Rare Metal Materials and Engineering Volume 42, Issue 12, December 2013 Online English edition of the Chinese language journal
Cite this article as: Rare Metal Materials and Engineering, 2013, 42(12): 2467-2471.
Received date: December 21, 2012 Foundation item: National Natural Science Foundation of China (50901030); Youth Foundation of Hebei Normal University (L2008Q07); Fund of Hebei Education Department (2010256) Corresponding author: Chai Yujun, Ph. D., School of Chemistry and Materials Science, Shijiazhuang 050024, P. R. China, E-mail: [email protected]
Hydrogen Storage Property of Porous/Hollow TiO2 Using Yeast as Template Zhao Zhongyi, Chen Huimiao, Wang Ning, Chai Yujun Hebei Normal University, Shijiazhuang 050024, China
Abstract: The hydrogen storage property of the porous/hollow TiO2 using yeast as template was investigated. It is demonstrated that crystalline TiO2 forms until the temperature reaches 620 °C. The product is composed of TiO2 and minor amount of H2Ti3O7 after heat treatment. The porous/hollow TiO2 with a specific surface area of 252.6 m2/g is obtained due to the removal of yeast. Pres-sure-composition isothermal (P-C-I) curve of TiO2 shows that a reversible hydriding/dehydridng process occurs at 30 °C, suggesting an obvious physisorption. The change of the chemical bond in Fourier-transform infrared (FT-IR) spectrum in hydriding/dehydridng indicates that hydrogen reacts with OH group on TiO2 surface even at 30 °C. The porous/hollow TiO2 collapses as cycles increase and the corresponding specific surface area decreases dramatically. Contrary, the hydrogen uptake increases with increasing of tem-perature, which is caused by chemisorption.
Nanostructured materials such as nanotubes, nanowires and porous/hollow microspheres have been investigated as the hydrogen storage media due to the potential high hydro-gen uptakes and the quick reversible sorption/desorption process[1-4]. Hydrogen is stored in the nanostructured materi-als by physisorbed, chemisorbed or defects. Therefore, the specific surface area, the size of pore and the surface prop-erty are important for their hydrogen storage[5-9]. As for the nanotubes of BN, TiS2 etc. their investigations are generally centered on the pore structure and the large surface area
[2,10-12]. Lattice defects may be more important for nanowires[3,13]. Recently, porous/hollow spherical structure materials with the high surface area, micropores/interstices or the defect show the possible high hydrogen uptakes[4,14]. Gao reported that hollow nitrogen-containing carbon spheres material exhibited high hydrogen uptakes of 2.21 wt% at room-temperature[15]. Hierarchical nanostructured hollow site/mesopore shell carbon showed the discharge capacity up to 586 mAh/g with the discharge current of 25 mA/g, corre-
sponding to 2.17 wt% hydrogen uptake[16]. ~2 wt% H2 was stored in TiO2 nanotubes by physical ab-
sorption and binding with OH bonds [17]. Bavykin reported t h a t hydrogen banded with TiO2 to form host-guest compounds TiO2·xH2 [18,19]. In addition, TiO2 acted as the additive com-position to improve the hydrogen storage kinetics of Mg[20]. However, only a little attention was paid to hydrogen storage properties of porous/hollow TiO2 as well as its collapse in hydriding/dehydridng process.
Porous/hollow TiO2 was commonly prepared by the tem-plate method[21,22]. In this preparation, yeast was chosen as the template to synthesize porous/hollow TiO2 because of its friendly environmental and nontoxic[23]. And the structure and hydrogen storage properties were described in detailed.
1 Experiment
In a typical preparation, (a) 20 mL tetrabutyl titanate (Ti[OC4H9]4) was added dropwise into 26 mL ethanol. Then,
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22 mL mixture composed of water and ethanol was added into the above solution drop by drop and stirred for 1 h to obtain the sol solution. (b) Edible yeast powders were dis-solved in the water and continuously stirred for several min-utes. After that, they were filtrated and dried at 60 °C. The dried powders were ground to be less than 100 mesh. Then, 0.2 g yeast powders were poured into the above-mentioned sol solution containing Ti. The mixture was stirred for sev-eral h and placed for several days for hydrolysis reaction. The filtrated mixture was heated at 150 °C for 1h and then at 650 °C for 5 h to remove the yeast to obtain TiO2.
Differential scanning calorimetry–thermogravimetric measurement (DSC-TG) was performed using NETZCH STA 449F3 by heating the sample to 900 °C in air at a ramp rate of 10 K/min. FT-IR Fourier-transform infrared (FT-IR) spectroscopy measurements were recorded on a Shimadzu FTIR-8900 instrument to describe the change of chemical bond in hydriding/dehydriding. The structure of TiO2 was determined by D8 ADVANCE Bruke X-ray diffractometer with Cu Kα radiation. The data were collected in the range of 10º and 80º at 0.02º step size. The morphology of TiO2 was investigated by means of an S-4800 field-emission scanning electron microscope (FE-SEM) operating at an ac-celeration voltage of 80 kV. The samples were directly stuck on the conductive black adhesive tape without any ultrasonic pretreatment and then were coated with gold. Specific sur-face area and pore volumes of TiO2 were characterized with N2 absorption using Brunauer-Emmet-Teller (BET) meas-urements at 77 K.
2 Results and Discussion
2.1 Structure characterization The thermal behavior analysis of 7.865 mg sample prior to
heat treatment was carried out in air atmosphere, as shown in Fig.1. The mass loss of ca. 8.1wt% occurs, around 100 °C
Fig.1 DSC and DTG curves of TiO2 prior to calcinations
due to the evaporation of water and ethanol. Upon raising the temperature to 280 °C, a weak exothermic peak in DSC curve and a sharp peak in DTG curves are observed. This is attributed to the loss of crystal water and the further decom-position of organic compound in gel[24]. Two exothermic peaks in DSC curve and one strong exothermic peak in DTG curve in the range from 400 to 620 °C are observed. They are recognized as the further decomposition of organic com-pound as well as the phase transformation from amorphous to anatase, corresponding the mass loss of 55 wt%. There is almost no further mass loss when temperature increases, im-plying a final crystalline product of TiO2.
In order to obtain crystalline TiO2, the heat treatment tem-perature (650 °C) is higher than DSC data (620 °C). XRD pattern of TiO2 after heat treatment is shown in Fig.2. ac-cording to the peaks of 2θ=25.2°, 38° and 48.5°, it can be seen that TiO2 is the mixture of anatase (ICDD-JCPDS card no. 21-1272) and brookite(29-1360). According to the peaks of 2θ=16º, 29º, 32º, it is regarded that minor amount of H2Ti3O7 forms (47-0561). This suggests that the decomposi-tion of Ti-(OH)n does not complete as even heated at 650 °C. However, no obvious peaks of organic compound are ob-served in XRD pattern, indicating that the yeast has decom-posed after heat treatment.
The original morphology of yeast is closed to ellipsoid [25]. In the hydrolytic process, Ti-(OH)n is deposited on the sur-face of yeast due to the interaction of the electrostatic force. Therefore, the irregular TiO2 ellipsoids can be observed, as shown in Fig.3. In heat treatment, Ti-(OH)n decomposes to TiO2 and yeast decomposes to CO2. The removal of CO2 from the inner produces a lot of pores/interstices and the hollow site in the bulk, resulting in the formation of po-rous/hollow TiO2.
Fig.4 shows N2 adsorption/desorption isotherms and the corresponding pore size distributions for TiO2. The isotherms of these samples are of typical IV classification. The calcu-
Fig.2 XRD pattern of TiO2 after calcination at 650 °C
0 200 400 600 800-14-12-10
-8-6-4-202
Hea
t Flo
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-5
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-3
-2
-1
0
1
DTG
67.7
527.2
Mas
s Cha
nge/
%·m
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Temperature/℃
279.2
DSC
Exo
–
––
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–
–
–
–
–
–
–
– 10 20 30 40 50 60 70 802θ/(°)
300
250
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Inte
nsity
/cps
TiO2 H2Ti3O7
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Fig.3 SEM images of TiO2
Fig.4 N2 adsorption-desorption isotherms (* standard temperature pressure)
lated BJH (Barret-Joyer-Halenda) specific surface area, a pore volume and the average pore radius are 252.6 m2/g, 0.358 cm3/g and 18 nm, respectively. 2.2 Hydrogen storage performance of porous/hollow
TiO2 spheres Pressure-composition isothermal (P-C-I) curves of TiO2 at
30, 70 and 100 °C were measured, as shown in Fig.5. Firstly, the hydriding/dehydriding process was performed at 30 °C, then, continually at the evaluated temperature using the same sample. The hydrogen uptake reaches ~0.36 wt% at 30 °C under the pressure of ~5.5 MPa. ~0.24 wt% hydrogen is re-leased with reducing hydrogen pressure, suggesting that phy-sisorption caused by van der Waals is the main mean of hy-drogen storage. Upon increasing of temperature, the hydro-gen storage capacity increases. It reaches ~0.65 wt% at 70 °C and 0.67 wt% at 100 °C. Corresponding, ~0.47 wt% hy-drogen at 70 °C and ~0.56 wt% hydrogen at 100 °C are re-leased. In addition, each of the desorption curve does not follow the absorption one, especially that at 70 °C. It is as-sumed that the obvious increase of hydrogen capacity may be related to chemisorption.
FT-IR spectrum of TiO2 before and after hydriding, as shown in Fig.6, shows that the typical peaks for Ti-O bonds are observed at 1049~976 cm-1 and 478.5 cm-1. Because TiO2
Fig.5 P-C-I curves of TiO2 at different temperature
Fig.6 FT-IR spectrum of TiO2 is directly taken out from the chamber after hydriding at 30 °C, the fraction of hydrogen controlled by the physisorption is released. However, it is found that the peak around 1004.8 cm-1 becomes sharp and another shifts to 474.5 cm-1 after hydriding. No variation is observed after dehydriding in the spectrum, which indicates that the formed weak bond does not decompose after dehydriding at 30 °C.
Fig.7 shows SEM images after hydriding/dehydriding process. It can be seen that porous/hollow TiO2 partly col-lapses after first cycle, and most of porous/hollow TiO2 fall down after three cycles. BET results (Fig.8) show that the specific surface area dramatically decreases from the initial 252.6 m2/g (TiO2) to 24.7m2/g (after 1st) and 23.8m2/g (after 2nd), respectively. The corresponding pore volume is 0.358, 0.067 and 0.055 cm3/g. XRD patterns show that no new peaks appear, as shown in Fig.9. It suggests that no any phase forms in hydriding process even it is reported that TiO2·xH2 formed[19].
In present work, porous/hollow TiO2 gives the specific surface area of 252.6 m2/g, mainly coming from the porous and the hollow sites. TiO2, ZnO etc. nanostructured materials can absorb massive of hydrogen into the pores/interstices under higher hydrogen pressure[5,9]. Therefore, it is consid-e red tha t pa r t i a l hydrogen d i ffuses through the
15 μm
2.5 μm
0.0 0.2 0.4 0.6 0.8 1.020406080
100120140160180200220
Vol
ume
(STP
)* /cm
3 ·g-1
Relative Pressure, p/p0
0 1 2 3 4 50.0
0.1
0.2
0.3
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0.7
30 °C 70 °C 100 °C
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roge
n C
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t%
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2000 1800 1600 1400 1200 1000 800 600 400
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c
ba
474.5474.5
478.3
1004.8
1004.8
976
Tran
smitt
ance
/%
Wavenumber/cm-1
1049a- TiO2
b- After hydridingc- After dehydriding
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Fig.7 SEM image of TiO2 after cycles
Fig.8 N2 adsorption-desorption isotherms after cycles for TiO2
Fig.9 XRD patterns after hydriding/dehydriding process for TiO2 pores/interstices at elevated hydrogen pressures, which is then trapped in the pores/interstices or the hollow site. Meanwhile, partial hydrogen binds to the surface of enriched OH group on the surface of TiO2
[19] at high temperature, which is beneficial to the hydrogen uptake. That’s to say that the stored hydrogen is involved in the pores/interstices or the hollow site and forming H-OH binding with OH- [17]. In ad-dition, a part weak bonds breaks at higher temperature with decreasing of hydrogen pressure. Hence, the discharge capac-ity slightly increases. Unfortunately, a part pores/interstices crash due to the high hydrogen pressure, which leads to the obvious decrease of the specific surface area. This result
seems to be contrary to the literature report that the large spe-cific surface area is important to improve the hydrogen storage. The phenomena can be explained that pores/interstices mainly stores hydrogen by physisorption at room temperature and the effect of chemisorption obviously enhances at high tempera-ture.
3 Conclusions
1) The porous/hollow TiO2 can be obtained using yeast as the template. Yeast heated at 650 °C for 5 h decomposes al-most completely. Except TiO2, a small quantity of H2Ti3O7.
2) The porous/hollow TiO2 with a specific surface area of 252.6 m2/g is obtained. The hydrogen uptake is ~0.36 wt% under the pressure of 5.5 MPa at 30 ° C. ~67.6% hydrogen is released with decreasing of hydrogen pressure. The peak in FT-IR spectrum around 1004.8 cm-1 becomes sharp and an-other shifts from 478.5 to 474.5 cm-1.
3) Upon increasing temperature, the hydrogen uptakes in-creases to ~0.65 wt% at 70 °C. Accordingly, the specific sur-face area dramatically decreases to 23.8 m2/g within three cy-cles.
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a
bc
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b- After hydridingc- After dehydriding
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