BR112018008436B1 - MIXED OXIDE BASED ON CERIUM-ZIRCONIUM, CATALYTIC SYSTEM, PROCESS FOR TREATING AN EXHAUST GAS FROM A VEHICLE ENGINE AND PROCESS FOR PREPARING A MIXED HYDROXIDE OR MIXED OXIDE BASED ON CERIUM-ZIRCONIUM - Google Patents

Abstract

COMPOSITIONS BASED ON ZIRCONIA FOR USE AS THREE-WAY CATALYST. This invention relates to a cerium-zirconium based mixed oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of 10 to 50% by weight, wherein the composition has (i) a surface area of at least 18 m2/g, and a total pore volume as measured by N2 physiosorption of at least 0.11 cm3/g, after aging at 1100 (degrees Celsius ) in an air atmosphere for 6 hours, and (ii) a surface area of at least 42 m2/g, and a total pore volume as measured by N2 physiosorption of at least 0.31 cm3/g, after aging at 1000 (degrees Celsius) in an air atmosphere for 4 hours. The invention also relates to a catalytic system comprising the cerium-zirconium-based mixed oxide, as well as to a process for treating an exhaust gas from a vehicle engine comprising contacting the exhaust gas with the cerium-based mixed oxide. -zirconium. Furthermore, the invention relates to a process for preparing a mixed hydroxide or mixed oxide (...).

Description

[001] This invention relates to processes for preparing mixed hydroxides and mixed oxides based on cerium-zirconium, compositions comprising zirconium hydroxide/oxide and cerium hydroxide/oxide, as well as to the use of the mixed oxide in catalysis such as for treat vehicle exhaust gases.

Fundamentals

[002] It is well known to fit catalytic converters to vehicle exhaust systems. A catalytic converter is an emissions control device that converts toxic pollutants in exhaust gas to less toxic pollutants by catalyzing a redox (oxidation or reduction) reaction.

[003] A known type of catalytic converters are three-way catalytic converters (TWCs). A three-way catalytic converter has three simultaneous tasks: (i) Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2 (ii) Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2 (iii) Oxidation of unburned hydrocarbons (HC) to carbon dioxide and water: CxH2x+2+ [(3x+1)/2]O2 → xCO2 + (x+1)H2O.

[004] Compositions comprising zirconium oxide and cerium oxide (also referred to as cerium-zirconium-based mixed oxides) are known for use in TWCs. Such TWC materials need to have a minimum level of thermal stability in addition to good redox properties in order to satisfy legislative requirements in various countries.

[005] Thermal stability is normally tested by known analytical tests that demonstrate the existence and retention of a desired porous structure during thermal aging. This aspect is important in the area of TWCs since the retention of good aging PGM dispersion is essential to the durability and activity requirements of a TWC material in an exhaust stream, especially when accelerated aging in hydrothermal conditions is used.

[006] Another desirable property of compositions for use in TWCs is that they have good oxygen diffusion kinetics. A primary role of the composition for use in TWCs is to age as an oxygen store and release the material throughout the lean-rich cycle in a gasoline internal combustion engine. For a composition comprising zirconium oxide and cerium oxide, this is by virtue of the variable oxidation state of the cerium cations in the zirconia lattice. As emissions regulations become more stringent, catalysts that work more efficiently as well as under more brutal thermal conditions are desired. A large majority of emissions are released before catalyst start-up, so a catalyst that can operate at lower temperatures is of interest in the field.

[007] There is also a need for catalysts to be efficient in more dynamic situations. Real World Driving Cycles and Onboard Emissions Monitoring are driving the need for catalytic converters to operate at lower temperatures and more dynamic cycles than previous test protocols. In parallel, engine development is also demanding different behavior from catalysts so that they work efficiently in terms of fluctuations in temperature, emission levels out of the engine and/or lambda value (i.e. air:fuel ratio).

[008] Compositions for use as TWCs, comprising zirconium oxide and cerium oxide, having improved thermal stability and superior oxygen diffusion characteristics, have therefore been sought. Supplementary to the superior oxygen diffusion characteristics, the improved interaction of oxygen storage function and PGM can result in maintaining more effective dispersion of the active metal after aging. This enhanced PGM coupling allows for the potential for efficient operation over a wider range of dynamic conditions.

[009] The evaluation of the redox properties of these materials commonly uses a Temperature Programmed Reduction Technique. This is a measure of hydrogen consumption of a cerium and zirconium compound as a function of temperature and a total amount of hydrogen consumed can be related to the Total Oxygen Storage Capacity (OSC). A high Total Oxygen Storage Capacity is desired when using the materials in TWC materials.

[010] Although hydrogen is commonly used as the probe molecule, other gases or other gas mixtures can be used to investigate the Oxygen Storage behavior of mixed oxides based on cerium and zirconium. Hydrogen TPR has the advantage of being fast, cheap and widely available without large capital investment.

[011] Although the total amount of oxygen capable of being stored (and released) is important, the kinetics of oxygen storage and release through lean-rich cycles is demonstrably more important. The dynamic switching of the exhaust stream over the catalyst means that an equilibrium is unlikely to be reached. At low temperatures, it would be desirable to have a greater proportion of the total oxygen storage capacity available for reaction with the exhaust gases. The commonly used Temperature Programmed Reduction technique has limited ability to provide kinetic data. Instead, a laboratory protocol can be used whereby a high concentration of a reducing gas is pulsed onto the oxidized solid. At low temperatures and high reductant concentrations, the ability to react a greater amount of reductant in the first pulse provides an indication of the ‘dynamic’ activity of the materials.

[012] Alternatively, a reduced sample can be used and a high concentration of oxidizing gas pulsed over it at low temperatures to also demonstrate the relative activity of the solid in a catalytic system.

[013] Materials under test may be as prepared, subjected to a heat treatment or a hydrothermal treatment, and with or without a PGM (a Platinum Group Metal, i.e., palladium, platinum, rhodium, ruthenium, iridium and/or osmium). It is useful to know the ability of the solid to retain OSC function after an appropriate aging condition with PGM dispersed in the material. This will more closely resemble conditions in practical use. Retention of OSC function available for catalysis after aging is desirable. For example, a test method such as an H2 pulse technique can be used. This involves taking 100 mg of a powdered sample (typically loaded with 1% Pd, but no PGM or other PGMs and fillers may be chosen). This is pre-oxidized initially by pulsing 20% O2/He at 100°C followed by a flow of 20% O2/He at 500°C for 30 min. The temperature is then decreased to the desired experimental conditions (e.g. 70 °C in our case) under air flow. A series of 521 microliter pulses (15 in total) of 70% H2/Ar are passed over the sample, and its reaction monitored by TCD. The sample becomes ‘saturated’ and the amount of reaction in the first pulse is compared against this saturation limit to provide a ‘dynamic’ first pulse/OSC value. A low temperature and high H2 concentration are used to stress the system. There is a similar testing method that uses pulses of O2. This involves taking 100 mg of a powdered sample (typically no PGM, but PGM’s in various charges can be used). This is pre-reduced in a flow of 5% H2/Ar at 850 °C for 30 min. The temperature is then decreased to the desired experimental conditions (e.g., 50 °C in our case) under air flow. A series of 521 microliter pulses (14 in total) of 20% O2/He is passed over the sample, and its reaction is monitored by a Thermal Conductivity Detector (TCD). The sample becomes ‘saturated’ and the amount of reaction in the first pulse is compared against this saturation limit to provide a ‘dynamic’ first pulse/OSC value. When no PGM is loaded into the mixed metal oxide, the total OSC is calculated by summing the results of each pulse until the saturation point. A low temperature and high O2 concentration are used to stress the system.

[014] In parallel with the above assessment protocols, there are more advanced techniques through which the oxygen exchange rate can be estimated/determined. For example, Isothermal Temperature Reduction and Isothermal Isotopic Temperature Exchange (TIIE) techniques.

[015] Isothermal Temperature Reduction Techniques involve determining the kinetics of reduction of a solid metal oxide given by a probe gas under isothermal conditions. This is a dynamic technique, similar to the TPR technique except that the reduction kinetics are a function of time at constant temperature. At any given temperature, the reduction kinetics of a solid is characterized by a linear profile rather than a single point in the TPR technique. Therefore, it is suggested that this technique is more advantageous for comparing the reduction kinetics of metal or mixed metal oxides.

[016] WO2014/122140 and US6171572 describe mixed oxides based on cerium-zirconium and methods for preparing such materials. However, the disclosed compositions do not have the pore volume properties after aging that are obtained with the present invention.

Declaration of invention

[017] According to one aspect of the invention, there is provided a process for preparing a mixed hydroxide or mixed oxide based on cerium-zirconium, the process comprising the steps of: (a) dissolving a zirconium salt in an aqueous acid, (b) adding one or more complexing agents to the resulting solution, the one or more complexing agents being an organic compound comprising at least one of the following functional groups: an amine, an organosulfate, a sulfonate, a hydroxyl, an ether or an acid group carboxylic acid, (c) heating the solution or sol formed in step (b), (d) adding a cerium salt, and adding a sulfating agent before or after adding the cerium salt, and (e) adding a base to form a mixed hydroxide based on cerium-zirconium.

[018] When cerium-zirconium-based mixed hydroxides and oxides produced by this process are aged, especially using hydrothermal aging conditions at high temperatures, the pore volume in the mesoporous region can be advantageously retained. This can provide two benefits: (i) retaining a pore size that minimizes any gas diffusion limitations in the resulting solid; and (ii) retain sufficient volume of pores of an appropriate size such that reduction of catalytic activity due to loss of PGM dispersion is minimized.

[019] In some embodiments, the zirconium salt may be basic zirconium carbonate or zirconium hydroxide. In certain embodiments, zirconium basic carbonate (ZBC) is preferred because it dissolves easily in mineral acids, is commercially available, and the carbonate anions produced are fugitive and so that they do not participate in complicated subsequent reactions. Some alternative anions may not be environmentally favorable. In some embodiments, the aqueous acid may be hydrochloric acid, sulfuric acid, nitric acid or acetic acid, in particular the aqueous acid is nitric acid. Without wishing to be bound by any theory, although other acids can be used it is considered that the nitrate ions provided by nitric acid coordinate particularly well with the zirconium ions in the aqueous solution.

[020] In particular, in step (a) the molar ratio of zirconium ions to nitrate ions in the solution or sol may be 1:0.8 to 1:1.5, more particularly 1:1.0 to 1:1, 3.

[021] In the context of the invention, the term complexing agent is used to mean a ligand that binds to zirconium. In some embodiments, in step (b) the complexing agent may be a carboxylic acid, a dicarboxylic acid, an alpha hydroxycarboxylic acid, an amino acid, an organosulfate or a polyol. In particular, the complexing agent may be a multidentate, more particularly a bidentate, ligand. The polyol may be a polysaccharide, for example starch. In particular, the complexing agent may be an alpha hydroxycarboxylic acid. The complexing agent generally has a polar group (i.e., an amine, an organosulfate, a sulfonate, a hydroxyl, an ether, or a carboxylic acid group) that coordinates the zirconium, and one or more hydrocarbon groups. In some embodiments, the one or more hydrocarbon groups may comprise one or more aromatic substituents, more particularly one or more phenyl substituents. Without wishing to be bound by any theory, multidentate ligands effectively coordinate metal ions. The combination of different functional groups within the same molecule can be advantageous to interact with different coordination environments in the metal ion; provide both steric and electronic effects. Thus, depending on the nature of the pore size and pore network, complexing agents with different hydrocarbon groups can be used. For example, the alpha hydroxy carboxylic acid may be an aromatic (e.g. phenyl) or non-aromatic alpha hydroxycarboxylic acid, more particularly mandelic or benzylic or lactic acid.

[022] In particular, in step (a) the formed solution can be heated. In particular, the solution may be heated to a temperature above 25°C, more particularly to at least 40°C, even more particularly to at least 50°C, more particularly to a temperature in the range of 50 to 70°C. More particularly, the solution may be heated to about 60°C.

[023] Optionally, in step (a) the pH of the solution can be increased (i.e., partially neutralized) by adding a base. This increase in pH can also be described as a reduction in free acidity. In particular, the pH increase can be carried out before heating the solution. More particularly, the base may be sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, ammonium hydroxide, ammonium carbonate, ammonium hydrogen carbonate, potassium hydroxide, potassium carbonate, and/or potassium hydrogen carbonate.

[024] In particular, step (b) may further comprise adding water, typically deionized water, to the heated solution. More particularly, in step (b), after addition of the complexing agent, and optional water, the solution has an equivalent zirconium content of 5 to 25 wt% expressed as ZrO2, more particularly 10 to 20 wt%, even more particularly 12 to 16 wt%, expressed as ZrO2. The equivalent zirconium content expressed as ZrO2 means that, for example, 100 g of a 15 wt% solution would have the same zirconium content as 15 g of ZrO2.

[025] More particularly, in step (c) heating may comprise heating the solution or sol to a temperature of 60 to 100 °C, more particularly 80 to 100 °C, for 1 to 15 hours. In particular, heating can be carried out for 1 to 5 hours. More particularly, in step (c) the temperature of the solution or sol may be increased at a rate of 0.1 to 1.5 °C/min.

[026] In particular, in step (d) the solution or sol can be allowed to cool, or cooled, before adding the sulfating agent. More particularly, the solution or sol may be allowed to cool, or cooled, to a temperature of less than 40 °C, even more particularly less than 30 °C. Possible sulfating agents are water-soluble salts of sulfate, bisulfate, sulfite, bisulfite. In particular, the sulfating agent may be sulfuric acid. The sulfating agent may be added such that the molar ratio of zirconium ions to sulfate ions is 1:0.05 to 1:1. After addition of sulfate in step (d), the process may comprise the step of isolating the solid from the solution or sol, for example by filtration.

[027] More particularly, step (d) may further comprise adding an aqueous electrolyte before adding the sulfating agent. The aqueous electrolyte can be added before adding the cerium salt. In particular, the aqueous electrolyte may be completely or partially neutralized hydrochloric acid, completely or partially neutralized nitric acid, or completely or partially neutralized acetic acid. Partially neutralized nitric acid is also referred to as acidified sodium nitrate.

[028] More particularly, the cerium salt can be cerium carbonate, cerium chloride, cerium nitrate (for example, waxy nitrate, ceric nitrate or a mixture thereof) or ammonium cerium nitrate. In particular, step (d) may further comprise adding one or more salts of: silica, aluminum, strontium, a transition metal (more particularly tin, niobium, tungsten, manganese and/or iron), or a rare earth element. (more particularly scandium, lanthanum, neodymium, praseodymium, yttrium, gadolinium and/or samarium). In the context of the invention, yttrium is considered to be a rare earth element.

[029] In step (e), the base can be sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, ammonium hydroxide, ammonium carbonate, ammonium hydrogen carbonate, potassium hydroxide, potassium carbonate and/or hydrogen Potassium carbonate. More particularly, in step (e) the addition of the base is to form a cerium-zirconium based mixed hydroxide precipitate. Step (e) can be carried out at any temperature at which the solution or sol is not frozen, that is, from -5 ° C to 95 ° C, more particularly, 10 ° C to 80 ° C.

[030] In some embodiments, the process may comprise after step (e) the step of (f) thermally treating the cerium-zirconium-based mixed hydroxide. Heat treatment can be hydrothermal treatment. Hydrothermal treatment may comprise heating the solution or sol to a temperature of 80 to 250 °C, more particularly 100 to 250 °C, for 1 to 15 hours in an autoclave.

[031] More particularly, between steps (e) and (f) the process may comprise the steps of isolating, for example filtering and/or washing the mixed hydroxide based on cerium-zirconium. These steps can be performed to remove chloride ions, sulfate ions, nitrate ions, acetate ions, sodium ions, potassium ions, ammonium ions and/or organic residue if desired. Levels of sulfate ions can be reduced to 0.3% by weight or less, more particularly 0.1% by weight or less. Levels of sodium, potassium and chloride ions can be reduced to 0.05% by weight or less of each, more particularly 0.01% by weight or less of each.

[032] In some embodiments, the process may comprise after step (f), or after step (e) if step (f) is not performed, the step of (g) drying the mixed cerium-zirconium hydroxide. In particular, this may be by oven drying, spray drying or vacuum drying. Drying can be carried out in an oxidizing, inert (e.g. N2) or reducing atmosphere. More particularly, the cerium-zirconium based mixed hydroxide can be dried at a temperature of 50 to 200 °C. If a vacuum is used, the drying temperature may be at the lower end of this range. Without a vacuum, temperatures at the upper end of this range may be required, for example 100 to 150 °C.

[033] In some embodiments, the process may comprise after step (g), or after step (e) or (f) if step (f) and/or (g) are not carried out, the step of (h ) calcinate the cerium-zirconium mixed hydroxide to form a cerium-zirconium based mixed oxide. More particularly, the calcination step can be carried out at a temperature of 500 to 1300 °C, even more particularly 700 to 1100 °C. The calcination step can be carried out for 1 to 10 hours, more particularly 2 to 8 hours. The calcination step can be carried out in any gaseous atmosphere. In particular, the calcination step can be carried out in a flowing or static air atmosphere, although a reductive or neutral atmosphere can be used. In the process of the invention, an air atmosphere is generally preferred as it can assist in removing organic species. A neutral atmosphere is generally defined as one that neither oxidizes nor reduces the composition in this atmosphere. This can be done by removing air or removing oxygen from the atmosphere. Another example of a neutral atmosphere is a nitrogen atmosphere. Furthermore, the calcination atmosphere may be that of combustion gases generated from a gas heating furnace.

[034] The invention also relates to compositions obtainable by the above process.

[035] According to another aspect of the invention, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content cerium of at least 5% by weight, wherein the composition has a surface area of at least 48 m2/g after aging at 950 °C in an air atmosphere for 2 hours.

[036] In some embodiments, the composition has a surface area of at least 60 m2/g after aging at 950 °C in an air atmosphere for 2 hours, optionally at least 70 m2/g. In some embodiments, the composition has a surface area of less than 120 m2/g after aging at 950 °C in an air atmosphere for 2 hours.

[037] According to a third aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a total pore volume as measured by N2 physiosorption of at least 0.29 cm3/g after aging at 950 °C in an air atmosphere for 2 hours.

[038] In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of at least 0.37 cm3/g after aging at 950 °C in an air atmosphere for 2 hours, optionally at least 0 .41 cm3/g. In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of less than 1.0 cm3/g after aging at 950 °C in an air atmosphere for 2 hours.

[039] According to a fourth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 12 nm after aging at 950 °C in an air atmosphere for 2 hours.

[040] For all crystallite size measurements discussed here. The “relevant peak” is the diffraction peak for zirconia in a metastable tetragonal system or in a cubic system in X-ray Diffraction (XRD) scanning.

[041] It is preferred that the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 10 nm after aging at 950 ° C in an air atmosphere for 2 hours, more preferably not more than 9.5 nm.

[042] According to a fifth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a surface area of at least 42 m2/g after aging at 1000 °C in an air atmosphere for 4 hours.

[043] In some embodiments, the composition has a surface area of at least 50 m2/g after aging at 1000 °C in an air atmosphere for 4 hours, optionally at least 60 m2/g. In some embodiments, the composition has a surface area of less than 120 m2/g after aging at 1000 °C in an air atmosphere for 4 hours.

[044] According to a sixth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a total pore volume as measured by N2 physiosorption of at least 0.31 cm3/g after aging at 1000 °C in an air atmosphere for 4 hours.

[045] In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of at least 0.35 cm3/g after aging at 1000 °C in an air atmosphere for 4 hours, optionally at least 0 .40 cm3/g, and in some embodiments at least 0.45 cm3/g. In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of less than 1.0 cm3/g after aging at 1000 °C in an air atmosphere for 4 hours.

[046] According to a seventh aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 14 nm after aging at 1000 °C in an air atmosphere for 4 hours.

[047] It is preferred that the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 11 nm after aging at 1000 ° C in an air atmosphere for 4 hours, in some embodiments no more than 10 nm.

[048] According to an eighth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a surface area of at least 33 m2/g after aging at 1050 °C in an air atmosphere for 2 hours. In some embodiments, the composition has a surface area of less than 120 m2/g after aging at 1050 °C in an air atmosphere for 2 hours.

[049] In some embodiments, the composition has a surface area of at least 38 m2/g after aging at 1050 °C in an air atmosphere for 2 hours, optionally at least 40 m2/g.

[050] According to a ninth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a total pore volume as measured by N2 physiosorption of at least 0.20 cm3/g after aging at 1050 °C in an air atmosphere for 2 hours. In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of less than 1.0 cm3/g after aging at 1050 °C in an air atmosphere for 2 hours.

[051] In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of at least 0.25 cm3/g after aging at 1050 °C in an air atmosphere for 2 hours, optionally at least 0 .28 cm3/g.

[052] According to a tenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 15 nm after aging at 1050 °C in an air atmosphere for 2 hours.

[053] It is preferred that the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 13 nm after aging at 1050 ° C in an air atmosphere for 2 hours, in some embodiments no more than 12 nm.

[054] According to an eleventh aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a surface area of at least 18 m2/g after aging at 1100 °C in an air atmosphere for 6 hours. In some embodiments, the composition has a surface area of less than 120 m2/g after aging at 1100 °C in an air atmosphere for 6 hours.

[055] It is preferred that the composition has a surface area of at least 20 m2/g after aging at 1100 °C in an air atmosphere for 6 hours, in some embodiments at least 23 m2/g, optionally at least 25 m2/g.

[056] According to a twelfth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a total pore volume as measured by N2 physiosorption of at least 0.11 cm3/g after aging at 1100 °C in an air atmosphere for 6 hours. In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of less than 1.0 cm3/g after aging at 1100 °C in an air atmosphere for 6 hours.

[057] In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of at least 0.14 cm3/g after aging at 1100 °C in an air atmosphere for 6 hours, optionally at least 0 .17 cm3/g.

[058] According to a thirteenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 26 nm after aging at 1100 °C in an air atmosphere for 6 hours.

[059] In some embodiments, the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 20 nm after aging at 1100 ° C in an air atmosphere. for 6 hours, optionally no more than 18 nm.

[060] According to a fourteenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a surface area of at least 18 m2/g after hydrothermal aging at 1100 °C for 12 hours in an atmosphere of air comprising 10% by volume of water.

[061] In some embodiments, the composition has a surface area of at least 19 m2 / g after hydrothermal aging at 1100 ° C for 12 hours in an air atmosphere comprising 10 wt % volume, optionally at least 20 m2 /g.

[062] According to a fifteenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a total pore volume as measured by N2 physiosorption of at least 0.11 cm3/g after aging hydrothermal at 1100 °C for 12 hours in an air atmosphere comprising 10% by volume water. In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of less than 1.0 cm3/g after aging at 1100 °C for 12 hours in an air atmosphere comprising 10 volume % water.

[063] In some embodiments, the composition has a total pore volume as measured by N2 physiosorption of at least 0.13 cm3 / g after hydrothermal aging at 1100 ° C for 12 hours in an air atmosphere comprising 10% in volume of water, optionally at least 0.15 cm3/g.

[064] According to a sixteenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no greater than 25 nm after hydrothermal aging at 1100 °C for 12 hours in an air atmosphere comprising 10 volume % water.

[065] In some embodiments, the composition has a crystallite size as measured by applying the Scherrer equation to the relevant peak in its XRD scan of no more than 22 nm after hydrothermal aging at 1100 ° C for 12 hours in an air atmosphere comprising 10% by volume water, more preferably not more than 19 nm.

[066] According to a seventeenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the composition has a Dynamic-Oxygen Storage Capacity (D-OSC) value as measured by H2-TIR of more than 500 μmol/g at 600 °C after aging at 800 °C in an air atmosphere for 2 hours. In some embodiments, the composition has a D-OSC value as measured by H2-TIR of less than 1500 μmol/g at 600 ° C after aging at 800 ° C in an air atmosphere for 2 hours.

[067] It is preferred that the composition has a D-OSC value as measured by H2-TIR of more than 875 μmol/g at 700 ° C after aging at 800 ° C in an air atmosphere for 2 hours, more preferably greater than 950 μmol/g at 800 °C.

[068] According to an eighteenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein the increase in the average pore diameter of the composition as measured by N2 physiosorption after hydrothermal aging at 1100 °C for 12 hours in an atmosphere of air comprising 10% by volume of water, it is no more than 50%.

[069] It is preferred that the increase in the average pore diameter of the composition as measured by N2 physiosorption after hydrothermal aging at 1100 ° C for 12 hours in an air atmosphere comprising 10% by volume of water, is no more than 30%, more preferably not more than 10%.

[070] According to a nineteenth aspect of the invention, and/or in combination with the compositional characteristics defined above, there is provided a composition comprising zirconium oxide and cerium oxide having: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of at least 5% by weight, wherein more than 80% of the total pore volume as measured by N2 physiosorption consists of pores with an average diameter between 18 nm and 78 nm after aging at 1100 °C in an air atmosphere for 6 hours. It is preferred that more than 85% of the total pore volume as measured by N2 physiosorption consists of pores with an average diameter between 18 nm and 78 nm after aging at 1100 ° C in an air atmosphere for 6 hours.

[071] It is preferred that the compositions defined here, or manufactured by the process defined above, comprise one or more rare earth oxides other than cerium oxide. Each of these one or more rare earth oxides except cerium oxide is preferably individually present in an amount of 1 to 15% by weight, in some embodiments 1.5 to 10% by weight, in other embodiments 2 to 6% by weight. . The rare earth elements are scandium, yttrium, lanthanum, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Preferred rare earth oxides other than cerium oxide are yttrium (yttria), neodymium oxide, praseodymium oxide and lanthanum oxide. In a preferred embodiment of the invention, the composition comprises 3 to 7% by weight, preferably about 5% by weight, of praseodymium oxide and 3 to 7% by weight, preferably about 5% by weight, of lanthanum oxide. In some embodiments, the composition may also comprise one or more tin oxide, niobium oxide, tungsten oxide, silica and iron oxide. The total amount of rare earth oxides other than cerium oxide is preferably less than 30% by weight. In some embodiments, the total amount of rare earth oxides other than cerium oxide is less than 20% by weight, optionally less than 15% by weight.

[072] Preferably, the compositions defined here, or manufactured by the process defined above, comprise 5 to 50% by weight of cerium oxide, more preferably 10 to 50% by weight of cerium oxide, even more preferably 20 to 45% by weight. weight, in some embodiments about 40% by weight of cerium oxide.

[073] It is preferred that the compositions derived here, or manufactured by the process defined above, comprise at least 20% by weight of zirconium oxide, more preferably at least 30% by weight.

[074] Preferably, in the compositions defined here, or manufactured by the process defined above, the total amount of cerium oxide and zirconium oxide is at least 80% by weight, more preferably at least 85% by weight.

[075] The compositions defined here, or manufactured by the process defined above, generally comprise hafnium oxide (hafnia) as an impurity. This is normally derived from the material that is used as the source of zirconium. The amount of hafnia generally depends on the zirconium level, but is typically less than 2% by weight, and often less than 1% by weight.

[076] It is preferred that the compositions defined here, or manufactured by the process defined above, comprise less than 0.3% by weight SO4, preferably less than 0.2% by weight, more preferably less than 0.1 % by weight. An upper limit of 0.1% by weight is acceptable for most uses, although the SO4 content can be further reduced by repeating the relevant washing steps of the preparative method described below. More generally, the compositions defined herein, or manufactured by the process defined above, preferably comprise incidental impurities (i.e., those not deliberately added) in an amount of up to 0.5% by weight. The term “incidental impurities” does not include, for example, carbonate, sulfate or nitrate ions as these may be deliberately added.

[077] Furthermore, in some embodiments the compositions defined here, or manufactured by the process defined above, comprise less than 0.10% by weight of Cl, preferably less than 0.05% by weight, more preferably less than 0.02% by weight. An upper limit of 0.02 wt % is acceptable for most uses, although the Cl content can be further reduced by repeating the relevant washing steps of the preparative method described below.

[078] It is preferred that the compositions defined here, or manufactured by the process defined above, comprise less than 250 ppm of Na or K, preferably less than 200 ppm, more preferably less than 125 ppm. An upper limit of 125 ppm is acceptable for most uses, although the sodium or potassium content can be further reduced by repeating the relevant washing steps of the preparative method described below.

[079] In some embodiments, the compositions defined here, or manufactured by the process defined above, may include up to 5% by weight of a Platinum Group Metal, typically up to 2% by weight, in some embodiments around 1% by weight. weight of a Platinum Group Metal (PGM). As noted above, PGMs are palladium, platinum, rhodium, ruthenium, iridium, and/or osmium. Palladium, rhodium and platinum and the most commonly used PGMs. These metals are normally added to the composition as an aqueous solution, usually in a formulation with other components, coated on a monolith and then calcined.

[080] Preferably, for the compositions defined here, or manufactured by the process defined above, a type of diffraction peak for zirconia in a tetragonal system or in a cubic system is observed in an XRD scan.

[081] When the compositions defined here, or manufactured by the process defined above, are aged, especially under hydrothermal aging conditions at high temperatures, we observe that the pore volume in the mesoporous region can be impressively retained. This effect can have two benefits, one is to retain a pore size that minimizes any gas diffusion limitations into the resulting solid, the second is to retain sufficient volume of pores of an appropriate size such that the reduction of catalytic activity by loss of gas dispersion PGM is minimized. The lack of change in pore size distribution and pore volume is indicative of inhibition of solid state sintering processes, which may thus lead to a small desirable change in PGM dispersion through encapsulation. It is not unreasonable to propose that such behavior could allow catalysts to achieve the same activity with less PGM, thus reducing the cost of the catalyst system.

[082] According to another aspect of the invention, an alternative process is provided for preparing a composition as defined herein, the process comprising the steps of: (a) preparing a zirconium hydroxynitrate solution or a zirconium oxynitrate solution, (b) heat treat the solution, (c) cool the solution, (d) add a bidentate or polydentate ligand, (e) add a solution containing cerium, and (f) add a base to adjust the pH of the solution to >8 so as to precipitate a mixed cerium-zirconium hydroxide.

[083] In step (a) it is preferred that the molar ratio of nitrate to zirconium is less than 1.6. Step (b) is performed in order to ensure optimal polymer/oligomer size for mesoporous powder preparation. Heat treatment typically involves heating the solution to a temperature above room temperature.

[084] During step (b), or between steps (a) and (b), the process may comprise the optional step of adding surfactants or organic molding molecules such as polyols, amino acids, a-hydroxy acids, polymers of carbohydrate, and/or sulfate derivatives to the solution. In step (c), the solution is preferably cooled to below 40 °C. In step (d) the bi/polydend ligand may be phosphate, nitrate or sulfate, or a mixture thereof. In step (e), soluble solutions comprising rare earth metals other than cerium can be added. This is in order to provide a mixed zirconium-rare earth dispersion with intimate mixing of the zirconium and rare earth elements. Alternatively, rare earth sulfate and nitrate ions can be added simultaneously to the zirconium nitrate solution/sol in order to provide good mixing of the components. In this case, zirconium oxychloride can also be used. It is also possible to add rare earth metals or other elements such as tin, niobium, tungsten, silica, strontium or iron at any of the previously mentioned stages, or during or after step (f).

[085] Once zirconium and rare earths are mixed properly, as presented in step (f) above, the addition of base until pH >8 is necessary to precipitate a mixed hydrated suspension of zirconium and rare earths. In an alternative embodiment, the addition of base to adjust the pH of the solution to >8 can be performed before step (e). In this embodiment, zirconium hydroxide is precipitated before adding the solution containing cerium (and optionally, other rare earths). Even the pH that the solution can be adjusted to depends on the base used. The base may be ammonium hydroxide or an alkali metal hydroxide, preferably sodium hydroxide. For ammonium hydroxide, the maximum pH that can be obtained is typically about pH 10. For alkali metal hydroxides, the pH can be adjusted to pH 11 to 13 or higher. Hydrogen peroxide may also be added to the precipitate (i.e., after step (f) or before addition of base (i.e., before step (f)). The precipitate may optionally be heated to a minimum of 50 °C. , for 30 minutes to 24 hours. After step (f) the process may comprise another step of filtering and/or washing the precipitate. This is done in order to remove impurity ions such as sodium, potassium, sulfate, phosphate and /or nitrate. Alkali metal ions can be removed by an additional step of re-pasting the washed precipitate cake and adding a mineral acid. The mineral acid is preferably nitric acid of about 10% to 60% by weight concentration. pH of the solution is generally adjusted to a pH of less than 9, preferably adjusted to between pH 7 and 9. After an optional additional filtration step the process may comprise the optional step of redispersing the precipitate in an aqueous medium and heating the resulting dispersed slurry or wet cake up to between 100 °C and 350 °C, preferably between 100 °C and 200 °C. This can be in a sealed reaction vessel such as an autoclave or up to 100°C in an open vessel.

[086] The process afterwards may include the optional step of drying the dispersed slurry or wet cake. This may be in a drying device such as a spray dryer, static oven, indirectly heated coated vessel, or in fact, any laboratory or commercial scale dryer. Optionally, the slurry or cake can be directly introduced into a furnace for calcination.

[087] The calcination step is carried out by calcination at around 800 °C to 1000 °C, preferably around 900 °C in a gas or electrically heated heating furnace, normally in air. Generally, the time at temperature is at least 30 minutes, more usually 2 to 3 hours. The time at temperature may depend on the thermal mass being calcined and it is necessary for consistency that adequate time at temperature is used to ensure the required degree of crystallinity, homogeneity and microstructure development of the solid.

[088] The calcined powder can be optionally deagglomerated or ground using known methods such as sieving, filtration, counter-air grinding, impact grinding, ball grinding, pearl grinding and the like. The powder can also be ground into a fluid (i.e., “wet”) paste in an aqueous or non-aqueous liquid.

[089] Calcination is preferably carried out at 800 to 1000 °C for 2 to 4 hours, more preferably at around 920 °C for around 3 hours. Optionally, the solid can be ground.

[090] According to another aspect of the invention, a catalytic system is provided comprising a composition as defined here, or manufactured by the processes defined above. In some embodiments, one or more PGMs and/or transition metals may be added to the composition. The composition can then be used as a catalytic converter. In some embodiments, the composition may be mixed with other active components (i.e., other catalytically active materials) to produce a fully formulated catalyst. According to another aspect of the invention, there is provided a process for treating a vehicle engine exhaust gas comprising contacting the exhaust gas with a composition as defined herein, or manufactured by the processes defined above. In some embodiments, the process for treating comprises one or more of (a) reduction of nitrogen oxides to nitrogen, (b) oxidation of carbon monoxide to carbon dioxide, and (c) oxidation of hydrocarbons, in the exhaust gas. In some embodiments, the process for treating comprises (a) reducing nitrogen oxides to nitrogen, (b) oxidizing carbon monoxide to carbon dioxide, and (c) oxidizing hydrocarbons in the exhaust gas. The invention also relates to a diesel oxidation catalyst, a NOx capture, a passive NOx absorber, a gasoline particulate filter coating or a probe NOx capture comprising the composition as defined herein, or manufactured by the processes defined above. According to another aspect of the invention, there is provided a process for treating a diesel engine exhaust gas comprising contacting the exhaust gas with a composition as defined herein, or manufactured by the processes defined above, or a mixture of the composition with a zeolite. or a metal-exchanged zeolite. This process may be a Selective Catalytic Reduction (SCR) process. According to another aspect of the invention, there is provided a process for oxidizing soot in a vehicle engine exhaust stream comprising contacting the exhaust stream with a composition as defined herein, or manufactured by the processes defined above.

[091] This invention will be further described by reference to the following Figures which are not intended to limit the scope of the claimed invention, in which:

[092] Figure 1 shows air-aged porosity data (1100 °C/6 h) for the compositions of Preparative Examples 6, 9 and 11 to 14,

[093] Figure 2 shows additional pore volume data aged in air (1100 ° C/6 h) for the compositions of Preparative Examples 6, 9 and 11 to 14,

[094] Figure 3 shows additional pore volume data aged in air (1100 ° C/6 h) for the compositions of Preparative Examples 6, 9 and 11 to 14,

[095] Figure 4 shows H2 pulse data for Preparative Examples 3, 5 and 8 and Comparative Example 1,

[096] Figure 5 shows O2 pulse data for Preparative Examples 3, 5 and 8 and Comparative Example 1.

[097] The invention will now be described by way of example with reference to the following examples.

Comparative Example 1 - 40Ce/5La/5Pr

[098] A sample was prepared according to the composition defined above, that is, 40% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of praseodymium oxide and the remainder ( that is ~50 wt %) zirconium dioxide.

[099] 118.8 g of basic zirconium carbonate (ZBC, 42.1% ZrO2) were dissolved in 126.9 g of nitric acid. This solution was then heated to 60°C. 119.5 g of water were then added. In this example, a complexing agent was not added to the solution. This solution was then heated to boiling and boiled for 2 hours.

[0100] After cooling to room temperature, solutions of 156.3 g of cerium (III) nitrate (25.6% CeO2), 23.3 g of lanthanum nitrate (21.5% La2O3), 25.6 g of praseodymium nitrate (19.5% Pr6O11) and 355.6 g of deionized water were added. 98.7 g of sulfuric acid were then added.

[0101] A 10% by weight aqueous solution of NaOH was then added dropwise to the mixture with stirring. Stirring and addition of the 10 wt % aqueous NaOH solution were continued until the pH became approximately 8. At this point, a 28 wt % aqueous NaOH solution was substituted in place of the 10 wt % solution and addition to the drops was continued with stirring until the pH became approximately 13.

[0102] The resulting slurry was then filtered. The filter cake was washed with deionized water at 60 °C. The cake was then redispersed and then adjusted to pH 8.0 with a 30% by weight solution of nitric acid. The resulting slurry was then filtered. The filter cake was washed with deionized water at 60 °C.

[0103] The final filter cake was heated in an autoclave to 127 ° C for 1 hour. The resulting suspension was then filtered and the resulting filter cake was calcined in air for 3 hours at 930 ° C, and milled, to provide a cerium-zirconium based mixed oxide.

Preparative Example 2 - 40Ce/5La/5Pr

[0104] A sample was prepared according to the composition defined above, that is, 40% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of praseodymium oxide and the remainder ( that is ~50 wt %) zirconium dioxide.

[0105] 119.9 g of basic zirconium carbonate (ZBC, 41.7% ZrO2) were dissolved in 126.9 g of nitric acid. This solution was then heated to 60°C. 3.0 g of soluble starch was added to the solution, along with 108.0 g of water. This solution was then heated to boiling and boiled for 2 hours.

[0106] After cooling to room temperature, solutions of 156.3 g of cerium (III) nitrate (25.6% CeO2), 23.3 g of lanthanum nitrate (21.5% La2O3), 25.6 g of praseodymium nitrate (19.5% Pr6O11) and 355.6 g of deionized water were added. 98.7 g of sulfuric acid were then added.

[0107] The same procedure as Preparative Example 3 below was then followed, until the formation of the final filter cake.

[0108] The final filter cake was calcined in air for 2 hours at 850 °C and then ground.

Preparative Example 3 - 40Ce/5La/5Pr

[0109] A sample was prepared according to the composition defined above, that is, 40% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of praseodymium oxide and the remainder ( that is ~50 wt %) zirconium dioxide.

[0110] 118.8 g of basic zirconium carbonate (ZBC, 42.1% ZrO2) were dissolved in 126.9 g of nitric acid. This solution was then heated to 60°C. 0.92 g of mandelic acid was added to the solution, along with 110.2 g of water. This solution was then heated to boiling and boiled for 2 hours.

[0111] After cooling to room temperature solutions of 161.9 g of cerium (III) nitrate (24.7% CeO2), 24.2 g of lanthanum nitrate (20.7% La2O3), 23 .4 g of praseodymium nitrate (21.4% Pr6O11) and 362.9 g of deionized water were added. 98.7 g of sulfuric acid were then added.

[0112] The pH of the solution was then adjusted to pH 13.0 with a dilute sodium hydroxide solution. 45.2 g of a 35 wt% hydrogen peroxide solution were then added.

[0113] The resulting slurry was then filtered. The filter cake was washed with deionized water at 60 °C. The cake was then redispersed and then adjusted to pH 8.0 with a 30% by weight solution of nitric acid. The resulting slurry was then filtered. The filter cake was washed with deionized water at 60 °C.

[0114] The precipitate was calcined in air for 3 hours at 920 °C and ground.

Preparative Examples 4a and 4b- 40Ce/5La/5Pr

[0115] A sample was prepared according to the composition defined above, that is, 40% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of praseodymium oxide and the remainder ( that is ~50 wt %) zirconium dioxide.

[0116] The same procedure as Preparative Example 3 was followed, except that 124.4 g of nitric acid was used to dissolve the ZBC, and 96.7 g of sulfuric acid was subsequently added. Preparative Example 4a was thus prepared.

[0117] Preparative Example 4b was prepared by subjecting 4a to an additional grinding step.

Preparative Example 5 - 40Ce/5La/5Pr

[0118] The same procedure as Preparative Example 3 was followed, until the formation of the final filter cake.

[0119] The final filter cake was heated in an autoclave to 127 ° C for 1 hour. The resulting suspension was then dried at 110 °C in a static air oven. The solid was calcined in air for 3 hours at 930 °C and ground.

Preparative Example 6 - 40Ce/5La/5Pr

[0120] A sample was prepared according to the composition defined above, that is, 40% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of praseodymium oxide and the remainder ( that is ~50 wt %) zirconium dioxide.

[0121] 117.1 g of basic zirconium carbonate (ZBC, 42.7% ZrO2) were dissolved in 120.6 g of nitric acid. This solution was then heated to 60°C. 0.92 g of mandelic acid was added to the solution, along with 118.5 g of water. This solution was then heated to boiling and boiled for 2 hours.

[0122] After cooling to room temperature, 296.5 g of sodium nitrate solution, 233.1 g of deionized water and 98.7 g of sulfuric acid were then added. This was followed by solutions of 161.9 g of cerium(III) nitrate (24.7% CeO2), 22.8 g of lanthanum nitrate (21.9% La2O3) and 25.6 g of sodium nitrate. praseodymium (19.5% Pr6O11).

[0123] The same procedure as Preparative Example 3 was then followed, until the formation of the final filter cake.

[0124] The final filter cake was calcined in air for 3 hours at 910 °C and then ground.

Comparative Example 7- 40Ce/5La/5Pr

[0125] Mixed solution A was prepared by combining 241.5 g of ZOC (20.7% ZrO2), 156.3 g of cerium (III) nitrate (25.6% CeO2), 23.3 g of lanthanum nitrate (21.5% La2O3), 26.7 g of praseodymium nitrate (18.8% Pr6O11), 121.0 g of sulfuric acid and 98.0 g of deionized water.

[0126] A reaction vessel was charged with 765 g of deionized water, at room temperature. To this, 45.2 g of 35% hydrogen peroxide solution was added, and the pH was adjusted to ~9.75 with sodium hydroxide solution.

[0127] Mixed solution A was titrated against 27% sodium hydroxide solution in the reaction vessel at such a rate as to complete the addition over ~80 min while maintaining a system pH of 9.5 to 10. Following this, the pH was increased to 13 with another addition of 27% sodium hydroxide. In this example, a complexing agent was not added to the solution.

[0128] The resulting slurry was then filtered. The filter cake was washed with deionized water at 60 °C. The cake was then redispersed and then adjusted to pH 8.0 with a 30% by weight solution of nitric acid. The resulting slurry was then filtered. The filter cake was washed with deionized water at 60 °C.

[0129] The precipitate was hydrothermally treated at 127 °C for 1 hour. The resulting suspension was then dried at 110 °C in a static air oven and calcined in air for 2 hours at 800 °C and ground.

Comparative Example 8 - 40Ce/5La/5Pr

[0130] A sample was prepared according to the composition defined above, that is, 40% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of praseodymium oxide and the remainder ( that is ~50 wt %) zirconium dioxide. The sample was prepared according to patent EP1444036B1 (i.e. no complexing agent).

Preparative Example 9 - 40Ce/5La/5Pr

[0131] This was prepared in the same way as Preparative Example 6, except that 126.9 g of nitric acid was used to dissolve the ZBC. The precipitate was heated in an autoclave to 127 °C for 1 hour. The resulting suspension was then filtered and the resulting filter cake was calcined in air for 3 hours at 930 ° C and ground.

Comparative Example 10 - 40Ce/5La/5Pr

[0132] A sample was prepared according to the composition defined above, that is, 40% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of praseodymium oxide and the remainder ( that is ~50 wt %) zirconium dioxide. A basic zirconium sulfate precursor was prepared according to the following reference [S.M. Flask, I.A. Sheka, “Interaction of zirconium oxychloride and sulfuric acid in aqueous solution”, Russ. J. Inorg. Chem. 1969, 17 (1), 60 - 65]. A sample of this precursor containing 50 g of ZrO2 equivalent was mixed with solutions of 161.9 g of cerium (III) nitrate (24.7% CeO2), 22.8 g of lanthanum nitrate (21.9% of La2O3) and 25.6 g of praseodymium nitrate (19.5% Pr6O11). In this example, a complexing agent was not added to the solution. The pH of the solution was then adjusted to pH 13.0 with 27% sodium hydroxide solution. 45.2 g of a 35 wt% hydrogen peroxide solution were then added. The sample was calcined in air for 2 hours at 600 °C.

Preparative Example 11 - 45Ce/5La/5Y

[0133] A sample was prepared according to the composition defined above, that is, 45% by weight of cerium (IV) oxide, 5% by weight of lanthanum oxide, 5% by weight of yttrium oxide and the remainder ( that is ~45 wt %) zirconium dioxide.

[0134] 109.0 g of basic zirconium carbonate (ZBC, 41.3% ZrO2) were dissolved in 107.4 g of nitric acid. This solution was then heated to 60°C. 0.83 g of mandelic acid was added to the solution, along with 104.2 g of water. This solution was then heated to boiling and boiled for 2 hours.

[0135] After cooling to room temperature, 266.8 g of sodium nitrate solution, 248.3 g of deionized water and 88.8 g of sulfuric acid were then added. This was followed by solutions of 182.2 g of cerium(III) nitrate (24.7% CeO2), 22.8 g of lanthanum nitrate (21.9% La2O3) and 26.6 g of sodium nitrate. yttrium (18.8% Y2O3).

[0136] The same procedure as Preparative Example 3 was then followed, until the formation of the final filter cake.

[0137] The final filter cake was hydrothermally treated at 127 °C for 1 hour. The resulting suspension was then dried at 110 °C in a static air oven and calcined in air for 3 hours at 900 °C and then ground.

Preparative Example 12 - 45Ce/5La/5Y

[0138] This was prepared in the same way as Preparative Example 10, except that 116.8 g of nitric acid was used to dissolve the ZBC, and 1.07 g of mandelic acid was added. The final filter cake was not hydrothermally treated. It was calcined in air for 3 hours at 900 °C and then ground.

Preparative Example 13 - 35.5Ce/5.5La

[0139] A sample was prepared according to the composition defined above, i.e. 35.5% by weight cerium (IV) oxide, 5.5% by weight lanthanum oxide and the remainder (i.e. ~59% by weight) of zirconium dioxide.

[0140] 142.9 g of basic zirconium carbonate (ZBC, 41.3% ZrO2) were dissolved in 149.8 g of nitric acid. This solution was then heated to 60°C. 1.09 g of mandelic acid was added to the solution, along with 127.6 g of water. This solution was then heated to boiling and boiled for 2 hours.

[0141] After cooling to room temperature, solutions of 138.7 g of cerium (III) nitrate (25.6% CeO2), 25.6 g of lanthanum nitrate (21.5% La2O3) and 306.7 g of deionized water was added. 98.7 g of sulfuric acid were then added.

[0142] The same procedure as Preparative Example 3 was then followed, until the formation of the final filter cake.

[0143] The final filter cake was calcined in air for 3 hours at 900 °C and then ground.

Preparative Example 14 - 25Ce/3.5La/4Y

[0144] A sample was prepared according to the composition defined above, that is, 25% by weight of cerium (IV) oxide, 3.5% by weight of lanthanum oxide, 4% by weight of yttrium oxide and the remaining (i.e. ~67.5 wt%) zirconium dioxide.

[0145] 163.4 g of basic zirconium carbonate (ZBC, 41.3% ZrO2) were dissolved in 188.5 g of nitric acid. This solution was then heated to 60°C. 0.54 g of mandelic acid was added to the solution, along with 129.7 g of water. This solution was then heated to boiling and boiled for 2 hours.

[0146] After cooling to room temperature, 400.3 g of sodium nitrate solution, 444.4 g of deionized water and 133.3 g of sulfuric acid were then added. This was followed by solutions of 101.2 g of cerium(III) nitrate (24.7% CeO2), 16.0 g of lanthanum nitrate (21.9% La2O3) and 21.3 g of sodium nitrate. yttrium (18.8% Y2O3).

[0147] The same procedure as Preparative Example 3 was then followed, until the formation of the final filter cake.

[0148] The final filter cake was calcined in air for 3 hours at 900 °C and then ground.

Preparative Example 15 - 20Ce/1.5La/5Nd

[0149] A sample was prepared according to the composition defined above, that is, 20% by weight of cerium (IV) oxide, 1.5% by weight of lanthanum oxide, 5% by weight of neodymium oxide and the remaining (i.e. ~73.5 wt%) zirconium dioxide.

[0150] 178.0 g of basic zirconium carbonate (ZBC, 41.3% ZrO2) were dissolved in 205.3 g of nitric acid. This solution was then heated to 60°C. 0.59 g of mandelic acid was added to the solution, along with 141.1 g of water. This solution was then heated to boiling and boiled for 2 hours.

[0151] After cooling to room temperature, 491.1 g of sodium nitrate solution, 302.3 g of deionized water and 145.1 g of sulfuric acid were then added. This was followed by solutions of 81.0 g of cerium(III) nitrate (24.7% CeO2), 6.9 g of lanthanum nitrate (21.9% La2O3) and 23.5 g of neodymium (21.3% Nd2O3).

[0152] The same procedure as Preparative Example 3 was then followed, until the formation of the final filter cake.

[0153] The final filter cake was calcined in air for 3 hours at 900 °C and then ground.

Example A

[0154] The samples prepared above were analyzed as prepared (i.e. “fresh”) for their surface area (SA), total pore volume (TPV, by N2 physiosorption), crystallite size (CS, applying if the Scherrer equation to the relevant peak in your XRD scan) and average pore diameter (APD, by N2 physiosorption). This data is shown in Table 1 below.

[0155] The results for Preparative Examples 6, 9 and 12 to 15 aged in air at 1100 °C for 6 hours are shown graphically in Figure 1. The measurement of dV/dDiameter on the y-axis of this graph is effectively a measurement of the number of pores of a particular size, with the average pore diameter shown along the x-axis. These data show the improved porosity of the compositions of the invention after aging in air at 1100 °C for 6 hours. Figure 2 shows the additional pore volume for these samples, and Figure 3 shows the cumulative pore volume. Figures 4 and 5 show H2 and O2 pulse data respectively for Preparative Examples 3 and 5, as well as Comparative Examples 8 and 10. In Figure 5, the O2 pulse data for Comparative Example 10 is zero in both aging conditions. Table 1 Continuation of Table 1

Claims (24)

1. Cerium-zirconium based mixed oxide CHARACTERIZED by the fact that it has: (a) a Ce:Zr molar ratio of 1 or less, and (b) a cerium oxide content of 10 to 50% by weight, wherein the composition has (i) a surface area of at least 18 m2/g, and a total pore volume as measured by N2 physiosorption of at least 0.11 cm3/g, after aging at 1100 °C in an air atmosphere for 6 hours, and (ii) a surface area of at least 42 m2/g, and a total pore volume as measured by N2 physiosorption of at least 0.31 cm3/g, after aging at 1000 °C in an air atmosphere for 4 hours. 2. Cerium-zirconium based mixed oxide according to claim 1, CHARACTERIZED by the fact that it has a surface area of at least 33 m2/g, and a total pore volume as measured by hair N2 physiosorption minus 0.20 cm3/g, after aging at 1050 °C in an air atmosphere for 2 hours. 3. Cerium-zirconium based mixed oxide according to claim 1 or 2, CHARACTERIZED by the fact that it has a surface area of at least 48 m2/g, and a total pore volume as measured by N2 physiosorption of at least 0.29 cm3/g, after aging at 950 °C in an air atmosphere for 2 hours. 4. Cerium-zirconium based mixed oxide according to any one of claims 1 to 3, CHARACTERIZED by the fact that it has a surface area of at least 18 m2/g, and a total pore volume as measured by physiosorption of N2 of at least 0.11 cm3/g, after hydrothermal aging at 1100 °C for 12 hours in an air atmosphere comprising 10% by volume of water. 5. Mixed oxide based on cerium-zirconium, according to any one of claims 1 to 4, CHARACTERIZED by the fact that it comprises one or more rare earth oxides other than cerium oxide in an amount of 1 to 15% by weight individually , and 1 to 20% by weight in total. 6. Mixed oxide based on cerium-zirconium, according to any one of claims 1 to 5, CHARACTERIZED by the fact that the total amount of cerium oxide and zirconium oxide is at least 80% by weight. 7. Catalytic system CHARACTERIZED by the fact that it comprises (i) a mixed oxide based on cerium-zirconium as defined in any one of claims 1 to 6 and (ii) one or more PGMs (Platinum Group Metals), metals of transition or combinations thereof. 8. Process for treating an exhaust gas from a vehicle engine CHARACTERIZED by the fact that it comprises contacting the exhaust gas with a mixed oxide based on cerium-zirconium as defined in any one of claims 1 to 6. 9. Process for preparing a mixed hydroxide or mixed oxide based on cerium-zirconium, as defined in any one of claims 1 to 6, the process CHARACTERIZED by the fact that it comprises the steps of: (a) dissolving a zirconium salt in an aqueous acid, (b) adding one or more complexing agents to the resulting solution, the one or more complexing agents being an organic compound comprising at least one of the following functional groups: an amine, an organosulfate, a sulfonate, a hydroxyl, an ether or a carboxylic acid group, (c) heating the solution or sol formed in step (b), (d) adding a cerium salt, and adding a sulfating agent before or after adding the cerium salt, and (e) adding a base to form a mixed hydroxide based on cerium-zirconium. 10. Process, according to claim 9, CHARACTERIZED by the fact that the zirconium salt is basic zirconium carbonate or zirconium hydroxide. 11. Process, according to claim 9 or 10, CHARACTERIZED by the fact that the aqueous acid is hydrochloric acid, sulfuric acid, nitric acid or acetic acid. 12. Process, according to claim 11, CHARACTERIZED by the fact that the aqueous acid is nitric acid. 13. Process, according to claim 12, CHARACTERIZED by the fact that in step (a) the molar ratio of zirconium ions to nitrate ions in the solution or sol is 1:0.8 to 1:1.5. 14. Process according to any one of claims 9 to 13, CHARACTERIZED by the fact that the complexing agent is an alpha hydroxy carboxylic acid. 15. Process, according to claim 14, CHARACTERIZED by the fact that the alpha hydroxy carboxylic acid is mandelic acid. 16. Process according to any one of claims 9 to 15, CHARACTERIZED by the fact that in step (a) the solution is heated to at least 40 °C. 17. Process according to any one of claims 9 to 16, CHARACTERIZED by the fact that in step (c) the solution or sol is heated to a temperature of 80 to 100 °C. 18. Process, according to any one of claims 9 to 17, CHARACTERIZED by the fact that the sulfating agent is sulfuric acid. 19. Process, according to claim 18, CHARACTERIZED by the fact that in step (d) the solution is allowed to cool, or cooled, to a temperature of less than 40 ° C before adding the sulfuric acid. 20. Process according to any one of claims 9 to 19, CHARACTERIZED by the fact that step (d) additionally comprises adding an aqueous electrolyte. 21. Process according to claim 20, CHARACTERIZED by the fact that the aqueous electrolyte is completely or partially neutralized hydrochloric acid, nitric acid or acetic acid. 22. Process according to any one of claims 9 to 21, CHARACTERIZED by the fact that step (d) additionally comprises adding one or more salts of: silica, aluminum, strontium, a transition metal or a rare earth element including yttrium. 23. Process according to any one of claims 9 to 22, CHARACTERIZED by the fact that step (e) is carried out at a temperature of -5 °C to 95 °C. 24. Process according to any one of claims 9 to 23, CHARACTERIZED by the fact that the mixed hydroxide or mixed oxide based on cerium-zirconium comprises 10 to 50% by weight of cerium oxide and at least 20% by weight of zirconium oxide.