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Catalytic and inhibitive effects of Pd and Pt decorated MWCNTs on the dehydrogenation behavior of LiAlH4
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Catalytic and inhibitive effects of Pd and Pt decorated MWCNTs on the dehydrogenation behavior of LiAlH4 Chia-Yen Tan a, Wen-Ta Tsai a,b,* a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan
article info
abstract
Article history:
A novel catalyst was developed to modify the dehydrogenation behavior of LiAlH4 (lithium
Received 1 May 2015
alanate). Using a chemical reduction method, Pd and Pt nano-particles were decorated on
Received in revised form
multi-walled carbon nanotubes (MWCNTs). The homogeneity and surface morphology of
8 June 2015
Pd and Pt decorated MWCNTs were examined by using transmission electron microscopy
Accepted 22 June 2015
and scanning electron microscopy. The dehydrogenation behavior of LiAlH4 admixed with
Available online 11 July 2015
Pd and Pt decorated MWCNTs was investigated by thermal gravimetric analysis (TGA) and the in-situ synchrotron X-ray diffraction (XRD) technique. The TGA results revealed that the addition of Pd or Pt decorated MWCNTs would have either catalytic or inhibitive effects on
Keywords: Lithium alanate (LiAlH4)
the dehydrogenation of LiAlH4, which were concentration dependent. The in-situ XRD
Multi-walled carbon nanotubes
analysis confirmed the catalytic power of Pd or Pt decorated MWCNTs and the corre-
(MWCNTs)
sponding dehydrogenation reaction pathway of LiAlH4. The TGA results showed that both
Dehydrogenation reaction
Pd and Pt decorated MWCNTs acted not only as a catalyst for LiAlH4, but also a hydrogen
In-situ synchrotron X-ray diffraction
absorber. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
3LiH/3Li þ 3=2H2
Introduction Lithium aluminum hydride (LiAlH4) is a promising complex metal hydride used for solid state hydrogen storage due to its high gravimetric hydrogen density. The dehydrogenation reactions and the corresponding temperatures of LiAlH4 are as follows [1e4]: 3LiAlH4 /Li3 AlH6 þ 2Al þ 3H2 Li3 AlH6 /3LiH þ Al þ 3=2H2
ð160 180 C; 5:3 wt%H2 Þ ð180 220 C; 2:6 wt%H2 Þ
(1) (2)
ðAbove 400 C; 2:6 wt%H2 Þ
(3)
The theoretical amount of hydrogen released from LiAlH4 resulting from both the first and the second dehydrogenation reactions is 7.9 wt%. Nevertheless, the dehydrogenation temperature of the LiAlH4 is still far from the target operation temperature set by the U.S. Department of Energy (DOE) [5]. Therefore, a number of studies have aimed to develop catalysts to improve the dehydrogenation properties (such as operation temperature and reaction kinetics) of LiAlH4. Some of the effective catalysts are carbon nano materials, such as carbon nano-fibers (CNFs) [6,7], single-walled carbon
* Corresponding author. 1, Ta Hsueh Road, Tainan 701, Taiwan. Tel.: þ886 62757575x62927; fax: þ886 62754395. E-mail address: [email protected] (W.-T. Tsai). http://dx.doi.org/10.1016/j.ijhydene.2015.06.106 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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nanotubes (SWCNTs) [8] and multi-walled carbon nanotubes (MWCNTs) [9e11]. These carbon nano-materials have been confirmed to possess the property of nano-confinement [12e16], have destabilization effects [17e19] and are able to serve as effective forms of transit that provide an efficient pathway for hydrogen diffusion [20]. Moreover, the electron affinity of carbon materials can affect the H-removal energy during dehydrogenation reactions [21]. Previous studies have shown that the addition of MWCNTs would modify the thermodynamic and kinetic properties of LiAlH4, lowering its dehydrogenation temperature to as low as 120 C [11]. It has been reported that MWCNTs is a promising solid state hydrogen storage material if decorated with Pt or Pd [22e28]. The Pd or Pt decorated MWCNTs can absorb up to 2.7 wt% of hydrogen at room temperature under 7 MPa of hydrogen pressure [28]. The “spill-over” effect induced by the decorated metal particles provides an efficient pathway for hydrogen molecules to dissociate and the atoms to diffuse in and out of the carbon material [22,29,30]. Since MWCNTs can serve as an effective catalyst for the dehydrogenation of LiAlH4, it is of interest to examine whether the decoration of Pt or Pd can further affect the catalytic power. A recent study proposed decorating Fe, Ni and Pd nano-particles on carbon nano-materials to produce an effective catalyst for dehydrogenation of LiAlH4 [31]. The aim of the current study is to further clarify the effects of Pd or Pt decorated MWCNTs on the dehydrogenation behavior of LiAlH4, and especially the catalytic and/or inhibitive effects. Using a chemical reduction method, both Pd and Pt particles can be successfully decorated on MWCNTs. Thermogravimetric analysis (TGA) was utilized to reveal the dehydrogenation behaviors of LiAlH4 with various additions of Pd or Pt decorated MWCNTs. In-situ XRD analysis was used to identify the reaction pathways of the dehydrogenation reactions.
methanol through a centrifugal sedimentation process and then vacuum dried for 12 h. The vacuum dried MWCNTs/Pt or MWCNTs/Pd with specific contents (0e20 wt%) was admixed with lithium alanate (LiAlH4, Chemetall, 97% purity) and sealed in a 75 ml stainless steel vessel for milling and mixing treatment. The stainless steel balls contained in this vessel had an average diameter of 4.8 mm. The ball to powder weight ratio was maintained at 10:1. The above sample loading process was carried out in a N2 purified glove box. The sealed stainless steel vessel was then cooled in liquid N2 before ball milling. The ball milling was performed in a ball milling machine (SPEX 800) at 1700 rpm for 10 min. The cooling and milling processes were repeated three times to ensure full mixing of the powders.
Thermal dehydrogenation analysis The dehydrogenation behavior of the LiAlH4 admixed with MWCNTs/Pd or MWCNTs/Pt was evaluated by thermogravimetric analysis (TGA) with a high pressure microbalance (Cahn D-110). The sample with an initial weight of ca. 200 mg was loaded in a quartz crucible and transferred into a high pressure chamber. The chamber was evacuated to 1 104 torr, followed by charging H2 gas (99.99% purity) to ambient pressure. As soon as the microbalance system stabilized, heating processes were applied. Two heating processes were adopted in this study, namely (1) continuous heating from room temperature to 350 C at a heating rate of 5 C min1, and (2) isothermal heating at 140 C for 10 h, respectively. The weight change of the sample was continuously measured during each heating process. The weight percentage of hydrogen released from each sample at the specific temperature was then calculated.
Material characterization
Experimental procedure Sample preparation The Pt and Pd particles were decorated separately on MWCNTs (purity > 95%, outer diameter 20e30 nm; length 5e15 mm, Uni Region, Bio-Tech) by a chemical reduction process. First, 0.3 g of MWCNTs was successively immersed in 0.31 M SnCl2 (Merck, >99%) and 1.4 mM PdCl2 (Seedchem, >95%) solution and subjected to ultrasonic vibration (Ultrasonic Steri-Cleaner LEO-150; 150 W, 46 kHz) for 10 and 2 min, respectively, for the sensitization and the activation processes. The activated MWCNTs were then added into 150 mg/L of H2PtCl6$6H2O (Acros Organics, 99.9%) or palladium bishexafluoracetylacetonate (Pd(hfa)2), (Strem Chemicals, 95%) solution, with methanol as the solvent. Boranedimethylamine complex (DMAB, Acros Organics, 98%) was used as a reducing agent. By slowly adding an adequate amount of DMAB into H2PtCl6$6H2O and Pd((hfa)2) solutions, the Pt4þ and Pd4þ ions were reduced to form Pt and Pd particles, respectively, and precipitated on MWCNTs. The treated Pt or Pd decorated MWCNTs (hereafter referred as MWCNTs/Pt or MWCNTs/Pd) were later cleansed by
The morphological aspects of MWCNTs/Pt, MWCNTs/Pd and LiAlH4 admixed with MWCNTs/Pt or MWCNTs/Pd were examined by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The chemical compositions of MWCNTs/Pt and MWCNTs/Pd were analyzed by an energy dispersive spectrometer (EDS). The transition of the crystal structure during the dehydrogenation reaction was identified by in-situ synchrotron X-ray diffraction (in-situ XRD) employing beamline 17A, at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. To perform in-situ XRD analysis, the sample was first loaded in a 0.5 mm diameter glass capillary tube, and then mounted to the specimen holder. N2 gas was introduced at one end of the capillary tube to form a protective atmosphere. The sample was heated from room temperature to 350 C at a heating rate of 5 C min1 with a hot air blower. During heating, the sample was repeatedly exposed to the ˚ at insynchrotron X-ray with a wavelength of 1.033105 A tervals of 240 s. The 2-D diffraction patterns were collected continuously and converted to 1-D patterns by Fit2D software [32]. The transition of the crystal structure during the heating process was thus analyzed.
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Results and discussion Material characterization Fig. 1(a) and (b) show SEM images of MWCNTs decorated with Pt and Pd, respectively. The MWCNTs were around 20e30 mm
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long, and they tended to tangle with each other after the vacuum drying process. Fig. 1(c) and (d) show the morphologies of MWCNTs/Pt and MWCNTs/Pd, as obtained by TEM. The Pt and Pd nano-particles were homogeneously distributed throughout the MWCNTs. The size of each Pt or Pd particle was in the range of 5e20 nm. However, inevitable agglomeration of Pt or Pd particles occurred during the chemical
Fig. 1 e SEM images of (a) MWCNTs/Pt and (b) MWCNTs/Pd; TEM images of (c) MWCNTs/Pt and (d) MWCNTs/Pd; EDS results for (e) MWCNTs/Pt and (f) MWCNTs/Pd. SEM images of LiAlH4 eadmixed with (g) 20 wt% MWCNTs/Pt and (h) 20 wt% MWCNTs/Pd.
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Fig. 2 e TGA results of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/ Pd, respectively.
reduction process, and some clusters grew as large as 50e100 nm. The EDS results, as revealed in Fig. 1(e) and (f), confirmed the existence of Pt and Pd particles. The Sn signal, which originated from the sensitization process, also existed in both MWCNTs/Pt and MWCNTs/Pd. The Sn ions played a key role in the chemical reduction process. It is noted that there was some Cl residue left from the H2PtCl6$6H2O in the MWCNTs/Pt after chemical reduction process. As shown in Fig. 1(e) and (f), after 30 min of ball milling, LiAlH4 was able to mix with MWCNTs/Pt and MWCNTs/Pd homogeneously, resulting in the size of the LiAlH4 particles reduced to 5e50 mm.
Effect of Pd decorated MWCNTs The dehydrogenation behaviors of LiAlH4 and those with the addition of various forms of MWCNTs were evaluated quantitatively by TGA. As revealed in Fig. 2, the weight loss of each sample was measured progressively during heating at a rate of 5 C min1, from room temperature to 350 C in 0.1 Mpa hydrogen atmosphere. Based on the TGA results, the onset dehydrogenation temperature was determined as the temperature at which a 0.25 wt% weight loss occurred to the sample being measured. The as-milled pristine LiAlH4 exhibited a two-step dehydrogenation process. The onset
dehydrogenation of LiAlH4 is approximately 170 C, in agreement with a previous report [11]. The 2nd dehydrogenation temperature was detected when the sample was continuously heated to 200 C. The total amount of hydrogen released was 6.5 wt% when the temperature was raised to 260 C, and this then remained nearly unchanged thereafter. As revealed in Fig. 2, when 20 wt% of pure MWCNTs was added, the onset dehydrogenation temperature of LiAlH4 decreased to 140 C, while the second stage hydrogenation temperature started at 190 C. The catalytic effect of MWCNTs on accelerating the dehydrogenation of LiAlH4 was thus verified. However, the addition of MWCNTs into the LiAlH4 caused a decrease in the total amount of hydrogen released to 4.6 wt%. Decorating MWCNTs with Pd particles had a significant influence on their catalytic power. As shown in Fig. 2, admixing 5 and 10 wt% of MWCNTs/Pd in LiAlH4 caused a delay of the onset dehydrogenation temperature to nearly 200 C. When these amounts of MWCNTs/Pd were added, the hydrogen desorption rate of LiAlH4 was slower compared with the as-milled pristine LiAlH4. Furthermore, the boundary between the 1st and the 2nd dehydrogenation reactions of LiAlH4 became less distinguishable. The results showed that the addition of 5 or 10 wt% of MWCNTs/Pd had an inhibitive effect on the dehydrogenation of LiAlH4, as far as the 1st step dehydrogenation temperature was concerned. It is worth noting, however, the onset dehydrogenation temperature of LiAlH4 was lowered to 140 C when the addition of MWCNTs/ Pd increased to 20 wt%, indicating a reversion from an inhibitive to catalytic role of the MWCNTs/Pd. The TGA results show that the effects of MWCNTs/Pd on the dehydrogenation of LiAlH4, either inhibitive or catalytic, were concentration dependent. The onset dehydrogenation temperature and the total amount of hydrogen desorbed from the LiAlH4 admixed with 5e20 wt% of MWCNTs/Pt and MWCNTs/Pd measured in TGA tests are summarized in Table 1. The isothermal TGA test provides a different approach to demonstrate the catalytic effects of MWCNTs/Pd on the dehydrogenation of LiAlH4. As shown in Fig. 3, the weight loss of LiAlH4 with and without MWCNTs/Pd addition was continuously measured at 140 C under 0.1 Mpa of hydrogen atmosphere. The pristine LiAlH4 did not release any hydrogen at 140 C, while a notable amount of weight loss due to hydrogen desorption was observed when MWCNTs or MWCNTs/Pd were admixed. Each isothermal TGA curve (except that for pristine LiAlH4) shown in Fig. 3 could be
Table 1 e The onset hydrogen desorption temperatures and the amounts of hydrogen released from LiAlH4, and from those admixed with various amount of MWCNTs decorated with Pd and Pt, respectively. Composition LiAlH4 LiAlH4-20 wt% MWCNTs LiAlH4-5 wt% MWCNTs/Pd LiAlH4-10 wt% MWCNTs/Pd LiAlH4-20 wt% MWCNTs/Pd LiAlH4-5 wt% MWCNTs/Pt LiAlH4-10 wt% MWCNTs/Pt LiAlH4-20 wt% MWCNTs/Pt
Onset temperature ( C)
Hydrogen released (wt%, including MWCNTs)
Hydrogen released (wt%, excluding MWCNTs)
171 148 198 194 143 187 202 156
6.2 4.6 6.2 5.8 5.1 5.6 5.5 5.1
6.2 5.7 6.5 6.4 6.4 5.9 6.1 6.4
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To further explore the dehydrogenation mechanism, in-situ XRD analysis was performed at a heating rate of 5 C min1. Fig. 4(a) shows the XRD patterns of LiAlH4 admixed with 20 wt % MWCNTs/Pd as a function of temperature. As shown in this figure, the XRD pattern of LiAlH4 admixed with 20 wt% MWCNTs/Pd consisted of only the diffraction peaks of LiAlH4 at 30 C. When the temperature was raised to 150 C, the 1st dehydrogenation reaction occurred, which was specified as reaction (1). The LiAlH4 started to decompose to form Li3AlH6 and Al with the liberation of H2. As the temperature was raised to 230 C, the diffraction peaks of Li3AlH6 diminished and those of to Al and LiH intensified, indicating the occurrence of the 2nd dehydrogenation reaction, as specified in reaction (2). The in-situ XRD results suggested that the reaction route of LiAlH4 was not altered by addition of MWCNTs/Pd. However, Fig. 3 e Isothermal TGA results at 140 C of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/Pd, respectively.
divided into three regions according to the hydrogen desorption rate (or the slope of the curve). The first region exhibited a low hydrogen desorption rate, which was followed by a fast but almost constant dehydrogenation rate in the intermediate (or the second) region. In the third region, the dehydrogenation reaction ceased and no further weight loss was detected. The maximum desorption rates determined from the second region (the linear part at each curve) are summarized in Table 2. The results show that the maximum dehydrogenation rate of LiAlH4 was enhanced when the MWCNTs were decorated with Pd. However, the beneficial effect of Pd decoration decreased as the amount of MWCNTs/Pd addition increased. More specifically, the isothermal TGA results shown in Fig. 3 indicate that MWCNTs/Pd played a role in catalyzing the dehydrogenation reaction of LiAlH4, the total amount of hydrogen and its desorption rate decreased as the amount of MWCNTs/Pd addition increased. The reason for the decreasing amount of H2 desorbed from LiAlH4 with an increasing amount of MWCNTs/Pd addition will be discussed later.
Table 2 e Isothermal TGA results showing the linear hydrogen desorption rate and the total amount of hydrogen released at 140 C for LiAlH4, and those admixed with various amount of MWCNTs decorated with Pd and Pt, respectively. Composition
LiAlH4 LiAlH4-20 wt% MWCNTs LiAlH4-5 wt% MWCNTs/Pd LiAlH4-10 wt% MWCNTs/Pd LiAlH4-20 wt% MWCNTs/Pd LiAlH4-5 wt% MWCNTs/Pt LiAlH4-10 wt% MWCNTs/Pt LiAlH4-20 wt% MWCNTs/Pt
Hydrogen Total hydrogen desorption rate released (wt%) (wt% min1, linear region) 0 0.015 0.023 0.017 0.009 0.030 0.018 0.014
0 2.62 3.66 3.44 2.28 3.10 2.87 2.00
Fig. 4 e (a) In-situ synchrotron XRD patterns of LiAlH4 eadmixed with 20 wt% MWCNTs/Pd heated from room temperature to 350 C; (b) variations of the highest peak intensity with temperature for the major species appearing during heating.
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as the temperature rose to above 330 C, some peaks corresponding to the AlePd intermetallic compound (Al21Pd8) emerged, indicating the reaction of Pd particles with Al residue from the 2nd dehydrogenation reaction. In addition, two diffraction peaks appeared at the 2-theta values of 23.11 and 33.35 after the 2nd dehydrogenation reaction at 230 C. However, these two peaks could not be identified based on the diffraction data of known inorganic compounds. The temperature dependent X-ray diffraction peak intensity for various identifiable phases are summarized in Fig. 4(b). As shown in this figure, the Li3AlH6 phase emerged at 150 C and diminished at 230 C, indicating that admixing 20 wt% of MWCNTs/Pd would lower the onset dehydrogenation temperature of LiAlH4 to around 150 C without altering its twostep reaction pathway. Generally, the in-situ XRD analysis did verify the catalytic power of MWCNTs/Pd on LiAlH4.
Effect of Pt decorated MWCNTs The Pt decorated MWCNTs also exhibited either catalytic or inhibitive effects on the dehydrogenation properties of LiAlH4, depending on the amount added. As shown in Fig. 5, with 5 wt % of MWCNTs/Pt addition, the onset dehydrogenation temperature of LiAlH4 rose to 190 C. The hydrogen desorption rate was slower than that of the pristine LiAlH4. As the addition of MWCNTs/Pt increased to 10 wt%, a further delay of hydrogen desorption was observed. Meanwhile, the boundary between the 1st and the 2nd dehydrogenation reactions also became less distinguishable. The above results clearly indicate that MWCNTs/Pt played an inhibitive role in the dehydrogenation reaction of LiAlH4, contrary to that of pure MWCNTs. The MWCNTs/Pt regained its catalytic role when its addition was increased to 20 wt%, where the onset dehydrogenation temperature of LiAlH4 dropped to 160 C. It is apparent that MWCNTs/Pt exhibited both inhibitive and catalytic effects with regard to the dehydrogenation of LiAlH4, similar to the results found with MWCNTs/Pd addition. The amount of MWCNTs/Pt addition also played a critical role in
Fig. 5 e TGA results of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/ Pt, respectively.
determining whether the MWCNTs/Pt had inhibitive or catalytic effects on LiAlH4. The isothermal TGA measurement at 140 C was also adopted to investigate the hydrogen desorption rate of LiAlH4 with and without the addition of MWCNTs/Pt. The results are shown in Fig. 6, and also summarized in Table 2. Comparing with the role of pure MWCNTs, the addition of MWCNTs/Pt had an enhanced and concentration dependent catalytic effect on the dehydrogenation of LiAlH4 at 140 C, similar to that found for the LiAlH4 admixed with MWCNTs/Pd. The hydrogen desorption rate (in the linear region) of the LiAlH4 admixed with 5 wt% MWCNTs/Pt was around 0.03 wt% min1, which was two-fold faster compared with LiAlH4 admixed with 20 wt% MWCNTs. In addition, a moderate weight gain of 0.5 wt% appeared after releasing 3.1 wt% of hydrogen in a holding time longer than 2 h. This increase in the sample weight was attributed to the adsorption of H2 by the MWCNTs, as will be discussed latter. As the amount of MWCNTs/Pt added increased from 5 wt% to 20 wt%, the hydrogen desorption rate of LiAlH4 decreased and the weight gain originating from H2 adsorption by the MWCNTs became even more significant. In the isothermal TGA, hydrogen adsorption by MWCNTs not only occurred after fully discharging from LiAlH4, but was also observed in the early stages of the isothermal TGA analysis, where the system was exposed under 0.1 Mpa H2 atmosphere. As revealed in Fig. 6, an initial weight gain of 0.2 wt% was observed for LiAlH4 admixed with 20 wt% MWCNTs/Pt, then a weight loss of 2 wt% occurred at a holding time of 4.5 h. Beyond this point, the sample gained 1 wt% of weight as the holding time was further prolonged in the next 5 h. The latter stage weight gain is also believed to be due to H2 adsorption by MWCNTs/Pt. In-situ synchrotron X-ray diffraction analysis was employed to qualitatively examine the dehydrogenation behavior of LiAlH4 admixed with 20 wt% of MWCNTs/Pt. As shown in Fig. 7(a), the XRD patterns obtained at room temperature revealed the existence of LiAlH4, Li3AlH6 and Al phases. It should be mentioned that this sample was stored at
Fig. 6 e Isothermal TGA results at 140 C of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/Pt, respectively.
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room temperature for 30 days after mixing. Since Li3AlH6 and Al are the reaction products of the 1st dehydrogenation reaction of LiAlH4, part of the LiAlH4 admixed with 20 wt% MWCNTs/Pt might have already decomposed prior to the insitu XRD analysis. This observation indicated that the 1st dehydrogenation reaction of LiAlH4 admixed with 20 wt% MWCNTs/Pt could occur even during storage at room temperature, which was catalyzed by MWCNTs/Pt, leading to its early and progressive decomposition. The peak intensity corresponding to LiAlH4 gradually decreased and disappeared when the temperature was raised to 110 C, accompanied by increasing peak intensities of Li3AlH6 and Al. Although this sample was already partially decomposed, the results suggest that a fast 1st dehydrogenation reaction started at around 110e130 C. As the temperature continued to rise to 190 C, the
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peak corresponding to Li3AlH6 started to fade while the peak intensity of Al phase continued to grow, which indicated the occurrence of the 2nd dehydrogenation reaction. When the temperature reached around 300 C, the gradual appearance of Li5PtH3 indicated the occurrence of the following reaction: 5LiH þ Pt/Li5 PtH3 þ H2
ð < 300 CÞ
(4)
This differed from reaction (3), and released additional hydrogen from the residual LiH at a lower temperature. The TGA results shown in Fig. 5 actually reveal that a higher amount of hydrogen could be desorbed from LiAlH4 (5.1 wt%) when 20 wt% MWCNTs/Pt was admixed, as compared with that of 20 wt% MWCNTs, which only released 4.6 wt% hydrogen before the start of reaction (3). The temperature-dependent X-ray diffraction peak intensities for various phases are summarized in Fig. 7(b). The results show that the temperatures where the dehydrogenation reactions commenced were lowered to 110 C for the 1st dehydrogenation reaction and 190 C for the 2nd dehydrogenation reaction. The catalytic power of MWCNTs/Pt, as demonstrated from the in-situ XRD analysis, was thus quite straight forward.
Comparison between the effects of Pt and Pd decorated MWCNTs on hydrogen absorption
Fig. 7 e (a) In-situ synchrotron XRD patterns of LiAlH4 eadmixed with 20 wt% MWCNTs/Pt heated from room temperature to 350 C; (b) variations of the highest peak intensity with temperature for the major species appearing during heating.
Previous studies [27,28] have shown that MWCNTs are a promising solid state hydrogen storage material. With the proper decoration of transition metal particles, such as Pt and Pd, the MWCNTs could absorb nearly 2.7 wt% of hydrogen under 7 MPa of hydrogen pressure at room temperature via the so-called “spill-over” mechanism. In addition to physical absorption of hydrogen, other studies indicate that carbon materials also exhibit a chemical absorption capability for hydrogen [33e35]. At high temperature conditions, the H2 molecules may permeate into a specific carbon material, dissociate into hydrogen atoms and react with dangling bonds of C atoms, forming CeH bonding [34,35]. In addition to pure CeH bonding, the hydrogen molecules may also be chemically absorbed by the metal particles decorated on the carbon material. Kim et al. used a molecular dynamic simulation to demonstrate that H2 molecules would be absorbed by the Ni particles decorating carbon nanotubes [36]. The TGA results obtained in this study indicate that MWCNTs/Pd and MWCNTs/Pt could act as hydrogen sinks during the dehydrogenation of LiAlH4. To further compare and differentiate the hydrogen absorption capabilities of MWCNTs/Pd and MWCNTs/Pt, isothermal TGA tests were employed to measure the weight gains of these two different noble metal decorated MWCNTs under hydrogen atmosphere. Fig. 8 shows the isothermal TGA results with the addition of Pt and Pd decorated MWCNTs under 1.7 Mpa of hydrogen pressure, held at 200 C for 210 min. The Pt decorated MWCNTs started to absorb hydrogen at a rate of 0.033 wt% min1 in the first 50 min. When the amount of hydrogen reached 1.5 wt%, the absorption rate slowed down, with insignificant H2 pickup thereafter. In contrast, the Pd decorated MWCNTs only absorbed 0.7 wt% of hydrogen at an initial rate of 0.016 wt% min1, and then became saturated. The TGA results clearly
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their contents in lowering the onset dehydrogenation temperature, as shown in Figs. 2 and 5, was mainly attributed to the amount of MWCNTs added as addressed previously [11]. The slower dehydrogenation rates of LiAlH4 with higher contents of MWCNTs/Pd and MWCNTs/Pt composite additions (say 20 wt%), as determined in the isothermal TGA test (Figs. 3 and 6), as compared with that of 5 wt% additions were associated with Pd and Pt facilitated “spill-over” mechanism.
Conclusions
Fig. 8 e Isothermal TGA results of Pt and Pd decorated MWCNTs under 1.7 Mpa hydrogen pressure, holding at 200 C for 210 min.
indicated that Pt decorated MWCNTs were more effective with regard to the “spill-over” mechanism and served as a greater hydrogen sink than Pd decorated MWCNTs. When mixing with LiAlH4, the Pd and Pt decorated MWCNTs not only catalyzed the dehydrogenation of LiAlH4, but also acted as a hydrogen sink that absorbed a nonnegligible amount of hydrogen released from LiAlH4, leading to a delay in the hydrogen desorption of LiAlH4. Fig. 9 is a schematic diagram that illustrates the hydrogen desorption process of LiAlH4 admixed with MWCNTs/Pt and MWCNTs/Pd. The Pt and Pd particles reinforce the catalytic power of MWCNTs with regard to accelerating the dehydrogenation reaction of LiAlH4. They can further activate the “spill-over” mechanism and assist the absorption of hydrogen released from LiAlH4 by MWCNTs, not only by physical-sorption but also by chemisorption. This fact could explain the delayed dehydrogenation reaction of LiAlH4, as revealed in the TGA results. Specifically, the increasing catalytic power of MWCNTs/Pd and MWCNTs/Pt composites with increasing
The mixing of MWCNTs with and without Pd and Pt nano particles decoration could significantly affect the dehydrogenation behavior of LiAlH4. Both catalytic and inhibitive effects of MWCNTs/Pd or MWCNTs/Pt on the dehydrogenation of LiAlH4 were observed, and these were concentration dependent. The TGA results obtained under continuous heating conditions showed that the addition of MWCNTs could lower the onset dehydrogenation temperature of LiAlH4 from 170 to 140 C. However, the onset dehydrogenation temperature was delayed to 190e200 C if MWCNTs were decorated with either Pd or Pt. This inhibitive behavior with regard to the dehydrogenation process is attributed to the absorption of hydrogen by MWCNTs, which was assisted by the “spill-over” mechanism due to Pd and Pt decoration. The role of MWCNTs/Pt as a hydrogen sink was more significant than that of MWCNTs/Pd. The isothermal TGA tests demonstrated that the catalytic power of either MWCNTs/Pd or MWCNTs/Pt was superior to that of pure MWCNTs, with the former hydrogen desorption rate of LiAlH4 being about two-fold faster than the latter. The in-situ XRD analysis further confirmed the changes of the dehydrogenation temperature with various MWCNTs-based additions. A pathway change was identified when MWCNTs/ Pt was admixed with LiAlH4, leading to the formation of Li5PtH3 and altering the sequence of H2 desorption at temperatures higher than 300 C.
Acknowledgments The authors gratefully acknowledge the financial support received for this study from the Ministry of Science and Technology (Republic of China, Taiwan), under grant 1022221-E-006-159-MY2, and the Research Center for Energy Technology and Strategy, National Cheng Kung University, under grant D100-23003.
references
Fig. 9 e Schematic diagram showing the complicated hydrogen desorption and absorption processes in the system of LiAlH4 admixed with MWCNTs/Pt and/or MWCNTs/Pd.
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Catalytic and inhibitive effects of Pd and Pt decorated MWCNTs on the dehydrogenation behavior of LiAlH4 Chia-Yen Tan a, Wen-Ta Tsai a,b,* a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan
article info
abstract
Article history:
A novel catalyst was developed to modify the dehydrogenation behavior of LiAlH4 (lithium
Received 1 May 2015
alanate). Using a chemical reduction method, Pd and Pt nano-particles were decorated on
Received in revised form
multi-walled carbon nanotubes (MWCNTs). The homogeneity and surface morphology of
8 June 2015
Pd and Pt decorated MWCNTs were examined by using transmission electron microscopy
Accepted 22 June 2015
and scanning electron microscopy. The dehydrogenation behavior of LiAlH4 admixed with
Available online 11 July 2015
Pd and Pt decorated MWCNTs was investigated by thermal gravimetric analysis (TGA) and the in-situ synchrotron X-ray diffraction (XRD) technique. The TGA results revealed that the addition of Pd or Pt decorated MWCNTs would have either catalytic or inhibitive effects on
Keywords: Lithium alanate (LiAlH4)
the dehydrogenation of LiAlH4, which were concentration dependent. The in-situ XRD
Multi-walled carbon nanotubes
analysis confirmed the catalytic power of Pd or Pt decorated MWCNTs and the corre-
(MWCNTs)
sponding dehydrogenation reaction pathway of LiAlH4. The TGA results showed that both
Dehydrogenation reaction
Pd and Pt decorated MWCNTs acted not only as a catalyst for LiAlH4, but also a hydrogen
In-situ synchrotron X-ray diffraction
absorber. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
3LiH/3Li þ 3=2H2
Introduction Lithium aluminum hydride (LiAlH4) is a promising complex metal hydride used for solid state hydrogen storage due to its high gravimetric hydrogen density. The dehydrogenation reactions and the corresponding temperatures of LiAlH4 are as follows [1e4]: 3LiAlH4 /Li3 AlH6 þ 2Al þ 3H2 Li3 AlH6 /3LiH þ Al þ 3=2H2
ð160 180 C; 5:3 wt%H2 Þ ð180 220 C; 2:6 wt%H2 Þ
(1) (2)
ðAbove 400 C; 2:6 wt%H2 Þ
(3)
The theoretical amount of hydrogen released from LiAlH4 resulting from both the first and the second dehydrogenation reactions is 7.9 wt%. Nevertheless, the dehydrogenation temperature of the LiAlH4 is still far from the target operation temperature set by the U.S. Department of Energy (DOE) [5]. Therefore, a number of studies have aimed to develop catalysts to improve the dehydrogenation properties (such as operation temperature and reaction kinetics) of LiAlH4. Some of the effective catalysts are carbon nano materials, such as carbon nano-fibers (CNFs) [6,7], single-walled carbon
* Corresponding author. 1, Ta Hsueh Road, Tainan 701, Taiwan. Tel.: þ886 62757575x62927; fax: þ886 62754395. E-mail address: [email protected] (W.-T. Tsai). http://dx.doi.org/10.1016/j.ijhydene.2015.06.106 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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nanotubes (SWCNTs) [8] and multi-walled carbon nanotubes (MWCNTs) [9e11]. These carbon nano-materials have been confirmed to possess the property of nano-confinement [12e16], have destabilization effects [17e19] and are able to serve as effective forms of transit that provide an efficient pathway for hydrogen diffusion [20]. Moreover, the electron affinity of carbon materials can affect the H-removal energy during dehydrogenation reactions [21]. Previous studies have shown that the addition of MWCNTs would modify the thermodynamic and kinetic properties of LiAlH4, lowering its dehydrogenation temperature to as low as 120 C [11]. It has been reported that MWCNTs is a promising solid state hydrogen storage material if decorated with Pt or Pd [22e28]. The Pd or Pt decorated MWCNTs can absorb up to 2.7 wt% of hydrogen at room temperature under 7 MPa of hydrogen pressure [28]. The “spill-over” effect induced by the decorated metal particles provides an efficient pathway for hydrogen molecules to dissociate and the atoms to diffuse in and out of the carbon material [22,29,30]. Since MWCNTs can serve as an effective catalyst for the dehydrogenation of LiAlH4, it is of interest to examine whether the decoration of Pt or Pd can further affect the catalytic power. A recent study proposed decorating Fe, Ni and Pd nano-particles on carbon nano-materials to produce an effective catalyst for dehydrogenation of LiAlH4 [31]. The aim of the current study is to further clarify the effects of Pd or Pt decorated MWCNTs on the dehydrogenation behavior of LiAlH4, and especially the catalytic and/or inhibitive effects. Using a chemical reduction method, both Pd and Pt particles can be successfully decorated on MWCNTs. Thermogravimetric analysis (TGA) was utilized to reveal the dehydrogenation behaviors of LiAlH4 with various additions of Pd or Pt decorated MWCNTs. In-situ XRD analysis was used to identify the reaction pathways of the dehydrogenation reactions.
methanol through a centrifugal sedimentation process and then vacuum dried for 12 h. The vacuum dried MWCNTs/Pt or MWCNTs/Pd with specific contents (0e20 wt%) was admixed with lithium alanate (LiAlH4, Chemetall, 97% purity) and sealed in a 75 ml stainless steel vessel for milling and mixing treatment. The stainless steel balls contained in this vessel had an average diameter of 4.8 mm. The ball to powder weight ratio was maintained at 10:1. The above sample loading process was carried out in a N2 purified glove box. The sealed stainless steel vessel was then cooled in liquid N2 before ball milling. The ball milling was performed in a ball milling machine (SPEX 800) at 1700 rpm for 10 min. The cooling and milling processes were repeated three times to ensure full mixing of the powders.
Thermal dehydrogenation analysis The dehydrogenation behavior of the LiAlH4 admixed with MWCNTs/Pd or MWCNTs/Pt was evaluated by thermogravimetric analysis (TGA) with a high pressure microbalance (Cahn D-110). The sample with an initial weight of ca. 200 mg was loaded in a quartz crucible and transferred into a high pressure chamber. The chamber was evacuated to 1 104 torr, followed by charging H2 gas (99.99% purity) to ambient pressure. As soon as the microbalance system stabilized, heating processes were applied. Two heating processes were adopted in this study, namely (1) continuous heating from room temperature to 350 C at a heating rate of 5 C min1, and (2) isothermal heating at 140 C for 10 h, respectively. The weight change of the sample was continuously measured during each heating process. The weight percentage of hydrogen released from each sample at the specific temperature was then calculated.
Material characterization
Experimental procedure Sample preparation The Pt and Pd particles were decorated separately on MWCNTs (purity > 95%, outer diameter 20e30 nm; length 5e15 mm, Uni Region, Bio-Tech) by a chemical reduction process. First, 0.3 g of MWCNTs was successively immersed in 0.31 M SnCl2 (Merck, >99%) and 1.4 mM PdCl2 (Seedchem, >95%) solution and subjected to ultrasonic vibration (Ultrasonic Steri-Cleaner LEO-150; 150 W, 46 kHz) for 10 and 2 min, respectively, for the sensitization and the activation processes. The activated MWCNTs were then added into 150 mg/L of H2PtCl6$6H2O (Acros Organics, 99.9%) or palladium bishexafluoracetylacetonate (Pd(hfa)2), (Strem Chemicals, 95%) solution, with methanol as the solvent. Boranedimethylamine complex (DMAB, Acros Organics, 98%) was used as a reducing agent. By slowly adding an adequate amount of DMAB into H2PtCl6$6H2O and Pd((hfa)2) solutions, the Pt4þ and Pd4þ ions were reduced to form Pt and Pd particles, respectively, and precipitated on MWCNTs. The treated Pt or Pd decorated MWCNTs (hereafter referred as MWCNTs/Pt or MWCNTs/Pd) were later cleansed by
The morphological aspects of MWCNTs/Pt, MWCNTs/Pd and LiAlH4 admixed with MWCNTs/Pt or MWCNTs/Pd were examined by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The chemical compositions of MWCNTs/Pt and MWCNTs/Pd were analyzed by an energy dispersive spectrometer (EDS). The transition of the crystal structure during the dehydrogenation reaction was identified by in-situ synchrotron X-ray diffraction (in-situ XRD) employing beamline 17A, at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. To perform in-situ XRD analysis, the sample was first loaded in a 0.5 mm diameter glass capillary tube, and then mounted to the specimen holder. N2 gas was introduced at one end of the capillary tube to form a protective atmosphere. The sample was heated from room temperature to 350 C at a heating rate of 5 C min1 with a hot air blower. During heating, the sample was repeatedly exposed to the ˚ at insynchrotron X-ray with a wavelength of 1.033105 A tervals of 240 s. The 2-D diffraction patterns were collected continuously and converted to 1-D patterns by Fit2D software [32]. The transition of the crystal structure during the heating process was thus analyzed.
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Results and discussion Material characterization Fig. 1(a) and (b) show SEM images of MWCNTs decorated with Pt and Pd, respectively. The MWCNTs were around 20e30 mm
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long, and they tended to tangle with each other after the vacuum drying process. Fig. 1(c) and (d) show the morphologies of MWCNTs/Pt and MWCNTs/Pd, as obtained by TEM. The Pt and Pd nano-particles were homogeneously distributed throughout the MWCNTs. The size of each Pt or Pd particle was in the range of 5e20 nm. However, inevitable agglomeration of Pt or Pd particles occurred during the chemical
Fig. 1 e SEM images of (a) MWCNTs/Pt and (b) MWCNTs/Pd; TEM images of (c) MWCNTs/Pt and (d) MWCNTs/Pd; EDS results for (e) MWCNTs/Pt and (f) MWCNTs/Pd. SEM images of LiAlH4 eadmixed with (g) 20 wt% MWCNTs/Pt and (h) 20 wt% MWCNTs/Pd.
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Fig. 2 e TGA results of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/ Pd, respectively.
reduction process, and some clusters grew as large as 50e100 nm. The EDS results, as revealed in Fig. 1(e) and (f), confirmed the existence of Pt and Pd particles. The Sn signal, which originated from the sensitization process, also existed in both MWCNTs/Pt and MWCNTs/Pd. The Sn ions played a key role in the chemical reduction process. It is noted that there was some Cl residue left from the H2PtCl6$6H2O in the MWCNTs/Pt after chemical reduction process. As shown in Fig. 1(e) and (f), after 30 min of ball milling, LiAlH4 was able to mix with MWCNTs/Pt and MWCNTs/Pd homogeneously, resulting in the size of the LiAlH4 particles reduced to 5e50 mm.
Effect of Pd decorated MWCNTs The dehydrogenation behaviors of LiAlH4 and those with the addition of various forms of MWCNTs were evaluated quantitatively by TGA. As revealed in Fig. 2, the weight loss of each sample was measured progressively during heating at a rate of 5 C min1, from room temperature to 350 C in 0.1 Mpa hydrogen atmosphere. Based on the TGA results, the onset dehydrogenation temperature was determined as the temperature at which a 0.25 wt% weight loss occurred to the sample being measured. The as-milled pristine LiAlH4 exhibited a two-step dehydrogenation process. The onset
dehydrogenation of LiAlH4 is approximately 170 C, in agreement with a previous report [11]. The 2nd dehydrogenation temperature was detected when the sample was continuously heated to 200 C. The total amount of hydrogen released was 6.5 wt% when the temperature was raised to 260 C, and this then remained nearly unchanged thereafter. As revealed in Fig. 2, when 20 wt% of pure MWCNTs was added, the onset dehydrogenation temperature of LiAlH4 decreased to 140 C, while the second stage hydrogenation temperature started at 190 C. The catalytic effect of MWCNTs on accelerating the dehydrogenation of LiAlH4 was thus verified. However, the addition of MWCNTs into the LiAlH4 caused a decrease in the total amount of hydrogen released to 4.6 wt%. Decorating MWCNTs with Pd particles had a significant influence on their catalytic power. As shown in Fig. 2, admixing 5 and 10 wt% of MWCNTs/Pd in LiAlH4 caused a delay of the onset dehydrogenation temperature to nearly 200 C. When these amounts of MWCNTs/Pd were added, the hydrogen desorption rate of LiAlH4 was slower compared with the as-milled pristine LiAlH4. Furthermore, the boundary between the 1st and the 2nd dehydrogenation reactions of LiAlH4 became less distinguishable. The results showed that the addition of 5 or 10 wt% of MWCNTs/Pd had an inhibitive effect on the dehydrogenation of LiAlH4, as far as the 1st step dehydrogenation temperature was concerned. It is worth noting, however, the onset dehydrogenation temperature of LiAlH4 was lowered to 140 C when the addition of MWCNTs/ Pd increased to 20 wt%, indicating a reversion from an inhibitive to catalytic role of the MWCNTs/Pd. The TGA results show that the effects of MWCNTs/Pd on the dehydrogenation of LiAlH4, either inhibitive or catalytic, were concentration dependent. The onset dehydrogenation temperature and the total amount of hydrogen desorbed from the LiAlH4 admixed with 5e20 wt% of MWCNTs/Pt and MWCNTs/Pd measured in TGA tests are summarized in Table 1. The isothermal TGA test provides a different approach to demonstrate the catalytic effects of MWCNTs/Pd on the dehydrogenation of LiAlH4. As shown in Fig. 3, the weight loss of LiAlH4 with and without MWCNTs/Pd addition was continuously measured at 140 C under 0.1 Mpa of hydrogen atmosphere. The pristine LiAlH4 did not release any hydrogen at 140 C, while a notable amount of weight loss due to hydrogen desorption was observed when MWCNTs or MWCNTs/Pd were admixed. Each isothermal TGA curve (except that for pristine LiAlH4) shown in Fig. 3 could be
Table 1 e The onset hydrogen desorption temperatures and the amounts of hydrogen released from LiAlH4, and from those admixed with various amount of MWCNTs decorated with Pd and Pt, respectively. Composition LiAlH4 LiAlH4-20 wt% MWCNTs LiAlH4-5 wt% MWCNTs/Pd LiAlH4-10 wt% MWCNTs/Pd LiAlH4-20 wt% MWCNTs/Pd LiAlH4-5 wt% MWCNTs/Pt LiAlH4-10 wt% MWCNTs/Pt LiAlH4-20 wt% MWCNTs/Pt
Onset temperature ( C)
Hydrogen released (wt%, including MWCNTs)
Hydrogen released (wt%, excluding MWCNTs)
171 148 198 194 143 187 202 156
6.2 4.6 6.2 5.8 5.1 5.6 5.5 5.1
6.2 5.7 6.5 6.4 6.4 5.9 6.1 6.4
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To further explore the dehydrogenation mechanism, in-situ XRD analysis was performed at a heating rate of 5 C min1. Fig. 4(a) shows the XRD patterns of LiAlH4 admixed with 20 wt % MWCNTs/Pd as a function of temperature. As shown in this figure, the XRD pattern of LiAlH4 admixed with 20 wt% MWCNTs/Pd consisted of only the diffraction peaks of LiAlH4 at 30 C. When the temperature was raised to 150 C, the 1st dehydrogenation reaction occurred, which was specified as reaction (1). The LiAlH4 started to decompose to form Li3AlH6 and Al with the liberation of H2. As the temperature was raised to 230 C, the diffraction peaks of Li3AlH6 diminished and those of to Al and LiH intensified, indicating the occurrence of the 2nd dehydrogenation reaction, as specified in reaction (2). The in-situ XRD results suggested that the reaction route of LiAlH4 was not altered by addition of MWCNTs/Pd. However, Fig. 3 e Isothermal TGA results at 140 C of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/Pd, respectively.
divided into three regions according to the hydrogen desorption rate (or the slope of the curve). The first region exhibited a low hydrogen desorption rate, which was followed by a fast but almost constant dehydrogenation rate in the intermediate (or the second) region. In the third region, the dehydrogenation reaction ceased and no further weight loss was detected. The maximum desorption rates determined from the second region (the linear part at each curve) are summarized in Table 2. The results show that the maximum dehydrogenation rate of LiAlH4 was enhanced when the MWCNTs were decorated with Pd. However, the beneficial effect of Pd decoration decreased as the amount of MWCNTs/Pd addition increased. More specifically, the isothermal TGA results shown in Fig. 3 indicate that MWCNTs/Pd played a role in catalyzing the dehydrogenation reaction of LiAlH4, the total amount of hydrogen and its desorption rate decreased as the amount of MWCNTs/Pd addition increased. The reason for the decreasing amount of H2 desorbed from LiAlH4 with an increasing amount of MWCNTs/Pd addition will be discussed later.
Table 2 e Isothermal TGA results showing the linear hydrogen desorption rate and the total amount of hydrogen released at 140 C for LiAlH4, and those admixed with various amount of MWCNTs decorated with Pd and Pt, respectively. Composition
LiAlH4 LiAlH4-20 wt% MWCNTs LiAlH4-5 wt% MWCNTs/Pd LiAlH4-10 wt% MWCNTs/Pd LiAlH4-20 wt% MWCNTs/Pd LiAlH4-5 wt% MWCNTs/Pt LiAlH4-10 wt% MWCNTs/Pt LiAlH4-20 wt% MWCNTs/Pt
Hydrogen Total hydrogen desorption rate released (wt%) (wt% min1, linear region) 0 0.015 0.023 0.017 0.009 0.030 0.018 0.014
0 2.62 3.66 3.44 2.28 3.10 2.87 2.00
Fig. 4 e (a) In-situ synchrotron XRD patterns of LiAlH4 eadmixed with 20 wt% MWCNTs/Pd heated from room temperature to 350 C; (b) variations of the highest peak intensity with temperature for the major species appearing during heating.
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as the temperature rose to above 330 C, some peaks corresponding to the AlePd intermetallic compound (Al21Pd8) emerged, indicating the reaction of Pd particles with Al residue from the 2nd dehydrogenation reaction. In addition, two diffraction peaks appeared at the 2-theta values of 23.11 and 33.35 after the 2nd dehydrogenation reaction at 230 C. However, these two peaks could not be identified based on the diffraction data of known inorganic compounds. The temperature dependent X-ray diffraction peak intensity for various identifiable phases are summarized in Fig. 4(b). As shown in this figure, the Li3AlH6 phase emerged at 150 C and diminished at 230 C, indicating that admixing 20 wt% of MWCNTs/Pd would lower the onset dehydrogenation temperature of LiAlH4 to around 150 C without altering its twostep reaction pathway. Generally, the in-situ XRD analysis did verify the catalytic power of MWCNTs/Pd on LiAlH4.
Effect of Pt decorated MWCNTs The Pt decorated MWCNTs also exhibited either catalytic or inhibitive effects on the dehydrogenation properties of LiAlH4, depending on the amount added. As shown in Fig. 5, with 5 wt % of MWCNTs/Pt addition, the onset dehydrogenation temperature of LiAlH4 rose to 190 C. The hydrogen desorption rate was slower than that of the pristine LiAlH4. As the addition of MWCNTs/Pt increased to 10 wt%, a further delay of hydrogen desorption was observed. Meanwhile, the boundary between the 1st and the 2nd dehydrogenation reactions also became less distinguishable. The above results clearly indicate that MWCNTs/Pt played an inhibitive role in the dehydrogenation reaction of LiAlH4, contrary to that of pure MWCNTs. The MWCNTs/Pt regained its catalytic role when its addition was increased to 20 wt%, where the onset dehydrogenation temperature of LiAlH4 dropped to 160 C. It is apparent that MWCNTs/Pt exhibited both inhibitive and catalytic effects with regard to the dehydrogenation of LiAlH4, similar to the results found with MWCNTs/Pd addition. The amount of MWCNTs/Pt addition also played a critical role in
Fig. 5 e TGA results of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/ Pt, respectively.
determining whether the MWCNTs/Pt had inhibitive or catalytic effects on LiAlH4. The isothermal TGA measurement at 140 C was also adopted to investigate the hydrogen desorption rate of LiAlH4 with and without the addition of MWCNTs/Pt. The results are shown in Fig. 6, and also summarized in Table 2. Comparing with the role of pure MWCNTs, the addition of MWCNTs/Pt had an enhanced and concentration dependent catalytic effect on the dehydrogenation of LiAlH4 at 140 C, similar to that found for the LiAlH4 admixed with MWCNTs/Pd. The hydrogen desorption rate (in the linear region) of the LiAlH4 admixed with 5 wt% MWCNTs/Pt was around 0.03 wt% min1, which was two-fold faster compared with LiAlH4 admixed with 20 wt% MWCNTs. In addition, a moderate weight gain of 0.5 wt% appeared after releasing 3.1 wt% of hydrogen in a holding time longer than 2 h. This increase in the sample weight was attributed to the adsorption of H2 by the MWCNTs, as will be discussed latter. As the amount of MWCNTs/Pt added increased from 5 wt% to 20 wt%, the hydrogen desorption rate of LiAlH4 decreased and the weight gain originating from H2 adsorption by the MWCNTs became even more significant. In the isothermal TGA, hydrogen adsorption by MWCNTs not only occurred after fully discharging from LiAlH4, but was also observed in the early stages of the isothermal TGA analysis, where the system was exposed under 0.1 Mpa H2 atmosphere. As revealed in Fig. 6, an initial weight gain of 0.2 wt% was observed for LiAlH4 admixed with 20 wt% MWCNTs/Pt, then a weight loss of 2 wt% occurred at a holding time of 4.5 h. Beyond this point, the sample gained 1 wt% of weight as the holding time was further prolonged in the next 5 h. The latter stage weight gain is also believed to be due to H2 adsorption by MWCNTs/Pt. In-situ synchrotron X-ray diffraction analysis was employed to qualitatively examine the dehydrogenation behavior of LiAlH4 admixed with 20 wt% of MWCNTs/Pt. As shown in Fig. 7(a), the XRD patterns obtained at room temperature revealed the existence of LiAlH4, Li3AlH6 and Al phases. It should be mentioned that this sample was stored at
Fig. 6 e Isothermal TGA results at 140 C of the as-milled LiAlH4, and those admixed with 20 wt% MWCNTs and 5e20 wt% MWCNTs/Pt, respectively.
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room temperature for 30 days after mixing. Since Li3AlH6 and Al are the reaction products of the 1st dehydrogenation reaction of LiAlH4, part of the LiAlH4 admixed with 20 wt% MWCNTs/Pt might have already decomposed prior to the insitu XRD analysis. This observation indicated that the 1st dehydrogenation reaction of LiAlH4 admixed with 20 wt% MWCNTs/Pt could occur even during storage at room temperature, which was catalyzed by MWCNTs/Pt, leading to its early and progressive decomposition. The peak intensity corresponding to LiAlH4 gradually decreased and disappeared when the temperature was raised to 110 C, accompanied by increasing peak intensities of Li3AlH6 and Al. Although this sample was already partially decomposed, the results suggest that a fast 1st dehydrogenation reaction started at around 110e130 C. As the temperature continued to rise to 190 C, the
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peak corresponding to Li3AlH6 started to fade while the peak intensity of Al phase continued to grow, which indicated the occurrence of the 2nd dehydrogenation reaction. When the temperature reached around 300 C, the gradual appearance of Li5PtH3 indicated the occurrence of the following reaction: 5LiH þ Pt/Li5 PtH3 þ H2
ð < 300 CÞ
(4)
This differed from reaction (3), and released additional hydrogen from the residual LiH at a lower temperature. The TGA results shown in Fig. 5 actually reveal that a higher amount of hydrogen could be desorbed from LiAlH4 (5.1 wt%) when 20 wt% MWCNTs/Pt was admixed, as compared with that of 20 wt% MWCNTs, which only released 4.6 wt% hydrogen before the start of reaction (3). The temperature-dependent X-ray diffraction peak intensities for various phases are summarized in Fig. 7(b). The results show that the temperatures where the dehydrogenation reactions commenced were lowered to 110 C for the 1st dehydrogenation reaction and 190 C for the 2nd dehydrogenation reaction. The catalytic power of MWCNTs/Pt, as demonstrated from the in-situ XRD analysis, was thus quite straight forward.
Comparison between the effects of Pt and Pd decorated MWCNTs on hydrogen absorption
Fig. 7 e (a) In-situ synchrotron XRD patterns of LiAlH4 eadmixed with 20 wt% MWCNTs/Pt heated from room temperature to 350 C; (b) variations of the highest peak intensity with temperature for the major species appearing during heating.
Previous studies [27,28] have shown that MWCNTs are a promising solid state hydrogen storage material. With the proper decoration of transition metal particles, such as Pt and Pd, the MWCNTs could absorb nearly 2.7 wt% of hydrogen under 7 MPa of hydrogen pressure at room temperature via the so-called “spill-over” mechanism. In addition to physical absorption of hydrogen, other studies indicate that carbon materials also exhibit a chemical absorption capability for hydrogen [33e35]. At high temperature conditions, the H2 molecules may permeate into a specific carbon material, dissociate into hydrogen atoms and react with dangling bonds of C atoms, forming CeH bonding [34,35]. In addition to pure CeH bonding, the hydrogen molecules may also be chemically absorbed by the metal particles decorated on the carbon material. Kim et al. used a molecular dynamic simulation to demonstrate that H2 molecules would be absorbed by the Ni particles decorating carbon nanotubes [36]. The TGA results obtained in this study indicate that MWCNTs/Pd and MWCNTs/Pt could act as hydrogen sinks during the dehydrogenation of LiAlH4. To further compare and differentiate the hydrogen absorption capabilities of MWCNTs/Pd and MWCNTs/Pt, isothermal TGA tests were employed to measure the weight gains of these two different noble metal decorated MWCNTs under hydrogen atmosphere. Fig. 8 shows the isothermal TGA results with the addition of Pt and Pd decorated MWCNTs under 1.7 Mpa of hydrogen pressure, held at 200 C for 210 min. The Pt decorated MWCNTs started to absorb hydrogen at a rate of 0.033 wt% min1 in the first 50 min. When the amount of hydrogen reached 1.5 wt%, the absorption rate slowed down, with insignificant H2 pickup thereafter. In contrast, the Pd decorated MWCNTs only absorbed 0.7 wt% of hydrogen at an initial rate of 0.016 wt% min1, and then became saturated. The TGA results clearly
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their contents in lowering the onset dehydrogenation temperature, as shown in Figs. 2 and 5, was mainly attributed to the amount of MWCNTs added as addressed previously [11]. The slower dehydrogenation rates of LiAlH4 with higher contents of MWCNTs/Pd and MWCNTs/Pt composite additions (say 20 wt%), as determined in the isothermal TGA test (Figs. 3 and 6), as compared with that of 5 wt% additions were associated with Pd and Pt facilitated “spill-over” mechanism.
Conclusions
Fig. 8 e Isothermal TGA results of Pt and Pd decorated MWCNTs under 1.7 Mpa hydrogen pressure, holding at 200 C for 210 min.
indicated that Pt decorated MWCNTs were more effective with regard to the “spill-over” mechanism and served as a greater hydrogen sink than Pd decorated MWCNTs. When mixing with LiAlH4, the Pd and Pt decorated MWCNTs not only catalyzed the dehydrogenation of LiAlH4, but also acted as a hydrogen sink that absorbed a nonnegligible amount of hydrogen released from LiAlH4, leading to a delay in the hydrogen desorption of LiAlH4. Fig. 9 is a schematic diagram that illustrates the hydrogen desorption process of LiAlH4 admixed with MWCNTs/Pt and MWCNTs/Pd. The Pt and Pd particles reinforce the catalytic power of MWCNTs with regard to accelerating the dehydrogenation reaction of LiAlH4. They can further activate the “spill-over” mechanism and assist the absorption of hydrogen released from LiAlH4 by MWCNTs, not only by physical-sorption but also by chemisorption. This fact could explain the delayed dehydrogenation reaction of LiAlH4, as revealed in the TGA results. Specifically, the increasing catalytic power of MWCNTs/Pd and MWCNTs/Pt composites with increasing
The mixing of MWCNTs with and without Pd and Pt nano particles decoration could significantly affect the dehydrogenation behavior of LiAlH4. Both catalytic and inhibitive effects of MWCNTs/Pd or MWCNTs/Pt on the dehydrogenation of LiAlH4 were observed, and these were concentration dependent. The TGA results obtained under continuous heating conditions showed that the addition of MWCNTs could lower the onset dehydrogenation temperature of LiAlH4 from 170 to 140 C. However, the onset dehydrogenation temperature was delayed to 190e200 C if MWCNTs were decorated with either Pd or Pt. This inhibitive behavior with regard to the dehydrogenation process is attributed to the absorption of hydrogen by MWCNTs, which was assisted by the “spill-over” mechanism due to Pd and Pt decoration. The role of MWCNTs/Pt as a hydrogen sink was more significant than that of MWCNTs/Pd. The isothermal TGA tests demonstrated that the catalytic power of either MWCNTs/Pd or MWCNTs/Pt was superior to that of pure MWCNTs, with the former hydrogen desorption rate of LiAlH4 being about two-fold faster than the latter. The in-situ XRD analysis further confirmed the changes of the dehydrogenation temperature with various MWCNTs-based additions. A pathway change was identified when MWCNTs/ Pt was admixed with LiAlH4, leading to the formation of Li5PtH3 and altering the sequence of H2 desorption at temperatures higher than 300 C.
Acknowledgments The authors gratefully acknowledge the financial support received for this study from the Ministry of Science and Technology (Republic of China, Taiwan), under grant 1022221-E-006-159-MY2, and the Research Center for Energy Technology and Strategy, National Cheng Kung University, under grant D100-23003.
references
Fig. 9 e Schematic diagram showing the complicated hydrogen desorption and absorption processes in the system of LiAlH4 admixed with MWCNTs/Pt and/or MWCNTs/Pd.
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