5.1. ICCC with Methanation Reaction
The majority of studies on ICCC using DFMs focus on the methanation of CO
2, which takes place primarily via the Sabatier reaction (Equation (9)):
The reaction is typically carried out at temperatures around 300–350 °C and pressures of 5–20 MPa [
119]. Given its highly exothermic nature, the reaction releases copious amounts of heat, which has to be managed accordingly to prevent the thermal degradation (e.g., sintering) of the DFMs. This can be achieved via the engineering of more thermally resistant DFMs, or by harnessing the heat of the reaction via efficient heat transfer and heat integration, e.g., powering endothermic reactions. In this view, CO
2 methanation offers an isothermal solution to the DFMs system because the exothermic reaction can supply the required heat for the decarbonation reaction. Additionally, the usage of real flue gas in industrial settings might lead to the poisoning and deactivation of the Ni-based DFMs typically used for methanation [
119,
120]. Practical solutions include the pre-scrubbing of the flue gas or considering the usage of alternative catalysts like Ru, which can be activated via pre-reduction at lower temperatures in a H
2 environment. The vast body of work studying the ICCC–methanation process is summarized in
Table 3. It can be seen that Ru- and Ni-based DFMs supported on alkaline metal oxides, such as Na
2O, MgO, and CaO, are the most commonly studied materials. In fact, as shown in
Figure 15, Na
2O, MgO, and CaO are among the most common sorbents for ICCC with methanation [
14], due to their high CO
2 sorption capacity and methanation activity at moderate temperatures, given the thermodynamic limitation of the reaction.
With a wide range of variables that are at play in the synthesis of a DFM, an extensive amount of work is naturally devoted to finding the best synthetic routes that produce DFMs with optimized makeups [
11,
95,
101,
110,
117,
121,
122,
123,
125,
126,
127,
128,
129,
130,
131,
134,
135,
136]. In a pioneering work by Duyar et al., CaO-based DFMs with Ru as the methanation catalyst supported on alumina were synthesized via incipient wetness impregnation (IWI) with varying adsorbent/catalyst loadings [
11]. It was observed that the order of impregnation (i.e., Ru impregnated on CaO-alumina vs. CaO impregnated on Ru-alumina) affected the CO
2 capture and methanation performances at 320 °C. In particular, a DFM comprising 5 wt% Ru impregnated on 10 wt% CaO-γAl
2O
3 showed a single-pass methanation capacity of 0.5 mmol/g. Upon subjecting to cyclic testing, an average of 0.41 mmol/g of CO
2 capture could be obtained, with an average CO
2 conversion of 76.17% and a methanation capacity of 0.31 mmol CH
4/g over 19 cycles. The authors concluded that the order of impregnation was significant where the impregnation of Ru was found to be better than that of CaO due to the good dispersion of Ru. Should the order be reversed, the active Ru sites could be blocked by CaO particles, thus decreasing the catalytic activity of the DFM. Upon varying the CaO:Ru ratio, a high methane turnover (CH
4 yield/Ru) was observed at a high CaO:Ru ratio, indicating CO
2 spillover from the CaO to Ru sites where the methanation reaction takes place. It also suggests that the close proximity of CaO and Ru was an important factor in the DFM performance. However, low Ru loadings might not generate enough heat from the exothermic methanation reaction to liberate CO
2 chemisorbed to CaO, since methanation begins primarily where CO
2 is chemisorbed to Ru, thus compromising the overall CH
4 yield. Lastly, testing under simulated flue gas conditions showed that, in the presence of steam, the methanation capacity decreased to 0.27 mmol CH
4/g over 20 cycles, although the purity of the effluent gas improved to 99.9% CH
4 by volume, as compared to 83.6% CH
4 (balance CO
2) in the absence of steam, suggesting the competing adsorption of H
2O and CO
2 on the DFM surface. A follow-up study by Duyar et al. presented two new DFMs with similar adsorbent/catalyst loadings, but with K
2CO
3 or Na
2CO
3 substituting CaO as the adsorbent component [
93]. These DFMs showed improved methanation capacities of 0.91 and 1.05 mmol CH
4/g for K and Na, respectively, owing to the increasing CO
2 adsorption capacity in the same order: Na
2CO
3 > K
2CO
3 > CaO. The results, therefore, show the importance of optimizing the adsorbent component of DFMs in future attempts to engineer more efficient materials.
In another study, Porta et al. assessed the CO
2 capture and methanation performances at 350 °C of Ru-based DFMs with six different adsorbent components (Li/Na/K/Mg/Ca/Ba) supported on alumina [
123]. They found that metals capable of forming carbonates with high thermal stability led to the production of the highest amounts of CH
4, namely, DFMs promoted with K, Ca, and Ba. On the other hand, Arellano-Treviño et al. evaluated Ru-based DFMs supported on γ-Al
2O
3 containing different alkali and alkaline earth oxides, such as Na
2O, K
2O, CaO, and MgO, and found that Na
2O-promoted DFM (5%Ru-6.1%Na
2O/Al
2O
3) exhibited a high CO
2 capture capacity (0.651 mmol/g) and methanation rate (0.614 mmol CH
4/g) when tested in 10%CO
2/N
2 atmosphere [
92]. Although the CH
4 yield significantly decreased upon exposure to a simulated flue gas condition (7.5%CO
2, 4.5%O
2, 15%H
2O, balance N
2), the DFM remained active, producing 0.291 mmol CH
4/g at 320 °C. In another experiment using a Rh-Na
2O-based DFM, the CO
2 capture capacity and catalytic activity of 0.5%Rh was compared with those of 5%Ru in order to obtain a similar material price. While both DFMs exhibited a similar CO
2 capture capacity (0.625 vs. 0.651 mmol/g, respectively), 5%Ru yielded higher methanation (0.614 mmol/g) than the 0.5%Rh counterpart (0.422 mmol/g), although one could argue that the methane yield per unit of metal is higher for Rh than that for Ru. As an alternative to noble metals, Ni-Na
2O-based DFM was also synthesized and tested, which produced 0.276 mmol CH4/g under a 10%CO
2/N
2 atmosphere. However, the material was deactivated when exposed to flue gas containing H
2O, with no methane being produced, as shown in
Figure 16. Further analysis suggested that Ni atoms could not be completely reduced to the active metallic state in the presence of O
2 and H
2O, thus losing its catalytic activity.
Further possible variables during the synthesis of a DFM include the choice of adding promoters that can improve the CO
2 capture capacity and CH
4 selectivity, or the choice of precursor chemicals, which might affect the interactions between different components in the synthesized DFM. One study by Cimino et al. [
135] found that promoting Ru/Al DFM with Li resulted in a four-to-five-fold increase in the CO
2 capture capacity compared to the unpromoted material. In addition, stable cyclic carbonation/methanation was possible at temperatures around 230 °C, instead of the commonly utilized temperatures of 300 to 320 °C. In another study, the incorporation of Cs into Ni-hydrotalcite-based DFMs was found to increase the CO
2 capture capacity up to 0.48 mmol/g and a CH
4 yield of 0.33 mmol/g at 350 °C [
130]. These examples show the promotional effect of alkali metal as it increases the material’s basicity, in particular, the number of medium and strong basic sites. Besides enhancing the basicity of the material, the addition of a promoter can also improve the stability of the material. For example, Ma et al. investigated the effect of doping Ni-CaO-based DFMs with various metal oxides, such as Mg, Al, Mn, Y, Zr, La, and Ce [
110]. Among the synthesized DFMs, Zr-doped Ni/CaO exhibited the best performance by maintaining a stable CO
2 capture capacity of 9 mmol/g and 74% CH
4 selectivity after 20 cycles. It was attributed to the formation of CaZrO
3, which helped improve the thermal resistance to sintering.
To investigate the effect of catalyst loading, Bermejo-López et al. varied the Ni content on CaO- or Na
2CO
3-based DFMs with alumina support through the impregnation method [
101]. Notably, it was found that increasing Ni loading would increase the particulate sizes, catalyst reducibility, and consequently, methanation rates, with 0.142 mmol CH
4/g produced by a DFM comprising 15 wt% Ni on 15% CaO/Al
2O
3 at 520 °C.
Lastly, the choice of the synthetic method has also been known to significantly influence the final microstructures and other physicochemical characteristics of a DFM, and a range of works have been devoted to comparatively evaluating the various possible synthetic routes [
105,
112,
117,
125]. For example, Zhang et al. noted that sol–gel synthetic methods tend to produce DFMs with physicochemical characteristics more favorable for ICCC processes, including an optimal balance of the BET surface area and crystallite sizes [
112]. Sun et al. noted that the dispersion between catalyst and sorbent particles could even be tuned via different synthetic methods, thus giving rise to varying carbonation and methanation capabilities [
125].
In addition to optimizing the materials design and synthesis, careful attention should also be given to the experimental parameters of the carbonation and methanation reaction, as well as the simulated flue gas conditions, as they can help uncover the possibility of translating ideal laboratorial conditions to a more real-life (e.g., industrial) setting, and provide plausible avenues for future work [
98,
100,
122,
124,
137]. In one such study by Wang et al., a DFM consisting of Ru-Na
2O impregnated on an alumina support was subjected to parametric and cyclic tests [
98]. Specifically, the effect of three parameters—gas space velocity, reaction temperature, and exposure to oxygen during adsorption—on performance indicators such as CO
2 uptake and CH
4 generation, were investigated. From the cyclic studies, a CO
2 uptake and CH
4 generation capacities of 0.40 mmol/g and 0.32 mmol/g, respectively, were reported after 50 cycles at 300 °C. Through parametric studies, it was found that gas space velocities and temperature would affect the rates and the extents of carbonation and methanation in accordance with kinetic and thermodynamic principles. For instance, increasing the gas space velocity would increase the rates of CO
2 uptake and CH
4 production due to a more efficient mass transfer, whereas increasing the temperature would decrease CO
2 adsorption and CH
4 generation due to the exothermic nature of the reactions. The oxygen exposure tests also showed that, minimally, 15% H
2/N
2 for methanation was necessary for the reduction of oxidized Ru due to exposure to oxygen-containing flue gas during adsorption. The presence of precious metals such as Ru in overcoming the limitations of Ni as a catalytic metal is further displayed by studies such as one by Arellano-Treviño et al., which suggests that Ru doping aids in the reduction and re-activation of Ni after oxygen exposure, leading to the possibility of developing DFMs which are more resistant to industrial reaction conditions at cheaper costs [
100].
Meanwhile, other authors have studied reaction parameters such as optimal carbonation/methanation durations, feed gas concentrations, and process pressures. Kosaka et al., for example, investigated Ni-based DFMs promoted with alkali metals including Na/K/Ca supported on alumina for their carbonation and methanation at 450 °C [
122]. It was found that increasing operational pressures (a range of 0.1 MPa to 0.9 MPa was used) could increase the CO
2 capture capacity of a Ni/Na-Al
2O
3 DFM from 0.209 mmol CO
2/g to 0.299 mmol CO
2/g, and its methanation capacity from 0.188 mmol CH
4/g to 0.266 mmol CH
4/g, in agreement with thermodynamics as the methanation reaction involves a decrease in the number of moles. For these tests, a gas feed containing 5% CO
2/N
2 was used, but it was additionally reported that increasing pressures enhanced carbonation and methanation at lower CO
2 concentrations of 100 and 400 ppm CO
2/N
2 as well. Jeong-Potter and Farrauto also attempted an investigation of the effectiveness of a Ru/Na-Al
2O
3 DFM at a CO
2 concentration of 400 ppm in air to assess the feasibility of utilizing the DFM for direct air capture purposes [
124].
Considering that ICCC and methanation have been extensively studied, a good understanding of the kinetics and mechanisms of carbonation and methanation is equally important [
94,
95,
96,
138,
139,
140]. The mechanistic study is typically performed via investigative techniques including Fourier transform–infrared (FT-IR) spectroscopy, which is capable of measuring the gas composition at the reactor outlet and its changes with time, or by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which detects the presence of chemical bonds that suggest the occurrence of possible reaction intermediates.
For instance, a study by Bermejo-López et al. uses FT-IR to study the temporal evolution of the gas composition within a reactor [
96]. Their work involved the usage of Ru-based CaO and Na
2CO
3 DFMs supported on alumina. They proposed a mechanism where CaCO
3 formed through the carbonation of Ca(OH)
2 (Equations (10) and (11)) is decomposed upon the addition of H
2 during the methanation step (Equation (12)), releasing CO
2 which is subsequently converted on Ru sites according to the Sabatier reaction (Equation (13)). Additional observations include the delayed evolution of CO
2 itself during the carbonation step, suggesting the saturation of the CO
2 uptake sites, and the delayed evolution of H
2O during methanation (even though the Sabatier reaction produces H
2O), suggesting the hydration of CaO to form Ca(OH)
2 (Equation (14)). A similar reaction scheme is suggested for Na
2CO
3-based DFMs.
Conversely, other studies utilize DRIFTS to detect the presence of reaction intermediates, and suggest possible reaction mechanisms [
94,
95,
140]. One such study by Proaño et al. with Ru-based DFM [
94,
95] suggests the formation of bidentate carbonate species during carbonation and formate species during methanation (see
Figure 17 for their detailed mechanism), but there exists a range of works that find corroborating [
139] and disputing [
140] results. Proaño’s group has since conducted further studies into DFMs with a similar makeup (10% Ni, 6.1% Na
2O/Al
2O
3 DFMs enhanced with 1% Pt/Ru), utilizing in situ DRIFTS to study carbonation and methanation under oxidizing and non-oxidizing conditions, yielding largely corroborative results.
5.2. ICCC with Reverse Water–Gas Shift (RWGS) Reaction
RWGS is another key reaction in C1 chemistry that takes place according to the following reversible reaction (Equation (15)):
RWGS is an important reaction for producing syngas with desired H
2/CO ratios, where CO can be further valorized into various chemicals, such as methanol via the CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water–gas-shift reaction) process or hydrocarbons via Fischer–Tropsch synthesis. However, although CO is industrially more useful than methane, the development of an effective DFM system to generate CO has been much less explored than that for methane generation. This is partly because the ability of DFMs to achieve a practical H
2/CO ratio remains a major challenge. The accurate tuning of syngas composition can be tricky because of other secondary reactions that may occur in parallel with the RWGS reaction, such as the Sabatier reaction (Equation (10)) and the methanation of CO (Equation (16)):
Considering the endothermic nature of RWGS, increasing operating temperatures might reduce the extent of the exothermic side reactions and favor the RWGS reaction. Indeed, in a review by González-Castaño et al. [
141], where the equilibrium concentrations of chemical species involved in the RWGS, Sabatier, and CO methanation reactions were investigated, it was found that the equilibrium concentration of CH
4 decreases to near zero above 700 °C, as shown in
Figure 18. In addition, the CO
2/H
2 molar ratio also plays a critical role in adjusting the CO composition, with a low CO
2/H
2 ratio being thermodynamically favorable for the RWGS reaction. These considerations are therefore important in maximizing the selectivity of CO, which turns out to be a key metric in evaluating DFMs used in ICCC processes involving RWGS.
Apart from thermodynamic considerations, ensuring a high selectivity of CO is also dependent on a prudent catalyst design, which is aided by the understanding of the kinetics and mechanisms of RWGS. Conventionally, Cu is favored for its CO selectivity and low methanation rates, while Pt is preferred for higher CO
2 conversions [
78]. With regards to DFM selection for the RWGS reaction, a variety of materials have been developed, including those containing transition metals, such as Fe, Co, Ni, or combinations thereof [
142,
143]. Recently, there have also been studies that do not involve any transition metals, such as supported Na on CaO and Al
2O
3 [
144,
145].
Table 4 summarizes the DFMs that catalyze the RWGS reaction.
In a recent work by Wu et al., Ni-CaO-CeO
2 DFMs were synthesized via their acetate/nitrate precursors, using several methods including wet mixing, co-precipitation, and a citric acid-based sol–gel pathway [
105]. Among the synthesized samples, DFMs prepared by sol–gel method exhibited superior physicochemical properties and hence displayed the highest CO
2 capture capacities and conversion, as well as CO yields and selectivity. In particular, the sol–gel-derived material with acetate precursors showed a CO
2 uptake of 15.34 mmol/g and CO
2 conversion of 92.4%, with CO selectivity of 89.1% at 650 °C. Meanwhile, the material prepared with nitrate precursors displayed a lower CO
2 uptake of 13.15%, but higher CO
2 conversion of 96% and higher CO selectivity (87–96%). It was concluded that, firstly, the sol–gel synthesis resulted in an optimal morphology and internal structure for the CO
2 capture and conversion. For instance, large BET surface areas and pore volumes facilitated CO
2 diffusion and adsorption, whereas an enhanced surface basicity promoted CO
2 affinity. Meanwhile, a smaller NiO grain size, uniform Ni dispersion, and higher reducibility could have led to improved catalytic activity. Secondly, the choice of acetate/nitrate precursors could affect catalyst–support interactions, which in turn result in variations in CO
2 uptake and conversion, CO selectivity, as well as the material’s stability upon recycle.
Similar DFMs consisting of Ni over Ce-modified CaO were reported earlier by Sun et al., who employed a sol–gel method with citric acid [
111]. The incorporation of CeO
2 was found to enhance the material’s stability due to the formation of well-dispersed CeO
2 that can effectively prevent the growth and agglomeration of CaO and NiO crystallites, resulting in a stable performance after 20 cycles. This is reflected by the synthesized material with Ca/Ni/Ce molar ratio of 1:0.1:0.033, which exhibited a CO
2 uptake of 14.1 mmol/g, 51.8% CO
2 conversion, and almost 100% CO selectivity at 650 °C. The authors proposed possible reaction mechanisms which are based on a redox (Scheme 1) or associative formate (Scheme 2) mechanism, as illustrated in
Figure 19. According to the redox mechanism, upon switching to the hydrogenation step, the adsorbed CO
2 could spill over to the Ni active sites, which may result in the formation of CO and NiO. In the presence of CeO
2, the oxygen vacancy on the CeO
2 surface may also interact with CO
2, leading to the formation of CO and the oxidation of Ce
3+ species. The introduction of H
2 into the reactor would subsequently reduce NiO back to Ni and release H
2O molecules. Likewise, the adsorbed H
2 species on the CeO
2 surface extracts the lattice oxygen from ceria, releasing a H
2O molecule along with the reduction of the catalyst. Alternatively, the associative formate mechanism suggests that the adsorbed CO
2 on the CeO
2 surface may form bidentate formate as a reaction intermediate, which decomposes to form CO and terminal hydroxyl groups. Subsequently, the adsorbed H
2 reacts with the hydroxyl group, releasing a H
2O molecule.
In another study by Guo et al., ZrO
2 was used as a dopant for Ni/CaO, where it enhanced the surface basicity and reducibility of the DFM, resulting in an increased CO
2 uptake and catalytic activity [
146]. At the optimum 12 wt% ZrO
2 loading, the material exhibited a high CO
2 uptake of 16.7 mmol/g with 63.2% CO
2 conversion and a 10.5 mmol/g CO yield at 650 °C. The formation of CaZrO
3 was also found to reduce the sintering of CaO and NiO crystals. The further incorporation of CeO
2 into Ni/CaO-ZrO
2 resulted in increased CO
2 conversion to 72.1% due to the increase of an oxygen vacancy, although the CO
2 uptake capacity decreased to 11.9 mmol/g, which could be attributed to a lower surface area. A similar strategy was also implemented by Sun et al. who incorporated Fe into a Ni/CaO DFM. An optimum CO yield of 11.3 mmol/g was obtained by using Ni
1Fe
9-CaO at 650 °C. It was observed that the formation of Ca
2Fe
2O
5 acted as an oxygen carrier that promoted CO production and helped improve the stability of the DFM by acting as a physical barrier that slowed down CaO sintering [
143].
As alternatives to transition-metal-based materials, Sasayama et al. prepared DFMs that only contain alkali/alkaline earth metals (e.g., Na, K, and Ca) supported on γ-Al
2O
3 via the impregnation method [
144]. They were then tested with 5 vol% and 400 ppm CO
2 in N
2 to investigate the production of syngas under model flue gas or direct air capture modes, respectively. Upon testing with 5 vol% CO
2 at 500 °C, Na/Al
2O
3 exhibited a 0.135 mmol/g CO
2 uptake with 77% CO
2 conversion and 99.8% CO selectivity. On the other hand, with 400 ppm CO
2, its CO
2 uptake was about 0.09 mmol/g with 77.7% CO
2 conversion and 94.3% CO selectivity. The results therefore indicate the potential use of transition-metal-free DFMs for ICCC process, although further works are needed to improve the materials’ performance and to study the reaction mechanism as well as to understand the catalytic active sites.
In another study, the further incorporation of Pt on Na/Al
2O
3 was found to be selective for CO formation at lower temperatures of 350 °C [
145], which is advantageous in reducing the typically high temperature requirement for the RWGS reaction. The DFM was prepared via the sequential impregnation method of Al
2O
3, first with Na, followed by Pt. The CO
2 capture was evaluated with 1% CO
2/10% O
2 in N
2, and its uptake capacity was found to be 0.19 mmol/g, comparable to that of Na/Al
2O
3 (0.21 mmol/g), and much higher than that of Pt/Al
2O
3 (0.08 mmol/g), indicating that Na serves as the CO
2 capture site. Upon hydrogenation, CO
2 was converted to CO with 89% conversion and 93% CO selectivity. In a control experiment, a physical mixture of Na/Al
2O
3 and Pt/Al
2O
3 was tested, and it was found that CO
2 conversion and CO selectivity decreased significantly to only 57% and 41%, respectively. The EDX spectroscopy (
Figure 20) suggests that a core–shell structure of Pt-Na nanoparticles was formed on Pt-Na/Al
2O
3, with Pt as the core. As a result of the close interaction between the two metals, the Na species not only serves as the CO
2 capture site, but also as a promoter to enhance CO formation. Through in situ FTIR measurements, it was found that the adsorbed CO species was hardly observed over the material’s surface, indicating that the presence of the Na species inhibited the adsorption of the generated CO, leading to high selectivity for CO.
5.3. ICCC with Dry Reforming of Methane (DRM)
Besides RWGS, syngas can also be produced via the dry reforming of methane (DRM), where two potent greenhouse gases, CH
4 and CO
2, react with each other according to the following equation (Equation (17)):
Despite being favored for its “greening” capability, ICCC with DRM faces several disadvantages at industrial scales. First, the highly endothermic reaction requires a large energy input (Equation (17)). Second, the DFMs that catalyze the DRM reaction typically contain Ni in the catalytic component, which is susceptible to poisoning and deactivation via coking in the absence of steam. It has, therefore, been proposed that continued research in this field would focus on developing Ni-based DFMs that exhibit stronger coking resistance, e.g., by including alkaline promoters like Mg, or considering the use of Rh- or Ru-based DFMs to catalyze the reaction, in addition to uncovering further insights into the mechanisms of DRM so as to strengthen and optimize its industrial applicability [
134,
148]. The summary of DFMs that catalyze the DRM reaction is presented in
Table 5 and
Table 6. The two tables categorize works according to their presentation of the ICCC performances of DFMs. In the former, the conversions of CO
2 and CH
4 are presented, whereas the yields of H
2 and CO are presented in the latter.
In an earlier work, Kim et al. first demonstrated the calcium looping process combined with a catalytic system by using a physical mixture of CaO and Ni/MgO-Al
2O
3 as the CO
2 sorbent and DRM catalyst, respectively, in a single fluidized bed reactor [
150]. The two-step process was operated in a cyclic manner where CO
2 was first captured on CaO during the carbonation step. Upon gas switching to CH
4, the CaO sorbent was regenerated while releasing CO
2, which instantaneously reacted with CH
4 on the Ni/MgO-Al
2O
3 catalyst, as depicted in
Figure 21. CaO was derived from limestone via calcination at 800 °C whereas the Ni catalyst was derived from a hydrotalcite precursor that was synthesized via the co-precipitation method. At 720 °C, the CO
2 capture capacity was 14.1 mmol/g in the first cycle with an almost 100% conversion of CO
2 and CH
4. The resulting syngas thus had a H
2:CO ratio of 1.06:1. Due to the sintering of CaO, there was a significant loss of CO
2 uptake, which decreased to 9 mmol/g after 10 cycles of the reaction. Although, initially, Ni catalyst deactivation was observed due to coke formation, the carbon deposition amount was decreasing with increasing cycle numbers. It was found that the deposited carbon was removed during the carbonation step while the Ni metallic state was preserved, which could be due to the reverse Boudouard reaction
that is favorable at high temperatures [
152]. Therefore, CO
2 and CH
4 conversion can be maintained above 95% throughout the 10 cycles.
In subsequent works by Tian et al., a Ni/CaO-based DFM was prepared via citrate-based sol–gel synthesis which produced an average syngas yield of around 9 mmol/g throughout 10 cycles [
152] of CO
2 capture (at 600 °C) and conversion with CH
4 (at 800 °C). The experimental H
2:CO ratio was about 1.1, higher than the theoretical ratio of 1. As the average CH4 conversion (86%) was higher than that of CO
2 (65%), it could be suggested that the Ni-CaO interface in the DFM was more active in dissociating CH
4 to yield H
2 than reducing CO
2 to yield CO. On the other hand, despite using a Ni/CaO DFM prepared with a similar method, Jo et al. obtained syngas with a high H
2:CO ratio of 6.52 at 700 °C, indicating that the CO
2 reduction reaction was suppressed [
153]. It was observed that a large amount of carbon deposit was formed on the DFM, as well as the hydration of CaO to form Ca(OH)
2.
A further improvement of the Ni/CaO DFM was reported by Hu et al. where porous CeO
2-modified CaO microparticles were used as the support for Ni impregnation [
154]. The introduction of CeO
2 into the material enhanced the DFM performance in two aspects, in that it acted as (i) a promoter that enhanced the CO
2 affinity towards CaO through the increase in lattice oxygen, thus enabling a high CO
2 uptake, and (ii) a physical stabilizer that enhanced the sintering resistance of CaO, improved Ni particle dispersion through the formation of small Ni crystallites, as well as increased Ni reducibility, thus enabling high and stable catalytic activity. Among the investigated samples, the material with 85%CaO:15%CeO
2 (Ni/Ca
85Ce
15) showed a constant CO
2 uptake and retained about 80–90% of its initial syngas time-averaged space time yield (STY) over nine cycles of CO
2 sorption and conversion at 650 °C, as shown in
Figure 22. In another experiment, a layer of ZrO
2 was coated on CaCO
3 nanoparticles, followed by the co-impregnation of Ni and Ce [
149]. During the CO
2 capture–DRM cycle tests at 720 °C, 5% CO
2 was used as the gas feed to mimic flue gas composition where over 40% conversion of CO
2 and CH
4 can be achieved. The presence of ZrO
2 was also found to enhance material stability by preventing sintering. Through the in situ DRIFTS analysis (
Figure 23), monodentate carbonate was observed on the CaO surface during the carbonation step, whereas polydentate carbonate was observed on ZrO
2 and bidentate carbonate on the CeO
2 surface. CO
2 dissociation to CO was also detected on NiCe/Ca-Zr oxide due to the presence of abundant oxygen vacancies on the reduced CeO
2. Upon gas switching to CH
4, the characteristic IR peaks of formate species and gaseous CO were observed, while carbonate peak intensities were decreased. These suggested that the occurrence of CaCO
3 decarbonation was coupled with the DRM reaction, during which the adsorbed
species produced from CH
4 decomposition reacted with the
species to form formate species, which further decomposed to CO [
149].
The combination of Ni with Ru supported on CeO
2/Al
2O
3 containing Na
2O, K
2O, or CaO was reported by Merkouri et al. [
155]. The DFMs contained 15%Ni and 1%Ru, and were mostly active for ICCC with DRM at 650 °C, where the CaO-containing material produced 0.338 mmol/g CO and 32.6 mmol/g H
2, yielding H
2-rich syngas. The large formation of H
2 during DRM was mainly due to methane cracking and its decomposition into carbon, evidenced by CH
4-TPSR experiments which showed that methane cracking peaked at around 635 °C. By using an operando DRIFTS-MS method, the authors elucidated the CO
2 capture and reaction mechanism over the Ni-Ru-Na
2O/CeO
2-Al
2O
3 DFM through alternating successive CO
2 and CH
4 cycles at 550 °C. In agreement with Tian et al.’s previous work [
152], CH
4 reduction on the metallic sites produced a H
2 and graphitic carbon layer, which further gasified with CO
2 during the capture step to yield CO through a reverse Bourdouard reaction. However, it was noted that the coke gasification kinetics were slower than the coke formation, resulting in the rapid deactivation of the catalyst. This mechanistic study thus highlights the importance of controlling CH
4 cracking during the DRM reaction via optimizing the material’s design and reaction engineering in order to slow down the catalyst deactivation, as well as to obtain syngas with a favorable H
2:CO ratio.
Further alternatives that include the dry reforming of other hydrocarbons, such as ethane (DRE), to produce syngas (Equation (18)) was reported by Al-Mamoori et al. [
91], who investigated the use of CaO- and MgO-based double salts promoted by K/Ca, supported on alumina, and impregnated with Ni as the catalyst at 650 °C.
The material’s synthetic methods consisted of a sol–gel method for the alumina support, while the adsorbent and catalyst components were incorporated via wet impregnation. The CaO-based DFMs comprising 20 wt% Ni and a 1:1 ratio of adsorbent to support showed the best ICCC performance, with CO2 adsorption capacities of 0.99 mmol/g and 0.63 mmol/g for the K-Ca-based and Na-Ca-based DFMs, respectively, a 65% and 75% conversion of CO2 for the K-Ca-based and Na-Ca-based DFMs, respectively, and a 100% C2H6 conversion for both DFMs. Additionally, yields of approximately 45% and 37.5% for CO and H2, respectively, were reported for the K-Ca-based DFM when subjected to a sustained DRE process (up to 10 h) at 650 °C. Despite a relatively stable performance, coke formation (9 wt%) was observed due to a high Ni content. From their investigations, it was concluded that the adsorbent/catalyst contents directly influenced CO2 capture and conversion performances, and that reaction conditions strongly influenced the selectivity toward DRE or the occurrence of side reactions, such as oxidative dehydrogenation, ethane cracking, RWGS, and coking. Therefore, it is imperative to optimize their chemical and physical properties.