Carbon Nitride-Based Nanocomposites For High-Efficiency Hydrolysis Of Ammonia Borane

In recent years,green energy has received extensive attention.Among many green energy,hydrogen energy has become a hot research topic due to its many advantages.So far,there is still much room for improvement in the production and storage of hydrogen energy.There are various drawbacks in the storage of hydrogen in both gaseous and liquid states.Therefore,chemical storage of hydrogen is an ideal choice for the future.Studies have shown that ammonia borane is an ideal chemical hydrogen storage material.Ammonia borane has the advantages of high hydrogen storage rate,good water solubility and non-toxicity.At room temperature,ammonia borane can be hydrolyzed with water.1 mol ammonia borane can completely react with water to form 3 mol hydrogen gas.However,this reaction proceeds very slowly and the reaction rate can be greatly increased by adding suitable catalysts.For the past few years,a large number of highly efficient catalysts have been studied to catalyze the hydrolysis of ammonia borane.At the beginning,most of the articles reported noble metal catalysts mainly composed of silver,platinum and rhodium.However,since it is rare and expensive,it's difficult to be widely used.Therefore,people started to study the high-efficiency non-precious metal catalysts such as nickel,cobalt and copper.In this thesis,various types of metals are deposited on various types of carbon nitride via different reduction methods.By using the interfacial interaction between metal oxide nanoparticles and carbon nitride-based composite materials,the catalyst can achieve remarkable catalytic efficiency and good catalytic stability.This thesis mainly shows three carbon nitride-based nanocomposites as efficient catalysts,which are nickel-cobalt oxide nanoparticles supported on nitric acid-treated carbon nitride(Ni0.5Co0.5O-NCN),copper-cobalt metal oxide nanoparticles supported on graphene and nitric acid-treated carbon nitride composite(Cu0.5Co0.5O-GNCN)and platinum-cobalt nanoparticles supported on exfoliated carbon nitride(Pt0.1Co0.9O-ECN).The main research results of this thesis are shown as below:(1)Ni0.5Co0.5O was synthesized by in-situ reduction method and loaded onto nitric acid treated carbon nitride,which is used to catalyze the hydrolysis of ammonia borane to hydrogen evolution.After test,the catalyst shows excellent catalytic performance.Its TOF value is 76.1(H2)mol/(Cat-M)mol·min.Its stability is also very good.After 6 cycles,its catalytic activity can maintain 83.2%.Synchrotron radiation X-ray absorption spectroscopy shows that the carbon nitride substrate has two reaction centers to form a stable interfacial interaction with the nanoparticles,wherein carbon acts as an electron acceptor and nitrogen acts as an electron donor.Therefore,a stable interfacial interaction in Ni0.5Co0.5O-NCN can be formed,thereby accelerating the hydrolysis of ammonia borane.The synergistic effect between two metallic elements in the nanoparticles also improves the reaction activity.The two reaction center structure in carbon nitride can also be used for the mechanism study in various catalytic applications using carbon nitride as a substrate.(2)Using high-pressure hydrothermal reaction method,graphene and various types of carbon nitride were successfully mixed.The composite was used as a good substrate.Cobalt and copper bimetallic oxide nanoparticles are supported on it to catalyze the hydrogen evolution of ammonia borane.After comparison,we found that when the mass ratio of graphene oxide and nitic acid treated carbon nitride is 1:4,TOF reaches the highest value for the hydrogen evolution of ammonia borane,which is 51.7(H2)mol/(Cat-M)mol min.(3)By direct reduction of sodium borohydride,cobalt is mixed with precious metal elements such as platinum,palladium and ruthenium to be deposited on various types of carbon nitrides as catalyst to catalyze the hydrogen evolution of ammonia borane.When the molar ratio of platinum to non-precious metal is 1:9,the platinum-cobalt bimetallic nanomaterial shows the highest TOF value of 448.5(H2)mol/(Cat-Pt)mol min.After 6 cycles,the catalytic efficiency can maintain at 75.6%.From the XPS and XRD results,we can confirm that the bimetallic nanoparticles can partly maintain oxidation state after reduction by sodium borohydride.