Synergistic effects of co-dopants on the dehydrogenation kinetics of sodium aluminum hydride

Abstract

A systematic analysis of the effect of co-dopants on the dehydrogenation kinetics of freshly doped and ball milled NaAlH 4 samples was carried out with chlorides of Ti, Zr and Fe as the catalysts. Numerous samples of NaAlH 4 when co-doped with binary and ternary combinations of Ti, Zr and Fe at 4 mol% total catalyst content exhibited synergistic behavior, with respect to improving the dehydrogenation kinetics of the first decomposition reaction (i.e., NaAlH 4 → Na 3AlH 6) over that of a sample of NaAlH 4 doped with 4 mol% Ti or Zr as single catalysts. In general, the dehydrogenation kinetics improved with the amount of Ti present in a co-doped sample, whether it was a binary or ternary system, with the top five performers all having at least 2 mol% Ti as one of the co-dopants. The binary combination of 3 mol% Ti–1 mol% Fe exhibited the best synergistic performance, with dehydrogenation rates 3.7, 2.0 and 1.5 times that of 4 mol% Ti alone at 90, 110 and 130 °C, respectively. The binary co-doped Zr–Fe system exhibited more pronounced synergistic effects than did the binary co-doped Ti–Fe system; however, their performance was always worse because Ti is a better single catalyst than Zr. The least synergism was exhibited by the binary co-doped Ti–Zr system, where it was surmised that the superior electron sharing ability of electron rich Fe was responsible for it being a better promoter of Ti and Zr than Zr was of Ti. This supposition was further supported by the systematic trends observed with the ternary co-doped systems, with their synergistic effects seemingly limited to binary combinations of the Ti–Fe and Zr–Fe systems. The effects of Ti, Zr and Fe as co-dopants on the second decomposition reaction (i.e., Na 3AlH 6 → NaH) were not as pronounced as their effects on the first reaction; but synergisms were still observed, especially with all three binary Zr–Fe co-doped systems and to a lesser extent only with the 3 mol% Ti–1 mol% Fe system. A future study will consider the effects of these co-dopants on the dehydrogenation/rehydrogenation kinetics after cycling.