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Cite this: DOI: 10.1039/c4nr03357d
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Metal/graphene nanocomposites synthesized with the aid of supercritical fluid for promoting hydrogen release from complex hydrides† De-Hao Jiang, Cheng-Hsien Yang, Chuan-Ming Tseng, Sheng-Long Lee and Jeng-Kuei Chang* With the aid of supercritical CO2, Fe-, Ni-, Pd-, and Au-nanoparticle-decorated nanostructured carbon materials (graphene, activated carbon, carbon black, and carbon nanotubes) are synthesized for catalyzing the dehydrogenation of LiAlH4. The effects of the metal nanoparticle size and distribution, and the type of carbon structure on the hydrogen release properties are investigated. The Fe/graphene nanocomposite,
Received 17th June 2014, Accepted 4th August 2014 DOI: 10.1039/c4nr03357d www.rsc.org/nanoscale
1.
which consists of ∼2 nm Fe particles highly dispersed on graphene nanosheets, exhibits the highest catalytic performance. With this nanocomposite, the initial dehydrogenation temperature can be lowered (from ∼135 °C for pristine LiAlH4) to ∼40 °C without altering the reaction route (confirmed by in situ X-ray diffraction), and 4.5 wt% H2 can be released at 100 °C within 6 min, which is faster by more than 135-fold than the time required to release the same amount of H2 from pristine LiAlH4.
Introduction
Ideal energy sources are clean, affordable, and renewable. The move from coal (C) to oil (–CH2–) and natural gas (CH4), which can produce more heat with less pollution, was prompted by the advantages of using hydrogen-rich fuels.1 Hydrogen is the ultimate energy carrier that can reduce our dependence on fossil fuels and eliminate carbon dioxide emissions.2,3 To bridge hydrogen production with utilization, hydrogen storage is required.4 Finding an efficient storage medium is currently the key challenge for the use of hydrogen in both mobile and stationary applications. For solid-state hydrogen storage, complex metal hydrides, consisting of alkali/alkali earth metal cations and complex anions (including [AlH4]−, [BH4]−, and [NH2]−), are promising materials due to their high gravimetric hydrogen densities.5,6 However, further improvement in the hydrogen adsorption/desorption properties (such as operation temperature, reactions kinetics, and reversibility) of complex hydrides is necessary for their practical application. Halogen-,7–9 oxide-,10–12 and metal-based12–16 catalysts have been used to overcome the sluggish hydrogen release kinetics and relatively high dehydrogenation temperature of LiAlH4, which has a high hydrogen content (theoretically, 10.5 wt% and 96.3 g H2 L−1). Although positive results have been
Institute of Materials Science and Engineering, National Central University, 300 Jhongda Road, Taoyuan, 32001, Taiwan. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr03357d
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reported, more effective catalysis at a lower cost is desirable. A literature review indicates that a species with a given chemical composition can show significant differences in catalytic performance depending on its size.10,13,14,17 For instance, the dehydrogenation temperature of LiAlH4 was lowered by 25 and 40 °C, respectively, when 5 μm and 80 nm diameter Cr2O3 was incorporated.10 In addition, 100 nm Ni decreased the LiAlH4 dehydrogenation temperature by 55 °C,14 much better than the 10 °C reduction obtained with 100 μm Ni.13 Although catalyst dimensions seem to play a substantial role, there is a lack of systematic investigation. The present study synthesizes catalysts even smaller than those reported in the literature in order to improve their catalytic capability; moreover, the size effects were further explored. Another attractive category of additives for complex hydrides is nanostructured carbon materials (including C60, carbon nanotubes (CNTs), and nanoporous carbon), which have been confirmed to possess destabilization,18,19 nanoconfinement,20,21 and catalysis effects,22,23 improving the hydrogen release properties. Graphene nanosheets (GNSs), characterized by a two-dimensional (2D) honeycomb lattice structure, large surface area, high conductivity, excellent stability, and high strength,24,25 are a novel type of dehydrogenation promoter for complex hydrides.26–28 A recent study indicated that GNSs can effectively lower the temperature and increase the rate of LiAlH4 dehydrogenation.28 The nanostructured carbon materials mentioned beforehand can also support other types of catalyst, preventing their aggregation and increasing utilization; as a result, synergistic catalytic effects can be obtained.
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If this idea is valid, the dehydrogenation performance of complex hydrides can be further enhanced. Nevertheless, this approach has never been attempted for LiAlH4. In the present study, nano-sized Fe, Ni, Pd, and Au particles were synthesized on various carbon supports (GNSs, activated carbon (AC), carbon black (CB), and multi-walled CNTs) using a supercritical CO2 (SCCO2)-assisted deposition technique. SCCO2, which has gas-like diffusivity, near-zero surface tension, and excellent mass-transfer properties, is ideal for uniformly dispersing nanoparticles onto high-surface-area supporting materials.29–31 The metal/carbon composites were prepared using both a conventional chemical deposition method and a physical mixing method for comparison. To the best of our knowledge, this is the first work reporting the fabrication of dehydrogenation catalysts using SCCO2, to develop low cost catalysts that exploit size effects, and to explore how the type of carbon support affects catalytic performance. The experimental results indicate that the Fe/GNS nanocomposite synthesized using SCCO2 is a promising catalyst for promoting the dehydrogenation of LiAlH4. With Fe/GNS incorporation, the hydrogen release rate can be increased by ∼135-fold at 100 °C.
2. Experimental procedure 2.1.
Materials and preparation
LiAlH4 powder (97%, ∼50 μm) was purchased from Alfa Aesar. Fe powder (Alfa Aesar; 99%, ∼50 μm), Ni powder (Alfa Aesar; 99.8%, ∼50 μm), Pd powder (Alfa Aesar; 99.95%, ∼50 μm), Au powder (Alfa Aesar; 99.9%, ∼50 μm), AC (Aldrich; 98%), CB (Nippon Shiyaku Kogyo; 99.9%), CNTs (Nanostructured & Amorphous Materials; 99%), TiO2 (Aldrich; 99%), and VCl3 (Aldrich; 99.999%) were used as received. GNSs were prepared using a modified Staudenmaier method.32,33 Natural graphite powder (Alfa Aesar; particle size: ∼70 μm; purity: 99.999%) was chemically oxidized to form graphite oxide (GO) at room temperature. The graphite (5 g) was continuously stirred in a mixed solution of sulfuric acid (100 mL), nitric acid (50 mL), and potassium chlorate (50 g) for approximately 100 h. The resulting GO was rinsed with a 5 wt% aqueous solution of HCl and then repeatedly washed with deionized water until the pH of the filtrate was neutral. The product was dried in air and pulverized. Finally, the GO was exfoliated by rapid heating (∼30 °C min−1) to 1050 °C under an inert Ar atmosphere. After thermal reduction (held at 1050 °C for 30 min), the GNSs were obtained. To prepare metal (Fe, Ni, Pd, Au)/carbon (GNS, CNT, CB, AC) composites, 40 mg of a given carbon material together with the required amount of iron(III) acetylacetonate (Aldrich; 99.9%), nickel(II) acetylacetonate (Aldrich; 99.9%), palladium(II) hexafluoroacetylacetonate (Aldrich; 99.9%), or gold(III) chloride (Aldrich; 99.9%) were loaded into a stainless steel autoclave (500 mL). Methanol (50 mL, TEDIA; >99.9%) was added as the solvent, which is miscible with SCCO2. Dimethyl amineborane (TCI; >95%) was used as the reducing agent. The
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autoclave was pressurized with CO2 up to 10 MPa at 50 °C, at which a supercritical state was achieved. The system was stirred vigorously for 2 h before the vessel was depressurized. The resulting metal/carbon composites were repeatedly washed with deionized water and methanol, and then collected via centrifugation. For comparison, the composites were also synthesized in the absence of SCCO2, with all other preparation parameters identical to those described above. A ball-milling machine (SPEX 8000D) was used to mix LiAlH4 with the additives (2.5 wt%); the ball-to-powder weight ratio was 10 : 1. The mixtures were weighed and handled in an Ar-purified glove box (Innovation Technology Co.), where both the moisture content and oxygen content were maintained below 0.5 ppm. After being sealed in the glove box, the ballmilling vessel was transferred to the milling machine. The milling time was 30 min. 2.2. Material characterization and evaluation of dehydrogenation performance The microstructures of the samples were examined using a transmission electron microscope (TEM, JOEL 2100F) operated at 200 kV. The EELS and elemental mapping were performed using a Gatan energy filter (Tridem 863) with an energy resolution of 1.05 eV. The weight ratios of the metals in various samples were quantified using an atomic absorption spectrometer (AAS, SOLAAR M6). A Raman spectrometer (UniRAM MicroRaman) was employed to study the bonding structure of [AlH4]−; the spectra were excited using a diode-pump solidstate laser with a wavelength of 532 nm. A temperature-programmed desorption (TPD) analyzer was used to evaluate the hydrogen desorption properties of the samples. The analysis was conducted using an argon gas flow rate of 90 mL min−1 under ambient pressure. The temperature was programmed from room temperature to 250 °C at a heating rate of 2 °C min−1. The gas thermal conductivity change, corresponding to hydrogen release, was recorded as a function of temperature. In addition, hydrogen desorption profiles at 100 °C were measured using the TPD analyzer to study the dehydrogenation kinetics. The composition of the released gas was checked using an Agilent 5975 gas chromatograph/ mass selective detector (GC/MSD). The total amount of hydrogen desorbed was calibrated using Mg2NiH4, which has a known hydrogen capacity of 3.6 wt% (confirmed by a Sievert’s measurement). In situ X-ray diffraction (XRD) analyses were performed with the aid of synchrotron radiation (beamline 01C2 in National Synchrotron Radiation Research Center, Taiwan) to explore the dehydrogenation mechanism. In the experiment, the sample powder was loaded into a glass capillary tube (1 mm in diameter), which was mounted on the specimen holder. Inert N2 gas was introduced into the tube to prevent oxidation. Then, the sample was heated from room temperature to 250 °C at a rate of 5 °C min−1. The 2D diffraction patterns were collected by a Mar345 image plate detector. The 2D data were then converted into one-dimensional diffraction patterns using Fit2D software.
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3. Results and discussion
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3.1. The effect of catalyst size on the dehydrogenation properties Fig. 1(a) and (b) show the TEM bright-field images of the Fe/ GNS composites (with a Fe/carbon weight ratio of 1 : 9) synthesized with and without the aid of SCCO2. The conventional chemical reduction method (no SCCO2) produced aggregated Fe clusters with dimensions up to the sub-micron scale (the sample is denoted as Fe/GNS-air). Moreover, since the plating solution had a high surface tension and the carbon surface was hydrophobic (and may have uneven surface conditions in various portions), the deposited Fe was non-uniform on the GNSs. In contrast, the size of the Fe particles decreased considerably to ∼2 nm in diameter when SCCO2 was used. In addition, the nanoparticles were highly dispersed on the GNSs. This can be attributed to the extremely low viscosity and excellent penetration ability of SCCO2,29,30 which can help debundle the GNSs and effectively transport the precursors throughout the sample.34,35 Fig. 1(c) shows the electron energy
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loss spectroscopy (EELS) spectrum of a deposited particle in Fig. 1(b). The Fe L-edge and C K-edge (results from the underlying carbon support) signals can be clearly recognized. The small O K-edge peak was mainly attributed to the GNS (although partial oxidation of Fe upon exposure to air cannot be excluded), since the plain GNS region showed a similar signal. This is associated with the residual oxygen surface functional groups on the GNSs that were not completely removed during the thermal reduction process. The EELS Fe Ledge mapping over the SCCO2-synthesized composite is shown in Fig. 1(d), confirming that the well-distributed deposits on the GNS are Fe nanoparticles. The Ni/GNS composites prepared with and without SCCO2 are shown in Fig. S1 (see ESI†). Similar to the results for Fe, the latter sample showed a smaller particle size and much better dispersion of Ni. The results clearly demonstrate the advantages of using SCCO2 to fabricate highly uniform nanocomposites. A TPD analyzer was used to evaluate the hydrogen desorption properties of the samples. Fig. 2 shows the TPD profiles of LiAlH4 with and without 2.5 wt% of various additives
Fig. 1 TEM bright-field images of the Fe/GNS composites synthesized (a) without and (b) with SCCO2. (c) EELS spectrum of a deposited particle in (b). (d) EELS Fe L-edge mapping over Fe/GNS composite synthesized using SCCO2.
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Fig. 2 TPD signals of LiAlH4 with and without 2.5 wt% of various additives.
(mixed by ball milling). The pristine LiAlH4 (subjected to the same ball milling process but without an additive) exhibited two dehydrogenation signals, which were initiated at approximately 135 and 175 °C, in the plot. There is another desorption reaction at ∼480 °C; however, it is relatively difficult to be utilized and thus not considered in this study. As shown, the addition of plain GNSs and commercial Fe powder (50 μm in diameter) have an insignificant influence on the TPD curves. With the incorporation of Fe/GNS-air, the initial desorption temperature was significantly lowered to ∼60 °C. The most effective additive was found to be Fe/GNS synthesized using SCCO2; the initial dehydrogenation occurred at temperatures as low as 40 °C under ambient pressure. The well-dispersed nano-sized Fe fabricated via SCCO2 maximizes the interaction with [AlH4]−, leading to a superior catalytic effect. LiAlH4 was also milled with a combination of the commercial Fe powder and GNSs (in a weight ratio of 1 : 9 to mimic the composition of the Fe/GNS composites); the data (curve iv) in Fig. 2 excludes the possible contribution of the Fe/C reaction product (which may be formed during the ball milling process) to the improvement in LiAlH4 dehydrogenation performance. The hydrogen desorption properties of LiAlH4 incorporated with a mixture of Ni powder (∼50 μm) and GNSs, and the Ni/GNS composites synthesized with and without SCCO2 are shown in Fig. S2;† the size effects of the catalytic particles were again confirmed. The experimental results also reveal that the SCCO2-based technique is an effective method for synthesizing catalysts with improved properties. To our knowledge, the 2 nm diameter (for the SCCO2-derived Fe) is the smallest size (and corresponding best performance) ever reported for LiAlH4 dehydrogenation catalysts. 3.2. The effect of metal particle type on the dehydrogenation properties Pd and Au particles were also synthesized on GNSs using an analogous SCCO2-assisted synthesis. As shown in the TEM
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Fig. 3 TPD signals of LiAlH4 incorporated with 2.5 wt% of GNSs and various metal/GNS nanocomposites.
micrographs in Fig. S3,† similar to the cases for Fe and Ni, nano-sized Pd and Au are uniformly distributed on the GNSs. This indicates that the SCCO2-based technique is generally applicable for the preparation of various nanoparticles. The weight ratios of Fe, Ni, Pd, and Au to carbon were fixed at 10 (±2) wt% to enable performance comparisons between the various metals. Fig. 3 compares the TPD data of LiAlH4 incorporated with 2.5 wt% of various metal/GNS nanocomposites. As shown, Au/GNS has similar performance to that of plain GNSs, indicating that the Au particles do not really promote hydrogen release. The ability to lower the dehydrogenation temperature increased in the sequence of Au, Pd, Ni, and Fe. It is noted that the noble elements (Au and Pd) are less effective than the active elements (Ni and Fe), which could donate electrons to [AlH4]−,36 making this complex anion unstable and thus capable of easily releasing hydrogen. The fewer nuclear charges of Fe (and thus lower attraction for valence electrons) compared to those of Ni and the high energy of its sixth electron in the 3d orbital (favoring its removal) at least partially explain the better performance for Fe compared to that of Ni. Varin et al. suggested that Fe and Ni are able to dissolve into the [AlH4]− lattice, causing lattice expansion (i.e., structural instability) and thus facilitating dehydrogenation.17 The larger ionic radius of Fe (77 pm) than that of Ni (69 pm), compared to 53 pm for Al, is another reason for the superior catalytic effects of Fe. Fig. S4† shows the TPD data of LiAlH4 incorporated with micron-scale powders of Fe, Ni, Pd, and Au. Of note, the four additives exhibited similar but insignificant dehydrogenation promotion capabilities. The results indicate that the effects of metal type on hydrogen release are more pronounced with a decrease in particle size. With the aid of SCCO2, inexpensive Fe nanoparticles can be synthesized as an excellent dehydrogenation catalyst.
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3.3. The effect of the carbon support type on the dehydrogenation properties To clarify the effect of the type of carbon support, Fe nanoparticles were deposited via SCCO2 on AC, CB, and CNTs (besides GNSs). The TEM micrographs of the composites obtained are shown in Fig. 4. The Fe to carbon ratios were controlled at 10 (±2) wt% for all the samples. When compared to Fe/GNS (in Fig. 1(b)), a similar size (∼2 nm) and uniform distribution of Fe particles was found on the other carbon supports. Fig. 5 shows the TPD data for LiAlH4 with 2.5 wt% of various Fe/carbon composites. As clearly shown, the type of carbon support underlying the Fe particles affects the catalytic performance. The TPD peaks shifted towards lower temperatures in the sequence of Fe/AC-, Fe/CB-, Fe/CNT-, and Fe/GNSincorporated LiAlH4 samples. Although AC has a higher surface area (∼2500 m2 g−1) than those of CB and CNT (both ∼200 m2 g−1), a large fraction of the AC area is attributed to its internal cavities (within the particles). The pore size of AC is shown in Fig. S5,† which indicates that the distribution is mainly in the range of 1.5–4 nm. The Fe deposited inside these cavities can hardly contact LiAlH4 (∼50 μm in diameter), resulting in the poorer catalytic activity of Fe/AC when compared to those for Fe/CB and Fe/CNT, whose Fe nanoparticles are on the open surface of the carbon support. The SEM micro-
Fig. 4
Fig. 5 TPD signals of LiAlH4 with and without 2.5 wt% of various Fe/ carbon composite additives.
graphs shown in Fig. S6† clearly reveal that the bulky AC particles have the least exposed area (to interact with LiAlH4) among the four kinds of carbon support. It was found that the Fe/GNS composite had the best capability to lower the
TEM micrographs of (a) Fe/AC, (b) Fe/CB, and (c) Fe/CNT composites synthesized using SCCO2.
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dehydrogenation temperature. GNSs, with a large accessible area of ∼600 m2 g−1, provides a desirable platform for loading Fe nanoparticles and for catalyzing the dehydrogenation of LiAlH4. It was confirmed that the GNSs are effective grinding agents that enhance the mixing of catalysts and reduce the particle and crystal size of the hydride.28 The high thermal conductivity of GNSs also explains their superior performance, as the dehydrogenation reaction requires heat transport.37 Moreover, the GNSs have a long-range-ordered sp2 structure and delocalized π bonds, which helps promote the electronic interactions between Fe and [AlH4]−, facilitating hydrogen release from the Al–H bonds. This is the first study to verify that GNSs are a superior type of carbon support for dehydrogenation catalysts. The proposed Fe/GNS remarkably outperforms Ni and Y/CNT,14 TiO2/carbon aerogel,38 and TiCl3/carbon aerogel38 composite catalysts for LiAlH4 previously reported in the literature. 3.4. The dehydrogenation mechanism of LiAlH4 with Fe/GNS nanocomposites To explore the dehydrogenation mechanism, in situ XRD analyses were performed. Fig. 6(a) shows the obtained diffraction patterns of pristine LiAlH4 as a function of temperature. As shown, the LiAlH4 begins to decompose and transforms into Li3AlH6 and Al at 150 °C. The following dehydrogenation reaction occurs: 3LiAlH4 ! Li3 AlH6 þ 2Al þ 3H2
ð1Þ
At 210 °C, the intensity of Li3AlH6 diminishes and a new LiH phase forms, indicating a second hydrogen release, as shown in the reaction below: Li3 AlH6 ! 3LiH þ Al þ 1:5H2
ð2Þ
The reaction temperatures are slightly higher than those found in the TPD data since a higher temperature increase rate (5 °C min−1 vs. 2 °C min−1) was used in the in situ XRD analyses. Fig. 6(b) shows the in situ XRD data for Fe/GNS-incorporated LiAlH4. The observed transition temperatures for the above two reactions significantly decreased to 60 and 150 °C, respectively, confirming that hydrogen release was indeed facilitated. According to the literature, additive-induced promotion of dehydrogenation can be attributed to two possible mechanisms, namely catalysis22,23,39 and destabilization.18,19,40 The latter effect is associated with the formation of intermediate species during dehydrogenation, decreasing reaction enthalpy and thus thermodynamically promoting hydrogen release. As shown in Fig. 6(b), there were no Fe- or Ccontaining compounds after ball milling and throughout the dehydrogenation processes, indicating that the reaction routes were not altered. Accordingly, the catalysis effect seems to be the dominant mechanism for Fe/GNS promoted hydrogen release. The influence of the added Fe/GNS on the bonding structure of LiAlH4 was examined using Raman spectrometry; the obtained data are shown in Fig. 7. As demonstrated, pristine
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Fig. 6 In situ synchrotron XRD patterns of (a) pristine LiAlH4 and (b) 2.5 wt% Fe/GNS-incorporated LiAlH4 recorded as a function of temperature.
Fig. 7 Raman spectra of pristine LiAlH4 and 2.5 wt% Fe/GNS-incorporated LiAlH4.
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proposed dehydrogenation catalyst for complex hydrides. Of note, gas analysis confirmed that the product released from the Fe/GNS-catalyzed LiAlH4 was hydrogen without any detectable impurities. The aforementioned results indicate that the waste heat of PEMFCs (∼100 °C) was able to make the proposed Fe/GNS-catalyzed LiAlH4 readily release hydrogen (4.5 wt % in 6 min), which is of high purity and thus can be directly introduced into the fuel cells.
4.
Fig. 8 Hydrogen desorption profiles (at 100 °C) of LiAlH4 with and without various additives.
LiAlH4 displayed two Raman shift signals, at frequencies of 1755 cm−1 and 1830 cm−1, respectively, attributed to the Al–H stretching vibrations.41 However, with the incorporation of 2.5 wt% Fe/GNS, the signals disappeared. Presumably, the Fe/ GNS composite donated electrons to [AlH4]−, leading to distortion of the covalent Al–H bonds. This explains the improved dehydrogenation performance of Fe/GNS-catalyzed LiAlH4. 3.5.
Hydrogen desorption performance at 100 °C
Hydrogen discharge kinetics at accessible temperatures is a crucial concern for practical applications. Fig. 8 shows the hydrogen desorption profiles of various samples at 100 °C, which is close to the operation temperature of proton exchange membrane fuel cells (PEMFCs). All the samples eventually released ∼5.3 wt% of hydrogen (normalized to the weight of LiAlH4), suggesting that the catalysts did not change the quantity of hydrogen released, with only the reaction in eqn (1) triggered at such temperature. Nevertheless, this hydrogen density is already sufficient for many applications.42 As shown, pristine LiAlH4 showed sluggish dehydrogenation behavior; it took ∼12 h to achieve 85% of the hydrogen release (∼4.5 wt%); this sample took ∼21 h to completely dehydrogenate (5.3 wt%). With the incorporation of 2.5 wt% of Fe/GNS nanocomposite, less than 0.3 h was needed to reach the same quantity of hydrogen desorption (4.5 wt%). When Fe/GNS addition was increased to 10 wt%, only 6 min was required, which is shorter than the time required with pristine LiAlH4 by more than 135fold. The catalytic properties of Fe/GNS were compared to those of the two best catalysts, namely VCl3 and TiO2, reported in the literature;11,12,43 the results are also shown in Fig. 8. As demonstrated, the hydrogen release rate for the Fe/GNS-catalyzed LiAlH4 was significantly faster than those obtained with the other two additives. This verifies the effectiveness of the
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Conclusion
The size and distribution of metal particles significantly affect the capability for promoting the dehydrogenation of LiAlH4. With the aid of SCCO2, nano-sized metals can be highly dispersed on GNSs. The catalytic properties of the resulting nanocomposites were significantly superior to those of their counterparts synthesized via a conventional chemical deposition method (without SCCO2). Using the SCCO2 synthetic technique, low cost Fe can be made to exhibit better catalytic performance than that of Ni, Pd, and Au, while these four kinds of powder on a micron scale show similar (but insignificant) capability. Owing to their high accessible area (thus high number of catalytic sites), high mechanical strength (which improves mixing and refining), high thermal conductivity (which facilitates heat transport during dehydrogenation), and high electronic conductivity (which promotes electron exchange between Fe and [AlH4]−), GNSs are a superior carbon support to CNTs, CB, and AC for Fe nanoparticles for enhancing catalytic effects. With the incorporation of 2.5 wt% of Fe/GNS, which results in a distortion of the Al–H bonds, the initial dehydrogenation temperature of LiAlH4 was lowered to ∼40 °C. Moreover, the addition of 10 wt% of Fe/GNS can lead to 4.5 wt% H2 release (from LiAlH4) in 6 min at 100 °C; this rate is faster than that of pristine LiAlH4 by more than 135fold. The same approach should be applicable for synthesizing other nanoparticle/GNS composites. Exceptional catalytic capabilities for various complex hydrides to improve their hydrogen storage properties are expected. These studies are currently in progress.
Acknowledgements Financial support of this work by the National Science Council of Taiwan is gratefully appreciated.
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22 P. A. Berseth, A. G. Harter, R. Zidan, A. Blomqvist, C. M. Araújo, R. H. Scheicher, R. Ahuja and P. Jena, Nano Lett., 2009, 9, 1501. 23 M. S. Wellons, P. A. Berseth and R. Zidan, Nanotechnology, 2009, 20, 204022. 24 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183. 25 D. Li, M. B. Müller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101. 26 J. Xu, R. Meng, J. Cao, X. Gu, Z. Qi, W. Wang and Z. Chen, Int. J. Hydrogen Energy, 2013, 38, 2796. 27 L. H. Kumar, C. V. Rao and B. Viswanathan, J. Mater. Chem. A, 2013, 1, 3355. 28 C. P. Hsu, D. H. Jiang, S. L. Lee, J. L. Horng, M. D. Ger and J. K. Chang, Chem. Commun., 2013, 49, 8845. 29 X. R. Ye, Y. H. Lin, C. M. Wang, M. H. Engelhard, Y. Wang and C. M. Wai, J. Mater. Chem., 2004, 14, 908. 30 Y. Zhang and C. Erkey, J. Supercrit. Fluids, 2006, 38, 252. 31 C. Y. Chen, J. K. Chang, W. T. Tsai and C. H. Hung, J. Mater. Chem., 2011, 21, 19063. 32 J. W. Wu, C. H. Wang, Y. C. Wang and J. K. Chang, Biosens. Bioelectron., 2013, 46, 30. 33 C. H. Wu, C. H. Wang, M. T. Lee and J. K. Chang, J. Mater. Chem., 2012, 22, 21466. 34 W. Grochala and P. P. Edwards, Chem. Rev., 2004, 104, 1283. 35 J. A. Dilts and E. C. Ashby, Inorg. Chem., 1972, 11, 1230. 36 P. Rangsunvigit, P. Purasaka, T. Chaisuwan, B. Kitiyanan and S. Kulprathipanja, Chem. Lett., 2012, 41, 1368. 37 T. J. Frankcombe, Chem. Rev., 2012, 112, 2164. 38 J. J. Vajo and G. L. Olson, Scr. Mater., 2007, 56, 829. 39 R. S. Chellappa, D. Chandra, S. A. Gramsch, R. J. Hemley, J. F. Lin and Y. Song, J. Phys. Chem. B, 2006, 110, 11088. 40 J. Yang, A. Sudik, C. Wolverton and D. J. Siegelw, Chem. Soc. Rev., 2010, 39, 656. 41 D. Blanchard, H. W. Brinks, B. C. Hauback and P. Norby, Mater. Sci. Eng., B, 2004, 108, 54. 42 L. Staudenmaier, Ber. Dtsch. Chem. Ges., 1898, 31, 1481. 43 C. Y. Chen, C. Y. Fan, M. T. Lee and J. K. Chang, J. Mater. Chem., 2012, 22, 7697.
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Cite this: DOI: 10.1039/c4nr03357d
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Metal/graphene nanocomposites synthesized with the aid of supercritical fluid for promoting hydrogen release from complex hydrides† De-Hao Jiang, Cheng-Hsien Yang, Chuan-Ming Tseng, Sheng-Long Lee and Jeng-Kuei Chang* With the aid of supercritical CO2, Fe-, Ni-, Pd-, and Au-nanoparticle-decorated nanostructured carbon materials (graphene, activated carbon, carbon black, and carbon nanotubes) are synthesized for catalyzing the dehydrogenation of LiAlH4. The effects of the metal nanoparticle size and distribution, and the type of carbon structure on the hydrogen release properties are investigated. The Fe/graphene nanocomposite,
Received 17th June 2014, Accepted 4th August 2014 DOI: 10.1039/c4nr03357d www.rsc.org/nanoscale
1.
which consists of ∼2 nm Fe particles highly dispersed on graphene nanosheets, exhibits the highest catalytic performance. With this nanocomposite, the initial dehydrogenation temperature can be lowered (from ∼135 °C for pristine LiAlH4) to ∼40 °C without altering the reaction route (confirmed by in situ X-ray diffraction), and 4.5 wt% H2 can be released at 100 °C within 6 min, which is faster by more than 135-fold than the time required to release the same amount of H2 from pristine LiAlH4.
Introduction
Ideal energy sources are clean, affordable, and renewable. The move from coal (C) to oil (–CH2–) and natural gas (CH4), which can produce more heat with less pollution, was prompted by the advantages of using hydrogen-rich fuels.1 Hydrogen is the ultimate energy carrier that can reduce our dependence on fossil fuels and eliminate carbon dioxide emissions.2,3 To bridge hydrogen production with utilization, hydrogen storage is required.4 Finding an efficient storage medium is currently the key challenge for the use of hydrogen in both mobile and stationary applications. For solid-state hydrogen storage, complex metal hydrides, consisting of alkali/alkali earth metal cations and complex anions (including [AlH4]−, [BH4]−, and [NH2]−), are promising materials due to their high gravimetric hydrogen densities.5,6 However, further improvement in the hydrogen adsorption/desorption properties (such as operation temperature, reactions kinetics, and reversibility) of complex hydrides is necessary for their practical application. Halogen-,7–9 oxide-,10–12 and metal-based12–16 catalysts have been used to overcome the sluggish hydrogen release kinetics and relatively high dehydrogenation temperature of LiAlH4, which has a high hydrogen content (theoretically, 10.5 wt% and 96.3 g H2 L−1). Although positive results have been
Institute of Materials Science and Engineering, National Central University, 300 Jhongda Road, Taoyuan, 32001, Taiwan. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr03357d
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reported, more effective catalysis at a lower cost is desirable. A literature review indicates that a species with a given chemical composition can show significant differences in catalytic performance depending on its size.10,13,14,17 For instance, the dehydrogenation temperature of LiAlH4 was lowered by 25 and 40 °C, respectively, when 5 μm and 80 nm diameter Cr2O3 was incorporated.10 In addition, 100 nm Ni decreased the LiAlH4 dehydrogenation temperature by 55 °C,14 much better than the 10 °C reduction obtained with 100 μm Ni.13 Although catalyst dimensions seem to play a substantial role, there is a lack of systematic investigation. The present study synthesizes catalysts even smaller than those reported in the literature in order to improve their catalytic capability; moreover, the size effects were further explored. Another attractive category of additives for complex hydrides is nanostructured carbon materials (including C60, carbon nanotubes (CNTs), and nanoporous carbon), which have been confirmed to possess destabilization,18,19 nanoconfinement,20,21 and catalysis effects,22,23 improving the hydrogen release properties. Graphene nanosheets (GNSs), characterized by a two-dimensional (2D) honeycomb lattice structure, large surface area, high conductivity, excellent stability, and high strength,24,25 are a novel type of dehydrogenation promoter for complex hydrides.26–28 A recent study indicated that GNSs can effectively lower the temperature and increase the rate of LiAlH4 dehydrogenation.28 The nanostructured carbon materials mentioned beforehand can also support other types of catalyst, preventing their aggregation and increasing utilization; as a result, synergistic catalytic effects can be obtained.
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If this idea is valid, the dehydrogenation performance of complex hydrides can be further enhanced. Nevertheless, this approach has never been attempted for LiAlH4. In the present study, nano-sized Fe, Ni, Pd, and Au particles were synthesized on various carbon supports (GNSs, activated carbon (AC), carbon black (CB), and multi-walled CNTs) using a supercritical CO2 (SCCO2)-assisted deposition technique. SCCO2, which has gas-like diffusivity, near-zero surface tension, and excellent mass-transfer properties, is ideal for uniformly dispersing nanoparticles onto high-surface-area supporting materials.29–31 The metal/carbon composites were prepared using both a conventional chemical deposition method and a physical mixing method for comparison. To the best of our knowledge, this is the first work reporting the fabrication of dehydrogenation catalysts using SCCO2, to develop low cost catalysts that exploit size effects, and to explore how the type of carbon support affects catalytic performance. The experimental results indicate that the Fe/GNS nanocomposite synthesized using SCCO2 is a promising catalyst for promoting the dehydrogenation of LiAlH4. With Fe/GNS incorporation, the hydrogen release rate can be increased by ∼135-fold at 100 °C.
2. Experimental procedure 2.1.
Materials and preparation
LiAlH4 powder (97%, ∼50 μm) was purchased from Alfa Aesar. Fe powder (Alfa Aesar; 99%, ∼50 μm), Ni powder (Alfa Aesar; 99.8%, ∼50 μm), Pd powder (Alfa Aesar; 99.95%, ∼50 μm), Au powder (Alfa Aesar; 99.9%, ∼50 μm), AC (Aldrich; 98%), CB (Nippon Shiyaku Kogyo; 99.9%), CNTs (Nanostructured & Amorphous Materials; 99%), TiO2 (Aldrich; 99%), and VCl3 (Aldrich; 99.999%) were used as received. GNSs were prepared using a modified Staudenmaier method.32,33 Natural graphite powder (Alfa Aesar; particle size: ∼70 μm; purity: 99.999%) was chemically oxidized to form graphite oxide (GO) at room temperature. The graphite (5 g) was continuously stirred in a mixed solution of sulfuric acid (100 mL), nitric acid (50 mL), and potassium chlorate (50 g) for approximately 100 h. The resulting GO was rinsed with a 5 wt% aqueous solution of HCl and then repeatedly washed with deionized water until the pH of the filtrate was neutral. The product was dried in air and pulverized. Finally, the GO was exfoliated by rapid heating (∼30 °C min−1) to 1050 °C under an inert Ar atmosphere. After thermal reduction (held at 1050 °C for 30 min), the GNSs were obtained. To prepare metal (Fe, Ni, Pd, Au)/carbon (GNS, CNT, CB, AC) composites, 40 mg of a given carbon material together with the required amount of iron(III) acetylacetonate (Aldrich; 99.9%), nickel(II) acetylacetonate (Aldrich; 99.9%), palladium(II) hexafluoroacetylacetonate (Aldrich; 99.9%), or gold(III) chloride (Aldrich; 99.9%) were loaded into a stainless steel autoclave (500 mL). Methanol (50 mL, TEDIA; >99.9%) was added as the solvent, which is miscible with SCCO2. Dimethyl amineborane (TCI; >95%) was used as the reducing agent. The
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autoclave was pressurized with CO2 up to 10 MPa at 50 °C, at which a supercritical state was achieved. The system was stirred vigorously for 2 h before the vessel was depressurized. The resulting metal/carbon composites were repeatedly washed with deionized water and methanol, and then collected via centrifugation. For comparison, the composites were also synthesized in the absence of SCCO2, with all other preparation parameters identical to those described above. A ball-milling machine (SPEX 8000D) was used to mix LiAlH4 with the additives (2.5 wt%); the ball-to-powder weight ratio was 10 : 1. The mixtures were weighed and handled in an Ar-purified glove box (Innovation Technology Co.), where both the moisture content and oxygen content were maintained below 0.5 ppm. After being sealed in the glove box, the ballmilling vessel was transferred to the milling machine. The milling time was 30 min. 2.2. Material characterization and evaluation of dehydrogenation performance The microstructures of the samples were examined using a transmission electron microscope (TEM, JOEL 2100F) operated at 200 kV. The EELS and elemental mapping were performed using a Gatan energy filter (Tridem 863) with an energy resolution of 1.05 eV. The weight ratios of the metals in various samples were quantified using an atomic absorption spectrometer (AAS, SOLAAR M6). A Raman spectrometer (UniRAM MicroRaman) was employed to study the bonding structure of [AlH4]−; the spectra were excited using a diode-pump solidstate laser with a wavelength of 532 nm. A temperature-programmed desorption (TPD) analyzer was used to evaluate the hydrogen desorption properties of the samples. The analysis was conducted using an argon gas flow rate of 90 mL min−1 under ambient pressure. The temperature was programmed from room temperature to 250 °C at a heating rate of 2 °C min−1. The gas thermal conductivity change, corresponding to hydrogen release, was recorded as a function of temperature. In addition, hydrogen desorption profiles at 100 °C were measured using the TPD analyzer to study the dehydrogenation kinetics. The composition of the released gas was checked using an Agilent 5975 gas chromatograph/ mass selective detector (GC/MSD). The total amount of hydrogen desorbed was calibrated using Mg2NiH4, which has a known hydrogen capacity of 3.6 wt% (confirmed by a Sievert’s measurement). In situ X-ray diffraction (XRD) analyses were performed with the aid of synchrotron radiation (beamline 01C2 in National Synchrotron Radiation Research Center, Taiwan) to explore the dehydrogenation mechanism. In the experiment, the sample powder was loaded into a glass capillary tube (1 mm in diameter), which was mounted on the specimen holder. Inert N2 gas was introduced into the tube to prevent oxidation. Then, the sample was heated from room temperature to 250 °C at a rate of 5 °C min−1. The 2D diffraction patterns were collected by a Mar345 image plate detector. The 2D data were then converted into one-dimensional diffraction patterns using Fit2D software.
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3. Results and discussion
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3.1. The effect of catalyst size on the dehydrogenation properties Fig. 1(a) and (b) show the TEM bright-field images of the Fe/ GNS composites (with a Fe/carbon weight ratio of 1 : 9) synthesized with and without the aid of SCCO2. The conventional chemical reduction method (no SCCO2) produced aggregated Fe clusters with dimensions up to the sub-micron scale (the sample is denoted as Fe/GNS-air). Moreover, since the plating solution had a high surface tension and the carbon surface was hydrophobic (and may have uneven surface conditions in various portions), the deposited Fe was non-uniform on the GNSs. In contrast, the size of the Fe particles decreased considerably to ∼2 nm in diameter when SCCO2 was used. In addition, the nanoparticles were highly dispersed on the GNSs. This can be attributed to the extremely low viscosity and excellent penetration ability of SCCO2,29,30 which can help debundle the GNSs and effectively transport the precursors throughout the sample.34,35 Fig. 1(c) shows the electron energy
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loss spectroscopy (EELS) spectrum of a deposited particle in Fig. 1(b). The Fe L-edge and C K-edge (results from the underlying carbon support) signals can be clearly recognized. The small O K-edge peak was mainly attributed to the GNS (although partial oxidation of Fe upon exposure to air cannot be excluded), since the plain GNS region showed a similar signal. This is associated with the residual oxygen surface functional groups on the GNSs that were not completely removed during the thermal reduction process. The EELS Fe Ledge mapping over the SCCO2-synthesized composite is shown in Fig. 1(d), confirming that the well-distributed deposits on the GNS are Fe nanoparticles. The Ni/GNS composites prepared with and without SCCO2 are shown in Fig. S1 (see ESI†). Similar to the results for Fe, the latter sample showed a smaller particle size and much better dispersion of Ni. The results clearly demonstrate the advantages of using SCCO2 to fabricate highly uniform nanocomposites. A TPD analyzer was used to evaluate the hydrogen desorption properties of the samples. Fig. 2 shows the TPD profiles of LiAlH4 with and without 2.5 wt% of various additives
Fig. 1 TEM bright-field images of the Fe/GNS composites synthesized (a) without and (b) with SCCO2. (c) EELS spectrum of a deposited particle in (b). (d) EELS Fe L-edge mapping over Fe/GNS composite synthesized using SCCO2.
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Fig. 2 TPD signals of LiAlH4 with and without 2.5 wt% of various additives.
(mixed by ball milling). The pristine LiAlH4 (subjected to the same ball milling process but without an additive) exhibited two dehydrogenation signals, which were initiated at approximately 135 and 175 °C, in the plot. There is another desorption reaction at ∼480 °C; however, it is relatively difficult to be utilized and thus not considered in this study. As shown, the addition of plain GNSs and commercial Fe powder (50 μm in diameter) have an insignificant influence on the TPD curves. With the incorporation of Fe/GNS-air, the initial desorption temperature was significantly lowered to ∼60 °C. The most effective additive was found to be Fe/GNS synthesized using SCCO2; the initial dehydrogenation occurred at temperatures as low as 40 °C under ambient pressure. The well-dispersed nano-sized Fe fabricated via SCCO2 maximizes the interaction with [AlH4]−, leading to a superior catalytic effect. LiAlH4 was also milled with a combination of the commercial Fe powder and GNSs (in a weight ratio of 1 : 9 to mimic the composition of the Fe/GNS composites); the data (curve iv) in Fig. 2 excludes the possible contribution of the Fe/C reaction product (which may be formed during the ball milling process) to the improvement in LiAlH4 dehydrogenation performance. The hydrogen desorption properties of LiAlH4 incorporated with a mixture of Ni powder (∼50 μm) and GNSs, and the Ni/GNS composites synthesized with and without SCCO2 are shown in Fig. S2;† the size effects of the catalytic particles were again confirmed. The experimental results also reveal that the SCCO2-based technique is an effective method for synthesizing catalysts with improved properties. To our knowledge, the 2 nm diameter (for the SCCO2-derived Fe) is the smallest size (and corresponding best performance) ever reported for LiAlH4 dehydrogenation catalysts. 3.2. The effect of metal particle type on the dehydrogenation properties Pd and Au particles were also synthesized on GNSs using an analogous SCCO2-assisted synthesis. As shown in the TEM
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Fig. 3 TPD signals of LiAlH4 incorporated with 2.5 wt% of GNSs and various metal/GNS nanocomposites.
micrographs in Fig. S3,† similar to the cases for Fe and Ni, nano-sized Pd and Au are uniformly distributed on the GNSs. This indicates that the SCCO2-based technique is generally applicable for the preparation of various nanoparticles. The weight ratios of Fe, Ni, Pd, and Au to carbon were fixed at 10 (±2) wt% to enable performance comparisons between the various metals. Fig. 3 compares the TPD data of LiAlH4 incorporated with 2.5 wt% of various metal/GNS nanocomposites. As shown, Au/GNS has similar performance to that of plain GNSs, indicating that the Au particles do not really promote hydrogen release. The ability to lower the dehydrogenation temperature increased in the sequence of Au, Pd, Ni, and Fe. It is noted that the noble elements (Au and Pd) are less effective than the active elements (Ni and Fe), which could donate electrons to [AlH4]−,36 making this complex anion unstable and thus capable of easily releasing hydrogen. The fewer nuclear charges of Fe (and thus lower attraction for valence electrons) compared to those of Ni and the high energy of its sixth electron in the 3d orbital (favoring its removal) at least partially explain the better performance for Fe compared to that of Ni. Varin et al. suggested that Fe and Ni are able to dissolve into the [AlH4]− lattice, causing lattice expansion (i.e., structural instability) and thus facilitating dehydrogenation.17 The larger ionic radius of Fe (77 pm) than that of Ni (69 pm), compared to 53 pm for Al, is another reason for the superior catalytic effects of Fe. Fig. S4† shows the TPD data of LiAlH4 incorporated with micron-scale powders of Fe, Ni, Pd, and Au. Of note, the four additives exhibited similar but insignificant dehydrogenation promotion capabilities. The results indicate that the effects of metal type on hydrogen release are more pronounced with a decrease in particle size. With the aid of SCCO2, inexpensive Fe nanoparticles can be synthesized as an excellent dehydrogenation catalyst.
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3.3. The effect of the carbon support type on the dehydrogenation properties To clarify the effect of the type of carbon support, Fe nanoparticles were deposited via SCCO2 on AC, CB, and CNTs (besides GNSs). The TEM micrographs of the composites obtained are shown in Fig. 4. The Fe to carbon ratios were controlled at 10 (±2) wt% for all the samples. When compared to Fe/GNS (in Fig. 1(b)), a similar size (∼2 nm) and uniform distribution of Fe particles was found on the other carbon supports. Fig. 5 shows the TPD data for LiAlH4 with 2.5 wt% of various Fe/carbon composites. As clearly shown, the type of carbon support underlying the Fe particles affects the catalytic performance. The TPD peaks shifted towards lower temperatures in the sequence of Fe/AC-, Fe/CB-, Fe/CNT-, and Fe/GNSincorporated LiAlH4 samples. Although AC has a higher surface area (∼2500 m2 g−1) than those of CB and CNT (both ∼200 m2 g−1), a large fraction of the AC area is attributed to its internal cavities (within the particles). The pore size of AC is shown in Fig. S5,† which indicates that the distribution is mainly in the range of 1.5–4 nm. The Fe deposited inside these cavities can hardly contact LiAlH4 (∼50 μm in diameter), resulting in the poorer catalytic activity of Fe/AC when compared to those for Fe/CB and Fe/CNT, whose Fe nanoparticles are on the open surface of the carbon support. The SEM micro-
Fig. 4
Fig. 5 TPD signals of LiAlH4 with and without 2.5 wt% of various Fe/ carbon composite additives.
graphs shown in Fig. S6† clearly reveal that the bulky AC particles have the least exposed area (to interact with LiAlH4) among the four kinds of carbon support. It was found that the Fe/GNS composite had the best capability to lower the
TEM micrographs of (a) Fe/AC, (b) Fe/CB, and (c) Fe/CNT composites synthesized using SCCO2.
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dehydrogenation temperature. GNSs, with a large accessible area of ∼600 m2 g−1, provides a desirable platform for loading Fe nanoparticles and for catalyzing the dehydrogenation of LiAlH4. It was confirmed that the GNSs are effective grinding agents that enhance the mixing of catalysts and reduce the particle and crystal size of the hydride.28 The high thermal conductivity of GNSs also explains their superior performance, as the dehydrogenation reaction requires heat transport.37 Moreover, the GNSs have a long-range-ordered sp2 structure and delocalized π bonds, which helps promote the electronic interactions between Fe and [AlH4]−, facilitating hydrogen release from the Al–H bonds. This is the first study to verify that GNSs are a superior type of carbon support for dehydrogenation catalysts. The proposed Fe/GNS remarkably outperforms Ni and Y/CNT,14 TiO2/carbon aerogel,38 and TiCl3/carbon aerogel38 composite catalysts for LiAlH4 previously reported in the literature. 3.4. The dehydrogenation mechanism of LiAlH4 with Fe/GNS nanocomposites To explore the dehydrogenation mechanism, in situ XRD analyses were performed. Fig. 6(a) shows the obtained diffraction patterns of pristine LiAlH4 as a function of temperature. As shown, the LiAlH4 begins to decompose and transforms into Li3AlH6 and Al at 150 °C. The following dehydrogenation reaction occurs: 3LiAlH4 ! Li3 AlH6 þ 2Al þ 3H2
ð1Þ
At 210 °C, the intensity of Li3AlH6 diminishes and a new LiH phase forms, indicating a second hydrogen release, as shown in the reaction below: Li3 AlH6 ! 3LiH þ Al þ 1:5H2
ð2Þ
The reaction temperatures are slightly higher than those found in the TPD data since a higher temperature increase rate (5 °C min−1 vs. 2 °C min−1) was used in the in situ XRD analyses. Fig. 6(b) shows the in situ XRD data for Fe/GNS-incorporated LiAlH4. The observed transition temperatures for the above two reactions significantly decreased to 60 and 150 °C, respectively, confirming that hydrogen release was indeed facilitated. According to the literature, additive-induced promotion of dehydrogenation can be attributed to two possible mechanisms, namely catalysis22,23,39 and destabilization.18,19,40 The latter effect is associated with the formation of intermediate species during dehydrogenation, decreasing reaction enthalpy and thus thermodynamically promoting hydrogen release. As shown in Fig. 6(b), there were no Fe- or Ccontaining compounds after ball milling and throughout the dehydrogenation processes, indicating that the reaction routes were not altered. Accordingly, the catalysis effect seems to be the dominant mechanism for Fe/GNS promoted hydrogen release. The influence of the added Fe/GNS on the bonding structure of LiAlH4 was examined using Raman spectrometry; the obtained data are shown in Fig. 7. As demonstrated, pristine
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Fig. 6 In situ synchrotron XRD patterns of (a) pristine LiAlH4 and (b) 2.5 wt% Fe/GNS-incorporated LiAlH4 recorded as a function of temperature.
Fig. 7 Raman spectra of pristine LiAlH4 and 2.5 wt% Fe/GNS-incorporated LiAlH4.
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proposed dehydrogenation catalyst for complex hydrides. Of note, gas analysis confirmed that the product released from the Fe/GNS-catalyzed LiAlH4 was hydrogen without any detectable impurities. The aforementioned results indicate that the waste heat of PEMFCs (∼100 °C) was able to make the proposed Fe/GNS-catalyzed LiAlH4 readily release hydrogen (4.5 wt % in 6 min), which is of high purity and thus can be directly introduced into the fuel cells.
4.
Fig. 8 Hydrogen desorption profiles (at 100 °C) of LiAlH4 with and without various additives.
LiAlH4 displayed two Raman shift signals, at frequencies of 1755 cm−1 and 1830 cm−1, respectively, attributed to the Al–H stretching vibrations.41 However, with the incorporation of 2.5 wt% Fe/GNS, the signals disappeared. Presumably, the Fe/ GNS composite donated electrons to [AlH4]−, leading to distortion of the covalent Al–H bonds. This explains the improved dehydrogenation performance of Fe/GNS-catalyzed LiAlH4. 3.5.
Hydrogen desorption performance at 100 °C
Hydrogen discharge kinetics at accessible temperatures is a crucial concern for practical applications. Fig. 8 shows the hydrogen desorption profiles of various samples at 100 °C, which is close to the operation temperature of proton exchange membrane fuel cells (PEMFCs). All the samples eventually released ∼5.3 wt% of hydrogen (normalized to the weight of LiAlH4), suggesting that the catalysts did not change the quantity of hydrogen released, with only the reaction in eqn (1) triggered at such temperature. Nevertheless, this hydrogen density is already sufficient for many applications.42 As shown, pristine LiAlH4 showed sluggish dehydrogenation behavior; it took ∼12 h to achieve 85% of the hydrogen release (∼4.5 wt%); this sample took ∼21 h to completely dehydrogenate (5.3 wt%). With the incorporation of 2.5 wt% of Fe/GNS nanocomposite, less than 0.3 h was needed to reach the same quantity of hydrogen desorption (4.5 wt%). When Fe/GNS addition was increased to 10 wt%, only 6 min was required, which is shorter than the time required with pristine LiAlH4 by more than 135fold. The catalytic properties of Fe/GNS were compared to those of the two best catalysts, namely VCl3 and TiO2, reported in the literature;11,12,43 the results are also shown in Fig. 8. As demonstrated, the hydrogen release rate for the Fe/GNS-catalyzed LiAlH4 was significantly faster than those obtained with the other two additives. This verifies the effectiveness of the
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Conclusion
The size and distribution of metal particles significantly affect the capability for promoting the dehydrogenation of LiAlH4. With the aid of SCCO2, nano-sized metals can be highly dispersed on GNSs. The catalytic properties of the resulting nanocomposites were significantly superior to those of their counterparts synthesized via a conventional chemical deposition method (without SCCO2). Using the SCCO2 synthetic technique, low cost Fe can be made to exhibit better catalytic performance than that of Ni, Pd, and Au, while these four kinds of powder on a micron scale show similar (but insignificant) capability. Owing to their high accessible area (thus high number of catalytic sites), high mechanical strength (which improves mixing and refining), high thermal conductivity (which facilitates heat transport during dehydrogenation), and high electronic conductivity (which promotes electron exchange between Fe and [AlH4]−), GNSs are a superior carbon support to CNTs, CB, and AC for Fe nanoparticles for enhancing catalytic effects. With the incorporation of 2.5 wt% of Fe/GNS, which results in a distortion of the Al–H bonds, the initial dehydrogenation temperature of LiAlH4 was lowered to ∼40 °C. Moreover, the addition of 10 wt% of Fe/GNS can lead to 4.5 wt% H2 release (from LiAlH4) in 6 min at 100 °C; this rate is faster than that of pristine LiAlH4 by more than 135fold. The same approach should be applicable for synthesizing other nanoparticle/GNS composites. Exceptional catalytic capabilities for various complex hydrides to improve their hydrogen storage properties are expected. These studies are currently in progress.
Acknowledgements Financial support of this work by the National Science Council of Taiwan is gratefully appreciated.
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