Hydrogen Generation Via Hydrolysis Of Mg/MgH₂–based Composites

Metallic Mg and its hydride have received great attention as promissing hydrogen production materials due to their high hydrogen capacity,abundance and mature regeneration technology,which can be applied as hydrogen sources for protable devices.However,the hydrolysis reaction is easily interrupted by the formation of Mg(OH)2 passive film on Mg surface during hydrolysis process.As a resust,the conversion yield is quite low and the kinetics is poor,which limits its practical application.In this thesis,Mg or its hydride is used as starting material and the key scientific issues,such as promoting hydrolysis kinetics,increasing conversion yield et al,were studied.The improved appoaches in terms of combination with catalysts and changing solution composition were used to accelerate the hydrolysis reaction.Characterization,such as XRD,SEM,XPS etc.,were adopted to analyze and clarify the catalytic mechanism.The main contents are as below:Molybdenum disulfide(MoS2)was applied to catalyze Mg hydrolysis for the first time in this thesis and Mg-MoS2 composites were prepared by a simple planetary ball mill technique.The hydrolysis results demonstrate that small amounts of MoS2 added could significantly accelerate and enhance the hydrolysis reaction of Mg in seawater.Mg-0.2 wt% MoS2 composite milled for 1 h could release 81.8%(763 mL g-1)H2 within 10 min in seawater,which is 4.1 times as pure Mg.Besides,the hydrogen generation yield of the composites first increases with increasing the milling time and the addition amount of MoS2 and then decrease.Moreover,flower-like MoS2 spheres prepared via a hydrothermal method were also used to catalyze Mg hydrolysis.SEM images reveal that the as-prepared MoS2 consists of large numbers of intercrossed curved nanoflakes with a thickness of several nanometers.Due to the unique hierarchical structures,the as-prepared MoS2 show a higher catalytic activity than bulk MoS2 on the hydrolysis of Mg.In addition,the effects of Mo and its compounds and transition metal oxides on the hydrolysis properties of Mg were also studied.Among these oxides,Fe2O3 and MoO3 are ideal additives.At 25?,the 1 h milled Mg-5 wt% Fe2O3 and Mg-5 wt% MoO3 composite reached a yield of 93.1% and 95.2%,respectively.In particular,the oxidation states of Mo and Fe ion were found to play an important role in Mg hydrolysis.That is,a higher oxidation number leds to a better hydrolysis performance.Apart from Mg-based materials,the influence of MoS2 addition and solution composition on hydrolysis of MgH2 are also studied.Compared to MoS2 addition,using NH4 Cl solution as hydrolysis medium is more effective to promote reaction kinetics.The hydrogen yield of MgH2 reached to 100% in saturated NH4 Cl solution,which can be attributed to the fact that NH4+ could destroy the Mg(OH)2 passive film to produce a new soluble compound MgCl2NH4Cl·6H2O.