WO2014202149A1

WO2014202149A1 - Composite oxide based on cerium oxide, silicon oxide and titanium oxide

Info

Publication number: WO2014202149A1
Application number: PCT/EP2013/062977
Authority: WO
Inventors: Naotaka Ohtake; Toshihiro Sasaki
Original assignee: Rhodia Operations SAS
Current assignee: Rhodia Operations SAS
Priority date: 2013-06-21
Filing date: 2013-06-21
Publication date: 2014-12-24
Anticipated expiration: 2015-12-21
Also published as: JP6463348B2; WO2014202725A1; CN105431228B; CN105431228A; JP2016530183A

Abstract

The present invention concerns a composite oxide based on cerium oxide, silicon oxide and titanium oxide. The present invention also concerns a process to obtain these composites, a catalytic system comprising said composite oxides and their use for the treatment of exhaust gases from internal combustion, notably by bringing into contact exhaust gases from internal combustion engines with these catalytic systems.

Description

COMPOSITE OXIDE BASED ON CERIUM OXIDE, SILICON OXIDE AND TITANIUM OXIDE

PRIOR ART

"Multifunctional" catalysts are currently used for the treatment of exhaust gases from internal combustion engines (automobile afterburning catalysis). The term "multifunctional" is understood to mean catalysts capable of carrying out not only oxidation, in particular of carbon monoxide and hydrocarbons present in exhaust gases, but also reduction, in particular of nitrogen oxides also present in these gases. Cerium oxides today appear as constituents which are particularly important and advantageous for this type of catalyst, notably for the conversion of noxious gases released by diesel engines either from mobiles sources or stationary sources. The reduction of nitrogen oxide (NOx) such as nitric oxide (NO), nitrogen dioxide (N0₂), and nitrous oxide (N₂0), in exhaust gas is a widely addressed problem as a result of environmental concerns and mandated government emissions regulations, particularly in the transportation industry. One approach uses catalysts to treat the exhaust gases of spark-ignited gasoline internal combustion engines for the reduction of NOx emissions, since the exhaust contains minimal oxygen.

5 In addition to the NOx, the exhaust gases typically contain sulfur compounds such as sulfur dioxide (S0₂) gas, which are produced by the burning of sulfur contained in the fuel. The NOx catalyst oxidizes sulfur dioxide to sulfur trioxide in oxygen-rich atmospheres. Still further, S0₃ reacts readily with water vapor, which is also contained in l o the exhaust gases, to produce sulfite ions and sulfate ions. The sulfite ions and sulfate ions react with the NOx catalyst to produce sulfites and sulfates. The resulting sulfites and sulfates adversely affect the NOx storage reaction. Thus, such sulfites and sulfates are commonly referred to as NOx poisons that degrade exhaust purification as they

15 are irreversibly adsorbed at the surface of the washcoat materials, such as alumina, ceria or barium oxides.

Ti0₂ containing support materials were also reported to suppress the sulfur poisoning (H.Y. Huang, R.Q. Long, R.T. Yang, Appl. Catal. B

20 33 (2001) 127). This effect is associated with the surface acidity of Ti0₂, inhibiting the adsorption of acidic SOx species (T. Takahashi, A. Suda, I. Hachisuka, M. Sugiura, H. Sobukawa, H. Shinjoh, Appl. Catal. B 72 (2007) 187). In order to prevent SOx poisoning, it would be then possible to provide dopants to increase acidity on the Ce0₂ surface by

25 using for instance Ti0₂ (Wenqing Xu, Yunbo Yu, Changbin Zhang, Hong He, Catal. Comm. 9 (2008) 1453). However it appears that CeTi exhibits a dramatically drop of the specific surface area (SBET) at high temperatures and can not be used in said application. Otherwise, it is also known that a silicon-containing cerium composite oxide comprising 2 to 20 mass % silicon in terms of Si0₂, exhibits a higher specific surface area even in use in a high temperature environment as expressed in Patent Publication US2012/0316059. However it appears that this CeSi does not provide a sufficient acidity on the Ce0₂ surface.

Therefore, there is a need to develop sulfur resistant materials which show the best trade-off between thermal stability and sulfur resistance in order to achieve the most severe regulation limits of diesel engines.

INVENTION

The subject matter of the invention is thus the development of a composite oxide with simultaneously improved high specific surface area at high temperature and a high acidity leading to low SOx adsorption and easier "desulfation properties" - ie. a SOx desorption in a lower temperature range than the existing materials. Indeed, it appears that the cerium composite oxide comprising silicon oxide and titanium oxide of the invention is sufficiently acid to provide a sulfur resistance and may notably be used in the formulation of sulfur resistant diesel catalyst. The above-described and other disadvantages of the prior art are then overcome by the composition of the invention.

The present invention then concerns a cerium composite oxide comprising at least:

- silicon oxide in a proportion comprised between 1 and 15 % by weight of oxide, preferably in a proportion comprised between 5 and 15 % by weight of oxide; and - titanium oxide in a proportion comprised between 1 and 20 % by- weight of oxide, preferably in a proportion comprised between 5 and 15 % by weight of oxide. The present invention also concerns a process to obtain these composite oxides, a catalytic system comprising said composite oxides and the use of them for the treatment of exhaust gases from internal combustion, notably by bringing into contact exhaust gases from internal combustion engines with these catalytic systems.

Preferably, the present invention concerns a precipitated and calcined composition based on cerium oxide, silicon oxide and titanium oxide as described. Composite oxides of the invention may exhibit a pH inferior or equal to 7, more preferably inferior or equal to 6, more preferably a pH comprised between 1 and 6; wherein pH is measured in an aqueous solution comprising 3 % by weight of this composition, at 25°C pH may notably be measured according to the following protocol: a composite oxide powder is dried at 200°C for 1 hour and the dried oxide powder is hold in a desiccator for 30 minutes. 3.0 g of thus obtained oxide powder is then added into a 100 ml of deionized water under stirring. After 1 minute of stirring, a pH meter (HORIBA D-51) is put into the slurry. The pH value is collected after 3 minutes of putting the pH meter.

Preferably said oxide composites exhibit a specific surface area (SBET), after calcination at 800°C for 2 hours, comprised between 70 and 120 m²/g ; notably comprised between 85 and 110 m²/g. Said oxide composites may also exhibit a specific surface area (SBET), after calcination at 900°C for 5 hours, comprised between 40 and 85 m 2 /g, notably comprised between 50 and 70 m 2 /g.

This specific surface area may be obtained as follows by using a MOU TECH Co., LTD. Macsorb analyzer with a 200 mg sample which has been calcined beforehand at subjected temperature under air. In the continuation of the description, the term "specific surface" is understood to mean the BET specific surface determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 laid down from the Brunauer-Emmett-Teller method described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)".

It is specified that, in the continuation of the description, unless otherwise indicated, the values at the limits are included in the ranges of values which are given. The contents are given as oxides, unless otherwise indicated. The cerium oxide is in the form of eerie oxide (Ce0₂). The silicon oxide is Si0₂ and the titanium oxide is Ti0₂.

Process

The composite oxides of the present invention may be obtained according to several possible processes.

Usually, the process consists in a calcination of a precipitate comprising compounds of cerium and if appropriate other compounds. Such a precipitate is generally obtained by the addition of a basic compound. It is notably possible to heat the precipitate in an aqueous medium before to dry and calcine the precipitate. Compounds of silicon and titanium may notably be added before or after the precipitation of the cerium compound.

The composition of the invention may notably be obtained by a method for producing a composition comprising the steps of:

(a) providing a cerium solution, in which preferably not less than 90 mol % of which cerium ions are tetravalent,

(b) heating and maintaining said cerium solution obtained from step (a) up to and at not lower than 60°C,

(c) adding a precipitant to a cerium suspension obtained through said heating and maintaining to obtain a precipitate,

(d) calcining said precipitate to obtain a cerium oxide,

(e) impregnating said cerium oxide obtained through said calcination with a solution of a silicon oxide precursor and titanium oxide precursor, and

(f) calcining said cerium oxide impregnated with said solution of a silicon oxide precursor and titanium oxide precursor.

It is also possible to produce the composition according to a method for producing a composition comprising the steps of:

(C) adding a silicon oxide precursor and a titanium oxide precursor to a cerium suspension obtained through said heating and maintaining, (D) heating and maintaining said cerium suspension containing said silicon oxide precursor and titanium oxide precursor up to and at not lower than 100°C,

(E) adding a precipitant to said cerium suspension containing said silicon oxide precursor and titanium oxide precursor obtained through said heating and maintaining, to obtain a precipitate, and

(F) calcining said precipitate.

A water-soluble cerium compound which may be used in step (a) may be, for example, a eerie nitrate solution or ammonium eerie nitrate, with the eerie nitrate solution being particularly preferred.

In step (a), the initial concentration of the cerium solution not less than 90 mol % of which cerium ions are tetravalent, may be adjusted to usually 5 to 100 g/L cerium, preferably 5 to 80 g/L, more preferably 10 to 70 g/L in terms of Ce0₂. Usually water is used for the adjustment of the concentration of the cerium solution, and deionized water is particularly preferred. If the initial concentration is too high, the crystallinity of the precipitate is not sufficiently high and sufficient pores for impregnation with the solution of silicon oxide precursor and titanium oxide precursor cannot be formed, resulting in insufficient heat resistance and reducibility of the ultimate composite oxide. Too low an initial concentration leads to low productivity, which is not industrially advantageous.

In the first method, step (b) of heating and maintaining the cerium solution obtained from step (a) up to and at not lower than 60° C is carried out to cause reaction of the cerium solution. A reactor to be used in step (b) may either be a sealed- or open-type vessel. An autoclave reactor may preferably be used.

In step (b), the temperature at which the cerium solution is heated and maintained is not lower than 60°C, preferably 60 to 200°C, more 5 preferably 80 to 180°C, most preferably 90 to 160°C. The duration of heating and maintaining is usually 10 minutes to 48 hours, preferably 30 minutes to 36 hours, more preferably 1 hour to 24 hours. With insufficient heating and maintaining, the crystallinity of the precipitate is not sufficiently high and a sufficient volume of pores for l o impregnation with the solution of silicon oxide precursor and titanium oxide precursor cannot be formed, resulting in insufficient heat resistance of the ultimate composite oxide. Too long a period of heating and maintaining affects little the heat resistance and is not industrially advantageous.

15

The first method further includes step (c) of adding a precipitant to the cerium suspension obtained through the heating and maintaining in step (b) to obtain a precipitate.

20 The precipitant used in step (c) may be a base, for example, sodium hydroxide, potassium hydroxide, aqueous ammonia, ammonia gas, or a mixture thereof, with the aqueous ammonia being particularly preferred.

25 The precipitant maybe added, for example, by preparing an aqueous solution of the precipitant at a suitable concentration and adding the solution to the cerium suspension obtained from step (b) under stirring, or in the case of ammonia gas, by bubbling the cerium suspension with the ammonia gas in the reactor under stirring. The amount of the precipitant to be added may easily be determined by monitoring the pH change of the suspension. Usually, the amount for generating a precipitate in the cerium suspension at pH 7 to 9, preferably pH 7 to 8.5, is sufficient.

Step (c) may be carried out after the cerium suspension obtained through the heating and maintaining in step (b) is cooled. Such cooling may usually be carried out under stirring according to a commonly known method. The cooling may either be natural cooling by leaving the suspension to stand, or forced cooling with cooling tubes. The cooling may be carried out down to usually 40°C or lower, preferably a room temperature of 20 to 30°C.

Through the precipitation reaction in step (c), a slurry containing a precipitate of cerium oxide hydrate with grown crystals is obtained. The precipitate may be separated by, for example, the Nutsche method, centrifugation, or filter-pressing. The precipitate may optionally be washed with water as needed. Further, in order to improve the efficiency in the following step (d), the precipitate may optionally be dried to a suitable level.

The first method includes step (d) of calcining the precipitate to obtain a cerium oxide. The temperature for the calcining is usually 250 to 500°C, preferably 280 to 450°C.

The cerium oxide obtained through calcination in step (d) is in the form of a porous body having pores of sufficient volume for impregnation with a solution of a silicon oxide precursor and titanium oxide precursor. This facilitates impregnation with a solution of a silicon oxide precursor and titanium oxide precursor and improves the heat resistance of the ultimate composite oxide.

The duration of the calcination may usually be 30 minutes to 36 hours, preferably 1 hour to 24 hours, more preferably 3 hours to 20 hours.

The first method includes step (e) of impregnating the cerium oxide obtained through calcination with a solution of a silicon oxide precursor and a titanium oxide precursor.

The silicon oxide precursor used in step (e) may be any compound which may be converted to a silicon oxide through an oxidation treatment, such as calcining, as long as the calcined cerium oxide porous body may be impregnated with the compound dissolved in a solvent. Examples of the precursor may include silicates, such as sodium silicate, silane compounds, such as tetraethyl orthosilicate, silyl compounds, such as trimethylsilyl isocyanate, quaternary ammonium silicates, such as tetramethyl ammonium silicate, and colloidal silica. The colloidal silica is a commercially-available product, such as for example AT-20Q provided by ADEKA with the following characteristics: acid stabilized type, %Si0₂ = 20, primary particle size = 10-15 nm. The titanium oxide precursor used in step (e) may be any compound which may be converted to a titanium oxide through an oxidation treatment, such as calcining, as long as the calcined cerium oxide porous body may be impregnated with the compound dissolved in a solvent. Examples of the precursor may include titanium sulfate, titanium oxysulfate, titanium tetrachloride, titanium oxychloride, titanium nitrate, titanium tetra-methoxide, titanium tetra-ethoxide, titanium tetra-propoxide, titanium tetra-butoxide, titanium tetra- acetylacetonate, quaternary ammonium titanate sol and colloidal titania.

The colloidal titania is a commonly-available product, such as for example TKS-202 provided by TAYCA with the following characteristics: acid stabilized type, % Ti0₂ = 33%, primary particle size = 6 nm.

The solvent to be used for dissolving the silicon oxide precursor and the titanium oxide precursor may be selected depending on the kind of the precursor to be used, and may be, for example, water or organic solvents, such as alcohol, xylene, hexane, or toluene.

The concentration of the solution of the silicon oxide precursor is not particularly limited as long as the cerium oxide may be impregnated with the solution, and may usually be 1 to 300 g/L, preferably about 10 to 200 g/L of the silicon oxide precursor in terms of Si0₂ for workability and efficiency.

The concentration of the solution of the titanium oxide precursor is not particularly limited as long as the cerium oxide may be impregnated with the solution, and may usually be 1 to 300 g/L, preferably about 10 to 200 g/L of the silicon oxide precursor in terms of Ti0₂ for workability and efficiency.

In step (e), the amount of the silicon oxide precursor is usually 1 to 15 mass %, preferably 5 to 15 mass % of silicon oxide precursor in terms of Si0₂ with respect to the total amount of the silicon oxide precursor in terms of Si0₂, the titanium oxide precursor in terms of Ti0₂ and the cerium in terms of Ce0₂.

In step (e), the amount of the titanium oxide precursor is usually 1 to 20 mass %, preferably 5 to 15 mass % of titanium oxide precursor in terms of Ti0₂ with respect to the total amount of the silicon oxide precursor in terms of Si0₂, the titanium oxide precursor in terms of Ti0₂ and the cerium in terms of Ce0₂.

In step (e), the impregnation of the cerium oxide with the solution of the silicon oxide precursor and titanium oxide precursor may be carried out, for example, by pore-filling, adsorption, or evaporation to dryness. The pore-filling maybe effected by measuring in advance the total pore volume of the cerium oxide, and adding the same volume of the solution of the silicon oxide precursor and titanium oxide precursor so that the surface of the cerium oxide is evenly wetted. The first method includes step (f) of calcinating the cerium oxide thus impregnated with the solution of the silicon oxide precursor and titanium oxide precursor. The temperature of the calcination is usually 300 to 900°C, preferably 450 to 750°C. The duration of calcination in step (f) may suitably be determined in view of the calcination temperature, and may usually be 1 to 10 hours.

In the first method, after step (e) and before step (f), the cerium oxide impregnated with the solution of the silicon oxide precursor and titanium oxide precursor may optionally be dried at about 60 to 200°C. With such a drying step, the efficiency of the calcination in step (f) may be improved.

In the first method, after step (f), the cerium oxide impregnated with the solution of the silicon oxide precursor and titanium oxide precursor may optionally be milled. The milling may usually be carried out by commonly known method such as hammer milling or jet milling.

The second method according to the present invention includes step (A) of providing a cerium solution not less than 90 mol % of which cerium ions are tetravalent.

In step (A), the initial concentration of the cerium solution not less than 90 mol % of which cerium ions are tetravalent, may be adjusted to usually 5 to 100 g/L cerium, preferably 5 to 80 g/L, more preferably 10 to 70 g/L in terms of Ce0₂. Usually water is used for the adjustment of the concentration of the cerium solution, and deionized water is particularly preferred.

In the second method, step (B) of heating and maintaining the cerium solution obtained from step (A) up to and at not lower than 60°C is carried out next.

A reactor to be used in step (B) may either be a sealed- or open-type vessel, and an autoclave reactor may preferably be used. In step (B), the temperature at which the cerium solution is heated and maintained is not lower than 60°C, preferably 60 to 200°C, more preferably 80 to 180°C, most preferably 90 to 160°C. The duration of heating and maintaining is usually 10 minutes to 48 hours, preferably 15 minutes to 36 hours, more preferably 30 minutes to 10 hours.

The second method further includes step (C) of adding a silicon oxide precursor and a titanium oxide precursor to a cerium suspension obtained from step (B).

In step (C), the silicon oxide precursor to be added to the cerium suspension may be any compound which may be converted to a silicon oxide through an oxidation treatment, such as calcination, and may be, for example, colloidal silica, siliconate, or quaternary ammonium silicate sol, with the colloidal silica being particularly preferred in view of the production cost and reduction of environmental burden.

In step (C), the titanium oxide precursor to be added to the cerium suspension may be any compound which may be converted to a titanium oxide through an oxidation treatment, such as calcination, and may be, for example, colloidal titania, titanium sulfate, titanium oxysulfate, titanium tetrachloride, titanium oxychloride, titanium nitrate, titanium tetra-methoxide, titanium tetra-ethoxide, titanium tetra-propoxide, titanium tetra-butoxide, titanium tetra-acetylacetonate or quaternary ammonium titanate sol, with the colloidal titania being particularly preferred in view of the production cost and reduction of environmental burden. In step (C), the amount of the silicon oxide precursor is usually 1 to 15 mass % of the silicon oxide precursor, preferably 5 to 15 mass %, in terms of Si0₂ with respect to the total amount of the silicon oxide precursor in terms of Si0₂, the titanium oxide precursor in terms of Ti0₂ and the cerium in terms of Ce0₂.

In step (C), the amount of the titanium oxide precursor is usually 1 to 20 mass % of the titanium oxide precursor, preferably 5 to 15 mass %, in terms of Ti0₂ with respect to the total amount of the silicon oxide precursor in terms of Si0₂, the titanium oxide precursor in terms of Ti0₂ and the cerium in terms of Ce0₂.

In step (C), before adding the silicon oxide precursor and the titanium oxide precursor, the salt concentration of the cerium suspension may be adjusted by removing the mother liquor from the cerium suspension or by adding water. The removal of the mother liquor maybe effected, for example, by decantation, Nutsche method, centrifugation, or filter- pressing. In this case, a slight amount of cerium is removed with the mother liquor, so the amount of the silicon oxide precursor and titanium oxide precursor and water to be added next may be adjusted, taking this removed amount of cerium into consideration.

Step (C) may be carried out after the cerium suspension obtained through the heating and maintaining in step (B) is cooled. Such cooling may usually be carried out under stirring according to a commonly known method. The cooling may either be natural cooling by leaving the suspension to stand, or forced cooling with cooling tubes. The cooling may be carried out down to usually 40°C or lower, preferably a room temperature of 20 to 30°C. The second method includes step (D) of heating and maintaining the cerium suspension containing the silicon oxide precursor and titanium oxide precursor up to and at not lower than 100°C, preferably 100 to 200°C, more preferably 100 to 150°C.

In step (D), the duration of the heating and maintaining may be usually 10 minutes to 6 hours, preferably 20 minutes to 5 hours, more preferably 30 minutes to 4 hours. In step (D) of heating and maintaining, at lower than 100°C, the crystallinity of the precipitate is not sufficiently high, resulting in insufficient heat resistance of the ultimate composite oxide.

The second method includes step (E) of adding a precipitant to the cerium suspension containing the silicon oxide precursor and titanium oxide precursor obtained through the heating and maintaining to obtain a precipitate.

The precipitant used in step (E) may be a base, for example, sodium hydroxide, potassium hydroxide, aqueous ammonia, ammonia gas, or a mixture thereof, with the aqueous ammonia being particularly preferred. The amount of the precipitant to be added in step (E) may easily be determined by monitoring the pH change of the cerium suspension containing the silicon oxide precursor and titanium oxide precursor. Usually, the amount for generating a precipitate in the cerium suspension at pH 7 to 9, preferably pH 7 to 8.5, is sufficient.

Step (E) may be carried out after the cerium suspension obtained through the heating and maintaining in step (D) is cooled. Such cooling may usually be carried out under stirring according to a commonly known method. The cooling may either be natural cooling by leaving the suspension to stand, or forced cooling with cooling tubes. The cooling may be carried out down to usually 40°C or lower, preferably a room temperature of 20 to 30°C.

The precipitate may be separated by, for example, the Nutsche method, centrifugation, or filter-pressing. The precipitate may optionally be washed with water as needed.

The second method includes step (F) of calcining the precipitate thus obtained. The temperature for the calcining is usually 300 to 900°C, preferably 450 to 750°C. The duration of the calcination may usually be 1 to 48 hours, preferably 1 to 24 hours, more preferably 3 to 20 hours.

In the second method, after step (F), the cerium based composite oxide may optionally be milled. The milling may usually be carried out by commonly known method such as hammer milling or jet milling.

The composite oxides of the invention as described above or as obtained by means of the preparation process previously described are in the form of powders, but they can optionally be formed so as to be in the form of granules, pellets, foams, beads, cylinders or honeycombs of variable dimensions.

These composite oxides can be applied to any support commonly used in the field of catalysis, that is to say in particular thermally inert supports. This support can be chosen from alumina, titanium oxide, cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates, crystalline silicoaluminum phosphates or crystalline aluminum phosphates. The present invention also concerns a composite oxide susceptible to be obtained according to the above mentioned processes of the invention.

Application

The composite oxides of the invention may be used in catalytic systems. These catalytic systems can comprise a coating (wash coat), based on these composite oxides and with catalytic properties, on a substrate of the metal or ceramic monolith type, for example. Such a monolith type can be a filter type based on silicon carbide, cordierite or aluminium titanate, for instance. The coating can itself also comprise a support of the type of those mentioned above. This coating is obtained by mixing the composite oxides with the support, so as to form a suspension which can subsequently be deposited on the substrate. These catalytic systems and more particularly the composite oxides of the invention can have a great many applications. They are therefore particularly well suited to, and thus usable in, the catalysis of various reactions, such as, for example, dehydration, hydrosulfurization, hydrodenitrification, desulfurization, hydrodesulfurization, dehydrohalogenation, reforming, steam reforming, cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization, dismutation, oxychlorination, dehydrocyclization of hydrocarbons or other organic compounds, oxidation and/or reduction reactions, the Claus reaction, treatment of exhaust gases from internal combustion engines, demetallation, methanation, the shift conversion, oxidation of CO, purification of air by low-temperature oxidation (<200°C, indeed even <100°C), catalytic oxidation of the soot emitted by internal combustion engines, such as diesel engines or petrol engines operating under lean burn conditions.

In the case of these uses in catalysis, the composite oxides of the invention can be employed in combination with precious metals. The nature of these metals and the techniques for the incorporation of the latter in these compositions are well known to a person skilled in the art. For example, the metals can be platinum, rhodium, palladium, gold or iridium and they can, in particular, be incorporated in the compositions by impregnation. Among the uses mentioned, the treatment of exhaust gases from internal combustion engines (automobile afterburning catalysis) is a particularly advantageous application. The compositions of the invention can thus be used in this case for catalysis. More particularly still in the case of this use in catalysis, the compositions can be employed in combination with an NOx (nitrogen oxides) trap for the treatment of exhaust gases from petrol engines operating with a lean burn mixture, for example in the catalysis layer of such a trap. The composite oxides of the invention can be incorporated in oxidation catalysts for diesel engines.

For this reason, the invention also relates very particularly to a process for the treatment of exhaust gases from internal combustion engines which is characterized in that use is made, as catalyst, of a composite oxide or of a catalytic system as described above. Another advantageous use is the purification of air at temperatures of less than 200°C, indeed even of less than 100°C, this air comprising at least one compound of the carbon monoxide, ethylene, aldehyde, amine, mercaptan or ozone type and generally of the type of the volatile organic compounds or atmospheric pollutants, such as fatty acids, hydrocarbons, in particular aromatic hydrocarbons, and nitrogen oxides (for the oxidation of NO to give N0₂), and of the malodorous compounds type.

The present invention also concerns then a process for the purification of air, said air comprising carbon monoxide, ethylene, aldehyde, amine, mercaptan, ozone, volatile organic compounds, atmospheric pollutants, fatty acids, hydrocarbons, aromatic hydrocarbons, nitrogen oxides or malodorous compounds, comprising the step of bringing into contact said gases with the catalytic system of the invention.

Mention may more particularly be made, as compounds of this type, of ethanethiol, valeric acid and trimethylamine. This treatment is carried out by bringing the air to be treated into contact with a composite oxide or a catalytic system as described above or obtained by the processes described in detail above.

Concrete but non limiting examples will now be given. EXPERIMENTAL PART

Example 1

This example relates to a composite oxide of cerium oxide, silicon oxide and titanium oxide at a mass ratio of 92.8:5r2 5.1 :2.1.

5

50 g of a eerie nitrate solution in terms of Ce0₂ containing not less than 90 mol % tetravalent cerium ions was measured out, and adjusted to a total amount of 1 L with deionized water. The obtained solution was heated to 100°C, maintained at this temperature for 30 minutes, o and allowed to cool down to the room temperature, to thereby obtain a cerium suspension. After the mother liquor was removed (2.6 g of cerium in terms of Ce0₂ was removed-with the mother liquor) from the cerium suspension thus obtained, 12.9 g of a colloidal silica (2.6 g in terms of Si0₂) and 3.2 g of a colloidal titania (1.1 g in terms of Ti0₂)5 were added, and the total volume was adjusted to 1 L with deionized water.

Then the cerium suspension containing a precursor of silicon oxide and titanium oxide was maintained at 120°C for 2 hours, allowed to cool, o and neutralized to pH 8.5 with aqueous ammonia.

The obtained slurry was subjected to solid-liquid separation through a Nutsche filter to obtain a filter cake. The cake was calcined in the air at 700°C for 10 hours to obtain composite oxide powder mainly 5 composed of cerium oxide with 5.1 mass % of silicon oxide and 2.1 mass % of titanium oxide.

The obtained composite oxide powder was measured of the specific surface area by the BET method after calcination at 800°C for 2 hours and at 900°C for 5 hours. Further, the pH in the aqueous solution comprising 3 % by weight of this composite oxide was measured.

The results are shown in Table 1.

Example 2

This example relates to a composite oxide of cerium oxide, silicon oxide and titanium oxide at a mass ratio of 90.0:5.0:5.0. A composite oxide powder mainly composed of cerium oxide with 5.0 mass % of silicon oxide and 5.0 mass % of titanium oxide was prepared in the same way as in Example 1 except that the amount of a colloidal titania was 8.0 g (2.6 g in terms of Ti0₂). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1 and the results are shown in Table 1.

Example 3

This example relates to a composite oxide of cerium oxide, silicon oxide and titanium oxide at a mass ratio of 85.7:4.8:9.5.

A composite oxide powder mainly composed of cerium oxide with 4.8 mass % of silicon oxide and 9.5 mass % of titanium oxide was prepared in the same way as in Example 1 except that the amount of a colloidal titania was 16.0 g (5.3 g in terms of Ti0₂). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1 and the results are shown in Table 1. Example 4

This example relates to a composite oxide of cerium oxide, silicon oxide and titanium oxide at a mass ratio of 90.0:5.0:5.0 and prepared by a method different from Example 2. A cerium oxide powder was prepared in accordance with the method disclosed in Patent Publication WO2003/022740.

20 g of a eerie nitrate solution in terms of Ce0₂ containing not less than 90 mol % tetravalent cerium ions was measured out, and adjusted to a total amount of 1 L with deionized water. The obtained solution was heated to 100°C, maintained at this temperature for 24 hours, and allowed to cool down to the room temperature. Then aqueous ammonia was added to neutralize to pH 8 to obtain cerium oxide hydrate in the form of the slurry. The slurry was then subjected to solid-liquid separation with a Nutsche to obtain a filter cake. The cake was calcined in the air at 300°C for 10 hours to obtain cerium oxide powder.

47.4 g of the cerium oxide powder (45.0 g in terms of Ce0₂) thus obtained was placed in a beaker, to which an ethanol solution of 8.39 g of tetraethylorthosilicate (2.5 g in terms of Si0₂) and 8.90 g of titanium n-Propoxide (2.5 g in terms of Ti0₂) in a total amount of 28.5 mL was added to impregnate the cerium oxide with a solution of silicon oxide precursor and titanium oxide precursor by pore-filling. Then the cerium oxide impregnated with the solution of silicon oxide precursor and titanium oxide precursor was dried at 120°C for 10 hours, and calcined in the air at 700°C for 10 hours to obtain composite oxide powder mainly composed of cerium oxide with 5.0 mass % of silicon oxide and 5.0 mass % of titanium oxide.

The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1 and the results are shown in Table 1.

Comparative example 1

This example relates to a cerium oxide without silicon oxide and titanium oxide, which was obtained by calcination at 700°C for 10 hours before the impregnation with the solution of a silicon oxide precursor and a titanium oxide precursor in Example 4.

The properties of the oxide powder thus obtained were evaluated in the same way as in Example 1 and the results are shown in Table 1.

Comparative example 2

This example relates to a composite oxide of cerium oxide and titanium oxide at a mass ratio of 98.0:2.0. A composite oxide powder mainly composed of cerium oxide with 2.0 mass % of titanium oxide was prepared in the same way as in Example 1 except that the amount of a colloidal titania was 2.9 g (1.0 g in terms of Ti0₂) and colloidal silica was not added. The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1 and the results are shown in Table 1. Comparative example 3

This example relates to a composite oxide of cerium oxide and titanium oxide at a mass ratio of 95.0:5.0.

A composite oxide powder mainly composed of cerium oxide with 5.0 mass % of titanium oxide was prepared in the same way as in Example 1 except that the amount of a colloidal titania was 7.6 g (2.5 g in terms of Ti0₂) and colloidal silica was not added.

Comparative example 4

This example relates to a composite oxide of cerium oxide and titanium oxide at a mass ratio of 90.0: 10.0.

A composite oxide powder mainly composed of cerium oxide with 10.0 mass % of titanium oxide was prepared in the same way as in Example 1 except that the amount of a colloidal titania was 16.0 g (5.3 g in terms of Ti0₂) and colloidal silica was not added.

The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1 and the results are shown in Table 1. Comparative example 5

This example relates to a composite oxide of cerium oxide and silicon oxide at a mass ratio of 95.0:5.0.

A composite oxide powder mainly composed of cerium oxide with 5.0 mass % of silicon oxide was prepared in the same way as in Example 1 except that colloidal titania was not added. The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1 and the results are shown in Table 1.

Table 1

Description of analysis method

SBET: The Specific surface area is measured by BET method in the following way. Use is made of a MOUNTECH Co., LTD. Macsorb analyzer with a 200 mg sample which has been calcined beforehand at 800°C for 2 hours or 900°C for 5 hours under air. pH in the aqueous suspension: The pH in the aqueous solution comprising 3% by weight of the oxide powder is measured in the following way. An oxide powder is dried at 200°C for 1 hour. The dried oxide powder is hold in the desiccator for 30 minutes. 3.0 g of 5 thus obtained oxide powder is added into a 100 ml of deionized water under stirring. After 1 minute of stirring, a pH meter (HORIBA D-51) is put into the slurry. The pH value is collected after 3 minutes of putting the pH meter. l o While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the system and method have been described by way of illustration only, and such illustrations and embodiments as have been

15 disclosed herein are not to be construed as limiting to the claims.

Claims

1. A cerium composite oxide comprising at least:

- silicon oxide in a proportion comprised between 1 and 15 % by weight of oxide; and

- titanium oxide in a proportion comprised between 1 and 20 % by- weight of oxide.

2. Cerium composite oxide according to claim 1 wherein it comprises silicone oxide in a proportion comprised between 5 and 15 % by weight of oxide.

3. Cerium composite oxide according to claim 1 or 2 wherein it comprises titanium oxide in a proportion comprised between 5 and 15 % by weight of oxide.

4. Cerium composite oxide according to claim anyone of claims 1 to 3 wherein it exhibits a pH inferior or equal to 7, wherein pH is measured in an aqueous solution comprising 3 % by weight of this composition, at 25°C.

5. Cerium composite oxide according to claim anyone of claims 1 to 4 wherein it exhibits a specific surface area, after calcination at 800°C for 2 hours, comprised between 70 and 120 m²/g.

6. Cerium composite oxide according to claim anyone of claims 1 to 5 wherein it exhibits a specific surface area, after calcination at 900°C for 5 hours, comprised between 40 and 85 m /g.

7. Process to produce a cerium composite oxide according to anyone of claims 1 to 6 comprising the steps of:

(a) providing a cerium solution,

(d) calcining said precipitate to obtain a cerium oxide,

8. Process to produce a cerium composite oxide according to anyone of claims 1 to 6 comprising the steps of:

(A) providing a cerium solution,

(F) calcining said precipitate.

9. Process according to claim 7 or 8 wherein step (a) or (A) provides a cerium solution in which not less than 90 mol % of which cerium ions are tetravalent.

5 10. A catalytic system comprising a cerium composite oxide according to anyone of claims 1 to 6

11. A process for the treatment of exhaust gases from internal combustion engines, comprising the step of bringing into contact said l o gases with the catalytic system as claimed in claim 10.

12. A process for the purification of air, said air comprising carbon monoxide, ethylene, aldehyde, amine, mercaptan, ozone, volatile organic compounds, atmospheric pollutants, fatty acids, hydrocarbons,

15 aromatic hydrocarbons, nitrogen oxides or malodorous compounds, comprising the step of bringing into contact said gases with the catalytic system as claimed in claim 10.