Ammonia synthesis catalyst and preparation method thereof

Abstract

Disclosed is a method for synthesizing an ammonia synthesis catalyst, comprising the steps of providing a transition metal precursor and contacting the transition metal precursor with a solution to form a modified solution, providing a catalyst support and contacting the catalyst support with the modified solution to form a suspension; dehydrating 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 invention relates to an ammonia synthesis catalyst and a preparation method thereof and application thereof in the ammonia industry. Background Art

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

[0003] The catalyst must perform two functions during the synthesis process, namely, activation of molecular hydrogen and nitrogen to form reactive atomic species. This activation usually involves an electron transfer reaction promoted by high temperatures and the presence of a co-catalyst with electron-donating properties.

[0004] Several catalysts are active for the synthesis of ammonia. Historically, iron (Fe) based catalysts containing Fe3O4 and small proportions of Al2O3 and K2O have been used for ammonia synthesis in the temperature range of 400°C to 500°C and pressure range of 150 bar to 300 bar. More recently, ruthenium (Ru) based catalysts have made it possible to synthesize ammonia under milder conditions, for example at 250°C to 400°C and pressures of 110 bar or less.

[0005] Nowadays, the commonly used catalysts for ammonia synthesis are supported catalysts, in which the active catalytic elements are dispersed on a support to reduce their manufacturing 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. In addition to ruthenium, other transition metals may also be used, including Fe, Co, Co-Mo, or Ni-Mo.

[0007] There is a growing interest in the art to find ammonia synthesis catalysts that are characterized by high activity, high resistance to poisoning and the ability to operate over a wide temperature range, in particular temperatures below those typical of Fe-based industrial catalysts.

[0008] Catalysts based on metal and hydride coupling are considered promising for the above purposes. Examples of such metal-hydride catalysts are disclosed in US2016 / 0361712 A1 and US2018 / 0327272 A1.

[0009] Unfortunately, the performance of current metal-hydride catalysts is not entirely satisfactory, since hydride compounds are often unstable in air. In fact, they oxidize in the presence of oxygen and also react strongly with water. In addition, hydrides thermally degrade at high temperatures, so their operating window is limited to temperatures below 400 °C.

[0010] Due to the above reasons, another problem with metal-hydride catalysts relates to their preparation methods, which require synthesis in a chemically and thermally controlled atmosphere and / or the use of expensive equipment, i.e., ball milling jars.

[0011] Therefore, it is highly desirable to provide an improved hydride-type ammonia synthesis catalyst which can be prepared and handled in air under mild conditions. It is also highly desirable to provide a simpler, cheaper and less energy intensive method for synthesizing the catalyst. Summary of the invention

[0012] The object of the present invention is to overcome the above-mentioned drawbacks of the background art, namely the unsatisfactory resistance of conventional hydride catalysts to high temperatures and environments containing O 2 and H 2 O, which usually limits the choice of preparation methods.

[0013] A further object is to provide a method for producing an ammonia catalyst that can be easily scaled up for industrial production.

[0014] Therefore, one aspect of the present invention is a method for synthesizing an ammonia synthesis catalyst according to claim 1.

[0015] The method comprises the following steps: providing a transition metal precursor preferably selected from Fe, Co, Ru, Mn or V (and optionally, a mixture thereof), and contacting the transition metal precursor with a solution or a solvent (preferably, an aqueous solvent or an organic solvent, more preferably an aqueous solvent, and even more preferably water or distilled water) to form a modified solution; providing a catalyst support, and contacting the catalyst support with the modified solution to form a suspension; dehydrating the suspension to obtain a solid powder; optionally, subjecting the solid powder to a purification step to obtain a purified solid powder; mixing the solid powder or the high-purity solid powder with a hydride compound.

[0016] The hydride catalyst of the present invention is synthesized with low energy consumption because the solid powder and the hydride compound can be mixed in the open air and at room temperature, thus eliminating the need to use a pressure-tight container and to synthesize in a chemically and thermally controlled environment.

[0017] This method is particularly suitable for large-scale production because it does not require a pressure-tight vessel for the synthesis and also provides excellent control of metal loading, which prevents metal leaching and achieves an optimal distribution of the transition metal on the catalytic support.

[0018] A second aspect of the present invention is an ammonia synthesis catalyst obtainable by the process of the present invention.

[0019] 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 selected from one or more of the following oxides: CeO2, SiO2, doped SiO2, TiO2, doped TiO2, ZrO2, doped ZrO2, ZnO, Pr2O3, Nb2O5, La2O3, CaO.Al2O3, calcium aluminum, LaCeO x 、BaTiO3、BaCeO3、BaCe x Y 1-x O3, SrTiO3, CaTiO3, LaCoO3, BaZrO3, Y2O3, LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite), and optionally, mixtures thereof.

[0020] In the prior art, it is believed that close contact between metal nanoparticles and hydrides is required to ensure that activated nitrogen on the metal is transferred to the hydride, thereby achieving high ammonia productivity, as supported by the work done 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.

[0021] In the present invention, contrary to the teachings of the prior art, the applicant has found that surprisingly enhanced catalytic activity can be obtained by depositing metal nanoparticles on a support and then adding the hydride by simple mechanical mixing.

[0022] Furthermore, the obtained catalysts have been found to exhibit improved in-production stability, as the support can promote resistance to sintering, which typically occurs when the metal is deposited directly on the hydride.

[0023] Another advantage is that lower hydride content is required to achieve comparable ammonia productivity compared to conventional hydride catalysts. In addition, partial replacement of hydride with inexpensive supports further reduces its manufacturing cost.

[0024] Furthermore, mixing the hydride compound with the powder comprising the metal compound deposited on the metal oxide support in a predetermined sequence, and in particular mixing the hydride with the powder only after the preparation of the powder is complete, prevents degradation of the hydride during the synthesis process.

[0025] Furthermore, during the initial preparation of the ammonia catalyst, a powder with the desired mechanical and structural properties can be produced before the hydride compound is added. This is advantageous because once the hydride is added, processing the powder to obtain the desired properties is more challenging.

[0026] Another aspect of the invention is the use of the catalyst of the invention for synthesizing ammonia by reacting a make-up gas containing nitrogen and hydrogen over the catalyst. Advantageously, due to the enhanced activity of the catalyst, ammonia can be synthesized at a high productivity.

[0027] In the following, if not specified otherwise, the term "transition metal" denotes a chemical element in the d-block of the Periodic Table of the Elements (Groups 3 to 12, preferably Groups 5 to 9, even more preferably Groups 5, 7, 8 or 9).

[0028] The term "catalyst precursor" or simply "precursor" refers to a substance that requires activation or reaction to produce an active catalyst. It can be seen that the combined term "transition metal precursor" indicates a chemical element in the d block of the periodic table of elements, which, according to the above definition, is not yet catalytically active, for example, it does not show substantial catalytic activity, but requires further activation or reaction to be converted into an active catalyst.

[0029] In this specification, the modified solution is a solution in which the transition metal precursor is dissolved after contacting with a solution or a solvent. In other words, the modified solution is a transition metal-containing solution.

[0030] Furthermore, the term catalyst support refers to a material, usually in solid form, characterized by a high surface area, allowing the catalyst to attach or deposit on its surface.

[0031] The term reducing agent indicates a substance that loses electrons to another substance in a redox reaction and is oxidized to a higher valence state.

[0032] The term precipitant denotes a substance added to a medium in order to precipitate another substance present in the medium. This precipitation may be a selective precipitation of the substance that must be precipitated from the medium.

[0033] The term inert atmosphere refers to an atmosphere without reactive gases (e.g., oxygen). More specifically, in the present invention, an inert atmosphere indicates an atmosphere that does not affect catalyst synthesis, for example, it does not cause a chemical or electrochemical reaction or physical interaction between the gas 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.

[0034] In this specification, the term "nanoparticles" refers to particles having an average particle size distribution of 1 nanometer (nm) to 1000 nm. This average particle size distribution can be determined by known analytical methods, such as laser diffraction. DETAILED DESCRIPTION

[0035] The process of the invention is very versatile since it can be implemented to synthesize ammonia catalysts with various support types and various hydride compounds.

[0036] For example, the catalyst support may be selected from one or more of the following oxides: CeO2, SiO2, doped SiO2, TiO2, doped TiO2, ZrO2, doped ZrO2, ZnO, Pr2O3, Nb2O5, La2O3, CaO.Al2O3, calcium aluminum oxide, LaCeO x 、BaTiO3、BaCeO3、BaCe x Y 1-x O3, SrTiO3, CaTiO3, LaCoO3, BaZrO3, Y2O3, LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite), and optionally, mixtures thereof.

[0037] According to a particularly preferred embodiment, the catalyst support is TiO2 or SiO2.

[0038] The hydride compound can be represented by the formula XH n denoted as , wherein 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.

[0039] The mixing between the solid powder or the purified solid powder and the hydride compound can be carried out under any suitable conditions using any suitable method known to those skilled in the art, but according to particularly interesting applications, the mixing of the solid powder or the purified solid powder and the hydride compound does not need to be carried out in a controlled inert atmosphere, for example, it can be carried out at room temperature in the open air or in ambient air.

[0040] Ambient air refers to the atmosphere in its natural state, and typically contains 78% nitrogen, 21% oxygen, and trace components. The possibility of mixing a solid powder or a purified solid powder with a hydride compound in ambient air can significantly reduce the synthesis cost of the catalyst, which is particularly advantageous for large-scale production applications.

[0041] The applicant has found that better catalytic performance is obtained when the powder or the purified solid powder is mixed with the hydride compound in a weight ratio of 60:40. Other less preferred weight ratios are from 100:1 to 1:100.

[0042] The method of the present invention may further comprise the step of contacting the modified solution with a reducing agent or a precipitating agent before contacting the modified solution with the catalyst support.

[0043] Alternatively, the method of the present invention may further comprise the step of contacting the suspension obtained by contacting the catalyst support with the modifying solution with a reducing agent or a precipitating agent before dehydration.

[0044] According to a preferred embodiment, when a precipitant instead of a reducing agent is used in the process of the invention, the reduction of the ammonia catalyst is carried out in situ in the ammonia synthesis converter.

[0045] Suitable reducing agents for reducing the modified solution or suspension are sodium borohydride NaBH4 or potassium borohydride KBH4. Suitable precipitating agents are aqueous solutions of carbonates or urea. Preferably, the carbonate is (NH4)2CO3, Na2CO3 or K2CO3.

[0046] It is known in the art that boron can be incorporated into the catalyst during the catalyst preparation process using, for example, NaBH4 to improve the catalytic activity of the synthesized catalyst. In the present invention, when a boron-containing reducing agent is used, the reducing agent is not used to incorporate boron into the catalyst, but is only used to cause the precipitation of the transition metal on the support. In other words, the reducing agent is only used to incorporate the metal ions (e.g., Co) on the surface of the support. 2+ ) into metallic nanoparticles (e.g. metallic cobalt) rather than incorporating boron into them.

[0047] Fe and Co are preferred transition metals. Preferably, the transition metal precursor is a metal salt and / or a metal complex, particularly preferably selected from FeCl3, Fe(NO3)3 or Fe(acac)3 [iron(III) acetylacetonate] or CoCl2.

[0048] Preferably, the purification step comprises at least one washing operation. The washing operation can be carried out using distilled water as a washing agent at a temperature ranging from 80° C. to 95° C., preferably at a temperature equal to 90° C. Particularly preferably, the washing step is carried out to remove excess boron that may have been introduced during the treatment with the boron-containing reducing agent. The washing operation can be carried out to remove excess boron from the surface of the support, leaving only the metallic nanoparticles. Preferably, the boron content remaining in the purified solid powder after purification is equal to or less than 0.5 wt %. This washing step in the catalyst preparation has been shown to be beneficial to the catalyst performance during ammonia synthesis.

[0049] According to the above, embodiments of the purification step include reducing the boron content of the solid powder. The residual boron content remaining in the purified solid after purification preferably does not exceed 0.5wt%, such as 0 to 0.5wt% or 0.01wt% to 0.5wt% or 0.1wt% to 0.5wt%.

[0050] The dehydration step may be performed by filtration on a membrane having a pore size ranging from 0.10 μm to 0.40 μm, preferably ranging from 0.20 μm to 0.30 μm, or even more preferably equal to or about 0.22 μm.

[0051] The present invention provides a preparation method which is less energy intensive and provides good control of metal loading with no metal leaching. The additional support brings enhanced activity and improved in-service stability due to sintering resistance.

[0052] Without being bound by theory, applicants believe that a synergistic effect due to the interconversion of oxide and hydride phases, the formation of an in situ (oxy)nitride phase and improved metal dispersion is responsible for the observed activity enhancement of the investigated catalysts.

[0053] According to 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.

[0054] In a particularly preferred embodiment, the catalyst support is TiO2 or SiO2.

[0055] In a particularly preferred embodiment of the present invention, no capping agent is used to prevent the aggregation of nanoparticles during the catalyst synthesis process. In fact, the applicant has found through electron microscopy studies that the formation of metal nanoparticles in the catalyst does not require the use of a capping agent. Capping agents widely used in the literature are hexadecyltrimethylammonium chloride and citric acid, but are not used in the present invention.

[0056] According to a particularly preferred embodiment, the method for synthesizing an ammonia synthesis catalyst comprises the following steps:

[0057] 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 the salt and / or complex with an aqueous solvent to form a transition metal-containing solution;

[0058] Preferably, the salt and / or complex is selected from FeCl3, Fe(NO3)3, Fe(acac)3 or CoCl2;

[0059] b) providing a catalyst support and contacting the catalyst support with the transition metal-containing solution to form a suspension;

[0060] The catalyst carrier is selected from CeO2, SiO2, doped SiO2, TiO2, doped TiO2, ZrO2, doped ZrO2, ZnO, Pr2O3, Nb2O5, La2O3, CaO.Al2O3, calcium aluminum oxide, LaCeO x 、BaTiO3、BaCeO3、BaCe x Y 1-x O3, SrTiO3, CaTiO3, LaCoO3, BaZrO3, Y2O3, LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica), ZSM-5 (zeolite), and mixtures thereof;

[0061] Preferably, the catalyst support is selected from CeO2, SiO2, doped SiO2, TiO2, doped TiO2 and mixtures thereof;

[0062] c) dehydrating the suspension, preferably by filtering on a membrane, to obtain a solid powder;

[0063] d) optionally, subjecting the solid powder to a purification step, preferably at least one washing operation step, to obtain a purified solid powder;

[0064] e) mechanically mixing the solid powder or the purified solid powder with an alkali metal or alkaline earth metal hydride in ambient air to obtain the ammonia synthesis catalyst, preferably, wherein the transition metal is in the form of nanoparticles.

[0065] Example

[0066] The present invention will now be described with reference to the Examples and Comparative Examples reported below.

[0067] Examples 1 to 5 describe the preparation of an ammonia oxidation catalyst obtained according to the process of the present invention, while Comparative Examples 1 to 6 describe the preparation of a conventional catalyst for ammonia synthesis.

[0068] Example 1: Synthesis of Fe / TiO2-LiH

[0069] In a container, 7.14 g of tetradecyl trimethyl ammonium bromide (TTAB, TCI>98%) was dissolved in 47.34 g of distilled water. The solution was magnetically stirred at room temperature for 20 minutes. 0.74 g (20 wt.% of Fe) FeCl3 was added to the TTAB aqueous solution, and the mixture was magnetically stirred at room temperature for 10 minutes. Subsequently, 1.0 g of TiO2 (P25, manufactured by Degussa) was added as a carrier, and the suspension was magnetically stirred at room temperature for 5 minutes.

[0070] In a separate container, 1.6 g of NaBH4 (Merck) was dissolved in 52 g of distilled water and stirred magnetically at room temperature for 5 minutes. Afterwards, the NaBH4 solution was added dropwise to an aqueous suspension containing TTAB, FeCl3 and TiO2 carriers. Under magnetic stirring, the resulting mixture was reacted at room temperature for 1 hour. 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 of about 90° C.). The resulting solid was finally dried in air for 48 hours. The Fe / TiO2 powder obtained was about 1.4 g.

[0071] Subsequently, the Fe / TiO2 powder was mechanically mixed with the LiH powder in a porcelain mortar at a weight ratio of 50:50 for 2 minutes, and the resulting powder was stored in a moisture absorber before use. The Fe loading obtained in the Fe / TiO2-LiH powder was 10 wt.%.

[0072] Example 2: Synthesis of Fe / SiO2-LiH

[0073] The material was prepared according to the procedure reported above in Example 1. In this case, the TiO2 support was replaced by SiO2.

[0074] Example 3: Synthesis of Co / TiO2-LiH

[0075] In a container, 7.14 g of tetradecyl trimethyl ammonium bromide (TTAB, TCI>98%) was dissolved in 47.34 g of distilled water. The solution was stirred magnetically at room temperature for 20 minutes. Subsequently, 0.55 g (20 wt.% Co) CoCl2 (Alfa Aesar, 99.7%, anhydrous) was added to the TTAB aqueous solution, and the mixture was stirred magnetically at room temperature for 10 minutes. Subsequently, 1 g of TiO2 (P25, manufactured by Degussa) was added as a carrier, and the suspension was stirred magnetically at room temperature for 5 minutes.

[0076] In a separate container, 1.52g NaBH4 (Merck) was dissolved in 52g distilled water and magnetically stirred at room temperature for 5 minutes. Afterwards, the NaBH4 solution was added dropwise to an aqueous suspension containing TTAB, CoCl2 and TiO2 carriers. Under magnetic stirring, the resulting mixture was reacted at room temperature for 1 hour. Then, the suspension was filtered using a 220nm membrane filter and the solid was washed three times with 25mL hot distilled water (temperature was about 90°C). Finally, the resulting solid was dried in air for 48 hours and further vacuum dried for 3 hours at room temperature. The Co / TiO2 powder obtained was about 1.4g.

[0077] After that, the Co / TiO2 powder was mechanically mixed with the LiH powder in a porcelain mortar at a weight ratio of 50:50 for 2 minutes. The obtained powder was stored in a moisture absorber before use. The Co loading obtained in the Co / TiO2-LiH powder was 10 wt.%.

[0078] Example 4: Synthesis of Co / TiO2-LiH at 60:40

[0079] The material was prepared according to the procedure reported above in Example 4. In this case, Co / TiO2 powder was mechanically mixed with LiH powder in a weight ratio of 60:40.

[0080] Example 5: Synthesis of Co / TiO2-LiH DP

[0081] In a container, 0.55g (20wt.% Co) CoCl2 was stirred and dissolved in 32g distilled water. Then, 1g TiO2 (P25, manufactured by Degussa) was added. Subsequently, 12.75mL of 0.5M (NH4)2CO3 aqueous solution was added dropwise to the suspension, and the resulting mixture was stirred in air at room temperature for 2 hours. The suspension was filtered using a 220nm membrane filter, and the solid was washed three times with 15mL hot distilled water (temperature was about 90°C). The resulting solid was dried in air at 110°C for 16 hours and finally calcined in air at 500°C for 3 hours. The Co / TiO2 DP (deposition-precipitation) powder obtained was about 1.1g.

[0082] Comparative Example 1: Synthesis of Fe / LiH

[0083] Fe / LiH powder was synthesized according to the scheme described in Nature Chemistry, September 2017, pp. 64-70. 2 g of LiH (Alfa Aesar, >97%) was mixed with 0.659 g (10 wt.% Fe) FeCl3 (Alfa Aesar, 98%, anhydrous) in a 45 mL stainless steel grinding bowl. Five beads with a diameter of 10 mm (the mass ratio of beads to sample was 10) were then added and the bowl was sealed. The bowl was then introduced into a Pulverisette 7 Premium Line (Fritsch) planetary ball mill running at 200 rpm. The sample was ball milled for 3 hours (6 cycles of 30 minutes, reversing the direction of rotation between each cycle), followed by washing three times with 50 mL THF (Roth, 99.5%) to remove LiCl. The sample was vacuum dried at room temperature for 3 hours and stored in a desiccator before use. The Fe / LiH powder obtained was approximately 2.3 g.

[0084] Comparative Example 2: Synthesis of Co / LiH

[0085] Co / LiH powder was synthesized according to the procedure reported in Comparative Example 1, except that CoCl 2 (Alfa Aesar, 99.7%, anhydrous) was used as metal precursor instead of FeCl 3 .

[0086] Comparative Example 3: Synthesis of Cs-Ru / CeO2

[0087] Ru / CeO2 powder was synthesized in the same manner as Fe / TiO2 in Example 1, except that CeO2 was used as a support instead of TiO2, and Ru(NO)(NO3)3 was used as a metal precursor instead of FeCl3.

[0088] The CeO2 support was prepared according to the procedure described in ACS Sustainable Chemistry and Engineering, June 2018, 13867-13876. In a glass container, 63.70 g of NH4Ce(NO3)6 was dissolved in 212.35 g of distilled water. In a separate container, 106.17 g of urea was dissolved in 212.35 g of distilled water. The two solutions were magnetically stirred at room temperature for 10 minutes. The urea aqueous solution was then added to the NH4Ce(NO3)6 aqueous solution, and the resulting mixture was heated at 100°C for 6 hours under magnetic stirring. Next, the suspension was filtered and the solid was washed four times with 100 mL of hot distilled water (temperature was about 90°C). The powder was then vacuum dried overnight at 110°C and finally calcined in air at 550°C for 3 hours (5°C / min temperature increase). The obtained CeO2 powder was about 20 g and was stored in a desiccator before use.

[0089] 0.17 g of Ru(NO)(NO3)3 (5 wt.% of Ru) was introduced into the TTAB aqueous solution, and the resulting mixture was magnetically stirred at 30°C for 30 minutes. After adding 1 g of CeO2, a solution made of 0.89 g of NaBH4 dissolved in 52 g of distilled water was added dropwise, and the resulting suspension was stirred at room temperature for 1 hour. The rest of the synthesis was the same as in Example 1. The Ru / CeO2 powder obtained was about 1.05 g.

[0090] The Cs co-catalyst was introduced by a wet impregnation method with a Cs-Ru molar ratio of 2. First, 0.20 g of CsNO3 (Cs to Ru molar ratio of 2) was dissolved in 50 g of distilled water and the solution was magnetically stirred at room temperature for 10 minutes. Then, Ru / CeO2 powder was added to the CsNO3 aqueous solution, and the resulting suspension was magnetically stirred at room temperature for 15 minutes and then ultrasonicated at room temperature for 30 minutes. The solvent was then removed by evaporation using a Büchi Rotavapor R-200 device running at 80 rpm at 50°C. The obtained Cs-Ru / CeO2 powder was approximately 1.2 g and was stored in a desiccator before use.

[0091] Comparative Example 4: Synthesis of Cs-Ru / Al2O3

[0092] Cs-Ru / Al2O3 powder was synthesized in the same manner as Comparative Example 3, except that the CeO2 support was replaced by Al2O3 and the Cs-Ru molar ratio was adjusted to 10.

[0093] Comparative Example 5: Ru / C

[0094] Ru / C is a standard commercial catalyst.

[0095] Comparative Example 6: Fe / CeO2-Li3N

[0096] Fe / CeO2-Li3N powder was synthesized in the same manner as in Example 1, except that CeO2 was used as a support instead of TiO2, and LiH was replaced by Li3N.

[0097] Ammonia production rate calculation

[0098] After synthesis, the catalysts were tested in a reactor to produce ammonia and the ammonia productivity of each catalyst was calculated according to the procedure reported below.

[0099] 0.12 mL of catalyst powder was mixed with 0.12 mL of glass beads (350-500 μm in diameter) and arranged into a stainless steel fixed bed reactor.

[0100] Nitrogen (N2, Air Liquide, purity 5.0) and hydrogen (H2, Air Liquide, purity 5.0) were supplied to the fixed bed reactor. The gas flow rates were set to N2: 20 mL / min and H2: 60 mL / min, and the gas hourly space velocity (GHSV) was 40000 h -1 .

[0101] The pressure was maintained at 5 bar and the catalyst was first treated under a N2:H2 gas mixture at 350°C for 3 hours. The temperature was then lowered sequentially to 300°C and 250°C while keeping the temperature constant for at least 1 hour at each temperature plateau.

[0102] The off-gas of the reactor of the flow system was analyzed by gas chromatography (GC), and then the productivity was calculated at 350°C.

[0103] The productivity value calculated for Comparative Example 1 (Fe / LiH) was then used to normalize the productivity values ​​for the other samples, the latter percentages of which are reported in Table 1.

[0104] In more detail, the productivity value in percent was calculated for each catalyst according to the following formula:

[0105]

[0106] in:

[0107] productivity i is the calculated catalyst productivity;

[0108] productivity 对比例 is the calculated productivity of the catalyst reported in Comparative Example 1.

[0109] Table 1

[0110]

[0111] Table 1 shows that the productivity percentages of the catalysts produced according to the process of the present invention are significantly higher than the productivity values ​​calculated for conventional catalysts synthesized according to known processes (Comparative Examples 1 to 6).

[0112] The applicant strongly believes that this unexpected enhancement of the catalytic activity observed cannot be explained solely by variations in the metal dispersion on the catalytic support, since different ammonia production rates were observed when studying the performance of different catalytic supports with similar structural characteristics. In summary, all the evidence reported above seems to indicate an unexpected active role of the support in the catalytic mechanism.

Claims

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

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

3. The method according to claim 1 or 2, wherein the transition metal precursor is a metal salt and / or a metal complex, preferably selected from FeCl3, Fe(NO3)3, Fe(acac)3 or CoCl2.

4. The method according to any one of the preceding claims, wherein the catalyst support is selected from one or more of the following: CeO2, SiO2, doped SiO2, TiO2, doped TiO2, ZrO2, doped ZrO2, ZnO, Pr2O3, Nb2O5, La2O3, CaO.Al2O3, calcium aluminum, LaCeO x 、BaTiO3、BaCeO3、BaCe x Y 1-x O3, SrTiO3, CaTiO3, LaCoO3, BaZrO3, Y2O3, LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica) or ZSM-5 (zeolite); preferably CeO2, TiO2, doped TiO2, SiO2 or doped SiO2.

5. A method according to any one of the preceding claims, wherein the hydride compound is of the formula XH n It represents, wherein X is an alkali metal or an alkaline earth metal, preferably Li, Na, K, Ca, Ba or Sr.

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

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

40.

8. The method according to any one of claims 2 to 8, wherein the reducing agent is sodium borohydride NaBH4 or potassium borohydride KBH4, and the precipitating agent is an aqueous solution of a carbonate or urea, wherein the carbonate is preferably (NH4)2CO3, Na2CO3 or K2CO3.

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

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

11. The method according to any one of the preceding claims, wherein the dehydration of the suspension of step c) comprises the following steps: The suspension is filtered on a membrane having a pore size ranging from 0.10 μm to 0.40 μm, preferably ranging from 0.20 μm to 0.30 μm, or even more preferably equal to or about 0.22 μm.

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

13. The method according to claim 12, wherein the residual content of boron remaining in the purified solid after purification is not more than 0.5 wt%.

14. The method according to claim 1, comprising 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 the salt and / or complex with an aqueous solvent to form a transition metal-containing solution; Preferably, the salt and / or complex is selected from FeCl3, Fe(NO3)3, Fe(acac)3 or CoCl2; b) providing a catalyst support and contacting the catalyst support with the transition metal-containing solution to form a suspension; The catalyst carrier is selected from CeO2, SiO2, doped SiO2, TiO2, doped TiO2, ZrO2, doped ZrO2, ZnO, Pr2O3, Nb2O5, La2O3, CaO.Al2O3, calcium aluminum oxide, LaCeO x 、BaTiO3、BaCeO3、BaCe x Y 1-x O3, SrTiO3, CaTiO3, LaCoO3, BaZrO3, Y2O3, LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica), ZSM-5 (zeolite), and mixtures thereof; Preferably, the catalyst support is selected from CeO2, SiO2, doped SiO2, TiO2, doped TiO2 and mixtures thereof; c) dehydrating the suspension, preferably by filtering on a membrane, to obtain a solid powder; d) optionally, subjecting the solid powder to a purification step, preferably at least one washing operation step, to obtain a purified solid powder; e) mechanically mixing the solid powder or the purified solid powder with an alkali metal or alkaline earth metal hydride in ambient air to obtain the ammonia synthesis catalyst, wherein the transition metal is in the form of nanoparticles.

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

16. An ammonia synthesis catalyst comprising a transition metal, a catalyst support and a hydride compound, wherein: The transition metal is preferably selected from Fe, Co, Ru, Mn or V; The catalyst carrier is selected from one or more of the following: CeO2, SiO2, doped SiO2, TiO2, doped TiO2, ZrO2, doped ZrO2, ZnO, Pr2O3, Nb2O5, La2O3, CaO.Al2O3, calcium aluminum oxide, LaCeO x 、BaTiO3、BaCeO3、BaCe x Y 1-x O3, SrTiO3, CaTiO3, LaCoO3, BaZrO3, Y2O3, LaScSi, MCM-41 (ordered mesoporous silica), silicalite-1 (crystalline silica), or ZSM-5 (zeolite); The hydride compound is represented by the formula XH n , wherein X is Li, Na, K, Ca, Ba or Sr.

17. The ammonia synthesis catalyst according to claim 16, wherein the transition metal consists of nanoparticles dispersed and supported on the catalyst carrier.

18. The ammonia synthesis catalyst according to claim 16 or 17, wherein the catalyst support is CeO2, TiO2, doped TiO2, SiO2 or doped SiO2.

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

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