Investigation on the nature of active species in the CeCl 3-doped sodium alanate system

Abstract

CeCl 3-doped NaAlH 4 was directly synthesized in hydrogenation process using NaH/Al with 2 mol% CeCl 3 under ball-milling. X-ray diffraction was utilized to unveil the nature of cerium during NaAlH 4 synthesis process and succedent cycling. It is found that, CeCl 3 is reduced in the ball-milling process and following cycles, causing the formation of NaCl and Al–Ce alloy with a structure of CeAl 4. The catalytic enhancement arising upon doping the ball-milled CeAl 4 alloy is quite similar to that achieved in the CeCl 3-doped sodium alanate. Because the CeAl 4 dopant does not consume the effective hydrogen storage component, the CeAl 4-doped NaAlH 4 exhibits more hydrogen storage capacity. Moreover, CeCl 3-doped NaAlH 4 and CeAl 4-doped NaAlH 4 exhibit similar apparent activation energies estimated from Kissinger's method, suggesting the reactions are all determined by the same rate-limiting step. These results clearly demonstrate that the in situ formed CeAl 4 acts as active species to catalyze the reversible dehydriding/rehydriding of NaAlH 4.