Reduction process assisted by alternating magnetic fields

Abstract

The present invention proposes a process for the direct reduction of reducible materials at low temperatures (< 600 °C approx.) by means of alternating magnetic fields. No chemical-reducing agents or contact electrodes might be further required. The invention also refers to the equipment used for carrying out said reduction, to the materials obtained by said reduction process, and to the use of such materials to produce chemicals of industrial interest and / or as energy carrier.

Description

[0001] REDUCTION PROCESS ASSISTED BY ALTERNATING MAGNETIC FIELDS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of energy storage and conversion, in particular through the use of alternating magnetic fields in a low temperature method that allows the easy and economical production of thermochemical reactive materials for thermal energy storage or the production of valuable chemicals and fuels.

[0004] BACKGROUND

[0005] Current technological activities are directly or indirectly linked to the production and use of energy. The depletion of fossil fuel, the ever-increasing energy demand as well as the major environmental issues originating from CO2 emissions require a radical change to ensure the long-term sustainability of the global energy and production systems. In this context, renewable energies, particularly renewable electricity, will be a key element of the energy transition underway. Free of CO2 emissions, renewable electricity will play an important role both in the decarbonization of the automotive sector through electric vehicles, and in the decarbonization of industry through the electrification of thermal processes that are currently based on hydrocarbons combustion or oxidation.

[0006] Due to the variability and intermittent nature of solar and wind energy, the availability of electrical energy storage systems is essential to ensure the stability and proper functioning of the grid and the energy system. There are many possible ways of storing electrical energy. Apart from mechanical (e.g., flywheels, CAES, LAES) and electrochemical storage (e.g., batteries and capacitors), thermal energy storage with prior conversion of electricity into heat (Power-to-Heat) and chemical energy storage through the production of fuels or chemicals (Power-to-X) are becoming increasingly popular.

[0007] Thermal energy storage is a cheap way of storing renewable energy that may assist the decarbonization of both industrial and domestic heat. On the other hand, storage in chemical energy carriers opens a huge range of possibilities. There are currently several technologies that allow the conversion of electricity (or solar thermal energy) into chemical energy carriers (and vice versa) without emitting CO2: - Electrochemical cells that work as electrolyzers (e.g. alkaline electrolyzers, PEM or Proton-Exchange Membrane electrolyzer, SOEC or Solid-Oxide Electrolyzers), which produce fuels and feedstocks from electricity;

[0008] - electrochemical cells that work as fuel cells (e.g., SOFC or Solid-Oxide Fuel Cells), which produce electricity from renewable fuels;

[0009] - O2 separators based on solid electrolytes, as well as electrically driven O2 generators;

[0010] - concentrating solar thermal power-driven thermochemical cycles, which produce fuels and feedstocks from solar radiation.

[0011] These Power-to-X transformations currently focus on the production of H2, O2 and hydrocarbons, but could be extended to the synthesis of ammonia or nitrates. They could also be used in CO2 valorization processes, converting CO2 into CO, syngas or hydrocarbons.

[0012] The Power-to-X transformations mentioned above are based on reduction-oxidation (Redox) processes where the reduction of suitable substances is usually carried out at high temperatures and / or using contact electrodes. This results in complex installations, high investments, significant operating costs, and low energy and mass efficiencies.

[0013] Usually, the use of microwave and / or suitable reducing agents may facilitate the reduction process in Power-to-X transformations. Particularly, the use of reducing agents is a widespread method to lower the working temperature in reduction processes. Commonly used reducing agents are hydrogen, carbon monoxide, gaseous ammonia, and hydrocarbons. By using gaseous streams of H2 or CO, the reduction process may take place at temperature which are considerably lower than in absence of reducing agents. For example, Seo et al. (Continuous hydrogen regeneration through the oxygen vacancy control using microwave irradiation, RSC Advances 2018, 8, 37958) reported the reduction of a porous copper oxide macrostructure, the surface of which is impregnated with a PVP (polyvinylpyrrolidone) solution which acts as a reducing agent, before irradiating the material with microwaves. Likewise, S. Fujii et al. (Reduction of metal oxides using thermogravimetry under microwave irradiation, AIP Advances 2021, 11, 065207) reported the reduction of copper oxide using graphite microparticles as reducing agent and with a single-mode microwave cavity; the effect of both the magnetic field and the electric field on the reduction temperature is further evaluated. P.A. Chemavsky et al. (The influence of an external magnetic field on the dynamics of magnetite reduction with hydrogen, RSC Advances 2021, 11, 15422) have investigated the effect of constant magnetic fields on the kinetics of magnetite reduction to iron using H2 as reducing agent. The same reductant (H2) was also used by M-W. Rowe et al. (Effect of magnetic field on reduction of iron oxides: magnetite and wiistite, Nature 1977, 266, 612-614) to study the reduction kinetics of magnetite and wiistite under constant and intense magnetic fields. The reduction of metal oxides with different carbon materials is also known and may accomplish a comparable decrease in the reduction temperature of oxides. For example, document US4494984A discloses the reduction of iron oxides using carbon materials as the reducing agent. In particular, induction is used to heat the walls of the vessel containing the oxides to be reduced.

[0014] It becomes clear that the avoidance of reducing agents might represent a remarkable practical advantage from a sustainable viewpoint. Particularly, microwave-assisted reduction is a strategy where microwaves (frequency range: 0.9-25 GHz) interact with the material to be reduced modifying its electrical conductivity and promoting its reduction at low to moderate temperatures without the use of reducing agents or contact electrodes.

[0015] In this regard, the direct reduction at “low temperature” (<600°C) of a material by microwave radiation without the need to use chemical reducing agents or electrical contacts has been recently reported (ES2726028B2 and J.M. Serra et al., Nature Energy 2020, 5, 910-919). In the latter documents, ceria-based oxides have been reduced at temperatures of 250-400 °C instead of typical temperatures of above 1000 °C.

[0016] Document JP2010173930A discloses a method for generating hydrogen at a temperature as low as at most 500°C. In such method, a magnetic raw material (Mn-Zn ferrite or the like) is irradiated with microwaves while controlling the temperature so as to convert the microwaves into wavelengths of IR (infrared) rays or far-IR rays. When methane, ammonia or overheated steam is irradiated with the converted radiation, oscillation wavelength is amplified, which induces resonance of hydrogen, carbon, nitrogen and oxygen molecules included in the gas with the surface molecules of the magnetic raw material. Hydrogen is then generated from the methane, ammonia or steam.

[0017] Despite the progress in the reduction of materials for energy storage, more strategies are sought for that simplify current technologies by, for example, not only avoiding reducing agents and / or contact electrodes but also avoiding the need for setups which may hamper the upscaling of the reduction processes (such as microwave setups).

[0018] BRIEF DESCRIPTION OF THE INVENTION

[0019] The inventors have devised a new method for the reduction of materials based on the surprising finding that reducible materials (such as a magnetic oxide) exposed to the effects of an alternating magnetic field enables the low temperature reduction of said materials. Remarkably, the new reduction process may not require the presence of reducing agents and / or contact electrodes, solving the issues that have been, so far, associated with current technologies, such as high temperatures, complex installations, high investments and operating costs, low energy and mass efficiencies. Particularly, the present invention takes advantage of the interaction of alternating magnetic fields with the magnetic nature of target materials for their reduction at low temperatures, in some cases below 100 °C, whereas temperatures above 1000 °C would be required by conventional heating techniques. This is also due to the magnetic field having a nonthermal effect on the reducible material, so that the reduction starts at temperatures well below those of conventional heating methods. In addition, the reduction process is accelerated, uniform throughout the mass of the material, and requires less energy input. Furthermore, the material reduced by this process is activated toward the production of chemicals of industrial interest (e.g., CO) and energy carriers (e.g., H2).

[0020] Thus, in a first aspect the invention relates to a method for reducing a reducible material, wherein the reducible material is a magnetic material, said method comprising the following steps: i) introducing the reducible material into a reactor cavity; ii) subjecting the reducible material to an alternating magnetic field; iii) heating the reducible material to at least the temperature at which the reduction of said material starts, thereby forming a reduced material and oxidized products; and iv) releasing from the reactor the generated oxidized products and, optionally, the reduced material, provided that only the reducible material is responsive to the alternating magnetic field and the reducible material is not subjected to an electric field. DESCRIPTION OF THE FIGURES

[0021] Figure 1. Apparatus for carrying out the induction-assisted reduction of the present invention: A) Fixed bed reactor; B) Moving bed reactor.

[0022] Figure 2. Experimental set-up suitable for the induction-assisted reduction method of the present invention.

[0023] Figure 3. Graph representing the dependence of temperature of the sample and oxygen release evolution over time for the induction-assisted reduction of CoFe2O4.

[0024] Figure 4. A) XRD spectrum of the sample after the induction-assisted reduction; B) Zoomed-in insert of the XRD spectrum of panel A.

[0025] Figure 5. Water splitting process. Upper panel: hydrogen release over time; lower part: time dependence of material weight (solid line) and temperature (dotted line).

[0026] DETAILED DESCRIPTION OF THE INVENTION

[0027] All terms as used herein in this application, unless otherwise stated, shall be understood in their ordinary meaning as known in the art. Other more specific definitions for certain terms as used in the present application are as set forth below and are intended to apply uniformly throughout the specification and claims unless an otherwise expressly set out definition provides a broader definition.

[0028] Throughout the description and claims the word “comprises" and variations of the word, are not intended to exclude other technical features, additives, components or steps. Furthermore, the word “comprise” encompasses the cases of “consist of’ and “consists essentially of’. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention.

[0029] For the purposes of the invention, any ranges given include both the lower and the upper end-points of the range. Ranges or values given, such as temperatures, times, molar ratio, volume ratio and the like, should be considered approximate when they are defined by the term “about” (i.e. with a 5% margin of variation around indicated point).

[0030] Through the description, reference will be made to “low temperature”, “high temperature” or “low temperature reduction”: in the context of the invention, “low temperature”, when used, refers to a temperature comprised between room temperature and 600 °C, while “high temperature” refers to temperatures higher than 600 °C.

[0031] Reduction method

[0032] As mentioned above, in a first aspect the invention relates to a method for reducing a reducible material, wherein the reducible material is a magnetic material, said method comprising the following steps: i) introducing the reducible material into a reactor cavity; ii) subjecting the reducible material to an alternating magnetic field; iii) heating the reducible material to at least the temperature at which the reduction of said material starts, thereby forming a reduced material and oxidized products; and iv) releasing from the reactor the generated oxidized products and, optionally, the reduced material, provided that only the reducible material is responsive to the alternating magnetic field and the reducible material is not subjected to an electric field.

[0033] The method has the advantage that no other elements other than the reducible material are “responsive” to, i.e. are inductively heated by, the alternating magnetic field.

[0034] In particular, the magnetic material may be a magnetic susceptor, that is a material or component which absorbs magnetic energy and converts it into heat; more particularly, no other magnetic susceptor materials are employed in the method. Thus, in a particular embodiment, the magnetic material is inductively heated by the alternating magnetic field and reduced, provided that no other magnetic susceptor materials are employed in the method.

[0035] In the method, no electric field is used. In a preferred embodiment, no microwave radiation is used. Thus, in more particular embodiment, the reducible material is the only material being a magnetic susceptor responsive to the alternating magnetic field, being inductively heated by the alternating magnetic field and then reduced, while is not subjected to an electric field. In a preferred embodiment, the reducible material of the first aspect of the invention is selected from a ferromagnetic, ferromagnetic and superparamagnetic material. The ferromagnetic, ferrimagnetic or superparamagnetic properties of the reducible material ensure that, when exposed to an alternating magnetic field, the reducible material will start heating up due to the rotation of the magnetic moments, appearance of hysteresis losses, or the generation of induction currents.

[0036] In an embodiment, the reducible material comprises inorganic cations susceptible to reduction.

[0037] In an embodiment, the reducible material may be in a solid state, in a molten state or suspended or dissolved in a fluid. In the case of solid state materials, they may be in powder form, porous structures or even mechanically supported on other non-active materials. In a preferred embodiment, the reducible material is solid.

[0038] In an embodiment, the reducible material with magnetic properties is selected from a metal oxide, a mixed metal oxide, a metal nitride (such as iron nitride, FexNy), a metal oxyfluoride (such as XQFeOF; wherein X is a trivalent metal and Q is a divalent metal) or a double perovskite halide (e.g., Cs2GeMXe with M=V, Mn or Ni and X=C1, Br or I) that can be reduced by the method of the invention.

[0039] In a particular embodiment, the structure of the reducible material may comprise dopants, such as transition metals, which alter the original physical-chemical properties of the reducible material. Preferred dopants include Fe, Co, V, Mn, Ni, Cr.

[0040] Examples of metal oxides and mixed metal oxides with magnetic properties include, but are not limited to, those having general formula MO, AB2O4 or ABO3, where M, A and B refer to metals or combination of metals with the particularly defined stoichiometry. Metal oxides also comprise M-MO (metal-metal oxide) and MO-MO (metal oxide-metal oxide) combinations. Reducible oxides are solid state materials that are characterized by the reversible oxidation state of the metal. Because of the reversibility, these materials are promising for storing and releasing oxygen, as well as, for a huge variety of catalytic processes.

[0041] In a preferred embodiment, the metal oxides and mixed metal oxides are selected from FesO4, CrO2, TiO2, SnO2, ZnO, CeO2, ImOs, MmOs-MnO, CoFe2O4, NiFe2O4, CuFe2O4, MnFe2O4, Ni-MgFe2O4, ZnFe2O4, ZrO2-supported NiFe2O4, Sr / Ca-LaFeOs, YsFesOn, BaFenOw, Lao.vSro.sMnOs, S^FeMoOe, combinations and doped variants thereof. The following table summarizes the preferred metal oxides and mixed metal oxides and specifies their magnetic properties. In a preferred embodiment, the reducible material is a ferrimagnetic material. More preferably, the reducible material is a mixed metal oxide AB2O4 selected from the table above, i.e. selected from CoFe2O4, NiFe2O4, CuFe2O4, MnFe2C>4, Ni-MgFe2O4 and ZnFe2O4. Even more preferably, the reducible material is CoFe2O4.

[0042] In a first step, the reducible material is introduced into a reactor cavity. The reactor may be a fixed bed reactor (e.g. the reducible material stays in a fixed position and does not undergo any translational or rotational movement) or a moving bed reactor (the reducible material undergoes translational and / or rotational movements).

[0043] In a particular embodiment, the reducible material is introduced in a fixed bed reactor, such as a quartz reactor. Preferably the reducible material is placed on top of a mechanical and inert support, such as quartz wool or other porous materials (e.g., porous ceramic membranes, ceramic foams, fiber meshes or filters, micro-perforated plates) which is placed at the bottom of the reactor.

[0044] In another particular embodiment, the reducible material is introduced in a moving bed reactor, along with a carrier gas through an inlet opening, so as the reducible material is circulating within the reactor. This can be made, for example, by means of a conveyor belt, a pneumatic conveyor or a screw conveyor.

[0045] The amount of reducible material and, optionally, of mechanical support material, will depend on the size of the reactor and on the desired amount of reduced material and oxidized products to be obtained. A skilled person would possess the required knowledge to decide what the suitable amount of reducible material is. The weight of the reducible material may be pre-determined before its introduction into the reactor cavity.

[0046] In a particular embodiment, a gas flow, such as argon, is passed through suitable openings of the reactor prior to or after introducing the reducible material with the aim of removing air and moisture from the reactor or calibrating and / or sensing a certain gas (e.g. oxygen). An inert gas flow can be kept constant or varied throughout the method of the invention; in a particular embodiment, argon is used at a constant flow, more particularly the flow is kept at about 75 mL / min.

[0047] In an embodiment, the reduction process may further comprise chemical reductants. A chemical reductant is a substance that can donate electrons to another chemical species (such as the reducible material), thereby causing the reduction of the latter. In a particular embodiment, the chemical reductants is selected from the group consisting of H2, CO, ammonia, hydrocarbons, methanol, ethanol and any combination thereof.

[0048] In an embodiment, graphite is not used in the reduction method. In an embodiment, graphite and coal are not used in the reduction method. In a more particular embodiment, solid carbon-containing chemical reductants are not used in the reduction method (wherein “solid” refers to ambient conditions). In a preferred embodiment, the reduction process is carried out without using graphite, CO, H2, CH4, natural gas, coal, brown-coal dust, hydrocarbons, ammonia and combinations thereof.

[0049] Preferably the reduction process is carried out without any additional chemical reductant; this means that the reducible material is heated by the alternating magnetic fields to at least the temperature at which the reduction of said material starts, said reduction taking place without the assistance of any further chemical reductant. The reducible material is then subjected to an alternating magnetic field. This is generated by an AC (alternating current) power generator which is activated at the desired power. In a preferred embodiment, the power of the AC generator is comprised between 2 kW and 30 MW, more preferably between 2 kW and 1 MW, even more preferably between 2 and 4 kW. In the most preferred embodiment, the power of the AC generator is about 3.5 kW. The power unit can also be expressed as 1 joule per second or 1 kg-m2-s3.

[0050] In an embodiment, the frequency of the alternated magnetic field is lower than 30 MHz. The present invention requires that no electric field is applied in the reduction method and a skilled person would be familiar with induction heating techniques and, therefore, how to apply an alternating magnetic field without, at the same time, applying an electric field. Among known techniques, the following scenarios may apply:

[0051] - In the far-field region of an oscillating magnetic dipole, the magnetic field can be predominantly alternating with the electric field components being relatively weak and at right angles (Far-Field Approximation);

[0052] - an alternating magnetic field can be created inside a shielded environment (e.g., using mu-metal or superconducting shields) that prevents the electric field components from entering or exiting (Shielded Environment);

[0053] - when the frequency of oscillation is very low, the induced electric fields are totally negligible compared to the magnetic fields (i.e. the reducible material is heated purely by the effect of the alternating magnetic field). This is often seen in practical applications like transformers or inductors at low frequencies where the displacement currents are minimal (Magnetostatic Approximation).

[0054] - in certain superconducting materials, magnetic fields can penetrate and oscillate without inducing electric fields within the superconductor due to the Meissner effect and the nature of superconductivity (superconductors). For example, in a preferred embodiment, the frequency of the alternated magnetic field is very low, i.e. equal to or lower than 800 kHz, preferably comprised between 100 kHz and 800 kHz, more preferably between 200 kHz and 600 kHz; even more preferably, the frequency is about 400 kHz. Among other options known to a skilled person (see above), such low frequencies ensure that the reducible material is subjected to an alternating magnetic field in absence of an electric field.

[0055] A sensor for the detection of signals due to oxidized products may be present and activated prior to or simultaneously with the AC generator.

[0056] The alternating magnetic field causes the heating of the reducible material by induction heating. Alternatively, a further source of heating can be used in combination with induction heating originated from the applied magnetic field; in this regard, conventional sources of heating, such as industrial waste heat, solar or electrical heating, combustion, are known to a person skilled in the art. Alternatively, heating sources selected from microwave irradiation, plasma / electric arc, metallurgical furnace, or heating by oxidation reactions are not used in the method.

[0057] In a preferred embodiment, no further heating sources other than the alternating magnetic field is used in the method.

[0058] Thus, in an embodiment, the reducible material is inductively heated by the alternating magnetic field and reduced with no further chemical reductants. This is also caused by non-thermal effects of the alternating magnetic field on the reducible material.

[0059] In an embodiment, no further heating sources other than the alternating magnetic field and no further chemical reductants, as defined above, are used in the method. The temperature of the reducible material and the amount of oxidized products released can be continuously monitored. The reduction of the reducible material takes place at low temperature, particularly at a temperature of less than 250 °C, less than 150 °C, less than 100 °C. In another embodiment, the reduction of the reducible material starts at a temperature comprised between 50 and 100 °C, more preferably between 60 and 90 °C, even more preferably between 80 and 90 °C.

[0060] In an embodiment, the method of the invention comprises steps i-iv, provided that: only the reducible material is responsive to the alternating magnetic field, the frequency of the alternating magnetic field is lower than 30 MHz, preferably, less than or equal to 800 KHz, more preferably between 200 and 600 KHz; and the alternating magnetic field has also a non-thermal effect on the reducible material, so that the reduction starts at a temperatures of less than 250 °C, preferably less than 150 °C, more preferably less than 100 °C.

[0061] In another embodiment, the reduction of the reducible material starts shortly after subjecting it to an alternating magnetic field, preferably after at least 5, at least 10, at least 15, at least 20, at least 25, at least 30 seconds; more preferably, the reduction starts after an amount of time between 5 and 60 seconds, more preferably between 10 and 50 seconds, even more preferably between 20 and 40 seconds, yet even more preferably after about 30 seconds from applying an alternating magnetic field.

[0062] In a preferred embodiment, the reduction starts at a temperature between 50 and 100 °C and after an amount of time between 5 and 60 seconds from applying an alternating magnetic field.

[0063] The heating goes up to a maximum which reflects the peak of reduction of the reducible material (hence the maximum formation of reduced materials) and release of oxidized products. In an embodiment, the maximum reduction temperature is lower than or equal to 600 °C. In a preferred embodiment, the maximum reduction temperature is comprised between 50 °C and 600 °C, more preferably between 100 °C and 500 °C, even more preferably between 50 °C and 400 °C. As derivable from above, the term “maximum temperature” refers to the temperature at which the maximum amount of reduced material is reached.

[0064] The method of the invention may afford nearly quantitative reduction of the reducible material. In an embodiment >20 wt% of the material is reduced; preferably >50 wt% of the material is reduced; more preferably, > 70 wt% of the material is reduced; even more preferably, >80 wt% of the material is reduced.

[0065] In a particular embodiment, the resulting reduced material comprises at least a metal oxide, i.e., a metal oxide having a lower valence than the corresponding reducible material from which it originates; preferably, a mixture of metal oxides.

[0066] Preferably, the reduced material comprises at least a metal in an elemental state; preferably, wherein more than 80 wt.% of the reducible material has been converted into at least a metal in an elemental state. The oxidized products are preferably in the form of a gas stream, and preferably comprise molecules such as O2, Ch, F2, Br2, S, H2, CO, CO or syngas; more preferably the oxidized products essentially consist of O2 molecules or syngas.

[0067] Once the reduced and oxidized products have been formed, the process of the invention further comprises the release of the oxidized products from the reactor.

[0068] In a particular embodiment, the release of the oxidized products is made by one of the following means or a combination thereof: application of vacuum, the use of a carrier gas, the use of a reactive stream that can react with the generated oxidized products, the use of a selective separator for separating the reduced material from the oxidized product.

[0069] In a particular embodiment, the release of the oxidized products is made by using a carrier gas. Examples of carrier gases include argon and nitrogen.

[0070] Said carrier gas is introduced in the reactor through at least an inlet opening. As the reaction takes place, the resulting oxidized products are released from the reactor by the carrier gas through at least an outlet opening.

[0071] In a particular embodiment, when the reaction takes place in a moving bed reactor, the reducible material can be introduced into the reactor along with the carrier gas through the same or different inlet openings. Within this particular embodiment, and once the reaction takes place, the carrier gas allows releasing both the reduced material and the oxidized products from the reactor through at least an outlet opening.

[0072] In a preferred embodiment, when the reduced material and the oxidized products are both released from the reactor, the resulting mixture is then subjected to a separation step in order to separate the reduced material from the oxidized products.

[0073] For example, cyclone separators or porous filters can be used for such separation.

[0074] In another embodiment, when the process takes place in a fixed bed reactor, the release of the oxidized products from the reactor can also be made by applying vacuum. For example, vacuum pumps can be placed connected to an outlet opening provided in the reactor.

[0075] When the reduction reaction is complete, the AC-power generator may be turned off while the reduced material can be cooled down under an inert gas flow. The completion of the reaction can be monitored by sensors, e.g. when the signal of the sensor detecting the oxidized products returns to its baseline. The process of the invention may also be accompanied by in-situ measurement of the temperature of the reducible material, the induced magnetic field, and the composition of the oxidized products generated. Likewise, the process can include the continuous adjustment of the intensity of the applied external magnetic field to improve the efficiency of the process.

[0076] Apparatus for carrying out the method of the invention

[0077] In a disclosure of the first aspect of the invention, the reduction process is carried out in an apparatus comprising:

[0078] - an induction furnace comprising an alternating current power generator, and an applicator (2);

[0079] - a reactor selected from fixed bed reactor and moving bed reactor, said reactor comprising means for introducing a reducible material into a reactor cavity and at least an outlet opening.

[0080] - optionally, process monitoring and control elements.

[0081] The induction furnace is responsible for the generation and application of the alternating magnetic fields to the reducible materials and may be assembled in different ways, provided that the induction furnace comprises an alternating current (AC) power generator and an applicator.

[0082] The induction furnace may also comprise power control elements and a cooling station. Power control elements may be present in order to tune the generated AC current, while a cooling station may be needed in order to cool down the electrical elements of the induction furnace and thereby prevent overheating of said elements.

[0083] The operating frequency range can go from 50 Hz to 1 MHz. The generated AC current is then transmitted to the applicator. By the term “applicator” can be understood a element that allows conducting the current to the reactor. Applicators are known to have different geometries (i.e. planar, helicoidal) depending on the design of the reactor, however a skilled person would recognize suitable geometries among the various possibilities. In a preferred embodiment, the applicator is made of metallic elements, preferably the applicator is a metal coil, more preferably is a copper coil.

[0084] The diameter of the applicator may be comprised between 1 and 5 cm, preferably between 2 and 4.0 cm, more preferably the diameter is 2.5 cm. In a preferred embodiment, the reactor is made from an inert material that i) does not interact with the magnetic field; ii) does not react with the reducible material or with the oxidized products generated during the reduction; and iii) withstands the expected working temperatures. Preferably the reactor is a quartz or alumina reactor; even more preferably, the reactor is a quartz tube; such tube may have different diameters depending on the application scale.

[0085] The reactor has means for introducing a reducible material into a reactor cavity and at least an outlet opening.

[0086] The means for introducing the reducible material into the reactor can be varied and include, for example, an opening gate or at least one inlet opening.

[0087] In a particular embodiment, said at least one inlet opening also allows introducing a carrier and / or inert gas and / or optional chemical reductants into the reactor.

[0088] On the other hand, the at least one outlet opening allows releasing the oxidized products generated in the reduction reaction and, optionally, the generated reduced materials. When a carrier and / or inert gas is introduced in the reactor through the at least one inlet opening, the carrier and / or inert gas can also be released through the at least one outlet opening along with the oxidized products.

[0089] The reactor may be a fixed bed reactor or a moving bed reactor.

[0090] In a preferred embodiment, the reactor is a fixed bed reactor. Figure 1(A) schematically depicts the components of the apparatus of the invention in a fixed bed configuration.

[0091] Within this embodiment, the apparatus comprises an induction furnace comprising an alternating current (AC) power generator (1) and an applicator (2) as defined herein above, wherein the applicator (2) is preferably in the form of metallic coils. The induction furnace may also comprise power control elements and a cooling station (not shown) such as also mentioned above.

[0092] In such configuration, the reducible material is introduced in the reactor (3), for example through an opening gate of said reactor, thus forming a reducible material bed (6). The reducible material can be placed on an inert support (7) inside the reactor (3), preferably homogeneously distributed as a layer with varying thickness. The inert support can be quartz wool or other porous mediums (e.g., porous ceramic membranes, ceramic foams, fiber meshes or filters, micro-perforated plates) and is placed in the bottom part of the reactor (3). Within this configuration, the reactor (3) may be provided with at least one inlet opening (4) which allows the introduction of a carrier and / or inert gas, such as argon, in the reactor. The fixed bed reactor also includes at least one outlet opening (5) for releasing the oxidized products resulting from the reduction reaction. When a carrier and / or inert gas is introduced in the reactor through the at least one inlet opening (4), the at least one outlet opening also allows releasing said carrier and / or inert gas, along with the oxidized products.

[0093] A fixed bed type reactor may additionally comprise elements to evacuate the oxidized products generated during the reduction process, such as vacuum pumps or pumps for the dragging and circulation of fluids.

[0094] In another embodiment, the reactor is a moving bed reactor.

[0095] Figure 1(B) schematically depicts the components of the apparatus of the invention in a moving bed configuration.

[0096] Within this embodiment, the apparatus also comprises an induction furnace comprising an alternating current (AC) power generator (1) and an applicator (2) as defined herein above, wherein the applicator (2) is preferably in the form of metallic coils. The induction furnace may also comprise power control elements and a cooling station (not shown) such as also mentioned above.

[0097] In such configuration, the reducible material is introduced in the reactor (3) along with a carrier gas through at least an opening inlet (4) of said reactor, thus allowing the reducible material (6) to circulate within the reactor (3).

[0098] In this particular embodiment, the reactor comprises means of conveying the reducible material to the inlet opening (4) of the reactor (3). When the reducible material is solid, said means of conveying include, for example, conveyor belt, a pneumatic conveyor or a screw conveyor.

[0099] The moving bed reactor also includes at least one outlet opening (5) for releasing the oxidized products resulting from the reduction reaction, along with the carrier gas.

[0100] Within this particular configuration, the apparatus of the invention further comprises a separator (8) at the reactor outlet opening (5) where the reduced material is separated from the oxidized products generated during the reduction process.

[0101] Cyclone separators or porous filters can be used for the separation of the reduced material from the oxidized products generated during the reduction process. Regardless of the configuration of the reactor, optional sensing means placed at the outlet opening of the reactor may allow the quantification of the oxidized products. In a particular embodiment, the sensing means are O2 sensors which detect variations in the O2 released at the outlet opening.

[0102] In a particular embodiment, the apparatus may further comprise temperature sensing means (such as a pyrometer) which provide contactless external measurement of the temperature inside the reactor. The temperature sensing means are preferably connected to the AC generator so to regulate the output power to reach / maintain the programmed temperature.

[0103] In one embodiment, the temperature sensing means are placed on one side of the reactor and perpendicular to it. The sensor is directed on one side of the reduced material inside the reactor. The temperature measured externally can be significantly lower than the one inside of the reactor. In fact, the external layer of the reducible material may be more susceptible to heat-losses, particularly:

[0104] • at the beginning of the exposure to the AC field, when the reducible material rapidly heats up but the reactor walls, which do not interact with the AC filed, stay cold, thereby forming a thermal gradient between the internal and external portions of the heated material; and / or

[0105] • when working at low power (i.e. low magnetic field strength) or with reducible materials that are poor absorbers; in fact, the rate of the heat flow dissipated through the reactor can be comparable with the rate of the energy flow absorbed by the reducible material.

[0106] Thus, in order to circumvent the potential above-mentioned issues, the reactor may be equipped with a top opening with a gas-tight connection that allows the introduction of temperature sensing means along the vertical axis of the reactor and permits the direct internal temperature measurement.

[0107] Optionally, the apparatus of the invention can be further equipped with process monitoring and control elements selected from temperature sensors (e.g., infrared thermometer), video cameras for the observation of the material during the reduction process, devices for the analysis of the composition of the products of the reduction method. Reduced materials

[0108] Another disclosure of the invention refers to a reduced material obtainable by the method of the first aspect of the invention.

[0109] The reduced materials are characterized by comprising metals or a mixture of metals at a lower oxidation state or even elemental state. Preferably, when the reducible material is CoFe2O4, the reduced material may comprise elemental iron, cobalt and unconverted CoFe2O4; more preferably, elemental iron ranges from 60 to 90 wt%, while elemental cobalt ranges from 1 to 10 wt%. Most preferably, the reduced material comprises about 80 wt% Fe, about 7 % Co and unconverted CoFe2O4.

[0110] The reduced material can be characterized by XRD, particularly, using a monochromatic Cu Kai radiation source of = 1.54056 A. The reduced material displays a specific diffractogram. In a specific embodiment, reduced material obtained by reduction of CoF e2O4 according to the method of the invention displays peaks at 30.2°, 35.5°, 43.2°, 53.5°, 57.1°, 62.7° and 73.9° corresponding to CoFe2O4, peaks at 44.9° and 65.2° corresponding to metal iron and peaks at 44.1° and 51.5° corresponding to cobalt.

[0111] Uses of the reduced materials

[0112] The reduced material obtained by the process of the invention, either removed from or remaining in the reactor, can be subjected to subsequent steps in order to further produce a variety of new reduced products or be used in different applications in the energy industry.

[0113] In fact, the invention also relates to the use of the reduced material obtained according to the process of the invention, in thermal energy storage, in the production of hydrogen, syngas and / or hydrocarbons, and as selective absorbent of impurities in gaseous streams. Most of these uses involve the re-oxidation of the reduced material, mainly for producing new reduced materials. Thus, in a particular embodiment, the process of the invention further comprises the re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, in the presence of at least an oxidizing agent or of a molecule susceptible of being reduced.

[0114] Depending on the application of the reduced material, the oxidizing agent or the molecule susceptible of being reduced will be selected accordingly. Thermal energy storage. For example, the reduced materials find use in thermal energy storage involving their redox cycling. A redox cycle of reducible materials involves two steps: 1) the charging step of the storage system, in which heat is supplied to reducible materials and both reduced materials and fluid oxidized products are generated (endothermic step, corresponding to the method of the invention); and 2) the discharging step of the storage system, in which the reduced material is then re-oxidized in the presence of O2 releasing heat (exothermic step).

[0115] Remarkable advantages of thermal energy storage are the compactness (high energy densities) and the possibility of storing energy without losses for long periods of time. Therefore, in a particular embodiment, the invention also relates to the use of the reduced material obtained by the process of the invention in thermal energy storage.

[0116] Accordingly, in a particular embodiment, the process of the invention further comprises the re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, in the presence of oxygen as oxidizing agent.

[0117] The reduced materials of the invention are obtained at lower temperature compared to those obtained in the prior art requiring very high temperatures (i.e. > 1000 °C to reduce metal oxides). High reduction temperature lead to problems related to the stability of the storage material (e.g., sintering over successive cycles of use) and to the cost of the materials needed to build the tank and heat / mass exchangers, which must withstand very high working temperatures. Thus, the use of the reduced materials of the present invention represents a considerable practical and economical advantage.

[0118] H2 production. In another embodiment, the invention relates to the use of the reduced materials obtained by the process of the invention in H2 production.

[0119] In a preferred embodiment, the production of H2 is made by thermochemical watersplitting cycles.

[0120] Accordingly, and within this preferred embodiment, the process of the invention further comprises the re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, by bringing it into contact with water vapor to produce H2. In another preferred embodiment, the production of H2 is made by using H2S as oxidizing agent. Accordingly, and within this preferred embodiment, the process of the invention further comprises the re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, in the presence of H2S as oxidizing agent to produce H2. Again, the method of the present invention achieves the reduction of reducible materials at low-to-moderate temperatures (< 600 °C), which is very competitive with the high temperatures employed in the prior art, where the heat sources needed are limited in number (e.g., solar concentrators, waste heat from intensive industry) and the methods involve problems with durability and cost of materials. In addition, when powered by renewable electricity, the present invention may compete with electrolyzers in the production of green H2.

[0121] CO2 valorization. In another embodiment, the invention relates to the use of the reduced material obtainable by the process of the invention for CO2 splitting or co-splitting of water and CO2.

[0122] The reduced materials of the present invention can be used in thermochemical cycles for CO2 splitting, or co-splitting of water and CO2.

[0123] Accordingly, the process of the invention further comprises the re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, by bringing it into contact with CO2 or with a mixture of CO2 and water, to produce CO and / or syngas.

[0124] In particular, the reduced material is brought into contact with CO2, being reoxidized to a reducible material and simultaneously producing CO (or syngas). Alternatively, the reduced material is brought into contact with a mixture of CO2 and H2O, being reoxidized to a reducible material and simultaneously producing syngas (CO and H2). CO is a powerful reducing agent with multiple uses in the industry. In addition to being an energy carrier and a reducing agent, syngas is commonly used to produce methanol or hydrocarbons (e.g., alkanes, olefins, aromatic compounds, waxes, etc.). Thus, the reduced materials of the present invention allow low temperature thermochemical cycles for CO2 valorization as opposed to prior art methods which require high temperatures required in the reduction of the reducible material.

[0125] Production of chemicals. The reduced materials obtained according to the process of the invention can also be used to produce reduced products with new functionalities while also being oxidized. In this particular case, the reduced material is brought into contact with a molecule susceptible of being reduced. Molecules such as alkanes, alkenes, naphthene or aromatic hydrocarbons could be suitable substrates to form reduced products with new functionalities. For example, reaction of the reduced material with methane or ethane will give olefins, hydrogen, synthesis gas or aromatic hydrocarbons. Selective absorption of a gaseous stream. In another embodiment, the invention relates to the use of the reduced material obtainable by the process of the invention as a selective absorbent of impurities in gaseous streams. The reduced material obtained by the process of this invention can react with and / or remove impurities from a gaseous stream selectively by fixing them in their structure. In a particular embodiment, the impurities are selected from O2, O3, Ch, F2, Br2, HC1, H2S, N2O, NOXor mixtures thereof. Thus, the reduced material acts as an absorbent material and can be regenerated when subjected to the action of an alternating magnetic field according to the method of the first aspect of the invention.

[0126] Further embodiments

[0127] Further embodiments are also disclosed in the present application:

[0128] 1. A method for reducing a reducible material, wherein the reducible material is a magnetic material, said method comprising the following steps: i) introducing the reducible material into a reactor cavity; ii) subjecting the reducible material to an alternating magnetic field; iii) heating to at least the temperature at which the reduction of the reducible material starts, thereby forming a reduced material and oxidized products; and iv) releasing from the reactor the generated oxidized products and, optionally, the reduced material.

[0129] 2. The method according to embodiment 1, wherein the reducible material is a ferromagnetic, ferrimagnetic or superparamagnetic material selected from a metal oxide, a mixed metal oxide, a metal nitride, a metal oxyfluoride or a double perovskite halide.

[0130] 3. The method according to any of embodiments 1 or 2, wherein the reducible material is selected from CoFe2O4, NiFe2O4, CuFe2O4, MnFe2C>4, Ni-MgFe2O4, ZnFe2C>4 or ZrCh supported NiFe2O4; preferably, the reducible material is CoFe2O4. 4. The method according to any one of the preceding embodiments, wherein the frequency of the alternated magnetic field in step ii) is comprised between 100 kHz and 800 kHz, more preferably between 200 kHz and 600 kHz; even more preferably, the frequency is about 400 kHz.

[0131] 5. The method according to any one of the preceding embodiments, wherein the maximum reduction temperature is comprised between 50 °C and 600 °C.

[0132] 6. The method according to any one of the preceding embodiments, wherein the oxidized products comprise at least a molecule selected from O2, Ch, F2, E and S.

[0133] 7. The method according to any one of the preceding embodiments, wherein the reduced material comprises at least a metal oxide; preferably, a mixture of metal oxides.

[0134] 8. The method according to any one of the preceding embodiments, wherein the reduced material comprises at least a metal in an elemental state; preferably, wherein more than 80 wt.% of the reducible material has been converted into at least a metal in an elemental state.

[0135] 9. The method according to any of embodiments 1 to 8, which further comprises:

[0136] - re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, in the presence of oxygen as oxidizing agent; or

[0137] - re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, by bringing it into contact with water vapor to produce H2; or

[0138] - re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, in the presence of H2S as oxidizing agent to produce H2; or

[0139] - re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, by bringing it into contact with CO2 or with a mixture of CO2 and water, to produce CO and / or syngas 10. An apparatus for carrying out the method of any of embodiments 1 to 8, said apparatus comprising:

[0140] - an induction furnace comprising an AC power generator (1) and an applicator (2);

[0141] - a reactor (3) selected from a fixed bed reactor and a moving bed reactor, said reactor comprising means for introducing a reducible material into a reactor cavity, and an outlet opening (5); and

[0142] - optionally, process monitoring and control elements.

[0143] 11. The apparatus according to embodiment 10, wherein the reactor (3) is a fixed bed reactor which further comprises at least one inlet opening (4) which allows the introduction of a carrier and / or inert gas in the reactor (3), and wherein the apparatus optionally further comprises elements to evacuate oxidized products, said elements being selected from vacuum pumps or pumps for the dragging and circulation of fluids.

[0144] 12. The apparatus according to embodiment 10, wherein the reactor is a moving bed reactor, wherein the means for introducing the reducible material to the inlet opening (4) of the reactor (3) are selected from a conveyor belt, a pneumatic conveyor and a screw conveyor, and wherein the apparatus further comprises a separator (7) at the reactor outlet opening (5).

[0145] 13. The apparatus according to any of embodiments 9 to 12, wherein the control elements are temperature sensors outside the reactor providing external measurement of the temperature inside the reactor.

[0146] 14. Reduced materials obtainable by the method according to any of embodiments 1 to 8, said materials preferably comprising elemental Fe, elemental Co and unconverted CoFe2O4.

[0147] 15. Use of the reduced materials according to embodiment 14 in thermal energy storage; in the production of hydrogen, syngas and / or hydrocarbons; and as selective absorbents of impurities in gaseous streams. EXAMPLES

[0148] The following examples are intended to illustrate but not to limit the disclosed embodiments.

[0149] Reagents and starting materials

[0150] CoFe2O4 supplied by Inframat® Advanced MaterialsTM LLC (USA) has been used. The main features of CoFe2O4 are the following:

[0151] Purity: 99.9 %

[0152] Particles size: 20-50 nm

[0153] Cristal size (XRD): 25 nm

[0154] Surface Area (BET): 46 m2 / g

[0155] Average pore size: 8.75 A

[0156] Coercivity: 44.21 Oe

[0157] Remanent magnetization: 0.42

[0158] Saturation magnetization 2.14 pBN

[0159] Material characterization.

[0160] The as-synthetized CoFe2O4, the reduced CoFe2O4, and CoFe2O4 after water splitting experiments were characterized by X-ray diffraction analysis using a Bruker D8 Discover equipped with a LYNXEYE XE detector and a monochromatic Cu Kai radiation source of = 1.54056 A. Phase identification was performed using the EVA diffraction commercial software. The collected patterns were then analysed according to the Rietveld method by using the MAUD software. Magnetic properties were carried out in a Quantum Design MPMS-XL5 SQUID magnetometer. The magnetization of the as-synthesized CoFe2C>4, as a function of the applied magnetic field was recorded from -1 T to +1 T at 300 K.

[0161] Description of the equipment

[0162] The equipment for carrying out the reduction process can be depicted in figure 2.

[0163] The apparatus included a power generator of alternated current (not shown in the figure) with maximum power of 3500 W and frequency range from 375 to 575 KHz (Ceia). The generated AC current is transmitted to a copper coil (2) of around 2.5 cm of diameter. The material to be reduced (6) is placed inside a quartz tube reactor (3) with external diameter of 2 cm. To keep the material particles in place at the desired height, a portion of quartz wool (7) is placed in the bottom part of the quartz reactor.

[0164] The quartz reactor is equipped with:

[0165] • an inlet opening (4) with a gas-tight connection for the introduction of a carrier gas (9); and

[0166] • an outlet opening (5) with a gas-tight connection for the evacuation of the gases (10), which allows the quantification of the O2 released by means of an O2 microsensor (11) (detection limit ~ 10'3vol.%, Unisense, Denmark).

[0167] Typically, the experimental setup for induction heating relies on the contactless external measurement of the temperature, which can be performed by placing a pyrometer on one side of the reactor and perpendicular to it. In the present case, the quartz reactor is equipped of a top opening (12) with a customized gas-tight connection that allows introducing a pyrometer (13) along the vertical axis of the reactor and permits the direct internal temperature measurement.

[0168] Example 1 : Procedure for the reduction process.

[0169] The induction-assisted reduction process of the sample was carried out in the experimental device according to figure 2 under the following experimental conditions:

[0170] - Mass of the CoFe2O4 sample: 1.5 g

[0171] - Quartz reactor with external diameter of 2 cm

[0172] - Sweeping gas used: Argon (75 ml / min)

[0173] - Power of the induction generator: 3.5 kW

[0174] - Frequency of the alternated magnetic field: 400 kHz

[0175] The experimental procedure for the magnetic-induced reduction involved the following steps:

[0176] 1) a portion of quartz wool of about 2 cm high was introduced inside the quartz reactor and pushed to the bottom to form a flat support for the material to be reduced. 2) 1.5 grams of CoFe2O4 powder were introduced inside the reactor on top of the quartz wool and the reactor was gently shaken to arrange the powder in a flat layer.

[0177] The inlet and the outlet openings of the reactor were closed and connected to the Ar reservoir and to the O2 microsensor, which was previously calibrated by using a two-point calibration. To this aim, pure argon and synthetic air (20 vol.% O2 in N2) were used.

[0178] 3) The distance between the pyrometer and the top surface of the CoFe2O4 powder was adjusted to ensure the right focus of the sensor.

[0179] 4) An argon flow of 75 mL / min was run through the reactor for 5 minutes to remove air and moisture from the reactor.

[0180] 5) The AC power generator was activated at the desired power and the acquisition of the O2 sensor signal was started simultaneously while the argon flow was kept at 75 mL / min.

[0181] 6) When the reduction was complete and the oxygen signal came back to the baseline value, the AC-power generator was turned off and the reduced CoFe2O4 was let cool down inside the reactor by maintaining the argon flow at 75 mL / min.

[0182] During the experiment, the temperature of the sample and the amount of O2 released were continuously monitored. The results achieved are shown in Fig. 3, where the sample started to heat-up as soon as the alternating magnetic field was applied (power on) and progresses steadily and smoothly until the Curie temperature of the material was reached. At such temperature, the material loses its magnetic properties, and it does not continue to heat up. During heating, oxygen was generated, meaning that the reduction of the material was taking place. As shown in Fig. 3, the reduction started after about 0.5 min and at very low temperature (~ 88 °C). O2 release occured from the beginning of the reduction process until a maximum value of 0.65 pmol / gcoFe2O4 at 170 °C was reached. Then, it slowed down. The reduction process was fast, ending after ca. 9 min.

[0183] The reduced sample was analyzed by XRD to identify and quantify the crystalline phases. The experimental conditions for XRD analysis are described in the “Material characterization” section. The quantification was performed using the Rietveld method. As shown in Fig. 4, the major phase of the sample was metallic iron (80.1 wt.%). Also 6.6 wt.% cobalt and 13.3 wt.% of original CoFe2O4were present. This indicates that an extensive reduction of the cobaltite was achieved, which is unusual when the reduction is performed by a conventional heating method. This is further highlighted in Table 1, which shows the advantages of the reduction method proposed in the present invention compared with a conventional heating method. In the latter case, results may vary slightly depending on the synthesis method and experimental conditions. The results obtained by Goikoetxea et al. (Int. J. of Hydrogen Energy 44(33): 17578-17585, 2019) are representative enough of the reduction by conventional heating and have been chosen for comparison. From Table 1, it is apparent that the use of an alternate magnetic field to heat up the material leads to:

[0184] • Substantial decrease of the working temperature: the material started being reduced at 88 °C, far below the usual reduction temperature needed by conventional heating (> 1400 °C).

[0185] • Substantial decrease of the process duration: oxygen release is completed in 9 minutes instead of hours.

[0186] • Higher reduction yield of the material: after the reduction, the main phases present in the sample are metallic phases instead of oxide phases.

[0187] Table 1. Comparison between reduction by conventional heating method and the reduction method disclosed herein (reduction assisted by alternating magnetic fields or induction-assisted reduction).

[0188] Example 2, Equipment and procedure for water splitting with the reduced material.

[0189] The reduced CoFe2O4 was removed from the quartz reactor to test the H2 production in a water splitting reaction. Water splitting experiments were performed using an STA 449 F3 Jupiter (NETZSCH) thermobalance coupled with a water vapour generator provided by aDROP GmbH. The outlet gas of the STA oven was connected to an H2 Clark-type microsensor (detection limit ~ 10'3vol.%) interfaced with an amplifier (Unisense, Denmark). The sensor was calibrated before each measurement by using a two-point calibration. To this aim, pure Ar and an Ar with 2 vol.% H2 standards were used.

[0190] The samples were heated at 10 °C min'1up to 650 °C under a 75 mL / min argon flow. Water vapour was then introduced into the gas flow at a rate of 2 g H2O / hour for 3 hours. When the water supply was stopped, the argon flow was maintained, and the sample was cooled to room temperature.

[0191] The sample was first heated at 10 K / min up to 650 °C under a 75 mL / min Ar flow. Steam was then introduced into the Ar flow at a rate of 2 g H2O / I1 while the temperature is keeping constant (isothermal conditions) and the H2 content in the outlet gas of the thermobalance oven is continuously monitored.

[0192] The lower part of figure 5 shows the temperature evolution of the sample as well as its weight changes, whereas H2 generation over time is plotted in the upper part. The total amount ofH2 released was equal to 3.9 mmol / g (8 gH2 / g CoFe2O4 approx.), which largely surpassed the values obtained when CoFe2O4 is reduced by conventional heating at high temperature (0.83 mmol / g) and under comparable water splitting conditions. Also, the time required to complete the re-oxidation of the material is much lower. As shown in Fig. 5, the duration of the water dissociation process was less than 60 min, whereas it takes several hours for the material reduced by conventional heating method at high temperature.

Claims

CLAIMS1. A method for reducing a reducible material, wherein the reducible material is a magnetic material, said method comprising the following steps: i) introducing the reducible material into a reactor cavity; ii) subjecting the reducible material to an alternating magnetic field; iii) heating the reducible material to at least the temperature at which the reduction of the reducible material starts, thereby forming a reduced material and oxidized products; and iv) releasing from the reactor the generated oxidized products and, optionally, the reduced material, provided that only the reducible material is responsive to the alternating magnetic field and the reducible material is not subjected to an electric field.

2. The method according to claim 1, wherein the reducible material is a ferromagnetic, ferrimagnetic or superparamagnetic material selected from a metal oxide, a mixed metal oxide, a metal nitride, a metal oxyfluoride or a double perovskite halide.

3. The method according to any of claims 1 or 2, wherein the reducible material is selected from CoFe2O4, NiFe2O4, CuFe2O4, MnFe2O4, Ni-MgFe2O4, ZnFe2C>4 or ZrCh supported NiFe2O4; preferably, the reducible material is CoFe2O4.

4. The method according to any one of the preceding claims, wherein a chemical reductant is further used in the method, preferably being the chemical reductant selected from a gaseous stream comprising, preferably consisting of, hydrogen, carbon monoxide, ammonia, hydrocarbons, methanol, ethanol or a combination thereof.

5. The method according to any one of the preceding claims, wherein the frequency of the alternated magnetic field in step ii) is equal to or lower than 800 kHz, preferably comprised between 100 kHz and 800 kHz, more preferably between 200 kHz and 600 kHz; even more preferably, the frequency is about 400 kHz.

6. The method according to any one of the preceding claims, wherein the maximum reduction temperature is comprised between 50 °C and 600 °C.

7. The method according to any one of the preceding claims, wherein the oxidized products comprise at least a molecule selected from O2, Ch, F2, Br2, S, H2, CO and CO2.

8. The method according to any one of the preceding claims, wherein the reduced material comprises at least a metal oxide; preferably, a mixture of metal oxides.

9. The method according to any one of the preceding claims, wherein the reduced material comprises at least a metal in an elemental state; preferably, wherein more than 80 wt.% of the reducible material has been converted into at least a metal in an elemental state.

10. The method according to any one of the preceding claims, wherein additional heating sources selected from microwave irradiation, plasma / electric arc, metallurgical furnace, or heating by oxidation reactions are not used in the method; preferably, no heating sources other than the alternating magnetic field are used.

11. The method according to any one of the preceding claims, wherein the reduction process is carried out without using chemical reductants selected from graphite, CO, H2, CH4, natural gas, brown-coal dust, hydrocarbons, ammonia and combinations thereof; preferably without any chemical reductant.

12. The method according to any one of the preceding claims, which further comprises:- re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, in the presence of oxygen as oxidizing agent; or- re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, by bringing it into contact with water vapor to produce H2; or- re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, in the presence of H2S as oxidizing agent to produce H2; or- re-oxidation of the reduced material obtained in step iii), and separated from the oxidized products, by bringing it into contact with CO2 or with a mixture of CO2 and water, to produce CO and / or syngas13. The method of any one of the preceding claims, wherein the reduction is carried out in a reactor (3) selected from a fixed bed reactor and a moving bed reactor, said reactor comprising means for introducing a reducible material into a reactor cavity, and an outlet opening (5); and wherein said reactor (3) is comprised in an apparatus further comprising:- an induction furnace comprising an AC power generator (1) and an applicator (2); and- optionally, process monitoring and control elements.

14. The method according to claim 12, wherein the reactor (3) is a fixed bed reactor which further comprises at least one inlet opening (4) which allows the introduction of a carrier and / or inert gas in the reactor (3), and wherein the apparatus optionally further comprises elements to evacuate oxidized products, said elements being selected from vacuum pumps or pumps for the dragging and circulation of fluids.

15. The method according to claim 12, wherein the reactor is a moving bed reactor, wherein the means for introducing the reducible material to the inlet opening (4) of the reactor (3) are selected from a conveyor belt, a pneumatic conveyor and a screw conveyor, and wherein the apparatus further comprises a separator (7) at the reactor outlet opening (5).

16. The apparatus according to any of claims 12 to 14, wherein the control elements are temperature sensors outside the reactor providing external measurement of the temperature inside the reactor.

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