Low temperature synthesized H2Ti3O7 nanotubes with a high CO2

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Low temperature synthesized HTiO nanotubes with a high CO adsorption property by amine-modification 2

Misaki Ota, Yuichiro Hirota, Yoshiaki Uchida, Yasuhiro Sakamoto, and Norikazu Nishiyama Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00317 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Langmuir 1 Low temperature synthesized H2Ti3O7 nanotubes with a high CO2 adsorption property by amine-modification Misaki Otaa*, Yuichiro Hirotaa, Yoshiaki Uchidaa, Yasuhiro Sakamotob, c and Norikazu Nishiyamaa a

Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3

Machikaneyama, Toyonaka, Osaka 560-8531, Japan b

PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

c

Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka

560-0043, Japan *Corresponding author: [email protected]

Keywords: titanium dioxide, hydrogen titanate, nanotubes, amine modification, CO2 adsorption Abstract Carbon dioxide (CO2) capture and storage (CCS) technologies have been attracted attention in terms of tackling with global warming. To date, various CO2 capture technologies including solvents, membranes, cryogenics and solid adsorbents have been proposed. Currently, a liquid adsorption method for CO2 using amine solution (monoethanolamine) has been practically used. However, this liquid phase CO2 adsorption process requires heat regeneration, and it can cause many problems such as corrosion of equipment and degradation of the solution. Meanwhile, solid adsorption methods using porous materials are more advantageous over the liquid method at these points. In this context, we here evaluated if hydrogen titanate (H2Ti3O7) nanotubes and the surface modification effectively capture CO2. For this aim, we first developed a facile synthesis method of H2Ti3O7 nanotubes different from any conventional methods. Briefly, they were converted from the precursors, amorphous TiO2 nanoparticles at room temperature (25°C). We then determined the outer and the inner diameters of the H2Ti3O7 nanotubes as 3.0 nm and 0.7 nm, respectively. It revealed that both values were much smaller than the reported ones; thus the specific surface area showed the highest value (735m2/g). Next, the outer surface of H2Ti3O7 nanotubes was modified using ethylenediamine to examine if CO2 adsorption capacity increases. The ethylendiamine-modified H2Ti3O7 nanotubes showed a higher CO2 adsorption capacity (50 cm3/g at 0°C, 100 kPa). We finally concluded that the higher CO2 adsorption capacity could be explained, not only by the high specific surface area of the nanotubes, but also by tripartite hydrogen bonding interactions among amines, CO2 and OH groups on the surface of H2Ti3O7. 1. Introduction Carbon dioxide capture and storage (CCS) technologies have been well studied over the last decade. This is because the increase of CO2 emission in the atmosphere might contribute to global warming. Various CO2 capture technologies including solvents, membranes, cryogenics and solid adsorbents have been proposed1-4. Currently, a liquid phase adsorption method using amine solution (monoethanolamine) has been put into practical use5. However, this process has problems such as corrosion of equipment, degradation of the solution, and, in addition, it

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requires heat regeneration. Meanwhile, solid adsorption methods have attracted more attention over the liquid method4. This is because the solid adsorption method has higher cycle stability and does not cause corrosion of equipment. Moreover, the solid adsorption by using pressure differences can reduce energy consumption for CO2 regeneration. Porous materials including mesoporous alumina, silica, activated carbon and zeolites6-9 have been well known as solid absorbent materials with high surface areas. Moreover, recent studies have shown that surface amine-modification of the porous supporting materials can improve CO2 adsorption capacity10-12. We therefore attempted to combine the both advantages of the chemical adsorption and the physical adsorption: chemical reactions between the amines and CO2, and CO2 immobilization to the porous material with a high surface area. Titanium dioxide and titanate materials have attracted the attention to researchers because of their remarkable properties among porous materials. Titanium dioxide has been applied to photocatalyst and dye-sensitized solar cells13-14. Titanate materials such as strontium titanate (SrTiO3) and lithium titanate (Li4Ti5O12) have also been used as photocatalyst and anode for lithium ion battery, respectively15-16. Recently, nanomaterials made of titanium dioxide and titanate materials have been studied because their higher surface areas with high reactivity enhance the catalytic activity and the performance of electrical devices17-18. So far, various synthesis methods of titanium dioxide and titanate nanomaterials have been reported19-21. However, they have not been suitable for mass production and for fabrication of nanomaterials because they require prolonged treatments at high temperature. Accordingly, the development of a simple low-temperature synthesis has been expected to prepare titanium dioxide and titanate nanomaterials. Hydrogen titanate (H2Ti3O7) nanotubes have been well studied as one of porous materials. This is because they are usefully applicable as supporting materials such as ion-exchangers22-25, batteries26 and catalysts27-28. However, there has been only a few reports on gas adsorption29-30. Liu et al. reported that amine-modified H2Ti3O7 nanotubes enhances CO2 adsorption capacity31. So far, H2Ti3O7 nanotubes have been synthesized in hydrothermal method32. However, the hydrothermal synthesis requires severe conditions at 100-150°C with high pressure. This is because the nanotube can form only in the temperature range. In addition, the length and the diameter are also limited to about 5 nm and a few hundred nm. There has been no report on the synthesis of microporous H2Ti3O7 nanotubes. In our previous work, we have developed a low temperature synthesis method of pure Li4Ti5O12 nanoparticles with high surface areas33. The synthesis of Li4Ti5O12 consists of two steps: the synthesis of amorphous titanium dioxide (TiO2) nanoparticles from titanium tetraisopropoxide (TTIP) by using tetrahydrofuran (THF) as a solvent at room temperature, and the treatment of the amorphous TiO2 nanoparticles with LiOH(aq) at room temperature to produce pure Li4Ti5O12. The amorphous TiO2 can be used as a precursor for synthesizing other various titanate nanomaterials because of its high surface area and high reactivity. In these contexts, we aimed to develop a facile synthesis method for H2Ti3O7 nanotubes using the amorphous TiO2 nanoparticles. In this study, we report a room temperature synthesis of H2Ti3O7 nanotubes by using the amorphous TiO2 as a raw material. The size and morphology of the H2Ti3O7 nanotube were quite different from the ones obtained by the conventional methods. The H2Ti3O7 nanotube modified with ethylenediamine provided higher CO2 adsorption capacity. Accompanied by this finding, we also developed a low temperature synthesis method of amorphous TiO2 nanoparticles, which confers a higher specific surface area and higher reactivity on them. As the amorphous TiO2


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Langmuir 3 can be useful as a precursor for synthesizing various titanate nanomaterials with high surface area and high reactivity, hereafter, we anticipate that this work will contribute to the future development and implementation of synthesizing various titanate nanomaterials.

2. Experimental 2.1. Synthesis of amorphous TiO2 According to our previous report33, 1.4 mL of TTIP was mixed with 30 mL of THF and the mixture was stirred at room temperature for 1 h. Next, 1.6 mL of water was added to cause hydrolysis reactions and white precipitates of TiO2 was immediately formed. Finally, amorphous TiO2 was collected by centrifugation and drying at 90°C. 2.2. Synthesis of H2Ti3O7 nanotubes H2Ti3O7 nanotubes were synthesized via a simple route using the amorphous TiO2 as a raw material which is a wet chemical treatment at room temperature. The synthetic procedure is as follows: firstly, 2.0 g of the amorphous TiO2 was mixed with 120 mL of 10 M NaOH aqueous solution for 48 h. The precipitate, sodium titanate (Na2Ti3O7) was acid-washed with 0.1 M HCl solution until the pH value reached one by ion exchanges between Na+ ions and H+ ions, and then washed with distilled water until pH reached a neutral value. Next, it was washed with ethanol twice. Finally, the sample was dried at 90°C for 12 h. As a reference, H2Ti3O7 nanotubes were obtained via a conventional hydrothermal treatment in teflon-lined autoclave at 120°C for 48 h by using commercially available P25-TiO2 as a raw material. The H2Ti3O7 samples prepared from the amorphous TiO2 and P25-TiO2 were labeled as HTO-RT-A and HTO-HT-P, respectively. 2.3. Preparation of amine-modified H2Ti3O7 nanotubes The titanate nanotubes were modified with amines by an impregnation process. The HTO samples (0.2 g) were added into 15 mL of 25 wt% ethylenediamine (EDA) solution in ethanol and stirred at room temperature for 3 h. Then the amine-modified samples were centrifuged and washed by using 10 mL of ethanol. After that, they were dried at 90°C. The final products were labeled as HTO-RT-A-EDA and HTO-HT-P-EDA, respectively. In addition, mesoporous silica (MCM-41), which had been well studied as a porous support, was also modified with EDA by the same process and it was labeled as MCM-41-EDA. 2.4. Characterization X-ray diffraction (XRD) pattern was measured to characterize their crystal structures of the amorphous TiO2 and HTO samples using Bruker D8 ADVANCE with Cu Kα X-ray (1.54 Å). The transmission electron microscopy (TEM) images of the raw materials and HTO-HT-P were recorded on Hitachi H800 electron microscope at an acceleration voltage at 200 kV. The TEM image of HTO-RT-A was recorded on JEOL JEM-3010 electron microscope at an acceleration voltage at 300 kV. Nitrogen adsorption-desorption isotherms were measured at 77 K using

BELSORP-max

(MicrotracBEL

Corp.).

The

specific

surface

area

was

calculated

by

the

Brunauer-Emmett-Teller (BET) method from nitrogen adsorption isotherms. The pore size distribution and pore volume were calculated by the Brunauer-Joyner-Halenda (BJH) method. Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu IRAffinity-1 in transmission mode with a scan number of 100. The amount of EDA loaded on the samples were measured by thermogravimetry (TG) analysis under air atmosphere to 800°C at a heating rate of 5°C/min. The adsorption isotherms of CO2 were measured at 0°C with BELSORP-max.


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3. Results and discussion The TEM images are shown in Figure 1. The particle size of the amorphous TiO2 is about 3.0 nm (Figure 1c) which is smaller than the commercially available P25-TiO2 of 20-30 nm (Figure 1a). The HTO-HT-P showed a nanotube structure with an outer diameter of 8.0 nm and an inner diameter of 3.5 nm (Figure 1b), while a nanotube with an outer diameter of 3.0 nm and an inner diameter of 0.7 nm was observed for HTO-RT-A (Figure 1d) calculated by 1D profile analysis (Figure 1e). The size of H2Ti3O7 nanotubes can depend on the particle sizes of the raw material of TiO2, which can be explained based on the following proposed formation mechanism. The formation of nanotubes involves peeling and scrolling processes34-35. First, nanosheets of Na2Ti3O7 are formed on TiO2 particles, and then the nanosheets of Na2Ti3O7 delaminate from TiO2 particles in NaOH solution. Finally, the nanosheets are rolled up into nanotubes during the ion-exchange reactions between Na+ ions and H+ ions. It has been also reported that the interlayer distances of nanotubes which are synthesized by the conventional hydrothermal method are 0.78 nm on average and the number of layers is 3-5 as shown in the TEM image of HTO-HT-P, which was synthesized by the conventional method36. On the other hand, a monolayer nanotube structure could be formed for HTO-RT-A, using the nano-sized amorphous TiO2 particles as a raw material. Figure 2 shows the XRD patterns of the raw materials and the HTO samples. The commercial P25-TiO2 is composed of a mixture of anatase and rutile crystal structures. On the other hand, the TiO2 sample synthesized with THF did not show any peaks, which indicated that it was low crystallinity and amorphous phase. The peaks at 2θ of 24.4° and 48.2° in both the XRD patterns of HTO-RT-A and HTO-HT-P are attributed to the H2Ti3O7 crystal structure. This result indicates that H2Ti3O7 could be successfully synthesized using amorphous TiO2 as a raw material without high pressure and heat treatment, showing the high reactivity of the amorphous TiO2. On the other hand, when the P25-TiO2 was used as raw material under the same reaction condition, the H2Ti3O7 crystal structure was not formed without heat treatment and the pressure, and its nanostructure did not change. When the P25-TiO2 was used, hydrothermal conditions at 120°C and autogenous pressure were required for the phase transition from TiO2 to the H2Ti3O7 structure (Figure 2b, 2d). There are two reasons why H2Ti3O7 could be synthesized from only amorphous TiO2. First, amorphous TiO2 has a high specific surface area, SBET = 680 m2/g which is 10 times higher than P25-TiO2 (SBET = 79 m2/g). Small particles with sizes of less than 3.0 nm were observed by TEM measurement as shown in Figure 1c. The THF solvent stabilizes TTIP and slows down hydrolysis reactions. Second, there are many defects within the TiO2 structure derived from amorphous phase which contribute to its high reactivity as we discussed in the previous report33. Since crystal TiO2 is stable, it was difficult to react with NaOH and H2Ti3O7 was not produced from P25-TiO2. The peak at 2θ of about 10° was observed for HTO-HT-P, while the sample of HTO-RT-A did not show the peak. It has been proposed that the peak at 2θ of about 10° was assigned to interlayer distance of 0.78 nm (d100) of rolled nanotube structure37. Therefore, HTO-HT-P was multilayered nanotubes while HTO-RT-A could be monolayered nanotubes. The peak at 2θ of about 48.2° was assigned to b-axis length of nanotube. The peak intensity of HTO-RT-A was weaker than HTO-HT-P, indicating that the length of HTO-RT-A nanotube was shorter than HTO-HT-P. These results are consistent with the results of TEM observations (Figure 1b, 1d).


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Langmuir 5

Figure 3a shows the nitrogen adsorption/desorption isotherms of the HTO samples. The hysteresis loop of adsorption/desorption isotherms of HTO-RT-A indicated the presence of mesopores in addition to micropores. The isotherm of HTO-RT-A had an initial rise derived from the micropores at low pressure area. From the TEM images, the HTO-RT-A hydrogen titanate nanotubes possess a micro-meso bimodal porous structure. The interstitial spaces observed between the nanotubes correspond to the above-mentioned mesopores and the holes within nanotubes are micropores. The specific surface areas of HTO-RT-A and HTO-HT-P calculated from the N2 adsorption isotherms were SBET = 735 and 408 m2/g, respectively. To confirm the amine-modification onto H2Ti3O7 nanotubes, FT-IR spectra of HTO and HTO-EDA samples were measured. According to Figure 4, the adsorption peaks at 400-1000 cm-1 of all the samples were attributed to lattice vibration of TiO6 octahedral crystal. The strong adsorption at 1630 and around 3400 cm-1 were ascribed to the bending vibration of O-H bonds and the stretching vibration respectively, indicating that the HTO samples had a massive amount of OH groups on the surface. The possible structure of the HTO nanotube is shown in Figure S1. After EDA modification, the new peaks at 1136, 1531 cm-1 were assigned to C-N stretching vibration and N-H2 vibration in the primary amine group (RNH2) respectively, indicating the presence of EDA on the surface of HTO samples. On the other hand, the intensity of the adsorption peaks ascribed to OH groups slightly decreased after EDA modification, suggesting that OH groups still remained. The peaks at 1331 cm-1 could be ascribed to skeletal vibration of -NCOO by adsorbed gaseous CO2 in the atmosphere38. The specific surface area, pore volume and the amount of EDA loaded on the samples measured by TG analysis were summarized in Table 1 and compared with those from MCM-41. The loaded amount of EDA was 7.6 wt% (HTO-RT-A-EDA), 8.3 wt% (HTO-HT-P-EDA), and 7.8 wt% (MCM41-EDA), respectively. The specific surface area and pore volume decreased with increasing of amount of loaded amine (see supporting information Table S1). According to pore size distributions calculated from the nitrogen adsorption isotherms after amine-modification, the peak did not shift and only the pore volume was reduced (see supporting information Figure S2). This results show that EDA was not uniformly modified on the surface of HTO, but it was rather modified as if it blocked some of the pores.

The adsorption isotherms of CO2 at 0°C are shown in Figure 5. The effect of the modification with amine on the enhancement of CO2 adsorption capacity was more largely for HTO-RT-A than HTO-HT-P. Three schemes of enhanced CO2 adsorption onto amine-modified H2Ti3O7 have been proposed (Figure 6)31. First, -NH2 groups of amines react with CO2 to form carbamate species according to the equation shown below (Figure 6a). CO2 + 2RNH2 ↔ RNH3+ + RNHCOOSecond, the OH groups can donate protons to CO2 to convert CO2 into bicarbonate species39 (Figure 6b). Third, electrostatic force between amine molecules, -OH groups of H2Ti3O7 and CO2 molecules promotes adsorption capacity (Figure 6c). According to Figure 7, the DRIFT peak at 2627 cm-1 after CO2 adsorption was attributed to such hydrogen bonded CO2 species31. The high CO2 adsorption capacity of HTO-RT-A-EDA is possibly due to the


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enhancement of the third scheme because of its high surface area and a high surface concentration of -OH groups. In addition, compared to MCM-41-EDA, HTO-RT-A-EDA exhibited the highest CO2 adsorption capacity even though the specific surface area of HT-RT-A was lower than that of MCM-41 (978 m2/g). In the CO2 adsorption isotherm, the rise at the low-pressure side is due to absorption by the reaction of amines and CO2 described above as the first scheme. Generally, the difference between absorption volume at 100 kPa and the volume caused by the reaction is considered as physical adsorption volume. In that case, the adsorption isotherm before and after amine-modification should be parallel at the high pressure side. However, the slope of the adsorption isotherm of MCM-41-EDA was smaller than the slope of the MCM-41, which were not parallel. This is because the surface area for the physical adsorption was decreased by amine-modification. Thus, the slope of the adsorption isotherm should decrease with a decrease in the specific surface area after the amine-modification. However, the slope of the adsorption isotherm of HTO sample was not reduced and the one of HTO-RT-A rather increased. This could be due to an improvement of adsorption capacity by the above third scheme (Figure 6c). In the adsorption process of the third scheme, the OH group should be exposed on the surface. Therefore, with regard to the MCM-41which had few OH groups, the process of the third scheme didn't occur. On the other hand, as described above, according to FT-IR measurement of the HTO sample, a part of OH groups was exposed on the surface of H2Ti3O7, which caused adsorption of the third scheme process. Along with increasing the loaded EDA amount, CO2 adsorption capacity decreased (see supporting information Figure S3). Excessive amine-modification caused the pores to be blocked and the surface area to be reduced. In addition, OH groups on the surface was covered with amine molecules. These facts suggest that the adsorption as shown in Figure 6c contributes to the enhancement of adsorption capacity.

4. Conclusions This work demonstrates facile synthesis methods of both amorphous TiO2 nanoparticles and H2Ti3O7 nanotubes in normal temperature and pressure: firstly, highly active amorphous TiO2 nanoparticles were synthesized by using THF as a solvent; then H2Ti3O7 nanotubes were synthesized by introducing Na+ into the highly active amorphous TiO2 nanoparticles, and finally, Na+ ions were exchanged for H+ ions at room temperature (25°C). According to the results of the TEM observation, the H2Ti3O7 nanotubes had micropores with an inner diameter of 0.7 nm, which is the smallest size among the conventional ones. The nitrogen adsorption isotherm had an initial rise derived from the micropores at a low-pressure area. The specific surface area calculated from the nitrogen adsorption isotherm was 735m2/g, which was 2 times larger than the conventional ones. Next, our H2Ti3O7 nanotubes were modified by ethylenediamine. The subsequently amine-modified H2Ti3O7 nanotubes showed the highest CO2 adsorption capacity among those of the amine-modified conventional porous supports, not only H2Ti3O7 nanotubes but also MCM-41. We here propose three possible reasons of the increased CO2 adsorption capacity as follows: 1) the higher specific surface area of our synthesized H2Ti3O7 nanotubes increased the physical immobilization with CO2; 2) the modified amine molecules reacted with CO2 and effectively absorbed it; and 3) the tripartite hydrogen bonding interactions among the amine molecules, CO2 and OH groups on the H2Ti3O7 surface created new adsorption processes. We anticipate that this work will contribute to the future development and implementation of synthesizing various titanate nanomaterials hereafter.


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Langmuir 7 5. Acknowledgement The TEM measurements were carried out by using a facility in Research Center for Ultrahigh Voltage Electron Microscopy, Osaka University. This work was supported by JSPS KAKENHI Grant Number 16K14458.

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Langmuir 9 amine-functionalized TiO2 nanotubes. Applied Surface Science 2013, 268, 124-128. 31. Liu, Y.; Liu, J.; Yao, W. Y.; Cen, W. L.; Wang, H. Q.; Weng, X. L.; Wu, Z. B. The effects of surface acidity on CO2 adsorption over amine functionalized protonated titanate nanotubes. Rsc Advances 2013, 3 (41), 18803-18810. 32. Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Formation of titanium oxide nanotube. Langmuir 1998, 14 (12), 3160-3163. 33. Ota, M.; Dwijaya, B.; Hirota, Y.; Uchida, Y.; Tanaka, S.; Nishiyama, N. Synthesis of Amorphous TiO2 Nanoparticles with a High Surface Area and Their Transformation to Li4Ti5O12 Nanoparticles. Chemistry Letters 2016, 45 (11), 1285-1287. 34. Chen, Q.; Du, G. H.; Zhang, S.; Peng, L. M. The structure of trititanate nanotubes. Acta Crystallographica Section B-Structural Science 2002, 58, 587-593. 35. Zhang, S.; Chen, Q.; Peng, L. M. Structure and formation of H2Ti3O7 nanotubes in an alkali environment. Physical Review B 2005, 71 (1), 11. 36. Wu, D.; Liu, J.; Zhao, X. N.; Li, A. D.; Chen, Y. F.; Ming, N. B. Sequence of events for the formation of titanate nanotubes, nanofibers, nanowires, and nanobelts. Chemistry of Materials 2006, 18 (2), 547-553. 37. Chen, Q.; Zhou, W. Z.; Du, G. H.; Peng, L. M. Trititanate nanotubes made via a single alkali treatment. Advanced Materials 2002, 14 (17), 1208. 38. Su, F. S.; Lu, C. Y.; Kuo, S. C.; Zeng, W. T. Adsorption of CO2 on Amine-Functionalized Y-Type Zeolites. Energy & Fuels 2010, 24, 1441-1448. 39. Pan, Y.; Liu, C.; Ge, Q. Effect of surface hydroxyls on selective CO2 hydrogenation over Ni4/γ-Al2O3: A density functional theory study. Journal of Catalysis 2010, 383, 227-24.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1 TEM images of raw materials and HTO samples (a) commercially available P25-TiO2 (b) H2Ti3O7 nanotubes synthesized by conventional hydrothermal method from P25-TiO2 (c) amorphous TiO2 (d) H2Ti3O7 nanotubes synthesized at room temperature from amorphous TiO2 (e) 1D profile of HTO-RT-A

Figure 2 XRD patterns of raw materials and HTO samples (a) amorphous TiO2 (b) commercially available


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Langmuir 11 P25-TiO2 (c) H2Ti3O7 nanotubes synthesized at room temperature from amorphous TiO2 (d) H2Ti3O7 nanotubes synthesized by conventional hydrothermal method from P25-TiO2

Figure 3 N2 adsorption/desorption isotherms and pore size distributions of HTO samples (a) H2Ti3O7 nanotubes synthesized at room temperature from amorphous TiO2 (b) H2Ti3O7 nanotubes synthesized by conventional hydrothermal method from P25-TiO2

Figure 4 FT-IR spectra of HTO samples before amine-modification (a) HTO-RT-A (b) HTO-HT-P, and after


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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amine-modification (c) HTO-RT-A-EDA (d) HTO-HT-P-EDA

Figure 5 CO2 adsorption isotherms of HTO samples and MCM-41 at 0°C (open symbols) before amine-modification (closed symbols) after amine-modification

Figure 6 Proposed reaction schemes of CO2 adsorption process on amine-modified H2Ti3O7


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Langmuir 13

Figure 7 Time dependent DRIFT spectra of HTO-RT-A for CO2 adsorption (a) 0 min (b) 10 min

Table 1 Specific surface area, pore volume and amount of loaded amine of HTO samples and MCM-41 SBET (m2/g)

V (cm3/g)

Amount of loaded amine (wt%)

HTO-RT-A

735

1.63

-

HTO-RT-A-EDA

638

1.43

7.6

HTO-HT-P

408

2.43

-

HTO-HT-P-EDA

266

1.55

8.3

MCM-41

978

0.25

-

MCM-41-EDA

603

0.22

7.8


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table of Contents


Page 15 of 21

(a) P25-TiO2

(b) HTO-HT-P

(d)

10 nm 100 nm

(c) Amorphous TiO2

20 nm

20 nm

(d) HTO-RT-A

(e)

Pixel Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Langmuir

20 nm

0.7 nm

0


5 Distance (nm)

10

Langmuir

● TiO2 ▼ H2Ti3O7

▼

▼

▼ ▼

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

▼

(d) HTO-HT-P

▼

●

(c) HTO-RT-A ●

●

●

●●

●

●●

●

(b) P25-TiO2 ×0.1 (a) Amorphous TiO2 20

40 2θ (degree)

60

80


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Page 17 of 21

1500

● (a) HTO-RT-A ○ (b) HTO-HT-P

1000

500

0 0

0.5 Relative Pressure P/P0 (-)

● (a) HTO-RT-A ○ (b) HTO-HT-P

0.15

dVp/ddp (cm3/g/nm)

Volume (cm3-STP/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Langmuir

0.1

0.05

0 0 10 ACS Paragon Plus Environment 1

1

10 Pore diameter (nm)

2

10

Langmuir

(a) HTO-RT-A

(b) HTO-HT-P Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(c) HTO-RT-A-EDA

(d) HTO-HT-P-EDA

O-H

4000

C-N -NCOO N-H2 -OH

3000 2000 Wavenumber (cm-1)

Ti-O

1000 ACS Paragon Plus Environment

Page 18 of 21

Page 19 of 21

60

○ HTO-RT-A □ HTO-HT-P ▽ MCM-41 Volume (cm3-STP/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Langmuir

● HTO-RT-A-EDA ■ HTO-HT-P-EDA ▼ MCM-41-EDA

40

20

0 0

50 Pressure (kPa)

100 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Page 21 of 21

(a) CO2 0 min

(b) CO2 10 min

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

4000

3000 2000 -1 Wavenumber (cm )


1000

Low temperature synthesized H2Ti3O7 nanotubes with a high CO2

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