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Anode graphite was recovered from a spent lithium ion battery (LIB) and reutilized as a carbon precursor to obtain graphene-based materials. Characterization results revealed that impurities were removed from the obtained graphite powder by cleansing processes. The as-synthesized reduced graphene oxide (rGO) from the purified graphite (LIB-rGO) demonstrated excellent catalytic ozonation activity against organic pollutants removal. To probe potential catalytic active sites, LIB-rGOs with different defective levels but similar oxygen contents were synthesized. Catalytic ozonation tests revealed that a higher defective level resulted in a greater catalytic activity. Density functional theory (DFT) calculation further demonstrated that ozone molecules could spontaneously decompose into active oxygen species on graphene structural vacancies and edges, which consolidated the role of defective structure in catalytic ozonation activity. Meanwhile, we discovered the pollutant-structure-dependent behavior of dominant reactive oxygen species (ROS) with the aid of radical scavenging tests and electron paramagnetic resonance (EPR) spectra. For phenolic pollutants vulnerable to direct ozone attacking, superoxide radicals (O 2 − ) and singlet oxygen ( 1 O 2 ) were found to be responsible ROS, whereas hydroxyl radicals ( OH) were identified as the principle ROS for aliphatic organic pollutants destruction. This study not only put forward a possible way for reutilization of waste LIB anode, but also stepped further for investigating the catalytic ozonation mechanism towards the graphene-based materials including the active sites and the generation of ROS. [ABSTRACT FROM AUTHOR]

Highlights: [•] Nitration treatment enhanced the activity of activated carbon in catalytic ozonation. [•] The amination modification enhanced the activity of activated carbon in catalytic ozonation. [•] Acid oxygenated groups play a positive role in catalytic ozonation of oxalic acid. [•] This provides a new choice to promote the activity of activated carbon in catalytic ozonation. [Copyright &y& Elsevier]

[Display omitted] • Dramatic activity enhancement was observed over Fe-doped g-C 3 N 4. • Two-photoelectron reduction pathway was verified in Fe/g-C 3 N 4 Vis-O 3 system. • The different roles of single Fe atoms and nanoparticles was explored. • Fe(II)N x sites promoted the adsorption and reduction of O 2 rather than O 3. The identification of active sites and the establishment of structure-reactivity relationship at atomic scale is important for designing efficient catalysts and understanding the reaction mechanism. Here, single atom catalysts (SACs) of Fe anchored on g-C 3 N 4 were synthesized. The catalysts have larger surface area, more photo-generated electrons with stronger reduction ability and faster charge carrier migration than g-C 3 N 4 , thereby exhibited dramatically enhanced activity and stability in electron reduction of ozone (O 3) under visible light. However, lower catalytic performance was observed when Fe nanoparticles coexisted, which regulated more photoelectrons to reduce O 2 to OH via two or three-electron reduction pathways rather than one-electron reduction of O 3. Mössbauer spectrum and X-ray adsorption spectroscopy indicated that the presence of Fe nanoparticles led to higher proportion of Fe(II)N x , which played more significant role in the adsorption and reduction of O 2. The insight on the contribution of different Fe sites with well-defined local coordination structure is beneficial for rational design of efficient catalysts in photocatalytic ozonation. [ABSTRACT FROM AUTHOR]

• Hierarchical sphere-like LMO perovskite hybrids were prepared for O 3 activation. • Oxygen vacancies (OV) and multi-valence Mn were the main Lewis acidic sites. • DFT studies revealed OV and multi-valence Mn as active sites for O 3 decomposition. • Generated reactive species, O 2 −, OH and 1O 2 , were possible for BTA degradation. La-based perovskites are catalytically active owing to the oxygen vacancies, redox metal centers of B sites and surface hydroxyl groups. Nevertheless, the insights into these active centers on environmental catalysis are still insufficient. In this study, hierarchical mixed oxides perovskite microspheres were synthesized for catalytic ozonation over oxalic acid and benzotriazole. LaMn 4 O x , with LaMnO 3-δ as the dominant crystal phase, demonstrated superior catalytic activity to Mn 2 O 3 and LaMnO 3 synthesized from citric acid sol-gel method. Temperature-programmed desorption of NH 3 (NH 3 -TPD) and pyridine-Fourier transform infrared spectroscopy (pyridine-FTIR) tests proved Lewis acid as the main acid type. Temperature-programmed reduction of H 2 (H 2 -TPR), O 2 -TPD and X-ray photoelectron spectroscopy (XPS) analysis indicated the presence of oxygen vacancies and mixed valences of Mn in the crystal structure facilitated the catalytic process. Moreover, the content of oxygen vacancy was calculated by iodometric titration method. With the aid of theoretical calculations, oxygen vacancies were found to exhibit a strong affinity toward ozone adsorption, where ozone molecules spontaneously dissociated into reactive oxygen species (ROS) such as O 2 − and 1O 2. The B site of Mn facilitated ozone decomposition by extending the O O bond of ozone due to the electron transfer from Mn3+/Mn4+ redox cycle. In-situ EPR and quenching tests confirmed the contribution of O 2 − and 1O 2 in benzotriazole degradation along with OH. This study stepped further to unveil the ozone adsorption/decomposition and ROS generation on nanoscale perovskite-based composites. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Atomic Co/Ni dual sites in N-doped porous Janus-like carbon frameworks is successfully constructed. • Co/Ni dual sites electrocatalysts exhibit good ORR and OER bifunctional performance. • The formation of Ni N bond increases the electronic activity of N atoms. • Different active sites in Janus-like electrocatalysts for ORR and OER. Abstract Single-atom electrocatalysts have attracted board interest in the recent years as they combine the advantages of heterogeneous and homogeneous electrocatalysts. Nevertheless, single-atom electrocatalysts with single metal component cannot further satisfy the demand of catalytic properties. This work developed atomic Co/Ni dual sites in N-doped porous carbon Janus-like frameworks through epitaxial growth of cobalt based MOFs on nickel complexes. Structural characterization and atomic-scale transmission electron microscopy revealed the homogeneously dispersed active sites of Co-Ni alloy and single Co/Ni atoms. Electrochemical data strongly demonstrated the advantages of integrating Co-MOF and Ni complex with different topological structures to form a Janus-like structure. The resultant catalysts afforded onset potential of 0.93 V and half-wave potential of 0.84 V for oxygen reduction reaction in alkaline media, and 0.86 V and 0.73 V in acid media, which is better than single noble-metal-free catalysts, even close to commercial Pt/C. Besides, the catalysts also exhibited good oxygen evolution reaction performance (a current density of 10 mA cm−2 at a potential of 1.59 V) and overvoltage between ORR and OER is 0.78 V. Density functional theory calculations indicated the high electrocatalytic activities are originated from the synergetic effect of atomic Co/Ni -N- C bonds and microstructure of the prepared materials. This work paves a new avenue for the development of multiatomic electrocatalysts for energy conversion. [ABSTRACT FROM AUTHOR]

Photocatalytic ozonation offers a potential approach for highly efficient organic degradation by in-situ generation of reactive •OH via electron reduction of ozone. Herein, we synthesized a Mn single atom photocatalyst for promoting the •OH generation from O 3 activation under visible light irradiation. Theoretical calculation and experimental studies reveal that Mn single atoms on carbon nitride significantly promote the excitation and transfer of photo-generated electrons, making charge carriers enriched around MnN 4 sites and shortening their migration path to participate in surface reactions. Meanwhile, Mn single atoms serve as adsorption and catalytic sites for O 3 , favoring a fast one-electron-reduction reaction kinetics. More importantly, the trade-off between the number of available electrons and active sites in photocatalytic performance was observed from regulating the density of Mn single atoms. This work offers an intriguing regulation strategy for enriching surface charges on single atom catalyst interface toward efficient •OH production and removal of aqueous organics. [Display omitted] • Mn single atom catalysts demonstrate high performance in photocatalytic ozonation. • The anchored Mn single atoms enrich photo charge carriers for promoting O 3 adsorption. • One-electron reduction path ensures efficient O 3 activation to hydroxyl radicals. • A higher Mn single atom density favors higher photocatalytic performance. [ABSTRACT FROM AUTHOR]

Porous g-C 3 N 4 (PGCN) has attracted enormous attention due to its accessible nanoporous framework benefiting photocatalytic reactions. Here, we reported a one-pot template-free method to fabricate honeycomb-like PGCN by simply mixing ammonia chloride with the precursor of g-C 3 N 4 before calcination. The resulting PGCN exhibited obviously improved photocatalytic activity for p -hydroxybenzoic acid (PHBA) degradation under visible light due to its high surface area and enlarged band gap, but PHBA can hardly be mineralized in this process. Hence, for the first time, Vis/PGCN was coupled with ozone in this paper and the results showed that PGCN could trigger a vigorous synergy between photocatalysis and ozonation. Vis/O 3 /PGCN led to almost complete mineralization of PHBA with an ozone dosage of 1.5 mg/min, and the process could be further accelerated by increasing the ozone dosage. Such a remarkable mineralization enhancement was mainly attributed to the systematically promoted generation of non-selective hydroxyl radicals ( OH). The high CB level of PGCN benefited electron capture by ozone molecules, thus significantly enhanced charge separation and the decay of ozone into abundant OH. OH could vigorously react with PHBA and its ozone-recalcitrant intermediates such as the identified carboxylic acids, finally leading to thorough mineralization. Electrospray ionization–mass spectrometry was adopted to detect the evolution of degradation intermediates in ozonation and Vis/O 3 /PGCN, and the mineralization procedure from the original PHBA to CO 2 and H 2 O was comprehensively proposed. This study contributes to the integration of sunlight/PGCN with ozone as an efficient metal-free advanced oxidation process for water treatment. [ABSTRACT FROM AUTHOR]

The recent thrust in utilizing g-C 3 N 4 as a metal-free photocatalyst has led to accelerated attention in environmental decontamination. Despite of enormous efforts paid on material modification, the overall efficiency is still limited, especially for decomposition of chemically stable contaminants and mineralization of organics. Here, we coupled bulk g-C 3 N 4 (GCN-T and GCN-D synthesized from thiourea and dicyandiamide, respectively) photocatalysis with ozonation for mineralization of oxalic acid (OA) and p -hydroxybenzoic acid (PHBA) under UV and visible-light irradiation. The compelling experiments confirmed that bulk g-C 3 N 4 could trigger a super synergy between photocatalysis and ozonation under visible light. The apparent rate constant of OA removal by Vis/O 3 /GCN-D was 20.6 times as great as the sum of that in Vis/GCN-D and ozonation. The TOC removal of PHBA by Vis/O 3 /GCN-D was 98.0%, which was about 39.3% higher than the sum of that in Vis/GCN-D and ozonation. The high conduction band (CB) position of g-C 3 N 4 proved to be crucial for the synergy between photocatalysis and ozonation. GCN-D possessed a ∼0.10 V upshifted CB level compared to GCN-T, resulting in a 1.30 times higher photocatalytic ozonation activity. More negative CB potential benefited photoinduced electron capture by ozone molecules, thus significantly enhanced charge separation as well as the decay of ozone eventually with generation of abundant hydroxyl radicals ( OH). Exceptionally, Vis/O 3 /GCN-D exhibited stronger oxidizing ability than UV/O 3 /GCN-D for OA degradation with the same light intensity. The reason was that the photolysis of ozone occurred under UV and partly inhibited ozone capturing photoinduced electrons. The amount of generated OH thus decreased in UV/O 3 /GCN-D instead with the formation of a few atomic oxygen species (O ( 1 D)). This study puts forward new application of the easily obtained bulk g-C 3 N 4 and may drive the advance of sunlight/O 3 /g-C 3 N 4 method for efficient and green water decontamination using sunlight as energy source. [ABSTRACT FROM AUTHOR]

Highlights: [•] We synthesized branched TiO2 NRs owning the surface anatase/rutile junctions. [•] The flower-like branched TiO2 NRs showed excellent PEC performance response. [•] The surface anatase/rutile junctions formed on the boundary of branches and trunks improved the PEC performance. [Copyright &y& Elsevier]