Porous carbons for use in electro-Fenton and Fenton-like reactions

PAN Zhe-lun, QIAN Xu-fang

（School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China）

Abstract： Electro-Fenton，photo-Fenton and Fenton-like reactions are important advanced oxidation processes for waste water treatment, which overcome the limitations of a narrow pH range and excessive iron sludge production in the classic Fenton reaction and have received great attention in the last few decades.The porous carbons in these Fenton-like reaction systems act as catalyst carriers to disperse active species, and as adsorbents to enrich reactants.They promote electron and mass transfer, prevente metal leaching and improve the efficiency of contaminant removal.They also promote the production and activation of hydrogen peroxide in electro-Fenton reactions and inhibit the recombination rate of electron/hole pairs in photo-Fenton reactions.There are well-developed synthesis methods for porous carbons, giving them different functionalities, and a high chemical and thermal stability, making them favored materials for use in these reactions.Recent developments in these fields are discussed.

Key words: Porous carbon materials；Fenton-like reaction；H2O2 production；Adsorbent

1 Introductions

Due to its high efficiency and cost effectiveness,the Fenton reaction is of great importance in water treatment[1,2], with efficient radical production by chain reaction of hydrogen peroxide and ferrous ions(Eq.1).However, the large scale adhibition of classical Fenton reaction (Fe(II)/H2O2) is hampered by two inherent drawbacks, a limited pH range, only from 2 to 3, and excessive iron sludge.The narrow initial pH range is restricted by the hydrolysis of Fe(III) as well as the limitation of H2O2[2-4](Eq.2).The iron sludge as solid waste needs extra cost to dispose[2].

For decades, efforts have been focused on addressing the two issues mentioned above.To get rid of the hydrolysis of Fe(III), Fenton-like systems containing transition metals with multiple oxidation states,such as Cu+, Co3+and Ce3+, have been employed at a wider pH range.However, these metal ions are of relatively low reaction rates and high toxicity[4].

A Fenton-like reaction based on solid catalysts,such as ferrihydrite, hematite and goethite, could broaden the pH limits with little iron residue[5].However, the reaction rates of solid catalysts are one order of magnitude slower than those of ferrous ions[6].The easy agglomeration of metal particles and infection by other ions also restricts their practical applications[7,8].According to the previous research, the sluggish reaction rate to generate Fe(II) (Eq.2) is usually the rate-determining step[9].Hence, how to boost the circulation of Fe(III)/Fe(II) is a critical issue in improving the reaction efficiency.

The introduction of chelating agents, such as citrate, succinate, and oxalate, could promote the dissolution of Fe species at a wide pH range, from 3 to 8[10,11],remitting the hydrolysis of iron.Nevertheless, the added organic chelating agents are usually undesirable for effluent because they could be hardly degraded by bacteria[12].Meanwhile, environmentally friendly chelating agents like fluvic acid are also applied to promote iron circulation.But these organic matters would be degraded during the Fenton-like reaction, which shows little influence.

Another approach is to increase the number of active sites by dispersing catalytic composition in supporting materials.Designing supporting materials of well-controlled morphology and structure with external fields such as photonic and electrical driving forcesis thought to be a rational choice to optimize the density and accessibility of active sites.Taking these conditions into consideration, porous silica and carbons with large surface areas are favored materials.For both materials, the preparation technology is very mature.While, compared to porous silica, the porous carbons are more stable and resistant to structural changes from hydrolytic effects in water.Especially,in alkaline solutions, the structure of porous silica is more prone to change[13].

Hence, with multiple advantages, including rich porosity, electron transfer ability, diverse functionality, well-developed synthetic methods, controllable surface structure, high stability to physical and chemical effects[14], porous carbons in advanced oxidation technologies have attracted wide attention, most of which are focused on graphene-based materials in activating persulfate[15-17].However, the application of porous carbons in Fenton-like processes is lacking, the reason for which may be the poor activity of porous carbons to hydrogen peroxide.Actually, porous carbons could play different roles in the O2and H2O2activation processes.

2 Adsorption

The characteristic pore structure offers the ability to adsorb pollutants, shortening the distance between contaminants and radicals generated later.The subsequent Fenton-like reaction is applied to degrade the adsorbed pollutants and regenerate the adsorbent[18].The decorated metal ions could either act as adsorption centers or active centers on the surface.Meanwhile, adsorption sites are also found to be beneficial for Fenton-like reaction[19].

Highly efficient adsorption of different pollutants was achieved by the cooperative adsorption of carbon and iron hybrid derived from MIL-101(Fe)[20].This material showed high adsorption capacity for tetracycline, oxytetracycline, chlortetracycline hydrochloride and tetracycline hydrochloride.The affluent meso- and micropores were conducive to the transport and enrichment of pollutants.The surface complexation and electrostatic attraction between iron nanoparticles and pollutants facilitated the adsorption.After adsorption equilibrium was reached, an oxidizing agent was added to the solution to regenerate the catalyst.Through this method, a multi-cycle reutilization was realized.The whole process is shown in Fig.1.

Magnetically-responsive mesoporous carbons were synthesized from alkyne-functionalized ionic liquids comprising paramagnetic anions[21].Both cationic dye and anionic dye from a contaminated aqueous stream were selectively adsorbed and subsequently degraded via Fenton process.Liu et al.[22]used a biomass precursor to fabricate a N, O-doped magnetic porous carbon framework embedded with Au nanoparticles.The adsorbent exhibited outstanding performance in tetracycline degradation with the help of H2O2.The pore structure provided the enriched tetracycline molecules with an environment surrounded by?OH, leading to an impressive degradation efficiency of 96%.The catalyst maintained good activity and reusability during seven consecutive cycles, with an average tetracycline degradation efficiency of over 82%.

Hence, this scheme could make better utilization of temporary reactive species, like?OH, by increasing the exposure of pollutants to reactive species.This phenomenon may be closely related to pore size and surface functional groups[23].While without the embraced metal particles, the porous carbon showed little or no activity, suggesting the importance of the metal component.

3 Catalyst carriers

Carbon-based materials with rich pore structures could offer space not only for the pollutants but also different catalysts, expanding their application.As early as the 1960s, R.W.Coughlin[24]raised the theory that the catalytic activity of carbon catalysts was related to their surface chemistry.Both defect sites and mesoporous structures can help to anchor metal and suppress leaching.Mesoporous carbons, in particular, provide an opportunity for selective adsorption of special particles, proteins, and other biomolecules,because of their tunable textural parameters[13,23,25].As a too small pore size may lead to a blocking and poormass transfer.The low surface charge shows less interference to the structure and orientation of active centers in enzymes than porous silica.The electron distribution and surface functional groups help to immobilize the catalytic component via π-stacking, hydrogen bonding and physical absorption, avoiding catalyst leakage and the following risks[26-29].Meanwhile, the pore structure not only endows the material with a continuous electron pathway but also facilitates mass transport owing to shorter diffusion distances[30,31].

3.1 Recent development in porous carbon supported catalysts

Despite the poor catalytic activity of bare carbon toward H2O2, it has been found that porous carbon materials could promote the catalytic effect of metals with an ordered pore structure.Fenton-like reactions highly depend on interfaces, which could be improved by a larger specific surface area[32,33].An ordered structure is good for a better distribution of metal particles, improving the crystallinity and catalytic activity, as a result[19].

It can be concluded that activity and selectivity mainly depend on how well the catalyst dispersion and their size distribution can be controlled.The phenomenon was concluded in Fig.2.These conditions are directly related to the pore size distribution.And the control of surface chemistry also counts, because of the importance of functionalities on electron transport[13].Therefore, among all the methods, doped elements and highly dispersed metal catalysts have shown huge potential.

The nitrogen and FeSxco-doped carbon was prepared by a hydrothermal method[34].The composite displayed high degradation efficiency towards organic dyes with excellent reusability even in alkaline solutions.The special result may be attributed to the introduction of Sxor specific radicals, except for?OH.A pathway of?OH generation by Sxwas found and the signal of?O2?was detected, proving that the introduction of other elements could offer several reaction routes.

Relatively speaking, how to control metal particle dispersion has attracted more interest.The Fe0-Fe2O3composite embedded in an ordered mesoporous carbon (OMC) was designed by Wang et al[35].During the reaction, the generation of Fe2+would prefer to occur at the interface of Fe0and Fe2O3[36],due to the co-existence of Fe0and Fe2O3.Furthermore,the Fe0-Fe2O3composites with an ultra-small particle size, which led to higher activity, were obtained with the OMC as a support.Moreover, the synergistic effect between the enrichment of containments and the catalysis of Fe0-Fe2O3composites could speed up the reaction kinetics.Based on the previous research,single-atom catalyst was proposed.In the study by Yang et al[37], single-atom iron fixed on a nitrogendoped porous carbon was synthesized to activate H2O2for the degradation of sulfadiazine.The material exhibited remarkable performance in H2O2activation and cyclic tests at a wide pH range.The degradation rate of sulfadiazine could reach 80%, even after 5 cycles.

Besides highly dispersed iron materials, the catalytic properties of other elements have been tried,such as copper, manganese, and cobalt.The process of Cu-containing Fenton-like is similar to the typical Fenton reaction.In Cu(I)-containing solution, the?OH could be produced directly[38].In the absence of reducing species, the Cu-based Fenton-like process has been reported to be initiated from the reduction of Cu(II) by H2O2[39], constructing the reaction cycle.In a work by Xiong[19], a CuxO/C composite was prepared from the Cu-MOF template.On the one hand, the high surface area and good adsorption ability of carbon component could provide more junction points.On the other hand, the well-crystallized framework could improve electron transport, promoting the reduction of Cu(II) and yield of?OH.The MnOxnanoparticles and porous carbon composite were fabricated by mixing followed by carbonization[40].MnOxparticles were able to be homogeneously dispersed in the carbon matrix, showing superior catalytic performance toward methylene blue degradation.A cobalt oxide decorated nanoporous activated carbon was synthesized and used for the removal of organic dye chemicals.The maximum COD removal rate in treating tannery dyeing reached 77%[21].

And others tried to adopt composite metal.For example, Dan et al.[41]prepared a SC@CuFeO2catalyst for removing sulfamethoxazole in a basic solution with a degradation efficiency of about 93% and prevented iron and copper ions from leaching.The introduction of sulfur element offers the opportunity to form Cu-S bonding, which is advantageous to electron transfer.Under alkaline conditions,1O2was transformed from sufficient hydroxyl radicals and superoxide radicals, while CO3??was formed from?OH and HCO3?.Radical process and non-radical process both contribute to pollutant degradation, alleviatingthe influence of real water matrix.In another case, a porous carbon was evenly coated by Co/Fe nanoparticles, showing good activity in 100 mg L-1rhodamine B degradation with a color and TOC removal rates of 99.41% and 64.6%, respectively, within 30 min[42].

3.2 Accelerating reduction of ferric ions

Given the sluggish kinetics of ferric ion to ferrous ion, improving ferric ions reduction is also very important for Fenton reaction.Except for the addition of electron donors, which is unsustainable and costly,promoting the reaction between hydrogen peroxide and ferric ions (Eq.2) is an effective pathway.Yang et al.[43]introduced functionalized multi-walled carbon nanotubes (FCNT-H) to the traditional Fenton reagents.Fast Fe(III)/Fe(II) cycling was realized by carboxyl groups on the surface of FCNT-H.Ferric ion was complexed with FCNT-H and partly reduced through the carboxyl groups.It was reported that only 11% of total generated ferrous iron was derived from the classical reactions with H2O2, while the carboxyl groups accounted for 8% ferrous iron generation.It was obvious that the cooperation of H2O2and FCNTH increases the Fe(III)/Fe(II) cycling.The FCNT-H mediated heterogenous Fenton-like reaction had good performance in atrazine degradation, showing a 90%decomposition rate in 30 min.

Besides carbon nanotubes, the reduction ability of carbon materials by functional groups was studied,too.A hydrothermal carbon was synthesized to promote the Fenton-like reaction by offering electron to Fe(III)[44].Carbon-centered persistent free radicals endow the hydrothermal carbon with abundant electrons.Hydroxyl groups on the hydrothermal carbon surface contribute to the high reaction rate.

3.3 Photo-Fenton

Some researchers found that the introduction of UV light irradiation to photosensitive materials could generate electrons to reduce metal ions and promote the production of HO˙[47,48].Most iron oxides and ironrelated minerals can be activated by visible light, due to the narrow band gaps[49,50].However, the extremely fast recombination rate would happen on photo-generated charge carries generated from these iron-containing compounds, leading to an extremely slow surface reduction rate of solid ferric ions.

Thus, it is clear that Fe(III)/Fe(II) circulation is heavily reliant on electrons generated from light irradiation.And, it is necessary to develop feasible strategies to achieve efficient separation of electron-hole pairs and rapid electron transfer.To achieve these purposes, a good dispersion of iron species on suitable supports is essential.The intimate interfacial configuration between iron species and supports is a prerequisite for facilitating timely charge migration inside the catalysts.The supports would also contribute to the formation of highly dispersed iron species, which directly influences the active site density and accessibility.

To sum up briefly, a superior catalyst carrier plays a significant role in increasing the electron transfer for Fe(II) regeneration.Combining iron-containing composites with defective carbon materials such as graphene-based materials or other carbonbased materials is an effective approach[51-53].Thanks to the materials with three-dimensional or lamellar hexagonal networks of carbon atoms, electrons are able to transfer rapidly to the catalyst surface as active species.The separated electrons can then directly react with pre-absorbed containments and pre-enriched hydrogen peroxides in carbon materials[54].

Previous work by Qian et al.[55]developedα-FeOOH/mesoporous carbon composite by in situ crystallization of adsorbed ferric ions on mesoporous carbon, which could be applied in photo-Fenton reaction.The iron oxides encapsulated stably in the mesoporous frameworks via C―O―Fe bonding were active to visible light.Assisted with visible light irradiation,the mineralization rate increased as photogenerated charges boosted the activation of H2O2and the cycle of Fe(III)/Fe(II).The mechanism was shown in Fig.3.It is mentioned that oxygen containing groups emerged between iron-containing particles and mesoporous carbon during reaction, forming extra active sites and further promoting the Fenton-like process.

Another work[56]reported a visible light activematerial derived from ferric ions anchored to a hydrophilic mesoporous carbon with graphene domains and abundant oxygen containing groups.It was found that the carboxyl groups complexing with ferric ions on the mesoporous carbon themself could absorb the light energy and generate electrons and holes for reaction.The composite showed obvious enhancement in phenol degradation with visible light.The generated phenoxyl radicals cooperated with ligand to metal charge transfer to facilitate the iron cycling as well as inhibit the side reaction through the Haber-Weiss reaction.

Compared to carbon-based materials, the graphene ingredient may show better behavior.And with the help of anchored semiconductor materials,like TiO2, light could be absorbed in a wider range wavelength.However, most experiments were carried out on the graphene-based supports.As reported by Wang et al.[53], the TiO2andγ-Fe2O3co-doped graphene oxide (GO) nanosheets own narrow band gap energies and high structural stability.The supporting material GO could serve as an electron reservoir,rapidly capturing or shuttling the electrons generated under irradiation.Afterwards, electrons migrated to the conduction band of TiO2through the conductive network of GO, accelerating Fe(II) regeneration.In another work[57], a core-shell structured graphene oxide decorated with Fe3O4magnetic nanoparticles was constructed.The metal anchored material showed high catalytic activity, which could be ascribed to the efficient transfer of photo-generated electrons facilitated by GO.Benefited from strong magnetic character of Fe3O4, the composites could be recycled conveniently.

3.4 Hydrogen peroxide generation and application in catalytic medicals

H2O2as a critical reactant in the Fenton reaction brings a series of problems when applied in practice,such as high energy consumption, and difficulty in transportation and storage.Different methods have been tried to in situ produce H2O2, for example, generating H2O2from catalytical oxidation of glucose by glucose oxidase or photo-generating H2O2through O2reduction on the photocatalyst surface[58].

Li et al.[58]developed a system, including light energy as an energy source, g-C3N4-based photocatalyst as an in situ H2O2generator, and surface-decorated Fe3+as a trigger of H2O2conversion.Other cases of photo generation of H2O2by graphene-based materials were also reported[59,60].However, the porous carbon fails to generate H2O2.But thanks to the pore structure, especially the mesopores, natural and artificial enzymes could be loaded, accelerating the Fenton process.

While AOP technologies have long been associated with water treatment, an innovative adhibition was found by Wu et al.[46]as a chemodynamic therapy for cancer.In this study, a kind of nano-catalyst containing hollow porous carbon with FeS2coating was constructed.The catalyst was covered by a carbon shell, which was designed to hold FeS2as well as carriers for tannic acid and glucose oxidase.Carboxyl groups acted as a “switch” to seal the tannic acid on the carbon shell.Integration of negatively charged glucose oxidase was immobilized by electrostatic and covalent interaction.The carbon shell can efficiently convert the near-infrared light to heat, speeding up the Fenton reaction.Thanks to the hollow and porous structure, tannic acid was enclosed in hollow carbon shell for reducing Fe3+, accelerating the reaction kinetics.The loaded glucose oxidase could consume glucose around tumor and in situ produce H2O2for Fenton reaction at the same time.Furthermore, nano-catalysts were liable to aggregate around cancer cells.The complete consumption of glucose could lead to cancer starvation therapy.Yang et al.[61]used an iron-con-taining organic framework to catalyze the production of ?OH radicals in microenvironment surrounding cancer cells.Meanwhile, chloroquine was added to deacidify lysosomes and inhibit autophagy, preventing cancer cells from self-protecting, which could be seen in Fig.4.

The catalytic medical pioneered the introduction of traditional catalytic techniques into the human body, which brought development to the medical.While the catalytic technique may be dangerous because of the strong reactive activity.

3.5 Diverse carbon sources

Although the cost of carbon-based materials is quite low, different methods have been developed to prepare porous carbons, using sludge or abandoned biomass as materials[62,63].Biochar derivatives have been widely applied as carriers for different catalysts because of their abundant surface groups, large surface area, and strong structure stability[64]in order to develop environmentally-friendly, and sustainable catalytic systems for contaminant degradation.Shinta et al.[65]obtained a biomass-derived carbon with micropores.Although, the reaction rate was smaller than that of the polymer-derived carbon with mesopores,the controllable preparation method and raw materials offered the biomass-derived carbon a promising future.And others tried to make full utilization of not only discarded biomass but also dangerous waste.The disposal of sludge generated in wastewater treatment has aroused extensive concern.Yang et al.[63]constructed a kind of stable Fenton-like catalyst by synthesizing a three-dimensional hierarchical porous carbon from sludge in FeSO4solution on silicon carbide foams.Through this method, ferrous ions were imbedded into the porous catalyst, which showed a remarkable catalytic effect on CH3SH elimination, with a de-gradation rate of approximately 99% after activation.

Researchers have made great progress in synthesizing porous carbons from biomass and waste.However, the complex components in different biomass led to a huge challenge in practical production processes.The carbon content, impurities, metal components and other differences made it difficult to obtain products with constant quality.A technology which could handle various raw materials was needed instead of a successful case from specific biomass or waste.

4 Carbon materials offering active sites

In order to improve the H2O2activation, besides the supported metal component, turning the carbon materials from pure carriers or adsorbents into catalysts is a rational pathway.Graphene has recently been proved to be efficient metal-free catalysts for Fenton reaction[66].According to the researches before, activation of H2O2by graphene-based materials is highly related to their surface chemistry such as the oxygenated functional groups and defective sites[66-68].However, bare porous carbon materials show poor activity properties for the lack of active sites, in spite of the similarities shared with graphene, which is still not clear.Fortunately, with the electric driving force,carbon materials could have significant effects.And in the presence of electricity, porous carbons could further boost?OH generation as an active part.

Electro-Fenton (EF) technique relies on hydrogen peroxide produced by the cathode to facilitate the reaction cycle.It is the carbon-based material that could selectively turn oxygen into hydrogen peroxide instead of water.The large specific surface area provides not only the convenience for gas molecule transfer but also a huge number of active sites.Compared with the easily blocked nanopores and micropores with higher surface area, the mesopores are preferred for their suitable size.And the carbon materials themselves are good conductors which hardly hinder the electron transfer, and they could prevent the supported particles from leaching.

A variety of porous carbon monoliths were synthesized as a “one-stop” platform for a two-step reaction.The 2-electron oxygen reduction reaction (ORR)firstly happened on the carbon monolith, followed by further catalysis of H2O2to generate?OH[69].A best H2O2production performance that was 374% higher than that obtained by commercial carbon black.Besides H2O2production, obtained composites were also capable of simultaneously catalyzing H2O2into?OH under ?0.146 V, through reductive cleavage processes (Eq.3 and Eq.4).

The monolith electrode achieved an (80 ± 2)%degradation of napropamide in 60 min over a wide pH range, from 4 to 10, at a mildly reducing potential,overcoming the drawbacks of the conventional EF processes, including complex system and pH limitation.While the pure porous carbon materials still suffer from low current density, different methods have been carried out.And a practical system merely using a porous-carbon with mesopores as a cathode was designed to treat pollutants (As shown in Fig.5).The degradation efficiency of phenol, sulfamethoxazole or atrazine, with an original concentration of 20 mg L-1,was at least 85%[70].Carbon materials show high selectivity in ORR reaction but relatively low activity to H2O2, which is another emphasis of recent researches.

4.1 Optimization strategy

One of the strategy to boost Feton reaction is to improve the activity of hydrogen peroxide to further increase the reaction kinetics of hydrogen peroxide production.It was found that the selectivity and activity of ORR on mesoporous carbons were higher than those on microporous carbons[71,72].And the electronic properties were highly related to activity.Surface functionalization and pore creation are two strategies to optimize the electrocatalytic activity of carbon materials.Thus, various methods are applied to generate function groups and uniform pores.The optimization strategies were collected in Fig.6.

4.1.1 Structure modification

Modification methods, like electrophoretic deposition, chemical modification or thermal treatment of carbon materials have been quite mature, offering the potential to fulfill different requirements, including controllable pore size and hydrophilia property.The work by Ganiyu et al.[73]prepared a carbon cathode by polymerization and carbonization of sucrose at a high temperature.The as-prepared cathode exhibited excellent electrocatalytic properties such as high conductivity, relatively high redox current, and several active-sites for producing oxidizing species.Gao et al.[74]modified a porous carbon cathode by cyclic voltametric electrodepositing polypyrrole and anthraquinone 2-sulfonate, enlarging the specific surface area and ensuring more active sites available for oxygen reduction reaction.Compared to the bare carbon felt cathode, the modified cathode achieved an improvement of approximately 24% in the removal rate of Rhodamine B.A porous carbon cathode was prepared by sintering a composite of graphite, polytetrafluoroethylene, and degreasing cotton by Su et al[75].The utilization of degreasing cotton and controlled low pressure provided the cathode with more pores,which not only enhanced the transmission of O2but also endowed the electrode with a rough surface.The increase in roughness led to the increasement in the surface hydrophobicity and aerophily, which was beneficial to the adsorption of oxygen on the cathode surface, accelerating the oxygen reduction.

4.1.2 Metal element introduction

In consideration of too few active sites on carbon materials, introduction of metal elements greatly improved the catalytic activity.Similar to the applications in the Fenton-like process, carbon materials could offer an ordered structure for metal elements,promoting their distribution and catalytic performance.The uniform-sized cerium dioxide hollow spheres were evenly loaded on a porous carbon[76]derived from skimmed cotton.A material with a large surface area and a uniform pore size was obtained.The CeO2loaded on the surface improved redox ability of the material and catalytic activity to Fenton-like reaction, achieving a 97.6% degradation efficiency on phenol in 120 min.

The biochar was activated by a molten salt method under different temperatures to construct a hierarchically porous architecture[77].Results showed that high-temperature carbonization improved selectivity for four-electron ORR due to the rich carbon defects,while the mild-temperature treatment regulated species and distribution of oxygen functional groups, increasing the selectivity toward producing hydrogenperoxide.Furthermore, hydroxyl radical was generated in the electro-Fenton system to attain fast decomposition of various organic pollutants, with addition of ferrous ions, realizing a satisfactory mineralization efficiency toward phenolic pollutants.Sun et al.[78]developed ordered meso-NiMn2O4nanoparticles as an efficient electro-Fenton-like catalyst.The mesoporous architecture of this Fenton-like catalyst provided more activated sites and interconnected channels.The bifunctional catalyst could provide two redox couples(Mn4+/Mn3+and Ni3+/Ni2+) to enhance the electro-Fenton-like reaction by increasing the generation rate of active radicals.To overcome the limitation of slow Fe(II) regeneration, Cao et al.[79]encapsulated FeOxnanoparticles into a N-doped porous carbon.The bifunctional catalyst owned high activity and selectivity for H2O2production with a low overpotential because of the regulated N doping configurations and contents.The strong interfacial interaction between FeOxand the N-doped porous carbon could even contribute to the circulation of iron.

Ignoring the cost factor, noble metals could show better performance than transition metals.Zhang et al.[80]loaded not only mimicking Au nanoparticles as artificial glucose oxidase via in situ growth but alsoα-FeOOH via crystallization of adsorbed Fe(III) onto the hierarchically porous carbon, making full use of mesopores.The nanozyme-modified biomimetic catalyst was capable of oxidizing glucose to gluconic acid and H2O2.The byproduct gluconic acid could properly adjust pH value in favor of iron cycle, while main product H2O2of the enzymatic reaction was immediately utilized in the following reaction to generate redical?OH.The hierarchically porous carbon attached ferric ions by C―O―Fe bonding.Meanwhile, abundant pores endowed the materials with abilities to facilitate the electron and H2O2transfer as well as suppress iron leaching.The noble metals showed relatively low over potential, as compared with transition metals.While the unbearable price was the barrier to actual use.

4.1.3 Doping

Furthermore, decorating carbon catalyst with metal-free elements, like nitrogen[81], sulfur[82], fluorine[83], or N, S co-doping[84]can also play a decisive role in ORR.Heteroatom doping could effectively alter the electroneutrality of carbon matrix as well as regulate the electron/spin density of the adjacent carbons.The adventitious atoms tailored binding strength and electron transfer to dioxygen molecules and facilitated the two-electron ORR, due to different electronegativities.

Among the doping species, oxygen element is commonly produced in carbon materials during the synthesis and plays vital roles in determining selectivity and reactivity of carbon catalysts, such as carbon nanotubes[85]and graphene[86].A N, P, S tri-doped porous carbon with enriched defects was developed by Chen et al.[87]by controlled pyrolysis of a covalent triazine polymer precursor.In view of the introduction of N, P, S heteroelements, porous texture was formed.Special functionalities and the pore structure demonstrate abundant active sites and electrocatalytic capabilities.The surface defects contribute to versatile performances in capacitive charge storage, ORR as well as high EF degradation efficiency for mixed dyes.The nitrogen and oxygen co-doped porous carbons were synthesized from black soya beans for the degradation of chloramphenicol[88].The cathode facilitated dissolved O2diffusion and enhanced the EF activity, achieving a thorough removal of chloramphenicol in 80 min at the optimum condition.

4.2 Sustainable self-powered system

As different promotions have been made in electrode, some efforts were paid on designing specific systems which could be put into large-scale application.To get rid of the energy concern, self-powered electro-Fenton system was proposed.Gao et al.[94]proposed the concept with a primary design, including a biomass-derived carbon cathode and a multilayer triboelectric nanogenerator.After that, several promotions have been made on this system.

According to the latest paper, Chen et al.[95]applied the adsorption-pyrolysis-doping strategy to tailor the content and pore sizes of a N, S-doped porous carbon catalyst.More importantly, they developed arevolving roller-compacted triboelectric nanogenerator as an electricity supply to EF system to degrade mixed dyes, achieving a decolorization rate of 97.8%within 45 min.In this equipment, there are seven specially designed pairs of elastic warped edges made from aluminum foils and PTFE films and a rotary shaft connected to blades, the detailed structure was exhibited in Fig.7.The rotation of the blades collected wind energy.Then, the energy was transferred to the elastic warped edges by rotation.The current occurred during the repetitive contact and separation between aluminum foils and PTFE films, based on the coupling of the contact electrification and electrostatic induction[96].And they also designed other selfpowered system[97].

It was a breakthrough in practical utilization, as this system succeeded in getting rid of stationary electricity equipment.The flexible device may be packaged into an independent cell, which could be taken everywhere easily with a car or a boat.A mobile water treatment station might be realized.And there was still room for improvement, such as the energy resource.The electricity was derived directly from friction, which was originated from other energy forms.Maybe a self-powered EF system based on hydraulic energy from a boat would be a convenient design.

5 Conclusion

Fenton-like technology is expected to be applied on a large scale because of its low secondary pollution.In order to make up for its low reactivity, porous carbon materials with weak catalytic activity towards hydrogen peroxide have attracted attention in Fentonlike processes without other driving forces.As the carrier of metal materials, porous carbon materials could provide an ordered porous structure for highly dispersed metal active sites and expose the enriched pollutants to the reactive oxygen species, thereby improving the catalytic effect.And porous carbon materials at the same time effectively embed metal materials, preventing loss of active particles.In Photo-Fenton reaction, carbon materials also play the role of catalyst carriers.Due to their electron transport properties, they can effectively prolong the electron transport distance to avoid recombination of photogenerated electrons and holes.While in the process of Electro-Fenton, excellent oxygen activation selectivity and the mass transfer performance enable porous carbon electrodes to become the providers of the active sites,realizing a ‘one-stop’ system to produce and activate hydrogen peroxide.With the help of surface modification, heteroatom doping and metal loading, the porous carbons can show better performance.Based on the existed researches about pore structure, the mesopores could meet most demands in different circumstances.The controllable pore structure of carbon materials may be one of the greatest advantages over other materials.

Except for the typical research fields, researchers have raised new ideas in organic waste reutiliza-tion, like synthesizing porous carbons from discarded biomass or sludge.Different performances were found between porous graphene and carbon materials.But the true source of active sites to activate H2O2on graphene is still unclear, restricting the application of carbon materials.The innovative use of Fenton process into cancer treatment was also impressive.The new self-powered system made practical application of EF more realizable.However, there still remained some problems to be solved, like difficulty in largescale equipment production.

Acknowledgements

This work was supported by Natural Science Foundation of China (21777097), the Ministry of Science and Technology of China (2018YFC1802001),IJLRC-Ministry of Education.