Preparation Of Ti-V-based Catalysts And Its Effects On Hydrogen Storage Properties Of MgH₂

MgH₂ hydrogen storage materials have attracted considerable attentions because of its high hydrogen storage capacity and reversible hydrogen properties.However,the poor thermodynamic and kinetics performance for hydrogen storage reaction prevent their practical applications.In this work,to reduce the operating temperature and improve the kinetics of MgH₂,the effects of?Ti0.5V0.5?3AlC2,TiVAlC,(Ti_0.5V_0.5)₃C₂ and TiVOx on hydrogen storage behaviors were systematically studied,and their action mechanisms were investigated.The MgH_2-x wt%?Ti0.5V0.5?3AlC2/TiVAlC?x=5,7,10,12?composites were prepared by ball milling,and hydrogen storage properties and mechanisms of the as-prepared samples were investigated.It was found that hydrogen storage performances depended strongly on the content of?Ti0.5V0.5?3AlC2/TiVAlC,and the sample with 10 wt%?Ti0.5V0.5?3AlC2/TiVAlC behaved the optimum hydrogen storage performances.The MgH₂-10 wt%?Ti0.5V0.5?3AlC2/TiVAlC composite started to dehydrogenation at 230 ?,and desorbed 6.9 wt%of hydrogen while being heated to 350 0C.At 275 ?,3.1 wt%and 3.8 wt%of hydrogen was released respectively from the composite within 10 min,and the dehydrogenation rate are deteremined to be 0.35 wt%min-1 and 0.44 wt%min-1,which is 4-5 times faster than that of the pristine sample.The dehydrogenated?Ti0.5V0.5?3AlC2/TiVAlC-added samples absorbed 6.5 wt%of hydrogen at 50 bar hydrogen pressure and 250 ?.At 150 ?,the samples can absorb 4.7 wt%and 4.5 wt%of hydrogen respectively in 60 s.Thermodynamic and kinetic measurements suggested that adding?Ti0.5V0.5?3AlC2/TiVAlC decreased the activation energy of dehydrogenation of MgH₂,but the enthalpy change remained almost unchanged.Further XRD analyses revealed that the?Ti0.5V0.5?3AlC2/TiVAlC still preserved during dehydrogenation.The preparation of the(Ti_0.5V_0.5)₃C₂ and its effects on hydrogen storage of MgH₂ were investigated.Two-dimensional(Ti_0.5V_0.5)₃C₂ were synthesized by the exfoliation of?Ti0.5V0.5?3AlC2 with a HF solution,its crystal structure and lattice constants are determined.The prepared(Ti_0.5V_0.5)₃C₂ was added in MgH₂ by ball milling.The results showed that the catalytic activity of(Ti_0.5V_0.5)₃C₂ was superior to?Ti0.5V0.5?3AlC2.The onset temperature for the dehydrogenation of the MgH₂-10 wt%(Ti_0.5V_0.5)₃C₂ composite was reduced from 290 0C?the pristine MgH₂?to 210 0C,representing a 80 ? reduction.At 300 ?,approximately 6.1 wt%of hydrogen was released from the composite within 7 min.Further hydrogenation examinations indicated that the dehyrogenated(Ti_0.5V_0.5)₃C₂-added sample showed a good hydrogen storage reversibility.It absorbed 4.7 wt%of hydrogen within 5 s at a temperature low as 120 ?.After 10 cycles,99%of capacity was remained,representing improved cycling stability.The apparent activation energy was calculated to be 77.3 kJ mol-1 for hydrogen desorption from the MgH₂-10 wt%(Ti_0.5V_0.5)₃C₂ composite,which is reduced by 50%in comparison with the pristine MgH₂.Further XRD and XPS analyses indicated that the in situ formed nano Ti and V,which derived from the redox reaction of(Ti_0.5V_0.5)₃C₂ and MgH₂ in the ball milling process,played an important role to improve hydrogen storage properties of MgH₂.The preparation of the TiVOx and its effects on hydrogen storage of MgH₂ were investigated.The prepared TiVOx was added in MgH₂ by ball milling.It is found that the TiVOx prepared at 300 ??TiVO-300?has superior catalytic performance for MgH₂.The onset temperature for the dehydrogenation of the MgH₂-10 wt%TiVO-300 composite was reduced from 290 ??the pristine MgH₂?to 204 ?,representing a 86 ? reduction,and 6.8 wt%of hydrogen were desorbed while being heated to 350 ?.At 275 ?,approximately 5.3 wt%of hydrogen was released from the composite within 5 min.Further hydrogenation examinations indicated that the dehyrogenated TiVO-300-added sample showed a good hydrogen storage reversibility.It absorbed 3.7 wt%of hydrogen within 5 s at a temperature low as 100 ?.The apparent activation energy was calculated to be 62.4 kJ mol-1 for hydrogen desorption from the MgH₂-10 wt%TiVO-300 composite,which is reduced by 59%in comparison with the pristine MgH₂.Further XRD and XPS analyses indicated that the in situ formed nano Ti and V,which derived from the redox reaction of TiVO-300 and MgH₂ in the ball milling process,played an important role to improve hydrogen storage properties of MgH₂.