EP1715949A1

EP1715949A1 - Gold and reducible oxide-based composition, method for the preparation and the use thereof in the form of a catalyst, in particular for carbon monoxide oxidation

Info

Publication number: EP1715949A1
Application number: EP05717645A
Authority: EP
Inventors: Franck Fajardie; Stephan Verdier; Kazuhiko Yokota
Original assignee: Rhodia Acetow GmbH
Current assignee: Cerdia Produktions GmbH
Priority date: 2004-02-18
Filing date: 2005-02-17
Publication date: 2006-11-02
Also published as: FR2866248B1; CN1921936A; AU2005224127A1; JP2007522932A; WO2005089936A1; BRPI0507556A; AU2005224127B2; US20070134144A1; RU2006133316A; FR2866248A1; CA2553284A1; RU2339447C2

Abstract

The invention relates to a gold-based composition on a reducible oxide-based support characterised in that the halogen content thereof with respect to a molar halogen/gold ratio is equal to or less than 0.05, wherein the gold is embodied in the form of particles whose size is equal to or less than 10 nm and the composition is exposed to reduction treatment. Said composition is obtainable by a method consisting in bringing a reducible oxide-based compound into contact with a gold halide-based compound, thereby forming the suspension thereof, the thus obtained medium pH being fixed to a value of at least 8, subsequently, in separating a solid from a reaction medium and in washing said solid with a basic solution. Said method also involves a reduction treatment which is carried out prior to washing or thereafter. The inventive composition can be used in the form of a catalyst in carbon monoxide oxidation methods, for treating tobacco smoke and air.

Description

COMPOSITION BASED ON GOLD AND A REDUCABLE OXIDE, METHOD

The present invention relates to a composition based on gold and of a reducible oxide, to its preparation process and to its use as a catalyst, in particular for the oxidation of the carbonate monoxide, in particular for the oxidation of carbon monoxide. carbon monoxide. Gold catalysts already exist which are used in particular in CO oxidation processes. Furthermore, a certain number of these oxidation processes take place at relatively low temperatures, for example below 250 ° C., in particular in gas-to-water conversion reactions. We even try to oxidize CO at room temperature, for example in air treatment processes, and / or under harsh conditions such as very high hourly volume velocities (hvv), this is the case for example treatment of cigarette smoke. The catalysts currently available and which can be used from an economic point of view do not have sufficient performance to meet this need. The object of the invention is to provide effective catalysts at low temperatures and / or high hvv. For this purpose, the composition of the invention is based on gold on a support based on at least one reducible oxide, and it is characterized in that its halogen content expressed by the halogen / gold molar ratio is d '' at most 0.05, in that the gold is in the form of particles of size at most 10 nm and in that it has undergone a reduction treatment, being excluded compositions with supports in which the the only or the only reducible oxides are cerium oxide, cerium oxide in combination with zirconium oxide, cerium oxide in combination with praseodymium oxide, cerium oxide in combination with titanium oxide or tin oxide in an atomic proportion Ti / Ce or Sn / Ce of less than 50%. The invention also relates to the process for the preparation of this composition which, according to a first embodiment, is characterized in that it comprises the following steps:

- A compound based on at least one reducible oxide and a compound based on a gold halide are brought into contact by forming a suspension of these compounds, the pH of the medium thus formed being fixed at a value of at least minus 8; - The solid is separated from the reaction medium;

- washing the solid with a basic solution; the method further comprising a reduction treatment either before or after the above-mentioned washing step. The invention also relates to a method according to a second embodiment which is characterized in that it comprises the following steps:

- Gold is deposited on a compound based on at least one oxide reducible by impregnation or by ion exchange;

- washing the solid from the previous step with a basic solution having a pH of at least 10; the method further comprising a reduction treatment either before or after the above-mentioned washing step. The compositions of the invention are effective at low temperatures, high hvvs and furthermore with low gold contents. Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as various concrete but nonlimiting examples intended to illustrate it. The periodic classification of the elements to which reference is made in this description is that published in the Supplement to the Bulletin of the Société Chimique de France No. 1 (January 1966). By rare earth is meant the elements of the group constituted by ryttrium and the elements of the periodic classification of atomic number included inclusively between 57 and 71. By specific surface is meant the BET specific surface determined by nitrogen adsorption in accordance with ASTM standard D 3663-78 established from the BRUNAUER - EMMETT-TELLER method described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)". As indicated above, the composition of the invention comprises gold and a reducible oxide. The reducible oxide forms a support. The term “support” must be taken in a broad sense to designate, in the composition of the invention, the constituent (s) which are predominant in the composition, the supported element being present essentially on the surface of these constituents. To simplify, we will speak in the following description of support and supported phase but it will be understood that it would not depart from the scope of the present invention in the case where an element described as belonging to the supported phase would be present in the support, for example by having been introduced into it during the preparation of the support itself. By reducible oxide is meant an oxide of a metal which may have several degrees of oxidation. It will be noted that the metal which enters into the constitution of the support is in a form which consists essentially or solely of oxide of said metal. By “consists essentially” here is meant that amorphous species of the type for example hydroxide or oxyhydroxide are present only in the trace state, By defining by amorphous any product whose diffractogram RX does not have diffraction lines centered on the oxide phase or whose RX diffractogram has halos centered on the oxide phase but whose width at mid-height would make it possible to calculate, according to the Debye-Scherrer method, crystallite sizes less than 2 nm, it should be understood, in the context of the present invention, by the terms: amorphous species are only present in trace amounts, the fact that the comparison of an RX diagram of a pure metal oxide with that of an oxide of the same metal but containing these species does not reveal detectable differences and in particular does not reveal halos. As reducible oxides which are suitable in the context of the present invention, there may be mentioned the oxides of transition metals and the oxides of rare earths. By transition metals is meant the elements of groups IIIA to MB of the periodic table. Mention may more particularly be made of the oxides of titanium, manganese, iron, copper, cobalt or tin. The support can therefore advantageously be based on at least one of these oxides. As indicated above, there are no particular supports within the scope of the present invention. These are carriers based on cerium oxide, cerium oxide and zirconium oxide, cerium oxide and praseodymium oxide, cerium oxide in combination with oxide of titanium or tin oxide in an atomic proportion Ti / Ce or Sn / Ce of less than 50%, insofar as these oxides are the only reducible oxides present in the support. It will therefore be noted that a support based on the abovementioned oxides but also containing another reducible oxide, for example manganese oxide, is not excluded from the present invention. The compound used for the support must moreover have a specific surface sufficiently high to allow a dispersion of the gold at its suitable surface so that the gold can have a sufficient catalytic activity. Finally, the composition of the invention must have undergone a reduction treatment. By reduction treatment is meant a treatment which is carried out under conditions such that the support (reducible oxide) and the supported phase (gold) are both reduced. The fact that the composition has undergone such a treatment may result in the presence of an oxygen defect in the support, that is to say that the amount of oxygen in the oxide forming the support is less than the stoichiometric quantity. This oxygen defect can for example be demonstrated by X-ray diffraction or by an analysis according to the XPS technique. It should be noted that the composition of the invention may contain gold with, in addition, at least one other metallic element chosen from silver, platinum, palladium and copper. In this case, this or these other metallic elements may be present for example in an amount of at most 400%, more particularly at most 120% and in particular between 5% and 50% relative to gold, this quantity being expressed in molar% metallic element (s) / gold. Compositions of this type, in the case of use at high hvv, can reach their maximum effectiveness even more quickly. The contents of gold, or of gold and aforementioned metallic element, of the composition are not critical, they correspond to the contents generally used in catalysts to obtain a catalytic activity. By way of example, this content is at most 5%, in particular at most 1%. It can more particularly be at most 0.5% and even at most 0.25%. Grades above 5% are generally of no economic interest. These contents are expressed as a percentage by mass of gold, possibly with the metallic element, relative to the oxide (or to the oxides) which constitutes the support. The composition of the invention has two other specific characteristics. The first is its halogen content. The halogen can more particularly be bromine or chlorine. This content, which is expressed by the halogen / gold molar ratio, is at most 0.05. More particularly, it is at most 0.04 and even more particularly at most 0.025. The halogen determination can be carried out using the following method. The quantity of catalyst required for the analysis is vaporized in the flame of an oxyhydrogen torch (H ₂ / O ₂ mixture at approximately 2000 ° C). The resulting vapor is trapped in an aqueous solution containing hydrogen peroxide. In the case where a solid residue is obtained at the end of the treatment under an oxyhydrogen torch, it is suspension in the solution where the combustion gases were collected (water + H ₂ 0 ₂ ) then filtered. The collected filtrate is then analyzed by ion chromatography and the halogen content calculated by integrating the appropriate dilution factor. Finally, the halogen content of the catalyst is calculated taking into account the mass of catalyst used for the analysis. The other characteristic is the size of the gold particles present in the composition. These particles have a size of at most 10 nm. Preferably, it is at most 3 nm. Here, and for the whole of the present description, this size is determined from the analysis of the RX spectra of the composition, using the width (I) at half height of the gold diffraction peak. The size of the particles is proportional to the inverse (1/1) of the value of this width I. It will be noted that the X-ray analysis neither makes it possible to detect a phase corresponding to gold for particles whose size is smaller at 3nm nor to detect gold for gold contents lower than 0.25%. In these two cases, we can then use MET analysis. The process for preparing the composition of the invention will now be described. This method can be implemented according to a first embodiment. In this first mode, the first step of the process consists in bringing into contact a compound based on a reducible oxide and a compound based on a gold halide and, if necessary of a compound based on platinum, palladium or copper. This contacting is done by forming a suspension which is generally an aqueous suspension. This starting suspension can be obtained from a preliminary dispersion of a support based on a reducible oxide of the type described above, prepared by dispersing this support in a liquid phase, and by mixing with a solution or a dispersion of the gold compound. As compound of this type, it is possible to use chlorinated or brominated compounds of gold, for example chlorauric acid HAuCI ₄ or its salts such as NaAuCI ₄ which are the most common compounds. In the case of the preparation of a composition also comprising silver, platinum, palladium or copper, it is possible to choose as compound of these elements the salts of inorganic acids such as nitrates, sulfates or chlorides. It is also possible to use the salts of organic acids and in particular the salts of saturated aliphatic carboxylic acids or the salts of acids. hydroxy. By way of examples, mention may be made of formates, acetates, propionates, oxalates or citrates. Finally, mention may be made in particular for platinum of platinum (II) tetramine hydroxide. For the remainder of the description of the process, only the compound based on gold halide will be mentioned, but it should be understood that the description applies likewise also in the case where a silver compound is used, platinum, palladium or copper as described above. The starting suspension can be obtained for example by introducing the solution or dispersion of the gold compound into the dispersion of the support. According to a specific characteristic of the process, the pH of the suspension thus formed is brought to a value of at least 8, more particularly at least 8.5 and even more particularly at least 9. Preferably, the pH at the value of at least 8 during the formation of the suspension, when the compound based on a reducible oxide is brought into contact with the compound based on gold halide by concomitant introduction of a compound basic. For example, when one proceeds by introducing the solution or dispersion of the gold compound into the dispersion of the support, a basic compound is added at the same time. The flow rate of basic compound can be adjusted so as to maintain the pH of the medium at a constant value, that is to say a value varying by plus or minus 0.3 pH units relative to the fixed value. As basic compound, it is possible in particular to use products of the hydroxide or carbonate type. Mention may be made of alkali or alkaline hydroxides and ammonia. It is also possible to use secondary, tertiary or quaternary amines. Mention may also be made of urea. The basic compound is generally used in the form of a solution. According to a variant of the method, it is possible to implement a dispersion of the support and a solution or dispersion of the gold compound which have both been previously brought to a pH of at least 8 so that it does not is not necessary, when brought into contact, to add a basic compound. The contacting of the compound based on cerium oxide and the compound based on gold halide is generally done at room temperature but it is possible to do it hot for example at a temperature of at least 60 ° vs. The suspension formed during the first step of the process is generally kept under stirring for a period of a few minutes. In a second step, the solid is separated from the reaction medium, by any known means. The solid thus obtained is then washed with a basic solution. Preferably, this basic solution has a pH of at least 8, more particularly at least 9. The basic solution can be based on the same basic compounds as those which have been mentioned above. This washing can be carried out by any suitable method, for example using the piston washing technique or by redispersion. In the latter case, the solid is redispersed in the basic solution and then, generally after stirring, the solid is separated from the liquid medium. Washing with the basic solution can be repeated several times if necessary. It can be followed, possibly by washing with water. After washing, the solid obtained is generally dried. Drying can be done by any suitable method, for example in air or by freeze-drying. The method of the invention also includes a reduction treatment. This reduction treatment can take place either before washing with the basic solution which has just been described, or after this washing. In the latter case, this reduction treatment can also be carried out before washing with water or after it, in the case of such washing with water, and before or after optional drying. This treatment is carried out in such a way that the whole of the gold element has a degree of oxidation lower than its degree of oxidation before treatment, this degree of oxidation before treatment generally being 3. The degree of oxidation of gold can be determined by techniques known to those skilled in the art, for example by the programmed temperature reduction method (RTP) or by X-ray photoelectron spectroscopy (XPS). Different types of reduction treatment can be considered. First of all, a chemical reduction can be carried out by bringing the product into contact with a reducing agent such as ferrous, citrate or stannous ions, oxalic acid, citric acid, hydrogen peroxide, hydrides such as NaBH_4, l hydrazine (NH2-NH ₂ ), formaldehyde in aqueous solution (H ₂ CO), phosphorous reducers including tetrakis (hydroxymethyl) phosphonium chloride or NaH ₂ P0 ₂ . This treatment can be done by suspending the product in an aqueous medium containing the reducing agent or also on the product in the reaction medium after the deposition of gold. One can also achieve a reduction under ultraviolet rays; the treatment can be done, in this case, on a solution or suspension of the product or even on a powder. This treatment can be done before or after the washing step described above. In addition, the reduction treatment can be carried out by gas using a reducing gas which can be chosen from hydrogen, carbon monoxide or hydrocarbons, this gas can be used in any volume concentration. It is particularly possible to use hydrogen diluted in argon. In the case of a reduction treatment according to the latter type, this is done after the aforementioned washing step. In this case, the treatment is carried out at a temperature which is at most 200 ° C., preferably at most 180 ° C. The duration of this treatment can be between 0.5 and 6 hours in particular. At the end of the reduction treatment, it is generally not necessary to carry out a calcination. However, such calcination is not excluded, preferably at low temperature, that is to say at most 250 ° C for a period of at most 4 hours for example and in air. It may be advantageous to carry out such a calcination in the case of the reduction treatment of the chemical type described above. The method of the invention can also be implemented according to a second embodiment which will now be described. The first step consists in depositing gold and, where appropriate silver, platinum, palladium or copper, on the compound based on an oxide reducible by impregnation or by ion exchange. The impregnation method is well known. Preferably dry impregnation is used. Dry impregnation consists in adding to the product to be impregnated, here the support based on a reducible oxide, a volume of a solution of the gold compound which is equal to the pore volume of the solid to be impregnated. The gold compound is here of the same type as that which has been described above for the first embodiment. Ion exchange deposition is also a known method. The same type of gold compound can also be used there as above. In the second step of the process, the product from the previous step is then washed with a basic solution whose pH is at least 10, preferably at least 11. This washing can be done in the same way and with the same basic compounds as described for the method according to the first mode. Furthermore, it is also possible to implement in this second embodiment a reduction and drying treatment in the same manner as that which has been described above. Finally, it should be noted that it is also possible, in the case of the preparation of a composition based, in addition to gold, on another metallic element, to deposit this metallic element firstly on the support, for example by impregnation, then, in a second step, to deposit the gold by following the methods which have just been described. The compositions of the invention as obtained by the process described above are in the form of powders but they can optionally be shaped to be in the form of granules, beads, cylinders, extrusions or honeycombs. variable dimensions. They can be used in catalytic systems comprising a coating (wash coat) based on these compositions, on a substrate of the type, for example metallic or ceramic monolith. The coating may for example include alumina. It may be noted that the deposition of gold can also take place on a support previously put into a form of the type given above. The compositions of the invention, as described above or obtained by the process detailed above, can be used more particularly, as catalysts, in the processes implementing an oxidation of carbon monoxide. They are very particularly effective for processes of this type which are carried out at low temperatures, by which is meant temperatures of at most 250 ° C. They are even effective at room temperature. By ambient temperature is meant here and for the whole of the description, unless otherwise indicated, a temperature of at most 35 ° C., more particularly in a range from 10 ° C. to 25 ° C. Finally, they can also be effective under high hvv conditions which, for example, can range at least up to 1,500,000 cm ³ / g _cata / h. In addition, the compositions of the invention can be used for the oxidation of carbon monoxide at even lower temperatures, that is to say below 0 ° C., for example comprised at -10 ° C. and 0 ° C and for the treatment of a gas or medium with a very low CO content, for example of at most 1 000 ppme and for extremely high vvh which can go up to 30,000,000 cm ³ / g _cata / h. Thus, by way of example of use in processes implementing an oxidation of carbon monoxide, they can be used in the treatment of cigarette smoke, in the reaction of conversion of gas to water (CO + H ₂ 0- »C0 ₂ + H ₂ ) at a temperature below 100 ° C in particular, or also in the treatment of reforming gases at a temperature below 150 ° C, treatment of the PROX type (preferential oxidation of CO in the presence of 'hydrogen). In the particular case of the treatment of cigarette smoke, the catalytic composition can be in the form of a powder. It can also undergo an adequate shaping, for example, it can be in the form of granules or flakes. In the case of a powder, the particle size of the composition can be between 1 μm and 200 μm. In the case of granules, this size can be between 700 μm and 1500 μm, for the pearls, the size can be between 200 μm and 700 μm and between 100 μm and 1500 μm for the flakes. The catalytic composition can be incorporated by mixing or bonding with the fiber which constitutes the filter of the cigarette (for example cellulose acetate) during the manufacture of the filter, in particular in the case of so-called “Dual filter” or “Triple filter” filters. ". The catalytic composition can also be deposited on the internal part of the paper wrapping the cable constituting the filter ("tipping paper") in the case of a filter of the "Patch filter" type. The catalytic composition may also be introduced into the cavity of a "Cavity filter" type filter. In the case of the use of the catalytic composition of the invention in a cigarette filter, the reduction treatment of the composition can be carried out once it has been incorporated into the filter. The reduction treatment is then carried out according to the methods which have been described above. The amount of catalyst composition used is not critical. It is limited in particular by the dimensions of the filter and the pressure drop due to the presence of the composition in the filter. It is generally at most

350mg per cigarette, preferably it is between 20mg and 100mg per cigarette. The invention therefore relates to a cigarette filter, which contains a composition as described above or obtained by the method detailed above. It will be noted here that the term "cigarette" must be taken in the broad sense to cover any article intended to be smoked and containing tobacco wrapped in a tube for example made from paper or tobacco. This term therefore also applies here to cigars and cigarillos. Finally, the compositions of the invention can also be used in air purification treatments in the case of air containing at least one compound of the type carbon monoxide, ethylene, aldehyde, amino, mercaptan, ozone and, generally, of the type of volatile organic compounds or atmospheric pollutants such as fatty acids, hydrocarbons, in particular aromatic hydrocarbons, and nitrogen oxides (for the oxidation of NO to N0 ₂ ) and of like smelly compounds. Mention may more particularly be made, as compounds of this kind, of ethanethiol, valeric acid and trimethylamine. This treatment is carried out by bringing the air to be treated into contact with a composition as described above or obtained by the process detailed above. The compositions of the invention make it possible to carry out this treatment at ambient temperature. Examples will now be given. In these examples, results are given for the oxidation of CO. These results were obtained by implementing the catalytic CO oxidation test which is described below. The catalytic compound is tested in the form of flakes of 125 to 250 μm which are obtained by tableting, grinding and sieving the powder of catalytic compound. The catalytic compound is placed in the reactor on a frit which acts as a physical support for the powder. In this test, a synthetic mixture containing 1 to 10% vol of CO, 10% vol of C0 ₂ , 10% vol of 0 ₂ , 1.8% vol of H ₂ 0 in N ₂ is passed over the catalyst. The gas mixture circulates continuously in a quartz reactor containing between 25 and 200 mg of catalytic compound with a flow rate of 30 L / h. When the mass of catalytic compound is less than 200 mg, silicon carbide SiC is added so that the sum of the masses of the catalytic compound and SiC is equal to 200 mg. SiC is inert with respect to the CO oxidation reaction and plays here the role of diluent making it possible to ensure the homogeneity of the catalytic bed. The conversion of CO is first of all measured at ambient temperature (T = 20 ° C. in the examples) and it is only when this conversion is not complete at this temperature that it is increased to using an oven from room temperature to 300 ° C with a ramp of 10 ° C / min. The gases leaving the reactor are analyzed by infrared spectroscopy at intervals of approximately 10 s in order to measure the conversion of CO to C0 ₂ . When the CO conversion is not complete at room temperature, the results are expressed as the half-conversion temperature (T50%), the temperature at which 50% of the CO present in the gas stream is converted to C0 ₂ . In the examples which follow, the catalytic compounds were evaluated for the reaction for the oxidation of CO to C0 ₂ under the following conditions: Gas mixture: 3% voiCO, 10% volCO ₂ , 10% volO ₂ , 1, 8% volH ₂ 0 in N ₂ Total flow rate: 30 L h Mass of catalyst: 100 mg WH: 300,000 cc / g _cata / h Conditions B : 3% vol CO - WH = 600,000 cm ³ / q, h Gas mixture: 3% volCO, 1 0% volCO _2l 10% volO ₂ , 1, 8% volH ₂ 0 in N ₂ Total flow: 30 L / h Mass catalyst: 50 mg WH: 600,000 cc / g _ca ta / h Conditions C: 3% vol CO - WH = 900.Q00 cm ^z / g _ra h Gas mixture: 3% volCO, 1 0% volCO ₂ , 10% volO ₂ , 1, 8% volH ₂ 0 in N ₂ Total flow: 30 L / h Mass of catalyst: 33 mg WH: 900,000 cc / g _cat a / h Conditions D: 3% vol CO - WH = 1,200,000 cn ^ Λ h Gas mixture: 3% volCO, 10% volCO ₂ , 10% volO ₂ , 1, 8% volH ₂ 0 in N ₂ Total flow: 30 L / h Mass of catalyst: 25 mg WH: 1,200,000 cc / g _cata / h Conditions E: 3% vol CO - WH = 1.500.000 cm ^ q ^ / h Gas mixture: 3% volCO, 10% volCO ₂ , 10% volO ₂ , 1, 8% volH ₂ 0 in N ₂ Flow total: 30 L / h Catalyst mass: 20 mg WH: 1,500,000 cc / g _cata / h Gas mixture: 3% volCO, 10% volCO ₂ , 10% volO ₂ , 1, 8% volH ₂ O in N ₂ Total flow: 12 L / h Mass of catalyst: 120 mg WH: 150,000 cc / g _ca ta / h

EXAMPLE 1 40 g of a surface titanium oxide powder 75 m ² / g are dispersed with stirring in 250 ml of water. The pH of the suspension is then adjusted to 9 by adding a solution of Na ₂ C0 ₃ 1 M. At the same time 0.8 g of HAuCl ₄ -3H ₂ 0 (Sigma-Aldrich) are dissolved in 250 ml of water. The gold solution is then added in one hour to the suspension of titanium oxide. The pH of the suspension is maintained between 8.7 and 9.3 during the addition of the gold solution by adding a solution of Na ₂ C0 ₃ 1 M. The resulting suspension is kept under stirring 20 minutes before d '' be vacuum filtered. The cake obtained is redispersed in a solution of Na ₂ C0 ₃ at pH 9 whose volume is equivalent to that of the mother liquors eliminated during the first filtration step. The suspension is kept stirring for 20 minutes. This basic washing procedure is repeated 2 more times. The cake obtained is finally redispersed in a volume of water equivalent to the volume of mother liquor eliminated during the first filtration and then filtered under vacuum. The washed cake is lyophilized and then reduced for 2 hours at 170 ° C. with a gas mixture composed of 10 vol% of dihydrogen diluted in argon. The analyzes carried out on the catalyst give the results which appear in Table 1 below. EXAMPLE 2 The catalyst is prepared according to the same protocol as that described in Example 1 except that the titanium oxide powder used develops an area of 105 m ² / g and that the washed cake is air dried for 2 hours at 100 ° C instead of being lyophilized before treatment under dilute hydrogen. The analyzes carried out on the catalyst give the results which appear in Table 1 below. COMPARATIVE EXAMPLE 3 The catalyst is prepared according to the same protocol as that described in Example 1 except that the dried product is not treated under dilute hydrogen. The analyzes carried out on the catalyst give the results which appear in Table 1 below.

EXAMPLE 4 40 g of a surface titanium oxide powder of 105 m ² / g are dispersed with stirring in 250 ml of water. The pH of the suspension is then adjusted to 9 by adding a 1 M NaOH solution. At the same time 0.8 g of HAuCI ₄ .3H ₂ 0 (Sigma-Aldrich) are dissolved in 250 ml of water. The solution is heated to 70 ° C. and then its pH is brought to pH 9 by addition of a 1 M NaOH solution. The gold solution is then added in 30 minutes to the suspension of titanium oxide. The resulting suspension is kept at 70 ° C with stirring for 1 hour before being filtered under vacuum. The cake obtained is redispersed in a NaOH solution at pH 9, the volume of which is equivalent to that of the mother liquors eliminated during the first filtration step. The suspension is kept stirring for 20 minutes. This basic washing procedure is repeated once more. The cake obtained is finally redispersed in a volume of water equivalent to the volume of mother liquor eliminated during the first filtration and then filtered under vacuum. The washed cake is lyophilized and then reduced for 2 hours at 170 ° C. with a gas mixture composed of 10 vol% of dihydrogen diluted in argon. The analyzes carried out on the catalyst give the results which appear in Table 1 below.

EXAMPLE 5 An example is now given of the preparation of a catalyst in the form of granules. 21 g of titanium oxide (Ti0 ₂ ) granules with a specific surface of 90 m ² / g are placed in a column. This column is connected by a circulation system to a reactor (1) containing 125 g of water. At the same time, 0.4 g of HAuCI ₄ , 3H ₂ C> is dissolved in a reactor (2) containing 125 g of water. The gold solution contained in the reactor (2) is heated to 70 ° C and the pH is adjusted to 9 using a solution of Na ₂ C0 ₃ 1 M. The solution contained in the reactor (1) is circulated through the column containing the Ti0 ₂ granules with a flow rate of

10mL / min. Once the circulation has been established between the reactor (1) and the column, the reactor (1) is heated to 70 ° C. and the pH is adjusted to 9 using a solution of Na ₂ C0 ₃ 1 M. The gold solution is introduced with stirring into the reactor (1) in 30 min. The pH is maintained at 9 in the reactor (1) with a solution of Na ₂ C0 ₃ 1 M. The solution is kept stirring 1 h after the addition of the gold solution. The circulation is stopped between the reactor (1) and the column. The mother solution is drawn off, then replaced with 250 g of water (pH adjusted to 9 with 1 M Na ₂ C0 ₃ at room temperature). Circulation is resumed between the reactor (1) and the column for 10 min. This operation is repeated twice before a last wash with 250 g of water. The granules are separated from the washing solution and then lyophilized. They are then reduced for 2 h at 170 ° C. by a gas mixture composed of 10 vol% of dihydrogen diluted in argon. The analyzes carried out on the catalyst give the results which appear in Table 1 below. The two examples which follow relate to a chemical reduction treatment in situ, that is to say in the reaction medium after the phase of deposition of gold in aqueous solution

EXAMPLE 6 21 g of a surface titanium oxide powder 75 m ² / g are dispersed with stirring in a reactor (1) containing 125 g of water. Parallel 0.4g HAuCI ₄ .3H ₂ 0 (Sigma-Aldrich) were dissolved with stirring in a reactor (2) containing 125 g of water. The two reactors are heated to 70 ° C. and, in addition, their pHs are adjusted to 9 using a solution of Na ₂ C0 ₃ 1 M. The gold solution is then added in 30 min to the reactor (1). During the addition of the gold solution, the pH of the reactor (1) is maintained at 9 if necessary by adding a solution of Na ₂ C0 ₃ 1M. The resulting suspension is stirred at 70 ° C for 30 min after the addition of the gold solution. 0.32 g of THPC (tetrakis (hydroxymethyl) phosphonium chloride) at 80% in aqueous solution, Aldrich) previously diluted in 5 ml of water are added dropwise to the reactor (1) in a few minutes. The amount of THPC used corresponds to a THPC / Au molar ratio of 1.35. After this addition, the reactor (1) is kept stirring for 30 min at 70 ° C. Then after cooling, the resulting suspension is centrifuged (10 min at 4500 rpm). The cake obtained is redispersed in a solution of Na ₂ CC> ₃ at pH 9, the volume of which is equivalent to that of the mother liquors eliminated during the first centrifugation. The suspension is stirred for 10 min before a further centrifugation. This procedure is repeated two more times. The cake obtained is finally redispersed in a volume of water equivalent to the volume of mother liquor eliminated during the first centrifugation. The washed cake is dried overnight at 80 ° C. and then calcined for 2 hours at 200 ° C. in air. The analyzes carried out on the catalyst give the results which appear in Table 1 below.

EXAMPLE 7 21 g of titanium oxide (Ti0 ₂ ) granules with a specific surface of 90 m ² / g are placed in a column. This column is connected by a circulation system to a reactor (1) containing 125 g of water. At the same time, 0.4 g of HAuCI ₄ , 3H ₂ 0 is dissolved in a reactor (2) containing 125 g of water. The gold solution contained in the reactor (2) is heated to 70 ° C and the pH is adjusted to 9 using a solution of Na ₂ C0 ₃ 1 M. The solution contained in the reactor (1) is circulated through the column containing the Ti0 ₂ granules with a flow rate of

10mL / min. Once the circulation has been established between the reactor (1) and the column, the reactor (1) is heated to 70 ° C. and the pH is adjusted to 9 using a solution of

Na ₂ C0 ₃ 1M. The gold solution is introduced with stirring into the reactor (1) over 30 min. The pH is maintained at 9 in the reactor (1) with a solution of Na ₂ C0 ₃ 1

M. The solution is stirred for 1 hour after the addition of the gold solution. 0.32 g of THPC (tetrakis (hydroxymethyl) phosphonium) chloride 80% in aqueous solution, AIdrich) previously diluted in 5 ml of water are added dropwise to the reactor (1) in a few minutes. The amount of THPC added corresponds to a THPC / Au molar ratio of 1.35. After this addition, the reactor (1) is kept stirring for 30 min at 70 ° C. then the circulation is stopped between the reactor (1) and the column. The mother solution is drawn off, then replaced with 250 g of water (pH adjusted to 9 with 1 M Na ₂ C0 ₃ at room temperature). Circulation is resumed between the reactor (1) and the column for 10 min. This operation is repeated twice before a last wash with 250 g of water. The granules are separated from the washing solution and then dried at 80 ° C overnight and are finally calcined in air at 200 ° C for 2 h. The analyzes carried out on the catalyst give the results which appear in Table 1 below. EXAMPLE 8 40 g of an iron oxide powder (Fe ₂ 0 ₃ ) with a surface area of 225 m ² / g are dispersed with stirring in 250 ml of water. The pH of the suspension is then adjusted to 9 by adding a solution of Na ₂ C0 ₃ 1 M. At the same time 0.8 g of HAuCI ₄ , 3H ₂ 0 (Sigma-Aldrich) are dissolved in 250 ml of water. The gold solution is then added in one hour to the suspension of iron oxide. The pH of the suspension is kept at 9 during the addition of the gold solution by adding a solution of Na ₂ CO ₃ 1 IN I. The resulting suspension is kept stirring 20 minutes before being filtered under vacuum . The cake obtained is redispersed in a solution of Na ₂ C0 ₃ at pH 9 whose volume is equivalent to that of the mother liquors eliminated during the first filtration step. The suspension is stirred for 20 minutes. This basic washing procedure is repeated 2 more times. The cake obtained is finally redispersed in a volume of water equivalent to the volume of mother liquor eliminated during the first filtration and then filtered under vacuum. The washed cake is lyophilized and then re used for 2 hours at 170 ° C. with a gas mixture composed of 10 vol% of dihydrogen diluted in argon. The analyzes carried out on the catalyst give the results which appear in Table 1 below.

Table 1

The results obtained with the catalysts of the examples for the conversion of CO (3% vol of CO) are given in table 2 below.

Table 2

Ta: room temperature = 20 ° C

It can be seen that the catalyst of example 3 only converts 50% of the CO and this at a temperature above 35 ° C. whereas that of example 1 oxidizes CO to CO ₂ at 100% and at room temperature for WH of at least up to 1,500,000 cm ³ / g _cata / h. The results are now given for the oxidation of low CO _{2 to} CO ₂ contents using the test described above. The oxidation reaction is carried out at low temperature, -10 ° C, under the following conditions:

Conditions G: 50vpm CO - WH = 900,000 cm ³ / q _r . _a t _a / h Gas mixture: 50vpmCO, 20% volO ₂ in N ₂ Total flow: 30 L / h Catalyst mass: 33 mg WH: 900,000 cm ³ / g _cata / h H conditions: 50vpm CO - WH = 3,000. 000 cm ³ / g _πa t _a / h Gas mixture: 50vpmCO, 20% volO ₂ in N ₂ Total flow: 30 L / h Catalyst mass: 10 mg WH: 3,000,000 cm ³ / g _ca ta / h Conditions I : 50vpm CO - WH = 6,000,000 cm ³ / q ^ h Gas mixture: 50vpmCO, 20% volO ₂ in N ₂ Total flow: 30 L / h Mass of catalyst: 5 mg WH: 6,000,000 cm ³ / g _cata / h

The results obtained with the catalyst according to Example 1 for the conversion of 50vpm CO at low temperature are given in table 3 below.

Table 3

The results are now given for the oxidation of low CO ₂ CO ₂ to very high WH contents using the following test. 2 gas bags of 30L are connected respectively to the inlet and the outlet of a pump via rubber tubing with an internal diameter of 8 mm. Between the outlet of the pump and the gas bag, the catalytic compound is placed in the rubber tubing in the form of flakes of 125 to 250 μm which are obtained by tableting, grinding and sieving the powder of catalytic compound. The catalytic compound is immobilized by two pads of quartz wool. While the gas bag: connected to the outlet of the pump is empty, one creates in the one connected to its inlet an atmosphere containing 100vpm of CO in the air. At t = 0, the pump is started with a flow rate of δOL / min and the content of the gas bag connected to the inlet is transferred via the catalytic bed into the initially empty gas bag. 0 The CO content of the gas bag is then measured using a Draeger CO reagent tube. This test was carried out at room temperature under the following conditions:

Conditions J: lOOvpm CO - WH = 10,000,000 cm ^ / h δ Gas mixture: 10OvpmCO, 20% vol ₂ in N ₂ Total flow: δO L / min Mass of catalyst: 300 mg WH: 10,000,000 cc / gcata / h K conditions: 100vpm CO - WH = 1δ.000.0OQ cm ^ g ^ / h0 Gas mixture: 100vpmCO, 20% vol0 ₂ in N ₂ Total flow: δO Umin Mass of catalyst: 200 mg WH: 1 δ.000 000 cc / g _cata / h Conditions L .: 100vpm CO - WH = 30,000.00Q cm ^ q ^ / hδ Gas mixture: 100vpmCO, 20% vol0 ₂ in N ₂ Total flow: δO L / min Mass of catalyst: 100 mg WH: 30,000 .000 cm ³ / g _cat / h 0 Table 4 below gives the results obtained with the catalyst according to example 1 for the conversion of "1 OOvpm CO at room temperature. Table 4

δ The results in Tables 3 and 4 show that the catalyst according to the invention is capable of oxidizing CO to C0 ₂ at low CO contents and at very high WH.

The following example concerns the conversion of ozone (0 ₃ ) to oxygen 0 (0 ₂ ) by a decomposition reaction. This result was obtained by implementing the catalytic test which is described below. In this test, there is a closed polymer enclosure with a volume of δ, 3 L equipped with several orifices allowing the introduction of ozone, that of the catalyst and the sampling of the gas phase. an ozone generator which is regulated to supply a gas flow containing 12δg / m ³ of ozone in the air. A 100 ml gas bulb is filled with this gas flow and then 17 ml of this gas bulb is withdrawn using a gas syringe which are then injected into the closed enclosure to constitute an atmosphere containing 200 vpm of ozone clans of air.0 In a second step, 200 mg of catalytic compound in powder form are introduced into the enclosure using a device avoiding any contact with the atmosphere outside the enclosure. The origin of the times is determined by the introduction of the catalyst into the enclosure. The gas phase is homogenized using a recirculation pump, the flow rate of which is set at δ 13.5 L / min. The disappearance of the ozone present in the enclosure is monitored over time using Draeger reagent tubes for ozone. The conversion to molecule (M) of ozone to be oxidized is calculated as follows thanks to the concentrations determined with the Draeger reagent tubes: Conv (M) = [concιvι (t) -

EXAMPLE 9 The catalyst according to example 1 is used in the test which has been described above. The results obtained at room temperature for the conversion of 200 vpm of ozone are given in table δ below.

Table δ

These data show that 200 vpm of ozone is broken down into oxygen in less than 10 min at room temperature. 0 EXAMPLE 10 An example is now given of the preparation of a catalyst in the form of granules containing, in addition, silver gold. 40 g of granules of titanium oxide (Ti0 ₂ ) with a specific surface of 90 m ² / g are impregnated with 25.8 ml of an aqueous solution at 6.7 x 10 ^"2 M of AgN0 ₃ .δ The paste is then dried in an oven overnight at 120 ° C. and then calcined in air at 500 ° C. for 2 hours. Then, according to Example 5, the gold is deposited on 21 g of granules thus obtained. The analyzes carried out on the catalyst give the results which are shown in Table 6 below.

Table 6

δ The results obtained with the catalysts of the examples for the conversion of 3% vol of CO are given in table 7 below. Table 7

Ta: room temperature = 20 ° C

It can be seen that the catalyst of Example 10 has the particularity of reaching its maximum level of CO conversion more quickly than that of Example 5.

The examples which follow relate to the oxidation of different volatile organic compounds (VOCs) such as acetaldehyde (CH ₃ CHO), methanol (CH ₃ OH), ethanethiol (CH ₃ CH ₂ SH), valeric acid (CH ₃ (CH ₂ ) ₃ C0 ₂ H) and trimethylamine ((CH ₃ ) ₃ N). These results were obtained by implementing the catalytic oxidation test which is described below. In this test, there is a closed polymer enclosure with a volume of δ, 3 L equipped with several orifices allowing the introduction of the molecule to be oxidized, that of the catalyst and the sampling of the gas phase. First, a volume of liquid molecule is introduced using a syringe into the closed enclosure. The volumes injected are -2, δ, 2, 3, δ, δ and 6 μL respectively for acetaldehyde, methanol, ethanethiol, valeric acid and trimethylamine (in δ0% aqueous solution). At room temperature (T = 20 to 30 ° C), all of the injected liquid is vaporized in the enclosure to create an atmosphere consisting of 200 vpm of molecule to be oxidized in air. In a second step, 200 mg of catalytic compound in powder form are introduced into the enclosure using a device avoiding any contact with the atmosphere outside the enclosure. The origin of the times is determined by the introduction of the catalyst into the enclosure. The used gauze phase is homogenized using a recirculation pump, the flow rate of which is set at 13 δ L / min. To monitor the oxidation reaction, the gaseous phase of the enclosure is removed through a septum and then analyzed by gas chromatography. H ₂ 0, CO, CO2, CH ₃ CHO, CH ₃ OH and CH ₃ CH ₂ SH are analyzed on a Hewlett Packard Micro GC HP M200 chromatograph using the sampling device with which this analyzer is equipped. Valeric acid (CH ₃ (CH ₂ ) ₃ C0 ₂ H) and trimethylamine ((CH ₃ ) ₃ N) are analyzed on a Varian 3200 chromatograph using a syringe for sampling the gaseous phase of the enclosure closed. The gas phase is analyzed before introduction of the catalyst and then after introduction at regular intervals between a few minutes and a few hours depending on the tests. The conversion to the molecule to be oxidized (M) is calculated as follows using the surfaces of the chromatograms: Conv (M) = [surface _M (t) - surfaceιvι (t = 0)] / surfaceivι (t = 0)

For each molecule to be oxidized studied, a blank test without catalyst is carried out under the same conditions and for which no change in the concentration of molecule to be oxidized is observed over time.

EXAMPLE 11 The catalyst according to Example 1 is used in the test which has been described above. The results obtained in table 8 below are given at room temperature for the conversion of 200 vpm of acetaldehyde.

Table 8

These data show that 200 vpm of acetaldehyde are completely converted in less than 1 hour of reaction. 2δ Chromatographic analysis makes it possible to verify that the quantities of C0 ₂ and H ₂ 0 produced indeed correspond to a total oxidation reaction which leads to the elimination of acetaldehyde according to: CH ₃ CHO + δ / 20 ₂ → 2C0 ₂ + 2H ₂ 0

EXAMPLE 12 The catalyst according to example 1 is used in the test which has been described above. The results given in table 9 below are given at room temperature for the conversion of 200 vpm of methanol.

Table 9

These data show that 200 vpm of methanol is more than 90% converted in 20 hours of reaction. Chromatographic analysis makes it possible to verify that the quantities of C0 ₂ and H ₂ 0 produced correspond well to a total oxidation reaction which leads to the elimination of methanol according to: CH ₃ OH + 3/20 ₂ → C0 ₂ + 2H ₂ 0

EXAMPLE 13 The catalyst according to example 1 is used in the test which has been described above. The results given in table 10 below are given at room temperature for the conversion of 200 vpm of ethanethiol. Table 10

These data show that 200 vpm of ethanethiol are more than 70% converted after 1 hour of reaction. Analysis of the gas phase with a Draeger sulfur dioxide S0 ₂ tube at t = 50 min shows that more than 100 vpm of S0 ₂ are present in the enclosure. The evolution of the concentrations of C0 ₂ , H ₂ 0 and the presence of S0 ₂ makes it possible to attribute the disappearance of ethanethiol to its partial oxidation.

EXAMPLE 14 The catalyst according to example 1 is used in the test which has been described above. The results given at room temperature for the conversion of valeric acid are given in table 11 below.

Table 11

These data show that each of the 200 vpm injections of valeric acid are converted in less than 60 minutes. Analysis of the gas phase shows that on balance 400 vpm of valeric acid are converted and that 200 vpm of C0 ₂ and 1000 vpm of H ₂ 0 are formed. The evolution of the concentrations of C0 ₂ , H ₂ 0 and valeric acid makes it possible to attribute the disappearance of valeric acid to its partial oxidation.

EXAMPLE 15 The catalyst according to Example 1 is used in the test which has been described above. The results given in table 12 below are given at room temperature for the conversion of 200 vpm of trimethylamine.

Table 12

These data show that 200 vpm of trimethylamine are converted to more than 80%> after 30 min of reaction. Analysis of the gas phase shows that δO vpm of C0 ₂ and 1000 vpm of H ₂ 0 are also formed. The evolution of the concentrations of C0 ₂ , H ₂ O and valeric acid makes it possible to attribute the disappearance of the trimethylamine to its partial oxidation.

Claims

δ 1- Composition based on gold on a support based on at least one reducible oxide, characterized in that its halogen content expressed by the halogen / gold molar ratio is at most 0.0δ, in that that the gold occurs in the form of particles with a size of at most 10 nm and in that it has undergone a reduction treatment, being excluded compositions with supports0 in which the only or the only reducible oxides are l cerium oxide, cerium oxide in combination with zirconium oxide, cerium oxide in combination with praseodymium oxide, cerium oxide in combination with titanium oxide or l tin oxide in an atomic proportion Ti / Ce or Sn / Ce of less than δ0% .5 2- Composition according to claim 1, characterized in that the support is based on at least one oxide chosen from oxide titanium, manganese oxide, ferric oxide or tin oxide. 0 3- Composition according to claim 1 or 2, characterized in that its halogen content is at most 0.04 and more particularly at most 0.025.

4- Composition according to one of the preceding claims, characterized in that the gold is in the form of particles of size at most 3 nm. 5- Composition according to one of the preceding claims, characterized in that l halogen is chlorine.

6- Composition according to one of the preceding claims, characterized in that the gold content is at most δ%, more particularly at most 1%.

7- Composition according to one of the preceding claims, characterized in that it also comprises at least one other metallic element chosen from silver, platinum, palladium and copper.δ 8- Composition according to claim 7, characterized in that the other aforementioned metallic element is present in an amount of at most 400%, more particularly between δ% and δ0%, relative to the gold. 9- Process for preparing a composition according to one of the preceding claims, characterized in that it comprises the following stages:

a compound based on at least one reducible oxide and a compound based on a gold halide and, where appropriate, a compound based on silver, platinum, palladium or copper, are brought into contact, by forming a suspension of these compounds, the pH of the medium thus formed being fixed at a value of at least 8;

- The solid is separated from the reaction medium; - washing the solid with a basic solution; the method further comprising a reduction treatment either before or after the above-mentioned washing step.

10- A method according to claim 9, characterized in that the pH of the medium formed is maintained at the value of at least 8 during the formation of the suspension of the compound based on at least one reducible oxide and the compound based gold halide, and optionally the compound based on silver, platinum, palladium or copper, by addition of a basic compound.

11- Method according to claim 9 or 10, characterized in that the solid obtained is washed with a basic solution having a pH of at least 8, preferably at least 9.

12- A method of preparing a composition according to one of claims 1 to 8, characterized in that it comprises the following steps:

- Gold and, where appropriate, silver, platinum, palladium or copper are deposited on a compound based on at least one oxide which can be reduced by impregnation or by ion exchange; - washing the solid from the previous step with a basic solution having a pH of at least 10; the method further comprising a reduction treatment either before or after the above-mentioned washing step.

13- Method according to one of claims 9 to 12, characterized in that the reduction treatment is carried out with a reducing gas at a temperature of at most 200 ° C, preferably at most 180 ° C. 14- Method according to one of claims 9 to 13, characterized in that the solid obtained after the reduction treatment is subjected to calcination at a temperature of at most 250 ° C.

15- A process for the oxidation of carbon monoxide, characterized in that a composition according to one of claims 1 to 8 or a composition obtained by the process according to one of claims 9 to 6 is used as catalyst 14.

16- Method according to claim 15, characterized in that it is implemented for the treatment of cigarette smoke, in the reaction of conversion of gas to water (water gas shift), in the treatment of gases reforming (PROX).

17- Process for purifying the air, this air containing at least one compound of the carbon monoxide, ethylene, aldehyde, amino, mercaptan, ozone type, of the type of volatile organic compounds or of air pollutants and of the type of smelly compounds, characterized in that the air is brought into contact with a composition according to one of claims 1 to 8 or a composition obtained by the process according to one of claims 9 to 14.

18- Cigarette filter, characterized in that it contains a composition according to one of claims 1 to 8 or a composition obtained by the process according to one of claims 9 to 14.