POROUS CERAMIC STRUCTURE

Abstract

Porous ceramic structure (1), comprising: a porous structural body consisting primarily of cordierite; and Manganese and tungsten, which are firmly bonded to the structural body, whereby the manganese and the tungsten are components of a metal oxide particle (2) that is firmly connected to the interior of a pore (121) of the structural body, and the metal oxide particle (2) has: a firmly connected section (21) located within the structural body; and a protrusion (22) adjacent to the firmly connected section (21) and extending into the pore.

Description

[0001] This application claims priority over Japanese patent application number JP 2019-047594 (filed on March 14, 2019). Technical field

[0002] The present invention relates to a ceramic structure. State of the art

[0003] Japanese patent application no. JP 2017-186 220 A (Document 1) proposes cerium dioxide particles containing a transition metal oxide with iron or manganese in or on them. It is suggested that such cerium dioxide particles could be used as an oxidation catalyst, for example, in a diesel particulate filter (DPF) that incorporates a diesel oxidation catalyst (DOC) and a catalyzed soot filter (CSF).

[0004] Japanese patent applications JP 2018-30105A (Document 2) and JP 2017-171543A (Document 3) propose methods for applying a sufficient quantity of a catalyst to porous ceramic structures used in DPFs or other devices to maintain catalytic activity. In these porous ceramic structures, portions of the cerium dioxide particles are incorporated into the structure, while other portions are exposed to the pore surfaces. In the porous ceramic structure described in Document 2, the portions of the cerium dioxide particles exposed to the pore surfaces contain iron oxide. In the porous ceramic structure described in Document 3, the portions of the cerium dioxide particles exposed to the pore surfaces carry fine catalyst particles of a platinum group element.

[0005] The porous ceramic structures used in DPFs or other devices must both reduce pressure loss and improve catalytic performance.

[0006] Further state of the art is known from JP 2006 - 68 661 A, JP 6 329 245 B2 and DE 10 2017 006 390 A1. Brief description of the invention

[0007] The present invention relates to a porous ceramic structure, and it is an objective of the present invention to provide a porous ceramic structure with low pressure loss and high catalytic performance.

[0008] A porous ceramic structure according to a preferred embodiment of the present invention comprises a porous structural body consisting predominantly of cordierite, and manganese and tungsten which are firmly bonded to the structural body.

[0009] Accordingly, it is possible to provide a porous ceramic structure with low pressure loss and high catalytic performance.

[0010] The manganese and tungsten are components of a metal oxide particle that is firmly bonded to the interior of a pore within the structural body. The metal oxide particle has a firmly bonded section located inside the structural body and a protrusion adjacent to the firmly bonded section and extending into the pore.

[0011] Preferably, the ceramic structure has a tungsten content greater than or equal to 0.1 wt% and less than or equal to 1.5 wt% with respect to WO3.

[0012] Preferably, the ceramic structure has a manganese content greater than or equal to 0.5 wt% and less than or equal to 3.0 wt% with respect to WO2.

[0013] Preferably, the metal oxide particle comprises a MnWO4 particle.

[0014] Preferably, the ceramic structure has a MnWO4 content greater than or equal to 0.2 wt% and less than or equal to 2.0 wt%.

[0015] Preferably, the MnWO4 particle has an aspect ratio greater than or equal to 5.5.

[0016] Preferably, the MnWO4 particle is particulate or fibrous. The MnWO4 particle has the firmly bonded section located at a grain boundary of cordierite crystals in the structural body, and the projection extending from the grain boundary into the pore.

[0017] These and other items, features, aspects and advantages of the present invention will become apparent from the following detailed description of the present invention in conjunction with the accompanying drawings. Brief description of the drawings Fig. Figure 1 illustrates the structure of an exhaust gas purification system; Fig. Figure 2 illustrates a porous ceramic structure; Fig. Figure 3 is a cross-sectional view of the porous ceramic structure; Fig. Figure 4 is an enlarged view of part of a partition wall; Fig. Figure 5 shows a SEM image of the surface of a pore in a honeycomb structure; Fig. Figure 6 shows a SEM image of the surface of a pore in the honeycomb structure; Fig. Figure 7 is a cross-sectional view of an area near a metal oxide particle; Fig. Figure 8 is a flowchart of the process for producing the porous ceramic structure; Fig. Figure 9 shows an enlarged SEM image of the surface of a pore in the porous ceramic structure according to an example; and Fig. Figure 10 shows a SEM image of the surface of a porous ceramic structure according to a comparative example. Description of the embodiments

[0018] Fig.Figure 1 illustrates the structure of an exhaust gas purification system 8. The exhaust gas purification system 8 is designed to clean exhaust gas from an engine. The exhaust gas purification system 8 comprises a diesel particulate filter (DPF) 81, a selective catalytic reduction (SCR) catalyst 85, and a urea injection system 86. The DPF 81, the urea injection system 86, and the catalyst 85 are arranged in this order in the direction of exhaust gas flow.

[0019] The DPF 81 comprises a diesel oxidation catalyst (DOC) 82 and a catalytic soot filter (CSF) 83. The DOC 82 comprises a honeycomb structure, the interior of which is divided into a multitude of cells by a partition, and a precious metal catalyst supported by the partition. The CSF 83 comprises a honeycomb structure similar to the one described above and a metal oxidation catalyst supported by a partition within the honeycomb structure. The structure of the CSF 83 will be described in more detail later. The urea injection system 86 is arranged in an exhaust gas path between the DPF 81 and the SCR catalyst 85. The SCR catalyst 85 comprises a honeycomb structure similar to the one described above and an SCR catalyst supported by a partition within the honeycomb structure.

[0020] The exhaust gas emitted by the engine flows into the DOC 82 of the DPF 81. The exhaust gas contains nitrogen monoxide (NO), oxygen (O2), and nitrogen (N2) and undergoes the reactions described in Equations 1 and 2 below within the DOC 82. The reaction described by Equation 1 produces nitrogen dioxide (NO2). Equation 2 below includes a soluble organic fraction (SOF) in the particulate matter of the exhaust gas. 2 NO + O2 = 2 NO2 (1) SOF + O2 = CO, CO2, H2O (2)

[0021] The CSF 83 collects carbon (soot) contained in the exhaust gas. In the CSF 83, the soot and NO2 undergo reactions (combustion reactions) described by equations 3, 4, and 5 below, resulting in the production of NO from NO2. C (soot) + 2 NO2 = CO2 + 2 NO ) (3 C (soot) + NO2 = CO + NO (4) C (soot) + ½ O2 + NO2 = CO2 + NO (5)

[0022] The urea injection system 86 mixes urea into the exhaust gas emitted by the CSF 83, and the exhaust gas, which contains ammonia (NH3) produced by the breakdown of the urea, flows into the SCR catalyst 85. In the SCR catalyst 85, the reactions described by equations 6, 7, and 8 below take place, such that NO contained in the exhaust gas is reduced to 10⁻⁶. x is being cleaned. 4NO + 4NH3 + O2 = 4N2 + 6H2O (6) NO + NO2 + 2 NH3 = 2 N2 + 3 H2O (7) 6 NO2 + 8 NH3 = 7 N2 + 12 H2O (8)

[0023] The reaction described by Equation 7 is called the fast SCR reaction and proceeds at a higher reaction rate than the reactions described by Equations 6 and 8. To improve the efficiency of the reactions taking place in the SCR catalyst 85 according to Equation 7, the ratio between the amounts of NO and NO2 flowing into the SCR catalyst 85 must be 1:1. Meanwhile, the CSF 83 consumes a large amount of NO2 during the combustion of soot and produces NO, as previously described by Equations 3, 4, and 5.

[0024] Against this background, the exhaust gas purification system 8 according to the present invention comprises, as CSF 83, a porous ceramic structure (described below) which includes an oxidation catalyst. The porous ceramic structure oxidizes a portion of the NO to generate NO2, or converts NO to NO2. This allows the ratio of the amounts of NO and NO2 flowing into the SCR catalyst 85 to be brought closer to 1:1, thus improving the efficiency of the reactions taking place in the SCR catalyst 85.

[0025] If a certain amount or more soot is deposited on the CSF 83, the exhaust gas purification system 8 carries out a soot combustion process (or regeneration). In this case, too, the reactions (combustion reactions) described by equations 3, 4, and 5 take place in the CSF 83. If a large amount of carbon monoxide (CO) produced in the reaction flows into the SCR catalyst 85, the efficiency of the NOₓ reduction may be reduced. x-Reduce the cleaning of the SCR catalyst 85. The same applies if hydrocarbons (HC) in the fuel supplied to the CSF 83 flow into the SCR catalyst 85 in large quantities during the soot combustion process.

[0026] Since the porous ceramic structure with the aforementioned oxidation catalyst as CSF 83 is provided in the exhaust gas purification system 8 according to the present invention, a portion of the CO is oxidized to carbon dioxide (CO2), and a portion of the HC is oxidized to CO2 and H2O. This suppresses the flow of CO, HC, and other substances into the SCR catalyst 85 and prevents a decrease in NO. x -Cleaning efficiency of the SCR catalyst 85.

[0027] Fig. 2 and Fig. Figure 3 shows simplified diagrams of a porous ceramic structure used as CSF 83 (see Figure 1). Fig. 1) The porous ceramic structure 1 is a tubular component that is long in one direction, and Fig.Figure 2 illustrates the end surface on one side of the porous ceramic structure 1 in the longitudinal direction. Fig. Figure 3 is a sectional view of the porous ceramic structure 1 and illustrates part of a section along the longitudinal direction of the porous ceramic structure 1.

[0028] The porous ceramic structure 1 comprises a honeycomb structure 10, which serves as the porous structural body, and an oxidation catalyst that is rigidly bonded to the honeycomb structure 10. The oxidation catalyst preferably consists of metal oxide particles (e.g., fine particles consisting predominantly of a metal oxide) that are rigidly bonded to the honeycomb structure 10. The metal oxide particles contain manganese (Mn) and tungsten (W) elements as constituents. In addition to the aforementioned metal oxide particles, other fine particles, different from the metal oxide particles, can also be rigidly bonded to the honeycomb structure 10 of the porous ceramic structure 1.

[0029] The honeycomb structure 10 comprises a tubular outer wall 11 and a partition 12. The tubular outer wall 11 has a longitudinal shape. Its cross-sectional shape perpendicular to the longitudinal direction can be, for example, circular, polygonal, or any other shape. The partition 12 is located inside the tubular outer wall 11 and divides the interior into a plurality of cells 13. The honeycomb structure 10 is a cellular structure whose interior is divided into a plurality of cells 13 by the partition 12. The tubular outer wall 11 and the partition 12 are made of a porous material. As described below, the exhaust gas flows through pores of the partition wall 12. To increase the strength of the porous ceramic structure 1, the thickness of the partition wall 12 is, for example, greater than or equal to 50 micrometers (µm), preferably greater than or equal to 100 µm and particularly preferably greater than or equal to 150 µm.To reduce the pressure loss in the partition 12, the thickness of the partition 12 is, for example, less than or equal to 500 µm and preferably less than or equal to 450 µm.

[0030] Each cell 13 is a space extending longitudinally. The cross-sectional shapes of the cells 13 perpendicular to the longitudinal direction can be, for example, polygonal (e.g., triangular, square, pentagonal, or hexagonal), circular, or any other shape. The cells 13 typically have the same cross-sectional shape. Alternatively, the cells 13 can have cells 13 with different cross-sectional shapes. To improve the oxidation performance of the porous ceramic structure 1, the cell density is, for example, greater than or equal to 8 cells per square centimeter (cells / cm²). 2 ) and preferably greater than or equal to 15 cells / cm² 2 To reduce pressure loss, the cell density is, for example, less than or equal to 95 cells / cm². 2and preferably less than or equal to 78 cells / cm² 2 .

[0031] In the porous ceramic structure 1 used in the CSF 83, the exhaust gas from the DOC 82 flows using one end longitudinally along the honeycomb structure 10 as an inlet and the other end as an outlet. A predetermined number of cells 13 are equipped with a seal 14 at their end on the inlet side, and the remaining cells 13 are each equipped with a seal 14 at their end on the outlet side. Thus, the exhaust gas flowing into the honeycomb structure 10 passes from the cells 13 with unsealed inlets through the partition 12 into the cells 13 with unsealed outlets (see arrows A1 in the figure). Fig.3) At this point, the exhaust gas is oxidized by the metal oxide particles (i.e., oxidation catalyst) on the partition 12. Preferably, the seals 14 are provided alternately at the inlet and outlet ends of the honeycomb structure 10 in the direction of the arrangement of the cells 13.

[0032] The honeycomb structure 10 consists predominantly of cordierite (2MgO·2Al2O3·5SiO2). The honeycomb structure 10 can consist solely of cordierite or contain materials other than cordierite (e.g., metal or a ceramic other than cordierite). The cordierite content in the honeycomb structure 10 is, for example, greater than or equal to 75 wt% and preferably less than or equal to 80 wt%. In the present embodiment, the honeycomb structure 10 consists essentially only of cordierite.

[0033] Fig.Figure 4 is an enlarged view of part of a partition 12 in the porous ceramic structure 1. The honeycomb structure 10 has a large number of cavities for gas (hereinafter referred to as "pores 121"). The aforementioned metal oxide particles 2 are firmly bound to the inner surfaces of the pores 121 (or the surfaces of the pores) in the honeycomb structure 10. Other particles (hereinafter referred to as "fine additive particles"), which differ from the metal oxide particles 2, may also be firmly bound to the inner surfaces of the pores 121. The fine additive particles are, for example, fine particles containing cerium (Ce) and / or iron (Fe) elements. The fine additive particles are, for example, fine iron oxide (Fe₂O₃) particles and / or cerium dioxide (CeO₂) particles. Fig.Figure 4 shows a schematic representation of the metal oxide particles 2 and the fine additive particles on the surface of the pores 121 as cross-hatching, without distinguishing between the two. The metal oxide particles 2 and the fine additive particles do not necessarily have to cover the entire surface of the pores 121.

[0034] To reduce the pressure loss in the porous ceramic structure 1, the open porosity of the partition 12 in the honeycomb structure 10 is, for example, greater than or equal to 25%, preferably greater than or equal to 30%, and particularly preferably greater than or equal to 35%. To ensure the strength of the porous ceramic structure 1, the open porosity of the partition 12 is, for example, less than or equal to 70%, and preferably less than or equal to 65%. The open porosity can be measured, for example, using the Archimedes method with deionized water as the medium.

[0035] The mean pore diameter of the partition 12 in the honeycomb structure 10 is, for example, greater than or equal to 5 µm and preferably greater than or equal to 8 µm. As the mean pore diameter of the partition 12 increases with the open porosity, the pressure drop in the porous ceramic structure 1 decreases. To improve the oxidation performance of the porous ceramic structure 1, the mean pore diameter of the honeycomb structure 10 is, for example, less than or equal to 40 µm, preferably less than or equal to 30 µm, and particularly preferably less than or equal to 25 µm. The mean pore diameter can be measured, for example, by mercury injection (according to JIS R1655). Depending on the design of the porous ceramic structure 1, the seals 14 can be omitted, and the metal oxide particles 2 can be arranged in a layer on the surface of the cells 13.

[0036] Fig.Figure 5 shows a scanning electron microscope (SEM) image of the surface of a pore 121 in the honeycomb structure 10. In the porous ceramic structure 1, a multitude of essentially particulate or fibrous metal oxide particles 2 are tightly bound to the surface (i.e., the inner surface) of the pore 121 in the honeycomb structure 10. The metal oxide particles 2 are tightly bound to the grain boundaries of a large number of essentially rectangular parallelepiped cordierite crystals 3 that form the pore 121 and grow from the surface of the pore 121 and project into the space within the pore 121.

[0037] The white metal oxide particles 2 are located approximately in the middle of Fig. 5 are file particles made of manganese tungstate (MnWO4). In the case of the Fig.In the example shown in Figure 5, the metal oxide particles 2 are essentially fibrous with anisotropy. The principal axes of the fine MnWO4 particles are preferably in the range of 50 nanometers (nm) to 5000 nm, and particularly preferably in the range of 1000 nm to 4000 nm. The minor axes of the fine MnWO4 particles are preferably in the range of 50 nm to 1000 nm, and particularly preferably in the range of 150 nm to 500 nm. The aspect ratio of the fine MnWO4 particles is preferably greater than or equal to 1.5, and particularly preferably greater than or equal to 5.5. Although no particular upper limit is specified for the aspect ratio, it is, for example, less than or equal to 100, and preferably less than or equal to 10.

[0038] The principal axes, minor axes, mean particle diameter, and aspect ratio of the metal oxide particles 2 are determined by a procedure described below. First, the porous ceramic structure 1 is processed with a cross-section polisher (CP) to expose a polished cross-section, and an image of this polished cross-section is acquired by a SEM at a predetermined magnification (e.g., 1000x). Then, a field of view is set such that at least five metal oxide particles 2 are contained within this field of view.

[0039] Then, a metal oxide particle 2 is focused in the acquired SEM image, and longitudinal lengths L1, L2, and L3 of the metal oxide particle 2 are measured at three points in the lateral direction as shown in Fig.Figure 6 shows the measurements. The lengths L1, L2, and L3 are measured at three arbitrary points (e.g., approximately in the middle and at approximately opposite ends in the lateral direction) along the width of the metal oxide particle 2. Then, an arithmetic mean of the lengths L1, L2, and L3 is defined as a presumed principal axis.

[0040] The widths L4, L5, and L6 in the lateral direction of the metal oxide particle 2 are also measured at three arbitrary points in the longitudinal direction, and an arithmetic mean of the widths L4, L5, and L6 is defined as the assumed minor axis. The arbitrary three points for measuring the widths L4, L5, and L6 include, for example, a point that is substantially at the center of the metal oxide particle 2 in the longitudinal direction, and two points that are located one-quarter to one-half of the aforementioned major axis from the center. These two two points are arranged on opposite sides in the longitudinal direction, with the point that is substantially at the center of the metal oxide particle 2 lying between them.

[0041] The assumed principal and minor axes of the five metal oxide particles 2 in the aforementioned SEM image are determined using the same procedure, and an arithmetic mean of the five assumed principal axes and an arithmetic mean of the five assumed minor axes are each determined as the principal and minor axes of the metal oxide particles 2. Furthermore, an arithmetic mean of the principal and minor axes is defined as a mean particle diameter of the metal oxide particles 2, and a value obtained by dividing the principal axis by the minor axis is determined as the aspect ratio of the metal oxide particles 2.

[0042] In the aforementioned measurements of the principal and minor axes of the metal oxide particles 2, the longitudinal directions of the metal oxide particles 2 are determined as follows. First, in the aforementioned SEM image, each metal oxide particle 2 is outlined by two parallel lines (hereinafter referred to as a "pair of lines") such that the metal oxide particle 2 lies between them. Then, the orientation of the pair of lines is changed while the metal oxide particle 2 remains outlined by the pair of lines. The direction in which the pair of lines extends when the distance between the pair of lines (i.e., the distance between the pair of lines in a direction perpendicular to the pair of lines) is minimized is then defined as the longitudinal direction. A direction perpendicular to this longitudinal direction is also defined as the latitude direction.

[0043] The MnWO4 content in the porous ceramic structure 1 is preferably greater than or equal to 0.2 wt% and less than or equal to 2.0 wt%. The MnWO4 content in the porous ceramic structure 1 is particularly preferably greater than or equal to 0.4 wt%. This content is particularly preferably less than or equal to 1.8 wt%.

[0044] Fig. Figure 7 is a sectional view of an area near a metal oxide particle 2 on the surface of a pore 121. As in Fig. Figure 7 shows that the metal oxide particle 2 has a shape that partially protrudes from the interior of the honeycomb structure 10 into the pore.

[0045] The metal oxide particle 2 has a firmly attached section 21 and a protrusion 22. The firmly attached section 21 is located inside the honeycomb structure 10. The expression "inside the honeycomb structure 10" refers to the cordierite surrounding the pore 121, and not to the interior of the pore 121 within the honeycomb structure 10 (i.e., the interior of the pore 121). The firmly attached section 21 is a connecting section of the metal oxide particle 2 that is bonded to the cordierite, which is the main component of the honeycomb structure 10, and is firmly bonded to the interior of the cordierite. In other words, the firmly attached section 21 is a section of the metal oxide particle 2 that creeps from the surface of the pore 121 in the honeycomb structure 10 into the cordierite to the opposite side of the pore 121 and is absorbed by the cordierite.In other words, the tightly bonded section 21 is a section of the metal oxide particle 2 whose surface is covered with cordierite. More precisely, the tightly bonded section 21 is located at a grain boundary of cordierite crystals 3 in the honeycomb structure 10 and is tightly bonded to the grain boundary.

[0046] The protrusion 22 is a section of the metal oxide particle 2 that rises in a particulate or fibrous form from the surface of the pore 121 into the pore 121. In other words, the protrusion 22 is a particulate or fibrous section exposed from the surface of the aforementioned cordierite. More precisely, the protrusion 22 rises in a particulate or fibrous form from the grain boundary of the cordierite crystals 3 into the pore 121. The protrusion 22 is continuously adjacent to the firmly connected section 21. The principal axis, the minor axis, the mean particle diameter, and the aspect ratio of the aforementioned metal oxide particle 2 are the principal axis, the minor axis, the mean particle diameter, and the aspect ratio of the protrusion 22, which can be observed with a SEM.

[0047] Of a large number of metal oxide particles 2 in the porous ceramic structure 1, for example, some metal oxide particles 2 are firmly bonded to the surfaces of pores 121 within the pores 121, as described above, and the other metal oxide particles 2 are entirely arranged within the honeycomb structure 10. Almost all metal oxide particles 2 can be firmly bonded to the surface of the pores 121 within the pores 121.

[0048] In the porous ceramic structure 1, the honeycomb structure 10 is not affected by any coating process (so-called wash coating) with γ-aluminum oxide or the like. Therefore, the surfaces of the pores 121 have no coating formed by the aforementioned coating process, and as a natural result, no coating hinders the attachment of the honeycomb structure 10 and the metal oxide particles 2.

[0049] A large number of metal oxide particles 2, firmly bound to the honeycomb structure 10, do not necessarily have to consist of fine MnWO4 particles, as long as they contain Mn and W as constituents, and may contain other metal oxide particles that differ from the fine MnWO4 particles, instead of or in addition to the fine MnWO4 particles. Even in this case, the major axes, minor axes, mean particle diameter, and aspect ratio of the metal oxide particles 2 can be determined using the same procedure as described above.

[0050] The Mn content in the porous ceramic structure 1 is preferably greater than or equal to 0.5 wt% and less than or equal to 3.0 wt% with respect to manganese oxide (Mn₂O₃). The Mn content used here with respect to Mn₂O₃ refers to the percentage of a value obtained by dividing the mass of Mn₂O₃ by the mass of the porous ceramic structure 1, assuming that all Mn components in the porous ceramic structure 1 are present as Mn₂O₃.

[0051] The tungsten content (W) in the porous ceramic structure 1 is preferably greater than or equal to 0.1 wt% and less than or equal to 1.5 wt% with respect to tungsten oxide (WO3). The W content in the porous ceramic structure 1 is preferably greater than or equal to 0.5 wt% with respect to WO3. This content (i.e., the W content with respect to WO3) is particularly preferably also less than or equal to 1.4 wt% with respect to WO3. The W content with respect to WO3 used here refers to the percentage of a value obtained by dividing the mass of MnWO3 by the mass of the porous ceramic structure 1, assuming that all W components in the porous ceramic structure 1 are present as WO3.

[0052] To achieve high catalytic performance of the porous ceramic structure 1 with the metal oxide particles, the content of metal oxide particles 2 in the porous ceramic structure 1 is, for example, higher than or equal to 0.1 wt%. To reduce the pressure drop in the porous ceramic structure 1, the content of metal oxide particles 2 in the porous ceramic structure 1 is, for example, less than or equal to 5.0 wt%.

[0053] In other words, the amount of metal oxide particles 2 carried by the porous ceramic structure 1 is, for example, greater than or equal to 3 grams per liter (g / L), preferably greater than or equal to 5 g / L, and particularly preferably greater than or equal to 8 g / L. The amount of metal oxide particles 2 carried by the porous ceramic structure 1 is, for example, less than or equal to 50 grams per liter (g / L), preferably less than or equal to 45 g / L, and particularly preferably less than or equal to 40 g / L. The amount (g / L) of carried metal oxide particles 2 denotes the amount (g) of metal oxide particles 2 carried per unit volume (L) of the honeycomb structure 10.

[0054] Next, an example of the process for producing the porous ceramic structure 1 will be given with reference to Fig.As described in section 8, the production of the porous ceramic structure 1 begins with the preparation of a raw material structure by weighing and mixing materials for the honeycomb structure 10, materials for the metal oxide particles 2, and materials for the fine additive particles. The materials for the honeycomb structure 10 consist primarily of a cordierite raw material, which serves as the mineral mixture for the honeycomb structure 10 and includes, for example, magnesium oxide (MgO), aluminum oxide (Al₂O₃), and silicon dioxide (SiO₂). The materials for the honeycomb structure 10 also include other components, such as a hole-forming material and a binder. The materials for the metal oxide particles 2 include, for example, Mn₂O₃ and WO₃. The materials for the fine additive particles include, for example, Fe₂O₃ and CeO₂.Then, after the raw material of the structure has been dry-mixed in a kneading machine, it is fed with water and further mixed and kneaded to prepare a kneaded clay (step S11).

[0055] The required time for the dry mixing and kneading described above is, for example, 15 minutes and 30 minutes, respectively. These dry mixing and kneading times can be varied in several ways. For example, instead of Mn₂O₃, salts such as manganese nitrate and a tungsten ammonium salt can be used for the metal oxide particles 2. Similarly, for the fine additive particles, salts such as iron nitrate and cerium nitrate can be used instead of Fe₂O₃ and CeO₂.

[0056] In step S11, the raw materials for the metal oxide particles 2 and the raw materials for the fine additive particles are added individually to the mineral mixtures or the like for the honeycomb structure 10, but the method for adding these raw materials can be modified in various ways. For example, a material produced by immersing the raw materials for the metal oxide particles 2 in CeO2 and drying and firing the raw materials can be added to the mineral mixtures or the like for the honeycomb structure 10. In this material, some of the raw materials for the metal oxide particles 2 may be present as dissolved solids or adhere to the CeO2.

[0057] The kneaded clay prepared in step S11 is formed into a column shape in a vacuum kneading machine or other machine and then subjected to an extrusion process to form a honeycomb-shaped mold (step S12). The honeycomb mold incorporates an internal grid-like partition that divides it into numerous cells, which serve as flow paths for a fluid. The honeycomb mold has a cell diameter of 30 mm, a partition thickness of 12 mil (approximately 0.3 mm), and a cell density of 300 cells per square inch (cpsi), i.e., 46.5 cells / cm². 2 and an outer wall thickness of approximately 0.6 mm. Alternatively, the honeycomb press body can be formed in step S12 by a different forming process than extrusion.

[0058] The honeycomb press body produced in step S12 is subjected to microwave drying until approximately 70% of the moisture evaporates, and then to hot air drying (80°C × 12 hours). The honeycomb press body is then placed in a degreasing oven operated at 450°C to remove organic components (i.e., degrease). Following this, the honeycomb press body undergoes a firing process (firing) to obtain the porous ceramic structure 1 with the honeycomb structure 10, the metal oxide particles 2, and the fine additive particles (step S13). The firing process in step S13 is carried out, for example, at a firing temperature of 1300°C to 1500°C for eight hours at atmospheric pressure. The firing temperature is preferably greater than or equal to 1350°C and particularly preferably greater than or equal to 1370°C.The firing temperature is preferably lower than or equal to 1450°C and particularly preferably lower than or equal to 1430°C. The firing conditions can be modified accordingly. The porous ceramic structure 1 produced by the manufacturing process described above contains no precious metals and can therefore be manufactured cost-effectively.

[0059] Next, the ratio of the content of the metal oxide particles 2 in the porous ceramic structure 1, the pressure drop and the catalytic performance are described with reference to Tables 1 to 3. Table 1 Material composition (wt%) MgO Al2O3 SiO2 Mn2O3 WO3 Fe2O3 CeO2 In total Example 1 7,2 40,7 44,2 2,0 0,1 2,2 3,6 100 Example 2 7,2 40,6 44,1 2,0 0,3 2,2 3,5 100 Example 3 7,1 40,5 44,0 2,0 0,6 2,2 3,5 100 Example 4 7,1 40,2 43,7 2,0 1,3 2,2 3,5 100 Example 5 7,6 43,3 47,1 0,6 1,4 0,0 0,0 100 Comparison example 1 7,8 44,2 48,0 0,0 0,0 0,0 0,0 100 Comparison example 2 7,2 40,7 44,3 2,0 0,0 2,2 3,6 100 Table 2 Composition of crystal phases (w%) Cordierite MnWO4 Other In total Example 1 88,0 ≤Detection limits 12,0 100 Example 2 87,0 0,3 12,7 100 Example 3 85,0 0,4 14,6 100 Example 4 83,0 0,8 16,2 100 Example 5 88,0 1,5 10,5 100 Comparison example 1 95,0 0,0 5,0 100 Comparison example 2 90,0 0,0 10,0 100 Table 3 Dimensions of MnWO4 Open-pore NOO oxidation temperature coefficient of thermal expansion Main axis (m) Secondary axle (nm) aspect ratio (%) (°C) 40-800 °C Example 1 0,1 0,1 1,4 62 500 0,4 Example 2 0,7 0,1 5,2 62 490 0,4 Example 3 1,2 0,2 6,7 62 480 0,4 Example 4 3,5 0,5 7,1 61 480 0,4 Example 5 1,7 0,3 5,7 60 460 0,3 Comparative example 1 No MnWO4 58 540 0,3 Comparative example 2 No MnWO4 60 480 0,6

[0060] The composition of the crystalline phases (i.e., the mass ratio of the constituents) in the porous ceramic structure 1 was identified and quantified as follows. For the fabricated samples, the crystalline phase of each particle was measured using an X-ray diffractometer (rotating anticathode X-ray diffractometer: RINT 2500, manufactured by Rigaku Corporation). The X-ray diffraction conditions were a CuKα radiation source, 50 kV, 300 mA, and 2θ = 10° to 60°, and the resulting X-ray diffraction data were evaluated using commercially available X-ray diffraction analysis software.

[0061] The major axis, minor axis, and aspect ratio of MnWO4, i.e., the metal oxide particles 2, were determined using the aforementioned method. The open porosity of the porous ceramic structure 1 was measured using the Archimedes method with deionized water as the medium. As described above, the pressure drop in the porous ceramic structure 1 decreases with increasing open porosity.

[0062] The NO oxidation temperature of the porous ceramic structure 1 was obtained as follows. First, the relationship between temperature and the NO₂ conversion rate of the porous ceramic structure 1 was determined. The NO₂ conversion rate was the rate of conversion of NO to NO₂ in a sampled gas containing NO, which traveled through the porous ceramic structure 1 at a space velocity (SV) of 24,400 h⁻¹. -1A gas was introduced and flowed through the porous ceramic structure 1. An initial gas contained 100 ppm NO, 1500 ppm CO, 5% CO2, 450 ppm propane (C3H8), and 2% H2O. The extracted gas was analyzed using Fourier transform infrared spectrometry (FT-IR). The NO2 conversion rate was approximately 0% at low temperatures and gradually increased to a maximum value with increasing temperature, then decreased stepwise. The porous ceramic structure 1 exhibited higher catalytic performance with increasing NO2 conversion rate. Once the relationship between the NO2 conversion rate and temperature was established, the temperature was increased from low temperature according to the above relationship, and a temperature at which the NO2 conversion rate was half its maximum value was determined to be the NO oxidation temperature. The porous ceramic structure 1 exhibited higher catalytic performance with decreasing NO oxidation temperature.

[0063] The coefficient of thermal expansion of the porous ceramic structure 1 was a value measured using a method according to JIS R 1618. Specifically, a sample with dimensions of 3 cells height, 3 cells width, and 50 mm length was cut from the honeycomb structure 10, and a coefficient of thermal expansion in the A-axis direction (i.e., a direction parallel to the flow path in the honeycomb structure) was measured at a temperature from 40°C to 800°C.

[0064] In Examples 1 to 5, the materials contained Mn₂O₃ and WO₃, and MnWO₄ was generated as the metal oxide particles 2 of the porous ceramic structure 1. In Example 1, the amount of WO₃ in the materials was too small to measure the mass percent (wt%) of MnWO₄.

[0065] Fig. Figure 9 shows an enlarged SEM image of the surface of a pore 121 in the porous ceramic structure 1 according to Example 4. Fig.9. There are metal oxide particles 2 (white areas in the figure) on the surface of pore 121 in the honeycomb structure 10.

[0066] In Examples 1 to 5, the porous ceramic structure 1 contained Mn and W. The Mn content of porous ceramic structure 1 ranged from 0.6 wt% to 2.0 wt% with respect to Mn₂O₃ and fell within the range of 0.5 wt% to 3.0 wt%. The W content of porous ceramic structure 1 ranged from 0.1 wt% to 1.4 wt% with respect to WO₃ and fell within the range of 0.1 wt% to 1.5 wt%. In Examples 2 to 5, the MnWO₄ content in porous ceramic structure 1 ranged from 0.3 wt% to 1.5 wt% and fell within the range of 0.2 wt% to 2.0 wt%.

[0067] In Examples 1 to 5, the open porosity was in the range of 60% to 62% and relatively high, thus keeping the pressure drop in the porous ceramic structure 1 low. In Examples 1 to 5, the NO oxidation temperature was in the range of 460°C to 500°C and low, indicating that the porous ceramic structure 1 had high catalytic performance. Examples 1 to 5 show that the NO oxidation temperature decreases as the MnWO4 content in the porous ceramic structure 1 increases.

[0068] In examples 3 to 5, the MnWO4 content in the porous ceramic structure 1 ranged from 0.4 wt% to 1.5 wt%. In examples 3 to 5, the NO oxidation temperature was less than or equal to 480°C and lower, which meant that the porous ceramic structure 1 had an even higher catalytic performance.

[0069] In Examples 1 to 5, the coefficients of thermal expansion at temperatures from 40°C to 800°C were in the range of 0.3 to 0.4 and were only slightly higher than the coefficient of thermal expansion according to Comparative Example 1, which is described later (and which contained only the cordierite honeycomb structure 10). Therefore, the porous ceramic structure 1 according to Examples 1 to 5 had a thermal shock resistance approximately the same as that in Comparative Example 1. The suppression of an increase in the coefficient of thermal expansion of the porous ceramic structure 1 is considered a result of the W component, which accelerates the crystallization of cordierite and thereby suppresses the formation of an amorphous material.

[0070] Comparative example 1 showed a test result only for a honeycomb structure 10 made of cordierite, which contained neither metal oxide particles 2 nor fine additive particles. Fig.Figure 10 shows a SEM image of the surface of a porous ceramic structure according to comparison example 1. Fig. In example 10, the porous ceramic structure 1, as described above, consists only of cordierite crystals. In comparative example 1, the NO oxidation temperature was 540°C, which was high, indicating that the porous ceramic structure 1 had a lower catalytic performance than examples 1 to 5.

[0071] In comparative example 2, the materials contained Mn₂O₃ but no WO₃. Therefore, no MnWO₄ was produced in the porous ceramic structure 1. In comparative example 2, the porous ceramic structure 1 contained essentially no W, so there were no effects of W components to accelerate the crystallization of cordierite and suppress the formation of amorphous materials. Accordingly, the coefficient of thermal expansion at temperatures in the range of 40°C to 800°C was 0.6 and was high, and the thermal shock resistance was lower than in examples 1 to 5 and comparative example 1.

[0072] As described above, the porous ceramic structure comprises a porous structural body (i.e., a honeycomb structure 10) consisting primarily of cordierite, as well as manganese (Mn) and tungsten (W) firmly bonded to the honeycomb structure 10. Accordingly, it is possible to reduce the pressure drop in the porous ceramic structure 1 and increase the NO₂ conversion rate. It is also possible to lower the NO combustion temperature in the porous ceramic structure 1. Furthermore, the conversion rate of CO to CO₂ and the conversion rate of CH₄ to CO₂ and H₂O can be increased. In other words, the above design of the porous ceramic structure 1 allows for low pressure drop and high catalytic performance.

[0073] As described above, Mn and W in the porous ceramic structure 1 are preferred components of the metal oxide particles 2, which are firmly bonded to the interior of the gas cavities (i.e., pores 121) in the honeycomb structure 10. The metal oxide particles 2 preferably include firmly bonded sections 21 located within the honeycomb structure 10 and protrusions 22 adjacent to the firmly bonded sections 21 and projecting into the pores 121. This allows for an increase in the contact ratio between molecules to be converted, such as NO, CO, or HC, and the Mn and W components in the pores 121. As a result, the porous ceramic structure 1 can achieve higher catalytic performance.Since the metal oxide particles 2 are directly and firmly bonded to the honeycomb structure 10, pressure loss can be reduced more than if the honeycomb structure 10 is subjected to a process such as a coating process to form a coating on the surfaces of the pores 121.

[0074] As described above, the W content in the porous ceramic structure 1 is preferably greater than or equal to 0.1 wt% and less than or equal to 1.5 wt% with respect to WO3. Accordingly, the porous ceramic structure 1 can achieve a higher catalytic performance.

[0075] As described above, the Mn content in the porous ceramic structure 1 is preferably greater than or equal to 0.5 wt% and less than or equal to 3.0 wt% with respect to Mn₂O₃. Accordingly, the porous ceramic structure 1 can achieve a higher catalytic performance.

[0076] As described above, the metal oxide particles 2 preferably contain MnWO4 particles. Accordingly, the porous ceramic structure 1 can achieve a higher catalytic performance. Particularly preferably, the MnWO4 content in the porous ceramic structure 1 is greater than or equal to 0.2 wt% and less than or equal to 2.0 wt%. Accordingly, the porous ceramic structure 1 can achieve an even higher catalytic performance.

[0077] The aspect ratio of the MnWO4 particles is preferably greater than or equal to 5.5. This increases the surface area of ​​the particles if the MnWO4 particles exhibit anisotropy, and it is possible to increase the contact ratio between MnWO4 and the aforementioned molecules, which are the target of the conversion in the pores 121. As a result, the porous ceramic structure 1 can achieve even higher catalytic performance.

[0078] As described above, the MnWO4 particles are preferably particulate or fibrous. The MnWO4 particles preferably comprise tightly bonded sections 21 located at the grain boundary of cordierite crystals 3 in the honeycomb structure 10, and projections 22 extending from the grain boundaries into the pores 121. This structure is achieved by the W components accelerating the crystallization of the cordierite and thus suppressing the formation of amorphous materials during the production of the porous ceramic structure 1. The structure of the porous ceramic structure 1 suppresses an increase in the coefficient of thermal expansion of the porous ceramic structure 1. As a result, it is possible to suppress a deterioration in the thermal shock resistance of the porous ceramic structure 1.

[0079] The porous ceramic structure 1 described above and the method for producing the porous ceramic structure 1 can be modified in various ways.

[0080] For example, the shapes of the fine MnWO4 particles are not limited to the particle or fiber form and can be modified in various ways. The tightly bonded sections 21 of the fine MnWO4 particles do not necessarily have to be located at the grain boundary of the cordierite crystals 3, and the protrusions 22 do not necessarily have to rise from the grain boundaries.

[0081] The aspect ratio of the fine MnWO4 particles can be lower than 5.5, and the fine MnWO4 particles do not necessarily exhibit anisotropy. The principal axes, minor axes, and mean particle diameters of the fine MnWO4 particles are not limited to the ranges described above.

[0082] The MnWO4 content in the porous ceramic structure 1 can be less than 0.2 wt% and greater than 2.0 wt%.

[0083] The Mn content in the porous ceramic structure 1 can be less than 0.5 wt% and greater than 3.0 wt% with respect to Mn2O3.

[0084] The W content in the porous ceramic structure 1 can be less than 0.1 wt% and greater than 1.5 wt% with respect to WO3.

[0085] In the porous ceramic structure 1, Mn and W do not necessarily have to be present as components of the metal oxide particles 2, which comprise the firmly bonded sections 21 and the protrusions 22, but can exist in other forms. For example, Mn and W can be present as components of the metal oxide particles 2, which are entirely embedded in the honeycomb structure 10, or they can be firmly bonded to the honeycomb structure 10 in a different form than as metal oxide particles 2.

[0086] In the porous ceramic structure 1, the shape of the aforementioned structural body is not limited to a honeycomb shape and can be any other shape (e.g., essentially circular cylindrical) instead of the honeycomb shape.

[0087] The manufacturing process of the porous ceramic structure 1 is not limited to the examples described above and can be modified in various ways.

[0088] The porous ceramic structure 1 can be used in applications other than CSF or DPF.

[0089] The designs of the preferred embodiments and variations described above can be combined appropriately, provided there are no mutual contradictions.

[0090] The invention has been shown and described in detail, but the foregoing description with all its aspects is to be understood as exemplary and not as limiting. It is therefore understood that numerous variations and modifications can be developed without deviating from the scope of the invention. Commercial applicability

[0091] The present invention is applicable to filters for collecting fine dust, for example DPFs that collect fine dust in exhaust gases from diesel engines. Reference symbol list 1 Porous ceramic structure 2 metal oxide particles 3 Cordierite crystal 8 Exhaust gas purification system 10 honeycomb structure 11 tubular outer wall 12 Partition wall 13th cell 14 Seal 21 Permanently connected section 22 Survey 81 Diesel particulate filters (DPF) 82 Diesel Oxidation Catalyst (DOC) 83 catalytic soot filter (CSF) 85 SCR catalyst 86 Urea injection 121 pores A1 Arrow L1 to L3 length L4 to L6 width Step S11 to S13

Claims

Porous ceramic structure (1) comprising: a porous structural body consisting predominantly of cordierite; and manganese and tungsten firmly bonded to the structural body, wherein the manganese and tungsten are components of a metal oxide particle (2) firmly bonded to an interior of a pore (121) of the structural body, and the metal oxide particle (2) having: a firmly bonded section (21) located within the structural body; and a protrusion (22) adjacent to the firmly bonded section (21) and projecting into the pore. Ceramic structure (1) according to claim 1 , with a tungsten content greater than or equal to 0.1 wt% and less than or equal to 1.5 wt% with respect to WO3. Ceramic structure (1) according to one of claims 1 or 2, having a manganese content greater than or equal to 0.5 wt% and less than or equal to 3.0 wt% with respect to Mn2O3. Ceramic structure (1) according to one of claims 1 to 3, wherein the metal oxide particle (2) comprises a MnWO4 particle. Ceramic structure (1) according to claim 4, with a MnWO4 content greater than or equal to 0.2 wt% and less than or equal to 2.0 wt%. Ceramic structure (1) according to claim 4 or 5, wherein the MnWO4 particle has an aspect ratio greater than or equal to 5.

5. Ceramic structure (1) according to one of claims 4 to 6, wherein the MnWO4 particle is particulate or fibrous, and the MnWO4 particle comprises: the firmly connected section (21) located at a grain boundary of cordierite crystals (3) in the structural body; and the elevation (22) projecting from the grain boundary into the pore (121).

Images