WO2024083850A1

WO2024083850A1 - Electrocatalyst

Info

Publication number: WO2024083850A1
Application number: PCT/EP2023/078865
Authority: WO
Inventors: Victor MOUGEL; Yuan-Zi ZU; Daniel Francis ABBOTT; Amrita Rani SINGH-MORGAN; Kim Maureen TROESCH
Original assignee: ETH Zürich
Priority date: 2022-10-18
Filing date: 2023-10-17
Publication date: 2024-04-25

Abstract

The present invention relates to a catalyst for the electrocatalytic reduction of nitrate, nitrite, and NO comprising an electrically conductive support having a hierarchical structure, wherein at least a part of the outer surface of the electrically conductive support is covered by a catalytically active molybdenum layer, wherein said molybdenum layer comprises both metallic molybdenum and oxidized molybdenum and containing less than 10% of other metals and metal oxides.

Description

Electrocatalyst

The present invention relates to an electrocatalyst that can be used for the reduction of NOx, nitrite, and nitrate.

About 80% of worldwide ammonia production is required in the fertilizer industry, while the remainder is largely used to manufacture cosmetics, dyes, plastics, textiles, explosives, and other chemicals.

The most commonly utilized ammonia production method is the Haber-Bosch process. The downside to this technology is the high greenhouse gas emissions and high amounts of energy usage mainly due to the strict operational conditions at high temperature and pressure. This vast carbon and energy footprint have created an urgent need for sustainable and environmentally benign technologies to produce NH₃ .

CN11223792A discloses a catalyst for electrocatalytic reduction of nitrate, and a preparation method and application thereof. Said electrocatalytic ammonia synthesis is recognized as a viable alternative and has even been predicted to be more efficient than the Haber-Bosch process since it eliminates the need to use H₂ derived from fossil fuels and can be integrated with power derived from renewable energy sources, such as wind and solar photovoltaic.

Fu-Hang Yuan et al ("electrodeposition of self-supported NiMo amorphous coating as an efficient and stable catalyst for hydrogen evolution reaction", Rare Met. (2022) 41(8):2624- 2632) disclose a self-supported NiMo amorphous coating as an efficient and stable catalyst for hydrogen evolution reaction (HER) in water-alkali electrolysis. The NiMo coating was electrodeposited on a Ni foam substrate.

Xiaopeng Li et al (Sequential Electrodeposition of Bifunctional Catalytically Active Structures in Mo03/Ni-Ni0 Composite Electrocatalysts for Selective Hydrogen and Oxygen Evolution, Adv. Mater. 2020, 32, 200341) disclose a composite material, Mo0₃/Ni-Ni0, synthesized through a sequential electrodeposition strategy. The composite is used for water electrolysis, specifically for catalyzing both the hydrogen and oxygen evolution reactions.

WO2022/157034 discloses compounds comprising molybdenum in the oxidation state (VI) which catalyze the reduction of nitrogen (N₂) to ammonia.

WO2022/034928 and JP H3 254816 disclose compounds comprising molybdenum in the oxidation state (VI) which catalyze the reduction of nitrate to nitrogen (N₂).

WO2018/154812 discloses an electrode catalyst capable of reducing nitric acid and nitrous acid to nitrogen (N₂), and provides an electrode with a molybdenum oxysulfide-containing catalyst layer, and a denitrification method using the same to perform reduction reactions of nitric acid ions and/or nitrous acid ions.

EP0399833 discloses a process for preparing an electronically- conductive membrane by mixing at least one electronically- conductive metal oxide with at least one oxygen ion-conductive material, reducing the metal oxide to metal, forming the reduced mixture into a desired shape, and heating the formed mixture to form a dense and solid membrane. It also mentions the use of molybdenum oxide as a catalyst for reducing oxides of sulfur.

WO2022171663 discloses an electrochemical device that can convert nitrogen oxides into ammonia and hydrogen. The device uses a proton donor compound and a nitride-based material containing a metal with an oxidation state of (+III), such as molybdenum. The reaction involves the reduction of nitrogen oxides to ammonia and hydrogen gas.

In addition to offering a green approach to ammonia synthesis, electrocatalytic reduction of nitrate addresses the problem of nitrate pollution in the environment. Over the past century, the widespread use of fertilizers combined with global industrialization has led to a drastic increase of the amount of reactive nitrate in the environment. In particular, large amounts of nitrate originating from overfertilization are being introduced into the soil, which then leaches into the groundwater and contaminates surrounding water sources. This is well known to cause eutrophication that severely damages surrounding aquatic and terrestrial ecosystems.

Furthermore, high levels of nitrate found in drinking water can pose a serious risk to human health that can lead to the development methemoglobinemia, cancer, and severe birth defects. Although there are several methods of wastewater treatment that are capable of removing nitrate, it is typically either displaced or converted into inert N₂ .

Nitrate electroreduction, on the other hand, allows for the direct conversion of nitrate into ammonia, which can be collected and recycled into valuable chemical products. To date, the largest hurdles in electrocatalyst development for nitrate reduction have been product selectivity and stability. The reason for that is the high complexity of the reaction pathway for nitrate electroreduction which proceeds through the formation of several intermediate species, often leading to the formation of multiple products. In addition, designing catalysts that favor the reduction of nitrate over the competing hydrogen evolution reaction (i.e. the reduction of water to H₂) is a major issue.

In addition to nitrate-contaminated wastewater, toxic NOx gasses released from various sources, such as flue gas from combustion plants, transportation exhaust, agricultural emissions, and tail gas from nitric acid synthesis, can be targeted as nitrogen sources for the electrochemical reduction to ammonia as well. The current approach for reducing the nitrogen-oxide content of industrial effluents requires the use of selective catalytic reduction (SCR) processes, which are typically costly and energy consuming processes due to the high temperatures required. Furthermore, SCR processes mainly generate N₂ and H₂O, which have little chemical value.

CN113862700A discloses a nanocomposite electrocatalyst consisting of spherical Mo0₂ particles dispersed onto a nitrogen doped carbon support for the preparation of ammonia. Said electrocatalyst is prepared via hydrothermal methods, which is very energy consuming.

WO2022056427 discloses a nitrogen-doped graphene substrate that has also been doped with molybdenum. Said electrocatalyst comprises isolated, single-atom molybdenum catalytic sites.

CN110015693A discloses a hydrothermally prepared electrocatalyst comprising a two-dimensional MoO_x material to reduce nitrogen to ammonia.

CN108754532A discloses a hydrothermally prepared electrocatalyst comprising a nickel-iron alloy that has been doped with single atoms of molybdenum.

CN105261489A discloses a hydrothermally prepared electrocatalyst comprising molybdenum sulfide material formed through a polymerization reaction.

The object of the present invention was to provide a highly selective and stable catalyst to reduce nitrate, nitrite, and NOx to NH₃.

The problem is solved by the catalyst according to the present invention. Further preferred embodiments are subject of dependent claims 2 to 16.

The catalyst according to the present invention comprises an electrically conductive support having a hierarchical structure, wherein at least a part of the outer surface of the electrically conductive support is covered by an electrodeposited, catalytically active molybdenum layer, characterized in that said molybdenum layer comprises both metallic molybdenum and oxidized molybdenum and less than 10% of other metals and metal oxides.

It was shown that the catalyst according to the present invention displays an exceptionally high activity and selectivity for the electrocatalytic reduction of nitrate to NH₃.In particular, it can be used for direct NO₃“ reduction to NH₃ as well as to convert nitrite and gaseous nitrogen oxides (NOx) such as NO and NO₂ to NH₃ . The electrochemical reduction of NOx gasses, and in particular of NO, has the potential to provide a convenient and energy efficient pathway to a value- added product, namely ammonia. Moreover, the catalyst according to the present invention can be used in a broad variety of media and preferably in water. In particular, the outstanding performances can be obtained at moderate pH values, i.e., between pH 2 and 12, and notably also at a pH around 7, i.e., pH 6 to 8. Furthermore, the catalyst according to the present invention has a very high stability and can be produced at low costs and easily scaled up, which makes it ideal for a production at the industrial scale. In addition, due to the direct synthesis of the catalytically active layer comprising metallic and oxidized molybdenum on the electrically conductive support by electrodeposition the ohmic losses can be minimized.

Within the context of the present invention the term "molybdenum layer" refers to a catalytically active layer that comprises molybdenum and molybdenum oxides and less than 10% by weight of other metals and metal oxides. The term "other metals and metal oxides" means that in addition to metallic molybdenum and molybdenum oxides, the layer can contain a small proportion (less than 10% by weight) of metals other than molybdenum and their respective oxides. For instance, said molybdenum layer can include minor quantities of cobalt and its corresponding oxides, then cobalt would be categorized as "other metals".

Preferably, the molybdenum layer is essentially free of other metals or metal oxides, thus it essentially consists of metallic molybdenum and molybdenum oxides. Such a catalyst provides an outstanding selectivity. Within the context of the present invention the term "metallic molybdenum" stands for molybdenum in the oxidation state 0. The term "oxidized molybdenum" stands for molybdenum in an oxidation state that is not 0, preferably in the oxidation state II, III, IV, V and/or VI. In particular, it means that molybdenum may be present in different oxidation states, for example as a mixture of molybdenum oxides in the oxidation states IV and V.

Within the context of the present invention "selectivity of the catalyst" means that the catalyst reduces nitrate directly into ammonia and not into other products such as N₂, N0₂“, NH₂OH or H₂ . The selectivity is preferably more than 80%, most preferably more than 90% and ideally about 95% or more. Such a high selectivity is uncommon for other electrocatalytic systems in this area. Furthermore, the combination of such a high selectivity with very high current densities of more than 100 mA/cm² and that can go up to over 1 A/cm² is unique. Surprisingly, it is possible to obtain such a high selectivity at pH values between 2 and 12, and in particular also in neutral water having a pH around 7. The performance of the catalyst according to the present invention allows to avoid highly basic media having a pH around or over 14.

The conductive support has a hierarchical structure which leads to a morphology with high roughness. Within the context of the present invention the term "hierarchical structure" refers to a multidimensional structure, or "architecture", comprising both micro- and nanostructuration, having features on two or more scales. The hierarchical structures thus have a surface containing microscale features, such as micropores or microwires, the structure of said microscale features further comprising nanoscale features. Hierarchically-structured surfaces are thus those made up of nano-scale structures that form part of a larger micro-scale structure; thereby generating a 'hierarchy' in the sense that the larger structure is made up of many smaller structures (Yoon, Y., Kim, D. & Lee, J B. Micro and Nano Syst. Lett., 2014, 2, 3). The hierarchical structure serves as a high surface area scaffold onto which the catalytically active molybdenum layer is deposited. Thus, the catalyst according to the present invention is not only a two-dimensional, but a three-dimensional catalyst. The three- dimensional structures of the catalyst according to the present invention provides a large electrochemically active surface area and therefore a high density of surface active sites leading to higher current densities and selectivity towards ammonia production.

In one aspect of the present invention the electrically conductive support comprises a core and at least one first intermediate layer arranged thereon. The core can be made of an inexpensive, electrically conductive material and essentially serves as a carrier material. In this embodiment, the intermediate layer has a hierarchical structure and serves to enlarge the surface to be at least partly covered by the catalytically active molybdenum layer. The hierarchical structure is preferably made of a metal and can be prepared for example by electrodeposition.

Examples of such metallic hierarchical structures are dendritic hierarchical structures . The term "dendritic hierarchical structure" refers to a specific fractal metallic hierarchical structure the shape of which results from favored growth along energetically favorable crystallographic directions. The dendritic hierarchical structure can for instance have a tree-like form having one or more trunks and a plurality of branches. Said dendritic hierarchical structure, can comprise a nanostructuration in the form of dendrites.

The metal of said hierarchical structure is preferably selected from the group consisting of iron, copper, zinc, tin, silver, and nickel or alloys thereof. Said metals have a good conductivity, are able to form hierarchical nanostructures upon simple electrodeposition procedures, have a relatively low cost and provide an excellent stability in electrochemical conditions. Preferably, said hierarchical structures are made of metals or alloys in the oxidation state 0. These metallic hierarchical structures are thus substantially free of metal oxides, but traces of these can exist due to unwanted oxidation of the surface.

Preferably, the at least one intermediate layer has a metallic hierarchical nanostructure. Said metallic hierarchical nanostructure provides a high surface on which the catalytically active molybdenum layer generated by electrodeposition displays an unprecedented selectivity. Although direct deposition of high surface area metallic molybdenum and molybdenum oxide is very challenging, it was surprisingly found that the metallic hierarchical nanostructure, in particular dendritic hierarchical structure, is an excellent support for the electrodeposition of molybdenum and molybdenum oxide. Typically, the at least one intermediate layer has a thickness around 50 pm, but is itself structured at the nanoscale, up to 10-20 nm. There may be one or several (such as two, three or four) intermediate layers arranged on the core. Several intermediate layers may be of interest to eventually stabilize the electrically conductive support towards corrosion, like for example having a second intermediate layer of another metal or metal alloy on the generated hierarchical nanostructure of the first intermediate layer to enhance the stability of the nanostructure towards corrosion and dissolution phenomena. This may be for example an inert metal layer selected from the group consisting of Au, Cr, Pt, Ag and Ru deposited by electrochemical, PVD or CVD processes on top of the first intermediate layer.

Preferably, said metal is nickel or a nickel containing alloy. It was shown that the presence of nickel or a nickel alloy in metallic hierarchical structure, and in particular in the dendritic hierarchical nanostructure ensures the adhesion of said structure to the core. Further, it results in a uniform catalytically active layer comprising molybdenum and oxidized molybdenum on the outer surface of the structure. In addition, such catalysts have a high current density, while preserving selectivity and displaying high stability. Preferably, the metallic hierarchical nanostructure has a thickness of 5 to 200 pm .

Preferably, the core of the electrically conductive support comprises a metallic foam or a conductive carbon foam. Especially good results could be obtained with a nickel foam which provides a high surface area and ensures good mechanical and chemical stability to the electrochemically grown hierarchical nanostructures.

The electrodeposited catalytically active layer comprises both metallic and oxidized molybdenum (also referred to as catalytically active molybdenum layer). Said molybdenum layer comprises less than 10% of other metals and metal oxides and is preferably essentially free of other metals and metal oxides. Preferably, it is a composite comprising metallic molybdenum (in the oxidation state 0) and molybdenum oxides, wherein said molybdenum oxides comprise at least 50% by weight molybdenum in the oxidation state IV and can comprise further molybdenum oxides such as Mo (V) oxide. Preferably, said composite comprises 1 to 60% by weight of Mo(0), more preferably 1 to 30% by weight of Mo(0) based on the total content of metallic and oxidized molybdenum.

Preferably, the electrodeposited catalytically active molybdenum layer has a thickness of 0.1 to 4 pm, preferably 1.5 to 2.5 pm. The layer thickness is measured by transmission electron microscopy (TEM).

Preferably, the catalytically active layer comprising molybdenum and molybdenum oxide covers the majority of the outer surface of the electrically conductive support, preferably covering at least 80%, more preferably at least 90%, even more preferably at least 95% and ideally at least 99% of the outer surface of the electrically conductive support.

The invention also relates to a process for preparing a catalyst, involving the step of covering an electrically conductive support with a catalytically active layer comprising metallic and oxidized molybdenum via electrodeposition. For the electrodeposition, the electrically conductive support is preferably placed in an electrochemical cell together with a counter electrode (such as a platinum electrode). A solution of comprising molybdenum ions is added to the cell and a current is applied resulting in the deposition of molybdenum and oxidized molybdenum onto the surface of the electrically conductive support. The solution comprising molybdenum ions is preferably an aqueous solution. The method according to the present invention constitutes a simple and convenient approach for the direct deposition of large surface area metal structures onto an electrically conductive support. It provides a catalyst with a high current density while preserving selectivity and displaying high stability.

Preferably, the molybdenum layer is produced by applying a current density of more than 200 mA cm^-2, preferably more than 0.5 A cm^-2, and most preferably more than 1 A cm^-2. Excellent results could be obtained by applying a high current density of about 1 Acm² in a 0.5 M (NH₄)₆Mo₇0₂4 solution in the presence of acetate. The presence of a carboxylic acid such as acetate or a bidentate ligand of the acetylacetonate family can help to obtain low valent molybdenum species, and in particular molybdenum in the oxidation state 0.

Preferably, the process also involves the preparation of the electrically conductive support. It is prepared by covering a core with an intermediate layer having a metallic hierarchical structure, such as a dendritic structure, via electrodeposition. The hierarchical structure serves as a high surface area scaffold onto which the active molybdenum catalyst layer is deposited. For the electrodeposition, the core is preferably placed in an electrochemical cell together with a counter electrode (such as a platinum electrode). A solution comprising the corresponding metal ions is added to the cell and a current is applied resulting in the deposition of metallic hierarchical structure onto the surface of the core. The solution comprising metal ions is preferably an aqueous solution. Excellent results could be obtained by applying the limiting current density (for example 1 A cm^-2) in a metal sulphate or chloride solution (for example in 0.2 M NiSO₄ or NiCl₂) onto the core.

A further aspect of the present invention relates to the use of the catalyst to reduce NOx, nitrate and nitrite. The catalyst according to the present invention cannot only effectively reduce nitrate to ammonia but also reduce other nitrogen oxides such as NO and NO₂ to ammonia. Thus, it can be used to purify motor exhaust, flue gas, or in nitric acid synthesis tail gas reprocessing.

The catalyst according to the present invention can be used in a broad variety of media and preferably in water. It shows high chemical and structural stability under continuous nitrate reduction conditions in neutral pH for over 100 days.

In one embodiment of the present invention, the catalyst is used to reduce nitrate, in particular to reduce the nitrate concentration in tap water. The recommended maximum contaminant level (MCL) N0₃“ in drinking water has been established at 50mg/L by the European Community and World Health Organization. It was shown that with the catalyst according to the present invention N0₃“ ions can be effectively reduced in tap water used without any additional treatment.

A further aspect of the present invention relates to the use of the catalyst to produce ammonia. The catalyst according to the present invention provides a highly selective, sustainable route to produce ammonia from waste sources. In particular, the catalyst according to the present invention displays an exceptional NH₃ selectivity of 98% in neutral media, which is higher than that of other reported catalysts. In addition, considerably higher NH₃ yield rates can be obtained (i.e. more ammonia is being produced per unit time) at comparable or even lower overpotentials (i.e. requiring the same or even less energy input). In addition, the high purity of the produced NH₃ also means that there will be less need for product purification.

A further aspect of the present invention relates to the use of the catalyst in the treatment of polluted groundwater and/or of wastewater. Due to the fact, that the catalyst according to the present invention is highly selective and produces high quantities of NH₃ at only a moderate overpotential in neutral media, it can be effectively used to treat polluted groundwater and wastewater and in particular industrial wastewater.

A further aspect of the present invention relates to the use of the catalyst to remove NOx from a gas stream. The catalyst according to the present invention can not only effectively reduce NOx in a liquid medium but also in gas phase. The gas stream to be treated may be for example a combustion product stream generated from any type of furnace employing air to burn a carbonaceous fuel which will generate a combustion products containing nitrogen oxides. Especially preferred it is used to reduce NOx in car exhausts, tail gas and flue gas feed. The catalyst according to the present invention can be used in an electrolysis cell comprising an anode and cathode electrode typically separated by a proton exchange membrane. When water containing nitrate is flowed through the cell and an electric current is applied, a commercially available catalyst at the anode oxidizes water molecules in order to produce O₂, H⁺, and electrons. The electrons are then passed in an external circuit while the protons (H+) are transported across the membrane to the cathode. At the cathode electrode, the nitrate in the contaminated water stream is transported to the surface of the catalyst according to the present invention where it binds to the catalytically active molybdenum layer and reacts with the protons being transported from the anode and is directly reduced to NH₃ . Electricity to power the cell can ideally be sourced from renewable energy sources (e.g. solar, wind, hydroelectric) such that a zero-emissions pathway to ammonia production is achieved. The heat generated by the electrochemical process and the pH gradient resulting from the electrolysis can be utilized to recover NH₃ as a gaseous product directly from the electrolyzer solution.

The catalyst according to the present invention can be used in an electrolysis cell for NOx reduction in a similar manner. In this case, the NOx-containing gas can be supplied to the cathode in the gas phase either by bubbling it through the cathode compartment of the cell or by directly feeding it to the back of the cathode electrode.

The catalyst according to the present invention can be used in an electrolysis cell comprising an anode and cathode electrode separated by other barriers as well, such as cation exchange membranes, anion exchange membranes (OH-transported from cathode to anode), bipolar membranes, or other simple separators.

A further aspect of the present invention relates to the use of the catalyst according to the present invention for dilute NO reduction to ammonia in gas phase using a membrane electrode assembly electrolyzer. The term "dilute NO" refers to a diluted concentration of NO, typically in the range of 0.05 to 5%. Membrane Electrode Assembly (MEA) electrolyzer with an Anion Exchange Membrane typically comprises the following components: an anode, a cathode, and an Anion Exchange Membrane. In the context of converting nitric oxide (NO) to ammonia (NH₃), at the cathode, nitric oxide (NO) undergoes reduction to produce ammonia. The Anion Exchange Membrane facilitates the transport of anions, such as hydroxide ions (OH”), from the anode to the cathode, supporting the electrochemical reduction of NO to NH₃ . Due to the catalyst according to the present invention, the efficiency and selectivity of this conversion is excellent. For example, applying a constant current of 10 mA for 3h allowed an ammonia production rate of 0.05 mmol/h cm². Ammonia is generated in the gas phase, which is a significant advantage for its subsequent use.

Examples

Example 1 - Preparation of the catalyst (Ni@Mo)

The typical preparation procedure for a catalyst according to the present invention is depicted in Figure 1. In the first step, a high surface area dendritic structure is produced via the electrodeposition of nickel above the limiting current density (1 A cur² applied in 0.2 M NiCl₂ for 10 minutes) onto the commercial nickel foam (NF). The high surface area nickel dendrites on nickel foam (Ni/NF) material then serve as a scaffold for the catalytically active Mo layer, which is electrodeposited from a 0.5 M (NH4)₆Mo₇0₂4 solution in the presence of acetate at high current density (1 A crrr² for 10 minutes) during the second step.

Example 2 - Composition of the catalyst as determined by X-ray photoelectron spectroscopy (XPS)

The Mo 3d XPS spectra are shown for a series of Mo reference compounds (M0O3, Mo0₂, and Mo foil (i.e. metallic Mo)) and are compared to that of the catalyst (Figure 2a). The spectrum of the catalyst before depth profiling shows that the surface of the catalyst comprises Mo in an oxidized state, with the peak positions indicating a mix of molybdenum oxide species. After depth profiling, the appearance of a peak at ca. 228 eV indicates the presence of metallic molybdenum. The progression of the depth profiling can be seen in (Figure 2b), which shows a shoulder appearing at 228 eV as the etch depth is increased and the metallic Mo beneath the surface is exposed, indicating that the deposited molybdenum layer is a composite containing both metallic and oxidized molybdenum species. Furthermore, it is shown that just after the synthesis, i.e. "before depth profiling", there is no Mo (VI) at the surface, but only Mo (V) and (IV), and upon depth profiling there are significant amounts of Mo metal. Even after a very long operation time (over IlOh), Mo(0) is still present (Figures 2e and 2f).

Example 3 - Composition of the catalyst as characterized by scanning and transmission electron microscopy (SEM/TEM) Scanning electron microscopy (SEM) shows that the electrodeposited catalyst features a porous, mossy structure that is characterized by visible, large macroscopic pores (Figure 3a). Taking a cross-section cut of the material using focused ion-beam (FIB) (bright-field STEM image of the cross section being displayed in Figure 3b) and analyzing it viaEDS- STEM elemental mapping (Figure 3c) reveal that the catalyst features a layered structure comprising the electrically conductive support and its shell-like catalytically active molybdenum layer. The elemental map images (Figure 3c) for Mo, Ni, 0, and their stacked images highlight that Ni and Mo are not mixed in their respective layer, as no Ni is visible in the external Mo oxide layer and no Mo is visible in the Ni core of the material. Thus, the electrically conductive support consists of a Ni foam which is at least partly covered by metallic Ni dendrites. Onto said metallic Ni dendrites an oxygen-rich molybdenum layer was deposited, as can be seen by the elemental map images (Figure 3c) for Mo, Ni, 0, and their stacked images. High resolution transmission electron microscopy (HRTEM) in the molybdenum layer (Figure 3d) also reveals that the Mo layer is at least partly amorphous, as can be seen by the lack of diffraction rings in the selected area electron diffraction (SAED) (shown in insert).

Example 4 - Activity of the catalyst for the electrochemical reduction of nitrate

The linear sweep voltammograms (LSVs) show the current density (j) as a function of the potential (E) of the catalyst. It is scanned in 1 M Na₂SO₄ electrolyte with and without nitrate in solution. It is shown in Figure 4 that much more negative current densities are achieved with 0.1 M NO₃“ added to the electrolyte, indicating that the catalyst is active for nitrate reduction reaction.

Example 5 - Selectivity of the catalyst for nitrate reduction to ammonia in IM Na₂SO₄

The electrolysis of nitrate is performed at a series of applied potentials in IM Na₂SO₄ containing 0.1 M NO₃“. After holding the catalyst at the designated potential (E, V vs. RHE) for an extended period of time, a sample of the electrolyte is collected and the amount of NH₃ produced is quantified via ^XH NMR. The Faradaic efficiency (FE_NH3) for the conversion of NO₃" to NH₃ is then calculated based on the measured current density and the amount of NH₃ detected. In Figure 5 it can be observed that the catalyst converts NO₃“ to NH₃ with >90% FE over a broad range of potentials from -0.2 to -0.7 V, indicating that >90% of the measured current is utilized for the electrochemical reduction of nitrate to ammonia. The catalyst reaches a maximum of nearly 100% FE at -0.5 V with a current density

of ca. 300 mA cm^-2 and continues to show ca. 80% FE at -1.0 V at a current density of over 1 A cm^-2.

Example 6 - High durability of the catalyst for NO₃~ reduction to ammonia for over 2400 hours

The electrolysis of NO₃“ is performed in 1 M Na₂SO₄ at a constant current density of -650 mA cm^-2 on a 1 cm² catalyst. The catalyst demonstrates an impressive stability, as is shown by the stability of the measured potential (E, V vs. RHE) for over 2400 h (Figure 6). The Faradaic efficiency for NH₃ (FE_NH3) was maintained above ~75% during continuous operation for 2400 h. Example 7 Ability of the catalyst to produce ammonium chloride at constant current density

The electrolysis of N0₃“ is performed in 1 M Na₂SO₄ containing 0.1 M NaNO₃ at a constant, high current density (-750 mA cm^-2 or -850 mA cm^-2) for several hours on the catalyst according to the present invention using a flow-cell setup. Ammonia produced at the catalyst is evaporated and passed through a trap containing HC1, where it is absorbed and forms NH₄C1. As clearly shown in Figure 7, the amount of ammonia absorption increases continuously as time progresses. More importantly, the very high selectivity of the catalyst at -850 mA cm^-2 enables a full conversion of the nitrate to pure NH₄C1 in about 6 hours.

Example 8 - High integrity of the catalyst structure and morphology after extended electrolysis

Scanning electron microscopy (SEM) shows that the porous, mossy structure of the catalyst is preserved after 2400 hours of use (Figures 8a-8c). A cross-section cut of the used electrode prepared using focused ion-beam (FIB) (Figure 8d) and analysis by SEM-EDS reveals that the 3-dimensional structure comprising the electrically conductive support and its shell-like catalytically active molybdenum layer is maintained, as is evidenced by the elemental maps of Mo, Ni, 0, and their stacked images (Figure 8d). Similar results were obtained from elemental mapping via HRTEM-EDS analysis (Figures 8e to 8f). Furthermore, HRTEM also reveals the crystallinity of the Ni dendrites is preserved (Figure 8g) and that the Mo layer remains at least partly amorphous (Figure 8h), as can be inferred from the corresponding selected area electron diffraction (SAED) inserts.

Example 9 - High Selectivity of the catalyst for NO_X reduction to ammonia in various media

The activity of the catalyst towards the reduction of various N0_x gasses (NO, NO₂, or N₂0) was evaluated on 1 cm² of the catalyst in 1 M Na₂SO₄ by holding at the applied potential (E, V vs RHE) for 30 minutes. In each test, the NO_X gas constantly bubbled through the electrolyte to ensure saturation of the electrolyte with the gas. The Faradaic efficiencies towards NH₃ or N₂ are shown in (Figure 9a) and the corresponding partial current densities are shown in (Figure 9b). It is shown that FE_NH3 remains above 80% from -0.1 to -0.5 V for NO reduction (NORR) from -0.2 to -0.6 V for N₂O reduction (N₂ORR). The main product for NO₂ reduction (NO₂RR) was found to be N₂, displaying FE_N2 of 95% under all tested potentials, however, NH₃ is formed as well. In a similar series of tests, the activity of the catalyst was evaluated for the reduction of nitrite (0.1 M NO₂" in 1 M Na₂SO₄), which shows an FE_NH3 of >90% at all tested potentials (Figure 9a) and NH₃ yield rates of up to 0.36 mmol h^-1 cm^-2 at -0.6 V (Figure 9b). The effectiveness of the catalyst to reduce nitrate (NO₃“) in tap water (containing 20ppm levels of NO₃“) and wastewater (151 mM NO₃“) is also demonstrated.

Example 10 - Comparison of the catalyst according to the present invention with known catalysts Table 1 shows a comparison of the catalyst according to the present invention with known catalyst.

A23703WO/17.10.2023

A23703WO/17.10.2023

References mentioned in the above table:

1. Li, J. et al. Efficient ammonia electrosynthesis from nitrate on strained ruthenium nanoclusters. Journal of the American Chemical Society 142, 7036-7046 (2020).

2. Li, P., Jin, Z., Fang, Z. & Yu, G. A single-site iron catalyst with preoccupied active centers that achieves selective ammonia electrosynthesis from nitrate. Energy & Environmental Science (2021).

3. Wang, Y. et al. Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption. Journal of the American Chemical Society 142, 5702-5708 (2020).

4. Guo, Y. et al. Pd Doping-weakened Intermediate adsorption to Promote Electrocatalytic Nitrate Reduction on TiO2 Nanoarrays for Ammonia Production and Energy Supply with Zinc-Nitrate Batteries. Energy & Environmental Science (2021).

5. McEnaney, J.M. et al. Electrolyte engineering for efficient electrochemical nitrate reduction to ammonia on a titanium electrode. ACS Sustainable Chemistry & Engineering 8, 2672-2681 (2020).

6. Wang, Y., Zhou, W., Jia, R., Yu, Y. & Zhang, B. Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia. Angewandte Chemie 132, 5388-5392 (2020). 7. Wang, C. et al. Metasequoia-like Nanocrystal of Iron-Doped Copper for Efficient Electrocatalytic Nitrate Reduction into Ammonia in Neutral Media. ChemSusChem (2021).

8. Yu, Y., Wang, C., Yu, Y., Wang, Y. & Zhang, B. Promoting selective electroreduction of nitrates to ammonia over electron-deficient Co modulated by rectifying Schottky contacts. Science China Chemistry 63, 1469-1476 (2020).

9. Chen, G.-F. et al. Electrochemical reduction of nitrate to ammonia via direct eight-electron transfer using a copper- molecular solid catalyst. Nature Energy 5, 605-613 (2020).

10. Jia, R. et al. Boosting selective nitrate electroreduction to ammonium by constructing oxygen vacancies in TiO2. ACS Catalysis 10, 3533-3540 (2020).

11. Wu, Z.-Y. et al. Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst. Nature communications 12, 1-10 (2021).

12. Daiyan, R. et al. Nitrate reduction to ammonium: from CuO defect engineering to waste NO x-to-NH 3 economic feasibility. Energy & Environmental Science (2021).

Example 11 - High Selectivity of the catalyst for dilute NO reduction to ammonia in gas phase using a Membrane Electrode Assembly electrolyzer.

The activity of the catalyst towards the reduction of a humidified 1% NO in Ar flowing in a gas phase at 20 mls/min was evaluated on 1 cm² of the catalyst according to the present invention directly pressed on an Anion Exchange Membrane (Sustanion X37-50) in a membrane electrode assembly electrolyzer, on by holding at a constant current of 10 mA for 3h. A stable faradaic efficiency towards NH₃ of 78% was observed throughout the experiment, corresponding to an ammonia production rate of 0.05 mmol/h*cm². The ammonia is 5 generated here in the gas phase, being a significant advantage for its subsequent use. These conditions correspond to a 11% single pass conversion efficiency.

Claims

1. A catalyst for the electrocatalytic reduction of nitrate, nitrite, and N0_x comprising an electrically conductive support having a hierarchical structure, wherein at least a part of the outer surface of the electrically conductive support is covered by an electrodeposited, catalytically active molybdenum layer, characterized in that said molybdenum layer comprises both metallic molybdenum and oxidized molybdenum and less than 10% of other metals and metal oxides.

2. Catalyst according to claim 1, wherein the electrically conductive support comprises a core and at least one first intermediate layer arranged thereon.

3. Catalyst according to claim 2, wherein the intermediate layer provides a metallic hierarchical structure, wherein the metal of said hierarchical structure is preferably selected from the group consisting of iron, copper, zinc, tin, silver, and nickel or alloys thereof.

4. Catalyst according to claim 3, wherein the metal is nickel or a nickel containing alloy.

5. Catalyst according to any of claims 3 or 4, wherein the metallic hierarchical structure comprises a hierarchical nanostructure .

6. Catalyst according to any of claims 2 to 5, wherein the core comprises a metallic foam or a conductive carbon foam, most preferably a nickel foam.

7. Catalyst according to any of the preceding claims, wherein the electrodeposited catalytically active layer has a thickness of 0.1 to 4 pm.

8. Process for preparing a catalyst according to any of the preceding claims, involving the step of at least partly covering the outer surface of an electrically conductive support with a catalytically active layer comprising both, metallic molybdenum and oxidized molybdenum via electrodeposition.

9. Process according to claim 8, wherein the catalytically active layer is produced by applying a current density of more than 200 mA cm^-2, preferably more than 0.5 A cm^-2, and most preferably more than 1 A cm^-2.

10. Process according to any of claims 8 or 9, wherein the conductive support is prepared by covering a core with an intermediate layer having a metallic hierarchical structure via electrodeposition.

11. Use of the catalyst according to any of claims 1 to 7 to reduce nitrate, nitrite, and NO_X.

12. Use of the catalyst according claim 11 to reduce nitrate.

13. Use of the catalyst according to any of claims 11 to 12 to produce ammonia.

14. Use of the catalyst according to claim 11 in the treatment of wastewater to reduce nitrate, nitrite, and N0_x.

15 . Use of the catalyst according to claim 11 to remove N0_x from a gas stream, in particular car exhausts, tail gas and flue gas feed.

16. Use of the catalyst according to claim 11 to reduce diluted NO to produce NH₃ in the gas phase by using a membrane electrode assembly electrolyzer.