Note: Descriptions are shown in the official language in which they were submitted.
1~567~7
PC-2821
BACKGROUND OF THE INVENTION
. . _
1. Field of the Invention:
This invention relates to a catalyst which is useful
for converting harmful constituents, such as organic compounds,
carbon monoxide and nitrogen oxides, in gaseous waste streams
to a non-objectionable form before emission to the environment.
The waste streams containing such noxious constituents may be,
for example, exhaust gases from industrial processes or motor
vehicles.
In recent years considerable attention has been focused
on the problem of air pollution abatement. Among the gaseous
contaminants most prevalent in the air are carbon monoxide,
hydrocarbons, and nitrogen oxides, each of which presents special
problems. Nitrogen oxides, for example, are harmful directly,
in that they are toxic to plants and animals, and indirectly in
that they participate in the formation of smog. Generally,
nitrogen oxides, referred to herein as NOX, are produced when
fossil fuels, e.g., coal, fuel oil, natural gas are burned in
air, and the major contributors to NOx-pollution are waste
streams of many industrial furnaces, electric utilities, and
exhaust gases from internal combustion engines. Of these, the
automobile is a slgnificant source of the NOX in the air, and
at present it is considered especially difficult to handle the
NOX in automobile exhaust streams.
Automobile exhaust streams contain CO and unburned
or partially burned hydrocarbons in addition to NOX (which is
present mainly as NO and NO2) as the principal gaseous air-
polluting constituents. The composition temperature and
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pressure of the exhaust stream depend on many and complex factors
such as engine design, age of the automobile, driving mode, fuel,
and air/fuel ratio. NOX formation is favored at higher temp-
eratures when the automobile is operated at high speed, high
load, and at near stoichiometric air-fuel ratios. Hence NOX
build-up in the exhaust occurs during cruising and accelerating
modes. Running the engine fuel-rich, fuel-lean and/or recycle
of the exhaust tend to reduce the NOX content, but these tech-
niques increase the concentration of CO and/or hydrocarbons. A
further complicating factor is that the gas stream is highly
complex and at high temperatures and high NOX concentrations a
large number of reactions may take place. Catalytic conversion
is one of the techniques used to reduce the undesirable gaseous
constituents in the exhaust, and many catalysts and catalytic
devices which contain one or more beds have been proposed for
this purpose.
This invention concerns catalysts which can be used,
generally, in high temperature reactions including the reduction
of NOX from waste streams from any source. The present catalysts
can further be used for the oxidation of CO and hydrocarbons,
making them particularly useful for the purification of auto-
mobile exhaust. For this reason and for convenience, the present
invention is described with particular reference to automobile
exhaust purification.
2. Description of Prior Art:
Many catalysts are known which are highly active for
the reduction of NOX. To be useful for the purification of
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automobile exhaust, a catalyst must not only be capable of
converting NOX, but the conversion must be to a form which is
not harmful and cannot be reconverted into a noxious material.
Moreover, the catalyst must meet stringent requirements of high
activity and selectivity while maintaining good chemical and
mechanical stability under extremely adverse conditions.
Numerous catalysts, catalytic devices, and catalytic
systems containing one or more catalytic beds have been proposed
for the purification of automotive exhaust gases. Published
materials~in this field show that research in this area has been
intensive and literally hundreds of catalysts have been tested.
Among the many patents and publications relating to catalysts
for removal of NOX from exhaust streams are U.S. Patents Nos.
3,398,101, 3,565,574, 3,669,906, 3,674,423, and SAE Reports Nos.
720480 (May 1972), 720209 ( January 1972), 710291 and 710014
(January 1971), publications of the Society of Automotive
Engineers. The reported catalysts include supported and unsupported
base and noble metals, oxides, and mixtures of oxides. Among the
promising catalysts are those containing chromium, copper, nickel,
or their oxides and combinations thereof. For example, oxides of
nickel, copper and chromium, and various combinations, such as
mixtures of copper oxide and chromium oxide, have been deposited
on refractory oxides. Unsupported copper and copper chromites
have been tried. Copper-plated stainless steel and alloys
such as MONEL* nickel-copper alloys and INCONEL* nickel-
chromium alloys have been suggésted. Supported and unsupported
MONEL nickel-copper alloys have been specifically proposed as
a NOX removal catalysts for a first stage NOX reduction in
a dual-bed automotive exhaust converter. It is known
*trademarks of International Nickel
~'
~r',
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that the method of preparation as well as the composition plays
a material part in the ultimate performance of the catalyst in
the exhaust purification system, and many methods of preparation
have been proposed. Typically, the copper, nickel, and chromium
catalysts for NOX removal have been produced as supported or
unsupported materials by steps including precipitating or co-
precipitating metals or their oxides from a solution thermally,
chemically or electrically. Alloys used as catalysts have been
formed by the usual metallurgical melting, casting and working
techniques. Despite vast research efforts expended in this field,
to date no catalyst has been found which is entirely satisfactory
for NOX removal from auto exhaust. The principal problems are
poor mechanical integrity and/or loss of catalytic activity.
It is the object of the present invention to provide
a catalyst which has good chemical and mechanical stability at
high temperatures.
Another object is to provide a catalyst for the
conversion of NOX which is present in a waste stream to a non-
objectionable product.
It is still another object to provide a catalyst which
is compatible with automotive exhaust streams, is effective for
removing NOX from exhaust streams at typical automotive exhaust
temperatures, is selective under these conditions for the re-
duction of NOX to N2, and is active for removing CO and hydro-
carbons as well as NOX from such streams.
A further object is to provide a catalyst which is
effective for NOX reduction in a single or multi-bed catalytic
converter for automobile exhau~t systems.
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These and other objects are achieved with the
catalyst of the present invention, as will be illustrated
by the following description and examples and by the
accompanying drawings.
SU~l~RY OF THE INVENTION
Briefly, in accordance with this invention a
catalyst is provided which is especially useful ~or high
temperature reactions. The catalyst is a material com-
prised of an alloy containing chromium and copper, e.g.,
consisting essentially of chromium and copper or of
chromium, nickel and copper. The alloy contains at least
about 5% chromium, and it is characterized by a uniform
microstructure which is substantially free of optically
observable heterogeneities of a size greater than about
20 microns, and preferably free of heterogeneities of a
size greater than about 10 or 15 microns. The alloy may
be close to full density. The material may be single or
multi-phase. The multi-phase material is further character-
ized in that the distance between heterogeneities is no
greater than about 20 microns, and preferably it is less
than about 10 or 15 microns.
The terms heterogeneity and theoretical density
are used herein as follows: Heterogeneity refers to a
region or area in the alloy which is compositionally or
structurally different from the matrix phase. Eor example,
one or more phases mav exist in the alloy which are dif-
ferent in composition or structure from the average or
bulk characteristics of the alloy. The different phases
and different compositions of a same phase which may be
present are explained in more detail below. Ho~Jever, the
alloys of this invention are characterized in that if such
heterogeneities are present they are finely and uniformly
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dispersed throughout the structure. Theoretical density of
an alloy is the maximum possible density for the given compo-
sition and phase distribution.
In one feature of this invention the chromium-
and copper-containing alloy of uniform composition is prepared
as a powder by a powder metallurgy technique which will give
the desired uniform microstructure and density, and the alloy,
which may first be formed into a macrostructure, is treated
at an elevated temperature in a gas phase atmosphere which is
oxygenating with respect to said alloy to develop an oxidation-
resistant, stratified,catalytically active surface on the alloy.
It will be noted that the components of the alloy may be present
at the surface of the catalytic material as elemental metals,
compounds such as oxides, salts, alloys, or mixtures thereof.
In another feature of this invention the catalyst is
a composition comprised of an alloy of chromium, nickel and
copper which is prepared by a method comprising a powder metal-
lurgy step to obtain a high density microstructurally uniform
alloy powder. Alloys formed in this manner and having the chromium,
nickel and copper composition, falling within the boundaries
described below haye high activity and selectivity for the
reduction of NOX to N2, and they exhibit mechanical and chemical
stability and in addition they are effective for the oxidation
of CO and hydrocarbons.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a diagrammatic representation, showing
the chromium-nickel-copper alloy compositions on a metal component
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basis which are effective for selectively reducing NOx, the
preferred compositions falling within the area defined by
A-B-C-D-E-A and the more preferred compositions within the
area defined by F-G-H-I-F.
Figure 2a is a photomicrograph at 1000 X magnification
of a catalyst of 35Cr-13Ni-52Cu, showing the uniform, fine
composition and phase distribution which characterize the multi-
phase alloys of this invention.
Figure 2b is a photomicrograph at 1000 X magnification
of an alloy composition of this invention viz. 35Cr-13Ni-52Cu,
which has been formed as a pellet from an alloy powder, etched,
and then treated in air at 1500F for 30 hours.
Figures 3a and 3b are photomicrographs at 1000 X
magnification of samples prepared from alloy powders and treated
in air at 1500F for 30 hours, but having Cr-Ni-Cu compositions just
on and outside the limits defined in Figure 1. The sample of
Figure 3a has the alloy composition 15Cr-15Ni-70Cu, and the
sample of Figure 3b has the alloy composition 53Cr-9Ni-38Cu.
Figure 4 is graph showing the NOX and CO removal from
a simulated automobile exhaust stream using as the catalyst
pellets of 35Cr-13Ni-52Cu prepared in accordance with this
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
COMPOSITION
The catalyst is a material comprised of an alloy
containing chromium and copper, e.g., the alloy may consist
essentially of chromium and copper, or of chromium, nickel
and copper.
105~
In a preferred embodiment of this invention the
catalyst, which is especially useful for purifying gas streams
containing NOX and CO is an alloy and/or developed from an
alloy of chromium, nickel, and copper. During the reaction at
the surface the component metals of the alloy may be present in
metallic form and/or as one or more oxides, for example cuprous
chromite (Cu2Cr2O4), cupric chromite (CuCr2O4), cuprous oxide
Cu2O, cupric oxide, (CuO), nickel oxide (NiO), and chromic oxide
(Cr2O3). In terms of the metallic components, the composition
preferably falls within the area defined by the letters
-A-~-C-D-E-A, and more preferably F-G-H-I-F of the accompanying
Figure 1. It will be noted that the nickel and chromium
composition must be correlated, i.e., where the chromium
concentration is as low as about 5% the nickel content must be
accordingly, greater than about 45%. Typically, and within
the bounds of the above proviso, the alloys may contain about
15% to about 45% chromium, about 10% to about 60% nickel, and
about 15% to about 70% copper.
Preferred alloys are comprised of about 25% to about
~0 35% chromium at nickel levels between about 5% and 25% or 30%,
and about 15 to about 25% chromium at nickel levels between
about 25% and 60%, and the balance copper. At a chromium level
of about 15% or less (and a nickel concentration of about 30%
or less) the catalyst will not withstand the exhaust environment
in that it is not sufficiently oxidation resistant. The minimum
chromium level is about 5% provided the nickel level is above
about 45%. Above a chromium level of about 45% at a nickel
level above about 10% the material does not exhibit sufficient
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catalytic activity. It is believed that at high chromium levels
the chromium tends to migrate to the surface under oxidizing
conditions. This condition is somewhat mitigated by decreasing
the nickel content. Thus, the chromium and nickel levels are
correlated, as shown in Figure 1.
With respect to the nickel content, at least about
5% nickel must be present, otherwise the NH3 formation is too
high. When the nickel content is greater than about 30% at a
chromium level of about 35% or greater, the material tends to
show little catalytic activity, apparently due to the predominance
of chromia formation. It will be noted that commercial nickel-
copper alloys, i.e., al-loys composed nominally of about 30Cu-
70Ni, have suitable activity; however, they are not especially
durable in the exhaust environment. In any event, the nickel con-
centration in the Cr-Ni-Cu catalysts of this invention may range
from about 5% to about 60%, within the relationships shown in Figure 1.
The copper concentration is typically from about 30%
to about 65%. At a copper concentration greater than about
65%, the copper oxides formed at the surface tends to make the
catalytic material formed at the surface non-adherent. At a
copper concentration below about 30%, and a chromium level of
about 35% the catalyst is not sufficiently active. It has been
found that Ni-Cr catalysts, which contain no copper have little
activity for the reduction of NOX, except under very reducing
conditions.
Thus according to one aspect of this invention an
alloy is provided having a Cr-Ni-Cu composition which consists
essentially of about 5% to about 50 chromium, about 5% to about
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1~5~i797
60~ nickel and the balance essentially copper, provided the
chromium, nickel and copper components are correlated such that
the upper limit of chromium concentration is defined, in weight
%, by
Cr = 50 - 0.286Ni, preferably 45 - 0.333 Ni, and the
lower limit of chromium concentration is defined by
Cr = 20 - 0.333Ni, preferably 29 - 0.333 Ni.
Compositions having a relatively high level of chromium,
which fall within the alloys of this invention and which consist
essentially of about 15% to about 50% chromium, less than about
25~ nickel and the balance essentially copper, are essentially
two phase systems. Exemplary of such compositicns is the alloy
consisting essentially of about 35% chromium, about 13% nickel
and the balance essentially copper. Compositions falling within
such a two phase system exhibit more stability with respect to
heterogeneities which exist therein. That is, there is a lower
tendency to phase and compos}tional rearrangement with time and
temperature than in the single or three phase or high nickel two
phase systems.
Single phase compositions, i.e., those which fall to
the right of the boundary W-X-Y-Z of Figure 1 are relatively
nickel-rich. The single phase materials which have a relatively
low chromium content, i.e., about 5% to about 15% or 20%, are
characteristically more easily fabricated than those having higher
chromium contents. Thus, although not preferred, it is possible
that the single phase alloys, illustrated by an alloy consisting
essentially of about 15~ chromium, about 50% nickel, and the
balance essentially copper may be prepared as a wrought material.
It will be noted that in addition to these essential
components, i.e. Cr, Ni and Cu, various elements or compounds
thereof may be present either for special effects or without a
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deleterious effect on their catalyst performance or durability.
For example, in addition to Cr, Ni and Cu, the present catalysts
may contain minor amounts e.g. up to about 3% or 5% each and
preferably no greater than 10% total, of La, Y, Ce, Ba, Ag, Pt,
Ru, Pd, Ir, Li, Pb, V, Sn, Si or compounds thereof. Also, certain
metals may be substituted for a part of the essential components.
For example, up to about 15% iron, e.g., 5% or 12% iron, may be
substituted for a part of the chromium or copper. Other additives
which may be present, e.g., up to about 10%, are cobalt, manganese,
and molybdenum. It will be appreciated that iron may be introduced
into the material to make the alloy less costly by using ferrochrome
since ferrochrome is considerably less expensive than pure chromium.
Cobalt may be included as an agent to increase activity of alloys
of the higher chromium-lower copper content. Manganese may be
useful where the catalyst is used mainly as an oxidation catalyst
to increase the oxidation-resistance of the material. The total
iron, cobalt, manganese, and molybdenum content will preferably
not exceed about 20~.
Alloys exemplary of those which may be used in accordance
with this invention for NOX removal are shown in Table I.
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TABLE I
Compo~ition (Weight %)
A~loy Cr . Ni . Other Cu
1 15 50 - Bal.
2 25 15 . - Bal.
3 25 . 30 - Bal.
4 35 13 . - Bal.
35 26 - Bal.
6 45 11 . - Bal.
7 : 30 : 13 : 5 Fe : Bal.
8 . 23 13 .12 Fe Bal.
9 34 13 1 Co Bal.
25 43 - Bal.
11 15 34 - Bal.
12 : 25 : 45 : - : Bal.
13 25 15 .1 La203 Bal.
14 15 50 Ø5 Y203 ~ Bal.
45 11 1 Pb304 . Bal.
16 : 35 : 13 : 1 Pd : Bal.
17 35 . 13 1 Ba Bal.
18 30 14 1 BaO Bal.
19 35 13 . 5 Mn Bal.
45 11 0.5 Li20 Bal.
21 25 30 l Misch Metal- Bal.
22 32 13 3 Co . Bal.
23 : 35 : 12 :15 Fe : Bal.
24 : 35 : 12 :~5 Fe Bal.
. ~1 Misch Metal-
18 48 6 Fe Bal.
26 : 25 50 ~ 5 Co . Bal.
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The components of the alloys in TABLE I, as in all
other alloy compositions described herein, are given in weight ~.
As will be explained below, it appears that in use,
the surface of the catalyst, typically to a depth of about 0.0005
to about 0.002 inch, is at least partially oxidized and the oxides
present may participate in the performance of the catalyst. It
is possible to add a portion of the catalyst as an oxide in the
initial step because of the unique method of preparation of these
catalysts. A purpose of adding some chromium as Cr203 powder,
for example, is to lower the cost of the catalyst and in some
cases to avoid harmful redistribution of components of the alloy.
MICROSTRUCTURE OF CATALYSTS
. _
The term catalyst, as used herein, includes the
precursor materials such as the alloy powders which are used to
form the catalytic structures as well as the active materials
containing various oxides developed from such alloys.
The precursor powders are dense, preferably at least
about 80 or 90% of full theoretical density, and characterized
by a uniform microstructure. Heterogeneities when present are
fine and uniformly distributed. As noted previously, the alloys
contain at least two immiscible metals, e.g., chromium and copper,
and may have one or more phases or phase compositions depending
on the presence of additional elements and their concentrations.
In other words, the alloys of this invention are normally subject
to compositional and/or phase separation into coarse or inhomogen-
eouo components. This occurs, for example, if the alloys consist
only of chromium and copper which are essentially immiscible, or
if the alloys consist of chromium, copper and nickel. In the
Cr-Ni-Cu alloys having a high level of nickel there is a tendency
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to phase and/or composition separation due to segregation and
coring. This inhomogeneous separation when coarse can be
particularly undesirable where the separate phases and/or
compositions in the alloy exhibit widely different properties,
for example with respect to deformation or oxidation resistance.
However, when prepared according to the method of the present
invention the composition of the phases show only relatively
small deviations from equilibrium. Thus, if an alloy can be
single phase, such as the high nickel-containing alloys, according
to the present invention the alloy will be substantially single
phase and any other phase which may appear, e.g., due to
segregation or coring will be minimized and distributed uniformly
and finely throughout the alloy. Such heterogeneities in the
single phase system are no greater than about 20 microns. Where
the alloy composition has a minimum of two or more phases, the
microstructure is characterized by a matrix phase and the uniform
and fine distribution of the dispersed phase or phases (also
referred to as heterogeneities) throughout the structure. In
the chromium-nickel-copper system of this invention shown in
Figure 1, the cross-hatched area W-~Y-Z represents the approximate
boundary between the materials forming more than one phase (viz.
on the left of W-X-Y-Z), and those in which only a single phase
may be present ( viz. on the right of W-X-Y-Z).
The principal metallic phases which may exist in the
chromium-nickel-copper alloys of this invention are:
Gamma one: a copper-rich face-centered cubic phase.
Alpha : a chromium-rich body-centered cubic phase.
Gamma two: a nickel-rich face-centered cubic phase.
In the alloys of this invention containing a minimum
of two phages, the microstructure consists essentially of a
copper-rich substantially continuolls matrix and a discontinuous
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chromium-rich phase essentially uniformly dispersed therein.
This microstructure is illustrated for the alloy 35Cr-13Ni-52Cu
in Figure 2a. The distribution of the uniformly dispersed alpha
phase is such that the average distance between the heterogeneities
is no greater than about 10 or 20 microns.
With respect to heterogeneities of the order of
magnitude of about 20microns or greater, these can be optically observed
at 100x magnification. Typically, in alloys of this invention
the heterogeneities are of a size less than about 10 or 12 microns
and are observable at a magnification of 1000x.
CATALYST PREPARATION
In accordance with this invention, catalysts especially
suitable for use at high temperatures are produced by a method
comprising: (a) providing an alloy in the form of a powder
comprised of copper and at least one other of the metals chromium
and nickel said alloy being or capable of being on heat treatment
either: (A) substantially a single phase system substantially
free of heterogeneities of a size greater than about 20 microns,
or (B) a multi-phase system having a matrix phase and hetero-
geneities consisting essentially of at least one phase differentfrom the matrix phase distributed in such structure, the hetero-
geneities being no greater in size than about 20 microns and the
average distance between such heterogeneities being less than
about 20 microns, and (b) treating the alloy at an elevated
temperature in a gas phase atmosphere which is oxygenating with
respect to said alloy to develop an oxidation resistant, stratified,
catalytically active surface on the alloy.
The expression "for catalytic use at high temperatures"
as used herein refers to temperatures of the order of about 500F
to about 1800F.
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It will be noted that for certain applications, e.g.,
for use as an automotive exhaust catalyst, the alloy powders are
formed into shaped macrostructure, and the resultant macrostructure
is heat treated according to step "b" to develop the stratified
catalytically active surface on the macrostructure.
As known to those skilled in powder metallurgy techniques,
certain methods of preparation will tend to produce alloys powders
of the type described above. Included in such methods are
mechanical alloying, atomization, splat cooling, plasma or flame
spraying. Very fine dry powders of elemental metals, e.g., less
than about 10 microns, may be blended and diffused to form alloys
having a uniform microstructure. However, this fine powder blend
method is not as satisfactory from a point of view of cost or
quality of alloys, or flexibility in composition as those produced
by e.g., mechanical alloying or atomization. Apart from forming
alloys having the desired compositions and microstructure, certain
of the powder metallurgy techniques have the additional advantage
that they permit incorporation into the catalyst materials a broad
range of ingredients which may serve to enhance the physical or
catalytic characteristics of the composition. Consequently, it is
possible to improve,e.g., durability,catalytic activity and/or
selectivity by incorporation of suitable additives.
Preferably, the alloys are first produced as a dry
powder by a powder metallurgy technique. For example, one method
by which suitable alloys can be prepared is by a mechanical alloying
technique such as described in U.S. Patent No. 3,591,362. The
alloy powder is formed into a shaped material and treated at an
elevated temperature. These steps may be performed simultaneously,
e.g., by a hot consolidation treatment such as hot extrusion or
isostatic compaction, to develop the proper equilibrium phase
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distribution and density in the alloy. Generally, temperatures
of about 1500F to about 2000F are required for consolidation.
Typically extrusion consolidation is effected by extrusion of
powder held in a container, e.g., a steel container, and
at a temperature in the range of about 1500F to 2050F (816C
to 1121C), depending on the alloy composition, with an extrusion
ratio of about 10 : 1 to 90 : 1, e.g., 80 : 1.
Accordingly, a mixture of fine powders of chromium,
nickel and copper, having a particle size of about 1 to 200
microns is subjec~ed to repeated application of compressive
forces, for example by agitation milling, under dry conditions,
in the presence of attritive elements, and maintained kinetically
in a highly active state of relative motion in a protective
atmosphere, the dry milling being continued for a time sufficient
to cause the constituents to comminute and bond or w~ld together
throughout the resulting metal matrix of the product powders.
This occurs when the mechanically alloyed powder particles reach
substantially the saturation hardness level. The mechanical alloy
produced in the manner is characterized metallographically by a
cohesive internal structure in which the constituents are intimately
united to provide an interspersion of comminuted fragments of the
starting constituents.
The chromium-nickel-copper alloy powders prepared by
such mechanical alloying technique may be further processed, e.g.,
by extrusion consolidation, as described above. The microstructure
of the resultant alloy is characterized by its uniformity and the
fineness of any dispersed phase which may be present.
The consolidated chromium-nickel-copper alloys may then
be fabricated in the form of macroparticles, such as pellets, chips,
spheres, and in some cases rods, wire, sheet and screen, which are
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the precursor alloy material from which the active catalysts are
developed. As shown in co-pending Canadian application Serial
No. 187,331, the alloy may be deposited on a structural support.
It is a major advantage of the method of preparation of this
invention that metal compounds can be readi~ly incorporated
in the alloy.
Alternatively, precursor alloys having the essential
uniform and dense microstructure described above can be provided
by a powder metallurgy technique known as atomization. Generally,
in this method a melt ofthe metals is fed a~ a controlled rate
through a nozzle of given dimensions and the stream of metal is
fragmented into very small droplets with a fluid jet stream such
a9 argon or water. The chromium-nickel-copper alloy powders,
formed by gas-atomization may be fabricated into a structure of
a desired shape.
To develop a durable, active, stratified catalytic
surface, the precursor alloy, preferably in the form of a shaped
material, is heated in an atmosphere which is oxygenating with
respect to the alloy. Suitably the atmosphere contains free oxygen,
for example, the atmosphere may be air, air containing up to about
10% moisture, or a nitrogen stream containing oxygen. However, it
has been found that the active catalyst may be developed in-situ,
e.g., in the exhaust stream itself where the NO, CO2 or H2O may
supply the oxygen needed to form the combination of oxides and/or
metal at the surface of the alloy which is active and resistant to
further oxidation. Thus, although stoichiometrically reducing in
respect to the CO, the stream may be oxidizing with respect to
constituents of the precursor alloys. The temperature for developing
the catalytic surface may range from about 1100F to about 1900F.
For preliminary oxidation alloys may suitably be treated in air at
a temperature in the range of about 1500F to 1700F for a period
of about 2 to 30 hours.
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Before the oxidizillg treatment the shaped alloy may
be treated so as to remove chromium from the immediate vicinity
of the surface. This is especially advantageous for Cr-Ni-Cu
alloy of low Ni (less than about 20 or 25~ Ni) and high Cr (more
than about 25% Cr) levels. In a typical treatment, the alloy
is washed with a mild acid, e.g., 20% HCl, which may be warmed,
for example, to 40C. Preferably, the acid is one which will not
attack the nickel or copper in the alloy, but only the chromium
at the surface. It has been found that Cr-Ni-Cu alloys containing
greater than about 25% Cr treated in this way have high catalytic
activity for the reduction of NOX and at the same time can withstand
severe oxidizing atmospheres.
Metallographic examination of catalysts prepared in
accordance with this invention shows the formation on the alloy
of a surface region of up to about 10 to about 50 microns in thick-
ness, composed of at least one of Cu, Ni, CuO, Cu2O, NiO, Cu2Cr2O4,
CuCr2O4, NiCr2O4, and Cr2O3 and between the alloy and surface region
an intermediate layer of about 1 to about 5 microns thickness of an
oxide of chromium, e.g., Cr2O3, or CuCr2O4, which i5 stable to the
environment under conditions where oxidation resistance is required.
The surface region may consist of aggregates, mixtures or one or
more layers of the various combinations. In this region there
may be considerable variation in the porosity, depending at
least in part on the manner in which the stratified layer is
developed. The intermediate sub-surface layer is substantially
continuous and dense. It is believed that this sub-surface
layer protects the alloy from degradation in severe oxidizing
atmospheres without completely preventing some migration of
copper, nickel and chromium to the surface for regeneration of
catalytically active species. However, if for example the
Cr2O3 layer predominates at the surface, the material
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.
will not exhibit catalytic activity for the reactions under
consideration except under very reducing conditions.
As noted above, the catalytic material can be used
as particles, e.g., pellets, chips, spheres, rods, wire or
sheet. If desired it can be used on a structural support or
preform. In effect, however, the catalyst itself can be
characterized as a monolithic structure which is developed from
an alloy into a catalytic surface material on the alloy which
serves as a support. Certain embodiments of this invention may
also be further characterized by self-regeneration in that the
inner alloy support material is available for further development
into an active catalytic surface material.
The following illustrative examples are given for the
purpose of providing those skilled in the art with a better
understanding of the invention.
EXAMPLE 1
This example illustrates the preparation of catalyst
pellets by hot consolidation from alloy powders prepared by a
mechanical alloying or gas atomization method. The various steps
in the preparation of the alloy pellets and subsequent treatments
are identified on the typical preparations shown for the alloy
35Cr-13Ni-52Cu.
Powder A
. _
An alloy powder is prepared by mechanical alloying by
charging to a Szegvari attritor, 3000 grams of metal powders
consisting of 2600 grams of a powder prepared by a gas atomization
technique, composed of, by weight, 25Cr-15Ni-60Cu (minus 60 to plus
200 mesh) and 400 grams of chromium powder (plus 200 mesh). The
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lQ5~797
powders are processed for 18 hours in the attritor with 70,000
grams of hardened steel balls at a speed of 288 rpm in an
atmosphere of 2 psi argon. Thereafter, the powders are drained
over a period of 2 hours.
Powder B
An alloy powder is prepared using essentially the same
method set forth for powder A, except that the charge to the
attritor is composed of elemental powders of chromium, nickel and
copper in proportions to provide an alloy composed of 35Cr-13Ni-52Cu,
the powders having the following particle size: minus 100 mesh
chromium powder, minus 325 mesh nickel powder and minus 325 mesh
copper powder.
Powder C
An alloy powder is prepared by inert gas atomization in
which a 35 kg. charge consisting of chromium, nickel, and copper
powders in amounts to give an alloy of the composition 35Cr-13Ni-
52Cu is melted in an induction furnace in an evacuated chamber.
The melt temperature is brought to about 2900F, that is super-
heated slightly (about 100F) to ensure dissolution of all con-
stituents in the bath. The melt is then poured into a heatedtundish having a bottom orifice through which the metal flows and
is impinged on by a jet of argon - which breaks the molten stream
into fine particles. The particles solidify as they fall.
To form the pellets, the alloy powder is sealed in a steel
can,compacted in an extrusion press at 1800F, and the resulting
billet is extruded to 3/16 inch diameter rods, which are centerless
ground to 1/8 inch diameter and cut to pellets 0.160 inch in
length. The pellets are used as prepared or subjected to various
preliminary etching and/or oxidation treatments as identified below.
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CATALYST ALLOY ACID OXIDATION
TYPE POWDER ETCH TREATMENT
A A No Yes
B B No Yes
C B No No
D B Yes Yes
E C No Yes
F B Yes No
The acid etch treatment consisted of immersion of the
pellets in a 20% aqueous solution of HCl for 20 minutes at
40 to 50C. The oxidation treatment consisted of subjecting the
material to an atmosphere of air at 1500F for 30 hours.
The catalyst materials described above were prepared
as pellets for use in screening tests in connection with their
use in synthetic exhaust. It will be appreciated that the alloy
powders may be used to develop catalysts of different configurations
and may be made using techniques other than hot consolidation.
Also, the alloy powders may be used as a catalytic coating on a
suitable substrate.
As noted above, a preliminary oxidation treatment is
not required for all catalysts. For example, for use as an
automobile exhaust catalysts, etched pellets prepared from 35Cr-
13Ni-52Cu alloys and 25Cr-15Ni-60Cu can be activated directly in
the exhaust stream without a preliminary oxidizing treatment. The
materials develop the stratified oxide coating in the exhaust,
however, it may be somewhat thinner than the oxide coating developed
under the preliminary activation conditions described above
The acid etch step described in connection with Catalyst D,
is especially useful for catalysts prepared from alloy powders
containing greater than about 25% chromium and less than 25% nickel.
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EXAMPLE 2
This example shows the microstructure of a multi-
phase alloy composition of this invention prepared by mechanical
alloying and the effect of an oxidation treatment thereon.
Samples were prepared of an alloy having the composition
35Cr-13Ni-52Cu in the form of pellets in accordance with the
method shown for Catalyst Type F (no subsequent oxidation
treatment) and Catalyst Type D (etching in 20% aqueous HCl at
about 45C for 20 minutes and treated in air at 1500F for 30
hours). Photomicrographs of the samples are shown in Figures
2a and 2b.
Figure 2a is a photomicrograph taken at 1000 X
magnification of the pellet prepared from an alloy powder having
the 35Cr-13Ni-52Cu composition prepared by mechanical alloying.
It shows the uniform fine dispersion of the chromium-rich alpha
phase in a continuous matrix of a copper-rich gamma phase. The
average distance between heterogeneities is not greater than about
5 microns. The figure shows the uniform distribution of hetero-
geneities over an area of about 2 x 104 square microns.
Figure 2b is a photomicrograph at 1000 X magnification
of a sample of the material subjected to the etching and oxidation
treatment. It shows the stratification into a thin chromia layer
of about 5 microns thickness adjacent to the alloy core and a
surface region consisting of an outer layer of CuO and sub-layers
containing one or more of copper, nickel, chromium oxides, and
cuprous and cupric chromite. The core material, as in Figure 2a,
is essentially a uniform and fine dispersion of the chromium-rich
alpha phase in the copper-rich gamma one matrix. The material
is active for the catalytic reduction of nitrogen oxides, for the
oxidation of CO and hydrocarbons and is oxidation-resistant.
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EXAMPLE 3
Pellets were prepared from an alloy composition
15Cr-50Ni-35Cu using a method similar to the gas atomization
method described for Catalyst E of Example 1. After being
treated in a 20% aqueous HCl solution at 40C for 20 minutes,
the pellets are subjected to a temperature of 1500F for 30
hours in air. A stratified oxide scale developed similar to
that shown in Figure 2b in that the oxide is stratified into an
outer region of CuO and sub-layers containing copper, nickel,
chromium oxides, and cuprous chromite, and a thin chromium-rich
intermediate layer of about 2-4 microns in thickness. The thin
intermediate layer is Cr203 and/or a chromium containing mixed
metal oxide. This material is active for the catalytic reduction
of nitrogen oxides, for the oxidation of CO and hydrocarbons and
it is oxidation resistant.
EXAMPLE 4
This example shows the effect of an oxidation treatment
on a Cr-Ni-Cu material prepared in accordance with the present
invention, one not falling within the composition boundary of
Figure 1 and the other falling just on the composition limit.
Samples were prepared by atomization and mechanical
alloying, respectively, and hot consolidation using a method
similar to that described in Example 1 but having the alloy
compositions 15Cr-lSNi-70Cu and 53Cr-9Ni-38Cu, and including
a treatment at 1500F in air for 30 hours. Samples were examined
microscopically, and typical photomicrographs (lOOOX magnification)
are shown in Figures 3a and 3b.
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105~797
Figure 3a, a photomicrograph of the alloy 15Cr-15Ni-
70Cu, shows a thick oxide layer developed on the surface of the
alloy. This material is active but the thick oxide layer spalls
easily. Figure 3b, a photomicrograph of the 53Cr-9Ni-38Cu, shows
a thin layer of Cr2O3 of approxlmately 2 microns thickness
developed on the surface of the alloy. The thin oxide layer is
adherent, but in this condition the material is not active for the
reduction of NOX except under very reducing conditions.
EXAMPLE_5
This example shows catalytic characteristics of
materials of this in~ention as catalysts for purifying a
simplified synthetic auto exhaust stream. The test for evaluating
the material is designed to simulate a first stage reactor of a
dual-bed catalytic converter.
In a dual-bed system the exhaust gas usually passes
sequentially through two beds with secondary air being introduced
downstream of the first bed. The first bed is posit:ioned so that
at acceleration and cruising modes it is rapidly hot and it is
reducing, and a NOX catalyst is used in such first bed. This
system permits the exhaust stream at the time it contains the
maximum NOX to contact the NOx reduction catalyst at high tempera-
ture and low 2 level, conditions conducive to NOX reduction.
In the tests, 15 cc of catalyst in the form of pellets,
0.125 inch diameter by 0.165 inch long, were used in a fixed-bed
downflow 0.8 inch diameter quartz reactor. The catalyst bed was
approximately 1.8 inch deep. Gaseous feeds, simulating automobile
exhaust, were prepared by mixing metered flows of the specific
constituents. The inlet gas had a composition of, by volume,
approximately 0.15% NO, 1.50% CO, 0.05% C3H8, 10.0% CO2 0-1.0% 2
1~)56797
and the balance N2. The gas feed was saturated to 10.0~ H2O
The oxygen content was varied from 0 to 1.0% at 1300F, then at
1500F, and again at 1300F so that the effect of the variation
of 2 level and temperature on the reactivity of the catalyst
could be determined. (About 1.0~ 2 represents the stoichiometric
amount required to oxidize all the CO and hydrocarbons in the stream.)
The gaseous mixture was fed at an hourly space velocity of 10,000
to lOO,OOOV/V/hr (volumes of gas measured at standard temperature
and pressure per volume of catalyst bed per hour).
The synthesized exhaust gas was preheated above the
catalyst bed by passing through heated quartz wool or pieces
of SiC, which have no catalytic effect on th~ gas stream.
The gas was analyzed for NO, NO2 and for NH3 using a
Thermoelectron Model lOA Chemiluminescent Analyzer equipped with
a gas dilution control. Analysis for NH3 was supplemented by
periodic checks using a wet chemical technique. Analysis of CO
was done by a Beckman*Model 315B NDIR Analyzer. The inlet oxygen
was checked periodically using a mass spectrometer. The effluent
NO, NH3 and CO were recorded graphically. The catalyst composition
preparation, and performance of typical tests run at a bed tempera-
ture of 1300F, and 2 level of 0.5% and 0.75% and a space velocity
of 40,000 V/V/hr are tabulated in TABLE II. The oxygen levels of
0.5% and 0.75% are believed to represent realistic operating levels
for an engine running slightly rich. Data for NH3 formation are
reported at zero oxygen since NH3 formation is maximum at this
level. For the purpose of comparision the ease of activation by
oxidation alone was rated, assigning the values 1 through 5. No. 1
*trademark of Beckman Instruments, Inc.
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lQ5~797
being assigned to the catalysts activated with the least difficulty
and No. 5 to those activated with the most difficulty. TABLE II
also shows the relative durability of the alloys (except for quaternary alloys)
based on the weight loss at 1700F experienced in synthetic exhaust atmospheres
exposure, as described in Example 12. Relative durability is
expressed as a rank from 1 to 15 with No. 1 having least weight
loss and No. 15 having greatest weight loss.
Table II shows that Cr-Ni-Cu catalysts prepared in
accordance with this invention and having the alloy compositions:
25Cr-15Ni-60Cu, 35Cr-13Ni-52Cu, 15Cr-34Ni-51Cu, 25Cr-30Ni-45Cu,
15Cr-50Ni-35Cu are especially effective for selective reduction
of NOX to N2 and oxidation of CO to C02 and they are easily activated
and durable. It will be noted that the catalytic profile with change
in 2 level showed that, generally, N0x reduction was favored by
low 2 concentration. Figure 4 is a graph showing the NO reduction
to N2 and to NH3 and CO oxidation to CO2at 2 concentrations from O to
1.0~ in reactions run at a catalytic bed temperature of 1300F, a
space velocity of 40,000 V/V/hr, using as catalyst pellets of the
alloy 35Cr-13Ni-52Cu, prepared in accordance with the method shownfor
Catalyst D in Example 1. Figure 4 illustrates that over a range f 2
levels which would be present in automobile exhaust, especially
in a first stage of dual-catalyst converter, the catalyst is
useful for the conversion of NO to N2 and CO to CO2. Moreover the
catalyst showed no signs of physical breakdown over a period of
50 hours.
EXAMPLE 6
Using a 35Cr-13Ni-52Cu alloy as catalyst pellets,
prepared similarly to Catalyst D of Example 1, tests were run as
described in Example 5, except that the space velocity was varied
'~
_.
1056797
TABLE II
PERFORMANCE OF CATALYST PELLETS*
o
% Composition% Conversion ~ ~ ~ ~
~) R
Alloy NO --~ N2 N~13 CO--~ CO2
Cr Nl Cu OTHER0.5 0.75 o---- 0.5 1.0 v ~ ~
0 85 67 71 82 79 76 1 14
0 75 65 47 57 67 63 1 12
0 65 60 43 74 67 60 2 5
0 55 87 75 20 63 75 5 4
84 72 24 73 79 1 11
71 15 73 90 2 6
13 52 94 93 13 74 92 3 2
11 44 93 93 16 78 94 5
34 51 91 91 13 78 93 1 10
93 63 13 85 95 1 9
26 39 88 63 17 66 68 4 3
93 15 11 77 94 1 7
0 70 30 96 90 5 74 83 1 8
0 67 30 N.D. N.D. N.D. N.D. N.D. _ 13
0 0 100 23 3 30 46 48 _ 15
~ 18 ^ 8 0 Bal Fe**1 1 13 21 17 _
13 47 5 Fe 91 87 15 78 91 3
13 51 1 La203 94 61 12 78 91 5
34 13 52 1 Co 89 88 29 80 94 3
34 13 52 1 Pb304 92 74 1 67 79
.
*Catalyst Type D except for 70Ni-30Cu prepared as pellets from a vacuum melt
heat drawn to rod and 67Ni-30Cu prepared as pellets from commercial MONEL alloy
400 rod.
**304 stainless steel.
***Estimat~d
N.D.=No data -28-
lQ56797
from 10,000 to 120,000 V/V/hr. As contemplated, NOX reduction
decreased with increase in space velocity. ~owever, it was found
that even at 100,000 V/V/hr, NOX reduction was about 80~ at an
2 level of 0.5%, even when the catalyst is in pellet form.
EXAMPLE 7
.
Using a 25Cr-15Ni-52Cu alloy, catalysts were prepared
to demonstrate the effect of a physical configuration. (This is
illustrated further in Example 9). For this purpose, alloy chips
were machined from a billet of consolidated atomized powder. For
comparison pellets of this same alloy were prepared. Both forms
we~e etched prior to testing but were not subjected to a preliminary
oxidation treatment. Samples of each catalyst were used in tests
described in Example 5. A 15cc catalyst bed containing 5 grams of
chips having an estimated geometric surface area of 252 sq. cm. gave
92, 92, and 50 percent conversion of NO to N2 at 0.25, 0.50, and
0.75 percent oxygen levels respectively. A 15cc catalyst bed of
about 60 grams of pellets having an estimated geometric surface
area of 136 sq. cm. resulted in only 86, 30, and 0 percent conver-
sions of NO to N2 at the same oxygen levels. Preoxidation treat-
ment would further improve the activity of each form.
EXAMPLE 8
. .
Using a 35Cr-13Ni-52Cu alloy catalyst prepared as
pellets in accordance with Catalyst D of E~le 1, tests w~re run to
determine the reproducibility of the catalytic activity. The
tests were run at temperatures of 1300F and 1500F., and at
space velocity of 40,000 V/V/hr. The results are tabulated in
TABLE III.
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.
_, _, . . .
56797
TABLE III
Reproducibility of 35Cr-13-Ni-52-Cu Alloy Catalyst
Catalyst Activity at 1300F.
%Oxygen NO~ N2 . CO- ~ C0
Catalyst ~ t 0.2S ~ 0.25 ~ 0.75 ;~~~ ~ 0.50 ~ 1.0 ~ O
C-l : 82 : 83 :86 : 87 : 52 : 73 : 81 : 1
:
C-2 : 88 : 92 :85 : 79 : 34 : 63 : 84 : 12
C-3 : 90 : 95 :90 : 77 : 46 : 67 : 87 : 10
C-4 : 87 : 94 :94 : 89 : 55 : 78 : 92 : 13
C-5 : 90 : 93 :85 : 8 : 55 : 84 : 95 : 12
C-6 : 82 : 88 :86 : 13 : 59 : 80 : 90 : 18
Catalyst Activity at 1500F.
:
C-l : 95 : 96 : 97 : 97 : 41 : 65 : 85 : 5
- : : : : : : : :
C-2 : 99+: 96 : 65 : 40 : 25 : 48 : 62 : 0
C-3 : 99+: 99 : 43 : 11 : 33 : 48 : 65 : 0
C-4 : 99 : 99 : 98 : 97 : 33 : 61 : 85
C-5 : 99 : 98 : 75 : 18 : 53 : 73 : 90
C-6 : 95 : 97 : 97 : 55 : 50 70 73 5
The results in TABLE III show that the performance
of 6 similarly prepared catalysts were especially consistent
at the low 2 levels, activity being higher for both NO to N2
(without NH3 formation) and CO to CO2 at 1500F than at 1300F.
At the higher 2 levels, although less consistent for NO reduction,
the catalyst was very active for both reactions.
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1~5~i797
EXAMPLE 9
This example shows catalytic characteristics of two
alloys of this invention used to purify a synthetic auto exhaust
stream. This test is similar to that of Example 5 except for the
composition of the inlet gas stream which was varied to represent
exhaust for engine operation at various air fuel ratios (AFR values).
The inlet gas compositions are based on an assumed fuel of C8H17
for which stoichiometric combustion occurs at an AFR of 14.7.
Catalysts were tested at 1300F at space velocities of 40,000 to
160,000 V/V/hr. The catalyst identified as D-l is a pelleted
material prepared substantially as described for Catalyst D of
Example 1 from a 35Ci-13Ni-52Cu alloy. Catalyst D-2 is prepared
from 35Ci-13Ni-52Cu but is deposited on expanded metal mesh.
Catalyst D-3 is a material prepared similarly to Catalyst D-2 but
from a 15Ci-50Ni-35Cu alloy.
Table IV shows that Cr-Ni-Cu catalysts prepared in
accordance with this invention and~having alloy compositions
35Cr-13Ni-52Cu and 15Cr-50Ni-35Cu are effective for the selective
reduction of NOX to N2 at AFR values just rich of stoichiometric
combustion. At lower AFR values more NH3 was formed (as at lower
oxygen concentrations in Example 5). Hiyher NH3 concentrations
are observed in this example than in Example 5 because the oxidation
potential is lower and hydrogen availability is higher. Where
objectionable, NH3 formation could be further reduced by injection
of a small quantity of air into the exhaust, operating at a higher
temperature, and/or selecting an alloy having higher Ni content as
illustrated by the data of Table IV.
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TABLE IV
CATALYTIC PURIFICATION OF SYNTHETIC AUTO EXHAUST
. . ~
% Conversion at Various AFR
Catalyst D-l at 1300F. and 40,000 V/V/hr
AFR : 13.0: 13.5: 14.0:14.5:14.7:15.0: 15.5
_
NO (to N2) : 41 : 54 : 82 95 9 2 :
NO(to NH3) : 59 :60 : 18 : 5 : 5 : - :
CO: ~30 : 30 : 41 : 73 :85 : 81 :
Catalyst D-l at 1300F. and 80,000 V/V/hr
10 AFR: 13.0~ 13.5: 14.0: 14.5: 14.7: 15.0: 15.5
NO(to N2) : 44 : 56 : 79 : 58 : 0 : 0 : -
NO (to NH3) : 56 : 44 : 17 : 8 : 0 : - :
CO: - : - : 19 : 49 : 45 : 50 :
Catalyst D-2 at_l300F. and 80,000 V/V/hr
AFR: 13.0: 13.5: 14.0: 14.5:14.7:15.0:15.5
NO(to N2) : 57 :61 : 70 : 87 0 : 0 : 0
NO(to NH3) : 43 :39 : 30 : 5 : 0 : 0 : 0
CO~44 : 38 : 37 : 78 :87 : 80 : 60
C3H852 : 40 : 39 : 28 :42 : 46 : 88
20Catalvst D-2 at 1300F. and 80,000 V/V/hr
. _ .
~FR : 13.0: 13.5: 14.0: 14.5: 14.7: 15~0: 15.5
NO(to N2) : 60 : 62 : 73 : 90 : 2 : 0 : 0
NO(to NH3) : 40 :38 : 27 : 9 : 0 : 0 : 0
CO: - : 35 : 32 : 77 :85 : 81 : 73
C3H6: 96 : 95~: 94 : 91 :74 : 84 : 93
Catalyst D-3 at 1300F. and 80,000 V/V/hr
AFR: 13.0: 13.5: 14.0: 14.5:14.7:15.0:15.5
_ ~
NO(to N2) : 74 : 81 : 89 : 80 : 0 : 0 : 0
NO(to NH3) : 26 :19 12 : 7 : 0 : 0 : 0
30 CO : 45 :39 :36 : 81 :87 : 80 : 59
C3H8 : 60 :45 :40 : 16 18 : 40 98
.
. ~
105G7~7
The results in Table IV show that increasing space
rate results in slightly decreased activity under otherwise
similar conditions, and that more efficient distribution of the
catalyst, results in improved activity.
It will be noted that the catalysts shown in Table IV
are also effective for oxidation of CO and hydrocarbons (HC)
under various conditions. Moreover, if the hydrocarbons are
unsaturated as represented by propylene (C3H6) of Table IV rather
than saturated as represented by propane (C3H8), the catalysts
are especially active for oxidation under both oxidizing
(AFR ~14.7)and reducing(AFR ~14.7) conditions. Actual auto exhaust
contains a mixture of saturated and unsaturated hydrocarbons and
hence conditions exist for which the catalysts of this invention
appear applicable for useful conversion of NO, CO and HC simultan-
eously.
EXA~PLE 10
This example shows the results of tests performed using
a prototype 1976 V-8 engine of 405 cubic inch displacement with a
compression ratio of about 8:1. The engine was equipped with
ceramic exhaust manifold and post-manifold-reactor liners to avoid
catalytic interference which might occur with conventional
metallic parts.
Catalyst beds of the 35Cr-13Ni-52Cu and 15Cr-50Ni-35Cu
alloy, deposited an expanded metal mesh, were tested at about
1300F and 120,000 V/V/hr. space velocity. The catalyst beds
compatable with the exhaust system were only 27 cubic inches in
volume and weighed about 500 grams. Because the catalyst volume
wa~ small with respect to the engine size, the engine was operated
at the rather slow speed of about 1500 rpm to provide exhaust flow
and temperatures considered realistic for catalyst operation. Air
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105~7~7
fuel ratios were varied between about 13.0 and 15.0; torque and
ignition timing were controlled to obtain the desired steady
state exhaust gas mass flow and temperature. To provide NOx
levels of about lOOOppm in the exhaust, NO was injected in the
intake manifold.
The results of these tests showed minimum peak net
efficiencies for conversion of NOx to N2 of roughly 65 and 80
percent at about 1300F and 120,000 V/V/hr space velocity,
respectively, for the 35Cr-13Ni-52Cu and 15Cr-SONi-35Cu alloys.
The lower conversion level for the 35Cr-13Ni-52Cu alloy was due
to its slightly higher NH3 formation. Additionally, these levels
of activity were maintained for the entire operating period which
was about 115 hours. This is estimated to be equivalent to about
5,000 miles of driving.
EXAMPLE 11
Various compositions prepared in the form of cylinders
of 0.300 inch diameter 0.750 inch length, using a method similar
to that shown for Catalyst E of Example 1, were subjected to an
oxidation resistance screening test by temperature cycling e~ery
24-hours in an air-5~ moisture atmosphere. The 24-hour temperature
cycle consisted of a 23-hour exposure to a temperature of 1700F
followed by a l-hour cool to room temperature in still air. The
qamples were weighed initially and at the 24-hour intervals. Prior
to weighing, the specimens are tapped against a hard surface to
knock-off any loose or moderately loose oxide scale.
The results showed oxidation resistance in this atmos-
phere to be largely a function of the copper content of the alloy
with minor exceptions. Weight losses were only 14% and 13%,
respectively for 35Cr-13Ni-52Cu and 15Cr-50Ni-35Cu alloys in 500
hours.
- 34 -
lOS67S7
Samples of commercial MONEL alloy 400 were subjected
to the screening test for oxidation resistance. The results
showed greater than 100% weight loss in 500 hours. A vacuum
melted 70Ni-30Cu alloy also showed greater than 100% weight
loss in this period.
Samples of the composition 25Cr-15Ni-60Cu were prepared by:
1) blending and consolidating "coarse" elemental
powders, i.e., plus 100 mesh,
2) blending and consolidating "fine" elemental
powders, i.e., minus 325 mesh,
3) atomizing and consolidating powder, i.e.,
comparable to Catalyst E of Example 1, but not
pre-oxidized,
4) mechanically alloying and consolidating powder,
i.e., comparable to Catalyst C of Example 1.
Blending refers to mechanical mixing of the powders and "consoli-
dation" refers to the compaction in a steel can at elevated
temperature, as described in Example 1.
A comparison of results of the tests for those materials
formed by simply blending powders with those prepared with this
invention represented by gas atomization and mechanical alloying
is shown in TABLE V. The results show the greater oxidation
resistance of the materials of this invention.
With respect to the multi-phase alloy material it is
believed that if the heterogeneity spacing is too great, i.e.
greater than about 20 microns, this prevents the formation of a
chromium-rich layer which is substantially continuous and, thus
protective. Also it is believed that the formation of a chromium-
rich layer which is substantially continuous is not liable to be
~ormed where the heterogeneities are coarse, i.e. greater than about
20 microns,and/or non-uniformly distributed in the alloy system.
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TABLE V
. ~
TYPE PREP :Alpha-Chromium Size % Weight loss
: (Microns) in 240 hours
. (10 cycles)
Coarse powder blending ; ,~ 120 . 90.0%
Fine powder blending ~, 25 69.7~
Gas atomization r- 8 49.1%
Mechanical alloying r~ 1.5 . 29.1%
,
EXAMPLE 12
Various materials, prepared in the form of cylinders
0.300 inch diameter by 0.750 inch length using the method of Catalyst
E of Example 1, were subjected to an atmosphere of synthetic
exhaust gas for a period of 448 hours under a cycle which consisted
of placing the specimen in a tube furnace, maintaining it at a
temperature of 1700F, for 16 hours, and withdrawing it slowly so
as to cool it in the synthetic exhaust atmosphere. A subsequent
cycle was run with the same temperature profile, however the 2
content of the synthetic gas was changed. The gas mixture was
composed of 1500 ppm NO, 1.50% CO, 0.05% C3H8, 10% CO2, 0.25~ and
1.5% 2' and the balance N2, saturated to a 10% H2O content. The
CO equivalent, i.e., CO + C3H8, in the synthetic gas mix was equal
to 2.0%, for which stoichiometric 2 was 1.0%.
Weight losses of 0.04 and 4.4% were found for 35Cr-13Ni-52Cu and
15Cr-50Ni-35Cu alloys, respectively. Commercial and vacuum melted
- 36 -
- lO5S797
composition of about 70Ni-30Cu s~owed weight losses of 77% and
46%, respectively. In general, the resistance to degradation was
found to be a function of the chromium content of the alloy.
EX~MPLE 13
This example demonstrates the activity of a catalyst
in accordance with this invention for oxidation of CO and hydro-
carbons.
Alloy pellets having the composition 35Cr-13Ni-52Cu
were prepared substantially according to the method described
for Catalyst D of Example 1. The pellets were screened for
catalytic activity for the oxidation of CO in an atmosphere
composed of 0.15% NO, 1.5% CO, 0.05% C3H8, 10% CO2, 2.5~ 2
and the balance N2, saturated to 10% H2O, at space velocity of
40,000 V/V/hr. On raising the temperature of the catalyst bed at
a rate of between l? and 15F/minute, a 50% conversion of CO was
reached at 545F or 610F, depending on whether the prior use of
the catalyst had been under oxidizing or reducing conditions
respectively.
At a temperature of 1300F in the aforementioned atmos-
phere; conversion of CO to CO2 was found to be about 90%.
Additionally, conversion of the C3H8 hydrocarbon was found tobe abou~ 66~. However, if the unsaturated hydrocarbon, propylene,
was substituted for C3H8 in this test, the hydrocarbon conversion
was about 84%. The type of hydrocarbon was also observed to
affect the hydrocarbon conversion during warm up of the catalyst
bed: propylene conversion closely followed CO conversion while
propane conversion trailed behind, requiring higher temperatures.
The activity of this catalyst for oxidation of CO under less
favorable conditions in so far as the 2 content of the strcam is
illustrated in Example 5.
~ -37-
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EXAMPLE 14
.
This example illustrates the activity of a catalyst
of this invention for the reduction of oxides of nitrogen using
various fuels.
The tests were performed on a simulated nitric acid
plant tail gas stream, typically compound of 0.3% NO + NO2, 2.0~ ;
2~ 1.0% H2O and the balance N2. In addition the tests were run
with no 2 to evaluate reduction activity directly and also with
H2O content at less than 0.3%. The fuels included H2, CO, C3H8, ii
and CH4. The catalyst was prepared in accordance with this
invention from a gas atomized powder essentially in accordance
wlth ~e method given for Catalyst E, except that instead of
forming pellets the gas atomized powder is deposited on an expanded
mesh screen as disclosed in the aforementioned co-pending
application.
The tests were run at a space velocity of 60,000-
70,000 V/V/hr and temperature measurements and gas inlet and out-
let analyses were made to determine the steady state response
at several temperatures and the ignition temperature at which
reactivity was detected.
Under the conditions tested, CO and H2 are superior
fuels to hydrocarbons. Typical results with H2 and CO are
tabulated in TABLE VI.
The data in TABLE VI show that the catalyst is effective
for purifying a stream contai~ing NO when contacted, at an ele-
vated temperature, with such stream in the presence of a fuel
which provides a reducing atmosphere.
From the foregoing description it is clear that the
catalysts of the present invention are u.seful not only for purifica-
tion of automobile exhaust but also for reduction of nitrogen oxide,
~"~ .
-38-
105~;797
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m
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-- 39 --
105~797
oxidation of carbon monoxide, oxidation of hydrocarbons, and
formation of ammonia in the absence of oxygen.
Although the present invention has been described
in conjunction with preferred embodiments, it is to be under-
stood that modifications and variations may be resorted to
without departing from the spirit and scope of the invention
as those skilled in the art will readily understand. Such
modifications and variations are considered to be within the
purview and scope of the invention and appended claims.
,,_a~
