WO2009118871A1 - ハニカム構造体 - Google Patents

ハニカム構造体 Download PDF

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Publication number
WO2009118871A1
WO2009118871A1 PCT/JP2008/055976 JP2008055976W WO2009118871A1 WO 2009118871 A1 WO2009118871 A1 WO 2009118871A1 JP 2008055976 W JP2008055976 W JP 2008055976W WO 2009118871 A1 WO2009118871 A1 WO 2009118871A1
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WIPO (PCT)
Prior art keywords
honeycomb structure
honeycomb
structure according
sox
inorganic
Prior art date
Application number
PCT/JP2008/055976
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English (en)
French (fr)
Inventor
貴彦 井戸
千鶴 笠井
Original Assignee
イビデン株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by イビデン株式会社 filed Critical イビデン株式会社
Priority to PCT/JP2008/055976 priority Critical patent/WO2009118871A1/ja
Priority to JP2008558988A priority patent/JPWO2009118871A1/ja
Priority to AT08015202T priority patent/ATE496680T1/de
Priority to DE602008004735T priority patent/DE602008004735D1/de
Priority to EP08015202A priority patent/EP2105194B1/en
Priority to US12/239,342 priority patent/US8323766B2/en
Publication of WO2009118871A1 publication Critical patent/WO2009118871A1/ja

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    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8603—Removing sulfur compounds
    • B01D53/8609—Sulfur oxides
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    • C04B2237/30—Composition of layers of ceramic laminates or of ceramic or metallic articles to be joined by heating, e.g. Si substrates
    • C04B2237/32—Ceramic
    • C04B2237/34—Oxidic
    • C04B2237/345—Refractory metal oxides
    • C04B2237/348—Zirconia, hafnia, zirconates or hafnates
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2237/00—Aspects relating to ceramic laminates or to joining of ceramic articles with other articles by heating
    • C04B2237/30—Composition of layers of ceramic laminates or of ceramic or metallic articles to be joined by heating, e.g. Si substrates
    • C04B2237/32—Ceramic
    • C04B2237/38—Fiber or whisker reinforced
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2237/00—Aspects relating to ceramic laminates or to joining of ceramic articles with other articles by heating
    • C04B2237/50—Processing aspects relating to ceramic laminates or to the joining of ceramic articles with other articles by heating
    • C04B2237/70—Forming laminates or joined articles comprising layers of a specific, unusual thickness
    • C04B2237/708—Forming laminates or joined articles comprising layers of a specific, unusual thickness of one or more of the interlayers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2570/00—Exhaust treating apparatus eliminating, absorbing or adsorbing specific elements or compounds
    • F01N2570/04—Sulfur or sulfur oxides
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/24—Structurally defined web or sheet [e.g., overall dimension, etc.]
    • Y10T428/24149—Honeycomb-like
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/24—Structurally defined web or sheet [e.g., overall dimension, etc.]
    • Y10T428/24149—Honeycomb-like
    • Y10T428/24157—Filled honeycomb cells [e.g., solid substance in cavities, etc.]

Definitions

  • the present invention relates to a honeycomb structure.
  • a honeycomb catalyst used for purifying automobile exhaust gas has a layer of a material having a large specific surface area such as activated alumina formed on the surface of a honeycomb structure made of cordierite, and a catalyst such as platinum on the surface. Is carried.
  • the honeycomb catalyst used for purifying the exhaust gas of the diesel engine further carries a NOx storage agent in order to treat NOx under an oxygen-excess atmosphere.
  • the NOx storage agent since the NOx storage agent has the property of storing SOx more stably than NOx, the NOx storage agent stores SOx, and SOx poisoning that makes it impossible to store NOx properly occurs.
  • a sulfur trap composed of a sulfur absorbent and a casing surrounding the sulfur absorbent is disposed in the exhaust gas passage upstream of the NOx storage agent.
  • the sulfur absorbent on an alumina support, at least one selected from alkali metals such as potassium, sodium, lithium and cesium, alkaline earth such as barium and calcium, and rare earth such as lanthanum and yttrium
  • alkali metals such as potassium, sodium, lithium and cesium
  • alkaline earth such as barium and calcium
  • rare earth such as lanthanum and yttrium
  • the honeycomb structure as described in Patent Document 2 that is, having a plurality of through holes, the first form of inorganic material (for example, ceramic particles), the second form of inorganic material (for example, It is conceivable to use a honeycomb structure in which a porous honeycomb unit including inorganic fibers or ceramic particles having a large particle diameter) and an inorganic binder is bonded to the outer surface through which no through-holes are opened via a sealing material layer. Since such a honeycomb structure has a large specific surface area, the size can be reduced. JP-A-6-58138 International Publication No. 05/063653 Pamphlet
  • the present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide a honeycomb structure capable of suppressing a decrease in the storage performance of SOx due to repeated application of heat by exhaust gas.
  • a honeycomb structure of the present invention is a honeycomb structure having a honeycomb unit in which a plurality of through holes are arranged in parallel in a longitudinal direction with a partition wall therebetween, and the honeycomb unit includes an SOx storage agent, inorganic particles, and an inorganic binder.
  • the specific surface area when SOx is occluded X [g] is Y [m 2 / L], the formula Y ⁇ ⁇ 26X + 40000 (0 ⁇ X ⁇ 500) Meet.
  • the SOx storage agent preferably contains at least one of an alkali metal and an alkaline earth metal, and particularly preferably contains at least one of sodium, potassium, magnesium, calcium and barium.
  • the partition wall preferably contains the SOx storage agent in the range of 1.0 mol / L to 2.5 mol / L.
  • the inorganic particles are preferably at least one selected from the group consisting of alumina, titania, silica, zirconia, ceria and mullite. *
  • the inorganic binder is preferably a solid content contained in one or more selected from the group consisting of alumina sol, silica sol, titania sol, water glass, sepiolite, and attapulgite. *
  • the honeycomb unit preferably further includes an inorganic fiber, and the inorganic fiber is at least one selected from the group consisting of alumina, silica, silicon carbide, silica alumina, glass, potassium titanate, and aluminum borate. It is particularly desirable to be. *
  • a noble metal catalyst is supported on the partition wall, and it is particularly desirable that the noble metal catalyst is at least one of platinum, palladium and rhodium.
  • the present invention it is possible to provide a honeycomb structure capable of suppressing a decrease in SOx storage performance due to repeated application of heat by exhaust gas.
  • FIG. 1A It is a perspective view which shows an example of the honeycomb structure of this invention. It is a perspective view which shows the honeycomb unit of FIG. 1A. It is a perspective view which shows the other example of the honeycomb structure of this invention. It is a graph which shows the relationship of the specific surface area per unit volume with respect to the SOx occlusion amount of a honeycomb structure.
  • FIG. 1A and 1B show an example of the honeycomb structure of the present invention.
  • the honeycomb structure 10 a plurality of honeycomb units 11 in which a plurality of through holes 12 are arranged in parallel in the longitudinal direction with partition walls are bonded via an adhesive layer 13, and an outer peripheral surface is covered with an outer peripheral coat layer 14.
  • the honeycomb unit 11 includes a SOx storage agent, inorganic particles, and an inorganic binder. Further, assuming that the specific surface area when SOx of the honeycomb structure 10 is occluded by X [g] is Y [m 2 / L], the formula Y ⁇ ⁇ 26X + 40000 (0 ⁇ X ⁇ 500) Meet.
  • the SOx poisoning of the NOx storage agent can be suppressed by disposing the honeycomb structure 10 on the upstream side of the honeycomb structure including the NOx storage agent with respect to the flow of the exhaust gas.
  • Y is preferably 70000 m 2 / L or less.
  • the SOx occlusion agent is not particularly limited as long as it can react with SOx and occlude as a sulfate, and examples thereof include alkali metals such as sodium and potassium, and alkaline earth metals such as magnesium, calcium and barium. Two or more kinds may be used in combination.
  • the SOx storage agent may be included in the partition walls of the honeycomb unit 11 or may be supported on the partition walls.
  • the SOx storage agent may be partly contained in the partition wall and partly supported on the partition wall.
  • the SOx occlusion agent contained in the partition walls and the SOx occlusion agent carried on the partition walls may be the same or different.
  • the honeycomb structure 10 preferably has a SOx storage agent content in the partition walls of 1.0 to 2.5 mol / L.
  • the honeycomb structure 10 may not be able to be reduced in size in order to sufficiently maintain the SOx storage performance.
  • the content of the SOx storage agent in the partition walls exceeds 2.5 mol / L, it may be difficult to manufacture the honeycomb structure 10.
  • the inorganic particles are not particularly limited as long as the specific surface area of the honeycomb structure 10 can be increased in order to make the SOx occlusion agent easy to occlude SOx, as the inorganic particles are made of an inorganic compound excluding the SOx occlusion agent.
  • Alumina, titania, silica, zirconia, ceria, mullite, zeolite and the like may be mentioned, and two or more kinds may be used in combination. Of these, alumina is particularly preferable.
  • the inorganic particles preferably have an average particle size of 0.1 to 10 ⁇ m.
  • the average particle size is less than 0.1 ⁇ m, it is necessary to add a large amount of an inorganic binder. As a result, extrusion molding may be difficult, and when it exceeds 10 ⁇ m, the specific surface area of the honeycomb structure 10 is increased. The effect may be insufficient.
  • the honeycomb unit 11 preferably has an inorganic particle content of 30 to 90% by weight, more preferably 40 to 80% by weight, and particularly preferably 50 to 75% by weight.
  • the content of the inorganic particles is less than 30% by weight, the specific surface area of the honeycomb unit 11 may be reduced.
  • the strength of the honeycomb unit 11 may be lowered.
  • the inorganic binder is not particularly limited, but includes solids contained in alumina sol, silica sol, titania sol, water glass, sepiolite, attapulgite, etc., and two or more kinds may be used in combination.
  • the honeycomb unit 11 preferably has an inorganic binder content of 5 to 50% by weight, more preferably 10 to 40% by weight, and particularly preferably 15 to 35% by weight.
  • the content of the inorganic binder is less than 5% by weight, the strength of the honeycomb unit 11 may be reduced, and when it exceeds 50% by weight, molding may be difficult.
  • the honeycomb unit 11 preferably further includes inorganic fibers. Thereby, the strength of the honeycomb unit 11 can be improved.
  • the inorganic fiber is not particularly limited as long as the strength of the honeycomb unit 11 can be improved, and examples thereof include alumina, silica, silicon carbide, silica alumina, glass, potassium titanate, and aluminum borate. Two or more species may be used in combination.
  • the inorganic fibers preferably have an aspect ratio of 2 to 1000, more preferably 5 to 800, and particularly preferably 10 to 500. If the aspect ratio is less than 2, the effect of improving the strength of the honeycomb unit 11 may be reduced. On the other hand, when the aspect ratio exceeds 1000, clogging or the like may occur during molding such as extrusion molding, and the effect of improving the strength of the honeycomb unit 11 may be reduced due to breakage of inorganic fibers during molding. is there.
  • the honeycomb unit 11 preferably has an inorganic fiber content of 3 to 50% by weight, more preferably 5 to 40% by weight, and particularly preferably 8 to 30% by weight.
  • the inorganic fiber content is less than 3% by weight, the effect of improving the strength of the honeycomb unit 11 may be reduced, and when it exceeds 50% by weight, the specific surface area of the honeycomb unit 11 may be reduced.
  • the honeycomb unit 11 preferably has a cross-sectional area of 5 to 50 cm 2 in a cross section perpendicular to the longitudinal direction, that is, a cross section perpendicular to the through hole 12.
  • the cross-sectional area is less than 5 cm 2 , the specific surface area of the honeycomb structure 10 may be reduced and the pressure loss may be increased.
  • the cross-sectional area exceeds 50 cm 2 , the strength against thermal stress generated in the honeycomb unit 11 is increased. May become insufficient.
  • the partition walls separating the through holes 12 of the honeycomb unit 11 preferably have a thickness of 0.05 to 0.35 mm, more preferably 0.10 to 0.30 mm, and particularly preferably 0.15 to 0.25 mm. If the partition wall thickness is less than 0.05 mm, the strength of the honeycomb unit 11 may decrease. If the partition wall thickness exceeds 0.35 mm, the exhaust gas hardly penetrates into the partition wall, and the SOx occlusion performance decreases. There are things to do.
  • the number of through holes 12 per 1 cm 2 of the cross section perpendicular to the longitudinal direction is preferably 15.5 to 186, more preferably 46.5 to 170.5, and 62.0 Particularly preferred is ⁇ 155. If the number of through-holes 12 per 1 cm 2 is less than 15.5, the strength of the honeycomb unit 11 may decrease, and if it exceeds 186, the pressure loss of the honeycomb unit 11 may increase.
  • the adhesive layer 13 to which the honeycomb unit 11 is bonded is preferably 0.5 to 2 mm in thickness. If the thickness of the adhesive layer 13 is less than 0.5 mm, the adhesive strength may be insufficient. On the other hand, if the thickness of the adhesive layer 13 exceeds 2 mm, the specific surface area of the honeycomb structure 10 may decrease and the pressure loss of the honeycomb structure 10 may increase.
  • the outer peripheral coat layer 14 preferably has a thickness of 0.1 to 3 mm.
  • the thickness of the outer peripheral coat layer 14 is less than 0.1 mm, the effect of improving the strength of the honeycomb structure 10 may be insufficient.
  • the thickness exceeds 3 mm the specific surface area of the honeycomb structure 10 decreases. Sometimes.
  • the honeycomb structure 10 has a cylindrical shape, but the shape of the honeycomb structure of the present invention is not particularly limited, and examples thereof include a prismatic shape and an elliptical columnar shape.
  • the honeycomb unit 11 has a quadrangular prism shape.
  • the shape of the honeycomb unit is not particularly limited, and is preferably a shape in which the honeycomb units are easily bonded to each other, such as a hexagonal column shape. It is done.
  • the shape of the through hole 12 is a quadrangular prism shape
  • the shape of the through hole is not particularly limited, and examples thereof include a triangular prism shape and a hexagonal prism shape.
  • a noble metal catalyst may be supported on the partition walls of the honeycomb unit 11.
  • the noble metal catalyst is not particularly limited as long as it can oxidize SO 2 to SO 3, and examples thereof include platinum, palladium, rhodium and the like, and two or more kinds may be used in combination.
  • the honeycomb structure 10 includes inorganic particles and an inorganic binder, and if necessary, molding such as extrusion molding is performed using a raw material paste further containing SOx occlusion agent and inorganic fibers, and a plurality of through holes are separated in the longitudinal direction with partition walls. A raw honeycomb formed body arranged side by side is manufactured. Thereby, even if the firing temperature is lowered, the honeycomb unit 11 having sufficient strength can be obtained.
  • the inorganic binder is added to the raw material paste as alumina sol, silica sol, titania sol, water glass, sepiolite, attapulgite, etc., and two or more kinds may be used in combination.
  • an organic binder, a dispersion medium, a molding aid, and the like may be appropriately added to the raw material paste as necessary.
  • the organic binder is not particularly limited, and examples thereof include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene glycol, phenol resin, and epoxy resin, and two or more kinds may be used in combination.
  • the addition amount of the organic binder is preferably 1 to 10% with respect to the total weight of the inorganic particles, the inorganic fibers, and the inorganic binder.
  • the dispersion medium is not particularly limited, and examples thereof include water, organic solvents such as benzene, alcohols such as methanol, and the like.
  • the molding aid is not particularly limited, and examples thereof include ethylene glycol, dextrin, fatty acid, fatty acid soap, polyalcohol and the like, and two or more kinds may be used in combination.
  • the raw material paste it is preferable to mix and knead, and it may be mixed using a mixer, an attritor or the like, or may be kneaded using a kneader or the like.
  • the obtained honeycomb formed body is dried using a dryer such as a microwave dryer, a hot air dryer, a dielectric dryer, a vacuum dryer, a vacuum dryer, or a freeze dryer.
  • a dryer such as a microwave dryer, a hot air dryer, a dielectric dryer, a vacuum dryer, a vacuum dryer, or a freeze dryer.
  • the obtained honeycomb formed body is degreased.
  • the degreasing conditions are not particularly limited and can be appropriately selected depending on the type and amount of the organic substance contained in the molded body, but it is preferably 400 ° C. for 2 hours.
  • the honeycomb unit 11 is obtained by firing the obtained honeycomb formed body.
  • the firing temperature is preferably 600 to 1200 ° C, particularly preferably 600 to 1000 ° C. If the firing temperature is less than 600 ° C., the sintering is difficult to proceed, and the strength of the honeycomb structure 10 may be lowered. If the firing temperature exceeds 1200 ° C., the sintering proceeds too much, and the honeycomb structure 10 The specific surface area may be reduced.
  • an adhesive layer paste is applied to the outer peripheral surface of the honeycomb unit 11, the honeycomb units 11 are sequentially adhered, and dried and solidified, thereby producing an aggregate of the honeycomb units 11.
  • the aggregate of the honeycomb units 11 may be cut into a cylindrical shape and polished.
  • a honeycomb unit 11 having a columnar shape may be manufactured by bonding the honeycomb units 11 having a cross-section formed in a fan shape or a square shape.
  • the adhesive layer paste is not particularly limited, and examples thereof include a mixture of inorganic binder and inorganic particles, a mixture of inorganic binder and inorganic fibers, a mixture of inorganic binder, inorganic particles, and inorganic fibers.
  • the adhesive paste may contain an organic binder.
  • an organic binder Polyvinyl alcohol, methylcellulose, ethylcellulose, carboxymethylcellulose, etc. are mentioned, You may use 2 or more types together.
  • the outer peripheral coat layer paste is applied to the outer peripheral surface of the aggregate of the cylindrical honeycomb units 11 and dried and solidified.
  • the outer periphery coating layer paste is not particularly limited, it may contain the same material as the adhesive layer paste or may contain a different material. Further, the outer peripheral coat layer paste may have the same composition as the adhesive layer paste.
  • the honeycomb structure 10 is obtained by drying and solidifying the aggregate of the honeycomb units 11 to which the outer peripheral coat layer paste has been applied.
  • an organic binder is contained in the adhesive layer paste and / or the outer peripheral coat layer paste, it is preferable to degrease.
  • the degreasing conditions can be appropriately selected depending on the type and amount of the organic substance, but is preferably 700 ° C. for 2 hours.
  • the SOx storage agent and / or the noble metal catalyst is supported on the partition walls of the honeycomb structure 10 as necessary.
  • the method for supporting the SOx storage agent and / or the noble metal catalyst is not particularly limited, and examples thereof include an impregnation method.
  • FIG. 2 shows another example of the honeycomb structure of the present invention.
  • the honeycomb structure 20 is the same as the honeycomb structure 10 except that the plurality of through-holes 12 are constituted by a single honeycomb unit 11 arranged in parallel in the longitudinal direction with partition walls therebetween.
  • the outer peripheral coat layer may be formed or may not be formed.
  • Example 1 First, 1070 g of magnesium oxide as a SOx storage agent, 1180 g of ⁇ -alumina having an average particle diameter of 2 ⁇ m as inorganic particles, 680 g of alumina fibers having an average fiber diameter of 6 ⁇ m and an average fiber length of 100 ⁇ m as inorganic fibers, an inorganic binder A raw material paste was obtained by mixing and kneading 2600 g of an alumina sol having a solid content of 20% by weight as a component and 320 g of methylcellulose as an organic binder. Next, the raw material paste was extruded using an extrusion molding machine to obtain a raw honeycomb molded body.
  • the honeycomb formed body was dried using a microwave dryer and a hot air dryer, and then degreased at 400 ° C. for 2 hours. Next, it is fired at 700 ° C. for 2 hours, and is a regular rectangular column having a length of 35 mm, a width of 35 mm, a height of 68 mm, a number of through-holes per 1 cm 2 of a cross section perpendicular to the longitudinal direction, and 93 Obtained honeycomb unit.
  • the adhesive layer paste was applied so that the thickness of the adhesive layer was 1 mm, the honeycomb unit was adhered, dried and solidified at 120 ° C., and an aggregate of honeycomb units was prepared, then using a diamond cutter, It was cut into a cylindrical shape so that the cross section perpendicular to the longitudinal direction was substantially point-symmetric. Furthermore, after the adhesive layer paste was applied to the outer peripheral surface so that the thickness of the outer peripheral coat layer was 0.5 mm, it was dried and solidified at 120 ° C. using a microwave dryer and a hot air dryer, and at 400 ° C. Degreasing was conducted for 2 hours to obtain a cylindrical honeycomb structure having a diameter of 138 mm and a height of 68 mm (volume 2 L).
  • honeycomb structure was impregnated with a platinum nitrate solution, and then held at 600 ° C. for 1 hour to support 3 g / L of platinum as a noble metal catalyst. Note that the honeycomb structure on which platinum is supported contains 2.5 mol / L of magnesium oxide.
  • Example 2 A honeycomb structure carrying platinum was obtained in the same manner as in Example 1 except that the average particle diameter of ⁇ -alumina used for preparing the raw material paste was 10 ⁇ m. Note that the honeycomb structure on which platinum is supported contains 2.5 mol / L of magnesium oxide.
  • Example 3 A honeycomb structure carrying platinum was obtained in the same manner as in Example 1 except that the average particle diameter of ⁇ -alumina used for preparing the raw material paste was 0.1 ⁇ m. Note that the honeycomb structure on which platinum is supported contains 2.5 mol / L of magnesium oxide.
  • Example 4 A honeycomb structure on which platinum was not supported was obtained in the same manner as in Example 1 except that the content of magnesium oxide used for preparing the raw material paste was 650 g and the content of ⁇ alumina was 1600 g. Further, the obtained honeycomb structure was impregnated with a dispersion of magnesium oxide, and then held at 600 ° C. for 1 hour to carry 1.0 mol / L of magnesium oxide.
  • the honeycomb structure carrying magnesium oxide was impregnated with a platinum nitrate solution and then held at 600 ° C. for 1 hour to carry 3 g / L of platinum as a noble metal catalyst. Note that the honeycomb structure on which magnesium oxide and platinum are supported contains 2.5 mol / L of magnesium oxide.
  • a layer made of alumina is formed on the surface of the partition wall, the diameter is 138 mm, the height is 68 mm (volume 2 L), the number of through-holes per 1 cm 2 cross section perpendicular to the longitudinal direction is 93, and the partition wall thickness is 0.2 mm.
  • a columnar honeycomb structure made of cordierite was obtained. Next, the obtained honeycomb structure was impregnated with a dispersion of magnesium oxide, and then held at 600 ° C. for 1 hour, thereby supporting 2.5 mol / L of magnesium oxide.
  • Example 2 A honeycomb structure carrying platinum was obtained in the same manner as in Example 1 except that the average particle diameter of ⁇ -alumina used for preparing the raw material paste was 20 ⁇ m. Note that the honeycomb structure on which platinum is supported contains 2.5 mol / L of magnesium oxide.
  • the specific surface area per unit volume of the honeycomb structure when the SOx occlusion amount X was 0 g was measured.
  • a simulated gas at 400 ° C. was flowed through the honeycomb structure at a space velocity (SV) of 50000 / hr, and when the SOx occlusion amount X reached 150 g, 300 g, and 500 g, the honeycomb structure was taken out per unit volume.
  • the specific surface area of was measured.
  • the constituent components of the simulated gas are nitrogen, carbon dioxide (10% by volume), oxygen (10% by volume), nitric oxide (200 ppm), carbon monoxide (0% by volume), hydrocarbon (200 ppm). , Sulfur dioxide (125 ppm).
  • the ratio A [volume%] of the volume of the honeycomb structure to the volume of the outer shape of the honeycomb structure including the volume of the through holes was calculated.
  • the BET specific surface area B [m 2 / g] of the honeycomb structure was measured.
  • the BET specific surface area was measured by a one-point method according to JIS-R-1626 (1996) defined by Japanese Industrial Standards using a BET measuring device Micromeritics Flowsorb II-2300 (manufactured by Shimadzu Corporation).
  • 2 g of powder obtained by pulverizing a sample cut out from the honeycomb structure was used.
  • FIG. 3 shows the relationship of the specific surface area Y per unit volume to the SOx occlusion amount X of the honeycomb structure.
  • the honeycomb structures of Examples 1 to 4 that satisfy the above conditions have a sulfur dioxide purification rate of about 90% or more, and can suppress a decrease in SOx occlusion performance due to heating.

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Abstract

　本発明のハニカム構造体は、複数の貫通孔が隔壁を隔てて長手方向に並設されたハニカムユニットを有するハニカム構造体であって、ハニカムユニットは、ＳＯｘ吸蔵剤と、無機粒子と、無機バインダとを含み、ＳＯｘがＸ［ｇ］吸蔵されたときの比表面積をＹ［ｍ２／Ｌ］とすると、式 　Ｙ≧－２６Ｘ＋４００００（０＜Ｘ≦５００） を満たす。

Description

ハニカム構造体
　本発明は、ハニカム構造体に関する。
　従来、自動車の排ガスを浄化するために用いられるハニカム触媒は、コージェライトからなるハニカム構造体の表面に、活性アルミナ等の比表面積が大きい材料からなる層が形成され、その上に白金等の触媒が担持されている。また、ディーゼルエンジンの排ガスを浄化するために用いられるハニカム触媒は、酸素過剰雰囲気下で、ＮＯｘを処理するために、ＮＯｘ吸蔵剤がさらに担持されている。
　しかしながら、ＮＯｘ吸蔵剤は、ＮＯｘよりもＳＯｘをより安定に吸蔵するという性質を有するため、ＮＯｘ吸蔵剤がＳＯｘを吸蔵して、ＮＯｘを適正に吸蔵できなくなるＳＯｘ被毒が発生する。
　そこで、特許文献１では、ＮＯｘ吸蔵剤の上流の排ガスの通路内に、イオウ吸収剤と、イオウ吸収剤を包囲するケーシングから構成されているイオウ捕獲装置が配置されている。なお、イオウ吸収剤としては、アルミナ担体上に、カリウム、ナトリウム、リチウム、セシウムのようなアルカリ金属、バリウム、カルシウムのようなアルカリ土類、ランタン、イットリウムのような希土類から選ばれた少なくとも一つと、白金Ｐｔのような貴金属とが担持されている構成が例示されている。
　しかしながら、このようなイオウ捕獲装置を使用する場合は、多量のイオウを吸収する必要があり、それに伴ってサイズを大きくする必要がある。このため、特許文献２に記載されているようなハニカム構造体、即ち、複数の貫通孔を有し、第１の形態の無機材料（例えば、セラミック粒子）、第２の形態の無機材料（例えば、無機繊維や粒径の大きなセラミック粒子）と無機バインダとを含む多孔質ハニカムユニットがシール材層を介して貫通孔が開口していない外面で接合したハニカム構造体を使用することが考えられる。このようなハニカム構造体は、比表面積が大きいため、サイズを小さくすることができる。
特開平６－５８１３８号公報 国際公開第０５／０６３６５３号パンフレット
　しかしながら、このようなハニカム構造体を、ＳＯｘを吸蔵する用途で使用すると、排ガスにより繰り返し熱が加わることにより、比表面積が低下し、ＳＯｘの吸蔵性能が低下するという問題がある。
　本発明は、上記の従来技術が有する問題に鑑み、排ガスにより繰り返し熱が加わることによるＳＯｘの吸蔵性能の低下を抑制することが可能なハニカム構造体を提供することを目的とする。
　本発明のハニカム構造体は、複数の貫通孔が隔壁を隔てて長手方向に並設されたハニカムユニットを有するハニカム構造体であって、ハニカムユニットは、ＳＯｘ吸蔵剤と、無機粒子と、無機バインダとを含み、ＳＯｘがＸ［ｇ］吸蔵されたときの比表面積をＹ［ｍ２／Ｌ］とすると、式
　Ｙ≧－２６Ｘ＋４００００（０＜Ｘ≦５００）
を満たす。
また、上記のＳＯｘ吸蔵剤の少なくとも一部が上記の隔壁に担持されていることが好ましい。　
また、上記のＳＯｘ吸蔵剤は、アルカリ金属及びアルカリ土類金属の少なくとも一方を含むことが望ましく、ナトリウム、カリウム、マグネシウム、カルシウム及びバリウムの少なくとも一つを含むことが特に望ましい。　
また、上記の隔壁は、上記のＳＯｘ吸蔵剤を１．０ｍｏｌ／Ｌ以上２．５ｍｏｌ／Ｌ以下含むことが望ましい。　
また、上記の無機粒子は、アルミナ、チタニア、シリカ、ジルコニア、セリア及びムライトからなる群より選択される一種以上であることが望ましい。　
また、上記の無機バインダは、アルミナゾル、シリカゾル、チタニアゾル、水ガラス、セピオライト及びアタパルジャイトからなる群より選択される一種以上に含まれる固形分であることが望ましい。　
また、上記のハニカムユニットは、無機繊維をさらに含むことが望ましく、無機繊維は、アルミナ、シリカ、炭化ケイ素、シリカアルミナ、ガラス、チタン酸カリウム及びホウ酸アルミニウムからなる群より選択される一種以上であることが特に望ましい。　
また、複数の上記のハニカムユニットが接着層を介して接着されていることが望ましい。　
また、上記の隔壁に貴金属触媒が担持されていることが望ましく、貴金属触媒は、白金、パラジウム及びロジウムの少なくとも一つであることが特に望ましい。
　本発明によれば、排ガスにより繰り返し熱が加わることによるＳＯｘの吸蔵性能の低下を抑制することが可能なハニカム構造体を提供することができる。
本発明のハニカム構造体の一例を示す斜視図である。 図１Ａのハニカムユニットを示す斜視図である。 本発明のハニカム構造体の他の例を示す斜視図である。 ハニカム構造体のＳＯｘ吸蔵量に対する単位体積当たりの比表面積の関係を示すグラフである。
符号の説明
１０、２０　　ハニカム構造体１１　　ハニカムユニット１２　　貫通孔１３　　接着層１４　　外周コート層
　次に、本発明を実施するための最良の形態を図面と共に説明する。
　図１Ａ及び図１Ｂに、本発明のハニカム構造体の一例を示す。ハニカム構造体１０は、複数の貫通孔１２が隔壁を隔てて長手方向に並設されたハニカムユニット１１が接着層１３を介して複数個接着されており、外周面が外周コート層１４で覆われている。このとき、ハニカムユニット１１は、ＳＯｘ吸蔵剤と、無機粒子と、無機バインダとを含む。また、ハニカム構造体１０のＳＯｘがＸ［ｇ］吸蔵されたときの比表面積をＹ［ｍ２／Ｌ］とすると、式
　Ｙ≧－２６Ｘ＋４００００（０＜Ｘ≦５００）
を満たす。これにより、ハニカム構造体１０の加熱によるＳＯｘの吸蔵性能の低下を抑制することができる。このため、ハニカム構造体１０を、排ガスの流れに対して、ＮＯｘ吸蔵剤を含むハニカム構造体の上流側に配置することにより、ＮＯｘ吸蔵剤のＳＯｘ被毒を抑制することができる。
　なお、ＳＯｘ吸蔵剤等の分散の限界を考慮すると、Ｙは７００００ｍ２／Ｌ以下であることが好ましい。
　ＳＯｘ吸蔵剤としては、ＳＯｘと反応し、硫酸塩として吸蔵することが可能であれば、特に限定されないが、ナトリウム、カリウム等のアルカリ金属、マグネシウム、カルシウム、バリウム等のアルカリ土類金属が挙げられ、二種以上併用してもよい。
　なお、ＳＯｘ吸蔵剤は、ハニカムユニット１１の隔壁に含まれていてもよいし、隔壁に担持されていてもよい。また、ＳＯｘ吸蔵剤は、一部が隔壁に含まれ、一部が隔壁に担持されていてもよい。この場合、隔壁に含まれるＳＯｘ吸蔵剤と、隔壁に担持されるＳＯｘ吸蔵剤は、同一であってもよいし、異なっていてもよい。
　ハニカム構造体１０は、隔壁中のＳＯｘ吸蔵剤の含有量が１．０～２．５ｍｏｌ／Ｌであることが好ましい。隔壁中のＳＯｘ吸蔵剤の含有量が１．０ｍｏｌ／Ｌ未満であると、ＳＯｘの吸蔵性能を十分に保つために、ハニカム構造体１０の寸法を小さくすることができないことがある。一方、隔壁中のＳＯｘ吸蔵剤の含有量が２．５ｍｏｌ／Ｌを超えると、ハニカム構造体１０を製造することが困難になることがある。
　無機粒子としては、ＳＯｘ吸蔵剤を除く無機化合物からなり、ＳＯｘ吸蔵剤にＳＯｘを吸蔵させやすくするために、ハニカム構造体１０の比表面積を増大させることが可能であれば、特に限定されないが、アルミナ、チタニア、シリカ、ジルコニア、セリア、ムライト、ゼオライト等が挙げられ、二種以上併用してもよい。中でも、アルミナが特に好ましい。
　無機粒子は、平均粒径が０．１～１０μｍであることが好ましい。平均粒径が０．１μｍ未満であると、無機バインダを多量に添加する必要があり、その結果、押出成形しにくくなることがあり、１０μｍを超えると、ハニカム構造体１０の比表面積を増大させる効果が不十分になることがある。
　ハニカムユニット１１は、無機粒子の含有量が３０～９０重量％であることが好ましく、４０～８０重量％がさらに好ましく、５０～７５重量％が特に好ましい。無機粒子の含有量が３０重量％未満であると、ハニカムユニット１１の比表面積が低下することがある。一方、無機粒子の含有量が９０重量％を超えると、ハニカムユニット１１の強度が低下することがある。
　無機バインダとしては、特に限定されないが、アルミナゾル、シリカゾル、チタニアゾル、水ガラス、セピオライト、アタパルジャイト等に含まれる固形分が挙げられ、二種以上併用してもよい。
　ハニカムユニット１１は、無機バインダの含有量が５～５０重量％であることが好ましく、１０～４０重量％がさらに好ましく、１５～３５重量％が特に好ましい。無機バインダの含有量が５重量％未満であると、ハニカムユニット１１の強度が低下することがあり、５０重量％を超えると、成形が困難になることがある。
　ハニカムユニット１１は、無機繊維をさらに含むことが好ましい。これにより、ハニカムユニット１１の強度を向上させることができる。
　無機繊維としては、ハニカムユニット１１の強度を向上させることが可能であれば、特に限定されないが、アルミナ、シリカ、炭化ケイ素、シリカアルミナ、ガラス、チタン酸カリウム、ホウ酸アルミニウム等が挙げられ、二種以上併用してもよい。
　無機繊維は、アスペクト比が２～１０００であることが好ましく、５～８００がさらに好ましく、１０～５００が特に好ましい。アスペクト比が２未満であると、ハニカムユニット１１の強度を向上させる効果が小さくなることがある。一方、アスペクト比が１０００を超えると、押出成形等の成形時に目詰まり等が発生することがあり、また、成形時に無機繊維が折れて、ハニカムユニット１１の強度を向上させる効果が小さくなることがある。
　ハニカムユニット１１は、無機繊維の含有量が３～５０重量％であることが好ましく、５～４０重量％がさらに好ましく、８～３０重量％が特に好ましい。無機繊維の含有量が３重量％未満であると、ハニカムユニット１１の強度を向上させる効果が小さくなることがあり、５０重量％を超えると、ハニカムユニット１１の比表面積が低下することがある。
　ハニカムユニット１１は、長手方向に垂直な断面、即ち、貫通孔１２に垂直な断面の断面積が５～５０ｃｍ２であることが好ましい。断面積が５ｃｍ２未満であると、ハニカム構造体１０の比表面積が低下すると共に、圧力損失が増大することがあり、断面積が５０ｃｍ２を超えると、ハニカムユニット１１に発生する熱応力に対する強度が不十分になることがある。
　ハニカムユニット１１の貫通孔１２を隔てる隔壁は、厚さが０．０５～０．３５ｍｍであることが好ましく、０．１０～０．３０ｍｍがより好ましく、０．１５～０．２５ｍｍが特に好ましい。隔壁の厚さが０．０５ｍｍ未満であると、ハニカムユニット１１の強度が低下することがあり、０．３５ｍｍを超えると、排ガスが隔壁の内部まで浸透しにくくなって、ＳＯｘの吸蔵性能が低下することがある。
　また、ハニカムユニット１１は、長手方向に垂直な断面１ｃｍ２当たりの貫通孔１２の数が１５．５～１８６個であることが好ましく、４６．５～１７０．５個がより好ましく、６２．０～１５５個が特に好ましい。１ｃｍ２当たりの貫通孔１２の数が１５．５個未満であると、ハニカムユニット１１の強度が低下することがあり、１８６個を超えると、ハニカムユニット１１の圧力損失が増大することがある。
　ハニカムユニット１１を接着させる接着層１３は、厚さが０．５～２ｍｍであることが好ましい。接着層１３の厚さが０．５ｍｍ未満であると、接着強度が不十分になることがある。一方、接着層１３の厚さが２ｍｍを超えると、ハニカム構造体１０の比表面積が低下すると共に、ハニカム構造体１０の圧力損失が増大することがある。
　外周コート層１４は、厚さが０．１～３ｍｍであることが好ましい。外周コート層１４の厚さが０．１ｍｍ未満であると、ハニカム構造体１０の強度を向上させる効果が不十分になることがあり、３ｍｍを超えると、ハニカム構造体１０の比表面積が低下することがある。
　ハニカム構造体１０は、円柱状であるが、本発明のハニカム構造体の形状としては、特に限定されず、角柱状、楕円柱状等が挙げられる。
　また、ハニカムユニット１１は、四角柱状であるが、本発明において、ハニカムユニットの形状としては、特に限定されず、ハニカムユニット同士を接着しやすい形状であることが好ましく、例えば、六角柱状等が挙げられる。
　さらに、貫通孔１２の形状は、四角柱状であるが、本発明において、貫通孔の形状としては、特に限定されず、三角柱状、六角柱状等が挙げられる。
　なお、ハニカムユニット１１の隔壁に、貴金属触媒が担持されていてもよい。貴金属触媒としては、ＳＯ２をＳＯ３に酸化することが可能であれば、特に限定されないが、白金、パラジウム、ロジウム等が挙げられ、二種以上併用してもよい。
　次に、ハニカム構造体１０の製造方法の一例について説明する。まず、無機粒子、及び無機バインダを含み、必要に応じて、ＳＯｘ吸蔵剤及び無機繊維をさらに含む原料ペーストを用いて押出成形等の成形を行い、複数の貫通孔が隔壁を隔てて長手方向に並設された生のハニカム成形体を作製する。これにより、焼成温度を低くしても、十分な強度を有するハニカムユニット１１が得られる。
　なお、無機バインダは、原料ペースト中に、アルミナゾル、シリカゾル、チタニアゾル、水ガラス、セピオライト、アタパルジャイト等として添加されており、二種以上併用されていてもよい。
　また、原料ペーストは、有機バインダ、分散媒、成形助剤等を、必要に応じて、適宜添加してもよい。
　有機バインダとしては、特に限定されないが、メチルセルロース、カルボキシメチルセルロース、ヒドロキシエチルセルロース、ポリエチレングリコール、フェノール樹脂、エポキシ樹脂等が挙げられ、二種以上併用してもよい。なお、有機バインダの添加量は、無機粒子、無機繊維及び無機バインダの総重量に対して、１～１０％であることが好ましい。
　分散媒としては、特に限定されないが、水、ベンゼン等の有機溶媒、メタノール等のアルコール等が挙げられ、二種以上併用してもよい。
　成形助剤としては、特に限定されないが、エチレングリコール、デキストリン、脂肪酸、脂肪酸石鹸、ポリアルコール等が挙げられ、二種以上併用してもよい。
　原料ペーストを調製する際には、混合混練することが好ましく、ミキサー、アトライタ等を用いて混合してもよく、ニーダー等を用いて混練してもよい。
　次に、マイクロ波乾燥機、熱風乾燥機、誘電乾燥機、減圧乾燥機、真空乾燥機、凍結乾燥機等の乾燥機を用いて、得られたハニカム成形体を乾燥する。
　また、得られたハニカム成形体を脱脂する。脱脂条件は、特に限定されず、成形体に含まれる有機物の種類や量によって適宜選択することができるが、４００℃で２時間であることが好ましい。
　さらに、得られたハニカム成形体を焼成することにより、ハニカムユニット１１が得られる。焼成温度は、６００～１２００℃であることが好ましく、６００～１０００℃が特に好ましい。焼成温度が６００℃未満であると、焼結が進行しにくくなって、ハニカム構造体１０の強度が低くなることがあり、１２００℃を超えると、焼結が進行しすぎて、ハニカム構造体１０の比表面積が低下することがある。
　次に、ハニカムユニット１１の外周面に接着層用ペーストを塗布して、ハニカムユニット１１を順次接着させ、乾燥固化することにより、ハニカムユニット１１の集合体を作製する。このとき、ハニカムユニット１１の集合体を作製した後に、円柱状に切削加工し、研磨してもよい。また、断面が扇形状や正方形状に成形されたハニカムユニット１１を接着させて円柱状のハニカムユニット１１の集合体を作製してもよい。
　接着層用ペーストとしては、特に限定されないが、無機バインダ及び無機粒子の混合物、無機バインダ及び無機繊維の混合物、無機バインダ、無機粒子及び無機繊維の混合物等が挙げられる。
　また、接着ペーストは、有機バインダを含有してもよい。有機バインダとしては、特に限定されないが、ポリビニルアルコール、メチルセルロース、エチルセルロース、カルボキシメチルセルロース等が挙げられ、二種以上併用してもよい。
　次に、円柱状のハニカムユニット１１の集合体の外周面に外周コート層用ペーストを塗布し、乾燥固化する。外周コート層用ペーストは、特に限定されないが、接着層用ペーストと同じ材料を含有してもよいし、異なる材料を含有してもよい。また、外周コート層用ペーストは、接着層用ペーストと同一の組成であってもよい。
　次に、外周コート層用ペーストが塗布されたハニカムユニット１１の集合体を乾燥固化することにより、ハニカム構造体１０が得られる。このとき、接着層用ペースト及び／又は外周コート層用ペーストに有機バインダが含まれている場合は、脱脂することが好ましい。脱脂条件は、有機物の種類や量によって適宜選択することができるが、７００℃で２時間であることが好ましい。
　さらに、ハニカム構造体１０の隔壁に、必要に応じて、ＳＯｘ吸蔵剤及び／又は貴金属触媒を担持させる。ＳＯｘ吸蔵剤及び／又は貴金属触媒を担持させる方法としては、特に限定されないが、含浸法等が挙げられる。
　図２に、本発明のハニカム構造体の他の例を示す。なお、ハニカム構造体２０は、複数の貫通孔１２が隔壁を隔てて長手方向に並設された単一のハニカムユニット１１から構成されている以外は、ハニカム構造体１０と同様である。
　本発明のハニカム構造体は、外周コート層が形成されていてもよいし、形成されていなくてもよい。
　［実施例１］
　まず、ＳＯｘ吸蔵剤としての、酸化マグネシウム１０７０ｇ、無機粒子としての、平均粒径が２μｍのγアルミナ１１８０ｇ、無機繊維としての、平均繊維径が６μｍ、平均繊維長が１００μｍのアルミナ繊維６８０ｇ、無機バインダ含有成分としての、固形分２０重量％のアルミナゾル２６００ｇ、有機バインダとしての、メチルセルロース３２０ｇを混合混練して、原料ペーストを得た。次に、押出成形機を用いて、原料ペーストを押出成形し、生のハニカム成形体を得た。そして、マイクロ波乾燥機及び熱風乾燥機を用いて、ハニカム成形体を乾燥させた後、４００℃で２時間脱脂した。次に、７００℃で２時間焼成し、縦３５ｍｍ、横３５ｍｍ、高さ６８ｍｍ、長手方向に垂直な断面１ｃｍ２当たりの貫通孔数が９３個、隔壁の厚さが０．２ｍｍの正四角柱状のハニカムユニットを得た。
　次に、平均粒径が２μｍのγアルミナ２６重量部、平均繊維径が０．５μｍ、平均繊維長が１５μｍのアルミナ繊維３７重量部、無機バインダ含有成分としての、固形分２０重量％のアルミナゾル３１．５重量部、有機バインダとしての、カルボキシメチルセルロース０．５重量部、水５重量部を混合混練して、耐熱性の接着層用ペーストを得た。
　接着層の厚さが１ｍｍになるように接着層用ペーストを塗布して、ハニカムユニットを接着させ、１２０℃で乾燥固化して、ハニカムユニットの集合体を作製した後、ダイヤモンドカッターを用いて、長手方向に垂直な断面が略点対称になるように円柱状に切削加工した。さらに、外周面に、外周コート層の厚さが０．５ｍｍになるように接着層用ペーストを塗布した後、マイクロ波乾燥機及び熱風乾燥機を用いて１２０℃で乾燥固化し、４００℃で２時間脱脂して、直径１３８ｍｍ、高さ６８ｍｍ（体積２Ｌ）の円柱状のハニカム構造体を得た。
　次に、得られたハニカム構造体を硝酸白金溶液に含浸させた後、６００℃で１時間保持することにより、貴金属触媒としての、白金を３ｇ／Ｌ担持させた。なお、白金が担持されたハニカム構造体は、２．５ｍｏｌ／Ｌの酸化マグネシウムを含む。
　［実施例２］
　原料ペーストを調製する際に用いるγアルミナの平均粒径を１０μｍとした以外は、実施例１と同様にして、白金が担持されたハニカム構造体を得た。なお、白金が担持されたハニカム構造体は、２．５ｍｏｌ／Ｌの酸化マグネシウムを含む。
　［実施例３］
　原料ペーストを調製する際に用いるγアルミナの平均粒径を０．１μｍとした以外は、実施例１と同様にして、白金が担持されたハニカム構造体を得た。なお、白金が担持されたハニカム構造体は、２．５ｍｏｌ／Ｌの酸化マグネシウムを含む。
　［実施例４］
　原料ペーストを調製する際に用いる酸化マグネシウムの含有量を６５０ｇ、γアルミナの含有量を１６００ｇとした以外は、実施例１と同様にして、白金が担持されていないハニカム構造体を得た。さらに、得られたハニカム構造体を酸化マグネシウムの分散液に含浸させた後、６００℃で１時間保持し、酸化マグネシウムを１．０ｍｏｌ／Ｌ担持させた。
　次に、酸化マグネシウムが担持されたハニカム構造体を硝酸白金溶液に含浸させた後、６００℃で１時間保持することにより、貴金属触媒としての、白金を３ｇ／Ｌ担持させた。なお、酸化マグネシウム及び白金が担持されたハニカム構造体は、２．５ｍｏｌ／Ｌの酸化マグネシウムを含む。
　［比較例１］
　隔壁の表面にアルミナからなる層が形成されている、直径１３８ｍｍ、高さ６８ｍｍ（体積２Ｌ）、長手方向に垂直な断面１ｃｍ２当たりの貫通孔数が９３個、隔壁の厚さが０．２ｍｍのコージェライトからなる円柱状のハニカム構造体を得た。次に、得られたハニカム構造体を酸化マグネシウムの分散液に含浸させた後、６００℃で１時間保持することにより、酸化マグネシウムを２．５ｍｏｌ／Ｌ担持させた。
　［比較例２］
　原料ペーストを調製する際に用いるγアルミナの平均粒径を２０μｍとした以外は、実施例１と同様にして、白金が担持されたハニカム構造体を得た。なお、白金が担持されたハニカム構造体は、２．５ｍｏｌ／Ｌの酸化マグネシウムを含む。
　［比表面積の測定］
　まず、ＳＯｘの吸蔵量Ｘが０ｇの時のハニカム構造体の単位体積当たりの比表面積を測定した。次に、４００℃の模擬ガスをハニカム構造体に空間速度（ＳＶ）５００００／ｈｒで流して、ＳＯｘの吸蔵量Ｘが１５０ｇ、３００ｇ及び５００ｇになった時に、ハニカム構造体を取り出し、単位体積当たりの比表面積を測定した。なお、模擬ガスの構成成分は、窒素（ｂａｌａｎｃｅ）、二酸化炭素（１０体積％）、酸素（１０体積％）、一酸化窒素（２００ｐｐｍ）、一酸化炭素（０体積％）、炭化水素（２００ｐｐｍ）、二酸化硫黄（１２５ｐｐｍ）である。
　具体的には、まず、貫通孔の体積を含むハニカム構造体の外形の体積に対する、ハニカム構造体の体積の比Ａ［体積％］を算出した。次に、ハニカム構造体のＢＥＴ比表面積Ｂ［ｍ２／ｇ］を測定した。ＢＥＴ比表面積は、ＢＥＴ測定装置ＭｉｃｒｏｍｅｒｉｔｉｃｓフローソーブＩＩ－２３００（島津製作所社製）を用いて、日本工業規格で定められるＪＩＳ－Ｒ－１６２６（１９９６）に準じて１点法により測定した。なお、ＢＥＴ比表面積の測定には、ハニカム構造体から切り出したサンプルを砕いた粉末２ｇを用いた。そして、ハニカム構造体の見かけ密度Ｃ［ｇ／Ｌ］をハニカム構造体の重量と外形の体積から算出し、ハニカム構造体の単位体積当たりの比表面積Ｙ［ｍ２／Ｌ］を式
　Ｙ＝Ａ／１００×Ｂ×Ｃ
から求めた。表１に測定結果を示す。
Figure JPOXMLDOC01-appb-T000001
 また、図３に、ハニカム構造体のＳＯｘ吸蔵量Ｘに対する単位体積当たりの比表面積Ｙの関係を示す。
　［ＳＯｘの漏れ量の測定］
　ＳＯｘの吸蔵量が５００ｇになるまで、４００℃の模擬ガスをハニカム構造体に空間速度（ＳＶ）５００００／ｈｒで流しながら、ＭＥＸＡ－７１００Ｄ及びＭＥＸＡ－１１７０ＳＸ（以上、ＨＯＲＩＢＡ社製）を用いて、ハニカム構造体から流出するガス中のＳＯｘの濃度を測定した（検出限界０．１ｐｐｍ）。測定結果を表１に示す。なお、ハニカム構造体から流出するガス中のＳＯｘの濃度が１２．５ｐｐｍ以下である場合を○、１２．５ｐｐｍを超える場合を×とした。このとき、ハニカム構造体から流出するガス中のＳＯｘの濃度が１２．５ｐｐｍであることは、模擬ガス中の二酸化硫黄の含有量が１２５ｐｐｍであるため、浄化率が約９０％であることを意味する。
　以上のことから、式
　Ｙ≧－２６Ｘ＋４００００（０＜Ｘ≦５００）
を満たす実施例１～４のハニカム構造体は、二酸化硫黄の浄化率が約９０％以上であり、加熱によるＳＯｘの吸蔵性能の低下を抑制できることがわかる。

Claims (12)

  1. 　複数の貫通孔が隔壁を隔てて長手方向に並設されたハニカムユニットを有するハニカム構造体であって、
    　該ハニカムユニットは、ＳＯｘ吸蔵剤と、無機粒子と、無機バインダとを含み、
    　ＳＯｘがＸ［ｇ］吸蔵されたときの比表面積をＹ［ｍ２／Ｌ］とすると、式
    　Ｙ≧－２６Ｘ＋４００００（０＜Ｘ≦５００）
    を満たすことを特徴とするハニカム構造体。
  2. 前記ＳＯｘ吸蔵剤の少なくとも一部が前記隔壁に担持されていることを特徴とする請求項１に記載のハニカム構造体。
  3. 前記ＳＯｘ吸蔵剤は、アルカリ金属及びアルカリ土類金属の少なくとも一方を含むことを特徴とする請求項１又は２に記載のハニカム構造体。
  4. 前記ＳＯｘ吸蔵剤は、ナトリウム、カリウム、マグネシウム、カルシウム及びバリウムの少なくとも一つを含むことを特徴とする請求項３に記載のハニカム構造体。
  5. 前記隔壁は、前記ＳＯｘ吸蔵剤を１．０ｍｏｌ／Ｌ以上２．５ｍｏｌ／Ｌ以下含むことを特徴とする請求項１乃至４のいずれか一項に記載のハニカム構造体。
  6. 前記無機粒子は、アルミナ、チタニア、シリカ、ジルコニア、セリア、ムライト及びゼオライトからなる群より選択される一種以上であることを特徴とする請求項１乃至５のいずれか一項に記載のハニカム構造体。
  7. 前記無機バインダは、アルミナゾル、シリカゾル、チタニアゾル、水ガラス、セピオライト及びアタパルジャイトからなる群より選択される一種以上に含まれる固形分であることを特徴とする請求項１乃至６のいずれか一項に記載のハニカム構造体。
  8. 前記ハニカムユニットは、無機繊維をさらに含むことを特徴とする請求項１乃至７のいずれか一項に記載のハニカム構造体。
  9. 前記無機繊維は、アルミナ、シリカ、炭化ケイ素、シリカアルミナ、ガラス、チタン酸カリウム及びホウ酸アルミニウムからなる群より選択される一種以上であることを特徴とする請求項８に記載のハニカム構造体。
  10. 複数の前記ハニカムユニットが接着層を介して接着されていることを特徴とする請求項１乃至９のいずれか一項に記載のハニカム構造体。
  11. 前記隔壁に貴金属触媒が担持されていることを特徴とする請求項１乃至１０のいずれか一項に記載のハニカム構造体。
  12. 前記貴金属触媒は、白金、パラジウム及びロジウムの少なくとも一つであることを特徴とする請求項１１に記載のハニカム構造体。
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