CATALYTIC BED COMPRISING A PARTICULAR PHOTOCATALYTIC CATALYST

Abstract

The present invention relates to a catalytic bed comprising a particular photocatalytic catalyst. The bed comprises structuring particlesa made of inorganic material, b, combined with at least one semiconductor material, a, with photocatalytic properties, the combination being produced by mixing structuring particles made of inorganic material, b, with the semiconductor material, a, in the form of particles, —and/or by chemical or physicochemical deposition of the semiconductor material, a, on the structuring particles made of inorganic material, b, the structuring particles, b, being of substantially spherical shape and of mean diameter between 22 nm and 8.0 μm.

Claims

1. A catalytic bed comprising a particulate photocatalytic catalyst, characterized in that said bed comprises structuring particles made of mineral material b which are combined with at least one semiconductor material a having photocatalytic properties, the combination being produced by mixing the structuring particles made of mineral material b with the semiconductor material a in the form of particles, and/or by chemical or physicochemical deposition of the semiconductor material a on the structuring particles made of mineral material b, the structuring particles b being essentially spherical in shape and having a mean diameter of between 22 nm and 8.0 μm, and preferably between 30 nm and 7.5 μm.

2. The catalytic bed as claimed in claim 1, characterized in that all of the particles within the bed are arranged in a disorganized manner.

3. The catalytic bed as claimed in claim 1, characterized in that, when the bed contains the semiconductor material a in the form of particles, said particles a exhibit a mean dimension of at most 100 nm, in particular of at most 50 nm, and of at least 5 nm, preferably of between 10 and 30 nm.

4. The catalytic bed as claimed in claim 1, characterized in that it exhibits a void ratio, equal to the ratio of the void volume in the photocatalytic bed to the total volume of the photocatalytic bed composed of voids and of particles, of at least 40%, preferably of at most 80% and in particular of between 40% and 70%.

5. The catalytic bed as claimed in claim 1, characterized in that it exhibits, in particular in the case of a chemical or physicochemical deposition of the semiconductor material a on the structuring particles made of mineral material b, a dilution ratio, equal to the ratio of the volume occupied by the structuring particles made of mineral material b to the volume occupied by the sum of the semiconductor material(s) a, a′ and of the structuring particles made of mineral material b, of at most 80%, in particular of between 5% and 70%, preferably of between 10% and 50%.

6. The catalytic bed as claimed in claim 1, characterized in that it comprises at least two distinct semiconductor materials, a first material a and a second material a′, and in that it is produced by mixing structuring particles made of mineral material b with the semiconductor material(s) each in the form of particles of the first material a and of particles of the second material a′, and/or by chemical or physicochemical deposition of the semiconductor materials a, a′ on the support particles b, either by deposition both of the first semiconductor material a and of the second semiconductor material a′ on the structuring particles b, or by deposition of the first semiconductor material a on a first part of the structuring particles b and of the second semiconductor material a′ on a second part of the structuring particles b.

7. The catalytic bed as claimed in claim 1, characterized in that the structuring particles made of mineral material b are made of metal oxide(s), in particular made of oxides of metals of groups II la and IVa of the periodic table, and preferably chosen from aluminum oxide, silicon oxide, a mixture of aluminum and silica oxides.

8. The catalytic bed as claimed in one of the preceding claims claim 1, characterized in that the/at least one of the semiconductor material(s) a, a′ comprises at least one of the following metal oxides: titanium oxide, tungsten oxide, cerium oxide, bismuth oxide, zinc oxide, copper oxide, vanadium oxide, iron oxide, cadmium oxide, and preferably is chosen from TiO.sub.2, Bi.sub.2O.sub.3, CdO, Ce.sub.2O.sub.3, CeO.sub.2, CeAlO.sub.3, CuO, Fe.sub.2O.sub.3, FeTiO.sub.3, ZnFe.sub.2O.sub.3, V.sub.2O.sub.5, ZnO, WO.sub.3 and ZnFe.sub.2O.sub.4, alone or as a mixture.

9. The catalytic bed as claimed in claim 1, characterized in that the/at least one of the semiconductor material(s) a, a′ is doped with one or more ions chosen from metal ions, in particular ions of V, Ni, Cr, Mo, Fe, Sn, Mn, Co, Re, Nb, Sb, La, Ce, Ta, Ti, or from non-metal ions, in particular C, N, S, F, P, or by a mixture of metal and non-metal ions.

10. The catalytic bed as claimed in claim 1, characterized in that the/at least one of the semiconductor material(s) a, a′ also comprises one or more element(s) in the metallic state chosen from an element of groups IVb, Vb, VIb, VIIb, VIIIb, Ib, IIb, IIIa, IVa and Va of the periodic table of the elements and in direct contact with said semiconductor material, preferably from platinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper, rhenium or rhodium.

11. A process for obtaining the catalytic bed as claimed in claim 1, characterized in that, on the one hand, the structuring particles of mineral material b and, on the other hand, the particles of semiconductor material a are mixed so as to produce a homogeneous distribution of the two types of particles within the bed.

12. A process for obtaining the catalytic bed as claimed in claim 1, characterized in that the or at least one of the semiconductor material(s) a, a′ is deposited on the structuring particles of mineral material b by impregnation of said structuring particles with a solution of at least one precursor of the semiconductor material, by ion exchange, by the electrochemical route of the type in particular with molten salts, then drying and optional calcination, by chemical vapor deposition, by spray drying or by atomic layer deposition.

13. A reactor (1) for the photocatalytic treatment of a feedstock in gaseous or liquid form and comprising at least one photocatalytic bed (2) as claimed in claim 1 and which is mounted in a fixed manner in said reactor.

14. A process for the photocatalytic treatment of a feedstock (7) in gaseous and/or liquid form, characterized in that: at least one photocatalytic bed (2) as claimed in claim 1 is arranged in a fixed manner in a reactor (1), said feedstock (7) is brought into contact in the reactor with the catalytic bed (2), and the photocatalytic bed (2), during the contacting operation, is irradiated with at least one irradiation source (6) emitting in the UVA-A range and/or the UV-B range and/or the visible range, in particular in the wavelength range of between 220 and 800 nm, preferably in the range of between 300 and 750 nm.

15. The process as claimed in claim 1, characterized in that the photocatalytic treatment is: a photo-oxidation of components present in a liquid or gaseous feedstock, in particular for the purposes of depollution/decontamination of the feedstock, or a photocatalytic reduction of the CO.sub.2 of a liquid or gaseous feedstock, or a photolysis of the water of a liquid or gaseous feedstock, for the purposes of producing H.sub.2.

Description

LIST OF THE FIGURES

[0046] FIG. 1 represents a diagrammatic re-emission pattern of an incident beam on particles according to a Rayleigh-type scattering and according to a Mie-type scattering.

[0047] FIG. 2 represents a transmission electron microscopy (TEM) image of the semiconductor particles made of titanium oxide used according to an embodiment of the photocatalytic material according to the invention.

[0048] FIG. 3 represents a scanning electron microscopy (SEM) image of the structuring particles made of silicon oxide used according to an embodiment of the photocatalytic material according to the invention.

[0049] FIG. 4 represents a simplified diagram of an installation targeted at measuring the performance qualities of a photocatalytic material according to the invention.

[0050] FIG. 5 represents a graph quantifying photocatalytic performance qualities of two examples of material according to the invention, with, on the abscissa, the fraction by volume of semiconductor made of titanium oxide of the material of the invention comprising this semiconductor and structuring particles made of silicon oxide and, on the ordinate, the overall consumption of electrons for 20 hours per square meter, expressed in μmol/m.sup.2.

DESCRIPTION OF THE EMBODIMENTS

[0051] The invention relates to the composition of a photocatalytic bed with mineral structuring particles, in this instance solid ones, which are calibrated according to the wavelength of the radiation emitted by a light source in order to activate a semiconductor material, so that the radiation scatters largely preferentially in the direction of the radiation incident to the surface of these spheres by making use of Mie scattering.

[0052] Thus, FIG. 1 diagrammatically represents simply the phenomenon of Mie scattering mentioned above: on the left is symbolically represented a light source S emitting radiation in a given wavelength λ. A spherical particle P1, the diameter of which is not calibrated according to the invention, and which is less than 0.1 λ, will fairly evenly re-emit the incident radiation in all directions; this is Rayleigh scattering. On the other hand, a particle P2, the diameter of which is calibrated to be between 0.1 λ and 10 λ, will re-emit the radiation in a favored manner along the direction of the incident radiation; this is Mie scattering. This is what the invention uses, so that the calibrated particles “lead” more radiation into the depth of the catalytic bed, that it facilitates its propagation, and that the semiconductor material is thus made better use of.

[0053] The semiconductor material combined with these particles then experiences an astonishing increase in its photocatalytic activity. This activity can be made use of in all the known fields of activity of photocatalysis of liquid and/or gaseous fluids. It can be the reduction of CO.sub.2, the photocatalytic production of H.sub.2 by photoconversion of water (which is also denoted under the term of “water-splitting”), or also the photocatalytic decontamination of air (conversion of VOCs) or of water.

[0054] The invention will be illustrated below by nonlimiting examples, using different photocatalytic materials and different structuring particles:

Photocatalytic Material

[0055] The photocatalytic material al is titanium oxide: it is TiO.sub.2 available under the trade name Aeroxide® P25 from Aldrich, with a purity of 99.5%. The titanium oxide is in the form of fine particles. Its particle size, measured by transmission electron microscopy (TEM), is 21 nm. Its specific surface, measured by the BET method, is 52 m.sup.2/g. BET is an abbreviated term: it is the Brunauer-Emmett-Teller method as defined in S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60 (2), pp 309-319.

[0056] Crystallographically, this titanium oxide is in the form of a mixture of rutile and anatase.

[0057] FIG. 2 is a representation obtained by TEM of these titanium oxide particles: it is seen that they are of irregular shape and that they tend to agglomerate. [0058] The photocatalytic material a2 is titanium oxide with the addition of platinum metal particles prepared by photodeposition in the following way:

[0059] 0.0712 g of H.sub.2PtCl.sub.6.6H.sub.2O (37.5% by weight of metal) is introduced into 500 ml of distilled water. ml of this solution are withdrawn and inserted into a jacketed glass reactor. 3 ml of methanol, followed by 250 mg of TiO.sub.2 of the al type (Aeroxide® P25, Aldrich™, purity >99.5%), are then added with stirring to form a suspension.

[0060] The mixture is then left with stirring and under UV radiation for two hours. The lamp used to supply the UV radiation is a 125 W HPK™ mercury vapor lamp. The mixture is subsequently centrifuged for 10 minutes at 3000 revolutions per minute in order to recover the solid. Two washing operations with water are subsequently carried out, each of the washing operations being followed by a centrifugation. The recovered powder is finally placed in an oven at 70° C. for 24 hours.

[0061] The photocatalytic material a2 is then obtained. The content of Pt element is measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) at 0.99% by weight. [0062] The photocatalytic material a3 is a commercial semiconductor based on WO.sub.3 (available from Sigma-Aldrich, exhibiting a particle size of less than 100 nm). The specific surface, measured by the BET method, is equal to 20 m.sup.2/g. The photocatalytic material particle size, measured by X-ray diffractometry (Debye-Scherrer method), is 50±5 nm. [0063] The photocatalytic material a4 is a mixture of titanium and copper oxides, with particles of platinum Cu.sub.2O/Pt/TiO.sub.2. It is prepared in the following way:

[0064] A solution of Cu(NO.sub.3).sub.2 is prepared by dissolving 0.125 g of Cu(NO.sub.3).sub.2.3H.sub.2O (Sigma-Aldrich™, 98%) in 50 ml of a 50/50 isopropanol/H.sub.2O mixture, i.e. a concentration of Cu.sub.2+ of 10.4 mmol/l.

[0065] The following were introduced into the reactor: 0.20 g of the photocatalytic material a2, 25 ml of distilled water and finally 25 ml of isopropanol. The system is purged in the dark under a stream of argon (100 ml/min) for 2 h. The reactor is thermostatically controlled at 25° C. throughout the synthesis.

[0066] The stream of argon is subsequently slowed down to 30 ml/min and the irradiation of the reaction mixture starts. The lamp used to provide the UV radiation is a 125 W HPK™ mercury vapor lamp. Then, the 50 ml of copper nitrate solution are added to the mixture. The mixture is left stirring and under irradiation for 10 hours. The mixture is subsequently centrifuged for 10 minutes at 3000 revolutions per minute in order to recover the solid. Two washing operations with water are subsequently carried out, each of the washing operations being followed by a centrifugation. The recovered powder is finally placed in an oven at 70° C. for 24 hours.

[0067] The photocatalytic material a4, Cu.sub.2O/Pt/TiO.sub.2, is then obtained. The content of Cu element is measured by ICP-AES at 2.2% by weight. By XPS (X-Ray Photoelectron Spectrometry) measurement, and copper oxide phases at 67% of Cu.sub.2O and 33% of CuO.

Structuring Particles

[0068] The structuring particles b1 chosen in some of the following examples are spherical particles made of silicon oxide based on commercial SiO.sub.2, which can be obtained from Alfa Aesar (CAS: 7631-86-9): these are beads with a purity of greater than 99.9%, and the mean diameter of which, measured by laser particle size analysis, is 0.4 μm.

[0069] FIG. 3 is a representation obtained by SEM of these beads, which are actually seen to be very homogeneous in their size and their shape. [0070] The structuring particles b2 chosen in other examples are particles made of silicon oxide based on commercial SiO.sub.2, which can be obtained from Sigma-Aldrich, under the commercial reference Davisil Grade 710, 10-14 μm: these are beads with a purity of greater than 99%, and the mean dimension of which, measured by laser particle size analysis, is 12.7 μm (distribution by volume).

[0071] The semiconductor particles a1 to a4 and the structuring particles b1 (SiO.sub.2 powder) or b2 (SiO.sub.2 powder with a greater particle size than that of b1) are mechanically mixed with a dilution ratio varying from 0% to 75% by volume, so as to obtain a homogeneous distribution of the two types of particles in the material. It is recalled that, within the meaning of the present invention, the “dilution ratio” is equal to the ratio of the volume occupied by the structuring particles made of mineral material to the volume occupied by the sum of the semiconductor material(s) and of the structuring particles.

[0072] Subsequently, as represented in FIG. 4, each sample 3 of photocatalytic material of each example is subjected to a test of photocatalytic reduction of CO.sub.2 in the gas phase in the following way: Use is made of a reactor 1, which operates continuously, with a fixed bed 2 arranged horizontally in its cavity, which bed comprises a sintered material 4 on which the sample 3 is placed. The reactor 1 exhibits, in its upper wall, an optical window made of quartz facing which is found the sample 3. Above the reactor, and facing the window 5, is arranged a source of UV-visible irradiation 6.

[0073] In operation, the reactor 1 is fed via an inlet in the top part with a stream 7 of gaseous CO.sub.2, which is bubbled beforehand into a container/saturator filled with water 8. The stream 7 passes through the sample 3 and is then discharged via an outlet in the bottom part in the form of a stream 9 which is analyzed in-line by a gas analyzer 10 of micro gas chromatograph type.

[0074] The UV-visible irradiation source 6 is a xenon lamp, available from Asahi under the trade name MAX 303.

[0075] The tests are carried out on samples 3 amounting to between 45 and 70 mg, their weight varying according to their chosen dilution ratio, the thickness of the catalytic bed 2, thus that of the sample 3, remaining fixed and equal to 0.3 mm.

[0076] The operating conditions are as follows: [0077] ambient temperature [0078] atmospheric pressure [0079] flow rate 7 of CO.sub.2 passing through the water saturator 8 of 18 ml/h [0080] duration of the test for each sample: 20 h [0081] irradiation power of the xenon lamp 6: kept constant at 80 W/m.sup.2, measured for a wavelength range of between 315 and 400 nm.

[0082] The targeted conversion of the CO.sub.2 corresponds to the following reaction:

CO.sub.2+H.sub.2O+hv.fwdarw.O.sub.2+H.sub.2, CO, CH.sub.4, C.sub.2H.sub.6

[0083] The measurement of the photocatalytic performance qualities of the samples is carried out by micro chromatography with the device 10, the production of H.sub.2, of CH.sub.4 and of CO which result from the reduction of CO.sub.2 and of H.sub.2 O being monitored by an analysis every 6 minutes. Products of the reduction of CO.sub.2 are identified, such as CO, methane or also ethane. The mean photocatalytic activities are expressed in μmol of photogenerated electrons which are consumed by the reaction over the duration of the test and per square meter of irradiated catalyst surface area.

EXAMPLES

[0084] All of the examples carried out and of the results appear in table 1 below:

TABLE-US-00001 TABLE 1 Fraction by Photocatalytic volume of the activity over 20 Catalytic semiconductor Dilution test hours Example bed material a1-a4 ratio (mmol/m.sup.2) 1 a1 1  0% 6 (comparative) (without solid b1) 2 a1 + b1 0.75 of solid a1 + 25% 27 0.25 of solid b1 3 a1 + b2 0.75 of solid a1 + 25% 5.6 0.25 of solid b2 4 a2 1  0% 65 (comparative) (without solid b1) 5 a2 + b1 0.75 of solid a2 + 25% 262 0.25 of solid b1 6 a3 1  0% 2.3 (comparative) (without solid B) 7 a3 + b1 0.75 of solid a3 + 25% 10 0.25 of solid b1 8 a4 1  0% 191 (comparative) (without solid b1) 9 a4 + b1 0.75 of solid a4 + 25% 765 0.25 of solid b1

[0085] From this table, it is found that the photocatalytic activity of the “mixed” material combining the semiconductor material with structuring particles according to the invention is very markedly greater than that of a material consisting solely of the semiconductor material responsible for the photocatalytic activity of the material:

[0086] If the results of example 1 (comparative) and of example 2 are compared, it is seen that, with 25% less semiconductor material (example 2), the photocatalytic activity jumps, being multiplied by 4.5. Starting from another semiconductor (materials a2, a3, a4), a photocatalytic activity at the “start” is higher for a material 100% made of semiconductor, and the invention still manages to multiply it by a factor of at least 4 by combining it with structuring particles: example 9 thus achieves an impressive level of photocatalytic activity.

[0087] FIG. 5 represents, in the form of a graph, the results of examples 2 and 3. The fraction by volume of the particles made of TiO.sub.2 is represented on the abscissa and the overall consumption of electrons over 20 h per square meter is represented on the ordinate. From this figure, it is seen that example 3 with the structuring particles b2 of too great a size gives results (the diamonds on the graph) which are much poorer than with example 2 using the structuring particles b1 (the circles on the graph), the size of which was calibrated to favor the Mie scattering.

[0088] This calibrating of the structuring particles is simple to choose and to obtain, and markedly more simple than to have to refine other parameters which are more complex to control of the macro- or microporosity of the material type.

[0089] It is seen that the invention is very flexible in its implementation: depending on the desired level of performance, depending on the items of equipment and the reactor chosen, it will be possible to adapt the composition of the material according to the invention by varying the choice of the materials, the dilution ratio and the way in which the mixing between the two materials will be carried out (mechanical mixing, chemical or physicochemical integration, and the like).