Hierarchical Construction of 2D Nitrogen-rich Carbon Materials for Catalytic Applications

Author:Zhang Bing

Supervisor:chen jie sheng


Degree Year:2018





Energy crisis and environmental pollution are severer issues we have to face up with,developing clean energy,fabricating cheap but efficient catalysts for green energy and improving the energy conversation efficiency are all promising ways to solve these problems.Among all the utilized green energy,solar energy,electricity and hydrogen energy are most popular and available.The key to acquiring or utilizing these above green energy predominitely lies on the design of efficient catalysts.Carbon and nitrogen elements are abundant in reservation on the earth which lead to the low cost of nitrogen-rich carbon based catalysts.Nitrogen-rich carbon based catalysts can also be easily prepared via simple thermal process and have been widely studied as metal-free catalysts or support materials for many catalytic reactions.Usually,by tuning the temperature,doping elements or exfoliation methods,the properties of nitrogen-rich carbon based catalysts can be easily changed,graphene,nitrogen-doped graphene or carbon nitride with different conductivity and catalytic activity can all be fabricated and can serve as phtotocatalysts or electrocatalysts for hydrogen evolution or other reactions.Graphene and carbon nitride as typical 2D materials possess larger specific area and higher electron transfer efficiency compared with bulk samples.However,these pristine 2D materials are easy to aggregate into condensed solids and bury the active sites which means the high dimensional construction of these materials are of great importance.In order to prepare catalysts with high activity,it’s crucial to maintain the merits of pristine 2D materials such as high surface area during the construction process.Based on the above considerations,we aim to prepare catalysts with unique structure and excellent activity via high dimensional construction of graphene or carbon nitride based 2D materials.In this dissertation we also further study the catalytic process of constructed catalysts triggered by photons or electricity which may help to reveal the relationship between the fine structure and catalytic activity of as-obtained catalysts.We summarize the detailed research contents as following:(1)Mesoporous Graphitic Carbon Nitride Supported Palladium Nanoparticles for Efficient Methylation of Anilines at Mild Conditions C-N coupling reactions are of great importance for medical or organic applications.N-Methylaniline is mostly used as a latent and coupling solvent or used as an intermediate for agrochemicals,dyes and other organic products.Here,we constructed Mott-Schottky catalyst via loading palladium nanoparticles onto carbon nitride and fulfilled efficient methylation of anilines with high conversion and selectivity efficiency under estimated solar light irradiation.The existence of carbon nitride could not only serve as light absorber but also affect the electron density of active Pd nanoparticles.Further study revealed that the existence of balanced oxidation and reduction zoons of the optimized Pd-3@mpgCN is key concern to fulfill such excellent catalytic performances.(2)Solution-phase process of g-C3N4/BiVO4 dyad to large-area photoanode: interfacial synergy for highly efficient water oxidation Oxygen evolution reaction(OER)is rate-limiting process for water splitting and highly efficient large-area OER photoanodes have been the essential part in photoelectrochemical water splitting reactors.The high hole-electron separation efficiency of photoanodes is highly required for real applications of photoanodes in sufficiently harvesting solar energy.Herein we show that inactive g-C3N4 nanolayers can be self-assemblied with BiVO4 into highly coupled BV/CN dyad to significantly enhance the charge separation efficiency of BiVO4 photoelectrodes for OER.The incident photon-to-current conversation efficiency(IPCE)of visible light(400 nm)provided by scalable BV/CN-5 photoanode was estimated to be 50%at 1.23 V vs.RHE in 0.5 M Na2SO4 solution and significantly increased to 97%at a bias voltage of 1.6 V vs.RHE.(3)Polarized g-C3N4 Fewlayers Composed Aerogel as Metal-free Electrocatalysts for Highly Efficient Reduction of CO2 Geen house effect and global warming are severe issues we have to face up with,because increasing demand of global fossil fuel use has led to rapid rise of greenhouse gas exhaust emission in the atmosphere and terrible climate.As a major contributor,CO2 has attracted much attention of scientists to convert it into useful products by means of electrochemical or photoelectrochemical reduction methods.Facile design of efficient but cheap and abundant catalysts to convert CO2 into fuels or valuable chemical products always remains at the heart of materials chemistry and catalysis for solving the global climate as well as energy crisis.g-C3N4 with sufficient pyridinic nitrogen atoms is promising for CO2 conversion but suffers from low active surface area,weak ability of adsorbing gas moleculars and relatively low reduction activity.Herein,we show that two-dimentional(2D)g-C3N4 fewlayers composed aerogel can function as efficient metal-free electrocatalysts for selective reduction of CO2 to CO and formate at low overpotentials with a high Faradaic efficiency of90%.Polarized surface with a more reductive conductive band of ultra-thin g-C3N4 layers(thickness:1 nm)boosts the excellent electrochemical activity for CO2 reduction.(4)Nitrogen-doped Graphene Microtubes with Opened Inner Voids:Highly Efficient Metal-free Electrocatalysts for Alkaline Hydrogen Evolution ReactionNitrogen-doped graphene with similar structure of g-C3N4 but enhanced eletron transfer efficiency is considered as promising metal-free electrocatalyst for high catalytic current.A facile method was proposed to fabricate nitrogen-doped graphene microtubes(N-GMT)with ultra-thin walls of 1-4 nm and large inner voids of 1-2μm.The success in the introduction of nitrogen dopants afforded N-GMT more active sites while large inver voids with ultrathin wall thickness avoided the overlap of typical 2D catalysts which significantly increased the active surface areas and enhanced hydrogen evolution reaction(HER)activity.N-GMT achieved 10 mA cm-2 current density at overpotential of 0.464 V and 0.426 V vs.RHE in 0.1M and 6M KOH solution,respectively.The HER performance surpassed the best metal-free catalyst reported in basic solution,further illustrating the great potential of N-GMT as efficient HER catalyst for real uses in water splitting and chlor-alkali processes.