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 F-containing 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.