Note: Descriptions are shown in the official language in which they were submitted.
,~ 3t7
Thls invention relates to exhau~t gas treatment ana,
more particularly, to the removal of suspended lead and
carbon partlcles and saseous pollutants from internal
combustion engine exhaust gases.
Filters have been used to remove suspended solids from
exhaust gases such as the lead and carbon particles in
internal combustion engine exhaust gases. As particles
accumulate in the filter, the resulting restriction of
exhaust gas flow increases the back pressure and reduces
filtration and engine efficiency. In order to restore
normal operation, the filter must be periodically
regenerated, for example, by mechanically cleaning the
filter or by heating it ana combusting the trapped carbon
particles.
Diesel particulate emissions are evidenced by the
occasional visible smoke discharges that occur during
acceleration or maximum power operation. The large
quantities of the very small and light carbon particles in
diesel exhaust gases present substantial difficulties in
achieving a high degree of particulate removal and
avoiding exce~sive back pressure.
In accordance ~ith the method of this invention,
carbon and lead particles are removeà from internal
combustion engine e~haust gases by passing the gases
through a coarse filter and then through a fine filter.
I'he filters comprise a refractory material effective to
trap the particles. The use of a fine filter permits
trapping of a high percentage of the particles and
substantially reduces particulate emissions. The use of a
relatively coarse filter to remove larger particles ~efore
-- 2
the gases reach the second fine filter extends ~he useful
life of t~e fine filter and reduces the rate at which the
back pressure increases as the particles accumulate in the
filters. The selective filtration and particulate
trapping distri~ution of this invention thus provide high
trapping efficiency with low increases in back press~re
and eftective ~iltration for longer periods before
regeneration is required.
The composition of this invention for convertinq one
or more pollutants in an exhaust gas to innocuous entities
and removing suspenaed particles from the gas comprises a
catalyst material effective for the conversion deposited
on a coarse filter and on a fine filter. The filters
comprise a refractory material effective to trap the
particles in the gas and are positioned so that the gas
flows in succession through the coarse filter and the fine
filter. W~en the catalyst material is effective for the
conversion of carbon and gaseous pollutants in the exhaust
gases, the exotherm from the conversion of gaseous
pollutants occurs in the fine filter because of lo~
conversion resulting from mass transfer limitations in the
coarse filter. The exotherm enhances combustion of carbon
particles trapped in the fine filter and regeneration of
the filtration capacity.
~5 The filters r,~ay comprise any material ~hich is
effective for trapping the particle~ in the gases, for
example, as a result of inertial impact or electrostatic
attraction. The filters are generally made of a porous,
refractory material which is resistant to the temperatures
of the gases and of catalytic conversion of the
pollutants. Suitable materials which have affinity for
t~
the partlcles an~ to ~nich the particles adhere inc~ude
refractory ceramic or metallic materials which have
sufficient thermal and mechanical stability for use in a
catalytic reactor. The metal may be, for example, steel,
stainless steel, aluminum, copper, or nickel. The ceramic
material may be a refractory metal oxide, such as alu~ina,
silica, magnesia, zirconia, titania, chromia, or
combinations thereof such as cordierite or a refractory
metal silicate or carbide.
The filters may be in the form of refractory inorganic
oxide beads, such as ceramic spheres or cylinders.
Preferably, the filters are unitary structures of
relatively large size such as ceramic monoliths, metal
wools, or metal meshes. An open cell filter structure
- 15 having a plurality of interconnected voids is especially
preferred. The continuous cells of such a structure
provide convoluted gas flow paths so that there is a
greater proba~ility that a particle will be trapped and
not pass through the filter. This structure has a larger
particulate retention capacity and higher filtration
efficiency than other filters.
An especially preferrea filter having a continuous
cellular structure is a ceramic foam. In addition to
larger particulate retention capacities and higher
filtration efficiencies, ceramic foams are particularly
useful in the treatment of diesel exhaust emissions
because of their lower pressure loss, higher
self-agitation, larger geometricai surface area, and lo~er
density than ceramic monoliths and other filters having
3n laterally extending flow paths. I'he ~oams also have
-- 4
~ ~'7~
substantial re~lstance to heat and chemical and physical
~eg r aa ation.
The ceran,ic foan filter preferably used in the present
invention is prepared from an open cell, flexible foam
material having a plurality of interconnected voids
surrounded by a web of the flexible foam material, such as
a polyurethane foam or cellulosic foam. The foam material
i~ impregnated with an aqueous ceramic slurry so that the
slurry coats the ~eb. The coated foam is dried and heated
to burn out the organic foam and sinter the ceran,ic
coating. The product is a fused ceramic foan, having a
plurality of interconnected voids surrounded by a ~eb of
bonoed or fusea ceramic in the configuration of the
organic foam. Suitable cordierite foams ot various cell
sizes are commercially available from Bridgestone Tire
Co., Ltd., Tokyo, Japan and may be prepared in accordance
with t~e method oescribed in Japanese Kokai 77/77,11~,
published June 29, 1977. Other ceramic foams which are
suitable for use in this invention are described in U. S.
Patents 3,893,917 and 3,962,081.
The degree to ~hich the filter permits the passage of
particles suspended in exhaust gases or traps them depends
upon the void volume or porosity and the pore size of the
filter. The porosity of the filter may comprise voids in
~5 a unitary structure or voids between individual components
of a particulate filter medium, such as ceramic beads.
The pore size and porosity of the filters used in this
invention may be varied to suit the particular gases being
~iltered. The filters generally have a pore size of from
about 2 to about 50 pores per 25 millimeters in length.
Generally, the filters have a porosity, i.e., a void volum~,
of from about 80 to about 95 percent of the total volume
occupied by the filter. The porosity is obtained from
measurements of the density of the filter material and
the weight using the formula
Porositv (~ weight of filter ~ x 100
density of filter material
x volume of filter
Suitable filters also generally have an air permeability
of from about 400 to about 8000 x 10 7 square
centimeters.
Tne coarse filter is located upstream in the flow of
the gases through the composition and the fine filter is
located downstream from the coarse filter in the flo~ of
gases through the composition. The fine filter has a
greater num~er of cells per unit length and a smaller cell
size than the coarse filter. The respective pore sizes
ana permeabilities may vary in accordance with the
particular nature of the gas under treatment. The cell
siæe~ o~ each filter are selecte~ to optimize the relative
degree of particulate trapping in each of the filters and
distribute the trappea particles between the filters so
that the pressure drop is minimized while good trappins
e~ficiency is maintained. The upstream tilter generally
has a relatively coarse pore size from about 2 to ab~ut 20
pores per 25 millimeters in length and an air permea~ility
of ~rom about 2500 to about 8000 x 10 7 square
centimeters. The downstream relatively fine filter
generally has a pore size of from about 15 to about 50
7 ~
pores per 25 milllmeters in length and an air permeabilit~
ot ~rom about 400 to about 2500 x 10 7 square
centimeters. Preferably, the coarse filter has from about
6 to about 2~ pores per 25 millimeters in length and the
fine filter has ~rom about 17 to about 30 pores per 25
millimeters in length.
Multiple coarse filters of the same or different cell
sizes may be employed in combination with multiple fine
filters of the same or different cell sizes to vary the
selective filtration and balance the pressure loss and
trapping efficiency needed for a particular qas treatment
application.
A catalyst material may be deposited on the filters.
The catalyst material is a catalytically active metal or
metal compound that is effective for the conversion of one
or more pollutants in the exhaust gases to innocuous
entltles. ~he pollutants may be the particulate and/or
gaseous pollutants present in the exhaust gases.
Generally, the catalyst material is an oxidation
catalyst. In internal combustion engine exhaust gas
treatment, the catalyst material ~,ay be effective for the
combustion of carbon particles. Suitable carbon
comb~stion catalyst materials include an element of the
first transition series, silver, hafnium, and mixtures
thereof. As used in this application, the elements of the
first transition series are vanadium, chromium, manganese,
iron, cobalt, nickel, copper, and zinc. The material may
be present in the form of the metal, metal o~ide, mixed
metal oxide, such as copper chromite or a perovskite, or
other catalytically-active metal compounds. Copper oxide
and chromium oxide are preferred.
- 7 -
ll )
When used in the treatment: of internal comhu~tion
engine exhaust gases~ the catalyst material is preferably
also effective for the convercion of hydrocarbons, carbon
monoxide, and/or nitrogen oxicle pollutants. Such catalyst
materials inclu~e a noble metal, an element of the first
transition series, and mixtures thereof. The noble metals
are gold, silver, and the platinum gro~p metals are
platinum, palladium, rhodium, ruthenium, iridium, and
osmium. The material may be in the form of the metal, the
metal oxide, or other catalytically active compounds of
the metal.
Platinum, palladi~m, and clhromium oxide are preferred
because of their high hydrocarbon oxidation activity at
relatively low temperatures. t'hromiu~ oxide is highly
preferred because it also is e~,pecially effective in the
combustion of diesel exhaust carbon particles. In an
especially preferred embodiment of this invention, the
catalyst material comprise~ a pl2tinum group metal suc~ as
platinum, palladium, or mixtures thereof and chromium
20 oxide. The com~ination of the platinum group metal and
chromium oxide catalyzes carbon combustion at a
significantly lo~er temperature lthan either component
alone.
The catalyst material may be deposited on the filters
in any desirable n,anner from aqueous or organic sol~tions
of a metal compound or complex or slurries of the me~al or
metal oxide~ Generally, deposiltion of this component is
effected by impregnating the fi:Lter ~ith an aqueous
solution of a water soluble, thermally decomposable
inorganic salt or complex of the particular metal or
- 8 ~
~.
7~
metals ana arying the impregnatea filter at a temperature
of from about 90 to about 250C. for about 2 to about 20
hours. The arlea filter may then be calcined at a
tem~erature of from about 300 to about 700C. for about 1
to about 3 hours. The calcination can be conducted in air
or other oxidizing gases or in a reducing gas such as
hyarogen if the metal form of the catalyst is ~esired.
Typical thermally decomposable ~water soluble metal
com~ounàs include the acetate, chloride, and nitrate.
Preferably, the platinum group metal component is
depositea in the ~orm of a sulfito complex as descrihed in
U. S. Patent 3,850,847 of Graham et al. to enhance its
disp~rsion ana surface area.
If a surface area higher than that of the fllter is
desired, the catalyst material may be supported on a
porous, refractory inorganic oxide. These oxides have a
high total pore volume and surface area. Generally, the
surface area of the refractory oxiae is at least about 75
square meters per gram, preferably from about 100 to about
300 square meters per gram, and the total pore volume i5
at least about 0.4 cubic centimeters per gram, preferably
from about 0.5 to ahout 2.0 cubic centimeters per gram.
The surface areas referred to throughout this
specification are determined by the nitrogen BET method.
The total pore volumes are determined by adding ~ater to a
po~cier sample to the point where incipient ~etness just
occurs.
Generally, the refractory oxide is composed
predominantly of oxides of one or more metals of Groups
II, III, and IV having atomic numbers not exceeding 40.
g
7.~3~
S~itable porous re~rac~ory inorganic oxides ca be
prepared by dehyarating, p~eferably s~b~tantially
co~,pletely, the hydrate form of th~ oxide by calcination
generally at temperatures ~f about 150 to about 800~C. for
perio~s of from about 1/2 to aboùt 6 ~o~rs. The preferred
refractory oxide is a transitional alumina, such as chi,
rho, kappa, gamma, delta, eta, and theta a~uminas,
especially gam~la alumina. A particularly pr~ferred gamma
al~mina may be prepared by calcining a boehmite-
pseudoboehn,ite intermediate al~mina prepared in accordance~ith U. S. Patent No. 4,154,812 of S3~chez et al~ at a
temperature of about 650C. for about 1 hour. Other
suitable oxides include, for example, calcined heryllia,
zirconia, magnesia, and mixtures of metal oxides such as
boria-alumina, silica-alumina, and the like.
In a highly preferred embodiment of t~is invention,
the catalyst ~,aterial compri~es the diesel exhaust
catalyst composition of U. S. Patent
No. 4,303,55~ by Ernest and Welsh
~0 entitled '`Composite Diesel Exhaust Catalyst'. This
catalyst comprise~ a mixture of catalytically-effective
amounts of at least one material selected from the group
consisting o~ a nobl~ metal, chromium, and catalytically-
active rompounds thereof supported on ~ porous refractory
~5 inorg~nic oxide and at least one bu~k material ~e1ected
from the group consi~ting o~ an element of the first
tran~ition series, silv~r, ~afnium, and catalytically-
active compounds thereof. The catalyst material ma~ compri~e
a mixture of from about 4n to about 60 weight percent of a
supported material comprising a platinum group metal, chromium
oxide, or mixtures thereof supported on a transitional alumina
and from about 40 to about 60 weight percent of a bulk material
comprising copper oxide. The bulk material may be prepared by
thermal decomposition of a compound of the desired metal.
35 Typically, the acetate, nitrate,
-- 10 --
carbonate, hycroxi~e, or chloride is heateo at a
temperature of from about 450 to about 800C. for a period
of from 1 to a~o~t 5 hours. The bulk material is slurried
with the supported rnaterial and deposited on the filters.
The refractory oxlde may be coated on the filter and
then the catalyst material deposited on the filter.
Pre~erably, ho~ever, the catalyst material is ~eposited on
the refractory oxide and then the supported catalyst is
deposited on the filter. For example, a suitable
catalytic component may be added to an aqueous slurry of
the oxide and the mixture deposited on the filter by
conventional methods, such as dipping or spraying.
The coatea tilter is then dried at a temperature of
from about 90C to about 250C. for about 1 to about 4
15 hours to remove the solvent and deposit the solids in an
adherent film on the filter. The dried filter may be
ca~cineu a~ Iro~, about 250C. to about 800C. for about 1
to about 4 hours.
The amount of the catalyst material that is coated on
20 the filter depends on economics, size limitations, and
design characteristics. The catalyst material generally
comprises about 1 to about 50 and preferably from about 2
to a~out 30 percent based upon the weight of the filter.
During use, the catalyst composition is typically
25 disposed so that it occupies the major part of the cross-
sectional area cf a housing having a gas inlet and a gas
outlet. The composition typically has the general shape
o~ the housing and is positioned in the housing with the
general direction of gas flo~ between the inlet and
-- 11 --
~g~ 7
.
outlet. The filters nay ~e adhered together or spaced
apart.
In employing the composltion of this invention in the
treatment of internal combustion engine exha~st gases, the
gases are contacted with the composition and the ~ead and
carbon particles are trapped in the filters. The carbon
particles are co~,b~sted along with the gaseo~s pollutants
in the exhaust gases. The accumulated carbon partic~late
deposits may be periodically removed by throttling the
engine to reduce the air flow with fuel flow remaining
constant an~ increase the exhaust temperature. At the
resulting higher exhaust temperatures, the combustion of
the particulates will be achievea quite rapidly in the
presence of the catalytic filter of this invention.
In addition to the filtration of lead and~or carbon
particles from internal combustion engine èxhaust
emissions, t~e ~ilter of this invention may he used, for
example, to reauce particulate emissions from other mobile
po~er plants as well as stationary sources, such as gas
turbine catalytic combustors, which utiliæe fuels which
produce partic~late pollutant~.
This invention is illustrated by the following
examples in which all parts and percentages are by weight
unless otherwise indicate~.
Example 1
A composition of t~is invention which comprised the
"Diesel Exhaust Catalyst" of U. S~ Patent
No. 4,303,552 by Ernest and ~elsh
deposited on comn,ercially available ceramic foam monoliths
o~ Bridgestone Tire Co., I,td,, Tokyo, Japan was prepared
as follows.
- 12 -
i
~ ~7~7
A boe~mite-pseuaoboeh~,ite intermediate hyarated
alumina powder prepared in accordance with the process of
U. S. Patent 9,154,812 of Sanchez et al. was calcined in
air at 649C. for one hour. The resulting gamma al~mina
had a total pore volume of 1.56 to 1.68 cubic centimeters
per gram and a total volatiles content (loss in weight
a~ter heating for 1 hour at 954C.) of 1.8 to 3.3 percent.
3000 grams of the calcined alumina po~der were
in,pregnated with 840 milliliters of a solution of 260.29
grams oE chromic acetate in 840 milliliters of deionized
water and 5 milliliters of glacial acetic acid. The
impregnated powder was allowed to dry in air for 1/2 hour
and then drie~ for 16 hours at 135C. The powder was
screened t~rough a 20 mesh UO S. Standard Sieve and
calcined at 843-871C. for 1 hour. The calcined
chromia-alumina powder had a surface area of 107 square
meters per gram and containea nominally 10 weight percent
chromia.
150 grams of copper oxi~e freshly prepared by
decomposing cupric acetate for 3 hours at 538C. in a
muffle furnace and 150 grams of the chromia-alumina powder
were separately ball milled with deionized water for 16
hours. The resulting slurries were combined in 3 1 to 1
ratio (solids basis) and homogenized. The pH of the
combined slurry was adjusted to 3.5 with nitric acid. The
procedure was repeated four times using a 1 to 1 weight
ratio of the copper oxide and the chromia-alumina but
varying the solids content of the slurry by adding
additional water. Each slurry was coated on a Bridgestone
ceramic foam monolith of 19.37 to 14.50 centimeters in
~_,9~
diameter anà 7.30 to 7.67 centimeters in length. The
excess slurry was blown out of the c~ated monolithc and
the monoliths were the~ ariea at 135CC. for 16 hours and
activated for 1 hour at 428C. The number of cells per 25
millimeters in lengt~ of the monoliths, the solids co~tent
of the slurries, and the amounts and percentages of the
catal~st material coatea on the monoliths are shown in
Table I.
Table I
10 Nominal No. ofSolids Pickup-
cells/25 mm. Content ~ qrams ~Coating
6 27 44.4 9.1
13 23 45.2 g.l
14 35.9 6.1
14 41.1 7.7
Three of the 13 size monoliths and one of the 30 size
monoliths were joined together and placed in a cy~indrical
container. The 13 size monoliths were positioned as the
20 inlet structure, the first cer,tral section adjacent the
ir,let structure, and the second central structure adjacent
the outlet structure and the 30 size monolith ~as
positioned as the outlet structure. The composition was
tested in the treatr,~ent of an exhaust gas from one bank
(4 cylinders) of a 5.7 liter Oldsmobile diesel engine at a
gas flow rate of 90 cubic feet per minute. I~he test was
run for about 6 hours and measurements of back pressure
and weight of emissions were taken approximately every
hour. Good average trapping efficiency and relatively low
30 pressure increase over the duration of the test were
observed.
- ~4 -
7~
Another confiq~ration was prepared in which the 6 size
monoliths were positioned as the inlet section and the
first and second central section and the 20 size was used
as the outlet section and tested ~y the same method. It
was Eound that there was a much lower pressure increase
but a much lower average trapping efficiency than in the
~irst conflguration.
Example 2
The proceaure of Example 1 was repeated except that
platinum and pallaàium were incorporated in the
chromia-alumina powder as follows.
320 grams of the chromia-alumina powder of Example 1
were impregnated with a mixed solution of platinu~ and
pallladium prepared by bubbling sulfur dioxide gas at 2
millimoles per minute into 250 milliliters of deionized
water for 11.0 minutes and adding 4.022 milliliters of
palla~ium nitrate solution having a titer o~ 129.27 grams
of pallaaium per liter of solution. 112.012 grams of
(NH4)6Pt(SO3)4 solution having a titer of 92.85
grams of platinum per kilogram of solution and 3.2 grams
of dibasic ammonium citrate were then added to the
solution. The total volume of the solution was increased
to 434 milliliters by addition of deionized water. ~he
powder was impregnated with this volume of solution, air
d;ied for one hour, and then oven dried for 16 hours at
135C. The powder was finally activated in air for one
hour at 538C. T~e powder nominally contained 3.30
percent of platinum and palladium in a 20 to 1 weight
ratio.
- 15 -
339 grarr's of the ac:ivated pow~er and 300 grarns of
freshly prepared copper oxide as in Example 1 were
separately hall n,illed with ~eionized water at 28~ solids
content for 16 hours. The two slips were combined in a 1
to 1 ratio (solids basis) and homogenized. Other slurries
were prepared using a 1 to 1 ratio of the powders but
varying the solids content by adding additional water.
The pH of the slurries was adjusted to 3.5 with nitric
acid an~ the slurries were coated on Bridqestone ceramic
foam monoliths of varying cell sizes. The coated
monoliths were dried at 135C. for 16 hours and then
activated for one hour at 428C. The pertinent data are
shown in Table II.
- 16 -
lrL O ~-L 60-~ ~1 0
5L-0 L-8 Z9-5~ ~1 0
9ZL'0 S-L 86-~ 91 oz
LL-0 S'L ~9'~ 91 oz
19L 0 ~-8 51-9~ ~Z 1
S9L 0 5 8 8-9~ ~Z 1
61L-0 9-L 55'~ 8Z 9
18L-0 L'8 9'Lb 8Z 9
;Inlpellea ~~ 6uT~eo;) s~e~g dn~ld ~; ~u~uo;~ S~llos ~ 5z/s~ o
wnul~ela ~o swe~g N leul~o~d
e,~
~7~
The monoliths may be joinea in the con~igurations of
Example 1 and used in the treatment of ~iesel exhaust
gases.
