Low-temperature denox catalyst for industrial flue gases

Abstract

The present invention comprises an active phase comprising manganese (Mn), at least one first group VIII element chosen from iron (Fe), cobalt (Co), and nickel (Ni), said catalyst comprising a porous support comprising at least one refractory oxide chosen from titanium dioxide, silica, alumina, silica-alumina, zirconia and ceria, used pure or as a mixture with one another, particularly intended for the reduction of nitrogen oxides (NOx). Said catalyst comprises between 0.1% and 15% by weight of manganese element and between 0.1% and 20% by weight of group VIII element relative to the total weight of the catalyst such that the molar ratio between the manganese and the group VIII element is between 0.1 and 30 mol / mol. The active phase is characterized in the form of nanoparticles having a size of between 1 nm and 30 nm, measured by transmission electron microscopy.

Description

Technical field The present invention relates to a manganese-based multi-metallic supported catalyst intended in particular for the reduction of nitrogen oxides (NOx), preferably in the presence of a reducing agent. Prior art Emissions from the burning of fossil fuels and the burning of biomass have environmental and health consequences at local, regional and global levels. Nitrogen oxides (NOx = NO + NO2), mainly emitted by power plants, factories and automobiles, are considered to be the main source of atmospheric contamination, which has a considerable influence on the global environment. Tropospheric chemistry has become the main cause of greenhouse effects and acid rain. In order to combat the serious harm caused by NOx, governments around the world have adopted increasingly stringent legislation and policies to control NOx emissions. The ever-increasing demand for improved air quality has created a strong drive to control NOx emissions from stationary sources and automobiles. Several denitrification (DeNOx) technologies, in particular selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) and non-selective catalytic reduction (NSCR) are available for NOx reduction. Currently, the SCR process using ammonia NH3 as reducing agent has been commercially applied in stationary-source combustion units that have a higher NOx removal efficiency than SNCR or combustion controls. In the United States alone, more than 1000 SCR systems have been constructed in order to reduce NOx from industrial boilers, process heaters, steel mills and chemical plants. For mobile sources, SCR systems are increasingly used in diesel vehicles because three-way catalysts (which convert three pollutants: hydrocarbons (HCs), carbon monoxide (CO) and nitrogen oxides (NOx)) are not suitable for NOx reduction in lean-burn engines. Catalysts are therefore a core part of SCR technologies for efficiently and selectively converting NOx into N2. V2O5-WO3(MoO3) / TiO2 catalysts have been used commercially in power plants due to their high NOx removal efficiency at a temperature of between 300 and 400°C. However, traditional vanadium(V)-based catalysts are progressively deactivated by poisoning with alkali / alkaline earth metals, phosphorus and heavy metals present in the combustion gases. Moreover, conventional vanadium(V)-based catalysts are not applicable in steelworks, cement plants and glassworks due to the low-temperature combustion gases (<250°C), nor are they suitable for diesel engines due to the requirement for a wider temperature window (150-500°C). Specifically, during steady state driving of a diesel engine, the temperature of the exhausted gas is of the order of 180-280°C, but it can reach 440°C when the engine is running at high speed. Increasing demands in terms of a wide operating temperature window, high resistance to SO2 / alkali / heavy metals and high hydrothermal stability have prompted the development of new NH3-SCR catalyst types. Another challenge is that of carrying out the NOx reduction (DeNOx) reaction at a temperature of less than 150°C in order to drastically reduce operating costs by avoiding heating the gaseous effluents. Among the vanadium(V)-free metal oxide-based catalysts, numerous articles and patent documents use certain transition metals such as the noble metals Pt or Pd, but also Mn, Ce, Cu, Fe, Ag, Rh and Co (cf. B. Tu et al. (ACS Sustainable Chem. Eng. 2017, 5, 5200)). G. Xu et al. (Applied Catalysis B: Environmental 2019, 244, 909) conducted work on the development of Pt / Al2O3 catalysts, but these solids operate at high temperatures (300-400°C). Recently, a certain number of studies have shown a benefit for solids based on manganese oxide (cf. Y. Qi et al., Water, Air, & Soil Pollution 2020, 231, 289), which have demonstrated good performance for low-temperature NH3-SCR application. Moreover, the NH3-SCR performance of manganese-containing catalysts are improved by the addition of other metals or of additives such as cerium (cf. Q. Yan et al. Applied Catalysis B: Environmental 2019, 255, 117749). The article by Zhang, et al. (Chemical Engineering Journal 2018, 348, 618) also shows that the performance of all of these catalysts can also be improved by changing the support. The catalyst support not only has advantageous intrinsic properties (and adjusts the acidity and alkalinity), but it leads to an appropriate particle size and shape. In 2016, W. Li et al. (Fuel Processing Technology 154) have shown that the deposition of manganese (Mn) on a support of TiO2 type enables the complete elimination of NOx at 150°C. Two other types of catalysts such as the solids MnCo-LDO (LDO for layered double oxide) and MnFe-LDO are interesting candidates for low-temperature NH3-SCR catalysts. Specifically, layered double oxide catalysts (clay family) not only have a unique two-dimensional structure, but also have large specific surface areas with large numbers of active sites. The most widely used procedure for the synthesis of LDOs is the simultaneous coprecipitation of two or more metals from the corresponding inorganic salts in an alkaline medium with a pH of approximately 10. This preparation is based on a laboratory-type procedure that cannot be extrapolated to the industrial scale, in particular as a result of numerous preparation steps, and double precipitation via the sol-gel route, a route that is very difficult to manage on an industrial scale. Moreover, in order to obtain a structure of LDO type, 30% by weight of manganese, 30% by weight of cobalt and 30% by weight of nickel are used, which results in a solid with a completely prohibitive production cost beyond the laboratory scale. Subject of the invention In order to solve the problems associated with the products and processes of the prior art in the field of catalysts for the selective catalytic reduction of ammonia (NH3-SCR) for reducing nitrogen oxides (NOx), the processes and products of the present invention have been developed. Surprisingly, the Applicant has discovered that a bimetallic or trimetallic catalyst comprising an active phase comprising manganese, and at least one element (e.g. one or two elements) from group VIII chosen from iron (Fe), cobalt (Co), nickel (Ni) and a porous support comprising at comprising a refractory oxide, in particular titanium dioxide, prepared by impregnation of said porous support with one or more solutions of metal precursors according to defined molar ratios and / or a highly specific protocol, makes it possible to obtain performance levels that are at least equally good, if not better, in terms of activity and selectivity in the context of DeNOx reactions within a wide temperature range and at low temperature, while using a smaller amount of metallic phase than that used in the prior art catalysts, enabling a lower manufacturing cost (low metal content). The selectivity appears to be provided via the modification of the electronic state of the manganese brought about by the addition of the at least one second metal from group VIII which is atomically close to the manganese (Mn) atoms. A first subject according to the invention relates to a catalyst comprising an active phase comprising manganese (Mn), at least a first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), said catalyst comprising a porous support comprising at least one refractory oxide chosen from titanium dioxide, silica, alumina, silica-alumina, zirconia, ceria, used pure or as a mixture with one another, said catalyst being characterized in that: - the manganese content is between 0.1% and 15% by weight of manganese element relative to the total weight of the catalyst; - the content of said first element from group VIII is between 0.1% and 20% by weight of element from group VIII relative to the total weight of the catalyst; - the molar ratio between the manganese and said first element from group VIII is between 0.1 mol / mol and 30 mol / mol; - the active phase is in the form of nanoparticles comprising manganese and at least said first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), said size of the nanoparticles being between 1 nm and 30 nm, measured by transmission electron microscopy. Preferably, the active phase is bimetallic, the metals being chosen from Mn-Fe, Mn-Co, Mn-Ni. Preferably, the active phase comprises a second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element from group VIII. Preferably, the active phase is in the form of nanoparticles comprising manganese, and at least said first element from group VIII and said second element from group VIII, said size of the nanoparticles being between 1 nm and 30 nm, measured by transmission electron microscopy. Preferably, the content of said second element from group VIII is between 0.1% and 20% by weight of element from group VIII relative to the total weight of the catalyst. Preferably, the molar ratio between the manganese and said second element from group VIII is between 0.1 mol / mol and 30 mol / mol. Preferably, the active phase is trimetallic, the metals being chosen from Mn-Co-Fe, Mn-Ni-Fe, Mn-Ni-Co. Preferably, said porous support comprises at least titanium dioxide. Preferably, said porous support has a specific surface area of between 10 m2 / g and 300 m2 / g. Preferably, said porous support is provided either in the form of powder or in the form of grains in the form of extrudates or beads. A second subject according to the invention relates to a process for preparing a catalyst according to the invention, comprising the following steps: a) a porous support comprising at least one refractory oxide chosen from titanium dioxide, silica, alumina, silica-alumina, zirconia, ceria, used pure or as a mixture with one another, is provided; b) the following substeps are carried out in order to obtain a catalyst precursor: b1) the porous support is brought into contact with at least one solution containing at least one manganese precursor; b2) the porous support is brought into contact with at least one solution containing at least one precursor of the first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni); substeps b1) and b2) being carried out separately, in any order, or at the same time; c) said catalyst precursor obtained on conclusion of step b) is dried at a temperature of less than 250°C in order to obtain a dried catalyst precursor; d) the dried catalyst precursor obtained in step c) is calcined at a temperature of between 250°C and 900°C in order to obtain said catalyst. Preferably, step b) comprises a substep b3) in which said porous support is brought into contact with at least one solution containing at least one precursor of the second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element from group VIII, said substep b3) being carried out either separately from substeps b1) and / or b2), in any order, or at the same time. Preferably, when said first element from group VIII is cobalt, substeps b1) and b2) are carried out separately in any order. Preferably, when said second element from group VIII is cobalt, substep b3) is carried out separately from substeps b1) and / or b2). A third subject according to the invention relates to the use of the catalyst for the catalytic reduction of nitrogen oxides (NOx). A fourth subject according to the invention relates to a process for the catalytic reduction of nitrogen oxides (NOx), using the catalyst according to the invention or prepared according to the invention, the process comprising bringing the catalyst and a nitrogen oxide into contact. Detailed description of the invention 1. Definitions The term "NOx" denotes nitrogen oxides (NOx = NO + NO2), x denotes the number of oxygens present in the molecule, it being possible for x to take the value 1 for NO and 2 for NO2. Hereinafter, the groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, published by CRC Press, editor-in-chief D.R. Lide, 81st edition, 2000-2001). For example, group VIII (or VIIIB) according to the CAS classification corresponds to the metals of columns 8, 9 and 10 according to the new IUPAC classification. The total pore volume is measured by mercury porosimetry according to the standard ASTM D4284-92 with a wetting angle of 140°, for example using an Autopore III™ model device of the Micromeritics™ brand. The BET specific surface area is measured by nitrogen physisorption according to the standard ASTM D3663-03, a method described in the work by Rouquerol F., Rouquerol J., Singh K., "Adsorption by Powders and Porous Solids: Principles, Methodology and Applications", Academic Press, 1999. The term "size of nanoparticles" is understood to mean the average size of one or more metallic nanoparticles (in the form of clusters or agglomerates of nanoparticles) that can be distinguished and measured by transmission electron microscopy (TEM). Analysis of the composition of the nanoparticles is possible via EDS (energy dispersion spectroscopy) analysis, during observation of the samples in TEM. The content of the metals manganese, iron, nickel and cobalt is measured by X-ray fluorescence and is given as a weight percentage relative to the total mass of the catalyst. The term "gas hourly space velocity" (GHSV) is understood to mean the volume flow rate of the gaseous feedstock at the reactor inlet in m3 / h divided by the volume of catalyst in m3 contained in the reactor. In the present patent application, the values in ppm are values in ppm by weight (unless defined otherwise). 2. Catalyst The catalyst comprises an active phase comprising manganese (Mn), at least a first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni) and a porous support comprising a refractory oxide chosen from titanium dioxide, silica, alumina, silica-alumina, zirconia, ceria, used pure or as a mixture with one another, said catalyst being characterized in that: - the manganese content is between 0.1% and 15% by weight of manganese element relative to the total weight of the catalyst; - the content of said first element from group VIII is between 0.1% and 20% by weight of element from group VIII relative to the total weight of the catalyst; - the molar ratio between the manganese and said first element from group VIII is between 0.1 mol / mol and 30 mol / mol; - the active phase is in the form of nanoparticles comprising manganese and at least a first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), said size of the nanoparticles being between 1 nm and 30 nm, measured by transmission electron microscopy. The manganese content in said catalyst according to the invention is advantageously between 0.1% and 15% by weight relative to the total weight of the catalyst, preferentially between 0.5% and 15% by weight, more preferentially between 2% and 12% by weight. The content of said first element from group VIII is advantageously between 0.1% and 20% by weight of element from group VIII relative to the total weight of the catalyst, preferentially between 0.5% and 15% by weight, more preferentially between 1% and 12% by weight. The molar ratio between the manganese and said first element from group VIII is between 0.1 mol / mol and 30 mol / mol, preferentially between 0.2 mol / mol and 20 mol / mol, more preferentially between 0.3 mol / mol and 15 mol / mol. According to a preferred embodiment of the present invention, the active phase is bimetallic, the metals being chosen from Mn-Fe, Mn-Co, and Mn-Ni, preferentially Mn-Fe and Mn-Ni, and more preferentially Mn-Fe. According to a preferred embodiment of the present invention, the active phase comprises a second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element from group VIII. The active phase is said to be trimetallic, the metals being chosen from Mn-Co-Fe, Mn-Ni-Fe, and Mn-Ni-Co, preferably Mn-Ni-Co and Mn-Fe-Co, and more preferentially Mn-Ni-Co. The content of said second element from group VIII is advantageously between 0.1% and 20% by weight of element from group VIII relative to the total weight of the catalyst, preferably between 1% and 15% by weight, and more preferentially between 2% and 14% by weight. According to one or more embodiments, when the catalyst is trimetallic, the molar ratio between the manganese and said first element from group VIII is between 0.1 mol / mol and 30 mol / mol, preferentially between 0.2 mol / mol and 20 mol / mol, more preferentially between 0.3 mol / mol and 15 mol / mol and the molar ratio between the manganese and said second element from group VIII is between 0.1 mol / mol and 30 mol / mol, preferentially between 0.2 mol / mol and 20 mol / mol, more preferentially between 0.3 mol / mol and 15 mol / mol. The active phase is in the form of isolated nanoparticles or clusters of nanoparticles comprising all of the metals present as active phase. When the catalyst is bimetallic, said nanoparticles comprise at least manganese, and said first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni). When the catalyst is trimetallic, said nanoparticles comprise at least manganese, said first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), and said second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element from group VIII. The contents obtained on the final catalyst are analyzed on the final catalyst by X-ray fluorescence. The multi-metallic (e.g. bi- or trimetallic) character within all of the nanoparticles observed in transmission electron microscopy (TEM) is confirmed by EDS (energy dispersion spectroscopy). Said nanoparticles have a size of between 1 nm and 30 nm, measured by transmission electron microscopy, preferably between 2 and 30 nm, and more preferentially still between 3 and 30 nm. 3. Support Preferably, the support comprises at least titanium dioxide (TiO2), a support known for being more resistant to sulfur poisoning due to the stability of the sulfates on the surface. Thus, the support protects the active phase. Optionally, the porous support comprising at least titanium dioxide (TiO2) may be sulfated according to techniques known to those skilled in the art in order to improve its resistance to sulfur. Preferably, the TiO2 is in its anatase and rutile forms, the rutile:anatase ratio by mass preferably being between 95:5 and 50:50. The specific surface area of the porous support is generally of between 10 m2 / g and 300 m2 / g, preferably between 10 m2 / g and 150 m2 / g, more preferably between 30 m2 / g and 120 m2 / g. The pore volume of the support is generally between 0.2 ml / g and 1.1 ml / g, preferably between 0.3 ml / g and 1 ml / g. According to one or more embodiments, the porous support is in the form of a powder with a particle size of between 20 pm and 500 pm. According to one or more embodiments, the porous support is in the form of grains shaped into the form of extrudates or beads with a diameter of between 0.5 mm and 10 mm. The "diameter" of the extrudates is understood to mean the diameter of the circle circumscribed in the cross section of these extrudates. 4. Process for preparing the catalyst A second subject according to the invention relates to a process for preparing the catalyst according to the invention, comprising the following steps: a) a porous support comprising a refractory oxide chosen from titanium dioxide, silica, alumina, silica-alumina, zirconia, ceria, used pure or as a mixture with one another, is provided; b) the following substeps are carried out in order to obtain a catalyst precursor: b1) the porous support is brought into contact with at least one solution containing at least one manganese precursor; b2) the porous support is brought into contact with at least one solution containing at least one precursor of the first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni); substeps b1) and b2) being carried out separately, in any order, or at the same time, c) said catalyst precursor obtained on conclusion of step b) is dried at a temperature of less than 250°C in order to obtain a dried catalyst precursor; d) the dried catalyst precursor obtained on conclusion of step c) is calcined at a temperature of between 250°C and 900°C. Intermediate steps may be introduced (in particular additional drying steps and maturation steps) and certain steps may be carried out several times in a row. Step b) impregnation of the support in order to obtain a catalyst precursor Said substep b1) may be carried out by dry impregnation or by excess impregnation according to methods well known to those skilled in the art. The pH of said solution may be modified by the optional addition of an acid or of a base. Preferably, said substep b1) is carried out by dry impregnation, which consists in bringing the porous support into contact with at least one solution preferably containing at least one manganese precursor, the volume of the solution of which is between 0.25 times and 1.5 times the pore volume of the support to be impregnated. According to one or more embodiments, the manganese precursor salt is selected from organic precursors of manganese such as manganese acetate, manganese oxalate and manganese citrate, and inorganic precursors of manganese, such as manganese nitrate, manganese carbonates and manganese sulfates. Preferably, the manganese precursor salt is manganese nitrate. The pore volume of the support and the manganese concentration are adjusted according to the type of impregnation (dry impregnation or excess impregnation) so as to obtain for the supported catalyst a manganese content of between 0.1% and 15% by weight of manganese element relative to the total weight of the catalyst, preferably between 0.5% and 15% by weight, more preferentially between 2% and 12% by weight. Said substep b2) may be carried out by dry impregnation or by excess impregnation according to methods well known to those skilled in the art. The pH of said solution may be modified by the optional addition of an acid or of a base. Preferably, said substep b2) is carried out by dry impregnation, which consists in bringing the porous support into contact with at least one solution containing, and preferably consisting of, at least one precursor of a first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), the volume of the solution of which is between 0.25 times and 1.5 times the pore volume of the support to be impregnated. According to one or more embodiments, the pore volume of the support and the concentration in solution of the first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), are adjusted according to the type of impregnation (dry impregnation or excess impregnation) so as to obtain for the supported catalyst a content of the first element from group VIII, chosen from iron (Fe), cobalt (Co), and nickel (Ni), of between 0.1% and 20% relative to the total weight of the catalyst, preferably between 0.5% and 15% by weight, more preferentially between 1% and 12% by weight. Likewise, the concentration of first element from group VIII, chosen from iron (Fe), cobalt (Co), and nickel (Ni), in solution is provided so that the atomic ratio between the manganese and the first element from group VIII, chosen from iron (Fe), cobalt (Co), and nickel (Ni), is between 0.1 mol / mol and 30 mol / mol, preferentially between 0.2 mol / mol and 20 mol / mol, more preferentially between 0.3 mol / mol and 15 mol / mol. According to one or more embodiments, when substeps b1) and b2) are carried out separately, in any order, an intermediate step of maturation followed by drying in which the catalyst precursor obtained on conclusion of substep b1), or of substep b2), is dried at a temperature of less than 250°C, preferably between 70°C and 200°C, is carried out between substeps b1) and b2), or between substeps b2) and b1). The drying time is generally between 0.5 hours and 20 hours. Longer periods of time are not ruled out, but do not necessarily afford any improvement. When the first element from group VIII is cobalt, step b2) is preferably carried out separately in any order. According to one or more embodiments, step b) additionally comprises a substep b3) in which said porous support is brought into contact with at least one solution containing at least one precursor of the second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element from group VIII. Said step b3) may be carried out by dry impregnation or by excess impregnation according to methods well known to those skilled in the art. The pH of said solution may be modified by the optional addition of an acid or of a base. Preferably, said substep b3) is carried out by dry impregnation, which consists in bringing the porous support into contact with at least one solution containing, and preferably consisting of, at least one precursor of a first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), the volume of the solution of which is between 0.25 times and 1.5 times the pore volume of the support to be impregnated. The pore volume of the support and the concentration in solution of the second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), are adjusted according to the type of impregnation (dry impregnation or excess impregnation) so as to obtain for the supported catalyst a content of second element from group VIII, chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element, of between 0.1% and 20% by weight relative to the total weight of the catalyst, preferably between 0.5% and 15% by weight, more preferentially between 1% and 12% by weight. Likewise, the concentration of element from group VIII, chosen from iron (Fe), cobalt (Co), and nickel (Ni), in solution is provided so that the atomic ratio between the manganese and the second element from group VIII, chosen from iron (Fe), cobalt (Co), and nickel (Ni) (different from the first element), is between 0.1 mol / mol and 30 mol / mol, preferentially between 0.2 mol / mol and 20 mol / mol, more preferentially between 0.3 mol / mol and 15 mol / mol. Said substep b3) being carried out either separately from substeps b1) and / or b2), in any order, or simultaneously. For example, substep b3) may be carried out alone, or simultaneously with substep b1) and / or substep b2), or simultaneously with steps b1) and b2). According to one or more embodiments, when substep b3) is carried out separately, in any order, from steps b1) and b2), an intermediate step of maturation followed by drying in which the catalyst precursor (obtained on conclusion of the preceding substep) is dried at a temperature of less than 250°C, preferably between 70°C and 200°C, is carried out between the substeps carried out separately. The maturation time is generally between 0.5 hours and 5 hours. Longer periods of time are not ruled out, but do not necessarily afford any improvement. The drying time is generally between 0.5 hours and 20 hours. Longer periods of time are not ruled out, but do not necessarily afford any improvement. When the first element from group VIII is cobalt, substep b3) is preferably carried out separately in any order. According to one or more embodiments, the iron precursor is chosen in the nitrate, carbonate, chloride, sulfate, hydroxide, hydroxycarbonate, formate, acetate or oxalate form, in the form of complexes formed with acetylacetonates, or also in the form of tetrammine or hexammine complexes, or in the form of any other inorganic derivative which is soluble in aqueous solution, which is brought into contact with said support. As iron precursor, preference is given to using iron nitrate, iron chloride, or iron hydroxide. Very preferably, the iron precursor is iron nitrate, iron sulfate or iron hydroxide. According to one or more embodiments, the cobalt precursor is chosen in the nitrate, carbonate, chloride, sulfate, hydroxide, hydroxycarbonate, formate, acetate or oxalate form, in the form of complexes formed with acetylacetonates, or also in the form of tetrammine or hexammine complexes, or in the form of any other inorganic derivative which is soluble in aqueous solution, which is brought into contact with said support. As cobalt precursor, preference is given to using cobalt nitrate, cobalt carbonate, cobalt chloride, cobalt hydroxide or cobalt hydroxycarbonate. Very preferably, the cobalt precursor is cobalt nitrate, cobalt carbonate or cobalt hydroxide. According to one or more embodiments, the nickel precursor is chosen in the nitrate, carbonate, chloride, sulfate, hydroxide, hydroxycarbonate, formate, acetate or oxalate form, in the form of complexes formed with acetylacetonates, or also in the form of tetrammine or hexammine complexes, or in the form of any other inorganic derivative which is soluble in aqueous solution, which is brought into contact with said support. As nickel precursor, preference is given to using nickel nitrate, nickel carbonate, nickel chloride, nickel hydroxide or nickel hydroxycarbonate. Very preferably, the nickel precursor is nickel nitrate, nickel carbonate or nickel hydroxide. Step c): drying of the catalyst precursor in order to obtain a dried catalyst precursor The catalyst precursor obtained on conclusion of step b) is dried at a temperature of less than 250°C, preferably between 70°C and 200°C. The drying time is generally between 0.5 hours and 20 hours. Longer periods of time are not ruled out, but do not necessarily afford any improvement. The drying is generally carried out under combustion air of a hydrocarbon, preferably methane, or under heated air comprising between 0 grams and 80 grams of water per kilogram of combustion air, an oxygen content of between 5% and 25% by volume and a carbon dioxide content of between 0% and 10% by volume. Optionally, the catalyst precursor obtained on conclusion of the intermediate drying step is subjected to a heat treatment under air, preferably combustion air, and more preferentially combustion air of methane, comprising between 40 grams and 80 grams of water per kg of air, an oxygen content of between 5% and 15% by volume and a CO2 content of between 4% and 10% by volume. Step d): calcination of the dried catalyst precursor in order to obtain said catalyst The dried catalyst precursor obtained on conclusion of step c) is then calcined. The calcination temperature is generally between 250°C and 900°C, preferably between 300°C and approximately 500°C. The calcination time is generally between 0.5 hours and 5 hours. The gas hourly space velocity (GHSV) is generally between 150 h-1 and 3000 h-1, preferably 300 h-1 and 1500 h-1. On conclusion of steps a) to d), the catalyst according to the invention is obtained. The formulation and the preparation process according to the present invention result in an active phase in the form of nanoparticles comprising at least manganese (Mn), a first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), and optionally a second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), when the catalyst is trimetallic. Said nanoparticles or clusters of nanoparticles have a size of between 1 nm and 30 nm, measured by transmission electron microscopy. 5. Use of the catalyst A third subject according to the invention relates to the use of the catalyst as a selective catalytic reduction (SCR) system in a process for lowering NOx in exhaust gases. The process for the catalytic reduction of nitrogen oxides (NOx), using the catalyst according to the invention or prepared according to the invention, comprises a step of bringing the catalyst and a nitrogen oxide into contact, preferably at a temperature of between 30°C and 600°C, preferably between 40°C and 500°C, and more preferably between 50°C and 400°C. The GHSVs employed are preferably between 6000 h-1 and 240 000 h-1. The amount of catalyst is adjusted by those skilled in the art according to the amount of NOx present in the gas to be treated. According to one or more embodiments, the gases to be treated by said catalyst comprise NOx at between 200 ppm and 3000 ppm by weight, oxygen O2 at between 2% and 8% by weight, CO at between 0 ppm and 500 ppm, H2O at between 0% and 10% by weight, a content of sulfur in SO2 form of less than 200 ppm, preferably less than 150 ppm by weight. An SCR system can be integrated into a variety of systems requiring NOx reduction. Applications include passenger vehicle and truck engine systems, stationary applications (utility boilers, industrial boilers, solid waste boilers), or maritime applications. Preferably, the catalyst is used in stationary applications. The reducing agents that may be used in the process for lowering NOx according to the invention are preferably chosen from hydrogen (H2-SCR), a hydrocarbon (HC-SCR) or ammonia (NH3-SCR). Preferably, the catalyst is used with ammonia and / or derivatives thereof as reducing agent. In this case, the catalytic system is preferably suitably connected to a source of ammonia so that the latter comes into contact with the exhaust gases. The ammonia may be supplied in the form of anhydrous ammonia, aqueous ammonia, urea, ammonium carbonate, ammonium formate or ammonium carbamate. According to one or more embodiments, the catalyst may be applied, deposited on an inert substrate in the form of a metal plate or foam, or a honeycomb. According to one or more embodiments, the catalyst may be combined with other solids such as fillers and binders so as to provide an extrudable paste that can be transformed into a porous structure such as a honeycomb. Examples Examples 1a to 1c: preparation of the impregnation solutions Example 1a: preparation of an aqueous solution S1 of Mn and Ni precursor The aqueous solution S1 used for the preparation of the catalysts A and C is prepared by dissolving 2.28 g of manganese nitrate (Mn(NO3^6H2O, supplier Strem Chemicals®) and 1.1 g of nickel nitrate (Ni(NO3^6H2O, supplier Strem Chemicals®) in a volume of 6 mL of distilled water. Example 1b: preparation of an aqueous solution S2 of Mn and Fe precursor The aqueous solution S2 used for the preparation of the catalyst B is prepared by dissolving 3.7 g of manganese nitrate (Mn(NO3^6H2O, supplier Strem Chemicals®) and 5.90 g of iron nitrate (Fe(NO3^9H2O, supplier Strem Chemicals®) in a volume of 6 mL of distilled water. Example 1c: preparation of an aqueous solution S3 of Co precursor The aqueous solution S3 used for the preparation of the catalyst C (second impregnation substep b3) is prepared by dissolving 1.9 g of cobalt nitrate (CoNO3^6H2O, supplier Strem Chemicals®) in a volume of 5 mL of distilled water. Examples 2 to 4: preparation of the catalysts In examples 2 to 4, the support used for preparing the catalysts A to C is the support in the form of TiO2 powder (TiO2-NP, P25) (CAS No. 13463-67-7) supplied by Aldrich. This support is composed of 80% by weight of rutile and 20% by weight of anatase relative to the total weight of the support and has a specific surface area SBET of 55 m2 / g and a pore volume of 0.5 ml / g. Example 2: preparation of the catalyst A In this example, steps b1) of manganese impregnation and b2) of impregnation of the first element from group VIII are carried out simultaneously. Specifically, a single solution S1 containing a mixture of manganese nitrate and nickel nitrate is impregnated on the support. 5 ml of solution S1 prepared according to example 1a are added dropwise to 10 g of support. The impregnated support obtained is statically matured in a saturator for 1 h and then is dried at 120°C for 16 h. The dried catalyst precursor is then calcined in a tube furnace at 400°C for 2 h with a ramp of 5°C / min under 1 L / h / g of air. A catalyst A comprising an Mn content of 5% by weight and an Ni content of 2.14% by weight relative to the total weight of catalyst A, measured by X-ray fluorescence, is obtained. The Mn-to-Ni molar ratio is 0.4 mol / mol. The size of the nanoparticles, measured by transmission electron microscopy, is centred on 15 nm. EDS (energy dispersion spectroscopy) analysis shows that all of the particles are bimetallic and comprise both Mn and Ni. Example 3: preparation of the catalyst B In this example, steps b1) of manganese impregnation and b2) of impregnation of the first element from group VIII are carried out simultaneously. Specifically, a single solution S2 containing a mixture of manganese nitrate and iron nitrate is impregnated on the support. 5 ml of solution S2 prepared according to example 1b are added dropwise to 10 g of support. The impregnated support is statically matured in a saturator for 1 h and then dried in an oven at 120°C for 16 h. The solid is calcined in a tube furnace at 400°C for 2 h with a ramp of 5°C / min under 1 L / h / g of air. A catalyst B comprising an Mn content of 8% by weight is obtained, the Fe content is 8% by weight relative to the total weight of catalyst B, measured by X-ray fluorescence. The Mn-to-Fe molar ratio is 1 mol / mol. The size of the nanoparticles, measured by transmission electron microscopy, is centred on 20 nm. EDS (energy dispersion spectroscopy) analysis shows that all of the particles are bimetallic and comprise both Mn and Fe. Example 4: preparation of the catalyst C In this example, substeps b1) of manganese impregnation and b2) of impregnation of the first element from group VIII are carried out simultaneously. Specifically, a single solution S1 containing a mixture of manganese nitrate and nickel nitrate is impregnated on the support. On the other hand, substep b3) of impregnation of the second element from group VIII is carried out separately since the solution S3 is introduced separately from steps b1) and b2) after an intermediate maturation and drying step. 5 ml of solution S1 prepared according to example 1a are added dropwise to 10 g of support. The impregnated support is statically matured in a saturator for 1 h and then dried in an oven at 120°C for 16 h. Next, 5 ml of cobalt nitrate-based solution S3 prepared according to example 1c is added dropwise to the impregnated support. The mixture obtained is statically matured in a saturator for 1 h and then dried in an oven at 120°C for 16 h. The solid is calcined in a tube furnace at 400°C for 2 h with a ramp of 5°C / min under 1 L / h / g of air. A catalyst C comprising an Mn content of 5% by weight, an Ni content of 2.14% by weight and a Co content of 5% by weight relative to the total weight of catalyst C, measured by X-ray fluorescence, is obtained. The Mn-to-Ni molar ratio is 0.4 mol / mol. The Mn-to-Mo molar ratio is 0.9 mol / mol. The average size of the nanoparticles, measured by transmission electron microscopy, is 20 nm. EDS (energy dispersion spectroscopy) analysis shows that all of the particles are trimetallic and comprise simultaneously Mn, Ni and Co. Example 5: characterization of the catalysts The characterization of the catalysts is presented in Table 1 below. The composition in terms of Mn, Fe, Ni and Co is measured by X-ray fluorescence and given as % by weight relative to the total weight of the catalyst. The molar ratios between elements, 5 expressed as mol / mol, are calculated from these compositions by mass. The size of the nanoparticles is measured by transmission electron microscopy (TEM). The term "size of nanoparticles" is understood to mean the average size of one or more metallic nanoparticles (in the form of clusters or agglomerates of nanoparticles). Analysis of the composition of the nanoparticles is carried out via EDS (energy dispersion 10 spectroscopy) analysis, during observation of the samples in TEM. Table 1 Characterization Catalyst A Catalyst B Catalyst C Support TiO2 TiO2 TiO2 Mn (% by weight) (X-ray fluorescence) 5 8 5 Fe (% by weight) (X-ray fluorescence) / 8 / Ni (% by weight) (X-ray fluorescence) 2.14 / 2.14 Co (% by weight) (X-ray fluorescence) / / 5 Mn / Fe (mol / mol) / 1 / Mn / Ni (mol / mol) 0.4 / 0.4 Mn / Co (mol / mol) / / 0.9 Composition of the nanoparticles particles (EDS) Mn, Ni Mn, Fe Mn, Ni, Co Sizes of the nanoparticles (nm) TEM 15 20 20 Example 6: Catalytic tests The catalysts are tested in a catalytic reactor. The gas to be purified is a reconstituted gas mixture having 500 ppm by weight of NO, 500 ppm by weight of NH3 and 5% O2 by weight 15 relative to the mass of gas to be purified, the remainder of the mixture being made up of a neutral carrier gas, in this case nitrogen. The gas hourly space velocity (GHSV) is 30 600 h-1. In addition to these operating conditions, a temperature rise was chosen with a rate of 10°C / min over a temperature range extending from 30°C to 450°C. The analysis of the gaseous effluents at the inlet and at the outlet of the reactor is carried out by chromatography and UV spectroscopy. The NO conversion, expressed in %, is calculated as the difference between the content by mass of NO in the inlet gas stream minus the content by mass of NO in the outlet gas stream; divided by the content by mass of NO in the inlet gas 5 stream. The results presented in Table 2 below are the conversion of NO, expressed in %, at 150°C. These results are compared with a commercial reference composed of titanium, tungsten and vanadium on TiO2 of the DNX-series range for DeNOx SCR sold by Umicore. Table 2 Catalyst NO conversion (%) Reference 20 Catalyst A 90 Catalyst B 85 Catalyst C 99 10 Compared to the reference catalyst, the three catalysts (A to C) in accordance with the invention are more active (greater NO conversion at 150°C). The most active catalyst being the trimetallic catalyst of formulation Mn-Ni-Co.

Claims

1. A catalyst comprising an active phase comprising manganese (Mn), at least a first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), said catalyst comprising a porous support comprising at least one refractory oxide chosen from titanium dioxide, silica, alumina, silica-alumina, zirconia, ceria, used pure or as a mixture with one another, said catalyst being characterized in that:- the manganese content is between 0.1% and 15% by weight of manganese element relative to the total weight of the catalyst;- the content of said first element from group VIII is between 0.1% and 20% by weight of element from group VIII relative to the total weight of the catalyst;- the molar ratio between the manganese and said first element from group VIII is between 0.1 mol / mol and 30 mol / mol;- the active phase is in the form of nanoparticles comprising manganese and at least said first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), said size of the nanoparticles being between 1 nm and 30 nm, measured by transmission electron microscopy.

2. The catalyst as claimed in claim 1, characterized in that the active phase is bimetallic, the metals being chosen from Mn-Fe, Mn-Co, Mn-Ni.

3. The catalyst as claimed in claim 1, characterized in that the active phase comprises a second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element from group VIII.

4. The catalyst as claimed in claim 3, characterized in that the active phase is in the form of nanoparticles comprising manganese, and at least said first element from group VIII and said second element from group VIII, said size of the nanoparticles being between 1 nm and 30 nm, measured by transmission electron microscopy.

5. The catalyst as claimed in either of claims 3 and 4, characterized in that the content of said second element from group VIII is between 0.1% and 20% by weight of element from group VIII relative to the total weight of the catalyst.

6. The catalyst as claimed in any one of claims 3 to 5, characterized in that the molar ratio between the manganese and said second element from group VIII is between 0.1 mol / mol and 30 mol / mol.

7. The catalyst as claimed in any one of claims 3 to 6, characterized in that the active phase is trimetallic, the metals being chosen from Mn-Co-Fe, Mn-Ni-Fe, Mn-Ni-Co.

8. The catalyst as claimed in any one of the preceding claims, characterized in that said porous support comprises at least titanium dioxide.

9. The catalyst as claimed in any one of the preceding claims, characterized in that said porous support has a specific surface area of between 10 m2 / g and 300 m2 / g.

10. The catalyst as claimed in any one of the preceding claims, characterized in that said porous support is provided either in the form of powder or in the form of grains in the form of extrudates or beads.

11. A process for preparing a catalyst as claimed in any one of claims 1 to 10, comprising the following steps:a) a porous support comprising at least one refractory oxide chosen from titanium dioxide, silica, alumina, silica-alumina, zirconia, ceria, used pure or as a mixture with one another, is provided;b) the following substeps are carried out in order to obtain a catalyst precursor:b1) the porous support is brought into contact with at least one solution containing at least one manganese precursor;b2) the porous support is brought into contact with at least one solution containing at least one precursor of the first element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni);substeps b1) and b2) being carried out separately, in any order, or at the same time;c) said catalyst precursor obtained on conclusion of step b) is dried at a temperature of less than 250°C in order to obtain a dried catalyst precursor;d) the dried catalyst precursor obtained in step c) is calcined at a temperature of between 250°C and 900°C in order to obtain said catalyst.

12. The process as claimed in claim 11, wherein step b) comprises a substep b3) in which said porous support is brought into contact with at least one solution containing at least one precursor of the second element from group VIII chosen from iron (Fe), cobalt (Co), and nickel (Ni), different from said first element from group VIII, said substep b3) being carried out either separately from substeps b1) and / or b2), in any order, or at the same time.

13. The process as claimed in either of claims 11 and 12, wherein, when said first element from group VIII is cobalt, substeps b1) and b2) are carried out separately in any order.

14. The process as claimed in any one of claims 12 and 13, wherein, when said second element from group VIII is cobalt, substep b3) is carried out separately from substeps b1) 5 and / or b2).

15. The use of the catalyst as claimed in any one of claims 1 to 10 or prepared as claimed in any one of claims 11 to 14 for the catalytic reduction of nitrogen oxides (NOx).