Superior catalytic effect of TiF3 over TiCl3 in improving the hydrogen sorption kinetics of MgH2: Catalytic role of fluorine anion

TiF3 shows a superior catalytic effect over TiCl3 in improving the hydrogen sorption kinetics of MgH2. Combined phase analysis and microstructure characterization suggest that both titanium halide additives react with host MgH2 in a similar way. However, systematic X-ray photoelectron spectroscopy studies reveal that the incorporated fluorine (F) differs significantly from its analog chlorine (Cl) in terms of bonding state. The asymmetry of F 1s spectra and the sputtering-induced peak shift suggest that, in addition to the Mg–F bond, a new Ti–F–Mg bonding is formed in the TiF3-doped MgH2. In contrast, only one stable binding state of Cl is identified in the form of MgCl2 for the TiCl3-doped MgH2. In combination with the designed experiments, these findings suggest that the generation of active Fcontaining species may be responsible for the advantage of TiF3 over TiCl3 in improving both the absorption and desorption kinetics of MgH2. Fundamentally, it emphasizes the functionality of F anion in tuning the activity of compound catalyst. 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.