Process for electrolytic production of ammonia from nitrogen using metal carbide 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 carbide, 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

PROCESS FOR ELECTROLYTIC PRODUCTION OF AMMONIA FROM NITROGEN USING METAL CARBIDE CATALYSTSFIELD OF INVENTIONThe 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 carbide catalysts for the cathode surface.BACKGROUNDAmmonia 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 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 catalysingthe NRR because of the facile H2 formation. [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 remains 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 W02020110155, respectively. Despite these efforts, most 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 0 ve rpote nti a Is / vo Itag e .References:

[0001] 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] Skulason, E., Bligaard, T., Gudmundsdottir, S., Studt, F., Rossmeisl, J., Abild-Pedersen, F., Vegge, T., Jonsson, H., and N0rskov, J.K. (2012). A theoretical evaluation of possible transition metal electrocatalysts 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 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, 290-296SUMMARYThe 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 at ambient conditions and at low to moderate overpotentials by using a highly specific, carefully selected, catalysts material. As discussed above, the competitive HER and the instability of previously mentioned promising catalyst candidates (i.e., theproneness of these materials to decompose) renders the search for viable and stable catalyst that is selective towards NRR extremely challenging. Transition metal carbides (TMCs) is a class of compounds that have stable rock salt bulk structures showing in general more stability than for example transition metal oxides and nitrides. Moreover, TMCs may sometimes show similar electrical and catalytic behaviours as Pt-group metals and may be employed as supports to minimize the total loading of the scarcer precious metals. The parent metals of TMCs are orders of magnitude more common and less costly than Pt-group metals in the earth's crust. Economically, any replacement of novel metals with TMCs can significantly reduce the cost of the catalyst. These characteristics of TMCs make them good supports for metals of the Pt group. 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. Therefore, the present invention relates to the identification and selection of specific transition metal carbide catalysts capable of catalysing the electrochemical NRR, wherein it is possible to efficiently form ammonia at ambient temperature and pressure using a low applied potential. 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.In the first aspect of the invention a process for producing ammonia is provided, said process comprising: 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 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 carbide; 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 carbide may be selected from a group of transition metal carbides comprising; Yttrium carbide, Scandium carbide, Hafnium carbide, Zirconium carbide, Titanium carbide, Chromium carbide, Niobium carbide, Molybdenum carbide, Tantalum carbide, Wolfram carbide (also referred to as Tungsten carbide).Therefore, in accordance with the present invention, a process for producing ammonia is provided, wherein said 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 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 carbide selected from the group consisting of Zirconium carbide, Chromium carbide, Niobium carbide and Tungsten carbide, 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 may comprise one or more transition metal carbides selected from the group consisting of Zirconium carbide, Niobium carbide and Tungsten carbide.In one embodiment, the catalyst surface may comprise at least one surface having a Rocksalt structure, i.e., the catalyst may comprise a Rocksalt crystal structure.In one embodiment, the catalyst surface may comprise at least one surface having a (100) facet. In another embodiment, the catalyst may comprise at least one surface having a (1 10) facet. In another embodiment, the catalyst surface may comprise at least one surface having a (11 1) facet. Other surface facets are also well encompassed by the present invention. In some embodiments, the catalyst surface may comprise a mixture of surface facets.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 formed by the formation and release of carbon containing molecular species such as, but not limited to, methane (CH4).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 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, preferably at an electrode potential of less than about -0.6 V or more preferably at an electrode potential of less than about -0.3 V, using a reversible hydrogen electrode (RHE) as a reference.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 catalyst 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.In one embodiment, the source of protons in the formation of ammonia is from water splitting at the anode or H2 oxidation 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 cellIn 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 electrode having at least one catalytic surface, wherein the at least one catalytic surface comprises at least one catalyst comprising one or more transition metal carbide(s).In an embodiment of the present invention, the at least one catalyst surface comprising at least one transition metal carbide may be selected from a group of transition metal carbides including; Yttrium carbide, Scandium carbide, Hafnium carbide, Zirconium carbide, Titanium carbide, Chromium carbide, Niobium carbide, Molybdenum carbide, Tantalum carbide, Wolfram carbide (also referred to as Tungsten carbide).Therefore, in accordance with the invention, a system for generating ammonia is 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: Zirconium carbide, Chromium carbide, Niobium carbide and Tungsten carbide.In a preferred embodiment, the at least one catalyst may be selected from the group consisting of Zirconium carbide, Niobium carbide and Tungsten carbide.In one embodiment, the at least one catalyst may comprise 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.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 .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, 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 FIGURESThe 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 side-view of a 5-layer TMC in the (100) facet, in particular WC, in the exposed (100) facet of the rock-salt structure. A top view of the surface is shown as an inset.Figure 2 shows a comparison of adsorption energies of different nitrogen and oxygen species on clean TMC surfaces as well as protonation of a metal and carbon site at zero potential. *N2 denotes the associative binding of N2(g), *2N denotes the dissociative binding of N2(g), and *NNH denotes the associative binding of NNH, C*H denotes the protonation of a surface carbon atom, M*H denotes the protonation of a metal atom, *O denotes the adsorption of a oxygen ion. The binding of different intermediates to the transition metal carbide is given by the order as specified by the legend.Figure 3(a) shows an unrestricted free energy diagram ZrC resulting in ammonia formation via a dissociative mechanism at zero potential. The labellingAX refers to the positioning of the adsorbed species in the unit cell, in this case, site A as defined in the inset figure in the top right. The inset figure found in the top right defines all relevant adsorptionsites as well as the unit cell (black square). The PDS, for the plot at zero potential, and the free energy change associated with it can be found under the relevant adsorbed species. In addition to the reaction pathway, additional adsorbates are shown for the initial step.Figure 3(b) shows unrestricted free-energy diagram for ZrC resulting in ammonia formation via a dissociative mechanism at zero potential for a given OP: -0.69 V. The labellingAX refers to the positioning of the adsorbed species in the unit cell, in this case, site A as defined in the inset figure in the top right. The inset figure found in the top right defines all relevant adsorption sites as well as the unit cell (black square). The PDS, for the plot at zero potential, and the free energy change associated with it can be found under the relevant adsorbed species. In addition to the reaction pathway, additional adsorbates are shown for the initial step.Figure 4 shows a comparison of binding energies of different nitrogen species onto TMC surfaces with a carbon vacancy at zero potential. *N2 denotes the associative binding of N2(g) horizontally in the vacancy, *2N is the dissociative binding of N2(g) where one N sits in the vacancy and the other on a neighbouring carbon atom, *NNH is the binding of NNH horizontally in the vacancy, C*H is the protonation of a surface carbon, and M*H is the protonation of a surface metal atom. The binding of different intermediates to the transition metal carbide is given by the order as specified by the legend.Figure 5 shows a comparison of the free energy diagrams for ZrC with a vacancy (the lower diagram) and without a vacancy (the upper diagram) at zero potential. The labellingAX refers to the positioning of the adsorbed species in the unit cell, in this case, site A as defined in the inset figure in the middle. The inset figure found in the middle defines all relevant adsorption sites as well as the unit cell (black square), black circles indicate the periodic vacancy formation. The PDS and the free energy change associated with it can be found under the relevant adsorbed species.Figure 6 shows non-restricted free energy diagram for WC at zero potential. The black base path describes the unconstrained mechanism, each reaction step proceeds via the most favourable intermediate, starting with the formation of a carbon vacancy on the surface. The steps labelled by (1) are adsorption energies onto the clean surface, also included is the adsorption of NNH and H into the vacancy. The two branching paths (given with a different format of dashes between states) from the base path describe pathways resulting from the formation of an alternative intermediate with slightly higher energy as compared to the base path. The activation free energy for the dissociation of adsorbed N2 is labelled AGa(AGa= AEa+ 0) for the appropriate reaction step. The labellingAX refers to the positioning of the adsorbed species in the unit cell, in this case, site A as defined in the inset figure in the top middle. The inset figure found in the top middle defines all relevant adsorption sites as well as the unit cell (black square). Thevalue of the required OP (defined by the PDS) is denoted next to the corresponding reaction step on each pathway.Figure 7 shows non-restricted free energy diagram for CrC at zero potential. The base path describes the unconstrained mechanism, each reaction step proceeds via the most favourable intermediate, starting with the formation of a carbon vacancy on the surface. The steps labelled by (1) are adsorption energies onto the clean surface, also included is the adsorption of NNH and H into the vacancy. The path toACH also describes the most favourable reaction step following the protonation of the vacancy (poisoning). The value of the required OP (defined by the PDS) is denoted next to the corresponding reaction step on the path, as well as the PDS for the formation of the first equivalent of ammonia along the reaction path denoted by brackets. The labellingAX refers to the positioning of the adsorbed species in the unit cell, in this case, site A as defined in the inset figure in the top middle. The inset figure found in the top right defines all relevant adsorption sites as well as the unit cell (black square).Figure 8 shows non-restricted free energy diagram for NbC at zero potential. The black base path describes the unconstrained mechanism, each reaction step proceeds via the most favourable intermediate, starting with the formation of a carbon vacancy on the surface. The steps labelled (1) are adsorption energies onto the clean surface, also included is the adsorption of NNH and H into the vacancy. The value of the required OP (defined by the PDS) is denoted next to the corresponding reaction step for each pathway. The three branching paths (given by a different format of dashing then the base path) describe pathways resulting from the formation of an alternative intermediate with slightly higher energy as compared to the black path. The three branching paths describe pathways resulting from the formation of an alternative intermediate with slightly higher energy as compared to the base path. The activation free energy for the dissociation of adsorbed N2 is labelled AGa(AGa= AEa+ 0.2) for the appropriate reaction step. The labellingAX refers to the positioning of the adsorbed species in the unit cell, in this case, site A as defined in the inset figure in the top middle. The inset figure found in the top middle defines all relevant adsorption sites as well as the unit cell (black square).Figure 9 shows the overpotential of each TMC surface in the presence of a carbon vacancy. The bars which are marked with a * proceed to ammonia formation along the reaction pathway used to obtain the overpotential value. The other bars indicate surfaces that do not proceed to form two equivalents of ammonia, but rather produce either H2 (g) or CH4 (g).Figure 10 comparison of binding energies of protons, hydroxyl, and oxygen species in the vacancy of the TMCs, only the TMCs that favourably bind nitrogen dissociative ly are shown. The energies are plotted as the difference between the adsorption energy ofthe relevant species, at the required OP for operation, and the dissociative adsorption of N2. The required OP for each material can be found next to their respective name under each column.Figure 11 The minimum energy path for dissociative N2 adsorption onto ZrC (without a vacancy), WC, NbC, and VC in the presence of a vacancy. The activation energy is marked as an inset.Figure 12 illustrates a sub-layer carbon atom migrating to a vacancy in the surface along with the minimum energy path calculated for WC, VC and NbC. The numerical value of the activation energy for each material is given the respective colour.Figure 13 the required potential to drive the reactions corresponding to the most favourable reaction pathway for the restricted formation of ammonia on the TMC surfaces in the presence of a vacancy as a function of the dissociative adsorption energy of nitrogen. All the reactions plotted are labelled by colour in the lower left corner of the plot. For each reaction, the linear best fit has been drawn as a dotted line with the corresponding R-value displayed. For each TMC the limiting reaction has been labelled with the appropriate molecular symbol.Figure 14 The required potential to drive the reactions as a function of the dissociative binding energy of nitrogen on the TMC surfaces in the presence of a vacancy. This is derived from the scaling relation plots shown in Figure 15. The individual data points are only displayed for the two limiting reactions (right side and left side), and the reaction with the highest required potential is labelled with the corresponding TMC.Figure 15 shows the adsorption of different intermediates on the pristine structures of TMCs with a (110) facet at zero potential.Figure 16(a) shows a free energy diagram for TaC (110) at zero potential.Figure 16(b) shows a free energy diagram for WC (110) at zero potential.Figure 17 shows a volcano plot and illustrates the relationship between the free energy change of nitrogen adsorption (AG*N2) and the applied voltage (U vs RHE) for various transition metal carbides.DETAILED DESCRIPTIONIn 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.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 itseffect. 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. 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.The present invention provides a process and a system for generating ammonia at ambient temperature and pressure, wherein 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, H2 and 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:H22(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 asThe 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 1-7 below describing the so-called associative mechanism, where an asterisk denotes a surface site:*NH + 2(H++ e-) *NH2+ (H++ e") [6A]*NH2NH2+ 2(H++ e") *NH2+ NH3(g) + (H++ e") [6Bb]*NH2+ (H++ e-) NH3(g) + * [7]For the dissociative mechanism the reaction mechanism is according to equations 8-14:Alternatively, formation of methane (CFU) 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 carbide catalysts or be intentionally prepared during the synthesis of the electrode and / or transition metal carbide 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 carbide 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 carbide selected from the group consisting of Yttrium carbide, Scandium carbide, Hafnium carbide, Zirconium carbide, Titanium carbide,Chromium carbide, Niobium carbide, Molybdenum carbide, Tantalum carbide, Wolfram carbide. Any mixtures and combinations of two or more of these are also applicable in the invention.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, or bromide such as, but not limited to, sodium chloride, potassium chloride, calcium chloride, ammonium chloride, and other suitable salts. The aqueous electrolyte solutions may also comprise any one, or a combination of, alkali or alkaline earth metal oxides, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, rubidium hydroxide and cesium 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 carbides 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 carbides are also affected by the coordination of the metal cation and carbon anion, 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 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 different crystal structures exist for transition metal carbides 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. The catalyst surface comprises in preferred embodiments at least one surface having a (100) facet, or a (110) facet or a (111) facet. 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 .02 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 operated to form ammonia using the above-mentioned electrode potentials.At high electrode potentials, e.g., -1 .2 V, or even -1 .0 V, a material may be prone to favourthe 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.In some of the embodiments, a mixture or combination of two or more of the catalyst materials in accordance with the present invention may be used to lower the required electrode potential.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 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. Forexample, 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.NOXand SOXare 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 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 carbide 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 (NH^SC . 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 SOXand / or NOXexhaust 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 SOXand / or NOXfrom 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, NOXand / or SOXare 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.EXAMPLESIn the present example, an investigation of several transition metal carbides (TMCs) catalysts for the electrochemical production of ammonia under ambient circumstances is provided, wherein the activity of TMCs on rock-salt (RS) (100) surfaces is studied using density functional theory (DFT) to collect thermodynamic data and other activity descriptors. The onset potentials necessary for ammonia production, free energy diagrams, poisoning tendencies, kinetic barriers, and surface stability are among the collected data used to determine the activity of TMC surfaces. The computational standard hydrogen electrode [1] is used to account for the influence of an external potential, which is considered in the context of activity and stability of the surface at the operating potential.The computational methodology used in this example is as follows:All calculations are performed using Density functional theory (DFT) with the RPBE exchangecorrelation functional. The Vienna ab-initio simulation package (VASP) was used with a 4 x 4 x1 Monkhorst-Pack K-point mesh and a 400 eV energy cutoff for all the surfaces. The Projector augmented wave (PAW) method is used as implemented in the VASP code; this gives access to the full wavefunction by utilizing computationally efficient pseudopotentials, significantly reducing computation time. The occupation of the Kohn-Sham orbitals is smeared according to a Fermi-Dirac distribution with a smearing parameter of ksT = 0.1 eV.In total, 11 different TMCs are considered in this example, namely, YC, ScC, HfC, ZrC, TiC, VC, CrC, NbC, TaC, MoC, and WC. All of the surfaces are considered in the rock-salt (RS) crystallographic structure and the (100) surface orientation. The TMC surfaces are modeled with a 5-layer 2x2 unit cell, containing 20 metal atoms and 20 carbon atoms, see Figure 1 . The2 bottom layers are fixed in place / constrained while the top layers, along with any adsorbates, are allowed to relax, i.e., to become optimized by minimizing the atom forces acting on the atoms. Boundary conditions are periodic in the x and y directions, and each surface slab is separated with at least 12 A of vacuum in the z-direction, to minimize the influence of neighboring slabs. The structural optimization is considered converged when the forces in any direction on all moveable atoms are below 0.01 eV A-1. Activation energies are calculated as the highest point along the minimum energy path (MEP) using the climbing image nudged elastic band method.The total reaction pathway for the formation of ammonia can be modeled via the following equations:N2+ 6(H++ e ) 2NH3H22(H++ e )In the electrochemical cell, the reactants required for the formation of ammonia at the cathode are supplied by nitrogen in the atmosphere, protons formed at the anode, and electrons from the applied potential. Since the protons required for the reaction are formed exclusively at the anode, it is possible to make a direct comparison of the applied potential to the standard hydrogen electrode (SHE) or the reversible hydrogen electrode (RHE). The reaction pathway was modeled via an unconstrained mechanism where for each protonation step the most thermodynamically favorable adsorption site is found and used for the next protonation until a product was formed. An unconstrained mechanism (i.e., sampling of adsorption sites) gives the best insight into the catalytic activity of the catalyst and which product(s) it tends to form. For each reaction step the free energy change, at U = 0 V vs. RHE and pH = 0, can be calculated via the following equation:AG(0) = AE + AEZPE - TASWhere AG(0) is the free energy change at pH = 0, AE is the thermodynamic value calculated via DFT, AEZPE and TAS are the calculated zero point energy and entropy corrections. To calculate the free energy change at a given potential and pH value (AG(U, pH)) the implicit computational hydrogen electrode approach is used:AG(U, pH) = AG(0) - neU - kBTln(10) x pH where n is the number of electrons involved in the reaction step, e is the charge of an electron, and U is the applied potential. All reported data is calculated at pH = 0. For each reaction pathway, the potential determining step (PDS) is defined to be the electrochemical reaction step with the highest positive free energy change. From the PDS, the overpotential (OP) is defined as the potential required to make the free energy change of the PDS zero,OP = -AG / eThe OP (in volts) is simply equal to the negative magnitude of the free energy change associated with the PDS in electron volts.In this example, the competition between adsorption of N2H and H on the surface is investigated, and based on this information, the catalytic properties of the carbides that show higher probability of binding NNH rather than H in the associative mechanism is studied. The possibility of ammonia formation on these surfaces via the dissociative mechanisms is studied as well as by looking into the adsorption free energies of 2*N (AGZ*N) on the surface and calculation of the activation energy for the N2 dissociation where AG2*N is exergonic. Moreover, the adsorption free energy of N and H is calculated on the clean surface of these carbides to investigate the competition between nitrogen reduction to ammonia and hydrogen evolution reaction. The possibility of ammonia formation on both clean carbide surfaces and the carbides with the presence of defect, namely carbon vacancy, is also investigated via both the associative and dissociative mechanisms and predicted the onset potentials needed forammonia formation. Free energy diagrams are constructed for all the possible intermediates being formed.The reactivity of TMC surfaces in the RS structure with the (100) crystallographic orientation for the NRR is tested by modelling adsorption energies and reaction pathways for each candidate.. These candidates are chosen based on reported stability on the Materials Project website [2], with the addition of MoC and WC which are reported to be meta-stable, possibly decomposing to other structures or stoichiometries. Firstly, for any reactivity to occur adsorption of a nitrogen species onto the surface should be investigated. For the adsorption of nitrogen three possible mechanisms are considered, the associative binding of nitrogen gas CN2), the dissociative binding of nitrogen gas (*2N), and the associative binding of NNH (*NNH). For each of these options, all possible adsorption sites are tested, and the most favorable adsorption is used for comparison, the results for the adsorption of nitrogen and possible active site poisoning by other species can be seen in Figure 2.Along with the adsorption energies of nitrogen species, Figure 2 shows the energy associated with the protonation of a metal site which is a good indicator of the likelihood of the hydrogen evolution reaction (HER), which is usually the main competing reaction to the NRR. As can be seen in Fig 2, only YC and ScC can favourably bind nitrogen, however, all the carbides studied here are expected to bind nitrogen stronger than protons onto the metal site. This eliminates all other candidates, although for ZrC the dissociative binding of nitrogen gas is only very slightly endergonic and so ZrC will also be considered. For YC and ScC, after the binding of nitrogen, the favoured reaction pathway was found by sequential protonation of the surface via an unconstrained mechanism. This means that for each protonation step all possible adsorption sites were considered and the most favorable chosen to continue the reaction. For both these surfaces in all three cases (the three different nitrogen species) the formation of ammonia (NH3) was not observed. The favorable protonation of carbon sites over nitrogen led to the formation of either methane (CH4) or hydrogen (H2) at which point calculations were terminated. The formation of methane or hydrogen indicates a tendency for either the decomposition of the surface or the prevalent presence of the HER, both of which are not desired for an NRR catalyst. This behavior is reflected in the exergonic protonation of carbon sites on some of these TMCs, YC, and ScC included. In addition to not forming the desired product, YC and ScC are the only two TMC surfaces that favorably bind the oxygen species O2-at zero potential. Such oxidation of the TMC surface can renderthe catalyst inactive and in the case of exergonic adsorption, partial or total oxidation is difficult to prevent when moving the catalyst from the growth chamber to the electrolytic cell. For ZrC, however, oxidation is endergonic, but it still suffers from exergonic protonation of a carbon site. This means the activity of ZrC will be hindered in an aqueous electrolyte but might be promising in a system using an ionic liquid / molten salts. For the full free energy diagram of ZrC see Figure 3(a).As can be seen in Figure 3(a) although the initial adsorption of nitrogen is slightly endergonic the surface proceeds to ammonia formation with a PDS of 0.69 eV. The PDS is the electrochemical reaction step with the largest free energy change, this will determine the potential required to make the entire path exergonic and the formation of ammonia spontaneous. The reaction pathway for ZrC at the requiredoverpotential (OP) is shown in Figure 3(b). Also, Figure 3(a) shows a comparison between the dissociative binding of nitrogen, associative binding of nitrogen, binding of NNH, the adsorption of hydrogen onto a metal site, and the adsorption of hydrogen onto a carbon site on the clean surface. ZrC is, therefore, a potential catalyst for the NRR, the required OP is modest (-0.69 V), and the reaction pathway only requires 9 protonation steps, meaning there is minimal proton coverage on the surface. Increased surface coverage means a higher possibility of the HER occurring or in the case of TMC surfaces, the formation of methane. If experimentalists have a way of circumventing the endergonic dissociative adsorption of nitrogen onto the ZrC surface (either by pressure or temperature tuning), it is recommended to explore this possible catalyst. In general, however, for the clean TMC surfaces, it is apparent that there is not much potential for catalytic activity, the main problem being the initial binding of the required nitrogen species.To better facilitate nitrogen adsorption the idea of creating a carbon vacancy on the surface may be promising. It should also be noted that in real experiments, TMC should have a large number of defects such as carbon vacancies, because usually a full stoichiometry is hard to reach. To test this hypothesis next, we will look at the TMC surfaces in the presence of a carbon vacancy, the vacancy is created by removing one carbon from the surface as CH4 and letting the resulting structure relax. Here, it is assumed that either the surface has pre-existing vacancies formed in the manufacturing of the material or that the surface will create CH4 at standard operating conditions. For all of the surfaces tested the free energy associated with the formation of CH4 was exergonic at the respective OP for each material. The presence of a vacancy on the surface provides an active site where nitrogen can more easily bind due to the lower coordination associated with the vacancy and potentially facilitates higher selectivity towards nitrogen adsorption and then protonation. First, the binding of nitrogen species in the vacancy of these TMC surfaces is inspected, see Figure 4.As can be seen in Figure 4 the adsorption energies of the required nitrogen species are vastly improved in the presence of a vacancy when, as is seen by a comparison of Figure 2 and Figure 4. Especially promising is the dissociative binding of nitrogen, which is exergonic for most of the TMCs, this does however eliminate MoC which does not bind any nitrogen species favorably. CrC is an outlier, having only slightly endergonic dissociative adsorption of nitrogen, and so as in the case for clean ZrC, CrC is also included in the list of promising candidates for now. The reaction pathways are calculated for each of the exergonic nitrogen adsorptions until the formation of either two molecules of ammonia or the first by-product (H2 or CH4) is achieved. Some surfaces that proceeded to ammonia formation are WC, NbC, and CrC when binding nitrogen dissociatively, see free energy diagrams in Figures 6, 7, 8 and 9, respectively. Since the dissociative binding of nitrogen is always favored over the other nitrogen adsorbates, the focus is now on the dissociative binding of nitrogen on the aforementioned TMC surfaces in the presence of a carbon vacancy. Although the presence of a vacancy gets around the problem of the endergonic dissociative N2 adsorption energy, some TMCs that produced ammonia with a clean surface (without the presence of C-vacancy) did not do so in the presence of a vacancy, see Figure 5. These surfaces are HfC, ZrC, TiC, and TaC. As an example, comparing the reaction pathways of ZrC with a vacancy to that of ZrC without a vacancy, it is possible to see the negative effect thepresence of a vacancy has on the selectivity of this catalyst, and conversely, the positive effect on the adsorption energy of nitrogen, see Figure 5. For ZrC, there is a significant lowering of the adsorption energy of nitrogen, and also a lowering of the required OP which would enhance activity, but the measure of selectivity due to the tightly bound nitrogen molecule in the vacancy. The protonation of the surface nitrogen,AN, proceeds without a hitch, and one molecule of ammonia is developed, but protonation of the nitrogen in the vacancy site seems to be hindered. It is believed that due to the very favorable adsorption of nitrogen, the stability gained by keeping the nitrogen atom in the vacancy is favored over protonation. This would mean that to achieve the desired activity the adsorption of nitrogen would have to be exergonic but below some threshold energy. Looking at Figure 4 it is possible to see that this threshold energy would be approximately up to -1 .0 eV since that would correctly predict that for example WC and NbC, would be active catalysts. There is one outlier, that is TaC, which does not produce ammonia despite having very similar adsorption energy to NbC, only a difference of 0.01 eV. For TaC, and the rest of the TMC that bind the vacancy nitrogen too strongly, it would be interesting to test these surfaces further by exploring the possibility of alternative reaction pathways. For example, investigating the activity in the case of additional nitrogen adsorption after the formation of one molecule of ammonia or the presence of a higher quantity of vacancies. The OP associated with each TMC surface in the presence of a vacancy can be found in Figure 9.As can be seen in Figure 9, all of the TMC vacancy surfaces require an OP of less than 1 eV, which is generally considered the threshold value for good activity. Not all of the TMC vacancy surfaces produce ammonia along the reaction pathway calculated, as previously mentioned, CrC, NbC, and WC produce 2 equivalents of ammonia, marked with a * in Figure 9. The other surfaces produce either H2 (g) or CPU (g) along the most favorable reaction pathway.The introduction of a vacancy not only provides a catalytic site for nitrogen adsorption but can also cause surface poisoning by other ions present in the electrolytic solution. To explore the possibility of poisoning the vacancy site and check the stability of the catalyst against poisoning, the adsorption energies of H+, O2- and OH- in the vacancy are calculated and compared to the dissociative binding of nitrogen at the operating potential of each TMC, see Figure 10.The poisoning of the catalytic site seems to be a concern in the case of CrC, which prefer to bind protons in the vacancy over the dissociative binding of nitrogen.Now considering CrC, it is observed that the rotation of NH s exothermic for CrC, and so is assumed to also have an insubstantial barrier. CrC, however, suffers from the same subsequent protonation of a neighbouring carbon atom which is much more favourable than the dissociative adsorption of nitrogen. What makes CrC interesting, however, is that if these branching protonation paths can be circumvented the formation of the first equivalent of ammonia only requires an OP of 0.19 V, see Figure 7. Although when the full path is considered, the required OP is -0.79 V, a mechanism utilizing the lower OP by forming one equivalent of ammonia and then regeneration of the surface orwith both nitrogen molecules adsorbed in vacancies would be interesting to investigate. Consequently, we recommend investigatingCrC further either in non-aqueous electrolyte or in the presence of a higher concentration of surface vacancies, but the surface will not be considered further in this example.Another kinetic barrier that must be considered is the activation energy of N2 adsorption. NEB calculations for some promising candidates: ZrC (without a vacancy), WC and NbC, are performed, and the results are shown in Figure 11 . The kinetic barrier for dissociative nitrogen adsorption onto the NbC surface in the presence of a vacancy is too high (1 .54 eV) to consider NbC a promising catalyst under ambient conditions. The catalyst must be able to adsorb nitrogen on a reasonable timescale for the production to be commercially viable, and a kinetic barrier over 1 .0 eV is certainly going to impede that process at ambient conditions. For the other candidates, the kinetic barrier is acceptable and surmountable in ambient conditions, but it is important to compare these values to the kinetic barriers associated with the migration of a sub-layer carbon to fill the surface vacancy. This comparison gives a measure of how likely the surface is to decompose under operating conditions and thus could be considered a good indicator of stability. If the barrier for migration (causing decomposition) is lowerthan the barrier for nitrogen adsorption (causing regeneration), surface vacancies are more likely to be filled by sub-layer carbon. Another carbon surface vacancy might subsequently form, and the process continues to erode the catalyst and decomposes it to its parental metal. The path of migration for a second-layer carbon to fill a surface vacancy along with the associated energy values is shown in Figure 12. In all cases, the barrier for carbon sub-layer migration is more than 1 eV higher than the barrier for dissociative N2 adsorption. This means the migration of sub-layer carbon to a surface vacancy should not be of concern, and these TMCs are expected to show stability against decomposition. It is important to note that for NbC, although the barrier for dissociative nitrogen adsorption is very high (approx. 1 .54 eV) since the barrier for sub-layer carbon migration is much higher (2.96 eV) the catalyst might still show good stability under the correct temperature and pressure conditions. For instance, tuning the temperature, increasing it above ambient conditions, could make the adsorption barrier of 1 .54 eV surmountable while keeping carbon migration kinetically hindered, for the full reaction pathway of NbC, see Figure 8.The activity of TM-based catalysts for the NRR has been shown to be limited by scaling relations [3], The linear scaling of certain reactions on the catalyst surface causes the thermodynamics of the optimal catalyst to be limited. These scaling relations can be a valuable way to predict the activity of a potential catalytic surface and to determine if any of the tested materials display the optimal energetics. To determine whether TMCs are limited by scaling relations, in the case of dissociative nitrogen adsorption in the presence of a vacancy, the energies of all reaction steps involved in the reaction pathways calculated are plotted as a function of dissociative nitrogen adsorption. This reaction pathway, TMC vacancy surfaces with dissociative nitrogen adsorption, is chosen due to displaying the most promising catalytic properties. These calculations, however, do not yield a volcano plot, as the reaction pathways for each TMC surface are too varied, and no discernible trend was obtained. There is no shared limiting reaction step for all of the TMC surfaces. The presence of scaling relations is also investigated in the case of a simplified reaction mechanism that only considers reaction steps leading to ammonia formation. This limits the reaction configurational space, making it more likely a trend can be obtained.All possible reaction pathways leading to ammonia formation are calculated via a restricted Mars van Krevelen (MvK) mechanism, and the pathway leading to the average lowest OP is plotted, see Figure 13. From the total reaction pathway, a volcano plot was extracted, see Figure 14. This volcano plot, however, provides limited information due to the simplified reaction mechanism but can be used to assess the relative tendency of a surface to produce ammonia in the absence of undesirable side reactions.Figure 6 shows that WC is a very promising catalyst that produces ammonia after 9 protonation steps at a low OP (-0.35 V), and is very selective towards NH3. For further analysis, the electronic properties of some of the promising candidates; CrC.NbC, and WC for the clean surface, the vacancy surface, and the vacancy surface with nitrogen dissociatively adsorbed are performed.To summarize, the outcome of the comprehensive analysis of the suitability of TMCs as catalysts for the NRR reveals that without the presence of a surface defect these TMCs are limited in their reactivity due to endergonic adsorption of nitrogen. ZrC is the only TMC that could semi-favorably adsorb nitrogen and subsequently produce ammonia. ZrC is, however, prone to protonate carbon sites over nitrogen and so not recommended as a selective NRR catalyst in aqueous solution. Since these surfaces are prone to form surface carbon vacancies at their respective operating conditions, the unfavorable nitrogen adsorption observed for these TMCs could be circumvented in the presence of a carbon vacancy, which should also be common on a real surface. When the vacancy is introduced, most of the TMC surfaces adsorbed nitrogen favorably and WC and NbC may subsequently produced ammonia. A lack of ammonia formation is observed for the TMCs that featured highly exergonic nitrogen adsorption; the lack of reactivity is likely due to the stability gained by having nitrogen in the vacancy outweighing the tendency for protonation. For the catalysts with promising activity, subsequent calculations on potential poisoning and kinetic barriers only ruled out NbC due to high activation energy for the dissociative adsorption of N2. NbC is therefore not a promising candidate under ambient conditions but might display good activity under the correct temperature and pressure conditions. WC was the most promising candidate, displaying good activity, stability, resistance to poisoning, low OP (- 0.35 V), and selectivity. We highly recommend WC to experimentalists to explore the possibility of utilization for the NRR in real-life applications and if promising then possibility of scale-up and commercialization. Although ZrC was promising when exploring the clean TMC surface, it did not display catalytic activity in the presence of a vacancy. More work is required to understand the viability of using surfaces such as CrC or ZrC in the presence of a vacancy, which display mostly favorable energetics but suffer either from slightly endergonic adsorption energies or binding nitrogen too tightly in vacancy sites. These surfaces might produce ammonia if more complex reaction pathways are considered, such as a reaction pathway involving a higher concentration of vacancies.In a second example, a computational screening technique was applied to assess a variety of transition metal carbide (TMC) catalysts. The catalysts include elements from groups III to VI and have a rocksalt (RS) structure with (110) facets. The extensive investigation analyzes a total of eleven transition metal carbide (TMC) surfaces, including ScC, YC, TiC, VC, CrC,YC, ZrC, NbC, MoC, HfC, TaC, andWC. The catalysts exhibit stability and are thus chosen for further comprehensive study as candidate catalysts for the nitrogen reduction process (NRR).The evaluation method for these TMC candidates, which aims to generate ammonia electrochemically at ambient conditions, consists of a comprehensive analysis of at least four important variables, (i) The mechanism of the NRR by examining the distinct paths and reactions facilitated by each TMC. (ii) The evaluation of surface poisoning is conducted to determine the vulnerability of each TMC surface to chemicals that may hinder their catalytic activity, (iii) An evaluation is conducted to assess the overall catalytic performance of each transition metal catalyst (TMC) to determine their level of efficiency and efficacy in promoting the nitrogen reduction reaction (NRR). (iv) volcano-plot analysis is performed to show the optimal performance (OP) of each TMC based on a reactivity descriptor.Mechanism of nitrogen reduction reactionIn this example, three different mechanisms for the nitrogen reduction reaction (NRR) are investigated. These mechanisms include the dissociative mechanism (DM), the associative mechanism (AM), and the Mars-van Krevelen mechanism (MvK). The strong triple bond that exists between nitrogen atoms in the DM is broken when N2 comes into touch with the surface of the catalyst, which could be an atom of carbon or metal. The end outcome of this procedure is that every single nitrogen atom binds to the catalyst on its own behalf.In the AM, N2 adsorbs onto the surface of the catalyst and goes through bond cleavage at a certain step of the hydrogenation reaction. This is the opposite to the direct cleavage that occurs in the DM. Within the confines of this framework, the possibility that adsorbed NNH (*NNH) should play the role of an intermediary in the reaction pathway was investigated. Incorporating the adsorption of NNH into the free energy diagrams was necessary in order to acquire a comprehension of the mechanism itself. It is possible to provide greater insights into the overall reaction kinetics and the efficiency of the AM in boosting ammonia synthesis as a result of this addition, which enables more precise definition of the energy environment and identification of steps that may be thermodynamically beneficial.In the MvK mechanism, lattice carbon atoms on the surfaces of TMCs are converted into methane. This conversion is dependent on whether proton adsorption takes place on carbon sites or metal site, respectively. During this process, surface vacancies are created, which are essential for the continuation of catalytic activity. It is possible for nitrogen molecules to fill these vacancies through either an associative (associative MvK) or dissociative (dissociative MvK) process, which will result in the catalyst being efficiently regenerated.Surface PoisoningBefore initiating any reaction, it is critical to thoroughly investigate how nitrogen species adhere to the catalyst surface. This is especially relevant when competing species such as H, oxygen (O), and hydroxyl (OH) are present. Three types of nitrogen adsorption are investigated: associative adsorption of N2 molecules, dissociative adsorption of nitrogen gas (*2N), and associative adsorption of NNH molecules. These scenarios were compared to the adsorption of H, O, OH, and H2O to determine howthey would compete with one another and how likely it is that these species will harm the active site. The results of these scenarios is summarized in Figure 15.The results show that some TMCs are better at attracting dissociated nitrogen (*2N) than other species. In addition, we investigated how protons attach to the metal surfaces to evaluate the probability that the hydrogen evolution process (HER) will occur. Interestingly, protons move to the carbon sites instead of staying on the metal sites. This pattern of activity suggests that proton adsorption on metal sites, which would lead to HER, is not likely for all the TMCs that were tested.The materials appear to prefer dissolved nitrogen adsorption, which may be interpreted as a possibility of being efficient catalysts for NRR, opening up a possible way to make ammonia quickly. But, to keep the high catalytic activity, it's important to keep an eye on the possibility of rival species like H, O, OH, and H2O poisoning the active site. The finding that protons move to carbon sites, making it less likely for HER to happen on metal sites, adds to the evidence that TMCs may be stable and effective in catalytic uses.Catalytic activityA thorough examination of the reactions that occur on the RS (110) facets of TMCs was carried out, assessing several processes previously described for all eleven TMCs under consideration. However, for the sake of clarity, only free energy diagrams for the most promising candidates are included.Figure 16(a) and Figure 16(b) show that the AM is the viable pathway for creating ammonia on TaC and WC, respectively. N2 is adsorbed exergonically onto the catalyst surface, triggering the protonation reaction. The initial proton preferentially binds to the surface carbon rather than the adsorbed nitrogen. When the surface has reached a half-monolayer coverage of protons, the fifth proton bonds with N2 to create NNH. After ten protonation steps, we have two ammonia molecules, with a total potential determining step (PDS) of 0.69 eV. Furthermore, adsorbing NNH on the surface tuned the PDS from 0.69 to 0.66 eV, favoring NH3 generation.The Mars-van Krevelen (MvK) mechanism was also investigated, which considers carbon vacancies as either CH4 or surface defects and fills them with nitrogen gas in several types of forms: associatively as N2, dissociatively as 2N, and associatively as NNH. Our study revealed that the MvK mechanism is ineffective at ambient temperatures due to the high PDS. While NH3 desorption is not an electrochemical step, in acidic electrolytes, it can be converted to NH4+and then released from the interface. However, in neutral or alkaline media, such a phase may act as a bottleneck.In the instance of WC, the adsorption of NNH is endergonic, thus we begin with the AM pathway. After a half-monolayer of protons, the fifth proton attaches to metal rather than N2. Following the ninth protonation step, the first ammonia molecule is released automatically, resulting in the synthesis of two ammonia molecules after twelve steps with a PDS of 0.82 eV. The MvK mechanism was also tested for WC. However, our study suggests that it may be ineffectual due to the high PDS, which prevents ammonia production.Exploring the scaling relations and formation of volcano plotsA "volcano plot" shows that catalytic activity peaks at an ideal binding energy and decreases on either side of this peak. The peak of the volcano represents the perfect equilibrium between the energies of adsorption and desorption: the catalyst binds intermediate strongly enough to promote reaction without binding so strongly that it prevents desorption. Reaction rates are lowered by too strong binding, which is seen in catalysts on the left slope of the volcano plot. On the other hand, too weakly bound intermediates by catalysts on the right slope cause insufficient adsorption to efficiently drive the process.At the top of the volcano binding energy is optimized for maximal catalytic activity, the volcano plot is an important tool in catalyst design. A volcano diagram may be built by using linear relations for different reaction steps as a function of the Gibbs free energy change (AG) of *N2 (used as a descriptor). This technique is well-known and has already been used to electrolyze water on oxide surfaces. With this method, electrocatalytic activity may be directly indicated by the potential connected with the potentiallimiting step. Every basic step in the reaction has its free energy stated as a function of the applied bias (U) and the AG of N2, represented by *N2.The volcano plot is shown in Figure 17. Notably, TaC and WC are shown to have better catalytic performance with low overpotentials by being at the top of the volcano plot. A logical design and selection of very effective electrocatalysts is made possible by this thorough knowledge of the relation between binding energy and catalytic activity.ConclusionUsing density functional theory calculations, the catalytic activity of the (110) facets of rocksalt structures is investigated for 11 different transition metal carbides. The catalytic activity of these materials is investigated using free energy diagrams of the Mars-van Krevelen, associative, and dissociative reaction mechanisms. Additionally, the materials' resistance to electrochemical poisoning and degradation into their constituent metals during operation is investigated. Under ambient conditions, tantalum carbide (TaC) and tungsten carbide (WC) exhibit exceptionally low onset potentials (-0.66 V and -0.82 V vs RHE, respectively) following the associative mechanism.References:

[0001] J. K. N0rskov et al. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. TheJournal of Physical Chemistry B, 2004. 108(46): p. 17886-17892.6.[2] A. Jain*; S.P. Ong* (*=equal contributions) et al.; The Materials Project: A materials genome approach to accelerating materials innovation. APL Materials, 2013. 1 (1), 011002[3] J. H. Montoya et al. The Challenge of Electrochemical Ammonia Synthesis: A New Perspective on the Role of Nitrogen Scaling Relations. Chem Sus Chem, 2015. 8(13): p. 2180-2186

Claims

CLAIMS1 . A process for producing ammonia, said process comprising: a. feeding N2 to an electrolytic cell that comprises a cathode, an anode, an electrolyte and at least one source of protons, b. allowing the N2 to 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 carbide selected from the group consisting of Zirconium carbide, Chromium carbide, Niobium carbide, Yttrium carbide, Scandium carbide, Hafnium carbide, Titanium carbide, Molybdenum carbide, Tantalum carbide and Tungsten carbide, 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 at least one catalyst surface comprises one or more transition metal carbides selected from the group consisting of Zirconium carbide, Niobium carbide and Tungsten carbide.

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 or a (1 10) facet, or a combination thereof.

5. The process according to any one of the preceding claims, wherein the catalyst surface may comprise at least one carbon vacancy.

6. The process according to any one 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.

8. The process according to any of the preceding claims, wherein less than 50% moles H2 are formed compared to moles NH3 formed, 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 H2 oxidation 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 0°C to 80°C, preferably in a range from 10°C to 50°C, more preferably in the range from 20°C to 30°C, even more preferably in the range from 20°C to 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 the 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, even more preferably the process being carried out at atmospheric pressure.

15. The process according to any of the proceeding 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: Zirconium carbide, Chromium carbide, Niobium carbide, Yttrium carbide, Scandium carbide, Hafnium carbide, Titanium carbide, Molybdenum carbide, Tantalum carbide and Tungsten carbide.

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

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

19. The system according to any one of the preceding claims 16 to 18, wherein the catalyst surface comprises at least one surface having a (100) facet or a (110) facet, or a combination thereof.

20. The system according to any one of the 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 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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