Process for electrolytic production of ammonia from nitrogen using metal carbonitride catalysts

Abstract

The invention relates to a process and system for electrolytic production ammonia. The process comprises feeding nitrogen to an electrolytic cell, where it comes in contact with a cathode surface, wherein said surface has a catalyst surface comprising at least one transition metal carbonitride, the electrolytic cell further comprising a proton donor, and running a current through said electrolytic cell, whereby nitrogen reacts with protons to form ammonia.

Description

[0001] P15459PC00 PROCESS FOR ELECTROLYTIC PRODUCTION OF AMMONIA FROM NITROGEN USING METAL CARBONITRIDE CATALYSTS FIELD OF INVENTION The disclosure is within the field of process chemistry, and specifically relates to the catalytic production of ammonia from nitrogen with electrolytic methods using novel transition metal carbonitride catalysts for the cathode surface. BACKGROUND Ammonia is a high value chemical owing to its use in fertilizers, with an annual production of 175 million tonnes, but its potential as an energy storage material and a carbon-free maritime fuel have recently gained attention. For more than a century, ammonia has been produced via the Haber-Bosch process, in which nitrogen and hydrogen gases react over a promoted Fe / Ru catalyst at a temperature exceeding 400°C and pressure upwards of 150 bar; N2(g)+ 3H2(g)→ 2NH3(g)This leads to substantial carbon emissions, owing mostly to the production of hydrogen gas via steam reforming, but also to the extreme reaction conditions rendering the process highly energy consuming. These are in stark contrast to the ambient conditions at which the enzyme nitrogenase in bacteria naturally catalyses nitrogen reduction from solvated protons, electrons and atmospheric nitrogen; N2(g) + 8H++ 8e- → 2NH3(g) + H2(g) This natural process of nitrogen fixation has inspired the search and development of catalysts that can work at (much milder) ambient conditions. However, as is common knowledge, the search for a suitable catalyst for a specific reaction is both challenging and complex, wherein a search needs to be based on factors such as availability, cost, stability, activity, and selectivity of the catalyst material under the desired operational conditions. In particular, the realisation of the electrochemical nitrogen reduction reaction (NRR) using solid catalytic electrodes in an aqueous solution, with the protons coming from water splitting at the anode, has been an active field of research, both computationally and experimentally [1]. Despite the research interest and recent efforts, little progress has been made in finding a suitable catalyst for NRR with the main culprit being the competing hydrogen evolution reaction (HER), as most of the materials that are active towards the NRR are more selective towards HER, especially at higher overpotentials. This leads to extremely low Faradaic efficiencies, i.e., most of the P15459PC00 electrical energy supplied to the system is being wasted on the HER rather than being utilized for the NRR. To emphasize the challenge of finding a suitable NRR catalyst, a few examples from the literature and from previous efforts by the applicants are discussed below. Out of the pure transition metals, namely, Sc, Y, Ti, Zr Re, Os, Co, Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, V, Nb, Ta, Cr, Mo, W, and Fe it has been predicted (with computational studies) that none of the pure transition metals is capable of catalysing the NRR because of the facile H2formation. [2] The linear scaling relations of adsorption energies for the pure metals in the periodic table predict that they are not able to catalyse this reaction. Some of these predictions have been verified in experiments, namely for Ru, Rh and Re, with strict protocols used to avoid false positive results. [3] In fact, it is safe to say that efficient ammonia synthesis in an aqueous solution verified with strict experimental protocols remain a challenge, with the literature littered with likely false positives [3,4,5]. Recent efforts by the present applicants have focused on novel ceramics materials, i.e., using transition metal nitride (TMN), transition metal oxide (TMO) and transition metal sulphide (TMS) catalyst surfaces, as disclosed in WO2015189865, WO2019053749 and WO2020110155, respectively. Despite these efforts, most investigated materials still catalyse HER over NRR. The identification of a promising catalyst is one of the steps needed to realise electrochemical NRR at low to moderate overpotentials. The present invention seeks to ameliorate these problems by providing novel catalyst compounds capable of catalysing the electrochemical nitrogen reduction reaction (NRR) at mild operational conditions such as in an aqueous solution, using low to moderate overpotentials / voltage. References: [1] Guo, W., Zhang, K., Liang, Z., Zou, R., and Xu, Q. (2019). Electrochemical nitrogen fixation and utilization: theories, advanced catalyst materials and system design. Chem. Soc. Rev.48, 5658-5716. [2] Skúlason, E., Bligaard, T., Gudmundsdóttir, S., Studt, F., Rossmeisl, J., Abild-Pedersen, F., Vegge, T., Jónsson, H., and Nørskov, J.K. (2012). A theoretical evaluation of possible transition metal electro-catalysts for N2 reduction. Phys. Chem. Chem. Phys.14, 1235-1245. [3] Andersen, S.Z., Colic, V., Yang, S., Schwalbe, J.A., Nielander, A.C., McEnaney, J.M., Enemark-Rasmussen, K., Baker, J.G., Singh, A.R., Rohr, B.A., et al. (2019). A rigorous P15459PC00 electrochemical ammonia synthesis protocol with quantitative isotope measurements. Nature. 570, 504-508. [4] Tang, C., and Qiao, S.-Z. (2019). How to explore ambient electrocatalytic nitrogen reduction reliably and insightfully. Chem. Soc. Rev.48, 3166-3180. [5] Suryanto, B.H.R., Du, H.-L., Wang, D., Chen, J., Simonov, A.N., and MacFarlane, D.R. (2019). Challenges and prospects in the catalysis of electroreduction of nitrogen to ammonia. Nat. Catal.2, 290-296 SUMMARY The present invention is specified in the claims as well as in the below description. In broad terms of the present invention, the invention relates to the electrochemical synthesis of ammonia from molecular nitrogen, i.e., via the electrochemical nitrogen reduction reaction. In one embodiment, the process may be carried out in an aqueous solution at ambient conditions and preferably at low to moderate overpotentials by using a highly specific, carefully selected, catalysts material. As discussed above, the competitive HER and the instability of promising catalyst candidates (i.e., the proneness of these materials to decompose) renders the search for viable and stable catalyst that is selective towards NRR extremely challenging. Transition metal carbonitrides is a class of compounds that exists in a variety of crystal structures such as, but not limited to, rock salt. These materials generally show high strength, hardness, melting temperature, corrosion resistance, conducting properties and most importantly high chemical stability. Furthermore, it is found by the applicants that this class of compounds may show higher catalytic activity of NRR compared to the respective parent transition metal due to the introduction of heteroatoms (carbon and nitrogen) that influences the d-band electronic structure of parent metals, in turn optimizing the bond strengths between metal and absorbed reactive intermediates. Therefore, the present invention relates to identification and selection of specific transition metal carbonitrides catalysts capable of catalysing the electrochemical NRR. This surprising discovery is obtained after rigorous experimentation (large scale calculations and modelling) and comprehensive analysis of the data as is discussed below in the example. Moreover, given the importance of ammonia production and the energy intensive and environmentally unfavourable conditions typically required to produce ammonia, the invention finds important applicability in a plethora of industries such as, but not limited to, the production of fertilizer. P15459PC00 In the first aspect of the invention a process for producing ammonia is provided, said process comprising feeding N2to an electrolytic cell that comprises a cathode, an anode, an electrolyte and at least one source of protons; allowing the N2to come into contact with an electrode surface of the cathode in the electrolytic cell, wherein said electrode surface comprises at least one catalyst surface comprising at least one transition metal carbonitride; and running a current through said electrolytic cell, whereby nitrogen reacts with protons to form ammonia. In an embodiment of the present invention, the at least one catalyst surface comprising at least one transition metal carbonitride may be selected from a group of transition metal carbonitrides comprising; Yttrium Carbonitride (YCN), Scandium Carbonitride (ScCN), Hafnium Carbonitride (HfCN), Zirconium Carbonitride (ZrCN), Titanium Carbonitride (TiCN), Vanadium Carbonitride (VCN), Chromium Carbonitride (CrCN), Niobium Carbonitride (NbCN), Tantalum Carbonitride (TaCN), Molybdenum Carbonitride (MoCN), and Tungsten Carbonitride (WCN) (also referred to as Wolfram Carbonitride). Therefore, in accordance with the present invention, a process for producing ammonia is provided, wherein the process comprises: feeding N2 to an electrolytic cell that comprises a cathode, an anode, an electrolyte and at least one source of protons, allowing the N2 to come into contact with at an electrode surface of the cathode in the electrolytic cell, wherein said electrode surface comprises at least one catalyst surface selected from the group consisting of: Vanadium carbonitride, Niobium carbonitride, Tungsten carbonitride, Titanium carbonitride, Tantalum carbonitride, Hafnium carbonitride and Zirconium carbonitride, and, running a current through said electrolytic cell, whereby nitrogen reacts with protons to form ammonia. In an embodiment, the at least one catalyst surface comprises one or more transition metal carbonitrides selected from the group consisting of Vanadium carbonitride, Niobium carbonitride and Tungsten carbonitride. In one embodiment, the at least one catalyst may comprise a Rocksalt structure. In one embodiment, the catalyst surface may comprise at least one surface having a (100) facet. In another embodiment, the catalyst surface may comprise at least one surface having an exposed (111) facet. Other surface facets are also well encompassed by the present invention. In one embodiment, the catalyst surface may comprise at least one defect such as vacancies formed during or prior to operation of the catalyst, i.e., either intentionally during the manufacturing of the catalyst or during operation of the catalyst. A carbon vacancy may be P15459PC00 formed by the formation and release of carbon containing molecular species such as, but not limited to, methane (CH4). A nitrogen vacancy may be formed by the formation and release of nitrogen containing molecular species such as, but not limited to, ammonia (NH3). In one embodiment, the catalyst surface may comprise at least one nitrogen vacancy and / or carbon vacancy. In one such embodiment, the surface coverage of nitrogen vacancies may range from about 1% to 30% (vacancies / atoms) or range from about 5% to 25% or range from about 10% to 20%. In one such embodiment, the surface coverage of carbon vacancies may range from about 1% to 30% (vacancies / atoms) or range from about 5% to 25% or range from about 10% to 20%. In one embodiment, ammonia may be formed in the electrolytic cell at an electrode potential at less than about -1.2 V, more preferably less than about -0.6 V and even more preferably less than about -0.3 V, using a reversible hydrogen electrode (RHE) as a reference. At high electrode potentials, e.g., -1.2 V, or even -1.0 V, a material may be prone to favour the hydrogen evolution reaction over the nitrogen reduction reaction. Accordingly, it is an advantage of the present invention that the materials may be operatable to form ammonia at low-to-moderate electrode potentials such as lower than -1.0 V, or lower than -0.8 V, or lower than -0.6 V, e.g. -0.5 V or lower. In one embodiment, a cyclic varied potential may be used, wherein the cyclic varied potential fluctuates between an active potential and a resting potential to generate a cyclic varied potential through the electrolytic cell. In one embodiment, less than 50% moles of H2 may be formed compared to moles NH3 formed, and preferably less than 20% and even more preferably less than 10%. In other words, the catalysts favours NRR over HER. In an embodiment, the electrolytic cell may comprise one or more aqueous electrolytic solution. The aqueous electrolytic solution may be acidic, neutral or alkaline. However, in the preferred embodiment, the aqueous solution is neutral. In one embodiment, the electrolytic solution may comprise an organic protic or aprotic solvent or a miscible mixture thereof, or preferably a water-miscible organic solvent. In one embodiment, the electrolytic cell may comprise an anode within one cell compartment and a cathode within another cell compartment. P15459PC00 In one embodiment, the source of protons in the formation of ammonia is from water splitting at the anode or H2oxidation reaction in the anode. In a standard embodiment, the process of producing ammonia is preferably carried out at a temperature in the range from about -10°C to about 80°C, preferably in a range from about 10°C to about 50°C, more preferably in the range from about 20°C to about 30°C, even more preferably in the range from about 20°C to about 25°C which may also be seen as a benefit of the present invention. In one embodiment, the process may be carried out at atmospheric pressure. Alternatively, the process may be carried out at a pressure in the range of 1 to 30 atmospheres, preferably in the range of 1-20 atmospheres, preferably in the range of 1-10 atmospheres, more preferably in the range of 1-5 atmospheres. In the preferred embodiment, the process is carried out at atmospheric pressure. In one embodiment, nitrogen may be fed to the electrolytic cell by bubbling nitrogen gas to the electrolytic solution in contact with said cathode surface. In another embodiment, feeding N2 to the electrolytic cell comprises feeding gaseous nitrogen or air or liquid with dissolved nitrogen to the electrolytic cell. In the second aspect of the invention, a system for generating ammonia is provided. The system comprising at least one electrochemical cell, which comprises at least one cathode having at least one catalytic surface, wherein the at least one catalytic surface comprises at least one catalyst comprising one or more transition metal carbonitride(s). In an embodiment of the present invention, the at least one catalyst surface comprising at least one transition metal carbonitrides may be selected from a group of transition metal carbonitrides including: Yttrium Carbonitride, Scandium Carbonitride, Hafnium Carbonitride, Zirconium Carbonitride, Titanium Carbonitride, Vanadium Carbonitride, Chromium Carbonitride, Niobium Carbonitride, Tantalum Carbonitride, Molybdenum Carbonitride and Tungsten Carbonitride. Therefore, in accordance with the invention, a system for generating ammonia may be provided, wherein the system comprises at least one electrochemical cell, which comprises at least one cathode having at least one catalytic surface, wherein the at least one catalytic surface comprises at least one catalyst selected from the group consisting of: Vanadium carbonitride, Niobium carbonitride, Tungsten carbonitride, Titanium carbonitride, Tantalum carbonitride, Hafnium carbonitride and Zirconium carbonitride. P15459PC00 In a preferred embodiment, the at least one catalyst may be selected from the group consisting of Vanadium carbonitride, Niobium carbonitride and Tungsten carbonitride. In one embodiment, the at least one catalyst may comprise a Rocksalt structure. In one embodiment, the catalyst surface may comprise at least one surface having a (100) facet exposed. Alternatively, or additionally, the catalyst surface may comprise at least one surface having a (110) facet. In one embodiment, the catalyst surface may comprise at least one C-vacancy and / or at least one N-vacancy. In an embodiment, the electrolytic cell may comprise one or more electrolytic solution, preferably an acidic, neutral or alkaline aqueous solution. In one embodiment, the electrolytic solution may comprise an aqueous water-miscible organic solvent. In one embodiment, the electrolytic cell comprises an anode within one cell compartment and a cathode within another cell compartment. In a standard embodiment, ammonia is formed in the electrolytic cell at an electrode potential at less than about -1.2 V, more preferably less than about -0.6 V and even more preferably less than about -0.3 V, using a reversible hydrogen electrode (RHE) as a reference, which may also be considered a benefit of the present invention. In one embodiment, a cyclic varied potential may be used, wherein the cyclic varied potential fluctuates between an active potential and a resting potential to generate a cyclic potential through the electrolytic cell. BRIEF DESCRIPTION OF FIGURES The skilled person will understand that the figures, described below, are for illustration purposes only and provide support to the example presented below. The figures are not intended to limit the scope of the present teachings in any way. Figure 1 shows a model diagram of a 5-layer transition metal carbonitride catalyst, used in the computer simulations. The atoms are labelled with “M” for transition metal, “C” for carbon and “N” for nitrogen. In Figure 1(a) a top-view of the surface is shown with the black-box representing the unit cell. In Figure 1(b) a P15459PC00 side-view of the surface is shown with the black-box representing the two bottom layers being fixed. Figure 2 shows a histogram that for comparison of the adsorption energies of various nitrogen and oxygen species onto clean TMCN surfaces, in addition to the protonation of metal, nitrogen, and carbon site at zero potential. *N2denotes the associative binding of N2, *2N denotes the dissociative binding of N2, and *NNH denotes the associative binding of NNH. Figure 3. shows a H*-descriptor-based free energy diagrams of the prediction of HER activity on the studied transition metal carbonitrides. Figure 4. shows calculated free energy diagram for all possible catalytic pathways for NH3formation, i.e., the MVK, associative MVK, dissociative MVK, associate mechanism, dissociative mechanism, adsorption of hydrogen on top of a metal atom and binding of NNH, on the (100) facets RS crystal structures of different transition metal carbonitride catalysts. Figure 4(a) shows free energy diagrams for VCN, Figure 4(b) shows free energy diagrams for NbCN, Figure 4(c) shows free energy diagrams for WCN, Figure 4(d) shows free energy diagrams for ZrCN, Figure 4(e) shows free energy diagrams for TaCN, Figure 4(f) shows free energy diagrams for TiCN, Figure 4(g) shows free energy diagrams for CrCN, Figure 4(h) shows free energy diagrams for MoCN, Figure 4(i) shows free energy diagrams for HfCN, Figure 4(j) shows free energy diagrams for YCN and Figure 4(k) shows free energy diagrams for ScCN. Figure 5 shows calculated free energy diagrams for NH3 formation via the most favourable pathway on the (100) facets RS crystal structures of different transition metal carbonitride catalysts. Figure 5(a) shows the free energy diagram for the favourable pathway on VCN, Figure 5(b) shows the free energy diagram for the favourable pathway on NbCN, Figure 5(c) shows the free energy diagram for the favourable pathway on WCN, Figure 5(d) shows the free energy diagram for the favourable pathway on ZrCN. Figure 6 shows a histogram for the free energy of binding for several nitrogen species to nitrogen / carbon vacancies on TMCN surfaces at zero potential. H denotes protonation of a surface metal atom, *N2 indicates associative binding of N2 vertically in the vacancy, *2N indicates dissociative binding of N2 with one N P15459PC00 sitting in the vacancy and the other on a neighbouring metal / nitrogen / carbon atom, *NNH indicates binding of NNH vertically in the vacancy. Figure 7 shows (approximate) minimum energy paths, obtained by the CI-NEB method, for dissociative binding of N2. In Figure 7(a), the minimum energy paths are shown for on WCN, NbCN, and VCN in the presence of a vacancy. In Figure 7(b), the minimum energy paths are shown for ZrCN and HfCN without a vacancy. Figure 8 shows a histogram with the free energy of adsorption of O, OH, or H (relative to 2N) compared to the surface vacancy of catalytically active carbonitrides (G(*2N-*X), in eV). The free energies are computed with respect to N2(g), 1 / 2H2(g), and H2O(g). All free energies are assessed at each carbonitride's predicted onset potential (based on the most favourable reaction mechanism and shown versus SHE). The adsorption energy of two N atoms is compared to that of one O, one OH, or one H, when the nitrogen originates from N2filling the vacancy and one of the N atoms adsorbs to the vacancy while the second N atom adsorbs on top of the neighbouring metal atom. Figure 9 shows how a sub-layer nitrogen or a carbon atom may migrate to a surface vacancy of a transition metal carbonitride. In Figure 9(a), a visual representation of the process is given. In Figure 9(b) the activation energy of vacancy migration (Ea,vac) is given on the y-axis and the ΔE for vacancy migration on the x-axis. The dotted line at Ea,vac = 1 eV denotes the threshold over which metal carbonitrides are regarded sufficiently stable for further investigation. C-mig represents carbon migration from sub-layer to the surface layer while N-mig means nitrogen migration from the sub-layer to the surface layer. The elements having * means they have nitrogen vacancy while all others represent carbon vacancy. Figure 10 A volcano plot for the formation of ammonia by using *NH as a descriptor on different transition metal carbonitride catalysts. DETAILED DESCRIPTION In the following, exemplary embodiments of the invention will be described. These embodiments are provided to provide further understanding of the invention, without limiting its scope. P15459PC00 In the following description, a series of steps are described. The skilled person will appreciate that unless required by the context, the order of steps is not critical for the resulting configuration and its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps. As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Throughout the description and claims, the terms “comprise”, “including”, “having”, and “contain” and their variations should be understood as meaning “including but not limited to”, and are not intended to exclude other components. The present invention also covers the exact terms, features, values and ranges etc. in case these terms, features, values and ranges etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., "about 3" shall also cover exactly 3 or "substantially constant" shall also cover exactly constant). The term “at least one” should be understood as meaning “one or more”, and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with “at least one” have the same meaning, both when the feature is referred to as “the” and “the at least one”. It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Features disclosed in the specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features. Use of exemplary language, such as “for instance”, “such as”, “for example” and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless so claimed. Any steps described in the specification may be performed in any order or simultaneously, unless the context clearly indicates otherwise. All of the features and / or steps disclosed in the specification can be combined in any combination, except for combinations where at least some of the features and / or steps are mutually exclusive. In particular, preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. P15459PC00 The present invention provides processes and systems for generating ammonia at ambient temperature and pressure. In the process and system of the present invention, an electrolytic cell is used which can be any of a range of conventional commercially suitable and feasible electrolytic cell designs that can accommodate a special purpose cathode in accordance with the invention. Thus, the cell and system may in some embodiments have one or more cathode cells and one or more anode cells. An electrolytic cell in the present context is an electrochemical cell that undergoes a redox reaction when electrical energy is applied to the cell. The skilled person will appreciate that chemical compounds as described herein are provided by their chemical formula irrespective of their phase or state. In particular, compounds that are present in their gaseous state when present in a pure and isolated form at room temperature (such as N2, H2and NH3) are herein described by their chemical formula. For example, dinitrogen is herein described as N2, whether present as nitrogen gas, as individual molecules, in clusters, bound to surfaces or present as solutes, and the same applies to other molecular species described herein. The proton donor can be any suitable substance that is capable of donating protons in the electrolytic cell. The proton donor can for example be an acid, such as any suitable organic or inorganic acid. The proton donor can be provided in an acidic, neutral or alkaline aqueous solution. The proton donor can also, or alternatively, be provided by H2 oxidation at the anode. i.e., hydrogen can be considered as a source of protons: H2 ^ 2(H++ e-). The electrolytic cell comprises at least three general parts or components, a cathode electrode, an anode electrode and an electrolyte. The overall cathode reaction can be presented as N2 + 6(H++ e-) ^ 2NH3 The catalyst surface can be hydrogenated by adding one hydrogen atom at a time, representing a proton from the solution and an electron from the electrode surface. The reaction mechanism can be shown in the equations below describing the so-called Mars- van Krevelen (MvK) mechanism, where an asterisk denotes a surface site: *NH2+ 4(H++ e−) ⇔ *NH3+ 3(H++ e−) [3] P15459PC00 *NH + 2(H++ e−) ⇔ *NH2 + (H++ e−) [7] *NH2 + (H++ e−) ⇔ *NH3 [8] *NH3⇔ * + NH3(g)[9] After formation of first ammonia (NH3) and nitrogen vacancy, atmospheric nitrogen molecule (N2) replenishes the vacancy and protonation of the surface results in formation of second ammonia molecule. Another reaction mechanism can be shown in the following chemical equations below describing the so-called associative mechanism, where an asterisk denotes a surface site: For the dissociative mechanism the reaction mechanism is according to the below equations: 2*N + 6(H++ e−) ⇔ *N + *NH + 5(H++ e−)

[0018] P15459PC00 *NH2 + (H++ e−) ⇔ * + NH3(g)

[0023] Alternatively, formation of methane (CH4) or other carbon containing species may occur, resulting in a carbon vacancy, wherein nitrogen molecule (N2) may then serve to bind to the vacated binding site. Then, subsequent protonation of the surface will result in formation ammonia molecules. Vacancies may occur during operation of a transition metal carbonitride catalysts or be intentionally prepared during the synthesis of the electrode and / or transition metal carbonitride catalyst. The different parts or components can be provided in separate containers, or they can be provided in a single container. The electrolyte can be an aqueous solution in which ions are dissolved. The aqueous solution can be a neutral, an alkaline or an acidic solution. In some embodiments, the aqueous solution is an acidic solution. In some embodiments, the electrolyte can also be a molten salt, for example a sodium chloride salt. In general terms, the catalyst on the electrode surface should ideally have the following characteristics: It should (a) be chemically stable, it should (b) not become oxidized or otherwise consumed during the electrolytic process, it should facilitate the formation of ammonia, and (d) use of the catalyst should lead to the production of minimal amount of hydrogen gas. As will be further described, the transition metal carbonitride catalysts according to the invention fulfil these characteristics. As further illustrated and discussed herein, the catalyst in the process and system of the invention comprises in some embodiments one or more transition metal carbonitride selected from the group consisting of Yttrium carbonitride, Scandium carbonitride, Hafnium P15459PC00 carbonitride, Zirconium carbonitride, Titanium carbonitride, Vanadium carbonitride, Chromium carbonitride, Niobium carbonitride, Molybdenum carbonitride, Tantalum carbonitride, Wolfram carbonitride. Any mixtures and combinations of two or more of these are also applicable in the invention. Accordingly, a mixture or combination of two or more of these catalyst materials may be used to lower the required electrode potential. An advantage of the present invention is that the process can be suitably operated using aqueous electrolytes, such as preferably aqueous solutions with dissolved electrolytes (salts). Thus, in preferred embodiments of the process and system, the electrolytic cell comprises one or more aqueous electrolytic solutions, in one or more cell compartments. Aqueous electrolyte solutions may comprise any of various typical inorganic or organic salts such as but not limited to soluble salts of chloride, nitrate, chlorate bromide, etc. e.g., sodium chloride, potassium chloride, calcium chloride, ammonium chloride, and other suitable salts. The aqueous electrolyte solutions may also comprise anyone, or a combination of, alkali or alkaline earth metal oxides, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, rubidium hydroxide and caesium hydroxide. The aqueous electrolyte solution can also further, or alternatively, comprise one or more organic or inorganic acids. Inorganic acids can include mineral acids that include but are not limited to, hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid, boric acid, hydrofluoric acid, hydrobromic acid, and perchloric acid. The electrolyte can also comprise an organic solvent, preferably an organic solvent miscible in water that is mixed in an aqueous electrolyte. As appears from herein, the essential feature of the present invention concerns the composition and structure of the cathode electrode. Transition metal carbonitride have a wide variety of surface structures which affect the surface energy of these compounds and influence their chemical properties. The relative acidity and basicity of the atoms present on the surface of metal carbonitride are also affected by the coordination of the metal cation and carbon and nitrogen anions, which alter the catalytic properties of these compounds. Gas diffusion electrodes (GDE) may be used to facilitate access of gases, in particular N2, to and from the electrodes of the present invention. In one embodiment, the cathode and anode may be a GDE. In another embodiment, only the cathode is a GDE. The GDE may comprise at least one catalyst layer for facilitating and / or catalysing the electrochemical NRR reaction. The GDE may further comprise a gas diffusion layer to provide a pathway for the gas to the electrode surface and hinder and / or delay flooding of the electrode. The GDE may further comprise a gas diffusion media to enable the transport of gases to and from the gas diffusion P15459PC00 layer. The gas diffusion media may be hydrophobic and / or microporous to allow passage of gas while repelling liquid electrolyte. Depending on the substance composition of the catalyst, a suitable surface crystal structure may be preferred. Various crystal structures exist for transition metal carbonitride and different structures can be obtained at different growth conditions. In some embodiments the catalyst surface comprises at least one surface having a Rocksalt structure, i.e., the catalyst comprises a Rocksalt crystal structure. The catalyst surface comprises in preferred embodiments at least one surface having a (100) facet or a (111) facet exposed. Other crystal structure surfaces are as well encompassed within the scope of the invention (see., e.g., International Tables for Crystallography; http: / / it.iucr.org). As described in more detail herein, running a current through the electrolytic cell leads to a chemical reaction in which nitrogen reacts with protons to form ammonia. The running of current is achieved by applying a voltage to the cell. The invention makes possible electrolytic production of ammonia at a low electrode potential, which is beneficial in terms of energy efficiency and equipment demands. In certain useful embodiments of the invention, ammonia can be formed at an electrode potential at less than about -1.2 V vs RHE, less than about -1.0 V vs RHE, less than about - 1.0 V vs RHE, less than about -0.9 V vs RHE, less than about -0.8 V vs RHE, less than about -0.7 V vs RHE, less than about -0.6 V vs RHE, less than about -0.5 V vs RHE or less than about -0.4 V vs RHE, less than about -0.3 V vs RHE. In some embodiments, ammonia can be formed at electrode potential in the range of about -0.2 V to about -1.2 V vs RHE, such as in the range of about -0.3 V to about -0.8 V vs RHE, such as in the range of about -0.4 V to about -1.2 V vs RHE, or in the range of about -0.5 V to about -1.2 V vs RHE. The upper limit of the range can be about -0.6 V, about -0.7 V vs RHE, about -0.8 V vs RHE, about -0.9 V vs RHE, about –1.0 V vs RHE, or about -1.1 V vs RHE, about -1.2 V vs RHE. The lower limit of the range can be about -0.2 V vs RHE, about -0.3 V vs RHE, about -0.4 V vs RHE, about -0.5 V vs RHE or about -0.6 V vs RHE. In other words, the process may be oeprated to form ammonia using the above-mentioned electrode potentials. The present invention may be used with a pulsing electrode potential by applying a cyclic varied potential that fluctuates between an active potential and a resting potential to said electrolytic cell to generate a cyclic varied potential through the electrochemical cell. In one embodiment, the cyclic varied potential is applied in the form of, but not limited to, sinusoidal P15459PC00 varied potential, step potential, and intermittent cyclic varied potential. Alternatively, in some embodiments, a pulsing electrode potential can be used by applying a cyclic varied current. The cyclic varied potential comprises active potential pulses in the range from about 0.001 sec. to about 30 sec. with intermittent system specific resting potential for a period of time in the range from 2 sec. to about 180 sec. The cyclic varied potential or current is configured to minimize or eliminate adsorption on electrodes of intermediate species, and the resting potential and resting potential time-period are selected to release adsorbed intermediate species adsorbed during the active potential time-period. In particular, the cyclic varied potential or current is configured to minimize or eliminate adsorption on electrodes of intermediate species and the resting potential and resting potential time-period are selected to release adsorbed intermediate species adsorbed during the active potential time-period. For example, at anodic potentials, hydrogen and / or protons adsorbed on the catalyst electrode surfaces can be removed from the surface thereby steering the selectivity of the materials towards the NRR. An advantage of the present invention is the efficiency of NH3 formation over H2 formation, which has been a challenge in prior art investigations and trials. In certain embodiments of the invention, less than about 50% moles H2 are formed compared to moles NH3 formed, and preferably less than about 40% moles H2, less than about 30% moles H2, less than about 20% moles H2, less than about 10% moles H2, less than about 5% moles H2, less than about 2% moles H2, or less than about 1% moles H2. The system of the invention is suitably designed in order to accommodate one or more of the above process features. It is an advantage of the invention that the system can be made small, robust and cheaply, such as for using locally for production of fertilizer close to the intended site of use. Ammonia can be used as such as fertilizer, by injecting into soil as gas, although this requires investment by farmers in pressurized storage tanks and injection machinery. Ammonia can also be used to form urea, typically by reacting it with carbon dioxide. Ammonia can be reacted to form nitric acid, which in turn is readily reacted to form ammonium nitrate. Accordingly, systems and processes of the present invention can be readily combined with present solutions for reacting the produced ammonia to other desired products such as but not limited to the abovementioned. NOx and SOx are generic terms for mono-nitrogen and mono-sulphur oxides, such as NO, NO2, SO, SO2 and SO3. These gases are produced during combustion, especially at high P15459PC00 temperatures. In areas of high motor vehicle traffic, the amount of these pollutants can be significant. Accordingly, a useful aspect of the invention relates to a system for removing NOxand / or SOxfrom a stream of gas, by reacting the stream of gas with ammonia that is generated in situ in the stream, or in a system that can be fluidly connected to the stream of gas. The system can comprise a system for generating ammonia as described herein, in particular a system that comprises an electrolytic cell containing a transition metal carbonitride catalyst as described herein. In this context, in situ should be understood as ammonia generation within the system, for example within the gas stream, or in a compartment within the system that is fluidly connected to the gas stream. The ammonia thus generated, when in contact with the stream of gas, will react with NOxand / or SOxin the stream of gas to convert these toxic species to other molecular species, such as N2, H2O and (NH4)2SO4. In some embodiments, the system can be for use in an automobile engine exhaust or in other engines, where ammonia can be generated in situ by a process according to the present invention, and then used to reduce SOx and / or NOx exhaust gases from the engine. Such system can suitably use electric current produced by conversion from the car engine. Thus, by using electric current from a car engine, ammonia can be generated in situ, and the generated ammonia can thus be allowed to react with SOx and / or NOx from the gas exhaust of the automobile. The ammonia can be generated in the automobile, and subsequently fed into the car exhaust. The ammonia can also be generated in situ within the automobile exhaust system. Thereby, NOx and / or SOx are removed from the car exhaust, reducing the number of pollutants in the exhaust. The invention will now be illustrated by the following non-limiting example that further describes particular advantages and embodiments of the present invention. EXAMPLE In the current example, TMCNs catalysts for the electrochemical generation of ammonia under ambient conditions are investigated using density functional theory (DFT), wherein thermodynamic data and other activity descriptors are collected for TMCNs in a rocksalt (RS) crystal structure using the energy lowest (100) facet as a model system. In total, 11 TMCNs are investigated in this example, wherein the 11 TMCNs include YCN, ScCN, HfCN, ZrCN, TiCN, VCN, CrCN, NbCN, TaCN, MoCN, and WCN. The onset potentials required for ammonia formation, free energy diagrams, poisoning tendencies from the aqueous solution, kinetic barriers for N2 adsorption, electronic properties and surface stability are among the data obtained for determining the activity of TMCN surfaces. The computational standard hydrogen electrode [1] is implemented to account for the effect of an external potential, which is P15459PC00 examined in the context of surface activity and stability at the operational potential or the overpotential. The computational methodology used is as follows; • Density functional theory (DFT) using the RPBE exchange-correlation functional was adopted for all simulations. The Vienna ab initio simulation program (VASP) was employed with a 4 x 4 x 1 Monkhorst-Pack K-point mesh and a 500 eV energy cutoff for each surface. The projector augmented wave (PAW) approach as implemented in the VASP code was used; this provides access to the whole wavefunction by exploiting computationally efficient pseudopotentials, hence drastically lowering computing time. The Kohn-Sham orbital occupancy was smeared using a Fermi-Dirac distribution with a smearing value of kBT = 0.1 eV. All surfaces were studied in the RS crystallographic structure and the (100) surface orientation. The TMCN surfaces were modeled with a 5-layer 2x2 unit cell consisting of 20 metal atoms, 10 carbon atoms and 10 nitrogen atoms as shown in Figure 1. The bottom two layers are fixed, while the upper layers and any adsorbates are allowed to relax. The boundary conditions are periodic in the x and y dimensions, and each surface slab was separated from its neighbours by at least 15 Å of vacuum in the z direction. When the forces in either direction on all movable atoms fell below 0.01 eV, it was determined that the structural optimization had converged. Using the climbing image nudged elastic band approach (CI-NEB), activation energies were determined as the highest point along the minimal energy path (MEP). The required protons for the reaction could be either supplied through H2 oxidation reaction or water splitting at the anode. In order to link the absolute potential to the standard hydrogen electrode (SHE), H2 is referred to here only as a convenient source of protons and electrons, H2 ⇌ 2(H++ e-) where the protons are solvated in the electrolyte. The overall reaction for N2 reduction is; the surface is hydrogenated by adding one hydrogen atom at a time, representing a proton from the solution and an electron from the electrode surface. The Mars-van- P15459PC00 Krevelen mechanism is studied here is based on the equations as shown above in the Detailed Description. In the electrochemical cell, the reactants necessary for the synthesis of ammonia at the cathode are atmospheric nitrogen, protons generated at the anode and electrons from the applied voltage. Since the needed protons are generated only at the anode, the applied voltage may be directly compared to the standard hydrogen electrode (SHE) or the reversible hydrogen electrode (RHE). The chemical pathway was modeled using an unrestricted mechanism in which, for each protonation step, the thermodynamically most favorable adsorption site was chosen by a sampling of adsorption sites and used for the subsequent protonation step until a product was created. The best insight into the catalytic activity of a catalyst and the products it tends to produce is provided by an unrestricted mechanism. The following equation used to compute the free energy change for each reaction step at U = 0 V vs. RHE and pH = 0, ΔG(0) = ΔE + ΔEZPE – TΔS Where G(0) is the change in free energy at pH = 0, E is the thermodynamic value obtained by DFT, and EZPE and TS are the zero point energy and entropy corrections calculated, see Table 1. To determine the free energy change at a given voltage and pH value (G(U,pH)), the implicit hydrogen electrode method is employed as follows: ^G(U, pH) = ^G - neU - kBTln(10) × PH where n represents the number of electrons engaged in the reaction step, e represents the electron's charge, and U represents the applied potential. All data presented is computed at pH = 0. The potential determining step (PDS) for any reaction pathway is the electrochemical reaction step with the biggest positive free energy change. From the PDS, the operational potential (OP) may be defined as the potential necessary to make the free energy change of the PDS zero and hence it follows that OP = - ^G / e. The OP (in volts) is simply equal to the electron-volt magnitude of the negative shift in free energy associated with the PDS. Initially, group III-XII possible transition metal carbonitride catalysts were subjected to computational screening in the (100) facets of the rocksalt structure (RS). However, only VCN, P15459PC00 NbCN, ZrCN, TiCN, TaCN, CrCN, MoCN, HfCN, WCN, YCN, and ScCN are selected for further investigation due to stable structure, i.e., a geometry that was not distorted. Five factors are studied in order to evaluate the viability of each candidate for electrochemically producing NH3under ambient conditions: i) surface poisoning (ii) the stability of a surface nitrogen or carbon-vacancy, (iii) catalytic activity, (iv) poisoning of the catalyst surface vacancy, and (v) calculated kinetic barrier needed for N2to fill the vacancy and dissociate. Three reaction mechanisms for nitrogen reduction reaction (NRR) are studied in this example, namely, • The dissociative mechanism: the N-N triple bind is broken during the adsorption phase, resulting in the adsorption of separate N atoms at a distance apart of two different adsorption sites. In subsequent phases, the hydrogenation of each N atom results in the creation of ammonia, which is then released from the surface. • The associative mechanism: N2 adsorbs to the surface in a vertical manner and is then subsequently hydrogenated, forming NNH, NNH2 and NNH3 intermediates and releasing NH3from the surface. The second adsorbed nitrogen is then hydrogenated until it leaves the surface as ammonia. • The Mars-Van-Krevelen (MvK / MVK) mechanism: the lattice N or C atoms on the surface of the transition metal carbonitrides (TMCNs) are reduced to ammonia or methane and, as a result, vacancies are produced. The vacancies are then chemically replenished by nitrogen molecules via an associative and / or dissociative mechanism to activate the catalyst again. The adsorption energies and reaction pathways are modelled and calculated for each TMCN candidate, namely, VCN, CrCN, YCN, HfCN, NbCN, ScCN, ZrCN, TiCN, TaCN, WCN, and MoCN. These metrics can be used to assess the reactivity of the TMCN surfaces in the RS structure with the (100) crystallographic orientation for the NRR. Before any reaction may occur, the adsorption of a nitrogen species onto the surface must be studied. The associative binding of nitrogen gas (*N2), the dissociative binding of nitrogen gas (*2N), and the associative binding of NNH (*NNH) are examined for the adsorption of nitrogen. For each of these alternatives, all potential adsorption sites are examined, and the most favourable adsorption is selected for comparison; the findings for the adsorption of nitrogen and potential active site poisoning by other species are shown in Figure 2. In addition to the adsorption energies of nitrogen species, Figure 2 shows the energy associated with the protonation of a metal site, which is a useful indicator of the probability of P15459PC00 the Hydrogen Evolution Reaction (HER), which is typically the most important competitive reaction to the NRR. According to Figure 2, only YCN and ScCN show stronger dissociative binding of nitrogen to the metal site than binding of protons to the metal site. At first sight, this might hint at eliminating all other candidates, however, ZrCN, CrCN, MoCN and HfCN are also evaluated further because the dissociative binding of nitrogen gas is only a little uphill in energy, or endergonic. After binding of nitrogen to YCN and ScCN, the preferred reaction route was identified as successive protonation of the surface via an unrestricted mechanism. For both of these surfaces, the formation of ammonia (NH3) is not detected for any of the nitrogen sites (N2bind on C-site). The protonation of carbon sites over nitrogen results in the creation of either methane (CH4) or hydrogen (H2). At this point, the simulations are terminated. Both methane and hydrogen production are undesirable by-products of an NRR catalyst because they indicate a tendency for surface breakdown or the dominance of HER. The exergonic protonation of carbon sites on several of these TMCNs, YCN and ScCN included, reflects this behaviour. In addition to failing to generate the desire product, there are a number of TMCN surfaces that preferentially bind the oxygen species O2-at zero potential but this may be addressed by applying onset potential, wherein this could be no problem at negative operating potentials because these oxygen species should be released from the negatively charged surface. On the RS(100) facets of earlier TMCNs, a full analyses of the reaction via different mechanism for the 11 TMCNs is carried out. The free energy diagrams for each TMCN along the different catalytic pathways is shown in Figures 4(a) to 4(k). VCN, NbCN, and WCN are potential candidates for the MVK mechanism, shown in Figures 4(a) to 4(c) respectively, in which, upon protonation of the surface (with consideration of all the feasible adsorption sites for H), one nitrogen atom is reduced to ammonia and nitrogen vacancy is created before the catalyst is replenished by solvated nitrogen molecules. As the initial stage in the formation of hydrogen, the free energy of proton adsorption on the metal site is determined and shown in Figure 3. For both VCN (0.7 eV) and NbCN (0.4 eV), proton adsorption on the pure TMCN metal site is considerably weaker, only ScCN, YCN and WCN binds proton strongly on the metal sites while the rest of the TMCNs do it endergonically and thus the HER is expected to be improbable. Except for WCN and TaCN, where the proton adsorbs to a Tungsten or Tantalum atom initially, the most energetically favourable site for proton adsorption during surface protonation was found to be on surface carbon atoms for all carbonitrides investigated. Except for WCN and TaCN, none of the metallic atoms on the surface bind hydrogen more strongly than the surface C atoms. This means that when considering 2 nitrogen atoms, 2 carbon atoms, and 4 metallic atoms on the surface per unit P15459PC00 cell, the most thermodynamically favourable intermediate is *CH, but more interestingly, after two protons cover the carbon site, the third protonation step results in *NH formation (in case of VCN and NbCN) and same for WCN after covering one metal site and one carbon site. After five protonation steps, ammonia is initially obtained from the surface of VCN (as is shown in figure 5a), of NbCN (as is shown in figure 5b) and of WCN (as shown in Figure 5c), and for all other materials and pathways, after various protonation steps, as described in Figure 4(a) to Figure 4(k), CH4(g)is obtained. But for ScCN and YCN, due to the high PDS step for MVK, the rest of the pathway was omitted from the study. While the dissociative pathway is exergonic for both YCN and ScCN, it preferably goes on to make NH or CH on the surface instead of making NNH. It is extremely difficult to go from surface NH to another NH in the case of YCN and from NH to CH in the case of ScCN. Furthermore, ScCN prefers to go for C-vacancy formation rather than ammonia formation. ScCN and YCN are therefore eliminated from the remainder of the study, as these candidates cannot be employed to convert nitrogen to ammonia. TaCN was also discovered to be distinct from other carbonitrides in that the initial proton binds to the Ta metal atoms of the surface more strongly than to the N or C atoms, while the subsequent proton binds to the surface C and produces *CH. The free energy diagrams for CrCN and MoCN are shown in Figure 4(g) and Figure 4(h), where a pathway to NH3 creation via MvK and all other mechanisms is depicted. It can be observed that none of the mechanisms are in favour of ammonia formation under ambient circumstances. The potential-determining step (PDS) and the measurement of the catalytic activity toward nitrogen activation to ammonia on each carbonitride is recognized as the elementary reaction step with the greatest increase in free energy (ΔG). If this elementary step is a proton-electron transfer step, it can be surmounted by providing a bias, so altering the free energy landscape in a way that all reaction steps have a negative free energy gradient. As illustrated in Figure 5(a), the PDS is the fourth protonation step following the formation of the first ammonia for VCN, with ΔGPDS = 0.52 eV. By providing an external bias of -0.52V vs. SHE, all the steps on the free energy path will be shifted in a way all subsequent electrochemical processes will have a negative free energy slope. However, there is no change in the free energy of the steps without electron transfer (such as N2 adsorption) when an external bias is applied. As illustrated in the Figure 5(a, b, c), VCN, NbCN, and WCN are the carbonitrides that release two ammonia molecules after 8(H++ e) under ambient conditions. In addition, Figure 4(d) for ZrCN compares the various pathways, i.e. dissociative binding of nitrogen, associative binding of nitrogen, binding of NNH, adsorption of hydrogen onto a metal site, adsorption of hydrogen onto a nitrogen site, and adsorption of hydrogen onto a carbon site on the clean surface. Although the initial adsorption of dissociative nitrogen is slightly P15459PC00 endergonic for ZrCN as seen in Figure 5(d), it progresses to ammonia production with a PDS of only 0.11 eV. The PDS will predict the needed potential to make the whole electrochemical path exergonic and the formation of ammonia spontaneous. Same is true for HfCN (as shown in Figure 4(i)), where dissociative N2adsorption on the surface is slightly uphill and yields a PDS of 0.36 eV. ZrCN and HfCN are therefore potential catalysts for the NRR, as the required OP for ammonia formation on their surface is low (-0.11 V and -0.36 V) and the reaction pathway only requires six protonation steps to release two ammonia, indicating that there is no proton coverage on the surface contributing to the HER. As shown in Figures 4(d) and 4(i) where N2is adsorbs dissociatively on the surface indicating that if the surface (supercell) of the simulations is increased it will go for NRR rather than HER and methane. However, the surface protonation is started via the MVK, but it has already been seen that a carbon vacancy is produced after a total of six protons. It is also interesting to see the proton that was originally placed at the metal site has now moved to the vacant position left by the discharge of the methane. The associative and dissociative nitrogen replenishment through MVK was found to be very exergonic. Evidence that the free energy associated with the generation of CH4 was exergonic at the appropriate OP for all tested surfaces can be seen in Figures 4(e) to Figure 4(k). The presence of a vacancy on the surface offers an active site where nitrogen may attach more easily due to the stronger coordination associated with the presence of vacancy, and this promotes a greater selectivity for nitrogen activation. The preliminary investigation of the binding of nitrogen species in the vacancy of these TMCN surfaces is depicted in Figure 6. Comparison of Figure 2 and Figure 6, reveals that the adsorption energies of the necessary nitrogen species are considerably enhanced in the presence of a vacancy. The dissociative binding of nitrogen, which is exothermic for the majority of TMCN, is particularly promising. However, this eliminates CrCN, which does not bind any nitrogen species strongly, while MoCN and ScCN only bind it associatively. ScCN and YCN's reaction paths are terminated after the third protonation due to their high PDS, however for the other TMCN compounds, the exergonic nitrogen adsorption was simulated until the creation of either two molecules of ammonia or until the first by-product (H2 or CH4). WCN, VCN, and NbCN are the only surfaces that proceed to ammonia formation when binding nitrogen dissociatively in a vacancy. Due to this, and the fact that dissociative binding of nitrogen is always preferred over other nitrogen adsorbates, the focus is now set on the dissociative binding of nitrogen on these TMCN surfaces in the presence of a nitrogen or carbon vacancy. It is necessary to calculate the kinetic barrier that is represented by the activation energy of N2 adsorption on the surface. Here, NEB calculations for the five candidates, i.e., VCN, NbCN, WCN in the presence of vacancy is shown in Figure 7(a) and ZrCN and HfCN without a vacancy in Figure 7(b). For P15459PC00 economically feasible production, the catalyst must be able to absorb nitrogen in a very short amount of time, and a kinetic barrier greater than 1 eV will undoubtedly inhibit this process at room temperature. It is safe to assume that due to the extremely preferential adsorption of nitrogen, maintaining the nitrogen atom in the vacant site is preferable to protonation. This would imply that in order to attain the appropriate activity, nitrogen adsorption must be exergonic yet below a certain threshold energy. Looking at Figure 7(a), it is evident hat this threshold energy is below 1.0 eV, as this would accurately predict that only WCN, NbCN, and VCN are active catalysts, whereas the kinetic barrier for dissociative nitrogen adsorption onto the ZrCN and HfCN surfaces is too high, or 2.99 eV and 1.81 eV, respectively, as shown in Figure 7(b), causing them to be less interesting at ambient temperatures. For the catalytic cycle to continue, the surface vacancies produced must be filled with N2from the electrolyte. There is a possibility that the N-vacancy is not filled with nitrogen because it is blocked by a proton, an oxygen atom, or a hydroxyl atom from the aqueous electrolyte. Therefore, the free energy of filling the vacancy with any of these species compared to nitrogen (ΔG(*2N-*X)), where X= O, H, or OH, is used to study the rivalry between N and O, H, and OH for filling the surface vacancy. Under operational circumstances, these ions are derived from the water in the electrolyte, which results in the creation of O2-, OH-, and H+. At the commencement of ammonia production, the difference in adsorption energy is computed between these species and N2 (where two N atoms adsorb to the N or C-vacancy and metal site as already shown in Figure 2). Therefore, these free energies pertain to the gaseous states of N2, H2O, and H2. A negative value of ΔG(*2N-*X) implies that nitrogen is thermodynamically preferable to the other three species for filling the vacancy. The values are shown in Figure 8 for the whole group of catalytically active carbonitrides. N atoms bind more strongly than O atoms to surface vacancies in all carbonitrides except CrCN and ZrCN, although for ZrCN this phenomenon might not be as critical as the favourable pathway towards ammonia is not via formation of the vacancy. It is predicated that the surface vacancies of most of these carbonitrides will not be poisoned by these species. For all carbonitrides besides CrCN, N atoms attach to surface vacancies with greater affinity than OH species. However, the surface vacancy in CrCN is more likely to be filled with H. O and OH are better than N but for CrCN and MoCN the vacancy will be fill by O and H but for the remaining carbonitrides, it is anticipated that nitrogen will fill the vacancy, and therefore the catalytic cycle may continue to produce NH3. When compared to nitrogen, oxygen, hydrogen, and hydroxyl all have lower binding energies to the surface vacancy. As a result, these other possible replacements shouldn't be able to fill the surface vacancy when an electrical potential is applied. Overall, VCN, NbCN, and WCN stand out as the most viable choices, both in terms P15459PC00 of activity (Figure 5) and stability against poisoning (Figure 8). These carbonitrides are expected to convert nitrogen to ammonia at potentials of -0.52 V, -0.53 V, and -0.72 V. In the Mars-van Krevelen mechanism considered in this example, a surface nitrogen or carbon atom is reduced to generate NH3or CH4, respectively, following which a gaseous N2molecule fills the ensuing vacancy. For this replenishment to occur, the surface N / C-vacancy must be stable. If this is not the case, the N / C-vacancy may migrate to the bulk of the catalyst, meaning that the reactive nitrogen / carbon on the surface is replaced with additional nitrogen / carbon from the catalyst, as opposed to gaseous N2. This process can theoretically continue until all of the nitrogen and carbon atoms of carbonitrides have reacted and created NH3 / CH4, leaving just the pure metal. The stability of the N / C-vacancy at the surface of the catalyst is evaluated by comparing the energy difference between a carbonitride slab with a single N / C-vacancy in the surface layer (Evac,1) and a carbonitride slab with a single N / C- vacancy in the first subsurface layer (Evac,2). The energy difference (ΔEvac= Evac,2- Evac,1) is used to estimate the thermodynamic stability of the vacancy at the carbonitride's surface. Kinetic barriers for vacancy migration (Ea,vac) are also calculated, and Figure 9(a) and Figure 9(b) display a comparison of both thermodynamics (∆Evac) and kinetics (Ea,vac) of this process. It is revealed that, for the majority of carbonitrides, it is thermodynamically favourable for the vacancy to migrate to the bulk. However, due to the large kinetic barrier for vacancy migration, these carbonitrides are likely to show a stable surface vacancy. All carbonitrides have a kinetic barrier for vacancy migration greater than 1 eV and are thus preserved for further screening, as a barrier of this size at room temperature is unlikely to be overcome. Using the linear relations for various reaction steps as a function of the ΔG of *NH (as descriptor), it is possible to generate volcano diagram to determine which value of ΔG of *NH would result in the lowest onset potential for electrochemical ammonia synthesis. In reference [2], this method was applied to the electrolysis of water on oxide surfaces. In this analysis, the reaction-free energy can be used as a direct indicator of electrocatalytic activity. The free energy of the reaction for every fundamental step can be expressed as a function of the applied bias U and the ΔG of the NH, denoted by *NH. To begin, the pathway may be considered as a Heyrovsky type that uses the MVK mechanism. In this reaction, solvated protons from a solution can directly react with an electron and N2, NNH2 species on a surface. Here, the free energy is expressed as the sum of an energy contribution proportional to the adatom binding energy, a constant derived from zero-point energy and entropy, and the energy shift caused by the applied potential. Using the linear scaling relationships, the energy of all the distinct species is now expressed in terms of G of the NH, then it is possible to construct a volcano plot as shown in Figure 9. It can be seen that VCN and NbCN are at the summit of the volcano, P15459PC00 and WCN is very close by; these are the most promising materials for the formation of ammonia under ambient conditions. Furthermore, TiCN and TaCN, which are also not so bad catalysts for the formation of ammonia with PDS values of 1.06 eV and 1.16 eV, are not far from the summit of the volcano. To conclude, by using state-of-the-art, accurate and computationally demanding quantum chemical calculations (i.e., DFT calculations), it was predicted that the onset potential required for ammonia formation via the Mars-van Krevelen (MVK), an associative mechanism (AM), and a dissociative mechanism (DM) on the clean surface on the (100) facets of the Rocksalt structure of eleven carbonitrides: ScCN, TiCN, VCN, CrCN, YCN, ZrCN, NbCN, MoCN, HfCN, TaCN, and WCN. The free energies of all stable intermediates along the pathway of the reaction were computed and the free energy diagrams for each carbonitride created. It was possible to determine the free energy change of the step that determines the operating potential. Additionally, the activation energy of N2dissociation was estimated and the free energy of adsorption of two N adatoms on the surface evaluated. The conclusion is that reducing nitrogen to ammonia under ambient circumstances using neither DM nor AM may not be advantageous. DM is inactive for ammonia formation due to the endothermic nature of the adsorption of two N atoms to the surface and the high kinetic barriers of N2 dissociation. On the pristine surface of these carbonitrides, it is often endothermic to adsorb the N2 molecule through AM. However, it results in CH4 instead of NH3 production, which is intriguing. In addition, it is evident that only the MvK mechanism provides the lowest PDS among the possibilities examined here. On these metal carbonitride catalysts, the MvK may always be the most favourable route for nitrogen electroreduction to ammonia. However, it should be emphasized that none of the electrochemical proton-electron transfer barriers have been accounted for in this work, therefore the rates for these various methods have not yet been assessed. In this work, thermochemical model was utilized to predict the onset potentials for these various processes. Except for ScCN, YCN, and TaCN, which were shown to be more selective for the Hydrogen Evolution Reaction (HER) in an electrochemical environment, all other carbonitrides were found to be more selective for the nitrogen or carbon electroreduction reaction. VCN, NbCN, and WCN were discovered to have the lowest onset potential (-0.52 V, -0.53 V, -0.72 V vs. RHE) of all the carbonitride compounds tested. However, it was shown that CrCN and MoCN are vulnerable to poisoning in electrochemical media. References [1] J. K. Nørskov et al., The Journal of Physical Chemistry B, 2004, 108, 17886-17892. P15459PC00 [2] Rossmeisl, J., et al., Electrolysis of water on oxide surfaces. Journal of Electroanalytical Chemistry, 2007.607(1-2): p.83-89.

Claims

P15459PC00 CLAIMS 1. A process for producing ammonia comprising: a. feeding N2to an electrolytic cell that comprises a cathode, an anode, an electrolyte and at least one source of protons, b. allowing the N2to come into contact with at an electrode surface of the cathode in the electrolytic cell, wherein said electrode surface comprises at least one catalyst surface selected from the group consisting of: Vanadium carbonitride, Niobium carbonitride, Tungsten carbonitride, Titanium carbonitride, Tantalum carbonitride, Hafnium carbonitride, Molybdenum carbonitride, Chromium carbonitride, Scandium carbonitride, Yttrium Carbonitride and Zirconium carbonitride, and, c. running a current through said electrolytic cell, whereby nitrogen reacts with protons to form ammonia.

2. The process according to claim 1, wherein the catalyst surface comprises one or more transition metal carbonitrides selected from the group consisting of Vanadium carbonitride, Niobium carbonitride and Tungsten carbonitride.

3. The process according to any one of the preceding claims, wherein the at least one catalyst comprises a Rocksalt structure.

4. The process according to any one of the preceding claims, wherein the catalyst surface comprises at least one surface having a (100) facet.

5. The process according to any one of the preceding claims, wherein the catalyst surface comprises at least one nitrogen vacancy and / or carbon vacancy.

6. The process according to any of the preceding claims, wherein ammonia is formed in the electrolytic cell at an electrode potential at less than about -1.0 V, more preferably less than about -0.6 V and even more preferably less than about -0.3 V using a reversible hydrogen electrode (RHE) as a reference.

7. The process according to any one of claims 1 to 6, wherein a cyclic varied potential is used that fluctuates between an active potential and a resting potential to generate a cyclic varied potential through the electrolytic cell.P15459PC00 8. The process according to any one of the preceding claims, wherein less than 50% moles H2are formed compared to moles NH3formed, and preferably less than 20% and even more preferably less than 10%.

9. The process according to any one of the preceding claims, wherein said electrolytic cell comprises one or more aqueous electrolytic solution.

10. The process according to any one of the preceding claims, wherein the source of protons in the formation of ammonia is from water splitting at the anode or H2oxidation reaction in the anode.

11. The process according to any one of the preceding claims, wherein the electrolytic cell comprises an anode within one cell compartment and a cathode within another cell compartment.

12. The process according to any one of the preceding claims, wherein the process is carried out at a temperature in the range from about -10°C to about 80°C, preferably in a range from about 10°C to about 50°C, more preferably in the range from about 20°C to about 30°C, even more preferably in the range from about 20°C to about 25°C.

13. The process according to any one of the preceding claims, wherein the process is carried out at atmospheric pressure.

14. The process according to any one of claims 1-12, wherein the process is carried out at a pressure in the range of 1 to 30 atmospheres, preferably in the range of 1-20 atmospheres, preferably in the range of 1-10 atmospheres, more preferably in the range of 1-5 atmospheres.

15. The process according to any of the preceding claims, wherein said feeding N2 to the electrolytic cell comprises feeding gaseous nitrogen or air or liquid with dissolved nitrogen to the electrolytic cell.

16. A system for generating ammonia, the system comprising at least one electrochemical cell, which comprises at least one cathode having at least one catalytic surface, wherein the at least one catalytic surface comprises at least one catalyst selected from the group consisting of: Vanadium carbonitride, Niobium carbonitride, TungstenP15459PC00 carbonitride, Titanium carbonitride, Tantalum carbonitride, Hafnium carbonitride, Molybdenum carbonitride, Chromium carbonitride, Yttrium carbonitride, Scandium carbonitride and Zirconium carbonitride.

17. The system according to claim 16, wherein said at least one catalyst is selected from the group consisting of Vanadium carbonitride, Niobium carbonitride and Tungsten carbonitride.

18. The system according to any one of claims 16 to 17, wherein the at least one catalyst comprises a Rocksalt structure.

19. The system according to any one of claims 16 to 18, wherein the catalyst surface comprises at least one surface having a (100) facet.

20. The system according to any one of claims 16 to 19, wherein said electrolytic cell further comprises one or more electrolytic solution, preferably an acidic, neutral or alkaline aqueous solution.

21. The system according to claim 20, wherein the electrolytic solution comprises an aqueous water-miscible organic solvent.

22. The system according to any one of claims 16 to 21, wherein the electrolytic cell comprises an anode within one cell compartment and a cathode within another cell compartment.

23. The system according to any one of claims 16 to 22, wherein ammonia is formed in the electrolytic cell at an electrode potential at less than about -1.0 V, more preferably less than about -0.6 V and even more preferably less than about -0.3 V using a reversible hydrogen electrode (RHE) as a reference.

24. The system according to any one of claims 16 to 23, wherein a cyclic varied potential is used that fluctuates between an active potential and a resting potential to generate a cyclic varied potential through the electrolytic cell.

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