Effects of nitrogen-, boron-, and phosphorus-doping or codoping on metal-free graphene catalysis

Graphene-based materials have been demonstrated as excellent alternatives to traditional metal-based catalysts in environmental remediation. The metal-free nature of the nanocarbons can completely prevent toxic metal leaching and the associated secondary contamination. In this study, nitrogen doped graphene (NG) at a doping level of 6.54 at.% was prepared at mild conditions. Moreover, B- and P-doping or codoping with N in graphene were also achieved by a simple route. The modified graphene can efficiently activate peroxymonosulfate (PMS) to produce sulfate radicals to oxidize phenol solutions. Kinetic studies indicated that initial phenol concentration, PMS dosage, and temperature presented significant influences on the degradation rates. Electron paramagnetic resonance (EPR) analysis provided further insights into the evolution of active radicals during the activation of PMS and SO4•− was believed to be the primary radicals in the oxidation reactions. This study demonstrated a metal-free material for green catalysis in environmental remediation

Low temperature combustion synthesis of nitrogen-doped graphene for metal-free catalytic oxidation

Nitrogen-doped reduced graphene oxide (N-rGO) was prepared by a simple process of simultaneous reduction and nitrogen doping on graphene oxide (GO) at low temperatures using ammonium nitrate as a N precursor. Characterization techniques indicated that N-rGO materials with a high N loading (5–8 at%) can be easily produced and that the crystal/micro-structures and chemical compositions of N-rGO materials are dependent on the calcination conditions. The metal-free catalysis of N-rGO was investigated by catalytic activation of peroxymonosulfate (PMS) for phenol oxidative degradation in water. It was found that N-rGO samples are promising green catalysts for phenol degradation. Kinetic studies showed that phenol degradation follows first order reaction kinetics on N-rGO-350 with an activation energy of 31.6 kJ mol−1. The mechanism of PMS activation and phenol oxidation was elucidated by employing both electron paramagnetic resonance (EPR) studies and quenching tests with ethanol and tert-butanol

Nanocarbons in different structural dimensions (0–3D) for phenol adsorption and metal-free catalytic oxidation

Metal-free nanocarbon materials in different structural dimensions, such as 0D fullerene (C60), 1D single-walled carbon nanotubes (SWCNTs), 2D graphene nanoplate (GNP), 3D hexagonally-ordered mesoporous carbon (CMK-3) and cubically-ordered mesoporous carbon (CMK-8) were investigated for adsorption and catalytic oxidation of phenol in water solutions. A variety of characterisation techniques were used to investigate the properties of the carbon samples. It was found that structural dimension and heat treatment would significantly affect the performance of the nanocarbons in adsorption and catalysis. Both GNP and CMK-3 showed better phenol adsorption with around 40% phenol removal in 500 mLof 20 ppm solutions. The nanocarbons were also used for metal-free activation of peroxymonosulfate(PMS) to produce sulfate radicals for catalytic phenol oxidation. Efficient catalysis was observed on CMK-3, CMK-8 and SWCNTs. Thermal treatment of the nanocarbons at 350?C in nitrogen was conducted to modulate the crystal and micro-structures and surface functional groups of the different nanocarbons. Enhancements at 2-fold in adsorption on SWCNTs and 7.5-fold in catalysis on CMK-8 were observed after the heat treatments. Mechanisms of adsorption and catalytic oxidation of phenol were discussed. This study contributes to the development of green materials for sustainable remediation of aqueous organic pollutants

Novel polyoxometalate@g-C3N4 hybrid photocatalysts for degradation of dyes and phenolics

Graphitic carbon nitride (g-C3N4) is an emerging metal-free catalyst, and has attracted considerate research interests in photocatalysis. For improving the low photocatalytic activity due to the polymeric nature, a variety of methods have been developed. In this study, polyoxometalate (POMs) functionalized g-C3N4 were synthesized using a facile hydrothermal method as novel photocatalysts. The photocatalysts were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), N2 sorption isotherms, thermogravimetric analysis (TGA), and UV–vis diffusion reflectance spectroscopy (UV–vis DRS). The photocatalytic properties were evaluated in photodecomposition of aqueous methylene blue (MB) and phenol under UV–visible light irradiations. Compared to pristine g-C3N4, POMs modified samples demonstrated enhanced efficiencies in photodegradation of MB and phenol. It was suggested that increased specific surface area, porous volume and efficient charge transfer would be responsible for the improved photocatalysis. This study proves the promising role of POMs in modification of novel photocatalysts

New insights into heterogeneous generation and evolution processes of sulfate radicals for phenol degradation over one-dimensional alpha-MnO(2) nanostructures

Heterogeneous activation of peroxymonosulfate (PMS) has become an attractive approach for catalytic oxidation since it can not only provide sulfate radicals as an alternative to hydroxyl radicals, but also avoid the metal toxicity in homogeneous catalysis. In this study, three one-dimensional (1D) α-MnO₂ nanostructures, nanorods, nanotubes and nanowires, were fabricated by a one-pot hydrothermal method without addition of any surfactants. Shape-dependent performance of 1D α-MnO₂ was observed in catalytic degradation of phenol solutions. The phenol oxidation can be described by a first-order kinetic model and the activation energies of phenol oxidation on three α-MnO₂ materials were estimated to be 20.3, 39.3 and 87.1 kJ/mol on nanowires, nanorods, and nanotubes, respectively. Both electron paramagnetic resonance (EPR) spectra and competitive radical tests were applied to investigate the PMS activation processes and to differentiate the major reactive species dominating the catalytic oxidation. The processes of PMS activation, evolution of sulfate radicals, and phenol degradation pathways were clearly illustrated.Yuxian Wang, Stacey Indrawirawan, Xiaoguang Duan, Hongqi Sun, Ha Ming Ang, Moses O. Tadé, Shaobin Wan

Insights into Heterogeneous Catalysis of Persulfate Activation on Dimensional-Structured Nanocarbons

A variety of dimensional-structured nanocarbons were applied for the first time as metal-free catalysts to activate persulfate (PS) for catalytic oxidation of phenolics and dyes as well as their degradation intermediates. Singlewalled carbon nanotubes (SWCNTs), reduced graphene oxide (rGO), and mesoporous carbon (CMK-8) demonstrated superior catalytic activities for heterogeneous PS activation, whereas fullerene (C60), nanodiamonds, and graphitic carbon nitride (g-C3N4) presented low efficiencies. Moreover, the carbocatalysts presented even better catalytic performances than activated carbon and metal oxides, such as Fe3O4, CuO, Co3O4, and MnO2. The activity of prepared rGO-900 was further competing to the most efficient electron donor ofzerovalent iron (ZVI). Both characterization and oxidation results suggested that the catalytic performances of the nanocarbons are determined by the intrinsic atom arrangements of carbon hybridization, pore structure, defective sites, and functional groups (especially the carbonyl groups). Electron paramagnetic resonance (EPR) spectra revealed that carbocatalysts might act as an excellent electron bridge in activation of PS to oxidize adsorbed water directly to generate hydroxyl radicals, distinct from homogeneous and metal-based catalytic activation. This study discovers several efficient nanocarbons for heterogeneous PS activation, and it presents new insights into the catalytic activation processes, providing a fascinating strategy to develop metal-free catalysts for green remediation

New insights into heterogeneous generation and evolution processes of sulfate radicals for phenol degradation over one-dimensional a-MnO2 nanostructures

Heterogeneous activation of peroxymonosulfate (PMS) has become an attractive approach for catalytic oxidation since it can not only provide sulfate radicals as an alternative to hydroxyl radicals, but also avoid the metal toxicity in homogeneous catalysis. In this study, three one-dimensional (1D) a-MnO2 nanostructures, nanorods, nanotubes and nanowires, were fabricated by a one-pot hydrothermal method without addition of any surfactants. Shape-dependent performance of 1D a-MnO2 was observed in catalytic degradation of phenol solutions. The phenol oxidation can be described by a first-order kinetic model and the activation energies of phenol oxidation on three a-MnO2 materials were estimated to be 20.3, 39.3 and 87.1 kJ/mol on nanowires, nanorods, and nanotubes, respectively. Both electron paramagnetic resonance (EPR) spectra and competitive radical tests were applied to investigate the PMS activation processes and to differentiate the major reactive species dominating the catalytic oxidation. The processes of PMS activation, evolution of sulfate radicals, and phenol degradation pathways were clearly illustrated

Synergy of carbocatalytic and heat activation of persulfate for evolution of reactive radicals toward metal-free oxidation

Persulfate (or peroxydisulfate, PDS) is one of green and low-cost sources of sulfate radicals (SO4radical dot−) in advanced oxidation processes (AOPs) for in situ remediation of contaminated soil and water. The key in AOPs is to develop an effective technique for PDS activation. In this paper, nitrogen-doped single-walled carbon nanotubes (N-SWCNTs) were employed as a metal-free catalyst to activate PDS for oxidation of a diversity of organic contaminants such as nitrobenzene (NB), phenol, benzoquinone and sulfachlorpyridazine. For the first time, the coupling effects of carbocatalysis and heat were investigated in a range of 5–75 °C on PDS activation, which indicated that organic oxidation efficiency was enhanced at elevated temperatures. The presence of the carbocatalyst impressively decreased the PDS activation energy from 53.4 (by heat) to 10.3–22.5 kJ/mol (heat/carbocatalysis). Intriguingly, the mechanisms of heat-assisted carbocatalysis were temperature-dependant and the synergy of the heat/carbon integrated system appeared to be phenomenal at a high temperature region (55–75 °C). The thermal stimulation promoted PDS to generate hydroxyl radicals via heterogeneous water oxidation and mutual transformation from sulfate radicals, evidenced by the selective radical quenching and spin trapping techniques. The carbocatalyst boosted the radical production by simultaneously activating the reactants (PDS and organics) with the N-doped carbon atoms and facilitating the electron transport as a conductive substrate. Therefore, this study advances the understanding in the effect of reaction temperature on persulfate activation and unveils the synergy of carbocatalysis with heat for enhanced PDS activation.Xiaoguang Duan, Stacey Indrawirawan, Jian Kang, Wenjie Tian, Huayang Zhang, Xuezhi Duan, Xinggui Zhou, Hongqi Sun, Shaobin Wan

Adsorption

Removing of wastewater pollutants by novel adsorption techniques is urgent as they are continuously defiling the limited freshwater resources, seriously affecting the terrestrial, ecosystems, aquatic, and aerial flora and fauna. Emerging carbon nanotube (CNT)-based adsorbent materials are effective for efficient handling of wastewater pollutants. This chapter describes the mechanisms of CNT, and its forces to host the wastewater pollutants. Such details would help to considerably improve the performance of classical adsorbent technologies. Additionally, the functionalization of CNT and adsorption isotherms are considered as they have been significantly used for achieving maximum adsorption capacity and disclosing the adsorption phenomena of CNT, respectively. Some multifunctional CNT-based adsorbent are also discussed with reusability phenomena which need to be addressed before large-scale implementation of CNTs for water purification. Some suggestions and research clues are given to inform investigators of potentially disruptive CNT technologies and/or optimize the CNT sorption performances that have to be investigated in more detail