Showing papers in "Applied Catalysis B-environmental in 2017"

Electrochemical advanced oxidation processes: A review on their application to synthetic and real wastewaters

Abstract: Over the last decades, research efforts have been made at developing more effective technologies for the remediation of waters containing persistent organic pollutants. Among the various technologies, the so-called electrochemical advanced oxidation processes (EAOPs) have caused increasing interest. These technologies are based on the electrochemical generation of strong oxidants such as hydroxyl radicals ( OH). Here, we present an exhaustive review on the treatment of various synthetic and real wastewaters by five key EAOPs, i.e., anodic oxidation (AO), anodic oxidation with electrogenerated H 2 O 2 (AO-H 2 O 2 ), electro-Fenton (EF), photoelectro-Fenton (PEF) and solar photoelectro-Fenton (SPEF), alone and in combination with other methods like biological treatment, electrocoagulation, coagulation and membrane filtration processes. Fundamentals of each EAOP are also given.

Doping of graphitic carbon nitride for photocatalysis: A reveiw

Abstract: As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has been the hotspot in the materials science as a metal-free and visible-light-responsive photocatalyst. Pure g-C3N4 suffers from the insufficient sunlight absorption, low surface area and the fast recombination of photo-induced electron-hole pairs, resulting in low photocatalytic activity. Element doping is known to be an efficient method to tune the unique electronic structure and band gap of g-C3N4, which considerably broaden the light responsive range and enhance the charge separation. This review summarizes the recent progress in the development of efficient and low cost doped g-C3N4 systems in various realms such as photocatalytic hydrogen evolution, reduction of carbon dioxide, photocatalytic removal of contaminants in wastewater and gas phase. Typically, metal doping, nonmetal doping, co-doping and heterojunction based on doped g-C3N4 have been explored to simultaneously tune the crystallographic, textural and electronic structures for improving photocatalytic activity by enhancing the light absorption, facilitating the charge separation and transportation and prolonging the charge carrier lifetime. Finally, the current challenges and the crucial issues of element doped g-C3N4 photocatalysts that need to be addressed in future research are presented. This review presented herein can pave a novel avenue and add invaluable knowledge to the family of element doped g-C3N4 for the develop of more effective visible-light-driven photocatalysts.

Hierarchical assembly of graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme photocatalyst: An efficient, sustainable and heterogeneous catalyst with enhanced visible-light photoactivity towards tetracycline degradation under visible light irradiation

Abstract: A novel graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme heterojunction with excellent visible-light-driven photocatalytic performance was fabricated using a facile in situ deposition method followed by photo-reduction. The as-obtained nanocomposite was employed to degrade tetracycline (TC) in water under visible light irradiation. Compared to pure BiVO4, Ag3PO4 and other nanocomposites, Ag/Ag3PO4/BiVO4/RGO displayed more superior photodegradation efficiency with 94.96% removal of TC (10 mg/L) in 60 min, where the optimal conditions was catalysis dosage 0.50 g/L and initial pH at ca. 6.75. The influences of TC concentrations, light irradiation condition, coexistence ions and water sources were also investigated in details. The enhanced photocatalytic activities could be attributed to the suppression of charge recombination, high specific surface area and desirable absorption capability of Ag/Ag3PO4/BiVO4/RGO, which were in sequence confirmed by PL, PC, EIS, BET and DRS tests. The synergistic effects of RGO and Ag/Ag3PO4 in the hybrid could also contribute to the improved photo-stability and recyclability towards TC decomposition. In addition, radical trapping experiments and ESR measurement revealed that the photo-induced active species superoxide radical ( O2−) and holes (h+) were the predominant active species in the photocatalytic system. The Ag/Ag3PO4/BiVO4/RGO nanocomposite also possessed desirable photocatalytic performance on the degradation of TC from real wastewater, further verifying its potential in practical industries. This work provides a promising approach to construct visible-light response and more stabilized nanocomposite photocatalysts applied in efficient treatment of persistent pollutants in wastewater.

Direct Z-scheme g-C_3N_4/WO_3 photocatalyst with atomically defined junction for H_2 production

Abstract: Mimicking the natural photosynthesis, artificial Z-scheme photocatalysis enables more efficient utilization of solar energy for sustainable chemical fuel production. Herein, a direct Z-scheme g-C_3N_4/WO_3 photocatalyst with host-guest architecture is rationally designed, demonstrating significantly enhanced activities of photocatalytic H_2 production. Unprecedented atomic-scale imaging of both the in-plane and interlayer structures in g-C_3N_4 revealed the well-defined interfaces in such architecture, where the 2D g-C_3N_4 layers stand vertically on the flat facets of WO_3 nanocuboids. Through both experimental and theoretical investigations, mechanistic insights regarding the direct Z-scheme electron transfer from WO_3 to g-C_3N_4 were obtained. The Z-scheme electron transfer was driven by the internal electric field at the interfacial junction, defined by the covalent W-O-N-(C)_2 interaction. Under simultaneous light excitation, this atomically defined junction induces a rapid electron injection from WO_3 to inhibit the fast recombination kinetics within g-C_3N_4 and prolong the charge carrier lifetime of g-C_3N_4, thereby liberating more excited electrons with high reducing power for H_2 production.

Homogeneous photo-Fenton processes at near neutral pH: A review

Abstract: 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.

Graphitic C3N4 based noble-metal-free photocatalyst systems: A review

Abstract: Many reviews are written on this interesting visible light active polymeric semiconductor material, the graphitic carbon nitride (g-C3N4). Yet the ever-expanding volume of the ongoing research on this materials has inspired us to compile this review, especially on its nanoscale architectures of noble-metal-free photocatalyst systems. From the viewpoint of sustainable development, an economical photocatalyst which is made up of abundant elements e. g. C and N has a good prospect for large scale applications. Stability of the photocatalyst material under the experimental conditions is essential for its repeated usage, however, many semiconductors sought for visible-light-driven reaction, particularly sulfides and nitrides are in a compromising situation. However, g-C3N4 has high chemical- and photo-stability besides its high activity under visible light irradiation. Furthermore, solely semiconductor materials have the intrinsic problem of recombination of photogenerated electron-hole pairs. To overcome this problem, loading of the semiconductor with a co-catalyst, usually a noble metal is a common practice for transfer of electron and thus avoiding the recombination. Development of a noble-metal-free photocatalyst systems is essentially important for sustainable applications. Hence, the construction of a hybrid composite structure is interesting in the separation of photogenerated charge carriers. Besides diminishing the rate of recombination, the heterostructures are constructed for harnessing a wider spectrum of sunlight. In contrast to bulk semiconductors, their nanoscale counterpart offers a larger number of active sites along with interesting electrical and optical properties. Importantly, construction of extensive junctions between nanomaterials greatly enhance the separation of charges and consequently improve their photocatalytic efficiency. Usually, the stability of materials is compromised with the reduction of size to nano level, however, g-C3N4 and its nanomaterials demonstrate exceptional recycling in photocatalytic testing. One of the most important interests in controlling nanoparticle size, shape and composition is to develop noble-metal-free photocatalyst systems. Here in this review, we have compiled research on all the various applications of noble-metal-free nanoscale photocatalyst systems based on g-C3N4. By the end, we conclude the research topic and put forward future perspectives for further developments in designing practicable photocatalyst systems.

Photocatalytic oxidation technology for indoor environment air purification: The state-of-the-art

Abstract: Inevitable presence of volatile organic compounds (VOCs) in indoor environment and their adverse impact on human health and productivity have encouraged the development of various technologies for air pollution remediation. Among these technologies, photocatalytic oxidation (PCO) is regarded as one of the most promising methods and has been the focus of many research works in the last two decades. Titanium dioxide (TiO 2 ) is by far the most investigated photocatalyst for photocatalytic degradation of gaseous VOCs. This review article is intended to provide a comprehensive overview of the application of commercial TiO 2 photocatalysts for removal of VOCs in air. First, the fundamentals of photocatalytic oxidation are briefly discussed and common TiO 2 -based photocatalysts are introduced. Then, the relations between the characteristics of photocatalysts (e.g. crystallinity, surface area and surface chemistry) and photocatalytic activity as well as the influence of key operating parameters on PCO processes are investigated. Afterwards, the reaction mechanisms and identified reaction intermediates/by-products for the most prevalent VOC families are reviewed. Finally, the paper discusses the deactivation of photocatalysts during PCO processes and some of the common regeneration techniques.

Insight into highly efficient simultaneous photocatalytic removal of Cr(VI) and 2,4-diclorophenol under visible light irradiation by phosphorus doped porous ultrathin g-C3N4 nanosheets from aqueous media: Performance and reaction mechanism

Abstract: Carbon nitride (g-C3N4) has attracted great attention for its wide applications in hydrogen evolution and photocatalytic degradation. In this study, phosphorus doped porous ultrathin carbon nitride nanosheets (PCN-S) were prepared successfully via the element doping and thermal exfoliation method. The prepared PCN-S was characterized by XRD, SEM, TEM, N2-adsorption-desorption measurement, FT-IR, XPS, UV–vis diffuse reflectance spectra, photoluminescence (PL), photocurrent response (I-t) and EIS. The results show that PCN-S owns regular crystal structure of g-C3N4, large specific surface areas and nanosheet structure with lots of in-plane pores on its surface, excellent chemical stability, and broad light response to the whole visible light region, which was attributed to the doping of phosphorus element. Under visible light irradiation, the photocatalytic reduction of Cr(VI) over different samples indicated that the P doping and porous nanosheet structure play an important role for the enhanced performance of PCN-S. The reason was that P element doping can broaden the visible light response region, and large specific surface areas from the porous nanosheet structure can provide quantities of active sites for the photocatalytic reaction. Then the detailed study on the PCN-S for simultaneous photocatalytic reduction of Cr(VI) and oxidation of 2,4-diclorophenol (2,4-DCP) was conducted. The experiments results show that low pH value and enough dissolved oxygen were found to promote Cr(VI) reduction and 2,4-DCP oxidation. The detailed photocatalytic mechanism was proposed. The strategies used in this study could provide new insight into the design of g-C3N4 based materials with high photocatalytic activity, and present potential for the treatment of Cr(VI)/2,4-DCP or other mixed pollutants in wastewater.

Peroxymonosulfate enhanced visible light photocatalytic degradation bisphenol A by single-atom dispersed Ag mesoporous g-C3N4 hybrid

Abstract: Single-atom dispersed Ag modified mesoporous graphitic carbon nitride (Ag/mpg-C3N4) hybrid was synthesized by co-condensation method and employed as a visible light photocatalyst. In the presence of peroxymonosulfate (PMS), Ag/mpg-C3N4 showed excellent performance for the degradation of bisphenol A (BPA). 100% BPA and 80% TOC could be removed with 0.1 g/L catalyst and 1 mM PMS under visible light (λ > 400 nm) within 60 min. The reason for the enhanced performance is possibly due to the synergistic effect of single-atom Ag and mpg-C3N4. On the one hand, more visible light could be captured with the introduction of Ag; on the other hand, the existence of PMS promotes the separation efficiency of photogenerated electron-hole pairs. Electron spin resonance (ESR) and free radicals quenching experiment suggest that the major reactive oxygen species (ROS) are sulfate radical (SO4 −), superoxide radicals (O2 −) and photogenerated holes, while the role of hydroxyl radicals ( OH) is insignificant in this process. The findings of this work highlighted the great potential of g-C3N4 as photocatalysts and elucidated a new opportunity for PMS remediation of contaminated water.

Accelerated photocatalytic degradation of organic pollutant over metal-organic framework MIL-53(Fe) under visible LED light mediated by persulfate

Abstract: Photocatalysis based on metal-organic frameworks (MOFs) is being actively investigated as a potential technology in visible light harvesting and utilizing processes. Herein we report that MIL-53(Fe), an earth-abundant Fe-containing MOF material, shows photocatalytic activity for the degradation of Acid Orange 7 (AO7) from aqueous solution under visible LED light irradiation, yet the photocatalytic performance of bare MIL-53(Fe) was not satisfactory due to the fast recombination of photoinduced electron-hole pairs. This can be effectively overcome by adding the external electron acceptor (e.g., persulfate, PS) to the catalytic process. The accelerated photocatalytic degradation of AO7 is demonstrated by the result that the degradation efficiency of AO7 in the MIL-53(Fe)/PS/Vis process reached almost 100% within 90 min as compared to only 24% under the identical experimental conditions for the MIL-53(Fe)/Vis process. To investigate the mechanism of the MIL-53(Fe)/PS/Vis process, photoluminescence (PL) spectra, electrochemical measurements and electron paramagnetic resonance (EPR) analysis were performed. It was concluded that the efficient separation of photogenerated electrons and holes by the introduced PS and the subsequent formation of reactive radicals resulting from the activation of PS by photogenerated electrons accounted for the accelerated photocatalytic degradation of AO7 in the MIL-53(Fe)/PS/Vis process. Furthermore, the applicability of MIL-53(Fe) used in the persulfate-mediated photocatalytic process was systematically investigated in terms of the identification of reactive radicals, the reusability and stability of the photocatalyst, as well as the effect of operating parameters. The findings of this work highlighted the great potential of MOFs as photocatalysts and elucidated a new opportunity for persulfate remediation of contaminated water.

Facile synthesis of N-doped carbon dots/g-C3N4 photocatalyst with enhanced visible-light photocatalytic activity for the degradation of indomethacin

Abstract: In this study, a novel visible-light-driven N-doped carbon dot (NCDs)/g-C 3 N 4 composite was successfully synthesized by loading NCDs nanoparticles onto the interlayers and surface of g-C 3 N 4 via a facile polymerized method. The photocatalytic activity of the NCDs/g-C 3 N 4 was remarkably higher than that of g-C 3 N 4 and CDs/g-C 3 N 4 toward the degradation of indomethacin (IDM) under visible light irradiation. With increasing NCDs loading volumes, the photocatalytic activity of NCDs/g-C 3 N 4 initially increased, and then decreased. A very low NCDs content of 1.0 wt% resulted in a 13.6 fold higher reaction rate than that of pristine g-C 3 N 4 . This enhanced photocatalytic activity might have been ascribed to the unique up-converted PL properties, efficient charge separation, as well as bandgap narrowing of the NCDs. Reactive species (RS) scavenging experiments revealed that superoxide radical anions (O 2 −) and photogenerated holes (h + ) played key roles during the photocatalytic degradation of IDM. The quantification of O 2 − showed that NCDs/g-C 3 N 4 formed a larger amount of O 2 − than that of pristine g-C 3 N 4 . Potential photocatalytic pathways of IDM were proposed through the identification of intermediates using HPLC–MS/MS and the prediction of reaction sites based on Frontier Electron Densities (FEDs) calculations, which involved decarboxylation, hydroxylation, as well as the addition and cleavage of indole rings. Toxicity and mineralization evaluations revealed that NCDs/g-C 3 N 4 provided a very desirable performance for the toxicity reduction and mineralization of IDM under longer exposures of visible light irradiation.

Cu2O/TiO2 heterostructures for CO2 reduction through a direct Z-scheme: Protecting Cu2O from photocorrosion

Abstract: Fil: Aguirre, Matias Ezequiel Consejo Nacional de Investigaciones Cientificas y Tecnicas Centro Cientifico Tecnologico Conicet - Mar del Plata Instituto de Investigaciones Fisicas de Mar del Plata Universidad Nacional de Mar del Plata Facultad de Csexactas y Naturales Instituto de Investigaciones Fisicas de Mar del Plata; Argentina

Cd0.2Zn0.8S@UiO-66-NH2 nanocomposites as efficient and stable visible-light-driven photocatalyst for H2 evolution and CO2 reduction

Abstract: Metal-organic frameworks (MOFs), a new class of porous crystalline materials, have attracted great interest as fascinating materials for sustainable energy and environmental remediation. However, the functionalization and diversification of MOFs are still challenging and imperative for the development of highly active MOF-based materials. In this study, a series of Cd0.2Zn0.8S@UiO-66-NH2 nanocomposites with different UiO-66-NH2 contents were fabricated via a facile solvothermal method. The photocatalytic performances of the obtained Cd0.2Zn0.8S@UiO-66-NH2 nanocomposites were evaluated by photocatalytic H2 evolution and CO2 reduction under visible-light irradiation. The resultant hybrids exhibit significantly enhanced photocatalytic activity for hydrogen evolution and CO2 reduction as compared with pristine components, and the optimal UiO-66-NH2 content is 20 wt%. The composite can show a hydrogen evolution rate of 5846.5 μmol h−1 g−1 and a CH3OH production rate of 6.8 μmol h−1 g−1. The remarkable enhancement of the photocatalytic activity should be attributed to the efficient charge separation and transfer on the interface between Cd0.2Zn0.8S and UiO-66-NH2. Furthermore, the Cd0.2Zn0.8S@UiO-66-NH2 photocatalysts show excellent stability during photocatalytic hydrogen evolution and CO2 reduction. This work demonstrates that MOF-based composite materials hold great promise for applications in the field of energy conversion and environmental purification.

Visible-light reduction CO2 with dodecahedral zeolitic imidazolate framework ZIF-67 as an efficient co-catalyst

Abstract: The nanomorphology of ZIFs strongly influences or even improves chemical properties of the metal-organic materials. Nanosized rhombic dodecahedral ZIF-67 crystals were successfully synthesized through a simple co-precipitation method at room temperature, and fully characterized by XRD, FT-IR, DRS, XPS, TEM, SEM, TGA and N 2 /CO 2 sorption measurements. The as-prepared ZIF-67 material was applied to be an efficient heterogeneous co-catalyst for the photocatalytic CO 2 reduction by cooperating with a ruthenium-based dye as a photosensitizer under mild reaction conditions. Under the optimal reaction conditions, the photocatalytic CO 2 reduction system achieved a superior catalytic performance with a CO generation rate of 37.4 μmol/30 min, which was much higher than that of other types of MOFs. The carbon source of the evaluated CO was confirmed by 13 CO 2 isotopic experiment. The stability and reusability of the ZIF-67 co-catalyst in the reaction system were also examined. The present work provides new insights in the developments of nanoscale ZIFs materials for photocatalytic application.

First principle investigation of halogen-doped monolayer g-C3N4 photocatalyst

Abstract: Element doping is an efficient strategy for tuning the electronic structure and improving the photocatalytic activity of graphitic carbon nitride (g-C3N4). Employing the density functional theory computation performed by CASTEP module, we investigated the band structures, electronic and optical properties of monolayer g-C3N4 doped with halogens (F, Cl, Br or I). First, the halogen atoms occupying the interstitial space enclosed by three tri-s-triazine units in the monolayer g-C3N4 unit cell was demonstrated to be the most stable configuration in terms of adsorption energy. On the basis of these interstitial-doped monolayer g-C3N4 systems, it is found that the introduction of halogen atoms leads to various density of states (DOS) and redistribution of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). The F atom tends to occupy the valance band and HOMO due to its extremely high electronegativity. By contrast, the Cl, Br and I atoms are involved in the conduction band and LUMO. In sum, the calculation results show that the halogen-doped monolayer g-C3N4 systems have narrowed band gap, increased light absorption and reduced work function, which are conducive to high photocatalytic activity. The conclusions presented in this work indicate the availability of halogen-doped monolayer g-C3N4 with considerable photocatalytic performance.

CO2 hydrogenation to methanol over Pd/In2O3: effects of Pd and oxygen vacancy

Abstract: CO2 hydrogenation to methanol has attracted increasing attention. Previous theoretical study suggested that Pd/In2O3 has a high activity for CO2 hydrogenation to methanol with the Pd-In2O3 interfacial sites being the active ones. However, the strong interaction between Pd and In2O3 during the catalyst preparation leads to the formation of Pd-In bimetallic species and, consequently, reduces methanol yield. In this work, the Pd/In2O3 catalyst was prepared by firstly mixing In2O3 powder with Pd/peptide composite, which was followed by thermal treatment to remove the peptide. The resulting Pd/In2O3 catalyst is In2O3 supported highly-dispersed Pd-nanoparticles exposing predominately the (111) facets with particle sizes of 3.6 nm. Such Pd nanoparticles have a better ability to dissociatively adsorb hydrogen, thereby supplying hydrogen for the hydrogenation steps and facilitating oxygen vacancy creation. The interfacial sites are also active for enhanced CO2 adsorption and hydrogenation. All these factors contribute to a superior performance of the Pd/In2O3 catalyst for CO2 hydrogenation to methanol with a CO2 conversion >20% and methanol selectivity >70%, corresponding to a STY up to 0.89 gMeOH h−1 gcat−1 at 300 °C and 5 MPa.

P-doped tubular g-C3N4 with surface carbon defects: Universal synthesis and enhanced visible-light photocatalytic hydrogen production

Abstract: Hetero-element doping or vacancy defects of g-C3N4 framework were found significantly to control its electronic structure and enhance photocatalytic activity under visible light. Herein, we fabricated P-doped tubular g-C3N4 (P-TCN) with surface carbon defects wherein the P-doping and carbon defects were conveniently introduced during thermal polymerization of a supramolecular precursor. The supramolecular precursor of rod-like morphology was obtained only from melamine molecules under a sodium pyrophosphate-assisted hydrothermal process. As contrast, similar P-doped g-C3N4 tubes were obtained using other phosphates, such as ammonium phosphate, sodium hypophosphite and sodium phosphite, thus highlighting the versatility of this method to tune the morphology and C/N ratio for g-C3N4 tubes. The photocatalytic activities of P-TCNs were evaluated using hydrogen evolution from water under visible light. Among these, P-TCN obtained by sodium pyrophosphate-assisted hydrothermal reaction showed the highest photocatalytic activity due to high P element doping, enhanced visible light absorption and improved charge separation. The novel synthetic method described here thus represents an effective way of non-metal doping and C/N ratio tuning of g-C3N4 with excellent photocatalytic performance.

Enhanced catalytic degradation of methylene blue by α-Fe2O3/graphene oxide via heterogeneous photo-Fenton reactions

Abstract: Novel hybrid nanostructures or nanocomposites are receiving increasing attention due to their newly evolved properties. In this work, α-Fe 2 O 3 anchored to graphene oxide (GO) nanosheet (α-Fe 2 O 3 @GO) was synthesized through a facile hydrolysis process and its photo-catalytic performances and durability in heterogeneous Fenton system were fully evaluated. The decolorization rates of methylene blue in α-Fe 2 O 3 @GO + H 2 O 2 + UV system within a wide pH range were approximately 2.9-fold that of classical Degussa P25 TiO 2 + UV and 2.4-fold that of α-Fe 2 O 3 + H 2 O 2 + UV. This enhanced decolorization of methylene blue (MB) in α-Fe 2 O 3 @GO + H 2 O 2 + UV system were attributed to the unique incorporation of GO into the catalyst which not only mediated the morphology of active sites α-Fe 2 O 3 nanoparticles but also offered high electron conductivity and electrostatic attraction between negatively charged GO with positively charged MB. High efficiencies of degradation were achieved on various surface charged organic pollutants (around 96–100%), such as cationic compounds of MB and rhodamine B (RhB), anionic compound Orange II (OII) and Orange G (OG), neutral compounds of phenol, 2-nitrophenol (2-NP) and endocrine disrupting compound 17β-estradiol (E2). The dominant reactive oxygen species (ROS) responsible for decolorization, such as hydroxyl radicals ( OH) and superoxide anion radicals (O 2 − ) generated by activation of H 2 O 2 on the surface of α-Fe 2 O 3 @GO were detected and quantified by free radical quenching methods. The possible degradation mechanism of MB involved the rupture of phenothiazine ring by desulfurization and the rupture of phenyl ring due to the attack of ROS, which was analyzed by LC/MS/MS. The reduction of MB and its intermediates was consistent with the decreasing trend of the acute toxicity towards luminous bacteria with the increasing irradiation time. The results lay a foundation for highly effective and durable photo-Fenton technologies for organic wastewater within wider pH ranges than the conventional photo-Fenton reaction.

Well-designed 3D ZnIn2S4 nanosheets/TiO2 nanobelts as direct Z-scheme photocatalysts for CO2 photoreduction into renewable hydrocarbon fuel with high efficiency

Abstract: A 3-dimensional (3D) ZnIn2S4/TiO2 Z-scheme system has been designed and constructed for photocatalytic reduction of CO2 into renewable hydrocarbon fuels without the use of a solid-state electron mediator. The unique 3D morphology, achieved by assembling 2D ZnIn2S4 nanosheets onto 1D TiO2 nanobelts, not only provides large surface area but also improves the separation and transfer efficiency of photogenerated electrons and holes. The 3D ZnIn2S4/TiO2 Z-scheme photocatalysts show excellent light-harvesting properties demonstrated in photocatalytic reduction of CO2, resulting in generation of desired hydrocarbons. The CH4 production rate of the 3D ZnIn2S4/TiO2 can reach up to 1.135 μmol g−1 h−1, which is about 39-times higher than that of bare ZnIn2S4 (0.029 μmol g−1 h−1). The enhanced photocatalytic activity is attributed to effective separation of the charge carriers between ZnIn2S4 and TiO2 through the direct Z-scheme instead of a type-II heterojunction. The photogenerated electrons in TiO2 nanobelts recombine with the holes in ZnIn2S4 nanosheets, and the unrecombined electrons/holes on different active sites have stronger reduction/oxidation abilities, leading to higher photocatalytic activity for CO2 reduction.

Noble-metal-free cobalt phosphide modified carbon nitride: An efficient photocatalyst for hydrogen generation

Abstract: Photocatalytic hydrogen generation from water is an important solar-to-chemical conversion process, and is also a sustainable and environment-friendly generation approach for energy source of hydrogen. Herein, for the first time, we report the noble-metal-free CoP/g-C 3 N 4 hybrid as a photocatalyst for the highly efficient hydrogen generation by water splitting under visible light irradiation, whose hydrogen generation rate is ∼131 times higher than that of pure g-C 3 N 4 , and is even better than that of Pt/g-C 3 N 4 . Based on the detailed analyses of photoluminescence (PL) spectra, UV–vis diffuse reflectance spectroscopy (UV–vis DRS), photocurrent-time ( i-t ) curves, and electrochemical impedance spectroscopy (EIS) Nyquist plots, the reason for the high efficiency of CoP/g-C 3 N 4 is found to be its good absorption ability of visible light, highly effective separation and low recombination rate of photo-generated electron-hole pairs due to the addition of CoP to g-C 3 N 4 .

One step synthesis of oxygen doped porous graphitic carbon nitride with remarkable improvement of photo-oxidation activity: Role of oxygen on visible light photocatalytic activity

Abstract: A novel metal-free oxygen doped porous graphitic carbon nitride (OA-g-C3N4) was synthesized by condensation of oxalic acid and urea. The 40% OA-g-C3N4 catalyst can degrade bisphenol A (15 mg L−1) in 240 min with a mineralization rate as high as 56%. The markedly higher visible-light-driven oxidation activity of OA-g-C3N4 is attributed to the porous morphology and unique electrical structure. The porous structure of OA-g-C3N4 provides more active sites for adsorption and degradation of pollutants. Moreover, oxygen atoms in the tri-s-triazine units help to extend sufficient light absorption range up to 700 nm, improve the separation of charge-carriers and alter the position of valence band (VB) and conduction band (CB). The VB edge shifts from 1.95 eV to 2.46 eV due to the incorporation of O atoms, which leads to the change of active species in the photocatalytic reaction. Trapping experiment shows that superoxide radicals play the major role in the photocatalytic degradation of BPA on g-C3N4, while hydroxyl radical is the dominant active species in the photocatalytic degradation process over 40% OA-g-C3N4. This study presents a simple, economical and environment-friendly method to synthesized oxygen doped porous graphitic carbon nitride.

Surface oxygen vacancy induced α-MnO2 nanofiber for highly efficient ozone elimination

Abstract: α-MnO2 nanofiber with high concentration of surface oxygen vacancy was obtained via vacuum deoxidation method. The activity of α-MnO2 strongly depends on the concentration and extent of oxygen vacancy, which can be adjusted by tuning the temperature and time of vacuum deoxidation. The formation of oxygen vacancy enhanced the ratio of Mn3+/Mn4+, which changed the charge distribution on the α-MnO2 nanofiber, resulting in a significant improvement of the adsorption of ozone on the surface of the catalyst. In the dry gas flow, the ozone removal rate at 20 h has increased from 32.6% to 95%. In the wet gas flow, the ozone removal rate was also enhanced thanks to more active sites offered by α-MnO2. What's more, we found that the deactivation caused by water vapor was temporary and the activity would recover once the humidity has decreased. Finally, DFT calculation revealed that surface oxygen vacancy was the adsorption and reaction site for ozone decomposition and a new mechanism of ozone decomposition in the presence of H2O also was proposed. This work developed a deeper understanding to the process of ozone decomposition and would promote manganese oxide catalyst for practical application.

Atomic scale g-C3N4/Bi2WO6 2D/2D heterojunction with enhanced photocatalytic degradation of ibuprofen under visible light irradiation

Abstract: Although photocatalytic degradation is an ideal strategy for cleaning environmental pollution, it remains challenging to construct a highly efficient photocatalytic system by steering the charge flow in a precise manner. In this study, a novel atomic scale g-C3N4/Bi2WO6 heterojunction (UTCB) constructed by ultrathin g-C3N4 nanosheets (ug-CN) and monolayer Bi2WO6 nanosheets (m-BWO) was successfully prepared by hydrothermal reaction. The UTCB heterojunctions were characterized by various techniques including XRD, TEM, AFM, BET measurements, UV–vis spectrometry, and XPS. The results indicated that UTCB heterojunctions were assembly of m-BWO on ug-CN and presented high separation efficiency of photogenerated carriers. Under visible light irradiation, the optimum molar ratio of ug-CN/m-BWO (1:4, UTCB-25) reached almost 96.1% removal efficiency of IBF within 1 h, which was about 2.7 times as that of pure m-BWO. The photocatalytic mechanisms of UTCB-25 were revealed, suggesting that the synergistic effect of UTCB-25 heterojunction with strong interfacial interaction promoted the photoinduced charge separation. According to the LC–MS/MS, five photodegradation pathways of IBF under visible light irradiation were proposed. This study could open new opportunities for the rational design and a better understanding of atomic scale two dimensions/two dimensions (2D/2D) heterojunctions in environmental or other applications.

Perovskite oxide ultrathin nanosheets/g-C3N4 2D-2D heterojunction photocatalysts with significantly enhanced photocatalytic activity towards the photodegradation of tetracycline

Abstract: Novel visible-light-driven 2D-2D heterojunction photocatalysts was constructed based on 2D g-C 3 N 4 and exfoliated ultrathin K + Ca 2 Nb 3 O 10 − nanosheets, which is derived from Dion-Jacobson phase layer perovskites. The photocatalytic performance of the 2D-2D g-C 3 N 4 /K + Ca 2 Nb 3 O 10 − nanosheet heterojunctions was evaluated by the degradation of tetracycline hydrochloride under visible light irradiation. Compared with bare K + Ca 2 Nb 3 O 10 − and g-C 3 N 4 , the g-C 3 N 4 /K + Ca 2 Nb 3 O 10 − nanosheet heterojunctions exhibited considerably enhanced photocatalytic activity towards the degradation of tetracycline hydrochloride, which is about 6.6 and 1.8 times higher than that of pure K + Ca 2 Nb 3 O 10 − and g-C 3 N 4 , respectively. This enhanced activity can be mainly attributed to the synergistic effect of g-C 3 N 4 /K + Ca 2 Nb 3 O 10 − nanosheet heterojunctions with strong interfacial interaction and abundant 2D coupling interfaces, which could efficiently promote the photo-induced charge separation. The results of work demonstrated that construction of 2D-2D heterojunctions is an effective strategy to obtain the enhanced photocatalytic activity towards the degradation of tetracycline hydrochloride.

The effect of manganese vacancy in birnessite-type MnO2 on room-temperature oxidation of formaldehyde in air

Abstract: Catalytic reaction active site tends to be the structural defects, such as edges, corners, ribs and other position that has low coordination number. Here, the potassium (K+) type birnessite (i.e. a layered-structure MnO2) was designed with different amounts of manganese vacancy (VMn) for catalytic oxidation of formaldehyde (HCHO). The content of VMn was determined by the ratio of Mn/O and coordination number of Mn–Mn edge-sharing structure. The VMn showed a dramatic promotion effect on the activity of birnessite for HCHO oxidation. The specific rate at 30 °C over the birnessite with the highest content of VMn was highest (0.052 μmol/m2 min) under 40 ppm of HCHO, 120,000 mL/g h of GHSV and ∼ 80% of relative humidity. The presence of VMn induced unsaturated oxygen species and K+ locating nearby VMn sites for charge balance facilitated the formation of active oxygen species, accordingly the activity for HCHO oxidation was greatly improved. This finding reveals a way to enhance the catalytic activity of metal oxides via adjusting metal vacancies.

Dry reforming of methane over Ni/La2O3 nanorod catalysts with stabilized Ni nanoparticles

Abstract: This paper describes the design of a Ni/La2O3 catalyst using La2O2CO3 nanorod as a support precursor (denoted as Ni/La2O3-LOC) via a wet impregnation method for dry reforming of methane (DRM). The results showed that La2O3 derived from the La2O2CO3 precursor maintained its initial morphology upon thermal treatment and could highly disperse Ni particles on it. Additionally, the nanorod-shaped support could provide more medium-strength basic sites to facilitate CO2 adsorption and activation on its surface. Consequently, the Ni/La2O3-LOC catalyst reached 70% of CH4 conversion and 75% of CO2 conversion at 700 °C after 50 h DRM reaction with a H2/CO ratio of 0.87. The enhanced metal-support interaction restricted the sintering of nickel particles under harsh reaction conditions. Coke evolution on the catalysts was also investigated to understand coke formation mechanism and the role of La2O2CO3 in coke elimination. It has been found that nickel dispersion can affect distribution of coke and La2O2CO3 on the surface of catalyst, both of which have a close relation with catalytic performance.

Review of recent trends in photoelectrocatalytic conversion of solar energy to electricity and hydrogen

Abstract: This work is a review of the recent trends in the photoelectrocatalytic conversion of solar energy into electricity or hydrogen. It focuses on photocatalytic fuel cells and photoelectrocatalytic water splitting systems and presents both the basic principles and the design of devices. It includes a broad review of materials employed for the construction of photoanodes, photocathodes and tandem cells and highlights the related research fields which are expected to be of interest in the near future. The review is intended to become a basic manual for new adepts to the field and at the same time a handy reference to experienced researchers.

Mechanistic investigation of the enhanced NH3-SCR on cobalt-decorated Ce-Ti mixed oxide: In situ FTIR analysis for structure-activity correlation

Abstract: A series of transition metals (Co, Cu and Fe) were selected to decorate Ce-Ti mixed oxide to elevate the low-temperature activity of selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) reaction, by adjusting the ratio of surface Ce 3+ species and oxygen vacancies. Among them, Co-Ce-Ti sample exhibited the excellent low-temperature activity and broadened temperature window, which could be attributed to the improvement of the physico-chemical properties and the acceleration of the reactions in the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. Owing to the different ionic sizes of Co 2+ and Ce 4+ , the lattice distortion of Ce-Ti mixed oxide was greatly aggravated and subsequently increased the ratio of Ce 3+ and the surface adsorbed oxygen, which benefited the generation of adsorbed NO x species and improved the reaction in the L-H mechanism. Meanwhile, the coordinatively unsaturated cationic sites over the Co-Ce-Ti sample induced more Lewis acid sites and enhanced the formation of the adsorbed NH 3 species bounded with Lewis acid sites, which were considered as the crucial intermediates in E-R mechanism, and therefore facilitating the reaction between the adsorbed NH 3 species and NO molecules. The enhancements in both the reactions from L-H and E-R mechanisms appeared to directly correlated with the improved deNO x performance on the Co-Ce-Ti sample, and the L-H mechanism could be the dominate one at low temperatures due to its rapid reaction rate.

Novel ternary heterojunction photcocatalyst of Ag nanoparticles and g-C3N4 nanosheets co-modified BiVO4 for wider spectrum visible-light photocatalytic degradation of refractory pollutant

Abstract: A novel and highly efficient three-component Ag@g-C 3 N 4 @BiVO 4 heterojunction was successfully synthesized and characterized in terms of structure, porosity, chemical composition and optical properties. The photocatalytic activities of as-prepared samples were evaluated by the photocatalytic decomposition of tetracycline (TC) in the aqueous phase. Compared with single semiconductor BiVO 4 and g-C 3 N 4 , binary composites Ag@BiVO 4 and g-C 3 N 4 @BiVO 4 , the ternary Ag@g-C 3 N 4 @BiVO 4 heterojunction exhibited the higher photocatalytic activity under wider light spectrum irradiation. Furthermore, we also investigated the effects of initial TC concentrations and coexisting ions were in simulated practical wastewater. The mechanism research showed that matching of band structure between BiVO 4 and g-C 3 N 4 induced an efficient photogenerated electrons and holes transfer from the CB of BiVO 4 and VB of g-C 3 N 4 , respectively. As a charge transfer center, Ag nanoparticles were well photodeposited onto the surface of BiVO 4 and g-C 3 N 4 and increased the visible light absorption, even near-infrared via the surface plasmon resonance. The synergy effects between Ag, g-C 3 N 4 and BiVO 4 with the aid of Z-scheme mechanism were the main reason for improved photocatalytic performance. The trapping experiments and ESR tests confirmed that the O 2 − , h + and OH were main active species in photocatalytic degradation of TC. From the viewpoint of practical application, Ag@g-C 3 N 4 @BiVO 4 ternary structure displayed superior photostability after four times recycle.

Synergistic effect of surface and bulk single-electron-trapped oxygen vacancy of TiO2 in the photocatalytic reduction of CO2

Abstract: Oxygen vacancies play an important role in many photocatalytic reaction, and have attracted enormous attention from the scientists and engineers. The surface or bulk oxygen vacancies have a different function in the photo-reaction process. Herein, three different TiO2 nanoparticles possessing surface oxygen vacancies (SO) and/or bulk single-electron-trapped oxygen vacancy (SETOV) were fabricated by dehydration or reduction of different titania precursors. The three kinds of TiO2 nanoparticles were characterized systematically by XRD, TEM, Raman, XPS, ESR, TG, UV–vis DRS, and PL techniques. The photocatalytic reduction results of CO2 indicated that both the bulk SETOVs and surface oxygen vacancies contributed to the enhancement of the light absorption, while the surface vacancies facilitated to the separation of the photo-generated charge carriers, and on the contrast, the bulk SETOVs acted as the recombination center. The co-existence of the surface and bulk oxygen vacancies exhibited a synergistic effect to improve the photoreduction efficiency of CO2 to CH4. Through adjusting the ratio of the surface and bulk oxygen vacancies and analyzing the positron lifetime and relative intensity by positron annihilation, the photoreduction efficiency of CO2 improved with the increase of the ratio of surface oxygen vacancies to bulk SETOVs.