EP4747008A1 - Supported metal catalyst for ammonia production and ammonia cracking - Google Patents

Abstract

The present invention provides a catalyst comprising metallic nanoparticles made of same or different metal atoms, anchored to an oxide-based support comprising metal ions, wherein said metal atoms comprise Ru(0), Fe(0), and optionally Cu(0) and/or Ag(0); and said metal ions comprise at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally Fe(III) and/or Fe(II). The catalyst is useful in a process for the production of ammonia, which may be carried out under conditions that greatly reduce the capital- and operational expenditures of ammonia production by comparison to the current Haber-Bosch process; and is assumed to be further useful in a process for ammonia cracking.

Description

SUPPORTED METAL CATALYST FOR AMMONIA PRODUCTION AND AMMONIA CRACKING

TECHNICAL FIELD

[0001] The present invention provides a catalyst comprising metallic nanoparticles anchored to a metal oxide-based support. Said catalyst is useful in a process for the production of ammonia, which may be carried out under temperature and pressure conditions that greatly reduce the capital- and operational expenditures of ammonia production by comparison to the current Haber-Bosch process; and is assumed to be further useful in a process for ammonia cracking.

BACKGROUND ART

[0002] Ammonia production is an essential process for the agricultural industry (Fenn and Hossner, 1985) and is becoming increasingly important in the energy sector as a potential hydrogen carrier (Chatterjee et al., 2021; Ishimoto et al., 2020). The production of ammonia is typically done thermochemically at high pressure using the well-known Haber-Bosch (H-B) process. There is significant interest in lowering the operational pressure (200-400 atm) and temperature (400-600°C) for the H-B process owing to concerns relating to energy consumption, emissions, safety, and distribution. As with many catalytic reactions, the low-pressure route for thermochemical ammonia synthesis contains a myriad of elementary steps, each of which requiring a different type of active site. This is convoluted by the fact that active sites comprised of metals with high sticking coefficients for hydrogen (Danielson et al., 1978) lead to hydrogen poisoning and undermines the performance of the catalyst.

[0003] Given the potential advantages of low-pressure thermochemical ammonia synthesis, there have been many Ru-based catalysts reported on in the past decade. Most of such catalysts are tested at atmospheric pressure (0.1 MPa) and near 400°C with the understanding that this condition is used for benchmarking; apractical low-pressure thermochemical ammonia synthesis plant is likely to operate between 1-5 MPa. For comparison, among the Ru-based catalysts that have been reported at 0.1 MPa and 400°C in a 3:1 H2:N2 atmosphere, the electride class of catalysts has gathered significant interest having shifted the rate-limiting step from the N2 scission step to the N-H_n association step (Kitano et al., 2015) and exceeding synthesis rates of 5 mmol/gcat/h (Hayashi et al., 2013). Other active ammonia catalysts include Ru supported on hydrides such as Ru/Ca(NH2)2 (Inoue et al., 2016) (15.8 mmol/gcat/h), BaO-CaH2 (10 mmol/gcat/h) (Hattori et al., 2018), and CaH2 (4 mmol/gcat/h) (Kitano et al., 2016). Importantly, these studies used up to 10 wt% of Ru; therefore, it is instructive to normalize the comparison of the ammonia production rate by the weight percent of ruthenium in the catalyst. This brings the normalized catalytic activity of the CaFh support to 2 mmol/g_cat/h/wt% Ru, the Ca(NH2)2 support to 1.58 mmol/gcat/h/wt% Ru and the BaO-CaFL support to 1 mmol/gcat/h/wt% Ru. Additional studies have shown that modifying the Ru-phase, rather than the support, can have beneficial effects including the RuFe (Liu et al., 2022), RuCo (Banisalman et al., 2022) homogeneous alloyed phases.

[0004] A unique opportunity exists resulting from the exsolution of heterostructures, particularly if formed by thermodynamically stable immiscible combinations of metals. To be sure, kinetically limited metallic heterostructures have been previously reported as catalysts for other reactions in bi-metallic exsolution between miscible metals (i.e., Fe-Ni, Ni-Pd (Buharon et al., 2018), but these structures are thermodynamically predicted to form solid solutions after extended use. Although several studies have emphasized the ability to form heterostructures via exsolution (Buharon et al., 2018), the vast majority of bimetallic exsolution are solid solutions (Joo et al., 2020; Kim et al., 2023). Furthermore, there are only several studies involving the coexsolution of mutually immiscible metal pairs such as Cu-Co forming exsolved core-shell structures (Adijanto et al., 2012) and Cu-Fe (Lan et al., 2016; Song et al., 2018).

[0005] The exsolution of multiple insoluble phases has many implications in multi-functional catalysts, since alloying sometimes removes the unique catalytic properties of each individual metal to make an alloyed particle with new, but uniform, properties. In hetero-structured multiphase exsolution, each phase may have a completely different contribution to the catalytic process (i.e., via a metallic dimer or trimer structure).

SUMMARY OF INVENTION

[0006] In one aspect, disclosed herein is a supported metal catalyst comprising nanoparticles made of same or different metal atoms, anchored to an oxide-based support comprising metal ions, wherein said metal atoms comprise ruthenium atoms (Ru(0)), iron atoms (Fe(0)), and optionally at least one of copper atoms (Cu(0)) and silver atoms (Ag(0)); and said metal ions comprise at least two of strontium ions (Sr(II)), lanthanum ions (La(III)), calcium ions (Ca(II)), barium ions (Ba(II)), praseodymium ions (Pr(III)), neodymium ions (Nd(III)), zirconium ions (Zr(III), yttrium ions (Y(III)), and cesium ions (Cs(I)), and optionally at least one of iron ions selected from ferric ions (Fe(III)) and ferrous ions (Fe(II)). [0007] The supported metal catalyst disclosed may be obtained by an exsolution process, i.e., by exposing a perovskite-oxide material comprising metal ions comprising Ru(III), Fe(III), at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Cu(II) and Ag(I), occupying A sites and B sites of said perovskite-oxide material, to a reducing environment or a reducible electrochemical potential, at a temperature of, e.g., from about 600°C to about 800°C, to thereby reduce said Ru(III), Fe(III), and at least one of Cu(II) and Ag(I) when present, which consequently form nanoparticles made of metal atoms comprising Ru(0), Fe(0), and optionally at least one of Cu(0) and Ag(0), anchored to an oxidebased support comprising at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Fe(III) and Fe(II).

[0008] The disclosed catalyst is highly effective in a process for the production of ammonia, which may be carried out at a temperature of about 400°C and under atmospheric pressure (0.1 MPa), greatly reducing the capital expenditures (CAPEX) and operational expenditures (OPEX) of ammonia production by comparison to the current Haber-Bosch (H-B) process.

[0009] In another aspect, thus disclosed herein are methods for production of ammonia, referred to herein as "Production Method A" and "Production Method B" .

[0010] In one particular such aspect, disclosed herein is Production Method A, comprising reacting nitrogen and hydrogen in the presence of a supported metal catalyst as defined hereinabove, to thereby obtain said ammonia.

[0011] In another particular such aspect, disclosed herein Production Method B, comprising feeding nitrogen to an electrolytic cell comprising at least one source of protons; allowing the nitrogen to come into contact with the surface of a cathode electrode in the electrolytic cell, wherein the surface of the cathode electrode comprises a supported metal catalyst as defined hereinabove; and running a current through said electrolytic cell, whereby the nitrogen reacts with the protons to form said ammonia.

[0012] As it is well-known that ruthenium is an active ammonia cracking catalyst, it is assumed that the Ru-based catalyst disclosed herein may also be useful in a process for ammonia cracking, i.e., catalytic thermal decomposition of ammonia into hydrogen and nitrogen.

[0013] In a further aspect, thus disclosed herein is a process for ammonia cracking, said process comprising the step of exposing gaseous anhydrous ammonia to a temperature of at least 200°C and a pressure of at least 0.01 bar (1 atm = 1.01325 bar), e.g., a temperature of 200-550°C, preferably 420°C-480°C, and/or a pressure of 0.01-1 bar, in the presence of a supported metal catalyst as defined hereinabove, to thereby obtain a gas mixture comprising hydrogen and nitrogen.

[0014] The process for ammonia cracking disclosed herein may further comprise a purification step to reduce the amount of uncracked ammonia, when present in said gas mixture; and/or a step for separating said hydrogen from said gas mixture. Such a separation step is typically required wherein hydrogen is needed as either an energy source or fuel for downstream manufacturing processes. In certain embodiments, the hydrogen mixture separated from said gas mixture, regardless of whether containing nitrogen or not, is then fed into a fuel cell to produce electrical power.

BRIEF DESCRIPTION OF DRAWINGS

[0015] Fig. 1 shows calculated ternary equilibrium Fe-Cu-Ru phase diagram at 700°C.

[0016] Fig. 2 shows STEM-EDX elemental mapping for the Lao.gSro.iFeo.ssRuo.osCuo.iOs-s after exsolution at 700°C for 5h in hydrogen (panel a); exposure to 5% hydrogen in nitrogen at 650°C for 96 hours after the exsolution process (panel b); and schematic diagram demonstrating the anti-agglomeration properties of co-exsolved Ru-Cu particles in contrast to catalysts prepared by Ru-only and Ru-Co co-exsolution (panel c). LSFO is the support oxide of un-exsolved La, Sr and Fe.

[0017] Fig. 3 shows HAADF and STEM-EDX elemental mapping for Lao.9Sro.1Feo.8Ruo.o5 CU0.15O3-5 after exsolution for 5h in hydrogen at 650°C (panels a-c) and 700°C (panels d-h), highlighting the segregation of strontium (panels b, e); the alloying of Fe into Ru particles (panel g); and the tri-metallic hetero structure achieved by Cu-Fe-Ru exsolution (panel h).

[0018] Fig. 4 shows calculated step diagram at 700°C showing the thermodynamically stable phases at a fixed Cu composition of 15 at%.

[0019] Fig. 5 shows calculated ternary equilibrium Fe-Cu-Ru phase diagram at 800°C.

[0020] Fig. 6 shows STEM-EDX image of LaFeo.soRuo.osCuo.isOs-s after exsolution in hydrogen at 800°C for 5h.

[0021] Fig. 7 shows XRD of Lao.9Sro.iFeo.8oRuo.o5Cuo.i503-5 after exsolution in hydrogen at 650°C (lower graph), 700°C (middle graph) and 800°C (upper graph) in hydrogen for 5h.

[0022] Fig. 8 shows arrhenius plot for ammonia synthesis at O.IMPa (3:1 H2:N2) for calculating the activation energy of the catalyst derived by exsolution of Lao.9Sro.iFeo.8oRuo.o5Cuo.i503-5in hydrogen at 700°C for 5h. [0023] Fig. 9 shows X-ray photoemission spectroscopy of the Fe 2p (right panel) and Cu 2p (left panel) core electron for the as-synthesized Lao.gSro.iFeo.soRuo.osCuo.isOs-s precursor.

[0024] Fig. 10 shows XRD of the as-synthesized Lao.gSro.iFeo.soRuo.osCuo.isOs-s precursor, the exsolved catalyst after 5h in hydrogen at 700°C, and oxidation of the exsolved catalyst in oxygen at 800°C for 12h (left panel); and enhanced view of the same XRD result focused between 39° and 50° in order to emphasize the presence of the metal peaks (right panel).

DETAILED DESCRIPTION

[0025] Exemplified herein is a highly active catalyst for ambient pressure thermochemical ammonia synthesis, prepared by exsolution of Ru, Cu, and Fe. As shown in the experimental section, each metal contributed towards the improved activity of the catalyst, which was found to be comprised of hetero- structured nanoparticles containing all three elements. The hetero structure was derived by exploiting regions of the ternary phase diagram where large immiscibility was to be expected. Calculations provide evidence that the strain associated with the exsolution process at the metal-oxide interface likely do not make a large influence on the expected immiscibility between Cu and Ru, and that the design of exsolution catalysts can be based largely on information derived from equilibrium phase diagrams. The particular catalyst derived by exsolution from the Lao.9Sro.iFeo.85Ruo.o5Cuo.i03-5 precursor showed both excellent activity towards ambient pressure ammonia at 400°C, as well as excellent geometric and antiagglomeration stability for extended periods of time under reducing conditions. This stability is imbued by the immiscibility of Cu and Ru nanoparticles on the surface which acted as diffusion barriers towards agglomeration. Cu was shown to be an important promoter towards catalytic activity by limiting the growth of Ru particles, increasing the density of oxygen vacancies, and possibly providing spillover sites immediately adjacent to the active site, thereby relieving hydrogen poisoning.

[0026] In one aspect, the present invention thus provides a supported metal catalyst comprising nanoparticles made of same or different metal atoms, anchored to an oxide-based support comprising metal ions, wherein said metal atoms comprise Ru(0), Fe(0), and optionally at least one of Cu(0) and Ag(0); and said metal ions comprise at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of iron ions selected from Fe(III) and Fe(II).

[0027] The term “nanoparticles” as used herein refers to spheres, spheroids, or quasi-spheres having a size of from about 0.5 nm to about 50 nm, preferably from about 1 nm to about 10, 15 or 20 nm, more preferably from about 1 nm to about 5 nm, and consisting of one or more metal atoms. As disclosed herein, such nanoparticles each independently may consist of one metal (single-metallic nanoparticle), i.e., Ru(0), Fe(0), Cu(0), and Ag(0), or more than one, e.g., two or three, of said metals (multi-metallic nanoparticle). In the case of multi-metallic nanoparticles, the composition of each one of said nanoparticles may change as a function of distance of the metal atoms from the surface of the particle.

[0028] In certain embodiments, the supported metal catalyst of the present invention comprises nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only. In other embodiments, said supported metal catalyst comprises nanoparticles made of both Ru(0) and Fe(0), i.e., an alloy of Ru(0) and Fe(0) (hereinafter "Ru-Fe alloy"). In further embodiments, said supported metal catalyst comprises nanoparticles made of Ru(0) only, nanoparticles made of Fe(0) only, and nanoparticles made of said Ru-Fe alloy. In particular such embodiments, the size of the Ru(0) nanoparticles, when present, is from about 0.5 nm to about 20 nm, e.g., from about 1 nm to about 18 nm, from about 2 nm to about 16 nm, from about 3 nm to about 14 nm, from about 4 nm to about 12 nm, from about 5 nm to about 10 nm, or from about 6 nm to about 8 nm; and/or the size of the Fe(0) nanoparticles, when present, is from about 1 nm to about 20 nm, e.g., from about 2 nm to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm; and/or the size of the nanoparticles made of said Ru-Fe alloy, when present, is from about 1 nm to about 20 nm, e.g., from about 2 nm to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0029] In particular embodiments, the supported metal catalyst of the invention comprises nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; and/or nanoparticles made of Ru-Fe alloy, as defined above, and each one of said nanoparticles, i.e., those made of Ru(0) only, those made of Fe(0) only, and those made of Ru-Fe alloy, independently, is further coated with Cu(0), i.e., comprises a Cu(0) coating. According to the present invention, the Cu(0) atoms coating said nanoparticles may constitute from about 0.1% to about 5%, e.g., from about 0.2% to about 4.8%, from about 0.4% to about 4.6%, from about 0.6% to about 4.4%, from about 0.8% to about 4.2%, from about 1.0% to about 4.0%, from about 1.2% to about 3.8%, from about 1.4% to about 3.6%, from about 1.6% to about 3.4%, from about 1.8% to about 3.2%, or from about 2.0% to about 3.0%, by weight, of said supported metal catalyst.

[0030] In particular embodiments, the supported metal catalyst of the invention comprises nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; and/or nanoparticles made of Ru-Fe alloy, as defined above, and further comprises nanoparticles made of Cu(0) only. According to the present invention, the size of the nanoparticle made of Cu(0) only may be from about 1 nm to about 20 nm, e.g., from about 2 to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0031] In certain embodiments, the supported metal catalyst of the present invention comprises nanoparticles made of Ru(0), Fe(0), and Cu(0), i.e., an alloy of Ru(0), Fe(0) and Cu(0) (hereinafter "Ru-Fe-Cu alloy"). In particular such embodiments, the size of the nanoparticles made of Ru-Fe-Cu alloy is from about 1 nm to about 20 nm, e.g., from about 2 to about 18 nm, from about 3 nm to about 16 nm, or from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0032] The oxide -based support comprised within the supported metal catalyst of the present invention comprises at least two, e.g., two, three, four, or more, of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Fe(III) and Fe(II). In some embodiments, said oxide -based support comprises at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III) and Cs(I), i.e., two or more of the structures SrO, La2Os, CaO, BaO, PnCh, INcbCh, ZrCh, Y2O3, and CS2O. In other embodiments, said oxide-based support comprises at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and at least one of Fe(III) and Fe(II), in the form of a perovskite structure comprising same or different metal ions selected from Sr(II), La(III) and Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), occupying A sites of said perovskite structure; and same or different metal ions selected from Fe(III) and Fe(II), occupying B sites of said perovskite structure. In further embodiments, said oxide -based support comprises a combination of two or more of the structures SrO, La2Os, CaO, BaO, PnOs, INcbOs, ZrO2, Y2O3, and CS2O; and a perovskite structure comprising same or different metal ions occupying A sites and B sites of said perovskite structure as defined hereinabove.

[0033] In certain embodiments, the oxide -based support comprised within the supported metal catalyst, according to any one of the embodiments above, comprises Sr(II), La(III) and Fe(III). In particular such embodiments, said oxide -based support comprises a combination of SrO, La2O3, and Lao.9Sro.iFeo.8-x03-5.

[0034] In certain embodiments, the Ru(0) atoms comprised within the supported metal catalyst, according to any one of the embodiments above, constitute from about 0.1% to about 1%, e.g., from about 0.2% to about 0.9%, from about 0.3% to about 0.8%, from about 0.4% to about 0.7%, or from about 0.5% to about 0.6%, by weight, of said supported metal catalyst.

[0035] As shown herein, the supported metal catalyst of the present invention may be obtained by an exsolution process, i.e., by exposing a perov skite-oxide (starting) material comprising metal ions comprising Ru(III), Fe(III), at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Cu(II) and Ag(I), occupying A sites and B sites of said perovskite-oxide material, to a reducing environment, e.g., hydrogen, or a reducible electrochemical potential, at a suitable temperature, e.g., at a temperature of from about 600°C to about 800°C, to thereby reduce said Ru(III), Fe(III), and at least one of Cu(II) and Ag(I) when present, which consequently form nanoparticles made of metal atoms comprising Ru(0), Fe(0), and optionally at least one of Cu(0) and Ag(0), anchored to an oxidebased support comprising at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Fe(III) and Fe(II). During said process, the perovskite-oxide material partially breaks down, such that the oxide-based support obtained may comprise a mixture of a perovskite structure and other oxides as defined hereinabove. Moreover, upon exposure to oxygen at a suitable temperature, e.g., at a temperature of from about 700°C to about 1000°C, the nanoparticles comprised within the supported metal catalyst may be ingressed back into the oxide-based support, thereby regenerating the perovskite-oxide starting material. The regenerated starting material may then be subject to an exsolution process as described above, to grow fresh nanoparticles as described above. Such an "ingression process" may be used in case the metal atoms-based nanoparticles comprised within the supported metal catalyst grow over time above a certain threshold which negatively impacts catalysis.

[0036] In certain embodiments, the supported metal catalyst according to any one of the embodiments above is obtained by exsolution, i.e., exposing a perovskite-oxide (starting) material comprising metal ions occupying A sites and B sites of said perovskite-oxide material to a reducing environment (such as hydrogen) or a reducible electrochemical potential, at a temperature of from about 600°C to about 800°C, wherein said metal ions comprise: Ru(III), Fe(III), at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Cu(II) and Ag(I), thereby reducing said Ru(III), Fe(III), and at least one of Cu(II) and Ag(I) when present, to obtain said supported metal catalyst. Following this process, all the Ru(III), as well as all the Cu(II) and/or Ag(I) when present in the perovskite- oxide starting material, are reduced to metal atoms at the entire range of temperature recited above. On the other hand, the amount of the Fe(III) that is reduced during said process is temperature dependent and increases with increasing the temperature, wherein all the Fe(III) comprised within the starting material may be exsolved at a temperature of about 800°C.

[0037] In particular embodiments, at least 80% of the A sites of said perovskite-oxide starting material exposed to a reducing environment or a reducible electrochemical potential according to the exsolution process above are occupied, i.e., up to 20% of the A sites of said perovskite- oxide material may be vacant; and 100% of the B sited of said perovskite-oxide material are occupied, i.e., there no vacant B sites.

[0038] In certain embodiments, the perovskite-oxide material exposed to a reducing environment or a reducible electrochemical potential according to the exsolution process above comprises: (i) Sr(II), La(III), Ru(III), and Fe(III); or (ii) Sr(II), La(III), Ru(III), Fe(III), and Cu(II). In specific such embodiments, said perovskite-oxide materials is (i) Lao.9Sro.1Feo.95Ruo.05O3; (ii) Lao.9Sro.1Feo.85Ruo.05Cuo.1O3; (iii) Lao.9Sro.1Feo.sRuo.05Cuo.15O3; or (iv) La0.9Fe0.85Ru0.05Cu0.1O3.

[0039] In particular such embodiments, said exsolution process comprises exposing a perovskite-oxide material comprising Lao.9Sro.1Feo.8Ruo.05Cuo.15O3 to a reducing environment or a reducible electrochemical potential, at a temperature of 680-720°C, e.g., at 690°C, 700°C, or 710°C.

[0040] As shown herein, the supported metal catalyst of the present invention may be used, and it is highly effective, in a process for the production of ammonia, which may be carried out at a temperature of about 400°C and under atmospheric pressure (0. 1 MPa), greatly reducing the capital expenditures (CAPEX) and operational expenditures (OPEX) of ammonia production by comparison to the current Haber-Bosch (H-B) process.

[0041] In another aspect, the present invention thus relates to methods for production of ammonia, referred to herein as "Production Method A" and "Production Method B" .

[0042] In one particular such aspect, the method disclosed herein is Production Method A, said method comprising reacting nitrogen and hydrogen in the presence of a supported metal catalyst as disclosed herein, according to any one of the embodiments above, i.e., a supported metal catalyst comprising nanoparticles made of same or different metal atoms, anchored to an oxidebased support comprising metal ions, wherein said metal atoms comprise Ru(0), Fe(0), and optionally at least one of Cu(0) and Ag(0); and said metal ions comprise at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of iron ions selected from Fe(III) and Fe(II), to thereby obtain said ammonia. [0043] In another particular such aspect, the method disclosed herein is Production Method B, said method comprising feeding nitrogen to an electrolytic cell comprising at least one source of protons; allowing the nitrogen to come into contact with the surface of a cathode electrode in the electrolytic cell, wherein the surface of the cathode electrode comprises a supported metal catalyst as disclosed herein, according to any one of the embodiments above, i.e., a supported metal catalyst comprising nanoparticles made of same or different metal atoms, anchored to an oxide-based support comprising metal ions, wherein said metal atoms comprise Ru(0), Fe(0), and optionally at least one of Cu(0) and Ag(0); and said metal ions comprise at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of iron ions selected from Fe(III) and Fe(II); and running a current through said electrolytic cell, whereby the nitrogen reacts with the protons to form said ammonia.

[0044] In certain embodiments, the catalyst used in each one of the Production Methods disclosed herein is a supported metal catalyst comprising nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; or nanoparticles made of Ru-Fe alloy; or nanoparticles made of Ru(0) only, nanoparticles made of Fe(0) only, and nanoparticles made of Ru-Fe alloy. As defined above, the size of the Ru(0) nanoparticles, when present, may be from about 0.5 nm to about 20 nm, e.g., from about 1 nm to about 18 nm, from about 2 nm to about 16 nm, from about 3 nm to about 14 nm, from about 4 nm to about 12 nm, from about 5 nm to about 10 nm, or from about 6 nm to about 8 nm; the size of the Fe(0) nanoparticles, when present, may be from about 1 nm to about 20 nm, e.g., from about 2 nm to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm; and the size of the nanoparticles made of Ru-Fe alloy, when present, may be from about 1 nm to about 20 nm, e.g., from about 2 nm to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0045] In certain particular embodiments, the supported metal catalyst used in each one of the Production Methods disclosed comprises nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; and/or nanoparticles made of Ru-Fe alloy, as defined hereinabove, and each one of said nanoparticles, i.e., those made of Ru(0) only, those made of Fe(0) only, and those made of Ru-Fe alloy, independently, is further coated with Cu(0). As defined above, the Cu(0) atoms coating said nanoparticles may constitute from about 0.1% to about 5%, e.g., from about 0.2% to about 4.8%, from aboutO.4% to about 4.6%, from about 0.6% to about 4.4%, from about 0.8% to about 4.2%, from about 1.0% to about 4.0%, from about 1.2% to about 3.8%, from about 1.4% to about 3.6%, from about 1.6% to about 3.4%, from about 1.8% to about 3.2%, or from about 2.0% to about 3.0%, by weight, of said supported metal catalyst.

[0046] In other particular embodiments, the supported metal catalyst used in each one of the Production Methods disclosed comprises nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; and/or nanoparticles made of Ru-Fe alloy, as defined hereinabove, and further comprises nanoparticles made of Cu(0) only. As defined above, the size of the nanoparticles made of Cu(0) only may be from about 1 nm to about 20 nm, e.g., from about 2 to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0047] In certain embodiments, the catalyst used in each one of the Production Methods disclosed is a supported metal catalyst comprising nanoparticles made of Ru(0), Fe(0) and Cu(0), i.e. Ru-Fe-Cu alloy. As defined hereinabove, the size of the nanoparticles made of Ru-Fe-Cu alloy may be from about 1 nm to about 20 nm, e.g., from about 2 to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0048] In certain embodiments, the oxide -based support comprised within the supported metal catalyst utilized in each one of the Production Methods disclosed, according to any one of the embodiments above, comprises Sr(II), La(III) and Fe(III), e.g., a combination of SrO, La2Os, and Lao.9Sro.iFeo.8-x03-5-

[0049] In certain embodiments, the Ru(0) atoms comprised within the supported metal catalyst utilized in each one of the Production Methods disclosed, according to any one of the embodiments above, constitute from about 0.1% to about 1%, e.g., from about 0.2% to about 0.9%, from about 0.3% to about 0.8%, from about 0.4% to about 0.7%, or from about 0.5% to about 0.6%, by weight, of said supported metal catalyst.

[0050] In certain embodiments, the supported metal catalyst utilized in each one of the Production Methods disclosed, according to any one of the embodiments above, is obtained by exsolution, i.e., exposing a perovskite-oxide (starting) material comprising metal ions occupying A sites and B sites of said perovskite-oxide material to a reducing environment (such as hydrogen) or a reducible electrochemical potential, at a temperature of from about 600°C to about 800°C, wherein said metal ions comprise: Ru(III), Fe(III), at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Cu(II) and Ag(I), thereby reducing said Ru(III), Fe(III), and at least one of Cu(II) and Ag(I) when present, to obtain said supported metal catalyst. In particular embodiments, at least 80% of the A sites of said perovskite-oxide starting material are occupied, i.e., up to 20% of the A sites of said perovskite - oxide material may be vacant; and 100% of the B sited of said perovskite-oxide material are occupied, i.e., there no vacant B sites. As shown in the Experimental section herein, the perovskite-oxide material exposed to a reducing environment or a reducible electrochemical potential according to the exsolution process above may comprise: (i) Sr(II), La(III), Ru(III), and Fe(III); or (ii) Sr(II), La(III), Ru(III), Fe(III), and Cu(II). In specific embodiments, said perovskite-oxide material is (i) Eao.9Sro.1Feo.95Ruo.05O3; (ii) Ea0.9Sr0.1Fe0.85Ru0.05Cu0.1O3; (iii) Lao.9Sro.1Feo.sRuo.05Cuo.15O3; or (iv) Lao.9Feo.85Ruo.05Cuo.1O3. In particular specific embodiments, said perovskite-oxide material comprises Lao.9Sro.1Feo.sRuo.05Cuo.15O3, and said exsolution process comprises exposing said perovskite-oxide material to a reducing environment or a reducible electrochemical potential, at a temperature of 680-720°C, e.g., at 690°C, 700°C, or 710°C.

[0051] In certain embodiments, the reaction of nitrogen with hydrogen according to Production Method A, as disclosed in any one of the embodiments above, is carried out at a pressure lower than about 10 MPa, e.g., from about 1 MPa to about 8 MPa, from about 2 MPa to about 6 MPa, or from about 3 MPa to about 5 MPa, and/or at a temperature of up to about 400°C, e.g., about 300°C, 320°C, 340°C, 360°C, 38O°C, or 400°C.

[0052] In certain embodiments, the reaction of nitrogen with protons according to Production Method B, as disclosed in any one of the embodiments above, is carried out at or near room temperature, e.g., at about 18-25°C, 18-24°C, 18-23°C, or 18-22°C, and/or using a voltage of up to about 3V.

[0053] Considering that Ru is an active ammonia cracking catalyst, it is postulated that the supported metal catalyst of the present invention may further be useful and highly effective in a process for ammonia cracking, i.e., catalytic thermal decomposition of ammonia into hydrogen and nitrogen according to the reaction: 2NH3 — N2 + 3th.

[0054] In a further aspect, the present invention thus relates to a process for ammonia cracking comprising the step of exposing gaseous anhydrous ammonia to a temperature of at least 200°C and a pressure of at least 0.01 bar, in the presence of a supported metal catalyst as disclosed herein, according to any one of the embodiments above, i.e., a supported metal catalyst comprising nanoparticles made of same or different metal atoms, anchored to an oxide-based support comprising metal ions, wherein said metal atoms comprise Ru(0), Fe(0), and optionally at least one of Cu(0) and Ag(0); and said metal ions comprise at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of iron ions selected from Fe(III) and Fe(II), to thereby obtain a gas mixture comprising hydrogen and nitrogen. Such a process may be carried out, e.g., within an ammonia cracker, which is a catalytic cracking furnace in which the ammonia synthesis reaction is reversed at elevated temperatures.

[0055] In certain embodiments, said gaseous anhydrous ammonia is exposed to a temperature of from 200°C to about 550°C, e.g., from 250°C, 300°C, 350°C, or 400°C to about 500°C, such as about 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, or 480°C, and/or a pressure of from 0.01 to about 1 bar, e.g., about 0.025, 0.05, 0.075, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 bar, but preferably from about 0.05 to about 0.15 bar.

[0056] The gas mixture obtained by the cracking process is composed of hydrogen and nitrogen in the proportion of 3:1 (75% of H2 and 25% of N2) with very little amount (e.g., 20- 100 ppm) of residual uncracked ammonia with dew point in the range of from about -50°C to about -30°C, and it may be further purified, e.g., by molecular sieves purifier, so as to reduce the uncracked ammonia to, e.g., about 1-3 ppm with dew point of about -80°C. Said gas mixture, regardless of whether purified or not, may also be subjected to a separation process aimed at separating said hydrogen. Such a separation step is typically required when hydrogen is needed as either an energy source or fuel for downstream manufacturing processes. In certain embodiments, the hydrogen mixture separated from said gas mixture, regardless of whether containing nitrogen or not, is thus fed into a fuel cell to generate electricity.

[0057] In certain embodiments, the ammonia cracking process of the present invention, according to any one of the embodiments above, further comprise a purification step to reduce the amount of uncracked ammonia, when present in said gas mixture; and/or a step for separating said hydrogen from said gas mixture.

[0058] In certain embodiments, the catalyst used in the ammonia cracking process disclosed herein is a supported metal catalyst comprising nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; or nanoparticles made of Ru-Fe alloy; or nanoparticles made of Ru(0) only, nanoparticles made of Fe(0) only, and nanoparticles made of Ru-Fe alloy. As defined above, the size of the Ru(0) nanoparticles, when present, may be from about 0.5 nm to about 20 nm, e.g., from about 1 nm to about 18 nm, from about 2 nm to about 16 nm, from about 3 nm to about 14 nm, from about 4 nm to about 12 nm, from about 5 nm to about 10 nm, or from about 6 nm to about 8 nm; the size of the Fe(0) nanoparticles, when present, may be from about 1 nm to about 20 nm, e.g., from about 2 nm to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm; and the size of the nanoparticles made of Ru-Fe alloy, when present, may be from about 1 nm to about 20 nm, e.g., from about 2 nm to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0059] In certain particular embodiments, the supported metal catalyst used in ammonia cracking process disclosed comprises nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; and/or nanoparticles made of Ru-Fe alloy, as defined hereinabove, and each one of said nanoparticles, i.e., those made of Ru(0) only, those made of Fe(0) only, and those made of Ru-Fe alloy, independently, is further coated with Cu(0). As defined above, the Cu(0) atoms coating said nanoparticles may constitute from about 0.1% to about 5%, e.g., from about 0.2% to about 4.8%, from aboutO.4% to about 4.6%, from about 0.6% to about 4.4%, from about 0.8% to about 4.2%, from about 1.0% to about 4.0%, from about 1.2% to about 3.8%, from about 1.4% to about 3.6%, from about 1.6% to about 3.4%, from about 1.8% to about 3.2%, or from about 2.0% to about 3.0%, by weight, of said supported metal catalyst.

[0060] In other particular embodiments, the supported metal catalyst used in ammonia cracking process disclosed comprises nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; and/or nanoparticles made of Ru-Fe alloy, as defined hereinabove, and further comprises nanoparticles made of Cu(0) only. As defined above, the size of the nanoparticles made of Cu(0) only may be from about 1 nm to about 20 nm, e.g., from about 2 to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0061] In certain embodiments, the catalyst used in ammonia cracking process disclosed is a supported metal catalyst comprising nanoparticles made of Ru(0), Fe(0) and Cu(0), i.e. Ru-Fe- Cu alloy. As defined hereinabove, the size of the nanoparticles made of Ru-Fe-Cu alloy may be from about 1 nm to about 20 nm, e.g., from about 2 to about 18 nm, from about 3 nm to about 16 nm, from about 4 nm to about 14 nm, from about 5 nm to about 12 nm, or from about 6 nm to about 10 nm.

[0062] In certain embodiments, the oxide -based support comprised within the supported metal catalyst utilized in ammonia cracking process disclosed, according to any one of the embodiments above, comprises Sr(II), La(III) and Fe(III), e.g., a combination of SrO, La2Os, and Lao.9Sro.iFeo.8-x03-5-

[0063] In certain embodiments, the Ru(0) atoms comprised within the supported metal catalyst utilized in ammonia cracking process disclosed, according to any one of the embodiments above, constitute from about 0.1% to about 1%, e.g., from about 0.2% to about 0.9%, from about 0.3% to about 0.8%, from about 0.4% to about 0.7%, or from about 0.5% to about 0.6%, by weight, of said supported metal catalyst.

[0064] In certain embodiments, the supported metal catalyst utilized in ammonia cracking process disclosed, according to any one of the embodiments above, is obtained by exsolution, i.e., exposing a perovskite-oxide (starting) material comprising metal ions occupying A sites and B sites of said perovskite-oxide material to a reducing environment (such as hydrogen) or a reducible electrochemical potential, at a temperature of from about 600°C to about 800°C, wherein said metal ions comprise: Ru(III), Fe(III), at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Cu(II) and Ag(I), thereby reducing said Ru(III), Fe(III), and at least one of Cu(II) and Ag(I) when present, to obtain said supported metal catalyst. In particular embodiments, at least 80% of the A sites of said perovskite-oxide starting material are occupied, i.e., up to 20% of the A sites of said perovskite- oxide material may be vacant; and 100% of the B sited of said perovskite-oxide material are occupied, i.e., there no vacant B sites. As shown in the Experimental section herein, the perovskite-oxide material exposed to a reducing environment or a reducible electrochemical potential according to the exsolution process above may comprise: (i) Sr(II), La(III), Ru(III), and Fe(III); or (ii) Sr(II), La(III), Ru(III), Fe(III), and Cu(II). In specific embodiments, said perovskite-oxide material is (i) Lao.9Sro.1Feo.95Ruo.05O3; (ii) La0.9Sr0.1Fe0.85Ru0.05Cu0.1O3; (iii) Lao.9Sro.1Feo.8Ruo.05Cuo.15O3; or (iv) Lao.9Feo.85Ruo.05Cuo.1O3. In particular specific embodiments, said perovskite-oxide material comprises Lao.9Sro.1Feo.8Ruo.05Cuo.15O3, and said exsolution process comprises exposing said perovskite-oxide material to a reducing environment or a reducible electrochemical potential, at a temperature of 680-720°C, e.g., at 690°C, 700°C, or 710°C.

[0065] For purposes of clarity, and in no way limiting the scope of the teachings, unless otherwise indicated, all numbers referring to, e.g., sizes, weight percentages, temperatures, and pressures, used in the present specification are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in this description and claims are approximations that may vary by up to plus or minus 10% depending upon the desired properties sought to be obtained by the invention.

[0066] The invention will now be illustrated by the following non-limiting Examples. EXAMPLES

Study 1. Multi-metallic supported catalyst for low pressure ammonia synthesis prepared by exsolution

Materials and Methods

[0067] Catalyst synthesis. Sol-Gel Synthesis of LaFcOs perovskite with various doping of Sr, Ru, and Cu or Co has been made. La(NO3)3-6H2O, Fe(NO3)3- 9H2O, Sr(NO3)2, RuCh, CU(NO3)2- 3H₂O, CO(NO3)2- 6H₂O, and citric acid (0.384 g, 2 mmol) were mixed by need in deionized water (5 mL). The sample was heated under stirring for 1 hour and was left at 80°C overnight until a gel formed, then left to dry at 120°C overnight. The sample was calcined and crystallized in air in two stages, 600°C for 2 hours and 800°C for 4 hours. Then, La(i-_x)Sr_xFe(i-_y- z)CuyRu_zO3-5 perovskite powder was heated to 650°C/700°C/800°C at reducing atmosphere of 100% H2 for 5 hours to achieve the exsolution process. La(i-_x)Sr_xFe(i-y-_z)CoyRu_zO3-5 and Lap. _x)Sr_xFe(i-_z)Ru_zO3-5 perovskites powders were heated to 650°C at reducing atmosphere of 100% H2 for 5 hours, also for exsolution process.

[0068] Catalyst activity. In order to calculate NH3 rate, 60 seem (standard cubic centimeters per minute) of H2/N2 (molar ratio of 3, H2 purity 99.9999%, N2 purity 99.9999%) flowed over the catalyst at 400°C. The outlet gas was trapped in a 5mM sulfuric acid solution and the amount of NH₄ ⁺ generated in the solution was determined using Nessler’s reagent, the concentration of NH₄ ⁺ in the solution was tested by UV-vis instrument using a standard curve method to calculate the rate of ammonia synthesis. The same process was done over at 370°C, 355°C, 340°C, 310°C to reveal the activation energy using Arrhenius equation.

[0069] Catalyst stability. A Stability test was conducted to better understand the stability of the CuRu catalysts. 50 seem (5% H2/N2) at 650°C flowed for 96 hours over LSFO-Ru, LSFO- CuRu and LSFO-CoRu. The Ru/Cu/CoRu particles of each catalyst were measured by transmission electron microscope (TEM) before and after the stability test. An average of the particles size has been made to compare the particle growth with time in each catalyst.

[0070] Catalyst reversibility. To test the reversibility of Lao.9Sro.1Feo.8Cuo.15Ruo.05O3 after 700°C exsolution, the catalyst powder was placed in a quartz boat in a quartz tube and heated to 800°C under an oxidizing atmosphere of 100% O2 for 12 hours. Before the oxidation, the quartz tube with the sample was purged by 100% N2 from room temperature (RT) to 800 °C. [0071] Catalyst characterization. Powder X-ray diffraction (XRD) measurements were taken on a Bruker AXSA D8 Advance with a Cu Ka (X = 1.5406 A) radiation source and analyzed using the Diffrac.EVA software by Bruker. Hydrogen temperature programmed reduction (H2- TPR) was carried out in a Quantachrome ChemBET Pulsar gs adsorption system. The H2 consumption by the catalyst was measured by a thermal conductivity detector (TCD). Calibration for the H2 consumption was done by CU2O powder. X-ray photoelectron spectroscopy (XPS) was performed using a ESCALAB QXi (Thermo Scientific, USA) system.

[0072] Ru concentration was measured using a dual-view High-Resolution (ICP-OES) spectrometer PlasmaQuant PQ9000, Analytik Jena, Germany. TEM analysis was done using double Cs-corrected HR-S/TEM Titan Themis G2 60-300 (FEI /Thermo Fisher, USA) operated in STEM mode with 200 KeV accelerating voltage. For energy dispersive X-ray (EDX) mapping we used Dual-X detector (Bruker Corporation, USA). EDX maps were acquired, post processed and analyzed using the Velox software (Thermo-Fisher, USA). TEM images were taken also in Philips Tecnai F20 TEM with Schottky FEG, and through JEM-2010F (JEOL, Japan) scanning and transmission electron microscope, equipped with a UHR pole piece and Noran Sistem Six EDS (0.13 sterad collection angle; Thermo Fisher Scientific, USA). Data analysis was performed in Velox and Gatan Microscopy Suite (GMS) software.

[0073] Catalysis characterization. To calculate NH3 rate, 60 seem of H2/N2 (molar ratio of 3, H2 purity 99.9999%, N2 purity 99.9999%) flowed over the catalyst at 400°C. The outlet gas was trapped in a 5mM sulfuric acid solution and the amount of NHA generated in the solution was determined using Nessler’s reagent, the concentration of NHA in the solution was tested by UV- vis spectrophotometer instrument (Shimazu 2600) in the range 350-500 nm. Calibration models of 10 ml DI water with different concentrations of NH4CI (1, 10, 20, 30, 40 ppm) were used to create a calibration curve. The absorption of the calibration samples and the measured sample was taken at a wavelength of 420 nm. Thus, the amount of ppm of ammonium in the sample was calculated. The same process was done over at 370°C, 355°C, 340°C, 310°C to reveal the activation energy using Arrhenius equation.

Results

[0074] Exsolution of ruthenium (Ru) and copper (Cu) was carried out from LaFcOs and Lao.gSro.iFeOs perovskite backbones. All perovskites were prepared using the solgel method, using nitrate- and chloride -based precursors as elucidated in Table 1. Cu was substituted at the expense of Fe in the B-site up to 15 at% and Ru was substituted in the same position up to 5%. X-ray diffraction shows that at these levels, a nearly-pure perovskite can be obtained, in broad agreement with the solubility limits of Cu and Ru in the B-site of LaFcOs as reported by others (Caronna et al., 2009; lervolino et al., 2017; Phan et al., 2018). HAADF-STEM overlayed with EDX elemental mapping shows an even elemental distribution in the precursors. Quantitative temperature programmed reduction (TPR) in 5% H2 of perovskite precursors containing 5 at% Cu or 5 at% Ru revealed that Ru began to exsolve at lower temperatures compared to Cu in the fully stoichiometric backbone (LaFeCh), the A-site deficient backbone (Lao.9 Feo.gsMo.osCF-s, M=Cu, Ru), and the backbone with the non-isovalent strontium-substituted in the A-site (Eao.9Sro.iFeo.95Mo.o503-5, M=Cu, Ru). Between the three stoichiometric variations, quantitative analysis of TPR peak associated with Ru⁶⁺ reduction to Ru° (confirmed by XPS) showed that exsolution was most efficient in the backbone variation with the Sr-substituted A-site. Here, non- isovalent substitutions change the oxygen stoichiometry and promote better exsolution by blocking the formation of residual stoichiometric perovskite as opposed to A-site oxides (i.e., Ea2Os) (Neagu et al., 2013). Consequently, most of the precursors discussed herein contain the 10 at% Sr substitution for Fa. Atomic -resolution high angle annular dark field microscopy with energy dispersive X-ray spectroscopy (HAADF-EDX) of Lao.gSro.iFeo.ssRuo.osCuo.iOs-s supports the findings from TPR showing that after TPR (ramping to 650°C in 5% H2), both 1 nm Ru and Cu particles were already formed.

Table 1: Sol-gel precursor compositions used for preparation of various LaFcOs and Eao.9Sro.iFe03 perovskites (values are given in grams) [0075] It is well known that Cu and Ru are insoluble according to their equilibrium phase diagram, but also that strain can influence miscibility). Furthermore, non-equilibrium growth processes have shown that matching the growth kinetics of Cu and Ru can force Cu-Ru alloy formation by Ostwald ripening (Huang et al., 2020; Cui et al., 2021). It is therefore instructive to determine whether reasonable strain or size effects during the exsolution of the commonly immiscible Ru and Cu metals would significantly modulate their solubility with one another, in contrast to the expected behavior in the bulk from an equilibrium phase diagram. The relative change in solubility of Ru in Cu from its stress-free value can be estimated by the ratio of a change in chemical energy due to a change in composition G^(c — c₀) due to the elastic energy density, oe, where c is the composition in the presence of stress, c₀ is the stress-free equilibrium composition, G^ is the second derivative of the molar free energy with respect to composition, <j is a characteristic shear stress, and e is a characteristic strain. For a dilute solution G^ = RT /c₀ and thus the relative change in composition due to stress is, (c — c₀)/c₀ ~ aeV_m/RT. At the temperature of the typical exsolution in this study (700°C), using a large stress of 1 GPA and strain of 0.1, gives an approximate upper bound on the relative shift in composition of ~ 2%. It is thus unlikely that stress can shift the equilibrium phase diagram to yield s considerable solubility of Ru and Cu, and that the calculated ternary Fe-Cu-Ru equilibrium phase diagram represents, to a good approximation, the miscibility of exsolved metallic nanoparticles.

[0076] ThermoCalc version 2022b with the SSOL4/SGTE alloy solutions database (Andersson et al., 2002) was used to predict the thermodynamically stable phases exsolved from Lao.9Sro.iFeo.85Ruo.o5Cuo.i03-5 (and similar stoichiometric variations) via calculation of the bulk equilibrium phase diagram for the Fe-Cu-Ru system at 700°C, shown in Fig. 1. While there is insufficient data for alloys in this system with Ru content above 40 mol.% to reliably predict all of the stable phases, step diagrams (Fig. 4) can predict the phase composition in such region when one of the component compositions is held constant. In both types of diagrams, the phases are denoted by their crystal structure using Strukturbericht designation.

[0077] According to the Fe-Cu-Ru ternary phase diagram, the addition of Fe to the Cu-Ru system can allow for a single-phase trimetallic particle with the HCP structure (HCP_A3). If no Fe is exsolved, the surface metal composition resulting from complete exsolution of Cu and Ru from precursors such as Lao.gSro.iFeo.ssRuo.osCuo.iOs-s should be 33 at% Ru and 67 at% Cu and unalloyed, in line with its ratio in the B-site of the precursor. These percentages decrease as Fe is exsolved from the perovskite with increasing exsolution temperature, bringing the surface metal composition closer to the bottom-left corner of the ternary phase diagram (calculated ternary diagram for Fe-Cu-Ru at 800°C is shown in Fig. 5). Under these conditions, multiple solid-solution phases are predicted based on the FCC lattice of Cu (FCC_A1), the BCC lattice of a-Fe (BCC_A2) and/or the HCP lattice of Ru (HCP_A3).

[0078] Previous reports of alloy exsolution have suggested that the exsolution take place first, following by alloying between the formed nanoparticles by surface diffusion (Kwon et al., 2018). This mechanism, together with the expectation that strain does not significantly modulate the equilibrium phase diagram, predicts that exsolved nanoparticles from a B-site with a composition inside the multi-phase region should have a multiphase heterogeneous structure with multiple nanoparticles in proximity (if not touching), each rich in one element.

[0079] Here, we show how the exsolution of largely immiscible phases made from Cu, Ru, and Fe can be used to catalyze ambient pressure thermochemical ammonia synthesis at 400° C. At high pressures, under conditions of the well-known Haber-Bosch process, N2 and H2 are believed to adsorb and chemically dissociate on Fe-based catalysts via the Langmuir- Hinshelwood mechanism (Sajid et al., 2022; Qian et al., 2019). While Fe shows some activity at lower pressures, Ru is typically used since Fe on its own does not perform as well (Hara et al., 2017; Inoue et al., 2016; Aika etal., 1986). Ruthenium sites for low-pressure ammonia synthesis are characterized by high activation for N2 cleavage (the rate limiting step (Kitano et al., 2012)), low activation for H2, and high sticking for H2 leading to hydrogen poisoning (Huang et al., 2020; Kadowaki and Aika, 1996; Lin et al., 2021). It is therefore likely that a system with Ru- Fe alloy sites would balance activity with poisoning leading to enhanced activity. Indeed, transition state potential energies for N2 dissociation on Fe/Ru (0001) was shown to be lower than Ru (0001) (Logadottir et al., 2001). Interestingly, Cu (0001) was shown to have an extremely high potential energy for the rate limiting step of this reaction, and therefore would not be expected to be a good catalyst; herein, the utility of adding Cu as a promoter is shown.

[0080] Table 2 shows the impact of precursor composition and exsolution temperature on the rate of thermal NH3 synthesis carried out at 0.1 MPa and 400°C. The stoichiometric lanthanum ferrite perovskite (LaFcOs) on its own, when treated in hydrogen at 650°C for 5h in H2 showed poor catalytic activity towards ammonia synthesis and fell below the Nessler calibration curve used for this study (10 mmolNH3/gcat/h). For this sample, only oxygen vacancies are formed (confirmed by steam formation during reduction and change in lattice parameter), without the exsolution of metallic Fe. When 5 at% Ru was substituted in the B-site, Ru exsolution at 650°C catalyzed ammonia synthesis at a rate of 0.54 mmolNH3/gcat/h. This rate tripled when the exsolution temperature was raised to 800°C owing to the increased amount of ruthenium which was exsolved from the backbone along with the co-exsolution of metallic Fe (characteristic STEM EDX is shown in Fig. 6).

[0081] The rate of ammonia production was significantly improved by the addition of copper to the B-site. This was shown by comparing the ammonia production rate to the copper-free variant at the same exsolution temperature (650°C) and ruthenium concentration (5 at%). Further Cu substitution up to 10 at% (Lao.gSro.iFeo.ssRuo.osCuo.iOs-s) and 15 at% Lao.9Sro.iFeo.8oRuo.o5Cuo.i503-5 improved the ammonia production rate compared to the 5 at% sample by 1.35-fold and 2.37-fold, respectively. This is of significant interest because Cu is not a known catalyst for either the high-pressure or low-pressure thermochemical synthesis of ammonia due to its ineffectiveness at cleaving the H-H bond or N = N triple bond. Consequently, exsolution of a Cu-only variant (Lao.9Sro.iFeo.9oCuo.i03-5) at 650°C did not produce ammonia above the Nessler detection limit. This indicates that Cu acts as a promoter for the Ru-catalyzed reaction and is not catalytic in and of itself.

Table 2: NH3 Synthesis rate as a function of precursor composition and exsolution temperature

^§ Reactor at 0.1 MPa and 400°C, measured using the Nessler reagent method.

[0082] Although improvement in catalytic activity was observed when the exsolution temperature was increased from 650°C to 800°C owing to increased metal content, this came at the expense of decomposing the perovskite backbone to Ea2O3 and FeO (Figs. 6-7). Both Ea2O3 and FeO are not capable of supporting the high concentrations of oxygen vacancies (OVs) compared to the perovskite phase (Ei and Metiu, 2010), which can be detrimental to ammonia synthesis since they can act as active sites for nitrogen activation and cleavage (Lin et al., 2018; Marakatti and Gaigneaux, 2020). The ideal catalyst should therefore provide sufficient metal Ru, Fe, and Cu sites, together with a high concentration of oxygen vacancies, all while still maintaining the perovskite backbone.

[0083] This balance was achieved when the exsolution temperature of the best-performing precursor (Lao.gSro.iFeo.soRuo.osCuo.isCE-g) was increased from 650°C to 700°C. Under this condition, the ammonia production rate increased by more than 5.9-fold to a rate of 7.54 mmolNH3/gcat/h. ICP-OES of all the Ru-containing catalysts in Table 2 showed the Ru content to be only 1 wt%, making this a cost-effective low-pressure ammonia synthesis catalyst. The activation energy for this highly active catalyst was found to be 71.2 kJ/mol (Fig. 8), in the same range as other highly active ambient pressure ammonia catalyst (Kitano et al., 2012).

[0084] The influence of the partial pressure of both H2 and N2 was determined by a kinetic study while fitting the reaction rate to the expression: r=kPH2hPN2npNH4a, where h, n, and a are the reaction orders for hydrogen, nitrogen, and ammonia respectively. The gas-hourly space velocity during the kinetic experiments was fixed at 280 L/gcatxhour, while the ratio between the hydrogen and nitrogen flows varied between 1.5 to 5. The order for nitrogen, n, was measured to be 1.06 for Lao.9Sro.1Feo.soRuo.05Cuo.15O3 s exsolved at 700°C, supporting previous reports that the first-order nature of the reaction with nitrogen is connected to the fact that nitrogen scission is the rate-limiting step (Imamura et al., 2019). The reaction orders for hydrogen, h, and ammonia, a, were found to be -0.35 and -0.405, respectively, for the same catalyst. The ammonia order was measured by the technique described by Chorkendorff (Hagen et al., 2003). By comparison, Ru- based catalysts under the same conditions supported by AI2O3, and MgO have been reported to have h values of between -0.1 and -0.7 with activation energies between 70 and 125 kJ/mol (Rosowski et al., 1997).

[0085] The unexpected role of Cu as a promoter may be explained by three possible mechanisms. The first mechanism is by modulating the size and stability of the exsolved Ru particles. TEM images showed that the Ru-rich particles were smaller when Cu was co-exsolved, compared their size to when Cu was not present. This can be explained by the lack of solubility between Ru and Cu from their binary phase diagram. When only Ru is exsolved (Lao.9Sro.iFeo.95Ruo.o503-5), particles will undergo agglomeration during the exsolution and catalytic process, which both occur at high temperatures under highly reducing conditions. This agglomeration requires the solid-state surface diffusion of Ru nanoparticles, contact between adjacent particles, and eventual coalescence. When Cu particles are co-exsolved, the surface is intermixed with mutually immiscible Cu and Ru particles as shown in Fig. 2a using STEM-EDX elemental mapping for Ru, Cu and Fe. [0086] This anti- agglomeration property of Cu on Ru is quantified in Table 3 by measuring the Ru and Cu particle sizes after 5h of exsolution at 650°C (initial), and again after maintaining the same material in a reducing environment (5 vol.% H2 in N2) for an additional 96 hours (growth +96h). When only Ru particles exsolved, their size grew by 143% over 96 hours at 650°C, whereas the same Ru particles remarkably showed no growth under the same conditions when Cu was co-exsolved. As a counterexample, this stabilization effect was not seen when Ru is co-exsolved with a metal such as cobalt (Co), with which it has mutual solubility over a wide compositional range. In the case of Ru-Co co-exsolution, alloy particles can form and likewise agglomerate similarly to the case of the pure Ru particles. The Cu particles therefore act as diffusion barriers (Fig. 2b) for the Ru nanoparticles, restricting the ability of Ru to agglomerate by convoluting the path that a Ru nanoparticle would have to move through to meet another Ru particle, shown in schematically Fig. 2c. This effect restricts the growth of Ru particles to 1.8 nm, an ideal size for Ru particles for ammonia synthesis. This is due to the fact that at 1.8 nm, the number of Bs-type active sites and edge site is maximized, compared to if the Ru particle were larger or smaller [49]. From this perspective, agglomeration of Ru particles to 3 nm would cut the number of active sites in half, making the addition of Cu profoundly important towards the activity and stability of the catalyst.

Table 3: Agglomeration of particles after 96h at 650°C in H2

N=400 particles

[0087] The second mechanism for Cu promotion of ammonia synthesis is by the formation of additional oxygen vacancies (OVs) in the backbone of the perovskite. OVs have been shown absorb 1V and promote electron exchange, facilitating the cleavage of the N = N bond [50], improving ammonia catalysis by improving this rate limiting step. XPS results of the as- synthesized showed that Fe and Cu were present in the precursor perovskite in the +3 and +2 oxidation state, respectively (Fig. 9). The non-isovalent substitution of Cu for Fe in the formation of oxygen vacancies to maintain charge balance under reduction conditions. The number of vacancies is further increased from this initial concentration during the exsolution process as metals were exsolved to their metallic state, provided that the perovskite structure did not decompose, as was observed when exsolution occurred at an exsolution temperature which was too high (800°C, Figs. 6-7).

[0088] A third possible mechanism for Cu promotion is by providing additional hydrogen spillover capacity. Elementary steps of hydrogen cleavage and H-N bond formation are reported to take place effectively on the Ru surface at low pressure (e.g., 0.1 MPa). The effectiveness of Ru to cleave the H-H bond (85 kJ/mol) compared to the N = N bond (+149 kJ/mol) leads to hydrogen poisoning and a limitation on the rate of ammonia production, exacerbated by the fact that the sticking coefficient for nitrogen on Ru is generally weaker than hydrogen on Ru (Danielson et al., 1978). Owing to immiscibility, Cu particles can exsolve nearly in contact with the Ru or Ru-Fe alloy active sites, whereby providing spillover sites for the excess hydrogen and allowing more active sites for NH3 formation. The spillover of hydrogen onto Cu (Feng et al., 2018; Goodman and Peden, 1985) and specifically from Ru (Goodman and Peden, 1985; King et al., 1986; Alvarez-Rodrfguez et al., 2006; Eiu et al., 2013; Floyd et al., 1997) is seldom reported, but shown by others through experiment and simulation to occur.

[0089] Fig. 3 emphasizes the subtle differences in structure between exsolution from Eao.9Sro.iFeo.85Ruo.o5Cuo.i03-5 at 650°C and 700°C, a small temperature gap over which a marked improvement of ammonia production was observed (Table 2). Local EDX measurements show that the exsolved Ru particles at 650°C contained a minimal amount of Fe (<2 at%) compared to exsolution at 700°C where FeRu alloy particles were observed with a Fe:Ru ration of about 2:1 (Figs. 3f, 3g). Despite the FeRu alloying, the local Cu content adjacent to the FeRu particle is low enough to maintain phase separation, forming a FeRu+Cu hetero structure (Fig, 3h) and in line with the FCC_A1 + HCP_A3 region of the phase diagram.

[0090] The other major change was that the Sr distribution dramatically changed within this 50°C gap. At 650°C, Sr was strongly segregated to the surface and grain boundaries (Fig. 3e), but at 700°C, segregation gives way to phase separation, and SrO particles are formed. (Fig. 3e). The improved catalytic activity between precursors exsolved at 650°C and 700°C is aided by the formation of OVs as Sr was removed entirely from the perovskite lattice to form SrO.

[0091] Finally, an important advantage of exsolution catalysts over impregnated catalysts is the ability to recover from the impact of agglomeration by oxidizing the catalysts to its original precursor form (ingress of metal nanoparticles back into the backbone) followed by fresh exsolution of fresh non- agglomerated particles (Danielson et al., 1978; Onn et <2/., 2018; Kwon et al., 2022). Fig. 10 shows XRD of the original perovskite Lao.gSro.iFeo.ssRuo.osCuo.iOs-s, the catalyst after exsolution at 700°C in hydrogen for 5h, and the oxidation of that catalyst by exposure to oxygen for 12h at 800°C. The higher temperature for the oxidation step accounts for the fact that the ingress kinetics of particles into the backbone is typically slower than the kinetics of the exsolution. After exsolution, the (202) plane of the perovskite shifts to lower angles by 0.07°, indicating lattice expansion due to the incorporation of oxygen vacancies. Furthermore, small peaks associated with Fe and Cu are clearly visible. After the oxidation procedure, the original precursor perovskite is restored as the hetero- structured metal particles were oxidized, and the metal ions were reincorporated back into the perovskite. This is of great interest because A-site deficient perovskites are less likely to exhibit re-dissolution and reversibility owing to the relative stability of stoichiometric and non-stoichiometric perovskites and the position of the exsolved nanoparticle relative to its socket (Neagu et al., 2013; Chanthanumataporn et al., 2019; Neagu et al., 2015). While the perovskite precursor studied here (Lao.9Sro.1Feo.sRuo.05Cuo.15O 3-d was initially stoichiometric (albeit substituted), the process of exsolution drove Sr segregation and eventually phase separation of Sr from the A-site (Fig. 3e). This means that at the time of exsolution, the nature of the precursor was in fact A-site deficient.

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Claims

1. A supported metal catalyst comprising nanoparticles made of same or different metal atoms, anchored to an oxide -based support comprising metal ions, wherein said metal atoms comprise Ru(0), Fe(0), and optionally at least one of Cu(0) and Ag(0); and said metal ions comprise at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Fe(III) and Fe(II).

2. The metal catalyst of claim 1, comprising nanoparticles made of Ru(0) only and nanoparticles made of Fe(0) only; and/or nanoparticles made of both Ru(0) and Fe(0).

3. The metal catalyst of claim 2, wherein the size of the nanoparticles made of Ru(0) only is from about 0.5 nm to about 20 nm; the size of the nanoparticles made of Fe(0) only is from about 1 nm to about 20 nm; and/or the size of the nanoparticles made of both Ru(0) and Fe(0) is from about 1 nm to about 20 nm.

4. The metal catalyst of claim 2 or 3, wherein each one of said nanoparticles independently is further coated with Cu(0).

5. The metal catalyst of claim 4, wherein the Cu(0) atoms constitute from about 0.1% to about 5% by weight of said supported metal catalyst.

6. The metal catalyst of claim 2 or 3, further comprising nanoparticles made of Cu(0) only.

7. The metal catalyst of claim 6, wherein the size of the nanoparticle made of Cu(0) only is from about 1 nm to about 20 nm.

8. The metal catalyst of claim 1, comprising nanoparticles made of Ru(0), Fe(0) and Cu(0).

9. The metal catalyst of claim 8, wherein the size of the nanoparticles made of Ru(0), Fe(0) and Cu(0) is from about 1 nm to about 20 nm.

10. The metal catalyst of any one of claims 1 to 9, wherein said oxide-based support comprises Sr(II), La(III) and Fe(III).

11. The metal catalyst of any one of claims 1 to 10, wherein said Ru(0) atoms constitute from about 0.1% to about 1% by weight of said supported metal catalyst.

12. The metal catalyst of claim 1, obtained by exposing a perovskite material comprising metal ions occupying A sites and B sites of said perovskite material to a reducing environment or a reducible electrochemical potential, at a temperature of from about 600°C to about 800°C, wherein said metal ions comprise: Ru(III), Fe(III), at least two of Sr(II), La(III), Ca(II), Ba(II), Pr(III), Nd(III), Zr(III), Y(III), and Cs(I), and optionally at least one of Cu(II) and Ag(I), thereby reducing said Ru(III), Fe(III), and at least one of Cu(II) and Ag(I), when present, to obtain said supported metal catalyst.

13. The metal catalyst of claim 12, wherein said reducing environment is hydrogen environment.

14. The metal catalyst of claim 12 or 13, wherein at least 80% of the A sites of said perovskite material are occupied and 100% of the B sited of said perovskite material are occupied.

15. The metal catalyst of any one of claim 12 to 14, wherein said perovskite material comprises: (i) Sr(II), La(III), Ru(III), and Fe(III); or (ii) Sr(II), La(III), Ru(III), Fe(III), and Cu(II).

16. The metal catalyst of claim 15, wherein said perovskite material is selected from: (i) Lao.9Sro.1Feo.95Ruo.05O3; (ii) Lao.9Sro.1Feo.85Ruo.05Cuo.1O3; (iii) Lao.9Sro.1Feo.sRuo.05Cuo.15O3; and (iv) La0.9Fe0.85Ru0.05Cu0.1O3.

17. The metal catalyst of claim 16, wherein said perovskite material is Lao.9Sro.1Feo.8Ruo.05Cuo.15O3, and said temperature is from 680°C to 720°C, e.g., about 700°C.

18. A method for production of ammonia, comprising reacting nitrogen and hydrogen in the presence of a supported metal catalyst according to any one of claims 1-17, to thereby obtain said ammonia.

19. The method of claim 18, wherein said reaction is carried out at a pressure lower than about 10 MPa, and/or at a temperature of up to about 400°C.

20. The method of claim 19, wherein said reaction is carried out at a pressure of from about 3 MPa to about 5 MPa, and/or at a temperature of about 400°C.

21. A method for production of ammonia comprising feeding nitrogen to an electrolytic cell comprising at least one source of protons; allowing the nitrogen to come into contact with the surface of a cathode electrode in the electrolytic cell, wherein the surface of the cathode electrode comprises a supported metal catalyst according to any one of claims 1-17; and running a current through said electrolytic cell, whereby the nitrogen reacts with the protons to form said ammonia.

22. The method of claim 21 , wherein said reaction is carried out at or near room temperature, and/or using a voltage of up to about 3 V.

23. The method of claim 22, wherein said reaction is carried out at a temperature of about 18-22°C.

24. A process for ammonia cracking comprising the step of exposing gaseous anhydrous ammonia to a temperature of at least 200°C and a pressure of at least 0.01 bar, in the presence of a supported metal catalyst according to any one of claims 1-17, to thereby obtain a gas mixture comprising hydrogen and nitrogen.

25. The process of claim 24, wherein said gaseous anhydrous ammonia is exposed to a temperature of 200-550°C, preferably about 450°C, and/or a pressure of 0.01-1 bar, preferably about 0.05-0.15 bar.

26. The process of claim 24, carried out within an ammonia cracker.

27. The process of any one of claims 24-26, further comprising (i) a purification step to reduce the amount of uncracked ammonia, when present in said gas mixture; and/or (ii) a step for separating said hydrogen from said gas mixture.