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CO2 capture from ambient air by β-NaFeO2 in the presence of water vapor at 25–100 °C
Powder Technology 348 (2019) 43–50
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CO2 capture from ambient air by β-NaFeO2 in the presence of water vapor at 25–100 °C Ikuo Yanase ⁎, Shuhei Onozawa, Yuri Ohashi, Taisetsu Takeuchi Saitama University, Faculty of Engineering, Department of Applied Chemistry, 255 Shimoohkubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan
a r t i c l e
i n f o
Article history: Received 4 January 2019 Received in revised form 8 February 2019 Accepted 15 February 2019 Available online 18 February 2019 Keywords: Sodium ferrite carbon dioxide Water vapor Ambient air
a b s t r a c t Sodium ferrite (β-NaFeO2) was synthesized by heating raw mixtures of α-Fe2O3 and NaNO3 at 800 °C. The CO2 absorption properties of the synthesized β-NaFeO2 powders were then examined at temperatures of 25–100 °C under an air flow in the presence of water vapor by X-ray diffractometry (XRD), thermogravimetry (TG), and scanning electron microscopy (SEM). XRD results clarified that β-NaFeO2 reacted with CO2 and H2O to produce α-Fe2O3 and Na2CO3⸱H2O. TG results clarified that the absorption rates of CO2 from the air by β-NaFeO2 at 25 and 50 °C were greater than those at 75 and 100 °C and that CO2 absorption was promoted at a higher relative humidity. Thus, adsorption of water vapor onto the surface of β-NaFeO2 promoted CO2 absorption from air. A basic water solution formed at the surface of β-NaFeO2 in the presence of water vapor and this solution rapidly reacted with CO2 at temperatures as low as 25 °C. We also confirmed that β-NaFeO2 could be regenerated and used repeatedly for CO2 absorption in the presence of water vapor at 25 °C. © 2019 Published by Elsevier B.V.
1. Introduction
2. Experimental
Recently, the need has emerged for CO2 sorbents capable of capturing CO2 at room temperature from ambient air at low CO2 concentrations, for applications to CO2 capture and storage (CCS) [1–3]. Although amine-containing materials [4–6] have been used at temperatures below 100 °C, almost all CO2 capture materials, such as inorganic oxides including alkaline ions [7–11] and alkaline earth ions [12], are applicable at high temperatures above 100 °C [13–15]. Thus, inorganic oxides with the ability to capture CO2 over low temperature ranges, including room temperature to 100 °C, have yet to be studied sufficiently. Monyoncho et al. [16] has reported that α-NaFeO2 decomposes and Na+ ions in α-NaFeO2 elute into aqueous solution, suggesting that a basic solution is produced by dissolution of α-NaFeO2 in water. Therefore, a basic solution is expected to form on the surface of NaFeO2 by adsorption of water vapor at low temperatures. Conversely, β-NaFeO2 consists of FeO4 tetrahedra and Na+ in a wurtzite-derived structure [17,18], unlike that of α-NaFeO2, which consists of a layered structure of FeO6 and Na+ [16]. However, β-NaFeO2 has not been sufficiently studied for applications in gas capture over the range from room temperature to 100 °C. In this study, we focused on the influence of temperature on CO2 capture from air by β-NaFeO2 over the temperature range of 25–100 °C in the presence of water vapor. β-NaFeO2 might be expected to react with water vapor to produce a basic solution that can promote capture of CO2 gas.
2.1. Synthesis of β-NaFeO2
⁎ Corresponding author. E-mail address: [email protected] (I. Yanase).
https://doi.org/10.1016/j.powtec.2019.02.028 0032-5910/© 2019 Published by Elsevier B.V.
Commercially available α-Fe2O3 or γ-Fe2O3 (Wako Chemical, Japan) and NaNO3 (Wako Chemical, Japan) powders were mixed in a molar ratio of Na/Fe = 1 in ion-exchanged water and mixed for 1 h by an ultrasonic treatment. After removing the water with an evaporator, the mixed powder was dried and then heated over a temperature range of 550–850 °C for 5 h in air to synthesize β-NaFeO2 powders. 2.2. Evaluation X-ray diffractometry (XRD; CuKα, 40 kV, 30 mA, Bruker AXS, Germany) was used to examine the products in the calcined powders. The powder morphologies of the sample powders were examined with a field-emission scanning electron microscope (FESEM, S4100, Hitachi, Japan). We investigated the mass increase of the synthesized powders at temperatures of 25–100 °C in air in the presence of water vapor at an air flow rate of 100 mL/min by thermogravimetry and differential thermal analysis (TG-DTA; Thermoplus TG8120, Rigaku, Japan). Here, the theoretical ratio of the mass increase is 19.9% for the CO2 reaction (1): 2NaFeO2 þ CO2 →Na2 CO3 þ Fe2 O3
ð1Þ
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Fig. 3(a) shows the XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under a CO2 flow with 27%
relative humidity (hereafter, 27%RH). A small amount of Na2CO3⸱H2O phase was produced at 25, 50, and 75 °C, indicating that there was almost no CO2 absorption at higher temperatures. Conversely, the amount of β-NaFeO2 phase clearly decreased at 100 °C and α-Fe2O3 and Na2CO3⸱H2O phases were observed. The small peaks of XRD patterns at 100 °C indicates that the CO2 absorption reaction of β-NaFeO2 was almost complete at 100 °C for 5 h and amorphous phases of α-Fe2O3 and Na2CO3・H2O were produced by the reaction of CO2 with β-NaFeO2 at 100 °C, which was very low for increasing the size of particles of αFeO3 and Na2CO3・H2O. Fig. 3(b) shows the mass increase of the TG curves of β-NaFeO2 at temperatures of 25 to 100 °C under a CO2 flow with 27%RH. At 100 °C, the mass increase reached approximately 20% of the theoretical ratio of the above reaction (1). The mass increase ratio after 3.5 h was greater than the theoretical ratio because Na2CO3⸱H2O was produced rather than Na2CO3 in the presence of water vapor, as shown in Fig. 3(a). Conversely, the mass increase of β-NaFeO2 at 25 and 50 °C was greater than that at 100 °C at times less than 1 h. This result indicates that the CO2 absorption processes of β-NaFeO2 at low temperatures (i.e., 25 and 50 °C) are different from those at high temperatures (i.e., 75 and 100 °C). The water vapor in air promoted CO2 absorption by β-NaFeO2 at low temperatures of 25 and 50 °C, indicating that a lower water vapor pressure at lower temperatures promoted adsorption of water onto the surface of β-NaFeO2, which increased the CO2 absorption rate of β-NaFeO2. Fig. 4(a) shows the XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under an air flow with 80% relative humidity (hereafter, 80%RH). From the XRD patterns, the Na2CO3⸱H2O and α-Fe2O3 phases were clearly identified at 25 and 50 °C; however, these phases were not so formed at 75 and 100 °C. The TG curves show the mass increase of β-NaFeO2 at temperatures in the range of 25–100 °C under an air flow at 65%RH (Fig. 4(b)). The mass increases from CO2 absorption by β-NaFeO2 at 25 and 50 °C were pronounced, whereas there were almost no increases at 75 and 100 °C. Both the mass increase rates from CO2 absorption of β-NaFeO2 at 25 and 50 °C were greater than the theoretical mass increase rate of 19.9% according to reaction (1), indicating that the β-NaFeO2 absorbed CO2 in the presence of water vapor according to reaction (2) with a theoretical mass increase of 27.0%. The produced Na2CO3⸱H2O, as shown in Fig. 3(a), was caused by CO2 absorption in the presence of water vapor. From the above results, we suggest that water vapor in the air promoted
Fig. 1. XRD patterns of powders obtained by heating raw mixtures of NaNO3 and γ-Fe2O3 at temperatures of 550 to 850 °C for 15 h in air.
Fig. 2. XRD patterns of powders obtained by heating raw mixtures of NaNO3 and α-Fe2O3 at temperatures of 550 to 800 °C for 15 h in air.
Furthermore, the theoretical ratio of the mass increase is 27.0% for the CO2 reaction in the presence of water vapor (2): 2NaFeO2 þ H2 O þ CO2 →Na2 CO3 ⸱H2 O þ Fe2 O3
ð2Þ
The N2 adsorption and desorption isotherms at −196 °C were measured on a BELSORP-mini II (MicrotracBEL, Japan) to determine the Brunauer–Emmett–Teller (BET) specific surface area. In this study, the relative humidity was controlled by bubbling CO2 in water. The obtained highest relative humidity was 80%. The relative humidity was controlled in the range 27–80% because the low relative humidity of 27% caused a significant difference in CO2 absorption from the high relative humidity of 80%. 3. Results and discussion 3.1. Synthesis of β-NaFeO2 by a solid-state reaction and CO2 absorption β-NaFeO2 powders were synthesized by heating raw mixtures of NaNO3 and γ-Fe2O3 at temperatures of 550 to 850 °C for 15 h in air. The results are shown in Fig. 1. The α-NaFeO2 (JCPDS: No.76–2299) was produced at temperatures ranging from 550 to 750 °C and β-NaFeO2 (JCPDS: No. 74–1856) was produced at 800 and 850 °C for 15 h in air. Thus, we confirmed that β-NaFeO2 is thermodynamically stable at higher temperatures than is α-NaFeO2 [19,20]. Fig. 2 shows the XRD patterns for powders obtained by heating raw mixtures of NaNO3 and α-Fe2O3 at temperatures from 550 to 800 °C for 15 h in air. Compared with a synthesis based on the use of γ-Fe2O3 as a raw material, when α-Fe2O3 was used, β-NaFeO2 was produced at lower temperatures. The difference in the produced phases implies that the hexagonal close packing of oxide ions in α-Fe2O3 is effective for producing β-NaFeO2, which has a wurtzite-derived structure [17,18]. Wurtzite-type compounds have a common hexagonal close package structure. In this study, the β-NaFeO2 powder synthesized at 800 °C was used to investigate the CO2 absorption properties of β-NaFeO2. 3.2. CO2 absorption of β-NaFeO2 in the presence of water vapor
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Fig. 3. (a) XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under a CO2 flow with 27% of relative humidity (27% RH). (b) TG curves of β-NaFeO2 showing mass increases at temperatures of 25–100 °C under a CO2 flow with 27% RH. The dash line indicates the theoretical rate of the reaction: 2NaFeO2 + CO2 → Na2CO3 + Fe2O3.
CO2 absorption by β-NaFeO2 at low temperatures (25 and 50 °C), whereas a greater water vapor pressure at higher temperatures suppressed adsorption of water on the surface of β-NaFeO2. Hence, the CO2 absorption of β-NaFeO2 was caused by dissolution of β-NaFeO2 into water adsorbed on the surface of β-NaFeO2, indicating that the presence of water vapor produced a basic solution by decomposition of β-NaFeO2 to NaOH and Fe2O3, based on the following reaction (3); 2NaFeO2 þ H2 O→2NaOH þ Fe2 O3
ð3Þ
We investigated the relationship between the remaining ratio of βNaFeO2, [NaFeO2] and holding time, t for the TG curves in Fig. 4(b),
according to the Nernst–Noyes–Whitney equation for dissolution of solids, as given by Eq. (4) [21,22]: Ln ½NaFeO2 ¼ −kt þ A
ð4Þ
where, [NaFeO2] is the remaining ratio of β-NaFeO2, t; the CO2 reaction time, k; the rate constant, k = DS / Vh, D; the diffusion constant, S; the surface area, V; the volume of solution, h; the thickness of diffusion layer, A; a constant. The results at 25 and 50 °C are shown in Fig. 5(a) and (b), respectively. The plots for the Nernst–Noyes–Whitney Eq. (4) were linear, indicating that β-NaFeO2 absorbed CO2 for approximately 2 h at 25 °C and 5 h at 50 °C. Therefore, we considered that β-NaFeO2 easily dissolved into the adsorbed water to produce a basic solution
Fig. 4. (a) XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under an air flow with 80% of relative humidity (80%RH). (b) TG curves of β-NaFeO2 showing mass increase over a temperature range of 25–100 °C under an air flow with 65%RH. The dash line indicates the theoretical rate for the reaction: 2NaFeO2 + H2O + CO2 → Na2CO3⸱H2O + Fe2O3.
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Fig. 5. Relationship between holding time, t and Ln [NaFeO2] for TG curves at (a) 25 °C and (b) 50 °C. (c) Dissolution test of β-NaFeO2 on a pH test paper at 25 °C in air under water vapor and schematic illustration of CO2 capture mechanism of β-NaFeO2.
on its surface, resulting in CO2 absorption from air by β-NaFeO2 at low temperatures. The XRD patterns of β-NaFeO2 absorbed CO2 in air at 50 °C for 5 h in the presence of water vapor with 27%–80%RH are shown in Fig. 6 (a). The products of Na2CO3⸱H2O and α-Fe2O3 increased as the relative humidity in air increased from 27% to 65%RH. Thus, the presence of water vapor in air promoted CO2 absorption by β-NaFeO2. Conversely, Fig. 6(b) indicates that CO2 absorption in air at a low relative humidity of 27%RH hardly occurred over the temperature range of 25 to 100 °C. Fig. 7(a) and (b) show SEM images of the β-NaFeO2 powders before and after CO2 absorption at 25 °C and 80%RH. The specific surface area (1.7 m2/g) of the β-NaFeO2 powder increased to 5.1 m2/g when CO2 was absorbed in the presence of water vapor. Figs. 7(c) and (d) show the pore size distributions of the β-NaFeO2 powders before and after CO2 absorption at 25 °C and 80%RH, respectively. The CO2absorbed β-NaFeO2 powder had a larger pore volume than that of
the as-synthesized β-NaFeO2 powder. Hence, the surface morphology of β-NaFeO2 was changed by CO2 absorption in the presence of water vapor. A schematic diagram of the pH testing of the β-NaFeO2 powder under an air flow with water vapor (80%RH) at 25 °C in the tube furnace is shown in Fig. 8. Where, the CO2 absorption of the β-NaFeO2 powders under an air flow with water vapor (80%RH) at temperatures of 25–100 °C was also performed in the same tube furnace, set on an aluminum boat. During the pH testing, the β-NaFeO2 powder had a dark green color, which changed to a dark red upon absorbing CO2, indicating that α-Fe2O3 was produced after CO2 absorption. Furthermore, the color of the pH paper changed to yellowish green, indicating that the pH changed to 9–10 after CO2 absorption in the presence of water vapor. The pH change was likely caused by formation of a basic solution on the surface of the β-NaFeO2 particles. As shown in the scheme of Fig. 8, water vapor was absorbed as water droplets and/or films to the
Fig. 6. (a) XRD patterns of β-NaFeO2 absorbed CO2 in air at 50 °C for 5 h in the presence of water vapor with 27%–80%RH. (b) XRD patterns of β-NaFeO2 absorbed CO2 in air at 25, 50, 75, and 100 °C for 5 h in the presence of water vapor.
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dVp / ddp / (
dVp / ddp / (
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dp / nm
dp / nm
Fig. 7. SEM images (a) and (b) of the β-NaFeO2 powders before and after CO2 absorption at 25 °C under 80%RH, respectively. Pore size distributions (c) and (d) of the β-NaFeO2 powders before and after the CO2 absorption at 25 °C under 65%RH, respectively.
CO2 CO2
Na2CO3·H2O
Fe2O3
Water Adsorption
-NaFeO2 -NaFeO2
-NaFeO2
-NaFeO2
Fig. 8. Schematic diagram of pH testing of the β-NaFeO2 powder under an air flow with water vapor at 25 °C in the tube furnace and the CO2 absorption mechanism of β-NaFeO2 in the presence of water vapor.
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Fig. 9. XRD patterns of the CO2 absorbed β-NaFeO2 powder in the presence of water vapor (80%RH) at 25 °C in air and the powders obtained by heating the CO2 absorbed powder at 150–750 °C for 5 h under an Ar gas flow.
surface of the β-NaFeO2 particles, leading to formation of basic NaOH solution by dissolution of the surface of the NaFeO2 particles. The produced basic solution reacted with CO2 in air, which promoted CO2 absorption from air by β-NaFeO2 in the presence of water vapor. The small water droplets produced fine α-Fe2O3 and Na2CO3⸱H2O particles, which increased the specific surface area and pore volume. The above results indicate that higher water vapor pressures at higher temperatures suppressed the adsorption of water to the surface of β-NaFeO2. The absorption at 50 °C effectively enhanced CO2 absorption in the presence of water vapor because at 50 °C the NaFeO2 dissolved into water adsorbed to the surface of the β-NaFeO2 particles to a greater degree than at 25 °C. 3.3. CO2 absorption of regenerated β-NaFeO2 Fig. 9 shows the XRD patterns of the CO2 absorbed β-NaFeO2 powder in the presence of water vapor (80%RH) at 25 °C in air and powders obtained by heating the CO2 absorbed powder at 150–750 °C for 5 h under an Ar gas flow. A β-NaFeO2 single phase was produced above 600 °C,
indicating that β-NaFeO2 was regenerated by heating the powder absorbed CO2 in the presence of water vapor. The powder morphologies of the β-NaFeO2 powder before and after CO2 absorption in the presence of water vapor at 25 °C are shown in Fig. 10. The particles of β-NaFeO2 increased on absorbing CO2 at 25 °C in the presence of water vapor, resulting in growth of β-NaFeO2 particles after the release of CO2 at 600 °C in Ar. We investigated cycling of the CO2 absorption and desorption of β-NaFeO2 and its influence on the CO2 absorption capacity of βNaFeO2 from air in the presence of water vapor at 25 °C. Here, the CO2 absorption of β-NaFeO2 in the presence of water vapor (80%RH) was performed at 25 °C for 5 h and the regeneration was performed at 600 °C for 5 h under an Ar gas flow. The CO2 absorption capacity of 1 g of β-NaFeO2 at 25 °C for 5 h in the presence of water vapor (80%RH) was approximately 0.11 g for the first absorption (Fig. 11(a)). The CO2 absorption capacity was estimated from TG curves of CO2-absorbed βNaFeO2 (Fig. 11(b)). Over the temperature range from 30 to approximately 250 °C, decrease (①) corresponded to desorption of H2O adsorbed to the surface of the sample and decrease (②) corresponded to the release of H2O from Na2CO3·H2O. In the temperature range of 400 to 800 °C, decrease (③) corresponded to the CO2 release from Na2CO3. Thus, we used the ratio of decrease ③ to calculate the CO2 absorption capacity of β-NaFeO2. The capacity of β-NaFeO2 gradually decreased to be approximately 0.08 g as the cycle number of CO2 absorption was extended to 5. We attribute this decrease in capacity to the increase in the particle size of β-NaFeO2 during the CO2 release at 600 °C, as shown in Fig. 9. For 1 g of α-NaFeO2, the CO2 absorption capacity at 25 °C for 5 h was in the range of 0.05–0.06 g, indicating that that α-NaFeO2 had a lower capacity than that of β-NaFeO2. A previous study [23] has reported that the CO2 absorption of α-NaFeO2 in the presence of water vapor produces Na-deficient NaFeO2 (Na1-xFeO2) after CO2 absorption, leading to a lower CO2 capacity than that of β-NaFeO2, because β-NaFeO2 reacted with CO2 to produce Na2CO3⸱H2O and α-Fe2O3 without Na-containing iron oxides, i.e., Na1-xFeO2. We investigated the absorption behavior of βNaFeO2 for a CO2 gas with low concentrations (approximately 2200 pm) at 25 °C in a desiccator with 80%RH. As shown in Fig. 12(a), the CO2 concentration in the desiccator gradually decreased to be approximately 1850 ppm at 900 min, indicating that β-NaFeO2 effectively controlled the low CO2 concentration in air with water vapor at RT. Similarly, we investigated the CO2 absorption behaviors of commercially available K2CO3 (Wako chemical) and zeolite (Wako chemical) powders (Fig. 12(b)). The decrease of the CO2 concentration was clearly smaller than that of β-NaFeO2 because CO2 also physically adsorbed to the zeolite. K2CO3 is a deliquescent substance, which decreases the ambient CO2 concentration in the presence of water vapor. However, β-NaFeO2 can be reused for CO2 capture, which is a notable advantage over K2CO3.
Fig. 10. Powder morphologies of β-NaFeO2 powders before and after CO2 absorption in the presence of water vapor at 25 °C.
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Fig. 11. (a) Influence of cycling CO2 absorption and desorption of β-NaFeO2 (α-NaFeO2) on CO2 absorption capacity of β-NaFeO2 (α-NaFeO2) in the presence of water vapor (80%RH) in air at 25 °C. (b) TG curves of CO2-absorbed β-NaFeO2.
Time / min
Time / min
Fig. 12. CO2 absorption behaviors in a desiccator at 25 °C of (a) β-NaFeO2 and (b) K2CO3 and zeolite at low CO2 concentrations (approximately 2200 pm) with 80%RH.
4. Conclusions Single-phase β-NaFeO2 powder was synthesized by heating mixtures of commercial α-Fe2O3 and NaNO3 powders at 800 °C for 5 h under an air flow. The CO2 absorption properties of β-NaFeO2 were influenced by the relative humidity in air. The CO2 capture from air by β-NaFeO2 in the presence of water vapor at 25 and 50 °C was promoted as the relative humidity in air increased. These results indicate that adsorption of water vapor onto the surface of β-NaFeO2 produced a basic water film on the surface of β-NaFeO2, which promoted CO2 absorption from the air by β-NaFeO2. We confirmed that β-NaFeO2 could be used repeatedly for CO2 absorption in the presence of water vapor. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research (C) (18K05269) of Japan Society of the Promotion of Science (JSPS). References [1] D. Krekel, R.C. Samsun, R. Peters, D. Stolten, The separation of CO2 from ambient air – a techno-economic assessment, Appl. Energy 218 (2018) 361–381.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
CO2 capture from ambient air by β-NaFeO2 in the presence of water vapor at 25–100 °C Ikuo Yanase ⁎, Shuhei Onozawa, Yuri Ohashi, Taisetsu Takeuchi Saitama University, Faculty of Engineering, Department of Applied Chemistry, 255 Shimoohkubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan
a r t i c l e
i n f o
Article history: Received 4 January 2019 Received in revised form 8 February 2019 Accepted 15 February 2019 Available online 18 February 2019 Keywords: Sodium ferrite carbon dioxide Water vapor Ambient air
a b s t r a c t Sodium ferrite (β-NaFeO2) was synthesized by heating raw mixtures of α-Fe2O3 and NaNO3 at 800 °C. The CO2 absorption properties of the synthesized β-NaFeO2 powders were then examined at temperatures of 25–100 °C under an air flow in the presence of water vapor by X-ray diffractometry (XRD), thermogravimetry (TG), and scanning electron microscopy (SEM). XRD results clarified that β-NaFeO2 reacted with CO2 and H2O to produce α-Fe2O3 and Na2CO3⸱H2O. TG results clarified that the absorption rates of CO2 from the air by β-NaFeO2 at 25 and 50 °C were greater than those at 75 and 100 °C and that CO2 absorption was promoted at a higher relative humidity. Thus, adsorption of water vapor onto the surface of β-NaFeO2 promoted CO2 absorption from air. A basic water solution formed at the surface of β-NaFeO2 in the presence of water vapor and this solution rapidly reacted with CO2 at temperatures as low as 25 °C. We also confirmed that β-NaFeO2 could be regenerated and used repeatedly for CO2 absorption in the presence of water vapor at 25 °C. © 2019 Published by Elsevier B.V.
1. Introduction
2. Experimental
Recently, the need has emerged for CO2 sorbents capable of capturing CO2 at room temperature from ambient air at low CO2 concentrations, for applications to CO2 capture and storage (CCS) [1–3]. Although amine-containing materials [4–6] have been used at temperatures below 100 °C, almost all CO2 capture materials, such as inorganic oxides including alkaline ions [7–11] and alkaline earth ions [12], are applicable at high temperatures above 100 °C [13–15]. Thus, inorganic oxides with the ability to capture CO2 over low temperature ranges, including room temperature to 100 °C, have yet to be studied sufficiently. Monyoncho et al. [16] has reported that α-NaFeO2 decomposes and Na+ ions in α-NaFeO2 elute into aqueous solution, suggesting that a basic solution is produced by dissolution of α-NaFeO2 in water. Therefore, a basic solution is expected to form on the surface of NaFeO2 by adsorption of water vapor at low temperatures. Conversely, β-NaFeO2 consists of FeO4 tetrahedra and Na+ in a wurtzite-derived structure [17,18], unlike that of α-NaFeO2, which consists of a layered structure of FeO6 and Na+ [16]. However, β-NaFeO2 has not been sufficiently studied for applications in gas capture over the range from room temperature to 100 °C. In this study, we focused on the influence of temperature on CO2 capture from air by β-NaFeO2 over the temperature range of 25–100 °C in the presence of water vapor. β-NaFeO2 might be expected to react with water vapor to produce a basic solution that can promote capture of CO2 gas.
2.1. Synthesis of β-NaFeO2
⁎ Corresponding author. E-mail address: [email protected] (I. Yanase).
https://doi.org/10.1016/j.powtec.2019.02.028 0032-5910/© 2019 Published by Elsevier B.V.
Commercially available α-Fe2O3 or γ-Fe2O3 (Wako Chemical, Japan) and NaNO3 (Wako Chemical, Japan) powders were mixed in a molar ratio of Na/Fe = 1 in ion-exchanged water and mixed for 1 h by an ultrasonic treatment. After removing the water with an evaporator, the mixed powder was dried and then heated over a temperature range of 550–850 °C for 5 h in air to synthesize β-NaFeO2 powders. 2.2. Evaluation X-ray diffractometry (XRD; CuKα, 40 kV, 30 mA, Bruker AXS, Germany) was used to examine the products in the calcined powders. The powder morphologies of the sample powders were examined with a field-emission scanning electron microscope (FESEM, S4100, Hitachi, Japan). We investigated the mass increase of the synthesized powders at temperatures of 25–100 °C in air in the presence of water vapor at an air flow rate of 100 mL/min by thermogravimetry and differential thermal analysis (TG-DTA; Thermoplus TG8120, Rigaku, Japan). Here, the theoretical ratio of the mass increase is 19.9% for the CO2 reaction (1): 2NaFeO2 þ CO2 →Na2 CO3 þ Fe2 O3
ð1Þ
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Fig. 3(a) shows the XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under a CO2 flow with 27%
relative humidity (hereafter, 27%RH). A small amount of Na2CO3⸱H2O phase was produced at 25, 50, and 75 °C, indicating that there was almost no CO2 absorption at higher temperatures. Conversely, the amount of β-NaFeO2 phase clearly decreased at 100 °C and α-Fe2O3 and Na2CO3⸱H2O phases were observed. The small peaks of XRD patterns at 100 °C indicates that the CO2 absorption reaction of β-NaFeO2 was almost complete at 100 °C for 5 h and amorphous phases of α-Fe2O3 and Na2CO3・H2O were produced by the reaction of CO2 with β-NaFeO2 at 100 °C, which was very low for increasing the size of particles of αFeO3 and Na2CO3・H2O. Fig. 3(b) shows the mass increase of the TG curves of β-NaFeO2 at temperatures of 25 to 100 °C under a CO2 flow with 27%RH. At 100 °C, the mass increase reached approximately 20% of the theoretical ratio of the above reaction (1). The mass increase ratio after 3.5 h was greater than the theoretical ratio because Na2CO3⸱H2O was produced rather than Na2CO3 in the presence of water vapor, as shown in Fig. 3(a). Conversely, the mass increase of β-NaFeO2 at 25 and 50 °C was greater than that at 100 °C at times less than 1 h. This result indicates that the CO2 absorption processes of β-NaFeO2 at low temperatures (i.e., 25 and 50 °C) are different from those at high temperatures (i.e., 75 and 100 °C). The water vapor in air promoted CO2 absorption by β-NaFeO2 at low temperatures of 25 and 50 °C, indicating that a lower water vapor pressure at lower temperatures promoted adsorption of water onto the surface of β-NaFeO2, which increased the CO2 absorption rate of β-NaFeO2. Fig. 4(a) shows the XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under an air flow with 80% relative humidity (hereafter, 80%RH). From the XRD patterns, the Na2CO3⸱H2O and α-Fe2O3 phases were clearly identified at 25 and 50 °C; however, these phases were not so formed at 75 and 100 °C. The TG curves show the mass increase of β-NaFeO2 at temperatures in the range of 25–100 °C under an air flow at 65%RH (Fig. 4(b)). The mass increases from CO2 absorption by β-NaFeO2 at 25 and 50 °C were pronounced, whereas there were almost no increases at 75 and 100 °C. Both the mass increase rates from CO2 absorption of β-NaFeO2 at 25 and 50 °C were greater than the theoretical mass increase rate of 19.9% according to reaction (1), indicating that the β-NaFeO2 absorbed CO2 in the presence of water vapor according to reaction (2) with a theoretical mass increase of 27.0%. The produced Na2CO3⸱H2O, as shown in Fig. 3(a), was caused by CO2 absorption in the presence of water vapor. From the above results, we suggest that water vapor in the air promoted
Fig. 1. XRD patterns of powders obtained by heating raw mixtures of NaNO3 and γ-Fe2O3 at temperatures of 550 to 850 °C for 15 h in air.
Fig. 2. XRD patterns of powders obtained by heating raw mixtures of NaNO3 and α-Fe2O3 at temperatures of 550 to 800 °C for 15 h in air.
Furthermore, the theoretical ratio of the mass increase is 27.0% for the CO2 reaction in the presence of water vapor (2): 2NaFeO2 þ H2 O þ CO2 →Na2 CO3 ⸱H2 O þ Fe2 O3
ð2Þ
The N2 adsorption and desorption isotherms at −196 °C were measured on a BELSORP-mini II (MicrotracBEL, Japan) to determine the Brunauer–Emmett–Teller (BET) specific surface area. In this study, the relative humidity was controlled by bubbling CO2 in water. The obtained highest relative humidity was 80%. The relative humidity was controlled in the range 27–80% because the low relative humidity of 27% caused a significant difference in CO2 absorption from the high relative humidity of 80%. 3. Results and discussion 3.1. Synthesis of β-NaFeO2 by a solid-state reaction and CO2 absorption β-NaFeO2 powders were synthesized by heating raw mixtures of NaNO3 and γ-Fe2O3 at temperatures of 550 to 850 °C for 15 h in air. The results are shown in Fig. 1. The α-NaFeO2 (JCPDS: No.76–2299) was produced at temperatures ranging from 550 to 750 °C and β-NaFeO2 (JCPDS: No. 74–1856) was produced at 800 and 850 °C for 15 h in air. Thus, we confirmed that β-NaFeO2 is thermodynamically stable at higher temperatures than is α-NaFeO2 [19,20]. Fig. 2 shows the XRD patterns for powders obtained by heating raw mixtures of NaNO3 and α-Fe2O3 at temperatures from 550 to 800 °C for 15 h in air. Compared with a synthesis based on the use of γ-Fe2O3 as a raw material, when α-Fe2O3 was used, β-NaFeO2 was produced at lower temperatures. The difference in the produced phases implies that the hexagonal close packing of oxide ions in α-Fe2O3 is effective for producing β-NaFeO2, which has a wurtzite-derived structure [17,18]. Wurtzite-type compounds have a common hexagonal close package structure. In this study, the β-NaFeO2 powder synthesized at 800 °C was used to investigate the CO2 absorption properties of β-NaFeO2. 3.2. CO2 absorption of β-NaFeO2 in the presence of water vapor
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Fig. 3. (a) XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under a CO2 flow with 27% of relative humidity (27% RH). (b) TG curves of β-NaFeO2 showing mass increases at temperatures of 25–100 °C under a CO2 flow with 27% RH. The dash line indicates the theoretical rate of the reaction: 2NaFeO2 + CO2 → Na2CO3 + Fe2O3.
CO2 absorption by β-NaFeO2 at low temperatures (25 and 50 °C), whereas a greater water vapor pressure at higher temperatures suppressed adsorption of water on the surface of β-NaFeO2. Hence, the CO2 absorption of β-NaFeO2 was caused by dissolution of β-NaFeO2 into water adsorbed on the surface of β-NaFeO2, indicating that the presence of water vapor produced a basic solution by decomposition of β-NaFeO2 to NaOH and Fe2O3, based on the following reaction (3); 2NaFeO2 þ H2 O→2NaOH þ Fe2 O3
ð3Þ
We investigated the relationship between the remaining ratio of βNaFeO2, [NaFeO2] and holding time, t for the TG curves in Fig. 4(b),
according to the Nernst–Noyes–Whitney equation for dissolution of solids, as given by Eq. (4) [21,22]: Ln ½NaFeO2 ¼ −kt þ A
ð4Þ
where, [NaFeO2] is the remaining ratio of β-NaFeO2, t; the CO2 reaction time, k; the rate constant, k = DS / Vh, D; the diffusion constant, S; the surface area, V; the volume of solution, h; the thickness of diffusion layer, A; a constant. The results at 25 and 50 °C are shown in Fig. 5(a) and (b), respectively. The plots for the Nernst–Noyes–Whitney Eq. (4) were linear, indicating that β-NaFeO2 absorbed CO2 for approximately 2 h at 25 °C and 5 h at 50 °C. Therefore, we considered that β-NaFeO2 easily dissolved into the adsorbed water to produce a basic solution
Fig. 4. (a) XRD patterns of β-NaFeO2 absorbed CO2 for 5 h at temperatures of 25, 50, 75, and 100 °C under an air flow with 80% of relative humidity (80%RH). (b) TG curves of β-NaFeO2 showing mass increase over a temperature range of 25–100 °C under an air flow with 65%RH. The dash line indicates the theoretical rate for the reaction: 2NaFeO2 + H2O + CO2 → Na2CO3⸱H2O + Fe2O3.
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Fig. 5. Relationship between holding time, t and Ln [NaFeO2] for TG curves at (a) 25 °C and (b) 50 °C. (c) Dissolution test of β-NaFeO2 on a pH test paper at 25 °C in air under water vapor and schematic illustration of CO2 capture mechanism of β-NaFeO2.
on its surface, resulting in CO2 absorption from air by β-NaFeO2 at low temperatures. The XRD patterns of β-NaFeO2 absorbed CO2 in air at 50 °C for 5 h in the presence of water vapor with 27%–80%RH are shown in Fig. 6 (a). The products of Na2CO3⸱H2O and α-Fe2O3 increased as the relative humidity in air increased from 27% to 65%RH. Thus, the presence of water vapor in air promoted CO2 absorption by β-NaFeO2. Conversely, Fig. 6(b) indicates that CO2 absorption in air at a low relative humidity of 27%RH hardly occurred over the temperature range of 25 to 100 °C. Fig. 7(a) and (b) show SEM images of the β-NaFeO2 powders before and after CO2 absorption at 25 °C and 80%RH. The specific surface area (1.7 m2/g) of the β-NaFeO2 powder increased to 5.1 m2/g when CO2 was absorbed in the presence of water vapor. Figs. 7(c) and (d) show the pore size distributions of the β-NaFeO2 powders before and after CO2 absorption at 25 °C and 80%RH, respectively. The CO2absorbed β-NaFeO2 powder had a larger pore volume than that of
the as-synthesized β-NaFeO2 powder. Hence, the surface morphology of β-NaFeO2 was changed by CO2 absorption in the presence of water vapor. A schematic diagram of the pH testing of the β-NaFeO2 powder under an air flow with water vapor (80%RH) at 25 °C in the tube furnace is shown in Fig. 8. Where, the CO2 absorption of the β-NaFeO2 powders under an air flow with water vapor (80%RH) at temperatures of 25–100 °C was also performed in the same tube furnace, set on an aluminum boat. During the pH testing, the β-NaFeO2 powder had a dark green color, which changed to a dark red upon absorbing CO2, indicating that α-Fe2O3 was produced after CO2 absorption. Furthermore, the color of the pH paper changed to yellowish green, indicating that the pH changed to 9–10 after CO2 absorption in the presence of water vapor. The pH change was likely caused by formation of a basic solution on the surface of the β-NaFeO2 particles. As shown in the scheme of Fig. 8, water vapor was absorbed as water droplets and/or films to the
Fig. 6. (a) XRD patterns of β-NaFeO2 absorbed CO2 in air at 50 °C for 5 h in the presence of water vapor with 27%–80%RH. (b) XRD patterns of β-NaFeO2 absorbed CO2 in air at 25, 50, 75, and 100 °C for 5 h in the presence of water vapor.
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dVp / ddp / (
dVp / ddp / (
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dp / nm
dp / nm
Fig. 7. SEM images (a) and (b) of the β-NaFeO2 powders before and after CO2 absorption at 25 °C under 80%RH, respectively. Pore size distributions (c) and (d) of the β-NaFeO2 powders before and after the CO2 absorption at 25 °C under 65%RH, respectively.
CO2 CO2
Na2CO3·H2O
Fe2O3
Water Adsorption
-NaFeO2 -NaFeO2
-NaFeO2
-NaFeO2
Fig. 8. Schematic diagram of pH testing of the β-NaFeO2 powder under an air flow with water vapor at 25 °C in the tube furnace and the CO2 absorption mechanism of β-NaFeO2 in the presence of water vapor.
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Fig. 9. XRD patterns of the CO2 absorbed β-NaFeO2 powder in the presence of water vapor (80%RH) at 25 °C in air and the powders obtained by heating the CO2 absorbed powder at 150–750 °C for 5 h under an Ar gas flow.
surface of the β-NaFeO2 particles, leading to formation of basic NaOH solution by dissolution of the surface of the NaFeO2 particles. The produced basic solution reacted with CO2 in air, which promoted CO2 absorption from air by β-NaFeO2 in the presence of water vapor. The small water droplets produced fine α-Fe2O3 and Na2CO3⸱H2O particles, which increased the specific surface area and pore volume. The above results indicate that higher water vapor pressures at higher temperatures suppressed the adsorption of water to the surface of β-NaFeO2. The absorption at 50 °C effectively enhanced CO2 absorption in the presence of water vapor because at 50 °C the NaFeO2 dissolved into water adsorbed to the surface of the β-NaFeO2 particles to a greater degree than at 25 °C. 3.3. CO2 absorption of regenerated β-NaFeO2 Fig. 9 shows the XRD patterns of the CO2 absorbed β-NaFeO2 powder in the presence of water vapor (80%RH) at 25 °C in air and powders obtained by heating the CO2 absorbed powder at 150–750 °C for 5 h under an Ar gas flow. A β-NaFeO2 single phase was produced above 600 °C,
indicating that β-NaFeO2 was regenerated by heating the powder absorbed CO2 in the presence of water vapor. The powder morphologies of the β-NaFeO2 powder before and after CO2 absorption in the presence of water vapor at 25 °C are shown in Fig. 10. The particles of β-NaFeO2 increased on absorbing CO2 at 25 °C in the presence of water vapor, resulting in growth of β-NaFeO2 particles after the release of CO2 at 600 °C in Ar. We investigated cycling of the CO2 absorption and desorption of β-NaFeO2 and its influence on the CO2 absorption capacity of βNaFeO2 from air in the presence of water vapor at 25 °C. Here, the CO2 absorption of β-NaFeO2 in the presence of water vapor (80%RH) was performed at 25 °C for 5 h and the regeneration was performed at 600 °C for 5 h under an Ar gas flow. The CO2 absorption capacity of 1 g of β-NaFeO2 at 25 °C for 5 h in the presence of water vapor (80%RH) was approximately 0.11 g for the first absorption (Fig. 11(a)). The CO2 absorption capacity was estimated from TG curves of CO2-absorbed βNaFeO2 (Fig. 11(b)). Over the temperature range from 30 to approximately 250 °C, decrease (①) corresponded to desorption of H2O adsorbed to the surface of the sample and decrease (②) corresponded to the release of H2O from Na2CO3·H2O. In the temperature range of 400 to 800 °C, decrease (③) corresponded to the CO2 release from Na2CO3. Thus, we used the ratio of decrease ③ to calculate the CO2 absorption capacity of β-NaFeO2. The capacity of β-NaFeO2 gradually decreased to be approximately 0.08 g as the cycle number of CO2 absorption was extended to 5. We attribute this decrease in capacity to the increase in the particle size of β-NaFeO2 during the CO2 release at 600 °C, as shown in Fig. 9. For 1 g of α-NaFeO2, the CO2 absorption capacity at 25 °C for 5 h was in the range of 0.05–0.06 g, indicating that that α-NaFeO2 had a lower capacity than that of β-NaFeO2. A previous study [23] has reported that the CO2 absorption of α-NaFeO2 in the presence of water vapor produces Na-deficient NaFeO2 (Na1-xFeO2) after CO2 absorption, leading to a lower CO2 capacity than that of β-NaFeO2, because β-NaFeO2 reacted with CO2 to produce Na2CO3⸱H2O and α-Fe2O3 without Na-containing iron oxides, i.e., Na1-xFeO2. We investigated the absorption behavior of βNaFeO2 for a CO2 gas with low concentrations (approximately 2200 pm) at 25 °C in a desiccator with 80%RH. As shown in Fig. 12(a), the CO2 concentration in the desiccator gradually decreased to be approximately 1850 ppm at 900 min, indicating that β-NaFeO2 effectively controlled the low CO2 concentration in air with water vapor at RT. Similarly, we investigated the CO2 absorption behaviors of commercially available K2CO3 (Wako chemical) and zeolite (Wako chemical) powders (Fig. 12(b)). The decrease of the CO2 concentration was clearly smaller than that of β-NaFeO2 because CO2 also physically adsorbed to the zeolite. K2CO3 is a deliquescent substance, which decreases the ambient CO2 concentration in the presence of water vapor. However, β-NaFeO2 can be reused for CO2 capture, which is a notable advantage over K2CO3.
Fig. 10. Powder morphologies of β-NaFeO2 powders before and after CO2 absorption in the presence of water vapor at 25 °C.
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Fig. 11. (a) Influence of cycling CO2 absorption and desorption of β-NaFeO2 (α-NaFeO2) on CO2 absorption capacity of β-NaFeO2 (α-NaFeO2) in the presence of water vapor (80%RH) in air at 25 °C. (b) TG curves of CO2-absorbed β-NaFeO2.
Time / min
Time / min
Fig. 12. CO2 absorption behaviors in a desiccator at 25 °C of (a) β-NaFeO2 and (b) K2CO3 and zeolite at low CO2 concentrations (approximately 2200 pm) with 80%RH.
4. Conclusions Single-phase β-NaFeO2 powder was synthesized by heating mixtures of commercial α-Fe2O3 and NaNO3 powders at 800 °C for 5 h under an air flow. The CO2 absorption properties of β-NaFeO2 were influenced by the relative humidity in air. The CO2 capture from air by β-NaFeO2 in the presence of water vapor at 25 and 50 °C was promoted as the relative humidity in air increased. These results indicate that adsorption of water vapor onto the surface of β-NaFeO2 produced a basic water film on the surface of β-NaFeO2, which promoted CO2 absorption from the air by β-NaFeO2. We confirmed that β-NaFeO2 could be used repeatedly for CO2 absorption in the presence of water vapor. Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research (C) (18K05269) of Japan Society of the Promotion of Science (JSPS). References [1] D. Krekel, R.C. Samsun, R. Peters, D. Stolten, The separation of CO2 from ambient air – a techno-economic assessment, Appl. Energy 218 (2018) 361–381.
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