Nanoscale structural characterization of Mg(NH3)6Cl2 during NH3 desorption: An in situ small angle X-ray scattering study

Article abstract of DOI:10.1016/j.cplett.2007.05.001

Complex metal hydrides progressively display improved hydrogen storage capacity, but they are still far from fulfilling the requirements of the transport sector. Recently, indirect storage of hydrogen as ammonia in Mg(NH₃)₆Cl₂ has shown impressive capacity and reversibility. Here, we present an in situ nanoscale structural characterization of the thermal decomposition of Mg(NH₃)₆Cl₂ using small angle X-ray scattering (SAXS). We observe the growth of polydisperse spherical Mg(NH₃)₂Cl₂ crystallites forming a skeletal structure, the subsequent agglomeration of MgCl₂ and formation of a nanoscale porosity consisting of 25-30 nm wide channels, which may account for the exceptional fast reloading of the material.

Full text of DOI:10.1016/j.cplett.2007.05.001

Chemical Physics Letters 441 (2007) 255–260
www.elsevier.com/locate/cplett
Nanoscale structural characterization of Mg(NH ) Cl during
3
6
2
NH desorption: An in situ small angle X-ray scattering study
3
a
b
c
Hjalte Sylvest Jacobsen , Heine Anton Hansen , Jens Wenzel Andreasen ,
a
a
d
Qing Shi , Anders Andreasen , Robert Feidenhans’l ,
Martin Meedom Nielsen , Kenny St a˚ hl , Tejs Vegge
e
f
a,
*
a
Materials Research Department, Risø National Laboratory, Technical University of Denmark, DK-4000 Roskilde, Denmark
Center for Atomic-scale Materials Design (CAMD), Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark
b
c
Polymer Department, Risø National Laboratory, Technical University of Denmark, DK-4000 Roskilde, Denmark
d
Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark
Centre for Molecular Movies, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark
e
f
Department of Chemistry, Technical University of Denmark, DK-2800 Lyngby, Denmark
Received 6 December 2006; in final form 25 April 2007
Available online 6 May 2007
Abstract
Complex metal hydrides progressively display improved hydrogen storage capacity, but they are still far from fulfilling the require-
ments of the transport sector. Recently, indirect storage of hydrogen as ammonia in Mg(NH Cl has shown impressive capacity and
reversibility. Here, we present an in situ nanoscale structural characterization of the thermal decomposition of Mg(NH Cl using small
angle X-ray scattering (SAXS). We observe the growth of polydisperse spherical Mg(NH ) Cl crystallites forming a skeletal structure,
3
)
6
2
3
)
6
2
3
2
2
the subsequent agglomeration of MgCl
account for the exceptional fast reloading of the material.
2007 Elsevier B.V. All rights reserved.
2
and formation of a nanoscale porosity consisting of 25–30 nm wide channels, which may
ꢀ
1
. Introduction
The decomposition of Mg(NH ) Cl occurs in steps [6]:
3 6 2
The use of hydrogen in the transport sector requires
MgðNH
3
Þ6Cl
2
! MgðNH
3
Þ2Cl
2
þ 4NH
3
novel materials with superior volumetric and gravimetric
hydrogen storage capacity, high reversibility and low
desorption temperatures.
Much interest has been focused on complex hydrides [1–
], but an equally promising approach binds hydrogen in
!
!
MgNH Cl þ 5NH
3
2
3
MgCl þ 6NH
ð1Þ
2
3
Mg(NH ) Cl can easily be compacted to 95% of the
4
3 6
2
3
bulk density (q = 1.24 g/cm ) and preserves its macro-
scopic shape during multiple de-/absorption cycles [6],
although the solid fraction shrinks to 25% of the initial vol-
ume during a full decomposition. A detailed structural
understanding at the nanoscale is, however, required to
improve the cyclic, kinetic and thermodynamic properties
of Mg(NH ) Cl . Here, we use in situ small angle X-ray
the form of ammonia stored in salts, e.g. Mg(NH ) Cl ; a
material also proposed for automotive selective catalytic
3
6
2
reduction (SCR) of NO [5]. Whereas storage of liquid
x
ammonia is highly problematic due to its poisonous and
corrosive nature, the NH vapor pressure of Mg(NH ) Cl
3
3 6
2
is non-toxic at ambient conditions.
3
6
2
scattering (SAXS) to investigate the thermal decomposition
of Mg(NH ) Cl , focusing specifically on the formation
*
3
6
2
Corresponding author. Fax: +45 4677 5758.
E-mail address: tejs.vegge@risoe.dk (T. Vegge).
and stability of intermediate phases and porosity.
0
009-2614/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.05.001

256
H.S. Jacobsen et al. / Chemical Physics Letters 441 (2007) 255–260
2
. Experimental
in steps of ꢁ20 ꢂC/12 min, and 5 min after the temperature
had stabilized, a 5 min measurement was performed. The
scattering data was subsequently corrected for background
and detector sensitivity (flat-field correction) followed by
an azimuthal average. Additional measurements in a high
q geometry (distance: 767 mm) covering the q range of
Time resolved in situ SAXS measurements of the ther-
mal decomposition of Mg(NH ) Cl have been performed
using a pinhole camera with a q range 3 · 10
3
6
2
ꢀ3
–
ꢀ
2
ꢀ1
˚
˚
8
· 10
rotating Cu anode was operated in fine focus mode at
0 kV and 40 mA and the beam diameter at the sample
A for Cu K radiation with k = 1.5418 A. The
a
ꢀ
1
˚
0.03–0.3 A were performed to investigate the reported
development of 2–4 nm pores [8].
4
position was 1.0 mm. An 18 · 18 cm 2D position sensitive
gas detector was used for collecting the scattering data,
and a 4 mm beamstop was placed in front of the gas detec-
tor, situated 4656 mm from the sample.
3. Results and discussion
3.1. Data presentation
The sample was loaded in a 0.7 mm quartz capillary
tube with 0.01 mm walls (Markr o¨ hrchen, Hilgenberg
GmbH), between quartz wool plugs. The tube was con-
When the sample was heated from 128 ꢂC to 146 ꢂC, the
observed total scattering intensity increased by a factor of
3.6 due to an increased surface area during decomposition
of Mg(NH ) Cl . A ‘knee’-like Guinier regime appears in
nected to gas pipes by graphite ferrules with gastight Swa-
ꢁ
gelok fittings. A gas system for supplying He and NH to
3
3 6
2
the sample was constructed for un-/reloading of the sam-
ples. The capillary tube was placed in a cylindrical heater
inside the in situ cell. The temperature was measured by a
thermocouple placed between the walls of the heater and
the capillary tube; see Andreasen et al. [7]. The cell was
filled with He at 2 bars to ascertain efficient heat transport
inside the cell. The collimation section and flight tubes were
the scattering data at 146 ꢂC (dashed/magenta curve in
Fig. 1a), and the position decreases in q-space as the tem-
perature is increased further (Fig. 1b). A second Guinier
regime (Fig. 1c) appears at 249 ꢂC and its position also
decreases in q-space with increasing temperature
(Fig. 1d). The observed decrease in q-space corresponds
to growth of the crystallites (movie 1) and pores (movie
2), respectively; see Section 3.3.
ꢀ
5
evacuated to 10 mbar. The sample was heated to 350 ꢂC
a1000
b 1000
T=146 o
C
T=183 o
C
1
00
100
Mg(NH ) Cl crystallites
3
2
2
Mg(NH ) Cl crystallites
3
2
2
1
0
10
1
1
0
.1
0.1
0.01
0
.01
0
.1
q / [nm-1
1
0.1
q / [nm-1
1
]
]
c1000
d 1000
T=289 o
C
T=348 o
C
1
00
100
Pores
Pores
1
0
1
10
1
0
.1
0.1
0.01
0
.01
0
.1
q / [nm-1
1
0.1
q / [nm-1
1
]
]
Fig. 1. The small angle scattering cross section as a function of q at four different temperatures. The unified equation (solid/green) is decomposed to show
the location of the Guinier regimes (dashed/magenta). The unified function is only fitted to the red data points. (For interpretation of the references in
colour in this figure legend, the reader is referred to the web version of this article.)

H.S. Jacobsen et al. / Chemical Physics Letters 441 (2007) 255–260
257
3
.2. Theory
where Q is the Porod invariant pertaining to the particular
structural level. The distribution of particle sizes can be de-
scribed by the polydispersity index (PDI) [11]
The scattering data is analyzed using the unified equa-
tion proposed by Beaucage and Schaefer [9], where the
scattering intensity of N non-interrelated structural levels
with radii of gyrations Rg,i can be approximated by
4
BR
PDI ¼ 1
;
ð6Þ
:62G
0
1
which is normalized to one for monodisperse spheres and
increases with dispersion and anisotropy of the scatterers.
!
ꢀPi
Nꢀ1
2
2
g;i
X
q R
q
@
A
IðqÞ ffi
G
i
exp
ꢀ
þ B
i
;
À
À
pffiffiÁÁ
3
3
i¼0
erf qkR = 6
g;i
3.3. Growth of Mg(NH ) Cl crystallites
3 2 2
ð2Þ
The Guinier regime appearing between 128 ꢂC and
46 ꢂC is not observed in the experimental q range after
where G is the exponential prefactor, B is a prefactor spe-
cific to the power law scattering and k is an empirical con-
stant of 1 for steep power law decays, P > 3 [10]. At high q
relative to Rg,i the second term decreases as q , i.e. a
power law regime, which describes surface scattering. Pi
is structure dependant, e.g. smooth spheres display
P = 4, whereas rods and discs have an intermediate region
between the two Guinier regimes from the length and the
radius, with P = 1 and P = 2, respectively [7]. Rugged
interfaces have different P ’s, e.g. surface fractals display
< P < 4 [9]. The first term, the Guinier function, de-
i
scribes the low q scattering of a structural level with a char-
acteristic length scale. SAXS data can generally be
described by using a small part of Eq. (2) due to the limited
experimental q range. From 146 ꢂC to 268 ꢂC, the data is
well described by
1
2
68 ꢂC. This R is too large for a pore structure, because
g
i
it is >10 nm already at 146 ꢂC and nitrogen adsorption
measurements by Hummelshøj et al. showed no significant
pore structure with sizes above 10 nm until 91.7% ammonia
ꢀ
Pi
desorption [8] (corresponding to ꢁ300 ꢂC). Here, R is
g
i
found to exceed 40 nm at 268 ꢂC (Fig. 2). The massive
increase in scattering intensity prior to the first observation
of the Guinier knee at 146 ꢂC strongly suggests the struc-
ture is phase segregated Mg(NH ) Cl . The ammonia
i
i
i
3
2
2
3
desorption and higher density of the Mg(NH ) Cl phase
3
2
2
3
(
q = 1.70 g/cm ) yield a substantial decrease of the material
volume, a significant increase in the void fraction between
the crystallites and the formation of a high surface area
skeletal structure (Fig. 3a and b).
To determine the shape and dispersion of the scattering
objects, X-ray powder diffraction (XRPD) data, collected
at beamline I711 at the MAX-II synchrotron in Lund, Swe-
den [12], is included in the analysis (Fig. 4). The simplest
possible shape of the Mg(NH ) Cl crystallites, monodis-
!
!ꢀ
4
2
2
g;1
q R
q
ꢀ
P0
IðqÞffiB
0
q
þG
1
exp ꢀ
þB
1
pffiffiffi
;
3
3
ðerfðqR = 6ÞÞ
g;1
ð3aÞ
3
2
2
1
/2
perse spheres, fulfills (5/3) R = 3 V/S, which is not
g
where the first term describes scattering from large agglom-
erates and the last two terms describe the scattering from
Mg(NH ) Cl crystallites
the formed Mg(NH ) Cl crystallites and later aggregated
3
2
2
3 2
2
MgCl . From 249 ꢂC to 348 ꢂC, the data is described by
50
40
30
50
40
30
20
10
0
2
Rg,1
!
!ꢀ
4
2
2
g;1
q R
q
IðqÞ ffi G exp
ꢀ
þ B1
pffiffi
1
3
3
ðerfðqR = 6ÞÞ
g;1
!
!ꢀ
4
2
2
g;2
q R
q
þ G exp
ꢀ
þ B2
pffiffi
:
2
3
3
ðerfðqR = 6ÞÞ
g;2
ð3bÞ
The first two terms describe scattering from aggregated
2
1
0
0
MgCl and the last two terms scattering from a porous
2
structure. P and P are fixed to a value of 4 (see Sections
1
2
3
.3 and 3.4).
From (3), the average surface ÆSæ to volume Æ Væ ratio of
a particular structural level can be obtained without assum-
ing a shape of the scattering level [11]
〈
V〉 / 〈S〉
0
1
40
160
180
200
220
240
260
280
T / [°C]
hSi pB
¼
;
ð4Þ
ð5Þ
hV i
Q
Fig. 2. The radius of gyration, Rg,1, and volume to surface ratio, ÆVæ/ÆSæ
between 140 ꢂC and 280 ꢂC, where each data-point represents a 5 min.
measurement Monodisperse spheres fulfill R
points should coincide if the crystallites were monodisperse spheres.
Z
1
2
Q ¼
q IðqÞdq;
g
= 2.324 Æ ÆVæ/ÆSæ and the
0

258
H.S. Jacobsen et al. / Chemical Physics Letters 441 (2007) 255–260
3
Fig. 3. 2 D schematic representation of the sample during decomposition. (a) is the initial Mg(NH
3
)
6
Cl
(q = 2.35 g/cm ) at 348 ꢂC. The size and shape of the crystallites in (a)–(c) are obtained from
XRPD having average diameters of ꢁ60 nm, ꢁ55 nm and ꢁ45 nm, respectively. The first structure observed by SAXS, with a median diameter of 59 nm at
28 ꢂC, is crystallites forming a skeletal network. The second structure with a radius of 15 nm at 348 ꢂC, is pores in the crystallites, where the MgCl
2
(q = 1.24 g/cm ) sample, (b) is the intermediate
3
3
Mg(NH
3
)
2
Cl
2
(q = 1.70 g/cm ) at 228 ꢂC and (c) is MgCl
2
2
2
crystallites have aggregated and grown larger than 70 nm. The volume of the voids/pores increases from 5% in (a) to 55% in (b) and 71% in (c). In (c), the
pores are randomly distributed in the material. The magnifications show arbitrary lattice planes, to illustrate the crystalline domains.
MgCl2
Size = ~45 nm
Size = ~55 nm
Size = ~60 nm
Mg(NH ) Cl
T=210 C
3
2
2
2
o
Mg(NH ) Cl
3
6
o
T=23 C
1
1.5
2
2.5
3
3.5
q / [Å
]
˚
Fig. 4. Synchrotron XRPD patterns of Mg(NH
3
)
2
Cl
2
(bottom) and Mg(NH Cl
3
)
6
2
(middle) using k = 1.2724 A. The patterns were recorded with a Huber
G670 powder diffractometer equipped with a Huber G670.3 capillary furnace. XRPD patterns of MgCl
2
(top) collected using a Bragg–Brentano STOE
˚
a
diffractometer (CuK with k = 1.5418 A). The crystallite sizes are determined by the Scherrer-equation.
!
2
observed (Fig. 2), either because the crystallites are polydis-
perse or anisotropic in shape. Synchrotron XRPD experi-
ments show the size of the orthorhombic [13] Mg(NH ) Cl
2
1
ðlnðR=mÞÞ
f ðRÞ ¼ pffiffiffiffiffiexp
ꢀ
:
ð7Þ
2
Rr 2p
2r
3
2
crystallites to be ꢁ55 nm without a preferred growth
direction (Fig. 4). The volume weighted average diameter
obtained from the SAXS data at 228 ꢂC is 59 nm, indicating
no agglomeration and the Mg(NH ) Cl are thus assumed
The standard deviation r and the median radius m of a
lognormal distribution of spheres may be obtained as [11]
ꢀ
ꢁ
1=2
lnðPDIÞ
3
2
2
r ¼
;
ð8Þ
ð9Þ
1
2
to be spherical and polydisperse crystallites.
Growth in equilibrium often displays a 2-parameter log-
normal distribution of sizes where the density function f is
a function of the radius R
!
1
=2
2
5
R
g
m ¼
:
2
3
exp ð14r Þ

H.S. Jacobsen et al. / Chemical Physics Letters 441 (2007) 255–260
259
Table 1
Pores
The median radius m and standard deviation r of lognormal size
distributed crystallites
2
1
0
5
10
7.5
5
Rg,2
3 2 2
Mg(NH ) Cl crystallites
T (ꢂC)
m (nm)
r
1
1
2
2
2
2
2
46
83
03
07
28
49
68
2.73
4.99
12.2
17.7
26.4
29.7
36.1
0.511
0.451
0.333
0.317
0.274
0.267
0.242
10
5
Above 249 ꢂC, the crystallites have started to decompose to MgCl
and r are for the aggregated MgCl
2
and m
2
.
2.5
0
From our SAXS experiments, we determine the median
radius and the standard deviation of lognormally distrib-
uted spherical Mg(NH ) Cl crystallites (Table 1). The
median radius increases with temperature until 268 ꢂC
where it is no longer observable (>70–80 nm), which
together with the XRPD data (Fig. 4) indicates some aggre-
PDI
0
2
40 260 280 300 320 340 360 380 400
3
2
2
T / [°C]
Fig. 5. Plot of the radius of gyration, Rg,2, and polydispersity index (PDI)
between 240 ꢂC and 400 ꢂC, where the second Guinier regime is observed.
The points at 390 ꢂC correspond to the degassed sample cooled to 65 ꢂC
gation of the MgCl crystallites (Fig. 3c). The standard
2
(
displaced for clarity).
deviation decreases at higher temperatures and stabilizes
at 228 ꢂC, where the decomposition to Mg(NH ) Cl is
3
2
2
complete [6]. At 268 ꢂC, the standard deviation decreases
the intermediate power law scaling and the Guinier regime
from the length of the pores. However, Rg,2 and the calcu-
lated PDI still display interesting features, which can be
related to the pore structure (Fig. 5). The radius of gyration
is stable at 9.3 nm from 249 ꢂC to 268 ꢂC and increase to
17 nm from 268 ꢂC to 318 ꢂC, where it re-stabilizes. The
PDI displays an analogous development being stable at
7.6 until 289 ꢂC, then decreasing to 6.0 and re-stabilizing
above 318 ꢂC. The pore structure is first observed during
the decomposition of Mg(NH ) Cl , and the average pore
further.
3
.4. Pore growth
Existing BJH data show a cylindrical pore structure with
a characteristic mean radius of 9.6 nm after 91.7% ammo-
nia desorption [8], corresponding to a R of 8.3 nm for
g
monodisperse cylinders. In our data, a second Guinier
regime is observed with Rg,2 of 12–17 nm after 83–100%
desorption. The discrepancy is likely due to the simplifica-
tion of the unified equation and the different probing meth-
ods. This second Guinier regime is thus expected to be
scattering from a porous structure, since no addi-
tional structural regimes are observed during continued
heating.
3 2
2
radius increases by a factor of two. The decrease in the
PDI could correspond to a decrease in the dispersion of
the pore radius, indicative of saturation at a maximal pore
diameter, although this is non-conclusive, due to the
restricted use of the unified equation. No additional Gui-
nier regimes are observed within the experimental temper-
ature and q range, signifying no nucleation and growth of
well structured, phase segregated MgNH Cl or MgCl ;
Scattering from a cylindrical structure displays one Gui-
nier regime from the radius and one from the length of the
cylinder, separated by a power law scaling of P = 1 and
scaling with P = 4 at high q. The intermediate power law
scaling and the Guinier regime from the length is not dis-
cernible in the SAXS data, because the signal drowns in
the scattering from Mg(NH ) Cl . Convergence between
3
2
2
indicating the skeletal structure of the Mg(NH ) Cl crys-
3
2
2
tallites is maintained. A subsequent reloading of the sample
with ammonia at 1.5 bars displayed a fast regeneration of
the Mg(NH ) Cl phase, which we attribute to fast ammo-
3
6
2
nia transport through the pores inside the MgCl2
aggregates.
3
2
2
the SAXS data and the unified equation can only be
obtained by describing the scattering from the pores by a
unified equation where the Guinier regime from the length
and the intermediate power law (P = 1) is omitted; result-
ing in a radius of gyration which slightly overestimates
the average radius of cylindrical pores. The existing BJH
analysis shows the cylindrical pore structure to have a log-
normal distribution of the radius, with m = 9.6 nm and
r = 0.20 [8]. To calculate the median radius and standard
deviation of the pore structure, it is necessary to resolve
ꢀ
1
˚
In our experiments in high q geometry (0.03–0.3 A ),
we observe no Guinier regime during heating of
Mg(NH ) Cl from 20 ꢂC to 350 ꢂC, which strongly indi-
3
6
2
cates that there is no porous structure with a characteristic
size between 0.5 nm and 5 nm. A possible explanation for
the pore structure with a diameter of 2–4 nm observed in
the BJH analysis [8] is the so-called tensile strength effect,
which has often been assigned to pores of approximately
3.8 nm, when using N at 77 K [14].
2

260
H.S. Jacobsen et al. / Chemical Physics Letters 441 (2007) 255–260
4
. Conclusions
Desorption of ammonia from Mg(NH ) Cl has been
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.cplett.
2007.05.001.
3
6
2
ꢀ
3
examined by in situ SAXS in the q range 3 · 10
0
–
ꢀ
1
˚
.3 A corresponding to structures from a few nanometers
up to ꢁ40 nm in radius. During heating, the formation of
Mg(NH ) Cl crystallites is first observed at 146 ꢂC. By
References
3
2
2
continued heating, these crystallites disappear, agglomerate
or grow beyond the experimental resolution. Secondly, a
pore structure develops at temperatures above 249 ꢂC.
The SAXS and XRPD data presented here are well
described by the crystallites as lognormal distributed
spheres and the pores as tubular with a radius of 12–15 nm.
This increased understanding of ammonia de-/absorp-
tion in Mg(NH ) Cl can lead to superior functionality of
[
[
[
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