JP5982293B2 - Gas sensor element, gas sensor and exhaust gas purification device - Google Patents

Description

  The present invention relates to a gas sensor element, a gas sensor, and an exhaust gas purification device.

  Conventionally, in order to purify exhaust gas exhausted from an internal combustion engine such as an automobile engine, an exhaust gas purification device is provided on the downstream side of the internal combustion engine. The exhaust gas purification apparatus includes, for example, a three-way catalyst for purifying exhaust gas and a gas sensor that detects an excess air ratio (λ) in the exhaust gas based on sensor output. The exhaust gas purification device feeds back the excess air ratio (λ) of the exhaust gas after passing through the three-way catalyst to the control unit of the internal combustion engine, and controls the operating conditions of the internal combustion engine so that the three-way catalyst functions effectively The exhaust gas is purified by

  Specifically, NOx contained in the exhaust gas is generally hardly exhausted from the rich region where the excess air ratio λ is 1 or less to the stoichiometric where the excess air ratio λ is 1, and the excess air ratio λ is It is mainly discharged in lean areas greater than one. Therefore, NOx emissions are suppressed by using a gas sensor that can accurately detect stoichiometry (λ = 1) and controlling the internal combustion engine so that the detected excess air ratio λ does not enter the lean region.

  As the gas sensor, one having a gas sensor element composed of an oxide ion conductive solid electrolyte body and a Pt (platinum) electrode is known. Patent Document 1 discloses that in the above gas sensor element, only Ba (barium), Al (aluminum), Ce (cerium), Cr (chromium), Fe (iron) is provided on the surface of the Pt electrode on the measurement gas detection side. In addition, a gas sensor element is proposed in which at least one metal selected from the elements consisting of Co and cobalt (Co) is added, and Pt and the added metal are directly bonded.

Japanese Patent No. 3534149

However, the gas sensor element of Patent Document 1 has the following points to be improved.
That is, the gas sensor element combines Pt and a specific different element on the surface of the Pt electrode on the gas to be measured side, and Pt and the different element have a strong interaction. Therefore, a gas sensor using this gas sensor element can generate a sensor output equivalent to a conventional gas sensor for H ₂ . However, when Ba and Cr are combined, a low sensor output is generated with respect to CO. Also, when Ba, Cr, Fe, and Ce are combined, a low sensor output is generated for paraffinic hydrocarbons.

In the gas sensor element exhibiting such characteristics, the catalytic performance (oxidation performance) of Pt with respect to CO and paraffinic hydrocarbons is significantly reduced. Therefore, the electrode cannot consume O ₂ in the rich region. As a result, the sensor output in the rich region is greatly reduced, and the curve of the excess air ratio vs. sensor output characteristic (hereinafter, referred to as λ curve as appropriate) also exhibits a shape in which the sensor output gradually changes in the rich region. . Therefore, the gas sensor element has a narrow sensor output range that can be used for control, and it is difficult to accurately detect the rich air excess ratio useful for NOx reduction.

  The present invention has been made in view of such a background, and an object of the present invention is to provide a gas sensor element capable of accurately detecting an excess air ratio in a rich region and a gas sensor using the same. The present invention also aims to provide an exhaust gas purification device using this gas sensor.

One aspect of the present invention is an oxide ion conductive solid electrolyte body (2),
A measured gas side electrode (3) provided on one surface of the solid electrolyte body (2) for contacting the measured gas;
A reference gas side electrode (4) provided on the other surface of the solid electrolyte body (2) for contacting a reference gas, wherein the gas to be measured is lean or rich with respect to the stoichiometric air-fuel ratio; A gas sensor element (1) used in a gas sensor for determining whether or not a state is present,
On the surface (300) of the measured gas side electrode ( 3) located on the opposite side of the surface of the reference gas electrode (4) via the solid electrolyte body (2), the measured gas side electrode ( The electrode covering portion (5) for suppressing the catalytic performance of the electrode material constituting 3) partially covers the surface (300) of the measured gas side electrode (3) and the entire surface (300). Are provided intermittently,
The electrode covering portion (5) is composed of a compound containing at least one element selected from Ba, Sr, Cs, Rb, K, Na and La,
The compound is present in the gas sensor element (1), wherein the compound exists without being chemically bonded to the electrode material constituting the gas side electrode (3) to be measured (Claim 1).

  Another aspect of the present invention is a gas sensor having the gas sensor element (claim 4).

Still another aspect of the present invention provides a catalyst for purifying exhaust gas,
A gas sensor having the gas sensor element,
The total amount of noble metal per vehicle used for the catalyst is 5.0 g or less.

  In the gas sensor element, Ba (barium), Sr (strontium), Cs (cesium), Rb (rubidium), K (potassium), Na (sodium), and La (lanthanum) are formed on the surface of the gas side electrode to be measured. An electrode covering portion made of a compound containing at least one element selected from is provided. And the said compound which comprises an electrode coating | coated part exists, without chemically couple | bonding with the electrode material which comprises a to-be-measured gas side electrode. For this reason, the λ curve of the gas sensor element has a shape in which the sensor output in the rich region does not significantly decrease and the sensor output changes abruptly in the rich region. This is presumed to be due to the following principle.

That is, the compound constituting the electrode covering portion is in a state where it is in contact with the electrode material (for example, Pt (platinum)) constituting the measurement gas side electrode without being chemically bonded. increase the binding property between the electrode material and O _2, it has the effect of oxidizing the electrode material in the weak-rich region-lean region. In the oxidized electrode material, the catalyst performance is moderately suppressed as compared with the non-oxidized state. In particular, the oxidation performance (purification performance) for paraffinic hydrocarbons is moderately reduced. Therefore, in the weak-rich region (despite rich region), the remainder is O ₂ was supposed consumed originally on the measured gas side electrode, the sensor output decreases by detecting the excess O ₂ .

  On the other hand, in the rich region, the reduction reaction proceeds to return the oxidized electrode material to the original state (non-oxidized state). For this reason, the above-mentioned electrode material exhibits its inherent catalytic performance (particularly oxidation performance (purification performance) for paraffinic hydrocarbons) and exhibits normal sensor output. Accordingly, it is presumed that a gas sensor element having a λ curve having a shape in which the sensor output changes sharply in the rich region without significantly decreasing the sensor output in the rich region can be obtained.

  As is conventionally known, a gas sensor element using a measured gas side electrode made of Pt or the like has a λ curve in which the sensor output changes sharply when the excess air ratio λ is 1. On the other hand, the gas sensor element has a rich excess air ratio (hereinafter referred to as λ point as appropriate) when the sensor output changes sharply while maintaining the sensor output in the rich region almost equal to that of the conventional gas sensor element. Can be shifted to the side. Further, by selecting the element contained in the compound, the degree of suppression of the catalyst performance of the electrode material can be changed, and the shift amount of the λ point to the rich side can be controlled. Therefore, the sensor output range that can be used for the control is widened, and the excess air ratio in the rich region useful for NOx reduction can be detected with high accuracy.

  In the gas sensor element, an electrode covering portion made of the compound is uniformly and intermittently provided on the surface of the measured gas side electrode made of the electrode material over the entire surface of the measured gas side electrode. Therefore, the gas responsiveness of the gas sensor element can be sufficiently secured, and the above-described effect of shifting the λ point to the rich side can be effectively exhibited.

  That is, the compound that constitutes the electrode covering portion exhibits an effect of suppressing the catalytic performance of the electrode material at a location where it is in contact with the electrode material that constitutes the measurement gas side electrode. Therefore, the more contact points between the electrode covering portion (the compound) and the measured gas side electrode (the electrode material), and the more uniformly the electrode covering portion is present on the measured gas side electrode surface, the measured gas side The effect which suppresses the catalyst performance of the said electrode material which comprises an electrode can be exhibited. Therefore, when the electrode covering portion is unevenly distributed on the measured gas side electrode surface, the effect of suppressing the catalytic performance of the electrode material cannot be stably exhibited, and the sensor output becomes unstable. There is a fear. On the other hand, if the electrode covering portion covers the entire surface of the measured gas side electrode, the diffusion of the measured gas to the measured gas side electrode may be hindered, leading to a decrease in gas responsiveness.

  Therefore, in the gas sensor element, the electrode covering portion made of the compound is made to exist uniformly without being unevenly distributed over the entire surface of the gas side electrode to be measured made of the electrode material, and so as not to cover the entire surface. ing. This makes it possible to stably shift the λ point to the rich side while ensuring sufficient gas responsiveness without reducing the gas responsiveness of the gas sensor element.

  Thus, it is possible to provide a gas sensor element capable of accurately detecting the excess air ratio in the rich region and a gas sensor using the gas sensor element.

Further, it has been found that an exhaust gas purification apparatus using a conventional gas sensor emits NOx even from the vicinity of stoichiometric (λ = 1) when the amount of noble metal used for a catalyst for purifying exhaust gas is reduced. This phenomenon is particularly noticeable after deterioration of durability.
On the other hand, the exhaust gas purification apparatus uses a gas sensor having the gas sensor element. Therefore, it is possible to control the operation of the internal combustion engine or the like in a rich region where NOx is difficult to be discharged. Even when the total amount of precious metals per vehicle used for the catalyst is reduced to 5.0 g or less, the NOx emission amount can be suppressed. Thereby, the exhaust gas purification apparatus can achieve both a reduction in the amount of noble metal and a reduction in the NOx emission amount.

FIG. 3 is an explanatory diagram showing a gas sensor element in Example 1. Sectional explanatory drawing which shows the structure of the gas sensor element in Example 1. FIG. Sectional explanatory drawing which expands and shows a part of gas sensor element in Example 1. FIG. Sectional explanatory drawing which shows the structure of the gas sensor in Example 1. FIG. BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the exhaust gas purification apparatus in Example 1. FIG. The figure which shows the relationship between the air fuel ratio A / F and sensor output in Example 1. FIG. The figure which shows the relationship between the electrode coverage, the (lambda) point rich side shift amount, and response time in Example 1. FIG. The figure which shows the spectrum of the binding energy of the Pt4f orbit of the sample 1 and a comparative sample in Example 1. FIG. (A) The example which shows the X-ray-diffraction pattern of the sample 1 and a comparative sample in Example 1. (b) The figure which expands and shows a part of X-ray-diffraction pattern of the sample 1 and a comparative sample. Sectional explanatory drawing which shows another example of the gas sensor element in Example 2. FIG.

The gas sensor element will be described.
In the gas sensor element, the compound constituting the electrode covering portion exists without being chemically bonded to the electrode material constituting the measurement gas side electrode.
Here, the state in which the compound and the electrode material are not chemically bonded is a state in which the compound and the electrode material are simply in contact with each other, for example, used as the electrode material. It means that the chemical state of an element such as Pt is the same as when the element exists alone.

Further, on the surface of the measured gas side electrode, an electrode covering portion made of the compound is intermittently provided over the entire surface of the measured gas side electrode.
Here, the electrode covering portion being provided over the entire surface of the gas side electrode to be measured means that the electrode covering portion is present uniformly over the entire surface of the gas side electrode to be measured, that is, without being unevenly distributed. Say. Further, that the electrode covering portion is provided intermittently means that the electrode covering portion does not cover the entire surface of the measured gas side electrode but is provided with an intermittent portion at least partially.

Moreover, the said compound which comprises the said electrode coating | coated part can be set as the structure containing at least 1 or more types selected from the carbonate, oxide, and complex oxide of the said element (Claim 2).
In this case, the gas sensor element having a λ curve (the λ point is shifted to the rich side) having a shape in which the sensor output does not sharply decrease in the rich region and the sensor output sharply changes in the rich region. It becomes easy to obtain. In addition, the shift amount of the λ point to the rich side can be controlled by selecting the carbonate, oxide and composite oxide of the element contained in the compound.

Examples of the compound constituting the electrode covering portion include Ba, Sr carbonate, Cs, Rb, K, Na carbonate, oxide, composite oxide, La carbonate, oxide, and the like. . Specifically, for _{example, BaCO 3, SrCO 3, Cs} 2 CO 3, Rb 2 CO 3, K 2 CO 3, Na 2 CO 3, La 2 O 3 and the like.

The electrode covering portion is preferably made of a compound containing Ba (hereinafter referred to as a Ba compound as appropriate) or a compound containing Sr (hereinafter referred to as a Sr compound as appropriate). The Ba compound preferably contains Ba carbonate (BaCO ₃ ), and the Sr compound preferably contains Sr carbonate (SrCO ₃ ).
In this case, the shift amount of the λ point to the rich side can be controlled within a suitable range. Furthermore, since any compound (carbonate) has a thermal decomposition temperature of 1100 ° C. or higher, high heat resistance can be obtained.

Moreover, it is preferable that the electrode coverage which is a ratio of the part coat | covered with the said electrode coating | coated part among the surfaces of the said to-be-measured gas side electrode is 10 to 90%.
In this case, it is possible to sufficiently obtain the effect of shifting the λ point to the rich side while sufficiently ensuring the gas responsiveness of the gas sensor element.
Moreover, in order to acquire the above-mentioned effect more fully, it is more preferable that the electrode coverage is 30 to 80%.

When the electrode coverage is less than 10%, it is difficult to stably exhibit the effect of suppressing the catalytic performance of the electrode material constituting the measured gas side electrode by the compound constituting the electrode covering portion. Thus, the sensor output may become unstable.
On the other hand, when the electrode coverage exceeds 90%, diffusion of the gas to be measured to the gas to be measured side electrode is hindered by the electrode covering portion, which may cause a decrease in gas responsiveness. That is, there is a possibility that sufficient gas responsiveness cannot be ensured.
In addition, the said electrode coverage can be calculated | required by analyzing the surface of the to-be-measured gas side electrode using the analyzer by X-ray photoelectron spectroscopy (XPS).

  Moreover, the said compound which comprises the said electrode coating | coated part can be made into a particulate form. In this case, the electrode material constituting the measured gas side electrode and the particulate compound are easily brought into direct contact. Therefore, the effect that the λ point can be shifted to the rich side while maintaining the sensor output in the rich region substantially the same as in the past can be ensured. Furthermore, when the electrode material is particulate, the particulate electrode material and the particulate compound are easily in direct contact. Therefore, the above effect can be made more reliable.

The average particle diameter of the particulate compound is preferably 0.01 to 0.5 μm, more preferably from the viewpoint of heat resistance, gas diffusibility, contact property with the particulate electrode material, and the like. , 0.02 to 0.2 μm.
In addition, the average particle diameter in the particulate electrode material is preferably 0.01 to 2 μm, more preferably 0.02 to 1 μm, from the viewpoints of heat resistance, electrical conductivity, gas diffusibility, and the like. can do.

  The average particle diameter is an average value of the diameter or equivalent circle diameter of the compound or the electrode material measured for any 10 points by taking a cross-sectional photograph of the gas-side electrode to be measured with a scanning electron microscope. . Here, the above-mentioned “equivalent circle diameter” is a concept applied when the particle outline is not a substantially circular shape but an irregular shape, and a circle having the same area as the inner area of the particle outline. Is the diameter.

  The thickness of the electrode covering portion can be, for example, about 10 μm or less, preferably about 3 μm or less. In this case, it is possible to sufficiently obtain the effect of suppressing the catalytic performance of the electrode material constituting the measurement gas side electrode by the compound constituting the electrode covering portion.

For example, Pt, Pd, Au, alloys thereof, or the like can be used as the electrode material constituting the measurement gas side electrode.
The electrode material is preferably Pt. In this case, sufficient oxidation performance (purification performance) for paraffinic hydrocarbons can be obtained.

The shape of the solid electrolyte body may be, for example, a bottomed cylindrical shape or a plate shape. The material of the solid electrolyte body is not particularly limited as long as it has oxide ion conductivity. Examples of the material of the solid electrolyte body include ZrO ₂ ceramics. Moreover, the thickness of the said solid electrolyte body can be about 0.2-3 mm.
Examples of the gas to be measured include nitrogen oxide gas, O ₂ gas, water vapor, hydrocarbon gas, CO gas, H ₂ gas, and a combination of these gases. For example, air or oxygen gas can be used as the reference gas.

Next, the gas sensor will be described.
The gas sensor has the gas sensor element. Specifically, the gas sensor can be configured to include the gas sensor element and a heater for heating and activating the gas sensor element.

  The gas sensor is installed, for example, in an exhaust system of an internal combustion engine such as an automobile engine, and can be applied to detection of exhaust gas. Specifically, for example, a catalyst such as a three-way catalyst for purifying exhaust gas exhausted from an internal combustion engine or the like, and a gas sensor installed downstream of the catalyst and outputting a signal based on the excess air ratio of the exhaust gas It can be suitably used as a gas sensor in an exhaust gas purification apparatus equipped with

Next, the exhaust gas purification apparatus will be described.
The exhaust gas purification device includes a catalyst for purifying exhaust gas and a gas sensor having the gas sensor element. The exhaust gas purification device is provided, for example, in an exhaust system of an internal combustion engine such as an automobile engine, and purifies exhaust gas exhausted from the internal combustion engine.

Examples of the noble metal used in the catalyst include Pt, Pd, Rh and the like. These catalysts can be supported on a carrier made of, for example, a ceramic honeycomb structure.
In the exhaust gas purification apparatus, the total amount of precious metal per vehicle used for the catalyst can be preferably 5.0 g or less from the viewpoint of cost reduction and the like. Here, the total amount of the noble metal is the total mass (g) of the noble metal supported on the carrier made of a honeycomb structure or the like used per vehicle.

  The exhaust gas purification apparatus is configured to control the operating conditions of the internal combustion engine so that the sensor output value approaches the specified value when the sensor output value of the gas sensor of the apparatus exceeds or falls below a specified value. be able to. In addition, the value of the excess air ratio when outputting the specified value as an index for the control is preferably 0.9863 or more and 0.9983 or less, more preferably from the viewpoint of easily obtaining the NOx emission reduction effect. May be 0.9880 or more and 0.9966 or less, more preferably 0.9897 or more and 0.9932 or less.

Example 1
Embodiments of the gas sensor element, the gas sensor, and the exhaust gas purification device will be described with reference to the drawings.

First, the gas sensor element of this example will be described.
As shown in FIGS. 1 to 3, the gas sensor element 1 of this example is provided on one surface of an oxide ion conductive solid electrolyte body 2 and the solid electrolyte body 2 for contacting a gas to be measured. A gas side electrode 3 to be measured and a reference gas side electrode 4 provided on the other surface of the solid electrolyte body 2 for contacting a reference gas are provided.

As shown in the figure, an electrode coating made of a compound containing at least one element selected from Ba, Sr, Cs, Rb, K, Na and La is formed on the surface 300 of the measured gas side electrode 3. The part 5 is provided intermittently over the entire surface 300 of the measured gas side electrode 3. The compound constituting the electrode covering portion 5 exists without being chemically bonded to the electrode material constituting the measured gas side electrode 3.
This will be described in detail below.

As shown in FIGS. 1 to 3, the gas sensor element 1 is incorporated in a gas sensor 7 shown in FIG. 4 described later, and is used in an exhaust gas purification device 8 provided in an exhaust system of an automobile engine shown in FIG. 5 described later.
As shown in FIGS. 1 and 2, in the gas sensor element 1, one side of the solid electrolyte body 2 is closed, and is formed in a bottomed cylindrical shape that serves as a reference gas chamber 100 by introducing the atmosphere into the interior. (So-called cup type). Of the outer surface of the solid electrolyte body 2, the range of the length L from the most distal end of the solid electrolyte body 2 toward the base end side is the contact surface of the gas to be measured. The solid electrolyte body 2 of this example is made of ZrO ₂ ceramics.

As shown in the figure, the gas side electrode 3 to be measured is provided on the outer side surface of the solid electrolyte body 2, and the reference gas side electrode 4 is provided on the inner side surface facing the reference gas chamber 100. The measured gas side electrode 3 and the reference gas side electrode 4 of this example are made of Pt. In addition, an electrically conductive lead electrode 101 and terminal electrode 102 are connected to the measured gas side electrode 3 and the reference gas side electrode 4 in order to apply a voltage to the gas sensor element 1. FIG. 1 shows a lead electrode 101 having one end connected to the measured gas side electrode 3 and a terminal electrode 102 connected to the other end of the lead electrode 101 on the outer surface of the solid electrolyte body 2.
As shown in FIG. 2, the measured gas side electrode 3 is covered with a porous protective layer 6. This protective layer 6 is mainly composed of MgAl ₂ O ₄ that is a spinel oxide, and has a role of improving the trap effect of harmful components in the exhaust gas that is the gas to be measured.

As shown in FIG. 3, the electrode covering portion 5 is provided on the surface 300 of the measured gas side electrode 3. That is, the electrode covering portion 5 is provided between the measured gas side electrode 3 and the protective layer 6. FIG. 3 is an enlarged view of a portion surrounded by a dotted line in FIG.
Further, the electrode covering portion 5 is provided uniformly over the entire surface 300 of the measured gas side electrode 3, that is, without being unevenly distributed. Further, the electrode covering portion 5 is provided intermittently without covering the entire surface 300 of the measured gas side electrode 3. In this example, the plurality of electrode covering portions 5 are present in a state of being uniformly dispersed over the entire surface 300 of the measured gas side electrode 3. The electrode coverage, which is the ratio of the portion of the surface 300 of the measured gas side electrode 3 that is covered with the electrode covering portion 5, can be 10 to 90%.

  The electrode covering portion 5 is made of a compound containing at least one element selected from Ba, Sr, Cs, Rb, K, Na, and La. This compound contains at least one or more selected from carbonates, oxides and composite oxides of the above elements (Ba, Sr, Cs, Rb, K, Na and La). Moreover, the compound which comprises the electrode coating | coated part 5 exists without chemically couple | bonding with the electrode material (Pt) which comprises the to-be-measured gas side electrode 3. FIG. Specifically, the compound constituting the electrode covering portion 5 is in contact with the electrode material (Pt) constituting the measured gas side electrode 3, and the chemical state of Pt as the electrode material is Pt alone. It is the same state as it exists.

Next, the gas sensor 7 of this example will be described.
As shown in FIG. 4, the gas sensor 7 includes a housing 103 and a gas sensor element 1 inserted into the housing 103. A double measured gas side cover 104 is provided at the front end side of the housing 103 to protect the front end portion of the gas sensor element 1, and the inside thereof is a measured gas chamber 105. On the other hand, double atmosphere side covers 106 and 107 are provided on the base end side of the housing 103.

  As shown in the figure, a rod-shaped ceramic heater 108 is inserted into the reference gas chamber 100 of the gas sensor element 1. The heater 108 is in contact with the inner surface of the solid electrolyte body 2 at the tip, and is inserted and arranged in a state in which a desired clearance is secured. An elastic insulating member 112 into which lead wires 109, 110, 111 are inserted is provided on the base end side of the atmosphere side covers 106, 107. A voltage is applied to the gas sensor element 1 by the lead wires 109 and 110, and the sensor output of the gas sensor element 1 is taken out to the outside. The lead wire 111 is for energizing the heater 108 to generate heat.

  As shown in the figure, connection terminals 113 and 114 are provided on the leading ends of the lead wires 109 and 110. The connection terminals 113 and 114 ensure electrical continuity with the terminals 115 and 116 fixed to the gas sensor element 1. The terminal 115 is fixed in contact with the terminal electrode 102 in the gas sensor element 1. Further, the terminal 116 is fixed in contact with a terminal electrode (the inner surface of the solid electrolyte body, not shown) in the gas sensor element 1.

Next, the exhaust gas purification device 8 of this example will be described.
As shown in FIG. 5, the exhaust gas purification device 8 includes a catalyst 81 for purifying the exhaust gas and a gas sensor 7 having the gas sensor element 1. The total amount of precious metal per vehicle used for the catalyst 81 is 5.0 g or less. The catalyst 81 is a three-way catalyst, and is arranged in the exhaust path while being supported on a ceramic carrier having a honeycomb structure.

  As shown in the figure, the exhaust gas purification device 8 is specifically provided in the exhaust system of the automobile engine E. The exhaust gas purification device 8 is provided in a catalyst 81 for purifying exhaust gas, an upstream oxygen sensor 83 provided in an exhaust pipe 82 upstream of the catalyst 81, and an exhaust pipe 84 downstream of the catalyst 81. A gas sensor 7 on the downstream side. The oxygen sensor 83 is an A / F sensor, and is disposed to measure the exhaust gas concentration discharged from the automobile engine E on the upstream side of the catalyst 81. The gas sensor 7 has the above-described configuration.

Further, when the sensor output value exceeds or falls below a specified value, the exhaust gas purification device 8 feeds back the detection result to the control unit 85 of the automobile engine E so that the sensor output value approaches the specified value. It is configured.
Hereinafter, a more detailed description will be given using specific experimental examples.

(Sample preparation)
First, a cup-type solid electrolyte body made of ZrO ₂ ceramics having the shape shown in FIG. 1 was manufactured. The thickness of the solid electrolyte body is 0.1 to 3 mm. And the paste which contains 0.0002 mass% of dibenzylidene Pt by Pt content was apply | coated to the outer surface of a solid electrolyte body by pad printing, and the printing part was formed. The printed portion has the same shape as the measured gas side electrode, lead electrode, and terminal electrode shown in FIG.

  Next, the printed part was heat treated at 40 ° C., and then electroless plating was performed at 50 ° C. using a plating solution containing a Pt complex. Thereby, the gas side electrode, the lead electrode, and the terminal electrode to be measured were formed on the outer surface of the solid electrolyte body. Similarly, a reference gas side electrode, a lead electrode, and a terminal electrode were formed on the inner surface of the solid electrolyte body, except that a dispenser was used instead of the pad printing. The measured gas side electrode, the reference gas side electrode, the lead electrode, and the terminal electrode are all made of Pt and have a thickness of about 1.0 μm.

Next, the gas side electrode to be measured on the outer surface of the solid electrolyte body was immersed in a 1.5 mass% barium nitrate aqueous solution and held for 10 minutes while reducing the pressure with a vacuum pump. Then, in synthetic air atmosphere, and heat-treated for 1 hour at 1000 ° C., oxide of Ba (BaO, BaO ₂₎ was deposited on the surface of the measured gas side electrode.

Next, heat treatment was performed at 600 ° C. for 1 hour in a stream of CO ₂ : 10%, O ₂ : 5%, N ₂ : 85%, and Ba oxide (BaO, BaO ₂ ) on the gas side electrode to be measured. Carbonated to Ba carbonate (BaCO ₃ ). As a result, an electrode covering portion made of Ba carbonate (BaCO ₃ ) and having a thickness of about 0.5 μm was formed on the surface of the measurement gas side electrode. The electrode coverage is about 10% (the same applies to samples 2 to 7 described later). Thus, a gas sensor element of Sample 1 was obtained.

Further, Sr carbonate (SrCO ₃ ) was deposited on the surface of the gas side electrode to be measured except that barium nitrate was changed to strontium nitrate. As a result, an electrode covering portion made of Sr carbonate (SrCO ₃ ) and having a thickness of about 0.5 μm was formed on the surface of the measured gas side electrode. Thus, a gas sensor element of Sample 2 was obtained.

Further, in the preparation of Sample 1, Na carbonate (Na ₂ CO ₃ ) was deposited on the surface of the gas side electrode to be measured in the same manner except that barium nitrate was changed to sodium nitrate. As a result, an electrode covering portion made of Na carbonate (Na ₂ CO ₃ ) and having a thickness of about 0.5 μm was formed on the surface of the measured gas side electrode. Thus, a gas sensor element of Sample 3 was obtained.

Further, in the production of Sample 1, K carbonate (K ₂ CO ₃ ) was deposited on the surface of the gas side electrode to be measured except that barium nitrate was changed to potassium nitrate. As a result, an electrode covering portion made of K carbonate (K ₂ CO ₃ ) and having a thickness of about 0.5 μm was formed on the surface of the gas side electrode to be measured. Thus, a gas sensor element of Sample 4 was obtained.

Further, in the production of Sample 1, Rb carbonate (Rb ₂ CO ₃ ) was precipitated on the surface of the gas side electrode to be measured except that barium nitrate was changed to rubidium nitrate. As a result, an electrode covering portion made of Rb carbonate (Rb ₂ CO ₃ ) and having a thickness of about 0.5 μm was formed on the surface of the measurement gas side electrode. Thus, a gas sensor element of Sample 5 was obtained.

Further, in the preparation of Sample 1, Cs carbonate (Cs ₂ CO ₃ ) was deposited on the surface of the gas to be measured electrode in the same manner except that barium nitrate was changed to cesium nitrate. As a result, an electrode covering portion made of Cs carbonate (Cs ₂ CO ₃ ) and having a thickness of about 0.5 μm was formed on the surface of the measurement gas side electrode. Thus, a gas sensor element of Sample 6 was obtained.

Further, in the preparation of Sample 1, the measurement gas side electrode on the outer surface of the solid electrolyte body was immersed in a 1.5% by mass lanthanum nitrate aqueous solution and held for 10 minutes while reducing the pressure with a vacuum pump. Then, in synthetic air atmosphere, and heat-treated for 1 hour at 1000 ° C., oxide of La and _(La 2 _{O 3)} is deposited on the surface of the measured gas side electrode. As a result, an electrode covering portion made of La oxide (La ₂ O ₃ ) and having a thickness of about 0.5 μm was formed on the surface of the gas side electrode to be measured. Thus, a gas sensor element of Sample 7 was obtained.

  Further, a gas sensor element of a comparative sample was obtained in the same manner as in the preparation of Sample 1 except that the electrode covering portion was not formed on the surface of the gas side electrode to be measured.

(Excess air ratio λ vs. sensor output characteristics)
First, a heater for heating was attached to the inside of the solid electrolyte body in the gas sensor element of each sample to constitute each evaluation gas sensor.
Subsequently, the heater in each gas sensor for evaluation was heated so that the surface temperature of the gas sensor element was 600 ° C. N ₂ and C ₃ H ₈ were supplied at a fixed flow rate to N ₂ : 3000 cc / min and C ₃ H ₈ : 12 cc / min to the gas side electrode to be measured of the heated gas sensor element. The reference gas side electrode of each evaluation gas sensor element was released to the atmosphere and supplied with air.

Subsequently, O ₂ was supplied to each gas sensor element for evaluation in such a manner that the air-fuel ratio A / F of the mixed gas composed of N ₂ , C ₃ H ₈ and O ₂ was changed from the rich region to the lean region. And the electromotive force (V) of the reference | standard gas side electrode and the to-be-measured gas side electrode in that case was measured continuously.
The measurement results are shown in FIG. In the figure, the horizontal axis represents the air-fuel ratio A / F, and the vertical axis represents the sensor output (V). Here, since the air-fuel ratio A / F = the excess air ratio λ × theoretical air-fuel ratio, the curve of the air-fuel ratio A / F vs. sensor output characteristic in FIG. (The above-mentioned λ curve).

As shown in FIG. 3, each compound (BaCO ₃ , SrCO ₃ , Cs ₂ CO ₃ , Rb ₂ CO ₃ , K ₂ CO ₃ , An electrode covering portion composed of Na ₂ CO ₃ , La ₂ O ₃ ) is intermittently provided over the entire surface of the gas to be measured electrode. Therefore, as shown in FIG. 6, the λ curve of each gas sensor element has a shape in which the sensor output in the rich region does not greatly decrease and the sensor output changes sharply in the rich region. This is presumed to be due to the following principle.

That is, the compounds constituting the electrode covering portion (BaCO ₃ , SrCO ₃ , Cs ₂ CO ₃ , Rb ₂ CO ₃ , K ₂ CO ₃ , Na ₂ CO ₃ , La ₂ O ₃ ) constitute the measured gas side electrode. Pt, which is in contact with Pt, which is an electrode material that does not chemically bond, enhances the bonding between Pt and O ₂ and has the effect of oxidizing Pt in the weak rich region to the lean region . Oxidized Pt has moderately suppressed catalyst performance compared to the unoxidized state. In particular, the oxidation performance (purification performance) for C ₃ H ₈ which is a paraffinic hydrocarbon is moderately lowered. Therefore, in the weak-rich region (despite rich region), the remainder is O ₂ was supposed consumed originally on the measured gas side electrode, the sensor output decreases by detecting the excess O ₂ . That is, the λ point at which the sensor output changes rapidly shifts to the rich side as compared with the gas sensor element of the comparative sample.

On the other hand, in the rich region, the reduction reaction in which the oxidized Pt returns to the original state (non-oxidized state) proceeds. Therefore, Pt comes to exhibit the original catalyst performance (particularly oxidation performance (purification performance) for C ₃ H ₈ which is a paraffinic hydrocarbon), and shows a normal sensor output.
From the above, it is presumed that the λ curves of the gas sensor elements of Samples 1 to 7 have a shape in which the sensor output in the rich region does not significantly decrease and the sensor output changes sharply in the rich region.

Thus, it can be said that the gas sensor elements of Samples 1 to 7 can shift the λ point to the rich side while maintaining the sensor output of the rich region substantially equal to the gas sensor element of the comparative sample.
Further, as shown in FIG. 6, the amount of shift of the λ point to the rich side varies depending on each sample, that is, depending on the type of compound constituting the electrode covering portion. Therefore, it can be said that the shift amount of the λ point to the rich side can be controlled by selecting the compound constituting the electrode covering portion.

Next, using the gas sensor element of sample 1 (compound composing the electrode covering portion: BaCO ₃ ), the electrode coverage (the ratio of the portion covered by the electrode covering portion of the surface of the gas side electrode to be measured) is calculated. The λ point rich side shift amount and gas responsiveness at a sensor output of 0.6 V when changed were examined.

  In addition, the electrode coverage was calculated | required by analyzing the surface of the to-be-measured gas side electrode using the analyzer (ESCALAB250, the product made by Thermo Fisher Scientific Co., Ltd.) by XPS (X-ray photoelectron spectroscopy). Specifically, the ratio of Pt and Ba was determined from the surface composition ratio calculated by XPS analysis, and was determined from the formula of electrode coverage (%) = Ba composition ratio / (Ba composition ratio + Pt composition ratio). . In XPS analysis, Mg Kα ray was used as the X-ray source.

  Further, the λ point rich side shift amount is the amount by which the λ point is shifted from the reference point to the rich side at the sensor output 0.6 V (the difference in the air-fuel ratio A / F value at the λ point) as the λ point rich side shift amount. Asked. The reference point was the λ point (air-fuel ratio A / F = 14.594) of the gas sensor element of comparative sample 1.

Gas responsiveness was evaluated by the following method. That is, first, N ₂ and CO are supplied to the gas sensor (gas sensor element) at a fixed flow rate under the conditions of N ₂ : 3000 cc / min and CO: 0.6 cc / min until the sensor output is stabilized. Hold for (100 seconds). Next, O ₂ was supplied at a fixed flow rate under the condition of 1.2 cc / min. Then, the O ₂ supply start time was taken as the measurement start point (0 second), and the time from the start of O ₂ supply until the sensor output decreased to reach 0.6 V was measured as the gas response time. Here, the element surface temperature of the gas sensor element was controlled to 600 ° C.

The result is shown in FIG. In the figure, the horizontal axis represents the electrode coverage (%), and the vertical axis represents the λ point rich side shift amount and the response time (seconds).
According to the figure, it can be seen that when the electrode coverage is 10% or more, the effect of shifting the λ point to the rich side starts to appear, and the electrode coverage increases and the λ point rich side shift amount also increases. It can also be seen that when the electrode coverage is 30% or more, the increase in the λ point rich side shift amount becomes moderate.
On the other hand, when the electrode coverage exceeds 80%, the electrode coverage increases and the response time becomes longer, and the gas responsiveness is lowered. Further, it can be seen that when the electrode coverage exceeds 90%, the response time becomes abruptly long and the gas responsiveness is significantly lowered.

  From the above results, the electrode coverage is preferably 10 to 90% in order to sufficiently obtain the effect of shifting the λ point to the rich side while sufficiently ensuring the gas responsiveness of the gas sensor element. It can be said that it is more preferable that it is 30 to 80%.

Next, using the gas sensor element of the sample 1 (compound composing the electrode coating part: BaCO ₃ ), the compound (BaCO ₃ ) composing the electrode coating part and the electrode material (Pt) composing the measured gas side electrode and The existence was confirmed without chemical bonding.
Specifically, the surface of the measured gas side electrode was analyzed using an XPS analyzer (ESCALAB 250, manufactured by Thermo Fisher Scientific Co., Ltd.), and the binding energy of the 4t orbit of Pt was measured. In XPS analysis, Mg Kα ray was used as the X-ray source. For comparison, the same measurement was performed on a gas sensor element of a comparative sample in which no electrode covering portion was provided.

The result is shown in FIG. This figure shows the spectrum of the binding energy of the 4f orbit of Pt on the surface of the gas side electrode of the sample 1 and the comparative sample.
According to the figure, the sample 1 is provided with an electrode covering portion on the surface of the gas side electrode to be measured, but is not provided with an electrode covering portion (the electrode material Pt constituting the gas side electrode to be measured is other It can be seen that the peak positions of the comparative sample and the spectrum are in agreement with each other.
From the above results, it can be said that in the sample 1, the compound (BaCO ₃ ) constituting the electrode covering portion is not chemically bonded to the electrode material (Pt) constituting the measurement gas side electrode.

Next, in the same manner as described above, using the gas sensor element of sample 1 (compound constituting the electrode covering portion: BaCO ₃ ), the compound (BaCO ₃ ) constituting the electrode covering portion constitutes the gas to be measured electrode. The presence of the material (Pt) without chemical bonding was examined.
Specifically, an X-ray diffraction pattern was measured on the surface of the gas side electrode to be measured using an X-ray diffractometer (XRD-6100, manufactured by Shimadzu Corporation). In addition, the Kα ray of Cu was used as the X-ray source, and measurement was performed under the conditions of a thin film method (incident angle 1 °) and 2θ: 20 to 70 °. For comparison, the same measurement was performed on a gas sensor element of a comparative sample in which no electrode covering portion was provided.

The result is shown in FIG. FIG. 9A shows the X-ray diffraction patterns on the surfaces of the measured gas side electrodes of the sample 1 and the comparative sample. FIG. 9B is an enlarged view of the diffraction peak position (portion surrounded by a dotted line in FIG. 9A).
According to the figure, the sample 1 is provided with an electrode covering portion on the surface of the measured gas side electrode, but not provided with an electrode covering portion (the electrode material constituting the measured gas side electrode is Pt alone. It can be seen that the comparison sample and the diffraction peak position coincide. This indicates that the crystal of the electrode material Pt constituting the measured gas side electrode has not changed at all.
From the above results, it can be said that in the sample 1, the compound (BaCO ₃ ) constituting the electrode covering portion is not chemically bonded to the electrode material (Pt) constituting the measurement gas side electrode.

(Example 2)
In the above-described first embodiment, the cup-type gas sensor element 1 and the gas sensor 7 having the gas sensor element 1 have been described as shown in FIGS. On the other hand, as shown in FIG. 10, even in the laminated gas sensor element 1, at least the configuration of the gas side electrode 3 to be measured and the electrode covering portion (not shown) is the same as that of the first embodiment. Thus, the same effects as those of the first embodiment can be achieved.

  The gas sensor element 1 of this example includes a measured gas side electrode 3 provided on one surface of a flat solid electrolyte body 2 and a reference gas side electrode provided on the other surface, as shown in FIG. 4. In addition, a heater 120 that integrally includes a heating element 119 is provided on the back surface of the spacer 118 that constitutes the reference gas chamber 117. Further, the measured gas side electrode 3 is covered with a first protective layer 121 and a second protective layer 122 having a two-layer structure. An electrode covering portion (not shown) is provided on the surface 300 of the measured gas side electrode 3.

  As mentioned above, although the Example of this invention was described in detail, this invention is not limited to the said Example, A various change is possible within the range which does not impair the meaning of this invention.

DESCRIPTION OF SYMBOLS 1 Gas sensor element 2 Solid electrolyte body 3 Gas side electrode 300 to be measured 300 Surface (surface of gas side electrode to be measured)
4 Reference gas side electrode 5 Electrode covering part

Claims (5)

An oxide ion conductive solid electrolyte body (2);
A measured gas side electrode (3) provided on one surface of the solid electrolyte body (2) for contacting the measured gas;
A reference gas side electrode (4) provided on the other surface of the solid electrolyte body (2) for contacting a reference gas, wherein the gas to be measured is lean or rich with respect to the stoichiometric air-fuel ratio; A gas sensor element (1) used in a gas sensor for determining whether or not a state is present,
On the surface (300) of the measured gas side electrode ( 3) located on the opposite side of the surface of the reference gas electrode (4) via the solid electrolyte body (2), the measured gas side electrode ( The electrode covering portion (5) for suppressing the catalytic performance of the electrode material constituting 3) partially covers the surface (300) of the measured gas side electrode (3) and the entire surface (300). Are provided intermittently,
The electrode covering portion (5) is composed of a compound containing at least one element selected from Ba, Sr, Cs, Rb, K, Na and La,
The gas sensor element (1), wherein the compound is present without being chemically bonded to the electrode material constituting the measured gas side electrode (3).   2. The gas sensor element (1) according to claim 1, wherein the compound constituting the electrode covering portion (5) contains at least one selected from carbonates, oxides, and composite oxides of the elements. The gas sensor element (1) characterized by these.   The gas sensor element (1) according to claim 1 or 2, wherein the electrode coating is a ratio of a portion of the surface (300) of the measured gas side electrode (3) covered by the electrode coating portion (5). The gas sensor element (1), wherein the rate is 10 to 90%.   A gas sensor (7) comprising the gas sensor element (1) according to any one of claims 1 to 3. A catalyst (81) for purifying exhaust gas;
A gas sensor (7) comprising the gas sensor element (1) according to any one of claims 1 to 3,
The exhaust gas purification device (8), wherein the total amount of precious metals per vehicle used for the catalyst (81) is 5.0 g or less.

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