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The influence of ball-milling time on the dehydrogenationproperties of the NaAlH4-MgH2 compositeCitation for published version (APA):Bendyna, J. K., Dyjak, S. M., & Notten, P. H. L. (2015). The influence of ball-milling time on the dehydrogenationproperties of the NaAlH4-MgH2 composite. International Journal of Hydrogen Energy, 40(11), 4200-4206.https://doi.org/10.1016/j.ijhydene.2015.01.026
DOI:10.1016/j.ijhydene.2015.01.026
Document status and date:Published: 01/01/2015
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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 4 2 0 0e4 2 0 6
Available online at w
ScienceDirect
journal homepage: www.elsevier .com/locate/he
The influence of ball-milling time on thedehydrogenation properties of the NaAlH4eMgH2
composite
Joanna K. Bendyna a,b, Sławomir Dyjak c,*, Peter H.L. Notten a,d
a Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, Den Dolech 2, Postbus
513, 5600 MB Eindhoven, The Netherlandsb EMPA Swiss Federal Laboratories for Materials Science and Technology, Dept. Energy, Environment and Mobility,
CH-8600 Dubendorf, Switzerlandc Military University of Technology, Institute of Chemistry, 2 Kaliskiego Street, 00-908 Warsaw, Polandd Forschungszentrum Julich IEK-9, D-52425 Julich, Germany
a r t i c l e i n f o
Article history:
Received 12 November 2014
Received in revised form
6 January 2015
Accepted 8 January 2015
Available online 21 February 2015
Keywords:
Metal hydrides
Dehydrogenation reaction pathways
Ball milling
Alloys
X-ray diffraction
* Corresponding author. Military University oWarsaw, Poland. Tel.: þ48 (22) 683 77 46.
E-mail addresses: [email protected] (Jhttp://dx.doi.org/10.1016/j.ijhydene.2015.01.00360-3199/Copyright © 2015, Hydrogen Energ
a b s t r a c t
The recently developed NaAlH4eMgH2 composite shows improved hydrogen-storage
properties compared to MgH2 and NaAlH4. However, the dehydrogenation reaction rates
are still too limited, hampering practical applications. Mechanical ball milling is broadly
used to improve the dehydrogenation reaction rates of hydrides. Therefore, the hydrogen-
storage properties of the NaAlH4eMgH2 (1:1) composite have been investigated as a func-
tion of ball-milling time. Expectedly, elongated milling led to a faster dehydrogenation
rates. New insights of the structural transformation pathways of the decomposition re-
action are provided. A number of AleMg alloys, including the only reported Al12Mg17, seem
to participate in the dehydrogenation. Thereby, complex alloying process of Al with Mg or
MgH2 has been proposed. Our data indicate the possibility that hydrides Na3AlH6 and
NaMgH3, which are the intermediate products of the dehydrogenation, coexist. The study
shed a light on the complexity of the decomposition pathways of hydride mixtures in
which the key role play alloys.
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
Nowadays, the growing energy demand and increasing levels
of green-house gasses urges the development of alternative
energy solutions [1,2]. Amongst many potential solutions a
hydrogen-based economy attracts considerable attention.
This is based on the so-called ‘green energy’ concept that is
f Technology, Institute o
.K. Bendyna), sdyjak@wa26y Publications, LLC. Publ
realised as a closed cycle process. At the start of this cycle,
hydrogen can directly be produced from water by using sun-
light, e.g. by photo-catalysis or indirectly by combining solar
cells with electrolysis of water [3]. Subsequently, hydrogen
must be stored efficiently for a safe transport. Finally, the
green cycle is closed when hydrogen is oxidized together with
oxygen to deliver energy and water [4,5]. However,
f Chemistry, Department of Explosives, 2 Kaliskiego Street, 00-908
t.edu.pl (S. Dyjak), [email protected] (P.H.L. Notten).
ished by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 4 2 0 0e4 2 0 6 4201
implementation of the hydrogen-based economy suffers of a
severe bottleneck. Efficient and safe hydrogen-storage is still
very challenging, especially for the automotive industry ap-
plications [6].
The most promising methods to store hydrogen are by
means of chemisorption and physisorption. In the latter case,
most attractive sorbents are used with a large surface area
that requires very low temperatures, as low as liquid nitrogen,
to storage the hydrogen. These methods reveal good sorption
kinetics and often reasonable storage capacities [7]. The
chemisorption method is mostly dominated by alkali- and
alkaline-earth metal hydrides that often require elevated
temperatures for sorption and show high storage capacity. In
this group a large number of compounds are composed of
light-weight metals like Li and Na.
Both metals are able to form so-called alanates, such as
LiAlH4 [8,9] and NaAlH4 [9]. These alanates have been inten-
sively investigated as prospective complex hydrides with a
high theoretical gravimetric storage capacity of 10.6 and
7.5 wt.% for LiAlH4 and NaAlH4, respectively [10].
Thermodynamic destabilization of complex hydrides can
be achieved by using an additional hydride that results in the
formation of intermediate phases. Attractive hydride com-
posites have been developed, like LiAlH4eMgH2 [11] and
NaAlH4eMgH2 [12e14], which have a high storage capacity of
9.4 and 7.6 wt.%, respectively. So far, much emphasis in the
hydrogen-storage community was dedicated to the
LiAlH4eMgH2 system. Regarding the NaAlH4eMgH2 system
only a few limited investigations have been carried out, which
leave much space for further improvements of the composite
and to get more in-depth knowledge.
The process of the thermodynamic destabilization of
LiAlH4eMgH2 and NaAlH4eMgH2 involves complex multi-step
reactions. Firstly, LiAlH4 and NaAlH4 are expected to form
intermediate complex hydrides (Li3AlH6 or Na3AlH6) during
the decomposition. Furthermore, hydrogen gas, Al and metal
hydrides (LiH or NaH) are formed, according to a two-step
reaction [11e14].
3Li=NaAlH4/Li=Na3AlH6 þ 3Alþ 3H2 (1)
Li=Na3AlH6/3Li=NaHþ Alþ 1:5H2 (2)
The studies in which various molar ratios of LiAlH4:MgH2
(1:1, 1:2 and 1:4) were used, revealed that the dehydrogena-
tion pathway follows always the reactions (1) and (2) [15]. It
has been described that Li0.92Mg4.08 and Al12Mg17 alloys are
formed as well [11,16], according to the following reactions
4.08 MgH2 þ 0.92 LiH / Li0.92Mg4.08 þ 4.5 H2 (3)
12 Al þ 17 MgH2 / Al12Mg17 þ 17 H2 (4)
Reaction (3) has been described in more general terms in
Ref. [17].
n AHx þ m BHy 4 AnBm þ 1/2(nx þ my) H2 (5)
It is believed that the above alloys play a dominant role in
the thermodynamic destabilization of the LiAlH4eMgH2
composites [11,15e17].
The investigation of NaAlH4eMgH2 [14] revealed two
possible reaction schemes (1:4 and 1:2 vs. 1:1), unlike
LiAlH4eMgH2 (1:1, 1:2 and 1:4) [11]. The NaAlH4eMgH2 (1:1)
composition [13,14] is expected to form the dehydrogenation
product Na3AlH6 and follow the anticipated reactions (1), (2)
and (4) [11,13,14]. Further decomposition of NaH and resid-
ual MgH2 leads to hydrogen gas and respectivemetals without
the formation of a NaeMg alloy.
The NaAlH4eMgH2 (1:4 and 1:2) composites are proposed to
dehydrogenate via intermediate products such as Al12Mg17(reaction (4)) and NaMgH3 [12e14], without the formation of
Na3AlH6. It is expected that the formation of the Al12Mg17 alloy
during dehydrogenation improves the thermodynamic prop-
erties of the NaAlH4eMgH2 system [12,14]. NaMgH3 is formed
according to
NaAlH4 þ MgH2 / NaMgH3 þ Al þ 1.5 H2 (6)
It is not clear yet why the reaction (6) was not observed in
the NaAlH4eMgH2 composites with the molar ratio of 1:1
[13,14] but only in the mixtures with excess MgH2 [12,14].
Furthermore, the decomposition of the excess MgH2 into Mg
and hydrogen as well as reaction (4) take place. At a later stage
of the dehydrogenation process, NaH and NaMgH3 decompose
according to
NaMgH3 / NaH þ Mg þ H2 (7)
NaH / Na þ 0.5 H2 (8)
Recently, it has been reported that during the dehydroge-
nation of the Na3AlH6eMgH2 mixture, NaMgH3 can also be
formed [18].
Na3AlH6 þ 3 MgH2 / 3 NaMgH3 þ Al þ 1.5 H2 (9)
The decomposition reactions of the NaAlH4eMgH2 and
Na3AlH6eMgH2 composites as presented above indicate the
possibility of a coexistence of the intermediate hydrides
Na3AlH6 and NaMgH3 since both can be formed from the same
hydride mixtures. So far, it was suggested that the reported
NaAlH4eMgH2 mixture dehydrogenates exclusively via one
intermediate hydride NaMgH3 or Na3AlH6 which depends on
the molar ratio of the primary hydrides. However, the X-ray
diffraction (XRD) reflections of these ball-milled samples may
show much overlap, since the crystallinity is typically signif-
icantly decreased by grinding, which may lead to issues with
the data interpretation.
The goals of the present study are twofold. Firstly, to
analyse the influence of the high-energy ball-milling on the
dehydrogenation properties as a function of time and sec-
ondly, to gain more knowledge about the structural trans-
formation pathways of the NaAlH4eMgH2 (1:1) composite.
Fig. 1 e The dehydrogenation characteristics of the NaAlH4eMgH2 (1:1) mixture ball-milled for 1e360 min with a high-
energy ball-mill: a) desorbed hydrogen as a function of the temperature and b) desorbed hydrogen and temperature as a
function of time.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 4 2 0 0e4 2 0 64202
Experimental methods
All samples were handled and manipulated in an argon filled
glove-box. Sodium Aluminium Hydride (NaAlH4, Sigma-
eAldrich, 95%) and Magnesium Hydride (MgH2, Sigma-
eAldrich, 95%) were used as-received from producer without
further purification.
2 g of powder mixtures of NaAlH4eMgH2 (1:1) were placed
in a stainless steel 50 ml vial and milled with a shaker SPEX
8000 mill which operates at 1200 rpm and subsequently,
placed in the glove-box. Stainless steel balls (6 of 13 mm and
16 of 6 mm diameter) were used for the milling experiments.
The ball-to-powder weight ratio was kept constantly at 16:1.
The samples were milled from 1 to 360 min.
The powder samples were characterized, before and after
milling, with X-ray diffraction (XRD) using a Bruker D4
Endeavor X-ray Diffractometer with Cu Ka radiation and 2q
range of 10e80� and 0.02� step. Thematerials were loaded in a
commercial steel sample holder equipped with a Kapton foil
to protect the sample fromair. The ICDDdata basewas used to
evaluate the phase formation.
Dehydrogenation experiments were conducted with a
Sievert-type apparatus HyEnergy PCTpro2000. The Sievert-
type of set-up was successfully applied before to investigate
temperature programmed desorption (TPD) [19]. The samples
were prepared and measured applying the standard proce-
dure for each step. This includes a leak check; purge gas and
volume calibration at 100 �C. The dehydrogenation experi-
ments were performed in a static vacuum (staring level
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 4 2 0 0e4 2 0 6 4203
~0.1 bar). The TPD measurements as conducted between 100
and 350 �C with a heating rate of 3 �C/min. After approxi-
mately 4 or 2 h the pressure was reverted to the starting level
due to the pressure increase caused by hydrogen release from
themixture and temperature change. Therefore, blank tests in
the static vacuum were performed under similar TPD condi-
tions to estimate the effect of the pressure change and effec-
tive wt.% changes of the performed experiments. The blank
tests have shown small wt.% changes of the order of
~0.01e0.05 wt.%.
Fig. 2 e XRD diffraction patterns collected at room
temperature of the NaAlH4eMgH2 mixtures after 1 min to
6 h of ball-milling: a) 2q range of 29e66� and b) 2q range of
29e41�.
Results
Dehydrogenation
The effect of ball-milling time on the dehydrogenation
behaviour of the NaAlH4eMgH2 (1:1) mixture is shown in
Fig. 1. Prolonged ball-milling (30e360min) has an effect on the
dehydrogenation reaction rates. For these samples the initial
temperature at which hydrogen starts to get released is 150 �C,which is approximately 30 �C lower compared to the sample
milled for only 1 min (Fig. 1a). This effect may be associated
with the impact of high energy ball-milling on the grinded
samples. Prolonged milling of hydrides such as MgH2 leads to
decreased crystallite and particle sizes, and of the introduc-
tion of various defects, which enable a faster hydrogen
diffusion process in the bulk of the ball-milled materials
[20,21].
A significant temperature decrease of 60e80 �C, dependingon the sample, is also more clearly observed during the
completed first dehydrogenation step in Fig. 1a and b. The first
step of the dehydrogenation is completed at 290, 230, 210 and
220 �C for the 1, 30, 120 and 360 min milled samples, respec-
tively. These data indicate that the first step was stretched
over a large temperature range (between 180 and 290 �C) in the
sample milled for 1 min, while other samples released
hydrogen much faster in a significantly shorter temperature
range (from 150 to 210e230 �C). Typically, this step is associ-
ated with the formation of intermediate hydrides (see
reactions (1) and (6) [11e14]).
Further stages of dehydrogenation can be observed in
Fig. 1b in which, at constant temperature (350 �C), all the
milled compositions show two more distinctive dehydroge-
nation processes. These represent stages of further phase
transformations of the composites. A detailed structural XRD
analysis will be discussed below.
Ball milling also shows an effect on the amount of
hydrogen released. The lowest amount of released hydrogen
was found to correspond to the sample which was milled
during 1 min. This sample showed a weight decrease of
~5.2 wt.% at themaximum temperature of 350 �C compared to
the samples, which were ball-milled for 30 (~5.8 wt.%),
120 (~5.8 wt.%) and 360 min (~5.6 wt.%). Obviously, the
hydrogen release flow of the samplemilled for 1 min seems to
be quite unsteady and not continuous. This may be attributed
to bad homogeneity of the mixture. In contrast, the samples
milled for longer times show smooth hydrogen release curves.
Note that both samples ball-milled for 120 and 360 min show
similar dehydrogenation reaction rates and storage capacities,
therefore the optimummilling time in these experiments was
120 min since extended ball milling was not beneficial for the
rates and consumes more electricity.
Structural analysis
Fig. 2 shows the XRD results of the NaAlH4eMgH2 (1:1) mix-
tures ball-milled for 1e360min in a SPEXmill. Themain effect
of the prolongedmilling is the decreasing size of particles and
crystallites. This resulted in significant broadening of all re-
flections and vanishing of the smallest peaks of the starting
materials. The main starting materials NaAlH4, MgH2 and Mg
(always present as traces in the commercial magnesium hy-
dride) could be confirmed in the mixture milled for 1 min. In
the samples milled elongated times the traces of Mg vanished
in XRD due to, firstly, decreasing crystallites size and, sec-
ondly, potential reactions of themetal with other components
of the mixture.
Furthermore, several additional very small and broad XRD
peaks could be detected in the samples milled elongated
times. These peaks could be assigned to two phases which
typically are observed during the dehydrogenation
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 4 2 0 0e4 2 0 64204
(decomposition) of the NaAlH4eMgH2 mixture. The first group
of the reflections might be attributed to b-Na3AlH6 or NaMgH3
(marked with &) while the second group of reflections may be
fitted to Al, alternatively to Al0.95Mg0.05 (marked with x). Thedecomposition reactions may be sometimes initiated during
high energy ball-milling of various complex hydridemixtures.
The XRD of the dehydrogenated NaAlH4eMgH2 mixtures
which were previously ball-milled for 1 and 120 min are
shown in Fig. 3. For simplicity, only these two XRD patterns
are shown because the phase composition of the 120 min
sample resembles the two remaining samples. It should be
noted that the overall phase composition of all samples seems
to be somewhat similar, and the main differences are related
to the peak intensity of the components.
In these multi-component samples, the most distinct
phases that only partially overlap with other compounds are
NaH and Al12Mg17. A clear distinction between b-Na3AlH6 and
NaMgH3 is again hindered since the identification is exclu-
sively based on a few strongest reflections (many smallest are
missing) which are broad and overlap with each other and
some other phases.
Next to the above mentioned Al12Mg17 alloy, at least two
other alloys can be identified, such as AlMg and Al3Mg2. Also
other alloy candidates, assigned using some low intensity
reflections (not indicated on the figure for simplicity), may be
Fig. 3 e XRD diffraction patterns collected at room
temperature of the NaAlH4eMgH2 mixtures ball-milled (for
1 min and 2 h) and after dehydrogenation at 350 �C: a) 2qrange of 29e66� and b) 2q range of 31e41�.
proposed, such as Al5.15Mg3.15, Al3.16Mg1.84, and Al2Mg. How-
ever, most reflections of these phases overlap with other
proposed compounds. Many reflections of the smallest in-
tensity belonging to various AleMg alloys are observedmostly
as some disturbance in the background (very small and broad
peaks), therefore these are not shown since their assignment
is hampered by a limited interpretation.
Among the expected single metal compounds, such as Mg,
Al, and Na, none of these could be observed.
Discussion
Proposed dehydrogenation pathway for ball-milledNaAlH4eMgH2 (1:1)
Based on the above presented data we would like to propose
the following possible decomposition pathway of the
NaAlH4eMgH2 (1:1) mixture, Phase 1, as shown in Fig. 4. The
evaluation of the XRD data leads to the conclusion that
decomposition of themixture is already initiated during high-
energy ball-milling for 30 min and longer times. This is
possible because the mechanical grinding results in elevated
pressures and temperatures at the impact point of balls.
However, the major decomposition of the composite takes
place during TPD above 150 �C (Phase 2 below). For the 1 min
ball-milled sample, it is expected that our proposed dehy-
drogenation pathway is only initiated at elevated tempera-
tures during the TPD experiment as could be judged from XRD
(Phase 1 and Phase 2 occurring only during TPD). It is worth-
while to note that only a small quantity of hydrogen was
released during milling for 30 min and longer times, as can be
judged from the dehydrogenation experiments.
The first major step in the decomposition process of the
NaAlH4eMgH2 (1:1) composite may be ascribed to the forma-
tion of the intermediate hydride b-Na3AlH6, alternatively
NaMgH3, or both simultaneously. Literature indicates one
more possibility, reaction (9) [18], in which Na3AlH6 react with
MgH2 to form NaMgH3. According to the literature, the two
hydrides b-Na3AlH6 and NaMgH3 [12e14] are formed in a
similar temperature range as the observed during our TPD
study (first step of the dehydrogenation process). The evalu-
ation of the XRD data, even if challenging, allowed concluding
that coexistence of both hydrides is possible. Therefore, it
cannot be excluded that these hydrides form as a result of
competing reactions.
Fig. 4 e Possible decomposition pathway of NaAlH4eMgH2
(1:1) initiated by high-energy ball-milling e Phase 1.
Fig. 5 e The potential decomposition pathway of the ball-
milled NaAlH4eMgH2 (1:1) mixture observed at high-
temperature TPD experiments e Phase 2.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 4 2 0 0e4 2 0 6 4205
Additionally, Al and Al0.95Mg0.05 (Al1�xMgx) are the dehy-
drogenation products of the composite on the above shown
Fig. 4. The general term Al1�xMgx relates exclusively to the
molar ratio of Al:Mg, but not to the crystal structures of the
observed alloys. The Al metal is detected when b-Na3AlH6 and
NaMgH3 are formed, while Al0.95Mg0.05 may be formed from Al
and Mg (present in the starting hydrides) or Al and MgH2.
Proposed dehydrogenation pathway for NaAlH4eMgH2
(1:1) at high temperatures
At high-temperatures (TPD experiments) further decomposi-
tion of the ball-milled NaAlH4eMgH2 (1:1) mixture occurs,
Phase 2, as indicated in Fig. 5. In particular, the decomposition
of the b-Na3AlH6 and NaMgH3 hydrides is taking place and
several AleMg alloys (described by a general term Al1-xMgx)
can be formed. Previous research only identified Al12Mg17 as
being responsible for the improvements of the NaAlH4eMgH2
composite thermodynamics (independently of themolar ratio
of NaAlH4:MgH2) [12e14].
We believe that the alloying process may take place at all
steps of the decomposition (Phase 1 and Phase 2), as proposed
in Fig. 6. It starts with Al that may be doped with small amount
of Mg (Al0.95Mg0.05). Further, the reaction continues at high
temperatures with the formation of the aluminum rich alloys,
such as Al3Mg2, Al5.15Mg3.15, and Al2Mg. Subsequently, the alloy
with a unity molar ratio of Al:Mg (1:1) might be formed. Finally,
the Mg-rich alloy, previously reported as Al12Mg17, could be
identified. Therefore, the simplified alloy formula Al1�xMgx is
Fig. 6 e Proposed alloying process based on the
dehydrogenation experiments of NaAlH4eMgH2 (1:1).
used with x ¼ 0e0.7. With x ¼ 0, Al1�xMgx is a pure Al metal,
while Al1�xMgx with x ¼ 0.7 relates to Al12Mg17.
We believe that the discrepancies between literature and
what was observed in this study regarding the formation of
the alloys may be attributed to the different temperature
ranges used in the TPD studies, reaction times, and challenges
to satisfactory identify the decomposition products in XRD
data due to multi-phase samples and often poor crystalline
materials. Our experiments end at approximately 350 �Cbecause of practical reasons and this therefore results in the
formation of multiple alloys, while experiments from the
literature mostly end at much higher temperatures between
450 �C and 500 �C, and result in the formation of only a single
alloy with the composition: Al12Mg17 [12e14,18]. The latter
experiments also lead to the complete decomposition of NaH,
to the Na metal and hydrogen gas, which was not observed in
this study.
The remaining questions, that can be derived based on the
above discussion, are how these various AleMg alloys form
exactly and how these influence the reversibility and ther-
modynamic stability of the NaAlH4eMgH2 composite. These
issues are the subject of our further study that is currently
underway. An attempt to evaluate the participation of the two
hydrides, b-Na3AlH6 and NaMgH3, in the dehydrogenation
process will be a subject of a detailed study using IR spec-
troscopy. Our further investigations include the studies on the
effect of the catalysts on the properties of the mixture.
Conclusions
NaAlH4eMgH2 is an attractive material for future hydrogen-
storage applications. The influence of high-energy ball-mill-
ing, as a function of time, on the structural phase trans-
formations and on the dehydrogenation properties of
NaAlH4eMgH2 (1:1) was investigated. Expectedly, the pro-
longed ball-milling of the mixtures leads to significantly
enhanced dehydrogenation reaction rates, i.e. desorption of
hydrogen atmuch lower temperatures. Novel insights into the
decomposition reactions of the material were provided. A
number of AleMg alloys seem to participate in the dehydro-
genation via a complex alloying process of Al withMg orMgH2.
The obtained results suggest the two hydrides Na3AlH6 and
NaMgH3 coexist during decomposition of the mixture. The
study shed a light on the complexity of the decomposition
pathways of hydride mixtures in which the key role play
alloys.
Acknowledgements
The authors thank the Eindhoven University of Technology
and Department of Chemical Engineering and Chemistry;
EMPA Swiss Federal Laboratories for Materials Science and
Technology and Empa International Postdoctoral Fellowship
programme funded by FP7: People Marie-Curie action
COFUND (PCOFUND-GA-2010-267161) for financial support of
the research. Many thanks to Dr. A. Remhof, Dr. M. Polanski
and M.Sc. R. Kregting for critical reading of the manuscript.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 4 2 0 0e4 2 0 64206
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