With the increasingly serious problem of climate warming in recent years, CO₂ capture technology has been widely concerned by countries all over the world. One of the effective methods to reduce CO₂ emissions is to capture CO₂ after combustion. Alkali metal salt modified MgO-based adsorbents were synthesized by hydration impregnation method to obtain high efficient adsorbents in this study. The CO₂ cyclic adsorption capacity test was carried out in a fixed bed experimental setup. The microstructure of the adsorbent was characterized by XRD, SEM and BET techniques. The adsorption properties and cycle stability of alkali metal salts modified MgO adsorbents were deeply studied. The results show that the adsorbent with 10 wt% K₂CO₃-MgO has the best adsorption performance and cyclic stability. The initial adsorption capacity is 89 mg/g·sorbent, and the adsorption capacity of the 15th cycle is 63 mgCO₂/ g·sorbent. 10 wt% K₂CO₃-MgO adsorbent has the largest specific surface area, and K₂CO₃ doping of alkali metal carbonate can effectively improve the CO₂ adsorption capacity and cyclic stability of the adsorbent.

Keywords: CO₂ adsorbent, MgO, Alkali metal salt, Adsorption capacity, Cyclic stability.

The majority of global carbon dioxide emissions come from coal-fired power plants and the transportation industry, making effective capture of this carbon dioxide a focal point of concern for governments, businesses, and academia both domestically and internationally. Within the existing coal-fired power systems, CO₂ capture at coal-fired plants primarily follows three routes: pre-combustion capture, post-combustion capture, and oxy-fuel combustion [1-5]. Figure 1 shows the post-combustion carbon capture process using MgO as an adsorbent. MgO captures CO₂ through cyclic carbonation/calcination process, and the following is the reaction equation.



However, the practical application of pure MgO is greatly limited due to its low CO₂ adsorption capacity and poor stability in adsorption-regeneration cycles [6]. Consequently, it is necessary to modify MgO to enhance its CO₂ adsorption performance.
Currently, research on enhancing the CO₂ adsorption capacity of MgO often involves altering its microstructure or incorporating a second metal oxide to modify its properties. Despite its high theoretical adsorption capacity, the continuous reaction of MgO with CO₂ leads to the formation of a dense layer of MgCO₃ on its surface, which inhibits further reactions and results in a very low adsorption capacity of pure MgO. The CO₂ adsorption capacity of commercial pure MgO ranges only from 0.0092 to 0.018 g/g [7]. To improve the adsorption ability of pure MgO, it is crucial to increase its specific surface area and pore volume and to reduce the impact of MgCO₃ formation on its CO₂ adsorption performance. Song et al. [8] synthesized MgO with varying microstructures by calcining different precursors (magnesium hydroxide, basic magnesium carbonate, and magnesium oxalate). Their results demonstrated that MgO derived from these precursors had significantly increased surface areas and pore volumes. Specifically, MgO obtained from magnesium hydroxide exhibited the largest specific surface area at 211.3 m²/g, while that derived from magnesium oxalate had the largest pore volume at 0.484 m³/g and also showed the highest CO₂ adsorption capacity at 0.061 g/g. Hanif et al. [9] produced nanoscale MgO with high surface area and large pore volume by calcining samples under vacuum at 400°C using ammonia as a precipitant, achieving a CO₂ adsorption capacity of 0.088 g/g at 200°C. Han et al. [10] employed a sol-gel process combined with supercritical CO₂ drying to prepare a series of nanoscale MgO-based CO₂ adsorbents. At a calcination temperature of 600°C, as the Mg/Al molar ratio varied from 0 to 3, the composition of the adsorbent transitioned from Al₂O₃ to MgAl₂O₄ through MgAl₂O₃. The adsorbent with a Mg/Al molar ratio of 0.5 exhibited a surface area of 409 m²/g and a pore volume of 1.4 cm³/g, and under conditions of 200 °C and 10% CO₂, its adsorption capacity was approximately 0.026 g/g. Li et al. [11] used a wet impregnation method to load magnesium acetate tetrahydrate and magnesium nitrate hexahydrate onto various carriers such as activated carbon and silica. The results indicated that the carbonization of magnesium acetate tetrahydrate prevented the agglomeration of MgO, thus enhancing the dispersion and consequently, the CO₂ adsorption performance of MgO/SiO₂ adsorbents. However, despite these modifications to MgO's microstructure or the incorporation of a second metal oxide, the CO₂ adsorption capacity of modified MgO adsorbents remains relatively low, and their suitable temperature range is narrow. Recently, some researchers have started incorporating alkali metal nitrates or carbonates to modify MgO, which not only improves its CO₂ adsorption capacity but also extends the applicable temperature range for CO₂ adsorption. Xiao et al. [12] modified MgO with K₂CO₃ and investigated the impact of the Mg/K molar ratio on the adsorbent's CO₂ adsorption capability. The results showed that at an adsorption temperature of 350°C and 100% CO₂, the highest adsorption capacity was achieved with a Mg/K molar ratio of 1:1.89. Hatton et al. [13] coated basic magnesium carbonate with a layer of mixed nitrates and then examined the effects of the composition of mixed nitrates, carbonation temperature, and nitrate loading on CO₂ adsorption capacity. The findings indicated that the 15 mol% (Li-Na-K)NO₃/MgO adsorbent did not decrease in adsorption capacity even after 40 adsorption-regeneration cycles; instead, it increased.
Therefore, the purpose of this study is to modify MgO with alkali metal salts to enhance its adsorption capacity for CO₂. By modifying MgO with chloride salt and carbonates, it is aimed to improve both the CO₂ adsorption performance and the cyclic stability of MgO-based adsorbent. In this study, the influence of different types of alkali metal salts and their mass ratios on the CO₂ adsorption performance of MgO was investigated. All adsorbents are prepared using a hydration impregnation method, and CO₂ adsorption tests are conducted in a fixed-bed reactor. In addition, the adsorbent underwent multiple cyclic adsorption and regeneration experiments, and the cyclic CO₂ adsorption capacity was analyzed and discussed.

Preparation method
In this study, hydration impregnation method was used to prepare MgO-based adsorbent. Firstly, calcine pure MgO in a muffle furnace at 650°C for 3 hours. Then, mix the calcined MgO thoroughly in a solution containing 80ml of alkali metal ions. Then heat the obtained mixture in a water bath at 80°C and stir continuously for 3 hours until the water has almost evaporated. Afterwards, dry the evaporated paste mixture in a drying oven at 100°C for 12 hours. Finally, remove and grind it to obtain the final powder sample. The mass ratio of each substance and the sample naming of the adsorbent are shown in Table 1.
CO₂ Cyclic Adsorption Experiment
The cyclical CO₂ capture performance of MgO-based adsorbent was studied in a fixed bed reactor. As shown in Fig. 2.
The calcination temperature was 500°C, and the calcination time was 15 min under the pure N₂ atmosphere. The carbonation temperature is 380°C and the carbonation time is 15 min. Firstly, the adsorption capacity and cycle stability of 9 groups of samples were tested by 5 adsorption-desorption cycles. Among them, 4 groups of samples with better adsorption performance and cycle stability were added to 10 cycles, and the cycle process was consistent with the atmosphere. Four groups of carbonate modified samples were calcined at 280°C, 330°C and 380°C for five cycles in order to explore the influence of temperature on the cyclic adsorption performance of adsorbents. The adsorbent is firstly calcined at high temperature, MgCO₃ or Mg(OH)₂ is completely decomposed into MgO, and the calcined adsorbent is carbonated. The mass of calcined and carbonated adsorbent samples was weighed by electronic balance, and the CO₂ adsorption capacity (C_n) was calculated by the change of sample mass. The performance of cycling CO₂ capture was evaluated by this parameter, and the calculation formula was shown in Formula (1). The precision of electronic balance is 1mg, and the balance error is relatively small. Moreover, the average value is taken as experimental data after many measurements in numerical processing, and the result is more accurate.



Where n is the number of cyclic reactions; C_n is the adsorption amount of CO₂ after the nth cycle, representing the mass of CO₂ adsorbed by the calcined adsorbent per unit mass, and the unit is mg/g; m_n is the mass of adsorbent after the nth cycle, and the unit is mg; m_n-1 is the mass of the sample after the calcination stage of the (n-1) cycle, and the unit is mg; m₀ is the mass of adsorbent after initial calcination, and the unit is g.
Characterization
Ultima IV X-ray diffractometer made by Rigaku Company of Japan was used to determine the phase and analyze the crystal phase of the adsorbent. Using copper target, the sample was laid on a glass slide and placed in the X-ray diffractometer. The scanning range was 10° ~90°, and the scanning speed was 5°/min.
SUPRA 55 SAPPHIRE field emission scanning electron microscope produced by ZEISS Company of Germany was used to analyze the appearance and element distribution of the samples. Scanning electron microscope (SEM) was imaged by backscattered electron beam or secondary electron, and gold was sprayed for 150s under vacuum condition before testing. The amplification factor is 10~100 K, and the acceleration voltage is 5~20 KV.
The adsorbents were adsorbed and desorbed by ASAP 2460 automatic specific surface area and porosity analyzer produced by Micromeritics Company of USA. The specific surface area and pore characteristics of adsorbents were analyzed by multi-molecular layer adsorption theory and pore size distribution calculation model.

Comparison experiment of the adsorbent performance
The CO₂ adsorption performance of MgO based adsorbents modified with different alkali metal salts (K₂CO₃, Na₂CO₃, KCl, NaCl) at 380°C was compared. The result is shown in Fig. 3. It can be seen that K₂CO₃, Na₂CO₃, and NaCl modified MgO adsorbents have better initial CO₂ adsorption performance than unmodified pure MgO, and the adsorption performance of these adsorbents will decrease to varying degrees with increasing cycle times. Figs. 3(a)~(d) show a comparison of MgO and modified adsorbents with different mass ratios. In the first cycle, all adsorbents except KCl-9MgO have good CO₂ capture capacity, with CO₂ adsorption capacity above 58 mg/g·adsorbent. Among them, K₂CO₃-9MgO adsorbent has the best adsorption performance, with a first adsorption capacity of 89 mg/g·adsorbent. As the number of cycles increases, the adsorption capacity of all adsorbents for CO₂ decreases to varying degrees. Among them, the adsorption capacity of NaCl-9MgO and NaCl-4MgO adsorbents decreased significantly, and their cycling stability was poor. After the fifth cycle, the adsorption capacity decreased to approximately 25 mg/g·adsorbent and 17 mg/g·adsorbent, respectively. And K₂CO₃-9MgO and Na₂CO₃-4MgO adsorbents exhibit strong cyclic adsorption capacity. After the fifth cycle, the adsorption capacity of CO₂ can still reach 71.9 mg/g·adsorbent and 60.2 mg/g·adsorbent. Therefore, adding alkali metal carbonates (K₂CO₃, Na₂CO₃) can effectively improve the adsorption performance of MgO based adsorbents for CO₂. Especially for MgO based adsorbents with a mass ratio of 1:9, the CO₂ adsorption capacity of the adsorbent remains relatively stable after 5 cycles with the addition of alkali metal carbonates. However, the adsorption performance of Na₂CO₃-9MgO adsorbent rapidly decreased after the third cycle. Therefore, three carbonate modified adsorbents with better adsorption performance, namely Na₂CO₃-4MgO, K₂CO₃-9MgO, and K₂CO₃-4MgO, were selected for subsequent characterization analysis and cycling stability research.
Select four adsorbents with better performance for CO₂ adsorption performance research at different temperatures. The CO₂ adsorption capacities of K₂CO₃-9MgO, K₂CO₃-4MgO, Na₂CO₃-9MgO, Na₂CO₃-9MgO and MgO at different temperatures (280°C, 330°C and 380°C) were investigated. The results are shown in Fig. 4. It can be seen from the figure that temperature has obvious influence on the adsorption capacity of adsorbent CO₂. In a certain temperature range, the adsorption capacity of adsorbent with the same cycle times will obviously increase with the increase of adsorption temperature [14-16]. At 280°C, K₂CO₃-4MgO and Na₂CO₃-4MgO adsorbents had better adsorption capacity of 60 mg/g·adsorbent and 61 mg/g·adsorbent respectively, while at 330°C and 380°C, K₂CO₃-9MgO adsorbents had better adsorption capacity of 80 mg/g·adsorbent and 89 mg/g·adsorbent respectively.
XRD Analysis
The XRD spectra of the fresh and the 15 cycles of adsorbents are shown in Figs. 5 and 6. The diffraction peaks of MgO were observed in XRD patterns of all adsorbents. Mg(OH)₂ diffraction peaks are present in fresh adsorbent samples which may be the reason for the existence of water vapor in the air.
The XRD pattern of the fresh Na₂CO₃-4MgO shows the crystalline phase of Na₂Mg (CO₃)₂, which is caused by the reaction of Na₂CO₃ and MgO with CO₂. The XRD spectra of K₂CO₃-9MgO and K₂CO₃-4MgO adsorbents show a new phase of K₂Mg (CO₃)₂. Lee et al. [17, 18] also mentioned the newly formed substance K₂Mg (CO₃)₂, which is the reaction product of MgO, K₂CO₃ and CO₂. The existence of K₂Mg (CO₃)₂ may be the reason for improving the adsorption performance of the adsorbent. The crystal phase of K₂CO₃ was also detected in K₂CO₃-4MgO adsorbent, which may be the reason why the adsorption performance of K₂CO₃-4MgO adsorbent is lower than that of K₂CO₃-9MgO adsorbent.
SEM and EDS Analysis
The morphology changes of Na₂CO₃-4MgO, K₂CO₃-9MgO and K₂CO₃-4MgO adsorbents before and after 15 cycles were analyzed by SEM. Fig. 7 is the SEM image of the fresh adsorbent. As shown in the figure, the main body of the nano-flake structure is MgO, and the tiny particles attached to the large layer are doped alkali metal salts. For MgO-based adsorbents modified by alkali metal salts, K₂CO₃-9MgO adsorbents have the thinnest lamellae and more pores.
Figure 8 is the SEM image of the adsorbent after 15 cycles. Compared with fresh adsorbents, the flaky structures of each adsorbent have different degrees of aggregation, which may be the reason for the gradual decline of adsorption performance of adsorbents. At the same time, however, pores with uniform distribution and different sizes are produced on the adsorbent lamellae. This may be due to the chemical reaction between MgO and CO₂ to form MgCO₃ during CO₂ adsorption. In the process of cyclic adsorption and release, this reaction may be reversible or partially reversible. Each time magnesium carbonate is formed and decomposed, the crystal structure of MgO may be destroyed, which leads to the change of surface and internal structure of the material and the formation of new holes and cracks [19-21]. Furthermore, during the cycle, the adsorbent adsorbs CO₂ at a relatively low temperature, and then releases CO₂ at a higher temperature. This periodic change in temperature may lead to thermal expansion and contraction of materials, resulting in physical stress. These stresses may lead to the formation of microscopic cracks or holes in the material structure [22-24]. Among them, K₂CO₃-9MgO adsorbent has relatively more pores due to its thinnest layer, and the increase in pores will lead to sufficient contact between the adsorbent and CO₂, which may also be the reason for the better adsorption performance of K₂CO₃-9MgO adsorbent.
Figure 9 shows the surface element distribution of fresh adsorbents Na₂CO₃-4MgO, K₂CO₃-9MgO and K₂CO₃-4MgO. It can be found that Mg, O, Na, C and K elements on the surface of MgO-based adsorbent modified by alkali metal salt prepared by this method are uniformly distributed. Especially for K₂CO₃-9MgO adsorbent, in which doped alkali metal salts are uniformly distributed among MgO lamellae, effectively inhibiting the formation of MgCO₃ layer during cyclic calcination, therefore K₂CO₃-9MgO adsorbent can still keep thin lamellae structure and obtain higher CO₂ adsorption capacity after multiple cycles.
BET Analysis
Typical BET isotherms and corresponding pore size distribution curves for various samples are shown in Figs. 10 and 11. Due to the best adsorption and cycling performance of K₂CO₃-9MgO adsorbent in this experiment, this study compared the N₂ adsorption-desorption curves and pore size distribution curves of fresh Na₂CO₃-4MgO, K₂CO₃-9MgO, and K₂CO₃-4MgO adsorbent samples and K₂CO₃-9MgO adsorbent samples after 15 cycles. It was found that K₂CO₃-9MgO adsorbent has the smallest pore size among the fresh adsorbents, which may be the reason for its best adsorption performance.
At the same time, it can be observed that the pore size of K₂CO₃-9MgO adsorbent will increase after 15 cycles. The pore size of fresh adsorbents is mainly distributed between 0-15 nm. After 15 cycles, the pore size of K₂CO₃-9MgO adsorbent is mainly distributed between 10-50 nm. This indicates that the pore size of K₂CO₃-9MgO adsorbent increases during circulation. This may be because the crystal structure of MgO may be destroyed during cyclic adsorption and release, and new pores and cracks may be formed. Furthermore, the periodic change of temperature may lead to thermal expansion and contraction of materials, which may lead to the formation of microscopic cracks or holes in the material structure [22-24]. This is consistent with the above SEM characterization results. This is also the reason why the amount of pores distributed between 10-50 nm increases after 15 cycles.
Table 2 shows the pore size, pore volume, and specific surface area of fresh samples of Na₂CO₃-4MgO, K₂CO₃-9MgO, and K₂CO₃-4MgO adsorbents and samples of K₂CO₃-9MgO adsorbents after 15 cycles. It can be seen from the table that K₂CO₃-9MgO adsorbent has the smallest pore size and the largest specific surface area among the fresh adsorbents, which may be the reason for its best adsorption performance. At the same time, it can be seen that after 15 cycles, the pore size of K₂CO₃-9MgO adsorbent will increase, while the pore volume and specific surface area will decrease, which may be the reason for the decline of its adsorption performance [25].
CO₂ Cyclic Adsorption Performance
Three groups of samples (Na₂CO₃-4MgO, K₂CO₃-9MgO and K₂CO₃-4MgO) with good adsorption performance were selected for multiple cyclic studies. The results are shown in Fig. 12. After 15 cycles, the CO₂ adsorption capacity of K₂CO₃-9MgO adsorbent can still reaches at 63 mg/g·adsorbent. This indicates that K₂CO₃-9MgO adsorbent has a relatively stable performance in cyclic CO₂ capture.

In this study, MgO-based adsorbents modified by alkali metal salts with different mass ratios were prepared and applied to capture CO₂ in the atmosphere. The cyclic adsorption capacity of CO₂ was tested in a fixed-bed reactor. XRD, SEM, and BET were used to characterize the physicochemical properties of the adsorbent samples. The influence of alkali metal salt doping on MgO-based adsorbent was deeply studied. The main conclusions are as follows:
Among the 8 modified samples, K₂CO₃-9MgO adsorbent has the largest adsorption capacity, the first adsorption capacity is 89 mg/g·adsorbent, and the adsorption capacity can still reach 63 mg/g·adsorbent after 15 cycles. Compared with other adsorbents prepared in this experiment, the adsorption performance of K₂CO₃-9MgO adsorbent is more ideal and the adsorption capacity is more stable.
The adsorption capacity and cyclic stability of MgO-based adsorbents modified by basic carbonate at different temperatures and different mass ratios were also studied. Overall, the adsorption capacity and cycle stability of modified samples are better than those of unmodified pure MgO-based adsorbents; The performance of K₂CO₃ modified adsorbent is better than that of Na₂CO₃ modified adsorbent. At 330°C and 380°C, the adsorption capacity and cycle performance of K₂CO₃-9MgO adsorbent are the best.
Due to the limitation of research depth, the carbon capture technology explored in this report is mainly based on theoretical research and laboratory verification, and more performance tests are needed to evaluate the practical application of the developed adsorbent in atmospheric CO₂ capture. The purpose of this study is to provide enlightenment for the design of atmospheric CO₂ recycling absorption and adsorption materials with higher efficiency and lower energy consumption. In the future, further work will be carried out to popularize this MgO-based carbon capture technology and apply it to actual carbon capture concentration.

This work was supported by the Science and Technology Projects of Liaoning Province (No. 2023JH1/10400077), and the National Natural Science Foundation of China (No. 52001045).

The authors declare no conflict of interest.