U.S. patent application number 14/079753 was filed with the patent office on 2014-03-13 for porous ceramic honeycomb articles and methods for making the same.
This patent application is currently assigned to Corning Incorporated. The applicant listed for this patent is Corning Incorporated. Invention is credited to Thorsten Rolf Boger, Weiguo Miao, Zhen Song, Jianguo Wang.
Application Number | 20140070441 14/079753 |
Document ID | / |
Family ID | 45218891 |
Filed Date | 2014-03-13 |
United States Patent
Application |
20140070441 |
Kind Code |
A1 |
Boger; Thorsten Rolf ; et
al. |
March 13, 2014 |
POROUS CERAMIC HONEYCOMB ARTICLES AND METHODS FOR MAKING THE
SAME
Abstract
A porous ceramic honeycomb article comprising a honeycomb body
formed from cordierite ceramic, wherein the honeycomb body has a
porosity P %.gtoreq.55% and a cell channel density CD.gtoreq.150
cpsi. The porous channel walls have a wall thickness T, wherein
(11+(300-CD)*0.03).gtoreq.T.gtoreq.(8+(300-CD)*0.02), a median pore
size.ltoreq.20 microns, and a pore size distribution with a
d-factor of .ltoreq.0.35. The honeycomb body has a specific pore
volume of VP.ltoreq.0.22. The porous ceramic honeycomb article
exhibits a coated pressure drop increase of .ltoreq.8 kPa at a flow
rate of 26.5 cubic feet per minute when coated with 100 g/L of a
washcoat catalyst and loaded with 5 g/L of soot.
Inventors: |
Boger; Thorsten Rolf; (Bad
Camberg, DE) ; Miao; Weiguo; (Horseheads, NY)
; Song; Zhen; (Painted Post, NY) ; Wang;
Jianguo; (Horseheads, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Assignee: |
Corning Incorporated
Corning
NY
|
Family ID: |
45218891 |
Appl. No.: |
14/079753 |
Filed: |
November 14, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12955268 |
Nov 29, 2010 |
8609032 |
|
|
14079753 |
|
|
|
|
Current U.S.
Class: |
264/44 |
Current CPC
Class: |
B01D 2255/20738
20130101; B01D 2046/2437 20130101; C04B 38/0009 20130101; B01J
37/0215 20130101; C04B 2235/3217 20130101; Y02A 50/20 20180101;
B01J 37/0246 20130101; B01D 46/2429 20130101; C04B 2111/0081
20130101; B01D 53/945 20130101; B01D 2258/012 20130101; B01D 53/944
20130101; B01D 2279/30 20130101; Y02T 10/12 20130101; C04B 35/195
20130101; B01D 2255/504 20130101; B01D 46/247 20130101; B01D
2046/2433 20130101; B01D 46/2425 20130101; B01D 46/2474 20130101;
C04B 2111/00793 20130101; B01D 2046/2496 20130101; B01D 2255/9205
20130101; B01J 35/04 20130101; B01J 29/46 20130101; C04B 2235/3418
20130101; B28B 11/00 20130101; C04B 2235/3445 20130101; C04B
38/0009 20130101; C04B 35/195 20130101; C04B 38/0054 20130101; C04B
38/0074 20130101 |
Class at
Publication: |
264/44 |
International
Class: |
B28B 11/00 20060101
B28B011/00 |
Claims
1. A method for making a porous ceramic honeycomb article, the
method comprising: mixing a batch of inorganic components with an
organic pore former and at least one processing aid to form a
plasticized batch, wherein: the batch of inorganic components has a
median inorganic particle size d.sub.50IP.ltoreq.15 microns and
comprise talc having d.sub.pt50.ltoreq.10 .mu.m, a silica-forming
source having d.sub.ps50.ltoreq.20 .mu.m, and an alumina-forming
source having a median particle diameter d.sub.pa50.ltoreq.10.0
.mu.m, wherein d.sub.ps50 is a median particle diameter of the
silica-forming source, d.sub.pa50 is a median particle diameter of
the alumina-forming source and d.sub.pt50 is a median particle
diameter of the talc; the organic pore former is present in the
plasticized batch in an amount greater than at least 30 wt. % of
the inorganic components, the organic pore former having
d.sub.pp50.ltoreq.25 .mu.m, wherein d.sub.pp50 is a median particle
diameter of the organic pore former; forming the plasticized batch
into a green honeycomb article; burning the organic pore former out
of the green honeycomb article; firing the green honeycomb article
under conditions effective to form the porous ceramic honeycomb
article comprising: a cordierite crystal phase having a microcrack
parameter (Nb.sup.3) of from about 0.04 to about 0.25; a porosity P
%.gtoreq.55%; a median pore size.ltoreq.20 microns; a wall
thickness T, wherein
(11+(300-CD)*0.03).gtoreq.T.gtoreq.(8+(300-CD)*0.02), wherein the
wall thickness T is in units of mils; and a pore size distribution
with a d-factor of .ltoreq.0.35, wherein the
d-factor=(d50-d10)/d50; and subsequent to firing, exposing the
porous ceramic honeycomb article to a microcracking condition,
wherein after exposure to the microcracking condition, the porous
ceramic honeycomb article comprises a microcrack parameter
(Nb.sup.3) is at least 20% greater than the microcrack parameter
prior to exposure to the microcracking condition.
2. The method of claim 1, wherein the microcracking condition
comprises a thermal cycle in which the porous ceramic honeycomb
article is heated to a peak temperature of at least 400.degree. C.
and after the porous ceramic honeycomb article reaches the peak
temperature, the porous ceramic honeycomb article is cooled at a
rate of at least 200.degree. C./hr.
3. The method of claim 1, wherein a bare initial filtration
efficiency and a coated initial filtration efficiency of the porous
ceramic honeycomb article are .gtoreq.50%.
4. The method of claim 1, wherein the porous ceramic honeycomb
article has a surface porosity of greater than 35%.
5. The method of claim 1, wherein the pore size distribution of the
porous ceramic honeycomb article has an absolute breadth
d.sub.Absb.ltoreq.10 microns, wherein
d.sub.Absb=d.sub.75-d.sub.25.
6. The method of claim 1, wherein the porous ceramic honeycomb
article exhibits: a coated pressure drop increase of .ltoreq.8 kPa
at a flow rate of 26.5 cubic feet per minute when coated with 100
g/L of a washcoat catalyst and loaded with 5 g/L of soot; and a
bare pressure drop increase of .ltoreq.4 kPa when loaded with 5 g/L
of soot.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of and claims the benefit
of priority to U.S. application Ser. No. 12/955,268 filed on Nov.
29, 2010, which is hereby incorporated by reference for all
purposes as if fully set forth herein.
FIELD
[0002] The present specification generally relates to porous
ceramic honeycomb articles and, more specifically, to cordierite
porous ceramic honeycomb articles for use as filter materials.
TECHNICAL BACKGROUND
[0003] Ceramic honeycomb articles are widely used as anti-pollution
devices in the exhaust systems of automotive vehicles, both as
catalytic converter substrates in automobiles, and as particulate
filters in diesel-powered vehicles. Ceramic honeycomb articles for
use in such applications are formed from a matrix of thin, porous
ceramic walls which define a plurality of parallel, gas conducting
channels. In ceramic honeycomb articles used as catalytic
substrates in automobiles with gasoline engines, the gas conducting
channels are open at both ends. A catalytic coating is applied to
the outer surfaces of the walls. Exhaust gasses flowing through the
channels come into contact with catalytic coatings on the surfaces
of the walls. These honeycomb articles are referred to as
flow-through substrates. In diesel systems, exhaust gasses also
come into contact with catalytic coatings on the surfaces of the
walls. In diesel applications, the ceramic honeycomb articles may
also have end-plugs in alternate gas conducting channels to force
exhaust gasses to pass through the porous channel walls in order to
capture and filter out soot and ash particulates prior to exhaust
discharge. These ceramic honeycomb substrates are referred to as
ceramic wall-flow particulate filters and, more specifically, as
diesel particulate filters.
[0004] Application of the catalyst washcoat to the honeycomb
substrate alters the properties of the honeycomb structure as the
washcoat is deposited on the walls and within the pores of the
honeycomb substrate. This results in an increase of backpressure
for exhaust gasses flowing through the honeycomb. Furthermore, the
extreme temperature fluctuations experienced by honeycomb articles
used in both automotive and diesel applications makes the ceramic
honeycomb articles susceptible to temperature-induced cracking
which leads to the degradation of the honeycomb articles.
[0005] Accordingly, a need exists for alternative porous ceramic
honeycomb structures with decreased back pressure gain after
coating with a catalyst washcoat.
SUMMARY
[0006] According to one embodiment, a porous ceramic honeycomb
article includes a honeycomb body formed from cordierite ceramic
and comprising a plurality of cell channels formed by porous
channel walls. The honeycomb body has a porosity P %.gtoreq.55%, a
cell channel density CD.gtoreq.150 cpsi, and a wall thickness T,
wherein (11+(300-CD)*0.03).gtoreq.T.gtoreq.(8+(300-CD)*0.02),
wherein the wall thickness T is in units of mils. The porous
channel walls of the honeycomb body have a median pore
size.ltoreq.20 microns. The porous channel walls of the honeycomb
body have a pore size distribution with a d-factor of .ltoreq.0.35,
wherein the d-factor=(d.sub.50-d.sub.10)/d.sub.50. The honeycomb
body has a specific pore volume per volume of the honeycomb body of
VP.ltoreq.0.22, wherein VP=(1-OFA)*P %, wherein OFA is an open
frontal area of the porous honeycomb body. The porous ceramic
honeycomb article exhibits a coated pressure drop increase of
.ltoreq.8 kPa at a flow rate of 26.5 cubic feet per minute when
coated with 100 g/L of a washcoat catalyst and loaded with 5 g/L of
soot.
[0007] In another embodiment, a method for making a porous ceramic
honeycomb article includes mixing a batch of inorganic components
with an organic pore former and at least one processing aid to form
a plasticized batch. The batch of inorganic components has a median
inorganic particle size d.sub.50IP.ltoreq.15 microns and comprise
talc having d.sub.pt50.ltoreq.10 .mu.m, a silica-forming source
having d.sub.ps50.ltoreq.20 .mu.m, and an alumina-forming source
having a median particle diameter d.sub.pa50.ltoreq.10.0 .mu.m,
wherein d.sub.ps50 is a median particle diameter of the
silica-forming source, d.sub.pa50 is a median particle diameter of
the alumina-forming source and d.sub.pt50 is a median particle
diameter of the talc. The organic pore former is present in the
plasticized batch in an amount greater than about 35 wt. %, the
organic pore former having d.sub.pp50.ltoreq.25 .mu.m, wherein
d.sub.pp50 is a median particle diameter of the organic pore
former. The plasticized batch is formed into a green honeycomb
article and the organic pore former is burned out of the green
honeycomb article. The green honeycomb article is fired under
conditions effective to form the porous ceramic honeycomb article
having: a cordierite crystal phase having a microcrack parameter
(Nb.sup.3) of from about 0.04 to about 0.25; a porosity P
%.gtoreq.55%; a median pore size.ltoreq.20 microns; a wall
thickness T, wherein
(11+(300-CD)*0.03).gtoreq.T.gtoreq.(8+(300-CD)*0.02), wherein the
wall thickness T is in units of mils; and a pore size distribution
with a d-factor of .ltoreq.0.35, wherein the
d-factor=(d50-d10)/d50. Subsequent to firing, the porous ceramic
honeycomb article is exposed to a microcracking condition, wherein
after exposure to the microcracking condition, the porous ceramic
honeycomb article comprises a microcrack parameter (Nb.sup.3) is at
least 20% greater than the microcrack parameter prior to exposure
to the microcracking condition.
[0008] Additional features and advantages will be set forth in the
detailed description which follows, and in part will be readily
apparent to those skilled in the art from that description or
recognized by practicing the embodiments described herein,
including the detailed description which follows, the claims, as
well as the appended drawings.
[0009] It is to be understood that both the foregoing general
description and the following detailed description describe various
embodiments and are intended to provide an overview or framework
for understanding the nature and character of the claimed subject
matter. The accompanying drawings are included to provide a further
understanding of the various embodiments, and are incorporated into
and constitute a part of this specification. The drawings
illustrate the various embodiments described herein, and together
with the description serve to explain the principles and operations
of the claimed subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 schematically depicts a porous ceramic honeycomb
article according to one or more embodiments shown and described
herein;
[0011] FIG. 2 schematically depicts a porous ceramic honeycomb
article according to one or more embodiments shown and described
herein;
[0012] FIG. 3 is an SEM micrograph of a polished cross section of a
porous cell channel wall of a porous ceramic honeycomb article
coated with a catalyst washcoat according to one or more
embodiments shown and described herein;
[0013] FIG. 4 is an SEM micrograph of a portion of the porous cell
channel wall of FIG. 3;
[0014] FIG. 5 is an SEM micrograph of a surface of a porous cell
channel wall of a porous ceramic honeycomb article coated with a
catalyst washcoat according to one or more embodiments shown and
described herein;
[0015] FIG. 6 is an SEM micrograph of a portion of the porous cell
channel wall of FIG. 5;
[0016] FIG. 7 is a plot of pressure drop (y-axis) as a function of
soot loading (x-axis) for bare porous ceramic honeycomb article and
porous ceramic honeycomb articles coated with different amounts of
a catalyst washcoat;
[0017] FIG. 8 is an SEM micrograph of a porous ceramic honeycomb
article prior to exposure to a microcracking condition according to
one or more embodiments shown and described herein;
[0018] FIG. 9 is an SEM micrograph of a porous ceramic honeycomb
article after exposure to a microcracking condition according to
one or more embodiments shown and described herein;
[0019] FIG. 10 graphically depicts an exemplary firing schedule for
producing a porous ceramic honeycomb body according to one or more
embodiments shown and described herein;
[0020] FIG. 11 is a plot of pressure drop (y-axis) as a function of
soot loading (x-axis) for an Inventive Example of a porous ceramic
honeycomb article coated with catalyst washcoat and Comparative
Examples of porous ceramic honeycomb articles coated with a
catalyst washcoat;
[0021] FIG. 12 is a plot of pressure drop (y-axis) as a function of
catalyst loading (x-axis) for an Inventive Example of a porous
ceramic honeycomb article coated with catalyst washcoat and
Comparative Examples of porous ceramic honeycomb articles coated
with a catalyst washcoat; and
[0022] FIG. 13 is a plot of filtration efficiency (y-axis) as a
function of soot loading (x-axis) for an Inventive Example of a
porous ceramic honeycomb article coated with catalyst washcoat.
DETAILED DESCRIPTION
[0023] Reference will now be made in detail to embodiments of
porous ceramic honeycomb articles, examples of which are
illustrated in the accompanying drawings. Whenever possible, the
same reference numerals will be used throughout the drawings to
refer to the same or like parts. One embodiment of a porous ceramic
honeycomb article is schematically depicted in FIG. 1. The porous
ceramic honeycomb article includes a honeycomb body formed from
cordierite ceramic and comprising a plurality of cell channels
formed by porous channel walls. The channel walls of the honeycomb
body have a porosity P %.gtoreq.55%, a median pore size.ltoreq.20
microns, and a cell channel density CD.gtoreq.150 cpsi. The porous
channel walls of the honeycomb body have a wall thickness T,
wherein (11+(300-CD)*0.03).gtoreq.T.gtoreq.(8+(300-CD)*0.02), where
the wall thickness T is in units of mils. The honeycomb body also
has a specific pore volume.ltoreq.0.22 which represents the ratio
of the total volume of the pores present in the porous channel
walls to the total volume of the honeycomb. The porous channel
walls of the honeycomb body have a pore size distribution with a
d-factor of .ltoreq.0.35, wherein the
d-factor=(d.sub.50-d.sub.10)/d.sub.50. The ratio of the surface
porosity to the total bulk porosity is greater than or equal to
0.5. The porous ceramic honeycomb article has a coated pressure
drop increase of .ltoreq.8 kPa at a flow rate of 26.5 cubic feet
per minute when coated with 100 g/L of a washcoat catalyst and
loaded with 5 g/L of soot. The porous ceramic honeycomb articles
and methods for making the porous ceramic honeycomb articles will
be described in more detail herein.
[0024] As used herein, the singular forms "a," "an" and "the"
include plural referents unless the context clearly dictates
otherwise. Thus, for example, reference to a "silica-forming
source" or an "alumina-forming source" may include aspects of
having two or more such forming sources, unless the context clearly
indicates otherwise.
[0025] As used herein, a "wt. %" or "weight percent" or "percent by
weight" of an organic component, unless specifically stated to the
contrary, is based on the total weight of the total inorganics in
which the component is included. All organic additions, such as,
for example, pore formers and binders, are specified herein as
superadditions based upon 100% of the inorganics used.
[0026] Referring now to FIG. 1, a porous ceramic honeycomb article
100 is schematically depicted. The porous ceramic honeycomb article
100 may be used as a wall-flow filter for particulate matter
filtration. For example, the porous ceramic honeycomb article 100
may be used in filtering particulate matter from a vehicle exhaust.
The porous ceramic honeycomb article 100 generally comprises a
porous cordierite ceramic honeycomb body having a plurality of cell
channels 101 extending between a first end 102 and a second end
104. The plurality of generally parallel cell channels 101 formed
by, and at least partially defined by, intersecting porous channel
walls 106 that extend from the first end 102 to the second end 104.
The porous ceramic honeycomb article 100 may also include a skin
formed about and surrounding the plurality of cell channels. This
skin may be extruded during the formation of the channel walls 106
or formed in later processing as an after-applied skin, by applying
a skinning cement to the outer peripheral portion of the cells.
[0027] In one embodiment, the plurality of parallel cell channels
101 are generally square in cross section and are formed into a
honeycomb structure. However, in alternative embodiments, the
plurality of parallel cell channels in the honeycomb structure may
have other cross-sectional configurations, including rectangular,
round, oblong, triangular, octagonal, hexagonal, or combinations
thereof.
[0028] The term "honeycomb" as used herein is defined as a
structure of longitudinally-extending cells formed from the channel
walls 106 and preferably having a generally repeating grid pattern
therein. For honeycombs utilized in filter applications, certain
cells are designated as inlet cells 108 and certain other cells are
designated as outlet cells 110. Moreover, in a porous ceramic
honeycomb article 100, at least some of the cells may be plugged
with plugs 112. Generally, the plugs 112 are arranged at or near
the ends of the cell channels and are arranged in some defined
pattern, such as in the checkerboard pattern shown in FIG. 1, with
every other cell being plugged at an end. The inlet channels 108
may be plugged at or near the second end 104, and the outlet
channels 110 may be plugged at or near the first end 102 on
channels not corresponding to the inlet channels. Accordingly, each
cell may be plugged at or near one end of the porous ceramic
honeycomb article only.
[0029] Referring now to FIG. 2, an alternative embodiment of a
porous ceramic honeycomb article 200 is schematically depicted. In
this embodiment, some cell channels may be flow-through channels
(unplugged along their entire length) while other channels may be
plugged thus providing a so-called "partial filter" design. More
specifically, the porous ceramic honeycomb article depicted in FIG.
2 generally comprises intersecting porous walls 206, inlet cells
208 plugged with plugs (not shown) at the outlet end 204, outlet
cells 210 plugged with plugs 212 at the inlet end and at least some
flow through (unplugged) channels 214 where flow passes directly
through the body of the porous ceramic honeycomb article without
passing through the porous channel walls 206. For example, in one
embodiment (not shown), every other cell in every other row is a
flow through channel. Thus, in this embodiment, less than 50% of
the channels may be unplugged.
[0030] While FIGS. 1 and 2 depict embodiments of porous ceramic
honeycomb articles 100, 200 in which some or all of the channels
are plugged, is should be understood that, in alternative
embodiments, all the channels of the porous ceramic honeycomb
articles may be unplugged, such as when the porous ceramic
honeycomb articles 100, 200 are used as catalytic through-flow
substrates for use with gasoline engines.
[0031] In one embodiment of the porous ceramic honeycomb articles
100, 200 described herein, the thickness T of the porous channel
walls 106, 206 in units of mils ( 1/1000 inch or 25.4 microns) is a
function of the cell density of the honeycomb article 100, 200 in
cpsi. Specifically, the thickness T of the porous channel walls may
be in a range from about (11+(300-CD)*0.03) to about
(8+(300-CD)*0.02), where CD is the density of the cells in cells
per square inch (cpsi). In other embodiments, the thickness of the
channel walls is in a range from about (10+(300-CD)*0.03) to about
(6+(300-CD)*0.02). In other embodiments, the thickness of the
channel walls is in range from about (12+(300-CD)*0.03) to about
(8+(300-CD)*0.02).
[0032] The cell density CD of the porous ceramic honeycomb articles
100, 200 may be less than or equal to about 400 cells/in.sup.2. In
another embodiment, the cellular density of the porous ceramic
honeycomb articles 100, 200 may be less than or equal to about 300
cells/in.sup.2. In yet another embodiment, the cellular density of
the porous ceramic honeycomb articles 100, 200 may be greater than
or equal to about 150 cells/in.sup.2. Accordingly, in the
embodiments described herein, it should be understood that the
cellular density of the porous ceramic honeycomb articles 100, 200
may be greater than or equal to about 150 cells/in.sup.2 and less
than or equal to about 400 cells/in.sup.2.
[0033] Reference may be made herein to the porous ceramic honeycomb
article having"geometry" of A/B where A is the cellular density of
the porous ceramic honeycomb article and B is the thickness of the
channel walls. By way of example and not limitation, a porous
ceramic honeycomb article having a 200/10 geometry has a cellular
density of 200 cells/in.sup.2 and a cell wall thickness of 10 mils.
In some embodiments described herein, the porous ceramic articles
have a geometry of 300/8. In other embodiments, the porous ceramic
articles have a geometry of 300/10. In still other embodiments the
porous ceramic articles have a geometry of 200/12. However, it
should be understood that other geometries are possible.
[0034] The porous ceramic honeycomb articles described herein
generally have a relatively high total porosity (% P). In the
embodiments of the porous ceramic honeycomb articles described
herein, the total porosity % P is greater than or equal to about
50% and less than or equal to about 70% as measured with mercury
porosimetry. In some embodiments, the total porosity % P is greater
than or equal to about 55% and less than or equal to about 65%. In
other embodiments, the total porosity % P is greater than or equal
to 58% and less than or equal to 62%. In yet other embodiments the
total porosity % P is greater than or equal to about 62% and less
than or equal to 65%.
[0035] Referring to FIGS. 3-6, the pores of the porous ceramic
honeycomb article are highly connected within the channel-like
domains of cordierite ceramic indicating an interpenetrated network
structure. Specifically FIGS. 3 and 4, depict SEM micrographs of
the pore morphology of a polished axial cross section of a cell
channel wall coated with a catalyst washcoat. FIGS. 5 and 6 depict
the surface pore morphology of a porous ceramic honeycomb article
coated with a catalyst washcoat. As shown in FIGS. 3 and 4, the
pores of the porous ceramic honeycomb are generally well connected
into channels. The surface pore morphology depicted in FIGS. 5 and
6 is generally similar to the pore morphology of the axial cross
section shown in FIGS. 3 and 4. Accordingly, the morphology of the
surface porosity taken in conjunction with the morphology of the
total body porosity is generally a bi-continuous morphology. In the
embodiments described herein the cordierite domain size is
generally greater than or equal to about 20 microns or even 40
microns. In some embodiments the cordierite domain size is greater
than 60 microns. In other embodiments, the cordierite domain size
within the porous ceramic honeycomb article is in the range from
about 20 microns to about 80 microns.
[0036] The specific pore volume of the honeycomb article
characterizes the total volume available inside the porous
structure of the channels walls as a function of the porosity % P
of the of the porous ceramic honeycomb article and the total volume
of the channel walls present in the porous ceramic article,
referred to herein as the open frontal area (OFA) of the porous
honeycomb article. More specifically, the specific pore volume VP
is related to the OFA and the porosity % P according to the
relation:
VP=(1-OFA)*(%P)
[0037] The porous ceramic honeycomb articles described herein
generally have a relatively low specific pore volume VP. In the
embodiments described herein, the porous ceramic honeycomb articles
have a specific pore volume less than 0.22. In some embodiments,
the specific pore volume may be less than 0.2 or even less than
0.185. In yet another embodiment, the specific pore volume may be
less than 0.18. In some other embodiments, the specific pore volume
may be in the range of 0.21.ltoreq.VP.ltoreq.0.14.
[0038] The bare surface porosity of the porous ceramic honeycomb
articles, as measured by image analysis of SEM micrographs of the
porous ceramic honeycomb articles prior to washcoating, is
generally .gtoreq.30% or even .gtoreq.35%. In some embodiments, the
surface porosity of the porous ceramic honeycomb articles is
.gtoreq.38% or even .gtoreq.40%. In other embodiments, the surface
porosity is .gtoreq.42%. The higher surface porosity yields a
porous ceramic honeycomb article with a higher permeability and a
corresponding lower backpressure drop when used as a particulate
filter in automotive and/or diesel applications. Based on the
surface porosity and the total porosity, embodiments of the porous
ceramic honeycomb articles have a surface porosity to total
porosity ratio of greater than or equal to 0.5 or even greater than
equal to 0.6. In some embodiments the surface porosity to total
porosity ratio is greater than or equal to 0.7.
[0039] The porous ceramic honeycomb articles described herein
generally have a median pore diameter d.sub.50 in the range from
about 12 microns to about 20 microns. In some embodiments, the
median pore diameter d.sub.50 of the porous ceramic honeycomb
article is less than or equal to about 20 microns or even less than
or equal to about 16 microns. In other embodiments, the median pore
diameter d.sub.50 of the fired porous ceramic honeycomb article is
in the range from about 12 microns to about 14 microns. Controlling
the porosity such that the median pore diameter d.sub.50 is within
these ranges limits the amount of very small pores and thereby
minimizes the washcoated backpressure of the fired porous ceramic
article.
[0040] In the embodiments described herein, the pore size
distribution of the porous ceramic honeycomb article comprises a
d.sub.10 value of greater than or equal to 5 microns or even
greater than or equal to 8 microns. The quantity d.sub.10, as used
herein, is the pore diameter at which 10% of the pore volume is
comprised of pores with diameters smaller than the value of
d.sub.10; thus, using mercury porosimetry techniques to measure
porosity, d.sub.10 is equal to the pore diameter at which 90% by
volume of the open porosity of the ceramic has been intruded by
mercury during the porosimetry measurement.
[0041] As used herein, the d-factor d.sub.f is a characterization
of the relative width of the distribution of pore sizes that are
finer than the median pore size d.sub.50. The d-factor d.sub.f is
defined as:
d.sub.f=(d.sub.50-d.sub.10)/d.sub.50,
where d.sub.50 and d.sub.10 are as defined hereinabove. In the
embodiments described herein, the pore size distribution of the
open interconnected porosity of the porous walls of the porous
ceramic honeycomb article is relatively narrow such that
d.sub.f.ltoreq.0.35, d.sub.f.ltoreq.0.3, d.sub.f.ltoreq.0.25,
d.sub.f.ltoreq.0.22, or even d.sub.f.ltoreq.0.2. In some
embodiments, the d-factor of the porous ceramic honeycomb articles
is in a range from about 0.15 to about 0.35.
[0042] In the embodiments described herein, the porous ceramic
honeycomb article has a pore size distribution with a d.sub.90
value of less than or equal to 45 microns or even less than or
equal to 35 microns. Some embodiments of the porous ceramic
honeycomb articles have a pore size distribution with a d.sub.90
value of less than or equal to 30 microns. The quantity d.sub.90,
as used herein, is the pore diameter at which 90% of the pore
volume is comprised of pores with diameters smaller than the value
of d.sub.90; thus, using mercury porosimetry techniques to measure
porosity, d.sub.90 is equal to the pore diameter at which 10% by
volume of the open porosity of the ceramic has been intruded by
mercury during the porosimetry measurement.
[0043] The ultra narrow pore size distribution of the porous
ceramic honeycomb articles may also be characterized by the breadth
d.sub.Absb of the distribution of pore sizes that are both finer
and coarser than the median pore size d.sub.50. As used herein,
d.sub.Absb is defined as:
d.sub.Absb=(d.sub.75-d.sub.25),
where the quantity d.sub.25, as used herein, is the pore diameter
at which 25% of the pore volume is comprised of pores with
diameters smaller than the value of d.sub.25 and the quantity
d.sub.75, as used herein, is the pore diameter at which 75% of the
pore volume is comprised of pores with diameters smaller than the
value of d.sub.75. The porous ceramic honeycomb articles described
herein may have a pore size distribution exhibiting a
d.sub.Absb.ltoreq.10 microns. In some embodiments, the porous
ceramic honeycomb articles exhibit a d.sub.Absb.ltoreq.8 microns or
even a d.sub.Absb.ltoreq.6 microns. Having a narrow breadth around
the median pore size value ensures that the majority of the pores
and pore space/pore volume are within a desired range and that
little volume of the porous ceramic honeycomb article is lost to
pores that are either too small or too large. It is believed that
this narrow absolute breadth is expected to provide improved
catalyst washcoat coatability as well as high permeability
following coating with a catalyst washcoat (i.e., high efficiency
in pore utilization for flow).
[0044] In the embodiments described herein, the combined properties
of the total porosity, the surface porosity, the median pore
diameter d.sub.50, the d-factor d.sub.f, and the specific pore
volume provide a porous ceramic honeycomb article with a relatively
high initial filtration efficiency in both the bare and coated
conditions. In some embodiments, the bare initial filtration
efficiency is greater than or equal to 50% or even greater than or
equal to 55%. In other embodiments of the porous ceramic honeycomb
articles described herein, the bare initial filtration efficiency
is greater than or equal to 60% or even greater than 70%. In still
other embodiments the bare initial filtration efficiency is greater
than or equal to 90%. Similarly, in some embodiments, the coated
initial filtration efficiency is greater than or equal to 50% or
even greater than or equal to 55% after coating with a catalyst
washcoat. In some embodiments of the porous ceramic honeycomb
articles described herein, the coated initial filtration efficiency
is greater than or equal to 60% or even greater than 70%. In other
embodiments the coated initial filtration efficiency is greater
than or equal to 90%.
[0045] Referring to FIG. 13 by way of example, a plot of the coated
initial filtration efficiency (FE) (y-axis) as a function of the
soot loading (x-axis) is graphically depicted for one Inventive
Example of a porous ceramic honeycomb article. The Inventive
Example had a 300/9 cell geometry, a porosity of 60%, a d-factor of
0.26, a median pore diameter of 19.3 microns, and a specific pore
volume of 0.144. The sample was coated with 120 g/L of catalyst
washcoat of 12 wt. % of Fe-ZSM-5 zeolite (3 micron particle size)
in water. As shown in FIG. 13, the initial filtration efficiency
(FE.sub.0) of the porous ceramic honeycomb article (i.e., the
filtration efficiency at a soot loading of 0 g/L) was greater than
about 65%.
[0046] Further, it has now been found that the combination of the
total porosity, the surface porosity, the median pore diameter
d.sub.50, the d-factor d.sub.f, and the specific pore volume
described herein generally provide a porous ceramic honeycomb
article which can be readily coated with a significant amount of
catalyst washcoat in a single washcoating step, thus exhibiting a
unique accessibility of the pore space provided within the porous
ceramic honeycomb. In the embodiments described herein, the
catalyst washcoat is generally present in the porous ceramic
honeycomb article in an amount greater than about 30 g/L. In some
embodiments described herein, the porous ceramic honeycomb articles
have a single-coat catalyst washcoat loading .gtoreq.50 g/L or even
greater than 60 g/L for a washcoat slurry containing 12 wt. % of
Fe-ZSM-5 zeolite in water with a peak particle size of 3.3 microns.
For some embodiments, it is contemplated that the single-coat
catalyst washcoat loading may be .gtoreq.60 g/L.
[0047] In the embodiments described herein the porous ceramic
articles exhibit a coated pressure drop increase .ltoreq.8 kPa at a
flow rate of 26.5 cubic feet per minute when coated with 100 g/L of
the washcoat catalyst and loaded with 5 g/L of soot. In some
embodiments the coated pressure drop increase is .ltoreq.7 kPa, or
even .ltoreq.6.5 kPa under the same conditions. In other
embodiments, the coated pressure drop increase is .ltoreq.6 kPa, or
even .ltoreq.5.5 kPa under the same conditions. In still other
embodiments the coated pressure drop increase is .ltoreq.5 kPa
under the same conditions.
[0048] In the embodiments described herein the porous ceramic
articles exhibit a bare pressure drop increase .ltoreq.4 kPa at a
flow rate of 26.5 cubic feet per minute when loaded with 5 g/L of
soot. In some embodiments the coated pressure drop increase is
.ltoreq.3.5 kPa, or even .ltoreq.3.0 kPa under the same conditions.
In other embodiments, the coated pressure drop increase is
.ltoreq.2.5 kPa, or even .ltoreq.2.0 kPa under the same conditions.
In still other embodiments the coated pressure drop increase is
.ltoreq.1.5 kPa under the same conditions.
[0049] Referring, now to FIG. 7, a plot of the pressure drop
(y-axis) as a function of soot loading is graphically depicted for
a bare (i.e., uncoated) porous ceramic article as well as for
porous ceramic articles loaded with different amounts of a catalyst
washcoat, specifically 60 g/L, 102 g/L and 119 g/L. Each porous
ceramic article had a 300/8 cell geometry (i.e., 300 cpsi and a
wall thickness of 8 mils), a median pore size of 11.7 microns, a
porosity of 62% prior to washcoating, and a d-factor d.sub.f of
0.25. The porous ceramic honeycombs used in the experiments had a
diameter of 2'' and a length of 6''. The soot was Printex U loaded
at a rate of 0.1 g/min to 0.3 g/min at a gas flow rate of 10-30
standard cubic feet per minute at room temperature. The curves were
obtained at a flow rate of 26.5 cubic feet per minute. Table 1
contains the pressure values in kPa for the clean back pressure
(i.e., the back pressure with no soot loading), the 5 g/L soot
loaded back pressure (i.e., the back pressure when the porous
ceramic article is loaded with 5 g of soot per liter of the porous
ceramic honeycomb), and the knee height for each of the porous
ceramic articles. The knee height, as used herein, is defined as
the pressure drop at the point Pk which may be determined
graphically as shown in FIG. 7.
TABLE-US-00001 TABLE 1 Catalyst Clean back pressure 5 g/L soot
loaded Height of knee loading (kPa) back pressure (kPa) (kPa) Bare
1.2 2.5 0.1 60 g/L 1.7 4.7 0.9 102 g/L 2.3 5.5 0.8 119 g/L 3.0 7.8
2.2
[0050] As graphically depicted in FIG. 7, the bare porous ceramic
honeycomb article had a clean back pressure increase of 1.2 kPa
while the porous ceramic honeycomb article coated with
approximately 100 g/L of catalyst washcoat comprising 12 wt. % of
Fe-ZSM-5 zeolite in water had a clean back pressure increase of 2.3
kPa. This value is unexpectedly low considering the low specific
pore volume of the porous ceramic honeycomb article (0.16 in the
examples) and low median pore size. Accordingly, the porous ceramic
honeycomb with catalyst washcoat of 100 g/L had a clean pressure
drop increase of approximately 91% after being coated with the 100
g/L of catalyst washcoat.
[0051] Similarly, the bare porous ceramic honeycomb article had a 5
g/L soot loaded back pressure increase of 2.5 kPa while the porous
ceramic honeycomb article coated with approximately 100 g/L of
catalyst washcoat comprising 12 wt. % of Fe-ZSM-5 zeolite in water
had a 5 g/L soot loaded back pressure increase of 5.5 kPa. This
value is unexpectedly low considering the low specific pore volume
of the porous ceramic honeycomb article (0.16 in the examples) and
low median pore size. Accordingly, the porous ceramic honeycomb
with catalyst washcoat of 100 g/L had a 5 g/L soot loaded pressure
drop increase of approximately 120% after being coated with 100 g/L
of catalyst washcoat.
[0052] Accordingly, it should be understood that, in some
embodiments described herein, the porous ceramic honeycomb articles
exhibit a clean pressure drop increase of .ltoreq.100% after
coating with 100 g/L of a washcoat catalyst relative to the porous
ceramic article prior to coating under identical test conditions.
In some other embodiments, the clean pressure drop increase after
coating with 100 g/L of a washcoat catalyst is .ltoreq.95% relative
to the uncoated porous ceramic article, or even .ltoreq.93%
relative to the uncoated porous ceramic article.
[0053] Further, it should also be understood that, in some
embodiments described herein, the porous ceramic honeycomb articles
exhibit a 5 g/L soot loaded pressure drop increase of .ltoreq.150%
after coating with 100 g/L of a washcoat catalyst relative to the
porous ceramic article prior to coating under identical test
conditions. In some other embodiments, the 5 g/L soot-loaded
pressure drop increase after coating with 100 g/L of a washcoat
catalyst is .ltoreq.140% relative to the uncoated porous ceramic
article, or even .ltoreq.130% relative to the uncoated porous
ceramic article.
[0054] Without being bound by theory, it is believed that the high
single-coat catalyst washcoat loading and relatively low pressure
drop increase in both clean and soot-loaded conditions can be
attributed to the selected range of median pore sizes, the narrow
pore size distribution, and the high ratio of surface porosity to
bulk porosity enabling an optimized utilization of the pore space
inside the wall structure as well as cell geometries that allow for
low resistance to flow across the filter. For example, a lower
resistance to flow in general correlates to porous ceramic
honeycomb articles with higher open frontal area, which have been
heretofore perceived as having inferior catalyst washcoat loading
capacities due to the lower specific pore volume. However, the
unique structure of the porous ceramic honeycomb articles described
herein compensate for these shortfalls by enabling high utilization
of the pore volume to accommodate more catalyst washcoat. As a
result, low pressure drop increases can be achieved at high
washcoat loadings despite the lower specific pore volume VP of the
porous ceramic honeycomb articles. For example, referring to FIGS.
3 and 4, the porous ceramic honeycomb article has a uniform
channel-like domain 250 (i.e., the lighter portions of the
micrograph) where all the catalyst washcoat is coated into the
pores rather than on the walls of the channels. More specifically,
the zeolite particles 260 in the catalyst washcoat preferentially
coat the small pores 265, and portions of the domain with small
radii of curvature as well as neck areas 266 due to higher
micro-capillary forces in these areas as the catalyst washcoat is
dried in the porous ceramic article. By comparison, portions with
relatively larger radii of curvature, such as relatively larger
pores 267 and/or flat areas of the domain structure, have much less
catalyst after drying due to the effective lowering of the boiling
point of the washcoat as a result of relatively lower
micro-capillary forces. As a consequence, the catalyst washcoat
with the zeolite preferentially flows in to the smaller pores
which, in turn, are dried last (i.e., after the larger pores)
thereby yielding a higher concentration of catalyst in the smaller
pores.
[0055] Further, the high density of uniformly distributed and well
connected pores allows for a greater amount of zeolite to be
washcoated into the porous ceramic article while still maintaining
the permeability of the porous ceramic article to gas, such as
exhaust gases, which flow through the porous ceramic article.
Similarly, because the porous ceramic article has a narrow pore
size distribution with a relatively small median pore size, the
pores have a high microcapillary force which assists in retaining
the zeolite in the pores. Accordingly, the zeolite deposited in the
pores during washcoating with the catalyst washcoat is not easily
dislodged from the pores during high-volume flow of gas through the
porous ceramic article compared to porous ceramic articles having
larger median pore sizes and broader pore size distributions.
[0056] In addition, the porous ceramic honeycomb articles described
herein have a set of physical properties (i.e., coefficient of
thermal expansion (CTE), thermal shock limit (TSL), microcrack
parameter (Nb.sup.3), etc.) which change when the porous ceramic
honeycomb articles are exposed to a microcracking condition thereby
producing a porous ceramic honeycomb article which has an improved
resistance to thermal shock. More specifically, it has been found
that the porous ceramic honeycomb articles described herein have a
relatively high CTE over the temperature range from about
25.degree. C. to 800.degree. C. and a corresponding low thermal
shock limit (TSL) after firing. However, following exposure to a
microcracking condition, the porous ceramic cordierite honeycomb
articles described herein have a relatively lower CTE over the
temperature range from about 25.degree. C. to 800.degree. C. and a
relatively higher thermal shock limit (TSL). It should be
understood that CTE, as used herein, is the coefficient of thermal
expansion in at least one direction of the article over the
specified temperature range, unless otherwise specified. The
improvement in the CTE and TSL following exposure to the
microcracking condition is due to the increase in the volume of
microcracks following exposure to the microcracking condition as
indicated by an increase in the microcrack parameter Nb.sup.3 after
exposure to the microcracking condition. More specifically, the
microcrack parameter Nb.sup.3 of the porous ceramic honeycomb
articles increases by at least 20 percent after the article is
exposed to a microcracking condition.
[0057] The microcrack parameter Nb.sup.3 is derived from the
modulus of elasticity (E.sub.mod) heating and cooling curve between
room temperature and 1200.degree. C. and is an indirect measure of
the microcrack volume of the article. Nb.sup.3 is calculated
as:
Nb 3 = [ E 0 E - 1 ] 1.8 , ##EQU00001##
where E is the elastic modulus of the article at room temperature
with microcracks (i.e., after exposure to a microcracking
condition), E.sub.0 is the elastic modulus of the article at room
temperature without microcracks (i.e., before exposure to a
microcracking condition), N is the number of microcracks and b is
the average length of a microcrack. The microcrack parameter
Nb.sup.3 is measured in units of volume given that the average
crack length b, is cubed.
[0058] Thermal Shock Limit (TSL), as used herein, is defined
as:
TSL=TSP+500.degree. C.,
where TSP is the Thermal Shock Parameter such that:
TSP=MOR/{[E.sub.mod][CTE.sub.H]} and
E.sub.mod is the elastic modulus of the article at 25.degree. C.
(i.e., room temperature (RT)), MOR is the modulus of rupture
strength at room temperature and is measured in psi, and CTE.sub.H
is the high temperature thermal expansion coefficient measured
between 500.degree. C. and 900.degree. C. As the TSP increases, the
ability of the article to withstand thermal gradients also
increases. MOR, E.sub.mod, and CTE.sub.H are all measured on a
cellular specimen parallel to the length of the channels which is
referred to herein as the axial direction. MOR was measured using
the four point bend method in the axial direction of a rectangular
cellular bar having dimensions of 4.times.1.times.0.5 inches.
[0059] Referring to FIG. 8, the porous ceramic honeycomb article
has a relatively low amount of microcracking prior to exposure to
the microcracking condition. Specifically, the SEM micrograph of
FIG. 8 depicts a portion of a low-microcracked porous ceramic
honeycomb article with very few microcracks 220 (one indicated in
FIG. 8). Accordingly, prior to exposure to the microcracking
condition, the porous ceramic honeycomb article may be
alternatively referred to as a low-microcracked (LMC) porous
ceramic honeycomb article. In the embodiments described herein, the
LMC porous ceramic honeycomb articles have a microcrack parameter
Nb.sup.3 from about 0.04 to about 0.25 after firing and prior to
exposure to a microcracking condition. Low microcrack parameters in
the range of 0.04 to about 0.25 generally correspond to a porous
ceramic honeycomb article with very few microcracks 220, as
depicted in FIG. 8. In some embodiments, the LMC porous ceramic
honeycomb articles have a CTE measured between room temperature and
800.degree. C. from about 7.0.times.10.sup.-7/.degree. C. to about
15.times.10.sup.-7/.degree. C. or even from about
8.0.times.10.sup.-7/.degree. C. to about
13.times.10.sup.-7/.degree. C. In other embodiments, the LMC porous
ceramic honeycomb articles have a CTE from about
9.0.times.10.sup.-7/.degree. C. to about
12.times.10.sup.-7/.degree. C. Due to the relatively low microcrack
parameter Nb.sup.3, LMC porous ceramic honeycomb articles have a
Thermal Shock Limits (TSL) which, in the embodiments of the LMC
porous ceramic honeycomb articles described herein, is in the range
from about 800.degree. C. to about 1100.degree. C. In the
embodiments described herein, the LMC porous ceramic honeycomb
articles have a modulus of rupture (MOR) of greater than 300 psi or
even greater than 400 psi at room temperature. For example, the MOR
of a porous ceramic honeycomb article with a 300/8 cell geometry
and a median pore size of 11.7 microns, a porosity of 62% prior to
washcoating, and a d-factor df of 0.25 is approximately 450 psi at
room temperature. In some embodiments, the MOR of the LMC porous
ceramic honeycomb articles is greater than about 500 psi.
[0060] LMC porous ceramic honeycomb articles made having a 200/10
geometry generally have a modulus of elasticity (E.sub.mod) at room
temperature of greater than or equal to 3.0.times.10.sup.5 psi or
even greater than 4.5.times.10.sup.5 psi. In some embodiments, the
modulus of elasticity of the LMC porous ceramic honeycomb articles
is in the range from about 3.0.times.10.sup.5 psi to about
5.5.times.10.sup.5 psi. Based on the MOR and E.sub.mod, embodiments
of the LMC porous ceramic honeycomb articles have a strain
tolerance (i.e., MOR/E.sub.mod) of at least 700 ppm. Other
embodiments have a strain tolerance of greater than or equal to 800
ppm, or even greater than 1000 ppm. In yet other embodiments the
LMC porous ceramic honeycomb articles have strain tolerance greater
than or equal to 1200 ppm.
[0061] Referring to FIG. 9, the microcrack parameter of the
substrate may be increased by exposing the LMC porous ceramic
honeycomb to a microcracking condition. Specifically, FIG. 9 is an
SEM micrograph depicting a portion of a microcracked (MC) porous
ceramic honeycomb article which is produced by exposing an LMC
porous ceramic honeycomb article to a microcracking condition.
Following exposure to a microcracking condition, the
now-microcracked porous ceramic honeycomb article has a relatively
greater number of microcracks 220 (a plurality of which are
indicated in FIG. 9) than the LMC porous ceramic honeycomb article
(i.e., the number of microcracks 220 in FIG. 9 is greater than the
number of microcracks 220 in FIG. 8). In the embodiments described
herein, the microcracking condition may include a thermal cycle or
an acid wash, as will be described in more detail herein. As a
result of being exposed to the microcracking condition, the
microcrack parameter Nb.sup.3 of the MC porous ceramic honeycomb
article is at least 20% higher than the microcrack parameter of the
LMC porous ceramic honeycomb article thus indicating that the MC
porous ceramic honeycomb articles have more microcracks per unit
volume than the LMC porous ceramic honeycomb articles. For example,
the microcrack parameter Nb.sup.3 of the MC porous ceramic
honeycomb articles may be in the range from at least 0.06 to at
least 0.3. The increase in the microcracking parameter Nb.sup.3 is
accompanied by a decrease in the CTE of the article relative to the
LMC porous ceramic honeycomb articles. For example, the CTE of the
MC porous ceramic honeycomb articles is generally in the range from
about 1.0.times.10.sup.-7/.degree. C. to about
10.times.10.sup.-7/.degree. C. over the range of from about
25.degree. C. to about 800.degree. C. In some embodiments, the CTE
of the MC porous ceramic honeycomb articles is less than or equal
to about 7.0.times.10.sup.-7/.degree. C. over the range of from
about 25.degree. C. to about 800.degree. C. or even less than or
equal to about 5.0.times.10.sup.-7/.degree. C. over the range of
from about 25.degree. C. to about 800.degree. C. The increase in
Nb.sup.3 is accompanied by an increase in the TSL of the porous
ceramic honeycomb articles. For example, the TSL of the MC porous
ceramic honeycomb articles is greater than or equal to 900.degree.
C. or even greater than or equal to 1000.degree. C. In some
embodiments, the TSL of the MC porous ceramic honeycomb articles is
greater than or equal 1100.degree. C.
[0062] While exposure to the microcracking condition generally
increases the microcrack parameter Nb.sup.3 and the TSL of the MC
porous ceramic honeycomb articles, the increase in the number of
microcracks per unit volume generally decreases the modulus of
rupture (MOR) at room temperature as well as the modulus of
elasticity at room temperature (E.sub.mod) compared to the LMC
porous ceramic honeycomb articles. Accordingly, in the embodiments
described herein, the MOR of the MC porous ceramic honeycomb
article is greater than or equal to about 200 psi or even greater
than about 300 psi. The E.sub.mod of the MC porous ceramic
honeycomb article is generally in the range from about
2.8.times.10.sup.5 psi to about 4.4.times.10.sup.5 psi for a MC
porous ceramic honeycomb article having a 200/10 geometry. In some
embodiments, the E.sub.mod of the MC porous ceramic honeycomb
article may be greater than or equal to 2.8.times.10.sup.5 psi for
a 200/10 geometry.
[0063] The porous ceramic honeycomb articles described herein are
formed by first mixing a cordierite precursor batch composition,
forming the cordierite precursor batch composition into a green
honeycomb article, drying the green honeycomb article and firing
the green honeycomb article under conditions suitable to initially
produce a low microcracked (LMC) porous ceramic honeycomb article.
In one embodiment, after the green honeycomb article is fired to
produce the LMC porous ceramic honeycomb article, the LMC porous
ceramic honeycomb article may be washcoated with a
catalyst-containing washcoat prior to being exposed to the
microcracking condition. Because the LMC porous ceramic honeycomb
article has relatively few microcracks, a separate passivation
coating is not needed prior to application of the washcoat.
[0064] In one embodiment, the cordierite precursor batch
composition comprises a combination of constituent materials
suitable for producing a ceramic article which predominately
comprises a cordierite crystalline phase. In general, the batch
composition comprises a combination of inorganic components
including a relatively fine talc, a relatively fine silica-forming
source, and an alumina-forming source. In still other embodiments
the batch composition may comprise clay, such as, for example,
kaolin clay. The cordierite precursor batch composition may also
contain organic components such as organic pore formers. For
example, the batch composition may comprise a starch which is
suitable for use as a pore former and/or other processing aids. In
the embodiments described herein, the organic pore former comprises
a single material as opposed to a mixture of different organic
materials thereby reducing the number of constituent materials in
the cordierite precursor batch composition.
[0065] In the embodiments described herein, the inorganic batch
components and the organic batch components are selected in
conjunction with a specific firing cycle so as to yield a porous
ceramic honeycomb article comprising a predominant cordierite
crystalline phase with a specific microstructure. However, it
should be understood that, after firing, the porous ceramic
honeycomb article may also include small amounts of mullite,
spinel, and/or mixtures thereof. For example, and without
limitation, in some embodiments, the porous ceramic honeycomb
article may comprise at least 90% by weight, or even at least 95%
by weight, or even at least 98%-99% by weight of a cordierite
crystalline phase, as measured by x-ray diffraction. The cordierite
crystalline phase produced consists essentially of, as
characterized in an oxide weight percent basis, from about 49% to
about 53% by weight SiO.sub.2, from about 33% to about 38% by
weight Al.sub.2O.sub.3, and from about 12% to about 16% by weight
MgO. Moreover, the cordierite crystalline phase stoichiometry
approximates Mg.sub.2Al.sub.4Si.sub.5O.sub.18. The inorganic
cordierite precursor batch composition may be appropriately
adjusted to achieve the aforementioned oxide weights within the
cordierite crystalline phase of the porous ceramic honeycomb
article.
[0066] In some embodiments described herein, the cordierite
precursor batch compositions comprise from about 35% to about 45%
by weight of talc. In other embodiments, the cordierite precursor
batch composition may comprise from about 38% to about 43% by
weight of talc. The talc may have a relatively fine particle size.
For example, in some embodiments, the talc has a median particle
diameter d.sub.pt50 of less than or equal to 10 microns, or even a
d.sub.pt50 of less than or equal to 9 microns. In other
embodiments, the talc has a median particle diameter d.sub.pt50
less than 8 microns or even a d.sub.pt50 less than 6 microns. In
still other embodiments the talc may have a median particle size
d.sub.pt50 of less than 5 microns. In one exemplary embodiment, the
talc has a median particle size d.sub.pt50 in the range from about
3 microns to about 10 microns. In another exemplary embodiment, the
talc has a median particle size d.sub.pt50 in the range from about
8 microns to about 10 microns. All particle sizes described herein
are measured by a particle size distribution (PSD) technique,
preferably by a Sedigraph by Micrometrics.
[0067] In some embodiments, the amount of the silica-forming source
in the cordierite precursor batch composition is from about 13% to
about 24% by weight. In other embodiments, the amount of the
silica-forming source in the cordierite precursor batch composition
may be from about 15% to about 18% by weight. The silica-forming
source generally has a fine particle size. For example, in some
embodiments, the silica-forming source has a median particle
diameter d.sub.ps50 of less than or equal to 20 microns, or even a
d.sub.ps50 of less than or equal to 15 microns. In other
embodiments, the silica-forming source has a median particle
diameter d.sub.ps50 less than 10 microns. In one embodiment, the
silica-forming source is a microcrystalline silica such as
Imsil.RTM. A-25. However, it should be understood that other
silica-forming sources may be used. For example, other suitable
silica-forming sources include fused silica; colloidal silica; or
crystalline silica such as quartz or crystobalite.
[0068] In some embodiments, the amount of the alumina-forming
source in the cordierite precursor batch composition is from about
20% to about 35% by weight while in other embodiments the amount of
the alumina-forming source in the cordierite precursor batch
composition is from about 22% to about 33% by weight. In still
other embodiments the amount of the alumina forming source in the
cordierite precursor batch composition is from about 26% to about
29% by weight. The alumina-forming source generally has a fine
particle size. For example, in some embodiments, the
alumina-forming source has a median particle diameter d.sub.pa50 of
less than or equal to 10 microns, or even a d.sub.pa50 of less than
or equal to 8 microns. In other embodiments, the silica-forming
source has a median particle diameter d.sub.pa50 less than 6
microns.
[0069] Exemplary alumina-forming sources may include any aluminum
oxide or a compound containing aluminum which, when heated to a
sufficiently high temperature, yields essentially 100% aluminum
oxide, such as alpha-alumina and/or hydrated alumina. Further
non-limiting examples of alumina-forming sources include corundum,
gamma-alumina, or transitional aluminas. The aluminum hydroxide may
comprise gibbsite and bayerite, boehmite, diaspore, aluminum
isopropoxide, and the like. If desired, the alumina-forming source
may also comprise a dispersible alumina-forming source. As used
herein, a dispersible alumina-forming source is one that is at
least substantially dispersible in a solvent or liquid medium and
that can be used to provide a colloidal suspension in a solvent or
liquid medium. In one aspect, a dispersible alumina-forming source
can be a relatively high surface area alumina source having a
specific surface area of at least 20 m.sup.2/g, at least 50
m.sup.2/g, or even at least 100 m.sup.2/g. A suitable dispersible
alumina source comprises alpha aluminum oxide hydroxide
(AlOOH.x.H.sub.2O) commonly referred to as boehmite,
pseudoboehmite, and as aluminum monohydrate. In alternative
embodiments, the dispersible alumina source can comprise the
so-called transition or activated aluminas (i.e., aluminum
oxyhydroxide and chi, eta, rho, iota, kappa, gamma, delta, and
theta alumina) which can contain various amounts of chemically
bound water or hydroxyl functionalities.
[0070] In some embodiments, the cordierite precursor batch
composition may further comprise clay. The amount of clay in the
cordierite precursor batch composition may be from about 0% to
about 20% by weight. In another embodiment, the amount of clay in
the cordierite precursor batch composition is from about 10% to
about 18% by weight or even from about 12% to about 16% by weight.
When included in the cordierite batch composition, the clay
generally has a median particle size d.sub.pc50 of less than or
equal to 10 microns. In some embodiments, the median particle size
d.sub.pc50 is less than or equal to 5 microns or even less than or
equal to 3 microns. Suitable clays which may be included in the
cordierite precursor batch composition include, without limitation,
raw kaolin clay, calcined kaolin clay, and/or mixtures thereof.
Exemplary and non-limiting clays include non-delaminated kaolinite
raw clay and delaminated kaolinite.
[0071] In the embodiments described herein, the inorganic
components of the cordierite batch composition (i.e., talc, silica,
alumina and clay) have a median inorganic particle size d.sub.50IP
less than or equal to 15 microns.
[0072] As described herein above, the cordierite precursor batch
composition further comprises organic components such as relatively
fine pore formers. In the embodiments described herein, an organic
pore former is added to the batch composition in an amount
sufficient to create a relatively high pore number density with a
relatively small median pore size and a relatively narrow pore size
distribution. In the embodiments described herein, the cordierite
precursor batch composition may comprise greater than or equal to
about 30% by weight of an organic pore former. In some embodiments,
the amount of pore former added to the batch composition is greater
than about 30% by weight. In other embodiments, the amount of pore
former added to the batch composition is greater than about 35% by
weight. In other embodiments, the amount of pore former added to
the batch composition is greater than about 40% by weight. In other
embodiments, the amount of pore former added to the batch
composition is greater than or equal to about 50% by weight or even
greater than or equal to about 55% by weight. In still other
embodiments the amount of pore former added to the batch
composition is greater than or equal to about 60% by weight. It
should be understood that, increasing the amount of pore former in
the batch composition increases the pore number density of the
porous ceramic honeycomb article after firing. In the embodiments
described herein the organic pore former generally has a median
particle size d.sub.pp50 less than or equal to 25 microns. In some
embodiments, the organic pore former has a median particle size
d.sub.pp50 less than or equal to 20 microns or even less than or
equal to 15 microns. In other embodiments, the median particle size
d.sub.pp50 is less than or equal to 10 microns. The organic pore
former may be a cross-linked pore former (i.e., cross-linked
starches and the like) or un-cross-linked pore former. Examples of
suitable pore forming materials include, without limitation,
cross-linked corn starch, cross-linked wheat starch, cross-linked
potato starch, un-cross-linked potato starch, un-cross-linked corn
starch, green bean starch, and pea starch.
[0073] The inorganic and organic components described above are
combined and mixed together with processing aids such as, for
example, a binder, and a liquid vehicle, to create a plasticized
batch mixture. These processing aids may improve processing and/or
reduce drying and/or firing cracking and/or aid in producing
desirable properties in the honeycomb article. For example, the
binder can include an organic binder. Suitable organic binders
include water soluble cellulose ether binders such as
methylcellulose, hydroxypropyl methylcellulose, methylcellulose
derivatives, hydroxyethyl acrylate, polyvinylalcohol, and/or any
combinations thereof. Preferably, the organic binder is present in
the composition as a super addition in an amount in the range of
from 0.1% to about 10.0% by weight of the inorganic powder batch
composition. In another embodiment, the organic binder can be
present in the composition as a super addition in an amount in the
range of from 2.0% to 8.0% by weight of the inorganic powder batch
composition. Incorporation of the organic binder into the
plasticized batch composition allows the plasticized batch
composition to be readily extruded.
[0074] One liquid vehicle for providing a flowable or paste-like
consistency to the batch composition is water, although it should
be understood that other liquid vehicles exhibiting solvent action
with respect to suitable temporary organic binders can be used. The
amount of the liquid vehicle component can vary in order to impart
optimum handling properties and compatibility with the other
components in the batch composition. In some embodiments, the
liquid vehicle content is present as a super addition in an amount
in the range from 20% to 50% by weight, and in other embodiments in
the range from 20% to 35% by weight. Minimization of liquid
components in the batch composition can lead to further reductions
in undesired drying shrinkage and crack formation during the drying
process.
[0075] In addition to the liquid vehicle and binder, the
plasticized batch composition may include one or more optional
forming or processing aids such as, for example, a lubricant.
Exemplary lubricants can include tall oil, sodium stearate or other
suitable lubricants. The amount of lubricant present in the
plasticized batch mixture may be from about 0.5% by weight to about
10% be weight.
[0076] It should be understood that the liquid vehicle, pore
formers, binders, lubricants and any other processing aids included
in the batch composition are added to the batch composition as
super additions based upon the weight % of 100% of the inorganic
materials.
[0077] The combination of inorganic batch components, pore formers,
binders, the liquid vehicle, lubricants and any other additives are
mixed together in a Littleford mixer and kneaded for approximately
5-20 minutes to produce a plasticized batch composition having the
desired plastic formability and green strength to permit the
plasticized batch composition to be shaped into a honeycomb
article.
[0078] The resulting plasticized cordierite precursor batch
composition is then shaped into a green body (i.e., a green
honeycomb article) by conventional ceramic forming processes, such
as, for example, extrusion. When the green honeycomb article is
formed by extrusion, the extrusion can be performed using a
hydraulic ram extrusion press, or alternatively, a two stage
de-airing single auger extruder, or a twin screw mixer with a die
assembly attached to the discharge end.
[0079] After the plasticized cordierite precursor batch composition
has been formed into a green honeycomb article, the green honeycomb
article is dried to remove excess liquid from the green honeycomb
article. Suitable drying techniques include microwave drying, hot
air drying, RF drying or various combinations thereof. After
drying, the green honeycomb article is placed in a kiln or furnace
and fired under conditions effective to convert the green honeycomb
article into a ceramic honeycomb article comprising a primary
cordierite crystalline phase, as described herein.
[0080] It should be understood that the firing conditions utilized
to convert the green honeycomb body into a ceramic honeycomb
article can vary depending on the process conditions such as, for
example, the specific composition, size of the green honeycomb
body, and nature of the equipment used. To that end, in one aspect,
the optimal firing conditions specified herein may need to be
adapted (i.e., slowed down) for very large cordierite structures,
for example.
[0081] The firing schedules utilized to produce porous ceramic
honeycomb articles having the properties described herein may ramp
quickly from 1200.degree. C. to a maximum hold temperature at or
above 1420.degree. C., or even at or above 1425.degree. C. The
quick ramp rate may be 50.degree. C./hr or higher. In one
embodiment, the ramp rate is 75.degree. C./hr or higher. In some
embodiments, the green honeycomb bodies may be held at the maximum
temperature (i.e., the soak temperature) for 5 to 20 hours. In
other embodiments the green honeycomb bodies may be held at the
soak temperature from about 10 hours to about 15 hours. In yet
other embodiments, the green honeycomb bodies can be fired at a
soak temperature in the range of from about 1420.degree. C. to
about 1435.degree. C. In still other embodiments, the green body
may be fired at a soak temperature in the range of from about
1425.degree. C. to about 1435.degree. C. In one embodiment, the
firing cycle includes a quick ramp rate of 50.degree. C./hr or
higher from about 1200.degree. C. and a soak temperature in the
range from about 1420.degree. C. to about 1435.degree. C. for a
sufficient time to form the cordierite crystalline phase in the
fired body.
[0082] The total firing time may range from approximately 40 to 250
hours, largely depending on the size of the honeycomb article
fired, during which time a maximum soak temperature is reached and
held for a sufficient time as described above. In one embodiment,
the firing schedule includes ramping from 1200.degree. C. at a rate
above 50.degree. C./hour and firing at a soak temperature of
between about 1425.degree. C. and 1435.degree. C. for between about
10 hours to about 15 hours.
[0083] Referring now to FIG. 10, one embodiment of a firing
schedule utilized to produce porous ceramic honeycomb articles
having the properties described herein is graphically illustrated.
In this embodiment, an average firing rate may be employed in the
first firing portion 120 of the firing schedule. The average firing
rate is between about 20.degree. C./hour and about 70.degree.
C./hour between room temp and about 1200.degree. C. The first
portion 120 of the firing schedule may include a pore former
burnout stage 125 which may be a hold or slight ramp within the
range of pore former burnout temperatures to minimize cracking and
temperature differentials between the skin and the core of the
honeycomb. In one embodiment, the burnout stage 125 may be followed
by an intermediate ramp 135 to about 1200.degree. C. The upper
portion 130 of the firing schedule includes a relatively faster
ramp rate at temperatures above 1200.degree. C. This fast ramp in
the upper portion 130 may be coupled with a hold portion 140 at a
temperature above 1420.degree. C., or even at or above 1425.degree.
C., and preferably between 1420.degree. C. and 1435.degree. C. The
cordierite crystalline phase of the porous honeycomb ceramic
article is formed during this hold portion 140. The ramp rate in
the upper portion 130 of the firing cycle may be 50.degree. C./hour
or more, 75.degree. C./hour or more, 100.degree. C./hour or more,
or even 120.degree. C./hour or more. By utilizing the faster ramp
rate in the upper portion 130 above about 1200.degree. C. and the
relatively high hold temperature (above 1420.degree. C.), unique
microstructure characteristics of the fired ceramic body may be
achieved, as will be described in more detail herein.
[0084] In particular, the firing cycle described herein aids in
reducing the relative amount of fine porosity present in the fired
ceramic honeycomb article to below about 4.0 microns. The reduction
mechanism is thought to be from the promotion of viscous flow of
the cordierite forming components such that fine pores are filled
by the viscous flow of the components during the initial formation
of the cordierite phase. Following the fast ramp, the honeycomb is
held at the soak temperature for a suitable time, such as 5 to 20
hours, to form the cordierite phase. After this, the honeycomb
article is cooled to room temperature in portion 150 of the firing
schedule. The cooling rate is slow enough to prevent cracking and
is dependent on the size of the part fired.
[0085] In some embodiments described herein, the LMC porous ceramic
honeycomb articles are washcoated with a catalyst washcoat after
firing. For example, a slurry of a particulate catalyst washcoating
composition can be applied to the surfaces (both internal and
external) of the LMC porous ceramic honeycomb article. For example,
in the embodiments described herein, the catalyst washcoat has a
catalytic function that promotes catalytic reactions involving the
reduction of NOx and/or the oxidation of CO, hydrocarbons and NO in
an exhaust gas stream which is directed through the porous ceramic
honeycomb article. Thus, it should be understood that, in addition
to acting as a particulate filter, the porous ceramic honeycomb
articles described herein may also exhibit catalyst functionalities
and, as such, may be utilized as a 4-way filter deNOx integrated
filter (NIF).
[0086] In some embodiments, the primary particulate component of
the washcoating slurry is alumina. In other embodiments, the
primary particulate component is a zeolite, such as Fe-ZSM-5 which
may be incorporated in water in an amount from about 7 wt. % to
about 12 wt. % to form a catalyst washcoat slurry. However, it
should be understood that, in other embodiments, the catalyst
washcoat may comprise a different primary particulate component. In
some embodiments, the catalyst washcoat may additionally comprise a
particulate catalyst such as, by way of example and not limitation,
platinum, palladium, rhodium, or any other catalytic material
and/or various alloys thereof.
[0087] Because the LMC porous ceramic honeycomb article contains
relatively few microcracks per unit volume (i.e., because the
microcrack parameter Nb.sup.3 is from about 0.04 to about 0.25), it
is not necessary to apply a preliminary passivation coating to the
porous ceramic honeycomb article to prevent the washcoating
material from becoming lodged in the microcracks, as is the case
for more highly microcracked articles.
[0088] Following application of the washcoat to the LMC porous
ceramic honeycomb article, the article is exposed to a
microcracking condition which increases the number of microcracks
per unit volume in the porous ceramic article as described above.
In one embodiment, the microcracking condition is a thermal cycle.
In this embodiment, the LMC porous ceramic article is heated to a
peak temperature and then rapidly cooled. The heating and rapid
cooling causes the LMC porous ceramic article to expand and
contract thereby causing microcracks to nucleate and grow in the
porous ceramic article. In some embodiments, the peak temperature
of the thermal cycle is greater than or equal to about 400.degree.
C. or even greater than or equal to about 600.degree. C. In
general, the peak temperature of the thermal cycle is in the range
from about 400.degree. C. to about 800.degree. C. After heating to
the peak temperature, the porous ceramic honeycomb article is
rapidly cooled at a rate of at least 200.degree. C./hr during which
time microcracks are formed throughout the volume of the porous
ceramic honeycomb article. By exposing the LMC porous ceramic
honeycomb article to the thermal cycle, the LMC porous ceramic
honeycomb article becomes a microcracked (MC) porous ceramic
honeycomb article.
[0089] In another embodiment, the microcracking condition is an
acid wash. In this embodiment, the LMC porous ceramic honeycomb
article is immersed in an acid solution which precipitates the
nucleation and growth of microcracks throughout the honeycomb
article. For example, in some embodiments the LMC porous ceramic
honeycomb article may be immersed in a solution having a pH of less
than 6 or even less than 5 to cause further microcracking in the
honeycomb article. By exposing the LMC porous ceramic honeycomb
article to the acidic solution, the LMC porous ceramic honeycomb
article becomes a microcracked (MC) porous ceramic honeycomb
article.
EXAMPLES
[0090] The following examples are offered to illustrate specific
embodiments of the porous ceramic honeycomb articles described
above. It should be understood that the following examples are for
purposes of description only and are not intended to limit the
scope of the claimed subject matter.
[0091] Table 2 lists the compositions of Comparative Example A.
Table 3 lists the compositions of Inventive Examples 1-3. As shown
in Table 2, Comparative Example A contains a multiple pore formers
(graphite and potato starch), both of which have a median particle
size of greater than 30 microns. Further, the median particle size
of the inorganic components (d.sub.IP50) is greater than 15
microns.
[0092] Inventive Examples 2-3 were formed with a single organic
pore former (corn starch) having a media particle size of 15
microns. The composition of the inorganic components of Inventive
Examples 1 and 2 were identical. However, Inventive Example 1
contained 30 wt. % pore former while Inventive Example 2 contained
50 wt. % of the same pore former. The alumina source used in
Inventive Example 3 had a greater median particle size than the
alumina forming sources used in Inventive Examples 1 and 2. The
median particle size of the inorganic components (d.sub.IP50) was
less than 15 microns. Specifically, Inventive Examples 1-3 had
median particle sizes of 6.3 microns, 6.3 microns and 7.1 microns,
respectively.
TABLE-US-00002 TABLE 2 Composition of Comparative Examples A and B.
Comp. Ex. A Comp. Ex. B Material Identifier (d50, wt %) Material
Identifier (d50, wt %) Inorganic Talc FCOR -325 mesh (21.5 .mu.m,
FCOR -325 mesh (21.5 .mu.m, 19.26%) 19.26%) Talc FCOR (25.4 .mu.m,
19.26%) FCOR (25.4 .mu.m, 19.26%) Silica Cerasil 300 (26.9 .mu.m,
15.38%) Cerasil 300 (26.9 .mu.m, 15.38%) Alumina A10 -325 mesh
(10.7 .mu.m, 12.27%) A10 -325 mesh (10.7 .mu.m, 12.27%)
Alumina-Hydrate Micral 6000 (5.2 .mu.m, 20.99%) Micral 632 (3.5
.mu.m, 20.99%) Hydrous Clay CHC-94 (7.3 .mu.m, 12.84%) CHC-94 (7.3
.mu.m, 12.84%) Additives Yttrium Oxide-Grade C (0.40%) Yttrium
Oxide-Grade C (0.40%) Pore former Graphite-4602 Graphite-460 (33
.mu.m, 22.00%) Graphite-460 (33 .mu.m, 22.00%) Xlinked starch
Potato starch (45 .mu.m, 22.00%) Potato starch (45 .mu.m, 22.00%)
Binder and Lubricant Binder-Methylcellulose F240 (7.00%) F240
(7.00%) lubricant Liga (1.00%) Liga (1.00%)
TABLE-US-00003 TABLE 3 Compositions of Inventive Examples 1-3. Inv.
Ex. 1 Inv. Ex. 2 Inv. Ex. 3 Material Identifier (d50, wt %)
Material Identifier (d50, wt %) Material Identifier (d50, wt %)
Inorganic Talc Barretts 93-37 (9.8 .mu.m, 41.54%) Barretts 93-37
(9.8 .mu.m, 41.54%) Barretts 93-37 (9.8 .mu.m, 41.54%) Talc Silica
Imsil A25 (5.4 .mu.m, 16.59%) Imsil A25 (5.4 .mu.m, 16.59%) Imsil
A25 (5.4 .mu.m, 16.59%) Alumina A3000 (3.2 .mu.m, 27.93%) A3000
(3.2 .mu.m, 27.93%) HVA-FG (5.9 .mu.m, 27.93%) Alumina-Hydrate
Hydrous Clay FHC-03 (3.4 .mu.m, 13.85%) FHC-03 (3.4 .mu.m, 13.85%)
FHC-03 (3.4 .mu.m, 13.85%) Additives Pore former Graphite-4602
Xlinked starch Corn starch (15 .mu.m, 30%) Corn starch (15 .mu.m,
50%) Corn starch (15 .mu.m, 30.00%) Binder and Lubricant
Binder-Methylcellulose F240 (7.00%) F240 (7.00%) F240 (7.00%)
lubricant Liga (1.00%) Liga (1.00%) Liga (1.00%)
[0093] Table 4 lists the pore structure for Comparative Example A
and Inventive Examples 1-3. As shown in Table 4, Comparative
Example A had a higher porosity than Inventive Examples 1-3. In
addition, Comparative Example A also exhibited a greater median
pore size d.sub.50 as well as a greater d-factor d.sub.f and
absolute breadth d.sub.AbsB. The cell geometries for the Inventive
Examples and the Comparative Examples are also listed in Table 4.
The specific pore volume was calculated from the open frontal area
and the porosity of each sample for the cell geometries listed. The
open frontal areas for the various geometries are also listed. The
bare surface porosities for the Inventive Examples prepared with
the listed compositions were expected to be .gtoreq.35%. The
Inventive Examples prepared according to the listed compositions
were also expected to have bare initial filtration efficiencies of
50% or greater.
TABLE-US-00004 TABLE 4 Pore Structure of Comparative Example A and
Inventive Examples 1-3. Comp. Ex. A Comp. Ex. B Inv. Ex. 1 Inv. Ex.
2 Inv. Ex. 3 Porosity (%) 63-66 65 57 62 60 d10 - .mu.m 13.1 9.42
10.0 8.8 14.3 d25 - .mu.m 18.48 13.53 11.2 10.1 16.6 d50 - .mu.m
23.02 18 12.8 11.7 19.3 d75 - .mu.m 29.33 22.55 15.4 13.6 24.3 d90
- .mu.m 43.71 32.21 19.3 19.4 31.7 d.sub.f = (d50 - d10)/d50 0.39
0.48 0.22 0.25 0.26 d.sub.AbsB = (d75 - d25) 10.85 9.01 4.1 3.5 7.6
Specific Pore 0.276/(300/14) 0.307/(300/13) 0.139/(300/8)
0.172/(300/9) 0.144/(300/9) Volume/(Geometry) 0.268/(200/15)
0.205/(200/12) Open Frontal Area 0.58/(300/14) 0.53/(300/15)
0.69/(300/8) 0.72/(300/9) 0.68/(300/9) (Geometry) 0.60/(200/15)
0.67/(200/12)
[0094] The single-coat catalyst washcoat loading of the Inventive
Examples and the Comparative Example were compared. Samples of the
Inventive Examples and the Comparative Example were prepared
utilizing the compositions set forth in Tables 2 and 3. The samples
were cylindrical with a 2 inch diameter and an axial length of 6
inches. Inventive Examples 1 and 2 were constructed with 300/8 and
300/9 cell geometries, respectively while the Comparative Example A
was constructed with a 300/14 cell geometry. Each sample was coated
with a single coating of a catalyst slurry which consisted of 12
wt. % of Fe-ZSM-5 zeolite in water using a "water fall" process.
The results of the study are presented in Table 5.
TABLE-US-00005 TABLE 5 Single-coat catalyst washcoat loading
Single-coat catalyst Sample washcoat loading (g/L) Inv. Ex. 2 63
g/L Inv. Ex. 3 50 g/L Comp. Ex. A 43 g/L
[0095] As shown in Table 5, Inventive Example 2 exhibited a
single-coat catalyst washcoat loading of 63 g/L while Inventive
Example 3 exhibited a single-coat catalyst washcoat loading of 50
g/L. However, despite having a greater porosity, median pore size
and higher specific pore volume VP, Comparative Example A only
exhibited a single-coat catalyst waschcoated loading of 43 g/L.
These results unexpectedly demonstrate that, while the Inventive
Examples have a thin wall thickness relative to the Comparative
Examples (8 and 9 mils compared to 14 mils) and a corresponding
lower specific pore volume (approximately 58% of the coating volume
of the Comparative Example), the Inventive Examples are capable of
coating more catalyst waschcoat per unit volume with a single
coating step than the comparative examples.
[0096] Referring, now to FIG. 11, a plot of the pressure drop
(y-axis) as a function of soot loading is graphically depicted for
Inventive Example 2 and Comparative Examples A and B. The porous
ceramic article of Inventive Example 2 had a 300/9 cell geometry
(i.e., 300 cpsi and a wall thickness of 9 mils), a median pore size
of 11.7 microns, a porosity of 62% prior to washcoating, and a
d-factor d.sub.f of 0.25. The porous ceramic article of Inventive
Example 2 was coated with approximately 101 g/L of a catalyst
washcoat containing 12 wt. % Fe-ZSM-5 in water. Comparative Example
A had a 300/14 cell geometry, a median pore size of 23 microns, a
porosity of 65% and a d-factor d.sub.f of 0.39. The porous ceramic
article of Comparative Example A was coated with approximately 106
g/L of a catalyst washcoat containing 12 wt. % Fe-ZSM-5 in water.
Comparative Example B had a 300/15 cell geometry, a median pore
size of 18 microns, a porosity of 65% and a d-factor d.sub.f of
0.48. The porous ceramic article of Comparative Example B was
coated with approximately 89 g/L of a catalyst washcoat containing
12 wt. % Fe-ZSM-5 in water. Each sample was 6'' in length and 2''
in diameter. The curves were obtained at a flow rate of 26.25 cubic
feet per minute. Table 6 contains the pressure values in kPa for
the clean back pressure (i.e., the back pressure with no soot
loading), and the 5 g/L soot loaded back pressure (i.e., the back
pressure when the porous ceramic article is loaded with 5 g of soot
per liter of the porous ceramic honeycomb). Printex U was used as
soot and was loaded into each sample at a constant flow rate.
TABLE-US-00006 TABLE 6 Back Pressure - clean and soot loaded Clean
back pressure 5 g/L soot loaded Sample (kPa) back pressure (kPa)
Inv. Ex. 2 2.3 5.5 Comp. Ex. A 2.4 8.0 Comp. Ex. B 2.8 8.9
[0097] As shown in Table 6, Inventive Example 2 exhibited a lower 5
g/L soot loaded back pressure drop than Comparative Examples A and
B at a higher catalyst washcoat loading despite having a lower
porosity, lower specific pore volume VP and smaller median pore
size.
[0098] Referring to FIG. 12, a plot of the 5 g/L soot loaded
pressure drop as a function of the amount of catalyst washcoat
loading is graphically depicted. Results are shown for samples
produced from the composition of Inventive Example 2 with 200/12
and 300/8 geometries. Results are also shown for sample produced
from the composition of Comparative Example A with a 200/15
geometry and a 300/13 geometry. As shown in FIG. 12, Inventive
Example 2 exhibits a lower pressure drop at a catalyst washcoat
loading of up to approximately 120 g/L. FIG. 12 illustrates a lower
soot loaded pressure drop for the Inventive Example 2 with a 300/8
geometry despite having a smaller pore size and narrower pore size
distribution. In addition, FIG. 12 illustrates that the thin wall
filter of Inventive Example 2 with a 300/8 geometry exhibits an
even lower pressure drop than a filter having the same composition
but with a 200/12 geometry, indicating that it is possible to
construct a thin wall filter with a small pore size distribution
without incurring a loss of filtration efficiency.
[0099] It will be apparent to those skilled in the art that various
modifications and variations can be made to the embodiments
described herein without departing from the spirit and scope of the
claimed subject matter. Thus it is intended that the specification
cover the modifications and variations of the various embodiments
described herein provided such modification and variations come
within the scope of the appended claims and their equivalents.
* * * * *