WO2013021395A1

WO2013021395A1 - Catalyst for after-treatment of exhaust gas from an internal combustion engine

Info

Publication number: WO2013021395A1
Application number: PCT/IN2011/000677
Authority: WO
Inventors: Rajan S. BOSCO; R. S. Senthilkumaran; Rustom Minocher CURSETJI; Touquire A. SIDDIQUIE
Original assignee: Süd-Chemie India Ltd.
Priority date: 2011-08-10
Filing date: 2011-09-29
Publication date: 2013-02-14

Abstract

The 3-way catalytic converter of the invention comprises first and second oxidation elements and a reduction element supported respectively on non-OSC(oxygen storage component) type, OSC-type and alumina-free carriers, for example, lanthana-stablilised alumina, ceria- zirconia stabilised alumina and ceria-zirconia solid solution composite respectively. The ceria-zirconia ratio in the alumina-free and alumina-based carriers is zirconia rich and ceria- rich respectively. Said first and second elements and the reduction element are selected from an oxidation elements group (e.g. Pt or Pd) and a reduction elements group(e.g Rh or Pd), the primary roles thereof being CO + HC oxidation, associate role of catalysing the activity of the reduction element, and of NOx reduction respectively. Three compositions: Pt/Rh, Pd-Pt/Rh and Pd-Pd/Rh are disclosed that exhibit increased conversion efficiencies, reduced catalyst metal loadings, better light-off performance, back-up capacities and offer other operational, manufacturing and techno-commercial advantages. Consideration of metal support reactions has brought down noble metal loadings and enhanced catalyst activity and durability. Pd-distribution between the two layers is an important parameter.

Description

Catalyst for After-Treatment of Exhaust Gas from an Internal Combustion Engine

Introduction

This invention relates to the three-way catalytic converters (TWC) such as are used in the treatment of engine exhaust gases in internal combustion(IC) engine vehicles and in other similar applications such as stationery IC engines and others. More particularly, the invention relates to a said three-way catalyst. Background to the invention

Catalyst converters are used in automotive vehicles to treat the exhaust gas coming from the engines. In said treatment, unburnt hydrocarbons(HCs) arising from the partial combustion of the fuel, are converted into C02 and H20, carbon monoxide(CO) into C02 and various nitrogen oxides collectively represented by the expression NOx into nitrogen. A catalyst converter that takes care of all the three said pollutants is referred to as a three-way catalyst or three-way catalytic converter (TWC for short).

Although other metals catalyse said conversions, the noble or precious metals

platinum(Pt), palladium(Pd) and rhodium(Rh) are the most commonly used. These metals are referred to herein by their abbreviated periodic table names shown in brackets adjacent their names. Said three metals are collectively referred to herein as the platinum or precious group of metals or PGMs for short. For the purposes of this specification, the term PGM is intended to cover all precious and non-precious metals and compounds that are suitable for catalytic purposes in TWCs for IC engines. The two terms 'platinum group of metals' and 'precious group of metals' are considered to be synonymous herein.

All the three PGMs catalyse the three said conversions(reactions) but in view of the differences in their activities and other characteristics and scope of application, a TWC commonly comprises at least two said PGMs and sometimes all three so as to take advantage of said activity differences and avail of cost benefits and other advantages in manufacturing, operation and others. A Pd-only TWC is also known in the art.

Although there are other metals in the platinum group, this invention is mainly concerned with said three PGMs named hereinabove. Said limitation is in the interests of conciseness and clarity of the description and is without limitation to the scope of the invention.

A TWC comprises:

(i) a substrate;

(ii) a carrier; and

(iii) one or more catalyst elements.

A substrate is the mechanical base or support upon which the catalyst architecture is built up. In automobile TWCs, the substrate comprises a ceramic or metal monolith having numerous pores and channels(passages) that provide a large surface area for the catalyst sites. One or more catalyst elements together with the carrier materials thereof are distributed/dispersed on said pore/channel surfaces and other surfaces of the monoliths. Silicon carbide monoliths are also used. Said substrates are also referred to by other terms such as monoliths, supports, carriers, catalyst carriers and others in the art.

A said carrier is the component over which the catalyst element(s) is distributed/dispersed or impregnated. The carrier is laid in the form of a layer on the external and internal surfaces of the monolith. In a completed catalyst assembly(TWC), said layer(s) extend over the outer surfaces of the monolith and over the walls of the said pores and channels thereof.

Catalyst element(s) catalyse said reactions mentioned hereinabove. They are dispersed or impregnated onto said carrier material generally before the latter is laid down in the form of layers on the monolith. Carriers are also sometimes referred to as 'supports' in the art. The practice of catalyst element(s) dispersal/impregnation on previously deposited said layers is also adopted in the prior art. There are known processes in the art for dispersing or impregnating the catalyst metal(s) (catalyst elements) on carrier materials such as the pore filling and incipient wetness methods.

For said layer formation, the loaded carrier material(loaded with the catalyst element(s)) is dispersed in a liquid, such as water giving a slurry that is referred to as a washcoat slurry. Said layers are formed by applying such washcoat slurries on the monoliths.

Layer(s)(coating(s)) of the desired specifications are generated either by controlled immersion of the monoliths in said washcoat slurries or by the incipient wetness method wherein a monolith is contacted with a pre-determined amount of said slurry to yield the desired catalyst loading.

Said layers are also referred to as washcoats, washcoat layers and other terms. The fixing of said layers on the substrate is done by means of calcination. Optionally, binders are used in said fixing processes. Fixing without resorting to calcination is also practised in the art.

A monolith may have more than one said layers(coatings) and each said layer(coating) may contain more than one said carriers. As mentioned, a said carrier may be loaded with one said catalyst element or a plurality thereof within the scope of the invention.

Said catalyst elements may be one or more of said PGMs mentioned hereinabove. They provide the primary catalytic action. They are also referred to as active elements and as catalysts herein. They may be supplemented by promoter compounds and other additives. Within the scope of the invention, said catalyst elements may be any of the other PGMs or other metals, oxides and other compounds that are catalytically active with regard to the said automobile exhaust gas conversion reaction(s). Within the scope of the invention, said catalyst elements may also be a mixture of one or more of said PGMs.

Carrier materials are commonly one or the other of the refractory inorganic oxides(RIOs) such as the one or the other of the alumina phases or the rare metal oxides or mixtures thereof. Aluminas, and in particular, gamma-alumina, or mixtures thereof with other alumina phases, are preferred because of their higher specific surface areas, that is, greater surface area per unit weight.

Carrier materials are also referred to as insulator oxides. Said gamma alumina, and sometimes the other alumina phases also, are referred to as active alumina in the art. Non-alumina carriers comprising oxides of metals other than aluminium, such as for example, the oxides of the rare earth metals are also used. Carriers may additionally be impregnated/dispersed with any of the other metal oxides or other compounds as required for carrying out other functions. Examples of such functions are oxygen storage, stabilisation of the PGM, stabilisation of the carrier structure, promoters for the said reactions, modifiers, doping and others.

The completed catalyst architecture comprising the substrate, the support and the catalyst element(s) is referred to herein as the 'catalyst assembly^* or as the three-way catalyst. Said assembly(that is the three way catalyst or TWC for short) may additionally contain one or more further components having functions such as described in the preceding para. A said three-way catalyst(TWC) may comprises a single catalyst element or a plurality thereof within the scope of the invention.

The term alumina-free, or non-alumina, is intended to mean a compound, composite or a component of the catalyst architecture, such as a carrier, that is substantially free of alumina. The term non-OSC, or non-OSC type, is intended to mean that the material referred to is substantially free of any oxygen storage component and that the oxygen storage capacity thereof is insignificant or insubstantial. An example of an oxygen storage component commonly used is ceria.

The term OSC-typeJs intended to mean that the material referred to comprises oxygen storage component(s) and the function thereof, is partly or fully, to provide oxygen storage capacity.

The considerations that have gone into the making of this invention and the objectives that have been set therefor are summarised hereinbelow.

The activity of rhodium for catalysing the NOx reaction is higher than that of the other two PGMs, Pt and Pd. On the other hand, both Pt and Pd are more active with regard to the oxidation reactions involving CO and HC than Rh. As far as the said oxidation reactions are concerned the highest activity is reported to be exhibited by Pt.

Said higher activity of Rh in respect of NOx conversion arises from its property of disassociative adsorption of NO and the subsequent reaction thereof with CO.

Rh is more active in catalysing the steam reforming reaction also. In this reaction, the unburnt HCs are steam reformed to CO and H2. The un burnt hydrocarbons(HCs) are high when the engine is running on low A/F(air/fuel ratios) such as in starting and high acceleration situations Under these conditions, the NOx level is also high and the contribution to NOx conversion through the said steam reforming route assumes importance. The ratio of the A/F value to the stoichiometric value is denoted by the greek letter 'lambda'. According to this invention, therefore, for the purposes of catalytic architecture design the PGMs can be classified into a first group and a second group: the said first group about 30:1 by wt. More preferably, the ratio should be from about 15:1 to about 6:1 by wt. In this range, there is optimum utilisation of the two metals which helps in minimising the metal requirement. The total catalyst element loading can be from about 10 g/cu.ft to about 100 g/ft3 but preferably from about 15 g/cu.ft to about 30 g/cu.ft. A proper division of the Pd component between the layers yields the advantage of very low metal requirement for the TWC as a whole while availing of the benefits of a lower light- off temp, and better conversion efficiency with regard to all the pollutants. The ratio of Pd in the inner layer to that in the outer layer is preferably about 2:3 by wt to about 1 :19 by wt. More preferably, the said ratio is from about 1 :3 by wt to about 1 :9 by wt.

The key advantages of this novel architecture in summary are: a) High oxidation activity of the outer layer where the Pd is on a non-OSC layer leading to lower light off temperature; b) Enhanced oxygen storage capacity of the catalyst from the inner layer where the Pd is on an OSC material, leading to higher activity at operating temperatures; c) The Pd of the inner layer is shielded from direct exposure to sulphur, present if any in the gases. Its location in the inner layer makes it less prone to sulphur poisoning.

The outer layer Pd which is in the said first oxidation role is supported on the non-OSC alumina carrier of the invention. Preferably, the bulk of the Pd component is provided in the outer layer rather than in the inner.

The two part Pd system of Ex. 3 exhibits certain unexpected performance advantages which are elaborated hereinbelow. The advantages and features of the systems of Examples 1 and 2 are also summarised hereinbelow. Within the scope of the invention, a mixture of Pt and Pd may be adopted for either the said associate role or the oxidative role or both.

- 21 - comprising PGMs that have a relatively higher activity for said reduction reaction(s) associated with NOx conversion than for said oxidation reactions associated with CO and HC conversion and the second group comprising those PGMs that have a relatively higher activity for the said oxidation reactions.

A PGM that has a reasonable activity for the reactions of a particular group may be classified in that group. A PGM may belong to both said groups provided it has reasonable level of activity for both said sets of reactions. Within the scope of the invention, the activity of a member of a said group for the reaction associated with the other said group may be negligible.

Said first group is referred to herein as the reduction catalysts group or the Rhodium group or Rh-group for short. It comprises Rh, Pd and Pt other metals and compounds yet to be classified. Similarly, said second group is referred to herein as the oxidation catalysts group or the Platinum group or the Pt-group in short. The latter is also referred to herein as the non-Rh group. It comprises Pt and Pd and other metals and compounds yet to be classified.

Of the three PGMs that are most commonly used and that are being considered in this description(without limitation to the scope of the invention), Rh falls into said first group and Pt in the second. The catalytic activities of Pd with regard to the two reaction groups are such that it can be considered to belong to both said groups.

The other PGMs can also be classified into one or the other of the abovementioned groups. Within the scope of the invention, said groups may include the platinum metals, other precious metals and also non-precious metals oxides or other materials that catalyse the respective reactions.

It may be noted that references to said reduction and oxidation catalysis groups has no connection with the periodic classification of elements. Said terms are as hereindefined

- 6 - and are a classification of the catalyst metals on the basis of their relative activity levels with respect to said oxidation and reduction reactions.

By adopting the classification disclosed hereinabove, it becomes possible to treat the TWC architecture design as an assembly of functions. Said functions provide precise criteria for the selection of suitable catalyst elements and the appropriate and optimum carriers therefor. Thus, said design becomes an operation of bringing together the element-carrier combinations of the invention in different arrangements. The use of said functions as the criteria in the selection of the appropriate catalyst elements and carriers results in architectures that are better performing and more optimised combinationsfTWC assemblies) of the respective catalyst elements than otherwise. It also affords surprisingly, considerable reduction in the catalyst noble metal requirements. This is novel.

The provision of said catalyst element groups and of suitable dedicated carriers(supports) therefor is apparently a novel concept and allows one to formulate numerous optimised architectures suiting different technical and techno-commercial factors, of cost, material availability, engine specifications, infrastructural factors, climatic factors, emission control requirements and others.

Another novel functional distinction is defined by this invention. A TWC requires two said oxidation elements. The requirement of at least two said oxidation catalyst elements arises because of the two functions they are required to perform. The first function is to catalyse the CO and HC conversion reactions. The second is to play an associative role in the functioning of the NOx conversion catalyst, namely Rh or other catalyst element of that group, said associative role being the catalysing of the Rh redox reaction. This is elaborated hereinbelow. Apart from its property of catalysing said CO and HC conversion reactions, Pt possesses the property of catalysing the reduction of Rh from a higher oxidation state to a lower, or

- 7 - the lowest, oxidation state thereof. This action is referred to herein as the associate role. Both Pt and Pd are capable of performing said associate role.

During operation, Rh undergoes a cyclical redox transformation swinging from a higher oxidation state to a lower one and back. This occurs in synchronisation with the cyclical swings in the A/F(air/fuel) ratio that arise from the action of the electronic control system of the engine. Such A/F oscillations occur mainly during cruising phases but may also occur in other driving phases. The catalysing of said Rh redox reduction reaction by Pt results in a higher conversion of the NOx component of the exhaust and better dynamical behaviour of the TWC. Pt is more preferable for this function than Pd because of the higher activity thereof for catalysing the said Rh reduction reaction. Thus, this invention lays down three functions to be considered when devising a catalyst architecture:

(i) reduction function of carrying out NOx conversion;

(ii) oxidation function of carrying out CO and HC conversion; and

(iii) associate role in supporting the said reduction function (i).

The features and properties required of the carriers for the PGMs for the said three functions are different. This invention finds that said required features and properties largely correlate better with the said functions rather than with the catalyst elements themselves. Thus, this invention offers another aspect of novel standardisation in the selection of the carrier materials and in the construction of catalyst architectures.

The object of the invention is to provide a suitable carrier for each said function such as results in better and optimised catalyst performance in any combination of the three functional element-carrier combinations. The object of the invention is also to provide flexibility and manouvrability in TWC architecture design arising from such

standardisation of functions and carriers.

- 8 - The carrier selected for a said function must preferably be such that it can be adopted with any of the catalyst elements that may be selected for the said function. Said combinations should be such as to permit the catalyst element-carrier combinations to be brought together in different arrangements to obtain different TWC architectures that are highly efficient catalysts and meet different technical and techno-commercial requirements.

The system of selecting elements from said groups and the said set of three carriers, of the invention is very versatile in so far as optimised TWC architectures can be more easily formulated to meet numerous combinations of technical and techno-commercial factors, demands and requirements.

Apart from the above, the various embodiments are found to have unexpected synergies and surprising and advantageous performance characteristics that are elaborated hereinbelow. Thus, more objectives and advantages of the invention will be apparent from the description and claims herein.

Brief description of the invention

According to the invention, therefore, there is provided a three-way catalytic converter (TWC) for treating the exhaust gases from internal combustion engines such as in automobile and other vehicles and for other applications, comprising inter alia, a monolith provided with one or more coatings(layers) of carrier material, with the catalyst dispersed thereupon and/or impregnated thereinto, said catalyst comprising:

(a) at least, one said reduction catalyst comprising one or more catalyst elements, or mixtures thereof, from the reduction catalyst group as hereindefined, for primarily treating the nitrogen oxides(NOx) gases in said exhaust and supported on a substantially alumina-free ceria-zirconia composite carrier; and

- 9 - (b) at least, a first and a second oxidation catalysts, each comprising one or more catalyst elements, or mixtures thereof, from the oxidation catalyst group as hereindefined, said first oxidation catalyst being primarily for CO and HC conversion and the said second being primarily in an associate role as hereindefined; and

(c) a first carrier of the non-OSC type as hereindefined such as, for example,

lanthana stabilised alumina and a second carrier of the OSC-type as hereindefined such as for example, ceria-zirconia stabilised alumina; said first oxidation catalyst being supported on a said non-OSC type first carrier and said second oxidation catalyst being supported on a said OSC type second carrier, and the metal oxides recited herein being substitutable by equivalent metal oxides, or mixtures thereof, belonging to the respective metal oxide series.

Detailed description of the invention

The various catalytic architectures according to the invention comprise at least one said reduction catalyst and at least a said first and second oxidation catalysts, each said catalyst comprising one or more catalyst elements of the respective said catalyst groups.

The terms 'catalyst' and 'catalyst matter' as used herein may refer to one said catalyst element or a mixture thereof. Said terms may also refer to the set of catalyst elements on a said carrier, or a said layer(coating) or may refer to the TWC as a whole. The meaning appropriate to the context may be taken.

In addition to referring to said PGMs, the term 'element' is intended to also mean a part or portion of a quantity of a said PGM. The question of 'part or portion' arises where a catalyst element is selected to play dual roles, for example, the said oxidation and associate roles.

- 10 - Thus, where a PGM is performing the said dual role, a part thereof is notionally considered to be performing the CO + HC oxidation role and the other part the said associate role. A catalyst element, as understood in this specification may also be a mixture of two or more said PGMs. The meaning appropriate to the context and representing the widest scope may be taken.

Preferably, this invention provides Rh in the said reduction catalyst role. Rh is adopted primarily for the conversion of NOx to nitrogen. Furthermore, two catalyst elements are selected from said oxidation catalyst group(Pt group) for the two functions: CO/HC oxidation and the said associative role.

Within the scope of the invention, said pair of two oxidation elements may be the same element(either Pt or Pd) or may be different. In case of the latter, both permutations, the pair Pt-Pd and the pair Pd-Pt are feasible and within the scope of the invention.

Pt is the preferred choice for the said Rh-associate role and so the pair Pd-Pt is the more preferred configuration as far as the choice of the said first and second oxidation elements respectively is concerned. The Pd is preferably in the inner layer with the Pt/Rh on the outer to give the configuration Pd-Pt/Rh. The Pd in this configuration is in the said first oxidation role.

Combination Pt-Pt for the said pair of roles is also feasible and within the scope of the invention. In this arrangement, the two parts of the Pt may be on different carriers, one an OSC carrier and the other a non-OSC one and installed on separate layers. The first oxidation part of the Pt preferably constitutes the inner layer and the Rh-associate part together with Rh, the outer layer. Preferably the Rh-associate part Pt and the Rh in the outer layer are housed in a common said layer but on separate dedicated supports (carriers). The different carriers of the invention are discussed further hereinbelow.

- 1 1 - It is also feasible, indeed preferable to merge the two Pt portions into a single mass and mount the same on a common OSC carrier. The Pt and Rh in this construction are in the same said layer but dispersed on their individual carriers. It may be noted that in the case where said first and second oxidation catalysts comprise the same element, that is, two portions of the element in a single mass such as in

Examples 1 and 2 hereinbelow, the preferred carrier therefor is the said second carrier, namely, ceria-zirconia stabilised alumina OSC type carrier of the invention. This is because in the absence of an OSC component, the TWC functions less efficiently. An OSC is an essential component of a TWC.

The use of Pt in the said dual role gives a single layer TWC(reference Example 1) wherein the lanthana stabilised alumina carrier of the invention is not utilised. This is not a drawback as the Pt interaction with the OSC component is less susceptible to sulphur poisoning than in the case of Pd.

Such a single entity arrangement can be combined with a said additional third oxidative component. Preferably, said third component is Pd. It is generally on the inner layer screened from the bulk flow of the exhaust gas. This gives the architecture configuration Pd-Pt/Rh. It will be observed that this Pd-Pt/Rh architecture considered here is different from the Pd-Pt/Rh configuration discussed further hereinabove.

The difference is that the Pd in one is in the role of said first oxidation catalyst whereas the status of Pd in the other is that of the said optional third oxidation element that is adopted to provide the extra benefit of a better light-off performance, back-up and increased working life for the TWC.

Apart from enhancing light-off performance and a longer life, the said additional Pd gives back-up capacity to the TWC and brings down the Pt loading.

- 12 - The Pd-Pt/Rh architecture embodiment of the invention wherein the Pd is in the said additional oxidation catalyst role and is housed in the inner layer of a two-layer system is Example 2 described hereinbelow. Another feasible embodiment is one wherein the two parts of the Pt component are separate and distinct and carried on separate carriers as mentioned hereinabove. This configuration is Pt-Pt/Rh. The two parts of Pt in this arrangement are in separate said layers. Within the scope of the invention, the two Pt parts with their separate carriers may be in a common layer to give the configuration Pt/Pt/Rh. Within the scope of the invention, the first Pt can be in the said back-up role, that is, said third oxidation role while the second Pt can be a two-part, single entity Pt component that performs the said first oxidation and associate roles. The two parts of Pt component may be on separate carriers and in separate layers. The arrangement wherein said inner and outer Pt are in the said first oxidation and associate roles is also feasible. In this case, the carriers to be adopted are non-OSC type and OSC type respectively.

As mentioned, the three major functions of a TWC are the oxidation function with respect to the CO and HC components, the reduction function in respect of the NOx component and the Rh-associate role.

The associate role Pt is preferably placed close to the Rh but not on a common carrier. Preferably, the associate Pt and the Rh are on segregated carriers on a common layer as provided in the embodiments(Examples 1 and 2) constructed by this invention. This is novel. Other arrangements are within the scope of the invention.

On the other hand, Pd in the associate role may be located farther away from the Rh and may be installed in a separate but adjacent layer as is provided in Example 3. In example 3, which has a Pd-Pd/Rh configuration, the inner layer Pd is in the said associate role while the outer Pd is in the first oxidation role. The Pd and Rh of the outer layer are on

- 13 - separate and dedicated carriers. The carriers for the inner and outer Pd components are OSC type and non-OSC type respectively in accordance with the invention. Preferably said OSC type and non-OSC type carriers are ceria-zirconia stabilised alumina and lanthana-stabilised alumina respectively of the invention.

It is clarified that the attribution of functions such as CO + HC conversion, the associate role and NOx conversion to the catalyst elements is not intended to be exclusive. It may be understood that the elements would be performing functions other than the primary functions assigned to them in the architecture. By associating elements with functions in this specification, it is intended to mean that the named function is the primary function of the element in the catalyst architecture under consideration and that it may very well be performing other functions to a greater or lesser degree in the architecture.

This invention provides that the associate role Pt is preferably located close to the Rh but not in intimate contact such as would arise if they are installed on a common carrier. Thus, this invention provides separate and dedicated carriers for the Pt and Rh while locating them in a common layer as will be observed in Examples 1 and 2. Within the scope of the invention said associate role Pt may be dual role Pt as is the case in

Examples 1 and 2 or otherwise.

The feature of placing the Pt and Rh in a common layer but on dedicated and separate carriers is novel.

Where Pd is in the said associate role, the Pd and Rh may be put together in proximity in a common layer but on dedicated carriers within the scope of the invention. But preferably they are located farther apart in separate layers, as is provided in Example 3 of the invention.

The preferred TWC configurations covered above are a single layer Pt/Rh and two dual layer architectures: Pd-Pt/Rh and Pd-Pd/Rh. These are preferred architectures in the context of certain sets of technical and techno-commercial factors. It will be observed,

- 14 - that within the scope of this invention, many other architectures can be formulated to meet other sets of technical and techno-commercial demands and factors. As mentioned, the three abovementioned preferred embodiments are elaborated in Examples 1-3 hereinbelow.

Two configurations that could meet the requirements of other technical and techno- commercial situations are: Pt-Pt/Rh, Pt-Pd/Rh. The inner Pt can be the said first oxidation element or a said third oxidation catalyst. In the second configuration indicated, the inner Pt could be in the associate role although associate Pt is preferably close to the Rh in a common layer. In each of the abovementioned configurations the individual PGM mentioned may be a mixture of PGMs within the scope of the invention.

In the catalyst architecture according to the invention, wherein the two said oxidation roles(first and second) are being performed by a single catalyst element, the same may be considered to notionally comprise of two parts, one said part(portion) primarily to attend to the oxidative function(item (ii) above) and the other primarily to play the said associative role(item (iii) above).

As mentioned, said two notional parts may be in a single mass or in separate layers or separated but on a common layer. If on a common layer they may be preferably installed on separate carriers. Other constructions are within the scope of the invention.

Said common catalyst element may also be a mixture of two or more oxidation catalyst elements within the scope of the invention. Thus, said first and second parts of a said dual role catalyst element may each comprise a mixture of the PGMs.

Where said two roles are performed by a single catalyst element(from the oxidation group) there may be an additional oxidative element, that is, a third oxidation element within the scope of the invention. The additional element also provides back-up capacity.

- 15 - In this arrangement, the said two notional parts of the said single catalyst element are a single entity as in the constructions of Example 1 and 2, but can be in two separate parts within a layer. The said third oxidation element is preferably Pd but can be any of the others within the scope of the invention. Said third element Pd is preferably located in an inner layer and more preferably in the innermost layer. The advantages of the third element Pd and the inner layer location thereof are elaborated hereinabove. The embodiments and examples presented herein demonstrate the versatility of the basic conception of the invention. It will be observed that numerous other embodiments are possible within the scope thereof.

In the first embodiment(Example 1), platinum has been selected to play the dual role(items (ii) and (iii)) above. Both said parts thereof are in the form of single consolidated mass dispersed on a single carrier. Said consolidated Pt mass together with the Rh are housed in a common said layer but on different and dedicated carriers.

In the second embodiment(Ex. 2) also Pt is in the said dual role. The architecture further includes Pd which is in the role of said third oxidation back-up catalyst.

Where Pt is adopted in the said dual role and the said two portions thereof are a single entity, the same is supported on an OSC type carrier such as the ceria-zirconia stabilised alumina carrier of the invention. In this case, the non-OSC carrier, that is, lanthana stabilised alumina of the invention is not utilised and is redundant. This is because if a selection has to be made between the two said carriers, OSC type and the non-OSC type, it has to be the OSC type as oxygen storage is a necessary requirement for the efficient performance of a TWC.

- 16 - If the two Pt portions are separate and are not a single entity, then the two portions may be supported one each on the OSC type and non-OSC type carriers of the invention according to the roles allotted for the two portions. The Pt(Examples 1 and 2) is supported on an OSC-type ceria-zirconia stabilised alumina carrier and the Pd(Ex. 2) on a non-OSC type lanthana stabilised alumina carrier. The Pt with the carrier thereof is in the outer layer together with the rhodium with the non- alumina carrier thereof. As would be noted, the Pt and Rh are on dedicated carriers. The Pd, with its carrier, comprises the inner layer. Within the scope of the invention, the said inner layer Pd can also be a mixture of Pt and Pd on a common carrier.

The Pd in the inner layer is screened from sulphur and lead poisons in the exhaust, if any, by the outer layer.

The Pt in the architecture of Ex. 1, although is a single entity, notionally it comprises two parts as discussed hereinabove.

If the two parts are separate, this invention provides for an OSC ceria-zirconia carrier and a non-OSC alumina carrier but as the two parts are a single entity the OSC carrier takes precedence and the single entity Pt is installed on the OSC carrier of the invention.

In Ex. 2, the Pt/Rh assembly of Ex. 1 has been adopted without any change and the same carrier retained. Within the scope of the invention, the inner layer Pd which is presently a said third oxidation catalyst could be made the associate catalyst and the Pt left to perform only the first oxidation role. However, Pt is better in the associate element role and as such, Ex. 2 configuration is to be preferred over the alternative mentioned.

It will be observed that Ex. 1 is a single layer TWC while Ex. 2 is a twin layer configuration.

- 17 - In the basic architecture of Example 1 , the Pt:Rh ratio by wt. can be any value within the scope of the invention. However, with a high Pt proportion, some of the said reduction role will fall on the Pt. The adoption of Rh in the said reduction role is desirable as it is more efficient at performing this role. A lesser amount of the Rh metal is required in contrast to the amount of Pt required for equivalent conversion of the NOx components of the exhaust gases. It will be observed that the overall metal requirement is brought down by the adoption of Rh and more specifically the requirement of Pt which is significant particularly if the availability of Rh is greater and the price lesser. Thus, this invention provides for said ratio(Pt:Rh) to be preferably from about 25: 1 by wt to about 1 : 1 by wt. More preferably, the said ratio is from about 18: 1 to about 2: 1 by wt.

While the overall metal loading in the architecture according to Example 1 can be any value within the scope of the invention, preferably the catalyst element loading is from about 5 g/cuft. to about 50 g/cu.ft. It will be observed that considerably reduced loadings as compared to prior art architectures are possible in the configuration of Ex. 1. This is true also of other configurations of the invention. In each of the examples of the invention, considerably higher conversion efficiencies together with other advantages are obtained in comparison with prior art catalyst models with equivalent noble metal loadings. The basic twin-layer configuration of Example 2, offers the flexibility of varying the proportions of Pt and Pd to suit different techno-commercial or operational requirement. For the reasons mentioned above in the discussion on the architecture of Ex. 1, having a very low Rh component defeats the purpose of adopting Rh. This invention observes that the significant ratio is therefore, the (Pt+Pd):Rh ratio. This invention preferably requires the said ratio[(Pt+Pd):Rh] to be from about 2: 1 by wt. to about 20: 1 by wt. For a specific value of this ratio, it will be observed that it is possible to vary the proportion Pt to Pd in the architecture to suit operational and techno- commercial factors. Thus, the Pt:Pd ratio within a specific value of (Pt+Pd):Rh can be from about 1 :0 by wt. to about 0: 1 by wt. In Ex. 2, the Pd is in the said third oxidation role to provide lower light-off temps, and back-up capacity.

- 18 - This invention observes that the above considerations apply even where the said Pd is, partly or fully, in another role such as for example, first oxidation or the said associate role. Within the overall (Pt+Pd) contribution, any distribution between the two components is feasible within the scope of the invention.

In this architecture also, said total metal loading may be any value within the scope of the invention but is preferably from about 10 g/cuft by wt. to about 200 g/cuft. by wt. From these figures, it will be observed that very low metal loadings are feasible in this architecture as in the other examples and embodiments of the invention. The low metal loadings of the invention do not require increases in monolith volumes, collateral or otherwise.

Within the scope of the invention, it is possible to put the two said parts of Pt on separate carriers to segregate the roles thereof. The segregated Pt parts may be in the same layer as the Rh or otherwise. The carriers in this configuration would be the OSC type, the non-OSC type and the non-alumina carriers of the invention respectively for the Pt,Pt,Rh combination.

Alternatively the carrier combination can be non-OSC, OSC and non-alumina carriers respectively within the scope of the invention corresponding to interchanged roles for the said two parts(portions).

In a system such as Pt-Pt/Rh the two Pt parts located on the inner and outer layers can be in the roles of either the third oxidation and the dual role or in the first oxidation and associate role respectively within the scope of the invention.

In the third embodiment according to the invention, Pd has been selected for said dual role. Here, the two parts of the Pd component are on separate carriers and in separate layers. The Pd-part constituting the said second oxidation catalyst, that is, intended primarily for the said associative role is housed in the inner layer and is on the OSC carrier of the invention, namely, ceria-zirconia stabilised alumina.

- 19 - The major component of the Pd which functions as primarily the first oxidation catalyst is supported on a non-OSC carrier and the minor component of the Pd that is primarily performing the said associate role is bound to an OSC type carrier. The main object of the minor Pd component is to enhance the oxygen storage capacity of the inner layer.

The Pd of the outer layer is primarily performing the first oxidation role with the major part of the oxygen for the CO+HC oxidation coming from the inner layer. It is by this division, with the majority of the Pd on the outer layer, that it is possible to get advantages of both the low light off from Pd on a non-OSC carrier and the increased catalyst activity on account of the OSC function kicking in at higher temperatures.

Therefore preferably the Pd is divided such that the inner Pd is considerably lower than outer Pd. Within the ratio ranges mentioned below, the outer Pd is able to exert a beneficial influence on the inner Pd.

The ratios in which the Pd content can be distributed between the inner and the outer layer according to this invention is typically from about 2:3 to about 1 : 19 by wt. and preferably from about 1 :3 by wt to about 1 :9 by wt. It is observed that when Pd is deposited on a ceria containing wash coat it gets oxidised by the ceria to the less active Pd + state whereas the Pd on the non-OSC alumina remains in the more active metallic Pd state. The Pd of the outer layer is thus in the more active zerovalent state and is ideal for the primary oxidation function which ensures low light off temperature as well as improved conversions at the operating temperatures due to the enhanced oxygen storage of the inner layer.

The Pd:Rh ratio can be any value but is preferably not less than about 2.T by wt. This is because below this value both the Pd functions, namely, first oxidation and the associate role suffer bringing down the conversion efficiency of all the reactions. As the proportion of the Pd is increased the risk of the formation of the Pd-Rh compound increases. This invention has determined that the ratio should preferably not exceed

- 20 - Three carriers are provided by this invention. They are:

(a) a non-alumina carrier comprising ceria-zirconia solid solution composite formed by a solution combustion method;

(b) an OSC type carrier comprising ceria-zirconia stabilised alumina composite; and

(c) a non-OSC type carrier comprising lanthana stabilised alumina.

Each said carrier mentioned may further optionally comprise one or more suitable dopants within the scope of the invention. They may also comprise other components for the various functions such as promotion of catalysis and others as outlined hereinabove.

In view of the propensity of Rh to react with alumina at high temperatures, this invention provides an alumina-free(non-alumina) carrier for Rh(or other reduction catalyst element such as Pd or any mixtures of the said reduction elements).

This carrier(item (a)) is exclusively for Rh, and the other reduction PGMs. It enhances the thermal stability of the Rh component and together with the other said element-carrier combinations enhances the high temperature performance of the TWC as a whole. The ceria-zirconia solid solution composite of the invention is preferably formed by a solution combustion process.

The CO and HC conversion function(first oxidation role) preferably requires a non-OSC carrier. This invention provides a non-OSC lanthana stabilised alumina carrier for this role.

The carrier provided for the associate role by this invention is an OSC-type ceria-zirconia stabilised alumina. Ceria is the main oxygen storage component herein and as such the composition is preferably ceria-rich. Whereas in the ceria-zirconia solution formed by the solution combustion method, which constitutes the non-alumina carrier of the invention, the composition is preferably zirconia-rich. A couple of other RE(rare earth) metal oxides have the capacity to enhance ceria's oxygen storage capacity or act as supplemental oxygen storage. Within the scope of the invention they may be employed in OSC compositions in which they add to oxygen storage capacity. The PGMs, particularly Pd, also exhibit a small amount of oxygen storage capacity by virtue of their variable valency.

In this type of combination, it is possible to design a distribution of the metal crystallite size over a defined range so as to facilitate both structure sensitive and non-structure sensitive reactions.

This invention adopts ceria for oxygen storage in the form of a ceria-rich ceria-zirconia stabilised alumina carrier(item (b)) for the associate role PGM. The stabilisation improves the high temperature performance and reduces high temperature aging. The zirconia contributes to stability in addition to enhancing ceria's oxygen storage capacity. Ceria catalyses the water-gas shift reactions and also stabilises the catalyst elements against sintering at high temperatures.

The noble metal interactions with the support play a significant role in optimising the design of the catalyst for activity and durability.

The non-OSC type Ianthana-stabilised alumina carrier of item (c) provided by this invention is meant for supporting the PGM that is in the said first oxidation role. It may be used in association with Pt or Pd or other oxidation catalysts. The interaction between the catalyst element and the support material can alter the catalytic activity of the elements in the CO-HC oxidative role and also possibly make them more prone to sulphur poisoning. Hence, the adoption of lanthana stabilised alumina as the carrier. Lanthana provides the structural stability to the alumina and minimises sintering. During operation under lean conditions, Pd forms Pd oxide which gets reduced to Pd metal during rich operating conditions. Pt, generally speaking, does not react in the same way as Pd. It is therefore, desirable to put Pd if it is in the first oxidation role on non-OSC lanthana stabilised alumina carrier to provide the redox potential, in accordance with the invention.

This invention provides for a balance between the requirement for oxygen storage capacity and for optimum redox potential of the oxidative components. The ceria- zirconia composite carrier provides adequate oxygen storage property for the purposes of NOx conversion.

The invention therefore preferably provides a Pt/Rh combination for said NOx reduction function where said Pt is in close proximity to the Rh component but not on a common carrier.

The alternative is a Pd/Rh combination. It is particularly necessary for Pd and Rh to be on separate dedicated carriers in view of the propensity of Pd and Rh to form

intermetallic compounds at high temperatures and affecting the high temperature stability of the TWC.

As for the said oxidation catalyst, two permutations are provided by the invention: Pt - Pd and Pd - Pt for the CO and HC conversion role and the Rh reduction associate catalysis role respectively. Two further combinations are Pt-Pt and Pd-Pd for said two roles. They may be on separate layers or in the same layer. Thus, single layer and twin layer configurations are possible as discussed hereinabove.

The possible single layer configurations are Pt/Rh and Pd/Rh wherein the two parts of both Pt and Pd are single entities. If the two parts are separate the single-layer configurations are depicted as Pt Pt/Rh and Pd/Pd/Rh. If Pd is provided in said two separate and physically isolated parts, the part in the said associative role is preferably on the OSC type ceria-zirconia stabilised alumina carrier of the invention. If the two Pd parts are together then the carrier has to be the OSC type for reasons mentioned hereinabove.

It is preferable that Pd is in two separate and isolated parts if it is adopted in the said dual role of said CO/HC oxidation and associative roles as dividing the Pd component offers a number of advantages. The ceria content in carrier (b) is more than in carrier (a). The zirconia content of carrier (a) is greater than that of carrier (b). The ceria and zirconia contents are expressed herein in molar ratios. The molar ceria-zirconia ratio is in favour of ceria in carrier (b) and in favour of zirconia in carrier (a). The catalyst elements may be installed on the carriers thereof by any of the known methods in the art. In this invention, pore filling procedure has been adopted. The dispersed material is fixed by means of calcination. Fixing by means other than calcination is known in the art. The art also optionally provides the use of suitable binders.

In the pore-filling method adopted in the examples hereinbelow, the PGM precursor and the respective carrier material powders are mixed in a planetary mixer for a

predetermined duration of time. The mixture is then dried and calcined in flowing air at about 550 C for about three hours. The precursor may be preferably in the liquid form. Preferably, the precursor material is a solution of a compound of the PGM employed.

This procedure is carried out with each of the PGMs going into a TWC architecture. If a PGM element in the TWC is in two parts, the procedure is separately carried out for each said part and each such part impregnated into the carrier thereof.

Other methods of said dispersal/impregnation are within the scope of the invention. The impregnated carrier is then slurried in water to give a washcoat slurry. Milling of the slurry is undertaken to reduce the particles to the desired size range. The slurry is then applied to the catalyst monolith such as to form a layer of the carrier material thereupon. The layer is formed both on the external surfaces of the monolith and the internal surfaces thereof, that is, on the walls of the pores and channels thereof. Said layers are also referred to as washcoats. This is the method adopted in the examples presented hereinbelow. There are known procedures in the art for the formation of said washcoats. In one procedure, the monolith is repeatedly dipped in said slurry. Said dippings are continued till the required catalyst loading on the monolith is achieved. Between dippings, the monolith is dried. Calcination may also be adopted between said dippings. In another method, a pre-determined amount of said slurry is taken corresponding to the desired said catalyst loading. The slurry is applied and the procedure repeated till the entire quantity is adsorbed on the monolith. Between dippings the formed layer may be dried or calcined or both. Where two PGMs(PGM mixtures) are required to be installed on a layer, the two impregnated carriers containing the two PGMs are mixed in the desired ratio and then slurried in water. The slurry may be applied to the monolith by any of the procedures mentioned hereinabove or other known methods. Binders may be optionally added to the slurry to assist layer formation.

After the layer is formed, the monolith is calcined. Where multiple layers need to be formed calcination is preferably adopted following the formation of each layer. Said size reduction of suspended material in the slurry may be carried out by any of the known methods. Calcination following each layer formation has been adopted in the examples give hereinbelow. Liquids other than water may also be adopted for slurry formation. Within the scope of the invention, one or more dopants may be added to the catalyst elements. Additional components such as stabilisers, promoters, modifiers, oxygen storage compounds, dopants and others are incorporated at the stage of mixing of the catalyst precursors and the carrier material powder.

If two said catalyst elements are required to be dispersed/impregnated on a single said carrier a mixture of the two precursor solutions is prepared and the same is then dispersed on the carrier by the pore filling method. The two elements may also be dispersed sequentially within the scope of the invention.

In summary, this invention provides three catalytic elements: Pt, Pd and Rh and three respective carrier compositions that can be combined to give different catalyst architecture configurations.

As mentioned hereinabove, only the three catalyst elements Pt, Pd and Rh have been incorporated in the discussion and description herein. This is in the interests of conciseness and no restriction of the scope of the invention is implied. The scope of the invention includes all said elements and compounds that can catalyse the said different reactions of the exhaust gases coming from internal combustion engines of vehicles or stationary systems.

The substantially alumina-free ceria-stabilised zirconia solid solution composite carrier adopted in the invention is produced by the solution combustion procedure and was found to exhibit surprising advantages such as increased layer stability and others.

The three TWC embodiments disclosed in detail herein are:

i. Single layer TWC comprising Pt/Rh, the former on an OSC-type ceria- zirconia stabilised alumina composite and the latter on an alumina-free ceria- zirconia solid solution composite produced by the solution combustion process. In the first carrier, namely the OSC carrier, the molar ceria-zirconia ratio is in favour of ceria in contrast to the alumina free ceria-zirconia carrier wherein the said ceria-zirconia ratio is in favour of zirconia.

Two layer TWC comprising Pd-Pt/Rh, the Pd of the inner layer being supported on a non-OSC type lanthana stabilised alumina carrier, the Pt and Rh of the outer layer having a configuration similar to the configuration of the Pt/Rh pair in the single layer TWC described in item (i). The Pd is the said third oxidation catalyst.

Two layer TWC comprising Pd-Pd/Rh, the Pd of the inner layer being the associate catalyst and supported on an OSC type ceria-zirconia stabilised alumina composite carrier of the type adopted for supporting Pt in items (i) and (ii), the Pd of the outer layer being said first oxidation catalyst and being supported on a non-OSC type lanthana stabilised alumina carrier. The Rh is supported on an alumina-free carrier similar to the one adopted for Rh in items (i) and (ii). Said primary oxidation catalyst is also referred to herein as the first oxidation catalyst.

The three carrier configurations of the invention are summarised hereinbelow:

a. an alumina-free ceria-zirconia solid solution composite for the rhodium in all the three abovementioned embodiments comprising a ceria-zirconia solid

solution composite produced by the solution combustion process. The molar proportion of zirconia is greater than that of ceria in this carrier composition. b. an OSC type ceria-zirconia stabilised alumina carrier also referred to herein as the first alumina-based support, said support incorporating ceria in an

oxygen storage capacity, for the said associate catalyst component, that is

for Pt in items (i) and (ii) and for the inner layer Pd in item (iii). The molar content ceria content is greater than that of zirconia in this composition. c. A non-OSC type lanthana stabilised alumina carrier also referred to herein as the second alumina-based carrier, said carrier being substantially ceria-free and adapted for supporting the said first oxidation catalyst element. It is also suitable for supporting Pd in a said first oxidation catalyst role in the Pd/Rh combination of the Pd-Pd/Rh embodiment. In this embodiment the inner Pd is in the said associate role.

Within the scope of the invention, numerous combinations of the elements and supports can be contemplated. A wide range of technical and techno-economic factors and requirements can be taken into consideration and catalytic architectures designed to meet such requirements. The said three PGMs and the said three carrier compositions of the invention can be juggled to give a versatile range of TWCs. Different architectures can be formulated from the above to take into account different requirements with regard to:

(i) emission levels;

(ii) engine types, capacities and configurations;

(iii) fuel specifications;

(iv) climatic and road conditions and user(driver) preferences,

(v) production and use patterns with respect to the three catalytic elements; and

(vi) situations as regards the availability and price levels of the catalytic elements.

The features/advantages of the 1 -layer Pt/Rh TWC embodiment described in Example 1 are:

(i) the Pt and Rh components enhance the oxygen storage (OSC) function.

(ii) the negative interactions between Rh and alumina are avoided. (iii) the combination of Pt and the ceria-zirconia stabilised alumina support enhances the catalytic activity with regard to CO and HC oxidation in a wider lambda window.

(iv) the interaction of Pt with the ceria-zirconia stabilised alumina support is

optimised by reference to TPR(Temperature programmed reduction) studies such as to avoid delayed light-off and hold down the propensity for sintering of the catalyst metal and carrier.

(v) providing a solid solution ceria-zirconia composite support wherein zirconia is the major component optimises the properties thereof with regard to promoting NOx reduction, providing large surface area and thermal stability to the catalyst assembly.

(vi) the tendency for the reduction of the ceria-zirconia by Rh is optimised in this architecture.

(vii) the provision of Rh and Pt in a single layer, although on different carriers maximises the conversion efficiencies of all three pollutants and minimises diffusion constraints.

The features/advantages of the 2-layer Pd-Pt/Rh TWC embodiment described in Example 2 are:

(i) the features/advantages associated with the Pt/Rh combination elaborated above also apply to this architecture.

(ii) the negative interactions between Pd-OSC component and Rh-alumina have been avoided in this architecture.

(iii) the Pd-alumina combination facilitates oxidation reactions.

(iv) the Pt and Rh component enhances the oxygen storage (OSC) function.

(v) the location of the Pd/lanthana stabilised alumina in the inner layer screens the Pd from poisoning by sulphur if any in the exhaust gases.

(vi) the reducing action of Pd promotes CO oxidation during operation at low A/F ratios resulting in a lower light-off temperature. (vii) the absence of ceria and zirconia in the carrier for the Pd ensures that sulphur adsorption on the support, and therefore S-poisoning of the Pd, is substantially absent. The features/advantages of the 2-layer Pd-Pd/Rh TWC embodiment described in Example 3 are:

(i) in a system having Pd distributed between two layers, the catalyst durability is enhanced by splitting of the catalyst metal between OSC and non-OSC washcoats located in separate layers.

(ii) the negative interactions between Rh and alumina have been avoided in this architecture.

(iii) the Pd distribution ratio between the two layers is optimised to give high

conversion efficiencies and durability of the catalyst.

(iv) the Pd in the outer layer supported on lanthana(La203) stabilised

alumina(A1203) enhances thermal stability, reduces light-off temperature and gives better CO and HC conversion efficiencies,

(v) a certain amount of Pd deactivation by the interaction of Pd with ceria- zirconia composite support thereof in the inner layer is built into this architecture. It is balanced by the increased catalyst durability and oxygen storage function provided by the architecture. Another balancing advantage is that the Pd/ceria-zirconia composite also catalyses the CO + NOx reaction which is important in low A/F ratio operation. In order to provide a clearer understanding of the invention and without limitation to the scope thereof, three embodiments thereof are described in detail hereinbelow with reference to the accompanying drawings wherein:

Fig. 1 is a graph giving the comparison between the conversion efficiencies of the embodiment of Example 1 with that of the composition of Example 1A which has the same catalyst element composition but wherein the Pt and Rh elements are installed on a common alumina-based carrier;

Fig. 2 is a graph giving the comparison between the conversion efficiencies of the embodiment of Example 2 with that of the composition of Example 2A which has the same catalyst element composition but wherein the Pt and Rh elements are installed on a common alumina-based carrier; and

Fig. 3 is a graph giving the comparison between the conversion efficiencies of two samples of the embodiment of Example 3, one plot being for the sample in a fresh condition and the other being for the sample subjected to aging.

Embodiments Example 1

Platinum sulphite acid and rhodium nitrate were the precursors used for Pt and Rh respectively.

A quantity of the Pt precursor was taken on the basis of about 28.6 g of Pt per ft3 of monolith volume. This precursor was impregnated on ceria-zirconia stabilised alumina carrier by the pore filling method using a planetary mixer. The impregnated powder was calcined in flowing air at about 550 C for three hours.

A quantity of the Rh precursor was taken on the basis of about 14.3 g of Rh per ft3 of monolith volume. This precursor was impregnated on ceria-zirconia solid solution composite made by a solution combustion method. The impregnation was carried out using the pore filling method using a planetary mixer.

The impregnated carrier was calcined in flowing air at about 550 C for three hours.

The two impregnated powders were mixed in the proportion of Pt.Rh of about 2: 1 by wt. and then slurried in distilled water.

The slurry was milled to obtain an average particle size of about 1 -10 microns(D50). This slurry was washcoated multiple times on the monolith to give about 2500 g. of layer material per cu. ft. of the monolith volume, after drying. The monolith was then calcined before taking up for evaluation.

The precious metal loading on the monolith was about 43 g/cu.ft. and the Pt/Pd/Rh ratio was about 2/0/1.

This TWC composition corresponds to Formulation 1 in the performance comparison graph shown in Fig. 1. Example lA(Based on prior art)

The parameters of this composition are substantially as in Example 1. The preparation method followed is as specified in Example I, the only difference is that the Pt and Rh in this composition are on a common carrier unlike in Example 1 wherein the Pt and Rh elements are on dedicated carriers.

The common carrier adopted in this example is an alumina-based carrier in contrast to the non-alumina ceria-zirconia composite Rh-carrier of Example 1.

The composition of this example is referred to as Formulation 2 in the performance comparison graph shown in Fig. 1.

Comparison of compositions 1 and 1A

The two formulations were evaluated on a modal gas test reactor bench. Fig. 1 compares the conversion efficiency. The graph(Fig. 1) shows clear advantages in adoption of dedicated carriers for the Pt and Rh and installing Rh on a zirconia-rich alumina free support. The segregation of the carriers, that is, adoption of dedicated carriers and the choice of the carriers for Pt and Rh appears to improve oxidation function with better CO and HC conversions. In particular, the provision of a zirconia-rich non-alumina carrier gives better reduction performance.

Example 2

Precursors of the three PGMs, Pt, Pd and Rh were taken. The precursors were Platinum sulphite acid, palladium nitrate and rhodium nitrate respectively. Each was individually impregnated on the respective carrier thereof. The carriers were ceria-zirconia stabilised alumina, lanthana stabilised alumina and ceria-zirconia solid solution composite produced by the solution combustion process respectively.

A quantity of the impregnated Pd precursor was taken on the basis of about 13.6 g of Pd per cu. ft. of monolith volume. This was impregnated on the lanthana stabilised alumina carrier by the pore filling method using a planetary mixer. The impregnated powder was calcined in flowing air at about 550 C for three hours.

The impregnated powder was slurried in distilled water.

The slurry was milled to obtain an average particle size of about 1-10 microns (D50).

This slurry was washcoated, multiple times on the monolith to give about 1 136 g. per cu. ft. of layer material on the monolith per cuft thereof, after drying.

A quantity of the Pt precursor was taken on the basis of about 13.6 g of Pt per cu. ft. of monolith volume. This was impregnated on ceria-zirconia stabilised alumina carrier by the pore filling method using a planetary mixer. The impregnated carrier was calcined in flowing air at about 550 C for three hours. A quantity of the Rh precursor was taken on the basis of about 2.72 g of Rh per cu. ft of monolith volume. This was impregnated on a ceria-zirconia composite using a planetary mixer. Said ceria-zirconia composite was made by the solution combustion method. The impregnated carrier was calcined in flowing air at about 550 C for three hours.

The two impregnated powders were mixed in the proportion of Pt:Rh of about 5: 1 by wt. and then slurried in distilled water.

The slurry was milled to obtain an average particle size of about 1 -10 microns(D50).

This slurry was washcoated multiple times on the monolith with the said Pd layer deposited thereon to yield about 1363 g. of deposited layer per cu. ft. of the monolith volume, after drying. The monolith was then calcined before taking up for evaluation. The precious metal loading on the monolith was about 30 g/cu.ft. and the Pt/Pd/Rh ratio was 5/5/1.

This composition is referred to as Formulation 1 in the comparison graph presented in Fig. 2. Fig. 2 provides a performance comparison of this composition with a substantially identical composition made by a generally identical procedure except that the Pt and Rh elements are on a common alumina-based carrier.

Example 2A(Based on prior art)

The parameters of this composition are substantially as in Example 2. The preparation method followed is as specified in Example 2, the only difference is that the Pt and Rh in this composition are on a common carrier unlike in Example 2 wherein the Pt and Rh elements are on dedicated carriers.

The common carrier adopted in this example is an alumina-based carrier in contrast to the non-alumina ceria-zirconia composite Rh-carrier in Example 2. The composition of this example is referred to as Formulation 2 in the performance comparison graph shown in Fig. 2.

Comparison of compositions NO. 2 and 2A

The comparison is shown in Fig. 2 which is a plot of conversion efficiency as measured by mass emissions with the two compositions. The two compositions were evaluated after hydrothermal ageing at 900 C with air and 10% steam.

Segregating the carriers for Rh and Pt and providing separate optimum carriers for the two metals appears to show improved thermal stability and better oxidation function. Pt supported on an optimised OSC-type carrier appears to enhance HC oxidation. Pd supported on a non-OSC carrier apparently leads to better oxidation than when supported on an OSC-type carrier support.

Example 3

Precursors of the two PGMs, Pd and Rh were taken. The precursors were palladium nitrate and rhodium nitrate respectively. The batch of the palladium precursor was divided into two parts to give totally three precursor batches: Pd, Pd and Rh.

The ratio of Pd in the two batches was about 1 :4.5. Each batch was individually impregnated on the respective carrier thereof. The carriers were ceria-zirconia stabilised alumina, lanthana stabilised alumina and ceria-zirconia solid solution composite produced by the solution combustion process, as described hereinabove.

Quantities of the said three precursor batches were taken on the basis of: 1 ^st Pd - about 5 g/ft3 2^nd Pd - about 22.5 g/ft3 3^rd Rh - about 2.5 g/ft3 The three precursor batches were prepared on the basis of the figures given above. The first precursor batch containing the first Pd was impregnated on the ceria-zirconia stabilised alumina composite by the pore filling method using a planetary mixer. The impregnated carrier was calcined in flowing air at about 550 C for three hours. The impregnated carrier was slurried in distilled water.

The slurry was milled to obtain an average particle size of about 1 -10 microns(D50). This slurry was washcoated multiple times on the monolith to deposit the inner Pd layer(first layer) thereon to give about 209 g. per cuft of layer per cu. ft. of monolith volume, after drying.

A quantity of the Pd precursor was taken on the basis of the said second batch figures, that is, about 22.5 g of Pd per ft3 of monolith volume. This was impregnated on lanthana stabilised alumina carrier by the pore filling method using a planetary mixer. The impregnated carrier was calcined in flowing air at about 550 C for three hours.

A quantity of the Rh precursor was taken on the basis of said third batch figures, that is, about 2.5 g of Rh per ft3 of monolith volume. This was impregnated on ceria-zirconia composite carrier formed by the solution combustion method by the pore filling method using a planetary mixer. The impregnated carrier was calcined in flowing air at about 550 C for three hours.

The two impregnated powders were mixed in the proportion of Pd:Rh of about 9: 1 by wt. and then slurried in distilled water.

The slurry was milled to obtain an average particle size of about 1-10 microns(D50).

The slurry was washcoated multiple times on the monolith with the previously deposited inner layer to yield about 2291 g. per cu. ft. of the second(outer) layer material per cuft. of monolith volume, after drying. The monolith was then calcined before being taken up for evaluation.

The precious metal loading on the monolith was about 30 g/cu.ft. and the Pt/Pd/Rh ratio was about 0/1 1/1.

This composition was evaluated for performance. Two samples were prepared. One was subjected to the performance test in the fresh condition while the second sample was subjected to aging and then tested. The performance comparison is presented in Fig. 3. The comparison shows that the composition of this example shows improved oxidation function and greater stability. Said advantages are believed to arise from the Pd-non- OSC support combination provided in the outer layer. The improved reduction function is believed to arise from the provision of the inner layer Pd. Evaluation of Composition of Ex. 3:

Improved oxidation and improved stability by supporting Pd on a non-OSC support. The Pd in the inner layer leads to better reduction performance. The extra catalyst metal corresponding to the inner layer Pd also improves thermal stability. Embodiments and variations other than described hereinabove are feasible by persons skilled in the art and the same are within the scope and spirit of this invention.

Claims

We claim:

1. A three-way catalytic converter (TWC) for treating the exhaust gases from internal combustion engines such as in automobile and other vehicles and for other applications, comprising inter alia, a monolith(s) provided with one or more coatings(layers) of carrier material, with the catalyst dispersed thereupon and/or impregnated thereinto, said catalyst comprising:

(a) at least, one said reduction catalyst comprising one or more catalyst elements, or mixtures thereof, from the reduction catalyst group as hereindefined, for primarily treating the nitrogen oxides(NOx) gases in said exhaust and supported on a substantially alumina-free ceria-zirconia solid solution composite carrier; and

(b) at least, a first and a second oxidation catalysts, each comprising one or more catalyst elements, or mixtures thereof, from the oxidation catalyst group as hereindefined, said first oxidation catalyst being primarily for CO and HC conversion and the said second being primarily in an associate role as hereindefined; and

(c) a first carrier of the non-OSC type as hereindefined such as, for example, lanthana stabilised alumina and a second carrier of the OSC-type as hereindefined such as for example, ceria-zirconia stabilised alumina;

said first oxidation catalyst being supported on a said non-OSC type first carrier and said second oxidation catalyst being supported on a said OSC type second carrier, and the metal oxides recited herein being substitutable by equivalent metal oxides, or mixtures thereof, belonging to the respective metal oxide series.

2. The three-way catalytic converter (TWC) as claimed in the preceding claim 1, wherein the said substantially alumina-free ceria-zirconia solid solution composite is made by the solution combustion method.

3. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 1 and 2, wherein the molar ratio of ceria to zirconia in the said substantially alumina-free ceria-zirconia composite is from about 1 :6 to about 6: 1 and preferably from about 1 :3 to about 3: 1.

- 1 -

4. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 1 to 3, wherein the said at least one reduction catalyst is rhodium.

5. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 1 to 4, wherein said first and second oxidation catalysts comprise first and second parts of palladium, said first and second parts being supported on said non-OSC type first carrier and OSC type second carrier respectively, and the said first part with the carrier thereof together with the said alumina-free carrier supported rhodium being housed in the outer said layer and said second part with the carrier thereof comprising the inner said layer.

6. The three-way catalytic converter (TWC) as claimed in the preceding claim 5, wherein said first and second carriers are lanthana-stabilised alumina and ceria-zirconia stabilised alumina respectively.

7. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 5 and 6, wherein the ratio of Pd in the said inner and outer layers ranges from about 2:3 by wt. to about 1 : 19 by w , preferably from about 1 :3 by wt. to about 1 :9 by weight.

8. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 5 to 7, wherein the ratio of palladium to rhodium therein is from about 30: 1 by wt. to about 2: 1 by wt. and is preferably from about 15:1 by wt to about 6: 1 by wt.

9. The three-way catalytic converter (TWC) as claimed in the preceding claims 8, wherein the total noble(catalyst) metal loading is from about 10 g cu.ft to about 100 g/cu.ft of the monolith volume, and preferably from about 15 g/cuft to about 30 g/cuft.

10. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 1 to 4, wherein both said first and second oxidation catalysts comprise first and second parts of platinum respectively, both said platinum parts being supported on a common

- 2 - said second OSC-type carrier and being housed together with said non-alumina carrier supported rhodium in a common said layer but segregated on separate carriers.

1 1. The three-way catalytic converter (TWC) as claimed in the preceding claim 10, wherein the said second OSC-type and the said non-alumina carriers are ceria-zirconia stabilised alumina and ceria-zirconia solid solution composite respectively, the said ceria- zirconia solid solution composite being preferably made by a solution combustion method.

12. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 10 and 1 1, wherein the ratio of the two catalyst element metals, namely platinum and rhodium therein, is from about 25: 1 to about 1 :1 by wt. and is preferably from about 18:1 by wt to about 2:1 by wt.

13. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 10 to 12, wherein, the total noble metal loading is from about 5 gm/cu. ft. to about 50 gm/cu.ft. of the monolith volume.

14. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 10 to 13, and further comprising a third oxidation catalyst.

15. The three-way catalytic converter (TWC) as claimed in the preceding claim 14, wherein the said third oxidation catalyst is palladium.

16. The three-way catalytic converter (TWC) as claimed in the preceding claim 15, wherein the said palladium is supported on said non-OSC type first carrier.

17. The three-way catalytic converter (TWC) as claimed in the preceding claim 16, wherein the said non-OSC type first carrier is lanthana stabilised alumina.

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18. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 16 and 17, wherein said palladium supported on said non-OSC type first carrier forms the inner layer and the said OSC-type first carrier supported platinum together with the said non-alumina carrier supported rhodium being housed together in the outer layer but being on segregated separate carriers.

19. The three-way catalytic converter (TWC) as claimed in the preceding claim 18, wherein the proportions of the three catalyst metals therein, namely, platinum, palladium and rhodium are distributed such that the ratio of (Pt + Pd) to Rh is from about 20: 1 by wt. to about 2: 1 by wt.

20. The three-way catalytic converter (TWC) as claimed in the preceding claim 19, wherein the ratio of Pt:Pd is from about 0:1 by wt. to about 1 :0 by wt.

21. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 19 and 20, wherein the total noble metal loading is from about 10 g/cu ft to about 200 g cu ft of the monolith volume.

22. The three-way catalytic converter (TWC) as claimed in any of the preceding claims 1 to 21 and further comprising one or more additional components such as promoters, modifiers, dopants, oxygen storage compounds, stabilisers and others.

23. A three-way catalytic converter (TWC) for treating the exhaust gases from internal combustion engines such as in automobile and other vehicles and for other applications, substantially as hereindescribed in the description and examples herein and illustrated with reference to the accompanying drawings.

24. A vehicle, or a non-vehicle system, comprising an internal combustion engine(s) and incorporating one or more three-way catalytic converter(s) (TWC) as claimed in any of the preceding claims 1 to 23.

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