Theoretical Simulation Study Of Nano-graphene As ORR Non-metallic Catalyst

At present,metals are usually used as catalysts for oxygen reduction reactions（ORR）,such as Au^[1],Co^[2],Fe^[3],Pt^[4,5],Pd^[6]and so on^[7].Although metal catalysts have good catalytic efficiency,disadvantages of high price,lack of resources and environmental pollution have to be considered.Mental-free catalysts as newly emerging green catalytic materials have attracted much attention in recent years.Compared with traditional metal catalysts,metal-free catalysts have the advantages of high efficiency,environmental protection,low energy consumption and corrosion resistance.In the fields of hydrocarbon conversion,chemical synthesis and energy catalysis,metal-free catalysts have shown excellent catalytic performance and development potential.People are expecting to find a green nonmetal catalyst which can replace metal catalysts and graphene is widely considered as a promising candidate as a two-dimension green catalytic material.However,the six-membered ring and?electronic structure of graphene causes graphene is too stable to find active sites as a catalyst in it.Instead,the decorated graphene and nanographene molecules in which the infinite?electronic structure is destroyed,are possible to be more suitable candidate catalytic materials.For nanographene,the band structure,optoelectronic properties can be controlled by adjusting the size,edge effect,and geometric shapes.The introduction of structural defects or atom doping to the pure graphene has also proven to be an effective method to regulate the surface reactivity and physical properties of graphene molecules.Therefore,nanographene molecules have better catalytic performance than pure graphene.Boron-and Nitrogen-atom-doped nano-graphene（BNG）molecule was successfully synthesized by Matthias Krieg’s team in 2015,and many of its properties and applications have not been reported.In this paper,we investigated the possibility of using BNG molecule as catalyst for CO oxidation with the first-principles approach.Combining the LH mechanism and the ER mechanism of the CO oxidation reaction,different reaction paths of the CO oxidation reaction are calculated with BNG as a catalyst.The results show that the energy barriers for dissociation and diffusion of O₂on the BNG surface are higher,1.28 e V and 1.17 e V,respectively,presenting that these reactions can not be carried out at room temperature.Thus,BNG molecules doped with Si atoms are considered to achieve better catalytic effect.Two optimized structures are obtained of Si-doped BNG,which were named as Si B₂N₃and Si B₃N₂.The calculation results show that the adsorption energy of O₂on the Si B₂N₃and Si B₃N₂is much larger than that of CO,indicating that the catalyst has oxygen affinity and will not be poisoned by CO.At the same time,it is found that Si B₂N₃and Si B₃N₂have respectively better catalytic effects in CO oxidation reaction by the simulation of the whole reaction path.The reaction mechanism is proved to be ER mechanism,and the reaction energy barrier of each step is lower than 0.91 e V.The low energy barrier states that the reaction can happen under mild conditions.Meanwhile,the Si B₂N₃and the Si B₃N₂are tried as catalysts for NO reduction reaction.The energy barrier of Si B₂N₃for NO reduction reaction was 0.61 e V（（NO）₂→N₂O+O）and 0.48 e V（N₂O→N₂+O）meeting the requirements of low temperature catalysis,which also proved that Si B₂N₃had the potential to be a catalyst for NO reduction reaction at room temperature.Therefore,the modified BNG molecule doped with Si atom can be used as an ideal mental-free catalyst.