Title of Invention

METHOD OF PRODUCING A POROUS SINTERED CERAMIC HONEYCOMB AND THE CERAMIC HONEYCOMB WALL FLOW FILTER

Abstract

The present invention relates to a method of producing porous sintered ceramic honeycomb comprising; (a) forming a mixture comprised of a dispersing liquid and ceramic powder, (b) inserting the mixture into at least one channel of a green ceramic honeycomb that is comprised of a clay to form a plugged green ceramic .honeycomb wherein the dispersing liquid fails to swell the clay, and (c) heating the plugged green ceramic honeycomb to a temperature sufficient to sinter the plugged green ceramic honeycomb to form a porous sintered plugged ceramic honeycomb.

Full Text

METHOD OF MAKING WALL-FLOW MONOLITH FILTER This application claims the benefit of U.S. Provisional Application Serial No. 6()/2!;5.810, filed April 24. 2001. The invention relates to ceramic wall-flow monolithic filters and methods of m.-k:iivg then. In pciiticular, the invention relates to particulate traps, such as diesel paitii:ulate iraps. As air quality standards become more stringem, considerable efforts have focu A honeycomb ceramic wall-flow through filter, such as described in U.S. Pacini No. ^•,276,071, has become the preferred type of particulate trap. These honeycomb filtcn ju-e made by extruding a paste comprised of water, binder and ceramic powders (for example, clay, mullite, silica and alumina) that form, for example, cordierite upon firing. Clays are generally used to make the paste sufficiently plastic to form useable honeycombs. Afi-;r ihe paste is extruded, the honeycomb is dried, debindered and sintered to form a honevcomb. The honeycomb is sintered typically to give sufficient strength to the thin cha:iro!l walls to survive insertion of a ceramic paste to plug the channels, as described nexi. Finally, to make thV wall-flow particulate trap or filter, one half of the openings of one end of the sintered honeycomb are plugged with a paste comprised of a suitable povv'der, dispersion medium and binder. Then on the other end, the channels not aire idy plugj^ed are plugged with the paste. Subsequently, the plugged honeycomb is sint Unfortunately, this method suffers from a number of problems. For example, the licuid in the paste may be drawn into the porous walls of the fired honeycomb prefer?ntialh' causing non-uniform drying shrinkage of the plug and ultimately cracks in the plujs;. A secc>nd problem is the necessity for multiple expensive steps (for example, at least two high terr.perat:ure firings) to manufacture the particulate trap. These multiple steps are typically needed because the walls of a green ceramic honeycomb are thin and fragile such that they tend to deform and/or break when inserting the paste. This is particularly true v^hen using a large senile process. Another problem, is the limited compositions that may be used for the plug due to the expansion of the fired honeycomb during the sintering shrinkage of the phag. Accordingly, it would be desirable to provide a method for making wall-flow trap.5, for example, that avoids one or more of the problems of the prior art, such as one of thos£t described above. A first aspect of the invention is a method of plugging channels in a ceramic he ncycomb comprising; (a) :orm:iog a mixture comprised of a dispersing liquid and ceramic powder, (b) inserting the mixture into at least one channel of a green ceramic honeycomb that is coinprised of a clay to fomj a plugged green ceramic honeycomb, wherein the dispersing liqui:( essentially fails to swell the clay, sind (c) heating the plugged green ceramic honeycomb to a temperature sufficient to sinter che plugged green ceramic honeycomb to form a porous sintered plugged ceramic hc;ie)'Comb. A second aspect of the invention is a method of plugging channels in a cexar-ic hor.eycomb comprising; (a) heating a green ceramic honeycomb that is comprised of clay, to a first ternpuature that is insufficient to\substantially sinter the green ceramic honeycomb, but is suf fit :ent to remove the binder and dehydrate the clay, such that the dehydrated clay substantially fails r.o be rehydrated when contacted with water co form a calcined ceramic honeycomb, (b) inserting a plug mixture comprised of a ceramic powder and dispersing liquid intc> ar least one channel of the calcined ceramic honeycomb to form a plugged calcined ceramic honeycomb, and (c) hsating the plugged calcined ceramic honeycomb to a temperature sufficient to form a sintered plugged honeycomb. A third aspect of the invention is a ceramic honeycomb wall-flow filter comprising a monolithic ceramic honeycomb body having an inlet end and outlet end connt:;2ted by adjacent inlet and outlet channels that extend from the inlet end to the outlet end of the ceramiic body, the inlet and outlet channels being defined by a plurality of interlaced thin gas filtering porous partition walls between the inlet and outlet channels and by ctrramic plugs, sjjch that the inlet channel has an inlet ceramic plug at the outlet end of the i:erami': body and the outlet channel ha5 an outlet ceramic plug at the inlet end of the ceramic bcdy st:ch "hat a fluid when entering the inlet end must pass through partition walls to exit the outlei: end, v/herein the ceramic honeycomb body has a discriminating layer on at leiis! one partition wall of the outlet channel the discriminating layer being a composition th;U is esseitially the same as the outlet ceramic plug of the outlet channel. A fourth aspect of the invention is a ceramic honeycomb wall-flow filter com /rising a monolithic ceramic honeycomb body having an inlet end and outlet end coinijcted by adjacent inlet and outlet channels that extend from the inlet end to the outlet end of the ceramic body, the inlet and ou;:let channels being defined by a plurality of inter aced thin gas filtering porous partition walls between the inlet and outlet channels and by ct ramie plugs such that the inlet channel has an inlet ceramic plug at the outlet end of the ceramic body and the outlet channel has an outlet ceramic plug at the inlet end of the ccrar'iic body such ihat a fluid, when entering the inlet end, must pass through partition walls to exir the cutlst end, wherein the outlet ceramic plugs have a different composition than :he inlet ceramic plugs. Each of the aforementioned methods is particularly useful, but not limited to, plug|!:ng of channels in honeycomb wall-ilow filters. Surprisingly, these methods may be used :D not only plug the channels, but also simultaneously provide a discriminating layer. for e;: ample, on the v/alls of the outlet channels of a ceramic honeycomb wall-flow filter. Th:: r:ethod may also be used to provide other useful materials on or in the walls of the filtE'.r channel (for example, a catalyst or nucleation agent), while simultaneously forming the plugs. FIG. I is a front view showing one embodiment of a ceramic honeycomb filt£'r :.ccording to the present invention. FIG. 2 is a side view of FIG. i with a part thereof cut away. FIG. 3 is an enlarged schematic view of adjacent channels in the filter of FIG. FIG. 4 is an enlarged cross-sectional view of a plugged portion of a channel and in-situ formed discriminating layer of the filter of FIG. 1. Throughout the different views of the drawings, numeral 2 is a ceramic honeycomb body, numeral 4 is an inlet channel, numeral 5 is an outlet channel, numeral 6 is a partition wall between adjacent channels, numeral 8 is an inlet plug, numeral 9 is an outlet plug, numeral 10 is an inlet end, numeral 11 is an outlet end and numeral 12 is a discriminating layer. Green ceramic honeycomb: A honeycomb body comprised of clay and, if necessary, other ceramics. Clay is a layered hydrated aluminosilicate that swells and absorbs water to form a plastic mass. Examples of clay minerals include kaolinite, montmorillonite, atapulgite, illite, bentonite, halloysite, pyrophyllite and mica. Calcined ceramic honeycomb: A green ceramic honeycomb that has been heated to a calcining temperature sufficient to dehydrate the clay present in the green ceramic honeycomb body, but insufficient to substantially sinter the ceramic constituents of the body. Green or calcined plugged ceramic honeycomb; A green ceramic honeycomb or calcined ceramic honeycomb that has at least one channel plugged with a mixture that forms a ceramic plug upon heating to form the sintered plugged ceramic honeycomb. Sintered plugged ceramic honeycomb: A green or calcined plugged ceramic honeycomb body that is heated to a sintering temperature sufficient to fuse (sinter) the ceramic constituents into a monolithic ceramic. Referring to FIG. 1 through FIG. 4, which depicts one preferred embodiment of the ceramic honeycomb filter comprising a ceramic honeycomb body 2 that has a plui ality of parallel inlet and outlet channels 4 and 5, respectively, extending there through (i:nat is, frc»m inkt end 10 to outlet end 11), defined by porous partition walls 6, inlet channel plugs 8 and outlet channel plugs 9. The outlet channels 9 also have disposed on the surface of he partition walls 6 a discriminating layer 12. V/hen a gas or liquid 14 ccniaining matter to be filtered is directed into inlet channels 4, the gas or liquid 14 passes th!"0..gh the partidon walls 6 and discriminating layer 12 and exits the outlet channels 5. Thus, the partition v/alls 6 and discriminating layer 12 filter out the matter from the gas or licuid 14. The ceramic honeycomb body 2 may be any useful ceramic that has sufficient porosity and strength to perfomn as a wall-flow filter. Examples of useful ceramics include si'ictjn carbide, silicon nitride, mullite, cordierite, beta spodumene, phosphate ceramics (for example, zirconiuo^i phosphate) or combinations thereof. Preferably, the ceramic is mullite or ccidierit(i. More preferably, the ceramic is mullite. Most preferably, the ceramic is a mull te formed in ti\c presence of a fluorine gas, such as those described in U.S. Patents 4,910.172; 4,911,902; 4,948,766; 5,098,455; 5,173,349; 5,19^,154; 5.198,007; 5,252,272 and 5.340,516. The grains of the mullite preferably have an average aspect ratio of at least 2, more preferably at least 5, and most preferably at least 10. Generally, the porosity of the ceramic honeycomb body 2 is from 30 percent to 80 percent porous. Preferably, the ceramic honeycomb body 2 is 40 percent to 70 percent poroi.s. Th€: plugs 8 and 9 may be any porosity sufficient to act effectively as a plug. Generally, the plugs 8 and 9 may be any ceramic composition including essentially the same cenniic composition as the ceramic honeycomb body 2. Essentially the same composition means the plugs S and/or 9 have essentially the same chemistry and microstrucrure as the ceramic honeycomb body 2. Examples of plug compositions include the same ceramics, as previ In one preferred embodiment of the ceramic wall-flow filter, the plug or pit: 5$ 8 and:? have the same composition as the ceramic honeycomb body 2. In this embodiiment, there may or may not be a discriminating layer 12. Preferably, there is a discriminating laj'er 12 that has essentially the same chemistry as the ceramic honeycomb body. hi another preferred embodiment, the sintered ceramic honeycomb body 2 h; s =11 inlet plug 8 at the outlet end 11 that has a different composition than an outlet plug 9 ai th(5 inlet end 10. Different composition means that, after sintering, the compositions have a i'e;;.di]y disceniable microslructural difference (for example, porosity, crystalline structure or grain size) or chemical difference by typically employed techniques for characterizing ce-amies. Essentially the same is when the differences are not readily discernable by the aforementioned techniques. Preferably, one half of all of the channels of the honeycomb bod)' 2 (that is, the inlet channels 4) are plugged at one end and the remaining channels not plugi^ed on the one end are plugged at the other end (that is, outlet channels 5 are plugged at thia inlet end). Even more preferably, the outlet plugs 9 have essentially the same chemistry bu'; different micro.:;tucture than inlet plugs 8. It is, further, preferred that this embodiment also have a discriminating layer 12. Most preferably, the outlet plugs ? have essentially the sane composition a.*; the discriminating layer 12 and the inlet plugs 8 have essentially the sane composition as the ceramic honeycomb body 2. The discriminating layer 12 may be any material useful for making a filter, so long ;)s the average pore size of the discriminating layer 12 is substantially less than the avera£;e ponj size of the ceramic honeycomb body. Suitable materials include those des::r: ocd for the ceramic honeycomb body. Substantially less generally means that the discriminatiiig layer average pore size is at most three quarter the average pore size of the ceran :c honeycomb body. Preferably, the average pore size of the discriminating layer 12 is at mo it one lalf and more preferably, at most one quarter the average pore size of the ceramic hon^iycomb body 2. In addition, the ceramic honeycomb wall-flow rllter 1 may have a catalyst on or v/ithin at least one partition wall 6 or discriminating layer 12. The catalyst may be any catalyst suitable to catalyze, for example, the combustion of soot particles or oxidation of CO (carbon monoxide) or NO^ (nitrogen oxides). Exemplary catalysts include the following. A first exemplary catalyst is directly bound^metal catalyst, such as noble meral;:;, base metals and combinations thereof. Examples of noble metal catalysts include plannum, rhodiurm, palladium, ruthenium, rhenium, silver and alloys thereof. Examples of base metal catalyst;; include copper, chromium, iron, cobalt, nickel, zinc, manganese, v;3n: Jium, titaniurn, scandium and combinations thereof. The metal catalyst preferably is in the iorm of a metal, but may be present as an inorganic compound, such as an oxide, nitride and carbidet, or as a defect srj"ucture within the ceramic grains of the porous catalyst support. Tlie metal may be applied by any suitable technique, such as those known in the art. For ex in:iple, the metal catalyst may be applied by chemical vapor deposition. A second exemplary catalyst is one that is incorporated into the lattice structure of the cerf. Thsse elements may be incorporated in any suitable manner, such as those known in iht cUt. A third exemplary catalyst is a combination of ceramic particles having metal dejio^ited thereon. These are typically referred to as wash coats. Generally, wash coats consi 3t of micrornster sized ceramic particles, such as zeoSitc, aluminosilicate, silica, ceria, zir<:o:ua baiium oxide barium carbonate and alumina panicles that have metal deposited ihej th may be any previously described for direciy metal. a pariicuiarly preferred wash coat catalyst coating is one comprised of particles havin=":" noble meraj thereon. it understood the more thar cne meial oxidi such as having oxides at least zirconium lanthanum rnagne>ium and cerium. A fourth exemplary catalyst is a pcrovskite-type catalyst comprising a metal oxicte compcsitior:. &uch as those describ€td by Golden in U.S. Patent No. 5,939,354. A fifth exemplary catalyst is one that is formed by and deposited on the catalyst support by calcining at a temperature of from 300^C to 3000°C, a composition that comprises (a) an aqueous salt solution containing at least one metal salt and (b) an amphiphilic ethylene oxide containing copolymer, wherein the copolymer has an average mot ec ular wejight of greater thjin 400, an ethylene oxide content of 5 to 90 percent and an HLB of between -15 and 15, as described by Gruenbauer, et al., PCT Patent Application No. WO99/18809. In addition, the catalyst may also be one as described by U.S. Patent No. 5,698,483 and PCT Patent Application No. WO99/03627. In performing a first prefeired method of plugging channels in a ceramic horKr/cornb, a mixture comprised of a dispersing liquid and a ceramic powder is formed. The mixture is then inserted into at least one channel of a gn^en ceramic honeycomb to form a pKigged green ceramic horeycomb, wherein the dispersing liquid substantially fails to i swell the clay. The plugged green ceramic honeycomb is then heated to a temperature si ff ioient lo sinter vhe plugged green ceramic honeycomb to form a porous sintered plugged a:r£;!nic hcneycomb. The mixture nay be formed by any suitable method, such as those known m th'i ;Ln:. Suitable methods include those described in Chapter 17 of Introduction to the Priniiples of Ceramic Processing, J. Reed, John Wiley and Sons, NY, 1988. li. this method, the dispersing liquid must be a liquid that fails to su DS.intiaJly sweJ the clay in the green ceramic honeycomb. "Substantially fails to swell thei clay" means the dispersing liquid fails to swell the clay sufficiently to cause the partition Wull of the green ceramic to deform or crack. Generally, substantially fails to swell is when the clay fails to absorb more than 1 percent by volume of the dispersing liquid. Since the clay ■; adversely affected by v/ater, the dispersing Hquic should have a water concentration insufficient to caustj the aforementioned swelling. Generally, the amount of water should be les.i than 5 perceni: by volume of the dispersing liquid. Tic amount of claV in the green ceramic honeycomb may var>' over a wide range dependent on rhe final csramic desired in the sintered plugged ceramic honeycomb. Generally, Lie amount of clay should be an amount sufficienr to extrude the green ceramic horie;'Comb. Typica'ly, the an:iount of clay -.s at least 1 percent by volume of the green ceriimic ho:neycornb. The dispersing liquid may be, for example, iiny organic liquid, such as an alcoh'jl, aliphatic, glycol, ketone, ether, aldehyde, ester, aromauc, alkene, alkyne, carboxyiic acid, :arbo:cylic acid chloride, amide, amine, nitrile, nitro, sulfide, sulfoxide, sulfone. organometaJlic oi" mixtures thereof that do not swell the clay, as described above. Preferably, the dispersing liquid is an aliphatic, alkene or alcohol. More preferably, the liq'-id is an alcohoL Preferably, the alcohol is a methanol, propanol, ethanol or conibinations thereof. Most preferably, the alcohol is propanol. The ceramic powder may be any ceramic powder useful to form the plugs, such as ce;-amic: powders that form ceramics, such as silicon carbide, silicon nitride, mullitCv cordieritc, beta spodumene, phosphate cenimics (for example, zirconium phosphate) or con filiations thereof. Preferably, the ceramic powders fonn mullite or cordierite. Preferred e.va:nples of ceramics include silica, alumina, aluminum fluoride, clay, fluorotopaz, zeolite, ard mixtures there<:f. more preferably ihe ceramic powder is comprised of powders that form fluorotopaz and mullite in a process has huorine gas present at some time the as previously described.> The mixture may contain other useful components, such as those known in th The mixture may also contain binders. Examples of binders include ceilu Dse edicrs, uich as those described in Chapter 11 of Introduction to tht; Principles of Ce aiiiic Process ng.. J. Reed, John Wiley and Sons, NY, NY, 1988. Preferably, the binder is s meihylcellulose or ethylcellulose, such as those available from The Dow Chemical Company under the trademark METHOCEL and ETHOCEL Preferably, the binder dis;;o ves in the dispersing liquid, but not water such as ETHOCEL. After forming the mixture, it is inserted into a channel of a green ceramic honeyoomb :o foim plugs (that is, form a plugged green ceramic honeycomb). Insertion intt: a chann»il may be accomplished by any suitable method such, as those known in the art. For example, the mixture may be poured, squirted, injected, squeezed, extruded or kneaded into tlie channel. In a. prsfetxed erabodiment, the mixture is fluid enough to be inserted into one end of a chanrel of the green ceramic honeycomb and subsequently flow through the cha.ine! and collect at the other end of the channel, for example, from the mere exertion of gravity. Thus, the mixture may deposit, for example, a discrimmating layer on the partition walis and foim the inlet or outlet plug, after removing a sufficient amount of disnersin? Wcnid to give the collected mixture enough integrity to form the plug. In this preferred ernb;'dinient, the vi 20 cp. The dispersing liquid may be removed by any suitable method, such as by dn'irij in ai% drying by application of heat or vacuum, or by removing it by blocking the chan:^el ends on one end of the ceramic honeycomb body with a porous medium that rencves the dispersing liquid by capillaj7 action. An example of such a porous medium is plastvtr of Paris, such as that used in slip casting ceramics. It is particularly preferred to seal off the channels lo be plugged, such that when pouring the mixture into all of the channels on on; end. the fluid flows through all of the channels and collects only at the sealed channels of the otier end to form the plugs, whereas the unsealed channels allow the mixtt:e to exit without forming plugs. After insertion of the mixture (that is, fomiation of the plugged green ceran:*<: honeycomb the plugged green ceramic is heated to a sintering tempt sufficisnt sinter form sinrer. honeycomb. in general sintered from percent tc porous and preferably porous.> The sintering temfJerature is dependent on the ceramic to be formed but, in general, is at least 900° C. Preferably, the sintering temperature is at least 1000°C and more preferably, ai leas". I LOO^C to preferably, at most 2000°C, more preferably, at most IVSO^'C and most preferably, at most MOO^'C. The heating to the sintering temperature may be canied out in any suitable manm:)' or heating apparatus and under any suitable atmosphere or combinations of atmosirheres, such t\s those known in the iut for making the particular sintered ceramic desired. In another method for fomiing the plugged sintered ceramic honeycomb, the green ;eramic honeycomb is heated to a calcining temperature that is insufficient to subttaritially sinter the green ceramic honeycomb, but is sufficient to dehydrate the clay, such tliat the dehych-ated clay substantially fails to be rehydrated when contacted with watei to focm a calcined ;:eramic honeycomb. The calcining temperature may be any temperature suitable to substantially deiiy ::rate die clay, such that the clay substantially fails to rehydrate. "Substantially fails to rehydrate," is gereraJIy when 90 percent by weight ofthti clay fails to rehydrate when placei in wi i:er for 24 hours. Preferably, the calcining temperature is great enough ihat 99 percent and rr-OVG preferably, all of the dehydrated clay fails to rehydrate when placed in water. Geneirally, the calcining tcnriperature is froin WO'^C to at most 1000^'C. More prefeiably, the calcining temperature is at least 500, more preferably at least 600. most pre "cribly at: least 6:50 to preferably at most 950, more preferably at most 900, and most pre:e:ably ai: most SSO'^C. The calcining atmosphere may be any atmosphere suitable for dehydrating the clay or both. Examples include air, vacuum, inert atmospheres (for example, noble gastrs nitrogen or combinations thereof. The method and apparatus for heating to the calcining ternper^LMre may be any suitablt^ apparatus such as those known in the art. After calcining, the mixture is inserted into a channel, as described pre\ iciisly, to form a plugged calcined ceramic honeycomb. In this method, the dispersing liquid may be water. This is so bifcause tie calcined cerarric honeycomb docs not contain enoug'i clay that can be rehydrated (that is, swell) to cause deleterious cracking of the parttiirn walls of the calcined ceramic hoaeycomb. The plugged ca;:cined ceramic honeycomb is then sintered in the same manner as previously described to form the sintered plugged ceramic honeycomb. EXAMPLES Exam])le 1 A 37.2 cells per cm^ green honeycomb was made by extruding and drying a pasty mixture of alumina, clay, binders and water at Advanced Ceramics Incorporated, Atlanta, GA). The green honeycomb was cut to a length of 150 mm. Half the channels on one tzd of the green honeycomb were plugged with the same pasty mixture used to make the fioneyc:omb .50 as to fomi a checkerboard pattern of plugs, on this end (first plugged end). Tie honeycomb was placed in a clamp with the first plugged end facing up (that is, the orhc r or second end was facing down). Mullite powder (Baikalox MULCR, Baikowski International, Charlotte, NC), h.?v;;ng an average particle size of 3 micrometers was mixed with 2 propanol and 3 percent b> veight of eth]^l cellulose (ETHOCEL, The Dow Chemical, Midland. MI) to form a slurry having 10 j)ercertt: by weight mullite. The slurry was fluid and easily poured. The slurry was then pw)ured into the unplugged channels of the first plugged end. The slurry flowed inio £tiid down the channels, coating the walls of the channels and collecting at the other end of tht: honeyconrib. The slurry collected at the second end and formed a checkerboard pateni of plugs cliae to capillarj' action in the channels not plugged in the first plugged end, to fo-rn a plugged green honeycomb filter. After drying, the plugged green honeycomb filler was heated to 1000°C to renove the binders and lightly sinter the oxides. The lightly siintered honeycomb was then cotiviirted to acicuhnr mullite using the process described by Moyer, et al„ U.S. Patent No. 5,1.9{: .007. The resultant honeycomb wall-flow filter had a discriminating layer of fine ne(=d]c;> of mullite v/here the mullite sluny contacted the walls of the honeycomb. The plugs on the first end had essentially the same mullite microstructure as the honeycomb, whereas the plugs on the second end had a mullite microstructure resembling the discriminating layer. Ex-mple 2 A jgreen honeycomb plugged on one end was prepared by the same method above. The .jreen lioneycomb plugged on one end was then heated to 1000°C to remove the bindei'5 and lightly sinter the oxides. The same mullite powder cf Example 1 was mixed with a 4 weight percent soliLiti-JH of METHOCEL in water to form a slurry having 10 percent by weight mullite. The slurry was poured down the open channels of the first end in the same manner as Example 1 to fonn plug;> at the second end. After drying, the honeycomb was heated to 600'C to ^emo^'5 the METHOCEL binder from the plugs at the second end. After this, the plugged hon'!)'conri!:) was converted to acicular mullite using the process described by Moyer, et aL, US Patent No. 5,198,007. The resultant honeycomb wall-flow filter had essentially the sixvni microstructure characteristics as the filter in Example 1. WHAT IS CLAIMED IS: 1. A method of plugging channels in a ceramic honeycomb comprising; (a) fomiing a mixture comprised of a dispersing liquid and ceramic powder, (b) inserting the mixture into at least one channel of a green ceramic honeycomb that :i is cornprisijd of a clay to form a plugged green ceramic honeycomb wherein the dispersing li()uid essentially fails to swell the clay, and (c) heati]ig the plugged green ceramic honeycomb to a temperature sufficient to sinter the plugged green ceramic honeycomb to form a porous sintered plugged ceramic honeycomb. 2. The method of Claim 1 wherein the dispersing liquid is an alcohoL 3. The method of Claim 2 wherein the dispersing liquid is methanol, propanol, ethancl or mixtures thereof 4. The method of Claim 3 wherein the dispersing liquid is propanol. 5. The method of Claim 1 wherein the inserting of the mixture is peifC'iTied by inserting the mixture at one end of the channel in the green ceramic honeycomb and allowing the mixture to flow to the other end, which is blocked, such that the mixture collects and forms a plug. 6. The method of Chiim 5 wherein the other end is blocked by a porous body capable of removing the dispersing liquid of the mixture. 7. The method of Claim 5 wherein the mixture, as it flows through the channel, deposits ceramic powder on a wall of the channel, such that upon heating of step (c) a liiscrirrinating layer is formed on the wall of the channel. 8. The method of Claim 1 wherein the mixture contains a catalyst. 9. A method of plugging channels in a ceramic honeycomb comprising; (a) hsating a green ceramic honeycomb, that is comprised of clay, to a first tempi:rature that is insufficient to substantially sinter the green ceramic honeycomb, but is sufficient to remove the binder and dehydrate the clay, such that the dehydrated clay suDiiiantiai'y raiis lo oe renycirated when contacted with water to form a calcined ceramic honeycomb, (b) inserting a mixture comprised of a ceramic powder and dispersing liquid into at least one channel of the calcined ceramic honeycomb to fonti a plugged calcined ceramic honeycomb, and (c) heating the plugged calcined ceramic honeycomb to a temperature sufficient to form a sintered plugged honeycomb. 10. The method of Claim 9 wherein the calcining temperature is from 400*^^:: to ftOO'^C and the temperature of step (c) is at least 1000°C. 11. The method of Claim 10 wherein the dispersing liquid is an alcohoL 12. The method of Claim 11 wherein the dispersing liquid is methanol, propanol, edianol or mixtures thereof. 13. The method of Chiim 12 wherein the dispersing liquid is propanol, 14. The mei:hod of Claim 9 wherein the inserting of the mixture is perlbi ined by inserting the mixture at one end of the channel in the calcined ceramic honsy: omb and aJJowing the mixture to flow to the other end, which is blocked, such that the m: xture collects and forms a plug at the other end, 15. The method of Claim 14 wherein the other end is blocked by a porous body capable of removing the dispersing liquid of the mixture. 16. The method of Claim 14 wherein the mixture, as it flows through the chajint!, deposits ceramic powder on a wall of the channel, such that upon heating of step (c) a d::5criminating layer is formed on the wall of the channel. 17. The metiiod of Claim 9 wherein the mixture contains a catalyst. 18. A ceramic honeycomb wall-flow filter comprising a monolithic ceramic honeycomb body having an inlet end and outlet end connected by adjacent inlet and outlet channels that extend from the inlet end to the outlet end of the ceramic body, the inlet and outlet channels being defined by a plurality of interlaced thin gas filtering porous partition \valls between the inlet and outlet channels and by ceramic plugs, such that the ink:t channel has an inlet ceramic plug at the outlet end of the ceramic body and the outlei channel has an cutlet ceramic plug at the inlet end of the ceramic body, such that a fluid Vi-hnsni entering ):he inlet end must pass through partition walls to exit the outlet end, where tl e ceramic honeycomb body has a discriminating layer on at least one partition wall of th outlet channel, the discriminating layer being a composition that is essentially the same as (he outlet ceramic plug of the outlet channel. 19. The ceramic honeycomb wall-flow filter of Claim 18 wherein the mortolitihic ceramic honeycomb body is muUite and the discriminating layer is mullite having a finer porosity than the mullite of the monolithic ceramic honeycomb. 20. The ceramic honeycomb wall-flow filter of Claim 18 wherein the in e^ ceramic plugs; and monolithic ceramic honeycomb have essentially the same composition. 21. A ceramic honeycomb wall-flow filter comprising a monolithic ce -a;nic honeycomb body having an inlet end and outlet end connected by adjacent inlet an^ oucU:: channels that extend from the inlet end to the outlet end of the ceramic body, the inle and cutlet channels being defined by a plurality of interlaced thin gas filtering porous paslition walls betv/een the inlet and outlet channels and by ceramic plugs, such that the inlet channcjl has an inlet ceramicJ plug at the outlet end of the ceramic body and the outlet channel has an outlet ceramic plug at the inlet end of the ceramic body, such that a fluid, when entering the inlet end, must pass through partition walls to exit the outlet end and, wherein the outlet ceramic plugs have a different composition than the inlet ceramic plugs. 22. The ceramic honeycomb wall-flow filter of Claim 21 wherein the cerar'tic horieycomb body has a discriminating layer on at least one partition wall of the outlet channels, the discriminating layer having a composition that is essentially the same as the outlet ccxamic plugs in the outlet channels. 23. The ceramic honeycomb wall-flow filter of Claim 22 wherein the discriminating laj^er has essentially the same chemistry as the ceramic honeycomb body, but a diflerent rnicrostructure than the ceramic honeycomb body. 24. The ceramic honeycomb wall-flow filter of Claim 18 wherein a catalyst is present ::n or on at least one partition wall, inlet plug or outlet plug. 2.5. The ceramic honeycomb wall-flow filter of Claim 21 wherein a cjtalyst is present in or on at least one piirtition wall, inlet plug or outlet plug. 26. A method of plugging channels in a ceramic honeycomb substantially as herem descnoea wun reference to the accompanying drawings.

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