JP5240432B2 - Hydrocarbon concentration measuring sensor element and hydrocarbon concentration measuring method - Google Patents

Hydrocarbon concentration measuring sensor element and hydrocarbon concentration measuring method Download PDF

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JP5240432B2
JP5240432B2 JP2008088030A JP2008088030A JP5240432B2 JP 5240432 B2 JP5240432 B2 JP 5240432B2 JP 2008088030 A JP2008088030 A JP 2008088030A JP 2008088030 A JP2008088030 A JP 2008088030A JP 5240432 B2 JP5240432 B2 JP 5240432B2
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hydrocarbon concentration
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則雄 三浦
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NGK Insulators Ltd
Kyushu University NUC
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本発明は、被検ガス中の炭化水素以外のガスに対してほとんど応答を示さず、炭化水素にはほぼ炭素数に応じて応答する炭化水素濃度測定用センサ素子、および炭化水素濃度測定方法に関する。本発明の炭化水素濃度測定用センサ素子、および炭化水素濃度測定方法は、例えば内燃機関の排ガス中の炭化水素濃度を測定するために用いられ、炭素数が２以上の炭化水素ガス濃度を測定する炭化水素濃度測定用センサ素子、および炭化水素濃度測定方法として使用される。 The present invention relates to a hydrocarbon concentration measuring sensor element that hardly responds to gases other than hydrocarbons in a test gas and responds to hydrocarbons almost according to the number of carbon atoms, and a hydrocarbon concentration measuring method. . Hydrocarbon concentration measuring sensor element, and the hydrocarbon concentration measuring method of the present invention is used, for example, to measure the concentration of hydrocarbon in the exhaust gas of an internal combustion engine, carbon number is to measure the two or more hydrocarbon gas concentration It is used as a hydrocarbon concentration measuring sensor element and a hydrocarbon concentration measuring method.

近年、自動車等の燃焼装置から排出される有害ガスは、燃料の改良や触媒等の後処理装置の開発により、その濃度は大幅に低減しているが、未だ人体や環境に大きな影響を及ぼしている。その中でも自動車排ガス中や産業プロセス中から発生する炭化水素は、光化学スモッグやグリーンハウス効果の原因となる典型的な大気汚染物質とされている。この炭化水素を系外に排出するのを最小限に制御するために小型で安価な高性能炭化水素センサの開発が必須とされている。また、自動車排ガス制御・監視用に使用される、より高性能な車載式故障診断システム（ＯＢＤシステム）の開発が進められている。   In recent years, the concentration of harmful gases emitted from combustion devices such as automobiles has been greatly reduced due to improvements in fuel and the development of post-treatment devices such as catalysts, but they still have a major impact on the human body and the environment. Yes. Among them, hydrocarbons generated from automobile exhaust gas and industrial processes are regarded as typical air pollutants that cause photochemical smog and the greenhouse effect. In order to control the discharge of this hydrocarbon out of the system to the minimum, development of a small and inexpensive high-performance hydrocarbon sensor is essential. In addition, development of a higher-performance in-vehicle fault diagnosis system (OBD system) used for automobile exhaust gas control / monitoring is underway.

このような炭化水素センサの候補として、固体電解質を主構成材料とした電気化学式センサが広く研究されており、これまでに半導体型センサや電流検出型センサ、複素インピーダンス応答型センサ、混成電位型センサがいくつか報告されている。その中の多くは、炭化水素の中でもプロピレンやメタンなどを単独で検知することを報告している。   As a candidate for such a hydrocarbon sensor, an electrochemical sensor using a solid electrolyte as a main constituent material has been widely studied, and so far a semiconductor type sensor, a current detection type sensor, a complex impedance response type sensor, a hybrid potential type sensor. Some have been reported. Many of them report that propylene and methane are detected alone among hydrocarbons.

例えば、金属酸化物半導体に対する被測定ガス中の可燃ガス成分の吸着に基づく電気抵抗の変化を検出して、この電気抵抗変化により、可燃ガス成分の濃度を測定する金属酸化物半導体型ガスセンサも、各種検討されているが、このタイプのガスセンサにあっても、その測定値が、酸素や湿度により影響を受けたり、炭化水素以外の可燃ガス成分の影響を受け、炭化水素のみを選択的に検出し得るものではなかった。   For example, a metal oxide semiconductor type gas sensor that detects a change in electric resistance based on adsorption of a combustible gas component in a gas to be measured with respect to a metal oxide semiconductor and measures the concentration of the combustible gas component by this electric resistance change, Various studies have been conducted, but even with this type of gas sensor, the measured value is affected by oxygen and humidity, or by a combustible gas component other than hydrocarbon, and only hydrocarbons are selectively detected. It was not possible.

通常、自動車排ガス中や産業プロセス中から発生する有害ガスは、ある炭化水素のみで構成されていることは少なく、様々な炭素数をもった炭化水素で構成されていることが多い。このため、複数の炭化水素が共存するガスの濃度を検知することが必要となる。検知極にある種の酸化物を用いた素子が炭化水素以外のガスに対してほとんど応答を示さず、炭化水素にはほぼ炭素数に応じて応答する炭化水素濃度測定用センサ素子が必要とされていた。   Usually, harmful gases generated from automobile exhaust gas and industrial processes are rarely composed of only certain hydrocarbons, and are often composed of hydrocarbons having various carbon numbers. For this reason, it is necessary to detect the concentration of a gas in which a plurality of hydrocarbons coexist. An element using a certain oxide at the sensing electrode hardly responds to gases other than hydrocarbons, and hydrocarbons require a sensor element for measuring hydrocarbon concentration that responds almost in accordance with the number of carbon atoms. It was.

また、米国特許第４１５８１６６号明細書には、酸素イオン伝導性固体電解質と一対の電極とを含んで構成される電気化学的酸素ポンプセルを用いて、被測定ガス中の可燃ガス成分を燃焼せしめ、その際の該ポンプセルに流れる電流を検出して、可燃ガス成分濃度を測定するセンサが明らかにされている。   In addition, in U.S. Pat.No. 4,158,166, using an electrochemical oxygen pump cell including an oxygen ion conductive solid electrolyte and a pair of electrodes, a combustible gas component in a measurement gas is burned, A sensor that detects the current flowing through the pump cell at that time and measures the combustible gas component concentration has been clarified.

米国特許第４１５８１６６号U.S. Pat.No. 4,158,166

しかし、特許文献１に記載の可燃ガス成分測定センサは、酸素の存在に大きく影響されるものであって、そこで対象とされる可燃性ガス雰囲気が、可燃物又は燃料成分が酸素よりも多い雰囲気を意味するものであるとされているように、酸素が可燃ガス成分と同程度の量か多く被測定ガス中に存在すると、可燃ガス成分は、ポンプセルのポンピングによる酸素供給がなくても、被測定ガス中の酸素と酸化反応を惹起し、このため、ポンプ電流値が大きく変化し、可燃ガス成分の正確な測定が困難であった。   However, the combustible gas component measuring sensor described in Patent Document 1 is greatly influenced by the presence of oxygen, and the target combustible gas atmosphere is an atmosphere in which combustible substances or fuel components are larger than oxygen. If oxygen is present in the gas to be measured in an amount equal to or larger than that of the combustible gas component, the combustible gas component is not subjected to oxygen supply by pump cell pumping. Oxidation reaction with oxygen in the measurement gas was caused, so that the pump current value changed greatly, and accurate measurement of combustible gas components was difficult.

また、炭化水素濃度測定用センサ素子は、自動車排ガス触媒のオンボード監視システムにおいて必要とされてきている。この場合、高速応答、小型、安価であることが求められ、さらに、厳しい環境下で高温でも作動する性能が求められていた。   Further, a sensor element for measuring hydrocarbon concentration has been required in an on-board monitoring system for automobile exhaust gas catalyst. In this case, a high-speed response, a small size, and a low cost are required, and further, a performance that operates at a high temperature in a severe environment is required.

本発明は、前記課題を解決するためになされたものであり、炭素数が２以上の炭化水素ガスに対して感度が大きく、選択性が高く、小型化が可能な炭化水素濃度測定用センサ素子、および炭化水素濃度測定方法を提供することを目的とする。   The present invention has been made to solve the above-mentioned problems, and has a high sensitivity to a hydrocarbon gas having 2 or more carbon atoms, a high selectivity, and a sensor element for measuring a hydrocarbon concentration that can be miniaturized. And a method for measuring hydrocarbon concentration.

即ち、本発明によれば、以下の炭化水素濃度測定用センサ素子、および炭化水素濃度測定方法が提供される。   That is, according to the present invention, the following hydrocarbon concentration measuring sensor element and hydrocarbon concentration measuring method are provided.

［１］イオン伝導性固体電解質からなる固体電解質基板と、前記固体電解質基板上に設けられたＺｎＯ含有複酸化物からなる検知極と、前記固体電解質基板上に設けられたＰｔ参照極と、前記固体電解質基板の温度調節をする温度調節部と、を含み、前記検知極と前記参照極との電位差が、炭素数が２以上のＣＨ ４ を除く総炭化水素ガスの濃度に実質的に対応することを特徴とする、炭素数が２以上の炭化水素濃度測定用センサ素子。 [1] A solid electrolyte substrate made of an ion conductive solid electrolyte, a detection electrode made of a ZnO-containing double oxide provided on the solid electrolyte substrate, a Pt reference electrode provided on the solid electrolyte substrate, seen including a temperature adjusting unit for the temperature adjustment of the solid electrolyte substrate, the potential difference between the sensing electrode and the reference electrode, substantially corresponds to the concentration of total hydrocarbon gas except two or more CH 4 carbon atoms A sensor element for measuring a hydrocarbon concentration having 2 or more carbon atoms.

［２］前記ＺｎＯ含有複酸化物がＺｎＣｒ２Ｏ４である前記［１］に記載の炭化水素濃度測定用センサ素子。 [2] The sensor element for measuring a hydrocarbon concentration according to [1], wherein the ZnO-containing double oxide is ZnCr 2 O 4 .

［３］前記固体電解質が安定化剤としてイットリアを３〜１５ｍｏｌ％添加したジルコニア固体電解質である前記［１］または［２］に記載の炭化水素濃度測定用センサ素子。   [3] The sensor element for measuring a hydrocarbon concentration according to [1] or [2], wherein the solid electrolyte is a zirconia solid electrolyte to which 3 to 15 mol% of yttria is added as a stabilizer.

［４］前記［１］〜［３］のいずれかに記載の前記炭化水素濃度測定用センサ素子を用い、前記検知極及び前記Ｐｔ参照極が設けられた区画に被検ガスを導入し、前記温度調節部を用いて前記固体電解質基板の温度を５５０〜６５０℃の範囲となるように制御し、前記検知極と前記Ｐｔ参照極との電極間の起電力差を測定し、前記検知極と前記Ｐｔ参照極との電極間の起電力差と、あらかじめ蓄積された前記検知極と前記Ｐｔ参照極との電極間の起電力差から得られた検量線データと、を比較することにより前記被検ガス中の炭素数が２以上の炭化水素濃度を測定する炭化水素濃度の測定方法。 [4] Using the hydrocarbon concentration measuring sensor element according to any one of [1] to [3], introducing a test gas into a section provided with the detection electrode and the Pt reference electrode, The temperature of the solid electrolyte substrate is controlled to be in a range of 550 to 650 ° C. using a temperature adjustment unit, and an electromotive force difference between the detection electrode and the Pt reference electrode is measured. By comparing the electromotive force difference between the electrode with the Pt reference electrode and the calibration curve data obtained from the electromotive force difference between the detection electrode and the electrode with the Pt reference electrode accumulated in advance, A method for measuring a hydrocarbon concentration, wherein the concentration of a hydrocarbon having 2 or more carbon atoms in the detected gas is measured.

本発明によれば、炭化水素ガスに対して感度が大きく、選択性が高く、小型化が可能な炭化水素濃度測定用センサ素子、および炭化水素濃度測定方法を提供することを目的とする。   An object of the present invention is to provide a hydrocarbon concentration measuring sensor element and a hydrocarbon concentration measuring method that are highly sensitive to hydrocarbon gas, have high selectivity, and can be miniaturized.

以下、図面を参照しつつ本発明の実施の形態について説明する。本発明は、以下の実施形態に限定されるものではなく、発明の範囲を逸脱しない限りにおいて、変更、修正、改良を加え得るものである。   Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments, and changes, modifications, and improvements can be added without departing from the scope of the invention.

本発明の炭化水素濃度測定用センサ素子の基本的な構成の一例を図１Ａ、１Ｂおよび図２Ａ、２Ｂに示す。図２Ａは図１Ａの炭化水素濃度測定用センサ素子１１のＸ−Ｘ’断面図を示す。図２Ｂは図１Ａの炭化水素濃度測定用センサ素子１１のＹ−Ｙ’断面図を示す。本発明の炭化水素濃度測定用センサ素子１１は、図１Ａ、１Ｂおよび図２Ａ、２Ｂに示すように、イオン伝導性固体電解質からなる固体電解質基板１２と、固体電解質基板１２上に設けられたＺｎＯ含有複酸化物１３からなる検知極１８と、固体電解質基板１２上に設けられたＰｔ参照極１５（参照極１９）と、固体電解質基板の温度調節をする温度調節部４３と、を含むように構成される。   An example of the basic configuration of the sensor element for measuring the hydrocarbon concentration of the present invention is shown in FIGS. 1A and 1B and FIGS. 2A and 2B. 2A shows an X-X ′ cross-sectional view of the sensor element 11 for measuring the hydrocarbon concentration of FIG. 1A. FIG. 2B shows a Y-Y ′ cross-sectional view of the hydrocarbon concentration measuring sensor element 11 of FIG. 1A. As shown in FIGS. 1A, 1B and 2A, 2B, the sensor element 11 for measuring the hydrocarbon concentration of the present invention includes a solid electrolyte substrate 12 made of an ion conductive solid electrolyte, and a ZnO provided on the solid electrolyte substrate 12. A detection electrode 18 made of the containing complex oxide 13, a Pt reference electrode 15 (reference electrode 19) provided on the solid electrolyte substrate 12, and a temperature adjusting unit 43 for adjusting the temperature of the solid electrolyte substrate. Composed.

また、本発明の炭化水素濃度測定用センサ素子１１においては、図２Ａに示すように、検知極１８は、Ｐｔ検知極リード１７により電気的に接続したＺｎＯ含有複酸化物１３と、を含むように構成することが好ましい。 Further, in the hydrocarbon concentration measuring sensor element 11 of the present invention, as shown in FIG. 2 A, the sensing electrode 18 includes a ZnO-containing complex oxide 13 was electrically connected by Pt sensing electrode leads 17 It is preferable to configure as described above.

本発明の炭化水素濃度測定用センサ素子１１においては、検知極１８を構成するＺｎＯ含有複酸化物１３は、ＺｎＣｒ２Ｏ４であることが好ましい。 In the hydrocarbon concentration measuring sensor element 11 of the present invention, the ZnO-containing complex oxide 13 constituting the detection electrode 18 is preferably ZnCr 2 O 4 .

また、本発明の炭化水素濃度測定用センサ素子１１においては、固体電解質基板１２は安定化剤としてイットリアを３−１５ｍｏｌ％添加したジルコニアからなることが好ましい。   In the hydrocarbon concentration measuring sensor element 11 of the present invention, the solid electrolyte substrate 12 is preferably made of zirconia to which 3-15 mol% of yttria is added as a stabilizer.

また、本発明の炭化水素濃度測定用センサ素子１１においては、固体電解質基板１２の形状は問わず、その表面に検知極１８と、参照極１９とが形成され、検知極１８と、参照極１９とを被検ガス（排ガス）に曝して用いられることが好ましい。   In the hydrocarbon concentration measuring sensor element 11 of the present invention, the detection electrode 18 and the reference electrode 19 are formed on the surface of the solid electrolyte substrate 12 regardless of the shape thereof, and the detection electrode 18 and the reference electrode 19 are formed. Are preferably used by being exposed to a test gas (exhaust gas).

また、本発明の炭化水素濃度測定用センサ素子１１を用いて炭化水素濃度測定装置（図示しない）を構成することができる。この場合、被検ガスを素子部分に導入する検知区画（図示しない）と、その検知区画に設けられた上述の本発明の炭化水素濃度測定用センサ素子１１と、検知極１８とＰｔ参照極１９との電極間の起電力差と、あらかじめ蓄積された検量線データとを比較することにより被検ガス中の炭化水素濃度を測定する演算部と、を含む炭化水素濃度測定装置を構成することができる。   Further, a hydrocarbon concentration measuring device (not shown) can be configured using the hydrocarbon concentration measuring sensor element 11 of the present invention. In this case, a detection section (not shown) for introducing a test gas into the element portion, the above-described hydrocarbon concentration measuring sensor element 11 of the present invention provided in the detection section, a detection electrode 18 and a Pt reference electrode 19. And a calculation unit that measures the hydrocarbon concentration in the test gas by comparing the electromotive force difference between the electrodes and the calibration curve data accumulated in advance. it can.

上述した本発明の炭化水素濃度センサ素子１１を用いて炭化水素濃度を測定する方法においては、炭化水素濃度センサ素子１１が設けられた区画（検知区画）に被検ガスを導入し、温度調節部４３を用いて固体電解質基板１２の温度を５５０〜６５０℃の範囲となるように制御し、検知極１８とＰｔ参照極１９との電極間の起電力差を測定し、検知極１８とＰｔ参照極１９との電極間の起電力差と、あらかじめ蓄積された検知極１８とＰｔ参照極１９との電極間の起電力差から得られた検量線データとを比較することにより被検ガス中の炭化水素濃度を測定することができる。   In the method of measuring the hydrocarbon concentration using the hydrocarbon concentration sensor element 11 of the present invention described above, a test gas is introduced into a section (detection section) in which the hydrocarbon concentration sensor element 11 is provided, and the temperature control unit 43 is used to control the temperature of the solid electrolyte substrate 12 to be in the range of 550 to 650 ° C., the electromotive force difference between the detection electrode 18 and the Pt reference electrode 19 is measured, and the detection electrode 18 and the Pt reference By comparing the electromotive force difference between the electrode with the electrode 19 and the calibration curve data obtained from the electromotive force difference between the electrode between the detection electrode 18 and the Pt reference electrode 19 accumulated in advance, The hydrocarbon concentration can be measured.

本発明の炭化水素ガスセンサ素子について推定されるメカニズムを以下に記す。（１）参照極（Ｐｔ参照極）中ではＣＯ、Ｈ２、ＣＨ４以外の炭化水素ガスも酸化し、３相界面では平衡電位を示す。ＣＨ４は３相界面においても反応せず、電位に影響しない。 The mechanism presumed about the hydrocarbon gas sensor element of this invention is described below. (1) In the reference electrode (Pt reference electrode), hydrocarbon gases other than CO, H 2 , and CH 4 are also oxidized, and show an equilibrium potential at the three-phase interface. CH 4 does not react at the three-phase interface and does not affect the potential.

（２）従って、ＣＯ、Ｈ２、ＣＨ４以外の炭化水素の燃焼に対して十分高い酸素濃度ならば、参照極ではほぼ元の酸素濃度に対応する電位を示す。 (2) Therefore, if the oxygen concentration is sufficiently high for combustion of hydrocarbons other than CO, H 2 , and CH 4 , the reference electrode shows a potential corresponding to the original oxygen concentration.

（３）検知極（ＺｎＯ含有複酸化物の層）ではＣＯ、Ｈ２は酸化するが、その他の炭化水素ガスは酸化しない。その３相界面では酸素濃度に対応する電位と炭化水素の燃焼との混成電位を示す（推定）。 (3) CO and H 2 are oxidized at the detection electrode (ZnO-containing double oxide layer), but other hydrocarbon gases are not oxidized. The three-phase interface shows a hybrid potential between the potential corresponding to the oxygen concentration and the combustion of hydrocarbons (estimation).

（４）従って、ＣＯ、Ｈ２、ＣＨ４以外の炭化水素の燃焼に対して十分高い酸素濃度ならば、参照極ではほぼ元の酸素濃度に対応する電位と、ＣＨ４以外の炭化水素ガスの燃焼との混成電位を示す（推定）。 (4) Therefore, if the oxygen concentration is sufficiently high for combustion of hydrocarbons other than CO, H 2 , and CH 4 , the potential corresponding to the original oxygen concentration at the reference electrode and the hydrocarbon gas other than CH 4 Indicates the hybrid potential with combustion (estimation).

（５）検知極および参照極の電位差はＣＨ４以外の炭化水素ガスの濃度に従う。 (5) The potential difference between the detection electrode and the reference electrode follows the concentration of hydrocarbon gas other than CH 4 .

（６）後述する実験結果から、酸素濃度に依存しない。   (6) From the experimental results described later, it does not depend on the oxygen concentration.

以下、本発明を実施例に基づいてさらに詳細に説明するが、本発明はこれらの実施例に限定されるものではない。   EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to these Examples.

本発明の炭化水素濃度測定用センサ素子の基本的な構成として平板状のセンサ素子の一例を図１Ａ、１Ｂおよび図２Ａ、２Ｂに示す。図２Ａは図１の炭化水素濃度測定用センサ素子１１のＸ−Ｘ’断面図を示す。図２Ｂは図１の炭化水素濃度測定用センサ素子１１のＹ−Ｙ’断面図を示す。本発明の実施形態の炭化水素濃度測定用センサ素子１１は、安定化剤としてイットリアを８ｍｏｌ％添加したジルコニアからなり、寸法が縦５０ｍｍ×横１０ｍｍ×厚さ５ｍｍである固体電解質基板１２の上に、ＺｎＯ含有複酸化物１３で構成される検知極１８とＰｔ参照極１５（参照極１９）を設け、固体電解質基板１２内部に温度調節をするための温度調節部４３を形成した。温度調節部４３は、図１Ｂに示すように、ヒーター４６およびヒーターリード４５とで構成した。   An example of a flat sensor element is shown in FIGS. 1A and 1B and FIGS. 2A and 2B as a basic configuration of the sensor element for measuring a hydrocarbon concentration of the present invention. 2A is a cross-sectional view taken along the line X-X ′ of the sensor element 11 for measuring the hydrocarbon concentration of FIG. 2B is a Y-Y ′ cross-sectional view of the hydrocarbon concentration measuring sensor element 11 of FIG. 1. The sensor element 11 for measuring hydrocarbon concentration according to the embodiment of the present invention is made of zirconia to which 8 mol% of yttria is added as a stabilizer, and is on a solid electrolyte substrate 12 having dimensions of 50 mm in length, 10 mm in width, and 5 mm in thickness. In addition, the detection electrode 18 composed of the ZnO-containing complex oxide 13 and the Pt reference electrode 15 (reference electrode 19) were provided, and the temperature adjustment unit 43 for adjusting the temperature inside the solid electrolyte substrate 12 was formed. As shown in FIG. 1B, the temperature adjustment unit 43 includes a heater 46 and a heater lead 45.

具体的にはヒーター４６を埋め込んだ固体電解質基板１２の表面に、Ｐｔペーストを検知極リード１７、参照極リード４１及び参照極１９、として塗布し、さらに検知極１８用のＺｎＯ複酸化物粉末を塗布し、温度１１００℃大気圧下で２時間焼成して得られた。   Specifically, a Pt paste is applied as the detection electrode lead 17, the reference electrode lead 41, and the reference electrode 19 to the surface of the solid electrolyte substrate 12 in which the heater 46 is embedded, and ZnO complex oxide powder for the detection electrode 18 is further applied. It was obtained by applying and baking at a temperature of 1100 ° C. under atmospheric pressure for 2 hours.

本発明の実施の形態として上述の平板状の炭化水素濃度測定用センサ素子１１の構成の他、円筒状の固体電解質を用いて以下に示す実施例１の構成の炭化水素濃度測定用センサ素子３１について説明する。また、実施例１、実施例２の炭化水素濃度測定用センサ素子３１、および以下に示す各比較例のセンサ素子を用いて具体的な特性について実験を行った。   As an embodiment of the present invention, in addition to the configuration of the flat hydrocarbon concentration measuring sensor element 11 described above, a hydrocarbon concentration measuring sensor element 31 having the configuration of Example 1 shown below using a cylindrical solid electrolyte is used. Will be described. In addition, experiments were performed on specific characteristics using the hydrocarbon concentration measuring sensor element 31 of Example 1 and Example 2 and the sensor elements of the following comparative examples.

（実施例１）
本発明の実施例１の炭化水素濃度測定用センサ素子３１を模式的に図３および図４に示す。図４は図３の炭化水素濃度測定用センサ素子３１の模式的断面図である。本発明の実施形態の炭化水素濃度測定用センサ素子３１は、固体電解質基板３２として市販のＹＳＺ管（８ｍｏｌ％、Ｙ２Ｏ３ｄｏｐｅｄ、ＮＫＴ製、内径５ｍｍ、外径８ｍｍ、長さ３００ｍｍ）を使用し、固体電解質基板３２の外側表面にＺｎＣｒ２Ｏ４ペースト（検知極３８）とＰｔペースト（参照極３９）を帯状に塗布した。その後、管状炉を用いて、大気雰囲気下、１１００℃で２時間焼成することにより、それぞれの検知極３８（ＳＥ）、参照極３９（ＲＥ）を形成して炭化水素ガス測定用センサ素子３１とした。参照極リード４１、検知極リード４４にはＰｔ線を用いた。 Example 1
FIG. 3 and FIG. 4 schematically show a sensor element 31 for measuring the hydrocarbon concentration of Example 1 of the present invention. FIG. 4 is a schematic cross-sectional view of the hydrocarbon concentration measuring sensor element 31 of FIG. The sensor element 31 for measuring the hydrocarbon concentration of the embodiment of the present invention uses a commercially available YSZ tube (8 mol%, Y 2 O 3 doped, made by NKT, inner diameter 5 mm, outer diameter 8 mm, length 300 mm) as the solid electrolyte substrate 32. The ZnCr 2 O 4 paste (detection electrode 38) and the Pt paste (reference electrode 39) were applied in a strip shape on the outer surface of the solid electrolyte substrate 32. Thereafter, by using a tubular furnace and firing at 1100 ° C. for 2 hours in an air atmosphere, the detection electrode 38 (SE) and the reference electrode 39 (RE) are formed, and the hydrocarbon gas measuring sensor element 31 and did. Pt wires were used for the reference electrode lead 41 and the detection electrode lead 44.

（実施例２）
検知極をＺｎＭｏＯ４とした以外は実施例１と同じ構成で実施例２の炭化水素濃度測定用センサ素子を作成し、実施例１と同じ条件で測定を行った。 (Example 2)
A sensor element for measuring the hydrocarbon concentration of Example 2 was prepared with the same configuration as Example 1 except that the detection electrode was ZnMoO 4, and measurement was performed under the same conditions as in Example 1.

（比較例１）
検知極をＺｎＦｅ２Ｏ４とした以外は実施例１と同じ構成で比較例１の炭化水素濃度測定用センサ素子を作成し、実施例１と同じ条件で測定を行った。 (Comparative Example 1)
A sensor element for measuring the hydrocarbon concentration of Comparative Example 1 was prepared with the same configuration as Example 1 except that the detection electrode was ZnFe 2 O 4, and measurement was performed under the same conditions as in Example 1.

（比較例２）
検知極をＺｎＷＯ４とした以外は実施例１と同じ構成で比較例２の炭化水素濃度測定用センサ素子を作成し、実施例１と同じ条件で測定を行った。 (Comparative Example 2)
A sensor element for measuring the hydrocarbon concentration of Comparative Example 2 was prepared with the same configuration as Example 1 except that the detection electrode was ZnWO 4, and measurement was performed under the same conditions as in Example 1.

（比較例３）
検知極をＺｎＯとした以外は実施例１と同じ構成で比較例３の炭化水素濃度測定用センサ素子を作成し、実施例１と同じ条件で測定を行った。 (Comparative Example 3)
A sensor element for hydrocarbon concentration measurement of Comparative Example 3 was prepared with the same configuration as Example 1 except that the detection electrode was ZnO, and measurement was performed under the same conditions as in Example 1.

実施例１、２および比較例１〜３のセンサ素子を、センサ素子に埋め込んだヒーターに代わる外部加熱のため電気炉が備え付けられたセンサ特性評価装置（ＳＳＨ−７、桜木理化学機械株式会社）に設置し、サンプルガスを流入させたときの検知極、参照極間の電位差をエレクトロメーター（Ｒ８２４０、アドバンテスト製）で測定した。ガス流速は１００ｃｍ３／ｍｉｎとし、主に加湿合成空気雰囲気下（＋５ｖｏｌ．％Ｈ２Ｏ）で行った。 The sensor elements of Examples 1 and 2 and Comparative Examples 1 to 3 were installed in a sensor characteristic evaluation apparatus (SSH-7, Sakuragi Riken Machine Co., Ltd.) equipped with an electric furnace for external heating instead of a heater embedded in the sensor element. The potential difference between the detection electrode and the reference electrode when the sample gas was installed and the sample gas was introduced was measured with an electrometer (R8240, manufactured by Advantest). The gas flow rate was 100 cm 3 / min, and was mainly performed in a humidified synthetic air atmosphere (+5 vol.% H 2 O).

図５に、６００℃における種々のガス（１００ｐｐｍ、乾燥空気希釈）に対する感度を測定し、比較して示した。この場合、感度比は、１００ｐｐｍＣ３Ｈ６に対する起電力差を基準（１００％）として示している。図５より、ＺｎＭｏＯ４またはＺｎＣｒ２Ｏ４を検知極に用いた素子が、Ｃ３Ｈ６に対して比較的良好な選択性を示すことがわかる。ただし、ＺｎＭｏＯ４を用いた素子の９０％応答は５分以上とかなり遅く、排ガス用センサとして用いるには更なる改良の余地がある。そこで、選択性と応答速度ともに良好であった実施例１のＺｎＣｒ２Ｏ４を、Ｃ３Ｈ６に対する最適な検知極材料として選んだ。 In FIG. 5, the sensitivity to various gases (100 ppm, dry air dilution) at 600 ° C. was measured and compared. In this case, the sensitivity ratio indicates the difference in electromotive force with respect to 100 ppm C 3 H 6 as a reference (100%). FIG. 5 shows that the element using ZnMoO 4 or ZnCr 2 O 4 as the detection electrode shows a relatively good selectivity for C 3 H 6 . However, the 90% response of the element using ZnMoO 4 is considerably slow as 5 minutes or more, and there is room for further improvement for use as an exhaust gas sensor. Therefore, ZnCr 2 O 4 of Example 1, which had good selectivity and response speed, was selected as the optimum sensing electrode material for C 3 H 6 .

以下に実施例１の構成の炭化水素濃度測定用センサ素子３１について、更に詳細な検討を重ねるべく、様々な特性について実験を行った結果をグラフを交えて記述する。   In the following, the results of experiments on various characteristics of the sensor element 31 for measuring the hydrocarbon concentration having the configuration of Example 1 will be described with graphs in order to make further detailed studies.

実施例１の炭化水素濃度測定用素子３１について、Ｏ２：２１ｖｏｌ．％、Ｈ２Ｏ：５ｖｏｌ．％共存下で、作動温度：５００〜７００℃まで５０℃毎に種々の被検ガス（Ｃ３Ｈ６、ＣＨ４、ＮＯ２、ＮＨ３、ＮＯ、ＣＯ、各濃度：１００ｐｐｍ）に対する応答感度を確認し、結果を図９に示す。その結果、選択性、応答感度の両方を考慮すると、作動温度５００℃での結果が最適だが、そのときの回復速度が約６０秒と遅い。また７００℃以上では十分な感度が得られなかった。このため、５００℃よりも応答・回復速度が早く、選択性、応答感度も満足な結果が得られる作動温度５５０℃を最適温度とする。 About the hydrocarbon concentration measuring element 31 of Example 1, O 2 : 21 vol. %, H 2 O: 5 vol. In the presence of%, operating sensitivity: response sensitivity to various test gases (C 3 H 6 , CH 4, NO 2 , NH 3 , NO, CO, each concentration: 100 ppm) every 50 ° C. from 500 to 700 ° C. The results are shown in FIG. As a result, when both selectivity and response sensitivity are taken into consideration, the result at an operating temperature of 500 ° C. is optimal, but the recovery speed at that time is as slow as about 60 seconds. Further, sufficient sensitivity could not be obtained at 700 ° C. or higher. For this reason, the operating temperature of 550 ° C., at which the response / recovery speed is faster than 500 ° C., and the selectivity and response sensitivity are satisfactory, is set as the optimum temperature.

図６には、５５０℃において加湿合成空気から種々の被検ガス（１００ｐｐｍ）に切り替えた時の起電力応答曲線のグラフを示す。これより、ＮＯ２には若干の応答を示しているが、自動車排ガス中に存在する可能性がある他の種々のガス（ＣＯ、ＮＯ、ＮＨ３、ＣＨ４）にはほとんど応答せず、Ｃ３Ｈ６だけに大きく応答し、しかも９０％応答時間も１０秒程度と短いことがわかる。また、ＮＯ２に対する感度もＣ３Ｈ６の約１／６と小さいため、本素子はＣ３Ｈ６に対して高感度、選択的かつ速い応答を示すといえる。 FIG. 6 shows a graph of an electromotive force response curve when switching from humidified synthetic air to various test gases (100 ppm) at 550 ° C. This shows a slight response to NO 2 , but hardly responds to various other gases (CO, NO, NH 3 , CH 4 ) that may be present in automobile exhaust gas, and C 2 It can be seen that it responds greatly only to 3 H 6 and the 90% response time is as short as about 10 seconds. Further, since the sensitivity to NO 2 is small and approximately 1/6 of the C 3 H 6, the device can be said to show a high sensitivity, selective and fast response to C 3 H 6.

また、同作動条件下で、種々の濃度のＣ３Ｈ６に対する応答を測定したところ、図７に示すように速い応答速度を示し、図８に示すように１０〜４００ｐｐｍの範囲においてΔｅｍｆはＣ３Ｈ６濃度の対数にほぼ比例することがわかった。 Further, when the response to various concentrations of C 3 H 6 was measured under the same operating conditions, it showed a fast response speed as shown in FIG. 7, and Δemf was C in the range of 10 to 400 ppm as shown in FIG. It was found to be approximately proportional to the logarithm of 3 H 6 concentration.

図１０は、実施例１の炭化水素濃度測定用素子３１を用いて、５ｖｏｌ．％Ｈ２Ｏ共存下、作動温度：５５０℃において、酸素濃度を２〜２１ｖｏｌ．％の範囲で変化させ、その際の濃度１００ｐｐｍのＣ３Ｈ６に対する起電力差Δｅｍｆ＝［Ｃ３Ｈ６のｅｍｆ値］−［ベース（Ａｉｒ）のｅｍｆ値］を測定した結果を示すグラフである。その結果、酸素濃度によらず起電力差Δｅｍｆの値が変化しないため、広い酸素濃度範囲で使用できることがわかる。 FIG. 10 shows a 5 vol. 5 using the hydrocarbon concentration measuring element 31 of Example 1. In the presence of% H 2 O, at an operating temperature of 550 ° C., the oxygen concentration is 2 to 21 vol. FIG. 5 is a graph showing a result of measuring an electromotive force difference Δemf = [emf value of C 3 H 6 ] − [emf value of base (Air)] with respect to C 3 H 6 with a concentration of 100 ppm at the time of changing in a range of%. is there. As a result, the value of the electromotive force difference Δemf does not change regardless of the oxygen concentration, so that it can be used in a wide oxygen concentration range.

図１１は、実施例１の炭化水素濃度測定用素子３１を用いて、作動温度：５５０℃、２１ｖｏｌ．％Ｏ２での水蒸気濃度を１．３５ｖｏｌ．％〜１０．８０ｖｏｌ．％の範囲で変化させたときのベースのｅｍｆ値と１００ｐｐｍＣ３Ｈ６のｅｍｆ値の測定を行った結果を示すグラフである。どちらともにほとんど変化しなかった。この結果から排ガス中の水蒸気濃度に対する応答特性の変動がほとんどないことがわかった。 FIG. 11 shows an operation temperature: 550 ° C., 21 vol. The water vapor concentration at% O 2 is 1.35 vol. % To 10.80 vol. % Based emf value when varied in the range between 100PpmC 3 result of measurement of emf values of H 6 is a graph showing a. Neither changed very much. From this result, it was found that there was almost no variation in the response characteristics with respect to the water vapor concentration in the exhaust gas.

図１２は、実施例１の炭化水素濃度測定用素子３１を用いて、２１ｖｏｌ．％Ｏ２、５ｖｏｌ．％Ｈ２Ｏ共存下、作動温度：５５０℃において、ＣＯ２濃度を２０ｖｏｌ．％まで５ｖｏｌ．％ずつ変化させ、その際の起電力値を測定した結果を示すグラフである。その結果、Ａｉｒの起電力値と１００ｐｐｍＣ３Ｈ６の起電力値ともにほとんど変化しなかった。この結果から排ガス中の二酸化炭素濃度に対する応答特性の変動がほとんどないことがわかった。 FIG. 12 is a graph showing 21 vol. Using the hydrocarbon concentration measuring element 31 of Example 1. % O 2, 5vol. In the presence of% H 2 O, at an operating temperature of 550 ° C., the CO 2 concentration is 20 vol. % Up to 5 vol. It is a graph which shows the result of having changed by% and measuring the electromotive force value in that case. As a result, the electromotive force value of Air and the electromotive force value of 100 ppm C 3 H 6 hardly changed. From this result, it was found that there was almost no variation in response characteristics to the carbon dioxide concentration in the exhaust gas.

図１３は、実施例１の炭化水素濃度測定用素子３１を用いて、Ｃ３Ｈ６以外の炭化水素、８種類に対しても応答感度の測定を行った結果を示すグラフである。その結果、炭素数が大きくなればなるほど、応答感度が大きくなる傾向を示した。また、一般に不飽和炭化水素のほうが大きな感度が得られるといわれるが、同じ炭素数の中で飽和、不飽和炭化水素といった結合の違いにもほとんど影響がなかった。さらに、図１４示すように測定した炭素数２〜４の範囲で、炭素数と応答感度に良好な直線関係が得られた。ここで、ＣＨ４が直線に乗っていない理由として、検知極の複酸化物と固体電解質（ＹＳＺ）の界面での電気化学反応がほとんど起こっていないために応答が検出されないのではと推測している。 FIG. 13 is a graph showing the results of measuring response sensitivity for 8 types of hydrocarbons other than C 3 H 6 using the hydrocarbon concentration measuring element 31 of Example 1. As a result, the response sensitivity tended to increase as the carbon number increased. In addition, it is generally said that unsaturated hydrocarbons can provide higher sensitivity, but there was little effect on the difference in bonds such as saturated and unsaturated hydrocarbons within the same carbon number. Furthermore, in the range of 2 to 4 carbon atoms measured as shown in FIG. 14, a good linear relationship between the carbon number and the response sensitivity was obtained. Here, it is assumed that CH 4 is not on a straight line because the electrochemical reaction at the interface between the double oxide of the sensing electrode and the solid electrolyte (YSZ) hardly occurs, so that no response is detected. Yes.

図１５は、実施例１の炭化水素濃度測定用素子３１を用いて、２１ｖｏｌ．％Ｏ２、５ｖｏｌ．％Ｈ２Ｏ共存下、作動温度：５５０℃において、１−Ｃ４Ｈ８、Ｃ３Ｈ６、Ｃ２Ｈ６の３種類の炭化水素ガスを１：１：１、２：１：１、１：２：１、１：１：２の濃度比となるように調製し、混合ガスをセンサ素子に流入させた際の検知極、参照極間の電位差をエレクトロメーターにより測定した結果を示すグラフである。得られた起電力値（Δｅｍｆ）をトータル炭化水素濃度（ｐｐｍＣ）に対してプロットした結果、比較的良好な直線性が得られた。 FIG. 15 is a graph showing 21 vol. Using the hydrocarbon concentration measuring element 31 of Example 1. % O 2, 5vol. In the presence of% H 2 O, at an operating temperature of 550 ° C., three types of hydrocarbon gases of 1-C 4 H 8 , C 3 H 6 , and C 2 H 6 were mixed at 1: 1: 1, 2: 1, 1: 1, A graph showing the results of measuring the potential difference between the sensing electrode and the reference electrode with an electrometer when the mixture is prepared to have a concentration ratio of 1: 2: 1, 1: 1: 2, and the mixed gas is allowed to flow into the sensor element. It is. As a result of plotting the obtained electromotive force value (Δemf) against the total hydrocarbon concentration (ppmC), relatively good linearity was obtained.

本発明によれば、炭化水素に対して感度が大きく、選択性が高く、小型化が可能で、耐熱性及び耐久性が高い炭化水素濃度測定用センサ素子、および炭化水素濃度測定方法を提供する。   According to the present invention, there are provided a hydrocarbon concentration measuring sensor element and a hydrocarbon concentration measuring method having high sensitivity to hydrocarbons, high selectivity, miniaturization, and high heat resistance and durability. .

本発明の実施の形態の炭化水素濃度測定用センサ素子の表側の概観図である。It is a general-view figure of the front side of the sensor element for hydrocarbon concentration measurement of an embodiment of the invention. 本発明の実施の形態の炭化水素濃度測定用センサ素子の裏側の概観図である。It is a general-view figure of the back side of the sensor element for hydrocarbon concentration measurement of embodiment of this invention. 図１に示す本発明の実施の形態の炭化水素濃度測定用センサ素子の模式的なＸ−Ｘ’断面図である。FIG. 2 is a schematic X-X ′ sectional view of the hydrocarbon concentration measurement sensor element according to the embodiment of the present invention shown in FIG. 1. 図１に示す本発明の実施の形態の炭化水素濃度測定用センサ素子の模式的なＹ−Ｙ’断面図である。FIG. 2 is a schematic Y-Y ′ sectional view of the hydrocarbon concentration measurement sensor element according to the embodiment of the present invention shown in FIG. 1. 本発明の他の実施の形態の炭化水素濃度測定用センサ素子の模式的側面図である。It is a typical side view of the sensor element for hydrocarbon concentration measurement of other embodiments of the present invention. 図３に示す本発明の他の実施の形態の炭化水素濃度測定用センサ素子の模式的断面図である。It is typical sectional drawing of the sensor element for hydrocarbon concentration measurement of other embodiment of this invention shown in FIG. 各実施例および各比較例の炭化水素ガス測定用センサ素子における複数のガスに対する選択性を示すグラフである。It is a graph which shows the selectivity with respect to several gas in the sensor element for hydrocarbon gas measurement of each Example and each comparative example. 本発明の実施形態の炭化水素ガス測定用センサ素子における複数のガスに対する起電力応答曲線を示すグラフである。It is a graph which shows the electromotive force response curve with respect to several gas in the sensor element for hydrocarbon gas measurement of embodiment of this invention. 本発明の実施形態の炭化水素ガス測定用センサ素子におけるＣ３Ｈ６の濃度変化に対する応答特性を示すグラフである。It is a graph showing the response characteristics with respect to changes in the concentration of C 3 H 6 in hydrocarbon gas measurement sensor element embodiments of the present invention. 本発明の実施形態の炭化水素ガス測定用センサ素子における種々の濃度のＣ３Ｈ６に対する応答特性を示すグラフである。Is a graph showing the response characteristics for C 3 H 6 of various concentrations of hydrocarbon gas measurement sensor element embodiments of the present invention. 本発明の実施形態の炭化水素ガス測定用センサ素子における複数のガスに対する温度による応答特性を示すグラフである。It is a graph which shows the response characteristic by temperature with respect to several gas in the sensor element for hydrocarbon gas measurement of embodiment of this invention. 本発明の実施形態の炭化水素ガス測定用センサ素子におけるＣ３Ｈ６に対する酸素濃度による応答特性を示すグラフである。It is a graph showing the response characteristics of the oxygen concentration to the C 3 H 6 in hydrocarbon gas measurement sensor element embodiments of the present invention. 本発明の実施形態の炭化水素ガス測定用センサ素子におけるＣ３Ｈ６に対する水蒸気濃度による応答特性を示すグラフである。It is a graph showing the response characteristics of the water vapor concentration on C 3 H 6 in hydrocarbon gas measurement sensor element embodiments of the present invention. 本発明の実施形態の炭化水素ガス測定用センサ素子におけるＣ３Ｈ６に対する二酸化炭素濃度による応答特性を示すグラフである。It is a graph showing the response characteristics of the carbon dioxide concentration on C 3 H 6 in hydrocarbon gas measurement sensor element embodiments of the present invention. 本発明の実施形態の炭化水素ガス測定用センサ素子における種々の炭素数に対する応答特性を示すグラフである。It is a graph which shows the response characteristic with respect to various carbon number in the sensor element for hydrocarbon gas measurement of embodiment of this invention. 本発明の実施形態の炭化水素ガス測定用センサ素子における炭化水素の炭素数に対する応答特性を示すグラフである。It is a graph which shows the response characteristic with respect to carbon number of the hydrocarbon in the sensor element for hydrocarbon gas measurement of embodiment of this invention. 本発明の実施形態の炭化水素ガス測定用センサ素子における種々の炭化水素のトータル炭素水素濃度に対する応答特性を示すグラフである。It is a graph which shows the response characteristic with respect to the total carbon hydrogen concentration of various hydrocarbons in the sensor element for hydrocarbon gas measurement of embodiment of this invention.

符号の説明Explanation of symbols

１１：炭化水素濃度測定用センサ素子、１２：固定電解質基板、１３：ＺｎＯ含有複酸化物、１５：Ｐｔ参照極、１７：Ｐｔ検知極リード、１８：検知極、１９：参照極、３１：炭化水素濃度測定用センサ素子、３２：固定電解質基板、３８：検知極、３９：参照極、４１：参照極リード、４３：温度調節部、４４：検知極リード、４５：ヒーターリード線、４６：ヒーター、５１：センサ素子表面、５２：センサ素子裏面。 11: Sensor element for measuring hydrocarbon concentration, 12: Fixed electrolyte substrate, 13: ZnO-containing double oxide, 15: Pt reference electrode, 17: Pt detection electrode lead, 18: Detection electrode, 19: Reference electrode, 31: Carbonization Sensor element for hydrogen concentration measurement, 32: fixed electrolyte substrate, 38: detection electrode, 39: reference electrode, 41: reference electrode lead, 43: temperature control part, 44: detection electrode lead, 45: heater lead wire, 46: heater 51: Sensor element front surface, 52: Sensor element back surface.

Claims (4)

イオン伝導性固体電解質からなる固体電解質基板と、
前記固体電解質基板上に設けられたＺｎＯ含有複酸化物からなる検知極と、
前記固体電解質基板上に設けられたＰｔ参照極と、
前記固体電解質基板の温度調節をする温度調節部と、を含み、
前記検知極と前記参照極との電位差が、炭素数が２以上のＣＨ ４ を除く総炭化水素ガスの濃度に実質的に対応することを特徴とする、
炭素数が２以上の炭化水素濃度測定用センサ素子。 A solid electrolyte substrate made of an ion conductive solid electrolyte;
A sensing electrode comprising a ZnO-containing double oxide provided on the solid electrolyte substrate;
A Pt reference electrode provided on the solid electrolyte substrate;
Look including a temperature adjusting unit for the temperature adjustment of the solid electrolyte substrate,
The potential difference between the detection electrode and the reference electrode substantially corresponds to the concentration of total hydrocarbon gas excluding CH 4 having 2 or more carbon atoms ,
A sensor element for measuring a hydrocarbon concentration having 2 or more carbon atoms. 前記ＺｎＯ含有複酸化物がＺｎＣｒ２Ｏ４である請求項１に記載の炭化水素濃度測定用センサ素子。 The sensor element for measuring a hydrocarbon concentration according to claim 1, wherein the ZnO-containing double oxide is ZnCr 2 O 4 . 前記固体電解質が安定化剤としてイットリアを３〜１５ｍｏｌ％添加したジルコニア固体電解質である請求項１または２に記載の炭化水素濃度測定用センサ素子。   The sensor element for measuring a hydrocarbon concentration according to claim 1 or 2, wherein the solid electrolyte is a zirconia solid electrolyte to which 3 to 15 mol% of yttria is added as a stabilizer. 請求項１〜３のいずれか１項に記載の前記炭化水素濃度測定用センサ素子を用い、
前記検知極及び前記Ｐｔ参照極が設けられた区画に被検ガスを導入し、
前記温度調節部を用いて前記固体電解質基板の温度を５５０〜６５０℃の範囲となるように制御し、
前記検知極と前記Ｐｔ参照極との電極間の起電力差を測定し、
前記検知極と前記Ｐｔ参照極との電極間の起電力差と、あらかじめ蓄積された前記検知極と前記Ｐｔ参照極との電極間の起電力差から得られた検量線データと、を比較することにより前記被検ガス中の炭素数が２以上の炭化水素濃度を測定する炭化水素濃度の測定方法。 Using the hydrocarbon concentration measuring sensor element according to any one of claims 1 to 3,
A test gas is introduced into a section provided with the detection electrode and the Pt reference electrode;
Controlling the temperature of the solid electrolyte substrate to be in the range of 550 to 650 ° C. using the temperature adjusting unit;
Measure the electromotive force difference between the detection electrode and the Pt reference electrode,
The electromotive force difference between the detection electrode and the Pt reference electrode is compared with the calibration curve data obtained from the previously accumulated electromotive force difference between the detection electrode and the Pt reference electrode. Thus, a hydrocarbon concentration measuring method for measuring a hydrocarbon concentration having 2 or more carbon atoms in the test gas.
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