Fabrication And Catalytic Applications Of Heteroatoms-Modified Nanodiamond And Its Derivative Materials

Current established catalytic processes in chemical industries involve the use of metals as catalysts, in many cases precious metals, and metal oxides are used as catalysts. The processes are often energy-inefficient and poor in product selectivity, which can lead to squander of resources and production of greenhouse gases. Metal-free catalysis is recognized as an effective method that can reduce the dependence on conventional metals or metal-based catalysts, and carbon materials have been proven to be potential alternatives to meet the requirements of sustainable chemistry and energy. Compared with other carbon materials, sp3-bonded ultra-dispersed nanodiamond and its derivative materials (UNDM), including bucky nanodiamond (BND) and onion-like carbon (OLC), receive increasing attention because of their interesting physicochemical properties at nanoscale. In this thesis, the difference of conventional nanocarbon materials and UNDM on the physicochemistry properties, the fabrication of heteroatom-modified UNDM, and the influence of doped heteroatoms on the electronic structure and microstructure of the materials are studied systematically. Five kinds of catalytic reactions are selected as probe reactions. The active sites of the catalysts and the plausible reaction routes are investigated systemically. The main results are summarized as follows:(1) Based on its specific sp/sp core-shell structure and surface oxygen functional groups, nitrogen-modified BND (N-BND) powders have been synthesized via three different preparation methods using a calcinations process. The detailed formation and dynamic behaviors of the nitrogen species on the N-BND are revealed by elemental analysis, X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption. Moreover, we study the catalytic performance of metal-free N-BND catalysts by means of selective oxidation of benzylic alcohols as a probe reaction. The results indicate that the N-BND catalysts exhibit a higher catalytic activity than pristine BND and some conventional nanocarbon materials. By correlating the concentration of nitrogen species with substrates conversion rates and using molecular model catalysts with well-defined nitrogen and oxygen unit, it is concluded that the pyridinic nitrogen defect species plays a pivotal role in the catalytic reaction.(2) The controllable synthesis of nitrogen-doped OLC (N-OLC) is achieved via nitric acid pretreatment, followed by different temperature calcination treatment under an ammonia atmosphere. The detailed physicochemical properties of the N-OLC samples are investigated. Moreover, the catalytic performance of such doped samples for styrene epoxidation reaction is studied, which is a kind of typical metal-related catalytic reaction. The results indicate that the N-OLC samples exhibit more excellent catalytic performance than pristine OLC and some reported metal-related catalysts. The active site on N-OLC is identified by comparative experiments, Raman and model catalysts. A plausible catalytic mechanism of stytene epoxidation is proposed.(3) We present a facile fabrication of doped OLC samples with a reasonable concentration of boron (0.63-4.57 at%) via a high temperature thermal diffusion method. The influence of boron on the graphene-like layers and electronic properties (e.g. work function, density of states and valence band edge) of OLC is systematically investigated using Raman spectroscopy with different excitation energies (1.58-3.8 eV) and ultraviolet photoelectron spectroscopy. The as-prepared boron-doped OLC (B-OLC) samples exhibit a four electron process for the oxygen reduction reaction, which is similar to commercial Pt/C. It is worth noting that the intrinsic relationship between the electronic structure and catalytic performance of doped samples is explored by using experimental studies instead of theoretical calculations.(4) B-OLC and boron-doped carbon nanotubes as metal-free catalysts exhibit excellent catalytic activity and stability in nitroarene (20 examples) reduction under a stoichiometric amount of reductant. The results show that substitutional boron species BC3 may play an important role in improving the catalytic performance and the efficient utilization of N2H4.(5) We describe a metal-free reaction pathway in which OLC as a low-cost catalyst exhibits excellent catalytic activity and stability in the selective oxidation of mono-, di-and trisubstituted phenols to their corresponding p-benzoquinones, even better than the reported metal-based catalysts (e.g., yield, stability) and industrial catalysts for particular substrates. Together with XPS, Raman, DFT calculations, and a series of comparative experiments, we demonstrate that the zigzag configuration as a type of carbon defects may play a crucial role in these reactions by stabilizing the intermediate phenoxy radicals.