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Curious Catalytic Characteristics of Al-CuFe Quasicrystal for De/Rehydrogenation of MgH

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Sunita Kumari Pandey, Ashish Bhatnagar, Shashank Shekhar Mishra, TP Prasad Yadav, Mohammad A. Shaz, and Onkar Nath Srivastava J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07336 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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The Journal of Physical Chemistry

Curious Catalytic Characteristics of Al-Cu-Fe Quasicrystal for De/Rehydrogenation of MgH2 Sunita K. Pandey, Ashish Bhatnagar, S.S. Mishra, T.P. Yadav, M.A. Shaz and O.N. Srivastava* Hydrogen Energy Centre, Department of Physics, Banaras Hindu University, Varanasi, 221005, India ABSTRACT The present study reports the curious catalytic action of a new class of catalyst; quasicrystal of Al65Cu20Fe15 on de/rehydrogenation properties of magnesium hydride (MgH2). Catalyzed through this catalyst, the onset desorption temperature of MgH2 gets reduced significantly from ~345˚C (for ball milled MgH2) to ~215˚C. More dramatic effect of the above catalyst has been observed on rehydrogenation. Here, 6.00 wt.% of hydrogen storage capacity is observed in just 30 seconds at 250˚C. Improved rehydrogenation kinetics has been found even at lower temperatures of 200 & 150˚C by absorbing ~ 5.50 and ~ 5.40 wt.% of H2 respectively, within one minute and ~5.00 wt.% at 100˚C in 30 minutes. These are one of the lowest desorption temperature and the rehydrogenation kinetics obtained for MgH2 through any other known catalyst. The storage capacity of MgH2 catalyzed with leached version of Al65Cu20Fe15 quasicrystalline alloy degrades negligibly even after 51 cycles of de/rehydrogenation. The feasible reason for catalytic action has been described and discussed based on structural, micro structural, Fourier transform infrared and X-ray photoelectron spectroscopic studies.

*Corresponding author: Email: [email protected], Tel: +91 542 2368468; Fax: +91 542 2369889

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INTRODUCTION For hydrogen to become a viable energy carrier, there is a clear need for safe,

lightweight and affordable hydrogen storage media.1,2 This has led to extensive search for potential hydrogen storage materials.3-6 There are large number of published results and review articles which reveal that solid state hydrogen storage in the form of hydrides offers high storage capacity and practical operating pressures.5,6,9 Therefore, most of the researches on storage are being focussed on this type of storage mode.7-12 Magnesium and magnesium based alloys are attractive option and they have become the center of extensive investigations in recent years.13-16 MgH2 is considered to be one of the most suitable candidates for hydrogen storage due to its abundance (eighth most abundant element on the earth crust), low cost, nontoxicity, reversibility, high gravimetric and volumetric hydrogen capacity of 7.6 wt.% and (110 gL-1), respectively.17-18 However, the practical use of magnesium hydride as a hydrogen storage medium has been hindered due to its high thermal stability and sluggish de/rehydrogenation kinetics which makes hydrogen release at moderate temperature very difficult.17 Also, high temperature (>400˚C) is required for hydrogen de/rehydrogenation processes.19 To overcome these limitations, different approaches like nanostructuring, alloying: targeting thermodynamic issues (i.e. to reduce hydride stability) and/or use of catalysts like transition metals, their oxides, alloys, carbon nanostructures etc. have been carried out.20-28 It should be noted that during absorption and desorption, the rate of reaction is controlled by the diffusion of H2 through the hydride phase (β-phase) and growth of magnesium phase, respectively29. Mechanical milling provides reduced particle size which decreases path length for hydrogen diffusion and enhances fresh specific surface area.30 Several groups have reported improvement in kinetics upon mechanical milling which leads to the formation of nanoparticles.17, 31 Xia et. al. reported a simple solvothermal method for the synthesis of wellcontrolled, ordered structures of MgH2 nanoparticles with good dispersion that self-assemble on graphene. The resulting graphene supported monodisperse MgH2 nanoparticles exhibited high H2 capacity, remarkably improved hydrogen storage performance, cycle life, and high thermal conductivity. 32 In regard to improving kinetics there have been several studies including doping of suitable catalysts, where even though enthalpy does not change appreciably, but the kinetics gets improved and de/rehydrogenation temperature is reduced appreciably. As for example Zhang et. al. in his recent publication have also shown that MgH2–Na2Ti3O7 nanotubes (NTs) and MgH2–Na2Ti3O7 nanorod (NR) composites can desorb 6.50 wt% H2 within 6 and 16 min respectively at 300˚C but the change in enthalpy of the MgH2–Na2Ti3O7 NT composite is 2


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~78.90 kJ (mol-1 H2) which is close to the standard value of bulk MgH2 (~75 kJ mol-1 H2). This indicates that the addition of MgH2–Na2Ti3O7 NTs could not alter the thermodynamic stability of MgH2.33 Ouyang et. al. also observed improved kinetics and reduced desorption activation energy but couldn’t observe any change in enthalpy even after several cycles for CeH2.73MgH2-Ni nanocomposites.34 Shao et. al. have shown that the desorption temperature of MgH2/0.1TiH2 nano-composite significantly decreased to 269˚C without any change in enthalpy. Significant improvement in kinetics compared to commercial MgH2 has been attributed to the catalyst TiH2 and the nanostructure produced during high pressure ball milling.35 On the other hand for improving thermodynamics which implies lowering of reaction enthalpy, the studies have generally utilized the process of changing reaction path through the formation of composites in the dehydrogenation process. Some excellent work has been done by forming Mg-Al alloys36 and also by forming Mg0.95In0.05 solid solution.37 Significant decrease in enthalpy has been observed in Mg2In0.1Ni solid solution proving the effect of changing reaction path in improving the thermodynamics of de/rehydrogenation. There has been a recent effort of in-situ catalyization during de/rehydrogenation for enhancing the kinetics of MgH2 powders by deployment of Ni particles.17 As regard the hydrogen release mechanism in bulk MgH2, it has been found that hydrogen release from the bulk particles of MgH2 (2µm) is based on the growth of multiple pre-existing Mg crystallites embodied in the MgH2 matrix. This is due to the difficulty of fully transforming whole of Mg into MgH2 in hydrogenation process. However, in their samples analogous to nano powders, dehydriding occurs through a shrinking core mechanism.7 It has been reported that the effects related to interfaces or surfaces may significantly influence the thermodynamics and kinetics that determine the hydrogen storage performance of the material.38 Hanada et. al. have reported catalytic effect of nanometer-sized 3d-transition metals Fe, Co, Ni, and Cu on H2 storage properties in MgH2 prepared by mechanically milling method in which the composite with 2.00 mol % Ni desorbed 6.50 wt.% of hydrogen in the temperature range from 150 to 250˚C while the hydrogenation was performed under pure hydrogen gas up to 3 MPa at 150 and 200˚C for 12 hrs.39 Shahi et. al. have shown that combined effect of different transition metals such as Ti, Fe and Ni on the de/rehydrogenation characteristics of MgH2 (i.e.MgH2-Ti5Fe5Ni5) offers superior hydrogen storage properties as compared to MgH2 catalyzed with metal catalysts Ti, Fe and Ni separately. They reported that the MgH2-Ti5Fe5Ni5 absorbs 5.30 wt.% within 15 min at 270˚C and 12 atm. 40 3



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However, inspite of above mentioned and other extensive studies, maintaining ~6.00 wt% of storage capacity coupled with reversibility for MgH2 is still a challenging problem which needs to be solved. Also, it is clear from the above studies that metals and combination of metals both act separately as a good catalyst for MgH2 but still some better approach is needed to make MgH2 a viable hydrogen storage material. Here, we report new catalyst which improves dramatically the de/rehydrogenation behavior of MgH2. We have used crystallographically forbidden quasicrystalline (QC) materials and its leached version as catalyst (leaching here implies partial removal of Al through chemical treatment with NaOH). QC are unusual materials since they are neither crystalline nor amorphous. They exhibit orientational periodicity but lack translational periodicity. Only the average structure of QC can be made discernible, exact structure is not known. Besides unusual lattice-structural features, QC also has unusual electronic structure. It is known that for QC material near to Fermi level, a pseudo gap opens and the relation of this gap with respect to Fermi level is very sensitive to composition where a small change in composition may result in doping like effect.41 It is known that the catalytic activity of any catalyst depends upon its structure, particle size and electronic structure. Thus it is expected that QC materials particularly those embodying transition metals may form a new class of catalyst for hydrogen sorption in MgH2. To the best of our knowledge, there have been no reports on using this type of comparatively new catalyst for improving the de/rehydrogenation properties of MgH2. Here, we have used to QC material Al65Cu20Fe15 as a catalyst which can be easily synthesized and leached. Feasible mechanism of catalysis by Al65Cu20Fe15 QC has been put forward. The results obtained show that the leached version of ball-milled (i)Al65Cu20Fe15 QC catalyst indeed works much efficiently as compared to other known catalysts for de/rehydrogenation of MgH2.

2. EXPERIMENTAL DETAILS 2.1 Synthesis of as-cast and leached (i)-Al-Cu-Fe QC The QC alloy Al65Cu20Fe15 was prepared by melting the appropriate ratio of constituent elements in a radio frequency (RF) induction furnace and solidifying by cold water cooling for 30 min. Since, the catalysts are most effective in nano-particle form therefore; the QC ingot was crushed into powder and then ball-milled through high energy ball milling at 200rpm (Retsch Germany, ball-miller) with ball to powder ratio of 40:1 for 40hrs duration. This ballmilled Al65Cu20Fe15 powder was leached with 10 mole of NaOH aqueous solution. After leaching for 2 hrs, the samples were cleaned with de-ionized water for 30 min followed by 4


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sonication (ultrasonic bath) in methanol for 30 min. Leached samples were then dried in oven for 30 min at 75˚C. For the sake of convenience, we have abbreviated different samples as shown in Table1. Here “ball-milled” has been for brevity denoted simply as “milled”. The dried form of both leached Al-Cu-Fe as well as leached version of ball-milled (milled) Al-CuFe and their native (unleached) versions were then employed as catalyst for de/rehydrogenation studies of hydrogen in MgH2. Table 1: Abbreviated names of the processed Al-Cu-Fe samples S No.

2.2.

Sample name

Abbreviated name

1

Al-Cu-Fe

ACF

2

Milled Al-Cu-Fe

MACF

3

Leached Al-Cu-Fe

LACF

4

Leached version of milled Al-Cu-Fe

LMACF

Sample preparation

Magnesium hydride (MgH2) powder with purity 99.99 % was purchased from Alfa Aesar. Mixture of 2 gm of MgH2 and 5.00 wt.% of synthesized catalysts (ACF, MACF LACF & LMACF) were loaded in 250cc stainless steel milling vial fabricated in our laboratory which can sustain high pressure upto 100 atmosphere.42 The samples with ball to powder ratio 40:1were ball-milled for 24 hrs at an operating speed of 200 rpm under hydrogen atmosphere (5atm) using Retsch (PM 400,Germany) planetary ball milling apparatus. The hydrogen pressure inside the vials was maintained throughout the ball milling to prevent the samples from moisture and formation of MgO. Handling of the samples before and after ball milling was always performed under inert atmosphere in an argon gas filled glove box (mBRAun MB 10 Compact, Germany) with H2O and O2 level less than 1ppm to prevent the oxidation and contamination of the samples. 2.3. De/rehydrogentaion measurements and characterization techniques De/rehydrogentaion behaviour of the samples (MgH2) was analysed through temperature programmed desorption (TPD) and pressure composition isotherm (PCI) using an automated four channel Sieverts type apparatus designed by Advanced Materials Corporation, Pittsburgh, USA. For each measurement about 120 mg of sample was loaded in the specimen chamber. Hydrogen absorption (with an initial pressure of 20 atm), desorption measurements and isothermal desorption kinetics (under 1 atm H2 pressure) were performed at various temperatures using the programmable furnace (Thermcraft, Germany). 5



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Structural characterization of the samples were carried out by using X-ray diffraction (XRD) (PANanalytical, Empyrean with 2D area detector) employing CuKα radiation (λ=1.541 Å) under ambient conditions with 0.05˚ step size. During XRD, the sample holder containing sample was covered by a thin layer of parafilm (Pechiney plastic packing) to prevent the samples from oxidation and moisture contamination. Fourier transformation infrared (FTIR) spectroscopic analysis was carried out using Spectrum 100 (Perkin Elmer). Samples were mixed uniformly with potassium bromide (KBr) powder with an agate mortar & pestle and then its pellet was put into the sample holder. KBr background spectrum was recorded before analysis of the samples. Surface morphological characterization and energy dispersive x-ray analysis (EDX) with color mapping of the elements in the samples were performed by scanning electron microscopy (SEM) (FEI-QUANTA 200) at 30kV. Microstructural characterizations with selected area electron diffraction (SAED) analysis were performed using transmission electron microscopy (TEM) (FEI-TECHNAI 20G2) operating at an accelerating voltage of 200 kV. The electronic states of various samples were measured by X-ray photoelectron spectrometer (OMICRON, France) with a hemispherical electron energy analyser & MgK@ 1253.6 eV source with base pressure 5x10-11 torr. 3. RESULTS AND DISCUSSION 3.1. Structural and morphological characterizations We examined XRD patterns of as synthesized QC and Figure-1(a) brings out the representative XRD pattern of the as-cast Al65Cu20Fe15 phase. This pattern represents the known structural features of QC phase of Al65Cu20Fe15.43 Figure-1(b) shows the XRD of leached version of ballmilled (i)-QC phase (LMACF). Figure-1(c-f) shows typical XRD patterns of (c) MgH2+LMACF, (d) Mg+LMACF (i.e. dehydrogenated sample), (e) MgH2+LMACF (i.e. rehydrogenated sample) and (f) Mg+LMACF after 51 cycles of absorption-desorption. As can be seen from Figure 1(c-f), the dominant peaks are from MgH2/Mg however; the presence of QC can be seen by weak and broad diffraction peak (marked by arrow). Furthermore, in the XRD patterns (Figure-1(d) & (f)), there is no evidence of formation of any new metastable/intermediate phase. The detailed elemental analysis has been carried out using EDX along with color mapping for the distribution of elements on the alloy surface before leaching (MACF) and after leaching (LMACF) from several areas. Figure-2(a) & (c) show the representative scanning electron micrograph of MACF & LMACF and Figure-2(b) & (d) shows their corresponding color mapping. It is apparent from Figure-2(b) that the Al, Cu and Fe elemental maps can be utilized to differentiate their concentration on the alloy surface and measure their general 6


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concentration. These results can be treated as quantitative analysis to the first order of approximation mainly because of the 3hrs. mapping was done in continuous scanning mode. Similar type of observation has been carried out for the leached surface as shown in Figure2(c) & (d). It is clear from the color mapping of the leached sample (Figure-2(d)) that Al is preferentially removed from the surface during leaching and the concentration of Cu and Fe becomes dominant producing a capping layer of Cu and Fe on Al-Cu-Fe matrix. Additionally, Cu and Fe are nearly homogeneously distributed on leached surface. It also has to be noted that due to the overlapping of precipitate projections, the measurement of volume fraction using EDX maps would not provide explicit results on concentration. Figure-3(a) and (c) show the representative SEM micrograph of ball-milled MgH2+LMACF and Figure-3(b) shows it’s corresponding EDX color mapping. Since, the distribution of the catalyst LMACF in MgH2 is not clearly visible (due to low content of catalyst) so, to visualize it, we magnified the selected region of Figure-3(b) and labeled it as Figure 3(d). It should be noted here that the small dotted circles with color coding in Figure-3(d) are just guide to eye to visualize the distribution of catalyst over MgH2. From the selected small portion of the EDX color mapping (Figure-3(d)), we can clearly see how LMACF is distributed over MgH2 sample. Further microstructural analysis of all the samples were carried out using TEM. Since QC can be unambiguously characterized through single crystal pattern, therefore, to clearly record typical five fold diffraction pattern of Al65Cu20Fe15 (i)-QC alloy, we ball milled the ascast (i)-QC alloy for 1 hr at 200 rpm. Figure-4(a-c) show TEM image, SAED pattern of typical five fold Al65Cu20Fe15 (i)-QC and elemental analysis EDX of single crystal Al65Cu20Fe15 (i)QC alloy (ACF), respectively. The indexing of Figure-4(b) has been carried out using Bancel scheme.44 Since, (i)-QC alloy is brittle in nature so on ball milling ACF for 40hrs followed by leaching treatment, we received fine powder of leached (i)-QC alloy (LMACF). TEM image, SAED pattern and EDX of the synthesized catalyst LMACF is shown in Figure-4(d-f). The synthesized catalyst LMACF was then ball-milled with MgH2 for 24 hrs. Figure-4(g-i) represents the TEM image and SAED pattern along with the elemental analysis EDX of MgH2+LMACF, respectively. As can be seen from SAED patterns (Figure-4(h)), the diffraction rings from both MgH2 and QC (catalyst) are easily discernible and the presence of Al (leached out), Cu & Fe from the catalyst portion is evident from EDX taken from selected region. Often the diffraction lines from catalyst in XRD may not be present but they may be present in SAED pattern because of higher atomic scattering factor for electrons. Therefore, the chances of finding catalyst in SAED pattern are higher. In order to see whether the catalyst is present and does not 7



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react to form any new compound, we increased the weight percent (8 wt% instead of 5.00 wt%) of catalyst in MgH2. Figure-4(h), shows representative SAED pattern of this MgH2 with 8.00 wt % catalyst and besides MgH2 diffraction rings, those from leached (i)-QC (catalyst) can also be seen.

3.2.

Hydrogen storage performance

3.2.1. Dehydrogenation and rehydrogenation studies Figure-5 shows the temperature programmed desorption (TPD) curves of (a) as-received MgH2, (b) Milled MgH2, (c) MgH2+ACF, (d) MgH2+MACF, (e) MgH2+LACF and (f) MgH2+LMACF respectively, which was carried out at the heating rate of 2˚C/min from room temperature. It is clearly observed from the TPD profile that the onset dehydrogenation temperature of as-received MgH2, ball-milled MgH2 and MgH2 catalyzed by 5.00 wt.% of ACF, MACF, LACF & LMACF is ~400, ~345, ~301, ~293, ~225 & ~215˚C, respectively. The difference in onset desorption temperature of various samples by taking MgH2+LMACF as reference is shown in Table 2 which clearly indicates that lowest onset desorption temperature has been found for MgH2+LMACF.

Table 2: Difference in desorption temperature of different samples S No.

Sample name

Onset desorption temp.

Difference in onset desorption temp. (Taken MgH2+LMACF as reference)

˚

1

As-received MgH2

~400 C

~185 ˚C

2

Milled MgH2

~345 ˚C

, ~130 ˚C

3

MgH2+ACF

~301 ˚C

~86 ˚C

4

MgH2+MACF

~293 ˚C

~78 ˚C

5

MgH2+LACF

~225 ˚C

~10˚C

6

MgH2+LMACF

~215˚C

-

For checking reversibility of catalyzed MgH2, rehydrogenation was carried out at 250˚C under 20 atm of H2 pressure and the results are embodied in Figure-6(i). As can be seen from Figure-6(i) that Mg+ACF, Mg+MACF and Mg+LACF could absorb ~4.50, ~4.60 and ~5.20 wt.% of H2, respectively in five minutes, whereas Mg+LMACF absorbed ~6.00 wt.% of H2 in just 30 seconds under similar conditions of temperature and pressure. This is one of the fastest absorption kinetics ever observed for MgH2. From both dehydrogenation and rehydrogenation studies, we can say that LMACF is a better catalyst as compared to ACF, MACF & LACF. We therefore, further performed rehydrogenation studies at lower temperatures only with 8


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Mg+LMACF and obtained interesting results. As can be seen (Figure-6(ii)), improved rehydrogenation kinetics has been observed even at lower temperatures of 200 & 150˚C by absorbing ~5.50 and~5.40 wt.%, respectively within 1 minute and ~5.00 wt.% at 100˚C in 30 minutes under 20 atm of H2 pressure. The rehydrogenated Mg+LACF & Mg+LMACF were then dehydrogenated at 300˚C under 1 atm of hydrogen pressure. It can be clearly seen from Figure-6(iii) that MgH2+LMACF released ~6.30 wt.% of hydrogen within 2.50 minutes at ~320˚C under 1 atm of hydrogen pressure. Also, the comparison of the dehydrogenation kinetics of 1st cycle and 51st cycle of MgH2+LMACF is shown in Figure-S1 where minor kinetic degradation can be seen after 51 cycles at temperature 300oC and 1 atm H2 pressure. To understand the improved dehydrogenation kinetics of the rehydrogenated MgH2+LMACF, apparent activation energy for desorption has been estimated and evaluated quantitatively using Arrhenius equation and the temperature dependence of the rate constant k. k= k0 exp (-Ea/RT)

....................

1

Where Ea is the apparent activation energy of the reaction process and k0 is a pre-exponential frequency factor. The desorption activation energy of MgH2+LMACF evaluated from Arrhenius plot of (ln (k)) versus (1/RT) shown as inset of Figure-6(iii) is found to be ~64.25 kJ/mol. This is significantly lower as compared to other studies.17,21,32 For example, ElEskandarany et. al. reported that the synthesized MgH2/5.5 wt.% Ni composite powders have decomposition temperature and the corresponding decomposition activation energy to be 218°C and 75 kJ/mol., respectively.17 3.2.2. Cycling study of MgH2 It is clear that LMACF as a catalyst has an interesting role in de/rehydrogenation of MgH2. Since reversibility/cycling are another important characteristic of any viable hydride so, we checked the reversibility of the sample by performing cyclic de/rehydrogenation experiments. Continuous dehydrogenation (1atm H2 pressure at 300˚C) followed by rehydrogenation (20 atm H2 pressure at 300˚C) upto 51 cycles were performed to check the cyclic stability of MgH2+LMACF. Figure-6(iv) shows the hydrogen storage capacity of MgH2 + LMACF during 51 cycles of absorption and desorption. It can be seen that even after 51 cycles, MgH2+LMACF shows good cyclic stability and the capacity degrades slightly from ~6.00 wt.% to ~5.60 wt.% of H2.

3.2.3. Pressure composition isotherms (PCI) The representative pressure composition isotherm for absorption and desorption of MgH2 catalyzed with LMACF is shown in Figure-7. The regions observed were found to be nearly 9



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flat with equilibrium pressures of 0.36, 0.70, 3.24 & 5.28 atm for PCI-absorption and 0.20, 0.47, 2.58 & 4.45 atm for PCI-desorption at 225, 250, 300 and 320˚C, respectively. The change in enthalpy for both absorption and desorption of MgH2 catalyzed with LMACF was calculated using van’t Hoff equation (shown below) …………………(2)

This calculated enthalpy is found to be ~71.12 and ~73.66 kJ mol-1 for absorption and desorption, respectively. The corresponding representative van’t Hoff plot between (ln P) and (1/T) is shown as an inset of Figure-7. It is observed that there is no significant enthalpy change as compared to ball-milled and pristine MgH2, which further suggests that there is no new phase formation. In the present work as has been observed, the enthalpy doesn’t change appreciably (~73.66 kJ/mole H2) but desorption temperature decreases from ~345 to ~215˚C (see Figure-5). The reason for this behavior is discussed in the catalysis mechanism section below.

4.

MECHANISM OF CATALYSIS

The catalytic action of leached QC material has been proposed with the help of XRD, electron microscopy, FTIR & XPS studies. The mechanism of catalysis is based on structural and microstructural changes of the QC and MgH2 on leaching. The effect of leaching treatment on QC may be understood by the schematic model as shown in Figure-8. The bulk structures of Al-Cu-Fe alloy (ACF) and MACF are shown in Figure-8(a) and 8(b), respectively. Further the structures of the leached versions of ACF & MACF i.e. LACF & LMACF are shown in Figures-8(c) and 8(d), respectively. Before leaching, the atoms of Al, Cu and Fe occupied their respective positions in QC but after leaching, most of the Al atoms get leached off and the matrix of Cu (and Fe) atoms remains on the QC surface. This is clearly evident from the SEM micrograph (Figure-2(a) & (c)) along with its corresponding EDX color mapping (Figure-2(b) & (d)) of Figure-2 that leaching treatment preferentially removes Al atoms resulting in a capping layer of Cu and Fe and can also be visualized from schematic model as shown in Figure-8. Recent studies,45-47 have revealed that even after leaching no basic change in the QC phase occurs and the starting structure (before leaching) remains intact. In the present investigation, subsurface layer contains transition metal along with Al; therefore, the quasicrystal structure beneath remains unchanged. The surface gradually becomes rich in Cu and Fe with increasing leaching time. This is because of surfaces always terminate at the layer with a high atomic density and Al content is much higher than Cu and Fe in the case of Al65Cu20Fe15 quasicrystal. Since the concentration of Al dominates the leaching process of Al10


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Cu-Fe alloys, therefore, the leaching would be blocked on the layers with poor Al content in the Al-Cu-Fe QC structure and the overall dissolution rate would be slowed down. The AlCu-Fe QC structure contributes to a decrease in the dissolution rate of Al for the quasicrystalline phase. The interface is formed between high atomic density lattice planes of the Cu/Fe particles and the quasicrystal. The formation of such interface may provide a greater stability of the Cu and Fe during catalytic reactions. It has been reported by several groups that the Cu and Fe catalysts enhances catalytic activity and stability.39,47,48 Another relevant point which needs to be taken into account is that Cu and Fe have negligible solubility and intermetallic phase has not been found to exist in the Cu-Fe phase diagram.49 Hence, removal of Al from Al-Cu-Fe (i)-QC does not result in the formation of a bimetallic Cu-Fe alloy infact, the leached Al-Cu-Fe remain intact on Al65Cu20Fe15 QC cluster with Cu and Fe atoms on its surface. Therefore, it can be taken in our present investigation that the distribution of nanoparticles of Cu-Fe-Cu-Fe metals localized on top of Al-Cu-Fe frame work provides useful catalytically active metals on QC surfaces. It may be pointed out that the Al65Cu20Fe15 QC is harder (hardness 7.5 Mohs)50 than MgH2 (hardness 4.0 Mohs).51,52 Thus, it is anticipated that during ball milling, QC pulverized the MgH2 particles. After pulverization the crystallite size of MgH2 is found to be ~50 nm on the other hand, the crystallite size of the stand-alone ball-milled MgH2 is found to be ~300nm. Consequently, exposed area of MgH2 for catalysis increases and diffusion pathways for hydrogen gets shortened. Also QC catalyst becomes fine powder (10-20 nm) on ball-milling which increases its catalytic activity. Thus, QC will work here like a catalyst cum pulverizer for MgH2. Additionally the catalytic mechanism of LMACF for de/rehydrogenation from/in MgH2/Mg in the form of schematic diagram is presented in Figure-9. The Cu (and Fe) atoms from neighboring polyhedra will form Cu (or Fe) species; however, they are not quite free but are part of the polyhedra as shown in Figure-8. Therefore, Cu (or Fe) atoms will not agglomerate and thus the catalytic activity will not degrade. It can thus be taken that catalytic activity is produced by Cu (and Fe) species which weaken the H-Mg-H bond resulting in lowering of desorption temperature of hydrogen from MgH2. Similar but reverse will be the case for absorption of hydrogen. Further investigation on the effect of LMACF on catalyzing MgH2 was carried out through changes in bond configuration employing Fourier Transform Infrared Radiation (FTIR) spectroscopy. The FTIR spectra for the (i) ball-milled MgH2+LMACF (ii) Mg+LMACF (dehydrogenated) and (iii) MgH2+LMACF (rehydrogenated) are shown in Figure-S2 where signature at 1236 cm-1 is assigned to bending band for the as-milled 11



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MgH2+LMACF

and signature at

717 cm-1 is assigned to stretching band for the

MgH2+LMACF, respectively. As Wang et. al.53 have also shown that there are two main regions with active infrared vibrations of the Mg–H bonds in the region 400-800 and 900-1300 cm-1. The spectra in the 400-800 cm-1 region corresponds to the Mg-H bending and the spectra in the region 900–1300 cm-1 corresponds to the Mg – H stretching bands. However, for the sample Mg+LMACF any FTIR signatures could not be found to be present. This implies that no oxides or hydrides are formed during the desorption studies. It is expected that after desorption of MgH2+LMACF, only Mg element is left along with the material LMACF which consists of a capping layer of transition metals Cu/Fe on Al-Cu-Fe alloy. There was no evidence of metal-hydrogen or metal-oxygen bond. As expected after rehydrogenation, stretching and bending bands of Mg–H reappears. After employing catalyst LMACF no difference was found in the FTIR spectra apparently, indicating that the MgH2 lattice is essentially unaffected by the presence of catalyst. In order to further investigate the details of catalytic activity, the change in the electronic state of Cu and Fe in ball-milled MgH2+LMACF (before desorption) and Mg+LMACF (after desorption) has been recorded by XPS (Figure-S3 (a-b)). Figure-S2(a) shows the Cu3p XPS spectra of MgH2+LMACF where the peak at binding energy (B.E.) 77.87 eV has been assigned to Cu3p1/2. On the other hand, considerable shift has been observed in the Cu3p XPS of Mg+LMACF, thus the B.E. value shifts to 75.30 eV (Cu3p3/2). Similarly for the case of iron, Fe3p1/2 peak at binding energy (B.E.) 53.60 eV (for MgH2+LMACF) gets shifted to 52.60 eV (Fe3p3/2) after desorption (Mg+LMACF). The XPS data clearly indicates the change of Cu and Fe valency. Moreover, it is important to note that Cu and Fe have electronegativity of 1.90 and 1.83, respectively which lies between the electronegativity of Magnesium (1.31) and Hydrogen (2.20). Thus, both can gain and loose electron with ease as compared to Mg and H. The gain and loss of electrons have been verified in XPS study shown in Figure-S3. Hence, Cu and Fe because of variable valency can act as an intermediate carrier and enhance the electron transfers between Mg2+ and H− during the de/rehydrogenation reaction.

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5.

CONCLUSIONS

The present investigation brings out important results that on leached version of ball-milled quasicrystalline alloy Al65Cu20Fe15 possess superior catalytic action on the de/rehydrogenation of MgH2. Rehydrogenation experiments have revealed that MgH2+LMACF absorbed ~6.00 wt.% of H2 in just 30 seconds under 20 atm H2 pressure at 250oC. Under the same pressure, improved rehydrogenation kinetics has been found even at lower temperatures of 150˚C by absorbing ~ 5.40 wt.% of H2 within one minute and ~5.00 wt.% of H2 at 100˚C in 30 minutes. In regard to dehydrogenation at 300˚C, the desorption kinetics corresponds to ~6.20 wt.% in 3.00 min. and the desorption activation energy is found to be ~64.25 kJmol-1. Based on EDX mapping, FTIR and XPS studies, it is proposed that the mechanism of catalysis in the present case corresponds to flow of electrons back and forth from MgH2 to Cu and Fe of QC. The present studies suggest that further studies on the catalytic behaviour of Al based QC materials for other light weight hydrides should be investigated.

6. AUTHOR INFORMATION Corresponding author: Prof. O. N. Srivastava E-mail address: [email protected] Tel: +91 542 2368468; Fax: +91 542 2369889 Notes: The authors declare no competing financial interest.

7. ACKNOWLEDGEMENTS We are thankful to Ministry of New and Renewable Energy, Department of Science & Technology (Nano Mission Project) and University Grants Commission, India for the financial support (Obama Singh 21st century knowledge initiative (OSI) project. The authors would like to thank Dr. Govind Gupta of NPL for help in recording XPS spectra. The authors wish to thank Prof. A. P. Tsai (Japan), Prof. Ranon McGrath and Dr. H. R Sharma, (The University of Liverpool, UK) for discussions. One of the authors AB would like to acknowledge CSIR-New Delhi for SRF fellowship. We would like to thank Dr. V. S. Subrahmanyam and Dr. M. S. L. Hudson for a critical reading of the manuscript and valuable suggestions.

8. SUPPOTING INFORMATION Dehydrogenation kinetics, FTIR spectra and XPS spectra (Cu and Fe) of MgH2+LMACF are provided as supporting information files. This material is available free of charge via the Internet at http://pubs.acs.org/. 13



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FIGURE CAPTIONS Figure 1: X-ray diffraction patterns of (a) as-cast (i)-Al65Cu20Fe15 (ACF), (b) Leached version of ball-milled Al65Cu20Fe15 (LMACF), (c) MgH2+LMACF, (d) Mg+LMACF (i.e. dehydrogenated sample), (e) MgH2+LMACF (i.e. rehydrogenated sample) and (f) Mg+LMACF after 51 cycles of absorption-desorption. Figure 2: (a & c) SEM micrograph of MACF & LMACF, and (b & d) their corresponding EDX color mapping. Figure 3: (a & c) SEM micrograph of MgH2+LMACF, (b) EDX color mapping of MgH2+LMACF, and (d) Magnified view of selected area of Figure 3(b) (Note: Small dotted circles in Figure 3(d) are guide to eye to visualize the presence of catalyst). Figure 4: (a-c) TEM image, typical five fold SAED pattern & EDX of single crystal Al65Cu20Fe15 (i)-QC alloy (ACF), (d-f) TEM image, SAED pattern & EDX of LMACF and (gi) TEM image, SAED pattern & EDX of MgH2+LMACF, respectively. Figure 5: TPD curves of (a) as-received MgH2, (b) ball-milled MgH2, (c) MgH2+ACF, (d) MgH2+MACF,(e) MgH2+LACF and (f) MgH2+LMACF. Figure 6: (i) Rehydrogenation kinetics curve of˚(a)Mg+ACF, (b)Mg+MACF, (c)Mg+LACF and (d) Mg+LMACF, 5(ii) Rehydrogenation kinetics curve of Mg+LMACF at lower temperatures of 100, 150, 200 and 250˚C, 5(iii) Dehydrogenation Kinetics curves of MgH2+LMACF(estimation of desorption activation energy (Ea) and Arrhenius plot of (ln k) versus (1/RT) is shown as an inset and 5(iv) Number of Cycles Vs. hydrogen storage capacity of MgH2+LMACF upto 51 cycles of absorption and desorption at 300˚C Figure 7:

Pressure composition isotherm (PCI) for absorption and desorption of

MgH2+LMACF (van’t Hoff plot is shown as inset). Figure 8: The tentative atomic clusters model and the effect of leaching illustrating the formation of Cu (and Fe) on top surface of quasicrystal. Figure 9: Schematic representation: Catalytic mechanism of LMACF for de/rehydrogenation from/in MgH2/Mg. TABLE CAPTIONS Table 1: Abbreviated names of the processed Al-Cu-Fe sample. Table 2: Difference in onset desorption temperature of various samples (MgH2+LMACF is taken as reference). 19



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Figure 1: X-ray diffraction patterns of (a) as-cast (i)-Al65Cu20Fe15 (ACF), (b) Leached ballmilled) Al65Cu20Fe15 (LMACF), (c) MgH2+LMACF, (d) Mg+LMACF (i.e. dehydrogenated sample), (e) MgH2+LMACF (i.e. rehydrogenated sample) and (f) Mg+LMACF after 51 cycles of absorption-desorption.

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Figure 2: (a & c) SEM micrograph of MACF & LMACF, and (b & d) their corresponding EDX color mapping.

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Figure 3: (a & c) SEM micrograph of MgH2+LMACF, (b) EDX color mapping of MgH2+LMACF, and (d) Magnified view of selected area of Figure 3(b) (Note: Small dotted circles in Figure 3(d) are guide to eye to visualize the presence of catalyst).

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Figure 4: (a-c) TEM image, typical five fold SAED pattern & EDX of single crystal Al65Cu20Fe15 (i)-QC alloy (ACF), (d-f) TEM image, SAED pattern & EDX of LMACF and (g-i) TEM image, SAED pattern & EDX of MgH2+LMACF, respectively.

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Figure 5: TPD curves of (a) as-received MgH2, (b) ball-milled MgH2, (c) MgH2+ACF, (d) MgH2+MACF, (e) MgH2+LACF and (f) MgH2+LMACF.

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Figure 6: (i) Rehydrogenation kinetics curve of˚(a)Mg+ACF, (b)Mg+MACF, (c)Mg+LACF and (d) Mg+LMACF, 5(ii) Rehydrogenation kinetics curve of Mg+LMACF at lower temperatures of 100, 150, 200 and 250˚C, 5(iii) Dehydrogenation Kinetics curves of MgH2+LMACF(estimation of desorption activation energy (Ea) and Arrhenius plot of (ln k) versus (1/RT) is shown as an inset and 5(iv) Number of Cycles Vs. hydrogen storage capacity of MgH2+LMACF upto 51 cycles of absorption and desorption at 300˚C

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Figure 7:

Pressure composition isotherm (PCI) for absorption and desorption of

MgH2+LMACF (van’t Hoff plot is shown as an inset).

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Figure 8: The tentative atomic clusters model and the effect of leaching illustrating the formation of Cu (and Fe) on top surface of quasicrystal.

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Figure 9: Schematic representation: Catalytic mechanism of LMACF for de/rehydrogenation from/in MgH2/Mg.

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TOC Graphic

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