Process for converting CO2 into methane

Abstract

The invention relates to a process for converting CO2 into methane in which hydrogen and a gaseous feedstock comprising CO2 are brought into contact in at least one methanation reactor comprising a catalyst bed at a temperature in the catalytic bed of between 160 and 550°C, at a pressure of between 0.1 and 1 MPa, with a gas hourly space velocity of between 10 and 50 m3 / kg / h and with an H2 / CO2 molar ratio of between 1 and 8 and in which the catalyst contains Ni metal deposited on a uranium oxide support of formula UO2+x, with x being between 0.01 and 0.6, and the mass content of which is between 5 and 40% of nickel metal relative to the total mass of the catalyst.

Description

Title of the invention: Process for converting CO2 into methane

[0001] The present invention relates to a process for the catalytic conversion of carbon dioxide in the presence of hydrogen into methane (methanation process) using a nickel-based catalyst dispersed on a uranium oxide-based support. State of the art

[0002] Sustainable energy production, combined with moderate consumption practices, represents a challenge for our civilization. Over the last decade, the continuous increase in global energy demand and the collective awareness of the problem of global warming have led to the development of means of producing electrical energy from renewable sources.

[0003] This massive integration of renewable energy sources into the energy landscape, however, comes up against the problem of managing electricity networks. The intermittent and localized nature of most of these sources, particularly wind and solar, complicates the balancing of these networks at all times, between production and demand for electrical energy. Added to this is the need to be able to manage the storage of surplus electricity so that it can be easily used in the event of a drop in the efficiency of renewable energy production methods.

[0004] Current storage methods only allow limited quantities of energy to be stored and can therefore only meet needs that extend over short periods, a few days at most.

[0005] In recent years, the idea of ​​an alternative storage method to traditional technologies has emerged and proposes using energy in chemical form. This storage method, called Power-to-Gas, proposes converting electrical energy into gas, for example hydrogen or methane, which is used as a storage vector. This chemical storage has the advantage that it can be maintained over long periods compared to those of the systems currently implemented.

[0006] The conversion of electrical energy into gas also offers numerous possibilities for the final use of this energy, such as domestic heating, industrial use or even personal mobility.

[0007] The Power-to-Gas process consists in particular of implementing one or two conversion stages depending on the chosen storage gas. This vector can be either hydrogen, produced by the electrolysis of water powered by solar or wind energy, or methane, produced in a second stage called methanation which carries out the conversion of hydrogen and carbon dioxide into methane. In addition to allowing the recovery CO2 considered as a by-product of industry, the achievement of this additional step makes it possible to limit the constraints and the cost of adapting storage, distribution and final use of the energy vector.

[0008] The methanation reaction converts CO2 in the presence of hydrogen into methane and water (Equation 1).

[0009] COj + 4H2 CH4 + 2Hî0 Equation 1

[0010] This reaction is highly exothermic and releases significant heat (AHO298K = -165.0 kJ / mol). According to Le Chatelier's principle, the formation of methane is favored at low temperature and high pressure. However, the CO2 molecule is a stable linear molecule composed of two O=C double bonds, hence the need to provide excess energy to activate this molecule and to use a catalyst to overcome the significant kinetic limitations of this reaction.

[0011] Numerous researches carried out to develop catalysts to reduce the activation energy barrier of this reaction have shown that noble metal catalysts are particularly active but their high prices constitute a limit to their uses. It appears that catalysts based on Ni dispersed on a support are the most promising for the methanization of COX (x = 1, 2) due to their good catalytic performances and their relatively low price. The supports of commonly used nickel-based catalysts are oxides such as Al2O3, SiO2, TiO2, ZrO2 and CeO2.Among them, Ni / SiO2 and Ni / Al2O3 have been widely studied due to their good initial activities but suffer, when implemented at high temperature, from deactivation phenomena due to sintering of the particles of the active phase (reducing the number of active sites) and significant carbon deposits (coke) thus blocking the access of the reactants to the active sites. Currently, catalysts based on Ni supported on an Al2O3 support are available from manufacturers such as Johnson Matthey, Haldor-Topspe or Clariant-Süd Chemie.

[0012] Also known in the state of the art is the publication by Berry et al. (Applied Catalysis A: General 100 (1993) 131-143) which focuses on catalysts based on Ni and uranium oxide useful for the CO2 methanation reaction. The catalysts are prepared by evaporation of an aqueous solution containing nickel nitrate and uranyl nitrate until the formation of a viscous residue which solidifies at room temperature. The solid thus obtained is then calcined in air at a temperature of 1000°C.

[0013] An aim of the present invention is to propose a CO2 methanation process which meets several criteria, in particular in terms of CO2 conversion rate, CH4 selectivity, productivity and which can be operated in particular at temperatures temperatures below 350°C and preferably below 300°C, or even below 260°C. Summary of the invention

[0014] The invention therefore relates to a process for converting CO2 into methane in which hydrogen and a gaseous feedstock comprising CO2 are brought into contact in at least one methanation reactor comprising a catalyst bed, at a temperature in the catalytic bed of between 160 and 550°C, at a pressure of between 0.1 and 1 MPa, with an hourly gas space velocity of between 10 and 50 m3 / kg / h and with an H2 / CO2 molar ratio of between 1 and 8 and in which the catalyst contains Ni metal deposited on a uranium oxide support of formula UO2+X, with x being between 0.01 and 0.6, and the mass content of which is between 5 and 40% of nickel metal relative to the total mass of the catalyst.

[0015] Surprisingly, the applicant has found that the use of a catalyst comprising nickel metal dispersed on a support based on uranium oxide UO2+x with x being between 0.01 and 0.6 exhibits an increased activity for the conversion of CO2 into methane, so that the process can be carried out at lower temperatures than that of the prior art, while maintaining a high yield and selectivity in CH4, i.e. respectively greater than 60% and close to 100%.

[0016] The catalyst is advantageously prepared by a process comprising the following steps: a) a support precursor consisting essentially of a uranium (IV) and / or uranium (VI) oxide is impregnated with a solution containing a nickel precursor and a polar solvent; b) the impregnated support precursor is calcined in air and at a temperature of at least 250°C; c) the impregnated and calcined support precursor is reduced under hydrogen at a temperature of at least 300°C.

[0017] The term "consisting essentially of an oxide of uranium (IV) and / or uranium (VI)" is understood to designate a support whose oxide content of uranium (IV) and / or uranium (VI) is at least 90% by mass.

[0018] The measurement of the temperature of the catalytic bed can be carried out by any method known to those skilled in the art, such as for example by means of one or more thermocouples arranged in said bed or by laser pyrometry.

[0019] According to particular embodiments, the method comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations.

[0020] The process for preparing the catalyst according to the invention may comprise, before step b) of calcination, a step in which the impregnated support precursor is dried at a temperature below 200°C in order to remove in particular the solvent from the impregnation solution. Preferably, the solution contains a polar solvent in which the nickel precursor is solubilized. Preferably, it is an aqueous solution containing the nickel precursor.

[0021] Preferably, calcination step b) is carried out at a temperature of at least 300°C for at least 1 hour, preferably for at least 2 hours.

[0022] Preferably, reduction step c) is carried out at a temperature of at least 350°C for at least 1 hour, preferably for at least 2 hours. The hydrogen required for the reduction is supplied in the form of pure hydrogen. This reduction step not only converts uranium (VI) oxide or uranium (IV) and (VI) oxide (eg U3O8) into UO2+X and forms nickel metal.

[0023] The support precursor based on uranium (IV) and / or uranium (VI) oxide may be chosen from UO2, UO3, UO4 and U3O8. Preferably the support precursor is U3O8.

[0024] According to the invention, the support precursor can take any form. For example, the support precursor has a morphology in the form of cylindrical or multi-lobed extrudates, a sphere or a powder of variable particle size. According to one embodiment, when the support precursor is a powder, it can advantageously be shaped (e.g., ball or extrudate) after the step of impregnating the nickel precursor.

[0025] For the impregnation step, the nickel precursor may be chosen from nickel hydroxide, hydroxycarbonate, carbonate and nitrate, with a preference for nickel nitrate. Step a) of impregnation of the nickel precursor may be carried out according to dry or excess impregnation methods.

[0026] Step c) can be carried out ex situ or in situ, i.e. directly in the methanation reactor.

[0027] To implement the method, the hydrogen and CO2 are sent separately into the methanation reactor, for example in a downward direction. Alternatively, the two gaseous reactants are mixed beforehand before being sent into the methanation reactor.

[0028] The methanation reactor is an adiabatic, isothermal or hybrid type reactor. Preferably, the methanation reactor is an adiabatic reactor with a fixed or fluidized catalyst bed. The heating of the reactor can be carried out by any method known to those skilled in the art, for example by means of a resistor arranged in the catalytic bed, an internal (coil type) or external heat exchanger system.

[0029] According to one embodiment, when the catalyst bed is a fixed bed, the catalytic bed is subjected to an alternating electromagnetic field so as to heat the catalytic bed by induction.

[0030] In order to promote induction heating, the catalyst bed may advantageously comprise a susceptor, i.e. an element which, when subjected to an alternating electromagnetic field, is capable of converting electromagnetic energy into heat and communicating it to the catalyst. This may be the result of hysteresis losses and / or eddy currents induced in the susceptor which depend in particular on the electrical and magnetic properties of the susceptor material. Hysteresis losses occur in ferromagnetic or ferrimagnetic susceptors and result from the switching of magnetic domains within the material when the latter is subjected to the influence of an alternating electromagnetic field. Eddy currents may be induced if the susceptor is electrically conductive.In the case of an electrically conductive ferromagnetic or ferrimagnetic susceptor, heat can be generated both by eddy currents and by hysteresis losses. In this case, the heating is carried out essentially at the surface of the susceptor, which can transmit the heat to the surface of the catalyst with which it is in contact. For example, the susceptor can be chosen from carbonaceous / graphitic materials, metals or metal alloys that are not reactive for the intended reaction, such as, for example, aluminum, iron, copper, bronze, stainless steel, ferritic stainless steel, martensitic stainless steel and austenitic stainless steel. The susceptor can be either in direct contact with the catalyst or separated from the catalyst by a non-thermally insulating wall so as to allow rapid and homogeneous transfer of heat to the catalyst.

[0031] Advantageously, in order to meet the challenges of the energy transition, the method according to the invention is operated with a renewable energy source in order to store the latter in chemical form.

[0032] According to one embodiment, the conversion process uses a reactor with a fixed catalyst bed and in which the gas hourly space velocity is fixed at a value of at least 15 m3 / kg / h so as to maintain a temperature in the catalytic bed at at least 200°C, whereby the conversion reaction is carried out without external heat input.

[0033] The invention also relates to a CO2 methanation catalyst comprising nickel metal deposited on a uranium oxide support of formula UO2+x with x being between 0.01 and 0.6 and in which the mass content of nickel metal is between 5 and 40% of Ni relative to the total mass of the catalyst. Preferably, the catalyst according to the invention consists of nickel metal deposited on a uranium oxide support of formula UO2+x with x being between 0.01 and 0.6.

[0034] The content of nickel metal is preferably between 10 and 20% by mass of nickel relative to the total mass of catalyst.

[0035] Finally, the invention relates to a CO2 methanation catalyst comprising nickel metal deposited on a uranium oxide support of formula UO2+x with x being between 0.01 and 0.6 and in which the mass content of nickel metal is between 5 and 40% of Ni relative to the total mass of the catalyst, the catalyst being capable of being obtained by a process which comprises the following steps: a) a support precursor consisting essentially of a uranium (IV) and / or uranium (VI) oxide is impregnated with a solution containing a nickel precursor and a polar solvent; b) the impregnated support precursor is calcined in air and at a temperature of at least 250°C; c) the impregnated and calcined support precursor is reduced under hydrogen at a temperature of at least 300°C. Detailed description of the invention Description of gaseous charges

[0036] The method according to the invention makes it possible to treat a gaseous feedstock having a CO2 volume content greater than 30%, preferably greater than 50% and even better greater than 90%. Alternatively, the gaseous feedstock containing CO2 may be mixed with methane which may have a volume content of at most 50%.

[0037] This gaseous CO2 feedstock is, for example, a gaseous effluent from a biomass methanization unit, a gasification unit, an oil refining unit or a cement plant. The CO2 can also come from CO2 capture units.

[0038] As for the gaseous charge of hydrogen (H2), it can be obtained from the electrolysis of water or come from a catalytic reforming unit of heavy petroleum cuts.

[0039] According to a preferred embodiment which is part of the concept of Power-to-Gas processes allowing the storage of renewable energies in chemical form, hydrogen is produced in water electrolysis units powered by solar or wind power plants.

[0040] Preferably the volume content of H2 in this charge is greater than 90% and preferably greater than 95%.

[0041] According to the invention, the CO2 and dihydrogen feeds can be mixed before being sent to the methanation reactor (preferred mode of operation) or else can be distributed separately in the methanation reactor. The operation is carried out conversion reaction in the presence of a gas mixture whose H2 / CO2 molar ratio is between 1 and 8, preferably between 1 and 4, and more preferably equal to 4.

[0042] Description of the methanation catalyst according to the invention

[0043] The methanation process according to the invention uses a heterogeneous catalyst comprising metallic nickel supported on a uranium oxide corresponding to the formula UO2+X with x being between 0.01 and 0.6.

[0044] Nickel is present in the catalyst at a mass content of between 5 and 40% of Ni metal relative to the total mass of the catalyst. Preferably, the mass content of Ni metal is between 10 and 20% relative to the total mass of the catalyst.

[0045] The catalyst according to the invention differs from that of the prior art described in Applied Catalysis A: General 100 (1993) 131-143, in that it is obtained in particular by impregnation of a precursor of the support with a solution containing a nickel salt, then calcination of the impregnated support and finally reduction under hydrogen of the calcination product. Advantageously, in particular in order to reduce the calcination treatment time, the impregnated support is subjected to a drying step before the calcination step.

[0046] In the context of the invention, the uranium oxide-based support precursor may come from the nuclear process for enriching natural uranium into uranium 235 (U235), which provides so-called "depleted" uranium, i.e. one whose mass content of U235 is less than 0.7%, generally between 0.2 and 0.4%.

[0047] In a first step of the catalyst synthesis process, the uranium oxide-based support precursor is impregnated with a solution containing a soluble nickel salt. The solution contains a polar solvent, which is preferably water, in which a nickel salt is dissolved, which may be chosen from nickel hydroxide, hydroxycarbonate, carbonate and nitrate. The nickel content of the impregnation solution may take any value, but preferably this value is less than the saturation of the salt in the solvent used. Preferably, a solution with the highest concentration of soluble nickel salt is used while avoiding saturation of the impregnation solution. In the case where the catalyst according to the invention contains high Ni contents (e.g. greater than 20%), it is possible to proceed by successive impregnations of the support with optionally intermediate drying and calcination steps.

[0048] The precursor of the support can be in the form of small diameter extrudates, cylindrical or multilobed (trilobed, quadrilobed, etc.), spheres, rings, monoliths in honeycomb structure or in the form of a powder.

[0049] The BET specific surface area of ​​the precursor is generally between 1 and 10 m2 / g, preferably between 1 and 5 m2 / g. The specific surface area is determined by nitrogen porosimetry.

[0050] The step of bringing said support precursor into contact with an impregnation solution containing nickel can be carried out either by slurry impregnation, or by excess impregnation, or by dry impregnation, or by any other means known to those skilled in the art. Equilibrium (or excess) impregnation consists of immersing the support in a volume of solution (often largely) greater than the pore volume of the support while maintaining the system under stirring to improve the exchanges between the solution and the support or catalyst. An equilibrium is finally reached after diffusion of the different species in the pores of the support. Control of the quantity of elements deposited is ensured for example by the prior measurement of an adsorption isotherm which makes it possible to relate the concentration of the elements to be deposited contained in the solution to the quantity of elements deposited on the solid in equilibrium with this solution.

[0051] Dry impregnation consists of introducing a volume of impregnation solution equal to the pore volume of the support. Dry impregnation makes it possible to deposit all of the additives contained in the impregnation solution on a given support or catalyst.

[0052] The step of impregnating the nickel solution can advantageously be carried out by one or more excess solution impregnations or preferably by one or more dry impregnations.

[0053] This impregnation step can be carried out at a temperature between 18 and 50°C, preferably between 20 and 30°C.

[0054] At the end of step a), the impregnated support can advantageously be allowed to mature so as to allow homogeneous dispersion of the impregnation solution within the support. Any maturation step is advantageously carried out at atmospheric pressure,

[0055] at a temperature between 18°C ​​and 50°C and preferably at room temperature. Generally a maturation time of between 10 minutes and 48 hours and preferably between 30 minutes and 6 hours is sufficient.

[0056] When the support precursor is a powder, after the step of impregnating the nickel precursor, the latter is advantageously shaped, for example by spheronization or by extrusion.

[0057] The support precursor after impregnation is optionally subjected to a drying step at a temperature below 200°C, advantageously between 50°C and 150°C, preferably between 70°C and 150°C, very preferably between 75°C and 130°C. The drying step is preferably carried out under an atmosphere containing oxygen, preferably in air. The drying step can be carried out by any technique known to those skilled in the art. It is advantageously carried out at atmospheric pressure or reduced pressure. Preferably, this step is carried out at atmospheric pressure. It is advantageously carried out in a traversed bed using air or any other hot gas. Preferably, when the drying is carried out in a fixed bed, the gas used is either air or an inert gas such as argon or nitrogen. Very preferably, the drying is carried out in a traversed bed under air. Preferably, the drying step has a duration of between 5 minutes and 15 hours, preferably between 2 hours and 12 hours.

[0058] It should be noted that the impregnation, maturation and / or drying step can be repeated several times in a row until the desired quantity of nickel is deposited on the support precursor.

[0059] The impregnated support precursor, optionally matured and dried, is then subjected to a calcination step under an oxidizing atmosphere, preferably under air or under diluted oxygen and at a temperature of at least 250°C and preferably at least 300°C. Typically, the impregnated precursor is calcined at a temperature between 300 and 500°C under air and for 1 to 5 hours.

[0060] The implementation of the methanization catalyst according to the invention requires that it is then activated in a reduction step in order to convert at least a portion of oxidized forms of nickel generated during the calcination step into nickel metal and to reduce the uranium (VI) oxide or the uranium (IV) and (VI) oxide into UO2+X. To this end, the catalyst is brought into contact with pure or diluted hydrogen at a temperature at least equal to 300°C, preferably between 350 and 500°C, for a period of at least one hour and preferably between 2 and 5 hours. This activation by reduction can be carried out ex situ in a dedicated reduction reactor or in situ, i.e. directly in the methanation reactor after it has been loaded into said reactor.

[0061] The activated catalyst generally has a BET specific surface area, measured by a nitrogen adsorption isotherm, of between 1 and 10 m2 / g and preferably of between 2 and 6 m2 / g.

[0062] The catalyst according to the invention has an excellent catalytic activity for the methanation of CO2 which results in a CH4 yield of at least 60%, preferably at least 80% and with a CH4 selectivity close to 100%.

[0063] Implementation of the methanation process according to the invention

[0064] The CO2 methanation process according to the invention consists of bringing into contact, in a reactor, the gaseous feedstock containing mainly CO2, hydrogen and the catalyst described above. In the context of the invention, the gaseous feedstock containing CO2 can be mixed with a flow of hydrogen and the mixture is then sent to the reactor containing the catalyst.

[0065] Contact with the catalyst can be carried out in an adiabatic, isothermal or hybrid reactor. The term "adiabatic reactor" means a reactor which does not exchange heat with the external environment; the heat released by the exothermic methanation reaction is then used to fuel the reaction and the excess heat is removed by the effluents withdrawn from the reactor.

[0066] The term "isothermal reactor" means a reactor which is cooled by the circulation of a fluid making it possible to counterbalance the local release of reaction heat.

[0067] Finally, the term “hybrid reactor” means a reactor combining the two aforementioned characteristics in which a cooling flow is applied to partially counterbalance the local release of heat from the reaction but in which there is still a significant temperature gradient within the reactor.

[0068] The contacting of the gaseous reactants with the catalyst according to the invention is carried out at a temperature in the catalytic bed of between 160 and 550°C, preferably of between 180 and 350°C.

[0069] The process according to the invention is operated with an hourly space velocity of gas, ratio between the gas flow rate in m3 / h and the mass of catalyst in kg, of between 10 and 50 m3 / kg / h, preferably between 15 and 30 m3 / kg / h and with a molar ratio H2 / CO2 of between 1 and 8 (mol / mol), preferably between 1 and 4 (mol / mol), or even between 3 and 4 and even better equal to 4.

[0070] The gaseous feedstock can be sent into the reactor in a descending or ascending direction, preferably in a descending direction with withdrawal of the reaction products at the bottom of the reactor.

[0071] The catalyst according to the invention can be used in a fixed bed or fluidized bed reactor.

[0072] When the reactor is of the fixed bed type, it may comprise a plurality of perforated tubes in which the catalyst is arranged, possibly with packing elements. Alternatively, the fixed catalytic bed may be delimited by perforated lower and upper plates whose diameters correspond to the internal diameter of the reactor and between which the catalyst is arranged, possibly with packing elements.

[0073] The thermal energy required to be supplied to the reactor can be provided by any method known to those skilled in the art, in particular with an internal (coil type) or external heat exchanger system, by Joule effect, by microwaves and by inductive heating.

[0074] Unexpectedly, it has been observed that the catalysts according to the invention are capable of being heated under the action of an alternating electromagnetic field.

[0075] According to a preferred embodiment, the methanation reactor uses a bed fixed catalyst that is subjected to an alternating electromagnetic field that causes it to heat up without contact with the energy source. This heating method allows the energy required for the reaction to be supplied only to the catalyst while the gaseous reactants entering and leaving the catalytic bed are neither heated nor cooled.

[0076] According to the invention, in order to improve the efficiency of induction heating, the catalytic bed may comprise a mixture of catalyst according to the invention with an electrically conductive material (susceptor), the role of which is to communicate additional heat to the catalyst. The use of a susceptor is recommended when the catalyst has a nickel mass content of less than 15% relative to the total mass of catalyst. By way of example, the susceptor material may be chosen from carbonaceous / graphitic materials, metals or metal alloys which are not reactive for the targeted reaction such as, for example, aluminum, iron, copper, bronze, stainless steel, ferritic stainless steel, martensitic stainless steel and austenitic stainless steel.According to an alternative embodiment, the susceptor is separated from the catalytic bed through a non-thermally insulating wall allowing the transfer of heat to the catalyst.

[0077] This method of implementation by induction heating has several advantages: • precise adjustment of the temperature within the catalytic bed; • extremely rapid regulation, both in rise and fall, of the temperature in the catalytic bed; • the incoming reactant is not heated, thus helping to efficiently extract the thermal energy released by the reaction; • since only the catalyst is heated and not its immediate environment, the evacuation and maintenance of the temperature in the catalytic bed are encouraged and therefore minimize the risks of thermal runaway; • improved energy efficiency; • the water vapor generated by the reaction can be partly condensed in the empty space between the catalyst grains, because only the solid is heated, and thus promote the conversion of the reactants by reducing competitive adsorption problems on the catalyst surface.

[0078] To operate the induction heating, the reactor comprises an inductor device capable of generating an electromagnetic field. The inductor device can be arranged inside the reactor so as to encircle the catalytic bed so that the magnetic field it generates is essentially perpendicular to the thickness of the catalytic bed. According to a second embodiment, the inductor device is arranged at the level of the catalytic bed but outside or in the wall of the reactor. This second embodiment has the advantage that the inductor is decoupled from the chemical environment and thus allows easier control of the inductor. However, in this second embodiment, preference will be given to the use of a reactor made of a non-electrically conductive material such as glass or ceramic. The assembly can be protected by another external enclosure.

[0079] The inductor device is, for example and in a non-limiting manner, a helical induction coil extending over the thickness of the fixed catalytic bed or a part forming a ring whose height corresponds substantially to the thickness of the catalytic bed.

[0080] It should be noted that the catalyst according to the invention is particularly suitable for this type of induction heating because it has a high density, of the order of 6 to 12 g / cm3 and a relatively low specific surface area of ​​the order of 2 to 6 m2 / g.

[0081] Another advantage provided by induction heating is that it allows easy separation of the water formed during the conversion of CO2 without the need for a dedicated condensation unit using a heat exchanger system. Indeed, since only the catalytic bed is subjected to heating, the gaseous effluent containing a mixture of methane and water undergoes sudden cooling at the outlet of the reactor (the "quench" phenomenon according to English terminology), thus causing at least partial condensation of the water in the liquid state. The effluent withdrawn from the reactor can be sent to a separation tank in order to separate a gaseous phase containing the methane mixed with possibly unreacted CO2 and hydrogen and a liquid phase consisting of water.

[0082] Due to the increased catalytic activity of the catalyst according to the invention, the inventors have surprisingly found that the CO2 conversion process can be carried out under specific operating conditions according to a mode known as "auto-methanation", in which the heat released within the catalytic bed is sufficient to maintain the reaction without the need for an external energy supply.

[0083] This “auto-methanation” operating mode is possible when a reactor is used with a fixed catalyst bed, with an hourly gas space velocity of at least 15 m3 / kg / h in the presence of a gas mixture having an H2 / CO2 molar ratio of between 3 and 4, preferably equal to 4, and with a temperature within the catalytic bed of between 200 and 450°C, preferably of between 220 and 300°C, after stabilization of the thermal flows in the catalytic bed.

[0084] In order to limit the risks of thermal runaway in the reactor, several measures can be taken alone or in combination. Thus, one can choose to: i work on the dilution of the reaction flow: when starting the reaction, a more dilute flow (by adding an inert gas) could be used in order to reduce the heat released by the reaction, then, when the optimal temperature of the reaction is reached, the concentration of the reactants is increased step by step in order to promote the conversion of the reactants while maintaining a balance between the heat released by the reaction and heat evacuated so as not to cause a sudden thermal runaway which is difficult to control; ii replace Joule effect heating with high thermal inertia in terms of regulation with inductive heating as described above; iii apply forced cooling of the reactor by means of circulation of a cooling fluid to remove heat and thus maintain the reaction temperature. Brief description of the figures

[0085] [Fig.lA] schematically illustrates the assembly used by the Inventors to carry out the catalytic tests under indirect heating by the Joule effect.

[0086] [Fig. IB] schematically illustrates the assembly allowing induction heating of the catalytic bed.

[0087] [Fig.2A] and [Fig.2B] and [Fig.2C] respectively represent the methane yield as a function of furnace temperature, the variation of the catalyst bed temperature as a function of furnace temperature and the methane yield as a function of the catalyst bed temperature during tests with a Nilo / U02+x catalyst in the presence of a gas mixture with a molar ratio of H2 / CO2 of 4, with a gas hourly space velocity of 10 L / g / h.

[0088] [Fig.3A] and [Fig.3B] and [Fig.3C] and [Fig.3D] represent the variation of the temperature of the catalytic bed as a function of the temperature of the furnace during tests with a NilO / U02+x catalyst in the presence of a gas mixture with a molar ratio of H2 / CO2 of 4, for an hourly gas space velocity of 15, 20, 25 and 30 L / g / h.

[0089] [Fig.4A] and [Fig.4B] and [Fig.4C] and [Fig.4D] represent the methane yield as a function of the temperature of the catalytic bed during tests with a NilO / U02+x catalyst in the presence of a gas mixture with a molar ratio of H2 / CO2 of 4, for an hourly gas space velocity of 15, 20, 25 and 30 L / g / h.

[0090] [Fig.5A] and [Fig.5B] and [Fig.5C] respectively represent the methane yield as a function of the temperature furnace temperature, the variation of the catalyst bed temperature as a function of the furnace temperature and the methane yield as a function of the catalyst bed temperature during tests with the catalysts NilO / U02+x and NilO / Al2O3 in the presence of a gas mixture with a molar ratio of H2 / CO2 of 4, with a gas hourly space velocity of 20 L / g / h.

[0091] [Fig.6A] and [Fig.6B] and [Fig.6C] present the methane yield as a function of the catalyst bed temperature during tests with a Ni20 / UO2+x catalyst in the presence of a gas mixture with a H2 / CO2 molar ratio of 4, with a gas hourly space velocity of 20 L / g / h.

[0092] [Fig.7] gives the methane yield as a function of time for a catalytic test carried out with a Ni20 / UO2+x catalyst, in the presence of a gas mixture with a H2 / CO2 molar ratio of 4 and with an hourly gas space velocity of 20 L / g / h, in which the catalytic bed was heated by induction using a coil.

[0093] [Fig.8] illustrates the efficiency in operation during a conversion of CO2 into methane in “auto-methanation” mode in the presence of a Ni20 / UO2+x catalyst with a gas mixture of H2 / CO2 molar ratio of 4, under an hourly gas space velocity of 20 L / g / h, in which the catalytic bed has been previously heated by induction using a coil.

[0094] [Fig. 1 A] is a representation of the micropilot device used for the study of the catalytic CO2 conversion reaction. The device 1 comprises a glass enclosure 2 with an internal diameter DI of 6 mm containing a catalyst bed 3 between an upper layer 4 and a lower layer 5 of quartz wool. The enclosure 2, comprising an inlet 6 for the reactive gases and an outlet 7 for the effluent, is received in a furnace 8 equipped with heating structures 9 surrounding the wall of the furnace.

[0095] The device is also equipped with a first thermocouple 10 placed in the wall of the furnace and a second thermocouple 11 immersed in the catalytic bed making it possible to monitor respectively the temperature profile of the furnace TF and that of the catalytic bed Tc during the tests.

[0096] The micropilot is supplied by gas cylinders: hydrogen, carbon dioxide and argon as purge gas. The gas flow rates are measured and regulated by mass flow meters coupled with solenoid valves. The temperature of the incoming gases is adjusted by passing them through a preheater, the temperature of which can be regulated up to 400°C, before feeding the catalytic reactor. The operating mode described here is intended for a methanation reaction under indirect heating by the Joule effect.

[0097] The gaseous effluent recovered at the outlet of the reactor enclosure is cooled via a glass condenser then a Peltier effect condenser (T«10°C). The water is therefore considered to be fully condensed when the outlet gaseous effluent is analyzed.

[0098] The operating pressure in the micropilot is regulated by a control valve located downstream of the condenser.

[0099] The dry gaseous effluent is thus sampled downstream of the control valve and analyzed using a micro-gas chromatograph (R3000, SRA Instrument) which is equipped with two different columns: a molecular sieve column (MS5A) allowing the separation of CO, CH4 and H2 and a polymer adsorbent column (PPU) allowing, among other things, the analysis of CO2, methane, ethane, ethylene and acetylene.

[0100] For the conduct of catalytic conversion tests, hydrogen is always introduced into the reactor before carbon dioxide. When changing the reaction temperature, the operating conditions are maintained until the temperature within the reactor and the composition of the gas mixture at the reactor outlet stabilize. The time required for this stabilization is approximately 40 minutes. From the steady state of the operating conditions, the catalytic reaction is then maintained for at least one hour.

[0101] At the end of the tests, the reactor temperature is lowered to room temperature under a flow of argon before discharging the catalyst. Analysis of the composition of the dry gas and measurement of its flow rate make it possible to calculate the CO2 conversion rate and the CH4 selectivity obtained.

[0102] The conversion rate of CO2 and hydrogen are defined as follows: ... / A \ nn ^£»2 i - 777^—--- % . -------I* y

[0103] With: • ACo2, inlet = area of ​​the CO2 peak of the gas flow entering the reactor measured by the micro-chromatograph; • ACo2, outlet = area of ​​the CO2 peak of the gaseous effluent at the outlet of the reactor measured by the micro-chromatograph; • Outlet flow rate = flow rate of gaseous effluent leaving the reactor • Inlet flow rate = flow rate of the gas stream (H2+CO2) entering the reactor.

[0104] When the methane selectivity is equal to 100%, the CO2 conversion rate can be calculated from the following equation: * ICC

[0105] XCh4,output being the molar concentration of methane in the effluent.

[0106] The above relationship can also be written, considering that the water formed by the reaction is completely condensed in the output capacitor and that there is no formation of CO: - "case 11 T'

[0107] with R the molar ratio H2 / CO2 in the reaction mixture.

[0108] The last equation uses only one experimental parameter XCH4, output which is the molar concentration of methane formed and whose relative uncertainty is the lowest (less than 5%).

[0109] As long as the methane selectivity is 100%, the methane yield of the reaction is equal to the CO2 conversion rate determined by the above formulas.

[0110] The catalysts according to the invention were prepared from a U3O8 powder (Prolabo) which is shaped by spheronization in the presence of distilled water.

[0111] The obtained beads are then impregnated with an aqueous solution of nickel nitrate by the dry impregnation technique. The total pore volume of the U3O8 beads was determined by dry impregnation of a distilled water solution.

[0112] The incorporation of nickel onto the support by impregnation from an aqueous solution of nickel nitrate was carried out in a single step to provide catalysts whose mass content is 10, 15 and 20% nickel relative to the total mass of catalyst.

[0113] At the end of the impregnation step or steps, the solid is air-dried for 3 hours and then calcined in air in a sealed tubular reactor at 350°C for 2 hours.

[0114] The calcined balls are sieved in order to recover only the fraction having a grain size between 0.2 and 0.8 mm.

[0115] Before being used in the conversion reaction, the calcined beads are activated by reduction under a flow of pure hydrogen (50 mL / min) at 350°C for two hours.

[0116] X-ray diffraction analysis of the catalysts after reduction under hydrogen indicates the presence of a majority UO2 phase, minority U3O8 and nickel metal. Examples

[0117] In the following examples, the hourly space velocities are expressed relative to the quantity of catalyst used.

[0118] Example 1: Evaluation of the Nil0 / UO2+x catalyst at a gas hourly space velocity of 10 L / g / h under indirect heating by Joule effect

[0119] The catalytic activity of the Ni catalyst (10% by mass) on a UO2+X support prepared according to the method described above.

[0120] 400 mg of catalyst was premixed with silicon carbide (SiC) as an inert diluent. The mixture is introduced into the glass reactor and forms a catalytic bed approximately 12 mm thick.

[0121] The tests were carried out with a gas mixture whose H2 / CO2 ratio (mol / mol) is equal to 4 for an hourly gas space velocity of 10 L / g / h, at a pressure of 0.1 MPa and following the temperature of the furnace TF and that of the catalytic bed Tc.

[0122] With reference to [Fig.2A], it can be seen that the CO2 conversion remains low for TF furnace temperatures below 197°C. Beyond this value, the catalytic reaction starts abruptly and the determined CO2 conversion is approximately 90% for a TF temperature of 200°C. Beyond 200°C, the CO2 conversion rate reaches an asymptote which tends towards the value of 95% for a temperature above 200°C, close to thermodynamic equilibrium.

[0123] With reference to [Fig.2B] which represents the variation of the temperature in the catalytic bed Tc as a function of the temperature of the furnace TF, it is noted that when the temperature of the furnace Tf reaches 197°C, the measured temperature of the catalytic bed Tc is of the order of 270°C. This temperature difference AT (TC-TF) of approximately 70°C corresponds to the heat released by the exothermicity of the reaction which cannot be evacuated from the catalytic bed by exchanges with the gas flow. This temperature difference makes it possible to understand the jump in CO2 conversion observed in [Fig.2A].

[0124] Monitoring of the catalytic reaction was also carried out during the reactor cooling process by measuring the CO2 conversion as a function of the temperature in the catalytic bed Tc. Cooling was carried out by reducing the set temperature of the furnace, while maintaining the injection of the H2 and CO2 gas stream. The objective is to determine whether hysteresis in the catalytic process is observable when reducing the temperature of the TF furnace, which would result in maintaining the CO2 conversion solely due to the heat released by the reaction.

[0125] With reference to [Fig.2C] which gives the CH4 yield as a function of the temperature measured in the catalytic bed Tc during the rise and fall in temperature in the catalytic bed, it can be seen that the exothermic reaction releases a high level of heat in the catalytic bed. However, under these reaction conditions, the heat released within the catalytic bed is insufficient to achieve a state of equilibrium between the heat produced by the reaction and that which is evacuated by the gaseous effluent at the reactor outlet. Indeed, during the slow fall in the set temperature of the furnace TF, the CO2 conversion falls with the temperature Tc. No catalytic hysteresis is observed under the operating conditions of Example 1.

[0126] Example 2: Evaluation of the Nil0 / UO2+x catalyst at gas hourly space velocities of 15, 25 and 30 L / g / h under indirect heating by Joule effect

[0127] The tests of Example 2 were carried out with the same reactor configuration as in Example 1 in order to study the catalytic behavior of the catalyst containing 10% by mass of Ni relative to the total mass of catalyst, at higher hourly gas space velocities during the phases of temperature rise and fall of the furnace. These tests make it possible in particular to determine whether a hysteresis (conversion difference during the temperature rise and fall) can be observed.

[0128] Figures 3A to 3D represent the evolution of the temperature of the catalytic bed Tc as a function of the temperature of the furnace TF during the phase of increase and decrease in temperature of the furnace for the different values ​​of hourly space velocity of gas equal to 15, 20, 25 and 30 L / g / h in the presence of a gas flow containing a molar ratio H2 / CO2 equal to 4.

[0129] As for Figures 4A to 4D, they indicate the methane yield, representative of the methanation activity of the catalyst, as a function of the temperature of the catalytic bed and at the different hourly space velocities of gas during the phases of temperature increase and decrease of the furnace.

[0130] With the Nilo / U02+x catalyst and for a gas hourly space velocity greater than or equal to 15 L / g / h, a hysteresis appears when the furnace temperature decreases. This phenomenon is all the more notable when the gas hourly space velocities are high (Figures 3A to 3D). Indeed, the methanation activity decreases progressively but nevertheless still remains high for temperatures measured in the catalytic bed greater than 200°C.

[0131] It is further observed that the maximum temperature measured in the catalytic bed Tc increases with the hourly space velocity and that the drop in temperature in the catalytic bed during the temperature reduction of the furnace is all the less rapid as the hourly space velocity of the gas is high. This therefore reflects that the heat generated by the high conversion of CO2 makes it possible to maintain the temperature of the catalytic bed (Figures 4A to 4D).

[0132] In the case of the test carried out at a gas hourly space velocity of 30 L / g / h ([Fig.3D]), it was noted that even when the furnace was switched off, the temperature displayed by the furnace thermocouple remained above 90°C, indicating that part of the heat generated in the catalytic bed (whose temperature is maintained at approximately 220°C thanks to the exothermicity of the reaction) contributes to maintaining the furnace temperature thanks to heat exchanges.

[0133] The catalytic system according to the invention is capable of operating in “auto-methanation” mode when the hourly space velocity of gas is at least 25 L / g / h (i.e. 25 m3 / kg / h) and as long as the temperature of the catalytic bed remains at a value greater than or equal to 200°C, thanks to the establishment of a balance between the heat released by the reaction and the heat exchanged with the flow of reactants.

[0134] Example 3 (comparative): Evaluation of the NilO / gamma-Alumina catalyst compared to the NilO / UO2+x catalyst at 20 L / g / h under indirect heating by Joule effect

[0135] Comparative tests were carried out under the same operating conditions as those of example 1, i.e. with a gas mixture whose H2 / CO2 molar ratio is 4 but in the presence of a catalyst comprising a mass content of nickel on an alumina type support (A12O3).

[0136] The comparative catalyst was prepared as follows: the gamma-A12O3-based support, subsequently referred to as alumina, is in the form of extrudates (1 mm in diameter and 3 mm in length, Ketjen 300B supplied by Akzo Nobel) and is dry-impregnated with an aqueous solution of nickel nitrate at room temperature. The material is allowed to mature at room temperature for 3 hours and then dried at 110°C in air for 3 hours to remove the solvent. The dry material is calcined in air at 350°C for 2 hours (with a temperature rise gradient of 3°C / min). The catalyst precursor thus obtained is reduced directly in the catalytic reactor under a flow of pure hydrogen (20 mL / min) at 350°C for 2 hours.

[0137] The experimental results are reported in Figures 5A and 5C which give the methane yield as a function of the temperature of the TF furnace and the Te catalytic bed during the phases of temperature increase and decrease of the furnace. [Fig.5A] confirms the excellent methanation activity of the NilO / U02+x catalyst compared to its counterpart NilO / Al2O3.

[0138] [Fig.5B] shows the evolution of the temperature in the catalytic bed Tc as a function of the temperature of the furnace TF during the phases of increase and decrease in temperature of the furnace. For the Ni-10 / UO2+x catalyst, when the temperature of the furnace exceeds 190°C a significant jump in the temperature in the catalytic bed is measured (320°C). This jump is explained by a higher conversion of CO2 into CH4. In the case of the Ni-10 / Al2O3 catalyst a very small difference is observed between the two temperatures, which reflects a lower conversion.

[0139] With reference to [Fig.5C], it can be seen that at CH4 iso-yield, the temperature in the Ni10 / Al2O3 catalytic bed is always higher than that measured in the Ni10 / UO2+x catalytic bed. For example, as indicated in [Fig.5C], for a CH4 yield of 75%, the temperature measured in the catalytic bed containing Ni / Al2O3 catalyst is 320°C while that measured in the catalytic bed containing Ni-10 / UO2 catalyst is 220°C. These results therefore confirm the high reactivity of the Ni-10 / UO2+x catalyst.

[0140] The lower catalytic activity of the Nilo / Al2O3 catalyst is also manifested by the fact that the catalytic activity is not maintained during the decrease in temperature of the furnace and therefore by the absence of hysteresis in the temperature profile.

[0141] Example 4: Evaluation of the Ni20 / UO2+x catalyst at gas hourly space velocities of 20 and 30 L / g / h under indirect heating by Joule effect

[0142] Tests were carried out with a catalyst comprising 20% ​​by mass of nickel relative to the total mass of catalyst on a UO2+X support. This catalyst was prepared according to the same protocol as that of Example 1 with a double dry impregnation of an aqueous solution of nickel nitrate.

[0143] The catalyst after reduction under hydrogen was tested under two hourly gas space velocities of 20 and 30 L / g / h and with a gas mixture with a molar ratio of H2 / CO2 of 4.

[0144] Figures 6A, 6B and 6C show respectively, for the furnace temperature reduction phase, the CH4 yield as a function of the furnace temperature TF, the variation in the temperature of the catalytic bed as a function of the furnace temperature and finally the CH4 yield as a function of the catalytic bed temperature Tc.

[0145] It can be observed that the catalyst according to the invention containing 20% ​​by mass of Ni exhibits excellent catalytic activity. The CH4 yield remains greater than 80% for a temperature range in the catalytic bed between 260 and 360°C. With reference to Figures 6A, 6B and 6C, it is also noted that the catalyst is capable of operating according to the self-methanation mode during the temperature reduction of the furnace. For example, for an hourly gas space velocity of 30 L / g / h and a temperature of 90°C measured in the furnace, the CH4 yield remains greater than 80% thanks to maintaining the temperature of the catalytic bed at a value of approximately 260°C.

[0146] Example 5: Evaluation of the Ni20 / UO2+x catalyst at low temperature gas hourly space velocities of 20 L / g / h under inductive heating

[0147] Tests were carried out with a catalyst comprising 20% ​​by mass of nickel relative to the total mass of the catalyst on a UO2+X support.

[0148] The device 12 used to carry out the heating is shown in [Fig. 1B].

[0149] The catalytic bed 13 which is contained in a reactor 14 made of 8 mm glass internal diameter comprises a layer 15 consisting of 800 mg of catalyst mixed with 200 mg of silicon carbide as thermal diluent (SiC). The catalyst and SiC mixture is deposited above a graphite felt 16 acting as a susceptor. The thickness of the catalytic bed 13, which is between two layers of quartz wool 17, 18, measures approximately 17 mm.

[0150] The reactor 14 is heated by means of an EasyHeat 8310, 10kW induction heating system marketed by Ambrell Ltd. This system is equipped with a six-turn copper induction coil 19 which is cooled by water. circulating inside the coil. The reactor is arranged inside the coil so that the catalyst bed is encircled by the induction coil. Real-time temperature control is provided by a Eurotherm 3504 controller connected to an Optris® laser pyrometer pointing at the catalyst bed.

[0151] After an in situ reduction step of the catalyst at 300°C for 30 min under H2, the latter is allowed to cool to 160°C under Ar. The reaction mixture (1 mol of CO2 for 4 mol of H2) is introduced into the reactor with an hourly gas space velocity of 20 L / g / h.

[0152] The catalytic bed is gradually heated by induction to a temperature of 190°C measured by the laser pyrometer. During the test, the temperature in the catalytic bed is fixed at the set temperature of 190°C.

[0153] The results are presented in [Fig.7]. It can be seen that the Ni-20 / UO2+x catalyst has a stable CH4 yield of approximately 62%, for a temperature in the catalytic bed of 190°C. These results indicate that the Ni-20 / UO2 catalyst allows the methanation reaction to be carried out at a relatively low temperature compared to the results reported in the literature. In addition, induction specifically heats only the solid catalyst and the susceptor and not the gaseous reactants passing through the catalytic bed, hence a significant gain in terms of energy input to the process. It should be noted that the catalyst does not show any deactivation during the test, thus confirming its excellent stability.

[0154] Example 6: Evaluation of Ni20 / UO2+x catalyst at gas hourly space velocities of 20 L / g / h under inductive heating

[0155] Tests were conducted with the same catalyst and reactor configuration as in Example 5.

[0156] Example 6 differs from Example 5 in the conditions for heating the catalytic bed. The catalytic bed is gradually heated by induction to the set temperature of 170°C with a current of 120 amperes applied to the inductor. Then the set temperature is suddenly increased to 190°C, which is accompanied by a temperature jump within the bed, which reaches 230°C. This exothermicity in the catalytic bed reflects a runaway of the catalytic reaction for the conversion of CO2. The current applied to the inductor is then reduced to 23 amperes, then completely cut off, and finally the induction coil is moved so that the catalytic bed is outside its environment.

[0157] The results of the conversion of CO2 into methane after stopping the inducer are shown in [Fig.8]. The catalyst according to the invention achieves a conversion of approximately 80% of CO2 into CH4 in auto-methanation mode (without external heat input) for 140 min. During this test it was also noted that the temperature measured in the catalytic bed oscillates between 220 and 230°C thanks to the heat released by the catalytic reaction.

Claims

Claims

1. A process for converting CO2 into methane in which: • hydrogen and a gaseous feed comprising CO2 are brought into contact in at least one methanation reactor comprising a catalyst bed at a temperature in the catalytic bed of between 160 and 550°C, at a pressure of between 0.1 and 1 MPa, with a gas hourly space velocity of between 10 and 50 m3 / kg / h and with an H2 / CO2 molar ratio of between 1 and 8; ❖ the catalyst contains Ni metal deposited on a uranium oxide support of formula UO2+X, with x being between 0.01 and 0.6, and the mass content of which is between 5 and 40% of nickel metal relative to the total mass of the catalyst; and ❖ the catalyst is prepared by a process comprising the following steps a) to c): a.a support precursor consisting essentially of a uranium (IV) and / or uranium (VI) oxide is impregnated with a solution containing a nickel precursor and a polar solvent; b. the impregnated support precursor is calcined in air and at a temperature of at least 250°C; and c. the impregnated and calcined support precursor is reduced under hydrogen at a temperature of at least 300°C.

2. A method according to claim 1, wherein before the calcination step the impregnated support precursor is dried at a temperature below 200°C.

3. A method according to either of claims 1 or 2, wherein the support precursor is selected from U3O8, UO2, rUO4 and UO3.

4. A method according to any one of claims 1 to 3, wherein the support precursor is in the form of cylindrical or multi-lobed extrudates, spheres, rings, honeycomb-structured monoliths or a powder.

5. Method according to one of claims 1 to 4, in which the nickel precursor is chosen from hydroxide, hydroxy-carbonate, nickel carbonate and nitrate.

6. Method according to one of claims 1 to 5, in which step a) of impregnation implements dry or excess impregnation.

7. Method according to one of claims 1 to 6, in which reduction step c) is carried out in situ in the methanation reactor.

8. Method according to one of claims 1 to 7, in which the gaseous feed comprising CO2 and hydrogen are sent separately or as a mixture into the methanation reactor in a downward direction.

9. A method according to one of claims 1 to 8, wherein the methanation reactor is an adiabatic reactor.

10. Method according to one of claims 1 to 9, in which the methanation reactor is a fixed or fluidized catalyst bed reactor.

11. The method of claim 10, wherein the catalyst bed is a fixed bed and the catalyst bed is subjected to an alternating electromagnetic field so as to heat the catalyst bed by induction.

12. The method of claim 11, wherein the fixed catalyst bed further comprises susceptor particles.

13. Method according to one of claims 1 to 12, in which the method is carried out with a renewable energy source.

14. Process according to one of claims 1 to 13, in which the reactor is a fixed catalyst bed reactor and in which the hourly space velocity of gas is fixed at at least 15 m3 / kg / h so as to maintain a temperature of the catalytic bed Tc at at least 200°C, whereby the conversion reaction is carried out without external heat input.

15. A process for preparing a CO2 methanation catalyst comprising nickel metal deposited on a uranium oxide support of formula UO2+x with x being between 0.01 and 0.6, and in which the mass content of nickel is between 5 and 40% of nickel metal relative to the total mass of the catalyst, which process comprises the following steps a) to c): a) a support precursor consisting essentially of a uranium (IV) and / or uranium (VI) oxide is impregnated with a solution containing a nickel precursor and a polar solvent; b) the impregnated support precursor is calcined in air and at a temperature of at least 250°C; and c) the impregnated and calcined support precursor is reduced under hydrogen at a temperature of at least 300°C.

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