Third Day Poster Session, 17 June 2010 - NanoTR-VI

P
Poster Session, Thursday, June 17
Theme F686 - N1123
Crystal Structure Predictions for Hydrogen Storage Materials and Ammonia Dynamics in
Magnesium Ammine from DFT and Neutron Scattering
1
UAdem TekinUP P*
1
PInformatics Institute, Istanbul Technical University, 34469 Maslak Istanbul Turkey
Abstract- By combining several computational methods, the lowest energy crystal structures of Mg(NHR3R)RnRClR2R with n=6,2,1,
Mg(BH4)2, LiBH and MgNH were searched. Furthermore, NHR3R ab- and desorption mechanisms involved in metalammines were
investigated using a combination of DFT and quasi-elastic neutron scattering measurements.
Hydrogen and ammonia both have great potential as
carbon-neutral energy carriers for the future. However, there
are still some major challenges waiting to be addressed
concerning the production, storage, and the everyday use of
hydrogen and ammonia. In addition to gas or liquid forms of
storage (which are not efficient), hydrogen can also be stored
with high capacity in the condensed phase in the form of
metal hydrides, carbon nanotubes, metal–organic
frameworks, metal borohydrides and metalammines.
Details of absorption and desorption mechanisms of
NHR3R/HR2R in different storage mediums are based on the crystal
structure. This point becomes more delicate if the crystal
structure is unknown, as in the case of the low temperature
structure of Mg(NHR3R)R6RClR2R. Therefore, a new crystal structure
prediction method based on Simulated Annealing (SA) [1] is
implemented and first applied to Mg(NHR3R)RnRClR2R with n=6,2,1
[2]. In metal ammines, hydrogen bonds between NHR3R's
hydrogens and chlorine atoms are important to stabilize the
metal complex. This fact is exploited in the SA search to
construct crystal structures by maximizing the number of
hydrogen bonds within a (2×2×2) cut-through lattice using
only several bond length constraints. SA optimizations found
all the experimentally known structures and predicted the
C2/m structure for the uncharacterized low temperature phase
of Mg(NHR3R)R6RClR2R.
Then the SA method applied to one of the promising metal
borohydride, Mg(BHR4R)R2R [3], which stores 14.9 % wt of
hydrogen. These SA optimizations successfully yielded
previously proposed I4m2 and F222 symmetry structures of
Mg(BHR4R)R2R. Further optimizations the Density Functional
Theory (DFT) level indicated that the ground state structure
of Mg(BHR4R)R2R is the one with I4m2 symmetry.
In the last decade, LiBHR4R has been proposed as a promising
hydrogen storage medium due to its high gravimetric (18.5 %
3
wt hydrogen) and volumetric (121 kg H/mP P) hydrogen
density. Although a considerable amount of papers have been
published on LiBHR4R, a clear theoretical structure
determination seems to suffer from a lack of methodological
approach. Therefore, the potential energy surface of LiBHR4R
was investigated by the SA method and DFT calculations. A
new stable orthogonal structure with Pnma symmetry was
found [4], which is 9.66 kJ/mol lower in energy than the
proposed Pnma structure [5]. For the high temperature
structure, a new orthorhombic P2/c structure was proposed,
which is 21.26 kJ/mol over the ground-state energy and
showed no lattice instability.
Li – Mg – N – H systems composed of Mg(NHR2R)R2R and LiH
with various ratios can reversibly store hydrogen at moderate
operating conditions. Depending on the Mg/Li ratio different
products may be formed. Amongst them, the crystal structure
of magnesium imide (MgNH) is unknown. Therefore, the SA
method was also applied to find the ground-state structure of
MgNH. A new stable tetragonal phase with P4/nmm
symmetry was found as the lowest-energy structure of MgNH
[6].
Using the structures of Mg(NHR3R)RnRClR2R with n=6,2,1 found
by the SA method, NHR3R rotation and diffusion processes in
these metalammines were investigated using a combination
of DFT and quasi-elastic neutron scattering measurements.
DFT calculations involving bulk diffusion of NHR3R correctly
reproduced the trends observed in the experimental
desorption enthalpies. In particular, for n = 6, 2, 1, there is a
good agreement between activation barriers and experimental
enthalpies. The release of NHR3R in magnesium ammine is thus
found to be limited by bulk diffusion.
Figure 1. Calculated (dotted line) versus experimental (solid line)
desorption enthalpies for the different desorption steps, 6 2, 2
1, and 1 0, of magnesium ammine. The lowest activation
barriers obtained for NHR3R diffusion are shown in squares [2].
Ammonia dynamics study was supported by European
Commission DG Research (contract MRTN-CT-2006-
032474/Hydrogen). I thank Riccarda Caputo (from EMPA)
and Deniz Cakir (from University of Twente) for their DFT
calculations for Mg(BHR4R)R2R and LiBHR4R and MgNH,
respectively.
HT*Corresponding author: adem.tekin@be.itu.edu.trT
[1] Corona A, Marchesi M, Martini C, Ridella S., 1987.
Minimizing Multimodal Functions of Continuous Variables with
the ``Simulated Annealing'' Algorithm, Assoc. Comput. Mach.,
Trans. Math. Software, 13: 262-280.
[2] Tekin A, Hummelshøj J. S., Jacobsen H. S., Sveinbjörnsson
D, Blanchard D, Nørskov J. K., Vegge T., 2010. Ammonia
dynamics in magnesium ammine from DFT and neutron
scattering, Energy Environ. Sci., DOI: 10.1039/b921442a.
[3] Caputo R., Tekin A., Sikora W., Züttel A., 2009. Firstprinciples
determination of the ground-state structure of
Mg(BH4)2, Chem. Phys. Lett., 480: 203-209.
[4] First principles determination of ground-state structure of
LiBHR4R, Tekin A, Caputo R., Züttel A., 2010. Submitted to Phys.
Rev. Lett.
[5] Soulié J-Ph., Renaud G., erny R., Yvon K., 2002. Lithium
boro-hydride LiBHR4R I. Crystal structure, J. Alloys. Compd.
346:200-205.
[6] Cakir D, Tekin A, Brocks G., 2010. The crytsal structure of
MgNH: a computational study, Submitted to Phys. Rev. B.
6th Nanoscience and Nanotechnology Conference, zmir, 2010 771