RU2803599C2 - Electrolytic ammonia production using catalysts based on transition metal oxides - Google Patents

Abstract

FIELD: electrical chemistry.

SUBSTANCE: method for producing ammonia, including: supplying N₂ to an electrolytic cell containing at least one source of protons; enabling N₂ contact with the surface of the cathode electrode in an electrolytic cell, where the surface of the cathode electrode contains a catalytic surface containing at least one transition metal oxide selected from the group consisting of niobium oxide, tantalum oxide, osmium oxide, rhenium oxide and iridium oxide, where at least one transition metal oxide contains at least one surface having a face; and passing a current through this electrolytic cell, whereby nitrogen reacts with protons to form ammonia. The invention also relates to a system.

EFFECT: obtaining ammonia at room ambient temperature and atmospheric pressure.

22 cl, 44 dwg, 1 tbl, 2 ex

Description

Field of technology

This invention relates to the field of chemistry of technological processes and, in particular, concerns the production of ammonia by electrolytic methods and new catalysts intended for this.

Introduction

Ammonia is one of the chemicals produced in the world in the highest volume. Industrial ammonia synthesis, now called the Haber-Bosch process, is the first heterogeneous catalytic system, a key element in the global industrial production of nitrogen fertilizers. Today, ammonia is also attracting attention as a possible energy carrier and a potential fuel for transport with high specific energy intensity, but without CO ₂ emissions. The centralized and energy-intensive Haber-Bosch process requires high pressure (150-350 atm) and high temperature (350-550 °C) to directly dissociate and combine nitrogen and hydrogen gas molecules over a ruthenium or iron catalyst to form ammonia by the following reaction:

The disadvantage of this industrial approach is the high temperature and pressure required due to kinetic and thermodynamic reasons. Another and more serious disadvantage is that hydrogen gas is obtained from natural gas. This multi-step process takes up the majority of all chemical production and is the most expensive and environmentally unfriendly. This fact is the main reason why it is necessary to develop a self-sustaining process, since at some point natural gas reserves will be exhausted. In this regard, a small-scale system for decentralized ammonia production that uses less energy and environmental conditions will be of great importance. In addition, to optimize the efficiency of ammonia synthesis, new catalysts capable of hydrogenating molecular nitrogen at a reasonable rate, but under milder conditions, will be highly valued.

The triple bond in molecular nitrogen _N2 is very strong and, as a result, nitrogen is very inactive and is often used as an inert gas. It ruptures under the harsh conditions of the Haber-Bosch process, but it also ruptures under environmental conditions through a natural process carried out by microorganisms using the enzyme nitrogenase. The active center of nitrogenase is the MoFe ₇ S ₉ N cluster, which catalyzes the formation of ammonia from solvated protons, electrons and atmospheric nitrogen through an electrochemical reaction

N ₂ + 8H ⁺ + 8e- → 2NH ₃ + H ₂

Inspired by nature, biological nitrogen fixation as an alternative to the Haber-Bosch process for ammonia synthesis under ambient conditions has attracted much attention. Much effort has been invested in research aimed at developing similar electrochemical processes. In recent decades, various methods for the synthesis of ammonia under ambient conditions have been explored. (Giddey S, Int J Hydrogen Energy 2013, 38, 14576-14594; Amar A, J Solid State Electrochem 2011, 15, 1845-60; Shipman MA, Catalysis Today 2017, 286, 57-68). Although such studies have provided insight into the formation of ammonia, their kinetics are still too slow for practical use, and in most cases hydrogen gas is generally produced faster than nitrogen protonation occurs. Reducing molecular nitrogen with protons and electrons to selectively form ammonia at room temperature and pressure has proven to be much more difficult than expected.

The inventors have previously found (WO 2015/189865) that certain metal nitride catalysts can be used in electrochemical ammonia processes. However, it remains unclear whether other or additional metal compounds may be effective in producing ammonia. Other attempts to artificially synthesize ammonia using electrochemical methods have resulted in relatively low current efficiency (CE). For many of them, regeneration of the active nitrogen-fixing complex has proven problematic, and production rates are far from cost-effective.

Brief description of the invention

The above features, together with additional details of the invention, are further described in the following examples, which are intended to further illustrate the invention, but are not intended to limit its scope in any way.

The present inventors have discovered that certain transition metal oxide catalysts can be used in electrochemical processes for the production of ammonia. This has led to the present invention, which makes it possible to produce ammonia at ambient room temperature and atmospheric pressure.

The present invention relates to a method for producing ammonia, comprising supplying N ₂ to an electrolytic cell containing at least one proton source; allowing N ₂ to contact the surface of a cathode electrode in an electrolytic cell, wherein the surface of the cathode electrode comprises a catalytic surface containing at least one transition metal oxide; and passing a current through said electrolytic cell, causing nitrogen to react with protons to form ammonia.

The invention also relates to a system for generating ammonia, in particular, a system that implements the process for producing ammonia disclosed herein. Thus, the invention relates to a system for generating ammonia comprising at least one electrochemical cell containing at least one cathode electrode having a catalytic surface, wherein said catalytic surface is filled with at least one catalyst containing one or more transition metal oxides.

In the method and system according to the invention, the transition metal oxide may be selected from the group consisting of titanium oxide, chromium oxide, manganese oxide, niobium oxide, tantalum oxide, ruthenium oxide, rhodium oxide, platinum oxide, osmium oxide, rhenium oxide and iridium. In some preferred embodiments, the oxide is selected from the group consisting of rhenium oxide, tantalum oxide and niobium oxide.

The catalytic surface may comprise at least one surface having a rutile structure, in particular a rutile structure having a (110) face on which catalytic reactions occur. In some embodiments, the catalyst comprises a surface having bridging regions between sixfold coordinated transition metal atoms covered with hydrogen atoms.

BRIEF DESCRIPTION OF GRAPHIC MATERIALS

One skilled in the art will appreciate that the figures described below are for illustrative purposes only. The figures are in no way intended to limit the scope of the present invention.

In FIG. Figure 1 shows three different surfaces on the (110) face. (a) The restored surface is on the left, where the bridging regions between the sixfold coordinated metal atoms remain vacant. The bridge and cus areas are marked in the drawing of the restored surface. (b) The surface modified with 0.5 ML (monolayer) of hydrogen presents a structure in the center where hydrogen atoms occupy br sites. (c) Surfaces modified with 0.5 ML oxygen, where oxygen occupies br sites instead of hydrogen, are depicted on the right.

In FIG. Figures 2-12 show the relative stability of adsorbates formed by proton reduction and water oxidation on various transition metal (110) oxide surfaces in the rutile structure. The top (flat) line in each diagram shows the reduced surface used as a reference, where all bridge oxygen atoms have been reduced to H ₂ O. The relative stability of the adsorbates for the (110) faces of NbO ₂ , TiO ₂ , TaO _2, ReO _2, IrO _2, OsO _2, CrO _2, MnO _2, RuO _2, RhO ₂ and PtO ₂ , respectively.

In FIG. Figure 13 shows the free energy diagram for the formation of NH ₃ on the (110) face of rutile NbO ₂ . The potential determination step (PDS) for the hydrogen-modified surface is the first protonation step, while the PDS for the oxygen-modified and reduced surfaces is the last protonation step from *NH ₂ to NH ₃ . All reaction steps refer to a clean surface and N ₂ and H ₂ in the gas phase. Desorption of ammonia gas is indicated by arrows. A black intermediate refers to surfaces where no other intermediate is specified, while a different color intermediate refers only to that specific surface.

In FIG. 14-23 show free energy diagrams for the electrochemical formation of ammonia on the (110) faces of TiO ₂ , TaO _2, ReO _2, IrO _2, OsO _2, CrO _2, MnO _2, RuO _2, RhO ₂ and PtO ₂ , respectively, in the structure rutile.

In FIG. Figure 24 shows the predicted initial potentials (ΔG = -U) for each of the oxide surfaces modified in different ways; modified with hydrogen, modified with oxygen and reduced. The wider bars (to the left of the initial potentials for each oxide) reflect the preferred surface modification (SM) as the applied potential changes, with the potential shown on the y-axis. The oxides are arranged from left to right in increasing Potential Determination Stage (PDS) magnitude on the hydrogen-modified surface.

In FIG. 25-35 show free energy diagrams for the formation of NH ₃ on the H-modified surface of 11 different rutile oxides. All reaction steps refer to a clean surface and N ₂ and H ₂ in the gas phase. All stable intermediates are shown for each reaction step.

In FIG. 36 shows the step of determining the electrochemical formation potential of ammonia on each metal oxide as a function of the N ₂ H binding energy on the metal. The lines are calculated using the scaling relationships shown in FIG. 26-28.

In FIG. 37 shows the step of determining the electrochemical formation potential of ammonia on each metal oxide as a function of the N ₂ H binding energy on the metal. The lines are calculated using the scaling relationships shown in FIG. 38-43. The figure shows a volcano-shaped diagram similar to FIG. 36, but in FIG. Figure 37 shows all stages of the reaction.

In FIG. 38-41 show the adsorption energies of NHNH, NNH ₂ , NHNH ₂ , NH ₂ NH ₂ , NH ₂ +NH ₂ , NH+NH ₃ and NH ₂ depending on the chemisorption energy of NNH. For each adsorbate, the best line equation for the data set is given.

In FIG. Figure 42 shows the N adsorption energies as a function of the N ₂ H chemisorption energies. For each adsorbate, the best line equation for the data set is given.

In FIG. Figure 43 shows the adsorption energies of NH and NH ₂ as a function of the chemisorption energies of N ₂ H. For each adsorbate, the best line equation for the data set is given.

In FIG. 44 shows the step of determining the potential for each stage of the reaction of electrochemical formation of ammonia on metal oxides, plotted as a function of the N ₂ H binding energy on the metal.

Description of the invention

Illustrative embodiments of the invention will now be described with reference to the drawings. These examples are provided to provide a better understanding of the invention without limiting its scope.

The following description describes the sequence of steps. It will be clear to one skilled in the art that, unless the context requires it, the order of the steps is not essential to the resulting configuration and its effect. In addition, it will be apparent to one skilled in the art that regardless of the order of steps between all or some of the described steps may be carried out with or without a time delay.

As used herein, including in the claims, terms in the singular should be construed to also include the plural and vice versa, unless the context indicates otherwise. Accordingly, it should be noted that, as used herein, terms in the singular include the corresponding plural terms unless the context clearly requires otherwise.

Throughout the specification and claims, the terms “comprising,” “including,” and “having,” and variations thereof, are to be construed to mean “including, but not limited to,” and are not intended to exclude other components.

The present invention also covers the precise terms, features, meanings and ranges, etc., when these terms, features, meanings, and ranges, etc. are used in conjunction with such terms as about, about, generally, according to essentially, in principle, at least, etc. (that is, "approximately 3" must also cover exactly 3, or "essentially constant" must also cover exactly constant).

The term “at least one” should be interpreted to mean “one or more” and therefore includes both embodiments including one or more components. In addition, dependent claims that refer to independent claims that describe features using the expression “at least one” have the same meaning when the element is referred to as “specified” and “at least one”.

It should be understood that changes may be made to the above-described embodiments without departing from the scope of the invention. The features disclosed in the specification, unless otherwise indicated, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless otherwise indicated, each feature disclosed represents one example of a general series of equivalent or similar features.

References to examples such as “for example,” “such as,” “for example,” and the like are intended only to better illustrate the invention and are not intended to be limiting as to the scope of the invention unless so stated in the claims. Any of the steps described herein may be performed in any order or simultaneously unless the context clearly indicates otherwise.

All features and/or steps disclosed herein may be combined in any combination, except combinations in which at least some of the features and/or steps are mutually exclusive. In particular, the preferred features of the invention apply to all aspects of the invention and can be used in any combination.

The present invention is based on the unexpected discovery that ammonia can form on the surface of certain transition metal oxide catalysts at ambient temperature and pressure with low applied potential. Given the importance of ammonia, in particular in the production of fertilizers, and the energy-intensive and environmentally unfavorable conditions that are typically used in its production, the invention has important applications in various industries.

Thus, the invention provides methods and systems for generating ammonia at ambient temperature and pressure. Ambient temperature can generally refer to the normal temperatures of both indoor and outdoor air. Accordingly, in some embodiments, the method and system of the invention is used at a temperature of from about minus 10° to about 40°C, such as from about 0° to about 40°C, such as from about minus 10°, or from about minus 5 °C, or from about 0°, or from about 4 °C, or from about 5 °C, or from about 8 °C, or from about 10 °C to about 40 °C, or to about 30 °C, for example , in the range of approximately 20-30 °C or in the range of approximately 20-25 °C. Ambient pressure usually refers to atmospheric pressure. The method and system of the present invention uses an electrolytic cell, which can be any of a number of conventional commercially available and practicable electrolytic cell designs that can accommodate the special purpose cathode of 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 this context is an electrochemical cell that undergoes a redox reaction when electrical energy is applied to the cell.

It will be clear to one skilled in the art that chemical compounds as described herein have their own chemical formula regardless of their phase or state. In particular, compounds that are present in their gaseous state, being pure and isolated at room temperature (such as N ₂ , H ₂ and NH ₃ ), are described herein by their chemical formula. For example, molecular nitrogen is described herein 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 particles described herein.

The proton donor can be any suitable substance capable of donating protons in an electrolytic cell. The proton donor may, for example, be an acid, such as any suitable organic or inorganic acid. The proton donor can be present in acidic, neutral or alkaline aqueous solutions. A proton donor may also, or alternatively, be provided by H ₂ oxidation at the anode. That is, hydrogen can be considered as a source of protons:

An electrolytic cell contains at least three main parts or components: a cathode electrode, an anode electrode, and an electrolyte. The overall reaction at the cathode can be represented as

.

The catalyst surface can be hydrogenated by adding one hydrogen atom at a time, representing a proton from solution and an electron from the electrode surface. The reaction mechanism can be shown in the equations below, where the asterisk indicates the area on the surface:

After adding 3(H ⁺ + e ^- ), one molecule of ammonia is formed, and the second is formed after adding 6(H ⁺ + e ^- ).

Different parts or components may be provided in separate containers, or they may be provided in a single container. An advantage of the present invention is that the method and system can suitably be applied using aqueous electrolytes, such as preferably aqueous solutions of dissolved electrolytes (salts). Thus, in preferred embodiments of the method and system, the electrolytic cell contains one or more aqueous electrolytic solutions in one or more compartments of the cell. The electrolyte solutions may contain any of various conventional inorganic or organic salts, such as, but not limited to, soluble chlorides, nitrates, chlorates, bromides, etc., for example, sodium chloride, potassium chloride, calcium chloride, ammonium chloride and other suitable salt. Aqueous electrolyte solutions may also contain any alkali or alkaline earth metal hydroxide or combination thereof, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, rubidium hydroxide and cesium hydroxide. The aqueous electrolyte solution may also additionally or alternatively contain one or more organic or inorganic acids. Inorganic acids may include mineral acids including, but not limited to, hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, boric acid, hydrofluoric acid, hydrobromic acid, tetrafluoroacetic acid, acetic acid and perchloric acid. Accordingly, the aqueous solution may be a neutral, alkaline or acidic solution. In some embodiments, the aqueous solution is an acidic solution. The electrolyte may also be a molten salt, such as sodium chloride salt. In some embodiments, the electrolytic cell contains an electrolytic solution containing an organic protic or aprotic solvent or a miscible mixture thereof, preferably a water-miscible organic solvent such as, for example, but not limited to, ethanol, ethylene glycol, butanediol, glycerol, diethanolamine, dimethoxyethane, 1 ,4-dioxane and mixtures thereof. The electrolytic solution may be a solution containing water and one or more water-miscible organic solvents, such as, but not limited to, one or more of the above solvents.

In general terms, the catalyst on the electrode surface should ideally have the following characteristics: it should (a) be chemically stable, (b) not oxidize or otherwise be consumed during the electrolytic process, it should promote the formation of ammonia, and (d) use catalyst should result in the production of a minimum amount of hydrogen gas. As will be described below, the catalytic oxides according to the invention satisfy these characteristics.

An advantage of the present invention is that the method can suitably be applied using aqueous electrolytes, such as preferably aqueous solutions of dissolved electrolytes (salts). Thus, in preferred embodiments of the method and system, the electrolytic cell contains one or more aqueous electrolytic solutions in one or more compartments of the cell. The aqueous electrolyte solutions may contain any of various conventional inorganic or organic salts, such as, but not limited to, soluble chlorides, nitrates, chlorates, bromides, etc., for example, sodium chloride, potassium chloride, calcium chloride, ammonium chloride and other suitable salt. Aqueous electrolyte solutions may also contain any alkali or alkaline earth metal oxide or combination thereof, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, rubidium hydroxide and cesium hydroxide. The aqueous electrolyte solution may also additionally or alternatively contain one or more organic or inorganic acids. Inorganic acids may include mineral acids, including, but not limited to, hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, boric acid, hydrofluoric acid, hydrobromic acid and perchloric acid.

As can be seen herein, an essential feature of the present invention relates to the composition and structure of the cathode electrode. Transition metal oxides are characterized by a wide range of surface structures, which affect the surface energy of these compounds and affect their chemical properties. The relative acidity and basicity of the atoms present on the surface of metal oxides is also influenced by the coordination of the metal cation and oxygen anion, which changes the catalytic properties of these compounds.

In certain embodiments, the transition metal oxide catalyst on the surface of the cathode electrode is selected from one or more of the following: titanium oxide, chromium oxide, manganese oxide, niobium oxide, tantalum oxide, ruthenium oxide, rhodium oxide, platinum oxide, osmium oxide, oxide rhenium and iridium oxide. Preferably, the catalyst includes one or more of rhenium oxide, tantalum oxide and niobium oxide.

Depending on the composition of the catalyst material, a suitable surface crystal structure may be preferred. Various crystal structures exist for transition metal oxides, and different structures can be obtained under different growth conditions. The selection of suitable surface crystal structures is within the skill of the person skilled in the art.

It may be preferred that the catalyst contains at least one surface having a rutile structure. Other crystal structures known in the art (eg, halite structure, sphalerite structure, anatase structure, perovskite structure) are also possible (see, for example, International Tables for Crystallography; http://it.iucr. org) .

For a given crystal structure, there may be several different surface faces (polycrystalline surfaces). The (110) face of rutile has the lowest free surface energy and therefore, in general, it is thermodynamically the most stable. Accordingly, in some embodiments, transition metal oxides may have a rutile structure with a (110) face forming the catalytic surface. Alternatively, the (100) and/or (111) faces of the halite structure may be selected.

Thus, in some embodiments, the catalytic surface is a rutile transition metal surface. The surface may have any suitable edge, including, but not limited to, edge (110). In some embodiments, the surface facet comprises or consists of a facet (110) of a transition metal oxide selected from the group consisting of titanium oxide, chromium oxide, manganese oxide, niobium oxide, tantalum oxide, ruthenium oxide, rhodium oxide, platinum oxide, osmium oxide , rhenium oxide and iridium oxide.

In some preferred embodiments, the catalyst comprises a rutile structure face (110) of one or more oxides selected from rhenium oxide, tantalum oxide and niobium oxide.

The surface of a rutile metal oxide having a (110) face contains metal atoms of two different coordination environments, where rows of sixfold coordinated metal atoms alternate with rows of fivefold coordinated metal atoms along the [001] direction. While sixfold coordinated metal atoms have approximately the same geometry as the bulk, fivefold coordinated metal atoms have an unsaturated bond perpendicular to the surface. Thus, there are two different surface areas: coordinatively unsaturated areas (cus-sites) on top of the 5-fold coordinated metal atoms and bridging areas (br-sites) between two six-fold coordinated metal atoms. It was found that br sites tend to bind adsorbates more strongly than cus sites. On the stoichiometric (110) surface, the br sites are occupied by oxygen, while the cus sites are vacant. The (110) surface may be referred to as oxygen-modified (O-modified). The bridging oxygen atoms are not sufficiently coordinated and can be reduced to the surface:

leaving br-sites vacant. These vacant br sites can then be further protonated:

As a consequence, the surface 110 can be restored, leaving the bridging regions between the sixfold coordinated metal atoms vacant. The bridging regions may also be partially or completely occupied by other atoms, such as oxygen or hydrogen atoms, as shown in FIG. 1 in this document.

It was found that the catalytic activity of the (110) surface may also depend on the occupancy of br sites. Thus, it may be thermodynamically advantageous to reduce the bridging oxygen atoms on transition metal oxides to H ₂ O and additionally cover the br sites with hydrogen atoms.

Accordingly, in some embodiments, the bridging regions between the sixfold coordinated transition metal atoms on the transition metal oxide surfaces are covered with hydrogen atoms.

As will be apparent to one skilled in the art, the catalyst of the invention may contain a single transition metal oxide. The catalyst may also contain or consist of a mixture of two or more such oxides. Such mixed oxides may contain a single structure, for example, a rutile structure. Mixed metal oxides may also contain a mixture of oxides having different crystal structures and/or oxides with different catalytic facets. Accordingly, such mixed oxides may further comprise one facet or a mixture thereof. Mixed oxide catalysts can be grown or manufactured separately and then assembled into mixed catalysts containing different metal oxides, where the oxides in the mixture have the same or different crystal structures.

As described in more detail herein, passing current through an electrolytic cell results in a chemical reaction in which nitrogen reacts with protons to form ammonia. Current passing is achieved by applying voltage to the cell. The invention makes it possible to electrolytically produce ammonia at a low electrode potential, which is beneficial from the point of view of energy efficiency and equipment requirements.

Without being bound by any particular theory, it is believed that oxide catalysts are capable of shifting the bottleneck of ammonia synthesis from the breakdown of N ₂ to the sequential formation of nitrogen-hydrogen species (*NH, *NH ₂ or *NH ₃ ), which is expected to simpler, but at the same time higher yield, formation of ammonia.

In some suitable embodiments, ammonia may be formed at an electrode potential of less than about minus 1.1 V, less than about minus 1.0 V, less than about minus 0.9 V, less than about minus 0.8 V, less less than about minus 0.7 V, less than about minus 0.6 V, less than about 0.5 V, or less than about minus 0.4 V. In some embodiments, ammonia can be formed at an electrode potential of about minus 0. 2 V to about minus 1.0 V, for example, from about minus 0.3 V to about minus 0.8 V, for example, from about minus 0.4 V to about minus 1.0 V, or from about minus 0.5 V to approximately minus 1.0 V. The upper range limit may be approximately minus 0.6 V, approximately minus 0.7 V, approximately minus 0.8 V, approximately minus 1.0 V, or approximately minus 1.1 V. Lower the range limit may be approximately minus 0.2 V, approximately minus 0.3 V, approximately minus 0.4 V, approximately minus 0.5 V, or approximately minus 0.6 V.

In some embodiments, ammonia can be formed using a niobium oxide catalyst at an electrode potential of from about minus 0.4 V to about minus 0.5 V. In some embodiments, ammonia can be formed using a rhenium oxide catalyst at an electrode potential from about minus 0.8 V to about minus 0.9 V. In some embodiments, ammonia can be formed using a tantalum oxide catalyst at an electrode potential of minus 1.0 V to about minus 1.1 V.

An advantage of the present invention is the efficiency of NH ₃ formation compared to H ₂ formation, which has been a problem in prior art research and testing. In certain embodiments, less than about 50% moles of H ₂ are formed compared to the number of moles of NH ₃ formed, and preferably less than about 40% moles of H ₂ , less than about 30% moles of H ₂ , less than about 20% moles of H ₂ , less than about 10% moles of H ₂ , less than about 5% of moles of H ₂ , less than about 2% of moles of H ₂ or less than about 1% of moles of H ₂ .

The system according to the invention is suitably designed to comply with one or more of the above listed features of the method. An advantage of the invention is that the system can be made small, reliable and inexpensive, for example for on-site application for the production of fertilizers near the intended place of application.

Ammonia can be applied as a fertilizer by injecting it into the soil as a gas, although this requires farmers to invest in pressurized storage tanks and injection equipment. Ammonia can also be used to form urea, usually by reacting with carbon dioxide. Ammonia can react to form nitric acid, which in turn readily reacts to form ammonium nitrate. Accordingly, the systems and methods of the present invention can be easily combined with existing solutions for reacting the resulting ammonia with other desired products, such as, but not limited to, those mentioned above.

NO _x and SO _x are general terms for mononitrogen and monosulfur axoids such as NO, NO ₂ , SO, SO ₂ and SO ₃ . These gases are formed during combustion, especially at high temperatures. In areas with heavy traffic, the amounts of these pollutants can be significant.

Accordingly, an advantageous aspect of the invention relates to a system for removing NO _x and/or SO _x from a gas stream by reacting the gas stream with ammonia that is generated in situ in the stream or in a system that may be in fluid communication with the gas stream. The system may comprise a system for generating ammonia as described herein, particularly a system comprising an electrolytic cell containing a transition metal oxide catalyst as described herein. In this context, in situ should be interpreted to mean that ammonia is generated within the system, for example within a gas stream or in a compartment within the system that is in fluid communication with the gas stream. The ammonia generated in this way, when in contact with the gas stream, will react with NO _x and/or SO _x in the gas stream, causing these toxic compounds to be converted into other molecular compounds such as N ₂ , H ₂ O and (NH ₄ ) ₂ SO ₄ . In some embodiments, the system may be for use in automotive engine exhaust or other engines where ammonia can be generated in situ by the method of the present invention and then used to reduce SO _x and/or NO _x in engine exhaust. Such a system can suitably utilize electric current obtained by conversion from an automobile engine. Thus, using electric current from an automobile engine, ammonia can be generated in situ, and the ammonia thus generated can react with SO _x and/or NO _x from the exhaust gases of the automobile. Ammonia can be generated in a vehicle and then released into the vehicle's exhaust. Ammonia can also be generated in situ within a vehicle's exhaust system. In this way, NO _x and/or SO _x are removed from the vehicle's exhaust gases, reducing the amount of pollutants in the exhaust gases.

Embodiments of the invention include the following non-limiting points:

1. A method for producing ammonia, including:

supplying N ₂ to an electrolytic cell containing at least one proton source;

allowing N ₂ to contact the surface of a cathode electrode in an electrolytic cell, wherein the surface of the cathode electrode comprises a catalytic surface containing at least one transition metal oxide; And

passing a current through said electrolytic cell, causing nitrogen to react with protons to form ammonia.

2. The method according to claim 1, where the catalyst contains one or more transition metal oxides selected from the group consisting of titanium oxide, chromium oxide, manganese oxide, niobium oxide, tantalum oxide, ruthenium oxide, rhodium oxide, platinum oxide, osmium oxide , rhenium oxide and iridium oxide.

3. The method according to claim 1 or claim 2, where the catalyst contains one or more oxides selected from the group consisting of rhenium oxide, tantalum oxide and niobium oxide.

4. Method according to any one of paragraphs. 1-3, where the catalytic surface contains at least one surface having a rutile structure.

5. Method according to any one of paragraphs. 1-4, where the catalytic surface comprises at least one surface having an edge (110).

6. Method according to any one of paragraphs. 1-5, wherein ammonia is generated in the electrolytic cell at an electrode potential of less than about minus 1.0 V, more preferably less than about minus 0.8 V, and even more preferably less than about minus 0.5 V.

7. Method according to any one of paragraphs. 1-6, where the catalyst contains niobium oxide, and where ammonia is formed in the electrolytic cell at an electrode potential of less than about minus 0.5 V.

8. Method according to any one of paragraphs. 1-7, where the catalyst contains rhenium oxide, and where ammonia is formed in the electrolytic cell at an electrode potential of less than about minus 0.9 V.

9. Method according to any one of paragraphs. 1-8, wherein the catalyst comprises tantalum oxide, and wherein ammonia is generated in the electrolytic cell at an electrode potential of less than about minus 1.0 V.

10. Method according to any one of paragraphs. 1-9, where less than 50% of the moles of H ₂ are formed compared to the number of moles of NH ₃ formed, preferably less than 20% and even more preferably less than 10%.

11. Method according to any one of paragraphs. 1-10, wherein said electrolytic cell contains one or more aqueous electrolytic solutions.

12. Method according to any one of paragraphs. 1-11, where the source of protons in the formation of ammonia is the splitting of water at the electrode or the oxidation reaction of H ₂ at the anode.

13. A system for generating ammonia, comprising at least one electrochemical cell containing at least one cathode electrode having a catalytic surface, where the catalytic surface is filled with at least one catalyst containing one or more transition metal oxides.

14. The system of claim 13, wherein the at least one transition metal oxide is selected from the group consisting of titanium oxide, chromium oxide, manganese oxide, niobium oxide, tantalum oxide, ruthenium oxide, rhodium oxide, platinum oxide, osmium oxide, oxide rhenium and iridium oxide.

15. The system of claim 13 or claim 14, wherein the at least one transition metal oxide is selected from the group consisting of rhenium oxide, tantalum oxide and niobium oxide.

16. The system of claim 14 or claim 15, wherein the catalytic surface comprises at least one surface having a rutile structure.

17. The system according to any one of paragraphs. 14-16, where the catalytic surface includes at least one surface having an edge (110).

18. The system according to any one of paragraphs. 14-17, wherein said electrolytic cell further contains one or more electrolytic solutions.

19. The system according to claim 18, where the electrolytic solution is an acidic aqueous solution.

The invention will be further illustrated by the following non-limiting examples, which further describe specific advantages and embodiments of the present invention.

Example 1

The zero-point energy correction and the entropy difference between the adsorbed species and the gas molecule are calculated for all intermediates in the reaction in the harmonic approximation. These values are shown in Table 1.

Table 1. Zero point energy (ZPE) and entropy contributions to the free energy of the gas phase and adsorbed molecules at 300 K for rutile oxides (in eV).

Rutiles (110)
restored surface MP ZPE NH ₃ 0.74 0.89 H ₂ 0.41 0.27 N ₂ 0.60 0.15 *H 0.01 0.19 *NNH 0.15 0.48 *NHNH 0.12 0.81 *NNH ₂ 0.10 0.86 *NHNH ₂ 0.12 1.18 *NH ₂ NH ₂ 0.15 1.48 *NHNH ₃ 0.15 1.47 *N 0.02 0.11 *NH 0.05 0.36 *NH ₂ 0.07 0.70 *NH ₃ 0.16 1.00 Rutiles (110)
O-modified/H-modified surfaces MP ZPE *H 0.01 0.19 *NNH 0.09 0.52 *NHNH 0.10 0.82 *NNH ₂ 0.11 0.82 *NHNH ₂ 0.14 1.15 *NH ₂ NH ₂ 0.17 1.52 *NHNH ₃ 0.17 1.49 *N 0.06 0.08 *NH 0.07 0.37 *NH ₂ 0.11 0.66 *NH ₃ 0.15 1.02

Values for gas phase molecules are taken from Weast (Handbook of Chemistry and Physics, 49th edn., p. D109, The Chemical Rubber Company, Cleveland 1968-1969) and Atkins (Physical Chemistry, Oxford University Press, 6th edn., 1998), and values for adsorbed molecules are obtained from DFT (density functional theory) calculations of vibrational normal modes.

Example 2

Introduction

Various electrolytes and electrode materials have been tested in order to relax the thermodynamic requirements and optimize the rate of ammonia formation using heterogeneous catalysis. (Amar 2011; Denvir 2008 (US7314544 B2); Ouzounidou 2007; Pappenfus 2009; Amar 2011; Song 2004). Electrolytic cells based on solid electrolytes and polymer electrolytic membranes (PEMs) have been the subject of many of these studies because they facilitate the separation of the hydrogen gas feed and the ammonia product. The highest ammonia production rate obtained with a Nafion membrane was 1.13 x 10 ^-8 mol s ^-1 cm ^-2 with a Faraday efficiency of approximately 90% using wet H ₂ as the feed gas. (Xu 2009) Using air and water as feed gases is an interesting possibility, however, the highest ammonia production rate achieved in this way was much lower, 1.14×10 ^-9 mol s ^-1 cm ^-2 . (Lan 2013) Only 1% CE was obtained, which required an overvoltage of minus 1.6 V. While these results are promising, they are still far from production rates close to commercially viable (4.3-8.7× 10 ^-7 mol s ^-1 cm ^-2 ). (Giddey, Badwal, 2013)

Until now, experimenters continue to search for a new way to synthesize ammonia. However, electrocatalytic _N2 reduction has recently become the subject of theoretical research, where materials that promote _N2 reduction and suppress hydrogen evolution have been proposed using density functional theory (DFT) calculations. (Skúlason 2012, Abghoui 2015 enabling, 2016 ACS Catalysis, 2016 Catalysis Today, 2016 Catalysis Today, 2017 Phys. Chem. C, J Howalt 2013, J Howalt 2014, Hargreaves 2014, Zelinapour-Yazdi 2016) The use of computational methods makes it possible to scan large groups potential catalysts without the need to produce them in a laboratory, saving time and money. In 2012, Skúlason et al. performed a detailed DFT analysis of the catalytic activity of both planar and stepped pure transition metal surfaces, in which they identified trends and calculated the free energy profile for nitrogen reduction. So-called volcano plots are useful in assessing trends in catalytic reactivity, and Skúlason et al. found that early transition metals were more active in reducing N ₂ than later transition metals and were also less prone to hydrogen evolution. (Skúlason 2012) Abghoui et al. conducted a similar study on transition metal mononitrides and found that early transition metal nitrides were promising candidates for _N2 reduction, predicting that ZrN and VN would form ammonia at potentials of minus 0.76 V and minus 0.51 V compared to SHE (standard hydrogen electrode), respectively. (2015 PCCP, 2015 Proceedia computer science, 2016 ACS Catalysis). Later they also suggested that NbN and CrN might behave similarly. (Abghoui Skulason, 2017, MVK). Further studies yielded promising results in which the (110) facets of the RuN sphalerite structure were reported to catalyze the formation of ammonia at a very low initial potential of approximately minus 0.23 V compared to SHE. (Abghoui 2017) Comparison of the Mars-van Krevelen (MvK) mechanism with traditional associative and dissociative mechanisms indicated that MvK should be a more favorable reaction mechanism at the surface of these nitrides. (Abghoui and Skulason, 2017)

In the present study, transition metal oxides in the rutile structure are investigated as possible candidates for catalyzing the electrochemical formation of ammonia under ambient conditions. DFT calculations were used to construct a stability diagram for each metal oxide, where the stability of the (110) facet covered with various adsorbed species was calculated as a function of potential and the most stable facet was determined for each potential. The authors further studied the thermodynamics of the cathodic reaction and constructed free energy diagrams for the electrochemical protonation of adsorbed nitrogen compounds. The energy balance between the adsorption of protons and N ₂ H species was examined for all potential catalysts to see the tendency for N ₂ reduction compared to the competing hydrogen evolution reaction (HER). The authors then estimated the initial potentials required for ammonia formation on the surface of these oxides. The influence of external potential was taken into account using a standard hydrogen electrode (SHE) calculation (Norskov, Jonsson 2004), and for each metal oxide the lowest initial potential required to reduce _N2 to ammonia was estimated. Finally, a volcano-shaped diagram was obtained showing the catalytic activity of the various oxides using the N ₂ H binding energy as an overall indicator (Rossmeisl 2007, Skúlason 2012).

Methodology

DFT Calculations: In the present study, the (110) face of eleven transition metal oxides naturally present in the rutile structure was considered with an emphasis on the electrochemical formation of ammonia, since the (110) face is the most stable of the low-index facets of the rutile structure. The oxides that are stable in the rutile structure under ambient conditions are the following: TiO ₂ , NbO ₂ , TaO ₂ , ReO ₂ , IrO 2 , OsO ₂ , CrO ₂ , MnO ₂ , RuO _{2 ,} RhO ₂ and PtO ₂ . (Harold Kung) The oxide surfaces were modeled with 48 atoms in four layers, each of which consisted of 4 metal atoms and 8 oxygen atoms. The two lower layers remained stationary, while the upper two layers and the adsorbed particles were in a relaxed state. The boundary conditions were periodic in the x and y directions, and the surfaces were separated by a vacuum of 12 Å in the z direction. Structure optimization was considered approximate when the forces in any direction on all mobile atoms were less than 0.01 eV/Å. The RPBE lattice constants were optimized for each oxide and took spin polarization into account. The RPBE lattice constants optimized for the oxides in this study are as follows: TiO ₂ : a=4.65 Å, c=2.98 Å; NbO ₂ : a=5.11 Å, c=3.00 Å; TaO ₂ : a=4.98 Å, c=3.02 Å; ReO ₂ : a=4.77 Å, c=3.14 Å; IrO ₂ : a=4.58 Å, c=3.13 Å; OsO ₂ : a=4.58 Å, c=3.18 Å; CrO ₂ : a=4.50 Å, c=2.96 Å; MnO ₂ : a=4.86 Å, c=2.85 Å; RuO ₂ : a=4.58 Å, c=3.2 Å; RhO ₂ : a=4.55 Å, c=3.06 Å and PtO ₂ : a=4.65 Å, c=3.19 Å.

Calculations were performed using DFT using the RPBE exchange correlation functional. A plane wave basis with an energy cutoff of 350 eV was used to represent the valence electrons with a PAW representation of the inner shell electrons, as implemented in the VASP code (VASP1-VASP4). The self-consistent electron density was determined by iterative diagonalization of the Kohn-Sham Hamiltonian, and the occupancy of the Kohn-Sham states was smeared along the Fermi-Dirac distribution with the smearing parameter kBT = 0.1 eV. A 4 × 4 × 1 k-point sample obtained using the Monkhorst-Pack scheme was used for all surfaces, and maximum symmetry was used to reduce the number of k-points in the calculations.

Electrochemical reactions and modeling: The source of protons in the reaction can be either the splitting of water or the oxidation reaction of H ₂ at the anode. To relate absolute potential to SHE, the authors herein refer to _H2 only as a convenient source of protons and electrons.

where protons are solvated in an electrolyte and electrons are transferred to the cathode through a wire. The general _N2 reduction reaction is as follows:

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 studied associative reaction mechanism is shown in the equations below, where the asterisk indicates the area on the surface:

.

The first ammonia molecule is formed after the addition of three to five protons, depending on the preferred reaction path. A second ammonia molecule is then formed after the addition of six protons. The free energy of N ₂ H adsorption is compared with the adsorption energy of protons to determine whether the surface is selective for ammonia formation or hydrogen evolution.

As a first approximation, it is assumed that the activation barriers between stable minima are low, or that they follow the Brønsted-Evans-Polyany relationship and can therefore be neglected during electrochemical reactions in the context of the present invention. The free energy of each elemental stage is estimated at pH = 0 and T = 298K according to the following:

where ΔE represents the energy calculated using DFT. ΔE _ZPE and ΔS represent the differences in zero point energy and entropy, respectively, between adsorbed species and gas phase molecules. They were calculated in the harmonic approximation and the values are given in ESI. The effect of the applied bias U is included for all stages of an electrochemical reaction by shifting the free energy for a reaction involving n electrons by minus neU, so that the free energy of each elementary step is given by:

.

Explicitly including water in the simulation would have significantly increased the computational effort and was therefore not included in the present study. The presence of water is known to stabilize some particles through the formation of hydrogen bonds. For example, *NH ₂ is expected to be somewhat more stable near water, while *N is expected to be unaffected by the aqueous layer. Previous publications have estimated the stabilizing effect of water to be less than 0.1 eV per hydrogen bond (Montoya 2015). Thus, the authors estimated that the inclusion of hydrogen bonding would change the initial potentials calculated in this study by less than 0.1 eV, representing a correction not included in the present study.

Results and discussion

Stability: The rutile (110) surface contains metal atoms of two different coordination environments, see FIG. 1. Rows of sixfold coordinated metal atoms alternate with rows of fivefold coordinated metal atoms in the [001] direction. While sixfold coordinated metal atoms have approximately the same geometry as the bulk, fivefold coordinated metal atoms have an unsaturated bond perpendicular to the surface. (Morgan 2007) Thus, there are two distinct surface regions: coordinatively unsaturated sites (cus sites) on top of fivefold coordinated metal atoms and bridging sites (br sites) between two sixfold coordinated metal atoms. According to the authors' observations, br sites usually bind adsorbates more strongly than cus sites, and the catalytic activity of the surface depends on the occupancy of br sites. Therefore, for each rutile oxide, a systematic study of different oxygen and hydrogen coatings on the br-sites was carried out, and the stability diagrams show the relative stability of the (110) face with different coatings, see FIG. 2 as an example (but all others are shown in ESI). On the stoichiometric (110) surface, the br sites are occupied by oxygen, while the cus sites are vacant. We refer to the stoichiometric (110) surface as an oxygen-modified (O-modified) surface, see FIG. 1c. The bridging oxygen atoms of the O-modified surface are not sufficiently coordinated and can be recovered from the surface under experimental and operational conditions. To calculate the free energy of reduction of an O-modified surface, the authors followed the methodology developed by Nørskov et al., where surface reduction occurs through the exchange of water and protons with the electrolyte:

,

leaving br-sites vacant. The authors call this surface restored, see FIG. 1a. These vacant br sites can then be further protonated:

.

We refer to this surface as a hydrogen-modified (H-modified) surface, see FIG. 1b. The authors also considered a surface configuration where half of the br sites are covered with either hydrogen or oxygen, and half of the sites are vacant. These surfaces are called 0.25 monolayer (ML) surfaces.

In FIG. Figure 2 shows an example of a stability diagram constructed for NbO ₂ . The x-axis represents the free energy of the reconstructed surface, which is used as a reference to which other surfaces are normalized. From the stability chart in FIG. 2 it can be seen that at potentials below minus 0.5 V relative to the SHE, it is thermodynamically advantageous to reduce the bridging oxygen atoms on NbO ₂ to H ₂ O and then protonate the br sites to a coverage of 0.5 ML H. Stability diagrams for other metals are shown in FIG. 3-12. For all oxide coatings, 0.25 ML H or O are not preferred at any relevant potential and were therefore excluded from all further calculations. It should be noted that this analysis is based on thermodynamics and does not include activation energies.

Catalytic activity: The catalytic activity of all rutile oxides towards electrochemical ammonia formation was calculated using DFT. For each rutile, the free energy profile was calculated for three different surfaces: reduced, H-modified, and O-modified surfaces. The free energy of each intermediate was found using equation (10) relating to N ₂ and H ₂ in the gas phase. As can be seen from the stability diagrams, the 0.25 ML surfaces are not favored over any significant potential range and were therefore excluded from all further calculations.

In FIG. 13 shows the free energy profile of ammonia formation on NbO ₂ , where the corresponding potential determination step (PDS) for each surface is determined, that is, the step with the largest free energy change, which determines the initial potential required for all reaction steps to have a decreasing free energy. energy. The authors define this stage as a measure of the activity of the oxide towards the formation of ammonia. Free energy diagrams for other rutile oxides are presented in FIG. 14-23. In FIG. Figure 13 shows that the PDS free energy change predicted for ammonia formation on the O-modified and reduced surfaces is −0.53 eV and −2.01 eV, respectively.

Considering the stability diagram shown in FIG. 2 for NbO ₂ , the preferred surface with the initial potential required for ammonia formation is, for both cases, an H-modified surface. This eliminates the possibility of ammonia formation on the O-modified and reduced NbO ₂ surface. A similar analysis was performed for all oxides and the results are presented in FIG. 24. For all rutile oxides, the required applied potential is in the potential range where the H-modified surface is preferred and appears to eliminate the possibility of ammonia formation on the oxygen-modified and reduced surfaces. The PDS free energy change for hydrogen-modified IrO ₂ , NbO ₂ , OsO ₂ , ReO ₂ , RuO ₂ , TaO ₂ , PtO ₂ , RhO ₂ , CrO ₂ , TiO ₂ and MnO ₂ surfaces is 0.36 eV, 0.57 eV , 0.60 eV, 1.07 eV, 1.14 eV, 1.21 eV, 1.29 eV, 1.61 eV, 2.04 eV, 2.07 eV and 2.35 eV, respectively. Three of these surfaces, IrO ₂ , NbO ₂ and OsO ₂ , have a required overvoltage similar to or lower than that required for nitrogen reduction by nitrogenase, but is thought to be approximately 0.63 V. Free energy diagrams showing all possible intermediates for the H-modified surface of all candidates are shown in FIG. 25-35.

Scale Relationships and Volcano Plot: Using the scale relationships between the binding energies of the various N ₂ reduction mechanism intermediates, a volcano plot was constructed as shown in FIG. 36. This graph shows only three of the six electrochemical reaction steps in the ammonia formation mechanism. The three stages that are not included have a y-intercept greater than zero and therefore do not influence the conclusions drawn from the graph.

The scaling relationships used to calculate the lines in FIG. 36-37 are shown in FIG. 38-43, and a graph showing all reaction steps of the ammonia generation mechanism is shown in FIG. 37.

Similar diagrams were previously constructed for pure transition metals, where the binding energy of N to the surface was used, rather than the binding energy of N ₂ H. (Skulason 2012, Montoya 2015)

The *N binding energy is endothermic for all candidates in this study (except for ReO ₂ and TaO ₂ ), and therefore the dissociation of N ₂ will be endothermic and also have a high activation energy. However, the binding energy of *N on ReO ₂ and TaO ₂ is approximately minus 1 eV, and therefore the dissociation of N ₂ on these candidates may have a low barrier since the reaction energy in this case is approximately minus 2 eV. However, including the dissociation pathway will not change the reported PDS for either candidate because *NH ₂ → NH ₃ is the PDS on the left "leg" of the volcano where ReO ₂ and TaO ₂ are located. Reaction steps *N → *NH and *NH → * NH ₂ is also included in the authors' proposed pathways, and these steps never become PDS (see FIG. 43).

First protonation: Up to this point, the authors have not considered competing reactions, such as the hydrogen evolution reaction. As a first step toward incorporating competing reactions, the authors calculated the free energy of the first step of the ammonia formation reaction, which involves the adsorption of N ₂ H, and compared it with the free energy of hydrogen adsorption on a surface. The hydrogen atom was allowed to find the most favorable binding site on the surface. This was done only for H-modified surfaces, since otherwise modified surfaces were already excluded due to instability under service conditions. The results of this analysis are shown in FIG. 6. Apparently, the two oxides ReO ₂ and TaO ₂ promote the adsorption of N ₂ H over proton and are therefore promising for the electrochemical formation of ammonia with higher yields. NbO ₂ binds N ₂ H and H with equal strength, so both compounds are expected to be present at this surface, and therefore both ammonia and hydrogen gas can be expected to form on NbO ₂ . IrO ₂ highly favors proton adsorption compared to NNH, which is unfortunate since IrO ₂ actually has the lowest predicted onset potential of the eleven oxides examined in this study. ReO ₂ and TaO ₂ , however, can be active and selective for N ₂ electroreduction, but with a slightly higher overpotential.

Conclusion

DFT calculations were used to study the possibility of nitrogen activation for electrochemical production of ammonia under ambient conditions on the (110) face of the rutile structure NbO ₂ , RuO ₂ , RhO ₂ , TaO ₂ , ReO ₂ , TiO ₂ , OsO ₂ , MnO ₂ , CrO ₂ , IrO ₂ and PtO ₂ . The relative stability of the faces with different adsorbates was calculated as a function of the applied potential. The catalytic activity of each surface was studied and the stage that determines the potential was discovered. At the applied potential required for all electrochemical steps to have decreasing free energy, all oxides were found to be the most stable with hydrogen adsorbed at the sites. Of these oxides, ReO ₂ and TaO ₂ promote the adsorption of N ₂ H.

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Claims (25)

1. A method for producing ammonia, including: supplying N ₂ to an electrolytic cell containing at least one proton source; allowing N ₂ to contact the surface of a cathode electrode in an electrolytic cell, wherein the surface of the cathode electrode comprises a catalytic surface comprising at least one transition metal oxide selected from the group consisting of niobium oxide, tantalum oxide, osmium oxide, rhenium oxide and iridium oxide wherein the at least one transition metal oxide comprises at least one surface having a (110) facet; And passing a current through said electrolytic cell, causing nitrogen to react with protons to form ammonia. 2. The method according to claim 1, where the catalytic surface contains at least one surface having a rutile structure. 3. The method of claim 1 or 2, wherein ammonia is generated in the electrolytic cell at an electrode potential of less than about minus 1.0 V, more preferably less than about minus 0.8 V, and even more preferably less than about minus 0.5 V compared to a standard hydrogen electrode (SHE). 4. The method of claim 1 or 2, wherein the catalyst comprises niobium oxide, and wherein ammonia is generated in the electrolytic cell at an electrode potential of less than about minus 1.1 V compared to the SHE. 5. The method of claim 1 or 2, wherein the catalyst comprises rhenium oxide, and wherein ammonia is generated in the electrolytic cell at an electrode potential of less than about minus 0.7 V compared to the SHE. 6. The method of claim 1 or 2, wherein the catalyst comprises tantalum oxide, and wherein ammonia is generated in the electrolytic cell at an electrode potential of less than about minus 0.7 V compared to the SHE. 7. Method according to any one of paragraphs. 1-6, where less than 50% of the moles of H ₂ are formed compared to the number of moles of NH ₃ formed, preferably less than 20% and even more preferably less than 10%. 8. Method according to any one of paragraphs. 1-7, wherein said electrolytic cell contains one or more aqueous electrolytic solutions. 9. Method according to any one of paragraphs. 1-7, wherein said electrolytic cell contains an electrolytic solution containing an organic protic or aprotic solvent or a miscible mixture thereof, preferably a water-miscible organic solvent. 10. The method according to claim 8 or 9, wherein said nitrogen is supplied to the electrolytic cell by bubbling nitrogen gas into the electrolytic solution in contact with said surface of the cathode electrode. 11. Method according to any one of paragraphs. 1-10, where the source of protons in the formation of ammonia is the splitting of water at the anode or the oxidation reaction of H ₂ at the anode. 12. Method according to any one of paragraphs. 1-11, which is carried out at a temperature of from about minus 10 °C to about 40 °C, and preferably from about 0 to 40 °C. 13. Method according to any one of paragraphs. 1-12, which is carried out at room ambient temperature and atmospheric pressure. 14. A system for generating ammonia, comprising at least one electrochemical cell containing at least one cathode electrode having a catalytic surface, where the catalytic surface is filled with at least one catalyst containing one or more transition metal oxides selected from the group consisting of niobium oxide, tantalum oxide, osmium oxide, rhenium oxide and iridium oxide, where the transition metal oxide contains at least one surface having a (110) face. 15. The system of claim 14, wherein the catalytic surface comprises at least one surface having a rutile structure. 16. The system according to claim 14 or 15, wherein said electrolytic cell further contains one or more electrolytic solutions. 17. The system according to claim 16, wherein said electrolytic cell contains an acidic, neutral or alkaline aqueous solution. 18. The system of claim 17, wherein said electrolytic cell contains an electrolytic solution containing an organic protic or aprotic solvent or a miscible mixture thereof, preferably a water-miscible organic solvent. 19. The system according to any one of paragraphs. 14-18, designed to produce ammonia in an electrolytic cell at an electrode potential of less than about minus 1.0 V, more preferably less than about minus 0.8 V, and even more preferably less than about minus 0.5 V compared to SHE. 20. The system of claim 19, wherein the catalyst comprises niobium oxide, and wherein the system is designed to produce ammonia in an electrolytic cell at an electrode potential of less than about minus 1.1 V compared to SHE. 21. The system of claim 19, wherein the catalyst comprises rhenium oxide, and wherein the system is designed to produce ammonia in an electrolytic cell at an electrode potential of less than about minus 0.7 V compared to SHE. 22. The system of claim 19, wherein the catalyst comprises tantalum oxide, and wherein the system is designed to produce ammonia in an electrolytic cell at an electrode potential of less than about minus 0.7 V compared to SHE.