin film electrode 13.

  On the lower surface of the substrate 11 (FIG. 2), the thin film electrode 15 is disposed at a position facing the thin film electrode 13 on the upper surface. The lead wire 16 extends from the lower surface side of the through hole 14 toward the thin film electrode 15.

  The antenna coil 12 and the thin film electrodes 13 and 15 are formed on the substrate 11 by laminating a platinum (Pt) (about 3000 Å) film on a base of about 1000 チ タ ン titanium (Ti). The interval between the rectangles constituting the antenna coil 12 is, for example, 500 μm or less.

  Since the thin film electrodes 13 and 15 are thus arranged on both the upper and lower surfaces of the substrate 11, a capacitor is electrically formed. Therefore, the hydrogen gas detection element 1 is electrically equivalent to an LC resonance circuit.

  FIG. 4 shows an equivalent circuit of the hydrogen gas detection element 1. As shown in FIG. 4, the hydrogen gas detection element 1 is electrically expressed as an antenna coil 12 and a capacitor 31 including a substrate 11 and thin film electrodes 13 and 15 connected in parallel.

  Note that the resonance frequency of an equivalent circuit composed of a coil and a capacitor has a relationship represented by the equation (3) with the inductance of the coil and the capacitance of the capacitor.

  In Expression (3), F is the resonance frequency of the LC resonance circuit. L is the inductance of the coil. C is the capacitance of the capacitor.

  Returning to FIG. 3, the upper surface of the substrate 11 (that is, the antenna coil 12 and the thin film electrode 13) is covered with a resin material 21. The resin material 21 is mixed with a metal oxide (in this example, tungsten oxide). The tungsten oxide contains hydrogen that dissociates hydrogen molecules such as platinum (Pt) or palladium (Pd) into hydrogen atoms. A noble metal medium having dissociation ability is supported.

  The film formation of the resin material 21 is performed by, for example, sol-gel dip coating. The sol-gel method is a method in which, for example, a sol made of a metal alkoxide is converted into a gel that loses fluidity by hydrolysis and polycondensation reaction, and the gel is heated to obtain an oxide. Dip coating is a method of forming a thin film by preparing a solution containing a metal cation, immersing the substrate in it, pulling it up, and then drying and heat-treating it.

  When hydrogen gas comes into contact with tungsten oxide carrying a noble metal medium having hydrogen dissociation ability, the reaction is promoted by effectively drawing out the reducing power of hydrogen as described later. Is reduced to produce tungsten bronze. As a result, since the resin material 21 is made conductive and the antenna coil 12 is short-circuited, the inductance L of the equivalent circuit of the hydrogen gas detection element 1 is changed, thereby causing the resonance of the LC resonance circuit shown in Expression (3). The frequency itself changes.

  As described above, the hydrogen gas detection element 1 is configured. That is, in this hydrogen gas detection element 1, an antenna coil 12 which is a part of a circuit element constituting an LC resonance circuit having a predetermined resonance frequency is covered with tungsten oxide carrying a noble metal catalyst having hydrogen dissociation ability, The conductivity of tungsten oxide is changed by dissociative adsorption of hydrogen atoms, and the resonance frequency of the LC resonance circuit is changed in accordance with the change in conductivity of tungsten oxide.

  FIG. 5 is a diagram illustrating a usage example of the hydrogen gas detection element 1.

  The radio wave transmitter 51 and the radio wave receiver 52 are installed so as to face each other with a detection space to be detected interposed therebetween. It is assumed that the hydrogen gas detection element 1 is placed in the detection space.

  The radio wave transmitter 51 transmits a radio signal having a frequency corresponding to the resonance frequency of the hydrogen gas sensing element 1 toward the detection space, and the radio wave receiver 52 transmits the radio signal that has reached itself through the detection space. Measure strength.

  When hydrogen does not exist in the detection space, the LC resonance circuit of the hydrogen gas detection element 1 resonates and an alternating current is induced, so that the intensity of the radio signal reaching the radio wave receiver 52 is reduced. On the other hand, when hydrogen is present in the detection space, resonance does not occur because the resonance frequency of the LC resonance circuit of the hydrogen gas detection element 1 changes, and the intensity of the radio signal reaching the radio wave receiver 52 does not decrease. .

  That is, in the example of FIG. 5, it is possible to detect the presence or absence of hydrogen in the detection space by a change in the intensity of the radio signal reaching the radio wave receiver 52.

  In this way, the presence or absence of hydrogen can be detected using the hydrogen gas detection element 1.

  Next, the reduction reaction of tungsten oxide in the resin material 21 will be described.

  As described above, the tungsten oxide carries a noble metal medium having hydrogen dissociation ability such as platinum (Pt) or palladium (Pd).

Noble metal catalysts such as platinum (Pt) and palladium (Pd) dissociate and adsorb hydrogen molecules into hydrogen atoms. For example, when a platinum catalyst is used, dissociative adsorption of hydrogen atoms is performed as shown in Formula (4). In formula (4), Had-Pt represents a hydrogen atom dissociated and adsorbed on the platinum catalyst surface.
H ₂ + 2Pt ⇔ ₂ Had-Pt (4)

In addition, hydrogen atoms dissociated and adsorbed on the platinum catalyst (Had-Pt) are diffused on the surface by the spillover effect (so-called overflow of atomic hydrogen dissociated from molecular hydrogen) as shown in formula (5), and tungsten oxide. Also reach. In the formula (5), Had-WO ₃ represents a dissociated and adsorbed hydrogen atom that is surface diffused in tungsten oxide.
Had-Pt ⇔ Had-WO ₃ (5)

When the hydrogen atom reaches the tungsten oxide in this way, the hydrogen atom has a higher reducing ability than the molecular hydrogen. Therefore, between the dissociated and adsorbed hydrogen atom (Had-WO ₃ ) and the tungsten oxide, the formula ( The reaction shown in 6) occurs, and tungsten bronze is produced.
WO ₃ + xHad-WO ₃ ⇔ H _x WO ₃ (6)

  Thus, the tungsten oxide contained in the resin material 21 is reduced. That is, the reduction reaction of tungsten oxide is based on a chemical reducing power that does not require the help of so-called electrochemical reaction and photochemical reaction.

In other words, this reaction is also considered to be due to the fact that the adsorbed hydrogen (Had) finally becomes protons and is injected into tungsten oxide together with the released electrons in the oxidation process, as shown in the equation (7). .
Had ⇔ H ⁺ + e ⁻ (7)

Protons injected into tungsten oxide combine with the closest O ²⁻ ions in WO ₃ , and electrons injected into tungsten oxide are trapped by central W ⁶⁺ ions, and W ⁶⁺ is partially W Reduced to ⁵⁺ . As a result, tungsten bronze (H _x WO ₃ ) is formed as shown in equation (8).
WO ₃ + xH ⁺ + xe ⁻ ⇔ HxWO ₃ (0 <x <1) (8)

  This expression is the same as Expression (1) when A = H. That is, the reduction reaction of tungsten oxide by the chemical reduction force in the resin material 21 is performed in the same manner as the reduction reaction of tungsten oxide performed with the aid of the electrochemical reaction.

  As described above, by supporting a noble metal catalyst having hydrogen dissociation ability and introducing it into the reaction system, a hydrogen spillover effect occurs, and a reduction reaction of tungsten oxide with hydrogen occurs. This reduction reaction due to the spillover effect occurs even at room temperature because the reducing power of hydrogen is effectively extracted and the reaction is promoted. That is, the reduction reaction of tungsten oxide by the chemical reducing power of hydrogen in the resin material 21 can be performed at room temperature, and as a result, hydrogen can be detected at room temperature. That is, the hydrogen gas detection element 1 can be configured to be safe and simple.

  Next, the hydrogen sensitivity characteristic of the hydrogen gas detection element to which the present invention is applied will be described.

  FIG. 6 shows an experimental example for evaluating the hydrogen sensitivity of the hydrogen gas sensing element.

  In this experiment, the hydrogen gas detection element 101 is placed in the chamber 102, hydrogen gas is introduced into the chamber 102, and the electrical properties of the hydrogen gas detection element 101 before and after introduction of hydrogen gas (that is, before and after hydrogen exposure) are introduced. Characteristics are measured.

A tube 131 </ b> A for introducing hydrogen gas and a tube 131 </ b> B for exhausting the gas in the chamber 102 are drawn into the lid portion of the chamber 102. In this experiment, N ₂ : H ₂ = 90%: 10% hydrogen and air gas are introduced into the chamber 102 from the tube 131A.

  Leads 121 </ b> A and 121 </ b> B are electrically connected to the hydrogen gas detection element 101. The leads 121A and 121B are pulled out from the lid portion of the chamber 102 and connected to an LCR meter (not shown). The hydrogen gas detection element 101 has a predetermined frequency (100 Hz, 200 Hz, 500 Hz, 700 Hz, 1000 Hz, 2000 Hz, 5000 Hz, 7000 Hz, 10000 Hz, 20000 Hz, 50000 Hz, 70000 Hz, 100000 Hz, 200000 Hz) via the lead 121 by an LCR meter. An AC voltage is applied, and the impedance and phase of a circuit element (described later) of the hydrogen gas detection element 101 are measured for each frequency.

  FIG. 7 shows a configuration example of the hydrogen gas detection element 101. The hydrogen gas detection element 101 includes a quartz substrate 111 (hereinafter simply referred to as a substrate) 111, two comb-shaped electrodes 112A and 112B having comb-like portions, and a coating 113.

  The two comb-shaped electrodes 112A and 112B (hereinafter simply referred to as the comb-shaped electrode 112 if they do not need to be distinguished from each other, the same applies to other ones) so that the comb-like portions are separated by a certain distance. Are combined. The comb electrode 112 is formed by sputtering gold or platinum.

The comb electrode 112 is formed with a coating 113 made of tungsten oxide (Pt / WO ₃ ) carrying a platinum catalyst by dip coding so as to cover the tooth-like portion.

  In this way, the comb-shaped electrode 112 is combined so that the teeth of the comb are spaced apart from each other, and the portions are covered with the coating 113, so that the insulating layer of the coating 113 is sandwiched between the teeth. A capacitor is formed. Therefore, the hydrogen gas detection element 101 is electrically equivalent to a circuit in which a resistor and a capacitor are connected in parallel as shown in FIG.

  When hydrogen gas comes into contact with the tungsten oxide carrying the platinum catalyst, as described above, tungsten bronze is produced from tungsten oxide at normal temperature, and the coating 113 is made conductive. As a result, the capacitance of the capacitor in which the insulating layer of the film 113 is sandwiched between the tooth-shaped portions of the comb-shaped electrode 112 changes, so that the impedance of the equivalent circuit changes.

  That is, in the hydrogen gas detection element 101, basically, like the hydrogen gas detection element 1, the comb-shaped electrode 112 which is a part of a circuit element constituting an equivalent circuit having a predetermined impedance has hydrogen dissociation ability. Covered with tungsten oxide supporting a noble metal catalyst, the conductivity of tungsten oxide changes due to dissociative adsorption of hydrogen atoms, and the impedance of the equivalent circuit changes according to the change in conductivity of tungsten oxide. It has become.

  9 to 14 show the absolute value and phase measurement value of the impedance of the hydrogen gas detection element 101 after the introduction of the hydrogen gas and after the introduction (that is, before and after the hydrogen exposure). Here, the measurement was performed in each of the case where the comb-shaped electrode 112 was formed of gold and the case where it was formed of platinum.

  9 to 11 show measurement results when the comb-shaped electrode 112 is formed of gold.

  FIG. 9 shows the measurement results of the absolute value and phase of the impedance before introducing hydrogen gas (ie, before hydrogen exposure) and the impedance after introduction of hydrogen gas (ie, after hydrogen exposure) for each frequency of the applied AC voltage. The absolute value and phase measurement results are shown respectively.

  FIG. 10 is a diagram in which the impedance measurement results shown in FIG. 9 are plotted for each frequency. In FIG. 10, a curve A represents the measurement result of the impedance before introducing hydrogen gas, and a curve B represents the measurement result of the impedance after introduction of hydrogen gas. FIG. 11 is a diagram in which the phase measurement results shown in FIG. 9 are plotted for each frequency. In FIG. 11, a curve A represents the measurement result of the phase before the introduction of hydrogen gas, and a curve B represents the measurement result of the phase after the introduction of hydrogen gas.

  12 to 14 show measurement results when the comb-shaped electrode 112 is made of platinum.

  FIG. 12 shows the measurement results of the absolute value and phase of the impedance before hydrogen gas introduction (ie before hydrogen exposure) and the impedance after hydrogen gas introduction (ie after hydrogen exposure) for each frequency of the applied AC voltage. The absolute value and phase measurement results are shown respectively.

  FIG. 13 is a diagram in which the impedance measurement results shown in FIG. 12 are plotted for each frequency. In FIG. 13, a curve A represents the measurement result of the impedance before introducing hydrogen gas, and a curve B represents the measurement result of the impedance after introduction of hydrogen gas. FIG. 14 is a diagram in which the phase measurement results shown in FIG. 12 are plotted for each frequency. In FIG. 14, a curve A represents the measurement result of the phase before the introduction of hydrogen gas, and a curve B represents the measurement result of the phase after the introduction of hydrogen gas.

  As described above, in FIGS. 9 to 14, the curve A representing the measurement result before introducing hydrogen gas and the curve B representing the measurement result after introducing hydrogen gas are greatly separated. That is, regardless of whether the comb electrode 112 is made of gold or platinum, the impedance and phase are greatly changed by hydrogen exposure in a wide frequency range. Therefore, the electrical characteristics of the circuit element covered with tungsten oxide carrying a noble metal medium having hydrogen dissociation ability are greatly changed by hydrogen exposure.

  15 to 22 show changes in impedance and phase over time due to hydrogen exposure. In this example, the frequency is fixed at 200 kHz.

  FIG. 15 shows changes with time of impedance due to hydrogen exposure when the comb-shaped electrode 112 is made of gold. The arrows in FIG. 15 indicate the timing when hydrogen gas is introduced. The same applies to the arrows shown in FIGS. 16 to 22 described later.

  FIG. 16 is an enlarged view of the range from 0 s to 350 s in FIG.

  FIG. 17 shows the phase change with time due to hydrogen exposure when the comb-shaped electrode 112 is made of gold. FIG. 18 is an enlarged view of the range of 0s to 120s in FIG.

  FIG. 19 shows changes with time of impedance due to hydrogen exposure when the comb-shaped electrode 112 is made of platinum. FIG. 20 is an enlarged view of the range of 50s to 150s in FIG.

  FIG. 21 shows the phase change with time due to hydrogen exposure when the comb-shaped electrode 112 is made of platinum. FIG. 22 is an enlarged view of the range of 60s to 75s in FIG.

  As described above, even when the comb-shaped electrode 112 is made of gold or platinum, changes in impedance and phase are started almost simultaneously with the introduction of hydrogen gas. Therefore, the electrical characteristics of the circuit element covered with tungsten oxide carrying a noble metal medium having hydrogen dissociation ability change rapidly by hydrogen exposure.

  In comparison between the case where the comb-shaped electrode 112 is made of gold and the case where it is made of platinum, the response speed is faster when it is made of platinum, and the response speed differs depending on the underlying metal. ing.

  As described above, the electrical characteristics of the circuit element covered with the tungsten oxide supporting the noble metal medium that dissociates and adsorbs hydrogen atoms change greatly and rapidly due to hydrogen exposure. Therefore, the hydrogen gas detection element 101 (hydrogen gas detection If the element 1 is also used, hydrogen gas can be detected accurately and quickly.

  In the above description, tungsten oxide has been described as an example, but other metal oxides can also be used.

It is a top view which shows the structural example of the hydrogen gas detection element to which this invention is applied. It is a bottom view which shows the structural example of the hydrogen gas detection element to which this invention is applied. It is sectional drawing of the A-A 'line | wire of the hydrogen gas detection element shown in FIG. It is a figure which shows the equivalent circuit of the hydrogen gas detection element shown in FIG. It is a figure which shows the usage example of the hydrogen gas detection element shown in FIG. It is a figure which shows the experiment example for the hydrogen sensitive characteristic evaluation of a hydrogen gas detection element. It is a figure which shows the structural example of the hydrogen gas detection element shown in FIG. It is a figure which shows the equivalent circuit of the hydrogen gas detection element shown in FIG. It is a figure which shows the measurement result of an impedance and a phase in case the comb-shaped electrode of the hydrogen gas detection element shown in FIG. 7 is formed with gold. It is the figure which showed the measurement result of the impedance shown in FIG. 9 for every frequency. It is the figure which showed the measurement result of the phase shown in FIG. 9 for every frequency. It is a figure which shows the measurement result of an impedance and a phase in case the comb-shaped electrode of the hydrogen gas detection element shown in FIG. 7 is formed with platinum. It is the figure which showed the measurement result of the impedance shown in FIG. 12 for every frequency. It is the figure which showed the measurement result of the phase shown in FIG. 12 for every frequency. It is a figure which shows the time-dependent change of the impedance by hydrogen exposure in case the comb-shaped electrode of the hydrogen gas detection element shown in FIG. 7 is formed with gold. FIG. 16 is an enlarged view of a predetermined range in FIG. 15. It is a figure which shows the time-dependent change of the phase by hydrogen exposure in case the comb-shaped electrode of the hydrogen gas detection element shown in FIG. 7 is formed with gold. FIG. 18 is an enlarged view of a predetermined range in FIG. 17. It is a figure which shows the time-dependent change of the impedance by hydrogen exposure in case the comb-shaped electrode of the hydrogen gas detection element shown in FIG. 7 is formed with platinum. FIG. 20 is an enlarged view of a predetermined range in FIG. 19. It is a figure which shows transition of the phase by hydrogen exposure in case the comb-shaped electrode of the hydrogen gas detection element shown in FIG. 7 is formed with platinum. It is an enlarged view of the predetermined range in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Hydrogen gas detection element, 11 Substrate, 12 Antenna coil, 13 Thin film electrode, 14 Through hole, 15 Thin film electrode, 16 Lead wire, 21 Resin material, 51 Radio wave transmitter, 52 Radio wave receiver, 101 Hydrogen gas detection element, 111 Substrate, 112 comb electrode, 113 coating, 121 lead

Claims (4)

A circuit element having predetermined electrical characteristics;
In a hydrogen gas sensing element comprising a metal oxide supporting a noble metal catalyst having hydrogen dissociation ability,
At least a portion of the circuit element is coated with the metal oxide;
The conductivity of the metal oxide is changed by dissociative adsorption of hydrogen atoms to the metal oxide,
An electrical characteristic of the circuit element changes according to a change in conductivity of the metal oxide. The metal oxide is tungsten oxide supporting a noble metal catalyst having hydrogen dissociation ability,
The hydrogen gas detection element according to claim 1, wherein the tungsten bronze is generated by the reduction force of the hydrogen atoms dissociated and adsorbed on the tungsten oxide, whereby the conductivity of the tungsten oxide changes. The circuit element is represented by an equivalent circuit having a predetermined impedance,
The hydrogen gas detection element according to claim 1, wherein the impedance or phase changes according to a change in conductivity of the metal oxide. The equivalent circuit has a predetermined resonance frequency,
The hydrogen gas detection element according to claim 3, wherein a resonance frequency of the equivalent circuit changes according to a change in the impedance according to a change in conductivity of the metal oxide.

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