First-Principles Studies Of Hydrolytic Mechanism Of Ammonia Borane

With the enhancement of the performance of supercomputers,simulations based on density functional theory(DFT)have become more and more popular for studying the properties of materials and designing new functional materials.DFT calculations help to explore the catalytic activity on various materials at the atomic level,which is of great significance to design and predict new novel catalysts.Ammonia Borane(NH3BH3,AB),an excellent hydrogen storage material,has attracted much attention for its characteristic of hydrolytic dehydrogenation.In the presence of suitable catalysts,1 equiv of NH3BH3 can fully hydrolyze to release 3 equiv of hydrogen under ambient conditions.Based on DFT calculations,we mainly investigate the hydrolysis mechanism of NH3BH3 on different catalysts and design catalysts with low cost and outstanding performance.This thesis consists of six chapters.In the first chapter,the theoretical methods used in this thesis are introduced.Firstly,we introduce the framework of DFT,including Thomas-Fermi-Dirac approximation,the Hohenberg-Kohn theorem,the Kohn-Sham equation,and the exchange-related functionals.Then the development of DFT is briefly introduced.At last,the methods used to locate transition states especially synchronous transit method(ST)are introduced.The second chapter mainly introduces the state-of-the-art research of NH3BH3 hydrolysis,including the exploration of catalytic activity of various nanoparticle catalysts for the hydrolytic dehydrogenation of NH3BH3,and the experimental and theoretical studies on the hydrolysis mechanism of NH3BH3.Meanwhile,we briefly introduce the issues and prospects of NH3BH3 hydrolysis.In the chapter 3,we take face centered cubic Ru nanoparticles as an example of metal nanoparticles,which exhibit excellent catalytic activity for the hydrolysis of NH3BH3.Then the full hydrolysis of NH3BH3 catalyzed by the typical Ru(111),Ru(100)and Ru(211)surfaces is investigated by DFT calculations.It is found that the activation of the first H2O molecule and the combination of hydroxyl and NH3B group could be the rate determining step for the full hydrolysis of NH3BH3.Among these three surfaces,the stepped Ru(211)has the highest activity for the full hydrolysis of NH3BH3,which can proceed the hydrolysis reaction under ambient conditions.In the chapter 4,We adopt a structure of a Pt monolayer supported on the(0001)surface of tungsten carbide(PtML/WC(0001))to maximize the utilization of noble metal.Based on DFT calculations,a comparatively investigation is performed on the catalytic performance of NH3BH3 hydrolysis over PtML/WC(0001)and Pt(111).It is indicated that the catalytic performance of PtML/WC(0001)for NH3BH3 hydrolysis is comparable with that of Pt(111),and PtML/WC(0001)has the potential to be a good catalyst for the hydrolysis of NH3BH3 at room temperature.In the chapter 5,we collaborate with experimental group to use the supported single(double)atom catalysts to maximize the efficiency of noble metal atoms.Based on theoretical simulations combined with experimental characterization,we determined the atomic structures of Pt1/graphene and Pt2/graphene catalysts.And we provide a theoretical explanation of the reason of higher activity of Pt2/graphene than Pt1/graphene.In the last chapter,we design a structure of single Pt atoms supported on oxidized Graphene(Pt1/Gr-O)and investigate the full hydrolysis of per NH3BH3 molecule to produce three hydrolysis molecules by DFT calculations.It is suggested that the rate-limiting step of the whole process is the combination of one H2O molecule and BHNH3 with an energy barrier of 16.1 kcal/mol.Thus,Pt1/Gr-O is suggested to be a promising catalyst for hydrolysis of NH3BH3 at room temperature.In order to reduce the use of noble metals and the cost of catalysts,non-noble metal catalysts that can replace noble metals should be designed and prepared.In the next work,we will focus on the design of low-cost,high-activity,and high-stability non-noble metal nanoparticle catalysts for the hydrolysis of NH3BH3 by theoretical calculations.To maximize the use efficiency of non-noble metal atoms,we also attempt to design the single non-noble metal atoms supported on suitable substrates with the outstanding catalytic activity for NH3BH3 hydrolysis.