Photonic crystals in biology - NanoTR-VI

PPoster Session, Thursday, June 17Theme F686 - N1123Crystal Structure Predictions for Hydrogen Storage Materials and Ammonia Dynamics inMagnesium Ammine from DFT and Neutron Scattering1UAdem TekinUP P*1PInformatics Institute, Istanbul Technical University, 34469 Maslak Istanbul TurkeyAbstract- 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 wereinvestigated using a combination of DFT and quasi-elastic neutron scattering measurements.Hydrogen and ammonia both have great potential ascarbon-neutral energy carriers for the future. However, thereare still some major challenges waiting to be addressedconcerning the production, storage, and the everyday use ofhydrogen and ammonia. In addition to gas or liquid forms ofstorage (which are not efficient), hydrogen can also be storedwith high capacity in the condensed phase in the form ofmetal hydrides, carbon nanotubes, metal–organicframeworks, metal borohydrides and metalammines.Details of absorption and desorption mechanisms ofNHR3R/HR2R in different storage mediums are based on the crystalstructure. This point becomes more delicate if the crystalstructure is unknown, as in the case of the low temperaturestructure of Mg(NHR3R)R6RClR2R. Therefore, a new crystal structureprediction method based on Simulated Annealing (SA) [1] isimplemented and first applied to Mg(NHR3R)RnRClR2R with n=6,2,1[2]. In metal ammines, hydrogen bonds between NHR3R'shydrogens and chlorine atoms are important to stabilize themetal complex. This fact is exploited in the SA search toconstruct crystal structures by maximizing the number ofhydrogen bonds within a (2×2×2) cut-through lattice usingonly several bond length constraints. SA optimizations foundall the experimentally known structures and predicted theC2/m structure for the uncharacterized low temperature phaseof Mg(NHR3R)R6RClR2R.Then the SA method applied to one of the promising metalborohydride, Mg(BHR4R)R2R [3], which stores 14.9 % wt ofhydrogen. These SA optimizations successfully yieldedpreviously proposed I4m2 and F222 symmetry structures ofMg(BHR4R)R2R. Further optimizations the Density FunctionalTheory (DFT) level indicated that the ground state structureof Mg(BHR4R)R2R is the one with I4m2 symmetry.In the last decade, LiBHR4R has been proposed as a promisinghydrogen storage medium due to its high gravimetric (18.5 %3wt hydrogen) and volumetric (121 kg H/mP P) hydrogendensity. Although a considerable amount of papers have beenpublished on LiBHR4R, a clear theoretical structuredetermination seems to suffer from a lack of methodologicalapproach. Therefore, the potential energy surface of LiBHR4Rwas investigated by the SA method and DFT calculations. Anew stable orthogonal structure with Pnma symmetry wasfound [4], which is 9.66 kJ/mol lower in energy than theproposed Pnma structure [5]. For the high temperaturestructure, a new orthorhombic P2/c structure was proposed,which is 21.26 kJ/mol over the ground-state energy andshowed no lattice instability.Li – Mg – N – H systems composed of Mg(NHR2R)R2R and LiHwith various ratios can reversibly store hydrogen at moderateoperating conditions. Depending on the Mg/Li ratio differentproducts may be formed. Amongst them, the crystal structureof magnesium imide (MgNH) is unknown. Therefore, the SAmethod was also applied to find the ground-state structure ofMgNH. A new stable tetragonal phase with P4/nmmsymmetry was found as the lowest-energy structure of MgNH[6].Using the structures of Mg(NHR3R)RnRClR2R with n=6,2,1 foundby the SA method, NHR3R rotation and diffusion processes inthese metalammines were investigated using a combinationof DFT and quasi-elastic neutron scattering measurements.DFT calculations involving bulk diffusion of NHR3R correctlyreproduced the trends observed in the experimentaldesorption enthalpies. In particular, for n = 6, 2, 1, there is agood agreement between activation barriers and experimentalenthalpies. The release of NHR3R in magnesium ammine is thusfound to be limited by bulk diffusion.Figure 1. Calculated (dotted line) versus experimental (solid line)desorption enthalpies for the different desorption steps, 6 2, 21, and 1 0, of magnesium ammine. The lowest activationbarriers obtained for NHR3R diffusion are shown in squares [2].Ammonia dynamics study was supported by EuropeanCommission DG Research (contract MRTN-CT-2006-032474/Hydrogen). I thank Riccarda Caputo (from EMPA)and Deniz Cakir (from University of Twente) for their DFTcalculations 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 withthe ``Simulated Annealing'' Algorithm, Assoc. Comput. Mach.,Trans. Math. Software, 13: 262-280.[2] Tekin A, Hummelshøj J. S., Jacobsen H. S., SveinbjörnssonD, Blanchard D, Nørskov J. K., Vegge T., 2010. Ammoniadynamics in magnesium ammine from DFT and neutronscattering, Energy Environ. Sci., DOI: 10.1039/b921442a.[3] Caputo R., Tekin A., Sikora W., Züttel A., 2009. Firstprinciplesdetermination of the ground-state structure ofMg(BH4)2, Chem. Phys. Lett., 480: 203-209.[4] First principles determination of ground-state structure ofLiBHR4R, Tekin A, Caputo R., Züttel A., 2010. Submitted to Phys.Rev. Lett.[5] Soulié J-Ph., Renaud G., erny R., Yvon K., 2002. Lithiumboro-hydride LiBHR4R I. Crystal structure, J. Alloys. Compd.346:200-205.[6] Cakir D, Tekin A, Brocks G., 2010. The crytsal structure ofMgNH: a computational study, Submitted to Phys. Rev. B.6th Nanoscience and Nanotechnology Conference, zmir, 2010 771