CO2 capture and release of Na0.7MnO2.05 under water vapor at 25–150?°C

Article abstract of DOI:10.1016/j.inoche.2019.01.029

Sodium manganates with a layered structure, Na_0.7MnO_2.05, have been applied to a novel material for CCUS (CO₂ capture, utilization, and storage), capable of capturing CO₂ at 25 °C in the presence of water vapor and releasing CO₂ at 150 °C. The temperatures of capturing and releasing CO₂ of Na_0.7MnO_2.05 were remarkably lower than those of other traditional metal oxides. The CO₂ absorption and desorption properties of Na_0.7MnO_2.05 were investigated by various methods, such as thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and gas chromatography. These investigations confirmed that Na_0.7MnO_2.05 absorbed CO₂ at 25 °C in the presence of water vapor to produce NaHCO₃ and a birnessite and the CO₂ absorption was promoted by increasing relative humidity and CO₂ concentration. The CO₂ absorption at 25 °C of Na_0.7MnO_2.05 was promoted by the formation of a strong basic solution on Na_0.7MnO_2.05, caused by the elution of Na ions from the interlayer of Na_0.7MnO_2.05 into water, adsorbed on the Na_0.7MnO_2.05 surface. Furthermore, Na_0.7MnO_2.05 was regenerated by heating the CO₂-absorbed Na_0.7MnO_2.05 at temperatures as low as 150 °C. The low-temperature regeneration indicates that Na_0.7MnO_2.05 can be a low-energy consumption material for capturing and releasing CO₂ at low temperatures.

Full text of DOI:10.1016/j.inoche.2019.01.029

Accepted Manuscript
Reversible CO2 capture/release of sodium manganate with a
layered structure in the presence of water vapor at 25–150°C
Ikuo Yanase, Takano Takuya
PII:
S1387-7003(18)31117-1
DOI:
https://doi.org/10.1016/j.inoche.2019.01.029
INOCHE 7245
Reference:
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
17 December 2018
17 January 2019
22 January 2019
Please cite this article as: I. Yanase and T. Takuya, Reversible CO2 capture/release of
sodium manganate with a layered structure in the presence of water vapor at 25–150°C,
Inorganic Chemistry Communications, https://doi.org/10.1016/j.inoche.2019.01.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.

ACCEPTED MANUSCRIPT
Reversible CO capture/release of sodium manganate
2
with a layered structure in the presence of water vapor
at 25-150℃
Ikuo Yanase*, Takano Takuya
Saitama University, Faculty of Engineering, Department of Applied Chemistry, 255
Shimoohkubo, Sakura-ku, Saitama-shi, Saitama, 338-8570 Japan
*
Corresponding Author
Tel/Fax: +81-48-858-3720
yanase@apc.saitama-u.ac.jp

ACCEPTED MANUSCRIPT
Abstract
Sodium manganates with a layered structure, Na0.7MnO2.05, have been applied to a novel
material for CCUS (CO
2
capture, utilization, and storage), capable of capturing CO at 25 ℃ in
2
the presence of water vapor and releasing CO at 150 ℃. The temperatures of capturing and
2
releasing CO of Na0.7MnO2.05 were remarkably lower than those of other traditional metal
2
oxides. The CO absorption and desorption properties of Na0.7MnO2.05 were investigated by
2
various methods, such as thermogravimetry, Fourier transform infrared spectroscopy, X-ray
diffractometry, and gas chromatography. These investigations confirmed that Na0.7MnO2.05
absorbed CO
2
at 25 ℃ in the presence of water vapor to produce NaHCO and a birnessite and
3
the CO
2
absorption was promoted by increasing relative humidity and CO concentration. The
2
CO absorption at 25 ℃ of Na0.7MnO2.05 was promoted by the formation of a strong basic
2
solution on Na0.7MnO2.05, caused by the elution of Na ions from the interlayer of Na0.7MnO2.05
into water, adsorbed on the Na0.7MnO2.05 surface. Furthermore, Na0.7MnO2.05 was regenerated by
heating the CO -absorbed Na0.7MnO2.05 at temperatures as low as 150 °C. The low-temperature
2
regeneration indicates that Na0.7MnO2.05 can be a low-energy consumption material for
capturing and releasing CO at low temperatures.
2
Keywords:carbon dioxide; sodium manganate; water vapor; ambient temperature.

ACCEPTED MANUSCRIPT
1
. Introduction
Recently, inorganic materials capable of capturing and releasing CO at low temperatures
2
have been noted from the viewpoint of suppression of energy consumption in CCUS (CO
2
capture, utilization, and storage). However, various inorganic metal oxides, such as lithium
silicate [1-4] and lithium ferrite [5-8] are mainly used for CO captures under moderate and high
2
temperatures and they are not appropriate for capturing and releasing CO at low temperatures,
2
except for amine-induced inorganic compounds [9,10] and metal oxides including alkaline ions
[
11,12].
On the other hand, it has been reported that water vapor in a CO gas was effective for
2
capturing CO
2
at low temperatures of Li
4
SiO
4
and Li
2
SiO
3
[13]. Furthermore, O3-LiFeO ,
2
[
14-15] absorbs CO
2
from the air to produce carbonates [11] and α-NaFeO
2
[17-19] absorbs CO
2
from air including water vapor at low temperatures. The CO absorption under water vapor at
2
low temperatures such as 25 ℃ was promoted by an alkaline solution formed on the surface of
the inorganic compounds.
+
Inorganic compounds, such as O3-LiFeO
2
and NaFeO
2
, are expected to elute Na ions
under water vapor at low temperatures because these compounds have alkaline ions in the
interlayer [19]. However, the above compounds are needed to release CO at high temperatures
2
after they absorbed CO . Therefore, it is important to develop metal oxides with a layered
2

ACCEPTED MANUSCRIPT
structure, having low regeneration-temperatures. In this study, we focused on the layered
+
structure of sodium manganates, Na0.70MnO2.05, having Na ions in the interlayer. Hence, we
examined the CO
2
absorption of the synthesized Na0.7MnO2.05 powder at 25 ℃ under a CO gas
2
flow in the presence of water vapor and its CO desorption at low temperatures.
2
2
. Experimental
Commercially available MnO
2
(>99% purity, Wako) and Na
2
CO (>99% purity, Wako)
3
powders were mixed in ion-exchanged water for 1 h by an ultrasonic treatment. After removing
the water with an evaporator, the mixed powder was dried and then heated at 850 °C for 5 h in
air, following by cooling from 500 ℃ to room temperature (RT) under an Ar gas flow to
synthesize Na0.7MnO2.05 powders.
The CO absorption of the synthesized Na0.7MnO2.05 powder was carried out in a mullite
2
tube at 25 ℃ for 5 h under a gas flow with various CO
2
concentrations. Where, the CO
2
concentration was controlled by mixing Ar gas and the gas pressure was 0.1 MPa. The relative
humidity (RH) in the gas was controlled by bubbling the gases into water at 25 ℃. Both the
thermal decomposition behaviors under an Ar gas flow and the CO uptake behaviors of the
2
CO -absorbed Na0.7MnO2.05 powders were investigated by thermogravimetry and differential
2
thermal analysis (TG-DTA; Thermoplus TG8120, Rigaku). Furthermore, the CO
2
and H O
2

ACCEPTED MANUSCRIPT
released amounts of the Na0.7MnO2.05 powders after the CO
2
absorption in the presence of water
vapor were investigated by gas chromatography attached to an electrical furnace.
The produced phases were investigated by X-ray diffractometry (XRD; D8Advance,
BrukerAXS) in the temperature range of 25 － 200 ℃ and Fourier transform infrared
spectroscopy (FTIR; FT/IR4200, Jasco) with a KBr method. The powder morphologies of the
Na0.7MnO2.05 powder before and after CO absorption were examined with a field-emission
2
scanning electron microscope (FESEM, S4100, Hitachi). The N
2
adsorption/desorption
isotherms at −196 °C, examined by Belsorp-mini II (MicrotracBEL), were used to determine the
+
Brunauer–Emmett–Teller (BET) specific surface area. The concentrations of Na ions in water
+
were measured with a Na ion meter (Horiba, LAQUA B-722) and a pH meter (Horiba, LAQUA
twin).
3
3
. Results and discussion
.1 CO absorption ofNa0.7MnO2.05
2
The XRD patterns of as-synthesized NMO and NMO absorbed CO at 25 ℃ for 5 h at 15%
2
and 75% in relative humidity (hereafter, 15% RH and 75% RH) are shown in Fig. 1(a) and (b),
respectively. At 15% RH, the NMO phase only existed and birnessite and carbonates were not
produced. Conversely, at 75% RH, the NMO phase disappeared to produce birnessite

ACCEPTED MANUSCRIPT
(PDF#43-1456), NaHCO
3
(PDF#15-0700) and Mn
7
O
13 ・ 5H O (PDF#23-1239) phases,
2
indicating that the NMO reacted with CO
2
and H O, and the presence of water vapor promoted
2
the reaction of NMO and CO . Fig. 2 shows the results of pH testing of NMO with pH test
2
paper in a tube furnace, under an air flow in the presence of water vapor (75% RH) at 25 ℃. The
color of the pH paper changed to yellowish green (the pH changed to 9–10), indicating the
production of a basic solution on the surface of the NMO particles in the presence of water
vapor. Where, the CO
2
absorption of NMO set on an Al
2
3
O boat, was also performed at 25 ℃
under a CO
2
flow in the same tube furnace. A commercial CaCl powder was used instead of
2
water in the flask (Fig. 2) to prepare CO
2
gas with 15% RH. Conversely, CO gas with 75% RH
2
was prepared by bubbling a dry CO gas into water.
2
To investigate the solubility of NMO in water, 0.14 g of NMO was dispersed into 100 ml
+
of distilled water to prepare an NMO slurry. The result is shown in Fig. 3(a). The Na ion
concentration and pH of the NMO slurry rapidly increased for a short time and then gradually
increased to approximately 120 ppm, which corresponds to approximately 50% of the maximum
+
Na-dissolution ratio for NMO (the theoretical concentration is 218 ppm when all Na ions are
+
eluted from 0.14 g of NMO into 100 mL of water). This result suggests that Na in the interlayer
of NMO easily eluted in the presence of water, leading to the formation of a NaOH solution on
+
the surface of the NMO particles. We investigated the phase produced by elution of Na of

ACCEPTED MANUSCRIPT
NMO. The result shown in Fig. 3(b) confirmed that NMO changed to birnessite (PDF#43-1456)
+
+
including H
2
O molecules and Na ions in the interlayer. The amount of Na ions dissolved in
+
water means that the amount of Na ions decreased from approximately 0.70 in NMO to 0.31 in
birnessite. The chemical composition of Na in the birnessite was almost corresponded with the
other report. [20]
Fig. 4(a) shows the mass increasing behavior of NMO at 25 ℃ under a CO gas flow at
2
80% RH, obtained by TG. The TG curve confirmed that the mass increase of NMO was
increased with increasing the holing time to approximately 3 h, and it reached an almost
constant level above 3 h. In the presence of water vapor, the rate-determining process is the
+
elution of Na ions into water adsorbed on the NMO surface. NMO dissolved into water
droplets to produce carbonates, according to reactions (1) and (2);
－
2－
CO2 + 2OH
→
CO3
+ H O
(1)
2
+
2－
2
Na + CO
3
+ H
2
O
+ CO
2
→
2NaHCO
3
(2).
Hence, it was considered that the CO absorption of NMO at 25 ℃ in the presence of water
2
vapor proceeded according to the primary rate reaction equation (3);
Ln [NMO] = －kPCO
2
t
+ A
(3).
Where, [NMO] is the remaining ratio of NMO, t; the CO
2
reaction time, k; the rate constant,
k = DS / Vh, D; the diffuse constant, S; the surface area, V; the volume of solution, h; the

ACCEPTED MANUSCRIPT
thickness of diffusion layer, PCO
2
; the partial pressure of CO , A; a constant. The parameter k
2
refers to the Nernst-Noyes-Whitney equation for dissolution of solids [21,22]. Fig. 3(b) shows
the relationship between the remaining ratio of NMO, [NMO] and holding time, t for the TG
curves in Fig. 3(a). Here, the partial pressure, PCO
2
= 1 atm. The result indicates that the CO
2
+
absorption rate of NMO was influenced by elution of Na ions into water adsorbed on the NMO
surface below approximately 1 h. On the other hand, the rate-determining process above 1 h was
the diffuse reaction of CO because carbonates and a birnessite increased on the surface as the
2
CO absorption proceeded.
2
The morphologies of the as-synthesized NMO and NMO absorbed CO at 25 ℃ for 5 h
2
under a CO flow at 75% RH, are shown in Figs. 5(a) and (b), respectively. Compared to the
2
as-synthesized NMO, small particles of NaHCO were observed on the surface of the
3
CO
2
-absorbed NMO, which was formed during CO
2
absorption under water vapor. The CO
2
absorption of NMO at 25 ℃ under water vapor occurred according to the schematic of Fig. 5(c).
+
Thus, a basic solution formed on the surface of NMO, owing to Na elution from the layered
structure of NMO, which promoted the reaction of CO and NMO, resulting in production of
2
birnessite and NaHCO
3
. The XRD patterns and FTIR spectra of the NMO powder absorbed CO
2
with concentrations of 20%, 50% and 100% under 75% RH, together with the as-synthesized
NMO powder are shown in Figs. 6(a) and (b), respectively. The XRD patterns show that the

ACCEPTED MANUSCRIPT
NMO phase decreased and the birnessite phase increased as the CO concentration increased
2
from 20% to 100%. The FTIR spectra confirmed that the largest absorption peaks at around
-
1
-1
1
350 cm of NaHCO
3
and those at around 1400 cm of Na
2
CO
3
・H O were assigned to
2
-
2-
asymmetric vibrations of HCO
3
and CO
3
, respectively [23,24] and NaHCO was clearly
3
produced at 50% and 100%-CO concentrations.
2
3
.2 Decomposition ofCO -absorbed Na0.7MnO2.05
2
The TG (a) and DTA (b) curves of NMO absorbed CO at concentrations from 20% to
2
100% at 25 ℃ for 5 h in the presence of water vapor are shown in Fig. 7. The mass decrease
rates for CO
2
-absorbed NMO increased as the CO concentration increased. The mass decrease
2
was mainly caused by decomposition of NaHCO
3
. The endothermic peaks for CO -absorbed
2
NMO were mainly observed at temperatures below 90 °C for the case of 20%-CO
2
concentration. This result indicates that production of NaHCO
3
requires a higher CO
2
concentration because NaHCO
3
was not produced at a CO concentration of 20%, as shown in
2
Fig. 6. The CO
2
and H
2
O absorption amounts of the CO -absorbed NMO was investigated. The
2
results are shown in Fig. 7(c) and (d). The absorption of CO
2
for was greater than that of H O,
2
+
indicating that NMO absorbed CO
2
together with H
2
O and Na ions in the interlayer was
effective for absorbing CO at 25 ℃in the presence of water vapor.
2

ACCEPTED MANUSCRIPT
3
.3 Regeneration ofNa0.7MnO2.05
The high-temperature XRD patterns of the powders obtained by heating the CO -absorbed
2
NMO at temperatures from 25 ℃ to 200 °C in air are shown in Fig. 8. The birnessite phase
decreased and the NMO phase increased at 100 °C, although the birnessite phase was identified
at 50 °C. The birnessite phase disappeared and the NMO phase only appeared at 150 °C. Thus,
NMO was regenerated at temperatures as low as 150 °C, which is considerably lower than the
regeneration temperatures of other CO -absorbing materials containing alkaline ions, such as
2
7
00 °C for Li
4
SiO
4
[1,13], 500 °C for LiFeO [6,7]. We attribute the low-temperature
2
regeneration of NMO to the low-temperature reactions of birnessite and NaHCO . The
3
+
low-temperature regeneration indicates that the Na ions could be induced into the interlayer of
+
birnessite, which have fewer Na ions, by the thermal decomposition of NaHCO
3
to produce
+
NMO having a layered structure containing any interlayer Na ions.
SEM images and N adsorption/desorption isotherms of the as-synthesized NMO and
2
regenerated NMO powders are shown in Fig. 9. The SEM images showed that the regenerated
NMO particles were smaller than those of the as-synthesized NMO. The N
2
adsorption/desorption isotherms confirmed that the specific surface area of the regenerated
2
2
NMO (12.5 m /g) was much higher than that of the as-synthesized NMO (0.8 m /g). This is

ACCEPTED MANUSCRIPT
because the small particles (Fig. 9(b)) on the regenerated NMO ware produced by the thermal
decomposition of the small NaHCO
3
particles formed by the reaction of CO with small NaOH
2
droplets on the NMO surface. Fig. 10 shows CO
2
and H O amounts released from the
2
as-synthesized NMO, the regenerated NMO at the 1st cycle (hereafter, 1st-reNMO), 2nd cycle
(
hereafter, 2nd-reNMO), and 3rd cycle (hereafter, 3rd-reNMO). The mass ratios of CO for the
2
1st-reNMO, 2nd-reNMO, and 3rd-reNMO were larger than that of the as-synthesized NMO.
The increase of the released amounts of CO
2
and H O was caused by the increase of the specific
2
surface area of the regenerated NMO, as shown in Fig. 9. Thus, the regenerated NMO is also
capable of capturing and stabilizing CO in the presence of water vapor.
2
4
. Conclusion
Na0.7MnO2.05 (NMO) with a layered structure was synthesized by a solid-state reaction
method. NMO absorbed CO
2
at 25 ℃ under CO
2
flows with various CO concentrations in the
2
presence of water vapor to produce NaHCO
3
and birnessite. The CO absorption ratio at 25 ℃ of
2
NMO increased with increasing the CO
2
concentration in the presence of water vapor. The CO
2
absorption of NMO at 25 ℃ was caused by the formation of a basic solution on the surface of
NMO by adsorption of water vapor on the surface. NMO was regenerated by heating the
CO -absorbed NMO at temperatures as low as 150 °C. The low-temperature regeneration of
2

ACCEPTED MANUSCRIPT
NMO indicates that NMO can be used as a CO -capture material at 25 ℃ in the presence of
2
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] K. Essaki, M. Kato, K. Nakagawa, CO removal at high temperature using packed bed of
2
lithium silicate pellets, J. Ceram. Soc. Jpn. 114 (2006) 739-742.
2] Q. Zhang, X. Liang, D. Peng, X. Zhu, Development of a fly ash derived Li
4
SiO -based
4
sorbent for CO capture at high temperatures, Thermochimica Acta 669 (2018) 80-87.
2
3] S. Shan, Q. Jia, L. Jiang, Q. Li, Y. Wang, J. Peng, Novel Li
4
SiO -based sorbents from
4
diatomite for high temperature CO capture, Ceramics Int. 39 (2013) 5437-5441.
2
4] K. Wang, Z. Yin, P. Zhao, Synthesis of macroporous Li
4
SiO via a citric acid-based sol-gel
4
route coupled with carbon coating and its CO chemisorption properties, Ceramics Int. 42
2
(2016) 2990-2999.
[5] M. Kato, K. Essaki, K. Nakagawa, Y. Suyama, K. Terasaki, Carbonation properties of
lithium ferrite for application as a high-temperature CO absorbent, J. Ceram. Soc. Jpn. 113
2

ACCEPTED MANUSCRIPT
(2005) 684-686.
[
[
[
[
[
6] I. Yanase, A. Kameyama, H. Kobayashi, Carbonation and structural phase transformation of
α-LiFeO , J. Ceram. Soc. Jpn. 118 (2010) 48-51.
2
7] I. Yanase, H. Otsuka, H. Kobayashi, Carbonation of CeO
Jpn. 119 (2011) 933-938.
2
-coated α-LiFeO , J. Ceram. Soc.
2
8] I. Yanase, J. Miura, H. Kobayashi, Carbonation and phase transformation of LiMO (M=Fe,
2
Co, Ni) under CO atmosphere, Materials Chemistry and Physics 199 (2017) 18-22.
2
9] Q. Lai, Z. Diao, L. Kong, H. Adidharma, M. Fan, Amine-ompregnated silicic acid
composites as an efficient adsorbent for CO capture,Applied Energy 223 (2018) 293-301.
2
10] A. Wells, Y. Fang, L.T.-Murciano, A. Nearchou, Z. Dong, T.J. White, A. Sartbaeva, V.P.
Ting, Mechanism of CO capture in nanostructured sodium amide encapsulated in porous
2
silica, Surface and Coatings Technology 350 (2018) 227-233.
[11] M.S. Cho, C. Lee, H.J. Chae, Y.M. Kwon, H.J. Kim, Y. Ryu, B. Lee, J.C. Kim, Optimum
design and characteristics of potassium-based sorbents using SiO
capture, Renewable EnergyAvailable online, 2018.
2
for post-combustion CO
2
[
12] R. R.-Mosqueda, E.A. Bramer, G. Brem, CO capture from ambient air using hydrated
2
Na
2
CO
3
supported on activated carbon honeycombs with application to CO enrichment in
2
greenhouses, Chemical Engineering Science 189 (2018) 114-122.

ACCEPTED MANUSCRIPT
[
13] J.O.-Landeros, R. L.-Juarez, I.C. R.-Ibarra, H. Pfeiffer, H.B.-Ramirez, C.G.-Yanez, Li
2
SiO
3
fast microwave-assisted hydrothermal synthesis and evaluation of its water vapor and CO
absorption properties, Particuology 24 (2016) 129-137.
2
[
[
14] M. Barre, M. Catti, Neutron diffraction study of the β’ and γ phases of LiFeO , J. Solid
2
State Chem. 182 (2009) 2549-2554.
15] M. Tabuchi, S. Tsutsui, C. Masquelier, R. Kanno, K. Ando, I. Matsubara, S. Nasu, H.
Kageyama, Effect of cations arrangement on the magnetic properties of lithium ferrites
(
LiFeO ) prepared by hydrothermal reaction and post-annealing method, J. Solid State Chem.
2
140 (1998) 159-167.
[
16] Y.S. Lee, S. Sato, Y.K. Sun, K. Kobayakawa, Y. Sato, A new type of orthorhombic LiFeO
2
with advanced battery performance and its structural change during cycling, J. Power Sources
110-121 (2003) 285-289.
[
[
[
17] H. Yoshida, N. Yabuuchi, S. Komaba, NaFe0.5Co0.5
O as high energy and power positive
2
electrode for Na-ion batteries, Electrochemistry Comm. 34 (2013) 60-63.
18] P. Vassilaras, A.J. Toumar, G. Ceder, Electrochemical properties of NaNi1/3Co1/3Fe1/3
cathode material for Na-ion batteries, Electrochemistry Comm. 38 (2014) 79-81.
2
O as a
19] E. Monyoncho, R. Bissessur, Unique properties of α-NaFeO : De-intercalation of sodium
2
via hydrolysis and the intercalation of guest molecules into the extra solution, Materials

ACCEPTED MANUSCRIPT
Research Bulletin 48 (2013) 2678-2686.
[20] Kh.S. A.-E.-Sherbini, M.H. Askar, R. Schollhorn, Hydrated layered manganese dioxide
Part I. Synthesis and characterization of some hydrated layered manganese dioxides from
α-NaMnO , Solid State Ionics 150 (2002) 407-415.
2
[
[
21] J.J. L-Quintana, J.K. R.-Guerrero, P.G. Valenca, Carbonate hydroxyapatite as a catalyst for
ethanol conversion to hydrocarbon fuels, Applied Catalyst A, General 542 (2017) 136-145.
22] S. Garcia, E.S. Fernadez, A.J. Stewart, M.M. M.-Valer, Process integration of
post-combustion CO
2
capture with Li
4
SiO
4
/Li
2
CO looping in a NGCC plant, Energy
3
Procedia 114 (2017) 2611-2617.
[
[
23] Z.S. Nickolov, O. Ozcan, J.D. Miler, FTIR analysis of water structure and its significance
in the flotation of sodium carbonate and sodium bicarbonate salts, Colloids and Surfaces A:
Physicochem. Eng. Aspects 224 (2003) 231-230.
24] B. Dutcher, M. Fan, B. Leonard, M.D. Dyar, J. Tang, E.A. Speicher, P. Liu, Y. Zhang, Use
of nanoporous FeOOH as a catalysis support for NaHCO decomposition aimed at reduction
3
of energy requirement of Na
C 115 (2011) 15532-15544.
2
CO
3
/NaHCO
3
based CO separation technology, J. Phys. Chem.
2

ACCEPTED MANUSCRIPT
Figure captions
Fig. 1 XRD patterns of powders of as-synthesized NMO and CO
2
-absorbed NMO under CO
2
flow at 25 ℃under (a) 15 and (b) 75% RH.
Fig. 2 Schematic showing the pH testing of NMO with pH test paper in the tube furnace, under
an air flow in the presence of water vapor (75% RH) at 25 ℃.
+
Fig. 3 (a) Na ion concentration and pH of NMO slurry prepared by dispersion of NMO into
+
distilled water and (b) XRD pattern of dried powder obtained after measurements of Na
ion concentration and pH.
Fig. 4 (a) TG curve for the mass increasing behavior of NMO at 25 ℃ under a CO gas flow at
2
80% RH. (b) Relationship between the remaining ratio of NMO, [NMO] and holding time,
t.
Fig. 5 Powder morphologies of (a) as-synthesized NMO and (b) CO
2
-absorbed NMO at 25 ℃
for 5 h under a flow of CO
2
with 75% RH. (c) Scheme for the CO
2
absorption mechanism at
25 ℃of NMO in the presence of water vapor.
Fig. 6 (a) XRD patterns and (b) FTIR spectra of the NMO powders absorbed CO for 5 h at
2
concentrations of 20%, 50% and 100% under 75% RH.

ACCEPTED MANUSCRIPT
Fig. 7 (a) TG and (b) DTA curves for thermal decomposition under an Ar gas flow for NMO.
CO
2
and H
2
O absorption amounts of CO
2
-absorbed NMO (c, d). CO absorption of NMO
2
was carried out at 25 ℃for 5 h under 75% RH.
Fig. 8 High-temperature XRD patterns of samples powders obtained by heating the
CO -absorbed NMO at temperatures from 25 ℃ to 200 °C in air, with a heating rate of
2
5
°C/min.
Fig. 9 SEM images and N adsorption/desorption isotherms of as-synthesized NMO and
2
regenerated NMO powders.
Fig. 10 CO
2
and H O released from as-synthesized NMO, regenerated NMO for the 1st cycle
2
(
1st-reNMO), 2nd cycle (2nd-reNMO), and 3rd cycle (3rd-reNMO). CO absorption of
2
NMO was carried out at 25 ℃for 5 h under 75% RH.

ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Highlights
・
・
Na0.7MnO2.05 wasapplied to a novel capturing/releasing CO
2
material at low temperatures.
The CO
CO
Adsorption of water vapor on the surface of Na0.7MnO2.05 caused the low-temperature CO
2
capture of Na0.7MnO2.05 was promoted at 25 ℃ by increasing relative humidity and
2
concentration.
・
・
2
absorption.
Na0.7MnO2.05 wasregenerated at temperatures as low as 150 °C.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10