Hydrogen Desorption and Absorption Process of NaAlH4 and Modified NaAlH4

Chemistry, Chulalongkorn* Petrolium and Petrochemical College, Chulalongkorn University** UOP LLC***
â—‹Chattree Phurat* Nongnuj Muangsin* Pramoch Rangsunvigit** Santi Kulprathipanja*** Yindee Suttisawat**

NaAlH4 doped with 4mol% of HfCl4 showed the maximum hydrogen capacity of 5.5wt%. It was obtained from the first desorption and dropped to 2.2-2.6wt% after that. The drawback of the doped-NaAlH4 is the reported loss of stability upon cycling. The reason for this seems to be due to the incomplete rehydrogenation in the second step (Na3AlH6 to NaAlH4). This, in turn, might be explained by (i) the reduction availability of Al due to the formation of large Al crystallites, possibly accompanied by coating of the Al particles by NaAlH4 and (ii) the formation of Hf-Al alloy, resulting in its reduced effectiveness. In this work, we purpose to reduce the formation of Hf-Al alloy and large Al crystallites by adding porphyrins Al/Hf complex into the doped sodium alanate system. The mechanism and result of hydrogen desorption/absorption were investigated by X-ray diffraction and Rietveld refinement method.

3NaAlH4 = Na3AlH6 + 2Al + 3H2(1)
2Na3AlH6 = 6NaH + 2Al + 3H2(2)
6NaH = 6Na + 3H2(3)

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