Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to catalysts. More particularly, the
invention is concerned with catalysts suitable for the catalytic
oxidation of hydrocarbons and carbon monoxide and for the catalytic
reduction of oxides of nitrogen in the presence, respectlvely, of
suitable oxidising and reducing means.
The invention is especially concerned with the cataiytic purification
of the exhaust gases of petrol driven internal combustion engines and of
diesel engines but is by no means so limited.
The words "gas" and "gases" in this specification are to be taken
as nneaning a gas or gases per se, a vapour or vapours or a mixture or
mlxtures containing one or more gases and one or more vapours.
The components of exhaust gases of the type just referred to may
generally for convenience be divided into two groupsO The first group
contains cornponents which are, or will act as reducing agents and can
hence be oxidised and the second, components which are or will act
as oxidising agents and can hence be reduced~ By "purification" of
exhaust gases in thls specification is meant the oxidation of at least
a proportion of any components of the said first group and the reduction
of at least a proport'on of any components of the said second group so
20 ~ as to form one or more of at least the products water, carbon dioxide
anu nitrogen.
Typical members of the first group are carbon monoxide and
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hydrocarbons formed by the incomplete combustion of fuel in the
engine, hydrocarbons in the shape of unburnt but vapourised fuel
and hydrogen gas. The principal members of the second group are
oxides of nitrogen, often referred to collectively as "NOx'i, and oxygen.
Ideally the components of the first and second groups are in
stoichiometric balance in an exhaust stream so that by passing such
an exhaust over a suitable catalyst, the said components may be
caused to interact and to effect mutual oxidation and reduction with the
formation, for example, of carbon dioxide, water and nitrogen and
without leaving any residual unreacted gas or gases.
Such a stoichiometric balance hardly ever occurs in practice,
however, except perhaps momentarily, although it could possibly be
achieved if means were provided for continuously monitoring the
cornposition of the exhaust and continuously adjusting the ratio of
air and fuel being fed to the engine so as to counteract any deviations
from exhaust stoichiometry. Possibly means would also be required
for introducing, when necessary, additional oxygen into the exhaust
gases upstream of the catalyst.
In the absence of means for maintaining stoichiometric balance
as just described, the relative proportions of the various components in
the exhaust stream from an internal combustion or diesel engine will
vary from engine to engine depending upon, among other things~ the
design of the engine, the actual operating temperature of the engine and
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its state of tune. In addition, the composition oF the exhaust frorn
any given engine will vary during the operation of the engine and will
be governed by such factors as the degree of throttle opening~ In
fact the variation in exhaust composition in practice is such that there
are occasions when components of the first (the oxidisable) group
preponderate and occasions when those of the second (the reducible)
0roup are in excess.
In general, the variations in exhaust composition just described
will correspond to excursions firstly on the "rich" and secondly on the
"lean" side of a stoichiometric input rnixture of air and fuel to the
engine, so that firstly there will be an excéss of fuel and secondly an
excess of air in the input. This means that a "rich" input mixture will
lead to an exhaust stream which ;s relatively deficient in oxygen and
a "lean" mixture which will lead to an exhaust stream containing a
relative excess of oxygen.
If a catalyst of the type currently available for the purification of -
exhaust 0ases, such as a supported rhodium-platinum catalyst, is
used in an exhaust stream for the oxidation of such components as
carbon monoxide, hydrocarbons and hydrogen gas and for the reduction
20 of oxides of nitrogen, a relative deficiency of oxygen in the exha~;t
stream as described above, will reduce the efficiency of the catalytic
oxidation process. Conversely? a relative excess of oxygen will
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militate against the catalytic reduction process. This latter effect
is particularly rnarked with rhodium-platinum catalysts with which NOx
reduction seems to be especially easily "poisoned" by an excess of
oxygen .
It is an object of this invention to provide catalysts in which the
foregoing disadvantages of prior art catalysts are overcome.
According to a first aspect of this invention, a catalyst suitable
for catalysing oxidation and reduction processes in a gas (as herein
defined) is such that the catalyst is capable of removing oxygen fro~
a gas containing an excess of oxygen and restoring such removed oxygen
to a gas in which there is an oxygen deficiency.
Reference herein to "excess" and "deficiency" relates, in general,
to an oxidation process which the catalyst is required to catalyse.
Accordingly, an "excess" of oxygen means that more oxygen is present
in the gas than is required, in conjunction with any other oxidising agents
present, to effect the desired oxidation. Conversely, under conditions
of oxygen "deficiency" insufficient oxygen will be present to enable the
desired oxidation in conjunction with any other oxidising agents present,
to be èffected.
We have now found that a _metal catalyst in which
is incorporated or with which is associated one or more base metals,
,
at least one of which has two or more oxidation states, is suitable for
the purposes of this invention.
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According to further aspects of the invention, a catalyst suitable
for catalysing oxidation and reduction processes in accordance with the
first apsect of the invention has the Eollowing features taken together
or singly:-
(a) the catalyst co~nprises a support carrying at least one metal
selected from gold, si.lver, platinum, rhoclium, ruthenium,
palladium~ iridium and osmium, and at least one base metàl
selected from cobalt, manganese, nickel, zinc, tin, tungsten,
cerium, iron, copper, praseodymium, rhenium, chromium, molyb-
denum, lanthanum, calcium, strontium, barium, zirxonium and
gadolinium, at least two of the said metals in the catalyst be-
ing in theform of a perovskite, said base metal being between 5
and 99.9 weight per cent of the total weight of metal in the
catalyst, the support being stabil.ised by the inclusion therein
of at least one of sa:ld base meta:ls; and, optionally, one or .
more base metals from the group consisting of the rare earth
metal other than praseodymium and lanthanum and those base
metals of the first, second and third transition series which
are not listed above;
(b) the base metals of the cata:Lyst of feature (a) constitute not
less than S wt.%, preferably not less than 25 wt.% of the total
metal content of the catalyst. By total metal content of the
catalyst is meant the total content of base metal and other
metals as defined above, in the catalyst;
(c) the base metals of the catalyst of feature (a) constitute be-
tween 30 and 99.9 wt.% and-preferably bet~een 70 and 97 wt.% ~-
of the total metal content of the cata].yst;
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(d) -tl~e base metals of tlle catalyst o~ feature (a) consti-tute
94 wt,% of -the tota~l metal content of the catalys-t;
(e) -the lneta]s referred to in feature (a) may be in the Iorm
o~ a mixture or alloy;
(f) one or morc ol the ineta:ls referred to in feature (a)
- may be in tlle form of the oxide, e g copper chroluite
CuCrO
(g) two or more of the ~netals referred to in feature (a)
are in the form of a perovsl~ite;
Particularly preferred base metals are cobalt and cerium
which may be used separa-tely or togetller.
By a pexovskite is mean-t a mixed oxide system of the form
A]303, ~ ere A and B each represent one or more metals
such tllat the ~ cation is coordinated ~i-th 12 oxygen ions
and the B cation wi-th 6 oxygen ions. It follo~s that
the A cation will normally be found to be somewhat
larger than the B cation. In order for there to be
- ¢ontact bet~een the ~5B and 0 ions9 R~ ~ P~ should be equal
to ~¦2(R~ ~Ro) where ~ RB and Ro are the ionic radii
In practice, ~ may represen-t one or more rare earth me-tals
and B may represent one or more metals of the first,
second and third transition series of the Periodic Table.
(hj -the ~support or carrier of -feature (a) l~ay compromise one
or ~ore reLractory oxides and is preferably applied as a
continuous or discontinuous film, coating1 layer or
deposit on a substrate;
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(i) the substrate of feature (h) may be of ceram;c material
or metal and is preferably in the form of an inert, rigid
honeycomb structure to the surfaces of the channels or
passageways of which the support or carrier is applied.
Suitable alloys from which a metallic substrate may be
formed are chromium, alum inium, cobalt alloys, such
as KANTHAL (Registered Trade Mark) alloys and an iron
chromium, aluminium, yttrium alloy known as a FECRALLOY
(Registered Trade Mark) alloy. This latter alloy is
desc'ribed and claimed in British Patent No.1,0~5,993;
(j) the support or carrier of feature (h) is preferably selected
from the group consisting of oxides of aluminium, magnesium,
calcium, strontium, barium, scandium, yttr;um, the
lanthanides, gallium, indium, thallium, silican, titanium,
zirconium, hafnium, thorium, germ anium, tin, lead,
vanadium, n;obium, tantalum, chromium, molybdenum,
tungsten and rhenium;
(k) the support or carrier of feature (a) may be applied to the
substrate of features (h) or (i) by dipping the substrate into
' a suspension or dispersion of the support or carrier material
or other medium containing the support or carrier material
and having appropriate rheological properties, and then
calcining the so-treated substrate. Alternatively, the
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suspension or dispersion may be poured over the substrate
prior to calcination. In both cases, when a honeycomb
substrate is used in accordance with feature (i), the
surplus suspension or dispersion rnay be removed by
draining and/or blowing with compressed air before the
substrate is calcined.
(I) the support or carrier of feature (a) includes alunninium
oxide and the oxide of at least one other of the metals
Iisted in feature (j), such as Ianthanum oxide and/or
barium oxide so as to improve the surface area stability
of the support or carrier at high temperature;
(m! one of the "other" metals referred to in feature
(I) which will improve the high temperature stability of
the support or carrier is the, or one of the base metals
included in the catalyst of feature (a).
One way of preparing a catalyst according to the invention is as
fol lows: -
1. An alumina suspension suitable for the "coating" of a ceramic,
honeycomb monolith with a Iayer of alumina which, on firing,
will constitute the support or carrier previously referred to,
is prepared by slurrying sufficient alumina monohydrate with
water so that the slurry contains 40-46 wt,% of solids,
following which the pH of the slurry is adjusteu to 3.5 of the
addition of nitric acid. A ceramic honeycomb substrate
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is dipped into the slurry, allo~Ned to drain so that the
passageways of the honeycomb become clear and fired
at 550 C for 30 minutes.
2. The fired "washcoated" monolith is next immersed in an
aqueous solution of barium nitrate, dried and then fired
at 550 C for 30 rninutes. The concentration of barium
nitrate in the solution is such that, after immersion and
firing, the alumina coating will contain 5 wto%Of bariurn.
3. The process of stage 2 above is repeated except that the
solution contains, in place of the barium nitrate, the
nitrate of one or more of the base meta!s listed in feature
(a) of the invention except that, in the case of tungsten,
ammonium metatungstate is used. The quantity of the
base rnetal salt in solution is defined in sta~e 4 below.
4. The process of stage 3 above is repeated except that the
solution contains platinum tetrammine chloridel Pt(Ni 13)4C12,
and Claus' salt rRh(NH3)5C3 C12 and the ceramic monolith
is fired at 650 C for 30 minutes. The relative quantities of
the platinurn and rhodium salts are such as correspond to a
ratio of platinum to rhodium of 10 to 1; the total quantity of
platinum and rhodium salts is such as to give a precious
metal concentration in the washcoated and fired monolith
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of 40 grams per cubic foot of the so-treated monolith and
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the quantity of base metal salt used in stage 3 is such
that the amount of this base metal in the washcoated and
fired monolith is 87 wt.% of the total metal content. The
abo~e produces a catalyst accorcling to the invention in
which the washcoat which constitutes the support or carrier
is "stabllised" with the base metal.
Base metals incorporated in the catalyst in stage 3 above which
have been found to be especially suitable for the purposes of the invention
are cobait, cerium, chromium, manganese, nickel and praseodym ium and
our experiments have shown that the optin-~um ~uantity of base metal is
about 15~ grams per cubic foot of washcoated and fired monolith. In a
modification of this process, stayes 3 and 4 may be combined so that
the base metal nitrate~s) and platinum group metal compounds are
applied to the washcoated monolith from a mixture of the solutions used
for stayes 3 and 4. In this case, the monolith after immersion and
drying is fired at 650 C for 30 minutes.
In an alternative way of prepariny a catalyst, lanthanum nitrate is
used in place of the barium nitrate, the quantity of lanthanUm nitrate
present being sufficient to introduce 5 wt.% of lanthanum into the ~ired
washcoat. The process just described produces a catalyst in which the
washcoat lS stabilised with lanthanum.
Yet another way of prepariny a catalyst in accordance with the
invention in which three of the base metals such as lanthanum, strontium
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and cobalt for example, or lanlhanum, barium and cobalt are in the
form of a perovskite, such as LaO 8SrO 2CoO3 or LaO E3BaO ~CoO
(see feature (g) above), comprises the following stages:
~i) the nitrates of the respective metals are first
dissolved in water in the correct proportions following
which the metals are precipitated as the carbonates by
treatrnent with a solution of potassium carbonate;
. (ii) the carbonates are Filtered, dried and fired at 1000 C
for 12 hours to form the required perovskite;
(iii)the perovskite material formed in (ii) and alumina as
used in stage 1 (above) are slurried in water in the
proportions by weight of 1 to S, following which the
monolith is coated with the slurry, dried and fired at
550 C for 30 minutes;
~iv) the washcoated and fired monolith prepared in (iii)
is dipped into a solution containing barium nitrate,
platinum tetrammine chloride and Claus' salt and
.. then dried and Fired at 650 C for 30 minutes. .
In yet further ways~of preparing catalysts in accordance with the
invent i on:
(a) rhodium nitrate is included with the other nitrates in stage (i)
above so that on completion of stages (i) and (ii) a perovskite
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of the form ~ aO ~BaO 2CoO 9~7RhO.013 3
place of one of the perovskites previously mentioned.
Thereafter stages ( i i i ) and ~ i v) may be carried out as
described above. When rhodium is present in the
perovskite it is not st;ictly necessary for it to be added as
one of the catalyst metals in stage (iv). Accordingly,
Clausl salt may be omitted from the stage (iv) solution; and
(b) in place of the materials used to form a perovskite as in
stage (i) above there are used rnaterials which will form
barium mesoperrhenate; Ba3(ReO5)2. This substance is
conveniently made by mixing together soluti~ons of ammonium
- perrhenate and barium hydroxide in a molar ratio of 2:1
respectively and then boiling the solution to drive off the
ammonia and allowing the tiquid to cool whereupon crystals
of barium perrhenate are formed. Following this the crystals
are removed, washed and then dried at about 100 C. The
formation of the perrhenate is represented by the equation
2NH4ReO4 + Ba(OH)2~8H20 = Ba(ReO4)2 ~ 2NH3 ~ 10H20.
Next the white crystalline perrhenate is ground up with
barium carbonate In a molar ratio of 1: 2 respectively and
the mixture fired in R ctosed platinum crucible for 2 hours
at 700 C and then for 2 hours at 850 C tc form the bright
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yellow mesoperrhenate as follows:-
a( )4)2 3 3 5 2 2
: Finally stages (iii) and (iv) are carried out as describedabove except that the first line of section (iii) "the
perovskite material formed in (ii)" should be replaced
by "the barium mesoperrhenate formed as just described".
We have found that catalysts according to the invention have
particularly good oxygen removal and restoration properties as defined ^
in the statement of the first aspect of this invention when the metal,
the oxide of which is added to aluminium oxide to form the support or
carrier in accordance with feature (1) of the "further aspects" of the
invention, is also the or one of the base rnetals of the catalyst as
specified in feature (a). Thus when the support or carrier is stabilised
wlth barium oxide, particularly good results have been obtained when
the cataly,st layer includes the barium oxide-containing perovskite
LaO 8BaO 2C3- '
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