Molecular sieve abstract
A catalytic converter for treating exhaust gas from an internal
combustion engine containing a catalyst characterized by having
platinum or a platinum alloy dispersed on alumina particles and
predominantly on the exterior surface of molecular sieve crystals.
Also provided is a catalyst for such a converter and processes for
manufacture of such catalyst. In addition, methods for treating
exhaust gas from an internal combustion engine to reduce the amounts
of hydrocarbon, carbon monoxide and nitrogen oxides emitted to the
atmosphere are included.
Molecular sieve claims
We claim:
1. A catalytic converter for treating exhaust gas from an internal
combustion engine, comprising: an exhaust gas processing vessel
of suitable size and shape, adapted to receive exhaust directly
from the engine; and an effective amount, contained in said vessel,
of a catalyst coating uniformly dispersed on a substrate adapted
to fit within said vessel, said coating comprising: at least one
type of molecular sieve crystals having diameters within a range
of 0.5 .mu.m. to 50 .mu.m., said crystals having a catalytically
effective amount of noble metal dispersed thereon in a manner such
that at least 90% of the noble metal on said crystals is located
within 500 .ANG. of the exterior surface of said crystals, and alumina
particles between about 1 .mu.m. and about 15 .mu.m. in diameter
having a catalytically effective amount of noble metal dispersed
thereon.
2. The catalytic converter of claim 1 wherein said crystals and
alumina particles are evenly dispersed throughout said coating and
said coating is between 1 .mu.m. and 200 .mu.m. thick.
3. The catalytic converter of claim 1 wherein the substrate is
a monolith.
4. The catalytic converter of claim 1 wherein the noble metal is
selected from the group consisting of Pt and Pt alloyed with Rh,
Ir, Pd, Ag, or Au.
5. The catalytic converter of claim 1 wherein only one type of
molecular sieve crystals is employed.
6. The catalytic converter of claim 1 wherein said coating comprises:
between about 10 wt. % and about 90 wt. % of said molecular sieve
crystals; between about 10 wt. % and about 90 wt. % of said alumina
particles; and wherein the amount of said noble metal on said molecular
sieve crystals is between about 10%and about 80%of the amount of
said metal in said coating.
7. A catalyst for treating exhaust gas from internal combustion
engines, comprising a catalyst coating uniformly dispersed on a
substrate, said coating comprising: at least one type of molecular
sieve crystals having diameters within a range of 0.5 .mu.m. to
50 .mu.m., said crystals having a catalytically effective amount
of noble metal dispersed thereon in a manner such that at least
90%of the noble metal is located within 500 .ANG. of the exterior
surface of said crystals, and alumina particles between about 1
.mu.m. and about 15 .mu.m. in diameter having a catalytically effective
amount of noble metal dispersed thereon.
8. A process for making a catalyst containing alumina particles
impregnated with noble metal and at least one type of molecular
sieve crystals having noble metal predominantly on the exterior
surface of said molecular sieve crystals, comprising: a) preparing
an aqueous suspension of at least one type of molecular sieve crystals
and a noble metal reagent, said reagent being of the type that,
when suspended in water, will chemisorb onto the exterior 500 .ANG.
of said crystals by forming a covalent bond between said reagent
and said crystals, said crystals having been subjected to pre-treatment
to establish an alumina film on their exterior surfaces; b) adding
sufficient base to the suspension to raise its pH to above about
7 said base being of a type that will decompose to yield gaseous
products when its temperature is raised above about 150.degree.
C.; c) stirring for at least one hour; d) optionally adding an aqueous
suspension of at least one type of molecular sieve crystals different
from the type employed in step (a) to the product of step (c); e)
stirring the product of step (d) for at least one hour; f) milling
alumina particles, thereby reducing said particles' mean diameter
to between about 1 .mu.m. and about 15 .mu.m.; g) preparing an aqueous
suspension of said alumina particles and a noble metal reagent;
h) stirring the product of step (g) for at least one hour and then
mixing it with the product of step (e); i) coating a substrate with
the product of step (h) thereby preparing a coated substrate; and
j) drying and calcining said coated substrate.
9. A process for making a catalyst containing alumina particles
impregnated with noble metal and at least one type of molecular
sieve crystals having noble metal predominantly on the exterior
surface of said molecular sieve crystals, comprising: a) preparing
an aqueous suspension of at least one type of molecular sieve crystals,
said suspension having a pH above 10 and a noble metal reagent,
said reagent being of the type that, when suspended in water, will
chemisorb onto the exterior 500 .ANG. of said crystals by forming
an ionic bond between said reagent and said crystals; b) stirring
for at least one hour; c) optionally adding an aqueous suspension
of at least one type of molecular sieve crystals different from
the type employed in step (a) to the product of step (b); d) stirring
the product of step (c) for at least one hour; e) milling alumina
particles, thereby reducing said particles' mean diameter to between
about 1 .mu.m. and about 15 .mu.m.; f) preparing an aqueous suspension
of said alumina particles and a noble metal reagent; g) stirring
the product of step (f) for at least one hour and then mixing it
with the product of step (d); h) coating a substrate with the product
of step (g) thereby preparing a coated substrate; and i) drying
and calcining said coated substrate.
10. A process for making a catalyst containing alumina particles
impregnated with noble metal and at least one type of molecular
sieve crystals having noble metal predominantly on the exterior
surface of said molecular sieve crystals, comprising: a) preparing
an aqueous suspension of at least one type of molecular sieve crystals
and an amount of a hydrophilic polymer having a density of between
about 0.8 and about 1.0 gm/cm.sup.3 said amount of hydrophilic
polymer being sufficient to fill the pores of said molecular sieve
crystals; b) adding a noble metal reagent to the product of step
(a); c) adding sufficient base to the suspension to raise its pH
to above about 7 said base being of a type that will decompose
to yield gaseous products when its temperature is raised above about
150.degree. C.; d) stirring for at least one hour; e) optionally
adding an aqueous suspension of at least one type of molecular sieve
crystals different from the type employed in step (a) to the product
of step (d); f) stirring the product of step (e) for at least one
hour; g) milling alumina particles, thereby reducing said particles'
mean diameter to between about 1 .mu.m. and about 15 .mu.m.; h)
preparing an aqueous suspension of said alumina particles and a
noble metal reagent; i) stirring the product of step (h) for at
least one hour and then mixing it with the product of step (f);
j) coating a substrate with the product of step (i) thereby preparing
a coated substrate; and k) drying and calcining said coated substrate.
11. A process for making a catalyst containing alumina particles
impregnated with noble metal and at least one type of molecular
sieve crystals having noble metal predominantly on the exterior
surface of said molecular sieve crystals, comprising: a) preparing
an aqueous suspension of at least one type of molecular sieve crystals
and colloidal particles of noble metal, the size of said noble metal
particles being at least twice the largest crystallographically-defined
pore diameter of the molecular sieve crystals; b) adjusting the
suspension pH to between about 7 and about 8.; c) stirring for at
least one hour; d) optionally adding an aqueous suspension of at
least one type of molecular sieve crystals different from the type
employed in step (a) to the product of step (c); e) stirring the
product of step (d) for at least one hour; f) milling alumina particles,
thereby reducing said particles' mean diameter to between about
1 .mu.m. and about 15 .mu.m.; g) preparing an aqueous suspension
of said alumina particles and a noble metal reagent; h) stirring
the product of step (g) for at least one hour and then mixing it
with the product of step (e); i) coating a substrate with the product
of step (h) thereby preparing a coated substrate; and j) drying
and calcining said coated substrate.
12. A method for treating exhaust gas from a motor vehicle diesel
engine to reduce the amounts of hydrocarbon, carbon monoxide and
NO.sub.x emitted to the atmosphere which comprises passing the exhaust
gas through the catalytic converter of claim 1.
13. A method for treating exhaust gas from a motor vehicle diesel
engine to reduce the amounts of hydrocarbon, carbon monoxide and
NO.sub.x emitted to the atmosphere comprising passing the exhaust
gas through a converter containing the catalyst made by the process
of claim 8 9 10 or 11.
Molecular sieve description
[0001] This application is a continuation-in-part of application
Ser. No. 09/012095 filed Jan. 22 1998 pending, which is a continuation-in-part
of application Ser. No. 08/788214 filed Jan. 27 1997 pending.
FIELD OF THE INVENTION
[0002] This invention relates to a catalytic converter containing
therein a catalyst having noble metal dispersed on alumina and on
the surface of molecular sieve crystals. This invention further
relates to a catalyst suitable for such converter, a process for
treating the exhaust gas from an internal combustion engine, and
processes for making a catalyst having noble metal dispersed on
alumina and on the exterior surface of molecular sieve crystals.
BACKGROUND OF THE INVENTION
[0003] Catalytic converters containing various catalysts have been
employed for years by automobile manufacturers to meet the ever-more-stringent
regulations on emissions of hydrocarbon, carbon monoxide, and particularly,
nitrogen oxides from internal combustion engines. Concurrently with
the promulgation of these ever-tightening regulations on emissions,
automotive engine manufacturers have worked to improve the fuel
economy of such engines. This effort has led to the development
of engines that operate with an excess of air beyond that which
is required to consume stoichiometrically the fuel admitted to such
engines ("lean-burn engines"). The advent of lean-burn
engines has, in turn, exacerbated the problem of reducing emission
of nitrogen oxides from internal combustion engines.
[0004] Various techniques have been developed to treat exhaust
gas from internal combustion engines. One approach has been to provide
multiple catalyst chambers in the exhaust system filled with different
catalysts suited to particular tasks, such as that taught by Takeshima,
et al. in U.S. Pat. No. 5233830.
[0005] Another approach is to provide a control system operating
in conjunction with a catalytic converter, such as taught by Hirota,
etal. in U.S. Pat. Nos. 5201802 and 5189876 and also by Takeshima
in U.S. Pat. No. 5029061.
[0006] Clearly, there is a need for a simple system capable of
treating exhaust gas from internal combustion engines, particularly
the exhaust gas from lean-burn engines. The present invention is
such a system, employing as it does a single catalytic converter
having a single, unique catalyst contained therein which is capable
of reducing the amount of hydrocarbon, carbon monoxide and, particularly,
nitrogen oxides contained in exhaust gas prior to such gas's discharge
to the atmosphere.
[0007] As stated above, the key element in such a system is the
discovery of a catalyst which is effective in oxidizing hydrocarbon
and carbon monoxide while being surprisingly effective at concurrently
reducing nitrogen oxides in the environment presented by exhaust
gas from internal combustion engines. Numerous inventions in this
area have been taught by others skilled in the art, but none of
these resembles more than superficially the catalyst of the present
invention. For instance, U.S. Pat. No. 5376610 (Takahata, et al.)
teaches, as part of a multi-catalyst system for exhaust gas purification,
a catalyst consisting of alumina, and optionally zeolite, with Pt
and/or Pd impregnated therein.
[0008] U.S. Pat. No. 5292991 (Lachman, et al.) teaches a catalyst
consisting of a mixture of alumina and zeolite on a substrate with
noble metal predominately dispersed on the alumina.
[0009] The teachings of U.S. Pat. No. 4082699 (Petrow, et al.)
superficially resemble the present invention. Petrow, et al. teach
a catalyst comprising pellets of alumina impregnated with platinum
and assert that the platinum is deposited close to the pellet's
surface, with alumina particles inside the pellet having no platinum.
This teaching is, however, very different from that of the present
invention wherein noble metal is deposited on virtually all alumina
particles and on the external surfaces of the molecular sieve particles.
[0010] Patents have also been granted for catalysts intended for
other uses, and some of these also closely resemble, at least superficially,
the catalyst of the present invention. U.S. Pat. No. 5314854 (Galperin)
teaches a catalyst designed primarily for the dehydrocyclization
of paraffins to aromatics that consists of large pore molecular
sieves mixed with binder and formed into pellets, spheres, or other
such particles, said particles then having a surface-layer palladium
component and uniformly dispersed platinum.
[0011] U.S. Pat. No. 4992158 (Schweizer) teaches a catalyst for
reforming hydrocarbons having boiling points in the range of motor
gasoline that is characterized as L-type zeolite not fully impregnated
with Pt or Pd, using Pt- or Pd- acetylacetonate as the noble metal
compound to treat the L-zeolite.
[0012] U.S. Pat. No. 5017541 (Schmidt, et al.) teaches a catalyst
for the isomerization of alkanes consisting of particles of refractory
inorganic oxide having a slightly higher concentration of a metal
from the platinum group at the surface of such particles than at
their center. The width of the surface-enriched band is 100-150
.mu.m while the particles themselves have diameters of about 0.7-3.3
.mu.m.
[0013] Still more patents have been granted for the preparation
of noble metal/zeolite catalysts without regard to the use of same.
U.S. Pat. No. 4556646 (Bezman) teaches a method for the preparation
of a catalyst consisting of an inorganic oxide, a zeolite, and a
noble metal in which ammonium nitrate is employed as an impregnating
agent for Pd to produce a uniform distribution of the metal throughout
the catalyst particles. Data are presented showing that, if ammonium
nitrate is not used, Pd is concentrated in the outer shell of the
catalyst particles. In this case, however, the Pd is deposited on
the inorganic oxide as well as the zeolite, and the inventor suggests
that deposition of Pd in the outer shell is a condition to be avoided.
[0014] U.S. Pat. No. 4882307 (Tsao) teaches the use of various
salts, including sulfites, of cationic Pt complexes as an agent
for the deposition of Pt into zeolite as a way of yielding highly
dispersed Pt crystallites, after appropriate treatments.
[0015] U.S. Pat. No. 4683214 (Angevine, et al.) also teaches
the use of sulfite salts of cationic noble metal complexes as a
means of depositing noble metal on zeolite to yield, after appropriate
treatment, very highly dispersed metal crystallites.
[0016] And, finally, U.S. Pat. No. 5290534 (Tsao) teaches that
when a noble metal precursor is placed into a reactor during the
production or synthesis of ZSM-18 substantially all of the noble
metal remains on the outside of the synthesized ZSM-18. No indication
is given that this is a desirable result, however, since the point
of this invention is to provide a method of removing organic material
blocking the pores of a zeolite.
SUMMARY OF THE INVENTION
[0017] It is a feature of the invention to provide superior catalytic
converters for removing NO.sub.x from the exhaust of internal combustion
engines that are operating with the air-to-fuel ratio in the lean
range, that is, where the amount of oxygen supplied is greater than
the stoichiometric amount needed to support combustion.
[0018] This invention provides for a catalytic converter suitable
for treating exhaust gas from an internal combustion engine, and
particularly for treating the gas from a lean-bum motor vehicle
engine. The catalytic converter of the present invention comprises
an exhaust gas processing vessel of suitable size and shape that
is adapted to receive exhaust directly from the engine. Contained
in this vessel is an effective amount of a catalyst coating uniformly
dispersed on a suitable substrate. The substrate, in turn, is adapted
to fit within the vessel in such a way as to assure that the exhaust
gas adequately and effectively contacts the catalyst coating on
the substrate.
[0019] The catalyst coating for use in this invention is characterized
as comprising molecular sieve crystals on the surface of which has
been uniformly dispersed a catalytically effective amount of a noble
metal, preferably platinum or a platinum alloy and alumina impregnated
with a catalytically effective amount of a noble metal, preferably
platinum or a platinum alloy. The molecular sieve crystals on which
the noble metal has been dispersed have diameters within a range
of about 0.5 to about 50 .mu.m. The portion of the noble metal that
is dispersed on the surface of the molecular sieve crystals is done
so in such a manner as to locate at least 90% of the noble metal
within 50 nm (500 .ANG.) of the exterior surface of the crystals.
The alumina particles upon which the remainder of the noble metal
is dispersed are also small, preferably about 5 .mu.m in size.
[0020] A wide range in the relative amounts of molecular sieve
and alumina, and in the proportion of noble metal dispersed on each,
is permitted in the catalyst coating for use in this invention.
The finished coating may contain amounts of molecular sieve and
alumina such that the weight ratio of molecular sieve to alumina
is between about 0.1 and about 10 with the preferred ratio being
between about 1 and about 3. The molecular sieve in the finished
coating may contain from about 10 wt. % to about 90 wt. % of the
total amount of noble metal dispersed in the finished coating, preferably
from about 20 wt. % to about 60 wt. %, with the balance of the noble
metal dispersed on the alumina contained in the coating.
[0021] This invention also provides a catalyst for such a catalytic
converter and processes for making such catalyst. In one process
for making the catalyst according to this invention such that part
of the noble metal is dispersed predominately on the exterior surface
of the molecular sieve crystals and the remainder of the noble metal
is dispersed on the alumina particles, the first step is to prepare
two aqueous suspensions, one such suspension comprising molecular
sieve crystals and a noble metal reagent that will chemisorb onto
the exterior 50 nm (500 .ANG.) of the said crystals and the other
aqueous suspension comprising alumina particles previously milled
to a suitable size and a noble metal reagent.
[0022] The noble metal reagent mixed with the alumina particles
in aqueous suspension need not be the same reagent as that used
in admixture with the molecular sieve, but since using the same
reagent is usually more convenient, such is preferred. Prior to
preparing the suspension, the alumina should be milled until its
mean particle diameter is reduced to the desired size. An aqueous
suspension comprising the milled alumina, without the normal addition
of acid that partially peptizes the alumina, and the desired amount
of noble metal reagent is then formed and stirred for sufficient
time to disperse the reagent evenly on the alumina particles, usually
about two hours. This alumina suspension is then later combined
with an aqueous suspension comprising molecular sieve crystals and
a noble metal reagent that will chemisorb onto the exterior 50 nm
(500 .ANG.), coated onto a suitable substrate, dried and calcined
at a temperature sufficient to decompose the base reagent material.
[0023] Depending on the noble metal reagent used, bonding of the
reagent to the sieve will be either covalent or ionic. If the reagent
chosen forms a covalent bond, it is important that the molecular
sieve crystals used in this process be modified to have a very thin
film of aluminum oxide on their exterior surfaces. The choice of
method for forming this thin alumina film depends on the Si/Al ratio
of the zeolitic precursor. Sufficient acid or base may be added
to the suspension to adjust the pH to between about 7-8. Any pH
adjustment should be done using a material that will decompose to
yield gaseous products when its temperature is raised above about
150.degree. C. If the reagent chosen forms an ionic bond to the
sieve, no modification to the zeolite to form a thin alumina film
is necessary, but the pH of the aqueous suspension of molecular
sieve crystals and noble metal reagent should be above 10 and the
chosen reagent must be a cationic noble-metal complex whose kinetic
radius of its smallest conformation is larger than the largest crystallographically-defined
pore of the zeolite chosen. Appropriate noble metal reagents of
the type that form a covalent bond that, when suspended in water,
will chemisorb onto the exterior 50 nm (500 .ANG.) of said crystals
include Pt sulfite and [N(C.sub.2H.sub.5).sub.4].sub.2 [pt(OH).sub.6].
Appropriate noble metal reagents of the type that form anionic bond
that, when suspended in water, will chemisorb onto the exterior
50 nm (500 .ANG.) of said crystals, include [Pt(NH.sub.2CH.sub.2CH.sub.2NH.sub.2).sub.2][OH].sub.2
and [Pt(NH.sub.2CH.sub.2CH.sub.2NH.sub.2).sub.2][CO.sub.3]. After
mixing the reagent with the molecular sieve the suspension is stirred
for sufficient time to disperse the reagent evenly on the molecular
sieve surface, usually about two hours.
[0024] Alternatively, an aqueous suspension of molecular sieve
crystals of at least one type having noble metal dispersed predominantly
on the exterior surface of the crystals can be made by a process
in which the first step is to prepare an aqueous suspension of the
crystals with a hydrophilic polymer having a density between about
0.8 gm/cm.sup.3 and about 1.0 gm/cm.sup.3 the amount of polymer
used being sufficient to fill the pores of the molecular sieve crystals
used. Thereafter a suitable noble metal reagent (which need not
be chemisorbing) is added to the suspension and the pH is adjusted
to between about 7 and 8. To form the finished coating of this invention,
the molecular sieve suspension is combined with the alumina suspension
as above, then coated on a suitable substrate, dried and calcined.
In the process of heating, the material used to adjust the pH and
the hydrophilic polymer are completely burned off and vaporized,
thereby leaving the noble metal dispersed uniformly on the first
50 nm (500 .ANG.) of the molecular sieve crystals.
[0025] A third method for making an aqueous suspension containing
molecular sieve crystals of at least one type having noble metal
dispersed predominantly on the exterior surface of the crystals
employs the use of colloidal particles of platinum or a platinum
alloy of a size substantially greater, by at least a factor of two,
than the largest crystallographically defined pore diameter of the
molecular sieve crystals onto which the noble metal is to be deposited.
[0026] Finally, this invention provides processes for treating
exhaust gas from an internal combustion engine using the catalytic
converter and catalyst of the present invention.
BRIEF DESCRIPTION OF THE FIGURES
[0027] FIG. 1 is a micrograph of the molecular sieve powder used
to coat the monolith in one embodiment of the present invention.
[0028] FIG. 2 is a micrograph of a thin section of the inside,
together with portions of their exterior edges, of two molecular
sieve crystals taken from the coating of a catalyst prepared by
one of the methods presented herein but using a platinum reagent
that does not chemisorb onto the exterior surface of molecular sieve
crystals. This catalyst is typical of platinum-containing molecular
sieve catalysts that have been suggested in the prior art for use
in treating exhaust gas from an internal combustion engine.
[0029] FIG. 3 presents data comparing the performance (shown as
NO.sub.x conversion) of a catalytic converter loaded with a catalyst
having noble metal on the surface of molecular sieve crystals (but
with no noble metal on alumina) against that of a typical catalytic
converter using a typical catalyst when used to treat exhaust gas
from the same lean-bum diesel engine.
[0030] FIG. 4 presents data comparing the performance (shown as
NO.sub.x conversion) of three catalysts having noble metal on the
surface of molecular sieve crystals (but with no noble metal on
alumina) against that of a similar catalyst prepared without using
the process taught by the present invention.
[0031] FIG. 5 presents data comparing the performance (shown as
NO.sub.x conversion) of a catalytic converter loaded with a catalyst
having noble metal on the surface of molecular sieve crystals (but
with no noble metal on alumina) against that of a typical catalytic
converter using a catalyst similar to that taught in Lachman '991
when used to treat exhaust gas from the same lean-bum diesel engine.
[0032] FIG. 6 presents data comparing the performance (shown as
NO.sub.x conversion) of a catalyst of the present invention (using
Y zeolite) wherein about 75% of the platinum is dispersed on the
alumina against that of the same catalyst wherein an equivalent
amount of platinum is dispersed entirely on the Y zeolite.
[0033] FIG. 7 presents data comparing the performance (shown as
NO.sub.x conversion) of a catalyst of the present invention (using
.beta. zeolite) wherein about 75% of the platinum is dispersed on
the alumina against that of the same catalyst wherein a higher amount
of platinum is dispersed entirely on the .beta. zeolite.
DETAILED DESCRIPTION OF THE INVENTION
[0034] The catalytic converter of the present invention comprises
a vessel and a catalyst contained in the vessel.
Vessel
[0035] The exhaust gas processing vessel may be any suitable shape
and configuration, with cylindrical or "flattened" cylindrical
(i.e., an extended oval) being the preferred shapes, that provides
a substantially uniform distribution of exhaust gas throughout and
an efficient flow over the catalyst to be contained therein. The
vessel should have sufficient void volume to contain a catalyst
sized so that the gas hourly space velocity ("GHSV") over
the catalyst is no greater than about 150000 hr.sup.-1 when the
engine producing the exhaust gas to be treated is operating at its
maximum sustainable load. The vessel should be fitted at its inlet
end with a tubular conduit adapted to receive exhaust from the engine
exhaust manifold, and at its outlet end with a tubular conduit adapted
to transmit treated exhaust to a muffler or directly to the atmosphere.
Materials of construction of the vessel and inlet and outlet conduits
are well known to those skilled in the art and may be of any metal
or composite able to withstand temperatures of up to about 1200.degree.
C. with minimal corrosion from the compounds normally contained
in internal combustion engine exhaust gas.
Catalyst
[0036] The catalyst is comprised of a substrate on which has been
suitably applied an effective amount of a catalytically active coating
comprising at least one type of molecular sieve crystals, with platinum
or a platinum alloy uniformly dispersed on the exterior surface
of such crystals, and alumina with platinum or a platinum alloy
dispersed thereon. The coating may also contain a binder and one
or more types of molecular sieve crystals.
Substrate
[0037] The substrate may be any of the shapes employed by those
skilled in the art such as extrudates, spheres, beads, tablets,
and the like, but monoliths, either metallic or ceramic, are preferred.
The substrate should be inert in the sense that it does not react
with the coating or with the exhaust gas to which it is exposed
during engine operation.
[0038] If a monolith is employed as the substrate, it should be
sized and shaped so that it fits within the vessel and leaves no
gaps between the vessel interior wall and the outside surface of
the monolith which might result in blow-by of the exhaust gas past
the monolith (i.e., bypassing of the catalyst). Ceramic monoliths,
if employed, should be provided with a mat-mount between the exterior
surface of the monolith and the vessel wall, as is customary in
the industry, to cushion vibration during engine operation and also
to prevent blow-by of the engine exhaust gas past the catalyst.
Metallic monoliths should be attached to the vessel wall, as is
customary. There should be provided sufficient monolith to result
in a GHSV as defined above in the discussion on Vessel.
Coating
[0039] The catalyst coating for use in this invention is characterized
as comprising molecular sieve crystals on the surface of which has
been uniformly dispersed a catalytically effective amount of a noble
metal, preferably platinum or a platinum alloy, and alumina impregnated
with a catalytically effective amount of a noble metal, preferably
platinum or a platinum alloy. The molecular sieve crystals on which
the noble metal has been dispersed have diameters within a range
of about 0.5 to about 50 .mu.m. The portion of the noble metal that
is dispersed on the surface of the molecular sieve crystals is done
so in such a manner as to locate at least 90% of the noble metal
within 50 nm (500 .ANG.) of the exterior surface of the crystals.
The alumina particles upon which the remainder of the noble metal
is dispersed are also small, preferably about 5 .mu.m in size. It
is important that the alumina and molecular sieve crystals be thoroughly
mixed so that both are uniformly dispersed throughout the coating.
[0040] The coating should be uniformly applied to the substrate.
The coating's thickness should be no more than about 200 microns
and no less than about 1 micron, with a coating thickness averaging
between about 20 microns and 40 microns being preferred. Any method
known to those skilled in the art including, without limitation,
immersion of the substrate in an aqueous suspension of the desired
coating or spraying of the suspension onto the substrate, may be
employed to achieve the desired coating thickness and uniformity
of application. The preferred method of application onto monoliths
is "piston coating", which involves placing the monolith
above a reservoir containing an amount of the coating, compressing
the coating up into the monolith with a piston, withdrawing the
piston to permit the coating to drain back into the reservoir, and
then immediately clearing the channels of the monolith with a slotted
knife that emits compressed air. This method is particularly useful
when applying coatings containing large mass fractions of molecular
sieves since the sieve density is quite low.
[0041] Following application of the coating to the substrate, the
coated substrate is dried and then calcined at a temperature of
between about 400 and about 550.degree. C. for 2 to 4 hours.
Molecular Sieve
[0042] For the molecular sieve component of the catalytic coating
of the present invention, only a single type of molecular sieve
crystal is necessary. Any type of molecular sieve having an average
pore size between about 0.4 nm (4 .ANG.) and about 1 nm (10 .ANG.)
may be used, with the preferred type being Beta zeolite. The size
of the crystals, regardless of the type chosen, should be between
0.5 and 50 microns, however, and a crystal size between about 1
and about 2 microns is preferred. If noble metal is to be deposited
by covalent chemisorption, the silica/alumina ratio should be between
about 10 and about 100000 preferably between about 10 and about
100. If noble metal is to be deposited by ionic chemisorption, the
silica/alumina ratio should be between about 20 and about 100000
preferably between about 20 and about 100.
[0043] If the method to be used for noble metal deposition is covalent
chemisorption, the molecular sieve should be modified to establish
a thin alumina film on its exterior surfaces, either by steaming
to mildly de-aluminate sieve crystals having a low Si/Al ratio or
by treatment with an aqueous solution of very small pseudo-boehmite
crystallites for sieves with a high Si/Al ratio. Optionally, the
sieve crystals may be treated to expose acid sites on the crystal
surface.
[0044] Some applications, however, such as those in which it is
desirable to adsorb hydrocarbon during cold start of an engine,
might benefit from incorporation into the coating a second, or even
a third, type of sieve having characteristics desirable to that
application. For instance, in a catalytic converter is intended
for treatment of diesel exhaust, it might be desirable to employ
two types of sieve, one with an average pore size less than 6 .ANG.
on which platinum or a platinum alloy has been dispersed and another
(with an average pore size greater than 0.6 nm) which is capable
of adsorbing the large hydrocarbon molecules present in the exhaust
gas from such diesel engines. This larger-pore sieve would not necessarily
require platinum or a platinum alloy dispersed thereon since conversion
of hydrocarbon, carbon monoxide, and nitrogen oxides would be catalyzed
primarily by the smaller-pore sieve.
Alumina
[0045] The alumina particles of the catalytic coating of the present
invention may be of any of the various phases of alumina, with the
theta phase being preferred.
[0046] Alumina particles of suitable size may readily be made by
methods known to those skilled in the art, with the preferred method
being wet milling. Although alumina particles of mean diameter between
about 1 .mu.m and about 15 .mu.m are acceptable, particles having
a mean diameter of about 5 .mu.m are preferred. However, the normal
addition of acid that partially peptizes the alumina should be avoided.
[0047] The relative proportions of finely divided zeolite crystals
and alumina particles may vary widely with the zeolite content ranging
from about 10 to about 90 weight percent, and more usually in the
range of about 10 to about 50 weight percent of the composite.
Noble Metal
[0048] Platinum used alone is the preferred noble metal for use
in this invention. In addition, however, platinum alloyed with rhodium,
iridium, palladium, silver, or gold may be employed as the noble
metal. The finished coating should contain between about 30 g/m.sup.3
and about 7000 g/m.sup.3 of noble metal divided between the alumina
particles and molecular sieve crystals. Of the total amount of noble
metal dispersed in the finished coating, about 10 wt. % to about
80 wt. %, preferably about 20 wt. % to about 40 wt. %, should be
dispersed on the molecular sieve crystals, with the balance of the
noble metal dispersed on the alumina particles contained in the
coating.
Preparation of Catalyst
[0049] The first step in the preparation of catalyst of the present
invention is the preparation of an aqueous suspension of alumina
particles and noble metal reagent. Prior to preparing the suspension,
the alumina should be milled until its mean particle diameter is
reduced to the desired size, preferably about 5 .mu.m. An aqueous
suspension comprising the milled alumina and the desired amount
of noble metal reagent, without the normal addition of acid that
partially peptizes the alumina, is then formed and stirred for sufficient
time to disperse the reagent evenly on the alumina particles, usually
about two hours. This alumina suspension is then later combined
with an aqueous suspension comprising molecular sieve crystals and
a noble metal reagent that will chemisorb onto the exterior 50 nm
(500 .ANG.), as follows.
[0050] The key element of the present invention is that the portion
of platinum or the platinum alloy to be dispersed on the molecular
sieve crystals be dispersed predominantly on the exterior surface
of the zeolite crystals chosen for the particular application, and
preferably so that a very high proportion, above 90 %, of the platinum
or the platinum alloy is sited within 50 nm (500 .ANG.) of the exterior
crystal surfaces. It has been discovered that there are a number
of methods of dispersing platinum or a platinum alloy on zeolite
in a suitable manner. These methods involve contacting a solution
of a soluble compound or a suspension of colloidal platinum or platinum
alloy with the zeolite crystals on which the platinum or a platinum
alloy is to be dispersed in the absence of any other type of zeolite
crystals or any inorganic oxide binder which is or are to be included
in the catalyst.
[0051] One such method employs, as the depositing agent for platinum,
compounds that will chemisorb predominantly on and near the exterior
surface of the zeolite crystals by forming either a covalent or
an ionic bond between the noble metal and the zeolite. This method
is particularly preferred. Compounds that form a covalent bond include
Pt sulfite, [N(C.sub.2H.sub.5).sub.4].sub.2 [Pt(OH).sub.6], and
Pt ethanolamine, with Pt sulfite being preferred; compounds that
form an ionic bond include [Pt(NH.sub.2CH.sub.2CH.sub.2NH.sub.2).sub.2][OH].sub.2
and [Pt(NH.sub.2CH.sub.2CH.sub.2NH.sub.2).sub.2][CO.sub.3]. The
Pt sulfite employed in this invention should be a non-cationic square-planner
Pt complex in which each Pt has two Pt--S bonds and whose charge
is believed to be between -4 and zero, depending on pH.
[0052] Another method involves pretreatment of the zeolite crystals
on which platinum or a platinum alloy is to be dispersed with a
sufficient amount of hydrophilic polymer prior to exposure of the
crystals to a platinum or a platinum alloy salt suspended in water.
The hydrophilic polymer should be applied so as to fill up or block
a sufficient number of the zeolite pores to intrusion of platinum
species or a platinum alloy colloid into the crystal interior. The
hydrophilic polymer will be subsequently removed completely from
the catalyst during calcining of the finished catalyst. Some hydrophilic
polymers suitable for this purpose include polyethylene glycol,
polyvinyl glycol and polyvinyl pyrollidone, with polyethylene glycol
being particularly preferred.
[0053] A third method employs, as the depositing agent for platinum
or a platinum alloy, metallic colloidal particles of platinum or
a platinum alloy of a size substantially greater, by at least a
factor of two, than the largest crystallographically defined pore
diameter of the molecular sieve crystals onto which the noble metal
is to be deposited.
[0054] A fourth method, involving placing a noble metal precursor
into the reactor during the production or synthesis of a zeolite,
is taught in U.S. Pat. No. 5290534 (Tsao).
Preparation Using Pt Reagents that Chemisorb onto Zeolite Crystal
Exterior Surface
[0055] In this preferred method of preparation, an amount of platinum
or a platinum alloy reagent capable of being chemisorbed onto the
exterior of molecular sieve crystals is mixed with an aqueous suspension
of the type and size of molecular sieve crystals to be employed
in the catalyst.
[0056] If the reagent chosen forms a covalent bond with the zeolite,
it is important that the molecular sieve crystals used in this process
be modified to have a very thin film of aluminum oxide on their
exterior surfaces. The choice of method for forming this thin alumina
film depends on the Si/Al ratio of the zeolitic precursor. For molecular
sieves that have Si/Al ratios less than about 12 such as Zeolite
Y, the requisite alumina film can be established by de-aluminating
the zeolitic precursor to some degree by exposure to steam. For
molecular sieves with Si/Al ratios greater than about 12 such as
MFI-38 that do not have sufficient constituent aluminum to permit
formation by de-alumination of a useful alumina film while retaining
high acid site concentration, the requisite alumina film can be
established by treating the zeolitic precursor with an aqueous solution
of very small crystallite boehmite. The boehmite crystallites used
to promote formation of this thin alumina film should be less than
about 4 nm in size, with crystallites of between 2.5 nm and 3.5
nm being preferred. The solution employed should be such that significant
aggregation of the crystals has not occurred. Sols that are essentially
colorless and transparent but nevertheless contain alumina equivalent
to over 5 wt. % alumina have not had significant agglomeration of
the boehmite crystallites. The use of this procedure results in
the exterior surfaces of the zeolite being derivatized, with a thin
(approx. 3 to 5 nm) alumina film deposited thereon.
[0057] If the reagent chosen forms an ionic bond with the zeolite,
it is important that the zeolite chosen have a Si/Al ratio above
about 20 and that the pH of the aqueous suspension of zeolite be
above 10 prior to the addition of the noble metal compound, to
deprotonate the siliceous exterior surface of the zeolite.
[0058] The amount of noble metal reagent added should be determined
by the noble metal loading desired in the catalyst to be prepared.
The range of noble metal loadings should be between 0.05 % and 10%of
sieve weight, with the preferred range being between about 1 and
about 3 weight % of sieve weight.
[0059] If the reagent chosen forms a covalent bond with the zeolite,
sufficient base is then added to the suspension to adjust its pH
to between about 7 and 8. The base employed should be one that will
decompose to yield gaseous products when its temperature is raised
above about 150.degree. C., and the preferred base is tetramethylammonium
hydroxide. If the reagent chosen forms an ionic bond with the zeolite,
no further pH adjustment is required because the aqueous suspension
of zeolite should already be at a pH above 10. The suspension is
then stirred long enough to allow the noble metal reagent to be
chemisorbed onto the molecular sieve crystals, a period of at least
one hour.
[0060] If another type of molecular sieve crystals is to be included
in the catalyst being prepared, an aqueous suspension of this second
type of molecular sieve is then added to the suspension already
prepared. Along with this second type of molecular sieve, the aqueous
suspension of alumina impregnated with noble metal prepared above
should then be added to the mixture already prepared. The entire
mixture is then stirred for a period of time sufficient to thoroughly
mix the compounds that have been included in the suspension.
[0061] The suspension as prepared is then applied to a substrate,
preferably a monolith, by any of the methods known to those skilled
in the art, and preferably by piston coating, to achieve the desired
thickness of coating disposed on the substrate. It is then dried
and calcined by any of a number of methods known to those skilled
in the art.
Preparation Using Hydrophilic Polymer
[0062] In this method of catalyst preparation, a hydrophilic polymer
is employed to fill the pores of the molecular sieve crystals prior
to application of platinum or a platinum alloy to the crystals,
thereby preventing the platinum or a platinum alloy from being deposited
other than on the surface of the crystals.
[0063] Suitable hydrophilic polymers will be any such polymer having
a density of between about 0.8 and 1.0 gm./cm..sup.3 and that completely
volatilizes at calcining temperatures. Preferred polymers for this
application include, without limitation, polyethylene glycol, polyvinyl
glycol and polyvinyl pyrollidone. Polyethylene glycol is particularly
preferred.
[0064] In this method of catalyst preparation then, an amount of
hydrophilic polymer sufficient to fill the pores of the molecular
sieve crystals, without overfilling thereby leaving polymer on the
surface of the crystals, is added to an aqueous suspension of the
molecular sieve to be used, and the suspension is stirred for a
period of at least 30 minutes to promote adsorption of the polymer
in the pores of the crystals. A suitable method for determining
the amount of polymer to apply is:
[0065] 1) the pore volume per unit mass of the molecular sieve
to be employed is measured;
[0066] 2) a concentrated solution of a hydrophilic polymer is prepared
and the density of the solution is determined;
[0067] 3) the quantity of the hydrophilic polymer solution to be
added to the molecular sieve is set so that the product of the mass
of the sieve to be treated times the sieve's pore volume per unit
mass of sieve times the density of the hydrophilic polymer solution
divided by the mass of polymer solution is between 1 and 0.25.
[0068] Following adsorption of the polymer into the crystal pores,
a quantity of de-ionized water may be added to the solution to yield
an appropriate coating precursor.
[0069] Any platinum or a platinum alloy reagent known to those
skilled in the art is then added to the suspension in an amount
determined to provide the desired platinum or platinum alloy loading
on the final catalyst, in a similar manner to the method described
above. The remainder of this procedure is the same as for the procedure
above using those platinum or platinum-alloy reagents that chemisorb
preferentially on the surface of the molecular sieve crystals.
Preparation Using Colloidal Particles of Noble Metal
[0070] This method of preparation is superficially similar to the
method described above using Pt reagents that chemisorb onto the
exterior surface of molecular sieve crystals, although the nature
of the chemical interactions resulting in deposition of the colloidal
metal onto the zeolite is believed to be different.
[0071] An aqueous suspension of colloidal particles of platinum
or a platinum alloy is added to an aqueous suspension of molecular
sieve crystals. It is necessary that the size of the colloidal particles
be at least twice the largest crystallographically-defined pore
diameter of the molecular sieve crystals so that the colloidal particles
cannot enter the crystal pores, and are therefore adsorbed on the
surface of the crystals. The pH may optionally be adjusted to promote
chemisorption of the colloidal metal onto the molecular sieve. The
remainder of this procedure is the same as for the procedure above
using those platinum or platinum-alloy reagents that chemisorb preferentially
on the surface of the molecular sieve crystals.
EXAMPLE 1
[0072] This example describes the preparation of a catalyst comprising
Y molecular sieve on the exterior surface of which Pt is covalently
chemisorbed.
[0073] An aqueous suspension of ultra-stable Y molecular sieve
that had been partially de-aluminated by exposure to steam was prepared
and sufficient Pt sulfite was added to yield molecular sieve crystals
having about 2 wt. % Pt on sieve. Sufficient tetramethylammonium
hydroxide was added to raise the pH to about 7 and the suspension
was then stirred for several hours to permit chemisorption reactions
between the Pt sulfite and the molecular sieve crystals to occur.
Then a milled suspension of Condea SCFA-90 consisting predominately
of .theta.-alumina, was added without the normal addition of acid
that partially peptizes the alumina. The mixture was then stirred
for several hours, dried and calcined.
[0074] A sample of this material was analyzed by cutting thin sections
of particles contained in the sample. FIG. 1 is a micrograph of
these particles. It is clear from this figure that Pt is not found
throughout the bulk of the molecular sieve crystals; rather, Pt
does occur at the exterior surface of the molecular sieve crystals,
appearing as dark particles.
EXAMPLE 2
Comparative
[0075] This example describes the preparation of a catalyst that
has been suggested in the prior art for use in treating exhaust
gas from an internal combustion engine.
[0076] A catalyst sample was prepared using the same method as
employed in Example 1 except that [Pt(NH.sub.3).sub.4][OH].sub.2
was used as the platinum reagent instead of Pt sulfite. Again, a
sample of this material was analyzed by cutting thin sections of
particles contained in the sample. FIG. 2 is a micrograph of these
particles. In this figure is shown dark particles through the bulk
of the sieve crystals. An arrow points to a film on the surface
of the crystal. It should be noted that no platinum is present on
this film; rather, the platinum is present in regions away from
the edges. The possibility that those platinum particles are at
the exterior surface can be dismissed since, in the regions where
platinum is seen, the exterior surface has been cut away.
EXAMPLE 3
Comparative
[0077] This example compares the performance (as measured by NO.sub.x
conversion) of the catalyst prepared by the process described in
Example 1 against that of the system prepared in Example 2.
[0078] Catalytic converters were manufactured using the suspensions
prepared in Examples 1 and 2 applied to equivalent size and type
monoliths followed by drying and calcining. One converter at a time
was connected to the exhaust pipe of a Peugeot IDI, turbocharged
U60 engine. Catalyst inlet temperature was varied at constant speed
(1540 rpm) by variation of the load placed on the engine, i.e.,
as load increased, inlet temperature increased. Diesel fuel was
injected into the exhaust gas between the engine outlet and the
converter inlet to adjust the hydrocarbon/NO.sub.x ratio to 10 at
each load set point. NO.sub.x concentrations were determined using
a chemiluminescent NO.sub.x analyzer.
[0079] The results of this test are shown in FIG. 3. NO.sub.x conversion
versus converter inlet temperature is plotted for each of the converters,
with the performance of the converter using the catalyst prepared
as in Example 1 shown as solid circles, .circle-solid., and that
of the converter using the system prepared as in Example 2 as solid
squares, .box-solid.. It is clear that the converter of Example
1 demonstrates far superior NO.sub.x conversion compared to the
converter using a typical catalyst formulation. NO.sub.x conversion
begins at a lower temperature , reaches a higher maximum, and continues
at a higher level over the entire temperature range of interest.
EXAMPLE 4
[0080] This example describes preparation of catalysts comprising
MFI molecular sieve on the exterior surface of which Pt is is covalently
chemisorbed. It illustrates a variant of the method described in
Example 1 that is useful where the zeolite employed has a high Si/Al
ratio.
[0081] Three suspensions, each containing 20 grams of a molecular
sieve with the MFI structure and a Si/Al ratio of 19 in 100 grams
of de-ionized water, were prepared. Two grams of Condea Tondersol
10/2 a very fine, 10 wt. % dispersion of pseudo-boehmite in water,
was added to the first suspension, 10.5 grams to the second and
22.2 grams to the third. Catalytic coating precursors containing
1 %, 5%and 10%added Al.sub.1O.sub.3 respectively, were produced.
After stirring for 5 minutes, each mixture was quantitatively transferred
to a steam evaporator and the water evaporated away over a period
of three hours. The resulting solids were then separately calcined
at 500.degree. C. for 4 hours. Then, 10 gram samples of each of
these solids were suspended in 50 grams of de-ionized water. To
each suspension about 2 grams of a 9.94%Pt sulfite aqueous solution
was added, making three suspensions differing only in alumina content.
After stirring for one hour, each was filtered and the resultant
solid calcined at 500.degree. C. for 4 hours.
[0082] A fourth sample of catalyst was prepared according to the
procedure described above, but without the addition of Condea Tondersol
10/2.
[0083] The chemical compositions of the resulting catalysts are
shown in Table 1.
1TABLE 1 Designa- MFI MFI-1% Al.sub.2O.sub.3 MFI-5% Al.sub.2O.sub.3
MFI-10% Al.sub.2O.sub.3 tion: Added None 1% 5% 10% Al.sub.2O.sub.3:
Al, %: 2.458 2.982 4.352 7.493 Si, %: 42.233 42.646 41.931 38.737
Pt, %: 0.142 0.193 0.557 1.360
[0084] It should be noted that the platinum content of the samples
increases in rough proportion to the added alumina. Since the alumina
is size-excluded from the zeolite and therefore resides on exterior
surfaces of the WFI crystals, the Pt is supported on alumina that
is supported on the zeolite, in rough analogy to the Pt--Y material
prepared in Example 1.
EXAMPLE 5
[0085] The effectiveness for NO.sub.x reduction of the four catalysts
prepared in Example 4 is compared in this example.
[0086] In turn, a 1 gram sample of each of the materials was loaded
into a quartz reactor which was placed in a furnace and subjected
to the following program. A gas consisting of 1000 ppm NO.sub.x,
300 ppm CO, 8% H.sub.2O, 8% CO.sub.2 500 ppm octane (4000 ppm C.sub.1),
20 ppm SO.sub.2 12% O.sub.2 and the balance N.sub.2 was passed
over the sample at a flow rate of 5 liters per minute. The inlet
temperature to the reactor was increased from about 50.degree. C.
to 550.degree. C. at a rate of 7.degree. C. per minute, then held
constant for one hour, and finally cooled rapidly until the reactor
temperature returned to 50.degree. C. The heating, holding and cooling
regimen was then repeated.
[0087] FIG. 4 shows NO.sub.x conversion as a function of catalyst
inlet temperature during the first temperature ramp for each of
the samples. The data obtained during the second ramp were essentially
identical. The catalyst designated MFI, having no added alumina,
shows modest performance. Maximum NO.sub.x conversion is about 36%.
NO.sub.x conversion for the catalyst designated MFI-1%Al.sub.2O.sub.3
increases to 50 %, and the range of temperature over which significant
conversion occurs, the "conversion window", is considerably
broader than for the MFI catalyst. The modest increase in Pt content
(0.19%vs. 0.15 %) is quite effective at improving NO.sub.x conversion.
This is as expected for platinum situated on the exterior surfaces
of the molecular sieve. The additional alumina, and consequent increases
in Pt chemisorption, associated with the other two samples produces
modest incremental improvement.
EXAMPLE 6
[0088] This and the following example describe preparation of catalysts
comprising Beta zeolite using colloidal Pt particles to deposit
Pt on the sieve exterior surface.
[0089] 20 grams of NH.sub.4-Beta zeolite were suspended in 200
grams of de-ionized water. To this suspension, 22.3 grams of a stable
aqueous Pt colloid solution prepared by PGP Industries, Inc. that
contains 1.83 wt. % of Pt colloid particles roughly 2 nm. in size
were added. The suspension was then stirred for 2 hours and filtered.
The filter cake was dried at 100.degree. C. for about 60 hours.
Analysis of a sample of the filter cake showed that the Pt content
of fine powder was 1.29 %. Transmission Electron Microscopic (TEM)
examination of thin sections of the catalyst revealed homogeneously
distributed, apparently non-faceted Pt particles about 2 nm in size.
Since the largest micro-pore opening in Beta is about 0.7 nm, it
can be concluded that the Pt is on the exterior surface of the Beta.
Relatively high concentration of Pt occurs apparently due to the
disordered, mesoporous texture of Beta that results in a relatively
high exterior surface area.
EXAMPLE 7
[0090] A sample of catalyst was prepared using the same procedure
as Example 6 except that the zeolite used was H-MFI. TEM examination
revealed the same structure as the catalyst in Example 6 but the
resulting catalyst had a Pt content of only 0.06 %. The lower Pt
content is due to the nature of the MFI crystals which are about
1 micron in size with very little surface area available for metal
deposition. Since very little surface area is available for physical
adsorption, and apparently there is not a strong driving force for
chemisorption with this procedure, only a small amount of Pt is
deposited. Since the largest micro-pore opening in MFI is about
0.55 nm, it can be concluded that the Pt is on the exterior surface
of the MFI.
EXAMPLE 8
[0091] This example describes preparation of catalysts comprising
molecular sieve using bulky Pt complexes to deposit Pt on the sieve
exterior surface by ionic chemisorption.
[0092] A solution of bis(ethylenediamine)Pt(Hl) hydroxide containing
5.92 wt. % Pt was supplied by PGP. Three solutions were prepared,
each containing 6.895 grams of the PGP solution diluted with 185
grams of de-ionized water. To each of the solutions 20 grams of
a molecular sieve with the MFI structure and a Si/Al ratio of 19
were added. The pH of the resulting solutions was about 6. Tetraethylammonium
hydroxide was added to the solutions to raise the pH of one sample
to 8.5 another to 9.0 and the third to 11.0. The solutions were
stirred overnight, then filtered and calcined in air for 4 hours
at 400.degree. C.
[0093] Samples of fine powder from each catalyst were embedded
in epoxy, thin-sectioned and examined by TEM, which revealed Pt
particles from 10 to 50 nm. The particles decreased in size with
increasing pH of the synthesizing solution, which is consistent
with generation of the largest number of adsorption sites on the
siliceous surface of MFI at highest pH. As in Example 7 due to
the large size of the Pt complex relative to the zeolite pore size,
it can be concluded that the Pt is on the exterior surface of the
MFI.
EXAMPLE 9
Comparative
[0094] An aqueous suspension of Condea SCFA-90 alumina was mixed
with Pt sulfite for a period of time sufficient to result in chemisorption
of the platinum. The suspension was then mixed with ultra-stable
Y molecular sieve that had been partially dealuminated by exposure
to steam. However, since the Pt had already chemisorbed onto the
alumina, essentially no Pt was deposited onto the zeolite. The mixture
was wash-coated onto a monolithic substrate and then manufactured
into a catalytic converter. This procedure is the preferred procedure
taught by Lachman '991. The volumetric concentrations of Pt, alumina,
and molecular sieve were about the same as in the catalyst of Example
1.
[0095] The performance of this catalytic converter was then compared
to that of the converter of Example 3 made from the suspension of
Example 1. One converter at a time was connected to the exhaust
pipe of a Peugeot IDI, turbocharged U60 engine. Catalyst inlet temperature
was varied at constant speed (1540 rpm) by variation of the load
placed on the engine, i.e., as load increased, inlet temperature
increased. Diesel fuel was injected into the exhaust gas between
the engine outlet and the converter inlet to adjust the hydrocarbon/NO.sub.x
ratio to 10 at each load set point. NO.sub.x concentrations were
determined using a chemiluminescent NO.sub.x analyzer.
[0096] The results of this test are shown in FIG. 5. NO.sub.x conversion
versus converter inlet temperature is plotted for each of the converters,
with the performance of the converter using the catalyst prepared
as in Example 3 (using the suspension of Example 1) shown as solid
circles, .circle-solid., and that of the converter using the system
prepared as in this Example 9 as solid squares, .box-solid.. It
is clear that the converter having its noble metal dispersed on
the exterior surface of molecular sieve demonstrates far superior
NO.sub.x conversion compared to the converter using a catalyst formulation
similar to that taught by Lachman.
EXAMPLE 10
Comparative
[0097] This example describes the preparation of a catalyst comprising
Y molecular sieve, on the exterior surface of which Pt is covalently
chemisorbed, used in performance comparison against a catalyst of
the present invention prepared in Example 11.
[0098] An aqueous suspension of ultra-stable Y molecular sieve
that had been partially de-aluminated by exposure to steam was prepared
and sufficient Pt sulfite was added to yield a finished catalytic
coating having about 0.00247 g/cm.sup.3 (70 g/ft.sup.3) Pt. Sufficient
tetramethylammonium hydroxide was added to raise the pH to about
7 and the suspension was then stirred for two hours to permit chemisorption
reactions between the Pt sulfite and the molecular sieve crystals
to occur. Then a milled suspension of Condea SCFA-90 consisting
predominately of .theta.-alumina, was added without the normal addition
of acid that partially peptizes the alumina. The mixture was then
stirred for several hours, wash coated onto a 400 cell cordierite
monolith, dried and calcined. The resultant coating contained about
0.122 g/cm.sup.3 (2 g/in.sup.3) of the ultrastable Y zeolite, about
0.0183 g/cm.sup.3 (0.3 as g/in.sup.3) of the SCFA-90 alumina and
about 0.00247 g/cm.sup.3 (70 g/ft.sup.3) Pt.
EXAMPLE 11
[0099] This example describes the preparation of a catalyst of
the present invention using Y zeolite.
[0100] An aqueous suspension of ultra-stable Y molecular sieve
that had been partially de-aluminated by exposure to steam was prepared
and one-quarter of the amount of Pt sulfite used in Example 10 was
added. Sufficient tetramethylammonium hydroxide was added to raise
the pH to about 7 and the suspension was then stirred for several
hours to permit chemisorption reactions between the Pt sulfite and
the molecular sieve crystals to occur.
[0101] An amount of Condea SCFA-90 alumina was wet milled until
the mean particle size of the alumina was about 5 .mu.m. Another
aqueous suspension consisting of the milled alumina was prepared
and three-quarters of the amount of Pt sulfite used in Example 10
was added. The suspension was then stirred for two hours, then combined
with the sieve suspension and thoroughly mixed. It was then wash
coated onto a 400 cell cordierite monolith, dried and calcined.
The resultant coating contained about 0.122 g/cm.sup.3 (2 g/in.sup.3)
of the ultrastable Y zeolite, about 0.0183 g/cm.sup.3 (0.3 g/in.sup.3)
of the SCFA-90 alumina and about 0.00247 g/cm.sup.3 (70 g/ft.sup.3)
Pt.
EXAMPLE 12
Comparative
[0102] This example compares the NO.sub.x reduction effectiveness
of the catalysts prepared in Examples 10 and 11.
[0103] The NO.sub.x reduction effectiveness of a small cylindrical
sample (2.032 cm [0.8 inch] long and 2.2225 cm [7/8 inch] diameter)
taken from the monolith of the catalyst prepared in Example 11 (designated
Catalyst "A" on FIG. 6) was compared to that one taken
from the catalyst prepared in Example 10 (designated Catalyst "B"
on FIG. 6) using a synthetic gas bench. A gas consisting of 1000
vppm NO.sub.x, 500 vppm n-octane (4000 ppm C.sub.1), 300 vppm CO,
20 vppm SO.sub.2 8 vol. % H.sub.2O, 8 vol. % CO.sub.2 and the
balance N.sub.2 was passed through the samples at a space velocity
of 38000 hr.sup.-1 and NO.sub.x conversion was measured as a function
of temperature. The data are shown in FIG. 6. The catalyst of the
present invention (Catalyst "A") is more effective, showing
a maximum NO.sub.x conversion of 48%at 232.degree. C. versus a maximum
NO.sub.x conversion for the comparative catalyst (Catalyst "B")
of about 29%at 242.degree. C. Catalyst A exhibits higher NO.sub.x
conversion than does Catalyst B throughout the entire temperature
range where either is active for catalyzing NO.sub.x conversion.
EXAMPLE 13
Comparative
[0104] This example describes the preparation of a catalyst comprising
Beta zeolite, on the exterior surface of which Pt is covalently
chemisorbed, used in performance comparison against a catalyst of
the present invention prepared in Example 14.
[0105] A catalyst (designated Catalyst "C" in FIG. 7)
was prepared as in Example 10 except that Beta zeolite was substituted
for Y molecular sieve and an amount of platinum sulfite yielding
a finished catalyst containing 0.00226 g/cm.sup.3 (64 g/ft.sup.3)
Pt was used. The Beta zeolite had a Si/Al ratio of about 20 and
organic template remained within the zeolite even though alkali
metal cations initially present in the zeolite had been exchanged
with protons.
EXAMPLE 14
[0106] This example describes the preparation of a catalyst of
the present invention using Beta zeolite.
[0107] A catalyst (designated Catalyst "D" in FIG. 7)
was prepared as in Example 11 except that Beta zeolite was substituted
for Y molecular sieve and the finished catalyst contained 0.061
g/cm.sup.3 (1 g/in.sup.3) of the Beta zeolite, 0.079 g/cm.sup.3
(1.3 g/in.sup.3) of the SCFA-90 alumina and about 0.00177 g/cm.sup.3
(50 g/ft.sup.3) Pt.
EXAMPLE 15
Comparative
[0108] This example compares the NO.sub.x reduction effectiveness
of the catalysts prepared in Examples 13 and 14.
[0109] Catalytic converters containing 2.5 liters of the catalysts
made in Examples 13 and 14 were evaluated for NO.sub.x conversion
under the same test conditions as specified in Example 3. As shown
on FIG. 7 the catalyst of the present invention (Catalyst "D")
began reducing NO.sub.x at a lower temperature than did the comparative
catalyst (Catalyst "C"), in spite of the fact that catalyst
"C" has about 25% more platinum. Over the range of temperatures
where the two catalysts reduce NO.sub.x catalyst "D" is
at least as good or better than Catalyst "C". |