Method and system for capturing carbon monoxide with an electrically switchable adsorbent

FIELD OF THE INVENTION

The invention relates to a method for capturing carbon monoxide from a fluid mixture. The invention further relates to a system for capturing carbon monoxide from a fluid mixture. The invention further relates to a production method for providing an adsorbent. The invention further relates to the adsorbent. The invention further relates to a use of the adsorbent.

BACKGROUND OF THE INVENTION

Methods for capturing carbon monoxide are known in the art. For instance, US5529763A describes a composition, its synthesis, and a process for adsorptive separation of carbon monoxide from gas mixtures using adsorbents, which comprise cuprous compounds on amorphous oxide macroporous supports. The compositions are prepared by impregnating cupric compounds on supports followed by reduction of the cupric compound to the corresponding cuprous compound. The bulk nature of the active component (i.e., CuCl particles) and the resulting hysteresis in desorption provide adsorbents for a pressure swing adsorptive separation process.

WO20 16086234 describes non-noble element compositions of matter, structures, and methods for producing the catalysts that can catalyze oxygen reduction reactions (ORR). The described composition of matter can be comprised of graphitic carbon doped with nitrogen and associated with one or two kinds of transition metals.

EP0792684A2 describes an adsorbent for CO, comprising a composite comprised of a porous inorganic carrier and, carried thereon, a binary complex of a nitrogencontaining compound and a copper(I) halide, the nitrogen-containing compound being at least one member selected from at least one pyridine compound and a diamine represented by R ¹ R ² N(CH2) _n -NR ³ R ⁴ , wherein n is 2 or 3 and each of R ¹ , R ² , R ³ and R ⁴ independently represents a hydrogen atom or a Ci - C4 alkyl group, with the proviso that when n is 2, each of at least two of R ¹ , R ² , R ³ and R ⁴ represents a Ci - C4 alkyl group atoms and that when n is 3, at least one of R ¹ , R ² , R ³ and R ⁴ represents a Ci - C4 alkyl group, and a method for separating carbon monoxide by adsorption using the adsorbent.

W02017011873A1 describes a system for storing gas in an adsorbent, the system comprising a gas permeable graphitic material. The graphitic material comprises carbon and at least one other element such as nitrogen and/or boron. The system also comprises an electrical source which allows the application of a first and second potential to the graphitic material. It describes that upon application of the first potential, the gas is adsorbed to the graphitic material thereby producing a gas loaded material and that upon application of the second potential, at least some of the gas is desorbed from the gas loaded graphitic material.

SUMMARY OF THE INVENTION

The greenhouse gas carbon dioxide (CO2) may be one of the major factors driving climate change. Hence, there may be a desire to remove CO2 from the atmosphere to maintain natural ecosystems, and to reduce/prevent potentially catastrophic results from climate change. The photoelectrochemical reduction of CO2 using (renewable) electricity into value-added hydrocarbon fuels (e.g. formic acid (HCOOH), carbon monoxide (CO), ethanol (C2H5OH), methane (CH4), and ethylene (C2H4) etc.) can be a powerful solution as it not only directly consumes CO2, but may further also reduce dependence on fossil energy, which may result in a reduced CO2 production. However, a low reaction efficiency may discourage the near-future practical application of this photoelectrochemical CO2 reduction approach. The reaction yield may be improvable by (continuous) separation of the reaction product CO, such as from a liquid electrolyte, or such as from a gaseous mixture.

Further, carbon monoxide may be produced as a (by-)product in various industrial processes, such as in blast furnaces (when making steel), and such as in Fischer Tropsch synthesis, but may often not be recovered (efficiently), i.e., various industrial (waste) streams may comprise large amounts of unrecovered CO. Instead of recovering the CO, typically the CO may be converted to CO2, which may subsequently be emitted.

Hence, there may be a desire for efficient methods for CO capture, both to improve CO2-capture efforts, as well as to recover additional valuable resources from industrial processes.

Prior art processes for CO capture or separation may, however, be energy- intensive and/or may result in the production of waste products. Further, prior art processes for CO capture may require a high CO partial pressure for CO capture. Yet further, prior art processes for CO capture may be slow (with respect to a given volume for CO capture). In addition, prior art processes, such as for separating CO from N2, may be prohibitively expensive (in operating costs).

For example, the cryogenic distillation process, which may be a currently used industrial CO separation process, may require multiple heat exchangers and distillation columns leading to an (extremely) high energy need. In addition, because of the similar boiling points of CO and N2, it may be challenging to separate CO from large N2 contents using this method. Commercialized absorption processes such as COSORB and COPure ^SM , which may utilize CuAlCU in toluene or benzene to capture CO, may suffer from disadvantageous solvent degradation and poisoning of the copper with various chemical compounds, and may cause environmental issues. Moreover, these processes may comprise water removal steps prior to the cryogenic distillation and absorption processes, which water removal may add a further substantial energy requirement.

Hence, it is an aspect of the invention to provide an alternative method for CO capture, 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 method for capturing carbon monoxide from a fluid mixture (or “first fluid mixture”) using an adsorbent. The adsorbent may comprise a graphite plane comprising pyridinic nitrogen, especially wherein the adsorbent comprises (pyridinic) Fe-NU sites. In embodiments, the method may comprise an adsorption stage and a desorption stage. The adsorption stage may comprise exposing the adsorbent to the fluid mixture. The adsorption stage may further comprise imposing a first potential VI onto the adsorbent. The desorption stage may comprise exposing the adsorbent to a second fluid (or “desorption fluid”). The desorption stage may further comprise imposing a second potential V2 onto the adsorbent (100). In further embodiments, V2-V1 > 0.4 V, especially 0.4 V < V2- VI < 1 V, i.e., in embodiments, the second potential may be at least 0.4 V higher than the first potential, and especially at most 1.0 V higher than the first potential.

In particular, the adsorbent may be a conductive material in which metal centers can be switched between two redox states (such as Fe ²⁺ /Fe ³⁺ ), where one state has high adsorption affinity (adsorption state) to capture and remove carbon monoxide molecules, and the other state has low adsorption affinity (desorption state) to release CO. This material can further be used in an electrified separation process. In this process, CO molecules can be captured via binding to the developed high-affinity material, further by electrical switching the material to low-affinity, the CO can be released for further use. Hence, in embodiments, the adsorbent may especially be an electrically switchable adsorbent.

The invention may thus provide an "electrified" concept to separate CO from a fluid mixture, such as from a gaseous or liquid mixture resulting from a photoelectrochemical CO2 reduction reaction: CO will be removed via binding to a high-affinity material, followed by electrically switching the material to low-affinity to release CO for other applications, and thereby refreshing the material to capture CO from a (new) fluid mixture. In particular, the method of the invention may be suitable for the separation of CO even when CO is present in the fluid mixture at a relatively low concentration. The energy efficient separation of low concentrations of CO may facilitate providing, amongst other benefits, an efficient conversion of CO ₂ to CO.

Hence, the invention may provide an electrically conductive material in which the metal centers can be switched electrochemically between two redox states (such as Fe ²⁺ /Fe ³⁺ , and such as Cu ¹⁺ /Cu ²⁺ ), where one state has high adsorption affinity (adsorption state) to capture carbon monoxide molecules, and the other state has low adsorption affinity (desorption state) to release CO.

The invention may provide the benefit that CO may be captured from a fluid mixture in an energy-efficient way. In particular, CO may thereby be removed from a process where it is a (by-)product and may inhibit the ongoing process, and may be recovered from a waste stream to be used as a further chemical building block.

In specific embodiments, the invention may provide a method for capturing carbon monoxide from a fluid mixture using an adsorbent, wherein the adsorbent comprises a graphite plane comprising pyridinic nitrogen, wherein the adsorbent comprises Fe-NU sites, and wherein the method comprises: an adsorption stage comprising exposing the adsorbent to the fluid mixture, and imposing a first potential VI onto the adsorbent; and a desorption stage comprising imposing a second potential V2 onto the adsorbent, wherein 0.4 < V2-V1 < 1.

In further specific embodiments, the invention may provide a method for capturing carbon monoxide from a fluid mixture using an adsorbent, wherein a first electrode comprises the adsorbent, wherein the adsorbent comprises at least 3 wt% Fe, wherein the adsorbent comprises Fe-NU sites, and wherein the method comprises: an adsorption stage comprising exposing the adsorbent to the fluid mixture, and imposing a first potential VI onto the adsorbent by imposing a potential difference between the adsorbent and a second electrode, the first potential VI is selected such that at least 70 at.% of Fe in Fe-NU sites is ferrous; and a desorption stage comprising imposing a potential V2 onto the adsorbent by imposing a potential difference between the adsorbent and the second electrode, wherein the second potential V2 is selected such that at least 70 at.% of Fe in Fe-NU sites is ferric.

Hence, the invention may provide a method for capturing carbon monoxide (CO) from a fluid mixture, especially a gaseous mixture, or especially a liquid mixture, using an adsorbent. In further embodiments, the liquid mixture may comprise a liquid electrolyte. In further embodiments, the liquid electrolyte may comprise an aqueous electrolyte. In further embodiments, the liquid electrolyte may comprise an (organic) solvent-based electrolyte. In particular, in embodiments, the liquid electrolyte may comprise (dissolved) CO.

In further embodiments, the fluid mixture may comprise a heterogeneous mixture.

The fluid mixture may especially comprise a plurality of compounds. In particular, in embodiments wherein the fluid mixture comprises a gaseous mixture, the gaseous mixture may comprise a plurality of (gaseous) compounds. Similarly, in embodiments wherein the fluid mixture comprises a liquid mixture, the liquid mixture may comprise a solvent and one or more other compounds. In embodiments, the plurality of (gaseous) compounds may comprise compounds selected from the group comprising CO, CO2, N2, H2, H2S, H2, CH4, and water vapor, especially (at least) CO, and especially one or more (other) compounds selected from the group comprising CO2, N2, H2, H2S, H2, CH4, and water vapor, more especially CO and one or more (other) compounds selected from the group comprising N2 and H2. In further embodiments, the plurality of (gaseous) compounds may comprise at least CO, especially at least CO and CO2.

In particular, in embodiments wherein the fluid mixture comprises CO and CO2, the capturing of CO from the fluid mixture may improve the kinetics of converting CO2 to CO. Hence, in further embodiments, the method may comprise capturing CO from a fluid mixture from a photoelectrochemical CO2 reduction process.

The term “adsorbent” may herein especially refer to a substance to which other (gaseous) compounds, here especially CO, adsorb. In particular, CO may, depending on a state of the adsorbent, adsorb to the adsorbent, i.e., the adsorbent may (in a first state) comprise an adsorbent for CO.

The adsorbent may especially comprise an electrically switchable adsorbent, i.e., the adsorption properties of the adsorbent may be controlled by imposing a potential onto the adsorbent, such as by imposing a potential between the adsorbent and a second electrode (or: “counter electrode”). In particular, the adsorbent may comprise a transition metal, wherein the transition metal may be transitioned between a first oxidation state and a second oxidation state by imposing a potential on the adsorbent, and wherein CO adsorbs to the transition metal in the first oxidation state, and wherein CO desorbs from the transition metal in the second oxidation state. It will be clear to the person skilled in the art that the potential between the adsorbent and the second electrode also depends on the material of the second electrode. Hence, potentials for the adsorbent may herein be defined relative to a reference electrode, relative to vacuum, or relative to a redox potential of the adsorbent. In particular, in embodiments, the first potential may be selected such that at least a predetermined at.% of Fe in Fe-NU sites is ferrous, and, similarly, the second potential may be selected such that at least a predetermined at.% of Fe in Fe-NU sites is ferric (also see below).

In embodiments, the second electrode may function as counter electrode and/or reference electrode. In further embodiments, the second electrode may function as counter electrode. In particular, in embodiments, the second electrode may comprise a counter electrode.

In general, the first electrode and the second electrode may be different (spatially separated) electrodes.

However, in specific embodiments, the first electrode and the second electrode may be comprised by a single electrode, such as at opposite sides of the single electrode, or such as at different regions of an electrode. In particular, in such embodiments, the method may comprise imposing the first potential (and the second potential) between a first side (or first region) of the single electrode and a second side (or second region) of the single electrode, i.e., the first side of the single electrode may comprise the first electrode and the second side of the single electrode may comprise the second electrode. For instance, in embodiments, the single electrode may comprise a bipolar plate. Such embodiments may be particularly relevant in the context of the second electrode also comprising the adsorbent material (see below).

In embodiments, the adsorbent, especially the transition metal, may comprise one or more of iron (Fe) and copper (Cu), especially Fe, or especially Cu.

In particular, in embodiments wherein the adsorbent, especially the transition metal, comprises iron, the first oxidation state may comprise Fe(II) and the second oxidation state may comprise Fe(III), i.e., the adsorbent may be ferrous in the first oxidation state and ferric in the second oxidation state.

Further, in embodiments wherein the adsorbent, especially the transition metal, comprises copper, the first oxidation state may comprise Cu(I) and the second oxidation state may comprise Cu(II).

In embodiments, the adsorbent may comprise a carbon-based layer (or “carbon layer”), such as a graphene layer. In further embodiments, the carbon-based layer may comprise a 2D-layer, such as a graphene layer. The adsorbent may, in embodiments, comprise a graphite plane, i.e., the adsorbent may comprise a plane approximating a two-dimensional honeycomb lattice. The graphite plane may be particularly advantageous as it may support a high electron mobility as well as a good diffusion of CO molecules on the material surface.

In particular, the graphite plane may comprise nitrogen atoms present in the form of one or more of pyridinic, pyrrolic, graphitic and oxidized nitrogen. In embodiments, the graphite plane may comprise (at least) pyridinic nitrogen. In the pyridinic nitrogen species, a nitrogen atom is bound to two carbon atoms located on the edges of graphite planes. In graphitic-N, one nitrogen atom may be connected with three carbon atoms within a graphite plane.

In particular, the transition metal, such as iron, may be functionally connected to one or more nitrogen atoms, especially to one or more pyridinic nitrogen atoms (also see Fig. 1A).

Hence, in embodiments, the adsorbent may comprise M-N _x sites, wherein M is the transitional metal. In further embodiments, the M-N _x sites may especially be pyridinic M- N _x sites, i.e., (each of) the N in M-N _x may correspond to pyridinic nitrogen.

In particular, in further embodiments, the adsorbent may comprise (pyridinic) Fe-N sites.

In further embodiments, the adsorbent may comprise (pyridinic) Q1-N2 sites.

In further embodiments, wherein the transition metal comprises copper, the transition metals may be functionally coupled to nitrogen. In particular, the adsorbent may comprise Cu coordinatively bonded to nitrogen atoms inside the graphitic carbon structure.

In embodiments, the method may comprise an adsorption stage and a desorption stage. In particular, the method may comprise a plurality of alternating adsorption stages and desorption stages.

The adsorption stage may especially comprise exposing the adsorbent to the fluid mixture, especially wherein the fluid mixture comprises CO. The adsorption stage may further comprise imposing a first potential VI onto the adsorbent, especially onto the graphite plane, especially during a first adsorption phase of the adsorption stage. The first potential VI may especially be selected such that the transition metal, such as at least 55 at.% of the transition metal in the adsorbent, especially at least 70 at.%, such as at least 90 at.%, brought into the first oxidation state. Hence, the first potential VI may be selected such that at least 55 at.% of the transition metal in the adsorbent, especially at least 70 at.%, such as at least 90 at.%, is in the first oxidation state (during at least part of the adsorption stage). Hence, during (at least part of) the adsorption stage, at least 55 at.% of the transition metal in the adsorbent, especially at least 70 at.%, such as at least 90 at.%, may be in the first oxidation state.

In embodiments, the first potential VI may be selected from the range of -0.1 - 0.4 V vs. an Ag/AgCl reference electrode, such as from the range of -0.05 - 0.35 V vs. Ag/AgCl, especially from the range of 0 - 0.3 V vs. Ag/AgCl, or especially from the range of 0.05 - 0.35 V vs. Ag/AgCl.

In further embodiments, the first potential may be imposed between the adsorbent and a second electrode (or “counter electrode”), especially an inert (counter) electrode, such as a platinum electrode. In particular, the first potential VI may be imposed onto the adsorbent by imposing a potential difference between the adsorbent and the second electrode. For instance, in embodiments wherein the transition metal comprises iron, the first potential may be selected such that at least 55 at.% of Fe in the adsorbent, especially of Fe in Fe-NU sites, is ferrous (Fe(II)), such as at least 70 at.%, especially at least 90 at.%, such as at least 95 at.%, including 100%.

During the adsorption stage, the adsorbent, especially the transition metal, may have a (relatively) high affinity for CO. For instance, in embodiments wherein the transition metal comprises iron, the iron may be primarily present as Fe(II) during the adsorption stage, and CO may have a (relatively) high affinity for Fe(II), especially for Fe(II)-N4. In particular, for an Fe-NU cluster embedded in graphene, the adsorption energy between Fe(II) and the C atom in CO may be about -2.64 eV. Hence, in embodiments, and especially in the adsorption stage, an adsorption energy between the transition metal and CO may be < -1.5 eV, such as < - 2 eV, especially < -2.5 eV.

In embodiments, the first potential VI may be applied throughout (essentially) the entire adsorption stage. However, the first potential VI may also be applied during (only) part of the adsorption stage.

In particular, in embodiments, the first potential VI may (only) be applied during a part of the adsorption stage, especially at the start of the adsorption stage. By applying the first potential VI, the adsorbent may be brought in the first oxidation state, in which the adsorbent may have a high affinity for CO, but the first potential VI does not need to be continuously imposed to keep the adsorbent in the first oxidation state. Hence, in embodiments, the first potential VI may be applied onto the adsorbent at the start of the adsorption stage, such as to reduce Fe(III) to Fe(II). In further embodiments, the adsorption stage may have a first adsorption phase and a second adsorption phase, especially temporally arranged after the first adsorption phase, wherein the first adsorption phase comprises imposing the first potential VI onto the adsorbent and optionally exposing the adsorbent to the fluid mixture, and wherein the second adsorption phase comprises exposing the adsorbent to the fluid mixture.

Similarly, the second potential V2 may be applied throughout (essentially) the entire desorption stage. However, the second potential V2 may also be applied during (only) part of the desorption stage.

In further embodiments, the second potential V2 may (only) be applied during a part of the desorption stage, especially at the start of the desorption stage. By applying the second potential V2, the adsorbent may be brought in the second oxidation state, in which the adsorbent may have a low affinity for CO, but the second potential V2 does not need to be continuously imposed to keep the adsorbent in the second oxidation state. Hence, in embodiments, the second potential V2 may be applied onto the adsorbent at the start of the desorption stage, such as to oxidize Fe(II) to Fe(III). In further embodiments, the desorption stage may have a first desorption phase and a second desorption phase, especially temporally arranged after the first desorption phase, wherein the first desorption phase comprises imposing the second potential V2 onto the adsorbent and optionally exposing the adsorbent to the second fluid, and wherein the second desorption phase comprises exposing the adsorbent to the second fluid.

The desorption stage may comprise imposing a second potential onto the adsorbent, especially onto the graphite plane, especially during a first desorption phase of the desorption stage. The second potential may especially be selected such that the transition metal, such as at least 55 at.% of the transition metal in the adsorbent, especially at least 70 at.%, such as at least 90 at.%, is brought into the second oxidation state. Hence, the second potential may be selected such that at least 55 at.% of the transition metal in the adsorbent, especially at least 70 at.%, such as at least 90 at.%, is in the second oxidation state (during at least part of the desorption stage).

Hence, during (at least part of) the desorption stage, at least 55 at.% of the transition metal in the adsorbent, especially at least 70 at.%, such as at least 90 at.%, may be in the second oxidation state.

For instance, in embodiments wherein the transition metal comprises iron, the second potential may be selected such that at least 55 at.% of Fe in the adsorbent, especially of Fe in Fe-NU sites, is ferric (Fe(III)), such as at least 70 at.%, especially at least 90 at.%, such as at least 95 at.%, including 100 at.%. In embodiments, the second potential V2 may be selected from the range of 0.5 - 1.2 V vs Ag/AgCl, such as from the range of 0.6 - 1.1 V vs Ag/AgCl, especially from the range of 0.5 - 1 V vs Ag/AgCl. In further embodiments, the second potential may be selected from the range of 0.7 - 1.2 V, such as from the range of 0.7 - 1 V.

In further embodiments, the second potential may be imposed between the adsorbent and a second electrode (or “counter electrode”), especially an inert (counter) electrode, such as a platinum electrode. In particular, the second potential V2 may be imposed onto the adsorbent by imposing a potential difference between the adsorbent and the second electrode.

In embodiments, V2-V1 > 0.2 V, such as > 0.4 V, especially > 0.5 V. In further embodiments, V2-V1 < 1.2 V, such as < 1 V, especially < 0.9 V. In further embodiments, the adsorbent may coat a first electrode, i.e., the adsorbent may comprise a coating, wherein the coating is arranged on the first electrode. Hence, the first electrode may comprise the adsorbent, especially wherein the adsorbent is arranged on an electrode material. In such embodiments, the adsorption stage may comprise imposing the first potential onto the first electrode, especially by imposing a potential between the first electrode and a second electrode, such as a counter electrode. Further, in such embodiments, the desorption stage may comprise imposing the second potential onto the first electrode, especially by imposing a potential between the first electrode and a second electrode, such as a counter electrode.

In further embodiments, the first electrode may especially comprise a titanium- based electrode. In further embodiments, the first electrode may comprise a titanium plate.

In embodiments, the first electrode and the second electrode, such as a counter electrode, may be arranged in a (liquid) electrolyte. Especially, the first electrode may be arranged in a first electrolyte. Similarly, the second electrode may especially be arranged in a second electrolyte. In further embodiments, the first electrolyte and the second electrolyte may be separated by a separator, especially a membrane. In further embodiments, the separator may be configured to block transport of CO, while having permeability for an ion, such as a cation, especially a cation selected from the group comprising H ⁺ , Na ⁺ , K ⁺ , or such as an anion, especially an anion selected from the group comprising OH' and Cl'.

In embodiments, the liquid electrolyte may comprise an aqueous electrolyte. In further embodiments, the liquid electrolyte may comprise an (organic) solvent-based electrolyte, such as one or more of acetonitrile, an alkyl carbonate. In particular, in embodiments, the liquid electrolyte may comprise ethylene carbonate and a second (different) alkyl carbonate. In further embodiments, the first electrode and the second electrode, especially the counter electrode, may be separated by a separator. In particular, the separator may be arranged in physical contact with the first electrode and the second electrode.

However, in further embodiments, the separator may be arranged physically separated from the first electrode and the second electrode.

During the desorption stage, the adsorbent, especially the transition metal, may have a (relatively) low affinity for CO. For instance, in embodiments wherein the transition metal comprises iron, the iron may be primarily present as Fe(III) during the desorption stage, and CO may have a (relatively) low affinity for Fe(III), especially for Fe(III)-N4.

In embodiments, the affinity of CO for the transition metal in the first oxidation state may be at least 2 times as high as the affinity of CO for the transition metal in the second oxidation stage, such as at least 5 times as high, especially at least 10 times as high.

In embodiments, the desorption stage may further comprise exposing the adsorbent to a second fluid, different from the fluid mixture. In further embodiments, the second fluid may comprise a second liquid, especially a second liquid mixture, or especially a second liquid solution.

In embodiments, the second fluid may comprise a second gas, especially a second gaseous mixture. In embodiments, the second gas may have a lower CO pressure than the fluid mixture, especially than the gaseous mixture. However, in further embodiments, the second gas may have a higher CO pressure than the fluid mixture, especially than the gaseous mixture, such as to provide (essentially) pure CO. In further embodiments, the second gas may have an absolute gas pressure selected from the range of 1 mbara - 50 bara, especially from the range of 10 mbara and 5 bara, such as from the range of 50 mbara and 2 bara. In particular, a (relatively) high absolute gas pressure of the second gas may be advantageous in order to prevent compression. Hence, in embodiments, the second gas may have an absolute gas pressure of at least 0.5 bara, especially at least 1 bara, such as at least 2 bara.

In specific embodiments, the second fluid, especially the second gas, may comprise CO and or H2, especially CO, or especially H2. For instance, the second fluid may comprise H2 such that the method provides syngas (H2/CO mixture). Further, the second fluid may comprise CO, such that the method provides (essentially) pure CO.

In further embodiments, the second fluid, especially the second gas, may comprise water vapor.

Hence, the method of the invention may be suitable to capture CO from a fluid mixture, such as from a gaseous mixture, and to release the CO (to a second fluid). In particular, due to the high affinity of CO for the transition metal in the first oxidation state, the method of the invention may have a high specificity for CO and may efficiently capture CO from fluid mixtures, especially gaseous mixtures, with a broad range of partial pressure of CO. In particular, the method of the invention may facilitate capturing CO at a (relatively) low partial pressure. Hence, in embodiments, the fluid mixture, especially the gaseous mixture, may have a partial pressure of CO selected from the range at most 3 bara, such as at most 2 bara, especially at most 1 bara. In further embodiments, the fluid mixture, especially the gaseous mixture, may have a partial pressure of CO selected from the range of at most 500 mbara, such as at most 200 mbara, especially at most 100 mbara. In further embodiments, the fluid mixture, especially the gaseous mixture, may have a partial pressure of CO selected from the range of at most 70 mbara, such as at most 50 mbara, especially at most 30 mbara. In further embodiments, the fluid mixture, especially the gaseous mixture, may have a partial pressure of CO selected from the range of at most 10 mbara, such as at most 1 mbara, especially at most 0.5 mbara. In further embodiments, the fluid mixture, especially the gaseous mixture, may have a partial pressure of CO of at least 0.005 mbara, especially at least 0.01 mbara, such as at least 0.1 mbara. In further embodiments, the fluid mixture, especially the gaseous mixture, may have a partial pressure of CO of at least 1 mbara, especially at least 10 mbara, such as at least 100 mbara, especially at least 500 mbara.

As indicated above, the adsorbent may comprise a graphite plane, and may comprise N and Fe. Hence, in embodiments, the adsorbent may especially comprise at least C, N, and Fe. In particular, the carbon may provide the (planar) graphitic structure, the Fe may provide the electrically switchable affinity for CO, and the N may be arranged in the graphitic structure and may coordinate the Fe. Hence, there may be a balance in the structure with regards to the relative amounts of C, N, and Fe. For instance, if Fe is low, the capacity of the adsorbent for adsorbing CO may be low. However, if C is (too) low, the adsorbent may not have the graphitic structure. In particular, the graphitic structure may contribute to the conductivity of the material, which may be required for the method of the invention.

Hence, in further embodiments, the adsorbent may comprise at least 3 wt% Fe, such as at least 3.5 wt% Fe, especially at least 4 wt% Fe. In further embodiments, the adsorbent may comprise at least 4.1 wt% Fe, such as at least 4.3 wt% Fe, especially at least 4.5 wt% Fe. In further embodiments, the adsorbent may comprise at most 12 wt% Fe, such as at most 10 wt% Fe, especially at most 8 wt% Fe. In further embodiments, the adsorbent may comprise at most 7 wt% Fe, such as at most 6 wt% Fe, especially at most 5 wt% Fe. The wt% of Fe in the adsorbent may especially be determined using inductive coupled plasma optical emission spectrometry (ICP-OES). Especially, the content of Fe and Zn in the adsorbent may be quantified by degusting the adsorbent with a microwave from CEM model MARS 6 and measuring with an ICP-OES from SPECTRO model SPECTRO ARCOS.

In further embodiments, the adsorbent may comprise at most 25 wt% Fe, such as at most 24 wt%, especially at most 23 wt%. In further embodiments, the adsorbent may comprise at most 22 wt% Fe, such as at most 20 wt%, especially at most 18 wt%, such as at most 15 wt%.

In particular, in embodiments, at least part of the Fe may be present as Fe-NU sites. Hence, in embodiments, the adsorbent may comprise at least 0.02 wt% of Fe-NU sites, especially at least 0.03 wt%, such as at least 0.04 wt%. In further embodiments, the adsorbent may comprise at least 0.05 wt% of Fe-NU sites, especially at least 0.06 wt%, such as at least 0.07 wt%, especially at least 0.1 wt%. In further embodiments, the adsorbent may comprise at most 5 wt% of Fe-NU sites, such as at most 3 wt%, especially at most 2 wt%. In further embodiments, the adsorbent may comprise at most 1 wt% of Fe-NU sites, such as at most 0.8 wt%, especially at most 0.5 wt%.

In embodiments, the adsorbent may comprise at most 25 wt% of Fe-NU sites, such as at most 20 wt% Fe-NU sites. In further embodiments, the adsorbent may comprise at most 15 wt% of Fe-NU sites, such as at most 13 wt% of Fe-NU sites, especially at most 12 wt% of Fe-NU sites. In further embodiments, the adsorbent may comprise at most 10 wt% of Fe-NU sites, such as at most 8 wt% of Fe-NU sites, especially at most 7 wt% of Fe-NU sites.

In further embodiments, the adsorbent may comprise a C:N (atom) ratio of at most 10: 1, i.e., at most 10 C atoms per N atom, such as at most 9: 1, especially at most 8:1.

In further embodiments, the adsorbent may comprise O and/or S. However, the presence of O and S may reduce the prevalence of C-N bonds and, as Fe may coordinate to O and/or S, may reduce the affinity of the adsorbent to CO, and may thus be generally undesirable. Hence, in embodiments, the adsorbent may comprise at most 3 wt% O, such as at most 2 wt% O, especially at most 1 wt% O, including 0 wt% O. In further embodiments, the adsorbent may comprise at most 3 wt% S, such as at most 2 wt% S, especially at most 1 wt% S, including 0 wt% S.

In further embodiments, the adsorbent may further comprise Zn. In particular, the adsorbent may comprise at most 8 wt% Zn, such as at most 5 wt% Zn, especially at most 4 wt% Zn. In further embodiments, the adsorbent may comprise at most 3.5 wt% Zn, such as at most 3 wt% Zn, especially at most 2.5 wt% Zn. The wt% of Zn in the adsorbent may especially be determined using ICP-OES (also see above). In embodiments, the adsorbent may comprise at least 0.5 wt% Zn, such as at least 1 wt%, especially at least 2 wt%.

For the adsorption of CO to the adsorbent, the (external) surface of the adsorbent may be particularly relevant. Hence, in embodiments, the adsorbent may have (an external surface having) an external surface composition. The external surface composition may especially be determined using X-ray photoelectron microscopy (XPS). The XPS may especially be performed on a K-alpha Thermo Fisher Scientific (USA) spectrometer and a monochromatic Al Ka X-ray source with an X-ray spot size of 400 pm. The XPS measurements may be performed at ambient temperature and chamber pressure in the order of magnitude of 10 ^-7 mbar. A flood gun may be used for charge compensation. The sample powders may be put on a copper tape stuck onto a small metal plate. The plate may be attached to the sample holder using one or two spring clips. The spectra may be analyzed using the CasaXPS software. Background subtraction may be derived from Shirley background. Spectra may be deconvoluted using Lorentzian-Gaussian combination peaks. As will be clear to the person skilled in the art, the elemental ranges for the surface may differ from the bulk concentrations, and the presence of (organic) compounds on the surface may provide some contamination to the measurement, especially with regards to C and O.

In embodiments, the external surface composition may comprise 0.1 - 25 wt% of O, especially 3 - 25 wt% O, such as 5-20 wt% O, especially 7 - 15 wt% O. In further embodiments, the external surface composition may comprise 0.1 - 20 wt% O. In particular, the wt% O in the external surface composition may be based on x-ray photoelectron microscopy, especially based on an Ols peak obtained with photoelectron microscopy.

In further embodiments, the external surface composition may comprise 50 - 90 wt% C, such as 60 - 90 wt% C, especially 65 - 85 wt% C. In particular, the wt% C in the external surface composition may be based on x-ray photoelectron microscopy, especially based on an Cis peak obtained with photoelectron microscopy.

In further embodiments, the external surface composition may comprise 3-15 wt% N, such as 5 - 13 wt% N, especially 6 - 11 wt% N. In particular, the wt% N in the external surface composition may be based on x-ray photoelectron microscopy, especially based on an Nls peak obtained with photoelectron microscopy.

In further embodiments, the external surface composition may comprise 0.5 - 10 wt% S, such as 0.5-8 wt% S, especially 0.5 - 5 wt% S. In particular, the wt% S in the external surface composition may be based on x-ray photoelectron microscopy, especially based on an S2p peak obtained with photoelectron microscopy. In further embodiments, the external surface composition may comprise 0.1-5 wt% Fe, such as 0.2 - 4 wt% Fe, especially 0.25 - 3 wt% Fe. In particular, the wt% Fe in the external surface composition may be based on x-ray photoelectron microscopy, especially based on an Fe2p peak obtained with photoelectron microscopy.

In further embodiments, the external surface composition may comprise 0.3-5 wt% Zn, such as 0.5 - 3 wt% Zn, especially 0.6 - 2 wt% Zn. In particular, the wt% Zn in the external surface composition may be based on x-ray photoelectron microscopy, especially based on an Zn2p peak obtained with photoelectron microscopy.

Hence, in specific embodiments, the external surface composition may comprise: 5-20 wt% O; 60-90 wt% C; 3-15 wt% N; 0.5-8 wt% S; 0.1-5 wt% Fe; and 0.5-3 wt% Zn. Especially, based on x-ray photoelectron microscopy, the external surface composition may comprise 5-20 wt% O based on an Ols peak; 60-90 wt% C based on a Cis peak; 3-15 wt% N based on an Nls peak; 0.5-8 wt% S based on an S2p peak; 0.1-5 wt% Fe based on an Fe2p peak; and 0.5-3 wt% Zn based on an Zn2p peak.

The term “external surface”, and related terms, may herein especially refer to the surface as accessible using XPS, which may be about (the most external) 10 nm. However, the adsorbent may adsorb both to the external surface and to an internal surface. The term “internal surface” may herein especially refer to the surface of (micro and meso)pores. In general, the internal surface area may be substantially higher than the external surface area. The term “surface” may herein refer to both the internal and the external surface”.

As CO may adsorb to the (internal and external) surface of the adsorbent, it may be advantageous to provide a (relatively) large surface area relative to a weight of the adsorbent. Hence, in embodiments, the adsorbent may have a (total) surface area S _a selected from the range of 100 - 1500 m ² /g, especially selected from the range of 150 - 800 m ² /g, i.e., the adsorbent may have a surface area S _a of 150 - 800 m ² per gram of adsorbent. In further embodiments, the adsorbent may have a surface area S _a of at least 150 m ² /g, such as at least 300 m ² /g, especially at least 500 m ² /g. In further embodiments, the adsorbent may have a surface area S _a of at least 550 m ² /g, such as at least 600 m ² /g, especially at least 630 m ² /g. In further embodiments, the adsorbent may have a surface area S _a of at most 1500 m ² /g, such as at most 1200 m ² /g, especially at most 800 m ² /g, such as at most 700 m ² /g.

The surface area S _a may especially be a Brunauer-Emmett-Teller (BET) surface area, i.e., the surface area S _a may especially be determined using the method described in Rouquerol J. el al., “Is the BET equation applicable to microporous adsorbents?", Stud. Surf. Sci. Catal., 2007, 160, 49-56, which is hereby herein incorporated by reference. In particular, a porous structure may provide a high (effective) surface area. Hence, in embodiments, the adsorbent may have a (total) pore volume V selected from the range of 0.01 - 0.5 cm ³ /g, especially from the range of 0.02 - 0.4 cm ³ /g, such as from the range of 0.04 - 0.3 cm ³ /g, especially from the range of 0.1 - 0.25 cm ³ /g. The (total) pore volume may especially be a t-Plot micropore volume, i.e., the pore volume may especially be determined by volumetrically measuring nitrogen adsorption/desorption isotherms in a Tristar II 3020 Micromeritics instrument at 77K. The adsorbent may especially be degassed before the measurement at 433 K under N2 flow for 16 h. The pore volume values may be obtained from the data at P/Po=O.95.

Environmental conditions, such as temperature and (gas) pressure may further influence the adsorption and desorption behavior of CO on the adsorbent. In particular, adsorption may be higher at a lower temperature, and desorption may be higher at a higher temperature. Similarly, adsorption may be higher at a higher (gas) pressure, and desorption may be higher at a lower gas pressure. However, actively controlling the temperature at the adsorption stage and the desorption stage may require energy (for heating/cooling). Hence, there may be a trade-off between efficiency of adsorption/desorption and energy expenditure.

Hence, in embodiments, the adsorption stage may comprise exposing the adsorbent to an adsorption temperature, and the desorption stage may comprise exposing the adsorbent to a desorption temperature. In embodiments, the adsorption temperature may be at most 70 °C, such as at most 50 °C, especially at most 30 °C. In further embodiments, the adsorption temperature may be at least -100 °C, such as at least -10°C, especially at least 10 °C. In further embodiments, the desorption temperature may be at least 10 °C, such as at least 20 °C, especially at least 50°C. In further embodiments, the desorption temperature may be at most 500 °C, such as at most 400 °C, especially at most 300 °C. In further embodiments, the desorption temperature may be (about) room temperature. In particular, as the affinity of the transition metal for CO may be (relatively) low during the desorption stage, CO may be effectively desorbed at room temperature, which may be particularly energy efficient. However, the desorption (rate) may be enhanced by increasing the desorption temperature. Hence, the desorption temperature may be selected to tune desorption properties, particularly with regards to energy efficiency versus desorption rate.

In further embodiments, the adsorbent may (start to) deform at a deformation temperature, and the desorption temperature may be selected to be below the deformation temperature. The deformation temperature may, for instance, depend on environmental conditions such as an ambient gas. For instance, in embodiments, the adsorbent may be more stable under N2 than under air, so a higher desorption temperature may be selected under an N2 atmosphere compared to air.

In specific embodiments, the adsorption temperature may be selected from the range of 10 - 50 °C, and the desorption temperature may be selected from the range of 10 - 400 °C. In embodiments, the desorption temperature is higher than the adsorption temperature, such as at least about 10 °C higher.

In embodiments, the method may especially comprise controlling the temperature of the fluid mixture at the adsorption temperature. In further embodiments, the method may comprise controlling the temperature of the second fluid at the desorption temperature.

Similarly, in embodiments, the adsorption stage may comprise exposing the adsorbent to an adsorption pressure. In further embodiments, the desorption stage may comprise exposing the adsorbent to a desorption pressure. In embodiments, the adsorption pressure may be at most 30 bar, such as at most 10 bar, especially at most 2 bar. In further embodiments, the adsorption pressure may be at least 0.8 bar, such as at least 1 bar, especially at least 1.2 bar. In further embodiments, the desorption pressure may be at least 0.1 bar, such as at least 0.3 bar, especially at least 0.8 bar. In further embodiments, the desorption pressure may be at most 5 bar, such as at most 3 bar, especially at most 1.5 bar.

In further embodiments, the method may comprise controlling the (gas) pressure of the fluid mixture at the adsorption pressure. In further embodiments, the method may comprise controlling the (gas) pressure of the second fluid at the desorption pressure.

As indicated above, the method may comprise alternating between the adsorption stage and the desorption stage. In further embodiments, the adsorption stage may have an adsorption duration, and the desorption stage may have a desorption duration. Especially, in embodiments, the adsorption duration may be selected from the range of up to 2 hours, such as up to 1 hour, especially up to 0.5 hours. Similarly, in embodiments, the desorption duration may be selected from the range of up to 2 hours, such as up to 1 hour, especially up to 0.5 hours.

In particular, the adsorbent may have a maximal capacity Cmax for CO adsorption, and the adsorption duration may be selected such that the amount of CO adsorbed to the adsorbent is at least 0.6*C _ma x, such as at least 0.8*C _ma x, especially at least 0.9*C _ma x, including l*C _ma x. In particular, the adsorption duration may be (dynamically) selected based on a decreasing adsorption rate, i.e., an adsorption rate of CO to the adsorbent may decrease as the amount of adsorbed CO nears the maximal capacity C _max (the asymptote). Hence, in further embodiments, the method may comprise determining the adsorption rate of CO to the adsorbent, and selecting the adsorption duration based on the determined adsorption rate.

Similarly, the desorption duration may be selected in view of a threshold of desorbed CO. Hence, the desorption duration may be selected such that at least 60% of adsorbed CO (during the preceding adsorption stage) is desorbed from the adsorbent, especially at least 80%, such as at least 90%, including 100%. Further, the desorption duration may, in embodiments, be (dynamically) selected based on a decreasing desorption rate, i.e., the desorption rate of CO from the adsorbent may decrease as the amount of (remaining) adsorbed CO nears 0 (the asymptote).

Hence, in further embodiments, the method may comprise determining the desorption rate of CO to the adsorbent, and selecting the desorption duration based on the determined desorption rate.

In specific embodiments, the adsorbent may comprise 3-25 wt% Fe, such as 3.5- 12 wt% Fe, especially wherein the method comprises exposing the adsorbent to an adsorption temperature selected from the range of 10 - 50 °C during the adsorption stage, and wherein the method comprises exposing the adsorbent to a desorption temperature selected from the range of 10 - 400 °C during the desorption stage, wherein the method comprises alternating between the adsorption stage and the desorption stage, wherein the adsorption stage has an adsorption duration selected from the range of up to one hour, and wherein the desorption stage has a desorption duration selected from the range of up to one hour, wherein the first potential VI is selected from the range of 0 - 0.3 V vs. Ag/AgCl, and wherein the second potential is selected from the range of 0.7 - 1 V vs. Ag/AgCl, and wherein the desorption stage comprises contacting the adsorbent with a second fluid, which, not taking into account CO, has another composition than the fluid mixture.

In a second aspect, the invention may provide a system for capturing carbon monoxide from a fluid mixture using an adsorbent. In particular, the adsorbent may comprise a graphite plane, especially a graphite plane comprising pyridinic nitrogen. In embodiments, the adsorbent may comprise (pyridinic) Fe-NU sites. The system may especially comprise one or more of a first electrode, a fluid flow control unit, and a charge control unit. In embodiments, the first electrode may comprise the adsorbent. In embodiments, the adsorbent may be arranged on the first electrode. Especially, the adsorbent may be coated on the first electrode. In embodiments, the fluid flow control unit may be configured to control a fluid flow, especially to control a fluid flow past the first electrode. In particular, the fluid flow control unit may be configured to expose the first electrode to a fluid, especially to a fluid mixture during the adsorption stage, and especially to a second fluid during the desorption stage. The charge control unit may be configured to control a potential on the first electrode. In particular, the charge control unit may be configured to impose a potential onto the first electrode, especially a first potential VI during the adsorption stage, and especially a second potential V2 during the desorption stage. In embodiments, 0.4 < V2-V1 < 1 V.

In embodiments, the system may further comprise a second electrode, especially a counter electrode. In such embodiments, the charge control unit may be configured to control a potential (difference) between the first electrode and the second electrode. In particular, the charge control unit may be configured to control a potential on the first electrode by controlling a potential (difference) between the first electrode and the second electrode.

In embodiments, the fluid flow control unit may comprise a gas flow control unit. The gas flow control unit may especially be configured to control a gas flow, especially to control a gas flow past the first electrode.

In further embodiments, the fluid flow control unit may comprise a liquid flow control unit. The liquid flow control unit may especially be configured to control a liquid flow, especially to control a liquid flow past the first electrode.

In embodiments, the system may further comprise a functional unit, wherein the functional unit comprises a first cell, a second cell, and a separator, especially a membrane. In further embodiments, the first cell may especially comprise the first electrode, and the second cell may especially comprise the second, especially a counter electrode. The separator may, in embodiments, be arranged between the first cell and the second cell, i.e., the first cell and the second cell may share the separator. In further embodiments, the separator may be arranged between the first electrode and the second electrode. In further embodiments, the separator may be configured to block transport of CO, and to be permeable for ions, especially for one or more cations selected from the group comprising H ⁺ , Na ⁺ , and K ⁺ , and/or especially for one or more anions selected from the group comprising OH' and CT.

In embodiments, the system may be configured to operate without (liquid) electrolyte. In particular, during operation of the system, the first cell and the second cell may be devoid of (liquid) electrolyte. Such embodiments may be particularly suitable for recovering CO from gaseous mixtures.

In further embodiments, the system may comprise an electrolyte control system. The electrolyte control system may be configured to introduce a (liquid) electrolyte into the functional unit, especially to introduce a first (liquid) electrolyte into the first cell, and especially to introduce a second (liquid) electrolyte into the second cell.

In further embodiments, the electrolyte, especially the first electrolyte, or especially the second electrolyte, may be an aqueous electrolyte. In further embodiments, the electrolyte, especially the first electrolyte, or especially the second electrolyte, may be an (organic) solvent-based electrolyte.

In embodiments, the system may further comprise a control system. The control system may be configured to control the system, especially one or more elements of the system, such as the fluid flow control unit and/or the charge control unit.

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 operation mode (i.e. “on”, without further tunability).

In further embodiments, the operational mode may comprise an adsorption stage and a desorption stage. In particular, the operational mode may comprise alternating between the adsorption stage and the desorption stage.

In the adsorption stage, the fluid flow control unit may (be configured to) expose the adsorbent to a fluid mixture, i.e., to provide the fluid mixture to the adsorbent. Further, during (at least part of) the adsorption stage, especially during the first adsorption phase, the charge control unit may (be configured to) impose a first potential (difference) between the first electrode and the counter electrode. In further embodiments, the first potential may be selected from the range of -0.1 - 0.4 V vs. an Ag/AgCl reference electrode (“vs Ag/AgCl”), such as from the range of 0.05 - 0.35 V, especially from the range of 0 - 0.3 V.

In the desorption stage, the fluid flow control unit may (be configured to) expose the adsorbent to a second fluid, especially a second gas, i.e., to provide the second fluid, especially the second gas, to the adsorbent. Further, during (at least part of) the desorption stage, especially during the first desorption phase, the charge control unit may (be configured to) impose a second potential (difference) between the first electrode and the counter electrode. In further embodiments, the second potential may be selected from the range of 0.5 - 1.2 V, such as from the range of 0.6 - 1.1 V, especially from the range of 0.5 - 1 V, with respect to an Ag/AgCl reference electrode. In further embodiments, the second potential may be selected from the range of 0.7 - 1.2 V, such as from the range of 0.7 - 1 V.

The ranges of the first potential and the second potential mentioned herein may be indicated with regards to an Ag/AgCl reference electrode. It will be clear to the person skilled in the art that the invention is not limited to a specific reference electrode, but rather that the reference electrode may facilitate quantifying/setting the appropriate potential. For instance, where the first potential may be selected from the range of 0 - 0.3 V with respect to the Ag/AgCl reference electrode, it may also be selected from the range of 0.2 - 0.5 V with respect to a standard hydrogen electrode (SHE). Similarly, where the second potential may be selected from the range of 0.7 - 1 V with respect to the Ag/AgCl reference electrode, it may also be selected from the range of 0.9 - 1.2 V with respect to a standard hydrogen electrode (SHE). As a further example, where the first potential may be selected from the range of 0 - 0.3 V with respect to the Ag/AgCl reference electrode, it may also be selected from the range of (about) -5.1 - -4.7 eV with respect to vacuum. Similarly, where the second potential may be selected from the range of 0.7 - 1.2 V with respect to the Ag/AgCl reference electrode, it may also be selected from the range of (about) -6 - -5.4 eV with respect to vacuum.

It will be clear to the person skilled in the art that the first potential (and the second potential) may be selected in view of properties of the adsorbent. In particular, in embodiments, the first potential VI may be selected from the range of -0.7 - 0. V versus the (Fell/Felll) redox potential of the adsorbent, such as from the range of -0.55 - -0.05 V versus the (Fell/Felll) redox potential of the adsorbent, especially from the range of -0.5 - -0.1 V versus the (Fell/Felll) redox potential of the adsorbent. Similarly, in embodiments, the second potential V2 may be selected from the range of 0 - 0.9 V versus the (Fell/Felll) redox potential of the adsorbent, such as from the range of 0.05 - 0.75 V versus the (Fell/Felll) redox potential of the adsorbent, especially from the range of 0.10 - 0.6 V versus the (Fell/Felll) redox potential of the adsorbent.

Similarly, the ranges of the first potential and the second potential mentioned herein may be with respect to an aqueous electrolyte. It will be clear to the person skilled in the art that the suitable potential ranges may vary for different electrolytes.

Hence, in specific embodiments, the invention may provide a system for capturing carbon monoxide from a fluid mixture using an adsorbent, wherein the adsorbent comprises a graphite plane comprising pyridinic nitrogen, wherein the adsorbent comprises Fe- N4 sites, wherein the system comprises a first electrode, a fluid flow control unit, and a charge control unit, wherein the adsorbent is arranged on the first electrode, wherein the system comprises an operational mode comprising an adsorption stage and a desorption stage, wherein: the adsorption stage comprises the fluid flow control unit exposing the adsorbent to the fluid mixture, and the desorption stage may comprise the fluid flow control unit exposing the adsorbent to a second fluid.

In embodiments, in the operational mode, in the adsorption stage the charge control unit may (be configured to) impose a first potential onto the first electrode selected from the range of 0 - 0.3 V vs. Ag/AgCl. Similarly, in embodiments, in the desorption stage the charge control unit may (be configured to) impose a second potential onto the first electrode selected from the range of at least 0.7 V vs. Ag/AgCl, such as selected from the range of 0.7 - 1 V vs. Ag/AgCl.

In embodiments, the system may further comprise a temperature control system. The temperature control system may especially be configured to impose an adsorption temperature (to the adsorbent) during the adsorption stage, and to impose a desorption temperature (to the adsorbent) during the desorption stage. In further embodiments, the temperature control system may be configured to control the temperature of the fluid mixture at the adsorption temperature (during the adsorption stage), and to control the temperature of the second fluid at the desorption temperature (during the desorption stage). In further embodiments, the control system may be configured to control the temperature control system.

In further embodiments, the system may further comprise a pressure control system. The pressure control system may especially be configured to impose an adsorption pressure (to the adsorbent) during the adsorption stage, and to impose a desorption pressure (to the adsorbent) during the desorption stage. In further embodiments, the pressure control system may be configured to control the pressure of the fluid mixture at the adsorption pressure (during the adsorption stage), and to control the pressure of the second fluid at the desorption pressure (during the desorption stage). In further embodiments, the control system may be configured to control the pressure control system.

In a further aspect, the invention may provide a production method for providing the adsorbent, especially the electrically switchable adsorbent.

In particular, in embodiments, the production method may comprise a (high temperature) pyrolysis of a metal organic framework, especially an iron and zinc co-doped zeolitic imidazolate framework, to provide the adsorbent.

The production method may, in embodiments, comprise one or more of a preparation stage, a pyrolysis stage, an acid washing stage, and a reduction stage.

The preparation stage may comprise providing a metal-organic framework. In particular, the metal-organic framework may comprise metals, especially tetrahedrally- coordinated metals, such as metals tetrahedrally coordinated to nitrogen residues, especially to nitrogen residues of (aromatic) ligands. Hence, the tetrahedrally-coordinated metals may be (tetrahedrally) coordinated to nitrogen (residues), such as to nitrogen (residues) of (aromatic) ligands, especially to nitrogen (residues) of (aromatic) ligands of the metal-organic framework. In embodiments, 0.5-20 at.%, such as 1-15 at.%, especially 2-10 at.%, of the tetrahedrally- coordinated transition metals may comprise Fe and/or Cu, especially Fe, or especially Cu. In further embodiments, at least 75 at.%, such as at least 80 at.%, especially at least 90 at.%, such as at least 95 at.%, of the tetrahedrally-coordinated transition metals may comprise a metal selected from the group comprising Zn and Cd.

In further embodiments, at most 100 at.%, such as at most 99 at.%, especially at most 98 at.% of the tetrahedrally-coordinated transition metals may comprise Fe and/or Cu, especially Fe, or especially Cu. In further embodiments, at most 95 at.%, such as at most 90 at.%, especially at most 80 at.% of the tetrahedrally-coordinated transition metals may comprise Fe and/or Cu, especially Fe, or especially Cu. In further embodiments, at most 70 at.%, such as at most 60 at.%, especially at most 50 at.% of the tetrahedrally-coordinated transition metals may comprise Fe and/or Cu, especially Fe, or especially Cu.

In further embodiments, the tetrahedrally-coordinated transition metals may be devoid of a metal selected from the group comprising Zn and Cd, i.e., 0 at.% of the of the tetrahedrally-coordinated transition metals may comprise a metal selected from the group comprising Zn and Cd.

In further embodiments, at least 1 at.%, such as at least 2 at.%, especially at least 5 at.%, such as at least 10 at.%, of the tetrahedrally-coordinated transition metals may comprise a metal selected from the group comprising Zn and Cd. In further embodiments, at least 20 at.%, such as at least 30 at.%, especially at least 40 at.%, such as at least 50 at.%, of the tetrahedrally-coordinated transition metals may comprise a metal selected from the group comprising Zn and Cd.

In particular, metal organic frameworks based on Zn and Cd may provide a structure wherein the metal elements are tetrahedrally-coordinated, such as tetrahedrally coordinated to N. Hence, in embodiments, the metal-organic framework may comprise a Zn ²⁺ or Cd ²⁺ tetrahedrally coordinated and connected via the nitrogen atoms of aromatic ligands, especially a zeolitic imidazolate framework (ZIF), wherein the metal ions may be tetrahedrally coordinated to and connected via imidazolate based linkers, such as ZIF-8, ZIF -4 and ZIF-7, or a cadmium imidazolate framework (CdIF), such as CdIF-1.

In further embodiments, the metal-organic framework may be selected from the group comprising ZIF -4, ZIF-7, ZIF-8, and CdIF-1, especially from the group comprising ZIF- 4, ZIF-7, ZIF-8, such as ZIF-8, or especially CdIF-1. In further embodiments, the metal-organic framework may be Fe-doped, such as Fe doped ZIF-8 (or “Fe-ZIF-8”). In further embodiments, the metal-organic framework may be Cu-doped.

The metals may especially comprise the transition metal (see above), such as, in embodiments, especially iron. In further embodiments, at least 0.5 at.% of the metals may comprise the transition metal, such as at least 1 at.%, especially at least 2 at.%. In further embodiments, the metals may comprise at most 30 at.% of the transition metal, such as at most 20 at.%, especially at most 15 at.%, such as at most 10 at.%.

In further embodiments, the metals may further comprise a second metal, especially wherein the second metal comprises Zn and/or Cd, such as Zn, or such as Cd. The second metal may especially contribute to the overall structure of the metal-organic framework. In particular, the metals may comprise at least 70 at.% of the second metal, such as at least 75 at.%, especially at least 80 at.%. In further embodiments, the metals may comprise at most 99.5 at.% of the second metal, such as at most 99 at.%, especially at most 98 at.%, such as at most 95 at.%.

Hence, the second metal may be more abundant than the transition metal, despite the transition metal (later) providing the selective affinity for CO. In particular, the abundance of the transition metal may be lower to avoid the formation of by-products, such as the formation of metallic Fe or FesC after the pyrolysis in embodiments wherein the transition metal comprises iron.

In embodiments, the metal-organic framework may comprise a zeolitic imidazolate framework, especially a Fe-ZIF-8.

The pyrolysis stage may comprise subjecting the metal-organic framework to pyrolysis, especially at a pyrolysis temperature. In particular, the term “pyrolysis” may herein refer to the heating of the metal organic framework in the absence of oxygen. Due to the absence of oxygen, the exposure of the metal-organic framework to the pyrolysis temperature may cause the metal-organic framework to thermally decompose rather than combust.

In embodiments, the pyrolysis temperature may be at least 500 °C, such as at least 700 °C, especially at least 800 °C., such as at least 900 °C. In further embodiments, the pyrolysis temperature may be at most 3000 °C, such as at most 2500 °C, especially at most 2000 °C. In further embodiments, the pyrolysis temperature may be at most 1600 °C, such as at most 1500 °C, especially at most 1400 °C.

The pyrolysis stage may especially comprise subjecting the metal-organic framework to the pyrolysis temperature for a pyrolysis duration, and may especially provide a pyrolyzed material. In embodiments, the pyrolysis duration may be at least 0.5 hours, such as at least 1 hour, especially at least 1.5 hours. In further embodiments, the pyrolysis duration may be at most 5 hours, such as at most 3 hours, especially at most 2.5 hours.

In embodiments, the pyrolysis temperature and the pyrolysis duration may be selected such that at least 80 at.%, especially at least 90 at.%, of the Zn is removed and a graphene structure is formed. In particular, Zn may (start to) evaporate at about 907°C.

After the pyrolysis stage, metallic particles may be present in the pyrolyzed material. The presence of these metallic particles may be undesirable because they may interfere with the CO separation process. Hence, in embodiments, the pyrolyzed material may be subjected to acid washing in order to remove the metallic particles. The acid washing stage may further be beneficial as it may remove unreacted by-product. In embodiments, the acid washing stage may comprise exposing the pyrolyzed material to an acidic liquid, especially for a washing duration, especially to provide a washed pyrolyzed material. In particular, the acid washing stage may comprise washing the pyrolyzed material with the acidic liquid for the washing duration.

In embodiments, the acidic liquid may have a pH selected from the range of < 3.5, especially < 3, such as < 2.5, especially < 2.

In further embodiments, the washing duration may be at least 2 hours, such as at least 3 hours, especially at least 4 hours. In further embodiments, the washing duration may be at most 8 hours, such as at most 7 hours, especially at most 6 hours.

The transition metal, especially iron, in the washed pyrolyzed material may (at least partially) be in the second oxidation state, such as in the Fe(III) state. However, as described above, the first oxidation state, such as the Fe(II) state, may be preferable for adsorption of CO. Hence, in embodiments, the production method may comprise the reduction stage. The reduction stage may especially comprise exposing the washed pyrolyzed material to a reductive atmosphere at a reduction temperature to reduce the transition metal from the second oxidation state to the first oxidation state, such as to reduce Fe(III) to Fe(II). In addition, the surface of the washed pyrolyzed material may be (partially) covered with molecules such as H2O and O2, which may reduce access for CO during the adsorption stage, and which may be removed during the reduction stage (in the case of H2O primarily due to the increase in temperature).

The reduction stage may comprise exposing the washed pyrolyzed material to a (gaseous) reducing agent at a reduction temperature for a reduction duration. In embodiments, the reducing agent may be selected from the group comprising H2, KCs, and sodium naphthal eni de, especially H2. In further embodiments, the reducing agent may comprise CO.

In further embodiments, the reduction stage may comprise drying the washed pyrolyzed material, and electrochemically reducing the dried washed pyrolyzed material.

It will be clear to the person skilled in the art that the reduction temperature and/or the reduction duration may depend on the (selected) reducing agent.

For instance, in embodiments, especially with H2 as reducing agent, the reduction temperature may be selected from the range of 200 - 700°C, such as from the range of 250 - 500 °C, especially from the range of 300 - 450 °C. In further embodiments, the reduction temperature may be at least 250 °C, such as at least 270 °C, especially at least 300 °C. In further embodiments, the reduction temperature may be at most 700 °C, such as at most 600 °C, especially at most 500 °C. In further embodiments, especially with H2 as reducing agent, the reduction duration may be at least 10 minutes, such as at least 20 minutes, especially at least 30 minutes. In further embodiments, the reduction duration may be at most 2 hours, such as at most 1 hour, especially at most 45 minutes.

The reduction stage may further beneficially bring the transition metal, such as Fe, to the first oxidation state, such as Fe(II), which may be beneficial with regards during an adsorption stage during a (later) application (see method/system/use).

Thereby, especially after the reduction stage, the production method may provide the adsorbent.

Hence, in specific embodiments, the production method may comprise a preparation stage, a pyrolysis stage, an acid washing stage, and a reduction stage, wherein: the preparation stage comprises providing a metal-organic framework, wherein the metal-organic framework comprises Fe-ZIF-8, wherein the metal-organic framework comprises tetrahedrally- coordinated transition metals, wherein 1 - 100 at.% of the tetrahedrally-coordinated transition metals comprise Fe, such as wherein 1-15 at.% of the tetrahedrally-coordinated transition metals comprise Fe; the pyrolysis stage comprises subjecting the metal-organic framework to pyrolysis at a temperature selected from the range of 800 - 2000°C for a duration of 1-3 hours to provide a pyrolyzed material; the acid washing stage comprises washing the pyrolyzed material with an acidic liquid for a washing duration of 4-6 hours, wherein the acidic liquid has a pH selected from the range of < 3, especially < 2; and the reduction stage comprises exposing the washed pyrolyzed material to a reducing agent at a reduction temperature selected from the range of 250 - 500°C for a reduction duration of at least 20 minutes; thereby providing the adsorbent.

In embodiments, the reduction stage may comprise exposing the washed pyrolyzed material to a third fluid, especially a third gaseous mixture, wherein the third fluid comprises the reducing agent.

In particular, in embodiments, the third fluid may comprise the third gaseous mixture, especially wherein the third gaseous mixture comprises 3-30 mol.%, such as 5-20 mol.%, especially 7 - 15 mol.% of the reducing agent, such as of H2, or such as of NH3. In further embodiments, the reducing agent may comprise CO. In further embodiments, the third gaseous mixture may further comprise an inert gas, especially a noble gas, such as argon. In particular, in embodiments, the third gaseous mixture may comprise > 70 mol.% of the inert gas, such as > 80 mol.%, especially > 85 mol.%. In further embodiments, the third fluid may comprise a third liquid. In further embodiments, the third liquid may comprise sodium or lithium naphthalenide in tetrahydrofuran (THF).

In embodiments, the method may comprise a second pyrolysis stage. The second pyrolysis stage may especially be temporally arranged after the acid washing stage and prior to the reduction stage. In particular, after the acid washing stage, some H ⁺ may remain on the sample (e.g. on metal sites), and the second pyrolysis procedure may facilitate reactivating those sites. Further, the second pyrolysis stage may facilitate further removing Zn.

However, the second pyrolysis stage may also negatively affect properties of the adsorbent, such as the wt% of the transition metal, such as iron, in the adsorbent, and such as the CO uptake (see experiments below). Hence, in further embodiments, the production method may comprise a single pyrolysis stage, i.e., the production method may be devoid of the second pyrolysis stage.

In further embodiments, the production method may comprise arranging the adsorbent on a first electrode, i.e., the production method may be for providing a first electrode comprising the adsorbent, especially comprising the adsorbent as a coating.

The adsorbent obtainable with the production method of the invention may be particularly suitable for the method and the system of the invention, i.e., the adsorbent may be electrically switchable, and may provide a (relatively) high affinity for CO in a first oxidation state and a (relatively) low affinity for CO in a second oxidation state.

In a further aspect, the invention may provide the adsorbent obtainable with the production method of the invention.

Hence, in embodiments, the invention may provide an adsorbent comprising a graphite plane, especially a graphite plane comprising pyridinic nitrogen. In further embodiments, the adsorbent may comprise at least 0.02 wt% (pyridinic) M-N _x sites, such as M-N4 sites, wherein M comprises a transition metal, especially wherein M comprises Fe.

The adsorbent may especially be conductive, i.e., the (material of the) adsorbent may especially comprise a conductive material. The conductive material may especially have a conductivity (at room temperature) of at least 50 mS/cm, such as at last 100 mS/cm, especially at least 200 mS/cm. In further embodiments, the conductive material may have a conductivity (at room temperature) of at most 5000 S/m, especially at most 3000 S/m, such as at most 2000 S/m. In further embodiments, the conductive material may have a conductivity (at room temperature) of at most 2000 mS/cm, such as at most 1500 mS/cm, especially at most 1000 mS/cm. In a further aspect, the invention may provide a first electrode comprising the adsorbent of the invention. In particular, the first electrode may comprise a coating of the adsorbent.

In a further aspect, the invention may provide a use of the adsorbent for capturing carbon monoxide, especially for capturing carbon monoxide from a fluid mixture. In particular, the use may comprise adsorbing carbon monoxide from the fluid mixture to the adsorbent, and subsequently desorbing the carbon monoxide, especially to a second fluid.

The term “stage” and similar terms used herein may refer to a (time) period (also “phase”) of a method and/or an operational mode. It will be clear to the person skilled in the art how the stages may be beneficially arranged in time. In particular, the adsorption stage and the desorption stage may be temporally arranged in alternating fashion, such that CO may be adsorbed to the adsorbent during the adsorption stage and desorbed during the desorption stage.

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. Further embodiments of the adsorbent described in relation to the method and/or to the system may further relate to the adsorbent as such, and vice versa.

In yet further embodiments, the invention provides a system comprising a first reactor (part) containing the adsorbent, wherein the first reactor (part) can be used to adsorb CO from a (first) fluid in the reactor. Further, the first reactor (part) can be used to release the adsorbed CO to a (second fluid). Further, the system may comprise a second reactor (part), configured to convert at least part of the CO, or more especially at least part of the second fluid comprising CO to another material, such as synthetic natural gas, ammonia or methanol. The second reactor (part) may be configured downstream of the first reactor part.

Especially, the composition of the first fluid (i.e. the fluid mixture) is different from the second fluid. More especially, the first fluid is different from the second fluid even when not taking into account CO. For instance, the first fluid may be richer in EE or less rich in EE than the second fluid. In embodiments, the desorption stage may comprise contacting the adsorbent with a second fluid, which, not taking into account CO, has another composition than the fluid mixture.

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. 1 A schematically depicts the method of the invention; Fig. IB schematically depicts the system of the invention; Fig. 2 schematically depicts the production method of the invention; Fig. 3-8 schematically depict experimental results obtained in relation to the invention. The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1A schematically depicts an embodiment of the method for capturing carbon monoxide 30 from a fluid mixture 50 using a (partially depicted) adsorbent 100. In the depicted embodiment, the adsorbent 100 comprises a graphite plane 120 comprising pyridinic nitrogen. In particular, the adsorbent 100 comprises a (pyridinic) Fe- i site 110.

In the depicted embodiment, the method comprises an adsorption stage and a desorption stage.

The adsorption stage 1 may comprise exposing the adsorbent 100 to the fluid mixture 50, especially wherein the fluid mixture comprises CO. The adsorption stage may further comprise imposing a first potential onto the adsorbent 100. In embodiments, the adsorption stage comprise imposing a first potential (difference) between the adsorbent 100 and a second electrode 220 (see below), which second electrode 220 may work as counter electrode and/or as reference electrode. In particular, the first potential may be selected such that the Fe in the Fe- i site 110 comprises Fe(II) 10, which may have a high affinity for CO. During the adsorption stage, the carbon monoxide 30 may adsorb to the adsorbent, especially to Fe in the adsorbent, such as particularly to Fe(II) 10, more especially to the Fe(II) in the Fe- N4 site 110.

The desorption stage 2 may comprise exposing the adsorbent 100 to a second fluid 60. The desorption stage may further comprise imposing a second potential on the adsorbent 100, especially between the adsorbent 100 and the second electrode 220. In particular, the second potential may be selected such that the Fe in the Fe-N4 site 110 comprises Fe(III) 20, which may have a (relatively) low affinity to CO. Hence, during the desorption stage, the carbon monoxide 30 may desorb from the adsorbent, especially from Fe in the adsorbent, such as particularly from Fe(III) 10, more especially from the Fe(III) in the Fe-NU site 110.

In particular, in embodiments, during the adsorption stage 1, the first potential may be selected such that at least 90 at.% of Fe in Fe-NU sites 110 is ferrous. In further embodiments, during the desorption stage 2, the second potential may be selected such that at least 90 at.% of Fe in Fe-NU sites 110 is ferric.

In the depicted embodiment, the method may comprise alternating between the adsorption stage 1 and the desorption stage 2. In particular, the adsorption stage 1 may have an adsorption duration, and the desorption stage 2 may have a desorption duration. In particular, in embodiments, the adsorption duration and the desorption duration may be selected in dependence on the amount of CO adsorbed to the adsorbent, i.e., the adsorption duration may be selected such that the amount of adsorbed CO reaches or exceeds a predefined threshold, especially relative to a maximal capacity for CO adsorption of the adsorbent, and the desorption duration may be selected such that a predefined fraction of (previously) adsorbed CO is desorbed.

Fig. 1A further schematically depicts an embodiment of the adsorbent 100. In particular, in the depicted embodiment, the adsorbent comprises an electrically switchable adsorbent 100 comprising a graphite plane 120 comprising pyridinic nitrogen. In further embodiments, the adsorbent may comprise at least 0.02 wt% (pyridinic) Fe-NU sites.

Fig. 1 A further schematically depicts a use of the adsorbent 100 of the invention for capturing carbon monoxide 30 from a fluid mixture 50.

Fig. IB schematically depicts an embodiment of a system 200 for capturing carbon monoxide 30 from a fluid mixture 50 using the adsorbent 100. Especially, the adsorbent 100 may comprise a graphite plane 120 comprising pyridinic nitrogen, and may comprise (pyridinic) M-NU sites, especially Fe-NU sites 110. In the depicted embodiment, the system 200 further comprises a first electrode 210, a second electrode 220, a fluid flow control unit 230, and a charge control unit 240. In the depicted embodiment, both the first electrode 210 and the second electrode 220 comprise the adsorbent 100, especially wherein the adsorbent 100 comprises a coating, i.e., the adsorbent 100 is arranged as a coating on the first electrode 210 and is arranged as a coating on the second electrode 220 (or “counter electrode”). Hence, the second electrode 220 may (also) comprise the adsorbent 100. Thereby, when the transition metal of the first electrode 210 is being oxidized, a reduction half reaction may take place on the second electrode 220. When the transition metal of the first electrode 210 is being reduced, an oxidation reaction may take place on the second electrode 220. Thereby, at any moment one electrode may be reducing the (respective) transition metal, while the other electrode may oxidize the (respective) transition metal, allowing for an (essentially) (semi-)continuous process. Hence, while the first electrode 210 is exposed to the adsorption stage 1, the second electrode 220 may be exposed to the desorption stage 2, and vice versa. In particular, by changing the fluid feed to both cells, the cells may be switched with respect to in which cell adsorption and in which cell desorption is taking place.

In further embodiments, the second electrode 220 may comprise a Pt-based electrode, especially devoid of the adsorbent.

The fluid flow control unit 230 may be configured to control the exposure of the adsorbent to a gas, such as to the (first) fluid mixture 50, and such as to the second fluid 60. In particular, the fluid flow control unit 230 may be configured to expose the adsorbent 100 to the (first) fluid mixture 50 during the adsorption stage 1, and especially to expose the adsorbent 100 to a second fluid 60 during the desorption stage 2.

The charge control unit 240 may be configured to impose a potential on the adsorbent, especially on the first electrode, such as between the first electrode and the second electrode, which may function as the counter electrode, but which may also function as the reference electrode. In particular, in embodiments, the charge control unit 240 may be configured to impose a first potential onto the first electrode during the adsorption stage, and to impose a second potential onto the first electrode during the desorption stage.

In the depicted embodiment, the system 200 further comprises a control system 300, a temperature control unit 250, and a pressure control unit 260. The control system 300 may especially be configured to control the system 200, especially one or more of the fluid flow control unit 230, the charge control unit 240, the temperature control unit 250, and the pressure control unit 260.

In embodiments, the system may further comprise a functional unit 280. In the depicted embodiment, the functional unit 280 comprises a first cell 281, a second cell 282, and a separator 215, especially a membrane. In further embodiments, the first cell 281 may especially comprise the first electrode 210, and the second cell 282 may especially comprise the second electrode 220. The separator 215 may, in embodiments, be arranged between the first cell 281 and the second cell 282, i.e., the first cell 281 and the second cell 282 may share the separator 215. In particular, in the depicted embodiment, the separator 215 may be arranged between the first electrode 210 and the second electrode 220. Especially, the separator 215 may be arranged in physical contact with the first electrode 210 and the second electrode 220.

In the depicted embodiment, the first cell 281 and the second cell 282 may be devoid of an electrolyte 275, i.e., the system may be configured to operate in the absence of an electrolyte 275. Such configurations may be particularly suitable for embodiments wherein the fluid mixture comprises a gaseous mixture.

In further embodiments, the system 200 may further comprise an electrolyte control system 270. In such embodiments, the control system 300 may be configured to control the electrolyte control system 270. The electrolyte control system 270 may be configured to provide an electrolyte 275 to the functional unit 280, especially to provide a first electrolyte 275, 276 to the first cell 281, and especially to provide a second electrolyte 275, 277 to the second cell 282.

In embodiments, the system 200, especially the control system 300, comprises an operational mode. The operational mode may comprise an adsorption stage 1 and a desorption stage 2. Especially, the operational mode may comprise alternating between the adsorption stage 1 and the desorption stage 2.

The adsorption stage 1 may especially comprise the fluid flow control unit 230 providing the fluid mixture 50 to the adsorbent 100. The adsorption stage 1 may further comprise the charge control unit 240 imposing a first potential onto the first electrode 210, such as a first potential selected from the range of 0 - 0.3 V vs Ag/AgCl.

The desorption stage 2 may especially comprise the fluid flow control unit 230 exposing the adsorbent 100 to a second fluid 60. The desorption stage 2 may further comprise the charge control unit 240 imposing a second potential onto the first electrode 210, such as a second potential selected from the range of 0.7 - 1 V vs Ag/AgCl.

Fig. IB further schematically depicts an embodiment of the first electrode 210 comprising the adsorbent 100.

Fig. IB further schematically depicts a use of the first electrode 210 according to the invention for capturing carbon monoxide 30, especially from a fluid mixture 50.

Fig. IB further schematically depicts a combined system 1000 comprising the system 200 of the invention functionally coupled with a CO generating system 500, especially an electrochemical CO2 reduction system 501. The electrochemical CO2 reduction system 501 may be configured to reduce CO2 to CO 30, and may provide the CO 30 to the system 200 of the invention. In particular, the electrochemical CO2 reduction system 501 may be configured to provide the fluid mixture 50 to the system 200. In particular, in the depicted embodiment, the electrochemical CO2 reduction system 501 may comprise a second functional unit 580, wherein the second functional unit 580 comprises a third cell 583 comprising a third electrode 510, and wherein the second functional unit 580 comprises a fourth cell 584 comprising a fourth electrode 540. In the depicted embodiment, the third cell 583 and the fourth cell 584 share a second separator 515 configured to be permeable for at least H ⁺ . the electrochemical CO2 reduction system 500 further comprises a second charge control system 540 configured to control a potential between the third electrode 510 and the fourth electrode 520.

Hence, the invention may further provide a combined system 1000, wherein the combined system 1000 comprises a CO generating system 500 and the system 200, wherein the CO generating system 500 is configured to provide the fluid 50 (comprising CO) to the system 200.

Fig. 2 schematically depicts an embodiment of the production method for providing the adsorbent 100. In the depicted embodiment, the production method comprises a preparation stage 401, a pyrolysis stage 402, an acid washing stage 403, and a reduction stage 404. In particular, in the depicted embodiment, the preparation stage 401 comprises providing a metal-organic framework 410, wherein the metal-organic framework comprises Fe-ZIF-8, wherein the metal-organic framework 410 comprises tetrahedrally-coordinated transition metals, wherein 1-15 at.% of the tetrahedrally-coordinated transition metals comprise Fe. In the depicted embodiment, the pyrolysis stage 402 comprises subjecting the metal-organic framework 410 to pyrolysis at a temperature selected from the range of 800 - 2000°C for a duration of 1-3 hours to provide a pyrolyzed material 420. In the depicted embodiment, the acid washing stage 403 comprises washing the pyrolyzed material 420 with an acidic liquid 435 for a duration of 4-6 hours, thereby especially providing a washed pyrolyzed material 430, wherein the acidic liquid has a pH selected from the range of < 3, especially < 2.5, such as < 2. In the depicted embodiment, the reduction stage 404 comprises exposing the washed pyrolyzed material 430 to a (gaseous) reducing agent 445 at a reduction temperature selected from the range of 250 - 500°C for a reduction duration of at least 20 minutes. Thereby, the production method may provide the adsorbent 100.

In the depicted embodiment, the method comprises a single pyrolysis stage 402. However, in further embodiments, the method may comprise a second pyrolysis stage temporally arranged between the acid washing stage 403 and the reduction stage 404.

In embodiments, the provided adsorbent 100 may comprises an external surface 130 with an external surface composition comprising: 5-20 wt% O; 60-90 wt% C; 3-15 wt% N; 0.5-8 wt% S; 0.1-5 wt% Fe; and 0.5-3 wt% Zn; based on X-ray photoelectron microscopy. In further embodiments, the provided adsorbent 100 may comprises an external surface 130 with an external surface composition comprising: 3-20 wt% O; 50-90 wt% C; 2-15 wt% N; 0- 8 wt% S; 5-25 wt% Fe; and 0-0.5 wt% Zn; based on X-ray photoelectron microscopy.

Materials and methods

Unless specified otherwise, the experiments described herein (see below) are carried out according to the materials and methods specified hereinafter in this section.

Used chemicals: 2-methylimidazole (99% Sigma-Aldrich); Iron(II) sulfate heptahydrate (Sigma Aldrich); Iron(III) nitrate nonahydrate (98% Sigma-Aldrich); Zinc nitrate hexahydrate (98% Sigma-Aldrich); Methanol (>99.8% Sigma Aldrich); Sulfuric acid (98% Sigma-Aldrich), Carbon conductive cement adhesive (VWR International BV).

Characterization method 1 - Powder X-ray Diffraction - The equipment used was a Bruker-AXS D5005. All the samples were grinded with the use of a mortar and flattened to the standard height of the holder. The parameters used for the measurements are as follows: Wavelength - 1.789 A (Cobalt X-ray source); Operating range (20) - 5-90°; Step speed - 0.02 sec/step; Step size - 0.0197°. In the pattern(s) reported below, all 20 are converted to copper X-ray source to facilitate comparisons.

Characterization method 2 - Thermogravimetric Analysis (TGA) - The TGA equipment used was Mettler Toledo TGA/SDTA851e. The TGA patterns were recorded under 100 mL/min of N2 flow from 20 to 1000 °C. The sample mass employed is (about) 10 mg. A blank run with an empty crucible was performed first under the same conditions as for the test.

Characterization method 3 - Fb-Temperature Programmed Reduction (TPR) - The bb-Temperature Programmed Reduction was performed on a Thermo Scientific TPD/R/O 110 machine. A mixture of bb/Ar (30mL/min 10 vol.% H2) was flown over the samples (typically about 30 or 40 mg of sample was placed between two pieces of quartz wool and 200 mg of SiC in a quartz reactor) while the temperature was increased linearly from 25 to 800°C (10 °C/min). The sample was kept at 25 °C for 5 min, and then the temperature was ramped to 800°C at 10°C/min and held at that temperature for 5 min.

Characterization method 4 - CO pulse Chemisorption-Temperature Programmed Desorption (TPD) - CO pulse chemisorption and TPD were performed in a Thermo Scientific TPD/R/O 110 machine. The samples, with a weight of 40-50 mg, were placed between two pieces of quartz wool and SiC in the glass reactor. The samples were pretreated via an H2 reduction process at high temperature to reduce Fe(III) to Fe(II) and to remove adsorbed species, such as O2 or H2O, from the metal sites of the surface. The pretreatment of the samples was done with a mixture of H2 and Ar (10 vol.% H2, 30 mL/min) and ramping temperature from 25 to 350 or 500°C. (10°C/min, Ih hold time at the target temperature). Afterwards, the H2 line was closed and the reactor was cooled down to ambient temperature. Pulse chemisorption was started once the reactor reached a temperature of 35°C. An injection of 7.5 pL of CO for several pulses (ten min for each pulse) was performed. TPD (35 to 400°C, 10°C/min, hold time 10 min at 300 °C, Ar is the carrier gas) was performed followed by the cooling of the system to ambient conditions.

Characterization method 5 - N2-Adsorption/Desorption Isotherms - The machine used is TriStar II 3020 Version 3.02. The samples were placed in glass flasks and a 3-step degas process was started: The first one is a ramp to 25°C and held for 2 minutes. In the second step, temperature was increased to 90°C at 5°C/min and held for 60 minutes. Finally, temperature was increased to 150°C at 5°C/min and held for 900 min. Once the samples were degassed, they were cooled to 77K using liquid nitrogen. The equilibrium interval was every 12s.

Characterization method 6 - X-ray photoelectron spectroscopy (XPS) - The XPS was performed on a ThermoFisher K-Alpha with an X-ray gun using an Al Ka source. The sample powders were put on a copper tape stuck onto a small metal plate. The plate was attached to the sample holder using one or two spring clips. The spectra were analyzed using the CasaXPS software. Background subtraction was derived from Shirley background. Spectra were deconvoluted using Lorentzian-Gaussian combination peaks.

Characterization method 7 - Transmission electron microscopy (TEM) - A JEOL JEM-1400 plus TEM operated at 120 kV was used to characterize the morphology of the FeNC (see below) adsorbents. The adsorbents were prepared by suspending the adsorbents in ethanol and then depositing the suspension in a TEM grid.

Characterization method 8 - Inductive coupled plasma optical emission spectrometry (ICP-OES) - The content of Fe and Zn in the adsorbents was quantified using ICP-OES. The samples were degusted with a microwave from CEM model MARS 6 and measured with an ICP-OES from SPECTRO model SPECTRO ARCOS.

Characterization method 9 - Fe Mossbauer Spectroscopy - Transmission 57Fe Mossbauer spectra were collected at 300 K with a sinusoidal velocity spectrometer using a 57Co(Rh) source. Velocity calibration was carried out using an a-Fe foil at room temperature. The source and the samples were kept at the same temperature during the measurements. Regular experiments were done without any inert atmosphere protection. A detailed description of the experimental procedure is as follows: (1) A regular Mossbauer measurement on FeNCl- AW-2P sample (550 mg); (2) The FeNCl-AW-2P (550 mg) was heated at 400 °C for 2 hours with a heating rate of 10 °C/min' ¹ under 30 mL/min 10% H2/Ar (H2 3.0 mL/min and Ar 27.0 mL/min). After the reaction, the cell was cooled down and the gas switched back to Ar. A Mdssbauer measurement was done on FeNCl-AW-2P-R (reduced sample); (3) Flushing CO into the cell. First experiment: 100 pL of CO was introduced into the cell and the Mdssbauer spectrum of the sample was measured. Second experiment: 500 pL of CO was introduced into the cell and the Mdssbauer spectrum of the sample was measured. After that, the cell was flushed with Ar and another spectrum was taken. Third experiment: ImL of CO was introduced into the cell and the Mdssbauer spectrum of the sample was measured. After that, the cell was flushed with Ar and another spectrum was taken. Fourth experiment: 100 mL of CO was introduced into the cell and the Mdssbauer spectrum of the sample was measured. Fifth experiment: 100% of CO was introduced into the cell and the Mdssbauer spectrum of the sample was measured. After that, the cell was flushed with Ar and another spectrum was taken. The Mdssbauer spectra were fitted using the Mosswinn 4.0 program.

Characterization method 10 - Cyclic Voltammetry - The cyclic voltammetry measurement was conducted in a three-electrode-setup. The reference electrode was Ag/AgCl in KC1, and a platinum electrode was used as counter electrode (or “second electrode”). The first electrode (or “working electrode”) was a titanium plate with an area of 4 cm ² , wherein the first electrode was coated with the adsorbent using adhesive carbon glue. The electrochemical test was carried out in a potentiostat in 0.5 M H2SO4 aqueous solution, and the I/V curves were obtained with a scanning potential from 0 to 1.0 V at a sweeping rate of 50 mVs' ¹ . The solutions were degassed with N2 before the measurements.

Production method for producing the adsorbent - 68.1 mg (0.244 mmol) of FeSO4'7H2O and 0.887 g (2.98 mmol) of Zn(NC>3)2 6H2O were dissolved in 100 mL of methanol (Solution A). 3.225 g (39.3 mmol) of 2-methylimidazole were dissolved in 100 mL of methanol (Solution B). Solution A was poured into solution B and kept for one hour at room temperature under intensive stirring, thereby providing Fe-ZIF8-1. The Fe-ZIF8-1 was washed with methanol. This process was repeated three times. The white Fe-ZIF8-1 powder was dried in a vacuum oven overnight at 60 °C. The Fe-ZIF8-1 precursors were placed in a U-type glass reactor and heated to 900 °C with a heating rate of 2°C/min under 30 mL/min N2 flow and kept at that temperature for 2 hours. The black powder obtained was cooled down to room temperature and washed with 100 mL of 0.5 M H2SO4. The solution was heated to 80 °C under intensive stirring for 6 hours. Subsequently, a first adsorbent (hereinafter referred to as FeNCl- AW) was recovered by filtration and washed with deionized water. The samples were dried in the vacuum oven at 60 °C overnight. After that, a second heat treatment at 900 °C under N2 at the same condition was carried out to obtain a further adsorbent (FeNCl-AW-2P).

The sample obtained after acid washing will hereinafter be referred to as FeNCl- AW and the sample obtained after second pyrolysis will be referred to as FeNCl-AW-2P.

The adsorbent FeNCl-AW-2P (300 mg) was placed in a glass reactor and then heated at 500 °C for 1 h with a heating rate of 10 °C/min' ¹ under FF/Ar (H2 3.0 mL/min and Ar 27.0 mL/min). After the reaction, the glass reactor was cooled down; the resulting adsorbent is hereinafter referred to as FeNCl-AW-2P-R.

Pellet chunk preparation - After preparation of the adsorbent material (FeNCl- AW), the adsorbent material was ground together with gamma alumina as a binder to provide a sample comprising 20 wt% of FeNC-l-AW and 80 wt% gamma alumina. 5 wt% acetic acid in water was added to the sample until a runny paste was formed, which paste was further ground. The paste was allowed to dry slightly until it became sufficiently solid to handle and pelletize. The pelletization was performed under 4 tons of pressure using a pellet press die set. The pellets were broken into 9 mm chunks (to fit a degassing tube) and were sieved to a size > 212 pm to prevent dust from being sucked into a measurement instrument.

Characterization method 11 - CO adsorption on (reduced) pellet chunks - The pellet chunks were degassed under N2 flow in a micromeritics Smartprep Programmable Degas System by ramping to 25 °C for 2 minutes, then to 90 °C with a ramp rate of 5 °C/min and held at 90 °C for 60 minutes. Finally, the temperature was increased to 150 °C with a ramp rate of 5 °C/min and held at 150 °C for 900 minutes. The sample was then transferred to a micromeritics ASAP 2020 Surface Area and Porosity Analyzer in which the sample’s free space was measured using He gas at 25 °C for 30 minutes, then evacuated under vacuum at 150 °C with a ramp rate of 5 °C/min and held at 150 °C for 180 minutes. Finally, the system was left to cool down to 35 °C, after which it was evacuated under vacuum for 15 minutes, then leak tested, then evacuated under vacuum for 15 minutes at 35 °C again. Then CO adsorption of the (nonreduced) pellets was measured at 35 °C (see characterization method 12 below). The sample was then removed from the instrument and placed in a quartz activation tube in which the sample was reduced under a gas flow consisting of 27 ml/min Ar and 3 ml/min H2 at 350 °C with a ramp rate of 10 °C/min, and held at 350°C for 1 hour in an activation setup. The sample was cooled down under an Ar flow to prevent oxidation and was then quickly transferred back to the analysis instrument after cooling. The sample was then evacuated under vacuum at 35 °C with a ramp rate of 5 °C/min and held at 35 °C for 30 minutes, then evacuated under vacuum at 150 °C with a ramp rate of 5 °C/min and held at 150 °C for 120 minutes, then evacuated under vacuum at 350 °C with a ramp rate of 5 °C/min and held at 350 °C for 60 minutes. Finally, the sample was allowed to cool down to 35 °C and was evacuated under vacuum for 15 minutes, after which it was leak tested and evacuated for another 15 minutes under vacuum. Then CO adsorption of the (reduced) pellets was measured at 35 °C.

Characterization method 12 - CO Adsorption Isotherms - The machine used is micromeritics ASAP 2020 Surface Area and Porosity Analyzer. The samples were successively exposed to a known quantity of CO gas, which is dosed into the sample tube, and the pressure is left to equilibrate and then this equilibrium pressure is measured. The equilibrium interval was every 20s. Results

The structures of the synthesized FeNCl-AW and FeNCl-AW-2P samples were determined by a combination of characterization techniques including Powder X-ray Diffraction (pXRD), Thermogravimetric Analysis (TGA), Transmission electron microscopy (TEM), ICP Optical Emission Spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS) and Fe Mdssbauer Spectroscopy. Further studies, such as EE-Temperature Programmed Reduction (EE-TPR), CO-Temperature Programmed Desorption (CO-TPD) and measurements of CO adsorption isotherms were performed in order to further investigate the CO uptake abilities of the adsorbents.

XRD patterns were obtained for the precursor (Fe-ZIF8-1). Fig. 3 schematically depicts the pXRD data, wherein line LI corresponds to Fe-ZIF8-1, and wherein line L2 corresponds to ZIF-8. Hence, the diffraction pattern of the metal-organic framework 410 (or “the precursor”) matches with the one of the undoped ZIF-8 (from literature). The results suggest that the crystal structure of ZIF-8 is maintained in Fe-ZIF8-1, which may be beneficial for the formation of desired Fe-N-C structures. Hence, the ZIF-8 structure may remain intact at Fe doping of about 8 at.%(with respect to the metals), although part of the Zn atoms may be replaced by Fe atoms in the structure. Subsequently, the Fe-ZIF8-1 precursors were subjected to a pyrolysis stage at a pyrolysis temperature of 900 °C under N2. The resulting adsorbent is hereinafter referred to as FeNCl. FeNCl was subsequently subjected to an acid washing stage, providing adsorbent FeNCl-AW. FeNCl-AW was subsequently subjected to a second pyrolysis stage (see below), providing FeNCl-AW-2P.

The upper limit for Fe doping in ZIF-8 may be about 20 at.% (with respect to the metals), leading to a substantial part of the Zn atoms being replaced by Fe atoms in the structure. As indicated above, a high abundance of Fe in the structure may lead to the formation of (undesired) metallic Fe or FesC during pyrolysis. A presence of iron at about 8 at.% may lead to a particularly beneficial adsorbent (with respect to CO capturing). In alternative embodiments, the upper limit for Fe doping inZIF-8 may be about 100 at.% (with respect to the metals), such as about 90 at.%, especially about 80 at.%, leading to a substantial part of the Zn atoms being replaced by Fe atoms in the structure. A higher percentage of Fe may provide for a higher adsorption capacity. Hence, a presence of iron close to 100 at.% (with respect to the metals) may lead to a particularly beneficial adsorbent (with respect to CO capturing).

Fig. 4 schematically depicts the XRD patterns of the as-synthesized adsorbents, wherein line L3 indicates the XRD pattern of FeNCl, wherein line L4 indicates the XRD pattern of FeNCl-AW, and wherein line L5 indicates the XRD pattern of FeNCl-AW-2P. In particular, Fig. 4 schematically depicts two broad diffraction peaks centred at about 26° and 43° in all of the samples. These peaks may correspond to the (002) and (101) diffraction peaks of partially organised graphitic carbon. In particular, two small sharp peaks are observed in the sample before the acid washing treatment, which may belong to metallic particles. These peaks are not observed in the spectra for FeNCl-AW and FeNCl-AW -2P, indicating that (essentially) no inorganic iron-containing phase is present. Hence, the acid washing stage successfully removed the metallic particles.

Transmission electron microscope (TEM) images revealed that FeNCl-AW and FeNCl-AW-2P samples possess a carbonized structure with graphitic fragments.

The presence of Fe and Zn in the samples was assessed by ICP-OES, as shown in table 1 :

Sample Fe(wt%)

Fe-ZIF8-1 1.95 26.26

FeNC-1-AW-2P 4.12 2.37

FeNC-1-AW 4.52 2.34

Hence, compared to the metal organic framework 410, the adsorbents 100 may have a (relatively) increased Fe wt%, and a substantially decreased Zn wt%, indicating that Zn is effectively removed during pyrolysis at the pyrolysis temperature.

Hence, in embodiments, the adsorbent 100 may comprise at least 3.5 wt% Fe based on ICP-OES analysis, such as at least 4 wt% Fe. Similarly, in embodiments, the adsorbent 100 may comprise at most 5 wt% Zn based on ICP-OES analysis, such as at most 4 wt% Zn, especially at most 3 wt% Zn. In further embodiments, the adsorbent 100 may comprise at most 15 wt% Zn based on ICP-OES analysis, such as at most 10 wt% Zn, especially at most 8 wt% Zn.

To further analyze the chemical states of elements on the external surface of the FeNCl-AW and FeNCl-AW-2P samples, the adsorbents 100 were subjected to X-ray photoelectron spectroscopy (XPS). Fig. 5A-L schematically depicts an XPS spectra in counts (not to scale between spectra) vs. binding energy B (in eV), wherein Fig. 5A-F correspond to FeNCl-AW-2P, wherein Fig. 5G-L correspond to FeNCl-AW, and wherein Fig. 5A,G correspond to C Is, wherein Fig. 5B,H correspond to N Is, wherein Fig. 5 C,I correspond to O Is, wherein Fig. 5D,J correspond to S 2p, wherein Fig. 5E,K correspond to Zn 2p, and wherein Fig. 5F,L correspond to Fe 2p.

Specifically, the C Is spectra of FeNCl-AW -2P and FeNCl-AW revealed four different characteristic peaks corresponding to C-C at 284.8 eV, C-N and C-0 at 285.4 eV, O- C=O at 287.5 eV (285.9 eV for FeNCl-AW) and carbonate at 288.9 eV. The N Is spectrum of FeNC was deconvoluted into four peaks, centered at 398.4 eV, 399.4 eV, 400.9 eV and 401.5 eV. They were assigned to pyridinic N, pyrrolic N, graphitic N and oxidized N. Meanwhile, the N Is spectrum of FeNCl-AW was fitted into three peaks centered at 397.7 eV, 399.4, and 402.9 eV. These peaks were assigned to pyridinic, graphitic and oxidized N, respectively. Pyrrolic N was not identified in FeNCl-AW. The O ls spectrum of FeNCl-AW-2P was deconvoluted into three bonds, centered at 530.9 eV, 532.5 eV and 536.3 eV.3 The position of the three peaks for FeNCl-AW changed slightly compared with FeNCl-AW-2P. They were located at 530.5, 531.8 and 533.2 eV, respectively. Different sulfur species were identified in the external surface of the samples. The spectra were deconvoluted into four peaks. Two signals at 163 and 167.5 eV were attributed to S 2p3/2 and S 2pi/2. Also S-C may be present in both FeNCl-AW -2P and FeNCl-AW. The peaks at higher bonding energies (167.9-168.5 eV) may be attributed to sulfate (C-SO _X -C) species. There is no indication of the presence of FeS or FeS2 species (161.7 eV). The XPS spectra of Zn 2p of the adsorbents shows a distinct doublet (spinorbit splitting of 23.1 eV) located at 1021 and 1044 eV. The peaks were assigned to Zn 2p3/2 and Zn 2pi/2 of Zn(II). The fitted peaks at higher binding energies in both samples (1024 eV and 1047 eV) may correspond to zinc oxide. The Fe 2p spectra suggest the presence of both Fe(II) and Fe(III) on the external surface FeNCl-AW-2P and FeNCl-AW. The peaks centered at 710 and 723 eV are assigned to the 2p3/2 and 2pl/2 of Fe(II) species, respectively. The peaks centered at 714 and 727 eV belong to the 2p3/2 and 2pl/2 of Fe(III) species. The presence of metallic iron (Fe(0)) (at 707 eV) and hence iron nanoparticles is excluded. There is also no indication of the presence of FeS (707.5 eV). The external surface composition as determined by XPS is indicated in table 2

Name FeNC1-AW-2P FeNC1-AW

Weight (%) Weight (%)

O 8.49 13.94

C 76.82 71.24

N 10.13 8.24

S 1.56 5.08

Fe 2.12 0.31

Zn 0,87 1.17

Hence, in specific embodiments, the adsorbent 100 may comprises an external surface with an external surface composition comprising 5-20 wt% O; 60-90 wt% C; 3-15 wt% N; 0.5-8 wt% S; 0.1-5 wt% Fe; and 0.5-3 wt% Zn based on X-ray photoelectron microscopy.

Thermogravimetric analyses (TGA) were performed in an N2 atmosphere to evaluate the thermal stability of the samples. Three regions were distinguished where weight loss occurs. An initial weight loss at about 100°C was observed due to the evaporation of water and the removal of certain adsorbates, such as CO2 and O2, on the surface of the adsorbent. A second mass loss at a temperature > 100 °C was interpreted as the decomposition of organic functional groups that may be present in the structure. A mass fraction loss at high temperatures (T>800 °C) suggested carbon gasification. At high temperatures (>800 °C) nitrogen atoms may be released from the sample in the form of N2 and/or NH3.

Hence, both pXRD and TEM results imply that FeNCl-AW and FeNCl-AW- 2P are carbonaceous materials. According to the pXRD pattern, there is no obvious peak that can be attributed to metallic iron or Fe compounds in either FeNCl-AW or FeNCl-AW-2P samples, indicating that Fe atoms are incorporated into the carbon matrix. This is in agreement with the TEM results that no small black dots - which can be ascribed to metallic iron - are observed. Meanwhile, the sample is quite stable at high temperature, as observed with TGA analysis. Based on the XPS results, the nitrogen atoms appear to be present in the adsorbent in the form of pyridinic, pyrrolic, graphitic and oxidized nitrogen. In the pyridinic nitrogen species, a nitrogen atom is bound to two carbon atoms located on the edges of graphite planes. In graphitic-N, one nitrogen atom is connected with three carbon atoms within a graphite plane. The pyridinic nitrogen content was not observed to increase after second pyrolysis. The iron atoms in the material may be connected to N atoms and be present in both +2 and +3 oxidation states.

Subsequently, the samples were subjected to the reduction stage. In particular, H2 was used to reduce the Fe(III) sites in FeNC samples into Fe(II). The reducing temperature was evaluated by a H2-TPR measurement performed on both FeNCl-AW and FeNCl-AW -2P samples. The first peak, observed at 302 °C for FeNCl-AW and at 424 °C for FeNCl-AW-2P can be assigned to the reduction from Fe(III) to Fe(II). The second peaks, which are observed at higher temperatures of 411 °C for FeNCl-AW and 624 °C for FeNCl-AW-2P may belong to the reduction of Fe(II) to metallic Fe or (also) to the reduction of Fe(III) to Fe(II). Interestingly, the reduction temperature of FeNCl-AW-2P shifts to a higher temperature, indicating that the sample may be more difficult (and energy-demanding) to reduce. In particular, based on H2-TPR measurement a suitable reduction temperature may be selected for reducing Fe(III) to Fe(II).

Hence, the FeNCl-AW-2P sample was further reduced with 10%H2 in Ar for one hour at 500°C to give sample FeNCl-AW-2P-R.

FeNCl-AW -2P-R was subsequently subjected to XRD, TEM, and XPS analysis. The XRD analysis revealed peaks corresponding to partially organized graphitic carbon, including the (002) and (101) diffraction peaks. No metallic Fe was observed in the TEM images. The composition of the external surface of FeNCl-AW-2P-R as estimated by XPS is indicated in table 3 :

Elements O C N Zn Fe S

Weight (%) 11.07 78.99 6.26 1.58 0.96 1.15

To further analyse the pore structure and surface area of all the adsorbents, N2 adsorption/desorption isotherms were measured on FeNC-l-AW, FeNCl-AW -2P and FeNCl- AW-2P-R. In particular, the determined BET surface area and the pore volume of the studied adsorbents are displayed in Table 4:

Sample SBET (m ² /g) V _pO re(cm ³ /g)

Hence, substantial differences may be observed in the pore characteristics of the adsorbents. The highest surface area may be obtained after second pyrolysis and hydrogen reduction at 500°C (643.17 m ² /g). After the reduction treatment, the organics or gas molecules (H2O, O2) may be removed from the pores and the surface area may be exposed.

Hence, in embodiments, the adsorbent 100 may have a surface area S _a selected from the range of 150 - 800 m ² /g.

In further embodiments, the adsorbent 100 may have a (total) pore volume V selected from the range of 0.01 - 0.5 cm ³ /g. CO-pulse chemisorption-TPD was performed to test the CO adsorption uptake of the samples. Blank tests of CO pulses (with a time span of 10 min) were performed to serve as a comparison to estimate the CO uptake of the adsorbents.

Before introducing the CO gases, a H2 reduction step was performed on both FeNCl-AW-2P and FeNCl-AW at 500 °C for Ih (this temperature was chosen based on the H2-TPR results for FeNCl-AW-2P sample and as a comparison the same treatment was performed on FeNCl-AW sample), thus providing samples FeNCl-AW-2P-R and FeNCl- AW-500-R. Afterwards, the reactor was cooled down to room temperature and a series of CO pulses were injected. Subsequently, the gas was switched to Ar and a temperature-programmed- desorption (TPD) measurement was performed. In this step, the samples stayed in the reactor and were not exposed to air.

According to above-mentioned TPR profile, the temperature at which iron (III) is reduced to iron(II) in FeNCl-AW is 302 °C. Thus, a CO pulse chemisorption test was also performed after FeNCl-AW was reduced with a 10% FF/Ar mixture at 350°C, thus providing sample FeNCl-AW-350-R.

Fig. 6A-C schematically depict the ion current I against data points D (essentially corresponding to time), wherein Fig. 6A corresponds to FeCNl-AW-2P, Fig. 6B corresponds to FeNCl-AW-500-R, and Fig. 6C to FeNCl-AW-350-R, wherein for each line L6 corresponds to the blank, L7 corresponds to CO, and L8 corresponds to a baseline.

In the CO pulses experiments, two different types of CO adsorption behaviour were observed in the samples. FeNCl-AW-2P-R showed a stronger adsorption in the first CO pulse and a weaker adsorption in the subsequent pulses. The total CO uptake after 7 pulses is 11.84 nmol CO/mg FeNCl-AW-2P-R adsorbent. Strong adsorption is observed in the first two CO pulses of FeNCl-AW-500-R. Thus the sample shows higher CO uptake, up to 29.85 nmol CO/mg adsorbent after 7 pulses. For FeNCl-AW-350-R the total CO uptake after seven CO pulses is 5.86 nmol CO/ mg adsorbent.

The procedure to estimate the amount of CO adsorbed per mg of adsorbent is as follows:

1. Estimation of the baseline-peak area of the blank test and each peak in the sample.

2. Calculation of the differential peak area, corresponding to the adsorbed molar CO amount:

A l k.pcak ^— 4 blank ^— ^4k, sample Where iank is the average integrated peak area of CO pulses in absence of adsorbent, A sample corresponds to the area of a CO pulse in the presence of adsorbent.

3. A calibration constant cf, (mmol/unit area) can be derived from the blank measurement to convert between integral area and the injected amount of CO (7.5 pL in real experimental condition).

4. The amount of adsorbed CO per peak (Neo, ads) is calculated using the calibration constant (cf) together with the differential integral peak area A .pcak).

Neo, ads = cfA410 ⁶ [nmol]

5. The amount of CO adsorbed per sample (neo) is estimated by dividing by the grams of adsorbent employed in the experiment. neo = Nco,ad/m _a ds [nmol/mgads]

CO-Temperature Programmed Desorption (CO-TPD) took place right after the CO pulse chemisorption experiment (the gas was switched to Ar). The samples were heated from 25 °C to 300 °C (10°C/min). The amount of CO desorbed was 3.5 pL for FeNCl-AW- 2P-R and 7.6 pL for FeNCl-AW-500-R. This may correspond to about half a CO pulse and one CO pulse, respectively.

These measurements clearly show that both FeNCl-AW-2P-R and FeNCl-AW- 500-R can adsorb CO at room temperature. In order to gain further insight into the chemical nature of the iron compounds formed and to relate the structure to the CO adsorption process, Fe Mdssbauer spectroscopy was performed. The resulting spectra were fitted into three doublets. Each doublet was characterized by an isomer shift and a quadrupole splitting. The isomer shift (IS, mm/s) is the difference between the midpoint of the doublet and zero on the velocity scale and the quadrupole splitting (QS, mm/s) is the separation between the two components picks of a doublet. The values of the fitting parameters are summarized in Table 5, wherein each component is also assigned to a corresponding iron species:

The three doublets were assigned to Fe-N centers with different electronic and oxidation states. In FeNCl-AW-2P fresh sample, the first doublet shown in Table 5 (IS=0.36 mm/s, QS=0.96 mm/s) is assigned to Fe(II) low spin. The parameters are similar to the Fe-N4 structure with two axial ligands and Fe(II) in low spin state. The Mdssbauer parameters of this doublet remain almost unchanged after EE reduction treatment. Regarding the second configuration, the Fe center has been assigned to Fe(III) in low spin state. The original sample was dominated by the presence of Fe(III) sites (47%). The isomer and quadrupole shift of the second species increase after exposing the FeNCl-AW-2P sample to IC /oEE/Ar at 400°C for 2 hours, which indicates a change in oxidation state of this species. These parameters are similar to those FeN2+2 structure that have the Fe sites in their intermediate-spin state. It seems that this species was transferred from Fe(III) low spin to Fe(II) intermediate-spin states (S = 1) and Fe(II) low spin states after the H2 reduction. The last doublet was assigned to Fe(II) high spin in both original and reduced samples. The isomer shift of this structure is clearly larger than the one of the previous components. Its Mdssbauer parameters may be similar to those of N-FeN2+2 compounds.

The reduced FeNCl-AW-2P sample was exposed to different concentrations of CO flow, and the Mdssbauer spectrum was measured in CO atmosphere. Further, Mdssbauer spectra were taken after flushing Ar into the cell. Significant changes were observed after introducing CO and Ar.

The IS and QS values of the first Fe(II) low spin species decreased after adding CO into the cell. Those values are similar to the reported Fe-porphyrin complexes with CO coordinated to Fe(II) low spin centers. This species was not affected by the introduction of Ar into the cell, which suggests that the Fe(II) low spin sites have a strong affinity to the CO molecules. The second (intermediate-spin Fe(II)) and third (high-spin Fe(II)) contributions appear to be more affected by the CO and subsequent Ar introduction: a significant change of the IS and QS values can be observed due to CO addition. These parameters recovered after introducing Ar into the cell, suggesting that CO molecules were (relatively) weakly bound to those Fe sites.

Overall, all the results described above indicate that the CO molecules are indeed absorbed on the Fe sites of the samples.

Table 5 summarizes the results of the Mdssbauer spectroscopy experiments:

The above analysis suggests that the designed carbonaceous materials with atomically dispersed Fe coordinated to N have been synthesized successfully. This is achieved by a high temperature pyrolysis of a Fe, Zn co-doped zeolitic imidazolate framework. The XPS together with the Mdssbauer results indicate that the Fe atom is surrounded by 4 coordinated N atoms and the Fe centers are in +2 or +3 oxidation state. Meanwhile, the Fe(II) sites show different spin states. Fe(II) may be present in two different coordination modes in different spin states: FeN4 and FeN2+2. In the Fe-NU structure, the Fe centre is in its low spin (LS) state. Therefore, the d _xz , d _yz and dxy orbitals of the Fe atom may be fully filled, and the d _Z 2, d _X 2- _y 2 orbitals may remain empty. In particular, the empty d _Z 2 orbital may possess the appropriate symmetry to efficiently bind to CO molecules, thereby forming an c bond. In this case, CO may donate two electrons to the vacant d _Z 2 orbital of Fe. Meanwhile, the filled d _xz and d _yz orbitals of Fe may donate back its electrons to the orbitals of CO, especially thereby forming a 7t-bond. Hence, the FeN4 coordination state may facilitate a strong interaction between Fe(II) and CO.

If, however, iron is in its high spin(HS) state, its d _xz and d _yz orbitals may be partially filled and only (relatively) weak 7t-bonds may form between the metal atom and CO. Therefore, the Fe-N2+2 sites, which may have iron centres in high spin states, may (only) facilitate weak interactions with CO.

Hence, it may be preferable to have a (relatively) large proportion of Fe-NU sites. In particular, the FeNCl-AW sample may comprise (relatively) more Fe-NU sites, which may bind stronger to CO. When the FeNC sample is pyrolyzed twice (FeNCl-AW-2P), more Fe- N2+2 or N-Fe-N2+2 sites are created in the structure, which may lead to a weaker overall affinity for CO. Hence, in embodiments, the production method may comprise a single pyrolysis stage.

The amount of Fe(II) LS sites present in the samples as determined by CO pulses are shown in Table 6:

FeNC1-AW-2P FeNC1-AW

Fe(ll) doped (%) 0.066 0.167

The weight percentage of Fe(II) and Fe(III) on the external surface of the samples as determined by XPS analysis is shown in Table 7:

FeNC1-AW-2P FeNC1-AW-2P-R FeNC1-AW

Fe(ll) doped (%) 1.18 0.48 0.17

Fe(lll) doped (%) 0.94 0.48 0.14

The weight percentage of Fe(II) and Fe(III) for the bulk sample in FeNCl-AW- 2P were determined by a combination of ICP and Mdssbauer analyses, and are shown in Table 8:

FeNC1-AW-2P

Fe(ll) doped (%) _ 2.18 _

Fe(lll) doped (%) 1.93 Based on the Mdssbauer measurement (See above), the Fe(II) low-spin site may (primarily) absorb CO, and the spectral contribution of this site may be 56% in FeNCl-AW- 2P sample after the reduction treatment from the 100% CO-flushing experiment. Therefore, the Fe(II) low spin site, which is located in the sample as Fe(II)-N4, may have a weight percentage of 2.3 wt%.

Hence, in embodiments, the adsorbent 100 may comprise at least 0.03 wt% of Fe-NU sites, such as at least 0.1 wt%, especially at least 0.5 wt%. In further embodiments, the adsorbent 100 may comprise at least 1 wt% of Fe-NU sites, such as at least 1.5 wt%, especially at least 2 wt%.

In further embodiments, the adsorbent 100 may comprise at least 0.03 wt% of M-Nx sites, such as at least 0.1 wt%, especially at least 0.5 wt%. In further embodiments, the adsorbent 100 may comprise at least 1 wt% of M-N _x sites, such as at least 1.5 wt%, especially at least 2 wt%. In further embodiments, the M-N _x sites may especially comprise Cu-NU sites.

Cyclic voltammetry (CV), which investigates the reduction and oxidation processes of molecular species, was used to determine the reduction/oxidation potential of the iron sites in the material. The cyclic voltammetry measurements were conducted in a three- electrode-setup. The reference electrode is Ag/AgCl in KC1, and a platinum electrode was used as the counter electrode. The working electrode was a titanium plate with an area of 4 cm ² . The electrode was coated with the adsorbent using adhesive carbon glue (the influence of carbon glue for the CO adsorption process was tested, no CO adsorption was observed on the carbon glue). The electrochemical test was carried out in a potentiostat in 0.5 M H2SO4 aqueous solution and the I/V curves were obtained with a scanning potential from 0 to 1.0 V at a sweeping rate of 50 mVs' ¹ . The solution was degassed with N2 before the measurements. Fig. 7 schematically depicts the CV curves in current I (in A), vs. potential E (in V) vs. an Ag/AgCl reference electrode, wherein line L9 corresponds to FeNC-l-AW-2P, and wherein line L10 corresponds to FeNC-l-AW. In particular, redox peaks are observed in the CV curves with E1/2 = 0.48 V vs Ag/AgCl, which correspond to the Fe(II)/Fe(III) reduction/oxidation. Hence, the oxidation state of the Fe sites in the adsorbents can be tuned by applying a potential on the adsorbents.

Fig. 8 schematically depict CO adsorption on pellets (see characterization method 11) in adsorbed amount A (in mmol CO/g adsorbent material (excluding the gamma alumina)) versus CO pressure P (in kPa). In particular, in Fig. 8, line Li l corresponds to a CO isotherm of the pellets before reduction, and line L12 corresponds to a CO isotherm of the pellets after reduction. In particular, Fig. 8 schematically depicts that the pellets before reduction absorbed 0,07 mmol/g CO at 20 kPa and 0,275 mmol/g at 100 kPa, whereas the pellets after reduction adsorbed 0,19 mmol/g CO at 20 kPa and 0,50 mmol/g at 100 kPa. Hence, the reduction treatment may lead to improved CO adsorption.

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.