Switchable Dual Functional Material

Abstract

The disclosure provides a method of converting carbon dioxide into a reaction product. The method comprises providing a switchable dual function material (DFM) 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. The switchable DFM comprises an adsorbent, configured to adsorb carbon dioxide; and a switchable catalyst configured to catalyse the conversion of carbon dioxide into a reaction product. The disclosure extends to the switchable DFM per se.

Claims

1. A method of converting carbon dioxide into a reaction product, the method comprising: providing a switchable dual function material (DFM) 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, wherein the switchable DFM comprises an adsorbent, configured to adsorb carbon dioxide; and a switchable catalyst configured to catalyse the conversion of carbon dioxide into a reaction product.

2. The method of claim 1, wherein providing the switchable DFM loaded with carbon dioxide comprises contacting a gas comprising carbon dioxide and the switchable DFM, optionally wherein the carbon dioxide is present in the gas at a concentration of less than 80 vol %.

3. (canceled)

4. The method of claim 1, wherein the method comprises selecting a specific co-reactant to provide a desired reaction product, wherein the co-reactant is selected from the group consisting of a hydrocarbon; hydrogen; water and oxygen. The hydrocarbon may be as defined above.

5. The method of claim 1, wherein the method comprises contacting the switchable DFM loaded with carbon dioxide and the co-reactant at a temperature between 0 and 1,500 C.

6. 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.

7. The method of claim 1, wherein the switchable DFM comprises a support, and the adsorbent and the switchable catalyst are both disposed on the support.

8. The method of claim 7, wherein the support comprises or consists of a metal, a metal oxide, silica, a metal organic framework, a zeolite or a structured carbon, preferably wherein the support comprises or consists of cerium, aluminium, zirconium, titanium, silicon and/or an oxide thereof.

9. The method of claim 1, wherein the switchable catalyst is configured to catalyse two or more different chemical reactions selected from the group consisting of a dry reforming reaction; a CO.sub.2 methanation; an RWGS reaction; a bi-reforming reaction; a tri-reforming reaction; a dehydrogenation reaction; and a hydrogenation reaction.

10. The method of claim 1, wherein the switchable catalysts comprise one or more metals and/or a metal phosphide, preferably wherein the one or more metals is or comprises one or more transition metals and the metal phosphide is a transition metal phosphide, optionally wherein the switchable catalyst may comprise one or more of nickel (Ni), ruthenium (Ru), cerium (Ce), zirconium (Zr), iron (Fe), and/or a phosphide thereof.

11. (canceled)

12. The method of claim 10, wherein the catalyst comprises nickel, optionally wherein the switchable catalyst comprises nickel and one or more additional components or promoters, and the or each additional component or promoter is ruthenium (Ru) or iron (Fe).

13. The method of claim 10, wherein the switchable catalyst comprises or consists of a nickel phosphide.

14. The method of claim 1, wherein the adsorbent is or comprises a metal, an oxide of a metal, a carbonate of a metal, a metal organic framework, a zeolite, silica and/or carbon, optionally wherein the adsorbent is or comprises an alkali metal, an alkaline earth metal and/or an oxide or carbonate thereof, preferably wherein the adsorbent is or comprises sodium oxide, potassium oxide and/or calcium oxide.

15. (canceled)

16. The method of claim 1, wherein the switchable DFM has a surface area of at least 100 m.sup.2/g and/or a pore volume of at least 0.2 cm.sup.3/g.

17. 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.

18. The method of claim 17, wherein the method comprises disposing an adsorbent on the support, optionally wherein the method comprises disposing the adsorbent on the support prior to disposing the switchable catalyst on the support.

19. (canceled)

20. The method of claim 18, wherein disposing the adsorbent on the support comprises contacting the support with the adsorbent or an adsorbent precursor, optionally wherein 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 and subsequently drying the further suspension to remove the solvent therefrom and provide a support with the adsorbent or adsorbent precursor disposed thereon.

21. (canceled)

22. The method of claim 20, wherein the method comprises contacting the support with an adsorbent precursor, and subsequently calcining the adsorbent precursor to provide the adsorbent.

23. The method of claim 17, wherein disposing the switchable catalyst on the support comprises contacting the support with the switchable catalyst or a switchable catalyst precursor, optionally wherein 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 and subsequently drying the further suspension to remove the solvent therefrom and provide a support with the switchable catalyst or switchable catalyst precursor disposed thereon.

24. (canceled)

25. The method of claim 23, wherein the method comprises contacting the support with a switchable catalyst precursor, and subsequently calcining the switchable catalyst precursor to provide the switchable catalyst.

26. (canceled)

27. A method of capturing carbon dioxide, the method comprising contacting a gas comprising carbon dioxide and the switchable DFM defined by claim 6.

Description

[0119] 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:

[0120] FIG. 1 is a simplified process flow diagram for experimental set up;

[0121] FIG. 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;

[0122] FIG. 3 shows X-ray diffraction (XRD) patterns of fresh supported adsorbents on the left-hand side and of DFMs on the right-hand side;

[0123] FIG. 4 shows XRD patterns of reduced DFMs;

[0124] FIG. 5 shows the H.sub.2-TPR profiles of NRNa, NRK and NRCa FIG. 6 shows the CH.sub.4, H.sub.2O, CO and CO.sub.2 signals (m/z=15, 18, 28, 44) during H.sub.2-TPR for NRNa, NRK, NRCa and NR;

[0125] FIG. 7 shows CO.sub.2-TPD profiles of NRNa, NRK and NRCa on the left-hand side and of Na, K and Ca on the right-hand side;

[0126] FIG. 8 shows CO.sub.2-TPD profile of NR;

[0127] FIG. 9 shows CO.sub.2 Adsorption/desorption cycles of NRNa, NRK and NRCa at 350, 450, 550 and 650 C.

[0128] FIG. 10 shows TGA adsorption-desorption results at different temperatures for NRNa, NRK and NRCa;

[0129] FIG. 11 shows CO.sub.2 Adsorption/desorption cycles of Na, K and Ca at 350 and 650 C.;

[0130] FIG. 12 shows TGA adsorption-desorption results at different temperatures for Na, K and Ca;

[0131] FIG. 13 shows TGA adsorption-desorption results at different temperatures for NR;

[0132] FIG. 14 shows the results of feasibility study of flexible DFMs showing the amount of CO.sub.2 desorbed and products formation in each reaction for NRNa, NRK and NRCa;

[0133] FIG. 15 shows the CO.sub.2 capture and CO.sub.2 methanation, followed by the RWGS and DRM for NRNa;

[0134] FIG. 16 shows the CO.sub.2 capture and CO.sub.2 methanation, followed by the RWGS and DRM for NRK;

[0135] FIG. 17 shows the CO.sub.2 capture and CO.sub.2 methanation, followed by the RWGS and DRM for NRCa;

[0136] FIG. 18 shows XRD for post reaction NRNa, NRK and NRCa;

[0137] FIG. 19 shows TGA for post reaction NRNa, NRK and NRCa; and

[0138] FIG. 20 shows CH.sub.4-Temperature Programmed Surface Reaction (CH.sub.4-TPSR) profile of NiRuCa: A) H.sub.2O, B) CH.sub.4, C) CO.sub.2, D) CO and E) H.sub.2 signals.

EXAMPLE 1-DUAL FUNCTION MATERIALS SYNTHESIS

[0139] The three DFMs described herein were prepared by sequential impregnation. The adsorbents were impregnated onto the CeO.sub.2Al.sub.2O.sub.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 CeO.sub.2Al.sub.2O.sub.3 support (SCFa-160 Ce20 Puralox, Sasol) and NaNO.sub.3 (Fluka), KNO.sub.3 (Sigma Aldrich), and Ca(NO.sub.3).sub.2.Math.4H.sub.2O (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 120 C. and calcined at 400 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: [0140] Na.sub.2O/CeO.sub.2Al.sub.2O.sub.3 (identified hereinafter as Na), [0141] K.sub.2O/CeO.sub.2Al.sub.2O.sub.3 (identified hereinafter as K), and [0142] CaO/CeO.sub.2Al.sub.2O.sub.3(identified hereinafter as Ca).

[0143] The supported adsorbents had 11.9 wt. % Na.sub.2O, K.sub.2O, and CaO, respectively.

[0144] A similar impregnation procedure was performed when synthesising the three DFMs. The required amounts of Ni(NO.sub.3).sub.2.Math.6H.sub.2O (Acros Organics) and Ru(NO)(NO.sub.3).sub.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 120 C., and calcined at 500 C. for 3 hours (5 C./min). As a result, the prepared DFMs were: [0145] 15 wt. % Ni, 1 wt. % Ru-10 wt. % Na.sub.2O/CeO.sub.2Al.sub.2O.sub.3(identified hereinafter as NRNa), [0146] 15 wt. % Ni, 1 wt. % Ru-10 wt. % K.sub.2O/CeO.sub.2Al.sub.2O.sub.3 (identified hereinafter as NRK), and [0147] 15 wt. % Ni, 1 wt. % Ru-10 wt. % CaO/CeO.sub.2Al.sub.2O.sub.3 (identified hereinafter as NRCa).

[0148] In some experiments, the 15 wt. % Ni, 1 wt. % Ru/CeO.sub.2Al.sub.2O.sub.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 2Textural Properties

[0149] The textural properties of the supported adsorbents and DFMs are presented in Table 1, below.

TABLE-US-00001 TABLE 1 Textural properties of the supported adsorbents and DFMs Sample S.sub.BET (m.sup.2/g) V.sub.PORE (cm.sup.3/g) D.sub.PORE (nm) Na 109 0.29 10.5 K 136 0.36 10.4 Ca 170 0.39 8.7 NRNa 173 0.41 9.4 NRK 170 0.40 8.9 NRCa 194 0.37 6.9

[0150] 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. FIG. 2 shows the N.sub.2 adsorption-desorption isotherms of the materials. The isotherms generated corresponded to the type IV isotherms with a characteristic H1 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 3Crystalline Structure

[0151] FIG. 3 shows the crystalline phases of the fresh materials. All the supported adsorbents and DFMs had the characteristic peaks of -Al.sub.2O.sub.3 and CeO.sub.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 FIG. 4. Furthermore, in the fresh DFMs patterns, the peaks located at 2=37.2, 43.3, 62.9, and 75.4 (JCPDS 00-047-1049) were ascribed to NiO. The existence of nickel aluminate (NiAl.sub.2O.sub.4) spinels was discarded due to the low calcination temperature used [47]. Small peaks of RuO.sub.2 were also observed in the fresh samples (JCPDS 01-070-2662). No evidence of a NiRu alloy was detected in the fresh DFMs.

[0152] Upon reduction at 800 C., as observed in FIG. 4, the CeO.sub.2 peaks either disappeared or were significantly reduced. As a result, it was concluded that surface CeO.sub.2 species were mainly present in the samples, which was in agreement with the H.sub.2-TPR results [48]. No ceria aluminate (CeAlO.sub.3) species were seen due to the reduction temperature being 800 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 NiRu 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 4Morphological Structure

[0153] 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 5Reducibility profiles H.SUB.2.-TPR

[0154] The reduction properties and interaction between the species of the supported adsorbents and the DFMs were observed in the H.sub.2-TPR studies. FIG. 5 shows the H.sub.2 signals of the DFMs and FIG. 6 the CH.sub.4, H.sub.2O, CO and CO.sub.2 signals (m/z=15, 18, 28, 44) recorded during the H.sub.2-TPR experiments. In FIG. 6, NR was used as a reference material [42].

[0155] As seen in FIG. 5, which presented similar peaks to the H.sub.2O 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 350 C., but two peaks were seen over that temperature. By observing the H.sub.2O signals of the three DFMs in FIG. 6, it can be seen that the first peak at 200 C. corresponded to the reduction of Ru.sup.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 440 C. for the NRNa, another at 455 C. for the NRK, and another one at 520 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 CeO.sub.2 particles [52,53]. The slow decrease of the signal over 800 C. corresponded to the reduction of a small amount of bulk CeO.sub.2 species, confirming the XRD results in FIG. 4, which showed that surface CeO.sub.2 species were the main CeO.sub.2 species in the samples.

[0156] A striking finding was the detection of CH.sub.4 in the DFMs compared to NR, as seen in FIG. 6. By looking at the CO.sub.2, H.sub.2O, and H.sub.2 signals, besides the CH.sub.4, it can be seen that after their synthesis, these DFMs were able to adsorb CO.sub.2 directly from the atmospheric air and convert it into CH.sub.4 via the CO.sub.2 methanation reaction. Therefore, Ni and Ru, which were already reduced by these temperatures (i.e. 440 C. for NRNa, 455 C. for NRK, and 520 C. for NRCa) were used to produce CH.sub.4 and H.sub.2O from the atmospheric CO.sub.2 and the available H.sub.2 from the H.sub.2-TPR study. Desorption of CO.sub.2 and generation of other products of CO.sub.2 reduction were also detected mainly at low temperatures, in agreement with the CO.sub.2-TPD results and the different types of basic sites, as presented later. Consequently, the main peak of the DFMs in the H.sub.2O signal profiles corresponded to the H.sub.2O from the methanation reaction, holding promise for using DFMs for flexible chemicals synthesis from CO.sub.2 in the air.

Example 6Basicity Profiles CO.SUB.2.-TPD

[0157] The CO.sub.2-TPD was used so as to assess the basicity of the supported adsorbents and DFMs. In a CO.sub.2-TPD profile, different types of basic sites may be present, which can be categorised into weak, medium, and strong ones. Weak basic sites are present up to 250 C., as they are unstable, while medium basic sites are decomposed from 250 C. to 700 C. Finally, the strong basic sites, which are the most stable ones, do not desorb the CO.sub.2 until the temperature has reached 700 C. [54,55].

[0158] FIG. 7 demonstrates the CO.sub.2-TPD results of DFMs and supported adsorbents, while FIG. 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 CO.sub.2 at intermediate temperatures in agreement with literature [56]. Although the CO.sub.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 (300-700 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 CO.sub.2 signal profile in the H.sub.2-TPR study as well as the TGA study at high temperatures [55,57].

Example 7Adsorption and Desorption Studies in TGA

[0159] The main concern of selecting an adsorbent that carries out the CO.sub.2 capture step at different temperatures is its feasibility of efficiently adsorbing the CO.sub.2 in the 350-650 C. temperature range. CO.sub.2 adsorption is an exothermic process and it is favoured at lower temperatures, which is why CO.sub.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 CO.sub.2 adsorption and desorption were performed at different temperatures and the results of the DFMs are presented in FIGS. 9 and 10, those of the supported adsorbents in FIGS. 11 and 12, and the ones of the reference material in FIG. 13. Table 2 (below) summarises the adsorption capacities of the DFMs at different temperatures based on their first cycle.

[0160] FIG. 9 shows that the designed materials were able to reversibly adsorb CO.sub.2 in the temperature range of 350-650 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 650 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 CO.sub.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 350 C. and 650 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 650 C. was even better at 2nd-5th cycles than that at 350 C. was astonishing.

[0161] As can be seen in FIG. 9, a substantial amount of CO.sub.2 was adsorbed during the first cycle, which was not the case in the supported adsorbents or the switchable catalyst. Especially at 350 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 CO.sub.2 adsorption capacity. In other words, CO.sub.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 CO.sub.2 adsorption and reduction, rather than two different materials being mixed together. Additionally, as the role of CeO.sub.2 in CO.sub.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 CO.sub.2 was used to oxidise the reduced Ce species and form CO, probably explaining the small amounts of CO being formed during the CO.sub.2 capture step in their feasibility testing. After all, Ce is well known for its excellent redox properties [35,60].

[0162] FIGS. 10, 12 and 13 show the amount of CO.sub.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 CO.sub.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 CO.sub.2 onto the adsorbents' sites, meaning that they had managed to adsorb and desorb the same CO.sub.2 amounts, which was not the case with the DFMs. An interesting observation was that there were different slopes in the CO.sub.2 adsorption steps at 650 C. compared to those at 350 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 NiRu species, as can be seen by comparing FIGS. 10, 12 and 13. This result also showcased that the catalytic component of the DFMs contributed to the CO.sub.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-US-00002 TABLE 2 Summary of adsorption capacities of DFMs at different temperatures Temperature Adsorption Capacity mol/kg ( C.) NRNa NRK NRCa 350 0.49 0.29 0.39 450 0.32 0.17 0.17 550 0.14 0.15 0.20 650 0.16 0.18 0.26

Example 8Feasibility Study of Flexible DFMs

[0163] After proving that the materials had been able to adsorb CO.sub.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 CO.sub.2 methanation reaction first, followed by the RWGS and DRM reactions, by carrying out a CO.sub.2-capture step before each reaction. An N.sub.2 purge step was used in between the capture and the reaction steps until the CO.sub.2 reading in the gas analyser reached zero so as to prove that the produced gases were formed from the captured CO.sub.2. An N.sub.2 purge step was also used following the CO.sub.2 methanation reaction and prior to the heating-up step for 10 mins, as well as after the RWGS reaction and before the CO.sub.2 capture step in order to obtain zero readings in the gas analyser. The percentage of CO.sub.2 in the mixture was 10% so as to simulate the CO.sub.2 content in effluent streams [25]. FIG. 14 shows the amount of CO.sub.2 desorbed and products formation (CH.sub.4, CO or H.sub.2) in each reaction for the three DFMs and FIGS. 15, 16 and 17 show the volumetric flow rates of all gases vs time plots of NRNa, NRK, and NRCa, respectively.

[0164] FIG. 14 reveals the success of flexible DFMs. In all cases, the CO.sub.2 was captured at the desired temperature and was subsequently converted into CH.sub.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 CO.sub.2 was desorbed during all reactions. However, it seemed that this was not temperature dependent, because it was found out that approximately 10% of CO.sub.2 was desorbed in both CO.sub.2 methanation and DRM. This meant that this 10% of the adsorption sites was not in proximity with the catalytic sites, so the adsorbed CO.sub.2 ended up being desorbed. No CO.sub.2 was detected during the heating up step, proving that the entire amount of CO.sub.2 adsorbed had either been converted into CH.sub.4 or was desorbed.

[0165] Furthermore, it was observed that all DFMs were very active in DRM. NRK was the best DFM in CO.sub.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 CO.sub.2 desorption ability at the selected temperature, as observed in the CO.sub.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 CO.sub.2 capacity at 350 C., it had the worst performance in CO.sub.2 methanation reaction indicating an unsatisfactory interaction between the Na and NiRu species. Overall, their performance could be generalised as NRCa>NRK>NRNa.

[0166] In terms of product signals as observed in FIGS. 15-17, an initial spike in the products was detected, demonstrating a fast reaction between the captured CO.sub.2 and the in-excess 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 CO.sub.2 capture and reduction steps need to have the same duration and if this is not the case, alternative configurations are required.

[0167] An interesting finding was that of the CO formation during the capture steps, probably showing that CO.sub.2 reacted with Ce.sup.3+, or even with Ni and Ru species and formed CO. These species were re-reduced when H.sub.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 CO.sub.2 and some carbon species, resulting in CO production. In addition to the CO formation, there was also an amount of H.sub.2 produced during the capture step. This could have happened because of H.sub.2 being chemisorbed in a previous step and hence being displaced by CO.sub.2 and/or the oxidation of Ce.sup.3+ species with H.sub.2O to produce Ce.sup.4+ and H.sub.2. To sum up, it was demonstrated that several reactions may have taken place during the CO.sub.2 capture.

[0168] By observing the CO and H.sub.2 profiles of the NRNa during RWGS, an odd result emerged. It appeared that even when no CO and H.sub.2 were produced during that reaction, the H.sub.2 signal decreased and no CH.sub.4 was formed. However, during the subsequent CO.sub.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 CO.sub.2 during the capture step.

[0169] During DRM, the DFMs were able to initially convert the captured CO.sub.2 into syngas, as illustrated by the CO and H.sub.2 signals. However, methane cracking was observed for all DFMs as a significant amount of CH.sub.4 was used and H.sub.2 was produced. This was anticipated because the captured CO.sub.2 had initially been converted into syngas and, as time went by and its availability was limiting, the CH.sub.4 was decomposed into carbon and H.sub.2. This phenomenon had already been reported during the post-breakthrough stage [39]. As a result, the H.sub.2/CO ratio was higher than its stoichiometric value of 1 and a H.sub.2 rich syngas was produced which could be useful in CO.sub.2 applications that require a higher H.sub.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 CO.sub.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 CO.sub.2 capture step had occurred after the DRM step.

Example 9Post Reaction Characterisation

[0170] As shown in FIGS. 15-17, CH.sub.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.

[0171] FIG. 18 shows the XRD patterns of the post-reaction NRNa, NRK, and NRCa samples. The patterns were similar to their reduced ones presented in FIG. 4, displaying the characteristic peaks of Ni, CeO.sub.2, and Al.sub.2O.sub.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 2=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.

[0172] 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 CO.sub.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 FIG. 19. In all the samples, an initial weight loss took place up to 200 C. because of the weakly adsorbed atmospheric CO.sub.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 300-400 C. temperature range, which was associated with the oxidation of the remaining Ni and Ru particles. At temperatures higher than 400 C., a significant weight loss was detected, showing the carbon oxidation. In agreement with the XRD results, the coke oxidation was completed by 500 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.sub.4 cracking for the three DFMs.

Example 10CH.SUB.4.-Temperature Programmed Surface Reaction (CH.SUB.4.-TSPR)

[0173] A CH.sub.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 950 C. (10 C./min) with a pure CH.sub.4 feed of 30 ml/min. No N.sub.2 was used in the feed in order that mass to charge ratio (m/z) of 28 to be solely attributed to CO.

[0174] The aim was to observe the fresh DFM performance in a CH.sub.4 environment, similar to the H.sub.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.

[0175] FIG. 20 shows the CH.sub.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.sub.2-TPR results, atmospheric CO.sub.2 was adsorbed onto the DFM and was gradually released until a peak at 400 C. was observed. A peak at higher temperature analogous to the H.sub.2-TPR and CO.sub.2-TPD results was not identified, meaning that CO.sub.2 was fully consumed during low temperature reaction. A CO peak was also seen at this temperature (400 C.). Additionally, CH.sub.4 cracking took place starting at 350-400 C. and peaked at 635 C., as observed from the H.sub.2 and CH.sub.4 signals. Therefore, it was shown that in a CH.sub.4-rich environment and in the temperature range of 400-600 C., CH.sub.4 cracking supplied the H.sub.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.sub.4 and atmospheric CO.sub.2. However, after the consumption of CO.sub.2, excessive CH.sub.4 cracking took place, which was expected based on our isothermal experiment results.

[0176] Based on these results, it was confirmed that a DRM temperature reduction to 400 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. 650 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 900-1000 C. Investigating the DFMs behaviour at a temperature higher than 650 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 CO.sub.2 adsorption and sintering, let alone the increase of operating costs in a potential scale up.

CONCLUSIONS

[0177] The inventors have designed a series of advanced universal materials for the integrated CO.sub.2 capture and conversion. The combination of an adsorbent and a switchable catalyst gave birth to the flexible DFMs that were able to capture CO.sub.2 at various temperatures and, depending on the reaction conditions, to convert it into different added-value chemicals through the CO.sub.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 CO.sub.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.

[0178] 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.sub.2-TPR experiments demonstrated that these materials were able to adsorb the atmospheric CO.sub.2 and, upon Ni and Ru reduction, to convert it into synthetic natural gas. The CO.sub.2 adsorption-desorption studies of the selected DFMs surprisingly proved that they were able to reversibly adsorb CO.sub.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 CO.sub.2 adsorption capacities of the DFMs were in accordance with their basicity profiles, as revealed by the CO.sub.2-TPD results.

[0179] The feasibility study of performing CO.sub.2 capture and then either CO.sub.2 methanation, RWGS or DRM reactions was successful. The DFMs were capable of adsorbing CO.sub.2 from a CO.sub.2-containing stream and then, depending on the reaction temperature and the co-reactant used, CO.sub.2 was converted into CH.sub.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 CO.sub.2 emissions globally.

Methods

Characterisation

[0180] 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 250 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.

[0181] X-ray diffraction was performed on fresh supported adsorbents and on fresh, reduced, and spent DFMs in a XPert Powder from PANalytical apparatus. The diffraction patterns were obtained at 30 mA and 40 kV by using Cu K radiation (=0.154 nm). The 2 angle was increased every 450s by 0.05 in the range of 10-90.

[0182] 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.

[0183] H.sub.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.sub.2 consumption was observed in an online mass spectrometer (Omni-Star GSD 320). A 10% H.sub.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 950 C. with 10 C./min rate.

[0184] CO.sub.2-Temperature Programmed Reduction (CO.sub.2-TPD) was carried out on the same equipment as H.sub.2-TPR. In the CO.sub.2-TPD experiments, 50 mg of sample were initially reduced in situ at 800 C. with a 10% H.sub.2/N.sub.2 mixture and a total flow of 50 ml/min (10 C./min). After they had cooled down to 40 C. with a N.sub.2 purge, a flow 50 ml/min of a 10% CO.sub.2/N.sub.2 mixture was used for 45 mins to saturate the samples. Subsequently, 50 ml/min of N.sub.2 were flowed through the reactor to remove the weakly adsorbed CO.sub.2 for 30 mins and, finally, the temperature was raised to 800 C. at a 10 C./min rate. The mass to charge ratio (m/z) of 44 corresponding to CO.sub.2 was recorded during the temperature increase.

[0185] 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 100 mL/min of air was employed while the temperature was raised from room temperature to 950 C. at a 10 C./min rate.

Performance Testing

Adsorption and Desorption Study at Different Temperatures

[0186] 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 150 C. with 100 mL/min of Ar at a 10 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 350 C., 450 C., 550 C., or 650 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 CO.sub.2 adsorption and 20 mins of CO.sub.2 desorption. In each adsorption step, 20 ml/min of CO.sub.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 800 C. for 1 hr at a 50 ml/min total flow rate of a 10% H.sub.2/N.sub.2 mixture.

Feasibility Study of Flexible DFMs

[0187] 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 CO.sub.2, CH.sub.4, CO, and H.sub.2 in the outlet stream were monitored, using an ABB AO2020 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 FIG. 1.

[0188] Initially, 0.25 g of sample were reduced in situ at 800 C. for 1 hr at a 50 ml/min total flow rate of a 10% H.sub.2/N.sub.2 mixture (10 C./min). Then, the temperature was decreased to 350 C. with a N.sub.2 purge stream in order to perform a cycle of CO.sub.2 capture-N.sub.2 purge-CO.sub.2 methanation. The CO.sub.2 capture step was performed in the reactor mode with a 10% CO.sub.2/N.sub.2 mixture of 50 ml/min total flow rate for 20 mins. After the 20 mins had elapsed, a 10 mins N.sub.2 purge step was carried out, resulting in the removal of the weakly adsorbed CO.sub.2 and obtaining a zero CO.sub.2 reading in the gas analyser. Subsequently, in the bypass mode, a mixture containing 10% of H.sub.2 in N.sub.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 CO.sub.2 capture-N.sub.2 purge-CO.sub.2 methanation cycle was carried out, the temperature was increased to 650 C. at a 10 C./min rate. A cycle of CO.sub.2 capture-N.sub.2 purge-RWGS and a cycle of CO.sub.2 capture-N.sub.2 purge-DRM followed as described above, but in the case of DRM, CH.sub.4 instead of H.sub.2 was used in the reaction step.

[0189] It is worth noting that the N.sub.2 flow remained the same throughout the experiment as in all the CO.sub.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 CO.sub.2 capture and reaction steps were measured with the bubble meter, the exact flow rates of CO.sub.2 and co-reactant, either H.sub.2 or CH.sub.4, were calculated by subtracting the known N.sub.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 CO.sub.2 capture, N.sub.2 purge, and reaction steps.

[0190] In addition, the percentages of CO.sub.2, CH.sub.4, CO, and H.sub.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.sub.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. CO.sub.2, CH.sub.4, CO, and H.sub.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.

[00001]$F_i=\frac{\left[i\right]}{\left[N_2\right]}{F}_{N_2}$

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