Enhanced CO2 adsorption of MgO with alkali metal nitrates and carbonates

Applied Energy 263 (2020) 114681

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Enhanced CO2 adsorption of MgO with alkali metal nitrates and carbonates a

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Jing Ding , Chao Yu , Jianfeng Lu , Xiaolan Wei , Weilong Wang , Gechuanqi Pan a b c

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School of Materials Science and Engineering, Sun Yat-sen University, School of Intelligent Systems Engineering, Sun Yat-sen University, Guangzhou 510006, China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China School of Data and Computer Science, Sun Yat-sen University, Guangzhou 510006, China

H I GH L IG H T S

sorbents were synthesized by deposition of alkali metal salts. • MgO-based CO adsorption capacity of the sorbent is enlarged as 19.06 mmol·g • The • The adsorption enhancement mechanism is discussed in detail.

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A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 capture MgO adsorbent Moderate temperature Alkali metal salts

Carbon capture and storage is an effective way to mitigate the accumulation of greenhouse gases in the atmosphere. In this work, a series of MgO-basedadsorbents were synthesized by deposition of a mixed alkali metal nitrates and carbonates. The CO2 capture amount of the compound adsorbent is enlarged as 19.06 mmol·g−1 at 325 °C when loading 10% mole of [(Li0.44K0.56)NO3]2[(Na0.5K0.5)CO3]. The as-synthesized adsorbent also exhibits stable CO2 capture performance after long-term adsorption/desorption cycles. The effects of the molar ratio of alkali metal salts and adsorption conditions were investigated. The adsorption enhancement mechanism is discussed regarding the changes of composite microstructure during the reaction. It is found that the CO2 uptake curve has three adsorption stages corresponding to the interactions between CO2, MgO/metal nitrates and carbonates. The nitrite product plays a key role in the improvement of CO2 uptake since it not only yields more O2− but also reacts with MgO in molten nitrites to produce an intermediate nitrato compound, which leads to the rapid nucleation of MgCO3 by triggering lattice defects. It is found that the CO2 uptake decreased from 19.06 to 15.7 mmol·g−1 over 30 cycles, which proves that the new adsorbents have a good long-term adsorption/desorption stability.

1. Introduction Efficient CO2 capture is of great importance to control global climate change since CO2 is widely recognized as a major greenhouse gas [1]. Recently, the development of carbon capture and storage (CCS) technologies is considered as a practical way to mitigate CO2 emission. Among various CCS technologies, pre-combustion capture can improve energy efficiency by generating electricity and yielding high-purity H2 simultaneously [2]. Taking an integrated gasification combined cycle as an example, the syngas produced through coal gasification mainly consists of CO2, CO, and H2, which is provided for a water gas shift reactor in the range of 200–400 °C. Therefore, high-temperature solid CO2 adsorption has potential to replace low-temperature amine solution based technical method regarding cost, energy consumption, corrosion and efficiency penalty [3,4]. ⁎

In recent years, various high-temperature adsorption technique has been developed and employed for CO2 capture including layered double hydroxides (LDHs) and alkali and alkaline metal oxides [5]. Among them, LDHs based adsorption method still face a big challenge when using in a large-scale practical application due to their low adsorption amount [6]. Recently, MgO based adsorption technique are recognized as a potential candidate for industrial pre-combustion CO2 capture due to their unique properties, such as high adsorption amount, low-cost, eco-friendly, and noncorrosive [7]. In the adsorption process, chemical reaction takes place with low adsorption heat (MgO (s) + CO2 (g) → MgCO3 (s), ΔH = ~−106 kJ·mol−1). Furthermore, desorption process needs low energy consumption for adsorbent regeneration (MgCO3 (s) → CO2 (g) + MgO (s), ΔH = ~97.2 kJ·mol−1) in comparison with other representative methods (e.g., Li2CO3 (s) + Li2SiO3 (s) → CO2 (g) + Li4SiO4 (s), ΔH = ~142 kJ·mol−1; CaCO3 (s) = CO2

Corresponding author. E-mail address: [email protected] (W. Wang).

https://doi.org/10.1016/j.apenergy.2020.114681 Received 7 October 2019; Received in revised form 8 February 2020; Accepted 13 February 2020 0306-2619/ © 2020 Published by Elsevier Ltd.

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(g) + CaO (s), ΔH = ~170 kJ·mol−1) [8,9]. Low energy consumption is of importance for its industrial commercial application. However, this approach has several defects. Firstly, the reaction kinetic is mild due to the strong lattice enthalpy of solid metal oxides [10]. Secondly, CO2 diffusion becomes worse since the produced carbonates cover on the surface of metal oxides [11,12]. The theoretical CO2 adsorption capacity of MgO is 24.8 mmol·g−1 [13], while the actual uptake is reported less than 1 mmol·g−1 [14,15]. These defects will hinder the commercialization and application of this new technology Some work has been done to improve the CO2 uptake amount of MgO based CCS technology, including optimizing working conditions [16,17], doping promoters [18–20], and enlarging surface area [21,22]. By comparison, adding alkali molten salts is considered as an effective way. Zhang developed a NaNO3-modified MgO adsorbent via a ball milling and calcination, and the CO2 capture capacity is 15 mmol·g−1 at 330 °C [23]. Jo found that molten NaNO3 worked as a reaction medium by dissolving both MgO and CO2, leading to fast nucleation and growth of MgCO3 [24]. Amesoporous MgO-NaNO3-Na2CO3 composite shows a high CO2 uptake of 11.5 mmol·g−1 in a gas mixture (2.5% H2O, 10% CO2, and balanced N2) and 12.7 mmol·g−1 in pure CO2 at 325 °C. Vu believed that NaNO3 worked as a reaction promoter while Na2CO3 was a CO2 carrier during the adsorption process [25]. Harada obtained a new colloidal nanocluster of MgO by deposition of various alkali metal nitrates/nitrites. The maximum CO2 uptake was 15.7 mmol·g−1. Harada [26] and others [27,28] reported that molten alkali metal nitrates/nitrites can facilitate CO2 diffusion by providing a liquid channel. And the addition of nitrites is the key factor for adsorption enhancement since the formation of MgO-NOx may increase the critical thickness of the product-layer [9]. (Li,K)NO3−(Na,K)2CO3 salts could further improve the CO2 uptake capacity up to 16.6 mmol·g−1 at 350 °C [29]. So far, it makes a good progress in improving the CO2 adsorption performance of MgO. However, the adsorption capacity of the reported AMS-MgO adsorbents usually decreases rapidly after long-term adsorption and desorption cycles, which is a major block for their practical application. For instance, Qiao prepared a (Li, Na, K)NO3-MgO adsorbent whose uptake was decreased from 16.8 to 3.2 mmol·g−1 over 20 cycles [27]. Another example is that the uptake of (Li, K)NO3−(Na, K)2CO3-MgO adsorbent dramatically dropped by 45% after 30 cycles [29]. Therefore, it is still a big challenge to develop highly-efficient MgO compounds with a good cyclic stability. To our best knowledge, there is quite few work reported focusing on CO2 adsorption/desorption stability of solid adsorbents, and the role of molten salts during the adsorption/desorption process is still not completely understood, which plays a key role on its practical CCS application in industry. In this work, a new kind of MgO/[(Li0.44K0.56) NO3]2[(Na0.5K0.5)CO3]compound for CO2 capture were synthesized, and the CO2 uptake performance was studied. The influence of the molar ratio of alkali metal salts (AMS) and adsorption/desorption temperatures was investigated for the optimal operating conditions. The structure characterizations and surface morphology of the adsorbents were detected and analyzed during the adsorption process to find the structure-property relationship. The adsorption and desorption stability of adsorbents is also investigated experimently.

methanol evaporation-induced surface precipitation[9]. Firstly, a fixed amount of MgO was dispersed in the 80 ml methanol solution of the alkali metal nitrates and carbonates ([LiNO3]: [KNO3] = 0.44:0.56, [Na2CO3]: [K2CO3] = 0.5:0.5, and the molar ratio of nitrates and carbonates was fixed to be 2:1). Subsequently, the white suspension mixture was ultra-sonicated for 1 h after evaporated methanol in a rotary evaporator at 80 °C. Then, the solid sample was groundinto powder by mechanical grinding and then sealed in a desiccator. Before the adsorption test, all as-prepared samples were calcined in an oven at 450 °C for 4 h to eliminate the impact of moisture and CO2 on the surface of the adsorbents. The as-prepared MgO adsorbents were denoted as AMSX-MgO, where X represents the molar ratio of AMS to MgO is X:100, and the value is varied from 5 to 15 to examine the influence of the amount of mixed alkali metal nitrates and carbonates. Powder X-ray diffraction (XRD) method was used to explore the composition of samples on an Empyrean X-ray diffract meter with a copper target using Cu Kα radiation. XRD was recorded at a scanning speed of 1.2°·min−1 in the 2θ range of 20–80°. Bond vibration of adsorbents was investigated using a Fourier transform infrared spectroscopy (FT-IR) at room temperature. The size and morphologies of the as-prepared materials were measured using a high-resolution scanning electron microscopy (SEM) (JSM-6330F). 2.2. Evaluation of CO2 uptake ability The performance of CO2 capture at 275–375 °C and 1 bar was evaluated by a thermogravimetric method on the TGA/SDTA851e. The procedure was detailed as follows: Firstly, the temperature was increased to 450 °C and kept for 1 h to eliminate the effect of CO2 and moisture. The adsorption process was conducted by introducing high purity CO2 to a chamber at 275–375 °C for 4 h. The data were collected with the various adsorption/desorption temperatures on CO2 capture. 3. Results and discussion 3.1. Sample characterization analysis Fig. 1(a) and (b) displays XRD and FT-IR spectra of AMS10-MgO. The pattern of pure MgO is used as a reference. The distinct diffraction peaks of Mg(OH)2 (2θ = 37.98° and 58.67°, JCPDS 07–0239) are observed before calcination since nitrate salts adsorb H2O easily from the air. The synthesized product is identified as a composite of MgO (JCPDS 75-0447), K2CO3 (JCPDS 49-1093), Na2CO3 (JCPDS 37-0451), KNO3 (JCPDS 71-1558), NaNO3 (JCPDS 36-1474) and LiNaCO3 (JCPDS 341193) after calcinating at 450 °C for 4 h. The existence of NaNO3 and LiNaCO3 suggests that the ionic recombination takes place during AMS10-MgO calcination. LiNO3 is not detected, which might because some Li+ ions contribute to form LiNaCO3 [29]. After adsorbing CO2, the peak of MgCO3 (JCPDS 08-0479) and K2Mg(CO3)2 (2θ = 32.6°, JCPDS 33-1495) become larger gradually, which means that the final reaction products are MgCO3 and K2Mg(CO3)2. In the FT-IR spectrum, peaks at 825 cm−1 and 1384 cm−1contribute to out-of-plane bending vibration [31] and degenerated N-O stretch doubly of NO3− [27,32], respectively. It proves that nitrates exist in the whole process. After exposing to CO2 for 4 h, three new peaks appear at 749 cm−1, 886 cm−1, and 1440 cm−1corresponding to in-of-plane bending, outof-plane bending and an asymmetric stretch of the carbonate ion (CO32−), which proves that the reaction with CO2 has generated a large number of carbonates. The captured CO2 on the MgO surface leads to the appearance of the weak peaks, such as the peak at 862 cm−1 ascribes to various surface carbonate species [33–35]. The morphology change of AMS10-MgO is displayed in SEM and elemental mapping images (Fig. 2a–h). The adsorbents are granular shaped before calculation and the particle size becomes smaller and well dispersed after 4 h calcination at 450 °C. After adsorption, the samples show agglomerated and slice particles with larger sizes.

2. Experimental section 2.1. Sample preparation All the pure salts (KNO3, LiNO3, K2CO3,and Na2CO3) were bought with A.R. grade (99.0%, Sinopharm Chemical Reagent), and dried in an oven at 120 °C for 24 h to remove the moisture. The molten salt mixtures were prepared by melting blending method and then kept in a glove box [30]. The preparation of MgO (≥98.5%, Sinopharm Chemical Reagent)/ [(Li0.44K0.56)NO3]2[(Na0.5K0.5)CO3] adsorbents was carried out by deposition of mixed nitrates and carbonates on the surface of MgO via 2

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Fig. 1. (a) Powder XRD patterns of MgO and AMS10-MgO at different periods (b) FT-IR spectra of AMS10-MgO at different periods (Adsorption conditions: 325 °C, 1 atm, 100% pure CO2, 4 h. Calcination conditions: 450 °C, 1 atm, 100% pure N2, 4 h.)

Also, this new adsorbent has a good adsorption/desorption stability. After 30 cycles testing, the CO2 uptake is decreased by 17%.

Additionally, the elemental mapping images of Mg, Na, K and N (Fig. 1(e)–(h), respectively) reveal that the slice particles in Fig. 1(b) and (d) are NaNO3, which further confirms that the ionic recombination takes place during AMS10-MgO calcination.

3.3. Mechanism of sorption enhancement To further investigate the enhancement mechanism of CO2 adsorption, the crystalline structure of AMS10-MgOduring CO2 adsorption process was studied using XRD and FT-IR analyses. The patterns are depicted in Fig. 5. At the beginning of adsorption, the XRD patterns of AMS10-MgO shows the presence of MgCO3 (JCPDS 08-0479), Li2CO3 (2θ = 30.6°, JCPDS 22-1141), Na2CO3 (JCPDS 37-0451), LiNaCO3 (JCPDS 34-1193), KNO2 (2θ = 26.6°, JCPDS 79-1985) and K2C2O6 (2θ = 27.4°, JCPDS 22-0807). Subsequently, LiNaCO3 peaks disappear and Li2CO3 peaks become larger, indicating that LiNaCO3is transformed into Na2CO3 and Li2CO3. At the same time, KNO2 and K2C2O6 phases are also weakened gradually. The final reaction products are Na2Mg (CO3)2 (2θ = 34.4°, JCPDS 24-1227) and K2Mg(CO3)2 (2θ = 32.6°, JCPDS 33-1495), which is consistent with the previous study [25,32]. In the FT-IR spectrum, peaks at 749, 886 and 1440 cm−1corresponding to the carbonate ion (CO32−) grow rapidly, which illustrates that the products are various types of carbonates. Especially, the sharp peak at 3056 cm−1 belongs to the mono-coordinated hydroxyl groups, and the peak at 3421 cm−1 is ascribed to the stretching vibration caused by the interaction of surface OLC2− ions with four-coordinated hydroxyl groups [40,41]. Peaks at 2391 cm−1 are assigned to antisymmetric vibration of NO+ ions, while peaks at 1100 and 1764 cm−1 correspond to NeO stretching vibrations for nitrato compounds [9,36]. Regarding the formation of nitrato compounds, it is assumed that partial NO3− transforms NO2−. Then the nitryl ion provided by NO2− reacts with MgO to form some MgO-NOx surface species as alkali nitrites decompose into nitryl ion and O2− by adding oxide ion acceptors [42]. The disappearance of peaks at 1764 cm−1 suggests that MgO–NOx is an intermediate product. The peaks at 1384 cm−1 for NO3− prove that nitrates have remained in the whole process of the reaction. Based on the CO2 adsorption behavior of AMSX-MgO (in Fig. 3) and the XRD and FT-IR results in Fig. 5(a) and (b), the CO2 capture enhancement mechanism of salts is put forward: The CO2 uptake curve of AMSX-MgO evidently shows a “three-stage” adsorption process, which means that the kinetic characteristics of the reaction vary with the adsorption time. To better explain the affecting mechanism, the CO2 sorption process is divided into three stages by the reaction time:

3.2. CO2 capture performance analysis Fig. 3(a) compared the CO2 capture behavior of various adsorbents. Evidently, the amount of AMS and the adsorption temperature can influence CO2 sorption kinetic, and thus it is meaningful to optimize the operating condition to improve CO2 sorption performance of AMSXMgO. Among the adsorbents, AMS10-MgO possesses the highest CO2 uptake of 19.06 mmol·g−1 after contacting with CO2 for 4 h at 325 °C. The impact of temperature on sorption kinetics and CO2 uptake performance of AMS10-MgO is illustrated in Fig. 3(c)–(e). It is found that the initial adsorption rate is faster at lower temperatures since the pretransition time to the acceleration reaction stage is shorter. When the temperature further increases to 375 °C, the adsorption capacity sharply decreases to 9.42 mmol·g−1. The CO2 capture performance is greatly influenced by temperature. Positively, higher temperatures can accelerate the reaction between MgO and CO2 because of faster kinetics; However, when the temperature exceeds the optimal value, the dissolution of CO2 in molten promoter declines with the increase of temperature, resulting in less CO2 being dissolved in metal salts. Moreover, since the adsorption heat of the reaction is negative, the formation of MgCO3 is inhibited at higher temperatures. Thus, it is concluded that CO2 capture performance is determined by all these positive and negative factors. Finally, the optimal operating temperature of AMS10MgO is 325 °C. It is noticed that the CO2 uptake curve of AMS10-MgO can be divided into three stages in Fig. 3(d) and (e). It means that the kinetic characteristics of the reaction vary with the adsorption time. This phenomenon will be further explained in detail in the following mechanism discussion section. As for CO2 capture applications, the cyclic adsorption-desorption properties of adsorbents are typically taken into account. The stability of AMS10-MgO was examined by thirty consecutive adsorption/desorption cycles based on the static method. As shown in Fig. 4, the total CO2 adsorption capacity has a small decrease over cycle testing, remaining about 15.70 mmol·g−1 after 30 cycles, which implies the good long-term regenerability of the adsorbent. To contrast the CO2 adsorption/desorption stability of other adsorbents, the cyclic adsorption performance of AMS-MgO adsorbents is compared in Table 1. It can be seen that compared to other adsorbents, the new adsorbents in this work have a largest CO2 adsorption amount.

Stage I. Formation of carbonates on exposed MgO (0 < t < 7 min)

3

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Fig. 2. SEM images of AMS10-MgO (a) before calcination, (b) after calcination, (c) after CO2 adsorption, (d) after regeneration and (e–h) Na, N, Mg and K elemental mapping images ofAMS10-MgO after regeneration, respectively (Adsorption conditions: 325 °C, 1 atm, 100% pure CO2, 4 h. Calcination conditions: 450 °C, 1 atm, 100% pure N2, 4 h.)

4

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Fig. 3. (a) CO2 capture performance, (b) adsorption capacity of AMSX-MgO at 325 °C and 350 °C, (c) CO2capture performanceofAMS10-MgO at different temperatures and (d) Partial enlargement of (c); (e) CO2 sorption rate of AMS10-MgO at different temperatures (Adsorption conditions: 1 atm, 100% pure CO2, 4 h.)

reacts with the dissolved MgO to generate K2Mg(CO3)2. And the reaction between dissolved MgO, CO2 and NaCO3 forms Na2Mg(CO3)2, which is agreed well with the XRD patterns in Fig. 5(a) [25,32].

The CO2 uptake process takes place by the initial reaction between CO2dissolved in the coated molten salts layer and MgO to produce a carbonate layer, which is identified by the FT-IR analysis of AMS10MgOin Fig. 5(b). Then MgO is partially dissolved in the coated molten promoters layer to generate solvated [Mg2+···O2−] ionic pairs, which have much weaker interaction than the strong MgeO ionic bonds in bulk MgO. Meantime, K2CO3 reacts with CO2 and NO3− to form K2C2O6 and NO2−, and the nitryl ion provided by NO2− reacts with MgO to form several MgO-NOx surface species. K2C2O6 is unstable and further

Stage II. Restriction of the rigid carbonate layer and accumulation of O2− (7 < t < 10 min) As the reaction proceeds, the generated calcium carbonates adhere to the surface, which prevents the diffusion of carbon dioxide to the 5

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Fig. 4. (a) CO2capture performance, (b) adsorption capacity of AMS10-MgO over 30 sorption-desorption cycles (Adsorption conditions: 325 °C, 1 atm, 100% pure CO2, 3 h. Regeneration conditions: 450 °C, 1 atm, 100% pure N2, 1.5 h.)

CO2 adsorption capacity and capture kinetic are significantly affected by the amount of alkali metal salts and the adsorption temperature. The maximum adsorption amount of MgO adsorbent is up to 19.06 mmol·g−1 when loading 10% mole of [(Li0.44K0.56) NO3]2[(Na0.5K0.5) CO3]. The process of CO2 adsorption can be divided into three stages. The nitrite products play a key role on increasing CO2 uptake since it can provide more O2−, and reacts with MgO in molten nitrites to produce intermediate nitrato compounds, which results in the rapid nucleation of MgCO3 by triggering lattice defects. It is found that the new adsorbent has a good adsorption/desorption stability over 30 cycles, which is potential to be used in industrial CCS applications.

magnesium oxide layer and reduces the reaction rate [26]. Meantime, MgO continuously dissolves in the molten nitrates and carbonates layer to produce more solvated O2−. Also, alkali molten nitrates/nitrites can be decomposed to generate O2− [42]. Stage III. Rapid nucleation of MgCO3 crystals (10 < t < 60 min) In this stage, the CO2 adsorption rate is increased, which is mainly ascribed to the sufficient O2−and fast CO32− generation in the molten salts of mixed alkali metal nitrate and carbonate salts. Nitrates can promote the dissolving of carbonates by reducing the melting point of carbonate salts [29]. The produced nitrites in the first stage play a key role in the enhancement of CO2 capture ability of AMS10-MgO in that nitrite salts since the concentration of O2− can be increased in nitrites resulting in faster CO32− generation, which significantly contributes to the fast nucleation and growth of MgCO3. What’s more, MgO in molten nitrites can produce intermediate nitrato compounds, which result in the lattice defects and produce more CO32− [42,43]. Simultaneously, the dissolved Na2CO3 is able to capture CO2 via the reaction of MgO + Na2CO3 + CO2 → Na2Mg(CO3)2 [25,32]. The dissolved K2CO3 can be converted into the intermediate compound of K2C2O6, which can promote CO2 dissolution in the molten salts layer.

CRediT authorship contribution statement Jing Ding: Conceptualization, Methodology. Chao Yu: Writing original draft. Jianfeng Lu: Software. Xiaolan Wei: Methodology. Weilong Wang: Writing - review & editing, Project administration. Gechuanqi Pan: Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

4. Conclusions

Acknowledgements

A novel MgO-based CO2 adsorbents was synthesized via deposition of a design mixed alkali metal nitrates and carbonates, and their CO2 adsorption performance was investigated. The results show that the

The funding of Nature Science Foundation of China (51976237),

Table 1 Comparison of Different AMS-Promoted MgO-Based Adsorbents. Alkali metal salts

K2CO3 NaNO3 (Li, K)NO3 (Li, Na, K)NO3 NaNO3–NaNO2 NaNO3–NaNO2 LiNO3–(Na, K)NO2 NaNO3–NaCO3 NaNO3–NaCO3 (Li, Na)NO3–Na2CO3 (Li, K)NO3–(Na, K)2CO3 (Li, K)NO3–(Na, K)2CO3

Temperature (°C)/time (min)/atmosphere Sorption

Regeneration

350/30/CO2 330/90/CO2 350/60/CO2 300/30/CO2 350/30/85%CO2 350/30/85%CO2 340/60/CO2 360/90/CO2 325/60/CO2 325/10/CO2 350/20/CO2 325/120/CO2

500/60/N2 385/60/N2 500/10/N2 350/30/N2 400/20/N2 450/20/CO2 450/30/N2 400/60/N2 450/10/N2 425/5/N2 400/15/N2 450/90/N2

6

Number of cycles

CO2 uptake (1st/last) (mmol·g−1)

Ref.

10 9 20 20 15 15 20 30 14 30 30 30

2.09/1.11 15.0/7.0 10.5/7.2 16.8/3.2 18.4/8.6 18.4/7.3 15.7/12.0 14.3/6.3 12.7/5.9 9.1/5.9 16.6/9.09 19.06/15.7

[36] [23] [37] [27] [18] [18] [9] [38] [25] [39] [29] This work

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Fig. 5. (a) Powder XRD patterns, (b) FT-IR spectra of AMS10-MgOwith the CO2 adsorption time (Adsorption conditions: 325 °C, 1 atm, 100% pure CO2.)

Nature Science Foundation of China (U1707603), and Science and Technology Planning Project of Guang Dong Province (2015A010106006), and Nature Science Foundation of Guangdong (2016A030313362) supported this work.

[23]

References

[24]

[1] Mac Dowell N, Fennell PS, Shah N, Maitland GC. The role of CO2 capture and utilization in mitigating climate change. Nat Clim Change 2017;7:243–9. [2] Jansen D, Gazzani M, Manzolini G, Dijk Ev, Carbo M. Pre-combustion CO2 capture. Greem Gas Con 2015;40:167–87. [3] Manzolini G, Macchi E, Gazzani M. CO2 capture in integrated gasification combined cycle with SEWGS – Part B: Economic assessment. Fuel 2013;105:220–7. [4] Ji L, Yu H, Li K, Yu B, Grigore M, Yang Q, et al. Integrated absorption-mineralisation for low-energy CO2 capture and sequestration. Appl Energ 2018;225:356–66. [5] Wang J, Huang L, Yang R, Zhang Z, Wu J, Gao Y, et al. Recent advances in solid adsorbents for CO2 capture and new development trends. Energ Environ Sci 2014;7:3478–518. [6] Chuayplod P, Trakarnpruk W. Transesterification of rice bran oil with methanol catalyzed by Mg(Al)La hydrotalcites and metal/MgAl oxides. J Ind Eng Chem 2009;48:4177–83. [7] Kumar S, Saxena SK. A comparative study of CO2 sorption properties for different oxides. J Renew Sustain Energ 2014;3:1–15. [8] Duan Y, Zhang K, Li XS, King DL, Li B, Zhao L, et al. ab initio Thermodynamic study of the CO2 capture properties of M2CO3 (M = Na, K)- and CaCO3-promoted MgO adsorbents towards forming double salts. Aerosol Air Qual Res 2014;14:470–9. [9] Harada T, Hatton TA. Colloidal nanoclusters of MgO coated with alkali metal nitrates/nitrites for rapid, high capacity CO2 capture at moderate temperature. Chem Mater 2015;27:8153–61. [10] Gregg SJ, Ramsay JD. Adsorption of carbon dioxide by magnesia studied by use of infrared and isotherm measurements. J Chem Soc (A) 1970;1:2784–7. [11] Glasser L, Jenkins HDB. Volume-based thermodynamics: a prescription for its application and usage in approximation and prediction of thermodynamic data. J Chem Eng Data 2011;56:874–80. [12] Drese JH, Choi S, Lively RP, Koros WJ, Fauth DJ, Gray ML, et al. Synthesisâ structureâ property relationships for hyperbranched aminosilica CO2 adsorbents. Adv Funct Mater 2009;19:3821–32. [13] Fagerlund J, Highfield J, Zevenhoven R. Kinetics studies on wet and dry gas–solid carbonation of MgO and Mg(OH)2 for CO2 sequestration. RSC Adv 2012;2:10380–93. [14] Beruto D, Botter R, Searcy AW. Thermodynamics of two, two-dimensional phases formed by carbon dioxide chemisorption on magnesium oxide. J Phys Chem. 1987;91:3578–81. [15] Fujimoto RP. FTIR spectroscopic study of carbon dioxide adsorption/desorption on magnesia/calcium oxide catalysts. J Phys Chem 1992;96:9035–8. [16] Vu A-T, Park Y, Jeon PR, Lee C-H. Mesoporous MgO adsorbent promoted with KNO3 for CO2 capture at intermediate temperatures. Chem Eng J 2014;258:254–64. [17] Bhagiyalakshmi M, Lee JY, Jang HT. Synthesis of mesoporous magnesium oxide: Its application to CO2 chemisorption. Int J Greenhouse Gas Control 2010;4:51–6. [18] Zhao X, Ji G, Liu W, He X, Anthony EJ, Zhao M. Mesoporous MgO promoted with NaNO3/NaNO2 for rapid and high-capacity CO2 capture at moderate temperatures. Chem Eng J 2018;332:216–26. [19] Lee CH, Lee KB. Sorption-enhanced water gas shift reaction for high-purity hydrogen production: Application of a Na-Mg double salt-based adsorbent and the divided section packing concept. Appl Energ 2017;205:316–22. [20] Ma X, Li Y, Shi L, He Z, Wang Z. Fabrication and CO2 capture performance of magnesia-stabilized carbide slag by by-product of biodiesel during calcium looping process. Appl Energ 2016;168:85–95. [21] Li YY, Wan MM, Sun XD, Zhou J, Wang Y, Zhu JH. Novel fabrication of an efficient

[25]

[22]

[26]

[27]

[28]

[29]

[30] [31]

[32] [33]

[34] [35] [36]

[37]

[38] [39]

[40] [41]

[42] [43]

7

solid base: carbon-doped MgO–ZnO composite and its CO2 capture at 473 K. J Mater Chem A 2015;3:18535–45. Wang K, Hu X, Zhao P, Yin Z. Natural dolomite modified with carbon coating for cyclic high-temperature CO2 capture. Appl Energ 2016;165:14–21. Zhang K, Li XS, Li W-Z, Rohatgi A, Duan Y, Singh P, et al. Phase transfer-catalyzed fast CO2 absorption by MgO-based abadsorbents with high cycling capacity. Adv Mater Interfaces 2014;1:1400030. Jo SI, An YI, Kim KY, Choi SY, Kwak JS, Oh KR, et al. Mechanisms of absorption and desorption of CO2 by molten NaNO3-promoted MgO. Phys Chem Chem Phys 2017;19:6224–32. Vu A-T, Ho K, Jin S, Lee C-H. Double sodium salt-promoted mesoporous MgO adsorbent with high CO2 sorption capacity at intermediate temperatures under dry and wet conditions. Chem Eng J 2016;291:161–73. Harada T, Simeon F, Hamad EZ, Hatton TA. Alkali metal nitrate-promoted highcapacity MgO adsorbents for regenerable CO2 capture at moderate temperatures. Chem Mater 2015;27:1943–9. Qiao Y, Wang J, Zhang Y, Gao W, Harada T, Huang L, et al. Alkali nitrates molten salt modified commercial MgO for intermediate-temperature CO2 capture: optimization of the Li/Na/K ratio. Ind Eng Chem Res 2017;56:1509–17. Hwang BW, Lim JH, Chae HJ, Ryu H-J, Lee D, Lee JB, et al. CO2 capture and regeneration properties of MgO-based adsorbents promoted with alkali metal nitrates at high pressure for the sorption enhanced water gas shift process. Process Saf Environ 2018;116:219–27. Wang L, Zhou Z, Hu Y, Cheng Z, Fang X. Nanosheet MgO-based CO2 adsorbent promoted by mixed-alkali-metal nitrate and carbonate: performance and mechanism. J Ind Eng Chem 2017;56:5802–12. Peng Q, Yang X, Ding J, Wei X, Yang J. Design of new molten salt thermal energy storage material for solar thermal power plant. Appl Energ 2013;112:682–9. Prashar AK, Seo H, Choi WC, Kang NY, Park S, Kim K, et al. Factors affecting the rate of CO2 absorption after partial desorption in NaNO3-promoted MgO. Energ Fuel 2016;30:3298–305. Hadjiivanov KI. Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal Rev 2000;42:71–144. Du H, Williams CT, Ebner AD, Ritter JA. In situ FTIR spectroscopic analysis of carbonate transformations during adsorption and desorption of CO2 in K-promoted HTlc. Chem Mater 2010;22:3519–26. Ito T, Kobayashi H, Tashiro T. Boles of low-coordinated surface ions in adsorption of gases on MgO. Il Nuovo Cimento D 1997;19(11):1695–705. Hans HA, Paul FK. Infrared ansorption frequency trends for anhydrous normal carbonates. Am Mineral: J Earth Planet Mater 1963;48(1–2):124–37. Lee CH, Kwon HJ, Lee HC, Kwon S, Jeon SG, Lee KB. Effect of pH-controlled synthesis on the physical properties and intermediate-temperature CO2 sorption behaviors of K-Mg double salt-based adsorbents. Chem Eng J 2016;294:439–46. Lee H, Triviño MLT, Hwang S, Kwon SH, Lee SG, Moon JH, et al. In situ observation of carbon dioxide capture on pseudo-liquid eutectic mixture-promoted magnesium oxide. ACS Appl Mater Interf 2018;10:2414–22. Zhang K, Li XS, Chen H, Singh P, King DL. Molten salt promoting effect in double salt CO2 adsorbents. J Phys Chem C 2015;120:1089–96. Jin S, Ho K, Lee C-H. Facile synthesis of hierarchically porous MgO adsorbent doped with CaCO3 for fast CO2 capture in rapid intermediate temperature swing sorption. Chem Eng J 2018;334:1605–13. Hu J, Zhu K, Chen L, Kübel C, Richards R. MgO(111) nanosheets with unusual surface activity. J Am Chem Soc 2007;111:12038–44. Sutradhar N, Sinhamahapatra A, Roy B, Bajaj HC, Mukhopadhyay I, Panda AB. Preparation of MgO nano-rods with strong catalytic activity via hydrated basic magnesium carbonates. Mater Res Bull 2011;46:2163–7. Kust RN, Duke FR. A study of the nitrate ion dissociation in fused nitrates. J Am Chem Soc 1963;85(21):3338–40. Kust RN, Burke JD. Thermal decomposition in alkali metal nitrate melts. Inorg Nucl Chem Lett 1970;6(3):333–5.