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 predominately 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, a process for treating exhaust gas from an internal
combustion engine to reduce the amounts of hydrocarbon, carbon monoxide
and nitrogen oxides emitted to the atmosphere is included.
Molecular sieve claims
We claim:
1. A catalytic converter for treating exhaust gas from a motor
vehicle 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, and optionally a
binder, said sieve crystals having diameters within a range of 0.5
.mu.m to 50 .mu.m said at least one type of crystals further 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 such crystals.
2. The catalytic converter of claim 1 wherein said crystals 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 said binder is selected
from a group consisting of alumina, silica, alumina-silicates, titania,
magnesia, zirconia, beryllia, and mixtures thereof.
5. 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.
6. The catalytic converter of claim 1 wherein only one type of
molecular sieve crystals is employed.
7. A method for treating the exhaust 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
through the catalytic converter of claim 1.
8. The method of claim 7 further including the step of injecting
into the exhaust gases before they are passed through the converter
an amount of diesel fuel adequate to assure maximum NO.sub.x reduction.
9. A method for treating the exhaust 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 a converter containing a catalyst containing at least
one type of molecular sieve crystals having noble metal predominantly
on the exterior surface of at least one type of said molecular sieve
crystals made by a process 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;
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) 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) adding an aqueous suspension of binder to the product of step
(d);
f) stirring the product of step (e) for at least one hour;
g) coating a substrate with the product of step (f) thereby preparing
a coated substrate; and
h) drying and calcining said coated substrate.
10. A method for treating the exhaust 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 a converter containing a catalyst containing at least
one type of molecular sieve crystals having noble metal predominantly
on the exterior surface of at least one type of said molecular sieve
crystals made by a process 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) 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) adding an aqueous suspension of binder to the product of step
(e);
g) stirring the product of step (f) for at least one hour;
h) coating a substrate with the product of step (g) thereby preparing
a coated substrate; and
i) drying and calcining said coated substrate.
11. A method for treating the exhaust 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 a converter containing a catalyst containing at least
one type of molecular sieve crystals having noble metal predominantly
on the exterior surface of at least one type of said molecular sieve
crystals made by a process 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) 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) adding an aqueous suspension of binder to the product of step
(d);
f) stirring the product of step (e) for at least one hour;
g) coating a substrate with the product of step (f) thereby preparing
a coated substrate; and
h) drying and calcining said coated substrate.
Molecular sieve description
FIELD OF THE INVENTION
This invention relates to a catalytic converter containing therein
a catalyst having noble metal dispersed 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 the exterior surface of molecular
sieve crystals.
BACKGROUND OF THE INVENTION
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-bum engines").
The advent of lean-bum engines has, in turn, exacerbated the problem
of reducing emission of nitrogen oxides from internal combustion
engines.
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.
Another approach is to provide a control system operating in conjunction
with a catalytic converter, such as taught by Hirota, et al. in
U.S. Pat. No. 5201802 and No. 5189876 and also by Takeshima
in U.S. Pat. No. 5029061.
Clearly, there is a need for a simple system capable of treating
exhaust gas from internal combustion engines, particularly the exhaust
gas from lean-bum 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.
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.
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.
Patents have also been granted for catalysts intended for other
uses, and some of these more 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 disclosed to have diameters greater
than one millimeter. The particles are then provided with a surface-layer
palladium component about 25 microns in thickness and uniformly
dispersed platinum.
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.
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.
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.
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.
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.
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
It is a feature of the invention that 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.
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-burn 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.
The catalyst coating for use in this invention is characterized
as containing 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. 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 noble metal is
dispersed on the surface of the molecular sieve crystals in such
a manner as to locate at least 90% of the noble metal within 500
.ANG. of the exterior surface of the crystals.
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 the noble metal
is dispersed predominantly on the exterior surface of the molecular
sieve crystals, the first step is to prepare an aqueous suspension
of the molecular sieve crystals and a noble metal reagent that will
chemisorb onto the exterior 500 .ANG. of the said crystals. 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. After adequate stirring the
suspension is coated onto a suitable substrate, dried and calcined
at a temperature sufficient to decompose the base reagent material.
Appropriate noble metal reagents that, when suspended in water,
will chemisorb onto the exterior 500 .ANG. of said crystals include
Pt sulfite and [N(C.sub.2 H.sub.5).sub.4 ].sub.2 [Pt(OH).sub.6 ].
Alternatively, a catalyst containing 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 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, the pH adjusted to between about 7 and
8 and the suspension 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 500 .ANG. of the molecular sieve crystals. Finally, 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
FIG. 1 is a micrograph of the powder used to coat the monolith
in one embodiment of the present invention.
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 to treat exhaust gas
from an internal combustion engine.
FIG. 3 presents data comparing the performance (shown as NO.sub.x
conversion) of the present invention against that of a typical catalytic
converter using a typical catalyst when used to treat exhaust gas
from the same lean-burn diesel engine.
DETAILED DESCRIPTION OF THE INVENTION
The catalytic converter of the present invention comprises a vessel
and a catalyst contained in the vessel.
Vessel
The exhaust gas processing vessel may be any suitable shape and
configuration, with cylindrical and "flattened" cylindrical
(i.e., an extended oval) being the preferred shapes, to provide
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 in relation to size and
surface area of the catalyst 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 speed and 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
The catalyst is comprised of a substrate on which has been suitably
applied an effective amount of a catalytically active coating containing
at least one type of molecular sieve crystals, with platinum or
a platinum alloy uniformly dispersed on the exterior surface of
such crystals. The coating may also contain a binder and one or
more types of molecular sieve crystals.
Substrate
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.
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
The coating is comprised of suitable molecular sieve crystals,
onto the exterior surface of which crystals a catalytically effective
amount of platinum or a platinum alloy has been dispersed in a manner
such that at least 90% of the platinum or platinum alloy is deposited
within the exterior 500 .ANG. of the surface of such crystals. An
inorganic oxide binder may be included in the coating, but it is
an important aspect of this invention that little of the platinum
or platinum alloy be deposited on such inorganic oxide binder. If
a binder is employed it is important that the binder and molecular
sieve crystals be thoroughly mixed so that the crystals are uniformly
dispersed throughout the coating.
The coating should be applied uniformly 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.
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
Only a single type of molecular sieve crystal is necessary as the
catalyst support in the present invention. Any type of molecular
sieve having an average pore size between about 4 .ANG. and about
10 .ANG. may be used, with the preferred type being USY having a
pore size between 7 .ANG. and 8 .ANG.. The size of the crystals,
regardless of the type of sieve chosen, should be between 0.5 and
50 microns, however, and a crystal size between about 1 and about
2 microns is preferred. The silica/alumina ratio should be between
about 10 and about 100000 preferably between about 10 and about
100. Optionally, the sieve crystals may be treated to expose acid
sites on the crystal surface.
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 of a second, or even a third,
type of sieve having characteristics desirable to that application.
For instance, in a catalytic converter 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 6 .ANG.) which is capable of adsorbing the
larger 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.
Binder
As is the case with many catalysts, it is sometimes desirable to
incorporate with the zeolite another material resistant to the temperatures
and other conditions encountered in the environment to which the
catalyst is to be exposed. Often, these materials are inactive,
thereby serving as diluents so that the desired conversion can be
obtained economically without employing other means for controlling
the rate of reaction. At other times, a binder may serve to improve
the adhesion of the zeolite-containing coating to the substrate.
Binders useful for compositing with the zeolites of this invention
include inorganic oxides such as alumina, silica, alumina-silicates,
titania, magnesia, zirconia, beryllia, and mixtures thereof, with
alumina, titania, and zirconia being preferred, and with milled
alumina being particularly preferred.
The relative proportions of finely divided zeolite crystals and
inorganic oxide binder may vary widely with the zeolite content
ranging from about 1 to about 90 weight percent, and more usually
in the range of about 10 to about 50 weight percent of the composite.
Noble Metal
Platinum used alone is the preferred noble metal for use in this
invention. In addition, however, platinum alloyed with a minor amount
of rhodium, iridium, palladium, silver, or gold may be employed
as the noble metal.
Preparation of Catalyst
The key element of the present invention is that the platinum or
platinum alloy be dispersed predominately 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 500 .ANG. of the exterior
crystal surfaces. It has been discovered that there are a number
of methods of dispersing platinum or platinum alloy on zeolite in
a suitable manner. These methods involve contacting a suspension
of a platinum or a platinum alloy salt 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.
One such method employs, as the depositing agent for platinum or
a platinum alloy, compounds that will chemisorb predominately on
and near the exterior surface of the zeolite crystals. This method
is particularly preferred. These compounds include Pt sulfite, [N(C.sub.2
H.sub.5).sub.4 ].sub.2 [Pt(OH).sub.6 ], and Pt ethanolamine, with
Pt sulfite being preferred. 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.
Another method involves pretreatment of the zeolite crystals on
which platinum or a platinum alloy is to be dispersed with a sufficient
amount of a suitable hydrophilic polymer prior to exposure of the
crystals to a platinum species or a platinum alloy 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.
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.
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
In this preferred method of preparation, an amount of platinum
or 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. If Pt sulfite is to be used as the Pt reagent,
it is important that the molecular sieve crystals to be used have
been de-aluminated to some degree, as by exposure to steam, thereby
creating a very thin film of aluminum oxide on the crystalline surfaces.
This has been found to be necessary to promote deposition of Pt
sulfite on the crystalline surface.
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. Sufficient base is then added
to the suspension to raise its pH to above about 7. 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. The suspension is then stirred
long enough to allow the noble metal reagent to be chemisorbed onto
the molecular sieve crystals, typically a period of at least one
hour.
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, or instead
of it if only one type of molecular sieve is to be included in the
catalyst, an aqueous suspension of a suitable binder, preferably
one such as alumina, titania, or zirconia, may optionally 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.
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 Hydrolphilic Polymer
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.
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.
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:
1) the pore volume per unit mass of the molecular sieve to be employed
is measured;
2) a concentrated solution of a hydrophilic polymer is prepared
and the density of the solution is determined;
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.
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.
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
This method of preparation is very similar to the method described
above using Pt reagents that chemisorb onto the exterior surface
of molecular sieve crystals.
An aqueous suspension of colloidal particles of platinum or 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 crystals 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.
EXAMPLE
This example describes the preparation of a catalyst of the present
invention and characterizes the resultant coating.
An aqueous suspension of ultrastable 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.
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
This example describes the preparation of a catalyst that has been
suggested in the prior art for use to treat exhaust gas from an
internal combustion engine.
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 throughout 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
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.
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.
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 the present
invention 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. |