FIELD OF THE INVENTION

The invention relates to a system for converting a nitrogen oxide to a nitrogen product. The invention further relates to a method for converting a nitrogen oxide to a nitrogen product.

BACKGROUND OF THE INVENTION

Methods for the electrochemical reduction of nitrogen oxide to a nitrogen product are known in the art. For instance, Ko et al., “Electrochemical Reduction of Gaseous Nitrogen Oxides on Transition Metals at Ambient Conditions”, J. Am. Chem. Soc., 2022, describes the electrochemical reduction of NO and N2O in an alkaline or acidic electrolyte in a gas-fed three-compartment flow cell.

In another instance, US2019368398A1, describes an electrochemical reactor arranged in an exhaust passage of an internal combustion engine having a honeycomb member wherein a plurality of cells are formed. The honeycomb comprising an upstream and a downstream side partial honeycombs. The upstream side has a plurality of first and second cells arranged to at least partially adjoin the first cells through partition wall base members including an ion conductive solid electrolyte. The downstream side has a plurality of third and fourth cells arranged to at least partially adjoin the third cells through partition wall base members including an ion conductive solid electrolyte. The first and fourth cells have cathode layers, and second and third cells have anode layers. The electrochemical reactor is configured so all of the exhaust gas flowing through the first cells flows into the third cells and all of the exhaust gas flowing through the second cells flows into the fourth cells.

In another instance, US2009162257A1, describes a purification apparatus including a plurality of electrochemical devices. An anode of each electrochemical device is connected to a cathode of a battery, and a cathode of the electrochemical device is connected to an anode of the battery. An electrolyte layer containing an electrolyte is arranged between the anode and the cathode. The electrolyte layer contains a polymer film and a support body having a honeycomb structure and supporting the polymer film. The polymer film is an electrolyte that exhibits conductivity with respect to protons H ⁺ .

In yet another instance, DE4403367C2, describes an adsorbent for removing nitrogen oxides (NO _X : nitrogen monoxide and nitrogen dioxide), in particular nitrogen dioxide, from an exhaust gas which contains the nitrogen oxides in low concentrations and a process for removal of nitrogen oxides, in particular nitrogen dioxide, from an exhaust gas which contains the nitrogen oxides in low concentrations by using the adsorbent. The adsorbent has either at least one noble metal which is Pt, Au, Ru, Rh and Pd and/or a compound thereof applied to a support or has the noble metal constituent and an oxide of at least one heavy metal which is Mn, Fe, Co, Ni, Cu, Zn and Pb which, if necessary, can be applied to a support.

SUMMARY OF THE INVENTION

The anthropogenic perturbation of the natural nitrogen cycle represents a concerning threat to the environment. NO _X gases, such as NO, NO2 and N2O, emitted by fossil fuel power plants and automobile engines may contribute to the formation of acid rains and smog and may be dangerous for human health. The development of selective catalytic reduction (SCR) technology and the implementation of such units in power plants has significantly decreased NO _X emissions over the last decades. However, the capital costs of SCR may be high, and the resulting dinitrogen gas may not represent a valuable product from a commercial point of view. It may be desirable to convert NO _X gases to a useful chemical, such as ammonia. Currently, ammonia may primarily be produced via the energy -intensive Haber-Bosch process, which may be responsible for about 1% of the yearly global CO2 emissions.

The prior art may describe the use of metallic electrodes immersed in liquid electrolyte in batch systems for the conversion of NO to ammonia. However, such systems may typically be unsuitable for continuous operation, and may require an ammonia stripping unit to separate the ammonia from a liquid cathode feed.

Further, prior art systems may have low yield and/or low faradaic efficiency.

The prior art may further describe a hybrid system, wherein a nitrogen oxide is gas-fed to an electrode, but wherein a liquid catholyte is used to suppress hydrogen evolution and/or to act as absorbent/solvent for the nitrogen oxide. In particular, in such a system, the nitrogen oxide may dissolve in the liquid catholyte or may react at a triple phase boundary (gas, catholyte, catalyst). Nitrogen oxide reduction products may thus be present both in liquid and gas phase, which may complicate their recovery. Further, the catholyte solution may flood a gas channel in the cells, which may lead to a performance loss. In addition, the single pass conversion may be low, i.e., most of the fed gas may be channeled through the cell without reacting.

Further, prior art processes may involve harsh and energy-intensive reaction conditions, such as temperatures above 300 °C for selective catalytic reduction or NO _X storage and reduction. In addition, such processes may have an inefficient conversion during a cold start.

Further, prior art processes may require a sacrificial reducing agent, such as ammonia, a hydrocarbon or dihydrogen, which may be (relatively) expensive.

Hence, it is an aspect of the invention to provide an alternative system and/or method for the conversion of a nitrogen oxide to a nitrogen product, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

Hence, in a first aspect, the invention may provide a system for converting a nitrogen oxide to a nitrogen product, especially wherein the nitrogen oxide comprises one or more of NO, NO2, and N2O. Further, the nitrogen product may especially comprise one or more of N2, NH2OH and NH3. In embodiments, the system may comprise one or more of a proton exchange membrane cell (or “polymer electrolyte membrane cell” or “PEM cell”), a first fluid supply, and a second fluid supply. The proton exchange membrane cell may comprise a first compartment comprising a first electrode and a second compartment comprising a second electrode, especially wherein the first electrode and the second electrode are separated by a polymer electrolyte membrane. In particular, the polymer electrolyte membrane may be H ⁺ - conductive. In embodiments, a nitrogen oxide reduction catalyst may be arranged between the first electrode and the polymer electrolyte membrane. Further, in embodiments, an oxidation catalyst may be arranged between the second electrode and the polymer electrolyte membrane, especially wherein the oxidation catalyst is configured to catalyze the oxidation of a proton source, especially one or more of H2O and H2. In embodiments, the first fluid supply may be configured to provide a first gas to the first compartment, especially wherein the first gas comprises the nitrogen oxide. In further embodiments, the second fluid supply may be configured to provide a second fluid to the second compartment, especially wherein the second fluid comprises the proton source, such as one or more of H2 and H2O.

In particular, the system of the invention may provide oxidation of the proton source, especially H2 and/or H2O, in the second compartment, resulting in H ⁺ passing through the proton-conductive membrane to the first compartment, and may provide reduction of the nitrogen oxide in the first compartment, resulting in the formation of H2O and one or more of dinitrogen (N2), hydroxylamine (NH2OH), and ammonia (NH3), especially one or more of N2 and NH3. The formed H2O may be expelled from the cell in form of vapor together with unreacted NO and gaseous products. Generally, electrochemical cells may be operated with a liquid solution at the cathode (catholyte), which may typically be motivated by the ability to reach high concentrations of reactants at the cathode with a corresponding expectation of higher yields. In particular, the use of an aqueous solution may be thought to lead to better hydration of the membrane, which may lead to an enhanced proton conductivity, and thereby a better expected performance. Further, in the art, alkaline/acidic solutions may typically be used as catholytes to alter the pH during operation to favor or suppress the formation of specific reaction products as the selectivity of NO _X reduction products may be pH-dependent. For instance, hydrogen evolution may be suppressed with an alkaline catholyte.

Surprisingly, the inventors discovered that when feeding the nitrogen oxide to the cathode as a gas, without the presence of a liquid electrolyte, the process parameters improved and that the expected competing H2-evolution reactor was (essentially) not an issue.

In particular, the system of the invention may facilitate converting the nitrogen oxide to a nitrogen product with a high single-pass conversion rate, in a continuous process, while providing a high yield and high faradaic efficiency, and in mild conditions (such as temperatures below 120 °C). The system of the invention may thus apply advantages of PEM cells, such as low Ohmic losses, good product separation and relatively easy scalability to nitrogen oxide reduction. Further, as the nitrogen oxide reacts in the gas phase in the context of the present invention, issues in relation to separating products from a liquid solution, such as from catholyte components, are avoided, allowing for a more efficient downstream process. The PEM cell can be powered by renewable electricity, and may have a lower carbon-footprint than state of the art technologies. In contrast to state-of-the-art NO _X removal technologies, which may require the addition of chemicals like ammonia or urea, in embodiments of the system of the invention, water may be used as only reactant besides NO _X .

The system may facilitate both the abatement of the nitrogen oxide, such as by conversion to N2, which may be released into the atmosphere, as well as a value-added conversion to NH3 and/or NH2OH, which may be used in a variety of industrial and agricultural applications.

Hence, in specific embodiments, the invention provides a system for converting a nitrogen oxide to a nitrogen product, wherein the nitrogen oxide comprises one or more of NO, NO2, and N2O, wherein the nitrogen product comprises one or more of N2, NH2OH, and NH3, and wherein the system comprises a proton exchange membrane cell, a first fluid supply, and a second fluid supply, wherein: the proton exchange membrane cell comprises a first compartment comprising a first electrode and a second compartment comprising a second electrode, wherein the first electrode and the second electrode are separated by a polymer electrolyte membrane, wherein the polymer electrolyte membrane is H ⁺ -conductive, wherein a nitrogen oxide reduction catalyst is arranged between the first electrode and the polymer electrolyte membrane, and wherein an oxidation catalyst is arranged between the second electrode and the polymer electrolyte membrane, wherein the oxidation catalyst is configured to catalyze the oxidation of a proton source; the first fluid supply is configured to provide a first gas to the first compartment, wherein the first gas comprises the nitrogen oxide; the second fluid supply is configured to provide a second fluid to the second compartment, wherein the second fluid comprises the proton source, wherein the proton source comprises one or more of H2 and H2O.

Hence, the invention may provide a system for converting a nitrogen oxide to a nitrogen product.

The term “nitrogen oxide” may herein refer to any molecule consisting of nitrogen and oxygen. In particular, in embodiments, the nitrogen oxide may comprise one or more of nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), which may herein and in the art also be referred to as NO _x -compounds. In further embodiments, the nitrogen oxide may especially comprise NO. In further embodiments, the nitrogen oxide may especially comprise NO2. In further embodiments, the nitrogen oxide may especially comprise N2O.

The term “nitrogen product” may herein refer to a nitrogen-comprising molecule obtained from (electrochemical) reduction of the nitrogen oxide. In particular, in embodiments, the nitrogen product may comprise one or more of dinitrogen (N2), hydroxylamine (NH2OH) and ammonia (NH3), especially N2, or especially NH2OH or especially NH3.

The system may especially comprise one or more of a proton exchange membrane cell, a first fluid supply, and a second fluid supply. Especially, the system may comprise the proton exchange membrane cell, the first fluid supply, and the second fluid supply.

Hence, in embodiments, the system comprises a proton exchange membrane cell. The term “proton exchange membrane cell” (or “PEM cell”) may herein especially refer to an electrochemical cell comprising a solid electrolyte, especially a proton-conducting polymer electrolyte membrane. In particular, a PEM cell may be devoid of a liquid electrolyte. Generally, a PEM cell may comprise two (gas diffusion) electrodes separated by a polymer electrolyte membrane, wherein catalysts are arranged between gas diffusion layers and the membrane, such as deposited on the membrane and/or on the gas diffusion layer(s). The gas diffusion layer may comprise porous material that provides electric contact to the catalyst and provides transport of gases. The catalysts may especially be in direct contact with the membrane and the (respective) layer. In particular, the membrane may be nonconductive with respect to electrons.

It will be clear to the person skilled in the art that a variety of materials may be suitable to provide an electrode, especially a gas diffusion layer. In embodiments, the first electrode (or “second electrode”) may especially comprise a material selected from the group comprising carbon paper, carbon cloth (optionally with microporous layer), titanium felt, sintered titanium fibers, and sintered titanium powder (optionally with a platinum or a gold coating).

In embodiments, the proton exchange membrane cell may comprise a first compartment comprising a first electrode (or “cathode”) and a second compartment comprising a second electrode (or “anode”). In further embodiments, the first electrode and the second electrode may especially comprise gas diffusion electrode (layers). In particular, in embodiments, the first electrode may comprise a first gas diffusion electrode (layer). In further embodiments, the second electrode may comprise a second gas diffusion electrode (layer).

In the art, the term “gas diffusion electrode” may be used to refer to a gas diffusion layer or to a gas diffusion layer coated with a catalyst. Herein, the term “gas diffusion electrode” such as in “first gas diffusion electrode” and “second gas diffusion electrode” may especially refer to a gas diffusion layer.

The first compartment and the second compartment may especially be fluidically separated. In particular, in embodiments, the first electrode and the second electrode may be separated by a polymer electrolyte membrane, especially wherein the polymer electrolyte membrane is H ⁺ -conductive.

In embodiments, the first compartment may comprise a first inlet and a first outlet. The first inlet may especially be configured for receiving the first gas (from the first fluid supply). The first outlet may especially be configured for providing a first product stream comprising the nitrogen product.

Similarly, in embodiments, the second compartment may comprise a second inlet and a second outlet. The second inlet may be configured for receiving the second fluid (from the second fluid supply). The second outlet may be configured for providing an (oxygencontaining) second product stream.

The term “polymer electrolyte membrane” may herein especially refer to a solid proton-conductive electrolyte. Suitable polymer electrolyte membranes are known to the skilled person. For instance, in embodiments, the polymer electrolyte membrane may comprise a polymer with a hydrocarbon backbone or a tetrafluoroethylene backbone, such as Nation™, which may have a particularly good thermal and mechanical stability. In further embodiments, the polymer electrolyte membrane may be selected from the group comprising Aquivion® from Solvay, fumapem® from fumatech, and 725EW, 800EW ionomers from 3M, Toray Greenerity, and BASF Celtec.

In embodiments, a nitrogen oxide reduction catalyst may be arranged between the first electrode and the polymer electrolyte membrane. In particular, in embodiments, the nitrogen oxide reduction catalyst may be arranged on the polymer electrolyte membrane, such as on a first side of the polymer electrolyte membrane. For instance, the nitrogen oxide reduction catalyst may be arranged on the polymer electrolyte membrane in powder form. In further embodiments, the nitrogen oxide reduction catalyst may be deposited on the first electrode. The nitrogen oxide reduction catalyst may especially be arranged in direct (physical) contact with the first electrode and the (first side of the) polymer electrolyte membrane.

The term “nitrogen oxide reduction catalyst” may herein especially refer to a catalyst (material) configured to catalyze the reduction of a nitrogen oxide. In embodiments, the nitrogen oxide reduction catalyst may especially comprise a nitrogen oxide hydrogenation catalyst, i.e., a catalyst (material) configured to catalyze the hydrogenation of a nitrogen oxide.

It will be clear to the person skilled in the art that many catalyst materials may be suitable for catalyzing the reduction of a nitrogen oxide, such as for catalyzing the reduction of one or more of NO, N2O and NO2. For instance, the nitrogen oxide reduction catalyst may comprise a (suitable) metal by itself or supported on an electroconductive material such as a carbon black. In embodiments, the nitrogen oxide reduction catalyst may comprise a metal selected from the group comprising (metallic) Ru, (metallic) Cu, (metallic) Pt, (metallic) Rh, and (metallic) Pd, especially (metallic) Ru, or especially (metallic) Cu, or especially (metallic) Pt, or especially (metallic) Rh, or especially (metallic) Pd. In further embodiments, the nitrogen oxide reduction catalyst may (also) comprise (metallic) Ag. Hence, in embodiments, the nitrogen oxide reduction catalyst may comprise a metal selected from the group comprising (metallic) Ru, (metallic) Cu, (metallic) Pt, (metallic) Rh, (metallic) Pd, and (metallic) Ag, especially (metallic) Ru, or especially (metallic) Cu, or especially (metallic) Pt, or especially (metallic) Rh, or especially (metallic) Pd, or especially (metallic) Ag.

The nitrogen product (predominantly) produced) may depend on one or more of the choice of catalyst material, the cell voltage and/or the pressure in the cell.

In particular, in embodiments, the nitrogen oxide reduction catalyst may comprise Ru, i.e., the metal may comprise Ru. Such a catalyst may be particularly selective towards ammonia (generation). Hence, in further embodiments, the nitrogen oxide reduction catalyst may comprise Ru, and the nitrogen product may comprise NH3 and/or NH2OH, especially NH3, or especially NH2OH.

In further embodiments, the nitrogen oxide reduction catalyst may comprise a material selective towards hydroxylamine.

In further embodiments, the nitrogen oxide reduction catalyst may comprise a metal selected from the group comprising Pd and Cu. Such a catalyst may be particularly selective towards dinitrogen (generation). Hence, in further embodiments, the nitrogen oxide reduction catalyst may comprise a metal selected from the group comprising Pd and Cu, and the nitrogen product may comprise N2.

In particular, the nitrogen oxide reduction catalyst may be provided in a specific amount per geometric electrode area, which may also be referred to as the “catalyst loading”. A (too) low catalyst loading may lead to a low current density, and thus a low reaction rate, as well as in a high Ohmic resistance in the cell. However, a high catalyst loading may result in elevated current densities, which may increase the prevalence of the hydrogen evolution reaction, may decrease the faradaic efficiency of nitrogen oxidation, and may further be inefficient with regards to materials, and may be (relatively) expensive.

Hence, in embodiments, the nitrogen oxide reduction catalyst may be provided in an amount (or “catalyst loading”) selected from the range of 0.01 - 5 mg metal/cm ² , especially from the range of 0.5 - 2 mg metal/cm ² , such as (about) 1 mg metal/cm ² .

Similarly, in embodiments, an oxidation catalyst may be arranged between the second electrode and the polymer electrolyte membrane. In particular, in embodiments, the oxidation catalyst may be arranged on the polymer electrolyte membrane, such as on a second side of the polymer electrolyte membrane. For instance, the oxidation catalyst may be arranged on the polymer electrolyte membrane in powder form, optionally together with an ionomer, such as Nafion. In further embodiments, the oxidation catalyst may be deposited on the second electrode. The oxidation catalyst may especially be arranged in direct (physical) contact with the second electrode and the (second side of the) polymer electrolyte membrane.

Hence, in embodiments, the polymer electrolyte membrane may comprise a catalyst coated polymer electrolyte membrane, especially wherein the nitrogen oxide reduction catalyst is arranged on a first side of the polymer electrolyte membrane, and/or especially wherein the oxidation catalyst is arranged on a second side of the polymer electrolyte membrane. The term “oxidation catalyst” may herein especially refer to a catalyst (material) configured to catalyze the oxidation of a proton source, wherein the proton source comprises H2O and/or H2. In embodiments, the oxidation may especially be configured to catalyze the oxidation of H2O. In further embodiments, the oxidation may especially be configured to catalyze the oxidation of H2.

In embodiments, the oxidation catalyst may be provided in an amount selected from the range of 0.01 - 5 mg metal (oxide)/cm ² , especially from the range of 0.5 - 2 mg metal/cm ² , such as (about) 1 mg metal (oxide)/cm ² .

It will be clear to the person skilled in the art that many catalyst materials may be suitable for catalyzing the oxidation of the proton source, especially of H2O and/or H2. For instance, in embodiments, the oxidation catalyst may comprise one or more of IrCh, RuCh, Pt, Pd, and Ir, especially one or more of IrCh and RuCh, or especially one or more of (metallic) Pt, (metallic) Pd and (metallic) Ir.

In particular, an oxidation catalyst comprising IrCh and/or RuCh may be particularly suitable with respect to the oxidation of water.

Further, an oxidation catalyst comprising (metallic) Ir, (metallic) Pd and/or (metallic) Pt may be particularly suitable with respect to the oxidation of dihydrogen. In particular, in embodiments, the oxidation catalyst may comprise Ir, Pd and/or Pt in their reduced form, i.e., w.r.t. Ir, not as IrCh.

As indicated above, the term “polymer electrolyte membrane cell” may especially refer to an electrolytic cell with a solid electrolyte. Hence, in embodiments, the polymer electrolyte membrane cell, particularly the first compartment of the polymer electrolyte membrane cell, may be (essentially) devoid of a liquid electrolyte, such as devoid of a catholyte.

The system may, in embodiments, further comprise the first fluid supply. The first fluid supply may especially be configured to provide a first gas to (a first inlet of) the first compartment, especially to the first electrode, or especially to the nitrogen oxide reduction catalyst. The first gas may comprise the nitrogen oxide, such as one or more of NO, N2O and NO ₂ .

In embodiments, the first gas may especially comprise the nitrogen oxide in a gas mixture, especially wherein the nitrogen oxide is carried in a carrier gas. Hence, in embodiments, the first gas may comprise the nitrogen oxide and a carrier gas. For instance, the first gas may comprise one or more of NO, N2O and NO2 in helium. In particular, in embodiments, the first gas, especially the carrier gas, may comprise one or more of helium, argon, nitrogen, carbon dioxide, oxygen, hydrogen, methane, carbon monoxide, a sulfur oxide and water, especially one or more of helium and argon. In further embodiments, the first gas, especially the carrier gas, may comprise carbon monoxide.

Further, in embodiments, at least 0.001 vol% of the first gas may comprise the nitrogen oxide, such as at least 0.005 vol%, especially at least 0.01 vol%. In further embodiments, at least 0.1 vol% of the first gas may comprise the nitrogen oxide, such as at least 0.5 vol%, especially at least 1 vol%. In further embodiments, at least 10 vol% of the first gas may comprise the nitrogen oxide, such as at least 50 vol%, especially at least 90 vol%, including 100 vol%. Hence, in principle, the first gas may (essentially) consist of the nitrogen oxide. In further embodiments, at most 99 vol% of the first gas may comprise the nitrogen oxide, such as at most 95 vol%, especially at most 90 vol%.

Yet further, in embodiments, the first gas may beneficially comprise a supplemental gas. Such a supplemental gas may especially increase selectivity towards a particular nitrogen product, such as especially NH3, or especially, NH2OH, or especially N2. In embodiments, the first gas may comprise a supplemental gas selected from the group comprising carbon monoxide (CO), sulfur comprising gases, such as sulfur dioxide (SO2), oxygen (O2), chlorine comprising gases, bismuth comprising gases, and tin comprising gases. In particular, the first gas may comprise a supplemental gas suitable for selective surface modification, especially by strong binding of surface modifiers (which can be adsorbed molecules or surface atoms). In particular, in embodiments, at most 50 vol% of the first gas may comprise the supplemental gas, especially at most 10 vol%, such as at most 5 vol%, especially at most 3 vol%. In further embodiments, at most 2 vol% of the first gas may comprise a supplemental gas, especially at most 1 vol%, such as at most 0.5 vol%. In further embodiments, the first gas may comprise at least 0.001 vol% of the supplemental gas, such as at least 0.01 vol%, especially at least 0.1 vol.%. In further embodiments, the first gas may comprise at least 1 vol% of the supplemental gas, such as at least 2 vol%, especially at least 5 vol%.

For example, in embodiments, the first gas may comprise a combination of NO and CO, especially in a carrier gas, such as in helium. Such a first gas composition may be particularly selective towards ammonia (generation).

The term “supplemental gas” may herein also refer to a plurality of different supplemental gases.

In further embodiments, the first fluid supply may comprise a first container configured to host (or “store”) the first gas, especially the nitrogen oxide. The system may, in embodiments, further comprise the second fluid supply. The second fluid supply may be configured to provide a second fluid to (a second inlet of) the second compartment, especially to the second electrode, or especially to the oxidation catalyst. The second fluid may especially comprise (a proton source comprising) one or more of H2 and H ₂ O.

In embodiments, the second fluid may comprise a second gas comprising H2. For instance, the second gas may be an FF-comprising industrial waste stream. In embodiments, at least 0.1 vol% of the second gas may comprise H2, such as at least 0.5 vol%, especially at least 1 vol.%. In further embodiments, at least 5 vol% of the second gas may comprise H2, such as at least 10 vol%, especially at least 25 vol.%.

In further embodiments, the second gas may beneficially have a (relatively) high H2 concentration, and especially comprise (essentially) no CO. In particular, in embodiments, at least 60 vol% of the second gas may comprise H2, such as at least 80 vol%, especially at least 90 vol%. In further embodiments, at least 99.0 vol% of the second gas may comprise H2, especially at least 99.9 vol%, such as at least 99.97%.

In further embodiments, the second gas may comprise < 10 ppm CO, especially < 5 ppm CO, such as < 3 ppm CO, including (essentially) 0 ppm CO. In particular, as CO may poison the catalyst, the second gas may preferably comprise (essentially) no CO.

In further embodiments, the second fluid may comprise a second fluid comprising H2O, such as a second gas comprising H2O, or such as a second liquid comprising H2O. In further embodiments, the second fluid, especially the second liquid, may comprise deionized and/or demineralized water.

In further embodiments, the second fluid supply may comprise a second container configured to host (or “store”) the H2O and/or H2, especially the H2, or especially the H ₂ O.

The system may further comprise a charge control unit. The charge control unit may especially be configured to, during use, impose a potential difference between the first electrode and the second electrode. In particular, in an operational mode, the charge control unit may be configured to impose a potential difference between the first electrode and the second electrode, wherein the potential difference is suitable for converting the nitrogen oxide to the nitrogen product.

It will be clear to the person skilled in the art that the suitable potential difference may depend on whether the second fluid comprises H2 or H2O. Further, the suitable potential difference may depend on the nitrogen oxide. Further, the suitable potential difference may depend on the (desired) nitrogen product.

For instance, in embodiments wherein the second fluid comprises FfcO, the charge control unit may especially be configured to, in the operational mode, impose a potential difference between the first electrode and the second electrode selected from the range of 1.23

- 2.70 V, such as from the range of 1.6 - 2.2 V. In particular, in such embodiments, a potential difference selected from the range of 1.6 - 2.2 V may result in a particularly high efficiency.

In comparison, in embodiments wherein the second fluid comprises H2, the charge control unit may especially be configured to, in the operational mode, impose a potential difference between the first electrode and the second electrode selected from the range of -0.80

- 1.40 V, especially from the range of 0 - 1.20 V. In particular, in such embodiments, a potential difference selected from the range of 0 - 1.20 V may result in a particularly high efficiency.

Further, as also mentioned above, the system of the invention may facilitate continuous operation, i.e., the system of the invention may be operated as a continuous flow cell. Hence, in embodiments, the system may comprise a continuous flow cell. In particular, in embodiments, the system may be configured for continuous operation.

The system may further be configured to control one or more environmental parameters, such as one or more of temperature, pressure and humidity. In particular, the system may be configured to control one or more environmental parameters inside of the polymer electrolyte membrane cell.

In particular, it may be beneficial to hydrate the polymer electrolyte membrane, such as to increase the proton conductivity. The hydration may be particularly beneficial in embodiments wherein the second fluid comprises a second gas, such as H2. Hence, in embodiments, the system may comprise a humidifier configured to humidify the first compartment and/or the second compartment, especially (at least) the first compartment, or especially (at least) the second compartment. In particular, in embodiments, the system may comprise a humidifier configured to humidify the polymer electrolyte membrane.

Similarly, in embodiments, the system may comprise a liquid handling system. The liquid handling system may especially be configured to wet the polymer electrolyte membrane.

In further embodiments, the system may comprise a temperature control element. The temperature control element may be configured to control a temperature in (or “of’) the proton exchange membrane cell in the range of 0 - 230 °C, especially in the range of 50 - 100 °C. In particular, a (too) low temperature may result in a poor proton conductivity, whereas a (too) high temperature may result in degradation of the polymer electrolyte membrane. It will be clear to the person skilled in the art that the temperature may be selected in view of components of the system, such as in view of the temperature tolerance of the electrode (material), the catalyst(s) and the polymer electrolyte membrane.

In further embodiments, the system may comprise a pressure control element. The pressure control element may be configured to control a pressure in (or “of’) the proton exchange membrane cell in the range of (about) 1 - 130 bar, such as in the range of 0 - 129 barg. Here, “barg” may refer to a pressure in bar (105 N/m ² ) in excess of (local) atmospheric pressure. In further embodiments, the pressure control element may be configured to control a pressure in (or “of’) the proton exchange membrane cell in the range of at least 1 barg, such as at least 3 barg, especially at least 5 barg. In particular, a higher pressure may result in a higher availability of reactants at the electrode surface, which could lead to suppression of hydrogen evolution and/or higher product yields and thus an improved conversion of the nitrogen oxide to the nitrogen product.

In further embodiments, the polymer electrolyte membrane cell may be operated at (local) atmospheric pressure.

In embodiments, the system may further comprise a control system. The control system may especially be configured to control one or more of the first fluid supply, the second fluid supply and the polymer electrolyte membrane cell, especially at least the first fluid supply and the second fluid supply. In embodiments, the control system may further be configured to control the charge control unit. In further embodiments, the control system may be configured to control one or more of the temperature control unit, the pressure control element, the humidifier, and the liquid handling system, especially at least the temperature control unit, or especially at least the pressure control element, or especially at least the humidifier, or especially at least the liquid handling system.

The term “controlling” and similar terms herein may especially refer at least to determining the behavior or supervising the running of an element. Hence, herein “controlling” and similar terms may e.g. refer to imposing behavior to the element (determining the behavior or supervising the running of an element), etc., such as e.g. measuring, displaying, actuating, opening, shifting, changing temperature, etc.. Beyond that, the term “controlling” and similar terms may additionally include monitoring. Hence, the term “controlling” and similar terms may include imposing behavior on an element and also imposing behavior on an element and monitoring the element. The controlling of the element can be done with a control system. The control system and the element may thus at least temporarily, or permanently, functionally be coupled. The element may comprise the control system. In embodiments, the control system and the element may not be physically coupled. Control can be done via wired and/or wireless control. The term “control system” may also refer to a plurality of different control systems, which especially are functionally coupled, and of which e.g. one master control system may be a control system and one or more others may be slave control systems.

The system, especially the control system, may have an operational mode. The term “operational mode” may also be indicated as “controlling mode”. The system, or apparatus, or device (see further also below) may execute an action in a “mode” or “operational mode” or “mode of operation”. Likewise, in a method, an action, stage, or step may be executed in a “mode” or “operation mode” or “mode of operation”. This does not exclude that the system, or apparatus, or device may also be adapted for providing another operational mode, or a plurality of other operational modes. Likewise, this does not exclude that before executing the mode and/or after executing the mode one or more other modes may be executed. However, in embodiments, a control system (see further also below) may be available, that is adapted to provide at least the operational mode. Would other modes be available, the choice of such modes may especially be executed via a user interface, though other options, like executing a mode in dependence of a sensor signal or a (time) scheme, may also be possible. The operational mode may in embodiments also refer to a system, or apparatus, or device, that can only operate in a single operational mode (i.e. “on”, without further tunability).

In embodiments, in the operational mode, the first fluid supply may (be configured to) provide the first fluid to the first compartment.

Similarly, in embodiments, in the operational mode, the second fluid supply may (be configured to) provide the second fluid to the second compartment.

Further, in embodiments, in the operational mode, the charge control unit may (be configured to) impose the potential difference between the first electrode and the second electrode.

In yet further embodiments, in the operational mode, the temperature control system may (be configured to) control the temperature in the polymer electrolyte membrane cell.

In a further aspect, the invention may provide a method for converting a nitrogen oxide to a nitrogen product using a proton exchange membrane cell. The nitrogen oxide may especially comprise one or more of NO, NO2, and N2O. The nitrogen product may especially comprise one or more of N2, NH2OH and NH3. In embodiments, the proton exchange membrane cell may comprise a first compartment comprising a first electrode and a second compartment comprising a second electrode. In particular, the first electrode and the second electrode may be separated by a polymer electrolyte membrane, i.e., the proton exchange membrane cell may comprise a polymer electrolyte membrane configured to (fluidically and/or electrically) separate the first electrode and the second electrode. The polymer electrolyte membrane may especially be H ⁺ -conductive. In embodiments, a nitrogen oxide reduction catalyst may be arranged between the first electrode and the polymer electrolyte membrane, especially wherein the nitrogen oxide reduction catalyst is configured to catalyze the reduction of the nitrogen oxide. In further embodiments, an oxidation catalyst may be arranged between the second electrode and the polymer electrolyte membrane, especially wherein the oxidation catalyst is configured to catalyze the oxidation of a proton source, wherein the proton source comprises one or more of H2O and H2. The method may especially comprise providing a first gas to the first compartment, wherein the first gas comprises the nitrogen oxide. The method may further comprise providing a second fluid to the second compartment, wherein the second fluid comprises the proton source, especially one or more of H2 and H2O. In embodiments, the method may comprise imposing a potential difference between the first electrode and the second electrode, wherein the potential difference is selected for converting the nitrogen oxide to the nitrogen product.

Hence, in specific embodiments, the invention may provide a method for converting a nitrogen oxide to a nitrogen product using a proton exchange membrane cell, wherein the nitrogen oxide comprises one or more of NO, NO2, and N2O, wherein the nitrogen product comprises one or more of N2 and NH3, and wherein the proton exchange membrane cell comprises a first compartment comprising a first electrode and a second compartment comprising a second electrode, wherein the first electrode and the second electrode are separated by a polymer electrolyte membrane, wherein the polymer electrolyte membrane is H ⁺ -conductive, wherein a nitrogen oxide reduction catalyst is arranged between the first electrode and the polymer electrolyte membrane, and wherein an oxidation catalyst is arranged between the second electrode and the polymer electrolyte membrane, wherein the oxidation catalyst is configured to catalyze the oxidation of a proton source, wherein the proton source comprises one or more of H2O and H2; wherein the method comprises: providing a first gas to the first compartment, wherein the first gas comprises the nitrogen oxide; providing a second fluid to the second compartment, wherein the second fluid comprises one or more of H2 and H2O; and imposing a potential difference between the first electrode and the second electrode, wherein the potential difference is selected for converting the nitrogen oxide to the nitrogen product. In particular, the method may comprise converting a nitrogen oxide to a nitrogen product using the system of the invention.

In embodiments, the method may comprise providing a first gas to the first compartment, wherein the first gas comprises the nitrogen oxide, such as the NO, or such as the NO2, or such as the N2O. The first gas may especially comprise the nitrogen oxide in a gas mixture, such as in a carrier gas. For instance, the first gas may comprise one or more of NO, N2O and NO2 in a carrier gas, such as in helium. In particular, in embodiments, the carrier gas may comprise one or more of helium, argon, nitrogen, carbon dioxide, oxygen, hydrogen, methane, carbon monoxide, a sulfur oxide and water, especially one or more of helium and argon.

In further embodiments, the first gas may beneficially comprise a supplemental gas. Such a supplemental gas may especially increase selectivity towards a particular nitrogen product, such as especially NH3, or especially, NH2OH, or especially N2. In embodiments, the first gas may beneficially comprise one or more supplemental gases selected from the group comprising carbon monoxide (CO), sulfur comprising gases, such as sulfur dioxide (SO2), oxygen (O2), chlorine comprising gases, bismuth comprising gases, and tin comprising gases. In particular, the first gas may comprise a supplemental gas suitable for selective surface modification, especially by strong binding of surface modifiers (which can be adsorbed molecules or surface atoms).

In yet further embodiments, the method may comprise providing a second fluid to the second compartment, wherein the second fluid comprises the proton source, such as one or more of H2 and H2O. In particular, in embodiments, the method may comprise providing a second gas to the second compartment, wherein the second gas comprises H2 and/or H2O, especially H2, or especially H2O. In further embodiments, the method may comprise providing a second liquid to the second compartment, wherein the second liquid comprises H2O.

The method may further comprise imposing a potential difference between the first electrode and the second electrode, especially wherein the potential difference is selected for converting the nitrogen oxide to the nitrogen product.

In particular, especially in embodiments wherein the proton source comprises H2O, the potential difference may be selected from the range of 1.23 - 2.70 V, such as from the range of 1.6 - 2.2 V.

In further embodiments, such as embodiments wherein the proton source comprises H2, the potential difference may be selected from the range of -0.80 - 1.40 V, especially from the range of 0 - 1.20 V. In embodiments, the method may especially comprise humidifying the polymer electrolyte membrane cell.

In further embodiments, the method may especially comprise wetting the first electrode and/or the second electrode. In further embodiments, the method may especially comprise wetting the nitrogen oxide reduction catalyst and/or the oxidation catalyst.

In further embodiments, the method may comprise controlling the temperature of the polymer electrolyte membrane cell, especially in the range of 0 - 230 °C, such as in the range of 50 - 100 °C.

In further embodiments, the method may comprise controlling a pressure in (or “of’) the proton exchange membrane cell in the range of (about) 1 - 130 bar, such as in the range of 0 - 129 barg.

The embodiments described herein are not limited to a single aspect of the invention. For example, an embodiment describing the method may, for example, further relate to the system, especially to an operational mode of the system, or especially to the control system. Similarly, an embodiment of the system describing an operation of the system may further relate to embodiments of the method. In particular, an embodiment of the method describing an operation (of the system) may indicate that the system may, in embodiments, be configured for and/or be suitable for the operation. Similarly, an embodiment of the system describing actions of (a stage in) an operational mode may indicate that the method may, in embodiments, comprise those actions.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig. l schematically depicts an embodiment of the system of the invention. Fig. 2A-4B schematically depict experimental observations obtained using the system and the method of the invention. The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 schematically depicts an embodiment of the system 100 for converting a nitrogen oxide 11 to a nitrogen product 61. In particular, the nitrogen oxide 11 may especially comprise one or more of NO, NO2, and N2O, and the nitrogen product 61 may comprise one or more of N2, NH2OH and NH3. In the depicted embodiment, the system 100 comprises a proton exchange membrane cell 200, a first fluid supply 110, and a second fluid supply 120. The system 100 may especially be configured for continuous operation.

In particular, in the depicted embodiment, the proton exchange membrane cell 200 comprises a first compartment 210 comprising a first electrode 211 and a second compartment 220 comprising a second electrode 222. The first electrode 211 and the second electrode 222 are separated by a proton-conductive polymer electrolyte membrane 230. Further, a nitrogen oxide reduction catalyst 215 is arranged between the first electrode 211 and the polymer electrolyte membrane 230, and an oxidation catalyst 225 is arranged between the second electrode 222 and the polymer electrolyte membrane 230. In particular, the nitrogen oxide reduction catalyst 215 may be arranged on a first side 231 of the polymer electrolyte membrane 230, and the oxidation catalyst 225 may be arranged on a second side 232 of the polymer electrolyte membrane 230. The nitrogen oxide reduction catalyst may especially be configured to catalyze the reduction of the nitrogen oxide. The oxidation catalyst 225 may especially be configured to catalyze the oxidation of a proton source, especially wherein the proton source comprises one or more of H2O and H2.

In the depicted embodiment, the first fluid supply 110 is configured to provide a first gas 10 to the first compartment 210, wherein the first gas 10 comprises the nitrogen oxide 11. Similarly, the second fluid supply 120 is configured to provide a second fluid 20 to the second compartment 220, wherein the second fluid 20 comprises the proton source 21.

In particular, in the depicted embodiment, the first compartment 210 comprises a first inlet 216 and a first outlet 217, wherein the first inlet 216 is configured for receiving the first gas 10, and wherein the first outlet 217 is configured for providing a first product stream 60 comprising the nitrogen product 61. Hence, the first fluid supply 110 may especially be configured for providing the first gas 10 to the first inlet 216,

Similarly, in the depicted embodiment, the second compartment 220 comprises a second inlet 226 and a second outlet 227, wherein the second inlet 226 is configured for receiving the second fluid 20, wherein the second outlet 227 is configured for providing an (oxygen-containing) second product stream 70. Hence, the second fluid supply 120 may especially be configured for providing the second fluid 20 to the second inlet 226.

The system 100 may further comprise a charge control unit 130. In particular, in an operational mode, the charge control unit 130 may be configured to impose a potential difference between the first electrode 211 and the second electrode 222. The potential difference may especially be selected to be suitable (to stimulate) the conversion of the nitrogen oxide to the nitrogen product. In embodiments, such as wherein the proton source comprises H2O, the potential difference may be selected from the range of 1.23 - 2.70 V, such as from the range of 1.6 - 2.2 V.

In further embodiments, such as wherein the proton source comprises H2, the potential difference may be selected from the range of -0.80 - 1.40 V, especially from the range of O - 1.20 V.

In the depicted embodiment, the system further comprises a humidifier 150 configured to humidify the first compartment 210 and/or the second compartment 220. In addition, in the depicted embodiment, the system 100 comprises a liquid handling system 160 configured to wet the polymer electrolyte membrane 230.

The system may further comprise a temperature control element 140. The temperature control element 140 may especially be configured to control a temperature in the proton exchange membrane cell 200 in the range of 0 - 230 °C, such as in the range of 50 - 100 °C.

Fig. 1 further schematically depicts an embodiment of the method of the invention. In the depicted embodiment, the method may comprise: providing a first gas 10 to the first compartment 210, wherein the first gas 10 comprises the nitrogen oxide 11; providing a second fluid 20 to the second compartment 220, wherein the second fluid 20 comprises the proton source 21; and imposing a potential difference between the first electrode 211 and the second electrode 222, wherein the potential difference is selected for converting the nitrogen oxide 11 to the nitrogen product 61.

In further embodiments, such as wherein the proton source 21 comprises H2O, the potential difference may be selected from the range of 1.23 - 2.70 V, such as from the range of 1.6 - 2.2 V.

In yet further embodiments, such as wherein the proton source 21 comprises H2, the potential difference may be selected from the range of -0.80 - 1.40 V, especially from the range of 0 - 1.20 V.

Experiments

Materials & Methods

Unless specified otherwise, the materials and methods described hereinafter are used in the examples described below.

First fluid - a first fluid comprising 4.8 vol% NO in He was used in the examples 1-5 below. A first fluid comprising 2.5 vol% NO and 2.5 vol% CO in He was used in example 6 below. Ammonia collection and quantification - an acid trap was used to collect the formed ammonia. Specifically, a glass bubbler containing 0.2 M HC1 was used. The outlet gas was passed through the acid solution. The ammonia in the gas phase reacted with HC1 to produce NH4CI, which was quantified. This was implemented specifically for quantifying the ammonia generation. On a larger scale, other separation methods may typically be employed.

Catalyst synthesis - Electrically conductive active carbon Vulcan XC-72 (Cabot Corp) was used as catalyst support. RuCh xEEO (Sigma Aldrich) was used as Ru precursor. A sodium borohydride synthesis method was used for the preparation of cathode catalysts. In brief, the powders were prepared by suspending the carbon support in 100 ml water, to which the soluble metal precursors were added. The precursor concentration was selected so that the final metal loading in the catalyst would be 40 wt%. After stirring the mixture for 1 h at room temperature, sodium borohydride powder in six-fold excess was slowly added. The mixture was stirred for another 30 minutes until gas evolution stopped. The resulting catalysts were filtered and washed with 2 liters of water. Subsequently, the powders were dried at 90°C overnight and were further used for ink preparation and deposition over Nafion™ membranes.

Preparation of membrane electrode assembly (MEA) - Commercial Nafion™ membranes with a thickness of 127 pm were used as solid polymer acidic electrolyte in the PEM cell. The membranes were activated as follows: 1 h at 80°C in 3% H2O2 solution, Ih at 80°C in 1 M H2SO4, followed by 1 h boiling in type I ultrapure water. The cathode catalyst powders were incorporated in isopropanol-based inks, consisting of the catalyst powder (amounting to 2.5 mg/cm ² loading on membrane), 30 wt% Nafion™ ionomer and 2 ml of isopropanol. The inks were deposited over heated Nafion™ membranes at 60°C via spraycoating deposition. For experiments with water oxidation at the anode, commercial iridium oxide (Premion, Alfa Aesar) was used as anode catalyst and deposited in a similar fashion. The inks in this case consisted of the IrCE powder (amounting to 1 mg/cm ² loading on membrane, unless stated otherwise), 20 wt% Nafion™ ionomer and 2 ml of isopropanol. For experiments with hydrogen oxidation at the anode, a commercial 40 wt% Pt/C catalyst was used (Sigma Aldrich). The ink composition was similar to that of the NO reduction cathode catalyst; the amount of catalyst loading on the MEA was 1.5 mg/cm ² . The Nafion™ ionomer loading was 30 wt% and 2 ml of isopropanol was used for ink preparation. These were deposited in a similar fashion as the cathode catalysts, on the other side of Nafion™ membranes. After spraying, the catalyst-coated membranes (CCMs) were hot-pressed at 120°C at 1 MPa pressure for 3 minutes using a lab press. Subsequently, the CCMs were sandwiched between two porous platinum- coated titanium felt gas diffusion electrodes (0.45 mm, Bekaert) and assembled in an in-house built PEM electrolysis cell with titanium bipolar plates with serpentine flow channels and two aluminum cartridge-heated end plates. The cell was tightened using 8 stainless steel rods (4 mm 0) using a torque wrench set at 5 N m, to provide uniform compression across the cell area.

Nitric oxide reduction setup and product quantification - A mixture of 4.8% NO and 1% CEU in He was fed to the cathode side of the PEM cell via mass flow controllers (MFC). A flow of 20 ml/min was used during experiments, unless stated otherwise. He was used for purging the cathode compartment between experiments. The ammonia and hydroxylamine produced at the cathode were trapped in a 0.2 M hydrochloric acid (Sigma Aldrich) solution, which was further analyzed via ion chromatography and UV-vis spectrophotometry. In experiments with HOR at the anode, a condenser was used for removing ammonia from the outlet stream. A Metrohm 883 Basic IC Plus chromatograph equipped with a Metrosep C6 separation column was used for the quantification of ammonia and a Unicam UV 500 UV-Vis spectrometer was used for the quantification of hydroxylamine as described in Afkhami et al., “Indirect Kinetic Spectrophotometric Determination of Hydroxylamine Based on Its Reaction with Iodate”, Analytical Sciences, 2006, which is hereby herein incorporated by reference. A Bruker Alpha infrared spectrometer equipped with a transmission gas analysis module was used for the quantification of NO and N2O. CH4 was used as internal standard to account for volume changes during electrochemical experiments, which can arise due to the consumption of NO, as well as due to trapping of NH3 or the hydrogen evolution reaction (HER). Nitrogen was quantified using a CompactGC (Global Analyser Solutions). At the anode, a peristaltic pump (Ismatec) was used to feed type I ultrapure water at a flow of 1 ml/min in experiments with water oxidation at the anode. An MFC was used to feed hydrogen at a flow of 40 ml/min for experiments with hydrogen oxidation at the anode. For the latter, the hydrogen feed was humidified using a syringe pump (Isco), which fed water at a rate of 50 pl/min in a preheated tee fitting at 80°C. A Metrohm Autolab PGSTAT302N potentiostat was used for electrochemical measurements. The PEM cell was heated at 80°C using cartridge heaters and a temperature controller.

Catalyst characterization - The catalyst powders were characterized by nitrogen physisorption with a Micromeritics Tristar 3020 apparatus, after an overnight degassing step at 120°C. X-ray powder diffractograms were measured with a Bruker D8 ADVANCE X-ray diffractometer with a Co K-alpha radiation source. Transmission electron micrographs were acquired on a Jeol JEM-1400 plus transmission electron microscope (TEM). Conductive Vulcan XC-72 carbon black, typically used as a catalyst support both in PEM fuel cells and electrolysers, was chosen as the support of the active metal due to its good electrical conductivity, high surface area, inertness and facile dispersibility of the resulting catalyst in the inks to be deposited over the Nafion™ membrane. Powder X-ray diffraction was used to identify the crystalline phase of the metal in the catalysts. The Vulcan XC-72 support was observed only to exhibit a broad peak in the XRD pattern around 28° 29. Based on XRD, Ru is present in the metallic form. Based on TEM micrographs, the Ru/C formed spherical nanoparticles ranging from 2 to 6 nm, exhibiting a mostly uniform distribution, although some agglomerates as large as 20 nm were observed. The BET surface area of the Ru/C catalyst was observed to bel68 m ² /g.

Example 1

Experiments were performed with a system 100 of the invention, wherein the nitrogen oxide reduction catalyst comprises a Ru/C catalyst. A cell voltage (or “potential difference”) range between 1.7 and 2.1 V was investigated. Constant voltage experiments were carried out for 30 minutes. Considering the standard reduction potentials in Eq. 1 and Eq. 2 (see below), the thermodynamic potential of a cell with nitric oxide reduction taking place at the cathode and water oxidation at the anode may be E°cell = -0.394 V (Eq. 3; see below). In particular, a second liquid comprising H2O was provided to the second compartment.

2NO(g) + 6H ⁺ + 4e NH ₄ ⁺ + H ₂ O E° = 0.836 V (1)

2H ₂ O O ₂ + 4H ⁺ + 4e’ E° = 1.23 V (2)

E° _ce // = 0.836 V - 1.23 V = -0.394 V (3)

Fig. 2A schematically represents the faradaic efficiency FE (in %) towards different NO reduction products as function of cell voltage V, together with the average current density A (in mA cm' ² ) recorded during the experiments versus cell voltage V. Specifically, bars P1-P5 correspond to the FE, wherein Pl corresponds to EE, wherein P2 corresponds to N2, wherein P3 corresponds to N2O, wherein P4 corresponds to NH2OH, and wherein P5 corresponds to NH3. P1-P5 correspond to the same compounds in Fig. 2B and Fig. 4A-B. Further, line LI corresponds to the average current density A versus cell voltage V.

The ammonia faradaic efficiency exhibited a promising value of ca. 50% already at the lowest cell voltage tested, 1.7 V, with a current density of 32 mA cm' ² . The faradaic efficiency increased with cell voltage, exhibiting a maximum of ca. 78%, at 1.9 V, at a current density of ca. 64 mA cm' ² . At 2-2.1 V, the faradaic efficiency to ammonia decreased, and an increase in hydrogen faradaic efficiency was observed. Dinitrogen (or “nitrogen”) formation was observed at cell voltages as low as 1.7

V. Its faradaic efficiency remained relatively constant over the whole investigated cell voltage range.

N2O formation appeared more favorable at low cell voltage, showing a considerable faradaic efficiency of 28% at 1.7 V, which decreased to 2% at 2.1 V. Hydroxylamine formation was also detected, although the amounts were (relatively) low throughout the investigated cell voltage range. The highest recorded faradaic efficiency for hydroxylamine was 6.5% at 1.7 V cell voltage. At higher cell voltages, it dropped below 2%.

Fig. 2B schematically depicts the selectivity (in %) of nitric oxide conversion to P1-P5 (see Fig. A) as a function of cell voltage V, wherein line L2 corresponds to the total conversion rate C (in %). Specifically, Fig. 2B demonstrates that the conversion rate increased to a maximum of ca. 70% at 1.9 V, and decreased upon further voltage increase to 2 and 2.1 V. The nitrogen product selectivity trend may (essentially) follow the faradaic efficiency trend observed in Fig. 2A.

Example 2

In the experiments described in this section, the loading of the nitrogen oxide reduction catalyst Ru/C in the first compartment was kept constant at 2.5 mg cm' ² , while an iridium oxide (IrCh) loading in the second compartment was varied: 0.3, 1 or 2 mg cm' ² .

Fig. 3 A schematically depicts the one pass NO conversion rate C (in %) versus cell voltage V, wherein line L3 corresponds to anode catalyst loading of 0.3 mg cm' ² , line L4 corresponds to anode catalyst loading of 1 mg cm' ² and line L5 corresponds to anode catalyst loading of 2 mg cm' ² . Lines L3-L5 correspond to the same anode catalyst loadings in Fig. 3B- C. At cell voltages between 1.7 and 1.9 V, NO conversion was observed to increase with IrO2 loading. At 2.0 V, a similar performance for the three loadings was recorded. At 2.1 V, the 2 mg cm' ² IrO2 membrane electrode assembly (MEA) was found as the best performing. Nevertheless, it appears that, in the tested range, iridium oxide loading may not have a strong impact on NO conversion.

Fig. 3B schematically depicts the ammonia partial current density ANH3 at different cell voltages V. Specifically, Fig. 3B indicates that for 0.3 mg cm' ² IrO2, the proton generation rate at the anode is (relatively) low, which may explain the better NO conversion with the higher loadings. In particular, the ammonia partial current density may increase with iridium oxide loading. However, as depicted in Fig. 3C, the ammonia faradaic efficiency FENHS may decrease with the increase of iridium oxide loading from 1 to 2 mg cm’ ² at cell voltages above 1.8 V.

The highest ammonia yield was observed at 2.1 V cell voltage for the 1 mg cm’ ² MEA, where ammonia faradaic efficiency exhibited a modest value of 57%, but NO conversion was highest, at ca. 70%, along with the ammonia partial current density, at ca. 70 mA cm’ ² .

Unless specified otherwise, for further experiments described herein, the intermediate iridium oxide loading of 1 mg cm’ ² was used between the polymer electrolyte membrane and the second electrode.

Example 3

Flow variation experiments were conducted at 1.9 V cell voltage, where the best performance may be observed in terms of ammonia faradaic efficiency and NO conversion. Faradaic efficiencies (FENHS) and single pass conversion rates (XNO) were determined at NO/Fe flow rates (in ml min' ¹ ) of 8, 17, 24 and 53. The determined values are summarized in table 1 :

In particular, at 8 ml min' ¹ 4.8% NO in He flow, a 97% single-pass conversion of NO was observed in the PEM cell. The NO conversion was observed to decrease with higher flow rates, and the lowest value of 48% was recorded at 53 ml min' ¹ gas flow. The ammonia faradaic efficiency was observed to be highest at the flow rate of 17 ml min' ¹ .

Example 4

In this section, H2 was used as a proton source rather than H2O, and Pt supported on carbon black was used as oxidation catalyst. The catalyst loadings were 2.5 mg cm' ² 40 wt% Ru/C cathode catalyst (between the first electrode and the polymer electrolyte membrane), and 1.5 mg cm' ² 40 wt% Pt/C anode catalyst (between the first electrode and the polymer electrolyte membrane), with a flow rate of 50 ml min' ¹ H2 anode feed and 30 ml/min 4.8% NO in He cathode feed.

Considering the standard reduction potential of the nitric oxide to ammonia reaction (Eq. 4), coupled with the hydrogen oxidation reaction at the anode (Eq. 5), the theoretical cell potential of 0.836 V is (Eq. 6): 2N0 + 6H ⁺ + 4e’ NH ₄ ⁺ + H ₂ 0 E° = 0.836 V (4)

H ₂ 2H ⁺ + 2e' E° = 0 V (5)

E°ceii = 0.836 V - 0 V = 0.836 V (6)

Fig. 4A schematically depicts faradaic efficiencies FE as a function of the applied potential difference V (or “cell voltage V”), with respect to compounds P1-P5 (See description of Fig. 2A). Line L6 in Fig. 4A corresponds to the recorded current density A (in mA cm' ² ) as a function of the applied potential V. As for the second compartment of the cell, where a platinum catalyst and a dihydrogen feed were used, the assumption of pseudo-RHE reference electrode may hold at low current densities, the applied potentials are reported vs. pseudo-RHE.

The lowest faradaic efficiency towards ammonia (ca. 9%) was observed at the lowest applied potential of -0.2 V vs. pseudo-RHE. It reached a maximum of 30% at -0.4 V vs. pseudo-RHE. At more negative cell voltages, a decrease to ca. 20% was observed at -1 V vs. pseudo-RHE, accompanied by an increase in hydrogen formation.

Fig. 4B schematically depicts the NO conversion rate C (in %) versus the applied potential difference V. Line L7 in Fig. 4B corresponds to the total NO conversion rate C (in %) as a function of the applied potential V. The NO conversion rate C is observed to increase upon shifting the applied potential to more negative values, reaching a maximum of ca. 30% at -1 V vs. pseudo-RHE. Interestingly, the ammonia selectivity was observed to show a maximum at - 0.8 V vs. pseudo-RHE (ca. 85%), which is different from the potential at which the highest faradaic efficiency was observed (-0.4 V vs. pseudo-RHE).

In particular, a higher conversion of NO to ammonia was observed with H ₂ O as a proton source compared to H ₂ as a proton source. Hence, in embodiments, the proton source may especially comprise H ₂ O. In particular, in embodiments, the second fluid may comprise a second liquid.

In further embodiments, the second fluid may comprise a second gas, especially wherein the proton source comprises H ₂ . In such embodiments, the system may especially comprise a humidifier and/or liquid handling system.

Example 5

For example 5, the nitrogen oxide may be converted to ammonia, as the same catalyst was used. For this example, a first fluid comprising 983 ppm NO in He was used, and a condenser was used to remove water from the outlet gas stream. Ru/C was used as nitrogen oxide reduction catalyst. The second fluid comprised a second gas comprising H2 as proton source. The observations for example 5 are summarized in table 2:

As indicated in table 2, NO conversion was observed to exceed 98% already at 0 V vs. pseudo-RHE, while the current density exhibited a remarkably low value of ca. 1 mA/cm2. Although the current density increased between -0.2 and -0.6 V, the NO conversion showed a relatively stable value of around 98%. At -0.8 V a drop in current density and NO conversion was observed.

The performance obtained in these experiments indicate the great potential of this technology for nitrogen oxide abatement applications. In particular, the system and method of the invention may overcome state-of-the-art technologies in terms of energy efficiency.

Example 6

Experiments were performed with a system 100 of the invention, wherein the use of a first fluid comprising 2.5 vol% NO in He was compared to the use of a first fluid comprising 2.5 vol% NO and 2.5 vol% CO in He. A cell voltage (or “potential difference”) range between 1.7 and 2.1 V was investigated. Constant voltage experiments were carried out for 30 minutes. The nitrogen oxide reduction catalyst comprised was varied, such that a Ru/C catalyst, a Cu/C catalyst, a Pd/C catalyst, a Pt/C catalyst, and an Ag/C catalyst were used for different experiments. The experiments were observed in triplicate.

The observations for example 6 are summarized in tables 3a-j. The average values and corresponding standard deviations based on the triplicate experiments are indicated. 3a: 2.5 vol% NO in He with a Ru/C catalyst

3b: 2.5 vol% NO and 2.5 vol% CO in He with a Ru/C catalyst

Similarly, hydroxylamine formation appeared to increase when comparing NO to NO+CO, showing a faradaic efficiency of 11.4% at 1.7 V when using NO+CO, in comparison to a faradaic efficiency of 3.5% at 1.7 V when using NO.

Dinitrogen (or “nitrogen”) formation was observed at cell voltages as low as 1.7 V. Its faradaic efficiency remained relatively constant when comparing NO to NO+CO.

N2O formation appeared to slightly decrease when comparing NO to NO+CO.

The highest recorded faradaic efficiency for N2O was 15% at 1.7 V cell voltage when using NO. When using NO+CO, it dropped to 10%. 3c: 2.5 vol% NO in He with a Cu/C catalyst

3d: 2.5 vol% NO and 2.5 vol% CO in He with a Cu/C catalyst

As indicated in table 3c and 3d, the ammonia faradaic efficiency exhibited a promising increase in value when comparing NO to NO+CO. The increase is especially visible when comparing the highest value of faradaic efficiency of 61.1% at 1.7 V when using NO to the highest value of 89.1% at 2 V using NO+CO.

Formation of the other products (namely dinitrogen, hydroxylamine and N2O) appeared to remain relatively constant or even slightly decrease when comparing NO to NO+CO.

3e: 2.5 vol% NO in He with a Pd/C catalyst

3f: 2.5 vol% NO and 2.5 vol% CO in He with a Pd/C catalyst promising increase in value when comparing NO to NO+CO. The increase in value was clearly observed at all of the cell voltages 1.7 to 2.1 V.

Similarly, hydroxylamine formation and dinitrogen formation both appeared to slightly increase when comparing NO to NO+CO, at all of the cell voltages 1.7 to 2.1 V.

Ammonia formation exhibited an increase when comparing NO to NO+CO. The highest recorded faradaic efficiencies for ammonia were 7.3% at 2 V and 3.3% at 2.1 V when using NO. When using NO+CO, the efficiencies at both respective cell voltages increased to above 38%.

3g: 2.5 vol% NO in He with a Pt/C catalyst

3h: 2.5 vol% NO and 2.5 vol% CO in He with a Pt/C catalyst

As indicated in table 3i and 3j, the ammonia faradaic efficiency exhibited a promising increase in value at 1.7 V and 1.9 to 2.1 V when comparing NO to NO+CO. Hydroxylamine formation appeared to slightly increase at 1.7 and 1.8 V when comparing NO to NO+CO. Its faradaic efficiency showed an increase from 2.3% at 1.7 V and 2.5% at 1.8 V when using NO to a faradaic efficiency of 11.9% at 1.7 V and 8.2% at 1.8 V when using NO+CO.

Dinitrogen (or “nitrogen”) formation was observed at cell voltages as low as 1.7 V. Its faradaic efficiency remained relatively constant when comparing NO to NO+CO.

In contrast, N2O formation appeared to slightly decrease, especially at 1.7 V to 2.0 V, when comparing NO to NO+CO. The highest recorded faradaic efficiency for N2O was 24.6% at 1.7 V cell voltage when using NO. When using NO+CO, it dropped to 10.2%. 3i: 2.5 vol% NO in He with an Ag/C catalyst

3j : 2.5 vol% NO and 2.5 vol% CO in He with an Ag/C catalyst

As indicated in table 3a and 3b, the hydroxylamine faradaic efficiency exhibitec a promising increase in value when comparing NO to NO+CO. The increase in value was especially observed at the cell voltages of 1.8 to 2.1 V.

Similarly, dinitrogen (or “nitrogen”) formation appeared to slightly increase when comparing NO to NO+CO, showing a highest recorded faradaic efficiency of 34.4% at - 1.7 V when using NO+CO, in comparison to a faradaic efficiency of 4.9% at 1.7 V when using NO.

The faradaic efficiency for ammonia was observed remaining relatively constant when comparing NO to NO+CO. N2O formation appeared to slightly decrease when comparing NO to NO+CO, The highest recorded faradaic efficiency for N2O was 12.7% at 1.8 V cell voltage when using NO. When using NO+CO, it dropped to below 5% at every cell voltage tested.

The performance obtained in these experiments indicates a potential for selective modification using this technology. In particular, supplementing the NO feed with other gases, such as the CO used here, may improve product selectivity.

The term “plurality” refers to two or more. Furthermore, the terms “a plurality of’ and “a number of’ may be used interchangeably.

The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms ’’about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%-101% of the values(s) it refers to.

The term “comprise” also includes embodiments wherein the term “comprises” means “consists of’.

The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of' but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species".

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation. The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively. The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.