Application Of Light Metal Hydrides In The Preparation Of Clean Fuel Hydrogen And Methane

Currently,due to fossil fuel consumption and worsening of the global environment,humans are paying more attention to clean energy sources,and hydrogen is considered an ideal substitute for fossil fuel.Light-metal based H storage materials are considered as an efficient and convenient alternative to store hydrogen because of their high gravimetric and volumetric hydrogen contents,which also is a relatively safe method of solid-state storage and transport for hydrogen.Li₃N-H₂ hydrogen storage system has drawn a high degree of attention around the world since it was first reported by Chen et al in 2002.And then,LiNH₂-LiH as a classic hydrogen storage system has been widely investigated.However,the hydrogen desorption temperature of the reaction is too high for practical application.Therefore,it's need to explore high-efficiency additives to improve its hydrogen desorption properties and reduce the reaction temperatures.Light metal hydrides are often used as reductants and combined with other materials into high-energy-density hydrogen storage systems.Moreover,recent studies have also demonstrated that light metal hydrides can efficiently reduce gas small molecules,such as NH3 under mechanical ball milling condition producing H₂-included fuels.Inspired by these results,we expected that light metal hydrides can react with CO₂ generating hydrocarbon fuels.Firstly,the superior effect of RbF on reducing the hydrogen desorption operating temperature of the LiNH₂-LiH system has been discovered.By doping 5 mol%RbF,we obtain the lowest hydrogen desorption peak temperature?211 ??at a heating rate of 5 ?/min,which is approximately 69 ? lower than that of the pristine LiNH₂-LiH system.Moreover,it begins desorbing H₂ at 120 ?,which is the lowest onset temperature.The hydrogen desorption peak and onset temperatures can be further decreased by doping more RbF.Additionally,the reversibility of the LiNH₂-LiH system is enhanced by adding RbF.The decrease in activation energy is one of the important reasons for the decreased hydrogen desorption operating temperatures of the 5 mol%RbF doped LiNH₂-LiH sample.Detailed structural investigations reveal that during the heat treatment,the doped RbF reacts with LiH and is converted to RbH,which significantly improves the hydrogen desorption properties of the LiNH₂-LiH system.Secondly,this paper recounts for the first time our successful attempt to co-generate COx-free methane and hydrogen fuels at room temperature through solid/gas mechanochemical reaction of some light metal hydrides with CO₂.The yield and mole fraction of methane in the gas products mainly depend on the species of light metal hydrides,the strength and time of ball milling,and CO₂ pressure.The analysis of mechanism indicates that the amorphous carbon as the intermediate plays important roles in the process of the conversion of carbon dioxide to methane.This study opens a new and simple method for conveniently converting carbon dioxide to useful fuels under mild condition.The development of effective catalysts to accelerate the reaction rate and increase the yield of methane is the subject of current efforts.Lastly,in this paper,the thermochemical methanation interactions of the MgH₂-CO₂ system and the CaH₂-CO₂ system were studied.We succeed in obtianing methane and hydrogen fuels by convert CO₂ to green energy resource.The result of methanation influenced by the temperature and time of heating,and CO₂ pressure were examined.The optimum reaction conditions at a heating temperature 450 ? under 0.25 MPa CO₂ atmosphere pressure for 48 hours,and the yield of methane for the reaction of MgH₂ with CO₂ is 68%and the yield of methane for the reaction of CaH₂ with CO₂ is 88%.Alkaline earth metal hydrides?MgH₂ or CaH₂?can efficiently hydrogenation reduce CO₂ to methane and hydrogen fuels,which have a better combustion performance than pure methane gas.What is more,MgH₂ and CaH₂ are low-cost materials.Therefore,it's a conveniently promising and safe method for solid state storage of high energy density methane by use the MgH₂-CO₂ system and the CaH₂-CO₂ system converting carbon dioxide to H₂ and CH4 mixture fuels.