Catalytic Effects of Nanometric Ni (n-Ni) on the Dehydrogenation of Ball Milled Sodium Alanate (NaAlH_(4))

摘要

Mixtures of sodium alanate (NaAlH_(4)) with the nanometric Ni (n-Ni) additive were processed by controlled ball milling for 15 min in a magneto-mill and subsequently investigated by Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD) and volumetric hydrogen desorption in a Sieverts-type apparatus. The apparent activation energy of the decomposition of Na_(3)AlH_(6), which is the decomposition product in the first dehydrogenation step of NaAlH_(4), is equal to approx140 kJ/mol and approx101 kJ/mol for ball milled NaAlH_(4) and NaAlH_(4) + 5 wt(percent) n-Ni, respectively, as determined by the Kissinger method in DSC tests. The apparent activation energy for the decomposition of NaH, which is the decomposition product in the second dehydrogenation step of Na_(3)AlH_(6), is estimated as equal to approx143 kJ/mol and approx226 kJ/mol for ball milled NaAlH_(4) and NaAlH_(4)+5 wt(percent) n-Ni, respectively. It seems that the n-Ni additive accelerates the decomposition of Na_(3)AlH_(6) but decelerates the decomposition of NaH. Volumetric desorption tests indicate a substantial enhancement of the rate of hydrogen desorption at 170 deg C for the ball milled NaAlH_(4) + 5 wt(percent) n-Ni nanocomposite as a result of the catalytic action of n-Ni. No reaction of n-Ni with the NaAlH_(4) matrix is observed during dehydrogenation at 170-280 deg C. The n-Ni catalyst used in the present work compares very favorably with other catalytic additives reported in the literature (e.g., metal chlorides) which were used for the enhancement of dehydrogenation of NaAlH_(4).