The enhanced dehydrogenation performances of 17MgH2-12Al composite with additive TiH2

Abstract

Abstract In this work, we introduced TiH 2 into the 17MgH 2 -12Al composite by ball milling for improving the dehydrogenation performance of the reactive system. According to the measurements of non-isothermal dehydrogenation, the onset temperature of hydrogen release from 17MgH 2 -12Al-TiH 2 was reduced by 50 °C from that of 17MgH 2 -12Al. The investigations by Kissinger's method showed that the activation energy of hydrogen desorption of 17MgH 2 -12Al-TiH 2 (107 kJ mol −1 ) is much lower than 17MgH 2 -12Al (168 kJ mol −1 ), leading to enhanced dehydrogenation kinetics due to the TiH 2 additive. X-ray diffraction (XRD) analysis suggests that TiH 2 catalyzed the reaction between MgH 2 with Al of the system during the dehydrogenation below 280 °C. At temperatures between 280 °C to 400 °C, however, TiH 2 will thoroughly react with produced Mg 17 Al 12 and liberate hydrogen. The overall reaction of the system may be represented by 17 MgH 2 + 12 Al + TiH 2 → 3/4 Mg 17 Al 12 + Al 3 Ti + 17/4 Mg + 18 H 2 . Its theoretical dehydrogenation capacity (4.38 wt% H 2 ) agrees well with that achieved by temperature programmed dehydrogenation (TPD) measurements, demonstrating the occurrence of such reaction(s). The dehydrogenation enthalpy of 17MgH 2 -12Al-TiH 2 (71.7 kJ mol −1 H 2 ) proceeding at lower hydrogen desorption plateau was significantly lowered from that of 17MgH 2 -12Al (81.9 kJ mol −1 H 2 ) according to the measurements of pressure-composition isotherms (PCI) and van't Hoff plots. It means that TiH 2 was involved in the end reaction proceeding in the 17MgH 2 -12Al system. We believe that TiH 2 improves the dehydrogenation kinetics of 17MgH 2 -12Al initially and tailors its thermodynamics finally.