Conspectus 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. The journey starts with the discovery of peroxymonosulfate (PMS) and peroxydisulfate (PDS) activation by graphene-based materials. With the systematic investigations on most carbon allotropes, for the first time the carbocatalysis for PMS or PDS activation was correlated with the pristine carbon configuration, oxygen functionality (ketonic groups), defect degree (exposed edge sites and vacancies), and dimensional structure. Moreover, an intrinsic difference in catalytic oxidation does exist between PMS and PDS activation. For example, the PMS/carbon reaction is dominated by free radicals, while PDS/carbon catalysis was unveiled as a singlet oxygen- or nonradical-based process in which the surface-activated PDS complex directly degrades the organic pollutants without relying on the generation of free radicals. Nitrogen doping significantly enhances the carbocatalysis because of the positively charged carbon domains, which strongly bind with persulfates to form reactive intermediates toward organic reactions. More importantly, N doping substantially alters the catalytic oxidation from a radical process to a nonradical pathway in PMS activation. Codoping of sulfur or boron with nitrogen at a rational level will synergistically promote the catalysis as a result of the formation of more catalytic centers by improved charge/spin redistribution of the carbon framework. Furthermore, a structure–performance relationship was established for annealed nanodiamonds with a characteristic sp3/sp2 (core/shell) hybridization, where the catalytic pathways were intimately dependent on the thickness of the graphitic shells. Interestingly, the introduction of structural defects and N dopants into the well-defined graphitic carbon framework and alteration of graphene/diamond hybrids can transform the persulfate/carbon system from a radical oxidation pathway to a nonradical pathway. Encapsulation of metal nanoparticles within carbon layers further modulates the electronic states of the interacting carbon via charge transport to increase the electron density. Overall, this Account contributes to unveiling the mist of carbocatalysis in AOPs and to summarizing the achievements of metal-free remediation. We also present future research directions on underpinning the knowledge base to facilitate the applications of nanocarbons in sustainable catalysis and environmental chemistry.
Porous carbon materials have demonstrated exceptional performance in a variety of energy‐ and environment‐related applications. Over the past decades, tremendous efforts have been made in the ...coordinated design and fabrication of porous carbon nanoarchitectures in terms of pore sizes, surface chemistry, and structure. Herein, structure‐oriented carbon design and applications are reviewed. The unique properties of porous carbon materials that offer them promising design opportunities and broad applicability in some representative fields, including water remediation, CO2 capture, lithium‐ion batteries, lithium–sulfur batteries, lithium metal anodes, Na‐ion batteries, K‐ion batteries, supercapacitors, and the oxygen reduction reaction are highlighted. Then, the most up‐to‐date strategies for structural control and functionalization of porous carbons are summarized, toward tailoring microporous, mesoporous, macroporous, and hierarchically porous carbons with disordered or ordered, amorphous or graphitic structures. Meanwhile, the emerging features of these structures in various applications are introduced where applicable. Finally, insights into the challenges and perspectives for future development are provided.
Herein, the structure–property relationships of porous carbon materials in water remediation, carbon capture, lithium‐ion batteries, lithium–sulfur batteries, Li metal anodes, Na‐ion batteries, K‐ion batteries, supercapacitors, and oxygen reduction reactions are proposed. The recent progress in structural control and functionalization of porous carbons are also summarized, in terms of achieving microporous, mesoporous, macroporous, and hierarchically porous carbons.
•N-, B-, P-doping and codoping graphene was achieved by a simple method.•N-doped graphene exhibits high activity in metal-free oxidation of phenol.•N-doping induces positive effect on activity while ...B- or P-doping does not.•Electron paramagnetic resonance (EPR) demonstrates radical generation.
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 6.54at.% 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.
•A state of art review of photocatalysis for air purification.•Provides an in-depth analysis of photocatalyst development.•Discusses various aspects of photoreactor modelling.•Presents an overview of ...possible intensification pathways.
Photocatalysis has been extensively investigated for several decades, motivated by the fascinating applications in pollution remediation, chemical synthesis, and energy innovation. However, the practical/commercial/industrial applications of photocatalysis have been restricted in the field of building materials. The low quantum efficiency in solar energy conversion and limitation of low level of pollutants in photodegradation are very difficult to solve. Air purification by photocatalytic oxidation (PCO) of various pollutants, for example volatile organic compounds (VOCs) or inorganic gaseous (NOx, SOx, CO, H2S and ozone, etc) at reasonably low concentrations, appears to be more feasible for commercialization. This review firstly introduces the removal mechanism of these contaminations by PCO, and then provides detailed survey and discussion on both photocatalysts and reactor design. This paper aims to deliver fundamental and comprehensive information for paving the venue of gas-phase photodegradation to commercialized air purification.
Hydrocarbon fuels are the most important sources of energy in modern society because of their natural abundance, stability, and high energy density. However, the emissions of carbon dioxide from them ...and the associated global warming effect impose worldwide pressure on the use of sustainable solar energy and carbon dioxide transformation and sequestration. Photocatalytic conversion of CO2 to fuels using semiconductors is proposed as an effective solution. More recently, nanocarbons, such as carbon nanotube, graphene oxide, and graphene, possessing high thermal conductivity, high theoretical specific surface area, unique carrier mobility, low-dimensional structure, and sp2-hybridized carbon configuration, have shown promotion to photocatalysis. It has been proven that nanocarbon/semiconductor hybrids can be a competitive material compared to traditional metal oxides for CO2 reduction. This review summarizes the recent research advances in the synthesis of nanocarbon hybrid photocatalysts and their applications in photocatalytic reduction of CO2 to hydrocarbons. The roles of nanocarbons in extending light absorption, increasing separation of carriers, band gap engineering, and preferred CO2 adsorption in manipulating activity/selectivity are discussed. The studies in this topic will facilitate the design of advanced functional materials for energy innovation via solar energy use.
This paper reports the synthesis of Co3O4–reduced graphene oxide (rGO) hybrids and the catalytic performance in heterogeneous activation of peroxymonosulfate (PMS) for the decomposition of phenol. ...The surface morphologies and structures of the Co3O4–rGO hybrids were investigated by field emission scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Through an in situ chemical deposition and reduction, Co3O4–rGO hybrids with Co3O4 nanoparticles at an average size of 33 nm were produced. Catalytic testing showed that 20 mg/L of phenol could be completely oxidized in 20 min at 25 °C on Co3O4–rGO hybrids, which is mostly attributed to the generation of sulfate radicals through Co3O4-mediated activation of PMS. Phenol oxidation was fitted by a pseudo-zero-order kinetic model. The rate constant was found to increase with increasing temperature and PMS dosage, but to decrease with increasing initial phenol concentration. The combination of Co3O4 nanoparticles with graphene sheets leads to much higher catalytic activity than pure Co3O4. rGO plays an important role in Co3O4 dispersion and decomposition of phenol.
Low‐cost, nonprecious transition metal (TM) catalysts toward efficient water oxidation are of critical importance to future sustainable energy technologies. The advances in structure engineering of ...water oxidation catalysts (WOCs) with single TM centers as active sites, for example, single metallic molecular complexes (SMMCs), supported SMMCs, and single‐atom catalysts (SACs) in recent reports are examined. The efforts made on these configurations in terms of design principle, advanced characterization, performances and theoretical studies, are critically reviewed. A clear roadmap with the correlations between the single‐TM‐site‐based structures (coordination and geometric structure, TM species, support), and the catalytic performances in water oxidation is provided. The insights bridging SMMCs with SACs are also given. Finally, the challenges and opportunities in the single‐TM‐site catalysis are proposed.
Focusing on single transition metal (TM) site catalysis, a clear roadmap with the correlations between the catalytic performances in water oxidation and the single‐TM‐site‐based structures including coordination and geometric structure, TM species, and support is provided. Bridging remarks between single metallic molecular complexes and single‐atom catalysts are also given.
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► Graphene–titania composites (G–TiO2) were synthesized by a sol–gel method. ► G–TiO2 catalysts showed high efficiency in degradation of methylene blue under visible light. ► In situ ...prepared G–TiO2 presented higher activity than that of G–TiO2(P25). ► The mechanism of graphene in the enhanced visible light photocatalytic activity was proposed.
Several graphene–titania composites (G–TiO2) were synthesized by a sol–gel method using titanium isopropoxide (or P25) as Ti-precursors and reduced graphene oxide (RGO). The structural, morphological, and physicochemical properties of the samples were thoroughly investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), UV–vis diffuse reflectance (UV–vis DRS), and thermogravimetric-differential thermal analysis (TG-DTA). A significant increase in light absorption to visible light was observed by G–TiO2 compared with that of naked TiO2. The photocatalytic activity of G–TiO2 in methylene blue bleaching under visible light (>430nm) is much enhanced. G–TiO2 synthesized from titanium isopropoxide hydrolysis presented higher activity than that of G–TiO2(P25). Contribution of graphene on the enhancement of visible-light photocatalytic activity of the composite was discussed.
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