The invention relates to a dual function material (DFM), and more specifically a switchable DFM which may be used to capture carbon dioxide, and to catalyse the conversion of carbon dioxide into a reaction product. The invention also extends to a method of producing the switchable DFM and methods of using the switchable DFM.

Global warming is increasing at an alarming rate, as carbon dioxide (C0 ₂ ) emissions reached 33 Gt in 2021 [1]. The C0 ₂ concentration in the atmosphere has gone up from approximately 270 ppm in the pre-industrial era to 420 ppm in 2022, signifying a 50% increase [1-3]. C0 ₂ is associated with global warming and is considered the main greenhouse gas due to the global dependency on fossil fuels for energy, transportation, and industrial purposes. Efforts are continuously made to control greenhouse gas concentrations and thus the extent of global warming, leading to the Paris Agreement in 2015. Its goal was to hold the average temperature rise below 2°C above preindustrial levels and to take concerted action to keep the temperature increase below i.5°C above pre-industrial levels in order to reduce the anthropogenic impact on climate change [4]. Nonetheless, in 2021 the global average rise was i°C compared to 1880 as reported by NASA [5]. Consequently, the Intergovernmental Panel on Climate Change (IPCC) stresses the need to achieve net zero C0 ₂ emissions by 2050 and have a substantial decrease in C0 ₂ emissions after 2030 [6].

A solution to control C0 ₂ emissions is Carbon Capture and Storage (CCS). During this process, the C0 ₂ is captured from several industrial effluent streams, transported, injected into the lower earth layers, and stored in geological formations or into the oceans at great depths [7] . The current state-of-the-art technologies in terms of C0 ₂ capture includes the widely commercialised amine absorption and monoethanolamine (MEA) in particular. Due to their corrosive nature, amines have to be diluted with water (20-30% amine in water solution). In the regeneration step, during which it is necessary to increase the temperature in order to break the amine-C0 ₂ bonds, the high water content leads to a significant energy consumption [8-10]. Consequently, the high energy requirements of the CCS process are translated into high capital and operating costs, which, in turn, account for its slow commercialisation. More specifically, it is estimated that 75% of the total CCS cost is attributed to the C0 ₂ capture and compression [11]. An alternative to amine absorption is solid adsorption, e.g. metal organic frameworks MOFs, zeolites, and alkali/alkaline oxi des/ carbonates. During adsorption, C0 ₂ is either chemisorbed or physisorbed onto the adsorbent’s surface until breakthrough, or saturation, is reached. The desorption of a concentrated C0 ₂ stream usually occurs via a pressure or temperature swing process [8,12]. Another solution to control C0 ₂ emissions is Carbon Capture and Utilisation (CCU), during which the C0 ₂ is used to produce added-value chemicals and fuels instead of simply being sequestered. The CCU approach is considered a more active and sustainable way because C0 ₂ is an economically viable, safe, and renewable carbon source, which can be used as a Ci building block for the production of fuels and chemicals. These chemicals include synthetic natural gas, synthesis gas or syngas, urea, methanol, long chain hydrocarbons via Fischer-Tropsch synthesis, among many others. It is most commonly used in the production of urea (~i6o Mt/year). However, C0 ₂ being a highly stable molecule thermodynamically, it needs catalysts for its conversion into various products [11,13,14].

As far as the C0 ₂ catalytic upgrading routes are concerned, the dry reforming of methane (DRM) [15-17], the reverse water-gas shift (RWGS) [2,18-20] and the C0 ₂ methanation reactions [18,21,22] have attracted a lot of attention in the recent years. In DRM, two of the most harmful greenhouse gases react to obtain syngas, which is a mixture of carbon monoxide (CO) and hydrogen (H ₂ ). DRM can be used for biogas upgrading purposes because biogas is mainly a mixture of C0 ₂ and methane (CH ₄ ). Although research efforts are concentrated on finding the best catalytic formulation for DRM, deactivation via coke formation is inevitable at high operating temperatures [11,15]. RWGS, which is one of the C0 ₂ hydrogenation reactions, results in the formation of CO and water. CO is an important raw material for the production of methanol and hydrocarbons via the Fischer-Tropsch synthesis. Careful catalyst design is necessary, however, due to the occurrence of the forward reaction [11,13,18]. C0 ₂ methanation, which is another C0 ₂ hydrogenation reaction, can be used for the production of CH ₄ , or synthetic natural gas. This reaction attracted a lot of attention both in the 1970s, during the oil crisis, and nowadays, because of its possible use in power to gas schemes. Active catalysts are needed for C0 ₂ reduction and for overcoming significant kinetic limitations [11,13,23]. The aforementioned reactions are presented below.

DRM: C0 ₂ + CH ₄ 2CO + 2H ₂ AH298K = +247 kJ/mol (1)

RWGS: C0 ₂ + H ₂ CO + H ₂ 0 AH298K = +41 kJ/mol (2) C0 ₂ methanation: C0 ₂ + 4H2 CH ₄ + 2H ₂ 0 AH298K = -165 kJ/mol (3)

The use of Dual Function Materials (DFMs) for the integration of C0 ₂ capture and utilisation has been proposed recently as an alternative to the high energy demands and costs of CCU processes [24]. DFMs were introduced as a solution to the corrosive amines, the thermal swing process and the complex purification and transportation systems. They are designed to work in cyclic operation, i.e. in successive C0 ₂ capture and reduction cycles. They are composed of an adsorbent and a catalyst, both dispersed onto the same support. Consequently, DFMs are able to capture the C0 ₂ from an effluent stream and then to catalyse it to produce various chemicals based on the coreactant used. The adsorbents used are usually alkali/alkaline earth oxides/carbonates dispersed onto alumina oxide (A1 ₂ O ₃ ), resulting in an increased number of adsorption sites. DFMs make use of the mid-temperature chemisorbents, which can easily be regenerated by an inert gas purge stream. As regards the catalytic component, it depends on the targeted reaction. The most studied materials are the noble metals, like ruthenium (Ru) and rhodium (Rh) due to their increased catalytic activity in the DRM, RWGS and C0 ₂ methanation reactions. However, their high cost is prohibitive and efforts are made to use cheaper, but highly active, materials, like nickel (Ni) [25-28]. In a typical DFM configuration, as it was initially described for the first time in [29], two reactors are needed to run in parallel, one performing the C0 ₂ capture step and the other the C0 ₂ reduction step. Therefore, the DFMs are first exposed to a stream containing C0 ₂ until breakthrough is reached. Then, the co-reactant is introduced to the system and the C0 ₂ is spilled over from the adsorbents onto the catalyst and it is converted into the desired product [30,31].

The most studied reaction for the DFMs application to date is C0 ₂ methanation [25- 28]. This reaction offers an isothermal solution to the DFMs system because both C0 ₂ methanation and adsorption are exothermic processes. The exothermicity of the C0 ₂ methanation can supply the required heat for the C0 ₂ desorption and its spill-over onto the catalytic sites [24,31,32]. The development of DFMs in the RWGS and DRM is still in its infancy, but several studies in RWGS [33-37] and DRM [38-41] have shown great potential and are worthy of notice because the DFMs overcome the current limitations of cutting-edge technologies and can help in the development of carbonnegative technologies in the future. The present invention arose in attempting to overcome the problems associated with the prior art.

In accordance with a first aspect of the invention, there is provided a switchable dual function material (DFM) comprising: an adsorbent, configured to adsorb carbon dioxide; and a switchable catalyst configured to catalyse the conversion of carbon dioxide into a reaction product. Advantageously, the switchable DFM can be used to both capture carbon dioxide and convert it to an added-value chemical. This increases efficiency and lowers costs as carbon capture and conversion can be conducted in the same reactor. The inventors have also found that the presence of the catalyst synergistically improves the ability of the switchable DFM to capture carbon dioxide. Accordingly, the switchable DFM of the present invention can capture carbon dioxide from a plant exhaust and, surprisingly, can also passively capture carbon dioxide from atmospheric air. The claimed switchable DFM can therefore be used to provide carbon neutral or carbon negative processes, which are highly desirable. In some embodiments, the adsorbent defines a support, and the switchable catalyst is disposed on the support. In an alternative embodiment, the switchable DFM comprises a separate support, and the adsorbent and the switchable catalyst are both disposed on the support. It may be appreciated that in this embodiment, the support is separate to the adsorbent.

The switchable catalyst is preferably dispersed across the support.

The switchable catalyst disposed, and preferably dispersed, on the support may be configured to catalyse the conversion of carbon dioxide in the presence of a co-reactant.

The switchable catalyst may be configured to conduct the reduction of C0 ₂ .

The reduction of C0 ₂ may be conducted with a hydrocarbon as a co-reactant. In some embodiments, other co-reactants may be present. The other co-reactants may be water and/or oxygen. Accordingly, the switchable catalyst may be configured to conduct a dry reforming reaction, a bi-reforming reaction and/or a tri-reforming reaction. In some embodiments, the dry reforming reaction is a dry reforming of methane (DRM) reaction. Similarly, the bi-reforming reaction and/or a tri-reforming reaction may be a bi-reforming and/or tri-reforming reaction of methane. Alternatively, the dry reforming reaction, bi-reforming reaction and/or tri-reforming reaction maybe a dry reforming reaction, bi-reforming reaction and/or tri-reforming reaction of another hydrocarbon. Accordingly, the co-reactant may be or comprise a hydrocarbon. The hydrocarbon may be a C1-12 hydrocarbon, more preferably a Ci-6 hydrocarbon or a Ci- ₃ hydrocarbon. The hydrocarbon may be methane, ethane or propane. In some embodiments, the hydrocarbon is methane. The reaction product maybe or comprise carbon monoxide and hydrogen.

Alternatively, or additionally, the switchable catalyst may be configured to conduct a reverse water-gas shift (RWGS) reaction. Accordingly, the co-reactant may be or comprise hydrogen. The reaction product maybe or comprise carbon monoxide and water.

The switchable catalyst may be configured to conduct a C0 ₂ methanation reaction.

Accordingly, the co-reactant may be or comprise hydrogen. The reaction product may be or comprise methane and water.

The switchable catalyst may be configured to conduct oxidative dehydrogenation of a hydrocarbon. It may be appreciated that in this reaction the C0 ₂ could used as a soft oxidant. The co-reactant maybe or comprise a hydrocarbon. The hydrocarbon maybe a C _2-i2 hydrocarbon, a C ₂ -6 hydrocarbon or a C _2.3 hydrocarbon. The hydrocarbon may be ethane or propane. The reaction product may be or comprise carbon monoxide, an alkene and water. The alkene may have a corresponding number of carbons as the hydrocarbon used as the co-reactant. Accordingly, the alkene may be a C _{2 2} alkene, a C ₂ -6 hydrocarbon or a C _2.3 hydrocarbon. The alkene may be ethene or propene. The switchable catalyst may be configured to conduct a hydrogenation reaction of the C0 ₂ . The co-reactant may be H ₂ . The reaction product may be methanol and water.

A switchable catalyst may be understood to be a catalyst which can catalyse two or more different chemical reactions, wherein each reaction converts carbon dioxide into a product. The two or more different chemical reactions may be selected by varying the co-reactant and/or a reaction temperature. The two or more different chemical reactions may be two or more reactions selected from the group consisting of a dry reforming reaction; a C0 ₂ methanation; an RWGS reaction; a bi-reforming reaction; a tri-reforming reaction; a dehydrogenation reaction; and a hydrogenation reaction. More preferably, the two or more different chemical reactions may be two or more reactions selected from the group consisting of a dry reforming reaction; a C0 ₂ methanation; and an RWGS reaction. Preferably, the catalyst is able to switch between the different chemical reactions on demand.

In a preferred embodiment, the switchable catalyst is configured to be able to catalyse a dry reforming reaction and a C0 ₂ methanation. The switchable catalyst is preferably also configured to be able to catalyse an RWGS reaction.

The inventors have surprisingly found that the switchable DFMs described herein can switch between reactions by varying the co-reactant and/ or temperature, and maintain 100% selectivity. The inventors have also surprisingly found that despite the large variation in operating temperatures for the different reactions, the adsorbents in the DFMs are capable of functioning at the full range of temperatures over which the catalyst may be used. For instance, the DFM may be used to adsorb carbon dioxide and to catalyse the conversion of carbon dioxide into a reaction product at temperatures up to i,ooo°C. For instance, the DFM may be used to adsorb carbon dioxide and to catalyse the conversion of carbon dioxide into a reaction product at temperatures between too and i,ooo°C, between 250 and 75O°C or between 350 and 65O°C.

Advantageously, the same DFM can be used for two or three different reactions. This eliminates the need to have three different process and/or change materials when the process changes. Accordingly, the switchable DFM can be used in a more cost-effective manner than prior art catalysts and adsorbents. Furthermore, since the reaction may be varied if demand for the products changes, this ensures any plants incorporating the switchable DFMs of the invention are future proofed.

The switchable catalyst may comprise one or more metals and/or a metal phosphide.

The one or more metals may be or comprise one or more transition metals. The metal phosphide may comprise a transition metal phosphide. Accordingly, the switchable catalyst may comprise one or more of nickel (Ni), ruthenium (Ru), cerium (Ce), zirconium (Zr), iron (Fe), and/or an oxide or phosphide thereof. The catalyst preferably comprises nickel. The switchable DFM may comprise at least i wt%, at least 2 wt%, at least 4 wt%, at least 6 wt%, at least 8 wt%, at least 10 wt%, at least 12 wt%, at least 14 wt% or at least 15 wt% nickel. The switchable DFM may comprise less than 60 wt%, less than 50 wt%, less than 40 wt%, less than 30 wt%, less than 25 wt%, less than 20 wt%, less than 18 wt%, or less than 16 wt% nickel. The switchable DFM may comprise between 2 and 60 wt%, between 4 and 50 wt%, between 6 and 40 wt%, between 8 and 30 wt%, between 10 and 25 wt%, between 12 and 20 wt%, between 13 and 18 wt% or between 14 and 16 wt% nickel. The switchable DFM may comprise about 15 wt% nickel. In some embodiments, the catalyst consists of nickel. In alternative embodiments, the switchable catalyst comprises nickel and one or more additional components or promoters. The or each additional component or promoter may be a metal, and is preferably a transition metal. The transition metal may be ruthenium (Ru), cerium (Ce), zirconium (Zr) and/or iron (Fe).

In some embodiments, the switchable catalyst comprises an additional component or promoter, and the additional component or promoter is ruthenium. Accordingly, the switchable catalyst may comprise or consist of nickel (Ni) and ruthenium (Ru), more preferably a nickel ruthenium alloy.

Alternatively, or additionally, switchable catalyst comprises an additional component or promoter, and the additional component or promoter is iron. Accordingly, the switchable catalyst may comprise or consist of nickel (Ni) and iron (Fe), more preferably a nickel iron alloy.

In one embodiment, the switchable catalyst comprises two additional components or promoters. The additional components or promoters maybe ruthenium and iron. Accordingly, the switchable catalyst may comprise or consist of nickel (Ni), ruthenium (Ru) and iron (Fe), more preferably a nickel, ruthenium and iron alloy. The nickel and each additional component or promoter maybe present at any weight ratio. For instance, the nickel and the or each additional component or promoter may be present in a weight ratio of between 1:1 and 100:1, between 2:1 and 75:1, between 4:1 and 50:1, between 6:1 and 40:1, between 8:1 and 30:1, between 10:1 and 20:1, between 12:1 and 18:1 or between 14:1 and 16:1. The nickel and ruthenium maybe present in a weight ratio of about 15:1.

In an alternative embodiment, the switchable catalyst may comprise or consist of a nickel phosphide. Preferably, the nickel phosphide is a nickel rich nickel phosphide. Any ratio of nickel to phosphorus may be used. For instance, the molar ratio of nickel to phosphorous in the nickel phosphide maybe between 1:1 and 5:1, between 2:1 and 3:1 or between 11:5 and 13:5. The molar ratio of nickel to phosphorous in the nickel phosphide maybe about 12:5. Accordingly, the nickel phosphide maybe Nii ₂ P ₅ . Synthesis of a suitable catalyst is described in Zhang et al. (“Ni- Phosphide catalysts as versatile systems for gas-phase C0 ₂ conversion: Impact of the support and evidences of structure-sensitivity”, Fuel, 323 (2022) 124301).

The switchable catalyst may comprise at least 1 wt%, at least 2 wt%, at least 4 wt%, at least 6 wt%, at least 8 wt%, at least 10 wt%, at least 12 wt%, at least 14 wt% or at least 15 wt% of the switchable DFM. The switchable catalyst may comprise less than 60 wt%, less than 50 wt%, less than 40 wt%, less than 30 wt%, less than 25 wt%, less than 20 wt%, less than 18 wt%, or less than 17 wt% of the switchable DFM. The switchable catalyst may comprise between 2 and 60 wt% of the switchable DFM, between 4 and 50 wt% of the switchable DFM, between 6 and 40 wt% of the switchable DFM, between 8 and 30 wt% of the switchable DFM, between 10 and 25 wt% switchable DFM, between

12 and 20 wt% of the switchable DFM, between 14 and 18 wt% of the switchable DFM or between 15 and 17 wt% of the switchable DFM. The switchable catalyst may comprise about 16 wt% of the switchable DFM. The adsorbent is preferably dispersed across the support.

The adsorbent maybe or comprise a metal, an oxide of a metal, a carbonate of a metal, a metal organic framework, a zeolite, silica and/or carbon. The metal, the oxide of the metal or the carbonate of the metal may be an alkali metal, an alkaline earth metal, a transition metal, a p-block metal, a lanthanide or an oxide or carbonate thereof. In some embodiments, the adsorbent maybe or comprise an alkali metal, an alkaline earth metal and/ or an oxide or carbonate thereof. Accordingly, the adsorbent may be or comprise sodium oxide, potassium oxide and/or calcium oxide. In one preferred embodiment, the adsorbent comprises or consists of calcium oxide. The adsorbent may comprise at least i wt%, at least 2 wt%, at least 4 wt%, at least 6 wt%, at least 8 wt%, at least 9 wt% or at least 10 wt% of the switchable DFM. In some embodiments, the adsorbent may comprise at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt% or at least 80 wt% of the switchable DFM. Alternatively, the adsorbent may comprise less than 40 wt%, less than 30 wt%, less than 20 wt%, less than 15 wt%, less than 12 wt% or less than 11 wt% of the switchable DFM. Particularly in embodiments where the DFM comprises a separate support and adsorbent, the adsorbent may comprise less than 40 wt% of the DFM. The adsorbent may comprise between 2 and 40 wt% of the switchable DFM, between 4 and 30 wt% of the switchable DFM, between 6 and 20 wt% of the switchable DFM, between 8 and 15 wt% of the switchable DFM or between 9 and 11 wt% of the switchable DFM. The adsorbent may comprise about 10 wt% of the switchable DFM.

The support may comprise or consist of a metal, a metal oxide, silica, a metal organic framework, a zeolite or a structured carbon. The metal, metal oxide or metal organic framework may be a transition metal, a transition metal oxide, a transition metal organic framework, a p-block metal, a p-block metal oxide or a p-block metal organic framework. The support may comprise or consist of cerium, aluminium, zirconium, titanium, silicon and/or an oxide thereof. Accordingly, the support may comprise or consist of cerium(IV) oxide (Ce0 ₂ ), aluminium oxide (A1 ₂ O ₃ ), zirconium dioxide (Zr0 ₂ ), titanium dioxide (Ti0 ₂ ), and/or silica (Si0 ₂ ).

In some embodiments, the support comprises or consists Ce0 ₂ and A1 ₂ O ₃ . The support may comprise between 1 and 50 wt% Ce0 ₂ , between 5 and 40 wt% Ce0 ₂ , between 10 and 35 wt% Ce0 ₂ , between 15 and 30 wt% Ce0 ₂ , between 18 and 25 wt% Ce0 ₂ or between 19 and 21 wt% Ce0 ₂ . The support may comprise between 50 and 99 wt% A1 ₂ O ₃ , between 60 and 95 wt% A1 ₂ O ₃ , between 70 and 90 wt% A1 ₂ O ₃ or between 75 and 85 wt% A1 ₂ O ₃ .

Accordingly, the switchable catalyst may comprise or consist of Ni, a combination of Ni and Ru, a combination of Ni and Fe or a combination of Ni, Ru and Fe and the support may be or comprise Ce0 ₂ and A1 ₂ O ₃ . In alternative embodiments, the support comprises or consists Ce0 ₂ and Zr0 ₂ . The support may comprise between to and 95 wt% Ce0 ₂ , between 20 and 90 wt% Ce0 ₂ , between 30 and 80 wt% Ce0 ₂ , between 40 and 70 wt% Ce0 ₂ , between 50 and 65 wt% Ce0 ₂ or between 55 and 60 wt% Ce0 ₂ . The support may comprise between 5 and 90 wt% Zr0 ₂ , between 10 and 80 wt% Zr0 ₂ , between 20 and 70 wt% Zr0 ₂ , between 30 and 60 wt% Zr0 ₂ , between 35 and 50 wt% Zr0 ₂ or between 40 and 45 wt% Zr0 ₂ .

Accordingly, the switchable catalyst may comprise or consist of Ni, a combination of Ni and Ru, a combination of Ni and Fe or a combination of Ni, Ru and Fe and the support may be or comprise Ce0 ₂ and Zr0 ₂ .

The support may enhance the activity of the switchable catalyst and/or may also act as a catalyst.

The support may comprise at least 5 wt%, at least 10 wt%, at least 20 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt% or at least 73 wt% of the switchable DFM. The support may comprise less than 95 wt%, less than 90 wt%, less than 85 wt%, less than 80 wt% or less than 75 wt% of the switchable DFM. The support may comprise between 40 and 95 wt% of the switchable DFM, between 50 and 90 wt% of the switchable DFM, between 60 and 85 wt% of the switchable DFM, between 70 and 80 wt% of the switchable DFM or between 73 and 75 wt% of the switchable DFM. The adsorbent may comprise about 74 wt% of the DFM. The switchable DFM preferably has a surface area of at least too m ² /g, at least 120 m ² /g, at least 130 m ² /g, at least 140 m ² /g, at least 150 m ² /g, at least 160 m ² /g, at least 170 m ² /g, at least 180 m ² /g or at least 190 m ² /g. The switchable DFM may have a surface area between too and 400 m ² /g, between 120 and 350 m ² /g, between 140 and 300 m ² /g, between 160 and 250 m ² /g, between 180 and 220 m ² /g or between 190 and 200 m ² /g. The surface area may be calculated using the Brunauer-Emmett-Teller

(BET) equation, as described in the examples.

The switchable DFM is preferably porous. More preferably, the switchable DFM is a porous nanostructured material. The switchable DFM preferably has a pore volume of at least 0.2 cm3/g, at least 0.25 cm3/g, at least 0.3 cm3/g, at least 0.35 cm3/g or at least 0.4 cm3/g. The switchable DFM preferably has a pore volume of between 0.1 and 1.0 cm3/g, between 0.2 and 0.6 cm3/g, between 0.25 and 0.55 cm3/g, between 0.3 and 0.5 cm3/g, between 0.35 and 0.45 cm3/g or between 0.4 and 0.42 cm3/g. The pore volume maybe calculated using the Barett-Joyner-Halena (BJH) method. In accordance with a second aspect of the invention, there is provided a method of producing a switchable DFM, the method comprising: providing a support, wherein either the support is an adsorbent, and is configured to adsorb carbon dioxide, or the method comprises disposing an adsorbent on the support, wherein the adsorbent is configured to adsorb carbon dioxide; and disposing a switchable catalyst on the support, wherein the switchable catalyst is configured to catalyse the conversion of carbon dioxide into a reaction product; and thereby producing a switchable DFM.

The switchable DFM maybe a switchable DFM of the first aspect. Accordingly, the support, adsorbent and/or switchable catalyst maybe as defined in relation to the first aspect. In some embodiments, the method comprises disposing the adsorbent on the support.

The method may comprise disposing the adsorbent on the support prior to, at the same time or after disposing the switchable catalyst on the support. Preferably, the method comprises disposing the adsorbent on the support prior to disposing the switchable catalyst on the support.

Disposing the adsorbent on the support may comprise contacting the support with the adsorbent or an adsorbent precursor. Preferably, contacting the support with the adsorbent or an adsorbent precursor comprises contacting the support with a solution or suspension comprising a solvent and the adsorbent or adsorbent precursor, to provide a further suspension. The method may comprise subsequently drying the further suspension to remove the solvent therefrom and provide a support with the adsorbent or adsorbent precursor disposed thereon. The further suspension may be dried at an elevated temperature. The temperature maybe at least 3O°C, at least 4O°C, at least 6o°C, at least 8o°C, at least ioo°C or at least no°C. The temperature may be between 30 and 3OO°C, between 40 and 25O°C, between 60 and 200°C, between 80 and 175°C, between too and 15O°C or between no and 13O°C. The further suspension may be dried for at least i hour, at least 2 hours, at least 4 hours, at least 6 hours, at least 8 hours, at least 10 hours or at least 12 hours. It may be appreciated that in embodiments where the method comprises disposing the adsorbent on the support after disposing the switchable catalyst on the support then contacting the support with the adsorbent or adsorbent precursor may comprise contacting the support and switchable catalyst with the adsorbent or adsorbent precursor.

The solvent may be water.

In some embodiments, the method comprises contacting the support with a solution or suspension comprising the solvent and the adsorbent. The adsorbent maybe provided as a plurality of nanoparticles. Accordingly, the method may comprise contacting the support with a colloidal suspension comprising the solvent and the plurality of nanoparticles.

The adsorbent precursor may be a nitrate of an alkali metal, a nitrate of an alkaline earth metal, a carbonate of an alkali metal, a carbonate of an alkaline earth metal, a halide of an alkali metal, a halide of an alkaline earth metal, an acetate of an alkali metal or an acetate of an alkaline earth metal. The halide may be a chloride.

Accordingly, the adsorbent precursor maybe or comprise sodium nitrate (NaNO ₃ ), potassium nitrate (KNO ₃ ), calcium nitrate (Ca(NO ₃ ) ₂ ), sodium carbonate (Na ₂ CO ₃ ), potassium carbonate (K ₂ CO ₃ ), calcium carbonate (CaCO ₃ ), sodium chloride (NaCl), potassium chloride (KC1) and/or calcium chloride (CaCl ₂ ).

In embodiments which comprise contacting the support with an adsorbent precursor, the method may comprise calcining the adsorbent precursor to provide the adsorbent. The method may comprise contacting the support with the adsorbent precursor, and subsequently calcining the adsorbent precursor to provide the adsorbent. Calcining the adsorbent precursor may comprise heating the support with the adsorbent precursor disposed thereon to an elevated temperature. The elevated temperature may be at least ioo°C, at least 15O°C, at least 200 °C, at least 25O°C, at least 3OO°C, at least 325 °C, at least 35O°C or at least 375°C. The elevated temperature may be between too and i,ooo°C, between 150 and 8oo°C, between 200 and 7OO°C, between 250 and 6oo°C, between 300 and 500°C, between 325 and 475°C, between 350 and 45O°C or between 375 and 425°C. The support with the adsorbent precursor disposed thereon maybe heated to the elevated temperature for at least 30 minutes, at least 1 hour, at least 2 hours, at least 3 hours or at least 4 hours. The support with the adsorbent precursor disposed thereon may be heated to the elevated temperature for between 15 minutes and 48 hours, between 30 minutes and 24 hours, between 1 and 12 hours, between 2 and 6 hours, between 3 and 5 hours or between 3.5 and 4.5 hours.

In embodiments where the method comprises drying the support, the method may comprise calcining the adsorbent precursor after drying the support.

The amount of the adsorbent precursor used maybe sufficient to provide the resultant adsorbent at an amount as defined in the first aspect. Disposing the switchable catalyst on the support may comprise contacting the support with the switchable catalyst or a switchable catalyst precursor. Preferably, contacting the support with the switchable catalyst or switchable catalyst precursor comprises contacting the support with a solution or suspension comprising a solvent and the switchable catalyst or switchable catalyst precursor, to thereby provide a further suspension. The method may comprise subsequently drying the further suspension to remove the solvent therefrom and provide a support with the switchable catalyst or switchable catalyst precursor disposed thereon. The further suspension may be dried at an elevated temperature. The temperature may be at least 3O°C, at least 4O°C, at least 6o°C, at least 8o°C, at least ioo°C or at least no°C. The temperature may be between 30 and 3OO°C, between 40 and 25O°C, between 60 and 200°C, between 80 and 175°C, between too and 15O°C or between 110 and 13O°C. The further suspension may be dried for at least 1 hour, at least 2 hours, at least 4 hours, at least 6 hours, at least 8 hours, at least 10 hours or at least 12 hours. It may be appreciated that in embodiments where the method comprises disposing the switchable catalyst on the support after disposing the adsorbent on the support then contacting the support with the switchable catalyst or switchable catalyst precursor may comprise contacting the support and absorbent with the switchable catalyst or switchable catalyst precursor.

The solvent may be water. In some embodiments, the method comprises contacting the support with a solution or suspension comprising the solvent and the switchable catalyst. The switchable catalyst maybe provided as a plurality of nanoparticles. Accordingly, the method may comprise contacting the support with a colloidal suspension comprising the solvent and the plurality of nanoparticles.

The switchable catalyst precursor may be a transition metal nitrate, a transition metal nitrosyltrinitrate, a transition metal halide, a transition metal acetate, a transition metal carbonate and/or a hydrogen phosphate. The halide may be a chloride. Accordingly, the switchable catalyst precursor may be or comprise at least one of nickel(II) nitrate (NiNO ₃ ), ruthenium(III) nitrosyltrinitrate (Ru(NO)(NO ₃ ) ₃ ), iron nitrate (Fe(NO ₃ ) ₃ ), nickel chloride (NiCl ₂ ), ruthenium chloride (RUC1 ₃ ), iron chloride (FeCl ₃ ), nickel acetate, ruthenium acetate, iron acetate, nickel carbonate, ruthenium carbonate, iron carbonate and/or diammonium hydrogen phosphate [(NH ₄ ) ₂ HPO ₄ ]. In some embodiments, the switchable catalyst precursor comprises a combination of nickel(II) nitrate (NiNO ₃ ) and ruthenium(III) nitrosyltrinitrate (Ru(NO)(NO ₃ ) ₃ ). In some embodiments, the switchable catalyst precursor comprises a combination of nickel(II) nitrate (NiNO ₃ ) and iron nitrate (Fe(NO ₃ ) ₃ ). In some embodiments, the switchable catalyst precursor comprises a combination of nickel(II) nitrate (NiNO ₃ ), ruthenium(III) nitrosyltrinitrate (Ru(NO)(NO ₃ ) ₃ ) and iron nitrate (Fe(NO ₃ ) ₃ ). In alternative embodiments, the switchable catalyst precursor comprises a combination of nickel(II) nitrate (NiNO ₃ ) and diammonium hydrogen phosphate [(NH ₄ ) ₂ HPO ₄ ]. In embodiments which comprise contacting the support with a switchable catalyst precursor, the method may comprise calcining the switchable catalyst precursor to provide the switchable catalyst. The method may comprise contacting the support with the switchable catalyst precursor, and subsequently calcining the switchable catalyst precursor to provide the switchable catalyst. Calcining the switchable catalyst precursor may comprise heating the support with the switchable catalyst precursor disposed thereon to an elevated temperature. The elevated temperature may be at least ioo°C, at least 15O°C, at least 200 °C, at least 25O°C, at least 3OO°C, at least 35O°C, at least 4OO°C, at least 45O°C or at least 475°C. The elevated temperature maybe between too and i,ooo°C, between 150 and 9OO°C, between 200 and 8oo°C, between 250 and 7OO°C, between 400 and 6oo°C, between 550 and 55O°C or between 475 and 525°C. The support with the switchable catalyst precursor disposed thereon maybe heated to the elevated temperature for at least 30 minutes, at least 1 hour, at least 2 hours, at least 3 hours or at least 4 hours. The support with the adsorbent precursor disposed thereon may be heated to the elevated temperature for between 5 minutes and 48 hours, between 15 minutes and 24 hours, between 30 minutes and 12 hours, between 1 and 6 hours, between 2 and 4 hours or between 2.5 and 3.5 hours.

In accordance with a third aspect, there is provided use of the switchable DFM of the first aspect to capture carbon dioxide. In accordance with a fourth aspect, there is provided use of the switchable DFM of the first aspect to convert carbon dioxide into a reaction product.

In accordance with a fifth aspect, there is provided a method of capturing carbon dioxide, the method comprising contacting a gas comprising carbon dioxide and the switchable DFM of the first aspect.

The carbon dioxide maybe present in the gas at a concentration of at least 0.01 vol%, at least 0.02 vol% or at least 0.04 vol%. Accordingly, the gas comprising carbon dioxide may be atmospheric gas. Accordingly, the carbon dioxide may be present in the gas at a concentration of less than 80 vol%, less than 60 vol%, less than 40 vol%, less than 20 vol%, less than 10 vol%, less than 5 vol %, less than 1 vol%, less than 0.1 vol% or less than 0.05 vol%. The carbon dioxide may be present in the gas at a concentration of between 0.0001 and 40 vol%, between 0.0005 and 20 vol%, between 0.001 and 10 vol%, between 0.005 and 5 vol%, between 0.01 and 1 vol%, between 0.02 and 0.1 vol% or between 0.04 and 0.05 vol%.

Alternatively, the carbon dioxide may be present in the gas at a concentration of at least 0.1 vol%, at least 0.5 vol%, at least 1 vol%, at least 2 vol%, at least 4 vol%, at least 6 vol%, at least 8 vol% or at least 10 vol%. For instance, the gas maybe exhaust gas produced by an industrial plant. The carbon dioxide may be present in the gas at a concentration between 0.01 and 80 vol%, between 0.02 and 70 vol%, between 0.04 and 60 vol%, between 0.1 and 50 vol%, between 0.5 and 40 vol%, between 1 and 30 vol%, between 2 and 25 vol%, between 4 and 20 vol%, between 6 and 15 vol%, between 8 and 12 vol% or between 9 and 11 vol%. The method may comprise contacting the gas comprising carbon dioxide and the switchable DFM at room temperature. Alternatively, the method may comprise contacting the gas comprising carbon dioxide and the switchable DFM at an elevated temperature. Accordingly, the method may comprise contacting the gas comprising carbon dioxide and the switchable DFM at a temperature between o and i,5OO°C, between 20 and i,25O°C, between 50 and i,ooo°C, between too and 9OO°C, between 150 and 85O°C, between 200 and 8oo°C, between 250 and 75O°C, between 300 and 7OO°C or between 350 and 65O°C. In one embodiment, the method may comprise contacting the gas comprising carbon dioxide and the switchable DFM at a temperature between too and 6oo°C, between 150 and 55O°C, between 200 and 5OO°C, between 250 and 45O°C, between 300 and 4OO°C or between 325 and 375°C. In an alternative embodiment, the method may comprise contacting the gas comprising carbon dioxide and the switchable DFM at a temperature between 350 and i,ooo°C, between 400 and 9OO°C, between 450 and 85O°C, between 500 and 8oo°C, between 550 and 75O°C, between 600 and 7OO°C or between 625 and 675°C.

The method may be conducted under steady state, dynamic conditions or a combination thereof.

The method may be considered to be conducted under steady state if it is conducted at a fixed temperature. For instance, it may be appreciated that if the method is conducted at a fixed temperature then the temperature may remain substantially the same while the gas comprising carbon dioxide and the switchable DFM are contacted. The temperature may be understood to stay substantially the same if it does not vary by more than ioo°C, more preferably if it does not vary by more than 5O°C, if it does not vary by more than 25°C or if it does not vary by more than io°C. Alternatively, the method may be considered to be conducted under dynamic conditions if the temperature varies while the gas comprising carbon dioxide and the switchable DFM are contacted. The temperature may either increase or decrease as the gas comprising carbon dioxide and the switchable DFM are contacted. Preferably, the temperature increases as the gas comprising carbon dioxide and the switchable DFM are contacted. The temperature may vary at a rate of at least o.i°C/min, at least o.5°C/min, at least i°C/min, at least 2.5°C/min, at least 5°C/min, at least 7.5°C/min or at least io°C/min. The temperature may vary at a rate of between o.i and 50°C/min, between 0.5 and 40°C/min, between 1 and 30°C/min, between 2.5 and 20°C/min, between 5 and i5°C/min or between 7.5 and i2.5°C/min. When the method is conducted under dynamic conditions, the entire method may be conducted within a temperature range as defined above.

The captured carbon dioxide may be released from the switchable DFM. The captured carbon dioxide may be released by exposing the DFM to different conditions, such as purging with steam. The resulting carbon dioxide maybe stored. Alternatively, the carbon dioxide may be used in industry without the need for conversion. In particular, due to having been captured by the DFM, the carbon dioxide has been purified and so could be a valuable product in its own right. For instance, the purified carbon dioxide may be used in the beverage industry.

Alternatively, the carbon dioxide may be converted into a reaction product.

In accordance with a sixth aspect, there is provided a method of converting carbon dioxide into a reaction product, the method comprising: - providing a switchable DFM, as defined in the first aspect, loaded with carbon dioxide; and contacting the switchable DFM loaded with carbon dioxide and a co-reactant, thereby causing the carbon dioxide to react with the co-reactant to produce the reaction product.

Providing the switchable DFM loaded with carbon dioxide may comprise conducting the method of the fifth aspect. The step of contacting the switchable DFM loaded with carbon dioxide and the co-reactant may be conducted subsequent to the step of contacting a gas comprising carbon dioxide and the switchable DFM.

The method may comprise selecting a specific co-reactant and/or reaction temperature to provide a desired reaction product.

The co-reactant may be selected from the group consisting of a hydrocarbon; hydrogen; water and oxygen. The hydrocarbon may be as defined above. It maybe appreciated that when the co-reactant is a hydrocarbon, and optionally also water and/or oxygen, the switchable catalyst may be used to conduct a dry reforming reaction, a bi-reforming reaction, a tri-reforming reaction or dehydrogenation. The reaction product for the dry reforming reaction would be carbon monoxide and hydrogen. The reaction product for the dehydrogenation reaction would be carbon monoxide, an alkene and water. The alkene may be as defined above.

Alternatively, when the co-reactant is hydrogen, the switchable catalyst may be used to conduct the RWGS, C0 ₂ methanation or hydrogenation reactions. The reaction product for the RWGS reaction would be carbon monoxide and water. The reaction product for the C0 ₂ methanation reaction would be methane and water. The reaction product for the hydrogenation reaction would be methanol and water.

Accordingly, the reaction product may be selected from the group consisting of carbon monoxide; hydrogen; carbon monoxide; methane; water; an alkene; methanol; and a combination thereof. In one embodiment, the reaction product is carbon monoxide and hydrogen. In an alternative embodiment, the reaction product is carbon monoxide and water. In a further alternative embodiment, the reaction product is methane and water. In a further alternative embodiment, the reaction product is methanol and water.

The method may comprise contacting the switchable DFM loaded with carbon dioxide and a gas comprising the co-reactant. The co-reactant maybe present in the gas at a concentration of at least 0.1 vol%, at least 0.5 vol%, at least 1 vol%, at least 2 vol%, at least 4 vol%, at least 6 vol%, at least 8 vol% or at least 10 vol%. The co-reactant may be present in the gas at a concentration between 0.01 and 80 vol%, between 0.02 and 70 vol%, between 0.04 and 60 vol%, between 0.1 and 50 vol%, between 0.5 and 40 vol%, between 1 and 30 vol%, between 2 and 25 vol%, between 4 and 20 vol%, between 6 and 15 vol%, between 8 and 12 vol% or between 9 and 11 vol%.

The gas may comprise or consist of the co-reactant and nitrogen. Accordingly, nitrogen maybe present in the gas at a concentration between 1 and 99.9 vol%, between 10 and 99.5 vol%, between 20 and 99 vol%, between 30 and 98 vol%, between 40 and 97 vol%, between 50 and 96 vol%, between 60 and 95 vol%, between 70 and 94 vol%, between 80 and 93 vol%, between 85 and 92 vol% or between 89 and 91 vol%. The method may comprise contacting the switchable DFM loaded with carbon dioxide and the co-reactant at an elevated temperature. Accordingly, the method may comprise contacting the gas comprising carbon dioxide and the switchable DFM at a temperature between o and i,5OO°C, between 20 and i,25O°C, between 50 and i,ooo°C, between too and 9OO°C, between 150 and 85O°C, between 200 and 8oo°C, between 250 and 75O°C, between 300 and 7OO°C, between 350 and 65O°C or between 400 and 6oo°C.

In one embodiment, the method may comprise contacting the switchable DFM loaded with carbon dioxide and the co-reactant at a temperature between too and 6oo°C, between 150 and 55O°C, between 200 and 5OO°C, between 250 and 45O°C, between 300 and 4OO°C or between 325 and 375°C. In one embodiment, the temperature may be about 35O°C.

In an alternative embodiment, the method may comprise contacting the switchable DFM loaded with carbon dioxide and the co-reactant at a temperature between 350 and i,ooo°C or between 400 and 9OO°C. The method may comprise contacting the switchable DFM loaded with carbon dioxide and the co-reactant at a temperature between 450 and 85O°C, between 500 and 8oo°C, between 550 and 75O°C, between 600 and 7OO°C or between 625 and 675°C. The temperature maybe about 65O°C. Alternatively, the method may comprise contacting the switchable DFM loaded with carbon dioxide and the co-reactant at a temperature between 400 and 8oo°C, between 450 and 75O°C, between 500 and 7OO°C, between 550 and 65O°C or between 575 and 625°C. The temperature maybe about 6oo°C. In a further alternative embodiment, the method may comprise contacting the switchable DFM loaded with carbon dioxide and the co-reactant at a temperature between 450 and i,ooo°C, between 500 and 95O°C, between 600 and 9OO°C, between 650 and 85O°C or between 700 and 8oo°C. In a still further embodiment, the method may comprise contacting the switchable DFM loaded with carbon dioxide and the co-reactant at a temperature between 75 and 6oo°C, between too and 55O°C, between 125 and 5OO°C, between 150 and 45O°C, between 175 and 35O°C or between 200 and 3OO°C. The method may be conducted under steady state, dynamic conditions or a combination thereof. The method may be considered to be conducted under steady state if it is conducted at a fixed temperature. For instance, it may be appreciated that if the method is conducted at a fixed temperature then the temperature may remain substantially the same while the switchable DFM loaded with carbon dioxide and the co-reactant are contacted. The temperature may be understood to stay substantially the same if it does not vary by more than ioo°C, more preferably if it does not vary by more than 5O°C, if it does not vary by more than 25°C or if it does not vary by more than io°C. Alternatively, the method may be considered to be conducted under dynamic conditions if the temperature varies while the switchable DFM loaded with carbon dioxide and the co-reactant are contacted. The temperature may either increase or decrease as the switchable DFM loaded with carbon dioxide and the co-reactant are contacted. Preferably, the temperature increases as switchable DFM loaded with carbon dioxide and the co-reactant are contacted. The temperature may vary at a rate of at least o.i°C/min, at least o.5°C/min, at least i°C/min, at least 2.5°C/min, at least 5°C/min, at least 7.5°C/min or at least io°C/min. The temperature may vary at a rate of between 0.1 and 50°C/min, between 0.5 and 40°C/min, between 1 and 30°C/min, between 2.5 and 20°C/min, between 5 and i5°C/min or between 7.5 and i2.5°C/min.

When the method is conducted under dynamic conditions, the entire method may be conducted within a temperature range as defined above.

The switchable DFM loaded with carbon dioxide and the co-reactant may be contacted at atmospheric pressure. Alternatively, the switchable DFM loaded with carbon dioxide and the co-reactant maybe contacted at an elevated pressure. The elevated pressure maybe at least 2 bar, at least 5 bar, at least 10 bar, at least 20 bar, at least 30 bar, at least 40 bar or at least 50 bar. The switchable DFM loaded with carbon dioxide and the co-reactant maybe contacted at a pressure between 0.01 and 1,000 bar, between 0.1 and 100 bar or between 0.5 and 55 bar. In one embodiment, the switchable DFM loaded with carbon dioxide and the co-reactant maybe contacted at a pressure between 0.5 and 40 bar, between 0.6 and 20 bar, between 0.7 and 10 bar, between 0.8 and 5 bar or between 0.9 and 2 bar. In an alternative embodiment, the switchable DFM loaded with carbon dioxide and the co-reactant may be contacted at a pressure between 10 and 90 bar, between 20 and 80 bar, between 30 and 70 bar, between 40 and 60 bar or between 45 and 55 bar. The inventors have found that by selecting the co-reactant and the reaction temperature it is possible to maintain 100% selectivity of the reaction which is carried out, and the reaction product which is produced. In particular, when the reaction is carried out at 35O°C, and the co-reactant is hydrogen, the inventors have found that the C0 ₂ methanation reaction is conducted. Conversely, when the reaction is carried out at 65O°C, and the co-reactant is hydrogen, the inventors have found that the RWGS reaction is conducted. When the reaction is carried out between 400 and 65O°C, for instance at 65O°C, and the co-reactant is a hydrocarbon, the inventors have found that the dry reforming reaction is conducted. The inventors note that if the reaction were conducted between 700 and 8oo°C, and the co-reactant is a hydrocarbon and water then the bi-reforming reaction would take place. Similarly, if the reaction were conducted between 700 and 8oo°C, and the co-reactant is a hydrocarbon, water and oxygen then the tri-reforming reaction would take place. If the reaction were conducted at about 6oo°C and the co-reactant was a hydrocarbon, then the dehydrogenation reaction would take place. If the reaction were conducted between 2OO-3OO°C, optionally at an elevated pressure, and the co-reactant were hydrogen, then the hydrogenation reaction would take place. As explained above, since the reaction which is conducted may be varied if demand for the products changes, this ensures any plants incorporating the DFMs of the invention are future proofed.

The method may be repeated. Accordingly, the method may comprise: contacting a gas comprising carbon dioxide and the switchable DFM of the first aspect; - subsequently contacting the switchable DFM loaded with carbon dioxide and a co-reactant, thereby causing the carbon dioxide to react with the co-reactant to produce the reaction product; and subsequently repeating the above two steps at least once. Accordingly, the step of contacting a gas comprising carbon dioxide and the switchable DFM and the step of contacting the switchable DFM loaded with carbon dioxide and the co-reactant may be alternated. It may be understood that the method may comprise alternating streams of reactants (i.e. the gas comprising carbon dioxide and the co-reactant). The method could be repeated, by alternating the streams of reactants, multiple times. Accordingly, the method could be conducted continuously for at least a day, a week, a month or a year. It is noted that deactivation of a catalyst is a problem suffered by the prior art. Prior art steady state systems are not designed to cope with alternating streams of gases.

Instead, these catalysts tend to have a reactor feed comprising carbon dioxide and a co- reactant, delivered together. In these prior art systems, if a catalyst deactivates then the steady state operation must be interrupted to reactivate the catalyst. This is not desirable because it is expensive to shut down/stop/divert/delay the process.

However, due to the fact that the present method is designed to cope with alternating streams of gases, a catalyst can advantageously be reactivated without the need to shut down the entire system.

Accordingly, the method may comprise: contacting the DFM with a reactivating gas stream, and thereby regenerating and/or reactivating the catalyst.

The reactivating gas stream may be or comprise hydrogen and/or water. It may be appreciated that water will be in the form of steam. The above step may be conducted at the pressure and/ or temperature ranges described above.

The method may comprise sensing the activity of the DFM. Sensing the activity of the DFM may comprise sensing the concentration of the reaction product in an output gas stream. The above regenerating and/ or reactivating step may be conducted when the activity of the DFM falls below a predetermined activity. For instance, the above regenerating and/ or reactivating step may be conducted when the concentration of the reaction product in an output gas stream falls below a predetermined concentration. Subsequent to conducting the regenerating and/or reactivating step, the repeating steps of contacting the DFM with carbon dioxide and subsequently contacting the DFM with the co-reactant may be resumed.

All features described herein (including any accompanying claims, abstract and drawings), and/ or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: -

Figure 1 is a simplified process flow diagram for experimental set up;

Figure 2 shows nitrogen adsorption-desorption isotherms of supported adsorbents on the left-hand side and of dual functional materials (DFMs) on the right-hand side; Figure 3 shows X-ray diffraction (XRD) patterns of fresh supported adsorbents on the left-hand side and of DFMs on the right-hand side;

Figure 4 shows XRD patterns of reduced DFMs;

Figure 5 shows the H ₂ -TPR profiles of NRNa, NRK and NRCa

Figure 6 shows the CH ₄ , H ₂ 0, CO and C0 ₂ signals (m/z=i5, 18, 28, 44) during H ₂ -TPR for NRNa, NRK, NRCa and NR;

Figure 7 shows C0 ₂ -TPD profiles of NRNa, NRK and NRCa on the left-hand side and of Na, K and Ca on the right-hand side;

Figure 8 shows C0 ₂ -TPD profile of NR;

Figure 9 shows C0 ₂ Adsorption/desorption cycles of NRNa, NRK and NRCa at 350, 450, 550 and 650 °C’

Figure 10 shows TGA adsorption-desorption results at different temperatures for

NRNa, NRK and NRCa;

Figure 11 shows C0 ₂ Adsorption/desorption cycles of Na, K and Ca at 350 and 65O°C;

Figure 12 shows TGA adsorption-desorption results at different temperatures for Na, K and Ca;

Figure 13 shows TGA adsorption-desorption results at different temperatures for NR;

Figure 14 shows the results of feasibility study of flexible DFMs showing the amount of C0 ₂ desorbed and products formation in each reaction for NRNa, NRK and NRCa;

Figure 15 shows the C0 ₂ capture and C0 ₂ methanation, followed by the RWGS and DRM for NRNa;

Figure 16 shows the C0 ₂ capture and C0 ₂ methanation, followed by the RWGS and DRM for NRK;

Figure 17 shows the C0 ₂ capture and C0 ₂ methanation, followed by the RWGS and DRM for NRCa; Figure 18 shows XRD for post reaction NRNa, NRK and NRCa;

Figure 19 shows TGA for post reaction NRNa, NRK and NRCa; and Figure 20 shows CH ₄ -Temperature Programmed Surface Reaction (CH ₄ -TPSR) profile of NiRuCa: A) H ₂ 0, B) CH ₄ , C) C0 ₂ , D) CO and E) H ₂ signals.

Example 1 - Dual Function Materials synthesis The three DFMs described herein were prepared by sequential impregnation. The adsorbents were impregnated onto the Ce0 ₂ - A1 ₂ O ₃ support and then, the two transition metals, Ni and Ru, were impregnated onto the supported adsorbents, as the inventors have found that this order of impregnation resulted in a better DFM performance. Therefore, the three supported adsorbents were prepared by initially mixing the required amounts of the Ce0 ₂ - A1 ₂ O ₃ support (SCFa-160 Ce20 Puralox, Sasol) and NaNO ₃ (Fluka), KNO ₃ (Sigma Aldrich), and Ca(N0 ₃ ) ₂ -4H ₂ 0 (Sigma Aldrich) with distilled water. The suspensions were then mixed at room temperature with a magnetic stirrer and the excess water was removed in a rotary evaporator under reduced pressure. Afterwards, they were dried overnight at 12O°C and calcined at 4OO°C for 4 hours (5°C/min). The amount of the adsorbents’ precursors used was such that the final DFMs would have 10 wt.% of their respective oxides, and thus the resulting supported adsorbents were:

• N a ₂ 0 / Ce0 ₂ - A1 ₂ O ₃ (identified hereinafter as “Na”),

• K ₂ 0/Ce0 ₂ - A1 ₂ O ₃ (identified hereinafter as “K”), and • Ca0/Ce0 ₂ - A1 ₂ O ₃ (identified hereinafter as “Ca”).

The supported adsorbents had 11.9 wt.% Na ₂ 0, K ₂ 0, and CaO, respectively.

A similar impregnation procedure was performed when synthesising the three DFMs. The required amounts of Ni(N0 ₃ ) ₂ -6H ₂ 0 (Acros Organics) and Ru(NO)(NO ₃ ) ₃ solution (1.5 w/v Ru, Alfa Aesar) were mixed with the supported adsorbents in order to obtain 15 wt.% Ni and 1 wt.% Ru. These were mixed with excess distilled water, which was then removed in a rotary evaporator under reduced pressure, dried overnight at 12O°C, and calcined at 5OO°C for 3 hours (5°C/min). As a result, the prepared DFMs were: • 15 wt.% Ni, 1 wt.% Ru - 10 wt.% Na ₂ 0/Ce0 ₂ - A1 ₂ O ₃ (identified hereinafter as

“NRNa”),

• 15 wt.% Ni, 1 wt.% Ru - 10 wt.% K ₂ 0/Ce0 ₂ - A1 ₂ O ₃ (identified hereinafter as “NRK”), and

• 15 wt.% Ni, 1 wt.% Ru - 10 wt.% Ca0/Ce0 ₂ - A1 ₂ O ₃ (identified hereinafter as “NRCa”). In some experiments, the 15 wt.% Ni, 1 wt.% Ru/Ce0 ₂ - A1 ₂ O ₃ (identified hereinafter as “NR”), whose synthesis method was described in [42], was used as a reference material so as to show the impact of the addition of the adsorbents.

Example 2 - Textural properties

The textural properties of the supported adsorbents and DFMs are presented in Table 1, below. Table 1: Textural properties of the supported adsorbents and DFMs

It appears that the addition of Ni and Ru during impregnation did not cover the supported adsorbents and that high specific surface area materials were formed instead. Figure 2 shows the N ₂ adsorption-desorption isotherms of the materials. The isotherms generated corresponded to the type IV isotherms with a characteristic Hi hysteresis loop, according to IUPAC classification. This type of isotherms is linked to well-developed cylindrical mesoporous materials and the high steepness was indicative of the mesopores being homogeneously distributed throughout the structure of the samples, as observed in SEM as well [43,44].

Example - Crystalline structure

Figure 3 shows the crystalline phases of the fresh materials. All the supported adsorbents and DFMs had the characteristic peaks of y-Al ₂ O ₃ and Ce0 ₂ phases (JCPDS 00-004-0880 and JCPDS 03-065-5923, respectively). No peaks corresponding to any species of adsorbents appeared in the XRD patterns, meaning that the adsorbents were amorphous and/or highly dispersed [45,46]. In the fresh Na, K, and NRNa samples, some residual nitrates peaks were observed, which eventually disappeared upon reduction, as seen in Figure 4. Furthermore, in the fresh DFMs patterns, the peaks located at 20°=37.2°, 43.3 ⁰ , 62.9°, and 75.4° (JCPDS 00-047-1049) were ascribed to NiO. The existence of nickel aluminate (NiAl ₂ O ₄ ) spinels was discarded due to the low calcination temperature used [47]. Small peaks of Ru0 ₂ were also observed in the fresh samples (JCPDS 01-070-2662). No evidence of a Ni-Ru alloy was detected in the fresh DFMs.

Upon reduction at 8oo°C, as observed in Figure 4, the Ce0 ₂ peaks either disappeared or were significantly reduced. As a result, it was concluded that surface Ce0 ₂ species were mainly present in the samples, which was in agreement with the H ₂ -TPR results [48]. No ceria aluminate (CeA10 ₃ ) species were seen due to the reduction temperature being 8oo°C [43]. Metallic Ni peaks were also detected in the reduced DFMs XRD patterns (JCPDS 01-070-1849). In contrast, metallic Ru peaks were not observed, which led us to believe that Ru was well dispersed and/ or a Ni-Ru alloy was formed; however, due to the low Ru weight content (1 wt.%) compared to Ni (15 wt.%), no shift of Ni peaks took place in the XRD patterns [49].

Example 4 - Morphological structure The morphological structure of the fresh supported adsorbents and the DFMs was observed in SEM. This showed that after calcination, the materials were porous, which was in accordance with the BET results. The EDX mappings indicated that the materials were successfully impregnated as dispersed and homogenous materials were created. The high dispersion, porosity, and homogeneity of the DFMs were significant parameters that contributed to their impressive performance, as described later, due to the need to have a plethora of adsorption and catalytic sites in close proximity in order for them to work efficiently [24,50].

Example 5 - Reducibility profiles H -TPR The reduction properties and interaction between the species of the supported adsorbents and the DFMs were observed in the H ₂ -TPR studies. Figure 5 shows the H ₂ signals of the DFMs and Figure 6 the CH ₄ , H ₂ 0, CO and C0 ₂ signals (m/z=i5, 18, 28, 44) recorded during the H ₂ -TPR experiments. In Figure 6, NR was used as a reference material [42]. As seen in Figure 5, which presented similar peaks to the H ₂ 0 signal, the NRNa and NRK samples had a similar pattern since three distinct peaks were observed. The NRCa sample showed a broader reduction pattern up to 35O°C, but two peaks were seen over that temperature. By observing the H ₂ 0 signals of the three DFMs in Figure 6, it can be seen that the first peak at 200°C corresponded to the reduction of Ru ⁴⁺ to metallic Ru.

These were located at a higher temperature compared to the reference material, indicating a better interaction of the Ru species with the other species [51]. Moreover, a broad peak located at 44O°C for the NRNa, another at 455°C for the NRK, and another one at 52O°C for the NRCa were detected. In all those peaks, a shoulder at the lower temperature range was displayed, attributed to the medium-sized NiOx species interacting with the Ce0 ₂ particles [52,53]. The slow decrease of the signal over 8oo°C corresponded to the reduction of a small amount of bulk Ce02 species, confirming the XRD results in Figure 4, which showed that surface Ce0 ₂ species were the main Ce0 ₂ species in the samples.

A striking finding was the detection of CH ₄ in the DFMs compared to NR, as seen in Figure 6. By looking at the C0 ₂ , H ₂ 0, and H ₂ signals, besides the CH ₄ , it can be seen that after their synthesis, these DFMs were able to adsorb C0 ₂ directly from the atmospheric air and convert it into CH ₄ via the C0 ₂ methanation reaction. Therefore, Ni and Ru, which were already reduced by these temperatures (i.e. 44O°C for NRNa, 455°C for NRK, and 52O°C for NRCa) were used to produce CH ₄ and H ₂ 0 from the atmospheric C0 ₂ and the available H ₂ from the H ₂ -TPR study. Desorption of C0 ₂ and generation of other products of C0 ₂ reduction were also detected mainly at low temperatures, in agreement with the C0 ₂ -TPD results and the different types of basic sites, as presented later. Consequently, the main peak of the DFMs in the H ₂ 0 signal profiles corresponded to the H ₂ 0 from the methanation reaction, holding promise for using DFMs for flexible chemicals synthesis from C0 ₂ in the air.

Example 6 - Basicity profiles CO _? -TPD The C0 ₂ -TPD was used so as to assess the basicity of the supported adsorbents and DFMs. In a C0 ₂ -TPD profile, different types of basic sites maybe present, which can be categorised into weak, medium, and strong ones. Weak basic sites are present up to 25O°C, as they are unstable, while medium basic sites are decomposed from 25O°C to 7OO°C. Finally, the strong basic sites, which are the most stable ones, do not desorb the C0 ₂ until the temperature has reached 7OO°C [54,55]. Figure 7 demonstrates the C0 ₂ -TPD results of DFMs and supported adsorbents, while

Figure 8 the results of the reference material. All the materials evidently displayed mainly weak and medium basic sites. By comparing the DFMs with the NR sample, it was noted that the addition of the adsorbents contributed to the formation of medium basic sites. Therefore, it was proved that the adsorbents were in a dispersed form rather than bulk species, meaning that the DFMs would behave like mid-temperature adsorbents and reversibly adsorb C0 ₂ at intermediate temperatures in agreement with literature [56]. Although the C0 ₂ -TPD profiles of the DFMs had been mainly influenced by the supported adsorbents, it was observed that the addition of Ni and Ru to the supported adsorbents suppressed the medium basic sites. This meant that, to some extent, Ni and Ru covered a small amount of adsorption sites during impregnation [55]. Although based on the intensity of the signals, NRK displayed increased medium basic sites (300 - 7OO°C), in agreement with its better activity at intermediate temperatures as described later on, NRNa and NRK samples had similar profiles to each other. As regards the NRCa sample, a different profile was observed with a smaller amount of weak basic sites and more medium-strong ones, in agreement with its C0 ₂ signal profile in the H ₂ -TPR study as well as the TGA study at high temperatures [55,57].

Example 7 - Adsorption and desorption studies in TGA The main concern of selecting an adsorbent that carries out the C0 ₂ capture step at different temperatures is its feasibility of efficiently adsorbing the C0 ₂ in the 350- 65O°C temperature range. C0 ₂ adsorption is an exothermic process and it is favoured at lower temperatures, which is why C0 ₂ methanation is the most studied reaction [25]. The use of mid-temperature adsorbents in the flexible DFMs scenario was therefore considered necessary. Five (5) cycles of C0 ₂ adsorption and desorption were performed at different temperatures and the results of the DFMs are presented in Figures 9 and 10, those of the supported adsorbents in Figures 11 and 12, and the ones of the reference material in Figure 13. Table 2 (below) summarises the adsorption capacities of the DFMs at different temperatures based on their first cycle.

Figure 9 shows that the designed materials were able to reversibly adsorb C0 ₂ in the temperature range of 35O-65O°C, making them the ideal choice for the flexible DFMs scenario. They all displayed relatively high adsorption capacities without the requirement of high regeneration temperatures as the desorption was carried out by an inert gas purge. Concerning the NRNa sample, a drop in the adsorption capacity was demonstrated in respect of temperature, although that was not the case at 65O°C. A similar trend was also observed in the NRCa sample. A pattern was not easily detected in the NRK sample, but it was noted that it had the best performance at intermediate temperatures, in accordance with the C0 ₂ -TPD results. As a general outcome of this study, NRNa seemed to perform best at low temperatures, NRK at intermediate temperatures, and NRCa at high temperatures, in agreement with other findings [55,57]- However, the adsorption capacities of all DFMs at 35O°C and 65O°C, which were the temperatures used for their feasibility study, were not massively different, explaining the success of all the materials as flexible DFMs. Indeed, the fact that the adsorption capacity at 65O°C was even better at 2nd-5th cycles than that at 35O°C was astonishing.

As can be seen in Figure 9, a substantial amount of C0 ₂ was adsorbed during the first cycle, which was not the case in the supported adsorbents or the switchable catalyst. Especially at 35O°C, the adsorption capacity of the DFMs was higher than the supported adsorbents and the catalyst together, proving that a synergy between those materials took place, which enhanced the C0 ₂ adsorption capacity. In other words, C0 ₂ was not adsorbed only onto the adsorbents and the catalysts, a fact that has already been established in literature [58,59]. Consequently, these results demonstrate that when adsorbents and catalysts are in close proximity, they benefit from each other. The findings showcase the importance of having one material performing both the C0 ₂ adsorption and reduction, rather than two different materials being mixed together. Additionally, as the role of Ce0 ₂ in C0 ₂ adsorption is not yet understood, further mechanistic studies need to be done in the future. Nevertheless, it was firmly concluded that a small quantity of C0 ₂ was used to oxidise the reduced Ce species and form CO, probably explaining the small amounts of CO being formed during the C0 ₂ capture step in their feasibility testing. After all, Ce is well known for its excellent redox properties [35,6o].

Figures 10, 12 and 13 show the amount of C0 ₂ adsorbed per mg of sample during those five cycles at different temperatures. All the materials demonstrated that the adsorption was performed in two steps. An initial fast adsorption of C0 ₂ took place, accounting for the substantial weight gain at the beginning of each capture cycle. The second step was a slower one, evident from the change in slope. In addition, it was observed that the supported adsorbents were able to reach equilibrium and reversibly adsorb C0 ₂ onto the adsorbents’ sites, meaning that they had managed to adsorb and desorb the same C0 ₂ amounts, which was not the case with the DFMs. An interesting observation was that there were different slopes in the C0 ₂ adsorption steps at 65O°C compared to those at 35O°C, proving that a different adsorption mechanism had taken place. However, it was found that the change in slope was not attributed to the adsorbents, but to the Ni- Ru species, as can be seen by comparing Figures to, 12 and 13. This result also showcased that the catalytic component of the DFMs contributed to the C0 ₂ adsorption too and highlighted the importance of a synergy between these two components. Overall, it was concluded that the materials displayed an increased complexity, and thus further mechanistic studies could be carried out in the future so as to fully understand the adsorption as well as the reaction mechanisms.

Table 2: Summary of adsorption capacities of DFMs at different temperatures

Example 8 - Feasibility study of flexible DFMs

After proving that the materials had been able to adsorb C0 ₂ at different temperatures, the feasibility study of their flexibility scenario was carried out. In line to previous findings [42,61], it was decided to perform the C0 ₂ methanation reaction first, followed by the RWGS and DRM reactions, by carrying out a C0 ₂ -capture step before each reaction. An N ₂ purge step was used in between the capture and the reaction steps until the C0 ₂ reading in the gas analyser reached zero so as to prove that the produced gases were formed from the captured C0 ₂ . An N ₂ purge step was also used following the C0 ₂ methanation reaction and prior to the heating-up step for 10 mins, as well as after the RWGS reaction and before the C0 ₂ capture step in order to obtain zero readings in the gas analyser. The percentage of C0 ₂ in the mixture was 10% so as to simulate the C0 ₂ content in effluent streams [25]. Figure 14 shows the amount of C0 ₂ desorbed and products formation (CH ₄ , CO or H ₂ ) in each reaction for the three DFMs and Figures 15, 16 and 17 show the volumetric flow rates of all gases vs time plots of NRNa, NRK, and NRCa, respectively.

Figure 14 reveals the success of flexible DFMs. In all cases, the C0 ₂ was captured at the desired temperature and was subsequently converted into CH ₄ , CO, and syngas, depending on the temperature and the co-reactant used. It was demonstrated that all DFMs had 100% selectivity in all three reactions and the formation of by-products was prevented. A small amount of C0 ₂ was desorbed during all reactions. However, it seemed that this was not temperature dependent, because it was found out that approximately 10% of C0 ₂ was desorbed in both C0 ₂ methanation and DRM. This meant that this 10% of the adsorption sites was not in proximity with the catalytic sites, so the adsorbed C0 ₂ ended up being desorbed. No C0 ₂ was detected during the heating up step, proving that the entire amount of C0 ₂ adsorbed had either been converted into CH ₄ or was desorbed. Furthermore, it was observed that all DFMs were very active in DRM. NRK was the best DFM in C0 ₂ methanation followed by NRCa and NRNa. In both RWGS and DRM, NRCa was the best DFM and after that, it was NRK and NRNa. It was concluded that the DFMs performance strongly depended on the materials basicity and their C0 ₂ desorption ability at the selected temperature, as observed in the C0 ₂ -TPD profiles. Additionally, it was proved that the proximity of the adsorption and catalytic sites was a more vital parameter compared to the adsorption capacity. This was because even if NRNa had the highest C0 ₂ capacity at 35O°C, it had the worst performance in C0 ₂ methanation reaction indicating an unsatisfactory interaction between the Na and Ni- Ru species. Overall, their performance could be generalised as NRCa>NRK>NRNa.

In terms of product signals as observed in Figures 15-17, an initial spike in the products was detected, demonstrating a fast reaction between the captured C0 ₂ and the inexcess co-reactant, followed by a slower decrease in their signals. Additionally, it was observed that even though the reaction step lasted for 20 mins, the reaction steps had taken place in the first 10 min, allowing space for further optimisation of the process by minimising the reaction step time. It is worth noting, however, that in order for two reactors to run in parallel in the DFM technology, the C0 ₂ capture and reduction steps need to have the same duration and if this is not the case, alternative configurations are required.

An interesting finding was that of the CO formation during the capture steps, probably showing that C0 ₂ reacted with Ce ³⁺ , or even with Ni and Ru species and formed CO. These species were re-reduced when H ₂ was either flowing or being generated in the subsequent steps [35,41]. This would in turn mean that the materials were able to sequentially be oxidised and reduced throughout the duration of the experiment without any structure alteration, as proved by the post-characterisation of the spent materials. Another explanation of the CO formation during the capture step would have been the occurrence of the reverse Boudouard reaction between the C0 ₂ and some carbon species, resulting in CO production. In addition to the CO formation, there was also an amount of H ₂ produced during the capture step. This could have happened because of H ₂ being chemisorbed in a previous step and hence being displaced by C0 ₂ and/or the oxidation of Ce ³⁺ species with H ₂ 0 to produce Ce ⁴⁺ and H ₂ . To sum up, it was demonstrated that several reactions may have taken place during the C0 ₂ capture.

By observing the CO and H ₂ profiles of the NRNa during RWGS, an odd result emerged. It appeared that even when no CO and H ₂ were produced during that reaction, the H ₂ signal decreased and no CH ₄ was formed. However, during the subsequent C0 ₂ capture step, these gases were indeed detected. Therefore, it was assumed that there had not been enough heat produced during that step so as to release the products of the endothermic RWGS. Once a small amount of heat was produced during the next exothermic adsorption step, these gases were able to be released, and thus detected by the gas analyser. As a result, the DFM’s sites were once again free to adsorb C0 ₂ during the capture step.

During DRM, the DFMs were able to initially convert the captured C0 ₂ into syngas, as illustrated by the CO and H ₂ signals. However, methane cracking was observed for all

DFMs as a significant amount of CH ₄ was used and H ₂ was produced. This was anticipated because the captured C0 ₂ had initially been converted into syngas and, as time went by and its availability was limiting, the CH ₄ was decomposed into carbon and H ₂ . This phenomenon had already been reported during the post-breakthrough stage [39]- As a result, the H ₂ /C0 ratio was higher than its stoichiometric value of 1 and a H ₂ rich syngas was produced which could be useful in C0 ₂ applications that require a higher H ₂ / CO ratio. A good strategy for limiting the carbon formation would have been to decrease the DRM duration, as it had taken place predominantly in the first 5 to 10 mins, because allowing this step to occur for longer would serve no purpose. Nevertheless, previous reports [38-40] demonstrated that the carbon formed during the DRM reaction was able to be regenerated during the subsequent C0 ₂ capture step via the endothermic reverse Boudouard reaction. This phenomenon was not observed here as the DRM reaction was the last step in this set of experiments. However, the occurrence of the reverse Boudouard reaction would have been expected for these materials too if a C0 ₂ capture step had occurred after the DRM step. Example 9 - Post reaction characterisation

As shown in Figures 15-17, CH ₄ cracking took place in the feasibility study of the flexible DFMs that resulted in the formation of coke, and hence XRD and TGA were carried out on the spent catalysts to qualify and quantify the carbon species.

Figure 18 shows the XRD patterns of the post-reaction NRNa, NRK, and NRCa samples. The patterns were similar to their reduced ones presented in Figure 4, displaying the characteristic peaks of Ni, Ce0 ₂ , and A1 ₂ O ₃ . No carbon peak was observed in the spent NRNa and NRK XRD profiles, meaning that the type of carbon formed was soft with a poorer degree of crystallinity. However, this was not the case with the NRCa, as a carbon peak at 20°=26 was observed, signifying the formation of a harder carbon requiring higher regeneration temperatures. A slight shift of the Ni peak to the left appeared on the NRNa and NRCa samples indicating the formation of a NiRu alloy, which was not the case for their reduced materials. [22]. Hence, it was concluded that a NiRu alloy was formed during reduction even if it was not evident from their

XRD patterns.

Thermogravimetric analysis was also used to quantify the carbon formed during the DRM reaction. In order to accurately measure the amount of carbon, ex situ reduced DFMs were also tested in the same conditions. Therefore, the weight loss due to the atmospheric C0 ₂ and moisture adsorption and material degradation, as well as the weight gain due to the oxidation of the materials, were also taken into account. The results are presented in Figure 19. In all the samples, an initial weight loss took place up to 200°C because of the weakly adsorbed atmospheric C0 ₂ and moisture. A higher weight loss was seen in the reduced materials because they were able to absorb a higher amount of atmospheric impurities. Moreover, a weight gain was observed at the 300 - 4OO°C temperature range, which was associated with the oxidation of the remaining Ni and Ru particles. At temperatures higher than 4OO°C, a significant weight loss was detected, showing the carbon oxidation. In agreement with the XRD results, the coke oxidation was completed by 5OO°C for the NRNa and NRK samples, while a higher temperature was needed for the NRCa sample, confirming the existence of softer carbon species in the former samples and of harder ones in the latter. The formation of soft carbon was an intriguing result as it will likely allow an easier regeneration process in the future. Overall, the amount of carbon formed during the DRM reaction was 0.083 gC/gSample for NRNa, 0.072 gC/gSample for NRK and 0.075 gC/gSample for

NRCa, showing a similar extent of CH ₄ cracking for the three DFMs. Example 10 - CH _d -Temperature Programmed Surface Reaction TSPR)

A CH ₄ -TSPR was conducted on the fresh NiRuCa sample in a fixed bed quartz reactor. The data were logged by using the Quadera software package and the signals of gases were observed in an online mass spectrometer (Omni-Star GSD 320). In this experiment, 30 mg of fresh (after calcination) NiRuCa was used and the temperature was increased from room temperature to 95O°C (io°C/min) with a pure CH ₄ feed of 30 ml/min. No N ₂ was used in the feed in order that mass to charge ratio (m/z) of 28 to be solely attributed to CO.

The aim was to observe the fresh DFM performance in a CH ₄ environment, similar to the H ₂ -TPR experiment, and identify opportunities for a potential temperature reduction of the high-temperature reactions. This would ultimately be translated into energy requirements reduction, minimising the operating costs in a potential scale-up of the switchable DFMs.

Figure 20 shows the CH ₄ -TPSR results. Overall, it was demonstrated that the DFM’s oxidation (since the sample used was after calcination) did not shut down its activity for DRM which was remarkable. In agreement with the H ₂ -TPR results, atmospheric C0 ₂ was adsorbed onto the DFM and was gradually released until a peak at 4OO°C was observed. A peak at higher temperature analogous to the H ₂ -TPR and C0 ₂ -TPD results was not identified, meaning that C0 ₂ was fully consumed during low temperature reaction. A CO peak was also seen at this temperature (4OO°C). Additionally, CH ₄ cracking took place starting at 35O-4OO°C and peaked at 635°C, as observed from the H ₂ and CH ₄ signals. Therefore, it was shown that in a CH ₄ -rich environment and in the temperature range of 4OO-6oo°C, CH ₄ cracking supplied the H ₂ needed for the reduction of Ni and Ru which was feasible at these temperatures. Thus, DRM was able to take place because of the available CH ₄ and atmospheric C0 ₂ . However, after the consumption of C0 ₂ , excessive CH ₄ cracking took place, which was expected based on our isothermal experiment results.

Based on these results, it was confirmed that a DRM temperature reduction to 4OO°C could be feasible. It should be kept in mind that the DFMs operate under dynamic conditions, meaning that a deviation from the well-known steady state conditions is possible. The initial temperature of RWGS and DRM, i.e. 65O°C was considered useful for the validation study since higher temperatures are more representative of the current industrial reformers that operate close to equilibrium conversions at 900- iooo°C. Investigating the DFMs behaviour at a temperature higher than 65O°C was considered to be unnecessary, because the DFMs had a 100% selectivity in these reactions. In fact, it would have had a negative effect due to reduced C0 ₂ adsorption and sintering, let alone the increase of operating costs in a potential scale up.

Conclusions

The inventors have designed a series of advanced universal materials for the integrated C0 ₂ capture and conversion. The combination of an adsorbent and a switchable catalyst gave birth to the flexible DFMs that were able to capture C0 ₂ at various temperatures and, depending on the reaction conditions, to convert it into different added-value chemicals through the C0 ₂ methanation, RWGS, and DRM catalytic upgrading routes. Their proof of existence is a milestone in the development of carbon negative technologies to combat the increased C0 ₂ emissions and the subsequent global warming. Flexible DFMs can be adapted to the current infrastructure as a CCU unit, while offering a solution to the fragile energy sector.

The designed DFMs of this project were proved to be highly dispersed porous materials and their species, located in close proximity, showed a high degree of interaction with each other. The H ₂ -TPR experiments demonstrated that these materials were able to adsorb the atmospheric C0 ₂ and, upon Ni and Ru reduction, to convert it into synthetic natural gas. The C0 ₂ adsorption-desorption studies of the selected DFMs surprisingly proved that they were able to reversibly adsorb C0 ₂ at different temperatures, making them ideal for a flexible DFM scenario. A synergy between the species also took place, contributing to their increased adsorption capacities. Overall, the C0 ₂ adsorption capacities of the DFMs were in accordance with their basicity profiles, as revealed by the C0 ₂ -TPD results.

The feasibility study of performing C0 ₂ capture and then either C0 ₂ methanation, RWGS or DRM reactions was successful. The DFMs were capable of adsorbing C0 ₂ from a CO ₂ -containing stream and then, depending on the reaction temperature and the co-reactant used, C0 ₂ was converted into CH ₄ , CO, and syngas, respectively. The results showed that flexible DFMs exist and can be used as a tool in green technologies in the future, even if the optimisation of materials and conditions is necessary to combat issues, like the formation of coke during DRM. This work is undoubtedly the beginning of the development of advanced universal materials, and its findings are unambiguously a step forward in the fight against CO2 emissions globally.

Methods Characterisation

The Brunauer-Emmett-Teller (BET) equation and the Barett-Joyner-Halenda (BJH) methods were used to obtain the specific surface area and pore volume of the materials. Initially, degassing at 25O°C in vacuum for 4 hours took place and then, the textural properties of the materials were determined by nitrogen adsorption-desorption measurements at -195°C in a Micrometrics 3Flex apparatus.

X-ray diffraction was performed on fresh supported adsorbents and on fresh, reduced, and spent DFMs in a X’Pert Powder from PANalytical apparatus. The diffraction patterns were obtained at 30 mA and 40 kV by using Cu Ka radiation (X =0.154 nm). The 20° angle was increased every 450s by 0.05° in the range of 10-90°.

Scanning electron microscopy was carried out on the fresh supported adsorbents and DFMs by using a JEOL JSM-7100F instrument, which also had an Energy Dispersive X-ray Spectroscope (EDS) analyser. Carbon paint was used to fix the samples to the holder and coating was conducted to eliminate the charging effects.

H ₂ -Temperature Programmed Reduction (TPR) was performed on the fresh supported adsorbents and DFMs in fixed bed quartz reactor. The data were logged by using the Quadera software package and the H ₂ consumption was observed in an online mass spectrometer (Omni-Star GSD 320). A 10% H ₂ /Ar mixture with a total flow of 50 ml/min was passed through the reactor with 50 mg of sample while the temperature was raised from room temperature to 95O°C with io°C/min rate.

C0 ₂ -Temperature Programmed Reduction (C0 ₂ -TPD) was carried out on the same equipment as H ₂ -TPR. In the C0 ₂ -TPD experiments, 50 mg of sample were initially reduced in situ at 8oo°C with a 10% H ₂ /N ₂ mixture and a total flow of 50 ml/min (io°C/ min). After they had cooled down to 4O°C with a N ₂ purge, a flow 50 ml/min of a 10% C0 ₂ /N ₂ mixture was used for 45 mins to saturate the samples. Subsequently, 50 ml/min of N ₂ were flowed through the reactor to remove the weakly adsorbed C0 ₂ for 30 mins and, finally, the temperature was raised to 8oo°C at a io°C/min rate. The mass to charge ratio (m/z) of 44 corresponding to C0 ₂ was recorded during the temperature increase.

Thermogravimetric analysis (TGA) was performed on the reduced and spent DFMs after the reactor performance tests in a SDT650 apparatus from TA Instruments in order to measure the amount of carbon depositions. A flow of too mL/min of air was employed while the temperature was raised from room temperature to 95O°C at a io°C/min rate. Performance testing

Adsorption and desorption study at different temperatures

A SDT650 apparatus from TA Instruments was used so as to understand the adsorption and desorption behaviour of the materials at different temperatures. 10-15 mg of a reduced sample was used in each run. Firstly, the temperature was increased from room temperature to 15O°C with 100 mL/ min of Ar at a io°C/ min rate and was held at that temperature for 30 mins in order for all the weakly adsorbed gases to be desorbed. Next, it was raised to 35O°C, 45O°C, 55O°C, or 65O°C with the same Ar flow and temperature rate as before. Subsequently, the temperature was stabilised at the desired level for 200 mins, allowing 5 cycles to be conducted, each of which consisted of 20 mins of C0 ₂ adsorption and 20 mins of C0 ₂ desorption. In each adsorption step, 20 ml / min of C0 ₂ and 100 ml//min of Ar were used and in each desorption step, 100 ml/min of Ar. The test was carried out on the reduced supported adsorbents and DFMs. The reduction was performed ex situ at 8oo°C for 1 hr at a 50 ml/min total flow rate of a 10% H ₂ /N ₂ mixture.

Feasibility study of flexible DFMs

The three DFMs were tested in a tubular fixed bed quartz reactor (0.5 in OD) at atmospheric pressure and were supported on a quartz wool bed. The volumetric percentages of C0 ₂ , CH ₄ , CO, and H ₂ in the outlet stream were monitored, using an ABB A02020 online gas analyser, which was placed after the water was condensed and separated in a chiller. A bubble meter, which was a vertical tube of a known volume, was used to allow the accurate measurement of the total volumetric flow rate by simply measuring the time spent for a bubble to reach that known volume. Two different modes of experimental set up were used in those feasibility experiments: the ‘reactor’ mode and the ‘bypass’ mode. In the former, the desired gases were passing through the reactor, whereas in the latter, they were bypassing it, going to the condenser and then to the ABB gas analyser or the bubble meter. A simplified process diagram is shown in Figure 1.

Initially, 0.25 g of sample were reduced in situ at 8oo°C for 1 hr at a 50 ml/min total flow rate of a 10% H2/N2 mixture (io°C/min). Then, the temperature was decreased to 35O°C with a N ₂ purge stream in order to perform a cycle of C0 ₂ capture-N ₂ purge-C0 ₂ methanation. The C0 ₂ capture step was performed in the ‘reactor’ mode with a 10% C0 ₂ /N ₂ mixture of 50 ml/min total flow rate for 20 mins. After the 20 mins had elapsed, a 10 mins N ₂ purge step was carried out, resulting in the removal of the weakly adsorbed C0 ₂ and obtaining a zero C0 ₂ reading in the gas analyser. Subsequently, in the ‘bypass’ mode, a mixture containing 10% of H ₂ in N ₂ was flowing through the system until its reading in the analyser was stabilised and then in the ‘reactor’ mode, the reaction step was performed for 20 mins. Once the C0 ₂ capture-N ₂ purge-C0 ₂ methanation cycle was carried out, the temperature was increased to 65O°C at a io°C/ min rate. A cycle of C0 ₂ capture-N ₂ purge-RWGS and a cycle of C0 ₂ capture-N ₂ purge-DRM followed as described above, but in the case of DRM, CH ₄ instead of H ₂ was used in the reaction step.

It is worth noting that the N ₂ flow remained the same throughout the experiment as in all the C0 ₂ capture and reaction steps, it represented 90% of the mixture, i.e. 45 mL/min and its exact flow rate was measured at the beginning of the experiment. This meant that when the total flow rates in C0 ₂ capture and reaction steps were measured with the bubble meter, the exact flow rates of C0 ₂ and co-reactant, either H ₂ or CH ₄ , were calculated by subtracting the known N ₂ flow rate from the corresponding total flow rate. Moreover, all the total volumetric flow rate measurements and the stabilisation of the gases percentage were performed in the ‘bypass’ mode to make sure that the DFMs were exposed to the desired gases only during the C0 ₂ capture, N ₂ purge, and reaction steps. In addition, the percentages of C0 ₂ , CH ₄ , CO, and H ₂ in the outlet stream shown in the gas analyser were recorded every 5 sec throughout the experiment and it was assumed that the remaining volume percentage in the mixture was N ₂ . Hence, the volumetric flow rates of all gases were calculated according to the following formula, where the brackets represent the volumetric percentage of each gas, i.e. C0 ₂ , CH ₄ , CO, and H ₂ , the F the flow rate (mL/min), and the subscript ‘i’ the respective gas. The amount of products (in mL) was calculated based on the area under the curve at a flow rate (mL/min) vs time (min) graph.

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