Abstract
This study elucidates the role of transition metal (TM) additives in enhancing hydrogen (H) reversibility and hydrogenation kinetics for the NaAlH4 system. The isothermal hydrogen absorption kinetics of the planetary milled (PM) NaAlH4 + xTMCln (TM row 1 = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu; row 2 = Zn, Y, Zr, Nb, Mo, Ru, Rh Pd; row 3 = Pt; 2 < n < 5) and cryo-milled (CM) NaAlH4 + xTMCln (TM row 1 = Ti, V, Cr, Fe, Co, Ni, Cu; 2 < n < 3) systems have been measured at 140 °C and 150 bar system pressure. The variation in hydrogenation kinetics across the TM series for NaAlH4 + xTMCln is strongly dependent on the TM species and additive level, milling technique, and the type, structure, and morphological arrangement of nanoscopic Al1–xTMx phases that are embedded on the NaAlH4 surface. In the most interesting case, the surface-embedded Al1–xTix phases in the TiCl3-enhanced NaAlH4 system perform a dual catalytic function, where the outer Al1–xTix surface performs dissociation/recombination of molecular H2 and the inner Al1–xTix surface allows the distortion of a minor number of Al–H bonds from AlH4– tetrahedra in the vicinity of the subsurface Al1–xTix/NaAlH4 interface. The density of Ti atoms in the subsurface interface (which is Al:Ti composition- and H cycling temperature-dependent) shows the strongest effects on hydrogenation kinetics.