SENSOR ELEMENT, GAS SENSOR AND METHOD FOR PRODUCING A SENSOR ELEMENT

Abstract

The aim is to provide a sensor element that suppresses the reduction in catalytic performance of a catalyst supported on a porous carrier. The means for the solution comprises a sensor element including an oxygen ion-conductive solid electrolyte body, a detection electrode provided on one surface of the solid electrolyte body and in contact with a gas to be measured, and a reference electrode provided on the other surface of the solid electrolyte body and in contact with a reference gas. The sensor element further includes a catalyst layer covering the detection electrode. The catalyst layer includes a porous support formed from ceramic particles and catalyst particles deposited on the support and formed from one or more noble metals selected from a group consisting of Pt, Pd, Rh, and Au.The support contains oxide particles bonded to portions of the ceramic particle surfaces, wherein the oxide particles have a different composition than the ceramic particles, are smaller than the ceramic particles in terms of the equivalent diameter of a circle in a cross-section, and are formed from zirconium oxide, aluminum oxide, or lanthanum oxide. The catalyst particles are supported by at least one of the surfaces of the oxide particles and the surfaces of the ceramic particles.

Description

TECHNICAL FIELD

[0001] The present invention relates to a sensor element used in a gas sensor preferably used for detecting the concentration of a specific gas contained in, for example, combustion gas or exhaust gas discharged from a combustion chamber, an internal combustion engine, or the like to the gas sensor, and a method for manufacturing the sensor element. STATE OF THE ART

[0002] As a gas sensor for detecting the oxygen concentration in exhaust gas emitted from a motor vehicle or the like, a gas sensor is known that includes a sensor element in which a detection electrode and a reference electrode are provided on the surface of a tubular or plate-shaped solid electrolyte. Additionally, a porous electrode protective layer is formed on the surface of the detection electrode to prevent poisoning of the detection electrode.

[0003] In addition, a technique has been developed to improve gas detection accuracy and response or to stabilize the sensor output by forming the porous electrode protective layer such that catalyst particles made of a noble metal (e.g., Pt) are supported by the electrode protective layer and by allowing a specific component of the exhaust gas that has passed through the porous protective layer to react with the catalyst particles (Patent Literatures 1 and 2). Patent literature 1: JP 2002 - 071632 A Patent literature 2: JP 2019 - 117 135 A SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0004] However, there was a problem that the catalyst particles aggregate within the porous protective layer due to the heat and atmosphere in the exhaust gas during use of the gas sensor, resulting in coarsening, which reduces the surface area of ​​the catalyst and lowers the catalytic performance.

[0005] In view of the above, it is an object of the present invention to provide a sensor element that suppresses the reduction in catalytic performance of a catalyst supported on a porous carrier, a gas sensor containing the sensor element, and a method for manufacturing the sensor element. SOLUTION TO THE PROBLEM

[0006] To solve the above-described problem, a sensor element of the present invention comprises an oxygen ion-conductive solid electrolyte body, a detection electrode provided on one surface of the solid electrolyte body and in contact with a gas to be measured, and a reference electrode provided on the other surface of the solid electrolyte body and in contact with a reference gas. The sensor element is characterized by further comprising a catalyst layer covering the detection electrode, the catalyst layer including a porous support formed of ceramic particles and catalyst particles supported on the support and formed of one or more noble metals selected from a group consisting of Pt, Pd, Rh, and Au.The support contains oxide particles bonded to portions of the surfaces of the ceramic particles, wherein the oxide particles have a different composition than the ceramic particles, are smaller than the ceramic particles in terms of the equivalent diameter of a circle in a cross-sectional view, and are formed from zirconium oxide, aluminum oxide, or lanthanum oxide. The catalyst particles are supported on at least one of the surfaces of the oxide particles and the surfaces of the ceramic particles.

[0007] Because the catalyst layer support in this sensor element has a structure in which small oxide particles are bonded to portions of the ceramic particle surfaces, the catalyst particles can be prevented from aggregating, which would otherwise result in coarsening due to heat and atmosphere in the exhaust gas during use of the gas sensor. This prevents the surface area of ​​the catalyst particles from decreasing and the catalytic performance from deteriorating.

[0008] Although the reason for these effects is not clear, it is believed that because the oxide particles formed from zirconia, alumina or lanthanum oxide are bonded to the ceramic particles, the surface states (electric potential, etc.) of the ceramic particles and the oxide particles change and the catalyst particles become more strongly bonded to the ceramic particles and the oxide particles.

[0009] A gas sensor of the present invention comprises a sensor element and a housing body holding the sensor element, characterized in that the sensor element is the sensor element according to claim 1.

[0010] A method for manufacturing a sensor element according to a first variant of the present invention is a method for manufacturing the sensor element according to claim 1, characterized in that the support is prepared by applying a slurry for covering the detection electrode and firing the slurry, the slurry containing the ceramic particles and ions of zirconium oxide, aluminum oxide or lanthanum oxide for depositing the oxide particles.

[0011] A method for manufacturing a sensor element according to a second aspect of the present invention is a method for manufacturing the sensor element according to claim 1, characterized in that a porous body to serve as a support is prepared by applying a slurry containing the ceramic particles to cover the detection electrode and firing the slurry, and the porous body is impregnated with a solution containing ions of zirconium oxide, aluminum oxide or lanthanum oxide to deposit the oxide particles and fired. ADVANTAGEOUS EFFECT OF THE INVENTION

[0012] According to the invention, a sensor element is obtained which suppresses the reduction of the catalytic performance of a catalyst supported on a porous carrier. BRIEF DESCRIPTION OF THE CHARACTERS Fig. 1 is a cross-sectional view of a gas sensor (oxygen sensor) according to an embodiment of the present invention, the cross-sectional view being taken in the longitudinal direction of the gas sensor. Fig. 2 is a schematic exploded perspective view of a sensor element. Fig. 3 is an enlarged cross-sectional view of a portion of the sensor element at its front end side. Fig. Figure 4 is a schematic cross-sectional view of the sensor element perpendicular to its axial direction. Fig. Figure 5 is a schematic cross-sectional view of a catalyst layer. Fig. 6 is a schematic view showing a method for measuring the particle size of a ceramic particle. Fig. Figure 7 is a photograph showing an SEM cross-section through the catalyst layer. Fig. Figure 8 is a photograph showing the SEM cross-section through the catalyst layer at an enlarged scale. Fig. Figure 9 is a graph showing the results of gas sensitivity evaluation. Fig. 10 is a photograph showing a cross-sectional SEM image of a catalyst particle (Pt particle) grown in the catalyst layer of Example and Comparative Example. Fig. 11 is a photograph showing a cross-sectional SEM image of a catalyst particle (Pt particle) grown in the catalyst layer of Comparative Example. DESCRIPTION OF THE EMBODIMENT

[0013] An embodiment of the present invention will now be described.

[0014] Fig. 1 is a cross-sectional view of a gas sensor (oxygen sensor) 1 according to an embodiment of the present invention, the cross-sectional view being taken in the longitudinal direction (the direction of an axial line L) of the gas sensor 1. Fig. 2 is a schematic exploded perspective view of a sensor element 100. Fig. 3 is an enlarged cross-sectional view of a portion of the sensor element 100 at its front end side. Fig. 4 is a schematic cross-sectional view of the sensor element 100 perpendicular to the direction of the axial line L.

[0015] As in Fig. 1, the gas sensor 1 includes the sensor element 100, a case body (metal case) 30 which holds the sensor element 100, etc. therein, a protector 24 attached to a front end portion of the case body 30, etc. The sensor element 100 is arranged to extend in the direction of the axial line L.

[0016] In addition, a catalyst layer 20 is provided on the front end side of the sensor element 100 to cover a detection electrode (see Fig. 2).

[0017] As in Fig. As shown in Figure 2, the sensor element 100 includes an oxygen concentration detection cell 130 formed from a solid electrolyte body 105 and a reference electrode 104, as well as a detection electrode 106 formed on opposite sides of the solid electrolyte body 105. The reference electrode 104 has an electrode portion 104a and a reference lead portion 104L extending from the reference electrode portion 104a along the longitudinal direction of the solid electrolyte body 105. The detection electrode 106 has a detection electrode portion 106a and a detection lead portion 106L extending from the detection electrode portion 106a in the longitudinal direction of the solid electrolyte body 105.

[0018] In particular, Fig. 2 the catalyst layer 20 is not shown.

[0019] A protective layer 111 has a porous electrode protecting portion 113a and a reinforcing portion 112. The electrode protecting portion 113a prevents poisoning of the detecting electrode portion 106a by sandwiching the detecting electrode portion 106a between the electrode protecting portion 113a and the solid electrolyte body 105. The reinforcing portion 112 protects the solid electrolyte body 105, and at the same time, the detecting lead portion 106L is sandwiched between the reinforcing portion 112 and the solid electrolyte body 105. Specifically, the sensor element 100 of the present embodiment constitutes a so-called oxygen concentration electromotive gas sensor (λ sensor) capable of detecting the oxygen concentration using the voltage (electromotive force) generated between the electrodes of the oxygen concentration detecting cell 130.

[0020] Meanwhile, a lower surface layer 103 and an atmosphere introduction hole layer 107 are stacked on a lower surface of the reference electrode 104, so that the reference electrode 104 is sandwiched between the solid electrolyte body 105 and the lower surface layer 103 and the atmosphere introduction hole layer 107. The atmosphere introduction hole layer 107 has a generally square C-like shape with an opening at its rear end. An internal space surrounded by the solid electrolyte body 105, the atmosphere introduction hole layer 107, and the lower surface layer 103 forms an atmosphere introduction hole 107h. The reference electrode 104 is exposed to the atmosphere (reference gas) introduced into this hole 107h.

[0021] A stack of the lower surface layer 103, the atmosphere introduction hole layer 107, the reference electrode 104, the solid electrolyte body 105, the detection electrode 106, and the protective layer 111 forms an element body 300. In the present embodiment, the element body 300 has a plate-like shape.

[0022] One end of the reference lead portion 104L is electrically connected to a sensing element-side contact surface 121 on the solid electrolyte body 105 via a conductor formed in a through-hole 105a provided in the solid electrolyte body 105. At this time, the protective layer 111 is shorter in the axial direction L than the end of the sensing lead portion 106L, so that the end of the sensing lead portion 106L protrudes from the rear end of the protective layer 111 and appears on the upper surface. The end of the sensing lead portion 106L is connected to an external terminal (not shown) for connection to an external circuit.

[0023] The solid electrolyte body 105 is particularly oxygen ion-conductive and can contain, for example, a partially stabilized zirconium oxide (YSZ) solid solution as a main component, which is prepared by adding yttrium oxide as a stabilizer. The main component is defined here as a component whose amount amounts to more than 50 percent by mass of the solid electrolyte body 3s.

[0024] For example, the reference electrode 104 and the detection electrode 106 are each formed mainly of Pt. The term "mainly of Pt" means that "the component whose amount is more than 50 mass percent of the electrode is Pt."

[0025] Each of the lower surface layer 103, the protective layer 111, and the atmosphere introduction hole layer 107 may be formed of an insulating material such as alumina. The electrode protective portion 113a may be a porous body formed mainly of zirconium oxide. The porous body may be formed, for example, by bonding particles of one or more ceramic materials selected from the group consisting of alumina, spinel, zirconium oxide, mullite, zircon, and cordierite through firing or the like. When a slurry containing these particles is fired, an organic or inorganic binder present in the gaps between the ceramic particles and in the slurry burns and disappears, forming pores in the skeleton of the layer.

[0026] Back to Fig. 1: The housing body 30 is made of SUS430 and has an externally threaded portion 31 for attaching the gas sensor to an exhaust pipe and a hexagonal portion 32 into which a fastening tool engages when the gas sensor is attached to the exhaust pipe. The housing body 30 has a housing-side stepped portion 33 that protrudes radially inward. The housing-side stepped portion 33 supports a metal holder 34 that serves to accommodate the sensor element 100.

[0027] Inside the metal holder 34, a ceramic holder 35 and talc 36 are arranged in this order from the front end. The talc 36 is formed of a first talc 37 arranged inside the metal holder 34 and a second talc 38 arranged at the rear end of the metal holder 34.

[0028] The first talc 37 is compressed and compacted inside the metal holder 34, thereby securing the sensor element 100 to the metal holder 34. The second talc 38 is compressed and packed into the housing body 30, creating a seal between the outer surface of the sensor element 100 and the inner surface of the housing body 30.

[0029] A sleeve 39 made of alumina is disposed at the rear end of the second talc 38. This sleeve 39 is formed in a stepped cylindrical shape and has an axial hole 39a extending along the axial line, and the sensor element 100 is inserted into the axial hole 39a. A crimping portion 30a on the rear end of the housing body 30 is bent inward, so that the sleeve 39 is pressed against the front end of the housing body 30 via a ring portion 40 made of stainless steel.

[0030] The protector 24, formed of a metal and having a plurality of gas inlet ports 24a, is welded to the outer periphery of a front end portion of the case body 30 to cover a front end portion of the sensor element 100 protruding from the front end of the case body 30. This protector 24 has a dual structure including a cylindrical outer protector 41 with a closed end disposed on the outside and having a uniform outer diameter, and a cylindrical inner protector 42 with a closed end disposed on the inside and formed such that its rear end portion 42a has a larger outer diameter than its front end portion 42b.

[0031] A front end portion of an outer tube 25 formed of SUS430 is fitted into a rear end portion of the housing body 30. A front end portion 25a of the outer tube 25, which is enlarged in diameter on the front end side, is fixed to the housing body 30, for example, by laser welding. A partition member 50 is disposed in a rear end portion of the outer tube 25, and a holding member 51 is provided in the gap between the partition member 50 and the outer tube 25. This holding member 51 engages a protruding portion 50a, which will be described later, of the partition member 50. When the outer tube 25 is crimped, the holding member 51 is fixed by the outer tube 25 and the partition member 50.

[0032] An insertion hole 50b is formed in the separating element 50, into which connecting wires 11 and 12 (in Fig. 1, the lead wire 12 is not shown because it is hidden behind the lead wire 11) for the sensor element 100 is inserted so that it extends from the front end to the rear end. Connection terminals 16 for connecting the lead wires 11 and 12 to the sensing-element-side contact surfaces 121 of the sensor element 100 are accommodated in the insertion hole 50b. The lead wires 11 and 12 are externally connected to a connector (not shown). Electrical signals are transmitted between the lead wires 11 and 12 and an external device, such as a control device, via the connector (for input and output of the electrical signals). Although not shown in detail, each of the lead wires 11 and 12 has a structure in which a conductive wire is covered with an insulating resin coating.

[0033] On the rear end side of the separator 50, a nearly cylindrical rubber cap 52 is arranged to close a rear opening 25b of the outer tube 25. This rubber cap 52 is inserted into the rear end of the outer tube 25 and fixed to the outer tube 25 by radially crimping the outer circumference of the outer tube 25. Insertion holes 52a are formed in the rubber cap 52, into which the lead wires 11 to 15 are inserted so that they extend from the front end to the rear end.

[0034] Next, the catalyst layer 20 will be described. As in Fig. 3 and Fig. 4, the catalyst layer 20 is a porous layer covering the entire circumference of a front end portion of the sensor element 100 (the element body 300).

[0035] The catalyst layer 20 is formed to include a front end surface of the sensor element 100 (the element body 300) and extend along the direction of the axial line L toward the rear end side. As shown in Fig. As shown in Figure 4, the catalyst layer 20 is formed to completely surround the four surfaces (i.e., front and rear surfaces and opposite side surfaces) of the sensor element 100 (the element body 300). Viewed in the direction of the axial line L, the catalyst layer 20 covers at least a region of the sensor element 100 (the element body 300) including the reference electrode portion 104a and the detection electrode portion 106a (this region constitutes a detection portion), and extends from this region to the rear end.

[0036] The sensor element 100 may be exposed to a poisonous substance such as silicon and phosphorus contained in the exhaust gas, and water droplets in the exhaust gas may adhere to the sensor element 100. Since the outer surface of the sensor element 100 is covered with the catalyst layer 20, it is possible to trap the poisonous substance and prevent water droplets from coming into direct contact with the sensor element 100.

[0037] As in Fig. 5, the catalyst layer 20 contains a porous support 23 formed from ceramic particles and catalyst particles 60 applied to the support 23, which are formed from one or more noble metals from the group Pt, Pd, Rh and Au.

[0038] The catalyst particles 60 can improve the accuracy and response of gas detection and stabilize the sensor output by reacting with a specific component of the exhaust gas that has passed through the catalyst layer 20 (combustion of an unburned gas component). For example, the catalyst particles 60 can improve the response of the gas sensor in a high-velocity gas flow environment.

[0039] This will be briefly described here. When the gas flow rate becomes high, the unburned gas cannot combust sufficiently at the sensing electrode 106 and remains in the catalyst layer 20. As the electrode reaction approaches equilibrium, for example, CO gas (a type of unburned gas) remaining in the catalyst layer 20 reaches the sensing electrode 106 and reacts with it. In this case, the sensor output may not reflect the actual gas concentration.

[0040] To solve this problem, the catalyst particles 60 are introduced into the catalyst layer 20. Since a portion of the unburned gas reacts with the catalyst particles 60 and burns in the catalyst layer 20, it is possible to improve the response of the gas sensor in an environment with high gas flow velocity.

[0041] Of course, the effect achieved by incorporating the catalyst particles 60 into the catalyst layer 20 is not limited thereto.

[0042] Incidentally, the catalyst particles 60 aggregate within the catalyst layer 20 due to heat and atmosphere in the exhaust gas during use of the gas sensor 1, resulting in coarsening, which reduces the surface area of ​​the catalyst and reduces the catalytic performance.

[0043] The present invention has therefore made it possible to suppress the reduction in the catalytic performance of the catalyst particles 60 supported on the carrier 23 by configuring the carrier 23 as follows.

[0044] As in Fig. As shown in Figure 5, the support 23 has a structure in which oxide particles 22, which have a different composition than the ceramic particles 21 and are smaller than the ceramic particles 21, are bonded to portions of the surfaces of the ceramic particles 21. In this way, a portion of the surface of each ceramic particle 21 is exposed, and the remaining portion of the surface is covered by the oxide particles 22.

[0045] The catalyst particles 60 are formed scattered on at least one of the surfaces of the oxide particles 22 and the surfaces of the ceramic particles 21 forming the carrier 23.

[0046] The ceramic particles 21 preferably contain at least one or more species selected from, for example, alumina, alumina-magnesia spinel, zirconia and titania, and an example of a preferred ceramic material is alumina-magnesia spinel.

[0047] The oxide particles 22 are formed from zirconium oxide, alumina, or lanthanum oxide. Although zirconium oxide has a composition of, for example, ZrO2, it may contain a non-stoichiometric compound of Zr and oxygen, etc.

[0048] When the support 23 has a structure in which the small oxide particles 22 are bonded to portions of the surfaces of the ceramic particles 21, the catalyst particles 60 can be prevented from aggregating, which would otherwise result in coarsening due to heating and atmospheric exposure in the exhaust gas during use of the gas sensor. This can prevent the surface area of ​​the catalyst particles 60 from decreasing and the catalytic performance from decreasing.

[0049] Although the reason for these effects is not clear, it is considered that when the oxide particles 22 formed of zirconia, alumina, or lanthanum oxide are bonded to the ceramic particles 21, the surface states (electric potential, etc.) of the ceramic particles 21 and the oxide particles 22 change, and the catalyst particles 60 are more strongly bonded to the ceramic particles 21 and the oxide particles 22.

[0050] The ceramic particles 21 and the oxide particles 22 can be distinguished from each other by performing an elemental analysis of a cross-sectional sample of the catalyst layer 20 with an EPMA (electron beam microanalyzer) or EDS (energy dispersive X-ray spectrometry).

[0051] The particle sizes of the ceramic particles 21 and the oxide particles 22 are determined by obtaining the individual circle-equivalent diameters of the ceramic particles 21 and the oxide particles 22 identified by elemental analysis in the cross-sectional sample of the catalyst layer 20 (the EPMA image, EDS image, etc. described above).

[0052] The comparison between the particle sizes of the ceramic particles 21 and the oxide particles 22 is carried out for the oxide particles 22 that are bonded to the surfaces of three or more ceramic particles 21 in the sample in cross-section. Fig. 5, in the case marked E, that a first oxide particle 22 is bonded to the surface of a ceramic particle 21 and a second oxide particle 22 is bonded to the surface of the first oxide particle 22 (without mediation of the ceramic particle 21), the second oxide particle 22 is excluded.

[0053] In some cases, such as Fig. As shown in Figure 5, individual ceramic particles 21 are bonded and united together as a result of sintering, making the boundary AB between them unclear.

[0054] Therefore, the Fig. 6, where a ceramic particle 21x and an adjacent ceramic particle 21y are considered to be bonded as a result of sintering, the boundary therebetween is determined as follows.

[0055] First, in the case where the contour P of the ceramic particle 21x shows that the ceramic particle 21x narrows and forms a constricted section between points A and B, a direction parallel to a straight line C1 connecting points A and B is defined as the direction L. In this case, in the contours of the ceramic particle 21x and the ceramic particle 21y that are connected to each other, the lengths of the longest lines parallel to the direction L are represented by Lx and Ly, respectively. If the length of the straight line C1 is smaller than both lengths Lx and Ly, it is considered that the two ceramic particles 21x and 21y are connected to each other by sintering in the area between points A and B, and the straight line C1 is regarded as the boundary between the two ceramic particles 21x and 21y.

[0056] In the case where a portion of the ceramic particle 21x lies outside the field of view described above, the outer edge C2 of the field of view is used as a portion of the contour P of the ceramic particle 21x.

[0057] In the case where the outermost contour P of the ceramic particle 21x overlaps with oxide particles 22x and 22z, the contours P1 and P2 of the boundaries between the ceramic particle 21x and the oxide particles 22x and 22z are used as sections of the contour P of the ceramic particle 21x. Oxide particles 22y located within the contour P of the ceramic particle 21x are ignored.

[0058] Accordingly, the straight lines C1 and C2 are considered as parts of the contour P of the ceramic particle 21x, and the area surrounded by the entire contour P (a hatched section in Fig. 6), is considered as the circle equivalent diameter of the ceramic particle 21x.

[0059] A method for manufacturing a sensor element according to the embodiment of the present invention will be described below. In this method for manufacturing a sensor element, the support 23 of the catalyst layer 20 is formed as follows. A slurry containing the ceramic particles 21 and ions of zirconium oxide, alumina, or lanthanum oxide, which become the oxide particles 22, is applied to the surface of a front end portion of the sensor element 100 to cover the detection electrode 106 (the detection electrode portion 106a), and fired.

[0060] The ions that become the oxide particles are contained, for example, in an aqueous solution of oxyacetatozirconium, which is a complex. The slurry can be prepared by mixing this aqueous solution, the ceramic particles 21, a binder, and water or a solvent such as PGA. Upon firing this slurry, the ions form oxide particles 22 by deposition, which are bonded to portions of the surfaces of the ceramic particles 21, thereby obtaining the support 23.

[0061] In an alternative method, for example, a porous layer formed from the ceramic particles 21 is impregnated with a solution containing Zr ions (e.g., zirconium nitrate solution) and heated. This causes the oxide particles 22 to be deposited on the ceramic particles 21 and bonded to portions of the surfaces of the ceramic particles 21, thereby obtaining the support 23.

[0062] When the support 23 obtained by firing is immersed in a solution containing catalyst ions (e.g., dinitrodiamine Pt nitrate solution) and heated, tiny catalyst particles 60 are deposited on the surface of the support.

[0063] The present invention is not limited to the above-described embodiment. The sensor element only needs to include a solid electrolyte body, a detection electrode, and a reference electrode and can be applied to the oxygen sensor (oxygen sensor element) of the present embodiment. However, the present invention is not limited to these applications and includes various modifications and equivalents that fall within the spirit and scope of the present invention.

[0064] The present invention can be applied, for example, to a full-range oxygen sensor with an oxygen pumping cell, a NOx sensor (NOx sensor element) for detecting the NOx concentration in a gas to be measured, and an HC sensor (HC sensor element) for detecting the HC concentration. The sensor element can be tubular and can be a binary sensor or a linear sensor.

[0065] The gas sensor can be equipped with a heater that generates heat when energy is supplied. Example<Bewertung der Gasempfindlichkeit>

[0066] The Fig. 1 and Fig. 2, the plate-shaped sensor element (oxygen sensor element) 100 was manufactured.

[0067] A slurry was prepared containing alumina particles (the ceramic particles 21), an aqueous solution of oxyacetatozirconium (complex) containing a zirconium oxide ion structure (for depositing the oxide particles 22), a binder, and water. The prepared slurry was applied to the surface of a front end portion of the sensor element 100 to cover the detection electrode 106 (the detection electrode portion 106a) and fired. Thus, the support 23 of the catalyst layer 20 was obtained. The amount of the oxide particles 22 (zirconium oxide) was set to 5 mass percent of the support 23.

[0068] Furthermore, the support 23 obtained by firing was immersed in a catalyst solution containing Pt ions (e.g., dinitrodiamine Pt nitrate solution) and heated. This was used as an example.

[0069] As a comparative example, the support 23 of the catalyst layer 20 was formed in the same manner as described above, except that the slurry did not contain an aqueous solution containing zirconium oxide ions, and the support 23 obtained by firing was immersed in a catalyst solution containing Pt ions (e.g., dinitrodiamine Pt nitrate solution) and heated.

[0070] Next, the sensor element 100 described above was incorporated into the gas sensor 1, and the gas sensitivity was evaluated based on the difference between the sensor outputs for two predetermined different gas compositions (a gas in which H2 was predominant and a gas in which CO was predominant). Gas sensitivity refers to the degree of influence of the composition of the gas to be measured on the sensor output for the component to be measured, and the smaller the numerical value, the better the gas sensitivity.

[0071] Fig. Figures 7 to 11 show the results obtained.

[0072] Fig. 7 and Fig. 8 shows SEM cross-sectional images of the catalyst layer 20.

[0073] Fig. 9 shows the results of the gas sensitivity evaluation, and Fig. 10 and Fig. 11 show cross-sectional SEM images of grown catalyst particles 60 (Pt particles) in the catalyst layer 20 in the example and the comparative example, respectively.

[0074] Fig. 7 and Fig. 8 show that small particles of zirconium oxide (the oxide particles 22) were deposited on portions of the surfaces of alumina particles (the ceramic particles 21), and also show that, in the present example, tiny Pt particles (catalyst particles 60) were deposited on both the surfaces of the ceramic particles 21 and the surfaces of the oxide particles 22.

[0075] As in Fig. 9, in the case of the example using the carrier 23 in which small zirconia particles (the oxide particles 22) were deposited on portions of the surfaces of alumina particles (the ceramic particles 21), the gas sensitivity was satisfactory over a long period of time.

[0076] In the example where only aluminum oxide particles (the ceramic particles 21) were used as carrier 23, however, the gas sensitivity deteriorated over time.

[0077] As in Fig. 10 and Fig. 11, it was found that in the case of the example, the particle size of the grown Pt particles was at most about 20 nm and that in the case of the comparative example, the Pt particles grew to about 50 nm. LIST OF REFERENCE SYMBOLS 1 gas sensor 20 catalyst layer 21 ceramic particles 22 oxide particles 23 carriers 30 housing bodies 60 catalyst particles 100 sensor elements 104 Reference electrode 106 Detection electrode 105 solid electrolyte bodies QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] JP 2002 - 071632 A

[0003] JP 2019 - 117 135 A

[0003]

Claims

[1] A sensor element comprising: an oxygen ion conductive solid electrolyte body, a detection electrode provided on one surface of the solid electrolyte body and coming into contact with a gas to be measured, and a reference electrode provided on the other surface of the solid electrolyte body and coming into contact with a reference gas, where the sensor element characterized by is that it further comprises a catalyst layer covering the detection electrode, the catalyst layer comprising a porous support formed from ceramic particles and catalyst particles carried by the support formed from one or more noble metals selected from a group consisting of Pt, Pd, Rh and Au, wherein the support contains oxide particles bonded to portions of surfaces of the ceramic particles, wherein the oxide particles have a composition different from the composition of the ceramic particles, are smaller than the ceramic particles in terms of the circle-equivalent diameter in a cross-sectional image, and are formed from zirconium oxide, aluminum oxide, or lanthanum oxide, and the catalyst particles are supported by at least one of the surfaces of the oxide particles and the surfaces of the ceramic particles. [2] Gas sensor comprising a sensor element and a housing body holding the sensor element, characterized by that the sensor element is the sensor element according to claim 1. [3] Method for producing the sensor element according to claim 1, characterized bythat the carrier is prepared by applying a slurry to cover the detection electrode and firing the slurry, the slurry containing the ceramic particles and ions of zirconium oxide, aluminum oxide or lanthanum oxide to deposit the oxide particles. [4] Method for producing the sensor element according to claim 1, characterized by that a porous body to serve as a support is prepared by applying a slurry containing the ceramic particles to cover the detection electrode and firing the slurry, and the porous body is impregnated with a solution containing ions of zirconium oxide, aluminum oxide, or lanthanum oxide to deposit the oxide particles and fired.

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