Efficient catalyst for hydrogen generation from ammonia

Abstract

This invention relates to a catalyst, in particular a catalyst for producing hydrogen from ammonia, catalyst comprising a ternary metal imide of formula (I): X2Y(NH)2, wherein X is a metal selected from a group consisting of alkali metals and alkaline earth metals, and Y is a metal selected from a group consisting of transition metals and lanthanides.

Description

[0001] Efficient catalyst for hydrogen generation from ammonia

[0002] Technical Field

[0003] The present invention relates to a catalyst, in particular a catalyst used for producing hydrogen from ammonia, a device comprising a catalyst for producing hydrogen from ammonia, its use for producing hydrogen from ammonia and / or decomposing ammonia, and a method for producing hydrogen from ammonia.

[0004] Prior Art and the Problem Underlying the Invention

[0005] The future of energy system will rely on a diverse sustainable and fossil resources to fulfil all requirements for the energy supply concerning environment, sustainability, and quantity. One of these energy vectors is hydrogen. However, hydrogen as fuel source faces challenges in terms of efficient deployment (infrastructure costs, storage capacities) and distribution due to its unfavourable physical properties. The catalytic decomposition of ammonia to release hydrogen is an important step in enabling the use of ammonia as a potential replacement for fossil fuels.

[0006] The complete decomposition of NH3 to hydrogen and nitrogen: 2NH3 — N2 + 3H2 requires temperatures above 1000°C. Using current and commercially available catalyst technology, the kinetics of ammonia decomposition are slow and require very high operating temperatures that, in turn, compromise the efficiency of the ammonia-based energy cycle. The development of affordable catalysts with higher activity represents a significant step in facilitating the widespread use of ammonia as a sustainable fuel.

[0007] Among a wide range of studied catalytic materials, the most active metal-based catalysts for ammonia decomposition involve transition metals and use a range of support, such as porous carbon and metal oxides, along with alkali and alkaline earth-metal salts to enhance activity. Among the transition metals studied, ruthenium, nickel and iron have been the most widely investigated. All these catalytic materials actively decompose ammonia in hydrogen with better efficiencies than without catalysts. Ruthenium is generally considered to be the most active single metal catalyst but is expensive and rare metal with limited resources. Thus, different abundant and low-cost materials such as nickel or iron have been used. However, these transition metals, in particular nickel, suffer from low catalytic activity and are not material of choice for an in-vehicle technical application of producing hydrogen from ammonia.

[0008] Pure light metal amides and imides have recently emerged as a new class of catalyst for ammonia decomposition. This new class includes alkali metal amides and imides, alkali metal imides-amides, composites of alkali metal imide with transition metals and nitrides thereof, alkali metal ternary nitrides and alkali metal ternary imides (see Faraday Discuss., 2016, 188, 525). The generation of this new class of catalysts focuses on the use of lithium, magnesium, sodium and calcium as alkali metals since they are low-cost materials. Sodium amides and lithium imides as well as lithium-calcium ternary imides exhibit high catalytic activity for ammonia decomposition and would therefore be good candidates for ammonia decomposition in on-board technical applications or on-site hydrogen generation. However, the containment of these catalysts in a reactor is an issue due to the partial melting of these imide catalysts during the ammonia decomposition process above 350°C and even lower from 200°C to 460°C. To avoid this issue, the reaction of releasing hydrogen from ammonia must be performed in particular and very specific conditions to keep the catalysts in the solid state during the reaction. These conditions impede the kinetics of ammonia decomposition and involve low operating temperature, compromising the efficiency of the ammonia-based energy cycle.

[0009] The present invention addresses the problems depicted above, such as the problems of thermal instability, long-term sustainability, and durability of the catalysts, in particular the catalysts based on alkali metal imides or ternary alkali metal imide, while maintaining the efficiency of the ammonia-based energy cycle.

[0010] The present invention also addresses the problems associated with the use of metals with limited resources for the production of catalysts for the production of hydrogen from ammonia, in particular for on-site hydrogen production and on-board technical applications, as well as the problems associated with the costly and complex production of catalysts. of the Invention

[0011] The present invention aims to solve the problems depicted above in the catalytic production of hydrogen from ammonia, in particular for the in situ or on-site hydrogen production from ammonia, for on-board or in-vehicle technical application. In particular, the objective of the present invention is to stabilize catalysts based on alkali metal imides to avoid their melting and / or sintering during the ammonia decomposition process above 350°C while maintaining their highly catalytic activity in the ammonia decomposition or cracking. It is another objective of the present invention to provide a catalyst that is sustainable, long lasting, low-cost and straightforward to produce.

[0012] To this effect, the present invention proposes a catalyst comprising a ternary metal imide of formula (I): X2Y(NH)2 (I), wherein X is a metal selected from a group consisting of alkali metals and alkaline earth metals, and Y is a metal selected from a group consisting of transition metals and lanthanides or metals from the lanthanides group.

[0013] Preferably, the metal X of the ternary metal imide is selected from a group consisting of Li and Ca.

[0014] Preferably, the metal Y of the ternary metal imide is selected from a group consisting of V, Mn and La.

[0015] The inventors have found that ternary metal imides, wherein a metal of Group 1 or Group 2 elements is combined with a transition metal or a lanthanide, forms stable catalysts which do not melt, even partially, and do not sinter during the process of hydrogen production from ammonia at temperatures from about 300°C to about 600°C. Moreover, they demonstrate a high activity in the ammonia decomposition. Chemical modification, meaning addition of a transition metal or a lanthanide, in particular vanadium, manganese or lanthanum, into the alkali metal or alkaline-earth metal imide structure, in particular Li2NH or Ca2NH, forms thermically stable ternary alkali-transition metal or alkali-lanthanide metal imides. Unlike catalysts which are ternary alkali imides and comprise only alkali and alkaline earth metals, the present catalysts composed of ternary alkali-transition metal or alkali-lanthanide imides are stable throughout the ammonia decomposition process. The durability of the catalyst in the reaction and, in particular, when interacting with ammonia, is due to a change of electronegativity in their structure induced by the transition metal or the lanthanide on the alkali or alkaline-earth metal. Consequently, the catalysts composed of ternary alkali- transition metal or alkali-lanthanide imides, capable of high activity in the ammonia decomposition at relative low temperatures and pressures, are promising catalysts for producing hydrogen from ammonia onsite for use in on-board technical applications. The use of relatively abundant alkali and alkaline earth metal in the catalyst manufacturing combined to the stability and durability of the catalyst is a further advantage contributing to maintain the catalyst and hydrogen production costs low.

[0016] In another aspect, the invention provides a device for producing hydrogen from ammonia selected from a reactor or a reaction system for decomposing ammonia, comprising a catalyst of the invention.

[0017] Preferably, the device for producing hydrogen from ammonia is embedded and / or provides hydrogen in situ for use in a fuel cell and / or prime mover.

[0018] In one aspect, the invention further provides a use of a catalyst of the invention for producing hydrogen from ammonia and / or for decomposing ammonia in operating conditions, wherein an ammonia flow rate in the range of 10 ml / min to 400 L / min is maintained in a reaction system comprising the catalysts at an operating pressure of in the range of 2xl04to 7xl05Pa.

[0019] In a further aspect, the invention provides a method of producing hydrogen from ammonia comprising providing ammonia as a fuel source; providing a catalyst selected from a catalyst of the invention; and decomposing ammonia at a temperature in the range from 300 to 600 °C and at a pressure in the range of 2xl04to 7xl05Pa by catalytic reaction.

[0020] Other features and advantages of the present invention are mentioned in the dependent claims as well as in the following detailed description disclosing, with reference to the figures, preferred embodiments of the invention in more detail.

[0021] Brief Description of the Drawings

[0022] The attached figures exemplarily and schematically illustrate the principles as well as several embodiments of the present invention.

[0023] Figure 1 is graphic representations of ammonia (NHs) decomposition rate in function of temperature. Figure 1.1 is a graphic representation for ammonia decomposition in presence of different conventional metal-based and metal imide-amide catalysts: LiNFfc (square), NaNH2 (triangle), RusrowtAhOs (round) and Ni / AhCh (diamond). Figure 1.2 is a graphic representation for ammonia decomposition in presence of a catalyst of the present invention, in particular Li2V(NH)2.

[0024] Detailed Description of the invention

[0025] In the following, embodiments of the invention shall be described in detail with reference to the above-mentioned figures.

[0026] In one aspect, the invention concerns a catalyst comprising a compound selected from a ternary metal imide of formula (I)

[0027] X2Y(NH)2(I), wherein X is a metal selected from a group consisting of alkali metals and alkaline earth metals, and Y is a metal selected from a group consisting of transition metals and lanthanides metals.

[0028] In particular, X is a metal selected from metals of Group 1 elements and Group 2 elements and is combined with Y being a metal selected from transition metals of Group 5 and Group 7 elements and lanthanum. The preferred metal of Group 1 element is Li and the preferred metal of Group 2 element is Ca. As for transition metals of Group 5 and Group 7 elements, V and Mn are preferred metals. Preferably, X is a metal selected from a group consisting of Li and Ca, and Y is a metal selected from a group consisting of V, Mn and La.

[0029] According to one embodiment, the catalyst of the invention is a ternary metal imide of formula (I) which is selected from Li2V(NH)2, Li2Mn(NH)2, Li2La(NH)2, Ca2V(NH)2, Ca2Mn(NH)2, and Ca2La(NH)2.

[0030] The catalysts are synthesized by mixing a metal amide of Group 1 or Group 2 elements with a transition metal of Group 5 or Group 7 elements or lanthanum in form of hydride according to the molar ratio 2: 1. After being grounded, the mixture in powder form is submitted to pyrolysis at a selected temperature as disclosed hereunder and under flowing nitrogen gas with a selected flow rate as disclosed hereunder. The synthesis of a ternary alkali-transition metal imide Li2V(NH)2 is detailed as an example hereunder.

[0031] The pyrolysis for synthesizing new compounds is nowadays known in the art. Nevertheless, the specific conditions to obtain ternary alkali-transition metal or alkali-lanthanide imide or ternary alkaline earth-transition metal or alkaline earth-lanthanide are disclosed herein. The nitrogen gas flow rate is set in the range of 30 to 70 ml / min, preferably at 50 ml / min. The furnace, to which the quartz reactor containing the grounded metals mixture is connected, is set to be heated at a ramp rate in the range of 1 to 5 °C / min, preferably at 2°C / min. to a temperature in the range from 300°C to 450°C, preferably at 380°C. The pyrolysis temperature for the synthesis of the catalyst can be, for example, any of the following values, about any of the following values, at least any of the following values, no more than any of the following values, or within any range having any of the following values as endpoints (all values are in Celsius degree), though embodiments are not limited thereto: 300, 325, 330, 335, 340, 345, 350, 355, 360, 365, 370, 375, 380, 385, 390, 395, 400, 405, 410, 415, 420, 425, 430, 435, 440, 445 or 450. The pyrolysis duration time for the synthesis of the ternary alkali metal imide catalyst can be, for example, any of the following values, about any of the following values, at least any of the following values, no more than any of the following values, or within any range having any of the following values as endpoints (all values are in hours), though embodiments are not limited thereto: 1, 5, 10, 12, 20, 25, 30, or 35. As prepared by the above pyrolysis process, the catalyst is guaranteed to have optimum catalytic activity and durability.

[0032] Ammonia decomposition is favored at high temperatures and low pressures. Any technical application wherein ammonia decomposition is used to produce hydrogen for a subsequent process will require nearly full conversion of the ammonia to obtain a high yield of hydrogen and to keep low the ammonia content in the produced hydrogen. Ammonia is highly corrosive and reactive and could be a damaging impurity in the hydrogen for many subsequent processes or applications.

[0033] The catalysts comprising or consisting of a ternary metal imide of formula (I) are suitable for decomposing ammonia with almost complete conversion at temperatures below 580°C, or a temperature in the range from 400°C to 550°C at relatively low pressure from 104to 7x105Pa or from about 5xl04to 7xl05Pa as represented in Figure 1.2.

[0034] In one aspect, the invention also concerns a method of producing hydrogen from ammonia comprising providing ammonia as a fuel source, providing a catalyst of the invention in a reactor; reacting the catalyst with the ammonia; and decomposing ammonia at a temperature in the range from 300 to 600 °C and at a pressure in the range of IxlO4to 7xl05Pa. Further the produced hydrogen is removed from the reactor for subsequent use, e.g. in a combustion engine, a fuel cell or in a use mentioned herein. The method may be carried out in situ or in- vehicle.

[0035] In a further aspect, the invention concerns use of a catalyst of the invention for producing hydrogen from ammonia and / or for decomposing ammonia in operating conditions, wherein an ammonia flow rate in the range of 10 mL / min to 400 L / min is maintained in a reaction system comprising the catalysts and at an operating pressure of in the range of IxlO4to 7xl05Pa. The operating temperature range is from 300 to 600 °C, preferable from 400 to 500°C.

[0036] The catalyst may be provided to or loaded into a device for producing hydrogen from ammonia such as a reactor or a reaction system for decomposing ammonia.

[0037] The invention also concerns a device for producing hydrogen from ammonia selected from a reactor or a reaction system for decomposing ammonia, comprising a catalyst of the invention. In one embodiment, the device is connected to a fuel cell and / or a prime mover.

[0038] The catalyst may be provided in form of a powder to facilitate maximum reactive surface area. It may also be provided in form of pellets or beads and / or applied on a support suitable for the subsequent application being considered, such as a porous membrane to facilitate reactor design, e.g. The ammonia as fuel source, preferably stored in liquid form, is provided to the catalyst in the device in gaseous form to the reactor at a flow rate of at least 50 ml / min or in the range from 10 ml / min to 400 L / min, preferably from 100 L / min to 400 L / min. The reaction of ammonia catalytic decomposition is performed at a temperature in the range from 300 to 600°C, from 400 to 500°C, preferably about 450°C and at a pressure mentioned herein in the range from 104to 7xl05Pa. The primary products of the ammonia decomposition reaction in these conditions are hydrogen and nitrogen. Some remnants of ammonia below 20 ppm are possible.

[0039] After the reaction of ammonia decomposition in the reactor or the device suitable for decomposing ammonia in hydrogen and nitrogen, which is loaded with a catalyst of the invention, the post-reaction pressure is the same as the pressure used for the reaction, and the post-reaction flow rates of the outgases are respectively for hydrogen 1.5 times the ammonia flow rate and for nitrogen 0.5 times the ammonia flow rate. After the reaction, the outgases are cooled by a cooling system, such as but not limited to, an air-cooling system of a liquidcooling system. As mentioned herein, the ammonia decomposition for producing hydrogen may be performed in a reactor or a reaction system for decomposing ammonia. Such devices may be part of a further system for decomposing ammonia and / or for producing hydrogen which may contain a container, reservoir or tank, namely any container suitable for stocking ammonia, preferably in liquid form. Said container may be connected to a reactor loaded with a catalyst of the invention so as to supply the ammonia to the reactor in gaseous form, the ammonia being preheated or not before being provided to the reactor. The primary products produced by the ammonia catalytic decomposition, i.e., hydrogen and nitrogen, leave the reactor to be cooled down through a cooling system and / or heat exchanger. Depending on the subsequent use of hydrogen produced by ammonia decomposition, the mixture of hydrogen and nitrogen can be used directly in a further application, such as power generation through proton exchange membrane (PEM) to an alkaline or solid oxide hydrogen fuel cell, or further purified through pressure swing absorption (PSA) or metallic membrane hydrogen purification, for any subsequent application or use being considered.

[0040] So that, in an application or use as power supply, in particular as power supply for mobility, off-grid power supply or onsite power supply, the system for decomposing ammonia and / or producing hydrogen is connected directly or through a proton exchange membrane (PEM) to an alkaline or solid oxide hydrogen fuel cell to produce electricity from produced hydrogen. The efficiency of the electricity production is correlated to the purity of hydrogen, in particular in the use of hydrogen produced by decomposing ammonia in PEM hydrogen fuel cell for mobility.

[0041] Further applications or uses involve a purification of produced hydrogen from ammonia catalytic decomposition. In hydrogen combustion engine, the system for decomposing ammonia and / or producing hydrogen is connected to a hydrogen purification unit, such as pressure swing absorption (PSA) or metallic membrane hydrogen purification, wherein hydrogen is purified to obtain an efficient hydrogen combustion.

[0042] In a further use of the produced hydrogen in ammonia combustion engine for mechanical power in a vehicle, for power supply off-grid or for power supply onsite, the system can be connected to an ammonia combustion engine. The efficiency of the combustion depends on the mixture of feed gases, hydrogen helping to initiate the reaction while the ammonia is the main fuel. Reaction temperature is the key element in controlling the mixture of feed gases being ammonia, hydrogen and nitrogen.

[0043] The present invention will now be illustrated by way of examples. These examples do not limit the scope of this invention, which is defined by the appended claims.

[0044] Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other modifications and variations can be made without departing from the scope of the invention as hereinafter claimed.

[0045] Examples

[0046] Example 1: catalyst synthesis Li2V(NH)2

[0047] The catalyst is prepared by the following method, ensuring optimal activity and durability. In the glove box, 0.500 g of LiNH2 (lithium amide 95%, sigma Aldrich) and 0.561 g of VH2 (Vanadium hydride 99%, Nanochemazone) are mixed by hand using an agate mortar. 1.061 g of a resultant powder is obtained. In parallel, the vertical quartz reactor is placed in the glove box. Once, the reactor inside, the sample is placed in the quartz reactor. The reactor is then closed and moved outside of the glove box to be placed in the vertical furnace system. The duration is 2h, time to prepare the sample and prepare the entering of the glove box for the quartz reactor. Once the preparation is done, pyrolysis is done. The portion of the resultant powder is inside the quartz reactor which is ready to be placed in the vertical furnace. The quartz reactor is connected to the furnace which is placed. The sample is put under flowing nitrogen gas with a flow rate of 50 mL.min-1 and the furnace is set to be heated to 380 °C at a ramp rate of 2 °C.min-l and held at that temperature for 12 hours. Once the pyrolysis over, the quartz reactor is closed and transferred to the glove box. Once the reactor inside the glove box and ready to be opened, the resultant of the pyrolysis is weighted and placed into an airtight vial and place in the dedicated box waiting for further analysis and reaction.

[0048] Example 2: catalyst characterization

[0049] The catalyst is characterized thermally, structurally, and molecularly. Thermal characterization includes Differential Scanning Calorimetry (DSC) under ammonia atmosphere and Thermogravimetric Analysis (TGA). Structural characterization includes X-Ray diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS). Molecular characterization includes Infrared spectroscopy (IR), Raman spectroscopy (RAMAN), Inductively coupled plasma mass spectrometry (ICP) and Elemental Analysis (EA). Lifespan of the catalyst is characterized. The catalyst is designed to retain 99% activity after 200 hours of operation at 450°C. Further reaction shall retain 99% activity after 2000, 4000 and 6000h of operation at 450°C. The catalyst, as defined by Differential Scanning Calorimetry (DSC) under ammonia atmosphere, shows a melting point starting above 550 °C.

[0050] Example 3: Hydrogen production from ammonia

[0051] The reactor is loaded with the catalyst in a glove box under nitrogen inert atmosphere. The reactor is be equipped of opening and closing system allowing the transfer between from the glove box to the reaction system. The catalyst is in powder form to facilitate maximum reactive surface area. A catalyst amount of 0.5 g is used per reaction cycle at lab scale. Further reaction to scale up hydrogen production for subsequent application mentioned herein will be performed involving from 0.5 g to 50 kg of catalyst in the reactor.

[0052] During the decomposition reaction, the reactor operates at a pressure of 0.5 bar. An ammonia flow rate of 50 mL / min is maintained in the reactor. Representing the reactor loading, Gas Hourly Velocity (GHSV) in the present reaction is 5988.02 cm3 / g(cat)*hour, around 6000 cm3 / g(cat)*hour. The reactor temperature is set between 450 °C to optimize the decomposition process and at a temperature from 400 °C to 550 °C during the characterization.

[0053] The primary products out the ammonia decomposition reaction is hydrogen and nitrogen only. Traces of ammonia can remain.

[0054] In post-reaction, the system stabilizes to the same pressure than in the reactor, with a post-reaction pressure of 0.5 bar and the system stabilizes to the 1.5 times the flow rate of ammonia for hydrogen 75 mL / min and 0.5 times the flow rate of ammonia for nitrogen 25 mL / min, measured after the reactor.

[0055] The ammonia decomposition process is characterized by measuring ammonia, nitrogen, and hydrogen level after the ammonia decomposition reaction. Gas Chromatograph (GC 7890B) from Agilent is employed to monitor the decomposition rate of ammonia. The GC was equipped with two columns operating in tandem: a PoraPLOT Amines column, 25m in length, for ammonia separation, and a 2m ShinCarbon column tasked with the discernment of hydrogen and nitrogen. The temperature of the GC oven was maintained at 80°C to operate the separation process. The gaseous effluents respectively hydrogen (H2), nitrogen (N2) and traces of ammonia (NH3) emanating from the reactor were channeled directly into the GC. This system was programmed to collect gas samples from the reactor outflow every hour across the entire test duration.

Claims

Claims1. A catalyst comprising a ternary metal imide of formula (I)X2Y(NH)2(I), wherein- X is a metal selected from a group consisting of alkali metals and alkaline earth metals, and- Y is a metal selected from a group consisting of transition metals and lanthanides.

2. The catalyst according to claim 1, wherein X is a metal selected from a group consisting of Li and Ca.

3. The catalyst according to any one of the preceding claims, wherein Y is a metal selected from a group consisting of V, Mn and La.

4. The catalyst according to any one of the preceding claims, the ternary metal imide of formula (I) is selected from Li2V(NH)2, Li2Mn(NH)2, Li2La(NH)2, Ca2V(NH)2, Ca2Mn(NH)2, and Ca2La(NH)2.

5. A device for producing hydrogen from ammonia selected from a reactor, a catalyzer or a reaction system for decomposing ammonia, comprising a catalyst according to any one of claim 1 to 4.

6. The device of claim 5, characterized in that said device is embedded and / or provides hydrogen in situ for use in a fuel cell and / or prime mover.

7. Use of a catalyst according to any one of claims 1 to 4 for producing hydrogen from ammonia and / or for decomposing ammonia in operating conditions, wherein an ammonia flow rate in the range of 10 ml / min to 200 L / min is maintained in a reaction system comprising the catalysts and at an operating pressure of in the range of IxlO4to 7xl05Pa.

8. A method of producing hydrogen from ammonia comprising providing ammonia as a fuel source; providing a catalyst selected from a catalyst according to any one of claim 1 to 4; anddecomposing ammonia at a temperature in the range from 300 to 600 °C and at a pressure in the range of 2xl04to 7xl05Pa by catalytic reaction.

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