1. Introduction

Ever-growing increase of anthropogenic carbon dioxide emission and resultant climate change are the urgent that humankind has faced. CO₂ capture and storage (CCS) technologies have been arose as the near to midterm solution to the climate change[1-8]. It can be divided into three main types: (i) precombustion method, (ii) post-combustion method and (iii) oxy-fuel combustion method. Among them, post-combustion method is the most suitable one to the condition of existing power plants due to its simplicity and high efficiency, and the most mature of it in industrial scale is amine scrubbing. However, due to the high cost, involvement of corrosive chemicals and hazardous byproducts, alternatives to amine scrubbing are being vigorously sought. Widely developed alternatives are chemical looping combustion (CLC) and calcium looping (CaL: CaO_(s) + CO_2(g) ↔ CaCO_3(s))[9-11] (Fig. 1). Both processes are dependent on the cyclic reactions and the use of metal oxides. Yet, due to the cyclic nature of the process, cyclic stability decreases over multiple cycles largely due to sintering, which cause blockage, irreversible collapse of the structure, or increase the size of the constituent particles. The main cause of the sintering is the lower Tammann temperature (T_T).

In order to reduce the sintering-induced capacity decay, the incorporation of high Tammann temperature TT stabilizers have been proposed. Stabilizers work in two different ways upon incorporation especially in CaL: (i) forming solid solution with CaO and (ii) no solid solution formed with CaO. The effective incorporation of stabilizer depends on the two main aspects: (i) homogeneous distribution on the support, and (ii) minimized quantity. In that sense, atomic layer deposition (ALD) has rose as the attractive way of stabilizer incorporation since it can deposit thin films in atomic-level thickness with high conformity[12]. Moreover, the structure of the sorbent must contain certain features in order to enhance the performance capacity. Nano-structured morphology, certain level of porosity and multi-shelled nature of the structure are found to be desirable. Various new synthesis method such as hydrothermal approach, evaporation induced self-assembly (EISA) approach, utilizing hierarchical carbon nanosheet (CNS) and porous gas-separation membrane have been proposed[13]. Among them, templating synthesis is the most widely-employed method for developing structure with hollow interior and mesoporous outer shell. Templating method can be divided into soft and hard templating and the latter one is potentially more attractive due to its greater stability, predictability and controllability. In this review, various recent developments on the synthesis and enhancement methods of CO₂ capture and storage techniques, is discussed, concentrating on the use of metal oxide in the process.

2. Carbon Dioxide Capture and Storage

At higher pressure of the adsorption isotherms of gases at temperature near their condensation points, the isotherm convex towards the pressure axis, which may be the indication of the formation of multimolecular adsorbed layers[14]. DeBoer and Zwicker explained this multimolecular adsorption by the polarization theory[ 15]. However, this paper points out the limitation of the polarization theory then derives the isotherm equation of multimolecular adsorption by generalizing Langmuir’s treatment of the unimolecular layer and applies the equation to various experimental isotherms. Porous structure and large pore volume are the key of the performance of an amine-impregnated adsorbent [16] (Fig. 2). Since most porous materials can only interact weakly with CO₂, amine functionalization either by chemical grafting or physical impregnation is essential to overcome that shortcoming. Facile generation of hierarchical nitrogen-rich carbon nanosheet (CNS) with large pore width and pore volume is proposed in this study. CNS is synthesized by one-step carbonization of glucose and dicyandiamide. CNS is physically impregnated with pentaethylenehexamine (PEHA) for the enhancement of CO₂ adsorption. SEM and TEM images reveal the architecture of the synthesized CNS. 3-D Channels made up of micro-sized flakes, interconnecting uniform graphene-like layers of CNS, constitute the meso-macroporous structure of CNS. N₂ adsorption isotherm further suggests the presence of abundant macropores in CNS. BET (Brunauer-Emmett-Teller) method calculates specific surface area of CNS as 1,410 m²/g. Total pore volume of CNS is calculated as 8.41 cm³/g with meso-macropore volume of 8.29 cm³/g and micropore volume 0.12 cm³/g. Raman spectrum reveals the presence of sp³ and sp² carbon in CNS structure and abundant defects and disorders at carbon layer edges by strong D band. XRD reveals low crystalline and few-layered CNS structure. PEHA-CNS showed much higher CO₂ capture capacity than CNS due to the strong chemical interaction of PEHA with CO₂. The optimized temperature for CO₂ adsorption of PEHA-CNS composites is 75°C and optimal PEHA loading is 4 g/g. CO₂ uptake capacity at 75°C with 4 g/g PEHA is 5.45 mmol/g, which is the highest among known amine-impregnated absorbents. Due to the large pore width and volume of CNS, amine utilization of PEHA-CNS composites also shows competitive efficiency. Residual calcium chemicals (RCC) is an alternative sorbent to achieve cost-effective improvement of CaL process[17]. RCC are the hazardous wastes derived from dry processes for acid gas removal from flue gas in waste-to-energy (WtE) facilities. RCC is the feasible alternative sorbent for CaL process because they possess high content of available calcium since it is the product of the reaction between acid pollutants and Ca(OH)₂. Also, recycling RCC as the CaL sorbent is environmentally beneficial in a sense that it improves the sustainability of WtE flue-gas cleaning system since the gas-solid carbonation has been already suggested as a safe disposal method of RCC. Calcium is the most abundant element in RCC samples due to the injection of Ca(OH)₂ to the flue gas as a typical process of dry processes for acid gas removal. XRD, TGA and DTG data reveals the main component of the samples as Ca(OH)₂, CaOHCl as a product of reaction with HCl, CaSO₄ as a production of reaction with SO₂, and CaCO₃. The morphology of samples are shown by SEM. RCC samples show typical Ca(OH)₂ morphology of hexagonal plate-shaped nanograins and uniform coating of CaCO₃ product layer around Ca(OH)₂ nanograins. TG-FTIR then shows the point of generation of evolved gas including H₂O, CO₂, HCl and SO₂ gas during the thermal decomposition of residues. RCC samples were pre-calcined at 800°C in N₂ atmosphere then carbonated under 60 vol% CO₂ at three different temperatures: 500, 600, 700°C. Three RCC samples with different elemental composition were tested. When only amount of calcium is concerned, RCC-FC has the most abundant calcium, then RCC-FE and then RCC-RN. All RCC samples show a fast decay during the first 3 cycles, then recovery peaking, then gradual decline after multiple cycles. Electrochemical method is the new approach in carbon capture and storage (CCS) technique[18]. It usually involves applying electrical field across and electrochemical cell comprised of electrolyte and electrodes, however, new electrodeless high-temperature electrochemical cells have been developed in this work. Instead of the electrical potential, the chemical potential gradient of CO₂ and O₂ across the mixed conducting membrane is used as the driving force of the CO₂ transport. High temperature nature of the cell could fasten the surface reaction kinetics without the metal catalyst. Two mixed conducting membranes used are oxide-carbonate and metal-carbonate. The surface reactions enabling CO₂ separation in each membrane are respectively, CO_2(g) + O^2-_(oxide) ↔ CO₃^2-_(carbonate) for mixed oxide-ion and carbonate-ion membrane (MOCC), and CO_2(g) + 1/2O₂(g) + 2e^-_(metal) ↔ CO₃^2-_(carbonate) for mixed electron and carbonate-ion conductor (MECC). In MECC, since CO₂ and O₂ permeate through the membrane, further separation of those gases is necessary. If H₂ or syngas is used as a sweep gas, CO₂ and O₂ can be converted to pure H₂O and CO₂ stream which can be easily condensed as pure CO₂. Ag-molten carbonate (MC) MECC membrane with ALD-coated Al₂O₃ thin film was synthesized in this study to understand its high oxygen permeation rate. When the permeation flux of CO₂ and O₂ through the ALD-Al₂O₃-overcoated Ag-MC MECC membrane was calculated, remarkably high O₂ flux was recorded. For the gas separation membrane to be effective, it is required to have high flux and high selectivity[19]. High flux can be obtained by thin membrane and high selectivity can be obtained by small pores on the membrane with diameter less than 0.5 nm. As the pore size becomes smaller and smaller, it becomes extremely harder to manufacture, obtain size uniformity and reproducibility. Recently, “cross-linking” amphiphilic monomers in type 1 bicontinuous cubic (Q₁) lyotropic liquid crystal (LLC) phase with retention of phase microstructure has been suggested as the method for polymer membrane manufacture which has uniform sub-1-nm pores. The effective pore size of this microstructure has found to be 0.75 nm, which is still large for the light gas separation through size discrimination, noting average kinetic diameters of light gases are less than 0.4 nm with size differences within 0.1 nm. Post-treatments after first-generation Q₁ phase LLC polymer membranes generation by surface film deposition are suggested to decrease the effective pore size of membranes. Among thin film deposition techniques, atomic layer deposition (ALD) is almost the only method which can control the thickness of the coating at sub-nanometer scale. The experiment was done to modify nanoporous, supported Q₁-phase LLC polymer membranes by depositing ultra-thin ceramic films or clusters inside the pores via ALD. This work showed potential modification method of LLC nanoporous polymer membranes using ALD to successfully decrease effective pore size without creating any defect so that it can be applied in light gas separations via molecular size discrimination.

2.1. Calcium oxide

The structure of the CaO-based sorbent is critical in enhancement of CO₂ uptake capacity in calcium looping reaction[20] (Fig. 3). It has been reported that CaO-based sorbents with nanostructure and a certain level of porosity possess high and stable CO₂ uptake capacity. In this work, cage-like CaO hollow microspheres are successfully synthesized via template-assisted synthesis approach, where carbonaceous spheres (CSs) derived from hydrothermal reaction of starch are used as templates. SEM images show that hydrothermal treatment at 180°C with duration of 3 hours result in the desirable morphology of CSs with smooth surfaces and narrow particle size distributions. CaO-based sorbents are prepared from 0.5M Ca(NO₃)₂ solution, CSs template and precipitants. Types of precipitant determines the morphologies of the sorbents; among 3 different precipitants, sodium carbonate (SC), oxalic acid (OA) and urea(U), only urea results in the cage-like hollow microsphere structure. SEM and TEM images clearly show the cage-like hollow microsphere structure of CaO-U with highly porous shells consist of CaO nanoparticles. Polycrystallinity of obtained structure is shown by diffraction pattern (SAED). This completely different morphologies of synthetic CaO sorbents is due to (i) formation route and (ii) amount of the precipitated CaCO₃. Finally, via calcination, cage-like CaO hollow microspheres are obtained. Urea assists the deposition of Ca ions on the hydrophilic template surface in short period of time which provides the thick shell walls that does not collapse easily during calcination. Successful Ca ions deposition on the surface layer of the template is shown by increase of zeta-potential value. Synthetic CaO-based sorbents and reference limestone were tested for 15 cycles under both mild and harsh conditions. All synthetic sorbents show significantly higher initial CO₂ uptake capacity than that of limestone, especially at the initial rapid chemical controlled stage of carbonation. Cage-like CaO hollow microspheres possess the highest capacity of 98.2% under mild condition and 82.5% under harsh condition due to the highest specific surface area. During 15 repeated cycles, all of the sorbents experienced the decay in CO₂ uptake but cage-like CaO hollow microspheres show the lowest decay, maintaining 39.7% of CO₂ uptake under harsh condition. When it was put under 30 cycles, hard condition, it maintained 37.7% of CO₂ uptake. In order to understand the kinetic of carbonation reaction, two models have been used: (i) grain model for initial rapid chemical-controlled stage and (ii) three-dimensional diffusion model for diffusion-controlled stage. Reaction rate constant of each stage is calculated by the equation of respective model. It is found that the rate constant of chemical-controlled stage is always larger than the diffusion controlled stage and the rate constants for 15^th cycle are lower than those in the first cycle, showing the decrease in CO₂ capture capacity over the cycles. The rate constants and activation energy of cage-like CaO hollow microspheres are much larger than those of limestone. Performance of metal oxides are summarized in Table 1.

2.2. Modified calcium oxide

Deactivation of CaO-based sorbents has been generally attributed to the sintering-induced pore volume and surface area reduction, but it is unclear if these are the sole reason for deactivation[13]. This work identifies the deactivation mechanism of Ca₃Al₂O₆-stabilized CaO by studying structural and chemical properties of Ca₃Al₂O₆-stabilized CaO and their changes after repeating cycles with Al magic-angle spinning (MAS) solid- state NMR, dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS), in situ XRD, Raman spectroscopy, electron microscopy coupled EDX spectroscopy and N₂ physisorption. Ca₃Al₂O₆-stabilized CaO is prepared via an evaporation induced self-assembly (EISA) method using pluronic P-123 and metal oxide precursors, aluminum isopropoxide and calcium acetylacetonate (Ca : Al = 90 : 10). EISA method ensures the sorbent structure to obtain high pore volume in desired meso-porous range and homogeneous distribution between active CaO and the stabilizer, Ca₃Al₂O₆. It was calcined at 400°C and further temperature was raised to 700°C for CaO formation. XRD confirms the formation of metal oxides, Ca₁₂Al₁₄O₃₃ at 700~800°C and Ca₃Al₂O₆ at 800~900°C in the calcined sorbent. Homogeneous distribution of Al in as-synthesized sorbent and 400°C-calcined sorbent is shown by STEM with EDX. N₂ physisorption reveals comparatively high pore volume (0.19 cm³/gsorent) and surface area (29 m²/g_sorbent) of 900°C-calcined sorbent and hierarchical pore size distribution with rich amount of small mesopores (d < 10 nm). The cyclic CO₂ capture capacity of Ca₃Al₂O₆-stabilized CaO is assessed by TGA at realistic operating condition i.e., 900°C in pure CO₂. Significantly increased cyclic stability is observed for Ca₃Al₂O₆-stabilized CaO, retaining 0.34 gCO₂/g_sorbent after 30 cycles, when compared to that of limestone derived unsupported CaO (0.08 gCO₂/g_sorbent). Metal organic frameworks (MOF) are the porous, crystalline materials consist of self-assembled network structure of transition metal ions and organic ligands[21]. The structure, crystal size and morphology of the MOF precursor and synthesis method, calcination condition and surfactant types all significantly affect the morphology of the MOF-derived material. In this article, Al₂O₃-stabilized, MOF-derived, Ca-based CO₂ sorbents have been synthesized via a combined solvothermal and wet mixing method. 2-aminoterephthalic acid (H₂-ABDC), terephthalic acid (PTA) and isophthalic acid (IPA) are the three chosen organic ligands for MOF. FT-IR results showed the successful synthesis of MOFs driven by each organic ligand. XRD patterns then showed the chemical composition of Al-stabilized, MOF-driven sorbents as CaO, Ca(OH)₂, and calcium aluminum solid solution (Ca_xAl_yO_z), which were further identified as Ca₁₂Al₁₄O₃₃, Ca₅Al₆O₁₄, and Ca₃Al₁₀O₈. These calcium aluminum solids were formed by the reaction of CaO + Al₂O₃ → Ca_xAl_yO_z and they are the main cause of cyclic stability enhancement of the sorbents. The effect of the CaO/Al₂O₃ mass ratios on CO₂ sorption performance was analyzed. Increased stability of the stabilized sorbents is due to the prevention of agglomeration or sintering by the stabilizer, Al₂O₃ or Ca_xAl_yO_z, with high Tammann temperature. SEM images showed surface agglomeration and non-uniform pores on the unstabilized sorbents. It also showed the optimized micro- morphologies of MOF precursors with a blocky shape and MOF-derived sorbents with uniform pores and smoother surface. Even after 10 cycles, MOF-derived sorbents retained block-shape crystals. The highest sorption performance of 0.71 gCO₂/g_sorbent^-1 with 85.07% retention after 10 cycles was obtained with CaO/Al₂O₃ mass ratio of 30 : 1. Presence of calcium aluminum solids act as physical barriers, not participating in the cyclic reaction, successfully prevent agglomeration and sintering among particles. The self-activation phenomenon of MOF-derived sorbents also contributed to the sorption capacity enhancement. A hollow, microtubular Ca/Al CO₂ sorbent was synthesized via hard templating method, using absorbent cotton with a hollow microtubular structure as a template, limestone as a CaO precursor and Al(NO₃)₃ as an Al₂O₃ precursor, to be utilized in the calcium looping process (CLP)[22]. The XRD patterns show that the main components of the synthesized sorbent are CaO and Ca₁₂Al₁₄O₃₃, which is formed by the reaction between CaO and Al₂O₃. Ca₁₂Al₁₄O₃₃ hinders the sorbent’s deformation due to the sintering and therefore improve the cyclic stability. When comparing SEM images of Ca-Al-T and Ca-Al (sorbent without the absorbent cotton template), the effect of the addition of adsorbent cotton is evident. Ca-Al-T replicates the hollow microtubular morphology of the absorbent cotton, with diameters ranging from 0.5 to 5 μm, while Ca-Al do not contain hollow microtubes. Many mesopores are also present on the present of the Ca-Al-T. This structure of Ca-Al-T increases the contact area between CaO and CO₂ which enhance the CO₂ capture capacity. Electron diffraction (SAED) pattern indicates well-poly-crystallized structure and good stability of the sorbent structure. Different mass ratios of CaO to Al₂O₃ determines the mass ratios of CaO to Ca₁₂Al₁₄O₃₃ in the synthesized sorbent. The mass ratio of 90 : 10 results the highest CO₂ capture capacity of the sorbent, Ca-Al-T, over the cycles, retaining 0.38 g/g during 50 cycles. When the CO₂ capacity of Ca-Al-T was tested under mild (850°C, 100% N₂) and severe condition (920°C, 70% CO₂/30% N₂), it showed 7.91- and 8.93- times higher capacity respectively for each condition than that of the limestone over 50 cycles. Ca-Al-T showed the highest and most stable CO₂ capacity among Ca-Al-T, Ca-T, Ca-Al and limestone over 30 cycles. It showed 1.32 and 5.03 times greater capacity respectively for Ca-Al and limestone under mild condition and 1.30 and 8.24 times greater capacity respectively under sever condition. During 50 cycles, Ca-Al-T showed 7.91 and 8.93 times greater capacity respectively for limestone under mild and sever condition. This excellent CO₂ capture capacity of Ca-Al-T is attributed to (i) hollow microtubular structure and (ii) presence of Ca₁₂Al₁₄O₃₃ as a supporter. Even when the CO₂ capacity of Ca-Al-T, prepared by hard templating, is compared with the Ca/Al absorbents prepared by various methods, hard templating synthesized Ca-Al-T showed higher capacity than any other.

Double-shell, hollow microspheres structure of CaCO₃ was synthesized via hydrothermal reaction of sucrose, using carbonaceous microspheres (CMSs) as hard templates[ 23]. Different shrinking rates between CMSs and metal functional shell results in double-shelled structure. The spacing between shells is short since metal cations are mainly located in the surface of CMSs, which makes the structure more attractive since this close shell spacings further can shorten the transportation path of CO₂ and heat upon carbonation reaction. Four different sorbents were synthesized with different Ca/Mn molar ratios: pure CaCO₃, Ca₇₅Mn₁, Ca25Mn1 and Ca10Mn1. As determined by SEM and TEM images, all four sorbents obtained spherical structures, but as Ca/Mn molar ratio decreased, there was a slight increase in the diameter of the microspheres, as there are more metal cations available with increasing Mn dosage, which fastens the process of reaching critical concentration of forming a metal functional shell. Furthermore, Upon 25 adsorption-desorption cyclic tests, the importance of adequate amount of Mn doping was demonstrated more profoundly; with increasing amount of Mn doping, the adsorption stability and initial uptake capacity were notably promoted. Presence of agglomeration and coalescence among adjacent microspheres of the sorbents with insufficient Mn doping and decreased crystalline size further supports the fact of improved stability by Mn doping due to the microscopical separation of CaO nanocrystal and segregations of adjacent microspheres, forming effective physical barriers. In terms of reaction mechanism of CaO/CO₂, Mn doping promotes the adsorption performance by enhancing electron donating ability of CaO by increasing electron cloud density of Ca and O atom. In carbonation reaction, electrons in O atom of CaO are transferred to C atom in CO₂, so with enhanced electron cloud density, more electrons in O can be transferred to C. By XRD result, the presence of Mn multiple valence states is demonstrated, which can facilitate the occurrence of the oxygen vacancies. The Ca/Mn ratio of 12 : 1 resulted highest capacity. The facile synthesis method of highly porous, hollow, multi-shelled MgO-stabilized CaO microsphere sorbent for calcium looping is offered in this study[24]. MgO is the attractive stabilizer with high Tammann temperature in a sense that it does not form a solid solution under operating condition. The sorbent is synthesized via template-assisted method and the template used is carbonaceous spheres which are formed via hydrothermal treatment of an aqueous solution of xylose, urea and glycine with Ca and Mg precursor, which are Ca(NO₃)₂ and Mg(NO₃)₂ respectively. The template-assisted synthesis approach is found to be highly effective for two reasons: (i) yielding hollow microspheres with highly porous shells and (ii) simultaneously forming template and precipitation of compounds of interest, which are CaCO₃ and MgCO₃. When the template is calcined and removed completely, highly porous, multishelled microspheres which shells are composed of CaO and MgO nanoparticles are realized. SEM and TEM images show a formation of desired structure after thermal removal of carbonaceous template. The average size of shell-comprising CaO nanoparticles are ~85 ± 20 nm, fulfilling an requirement < 100 nm to avoid diffusion limitations, and the average size of MgO nanoparticles are 25 ± 5 nm. TGA study showed that when compared to CaO without a stabilizer, MgO stabilized- CaO exhibit more stable CO₂ uptake with less decay with increasing cycle numbers. Moreover, effectiveness of structure composed of primary particles with < 100 nm in diameter and void space is confirmed by the fact that CO₂ uptake performance of CaO without a stabilizer outperforms that of the limestone-derived pure CaO. The influence of MgO on the sorbent performance is also compared. Indeed, MgO-stabilized sorbents captures larger fraction of CO₂ particularly in the kinetically controlled regime of the carbonation, maintaining quantity of CO₂ to ~0.35 gCO₂/g_sorbent, while unstabilized sorbents all experience substantial decrease in CO₂ uptake. The transition to the diffusion-controlled reaction stage of the carbonation process is also more gradual in MgO-stabilized sorbents. The presence of steam also significantly enhances the rate of CO₂ uptake and hinders an early transition to diffusion-controlled reaction stage.

2.3. Magnesium oxide

Alkaline earth metal oxides are receiving increasing attention as the attractive solid sorbents for CCS technologies[ 25]. Magnesium oxide (MgO) is used in this work as the sorbent and promoted it with alkali metal nitrates (e.g., LiNO₃, NaNO₃, KNO₃) to further increase CO₂ uptake. The CO₂ capture reaction of MgO is carbonation and it is regenerated via calcination reaction: MgO(s) + CO_2(g) MgCO₃(s). Mg₅(CO₃)₄(OH)2⋅4H₂O, hydromagnesite (HM), is chosen as the MgO precursor. Salt mixtures of promoted MgO samples are followings: Li NO₃, NaNO₃, KNO₃, (Li,K)NO₃, (Na,K)NO₃, (Li,Na)NO₃ and (Li,Na,K)NO₃. MgO precursor and salt mixtures were mixed with deionized water, dried and calcinated. SEM showed the grain-like morphology and the morphology is maintained after carbonization in first cycle. XRD patterns showed conversion of MgO to MgCO₃ after carbonation. The peaks of MgCO₃ decreased after 10 cycles due to the decreased CO₂ uptake. Also, XRD patters of alkali metal nitrate promoted MgO showed sharper MgO peaks, indicating the acceleration of MgO crystal growth due to alkali metal nitrates. When the size is compared, MgO crystallite size of pure HM is 7~8 nm, while NaNO₃- or (Li,Na,K)NO₃-promoted MgO is ~20 nm. CO₂ uptake of unpromoted and promoted MgO can be compared with the TGA plot. Unpromoted MgO showed immediate CO₂ uptake at the very beginning while promoted MgOs showed abrupt acceleration of CO₂ uptake after a threshold temperature which is slightly lower than the melting point of the alkali metal nitrate. The promotion with eutectic mixtures of alkali metal nitrates further reduced the onset temperature of MgO carbonation. MgO promoted with LiNO₃, NaNO₃, (Na,K)NO₃, (Li,Na,K)NO₃ were analyzed with Diffuse reflectance infrared spectroscopy (DRIFTS). The analysis confiremd the temperature of which carbonation proceeds in each promoted MgO. Isothermal CO₂ sorption by promoted MgO at 300°C was done to compare the effect of different promoters on carbonation kinetics. The addition of alkali metal nitrates increased the amount of CO₂ uptake and ternary mixture, (Li,Na,K)NO₃ exhibited the fastest kinetics with highest CO₂ uptake of 474 mg CO₂/g sorbent. Integrated gasification combined cycle is the typical example of the precombustion capture of CCS[26] (Fig. 4). Layered double hydroxides and MgO are mainly included in this process but CO₂ capture capacity of the pure MgO is very small. Alkali metal salt-promoted MgO (AMS-MgO) is suggested as the modification of the pure MgO to enhance CO₂ capture capacity by decreasing activation energy of MgO ionic bond and increasing ion diffusivity which derives from the ability of molten nitrates. The operation temperature of the integrated gasification combined cycle which is between 200 to 400°C is enough for the incorporated nitrates to be dissolved. Indeed, AMS-MgO has much higher CO₂ capacity than pure MgO. Yet, slow sorption rate and poor stability of AMS-MgO must be improved. In this article, incorporation of CaCO₃ to MgO is suggested to develop AMS-MgO with high capacity, rapid sorption rated and good cyclic stability. Prepared MgO-CaCO₃ structure is denoted as Mg₉₈Ca₂, Mg₉₅Ca₅, Mg₉₀Ca₁₀, etc. according to the molar ration between MgO and CaCO₃. Following deposition of nitrates and carbonates on MgO-CaCO₃ support for the ultimate AMS supported structure was done using a solvent evaporation method. The structure was denoted as AMS-Mg_100-xCa_x. First, CaCO₃ incorporation was applied to the flower-like MgO structure. By the result of the field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM), the effect of CaCO₃ incorporation on MgO was studied. Even though the structure loses the its spherical morphology, flower-like structure constructed with nanoflakes was remained. Also, the specific surface area and the pore volume of the structure decreased, resulting in larger particles and pores size. The flower-like structure was well remained in the AMS-promoted MgO-based sorbents as well, also with the decrease in the specific surface area and the pore volume. The result of the XRD analysis confirmed the presence of AMS and that of the EDS mapping confirmed the homogeneous distribution of AMS on the support. Then, CO₂ capture performance on the structure was tested under two conditions: the mild condition and the severe condition. Under the mild condition, all sorbents, AMS-Mg₁₀₀, AMS-Mg₉₈Ca₂, AMS-Mg₉₅Ca₅ and AMS-Mg₉₀Ca₁₀ showed high capture capacity and excellent stability due to the incorporation of CaCO₃ on MgO with initial capacity of 0.72, 0.70, 0.65 and 0.57 to 0.66, 0.65, 0.61 and 0.57 gCO₂gMgO -1 after 50 cycles respectively.

2.4. Calcium looping

Along with the carbon capture and storage technology, the capture and subsequent utilization of captured CO₂, called CO₂ capture and utilization (CCU) technology is getting attention[27] (Fig. 5). In order the CCU to be effective, (i) sorbent regeneration and CO₂ conversion must be performed in an integrated fashion, (ii) yield high CO₂ conversion and (iii) avoid using expensive chemical resources. In this study, two-step calcium looping-based CCU process has been proposed. CaO derived from the limestone is used as the sorbent for CO₂ capture in the first step. Then, in the second step, sorbent regeneration and CO₂ conversion are performed simultaneously in a single step through: CaCO₃(s) + CH₄ → CaO_(s) + 2CO + 2H₂. The process is operated in a fluidized bed containing a mixture of an inexpensive limestone-derived CO₂ sorbent (CaO) and dry reforming catalyst, Ni/MgO-Al₂O₃, where dry reforming of methane (DRM) is demonstrated by the equation:

CH₄ + CO₂ → 2H₂ + 2CO. The morphological characteristics of limestone-derived CaO and DRM catalyst are shown by HR-SEM. It is found that active sites of DRM catalysts are metallic Ni nanoparticles on a periclase- type solid solution. Rates of CO₂ release and CO₂ conversion during sorbent regeneration must be very similar for the CCU to be effective. Regeneration temperature of 720°C shows stable CO₂ release which rate matches the activity of Ni-based DRM catalyst during sorbent regeneration, making the rate of CO₂ concentration trade-off between its release and conversion very alike. CO₂ with stable volume fraction of 2.4 vol% was sufficiently obtained under this condition. The CO₂ capture capacity of limestone-derived CaO is determined as 0.62 gCO₂/g_sorbent in the first cycle. In the second step of the process CH₄ : CO₂ in 1 : 1 ratio is introduced in the fluidized bed for CO₂ conversion via the DRM. Three reaction stages, prebreakthrough, breakthrough and postpreakthrough, are observed in this stage and prebreakthorugh is the stage in which almost full conversion of CO₂ into a synthesis gas is observed, with the ratio of H₂ to CO as ~1.06 : 1. In the last stage, almost no DRM or methane decomposition proceeds but only the Ni surface is significantly reduced due to coking.

2.4. Stabilized metal oxide by atomic layer deposition

One major drawback of limestone-derived CaO is its poor cyclic stability[28]. Significantly lower tamman temperature of CaCO₃ than operating condition results severe sintering, deforming surface irreversibly which decreases more than half of CO₂ uptake capacity of CaO within only 5 cycles. Therefore, high tammann temperature material such as Al₂O₃ and MgO is needed to stabilize CaO. In this study, Al₂O₃ was deposited by ALD. ALD process allows conformal film formation with atomic thickness of < 3 nm, which minimizes the quantity of the stabilizer as required to retain high CO₂ uptake capacity. Glucose and calcium were dissolved in water and then mixed with glucose. The solution was kept in an autoclave for 24 h at 170°C for hydrothermal treatment and then the precipitate was calcined at 800°C for 1 h to prepare spherical multiwall mesoporous CaO. Smaller CaO spheres (3~4 μm) resulted from the slow template removal comparing to those resulted from fast template removal by oxygen plasma treatment (> 5 μm). CO₂ uptake capacity of CaO-based CO₂ sorbent was analyzed with TGA and compared with the limestone-derived CaO. Even from the first cycle of calcination and carbonation, all synthesized CO₂ sorbents exceeded the performance of limestone-derived CaO by more than 30% and in the second cycle, synthesized sorbents almost reached their full theoretical capacity. But beyond 10 cycles CO₂ sorption capacity reduce drastically. In order to improve cyclic CO₂ uptake performance, the synthesized sorbents were stabilized by ALD-grown, nanometer-thick, conformal Al₂O₃ layers. CO₂ uptake in sorbents stabilized with ALD grown Al₂O₃ layer are lower than that of unstabilized material, and as thickness of the layer increases, uptake capacity decreases more. However, high capacity retention percentage of stabilized materials is noticed. The novel structure of the CaO-Al₂O₃ nanocomposite sorbents is successfully synthesized by a limited-space metal organic chemical vapor deposition (MOCVD) method in this work[29]. CaCO₃ substrate is mixed with the aluminum precursor, aluminum acetylacetonate and sealed in an ampoule. Temperature was raised to 280°C and then to 400°C and finally calcinated at 850°C in air to generate CaO-Al₂O₃ nanocomposite. TEM measurement showed that the CaO are coated by Al₂O₃ nanoparticles of around 4~8 nm (Fig. 6). It was observed that after 20 cylces the CaO in the presence of 10 % Al₂O₃ has 300% higher CO₂ adsorption than pristine CaO. Yuzbasi et al. reported a synthesis of a porous copper foam by electrochemical deposition method[ 30]. Al₂O₃ was deposited on the coppery foam by ALD and finally it was calcined at 800°C for 1 h to get rid of the PET substrate onto which the foam was developed. Copper oxide with 20 nm thick Al₂O₃ shows the 96% capacity retention upto 10 redox cycles.

3. Conclusions

Carbon capture and storage (CCS) technologies are crucial in mitigating constantly increasing anthropogenic carbon dioxide emission. Metal oxides used in recently developed CCS technologies, CLC and calcium looping, must undergo certain modifications to enhance performance capacity largely due to the sintering-induced capacity deterioration over the multiple cyclic reactions. Different types of stabilizers with high Tammann temperature, such as Al₂O₃, MgO, TiO₂, Mn, alkali metal nitrates, etc are incorporated into metal oxides via various methods including atomic layer deposition (ALD). Then, various synthesis method including templating method, hydrothermal approach, evaporation induced self-assembly (EISA) approach and use of carbon nanosheet and porous gas-separation membrane are used to develop overall metal oxide based-CO₂ capture and storage material. Good performing structures often contain a certain level of porosity, nanometer particle size and hollow interior. This review is classified into two parts: In part one CCS based on metal oxide and in part two thin film deposited metal oxide are discussed.