Abstract:
A honeycomb structure includes a plurality of honeycomb units and an adhesive layer. Each of the plurality of honeycomb units has a longitudinal direction and includes zeolite, an inorganic binder, a first end face, a second end face, and a cell wall. The second end face is located opposite to the first end face in the longitudinal direction. The cell wall extends from the first end face to the second end face along the longitudinal direction to define cells. The adhesive layer includes zeolite and is provided between the plurality of honeycomb units to connect the plurality of honeycomb units. A ratio of a coefficient of thermal expansion of the adhesive layer to a coefficient of thermal expansion of each of the plurality of honeycomb units is in a range from approximately 0.8 to approximately 1.2.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of PCT International Application No. PCT/JP2008/059269, filed on May 20, 2008, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to honeycomb structures. 
         [0004]    2. Description of the Related Art 
         [0005]    A large number of techniques have been developed in relation to conversion of automobile exhaust gas. With an increase in traffic, however, countermeasures taken against exhaust gas have hardly been satisfactory. Not only in Japan but also globally, automobile emission control is going to be further tightened. Especially, control on NOx in diesel emissions has been getting extremely strict. NOx reduction, which conventionally has been achieved by controlling the combustion system of an engine, has become too much to be handled by that alone. As a diesel NOx conversion system responding to such a problem, NOx reduction systems using ammonia as a reducing agent (referred to as “SCR [selective catalytic reduction] systems”) have been proposed. A honeycomb structure is known as a catalyst carrier used in such systems. 
         [0006]    This honeycomb structure has, for example, multiple cells (through holes) extending from one to another of the end faces of the honeycomb structure along its longitudinal directions, and these cells are separated from each other by cell walls supporting a catalyst. Accordingly, when exhaust gas is caused to flow through such a honeycomb structure, NOx included in the exhaust gas is converted by the catalyst supported by the cell walls. Therefore, it is possible to treat the exhaust gas. 
         [0007]    In general, the cell walls of such a honeycomb structure are formed of cordierite, and for example, zeolite (with iron or copper introduced through ion exchange) is supported by these cell walls as a catalyst. In addition, it has been proposed to form a honeycomb structure using zeolite for cell walls (for example, International Publication Number WO 06/137149 A1). 
         [0008]    The entire contents of International Publication Number WO 06/137149 A1 are incorporated herein by reference. 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect of the present invention, a honeycomb structure includes a plurality of honeycomb units and an adhesive layer. Each of the plurality of honeycomb units has a longitudinal direction and includes zeolite, an inorganic binder, a first end face, a second end face, and a cell wall. The second end face is located opposite to the first end face in the longitudinal direction. The cell wall extends from the first end face to the second end face along the longitudinal direction to define cells. The adhesive layer includes zeolite and is provided between the plurality of honeycomb units to connect the plurality of honeycomb units. A ratio of a coefficient of thermal expansion of the adhesive layer to a coefficient of thermal expansion of each of the plurality of honeycomb units is in a range from approximately 0.8 to approximately 1.2. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a schematic perspective view of a honeycomb structure according to an embodiment of the present invention; and 
           [0012]      FIG. 2  is a schematic perspective view of a honeycomb unit of the honeycomb structure of  FIG. 1  according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0013]    A description is given below, with reference to the accompanying drawings, of an embodiment of the present invention. 
         [0014]      FIG. 1  is a schematic diagram illustrating a honeycomb structure according to the embodiment of the present invention.  FIG. 2  is a schematic diagram illustrating a honeycomb unit that is the basic unit of the honeycomb structure illustrated in  FIG. 1 . 
         [0015]    As illustrated in  FIG. 1 , a honeycomb structure  100  according to the embodiment of the present invention has two open (end) faces  110  and  115 . Further, the honeycomb structure  100  has a peripheral coat layer  120  formed at its peripheral surface except for the end faces  110  and  115 . 
         [0016]    The honeycomb structure  100  is formed by, for example, joining multiple pillar ceramic honeycomb units  130  shown in  FIG. 2  (16 units in a four-by-four matrix in the case of  FIG. 1 ) by interposing an adhesive layer  150  and thereafter cutting the peripheral side along a predetermined shape (a cylindrical shape in the case of  FIG. 1 ). 
         [0017]    As illustrated in  FIG. 2 , the honeycomb unit  130  includes multiple cells (through holes)  121 , extending from one end to another end of the honeycomb unit  130  along its longitudinal directions and having two open end faces, and cell walls  123  separating the cells  121 . The honeycomb unit  130  contains zeolite, which contributes to conversion of NOx, as an SCR system. Accordingly, it is not always necessary to have the cell walls  123  support a noble metal catalyst in the case of using the honeycomb structure  100  according to the embodiment of the present invention as a catalyst support for NOx conversion. However, the cell walls  123  may further support a noble metal catalyst. 
         [0018]    The honeycomb structure  100  thus configured is used as, for example, the catalyst carrier of a urea SCR system having a urea tank. When exhaust gas is caused to flow through this urea SCR system, the urea contained in the urea tank reacts with water in the exhaust gas to generate ammonia: 
         [0000]      CO(NH 2 ) 2 +H 2 O→+2NH 3 +CO 2   (1) 
         [0019]    When this ammonia flows, together with exhaust gas including NOx, into each cell  121  from one of the open faces  110  and  115  (for example, the open face  110 ) of the honeycomb structure  100 , the following reactions occur in this gas mixture on zeolite included in the cell walls  123 : 
         [0000]      4NH 3 +4NO+O 2 →4N 2 +6H 2 O,  (2-1) 
         [0000]      8NH 3 +6NO 2 →7N 2 +12H 2 O,  (2-2) 
         [0000]      2NH 3 +NO+NO 2 →2N 2 +3H 2 O.  (2-3) 
         [0020]    Thereafter, the converted exhaust gas is discharged from another one of the open faces  110  and  115  (for example, the open face  115 ) of the honeycomb structure  100 . Thus, by causing exhaust gas to flow inside the honeycomb structure  100 , NOx in the exhaust gas can be treated. Here, a method is shown that supplies NH 3  by hydrolyzing urea water, but NH 3  may also be supplied by other methods. 
         [0021]    The conventional honeycomb structure described in International Publication Number WO 06/137149 A1 is manufactured by joining a predetermined number of ceramic units (honeycomb units) by interposing an adhesive layer and thereafter cutting this joined body into a desired shape. 
         [0022]    However, the conventional honeycomb structure has the problem of frequent occurrence of peeling or generation of cracks at the interface of the honeycomb units and the adhesive layer due to the difference in the coefficient of thermal expansion and the like between the honeycomb units and the adhesive layer during use of the honeycomb structure, that is, while high-temperature exhaust gas is being caused to flow through the honeycomb structure. 
         [0023]    It is often observed in the conventional honeycomb structure that peeling occurs at the interface of its honeycomb units and adhesive layer (hereinafter simply referred to as “interface”) because of an increase in internal stress at the interface due to the difference in the coefficient of thermal expansion between the honeycomb units and the adhesive layer during the period of service of the honeycomb structure. Here, it is possible to prevent the peeling at the interface at the initial stage of use of the honeycomb structure by matching the coefficients of thermal expansion of the honeycomb units and the adhesive layer in advance. Even in this case, however, after the honeycomb structure is used for a long time, a high internal stress can result between the honeycomb units and the adhesive layer because of the difference in the degradation behavior over time of material characteristics between the honeycomb units and the adhesive layer, for example, the difference between sintering shrinkage rates. Therefore, there is a problem in that peeling occurs at the interface after using the honeycomb structure over a long period even if such a measure is taken. 
         [0024]    On the other hand, the honeycomb structure  100  according to the embodiment of the present invention is formed so that its ratio of the coefficient of thermal expansion of the adhesive layer A a  to the coefficient of thermal expansion of the honeycomb unit A h  (the ratio A a /A h ) is within the range of approximately 0.8 to approximately 1.2. In addition, both the honeycomb units  130  and the adhesive layer  150  of the honeycomb structure  100  according to the embodiment of the present invention are formed of a material prepared mainly of zeolite. Therefore, in the honeycomb structure  100  according to the embodiment of the present invention, the material characteristics of the honeycomb units  130  and the adhesive layer  150  show equality in degradation behavior over time during the period of use of the honeycomb structure  100 . Accordingly, in the honeycomb structure  100  according to the embodiment of the present invention, the internal stress at the interface is controlled at the initial stage of its use, and an increase in the internal stress at the interface is also easily controllable even after its use over a long period. 
         [0025]    Based at least on these effects, it is more likely that the honeycomb structure  100  according to the embodiment of the present invention can prevent peeling and/or cracks from being caused at the interface of the honeycomb units  130  and the adhesive layer  150  for a long period of time. 
         [0026]    Each honeycomb unit  130  includes an inorganic binder in addition to zeolite. Further, the honeycomb unit  130  may further include inorganic particles other than zeolite and/or inorganic fibers. 
         [0027]    Preferably, zeolite is, for example, β-zeolite, zeolite Y, ferrierite, zeolite ZSM-5, mordenite, faujasite, zeolite A, or zeolite L. Alternatively, zeolite may have Fe, Cu, Ni, Co, Zn, Mn, Ag, or V introduced therein through ion exchange. 
         [0028]    Further, in zeolite, the ratio by weight of silica to alumina is preferably in the range of approximately 30 to approximately 50. 
         [0029]    Inorganic sol, a clay-based binder, etc., may be used as the inorganic binder. Examples of the inorganic sol include alumina sol, silica sol, titania sol, water glass and the like. Examples of the clay-based binder include clay, kaolin, montmonrillonite, clays of a double-chain structure type such as sepiolite and attapulgite, and the like. These may be used alone or in combination. 
         [0030]    Of these, at least one selected from the group consisting of alumina sol, silica sol, titania sol, water glass, sepiolite, and attapulgite is desirable. 
         [0031]    The inorganic particles other than zeolite are desirably of alumina, silica, zirconia, titania, ceria, mullite, precursors thereof or the like. These particles other than zeolite may be used alone or in combination. Of these, alumina and zirconia are particularly desirable. 
         [0032]    Further, in the case of adding inorganic fibers to the honeycomb unit  130 , alumina, silica, silicon carbide, silica-alumina, glass, potassium titanate, aluminum borate or the like is desirable as the material of the inorganic fibers. These may be used alone or in combination. Of the above-described materials, alumina is desirable. Whiskers are also included in inorganic fibers. 
         [0033]    With respect to the amount of inorganic particles (of zeolite and inorganic particles other than zeolite) included in the honeycomb unit  130 , a desirable lower limit is approximately 30 wt %, a more desirable lower limit is approximately 40 wt %, and a still more desirable lower limit is approximately 50 wt %, while a desirable upper limit is approximately 90 wt %, a more desirable upper limit is approximately 80 wt %, and a still more desirable upper limit is approximately 75 wt %. If the inorganic particles content (of zeolite and inorganic particles other than zeolite) is more than or equal to approximately 30 wt %, it is less likely that the amount of zeolite contributing to conversion is relatively reduced. On the other hand, if the inorganic particles content is less than or equal to approximately 90 wt %, the honeycomb unit strength is less likely to be reduced. 
         [0034]    The inorganic binder included is preferably more than or equal to approximately 5 wt %, more preferably more than or equal to approximately 10 wt %, and still more preferably more than or equal to approximately 15 wt % as solids content. On the other hand, the inorganic binder content is preferably less than or equal to approximately 50 wt %, more preferably less than or equal to approximately 40 wt %, and still more preferably less than or equal to approximately 35 wt % as solids content. If the amount of the inorganic binder is more than or equal to approximately 5 wt % as solids content, the manufactured honeycomb unit is less likely to be reduced in strength. On the other hand, if the amount of the inorganic binder is less than or equal to approximately 50 wt % as solids content, the moldability of the raw material composition is less likely to be degraded. 
         [0035]    In the case of the honeycomb unit  130  including inorganic fibers, the total amount of the inorganic fibers has a lower limit of desirably approximately 3 wt %, more desirably approximately 5 wt %, and still more desirably approximately 8 wt %, and has an upper limit of desirably approximately 50 wt %, more desirably approximately 40 wt %, and still more desirably approximately 30 wt %. If the inorganic fibers content is more than or equal to approximately 3 wt %, the contribution of the inorganic fibers to an increase in the honeycomb unit strength is less likely to be reduced. If the inorganic fibers content is less than or equal to approximately 50 wt %, it is less likely that the amount of zeolite contributing to NOx conversion is relatively reduced. 
         [0036]    The shape of a cross section of the honeycomb unit  130  perpendicular to its longitudinal directions is not limited in particular, and may be any shape as long as the honeycomb units  130  are joinable by interposing the adhesive layer  150 . The shape of the cross section of the honeycomb unit  130  perpendicular to its longitudinal directions may also be substantially square, substantially rectangular, substantially hexagonal, substantially sectorial, etc. 
         [0037]    Further, the shape of a cross section of each cell  121  of the honeycomb unit  130  perpendicular to its longitudinal directions is not limited in particular, and may be not only a substantially square shape but also a substantially triangular or substantially polygonal shape. 
         [0038]    The cell density of the honeycomb unit  130  is preferably in the range of approximately 15.5 cells/cm 2  to approximately 186 cells/cm 2  (approximately 100 cpsi to approximately 1200 cpsi), more preferably in the range of approximately 46.5 cells/cm 2  to approximately 170 cells/cm 2  (approximately 300 cpsi to approximately 1100 cpsi), and still more preferably in the range of approximately 62 cells/cm 2  to approximately 155 cells/cm 2  (approximately 400 cpsi to approximately 1000 cpsi). 
         [0039]    The thickness of the cell walls  123  of the honeycomb unit  130  is not limited in particular, but has a desirable lower limit of approximately 0.1 mm in terms of strength and has a desirable upper limit of approximately 0.4 mm in terms of conversion performance. 
         [0040]    The honeycomb structure  100  according to the embodiment of the present invention may have any shape. For example, in addition to a cylindrical shape shown in  FIG. 1 , the honeycomb structure  100  may also have a shape such as a substantial cylindroid, substantially square pillar, or substantially polygonal pillar. 
         [0041]    The adhesive layer  150  of the honeycomb structure  100  is formed using adhesive layer paste as a raw material. The adhesive layer paste includes zeolite. The adhesive layer paste may further include inorganic particles other than zeolite, an inorganic binder, inorganic fibers and/or an organic binder. 
         [0042]    The same materials as those forming the honeycomb unit  130  as described above may be used as the inorganic particles other than zeolite, inorganic binder, and inorganic fibers. The organic binder is not limited in particular, and may be one or more selected from, for example, polyvinyl alcohol, methylcellulose, ethylcellulose, carboxymethylcellulose, etc. Of the organic binders, carboxymethylcellulose is desirable. 
         [0043]    It is preferable that the adhesive layer  150  be approximately 0.3 mm to approximately 2.0 mm in thickness. This is because if the thickness of the adhesive layer  150  is more than or equal to approximately 0.3 mm, sufficient bonding strength is likely to be obtained. Further, if the thickness of the adhesive layer  150  is less than or equal to approximately 2.0 mm, pressure loss is less likely to increase. The number of honeycomb units to be joined is suitably determined in accordance with the size of the honeycomb structure. 
         [0044]    The peripheral coat layer  120  of the honeycomb structure  100  is formed using, as a raw material, paste including at least one of inorganic particles, an inorganic binder, and inorganic fibers. The paste forming the peripheral coat layer  120  may further include an organic binder. The same kinds of inorganic particles, inorganic binder, and/or inorganic fibers as the inorganic particles, inorganic binder, and/or inorganic fibers of a material forming the honeycomb unit  130  may be used for and included in the peripheral coat layer  120 . The materials forming the peripheral coat layer  120  may be either the same as or different from, but are preferably the same as those of the adhesive layer  150  because this makes occurrence of peeling or generation of cracks less likely the same as in the adhesive layer  150 . A pore-forming agent such as balloons, which are minute hollow balls whose component is oxide-based ceramic, spherical acryl particles, or graphite may be added as required to the paste serving as a raw material. The coat layer  120  preferably has a final thickness of approximately 0.1 mm to approximately 2.0 mm. 
         [0045]    [Method of Manufacturing Honeycomb Structure] 
         [0046]    Next, a description is given of a method of manufacturing a honeycomb structure according to the embodiment of the present invention. 
         [0047]    First, a honeycomb unit molded body is made by extrusion molding using raw material paste including inorganic particles including zeolite and an inorganic binder as principal components and further having inorganic fibers added as required. 
         [0048]    In addition to these, an organic binder, a dispersion medium, and a molding aid may be suitably added to the raw material paste in accordance with moldability. The organic binder is not limited in particular. The organic binder includes one or more organic binders selected from, for example, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyethylene glycol, phenolic resin, epoxy resin and the like. The amount of the organic binder blended is preferably approximately 1 to approximately 10 parts by weight to the total of 100 parts by weight of the inorganic particles, inorganic binder, and inorganic fibers. 
         [0049]    The dispersion medium is not limited in particular, and may be, for example, water, an organic solvent (such as benzene), alcohol (such as methanol), etc. The molding aid is not limited in particular, and may be, for example, ethylene glycol, dextrin, a fatty acid, fatty acid soap, polyalcohol and the like. 
         [0050]    The raw material paste is not limited in particular, and is preferably subjected to mixing and kneading. For example, the raw material paste may be mixed using a mixer, attritor, or the like, and may be well kneaded with a kneader or the like. The method of molding the raw material paste is not limited in particular. It is preferable, for example, to form the raw material paste into a shape having cells by extrusion molding or the like. 
         [0051]    Next, it is preferable to dry the obtained molded body. The drying apparatus used for drying is not limited in particular, and may be a microwave drying apparatus, hot air drying apparatus, dielectric drying apparatus, reduced-pressure drying apparatus, vacuum drying apparatus, freeze drying apparatus, or the like. Further, it is preferable to degrease the obtained molded body. The conditions for degreasing, which are not limited in particular and are suitably selected in accordance with the kind and amount of the organic matter included in the molded body, are preferably approximately 400° C. and approximately 2 hours. Further, it is preferable to subject the obtained molded body to firing. The condition for firing is not limited in particular, and is preferably approximately 600° C. to approximately 1200° C., and more preferably approximately 600° C. to approximately 1000° C. This is because sintering is likely to progress at firing temperatures more than or equal to approximately 600° C., so that the strength is less likely to be reduced as a honeycomb unit, and because sintering is less likely to progress excessively at firing temperatures less than or equal to approximately 1200° C., so that zeolite reaction sites are less likely to be reduced. 
         [0052]    Next, adhesive layer paste to later serve as an adhesive layer is applied with uniform thickness on the side surfaces of the honeycomb unit obtained by the above-described processes, and thereafter, other honeycomb units are stacked one after another on the corresponding sides with this adhesive layer paste being interposed therebetween. By repeating this process, a honeycomb structure of a desired size (for example, having honeycomb units arranged in a four-by-four matrix) is made. 
         [0053]    Next, this honeycomb structure is heated to dry and solidify the adhesive layer paste so as to form an adhesive layer and adhere and fix the honeycomb units to each other. Here, the coefficient of thermal expansion of the adhesive layer A a  is controlled to be approximately 0.8 to approximately 1.2 times the coefficient of thermal expansion of the honeycomb unit A h . The coefficient of thermal expansion of the adhesive layer A a  is easily controllable by controlling the material of the inorganic particles other than zeolite included in the adhesive layer paste and their blending ratio. 
         [0054]    Next, the honeycomb structure is cut into, for example, a cylindrical shape using a diamond cutter or the like, thereby making a honeycomb structure with a necessary peripheral shape. 
         [0055]    Next, after applying peripheral coat layer paste on the peripheral surface (side surface) of the honeycomb structure, the peripheral coat layer paste is dried and solidified to form a peripheral coat layer. 
         [0056]    It is preferable to degrease this honeycomb structure after joining the honeycomb units by interposing the adhesive layer (after forming the peripheral coat layer in the case of providing the peripheral coat layer). As a result of this treatment, if an organic binder is included in the adhesive layer paste and/or the peripheral coat layer paste, this organic binder can be removed by degreasing. The conditions for degreasing, which are suitably determined in accordance with the kind and amount of the included organic material, are usually approximately 700° C. and approximately 2 hours. 
         [0057]    By the above-described processes, the honeycomb structure  100  illustrated in  FIG. 1  can be manufactured. 
         [0058]    The cutting process may be omitted by manufacturing multiple differently-shaped honeycomb units and joining them into a predetermined shape by interposing an adhesive layer. 
       EXAMPLES 
       [0059]    A detailed description is given of the embodiment of the present invention based on examples. 
       Example 1 
       [0060]    First, 2250 parts by weight of Fe zeolite particles (2 μm in average particle size, a silica to alumina ratio by weight of 40), 960 parts by weight of zirconia particles (2 μm in average particle size), 2600 parts by weight of alumina sol (of a solids content of 30 wt %), 885 parts by weight of alumina fibers (100 μm in average fiber length and 6 μm in average fiber diameter), 410 parts by weight of methylcellulose, a plasticizer, and lubricant (UNILUB) were mixed and kneaded, so that a mixture composition (raw material composition) was obtained. The Fe zeolite particles had its part corresponding to 3 wt % of the zeolite weight exchanged with Fe ions. The exchange with Fe ions was performed by impregnating zeolite particles with an iron nitrate ammonium solution. The concentration of the iron nitrate ammonium solution was controlled so that zeolite included 3 wt % of Fe. β-zeolite was used as zeolite. The amount of ion exchange was determined by ICP (atomic emission spectrometry) using an apparatus ICPS-8100 (manufactured by Shimadzu Corporation). 
         [0061]    Next, this mixture composition was subjected to extrusion molding with an extruder, so that honeycomb unit molded bodies were obtained. 
         [0062]    Next, the molded bodies were sufficiently dried using a microwave drying apparatus and a hot air drying apparatus, and were degreased, being retained at 400° C. for 2 hours. Thereafter, the molded bodies were subjected to firing, being retained at 700° C. for 2 hours, so that honeycomb units (30 mm in height, 30 mm in width, and 150 mm in overall length) were obtained. The cell wall thickness was 0.25 mm, and the cell density was 78 cells/cm 2 . 
         [0063]    Next, 40 wt % of inorganic particles, 44 wt % of an inorganic binder, 9.5 wt % of inorganic fibers, and 6.5 wt % of methylcellulose were mixed and then 25 wt % of water was added to prepare adhesive layer paste. Zeolite particles (2 μm in average particle size) were used as inorganic particles, silica sol (of a solids content of 30 wt %) was used as an inorganic binder, and silica-alumina fibers (15 μm in average fiber length and 1 μm in average fiber diameter) were used as inorganic fibers. 
         [0064]    This adhesive layer paste was applied on side surfaces of the honeycomb units manufactured by the above-described method, and the honeycomb units were joined to obtain a honeycomb assembly of two honeycomb units in height and width, that is, a total of four honeycomb units. The adhesive layer paste was applied evenly on the honeycomb units so that a finished adhesive layer was 1 mm in thickness, and was then heated at 120° C. to be solidified. 
         [0065]    Next, peripheral coat layer paste (the same as the adhesive layer paste) was applied on the peripheral surface of the honeycomb assembly, and was heated and solidified to form a peripheral coat layer of 0.5 mm in thickness. By these processes, a honeycomb structure according to Example 1 (143.8 mm in outside diameter and 150 mm in overall length) was obtained. 
       Example 2 
       [0066]    A honeycomb structure was manufactured by the same method as in the case of Example 1. In Example 2, however, the adhesive layer paste used was prepared by mixing 40 wt % of zeolite particles (2 μm in average particle size), 44 wt % of alumina sol (of a solids content of 30 wt %), 9.5 wt % of silica-alumina fibers (15 μm in average fiber length and 1 μm in average fiber diameter), and 6.5 wt % of methylcellulose and further adding 25 wt % of water. 
       Example 3 
       [0067]    A honeycomb structure was manufactured by the same method as in the case of Example 1. In Example 3, however, the adhesive layer paste used was prepared by mixing 40 wt % of zeolite particles (2 μm in average particle size), 44 wt % of silica sol (of a solids content of 30 wt %), 9.5 wt % of mullite fibers (15 μm in average fiber length and 1 μm in average fiber diameter), and 6.5 wt % of methylcellulose and further adding 25 wt % of water. 
       Comparative Example 1 
       [0068]    A honeycomb structure was manufactured by the same method as in the case of Example 1. In Comparative Example 1, however, the adhesive layer paste used was prepared by mixing 40 wt % of zeolite particles (2 μm in average particle size), 44 wt % of alumina sol (of a solids content of 30 wt %), 9.5 wt % of aluminum borate fibers (15 μm in average fiber length and 1 μm in average fiber diameter), and 6.5 wt % of methylcellulose and further adding 25 wt % of water. 
       Comparative Example 2 
       [0069]    A honeycomb structure was manufactured by the same method as in the case of Example 1. In Comparative Example 2, however, the adhesive layer paste used was prepared by mixing 40 wt % of zeolite particles (2 μm in average particle size), 44 wt % of silica sol (of a solids content of 30 wt %), 9.5 wt % of alumina fibers (15 μm in average fiber length and 1 μm in average fiber diameter), and 6.5 wt % of methylcellulose and further adding 25 wt % of water. 
       Comparative Example 3 
       [0070]    A honeycomb structure was manufactured by the same method as in the case of Example 1. In Comparative Example 3, however, the adhesive layer paste used was prepared by mixing 40 wt % of alumina particles (2 μm in average particle size), 44 wt % of silica sol (of a solids content of 30 wt %), 9.5 wt % of aluminum borate fibers (15 μm in average fiber length and 1 μm in average fiber diameter), and 6.5 wt % of methylcellulose and further adding 25 wt % of water. 
       Comparative Example 4 
       [0071]    A honeycomb structure was manufactured by the same method as in the case of Example 1. In Comparative Example 4, however, the adhesive layer paste used was prepared by mixing 40 wt % of silica particles (2 μm in average particle size), 44 wt % of silica sol (of a solids content of 30 wt %), 9.5 wt % of silica-alumina fibers (15 μm in average fiber length and 1 μm in average fiber diameter), and 6.5 wt % of methylcellulose and further adding 25 wt % of water. 
         [0072]    Table 1 shows the composition of the adhesive layer of each of Examples 1 to 3 and Comparative Examples 1 to 4. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 COEFFICIENT 
                 RATIO OF COEFFICIENT OF 
                   
               
               
                   
                 OF THERMAL 
                 THERMAL EXPANSION OF 
               
               
                   
                 EXPANSION 
                 ADHESIVE LAYER Aa TO 
                 THERMAL 
               
             
          
           
               
                   
                 ADHESIVE LAYER COMPONENT 
                 OF ADHESIVE 
                 COEFFICIENT OF THERMAL 
                 SHOCK 
               
             
          
           
               
                   
                 INORGANIC 
                 INORGANIC 
                 INORGANIC 
                 LAYER Aa 
                 EXPANSION OF HONEYCOMB 
                 TEST 
               
               
                   
                 PARTICLES 
                 BINDER 
                 FIBERS 
                 (×10 −6 /K) 
                 UNIT Ah (Aa/Ah) 
                 RESULT 
               
               
                   
                   
               
             
          
           
               
                 EXAMPLE 1 
                 ZEOLITE 
                 SILICA SOL 
                 SILICA - 
                 2.8 
                 0.82 
                 ∘ 
               
               
                   
                   
                   
                 ALUMINA 
               
               
                 EXAMPLE 2 
                 ZEOLITE 
                 ALUMINA 
                 SILICA - 
                 3.1 
                 0.91 
                 ∘ 
               
               
                   
                   
                 SOL 
                 ALUMINA 
               
               
                 EXAMPLE 3 
                 ZEOLITE 
                 SILICA SOL 
                 MULLITE 
                 4.1 
                 1.19 
                 ∘ 
               
               
                 COMPARATIVE 
                 ZEOLITE 
                 ALUMINA 
                 ALUMINUM 
                 2.3 
                 0.68 
                 x 
               
               
                 EXAMPLE 1 
                   
                 SOL 
                 BORATE 
               
               
                 COMPARATIVE 
                 ZEOLITE 
                 SILICA SOL 
                 ALUMINA 
                 4.6 
                 1.35 
                 x 
               
               
                 EXAMPLE 2 
               
               
                 COMPARATIVE 
                 ALUMINA 
                 SILICA SOL 
                 ALUMINUM 
                 3.8 
                 1.12 
                 x 
               
               
                 EXAMPLE 3 
                   
                   
                 BORATE 
               
               
                 COMPARATIVE 
                 SILICA 
                 SILICA SOL 
                 SILICA - 
                 3.1 
                 0.91 
                 x 
               
               
                 EXAMPLE 4 
                   
                   
                 ALUMINA 
               
               
                   
               
               
                 (NOTE) 
               
               
                 COEFFICIENT OF THERMAL EXPANSION OF HONEYCOMB UNIT Ah IS 3.4 × 10 −6 /K 
               
             
          
         
       
     
         [0073]    [Measurement of Coefficient of Thermal Expansion] 
         [0074]    The coefficients of thermal expansion of the honeycomb unit and the adhesive layer of each of Examples 1 to 3 and Comparative Examples 1 to 4 were evaluated by the following method. 
         [0075]    First, a measurement sample simulating the honeycomb unit was manufactured in the same process as the above-described honeycomb unit manufacturing method. Likewise, a measurement sample simulating the adhesive layer was made by heating the adhesive layer paste of the above-described composition at 120° C. and solidifying it. The dimensions of each measurement sample were 3 mm in height, 3 mm in width, and 15 mm in length. 
         [0076]    Next, the measurement samples and an alumina reference sample (3 mm×3 mm×15 mm) were placed side by side in an airtight container with the longitudinal directions of the samples being horizontal directions. For each sample, a detection rod was provided so as to be in contact with the center portion of its upper surface (that is, the 3 mm×15 mm upper area). 
         [0077]    Next, in an argon atmosphere, the measurement samples and the reference sample had their temperatures increased from room temperature to 700° C. at a temperature increase rate of 5° C. per minute, and then were cooled naturally to room temperature after being retained at 700° C. for 20 minutes. In this case, the measurement samples and the reference sample expand thermally, and these changes are detected by the corresponding detection rods. Accordingly, the coefficient of thermal expansion of each measurement sample is determined from the difference in change between the measurement sample and the reference sample. 
         [0078]    A thermal dilatometer (DL-7000, manufactured by ULVAC-RIKO, Inc.) was used for the measurement. The minimum detection sensitivity of this apparatus is 0.1 μm. 
         [0079]    As a result of the measurements, the coefficient of thermal expansion of the honeycomb unit A h  was 3.4×10 −6 /K. On the other hand, the coefficients of thermal expansion of the adhesive layers A a  varied from 2.3×10 −6  to 4.6×10 −6 /K. Table 1 collectively shows the coefficient of thermal expansion of the adhesive layer A a  and the ratio of the coefficient of thermal expansion of the adhesive layer A a  to the coefficient of thermal expansion of the honeycomb unit A h  (A a /A h ) in each of Examples 1 to 3 and Comparative Examples 1 to 4. 
         [0080]    [Thermal Shock Test] 
         [0081]    A thermal shock test was conducted on each of the honeycomb structures of Examples 1 to 3 and Comparative Examples 1 to 4 manufactured in the above-described manner by the following method. 
         [0082]    An alumina mat of 15 cm in width and 46.5 cm in length (6 mm in thickness) (manufactured by Mitsubishi Chemical Corporation) was put around the entire peripheral surface of the honeycomb structure as a holding sealing material, and the honeycomb structure was attached to a metal casing. This metal casing was put in an electric furnace maintained at 600° C. and retained for 10 minutes. Thereafter, the honeycomb structure was extracted from the furnace and cooled to room temperature (natural cooling). This operation was repeated 10 times. 
         [0083]    After the thermal shock test, it was visually determined whether peeling or a crack had been caused in the honeycomb structure. Table 1 shows the thermal shock test result in each of Examples 1 to 3 and Comparative Examples 1 to 4. A circle indicates that neither peeling nor cracking was caused in the honeycomb structure after the thermal shock test, and a cross indicates that peeling or a crack was caused in the honeycomb structure after the thermal shock test. 
         [0084]    The thermal shock test saw generation of a crack in Comparative Examples 1 and 2. It is believed that this is due to a large difference in the coefficient of thermal expansion between the honeycomb unit and the adhesive layer. 
         [0085]    The thermal shock test saw occurrence of peeling in Comparative Examples 3 and 4. It is believed that peeling occurred because of an increase in the difference in the degree of degradation between the honeycomb unit and the adhesive layer caused by a repeated application of thermal shock because the honeycomb unit and the adhesive layer included different types of inorganic particles. 
         [0086]    These results show that the honeycomb structure presents good resistance to thermal shock in the case where the ratio of the coefficient of thermal expansion of the adhesive layer to the coefficient of thermal expansion of the honeycomb unit (A a /A h ) is in the range of 0.8 to 1.2 and the adhesive layer includes zeolite. 
         [0087]    The honeycomb structure according to the embodiment of the present invention can be used for conversion of exhaust gas, in particular, automobile exhaust gas, and can be used effectively in NOx conversion systems, in particular, SCR systems. 
         [0088]    The present invention is not limited to the specifically disclosed embodiment, and variations and modifications may be made without departing from the scope of the present invention.