H2 and CO2 uptake for hydrogen titanate (H2Ti3O7) nanotubes and nanorods at ambient temperature and pressure

H2 and CO2 uptake for hydrogen titanate (H2Ti3O7) nanotubes and nanorods at ambient temperature and pressure

Accepted Manuscript H2 and CO2 uptake for hydrogen titanate (H2Ti3O7) nanotubes and nanorods at ambient temperature and pressure Sora Sim, Eun-Bum Cho...

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Accepted Manuscript H2 and CO2 uptake for hydrogen titanate (H2Ti3O7) nanotubes and nanorods at ambient temperature and pressure Sora Sim, Eun-Bum Cho, Sriparna Chatterjee PII: DOI: Reference:

S1385-8947(16)30735-5 http://dx.doi.org/10.1016/j.cej.2016.05.099 CEJ 15259

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

29 March 2016 20 May 2016 22 May 2016

Please cite this article as: S. Sim, E-B. Cho, S. Chatterjee, H2 and CO2 uptake for hydrogen titanate (H2Ti3O7) nanotubes and nanorods at ambient temperature and pressure, Chemical Engineering Journal (2016), doi: http:// dx.doi.org/10.1016/j.cej.2016.05.099

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H2 and CO2 uptake for hydrogen titanate (H2Ti3O7) nanotubes and nanorods at ambient temperature and pressure

Sora Sima, Eun-Bum Cho a,*, Sriparna Chatterjeeb,*

a

Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul

01811, Korea b

CSIR-Institute of Minerals and Materials Technology, Acharya Vihar, Bhubaneswar, 751

013, India *Corresponding Authors: E.-B. Cho: e-mail: [email protected], Tel.: +82-2-970-6729 S. Chatterjee: e-mail: [email protected]

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ABSTRACT H2 and CO2 adsorption was investigated using pristine hydrogen titanate nanotube (HTNT) and nanorod (HTNR). HTNT and HTNR were modified with N-[3-(trimethoxysilyl)propyl]ethylenediamine (TPEDA). Chemical linkage between an amine-containing organosilane and titanium on the surface of hydrogen titanate was confirmed with

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Si CP-

MAS NMR spectrum. H2 and CO2 uptake on several hydrogen titanate samples was obtained by a TGA Q50 analyzer in flowing N2 gas of 40 mL/min and CO2 gas of 60 mL/min at 25 °C and 1 atm. Pristine HTNT sample showed the highest H2 uptake (i.e. ~ 12.5 mmol/g) on a TGA microbalance at 25 °C whereas amine-modified HTNT sample exhibited the highest CO2 uptake (i.e. ~ 1.2 mmol/g) among all samples studied. CO2/N2 and H2/N2 selectivity for pristine and amine-modified HTNT samples was investigated by comparing the weight of the sample in flowing pure N2 gas of 100 mL/min with above-mentioned mixed flows at 25 °C. Reversibility for CO2 and H2 uptake showed good performance for 9 cycles.

Keywords: Hydrogen titanate; Nanotube; Hydrogen storage; Carbon dioxide capture; Nanorod; Amine-modification

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1. Introduction In recent days search of alternative and sustainable green energy resource for environment safety is a major concern among researchers and lot of works are on-going in various direction which includes artificial photosynthesis [1], water splitting reaction [2], oxygen reduction reaction (ORR) [3] etc. Lot of efforts are also being given to use hydrogen as an alternative energy resource as it is considered to be promising one. However, storage of hydrogen is a difficult task. Either high-pressure tanks or cryogenic tanks are used to store H2. Unfortunately, both techniques are not economically viable. Even reversible storage of H2 in metals like palladium is not energetically and economically practicable [4]. Till date, widely acceptable strategies for H2 uptake are the use of metal–organic frameworks (MOFs) [5-7]. However, Goldsmith et al. discussed the limitation of using MOFs for H2 storage [8] and concluded that only improvement in surface area will not benefit the efficiency of H2 storage rather efforts should be given to achieve in controlling both mass density and surface area of the storage material and the at the same time, stability of the framework after solvent removal should be ensured. Other than MOFs, covalent organic frameworks (COFs) [9], porous aromatic frameworks (PAFs) [10], porous carbon based materials [11,12], CNTs [13], graphene based materials [14] have shown promising results. Newfangled trend is the use of nanostructured non-carbon materials for H2 uptake because of following reasons; (1) nanomaterials can strongly influence the thermodynamics and kinetics of the adsorption processes, (2) large-scale production of inorganic nanostructured materials are comparatively viable, (3) precursor materials are of low cost and earth-abundant etc. Different metal oxides [15-17], metal hydrides, metal and non-metal nitrides [18,19], metal borides [20], and boranes [21] are recently identified to be competent material for storage of hydrogen. Constant efforts have been given to explore nanotubular structures as H2 adsorbent material and alkali metal

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doped carbon nanotubes showed promising results [22]. Among pure inorganic nanotube systems, BN nanotubes could uptake 2.6 wt% of H2 [23]. But sincere observation reveals that BN nanotubes work in high-pressure condition (~ 10 MPa) and CNTs work in high temperature condition (200 – 400 oC). In comparison, TiO2 nanotubes uptake H2 at room temperature and comparatively low-pressure range (~ 6MPa) [24]. This observation attracted us to explore for a material that will uptake H2 at ambient pressure as well as ambient temperature. One dimensional nanostructures of protonated titanates are well known for wide range of application [25] including photocatalysis [26-29], ion-exchanger [30, 31], sorbent material [32, 33], nano-bio conjugate [34], and nano-welding material [35], metal ion adsorbent [36- 39], water splitter [40] etc. as they are having very high surface area as well as good stability. We find that pristine hydrogen titanate nanotube can uptake ~ 3% H2 at ambient pressure and temperature. It should be mentioned here that till date this is the first report on the H2 uptake capacity of hydrogen titanate nanotube. While most of the studied materials with nano-tubular structure works either in high pressure or high temperature condition (see Table S1 in supporting information), here, we have shown that pure hydrogen titanate nanotube can act as potential sorbent material for H2 uptake at room temperature and ambient pressure. In addition to this, we have also explored the CO2 adsorption capacity of one dimensional hydrogen titanate nanostructures. For CO2 adsorption also, mostly metal organic frameworks (MOFs) [41,42] are being used. Other than MOFs, porous organic molecules (POMs), microporous organic polymers (MOPs) [43], and microporous carbons [44, 45] have been explored for CO2 adsorption and in all of these systems physisorption of gases occurs through relatively weak van der Waals forces [46]. Among inorganic materials, zeolites [4749] and silicas [50] have shown notable CO2 capturing capacity. Other than zeolites,

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nanocrystal of MgO is reported to be voracious CO2 adsorbent [51,52]. Group I oxides like Li2O and Na2O have also been explored for CO2 capture [53]. However, both oxides are very sensitive to environment. Among transition metal oxides, CO2 is known to be chemisorbed on ZnO surfaces [54] and microporous pillared nanohybrids of self-assembled layered titanate nanosheets of Cr2O3/CdO etc [55]. CO2 adsorption of different types of amine-modified TiO2 nanotubes was explored by Song et al. [56] and found the enhancement in adsorption capacity of titania nanotube was observed when the surface was functionalized with tetraethylenepentamine. Aquino et al. assessed mesoporous TiO2 beads, Degussa P25, and composite beads of TiO2/ZrO2, and functionalized their surface with a range of amines (1, 2, and 3 carbon-chain primary amine) for CO2 adsorption at 30 °C and found that CO2 adsorption capacity increased with the carbon-chain length of the amine [57]. Aminefunctionalization with amino acids like L-glutamine or L-arginine was also investigated and L-arginine modified material showed the highest CO2 adsorption capacity [57]. Upendar et al., reported CO2 adsorption behavior of alkali metal titanates at low temperature [58]. Liu et al., studied role of surface acidity on CO2 adsorption for poly (ethyleneimine)-functionalized protonated titanate nanotubes [59]. Here, we have studied CO2 adsorption capacity of pristine and surface modified hydrogen titanate nanotubes at ambient pressure and temperature. The adsorption capacities of hydrogen titanate nanotubes are also compared with nanorod morphology of hydrogen titanate. This study contains detailed analysis on the correlation of physicochemical properties of pristine and amine-modified nanotube/nanorod system with their gas adsorption characteristics and we envisage that this work will open up a new direction of searching nanostructured materials that has application in both the fields of clean environment and green energy.

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2. Experimental section. 2.1. Materials TiO2 powder (99.99%), N-[3-(trimethoxysilyl)propyl]ethylene-diamine (TPEDA), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were obtained from Sigma-Aldrich and ethanol was obtained from Merck.

2.2. Synthesis of hydrogen titanate nanotube and nanorod Hydrogen titanate nanotube (HTNT) and nanorod (HTNR) samples have been synthesized as the protocol reported earlier by Ray et al. [34] and Chatterjee et al [26]. In a typical synthesis, 1 g of bulk anatase TiO2 powder (99.99%) was added in 35 mL of 10 M aqueous solution of NaOH under stirring magnetically at room temperature. The white suspension was kept in constant stirring for 72 h for nanotube, and the suspension was stirred for 24 h for nanorod, respectively. Then, both suspensions were transferred in two different pre-cleaned teflon lined autoclaves and inserted inside a convection oven simultaneously. HTNTs were grown at a temperature of 150 °C and HTNRs were grown at 200 °C. After 12 h, the reaction vessels were allowed to cool down to room temperature. The supernatant of strongly basic liquid was thrown away and the white colored solid residue was washed several times with 1:1 HCl and further treated with hot deionized water of 80 - 85 oC. The washing was done till the pH of the wash liquid was neutral and finally the residue was washed with high purity ethanol. After washing, the white colored residue was allowed to dry in air at 50 – 60 °C for at least 24 h to obtain free-flowing powder.

2.3. Amine modification on the surface of hydrogen titanate nanotube and nanorod

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N-[3-(trimethoxysilyl)propyl]ethylene-diamine (TPEDA) was used to modify the surface of HTNT and HTNR samples. Before using a diamine-containing organosilane precursor, HTNT and HTNR samples were kept in a vacuum oven for 24 h. In a typical preparation, 1 g of hydrogen titanate samples, 0.6 mL of diamine-containing organosilane precursor, and 50 mL of toluene as a solvent were mixed in a glass bottle of 125 mL. The mixture was stirred magnetically in oil bath at 110 oC for 8 h. The final product was collected after filtering using a suction flask with ethanol and acetone. The main amine-grafted sample names are A2HTNT and A2-HTNR, respectively, obtained by using TPEDA, and the modified sample obtained with 0.3 mL of TPEDA per 1 g HTNT samples is named as A1-HTNT, as listed in Table 1.

2.4. Characterization Morphology of both pristine and amine-modified HTNT and HTNR was investigated using a Zeiss Ultra-55 Plus scanning electron microscope (SEM) and the element-mapping image was obtained using a TESCAN VEGA3 SEM equipped with energy dispersive spectroscopy (EDS) operating at 15.0 kV. A Panalytical X’Pert Pro powder X-ray diffractometer was used to check the crystallinity of the as grown samples. With the help of high resolution TEM microscopy (CM12 PHILIPS Transmission Electron Microscope) the detailed structural and morphological characteristics of the nanostructures were established. Nitrogen adsorptiondesorption isotherms were measured at -196 oC on a Micromeritics ASAP 2420 analyzer. The samples were degassed at 110 oC under vacuum for at least 2 h before nitrogen gas adsorption. The BET (Brunauer–Emmet–Teller) specific surface area was calculated from adsorption data in the relative pressure (P/P0) range from 0.05 to 0.20. The single point pore volume was evaluated from the amount adsorbed at the relative pressure of 0.99. The pore size

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distributions (PSD) were calculated from adsorption branches of the isotherms by using the improved KJS (Kruk–Jaroniec–Sayari) method [60]. The pore diameter (Wmax) was obtained at the maximum of PSD. The solid-state 13C and 29Si CP-MAS NMR spectra were obtained with a Bruker AVANCE II+ (400 MHz) spectrometer using a 4 mm magic angle (MAS) spinning probe at the KBSI Seoul western center. Experimental conditions for

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C CP-MAS

NMR spectra were as follows: 8 kHz of spinning rate, 3 s of delay time, 100.4 MHz of radio frequency, and 2 ms of contact time.

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Si CP-MAS NMR spectra were obtained from the

following experimental conditions: 6 kHz of spinning rate, 3 s of delay time, and 79.488 MHz of radio frequency. The chemical shifts were obtained with respect to the tetramethylsilane (TMS) reference peak. The quantitative analysis of nitrogen, carbon, and hydrogen contents in an amine-modified sample was performed using a Flash EA 1112 series (CE Instruments) at the KBSI Seoul center.

2.5. Gas adsorption measurement CO2 and H2 gas uptakes were measured using a microbalance inside Q50 thermogravimetric analyzer (TA instrument). Before measuring the gas uptake using a Q50 analyzer, samples were kept in vacuum oven for 24 h. Approximately 10 mg of the pristine and modified HTNT/HTNR samples was loaded in the sample plate and sealed in the cylindrical chamber. As a first step, the cylindrical chamber was purged at 110 oC for 2 h in flowing N2 gas of 100 ml/min. Gas uptake was recorded for 3 h in flowing N2 gas of 40 ml/min and CO2 gas of 60 ml/min after cooling to 25 oC under a N2 flow. Analogous step was taken to analyze H2 gas uptake for pristine HTNT and amine-grafted HTNT samples in flowing N2 gas of 40 ml/min and H2 gas of 60 ml/min at 25 oC. Reversibility for CO2 adsorption was measured repeatedly without changing sample in a closed cylindrical chamber using amine-grafted HTNT sample

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(A2-HTNT). Reversibility was obtained up to 9 cycles in flowing N2 gas of 40 ml/min and CO2 gas of 60 ml/min at 25 oC after purging CO2 at 110 oC for 2 h in flowing N2 gas of 100 ml/min. Reversibility of H2 adsorption was also obtained for pristine HTNT sample using the similar procedure.

3. Results and discussions Pristine HTNT and HTNR samples, which synthesized by hydrothermal method were analyzed thoroughly using electron microscopy. Figure 1(a–c) represents the scanning and transmission electron micrographs of HTNT, which confirms the formation of 1D morphology of the as-grown samples. High-resolution TEM images (Figure 1(b) and (c)) shows that the 1-D units are actually hollow and the inner and outer diameter of the nanotubes are approximately 5 nm and 10 nm. On the other hand, formation of nanorod morphology is confirmed in the Figure 1 (d-e). Figure 1(d) represents the SEM image of a cluster of 1-D nanostructures. TEM analysis (Figure 1(e) and (f)) of these 1-D nanostructures reveals that the long cylindrical shape is not hollow but solid in nature with nanorod diameter of approximately 50 nm. In both the cases, i.e. HTNT and HTNR, the lengths of the nanostructures were in micron scale. The as-grown samples are found to be nano-crystalline in nature with X-ray diffraction lines at 11.2°, 24.8°, and 48.9° (see Figure S1) which corresponds to (200), (110), and (020) set of planes of hydrogen titanate, thereby, confirming the formation of hydrogen titanate phase [61]. The pristine HTNT and HTNR samples were modified with a diamine-containing organosilane (TPEDA) precursor by post-grafting method. The sample names and the corresponding amount of TPEDA are listed in Table 1. Nitrogen adsorption-desorption isotherms and pore size distributions of pristine and modified

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samples were obtained as shown in Figure 2 and the as-calculated physicochemical properties are listed in Table 1. Nitrogen adsorption-desorption isotherms for hydrogen titanate samples are type IV except for nanorod (i.e. HTNR and A2-HTNR) samples as shown in Figure 2(a). The isotherms depicts that the pristine hydrogen titanate nanotube are having the highest surface area of 306 m2g-1, which is in line with the earlier observations [26]. Comparatively the hydrogen titanate nanorods are showing much lesser surface area of 26 m2g-1 which is attributed to the solid microstructure of nanorod sample (Figure 1(e) and (f)) as compared to hollow nanotubes (Figure 1(b) and (c)). Upon surface modification with amines, the surface area of the nanotubes decreases at least by 3 times which is attributed to the probable blocking of pores on the nanotube wall by amines. It is observed that the surface area of A2HTNT is 122 m2g-1 and 93 m2g-1 for A1-HTNT sample. The surface area of amine modified hydrogen titanate nanorods are drastically decreased to 6 m2g-1. This drastic decrease of surface area of nanorods from 26 m2g-1 to 6 m2g-1 indicates that the amines are anchored to the walls of the one-dimensional nanostructures. Furthermore, the pore size distributions of all systems were measured and for pristine HTNT samples the maximum pore diameter is found to be around 13 nm, which matches closely to the earlier report as seen by Chatterjee et al [26]. The pore diameter was decreased from 13.6 nm down to 9.3 nm with amine-modification, which strongly suggests the modification is achieved effectively on the inner surface by diamine-containing organosilane precursors for aminemodified HTNT samples. The comparative pore volume and pore diameter for all samples are tabulated in Table 1. Elemental analysis was also investigated to confirm nitrogen content especially for amine-modified sample (i.e. A2-HTNT) as listed in Table 2. Elemental analysis shows the nitrogen content is about 3.04 mmol/g for A2-HTNT sample. Carbon content of 11.69 mmol/g as well as nitrogen is originated from diamine-containing

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organosilane modifier (i.e. TPEDA). Also, very high amount of hydrogen was observed as 34.9 mmol/g, which is mainly attributed to pristine hydrogen titanate. SEM-EDS analysis of A2-HTNT sample also showed the presence of carbon and nitrogen along with titanium and oxygen, which indicates the attachment of amine with the nanotube sample. EDS mapping of elements like C, N indicates the homogeneous attachment of amine on the surface of HTNT and HTNR. Figure 3(a-d) shows the representative EDS-mapping image of C, N, O, and Ti, respectively. The weight percentage of N and C calculated from SEM-EDS analysis matches quite well with the quantitative EA results, which is tabulated in Table 2. The solid-state

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Si CP-MAS NMR analysis was employed to investigate the chemical

linkages of amine-containing organosilane with titania inside the A2-HTNT sample as shown in Figure 4. Chemical shifts at -50, -58, and -68 ppm are indexed as T1 (C-Si-(OTi)1(OH)2), T2 (C-Si-(OTi)2(OH)), and T3 (C-Si-(OTi)3) peaks, respectively [62]. The appearance of T peaks strongly represents the formation of Si-O-Ti (or Si-O-Si) chemical linkages between titania inside hydrogen titanate nanotube and silica inside diamine-containing organosilanes. Moreover, the highest T3 peak represents the diamine-containing organosilanes are highly condensed on the surface of hydrogen titanate nanotube. From the results of EA, EDS, and solid-state 29Si CP-MAS NMR, it is confirmed that the silica inside TPEDA organosilane are linked chemically with titania on the surface of hydrogen titanate nanotube and the amine groups are distributed homogeneously with nanotubes. Amine group inside A2-HTNT was also confirmed by FT-IR analysis as shown in Figure S2 (see Supporting Information Figure S2). The CO2 uptake of both pristine and amine modified nanotube samples was investigated under atmospheric pressure at 25 °C. A mixture of CO2 and N2 gases was used for the analysis with fixed flow rate of 60 mL/min and 40 mL/min, respectively. Figure 5(a) shows

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the time-dependent molar amount of CO2 uptake based on 1 g of the pristine and aminemodified HTNT samples. From the isotherms, it is clearly observed that the A2-HTNT sample shows far better adsorption capacity than the other pristine and modified counterpart (HTNT and A1-HTNT). Within 3 h, A2-HTNT adsorbed 1.2 mmol of CO2 per 1 g sample whereas the adsorption capacity of pristine (HTNT) is 0.88 mmol/g and A1-HTNT shows adsorption capacity of 0.58 mmol/g. Figure 5(b) depicts the comparison of CO2 uptake of HTNR and A2-HTNR with time and provides similar conclusion that hydrogen titanate nanorods modified with 0.6 mL of TPEDA per 1 g of nanorod (i.e. A2-HTNR) shows better adsorption capacity (0.43 mmol/g in 3 h) than pristine nanorod (HTNR) sample (0.20 mmol/g in 3 h). It is concluded that among all samples including nanotubes and nanorods, the nanotube modified with 0.6 mL TPEDA per 1 g of nanotube (A2-HTNT) shows the most active adsorption capacity for CO2 (see Table S2 in supporting information). It should be emphasized here that the pristine hydrogen titanate nanotube sample prepared in this study shows better adsorption capacity than former studies made on amine functionalized titania based porous structures [57] and found to be comparable with other reported literatures. To investigate the trace of carbon dioxide before and after gas adsorption experiment, solidstate 13C CP-MAS NMR spectra were recorded. Figure 6(a,c) shows solid-state 13C CP-MAS NMR spectra of HTNT and A2-HTNT samples before gas adsorption. Figure 6(b, d) represents NMR spectra for HTNT and A2-HTNT samples after gas adsorption. Figure 6(e) is a scheme of N-[3-(trimethoxysilyl)propyl]ethylene-diamine (TPEDA) and numbers inside the scheme represent the chemical shifts (ppm) for respective carbons in amine-containing molecules, which were simulated by ChemDraw software package. Figure 6 (a, b) does not show any chemical shift before and after gas adsorption. It is obvious that the pristine HTNT sample without amine-modification does not contain any carbon even

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after flowing carbon dioxide gas. 13C CP-MAS NMR spectrum shown in Figure 6(b) clearly suggests that the gas uptake of 0.88 mmol/g obtained in Figure 5(a) is a kind of reversible physical adsorption inside nanotube and CO2 gas can be desorbed very easily from the nanotube. On the other hand, TPEDA-containing HTNT (Figure 6(c)) shows clearly 4 resonance peaks around at 11, 24, 39, and 51 ppm similar to the simulated values of 7.3, 25.8, 41.1, and 51.6-52.8 ppm, which represents that the hydrocarbon alkyl groups inside an amine-containing organosilane exist in A2-HTNT samples without any bond cleavage. A quite sharp resonance peak at 163 ppm indicated by red arrow in Figure 6(d) represents CO2 molecules adsorbed on the amine groups [62]. It is known that primary and secondary amines with lone pair can react with CO2 molecules with quadrupole and form carbamate complex, which is not easy to break the bond [63]. The

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C CP-MAS NMR spectra clearly show the

amine-modification using TPEDA ligand molecule is very effective to adsorb larger amount of CO2 gas molecules more tightly. Nitrogen adsorption reveals that nanotube samples (i.e. HTNT and A2-HTNT) are having high surface area by abundant pores. Therefore, these two materials were explored for storage of H2 gas at ambient temperature and normal pressure, which is itself a challenging task. Most of the prevailing literatures on H2 storage in porous materials is usually implemented under either high pressure or very low temperature which restricts any commercial applicability [63]. Figure 7 shows the time-dependent hydrogen uptakes for HTNT, A2HTNT, and HTNR samples for 3 h at 25 °C and ambient pressure. For HTNT, the adsorption capacity is higher than 102.5 wt% (12.5 mmol/g) and for A2- HTNT and HTNR, the adsorption capacities lies in the range of 100.5 – 101.0 wt%. The high hydrogen uptake for HTNT is attributed to well-developed cylindrical holes inside nanotube, while the low uptake for amine-modified A2-HTNT represents the basicity of amine groups gives negative effect

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on adsorbing hydrogen molecules without vacant orbital. In this study, gas adsorption experiments are conducted at rather hydrophobic non-polar flow conditions (i.e. mixed flows with nitrogen gas). In fact, the hydrophobic non-polar environment can enhance especially H2 uptakes on porous solids because the adsorption of water molecules can be suppressed by heavier N2 molecules. Thus, it is suggested that high amount of gas uptake observed in this study can be attributed to interplay of N2 molecules mixed simultaneously. To estimate experimentally the amount of N2 uptake over total amounts of gas uptakes for hydrogen titanate samples obtained in Figure 5 and Figure 7, pure N2 gas was adsorbed on HTNT sample. Line C of Figure 8(a) is an adsorption isotherm of pure N2 on HTNT at 25 oC and 1 atm, and the maximum uptake is obtained as 102.06 wt% (~ 0.736 mmol/g) for 3 h. Line A of Figure 8(a) is an isotherm for gas uptakes of CO2 and N2 mixed flow and line B of Figure 8(a) is for H2 and N2 mixed flow, respectively, at the same condition. Figure 8(b-A’ & b-B’) isotherms show selective CO2 (line A’ = line A – line C) and H2 (line B’ = line B – line C) uptakes calibrated simply by subtracting pure N2 uptake appeared in Figure 8(a-C). It is reasonable that the N2 uptake of Figure 8(a-C) is the maximum amount of uptake in mixed flows on the assumption of competitive adsorption between two kinds of gas molecules. Therefore, it is believed that the real uptakes of CO2 and H2 gas molecules are higher values than data line appeared in Figure 8(b). Figure 8(b) shows the maximums of CO2 and H2 uptakes are about 0.46 mmol/g within 50 min and 12.5 mmol/g within 15 min, respectively. The molar selectivity of CO2/N2 and H2/N2 for HTNT sample was calculated as 0.63 and 16.9, respectively. Analogous analysis was taken for A2-HTNT sample. Pure N2 uptake was shown in line C of Figure 9(a) and the maximum uptake is 100.63 wt% (~ 0.225 mmol/g) for 3 h at 25 oC and 1 atm. It is noted that pure N2 uptake is suppressed for amine-modified hydrogen titanate

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surface compared with pristine HTNT. Figure 9(a-A) is gas uptakes of CO2 and N2 mixed flow and Figure 9(a-B) is for H2 and N2 mixed flow, respectively. Figure 9(b-A’ & b-B’) isotherms show selective CO2 (line A’ = line A – line C) and H2 (line B’ = line B – line C) uptakes calibrated simply by subtracting pure N2 uptake appeared in Figure 9(a-C). Figure 9(b) shows the maximums of CO2 and H2 uptakes are about 1.04 mmol/g within 3 h and 4.43 mmol/g within 5 min, respectively. The molar selectivity of CO2/N2 and H2/N2 for A2-HTNT sample were calculated as 4.62 and 19.7, respectively. As A2-HTNT and HTNT samples showed the highest adsorption capacity for CO2 and H2 gases, respectively, A2-HTNT for CO2 gas and HTNT for H2 gas were selected to check the reversibility for gas adsorption-desorption. The adsorption-desorption was measured for 9 cycles up to 2.4 d. Figure 10(a) shows CO2 adsorption-desorption for A2-HTNT sample and Figure 10(d) is H2 adsorption-desorption for HTNT sample. Gas desorption was done using N2 gas flow of 100 mL/min at 110 oC for 2 h and adsorption was done in flowing N2 gas of 40 ml/min and CO2 (or H2) gas of 60 ml/min after cooling to 25 oC under a N2 flow. As can be seen in Figure 10, the reversibility of CO2 and H2 gas adsorption-desorption shows quite good performance for 9 cycles.

4. Conclusions Hydrogen titanate nanotube (HTNT) and nanorod (HTNR) were modified with N-[3(trimethoxysilyl)propyl]ethylene diamine (TPEDA). Amine groups were successfully linked chemically on the surface of hydrogen titanates, which was confirmed by appearance of T peaks between silica and titania through

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Si CP-MAS NMR spectra. Elemental analysis

(EA) showed N atom of 3.04 mmol/g was attached inside A2-HTNT sample. CO2 and H2 gas uptakes on several hydrogen titanate samples were obtained by a TGA Q50 analyzer in

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flowing N2 gas of 40 mL/min and CO2 (or H2) gas of 60 mL/min at 25 °C and 1 atm. Pristine HTNT showed the highest H2 uptake over 12.5 mmol/g within 15 min and TPEDA-modified HTNT (A2-HTNT) sample showed the highest CO2 uptake over 1.04 mmol/g within 3 h. The molar selectivity of CO2/N2 and H2/N2 for HTNT sample were calculated as 0.63 and 16.9, respectively. As for A2-HTNT sample, the molar selectivity was calculated as 4.62 and 19.7, respectively. The reversibility of gas adsorption-desorption also showed quite good performance for 9 cycles. At high temperature up to 800 °C, it was found that pristine HTNT sample is likely to desorb both CO2 and H2 gases (see Figure S2 and S3). It is concluded that hydrogen titanate nanotube prepared in this study is a very practical adsorbent for H2 gas storage and amine-modified one is also a good candidate for CO2 capture at ambient temperature and pressure conditions.

Acknowledgements E.-B. Cho acknowledges supports under the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF2012R1A1A2000855, NRF-2014R1A1A2059947). Sriparna Chatterjee is thankful to DST, India for support from project code no. IFA12-CH-65. Prof. S. Das (IIT Kharagpur, India) is acknowledged for extending TEM facility and Prof. B. K. Mishra is acknowledged for his support to undertake this work.

Supporting Information A representative comparison of H2 uptake of pristine and surface modified hydrogen titanate nanotubes (our results) with other nanotube structures reported earlier. XRD patterns for

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hydrogen nanotube and nanorod. FT-IR spectra for pristine hydrogen nanotube and aminemodified hydrogen nanotube. CO2 and H2 uptakes of hydrogen nanotube at high temperatures.

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Captions for Figures Figure 1. SEM and TEM images of pristine hydrogen titanate nanotubes (a-c) and pristine hydrogen titanate nanorods (d-f). Figure 2. Nitrogen adsorption isotherms (a) and pore size distributions (b) for pristine and amine-modified hydrogen titanate nanotubes and nanorods. Figure 3. SEM-EDS mapping images for C, N, O, and Ti for A2-HTNT sample. Figure 4. Solid State 29Si CP-MAS NMR for A2-HTNT sample. Figure 5. Carbon dioxide uptake for pristine and amine-modified hydrogen titanate nanotubes (a) and nanorods (b). Figure 6. Solid State

13

C CP-MAS NMR of pristine HTNT (a,b) and amine-modified

nanotube A2-HTNT samples (c,d) before and after carbon dioxide adsorption, respectively. (e) Schematic of N-[3-(trimethoxysilyl) propyl]ethylene-diamine. Numbers inside the sche- mes represent the chemical shifts (ppm) for respective carbons simulated by ChemDraw software. Figure 7. H2 uptake of pristine nanotube (HTNT), amine-modified nanotube (A2-HTNT), and pristine nanorod (HTNR) samples. Figure 8. Selectivity for CO2 and H2 gas uptake of pristine nanotube (HTNT) sample at 25 o

C and 1 atm. (a): Gas uptakes under mixed flows of CO2 and N2 (A) and H2 and

N2 (B), and pure N2 (C) for HTNT sample. (b): Selective gas uptakes of CO2 (A’ = A - C) and H2 (B’ = B - C) calibrated by subtracting N2 gas uptake (C) for HTNT sample. Figure 9. Selectivity for CO2 and H2 gas uptake of amine-modified nanotube (A2-HTNT) sample at 25 oC and 1 atm. (a): Gas uptakes under mixed flows of CO2 and N2 (A) and H2 and N2 (B), and pure N2 (C) for A2-HTNT sample. (b): Selective gas

26

uptakes of CO2 (A’ = A - C) and H2 (B’ = B - C) calibrated by subtracting N2 gas uptake (C) for A2-HTNT sample. Figure 10. Reversibility for CO2 gas adsorption-desorption of amine-modified A2-HTNT sample (a) and H2 gas adsorption-desorption of pristine HTNT sample (b) for 9 cycles, respectively.

27

Table 1. Physicochemical parameters determined from N2 sorption isotherms.a Nmod

S BET

Vsp

Wmax

(mL/g)

(m2/g)

(cc/g)

(nm)

HTNT

0

306

1.099

13.6

A1-HTNT

0.3

93

0.415

12.4

A2-HTNT

0.6

122

0.482

9.3

HTNR

0

26

0.134

2.5

A2-HTNR

0.6

6

0.029

6.3

Sample

a

Notation: Nmod = the amount of N-[3-(trimethoxysilyl)propyl]ethylene-diamine used to

modify 1 g of hydrogen titanate nanotude and nanorod; S BET = specific surface area calculated from adsorption data in relative pressure range 0.05-0.20; Vsp = single point pore volume calculated at P/P0 = 0.99; Wmax = pore diameter calculated at the maximum of PSD using improved KJS method [60].

28

Table 2. Elemental analysis for A2-HTNT sample. EA

SEM-EDS

Element wt %

mmol/g

wt %

C

14.03

11.69

11.27

H

3.49

34.9

n.d.

N

4.25

3.04

5.15

O

n.d.

n.d.

42.76

Ti

n.d.

n.d.

40.83

29

Figure 1

30

Figure 2

31

Figure 3

(a)

(b)

(c)

(d)

32

Figure 4

T3 T2

T3: - 68 ppm T2: - 58 ppm T1: - 50 ppm

T1

0

-20

-40

-60

-80 -100 -120 -140 -160

δSi (ppm)

33

Figure 5

34

Figure 6

35

Figure 7

36

Figure 8

37

Figure 9

38

Figure 10

39

Graphical Abstract

H2 and CO2 uptake for hydrogen titanate (H2Ti3O7) nanotubes and nanorods at ambient temperature and pressure a

a,

Sora Sim , Eun-Bum Cho *, Sriparna Chatterjeeb,* a

Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 01811, Korea b Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Acharya Vihar, Bhubaneswar 751013, India

Amine modified nanotube

1.8

20

1.6

18

1.4

16

1.2

14 12

1.0

10

0.8

8

0.6

6

0.4

4

0.2

H2 Uptake (mmol/g)

CO2 Uptake (mmol/g)

Pristine nanotube

2

0.0 0

20

40

60

80

100 120 140 160 180

Time (minute)

0

20

40

60

80

100

120

140

160

0 180

Time (minute)

CO2 and H2 adsorption characteristics of pristine and amine-modified one-dimensional nanostructures of hydrogen titanate

40

Highlights



Hydrogen titanate nanotube shows high H2 uptake of 12.5 mmol/g at 25 °C and 1 atm.



Amine-modified hydrogen titanate shows quite good CO2 uptake of 1.2 mmol/g.



Hydrogen titanate nanotube shows good reversibility for H2 and CO2 gas uptake.



Selectivity for gas uptake was observed under a gas flow mixed with N2 gas.

41