Hydrogen sorption in transition metal modified ETS-10

international journal of hydrogen energy 34 (2009) 888–896

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Hydrogen sorption in transition metal modified ETS-10 K.P. Prasantha, H.C. Bajaja, H.D. Chungb, K.Y. Choob, T.H. Kimb, R.V. Jasraa,* a Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), G.B. Marg, Bhavnagar, Gujarat 364 002, India b Advanced Process Research Center, KIER, Daejeon, South Korea

article info

abstract

Article history:

Hydrogen adsorption studies in nickel, rhodium and palladium exchanged and in situ

Received 24 January 2008

loaded titanosilicate ETS-10 were performed at 77.4 K using a static volumetric adsorption

Received in revised form

system up to 1 bar, and 303 K in a gravimetric adsorption system up to 5 bar. The hydrogen

2 July 2008

adsorption isotherms at 77.4 K were reversible with pressure but chemisorption of

Accepted 24 October 2008

hydrogen was noticed at 303 K. Rhodium exchanged ETS-10 showed the highest hydrogen

Available online 16 December 2008

adsorption capacity of 82.6 cc/g at 77.4 K. The hydrogen adsorption isotherm analysis at 303 K was repeated up to three adsorption runs to check the repeatability of hydrogen

Keywords:

uptake. At 303 K palladium loaded ETS-10 showed the highest hydrogen uptake capacity of

ETS-10

33.1 cc/g. The DRIFT spectra analysis of ETS-10 samples before and after hydrogen

Metal exchanged ETS-10

adsorption was conducted, which confirmed that the hydrogen adsorbed in transition

Hydrogen adsorption

metal modified ETS-10 at 303 K was due to the chemical interactions in the form of tran-

Hydrogen storage

sition metal hydrides inside ETS-10. The absorbed hydrogen at 303 K can be desorbed by

Nickel

heating the ETS-10 sample up to 413 K.

Rhodium

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Palladium

1.

Introduction

Efficient storage of hydrogen is very important for the utilization of hydrogen as alternative fuels for vehicles powered with fuel cells [1]. This is relevant at a time when increasing world wide demand for energy is placing considerable strain on the petroleum supplies. Various methods for storage of hydrogen have been developed: such as high pressure tanks for gaseous hydrogen, cryogenic vessel for liquid hydrogen and metal hydrides for solid state storage systems [2,3]. The first two methods could have safety issues if not handled properly and the latter one suffers from low gravimetric storage density. Hydrogen storage by adsorption represents one potential strategy for effective and relatively safe hydrogen storage. In recent years, various efforts have been made for the development of an efficient hydrogen storage

reserved.

material [4–12]. The development of a new material for this purpose is still a challenging task. Among the potential solutions being investigated, an attractive possibility is reversible hydrogen adsorption on a microporous solid. Porous materials such as carbons [4,5], aluminosilicate zeolites [6–10] and inorganic–organic hybrid materials, such as metal organic frameworks (MOFs) [11,12] have been studied as potential hydrogen sorbents. Microporous titanosilicate, ETS-10, built of TiO6-octahedra and SiO4-tetrahedra are a new class of materials, on which attention has been focused recently due to their interesting properties. The titanosilicate ETS-10 has a novel crystalline structure formed by orthogonal chains of corner sharing TiO6-octahedra linked by SiO4-tetrahedra through bridging oxygen atoms, generating a regular three dimensional arrangement of 12and 7-ring channels. ETS-10 is a large pore molecular sieve

* Corresponding author. Tel.: þ91 278 2471793; fax: þ91 278 2567562/2566970. E-mail addresses: [email protected], [email protected] (R.V. Jasra). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.10.078

international journal of hydrogen energy 34 (2009) 888–896

˚ . The presence of –Ti–O–Ti– comprising a pore opening of 8 A wires gives ETS-10 peculiar optical properties, while the presence of channel system makes it a molecular sieve. ETS-10 was first synthesised by Kuznicki in 1989 [13]. The presence of tetravalent titanium in octahedral coordination generates a formal negative charge of 2 on the unit cell, which is balanced by exchangeable cations Naþ or Kþ. ETS-10 first attracted attention as a high capacity ion exchange material [13]. It has also been used as a potential heterogeneous base catalyst [14], and to stabilize radical cations produced by photo-irradiation of organic molecules adsorbed in the pores [15]. The quantum wire concept proposed by Lamberti’s group [16,17] explains the optical properties of ETS-10. But only a few studies have been done to exploit its adsorption and separation properties. Ricchiardi et al. [18] have studied hydrogen adsorption in ETS-10 at 20 K both by experimental measurements and simulation technique. Kuznicki et al. reported the xenon gas adsorption in silver exchanged ETS-10 [19]. Gallo et al. have reported the Grand Canonical Monte Carlo simulation studies in the titanosilicate molecular sieve ETS-10 for separation of binary mixtures of hydrogen/methane and hydrogen/carbon dioxide at 298 K [20]. The properties of ETS-10 for the potential applications can be modified by incorporating transition metals into the structure. The modification can be done either by ion exchange or by in situ loading of the metal cation during the synthesis of the material. We have already reported hydrogen adsorption studies in transition metal exchanged zeolite X [21,22]. The transition metal cations inside the zeolite structural cavities play an important role in increasing the hydrogen uptake capacity of the zeolites. The very weak interactions of hydrogen with the materials such as columbic and van der Waal interactions are responsible for the adsorption of hydrogen at 77.4 K. Previous studies have shown that the Naþ and Kþ cations in the 12 member ring channels can act as Lewis acid centres and are able to form Mþ(H2) adducts at 77.4 K [18]. In the present study, we have synthesised ETS-10 sample, transition metal exchanged and in situ loaded ETS-10 samples. The transition metals such as nickel, rhodium and palladium are known to form metal hydrides; furthermore, these metals were reported as catalysts for the hydrogenation and hydroformylation reactions, wherein the interaction of these metals with hydrogen has been manifested [23–25]. The hydrogen adsorption capacities of the synthesized samples were performed at 77.4 K up to 1 bar and 303 K up to 5 bar pressure. Rhodium exchanged ETS-10 sample showed the highest hydrogen adsorption capacity at 77.4 K and 1 bar while palladium loaded ETS-10 sample showed the highest hydrogen uptake capacity at 303 K up to 5 bar pressure.

2.

Experimental

2.1.

Materials

Sodium silicate solution (Na2O 7.16 wt%, SiO2 24.33 wt%), sodium hydroxide, sodium chloride, potassium chloride, titanium dioxide powder (P-25 Degussa), nickel nitrate, rhodium chloride, tetraamine palladium (II) chloride (E Merck India Ltd., Mumbai, India) were used for the preparation of

889

ETS-10 samples. The ultra high pure hydrogen gas (Inox air products, India.), which was further purified by passing through a molecular sieve trap was used for the adsorption isotherm measurements.

2.2.

Synthesis of ETS-10

The synthesis procedure was adapted from Yang et al. [26]. In a typical synthesis, 40 g sodium silicate solution was diluted with 70 g of water, followed by the addition of 13.8 g of NaCl and 2.6 g KCl. The translucent thick gel formed was then stirred vigorously. Under vigorous stirring 2.6 g of P25 TiO2 were added. The slurry was stirred for another 40 min at room temperature and then transferred into a Teflon-lined autoclave, which in turn was heated statically at 473 K for 42 h. The starting gel had the composition 5.3SiO2:1.0TiO2:1.6Na2O:7.2NaCl:1.0KCl:110H2O, and the pH was maintained around 10.5. The white crystalline product obtained was washed with distilled water and dried at 333 K in an oven.

2.3.

Preparation of cation exchanged ETS-10

Nickel, rhodium and palladium cations were exchanged into the sodium/potassium form of ETS-10 by the conventional cation exchange method from aqueous solution. Typically, ETS-10 was treated with 0.05 M aqueous solution of nickel nitrate, rhodium chloride or tetra amine palladium (II) chloride with solid/liquid ratio of 1:80 at 353 K for four hours. The residue was filtered, washed with hot distilled water, until the washings were free from nitrate/chloride ions and dried in air at room temperature. The extent of nickel, rhodium or palladium exchange on ETS-10 was determined by Inductively Coupled Plasma (ICP) and Energy dispersive X-ray (EDX) analyses.

2.4.

Preparation of cation loaded ETS-10

Nickel, rhodium and palladium substituted ETS-10 samples were prepared by adding nickel nitrate, rhodium chloride or tetra amine palladium (II) chloride salts in situ during the gel preparation for ETS-10 preparation followed by stirring the slurry for another 30 min at room temperature respectively. The slurry was then transferred into a Teflon-lined autoclave, which in turn was heated statically at 473 K for 42 h in an oven. The cation loaded/substituted ETS-10 obtained was then washed with distilled water and dried at room temperature and then at 353 K in an oven. The dried samples are then calcined at 473 K for three hours. The following terminologies were used to describe the ion exchanged and in situ loaded samples: In case of ion exchanged samples the first two letters show the exchanged cation and for in situ loaded samples the last two letters in parentheses show the cation loaded into ETS-10.

3.

Characterization

3.1.

X-ray powder diffraction

The X-ray powder diffraction measurements at ambient temperature were carried out using a PHILIPS X’pert MPD

890

international journal of hydrogen energy 34 (2009) 888–896

system in the 2q range of 5–60 at a scan speed of 0.1 sec1 ˚ ). The diffraction patterns using CuKa1 radiation (l ¼ 1.54056 A of the materials show it is highly crystalline with reflections in the 2q range 5–60 typical of ETS-10. The relative percentage crystallinity of the transition metal ion exchanged/loaded titanosilicates were determined from the X-ray diffraction pattern by summation of the intensities of ten major peaks at 2q values 5.9, 12.3, 20.1, 24.7, 25.8, 26.6, 29.9, 31.7, 35.6 and 45.5. The as synthesised ETS-10 was considered as an arbitrary standard for the calculations.

3.2.

Thermo-gravimetric analysis (TGA)

Thermo-gravimetric analysis of as synthesised and the modified ETS-10 samples were carried out from room temperature to 900 K in a TGA/DTA analyzer (Mettler Toledo) at a heating rate of 10 K min1 under argon atmosphere.

3.3.

Diffuse reflectance UV/VIS spectroscopy

The diffuse reflectance UV/VIS spectra were recorded on a Shimadzu-UV 3101 PC spectrometer equipped with a diffuse reflectance attachment using a scan speed of 200 nm/min in a range of 200–800 nm. Barium sulphate was used as a white standard.

3.4. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy DRIFT spectroscopic studies of adsorbed hydrogen at 303 K in nickel, rhodium and palladium exchanged and in situ loaded ETS-10 samples were carried out in a Thermo Electron Corporation NICOLET 6700 FT-IR instrument equipped with the Selector DRIFT accessory incorporating an environmental chamber (EC) assembly (Madison WI USA, P/N 0030-090 series). The titanosilicate samples were activated in situ at 423 K under a vacuum of 5  103 mm Hg at a heating rate of 10 K min1 using an automatic temperature controller (Madison WI USA, P/N 0019-022e) connected with the EC. The sample was kept at 423 K for 60 min, then cooled to 303 K and the IR spectra was collected under vacuum, following this H2 was purged on the sample at 1 atm pressure, kept for 60 min and the IR spectra was collected. Typically 300 scans were co-added at a resolution of 4 cm1.

3.5.

Adsorption isotherm measurements

Hydrogen adsorption measurement at 77.4 K was done by static volumetric system (Micromeritics Instrument Corporation, USA, model ASAP 2010) up to 1 bar pressure. The samples were activated by heating at a rate of 1 K min1, to 423 K under vacuum (6.7  101 Pa) and the temperature and vacuum was maintained for about 8 h before the sorption measurements. The amount of activated sample was determined from the weight of the samples before as well after activation and prior to start of adsorption measurements. Hydrogen adsorption measurement at 303 K was performed in a gravimetric adsorption analysis system (Gravimetric Sorption Analyzer, VTI Co., USA) up to 5 bar pressure. The samples were activated in situ by controlled heating up to 423 K (heating rate 1 K min1) under high vacuum (1.3  101 Pa) for

8 h before the sorption measurements. After activation, the samples were allowed to cool down to the desired temperature and the temperature was maintained during the analysis using an external water circulator (PolyScience, USA). Hydrogen with definite pressure dosing was introduced into the sample chamber. The increase in the weight of the sample due to hydrogen adsorption was accurately measured using a microbalance (Cahn D-200, USA) connected to the sample holder. Desorption of hydrogen on heating from the ETS-10 titanosilicate samples was also performed in gravimetric sorption analyzer. The samples were heated at a rate of 1 K min1 up to 423 K after the adsorption analysis, without removing the samples from sample holder. The high pressure hydrogen adsorption isotherm measurements at 303 K up to 35 bar for the Pd loaded ETS-10 sample was performed on an automatic high pressure gas adsorption system BELSORP-HP, BEL Japan, Inc. Prior to the adsorption isotherm measurements, the sample was first dried at 383 K, followed by treatment under hydrogen flow at 423 K to reduce palladium cations. The reduced sample was again activated thermally up to 423 K under high vacuum (6.7  102 Pa). The activated sample was used for the repeated hydrogen adsorption measurements at 303 K up to 35 bar pressure.

3.6.

Surface area measurements

The Langmuir surface areas of ETS-10, ion exchanged and in situ metal loaded ETS-10 samples were calculated by fitting the hydrogen adsorption isotherm data at 77.4 K into Langmuir isotherm model. The cross-sectional area of one hydrogen molecule is taken as 0.142 nm2 [27] for the calculation of Langmuir surface area based on dry weight of adsorbent samples.

4.

Results and discussion

Powder X-ray diffraction patterns of ETS-10 and transition metal exchanged and in situ loaded ETS-10 samples are given in Fig. 1. The X-ray powder diffraction patterns of modified ETS-10 samples showed similar peaks and relative intensities as synthesized ETS-10. Transition metal exchanged ETS-10 samples showed a small decrease in percentage crystallinity but a larger decrease in crystallinity was found in case of in situ metal loaded ETS-10 samples. Palladium loaded ETS-10 sample shows 24% of loss in crystallinity while 50% crystallinity loss was observed for nickel and rhodium loaded ETS-10 samples. The percentage of cation exchanged/loaded, relative percentage crystallinity and Langmuir surface area of ETS-10 samples are shown in Table 1. The loss of crystallinity during transition metal exchange/ loading was reported for aluminosilicate zeolites [21,22,28]. The nickel, rhodium and palladium exchanged zeolites shows a considerable decrease in crystallinity during cation exchange. The decrease in crystallinity is mainly due to the hydrated transition metal ions, which hydrolyze within the zeolite cavity with the de-alumination of zeolite framework. In titanosilicate also, the loss of crystalline may be due to the hydrolysis of transition metal ions within the framework. The decrease in diffraction intensity could also be due to presence of transition metal cations impregnated on the surface of ETS-10.

891

international journal of hydrogen energy 34 (2009) 888–896

ETS-10(Rh) ETS-10(Pd)

Intensity (a.u.)

ETS-10(Ni)

RhETS-10 PdETS-10 NiETS-10

NaETS-10 10

20

30

40

50

60

2θ Fig. 1 – XRD Patterns of ETS-10 and transition metals exchanged and in situ loaded ETS-10 samples.

The Langmuir surface area of nickel and palladium exchanged ETS-10 samples decreases slightly while a small increase in Langmuir surface area was observed in case of rhodium exchanged ETS-10. The decrease in Langmuir surface area was also observed in the case of in situ transition metal loaded samples. This could be due to the presence of impregnated transition metal cations in the pores of ETS-10. The loss of crystallinity during the metal loading may also be another reason for this decrease. Diffuse reflectance UV/VIS spectra of ETS-10 and transition metal modified ETS-10 samples are shown in Fig. 2. Pure ETS10 sample shows absorption only in the UV region; showing three major bands centred at 250, 288 and 320 nm. The band centred at 250 nm is due to the charge transfer from the Si and Ti-linking oxygen atoms to the Ti (IV) centre atoms in directions perpendicular to the Ti–O–Ti–O chains [16]. The 288 and 320 nm bands were attributed to the charge transfer within the Ti–O–Ti–O chains of the ETS-10 structure [16]. The broad band around 380 nm in the prepared ETS-10 sample is due to TiO2 phases [29]. The amount of TiO2 in the ETS-10 sample was so small that it was undetectable by PXRD. Significant amount of absorption occurred in the visible range for the transition metal modified samples. The broad absorption bands from 500 to 800 nm could be ascribed to the presence of transition metals Ni, Rh, and Pd in ETS-10 framework. The

intensity of these absorption bands in the visible range are more for in situ metal loaded samples compared to the ion exchanged counter parts, which further confirms that the amount of metal content in the in situ loaded samples are higher than that of ion exchanged samples. Thermo-gravimetric analysis of ETS-10 and the transition metal modified ETS-10 samples were performed from room temperature to 900 K and are shown in Fig. 3. ETS-10 sample shows weight loss of 10.5 wt% in which 8 wt% loss was due to the dehydration of loosely bound water from the ETS-10 framework up to 523 K. The coupled DTA analysis curve of ETS-10 also shows an endothermic peak with a maximum at 370 K with a broad shoulder spread up to 523 K. After 523 K, the weight loss is mainly due to the removal of water from the pores of ETS-10. Rhodium and palladium loaded ETS-10 samples are more stable to thermal treatment up to 673 K than ETS-10 sample. ETS-10 (Rh) sample gave 5.5 wt% loss up to 523 K while ETS-10 (Pd) sample has a weight loss of 7.2 wt% on heating up to 523 K. Other transition metal modified ETS-10 samples have higher weight loss compared to ETS-10 in the corresponding temperature range. Hydrogen adsorption isotherms of as synthesized ETS-10, nickel, rhodium and palladium exchanged and in situ loaded ETS-10 samples were measured at 77.4 K are shown in Fig. 4. The adsorption isotherms of ETS-10 samples at 77.4 K are all Type I in nature, typical for adsorption in microporous frameworks. All the hydrogen adsorption isotherms at 77.4 K are reversible with pressure change. The adsorbed hydrogen can be easily desorbed by reducing the pressure. Nickel, rhodium and palladium metal loaded samples show a decrease in hydrogen adsorption capacity compared to that of ETS-10. The hydrogen adsorption capacity of rhodium exchanged ETS-10 was higher than ETS-10 sample while nickel and palladium exchanged ETS-10 samples showed a decrease in hydrogen adsorption capacity. The decrease in hydrogen adsorption in the case of transition metal loaded samples was probably due to the loss in crystallinity and pore occupancy by the transition metal cations sitting closer to the 12-ring channels as reflected by the decrease in surface area. The higher adsorption capacity of rhodium exchanged ETS-10 at 77.4 K could be due to the formation of Rh3þ(H2) adducts inside the ETS-10 channels [18]. Hydrogen adsorption studies in ETS-10, nickel, rhodium and palladium exchanged and in situ loaded ETS-10 samples were done at 303 K in a gravimetric sorption system up to 5 bar pressure and are shown in Fig. 5. As synthesized ETS-10

Table 1 – Percentage of cation exchanged/loaded, relative percentage crystallinity, Langmuir surface area of ETS-10 and modified ETS-10 materials. Sample name NaETS-10 NiETS-10 RhETS-10 PdETS-10 ETS-10(Ni) ETS-10(Rh) ETS-10(Pd)

Weight percentage of cation exchanged/loaded

Relative percentage crystallinity

Langmuir surface area (m2 g1)

– 7.5 wt% 6 wt% 4 wt% 10.2 wt% 10.5 wt% 8 wt%

100 92 79 77 51 50 76

279  3.29 235  4.35 282  3.67 236  2.56 155  1.75 193  2.48 129  2.52

892

international journal of hydrogen energy 34 (2009) 888–896

80 70

Volume adsorbed (cc/g)

Absorbance (a. u.)

ETS-10 NiETS-10 PdETS-10 RhETS-10 ETS-10(Ni) ETS-10(Rh) ETS-10(Pd)

60 50 40

NaETS-10 NiETS-10 RhETS- 10 PdETS-10 ETS-10(Ni) ETS-10(Rh) ETS-10(Pd)

30 20 10 0

200

300

400

500

600

700

800

0.0

0.2

0.4

Wavelength (nm)

sample show a hydrogen adsorption capacity of 19.1 cc/g at 303 K up to 5 bar pressure. This uptake could be mainly due to the encapsulation of hydrogen inside the pores of ETS-10 at high pressure as observed in aluminosilicate zeolites [10]. Palladium loaded ETS-10 sample showed the highest hydrogen uptake of 33.1 cc/g at 303 K up to 5 bar. The hydrogen adsorption isotherms at 303 K were not completely reversible, may be due the chemisorptions of hydrogen in modified ETS-10 samples. The hydrogen uptake in ETS-10 and modified ETS-10 samples at 77.4 K up to 1 bar and 303 K up to 5 bar pressure are shown in Table 2. In situ metal loaded ETS10 samples show higher hydrogen uptake capacities than metal exchanged samples at 303 K. The chemical interactions between the transition metal cation and the hydrogen molecules were predominant at 303 K. The hydrogen uptake in transition metal modified ETS-10 was due to the chemisorption of hydrogen on nickel, rhodium and palladium metals 0

ETS-10 NiETS-10 RhETS-10 PdETS-10 ETS-10(Ni) ETS-10(Rh) ETS-10(Rh)

-4

weight loss

0.8

1.0

-8

-12

Fig. 4 – Hydrogen Adsorption isotherms of transition metal exchanged and substituted ETS-10 samples at 77.4 K up to 1.2 bar pressure.

which were formed by the reduction of the corresponding cations inside the titanosilicate framework. The reduction of the transition metal ions to corresponding metals took place in presence of hydrogen before hydrogen is chemisorbed in the form of metal hydrides. The in situ metal loaded samples have significantly higher metal content than the ion exchanged counterpart (Table 1) and hence show higher hydrogen chemisorption capacities at 303 K than the ion exchanged ETS-10 samples. If we compare on an element to element basis of the in situ metal loaded and ion exchanged samples (e.g. ETS-10(Ni) vs. NiETS-10), the framework structural breakage and decrease in the surface area of the in situ metal loaded ETS-10 samples (Table 2) considerably decreases the accessibility of hydrogen to the transition metals inside the pores to form metal hydrides and thus decreases the uptake capacity. But in the case of ion exchanged samples, even though the metal content is lower compared to the in 35 30

Volume adsorbed (cc/g)

Fig. 2 – Diffuse reflectance UV/VIS spectra of ETS-10 samples (A) ETS-10, (B) nickel exchanged ETS-10, (C) rhodium exchanged ETS-10, (D) palladium exchanged ETS10, (E) nickel loaded ETS-10, (F) rhodium loaded ETS-10 and (G) palladium loaded ETS-10 samples.

-16

0.6

Pressure(bar)

25 20 15

ETS-10

NiETS-10 RhETS-10 PdETS-10 ETS-10(Ni) ETS-10(Rh) ETS-10(Pd)

10 5

ETS-10 DTA curve

0

0

-20 400

500

600

700

800

900

Temperature (K) Fig. 3 – TGA of ETS-10 and nickel, rhodium and palladium exchanged/loaded ETS-10.

1

2

3

4

5

6

Pressure (bar) Fig. 5 – Hydrogen adsorption isotherms of transition metal exchanged and in situ loaded ETS-10 samples at 303 K up to 5 bar pressure.

893

international journal of hydrogen energy 34 (2009) 888–896

40

100

First run Second run Third run

35

25

ETS-10(Pd)-I ETS-10(Pd)-II ETS-10(Pd)-III ETS-10(Pd) desorption ETS-10(Rh)-I ETS-10(Rh)-II ETS-10(Rh)-III ETS-10(Rh) desorption

20 15 10 5 0

0

1

2

3

4

5

Pressure (bar) Fig. 6 – Repeated hydrogen adsorption isotherms in palladium and rhodium loaded ETS-10 samples at 303 K up to 5 bar. The roman numerals denote the number of run.

situ loaded samples, there is not much loss of crystallinity due to the ion exchange process and hence they show higher hydrogen up take capacities comparable to the in situ metal loaded ETS-10 samples. The hydrogen adsorption studies in rhodium and palladium loaded ETS-10 samples have been done by repeated measurements of the hydrogen adsorption isotherms at 303 K up to 5 bar. A small decrease in the hydrogen uptake was observed in the second adsorption run, which could be due to the hydrogen consumed for the reduction of rhodium or palladium metal ions. The samples were then activated again at 423 K for 6 h under vacuum and the second hydrogen adsorption run was carried out. This was followed by third hydrogen adsorption run after activating the sample as in second adsorption run. The adsorption isotherm data obtained in the second and third runs were comparable as shown in Fig. 6. The repeatability of the chemisorption measurements was confirmed by hydrogen chemisorption studies in the pre-reduced Pd loaded ETS-10 sample at 303 K up to 35 bar pressure. Before the adsorption analyses the sample was reduced under flow of hydrogen for 5 h at 423 K and the reduced samples were thermally activated under

Table 2 – Hydrogen uptake values of ETS-10 and modified ETS-10 materials. ETS-10 sample

ETS-10 NiETS-10 RhETS-10 PdETS-10 ETS-10(Ni) ETS-10(Rh) ETS-10(Pd) ETS-10(Pd) pre-reduced

Hydrogen uptake values (cc/g) 77.4 K, 1 bar

303 K, 5 bar

69.8 66.8 82.6 67.2 44.9 56.1 61.9 –

17.4 20.0 28.1 30.1 23.7 31.6 33.1 34.4

Volume adsorbed (cc/g)

Volume Adsorbed (cc/g)

80 30

60 40 35

40

30 25 20 15

20

10 5 0 0

0

0

5

10

1

15 20 25 Pressure (bar)

2

3

30

4

5

35

40

Fig. 7 – Hydrogen adsorption isotherms in fully reduced ETS-10(Pd) sample at 303 K up to 35 bar.

vacuum. The first hydrogen adsorption analysis run was performed on this activated sample. After the completion of the first adsorption run, the sample was again thermally activated under vacuum and second hydrogen adsorption analysis run was performed, like that three consecutive hydrogen adsorption analyses runs were performed on the same sample and comparable hydrogen uptake capacities was obtained in each hydrogen adsorption run. Fig. 7 show the repeated hydrogen adsorption isotherms in fully reduced ETS10(Pd) sample at 303 K up to 35 bar. DRIFT spectra of ETS-10 and the modified ETS-10 samples before and after hydrogen adsorption have been shown in Fig. 8. The DRIFT spectra of bare ETS-10 sample didn’t show any additional peak. The formation of hydrides with transition metal and hydrogen molecule has been confirmed by the DRIFT analysis. The yM–H stretching frequencies observed in the range 1600–2200 cm1 confirms the formation of metal hydride [30]. The mechanism of hydride formation for the titanosilicate framework could be as follows: Mn þ þ nH2 % M–Hn þ nHþ

(1)

Based on this mechanism, metal hydride species and Hþ ions are formed following the dissociation of hydrogen molecule inside the titanosilicate channels. In our previous studies we have reported the mechanism for hydride formation inside aluminosilicate zeolite framework [21,22]. A similar mechanism was proposed by Baba et al. [31] for dissociative adsorption of hydrogen on silver exchanged zeolites. Serykh et al. have also proposed similar mechanism for the formation of Cd–H species inside Cd-ZSM-5 [30]. The Hþ ions formed during the dissociation and hydride formation balance the framework neutrality of the titanosilicate material. The reversibility of hydrogen adsorption in modified ETS10 materials was also studied by heating the previously hydrogen adsorbed modified ETS-10 samples. The hydrogen desorption studies were done by heating all the samples up to 473 K. The hydrogen desorption from Palladium loaded ETS-10 samples is shown in Fig. 9. From figure it is clear that desorption of hydrogen starts around 413 K.

894

international journal of hydrogen energy 34 (2009) 888–896

a

b

NiETS-10 NiETS-10 H2 adsorbed

transmittance (a. u.)

transmittance (a. u.)

ETS-10 ETS-10 H2 adsorbed

1727 cm-1

2400

2200

2000

1800

1600

1400

2400

1200

2200

c

1800

1600

1400

1200

d

PdETS-10 PdETS-10 H2 adsorbed

transmittance (a. u.)

RhETS-10 RhETS-10 H2 adsorbed

transmittance (a. u.)

2000

Wavenumber (cm-1)

Wavenumber (cm-1)

2100 cm-1

1810 cm-1 2400

2200

2000

1800

1600

1400

1200

2400

2200

Wavenumber (cm-1)

e

2000

1800

1600

1400

1200

Wavenumber (cm-1)

f

ETS-10(Rh) H2 adsorbed in ETS-10(Rh)

transmittance (a. u.)

transmittance (a. u.)

ETS-10(Ni) H2 adsorbed in ETS-10(Ni)

1950 cm-1

1736 cm-1

2400

2200

2000

1800

1600

1400

1200

Wavenumber (cm-1)

g

2200

2000

1800

1600

1400

1200

Wavenumber (cm-1)

ETS-10(Pd) H2 adsorbed in ETS-10(Pd)

transmittance (a. u.) 2400

2400

1877 cm-1

2200

2000

1800

1600

1400

1200

Wavenumber (cm-1) Fig. 8 – DRIFT spectra of ETS-10 samples recorded at 303 K. (a) ETS-10, (b) nickel exchanged ETS-10, (c) rhodium exchanged ETS10, (d) palladium exchanged ETS-10, (e) nickel loaded ETS-10, (f) rhodium loaded ETS-10 and (g) palladium loaded ETS-10 samples.

international journal of hydrogen energy 34 (2009) 888–896

0

Volume desorbed (cc/g)

-5 -10 -15 -20 -25 -30 -35 300

320

340

360

380

400

420

440

460

480

Temperature (K) Fig. 9 – Desorption of hydrogen by heating up to 473 K from ETS-10(Pd) sample, previously hydrogen adsorbed at 303 K and 5 bar.

5.

Conclusions

Hydrogen adsorption measurements were carried out at 77.4 K up to 1 bar and 303 K up to 5 bar pressures in ETS-10 and transition metals nickel, rhodium and palladium exchanged and in situ loaded ETS-10 samples using volumetric and gravimetric adsorption systems respectively. Physisorption of hydrogen was observed at 77.4 K for all the system studied and RhETS-10 showed the highest hydrogen adsorption capacity while chemisorption of hydrogen was observed at 303 K. The transition metal incorporation in ETS-10 was shown to increase the hydrogen adsorption capacity of ETS-10 at 303 K due to chemisorptions. Palladium loaded ETS-10 showed the highest hydrogen uptake at 303 K. The hydrogen adsorbed inside the titanosilicate channels can be easily desorbed by heating the titanosilicate up to 413 K. Transition metal modified ETS-10 materials are a promising candidate for room temperature hydrogen storage if one can increase the wt% loading of transition metals into the frame work without any further loss of crystallinity.

Acknowledgements This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-D00003). The authors are thankful to CSIR, New Delhi for financial support under CSIR Network Project NWP-022.

references

[1] Schlapbach L. Hydrogen as a fuel and its storage for mobility and transport. MRS Bull 2002;27:675–9. [2] Schlapbach L, Zuttel A. Hydrogen storage materials for mobile applications. Nature 2001;414:353–8.

895

[3] Zuttel A. Materials for hydrogen storage. Mater Today 2003;6: 24–33. [4] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999;286:1127–9. [5] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;36:377–9. [6] Palomino GT, Carayol MR, Arean CO. Hydrogen adsorption on magnesium-exchanged zeolites. J Mater Chem 2006;16: 2884–5. [7] Du X, Wu E. Physisorption of hydrogen in A, X and ZSM-5 types of zeolites at moderately high pressures. Chin J Chem Phys 2006;19:457–62. [8] Langmi HW, Book D, Walton A, Johnson SR, Al-Mamouri MM, Speight JD, et al. Hydrogen storage in ion exchanged zeolites. J Alloys Compd 2005;404–406:637–42. [9] Zecchina A, Bordiga S, Vitillo JG, Ricchiardi G, Lamberti C, Spoto G, et al. Liquid hydrogen in protonic chabazite. J Am Chem Soc 2005;127:6361–6. [10] Weitkamp J, Fritz M, Ernst S. Zeolites as media for hydrogen storage. Int J Hydrogen Energy 1995;20:967–70. [11] Rosi NL, Eckert J, Eddaoudi M, Vodak DT, Kim J, O’keefte M, et al. Hydrogen storage in microporous metal-organic frame works. Science 2003;300:1127–9. [12] Sun D, Ma S, Ke Y, Collins DJ, Zhou HC. An interweaving MOF with high hydrogen uptake. J Am Chem Soc 2006;128:3896–7. [13] Kuznicki SM. US Patent 4853202; 1989. [14] Philippou A, Rocha J, Anderson MW. The strong basicity of the microporous titanosilicate ETS-10. Catal Lett 1999;57:151–3. [15] Krishna RM, Prakash AM, Kurshev V, Kevan L. Photoinduced charge separation of methylphenothiazine in microporous titanosilicate M-ETS-10 (M ¼ Naþ þ Kþ, Hþ, Liþ, Naþ, Kþ, Ni2þ, Cu2þ, Co2þ) and Naþ, Kþ-ETS-4 molecular sieve at room temperature. Phys Chem Chem Phys 1999;1:4119–24. [16] Lamberti C. Electron-hole reduced effective mass in monoatomic. –O–Ti–O–Ti–O–.quantum wires embedded in siliceous crystalline matrix of ETS-10. Micropor Mesopor Mater 1999;30:155–63. [17] Bordiga S, Palomino GT, Zecchina A, Ranghino G, Giamello E, Lamberti C. Stoichiometric and sodium-doped titanium silicate molecular sieve containing atomically defined –OtiOTiO– chains: quantum ab initio calculations, spectroscopic properties, and reactivity. J Chem Phys 2000; 112:3859–67. [18] Ricchiardi G, Vitillo JG, Cocina D, Gribov EN, Zechina A. Direct observation and modelling of ordered hydrogen adsorption and catalyzed ortho-para conversions on ETS-10 titanosilicate material. Phys Chem Chem Phys 2007;9:2753–60. [19] Kuznicki SM, Anson A, Koenig A, Kuznicki TM, Haastrup T, Eyring EM, et al. Xenon adsorption on modified ETS-10. J Phys Chem C 2007;111:1560–2. [20] Gallo M, Nenoff TM, Mitchel MC. Selectivities for binary mixtures of hydrogen/methane and hydrogen/carbon dioxide in silicalite and ETS-10 by grand canonical Monte Carlo techniques. Fluid Phase Equilibria 2006;247:135–42. [21] Prasanth KP, Pillai RS, Bajaj HC, Jasra RV, Chung HD, Kim TH, et al. Adsorption of hydrogen in nickel and rhodium exchanged zeolite X. Int J Hydrogen Energy 2008;33(2):735–45. [22] Prasanth KP, Pillai RS, Sunil AP, Bajaj HC, Jasra RV, Chung HD, et al. Hydrogen uptake in palladium and ruthenium exchanged zeolite X. J Alloys Compd 2008;446(1-2):439–46. [23] Wang X, Andrews L. Infrared spectra of rhodium hydrides in solid argon, neon, and deuterium with supporting density functional calculations. J Phys Chem A 2002;106:3706–13. [24] Bauer HJ, Wagner FE. Hydride formation, magnetic and transport properties of nickel and nickel based alloys. Pol J Chem 2004;78:463–514.

896

international journal of hydrogen energy 34 (2009) 888–896

[25] Nishimiya N, Kishi T, Mizushima T, Matsumoto A, Tsutsumi K. Hyperstoichiometric hydrogen occlusion by palladium nanoparticles included in NaY zeolite. J Alloys Compd 2001;319:312–21. [26] Yang X, Paillaud JL, van Breukelen HFWJ, Kessler H, Duprey E. Synthesis of microporous titanosilicate ETS-10 with TiF4 or TiO2. Micropor Mesopor Mater 2001;46:1–11. [27] Zuttel A, Sudan P, Mauron P, Wenger P. Model for hydrogen adsorption on carbon nanostructures. Appl Phys A 2004;78:941–6. [28] Olson DH. Crystal structure of the zeolite nickel faujasite. J Phys Chem 1968;72:4366–73.

[29] Liu Z, Davis RJ. Investigation of the structure of microporous Ti–Si mixed oxides by X-ray, UV reflectance, FT-Raman and FT-IR spectroscopies. J Phys Chem 1994;98:1253–61. [30] Serykh AI. Nature of cadmium cationic sites in cadmiummodified ZSM-5 zeolite according to the DRIFT studies of molecular hydrogen adsorption. Micropor Mesopor Mater 2005;80:321–6. [31] Baba T, Komatsu N, Sawada H, Yamaguchi Y, Takahashi T, Sugisawa H, et al. 1H magic angle spinning NMR evidence for dissociative adsorption of hydrogen on Agþ-exchanged A and Y- zeolites. Langmuir 1999;15:7894–6.