Preparation Of Boron And Nitrogen Codoped Nanodiamond And Its Performance For The Electro-reduction Of CO₂

Since the development of industrial society, exhaustive exploitation of coal, oil and natural gas not only trigger a series of energy crisis but also emit large amounts of carbon dioxide which is considered to be one of the major contributor to the greenhouse effect and global warming. The conversion and utilization of CO₂ to renewable rules and commercial chemicals are important from the viewpoints of conservation of resources and the development of a sustainable society. Among the various approaches, electrochemical reduction is especially favored because it can be preceded at moderate conditions, the products can be tuned by varying reaction conditions and be powered by a renewable electricity source thus no further CO₂ will be emitted. Apparently, the performance of electrode material is crucial in electro-catalysis. Boron and nitrogen co-doped nanodiamond exhibits excellent properties, such as high overpotential for hydrogen evolution, low background capacitive current and a high chemical stability, which are processed by boron doped nanodiamond （BDD） and nitrogen doped nanodiamond （NDD）. These properties can help solve the problems including low efficiency due to hydrogen evolution, poor product selectivity and easy deactivation. Additionally, the codoped boron and nitrogen can creat the synergistic effect which will greatly enhance the conductivity and electrocatalytic activity of the diamond thus converting CO₂ and water to more valuable and energy-rich products. In this paper, BND was prepared to explore its electrocatalytic performance for CO₂ reduction, the content of B/N was adjusted to control the electrocatalytic activity. The main research contents and results are as follows:（1） HFCVD system was used to prepare the BND film on a low resistance silicon sheet at 2000℃ and 300 Pa, using CH4 as carbon source, N2 and B2H6 as nitrogen and boron dopant, respectively. BDD and NDD films were also synthesized by the same method in order to be compared with BND.（2） SEM, Ranan, XRD and XPS were employed to characterize the doped nanodiamond films. The SEM images show the morphology and crystalline structures of the BND films which cover the entire substrates without noticeable crack. The nanoparticles of the diamond are in the range of 50-150 nm, the thikness is about 2.5 μm. Raman spectrums show that the characteristic peak of the diamond is asymmetric and shifts to the lower frequencies due to the doping of boron. XRD spectrums show three main diffraction peaks associated with （111）, （220） and （311） crystal faces of the diamond. XPS spectrums demonstrate the successful incorporation of boron and nitrogen into the diamond and the the main content is sp3C, illustrating the good quality of the diamond.（3） The B/N content was controlled to optimize the performance for CO₂ reduction on BND, which showed highly catalytic activity thus converting CO₂ into C2 product ehthanol in aqueous solution. LSV indicated the negative potential for hydrogen evolution （-1.62 V）, which inhibited the reduction of electrolyte and resulted in high energy efficiency. The results showed that the incorporation of boron and nitrogen created synergistic effect which greatly enhanced the conductivity and electrocatalytic activity of the diamond. The energy-rich reduction product ethanol was produced, which required twelve electrons in process of electron transfer. The Faradaic efficiencies of ethanol increased with the nitrogen doping content and the maximum was 93.2%. The production rate of ethanol was 55.37 and 11.30 times as high as that of methanol and formic acid under the same condition. The maximum production rate of ethanol was acquired when the nitrogen doping content is moderate.The BND film exhibited no degradation over nearly 50 h of continuous CO₂ reduction, the production rate of ethanol maintained around 11.5 mg/L-h, suggesting the highly catalytic stability of the electrode material. The reaction pathway of CO₂ reduction on BND was illustrated by in situ FT-IR and DFT calculation.