Ammonia synthesis catalyst and process for its preparation

Abstract

A method for synthesizing an ammonia synthesis catalyst is described, comprising the steps of providing a transition metal precursor and contacting the transition metal precursor with a solution to form an improved solution; providing a catalyst support and contacting the catalyst support with the improved solution to form a suspension; drying the suspension to obtain a solid powder; and mixing the solid powder with a hydride compound to obtain the ammonia synthesis catalyst.

Description

[Technical field]

[0001] The present invention relates to an ammonia synthesis catalyst, a process for its preparation and its use in the ammonia industry. [Background technology]

[0002] Ammonia is synthesized industrially by reacting nitrogen and hydrogen at high temperature and pressure in the presence of a suitable catalyst.

[0003] During synthesis, the catalyst must accomplish two functions: activating hydrogen and nitrogen molecules to form reactive atomic species, which usually involves an electron transfer reaction that is facilitated by high temperature and the presence of an electron-donating promoter.

[0004] Several catalysts are active for the synthesis of ammonia. Historically, Fe 3 O 4 and a few percent Al 2 O 3 and K. 2 Iron (Fe)-based catalysts containing O have been used for ammonia synthesis at temperatures in the range 400-500 °C and pressures in the range 150-300 bar. More recently, ruthenium (Ru)-based catalysts have enabled ammonia synthesis under milder conditions, such as 250-400 °C and pressures up to 110 bar.

[0005] Currently, the catalysts commonly used in the synthesis of ammonia are supported catalysts in which active catalytic elements are dispersed on a support to reduce production costs, increase the dispersion of active species, and in some cases promote the activation of nitrogen by electron donation.

[0006] For example, known supported catalysts for ammonia synthesis include Ru / C, Ru / MgO, and Ru / CaO, etc. In addition to ruthenium, other transition metals can be used, including Fe, Co, Co-Mo, Ni-Mo, etc.

[0007] There is an increasing interest in the art to find ammonia synthesis catalysts that are characterized by high activity, high poisoning resistance, and the ability to operate over a wide temperature range, especially at temperatures lower than those typical for Fe-based industrial catalysts.

[0008] Catalysts based on the coupling of metals with hydrides are believed to be promising for the above purposes. Examples of such metal-hydride catalysts are disclosed in US Pat. No. 5,393,313 and US Pat. No. 5,493,326.

[0009] Unfortunately, the properties of metal-hydride catalysts at present are not entirely satisfactory, since the hydride compounds are unstable in air. In fact, hydrides are oxidized in the presence of oxygen and also react strongly with water. In addition, hydrides suffer from thermal degradation at high temperatures, so their operating window is limited to temperatures below 400 °C.

[0010] For the above reasons, another drawback of metal-hydride catalysts is their preparation process, which requires synthesis in a chemically and thermally controlled atmosphere and / or the use of expensive equipment, i.e. ball mill tanks.

[0011] It would therefore be highly desirable to provide improved ammonia synthesis catalysts of the hydride type that can be prepared and handled under mild conditions in air, and it would also be highly desirable to provide simpler, cheaper and less energy intensive processes for synthesizing said catalysts. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] US Patent Application Publication No. 2016 / 0361712 [Patent Document 2] US Patent Application Publication No. 2018 / 0327272 Summary of the Invention

[0013] The present invention is based on the high temperatures of conventional hydrogenation catalysts, which typically constrain the choice of preparation process, as well as the O 2 and H 2 The objective of the present invention is to overcome the drawback of the prior art, which is insufficient resistance to an O-containing environment.

[0014] A further object is to provide a process for the manufacture of an ammonia catalyst that is conveniently scalable for industrial production.

[0015] Therefore, one aspect of the present invention is a method for synthesizing an ammonia synthesis catalyst as claimed in claim 1.

[0016] The process comprises the steps of: providing a transition metal precursor, preferably selected from one of Fe, Co, Ru, Mn or V (and optionally mixtures thereof), and contacting said transition metal precursor with a solution or solvent (preferably an aqueous or organic solvent, more preferably an aqueous solvent, even more preferably water or distilled water) to form an improved solution; providing a catalyst support and contacting said catalyst support with said improved solution to form a suspension; drying said suspension to obtain a solid powder; optionally subjecting said solid powder to a purification step to obtain a purified solid powder; mixing said solid powder or said high purity solid powder with a hydride compound.

[0017] The hydride catalyst of the present invention is synthesized with low energy consumption since the mixing of solid powders with hydride compounds can be carried out in the open air and at room temperature, thus eliminating the need for the use of pressure-sealed vessels and synthesis in chemically and thermally controlled environments.

[0018] This process is particularly suitable for large-scale production since no pressure-sealed vessel is required to carry out the synthesis and the process allows for very good control over the routing of the metals, preventing the occurrence of metal leaching phenomena and allowing optimal distribution of the transition metals on the catalyst support.

[0019] A second aspect of the present invention is an ammonia synthesis catalyst obtainable by the process of the present invention.

[0020] The catalyst according to the present invention comprises a transition metal, a catalyst support and a hydride compound, wherein the transition metal is preferably selected from Fe, Co, Ru, Mn or V, and the catalyst support is an oxide of the following: CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO 2 , ZrO 2 , doped-ZrO 2 , ZnO, Pr 2 O 3 , Nb 2 O 5 , La 2 O 3 , CaO.Al 2 O 3 , Mayenite, LaCeO x , BaTiO 3 , BaCeO 3 , BaCe x Y 1-x O 3 , SrTiO 3 , CaTiO 3 , LaCoO 3 , BaZrO 3 , Y 2 O 3 , LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite), and optionally mixtures thereof.

[0021] In the prior art, as supported by the work conducted by Wang, Peikun et al. in "Breaking Scaling Relations to Achieve Low-Temperature Ammonia Synthesis through LiH-Mediated Nitrogen Transfer and Hydrogenation." Nature chemistry 9.1 (2017): 64-70, it is believed that intimate contact between the metal nanoparticles and the hydride is necessary to ensure the transfer of nitrogen activated on the metal to the hydride to enable high ammonia production rates.

[0022] In the present invention, and contrary to the teachings of the prior art, applicants have found that enhanced catalytic activity can surprisingly be obtained by depositing metal nanoparticles on a support and then adding a hydride via simple mechanical mixing.

[0023] Additionally, it has been discovered that the resulting catalysts exhibit improved on-stream stability because the support can promote resistance to sintering that typically occurs when metals are deposited directly on the hydride.

[0024] A further advantage is that, compared to conventional hydride catalysts, a lower amount of hydride content is required to achieve the same ammonia productivity. Moreover, the replacement of a portion of the hydride with a cheaper carrier further reduces its production cost.

[0025] Furthermore, the predetermined order of mixing of the hydride compound with the powder containing the metal compound deposited on the metal oxide support, in particular mixing the hydride with the powder only after the preparation of the latter is complete, prevents decomposition of the hydride during synthesis.

[0026] Furthermore, in the early stages of preparation of the ammonia catalyst, the powder can be manufactured to have the desired mechanical and structural properties prior to the addition of the hydride compound, which is advantageous because once the hydride is added, it becomes more difficult to process the powder to obtain the desired properties.

[0027] A further aspect of the present invention is the use of the catalyst of the present invention to synthesize ammonia by reacting a make-up gas containing nitrogen and hydrogen over said catalyst. Advantageously, due to the improved activity of the catalyst, ammonia can be synthesized with high productivity.

[0028] Hereinafter, the term "transition metal" refers to a chemical element in the d-block (groups 3 to 12, preferably groups 5 to 9, more preferably groups 5, 7, 8 or 9) of the periodic table, unless otherwise specified.

[0029] The term "catalyst precursor" or shortened "precursor" refers to a material that requires activation or reaction to produce an active catalyst. The combined term "transition metal precursor" refers to a chemical element in the d-block of the periodic table that is not yet catalytically active, i.e. does not exhibit substantial catalytic activity, according to the above definition, but requires further activation or reaction to be converted to an active catalyst.

[0030] As used herein, a modified solution is a solution in which a transition metal precursor has been dissolved after contacting the solution or solvent, in other words, a modified solution is a transition metal-containing solution.

[0031] Additionally, the term "catalyst support" refers to a material that is typically in solid form and characterized by a high surface area onto which a catalyst can be attached or deposited.

[0032] The term "reducing agent" refers to a substance that loses electrons to another substance in an oxidation-reduction reaction and is oxidized to a higher valence state.

[0033] The term "precipitation agent" refers to a substance that is added to a medium to cause the precipitation of another substance present in the medium. Such precipitation may be selective with respect to the substance that must be precipitated from the medium.

[0034] The term "inert atmosphere" refers to an atmosphere that does not contain reactive gases, such as oxygen. More specifically, in the present invention, an inert atmosphere refers to an atmosphere that does not affect the synthesis of the catalyst, for example, an atmosphere that does not induce chemical or electrochemical reactions or physical interactions between the gases contained in the atmosphere and the catalyst being synthesized. The inert atmosphere can be obtained by using nitrogen, argon or helium as the inert gas.

[0035] As used herein, the expression "nanoparticles" refers to particles having an average particle size distribution between 1 nanometer (nm) and 1000 nm. Such average particle size distribution can be determined using known analytical methods, such as laser diffraction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The process of the present invention is highly versatile as it makes it feasible to synthesize ammonia catalysts from a variety of support types and from a variety of hydride compounds.

[0037] For example, the catalyst support may be made of the following oxide: CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO 2 , ZrO 2 , doped-ZrO 2 , ZnO, Pr 2 O 3 , Nb2 O 5 , La 2 O 3 , CaO.Al 2 O 3 , Mayenite, LaCeO x , BaTiO 3 , BaCeO 3 , BaCe x Y 1-x O 3 , SrTiO 3 , CaTiO 3 , LaCoO 3 , BaZrO 3 , Y 2 O 3 , LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite), and optionally mixtures thereof.

[0038] In a particularly preferred embodiment, the catalyst support is TiO 2 or SiO 2 It is.

[0039] Hydride compounds have the formula XH n where X is an alkali metal or alkaline earth metal (preferably one of the following: Li, Na, K, Ca, Ba or Sr) and n is the number of hydrogen atoms in the hydride.

[0040] The mixing of the solid powder or the purified solid powder with the hydride compound can be carried out under any suitable conditions and by any suitable process known to the person skilled in the art, but according to the particular application of interest, the mixing of the solid powder or the purified solid powder with the hydride compound does not have to be carried out in an inert controlled atmosphere, but can be carried out, for example, in open air or in ambient air at room temperature.

[0041] Ambient air typically refers to the air in the atmosphere in its natural state, which contains 78% nitrogen and 21% oxygen, plus minor components. The ability to mix the solid powder or purified solid powder with the hydride compound in ambient air allows for a significant reduction in the synthesis cost of the catalyst, which is particularly beneficial for large-scale production applications.

[0042] The applicant has found that better catalytic performance is obtained when the powder or the purified solid powder is mixed with a hydride compound in a weight ratio of 60:40. Another less preferred weight ratio is 100:1 to 1:100.

[0043] The process of the present invention may further comprise the step of contacting the reformed solution with a reducing agent or with a precipitating agent prior to contacting the reformed solution with the catalyst support.

[0044] Alternatively, the process of the present invention may further comprise a step of contacting the suspension obtained by contacting the catalyst support with the modified solution with a reducing agent or with a precipitating agent before drying.

[0045] In a preferred embodiment, when a precipitating agent is used instead of a reducing agent in the process of the present invention, the reduction of the ammonia catalyst is carried out in situ in the ammonia synthesis converter.

[0046] A reducing agent that is suitable for reducing the improved solution or suspension is sodium borohydride NaBH 4 or potassium borohydride KBH 4 A suitable precipitating agent is an aqueous solution of carbonate or urea. Preferably, the carbonate is (NH 4 ) 2 CO 3 , Na 2 CO 3 or K 2 CO 3 It is.

[0047] In the art, boron has been added to the catalyst, for example, NaBH, to improve the catalytic activity of the synthesized catalyst.4 It is known that reducing agents containing boron can be incorporated during the preparation of the catalyst using the methods described above. In the present invention, when a reducing agent containing boron is used, the reducing agent is not utilized to incorporate boron into the catalyst, but is used only to induce the deposition of transition metals on the support. In other words, the reducing agent is a metal ion (e.g., Co 2+ ) to metal nanoparticles (e.g. metallic cobalt) on the support surface, but not to incorporate boron therein.

[0048] Fe and Co are preferred transition metals. Preferably, the transition metal precursor is a metal salt and / or a metal complex, particularly preferably FeCl 3 , Fe(NO 3 ) 3 , Fe(acac) 3 [iron(III) acetylacetonate], or CoCl 2 is selected from.

[0049] Preferably, the purification step comprises at least one washing operation. The washing operation can be carried out at a temperature in the range of 80-95 ° C, or preferably at a temperature equal to 90 ° C, using distilled water as washing agent. Particularly preferably, a washing step is carried out in order to remove excess boron potentially introduced during the treatment with the reducing agent containing boron. The washing operation can be carried out in order to remove excess boron from the surface of the support leaving only metal nanoparticles. Preferably, the content of boron remaining in the solid powder after purification is less than or equal to 0.5 wt %. This washing step in the catalyst preparation has proven beneficial for the catalytic performance during the synthesis of ammonia.

[0050] In accordance with the above, one embodiment of the purification step comprises reducing the boron content of the solid powder, the residual content of boron remaining in the purified solid after purification being preferably 0.5 wt% or less, for example 0-0.5 wt%, or 0.01-0.5 wt%, or 0.1-0.5 wt%.

[0051] The drying step may be carried out by filtration on a membrane having a pore size in the range of 0.10 to 0.40 μm, preferably in the range of 0.20 to 0.30 μm, or even more preferably equal to or about 0.22 μm.

[0052] The present invention provides a preparation process that is less energy consuming and offers excellent control over metal loading with no metal leaching. The additional support provides improved on-stream stability due to enhanced activity and resistance to sintering.

[0053] Without being bound by theory, the Applicant believes that the synergistic effects of the interconversion of oxide and hydride phases, the in situ formation of (oxy)nitride phases, and the improved metal dispersion contribute to the improved activity observed in the tested catalysts.

[0054] In the present invention, the ammonia synthesis catalyst comprises a transition metal, a catalyst support, and a hydride compound. Preferably, the transition metal is composed of nanoparticles dispersed and supported on the catalyst support.

[0055] In a particularly preferred embodiment, the catalyst support is TiO 2 or SiO 2 It is.

[0056] In a particularly preferred embodiment of the present invention, no capping agent is used to prevent nanoparticle aggregation during the synthesis of the catalyst.In fact, the applicant has discovered through electron microscopy examination that the use of a capping agent to form metal nanoparticles in the catalyst is not necessary.Capping agents that are widely used in the literature but are not used in the present invention are cetyltrimethylammonium chloride and citric acid.

[0057] In a particularly preferred embodiment, the process for synthesizing an ammonia synthesis catalyst comprises the steps of: a) providing a salt and / or complex of a transition metal selected from Fe, Co, Ru, Mn, V and mixtures thereof, preferably Fe and / or Co, and contacting said salt and / or complex with an aqueous solvent to form a transition metal containing solution, Preferably, the salt and / or complex is FeCl 3 , Fe(NO 3 ) 3 , Fe(acac) 3 , or CoCl 2 A step selected from the group consisting of: b) providing a catalyst support and contacting the catalyst support with the transition metal-containing solution to form a suspension, The catalyst support is CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO 2 , ZrO 2 , doped-ZrO 2 , ZnO, Pr 2 O 3 , Nb 2 O 5 , La 2 O 3 , CaO.Al 2 O 3 , Mayenite, LaCeO x , BaTiO 3 , BaCeO 3 , BaCe x Y 1-x O 3 , SrTiO 3 , CaTiO 3 , LaCoO 3 , BaZrO 3 , Y 2 O 3 , LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite), and mixtures thereof; Preferably, the catalyst support is CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO2 and mixtures thereof; c) drying said suspension, preferably by filtration on a membrane, to obtain a solid powder; d) optionally subjecting said solid powder to a purification step, preferably at least one washing operation step, in order to obtain a purified solid powder; e) mechanically mixing said solid powder or said purified solid powder with an alkali metal or alkaline earth metal hydride in ambient air to obtain said ammonia synthesis catalyst, preferably said transition metal being in the form of nanoparticles. Includes.

[0058] Working Example The present invention will now be described with reference to examples and comparative examples.

[0059] Examples 1-5 describe the preparation process of the ammonia oxidation catalyst obtained by the process of the present invention, while Comparative Examples 1-6 describe the preparation process of the conventional catalyst used in the synthesis of ammonia.

[0060] Example 1: Fe / TiO 2 Synthesis of -LiH In a container, 7.14 g of tetradecyltrimethylammonium bromide (TTAB, TCI>98%) was dissolved in 47.34 g of distilled water. 0.74 g (20 wt.% Fe) of FeCl 3 was added to the TTAB aqueous solution, and the mixture was magnetically stirred at room temperature for 10 min. Then, 1.0 g of TiO 2 (P25, Degussa) was added and the suspension was kept under magnetic stirring at room temperature for 5 min.

[0061] In a separate container, add 1.6 g of NaBH 4 (Merck) was dissolved in 52 g of distilled water and kept at room temperature under magnetic stirring for 5 minutes. 4 The solution was diluted with TTAB and FeCl. 3 and TiO 2The resulting mixture was reacted at room temperature for 1 hour under magnetic stirring. The suspension was then filtered using a 220 nm membrane filter, and the solid was washed three times with 25 mL of hot distilled water (temperature about 90°C). The resulting solid was finally dried in air for 48 hours. The resulting Fe / TiO 2 The powder weighed approximately 1.4 g.

[0062] Then, Fe / TiO 2 The powder was mechanically mixed with LiH powder in a 50:50 weight ratio in a porcelain mortar for 2 min, and the resulting powder was stored in a desiccator prior to use. 2 The amount of Fe supported in the -LiH powder was 10 wt.%.

[0063] Example 2: Fe / SiO 2 Synthesis of -LiH This material was prepared following the procedure reported in Example 1 above. In this case, TiO 2 The support is SiO 2 was replaced with.

[0064] Example 3: Co / TiO 2 Synthesis of -LiH In a vessel, 7.14 g of tetradecyltrimethylammonium bromide (TTAB, TCI>98%) was dissolved in 47.34 g of distilled water. The solution was magnetically stirred at room temperature for 20 minutes. Then, 0.55 g (20 wt.% Co) CoCl 2 (Alfa Aesar, 99.7%, anhydrous) was added to the TTAB aqueous solution, and the mixture was kept at room temperature under magnetic stirring for 10 min. Then, TiO, which was used as a support, was added to the TTAB aqueous solution. 2 (P25, Degussa) 1 g was added and the suspension was kept under magnetic stirring for 5 min at room temperature.

[0065] In a separate container, add 1.52 g of NaBH 4 (Merck) was dissolved in 52 g of distilled water and kept at room temperature for 5 minutes under magnetic stirring. 4 The solution was diluted with TTAB and CoCl.2 and TiO 2 The resulting mixture was reacted at room temperature for 1 hour under magnetic stirring. The suspension was then filtered using a 220 nm membrane filter, and the solid was washed three times with 25 mL of hot distilled water (temperature about 90°C). The resulting solid was finally dried in air for 48 hours and further dried under vacuum at room temperature for 3 hours. The resulting Co / TiO 2 The powder weighed approximately 1.4 g.

[0066] Then, Co / TiO 2 The powder was mechanically mixed with LiH powder in a 50:50 weight ratio in a porcelain mortar for 2 min. The resulting powder was stored in a desiccator prior to use. 2 The amount of Co supported in the -LiH powder was 10 wt.%.

[0067] Example 4: Co / TiO 2 Synthesis of -LiH 60:40 This material was prepared following the procedure described in Example 4 reported above. In this case, Co / TiO 2 The powder was mechanically mixed with LiH powder using a weight ratio of 60:40.

[0068] Example 5: Co / TiO 2 Synthesis of -LiH DP In the container, 0.55g (20wt.% Co) of CoCl 2 was dissolved in 32 g of distilled water under stirring. Next, 1 g of TiO 2 (Degussa, P25) was then added. Then, 12.75 mL of 0.5 M (NH 4 ) 2 CO 3 The aqueous solution was added dropwise to the suspension and the resulting mixture was stirred in air at room temperature for 2 h. The suspension was filtered using a 220 nm membrane filter and the solid was washed three times with 15 mL of hot distilled water (temperature about 90 °C). The resulting solid was dried in air at 110 °C for 16 h and finally calcined in air at 500 °C for 3 h. The resulting Co / TiO 2The DP (evaporation-precipitation) powder weighed approximately 1.1 g.

[0069] Comparative Example 1: Synthesis of Fe / LiH The Fe / LiH powder was synthesized according to the protocol described in Nature Chem 2017, 9, 64-70. 2 g of LiH (Alfa Aesar, >97%) was dissolved in 0.659 g (10 wt.% Fe) of FeCl 3 (Alfa Aesar, 98%, anhydrous) and Fe / LiH powder were mixed in a 45 mL stainless steel grinding bowl. Five beads with a diameter of 10 mm were added (bead to sample mass ratio of 10) and the bowl was sealed. The latter was then introduced into a Pulverisette 7 Premium Line (Fritsch) planetary ball mill operating at 200 rpm. The sample was ball milled for 3 h (6 cycles of 30 min with reversal of rotation direction between each cycle) and then washed three times with 50 mL THF (Roth, 99.5%) to remove LiCl. The sample was dried under vacuum at room temperature for 3 h and stored in a desiccator prior to use. The resulting Fe / LiH powder weighed approximately 2.3 g.

[0070] Comparative Example 2: Synthesis of Co / LiH Co / LiH powder is FeCl 3 Instead of CoCl 2 It was synthesized according to the procedure reported in Comparative Example 1, except that (Alfa Aesar, 99.7%, anhydrous) was used as the metal precursor.

[0071] Comparative example 3: Cs-Ru / CeO 2 Synthesis of TiO 2 Instead of CeO 2 was used as a support, and FeCl 3 Instead of Ru(NO)(NO 3 ) 3 The same procedure as in Example 1 was repeated except that Fe / TiO was used as the metal precursor. 2 Similarly, Ru / CeO 2 The powder was synthesized.

[0072] CEO 2The support was prepared according to the procedure described in ACS Sustainable Chem.Eng. 2018, 6, 13867-13876. In a glass container, 63.70 g of NH 4 Ce(NO 3 ) 6 In a separate container, 106.17 g of urea was dissolved in 212.35 g of distilled water. Both solutions were magnetically stirred at room temperature for 10 minutes. The urea solution was then diluted with NH 4 Ce(NO 3 ) 6 aqueous solution and the resulting mixture was heated at 100 °C for 6 h under magnetic stirring. The suspension was then filtered and the solid was washed four times with 100 mL of hot distilled water (temperature about 90 °C). The powder was then vacuum dried at 110 °C overnight and finally calcined in air at 550 °C for 3 h (5 °C / min ramp). The resulting CeO 2 The powder weighed approximately 20 g and was stored in a desiccator prior to use.

[0073] 0.17 g of Ru(NO)(NO 3 ) 3 (5 wt.% Ru) was introduced into the TTAB aqueous solution, and the resulting mixture was kept under magnetic stirring at 30 °C for 30 min. 1 g of CeO 2 After adding 0.89 g of NaBH 4 A solution of 52 g of distilled water was added dropwise and the resulting suspension was stirred at room temperature for 1 h. The rest of the synthesis was the same as in Example 1. The resulting Ru / CeO 2 The powder weighed approximately 1.05 g.

[0074] The Cs promoter was introduced via a wet impregnation process with a molar ratio of Cs to Ru of 2. First, 0.20 g of CsNO 3 (Cs to Ru molar ratio of 2) was dissolved in 50 g of distilled water and the solution was kept under magnetic stirring at room temperature for 10 min. Then, Ru / CeO 2 Powder CsNO 3The resulting suspension was kept under magnetic stirring for 15 min and then sonicated at room temperature for 30 min. The solvent was then evaporated off using a Buchi Rotavapor R-200 apparatus operating at 50 °C and 80 rpm. 2 The powder weighed approximately 1.2 g and was stored in a desiccator prior to use.

[0075] Comparative example 4: Cs-Ru / Al 2 O 3 Synthesis of CEO 2 The support is Al 2 O 3 The same procedure as in Comparative Example 3 was repeated except that the molar ratio of Cs to Ru was adjusted to 10. 2 O 3 The powder was synthesized.

[0076] Comparative example 5: Ru / C Ru / C is a standard commercial catalyst.

[0077] Comparative Example 6: Fe / CeO 2 -Li 3 N TiO 2 Instead of CeO as a support 2 is used, and LiH is Li 3 The same procedure as in Example 1 was repeated except that N was substituted. 2 -Li 3 N powder was synthesized.

[0078] Ammonia Productivity Calculation After synthesis, the catalysts were tested in a reactor to generate ammonia, and the ammonia productivity for each catalyst was calculated according to the procedure reported below.

[0079] 0.12 mL of catalyst powder was mixed with 0.12 mL of glass beads (diameter 350-500 μm) and placed in a stainless steel fixed-bed reactor.

[0080] In the fixed bed reactor, nitrogen (N2 , Air Liquide, purity 5.0) and hydrogen (H 2 The gas flow rate was N 2 : 20mL / min, H 2 : 60mL / min, and the gas hourly space velocity GHSV is 40,000h -1 It was.

[0081] The pressure was maintained at 5 bar, and the catalyst was first heated at 350 °C for 3 h with N 2 :H 2 The mixture was then treated under mixed gas, after which the temperature was successively decreased to 300° C. and 250° C., with each temperature plateau kept constant for at least 1 h.

[0082] The effluent gas from the reactor of the flow system was analyzed by gas chromatography (GC) and then the productivity was calculated at 350°C.

[0083] The calculated productivity value of Comparative Example 1 (Fe / LiH) was then used to normalize the productivity values ​​of the other samples, and the % productivity values ​​of the latter samples are shown in Table 1.

[0084] More specifically, the % productivity value for each catalyst was calculated according to the following formula: TIFF2025515817000001.tif11118

[0085] During the ceremony, Productivity i is the calculated productivity of the catalyst, Productivity 比較例1 is the calculated productivity of the catalyst reported in Comparative Example 1.

[0086] TIFF2025515817000002.tif110152

[0087] Table 1 shows that the % productivity of the catalysts produced according to the process of the present invention is significantly higher than the productivity values ​​calculated for the conventional catalysts (Comparative Examples 1-6) synthesized according to known processes.

[0088] Applicants strongly believe that such unexpected enhancement in catalytic activity cannot be explained solely by changes in metal dispersion on the catalyst support, since different ammonia production rates were observed when the performance of different catalyst supports with similar textural properties was tested. In conclusion, all the evidence reported here seems to suggest the existence of an unexpected active role of the support in the catalytic mechanism.

Claims

1. 1. A method for synthesizing an ammonia synthesis catalyst, comprising the steps of: a) providing a transition metal precursor, preferentially selected from Fe, Co, Ru, Mn or V, and contacting said transition metal precursor with a solution or solvent to form an improved solution (a transition metal-containing solution); b) providing a catalyst support and contacting said catalyst support with said modified solution to form a suspension; c) drying said suspension to obtain a solid powder; d) optionally subjecting said solid powder to a purification step to obtain a purified solid powder; e) mixing said solid powder or said purified solid powder with a hydride compound to obtain said ammonia synthesis catalyst; The method includes:

2. The following steps: prior to step b), contacting the improved solution of step a) with a reducing agent or with a precipitating agent; Prior to step c), contacting the suspension of step b) with a reducing agent or with a precipitating agent. The method of claim 1 further comprising one of:

3. The transition metal precursor is a metal salt and / or a metal complex, preferably FeCl 3 , Fe(NO 3 ) 3 Or Fe(acac) 3 , or CoCl 2 The method according to claim 1 or 2, wherein the

4. The catalyst support is CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO 2 , ZrO 2 , doped-ZrO 2 , ZnO, Pr 2 O 3 , Nb 2 O 5 , La 2 O 3 , CaO.Al 2 O 3 , Mayenite, LaCeO x , BaTiO 3 , BaCeO 3 , BaCe x Y 1-x O 3 , SrTiO 3 , CaTiO 3 , LaCoO 3 , BaZrO 3 , Y 2 O 3 , LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite), preferably CeO 2 , TiO 2 , doped-TiO 2 , SiO 2 or doped-SiO 2 The method according to any one of claims 1 to 3, wherein the method is selected from the group consisting of:

5. The hydride compound has the formula X-H n The method according to any one of claims 1 to 4, wherein X is an alkali metal or alkaline earth metal, preferably Li, Na, K, Ca, Ba or Sr.

6. 6. The method according to any one of claims 1 to 5, wherein step e) of claim 1 is not carried out in an inert controlled atmosphere, preferably step e) of claim 1 is carried out in ambient air.

7. 7. The method according to any one of claims 1 to 6, wherein step e) of claim 1 is carried out by mechanically mixing the solid powder or the purified solid powder with the hydride compound in a weight ratio of 60:

40.

8. The reducing agent is sodium borohydride NaBH 4 or potassium borohydride KBH 4 and the precipitant is an aqueous solution of carbonate or urea, the carbonate being preferably (NH 4 ) 2 CO 3 , Na 2 CO 3 Or K 2 CO 3 The method according to any one of claims 2 to 8, wherein

9. The method according to any one of claims 1 to 8, wherein the purification step according to claim 1 comprises a washing operation.

10. Method according to claim 10, wherein said washing operation is carried out with distilled water, at a temperature ranging from 80 to 95°C, preferably at a temperature equal to 90°C.

11. 11. The method according to any one of claims 1 to 10, wherein drying of the suspension in step c) comprises filtering the suspension on a membrane having a pore size in the range of 0.10 to 0.40 μm, preferably in the range of 0.20 to 0.30 μm, or even more preferably equal to or about 0.22 μm.

12. A method according to any one of claims 1 to 11, wherein the purification step comprises reducing the boron content of the solid powder.

13. 13. The method of claim 12, wherein the residual content of boron remaining in the purified solid is 0.5 wt% or less.

14. The following steps: a) providing a salt and / or complex of a transition metal selected from Fe, Co, Ru, Mn, V and mixtures thereof, preferably Fe and / or Co, and contacting said salt and / or complex with an aqueous solvent to form a transition metal-containing solution, Preferably, the salt and / or complex is FeCl 3 , Fe(NO 3 ) 3 , Fe(acac) 3 , or CoCl 2 A step selected from the group consisting of: b) providing a catalyst support and contacting said catalyst support with said transition metal-containing solution to form a suspension, The catalyst support is CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO 2 , ZrO 2 , doped-ZrO 2 , ZnO, Pr 2 O 3 , Nb 2 O 5 , La 2 O 3 , CaO.Al 2 O 3 , Mayenite, LaCeO x , BaTiO 3 , BaCeO 3 , BaCe x Y 1-x O 3 , SrTiO 3 , CaTiO 3 , LaCoO 3 , BaZrO 3 , Y 2 O 3 , LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite), and mixtures thereof; Preferably, the catalyst support is CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO 2 and mixtures thereof; c) drying said suspension, preferably by filtration on a membrane, to obtain a solid powder; d) optionally subjecting said solid powder to a purification step, preferably at least one washing operation step, in order to obtain a purified solid powder; e) mechanically mixing said solid powder or said purified solid powder with an alkali metal or alkaline earth metal hydride in ambient air to obtain said ammonia synthesis catalyst, said transition metal being in the form of nanoparticles. The method of claim 1 , comprising:

15. An ammonia synthesis catalyst obtainable by the method according to any one of claims 1 to 14.

16. 1. An ammonia synthesis catalyst comprising a transition metal, a catalyst support and a hydride compound, said transition metal is preferentially selected from Fe, Co, Ru, Mn or V; The catalyst support is CeO 2 , SiO 2 , doped-SiO 2 , TiO 2 , doped-TiO 2 , ZrO 2 , doped-ZrO 2 , ZnO, Pr 2 O 3 , Nb 2 O 5 , La 2 O 3 , CaO.Al 2 O 3 , Mayenite, LaCeO x , BaTiO 3 , BaCeO 3 , BaCe x Y 1-x O 3 , SrTiO 3 , CaTiO 3 , LaCoO 3 , BaZrO 3 , Y 2 O 3 , LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite); The hydride compound has the formula X-H n wherein X is Li, Na, K, Ca, Ba or Sr.

17. 17. The ammonia synthesis catalyst of claim 16, wherein the transition metal is comprised of nanoparticles dispersed and supported on the catalyst support.

18. The catalyst support is CeO 2 , TiO 2 , doped-TiO 2 , SiO 2 or doped-SiO 2 18. The ammonia synthesis catalyst according to claim 16 or 17,

19. Use of the ammonia synthesis catalyst according to any one of claims 15 to 18 for the synthesis of ammonia.

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