JP5772732B2 - Oxygen sensor - Google Patents

Description

The present invention relates to an oxygen sensor having an oxygen sensor element Pt film is coated, in particular, relates to excellent oxygen sensor detection accuracy.

Conventionally, an oxygen sensor (O ₂ sensor) includes an oxygen sensor element, and the oxygen sensor element is housed in a housing. For example, in the case of a bottomed cylindrical (cup-shaped) oxygen sensor element, the oxygen sensor element is provided on a cup-shaped solid electrolyte body having oxygen ion conductivity and an inner surface of the solid electrolyte body. A device including an inner electrode and an outer electrode provided on the outer surface of the solid electrolyte body is generally known.

  In such an oxygen sensor element, an inner electrode is brought into contact with the atmosphere as a reference electrode, while an outer electrode is brought into contact with a gas to be measured (exhaust gas) to be used as a measurement electrode, so that the oxygen concentration in the exhaust gas of the internal combustion engine Is to measure.

  In recent years, various studies have been made on the outer electrode in order to improve the performance of the oxygen sensor element. For example, an oxygen sensor element is disclosed in which the average particle diameter of the electrode material crystal of the outer electrode and the film thickness of the outer electrode are improved in order to improve the low temperature operability and gas responsiveness. Here, the solid electrolyte body constituting the oxygen sensor element is made of stabilized zirconia, and the surface of the solid electrolyte body is covered with a Pt (Pt) film as an electrode material.

  As a Pt film used for an electrode, for example, an oxygen sensor in which a nonporous film made of Pt cermet is coated on the surface of a solid electrolyte body by plating has been proposed (for example, see Patent Document 1). As another technique, an oxygen sensor including an electrode made of a noble metal selected from the group of platinum, gold, rhodium, ruthenium, and palladium or an alloy containing the noble metal has been proposed (for example, see Patent Document 2). .

JP 2000-105213 A JP 2007-248334 A

  However, in the above-described oxygen sensor, it is desirable that the output voltage of the oxygen sensor element changes rapidly in accordance with the switching of the rich / lean atmosphere regardless of the gas concentration. This element cannot be said to have sufficient followability (responsiveness). Stable supply of oxygen sensors equipped with highly sensitive and highly accurate oxygen sensor elements that excel in responsiveness (especially against lean gas) in terms of future exhaust gas regulations, low fuel consumption, and low cost. Is desired.

  In general, the oxygen sensor indicates that the measured gas atmosphere has changed as an output value based on the amount of oxygen in the atmosphere. Therefore, the ability to quickly change the oxygen state at the interface of the solid electrolyte body via the electrode on the measured gas side determines whether the output voltage of the oxygen sensor is good or bad. Here, since the oxygen state changes rapidly under a high-concentration gas atmosphere, the above-described response is not particularly problematic.

  However, in a low-concentration gas atmosphere of the gas to be measured, the reaction time of the oxygen sensor element tends to become longer as the oxygen state changes. For this reason, there may be a temporary shift between the oxygen concentration of the gas to be measured and the output voltage of the oxygen sensor. As a result, the controllability of the air-fuel ratio control is deteriorated, and as a result, the emission of the internal combustion engine may be reduced.

  By the way, in the air-fuel ratio control of the internal combustion engine of the automobile, the air-fuel ratio of the exhaust gas flowing into the catalyst becomes the target air-fuel ratio (for example, the stoichiometric air-fuel ratio) based on the output signal of the A / F sensor arranged upstream of the catalyst. The fuel injection amount is feedback-controlled so that

  On the other hand, the oxygen sensor is disposed on the downstream side of the catalyst. Since the oxygen sensor has a characteristic that the output voltage changes suddenly in the vicinity of the stoichiometric air-fuel ratio (stoichiometric), the control for correcting the output target value of the A / F sensor based on the output voltage of the oxygen sensor element is performed. The error of the output signal due to the deterioration of the / F sensor is corrected.

  By executing such feedback control, the fuel injection amount from the injector is adjusted so that the air-fuel ratio of the intake air becomes the target air-fuel ratio (for example, the theoretical air-fuel ratio), and the emission of exhaust gas is improved.

  Here, on the premise of the feedback control described above, when performing catalyst deterioration diagnosis control (on-board diagnosis: OBD control), techniques such as the Cmax method and the trajectory length method are generally used. These techniques use a hysteresis loop drawn by the air-fuel ratio and the output of the oxygen sensor to estimate the oxygen storage amount in the catalyst and diagnose the deterioration of the catalyst from the result. Therefore, a certain amount of response time is required for the characteristics of the oxygen sensor.

  However, as described above, from the viewpoint of emission, it is best that both the lean / rich response speed is fast. In such a case, the curve drawn by the air-fuel ratio and the output of the oxygen sensor has a hysteresis. Since it becomes a Z-shaped curve without a loop, application to OBD control becomes difficult.

  The present invention has been made from such a viewpoint, and an object of the present invention is to provide an oxygen sensor that is excellent in responsiveness and can be suitably used for diagnosing catalyst deterioration.

  In view of the above problems, the inventor has conducted intensive studies, and from the viewpoint of strengthening exhaust gas regulations, the responsiveness to lean gas (responsiveness when switching from rich gas to lean gas) is made faster, and responsiveness to rich gas ( It was considered desirable to have an oxygen sensor having sensor characteristics such that the response at the time of switching from lean gas to rich gas is equal to or delayed (conventional response time is relaxed to some extent as compared with the prior art).

  In order to obtain an oxygen sensor having such sensor characteristics, a specific metal is contained in a trace amount of Pt of the Pt electrode, so that a gas is controlled by controlling the electron density of Pt. It was thought that the adsorption / desorption property of the can be changed. Specifically, by controlling the electron density of Pt, the selectivity of each gas with respect to NO gas and CO gas, and the binding energy to these gases can be controlled, and the sensitivity to exhaust gas can be changed. I thought. As a result, the response speed from lean to rich and the response speed from rich to lean can be controlled.

  In this way, the deterioration diagnosis control of the catalyst is accurately performed by improving the response from rich to lean, improving the emission and fuel efficiency of the internal combustion engine, and reducing the response from lean to rich. I got new knowledge that I could do it.

  The present invention is based on this new knowledge, and an oxygen sensor according to the present invention is an oxygen sensor including an oxygen sensor element in which a Pt film is coated as a pair of electrodes on both surfaces of a solid electrolyte body. In the Pt film formed on at least the surface exposed to the gas to be measured among the both surfaces of the solid electrolyte body, at least one metal of Mo, Ru, or Nb is 0. It is contained in an amount of 01 to 0.5% by mass.

  According to the present invention, by containing 0.01 to 0.5% by mass of Mo, Ru, and Nb with respect to Pt of the Pt electrode, the electron density of Pt is reduced. As a result, the NO response in the exhaust gas is reduced. The speed can be increased, that is, the sensor response from rich to lean can be enhanced.

  That is, when at least one metal of Mo, Ru, or Nb exceeds 0.5 mass%, the electron density of Pt hardly changes, so it is difficult to increase the NO response speed in the exhaust gas. When the amount is less than 0.01% by mass, the effect of the added element cannot be obtained, the electron density of Pt hardly changes, and it is also difficult to increase the NO response speed in the exhaust gas.

  On the other hand, by containing 0.01 to 0.5% by mass of Rh with respect to Pt of the Pt electrode, the electron density of Pt is increased, and as a result, the CO response speed in the exhaust gas is decreased. Sensor response from low to rich can be relaxed.

  That is, when Rh exceeds 0.5 mass%, the electron density of Pt hardly changes, so it is difficult to increase the CO response speed in the exhaust gas. When the amount is less than 0.01% by mass, the effect of the added element cannot be obtained, the electron density of Pt hardly changes, and it is difficult to increase the CO response speed in the exhaust gas.

  As a more preferable aspect, the Pt film coated on the side exposed to the gas to be measured is formed by a plating solution in which at least one metal of Mo, Ru, Nb, and Rh is added to Pt. Then, it is heat-treated at 1000 ° C. to 1200 ° C.

  According to this aspect, the Pt film coated on the side exposed to the gas to be measured is formed by a plating solution in which at least one additional metal of Mo, Ru, Nb, and Rh is added to Pt. After the film formation, the Pt film heat-treated at 1000 ° C. to 1200 ° C. is burned off the additive metal contained on the surface of the Pt film. Therefore, the ratio of the additive metal contained on the surface of the Pt film is 0.01% by mass or less. In order to develop such a phenomenon, it is desirable that the heating time be in the range of 1 to 2 hours.

  That is, when the heating temperature is less than 1000 ° C. or the heating time is less than 1 hour, it is difficult to develop such a phenomenon, the heating temperature exceeds 1200 ° C., or the heating time exceeds 2 hours. In some cases, Pt aggregation may proceed.

  As a more preferred embodiment, closed pores are formed at least inside the Pt coating formed on the surface exposed to the gas to be measured, and oxygen gas is more contained in the closed pores than atmospheric gas. Filled with highly concentrated gas.

  According to the present invention, closed pores are formed inside the Pt film on the oxygen sensor element exposed to the gas to be measured, and the inside is filled with a gas having a higher oxygen gas concentration than the atmospheric gas. Therefore, when switching from a lean gas atmosphere to a rich gas atmosphere, oxygen in the closed pores of the Pt film acts as a buffer phase for the reaction at the three-phase interface. As a result, the response to the rich gas can be relaxed, and the catalyst deterioration diagnosis (OBD) control can be performed with high accuracy.

  On the other hand, when switching from a rich gas atmosphere to a lean gas atmosphere, the response (sensitivity) to the lean gas is improved, and the change in the atmosphere from the rich gas to the lean gas can be feedback-controlled at an early stage, further reducing the emissions of the internal combustion engine. Can be increased.

  ADVANTAGE OF THE INVENTION According to this invention, while being excellent in responsiveness, the oxygen sensor which can be utilized suitably when performing deterioration diagnosis of a catalyst can be obtained.

FIG. 2 is a schematic cross-sectional view of an oxygen sensor according to the present embodiment and an oxygen sensor element built in the oxygen sensor. The figure for demonstrating the elementary process of the electrode gas reaction in the outer electrode (Pt electrode) of the sensor at the time of the change from rich to lean, and at the time of change from lean to rich. The figure for demonstrating reaction control of the gas in the external electrode containing an additional metal. The figure which showed the relationship between the binding energy by XPS, and intensity | strength in order to measure the Pt4f photoelectron spectrum of the surface of the outer side electrode (Pt electrode) in Examples 1-4 and the comparative example 1. FIG. Relationship between the response time of the oxygen sensor when changing from rich gas to lean gas and the response time of the oxygen sensor when changing from lean gas to rich gas in the oxygen sensors of Examples 1 to 4 and the oxygen sensors of Comparative Examples 1 to 3 FIG. (A) is the figure which showed the response time of the oxygen sensor when changing from the rich gas of the comparative example 1 and comparative examples 4-7 to lean gas, (b) is the figure of the comparative example 1 and comparative examples 4-7. The figure which showed the response time of the oxygen sensor when changing from lean gas to rich gas. The figure which showed the relationship of the response time of the oxygen sensor of Examples 5-8 and the low concentration rich gas of a reference example, and lean gas.

An embodiment of manufacturing an oxygen sensor (O ₂ sensor) according to the following embodiment will be described.

  FIG. 1 is a schematic cross-sectional view of an oxygen sensor according to the present embodiment and an oxygen sensor element built in the oxygen sensor. As shown in FIG. 1, an oxygen sensor 1 according to this embodiment is installed in an exhaust pipe of an internal combustion engine, detects an oxygen concentration or an unburned gas concentration in exhaust gas of the internal combustion engine, and detects the oxygen concentration. Alternatively, the air-fuel ratio of the combustion chamber of the internal combustion engine is detected from the unburned gas concentration.

  Specifically, the oxygen sensor 1 incorporates an oxygen sensor element 11, and the oxygen sensor element 11 is inserted and fixed to the housing 10. The distal end side of the oxygen sensor element 11 is protected by a double-structured gas cover 12 to be measured. The measured gas cover 12 is provided with a measured gas inlet 12a for introducing the measured gas (exhaust gas). As a result, the measurement gas can be introduced into the outer electrode 23 disposed in the measurement gas cover 12 described later.

  The oxygen sensor element 11 includes, for example, a bottomed cylindrical (cup-shaped) solid electrolyte body 21 and at least a pair of electrodes 23 and 24 on both surfaces of the solid electrolyte body 21. The electrode of the oxygen sensor element 11 is positioned in the measured gas cover 12 when it is attached to the housing 10 of the oxygen sensor 1.

  More specifically, the outer surface of the gas sensor element 11 is covered with a Pt film that becomes the outer electrode 23, and a porous protective layer (or diffusion resistance layer) 25 that covers the outer electrode 23 is further formed. .

  On the other hand, the oxygen sensor element 11 is formed with an air chamber 22 for introducing air, and the inner surface of the solid electrolyte body 21 is covered with an inner electrode 24. In this manner, the oxygen sensor element 11 is configured such that the outer electrode 23 of the pair of electrodes 23 and 24 described above is exposed to the gas to be measured, and the inner electrode 24 is exposed to the reference gas (atmosphere). Yes.

  Here, yttria-stabilized zirconia (YSZ), which is generally known as an oxygen sensor element, can be mentioned here, but it is particularly limited as long as it is a material having ion conductivity and excellent heat resistance. is not. Furthermore, as will be described later, a closed hole 23a is formed inside the outer electrode 23, and a gas having a higher oxygen gas concentration than atmospheric gas is filled (enclosed) in the closed hole 23a. .

  The oxygen sensor 1 is attached to the exhaust pipe of the internal combustion engine. At this time, the inner electrode 24 exposed to the atmosphere serves as a reference electrode, the outer electrode 23 exposed to the exhaust gas serves as a measurement electrode, and due to the difference in oxygen concentration between the outer electrode 23 and the inner electrode 24. A concentration cell is formed, and the oxygen concentration can be measured by measuring the potential difference (voltage) between the electrodes.

  For example, when the exhaust gas changes to a rich gas, the oxygen concentration difference between the exhaust gas and the atmospheric gas increases, and the output voltage of the oxygen sensor increases. On the other hand, when the exhaust gas changes to lean, the oxygen concentration difference between the exhaust gas and the atmospheric gas becomes small, and the output voltage of the oxygen sensor decreases.

  With reference to FIG. 2, the response characteristics of the oxygen sensor and the influence of the additive element at the time of change from rich to lean and at the time of change from lean to rich will be described. FIG. 2 shows an elementary process of an electrode gas reaction in the outer electrode 23 (Pt electrode) of the sensor at the time of change from rich to lean and at the time of change from lean to rich.

  As shown in FIG. 2, the response speed of the oxygen sensor depends on the adsorption / desorption properties of the NO gas and CO gas, which are atmospheric gases, and the Pt electrode. That is, if (1) the Pt electrode is adsorbed and desorbed, NO gas or CO gas is controlled to be selected, and (2) the NO gas or CO gas adsorption / desorption force with respect to the Pt electrode, that is, the binding energy is controlled. The sensor response speed can be changed (controlled).

In concrete terms, as shown in i) of the upper part of FIG. 2, under a rich atmosphere, CO adsorbed on the surface of the Pt electrode. Here, when the rich atmosphere is changed to the lean atmosphere, as shown in ii), CO is desorbed from the surface of the Pt electrode, and NO is adsorbed. iii) Furthermore, when the atmosphere changes to a lean atmosphere, NO is adsorbed on the surface of the Pt electrode. Thereafter, as shown in iv), NO adsorbed on the surface of the Pt electrode increases, and O of NO diffuses between the Pt particles from the surface of the Pt electrode toward the inside. In this way, the output voltage of the oxygen sensor decreases.

  Here, in order to increase the sensor response speed from the rich atmosphere to the lean atmosphere, it is important to create a reaction site where NO in the Pt electrode reacts in the processes i) to iii), specifically, It is important to reduce the adsorptive power of CO adsorbed on the surface of the Pt electrode or increase the dissociative adsorptivity of NO.

On the other hand, as shown in i) in the lower part of FIG. 2, under the lean atmosphere, O is adsorbed on the surface of the Pt electrode. Here, when the lean atmosphere is changed to the rich atmosphere, as shown in ii), NO ₃ , CO, and O react on the surface of the Pt electrode, and O adsorbed on the surface of the Pt electrode is desorbed. Thereafter, as shown in iii), O diffused between the Pt particles moves from the inside of the Pt electrode toward the surface. In this way, the output voltage of the oxygen sensor increases.

  Here, in order to slow down the sensor response speed from the lean atmosphere to the rich atmosphere, it is important to increase the binding force of O adsorbed on the surface of the Pt electrode in the lean atmosphere in the processes i) and ii). is there.

  From this point of view, according to the inventors' experiment, the inventors focused on the electron density of Pt in the Pt electrode. Thus, it has been found that the electron density of Pt can be controlled by adding a very small amount of metal to Pt of the Pt electrode and diffusing it in the Pt electrode.

  That is, until now, when a metal such as Ru or Rh is added to Pt, the purpose is to cause the added metal itself to carry out the above-described oxidation-reduction reaction. That is, in this case, each of Pt and the additive element individually bears the oxidation-reduction reaction described above. However, in the case of the present invention, the purpose is to reform Pt itself with the additive element, and the oxidation-reduction reaction for the exhaust gas is carried out only by Pt. In that case, it has been found that such a modification of Pt can be achieved by adding a very small amount of the additive metal shown below.

  In this way, by changing the gas adsorption / desorption by controlling the electron density of Pt with a very small amount of added metal, the reactivity from rich to lean in the Pt electrode is improved as shown in the examples described later. To reduce the reactivity from lean to rich.

  Specifically, at least one metal of Mo, Ru, Nb and Rh is formed on the outer electrode (Pt film) 23 formed on at least the surface exposed to the gas to be measured among both surfaces of the solid electrolyte body. However, 0.01-0.5 mass% is contained with respect to Pt.

  Specifically, for the purpose of increasing the NO response speed (increasing the sensor response from rich to lean), Mo, Ru and Nb are added as additive metals for reducing the electron density of Pt and the electron density of Pt itself. Selected. By adding the above-mentioned amounts of Mo, Ru, and Nb to Pt, the electron density of Pt is lowered as compared with the case where Pt is not added.

  As a result, as shown in FIG. 3, it is possible to expect an effect of weakening the reverse electron donation between the CO gas and Pt due to the decrease in the electron density of Pt. It is possible to reduce the adsorption power of the adsorbed CO.

  Further, when at least one metal of Mo, Ru, or Nb exceeds 0.5 mass% with respect to Pt, the electron density of Pt hardly changes, so that the NO response speed in the exhaust gas is increased. It is difficult to let Further, when at least one metal of Mo, Ru, or Nb is less than 0.01% by mass with respect to Pt, the effect of the added element cannot be obtained, and the electron density of Pt is hardly changed. Similarly, it is difficult to increase the NO response speed in the exhaust gas.

  In addition, when transition metals, such as Sb and Pd, are used, the adsorption power of Pt with respect to CO will become high compared with Mo, Ru, or Nb. On the other hand, an additive metal such as Fe, Co, or Ni can also be used to obtain an adsorption power of Pt for CO equivalent to the case of using Mo, Ru, or Nb. However, when an additive metal is used, the additive metal itself is easily oxidized, which is not preferable for use as a Pt electrode of an oxygen sensor utilizing the stability caused by the oxidation level.

  On the other hand, for the purpose of reducing the CO response speed (relaxing the sensor response from lean to rich), Rh was selected as an additive metal for increasing the electron density of Pt higher than the electron density of Pt itself. As a result, as shown in FIG. 3, an increase in the electron density of Pt can be expected to increase the back donation of electrons between NO molecules and Pt atoms. As a result, the surface of the Pt electrode can be expected. It becomes possible to increase the binding force (adsorptive force) of O adsorbed on the substrate.

  Here, when Rh exceeds 0.5 mass% with respect to Pt in the outer electrode, the electron density of Pt hardly changes, so it is difficult to increase the CO response speed in the exhaust gas. In addition, when Rh is less than 0.01% by mass, the effect of the added element cannot be obtained, the electron density of Pt hardly changes, and similarly the CO response speed in the exhaust gas can be increased. difficult.

  A method for manufacturing the oxygen sensor element 11 will be described below. First, the solid electrolyte body 21 made of yttria-stabilized zirconia (YSZ) having a bottomed cylindrical shape in which the air chamber 22 is formed is formed.

  Next, a Pt film is coated on the outer surface of the solid electrolyte body 21 as an outer electrode 23 so that closed pores 23a are formed inside by an electroless Pt plating method in a non-oxidizing gas atmosphere such as hydrogen gas. To do.

  Specifically, a Pt aqueous solution such as dinitrodiamine Pt aqueous solution is mixed with a plating solution and an aqueous solution of rhodium chloride, niobium chloride, ruthenium chloride, or molybdenum ammonium salt as shown in Table 1, and this is brought to a predetermined liquid temperature. Additives such as heating and reducing agents are added to deposit Pt on the outer surface of the solid electrolyte body without rocking the solid electrolyte body and / or the jig holding the solid electrolyte body.

  Thereafter, the outer electrode (Pt film) 23 having a film thickness of 1 to 2 μm is formed by washing and drying. Thereby, the outer electrode 23 containing 0.01 to 0.5% by mass of at least one metal among Mo, Ru, Nb, and Rh with respect to Pt of the Pt film 23 can be obtained.

  Here, as a method for forming the closed pores 23a, conventionally, when the solid electrolyte body and / or the jig holding the same has been swung at the time of film formation, this rocking is performed. Do not perform. Furthermore, as another method for forming the closed pores 23a, a stabilizer for stabilizing plating in the plating solution is not reduced or added.

  Further, as another condition for forming the closed pores 23a inside the outer electrode (Pt film) 23, the Pt film is formed at a higher speed than the conventional film formation speed. For example, compared to conventional conditions, the concentration of the dinitrodiamine Pt aqueous solution is increased, the pH is adjusted, or the temperature of the plating solution is increased.

  By such a method, the hydrogen gas generated on the surface of the Pt film is not desorbed during the film formation, and the hydrogen gas remains as bubbles, so that the Pt film formed in this state has closed pores 23a ( The closed pores 23a that do not communicate with the outside air are formed, and the heat treatment thereafter allows the closed pores 23a to be filled with a gas having a higher oxygen gas concentration than the atmosphere. Note that the thickness of the Pt coating tends to be slightly thick due to the formation of crystal grains having closed pores and the influence of the generated hydrogen gas.

  In addition, the closed pores 23a may be formed inside the Pt coating by bubbling an electroless Pt plating solution with a non-oxidizing gas such as hydrogen gas. By bubbling with the non-oxidizing gas, bubbles of the non-oxidizing gas adhere to the surface of the Pt film in the middle of the film formation, and the Pt film is formed in this state. Therefore, the closed pores 23a are formed inside the Pt film. Can be easily formed.

  Here, by using hydrogen gas as the non-oxidizing gas, the molecular weight of the hydrogen gas is small, so that the hydrogen gas in the closed pores 23a can be easily released (replaced with a gas containing oxygen gas described later).

  Next, the outer electrode 23 is heated at a predetermined heating temperature (firing temperature) (1000 to 1200 ° C.) for one to two hours in an air atmosphere, and Pt of the outer electrode 23 is fired. As a result, a Pt electrode containing 0.01 to 0.05 mass of Mo, Ru, Nb, or Rh with respect to Pt in the electroless plating treatment plating solution is formed, and this is in the range of 1000 to 1200 ° C. And heat in 2 hours. In this way, a part of the added metal of Mo, Ru, Nb or Rh deposited on the surface of the outer electrode 23 is burned off, and a very small amount (0.005 ~ 0.01 mass) of Mo, Ru, Nb, or Rh can be diffused into Pt.

Next, in order to protect the outer electrode 23, a porous protective layer made of spinel (MgAl ₂ O ₄ ) is formed by plasma spraying. Further, the inner electrode 24 is formed on the inner surface of the solid electrolyte body (element) by electroless Pt plating. The inner electrode 24 made of a Pt film may be coated on the inner surface of the solid electrolyte body 21 when the outer electrode 23 is coated. , May be coated.

  Next, the oxygen sensor element 11 in which the outer electrode 23 and the inner electrode 24 are formed is contained in a gas atmosphere having a higher oxygen gas concentration than the atmospheric gas (preferably an atmosphere having an oxygen concentration of 50% by volume or more, or ozone gas). Heat (under atmosphere).

  Thereby, in the heating step, the Pt film coated on the side exposed to the gas to be measured is heated in a gas atmosphere having a higher oxygen gas concentration than the atmospheric gas, so that the closed pores 23a are formed from the surface of the Pt film. Oxygen gas penetrates and diffuses toward the inside. Further, after that, the gas inside the closed pores 23a is filled with a gas having a higher oxygen gas concentration than the atmospheric gas (permeated and diffused gas), and oxygen atoms are adsorbed by Pt atoms around the closed pores 23a. Conceivable.

  Here, heating is performed for 1 hour or more at a predetermined heating temperature (1100 to 1300 ° C.) higher than the firing temperature of the outer electrode 23 described above (aging the Pt film). Thereby, at least the crystal orientation of the Pt film of the outer electrode 23 can be aligned with the (001) plane.

  The oxygen sensor 1 can be obtained by incorporating such an oxygen sensor element 11 into the housing 10 as shown in FIG.

Here, in general, in a rich gas (exhaust gas containing HC, H ₂ , and CO) atmosphere, oxygen at a three-phase interface where the solid electrolyte body 21, the outer electrode (Pt coating) 23, and oxygen in the exhaust gas react with each other. The concentration is lower than in a lean gas (exhaust gas containing NOx) atmosphere. When the rich gas atmosphere is switched to the lean gas atmosphere, the oxygen concentration at the three-phase interface increases.

  In the present embodiment, 0.01 to 0.5 mass% of Mo, Ru, or Nb is added to the Pt electrode of Pt, and the Pt electrode is subjected to the heat treatment described above, thereby reducing the electron density of Pt. As a result, when the lean gas atmosphere is switched to the rich gas atmosphere, the NO response speed in the exhaust gas can be increased, and the response speed of the sensor can be increased.

  Further, since oxygen (atom or molecule) exists in and around the closed pores 23a of the outer electrode (Pt coating) 23, before and after the oxygen concentration at the three-phase interface decreases, the inside and around the closed pores 23a. The oxygen of the falls. That is, the closed pores 23a containing oxygen act as a buffer phase for the reaction at the three-phase interface. As a result, the response to the rich gas can be relaxed, and the catalyst deterioration diagnosis (OBD) control can be performed with high accuracy. Thereby, deterioration of a catalyst noble metal can be diagnosed appropriately and the cost merit of a catalyst noble metal can be improved.

  On the other hand, when the rich gas is switched to the lean gas, 0.01 to 0.5 mass% of Rh is added to Pt of the Pt electrode, and the Pt electrode is subjected to the heat treatment described above, thereby increasing the electron density of Pt. As a result, the CO response speed in the exhaust gas can be reduced and the sensor response speed can be reduced.

  Further, since oxygen is contained in and around the closed pores 23a of the outer electrode (Pt coating) 23, the concentration of oxygen gas at the three-phase interface can be increased more rapidly. Thereby, the responsiveness (sensitivity) with respect to lean gas can be improved. As a result, NOx contained in the lean gas can be detected more quickly. Thereby, the change in the atmosphere from rich gas to lean gas can be feedback-controlled at an early stage, and the emission of the internal combustion engine can be further increased.

  Further, when the outer electrode (Pt coating) 23 is coated on polycrystalline ceramics such as yttria stabilized zirconia by electroless plating, the crystal orientation of the crystal grains of the Pt coating is predicted to grow randomly or in the [111] direction. The However, since the outer electrode (Pt film) 23 is heated within the range of the heating temperature described above, the high temperature heat treatment is performed in a gas atmosphere having a higher oxygen gas concentration than the atmospheric gas. As a result, the crystal orientation of the outer electrode (Pt film) 23 is changed, and an electrode structure in which the ratio of the (001) plane advantageous for dissociation and adsorption of oxygen is increased is obtained.

  Further, since the outer electrode (Pt film) 23 formed by electroless Pt plating is formed by a fine Pt particle group, it melts under the above-described aging treatment conditions and then recrystallizes. It is easy to align the crystal orientation with the (001) plane.

  Since the oxygen sensor thus obtained can further improve the reactivity (sensitivity) to the extremely low concentration lean gas in the exhaust gas, it is possible to feed back the atmospheric change in the exhaust gas at an early stage, and a stable engine system. Control can be realized. As a result, the fuel consumption and emission of the internal combustion engine can be reduced as compared with the conventional oxygen sensor.

  The structure of the interface between the outer electrode 23 and the outer electrode 23 / solid electrolyte body 21 is smoothed by the aging of the outer electrode (Pt film) 23, and the structure is stabilized. As a result, the outer electrode 23 is stabilized in the active state (it becomes difficult for the state to change at the sensor operating temperature), so that the change in the sensor characteristics with time is reduced compared to the conventional one.

The present invention will be described below based on examples.
Example 1
An oxygen sensor was manufactured as shown below. Specifically, a solid electrolyte body made of bottomed cylindrical yttria stabilized zirconia (YSZ) (yttrium oxide 5 mol%) was molded. Next, an inner electrode made of a Pt film was coated on the inner surface of each solid electrolyte body by electroless Pt plating. Specifically, an aqueous solution (2 g / l) of dinitrodiamine Pt was used as a plating solution, and the plating solution was heated to 50 ° C.

  Next, as a reducing agent, the plating solution is adjusted to a range of 80% by mass hydrazine aqueous solution, 4 g / l, stabilizer, ammonia water pH 11 to 12, and the jig holding the solid electrolyte body is swung, Pt was deposited on the inner surface of the solid electrolyte body. Then, the inner electrode (Pt film) used as the film thickness of 1-2 micrometers was formed by washing with water and drying. This inner electrode was heated with a heater at 1000 ° C. for 1 hour in an air atmosphere to calcinate Pt.

  Next, an outer electrode made of a Pt film was coated on the outer surface of each solid electrolyte body by electroless Pt plating. Specifically, an aqueous solution (2 g / l) of dinitrodiamine Pt and an aqueous solution (0.5 g / l) obtained by dissolving ruthenium chloride in pure water were 0.01 to 0.5% by mass (specifically Was mixed so as to be within the range of 0.1% by mass), and the mixed plating solution was heated to 50 ° C.

  Next, in the plating solution, the reducing agent is adjusted to the range of 80% by mass hydrazine aqueous solution, 4 g / l, stabilizer, ammonia water pH 11-12, and the jig holding the solid electrolyte body is swung ( Pt and Ph were deposited on the inner surface of the solid electrolyte body. Then, the outer electrode (Pt film) which becomes a film thickness of the range of 1-2 micrometers was formed by washing with water and drying. This outer electrode was heated with a heater at 1000 ° C. for 1 hour in an air atmosphere, and Pt was fired to be alloyed.

  As a result of analysis by ICP, this outer electrode contained the amount of Ru described above with respect to Pt. Further, when measured by EDX, Ru was 0.01% by mass or less (detection limit or less) with respect to Pt on the surface of the external electrode. That is, by electroless plating, a Pt electrode was formed using a plating solution containing 0.01 to 0.05 mass of Ru with respect to Pt in the plating solution, and this was applied in the range of 1000 ° C. for 1 hour. It can be said that a very small amount of Ru is diffused with respect to Pt at the reaction site which becomes the surface of the Pd external electrode of the external electrode by burning a part of Ru (by a very small amount). .

Further, on this outer electrode, a porous protective layer made of spinel (MgAl ₂ O ₄ ) was formed to a thickness of 200 μm by plasma spraying. Furthermore, the inner electrode was formed on the inner surface of the solid electrolyte body (element) by the same method (electroless Pt plating method) as the formation of the outer electrode.

  The oxygen sensor element in which the outer electrode and the inner electrode are formed is heated at a heating temperature of 1100 ° C. for 1 hour in a gas (a gas having a higher oxygen gas concentration than atmospheric gas) in which nitrogen gas is mixed with 50% by volume of oxygen. Was heated (aged). The oxygen sensor element thus obtained was assembled in a housing to obtain an oxygen sensor.

(Example 2)
An oxygen sensor was produced in the same manner as in Example 1. The difference from Example 1 is that niobium chloride (0.5 g / l) falls within the range of 0.01 to 0.5 mass% (specifically, 0.1 mass%) with respect to Pt. The outer electrode (Pt film) was formed using the plating solution mixed in the above. As a result of analysis by ICP, this outer electrode contained the amount of Nb described above with respect to Pt. Further, when measured by EDX, Nb was 0.01% by mass or less (below the detection limit) with respect to Pt on the surfaces of all external electrodes.

(Example 3)
An oxygen sensor was produced in the same manner as in Example 1. The difference from Example 1 is that, for the outer electrode, molybdenum ammonium salt (0.5 g / l) is 0.01 to 0.5% by mass (specifically, 0.1% by mass) with respect to Pt. The outer electrode (Pt film) was formed using the plating solution mixed so as to be within the range of. As a result of analysis by ICP, it was confirmed that the outer electrode contained the amount of Mo described above with respect to Pt. Moreover, when measured by EDX, Mo was 0.01 mass% or less (below detection limit) with respect to Pt on the surface of all the external electrodes.

Example 4
A plurality of oxygen sensors were produced in the same manner as in Example 1. The difference from Example 1 is that rhodium chloride (0.2 g / l) is added to the outer electrode in an amount of 0.01 to 0.5% by mass (specifically 0.05% by mass) with respect to Pt. The outer electrode (Pt film) is formed using the plating solution mixed so as to be within the range. As a result of analysis by ICP, it was confirmed that this outer electrode contained the amount of Rh described above with respect to Pt. Moreover, when measured by EDX, Rh was 0.01 mass% or less (below detection limit) with respect to Pt on the surface of all the external electrodes.

(Comparative Example 1)
An oxygen sensor was produced in the same manner as in Example 1. The difference from Example 1 was that the outer electrode (Pt film) was formed using the same plating solution as that for the inner electrode without adding Ru as an additive metal to the outer electrode. Specifically, the porosity of the electrode was 18%, and the supported amount was 0.1% by mass with respect to the solid electrolyte body (porosity and ratio equivalent to those in Examples 1 to 4 described above). And 5% by weight of the solid electrolyte were prepared. In addition, the reference | standard plot of the comparative example 1 shown in FIG. 5 mentioned later, and the result of FIG. 6 (a), (b) are the porosity of an electrode 18%, and the load is 0.1 mass with respect to a solid electrolyte body. % Results.

(Comparative Example 2)
An oxygen sensor was produced in the same manner as in Example 1. The difference from Example 1 is that the outer electrode (Pt coating film) was added to the outer electrode by using a plating solution in which silver chloride (1 g / l) was added so that Ag was 0.2 mass% with respect to Pt. ) Was formed.

(Comparative Example 3)
An oxygen sensor was produced in the same manner as in Example 1. The difference from Example 1 is that the outer electrode (Pt) is prepared by using a plating solution in which gold sodium sulfite (1 g / l) is added to the outer electrode so that Au is 0.1% by mass with respect to Pt. Film) was formed.

(Comparative Examples 4-7)
An oxygen sensor was produced in the same manner as in Example 1. The difference from Example 1 is the measurement result of EDX, Comparative Example 4 is that Rh is contained by 2.2 mass% with respect to Pt of the outer electrode, and Comparative Example 5 is Pt of the outer electrode. In Comparative Example 6, Nb was contained in an amount of 1.5% by mass relative to Pt of the outer electrode, and the outer electrode of Comparative Example 7 was replaced with Pt. It is the point which made Mo contain 1.5 mass% with respect to.

<Analysis by XPS>
In order to measure the Pt4f photoelectron spectrum of the surface of the outer electrode (Pt electrode) in Examples 1 to 4 and Comparative Example 1, the relationship between the binding energy and the strength by XPS was measured. The result is shown in FIG. From this result, as in Examples 1 to 3, in the electrode added with Ru, Nb, or Mo, the intensity peak (peak value on the higher binding energy side) is shifted to the left side as compared with that in Comparative Example 1. Was. From this result, it can be seen that by adding Ru, Nb, or Mo to Pt in the above-described range and performing a heat treatment, the electron density of Pt is lower than the electron density of Pt itself.

  On the other hand, the intensity peak of the electrode to which Rh was added as in Example 4 was shifted to the right as compared with that of Comparative Example 1. From this result, it can be seen that by adding Rh to Pt in the above-described range, the electron density of Pt was increased more than the electron density of Pt itself.

<Performance evaluation>
A performance evaluation test was performed on the oxygen sensors of Examples 1 to 4 and Comparative Examples 4 to 7. Specifically, during exposure to a rich gas or a lean gas of about several hundred ppm by a model gas characteristic evaluation apparatus so that the gas environment changes from rich to lean in the vicinity of A / F 14.6 (stoichi), several hundreds at an arbitrary timing. The rectangle was switched to ppm lean gas or rich gas, and the time from the switching time until the predetermined sensor voltage was shown was measured.

  FIG. 5 shows the response time of the oxygen sensor when the oxygen sensor of Examples 1 to 4 and the oxygen sensor of Comparative Examples 1 to 3 change from rich gas to lean gas, and the response of the oxygen sensor when change from lean gas to rich gas. It is the figure which showed the relationship with time. 6A is a diagram showing the response time of the oxygen sensor when the rich gas in Comparative Example 1 and Comparative Examples 4 to 7 is changed from lean gas to lean gas. FIG. 6B is a diagram showing the response time of Comparative Example 1 and Comparative Example. It is the figure which showed the response time of the oxygen sensor when it changes from 4-7 lean gas to rich gas.

[Results and Discussion]
As shown from the results of Comparative Example 1 in FIG. 5, the line on the line indicated by the broken line is generally changed by changing the porosity and the ratio of the Pt electrode. Based on this line, the Pt electrodes of the oxygen sensors shown in Examples 2 to 4 contain Mo, Ru, and Nb in an amount of 0.01 to 0.5% by mass with respect to Pt, so that a lean gas atmosphere is changed from a lean gas atmosphere. It is considered that the NO response speed in the exhaust gas was increased and the response speed of the sensor could be increased when switching to.

  On the other hand, when compared with those of Examples 2 to 4, the oxygen sensor shown in Example 1 has an Rh of 0.01 to 0.5 mass relative to Pt of the Pt electrode when the gas is switched from rich gas to lean gas. As a result of increasing the electron density of Pt and reducing the CO response speed in the exhaust gas as a result, the sensor response speed can be moderated.

  On the other hand, as in Comparative Examples 2 and 3, the oxygen sensor in which Ag and Au were added to Pt gave the same result as that of Comparative Example 1.

  Further, as shown in FIGS. 6A and 6B, the comparative examples 4 to 7 are compared in either case when the gas is changed from rich gas to lean gas or when the gas is changed from lean gas to rich gas. It was found that the response speed of the oxygen sensor was further lowered than in Example 1, and the results were clearly different from those in Examples 1 to 4.

(Examples 5 to 8)
Examples 5 to 8 are examples corresponding to Examples 1 to 4, respectively, and oxygen sensors were produced in the same manner as Examples 1 to 4. Each embodiment is different in that an external electrode in which each additive metal (in order, Ru, Nb, Mo, Rh) is added to 0.01 to 0.5% by mass with respect to Pt of the outer electrode. This is that a plurality of oxygen sensors are prepared. And the performance evaluation test mentioned above was done with respect to this. The result is shown in FIG. In FIG. 7, the results of adding no added metal to Pt are also plotted as a reference example.

[Results and Discussion]
As shown in FIG. 7, the outer electrode is formed by adding Ru, Nb, Mo, or Rh to the plating solution in an amount of 0.01 to 0.5% by mass with respect to Pt. It was confirmed that the obtained oxygen sensor had the same characteristics as those shown in the result 1 described above.

  As mentioned above, although it explained in full detail using embodiment of this invention, a concrete structure is not limited to this embodiment and an Example, There exists a design change in the range which does not deviate from the summary of this invention. They are also included in the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Oxygen sensor, 11 ... Oxygen sensor element, 12 ... Cover for to-be-measured gas, 12a ... To-be-measured gas inlet, 21 ... Solid electrolyte body, 22 ... Atmospheric chamber, 23 ... Outer electrode (Pt film), 23a: Closed Pore, 24 ... inner electrode

Claims (3)

An oxygen sensor comprising an oxygen sensor element in which a Pt film is coated as a pair of electrodes on both surfaces of a solid electrolyte body,
In the Pt film formed on at least the surface exposed to the gas to be measured among both surfaces of the solid electrolyte body, at least one metal of Mo, Ru, and Nb is 0.01 to Pt. 0.5 oxygen sensor, characterized in that it is contained by mass%. The Pt film coated on the side exposed to the measurement gas after the deposition with respect to Pt, Mo, Ru, and the plating solution at least one metal is added among Nb, 1000 ° C. to 1200 ° C. The oxygen sensor according to claim 1, wherein the oxygen sensor is heat-treated.   Closed pores are formed at least inside the Pt film formed on the surface exposed to the gas to be measured. The closed pores are filled with a gas having a higher oxygen gas concentration than the atmospheric gas. The oxygen sensor according to claim 1 or 2, wherein the oxygen sensor is provided.

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