U.S. patent application number 11/143162 was filed with the patent office on 2006-12-07 for segregated catalyzed metallic wire filter for diesel soot filtration.
Invention is credited to Joseph Charles Dettling, Michael Patrick Galligan.
Application Number | 20060272319 11/143162 |
Document ID | / |
Family ID | 36928993 |
Filed Date | 2006-12-07 |
United States Patent
Application |
20060272319 |
Kind Code |
A1 |
Dettling; Joseph Charles ;
et al. |
December 7, 2006 |
Segregated catalyzed metallic wire filter for diesel soot
filtration
Abstract
A filter for removing soot particles from the exhaust gas of a
diesel engine comprises a plurality of hollow channels which
contain therein a metal mesh. The metal mesh can be coated with an
oxidation catalyst to promote ignition of the soot particles and
regeneration of the metal mesh for filtering the soot particles.
The metal mesh can optionally be removed from the hollow channels
and replaced with regenerated or new metal mesh if desired.
Inventors: |
Dettling; Joseph Charles;
(Howell, NJ) ; Galligan; Michael Patrick;
(Cranford, NJ) |
Correspondence
Address: |
ATTENTION: CHIEF PATENT COUNSEL;ENGELHARD CORPORATION
101 Wood Avenue
P.O. Box 770
Iselin
NJ
08830-0770
US
|
Family ID: |
36928993 |
Appl. No.: |
11/143162 |
Filed: |
June 2, 2005 |
Current U.S.
Class: |
60/295 ;
60/297 |
Current CPC
Class: |
F01N 3/035 20130101;
F01N 2330/12 20130101; F01N 3/0211 20130101; F01N 3/022 20130101;
F01N 2450/30 20130101 |
Class at
Publication: |
060/295 ;
060/297 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Claims
1. A filter for removing soot particles from the exhaust of a
diesel engine comprising: an enclosed area, a plurality of channels
disposed within said enclosed area and disposed longitudinally in
the same direction as gas flow through the filter, each of said
channels having a hollow interior and containing opposed open ends,
disposed within each of said channels and substantially filling
said hollow interior is an optionally removable metal mesh.
2. The filter of claim 1 including a void space between said
plurality of channels, said void space being open to said
exhaust.
3. The filter of claim 2 wherein the volume of said void space is
less than 25% of the volume of said enclosed area.
4. The filter of claim 1 wherein said metal mesh is a woven metal
mesh.
5. The filter of claim 1 wherein said metal mesh is non-woven.
6. The filter of claim 1 wherein said metal mesh contained in said
channels is of a single mass.
7. The filter of claim 1 wherein said channels are hollow
cylinders.
8. The filter of claim 1 wherein said metal mesh is coated with an
oxidation catalyst.
9. The filter of claim 8 wherein said oxidation catalyst is a
platinum group metal.
10. The filter of claim 8 containing a metal anchor coat disposed
between said metal mesh and said catalyst.
11. The filter of claim 10 wherein said anchor coat is applied by
electric arc spraying.
12. The filter of claim 8 wherein at least a portion of the
interior surfaces of said channels are coated with an oxidation
catalyst.
13. The filter of claim 2 wherein said void space is defined by a
plurality of void spaces between each of said channels.
14. The filter of claim 13 wherein said channels are supported by
opposed upstream and downstream end plates, said endplates
containing a plurality of orifices to direct exhaust gas to said
plurality of void spaces.
15. A method of removing soot from the exhaust gas of a diesel
engine comprising directing said exhaust gas through a diesel
filter comprising an enclosed area, a plurality of channels
disposed within said enclosed area and disposed longitudinally in
the same direction as gas flow through the filter, each of said
channels having a hollow interior end containing opposed open ends,
disposed within each of said channels and substantially filling
said hollow interior is an optionally removable metal mesh, whereby
soot particles contained in said exhaust gas are trapped on said
metal mesh.
16. The method of claim 15 including a void space between said
plurality of channels, said void space being open to said exhaust
gas flow.
17. The method of claim 16 wherein the volume of said void space is
less than 25% of the volume of said enclosed area.
18. The method of claim 15 wherein said channels are hollow
cylinders.
19. The method of claim 15 wherein said metal mesh is coated with
an oxidation catalyst, and said trapped soot particles are
continuously or periodically ignited and burned to regenerate the
filter.
20. The method of claim 19 wherein said oxidation catalyst is a
platinum group metal.
Description
FIELD OF THE INVENTION
[0001] This invention relates to diesel engine exhaust gas
treatment and more particularly to the filtering of particulates
from diesel engine exhaust gases using a catalyzed filter.
BACKGROUND OF THE INVENTION
[0002] Diesel engine exhaust is a heterogeneous mixture which
contains not only gaseous emissions such as carbon monoxide ("CO"),
unburned hydrocarbons ("HC") and nitrogen oxides ("NOx"), but also
condensed phase materials (liquids and solids) which constitute the
so-called particulates or particulate matter ("PM"). The total
particulate matter ("TPM") emissions are comprised of three main
components. One component is the solid, dry, solid carbonaceous
fraction or soot. This dry carbonaceous matter contributes to the
visible soot emissions commonly associated with diesel exhaust. A
second component of the TPM is the soluble organic fraction
("SOF"). The soluble organic fraction is sometimes referred to as
the volatile organic fraction ("VOF"), which terminology will be
used herein. The VOF may exist in diesel exhaust either as a vapor
or as an aerosol (fine droplets of liquid condensate) depending on
the temperature of the diesel exhaust, and are generally present as
condensed liquids at the standard particulate collection
temperature of 52.degree. C. in diluted exhaust, as prescribed by a
standard measurement test, such as the U.S. Heavy Duty Transient
Federal Test Procedure. These liquids arise from two sources: (1)
lubricating oil swept from the cylinder walls of the engine each
time the pistons go up and down; and (2) unburned or partially
burned diesel fuel.
[0003] The third component of the particulates is the so-called
sulfate fraction. Diesel fuel contains sulfur, and even the low
sulfur fuel available in the U.S. may contain 0.005% sulfur. Upon
combustion of the fuel in the engine, nearly all of the sulfur is
oxidized to sulfur dioxide which exits with the exhaust in the gas
phase. However, a small portion of the sulfur, perhaps 2-5%, is
oxidized further to SO.sub.3, which in turn combines rapidly with
water in the exhaust to form sulfuric acid which collects as a
condensed phase with the particulates as an aerosol, or is adsorbed
onto the other particulate components, and thereby adds to the mass
of TPM.
[0004] Emissions from diesel engines have been under increasing
scrutiny in recent years and standards, especially for particulate
emissions, have become stricter. In 1994 the particulate emission
standards in the U.S. for new engines allowed no more than a total
of 0.1 grams per brake horse power hour (g/BHP-h). For diesel
engines in buses operating in congested urban areas the particulate
emissions standard was even stricter, 0.07 g/BHP-h TPM. Both of
these standards were seen as significant reductions relative to the
prior particulate emission standard of 0.25 g/BHP-h which had been
in effect since 1991. Starting in 1994, for the first time, engine
technology developments alone were found to be incapable of meeting
the new standards, and for some engines after treatment technology,
for example, diesel oxidation catalyst (DOC) units, as discussed
further below, were necessary.
[0005] The question of how best to reduce the levels of particulate
matter expelled to the atmosphere in the exhaust gases of diesel
engines is currently of considerable interest as stricter emission
standards are constantly being legislated through the next decade.
In this connection, it is desired to develop efficient and
practical devices for removing substantial portions of particulates
from the exhaust gases in diesel engine exhaust systems before
permitting the exhaust gases to escape to the atmosphere.
[0006] It is known in the art to provide diesel engines with an
exhaust filter which traps particulates from the exhaust gas stream
during engine operation. The filters are generally made of porous,
solid materials having a plurality of pores extending therethrough
and having small cross-sectional size, such that the filter is
permeable to the exhaust gases which flow through the filters and
are capable of restraining most or all of the particulates from
passing through the filter with the gas. The restrained
particulates consist generally of carbonaceous particulates in the
form of soot particles and reference herein and in the claims to
"particulate" and "particulates" means such diesel engine-generated
particles. As the mass of collected particulates increases, the
flow rate of the exhaust gas through the filter is usually impeded,
whereby an increased back pressure is encountered within the filter
and reduced engine efficiency results.
[0007] There is a desire in the art to more simply regenerate the
particulate filter by continuous burn-off or incineration of the
soot particles as they are trapped in the filter. However,
experience has shown that in normal diesel engine operation, the
temperature in the exhaust system varies substantially under
different conditions of engine load and speed and that the
temperatures in the filter hardly ever reach the 510.degree. C.
temperature level required to incinerate the trapped
particulate.
[0008] In order to comply with the ever-increasing legislation both
in the United States and Europe to reduce the level of solid
emissions from both on- and off-highway diesel-powered vehicles,
exhaust after-treatment, such as a variety of soot filter media,
have been explored. The wallflow type ceramic honeycomb filter is
the most widely employed filtration technology used in current
systems for industrial applications. Wallflow filters provide an
answer to the filtration requirement, yet there remains the
residual problem of achieving a reliable and repeatable method of
cleaning the filter. This residual problem has been the source of
extensive engineering research and development. Wallflow filter
elements are particularly useful to filter particulate matter from
diesel engine exhaust gases. Many references disclose the use of
wallflow filters which can comprise catalysts on or in the filter
to filter and burn off filtered particulate matter. A common
ceramic wallflow filter construction is a multi-channel honeycomb
structure having the ends of alternate channels on the upstream and
downstream sides of the honeycomb structure plugged. This results
in a checkerboard-type pattern on either end. Channels plugged on
the upstream or inlet end are open on the downstream or outlet end.
This permits the gas to enter the open upstream channels, flow
through the porous walls and exit through the channels having open
downstream ends. The gas pressure forces the gas through the porous
structural walls into the channels closed at the upstream end and
open at the downstream end. Such structures are primarily disclosed
to filter particles out of the exhaust gas stream.
[0009] It is desired to remove the particulate matter from the
upstream sides of the wallflow filters. One method is to provide a
layer of catalyst on the wall to catalyze the ignition of the
particulate matter during operation of the filter. There are many
U.S. patents disclosing such wallflow structures.
[0010] A particularly useful particulate emission control filter
directed for use for diesel exhaust is presented in "3M Diesel
Filters for Particulate Emission Control, Designers Guide"
published by 3M Ceramic Materials Department, printed 1994 January
and hereby incorporated by reference. There is described a ceramic
filter comprising ceramic fiber specified to have 62%
Al.sub.2O.sub.3, 24% SiO.sub.2, and 14% B.sub.2O.sub.3. The filter
specification includes a white continuous fiber having a fiber
diameter of 10-12 microns with a fiber density of 2.7 grams per
cubic centimeter. The mechanical properties of the fiber include a
filament tensile strength of 1.72 GPA, a filament tensile modulus
of elasticity of 138 GPA, and elongation of 1.2%. The specified
thermal properties are continuous use temperature of 1204.degree.
C., short-term use temperature at 1371.degree. C., a lineal
shrinkage at 1093.degree. C. of 1.25%, a melting point of
1800.degree. C., a thermal expansion co-efficient (25-500.degree.
C.) of 3.0.times.10.sup.-6 .DELTA.L/L.degree. C., and a specific
heat of 1046.7 J/Kg.degree. K. The fiber is sold by the 3M Ceramic
Materials Department as NEXTEL.TM. FIBER. The above specified
properties are for NEXTEL.TM. 312 CERAMIC FIBER.
[0011] The NEXTEL.TM. fibers are used to make diesel filters. A
typical 3M diesel filter cartridge has a cylindrical support and a
continuous ceramic fiber woven in a diamond pattern on the support
to form a ceramic fiber winding, see U.S. Pat. No. 5,551,971, FIG.
1. The cylindrical support is an electric resistant heating element
that contains openings. The area of the openings can be used to
control the heat input along the support. Where less heat is
desired, the support can have larger openings or more openings at a
given location. The distribution of openings can be varied with the
most open area toward the center of the support. The cylindrical
support has an open end and a closed end. The filter is useful to
filter particulate matter from diesel engine exhaust. During engine
operation gas laden with particulate matter can pass through the
outer circumferential surface of the ceramic fiber windings through
the open areas of the cylindrical support and out through the open
end. Alternatively and preferably, the filter cartridge can be
operated in reverse. Particle laden gases can be fed into open end,
pass through the open areas of cylindrical support and then through
ceramic fiber windings depositing its particles in the ceramic
fiber windings.
[0012] During heating to regenerate the filter, an oxygen laden
gas, preferably air, is fed into open end. Electric energy is input
to heat the cylindrical support which acts as a heating element or
heater. The cylindrical heating element heats the ceramic fiber
windings to a temperature sufficient to oxidize particulate matter
trapped thereon.
[0013] The filter cartridge is used in a diesel engine exhaust
system. Typically, a plurality of filters are assembled within a
canister. The number of filter cartridges assembled in a canister
is sized to the exhaust flow rates and anticipated regeneration
intervals.
[0014] While a variety of soot filters are known in the art,
improvements are continually desired not only in the regeneration
of such filters, but for the ease of manufacture, retrofitting, and
replacement of such filters. Improvements are further desired in
maintaining gas flow through the filters even if soot accumulation
exceeds the soot-burning rate of the filtering media so as to keep
the vehicle running until cleaning can occur.
SUMMARY OF THE INVENTION
[0015] A diesel soot filter is provided comprising a plurality of
parallel channels that are composed of a metallic mesh to trap soot
particles as the diesel exhaust gas passes through the channels.
The filtering channels can be arranged such as in a canister such
that smaller by-pass gas channels are formed between the filtering
channels. The by-pass channels allow the exhaust gas to pass
therethrough in the event that soot accumulation in the main
filtering channels exceeds the burning rate of the accumulated
soot. The reduced gas flow through the by-pass channels reduces the
back pressure and keeps the vehicle running until a favorable
regeneration of the filter is achieved. The wire mesh can be
removed from the channels and replaced with new metal mesh, and/or
the channels containing the metal mesh may also be removed and
replaced if needed to renew soot removal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a longitudinal cross-sectional view of a canister
holding the soot filter channels of this invention and taken along
line 1-1 of FIG. 2.
[0017] FIG. 2 is a transverse sectional view of the canister and
soot filter device of the present invention taken along line 2-2 of
FIG. 1.
[0018] FIG. 3 is a perspective view of a soot filter channel of
this invention.
[0019] FIG. 4 is a plan view of one of the end plates which can be
used to hold the soot filter channels in place.
DETAILED DESCRIPTION OF THE INVENTION
[0020] In FIG. 1, the filtering element of this invention is shown
installed as a diesel particulate trap 10 which has a canister of
rectangular tubular casing 12, a pyramidal exhaust inlet 14, and a
pyramidal exhaust outlet 16. As installed, a plurality of hollow
channels 18 extend in the axial or longitudinal direction of the
filtering element which is also the primary direction of the flow
of exhaust through the diesel particulate trap. The metal sleeve 20
of the filtering element has been sealed to the casing 12 by an
intumescent mat 24 that expands when exposed to the heat of the
first use of the diesel particulate trap. Any such mat should be
selected to withstand temperatures encountered in use, especially
temperatures at which the filtering element is to be regenerated. A
particularly useful intumescent mat is provided by a
heat-expandable vermiculite mat.
[0021] The channel walls which form each of channels 18 are
impervious metal sheet in circular, square, trapezoid, rectangular,
etc., designs shown in FIGS. 1 and 2 as cylindrical channels 18
having a circular cross-section and open at each end 19 and 21. The
inside diameter of channel 18, regardless of shape, will be at
least 0.5 in, preferably at least about 1.0 inch wide. A plurality
of channels 18 are placed within enclosed area 26 of casing 12. End
plates 23 and 25 on opposite ends of area 26 and welded or
otherwise attached to metal sleeve 20 support the opposing ends of
channels 18. End plates 23 and 25 can include openings through
which the ends of channels 18 are supported. The channels 18 may be
permanently fixed to end plates 23 and 25 or temporarily fit so
that such channels can be replaced due to wear. Thus, the outside
diameters or perimeters of channels 18 can be such as to be
pressure fit within the openings 32 of the end plates 23 and 25,
see FIG. 4, or otherwise removably attached thereto such by bolts,
screws, etc., so as to remain in place during use. Upon wear, the
channels 18 can be removed from the respective openings 32 in the
end plates 23 and 25 and slid from the interior of enclosed area 26
for repair and/or replacement.
[0022] Within the interior of each channel 18 is placed a metal
mesh filtering element 28 which is capable of trapping the soot
particles contained within the diesel exhaust. The metal mesh can
be of various types and configurations so long as the mesh
filtering element 28 allows for gas flow therethrough, but forms a
barrier for soot particles. Woven metal mesh such as of steel wool
type, non-woven wire mesh in which individual wires are spot
soldered or the like with other wires to form a single mesh unit,
braided wire mesh in which a plurality of wire strands are twisted
together to form a mesh capable of the desired removal of soot from
a passing gas stream can be used. The metal mesh filtering element
28 is preferably placed as a single mass within the interior of
each channel 18 so as to substantially completely fill the channel
interior. More than one piece of metal mesh can be used if more
convenient to fill the channel interiors. The metal mesh filtering
element 28 can be readily removable from within each channel 18 so
as to be easily substituted with repaired, regenerated, or a new
mesh element to maintain optimum filtering capability. The metal
mesh filtering element 28 can be pressure fit within each channel
interior so as to remain in place during operation. Alternatively,
or in addition to pressure fitting, each open end 19 and 21 of
channel 18 may contain one or more cross pieces or mesh (not shown)
to keep the metal mesh filtering element 28 in place within the
interior of channels 18.
[0023] It is well known in the art of diesel soot filters to burn
off the soot particles from the filter so as to regenerate the
filter and again improve its capacity to filter the soot particles
from the exhaust gas. Unfortunately, the temperature generated by
the diesel engine and imparted to the exhaust gas is not high
enough to initiate ignition and burning of the soot particles.
Accordingly, oxidation catalysts have been incorporated onto the
filtering element so as to lower the ignition temperature of the
soot particles and allow the particles to be burned and the filter
element regenerated either on a continuous or alternating process
between filtering and regenerating. In accordance with the present
invention, the metal mesh elements 28 of this invention can be
coated with the oxidation catalysts well known in the art to
initiate the ignition of the soot particles from the diesel exhaust
gas that are trapped within the filtering element. The types of
catalysts and the methods of applying the catalysts to the metal
mesh filtering element 28 are more fully explained below. In
addition to coating the wire mesh filtering element 28, it is also
possible to coat the interior of the channels 18 with an oxidation
catalyst to initiate ignition of any soot particles attached to the
interior walls and/or initiate oxidation of gaseous contaminants
within the exhaust gas itself, such as CO, HC, and NOx. The
catalyst on the metal mesh filtering element 28 may be the same or
different than the catalyst that is coated on the interior walls of
channels 18. Further, any wire elements used to maintain the
filtering elements 28 in place within channels 18, such as any mesh
or the like, provided in openings 19 and 21 can also be provided
with a catalytic coating. Still further, it may be possible to
provide an oxidation catalyst on at least a portion of the exterior
of the channels 18, in particular, those areas of the channels
which are in contact with the interstitial voids 30, which are
formed between the channels 18 as the channels are stacked within
enclosed area 26 of casing 12. Thus, exhaust gas that passes
through the interstitial voids 30 can be treated so as to initiate
oxidation of the exhaust gas contaminants.
[0024] The filter device 10 of this invention will be placed in an
exhaust stream from a diesel engine. A diesel oxidation catalyst
(DOC) may or may not be placed in front of the filtering device 10
dependent upon the application. An exhaust gas from the diesel
engine containing HC, CO, NOx, and particulate matter passes
through the filter device 10 and, in particular, channels 18. Due
to the impaction of the soot particles on the catalyzed wire mesh
element 28, the soot particles are collected and burnt under
suitable exhaust regeneration conditions. If the application is
such that the soot accumulation rate on filter element 28 exceeds
the burning rate of the soot particles, exhaust gas flow from the
engine will be forced and diverted through the interstices 30
between the stacked channels 18 within casing 12. The interstitial
voids 30 are initially sized to permit only minor flow during most
diesel engine operating conditions since the back pressure in the
interstities 30 is higher than the back pressure through mesh
filtering element 28. Typically the interstitial void volume within
enclosed area 26 of casing 12 will comprise less than 25% of the
volume of enclosed area 26. Gas flow through the interstitial void
volume can be controlled or limited by use of orifice openings 34
in the endplates such as shown for endplate 23 in FIG. 4. When the
soot accumulates and starts to block the pores of the metal mesh
filtering element 28 and the back pressure rises, the interstitial
or bypass voids 30 still enable exhaust gas flow and allow the
diesel engine to continue operation. In this way the vehicle will
not stall due to a totally restricted flow path of the exhaust gas.
The density of the wire mesh (wire diameter and the amount of wire
weaved into the matrix) determines the back pressure.
[0025] The metal mesh can be made of any relatively high
temperature alloy, including most stainless steels, Fecralloy,
Hastalloy, etc.
[0026] In a preferred embodiment of the invention, the metal mesh
is pretreated prior to deposition of the catalyst composition to
improve the adherence of composition on the substrate. Pretreatment
of the substrate can be conducted by applying a metal anchor layer
to the substrate by known thermal spraying techniques before the
catalyst slurry is applied. These techniques include plasma
spraying, single wire spraying, high velocity oxy-fuel spraying,
combustion wire and/or powder spraying, electric arc spraying etc.
Preferably the metal anchor layer is applied by electric arc
spraying.
[0027] Electric arc spraying, e.g., twin wire arc spraying, of a
metal (which term, as used herein, includes mixtures of metals,
including without limitation, metal alloys, pseudoalloys, and other
intermetallic combinations) onto a metal foraminous substrate
yields a structure having superior utility as a substrate for
catalytic materials in the field of catalyst members. Twin wire arc
spraying (encompassed herein by the term "wire arc spraying" and by
the broader term "electric arc spraying") is a known process,
disclosed in U.S. Pat. No. 4,027,367 which is incorporated herein
by reference. Briefly described, in the twin wire arc spray
process, two feedstock wires act as two consumable electrodes.
These wires are insulated from each other as they are fed to the
spray nozzle of a spray gun in a fashion similar to wire flame
guns. The wires meet in the center of a gas stream generated in the
nozzle. An electric arc is initiated between the wires, and the
current flowing through the wires causes their tips to melt. A
compressed atomizing gas, usually air, is directed through the
nozzle and across the arc zone, shearing off the molten droplets to
form a spray that is propelled onto the substrate. Only metal wire
feedstock can be used in an arc spray system because the feedstock
must be conductive. The high particle temperatures created by the
spray gun produce minute weld zones at the impact point on a
metallic substrate. As a result, such electric arc spray coatings
(sometimes referred to herein as "anchor layers") maintain a strong
adhesive bond with the substrate.
[0028] Operating parameters for wire arc spraying for forming
anchor layer on foraminous substrates are disclosed in copending
U.S. patent application Ser. No. 09/301,626, filed Apr. 29, 1999
(the '626 application), now U.S. Publication No. 2002/0128151,
published Sep. 12, 2002, the disclosure of which is hereby
incorporated by reference in its entirety.
[0029] Anchor layers of a variety of compositions can be deposited
on a substrate by utilizing, without limitation, feedstocks of the
following metals and metal mixtures: Ni, Ni/Al, Ni/Cr, Ni/Cr/Al/Y,
Co/Cr, Co/Cr/Al/Y, Co/Ni/Cr/Al/Y, Fe/Al, Fe/Cr, Fe/Cr/Al,
Fe/Cr/Al/Y, Fe/Ni/Al, Fe/Ni/Cr, 300 and 400 series stainless
steels, and, optionally, mixtures of one or more thereof. One
specific example of a metal useful for wire arc spraying onto a
substrate in accordance with the '626 application is a
nickel/aluminum alloy that generally contains at least about 90%
nickel and from about 3% to 10% aluminum, preferably from about 4%
to 6% aluminum by weight. Such an alloy may contain minor
proportions of other metals referred to herein as "impurities"
totaling not more than about 2% of the alloy. A preferred specific
feedstock alloy comprises about 95% nickel and 5% aluminum and may
have a melting point of about 2642.degree. F. Some such impurities
may be included in the alloy for various purposes, e.g., as
processing aids to facilitate the wire arc spraying process or the
formation of the anchor layer, or to provide the anchor layer with
favorable properties.
[0030] Electric arc spraying a metal onto a metal substrate yields
a superior substrate for catalytic materials relative to substrates
having metal anchor layers applied thereto by other methods.
Catalytic materials have been seen to adhere better to a substrate
comprising an electric arc sprayed anchor layers than to a
substrate without an intermediate layer applied thereto and even
better than to a substrate having a metal layer deposited thereon
by plasma spraying. Catalytic materials disposed on metal
substrates, without intermediate layers between the substrate and
the catalytic material, often did not adhere sufficiently well to
the substrate to provide a commercially acceptable product. Metal
substrates having an intermediate layer applied by other thermal
spraying techniques typically suffer the same drawbacks. For
example, a metal substrate having a metal intermediate layer that
was plasma-sprayed thereon and having a catalytic material applied
to the intermediate layer failed to retain the catalytic material,
which flaked off upon routine handling, apparently due to a failure
of the intermediate layer to bond with the substrate. The catalytic
material on other substrates was seen to spall off upon normal use,
apparently as a result of being subjected to a high gas flow rate,
to thermal cycling, to the eroding contact of high temperature
steam and other components of the exhaust gas stream, vibrations,
etc. Application of the intermediate layer by electric arc spraying
therefore improves the durability of catalyst members comprising
catalytic materials carried on foraminous substrates by improving
their durability.
[0031] The metal mesh filter elements of this invention (also
referred to herein as foraminous substrates) useful for forming the
filtering elements include those metallic substrates which are able
to accommodate a high flow rate, are lightweight and have a low
thermal mass. The woven, non-woven, and braided wire mesh of this
invention as filter element 28 are suitable for application of a
metal anchor layer.
[0032] A suitable catalytic material for use on a foraminous
substrate can be prepared by dispersing a compound and/or complex
of any catalytically active component, e.g., one or more platinum
group metal compounds or complexes, onto relatively inert bulk
support material. As used herein, the term "compound", as in
"platinum group metal compound" means any salt, complex, or the
like of a catalytically active component (or "catalytic component")
which, upon calcination or upon use of the catalyst, decomposes or
otherwise converts to a catalytically active form, which is often,
but not necessarily, an oxide. The compounds or complexes of one or
more catalytic compounds may be dissolved or suspended in any
liquid which will wet or impregnate the support material, which
does not adversely react with other components of the catalytic
material and which is capable of being removed from the catalyst by
volatilization or decomposition upon heating and/or the application
of a vacuum. Generally, both from the point of view of economics
and environmental aspects, aqueous solutions of soluble compounds
or complexes are preferred. For example, suitable water-soluble
platinum group metal compounds are chloroplatinic acid, amine
solubilized platinum hydroxide, rhodium chloride, rhodium nitrate,
hexamine rhodium chloride, palladium nitrate or palladium chloride,
etc. The compound-containing liquid is impregnated into the pores
of the bulk support particles of the catalyst, and the impregnated
material is dried and preferably calcined to remove the liquid and
bind the platinum group metal into the support material. In some
cases, the completion of removal of the liquid (which may be
present as, e.g., water of crystallization) may not occur until the
catalyst is placed into use and subjected to the high temperature
exhaust gas. During the calcination step, or at least during the
initial phase of use of the catalyst, such compounds are converted
into a catalytically active form of the platinum group metal or a
compound thereof. An analogous approach can be taken to incorporate
the other components into the catalytic material. Optionally, the
inert support materials may be omitted and the catalytic material
may consist essentially of the catalytic component deposited
directly on the sprayed foraminous substrate by conventional
methods.
[0033] Preferred platinum group metal components for use in the
articles of the invention include platinum, palladium, rhodium,
ruthenium and iridium components. Platinum, palladium and rhodium
components are particularly preferred. When deposited on a
foraminous substrate (e.g., metal screen) such components are
generally deposited at a concentration of from 0.001 to 0.01
g/in.sup.2 for typical utility engine applications.
[0034] Suitable support materials for the catalytic component
include alumina, silica, titania, silica-alumina,
alumino-silicates, aluminum-zirconium oxide, aluminum-chromium
oxide, etc. Such materials are preferably used in their high
surface area forms. For example, gamma-alumina is preferred over
alpha-alumina. It is known to stabilize high surface area support
materials by impregnating the material with a stabilizer species.
For example, gamma-alumina can be stabilized against thermal
degradation by impregnating the material with a solution of a
cerium compound and then calcining the impregnated material to
remove the solvent and convert the cerium compound to a cerium
oxide. The stabilizing species may be present in an amount of from
about, e.g., 5 percent by weight of the support material. The
catalytic materials are typically used in particulate form with
particles in the micron-sized range, e.g., 10 to 20 microns in
diameter, so that they can be formed into a slurry and coated onto
a substrate.
[0035] A typical catalytic material for use on a filter member for
diesel engine exhaust comprises platinum, palladium and rhodium
dispersed on an alumina and further comprises oxides of neodymium,
strontium, lanthanum, barium and zirconium. Some suitable catalysts
are described in U.S. patent application Ser. No. 08/761,544 filed
Dec. 6, 1996, the disclosure of which is incorporated herein by
reference. In one embodiment described therein, a catalytic
material comprises a first refractory component and at least one
first platinum group component, preferably a first palladium
component and optionally, at least one first platinum group metal
component other than palladium, an oxygen storage component which
is preferably in intimate contact with the platinum group metal
component in the first layer. An oxygen storage component ("OSC")
effectively absorbs excess oxygen during periods of lean engine
operation and releases oxygen during periods where localized
concatenations of fuel produce a rich environment as seen in
light-off of the catalyst after prolonged idle condition. Bulk
ceria is known for use as a OSC, but other rare earth oxides may be
used as well. In addition, as indicated above, a co-formed rare
earth oxide-zirconia may be employed as a OSC. The co-formed rare
earth oxide-zirconia may be made by any suitable technique such as
co-precipitation, co-gelling or the like. One suitable technique
for making a co-formed ceria-zirconia material is illustrated in
the article by Luccini, E., Mariani, S., and Sbaizero, 0. (1989)
"Preparation of Zirconia Cerium Carbonate in Water With Urea" Int.
J. of Materials and Product Technology, vol. 4, no. 2, pp. 167-175,
the disclosure of which is incorporated herein by reference. As
disclosed starting at page 169 of the article, a dilute (0.1 M)
distilled water solution of zirconyl chloride and cerium nitrate in
proportions to promote a final product of ZrO.sub.2-10 mol %
CeO.sub.2 is prepared with ammonium nitrate as a buffer, to control
pH. The solution was boiled with constant stirring for two hours
and complete precipitation was attained with the pH not exceeding
6.5 at any stage.
[0036] Any suitable technique for preparing the co-formed rare
earth oxide-zirconia may be employed, provided that the resultant
product contains the rare earth oxide dispersed substantially
throughout the entire zirconia matrix in the finished product, and
not merely on the surface of the zirconia particles or only within
a surface layer, thereby leaving a substantial core of the zirconia
matrix without rare earth oxide dispersed therein. Thus,
co-precipitated zirconium and cerium (or one other rare earth
metal) salts may include chlorides, sulfates, nitrates, acetates,
etc. The co-precipitates may, after washing, be spray dried or
freeze dried to remove water and then calcined in air at about
500.degree. C. to form the co-formed rare earth oxide-zirconia
support. The catalytic materials of aforesaid application Ser. No.
08/761,544 may also include a first zirconium component, at least
one first alkaline earth metal component, and at least one first
rare earth metal component selected from the group consisting of
lanthanum metal components and neodymium metal components. The
catalytic material may also contain at least one alkaline earth
metal component and at least one rare earth component and,
optionally, at least one additional platinum group metal component
preferably selected from the group consisting of platinum, rhodium,
ruthenium, and iridium components with preferred additional first
layer platinum group metal components being selected from the group
consisting of platinum and rhodium and mixtures thereof.
[0037] A variety of deposition methods are known in the art for
depositing catalytic material on a foraminous substrate. These
methods of applying the catalytic component onto the substrate
constitute a separate step in the manufacturing process relative to
the application of any anchor layer (if applied) to the
substrate.
[0038] Methods for depositing catalytic material on the foraminous
substrate include, for example, disposing the catalytic material in
a liquid vehicle to form a slurry and wetting the foraminous
substrate with the slurry by dipping the substrate into the slurry,
spraying the slurry onto the substrate, etc. Alternatively, the
catalytic material may be dissolved in a solvent and the solvent
may then be wetted onto the surface of the foraminous substrate and
thereafter removed to leave the catalytic material, or a precursor
thereof, on the foraminous substrate. The removal procedure may
entail heating the wetted substrate and/or subjecting the wetted
substrate to a vacuum to remove the solvent via evaporation.
EXAMPLE 1
Preparation of a Catalyst Composition Containing Platinum and
Palladium in a 4:1 Ratio
[0039] First platinum and palladium compounds were dispersed on
high surface area gamma alumina and 5% lanthanum modified alumina
supports. Into 2104.5 g of gamma alumina (97% solids) and 2041 g of
5% lanthanum stabilized alumina was added an aqueous solution
containing 133.9 g of Pt as a 16% amine solubilized platinum
hydroxide diluted with 709 g of deionized water with mixing. After
mixing for additional 20 minutes a Pd solution was added containing
33.5 g Pd as a 19% palladium nitrate solution diluted with 700 g of
deionized water. This was mixed an additional 20 minutes to ensure
the powder was uniformly contacted with the precious metal
solution.
[0040] The resulting precious metal support mixture from above was
contacted with 6189 g of deionized water, 433.9 g of 90 acetic acid
and 18 g of octanol in a dispersion tank. This mixture was fed into
a continuous mill and ground until >90% of the material had a
particle diameter of less than 5 microns. A Ce/Zr composite oxide
was added with an additional 120 g of acetic acid and the resulting
slurry was further ground until the overall particle size was 90%
<1 micron. In a dispersion tank 583.3 g of zirconyl acetate
solution was added to the slurry and mixed vigorously. The final pH
of the slurry was in the range of 4.0-4.8.
EXAMPLE 2
Preparation of Wire Mesh Foraminous Catalytic Substrate
[0041] To prepare an article having the design as shown in FIG. 1,
a stainless steel wool mesh was wire arc spray-coated with a
nickel-aluminide alloy as described in Example 1 of the aforesaid
'626 application. The steel wool mesh was then coated with the
coating slurry described above (Example 1) at a washcoat loading of
0.05 to 0.1 g/in.sup.2. The mesh was then fitted into a cylindrical
channel having an inside diameter of 1.25 in.
[0042] While this invention has been described with an emphasis
upon preferred embodiments, it will be obvious to those of ordinary
skill in the art that variations in the preferred devices and
methods may be used and that it is intended that the invention may
be practiced otherwise than as specifically described herein.
Accordingly, this invention includes all modifications encompassed
within the spirit and scope of the invention as defined by the
claims that follow.
* * * * *