CA1062690A - Stable perovskite catalysts - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Perovskite catalysts of the general formula ABO3 having increased stability through the incorporation of metals having low first ionization potentials, and their use in oxidation and reduction reactions.

Description

-~" 106Z690 BACKGROU~lD OF T~IE INVENTION
Recent concern over the env:Lronrnental efIects Or industrial and automotive emissions has resulted in increased effort to develop catalysts whlch are effective in the oxida-tion or reduction of undesirable waste gases to innocuous products. In addition to the fundamental need for good catalytic activity for the desired conversions, catalysts have been sought which retain this catalytic activity over extended periods of time in environments which are normally detrlmental to the catalytic effect of many metals such as platinum. Most platinum catalysts, for example, are markedly depreciated in their catalytic activity in the presence of combustion residues from standard antiknock gasoline additives.
Moreover, many industrlal processes, such as those found ln petroleum refining, provide high temperatures or reducing conditions detrimental to most known catalysts.
SUMMARY OF THE INVENTION
The present invention relates to perovskite catalysts having improved stability in a wide variety of -chemical environments.
Specifically, the present invention provides, in a catalytic metal oxide having the general formula ABO3 and a perovskite crystal structure, in which A and B are each at least one metal cation, the improvement wherein (a) the ~attice Stability Index of the metal oxide is less than about 12.3 electron vol~s;
(~) about from 1% to 20% of the type B catlon sites are occupied by cations of at least one platinum metal; and (c) at least about lO~ of the type B catlon sites .. ':

- . .
. .

- 2 -~62690 are occupied b~ cations of at least one metal differing from the catal-~ic metal an~l having a ~-lrst i.onlzation potentlal of no ~reater tllan 7.1 electron volts.
The invention further pro-vides a process using these ca~alysts b~ bringing into contact at least one oxi-dizable an~ at least one reducible reactant in the presence of a catalyst and un~er such condltions as to effect a change in the o~idation state of at lea~t one reactant.

D~T~ILED DESCRIPTION OF THE INVEMTION
. .. . . .
The catalytic metal oxides to which the present invention relates have the general emplrical formula AB03 containing substantially equal numbers of metal cations occupying the A sites and the B sites in the perovskite crystalline structure. In the ideal perovskite structure such oxides contain cations of appropriate relative sizes and co-ordlnation properties and have cubic crystalline forms in which the corners of the unit cubes are occupied by the larger A site cations, each coordinated with twelve oxygen atoms, the centers of the cubes are occupied by the smaller B site cations, each coordinated with six oxygen atoms, and the faces of the cubes are occupied by oxygen atoms. Variations and distortions of this fundamental cubic crystal structure are known among materials commonly considered to be perovskites or perovskite-like. Distortions of the cubic crystal structure of perovskite and perovskite-like metal oxides include rhombo-hedral, orthorhombic, psuedocubic, tetragonal, and pseudo-tetragonal modifications. ~;

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.. .. . . . .. . . ... .

~06Z690 Thc ~ site metals used in the preparation of such perovskite compositions can be from the periodic table groups .
lA, lB, 2A, 2B, 3B, 4A, 5A lanthanide rare earth metals (atomic numbers 58 throu~h 71) and from the actinide rare earth metals (atornic numbers 90 through 104). Particularly satisfactory perovslcite compositions are obtained using A
site metals from Groups lA, 2A, 3B and the lanthanide rare earth metals.
The B site cations can be present in any amount and valence which are consistent with the perovskite crystal structure of the compounds. Accordingly, they can have ; , valences of 1 to 7 and can be from the periodic table groups lA, lB, 2A, 2B, 3A, I~A, 4~, 5A, 5B, 6B, 7B and 8 or from the lanthanide and actinide rare earth metals~ , The c,ations of type A generally have ionic radii of ', about from o.8 to 1.65A, while'the catlon~ of type B can have ionic radii of about from 0.4 to 1.4A. References to ionic : :
radii are based on the tabulations of Shannon and Prewitt, : '' ACTA CRYST. B25 925 (1969) and B26 1046 (1970). References ., ~
to the periodic table refer to that given at pa~es ~4~-449, ,,,"Handbook of Chemls~try and Physiçs," 40th edition, Chemical Rubber Publlshing Company (1958-59)~
The.present invention is based on the discovery .; .'~
that catalysts having excellent stability,can be obta~ned through the combined use of the perovskite crystal structure ' and the. inclusion of metals within the crystalline structure ~-havin~ a low :rirst lon.i.zation.potential such that the Lattice Stabillty Index of the compositions is less than about 12.3 , ' .. ' electron volts. Particularly stable compositions are obtained ~- , when the Lattice Stability Index is less than about 12.0 , electron volts. The Lattice Stability Index is the sum oi the products of the atomic fractions of each metal cation in a compound and the first ionization potential of the metal.
By first ionizatlon potent:ial is meant that given by Veneneyev et al. "Bond Energies, Ionization Potentials and Electron Affinities", St. Martin~s Press (1966).
The term "atomic fraction" is used in its usual sense, indicating the fraction of the type A or type B cation sites occupied by a metal Thus, for the composition [SrO 1 Laa 9]
[Alo g Ruo 1]3~ the atomic fractionsor the four metal cations are 0.1, 0.9, 0.9 and 0.1, respectively.
In calculating the Lattice Stability Index of the composition, the atomic fraction of each cation is multiplied by the first ionization potential of the metal from which the cation is formed. These products are then added together It is important that a si~nificant amount of a stabilizing metal be present in the type B cation sites of the present compositions for good stability of the products.
Accordingly, at least about 10% of the type B catlon sites should be occupied by cations of at least one stabilizing metal having a first ionization potential no greater than 7.1 electron volts. For maximum stability, B site stabilizing component should be separate from and in addition to the catalytic transition metal present in the B site. Repre-sentative stabilizing metals which can be used for this purpose include the following:
'.

.. . .

Metal First Ionization Potential Aluminum 5.984 Gallium 6.00 Indium 5.785 Titanium 6.82 Zirconium 6.84 Hafnium 7 Chromium 6.764 Vanadium 6.74 Molybdenum 7.10 :
l~iobium 6.88 In addition~ to satisfy the Lattice Stability Index requirements of the present compositions, at least a signi-ficanG amount, for example, about 20~ of the.cation sites of type A or type B are generally occupied by ions of at least one of the following metals:
Metal First Ionization Potential ~

Lithium 5.39 . . .:
SOdium 5.138 Potassium 4.339 Rubidium 4.176 Calcium 6.11 .
.- .
Strontium 5.692 ~ :

Barium 5.210 S¢andium 6.54 Yttrium 6.38 Lanthanum . 5.61 Lanthanide rare earth mlxtures 5.6-6.9 ¦ ~0The Lattice Stability Index of the present composi-Ition is particularly reduced by the inclusion of alkali metal ~062690 in amounts comprising at least about 10% of the type A or type B cation sites.
In the present catalytic compositions, at least about 1% of the type B cation sites are occupied by at least one of the platinum metals, rhodium, ruthenium, palladium, osmium, iridium, and platinum. The catalytic effect of the platinum metal generally increases until these metals occupy about 20% of the B cation sites. Incremen-tally less improvement is realized above these levels. `
The platinum metal ruthenium and platinum have been found to provide especially high catalytic activity. Transition metals having an atomic number of 24-29 can also be used ;
as catalytic B site components. It is preferred, for best catalytic properties effect, that at least about 5% of :.:. ~ .
such transition metals be present in a first valence and ' at least about 5% of the same metal be present in a second valence. It has also been found to be of catalytic benefit for these transition metals of atomic number 24-29 to be used in conJIunction with a platinum metal.
The first ionization potentials of the catalytic metals used in the invention are as follows. All catalytically active metals tend to increase the LSI
value:

V 6.74 Cu 7.724 Pt 9.0 Cr 6.764 Ru 7.364 Au 9.22 Mn 7.432 Rh 7.46 Re 7.87 W 7.98 Pd 8.33 Mo 7.10 Fe 7.87 Ag 7.574 - Nb 6.88 Co 7.86 OS 8.7 Ta 7.88 Ni 7.633 Ir 9 Tc 7.28 ~06Z690 The catalytic compound.s of the present invention can be prepared by heating mixtures of metal oxides, hydroxides;
metals and/or metal salts for sufficient times at temperatures which permit spontaneous formation of the compounds. The mixture of materials which are heated are preferably finely subdivided and intimately mixed before heating and are thoroughly ground and mixed by any conventional techniques several times during the heating period, since the compounds are in may instances formed by atomic diffusion, without melting of any of the starting or potential intermediate materials, and are subJect to coating of unreacted particles by reaction products.
The heating times and temperatures required for the formation of significant amounts of these compounds depend upon the particular compositions being formed, the required times usually being shorter at higher temperatures. Temperatures above about 800C; are usually suitable for the formation of these compounds but temperatues above about 900C. are often preferred with firing times of hours to days with occasional intermediate grinding and mixing, and temperatures of 1000C.
to 1500C. can generally be used.
In forming the compounds used in this invention, stoichiometric mixtures of starting materials are preferably heated in air or other oxygen-containing gas mixtures.
The perovskite compositions of the invention can be used as catalysts in the form of free-flowing powders, for example, in fluid-bed reaction systems, or in the form of shaped structures providing efficient contact between the catalyst and reactant gases. The catalyst compositions can contain minor or major amounts of catalytically inert 3o materials, with the catalytic compositions primarily on the , ., .. , , _ .. _ , _ _ . , ,-~

~6Z690 surrace of the inert materlal or dispersed throughout.
The catalytlc composition of this invention are prererably used in the form Or coatings on suitable refractory supports. ~uch supports can be composed solely or primarily of silica, of ceramic compositions having softening or meltlng ;
temperatures above the temperatures involved in forming or coating these catalytic compositions on such supports, of ~ ~
natural silicious materials such as diatomaceous earths and ~ ;
pumice, as well as of alundum, ~amma alumina, silicon carbide, , 10 titania, zirconia, and other such refractory materials.
I The catalytic compositions of the present invention are stable and durable at high temperatures and can be used for I a wide variety of liquid and gas-phase reactions. They are particularly effective in the catalyzation of the oxidation I of hydrocarbons and oarbon monoxide and also the reaction ,., ... .. . . .. _.
bet~leen nitro~en oxide (N0x) and carbon monoxide to glve nitro~en and carbon dioxide. They exhibit increased resis-tance to polsonin~ by the lead compounds present in the exhaust of internal compustion engines operated on leaded gasoline.
The metal catalysts o~ this invention are useful as catalysts for the oxidation o~ oxidizable carbon components to compounds o~ hi~her oxidation states, the reduction o~
carbon monox~de and o~ nitrogen oxides to compounds of lower - oxidation states and the reduction o~ hydrocarbyl mercaptans ; ancl sul~ides to su~stantially sulfur-free hydrocarbon compo-sitions.
~nong -the oxidation processes *or which the present catalysts can be used is the oxidation of carbon rnonoxidc to carbon dioxlcle an(l o~ hydrocarbons to carbon dioxide. Hydro-3 carbons.which can be used include those having 1-20 carbon _ g _ atoms, including those that are normally gaseous and those that can be entrained in a gaseous stream such as the liquefied petroleum gases and the volatile aromatic, ole~
finic and paraffinic hydrocarbons which are commonly in - -industrial solvents and in fuels for internal combustion engines. The oxidant for these processes can be oxygen nitrogen oxides, such as No and NO , which components are normally present in the exhaust gases of internal combustion engines. ;
The compounds of this invention can also be used to catalyzt the reduction of such oxides of nitrogen as nitric oxide, nitrogen dioxide, dinitrogen trioxide, dinitrogen ,~
tetroxide and the higher oxides of nitrogen such as may be present in waste gases from the production and use of nitric acid as well as in the exhaust gases of internal combustion engines. The reductant for these processes can be hydrogen, carbon monoxide and such hydrocarbons as described above and as present in said exhaust gases.
The metal catalysts of this invention containing ruthenium are particularly useful as catalysts for the re-duction of nitrogen oxides. They generally catalyze the reduction of these oxides to innocuous compounds (e.g., nitrogen) instead of to~ammonia. Metal catalysts contain-ing platinum and palladium are particularly useful as ~atalysts for the complete oxidation of carbon compounds to carbon dioxide.
Thus the compositions of this invention are useful -~;
for the oxidation of carbon monoxide and volatile hydrocar-bons and for the simultaneous reduction of oxides of nitro-:
gen under conditions typical of those involved in the `

-10- `' 1062690 :~
cleanup of the exhaust gases of automotive and other inter-nal combustion engines and are capable of effecting the substantially complete conversion of the obnoxious compo--nents of such gases to innocuous substances.
Still another hydrocarbon oxidation process that can `
be catalyzed by metal catalysts of this invention is the steam reforming of hydrocarbons. This process known also as hydrocarbon reforming involves reaction of methane or a homolog thereof such as those found in volatile naphthas with steam in the presence of a catalyst of the invention. Those containing Ni or Co or a platinum metal selected from Pd, Pt, Ir, Ru and Rh supported on alumina, magnesia, or a basic -oxide composition are particularly well suited for this application. The resulting product stream contains C0 and H2, normally accompanied by C02 formed by reaction of C0 with excess steam in the well-known water gas shift. Reac-tion temperatur~sare normally in the range 450 to 1000C., usually not above 900C., at pressures up to about 700 psi and usually at least about 100 to 200 psi for methane re-forming at reactant ratios of from about 1.5 to 6 moles of steam per carbon in the hydrocarbon feed stock.
The metal catalysts of this invention can also be used in the water gas shift reaction which involves reaction of ~0 with H20 (steam) at moderately elevated temperatures. -Particularly suitable are those catalysts containing cations of the first transition metal series, such as Fe, Co, Ni or Cu, preferably Fe or Cu. The resulting product-stream is depleted in C0 and containing C02 and H2. Temperatures in general are in the 200 to 500 C. range, with higher conver-sions favored at the lower temper~tures, higher reaction ~06Z690 rates at the higher temperatures. The process appears to be largely independent of pressure.
Still another hydrocarbon oxidation process that can be catalyzed by metal catalysts as described herein is the dehydrogenation of aliphatic, cycloaliphatic and alkylaro-two saturated (i.e., nonelefinic and nonaromatic) -CH-groups which are adjacent or in 1,6-positions relative to one another (corresponding to said first oxidation state) to hydrocarbons, usually of the same carbon content, formed by removal of the hydrogens from one or more pairs of said -CH-groups (corresponding to said second oxidation state). In-cluded are the dehydrogenation of such aliphatic hydrocarbons as butane and 2-methylbutane to such olefins and diolefins as butene, 2-methylbutene, butadiene and 2-methylbutadiene;
~cyclodehydorgenation of alkanes having removable hydrogens;
as defined and preferably having six -CH- groups in a chain, such as n-hexane, 2,3- and 4-methylhexane, n-heptane and various methylheptanes to the corresponding cyclohexanes, including methyl-substituted cyclohexanes; dehydroaromatiza-tion of cyclohexane and the methyl-substituted cyclohexanes to benzenoid hydrocarbons such as benzene, toluene and the xylenes; dehydroaromatization of decaline to naphthalenes; -dehydrogenation of alkyl side chains of alkylbenzenes such as ethylbenzene to form styrene.
Reaction conditions generally involve temperatures in the range of 400 to 700 C. and solid catalysts as described herein, particularly those containing Group VIII platinum metals, especially Pt. The reaction can be conducted in the presence of oxygen or in the absence of oxygen and in the presence of hydrogen gas as in the well-known catalytic re-Jforming process of the petroleium refining industry.

.~-- ,. .

In the important catalytic re~orming process of the petroleum refining indu~try, a relatively low octane value ~eed ~tream containing dehydrocyclizable and aromatizable hydrocarbons i8 converted into a relatively hlgh octane value exit stream containing aromatic hydrocarbons of the gasoline boiling range as the essential components resulting primarily ~rom dehydrocyclization o~ open-chain components to cyclohexanes and aromatization of cyclohexane~. Accom-panying reactions include hydrocracking to lower carbon content componentg and isomerization of straight-chain to higher octane value branched-chain components. The process is generally carried ln the presence of hydrogen to suppress side reactions leading to carbonization and to produce a composition which i8 largely saturated except ~or the aromatic hydrocarbon content.
The ~eed 6tream normally comprises alkanes and cyclo-alkanes having 4-12 carbons, preferably 5-10 carbons, and including (a) one or more open-chain compounds having 6-8 carbon~ and at least six -CH- groups in a chain, such a~
n-hexane, n-heptane and the methyl-substituted derivatives thereof described above, and/or preferably (b) one or more ~-cyclopentane8 having 1-3 methyl substituents on different ring carbons, such as methylcyclopentane, 1,2-dimethyl-, 1,3-dimethyl- and 1,2,4-trimethylcyclopentane, which are i80merizable into cyclohexanes and methyl-substitued cyclo-hexanes, hence aromatizable into the corresponding benzenoid hydrocarbons, A typical ~eed streàm composed as above will have a research octane number in the range 40-85, more usually 50-70.

,;, ., . . .: ~ . , , . . ~ . . ~

Thc rerormill~ reaction is normally conducted at about 450 to 550C. and at ~ressures Or about 200 to 900 psl and ' ,~, i~ deslred ln the prese'nce Or ~dded h,vdrogen ~as in amounts .
corresponding to 3-15 moles per mole o~ feed to minlmlze slde reactions. , .;
The product stream comprlse~ the so-called reformate rraction, rlch in hi~h octane value aromatics, such as ,' benzene, tolllene and the xylenes and having a t~pical re-search octane number ln the 88-103 range, accompanled by an essentlally saturated gaseous rractlon rlch ln Cl-C4 alkanes and hydrogen gas, a valuable by-product ror use ln varlous hydrotreating processes, e.g., hydrodesulfurlzatlon.
Still rurther processes that can be catalyzed ln ,', ' accordance wlth thls inventlon are those Flscher-Tropsch re-actlons involving the reductlon Or carbon monoxlde with hydrogen ln the presence Or a.metal catalyst as de~ined, , ~ .
particularly those contai~ing Fe, Co, Nl, or Ru at elevated tempe~atures (usuall,y 150 to 600C.) and pressures , (up to 15000 psl) .efrectlve to produce one or more products ,~:

20 contalnlng chemical'ly bound C and H wlth or without cheml- ~
cally bound O such as methane or one or more gaseous, liquld ' or solld hlgher hydrocarbons~ wlth or without alcohols, aldehydes, ketones and fatty aclds. One embodlment comprls~
'es the well~known methanation reaction generally,conducted at about 200 to 600C. at elevated pressures, typicall~

.
~:: about 50 to 500 psi, prererably over a Ni-contalning metal oxyhallde catalyst Or thls inventlon. Suitable ~eed streams lnclude the product stream rrom the steam rerormln~ Or : methane~ contalning CO, H2, unreacted steam and-some CO2 3 rormed ln the watcr gas shirt. Another embodiment widely .... .

... . ,: , .`. ~ ,.,. . ,.; , .... .

106Z6gO

used for tlle pl~oduction of llquid fu~ls in the ~asoline and diesel fuel ran~es involves reaction of CO with H2 at relatively low temperatures~ such as 150 to l~oooc. and pres-sures in thc range Or about 15 to 3OO psi, preferably over metal oxides containing Fe or Co ions, ~hich promote the formation of hydrocarbons higher than CII4~ especially the liquid fractions suitable as fuels for internal combustion engines. The reaction products may sometimes include par-tially rcduced, i.e., oxygenation products such as alcohols, aldehydes, ketones, and carboxy acids, as produced in accor-dance with the Fischer-Tropsch process variation known as the synthol process.
Another reduction process catalvzed by -ca'v~lJsts de-rined herein ls the catalytlc desulrurizatlon or hydrogenaly-sls Or organic divalent sulfur compounds, such as those naturally occurrlng in feed stocks used in the petroleum chemical lndustry, ror example, those used ~or the productlon of synthesis gas (CO and ~I2) bv steam reforminF. as descrlbed earlier, whicI- stocks include mercaptans, llnear sulfldes, cycllc sul~ideæ and the aromatic cyclic sulfide thiophene.
The feed stock desulrurization reaction is nor~ally conducted at temperatures of 150-500C., prererably 300-400 C., over a wide range of pressures, includlng atmospherlc, ln the presence Or a cobalt catalyst and in the presence of 2 large excess of H2 relatlve to the sulfur content of the feed stock~ typical proportions being 0,25 to l.0 mole of ]~2 per average mole of reed stock hydrocarbon correspondlng generall~ to 250-lO00 mole Or H2 per S atom in the reed.

.

~. -. : .
, ~O~Z690 The reaction product comprises hydro~en sulfide gas and a sub~tantially sulfur-free hydrocarbon composition.
The l-I~S can ~e removed by means descri~ed in the art, as by physical stripping or by chemical absorption, e.~., by ZnO
to produce ZnS.
In a reducin~ atmosphere and elevated temperature typical of many ~as phase reactions, the compositions o~ the present invention exhibit excellent resistance to structural decomposition, as indicated by an absence of a significant decrease in weight o~ the composition. X-ray diffraction analysis demonstrates little change in the crystalling struc-ture of the compounds o~ the present invention a~ter heating to 1000C., l~lile compounds havin~ a Lattice Stability Index in excess of 12.3 showed decomposition o~ the crystalline structure as shown ~ the presence of metallic nobel metal after heating. Moreover, the compositions o~ the present invention exhibit significantly less tendency to react with other composltions commonly used as catalyst supports at high temperature.

The invention is further illustrated by the following specific examples, in which parts and percentages are by weight unless otherwise indicated.
Exam~les 1-14 In Examples 1-14and comparative Examples A-Q, metal oxides were prepared by heating mixtures of precursor compounds containing appropriate stoichiometric amounts of the metals involved. The mixtures of precursor compounds were obtained by one of the following preparation procedures, as indicated in Table I:
Procedure A: Dry metal oxides and/or carbonates were ground together.

:

. - - . .......... : :- . . ~ . . - . -;.~ . . . ., .. ;.. . . . ...
.: . , . ~ . . . ,:,,. . , -lO~Z690 ;- -Procedure B: An aqueous potassium carbonate solution was added to a solution of soluble compounds (e.g. -metal nitrates, chloroplatinic acid, ruthenium chloride ~-~
hydrate) and the resulting insoluble materials were -separated, washed, and dried.
Procedure C: An aqueous potassium carbonate solution was added to an aqueous slurry of a powdered metal oxide (e.g. ruthenium oxide) in a solution of soluble compounds (e.g. metal nitrates) and the resulting insoluble materials were separated, washed, and dried. - ;
Procedure D: An aqueous potassium carbonate solution was added to an aqueous solution Or soluble compounds (e.g. metal nitrates), a powdered metal oxide (e.g.
platinum oxide, vanadium oxide) was added, and the insoluble materials were separated, washed, and dried.
Procedure E: An a~ueous potassium carbonate solution was added to an aqueous solution of soluble compounds (e.g.
metal nitrates), the resulting insoluble materials ~ ~ were separated, washed, dried and ground, an insoluble 20~ metal oxide (e.g. thorium oxide) was added, and the mixture was heated at 950C. for two hours, cooled, and ground.
rocedure F: An aqueous soIution of soluble compounds , ~ . .
(e.g. metal nitrates) was evaporated to dryness, the residue was ground and heated at 950C. for one hour, and the heated material was cooled, ground, washed with water, and dried.
The mixtures of precursor compounds were heated in air at 950 to 1300C. for several days with occasional cooling, grinding, and mixing. Each of the resulting metal oxide ' ''' , ' '' ' ' .

` - 106Z6~0 compositlons Na8 ~inely ground and passed through a 325-mesh Tyler standard sieve ~creen. The metal oxides uere identlfied as ha~ing the expected perov~kite structure. me metal oxides of the present invention prepared ln Examples 1-8 were applied to supports by the following procedure. One part of "Dispaln*
M alumina di~persant and binder (obtained ~rom the Continontal Oil Co~pany; sur~ace area about 164 square meters per gram, determlned with nitrogen by the Brunauer-Emmett-Tellet method) ~as m~ed with 17 parts oe water containing a few drops of 10 commercial concentrated hydrQchlorlc acld. To thls mixture was added 7.5 parts o~ the catalgtlc composltion to obtaln a stable thisotroplc slurry. A cyllnder Or "Torvesn* alumina ceramic honeycomb with stralght-through cells (obtalned rrOm E.I. du Pont de Nemours & Com~any) was soaked in water.
Thls cyllnder welghed about 6 to 7 grams, was about 2.5 centimeters ln dlameter and thickness and nominally had a cell BlZO or lh6 lnch, wall thicknes8 o~ 0.018 inch, open area Or 50%, 253 hexagonal holes per square lnch, and a nominal geometrlc surface area Or 462 square feet per cublc 20 foot. The ~ater-soaked cylinder was dlpped into the slurry of the catalytic composltlon, the gross escess Or slurry WaB
removed by blowing the cylinder with alr, the cyllnder was drled, and the cyllnder coated ~lth the catalytlc composition ~nd binder ~a8 heated for about 30 minutes in air in a muf~le furnace at about 700C, The cylinder wa~ again soaked ln ~ater, dipped into the ~lurry, blown rree of e~ces~ slur~y, and drled and then heated in air at about 700~C. ror two hours.
; The percentage increase in weight of the ~ylinder due to the ; adherent eatalytic eompo~ltion and binder i~ given ln Table I. ?

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The catalytic activity of these compositions in the reduction of nitric oxide by carbon monoxide was determined.
The "Torvex" ceramic honeycomb cylinder coated with the catalytic composition and binder was installed in a stainless steel chamber with a nominal internal diameter of 2.5 centi-meters, height of 2.5 centimeters, and volume Or 12.3 cubic centlmeters. Nitrogen containing about 2000 parts per million of nitric oxide and about 10,000 parts per million of carbon monoxide was passed through the chamber at a nominal hourly space velocity of about 40,000 hr. 1 and pressure of one pound per square inch gage while the feed gas and the catalyst chamber were heated so that the temperature of the gas entering the catalyst chamber increased from about 60C. to about 600C.
over about 90 minutes. Samples of the inlet and exit gases were obtained periodlcally. The nitric oxide in these samples was oxidized to nitrogen dioxide. The resulting gas mixture was analyzed and the percent reduction in the nitric oxide concentration of the gas upon passing through the catalyst chamber was calculated. A smooth plot was made of the degree of conversion of nitric oxide at different catalyst chamber lnlet temperatures for each catalytic composition. From a smooth curve through each plot, temperatures were estimated for "light-off" tthe intercept with the temperature axis of an extrapolation of the portion of the curve at which the degree of conversion changed rapidly with temperature) and for nitric oxide conversions of 25%, 50%, and 90%. The catalyst temperature was higher than the catalyst bed inlet temperature with all the catalytic compositions at nitr~c oxide conversions greater than about 25~. The estimated temperatures for "light-off" and for 25%, 50%, and 90%
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converslon of nitrlc oxide before and after hoating the catalyst-coated honeycomb cylinders for 100 hours at about 900C. are given in Table II. ~-The catalytic actlvity of the "Torve~" cylinder~ ~ - eoated ~ith the catalytic composltion and blnder in the oxidation of carbon monoside was determlned ln a ~lmilar apparatus and by a similar procedure. Nitrogen containing about lO 000 parts per million of earbon monoxide and 10 000 parts per million of oxygen ~as pa~ed through the catalyst -10 ehamber and the entering and exiting ga~ mlxtures were analyzed -chromatographically using a column eontaining granules of ~Linden* 13X moleeular sieve. me estimated temperatures for "llght-off~ and for 25%, 50%, and 90% eonversion o~ earbon mDnosido before and after heating the eataly~t-coated honey-eomb eylinders for lO0 hours at about 900-C. are given in ~able II.
The catalytle activlty of the "Torvex" eylinder~ eoatod ~ith the eatalytie eompo~ltion in the osidatlon of propane ~as determlnod in a similar apparatus and by a slmilar pro-....... .
20 eedure. Nltrogen eontaining about 1300 part~ per milllon ofpropane ~as determined in a similar apparatu~ and by a ~imilar proeedure. Nitrogen eontaining about 1300 parts per m~llion of propane and 880 parts per million Or o~ygen was passed ~-through the eatalyst eh~mber and the entering and e~itlng gase~ ~ere analy~ed ehromatographieally uslng a eolumn eo~
taining 80-lO0 mesh "Poropak"* Q. The temporatures for "light-ofr~ and for 25%, 50%, and 90% eonversion of propane before and after heating the eatalyst-eoated honeyeomb eylinders ~or lO0 hours at about 900~C. are given in Table II.

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The stability of catalytic compositions of the , present invention and of comparative examples was tested in a reducing atmosphere. Samples of about from 20 to 110 milli-grams of each of the metal oxides listed in Table III were heated to 1000C. in a Du Pont Model 950 Thermogravimetric Analyzer in an atmosphere containing 1% hydrogen, 4% carbon monoxide, and 95% nitrogen. The indicated atmospheric percentages are by volume and the atmosphere was flo~Jing at a rate of 30 milliliters per minute. The temperature was increased in a programmed manner at a rate of 10C. per minute. The resulting changes in weight shown in Table III
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Equal parts by weight of finely ground portions of the metal oxides listed in Table IV were mixed separately with finely ground cordierite (a magnesium aluminum silicate having the approximate composition 2MgO 2A1203-5SiO2) and with finely ground quartz silica. The mixtures were heated at 1000C. for one hour on alumina plates (separate experi-ments showed no reaction of the metal oxides with alumina)and then finely ground. The extent of reaction of the metal oxides with cordierite and with silica based on com-parisons of the X-ray diffraction patterns of the ground mixtures before and after heating is summarized in Table IV.
The color changes observed during heating of the mixtures containing silica are also shown in TabIe IV. Metal oxides having Lattice Stability Indexes of 11.86 and less showed no reaction with cordierite and less than total reaction with silic'a.

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Claims (13)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. In a catalytic metal oxide having the general formula ABO3 and a perovskite crystal structure, in which A
and B are each at least one metal cation in sites of type A
and of type B, respectively, wherein:
(1) the type A cation sites are occupied by metals each having an ionic radius between 0.8 and 1.65 A;
(2) the type B cation sites are occupied by metals each having an ionic radius between 0.4 and 1.4 .ANG.;
and (3) substantially equal numbers of metal cations occ-upy the sites of type A and of type B, the improvement wherein:
(a) the Lattice Stability Index of the metal oxide is less than about 12.0 electron volts;
(b) at least about 1% of the type B cation sites are occupied by cations of at least one cat-alytic metal of the platinum group or having an atomic number of from 24 to 29; and (c) at least about 10% of the type B cation sites are occupied by cations of at least one metal differing from the catalytic metal and having a first ionization potential of no greater than 7.1 electron volts.

2. A catalytic metal oxide of Claim 1 wherein the catalytic metal is a platinum group metal occupying no more than about 20% of the type B cation sites.

3, A catalytic metal oxide of Claim 1 wherein at least about 10% of the type B cation sites are occupied by cations of a metal having an atomic number of 24 to 29.

4. A catalytic metal oxide of Claim 1 wherein cations of an alkali metal occupy at least about 10% of the type A or type B cation sites.

5. A catalytic metal oxide of Claim 1 wherein 1%
to 20% of the type B cation sites are occupied by ruthenium or platinum.

6. A catalytic metal oxide of Claim 1 wherein at least 1% of the type B cation sites are occupied by at least one metal of the platinum group and at least 1% of the type B
cation sites are occupied by at least one metal having an atomic number of from 24 to 29.

7. A catalytic metal oxide of Claim 1 wherein aluminum ions occupy at least about 10% of the type B cation sites.

8. A catalytic metal oxide of Claim 1 having the formula [Sr0.1La0.9][Al0.9Ru0.1]O3

9. A catalytic metal oxide of Claim 1 having the formula [Ba0.1La0.9][Al0.9Pt0.1]O3.

10. A catalytic metal oxide Or Claim 1 having the formula [Sr0.06La0.94][Al0.8Co0.16Ru0.04]O3

11. A catalytic metal oxide of Claim 1 having the formula [Sr0.3La0.7][Al0.7ru0.3]O3

12. A catalytic composition of Claim 1 on a shaped support.

13. A catalytic composition of Claim 12 wherein the shaped support is alumina.