TY - JOUR
T1 - Ammonia dynamics in magnesium ammine from DFT and neutron scattering
AU - Tekin, Adem
AU - Hummelshøj, Jens S.
AU - Jacobsen, Hjalte S.
AU - Sveinbjörnsson, Dadi
AU - Blanchard, Didier
AU - Nørskov, Jens K.
AU - Vegge, Tejs
PY - 2010
Y1 - 2010
N2 - Energy storage in the form of ammonia bound in metal salts, so-called metal ammines, combines high energy density with the possibility of fast and reversible NH3 ab- and desorption kinetics. The mechanisms and processes involved in the NH3 kinetics are investigated by density functional theory (DFT) and quasielastic neutron scattering (QENS). The crystal structures of Mg(NH3)nCl2 with n = 6, 2, 1, which contains up to 9.19 wt % hydrogen and 0.115 kg hydrogen L-1, are first analyzed using an algorithm based on simulated annealing (SA), finding all the experimentally known structures and predicting the C2/m structure for the uncharacterized low temperature phase of Mg(NH3) 6Cl2. It is found from DFT that the rotation of ammonia in the hexammine complex (n = 6) requires an activation energy of 0.09 eV in the low temperature phase of Mg(NH3)6Cl2 and 0.002-0.12 eV in the high temperature phases; effectively having free rotors as observed experimentally. The findings are supported by the QENS data, which identify C3 rotations of NH3 in the low temperature phase with an activation energy of 0.09 eV. The calculated diffusion rates were found to be 106-107 Hz at the desorption temperatures for all n = 6, 2, 1 systems. DFT calculations involving bulk diffusion of NH3 correctly reproduces 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. These results indicate that the desorption of NH3 is likely to be diffusion limited.
AB - Energy storage in the form of ammonia bound in metal salts, so-called metal ammines, combines high energy density with the possibility of fast and reversible NH3 ab- and desorption kinetics. The mechanisms and processes involved in the NH3 kinetics are investigated by density functional theory (DFT) and quasielastic neutron scattering (QENS). The crystal structures of Mg(NH3)nCl2 with n = 6, 2, 1, which contains up to 9.19 wt % hydrogen and 0.115 kg hydrogen L-1, are first analyzed using an algorithm based on simulated annealing (SA), finding all the experimentally known structures and predicting the C2/m structure for the uncharacterized low temperature phase of Mg(NH3) 6Cl2. It is found from DFT that the rotation of ammonia in the hexammine complex (n = 6) requires an activation energy of 0.09 eV in the low temperature phase of Mg(NH3)6Cl2 and 0.002-0.12 eV in the high temperature phases; effectively having free rotors as observed experimentally. The findings are supported by the QENS data, which identify C3 rotations of NH3 in the low temperature phase with an activation energy of 0.09 eV. The calculated diffusion rates were found to be 106-107 Hz at the desorption temperatures for all n = 6, 2, 1 systems. DFT calculations involving bulk diffusion of NH3 correctly reproduces 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. These results indicate that the desorption of NH3 is likely to be diffusion limited.
UR - http://www.scopus.com/inward/record.url?scp=77950589382&partnerID=8YFLogxK
U2 - 10.1039/b921442a
DO - 10.1039/b921442a
M3 - Article
AN - SCOPUS:77950589382
SN - 1754-5692
VL - 3
SP - 448
EP - 456
JO - Energy and Environmental Science
JF - Energy and Environmental Science
IS - 4
ER -