The Effects Of Oxidation Structures During Adsorption And Catalytic Transformation Of Nitroaromatic Compounds By Graphene

Graphene is considered to be the next emerging environmental functional material,due to its large surface area,high hydrophobicity and accessibility of both the bottom and top surfaces.The potential applications of graphene material in organic pollution treatments have attracted wide interests,and become one of the hottest research areas in environment science.The molecular interactions between organic pollutant molecules and surfaces of graphene material are of great importance in both adsorption and catalytic transformation processes.However,the actual surface structure of graphene oxide(GO)remains unclear,and the existence of oxidation debris(OD)on GO nanosheets is still in dispute.On the other hand,a large number of studies have focus on using graphene-based material to remove polycyclic aromatic hydrocarbons(PAHs)through n-n interaction,but only very limited works have investigated the molecular-level mechanism of nitroaromatic compounds(NACs)adsorption onto graphene/graphene oxide nanosheets.NACs are widely used in pesticides,explosives,and as intermediates in the synthesis of dyes and other chemicals.They are typical organic pollutants in the environment.The correct understanding of the structures of graphene oxide nanosheets and interaction mechanisms of NACs on graphene materials are important for optimizing and designing new environmental functional materials for pratical applications.In this study,the surface properties of graphene-based material,the proposed interaction mechanisms of common pollutants on graphene/graphene oxide nanosheets,and the surface modification approaches of graphene-based material were firstly summarized.Subsequently,we confirmed the presence of oxidation debris(OD)for the first time using various characterization tools such as HRTEM,AFM,Q-TOF-MS,XPS,FTIR-ATR and Raman,and the molecular structure,location of OD were also initially proposed.Based on the research results of the micro-enviroment on graphene oxide nanosheets,adsorption isortherms and FTIR-ATR spectroscopy were selected to analysis the molecular interatction of different reduction degree graphene materials(GO,RGO and G)with NACs.A multiple molecular mechanism was first proposed in our study to explain the strong adsorption of NACs on nanosheets.The impacts of OD on the catalytic properties of graphene oxide nanosheets and its ability to stabilize metal nanocatalysts were also investigated.Combined with catalytic reaction kinetics and analysis methods(e.g.SEM,BET,and etc.),we found that OD had significant influence on the oxidation capability,hydrophobic property and surface conformation of graphene oxide nanosheets,and could regulate the stacking process of the nanosheets.The presence of OD on the nanosheets would restrict the application of graphene material significantly.Our original work provide a theoretical basis and an engineering reference for improving the performance of graphene material and designing novel functional materials.The main original conclusion of this work are drawn as follow:(1)The existence,location,and molecular structure of OD have been interpreted.It is the first time that OD(less than 5 nm)are observed by HRTEM and provides important new data for the current debate of the two-compoments structure of graphene oxide nanosheets.Combined with Q-TOF-MS,XPS,HRTEM images,FTIR-ATR,and AFM analyses,the hypothetical structure of a separated OD is initially proposed.Our results indicated that OD possesses a highly crystalline structure with a conderable number of oxygen-containing functional groups attached on the edges.OD were mainly adsorbed on the sp2 domains of nanosheets by ?-? interactions.(2)The removal of OD increases the adsorption capability of GO toward three common pollutants(i.e.phenanthrene,m-dinitrobenzene and Cd2+).The adsorption isotherms showed that the adsorption capacity of phenanthrene and Cd2+ on GO nanosheets without any OD is six-and two-fold greater than those on pristine nanosheets,respectively.The data emphasize the importance of considering the tiny OD in the environmental applications of graphene-based materials.(3)The surface properties and adsorption mechanisms of graphene materials are discussed deeply.FTIR-ATR spectroscopy and adsorption isotherms were applied to investigate the adsorption of m-dinitrobenzene,nitrobenzene,and p-nitrotoluene onto a serie of graphene materials(GO,RGO,and G).It was found that the Kd/KHW ratio of NACs reached up to 105 at the low sorbate concentration for RGO,which was almost three order of magnitude larger than that of naphthalene at the same sorbed concentration.In addition,the adsorption capability of NACs onto RGO and in this study was 10-50 times greater than that reported on CNTs.Therefore,a multiple molecular mechasnim was initially suggested to explain the distinct NACs adsorption,including the ?-? EDA interactions between the electron-deficient phenyl groups of the NACs and the electron-rich matrix of RGO and G,and the charge electrostatic interactions between the electron-withdrawing nitro groups of the NACs and defect/edge sites of graphene nanosheets.(4)We focused on the role of OD adsorbed on GO nanosheets in mediation of abiotic transformation of NACs and found that OD could modify the catalytic performance of GO efficiently.With the presence of OD,the transformation reaction of nitrobenzene was delayed for at least nine hours.By contrast,the reduction of nitrobenzene started immediately once the OD was removed from GO nanosheets.N-phenyhydroxylamine was more rapidly formed and transferred to aniline,and the transformation rate was more greatly enhanced(kobs increased from 1.17×10-2 h-1 to 2.5×10-2 h-1).The detachment of OD on GO nanosheets resulted in more convex and wrinkled regions with a high chemical reactivity which can be active catalytic sites on nitrobenzene reduction.Our researches imply the importance of the consideration of OD for a better optimization and design of GO nanosheets and other carbonaceous in environmental pollutant remediation.(5)Special attentions were paid to OD on the nucleation,growth and catalytic performance of metal nanocatalysts stabilized by GO nanosheets.According to the characteristic results of GO-based composites,the removal of OD significantly increases the amount of nucleation sites of Pt on GO nanosheets,and raises the loading capability for metal particles.In addition,OD could also efficiently regulate the aggregation and stacking process of graphene nanosheets.The appearance of more wrinkles and convexes on nanosheets as the detachment of OD prevented the restacking process of the reduced nanosheets,and created more micro pores in the GO-based catalysts.Catalytic reduction reactions showed that the performance of GO-based catalysts without OD on the nanosheets were four time larger than the pristine one.