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Synthesis of porous nitrogen doped carbon cage from carbide for catalytic oxidation
Abstract
Nitrogen doped porous carbon cage materials are successfully synthesized using a chemical vapor deposition (CVD) method. Several carbides are selected as templates and in situ removed by CCl4. NaNO3 is then used to create porous structure, and a very large surface area (SSA) (1224 m² g-1) could be obtained. The obtained materials can activate peroxymonsulfate (PMS) for degradation of various organic pollutants effectively. By NaNO3, the nitrogen doping styles could be regulated and the percentage of graphitic N and CO group could be increased, which will alter the PMS-based oxidation from a radical pathway to a non-radical dominated process. This study provides a novel way for synthesis of carbocatalysts, which should have great potential in environmental remediation.
... Wang et al. found that NPC with higher graphitic N content exhibited better ability to activate PMS to generate sulfate and hydroxyl radicals, which was attributed to the ability of graphitic N to activate adjacent carbon atoms (higher positive charge) enhanced PMS adsorption and dissociation [20]. In addition, it was shown that an increase in graphitic N could alter PMS-based oxidation from a radical pathway to a non-radical-dominated process and enhance catalytic activity [21]. As verified by Shi et al., among these species, pyrrolic N, pyridinic N and graphitic N were thought favorable for improving catalytic activity [22]. ...
Understanding the role of intrinsic defects and nonmetallic heteroatom doping defects in activating peroxymonosulfate (PMS) and subsequently degrading endocrine-disrupting compounds is crucial for designing more efficient carbon catalysts. Therefore, we synthesized N-rich carbon nanosheets (NCs) through pyrolysis of a glutamic acid and melamine mixture and utilized them to activate PMS for bisphenol A (BPA) degradation. Different weight ratios of the above mixtures were allowed for manipulating NCs' defect level and N configuration. The reaction rate constant (k) was significantly positively correlated with the pyridinic and pyrrolic N content, and negatively and weakly positively correlated with graphite N and intrinsic defects, respectively. These findings suggest pyridinic and pyrrolic N, rather than graphitic N and intrinsic defects, enhance PMS activation to generate reactive oxygen species (specifically O•–2 and 1O2) and oxidize BPA. The NC-activated PMS system with the highest N content (17.9 atom%) demonstrated a remarkably high k (0.127 min-1) using minimal concentrations of PMS (0.4 mM) and NC (0.15 g/L), highlighting the system's efficiency. Excess halide anions led to significantly increased k with only a limited formation of trichloromethane (disinfection byproducts) in presence of 100 mM Cl−. This study offers novel perspectives on identifying catalytic sites within N-doped carbonaceous materials.
... The C 1s spectrum of NCS/MnO 2 can be fitted into 4 peaks at 284.6, 285.2, 285.9, 288.3 and 292.5 eV (Fig. 2d), corresponding to C-C/C=C, C-N, C-O, C=O and O-C=O 59,60 . The N 1s spectrum of NCS/MnO 2 can be divided into three peaks (Fig. 2e), attributed to pyridinic-N (398.6 eV), pyrrolic-N (399.7eV) and graphitic-N (400.9 eV) 61 . It has been shown that pyridinic-N and pyrrolic-N can enhance the electronic conductivity of carbon and hence promote the battery performance 62,63 . ...
Rechargeable aqueous Zn-ion batteries (ZIBs) have obtained extensive attention owing to their high safety, low-cost, environmental friendliness and high energy density. Nevertheless, developing suitable cathode materials remains challenging due to...
... P11 (m/z = 319) was thought to generate intermediates by demethylation, dehydroxylation and ring-opening after TC dehydration [70,71]. Subsequently, small molecular weight intermediates, such as P111, P121 and P122 (m/z = 219, 207 and 179), were produced by further reactions [72]. In the second path, P21 was subsequently generated from N − dealkylation steps of TC. ...
... To further study the crystal structure of formed calcium-containing particles, XRD spectra were studied ( Fig. 1(f)). The two peaks with 2θ of 26 • and 43 • appearing in modified and unmodified CNTs well match the peak positions of CNTs of the (0 0 2) and (1 0 1) plans of carbon materials, respectively [41,42]. ...
Chromate and phosphate simultaneously exist in several types of waste streams (e.g., leather tannery). Herein, we developed a new carbon nanotube (CNT) modified by in-situ forming nano calcium peroxide particles for the treatment. The CNTs with different ratios were prepared and tested for the performance. The CNT (CNT-3) prepared by 0.5-M CaCl2 and 0.5-M H2O2 demonstrated with the best performance. It had a total -O-O- group content of 5.24±0.68 mmol/g, and was uniformly covered by synthesized particles with an average diameter of 20-50 nm. At a dosage of 200 mg/L, the removal efficiency of Cr(VI) and phosphate was achieved with the percentage of 81 and 99%, respectively, at contact time of 120 min. The maximum phosphate adsorption capacity was 126.74 mg-P/g, much higher than that of many reported adsorbents such as other Ca-based, Zr-based and La-based adsorbents. Commonly existing anions (namely SO4²⁻, CO3²⁻, SiO3²⁻, NO3⁻, and Cl⁻) showed negligible effects except arsenite and fluoride and simultaneous removal could be obtained on the removal of oxyanions. Acidic condition (e.g., pH 3.0) was beneficial for Cr(VI) removal, but caused the negative effect on phosphate removal. The chemical and physical properties of CNT-3 were studied via several analytical approaches (e.g., BET, Zeta potential, FESEM-EDX, XRD and XPS). Cr(VI) was removed via chemical reduction to Cr(III) by H2O2 released from CaO2 on CNT-3; phosphate was adsorbed via chemical precipitation (forming hydroxyapatite (Ca10(PO4)6(OH)2)) due to the presence of calcium ion and electrostatic interaction. This research indicates the CNT-3 has a great potential for treatment of wastewater containing chromate and/or phosphate oxyanions.
... The activation energy was calculated to be 29.0 kJ/mol, which was lower than the reported value of CNTs (43.8 kJ/mol) [22], N-CNTs (36.0 kJ/mol) [47], graphene (84.0 kJ/mol) [12] and recently reported nitrogen-doped carbon cages (P-NCC-0.8) (29.4 kJ/mol) [48], suggesting that CPS@ZIF-8 could be a promising catalyst at mild ambient temperatures. ...
Rational design of metal-free carbon-based heterogeneous catatlyst for wastewater remediation via peroxymonosulfate (PMS) activation is highly desirable. Here, hollow structured porous carbon with abundant N, a high graphitization degree, and a large specific surface area and pore volume (1301 m²/g and 1.12 cm³/g) was synthesized by the pyrolysis of core-shell structured composites consisting of polystyrene (PS) cores and Zeolitic imidazolate frameworks-8 (ZIF-8) shells. The hollow structured carbon ([email protected]) was characterized thoroughly and applied for phenol degradation by the activation of PMS. The effects of operation conditions such as the catalyst and PMS dose, phenol concentration, initial pH, and temperature on phenol removal were investigated comprehensively. Moreover, the main reactive species involved in phenol oxidation were investigated, and a plausible mechanism for the degradation of phenol is proposed. The results show that [email protected] exhibited an excellent phenol adsorption and degradation performance, which can be mainly ascribed to its large surface area, abundance of nitrogen and hollow porous structure. Moreover, both the nonradical pathway (involving ¹O2) and the radical pathway (involving SO4•− and •O2⁻) were found to be involved in the decomposition of phenol.
... In addition to the above methods, some engineering techniques that require specialized equipment can also be used for the preparation of hierarchically structured porous materials (Fig. 5), such as freeze drying [125,126], supercritical fluid [127,128], chemical vapor deposition [129], ink-jet printing [58] and 3D printing [130], etc. In order to obtain the ideal hierarchical porous structure, these technical means often have high requirements on operating experience and process parameters. ...
To address the growing energy demands of sustainable development, it is crucial to develop new materials that can improve the efficiency of energy storage systems. Hierarchically structured porous materials have shown their great potential for energy storage applications owing to their large accessible space, high surface area, low density, excellent accommodation capability with volume and thermal variation, variable chemical compositions and well controlled and interconnected hierarchical porosity at different length scales. Porous hierarchy benefits electron and ion transport, and mass diffusion and exchange. The electrochemical behavior of hierarchically structured porous materials varies with different pore parameters. Understanding their relationship can lead to the defined and accurate design of highly efficient hierarchically structured porous materials to enhance further their energy storage performance. In this review, we take the characteristic parameters of the hierarchical pores as the survey object to summarize the recent progress on hierarchically structured porous materials for energy storage. This is the first of this kind exclusively to survey the performance of hierarchically structured porous materials from different porous characteristics. For those who are not familiar with hierarchically structured porous materials, a series of very significant synthesis strategies of hierarchically structured porous materials are firstly and briefly reviewed. This will be beneficial for those who want to quickly obtain useful reference information about the synthesis strategies of new hierarchically structured porous materials to improve their performance in energy storage. The effect of different organizational, structural and geometric parameters of porous hierarchy on their electrochemical behavior is then deeply discussed. We outline the existing problems and development challenges of hierarchically structured porous materials that need to be addressed in renewable energy applications. We hope that this review can stimulate strong intuition into the design and application of new hierarchically structured porous materials in energy storage and other fields.
Metal-free carbon-based catalysts hold great promise for the degradation of organic pollutants by peroxymonosulfate (PMS) activation because they avoid the negative effects of metal catalysts such as harmful metal ions leaching. However, these carbon-based catalysts are limited by their high cost and complex synthesis, and the mechanisms for the activation of PMS are unclear. Herein, the N-rich carbon catalysts (GCN-x) derived from glucose and g-C3N4 were facilely synthesized by hydrothermal treatment and carbonization to explore the mechanism of PMS activation. The nitrogen content of catalysts could be adjusted by simply altering the ratio of glucose and g-C3N4. GCN-2.4 with a ratio of glucose and g-C3N4 of 2.4 displayed the highest efficiency for the degradation of pollutants represented by Levofloxacin. The electron paramagnetic resonance and quenching experiments demonstrated that the non-radical pathway was dominant in Levofloxacin degradation and singlet oxygen (1O2) was the main active specie. Further, we found 1O2 was generated from superoxide radical (• O2-) which has rarely been studied. Levofloxacin degradation rates were shown to be positively correlated with both the amount of graphitic N and pyridinic N. Graphitic N and pyridinic N were identified as the catalytic sites. The GCN-2.4/PMS system could also remove multifarious contaminants effectively. Overall, this research advances understanding of the role of N species in PMS activation and has potential practical application in wastewater treatment.
In this work, Fe@NC/B material is successfully synthesized and in-situ supported on the surface of amorphous boron (B) using a simple pyrolysis method. The interface between Fe species and B is improved by introducing N-doped carbon (NC) layers as intermediate, fast electron transfer from B to Fe@NC can therefore be achieved, thus could promote the fast redox cycle of Fe3+/Fe2+. The obtained material can therefore activate peroxymonosulfate (PMS) effectively to degrade Bisphenol A (BPA), a fast degradation rate and a very long lifetime in a continous tubular reactor are realized. Moreover, experiments and DFT calculation indicate that Fe2+ containing species are the dominated active sites, while the exposed B atoms and structure defect of B can also activate PMS directly to produce SO4•- and 1O2 species for BPA degradation. In addition, boric acid is the oxidation product of B, which can be dissolved into the aqueous solution and expose fresh B species again for PMS activation. The combination of B with Fe@NC provide novel materials for long term PMS activation, thus could promote the real application of persulfates on an industrial scale.
Nitrogen-doped carbon material is a promising oxygen reduction reaction (ORR) catalyst with great potential as a substitute for Pt-based electrocatalysts in fuel cells. Herein, N-doped porous carbon (N-PC) with a honeycomb-like structure was prepared by a simple solvent-free ball-milling method, in which the mixtures of waste biomass pine sawdust, melamine and K2CO3 were thermally annealed at 900 °C. The prepared N-PC catalyst exhibited a huge specific surface area (1504 m²/g), and a high N content (7.6 wt%). Comparable onset potential (0.96 V vs. RHE) and half-wave potential (0.84 V vs. RHE) to that of commercial Pt/C catalyst were achieved. Moreover, the N-PC demonstrated superior durability and methanol tolerance. The excellent catalytic performances of N-PC were correlated with the integrated effects of porous structure, large specific surface area, rich active sites, high defect level and great electrical conductivity. It provides an alternative way to construct metal-free materials as ORR electrocatalysts.
Catalytic oxidation plays important roles in energy conversion and environment protection. Boron-doped crystalline carbocatalyst has been demonstrated effective; however, the application potential of boron-doped amorphous carbocatalyst remains to be explored. For amorphous carbon material, finite-sized carbon clusters are the basic structural units, which exhibit unique activity due to edge and size effect. Herein, using sulfur dioxide (SO2) and carbon monoxide (CO) oxidation as probe thermal-catalysis reactions, we found the distribution and reactivity of active sites in boron-doped carbon clusters are simultaneously determined by dopants and edges. According to comparisons of oxygen (O2) chemisorption energy at different sites of symmetric and non-symmetric carbon cluster, the most active site is found to be the edge carbon atom with high electron donation ability, which can be accurately identified by electrophilic Fukui function. More importantly, the reactivity of boron-doped cluster is simultaneously influenced by doping configuration and the type of edge, based on which -O-B-O- configuration embedded into K-region edge (isolated carbon-carbon double bonds that do not belong to Clar sextet) is predicted to exhibit the highest reactivity among various boron doping configurations. This work clarifies unique activity origin of heteroatom-doped amorphous carbon materials, providing new insights into designing high-performance carbocatalysts.
The present work deals with simple hydrothermal preparation of magnetically separable BiVO4/N-rGO/CuFe2O4 Z-scheme hybrid photocatalyst for efficient detoxification of p-bromophenol (p-BP). BiVO4 is one of the most efficient visible light catalyst, however, suffers with photo-corrosion and high recombination of charge carriers that restricts its applications. Addition of graphene oxide doped with nitrogen are believed to overcome these issues. Further addition of CuFe2O4 enhances the catalytic degradation and helps in separating the catalyst. With these merits BiVO4/N-rGO/CuFe2O4 could be a promising photocatalyst for detoxification of p-BP. After 60 min of irradiation 94.1 ± 1.2% degradation was achieved by the BiVO4/N-rGO/CuFe2O4 (k = 0.01749 min⁻¹) while only 53.8 ± 2.3% was observed in BiVO4 (k = 0.00674 min⁻¹). The catalyst could be able to recover using external magnet and showed 86.4% of degradation efficiency even after five recycles, which suggested its good stability. The degradation products identification and pathway were proposed based on LC-ESI/MS analysis. Moreover, phytotoxicity assessment of the degradation products was investigated on Phaseolus vulgaris in which the germination index (GI) was about 11.4% for pure p-BP, while it was about 83.03% for degradation products. Thus, the results suggested that more efficient p-BP detoxification was achieved during this photodegradation.
Tetracycline (TC) is a commonly used antibiotic that has gained wide spread notoriety owing to its high environmental risks. In this study, rich carbonyl-modified carbon-coated Fe⁰ was obtained by pyrolysis of MIL-100(Fe) in an Ar atmosphere, and used to activate peroxymonosulfate (PMS) for the degradation of tetracycline in water. The roles of Fe⁰, carbon and surface carbonyl on PMS activation were investigated. Fe⁰ continuously activated PMS, acted as a sustained-release source of Fe²⁺, and could effectively activate PMS to produce SO4•−, O2•− and •OH. Carbon was found to do responsible for electron transportation during the activation of PMS and slow down the oxidation of Fe⁰. The carbonyl group on the carbon surface layer was the active site of ¹O2, which explains the enhanced performance for TC degradation. When Ca = 0.1 g/L and C0= 0.4 mM, TC degradation rate reached 96%, which was attributed to the synergistic effect of radicals (i.e., SO4•−, O2•−, •OH) and non-radical (i.e., ¹O2). Finally, the degradation pathway was proposed by combining density functional theory (DFT) calculations with liquid chromatography-mass spectrometry (LC-MS), toxicities of the intermediate products were also evaluated. All results show that carbonyl-modified carbon-coated Fe⁰ possesses promising capacity for the removal of antibiotics from water.
Fenton‐like reactions with persulfates as the oxidants have attracted increasing attentions for the remediation of emerging antibiotic pollutions. However, developing effective activators with outstanding activities and long‐term stabilities remains a great challenge in these reactions. Herein, a novel activator is successfully synthesized with single iron atoms anchored on porous N‐doped carbon (Fe‐N‐PC) by a facile chemical vapor deposition (CVD) method. The single Fe atoms are coordinated with four N atoms according to the XANES, and the Fe‐N4‐PC shows enhanced activity for the activation of peroxymonosulfate (PMS) to degrade sulfamethoxazole (SMX). The experiments and density functional theory (DFT) calculations reveal that the introduction of single Fe atoms will regulate the main active sites from graphite N into Fe‐N4, thus could enhance the stability and tune the PMS activation pathway from non‐radical into radical dominated process. In addition, the N atoms connected with single Fe atoms in the Fe‐N4‐C structure can be used to enhance the adsorption of organic molecules on these materials. Therefore, the Fe‐N4‐C here has dual roles for antibiotics adsorption and PMS activation. The CVD synthesized Fe‐N4‐C shows enhanced performance in persulfates based Fenton‐like reactions, thus has great potential in the environmental remediation field.
Peroxydisulfate (PDS) can be activated by graphitic carbon nitride (g-C3N4) under irradiation, and the generated oxygen reactive species (ROSs) can degrade organic pollutants effectively. However, the photocatalytic activity of pristine...
Iron oxide (FexOy) supported on porous nitrogen doped carbon is synthesized by a facile pyrolysis method. SiO2 and NaNO3 are used as the template and activation agent respectively for porous structure generation and large specific surface area (SSA) creation. The obtained materials show superior catalytic oxidation ability and can activate peroxymonosulfate (PMS) in a wide pH range (3–9) to degrade organic pollutants. The degradation process is a two-stage reaction, including a rapid initial decay and a following slow reaction stage. According to the free radical quenching experiments, electron paramagnetic resonance (EPR) spectroscopy analysis, and electrochemical tests, the superoxide radical (O2⁻) and singlet oxygen (¹O2) are proved to play crucial roles in organics degradation. The high SSA (773 m² g⁻¹), abundant of structural defects, and synergistic effect between FexOy and the nitrogen doped carbon are the key factors for the enhanced activity. The catalysts in this study can be synthesized easily and contain no toxic metals, thus should have great potential in the wastewater remediation.
Heteroatom-modified carbon-based catalysts (especially nitrogen-doped) have been reported to be effective in activating peroxymonosulfate (PMS). Nevertheless, here our study found that graphitized carbon nanomaterial (GC800) exhibited a similar catalytic capacity as the nitrogen-doped graphitized carbon nanomaterial (NGC800) in the degradation of bisphenol A (BPA) via activating PMS, however, the degradation rate constant of the former was approximately 1.72-times greater than that of the latter. Meanwhile, GC800 showed admirable versatility, wide application range of pH value and excellent stability. Furthermore, the connection between the difference in surface chemistry (surface functional groups, electronic properties) and reaction rate between GC800 and NGC800 was discussed. Quenching experiments combined with electron paramagnetic resonance (EPR) and electrochemical measurements revealed the existence of a multi-pathway synergy in GC800/PMS system dominated by the non-radical electron-transfer pathway and complemented by radical (O2•−)/non-radical (¹O2) pathway. The high proportion of carbonyl groups and sp²-hybridized carbon as the main parameters affected the electron density of GC800 in the process of non-radical electron transfer. In contrast, NGC800/PMS system only followed the non-radical oxidation pathway with singlet oxygen being the predominant ROS. Pyrrolic N affected the non-radical oxidation pathway of NGC800. This work verifies the significant role of surface chemistry of carbon materials and the synergy between non-radical electron transfer and radical/non-radical oxidation on the catalytic velocity. And it presents a novel thought in the design of high-efficiency carbon-based catalysts toward green and sustainable ecological restoration.
Nitrogen-doped graphene (NG) materials were successfully synthesized in one step by the restricted template method. Specifically, the use of different nitrogen sources and carbon sources makes the content of various nitrogen doping structures in NGs different and yields a higher nitrogen doping amount. In the lithium insertion process, nitrogen doping provides more active sites, which effectively improve the specific capacity. As anodes for lithium-ion batteries (LIBs), NGs exhibit excellent electrochemical performance with high reversible specific capacity, good cycle stability, and excellent rate performance. © 2021. The Authors. Published by ESG (www.electrochemsci.org). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).
In this work, γ-Al2O3-supported CuO (c-CuO/Al2O3) materials are successfully synthesized using a novel impregnation-precipitation-decomposition method. The obtained c-CuO/Al2O3 catalyst shows excellent catalytic activities for bisphenol A (BPA) degradation with sodium persulfate (PDS) as an oxidant. Radical quenching tests and electron paramagnetic resonance (EPR) studies indicate that PDS activation is a combined mechanism involving both free radical and nonfree radical pathways. In a continuous large-scale degradation process, about 1.78 L of 20 ppm BPA can be completely removed within 480 min. Although c-CuO/Al2O3 can be deactivated after several reaction cycles, the catalytic activity can be regenerated after simple aerobic calcination. X-ray photoelectron spectroscopy (XPS) and Raman analysis confirm that the deactivation of c-CuO/Al2O3 should be attributed to the conversion of Cu(II) to Cu(I). The aerobic calcination could oxidize Cu(I) back to Cu(II), thus recovering the catalytic activity. In addition, the density functional technology (DFT) and temperature-programmed oxidation (TPD) results reveal that γ-Al2O3 can not only serve as a carrier to anchor the CuO particles but also can adsorb and activate PDS by introducing more basic sites on the surface. c-CuO/Al2O3 has high activity and can be regenerated easily, thus having great potential applications for wastewater treatment.
Carbon materials are effective catalysts to activate peroxymonosulfate (PMS) for organic pollutant degradation. Both nitrogen doping and structural defects could enhance their catalytic performance for PMS activation. In this study, nitrogen-doped graphene (NG) is first synthesized by the calcination of graphene oxide (GO) with ammonium nitrate (NH4NO3). The obtained NG is then annealed further at a higher temperature under a N2 atmosphere to remove partially doped N atoms and create new structural defects. The obtained defective nitrogen-doped graphene (D-NG) can activate PMS for bisphenol A (BPA) degradation more effectively. Different annealing temperatures from 850 to 1150 °C are investigated, and D-NG synthesized at 1050 °C exhibits the highest activity. The enhanced catalytic performance is proposed to originate from the synergistic effect between doped N atoms and created structural defects. According to radical quenching, electron paramagnetic resonance (EPR), and electrochemical results, both radical and nonradical pathways are present during PMS activation, and the nonradical pathway plays the dominant role. This study provides a facile method for metal-free catalyst synthesis, which also enriches the synergistic mechanism between doped N and structural defects and thus should have great potential in wastewater remediation.
A facile eco-friendly co-precipitation synthesis at low temperature was employed to fabricate CuFe2O4-Fe2O3 for the oxidation of bisphenol A (BPA) via peroxymonosulfate (PMS) activation. The formation mechanism of CuFe2O4-Fe2O3 at low temperature is proposed. The FESEM and BET characterization studies revealed that the CuFe2O4-Fe2O3 has a quasi-cubic morphology and specific surface area of 63 m2 g-1. The performance of CuFe2O4-Fe2O3 as a PMS activator was compared with other catalysts and the results indicated that the performance was in the following order: CuFe2O4-Fe2O3> CuFe2O4> CoFe2O4> CuBi2O4> CuAl2O4> Fe2O3>MnFe2O4. A kinetic model with mechanistic consideration of the influence of pH, PMS dosage and catalyst loading was developed to model the BPA degradation. The intrinsic rate constant (ki) was obtained from the kinetic study. The relationship between the pseudo first-order rate constant and ki was established. The trend of ki revealed that increasing the catalyst loading decreased the BPA rem
Meeting the growing global energy demand is one of the important challenges of the 21st century. Currently over 80% of the world's energy requirements are supplied by the combustion of fossil fuels, which promotes global warming and has deleterious effects on our environment. Moreover, fossil fuels are non-renewable energy and will eventually be exhausted due to the high consumption rate. A new type of alternative energy that is clean, renewable and inexpensive is urgently needed. Several candidates are currently available such as hydraulic power, wind force and nuclear power. Solar energy is particularly attractive because it is essentially clean and inexhaustible. A year's worth of sunlight would provide more than 100 times the energy of the world's entire known fossil fuel reserves. Photocatalysis and photovoltaics are two of the most important routes for the utilization of solar energy. However, environmental protection is also critical to realize a sustainable future, and water pollution is a serious problem of current society. Photocatalysis is also an essential route for the degradation of organic dyes in wastewater. A type of compound with the defined structure of perovskite (ABX3) was observed to play important roles in photocatalysis and photovoltaics. These materials can be used as photocatalysts for water splitting reaction for hydrogen production and photo-degradation of organic dyes in wastewater as well as for photoanodes in dye-sensitized solar cells and light absorbers in perovskite-based solar cells for electricity generation. In this review paper, the recent progress of perovskites for applications in these fields is comprehensively summarized. A description of the basic principles of the water splitting reaction, photo-degradation of organic dyes and solar cells as well as the requirements for efficient photocatalysts is first provided. Then, emphasis is placed on the designation and strategies for perovskite catalysts to improve their photocatalytic activity and/or light adsorption capability. Comments on current and future challenges are also provided. The main purpose of this review paper is to provide a current summary of recent progress in perovskite materials for use in these important areas and to provide some useful guidelines for future development in these hot research areas.
Acetylene hydrochlorination is an important coal-based technology for the industrial production of vinyl chloride, however it is plagued by the toxicity of the mercury chloride catalyst. Therefore extensive efforts have been made to explore alternative catalysts with various metals. Here we report that a nanocomposite of nitrogen-doped carbon derived from silicon carbide activates acetylene directly for hydrochlorination in the absence of additional metal species. The catalyst delivers stable performance during a 150 hour test with acetylene conversion reaching 80% and vinyl chloride selectivity over 98% at 200 °C. Experimental studies and theoretical simulations reveal that the carbon atoms bonded with pyrrolic nitrogen atoms are the active sites. This proof-of-concept study demonstrates that such a nanocomposite is a potential substitute for mercury while further work is still necessary to bring this to the industrial stage. Furthermore, the finding also provides guidance for design of carbon-based catalysts for activation of other alkynes.
A considerable amount of sewage sludge (SS) is generated from wastewater treatment process, which is hazardous to the environment and in urge to be disposed. In this study, for the first time, we prepared carbocatalyst with abundant surface oxygen functional groups using the hazardous waste of SS as precursor via a facile hydrothermal coupled pyrolysis process. The hydrothermal treatment was found to be crucial for enhancing the oxygen content of sludge carbon (SC), most of which existed as ketonic groups. Catalytic performances of the developed SCs were examined by activating peroxymonosulfate (PMS) to degrade bisphenol A (BPA). Sample with more ketonic group performed better for BPA degradation. Under optimal reaction conditions, 100 % of BPA and 69.53 % of TOC could be removed in 20 min. Singlet oxygen (1O2) was suggested to be the main reactive oxygen species for degrading BPA and a BPA degradation pathway was proposed. The BPA solution showed decreased bio-toxicity after the oxidation process according to the acute ecotoxicity test. This study demonstrated the importance of surface functional groups on carbocatalyst for advanced oxidation process, which could be induced by a facile hydrothermal treatment. The feasibility of utilizing hazardous SS for advanced carbocatalyst fabrication was also revealed.
In this study, nitrogen-doped graphene quantum dots (N-GQDs, ∼5 nm) were synthesized and supported on graphite foam (GF) by a facile hydrothermal route. The obtained N-GQDs/GF composite demonstrates as an ultra-active free-standing electrode for electrochemical hydrogen evolution and activation of peroxymonosulfate to degrade phenol pollutant. An overpotential in the electrochemical hydrogen evolution is only −72 mV vs. RHE at 10 mA/cm² with a Tafel slope of 84 mV/Dec. The N-GQDs/GF electrode can also achieve 100% phenol removal in short time in electrochemical degradation.
This exhaustive review focuses on the fundamental principles and applications of heterogeneous electrochemical wastewater treatment based on Fenton’s chemistry reaction. The elementary equations involved in formation of hydroxyl radical in homogeneous electro-Fenton (EF) and photo electro-Fenton (PEF) processes was presented and the advantages of using insoluble solids as heterogeneous catalyst rather than soluble iron salts (heterogeneous electro-Fenton process) (Hetero-EF) was enumerated. Some of the required features of good heterogeneous catalysts were discussed, followed by the mechanisms of catalytic activation of H2O2 to reactive oxygen species (ROS) especially hydroxyl radical (●OH) by heterogeneous catalyst in Hetero-EF system. Extensive discussion on the two configuration of Hetero-EF system vis-à-vis added solid catalysts and functionalized cathodic materials were provided along with summaries of some relevant studies that are available in literature. The solid catalysts and the functionalized cathodic materials that have been utilized in Hetero-EF wastewater treatment were grouped into different classes and brief discussions on their synthesis route were given. Besides, the use of solid catalysts and iron-functionalized cathodic materials in bioelectrochemical system (BES) especially bioelectro-Fenton technology (BEF) using microbial fuel cells (MFCs) with concurrent electricity generation for Hetero-EF treatment of biorefractory organic pollutants was discussed. In the final part, emphasis was made on the challenges and future prospects of the Hetero-EF for wastewater treatment.
The thermolytic transformation of lignocellulosic spent coffee grounds to superior redox-active carbocatalyst (denoted as NBC) via nitrogen functionalization in a pyrolytic environment at various temperatures was investigated. The intrinsic (e.g. surface chemistry, degree of graphitization, etc.) and extrinsic (e.g. specific surface area, morphology, etc.) properties of the catalysts were systematically studied using various characterization techniques. The three main N configurations conducive to redox reactions, namely pyrrolic N, pyridinic N and graphitic N were present at different compositions in all the NBCs prepared at pyrolysis temperature ≥500 °C. The NBCs were used as peroxymonosulfate (PMS) activator for degrading bisphenol A. It was found that NBC-1000 (prepared at 1000 °C) has the highest catalytic performance (kapp = 0.072 min⁻¹) due to the relatively higher specific surface area (438 m² g⁻¹), excellent degree of graphitization, and optimum N bonding configuration ratio. Based on the radical scavenger and electron paramagnetic resonance studies, the nonradical pathway involving ¹O2 generation is identified as the prevailing pathway while the radical pathway involving SO4[rad]⁻and [rad]OH generation is the recessive pathway. Further investigation of the durability of surface active sites revealed that the active sites undergo N bonding configuration reconstruction and cannibalistic oxidation (increase in surface oxygen content) during PMS activation reaction. The graphitic N manifest greater catalytic activity and stability compared to pyridinic N and pyrrolic N under oxidizing environment. The results demonstrated that reaction optimization is critical to improve the durability of the catalyst. This study provides useful insights in converting lignocellulosic biomass waste into functional catalytic material, and the strategy to improve the durability of carbocatalysts for redox-based reactions.
Catalytic processes have remarkably boosted the rapid industrializations in chemical production, energy conversion, and environmental remediation. As one of the emerging applications of carbocatalysis, metal-free nanocarbons have demonstrated promise as catalysts for green remediation technologies to overcome the poor stability and undesirable metal leaching in metal-based advanced oxidation processes (AOPs). Since our reports of heterogeneous activation of persulfates with low-dimensional nanocarbons, the novel oxidative system has raised tremendous interest for degradation of organic contaminants in wastewater without secondary contamination. In this Account, we showcase our recent contributions to metal-free catalysis in advanced oxidation, including design of nanocarbon catalysts, exploration of intrinsic active sites, and identification of reactive species and reaction pathways, and we offer perspectives on carbocatalysis for future environmental applications.
Ferrous ion-based catalysts have been widely employed to oxidatively destruct major industrial pollutants such as phenolic compounds through the advanced oxidation processes (AOPs). These agents, however, inevitably show several drawbacks including the need of pH adjustment and further purification steps to remove residual salts. Here we report the use of chemical vapour deposition (CVD) graphene film as a novel metal-free catalyst for the AOP-based degradation of phenols in aqueous solution, which does not require additional steps for salt removal nor external energy to activate the process. We have also verified that the catalytic activity is strongly dependent on the surface area of graphene film and degradation efficiency can be markedly improved by exploiting an array of multiple graphene films. Finally, the recyclability of graphene film has been validated by performing repetitive degradation tests to ensure the practical use.
Sulfate radical-based advanced oxidation processes (AOPs) have been received increasing attention in recent years due to their high capability and adaptability for the degradation of emerging contaminants. Persulfate (PS, S2O8²⁻) and peroxymonosulfate (PMS, HSO5⁻) can be activated by thermal, alkaline, ultraviolet light, activated carbon, transition metal (such as Fe⁰, Fe²⁺, Cu²⁺, Co²⁺, Ag⁺), ultrasound and hydrogen peroxide to form sulfate radical (SO4⁻), which is strong oxidant and capable of effectively degrading emerging pollutants. Sulfate radical-based AOPs have a series of advantages in comparison with OH-based methods, for example: higher oxidation potential, higher selectivity and efficiency to oxidize pollutants containing unsaturated bonds or aromatic ring, wider pH range. Therefore, sulfate radicals are capable of removing the emerging contaminants more efficiently. In this review paper, various methods for the activation of PS and PMS were introduced, including, thermal, alkaline, radiation, transition metal ions and metal oxide, carbonaceous-based materials activation and so on; and their possible activation mechanisms were discussed. In addition, the application of activated PS and PMS for the degradation of emerging contaminants and the influencing factors were summarized. Finally, the concluding remarks and perspectives are made for future study on the activation of PS and PMS. This review can provide an overview for the activation and application of PS and PMS for the degradation of emerging contaminants, as well as for the deep understanding of the activation mechanisms of PS and PMS by various methods.
The occurrence of new emerging contaminants in surface waters has recently grabbed increased attention of the scientific community. The adoption of Advanced Oxidation Processes (AOPs) represents an efficient strategy to remove recalcitrant compounds from aqueous streams and achieve high mineralization levels. Amongst AOPs, the photo-Fenton process has been widely investigated due to the possibility of using a renewable energy source (i.e., solar energy) and low concentration of catalyst. On the other hand, the use of photo-Fenton process is restricted to acidic pH values, with associate high operating costs for industrial scale applications. To overcome these drawbacks, photo-Fenton processes modified by adding selected chelating agents can be successfully performed at neutral pH. The present review aims at examining and comparing the most relevant papers dealing with photo-Fenton processes at neutral pH that appeared in the literature so far. Such papers were classified by chelating species adopted. In particular, for each iron(III)-ligand complex, the mechanism of photolysis, the speciation diagram, the light absorption properties, the quantum yields, biodegradation and toxicity, and some example of applications are reported. As a conclusion, suitable criteria for choosing chelating agent and operating conditions in photo-Fenton processes at neutral pH are proposed.
Nitrogen doped nanocarbon materials have emerged as promising metal-free catalysts towards peroxymonosulfate (PMS) activation for environmental remediation. However, their catalytic efficiency for PMS activation still needs improvement. Moreover, the relationship between the catalytic efficiency and nitrogen content or species, which is important to clarify the catalytic mechanism, remains unclear. In this study, three nitrogen-rich metal-organic frameworks (ZIF-8, NH2-MIL-53 and IRMOF-3) with nitrogen content of 24.7 wt%, 6.28 wt% and 5.16 wt% respectively were chosen to prepare the nitrogen doped porous carbons (NPCs) with different nitrogen content. Several carbonization temperatures were employed to obtain the NPCs with varying nitrogen species. The PMS catalytic performance of NPCs and its relationship with nitrogen content or species were investigated. The results showed all the NPCs exhibited enhanced PMS activation for phenol degradation compared with the nitrogen-free porous carbon (obtained from MOF-5), and was even superior to the most effective PMS activator of homogeneous Co²⁺. The ZIF-8 derived NPC carbonized at 1000 °C, with the highest graphitic N content, displayed best performance with the kinetic constant of phenol degradation 4 times higher than that on porous carbon. The graphitic N plays a critical role for activating PMS to produce sulfate radical and hydroxyl radical.
Core-shell porous carbon represents an innovative concept for optimizing performances of carbon materials in a wide range application. Here, a novel route to produce carbons with spatially varying pore and microstructure is presented. Following the carbide-derived carbon method, a adjustable shell generation is achieved through mixing a limited amount of solid chlorine precursors with titanium carbide raw material. Chlorine is released in situ when reaching approx. 600 °C and consumed subsequently directly, resulting in the aspired materials. Various characterization methods proved that the amount of solid chlorine precursor directly adjusts the share of the shell for the core-shell material, while a range of 5–80% was studied. The amorphous, microporous shell can be converted to a mesoporous and graphitic shell (crystal sizes 20 nm), through a subsequent vacuum annealing, allowing to control the properties of the carbon shell. The carbon core results after a subsequent second chlorination step, where directly gaseous chlorine is used. The materials were characterized in detail after every step of this novel route. Adjusting the share between mesoporous/graphitic shell and microporous/amorphous core for these new materials is of interest in applications where competing processes like mass transfer and surface interaction need to be optimized.
Nitrogen-doped graphene is a promising candidate for the replacement of noble metal-based electrocatalysts for oxygen reduction reactions (ORRs). The addition of pores and holes into nitrogen-doped graphene enhances the ORR activity by introducing abundant exposed edges, accelerating mass transfer, and impeding aggregation of the graphene sheets. Herein, we present a straightforward but effective strategy for generating porous holey nitrogen-doped graphene (PHNG) via the pyrolysis of urea and magnesium acetate tetrahydrate. Due to the combined effects of the in situ generated gases and MgO nanoparticles, the synthesized PHNGs featured not only numerous out-of-plane pores among the crumpled graphene sheets, but also interpenetrated nanoscale (5–15 nm) holes in the assembled graphene. Moreover, the nitrogen doping configurations of PHNG were optimized by post-thermal treatments at different temperatures. It was found that the overall content of pyridinic and quaternary nitrogen positively correlates with the ORR activity; in particular, pyridinic nitrogen generates the most desirable characteristics for the ORR. This work reveals new routes for the synthesis of PHNG-based materials and elucidates the contributions of various nitrogen species to ORRs.
The hydrogen evolution reaction (HER) is a fundamental process in electrocatalysis and plays an important role in energy conversion through water splitting to produce hydrogen. Effective candidates for HER are often based on noble metals or transition metal dichalcogenides, while carbon-based metal-free electrocatalysts generally demonstrate poorer activity. Here we report evaluation of a series of heteroatom-doped graphene materials as efficient HER electrocatalysts by combining spectroscopic characterization, electrochemical measurements, and density functional theory calculations. Results of theoretical computations are shown to be in good agreement with experimental observations regarding the intrinsic electrocatalytic activity and the HER reaction mechanism. As a result, we establish a HER activity trend for graphene-based materials, and explore their reactivity origin to guide the design of more efficient electrocatalysts. We predict that by rationally modifying particular experimentally achievable physicochemical characteristics, a practically realizable graphene-based material will have the potential to exceed the performance of the metal-based benchmark for HER.
This study introduces graphited nanodiamond (G-ND) as an environmentally friendly, easy-to-regenerate, and cost-effective alternative catalyst to activate persulfate (i.e., peroxymonosulfate (PMS) and peroxydisulfate (PDS)) and oxidize organic compounds in water. The G-ND was found to be superior for persulfate activation to other benchmark carbon materials such as graphite, graphene, fullerene, and carbon nanotubes. The G-ND/persulfate showed selective reactivity toward phenolic compounds and some pharmaceuticals, and the degradation kinetics were not inhibited by the presence of oxidant scavengers and natural organic matter. These results indicate that radical intermediates such as sulfate radical anion and hydroxyl radical are not majorly responsible for this persulfate-driven oxidation of organic compounds. The findings from linear sweep voltammetry, thermogravimetric analysis, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy analyses suggest that the both persulfate and phenol effectively bind to G-ND surface and are likely to form charge transfer complex, in which G-ND plays a critical role in mediating facile electron transfer from phenol to persulfate.
Low-cost and highly efficient electrocatalysts for oxygen reactions are of high importance for oxygen-related energy storage/conversion devices (e.g. Metal-O2 batteries or fuel cells). In the present study, we synthesized NiCoMnO4 nanoparticles anchored on nitrogen-doped graphene nanosheets as highly efficient bifunctional electrocatalysts for the Oxygen Reduction Reaction (ORR) and for the Oxygen Evolution Reaction (OER). Proper anchoring of NiCoMnO4 nanoparticles on graphene layers was probed with various characterization techniques including X-ray diffraction, energy dispersive X-ray (EDX), Raman, and Fourier transform infrared (FTIR) spectroscopy as well as transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) was also employed to shed light on the oxidation states of metallic atoms and types of doped nitrogen on graphene layers. According to the obtained results, the NiCoMnO4/N-rGO hybrid showed excellent electrocatalytic activity towards ORR (Eonset = 0.92 V vs. RHE and high current density of 0.84 mA cm⁻²) and OER (Eonset = 1.5 V vs. RHE and high current density of 14 mA cm⁻²), much better than other evaluated catalysts. It has been shown that the NiCoMnO4/N-rGO catalyzes ORR mostly through 4e process, just as the commercial Pt based catalyst. Moreover, it outperforms the commercial catalyst with very little decay in ORR activity over long continuous operation and shows excellent catalytic selectivity and methanol tolerance.
Decahedral TiO2 decorated with bimetallic nanoparticles were synthesized via radiolysis and photodeposition method. The effect of bimetallic surface composition (Ag_Pt, Ag_Au, Au_Pd, Au_Pt) as well as deposition technique (simultaneous or sequential) on the photocatalytic activity in phenol degradation and efficiency of hydroxyl radicals generation under UV–vis light irradiation were investigated. Modified and pristine decahedral TiO2 anatase with exposed {001} were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM) with energy-dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM), inductively coupled plasma-mass spectrometry (ICP-MS) and X-ray photoelectron spectroscopy (XPS). The gas chromatography-mass spectrometry was employed to detect organic intermediates to establish degradation pathway of isotopically labeled (1-13C) phenol. The modification with Pt and Ag nanoparticles induced an increase in photocatalytic activity of phenol degradation under UV–vis light irradiation (79% of phenol was degraded after 90 min of irradiation). The main by-products detected in phenol oxidation were catechol, hydroquinone, malonic, fumaric and maleic acid. The results indicated the formation of isotopically labeled and unlabeled maleic acid. It was noticed that all samples sequentially photodeposited on TiO2 surface exhibited higher [radical dot]OH radicals generation compared to pristine TiO2. Our results suggest that synergistic effects between specifically engineered TiO2 nanocrystals and unique properties of loaded bimetallic nanoparticles can enhance the charge separation of photoinduced carriers.
Graphene-based materials have emerged as novel and green alternatives to metals/oxides for environmental catalysis. This study integrates deliberate material fabrication with density functional theory (DFT) calculations to probe intrinsic active sites, e.g. the defects and oxygen functionalities on graphene for activating OO bond in peroxymonosulfate (PMS) toward catalytic oxidation. The reaction rate constants of degradation efficiency were discovered to be closely related with the ID/IG values of thermally annealed reduced graphene oxides (rGOs). Three rGOs (rGO-CM, rGO-HH, and rGO-HT) with a similar oxygen level by different reduction methods were utilized to investigate the effect of different oxygen groups. The results indicate that rGO-HT with the highest contents of ketonic group (CO) presented the best activity. The theoretical calculations were applied to simulate the PMS chemisorption with all the possible active sites on rGO. The DFT results suggest that vacancies and defective edges are more reactive than the graphene basal plane with prolonged OO bond in PMS molecules, greater adsorption energy, and more electron transfer. Besides, the electron-rich ketonic groups may be the major active species among the oxygen functionalities. The findings will contribute to new insights in reaction mechanism and material design in heterogeneous carbocatalysis.
The active sites for metal-free carbocatalysis in environmental remediation are intricate compared to those for traditional metal-based catalysis. In this study, we report a facile fabrication of amorphous carbon spheres with varying oxygen functional groups by hydrothermal treatment of glucose solutions. With air/N2 annealing and regeneration in the glucose solution of the as-synthesized carbon spheres, the concentrations of oxygen-containing groups were tailored on the amorphous carbon spheres in an Excess-On-Off-On manner. Accordingly, an Off-On-Off-On catalytic behavior in peroxymonosulfate (PMS) activation using these amorphous carbon spheres was observed. To uncover the mechanism of catalytic activity, electron spin resonance (EPR) spectra were recorded to investigate the variation of the generated OH and SO4•–radicals. Moreover, density functional theory (DFT) studies were employed to identify the role of oxygen-containing groups on the amorphous carbon spheres in adsorptive O-O bond activation of PMS. Results revealed that ketone groups (CO) are the dominant active sites for PMS activation among oxygen-containing functional groups. In order to simulate real wastewater treatment, influences of chloride anions and humic acid on PMS activation for phenol degradation were further evaluated. This study provides an in-depth insight to discovering the role of oxygen-containing functional groups as the active sites in metal-free carbocatalysis.
Production of radicals by metal-free catalysis is expected to offer a promising oxidative reaction for remediation of emerging contaminants. In this study, novel metal-free activation of persulfate (PS) on annealed nanodiamonds (ANDs) was investigated, which demonstrated superior performances in decomposition of various pollutants to conventional metal-based catalysis. Comprehensive investigations on the effects of reaction parameters, such as solution pH, reaction temperature, initial phenol concentration, catalyst loading, PS usage, the presence of chlorine ions and humic acid, on phenol degradation were carried out. In addition, nanodiamond (ND) material optimization and reusability were also studied. Electron paramagnetic resonance (EPR) and selective organic degradation unraveled that the PS/AND system may produce both hydroxyl radicals (OH) and sulfate radicals (SO4−), initialized from oxidizing water molecules on the nanodiamond surface. The carbocatalysts served as an excellent electron tunnel to facilitate the charge transfer from water or hydroxide ions to PS, and the oxidized intermediates may play a crucial role in PS activation. Electrochemical analyses in PS oxidant solution and oxygen reduction reaction (ORR) were carried out to understand OO bond activation by the metal-free catalysis. This study provides an environmentally benign and highly efficient oxidative reaction system with reactive radicals along with insights into the metal-free PS activation process.
Sulfate radical-based advanced oxidation processes (SR-AOPs) employing heterogeneous catalysts to generate sulfate radical (SO4-) from peroxymonosulfate (PMS) and persulfate (PS) have been extensively employed for organic contaminant removal in water. This article aims to provide a state-of-the-art review on the recent development in heterogeneous catalysts including single metal, mixed metal, and nonmetal carbon catalysts for organic contaminants removal, with particular focus on PMS activation. The hybrid heterogeneous catalyst/PMS systems integrated with other advanced oxidation technologies is also discussed. Several strategies for the identification of principal reactive radicals in SO4--oxidation systems are evaluated, namely (i) use of chemical probe or spin trapping agent coupled with analytical tools, and (ii) competitive kinetic approach using selective radical scavengers. The main challenges and mitigation strategies pertinent to the SR-AOPs are identified, which include (i) possible formation of oxyanions and disinfection byproducts, and (ii) dealing with sulfate produced and residual PMS. Potential future applications and research direction of SR-AOPs are proposed. These include (i) novel reactor design for heterogeneous catalytic system based on batch or continuous flow (e.g. completely mixed or plug flow) reactor configuration with catalyst recovery, and (ii) catalytic ceramic membrane incorporating SR-AOPs.
While the synthesis of heterogeneous catalysts is well established, it is extremely challenging to fabricate complex hierarchical hollow structures with mixed transition metal oxides. Herein, we report a facile in situ growth process of SiO2@Fe3O4@MnO2, followed by etching method to synthesize a hierarchical hollow structure, namely Fe3O4@MnO2 ball-in-ball hollow spheres (Fe3O4@MnO2 BBHs). The as-prepared Fe3O4@MnO2 BBHs were applied to degrade methylene blue (MB) by catalytic generation of active radical from peroxymonosulfate (PMS), exhibiting the merits of excellent catalytic performance, easy separation, good stability and recyclability. In this architecture, the degradation process can be divided into three layers. The outer hierarchical MnO2 nanosheets could accumulate and transport the pollutants by electrostatic interactions and catalyze the generation of active radical for degradation. Both inner MnO2 nanosheets and outer Fe3O4 hollows could produce active radicals to accelerate the pollutant degradation. The active catalytic sites also existed in inner Fe3O4 hollows, which could further degrade the high concentrated pollutants in hollows. This work provided new strategies for the controllable synthesis of complex hollow structures and their application in environmental remediation.
Exploring highly-efficient and low-cost bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) in the renewable energy area has gained momentum but still remains a significant challenge. Here we present a simple but efficient method that utilizes ZIF-67 as the precursor and template for the one-step generation of homogeneous dispersed cobalt sulfide/N,S-codoped porous carbon nanocomposites as high-performance electrocatalysts. Due to the favourable molecular-like structural features and uniform dispersed active sites in the precursor, the resulting nanocomposites, possessing a unique core-shell structure, high porosity, homogeneous dispersion of active components together with N and S-doping effects, not only show excellent electrocatalytic activity towards ORR with the high onset potential (around -0.04 V vs. -0.02 V for the benchmark Pt/C catalyst) and four-electron pathway and OER with a small overpotential of 0.47 V for 10 mA cm(-2) current density, but also exhibit superior stability (92%) to the commercial Pt/C catalyst (74%) in ORR and promising OER stability (80%) with good methanol tolerance. Our findings suggest that the transition metal sulfide-porous carbon nanocomposites derived from the one-step simultaneous sulfurization and carbonization of zeolitic imidazolate frameworks are excellent alternative bifunctional electrocatalysts towards ORR and OER in the next generation of energy storage and conversion technologies.
While Metal Organic Frameworks (MOFs) have been extensively explored as a platform for developing porous metal oxides, another intriguing direction is to use MOFs as precursors to prepare carbonaceous materials. By simple one-step carbonization, MOFs can be turned into promising hierarchical carbon materials. Such a technique can be also used to convert MOF-composites to carbon-based composites with task-specific functionality other than the precursors. In this study, this strategy is adopted to prepare a magnetic cobalt-graphene (MCG) nanocomposite from carbonizing a self-assembly of a cobalt-based MOF, ZIF-67, and graphene oxide (GO). The preparation of MCG represents a simple alternative route to synthesize magnetic graphene materials and graphene-supported cobalt materials. By combining cobalt and reduced graphene oxide (RGO), the as-prepared MCG can be an effective catalyst to activate peroxymonosulfate (PMS) in the advanced oxidation process. Thus, the activation capability of MCG is evaluated by decolorizing Acid Yellow (AY) dye in water. MCG exhibited an enhanced catalytic activity to activate PMS compared to the carbonized ZIF-67 because RGO also activated PMS and improved electron transport ability. The kinetics of the decolorization of AY (10 mg L−1) was 0.0119 min−1 with PMS = 200 mg L−1 and MCG = 500 mg L−1. The activation energy of the decolorization using PMS activated by MCG was found to be 12 kJ mol–1. Factors influencing the PMS activation were also investigated including temperature, pH, UV, ultrasonication and inhibitors. To evaluate the long-term catalytic activity of MCG, a 50-cycle decolorization test was performed and the regeneration efficiency remained at 97.6% over 50 cycles, showing its stable and effective catalytic activity. These features make MCG a promising catalyst to activate PMS.
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 oC, showed the highest efficiency in degradation of phenol solutions by metal-free catalytic activation of peroxymonosulfate (PMS). The catalytic activity of the nitrogen doped rGO (NG-700) was about 80 times higher than un-doped rGO in phenol degradation. Moreover, the activity of NG-700 was 18.5 times higher than the most popular metal-based catalyst of nanocrystalline Co3O4 in PMS activation. Theoretical calculations using spinunrestricted density functional theory (DFT) were carried out to probe the active sites for PMS activation on N-doped graphene. Electron paramagnetic resonance (EPR) and classical competitive radical tests were employed to experimentally investigate the PMS activation and phenol degradation pathways on nanocarbons. It was observed that both •OH and SO4•- existed in the oxidation processes and played critical roles for phenol 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.
Metal-free materials have been demonstrated to be promising alternatives to conventional metal-based catalysts. Catalysis on nanocarbons comparable to that of cobalt- or manganese-based catalysts in peroxymonosulfate (PMS) activation has been achieved, yet the catalyst stability has to be addressed and the mechanism also needs to be elucidated. In this study, N-doped carbon nanotubes (NoCNTs) were employed as metal-free catalysts for phenol catalytic oxidation with sulfate radicals and, more importantly, a detailed mechanism of PMS activation and the roles of nitrogen heteroatoms were comprehensively investigated. For the first time, a nonradical pathway accompanied by radical generation (•OH and SO4•-) in phenol oxidation with PMS was discovered upon nitrogen heteroatom doping. The NoCNTs presented excellent stability due to the emerging nonradical processes. The findings can be used for the design of efficient and robust metal-free catalysts with both superior catalytic performance and high stability for various heterogeneous catalytic processes.Keywords: metal-free catalysis; carbon nanotubes; nitrogen doping; nonradical; sulfate radicals
In heterogeneous catalysis for water treatment, feasible recovery of nanocatalysts is crucial to make the process cost-effective and environmentally benign. In this study we applied two strategies, e.g. magnetic separation and hierarchical structure of solid catalysts, to ensure manganese catalysts are readily separable, meanwhile their catalytic performance was retained by the nanosized structure of MnO2 nanosheets or nanorods. ZnFe2O4 was used as a magnetic core and MnO2 corolla-like sphere consisting of nanosheets, and sea-urchin shaped structure made of nanorods, were fabricated by a hydrothermal method at 100 and 140 oC respectively. Crystalline structure, morphology and textual property of the materials were investigated. The prepared catalysts were able to effectively activate peroxymonosulfate (PMS) to generate sulfate radicals for catalytic oxidation of a typical organic pollutant of phenol. After the heterogeneous catalysis, the catalysts were easily recovered by applying an external magnetic field. The effects of temperature and repeated use on the degradation efficiencies were evaluated. The generation, evolution and oxidation of sulfate radicals were studied using both competitive radical tests and electron paramagnetic resonance (EPR).
Employing metal-free nanocarbons or carbonaceous materials as a catalyst for environmental water remediation is a promising clean approach because the green carbon materials can completely prevent the potential toxic metal leaching and secondary contamination to water body. This study reports that pristine multi-walled carbon nanotubes (MWCNTs) can effectively activate peroxymonosulfate (PMS) and peroxydisulfate (or persulfate, perdisulfate, PDS) to produce sulfate radicals for oxidation of phenol solutions. Surface nitrogen modification was conducted by a facile synthesis via annealing MWCNTs with ammonium nitrate at a low temperature and the nitrogen modified MWCNTs (N-CNT) was characterized by a variety of techniques. It was found that surface nitrogen modification of MWCNTs produced different effects on PMS and PDS activation. N-CNT can significantly improve the phenol degradation with PMS, but decrease the degradation efficiency with PDS. Reaction kinetics and the mechanism in catalytic oxidation of phenol solutions with sulfate radicals over CNT-based materials were discussed.
Nitrogen-doped carbon nanotubes (N-CNTs) were prepared by chemical vapor deposition method and employed as carbon-based catalysts for selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen as the terminal oxidant under the mild reaction conditions. The results showed that the N-CNTs exhibited much higher activity than the undoped CNTs, and the improved catalytic activity was probably attributed to the introduction of electron-rich nitrogen atoms in the graphitic domains enhanced electron transfer. Moreover, N-CNTs displayed excellent stability without an obvious loss in activity and selectivity for benzyl alcohol oxidation after eight cycling reactions. The results presented herein pave the way for the development of novel carbon catalyst for the liquid-phase oxidation of benzyl alcohol.
In the absence of a specific luminescent reagent, the chemiluminescence (CL) from the reaction of periodate with hydrogen peroxide in an aqueous alkaline solution was observed. The CL intensity was enhanced by the addition of potassium carbonate to the alkaline solution. The ratio of the signal-to-noise (S/N) is proportional to the concentration of hydrogen peroxide up to 1×10−5 M. The detection limit with the flow injection method is 5×10−9 M H2O2 (S/N=3). The relative standard deviation (RSD) for 4×10−8 M hydrogen peroxide is 2.8% (n=14). Sample throughput is ca. 100 h−1. The selectivity of this method is very high, and most of the transition metal ions have no effect on the determination. The proposed method was successfully applied to the determination of trace amounts of hydrogen peroxide.
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.
Recently, the application of microbial fuelcells (MFCs) with cost-effective and long durable cathodic catalysts to generate electricity sustainably, has drawn much attention. This study investigated the use of nitrogen-doped carbon nanotubes (NCNTs) as the cathodic catalyst for oxygenreduction in MFCs to produce electricity efficiently and durably. The obtained maximum power density was 1600 ± 50 mW m−2, which was higher than the commonly used platinum (Pt) catalyst (Pt/C) (1393 ± 35 mW m−2). Also, the drop percentage of power densities with NCNTs was lower than with Pt/C over 25 cycles, indicating that MFCs with NCNTs as the cathodic catalyst could generate electricity more durably than those with Pt/C. Further investigation of the mechanisms revealed that MFCs with the bamboo-shaped and vertically aligned NCNTs had lower internal resistance and higher cathode potentials. Rotating ring-disk electrodevoltammogram, Raman microspectroscopy and X-ray photoelectron spectroscopic analyses suggested that NCNTs possessed a higher electrocatalytic activity for the oxygenreduction reaction (ORR) via a four-electron pathway in neutral pH phosphate buffer solution (PBS). Cyclic voltammograms on NCNTs and Pt/C electrodes before and after a continuous potentiodynamic swept in neutral PBS demonstrated that NCNTs had a better durability for cathodic ORR than Pt/C, which drove MFCs with NCNTs to generate electricity durably.
Persulfate is the newest oxidant that is being used for in situ chemical oxidation (ISCO) in the remediation of soil and groundwater. In this review, the fundamental reactions and governing factors of persulfate relevant to ISCO are discussed. The latest experiences for ISCO with persulfate are presented, with a focus on the different activation methods, the amenable contaminants, and the reactions of persulfate with porous media, based primarily on a critical review of the peer-reviewed scientific literature and to a lesser extent on non-reviewed professional journals and conference proceedings. The last sections are devoted to identifying the best practices based on current experience and suggesting the direction of future research.
Carbon films have been produced on the surface of beta-SiC particles by reaction with Ar-Cl-2 and Ar-Cl-2-H-2 gas mixtures at atmospheric pressure and temperatures of 600-1000 degrees C. The structure and composition of the carbon films have been investigated using XRD, SEM, EDS, TEM, FTIR and Raman spectroscopy. BET and TG were also used for measuring the amount of carbon formed in the reaction. Uniform nanoporous carbon films with surface area exceeding 1000 m(2) g(-1) were obtained by reactions with Ar-Cl-2 gas at 600-1000 degrees C. Based on Raman spectroscopy and electron diffraction data, these films were identified as nanocrystalline graphite. An addition of hydrogen to the gas mixtures results in the etching of graphitic carbon. Traces of diamond were found along with amorphous carbon after treatment in Ar-Cl-2-H-2 gas mixtures at temperatures above 900 degrees C.
Highly disordered graphitic carbon layers were formed on various types of commercially available silicon carbide (SiC) ceramics by reaction with chlorine and chlorine-hydrogen gas mixtures at 1000 degreesC. The carbon was produced ranging from only a few micrometers to hundreds of micrometers thick. When a platinum sample holder was employed (instead of fused silica) platinum was found dispersed in the carbon layer concentrated near the SiC/C interface. This process can be used for incorporating platinum in porous carbon films for catalytic and other applications. In addition, the platinum resulted in a smoother physical interface between the SiC and carbon sublayer. The morphology of the platinum dispersion, its effect on the carbon layer, and its proposed formation mechanism are presented in this paper. (C) 2001 The Electrochemical Society.
This study explores the effect of ultraviolet (UV) light radiation and/or transition metals (M) for the activation of common oxidants (Ox) with the objective of treating recalcitrant organic contaminants in water. Hydrogen peroxide, potassium peroxymonosulfate and potassium persulfate were combined with iron, cobalt and silver, respectively, and/or with UV light (254nm) and were tested for the treatment of 2,4-dichlorophenol (2,4-DCP). Results from our previous studies indicated that these particular transition metals are the best catalysts for the activation of the respective oxidants [G.P. Anipsitakis, D.D. Dionysiou, Environ. Sci. Technol. 37 (2003) 4790; G.P. Anipsitakis, D.D. Dionysiou, Environ. Sci. Technol. 38 (2004) 3705]. From the combined use of UV, the oxidants and the transition metals, four general categories of advanced oxidation technologies were evaluated and compared for the degradation and mineralization of 2,4-DCP. Those were (i) the dark conjunction of each oxidant with its favorable metal activator (M/Ox), (ii) the use of UV alone, (iii) the combination of UV with each oxidant (UV/Ox) and (iv) the use of UV combined with each metal/oxidant systems (UV/M/Ox). In particular, the systems UV/KHSO5, UV/Co(II)/KHSO5 and UV/Ag(I)/K2S2O8 and the sulfate radicals generated thereby have never been tested before for water decontamination, as opposed to the extensively investigated hydroxyl radicals generated by UV/H2O2 and the photo-Fenton. The comparison of the results with respect to the transformation of 2,4-DCP and the extent of organic carbon removal led to the construction of the following order of efficiencies: UV/K2S2O8 > UV/KHSO5 > UV/H2O2 for the UV/Ox processes and UV/Fe(III)/H2O2 > UV/Fe(II)/H2O2 > UV/Co(II)/KHSO5 > UV/Ag(I)/K2S2O8 for the UV/M/Ox processes tested here. All experiments were homogeneous and conducted at ambient room temperature. The relative absorbance of the species participating in the reactions supports the former order of efficiency, since persulfate followed by peroxymonosulfate were proven more photosensitive than hydrogen peroxide. Among the metals tested, only iron species such as Fe(OH)2+ were found to absorb strongly at 254nm and to this is attributed the higher efficiencies obtained with the photo-Fenton reagents.
Wet hydrogen peroxide catalytic oxidation (WHPCO) is one of the most important industrially applicable advanced oxidation processes (AOPs) for the decomposition of organic pollutants in water. It is demonstrated that manganese functionalized silicate nanoparticles with interparticle porosity act as a superior Fenton-type nanocatalyst in WHPCO as they can decompose 80% of a test organic compound in 30 minutes at neutral pH and room temperature. By using X-ray absorption spectroscopic techniques it is also shown that the superior activity of the nanocatalyst can be attributed uniquely to framework manganese, which decomposes H2O2 to reactive hydroxyls and, unlike manganese in Mn3O4 or Mn2O3 nanoparticles, does not promote the simultaneous decomposition of hydrogen peroxide. The presented material thus introduces a new family of Fenton nanocatalysts, which are environmentally friendly, cost-effective, and possess superior efficiency for the decomposition of H2O2 to reactive hydroxyls (AOP), which in turn readily decompose organic pollutants dissolved in water.
Changing the carbon feedstock from pure ethanol to a 5 vol % mixture of acetonitrile in ethanol during the growth of vertically aligned single-walled carbon nanotubes (SWNTs) reduces the mean diameter of the emerging SWNTs from approximately 2 to 1 nm. We show this feedstock-dependent change is reversible and repeatable, as demonstrated by multilayered vertically aligned SWNT structures. The reversibility of this process and lack of necessity for catalyst modification provides insight into the role of nitrogen in reducing the SWNT diameter.
Don't be a dope: be a double dope! Graphene was doped with both boron and nitrogen at well-defined doping sites to induce a synergistic effect that boosts its catalytic activity for oxygen reduction. The excellent catalytic performance of the new metal-free catalyst is comparable to that of commercial Pt/C.
Two kinds of B&N codoped carbon nanotubes (CNTs) dominated by bonded or separated B and N are in-tentionally prepared, which present distinct oxygen reduc-tion reaction (ORR) performances. The experimental and theoretical results indicate that the bonded case cannot, while the separated one can, turn the inert CNTs into ORR electrocatalysts. This progress demonstrates the crucial role of the doping microstructure on ORR performance, which is of significance in exploring the advanced C-based metal-free electrocatalysts.