Preparation Of Binary Transition Metal Borides And Their Catalytic Properties For Hydrogen Storage In Liquid Phase

With the development of the society,the energy problem has gradually become the main difficulty that troubles human progress.Among them,hydrogen energy is a kind of clean energy with high calorific value and high energy density,and its efficient utilization is expected to become the key technology to solve the energy problem.However,the current development of hydrogen energy has some problems such as high cost,difficulty and unstable safety of hydrogen storage and transportation,which greatly limits the industrialization and large-scale application of hydrogen energy.Liquid hydrogen storage materials stable at room temperature can release hydrogen quickly and controllable with the help of catalysts.Meanwhile,liquid hydrogen storage materials have more advantages in safety and cost,which is considered to have broad application prospects.The theoretical hydrogen content of ammonioborite is up to 19.6 w.%,and its physical and chemical properties are stable at room temperature,which is conducive to storage.Ammonioborite aqueous solution can quickly release a large amount of hydrogen at room temperature with the assistance of catalyst.The theoretical hydrogen content of formaldehyde is 6.7 wt.%,and its aqueous solution is stable and not easy to decompose at room temperature,so it is a kind of high-quality hydrogen storage material.In order to achieve high efficiency hydrogen production in ammonia borane solution and formaldehyde solution,it is urgent to develop high performance catalyst for hydrogen production.Compared with traditional oxide catalysts,the boron in transition metal borides has lower electronegativity,electrons are more likely to gather in the transition metal with catalytic activity,and it is easier to reduce and release the hydrogen in the hydrogen storage material.Moreover,borides are stable at high temperature and have strong acid and alkaline resistance,so they are considered as promising hydrogen-producing catalysts.However,transition metal boride catalysts are faced with one difficulty:high synthesis temperature and difficult preparation of borides.In order to solve this difficulty,this paper carries out the following specific research:（1）The first project studied three preparation methods of borides and successfully prepared crystalline Co B at relatively low temperature by tin thermal reduction method.XRD,SEM,TEM and other characterization methods were used to determine the phase and study the relationship between the structure and electron interaction.Co B was selected as the research object of this project through the experiment of hydrolyzation of ammonia borane at room temperature to produce hydrogen.The activation energy of hydrogen production is 23 k J·mol^-1,and the conversion frequency（TOF）is as high as35.3 mol_H2·mol_Cat.^-1·min^-1,which is better than that of platinum catalysts under the same conditions.Through the study,it was found that the active site of Co B catalyst for hydrolyzed hydrogen production of ammonia borane was the Co species rich in electron on the surface.Through the cross hydrogen production experiment of control variables and the analysis of the groups and free radicals generated in the reaction process,the experimental mechanism was inferred as follows:Co⁰ activates water to produce hydrogen radical and hydroxide ion,and captures boroalkyl group in solution,then hydroxide ion attacks boroalkyl group to break boro-hydrogen bond to form metaborate radical and release hydrogen radical,and combines with hydrogen radical in water to produce hydrogen escape.（2）The second project uses the conclusion of the first work that boron can transfer electrons to the transition metal to design molybdenum boride catalyst.The innovative alkaline etching of Mo Al B avoids the problem of massive shedding of boron species on the surface of products after traditional acid etching.The layered two-dimensional transition metal boride（MBene）Mo₂B₂ is prepared,and Mo species are grown in situ on the surface of MBene by calcination in reducing atmosphere.The phase of Mo@MBene was determined by XRD,TEM,XPS and other characterization methods,and the strong interaction between Mo and MBene was studied.This property was used to apply Mo@MBene to formaldehyde reforming hydrogen production at room temperature.The low reaction energy barrier and high hydrogen efficiency were tested.Its E_a and TOF are 18.30 k J·mol^-1 and 79.05 h^-1,which are superior to most non-precious metals formaldehyde reforming catalysts.Mo⁰ can also activate water to form hydroxide ions and hydrogen radicals.O₂ is captured by Mo⁰ in the form of coordination bonds.Formaldehyde exists in the form of methyl diol in aqueous solution.CH₂OHO^—is formed by methyl diol dehydrogenation ion under alkaline conditions It is then oxidized to form formic acid and release·OOH,which combine to form hydrogen to escape.In conclusion,by controlling the reaction flux and reducing agent to reduce the synthesis temperature of borides,the unique electron transfer mode in borides was found in this paper,and the synergistic effect of metal and boron in borides was applied to the hydrolysis of ammonia borane and hydrogen production of formaldehyde hydrolysis,providing a new idea for the design of metal boride catalyst.