WO2015104025A1

WO2015104025A1 - Method of preparing a catalyst structure

Info

Publication number: WO2015104025A1
Application number: PCT/DK2015/050002
Authority: WO
Inventors: Christian KALLESØE; Henrik Fanø CLAUSEN; Leif Højslet CHRISTENSEN
Original assignee: Teknologisk Institut
Priority date: 2014-01-08
Filing date: 2015-01-08
Publication date: 2015-07-16
Also published as: EP3092067A1

Abstract

The present invention relates to a method of preparing a catalyst structure comprising a metallic catalyst and a metal oxide the method comprising the steps of: providing a solution of a metal oxide precursor; providing an aqueous reactive solvent in a supercritical or subcritical state; mixing the solution of the metal oxideprecurs or with the supercritical or subcritical aqueous reactive solvent to form an aqueous reactant mixture; injecting the aqueous reactant mixture into a reactor tube via a first inlet; re-establishing supercritical or subcritical conditions in the reactor tube if necessary; allowing a hydrothermal reaction of the metal oxide precursor in the supercritical or subcritical aqueous reactant mixture in the reactor tube to form metal oxide nanoparticles; providing at ambient conditions a solution of a metal precursor in a solvent; injecting the solution of the metal precursor into the reactor tube via a second inlet downstream of the first inlet; re-establishing supercritical or subcritical conditions in the reactor tube if necessary; allowing a reduction reaction of the metal precursor in the supercritical or subcritical reactant mixture to form metallic nanoparticles on the metal oxide nanoparticles; withdrawing the catalyst structure from the reactor tube via an outlet. The catalytic structuremay be an exhaust catalyst and is suitable for use in diesel oxidation catalysis or three way catalysis.

Description

Method of preparing a catalyst structure

Field of the invention

The present invention relates to a method of preparing a catalyst structure, such as an exhaust catalyst structure. The catalyst structure comprises a metallic catalyst and metal oxide, e.g. an oxygen buffer, in the form of nanoparticles. The catalyst structure may be employed in the catalytic conversion of exhaust gasses from the combustion of fossil fuels. The invention also relates to the catalyst structure and to a catalytic converter comprising the catalyst structure.

Prior art

In the field of catalytic conversion of compounds there is an ongoing desire to provide catalytic structures capable of more efficient conversion. A general trend has been to employ substrates of high specific surface area for deposition of nanosized catalytic particles with the aim of controlling the size and distribution of the particles on the support or substrate.

A specific area of relevance is the provision of catalytic structures in the purification of toxic exhaust gas emitted by automobiles engines and other combustion engines, including both mobile and stationary units. Gas pollution control in vehicles is a topic of ongoing interest, since vehicles are among the main responsibles of atmospheric pollution. Moreover, the demand for conventional fossil fuel-automobiles seems to be without limit, and in particular in countries with fast developing economies, such as the "BRICS countries", the demand for fossil fuel is projected to increase in the next decades. Automobile exhaust catalysts need to address two requirements simultaneously: ease of mass production, and efficient purification of exhaust gasses.

Automobile exhaust catalysts are solid catalyst structures for purifying exhaust gasses from automobile engines and promote a series of chemical reactions that clean up toxic gases, normally under ordinary pressure, in the temperature range from 300 to 1000°C. The automobile exhaust catalyst performs continuous processing of an exhaust gas, which varies in composition in a complex pattern depending on various factors, such as engine temperature, the composition of the gasoline or diesel and the power created by the engine. Furthermore, the exhaust catalyst is subject to oxidation-reduction cycles and mechanical vibration throughout its lifetime.

Furthermore, the requirements for exhaust purification are different for gasoline and diesel engines. Exhaust from gasoline and diesel engines generally contains N₂, 0₂, C0₂ and H₂0 as main components together with some other undesired by-products in much lower concentration, for example CO, unburned hydrocarbons (HC) and NO_x. Diesel engine exhausts further contains soot particles as pollutants. Other chemical species such as sulphur oxides (SO_x) and phosphorus oxides (PO_x) may also be present. Emissions of trace amounts of toxic chemical types need not cause significant physiological and environmental toxicity, but they may adsorb on the catalyst surface and act as catalytic poisons. An exhaust catalyst capable of oxidising carbon monoxide and hydrocarbons to carbon dioxide (and water) and reduce nitrogen oxides to molecular nitrogen is commonly referred to as a "three way catalyst" and the process to three way catalysis (TWC). For diesel engine exhausts, a separate exhaust catalyst for the oxidation process, e.g. a diesel oxidation catalyst (DOC), and a separate catalyst for the reduction process, e.g. a selective catalytic reduction (SCR) catalyst, are often required.

Exhaust catalysts typically comprise particles of platinum group metals acting as catalytic components. An exhaust catalyst may further comprise an oxygen buffer functionality where the Ce⁴⁺/Ce³⁺-redox couple of ceria allows for storing of oxygen during the oxidation and releasing during the reduction reactions taking place in the catalyst. Ceria may be deposited on a carrier material with rough, irregular surface, providing a high, e.g. 100 m²/g, specific surface area. The high specific surface area carrier material can be considered part of the active phase of a ceramic honeycomb structure or monolith on which the carrier material is deposited. The carrier material of the high specific surface area, with or without the ceria layer, is referred to as the "washcoat".

Catalytic structures comprising supports with immobilised metal nanoparticles are known from the prior art. For example, Schlange et al. (Beilstein J. Org. Chem., 7: 1412-1420, 2011) provide a process for the continuous preparation of carbon nanotube (CNT)-supported platinum catalysts in a flow reactor. In the process multiwalled CNT's (MWCNT) are initially pre-treated by washing in HCI and HNO₃. After ultrasonication a platinum precursor (H₂PtCI₆-6H₂0) is reacted in an ethylene glycol solvent, which serves to reduce the platinum precursor and deposit platinum nanoparticles on the MWCNT. This process provided platinum particles in the size range of 0.8 nm to 2.8 nm on the MWCNT.

Dong et al. (Carbon, 48: 781-787, 2010) produce graphene- supported platinum and platinum-ruthenium nanoparticles. The process of Dong et al. involves the dispersion of graphene oxide powder in an ethylene glycol (EG) solution followed by addition of hexachloroplatinic acid EG solution or hexachloroplatinic acid EG solution also containing ruthenium chloride and allowing a reaction to take place under alkaline conditions. In alternative processes graphite and carbon black were employed as carbon supports. The processes afforded formation of nanoparticles, e.g. smaller than 10 nm, on the support materials. However, the process was slow and may not be easily scalable, and furthermore, the size distribution of the nanoparticles prepared was not detailed.

Supercritical synthesis of the catalyst particles provides an approach to allow control of the size of the deposited particles. For example, WO 2005/069955 describes methods for preparing catalytic structures of nanostructures, e.g. CNT's, with catalytic metallic nanoparticles, e.g. with diameters between 2 and 12 nm. The methods of WO 2005/069955 generally involve mixing a precursor in a carrier, e.g. carbon dioxide, and transforming the precursor to form a metal. The transformation of the precursor can occur in the carrier or on the surface of a nanostructure substrate. The metal may be formed in the carrier and can then be transported to the surface of the nanostructure substrate in the carrier while the carrier is in supercritical fluid form. Alternatively, the transformation may occur on the surface of the nanostructure substrate while the carrier is in supercritical fluid form. The precursor is a complex that contains the metal precursor and a ligand or moiety that solubilises the compound in the carrier. US 2004/137214 discloses a method of manufacturing a material with surface nanometer functional structure. The process comprises the steps of providing a substrate and placing it in a high-pressure container; supplying a supercritical fluid into the high-pressure container; tuning the temperature and pressure inside the high-pressure container to their appropriate values; supplying a precursor of a target material to be formed with a surface nanometer functional structure to the high-pressure container; and releasing the pressure inside the high-pressure container after the fluid therein reaches its reaction balance point, bringing the precursor to adhere on the substrate surface to form the surface nanometer functional structure.

WO 2006/080702 describes a nanocomposite including carbon nanotubes with a metallic catalyst. The nanocomposite may be produced in a method using a supercritical C0₂ fluid deposition method, wherein a mesoporous carbon support is mixed with a precursor of the metallic catalyst and the mixture is reduced in a supercritical C0₂ fluid using hydrogen gas.

WO 2006/110822 provides processes for the preparation of a carbon aerogel supported catalyst, which may comprise metal particles having an average metal particle size of 2.5 nm or less. The structure of WO 2006/110822 may be prepared by contacting a support with a metal precursor dissolved in a supercritical fluid and reducing the metal precursor to a metallic state either by thermal reduction or hydrogen reduction at proper conditions.

The supercritical treatments of WO 2005/069955 and

WO 2006/110822 are performed batchwise, which makes the synthesis troublesome to scale up for industrial purpose. Furthermore, controlled high heating rates are problematic to obtain in batchwise synthesis, leading to an inhomogeneous heating, and hence the resulting particles may not be optimal for catalytic conversion processes.

Kim et al. (Applied Catalysis B: Environmental, 2007, 71 : 57-63) describe continuous hydrothermal synthesis in supercritical water of nanoparticulate ceria-zirconia mixed oxides and characterise the oxygen storage capacity of the produced particles. It is not shown how the produced particles may be implemented in a catalyst, and no platinum or other metal is deposited on the particles. In a further development of the method Kim et al. (Journal of Catalysis, 2009, 263: 123-133) Ce0₂-Zr0₂ mixed oxide prepared in supercritical water were modified by loading with rhodium in an incipient wetness impregnation method. Loading of rhodium was thus performed in a batch process and several expected advantages, e.g. regarding speed of manufacture and control of particle parameters, from the continuous process were lost by conducting the subsequent batch process. The prepared particles were not implemented in an actual catalytic converter.

JP 2011-183262 discloses a method of producing noble metal- reduced Pt/Ti0₂ nano-catalytic particles using a functional group, e.g. 3,4- dihydroxyhydrocinnamic acid, having affinity to Pt in a subcritical hydrothermal field. The nano-catalytic particles can be used in exhaust gas purification.

DE 9403581U and DE 29917118U disclose methods for producing nanoparticles with a core of an oxide, nitride or carbide of a metal, e.g. transition metals or lanthanides, which is surrounded by a sheath of metal compound which is different from the core.

FR 2991713 describes catalysts for purifying exhaust gas from cars. The catalyst comprise a crystalline ceramic structure with nanocrystals, e.g. Ce0₂ and Zr0₂, which are chemically and mechanically bound to an active phase of the metallic particles, e.g. platinum or rhodium. FR 2991713 addresses stability problems of prior catalysts.

US 2009/298683 relates to the production of a material comprising a mixture of noble metal nanoparticles and rare-earth oxide nanoparticles. The noble metal can be chosen from the group comprising the elements Ru, Rh, Ir, Ag, Au, Pd, Pt, Ni and Cu and the rare earth can be chosen from the group comprising the elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc. The material may be used for catalysis.

PCT/DK2013/050227 discloses a method for the continuous production of a catalytic structure in a supercritical flow. The catalytic structure comprises a support, e.g. a carbon support, onto which catalytic nanoparticles, e.g. platinum nanoparticles, are deposited. The catalytic structure is suited for fuel cells. PCT/DK2013/050227 does not disclose an oxygen buffer. Presently available exhaust catalysts are typically prepared in large scale batch processes. It is a further challenge that batch preparation of catalyst particles is generally inefficient, especially with respect to consumption of noble metals. In light of the above there is a need for an improved method for providing catalytic structures for the purification of exhaust gas emitted by combustion engines. In particular there is a need for a process for the manufacture of an exhaust catalyst, which is more efficient with respect to noble metals. It is an aim of the present invention to address this need.

Disclosure of the invention

The present invention relates to a method of preparing a catalyst structure, preferably an exhaust catalyst structure, comprising a metallic catalyst and a metal oxide, such as a transition metal oxide or a lanthanide oxide, e.g. a cerium oxide and optionally zirconium oxide, the method comprising the steps of:

providing a solution of a metal oxide precursor;

providing an aqueous reactive solvent in a supercritical or subcritical state;

mixing the solution of the metal oxide precursor with the supercritical or subcritical aqueous reactive solvent to form an aqueous reactant mixture;

injecting the aqueous reactant mixture into a reactor tube via a first inlet;

re-establishing supercritical or subcritical conditions in the reactor tube if necessary;

allowing a hydrothermal reaction of the metal oxide precursor in the supercritical or subcritical aqueous reactant mixture in the reactor tube to form metal oxide nanoparticles;

providing at ambient conditions a solution of a metal precursor in a solvent;

injecting the solution of the metal precursor into the reactor tube via a second inlet downstream of the first inlet; re-establishing supercritical or subcritical conditions in the reactor tube if necessary;

allowing a reduction reaction of the metal precursor in the supercritical or subcritical reactant mixture to form metallic nanoparticles on the metal oxide nanoparticles;

withdrawing the catalyst structure from the reactor tube via an outlet.

In another aspect the invention relates to a method of preparing a catalyst structure, preferably an exhaust catalyst structure, comprising a metallic catalyst and a metal oxide, such as a transition metal oxide or a lanthanide oxide, e.g. a cerium oxide and optionally zirconium oxide, the method comprising the steps of:

providing a solution of a metal precursor;

providing an aqueous reactive solvent in a supercritical or subcritical state;

mixing the solution of the metal precursor with the supercritical or subcritical aqueous reactive solvent to form an aqueous reactant mixture;

injecting the aqueous reactant mixture into a reactor tube via a first inlet;

allowing a reduction reaction of the metal precursor in the supercritical or subcritical reactant mixture to form metallic nanoparticles;

providing at ambient conditions a solution of a metal oxide precursor in a solvent;

injecting the solution of the metal oxide precursor into the reactor tube via a second inlet downstream of the first inlet;

allowing a hydrothermal reaction of the metal oxide precursor in the supercritical or subcritical aqueous reactant mixture in the reactor tube to form metal oxide nanoparticles on the metallic nanoparticles;

withdrawing the catalyst structure from the reactor tube via an outlet. Thus, in the first aspect the invention provides a method where metallic nanoparticles are deposited on metal oxide nanoparticles, and in the second aspect the invention provides a method where metal oxide nanoparticles are deposited on metallic nanoparticles. All features described below may be employed freely in either aspect, and the order of the steps in preparing the nanoparticles is generally not relevant. Where the present application mentions that metallic nanoparticles are deposited on the metal oxide particles, this is to be understood that metal oxide nanoparticles may also be deposited on the metallic nanoparticles. The order of the deposition is generally guided by the order of introduction into the reactor tube of the corresponding reactants.

The catalyst structure may be used for any catalysis and the metal and the metal oxide may be selected accordingly. Catalytic metals and appropriate metal oxides are well-known to the skilled person. In a preferred embodiment the catalyst structure is an exhaust catalyst structure, and the metal oxide is a transition metal oxide or a lanthanide oxide, e.g. a cerium oxide and optionally zirconium oxide. In the context of the invention the terms "metal oxide" and "oxygen buffer" may be used interchangeably, and whenever a "metal oxide" is mentioned it may always be a transition metal oxide or a lanthanide oxide, in particular a cerium oxide and optionally also a zirconium oxide, both in the context of metal oxide nanoparticles and in the context of metal oxide precursors. Likewise, the terms "catalyst structure" and "exhaust catalyst structure" may be used interchangeably.

In a preferred embodiment, the method of the invention provides an exhaust catalyst structure, where the catalytic effect of the structure is provided by metallic nanoparticles in combination with the oxygen buffering effect of the oxygen buffer nanoparticles. The exhaust catalyst structure is suitable for any exhaust catalytic process that can be catalysed via catalyst nanoparticles, in particular the exhaust catalyst structure is suited for a three way catalysis (TWC) process and/or for a diesel oxidation catalysis (DOC) process. In the context of the invention the term "catalyst nanoparticles" may be used to refer to an exhaust catalyst structure comprising both the oxygen buffer nanoparticles and also the metallic nanoparticles. The metal may be any metal capable of catalysing oxidation of carbon monoxide and hydrocarbons, e.g. to carbon dioxide and water, and reduce nitrogen oxides, e.g. to N₂. Preferred metals are selected from a transition metal or a lanthanide or a mixture thereof, for example Rh, Pd, or Pt. The oxygen buffer nanoparticles comprise a metal oxide, preferably cerium oxide, and the metal, e.g. cerium, may be in any oxidation level, e.g. Ce³⁺ or Ce⁴⁺ or Ce³⁺ and Ce⁴⁺ mixed in any ratio. The cerium oxide provides the "oxygen buffer" effect due to the Ce⁴⁺/Ce³⁺-redox couple. The cerium oxide may also be referred to as "ceria". It is however also contemplated in the invention that any metal, e.g. lanthanide or transition metal, having two or more oxidation levels above zero (i.e. the metal in its metallic form) may exist in an oxide form capable of providing an oxygen buffering effect in a DOC or TWC process.

The method of the invention employs a hydrothermal reaction to provide metal oxide nanoparticles, e.g. oxygen buffer nanoparticles, followed by a reduction to provide the metallic nanoparticles. The dielectric constant of water decreases with increasing temperature at a given pressure and inorganic salts soluble at ambient conditions may become insoluble under subcritical or supercritical conditions in an aqueous solvent. For example, transition metals and lanthanides at an oxidation level above zero may exist as hydrated metal ions in water at ambient conditions, which may be hydrolysed to precipitate as crystalline metal oxides through dehydration at a high temperature. This process is, in the context of the invention, referred to as "hydrothermal synthesis". In the context of the invention any solvent, which is liquid at ambient conditions and which is capable of precipitating hydrated metal ions as a metal oxide under subcritical or supercritical conditions is contemplated as the aqueous reactive solvent. Water is a preferred aqueous reactive solvent although the aqueous reactive solvent may also be a mixture of water with a water miscible solvent.

The oxygen buffer nanoparticles may comprise a single metal oxide or the oxide may comprise two or more transition metals and/or lanthanides. For example, oxygen buffer nanoparticles may comprise cerium and zirconium at any ratio, to form a "ceria-zirconia mixed oxide" that may be described by the formula Ce_xZri__x0₂; it is noted that this formula is not stoichiometric but serves to describe the ratio between Ce and Zr. The oxygen buffer precursor may comprise the transition metal or lanthanide in any form, e.g. as a complex with appropriate ligands, which is soluble in water at ambient conditions. Exemplary oxygen buffer precursor comprise (NH₄)₂Ce(S0₄)₃, (NH₄)₂Ce(N0₃)₆, Ce(N0₃)₃, (NH₄)₂Ce(N0₃)₅-4H₂0, Ce(C₂H₃0₂)₃- 1.5H₂0, Ce(C₃H₇0)₄, Ce(C₅H₇0₂)₃-xH₂0 and others are readily available to the skilled person. When the oxygen buffer precursor comprises a Zr-compound an exemplary precursor is ZrO(N0₃)₂-xH₂0.

The metallic nanoparticles are formed from a metal precursor comprising the metal in an oxidised form by reducing the oxidised metal to its metallic form. The reaction of the metal precursor is thus a reduction of the metal ion. The formation of metal oxide nanoparticles, such as oxygen buffer nanoparticles, e.g. ceria or ceria-zirconia mixed oxide nanoparticles, is performed in a subcritical or supercritical aqueous solvent, which in addition to providing the hydrothermal reaction conditions will also provide oxidising conditions. The present inventors have now surprisingly found that despite the oxidising conditions it is possible in the same subcritical or supercritical flow to reduce a metal precursor comprising the metal in an oxidised form to the metal in its metallic form without adding a separate stream of a subcritical or supercritical reducing solvent into the reactor tube. Thus, when a solution of a metal precursor, e.g. in a reducing solvent or in an aqueous solvent, is introduced into the reactor tube via an inlet downstream of the first inlet the metal component of the metal precursor will be reduced to form metallic nanoparticles on the metal oxide nanoparticles, e.g. the oxygen buffer nanoparticles. However, it is also possible to add a subcritical or supercritical reducing solvent to the flow, e.g. via a further inlet downstream of the first inlet. Thus, in an embodiment the method further comprises the steps of providing a reducing solvent in a supercritical or subcritical state, and injecting the reducing solvent in the supercritical or subcritical state into the reactor tube via a third, or further, inlet downstream of the first inlet to provide a supercritical or subcritical reducing reactant mixture. The third or further inlet may be downstream of the second inlet. In the method of the invention it is thereby possible to perform the whole process in a single reactor tube having appropriately positioned inlets. This removes the need to collect the metal oxide nanoparticles, e.g. the oxygen buffer nanoparticles, separate them from the oxidising conditions, and later reintroduce them in a second reactor tube under reducing conditions. Any solvent, which is liquid at ambient conditions and which provides a reducing effect under subcritical or supercritical conditions is considered a reducing solvent in the context of the invention. Preferred reducing solvents are ethanol, methanol, isopropanol, ethylene glycol and combinations thereof.

The metal precursor may be any metal salt or compound capable of forming the metal in its metallic form, and the metal precursor is preferably soluble, e.g. in the form of a complex with appropriate ligands, in the solvent. Preferred metals for an exhaust catalyst structure are platinum (Pt), palladium (Pd), rhodium (Rh) or a mixture of Pt, Pd, and Rh, and appropriate metal precursors are H₂PtCI₆-6H₂0, platinum(II) acetylacetonate (Pt(C₅H₇0₂)₂) (also known as Pt(acac)₂), RhCI₃-xH₂0, Rh(N0₃)₃-xH₂0, H₂PdCI₆-xH₂0 and (NH₄)₂PdCI₆-xH₂0. The metal may also be a lanthanide or a transition metal or a combination thereof, e.g. Re, Os, Ir, Pt, Au, Ru, Rh, Pd, Ag, Mn, Fe, Co, Ni, Cu, Ce. Selection of metal precursors is within the knowledge of the skilled person.

The present invention employs a metal oxide precursor, e.g. an oxygen buffer precursor, and a metal precursor. The precursors may be referred to collectively as "precursor compounds" and when a precursor compound is mentioned it may be a metal oxide precursor, an oxygen buffer precursor or a metal precursor as appropriate from the instant context.

The catalyst structure can advantageously be prepared directly in a one-step reaction in the flow synthesis reactor so that the final catalyst structure can be withdrawn from the reactor requiring only a minimum of additional processing steps, e.g. to purify the catalyst structure and deposit the catalyst structure on the substrate. The method enables the use of environmentally friendly solvents in the continuous flow production of catalyst nanoparticles, and offers laboratory- 1 ike control while providing high throughput for larger productions and scalability for industrial application. The advantages of avoiding agglomeration also allow a more efficient process with an increased yield from the expensive starting materials. The metal oxide nanoparticles and the metallic nanoparticles may be synthesised continuously in the supercritical or subcritical solvents, which gives excellent control of morphology, crystallinity, size and uniformity of the particles which are all important characteristics for catalytic properties of the nanoparticles.

The metal oxide nanoparticles will form in the subcritical or supercritical aqueous solvent without any need for nucleation points or the like, and the formed nanoparticles can advantageously be used as a carrier for formation of nanoparticles, e.g. the metallic nanoparticles in the reduction step, since the metal oxide nanoparticles, e.g. nanoparticles comprising cerium oxide, will provide nucleation points for formation of further nanoparticles, e.g. metallic nanoparticles. The same advantage is relevant when metallic nanoparticles are formed first and these are used as carriers for metal oxide nanoparticles. Formation of metallic nanoparticles on the metal oxide nanoparticles, e.g. the oxygen buffer nanoparticles, is considered particularly advantageous for an exhaust catalyst structure since this will ensure that the two materials, i.e. the oxygen buffer and the catalytic metal, are in reactive proximity for the TWC process and/or for the DOC process involving both oxidations and reductions with the aid of the oxygen buffer. Formation of metallic oxide nanoparticles on metal nanoparticles will provide the same advantage. It is possible to control the size and morphology of both the metal oxide nanoparticles, e.g. the oxygen buffer nanoparticles, and the metallic nanoparticles by adjusting parameters such as pressure, temperature and the residence time in the reactor tube, e.g. the residence time between the first inlet, the second inlet and the optional third inlet, and the outlet. The exhaust catalyst structure comprising the oxygen buffer nanoparticles and the metallic nanoparticles is considered to provide a particularly efficient catalyst due to the available control of the two types of nanoparticles and the better dispersion of the active catalytic component on the rough irregular surface of the carrier in the continuous supercritical or subcritical reaction, which cannot be obtained when exhaust catalyst structures are prepared in conventional batch processes. This allows that the consumption of expensive precious metals is minimised. In a specific embodiment, the metal nanoparticles comprise an alloy, e.g. a binary alloy, of platinum with a transition metal and/or a lanthanide (Pt_xM_y) likewise of a high electro catalytic efficiency, hence further minimising the amount of precious metal needed. When the catalyst structure comprises two or more metals the respective metal precursors may be added via separate inlets into the reactor tube so that a second metal precursor is added downstream of a first metal precursor. This allows formation of metallic particles comprising an inner metallic layer prepared from the first metal precursor and an outer metallic layer prepared from the second metal precursor. The layered metallic nanoparticles may be referred to as a "core-shell structure". The first and/or the second metal precursor may contain any number of metals, which still provides formation of layered metallic nanoparticles. Thus, in another embodiment the method of the invention further comprises the steps of: providing at ambient conditions a solution of a second metal precursor in a solvent; injecting the solution of the second metal precursor into the reactor tube via a further inlet downstream of the second inlet; allowing a reduction reaction of the second metal precursor in the supercritical or subcritical reactant mixture to form layered metallic nanoparticles on the metal oxide nanoparticles.

The method of the present invention is especially relevant for preparing catalyst structures for applications in oxidation catalysis in exhaust systems, such as DOC or TWC. It is however contemplated that the method of the present invention is also relevant for preparing catalyst structures for other applications, within conversion of exhaust gasses. For example, the method may also be used for preparing other catalyst structures for use with a diesel engine, e.g. for use in selective catalytic reduction (SCR), lean NOx trap or NOx adsorber as well as in diesel particulate filters (DPF). In addition to the oxygen buffer, or as an alternative to the oxygen buffer, the catalyst structure may also contain other metal oxides, which can be prepared in the hydrothermal reaction from appropriate precursor molecules. Such additional, or alternative, oxides may themselves provide a catalytic function, e.g. Ti0₂, V₂0₅, Fe₂0₃, or perovskite-type oxides such as La_xKi__xCo0₃ or La_xKi__xFe0₃ for NO_x reduction and simultaneous soot combustion.

In one embodiment additional catalytic metal oxides are employed together with catalytic metals and this combination advantageously reduces the consumption of precious metals in the production thereby reducing the overall cost. It is noted that when an additional or alternative catalytic metal oxide is employed in the method all parameters and variations of the parameters, e.g. for preparing the oxygen buffer nanoparticles, discussed throughout this document are relevant also for preparing a catalytic metal oxide. Catalytic metal oxides are especially relevant for core-shell structure catalysts, e.g. with an inner core of the catalytic metal oxide.

The catalyst nanoparticles are particles in the nanosize range, e.g. from about 1 nm to about 1000 nm, although it is also contemplated that the particles may be larger than nanosize, e.g. the particles may be of microsize with a size within the range of about 1 μηι to about 10 μηη. The sizes of the metal oxide nanoparticles and/or the metallic nanoparticles are generally in the range of about 1 nm to about 200 nm, e.g. to about 50 nm or about 20 nm, and the metal oxide nanoparticles and the metallic nanoparticles are preferably monodisperse. The indicated size ranges apply to the metal oxide nanoparticles or the metallic nanoparticles when considered individually as well as to nanoparticles comprising both the metal oxide nanoparticles and the metallic nanoparticles. A preferred size of the catalyst structure particles, e.g. the exhaust catalyst structure particles, is in the range from about 5 nm to about 70 nm, e.g. in the range of about 10 nm to about 50 nm.

In another embodiment the method further comprises the step of providing in a solvent at ambient conditions a suspension of a carrier material having a specific surface area of at least 1 m²/g, optionally sonicating the suspension of the carrier material and mixing the solution of the metal oxide precursor and the suspension of the carrier material with the supercritical or subcritical aqueous reactive solvent to form the supercritical or subcritical aqueous reactant mixture. In this embodiment the metal oxide nanoparticles are formed on the surface of the carrier material, which thus provides nucleation points for formation of the metal oxide nanoparticles. The carrier material allows that the metal oxide nanoparticles can be distributed evenly on the carrier material, and that the spacing of the nanoparticles can be controlled. It is also contemplated that the metallic nanoparticles may be formed on the carrier material prior to formation of the metal oxide nanoparticles. In this embodiment the metal precursor in a solvent, e.g. a reducing solvent, at ambient conditions is mixed with a suspension of the carrier material in the solvent before mixing with the solvent under subcritical or supercritical conditions and allowing the reducing reaction in the reactor tube; the aqueous solution of the metal oxide precursor and the reactive solvent in a supercritical or subcritical state are then injected into the reactor tube via the second and the third inlets.

In order to disperse the carrier material and optimise access to the large surface area of the carrier material, the method may comprise a step to improve the dispersion. Any technology allowing dispersion of a particulate material may be used. For example, the suspension of the carrier material may be sonicated. The sonication may be performed at any stage prior to or during the step of admixing the mixture of the solution of the precursor compound and the suspension of the carrier material in the supercritical or subcritical reactive solvent.

The use of a carrier makes handling of the prepared catalyst structure easier; for example the carrier material may be a highly porous material, e.g. a ceramic material, of microsized or larger particles, which may be settled and washed using simple filters or centrifugation at low g- force before coupling to a ceramic honeycomb structure or ceramic monolith to provide a catalytic converter. Likewise, the use of a carrier material also provides a further tool for coupling of the catalyst structure to a support structure, e.g. a ceramic honeycomb structure or monolith. Without being bound by any theory the present inventors believe that when the metal oxide or oxygen buffer nanoparticles are formed on a carrier material the metal oxide or oxygen buffer nanoparticles will also in this case serve as nucleation points for formation of the metallic nanoparticles so that the metallic nanoparticles will form on the metal oxide or oxygen buffer nanoparticles regardless of the presence of a carrier material. In particular, typical catalytic metals, e.g. Pt, Rh, Pd, preferentially attach to the surface of cerium oxide, and this preferential interaction of cerium oxides with noble metals promotes the dispersion, activity and stability of the metal nanoparticles. Any carrier material of a specific surface area above 1 m²/g, e.g. above 10 m²/g or above 100 m²/g, is appropriate for the method. In some embodiments the specific surface area is below 100 m²/g, e.g. from 1 to 10 m²/g- The carrier material should be insoluble in the solvents employed in the method of the invention, and the carrier material should also generally be insoluble under the conditions of the intended catalytic process, e.g. the DOC and/or the TWC process. Exemplary carrier materials comprise silicon containing oxides, aluminium containing oxides, transition metal oxides, lanthanide oxides, rnetals and alloys, earth alkaline oxides and compounds, perovskite structures, or even cerium containing oxides. The selection of a carrier material for a specific catalyst structure is well- known to the skilled person.

The method of the invention may also employ more than one precursor compound. For example, the method may employ two or more metal precursors in order to provide metallic catalyst nanoparticles comprising the corresponding two or more metals. It is likewise possible for the method to employ a mixture of one or more precursor compounds for providing metallic nanoparticles and one or more compounds for providing metal oxide, e.g. oxygen buffer, nanoparticles in order to provide catalyst nanoparticles comprising mixtures of metals and metal oxide compounds and/or metal oxide compounds with other compounds, such as zirconium in ceria-zirconia mixed oxide.

The reactive solvents, i.e. the aqueous reactive solvent and the optional reducing solvent, may also comprise another component, e.g. other solvents or dissolved components, for example to activate or enhance the activation of the carrier material or improve dispersion of the carrier material. Activation of the carrier may improve the dispersion of the carrier material or the activation may improve formation of the catalyst nanoparticles on the carrier, e.g. by improving physical or chemical binding of the catalyst nanoparticles or by providing nucleation points for formation of catalyst nanoparticles.

The concentrations of the metal oxide or oxygen buffer precursor, the metal precursor and the optional carrier material in the respective solvents may be chosen freely. In general, the ratio between the metal oxide or the oxygen buffer precursor and the metal precursor, e.g. expressed as a weight ratio of "metal: oxygen buffer", may be from 1 : 1000 to 100: 1. A ratio in the range between 1 : 100 to 1 : 10 is preferred. When an external carrier material is employed it is preferred that the concentration of the ca rrier material relative to the precursor com pound (s) is in the range of about 10 wt% to about 99 wt%, e.g . about 60 wt% to about 90 wt%, such as about 70 wt% to about 90 wt%. The concentrations of the precursor compounds may be expressed in molar concentrations, and the concentrations may be in the range of 0.001 to 10 M, e.g . at 1 M or 0. 1 M, althoug h concentrations outside these ranges are a lso contemplated .

The method em ploys an aq ueous reactive solvent in a supercritical or subcritical state when it is m ixed with the solution of the metal oxide precursor com pound, and further the method may em ploy a reducing solvent in a supercritical or subcritical state. In one em bodiment the aq ueous reactive solvent and/or the reducing solvent has a tem perature at or within 200°C below, or above the temperature of the critica l point (7_cr) of the respective solvent and the aq ueous reactive solvent and/or the reducing solvent is at a pressure at or within 60% below, or above the pressure of the critical point (P_cr) of the respective solvent. When both the tem perature a nd the pressure of a solvent are a bove the respective va lues of the critica l point the solvent is in a supercritica l state. When either the tem perature or the pressure of the reactive solvent are below the respective values of the critical point but within the indicated ranges the solvent is considered to be in a subcritical state. Both of the tem perature and the pressure of the reactive solvent may a lso be below the respective values of the critical point but within the indicated values; this is also considered to be a subcritica l state in the present invention. In one em bodiment, the aqueous reactive solvent and the optional reducing solvent have tem peratures above the T_cr of the respective solvent and the aqueous reactive solvent and the reducing solvent are at pressures above P_cr of the respective solvent. It is noted that when the solution of the oxygen precursor com pound is m ixed with the aq ueous reactive solvent in the supercritical or subcritical state the tem perature, e.g . the "m ixing tem perature" may drop to a value below the indicated range. However, in the method the tem perature will quickly be readjusted to be within the indicated ra nge allowing the reaction to take place as desired .

The ratio of the aqueous solution of the metal oxide precursor and the solution of the metal precursor may have any desired value. In pa rticular, the amounts of the metal oxide precursor and the metal precursor may be chosen freely. Likewise, the ratios of the precursor compounds, the optional carrier material and the solvents may also have any desired value. Preferred weight ratios of metal : metal oxide are from 1 : 100 to 1 : 1, such as 1 : 100 to 1 : 10, 1: 50 to 1 : 10, or 1 : 50 to 1 : 20, e.g. 1 : 100, 1 : 50, 1 :45, 1 :40, 1 : 35, 1 : 30, 1 : 25, 1 : 20, 1 : 15, 1 : 10. When expressed as a percentage of metal it is preferred that the metal is in the range of from about 1 wt% to about 5 wt%, e.g. about 2 wt% or about 3 wt%. For example, when the catalytic metal is PtPd it is preferred that the weight ratio is 2.5 wt% of metal to metal oxide, e.g. cerium oxide or cerium oxide/zirconium oxide. The mixture of the solution of the precursor compound and the optional suspension of the carrier material may be provided as cold reaction lines, e.g. at ambient conditions, which are mixed abruptly with the supercritical or subcritical reactive solvent. Alternatively, the pressure and/or temperature of the mixture of the solution of the precursor compound and/or the suspension of the carrier material may also be increased prior to admixing with subcritical or supercritical solvent. Likewise, when the method employs two or more metal oxide precursors and/or two or more metal precursors these may be mixed at any stage in the process. For example, metal oxide precursors may be mixed in ambient solution prior to mixing with the aqueous reactive solvent in a supercritical or subcritical state, or the second metal oxide precursor may be injected into the reactor tube via an inlet downstream of the first inlet. When the method employs two or more metal precursors these may be mixed in ambient solution prior to being injected into the reactor tube, or they may be injected into the reactor tube via distinct inlets. For example, the aqueous solution of an metal oxide precursor may be admixed with the aqueous reactive solvent, which is preheated at a pressure of ~200 bar resulting in a mixing temperature of ~200°C representing the subcritical regime of water. In a specific embodiment, the aqueous reactive solvent after mixing has a temperature in the range of about 100°C to about 450°C, and the pressure is in the range of about 100 bar to about 300 bar. Furthermore, either solution of the precursor compounds or the optional suspension of a carrier material may be brought to sub- or supercritical conditions before injecting into the reactor tube under sub- or supercritical conditions. High heating rates can be obtained by mixing a cold reaction line and the supercritical or subcritical solvent. The high heating rates can provide fast nucleation and reaction uniformity. In particular, the rapid increase in the temperature leads to fast homogenous nucleation resulting in monodisperse nanoparticles, which are further matured in the heater before being cooled down. The critical temperature and pressure are solvent dependent, and hence tuneable by using different aqueous reactive solvents or reducing solvents, e.g. in a pure form or as a mixture of solvents. The obtained products, i.e. the metal oxide nanoparticles and the metal nanoparticles, are tuneable by varying temperature and pressure, thus controllability of morphology, crystallinity, size, and uniformity of the particles are obtained. This results in homogenous nanoparticles with a narrow, e.g. monodisperse, size distribution, which is crucial for catalytic property of nanoparticles. The temperature and pressure of the solvents may be controlled and varied throughout the process. For example, the respective solvent may be at one set of temperature and pressure upon admixing with the solution of the precursor compound and the optional suspension of the carrier material, and subsequently the temperature and pressure may be increased or decreased in the reactor tube. The optional carrier material that is present in the super- or subcritical media may further prevent the catalyst nanoparticles from agglomerating, as these attach directly onto the carrier material. However, the metal oxide nanoparticles may also prevent agglomeration of the metallic nanoparticles without the need for an additional carrier material. It is moreover possible to control agglomeration of the formed nanoparticles by controlling the pH of the aqueous solvents. Particle agglomeration will depend on the zeta potential of the particles. The zeta potential is in general a term describing the electrokinetic potential of colloidal systems. Nanoparticles have a surface charge that attracts a thin layer of ions of opposite charge to the nanoparticle surface. The zeta potential is hence the potential difference between the dispersion medium used and the stationary layer of fluid or ions around the dispersed particles. The magnitude of the zeta potential is predictive of the colloidal stability. Nanoparticles with zeta potential values greater than +25 mV or less than -25 mV typically have high degrees of stability, whereas dispersions with a low zeta potential value will eventually aggregate due to Van Der Waal inter-particle attractions. Changes in the zeta potential can be accomplished by addition of anionic or cationic electrolytes. The change in pH can for one example be accomplished by adding aqueous solution of HCI, NH₄OH, or KOH, and hence controlled changes in zeta potential can be obtained.

The method of the invention is performed in a reactor tube so that the reaction can be described as a continuous process, e.g. the reaction takes place under continuous conditions. Operation under continuous conditions in a reactor tube provides advantages that cannot be realised in a batch type operation. For example, the continuous operation allows that relatively small portions of the mixture of the solution of the precursor compound and the suspension of the carrier material at a time are admixed with the super- or subcritical reactive solvent ensuring a fast and efficient change from ambient conditions to super- or subcritical conditions at which the catalyst nanoparticles will form. This allows good control of the size and uniformity of the nanoparticles, and furthermore it allows that the nanoparticle distribution on the carrier material is controlled when a carrier material is used. In one embodiment the catalyst nanoparticles are formed on a carrier material at a spacing between the catalyst nanoparticles which is in the range on about 0.1 nm to about 100 nm. The combined control of size, uniformity and distribution on the carrier material cannot be achieved in a batch process.

The reactor tube has an inlet, e.g. for the metal oxide precursor, and an outlet, for withdrawing the catalyst structure, and a second inlet, e.g. for the metal precursor, and optionally a third inlet downstream of the first inlet. When a subcritical or supercritical reducing solvent is employed this may be added via a third inlet, or the solution of the metal precursor in a solvent and the reducing solvent in a supercritical or subcritical state may be mixed outside the reactor tube, e.g. in a mixer or the like, before injecting into the reactor tube via the second inlet downstream of the first inlet. A third inlet may be employed when additional precursor compounds are employed, e.g. to prepare a core-shell structure. The reactor tube may have any number of inlets to allow introduction of additional metal oxide precursors, metal precursors and/or carrier materials or other precursor materials or the like and appropriate supercritical or subcritical solvents. The steps of mixing a solution of a precursor compound and/or a suspension of a carrier material with the appropriate supercritical or subcritical solvent in a reactor tube may be performed in an injector or a mixing chamber before injecting into the reactor tube via an inlet or the mixing may be directly in the reactor tube after injection into the reactor tube. The catalyst structure is withdrawn from the reactor tube at the outlet, so that the reactants will travel through the reactor tube from the inlet to the outlet. For example, the reactants may travel down a vertical reactor tube. It is preferred that the reactor tube is vertical with the first inlet at an upper section of the reactor tube and the outlet at a lower section of the reactor tube, so that the outlet is below the inlet. In another embodiment the first inlet may also be below the outlet so that the reactants travel upward in the reactor tube. The additional, e.g. the second and third, inlets may be at any position downstream of the first inlet and upstream of the outlet. Inlets in addition to the first allow for a more flexible process, since for example it is possible to supply the reaction solution with further precursor compounds allowing the formation of catalyst nanoparticles having a layered structure of different metals.

The reactor set-up may comprise a mixing chamber or the inlet tubes may contain a static mixer to improve mixing. For example, the solution of the precursor compounds and the optional suspension of the carrier material may be mixed using a static mixer prior to admixing with the respective solvents. Likewise, the step of mixing the mixture of the solution of the precursor compound and the suspension of the carrier material in the supercritical or subcritical solvent may be performed using a static mixer. Static mixers are well-known to the skilled person. In general, any mixing step where a solution of a precursor compound is mixed with a supercritical or subcritical solvent may be performed using cross-, vortex- or opposing flow-mixing, although other geometries are also contemplated.

The distance between the inlet, e.g. the first inlet or the second or further inlet, and the outlet coupled with the flow rate of the reactants in the reactor tube provides a residence time for the reactants flowing through the reactor tube. The reactor tube may have any cross-sectional area as desired. From the positioning of the second and third inlets may be defined residence times for the hydrothermal reaction and for the reduction reaction. The residence times may be selected freely. For example, the residence time may be up to 30 minutes, e.g. up to 5 minutes. In an embodiment, the reactant mixture has a residence time in the reactor tube between the first inlet and the outlet of the reactor tube in the range of from 2 seconds to 30 minutes. In another embodiment, the reactant mixture has a residence time in the reactor tube between the second inlet and the outlet of the reactor tube in the range of from 2 seconds to 30 minutes. It is noted that when additional volumes of supercritical or subcritical solvents, e.g. reducing solvents, are added to the reactor tube at the second inlet and the optional further, e.g. third, inlet and the reactor and the reactor tube may have the same cross-sectional area through the full length of the reactor tube or the cross-sectional area may for example be increased at the site of any additional inlet in order to provide that the linear flow-rate is the same through the full length of the reactor tube. The residence time in the reactor tube allows that the nanoparticles are matured further, e.g. to enhance crystallinity, thereby generating more well-defined nanoparticles. The fluid may be kept at supercritical or subcritical temperatures in the progress through the reactor tube, ensuring that all precursors may be used up. This provides better control of the process than is achievable in a batch process. Preferred residence times, e.g. for the hydrothermal reaction and/or for the reduction reaction, are in the range of about 2 seconds to about 60 seconds. However, the residence times are generally dependent on the scale of operation, and a residence time may also be shorter than 2 seconds or higher than 60 seconds. For example, a residence time may be 1 minute or more, such as 10 minutes or more. It is noted that the flow of the reactants in the reactor tube may also be stopped, so that the flow can be described as a stop-flow-operation; the flow does not need to be constant, and the flow-rate may be varied as desired.

The reactor tube may contain a cooling section at any site downstream of the inlets. For example, the supercritical or subcritical solvent comprising the catalyst structure may be cooled to liquefy the reaction solution. It is also possible to cool the supercritical or subcritical reaction solution, e.g. the reactant mixture, at any other stage, e.g. upstream of an additional inlet in order to control the temperature of the subsequent reaction and to provide better mixing of the added precursor and thus better distributed particles. Thus, the cooling may be performed at any stage in the process after formation of the metal oxide nanoparticles or the metal nanoparticles. The cooling may, for example be obtained indirectly by flowing water on the outside of the reaction tube, e.g. to a temperature where the reaction solution liquefies or to a temperature appropriate for the subsequent reaction. For example, the reaction solution may be exposed to a rapid cooling right before the exit via the outlet of the reactor tube. The reactant or precursor inlets may also comprise a cooled section, e.g. to prevent premature heating of the precursor compound. The pressure of the reactor tube may be relieved by a valve (pressure release valve or back pressure regulator), and the reaction solution, including the catalyst nanoparticles, can be continuously withdrawn or tapped.

The features of the embodiments described above may be combined freely as desired, and embodiments from such combinations are also considered within the scope of the invention.

In another aspect the invention relates to a catalyst structure, in particular an exhaust catalyst structure, obtainable in the method of the invention. In specific embodiments the catalyst structure is a TWC catalyst structure, a DOC catalyst structure, a SCR catalyst structure, or the catalyst structure is a lean NO_x trap or NO_x adsorber, or a DPF. The catalyst structure is preferably an exhaust catalyst structure comprising platinum or platinum-rhodium nanoparticles formed on oxygen buffer nanoparticles, which preferably comprise cerium oxide and even more preferably cerium oxide and zirconium oxide. In another embodiment the catalyst structure comprises two or more catalytic metals, in particular in a core-shell format as explained above. In a particular embodiment the core-shell structure comprises metallic nanoparticles having a core or inner layer of a first catalytic metal surrounded by an outer layer of a second catalytic metal. In another embodiment the inner layer is non-catalytic, e.g. a non-catalytic metal or a metal oxide.

The exhaust catalyst structure of the invention is suitable for a catalytic converter, e.g. for use in a gasoline or diesel powered vehicle, and in yet a further aspect the invention relates to a catalytic converter comprising a catalyst structure, in particular an exhaust catalyst structure, obtainable in the method of the invention, e.g. a TWC catalyst structure, a DOC catalyst structure, a SCR catalyst structure, a lean NO_x trap or NO_x adsorber, and/or a DPF. In certain embodiments the catalytic converter comprises two or more different catalyst structures. For example, the catalytic converter may comprise two or more distinct sections each having a different catalyst structures. When two or more distinct sections are present in a catalytic converter these will typically be in serial fluid communication, e.g. with a first section in an upstream position and second section downstream of the first section with optional additional sections downstream of the second section. The catalytic converter may also comprise a catalyst structure not of the invention in serial fluid communication with a catalyst structure of the invention. Catalytic converters having two or more distinct sections are especially relevant for the catalysis of diesel engine exhaust.

The catalytic converter will generally comprise a ceramic honeycomb structure or a ceramic monolithic structure and the catalyst structure may be immobilised onto the structure using any appropriate technology. For example, the catalyst structure may be adsorbed physically to the surface using a slurry of the ceramic material with post treatment steps, e.g. heating, or the catalyst structure may be bound chemically, e.g. using a binding agent. The exhaust catalyst structure of the invention is particularly useful for preparing a catalytic converter, since the oxygen buffer and the catalytic metal are combined into the nanoparticulate exhaust catalyst structure so that deposition onto the support, e.g. a ceramic material, is simplified. This observation is also relevant for other catalyst structures using other catalytic metals.

Brief description of the figures

In the following the invention will be explained in greater detail with the aid of an example and with reference to the figures, in which

Figure 1 shows a schematic drawing of a continuous supercritical reactor set-up.

Figure 2 shows various mixing geometries for proper mixing of the reactants with the hot solvent string; (a) and (b) cross-mixing, (c) opposing flow-mixing, and (d) vortex- mixing. Figure 3 shows a powder X-ray diffraction (PXRD) analysis of an exhaust catalyst of the invention.

Figure 4 shows a transmission electron microscopy (TEM) analysis of an exhaust catalyst of the invention.

Figure 5 shows particle size analysis of oxygen buffer nanoparticles prepared in the method of the invention.

Figure 6 shows particle size analysis of oxygen buffer nanoparticles prepared in the method of the invention.

Figure 7 shows particle size analysis of oxygen buffer nanoparticles prepared in the method of the invention.

Detailed description of the invention

The present invention relates to a method of providing a solution of a metal oxide precursor;

providing an aqueous reactive solvent in a supercritical or subcritical state;

injecting the aqueous reactant mixture into a reactor tube via a first inlet;

providing at ambient conditions a solution of a metal precursor in a solvent;

injecting the solution of the metal precursor into the reactor tube via a second inlet downstream of the first inlet;

re-establishing supercritical or subcritical conditions in the reactor tube if necessary; allowing a reduction reaction of the metal precursor in the supercritical or subcritical reactant mixture to form metallic nanoparticles on the metal oxide nanoparticles;

withdrawing the catalyst structure from the reactor tube via an outlet. In another aspect the order of preparing the metal oxide nanoparticles and the metallic nanoparticles is reversed so that the metal oxide nanoparticles are prepared on the metallic nanoparticles.

In the context of the invention an "exhaust catalyst structure" comprises a metallic catalyst for catalysing the oxidation of carbon monoxide and hydrocarbons to carbon dioxide and/or the reduction of nitrogen oxides to molecular nitrogen and an oxygen buffer for providing oxygen to the oxidation reactions and sequestering oxygen from the reduction reaction. The combined process may be referred to as "three way catalysis" (TWC). The exhaust catalyst is suited for purification of exhaust gasses from any combustion of a fossil fuel, including liquid hydrocarbons, solid hydrocarbons, coal etc. The invention is not limited to exhaust catalyst structures, and the catalyst structure may be employed for any catalytic process with appropriate selection of catalytic metals and metal oxides.

Any catalytic metal may be relevant for the catalyst structure, in particular transition metals, e.g. Ru, Rh, Pd, Os, Ir, Pt, and lanthanides. The metal may also be a mixture of two or more metals. It is also contemplated that a catalytic metal oxide is employed. The metal or mixture of metals may be selected based on the reaction to be catalysed using the catalyst structure. For example, the metal nanoparticles may be platinum nanoparticles, palladium nanoparticles, rhodium nanoparticles or nanoparticles containing any mixture of platinum, palladium and rhodium. These metals are particularly relevant for an exhaust catalyst structure. In general, metals of relevance comprise a transition metal, a lanthanide, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Gd, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Pt, Au, Ir, W, Sr, Ba. The metal may also be a mixture of two or more metals, such as Pt_xPd_y, Pt_xRh_y, Pd_xRh_y, Pt_xRu_y, Pt_xY_y, Pt_xGd_y, Pt_xSc_y, Pt_xTi_y, Pd_xTiy, Pd_xYy, Pt_xNby, Pt_xZn_y, Pt_xVy, Pt_xCd_y, Pd_xCd_y, Pt_xCuy, Pd_xCu_y, Pd_xNb_y, Pd_xV_y, Pt_xMO_y, Pt_xFe_y, Pt_xCr_y, Pd_xCr_y, Pt_xNi_y, Pt_xCo_y, Pd_xNi_y, Pd_xCo_y, Pt_xMn_y, Pt_xRhy, Pt_xIr_y, PtxRUyMOz, Pt_xRu_yW_z, Pt_xRu_yCo_z, Pt_xRu_yFe_z, Pt_xRu_yNi_z, Pt_xRUyCu_z, Pt_xRu_ySn_z, Pt_xRu_yAu_z, Pt_xRu_yAg_z, Pt_xPd_yRh_z, Pd_xRu_y. When two or more metals are employed the ratio between the metals, i.e. as represented by x and y and z in the listed combinations of metals, may be selected freely.

The exhaust catalyst may comprise an oxygen buffer. The oxygen buffer comprises an oxide of a metal having at least two oxidation levels above zero, such as cerium, where the redox couple of the two oxidation levels allows that the oxygen buffer may release or sequester oxygen as appropriate to the catalysed reactions. The oxygen buffer may also comprise other metal ions that form mixed metal oxides together with a metal oxide providing the oxygen buffering effect. A preferred metal for providing an oxygen buffer is cerium, and appropriate mixed oxides comprise Ceo.65Zro.35O2, AI₂0₃-CexZri-x02, Cei-x0₂,RUx0₂-5,

Cei-₍a₊b) r_aYb02-b/2, Cei-x-y-zZr_xAl_yNdz03-5, Cei-_x-y-zZrxLayAl_z0₃-5,

Ce02-Zr0₂-Nd₂03, Ce₀.3+xZr₀.6-xYo.iOi.95, Ce_xMni-_x0₂-y, Ce_xLayFe_zOx, where "La" denotes a lanthanide (e.g. different from cerium). The invention is not limited to these mixed metal oxides and others are known to the skilled person.

The catalyst nanoparticles may also comprise, in addition to the oxygen buffer nanoparticles, a mixture of a metal in its metallic form and a further active metal compound; the mixture may be random or the catalyst nanoparticles may comprise layers, e.g. distinct layers, of a metal and a metal compound. Layered catalyst nanoparticles may be prepared by initially forming a core particle and subsequently adding, e.g. via an inlet downstream in the reactor tube of the first inlet, a second, different precursor compound to the super- or subcritical reactive solvent and allowing the second precursor to react in the presence of the core catalyst nanoparticles. When the catalyst nanoparticles comprise more than one metal or a metal and more than one metal compound the catalyst nanoparticles may contain a first metal or metal compound representing the majority, e.g. more than 90 %w/w, more than 95 %w/w or more than 99 %w/w, of the mass of the catalyst nanoparticle and one or more minor components, e.g. a metal or a metal compound, present in e.g. less than 10 %w/w, less than 5 %w/w or less than 1 %w/w, of the mass of the catalyst nanoparticle. In this case the catalyst nanoparticle can be said to be "doped" with the minor component. Doped catalysts and the relative amount of their components are well-known to the skilled person.

The catalyst structure prepared in the method of the invention comprises metal oxide nanoparticies, e.g. oxygen buffer nanoparticies, and metallic nanoparticies. In the context of the invention a "nanoparticle" is a particle smaller than 1 μηη, e.g. in the range of about 1 nm to about 1000 nm, e.g. to about 200 nm, with the ranges of from about 1 nm to about 100 nm, or 3 nm to 50 nm, being preferred. Other preferred ranges are from about 1 nm to about 10 nm, e.g. about 1 nm to about 5 nm. A preferred size of the catalyst structure particles, e.g. comprising the oxygen buffer nanoparticies and the metallic nanoparticies, is in the range from about 5 nm to about 70 nm, e.g. in the range of about 10 nm to about 50 nm. The nanoparticies, e.g. the metal oxide nanoparticies, the metallic nanoparticies or the catalyst structure nanoparticies, formed in the method may be monodisperse having a narrow size distribution; samples of particles with standard deviations up to 50%, e.g. <40%, <30%, <20%, < 10%, e.g. <5%, in diameter are considered monodisperse. For example, according to one embodiment of the invention the nanoparticies are of about 5 nm or about 6 nm in size with the standard deviation of the particle size of one batch of nanoparticies being within 50% of 5 nm or 6 nm respectively. It is noted, however, that the smaller the nanoparticies, the larger the acceptable variation of the diameters for the nanoparticies to be considered monodisperse. It is further noted that the size of the metallic nanoparticies and the metal oxide nanoparticies are mutually independent, and in particular, when a carrier material is present, the sizes of the nanoparticies are generally not dependent on the carrier material, e.g. the specific surface area of the carrier material. For example, the size of the nanoparticies can be controlled via the temperature and control of the reaction solution. However, the carrier surface can also affect the catalyst particle size.

In the context of the invention a "carrier material" is a solid material, which may be inert regarding the reaction to be catalysed by the catalyst structure, e.g. the exhaust catalyst structure, and which has a high specific surface area allowing a high mass transfer rate in the catalysed reaction. Thus, the carrier material may have a specific surface area of at least 1 m²/g, such as at least 10 m²/g or at least 100 m²/g, although it may also be higher. When no external carrier material is employed the specific surface area of the catalyst structure may be at least 50 m²/g, e.g. about 100 m²/g or about 200 m²/g. In a specific embodiment the specific surface area is in the range of 10 m²/g to 250 m²/g, e.g. about 50 m²/g to about 1500 m²/g, about 100 m²/g to about 1000 m²/g. In general the higher the specific surface area the higher the mass transfer rate provided by the catalyst structure. Materials with specific surface areas relevant to the invention may be porous, or the high specific surface area may be due to the carrier material being present in an appropriately sized particulate form, or the specific surface area may be due to a combination of particle size and porosity of the carrier material. Determination of the specific surface area is well known to the skilled person. In specific embodiments it is possible to modify the surface of the carrier, e.g. to modify the hydrophilicity or hydrophobicity. For example a carrier material may be exposed to reducing or oxidising conditions prior to the reaction in which the catalyst nanoparticles are formed.

Preferred carrier materials comprise silicon containing oxides, e.g. Si0₂, ion-exchanged zeolites, such as BETA and ZSM-5 zeolites; transition metal oxides, e.g. Al₂0₃, Mg_xAl_yO_z, Ti0₂, Zr0₂,Y₂0₃, Y_xO_y, W0₃, VO, V₂0₃, V₂0₅, V0₂, V_n0_2n+i, W₂0₃, W0₂, W0₃; lanthanide oxides, e.g. La_xO_y, Nd₂0₃, La₂0₃/Zr0₂, La₂0₃, Lao.₆Zr₀.₄-xY_x0₂ (with "La" denoting a lanthanide); metals and alloys, e.g. Ti_xW_y; earth alkaline oxides and compounds, e.g. MgO, BaS0₄, Ba(OH)₂, BaC0₃, Ba(N0₃)₂, Ba_xO_y; perovskite structures, e.g. (^XIIA^2+VIB⁴⁺X^2" ₃) like the Pd-doped perovskite: LaFe₀.57Co₀.₃8 do.o₆0₃; and cerium containing oxides. The carrier may be in any appropriate form, e.g. aerogels; ceramic materials; metals; metal alloys, zeolites, tungsten carbide, metal oxides such as Al₂0₃, y-AIO(OH), Ti0₂, MgO, La₂0₃, Zr0₂, Si0₂, Si0₂-Al₂0₃, Ce0₂, ZnO, Ir0₂, Cr₂0₃, V₂0₅, MgAI₂0₄, BaS0₄, CaC0₃, SrC0₃ etc. The carrier material may be selected based on desired characteristics of the catalyst structure prepared. Other relevant carrier materials are any materials conventionally used in the field of heterogeneous catalysis, as are known to the skilled person. Exemplary carrier materials comprise ceramic materials, such as alumina, titania, silica, zirconia, metal oxides, metal sulphides or metals. When used the carrier material is provided as a suspension, which is mixed with the solution of a precursor compound. This should be understood broadly, and it is also contemplated that the carrier material and the precursor compound, e.g. in a dry form, are added to a solvent to suspend the carrier material and dissolve the precursor compound in order to provide the mixture of the solution of the precursor compound and the suspension of the carrier material. Thus, a specific embodiment of the method of the invention comprises the step of providing a suspension of a carrier material in a solvent at ambient conditions, which suspension contains a precursor compound. Likewise, the carrier material, e.g. in a dry form, may be added to a solution of the precursor compound, or the precursor compound, e.g. in a dry form, may be added to a suspension of the carrier material in order to provide the mixture of the solution of the precursor compound and the suspension of the carrier material. The same considerations apply when no carrier material is employed.

The metal oxide nanoparticles, e.g. oxygen buffer nanoparticles, and the metal nanoparticles are prepared from "precursor compounds". The metal precursor may be any metal salt or compound capable of forming a metal in its metallic form, and the metal oxide precursor allowing formation of the metal oxide, e.g. the oxygen buffering metal oxide. It is contemplated that the same precursor compound may be employed to form either or both metallic nanoparticles and metal oxide nanoparticles comprising the same metal. In particular, the hydrothermal reaction allows formation of particulate metal oxide which is stable towards the reducing conditions in the following step. It is preferred that the precursor compound is soluble in the respective solvent, and it is further preferred that the dissolved form of the precursor compound provides a solubilised metal ion or a metal ion solubilised as a complex with one or more partner atoms or ligands. For example, hexachloroplatinate may be used as a precursor compound for forming metallic platinum. The partner atom or ligand may be any molecule that can form a complex with the metal ion, and in particular the partner atom or ligand may be a molecule that can stabilise the metal ion, e.g. prevent spontaneous oxidation or reduction, and aid in solubilising the metal ion. The molecule may be a simple ion, e.g. chloride, or an organic compound or ion. Certain embodiments of the method of the invention employ more than one precursor compound, e.g. more than one precursor compound for forming the metal oxide nanoparticles, or more than one precursor compound for forming the metallic nanoparticles, which may be provided in a single solution, or individual precursor compounds may be provided as separate solutions, which may be mixed prior to or simultaneously with the mixing with a solvent in a supercritical or subcritical state. When multiple precursor compounds are employed the ratio between the metal ions, e.g. expressed in terms of mass or molarity, may be chosen freely. In one embodiment the metal precursor compound is H₂PtCI₆-6H₂0, Pt(acac)₂, RhCI₃-xH₂0, Rh(N0₃)₃-xH₂0, H₂PdCI₆-xH₂0 or (NH₄)₂PdCI₆-xH₂0, or a combination thereof is employed. These precursor compounds allow formation of metallic platinum and rhodium, respectively. These compounds are soluble in an appropriate solvent, e.g. ethanol or water or a mixture of ethanol and water, to prepare solutions that can be mixed with the reactive solvent in a supercritical or subcritical reactive solvent. Relevant precursor compounds containing platinum are H₂PtCI₆-6H₂0, H₂PtCI₆-xH₂0, PtCI₂, PtCI₄, Pt0₂, cis-dichlorobis(pyridine)platinum(II), platinum(II) acetylacetonate (Pt(C₅H₇0₂)₂) (also known as Pt(acac)₂), PtBr₂, Ptl₂, dichloro(ethylenediamine)platinum(II) (H₂NCH₂CH₂NH₂)PtCI₂), trans- platinum(II)diammine dichloride (Pt(NH₃)₂CI₂), platinum(IV) oxide hydrate (Pt0₂-xH₂0), ammonium hexachloroplatinate(IV) ((NH₄)₂PtCI₆), potassium hexachloroplatinate(IV) (K₂PtCI₆). Relevant precursor compounds containing palladium are hydrogen hexachloropalladate(IV) hydrate (H₂PdCI₆-xH₂0), ammonium hexachloropalladate(IV) hydrate ((NH₄)₂PdCI₆-xH₂0), potassium hexachloropalladate (IV), hydrogen tetrachloropalladate(II), Palladium(II) acetate Pd(OAc)₂, Pd(N0₃)₂, PdCI₂, Na₂PdCI₄, (Ethylenediamine)palladium(II) chloride (Pd(H₂NCH₂CH₂NH₂)CI₂),

Palladium(II) iodide (Pdl₂), Palladium(II) bromide (PdBr₂), PdO, OPd-xH₂0, Pd(OH)₂, Pd(OH)₄, Palladium(II) nitrate dihydrate (Pd(N0₃)₂-2H₂0), Palladium(II) nitrate hydrate (Pd(N0₃)₂-xH₂0), Palladium(II) trifluoroacetate ((CF₃COO)₂Pd), Palladium(II) hexafluoroacetylacetonate (Pd(C₅HF₆0₂)₂, Palladium(II) sulphate (PdS0₄), Palladium(II) cyanide (Pd(CN)₂), Palladium(II) propionate ((C₂H₅C0₂)₂Pd), Palladium(II) potassium thiosulphate monohydrate (K₂Pd(S₂0₃)₂-H₂0), Dichloro(l,5- cyclooctadiene)palladium(II) (C₈Hi₂CI₂Pd), Dichlorobis(triethylphosphine)palladium(II) ( [(C₂H₅)₃P]₂PdCI₂), Ammonium tetrachloropalladate(II) ((NH₄)₂PdCI₄), Potassium tetrachloropalladate(II) (K₂PdCI₄).

Some relevant precursor compounds containing rhodium are

RhCI₃-xH₂0, Rh(N0₃)₃-xH₂0, Rh(C₅H₇0₂)₃, RhCI₃, Rh₂(OOCCH₃)₄, Na₃RhCI₆, Rh(Cyclooctadiene)(Cyclooctatriene) (Rh(COT)(COD)), Rh(H₂C=CH-CH₂)₃ (Rh(Allyl)₃), (Rh[CH₃(CH₂)₆C0₂]₂)₂, [(CF₃COO)₂Rh]₂, [(CH₃COO)₂Rh]₂, [[(CH₃)₃CC0₂]₂Rh]₂, RhBr₃-xH₂0, RhP0₄, [Rh[CH₃(CH₂)₄C0₂]₂]₂, RhCi₂Hi₅0₂ (Acetylacetonato)(norbornadiene)rhodium(I), Ci₆H₂₆0₂Rh₂ Hydroxy(cyclooctadiene)rhodium(I) dimer, C₉Hi₅0₂Rh

Acetylacetonatobis(ethylene)rhodium(I), Ci₃Hi₉0₂Rh (Acetylacetonato)(l,5- cyclooctadiene)rhodium(I), Na₃RhCl₆ Sodium hexachlororhodate(III), H₃RhCI₆ hydrogen hexachlororhodate(III), Rh(CO)₂(C₅H₇0₂) (Acetylacetonato)dicarbonylrhodium(I), [Rh(CO)₂CI]₂ Di- -chloro- tetracarbonyldirhodium(I), (NH₄)₃RhCl₆ Ammonium hexachlororhodate(III), [Rh(C₂H₄)₂CI]₂ Di- -chlorotetraethylene dirhodium(I).

Some relevant precursor compounds containing cerium are ammonium cerium(IV) nitrate (NH₄)₂Ce(N0₃)₆, (NH₄)₂Ce(N0₃)₅-4H₂0, Ce(N0₃)₃, Ce(N0₃)₃-xH₂0, Ce(III) fluoride, Ce(III) iodide, Ce(III) chloride, Cerium(III) bromide, Ce(III) chloride hydrate, Ce(IV) sulphate, Ce(IV) sulphate hydrate, Cerium(IV) fluoride, Ammonium cerium(IV) sulphate dehydrate, Cerium(III) sulphate, Cerium(III) sulphate hydrate, Cerium(III) acetylacetonate hydrate, Cerium(III) acetate hydrate, Cerium(III) 2- ethylhexanoate, Cerium(III) carbonate hydrate, Cerium(IV) hydroxide, Cerium(III) oxalate hydrate, Cerium(III) bromide heptahydrate, Ammonium cerium(IV) sulphate hydrate.

Some relevant precursor compounds containing zirconium are ZrO(N0₃)₂ Zirconium(IV) oxynitrate hydrate, Zirconium(IV) acetylacetonate, Bis(cyclopentadienyl)zirconium(IV) dichloride, Zirconium acetate, Zirconium(IV) iodide, Zirconium(IV) fluoride, Zirconium(IV) bromide, Zirconium(IV) chloride, Zirconium(IV) iodide hydrate, Zirconium(IV) fluoride hydrate, Zirconium(IV) bromide hydrate, Zirconium(IV) chloride hydrate, Zr(CH₂CHC0₂)₄ Zirconium acrylate, Zr(CH₂CHC0₂CH₂CH₂C0₂)₄ Zirconium carboxyethyl acrylate, Zr(OCH₂CH₂CH₃)₄ Zirconium(IV) propoxide, ZrOCI₂-8H₂0 Zirconium(IV) oxychloride octahydrate, Zr(OC₄H₉)₄ Zirconium(IV) butoxide, Zr(OCH(CH₃)₂)₄-(CH₃)₂CHOH Zirconium(IV) isopropoxide isopropanol, Zr(OCH(CH₃)₂)₄ Zirconium(IV) isopropoxide, Zr[OC(CH₃)₃]₄ Zirconium(IV) tert-butoxide, Zr(OH)₄ Zirconium(IV) hydroxide, (CH₃C0₂)_xZr(OH)_y, x+y « 4 Zirconium(IV) acetate hydroxide, Zr(OC₂H₅)₄ Zirconium(IV) ethoxide, Zr(OCH₃)₄ Zirconium(IV) methoxide, Zr(HP0₄)₂ Zirconium(IV) hydrogenphosphate, Zr(S0₄)₂ Zirconium(IV) sulfate, Zr(S0₄)₂-xH₂0 Zirconium(IV) sulphate hydrate, Zr(OH)₂C0₃-Zr0₂ Zirconium(IV) carbonate hydroxide oxide, Zirconium(II) hydride, ZrOCI₂ ^■ 8H₂0 Zirconyl chloride octahydrate, ZrOCI₂ Zirconyl chloride, ZrOCI₂-xH₂0 Zirconyl chloride hydrate, H₂ZrF₆ Hexafluorozirconic acid.

The concentrations of the precursor compounds and the optional carrier material in their respective solvents is preferably in the range of about 1 wt% to about 10 wt%, or 0.001 to 5 M, e.g. 0.1 M or 1 M. The volumes of the solution of the precursor compound and the suspension of the carrier material are selected to provide the desired ratio of the precursor compound and the optional carrier material depending on their respective concentrations. It is, however, preferred that the volumes are of comparable size in order to ensure efficient mixing, and the concentrations will generally be selected to allow mixing of volumes of comparable size.

The method of the invention comprises steps where solvents are under "ambient conditions". In the context of the invention the term "ambient" should be understood broadly and in particular it means that the pressure is not increased or decreased relative to the pressure of the surroundings. The solvent under ambient conditions will be liquid, and for certain solvents the temperature may be decreased or increased relative to the temperature of the surroundings, in particular in order to ensure that the solvent is in a liquid state.

The present invention employs an "aqueous reactive solvent" that allows a hydrothermal reaction to take place and optionally a "reducing solvent". These solvents may be referred to collectively as "reactive solvents". A reactive solvent is any solvent that may form a supercritical or subcritical state, and which further comprises a reactive compound that may react with a precursor compound to form a metal oxide, such as an oxygen buffer compound, or a metal, as appropriate. The reactive solvent is preferably liquid at ambient conditions. It is however also contemplated that gaseous compounds, e.g. C0₂, may be employed as a supercritical solvent in the method of the invention. The reaction may be a reduction or a hydrothermal reaction, and the reactive compound may be molecules of the reactive solvent or the reactive solvent may comprise further, e.g. dissolved, reducing compounds. The reactive solvent may be selected from alcohols, ethers, ketones, aldehydes, amines, amides, water and other organic based liquids; preferred reactive solvents are ethanol, methanol, isopropanol, ethylene glycol, water and combinations thereof. Alcohols, e.g. ethanol, methanol and isopropanol, ethylene glycol are generally considered reducing solvents. The reactive solvent may also comprise a mixture of solvents, including reducing solvents with non-reducing solvents or aqueous solvents with other solvents. It is noted that certain solvents may be either reducing or oxidising depending on the conditions, e.g. regarding pressure and temperature.

In a certain embodiment, the suspension of the carrier material and/or the reactive solvent may also comprise a dispersion agent. In the context of the invention a "dispersion agent" is any compound that may aid in the dispersion of the carrier material and it may further improve the processing by minimising undesirable deposition of the carrier material or prepared catalyst in unit operations, such as valves, pumps, mixers, inlets, outlets etc. in the process stream. This is especially advantageous when the process is operating continuously since it allows the process to proceed for extended periods of time. A preferred dispersion agent is ethylene glycol, for example present at a concentration in the range of from about 0.1% to 10%, e.g. such as about 1%, about 2%, about 3%, about 4%, about 5%. Ethylene glycol is particularly advantageous as a dispersion agent when the reactive solvent is a reducing solvent, such as an alcohol, e.g. ethanol. Other dispersion agents comprise any non-ionic surfactant, e.g. Triton X- 100, or polymeric compounds, such as polyvinyl pyrrolidone, polyoxyethylene sorbitan monolaurate etc. Other dispersion agents may also be present at a concentration in the range of from about 0.1% to 10%. In a certain embodiment, the suspension of the carrier material and/or the reactive solvent may also comprise a pH neutraliser, e.g . NH₄OH.

Solvents generally have a critical point regarding temperature and pressure defining a supercritical regime, which is reached when exceeding the critical point in the phase diagram . The temperature value and the pressure value of the critical point are abbreviated "7_cr" and ⁿP_cr", respectively, in the context of this invention. In the supercritical regime distinct liquid and gas phases do not exist, and in this regime the fluid will have special properties which have many advantages for the synthesis of catalyst nanoparticles. Compared to conventional liquid solvents, the high diffusivities and low viscosities of supercritical or subcritical fluids result in enhanced mass-transfer. The low surface tension of supercritical or subcritical fluids can also help avoiding collapse of the carrier material. The present inventors have now surprisingly found that when the metallic nanoparticles are formed on the metal oxide nanoparticles, e.g. the oxygen buffer nanoparticles, prepared in the hydrothermal reaction, aggregation of the formed metallic nanoparticles can be avoided so that the catalyst nanoparticles comprising the metal oxide nanoparticles and the metallic nanoparticles can be distributed, as individual catalyst nanoparticles, on the ceramic honeycomb structure or ceramic monolith to form a catalytic converter of high efficiency. The properties of supercritical and subcritical fluids are tuneable by changing the pressure and temperature. In particular, density and viscosity change drastically at conditions close to the critical point, e.g. at a temperature at or within 200°C below T_cr, such as within 150°C, within 100°C or within 50°C below T_cr, and a pressure at or within 60% below P_cr, e.g. 50% below, 40% below or 30% below P_cr. There are generally no upper limits to the temperature and pressure in the method of the invention. However, it is contemplated that the temperature should generally be below 1000°C and the pressure generally be below 1000 bar. In certain embodiments the upper limit of the temperature is within 500°C, within 200°C or within 100°C above the T_cr, and the pressure has an upper limit of 2000%, 1000%, 500% or 200% of the P_cr.

In the context of the invention the terms "supercritical" or "supercritical state" refer to the state of a solvent above its critical point regarding temperature (7_cr) and pressure ( _cr). The reactive solvent may also be in a subcritical state. The term "subcritical state" generally refers to the state where one or both of the temperature and the pressure are below the critical point values T_cr and P_cr- In particular, in the context of the invention a subcritical state may be formed when a solvent is exposed to a temperature at or within 200°C, e.g. with 150°C, e.g. within 100°C, e.g within 50°C, e.g. within 40°C, 30°C, 20°C or 10°C, below the T_cr while the pressure is at or within 60%, e.g. within 50%, 40%, 30%, 25%, 20%, 15%, 10%, or 5%, below the P_cr- When either of the pressure or the temperature is within these ranges and the temperature or the pressure, respectively, is above the corresponding critical point value the solvent is also considered to be in a subcritical state. The super- and subcritical states may also be referred to as super- and subcritical conditions, respectively. In certain embodiments the state of the reaction solution may be changed between supercritical conditions and subcritical conditions and vice versa. When the supercritical or subcritical reactive solvent is admixed with the mixture of the solution of the precursor compound being under ambient conditions the temperature and pressure of the admixture will typically drop relative to the temperature and pressure of the reactive solvent due to ambient conditions of the solution of the precursor compound. However, due to the design of the apparatus the temperature and pressure of the reaction solution are quickly increased to the desired values. This allows that the initiation of the reaction of the precursor compound can be controlled further. As an example water at a temperature in the range of 250°C to 450°C at a pressure of 100 bar to 300 bar is mixed with the mixture of the solution of the precursor compound and the suspension of the carrier material providing a temperature at the mixing point in the range of about 100°C to about 350°C. In other embodiments, the pressure and/or temperature of the mixture of the solution of the precursor compound and the suspension of the carrier material is increased, e.g. to subcritical or supercritical conditions, in particular to the same pressure and temperature as the reactive solvent, prior to admixing with the reactive solvent under subcritical or supercritical conditions. The temperature and pressure values of the critical points of solvents are known to the skilled person. Specific examples of critical points of selected solvents are given in Table 1. Table 1 Critical points of selected solvents

An exemplary set-up of a supercritical synthesis reactor is illustrated in Figure 1. Figure 1 shows a set-up for preparing a catalyst structure of the invention. In Figure 1 two different solutions of oxygen buffer precursors, e.g. (NH₄)₂Ce(N0₃)6 or Ce(N0₃)₃, and ZrO(N0₃)₂, are provided via feed pumps 21 and 22, respectively, to a cooler 4 before being supplied to a mixer 6. An aqueous reactive solvent is provided via a solvent pump 3 to a heater 5. The oxygen buffer precursors are cooled in the cooler 4 before being supplied to the mixer 6, however, the cooler 4 is optional. The cooler 4 may serve to prevent that the pump or other heat sensitive parts are heated. In the mixer 6 the solutions of the oxygen buffer precursors are mixed with the supercritical or subcritical aqueous reactive solvent. The mixture is provided to the first section of the reactor tube 71 which first section comprises a heater. The reactor tube may comprise a cooling section for liquefying the aqueous reactant mixture, but the cooling section is optional. The set-up has a pressure release valve 9 allowing collection of prepared particles in a collection vessel 10. The set-up shown in Figure 1 comprises a second inlet for a metal precursor that may be supplied via reactant pump 23. The set-up may also comprise a cooler (not shown) after reactor tube 71, e.g. upstream of reactor tube 72, to mix reactant solvent and precursor from pump 23 before further heating and/or a third inlet (not shown) for a supercritical or subcritical reducing solvent to be introduced via an additional solvent pump (not shown). This introduces the reducing solvent at a location downstream or upstream of the inlet to the reactor tube of reactant pump 23, e.g. into a mixer (not shown) capable of mixing the stream from the reactor tube with the stream from reactant pump 23 and the optional additional solvent pump. The supercritical or subcritical stream in reactor tube 72 is cooled in cooler 8 before withdrawing via pressure release valve 9 into collection vessel 10.

The separate inlets prevent the reaction to occur prematurely and thus the precursor from solidifying on the carrier material before entering the reactor and the supercritical regime. In order for proper mixing of the precursor and carrier material, a static mixer is used before reaching the hot solvent. In order for proper mixing with the hot solvent, various mixing geometries can be used, such as cross-, vortex- or opposing flow-mixing, illustrated in Figure 2.

The invention will now be explained in the following non-limiting examples. As will be evident to the skilled person variations are possible without deviating from the invention. Example 1

An exhaust catalyst structure of the invention was prepared as follows.

The syntheses of the exhaust catalytic structure where platinum is deposited onto ceria-zirconia or ceria is here reported. The syntheses were performed in the supercritical regime or the subcritical regime, which for the solvent water is either at a temperature below 374°C or at a pressure below 221 Bar.

The reactions were carried out in a purpose built synthesis flow system which can withstand the harsh conditions of the supercritical fluids. The schematic shown in Figure 1 is a simplified version of the experimental set-up in which the general parts are illustrated. Firstly, the precursors for the oxygen buffer were precisely weighed in order to get the correct ratio between Zirconium and Cerium in the mixed oxide of Ceo.65Zro.35O2. The two precursor compounds, (NH₄)₂Ce(N0₃)6 and ZrO(N0₃)₂-xH₂0, were weighed in separate containers resulting in a mass of 1.78 g and 0.48 g, respectively. The two solid reactants were mixed in separate beakers with 65 ml_ and 35 ml_ respectively, of deionised water each creating a 0.05 M solution, and subsequently mixed to one 100 ml_ precursor solution. A mass of 715 mg of a platinum precursor (H₂PtCI₆-6H₂0) was precisely weighed on a micro scale in order to get precise concentration and weight ratio (Pt/Ce₀.65Zr₀.35O₂) in the final synthesis solution. The platinum precursor was dissolved in 100 ml_ of absolute ethanol, to create a 0.0138 M solution.

The compositions of the oxygen buffer nanoparticles were varied in alternative experiments where cerium oxide particles (without zirconium) were prepared from 0.05 M solutions of the respective cerium precursor compounds. In certain experiments oxygen buffer nanoparticles were prepared without platinum nanoparticles also using the 0.05 M solutions of oxygen buffer precursors. The concentration of platinum precursor was also varied in certain experiments. The experiments are summarised in Table 2.

The oxygen buffer precursor compound solution was pumped through reaction pump 21 into the pressurised system at 300 Bar. At the mixing point the cold reactant stream mix with the subcritical or supercritical preheated reactive solvent, Deionised Water from pump 3, heated to 300°C, leading to a mixing temperature of 150°C. The rapid increase in the temperature still leads to fast homogenous nucleation resulting in monodisperse nanoparticles of Ceo.65Zro.35C , which matured further down the column at 400°C (under supercritical conditions). The platinum precursor compound solution was pumped through reaction pump 23 into the pressurised system at 300 Bar mixing with the already formed oxygen buffer material. The rapid increase in the temperature of the cold platinum precursor solution leads to fast homogenous nucleation resulting in monodisperse platinum particles. The continuous flow of the produced nanoparticles, both ceria-zirconia and platinum, was cooled in the tube-in- tube cooler 8. The particles were subsequently withdrawn from the system using a pressure release valve 9, which also kept the system pressurised.

The synthesis products were characterised using powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM).

The PXRD of the crystalline exhaust catalyst materials results in d iff ractog rams. One example is shown in Figure 3, illustrating the result of the nanoparticies of Ceo.65Zro.35O2 and Platinum; a fit is shown along with the PXRD data for the size determination, where the black signal is correlated to Ceo.65Zro.35O2 and the grey to platinum. From the Bragg- angles, the material and crystal structure can be found while the line broadening provides information about the particle sizes.

Figure 4 shows TEM images of samples of the prepared exhaust catalyst material. Three different magnifications are shown; (a) and (b) show the distribution of the particles, where the darker particles indicate platinum as it is a heavier element than Ce and O, (c) shows a highly magnified image where the lattice planes of the particles can be seen, verifying that crystalline particles have been synthesised.

TEM provides high magnified images of the particles, and thus the particle size distribution can be found.

Table 2 presents the most important synthesis of oxygen buffer material with and without the deposition of Pt nanoparticies. Table 2 thus summarises 8 experiments and indicates, how the parameters were varied. Experiment 1 employs the conditions provided above, i.e. 65 ml_ (NH₄)₂Ce(N0₃)6 and 35 ml_ ZrO(N0₃)₂-xH₂0 each in 0.05 M concentration and 100 ml_ H₂PtCl₆-6H₂0 at 0.0138 M concentration with the temperatures shown in Table 2; the pressure was 300 bar. The sizes of the particles prepared in Experiments 2 to 8 are shown together with the parameters examined for each experiment. Other parameters were as for Experiment 1.

The stated weight percentage of the composition is given with respect to the amount of precursor used. TEM images have only been taken for a few of these experiments, while all have been analysed with PXRD.

Table 2 Important synthesis of oxygen buffer material with and without the deposition of Pt nanoparticies. All experiments performed at a pressure of 300 bar unless otherwise listed. The solvent was in all experiments Deionised Water (DI water) heated to 300°C unless otherwise stated.

Experi Oxygen Buffer/ Parameters Particle Particle size, ment metal size, metal oxygen buffer

(nm) (nm)

1 Ceo.65Zro.35O2 / Tsolvent = 300°C 4.8 4.9

Pt T_mix = 150°C Experi Oxygen Buffer/ Parameters Particle Particle size, ment metal size, metal oxygen buffer

(nm) (nm)

Tmature = 400°C

2 Ceo.65Zro.35O2 / 0.00138 M N.D. 4.8

Pt H₂PtCI₆«6H₂0

3 Ceo.65Zro.35O2 / (NH₄)₂Ce(N0₃)₆: -

- 0.05 M 6.4

0.5 M 7.9

1.0 M 8.3

4 Ceo.65Zro.35O2 / Tmature = 200°C - 2.5

- Tmature = 250°C 2.7

Tmature = 300°C 3.8

Tmature ' 350°C 5.4

Tmature = 400°C 6.1

5 Ceo.65Zro.35O2 / Tmix = 250°C - 3.5

- Tmix = 350°C 3.6

Tsoivent = 450°C for

both experiments

6 Ce0₂ / Tmature = 200°C - 3.2

- Tmature = 250°C 3.8

Tmature = 300°C 5.5

7 Ce0₂ / Tmature = 300°C -

- P = 300 bar 5.5

P = 200 bar 5.3

P = 100 bar 5.4

8 Ce0₂ / 0.025 M 5.2 8.6

Pt H₂PtCI₆-6H₂0

EtOH as solvent for both Ce0₂ and Ceo.65Zro.35O2 has also been tried, but the products contained too many impurities for a size to be determined.

The graph in Figure 5 shows the vertical heater (maturing) temperature (T_mature) dependence on the Ceo.65Zro.35O2 particle size measured by PXRD, and illustrates a very precise size control in the 2.5 - 6 nm range.

The graph in Figure 6 shows the vertical heater (maturing) temperature ( _msture) and pressure dependence on the Ce0₂ particle size measured by PXRD, and illustrates a very precise size control in the 3 - 6 nm range. It is noticeable that the pressure appears to have little effect on the final product, when both T_soi_ve„_t and T_matUre are below the critical temperature. Example 2

Further embodiments of exhaust catalyst structures of the invention, where platinum or platinum-palladium is deposited onto ceria- zirconia or ceria with the addition of a pH neutraliser, were prepared as follows. The syntheses of the exhaust catalytic structures were performed in the supercritical regime or the subcritical regime. The precursors used in the different experiments are given in Table 3 with their molar weights and solubility.

Table 3 Precursor used

The reactions were carried out in the flow system of Figure 1. Firstly, the precursors for the oxygen buffer were precisely weighed in order to get the correct ratio between Zirconium and Cerium in the mixed oxide of Ce₀.65Zr₀.35O₂. The two precursor compounds, Ce(N0₃)₃»6H₂0 and ZrO(N0₃)₂»xH₂0, were weighed in separate containers resulting in a mass of 14.1 g and 4.8 g, respectively. The two solid reactants were mixed in separate beakers with 65 mL and 35 mL respectively, of deionised water each creating a 0.5 M solution, and subsequently mixed to one 100 mL precursor solution. Masses of 0.33 g platinum precursor (H₂PtCI₆»6H₂0) and 0.11 g palladium precursor (PdCI₂) were precisely weighed on a micro scale in order to get precise concentrations and weight ratio (Pt: Pd 1 : 1 atom, and 2.5 wt% PtPd /Ce₀.65Zr₀.35O₂) in the final synthesis solution. The platinum and palladium precursors were dissolved in 100 ml_ deionised water and then mixed with the Cerium and Zirconium solution.

To ease the damage on the supercritical flow reactor and avoid contamination of the nanoparticle product a pH-neutraliser, NH₄OH, was used (often 2 M concentration).

The ratio between precursors and pH-neutraliser were varied in various experiments, and also ethanol was used as precursor solvent in alternative experiments. In certain experiments oxygen buffer nanoparticles were prepared without platinum/palladium nanoparticles. The concentrations of platinum and palladium precursors were also varied in certain experiments, including experiments without palladium. The experiments are summarised in Table 4. Other cerium-salts such as cerium ammonium nitrate ((NH₄)₂Ce(N0₃)₆) has also been used.

The supercritical flow synthesis reactor (SCF) has the basic setup consisting of four pumps, three heaters, five vessels (two for each reactant pump and one for the solvent pump) and a cooler (see Figure 1). The pumps feed reactant(s) and solvent into the system from the vessels. The reactants and solvents enter the system at certain flow rates, at pressures around 250 bars, which is controlled by an automatic back-pressure regulator. The solvent is deionised water or ethanol and is preheated to temperatures around 250 to 450°C. Reactant pump 23 was not used in this example, thus only three pumps were used.

The two reactant pumps 21, 22 each control two vessels. This gives the ability to change the solutions pumped into the system quickly. Often, one vessel contains water or ethanol to clean the system between experiments and the other contains the reactant(s). In these experiments, the reactant pumps contain precursors and pH neutraliser, possibly with additional carrier material. Pump 3 contains the solvent.

The two reactant flows are mixed before reaching the solvent flow. The solvent flow has travelled through a heater, thus being in a supercritical state (high pressure, high temperature) ensuring shock heating of the cold reactants, thus initiating the reaction. The mixing temperature can be sub- or supercritical and is controlled either by varying the flow rates or the solvent heater temperature. The rapid increase in the temperature leads to simultaneous nucleation resulting in mono-disperse nanoparticles, which are further matured down the vertical heater at 250 to 450°C, before being cooled.

In the specific experiment the oxygen buffer and precious metal precursor compound solution was pumped through one reaction pump 21 at 10 mLJmin into the pressurised system at 250 Bar. The pH neutraliser solution is pumped through the second reaction pump 22 also at 10 mL/min into the pressurised system, and is then mixed with the precursor solution. The cold reactant streams then mix with the supercritical preheated reactive solvent, deionised water (pump 3), heated to 450°C, leading to a mixing temperature of 300°C. The rapid increase in the temperature leads to fast homogenous nucleation resulting in monodisperse nanoparticles of Ceo.65Zro.35O2 with distributed PtPd nanoparticles which are matured further down the column at supercritical conditions (400°C). The continuous flow of the produced nanoparticles, both ceria-zirconia and platinum-palladium, was cooled in the tube-in-tube cooler 8. The particles were subsequently withdrawn from the system using the back-pressure regulator.

The synthesis products were characterised using PXRD and TEM. The PXRD of the crystalline exhaust catalyst materials results in d iff ractog rams. One example is shown in Figure 7, illustrating the result of the nanoparticles of Ceo.65Zro.35O2 and PtPd; a fit is shown along with the PXRD data for the size determination, where the black signal is correlated to Ceo.65Zro.35O2 and the grey to platinum (-palladium). From the Bragg- angles, the material and crystal structure can be found while the line broadening provides information about the particle sizes. In some experiments, a shoulder was seen on the right side of the Ceria Zirconia peaks indicating a size-broadening stemming from a distribution in composition (Ce- and Zr-rich particles).

Table 4 presents the most important synthesis series of oxygen buffer material with and without the deposition of PtPd/Pt nanoparticles, and comments on the experiments of Table 4 are presented in Table 5. Experiment series 1 employs the conditions provided above, i.e. 65 ml_ Ce(N0₃)₃ and 35 ml_ ZrO(N0₃)₂»xH₂0 each in 0.5 M concentration and 100 ml_ H₂PtCI₆»6H₂0 and PdCI₂ at 2.5wt% of the final product with the temperatures shown in Table 4; the pressure was 250 bar. For higher flow rates, larger volumes of the precursor solutions were used. The sizes of the particles prepared in the experiments are shown together with the parameters examined for each experiment. Other parameters were as for Experiment series 1. The parameters that have been varied and their impact analysed (desired control : particle sizes of both Ceo.65Zro.35O2 and PtPd) include flow rates, mixing temperature, maturing temperature, solvent temperature, pH value (controlled by pH neutraliser concentration), solvents and precursor concentrations.

Table 4 Standard conditions in experiments (unless other parameters are given in table) : T_solvent = 450°C, T_mix = 300°C, T_matUre = 400°C, Solvent: DI Water, P = 250 bar, Flow = 20 mLJmin, n_{N H40}H = 2M, n_Ce( 03)3,zro( 03)2 = 0.5M, n_H2Ptci6,Pdci2 0.0008M (2.5 wt%), pH_pr0duct = 10. Whenever two oxygen buffer particle sizes were found they are written as x(y) where x is the Ce- rich particle size and y the Zr-rich particle size.

(nm) (nm)

1 Ceo.65Zro.35O2 / Standard 21 6

2.5 wt% PtPd

2 Ceo.65Zro.35O2 / Tsolvent = 450°C

2.5 wt% PtPd 1 ^'mature = 300°C 19(5)

Tmature ⁼ 350°C 22(6)

Tmature = 450°C 20(6)

ΠΝΗ₄ΟΗ = 1M

Solvent: DI Water

3 Ceo.65Zro.35O2 / Tsolvent = 450°C

2.5 wt% PtPd T_mix = 300°C

Tmature = 300°C 10

Tmature ⁼ 350°C 10

Tmature = 450°C 10(3)

Solvent: ethanol

4 Ceo.65Zro.35O2 / Precursor solvent:

2.5 wt% PtPd Water 21 6

ethanol 21 6 Experi Oxygen Buffer/ Parameters Particle Particle size, ment metal size, metal oxygen buffer

(nm) (nm)

5 Ceo.65Zro.35O2 / pH = 3 12(6)

2.5 wt% PtPd pH = 5 13(7) pH = 7 14(5) pH = 9 14(4) pH = 10 20 6

6 Ceo.65Zro.35O2 pH = 5 - 13(7)

pH = 7 - 16(6) pH = 9 - 11(5) pH = 10 - 6

7 Ceo.65Zro.35O2 / pH = 5 11(5)

5 wt% Pt pH = 7 9(5) pH = 9 17 14(4) pH = 10 17 6

8 Ceo.65Zro.35O2 P = 300 bar - 8

P = 250 bar - 8

P = 200 bar - 6

9 Ceo.65Zro.35O2 Precursor & pH

neutraliser

Flow = 10 ml/min - 8

Flow = 20 ml/min - 7

Flow = 30 ml/min - 8

10 Ceo.65Zro.35O2 1 ^'mature = 300°C - 7

Tmature ⁼ 350°C - 8

Tmature = 400°C - 8

Tmature = 450°C - 8

Table 5 Parameters varied in experiments of Table 4 and comments on the results of the experiments

Experiment Parameter examined Comments

1 Standard experiment Large PtPd particles

2 Temperature dependence No size control, Oxygen buffer

(Water solvent) particle size/composition Experiment Parameter examined Comments

distribution.

3 Temperature dependence Little oxygen buffer particle size

(ethanol as solvent) /composition distribution.

4 Solvents Precursor solvent has no

influence on the particle size.

5 pH variation No crystalline Pt particles at low pHs. High pH narrows the oxygen buffer particle size/composition distribution.

6 pH value (no precious No oxygen buffer size control but metal) high pH (standard experiment,

2M pH neutraliser) narrows the oxygen buffer particle

size/composition distribution.

7 pH value No crystalline Pt particles at low

(no Pd) pHs. High pH narrows the oxygen buffer particle size/composition distribution. No Pt size control.

8 Pressure variation Going below the supercritical

(no precious metal) pressure seems to have influence on the Ceo.65Zro.35O2 particle size

9 Flow variation (no No flow influence on

precious metal) Ceo.65Zro.35O2 particle size

10 Temperature variation Maturing temperature has little or

(no precious metal) no effect on the particle size.

Claims

P A T E N T C L A I M S

1. A method of preparing a catalyst structure comprising a metallic catalyst and a metal oxide, the method comprising the steps of:

providing a solution of a metal oxide precursor;

providing an aqueous reactive solvent in a supercritical or subcritical state;

injecting the aqueous reactant mixture into a reactor tube via a first inlet;

providing at ambient conditions a solution of a metal precursor in a solvent;

withdrawing the catalyst structure from the reactor tube via an outlet.

2. A method of preparing a catalyst structure comprising a metallic catalyst and a metal oxide, the method comprising the steps of:

providing a solution of a metal precursor;

providing an aqueous reactive solvent in a supercritical or subcritical state;

mixing the solution of the metal precursor with the supercritical or subcritical aqueous reactive solvent to form an aqueous reactant mixture; injecting the aqueous reactant mixture into a reactor tube via a first inlet;

withdrawing the catalyst structure from the reactor tube via an outlet.

3. The method of preparing a catalyst structure according to claim 1 or 2, wherein the metal oxide is a transition metal oxide or a lanthanide oxide.

4. The method of preparing a catalyst structure according to any one of the preceding claims, wherein the metal oxide is an oxygen buffer comprising cerium oxide and optionally zirconium oxide.

5. The method of preparing a catalyst structure according to any one of the preceding claims, wherein the ratio between the metal oxide precursor and the metal precursor expressed as a weight ratio is in the range of from 1 : 100 to 100: 1.

6. The method according to any one of the preceding claims, wherein the metallic catalyst comprises a metal selected from the group consisting of Pt, Rh, Pd, a transition metal, a lanthanide, or a combination thereof.

7. The method of preparing a catalyst structure according to any one of the preceding claims, further comprising the steps of:

providing a reducing solvent in a supercritical or subcritical state; and injecting the reducing solvent in the supercritical or subcritical state into the reactor tube via a third inlet downstream of the first inlet to provide a supercritical or subcritical reducing reactant mixture.

8. The method of preparing a catalyst structure according to claim 7, wherein the reducing solvent is selected from the group consisting of ethanol, methanol, isopropanol, ethylene glycol or a combination thereof.

9. The method of preparing a catalyst structure according to any one of the preceding claims further comprising the steps of:

providing at ambient conditions a solution of a second metal precursor in a solvent;

injecting the solution of the second metal precursor into the reactor tube via a further inlet downstream of the second inlet;

allowing a reduction reaction of the second metal precursor in the supercritical or subcritical reactant mixture to form layered metallic nanoparticles on the metal oxide nanoparticles.

10. The method of preparing a catalyst structure according to any one of the preceding claims, further comprising the steps of:

providing in a solvent at ambient conditions a suspension of a carrier material having a specific surface area of at least 1 m²/g;

optionally sonicating the suspension of the carrier material;

mixing the solution of the metal oxide precursor and the suspension of the carrier material with the supercritical or subcritical aqueous reactive solvent to form the supercritical or subcritical aqueous reactant mixture.

11. The method of preparing a catalyst structure according to claim

10, wherein the carrier material is selected from the group consisting of silicon containing oxides, transition metal oxides, lanthanide oxides, metals and alloys, earth alkaline oxides and compounds, perovskite structures, and cerium containing oxides.

12. The method according to any one of the preceding claims further comprising the step of cooling the supercritical or subcritical reactant mixture.

13. The method according to any one of the preceding claims, wherein the aqueous reactive solvent and/or the reducing solvent has a temperature at or within 200°C below, or above the temperature of the critica l point (7_cr) of the respective solvent and the aq ueous reactive solvent and/or the reducing solvent is at a pressure at or within 60% below, or above the pressure of the critical point (P_cr) of the respective solvent.

14. The method according to any one of the preceding claims, wherein the reaction ta kes place under continuous cond itions.

15. The method according to any one of the preceding claims, wherein the reactant m ixture has a residence time in the reactor tube between the first inlet and the outlet of the reactor tube in the range of from 2 seconds to 30 m inutes.

16. The method according to any one of the preceding claims, wherein the reactant m ixture has a residence time in the reactor tube between the second inlet a nd the outlet of the reactor tube in the range of from 2 seconds to 30 m inutes.

17. A catalyst structure obtainable in the method according to any one of claims 1 to 16.

18. The catalyst structure according to claim 17, wherein the sizes of the metal oxide nanoparticles are in the ra nge of about 1 nm to about 200 nm .

19. The catalyst structure according to cla im 17 or 18, wherei n the sizes of the metallic nanoparticles are in the range of about 1 nm to about

200 nm .

20. The catalyst structure according to any one of claims 17 to 19, wherein the sizes of the catalyst structure particles are in the ra nge from about 5 nm to about 70 nm .

21. The catalyst structure according to any one of claims 17 to 20, wherein the metal oxide nanoparticles, the metallic nanoparticles or the catalyst structure nanoparticles are monod isperse having a size distribution with standa rd deviations in diameter of up to 50%.

22. The catalyst structure according to any one of claims 17 to 21, wherein the catalyst structure is a three way cata lysis catalyst structure, a diesel oxidation catalyst structure, a selective catalytic reduction catalyst structure, a lean NO_x trap catalyst structure, a NO_x adsorber catalyst structure, or a diesel particulate filter catalyst structure.

23. A catalytic converter comprising a catalyst structure according to any one of claims 17 to 22 immobilised on a ceramic honeycomb structure or a ceramic monolithic structure.

24. The catalytic converter according to claim 23 comprising two or more distinct sections, with each section having a different catalyst structure and with the sections being in serial fluid communication.