Hongqi Sun

Reduced Graphene Oxide for Catalytic Oxidation of Aqueous Organic Pollutants

We discovered that chemically reduced graphene oxide, with an I(D)/I(G) >1.4 (defective to graphite) can effectively activate peroxymonosulfate (PMS) to produce active sulfate radicals. The produced sulfate radicals (SO(4)(•-)) are powerful oxidizing species with a high oxidative potential (2.5-3.1 vs 2.7 V of hydroxyl radicals), and can effectively decompose various aqueous contaminants. Graphene demonstrated a higher activity than several carbon allotropes, such as activated carbon (AC), graphite powder (GP), graphene oxide (GO), and multiwall carbon nanotube (MWCNT). Kinetic study of graphene catalyzed activation of PMS was carried out. It was shown that graphene catalysis is superior to that on transition metal oxide (Co(3)O(4)) in degradation of phenol, 2,4-dichlorophenol (DCP) and a dye (methylene blue, MB) in water, therefore providing a novel strategy for environmental remediation.

Different Crystallographic One-dimensional MnO2 Nanomaterials and Their Superior Performance in Catalytic Phenol Degradation

Three one-dimensional MnO2 nanoparticles with different crystallographic phases, α-, β-, and γ-MnO2, were synthesized, characterized, and tested in heterogeneous activation of Oxone for phenol degradation in aqueous solution. The α-, β-, and γ-MnO2 nanostructured materials presented in morphologies of nanowires, nanorods, and nanofibers, respectively. They showed varying activities in activation of Oxone to generate sulfate radicals for phenol degradation depending on surface area and crystalline structure. α-MnO2 nanowires exhibited the highest activity and could degrade phenol in 60 min at phenol concentrations ranging in 25-100 mg/L. It was found that phenol degradation on α-MnO2 followed first order kinetics with an activation energy of 21.9 kJ/mol. The operational parameters, such as MnO2 and Oxone loading, phenol concentration and temperature, were found to influence phenol degradation efficiency. It was also found that α-MnO2 exhibited high stability in recycled tests without losing activity, demonstrating itself to be a superior heterogeneous catalyst to the toxic Co3O4 and Co(2+).

Nitrogen-doped graphene for generation and evolution of reactive radicals by metal-free catalysis.

N-Doped graphene (NG) nanomaterials were synthesized by directly annealing graphene oxide (GO) with a novel nitrogen precursor of melamine. A high N-doping level, 8-11 at. %, was achieved at a moderate temperature. The sample of NG-700, obtained at a calcination temperature of 700 °C, showed the highest efficiency in degradation of phenol solutions by metal-free catalytic activation of peroxymonosulfate (PMS). The catalytic activity of the N-doped rGO (NG-700) was about 80 times higher than that of undoped rGO in phenol degradation. Moreover, the activity of NG-700 was 18.5 times higher than that of the most popular metal-based catalyst of nanocrystalline Co3O4 in PMS activation. Theoretical calculations using spin-unrestricted density functional theory (DFT) were carried out to probe the active sites for PMS activation on N-doped graphene. In addition, experimental detection of generated radicals using electron paramagnetic resonance (EPR) and competitive radical reactions was performed to reveal the PMS activation processes and pathways of phenol degradation on nanocarbons. It was observed that both (•)OH and SO4(•-) existed in the oxidation processes and played critical roles for phenol oxidation.

Sulfur and nitrogen co-doped graphene for metal-free catalytic oxidation reactions

Sulfur and nitrogen co-doped reduced graphene oxide (rGO) is synthesized by a facile method and demonstrated remarkably enhanced activities in metal-free activation of peroxymonosulfate (PMS) for catalytic oxidation of phenol. Based on first-order kinetic model, S-N co-doped rGO (SNG) presents an apparent reaction rate constant of 0.043 ± 0.002 min(-1) , which is 86.6, 22.8, 19.7, and 4.5-fold as high as that over graphene oxide (GO), rGO, S-doped rGO (S-rGO), and N-doped rGO (N-rGO), respectively. A variety of characterization techniques and density functional theory calculations are employed to investigate the synergistic effect of sulfur and nitrogen co-doping. Co-doping of rGO at an optimal sulfur loading can effectively break the inertness of carbon systems, activate the sp(2) -hybridized carbon lattice and facilitate the electron transfer from covalent graphene sheets for PMS activation. Moreover, both electron paramagnetic resonance (EPR) spectroscopy and classical quenching tests are employed to investigate the generation and evolution of reactive radicals on the SNG sample for phenol catalytic oxidation. This study presents a novel metal-free catalyst for green remediation of organic pollutants in water.

Fabrication of Fe3O4/SiO2 core/shell nanoparticles attached to graphene oxide and its use as an adsorbent

Amino-functionalized Fe(3)O(4)/SiO(2) core/shell nanoparticles were synthesized by reacting Fe(3)O(4) nanoparticles with tetraethyl orthosilicate and (3-aminopropyl) triethoxysilane to introduce amino groups on the surface. The amino groups on the Fe(3)O(4)/SiO(2) were reacted with the carboxylic groups of graphene oxide (GO) with the aid of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinnimide to form Fe(3)O(4)/SiO(2)GO nanoparticles. The structural, surface, and magnetic characteristics of the material were investigated by scanning and transmission electron microscopy, energy-dispersive X-ray spectrometry, powder X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Adsorption equilibrium and kinetics of methylene blue on the Fe(3)O(4)/SiO(2)GO were studied in a batch system. The maximum adsorption capacities were found to be 97.0, 102.6, and 111.1 mg g(-1) at 25, 45, and 60°C, respectively. A second-order kinetic equation could best describe the sorption kinetics. Thermodynamic parameters indicated that the adsorption of methylene blue onto the material was thermodynamically feasible and could occur spontaneously.

Facile synthesis of nitrogen doped reduced graphene oxide as a superior metal-free catalyst for oxidation

Nitrogen (5.61 at%) doped reduced graphene oxide synthesized via a facile method was demonstrated as a superior metal-free catalyst for activation of peroxymonosulfate. Codoping with boron would further enhance the catalytic activity and the stability, providing a promising green material for environmental remediation.

Nano-Fe0 Encapsulated in Microcarbon Spheres: Synthesis, Characterization, and Environmental Applications

Nanoscaled zerovalent iron (ZVI) encapsulated in carbon spheres (nano-Fe⁰@CS) were prepared via a hydrothermal carbonization method, using glucose and iron(III) nitrate as precursors. The properties of the nano-Fe⁰@CS were investigated by X-ray diffraction (XRD), thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption/desorption isotherms. Nano-Fe⁰@CS was demonstrated, for the first time, as an effective material in activating Oxone (peroxymonosulfate, PMS) for the oxidation of organic pollutants. It was found that the efficiency of nano-Fe⁰@CS was higher than ZVI particles, iron ions, iron oxides, and a cobalt oxide. The mechanism of the high performance was discussed. The structure of the nano-Fe⁰@CS not only leads to high efficiency in the activation of PMS, but also good stability. This study extended the application of ZVI from reductive destruction of organics to oxidative degradation of organics by providing a green material for environmental remediation.

Synthesis of porous reduced graphene oxide as metal-free carbon for adsorption and catalytic oxidation of organics in water

Activation of reduced graphene oxide (RGO) using CO2 to obtain highly porous and metal-free carbonaceous materials for adsorption and catalysis was investigated. A facile one-pot thermal process can simultaneously reduce graphene oxide and produce activated RGO without introducing any solid or aqueous activation agent. This process can significantly increase the specific surface area (SSA) of RGO from 200 to higher than 1200 m2 g−1, and the obtained materials were proven to be highly effective for adsorptive removal of both anionic (phenol) and cationic (methylene blue, MB) organics from water. Moreover, the activated RGO materials exhibited much better activity in effective activation of peroxymonosulfate (PMS) to produce sulfate radicals for oxidative degradation of MB.

Recent advances in non-metal modification of graphitic carbon nitride for photocatalysis: a historic review

Photocatalysis is a green, feasible and versatile technology that has been widely used for energy conversion and environmental applications. As photocatalysis bears a great potential for solar energy utilization, enormous investigations have been implemented in the past decades. The fundamental mechanism and some applications were well addressed in the last century. Currently, the major focus in photocatalysis research is the design and development of photocatalyst materials. This review firstly introduces the historic milestones in photocatalysis studies and then a comprehensive survey is conducted on the metal-based photocatalysts, including TiO2-based photocatalysts, ZnO and other metal oxides, metal sulfides, metal nitrides, and plasmon photocatalysts. From a historical viewpoint, particular attention is paid to metal-free graphitic carbon nitride (g-C3N4), a novel visible-light photocatalyst. Various modification techniques for g-C3N4 are summarized and analyzed. In terms of its metal-free nature, the fabrication of a porous structure, shape-control synthesis and non-metal doping are discussed in detail. Photocatalytic studies on g-C3N4-based catalysts are introduced. Some emerging elemental photocatalysts are also introduced. Finally, perspectives on non-metal photocatalyst design and development are provided.

A New Metal-Free Carbon Hybrid for Enhanced Photocatalysis

Carbon nitride (C3N4) is a layered, stable, and polymeric metal-free material that has been discovered as a visible-light-response photocatalyst. Owing to C3N4 having a higher conduction band position, most previous studies have been focused on its reduction capability for solar fuel production, such as hydrogen generation from water splitting or hydrocarbon production from CO2. However, photooxidation ability of g-C3N4 is weak and has been less explored, especially for decomposition of chemically stable phenolics. Carbon spheres prepared by a hydrothermal carbonization of glucose have been widely applied as a support material or template due to their interesting physicochemical properties and the functional groups on the reactive surface. This study demonstrated that growth of carbon nanospheres onto g-C3N4 (CN-CS) can significantly increase the photooxidation ability (to about 4.79 times higher than that of pristine g-C3N4) in phenol degradation under artificial sunlight irradiations. The crystal structure, optical property, morphology, surface groups, recombination rate of electron/hole pairs, and thermal stability of CN-CS were investigated by a variety of characterization techniques. This study contributes to the further promising applications of carbon nitride in metal-free catalysis.