Method for producing a porous ceramic structure

Abstract

A method for producing a porous ceramic structure, comprising the following: a) Making kneaded clay by kneading a raw material; b) Obtaining a pressed body by shaping the kneaded clay and c) Firing of the pressed body, wherein the raw material contains the following: Cordierite; Cer and iron and / or manganese and / or cobalt, wherein the process c) forms a porous ceramic structure comprising a porous structural body formed essentially of cordierite, cerium-containing particles firmly attached to the structural body, and metal oxide particles each firmly attached to an inside of a pore in the structural body, wherein the metal oxide particles are particles of an oxide having a spinel structure containing iron and / or manganese and / or cobalt, The process c) includes the following: c1) Raising the temperature of the pressed body to a first temperature that is less than or equal to a liquid phase formation temperature; c2) Increasing the temperature of the pressed body from the first temperature to a second temperature that is higher than the liquid phase formation temperature and lower than a crystallization temperature of the metal oxide particles; c3) Increasing the temperature of the pressed body from the second temperature to a third temperature which is greater than the crystallization temperature and less than or equal to a maximum temperature; and c4) Maintaining the temperature of the press body at the maximum temperature for a specified period of time, and wherein a rate of temperature increase in process c2) in the range of 100 °C / h to 200 °C / h.

Description

Technical field

[0001] The present invention relates to a method for producing a porous ceramic structure. Cross-reference to related registration

[0002] This application claims priority over Japanese patent application No. 2020-57683, filed with the Japanese Patent Office on March 27, 2020, the disclosure of which is incorporated herein in full by reference. Technical background

[0003] Japanese patent publication JP 2017-186 220 A (Document 1) proposes cerium dioxide particles in or on which a transition metal oxide containing iron and manganese is incorporated. It is suggested that such cerium dioxide particles could be used, for example, as an oxidation catalyst in a diesel particulate filter (DPF) that includes a diesel oxidation catalyst (DOC) and a catalyst soot filter (CSF).

[0004] Japanese patent publication JP 2018-030 105 A (Document 2) and Japanese patent publication JP 2017-171 543 A (Document 3) propose techniques that enable a porous ceramic structure used in a DPF, or other measures, to carry a sufficient amount of catalyst to maintain catalytic activity. In the porous ceramic structure, sections of cerium dioxide particles are incorporated into the structure, with the remaining sections of the ceramic diode particles exposed at the surface of pores within the structure. In the porous ceramic structure according to Document 2, sections of cerium dioxide particles exposed at the surface of pores contain an iron oxide. In the porous ceramic structure according to Document 3, sections of cerium dioxide particles exposed at the surface of pores carry fine catalyst particles of a platinum group element.

[0005] The porous ceramic structures used in DPFs, or other measures, must achieve both a reduction in pressure loss and an improvement in catalytic performance.

[0006] Further state of the art is also known from the subsequently published DE 11 2020 000 288 T5. Summary of the invention

[0007] The present invention is directed towards the production of a porous ceramic structure and an object of the present invention is to create a porous ceramic structure with low pressure loss and high catalytic performance.

[0008] The present invention relates to a method for producing a porous ceramic structure. A method for producing a porous ceramic structure according to a preferred embodiment of the present invention comprises a) preparing kneaded clay by kneading a raw material, b) obtaining a molded body by shaping the kneaded clay, and c) firing the molded body. The raw material contains cordierite, cerium, and iron and / or manganese and / or cobalt. Process c) forms a porous ceramic structure comprising a porous structural body formed essentially of cordierite, cerium-containing particles firmly attached to the structural body, and metal oxide particles firmly attached to the inner surface of a pore in the structural body, wherein the metal oxide particles are particles of an oxide having a spinel structure containing iron and / or manganese and / or cobalt.Process c) comprises c1) raising the temperature of the press body to a first temperature less than or equal to a liquid phase formation temperature, c2) raising the temperature of the press body from the first temperature to a second temperature greater than the liquid phase formation temperature and less than a crystallization temperature of the metal oxide particles, c3) raising the temperature of the press body from the second temperature to a third temperature greater than the crystallization temperature and less than or equal to a maximum temperature, and c4) maintaining the temperature of the press body at the maximum temperature for a predetermined period. The temperature increase rate in process c2) is in the range of 100 °C / h to 200 °C / h.

[0009] Preferably, the rate of temperature increase in process c3) is in the range of 100 °C / h to 200 °C / h.

[0010] Preferably, the temperature of the press body is maintained at the crystallization temperature for a predetermined period during process c3).

[0011] A method for producing a porous ceramic structure according to a preferred embodiment of the present invention comprises a) preparing kneaded clay by kneading a raw material, b) obtaining a molded body by shaping the kneaded clay, and c) firing the molded body. The raw material contains cordierite, cerium, and iron and / or manganese and / or cobalt. Process c) forms a porous ceramic structure comprising a porous structural body formed essentially of cordierite, cerium-containing particles firmly attached to the structural body, and metal oxide particles firmly attached to the inner surface of a pore in the structural body, wherein the metal oxide particles are particles of an oxide having a spinel structure containing iron and / or manganese and / or cobalt.Process c) comprises c1) raising the temperature of the press body to a first temperature less than or equal to a liquid phase formation temperature, c2) raising the temperature of the press body from the first temperature to a second temperature greater than the liquid phase formation temperature and less than a crystallization temperature of the metal oxide particles, c3) raising the temperature of the press body from the second temperature to a third temperature greater than the crystallization temperature and less than or equal to a maximum temperature, and c4) maintaining the temperature of the press body at the maximum temperature for a predetermined period. The rate of temperature increase in process c2) is in the range of 1 °C / h to 10 °C / h.

[0012] Preferably, the temperature of the press body is maintained at the crystallization temperature for a predetermined period during process c3).

[0013] A method for producing a porous ceramic structure according to a further preferred embodiment of the present invention comprises a) preparing kneaded clay by kneading a raw material, b) obtaining a molded body by shaping the kneaded clay, and c) firing the molded body. The raw material contains cordierite, cerium, and iron and / or manganese and / or cobalt. Process c) forms a porous ceramic structure comprising a porous structural body formed essentially of cordierite, cerium-containing particles firmly attached to the structural body, and metal oxide particles firmly attached to the inner surface of a pore in the structural body, wherein the metal oxide particles are particles of an oxide having a spinel structure containing iron and / or manganese and / or cobalt.Process c) comprises c1) raising the temperature of the press body to a first temperature less than or equal to a liquid phase formation temperature, c2) raising the temperature of the press body from the first temperature to a second temperature greater than the liquid phase formation temperature and less than a crystallization temperature of the metal oxide particles, c3) raising the temperature of the press body from the second temperature to a third temperature greater than the crystallization temperature and less than or equal to a maximum temperature, and c4) maintaining the temperature of the press body at the maximum temperature for a predetermined period. In process c3), the temperature of the press body is maintained at the crystallization temperature for a predetermined period.

[0014] These and other tasks, features, aspects and advantages of the present invention will become clearer from the following detailed description of the present invention when it is considered in conjunction with the accompanying drawings. Brief description of the drawings Fig. Figure 1 is a diagram illustrating a configuration of an exhaust gas purification system; Fig. Figure 2 is a diagram illustrating a porous ceramic structure; Fig. Figure 3 is a cross-sectional view of the porous ceramic structure; Fig. Figure 4 is a diagram illustrating a section of a partition wall in enlarged dimensions; Fig. Figure 5 shows a SEM image of the surface of a pore in a honeycomb structure; Fig. Figure 6 shows an enlarged SEM image of metal oxide particles on the surface of a pore; Fig.Figure 7 is a cross-sectional view of an area near a metal oxide particle; Fig. Figure 8 is a diagram illustrating a procedure of a process for producing a porous ceramic structure; Fig. Figure 9 is a diagram illustrating one procedure of the process for producing a porous ceramic structure; Fig. Figure 10 is a diagram showing an example of a temperature profile during a firing step; Fig. Figure 11 is a diagram showing an example of the temperature profile during the firing step; Fig. Figure 12 is a diagram showing an example of the temperature profile during the firing step; Fig. Figure 13 is a diagram showing an example of the temperature profile during the firing step; Fig. Figure 14 is a diagram showing an example of the temperature profile during the firing step; Fig.Figure 15 shows a SEM image of the surface of a porous ceramic structure according to Example 3; Fig. Figure 16 shows an enlarged SEM image of the surface of a pore in the porous ceramic structure according to Example 3; Fig. Figure 17 shows a SEM image of the surface of a porous ceramic structure according to Example 4; and Fig. Figure 18 shows an enlarged SEM image of the surface of a pore in the porous ceramic structure according to Example 4. Description of the embodiments

[0015] Fig.Figure 1 is a diagram illustrating the configuration of an exhaust gas purification system 8. The exhaust gas purification system 8 is configured to purify exhaust gas emitted by a power engine. The exhaust gas purification system 8 includes a diesel particulate filter (DPF) 81, an exhaust gas catalyst 85 with selective catalytic reduction (SCR exhaust gas catalyst), and a urea injector 86. The DPF 81, the urea injector 86, and the SCR exhaust gas catalyst 85 are arranged in this order in the direction of exhaust gas flow.

[0016] The DPF 81 contains a diesel oxidation catalyst (DOC) 82 and a catalyst soot filter (CSF) 83. The DOC 82 has a honeycomb structure, the interior of which is divided into several cells by a partition, and a precious metal oxidation catalyst supported by the partition. The CSF 83 has a honeycomb structure similar to the one described above and a metal oxidation catalyst supported by a partition within the honeycomb structure. The details of the structure of the CSF 83 are described later. The urea injector 86 is located in an exhaust gas path between the DPF 81 and the SCR exhaust catalyst 85. The SCR exhaust catalyst 85 has a honeycomb structure similar to the one described above and an SCR catalyst supported by a partition within the honeycomb structure.

[0017] The exhaust gas emitted from the engine flows into the DOC 82 of the DPF 81. The exhaust gas contains nitric oxide (NO), oxygen (O2), and nitrogen (N2), causing reactions in the DOC 82 expressed by equations 1 and 2 below. The reaction expressed by equation 1 produces nitrogen dioxide (NO2). It should be noted that SOF (a soluble organic component) is contained in the particulate matter (PM) in the exhaust gas, as shown in equation 2 below. 2NO + O2 = 2NO2 (Equation 1) SOF + O2 = CO, CO2, H2O (Equation 2)

[0018] The CSF 83 collects carbon (soot) contained in the exhaust gas. In the CSF 83, the soot and NO2 cause reactions (combustion reactions) expressed by equations 3, 4 and 5 below, and NO is produced from NO2. C (soot) + 2NO2 = CO2 + 2NO (Equation 3) C (soot) + NO2 = CO + NO (Equation 4) C (soot) + 1 / 2O2 + NO2 = CO2 + NO (Equation 5)

[0019] The urea injector 86 mixes urea into the exhaust gas discharged by the CSF 83, and an exhaust gas containing ammonia (NH3), produced by the decomposition of the urea, flows into the SCR exhaust catalyst 85. In the SCR exhaust catalyst 85, reactions expressed by equations 6, 7, and 8 below occur, such that NOx contained in the exhaust gas is purified. 4NO + 4NH3 + O2 = 4N2 + 6H2O (Equation 6) NO + NO2 + 2NH3 = 2N2 + 3H2O (Equation 7) 6NO2 + 8NH3 = 7N2 + 12H2O (Equation 8)

[0020] The reaction expressed by Equation 7 is referred to as a fast SCR reaction and proceeds at a higher reaction rate than the reactions expressed by Equations 6 and 8. To improve the efficiency of the reactions occurring in the SCR exhaust catalyst 85, in accordance with Equation 7, the ratio between the amounts of NO and NO2 entering the SCR exhaust catalyst 85 must be 1:1. Meanwhile, the CSF 83 consumes a large amount of NO2 in the combustion of soot and produces NO, as expressed by Equations 3, 4, and 5, which were described previously.

[0021] In light of this, the exhaust gas purification system 8 according to the present invention, like the CSF 83, contains a porous ceramic structure (which will be described later) that includes an oxidation catalyst. The porous ceramic structure oxidizes a portion of the NO to produce NO2, i.e., converts NO to NO2. This allows the ratio between the amounts of NO and NO2 flowing into the SCR exhaust gas catalyst 85 to approach 1:1, thereby improving the efficiency of the reactions occurring in the SCR exhaust gas catalyst 85.

[0022] When a certain amount of soot or more is deposited in the CSF 83, the exhaust aftertreatment system 8 carries out a process to burn off the soot (i.e., a regeneration). In this case, the reactions (the combustion reactions) expressed by equations 3, 4, and 5 also occur in the CSF 83. If a large amount of carbon monoxide (CO) produced by these reactions flows into the SCR exhaust catalyst 85, the NOx removal efficiency of the SCR exhaust catalyst 85 may decrease. The same applies if a large amount of hydrocarbons (HC) contained in the fuel supplied to the CSF 83 flows into the SCR exhaust catalyst 85 during the soot combustion process.

[0023] In the exhaust gas purification system 8 according to the present invention, since the porous ceramic structure containing the aforementioned oxidation catalyst is provided as CSF 83, a portion of CO is oxidized to carbon dioxide (CO2) and a portion of HC is oxidized to CO2 and H2O. This suppresses the flow of CO, HC and other substances into the SCR exhaust gas catalyst 85 and prevents a decrease in the NOx purification efficiency of the SCR exhaust gas catalyst 85.

[0024] Fig. 2 and Fig. Figure 3 shows a simplified illustration of a porous ceramic structure 1. The porous ceramic structure 1 is a tubular element elongated in one direction, and Fig. Figure 2 illustrates the end face 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 taken in the longitudinal direction of the porous ceramic structure 1.

[0025] The porous ceramic structure 1 contains a honeycomb structure 10, which serves as a porous structural body, cerium-containing particles, and metal oxide particles, which act as an oxidation catalyst. The honeycomb structure 10 comprises a tubular outer wall 11 and a partition 12. The tubular outer wall 11 has a longitudinal shape. A cross-sectional view of the tubular outer wall 11, perpendicular to the longitudinal direction, can be, for example, circular, polygonal, or have another shape. The partition 12 is located inside the tubular outer wall 11 and divides the interior into several cells 13. The honeycomb structure 10 is a cellular structure whose interior is subdivided into several cells 13 by the partition 12. The tubular outer wall 11 and the partition 12 are made of a porous material. As will be described later, the exhaust gas passes through pores in the partition 12.To increase the strength of the porous ceramic structure 1, the partition 12 has, for example, a thickness greater than or equal to 50 micrometers (µm), preferably greater than or equal to 100 µm, and more preferably greater than or equal to 150 µm. To reduce pressure loss in the partition 12, the partition 12 has, for example, a thickness less than or equal to 500 µm, and preferably less than or equal to 450 µm.

[0026] Each cell 13 is a space running lengthwise. 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 some other shape. The cells 13 typically have the same cross-sectional shape. Alternatively, the cells 13 may contain cells 13 having 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. 2 and preferably less than or equal to 78 cells / cm² 2 .

[0027] In the porous ceramic structure 1 used in the CSF 83, the exhaust gas from the DOC 82 flows with one end along the longitudinal direction of the honeycomb structure 10 as the inlet and its other end as the outlet. Each of a predetermined number of cells 13 has a sealing device 14 located at the inlet end, and each of the remaining cells 13 has a sealing device 14 located at the outlet end. Therefore, the exhaust gas entering the honeycomb structure 10 flows from the cells 13 whose inlet sides are not sealed to the cells 13 whose outlet sides are not sealed as it passes through the partition 12 (see arrows A1 in Figure 1). Fig.3) At this point, the exhaust gas is oxidized by the metal oxide particles (i.e., the oxidation catalyst) at the partition 12. It is preferred that the sealing devices 14 are arranged alternately at each inlet and outlet end of the honeycomb structure 10 in the direction of arrangement of the cells 13.

[0028] The honeycomb structure 10 is essentially composed of cordierite. The honeycomb structure 10 can consist solely of cordierite or it can contain materials other than cordierite (e.g., a metal or a ceramic other than cordierite). The cordierite content in the honeycomb structure 10 is, for example, greater than or equal to 75% by mass and preferably greater than or equal to 80% by mass. In the present embodiment, the honeycomb structure 10 is essentially composed solely of cordierite.

[0029] Fig.Figure 4 is a diagram illustrating a magnified portion of the partition 12 within the porous ceramic structure 1. The honeycomb structure 10 contains a large number of gas cavities (hereinafter also referred to as "pores 121"). The metal oxide particles 2 and the cerium-containing particles 3 described above are firmly attached to the inner surfaces of the pores 121 (i.e., the pore surfaces) within the honeycomb structure 10. Fig. Figure 4 schematically illustrates metal oxide particles 2 and cerium-containing particles 3 on the surfaces of pores 121 by cross-hatching, without differentiating between them. It should be noted that the metal oxide particles 2 and the cerium-containing particles 3 do not necessarily have to cover the entire surface of the pores 121.

[0030] To reduce the pressure loss in the porous ceramic structure 1, the partition 12 in the honeycomb structure 10 has, for example, an open porosity greater than or equal to 25%, preferably greater than or equal to 30%, and more preferably greater than or equal to 35%. From the standpoint of ensuring 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, by the Archimedes method using deionized water as the medium.

[0031] The partition 12 in the honeycomb structure 10 has, for example, an average particle diameter greater than or equal to 5 µm and preferably greater than or equal to 8 µm. As in the case of open porosity, the pressure drop in the porous ceramic structure 1 decreases, while the average pore diameter of the partition 12 increases. To improve the oxidation performance of the porous ceramic structure 1, the average particle 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 more preferably less than or equal to 25 µm. The average pore diameter can be measured, for example, by mercury injection (in accordance with JIS R1655). Depending on the design of the porous ceramic structure 1, the sealing devices 14 can be omitted, and the metal oxide particles 2 can be retained in a layer on the surfaces of the cells 13.

[0032] Fig.Figure 5 shows a scanning electron microscope image (SEM image) of the surface of a pore 121 in the honeycomb structure 10. Fig.5. A large number of metal oxide particles 2, which have relatively small particle diameters, and a large number of cerium-containing particles 3, which have particle diameters larger than those of the metal oxide particles 2, are firmly attached to the surface of pore 121. For example, the metal oxide particles 2 have an average particle diameter greater than or equal to 10 nm. The particle diameters of the metal oxide particles 2 are, for example, less than or equal to 1 µm, preferably less than or equal to 100 nm, and more preferably less than or equal to 60 nm. The cerium-containing particles 3 have an average particle diameter larger than the average particle diameter of the metal oxide particles 2. For example, the average particle diameter of the cerium-containing particles 3 is greater than or equal to 0.5 µm, preferably greater than or equal to 1 µm, and more preferably greater than or equal to 2 µm.The particle diameters of the cerium-containing particles 3 are, for example, less than or equal to 30 µm, preferably less than or equal to 20 µm and more preferably less than or equal to 10 µm.

[0033] For example, the average particle diameter of the metal oxide particles 2 is obtained by calculating an average value of the particle diameters of the metal oxide particles 2 in an image of the metal oxide particles 2 acquired at a given magnification using a SEM. The same applies to the average particle diameter of the cerium-containing particles 3. Alternatively, the average particle diameters of the metal oxide particles 2 and the cerium-containing particles 3 can be obtained by laser diffractometry. As a further alternative, the diameters of crystallites of the metal oxide particles 2 and the cerium-containing particles 3, obtained by X-ray diffractometry (XRD), can be considered as the average particle diameters.

[0034] The metal oxide particles 2 are fine particles of an oxide possessing a spinel structure (i.e., a spinel crystal structure) containing iron elements (Fe elements) and / or manganese elements (Mn elements) and / or cobalt elements (Co elements). For example, the metal oxide particles 2 are formed solely from an oxide containing Fe and / or Mn and / or Co. Preferably, the metal oxide particles 2 are particles of an oxide possessing a spinel structure containing Fe, Mn, and oxygen (O). x Mn y O4, where x and y are positive numerical values ​​that satisfy x + y = 3), or particles of an oxide possessing a spinel structure containing Co and oxygen (O) (Co3O4). The cerium-containing particles 3 are fine particles containing cerium elements (Ce elements). For example, the cerium-containing particles 3 are cerium dioxide particles (CeO2 particles).

[0035] If the porous ceramic structure 1 contains Fe, the Fe content in the porous ceramic structure 1 is greater than or equal to 0.1 wt% and preferably greater than or equal to 1.5 wt% with respect to Fe₂O₃. The Fe content is also less than or equal to 3.0 wt% with respect to Fe₂O₃. The Fe content with respect to Fe₂O₃, as used herein, refers to the percentage of a value obtained by dividing the weight of Fe₂O₃ by the weight of the porous ceramic structure 1, assuming that all Fe components contained in the porous ceramic structure 1 are present as Fe₂O₃.

[0036] If the porous ceramic structure 1 contains Mn, the Mn content in the porous ceramic structure 1 is greater than or equal to 0.1 wt% and preferably greater than or equal to 1.5 wt% with respect to Mn₂O₃. The Mn content is also less than or equal to 3.0 wt% with respect to manganese oxide (Mn₂O₃). The Mn content with respect to Mn₂O₃, as used herein, refers to the percentage of a value obtained by dividing the weight of Mn₂O₃ by the weight of the porous ceramic structure 1, assuming that all Mn components contained in the porous ceramic structure 1 are present as Mn₂O₃.

[0037] If the porous ceramic structure 1 contains Co, the Co content in the porous ceramic structure 1 is as follows. If the porous ceramic structure 1 contains Co together with Fe or Mn (i.e., if the porous ceramic structure 1 contains Co and / or Fe and / or Mn), the Co content in the porous ceramic structure 1 is greater than or equal to 0.1 wt% and preferably greater than or equal to 1.5 wt% with respect to Co3O4. The Co content is also less than or equal to 3.0 wt% with respect to Co3O4.

[0038] Meanwhile, when the porous ceramic structure 1 contains Co without containing Fe and Mn, the Co content in the porous ceramic structure 1 is greater than or equal to 0.2 wt% and preferably greater than or equal to 3.0 wt% with respect to Co3O4. The Co content is also less than or equal to 6.0 wt% with respect to Co3O4. The Co content with respect to Co3O4, as used herein, refers to the percentage of a value obtained by dividing the weight of Co3O4 by the weight of the porous ceramic structure 1, assuming that all Co components contained in the porous ceramic structure 1 are present as Co3O4. In each of the cases described above, the majority of the Fe, Co, and Mn contained in the porous ceramic structure 1 are present as the metal oxide particles 2; however, it is also conceivable that some metal oxide particles 2 are dissolved in the honeycomb structure 10.

[0039] The Ce content in the porous ceramic structure 1 is greater than or equal to 0.1 wt% and preferably greater than or equal to 1.5 wt% with respect to CeO2. The Ce content is also less than or equal to 10 wt% and preferably less than or equal to 4.5 wt% with respect to CeO2. The Ce content with respect to CeO2, as used herein, refers to the percentage of a value obtained by dividing the weight of CeO2 by the weight of the porous ceramic structure 1, assuming that all Ce components contained in the porous ceramic structure 1 are present as CeO2. Although the majority of the Ce contained in the porous ceramic structure 1 is present as the cerium-containing particles 3, it is also conceivable that some of the Ce may be dissolved in the honeycomb structure 10.

[0040] A ratio of the sum of the Fe content with respect to Fe₂O₃, the Mn content with respect to Mn₂O₃, and the Co content with respect to Co₃O₄ to the Ce content with respect to CeO₂ (i.e., the value obtained by dividing the sum of the Fe content with respect to Fe₂O₃, the Mn content with respect to Mn₂O₃, and the Co content with respect to Co₃O₄ by the Ce content with respect to CeO₂) is greater than or equal to 0.8 and preferably greater than or equal to 1.0. This ratio is also less than or equal to 9.5 and preferably less than or equal to 4.0. In the following description, this ratio is referred to as the “Fe / Mn / Co ratio”.

[0041] The content of metal oxide particles 2 in the porous ceramic structure 1 is in the range of 0.3 wt% to 8.0 wt%. The content of metal oxide particles 2 as used herein refers to the mass ratio of the metal oxide particles 2 in the crystalline phase of the porous ceramic structure 1. Preferably, the content of metal oxide particles 2 is in the range of 2.0 wt% to 8.0 wt%.

[0042] The amount of metal oxide particles 2 carried in the porous ceramic structure 1 is, for example, greater than or equal to 2.0 grams per liter (g / L), preferably greater than or equal to 3.0 g / L, and more preferably greater than or equal to 5.0 g / L. The amount of metal oxide particles 2 carried in the porous ceramic structure 1 is also, for example, less than or equal to 50 g / L, preferably less than or equal to 45 g / L, and more preferably less than or equal to 40 g / L. The amount (g / L) of metal oxide particles 2 indicates the amount (g) of metal oxide particles 2 carried per unit volume (L) of the honeycomb structure 10.

[0043] Fig. Figure 6 shows a magnified SEM image of metal oxide particles 2 on the surface of a pore 121. As in Fig. As shown in Figure 6, the metal oxide particles have two forms, some of which protrude from within the honeycomb structure 10 into the pore 121. Fig. Figure 7 is a sectional view of an area near a metal oxide particle 2 in Fig. 6.

[0044] The metal oxide particle 2 has a fixed section 21 and a projection 22. The fixed section 21 is located within the honeycomb structure 10. The phrase "within the honeycomb structure 10" refers to the area within the cordierite surrounding the pore 121 and does not refer to the area within the pore 121 itself, which is provided for in the honeycomb structure 10. The fixed section 21 is a connecting section of the metal oxide particle 2 that is bonded to the cordierite, which serves as the main component of the honeycomb structure 10, and is fixed within the interior of the cordierite. In other words, the fixed section 21 is a section of the metal oxide particle 2 that creeps within the cordierite from the surface of the pore 121 to the side of the honeycomb structure 10 opposite the pore 121. In other words, the firmly attached section 21 is a section of the metal oxide particle 2 whose surface is covered with the cordierite.

[0045] The projection 22 is a section of the metal oxide particle 2 that protrudes from the surface of the pore 121 into the pore 121. In other words, the projection 22 is a section exposed from the surface of the cordierite. The projection 22 is associated with the firmly attached section 21.

[0046] In the porous ceramic structure 1, the honeycomb structure 10 is not subjected to any coating process using γ-aluminum oxide or the like (a so-called wash coating). Therefore, no coating intended to be formed by the aforementioned coating process is formed on the surfaces of the pores 121, and as a natural result, the metal oxide particles 2 are not firmly attached to the honeycomb structure 10 by means of such a coating.

[0047] Among a large number of metal oxide particles 2 contained in the porous ceramic structure 1, some metal oxide particles 2 are firmly attached to the surfaces of pores 121 within the pores 121, as described above, and the remaining metal oxide particles 2 are located entirely within the honeycomb structure 10. The same applies to the cerium-containing particles 3.

[0048] Then an example of the process for producing the porous ceramic structure 1 is given with reference to Fig.8. In the production of the porous ceramic structure 1, a structural raw material is first prepared by weighing and mixing a material for the honeycomb structure 10, a material for the cerium-containing particles 3 (e.g., CeO2), a material for the metal oxide particles 2, and ZnO, which serves as an aid. The material for the metal oxide particles 2 contains Fe and / or Mn and / or Co. For example, the material for the metal oxide particles 2 is Fe2O3, Mn2O3, or Co3O4. The main component of the material for the honeycomb structure 10 is cordierite, which serves as an aggregate of the honeycomb structure 10. The main component of the material for the honeycomb structure 10 can be, for example, kaolin, talc, or aluminum oxide, which is a raw material for cordierite. The material for the honeycomb structure 10 also contains other components such as a pore-forming agent and a binder.After the structural raw material has been subjected to dry mixing in a kneader, water is added and the structural raw material is further kneaded in the kneader into kneaded clay (step S11).

[0049] The time required for the specified dry mixing and the time required for the specified kneading are, for example, 15 minutes and 30 minutes, respectively. The dry mixing time and the kneading time can be modified in various ways. The specified raw material for the cerium-containing particles 3 can consist of salts such as cerium nitrate. The specified raw material for the metal oxide particles 2 can consist of salts such as iron nitrate, manganese nitrate, or cobalt nitrate.

[0050] In step S11, the raw material for the cerium-containing particles 3 and the raw material for the metal oxide particles 2 are added individually to the aggregate or the like of the honeycomb structure 10. However, the method of adding these raw materials can be modified in various ways. For example, the raw material for the metal oxide particles 2 can be immersed in CeO2, dried, and calcined to produce an additive, and this additive can be added to the aggregate or the like of the honeycomb structure 10. In the additive, a portion of the raw material for the metal oxide particles 2 is dissolved as a solid or adheres to CeO2.

[0051] The kneaded clay prepared in step S11 is formed into a column shape by a vacuum kneading machine or other machinery and then subjected to extrusion by an extruder to form a molded body with a honeycomb structure (hereinafter also referred to as a "honeycomb mold") (step S12). This honeycomb mold contains a grid-like partition that divides the mold into multiple cells, which serve as flow paths for a fluid. The honeycomb mold has a honeycomb diameter of 30 mm, a partition thickness of 12 mil (about 0.3 mm), and a cell density of 300 cells per square inch (cps), i.e., 46.5 cells / cm². 2 , and an outer wall thickness of approximately 0.6 mm. Alternatively, in step S12, the honeycomb press body can be formed using a molding process other than extrusion.

[0052] The honeycomb core produced in step S12 is then subjected to drying. There are no particular restrictions on the drying method. For example, the drying method can be hot air drying, microwave drying, dielectric drying, reduced-pressure drying, vacuum drying, or freeze-drying, or any combination of these methods. For example, the honeycomb core is microwave-dried to remove approximately 50 to 80% of its moisture content and then hot air-dried (for 6 to 20 hours at 60 to 100 °C). Preferably, the honeycomb core is microwave-dried to remove approximately 70% of its moisture content and then hot air-dried (for 12 hours at 80 °C).The honeycomb press body is then placed in a degreasing oven, which is kept at 450 °C, to remove (i.e., degrease) any organic components remaining in the honeycomb press body.

[0053] The honeycomb-pressed body is then subjected to a firing process (firing) to obtain the porous ceramic structure 1, which contains the honeycomb structure 10, the cerium-containing particles 3, and the metal oxide particles 2 (step S13). The porous ceramic structure 1 produced by the aforementioned manufacturing process contains no precious metals and can therefore be produced at low cost.

[0054] The firing process in step S13 is carried out, for example, with a predefined temperature profile under atmospheric pressure. Fig. Figure 9 is a diagram illustrating a detail of the firing step in step S13. Fig. 10 and Fig.Figure 11 are diagrams illustrating temperature profiles during a first firing step.

[0055] In the first firing step, the temperature of the honeycomb press body is initially reduced from a normal temperature (e.g. 20 °C) to a first temperature t1, which is less than or equal to a liquid phase formation temperature t L is raised (step S21). The liquid phase formation temperature t L As used herein, refers to a temperature at which the liquid phase formation of a raw material begins in the honeycomb press body, which is being heated. For example, the liquid phase formation temperature t L a temperature in the range of 1100 °C to 1170 °C. Fig. Figure 10 shows the case in which the first temperature t1 is equal to the liquid phase formation temperature t L As described above, the initial temperature t1 can be lower than the liquid phase formation temperature t. LThere are no particular restrictions on the rate at which the temperature is increased to the liquid phase formation temperature t. L before and e.g. this rate can be in the range of 100 °C / h to 200 °C / h.

[0056] Then the temperature of the honeycomb press body is raised from the first temperature t1 to a second temperature t2, which is higher than the first temperature t1 (step S22). The second temperature t2 is higher than the liquid phase formation temperature t. L and smaller than a crystallization temperature t C of the metal oxide particles 2. The crystallization temperature t C is a temperature at which the metal oxide particles 2 begin to crystallize and be deposited from the heated honeycomb core. For example, the crystallization temperature t COne temperature is in the range of 1250 °C to 1350 °C. The second temperature t2 is, for example, a temperature in the range of 1170 °C to 1250 °C. The rate of increasing the temperature of the honeycomb press body in step S22 is in the range of 100 °C / h to 200 °C / h.

[0057] Then the temperature of the honeycomb press body is raised from the second temperature t2 to a third temperature t3, which is higher than the second temperature t2 (step S23). The third temperature t3 is higher than the crystallization temperature t C and less than or equal to a maximum temperature t max For example, the maximum temperature t max a temperature in the range of 1350 °C to 1500 °C. Fig. Figure 10 shows the case in which the third temperature t3 is equal to the maximum temperature t max As described above, the third temperature t3 can be lower than the maximum temperature t. maxThere are no particular restrictions on the rate of increasing the temperature of the honeycomb press body in step S23, and this rate can, for example, be in the range of 25 °C / h to 200 °C / h and preferably in the range of 100 °C / h to 200 °C / h.

[0058] The temperature of the honeycomb press body is then monitored at the maximum temperature t for a specified period. max The temperature is held at the maximum temperature and then lowered to normal temperature (step S24). This completes the firing process of the honeycomb press body (step S13) and creates the porous ceramic structure 1. A period during which the temperature is held at the maximum temperature t max The temperature held in step S24 (hereinafter also referred to as a "maximum temperature holding period") is, for example, greater than or equal to five hours and less than or equal to 50 hours. If the third temperature t3 is less than the maximum temperature t maxThe temperature of the honeycomb press body changes from the third temperature t3 to the maximum temperature t in a period between steps S23 and S24. max raised. There are no particular restrictions on the rate at which the temperature is increased from the third temperature t3 to the maximum temperature t. max before and e.g. this rate can be in the range of 25 °C / h to 75 °C / h.

[0059] In the first firing step, the temperature rise of the honeycomb press body can be temporarily stopped in the middle of step S23, as shown in Fig. Figure 11 illustrates this. Specifically, when the temperature of the honeycomb press body is close to the crystallization temperature t. C Once the temperature has been raised in step S23, the temperature increase can be temporarily stopped, resulting in a state where the temperature of the honeycomb press body is at the crystallization temperature t. CThe remaining vehicle is held for a predetermined period (hereinafter also referred to as an "interim holding period"). The interim holding period is, for example, greater than or equal to one hour and less than or equal to 100 hours, and preferably greater than or equal to five hours and less than or equal to 75 hours. It should be noted that the interim holding period can be less than one hour and greater than 100 hours. The example given in Fig. Figure 10 illustrates a temperature profile where the holding time is zero. In the example shown in Fig. As illustrated in Figure 11, once the holding period has elapsed, the temperature of the honeycomb press body resumes its rise and is increased to the third temperature t3 (in the example shown in Figure 11). Fig. Figure 11 illustrates the same temperature as the maximum temperature t max ).

[0060] Then a second example of the firing step is given in step S13 with reference to Fig. 12 and Fig. 13 described. A precise procedure for the firing step is similar to that in Fig. Figure 9 illustrates this. Fig. 12 and Fig. Figure 13 are diagrams illustrating temperature profiles during a second firing step.

[0061] In the second firing step, as in the first firing step described above, the temperature of the honeycomb press body changes from a normal temperature to a first temperature t1, which is less than or equal to a liquid phase formation temperature t L is, raised (step S21 in Fig. 9) The liquid phase formation temperature t L For example, a temperature in the range of 1100 °C to 1170 °C. Fig. Figure 12 shows the case in which the first temperature t1 is equal to the liquid phase formation temperature t LAs described above, the initial temperature t1 can be lower than the liquid phase formation temperature t. L There are no particular restrictions on the rate at which the temperature is increased to the liquid phase formation temperature t. L before and, for example, this rate can range from 50 °C / h to 200 °C / h. In the example that is in Fig. As illustrated in Figure 12, the rate of temperature increase drops at the limit of approximately 900 °C.

[0062] Then the temperature of the honeycomb press body is raised from the first temperature t1 to a second temperature t2, which is higher than the first temperature t1 (step S22). The second temperature t2 is higher than the liquid phase formation temperature t. L and smaller than the crystallization temperature t C of the metal oxide particles 2. The crystallization temperature t CFor example, a temperature in the range of 1250 °C to 1350 °C. The second temperature t2, for instance, is in the range of 1170 °C to 1250 °C. In contrast to the first firing step, the rate of temperature increase of the honeycomb press body in step S22 is in the range of 1 °C / h to 10 °C / h.

[0063] Then the temperature of the honeycomb press body is raised from the second temperature t2 to a third temperature t3, which is higher than the second temperature t2 (step S23). The third temperature t3 is higher than the crystallization temperature t C and less than or equal to a maximum temperature t max The maximum temperature t max For example, a temperature in the range of 1350 °C to 1500 °C. Fig. Figure 12 shows the case in which the third temperature t3 is equal to the maximum temperature t max As described above, the third temperature t3 can be lower than the maximum temperature t. maxThere are no particular restrictions on the rate of increasing the temperature of the honeycomb press body in step S23, and this rate can, for example, range from 25 °C / h to 200 °C / h.

[0064] The temperature of the honeycomb press body is then monitored at the maximum temperature t for a specified period. max The temperature is held at the maximum temperature and then lowered to normal temperature (step S24). This completes the firing process of the honeycomb press body (step S13) and creates the porous ceramic structure 1. A period during which the temperature is held at the maximum temperature t max The temperature held in step S24 (i.e., a "maximum temperature holding period") is, for example, greater than or equal to five hours and less than or equal to 50 hours. If the third temperature t3 is less than the maximum temperature t max The temperature of the honeycomb press body changes from the third temperature t3 to the maximum temperature t in a period between steps S13 and 14. maxraised. There are no particular restrictions on the rate at which the temperature is increased from the third temperature t3 to the maximum temperature t. max and, for example, this rate can range from 25 °C / h to 75 °C / h.

[0065] In the second firing step, the temperature increase of the honeycomb press body can be temporarily stopped in the middle of step S23, as shown in Fig. Figure 13 illustrates this. Specifically, when the temperature of the honeycomb press body in step S23 is close to the crystallization temperature t. C Once the temperature has been raised, the increase is temporarily stopped, and a state is reached in which the temperature of the honeycomb press body is at the crystallization temperature t. CThe remaining vehicle is held for a predetermined period (i.e., an "interim holding period"). The interim holding period is, for example, in the range of one hour to 100 hours, and preferably in the range of five hours to 75 hours. It should be noted that the interim holding period can be less than one hour and greater than 100 hours. The example given in Fig. Figure 12 illustrates a temperature profile where the holding time is zero. In the example shown in Fig. As illustrated in Figure 13, once the holding period has elapsed, the temperature of the honeycomb press body resumes its rise and is increased to the third temperature t3 (in the example shown in Figure 13). Fig. Figure 13 illustrates the same temperature as the maximum temperature t max ).

[0066] Then a third example of the firing step is given in step S13 with reference to Fig.14 described. A precise procedure in the firing step is similar to that in Fig. 9. Fig. Figure 14 is a diagram illustrating a temperature profile during a third firing step.

[0067] In the third firing step, as in the first and second firing steps described above, the temperature of the honeycomb press body is reduced from a normal temperature to a first temperature t1, which is less than or equal to a liquid phase formation temperature t L is, raised (step S21 in Fig. 9) The liquid phase formation temperature t L For example, a temperature in the range of 1100 °C to 1170 °C. Fig. Figure 14 shows the case in which the first temperature t1 is equal to the liquid phase formation temperature t L As described above, the initial temperature t1 can be lower than the liquid phase formation temperature t. LThere are no particular restrictions on the rate at which the temperature is increased to the liquid phase formation temperature t. L and, for example, this rate can range from 50 °C / h to 200 °C / h. In the example given in Fig. As illustrated in Figure 14, the rate of temperature increase drops to a limit of about 900 °C.

[0068] Then the temperature of the honeycomb press body is raised from the first temperature t1 to a second temperature t2, which is higher than the first temperature t1 (step S22). The second temperature t2 is higher than the liquid phase formation temperature t. L and smaller than the crystallization temperature t C of the metal oxide particles 2. The crystallization temperature t CFor example, the temperature is in the range of 1250 °C to 1350 °C. The second temperature, t2, is in the range of 1170 °C to 1250 °C. Unlike the first and second firing steps, there are no specific restrictions on the rate of temperature increase of the honeycomb press body in step S22. The temperature increase rate is, for example, in the range of 10 °C / h to 100 °C / h.

[0069] Then the temperature of the honeycomb press body is raised from the second temperature t2 to a third temperature t3, which is higher than the second temperature t2 (step S23). The third temperature t3 is higher than the crystallization temperature t C and less than or equal to the maximum temperature t max The maximum temperature t max For example, a temperature in the range of 1350 °C to 1500 °C. Fig. Figure 14 shows the case in which the third temperature t3 is equal to the maximum temperature t maxAs described above, the third temperature t3 can be lower than the maximum temperature t. max There are no particular restrictions on the rate of increasing the temperature of the honeycomb press body in step S23, and this rate can, for example, range from 10 °C / h to 100 °C / h.

[0070] In the third firing step, the temperature rise of the honeycomb press body is temporarily halted in the middle of step S23. Specifically, this occurs when the temperature of the honeycomb press body in step S23 reaches the crystallization temperature t. C Once the temperature has risen, the increase is temporarily stopped, and a state is reached in which the temperature of the honeycomb press body is at the crystallization temperature t. CThe remaining temperature is held for a predetermined period (i.e., an "interim holding period"). For example, the intermediate holding period can range from one hour to 100 hours, and preferably from five hours to 75 hours. It should be noted that the intermediate holding period can be less than one hour and greater than 100 hours. Once the intermediate holding period has elapsed, the temperature of the honeycomb press body resumes its rise and is raised to the third temperature t3 (in the example shown in Fig. Figure 14 illustrates the same temperature as the maximum temperature t max ).

[0071] The temperature of the honeycomb press body is then monitored at the maximum temperature t for a specified period. maxThe temperature is held at the maximum temperature and then lowered to normal temperature (step S24). This completes the firing process of the honeycomb press body (step S13) and creates the porous ceramic structure 1. A period during which the temperature is held at the maximum temperature t max The maximum temperature holding period, as recorded in step S24, is, for example, in the range of five to 50 hours. If the third temperature t3 is lower than the maximum temperature t max The temperature of the honeycomb press body changes from the third temperature t3 to the maximum temperature t in a period between steps S23 and S24. max raised. There are no particular restrictions on the rate at which the temperature is increased from the third temperature t3 to the maximum temperature t. max and, for example, this rate can range from 25 °C / h to 75 °C / h.

[0072] The pressure drop and catalytic performance in the porous ceramic structures 1 according to Examples 1 to 10 are then described with reference to Tables 1 to 4. Comparative Example 1 is also described in the same manner. Among Examples 1 to 10, Examples 1 to 6 and Examples 7 to 10 differ in their material composition and crystalline phase composition, as described later (see Tables 2 and 3). It should be noted that the material composition shown in Table 2 indicates the composition of materials used to create the porous ceramic structure 1 (i.e., the fabricated composition). The crystalline phase composition shown in Table 3 is the composition of the crystalline phase in the fabricated porous ceramic structure 1. Table 1 Temperature rise rate C2 Temperature rise rate C3 Intermediate product holding period Maximum temperature holding period (°C / h) (°C / h) (h) (h) Example 1 12 60 10 7 Example 2 12 60 50 7 Example 3 3 60 0 7 Example 4 180 60 0 7 Example 5 200 200 0 7 Example 6 180 60 10 7 Example 7 12 60 10 7 Example 8 3 60 0 7 Example 9 180 60 0 7 Example 10 200 200 0 7 Comparative Example 1 12 60 0 7 Table 2 Material composition (mass %) FejMnjCo ratio MgO Al2O3 SiO2 Fe2O3 Mn2O3 CO3O4 CeO2 sum Example 1 12,7 31,0 46,3 2,6 2,4 0,0 5,0 100,0 1,0 Example 2 12,7 31,0 46,3 2,6 2,4 0,0 5,0 100,0 1,0 Example 3 12,7 31,0 46,3 2,6 2,4 0,0 5,0 100,0 1,0 Example 4 12,7 31,0 46,3 2,6 2,4 0,0 5,0 100,0 1,0 Example 5 12,7 31,0 46,3 2,6 2,4 0,0 5,0 100,0 1,0 Example 6 12,7 31,0 46,3 2,6 2,4 0,0 5,0 100,0 1,0 Example 7 12,7 31,0 46,3 0,0 0,0 5,5 4,5 100,0 1,2 Example 8 12,7 31,0 46,3 0,0 0,0 5,5 4,5 100,0 1,2 Example 9 12,7 31,0 46,3 0,0 0,0 5,5 4,5 100,0 1,2 Example 10 12,7 31,0 46,3 0,0 0,0 5,5 4,5 100,0 1,2 Comparative Example 1 12,7 31,0 46,3 2,6 2,4 0,0 5,0 100,0 1,0 Table 3 Composition of the crystalline phase (mass %) Cordierite Fe x Mn y O4 CO3O4 CeO2 More sum Example 1 87,5 2,5 0,0 2,2 1,8 100,0 Example 2 88,2 2,0 0,0 1,8 8,0 100,0 Example 3 85,3 3,8 0,0 3,6 7,3 100,0 Example 4 83,1 5,2 0,0 4,8 6,9 100,0 Example 5 81,8 6,0 0,0 5,8 6,4 100,0 Example 6 83,0 5,4 0,0 4,6 7,0 100,0 Example 7 87,3 0,0 2,6 2,4 7,7 100,0 Example 8 85,2 0,0 3,9 3,6 7,3 100,0 Example 9 82,1 0,0 5,2 4,9 7,8 100,0 Example 10 82,2 0,0 5,8 5,5 6,5 100,0 Comparative Example 1 87,0 1,8 0,0 3,0 8,2 100,0 Table 4 Average particle diameter (nm) Open-pore NO adsorption NO oxidation temperature Fe x Mn y O4 CO3O4 CeO2 (%) (µmol / g) (°C) Example 1 20 - 21 55 0,01 520 Example 2 22 - 23 52 0,01 520 Example 3 18 - 19 57 0,02 510 Example 4 20 - 22 62 0,04 490 Example 5 18 - 21 62 0,05 490 Example 6 19 - 22 60 0,04 490 Example 7 - 19 20 55 0,01 520 Example 8 - 19 21 57 0,02 510 Example 9 - 20 23 62 0,02 500 Example 10 - 15 20 62 0,02 500 Comparative Example 1 50 - 55 58 0,00 540

[0073] The porous ceramic structures 1 according to Examples 1, 2, and 7 were fired in the third firing step mentioned. The porous ceramic structures 1 according to Examples 3 and 8 were fired in the second firing step mentioned. The porous ceramic structures 1 according to Examples 4 to 6, 9, and 10 were fired in the first firing step mentioned. The temperature rise rate C2 in the table indicates the temperature rise rate (°C / h) in step S22, as described above (i.e., the temperature rise rate per hour). The temperature rise rate C3 indicates the temperature rise rate (°C / h) in step S23, as described above. The holding time (h) indicates the period during which the temperature remained at the crystallization temperature t. CThe maximum temperature holding period (h) indicates the time during which the temperature is maintained at the maximum temperature t. max is held in step S24, which is described above.

[0074] In Examples 1, 2, and 7, the holding time in the third firing step was changed. In Examples 1, 2, and 7, the temperature rise rate C2 was 12 °C / h and ranged from 1 °C / h to 10 °C / h as well as from 100 °C / h to 200 °C / h. The temperature profiles according to Examples 1, 2, and 7 were of the type described in Fig. Figure 14 illustrates this. In Examples 3 and 8, the temperature rise rate C2 during the second firing step was 3 °C / h (i.e., in the range of 1 °C / h to 10 °C / h). It should be noted that the holding period in Examples 3 and 8 was not set. The temperature profiles according to Examples 3 and 8 were of the type described in Fig.12 is illustrated.

[0075] In Examples 4 to 6, 9, and 10, the temperature rise rate C2 during the first firing step was in the range of 180 °C / h to 200 °C / h (i.e., greater than or equal to 100 °C and less than or equal to 200 °C / h). In Examples 4, 5, 9, and 10, the temperature rise rate C2 was in the range of 100 °C / h to 200 °C / h, and the temperature rise rate C3 was modified. It should be noted that the holding period was not set in Examples 4, 5, 9, and 10. The temperature profiles according to Examples 4, 5, 9, and 10 were of the type described in Fig. 10 is illustrated. In Example 6, the temperature rise rate C2 was in the range of 100 °C / h to 200 °C / h and the holding period was set to 10 hours. The temperature profile according to Example 6 was of the type described in Fig. 11 is illustrated.

[0076] The Fe / Mn / Co ratio in the table was obtained by dividing the sum of the Fe content (wt%) with respect to Fe2O3, the Mn content (wt%) with respect to Mn2O3 and the Co content (wt%) with respect to Co3O4 by the CeO2 content (wt%) in the material composition of the porous ceramic structure 1.

[0077] The composition of the crystalline phase in the porous ceramic structure 1 (i.e., the mass ratio of the crystalline phase components) was identified and quantified as follows. Measurements of the crystalline phase of each particle were performed on a prepared sample using an X-ray diffractometer (rotational counter-cathode X-ray diffractometer: RINT 2500, manufactured by Rigaku). The X-ray diffraction conditions were a CuKα radiation source, 50 kV, 300 mA, and 2θ in the range of 10° to 60°, and the resulting X-ray diffraction data were analyzed using commercial X-ray data analysis software. Fe was present in the crystalline phase. x Mn y O4 is a spinel oxide composed of Fe and Mn. In Fe x Mn y O4 were x and y positive numerical values ​​that satisfy x + y = 3. An average particle diameter of Fe x Mn yO4, where x and y were positive numerical values ​​satisfying x + y = 3, was assumed to be the diameter of crystallites. The diameter of crystallites was calculated based on the data obtained by the aforementioned X-ray diffractometry using the X-ray diffractometer, by applying these data to the Scherrer equation (τ = Kλ / βcosθ). Here, τ was the average size of crystallites, K was the form factor (a factor that establishes a relationship between the sizes of crystallites contained in a solid and the peak width of a diffraction pattern), λ was the X-ray wavelength, β was the total width at the peak value (in units of radians), and θ was the Bragg angle. In the crystalline phase, Co3O4 was a spinel oxide of Co. An average particle diameter of Co3O4 was calculated in the same way as the average particle diameter of Fe. x Mn yO4 was obtained. An average particle diameter of CeO2 was also obtained in the same way. Quantifications of Fe x Mn y O4, Co3O4, and CeO2 were analyzed by Rietveld analysis using the obtained X-ray diffraction data. The open porosity of the porous ceramic structure 1 was measured by the Archimedes method using deionized water as the medium. As described above, the porous ceramic structure 1 exhibited a lower pressure drop with increasing open porosity.

[0078] NO adsorption in the porous ceramic structure 1 was achieved as follows. First, an adsorption test was performed by introducing an NO-containing introductory gas to a sample corresponding to the porous ceramic structure 1. The introductory gas was a mixture containing 200 ppm by volume of NO and 10 vol% of oxygen (O2), with helium (He) as a balancing gas. The adsorption test was carried out at 250 °C for 60 minutes. After completion of the adsorption test, the sample was subjected to temperature-programmed desorption in He, and a portion of the resulting extracted gas was taken as a sample. This extracted gas was then analyzed using a mass spectrometer (GSD 320, manufactured by Pfeiffer Vacuum) to determine the amount of NO adsorbed in the sample.In general, there is a tendency for the porous ceramic structure 1 to exhibit greater catalytic performance with increasing NO adsorption.

[0079] The NO oxidation temperature of the porous ceramic structure 1 was obtained as follows. First, the relationship between the temperature and the NO₂ conversion rate in the porous ceramic structure 1 was determined. The NO₂ conversion rate was the ratio of the conversion of NO to NO₂ in an NO-containing sampled gas at a space velocity (SV) of 24,400 h⁻¹. -1A sample of NO was supplied to and passed through porous ceramic structure 1. The initial gas contained 100 ppm NO, 1500 ppm CO, 5% CO2, 450 ppm propane (C3H6), and 2% H2O. Analysis of the extracted gas was performed by Fourier transform infrared spectroscopy (FT-IR). The NO2 conversion rate was approximately 0% at low temperatures, gradually increasing to a peak with increasing temperature and then gradually decreasing. Porous ceramic structure 1 exhibited greater catalytic efficiency as the NO2 conversion rate increased. When the relationship between the NO2 conversion rate and temperature was established, the temperature was increased from the low-temperature side in accordance with the aforementioned relationship, and a temperature at which the NO2 conversion rate reached half its peak value was determined to be the NO oxidation temperature.The porous ceramic structure 1 exhibits greater catalytic performance with decreasing NO oxidation temperature.

[0080] In Examples 1 to 6, the Fe content is 2.6 wt% (i.e., in the range of 0.1 wt% to 3.0 wt%) with respect to Fe₂O₃, the Mn content is 2.4 wt% (i.e., in the range of 0.1 wt% to 3.0 wt%) with respect to Mn₂O₃, and the Ce content is 5.0 wt% (i.e., in the range of 0.1 wt% to 10 wt%) with respect to CeO₂. The porous ceramic structures 1 according to Examples 1 to 6 contain essentially no Co, and therefore the Co content therein is 0.0 wt% with respect to Co₃O₄. The Fe / Mn / Co ratio (i.e., the ratio of the sum of the Fe content with respect to Fe2O3, the Mn content with respect to Mn2O3, and the Co content with respect to Co3O4 to the Ce content with respect to CeO2) is 1.0 (i.e., in the range of 0.8 to 9.5).

[0081] The porous ceramic structures 1 according to Examples 7 to 10 contain essentially no Fe and Mn, and therefore the Fe content is 0.0 wt% with respect to Fe₂O₃ and the Mn content is 0.0 wt% with respect to Mn₂O₃. The Co content is 5.5 wt% (i.e., in the range of 3.0 wt% to 6.0 wt%) with respect to Co₃O₄ and the Ce content is 4.5 wt% (i.e., in the range of 1.5 wt% to 4.5 wt%) with respect to CeO₂. The Fe / Mn / Co ratio is 1.2 (i.e., in the range of 1.0 to 4.0). In this case, the Fe / Mn / Co ratio is essentially the ratio of the Co content with respect to Co₃O₄ to the Ce content with respect to CeO₂.

[0082] Fig. Figure 15 shows a SEM image of the surface of the porous ceramic structure 1 according to Example 3. Fig. Figure 16 shows an enlarged SEM image of the surface of a pore 121 in the porous ceramic structure 1 according to Example 3. Fig. Figure 17 shows a SEM image of the surface of the porous ceramic structure 1 according to Example 4. Fig.Figure 18 shows an enlarged SEM image of the surface of a pore 121 in the porous ceramic structure 1 according to Example 4. Fig. 15 and Fig. 17 black sections correspond to pores 121, gray sections correspond to the honeycomb structure 10, and white sections correspond to the metal oxide particles 2 and the cerium-containing particles 3. In Fig. 15 to Fig. 18 The metal oxide particles 2 and the cerium-containing particles 3 are present on the surfaces of the pores 121 in the porous ceramic structure 1.

[0083] In Examples 1 to 10, the content of metal oxide particles 2 ranges from 2.0 wt% to 6.0 wt% (i.e., from 0.3 wt% to 8.0 wt%). As described above, the content of metal oxide particles 2, as used herein, refers to the mass ratio of metal oxide particles 2 in the crystalline phase and the sum of the mass ratios of Fe. x Mn yO4 and Co3O4 in the crystalline phase are shown in Table 3. In examples 1 to 10, the average particle diameter of the metal oxide particles 2 is in the range of 15 nm to 22 nm (i.e., in the range of 10 nm to 1 µm).

[0084] In examples 1 to 10, the open porosity is in the range of 52% to 62% and relatively high, such that the pressure drop in the porous ceramic structure 1 is kept low. In examples 1 to 10, the NO absorption is in the range of 0.01 (µmol / g) to 0.05 (µmol / g) and the NO oxidation temperature is in the range of 490 °C to 520 °C and low. This indicates that the porous ceramic structures 1 exhibit high catalytic efficiency. Furthermore, in Example 5, with a temperature rise rate C3 of 200 °C / h (i.e., in the range of 100 °C / h to 200 °C / h), the NO absorption is 0.05 (µmol / g) and is the highest, which is greater than the NO absorption (0.04 µmol / g) in Example 4 with a temperature rise rate C3 of 60 °C / h.

[0085] On the other hand, in comparative example 1, the raw material composition is the same as in examples 1 to 6, but the temperature rise rate C2 is 12 °C / h. This means that the temperature rise rate C2 in comparative example 1 lies outside both the range of 1 °C / h to 10 °C / h (examples 3 and 8) and the range of 100 °C / h to 200 °C / h (examples 4 to 6, 9, and 10). Unlike examples 1, 2, 6, and 7, the holding period is not included in the temperature profile in comparative example 1. Therefore, the mass ratio of the metal oxide particles 2 in the crystalline phase is 1.8 wt% and lower than the mass ratios in examples 1 to 6 (in the range of 2.0 wt% to 6.0 wt%). Furthermore, the average particle diameter of the metal oxide particles is 250 nm and larger than the average particle diameters in examples 1 to 6 (in the range of 18 nm to 22 nm).Accordingly, in comparative example 1, the NO absorption is 0.00 (µmol / g) and the NO oxidation temperature is 540 °C and high.

[0086] As described above, the porous ceramic structure 1 contains the structural body (i.e., the honeycomb structure 10), the cerium-containing particles 3, and the metal oxide particles 2. The honeycomb structure 10 is a porous element composed primarily of cordierite. The cerium-containing particles 3 are tightly bound to the honeycomb structure 10. The metal oxide particles 2 are particles of an oxide possessing a spinel structure and containing Fe and / or Mn and / or Co. The metal oxide particles 2 are tightly bound to the inner surfaces of gas cavities (i.e., pores 121) within the honeycomb structure 10. Each metal oxide particle 2 contains the tightly bound section 21 and the projection 22. The tightly bound section 21 is located within the honeycomb structure 10. The projection 22 is connected to the tightly bound section 21 and projects into a pore 121.

[0087] If the porous ceramic structure 1 contains Fe, the Fe content ranges from 0.1 wt% to 3.0 wt% relative to Fe₂O₃, and if the porous ceramic structure 1 contains Mn, the Mn content ranges from 0.1 wt% to 3.0 wt% relative to Mn₂O₃. If the porous ceramic structure 1 contains Co along with Fe or Mn, the Co content ranges from 0.1 wt% to 3.0 wt% relative to Co₃O₄, and if the porous ceramic structure 1 contains only Co, without Fe or Mn, the Co content ranges from 0.2 wt% to 6.0 wt% relative to Co₃O₄. The Ce content ranges from 0.1 wt% to 10 wt% relative to CeO₂. The ratio of the sum of the Fe content (Fe2O3), the Mn content (Mn2O3), and the Co content (Co3O4) to the Ce content (CeO2) (i.e., the Fe / Mn / Co ratio) is in the range of 0.8 to 9.5. The content of metal oxide particles 2 is in the range of 0.3 wt% to 8.0 wt%.

[0088] Accordingly, as shown in Examples 1 to 10, it is possible to reduce the pressure drop in the porous ceramic structure 1 and to increase NO adsorption in the porous ceramic structure 1. It is also possible to increase the NO₂ conversion rate and to decrease the NO combustion temperature in the porous ceramic structure 1. Furthermore, it is possible to increase the rate of CO to CO₂ conversion and the rate of CH₄ to CO₂ and H₂O conversion. In other words, the use of the above-described configuration of the porous ceramic structure 1 makes it possible to create a porous ceramic structure 1 with low pressure drop and high catalytic efficiency.

[0089] In the porous ceramic structure 1, the Fe / Mn / Co ratio is in the range of 0.8 to 9.5, as described above. Therefore, it is possible to advantageously generate the metal oxide particles 2 of an oxide possessing a spinel structure during the fabrication of the porous ceramic structure 1. Since the content of the metal oxide particles 2 is in the range of 0.3 wt% to 8.0 wt%, as described above, the porous ceramic structure 1 can advantageously achieve both a reduction in pressure drop and an improvement in catalytic performance.

[0090] The porous ceramic structure 1 with low pressure loss and high catalytic performance, as described above, is particularly suitable for use in a diesel particulate filter that collects suspended particles in an exhaust gas emitted by a diesel engine.

[0091] As described above, if the porous ceramic structure contains Fe, the Fe content is preferably in the range of 1.5 wt% to 3.0 wt% relative to Fe₂O₃, and if the porous ceramic structure contains Mn, the Mn content is preferably in the range of 1.5 wt% to 3.0 wt% relative to Mn₂O₃. If the porous ceramic structure contains Co together with Fe or Mn, the Co content is preferably in the range of 1.5 wt% to 3.0 wt% relative to Co₃O₄, and if the porous ceramic structure contains Co without Fe and Mn, the Co content is preferably in the range of 3.0 wt% to 6.0 wt% relative to Co₃O₄. Furthermore, the Ce content is preferably in the range of 1.5 wt% to 4.5 wt% relative to CeO₂. The ratio of the sum of the Fe content with respect to Fe2O3, the Mn content with respect to Mn2O3 and the Co content with respect to Co3O4 to the Ce content with respect to CeO2 (i.e. the Fe / Mn / Co ratio) is preferably in the range of 1.0 to 4.0.This allows the porous ceramic structure 1 to have a greater catalytic performance.

[0092] As described above, the metal oxide particles 2 are more preferably particles of an oxide possessing a spinel structure containing Fe, Mn, and oxygen, or particles of an oxide possessing a spinel structure containing Co and oxygen. This allows the porous ceramic structure 1 to exhibit greater catalytic performance.

[0093] As described above, the average particle diameter of the metal oxide particles 2 is preferably in the range of 10 nm to 1 µm. This allows the porous ceramic structure 1 to advantageously achieve both a reduction in pressure loss and an improvement in catalytic performance.

[0094] The aforementioned process for producing the porous ceramic structure 1 comprises the step of preparing kneaded clay by kneading raw materials (step S11), the step of obtaining a pressed body by shaping the kneaded clay (step S12), and the step of firing the pressed body (step S13). The aforementioned raw materials contain cordierite, Ce, and Fe and / or Mn and / or Co. Step S13 forms the porous ceramic structure 1, which includes the porous structural body (i.e., the honeycomb structure 10), which is essentially composed of cordierite, the cerium-containing particles 3, which are firmly attached to the honeycomb structure 10, and the metal oxide particles 2, i.e., particles of an oxide possessing a spinel structure, containing Fe and / or Mn and / or Co, and which are firmly attached to the inner surfaces of gas cavities (i.e., pores 121) in the honeycomb structure 10.

[0095] Step S13 includes the step of increasing the temperature of the aforementioned press body to the first temperature t1 that is less than or equal to the liquid phase formation temperature t L is (step S21), the step of increasing the temperature of the press body from the first temperature t1 to the second temperature t2, which is greater than the liquid phase formation temperature t L and smaller than the crystallization temperature t C the metal oxide particle 2 is (step S22), the step of increasing the temperature of the press body from the second temperature t2 to the third temperature t3, which is greater than the crystallization temperature t C and less than or equal to the maximum temperature t max is (step S23), and the step of maintaining the temperature of the press body at the maximum temperature t maxfor a specified period (i.e., the maximum temperature holding period) (step S24). In the aforementioned first firing step, the temperature rise rate C2 in step S22 is in the range of 100 °C / h to 200 °C / h. This manufacturing process enables the simple production of the porous ceramic structure 1 with low pressure drop and high catalytic performance, as shown in Examples 4 to 6, 9 and 10.

[0096] As described above, the temperature rise rate C3 in step S23 is preferably also in the range of 100 °C / h to 200 °C / h. This allows the porous ceramic structure 1 to exhibit high catalytic performance.

[0097] Preferably, in step S23, the temperature of the aforementioned press body is measured for a specified period (i.e., the holding period) at the crystallization temperature t. CThis allows the porous ceramic structure 1 to exhibit high catalytic performance.

[0098] Step S13, described above, includes steps S21 to S24, and in the second firing step mentioned, the temperature rise rate C2 in step S22 is in the range of 1 °C / h to 10 °C / h. This manufacturing process enables the simple production of the porous ceramic structure 1 with low pressure drop and high catalytic performance, as shown in Examples 3 and 8.

[0099] Preferably, in step S23, the temperature of the aforementioned component is measured at the crystallization temperature t for a specified period (i.e., the holding period). C This allows the porous ceramic structure 1 to exhibit greater catalytic performance.

[0100] Step S13, described above, includes steps S21 to S24, and in the third firing step mentioned, the temperature of the pressed body is maintained at the crystallization temperature t for a specified period (i.e., the holding period). C This manufacturing process enables the simple production of the porous ceramic structure 1 with low pressure loss and high catalytic performance, as shown in Examples 1, 2 and 7.

[0101] The porous ceramic structure 1 and the method for producing the porous ceramic structure 1 described above can be modified in various ways.

[0102] For example, the average particle diameter of the metal oxide particles 2 can be in the range of 10 nm to 1 µm.

[0103] The metal oxide particles 2 can be particles of an oxide possessing a spinel structure containing only one of Fe and Mn. The metal oxide particles 2 can also be particles of an oxide possessing a spinel structure containing Fe and Co, but no Mn, or particles of an oxide possessing a spinel structure containing Mn and Co, but no Fe. The metal oxide particles 2 can also be particles of an oxide possessing a spinel structure containing all of Fe, Mn, and Co. The metal oxide particles 2 can contain a metal other than Fe, Mn, and Co.

[0104] In the porous ceramic structure 1, particles other than the cerium-containing particles 3 and the metal oxide particles 2 can be firmly attached to the honeycomb structure 10 (i.e., the structural body).

[0105] In the porous ceramic structure 1, the shape of the said structural body is not limited to a honeycomb shape and can be one of various shapes (e.g. a generally cylindrical shape) besides the honeycomb shape.

[0106] The method for producing the porous ceramic structure 1 is not limited to the method described above and can be modified in various ways.

[0107] The porous ceramic structure 1 can be used in applications other than for use in diesel particulate filters.

[0108] The configurations of the preferred embodiments and variations described above can be suitably combined, provided there are no mutual inconsistencies.

[0109] While the invention has been precisely demonstrated and described, the preceding description is illustrative in all aspects and not limiting. It is therefore understood that numerous modifications and variations can be developed without deviating from the scope of the invention. Industrial usability

[0110] The present invention is applicable to filters that collect suspended particles, e.g. diesel particulate filters that collect suspended particles in exhaust gases emitted by diesel engines. Reference symbol list 1 Porous ceramic structure 2 metal oxide particles 3 Cerium-containing particles 8 Exhaust gas purification system 10 honeycomb structure 11 Exterior wall 12 Partition wall 121 pores 13th cell 14 Sealing device 21 Fixed section 22 lead 81 DPF 82 DOC 83 CSF 85 SCR exhaust catalyst 86 Urea injector A1 Arrow S11 to S13, S21 to S24 step t1 to t3 temperature t C Crystallization temperature t L Liquid phase formation temperature t max Maximum temperature

Claims

[1] Method for producing a porous ceramic structure comprising: a) Making kneaded clay by kneading a raw material; b) Obtaining a pressed body by shaping the kneaded clay and c) Firing of the pressed body, wherein the raw material contains the following: Cordierite; Cer and iron and / or manganese and / or cobalt, wherein the process c) forms a porous ceramic structure comprising a porous structural body formed essentially of cordierite, cerium-containing particles firmly attached to the structural body, and metal oxide particles each firmly attached to an inside of a pore in the structural body, wherein the metal oxide particles are particles of an oxide having a spinel structure containing iron and / or manganese and / or cobalt, The process c) includes the following: c1) Raising the temperature of the pressed body to a first temperature that is less than or equal to a liquid phase formation temperature; c2) Increasing the temperature of the pressed body from the first temperature to a second temperature that is higher than the liquid phase formation temperature and lower than a crystallization temperature of the metal oxide particles; c3) Increasing the temperature of the pressed body from the second temperature to a third temperature which is greater than the crystallization temperature and less than or equal to a maximum temperature; and c4) Maintaining the temperature of the press body at the maximum temperature for a specified period of time, and wherein a rate of temperature increase in process c2) in the range of 100 °C / h to 200 °C / h. [2] Method for producing a porous ceramic structure according to claim 1, wherein the rate of increasing the temperature in process c3) is in the range of 100 °C / h to 200 °C / h. [3] Method for producing a porous ceramic structure comprising the following: a) Making kneaded clay by kneading a raw material; b) Obtaining a pressed body by shaping the kneaded clay and c) Firing of the pressed body, wherein the raw material contains the following: Cordierite; Cer and iron and / or manganese and / or cobalt, wherein the process c) forms a porous ceramic structure comprising a porous structural body formed essentially of cordierite, cerium-containing particles firmly attached to the structural body, and metal oxide particles each firmly attached to an inside of a pore in the structural body, wherein the metal oxide particles are particles of an oxide having a spinel structure containing iron and / or manganese and / or cobalt, The process c) includes the following: c1) Raising the temperature of the pressed body to a first temperature that is less than or equal to a liquid phase formation temperature; c2) Increasing the temperature of the pressed body from the first temperature to a second temperature that is higher than the liquid phase formation temperature and lower than a crystallization temperature of the metal oxide particles; c3) Increasing the temperature of the pressed body from the second temperature to a third temperature which is greater than the crystallization temperature and less than or equal to a maximum temperature; and c4) Maintaining the temperature of the press body at the maximum temperature for a specified period of time, and wherein a rate of temperature increase in process c2) in the range of 1 °C / h to 10 °C / h. [4] Method for producing a porous ceramic structure according to any one of claims 1 to 3, wherein the temperature of the press body is maintained at the crystallization temperature for a predetermined period of time in process c3). [5] Method for producing a porous ceramic structure comprising the following: a) Making kneaded clay by kneading a raw material; b) Obtaining a pressed body by shaping the kneaded clay and c) Firing of the pressed body, wherein the raw material contains the following: Cordierite; Cer and iron and / or manganese and / or cobalt, wherein the process c) forms a porous ceramic structure comprising a porous structural body formed essentially of cordierite, cerium-containing particles firmly attached to the structural body, and metal oxide particles each firmly attached to an inside of a pore in the structural body, wherein the metal oxide particles are particles of an oxide having a spinel structure containing iron and / or manganese and / or cobalt, The process c) includes the following: c1) Raising the temperature of the pressed body to a first temperature that is less than or equal to a liquid phase formation temperature; c2) Increasing the temperature of the pressed body from the first temperature to a second temperature that is higher than the liquid phase formation temperature and lower than a crystallization temperature of the metal oxide particles; c3) Increasing the temperature of the pressed body from the second temperature to a third temperature which is greater than the crystallization temperature and less than or equal to a maximum temperature; and c4) Maintaining the temperature of the press body at the maximum temperature for a specified period of time, and wherein the temperature of the press body in process c3) is maintained at the crystallization temperature for a specified period of time.

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