United States Patent (19) 11 Patent Number: 4,902,487

Cooper et al. (45) Date of Patent: Feb. 20, 1990

54 TREATMENT OF DIESELEXHAUST GASES 4,516,990 5/1985 Erdmannsdoerfer et al. ... 423/215.5
4,670,233 6/1987 Erdmannsdoerfer et al. ... 423/215.5
75 Inventors: Barry J. Cooper, Radnor; Hyun J. 4,759,918 7/1988 Homeier et al. ................. 423/213.5
Jung, Wayne; James E. Thoss, West
Chester, all of Pa. FOREIGN PATENT DOCUMENTS
73) Assignee: Johnson Matthey, Inc., Valley 1014498 12/1965. United Kingdom............. 423/213.2
Forge, Pa. 2188559 10/1987 United Kingdom ............. 423/213.7
21 Appl. No.: 193,529 Primary Examiner-Jeffrey E. Russel
Attorney, Agent, or Firm-Cushman, Darby & Cushman
22 Filed: May 13, 1988
57 ABSTRACT
51 Int. Cl.".............................................. B01D 53/34
52 U.S. Cl. .................................................. 423/215.5 In a process wherein diesel exhaust gas is passed
58 Field of Search ..................... 422/171; 423/213.2, through a filter to remove particulate therefrom before
423/213.5, 213.7, 215.5 discharge and particulate deposited on the filter is com
56) References Cited busted, the improvement which comprises combusting
the particulate with a gas containing NO2, which may
U.S. PATENT DOCUMENTS be catalytically generated in the exhaust system.
4,303,552 12/1981 Ernest et al. ..................... 423/213.5
4,451,441 5/1984 Ernest et al...................... 423/215.5 9 Claims, 5 Drawing Sheets
U.S. Patent Feb. 20, 1990 Sheet 1 of 5 4,902,487
U.S. Patent Feb. 20, 1990 Sheet 5 of 5 4.902,487

NN
 4,902,487 2. '
initially passing the diesel exhaust gas through a cata
TREATMENT OF DESELEXHAUST GASES lytic converter position upstream of the filter. The ex
haust gas usually includes a significant amount of NO,
The present invention is concerned with certain im O2, CO, CO2, H2O and SO2 as well as unburnt hydro
provements in the removal of particulate from diesel carbons and carbonaceous particulate. The important
exhaust gases. component, however, is the NO which, according to
BACKGROUND the invention, is converted to the NO2 oxidant neces
sary to combust particulate on the filter. Preferably, the
It is conventional in the art to pass exhaust gases from amount of N02 in the gas as it is fed to the filter will be
diesel engines through a filter to remove particulate O in the range 100-2000 ppm although it will be appreci
(soot or other carbonaceous solids) in the gas before the ated that the NO2 content can be widely varied within
gas is discharged into the atmosphere. This is generally and outside this range. The essential point is that there
accomplished by providing a filter, e.g., stainless steel is enough NO2 in the gas fed to the filter to effectively
wire mesh, which usually includes a catalyst, the pur combust the deposited carbon soots and like particu
pose of the catalyst being to facilitate combustion of the 15 lates.
particles which are collected on the filter. Such com In a preferred embodiment, the diesel exhaust gas is
bustion is desirable to avoid filter plugging and the passed through a low pressure drop monolith catalyst
buildup of undesirable back pressure as a result of the (e.g., a ceramic honeycomb) coated with platinum (Pt)
collection of soot or like particulate on the filter. or other platinum group metal (PGM) whereby NO in
Prior art filter systems have suffered from a number 20 the exhaust gas is catalytically converted to NO2 by
of disadvantages. Thus, for example, with currently reaction with oxygen. The thus treated gas is then
available systems, there is a problem of effective com passed downstream through a wire mesh particulate
bustion of diesel particulate at normal gas temperatures filter which, if desired, contains a catalyst. This catalyst
of 300° C. or below. While the temperature of diesel may simply comprise a conventional alumina washcoat.
exhaust gas may be as high as 500 C., it is generally 25 Alternatively, base metal catalysts may be used. Partic
considerably lower, e.g., 300 C. or below, and, as ularly useful results are obtained if the filter includes a
noted, conventional filter arrangements are not particu high temperature particulate combustion catalyst, typi
larly effective for combusting particulate at such low cally a catalyst comprising lanthanum, cesium and va
temperatures. Means may be provided to increase the nadium pentoxide (La/Cs/V2O5) or the like, to com
temperature of the exhaust gas to above 300 C. to 30 bust particulate collected on the filter.
facilitate combustion of the collected particulate but It is to be understood that the catalytically generated
this creates other difficulties. Additionally, prior ar oxidant is no necessarily all NO2. It may be, in whole
rangements have suffered from the substantial genera part N2O5, N2O or equivalent oxidant derived from NO
tion of sulphuric acid which in turn condenses te form in the diesel exhaust. For ease of reference, however,
undesired sulphate particulate. the oxidant is simply referred to herein as NO2. This gas
The principal object of the invention is to provide a apparently functions to combust the soot particulate on
method for effective combustion and removal of diesel the filter according to the following reactions:
particulate collected on the filter even at low tempera
tures, e.g., 225-300 C. Another object is to minimize
the formation of sulphuric acid and sulphate particulate. NO + O2 - GNO2 (a)
Other objects will also be hereinafter apparent.
BROAD DESCRIPTION OF THE INVENTION
Broadly state, the invention contemplates contacting (b)
particulate entrapped on the filter with NO2 gas so as to 45
cause combustion of the particulate and its consequent
removal from the filter. The NO2 required for this pur
pose is catalytically generated in the exhaust gas itself
N. -- co, (c)

before it is passed downstream to the filter on which It is to be noted that the combination of reactions (a)
diesel particulate is entrapped, the NO2 oxidant serving 50 and (c) provide for the removal of NO from the diesel
to effectively combust the collected particulate at low exhaust as well as providing the NO2 needed for soot
temperature and thus reduce the back pressure nor removal.
mally caused by particulate deposition on the filter. It is As noted earlier, particularly useful results are real
particularly useful to use the NO2 gas in combination ized if the combustion is carried out in the presence of
with H2O vapor as this combination appears to provide 55 small amounts of added water (e.g., 0.5% to 10%, based
especially effective results. on the total volume of the gas). It is believed that the
BRIEF DESCRIPTION OF THE DRAWING water may convert some of the NO2 to HNO3 which
also appears to facilitate combustion or removal of the
FIG. 1 is a perspective view with parts broken away particulate from the filter.
illustrating one embodiment of the invention while 60 The invention is based on the theory that if small
FIGS. 2-4 graphically illustrate advantages of the in amounts of NO2 and/or HNO3 can be provided in the
vention. diesel exhaust gas prior to the diesel particulate filter,
DETAILED DESCRIPTION OF THE the NO2 and/or HNO3 will be absorbed into the filter
INVENTION carbon deposits, which are then ignited, and removed as
harmless N2 and H2O more effectively and at lower
The NO2 oxidant gas used to combust particulate on temperatures than might otherwise be the case.
the filter may be catalytically generated in a variety of Known catalysts for producing NO2 from NO and
ways. However, this is preferably accomplished by O2 may be used to generate the NO2 oxidant for present
 4,902,487
3 4.
purposes Such catalysts are extensively used in the cata like carbonaceous particulate is combusted at a temper
lytic conversion of automotive exhaust gases. This in ature generally in the order of 250-400° C., or even
cludes, for example, Pt, Pd, Ru, Rh or combinations higher if desired, to convert carbon deposits to volatile
thereof, platinum group metal oxides such as RhCl3 and CO and/or CO2. The filter also effectively serves to
the like. support, e.g., a ceramic honeycomb. However, 5 trap any H2SO4 or sulfate generated in the system.
the catalyst may also be used in bead or particulate There may be sufficient water vapor in the gas being
form. processed in (1) to provide the desired amount of H2O
The filter may be in conventional form and structure. to improve combustion at the filter. However, water
Typically this comprises one or more wire meshes of may be added at (1) or at some point before the gas
appropriate metal (e.g., stainless steel or the like) O enters the filter (3).
through which the gas is passed. As noted earlier, the The invention is illustrated, but not limited, by the
mesh or equivalent may include a coating, such as alu following examples.
mina, which facilitates entrapment of the particulates or
a coating which facilitates higher temperature combus 5
EXAMPLE 1
tion, e.g., vanadium oxide or preferably La/Cs/V2O5. It The following series of experiments were run to de
will be appreciated, however, that the invention is not termine the effect of various gases on the low tempera
dependent on the use of a wire mesh filter, and that ture (<400 C.) gas phase oxidation of soot collected on
other available particulate filter alternatives, e.g., a a diesel particulate trap. Previous testing had shown
ceramic wall flow filter, or a ceramic foam filter, may that oxygen alone was not effective at temperatures
be used. below 400 C.
According to a preferred way of carrying out the The tests were run using a laboratory flow (SCAT
invention, diesel exhaust is passed through a low pres Synthetic Catalyst Activity Test) rig. This system is
sure drop platinum coated monolith catalyst (ceramic used to blend a synthetic exhaust gas using pure compo
honeycomb) containing a catalytically effective amount nent compressed gases and is capable of analyzing the
of platinum at a space velocity of, for example, up to 25 resulting gas mixture before and after it is passed
about 60,000 to generate NO2 from the combination of through a small experimental catalyst sample.
NO and O2 in the exhaust gas. The thus processed gas is For these experiments, two elements were installed in
then passed through a conventional wire mesh diesel the SCAT flow reactor system. The first element was a
particulate filter containing an alumina wash-coat or a 30 conventional ceramic monolith supported catalyst con
particulate combustion catalyst, preferably La/Cs/- taining approximately 80 gm/ft3 Pt. The monolith core
V2O5, located downstream of the monolith. The gas was 1' in diameter and 2.23' long which using a 20
passing through the filter serves to effectively combust l/min flow stream, provided a space velocity of approx
soot or like carbonaceous particles deposited on the imately 42,000 HR-1.
filter. 35 The second element, placed approximately 1' down
Space velocity of the gas through the NO2-forming stream of the monolith, was a l' diameter by 1' long
catalyst and through the filter can be widely varied wire mesh plug. The plugs so used were fabricated from
depending on other operating conditions, e.g., the na the same stainless steel mesh used for full size blocks,
ture of the catalyst. Generally speaking, however, space and were coated with a conventional dumina washcoat.
velocities of up to 200,000 per hour may be used to form No precious metal or any other catalyst was applied to
the NO2 while space velocities of up to 100,000 per hour these plugs. Soot was deposited on these plugs by in
are used through the filter. stalling them in a special fixture which was then
The system may be arranged so that the oxidant gen mounted in the exhaust system of an automotive diesel
eration step functions continuously during operation of engine running in a dynamometer test cell.
the engine or functions only periodically as soot accu 45 The SCAT flow reactor, containing the platinum
mulates beyond a desired point on the filter. An appro monolith followed by a heavily sooted plug, was then
priate arrangement for by-passing the NO2 generating heated to 300 C. in a flowing nitrogen stream. Differen
catalyst may be provided if the oxidant generation step tial pressure across the plug (measured in inches of
is to be performed only periodically. This may be desir water) was used as a measure of soot loading on the
able in the case where more sulphate is being formed 50 plug and the rate of reduction in this differential pres
than is desirable. sure (in inches of H2O per hour) was used as an indica
The invention is illustrated in the accompanying tor of the rate of burning of the trapped particulate.
drawing wherein the numeral (1) generally represents a Numerous combinations of gases were then intro
ceramic honeycomb monolith of conventional composi duced to the flow reactor and the rate of pressure drop
tion, the cells of which may be coated with an alumina 55 decrease was measured. The concentration of each gas
washcoat carrying, for example, a Pt catalyst. Con (indicated below) was held constant throughout this
nected to receive gas exiting from (1) via connecting series of tests:
passage (2) is a particulate filter or trap (3) of conven
tional design. The surfaces of (3) which contact gas may Oxygen 2%
carry an appropriate catalyst as noted earlier. A base 60 Water 4.5%
metal catalyst is usually preferred for this purpose, par Carbon Monoxide 200 ppm
ticularly one which is a sulfur- or sulfate-absorbing type Nitric Oxide 400 ppm
of catalyst. Sulfur Dioxide 50 ppm
Nitrogen Balance
In use, diesel exhaust enters the system through the
inlet (4), is processed in (1) to catalytically generate 65
NO2 from NO therein, the resulting gas with the en Three series of tests were run designated as SCAT
riched NO2 content being then fed through filter (3) run numbers 6797, 6802 and 6803. The results of these
after which it is discharged at (5). In filter (3), soot or tests are shown in Tables 1, 2 and 3.
 4,902,487
5 6
TABLE 1.
RUN #6797
TEST N2 NO O2 H2O SO2 CO "HO RATE
i BALANCE 400 ppm 12% 4.5% 50 ppm 200 ppm START "H2O/HR.
1 X 94 O
2 X X 93 10
3 X X X 87 32
4. X X 77 O
5 X X X 75 30
6 X X X X 68 64
7 X X X 59 28
8 X X 53 4.
9 X X X 51 O
10 X X X 34 O
11 X X X X X 35 16
2 X X X X 3. 16
13 X 29 O
14 X X X 28 O
S X . X X 28 O
16 X X X X 29 2
17 X X X 27 5
8 X X X X X X 27 17

TABLE 2
SCAT RUN 6802
TEST N2 SO2 O2 H2O NO 'H2O RATE
i BALANCE 50 ppm 12% 4.5% 400 ppm START "H2O/HR.
1 X 79 O
2 X X 79 4
3 X X X 76 4.
4. X X X X 74 4.
5 X X X 78 O
6 X X X X 78 O
7 X X X X X 69 53
8 X X X X 60 51
9 X X X 48 4
10 X X X 43 O --

TABLE 3
-- SCAT RUN i8803
N2 NO O2 H2O SO2 'H2O RATE
TEST BALANCE 400 ppm 12% 4.5% 50 ppm START 'H2O/HR
X X 72 O
2 X X 69 O
3 X X X 69 24
4. X X X X 58 49
5 X X X X X 46 30
6 X X X X 35 20
7 X X X 32 O
8 X X X X 3. 1.

4. the rate of combustion of the particulate, as indi

It will be appreciated that the differential pressure 50 cated by the rate of pressure change, decreases as
across the filter as shown in the second column from the the particulate loading decreases.
right in Tables 1-3 represents the starting pressure drop EXAMPLE 2
across the filter. The right-hand column in Tables 1-3
headed "Rate' shows the reduction in pressure drop Example 1 was repeated with the following changes:
resulting from use of the indicated gases. The greater 55 1. pure NO2 was introduced into the gas;
the reduction value shown in this column, the greater 2. oxygen was provided from one of two alternate
the combusting effect of the gas used. SOCS:
The following conclusions can be drawn on the basis a standard plant compressed air containing approxi
of the results obtained in the foregoing tests: mately 0.3% water; or
1. oxygen alone is incapable of oxidizing the trapped 60 b. zero grade air from a commercial compressed gas
particulate at a significant rate at 300 C.; cylinder containing less than 3 ppm water.
2. a combination of nitric oxide and oxygen passed 3, only non-catalyzed plugs with deposited diesel soot
over the Pt catalyst produces a gas phase oxidant were used, i.e., the platinum monolith catalyst used in
(presumably NO2) which is an effective particulate Example 1 was not employed in two of the tests (desig
oxidant at 300° C.; nated as 7169 and 7174) of the present example. One
3. the addition of water (along with NO2 and O2) experiment (7176) was conducted with a Pt monolith
substantially enhances the rate of pressure drop upstream of the particulate filter, and used NO rather
reduction; than NO2 in the gas stream.
 s

4,902,487
7 8
Three series of tests were run designated as SCAT ceramic monolith upstream of the uncatalyzed diesel
run numbers 7169, 7174 and 7176. The results of these filter is capable of significantly reducing the balance
tests are given in Tables 4, 5 and 6. The rate of decrease temperature of an uncatalyzed particulate filter.
in differential pressure was enhanced by the addition of The balance temperature for diesel particulate com
oxygen (plant air) to the NO2 in the first series of tests 5 bustion is defined within the context of a fixed engine
(#7169). This indicates that water played a key role in test procedure as the temperature at which the pressure
the NO2/soot reaction. differential across a trap stops increasing. That is, the
The second series of tests were designed to delay the rate of particulate oxidation balances the rate of particu
introduction of water into the system. It is considered late accumulation on the filter system. Below the bal
that the higher rate observed for the NO2 only reaction 10 ance temperature, pressure drop increases across the
in the first series of tests (36' H2O/hr) compared with filter and above the balance temperature, pressure drop
the lower rate of 13' H2O/hr measured in the second decreases as the particulate burns.
series was caused by residual water in the system during The diesel filter component of the two systems tested
the first series of tests. The effect of water was con- was identical. Each was comprised of six standard wire
firmed in the third series of tests where the Pt monolith 15 mesh blocks coated with an alumina washcoat only
was used to make NO2 by reaction of NO and oxygen, with no catalyst of any kind. The first system was the
the reaction being promoted by the water content of the uncatalyzed trap alone. The second system was made
oxygen feed stream. up of an identical uncatalyzed trap preceded by a cata
TABLE 4 lytic converter containing three 45 in ceramic monolith
o
20 bricks each containing approximately 2 grams of plati
SCAT RUN i7169
TEST N2 NO2 NO O2
RATE
H2O ' H2O/
These systems
s
were tested on a 2.5 liter Peugeot
i BALANCE 400 ppm 400 ppm 12% 4.5% HR diesel engine mounted in a dynamometer test cell. Stan
X O dard balance tests were run by holding engine speed
2 X X O 25 constant and gradually increasing the exhaust gas tem
3 X X O perature (4 C./minute) by increasing the engine load.
4 X X O Differential pressure across the filter was recorded as a
& X X X X s: function of the trap inlet temperature using an X-Y
7 X X X 40 recorder.
8 X X X 8 30 Results of the two balance tests are shown in FIGS. 2
and 3. As shown in the drawings, the trap alone (FIG.
2) gave a balance temperature of 410 C. whereas the
TABLE 5 platinum monolith plus trap system (FIG. 3) produced a
SCAT Run i7174 balance temperature of 275 C. These tests demonstrate
RATE 35 the feasibility of separating the catalyst from the diesel
TEST Bali,CE aS 5. NO H.O. "HO/ particulate filter contemplated herein. These tests also
A ppm 12% 400 ppm 4.5% HR verify the SCAT results and further demonstrate that
-- X 3 the invention is functional in a diesel exhaust environ
& & & X . ment. This system may be further improved by incorpo
4. X X X 5 40 rating a catalyst (precious metal, base metal, or other)
5 X X X 10 on the surface of the filter media.
6 X X X X 50
7 X X X 60 EXAMPLE 4
8 X X X X 23
9 X X X X 75 Example 3 was repeated except that the filters were
O X X X X X 24 45 exchanged as follows:
a. no Pt monolith catalyst was used upstream of a Pt
TABLE 6
Pt Monolith Upstream of an
Alumina Coated Filter
SCAT RUN 776
TEST N NO ZERO AIR PLANT AIR H2O RATE
if BALANCE 400 ppm (<3 ppm HO) (0.3% H2O) 4.5% "H2O/HR
1. X X X 12
2 X X X 24
3 X X X X 42

Conclusions which can be reached from the forego

ing results include the following:
i. NO2 alone is not necessarily sufficient to cause a
significant decrease in pressure drop; 60 catalyzed filter;
2. water and nitrogen dioxide together cause a rapid b. Pt monolith catalyst was used upstream of an alu
drop in pressure; mina coated filter; and
3 nitric oxide (NO) inhibits the NO2/H2O/soot reac- c. Pt monolith catalyst was used upstream of a
tion. La/Cs/V2O5 coated filter.
EXAMPLE 3 65 The engine was then run over a series of steady state
tests and rates of increase (--) or decrease (-) of pres
The purpose of the test described in this example was sure drop across the filter measured at various filter
to demonstrate that a platinum catalyst supported on a inlet temperatures with the following results:
 4,902,487
9 10
catalyst to facilitate combustion above 300 C. As an
Rate of Pressure Change (in H2O/hr)
other alternative, the filter may be provided with a
Filter Inlet System System System
catalyst to catalytically generate oxidant NO2 in situ
C. (a) (b) (c) with the particulate. Preferably, however, the NO2
225 --30 --0 --8
generating catalyst is positioned upstream from the
275 --14 --5 -- 10 filter to minimize fouling the NO2 generation catalyst
300 -6 -7 --l by diesel particulate.
400 - 5 -5 -9 As will be recognized, the invention offers a number
450 - 0 --3 -20 of advantages and particularly the possibility of effec
10 tively removing carbonaceous particulate deposits from
In addition, balance temperatures and emissions of the filter in a diesel exhaust gas system at low tempera
H2SO4, measured at 450 C. for the three systems gave tures. However, a further advantage is that the present
the following results: process permits effective NO removal from the exhaust
gas. It was not previously considered possible to effec
15 tively lower NO content in a diesel exhaust gas without
System Balance Temp. C. H2SO4 mg/ft increasing particulate deposit. The invention provides
2.
b
270° C.
245 C.
4.6
.3
means for reducing both NO content in the gas and
c 280 C. 2.8 carbon deposit on the filter.
FIG. 4 graphically shows that the present process
20 provides both particulate and NO removal as well as
The results obtained show that the combination of a reduced CO and hydrocarbon emission. The results
Pt catalyzed monolith to generate NO2 upstream of a shown in FIG. 4 were generated with a Pt-catalyzed
base metal catalyzed particulate filter gives an excellent metal monolith followed by a La/Cs/V2O5 wire mesh
combination of (a) low balance temperature, (b) low trap using a Cummins' L10 Heavy Duty Diesel Engine.
rate of pressure increased across the filter at low tem 25 Specific data for the test system was as follows:
peratures, and high rates of pressure decrease at high
temperatures, and (c) reduction of sulphuric acid emis Engine capacity
sions below the levels obtained with a Pt catalyzed Monolith Volune
0 liters
825 cu. in containing 38 g Pt
filter.
Trap Volume 49 annular wire mesh blocks
As noted earlier, it is believed that the presence of 30 4 o.d., 2" i.d. X 3' long.
H2O, by its addition to the gas or by its inherent pres
ence under the conditions used, results in the produc These results were compared to those obtained with a
tion of HNO3 in the exhaust gas. In the manufacture of
activated carbon, nitric acid is commonly used to oxi similar system, but with no Pt monolith and with the
dize and burn off carbon and it appears that the HNO3, 35 trap catalyzed with a 35% Rh/Pt catalyst. The system
formed in the exhaust gas prior to the filter by reaction of the invention showed lower CO, HC and particulate
between the catalytically generated NO2 and water removal, with about the same level of NO removal.
vapor, may function in a similar way in the present case The results illustrated in FIG. 4 were obtained over
to facilitate ignition and removal of carbon soots or like the Federal Test Procedure for Heavy Duty Engines on
particulate collected on the filter. a transient test bed.
While the invention contemplates the use of catalyti It will be recognized that some removal of particulate
cally generated NO2 to form HNO3, it is also possible, occurs via NO-NO2 generation over the monolith
as a further modification, to supply HNO3 to the filter in itself. However, use of the particulate filter or trap after
other ways, e.g., by controlled nitric acid injection into the monolith provides increased residence time and
the filter or into the exhaust gas before it reaches the 45 hence increased efficiency.
filter. FIG. 5 graphically shows the effect of the trap in
Other modifications in the invention are also contem reducing H2SO4 emission. The results shown were ob
plated. For example, as an alternative to the catalytic tained by processing exhaust gas successively over the
conversion of NO in the diesel exhaust to NO2 for diesel Pt monolith and La/Cs/V2O5 wire mesh measured at
soot removal, it is possible to use a metal nitrate which 50 225 C., 450° C., 300° C. and again at 450° C. The exper
decomposes at a temperature close to the required die iment was conducted, as in Example 3, on the Peugeot
sel soot combustion temperature to provide the diesel engine with the exhaust temperatures held at each of
NO2. Representative nitrates which may be mentioned these points. At 225 C., the trap was below the balance
for this purpose include potassium nitrate, magnesium temperature and was accumulating particulate. At this
nitrate, bismuth nitrate, lead nitrate. It appears that 55 low temperature, the engine was producing very little
metal nitrates which decompose at temperatures of up sulfate. After the monolith, the sulfate content increased
to about 470 C. can be used to supply NO2 at a temper slightly, but after the trap, the sulfate level was lower.
ature near 300° C. so as to effectively oxidize diesel On increasing the exhaust temperature to 450 C., the
SOOt. engine sulfate increased slightly, and there was a dra
It will be appreciated from the foregoing that the 60 matic increase in sulfate over the monolith, but again
invention involves in its preferred embodiment, the the sulfate level after the trap was reduced. In this con
catalytic generation of the oxidant NO2 which is carried dition, the trap was starting to regenerate.
downstream to a filter on which diesel particulate is Upon setting the engine exhaust to 300° C. (above
entrapped whereupon the oxidant attacks and combusts regeneration temperature), a similar pattern of results
the particulate and thereby reduces the pressure drop 65 was seen with lower sulfate emission after the trap than
across the filter. The filter also acts as a trap for any after the monolith. The engine exhaust was then in
sulfate which is generated so as to reduce sulfate emis creased to 450 C. and the trap fully cleaned. In this
sions. The filter may also include a high temperature case, the emissions after the trap were somewhat higher
 4,902,487
11. 12
than after the monolith probably due to saturation of the 3. The process of claim 2 wherein the particulate is
traps sulfate removal capability. Alternatively, there combusted at a temperature below 400° C.
could possibly be a reaction between the particulate on 4. The process of claim 3 wherein the gas containing
the trap and sulfate causing removal or conversion of NO2 is provided by initially passing diesel exhaust gas
5 containing NO over a catalyst to convert the NO to
sulfate to SO2, and if the trap reaches a fully clean state NO2.
then no particulate remains to meet with the sulfate. 5. The process of claim 4 wherein the catalyst com
Various modifications may be made in the invention. prises a platinum group metal.
Accordingly, the scope of the claims is defined in the 6. The process of claim 5 wherein the catalyst com
following claims wherein: 10 prises a platinum group metal on a monolithic honey
We claim: comb.
1. In a process wherein diesel exhaust gas is passed 7. The process of claim 6 wherein the filter includes a
through a filter to remove particulate therefrom before catalyst which facilitates particulate combustion.
discharge and particulate deposited on the filter is com 8. The process of claim 7 wherein the filter catalyst
busted, the improvement which comprises combusting
15 comprises a base metal.
9. The process of claim 7 wherein the fliter catalyst
the particulate with a gas containing NO2. comprises a combination of lanthanum, cesium and
2. The process of claim 1 wherein the gas containing vanadium pentoxide.:
NO2 also includes water vapor. 3k

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