EP3206788A1

EP3206788A1 - Photocatalytic carbon dioxide reduction method using a composite photocatalyst

Info

Publication number: EP3206788A1
Application number: EP15775204.9A
Authority: EP
Inventors: Dina LOFFICIAL; Antoine Fecant; Denis Uzio; Eric Puzenat; Christophe Geantet
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Claude Bernard Lyon 1 UCBL; IFP Energies Nouvelles IFPEN
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Claude Bernard Lyon 1 UCBL; IFP Energies Nouvelles IFPEN
Priority date: 2014-10-14
Filing date: 2015-10-06
Publication date: 2017-08-23
Also published as: WO2016058862A1; FR3026965B1; FR3026965A1

Abstract

The invention relates to a photocatalytic carbon dioxide reduction method carried out in liquid and/or gas phase under irradiation, using a photocatalyst containing a first semiconductor SC1, particles comprising one or more metallic-state elements M, and a second semiconductor SC2, wherein the method is carried out by contacting a feedstock containing the CO₂ and at least one sacrificial compound with the photocatalyst, then irradiating the photocatalyst such that the CO₂ is reduced, and oxidising the sacrificial compound in order to produce an effluent containing at least in part C1 or above carbon molecules other than CO₂.

Description

METHOD FOR PHOTOCATALYTIC REDUCTION OF CARBON DIOXIDE USING COMPOSITE PHOTOCATALYST

The field of the invention is that of the photocatalytic reduction of carbon dioxide (CO ₂ ) under irradiation by the use of a particular photocatalyst, preferably containing a supported core-layer type architecture.

Fossil fuels, such as coal, oil and natural gas, are the world's leading conventional sources of energy because of their availability, stability and high energy density. However, combustion produces carbon dioxide emissions which are considered to be the main cause of global warming. Thus, there is a growing need to mitigate C0 ₂ emissions, either by capturing it or by transforming it.

Although carbon capture and sequestration (CCS) are generally regarded as an effective method for reducing C0 ₂ emissions, other strategies should be considered, including C0 ₂ conversion strategies into products of economic value, such as fuels and industrial chemicals.

Such a strategy is the reduction of carbon dioxide into valuable products. The reduction of carbon dioxide can be carried out biologically, thermally, electrochemically or photocatalytically.

Among these options, photocatalytic C0 ₂ reduction is gaining increased attention as it can potentially consume alternative forms of energy by exploiting solar energy, which is abundant, cheap, and ecologically clean and safe.

The photocatalytic reduction of carbon dioxide makes it possible to obtain C1 carbonaceous molecules or more, such as CO, methane, methanol, ethanol, formaldehyde, formic acid or other molecules such as carboxylic acids, aldehydes, ketones or different alcohols. These molecules can find an energy utility directly, such as methanol, ethanol, formic acid or even methane and all hydrocarbons Ci ⁺ . The CO carbon monoxide can also be valorized energetically in a mixture with hydrogen for the formation of Fischer-Tropsch synthesis fuels. The molecules of carboxylic acids, aldehydes, ketones or different alcohols can be used in chemical or petrochemical processes. All these molecules are therefore of great interest from an industrial point of view.

PRIOR ART

Photocatalytic reduction processes of carbon dioxide in the presence of a sacrificial compound are known in the state of the art.

Halmann et al. (Solar Energy, 31, 4, 429-431, 1983) evaluated the performances of three semiconductors (Ti0 ₂ , SrTi0 ₃ and CaTiO ₃ ) for the photocatalytic reduction of CO ₂ in an aqueous medium. They notice the production of formaldehyde, formic acid and methanol.

Anpo et al. (J. Phys Chem B 101, pp. 2632-2636, 1997) have studied the photocatalytic reduction of CO ₂ with water vapor on TiO ₂ photocatalysts anchored in micropores of zeolites. These showed a very high selectivity in methanol gas.

Photocatalysts based on TiO ₂ on which platinum nanoparticles are deposited are known to convert a mixture of CO ₂ and H ₂ O in the gas phase to methane (QH, Zhang et al., Catal, Today, 148, p. 335-340, 2009).

Photocatalysts based on TiO ₂ loaded with gold nanoparticles are also known from the literature for the photocatalytic reduction of CO ₂ in the gas phase (SC Roy et al., ACS Nano, 4, 3, pp. 1259-1278, 2010) and in the aqueous phase (W. Hou et al., ACS Catal., 1, pp. 929-936, 201 1).

Wang et al. (J. Phys., Lett., 1, pp. 48-53, 2010) conducted a study on photoreduction of CO ₂ with H ₂ 0 in visible light catalyzed by heterostructured materials based on CdSe deposited on a solid composed of platinum nanoparticles on TiO ₂ surface. This type of implementation, however, differs from the invention in that the platinum metal is not at the heart of nanoparticles of CdSe or TiO ₂ . Indeed, as demonstrated in the publication by transmission electron microscopy and XPS analyzes, the solid is composed of CdSe particles on the one hand and platinum metal particles on the other hand, the two types of particles being deposited on the same TiO ₂ semiconductor substrate. It is also known that the photocatalytic reduction of C0 ₂ in methanol, formic acid and formaldehyde in aqueous solution can be carried out using different semiconductors such as ZnO, CdS, GaP, SiC or WO ₃ (Inoue). et al., Nature, 277, 637-638, 1979).

Liou et al. (Energy Sci., 4, pp. 1487-1494, 201 1) used NiO-doped InaO ₄ photocatalysts to reduce CO ₂ to CH ₃ OH.

Sato et al. (JACS, 133, pp. 15240-15243, 201 1) have studied a hybrid system combining a p-type InP semiconductor and a ruthenium-complexed polymer in order to achieve a selective reduction of CO ₂ .

Finally, a review and a book chapter from the open literature provide a comprehensive review of photocatalysts used in photocatalytic reduction of carbon dioxide: M. Tahir, N. S. Amin, Energy Conv. Manag., 76, p. 194-214, 2013 on the one hand, and Photocatalysis, Topics in current chemistry 303C.A. Bignozzi Editor, Springer, p. 151 -184,201 1 on the other hand.

The object of the invention is to propose a new, sustainable and more efficient way of producing carbon molecules that are valorized by photocatalytic conversion of carbon dioxide using an electromagnetic energy, implementing a photocatalyst containing a first semi -conductor SC1 in direct contact with particles comprising one or more element (s) M in the metal state selected from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, MB, NIA, IVA and VA of the periodic classification of the elements, said particles being in direct contact with a second semiconductor SC2 so that the second semiconductor SC2 covers at least 50% of the surface of the particles comprising one or more element (s) M to the metallic state. The implementation of this type of photocatalyst for the photocatalytic reduction of CO ₂ makes it possible to achieve improved performances compared to photocatalysts known for this reaction. Photocatalysts containing semiconductors, especially photocatalysts composed of core-layer particles on the surface of a semiconductor substrate are known in the state of the art. C. Li et al (J. Hydrogen Energy, 37, pp. 6431-6437, 2012) have unveiled the synthesis of TiO ₂ nanotube-based solids on which are deposited in a photoassisted manner metallic oxide copper particles on their surface. .

H. Lin et al. (Comm.Comm., 21, pp. 91-95, 2012) propose a composite prepared by coprecipitation composed of AgBr / Ag / AgI, AgBr and Agi being both semiconductors.

C. Wang et al. (Chem Eng J., 237, p.29-37, 2014) prepared by successive impregnations a material having contacts between W0 ₃ and Pt on the one hand and Pt and Ti0 ₂ on the other hand.

Finally, H. Tada (Nature Materials, 5, pp. 782-786, 2006) proposes a solid based on hemispherical particles having a layer of CdS around an Au core, which particles are deposited on the semiconductor Ti0 ₂ . It is also known from the open literature (L. Ding et al., Int.J. Hydrogen Energy, 38, pp. 8244-8253, 2013) to implement in photocatalytic conversion of H ₂ 0 in H ₂ a a solid having the CdS-Au-TiO _{2 structure} , CdS and Au being constitutive of nanoparticles such that the CdS is in the form of a layer around an Au core, these nanoparticles being deposited on a TiO ₂ support.

However, none of these documents discloses the use of a photocatalyst containing a first semiconductor SC1 in direct contact with particles comprising one or more element (s) M in the metallic state chosen from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, MB, NIA, IVA and VA of the Periodic Table of Elements, said particles being in direct contact with a second SC2 semiconductor in a photocatalytic carbon dioxide reduction process. More particularly, the invention describes a process for photocatalytic reduction of carbon dioxide carried out in the liquid phase and / or in the gas phase under irradiation using a photocatalyst containing a first semiconductor SC1, particles comprising one or more element (s) M in the metallic state selected from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, MB, NIA, IVA and VA of the periodic table of elements, and a second semiconductor SC2, said first semiconductor SC1 being in direct contact with said particles comprising one or more element (s) M in the metallic state, said particles being in direct contact with said second semiconductor SC2 so that the second semiconductor SC2 covers at least 50% of the surface of the particles comprising one or more element (s) M in the metallic state, said method comprising the following steps: a) a filler containing carbon dioxide and at least one sacrificial compound is contacted with said photocatalyst, b) the photocatalyst is irradiated with at least one irradiation source producing at least one wavelength less than the bandgap width of said photocatalyst so as to reduce the carbon dioxide and oxidize the sacrificial compound in the presence of said photocatalyst activated by said irradiation source, so as to produce an effluent containing at least partly C1 carbonaceous molecules or more, different from C0 _2.

According to a variant, and when the process is carried out in the gas phase, the sacrificial compound is a gaseous compound chosen from water, ammonia, hydrogen, methane and an alcohol.

According to a variant, and when the process is carried out in the liquid phase, the sacrificial compound is a soluble liquid or solid compound chosen from water, ammonia, an alcohol, an aldehyde or an amine.

Alternatively, a diluent fluid is present in steps a) and / or b). According to one variant, the irradiation source is a source of artificial or natural irradiation.

According to a preferred variant, the first semiconductor SC1 is in direct contact with the second semiconductor SC2.

According to a preferred variant, said first semiconductor SC1 forms a support, said support contains at its surface core-layer type particles, said layer being formed by said semiconductor SC2, said core being formed by said particles comprising one or several element (s) M in the metallic state.

According to one variant, the respective content of semiconductors SC1 or SC2 is between 0.01 and 50% by weight relative to the total weight of the photocatalyst. According to one variant, the content of element (s) M in the metallic state is between 0.001% and 20% by weight relative to the total weight of the photocatalyst.

According to one variant, the element M in the metallic state is chosen from platinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper, rhenium or rhodium.

According to one variant, the semiconductors SC1 and SC2 are independently chosen from an inorganic, organic or organic-inorganic semiconductor.

According to one variant, the semiconductor SC1 is chosen from TiO ₂ , ZnO, WO ₃ , Fe ₂ O ₃ , and ZnFe ₂ O ₄ .

According to a variant, the semiconductor SC2 is selected from Cu ₂ 0, Ce ₂ 0 ₃ , In ₂ 0 ₃ , SiC, ZnS, and In ₂ S ₃ .

According to one variant, the photocatalyst comprises a support composed of a SC1 semiconductor chosen from TiO ₂ , ZnO, WO ₃ , Fe ₂ O _{3 and} ZnFe ₂ O ₄ containing, on its surface, core-layer particles, said core being composed of one or more element (s) M in the metallic state selected from platinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper, rhenium or rhodium, said layer consisting of a semiconductor SC2, selected from Cu ₂ O, Ce ₂ O ₃ , In ₂ O ₃ .

In the following, the groups of chemical elements are given according to the classification CAS (CRC Handbook of Chemistry and Physics, publisher CRC press, editor-in-chief D. R. Lide, 81 st edition, 2000-2001). For example, group VIII according to the CAS classification corresponds to the metals of columns 8, 9 and 10 according to the new IUPAC classification.

DETAILED DESCRIPTION OF THE INVENTION

The invention describes a method for photocatalytic reduction of carbon dioxide carried out in the liquid phase and / or in the gas phase under irradiation using a photocatalyst containing a first semiconductor SC1, particles comprising one or more element (s) M to the metallic state selected from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, MB, NIA, IVA and VA of the periodic table of elements, and a second semiconductor SC2, said first semiconductor SC1 being direct contact with said particles comprising one or more element (s) M in the metallic state, said particles being in direct contact with said second semiconductor SC2 so that the second semiconductor SC2 covers at least 50% of the surface of the particles having one or more element (s) M in the metallic state, said method comprising the following steps: a) contacts a filler containing carbon dioxide and at least one sacrificial compound with said photocatalyst, b) the photocatalyst is irradiated with at least one irradiation source producing at least one wavelength less than the forbidden bandwidth of said photocatalyst; photocatalyst so as to reduce the carbon dioxide and oxidize the sacrificial compound in the presence of said photocatalyst activated by said irradiation source, so as to produce an effluent containing at least partly carbon molecules C1 or more, different from the C0 _2.

According to step a) of the process according to the invention, a feedstock containing said carbon dioxide and at least one sacrificial compound is contacted with said photocatalyst.

By sacrificial compound is meant an oxidizable compound. The sacrificial compound may be in gaseous or liquid form.

The term "C1 carbonaceous molecules or more" means molecules resulting from the reduction of CO ₂ containing one or more carbon atoms, with the exception of CO ₂ . Such molecules are, for example, CO, methane, methanol, ethanol, formaldehyde, formic acid or other molecules such as carboxylic acids, aldehydes, ketones or different alcohols.

The process according to the invention can be carried out in the liquid phase and / or in the gas phase. The filler treated according to the process is in gaseous, liquid or biphasic gas and liquid form.

When the filler is in gaseous form, the CO ₂ is present in its gaseous form in the presence of any gaseous sacrificial compounds alone or as a mixture. The gaseous sacrificial compounds are oxidizable compounds such as water (H ₂ O) _, ammonia (NH ₃ ), hydrogen (H ₂ ), methane (CH ₄ ) or alcohols. Preferably, the gaseous sacrificial compounds are water or hydrogen. When the feedstock is in gaseous form, the CO ₂ and the sacrificial compound may be diluted by a gaseous diluent fluid such as N ₂ or Ar. When the feedstock is in the liquid form, this can be in the form of an ionic liquid, organic or aqueous. The charge in liquid form is preferably aqueous. In an aqueous medium, the CO ₂ is then solubilized in the form of aqueous CO ₂ , hydrogen carbonate or carbonate. The sacrificial compounds are liquid or solid oxidizable compounds soluble in the liquid charge, such as water (H ₂ 0) _, ammonia (NH ₃ ), alcohols, aldehydes, amines. In a preferred manner, the sacrificial compound is water. When the liquid charge is an aqueous solution, the pH is generally between 2 and 12, preferably between 3 and 10. Optionally, and in order to modulate the pH of the aqueous liquid charge, a basic or acidic agent may be added to the charge. When a basic agent is introduced it is preferably selected from alkali or alkaline earth hydroxides, organic bases such as amines or ammonia. When an acidic agent is introduced, it is preferably selected from inorganic acids such as nitric, sulfuric, phosphoric, hydrochloric or hydrobromic acid or organic acids such as carboxylic or sulphonic acids.

Optionally, when the liquid charge is aqueous, it can contain in any quantity any solvated ion, such as for example K ⁺ , Li ⁺ , Na ⁺ , Ca ²⁺ , Mg ²⁺ , S0 ₄ ^{2 "} , CI ^" , F ^" , NO ₃ ^2" .

When the process is carried out in the liquid phase or in the gas phase, a diluent fluid, which may be liquid or gaseous, may be present in the reaction medium. The presence of a diluent fluid is not required for the realization of the invention, however it may be useful to add to the charge to ensure the dispersion of the charge in the medium, the dispersion of the photocatalyst, a control the adsorption of the reagents / products on the surface of the photocatalyst, a control of the absorption of photons by the photocatalyst, the dilution of the products to limit their recombination and other similar parasitic reactions. The presence of a diluent fluid also makes it possible to control the temperature of the reaction medium, thus being able to compensate for the possible exo / endothermicity of the photocatalyzed reaction. The nature of the diluent fluid is chosen in such a way that its influence is neutral on the reaction medium or that its possible reaction does not affect the achievement of the desired reduction of carbon dioxide. For example, nitrogen can be selected as the gaseous diluent fluid.

The contacting of the charge containing the carbon dioxide and the photocatalyst can be done by any means known to those skilled in the art. Preferably, the contacting of the carbon dioxide feedstock and the photocatalyst is in fixed bed crossed, fixed bed licking or suspension (also called "slurry" in the English terminology). The photocatalyst can also be deposited directly on optical fibers.

When the implementation is in fixed bed traversed, the photocatalyst is preferably deposited in a layer on a porous support, for example of ceramic or metallic sintered type, and the charge containing the carbon dioxide to be converted into gaseous and / or liquid form is sent through the photocatalytic bed.

When the implementation is in a fixed licking bed, the photocatalyst is preferably layered on a support and the feedstock containing the carbon dioxide to be converted into gaseous and / or liquid form is sent to the photocatalytic bed.

When the implementation is in suspension, the photocatalyst is preferably in the form of particles suspended in a liquid or liquid-gas feed containing carbon dioxide. In suspension, the implementation can be done in batch and continuously.

The photocatalyst comprises a first semiconductor SC1 in direct contact with particles comprising one or more element (s) M in the metallic state chosen from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, MB, NIA. , IVA and VA of the periodic table of the elements, said particles being in direct contact with a second semiconductor SC2.

Preferably, the photocatalyst consists of a first semiconductor SC1, particles comprising one or more element (s) M in the metallic state chosen from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB , MB, NIA, IVA and VA of the periodic table of elements, and a second semiconductor SC2. According to an important aspect of the invention, the first semiconductor SC1 is in direct contact with particles comprising one or more element (s) M in the metallic state, said particles being in direct contact with a second semiconductor SC2 of such so that the second semiconductor SC2 covers at least 50% of the surface of the particles comprising one or more element (s) M in the metallic state. Preferably, the first semiconductor SC1 is also in direct contact with the second semiconductor SC2.

According to a preferred variant of the invention, the photocatalyst used in the process according to the invention has a supported core-layer architecture. More particularly, said first semiconductor SC1 forms a support, said support contains on its surface core-layer type particles, said layer being formed by said semiconductor SC2, said core being formed by said particles comprising one or more element (s) M in the metallic state. The photocatalyst thus comprises a support containing a semiconductor SC1 containing at its surface core-layer type particles, said core comprising one or more of said element (s) M in the metallic state, said layer comprising a semiconductor SC2, said the core being in direct contact with said semiconductor SC1 of the support and the layer covers said core so that the layer covers at least 50% of the surface of the particles comprising one or more element (s) M in the metallic state.

The use of this type of photocatalyst in a photocatalytic reduction reaction of C0 ₂ surprisingly makes it possible to obtain improved photocatalytic performance compared to photocatalysts known from the state of the art which do not contain the core-type architecture. layer supported.

The layer covers an area greater than 50% of the metal core, and preferably greater than 60% and most preferably greater than 75%. The recovery rate is measured by XPS (X-ray photoelectron spectrometry or X-ray photoelectron spectrometry according to the English terminology), for example on an ESCA KRATOS® Axis Ultra device with a monochromatic Al source at 1486.6 eV, and an energy passing 40 eV, and expresses the covering of the total surface of the particles comprising one or more element (s) M in the metallic state. The layer has a thickness of 1 nm to 1000 nm, preferably 1 nm to 500 nm, and particularly preferably 2 to 50 nm.

The semiconductors SC1 and SC2 are independently selected from inorganic, organic or organic-inorganic semiconductors. The bandgap of inorganic, organic or hybrid organic-inorganic semiconductors is generally between 0.1 and 5.5 eV.

The semiconductors SC1 and SC2 may be identical or different in the photocatalyst used in the process according to the invention. Preferably, the semiconductor SC1 is different from the semiconductor SC2 in the photocatalyst used in the method according to the invention.

According to a first variant, the semiconductors SC1 and SC2 are independently selected from inorganic semiconductors. The inorganic semiconductors may be selected from one or more Group IVA elements, such as silicon, germanium, silicon carbide or silicon germanium. They may also be composed of elements of the NIA and VA groups, such as GaP, GaN, InP and InGaAs, or elements of groups MB and VIA, such as CdS, ZnO and ZnS, or elements of groups IB and VIIA, such as CuCl and AgBr, or elements of groups IVA and VIA, such as PbS, PbO, SnS and PbSnTe, or elements of groups VA and VIA, such as Bi ₂ Te ₃ and Bi ₂ 0 ₃ , or elements of groups MB and VA, such as Cd ₃ P2, Zn ₃ P ₂ and Zn ₃ As ₂ , or elements of groups IB and VIA, such as CuO, Cu ₂ O and Ag ₂ S, or of elements of groups VIIIB and VIA, such as CoO, PdO, Fe ₂ O ₃ and NiO, or elements of groups VIB and VIA, such as MoS ₂ and WO ₃ , or elements of groups VB and VIA, such as V ₂ O ₅ and Nb ₂ O ₅ , or elements of groups IVB and VIA, such as TiO ₂ and HfS ₂ , or elements of groups NIA and VIA, such as In ₂ O ₃ and In ₂ S ₃ , or elements of groups VIA and lanthanides, such as Ce ₂ O ₃ , Pr ₂ O ₃ , Sm ₂ S ₃ , Tb ₂ S ₃ and La ₂ S ₃ , or elements of groups VIA and actinides, such as UO ₂ and UO ₃ .

Preferably, the semiconductors SC1 and SC2 are independently selected from TiO ₂ , SiC, Bi ₂ S ₃ , Bi ₂ O ₃ , CdO, Ce ₂ O ₃ , CeO _2, CoO, Cu ₂ O, Fe ₂ O ₃ , FeTiO ₃ , In ₂ O ₃ , In (OH) ₃ , NiO, PbO, ZnO, Ag ₂ S, CdS, Ce ₂ S ₃ , Cu ₂ S, CulnS ₂ , In ₂ S ₃ , ZnFe ₂ O ₃ , ZnS, ZnO, WO ₃ , ZnFe ₂ O ₄ and ZrS ₂ . Particularly preferably, the semiconductor SC1 is selected from TiO ₂ , ZnO, WO ₃ , Fe ₂ O ₃ , and ZnFe ₂ O ₄ .

In a particularly preferred manner, the semiconductor SC2 is chosen from Ce ₂ 0 ₃ , ΓΙη ₂ 0 ₃ , Cu ₂ 0, SiC, ZnS, and In ₂ S ₃ .

According to another variant, the semiconductors SC1 and SC2 are chosen from organic semiconductors. Said organic semiconductors may be tetracene, anthracene, polythiophene, polystyrene sulphonate, phosphyrenes and fullerenes.

According to another variant, the semiconductors SC1 and SC2 are chosen from organic-inorganic semiconductors. Among the organic-inorganic semiconductors, mention may be made of crystalline solids of the MOF type (for Metal Organic Frameworks according to the English terminology). The MOFs consist of inorganic subunits (transition metals, lanthanides, etc.) connected to each other by organic ligands (carboxylates, phosphonates, imidazolates, etc.), thus defining crystallized hybrid networks, sometimes porous.

The semiconductors SC1 and SC2 may optionally be doped with one or more ions chosen from metal ions, such as, for example, ions of V, Ni, Cr, Mo, Fe, Sn, Mn, Co, Re, Nb, Sb. , La, Ce, Ta, Ti, non-metallic ions, such as, for example, C, N, S, F, P, or a mixture of metallic and non-metallic ions.

According to another variant, the semiconductors SC1 and SC2 may be surface-sensitized with any organic molecules capable of absorbing photons. The semiconductors SC1 and SC2 can be respectively in different forms (nanometric powder, nanoobjects with or without cavities, ...) or shaped (films, monolith, beads of micrometric or millimeter size, ...).

The respective content of semiconductors SC1 or SC2 is generally between 0.01 and 50% by weight, preferably between 0.5 and 20% by weight relative to the total weight of the photocatalyst, it being understood that the sum of the contents respective semiconductors SC1 or SC2 and particles having one or several element (s) M in the metallic state has 100% of photocatalysts when it consists of these 3 components.

The photocatalyst also comprises particles comprising one or more element (s) M in the metallic state chosen from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, MB, NIA, IVA and VA of the periodic table. elements. Said particles comprising one or more element (s) M are in direct contact with said semiconductor SC1 and SC2 respectively. Said particles can be composed of a single element in the metallic state or of several elements in the metallic state that can form an alloy.

The term "element in the metallic state" means an element belonging to the family of metals, said element being at the zero oxidation state (and therefore in the form of metal). Preferably, the element or elements M in the metallic state are chosen from a metal element of groups VIIB, VIIIB, IB and MB of the periodic table of elements, and particularly preferably from platinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper, rhenium or rhodium. Said particles comprising one or more element (s) M in the metallic state are preferably in the form of particles of sizes between 0.5 nm and 1000 nm, very preferably between 0.5 nm and 100 nm.

The content of element (s) M in the metallic state is between 0.001 and 20% by weight, preferably between 0.01 and 10% by weight relative to the total weight of the phtocatalyst.

The photocatalyst used in the process according to the invention may be in various forms (nanometric powder, nanoobjects with or without cavities, etc.) or shaped (films, monolith, beads of micrometric or millimetric size, etc. ). The photocatalyst is advantageously in the form of a nanometric powder. Preferably, the photocatalyst comprises a support composed of a semiconductor SC1 containing on its surface core-layer type particles, said core consisting of one or more of said element (s) M in the metallic state, said layer being consisting of a semiconductor SC2. Particularly preferably, the photocatalyst consists of a support composed of a semiconductor SC1 containing on its surface core-layer type particles, said core consisting of one or more of said element (s) M to metallic state, said layer consisting of a semiconductor SC2.

According to an even more preferred embodiment, the photocatalyst used in the method of the invention comprises, and preferably consists of a support made of a semiconductor selected from the SC1 ΤΊΟ2, ZnO, WO _3, Fe ₂ O ₃ and ZnFe ₂ O ₄ , containing on its surface core-layer type particles, said core consisting of one or more element (s) M in the metallic state chosen from platinum, palladium, gold , nickel, cobalt, ruthenium, silver, copper, rhenium or rhodium, said layer consisting of a semiconductor SC2 selected from Cu ₂ O, Ce ₂ O ₃ , ln ₂ O ₃ , SiC. ZnS and ln ₂ S ₃ .

The process for preparing the photocatalyst can be any preparation method known to those skilled in the art and adapted to the desired photocatalyst. In a preferred manner, the photocatalyst is prepared by successive photodepositions or by deposition-precipitation under irradiation. These preparation methods are known in the state of the art.

According to step b) of the process according to the invention, the photocatalyst is irradiated with at least one irradiation source producing at least one wavelength less than the band gap width of said photocatalyst so as to reduce carbon dioxide. and oxidizing the sacrificial compound in the presence of said photocatalyst activated by said irradiation source, so as to produce an effluent containing at least in part carbonaceous C1 molecules or more, different from CO ₂ .

Photocatalysis is based on the principle of activating a semiconductor or a set of semiconductors such as the photocatalyst used in the process according to the invention, using the energy provided by the irradiation . Photocatalysis can be defined as the absorption of a photon whose energy is greater than the forbidden bandgap or "bandgap" according to the English terminology between the valence band and the conduction band, which induces the forming an electron-hole pair in the semiconductor. We therefore have the excitation of an electron at the level of the conduction band and the formation of a hole on the valence band. This electron-hole pair will allow the formation of free radicals that will either react with compounds present in the medium or then recombine according to various mechanisms. Each semiconductor has a difference in energy between its conduction band and its valence band, or "bandgap", which is its own.

A photocatalyst composed of one or more semiconductors can be activated by the absorption of at least one photon. Absorbable photons are those whose energy is greater than bandgap, semiconductor. In other words, the photocatalysts can be activated by at least one photon of a wavelength corresponding to the energy associated with the bandgap widths of the semiconductors constituting the photocatalyst or of a lower wavelength. The maximum wavelength absorbable by a semiconductor is calculated using the following equation:

- _ h x c

max -

With A _max, the maximum wavelength absorbable by a semiconductor (in m), h the Planck constant (4,13433559.10 ^"15 eV.s), c the speed of light in a vacuum (299,792,458 m. ^"1 ) and Eg the bandgap or bandgap of the semiconductor (in eV).

Any irradiation source emitting at least one wavelength suitable for activating said photocatalyst, that is to say absorbable by the photocatalyst can be used according to the invention. For example, it is possible to use natural solar irradiation or an artificial irradiation source of the laser, Hg, incandescent lamp, fluorescent tube, plasma or light emitting diode (LED) type (LED or Light-Emitting Diode). Preferably, the irradiation source is solar irradiation.

The irradiation source produces a radiation of which at least a portion of the wavelengths is less than the maximum absorbable wavelength (A _max ) by the constituent semiconductors of the photocatalyst according to the invention. When the irradiation source is solar irradiation, it generally emits in the ultraviolet spectrum, visible and infra-red, that is to say it emits a wavelength range of 280 nm to 2500 nm about (according to ASTM G173-03). Preferably, the source emits at least one wavelength range greater than 280 nm, very preferably 315 nm to 800 nm, which includes the UV spectrum and / or the visible spectrum.

The irradiation source provides a photon flux that irradiates the reaction medium containing the photocatalyst. The interface between the reaction medium and the light source varies depending on the applications and the nature of the light source. In a preferred mode when it is solar irradiation, the irradiation source is located outside the reactor and the interface between the two may be an optical window pyrex, quartz, organic glass or any other interface allowing photons absorbable by the photocatalyst according to the invention to diffuse external medium within the reactor.

The realization of the photocatalytic reduction of carbon dioxide is conditioned by the provision of photons adapted to the photocatalytic system for the reaction envisaged and therefore is not limited to a specific pressure or temperature range apart from those allowing ensure the stability of the product (s). The temperature range employed for the photocatalytic reduction of the carbon dioxide containing feedstock is generally -10 ° C to + 200 ° C, more preferably 0 to 150 ° C, and most preferably 0 to 50 ° C. vs. The pressure range employed for the photocatalytic reduction of the carbon dioxide containing feedstock is generally from 0.01 MPa to 70 MPa (0.1 to 700 bar), more preferably from 0.1 MPa to 2 MPa (1 to 20 bar).

The effluent obtained after the photocatalytic reduction reaction of the carbon dioxide contains on the one hand at least one molecule at C1 or more, different from the carbon dioxide resulting from the reaction and secondly from the unreacted charge, as well as the possible diluent fluid, but also products of parallel reactions such as for example the dihydrogen resulting from the photocatalytic reduction of H ₂ 0 when this compound is used as a sacrificial compound.

The following examples illustrate the invention without limiting its scope. EXAMPLES

Example 1: Solid A (not in accordance with the invention) TiO ₂

A photocatalyst is a semiconductor-based shopping Ti0 ₂ (Aeroxide ^® P25, Aldrich ™, purity> 99.5%). The particle size of the photocatalyst is 21 nm and the specific surface area measured by BET method is equal to 52 m ² / g.

Example 2: Solid B (not in accordance with the invention) Pt / TiO ₂

0.0712 g of H ₂ PtCl ₆ , 6H ₂ O (37.5% by weight of metal) is added to 500 ml of distilled water. 50 ml of this solution are taken and inserted into a double jacketed glass reactor. 3 ml of methanol then 250 mg of TiO ₂ (P25, Degussa ™) are then added with stirring to form a suspension.

The mixture is then left stirring and under UV radiation for two hours. The lamp used to provide UV radiation is a 125W mercury vapor HPK ™ lamp.

The mixture is then centrifuged for 10 minutes at 3000 rpm to recover the solid. Two washings with water are then carried out, each washing being followed by centrifugation. The recovered powder is finally placed in an oven at 70 ° C. for 24 hours.

The solid B Pt / TiO _{2 is} then obtained. The content of Pt element is measured by plasma emission atomic emission spectrometry (or inductively coupled plasma atomic emission spectroscopy "ICP-AES" according to the English terminology) at 0.93% by mass.

Example 3: Solid C (in accordance with the invention) Cu ₂ O / Pt / TiO ₂

0.0712 g of H ₂ PtCl ₆ , 6H ₂ O (37.5% by weight of metal, Aldrich ™) is inserted into 500 ml of distilled water. 50 ml of this solution are taken and inserted into a double jacketed glass reactor. 3 ml of methanol then 250 mg of TiO ₂ (P25, Degussa ™) are then added with stirring to form a suspension.

The mixture is then centrifuged for 10 minutes at 3000 rpm to recover the solid. Two washes with water are then carried out, each of the washes followed by centrifugation. The recovered powder is finally placed in an oven at 70 ° C. for 24 hours.

A solid C Pt / TiO _{2 is} then obtained. The content of Pt element is measured by ICP-AES 0.93% by mass. A solution of Cu (NO ₃ ) 2 is prepared by dissolving 0.125 g of Cu (NO ₃ ) 2, 3H ₂ O (Sigma-Aldrich ™, 98%) in 50 ml of a 50/50 isopropanol / H ₂ O mixture. a concentration of Cu ²⁺ of 10.4 mmol / L.

In the reactor were introduced: 0.20 g of solid C, 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 hours. The reactor is thermostated at 25 ° C. throughout the synthesis. The argon flow is then 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 mercury vapor lamp HPK ™. Then the 50 ml of copper nitrate solution is added to the mixture. The mixture is left stirring for 10 hours and irradiation. The mixture is then centrifuged for 10 minutes at 3000 rpm to recover the solid. Two washings with water are then carried out, each washing being followed by centrifugation. The recovered powder is finally placed in an oven at 70 ° C. for 24 hours.

The solid C Cu ₂ O / Pt / TiO _{2 is} then obtained. The content of Cu element is measured by ICP-AES at 2.2% by mass. By measurement XPS (X-Ray Photoelectron Spectrometry according to the English terminology), a platinum particle coating greater than 77% and copper oxide phases were measured at 67% Cu ₂ O and 33% CuO. Transmission electron microscopy measured a 5 nm thick copper oxide layer thickness around the metal particles.

Example 4: Solid D (in accordance with the invention) Cu ₂ O / Au / TiO ₂

0.0470 g of HAuCl ₄ , xH ₂ O (52% by mass of metal, Aldrich ™) is inserted into 500 ml of distilled water. 50 ml of this solution are taken and inserted into a double jacketed glass reactor. 3 ml of methanol then 250 mg of TiO ₂ (P25, Degussa ™) are then added with stirring to form a suspension.

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

An Au / TiO ₂ solid is then obtained. The content of element Au is measured by ICP-AES at 0.96% by mass.

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

In the reactor were introduced: 0.20 g of the solid D ', 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 hours. The reactor is thermostated at 25 ° C. throughout the synthesis. The argon flow is then slowed down to 30 ml / min and the irradiation of the reaction mixture starts. The lamp used to provide UV radiation is a 125W mercury vapor HPK ™ lamp. Then, the 50 ml of copper nitrate solution is added to the mixture. The mixture is left stirring for 10 hours and irradiation. The mixture is then centrifuged for 10 minutes at 3000 rpm to recover the solid. Two washings with water are then carried out, each washing being followed by centrifugation. The recovered powder is finally placed in an oven at 70 ° C. for 24 hours.

The solid D Cu ₂ O / Au / TiO _{2 is} then obtained. The content of Cu element is measured by ICP-AES at 2.3% by weight. By XPS measurement, an overlap of platinum particles greater than 79% and phases of copper oxides at 76% Cu ₂ O and 24% CuO are measured. Transmission electron microscopy measured a mean copper oxide layer thickness of 7 nm around the metal particles.

Example 5: Solid E (in accordance with the invention) Cu ₂ O / Pt / ZnO

0.0714 g of H ₂ PtCl ₆ , 6H ₂ O (37.5% by weight of metal, Aldrich ™) is inserted into 500 ml of distilled water. 50 ml of this solution are taken and inserted into a double jacketed glass reactor. 3 ml of methanol then 250 mg of ZnO (Lotus Synthesis ™, surface area 50 m ² / g) are then added with stirring to form a suspension. The mixture is then left stirring and under UV radiation for six hours. The lamp used to provide UV radiation is a 125W mercury vapor HPK ™ lamp.

An E 'Pt / ZnO solid is then obtained. The content of Pt element is measured by ICP-AES at 0.77% by weight. A solution of Cu (NO ₃ ) 2 is prepared by dissolving 0.125 g of Cu (NO ₃ ) 2, 3H ₂ O (Sigma-Aldrich ™, 98%) in 50 ml of a 50/50 isopropanol / H ₂ O mixture. a concentration of Cu ²⁺ of 10.4 mmol / L.

In the reactor were introduced: 0.20 g of the solid E ', 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 hours. The reactor is thermostated at 25 ° C. throughout the synthesis. The argon flow is then 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 mercury vapor lamp HPK ™. Then the 50 ml of copper nitrate solution is added to the mixture. The mixture is left stirring for 10 hours and irradiation. The mixture is then centrifuged for 10 minutes at 3000 rpm to recover the solid. Two washings with water are then carried out, each washing being followed by centrifugation. The recovered powder is finally placed in an oven at 70 ° C. for 24 hours.

The solid E Cu ₂ O / Pt / ZnO is then obtained. The content of Cu element is measured by ICP-AES at 1, 9% by mass. By XPS measurement, a recovery of platinum particles greater than 83% and copper oxide phases 79% Cu ₂ O and 21% CuO. Transmission electron microscopy measured an average copper oxide layer thickness of 4 nm around the metal particles.

Example 6: Solid F (in accordance with the invention) Ce ₂ O ₃ / Pt / TiO ₂

0.0712 g of H ₂ PtCl ₆ , 6H ₂ O (37.5% by weight of metal) is added to 500 ml of distilled water. 50 ml of this solution are taken and inserted in a double reactor glass envelope. 3 ml of methanol then 250 mg of TiO ₂ (P25, Degussa ™) are then added with stirring to form a suspension.

The solid F 'Pt / TiO _{2 is} then obtained. The content of Pt element is measured by ICP-AES at 0.93% by mass.

A solution of Ce (NO ₃ ) 3 is prepared by dissolving 0.05 g of Ce (NO ₃ ) 3, 6H ₂ O (Sigma-Aldrich ™, 99%) in 50 ml of H ₂ O.

In the reactor were introduced: 0.10 g of the solid F ', 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 hours. The reactor is thermostated at 25 ° C. throughout the synthesis.

The argon flow is then 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 mercury vapor lamp HPK ™. Then, 5 ml of the cerium nitrate solution is added to the mixture. The mixture is left for 1 hour with stirring and irradiation. 1 ml of a 30% solution of NH _{3 is} then added. The mixture is again left for 1 hour with stirring and irradiation.

The solid F Ce ₂ O ₃ / Pt / TiO _{2 is} then obtained. The content of element Ce is measured by ICP-AES at 1.7% by mass. By XPS measurement, a recovery of platinum particles greater than 83% and cerium oxide phases are measured at 74% Ce ₂ O ₃ and 26% CeO ₂ . By transmission electron microscopy, an average layer thickness of cerium oxide of 4 nm is measured around the metal particles. Example 7: Solid G (in accordance with the invention) In ₂ O ₃ / Pt / TiO ₂

The solid G 'Pt / TiO _{2 is} then obtained. The content of Pt element is measured by ICP-AES at 0.93% by mass. A solution of In (NO ₃ ) 3 is prepared by dissolving 0.05 g of ln (NO ₃ ) 3, xH ₂ O (Sigma-Aldrich ™, 99.9%) in 50 ml of H ₂ O.

In the reactor were introduced: 0.10 g of the solid G ', 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 hours. The reactor is thermostated at 25 ° C. throughout the synthesis. The argon flow is then 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 mercury vapor lamp HPK ™. Then, 5 ml of the indium nitrate solution is added to the mixture. The mixture is left for 1 hour with stirring and irradiation. 1 ml of a 30% solution of NH _{3 is} then added. The mixture is again left for 1 hour with stirring and irradiation.

The solid G ln ₂ O ₃ / Pt / TiO _{2 is} then obtained. The content of element In is measured by ICP-AES at 1, 9% by mass. By XPS measurement, a recovery of platinum particles greater than 79% is measured. By transmission electron microscopy, an average layer thickness of 5 nm indium oxide is measured around the metal particles.

Example 8: Implementation of solids in photocatalytic reduction of C0 ₂ in the liquid phase

The solids A, B, C, D, E, F and G are subjected to a photocatalytic reduction test of C0 ₂ in the liquid phase in a semi-open stirred Pyrex reactor provided with a quartz optical window and a double jacket to regulate the test temperature.

100 mg of solid are suspended in 60 ml of an aqueous solution of potassium carbonate at 0.15 mol / l. The pH of the solution is 9. The tests are carried out at 25 ° C. under atmospheric pressure with an argon flow rate of 300 ml / h to entrain the gaseous products. The production of dihydrogen gas produced from the undesirable photocatalytic reduction of water is monitored and analyzed every 6 minutes by gas chromatography. The photocatalytic reduction of CO ₂ is followed by the production of formic acid by taking liquid samples every 30 minutes, which are analyzed by HPLC chromatography equipped with a UV detector. The UV-Visible irradiation source is provided by an Xe-Hg lamp (Asahi ™, MAX302 ™). The irradiation power is always maintained at 100%. The duration of the test is 20 hours.

The photocatalytic activities are expressed in μηιοΙ of dihydrogen and formic acid produced per hour and per gram of photocatalyst. The results are reported in Table 1. The activity values show that the implementation of the solids according to the invention systematically has the best photocatalytic performance and particularly better selectivities towards the photocatalytic reduction of CO ₂ . Activity Activity

photocatalyst

initial HCOOH initial H ₂ SC1 / M / SC2

(μηΊθΙ / h / g) (μηΊθΙ / h / g)

Solid A (non-compliant) Ti0 ₂ nd 12

Solid B (non-compliant) Pt / Ti0 ₂ nd 4230

Solid C (compliant) Cu ₂ 0 / Pt / Ti0 ₂ 207 5

Solid D (compliant) Cu ₂ 0 / Au / Ti0 ₂ 289 6

Solid E (compliant) Cu ₂ 0 / Pt / ZnO 146 2

Solid F (compliant) This ₂ 0 ₃ / Pt / Ti0 ₂ 102 nd

Solid G (compliant) In ₂ 0 ₃ / Pt / Ti0 ₂ 137 nd

n.d .: not determined

Table 1: Initial activity solids performances for the production of dihydrogen or formic acid from an aqueous solution of potassium carbonate

Example 9: Implementation of solids in photocatalytic reduction of C0 ₂ in the gas phase

The solids A, B, C, D, E, F and G are subjected to a photocatalytic CO ₂ gas phase reduction test in a continuous steel through-bed reactor equipped with a quartz optical window and a sintered in front of the optical window on which the photocatalytic solid is deposited.

100 mg of solid are deposited on the sinter. The tests are carried out at ambient temperature under atmospheric pressure. An argon flow rate of 300 ml / h and CO ₂ of 10 ml / hr passes through a water saturator before being dispensed into the reactor. The production of dihydrogen gas produced from the undesirable photocatalytic reduction of the water entrained in the saturator and of CH ₄ resulting from the reduction of carbon dioxide is followed by an analysis of the effluent every 6 minutes by micro-chromatography. gas phase. The UV-Visible irradiation source is provided by an Xe-Hg lamp (Asahi ™, MAX302 ™). The irradiation power is always maintained at 100%. The duration of the test is 20 hours.

The photocatalytic activities are expressed in μηιοΙ of dihydrogen and methane produced per hour and per gram of photocatalyst. The results are reported in Table 2. The activity values show that the implementation of the solids according to the invention systematically has the best photocatalytic performance and particularly better selectivities towards the photocatalytic reduction of C0 ₂ .

n.d .: not determined

Table 2: Performances of the initial activity solids for the production of dihydrogen or methane from a CO ₂ and H ₂ O mixture in the gas phase

Claims

1. Process for the photocatalytic reduction of carbon dioxide carried out in the liquid phase and / or in the gas phase under irradiation using a photocatalyst containing a first semiconductor SC1, particles comprising one or more selected metal element (s) M from an element of groups IVB, VB, VIB, VIIB, VIIIB, IB, MB, NIA, IVA and VA of the periodic table of elements, and a second semiconductor SC2, said first semiconductor SC1 being in direct contact with said particles comprising one or more elements M in the metallic state, said particles being in direct contact with said second semiconductor SC2 so that the second semiconductor SC2 covers at least 50% of the surface of the particles comprising one or more elements M in the metallic state, said process comprising the following steps: a) contacting a filler containing carbon dioxide and at least one sacrificial compound with said photocatalyst, b) the photocatalyst is irradiated with at least one irradiation source producing at least one wavelength less than the forbidden band width of said photocatalyst so as to reduce the carbon dioxide and oxidize the sacrificial compound in the presence of said photocatalyst activated by said irradiation source, so as to produce an effluent containing at least partly carbon molecules C1 or more, different from the C0 _2.

2. The method of claim 1, wherein, when carried out in the gas phase, the sacrificial compound is a gaseous compound selected from water, ammonia, hydrogen, methane and an alcohol.

The process according to claim 1, wherein when carried out in the liquid phase, the sacrificial compound is a soluble liquid or solid compound selected from water, ammonia, an alcohol, an aldehyde and an amine.

4. Method according to one of claims 1 to 3, wherein a diluent fluid is present in steps a) and / or b).

5. Method according to one of claims 1 to 4, wherein the source of irradiation is a source of artificial or natural irradiation.

6. Method according to one of claims 1 to 5, wherein the first semiconductor SC1 is in direct contact with the second semiconductor SC2.

7. Method according to one of claims 1 to 6, wherein said first semiconductor SC1 forms a support, said support contains on its surface core-layer type particles, said layer being formed by said semiconductor SC2, said core being formed by said particles having one or more element (s) M in the metallic state.

8. Method according to one of claims 1 to 7, wherein the respective content of semiconductors SC1 or SC2 is between 0.01 and 50% by weight relative to the total weight of the photocatalyst.

9. Method according to one of claims 1 to 8, wherein the content of element (s) M in the metallic state is between 0.001 and 20% by weight relative to the total weight of the photocatalyst.

10. Method according to one of claims 1 to 9, wherein the element M in the metallic state is selected from platinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper, rhenium and rhodium.

1 1. Method according to one of claims 1 to 10, wherein the semiconductors SC1 and SC2 are independently selected from inorganic, organic or organic-inorganic semiconductors.

12. The method of claims 1 to 1 1, wherein the semiconductor SC1 is selected from Ti0 ₂ , ZnO, WO ₃ , Fe ₂ 0 ₃ , and ZnFe ₂ 0 ₄ .

13. The method of claims 1 to 12, wherein the semiconductor SC2 is selected from Cu ₂ 0, Ce ₂ 0 ₃ , ln ₂ 0 ₃ , SiC, ZnS, ln ₂ S _3. .

14. Method according to one of claims 1 to 13, wherein the photocatalyst comprises a support composed of a semiconductor SC1 selected from Ti0 ₂ ,

ZnO, WO ₃ , Fe ₂ O _3, and ZnFe ₂ O ₄ , containing on its surface core-layer type particles, said core consisting of one or more element (s) M in the metallic state selected from platinum, palladium, gold, nickel, cobalt, ruthenium, silver, copper, rhenium or rhodium, said layer consisting of a semiconductor SC2 selected from Cu ₂ O, the Ce ₂ O ₃ and Πη ₂ Ο ₃ .