In-situ X-ray diffraction study of the decomposition of NaAlH4

Phase transitions and crystal structure modifications were observed during the thermal-desorption decomposition of the alanate NaAlH4. This was accomplished through the use of in-situ X-ray powder diffraction. A sequence of theta-2 theta scans were collected while heating the sample under a vacuum. The resulting diffraction patterns were assembled to provide a real-time representation of the decomposition reactions. It was found that upon heating, NaAlH4 initially experienced a lattice expansion principally in the c-axis direction. This was followed by continuous structural distortions observed as erratic variations in the Bragg intensities. Melting of NaAlH4 was observed at 180 degrees C followed by the rapid precipitation of a cubic Na3AlHx phase. This phase then underwent a slow transformation into a Na-rich cubic phase. The decomposition of NaAlH4 doped with a double catalyst (Ti + Zr) was also investigated. The uncatalyzed sample showed no decomposition when held under a vacuum at 150 degrees C for several hours. The catalyzed sample, on the other hand, began to decompose readily into the monoclinic (alpha)-Na3AlH6 phase when heated to 100 degrees C in vacuum. At 150 degrees C the (alpha)-Na3AlH6 phase decomposed in a second reaction according to alpha-Na3AlH6->3 NaH + Al + 3/2 H-2. The two decomposition reactions appear to be interdependent as the second transformation commenced only after the first reaction neared completion. The observed growth of a narrow Al (111) diffraction peak indicates the formation of aluminum crystallites (>100 nm) as a part of the decomposition reactions. This, and the fact that this solid state process is assisted through the interaction of a surface catalyst suggests long-range transport of a metal species. Some mechanisms are proposed to explain the catalytically enhanced kinetics of these materials. (C) 2000 Elsevier Science S.A. All rights reserved.