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Wahab, Mohammad A, Young, David J., Karim, Azharul, Fawzia, Sabrina,& Beltramini, Jorge N.(2016)Low-temperature hydrogen desorption from Mg(BH4)2 catalysed by Ultra-fine Ni nanoparticles in a Mesoporous Carbon Matrix.Low-temperature hydrogen desorption from Mg(BH4)2 catalysed by Ultra-fine Ni nanoparticles in a Mesoporous Carbon Matrix, 41(45), pp. 20573-20582.

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https://doi.org/http://dx.doi.org.ezp01.library.qut.edu.au/10.1016/j.ijhydene.2016.09.098

1

Low-temperature hydrogen desorption from

Mg(BH4)2 catalysed by Ultrafine Ni nanoparticles

in a Mesoporous Carbon Matrix

Mohammad A. Wahab*abc

, David J. Youngc, Azharul Karim

b, Sabrina Fawzia

d, and Jorge

N. Beltraminia

aARC Centre for Functional Nanomaterials, Australian Institute for Bioengineering and

Nanotechnology, The University of Queensland, Brisbane, St Lucia, QLD 4072, Australia

*E-mail: m.wahab@uq.edu.au; mawahab@gmail.com

bChemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland

University of Technology, 2 George Street, QLD 4001 Australia and

*Email: mdabdul.wahab@qut.edu.au

c Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast,

Maroochydore DC, Queensland 4558, Australia.

*Email: dyoung1@usc.edu.au

dScience and Engineering Faculty, Civil Engineering and Built Environmental Science.

Queensland University of Technology, 2 George Street, QLD 4001 Australia.

2

Abstract: Homogenously dispersed, ultrafine Ni nanoparticles (Ni NPs) have been grown in a

mesoporous carbon (MC) matrix (MC-Niinsitu) and efficiently catalyze low-temperature hydrogen

desorption from Mg(BH4)2. The onset desorption temperature (Tonset) of this MC-Niinsitu-

Mg(BH4)2 system is 44 oC compared to 275

oC (Tonset) for pure Mg(BH4)2. This is the lowest

value reported for hydrogen desorption from this metal hydride. The activation energy (Ea) of

MC-Niinsitu-Mg(BH4)2 was determined to be 21.3 kJ/mol which is less than half the value of 45.9

kJ/mol for pure Mg(BH4)2. We ascribe this reduction to the synergistic effects of

nanoconfinement and the homogeneously dispersed, ultrafine Ni NPs active sites in the

mesoporous carbon.

Keywords: Hydrogen storage, Mg(BH4)2, Porous carbon, Nanoconfinement, Catalysis,

Synergistic effects

3

Introduction

Utilizing hydrogen (H2) as a clean energy source has been intensively investigated because of the

adverse environmental effects of burning fossil fuels [1-9]. Various hydrogen storage systems

have been demonstrated, but the realization of an efficient catch-release system is elusive [1-15].

Magnesium borohydride (Mg(BH4)2) is a promising hydrogen storage material with a high

gravimetric capacity of 14.8 wt% H2 and volumetric capacity of 112.5 kgm-3

[5-14], which can

meet the targets of 7.5 wt% and 50 kg m-3

set by the US Department of Energy (DoE). However,

its slow kinetics and unfavorably high onset H2 desorption temperature (Tonset) (270 OC - 280

OC)

currently limits its potential for this application [7-14]. Mixing Mg(BH4)2 with other

compounds/dopants has been investigated as a possible solution [7-17]. For example, the

Mg(BH4)2 - LiNH2 system released hydrogen at 170-180 OC due to the combined reaction of

[BH4]- and [NH2]

- but unfortunately the formation of poisonous NH3 during the decomposition

reactions limits its practical application [11]. A mixture of LiBH4 and Mg(BH4)2 also prepared by

ball-milling exhibited fewer decomposition steps but the first Tonset of this mixture was 235 OC,

exhibiting no significant reduction compared to pure Mg(BH4)2 [12]. Previous studies involving

extensive ball-milling of Mg(BH4)2 with TiO2, TiH2 or Ti for 2h produced material that mostly

desorbed hydrogen from a temperature of 227 oC.

10 Similarly, the ball-milling of Mg(BH4)2 with

Al was found to desorb H2 at 150 OC with a total desorption of 3.6 wt.% H2 at 300

OC in 2 h

[13]. These desorption temperatures are still too high for Mg(BH4)2 to be a practical hydrogen

reservoir. Additionally, several strategies including doping catalysts, the use of binary mixtures

of two hydrides, creating reactive composites, employing gas-solid reactions, forming

ammoniates, nanoconfinement of hydrogen precursors, exploiting dual-tuning effects of metal

hydrides and their dopants with high energy ball-milling and post-addition methods have been

4

reported to improve the hydrogen storage properties of mostly magnesium hydride-based

systems [14-18].

Recently, it has been reported that the nanoconfinement of a metal hydride within a porous

scaffold (e.g. mesoporous carbon, carbon aerogel or catalyst functionalized porous carbon) with

high surface area can reduce the Tonset, improving desorption kinetics because the porous scaffold

can contribute to the efficiency of the catalytic system [6, 8, 14, 19 - 32]. The encapsulation of

LiBH4 in pure porous carbon, for example, decreased the Tonset to 100-150 OC from 400

OC for

bulk LiBH4 [19]. The Tonset for Mg(BH4)2 confined in a disordered activated carbon reduced the

Tonset to 170 OC

and was further reduced to 75

OC when Mg(BH4)2 was incorporated into Ni

particles containing mesoporous carbon [14]. The latter study incorporated Ni nanoparticles into

the pores of a mesoporous carbon framework post-synthesis, yielding Ni NPs in the range 10 -

20 nm in size [14]. No impurities were observed during the desorption process which is

consistent with other mesoporous carbon-based nanoconfined systems [14, 26, 50]. Liu et al, for

example, evaluated the decomposition behavior of eutectic LiBH4−Mg(BH4)2 confined in

ordered porous carbon [31]. The decomposition temperatures of this system were in the range

270 to 350 OC, essentially falling between that of pure LiBH4 and Mg(BH4)2. Nanoconfinement

of these hydride materials did not produce diborane and triborane impurities. Javadian et al [27]

have described the melt-infiltration of a mixture of LiBH4 and Mg(BH4)2 in an activated carbon

aerogel with the resulting Tonset of 115-125 OC, still well short of the target working temperatures

for hydrogen storage applications [7-14]. In-situ, nanoconfined growth allows ultrafine Ni

nanoparticle catalysts to be finely dispersed, in mesoporous carbon for better performance. These

Ni nanoparticles are found to be well-distributed in mesoporous channels and to be of uniform

5

size [22,23,25-30]. The use of this material could bring metal hydrides and catalysts into more

intimate contact than is possible with other scaffolds.

We herein report the in-situ growth of nanoconfined and highly dispersed Ni nanoparticles

throughout a mesoporous carbon framework (MC-Niinsitu) and compare the catalytic

dehydrogenation of Mg(BH4)2 in this matrix to that of the same metal hydride in mesoporous

carbon alone. Nanoconfinement not only limits particle growth and aggregation of particles, but

also creates a shorter diffusion path with a lower energy barrier between Mg(BH4)2 and the

homogeneously dispersed Ni NPs. We have found that this confined MC-Niinsitu-Mg(BH4)2

system releases hydrogen at ~ 44 oC with a peak temperature (Tmax) at 141

OC, a remarkable

reduction of about 226 OC relative to previously reported systems. The activation energy (Ea) of

MC-Niinsitu-Mg(BH4)2 and kinetics of dehydrogenation are also reported.

Experimental

Methods and materials: Tetraethylorthosilicate (TEOS), pluronic surfactant P123

(EO20PPO70EO20), magnesium borohydride (Mg(BH4)2), sucrose, and Ni(NO3)2·6H2O from

Sigma-Aldrich were used without further purification. Hydrofluoric acid (HF), tetrahydrofuran

(THF), diethyl ether, ethanol, 2M NaOH and sulphuric acid (H2SO4) were also used as received.

Preparation of mesoporous SBA15 silica template: A surfactant solution containing 2 g of

P123 in 60 ml of 2M HCl was stirred vigorously at 38 OC for 2 h. TEOS (4.2 g) was slowly

added to the surfactant solution and stirring was continued for another 6 - 8 min and then the

solution left without stirring for 24 h at 38 OC [14,23,33-35]. The mixture was then put into an

autoclave for 48 h at 100 OC, filtered and the precipitate dried. Finally, as-prepared SBA15 silica

was calcined at 550 OC for 6 h in air for producing pure SBA15 silica template (Fig.1a).

6

Synthesis of mesoporous carbon (MC): A mixture of sucrose (1.25 g), water (5.0 mL) and

H2SO4 (0.14 g) was prepared and mesoporous SBA15 silica (1.0 g) was added slowly with

continual stirring for 5-6 h. Thermal polymerization was conducted at 100 OC for 6 h and then at

160 OC for another 6 h. Another 5.0 mL of aqueous solution containing 0.8 g of sucrose and 0.09

g of conc. H2SO4 was added to the above polymerized sample. Thermal polymerization was

repeated using the same conditions [14,23]. Finally, mesoporous SBA-15 silica/carbon

nanocomposite was carbonized in a conventional furnace at 850 OC under a nitrogen flow. The

carbonized nanocomposite sample was stirred in an aqueous solution of 7-wt% hydrogen

fluoride (HF) to dissolve the SBA15 silica template, thereby creating the mesoporous carbon

(MC) framework.

Synthesis of MC-Niinsitu: A Ni source Ni(NO3)2·6H2O was added to SBA15 using a two-step

procedure. Sucrose (1.25 g) and a predetermined amount of Ni(NO3)2·6H2O precursor was added

into 7 mL of H2O containing 0.14 g of conc. H2SO4. The mixture was stirred for several hours

and then 1 g of SBA15 was added and the mixture stirred for another 6 h. Thermal

polymerisation was carried out at 100 oC for 6 h and then at 160

oC for 6 h. This procedure was

then repeated with the addition of a solution containing 0.8 g sucrose and a pre-determined

amount of Ni(NO3)2·6H2O precursor in 5 mL of H2O with 0.1 g of conc. H2SO4. Thermal

polymerization was repeated using the same conditions. The polymerized sample was carbonized

at 850 OC for 5 h under a nitrogen flow. The carbonized MC-Niinsitu sample was washed with 2

M NaOH solution for 7-8 h (Fig. 1e) to remove the SBA15 silica template. The final, dried Ni-

containing MC samples (MC-Niinsitu) were reduced by exposure to a 5% H2/N2 gas mixture at

500 oC for 2 h.

7

Synthesis of MC-Nipost: Ni(NO3)2·6H2O solution (10 wt%) was added slowly to the desired

amount of previously synthesized dried mesoporous carbon, followed by drying the slurry at

120-130 OC overnight. The final dried Ni-containing MC samples (MC-Nipost) were reduced by

by exposure to a 5% H2/N2 gas mixture at 500 oC for 2 h.

Infiltration of Mg(BH4)2 into MC (MC-Mg(BH4)2), MC-Nipost (MC-Nipost-Mg(BH4)2), and

MC-Niinsitu (MC-Niinsitu-Mg(BH4)2): All mesoporous carbon and Ni-containing mesoporous

carbon materials were thoroughly dried dried at 200 oC, cooled in a desiccator and transferred to

a glovebox. Mg(BH4)2 (45 wt%) solution (THF and diethyl ether) was slowly impregnated into

the MC, MC-Niinsitu, and MC-Nipost samples inside the glovebox [14,23,32]. The Mg(BH4)2

confined samples were dried again for 24 h at room temperature using a freeze-dryer at -77 - 80

oC with a pressure of 0.3 mbar to remove solvent and loosely bound materials from the surface.

All samples were stored inside the glove box.

Characterization and properties evaluation: Samples were prepared for XRD analysis by

smearing on a glass slide in an argon glove box and then being covered with Parafilm tape to

exclude moisture. X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex

diffractometer (Japan) with Cu Kα radiation at a scanning rate of 2 degree/min in the 2θ range

from 10 to 80° under identical conditions. An automated adsorption analyzer (Autosorb-1C,

Quantachrome, USA) was used to record the BET surface areas, pore volume and pore size

distribution (PSD) curves [36,37]. Porous carbon and related samples were ultrasonically

dispersed in ethanol for 10-15 mins. These samples were cast on carbon-coated Cu grids using a

dropper. TEM (FEI Tecnai 20, 200kV) was performed using a F20 microscope with an

accelerating voltage 200 kV. Hydrogen desorption behaviour of pure Mg(BH4)2 and Mg(BH4)2

confined samples was investigated using TPD data recorded by a Brooks 5850E mass flow

8

controller attached to a glass tube furnace. The temperature was varied from room temperature to

500 oC at a rate of 5

oC/min, with He flowing at a rate of 50 mL/min. An automated Sieverts’

apparatus (Suzuki Shokan PCT H2 Absorption Rig) was used to record hydrogen desorption

studies. Solid-state NMR spectroscopy was carried out on a Bruker Avance III spectrometer

fitted with a 300 MHz magnet. The solid-state 11

B NMR spectra were recorded using magic

angle spinning (9kHz), a single 2.5µs pulse and 10s relaxation delay, with high-power 1H

decoupling employing the tppm 15 sequence. Spectra are referenced to BF3OEt2 (δ = 0 ppm).

Other solid-state NMR spectra were also obtained on an Avance III spectrometer (Bruker),

operating at 300.1300 MHz for 1H, 59.627 MHz for

29Si and 75.468 MHz for

13C. Powdered

material was placed in the 4 mm zirconium rotor and rotated at the magic angle with 7 kHz

frequency. 13

C NMR spectra were recorded using the SP-hpdec technique (single pulse with

high-power proton decoupling) and CPMAS. The parameters included 42 ms acquisition time

with sweep width of 50 kHz; 2K data points were collected. High-power decoupling utilized the

tppm15 scheme at 100 kHz. Between 200 and 1000 scans were collected. The recycle times

ranged from 30s to 100s and were verified to be sufficient for relaxation without signal

saturation. 29

Si NMR spectra were recorded after a single pulse to allow for quantification of the

Si species. The 90° pulse was followed be high-power decoupling using the tppm15 scheme at

87 kHz.

Results and Discussion

The porous carbon used for this research was a highly ordered mesoporous carbon scaffold

characterized by large surface area, uniform pore size, ordered and interconnected pore structure,

with good thermal and mechanical stability [14,21-24,27-32, 34,35]. These characteristics

facilitated both high dispersion and high loading of hydrogen with fine control of the particles

9

within the pore channels. Figure 1 illustrates the synthesis of three types of ordered mesoporous

carbon scaffolds: (i) mesoporous carbon (MC), (ii) post-incorporated Ni NPs in MC (MC-Nipost),

and (iii) in-situ grown, nanoconfined Ni NPs in MC (MC-Niinsitu). In this work, Mg(BH4)2 was

slowly impregnated into the nanopores of previously well-dried MC, MC-Niinsitu, and MC-Nipost

matrices. All materials were thoroughly characterized by XRD and BET (supplementary

information, SI). The presence of three well-defined XRD reflections (100), (110), and (200)

(Fig. 1a, SI) confirmed the formation of a 2D hexagonal mesoporous structure (p6mm) for the

SBA15 silica template and it’s corresponding mesoporous carbon (MC) replica [14, 19-24,33-

35]. A small shift in the XRD diffraction for MC (Fig. 1a, SI) can be ascribed to the contraction

of both the mesoporous carbon framework and the silica template during the high-temperature

(850 OC) carbonization. The presence of broad peaks at higher angles (Fig. 1a, SI) for MC

supports the graphitic nature of the mesoporous carbon framework [14, 19-24,33-35]. Moreover,

the N2 isotherms and PSD curves (Figs. 1b and 1c, SI) for both the SBA15 silica template and

MC are in good agreement with previous reports on the formation of mesoporous structures [14,

19-24,34-37]. The structure of these matrices were also investigated by solid-state 29

Si and 13

C

NMR spectroscopy (Fig. 2, SI) which demonstrated that the structural composition was

dependent on the experimental conditions and final framework [38-42]. The absence of ‘Q sites’

(right side, Fig. 2, SI) confirmed the efficient removal of the silica template by HF washing. The

13C CP-MAS NMR spectra of carbonized mesoporous carbon samples (before (a) and after

removing silica template (b) (right side, Fig. 2, SI) clearly show the formation of mesoporous

carbon structures. NMR signals in the range 0 - 80 ppm are characteristic of sp3 carbon atoms,

while signals ranging from 100 - 160 ppm arise from sp2 carbon atoms, Signals between 140 and

10

160 ppm are assigned to oxygen bound, O-C=C sp2 carbons and those in the region of 165 - 200

ppm were ascribed to the C=O groups of the mesoporous carbon framework [38-42].

The incorporation of Ni NPs into the mesoporous carbon was observed in the XRD patterns and

TEM-EDX (Fig. 2) of MC-Nipost and MC-Niinsitu. The surface areas, pore sizes, and pore

volumes of the studied samples are given in Table 1 [14,19-24,43-52]. Incorporation of Ni NPs

into the MC framework post-synthetically decreased the pore size of MC-Nipost (Fig. 3, SI and

Table 1) whereas the pore size of MC-Niinsitu in Table 1 was slightly larger (by 0.85 nm) due to

the consumption of carbon during the nickel oxide/nickel nitrate reduction by surrounding

carbon [14,19-30,43]. The shapes of N2 isotherms and PSD curves (Fig. 3, SI) for MC-Nipost and

MC-Niinsitu are consistent with previous results [14,19-24,43-48]. The XRD peak intensities of

MC-Nipost-Mg(BH4)2 and MC-Niinsitu-Mg(BH4)2 in Fig. 2b are remarkably reduced due to the X-

ray scattering contrast between the mesoporous pore walls and the guest Mg(BH4)2 molecules

inside the pores of the mesoporous scaffold. Further, proof of confinement is provided by the

decrease in the surface areas, pore volumes and pore sizes of the impregnated matrices (Table 1),

consistent with previous reports on confinement-based systems [6,8,14,19-31,49-57]. TEM (Fig.

2d) confirmed the presence of homogeneously distributed, ultrafine Ni NPs of less than 5 nm

diameter compared to the post-synthesis incorporated Ni NPs (Fig. 2c) with average particle

sizes of about 20 nm that appear more aggregated, non-uniform and non-homogenously

distributed. TEM-EDX analysis of MC-Niinsitu (Fig. 2e) clearly indicates the presence of various

elements with a negligible amount of oxygen, consistent with previous studies [14,23,55-57].

The recorded small oxygen peak in Fig. 2e could be due to the adsorption of oxygen during the

dispersion of the samples in ethanol and/or by sample handling. Similar kinds of observations

have been reported previously [14,23].

11

Table 1 outlines the surface and structural properties before and after confining Mg(BH4)2 into

the various porous carbon matrices. Figures 3a and 3b show the XRD patterns of crystalline

Mg(BH4)2 [9] and graphitic MC, respectively, consistent with previous reports [41-43,48]. The

new diffraction peaks at 43.5°, 52°, 60°, and 74.08° in Fig. 3c confirm the incorporation of Ni

NPs [42,43,48]. Importantly, broadening of these Ni peaks also suggests the formation of small

Ni NPs as confirmed by the TEM image (Fig. 2d). Interestingly, a broad and weak XRD peak is

observed for Mg(BH4)2 in the confined sample MC-Niinsitu-Mg(BH4)2 (Fig. 3d) and for the pure

Mg(BH4)2 (Fig. 3a) in which peaks for the dehydrogenated MC-Niinsitu-Mg(BH4)2 sample at 350

oC (Fig. 3e) are even weaker and almost indistinguishable. These observations suggest that when

a guest molecule such as Mg(BH4)2 is confined within the pores of mesoporous carbon, there is a

change to the structure of the ordered pore channels. This is consistent with previous reports of

the nanoconfinement of metal hydride mixtures in porous scaffolds [14, 19-31,45-52]. An almost

negligible hump found at ~ 49.2o with evidence from the TEM-EDX (Fig. 3g) suggests the

formation of a small amount of MgO due to the presence of surface absorbed oxygen [14, 52].

The three broad hump peaks at 66, 68 and 75.8o could be consistent with the presence of MgH2.

The TEM (Fig. 3f) and TEM-EDS (Energy Dispersive Spectroscopy, Fig. 3g) images of sample

MC-Niinsitu-Mg(BH4)2 after desorption at 350 oC clearly show the presence of various elements

including a major signal for Mg.

The dehydrogenation behavior of pure Mg(BH4)2, MC-Mg(BH4)2, MC-Nipost-Mg(BH4)2, and

MC-Niinsitu-Mg(BH4)2 was systematically investigated by TPD analysis (Fig. 4).

Dehydrogenation of Mg(BH4)2, began at 270 oC (Fig. 4a) with a few additional peaks at higher

temperatures consistent with previous TPD studies [7-10,14,23]. By contrast, nanoconfinement

of Mg(BH4)2 in pure mesoporous carbon desorbed hydrogen at 150 oC (Tonset) (Fig. 4b) [26-32],

12

and the addition of Ni NPs in MC-Nipost-Mg(BH4)2 further reduced the Tonset to at ~ 76 oC with a

Tpeak at ~ 161 oC (Fig. 4c). The in-situ grown and highly dispersed Ni NPs (≤ 5 nm) in

mesoporous carbon exhibited a significant reduction of the Tonset to ~ 44 oC with a Tpeak at ~ 141

oC (Fig. 4d). This remarkably low value for Tonset is the lowest reported for Mg(BH4)2. [14]

These results indicate that the size, distribution, and aggregation of Ni NPs in mesoporous

carbon can remarkably influence the dehydrogenation kinetics of metal hydrides. These results

also suggest that synergistic effects between nanoconfinement, NP particle size, and dispersion

of catalyst throughout the mesoporous carbon (MC) is responsible for creating favorable

conditions that enable release of hydrogen at low temperatures. In addition,

compartmentalization by highly ordered porous carbon scaffolds also destabilizes the metal

hydride to release hydrogen at a significantly lower temperature with faster dehydrogenation

kinetics [14, 19-32,49-57]. However, the broad TPD curve profiles in Fig. 4 for the confined

systems, particularly at > 240-250 oC indicate slow release of H2. The formation of some large

metal hydride particles outside of the mesoporous carbon scaffold could be responsible for

producing such broad TPD profiles and Mg(BH4)2 particles buried in the deeper inside pores

may also be responsible for the slow release of hydrogen at higher temperatures.

The desorption profiles, the quantity of desorbed hydrogen (wt%) versus time (min), for the pure

Mg(BH4)2 and nanoconfined (MC-Mg(BH4)2, MC-Nipost-Mg(BH4)2 and MC-Niinsitu-Mg(BH4)2)

are compared in Fig. 5. The differences in hydrogen releasing behavior indicate that the size and

distribution of Ni NPs has a profound catalytic effect on hydrogen desorption from Mg(BH4)2

[14,23,48]. These results are consistent with previous reports in which the thermal hydrogen

desorption from MgH2 nanoparticles with primary particle size of <10 nm occurred at 350 K for

the Mg-based system [14, 19-31, 56-58]. These improvements were associated with the large

13

grain boundary and surface area achieved by small nanoparticle sizes, which also provides a

larger surface for reactions with the hydrogen [14, 19-31, 56-58]. Our results indicate that the

order of catalytic efficiency was MC-Niinsitu-Mg(BH4)2 > MC-Nipost-Mg(BH4)2 > (MC-

Mg(BH4)2. We also measured the dehydrogenation kinetics of the MC-Niinsitu-Mg(BH4)2 system

at different temperatures (Fig. 6). Confinement system MC-Niinsitu-Mg(BH4)2 delivered 7.21 wt%

of hydrogen at 350 OC (Fig. 6a), indicating that the higher the temperature, the higher the

capacity and the faster the kinetics of hydrogen release. An activation energy Ea of 21.3 kJ/mol

for this process (Figure 6b) was calculated using an Arrhenius plot (K = Aexp(-Ea/RT). This value

is less than half of the corresponding value of 45.9 kJ/mol for pure, bulk Mg(BH4)2 [14]. The

present results are consistent with those previously reported for nanoconfinement by porous

carbon scaffolds [22-32, 49-54].

Karger et al [50] have recently reported the altered reaction pathway of a nanoconfined mixture

of LiBH4–Mg(BH4)2 in which the formation of intermediate species such as [B12H12]2 was

inhibited by the influence of the porous carbon scaffold [50]. This altered reaction pathway

might be the origin of the enhanced desorption kinetics observed by us. Compartmentalization

not only limits the particle growth and aggregation of particles but also creates close contact

between Mg(BH4)2 and homogeneously dispersed ultrafine Ni NPs in mesoporous carbon

[14,19-32,54-58]. The enhanced desorption kinetics of MC-Niinsitu-Mg(BH4)2 is presumably the

synergy of (i) confinement/compartmentalization, (ii) homogeneously dispersed ultrafine

nanocatalyst with high surface area, (iii) the carbon matrix playing a catalytic role and (iv)

intimate contract between Mg(BH4)2 and MC-Niinsitu.

Conclusion

14

We have successfully demonstrated an in-situ growth nanoconfinement approach that has

reduced the Tonset of dehydrogenation of Mg(BH4)2 to an unprecedented 44 OC. MC-Niinsitu-

Mg(BH4)2) exhibited a superior dehydrogenation performance compared to pure Mg(BH4)2 or

confined Mg(BH4)2 without Ni catalyst. The activation energy (Ea) of MC-Niinsitu-Mg(BH4)2

system is 21.3 kJ/mol, less than half the value for pure Mg(BH4)2. We propose that this reduction

is due to nanoconfinement of the metal hydride with the finely divided and homogeneously

dispersed dehydrogenation catalyst within the mesoporous carbon scaffold. This approach could

be an important strategy for designing an efficient hydrogen storage system. The latter, of course

also requires modified thermodynamics to permit rehydrogenation/ reversibility and that will be

a major direction for future research.

Acknowledgements

The authors thank the AIBN-UQ for providing a new staff grant and research facilities at ARC

Centre for Functional Nanomaterials, AIBN UQ. We acknowledge the facilities and the scientific

and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the

Centre for Microscopy and Microanalysis, University of Queensland, Brisbane, Australia.

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21

Table 1. Surface and structural properties: specific surface area(SBET

), pore volume (cm3/g) and

the pore size (nm).

22

Fig. 1. Synthesis of MC/MC-Ni-insitu/MC-Ni-post using a mesoporous SBA15 silica template

followed by impregnation of Mg(BH4)2 in to MC/MC-Ni-insitu/MC-Ni-post. The structural

properties of all used materials are given in Table 1.

23

Fig. 2. (a) XRD patterns for MC-Niinsitu and MC-Nipost, (b) XRD patterns of Mg(BH4)2 confined

in MC-Niinsitu, (c) TEM image of MC-Nipost, (d) TEM image of MC-Niinsitu and (e) TEM-EDX of

(d).

24

Fig. 3. XRD patterns of (a) Pure Mg(BH4)2, (b) pure mesoporous carbon (MC), (c) MC-Niinsitu,

(d) MC-Niinsitu-Mg(BH4)2 (as-prepared and dried), (e) dehydrogenated MC-Niinsitu-Mg(BH4)2 at

350OC, (f) TEM image of MC-Niinsitu-Mg(BH4)2 at 350

OC, and (g) TEM-EDX of MC-Niinsitu-

Mg(BH4)2 at 350OC.

25

50 100 150 200 250 300 350 400 450 500

Temperature (OC)

(a) Pure Mg(BH4)2

(b) MC-Mg(BH4)2

(c) MC-Nipost

-Mg(BH4)2

(d) MC-Niinsitu

-Mg(BH4)2

(d)

(a)

(b)

(c)

Fig. 4. TPD-MS spectra of (a) Pure Mg(BH4)2, (b) MC-Mg(BH4)2, (c) MC-Ni-post-Mg(BH4)2,

and (d) MC-Niinsitu-Mg(BH4)2.

26

0 20 40 60 800

1

2

3(d)

(c)

(b)

(a) Pure Mg(BH4)

2

(b) MC-Mg(BH4)

2

(c) MC-Nipost

-Mg(BH4)

2

(d) MC-Niinsitu

-Mg(BH4)

2

H2 d

eso

rbed (

wt%

)

Time (min)

(a)

Fig. 5. PCT results of (a) Pure Mg(BH4)2, (b) MC-Mg(BH4)2, (c) MC-Ni-post-Mg(BH4)2, and (d)

MC-Niinsitu-Mg(BH4)2 at 145OC.

27

Fig. 6. (a) H2 desorption of pure Mg(BH4)2 and MC-Niinsitu-Mg(BH4)2 with temperature and (b)

activation energy from Arrhenius plot.

28

Supporting Information

Low-temperature hydrogen desorption from

Mg(BH4)2 catalysed by ultrafine Ni nanoparticles in

a mesoporous carbon matrix

Mohammad A. Wahab,*abc

David Jame Young,c Azharul Karim,

b Sabrina Fawzia

d and

Jorge N. Beltraminia

abARC Centre for Functional Nanomaterials, Australian Institute for Bioengineering and

Nanotechnology, The University of Queensland, Brisbane, St Lucia, QLD 4072, Australia

*E-mail: m.wahab@uq.edu.au; mawahab@gmail.com

bChemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland

University of Technology, 2 George Street, QLD 4001 Australia.

c Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast,

Maroochydore DC, Queensland 4558, Australia.

d Science and Engineering Faculty, Civil Engineering and Built Environmental Science.

Queensland University of Technology, 2 George Street, QLD 4001 Australia.

29

Fig. 1. (a) XRD patterns of mesoporous SBA15 silica template and its replicate MC. Inset shows

the higher angle XRD patterns of MC. (b) N2 isotherms of mesoporous SBA15 silica template

and its replicate MC, and pore size distribution curves of mesoporous SBA15 silica template and

its replicate MC.

30

Fig. 2. (left) Solid-state 29

Si NMR spectra of (a) mesoporous SBA15 silica template and (b) its

replicate MC after removing SBA15 silica template and (right) Solid-state 13

C NMR spectra of

(a) mesoporous SBA15 silica template/carbon (carbonized) and (b) its replicate MC after

removing SBA silica template.

Fig. 3. (a) N2 isotherms of MC-Niinsitu and MC-Nipost, (b) pore size distribution curves of MC-

Niinsitu and MC-Nipost..

31

0 -25 -50

(ppm)

(a) Pure Mg(BH4)

2

(b) MC-Niinsitu

-Mg(BH4)

2

(c) MC-Niinsitu

-Mg(BH4)

2

Fig. 4. Solid-state 11

B NMR spectra of (a) Pure Mg(BH4)2, (b) MC-Niinsitu-Mg(BH4)2 (as-

confined), and (c) MC-Niinsitu-Mg(BH4)2 (dehydrogenated at 350 oC).