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

A systematic analysis of the effect of co-dopants on the dehydrogenation kinetics of freshly doped and ball milled NaAlH4 samples was carried out with chlorides of Ti, Zr and Fe as the catalysts. Numerous samples of NaAlH4 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., NaAlH4 → Na3AlH6) over that of a sample of NaAlH4 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., Na3AlH6 → 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.