JP4375336B2 - Gas sensor and gas detection method - Google Patents

Description

The present invention relates to a gas sensor and a gas detection method for detecting the amount or concentration of a combustible gas suitable for detecting a gas component contained in exhaust gas discharged from an internal combustion engine such as automobile exhaust gas.

  One of the gas sensors is a contact combustion type gas sensor. The principle of this gas sensor is to detect the temperature rise associated with the catalytic combustion reaction of combustible gas on the surface of the platinum wire as a change in the electrical resistance of the platinum wire. It is. December 20, 1991 1st edition, 1st printing, “Catalyst Utilization Technology Collection” publisher, University Books, pages 229-231, describes the element structure and detection principle of this catalytic combustion gas sensor. .

This prior art document describes that this type of gas sensor is inferior in gas selectivity. The exhaust gas discharged from an internal combustion engine such as an automobile contains a plurality of types of combustible gases such as hydrocarbons, carbon monoxide, and hydrogen. Also, hydrocarbons include a plurality of types such as CH ₄ , C ₃ H ₈ and naphtha.

An object of the present invention is to provide a catalytic combustion type gas sensor and gas capable of detecting the amount or concentration of one or more specific types of combustible gases from a gas containing a plurality of combustible gases. It is to provide a detection method.

  The present invention comprises a catalyst for catalytically combusting a combustible gas and a catalyst temperature measuring device for measuring the temperature of the catalyst, and measures the temperature change of the catalyst when the combustible gas is contact-combusted on the catalyst, thereby combustible. In a catalytic combustion gas sensor that seeks at least one of the amount and concentration of flammable gas, a non-target flammable gas removal device that removes flammable gas that is not detected is provided, and flammable gas that is not detected is removed. The gas is brought into contact with the catalyst.

  The catalyst is preferably provided with a gas flow passage so that the gas flows in one direction. If it does in this way, it can prevent that gas other than the gas which flows through a gas flow path contacts a catalyst, and can raise the detection accuracy of a sensor.

  As the non-target flammable gas removal device, that is, a device that removes flammable gas other than the measurement target, a catalyst, a selectively permeable membrane, a filter, and the like can be used. In the present invention, it is particularly recommended to use a catalyst. .

  Catalyst containing noble metal such as platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh) as an active component, or iron (Fe), cobalt (Co) and manganese (Mn) The catalyst containing at least one selected from the above as an active component has the property that a specific combustible gas can be selectively removed by catalytic combustion by changing the heating temperature of the catalyst. In addition, it is possible to selectively remove specific combustible gas by catalytic combustion by changing the catalyst component or amount. In the present invention, it is recommended to use the non-target combustible gas removing device by utilizing such properties of the catalyst. In addition, when it is desired to individually detect the amount or concentration of a plurality of combustible gases, it can be handled by providing a plurality of catalysts. However, in this case, if the catalysts are in contact with each other, the heat generated in the catalytic combustion reaction is transferred to the adjacent catalyst, which may adversely affect the measurement accuracy.

The present invention will be described with reference to a catalyst in which Pd is supported on an alumina (Al ₂ O ₃ ) support. This catalyst has the lowest ignition temperature of H ₂ , and then the ignition temperature increases in the order of CO and CH ₄ . Therefore, when it is desired to detect CH ₄ from a gas containing H ₂ , CO, and CH ₄ in coexistence, two of the present catalysts are installed. First, CO is ignited first, but CH ₄ is not ignited. The gas is brought into contact with the catalyst whose heating temperature is set to 1, and then the gas is brought into contact with the catalyst whose heating temperature is set to a temperature at which CH ₄ is ignited. As a result, H ₂ and CO are burned and removed by the front catalyst, and only the gas containing CH ₄ can be circulated to the rear catalyst. Since the catalyst in the previous stage is heated to a temperature at which CO is ignited, not only CO but also H ₂ ignited at a lower temperature than CO is combusted and removed. If a catalyst temperature measuring device is installed in the catalyst that performs the combustion removal of CH ₄ , and the temperature change when CH ₄ is burned and removed, the amount or concentration of CH ₄ can be determined therefrom.

When it is desired to measure the amount or concentration of each combustible gas of H ₂ , CO, and CH ₄ , three catalysts are provided, and at a temperature at which H ₂ is first ignited but CO and CH ₄ are not ignited. Gas is circulated through the set catalyst to burn only CO. Next, CO is ignited, but gas is circulated through a catalyst set to a temperature set so that CH ₄ does not ignite, and CO is combusted. Finally, gas is circulated through the catalyst set to a temperature at which CH ₄ is ignited to burn CH ₄ . If a catalyst temperature measuring device is installed in each catalyst and the temperature change of each catalyst is measured, the amount or concentration of each combustible gas of H ₂ , CO, and CH ₄ can be determined. In order to heat the catalyst to a predetermined temperature, each catalyst is preferably provided with a heater.

  The gas sensor of the present invention can be used as a gas detection element by forming a catalyst layer on a resistance temperature detector made of a platinum wire or an iron-palladium alloy wire by coating or the like. When combustible gas burns on the catalyst, the temperature of the platinum wire or iron-palladium alloy wire rises due to the heat of combustion. Since the platinum wire or the iron-palladium alloy wire has different electric resistance depending on its temperature, the concentration or amount of the combustible gas can be obtained by measuring the change in the electric resistance during the combustion of the combustible gas. In this case, if a compensation circuit having the same structure as that of the gas sensing element but having no catalyst layer is used to form a bridge circuit having two sides of the gas sensing element and the compensation element, the electric resistance of both elements The gas concentration or amount can be measured more precisely. When using an iron-palladium alloy wire as a resistance temperature detector, the temperature coefficient is larger and the specific resistance is larger than that of platinum, so the applied voltage can be increased, the sensor output is increased, and the sensor sensitivity is increased. There is an effect that can be increased.

As means for measuring the temperature change of the catalyst, a thermoelectric conversion element can be used in addition to the resistance temperature detector. When the combustible gas burns on the catalyst, combustion heat is generated. At that time, the catalyst has a temperature gradient along the gas flow path. If the thermoelectric conversion element is installed along the temperature gradient, a thermoelectromotive force is generated in the thermoelectric conversion element according to the temperature gradient. By measuring the thermoelectromotive force, the generated combustion heat can be obtained, and the amount or concentration of the combustible gas can be detected. Any thermoelectric conversion element can be used as long as it can detect a temperature gradient. For example, Bi ₂ Te ₃ , BiSb ₄ Te _7.5 , Bi ₂ Te ₂ Se, a mixture of PbSnTe and MnTe, Si ₇₈ Ge ₂₂ , TeSbGeAg, An intermetallic compound made of Cu _1.97 Ag _0.03 Se, GdSe or the like can be used.

  As described above, a catalyst containing a transition metal such as a noble metal or Mn, Fe, or Co can be used as a catalyst for catalytic combustion of a combustible gas. These catalytically active components are desirably supported on a porous carrier in order to increase the dispersibility thereof. As the porous carrier, in addition to alumina, metal oxides such as titania, silica, silica-alumina, zirconia, and magnesia, or a composite oxide of La and Al can be used.

The combustible gas detection gas sensor of the present invention can be used in combination with a nitrogen oxide (hereinafter referred to as NOx) sensor or a sulfur oxide (hereinafter referred to as SOx) sensor. In this case, not only combustible gas but also NOx or SOx can be measured simultaneously. Further, when the combustible gas sensor of the present invention is used in combination with an oxygen (O ₂ ) sensor, the HC, CO, and O ₂ concentrations in the gas can be detected. (Weight ratio A / F ratio) can be calculated.

  The NOx gas sensor of the present invention captures nitrogen oxides contained in the exhaust gas when the inflowing exhaust gas is in an oxidizing atmosphere, and combusts the nitrogen oxides captured when the inflowing exhaust gas is in a reducing atmosphere. And a catalyst for measuring the temperature of the catalyst that measures the temperature of the catalyst. If the exhaust gas in the oxidizing atmosphere is first contacted with this catalyst, then the exhaust gas in the reducing atmosphere is contacted, and the degree of increase in the temperature of the catalyst when contacting the gas in the reducing atmosphere is measured, the exhaust gas in the oxidizing atmosphere The amount or concentration of NOx contained in can be detected. This NOx gas sensor can be used as a sensing element by forming a catalyst layer on a platinum wire or an iron-palladium alloy wire, similarly to the combustible gas sensor of the present invention.

  When it is desired to detect NOx contained in a reducing atmosphere gas, before bringing the gas into contact with the NOx gas sensor of the present invention, the gas is brought into contact with the combustible gas sensor of the present invention, where the combustible gas is burned and removed. Then, it is desirable that the gas be circulated through the NOx gas sensor after the oxidation atmosphere is made, or the gas containing oxygen is added to make the gas in the oxidation atmosphere and then contacted with the NOx gas sensor. Next, when the reducing atmosphere gas is circulated through the NOx gas sensor, the temperature of the catalyst of the flammable gas sensor is lowered so that the flammable gas does not ignite, or the flammable gas sensor is bypassed and the reducing atmosphere is passed to the NOx gas sensor. It is desirable to distribute gas.

  In this specification, the gas in an oxidizing atmosphere means an oxygen-excess gas, so-called lean burn exhaust gas, and the reducing atmosphere gas means a fuel-excess gas, so-called stoichiometric or rich air-fuel ratio. It means burned exhaust gas.

  In the NOx gas sensor of the present invention, the NOx trapping catalyst preferably includes a component having a NOx trapping function and a component having a NOx reduction function. The component having the NOx trapping function is preferably selected from alkali metals such as Na, K and Li or alkaline earth metals such as Sr, Ca and Ba. These 1 type (s) or 2 or more types can be used in combination. Moreover, it is desirable that the component having the NOx reduction function is selected from noble metals composed of Rh, Pt and Pd. These noble metals can also be used alone or in combination of two or more. If Mn is further contained in addition to these components, the NOx trapping ability in an oxidizing atmosphere can be further enhanced. These components are desirably supported on a porous carrier such as alumina in order to increase the dispersibility thereof.

  The sulfur oxide (SOx) gas sensor of the present invention includes a sulfur capture catalyst that captures a sulfur content in exhaust gas when the exhaust gas is in an oxidizing atmosphere and desorbs the captured sulfur content when the exhaust gas is in a reducing atmosphere; A catalyst temperature measuring device for measuring the temperature of the sulfur capture catalyst is provided. After contacting the exhaust gas in the oxidizing atmosphere with this gas sensor, the exhaust gas in the reducing atmosphere is contacted, and the heat generated when the sulfur content captured on the sulfur capturing catalyst is desorbed from the catalyst is measured. At least one of the amount and concentration of sulfur oxide contained in the exhaust gas is detected.

  When detecting sulfur oxides contained in the exhaust gas in the reducing atmosphere, a combustible gas sensor or an oxygen-containing gas addition means is provided in the same manner as described above for the NOx gas sensor, and the gas in the reducing atmosphere is provided. May be converted into a gas in an oxidizing atmosphere.

  It is desirable that the sulfur trapping catalyst includes a component having a sulfur trapping function and a component having a function of desorbing sulfur. The component having a sulfur trapping function is preferably composed of one or more selected from alkali metals such as Na, K and Li or alkaline earth metals such as Sr, Ca and Ba. In addition, the component having a function of desorbing sulfur is preferably composed of one or more kinds of noble metals selected from Rh, Pt and Pd. These components are desirably supported on a porous carrier such as alumina in order to increase the dispersibility thereof.

When PtNa / Al ₂ O ₃ is used as the sulfur trapping catalyst, it is presumed that SOx in the oxidizing atmosphere is taken into the catalyst by the following reaction formula.

SO ₂ + 1 / 2O ₂ + Na ₂ CO ₃ → Na ₂ SO ₄ + CO ₂

  Next, when a gas in a reducing atmosphere is circulated, sulfur is desorbed from the catalyst according to the following reaction formula.

Na ₂ SO ₄ + CO, HC → Na ₂ CO ₃ , SO ₂ , CO ₂ , H ₂ O, N ₂

Sulfur is mainly believed to be eliminated as SO _2. When the sulfur component is desorbed from the catalyst, heat is generated, and if the amount of sulfur trapped in the catalyst is large, the heat generation amount is increased accordingly. Therefore, by measuring the temperature rise of the catalyst during the reduction reaction, the amount of trapped sulfur can be known, and the amount or concentration of SOx in the exhaust gas can be detected.

  In the sulfur capture catalyst of the present invention, the same components as the catalyst components used for the NOx capture catalyst of the NOx gas sensor can be used. Therefore, if NOx and SOx coexist in the gas, both NOx and SOx may be taken in even with the SOx trapping catalyst. This is also true for the NOx trapping catalyst. For this reason, it may not be possible to determine whether the temperature increase of the catalyst during the reduction reaction is caused by NOx or SOx.

  In such a case, it is desirable that both the NOx gas sensor and the SOx gas sensor are installed, and the set temperature at the time of NOx reduction removal by the NOx gas sensor and the set temperature at the time of sulfur desorption by the SOx gas sensor are different. The sulfur desorption temperature or the NOx reduction temperature varies depending on the components and composition ratio of the catalyst used, but the SOx reduction temperature is generally higher than the two. For example, the catalyst containing alkali metal or alkaline earth metal described above has a NOx reduction temperature of 100 to 400 ° C., whereas a desorption temperature of sulfur is 500 to 800 ° C. Accordingly, sulfur desorption from the catalyst hardly occurs in the catalyst temperature range used in the NOx gas sensor, and only the temperature rise caused by NOx can be measured with almost no interference by SOx. On the other hand, in the temperature range used in the SOx sensor, NOx reduction and sulfur desorption occur simultaneously. Therefore, if the NOx concentration or amount is first measured using the NOx sensor and then SOx is detected by the SOx gas sensor, the temperature rise caused by SOx can be found from the following equation, and the concentration or amount of SOx can be determined. Can be requested.

(Temperature rise caused by SOx) = (Temperature rise detected by SOx sensor)
-(Temperature rise estimated from measured NOx concentration or amount)

  In the gas sensor according to the present invention, the cross-sectional shape of the gas flow passage can take various shapes such as a square shape, a round shape, and a hexagonal shape. In the case of a quadrangular or hexagonal shape, sensors can be stacked and used, which has the advantage of leading to space saving. On the other hand, in the case of a round cross section, there is an advantage that non-uniformity of gas diffusion is eliminated.

Sectional area of the gas sensor according to the invention is suitable 0.1mm ² ~30mm ^2. When smaller than this, the pressure loss in a gas flow path will become high, and it will become difficult to diffuse gas in a distribution furnace. On the other hand, if it is larger than this range, the combustible gas hardly reacts with the catalyst, and it becomes difficult to detect the gas accurately, and the responsiveness also deteriorates.

  In the NOx gas sensor, the temperature at which the catalyst captures NOx is preferably 50 ° C. to 400 ° C., and 100 ° C. to 350 ° C. is particularly suitable. If the temperature is too low or too high, NOx is difficult to be captured, and accurate NOx detection may not be possible. The temperature at which the catalyst used for the SOx sensor captures sulfur is preferably 50 ° C to 600 ° C, particularly preferably 100 ° C to 450 ° C. If the temperature is too low or too high, the sulfur content is difficult to be captured, and accurate SOx detection may not be possible.

  SV = 1000 / h to 100,000 / h is suitable for the amount of gas flowing through the gas sensor of the present invention with respect to the catalyst in the sensor. If it is smaller than this, gas diffusion into the sensor is delayed, and gas selectivity when detecting a specific gas from a gas containing a plurality of gas species is lowered. On the other hand, if it is larger than this, the reaction on the catalyst is difficult to proceed, and it becomes difficult to accurately detect the gas concentration or amount. The gas sensor of the present invention can detect the gas concentration or amount more accurately by combining with the amount of exhaust gas obtained using, for example, an air flow sensor.

  The method for preparing the catalyst used in the gas sensor of the present invention is not particularly limited, and a physical preparation method such as an impregnation method, a kneading method, a coprecipitation method, a sol-gel method, an ion exchange method, a vapor deposition method, or a preparation using a chemical reaction. Any method can be applied. Further, various compounds such as nitric acid compounds, acetic acid compounds, complex compounds, hydroxides, carbonic acid compounds, organic compounds, metals, or metal oxides can be used as starting materials for the catalyst.

Consider a case where the gas sensor of the present invention is applied to a lean burn engine vehicle as an example. The lean burn vehicle is provided with a lean NOx catalyst so that NOx in the exhaust gas can be removed even in the lean region. The lean NOx purification catalyst captures NOx in the exhaust gas by adsorption or absorption when the air-fuel ratio of the exhaust gas flowing into the catalyst is lean, and the air-fuel ratio of the exhaust gas flowing into the catalyst is rich or stoichiometric from the lean state. When switched to the state, NOx trapped in the catalyst is reduced to N ₂ . The lean burn vehicle is set to operate at a lean air / fuel ratio of 18 or more during normal urban driving, and to operate at a stoichiometric or rich air / fuel ratio of 14.7 or less during acceleration or start-up. If sulfur in the exhaust gas adheres and accumulates on the lean NOx purification catalyst, the NOx trapping performance in the lean operation deteriorates. At this time, if the NOx gas sensor of the present invention is installed before and after the lean NOx catalyst so as to detect the concentration or amount of NOx, the amount of NOx trapped by the lean NOx purification catalyst at the time of lean can be known. It can be judged how much the purification catalyst has deteriorated. That is, by monitoring the deterioration state of the lean NOx purification catalyst with the NOx gas sensor of the present invention, the timing for switching from the lean state to the stoichiometric or rich state, the time for operating in the stoichiometric or rich state, and EGR (exhaust gas recirculation) It is possible to apply feedback to control such as the timing of performing OBD, and also to respond to OBD (on-board-diagnostic system) laws and regulations.

  In addition to this, the gas sensor of the present invention can be used, for example, for monitoring the deterioration state of a three-way catalyst. If the NOx sensor of the present invention is installed in the EGR passage, not only the NOx concentration in the EGR passage but also the exhaust gas temperature can be detected at the same time, so that the EGR control can be performed accurately.

  Note that when the NOx sensor of the present invention comes into contact with the SOx-containing gas, sulfur content adheres and the sensitivity of the sensor may be lowered. In such a case, if the catalyst in the NOx sensor is heated to 600 ° C. or higher and a stoichiometric or rich gas is allowed to flow, the sulfur component adhering to the catalyst can be desorbed.

  In addition, when the SOx sensor of the present invention is installed before and after the catalyst mounted on the automobile, the amount of sulfur captured by the catalyst during lean can be known, so the degree of poisoning due to the sulfur content of the catalyst can be determined, The timing for desorbing sulfur from the catalyst can be determined by the rich spike mode. At this time, if the sulfur content desorbed from the catalyst is measured by the rich spike mode, it is possible to determine how much the catalyst performance has been recovered.

  According to the present invention, even when a plurality of gases coexist, the concentration or amount of a specific combustible gas can be selectively detected.

1 is a perspective view of a gas sensor according to an embodiment of the present invention. It is the graph which showed the combustion rate of the combustible gas by a catalyst. It is the schematic which showed the example of a gas detection in case several gas exists. It is a perspective view at the time of using a thermoelectric conversion element in order to detect the temperature change of a catalyst. It is a comparison figure with the case where a platinum (Pt) wire is used and the case where a thermoelectric conversion element is used in order to detect the temperature change of a catalyst. It is the graph which showed the combustion rate of the combustible gas by a catalyst. It is the graph which showed the temperature change of the catalyst before and behind NOx reduction reaction. It is the schematic which shows the example which installed the NOx sensor before and behind the lean NOx purification catalyst. It is the schematic which showed the example which detects NOx in gas. It is the schematic which shows the example which installed the SOx sensor before and behind the lean NOx purification catalyst.

  Examples will be described below.

  FIG. 1 shows a gas sensor according to an embodiment of the present invention. In this gas sensor, a catalyst layer 1 is formed on a Pt line 2, and a gas flow passage 3 having a rectangular cross section is provided at a substantially central portion of the catalyst layer 2. The gas detection mechanism of this gas sensor will be described. First, a current is always passed through both ends of the Pt line 2. When the combustible gas comes into contact with the catalyst layer 1, the combustible gas is burned by the catalytic action, and accordingly, the temperature of the Pt line increases and the electric resistance of the Pt line 2 increases. Then, a potential difference E is generated at both ends of the Pt line 2. This potential difference E is proportional to the electric resistance change ΔR of the Pt line caused by the combustion of the combustible gas, and ΔR is proportional to the temperature change ΔT of the Pt line 2 due to the combustion of the combustible gas. For this reason, the following formula is obtained.

    E = k1 × ΔR = k2 × ΔT

  Further, since the temperature change ΔT of the Pt line is proportional to the calorific value Q accompanying combustion of the combustible gas, and the calorific value Q is also proportional to the concentration C of the combustible gas and the molar combustion heat ΔH, the following equation is obtained.

    E = k2 × ΔT = k3 × Q = K × C × ΔH

  In the two equations, k1, k2 and k3 are constants.

  Accordingly, if the combustible gas is determined, the sensor output voltage is proportional to the combustible gas concentration C. Therefore, if the potential difference E across the Pt line is measured, the gas concentration C can be obtained. If the gas concentration and the flow rate of the entire gas are known, the amount of combustible gas can be calculated. In addition to this method, if the relationship between the change in the electrical resistance of the Pt line and the concentration of the combustible gas is mapped, the concentration of the combustible gas can be easily obtained using the map.

  A gas sensor having the structure shown in FIG. 1 was manufactured as follows, and ignition temperatures of various combustible gases were measured. First, a method for preparing the catalyst will be described.

While stirring a mixed solution of Al nitrate and La nitrate in water, NH ₃ was added to obtain a coprecipitate of La and Al. The coprecipitate was dried at 120 ° C. and then calcined at 600 ° C. for 1 hour. Further, it was fired at 900 ° C. for 1 hour to produce a composite oxide of La and Al. The composite oxide was used as a carrier, impregnated with a dinitrodiammine Pd nitric acid solution, dried at 120 ° C., and then calcined at 600 ° C. for 1 hour. Thus, a Pd / (La—Al) oxide catalyst containing 13 g of La in terms of element and 1 g of Pd with respect to 100 g of Al ₂ O ₃ was prepared. Hereinafter, this catalyst is referred to as catalyst A. A slurry prepared by adding alumina sol to catalyst A was coated with 190 g of catalyst in terms of 1 L of honeycomb on a cordierite honeycomb having only one cell (400 cells / inc ² ) and converted to 1 L of honeycomb. C. dried at 600.degree. C. and then calcined at 600.degree. C. for 1 hour. As described above, a gas sensor having the structure shown in FIG. 1 containing Pd as a catalytic active component was obtained. This gas sensor has a square cross section, and the length of one side of the cross section is approximately 1 mm. The length in the direction in which the Pt line 2 is installed is approximately 5 mm. A heat insulating material was installed around the gas sensor to improve heat insulation from the outside air.

With respect to the gas sensor having the structure shown in FIG. 1, heat was applied from the outside using a heater, the combustible gases shown in Table 1 were individually circulated, and the ignition temperature was evaluated. The concentration of flammable gas is 3% in CH ₄ (remaining is air) in the case of CH ₄ , and the concentration of other flammable gases is equal to the case where CH ₄ is 3% in terms of calorific value. It was adjusted. Table 1 shows the ignition temperatures of various combustible gases.

It can be seen that the ignition temperature varies depending on the type of combustible gas. Therefore, for example, when measuring the concentration of CH ₄ with respect to a gas containing H ₂ , CO, and CH ₄ , the same type of catalyst as that used in the gas sensor of this embodiment is used upstream of the gas sensor. If the temperature of the catalyst is set to 300 ° C. and gas is circulated, H ₂ and CO are burned and removed, and only CH ₄ flows into the gas sensor of the present invention, and CH ₄ is selectively used. Can be detected.

A sensor having a capacity of 6 c.c. was manufactured by stacking the sensors shown in FIG. However, the number of Pt lines installed in the sensor was only one. Through this gas sensor, H ₂ , CO, and CH ₄ gas were individually circulated, and the combustion rate of the catalyst before and after the gas sensor was circulated was measured. The circulating gas amount was 3 L / min. The gas concentration was the same as in Example 1. The burning rate of the combustible gas was calculated by the following formula.

    Combustion rate (%) = ((amount of combustible gas flowing into the catalyst) − (amount of combustible gas flowing out from the catalyst)) / (amount of combustible gas flowing into the catalyst) × 100

FIG. 2 shows the purification rates of H ₂ , CO ₂ , and CH ₄ with respect to the sensor temperature. From FIG. 2, it can be seen that, for example, when the sensor temperature is 150 ° C., H ₂ is almost burned, and CO and CH ₄ are not burned. It can also be seen that when the sensor temperature is 250 ° C., CO and H ₂ burn, and CH ₄ does not burn. Therefore, with respect to a gas in which H ₂ , CO, and CH ₄ coexist (such a gas is often found in automobile exhaust gas), a plurality of gas sensors are installed as shown in FIG. If the gas is set to selectively burn, the concentration or amount of H ₂ , CO, or CH ₄ gas can be selectively detected. In the sensor of FIG. 3, if the temperature of the sensor part (a) is set to 150 ° C., only H ₂ gas is combusted, and if the heat of combustion is evaluated from the change in electric resistance of the Pt line, the concentration of H ₂ or The amount can be determined. If the temperature of the sensor part (b) is set to 250 ° C., only CO is burned, and the concentration or amount of CO can be measured. Furthermore, if the temperature of the sensor part (c) is set to 550 ° C., the concentration or amount of CH ₄ can be measured.

In this embodiment, for example, the combustion heat generated when 1 mol of H ₂ , CO, and CH ₄ is burned each is 68.3 kcal for H ₂ , 67.6 kcal for CO, and 212.9 kcal for CH ₄ (high school) 3) If the heat insulation of the gas flow passage of the sensor is improved, the sensor unit shown in FIG. When H ₂ burns in (a), the gas temperature increases, so that it is possible to save heater power and the like required for heating the catalyst of the sensor parts (b) and (c).

  An example in which the thermoelectric conversion element 4 is used instead of the Pt line as an element for detecting a change in the catalyst temperature will be described. FIG. 4 is a conceptual diagram of the sensor of this embodiment. When the combustible gas is combusted by the catalyst layer 1 in the gas flow passage 3, the combustible gas is more easily combusted in the front part of the sensor, and therefore the combustion heat accompanying the gas combustion is larger in the front part of the sensor. For this reason, the gradient of the catalyst temperature T is made along the gas flow path. This is schematically shown in (I) of FIG. (I) of FIG. 5 shows how the difference ΔT between the temperature of the catalyst after flowing the rich gas and the temperature of the catalyst before flowing the rich gas changes depending on the distance from the sensor entrance. In that case, as shown in FIG. 5 (II), in the method of measuring the electrical resistance R of the Pt line, the electrical resistance R of the Pt line also has a gradient. (II) of FIG. 5 shows a state in which the difference ΔVr between the electric resistance of the Pt line after the rich gas flow and the electric resistance of the Pt line before the rich gas flow changes depending on the distance from the sensor entrance. Therefore, for example, in the method of measuring the electrical resistance change at points a and b, and points a and c representing the distance from the catalyst entrance, there is a difference between the two results. On the other hand, when the thermoelectric conversion element is previously installed along the temperature gradient as shown in FIG. 5 (III), a thermoelectromotive force is generated in the thermoelectric conversion element according to the temperature gradient. The difference ΔVs between the thermoelectromotive force after the rich gas circulation and the thermoelectromotive force before the rich gas circulation is small depending on the measurement point, and therefore there is little detection error of the gas concentration or amount due to the temperature gradient in the sensor.

  Catalysts B to F were prepared in the same manner as in catalyst A except that Pt and Ru, Mn, Co, and Fe were supported instead of Pd. FIG. 6 shows the results of measurement similar to Example 2 for Catalyst B. In addition, regarding the catalysts C to F, Table 2 shows temperatures at which the purification rate is 50% for each gas. The reason why the temperature at which the purification rate is 50% is that the purification rate of 50% is used as a reference for judging the catalyst performance. From FIG. 6 and Table 2, it can be seen that the combustion temperature varies depending on the catalyst components and gas types. Therefore, regarding the catalyst installed in the sensor, gas selectivity at the time of gas detection is enhanced by combining not only catalyst A but also catalysts B to F to optimize the heating temperature and the like.

A slurry made of alumina powder and an alumina precursor and prepared to be acidic with nitric acid is coated on a cordierite honeycomb (400 cells / inc ² ) attached with Pt wire, and then the alumina-coated honeycomb is mixed with nitric acid as a first impregnation component. After impregnating with the Ce solution, it was dried at 120 ° C. and then calcined at 600 ° C. for 1 hour. Next, as the second impregnation component, the Ce-supported honeycomb is impregnated with a dinitrodiammine Pt nitric acid solution, a dinitrodiammine Pd nitric acid solution, a nitric acid Rh solution, a mixed solution of Mn nitrate and acetic acid K, dried at 200 ° C., and subsequently 600 ° C. For 1 hour. Next, as a third impregnation component, a mixed solution of an acetic acid K solution, a sodium nitrate solution, a lithium nitrate solution, and a Ti sol was impregnated, dried at 200 ° C., and then fired at 600 ° C. for 1 hour. K contained in the second impregnation liquid and the third impregnation liquid was the same addition amount.

  As described above, 190 g of alumina per 1 L of honeycomb, and Ce27 g, Na12.4 g, K15.6 g, Li1.6 g, Ti4.3 g, Mn13.7 g, Rh0.139 g, Pt2.792 g, and Pd1.35 g in terms of elements. The contained catalyst G was obtained.

Using this catalyst, a NOx gas sensor having the structure shown in FIG. 1 was prepared, and the heat generated during NOx reduction was measured under the following conditions. First, the sensor shown in FIG. 1 was laminated to have a capacity of 6 c.c. was fixed in a quartz glass reaction tube. This reaction tube was introduced into an electric furnace, and the heating was controlled so that the gas temperature introduced into the sensor was 150 ° C. The gas introduced into the reaction tube is model gas (hereinafter referred to as stoichiometric model gas) that assumes exhaust gas when the automobile engine is operated at the stoichiometric air-fuel ratio, and when the automobile engine is performing lean burn operation. Model gas assuming exhaust gas (hereinafter referred to as lean model gas) was introduced by switching every 3 minutes. The composition of the stoichiometric model gas is NOx: 1000 ppm, C ₃ H ₈ : 1800 ppm, CO: 0.6%, CO ₂ : 10%, O ₂ : 0.5%, H ₂ : 0.3%, H ₂ O : 4%, N ₂ : The balance. The composition of the lean model gas is NOx: 160 ppm to 500 ppm, C ₃ H ₈ : 300 ppm, CO: 0.1%, CO ₂ : 4%, O ₂ : 11%, H ₂ O: 4%, N ₂ : balance It was. Experiments were conducted for three cases where the NOx concentration of the lean model gas was 160 ppm, 300 ppm, and 500 ppm.

  At this time, NOx trapped by the catalyst in the lean atmosphere is reduced when the stoichiometric gas is circulated, but heat is generated at that time, and the catalyst temperature rises. The change in the catalyst temperature due to the stoichiometric flow was calculated by the following equation.

Catalyst temperature change ΔT (° C) = (Catalyst layer temperature (° C) after 3 minutes switching to stoichiometry)
-(Catalyst layer temperature (° C) just before switching to stoichiometry)

  FIG. 7 shows the result of measuring the catalyst temperature change ΔT (° C.) by changing the NOx concentration by the above test method. The higher the NOx concentration in the lean atmosphere, the larger the change in the catalyst temperature during the stoichiometric distribution. Using this result, the NOx concentration or amount in the lean atmosphere can be measured by measuring ΔT. Accordingly, the NOx concentration or amount can be detected by installing this sensor in the exhaust gas flow path of the internal combustion engine into which the exhaust gas having a lean air-fuel ratio and the exhaust gas having a rich or stoichiometric air-fuel ratio flows.

  FIG. 8 is a schematic view in which the NOx gas sensor 5 used in Example 5 is mounted on a lean burn vehicle. As shown in FIG. 8, if the NOx sensor according to the present invention is installed before and after the lean NOx purification catalyst 6 mounted on the automobile and the change of each NOx concentration or amount with time is measured, the following equation is used for leaning: The amount of NOx trapped by the NOx purification catalyst is known.

    Captured NOx amount = (total NOx amount flowing into lean NOx purification catalyst) − (total NOx amount coming out from the downstream of the lean NOx purification catalyst)

  By measuring the amount of NOx trapped in the catalyst, it is possible to detect a change with time in the NOx purification rate of the catalyst during the lean operation, and therefore it is possible to determine the timing for switching to stoichiometric or rich. Furthermore, it is possible to detect the degree of deterioration of the catalyst.

A NOx sensor formed by combining the combustible gas oxidation catalyst D and the catalyst G is shown in FIG. The composition of the gas to be measured was NOx: 1000 ppm, CO: 0.6%, O ₂ : 1.0%, H ₂ : 0.3%, N ₂ : balance. As shown in (I) of FIG. 9, when the temperature of the catalyst D is set to 500 ° C. and the temperature of the catalyst G is set to 150 ° C., the combustible gases H ₂ and CO in the measurement gas in the catalyst D are all. It burns, and only NOx, O ₂ , CO ₂ , H ₂ O, and N ₂ are contained in the gas. By passing this gas through the catalyst G, NOx can be trapped on the catalyst. Next, as shown in FIG. 9 (II), when the heating temperature of the catalyst D is set to 50 ° C. or lower and a gas including a combustible gas is allowed to flow through the catalyst G, the catalyst D is trapped on the catalyst G. NOx is reduced, and NOx in the gas can be detected by measuring the heat of reduction at that time. If a gas sensor having a configuration as shown in FIG. 9 is used, NOx in the gas can be detected without switching the gas atmosphere.

A slurry made of alumina powder and an alumina precursor and prepared to be nitric acid is coated on a cordierite honeycomb (400 cells / inc ² ) to which a Pt wire is attached, and then the alumina-coated honeycomb is impregnated with a sodium nitrate solution as an impregnation component. After being impregnated, it was dried at 120 ° C. and then calcined at 600 ° C. for 1 hour. Thus, catalyst H containing 190 g of alumina and 42 g of Na ₂ CO ₃ was obtained with respect to 1 L of honeycomb.

Using the catalyst H, an SOx gas sensor having the structure shown in FIG. A sensor having a capacity of 6 c.c. was fixed in a quartz glass reaction tube by stacking the sensors shown in FIG. This reaction tube was introduced into an electric furnace, and the heating was controlled so that the gas temperature introduced into the sensor was 300 ° C. The gas introduced into the reaction tube was an SO ₂ -containing lean model gas (composition C ₃ H ₆ : 300 ppm, CO: 0.1%, CO ₂ : 4%, O ₂ : 11%, H ₂ O: 4%, SO ₂ : 300 ppm, N ₂ : balance). This lean model gas was circulated in the sensor. At this time, Na in the catalyst H is partially converted to Na sulfate by the reaction of the following formula.

Na ₂ CO ₃ + SO ₂ + 1 / 2O ₂ → Na ₂ SO ₄ + CO ₂

This sensor is kept at 600 ° C. and model gas (composition: C ₃ H ₆ : 600 ppm, CO: 0.6%, CO ₂ : 12%, O ₂ : 0.5%, H ₂ : 0.3%, The heat generated when H ₂ O: 11%, N ₂ : balance) was circulated through the gas flow passage in the sensor for 10 min was calculated by MALT2 (thermodynamic database for personal computers: The Japan Society for Thermometry). As a result, the generated heat was 3400 kJ. The generated heat is considered to be generated when Na ₂ SO ₄ contained in the catalyst H is changed to Na ₂ CO ₃ , SO ₂ , H ₂ S. Therefore, if the generated heat is measured, the amount of S contained in the sensor, that is, the SOx concentration or amount in the lean gas when the lean gas is circulated in the sensor can be detected.

  FIG. 10 is a diagram in which the SOx gas sensor 7 is mounted on a lean burn vehicle into which exhaust gas having a lean air-fuel ratio and exhaust gas having a rich or stoichiometric air-fuel ratio flows. As shown in FIG. 10, if the SOx sensors according to the present invention are installed in front of and behind the lean NOx purification catalyst 6 mounted on the automobile, and the time change of the respective SOx concentration or amount is measured, the lean condition is calculated by the following equation. The amount of SOx trapped by the NOx purification catalyst is known.

    Captured SOx amount = (total SOx amount flowing into the lean NOx purification catalyst) − (total SOx amount coming out from the downstream of the lean NOx purification catalyst)

  By measuring the amount of SOx trapped by the lean NOx purification catalyst, it can be determined how much the lean NOx purification catalyst is poisoned by SOx.

  According to the present invention, even when a plurality of gases coexist, the concentration or amount of a specific combustible gas can be selectively detected. Furthermore, the concentration or amount of NOx and SOx can be selectively detected.

As automobile exhaust gas regulations are tightened, more accurate HC sensors, NOx sensors, and SOx sensors are desired. Further, in a fuel cell vehicle using a PEFC (polymer electrolyte fuel cell), there is a demand for a sensor that can accurately detect each gas concentration in the coexisting gas of H ₂ and CO in order to detect H ₂ leakage. The present invention meets these needs.

DESCRIPTION OF SYMBOLS 1 Catalyst layer 2 Pt line 3 Gas flow path 4 Thermoelectric conversion element 5 NOx gas sensor 6 NOx purification catalyst 7 SOx gas sensor

Claims (7)

A catalyst for catalytically combusting combustible gas and a catalyst temperature measuring device for measuring the temperature of the catalyst are provided, and the amount of combustible gas is measured by measuring the temperature change of the catalyst when the combustible gas is catalytically combusted on the catalyst. In the contact combustion type gas sensor that seeks at least one of the concentration and
A non-target flammable gas removal device that removes flammable gas outside the detection target is provided, and the gas from which the flammable gas outside the detection target is removed is brought into contact with the catalyst ,
A plurality of catalytic combustion catalysts are provided so that the combustible gas sequentially comes into contact with each other, and each of the plurality of catalysts is provided with a catalyst temperature measuring device, and each catalyst ignites the target combustible gas, but more than that. Combustible gas that ignites at high temperature is set to a temperature that does not ignite, and a combustible gas that ignites at low temperature is detected by a catalyst on the upstream side in the flow direction of the combustible gas and becomes a downstream catalyst in the flow direction of the combustible gas. In accordance with the invention, a combustible gas that is sequentially ignited at a high temperature is detected .   2. The second catalytic combustion catalyst according to claim 1, wherein a second catalytic combustion catalyst is provided in addition to the first catalyst so that a gas passing through the second catalyst comes into contact with the first catalyst. Is set lower than the temperature at which the combustible gas to be detected is ignited, and the temperature of the first catalyst is combusted by the combustible gas to be detected, but is combusted at a higher temperature than the combustible gas to be detected. Is set to a temperature that does not ignite, and after the flammable gas that is ignited at a lower temperature than the combustible gas to be detected is burned and removed by the second catalyst, the gas is brought into contact with the first catalyst. The contact combustion type gas sensor characterized by the above-mentioned. 2. The catalytic combustion type gas sensor according to claim 1, wherein a gas flow passage is formed so that the gas is directed to the catalyst in one direction . In claim 1, when detecting hydrocarbons from a combustible gas in which H ₂ , CO and hydrocarbons coexist, at least two catalytic combustion catalysts having the highest hydrocarbon ignition temperature are provided, and the most downstream catalyst is The catalyst temperature measuring device is provided, and the temperature of the catalyst is set to a temperature at which hydrocarbons are ignited, and the temperature of other catalysts is set to a temperature at which H ₂ and CO are ignited but hydrocarbons are not ignited. Contact combustion type gas sensor. In the catalytic combustion type gas detection method in which at least one of the amount and concentration of the combustible gas is obtained based on the temperature change of the catalyst when the combustible gas is catalytically combusted on the catalyst,
Combustion gas that is not subject to detection is removed in advance, and then the combustible gas that is subject to detection on the catalyst is subjected to contact combustion,
A plurality of catalytic combustion catalysts are provided so that the combustible gas sequentially comes into contact with each other, and each of the plurality of catalysts is provided with a catalyst temperature measuring device, and each catalyst ignites the target combustible gas, but more than that. Combustible gas that ignites at high temperatures is set to a temperature that does not ignite, and a combustible gas that ignites at a low temperature is detected by an upstream catalyst in the flow direction of the combustible gas and becomes a downstream catalyst in the flow direction of the combustible gas In accordance with the present invention, a combustible gas that is sequentially ignited at a high temperature is detected. In detecting the target combustible gas from the gas in which plural kinds of combustible gases coexist, the gas in which the plural kinds of combustible gases coexist is brought into contact with the first catalyst. Before making contact with the second catalytic combustion catalyst, and setting the temperature of the second catalyst to be lower than that of the combustible gas to be detected is ignited, than on the combustible gas to be detected on the second catalyst. A catalytic combustion type gas detection method characterized in that combustible gas ignited at a low temperature is burned and removed . 7. The catalytic combustion type gas detection method according to claim 6, wherein the temperature of the first catalyst is set to a temperature at which the combustible gas to be detected is ignited but the combustible gas to be ignited at a higher temperature is not ignited. .

Images