Published on Web 12/09/2005
Generation of Nanopores during Desorption of NH3 from Mg(NH3)6Cl2
Jens S. Hummelshøj,† Rasmus Zink Sørensen,‡ Marina Yu. Kustova,§ Tue Johannessen,‡
Jens K. Nørskov,† and Claus Hviid Christensen*,§
Center for Atomic-Scale Materials Physics (CAMP), Department of Physics, Building 307, Technical UniVersity of
Denmark, DK-2800 Lyngby, Denmark, Amminex A/S, KemitorVet, Building 206, DK-2800 Lyngby, Denmark, and
Center for Sustainable and Green Chemistry, Department of Chemistry, Building 206, Technical UniVersity of
Denmark, DK-2800 Lyngby, Denmark
Received August 16, 2005; E-mail: chc@kemi.dtu.dk
Metal ammine complexes have been known for more than a
century,1 but they still turn out to have new and interesting
properties. Recently, they have, for example, been considered for
separation of NH3 from low-pressure ammonia plants2 and as
sorption systems for refrigeration.3,4 The latest suggestion is to use
them as solid hydrogen storage materials.5 Metal ammine com-
plexes, such as Mg(NH3)6Cl2, can store 9.1% hydrogen in the form
of NH3, which is relatively high compared to most solid hydrogen
storage materials currently studied.6,7 Hydrogen can be obtained
by combining the metal ammines with an NH3 decomposition
catalyst.8-10
One particularly interesting property of the Mg(NH3)6Cl2 material
is that it can be compacted into shaped objects essentially without
any void.5 Thus, very high volumetric storage capacities similar to
that of liquid ammonia can be achieved, and still the desorption
process is facile. Here, we investigate the reason for this behavior.
By a combination of decomposition rate and pore size distribution
measurements and density functional theory (DFT) calculations,
we provide new insight into the decomposition process. In
particular, we show that during decomposition of Mg(NH3)6Cl2 an
extended system of nanosized pores develops. This system of
channels facilitates the transport of desorbed NH3 away from the
interior of large volumes of compacted storage material.
Figure 1. Structures of selected Mg(NH3)xCl2 (x ) 6, 3, 1) and calculated
energies of coordinated NH3 relative to gas-phase ammonia. Black dots
are calculated by DFT from the K2PtCl6 structure reported by Hwang et
al.12
point was Mg(NH3)6Cl2, which crystallizes with the same structure
as K2PtCl6.11,12 The structure and lattice parameters were well
reproduced in the calculations. In Figure 1, the calculated NH3
binding energies are shown. They show interestingly that the first
two NH3 to desorb are bound by almost the same energy and
stronger than the next two. This immediately explains the almost
simultaneous desorption of four coordinated NH3 molecules. The
first two ammonia molecules will desorb at almost the same
temperature, after that, the next two must follow immediately. The
binding energy is higher for the fifth and highest for the sixth NH3
molecule, corresponding to the last two molecules of NH3 being
desorbed during the temperature ramp. For the last two structures,
we find another crystal structure to be more favorable than the K2-
PtCl6 structure, namely, the one which is also found experimentally
for x ) 2 by Leineweber et al.13 In the present analysis, this
structural change is not included since it is not clear that the system
has time to rearrange during the TPD experiment. The average
bonding enthalpy of the first four NH3 molecules is calculated to
be 44.5 kJ/mol. This should be compared to the measured value of
the bonding enthalpy of 55.7 kJ/mol.2,3
We now turn to the question of why decomposition kinetics
similar to that of a fine powder is observed from completely
compact samples of Mg(NH3)6Cl2 with a size of 1-2 cm3. To study
this, we have measured the pore size distribution during decomposi-
tion, as shown in Figure 2. During the desorption process, an
extended nanopore system develops. The average pore diameters
increase from initially 2-4 nm to roughly 20 nm.
In Figure 3, we compare the measured pore volumes to a model
based on the DFT calculations. In constructing the model, we have
used the fact that the calculations show that each crystal in the
The temperature-programmed desorption (TPD) of NH3 from
Mg(NH3)6Cl2 powder has already been reported.5 Desorption takes
place in three stages. The first desorption peak involves four
molecules of NH3, and the last two peaks each correspond to one
molecule of NH3. Here, we have conducted eight consecutive
desorption and readsorption experiments. From these, it is clear
that the absorption and desorption of NH3 is reversible, in agreement
with previous findings.2-4 In all runs, the integrated ammonia
content was 5.85 ( 0.15. X-ray powder diffraction (XRPD) showed
only pure Mg(NH3)6Cl2 and MgCl2 before and after decomposition.
During the first four runs, it is noteworthy that the first desorption
peak moves toward slightly lower temperatures and broadens. From
the X-ray diffraction patterns that show a successively increasing
line width, it is clear that this can be attributed to the relative large
crystals (>150 nm) of MgCl2 used in the first experiment gradually
transforming into smaller and smaller crystals. Already in the second
experiment, a crystal size of about 14 nm is estimated by the
Scherrer equation. From the fourth to the eighth absorption and
desorption, there is essentially no change in the TPD trace. Thus,
the exact position of the desorption peak depends on the partial
pressure of ammonia, the temperature ramp, and the crystal size.
To gain a more complete understanding of the decomposition
process, a series of DFT calculations were performed. The starting
† Department of Physics, Technical University of Denmark.
‡ Amminex A/S, Kemitorvet.
§ Department of Chemistry, Technical University of Denmark.
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J. AM. CHEM. SOC. 2006, 128, 16-17
10.1021/ja0556070 CCC: $33.50 © 2006 American Chemical Society
Hummelshoj, Jens S.
