CA2048748A1

CA2048748A1 - Catalysts

Info

Publication number: CA2048748A1
Application number: CA002048748A
Authority: CA
Inventors: William Carlysle Mackrodt; Martin Fowles; Michael Anthony Morris
Original assignee: William Carlysle Mackrodt; Martin Fowles; Michael Anthony Morris; Imperial Chemical Industries Plc
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1990-08-22
Filing date: 1991-08-08
Publication date: 1992-02-23
Also published as: ATE109682T1; DK0472307T3; JPH04250850A; DE69103360D1; ES2057776T3; EP0472307B1; EP0472307A1; GB9116780D0; GB9018409D0; US5137862A; DE69103360T2

Abstract

Abstract Catalysts Compositions suitable for use as oxidation catalysts essentially free from elements, or compounds thereof, of Group VIII of the Periodic Table, comprise an intimate mixture of oxides of at least three elements selected from Groups IIIa and IVa of the Periodic Table, including, of the total number of Group IIIa and IVa element atoms present, a) a total of at least 60% of atoms of at least one element X selected from cerium, zirconium, and hafnium, b) a total of at least 5% of atoms of at least one other element Y different from element X and selected from the variable valency elements titanium, cerium, praseodymium, and terbium; and c) a total of at least 5% of atoms of at least one element Z
differing from X and Y and selected from Group IIIa elements.
Such compositions, and similar two component compositions wherein X and Y are both cerium, have, after heating for 8 hours at 1200°C, a BET surface area of at least 1 m2.g-1 and are particularly useful for the oxidation of a feedstock with an oxygen-containing gas, eg catalytic combustion, especially under conditions such that the catalyst attains a temperature of at least 1000°C.

Description

874~
l B 35899 Catalysts This invention relates to catalytic oxidation and in particular to catalysts suitable for the oxidation of a-feedstock with an oxygen-containing gas, eg air.
One particular form of such oxidation is the combustion of a fuel with air, particularly with an excess of air to effect complete combustion.
In order to reduce the formation of oxides of nitrogen (NOx) when a fuel, eg gaseous hydrocarbons such as natural gas and/or hydrogen, is combusted with air, it is desirable to employ fuel/air mixtures of such composition that the adiabatic flame temperature is relatively low, desirably below about 1300C. For ~; many applications this means using a composition that is so rich in air that normal combustion is unstable and may not be self-sustaining. Catalytic combustion wherein a mixture of the fuel and air is passed through a bed of a combustion catalyst, enables ~; such problems to be overcome.
One application wherein catalytic combustion is desirable is in gas turbines. At initial start-up of a gas turbine, a mixture of the fuel and air, preheated, for example by a pilot burner, to a temperature typically of the order of 600-800C when the fuel is methane or natural gas, is fed, normally at superatmospheric pressure, eg at a pressure in the range 2 to 20 bar abs., to the inlet of the combustion catalyst bed. Combustion is effected at the catalyst surface forming a gas stream at elevated temperature. There is a rapid rise in the temperature of the catalyst bed to about the adiabatic flame temperature, typically about 1200C, when the catalyst lights-off.
The point at which this occurs is associated with the pre-heat temperature and the catalyst activity. Until light-off occurs, the solid temperature rises exponentially along the bed length.
The average temperature of the gas mixture increases more ` gradually as the gas mixture passes through the bed reflecting the increasing degree of combustion of the mixture. When the temperature of the gas mixture reaches a value, typically about " - 20~74~

9Q0C, at which homogeneous combustion commences, there is a rapid increase in the gas temperature to about the adiabatic flame temperature. When operating a gas turbine with catalytic combustion, when combustion has been established, it is usually desirable to decrease the preheating of the feed, eg to the temperature, typically about 300-400C, corresponding to the discharge temper~ture of the compressor compressing the air and fuel.
- It is seen therefore that the catalyst has to exhibit catalytic activity at a relatively lo~ feed temperature but has to withstand heating to relatively high temperatures of the order of 1000C or more without 1088 of that low temperature activity.
A]so, in gas turbine operation using catalytic ~ combustionr the catalyst not only has to be able to ~ithstand high i 15 temperatures, but also withstand the thermal shock of rapid ; temperature changes resulting from repeated stopping and starting of combustion. Also gas turbines ara usually operated using high gas flow rates. These conditions impose severe restraints on the materials that can be utilised as the catalyst.
~0 Combustion catalyts used under less severe conditions have commonly employed one Group VIII metals and/or oxides thereof supported on a suitable refractory support material. Examples of such metals and oxides that have be~en proposed include platlnum group metals, such as platinum, palladium, or rhodium, or mlxtures thereof, or iron, or nickel, in the metal or oxide form. We have found that for applications involving adiabatic $1ame temperatures above about 1000C, those catalysts are unsuitable. Thus in order to obtain a satisfactory activity the catalytically active material has to exhibit a high surface area; at ~he temperatures that are liable to be encountered, the aforementioned catalysts rapidly lose activity as a result o$ thermal slntering giving a decrease in the sur~ace area and/or as a result of the active material having an appreciable vapour pressure at such temperatures with consequential loss of active material through .

7 ~ g volatilisation, particularly where the gas stream has a high velocity gas stream.
We have found ~hat certain oxidic compositions that are essentially free from Group VIII metals or compounds thereof are particularly effective as combustion catalysts. Catalysts containing rare-earth oxides, ie the oxides of elements of atomic nu~ber 57-71, particularly ceria, have been proposed for catalytic combustion in numerous references, but those compositions generally also contain Group VIII metals as an active component, - 10 and 80 are unsuitable in applications where the Group VIII metal or oxide is liable to sinter and/or volatilise.
The rare earth oxides, ceria, terbia, and praseodymia are ionic oxides having the fluorite structure: this class of oxides also includes stabilisad zirconia, and hafnia. Ceria, terbiu, and praseodymia have defective structures and can be considered to be oxygen- deficient; terbia and praseodymia being more oxygen-deficient than ceria. It is believed that oxygen-deficient materials ~ive rise to catalytic activity, although ceria by itself has little catalytic combustion activity.
For use as a catalyst, one parameter of importance is its surface area, a high surface area being necessary. Materials of high surface area can be obtained, for example by careful evaporation of a solution of the corresponding nitrates or by precipitation of the oxide, or a precursor thereto. However we hsve found that exposure of high surface area oxidic materials having a high level of oxygen-deficiency to high temp?rAtures results in sintering with consequent loss o~ surface area and catAlytic activity. As a result the more highly oxygen-defic~ent fluorite oxides, praseodymia and terbia, are not themselves suitable as combustion catalysts.
We have found that certain ionic oxide compositions having the fluorite structure and containing two or more oxides of elements selected from Group IIIa (including the rare earths) or Group IVa of the Periodic Table (as published in the UK
"Abridgements of Patent Speciflcations~) 9 and including one or .

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more oxygen-deficient oxides, give catalysts that have significantly increased activity and that retain an adequa~e surface area after exposure to high temperatures.
It has been proposed by Machida et al in Kidorui 14 (1989) pl24-5 [Chem Abs 112 (9) 76121s] to employ a catalyst composition comprising ceria and ytterbia for the oxidative coupling of methane, but there is no indication that those catalysts would be of utility in at temperatures above 1000C.
It has been proposed in US-A-4940685 to stabilise high surface area ceria compositions for use as catalysts, or as supports for catalytic metals such as platinum, against sintering on exposure to high temperatures by the incorporation of up to 20%, particularly up to 5Z, of oxides of elements such as aluminium, silicon, ~irconium, thorium, or rare earths, such as lanthanum. There is however no suggestion that the stabiliser enhances the activity of ceria catalysts, nor that the stability is retained at temperatures above 1000C.
It has been proposed by Ter Maat et al in "Reactivity of Solids" (a Material Science Mono~raph edlted by Barrett & Dufour, published by Elsevier, 28B, 1984, pages 1021-1023) to employ certain compositions having a pyrochlore structure far the oxidation of carbon monoxide with oxygen. The pyrochlore ; compositions are said to have the structure A2B207 where A is a trivalent metal, eg a rare earth s~ch as neodymium, snd B is a tetravalent metal such as zirconiu~l. Ter Maat et al show that the replacement of up to 20Z of the zirconium atoms by cerium atoms increases the activity of the catalyst. It is seen that in these materials there are equal numbers of the ~etal A (eg neodymium) and metal B ~eg zirconium plus cerium) atoms.
The present invention provides an oxidatio~ process comprising reacting a feedstocX with an oxygen-containing gas in the presence of an oxidic catalyst under conditlons such that the catalyst attains a temperature of at least 1000C wherein the catalyst is essentially free from elements, or compounds thereof, of Group VIII of the Periodic Table, and, after heating for . .

' ~

' ' '' "''""'' ~, ` - ~0487~8 8 hours at 1200C, has a BET surface area of at least 1 m2.g~l, and comprises an intimate mixture of oxides of at least two elements selected from Groups IIIa and IVa of the Periodic Table, including, of the total number of Group IIIa and IVa element atoms present, a) a total of at least 60~ of atoms of at least one element selected from cerium, zirconium, and hafnium, and b) a total of at least 5~ of atoms of at least one Group IIIa element other than cerium; provid2d that a total of at least S~ are atoms of at least one variable valency element selected from titanium, cerium, praseodymium, and terbium; and the elemehts present include titanium and at least one Group IIIa element, or at least two Group IIIa elements; and the total amount of any cerium and any non-variable valency Group IIIa atoms is at least S~.
The catalysts used in the present invention notion~lly can be considered to consist of three components: an oxidic host material, an oxide of a variable valency element, and an oxide of a trivalent element that may be different from the variable valency element. As will be explained below, in some cases, oxides of the same element can perform two of these functions.
The catalytic oxidation reaction is thought to involve adsorption of oxygen atoms at the surface of the catalyst and the reaction of adsorbed oxygen with an electron to form a negatively charged oxygen species. It is believed that the variable valency element, herein designated Y, having a "defective" structure, provides a source of electrons, and an adjacent, different, trivalent element, designated Z, provides oxygen vacancies at the surface enabling oxygen to be adsorbed.
Cerium, praseodymium, titanium, and terbium are suitable variable valency elements Y. However praseodymia and terbia are so highly defective that they would rapidly loose surface area if unsupported. Titania does not have the fluorite structure and has a much lower melting point than the rare earths: also, under reducing conditions it forms sub-oxides. Consequently, while cerium can be used as both the host element, designated X, and as the variable valency element Y, in the case of praseodymium, ~37~8 terbium, or titanium as the variable valency element Y, a support is also required to provide the necessary thermal stability.
The host material is a selected from zirconia, ceria, and hafnia. These oxides have the fluorite structure which gives, and retains, an adequate high surface area in use. Since æirconia undergoes ph&se changes accompanied by significant volume expansiGn in the temperature range to which the catalyst is liable to be subjected in use, it has been conventional to incorporate stabilisers such as yttria, magnesia, or calcia, when using zirconia as a catalyst support. In the present invention, the oxides of the elements Y and/or Z will act as the necessary stabiliser. The support may comprise mixtures of oxides of different elements X.
As mentioned above, the second component is an oxlde of a variable valency element Y selected from praseodymium, terbium, cerium, and titanium. Mixtures may be employed. While cerium may be used as both the host element X and as the variable valency element Y, ceria does not have a very defective structure and so praseodymium and terbium are preferred as the variable valency element Y, especially where cerium is the host element X.
The third component of the catalyst is an oxide of a trivalent Group IIIa metal Z and is different to element Y.
Examples of such oxides are scandia, yttria, lanthana, ceria, praseodymia, neodymia, samaria, gadolinia, and terbia. While variable valency rare earths, vi~. cerium, praseodymium, and terbium, can be used as the third element Z, it is preferred that element Z is not a variable valency element, and, as it has been found that, at least for compositions containing only rare earth oxides, the catalytic activity increases as the trivalent ionic radius of the Group IIIa metal Z increases, lanthana is strongly preferred as the oxide of the element Z. M~x$ures of oxides of different Group IIla metals may be employed as element Z.
As noted above ceri~m can perform the function of host element X and varlable valency element Y or trivalent element Z, and likewise praseodymium and terbium can perfonm the functions of . ~
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variable valency element Y and trivalent element Z. It may not therefore be possible to distinguish between $he functions of the cerium, praseodymium and terbium.
The catalysts comprise an intimate mixture o oxides wherein, of the total number of Group IIIa and IVa element atoms present, a total of at least 60% are atoms of cerium, zirconium, or hafnium, ie the host element X, or, in the case of cerium, also the variable valency element Y. As a result the proportion of the trivalent Group IIIa element Z present is less than 40Z. In the catalysts of the invention a total of at least 5~ of the Group IIIa and Group IVa element atoms are at least one Group IIIa element other than cerium, thereby excluding catalysts comprising only ceria. To ensure that there is an adequate amount of the variable valency atoms, a total of at least SZ of the Group IIIa and Group IVa element atoms are atoms of the variable valency element selected from titanium, cerium, praseodymium, and terbium.
Since the compositions require Z atoms differing from Y atoms, the catalysts contain either oxides of at least two Group IIIa elements, at least one of which is a variable valency Group IIIa element, or oxides af titanium and at least o~e Group IIIa element. In order that there can be an adequate amount of Z
atoms, the total amount of cerium (which can act as Z atoms) and/or non-variable valency Group IIIa element present is at least 5Z of the total Group IIIa and Group IVa element atoms.
In preferred compositions containing praseodymia or terbia as the variable valency element, there are about 0.5 to 2.5 non-variahle valency Group IIIa element atoms for each variable valency element atam other than ceria.
During the production, or use, of the catalyst lt is subjected to high temperatures. It is believed that such heating gives rise to in m~gration of some species within the bulk of the catalyst with the formation of a different atomic composition and/or structure at the catalyst surface fsom that of the bulk catalyst. Although we do not wish to be limited by the following explanation, it is possible that the active species at the : ~;
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catalyst surface has the pyrochlors structure of the type described in the aforementioned paper by Ter Maat et al, 2g a composition of the form Z2.(Xl_X.Yx)2.07~ where x is typically up to 0.2, bulk compositions having such a large proportion of the element Z have been found to have a relatively low activity, possibly as a result of t~e element Z oxidej ie Z23~ rapidly segregating during heating during preparation of the catalyst and/or use to form discrete "islands" of the æ203 phase at the catalyst surface with consequent decrease in the area of active species accessible to the gaseous species to be adsorbed.
In preferred compositions the only oxides present are those of rare earths, so that cerium is the host element X, and element Y is cerium, terbium, and/or praseodymium, and element Z
is cerium or at least one other rare earth, provided that both Y
and Z are not both ceriu~. Preferably the components are present in such proportions that, of the total number of rare earth atoms present, the cerium atoms form 60-95%, the terbium atoms form 0-30%, the praseodymium atoms form 0-40%, and said other rare earth metal atoms form 0-40~.
Certain of the above compositions, those containing three or more components, are believed to be novel.
Accordingly the present invention also provides a composition that is essentially free from elements, or compounds thereof, of Group VIII of the Perlodic Table, and comprises an intimate mixture of oxides of at least three elements selected from Groups IIIa and IVa of the Periodic Table, including, of the total number of Group IIIa and IYa element atoms present, a) a total of at least 60~ of atoms of at least one element X selected from cerium, zirconium, and hafnium, b) a total of at least S~ of atoms of at least one other element Y different from element X and selected from the variable valency elements titanium, cerium, praseodymium, and terbium; and c) a total of at least SZ of atoms of at least one element Z differing from X and Y and selected from Group IIIa elements.

~referred compositions contain ceria, a) praseodymia and/or terbia, and b) at least one other rare earth oxide.
While the incorporation of terbia, and/or rare earths other than praseodymia or lanthana, gives some improvement to the activity of ceria based compositions, the most significant increases in acti~ity are found when the composition contains ceria, and praseodymia and/or lanthana.
Particularly preferred compositions comprise an intimate mixture in which cerium atoms form 60 to 90~ and a) praseodymium atoms form 5 to 35%, particularly 15-35Z, and/or b) lanthanum atoms form 5 to 40~, of the total number of rare earth metal atoms. Particularly preferred compositions contain ceria, and both praseodymia and lanthana.
Particularly preferred compositions comprise oxides o cerium, praseodymium, and at least one non-variable valency rare earth, especially lanthanum, in which 5-10~ of the total rare earth atoms are praseodymium atoms and there are 0.5 to 2.5 non-variable valency rare earth atoms for each praseodymium atom.
The compositions, after heating for 8 hours at 1200~C, have a BET surface area of st least 1 m2.g-1. Suitable compositions may be made by precipitation. Thus the intimate mixture of oxides may be formed by precipitating compounds of the relevant metals as compounds as oxides, or as compounds that decompose to oxides upon heating, from a solution of a suitable compound, eg nitrate, of the rel2vant element. The precipitation is conveniently effected from an aqueous solution using a precipitant such as an aqueous solution of an alkali metal, or ammonium, hydroxide or carbonate. The &ompounds required in the composition may be co-precipitated, eg by precipitation from a solution containing a miYture of compounds of the desired metals, or pre-formed precipitates may be intimately mixed, preferably before they are separated from the precipitation medium: for example a sequential precipitation procedure may be adopted - wherein one component desired in the composition is precipitated into a slurry &ontaining the previously formed precipitate of -` " 2 ~ 8 another component. After precipit tion, the precipitate or precipitates are washed to remove traces of the precipitant, dried, and then calcined if necessary to decompose the precipitated compounds to the oxides. By this method it is possible to obtain compositions which have a BET surface area above 1 m2.g~l even after heating the composition for 8 hours at 1200C.
Other methods of producing suitable intimate mixtures are known in the art and include evaporation o~ a solution containing a mixture of thermally decomposable compounds, especially nitrates, of the relevant metals to dryness followed by calcination to decompose the compounds to the oxides. Optionally the solution may contain an organic complexing acid, eg citric acid. Yet another method involves ball milling a mixture o~ the oxides or ccmpounds ther~tlly decomposable thereto.
The catalyst will generally be required in a supported form: a suitable refractory support, eg alumina, mullite, or sillcon carbide, preferably in the form of a honeycomb having a plurality of through passages, preferably at least 25 passages per cm2 of the honeycomb cross sectional area, may be coated with a slurry of the catalyst composition, followed by firing to form an adherent coating. Since there may be a tendency for some components of the catalyst to selectively migrate into the support, thereby depleting the catalyst coating of that component, it may be desirable to provide a barrier coat, for example of zirconia, between the support and the catalytic layer, to minimise such mi~ration, and/or to provide the support with a succession of coatings of differing composition such that negligible migration takes place from the outermost coating. Where a zirconia barrier coat is employed, this ls preferably of unstabilised zirconia: we have found that in the present compositions the usual sorts of stabilised ~irconia, eg yttria-stabilised zirconia, give less satisfactory results. Alternatively the catalyst may itself be formed into the desired shape, eg by extrusion into a honeycomb structure by the process described in ~B-A-1385907, particularly - - 2~7~8 ~ 11 B 35~9g using the technique described in EP-A-134138. However in order to provide R catalyst that can withstand the thermal shock that is liable to be encountered in some catalytic combustion applications, eg gas turbines, where the catalyst is to be used in a self-supporting form, it is preferred that it is produced in the form of a ceramic foam, for example by the processes described in GB-A-15375~9 and GB-A-20~7688. Alternatively the catalyst msy be a coating on such a foam made from a suitable support material.
~or catalytic combustion, typical operating conditions for the catalyst involve the passage of preheated fuel gas, eg natural gas, and air through a bed of the catalyst, eg through one or more honeycomb structures supporting or composed of the catalyst. During passage through the ca~alyst bedy combustion takes place with consequent increase in temperature. The outlet temperature is typically above 1000C, particularly above 1100C.
In gas turbine applications, the flow rate of the fuel gas and air ls high; typically the linear velocity of the fuel and air mixture through the cata]yst is in the range 25-150, particularly 5Q-100, m.s~l.
Another catalytic combustion application for which the catalysts are particulalrly suited as catalysts in radiant burners.
In addition to catalytic combustion, other oxidation processes are often operated at temperatures where stability of the catalyst is desirable. Examples of such other oxidation processes include partial oxidation of feedstocks such as propane, methane coupling, ammonia oxidation, the oxidative decomposition of nitrous oxide, and steam reforming of hydrocarbons. In addition, the catalysts of the present invention may be useful in oxidation reactions effected at relatively low temperatures, eg the oxidation of paraxylene to terephthalic acid.
The invention is illustrated by reference to the following Examples, some of the compositions of which are given by way of comparison.

20~ 48 Examples 1-45 In examples 1-21 and 36-40, the catalysts were prepared by forming an aqueous solution of nitrates of the desired elements in the desired proportions: the strength of the solution was such that the total metal content of the solution was about molar. The metals were precipitated from this solution by addition of 2M
ammonium bicarbonate solution at ambient temperature and the precipitate filtered, washed until free from alkali and then dried at lZ0C for 16 hours. The dried precipitate was formed into pellets.
The three and four component catalysts of Examples 22-35 and 41-44 were made by mixing solutions of nltrates of the relevant metals in the desired proportions followed by evaporation of the solutions to dryness, calcination at 450C.
Catalytic activity is assessed by the following technique. A sample of the catalyst is heated in a stream of air for 8 hours at 1200C to age the catalyst and is then crushed and sieved to obtain a size fraction in the range 1-1.4 mm diameter.
After ageing, the samples in accorclance with the present invention had a BET surface area in an excess of 1 m2.g-1. A known weight, occupying a volume of about 2.5 cm3, of the aged material is charged to an electrically heated reactor provided with an outlet gas analyser to monitor the carbon dioxide content of the outlet gas. A mixture of air containing 2% by volume of methane is passed through the reactor at atmospheric pressure at a rate of 0.5 m3.h~l and the temperature increased from ambient temperature to 400C at a rate of 400C.h-l, and then 8t a rate of 200C.h-until the monitored carbon dioxide content of the outlet gas indicates that the combustion is complete. To aid comparison between different catalysts, the rate of carbon dioxide formation per gram of catalyst at a temperature of 600C is deter~ined. The activity a~ thls temperature was chosen since at this temperature the extent of reaction is relatively small so that the bed approximates to isothermal conditions. At higher temperatures, '`. ::

` ~ 2Q~74~
. 13 B 35899 particularly with the more active catalysts, an appreciabl~
temperature rise would occur thus obscuring comparison.
The compositions and results are shown in the following Table 1.
Table 1 _______________________________________~___________________ Composition (~ by metal atoms) I Activity Example l------------------------------------l (mmol/h/g) Ce I Pr I Tb I La I Other 100 1 l l l l 1 100 1 l l I O
3 1 l l 1 100 1 1 0 . 1 5 l l l l I Gd 100 t : 15 1 6 1 95 1 5 1 1 I 1 5 7 1 75 125 1 l l 1 26 8 1 50 150 1 l l I lZ
I 9 1 25 175 1 l l 1 3 : 110 1 95 1 1 5 1 l l 8 14 1 87.5 1 1 1 12,5 1 1 26 116 1 50 l I 1 50 1 1 14 95 ~ Gd5 1 3 1 .
19 1 95 ~ Sm5 1 4 120 1 90 1 l l I Sm10 1 10 21 ~ 80 1 l l I Sm 20 ~ 7 122 1 90 1 5 ~ 1 5 1 1 13 24 1 75 1 8 1 ~ 17 1 1 44 : 125 1 70 115 1 1 15 1 1 41 : 30 126 1 60 130 1 1 10 1 1 21 27 ~ 60 120 1 1 20 1 1 29 : 131 1 25 150 1 ' 25 1 1 5 _ ______ , , 8 7 ~ ~
.

Table 1 (continued) ______ ___________________________~________________________ 7 I Composition (% by metal atomæ) I Activity I Example '------------------------------------' (m~ol/h/g) I
' I Ce I Pr I Tb I La I Other l l_________l______l______l______l______l________l_____ ______l ' 33 190 ' I 5 1 5 1 1 9 1 34 l80 1 110 1 10 1 ' 22 1 35 170 l 115 1 15 ' I 20 10 1 36 195 1 l l I Sc 5 ' 2 1 37 19S I ~ I I Yt 5 1 2 ' 38 '95 ' l l , Nd 5 ' 5 1 39 190 1 1 l I Nd 10 1 10 40 185 1 l I I Nd 15 , 7 I 1.' I I I I I

; 15 1 4~ 180 1a , 2 1 10 1 1 22 1 43 175 110 15 1 10 I j 23 1 44 175 120 15 1 l l 17 ______~____________________________________________________ Examples 1-5 show that the rare earths ceria, praseodymia, terbia, lanthana and gadolinla themselves have negligible activity. Examples 6-10 show that the addition of praseodymia or terbia to ceria gives a significant improvement to the activity, but at high praseodymia levels the improvement over ceria alone is only marginal. Examples 11 to 21 show that the modification of ceria by the incorporation of lanthana, gadolinia, or samaria, gives sn improvement i~ activity, and this is particularly marked in the case of lanthsna, although the improvement decreases at high lsnthana levels. Examples 22 to 32 show that particularly beneficial results are obtained by using ceria, praseodymia, lanthana mixtures. ~xamples 33 to 35 show ceria, terbia, lanthana mixtures. ~xamples 36 and 37 show the use of scandia or yttria in place of a rare earth as a modifier for ceria. Examples 38 ~o 40 show ceria/neodymia mixtures. Examples 41 to 43 show four component compositions and Example 44 shows a ceria/praseodymia/terbia mixture.

:' . ;. ~' . : "

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2~48~48 ~g~
A zirconia/yttria/praseod~mia composition was made up by evaporation of a mixted nitrates solution as described above to give the proportions, by metal atoms, Zr 66.4, Yt 16.7 and Pr 16.7. The activity when tested as above, was 7 mmoles/glh. The aged sample had a BET surface area of 3.2 m2/g.
xame~_46 To illustrate the segregation of lanthana from a ceriallanthana composition to the surface, a composition was made containing 5% lanthana and 95% ceria (by metal atoms) by the precipitation route described above. Samples of the composition, after calcining at 450C, were aged for 4 hours at different temperatures, and the composition of the surface, rather than the bulk, determined by X-ray photoelectron spectroscopy., The results are shown in the following Table 2.
Table 2 ______________________________________________ Ageing I La in 6urface composition temperature tC) I ~% by metal atoms) I llO0 1 41 1400 1 ~0 ~his example clearly demonstrates that considerable segregation of the lanthanum atoms takes place on ageing at high temperatures. It is seen that at about 1200-1300C, the surface has a composition approximating to a half monolayer covsrage of lanthana, but at higher ageing temperature~ a much higher proportion of the surface was lanthana.

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Example 47 A further series of ceriaj~raseodymia/lanthana compositions was made by $he e~aporation of nitrates solution method described abo~e, calcined at 450C, and then aged for 8 hours at different temperatures. The acti~ity of these compositions was tested as described above but at a temperature of 561C. The compositions, acti~i.ties, and surface sreas after ageing at the specified temperature (Tage) are as set out in the following Table 3.
Table 3 Composition I Activity (mmol/g/h) I Surfsce area (m2/g) ¦
(~ metal atoms) I at Tage (C) ¦ at Tage (C) I____________________I___________________ _I_____________________, I Ce I Pr ' La ~ 1100 1 1200 1 1300 ' 1100 1 1200 1 1300 l______l______l______l______l______l_______l______l______,_______l 1 75 1 0 125 113 1 7 1 3 1 7.2 1 4.5 1 2.1 75 1 8 117 131 110 1 5 1 6.8 1 4 4 1 1 8 I 75 1 17 1 8 111 ' 8 1 3 1 6.0 1 3.3 1 1.3 1 75 1 25 1 0 134 1 6 1 ~ ~ 9.0 1 2. 4 1 1 . 0 60 1 35 1 5 1 7 1 4 1 1 1 3.2 1 1.1 1 0.4 60 1 30 110 113 1 7 1 4 1 3.9 1 2. 5 1 1 . 2 60 1 20 120 112 110 1 4 1 4.9 1 3.2 1 1.7 60 1 10 130 114 1 3 1 5 1 5 . 1 1 3.5 1 2.0 1 60 1 5 135 113 1 6 1 2 1 4.4 1 2 . 2 1 0 . 7 1 75 112.5 112.5 123 110 1 4 1 6.9 1 3.5 1 1.4 1 96 1 2 1 2 1 6 1 2 1 1 1 4.1 1 1.7 1 0.8 ____________________________________________________________ ___ The surface composition of some of the above samples aged at 1200C was determined by X-ray photoelectron spectroscopy.
The bulk and surface oompositions were as shown in the following Table 4.

' 2 ~ 8 ,, Table 4 _________________________ ____________________________________ Bulk composition I Surface composition (% metal atoms) I (Z metal atoms) I-----------------------------------------1 1 Aged at lZ00C I Agsd at 1300C
l____________________l____________________l____________________l Ce ~ Pr I La I Ce I Pr I La I Ce I Pr I La l______l______l______l______l______l______l______l______l______l 1 60 1lO I30 140 1 4 1 56 143 1 8 1 49 75 117 1~3 162 127 111 1 63 126 111 75 1 8 117 1 - ~ - I - I 66 113 121 ______________________________________________________________ This again shows that there is substantial migration, . particularly of lanthanum atoms from the bulk to the surfa /
/

.

Claims

1. A composition that is essentially free from elements, or compounds thereof, of Group VIII of the Periodic Table, and comprises an intimate mixture of oxides of at least three elements selected from Groups IIIa and IVa of the Periodic Table, including, of the total number of Group IIIa and IVa element atoms present, a) a total of at least 60% of atoms of at least one element X selected from cerium, zirconium, and hafnium, b) a total of at least 5% of atoms of at least one other element Y different from element X and selected from the variable valency elements titanium, cerium, praseodymium, and terbium; and c) a total of at least 5% of atoms of at least one element Z differing from X and Y and selected from Group IIIa elements.

2. A composition according to claim 1 containing ceria together with praseodymia and lanthana.

3. A composition according to claim 2 wherein the cerium atoms form 60 to 90%, praseodymium atoms form 5 to 35%, and lanthanum atoms form 5 to 40%, of the total number of rare earth metal atoms in the composition.

4. A composition according to any one of claims 1 to 3 containing praseodymia or terbia as the variable valency element, having 0.5 to 2.5 non-variable valency Group IIIa element atoms for each variable valency element atom other than ceria.

5. A composition according to claim 4 comprising ceria, praseodymia, and a non-variable valency rare earth, in which 5-10% of the total rare earth atoms are praseodymium atoms and there are 0.5 to 2.5 non-variable valency rare earth atoms for each praseodymium atom.

6. A composition according to any one of claims 1 to 5 which, after heating at 1200°C for 8 hours, has a BET surface area of at least 1 m2.g-1.

7. A composition according to any one of claims 1 to 6 in the form of a coating on a refractory support.

8. The use of a composition according to any one of claims 1 to 7 as a catalyst for the oxidation of a feedstock with an oxygen-containing gas.

9. An oxidation process comprising reacting a feedstock with an oxygen-containing gas in the presence of an oxidic catalyst under conditions such that the catalyst attains a temperature of at least 1000°C wherein the catalyst is essentially free from elements, or compounds thereof, of Group VIII of the Periodic Table, and, after heating for 8 hours at 1200°C, has a BET surface area of at least 1 m2.g-1, and comprises an intimate mixture of oxides of at least two elements selected from Groups IIIa and IVa of the Periodic Table, including, of the total number of Group IIIa and IVa element atoms present, a) a total of at least 60% of atoms of at least one element selected from cerium, zirconium, and hafnium, and b) a total of at least 5% of atoms of at least one Group IIIa element other than cerium;
provided that a total of at least 5% are atoms of at least one variable valency element selected from titanium, cerium, praseodymium, and terbium; and the elements present include titanium and at least one Group IIIa element, or at least two Group IIIa elements; and the total amount of any cerium and any non-variable valency Group IIIa atoms is at least 5%.

10. A process according to claim 9 wherein the oxidation process is the combustion of a fuel with an excess of air.