Patent Application
20230151498
The invention is within the field of process chemistry, and specifically relating to production of ammonia with electrolytic methods, and new catalysts therefor.
Ammonia is one of the most highly produced chemicals worldwide. The industrial ammonia synthesis recognized nowadays as the Haber-Bosch process is the first heterogeneous catalytic system, a key element of the global industrial production of nitrogen fertilizer. Today, ammonia is also gaining attention as a possible energy carrier and a potential transportation fuel with high energy density but no CO2 emission. The centralized and energy demanding 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 based catalyst to form ammonia, by the following reaction:
A drawback of this industrial approach is the high temperature and pressure needed for kinetic and thermodynamic reasons. Another drawback and a more serious one is that the hydrogen gas is produced from natural gas. That multi-step process takes up the largest part of the whole chemical plant, and is the most costly and unfriendly with regards to the environment. That is the largest reason that a sustainable process is needed, since the natural gas will at some point be depleted. A small-scale system for decentralized ammonia production that uses less energy and ambient conditions would therefore be of great significance. Further, in order to optimise the efficiency of ammonia synthesis, new catalysts capable of hydrogenating dinitrogen at reasonable rate but at milder conditions will be much appreciated.
The triple bond in molecular nitrogen N2 is very strong and as a consequence nitrogen is very inactive and frequently used as an inert gas. It is broken down by the harsh conditions in the Haber-Bosch process, however, it is also broken down at ambient conditions in a natural process, by microorganisms through nitrogenase enzyme. The active site of nitrogenease is a MoFe7S9N cluster that catalyzes ammonia formation from solvated protons, electrons and atmospheric nitrogen through the electrochemical reaction
Inspired by nature, biological nitrogen fixation as an alternative approach to the Haber-Bosch process for synthesizing ammonia under ambient conditions has been attracting much attention. Much effort has been on research trying to develop similar electrochemical processes. Various methods for ammonia synthesis at ambient conditions have been explored in the last decades. (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). While such studies have provided insight into the process of ammonia formation, the kinetics are still too slow for practical applications and in most cases hydrogen gas is dominantly formed more readily than protonation of nitrogen. The reduction of dinitrogen by protons and electrons to selectively form ammonia at room temperature and pressure has proven much more challenging than expected.
The inventors have previously established (WO 2015/189865) that certain metal nitride catalysts may be employed in electrochemical processes for producing ammonia. It remains however unestablished whether other or additional metal compounds can be useful in the production of ammonia. Other efforts towards artificial synthesis of ammonia, using electrochemical methods, have yielded relatively low current efficiencies (CE). For many of these, regenerating the active nitrogen-fixing complex has proved problematic, and production rates far from reaching commercial viability.
The above features along with additional details of the invention, are described further in the examples below, which are intended to further illustrate the invention but are not intended to limit its scope in any way.
The present inventors have found that certain transition metal oxide catalysts may be employed in electrochemical processes for producing ammonia. This has lead to the present invention, that makes possible ammonia production at ambient room temperature and atmospheric pressure.
The present invention provides a process for producing ammonia comprising feeding N2 to an electrolytic cell that comprises at least one source of protons; allowing the N2 to come into contact with a cathode electrode surface in the electrolytic cell, wherein the cathode electrode surface comprises a catalyst surface comprising at least one transition metal oxide; and running a current through said electrolytic cell, whereby nitrogen reacts with protons to form ammonia.
The invention also provides a system for the generation of ammonia, in particular a system that carries out the method of generating ammonia as disclosed herein. Thus, the invention provides a system for generating ammonia, the system comprising at least one electrochemical cell, the at least one electrochemical cell comprising at least one cathode electrode having a catalytic surface, wherein the catalytic surface is charged with at least one catalyst comprising one or more transition metal oxide.
In the method and system in accordance with the invention, the transition metal oxide can 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 oxide. In some preferred embodiments, the oxide is selected from the group consisting of Rhenium oxide, Tantalum oxide and Niobium oxide.
The catalyst surface can comprises at least one surface having a rutile structure, in particular a rutile structure having a (110) facet on which the catalytic ractions take place. In some embodiments,. the catalyst comprises a surface having bridging sites between sixfold coordinated transition metal atoms that are covered by hydrogen atoms.
The skilled person will understand that the figures, described below, are for illustration purposes only. The figures are not intended to limit the scope of the present teachings in any way.
In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples 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 its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps.
As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.
Throughout the description and claims, the terms “comprise”, “including”, “having”, and “contain” and their variations should be understood as meaning “including but not limited to”, and are not intended to exclude other components.
The present invention also covers the exact terms, features, values and ranges etc. in case these terms, features, values and ranges etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., “about 3” shall also cover exactly 3 or “substantially constant” shall also cover exactly constant).
The term “at least one” should be understood as meaning “one or more”, and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with “at least one” have the same meaning, both when the feature is referred to as “the” and “the at least one”.
It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Features disclosed in the specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.
Use of exemplary language, such as “for instance”, “such as”, “for example” and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless so claimed. Any steps described in the specification may be performed in any order or simultaneously, unless the context clearly indicates otherwise.
All of the features and/or steps disclosed in the specification can be combined in any combination, except for combinations where at least some of the features and/or steps are mutually exclusive. In particular, preferred features of the invention are applicable to all aspects of the invention and may be used in any combination.
The present invention is based on the surprising discovery that on the surface of certain transition metal oxide catalysts, it is possible to form ammonia at ambient temperature and pressure with a low applied potential. Given the importance of ammonia, not least in the production of fertilizer, and the energy intensive and environmentally unfavourable conditions that are typically used during its manufacture, the invention finds important applicability in various industries.
Thus, the invention provides processes and systems for generating ammonia at ambient temperature and pressure. Ambient temperature can generally refer both to typical indoor and outdoor temperatures. Accordingly, in some embodiments the process and system of the invention is operated at a temperature in the range from about -10 ° to about 40° C., such as in the range from about 0 ° to about 40° C., such as in a range from about from about -10 ° or from about -5° C. or from about 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., such as in a range of about 20-30° C. or a range of about 20-25° C. Ambient pressure refers generally to atmospheric pressure. In the process and system of the present invention, an electrolytic cell is used which can be any of a range of conventional commercially suitable and feasible electrolytic cell designs that can accommodate a special purpose cathode in accordance with the invention. Thus, the cell and system may in some embodiment 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 solutions. The proton donor can also, or alternatively, be provided by H2 oxidation at the anode. I.e. hydrogen can be considered as a source of protons: H2 ⇔ 2(H+ + e-).
The electrolytic cell comprises at least three general parts or components, a cathode electrode, an anode electrode and an electrolyte. The overall cathode reaction can be presented as
The catalyst surface can be hydrogenated by adding one hydrogen atom at a time, representing a proton from the solution and an electron from the electrode surface. The reaction mechanism can be shown in the equations below where an asterisk denotes a surface site:
After the addition of 3(H+ + e-) one ammonia molecule is formed and the second one is formed after the addition of 6(H+ + e-).
The different parts or components can be provided in separate containers, or they can be provided in a single container. An advantage of the present invention is that the process and system 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. Electrolyte solutions may comprise any of various typical inorganic or organic salts such as but not limited to soluble salts of chloride, nitrate, chlorate, bromide, etc. e.g. sodium chloride, potassium chloride, calcium chloride, ammonium chloride, and other suitable salts. The aqueous electrolyte solutions may also comprise any one, or a combination of, alkali or alkaline earth metal hydroxides, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, rubidium hydroxide and cesiumcaesium 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, tetrafluoro-acetic-acid, acetic-acid and perchloric acid. Accordingly, the aqueous solution can be a neutral, an alkaline or an acidic solution. In some embodiments, the aqueous solution is an acidic solution. The electrolyte can also be a molten salt, for example a sodium chloride salt. In some embodiments the electrolytic cell comprises an electrolytic solution comprising a 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 thererof. The electrolytic solution can be a solution comprising water and one or more water-miscible organic solvent such as but not limited one or more of the above mentioned solvents.
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 catalyst oxides according to the invention fulfill these characteristics.
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 limited to soluble salts of chloride, nitrate, chlorate bromide, etc. e.g. sodium chloride, potassium chloride, calcium chloride, ammonium chloride, and other suitable salts. The aqueous electrolyte solutions may also comprise 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.
As appears from herein, the essential feature of the present invention concerns the composition and structure of the cathode electrode. Transition metal oxides 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 oxides are also affected by the coordination of the metal cation and oxygen anion, which alter the catalytic properties of these compounds.
In certain embodiments, the transition metal oxide catalyst on the cathode electrode surface 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, Rhenium oxide and Iridium oxide. Preferably the catalyst comprises one or more of Rhenium oxide, Tantalum oxide and Niobium oxide.
Depending on the substance composition of the catalyst, a suitable surface crystal structure may be preferred. Various different crystal structures exist for transition metal oxides and different structures can be obtained at different growth conditions. It is within scope of the skilled person to select appropriate surface crystal structures.
It may be preferable that the catalyst copmrise at least one surface having a rutile structure. Other crystal structures known in the art (e.g., rocksalt structure, zincblende structure, anatase structure, perovskite structure) are also possible (see., e.g., International Tables for Crystallography; http://it.iucr.org).
Several different surface facets may exist for a given crystal structure (polycrystalline surfaces). The (110) facet of rutile exhibits the lowest surface free energy and is therefore in general thermodynamically most stable. Accordingly, in some embodiments, the transition metal oxides can be of rutile structure with a (110) faced providing the catalytic surface. Alternatively, the (100) and/or the (111) facets of the rocksalt structure can be chosen.
Thus, in some embodiments, the catalyst surface is a transition metal rutile surface. The surface can have any suitable facet, including but not limited to the (110) facet. In some embodiments, the surface facet comprises, or consists of, the (110) facet 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 the (110) facet of the rutile structure of one or more oxide selected from Rhenium oxide, Tantalum oxide and Niobium oxide.
A rutile metal oxide surface having a (110) facet 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. Whereas the sixfold coordinated metal atoms have approximately the same geometry as bulk, the fivefold coordinated metal atoms have an unsaturated bond perpendicular to the surface. Thus two distinct surface sites are found, coordinatively unsaturated sites (cus-sites) on top of the 5-fold coordinated metal atoms and bridging sites (br-sites) between two sixfold coordinated metal atoms. It has been found that the br-sites generally bind adsorbates stronger than the cus-sites. On the stoichiometric (110) surface, the br-sites are occupied by oxygen, while the cus-sites are vacant. The (110) surface can be referred to as oxygen terminated (O-term). The bridging oxygen atoms are under-coordinated and can be reduced on the surface:
leaving the br-sites vacant. These vacant br-sites can then get further protonated:
As a consequence, the 110 surface may be reduced, leaving bridging sites between sixfold coordinated metal atoms vacant. The bridging sites may also be occupied, partially or completely, by other atoms such as oxygen or hydrogen atoms, as illustrated by
It has been found that the catalytic activity of the (110) surface may also be dependent on the occupancy of the br-sites. Thus, it can be favorable thermodynamically to reduce the bridging oxygen atoms on transition metal oxides to H2O and further cover the br-sites with hydrogen atoms.
Accordingly, in some embodiments, bridging sites between sixfold coordinated transition metal atoms on transition metal oxide surfaces are covered by hydrogen atoms.
As will be apparent to the skilled person, the catalyst according to the invention can comprise a single transition metal oxide. The catalyst can also comprise, or consist of, a mixture of two or more such oxides. Such mixed oxides can comprise a single structure, for example a rutile structure. The mixed metal oxides can also comprise a mixture of oxides that are of different crystal structures and/or oxides with different catalytic facets. Accordingly, such mixed oxides can further comprise a single, or a mixture of, facets. Mixed oxide catalysts can be grown or manufactured separately and then assembled into mixed catalysts comprising the different metal oxides, wherein the oxides in the mixture have the same or different crystal structures.
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.
Without intending to be bound by theory, it is believed that the oxide catalysts are capable of shifting the bottleneck of ammonia synthesis from N2 cleavage to the subsequent formation of nitrogen-hydrogen species (*NH,*NH2, or *NH3) due to which simpler but yet higher rate of ammonia formation is anticipated.
In certain useful embodiments of the invention, ammonia can be formed at an electrode potential at less than about -1.1 V, less than about -1.0 V, less than about -0.9 V, less than about -0.8 V, less than about -0.7 V, less than about -0.6 V, less than about 0.5 V or less than about -0.4 V. In some embodiments, ammonia can be formed at electrode potential in the range of about -0.2 V to about -1.0 V, such as in the range of about -0.3 V to about -0.8 V, such as in the range of about -0.4 V to about -1.0 V, or in the range of about -0.5 V to about -1.0 V. The upper limit of the range can be about -0.6 V, about -0.7 V, about -0.8 V, about -1.0 V, or about -1.1 V. The lower limit of the range can be about -0.2 V, about -0.3 V, about -0.4 V, about -0.5 V or about -0.6 V.
In some embodiments, ammonia can be formed using an Niobium oxide catalyst at an electrode potential that is in the range of about -0.4 V to about -0.5 V. In some embodiments, ammonia can be formed using an Rhenium oxide catalyst at an electrode potential of about -0.8 V to about -0.9 V. In some embodiments, ammonia can be formed using an Tantalum oxide catalyst at an electrode potential of about -1.0 V to about -1.1 V.
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 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 above mentioned.
NOx and SOx are generic terms for mono-nitrogen and mono-sulfur axoides, 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 NOx and/or SOx from 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 oxide catalyst as described herein. In this context, in situ should be understood as meaning that the ammonia is generated 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 NOx and/or SOx in the stream of gas so as to convert these toxic species to other molecular species, such as N2, H2O and (NH4)2SO4. In some embodiments, the system can be for use in an automobile engine exhaust or in other engines, where ammonia can be generated in situ by a process according to the present invention, and which is then used to reduce SOx and/or NOx exhaust gases from the engine. Such system can suitably use electric current produced by conversion from the car engine. Thus, by using electric current from a car engine, ammonia can be generated in situ, and the ammonia thus generated can be allowed to react with SOx and/or NOx from the gas exhaust of the automobile. The ammonia can be generated in the automobile, and subsequently fed into the car exhaust. The ammonia can also be generated in situ within the automobile exhaust system. Thereby, NOx and/or SOx are removed from the car exhaust, reducing the amount of pollutants in the exhaust.
Embodiments of the invention include the following non-limiting clauses:
The invention will now be illustrated by the following non-limiting examples that further describe particular advantages and embodiments of the present invention.
The zero-point energy correction and entropy difference between the adsorbed species and the gas molecule, are calculated for all reaction intermediates within a harmonic approximation. These values are given in Table 1.
The 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 the values for adsorbed molecules are obtained from DFT calculations of vibrational normal modes.
Various electrolytes and electrode materials have been tried in order to alleviate the thermodynamic requirements and optimize the ammonia formation rate using heterogeneous catalysis. (AMarch 2011; Denvir 2008 (US7314544 B2);Ouzoounidou 2007; Pappenfus 2009; AMarch 2011;Song 2004) Electrolytic cells based on solid-state electrolytes and polymer electrolyte membranes (PEM) have been the subjects of much of this research as the setup simplifies the separation of the hydrogen feed gas and the produced ammonia. The highest rate of ammonia formation reported with Nafion membrane is 1.13 x 10-8 mol s-1 cm-2 with faradaic efficiency of around 90%, where wet H2 was used as the feed gas. (Xu 2009) Using air and water as feed gases is an exciting possibility, however, the highest rate of ammonia formation obtained in that way was much lower, 1.14 x 10-9 mol s-1 cm-2. (Lan 2013) Only 1% CE was obtained, requiring an overpotential of -1.6 V. Although promising, these results are still far from the production rates nearing that of commercial viability (4.3-8.7 x 10-7 mol s-1 cm-2). (Giddey, Badwal, 2013)
The search for a new route towards ammonia synthesis has up until now been led by experimentalists. Recently however, electrocatalytic N2 reduction has been the subject of theoretical studies, where materials that favor N2 reduction and suppress hydrogen evolution are suggested by the help of 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 through large groups of potential catalysts without having to produce them in the laboratory, saving both time and money. In 2012 Skúlason et al. did a detailed DFT analysis on the catalytic activity of both flat and stepped pure transition metal surfaces where they identified trends and calculated the free energy profile for nitrogen reduction. The so-called volcano plots are useful when estimating trends in catalytic reactivity and Skúlason et al. found that the early transition metals were more active towards N2 reduction than the later transition metals, as well as being less prone to evolving hydrogen. (Skúlason 2012) Abghoui et al. did a similar study on transition metal mono nitrides and found that early transition metal nitrides were promising candidates towards N2 reduction, where ZrN and VN were predicted to form ammonia at potentials of -0.76 V and -0.51 V vs. SHE, respectively. (2015 PCCP, 2015 Proceedia computer science, 2016 ACS Catalysis). Later they also suggested that NbN and CrN could behave in a similar fashion. (Abghoui Skulason,2017,MVK). Further studies have given promising results where the (110) facets of the zincblende structure of RuN was reported to catalyze ammonia formation at very small onset potential of around -0.23 V vs. SHE. (Abghoui 2017) The comparison of the Mars-van Krevelen mechanism (MvK) with the conventional associative and dissociative mechanisms revealed that the MvK should be a more favorable reaction mechanism on 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 ammonia formation electrochemically at ambient conditions. DFT calculations are used to construct a stability diagram for each metal oxide where stability of the (110) facet covered with different adsorbed species is calculated as a function of potential, and the most stable facet at each potential is identified. Next we study the thermodynamics of the cathode reaction and construct free energy diagrams for the electrochemical protonation of the adsorbed nitrogen species. The energetics between adsorption of proton and N2H species were investigated for all the potential catalysts to see the trend toward N2 reduction compared to the competing hydrogen evolution reaction (HER). We then estimate the onset potentials required for ammonia formation on the surface of these oxides. The effect of external potential is included by using the computational standard hydrogen electrode (SHE) (Norskov, Jonsson 2004) and the lowest onset potential required to reduce N2 to ammonia is estimated for each metal oxide. Finally, a volcano diagram is obtained where the catalytic activity of the different oxides is plotted using the binding energy of N2H as a common descriptor. (Rossmeisl 2007, Skúlason 2012)
DFT calculations: In the present study the (110) facet of eleven transition metal oxides naturally occurring in the rutile structure is considered with focus on electrochemical ammonia formation, as the (110) surface is the most stable of the low-index facetss of the rutile structure. The oxides stable in the rutile structure at ambient conditions are: TiO2, NbO2, TaO2, ReO2, IrO2, OsO2, CrO2, MnO2, RuO2, RhO2 and PtO2. (Harold Kung) The oxide surfaces are modelled by 48 atoms in four layers, each layer consisting of 4 metal atoms and 8 oxygen atoms. The bottom two layers are kept fixed whereas the upper two layers and the adsorbed species are allowed to fully relax. Boundary conditions are periodic in the x and y directions and the surfaces are separated by 12 Å of vacuum in the z direction. The structural optimization is considered converged when the forces in any direction on all moveable atoms are less than 0.01 eV/Å. The RPBE lattice constants were optimized for each oxide and spin-polarization was accounted for. The RPBE lattice constants optimized for the oxides in this study are: TiO2: a=4.65 Å, c=2.98 Å; NbO2: a=5.11 Å, c=3.00 Å; TaO2: a=4.98 Å, c=3.02 Å; ReO2: a=4.77 Å, c=3.14 Å; lrO2: a=4.58 Å, c=3.13 Å; OsO2: a=4.58 Å, c=3.18 Å; CrO2: a=4.50 Å, c=2.96 Å; MnO2: a=4.86 Å, c=2.85 Å; RuO2: a=4.58 Å, c=3.2 Å; RhO2: a=4.55 Å, c=3.06 Å and PtO2: a=4.65 Å, c=3.19 Å.
The calculations are performed with DFT using the RPBE exchange correlation functional. A plane wave basis set with an energy cutoff of 350 eV is used to represent the valence electrons with a PAW representation of the core electrons as implemented in the VASP code (VASP1-VASP4). The self-consistent electron density is determined by iterative diagonalization of the Kohn-Sham Hamiltonian, with the occupation of the Kohn-Sham states being smeared according to a Fermi-Dirac distribution with a smearing parameter of kBT = 0.1 eV. A 4 x 4 x 1 Monkhorst-Pack k-point sampling is used for all the surfaces and maximum symmetry is applied to reduce the number of k-points in the calculations.
Electrochemical reactions and modelling: The source of protons in the reaction could either be water splitting or H2 oxidation reaction in the anode. In order to link our absolute potential to the SHE, we refer to H2 here only as a convenient source of protons and electrons.
where the protons are solvated in the electrolyte and the electrons are transferred to the cathode through a wire. The overall reaction for N2 reduction is
The surface is hydrogenated by adding one hydrogen atom at a time, representing a proton from the solution and an electron from the electrode surface. The reaction mechanism studied, the associative one, is shown in the equations below where an asterisk denotes a surface site:
The first ammonia molecule is formed after the addition of three to five protons, depending on the favored reaction pathway. The second ammonia is then formed after the addition of six protons. The free energy of N2H adsorption is compared to that of proton adsorption, to probe whether the surface is selective towards either ammonia formation or hydrogen evolution.
As a first approximation, it is assumed that activation barriers between stable minima are low or that they follow a Brønsted-Evans-Polanyi relationship and can therefor be neglected during the electrochemical reactions in the context of the present invention. The free energy of each elementary step is estimated at pH=0 and T=298 K according to:
where ΔE is the energy calculated using DFT. ΔEZPE and ΔS are the differences in zero point energy and entropy, respectively, between the adsorbed species and the gas phase molecules. They are calculated within a harmonic approximation and the values are given in the ESI. The effect of an applies bias U, is included for all electrochemical reaction steps by shifting the free energy for the reaction involving n electrons by -neU so the free energy of each elementary step is given by:
Explicit inclusion of water in the simulations would greatly increase the computational effort and has therefore not been included in the present study. The presence of water is known to stabilize some species via hydrogen bonding. For instance, *NH2 is expected to be slightly more stable in the vicinity of water whereas *N will not be affected by the water layer. Previous publications have estimated the stabilization effects of water to be smaller than 0.1 eV per hydrogen bond. (Montoya 2015) Therefore, we estimate that the inclusion of hydrogen bonding would change the onset potentials calculated in this study by less than 0.1 eV, a correction that has not been included here.
Stability: The rutile (110) surface contains metal atoms of two different coordination environments, see
leaving the br-sites vacant. We refer to that surface as the reduced surface, see
We refer to that surface as hydrogen terminated (H-term), see
Catalytic activity: The catalytic activity of all the rutile oxides towards electrochemical ammonia formation is calculated by the use of DFT. For each rutile the free energy landscape is calculated for the three different surfaces, the reduced, the H-term and the O-term surface. The free energy of each intermediate is found with equation (10), referenced to N2 and H2 in the gas phase. As seen by the stability diagrams, the 0.25 ML surfaces are not favored on any significant potential range, and are therefore left out of all further calculations.
Considering the stability diagram presented in
Scaling relations and volcano diagram: Using scaling relations between the binding energies of the various intermediates of the N2 reduction mechanism, a volcano plot was constructed as shown in
Scaling relations used to calculate the lines in
Similar diagrams have previously been constructed for the pure transition metals, where the binding energy of N to the surface was used rather than the binding energy of N2H. (Skulason 2012, Montoya 2015)
The binding energy of *N is endothermic on all candidates in this work (except ReO2 and TaO2) and thus dissociating N2 would be endothermic as well as having a high activation energy. However, the binding energy of *N on ReO2 and TaO2 is around -1 eV and therefore dissociating N2 might involve a low barrier on those candidates, since there the reaction energy is around -2 eV. The inclusion of the dissociative pathway would however not change the reported PDSs for any candidate, as the *NH2 ➔NH3 is the PDS on the left leg of the volcano where ReO2 and TaO2 are located. The reaction steps *N ➔ *NH and *NH ➔ *NH2 are also included in our pathways and those steps never become PDSs (see
First protonation: Up until this point we haven’t considered competing reactions such as the hydrogen evolution reaction. As a first step towards including competing reactions, we calculate the free energy of the first reaction step of the ammonia formation, which includes the adsorption of N2H, and compare it to the free energy of hydrogen adsorption on the surface. The hydrogen atom is allowed to find its most favorable binding site on the surface. This is only done for the H-term surfaces, as the other surface terminations have already been eliminated due to instability under operating conditions. The results of this analysis can be seen in
DFT calculations were used to explore the possibility of nitrogen activation for electrochemical ammonia at ambient conditions on the (110) facet of the rutile structure of NbO2, RuO2, RhO2, TaO2, ReO2, TiO2, OsO2, MnO2, CrO2, lrO2 and PtO2. The relative stability of the facets with different adsorbates was calculated as a function of applied potential. The catalytic activity of each surface was investigated and the potential determining step found. At the applied potential required to make all electrochemical steps downhill in free energy, all the oxides were found to be most stable with hydrogen adsorbed on the sites. Of those oxides, ReO2 and TaO2 favor N2H adsorption.
| Number | Date | Country | Kind |
|---|---|---|---|
| 50188 | Sep 2017 | IS | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IS2018/050008 | 9/10/2018 | WO |
