Chemistry, Chulalongkorn^* Petrolium and Petrochemical College, Chulalongkorn University^** UOP LLC^***
○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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