CA1100470A

CA1100470A - Catalysis

Info

Publication number: CA1100470A
Application number: CA273,815A
Authority: CA
Inventors: Barry J. Cooper; Gary J.K. Acres; Brian Harrison
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1976-03-12
Filing date: 1977-03-11
Publication date: 1981-05-05
Also published as: JPS52120293A; SE426787B; DE2710765A1; SE7702518L; FR2343505A1; FR2343505B1

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to catalysts, especially for the catalytic oxidation of hydrocarbons and carbon monoxide and for the catalyst reduction of oxides of nitrogen in the presence, respectively, of suitable oxidising and reducing means.
In more detail, the catalyst of the invention is capable of removing oxygen from a gas containing an excess of oxygen and restoring such removed oxygen to a gas having an oxygen deficiency, and the catalyst comprises a support carrying at least one metal selected from gold, silver, platnium, rhodium, ruthenium, palladium, iridium, and osmium, and at least one base metal selected from cobalt, manganese, nickel, zinc, tin, tungsten, cerium, iron, copper, preaseodium, rhenium, chromium, molybdenum, lanthanum, calcium, strontium, barium, zirconium, and gadolinium, at least two of the said metals in the catalyst being in the form of a perovskite, said base metal being between5 and 99.9 weight per cent of the total weight of metal in the catalyst, the support being stabilised by the inclusion therein of at least one of said base metals.

Description

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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 ~3-.
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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 ~4--~.
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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 .
of 40 grams per cubic foot of the so-treated monolith and .

-~0-' , 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 -12- ~ ' -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- ' ~ ' , - ~ 14-, , : ,: :
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A catalyst for the catalytic oxidation of a hydrocarbon or carbon monoxide and for the catalytic reduction of an oxide of nitrogen, the catalyst comprising a support carrying at least one metal selected from gold, silver, platinum, rhodium, ruthenium, palladium, iridium and osmium, and at least one base metal selected from cobalt, manganese, nickel, zinc, tin, tungsten, cerium, iron, copper, praseodium, rhenium, chromium, molybdenum, lanthanum, calcium, strontium, barium, zirconium and gadolinium, at least two of the said metals in the catalyst being in the form 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 stabilised by the inclusion therein of at least one of said base metals.

2. A catalyst according to Claim 1 including in addition to the said one or more base metals, one or more base metals selected from the group consisting of the rare earth metals (excluding lanthanum and praseodymium) and base metals of the first, second and third transition series of the Periodic Table and not specified in Claim 1.

3. A catalyst according to Claim 1 containing 25 to 99.9 wt.%
base metal.

4. A catalyst according to Claim 3 containing 70 to 97 wt.% base metal .

5. A catalyst according to Claim 3 containing 94 wt.% base metal.

6. A catalyst according to any one of Claims 1, 2 or 3 wherein the metals are present in the form of an alloy.

7. A catalyst according to any one of Claims 1, 2 or 3 wherein the metals are present in the form of a mixture.

8. A catalyst according to any one of Claims 1, 2 or 3 wherein at least one of the metals is present in the form of cm oxide.

9. A catalyst according to Claim 1 wherein the perovskite has the general formula AB03 where A and B each represent one or more metals with the A cation coordinated with 12 oxygen ions and with the B cation coordinated with 6 oxygen ions.

10. A catalyst according to Claim 9 wherein where RA, RB and RO are the ionic radii.

11. A catalyst according to Clalm 10 wherein A represents one or more rare earth metals and B represents one or more metals of the first, second and third transition series of the Periodic Table.

12. A catalyst according to Claim 1 wherein the perovskite is:
La0.8 Sr0.2 Co03, La0.8 Ba0.2 Co03, La0.8 Ba0.2 Co0.987 Rh0.01303 La0.8 Sr0.2 Co0.98 Rh0.01303

13. A catalyst according to Claim 1 wherein the base metals are barium and rhenium in the form of barium mesoperrhenate.

14. A catalyst according to Claim 1 wherein the catalyst is supported on a substrate made from a material selected from the group consisting of ceramic materials and metals.

15. A catalyst according to Claim 14 wherein the substrate is in the form of an inert, rigid honeycomb structure having a plurality of channels with the catalyst applied to the surfaces of the channels.

16. A catalyst according to Claim 14 including an intermediate support layer between the substrate and the catalyst metal, wherein the support layer is made from a material selected from the group consisting of oxides of aluminium, magnesium, calcium, strontium, barium, scandium, yttrium, the lanthanides, gallium, indium, thallium, silicon, titanium, zirconium, hafnium, thorium, germanium, tin, lead, vanadium, niobium, tantalum, chromium, molybdenum, tungsten and rhenium.

17. A catalyst according to Claim 16 wherein the intermediate support layer includes aluminium oxide and the oxide of at least one of the metals listed in Claim 16.

18. A catalyst according to any one of Claims 14, 15 or 16 wherein the base metal is intimately associated with the platinum group metal.

19. A catalyst according to any one of Claims 14, 15 or 16 wherein the base metal and the platinum group metal are located in dif-ferent areas of the substrate.