Nanoscale structural characterization of Mg(NH3)6Cl2
during NH3 desorption: An in situ small angle X-ray scattering study Hjalte Sylvest Jacobsen, Heine Anton Hansen, Jens Wenzel Andreasen, Qing Shi, Anders Andreasen, Robert Feidenhans’l, Martin Meedom Nielsen, Kenny Ståhl, and Tejs Vegge
Preprint submitted to Chem. Phys. Lett.
Published in Chem. Phys. Lett. 441 (2007) 255-260 http://dx.doi.org/10.1016/j.cplett.2007.05.001
Nanoscale structural characterization of Mg(NH3)6Cl2
during NH3 desorption: An in situ small angle X-ray
scattering study
Hjalte Sylvest Jacobsen,a Heine Anton Hansen,
b Jens Wenzel Andreasen,
c Qing Shi,
a Anders
Andreasen,a Robert Feidenhans’l,
d Martin Meedom Nielsen,
e Kenny Ståhl,
e and Tejs Vegge
a,*
a Materials Research Department, Risø National Laboratory, Technical University of Denmark, DK-
4000 Roskilde, Denmark.
b Center for Atomic-scale Materials design (CAMd), Department of Physics, Technical University of
Denmark, DK-2800 Lyngby, Denmark.
c Danish Polymer Centre, Risø National Laboratory, DK-4000 Roskilde, Denmark.
d Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen , Denmark.
e Centre for Molecular Movies, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen,
Denmark.
f Department of Chemistry, Technical University of Denmark, DK-2800 Lyngby, Denmark.
1
* Corresponding author: Tejs Vegge, Materials Research Department, Risø National Laboratory,
Technical University of Denmark, Frederiksborgvej 399, P.O. Box 49, DK-4000 Roskilde, Denmark.,
E-mail: [email protected], Fax: +45 4677 5758.
2
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.
3
1. Introduction
The use of hydrogen in the transport sector requires novel materials with superior volumetric- and
gravimetric hydrogen storage capacity, high reversibility and low desorption temperatures.
Much interest has been focused on complex hydrides [1,2,3], but an equally promising approach
binds hydrogen in the form of ammonia stored in salts, e.g. Mg(NH3)6Cl2; a material also proposed for
automotive selective catalytic reduction (SCR) of NOx [4]. Whereas storage of liquid ammonia is highly
problematic due to its poisonous and corrosive nature, the NH3 vapor pressure of Mg(NH3)6Cl2 is non-
toxic at ambient conditions.
The decomposition of Mg(NH3)6Cl2 is reported to occur via a series of steps [5]:
( ) ( )32323
3223263
NH6MgClNH5ClMgNH
NH4ClNHMgClNHMg
+→+
→+→ (1)
Mg(NH3)6Cl2 can easily be compacted to 95% of the bulk density (ȡ = 1.24 g/cm3) and preserves its
macroscopic shape during multiple de-/absorption cycles [5], although the solid fraction shrinks to 25 %
of the initial volume during a full decomposition. A detailed structural understanding of the system at
the nanoscale is, however, required to improve the cyclic, kinetic and thermodynamic properties of
Mg(NH3)6Cl2, where e.g. the desorption temperature of the last ammonia is very high, i.e. ~320 ºC [5].
Here, we use in situ small angle X-ray scattering (SAXS) to establish a structural understanding of the
thermal decomposition of Mg(NH3)6Cl2, focusing specifically on the formation and stability of
intermediate phases and porosity.
2. Experimental
Time resolved in situ SAXS measurements of the thermal decomposition of Mg(NH3)6Cl2 have been
4
performed using a pinhole camera with a q range 3·10-3-8·10-2 Å-1 for Cu Kg radiation with Ȝ=1.5418 Å.
The rotating Cu anode was operated in fine focus mode at 40 kV and 40 mA and the beam diameter at
the sample position was 1.0 mm. An 18x18 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 detector, situated
4656 mm from the sample.
The sample was loaded in a 0.7 mm quartz capillary tube with 0.01 mm walls (Markröhrchen,
Hilgenberg GmbH), between quartz wool plugs. The tube was connected to gas pipes by graphite
ferrules with gastight Swagelok® fittings. A gas system for supplying He and NH3 to the sample was
constructed for un-/reloading of the samples. 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. [6]. The cell was filled with He at 2 bars to ascertain
efficient heat transport inside the cell and to minimize background scattering. The collimation section
and flight tubes were evacuated to 10-5 mbar. The sample was heated from 100 °C to 350 °C in steps of
~20 °C/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 0.03-0.3 Å-1 were performed to investigate the reported development
of 2-4 nm pores [7].
3. Results and discussion
3.1 Data presentation
When the sample was heated from 128 °C to 146 °C, the total observed scattering intensity increased
by a factor of 3.6 due to an increased surface area during decomposition of Mg(NH3)6Cl2. A ‘knee’-like
5
Guinier regime appears in the scattering data at 146 °C (dashed/magenta curve in Figure 1a), and the
position decreases in q-space as the temperature is increased further (Figure 1b). A second Guinier
regime (Figure 1c) appears at 249 °C and its position also decreases in q-space with increasing
temperature (Figure 1d). The observed decrease in q-space corresponds to growth of the crystallites
(movie1.gif) and pores (movie2.gif), respectively; see section 3.3.
3.2 Theory
The scattering data is analyzed using the unified equation proposed by Beaucage [8], where the
scattering intensity of N non-interrelated structural levels with radii of gyrations Rg,i can be
approximated by
( ) ( )( )∑−
=
−
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛+⎟
⎟⎠
⎞⎜⎜⎝
⎛−≅
1
03
,
2,
2
63exp
N
i
P
ig
i
ig
i
i
qkRerf
qB
RqGqI , (2)
where G is the exponential prefactor, B is a prefactor specific to the power law scattering and k is an
empirical constant of 1 for steep power law decays, Pi>3 [9]. At high q relative to Rg,i the second term
decreases as q-Pi, i.e. a power law regime, which describes surface scattering. Pi is structure dependant,
e.g. smooth spheres display Pi = 4, whereas rods and discs have an intermediate region between the two
Guinier regimes from the length and the radius, with Pi = 1 and Pi = 2, respectively [9]. Rugged
interfaces have different Pi’s, e.g. surface fractals display 3<Pi<4 [8]. The first term, the Guinier
function, describes the low q scattering of a structural level with a characteristic 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 °C to 268 °C, the data is well described by
( ) ( )( )
4
3
1,
1
21,
2
10
63exp0
−
−
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛+⎟
⎟⎠
⎞⎜⎜⎝
⎛−+≅
g
gP
qRerf
qB
RqGqBqI (3a)
6
where the first term describes scattering from large agglomerates and the two last terms describe the
scattering from the formed Mg(NH3)2Cl2 crystallites and later aggregated MgCl2. From 249 °C to 348
°C, the data is described by
( ) ( )( )
( )( )
4
3
2,
2
22,
2
2
4
3
1,
1
21,
2
1
63exp
63exp
−
−
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛+⎟
⎟⎠
⎞⎜⎜⎝
⎛−
+⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛+⎟
⎟⎠
⎞⎜⎜⎝
⎛−≅
g
g
g
g
qRerf
qB
RqG
qRerf
qB
RqGqI
(3b)
The first two terms describe scattering from aggregated MgCl2 and the last two terms scattering from
a porous structure. P1 and P2 are fixed to a value of 4 (see sections 3.3 and 3.4).
From (3), the average surface <S> to volume <V> ratio of a particular structural level can be
obtained without assuming a shape of the scattering level [10]
Q
B
V
S π= , (4)
∫∞
=0
2 )( dqqIqQ , (5)
where Q is the Porod invariant pertaining to the particular structural level. The distribution of particle
sizes can be described by the Poly Dispersity Index (PDI) [10]
G
BRPDI
62.1
4
= , (6)
which is normalized to one for monodisperse spheres and increases with dispersion and anisotropy of
the scatterers.
3.3 Mg(NH3)2Cl2 growth
7
The Guinier regime appearing between 128 °C and 146 °C is not observed in the experimental q range
after 268 °C. This Rg is too large for a pore structure, because it is already >10 nm at 146 °C and
nitrogen adsorption measurements by Hummelshøj et al. showed no significant pore structure with sizes
above 10 nm until 91.7 % ammonia desorption [7] (corresponding to ~300 °C). Here, the Rg is found to
exceed 40 nm at 268 °C (Figure 2). The massive increase in scattering intensity prior to the first
observation of the Guinier knee at 146 °C strongly suggests the structure is phase segregated
Mg(NH3)2Cl2. The ammonia desorption and higher density of the Mg(NH3)2Cl2 phase (ȡ = 1.70 g/cm3)
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 (Figure 3a-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, Sweden [11], is included in the
analysis (Figure 4). The simplest possible shape of the Mg(NH3)2Cl2 crystallites, monodisperse spheres,
fulfills (5/3)½Rg = 3V/S, which is not observed (Figure 2), either because the crystallites are
polydisperse or of anisotropic shape. Synchrotron XRPD experiments show the size of the orthorhombic
[12] Mg(NH3)2Cl2 crystallites to be 48-58 nm without a preferred growth direction (Figure 4). The
median diameter of the Mg(NH3)2Cl2 obtain from the SAXS data at 228 °C is ~52 nm, indicating no
agglomeration and the Mg(NH3)2Cl2 are thus assumed to be spherical and polydisperse crystallites.
Growth in equilibrium often displays a 2-parameter lognormal distribution of sizes where the density
function f is a function of the radius R
( )( )⎟⎟⎠
⎞⎜⎜⎝
⎛−=
2
2
2
/lnexp
2
1)(
σπσmR
RRf . (8)
The standard deviation ı and the median radius m of a lognormal distribution of spheres may be
obtained as [10]
8
( ) 21
12
ln⎟⎠⎞
⎜⎝⎛=
PDIσ , (9)
( )2
1
2
2
14exp3
5
⎟⎟⎠
⎞⎜⎜⎝
⎛=
σgR
m . (10)
From our SAXS experiments, we determine the median radius and the standard deviation of
lognormally distributed spherical Mg(NH3)2Cl2 crystallites (Table 1). The median radius increases with
temperature until 268 °C where it is no longer observable (>70-80 nm), which together with the XRPD
data (Figure 4) indicates some aggregation of the MgCl2 crystallites (Figure 3c). The standard deviation
decreases at higher temperatures and stabilizes at 228 °C, where the decomposition to Mg(NH3)2Cl2 is
complete [5]. At 268 °C, the standard deviation decreases 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 % ammonia desorption [7], corresponding to a Rg of 8.3 nm for 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 simplification of the unified equation and the different probing methods.
This second Guinier regime is thus expected to be scattering from a porous structure, since no additional
structural regimes are observed during continued heating.
Scattering from a cylindrical structure displays one Guinier 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 visible in the SAXS data,
because the signal drowns in the scattering from the Mg(NH3)2Cl2. Convergence between 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
9
omitted. Applying this simplified equation results in a radius of gyration which slightly overestimates
the average radius of cylindrical pores. The existing BJH analysis shows the cylindrical pore structure to
has a lognormal distribution of the radius, with m = 9.6 nm and ı = 0.20 [7]. To calculate the median
radius and standard deviation of the pore structure, it is necessary to resolve the intermediate power law
scaling and the Guinier regime from the length of the pores. However, Rg,2 and the calculated PDI still
display interesting features, which can be related to the pore structure (Figure 5). The radius of gyration
is stable at 9.3 nm from 249 °C to 268 °C and increase to 17 nm from 268 °C to 318 °C, where it re-
stabilizes. The PDI displays an analogous development being stable at 7.6 until 289 °C, then decreasing
to 6.0 and re-stabilizing above 318 °C. The pore structure is first observed during the decomposition of
Mg(NH3)2Cl2, and the average pore radius increase 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; this is not conclusive due to the simplification of the unified equation. No additional Guinier
regimes are observed within the experimental temperature and q range, signifying no growth of well
structured, phase segregated MgNH3Cl2 or MgCl2, thus indicating the skeletal structure of the
Mg(NH3)2Cl2 crystallites is maintained. A subsequent reloading of the sample at room temperature with
ammonia at 1.5 bars displayed a fast regeneration of the Mg(NH3)6Cl2 phase, which we attribute to fast
ammonia transport through the pores inside the MgCl2 aggregates.
In our experiments in high q geometry (0.03-0.3 Å-1), we observe no Guinier regime during heating of
Mg(NH3)6Cl2 from 20 °C to 350 °C, which strongly indicates 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 [7] is the so-called tensile strength effect, which has
often been assigned to pores of approximately 3.8 nm, when using N2 at 77 K [13].
4. Conclusions
10
The desorption of ammonia from Mg(NH3)6Cl2 has been examined by in situ SAXS in the q range
3·10-3-0.3 Å-1 corresponding to structures from a few nanometers up to ~40 nm in radius. During
heating, the formation of Mg(NH3)2Cl2 crystallites is first observed at 146 °C. By continued heating,
these crystallites disappear, agglomerate or grow beyond the experimental resolution. Secondly, a pore
structure develops at temperatures above 249 °C. The SAXS data presented here, complemented by
BJH [7] and XRPD [11] data 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-/absorption in Mg(NH3)6Cl2 can lead to superior
functionality of metal ammines as indirect hydrogen carriers and SCR materials. The skeletal aggregate
structure with narrow and elongated pores could explain why Mg(NH3)6Cl2 is easily reloaded and keeps
its macroscopic structure during decomposition unlike most other hydrogen storage materials.
Acknowledgements
The present work has received funding from the Danish Research Council for Strategic Research
(NABIIT). The authors acknowledge Rasmus Zink Sørensen and Jens Strabo Hummelshøj for supplying
samples and valuable access to data.
References
11
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[3] Z. Łodziana, T. Vegge, Phys. Rev. Lett. 93 (2004) 145501.
12
[4] T.D. Elmøe, R.Z. Sørensen, U. Quaade, C.H. Christensen, J.K. Nørskov, T. Johannessen, Chem.
Eng. Sci. 61 (2006) 2618-2625.
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Køhler, J.K. Nørskov, J. Mater. Chem. 15 (2005), 4106-4108.
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Molenbroek, G. Goerigk, J. Appl. Cryst. 36 (2003) 812-813.
[7] J.S. Hummelshøj, R.Z. Sørensen, M.Y. Kostova, T. Johannessen, J.K. Nørskov, C.H. Christensen,
J. Am. Chem. Soc. 128 (2006) 16-17.
[8] G. Beaucage, D.W. Schaefer, J. Non-cryst Solids. 172 (1994) 797-805.
[9] G. Beaucage, J. Appl. Cryst. 29 (1996) 134-146.
[10] G. Beaucage, H.K. Kammler, S.E. Pratsinis, J. Appl. Cryst. 37 (2004) 523-535.
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Synchrotron Rad. 7, (2000) 203-208.
[12] A. Leineweber, M.W. Friedriszik, H. Jacobs, J. Solid State Chem. 147 (1999) 229-234.
[13] J.C. Groen, L.A.A. Peffer, J. Pérez-Ramírez, Micropor. Mesopor. Mater. 60 (2003) 1-17.
Figure 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.
Figure 2: The radius of gyration, Rg,1, and volume to surface ratio, <V>/<S> between 140 °C and 280
°C, where each data-point represents a 5 min. measurement Monodisperse spheres fulfill Rg =
2.324·<V>/<S> and the points should coincide if the crystallites were monodisperse spheres.
Figure 3: 2D schematic representation of the sample during decomposition. a) is the initial
Mg(NH3)6Cl2 (と = 1.24 g/cm3) sample, b) is the intermediate Mg(NH3)2Cl2 (と = 1.70 g/cm3) at 228 °C
and c) is MgCl2 (と = 2.35 g/cm3) at 348 °C. The size and shape of the crystallites in a), b) and 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 53 nm at 228 °C, is crystallites forming a
skeletal network. The second structure with a radius of 15 nm at 348 °C, is pores in the crystallites,
where the mgCl2 crystallites have aggregated and grown larger than 70 nm. The volume of the
voids/pores increase 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 the crystal planes (100) of the crystallites.
Figure 4: Synchrotron XRPD patterns of Mg(NH3)2Cl2 and Mg(NH3)6Cl2 using そ = 1.2724 Å. The
patterns were recorded with a Huber G670 powder diffractometer equipped with a Huber G670.3
capillary furnace. XRPD patterns of MgCl2 collected using a Bregg-Brantano STOE diffractometer
(CuKg with そ = 1.5418 Å). The crystallite sizes are determined by the Scherrer-equation.
Figure 5: Plot of the radius of gyration, Rg,2, and Poly Dispersity Index (PDI) between 240 °C and 400
°C, where the second Guinier regime is observed. The points at 390 °C correspond to the degassed
sample cooled to 65 °C (displaced for clarity).
Table 1: The median radius m and standard deviation ı of lognormal size distributed crystallites, above
249°C the crystallites have started to decompose to MgCl2 and m is associated to the aggregated
MgCl2..
Mg(NH3)2Cl2 crystallites
T / [°C] m / [nm] σ
146 2.73 0.511
183 4.99 0.451
203 12.2 0.333
207 17.7 0.317
228 26.4 0.274
249 29.7 0.267
268 36.1 0.242
(a)
0.01
0.1
1
10
100
1000
0.1 1
Inte
nsity
/ [a
rb.u
.]
q / [nm-1]
Mg(NH3)2Cl2 crystallites
T=146 Co
(b)
0.01
0.1
1
10
100
1000
0.1 1
Inte
nsity
/ [a
rb.u
.]
q / [nm-1]
Mg(NH3)2Cl2 crystallites
T=183 Co
(c)
0.01
0.1
1
10
100
1000
0.1 1
Inte
nsity
/ [a
rb.u
.]
q / [nm-1]
Pores
T=289 Co
(d)
0.01
0.1
1
10
100
1000
0.1 1
Inte
nsity
/ [a
rb.u
.]
q / [nm-1]
Pores
T=348 Co
140 160 180 200 220 240 260 2800
10
20
30
40
50
T / [°C]
Rg,
1 / [n
m]
R
g,1
0
10
20
30
40
50
⟨V⟩
/ ⟨S
⟩ ×2.
324
/ [nm
]
Mg(NH3)2Cl
2 aggregates
⟨V⟩ / ⟨S⟩
a) b) c)
1 1.5 2 2.5 3 3.5
Inte
nsity
/ [a
rb.u
.]
q / [Å−1]
MgCl2
Mg(NH3)2Cl2
Mg(NH3)6Cl2
Size = ~45 nm
Size = ~55 nm
Size = ~60 nm
240 260 280 300 320 340 360 380 4000
5
10
15
20
T / [°C]
Rg,
2 / [n
m]
R
g,2
0
2.5
5
7.5
10
PD
I
Pores
PDI
