Synergistic effects of bimetallic catalysis on the cycling behavior of NaAlH4 Co-Doped with Zr and Fe

Samples of NaAlH4 co-metal catalyzed with Zr and Fe exhibited significant, synergistic and sustained catalytic activity during hydrogenation (charge) and dehydrogenation (discharge) cycling. Before cycling, the discharge rate more than doubled for the 1 mol% Zr–3 mol% Fe bimetallic catalyst system compared to the 1 mol% Zr single metal catalyst system, and it increased by nearly 30% for the 3 mol% Zr–1 mol% Fe bimetallic system compared to the 3 mol% Zr single metal system. After cycling, the synergistic effects were even more pronounced, with the 3 mol% Zr–1 mol% Fe bimetallic system exhibiting faster discharge kinetics than the 4 mol% Zr single metal system, and the kinetics of the 1 mol% Zr–3 mol% Fe bimetallic system improving so much that they were nearly as fast as the 4 mol% Zr single metal system. This synergistic behavior of the Zr–Fe system persisted even during the more important charge process, with charge rates of the bimetallic systems (3 mol% Zr–1 mol% Fe and 1 mol% Zr–3 mol% Fe) approaching those of 4 mol% Zr. These results were surprising, since up to 4 mol% Fe was essentially inactive as a single metal catalyst. It is speculated that this synergistic behavior of Zr–Fe bimetallic catalysis might be associated with the metal-metal bond polarity concept that relates co-metal catalytic activity to the relative location of early (Zr) and late (Fe) transition metals in the periodic table.

► NaAlH4 co-doped with Zr and Fe exhibited sustained activity during charge and discharge cycling.
► After cycling 1 mol% Zr–3 mol% Fe bimetallic nearly as fast as 4 mol% Zr single metal.
► Synergistic behavior of Zr–Fe persisted even during more important charge process.
► Surprising results since up to 4 mol% Fe essentially inactive as single metal catalyst.
► Zr–Fe bimetallic catalysis explained in terms of metal-metal bond polarity concept.