Theoretical Simulation And Design Of Two-Dimensional Materials Supported Single-Atom Catalysts

The demand for energy is increasing.The ceaseless use of non-renewable energy sources has caused energy and environmental crisis.Developing renewable green energy sources is a pressing issue.Using electrocatalytic technology to conver molecules of H2O,N2 and CO2 to H2,NH3,CH4 or CH3OH products is a promising way to develop sustainable energy resouses.For instance,Hydrogen molecule has a high energy density,while its combustion product is pure water.Through the electrocatalytic hydrogen evolution reaction(HER),the water product can be reconverted into H2,achieving a recycling utilization.Ammonia(NH3)is an important chemical raw material with a wide range of applications.At present,the main source of ammonia is the industrial Haber-Bosch process technology,which normally requires high temperature and high pressure conditions(350-550 ?,150-350 atm)and thereby consumes much energy.While the electrocatalytic nitrogen reduction reaction(NRR)can transform N2 into NH3 under ambient conditions.Electrocatalytic carbon dioxide reduction reaction(CO2RR)also provides an excellent solution to the growing greenhouse effect problem.It not only reduces directly the carbon dioxide content,but also provides energy sources such as methane(CH4)or methanol(CH3OH).Hight performance electrocatalysts hold the key for driving electrocatalytic reaction efficiently.Developing stable and efficient electrocatalysts has long been a hot topic in scientific research.Recently,single-atom catalysts(SACs)have attracted extensive research interests due to their high catalytic efficiency and maximum atomic utilization.However,when the particle size is reduced to the atomic level,the highly exposed surface often holds high surface free energy.The single atom is thus easily diffused to form aggregates.The problem of stability has become a major bottleneck problem for the practical utilization of single atom catalysts.Using first-principles simulations,we have designed a series of single-atom catalysts supported by two-dimensional materials.This dissertation consists of five chapters.The contents of each chapter are as follows:In the first chapter,the background of single-atom catalysts,two-dimensional material and electrocatalytic reaction are introduced.Due to its extremely high catalytic efficiency and 100%atomic utilization rate,single-atom catalysts have attracted extensive research interests.When the particle size reaches a single atomic level,the properties of the catalyst change dramatically,such as surface free energy,quantum size effects,unsaturated coordination environments,and metal-support interactions.These thus reduce the amount of precious metal used while ensuring good catalytic activity.Meanwhile,single-atom catalysts also have limitations.For example,when the particle size is reduced to the atomic level,the high surface area will cause the surface free energy to rise sharply,which might redue the structural stability significantly.We have also introduced knowledge about two-dimensional materials.Since the advent of graphene in 2004,two-dimensional materials have attracted widespread attention due to their high stability,high specific surface area,excellent thermal conductivity,electrical conductivity,and strong modification.The two-dimensional materials involved in this dissertation are graphene and its derivatives,hexagonal boron nitride(h-BN)and carbon-nitrogen materials.In the second chapter,we introduced the quantum chemistry calculation methods and softwares.Density functional theory(DFT)is based on quantum mechanics.Using the Kohn-Sham equation,the interactive multi-particle system can be transformed into non-interactive single-particle system.By using the exchange-correlation functional approximation,the charge density of the ground state of the system can be obtained.In the actual simulations,according to the characteristics of different research objects,we can choose the most suitable exchange-related functional and quantitative calculation software packages.The next three chapters describe several designs of two-dimensional materials supported single-atom catalysts.In Chapter 3,based on first-principle calculations,we demonstrated a new design of stable and efficient transition-metal(TM)SAC that are protected by graphene(GR)or carbon-nitride(C3N)"chainmail".Our calculations found that stable sandwich structures of GR-TM-GR and C3N-TM-GR can be formed,holding high metal-GR/C3N binding energies and hight migration barriers for TM diffusion.Good HER catalytic activity is also found in C3N-Cu-GR,with very low |?GH*|values as 0.01 eV(being close to ideal free energy change for HER).Importantly,the GR and C3N surfaces hold relatively low adsorption energy with H atom leading to ease desorption of H2,which is otherwise very hard for the fully exposed TM SACs.Therefore,such sandwich design combines the high HER catalytic activities of TM SACs and good chemical stability of graphene or carbon-nitride materials.It not only prevents TM atoms from migrating,but also protects them from the direct attacks of reaction intermediates.This would pave the way for developing stable and efficient SAC systems towards practical utilization.In Chapter 4,we have designed two nitrogen fixing electrocatalysts:boron-carbon-nitrogen-supported single Mo catalyst(Mo@BCN)and graphene oxide supported transition metal catalyst(TM3@GO).(1)Mo@BCN.Based on spin-polarized density functional theory,we designed a model system of Mo SAC anchoring on BCN(Mo@BCN),as durable and highly efficient NRR catalyst.The hybrid BCN sheet support exhibits metallic property,facilitating the reduction of N2 by Mo SAC,and effectively suppress the competing hydrogen evolution.Three pathways for N2 reduction to NH3 on Mo@BCN,including distal,alternating,and enzymatic mechanisms,are discussed.The anchored Mo atom on BCN possesses excellent catalytic activity and high stability for NRR,with low overpotential of?0.42 V following the enzymatic pathway.Our results thus suggested a new strategy for making single atom NRR catalyst with high stability and activity.(2)TM3@GO.First-principles calculations have been employed to investigate the stability of TM(TM=Pt,Cu,Ni,and Co)atoms anchored on graphene(Gr)or GO,as well as their electrocatalytic activity for N2 fixation.The calculated results show that owing to the active sites provided by the epoxy functional group,TM atoms can strongly bind to the GO surface based on their large binding energies and high diffusion barriers.Compared to single TM atoms,TM3 trimers possess higher stability.Calculations of free energy pathways show that Ni3@GO is stable and exhibits high electrocatalytic activity for N2 fixation,compared to the Co,Cu,and Pt trimers.Our results open up a new strategy for the design of several metal atoms catalysts for N2 fixation.In Chapter 5,we have simulated a GO-supported single-ion catalyst where metal ions are directly immobilized on GO.Through first-principle calculations,we found that GO-supported cobalt ions is not only stable,but also can catalyze the chemiluminescence(CL)reaction of luminol as H2O2 acts as oxidizing agent.Our study revealed that the coupling between Co2+ and GO induces effective polarization charges.This improves chemical activity of reaction site,which accelerates the CL reactions.This work may be generalized to other GO-supported single-ion catalysts for a wide range of chemical reactions.