A New Metal-Free Carbon Hybrid for Enhanced Photocatalysis

Carbon nitride (C₃N₄) is a layered, stable, and polymeric metal-free material that has been discovered as a visible-light-response photocatalyst. Owing to C₃N₄ having a higher conduction band position, most previous studies have been focused on its reduction capability for solar fuel production, such as hydrogen generation from water splitting or hydrocarbon production from CO₂. However, photooxidation ability of g-C₃N₄ is weak and has been less explored, especially for decomposition of chemically stable phenolics. Carbon spheres prepared by a hydrothermal carbonization of glucose have been widely applied as a support material or template due to their interesting physicochemical properties and the functional groups on the reactive surface. This study demonstrated that growth of carbon nanospheres onto g-C₃N₄ (CN-CS) can significantly increase the photooxidation ability (to about 4.79 times higher than that of pristine g-C₃N₄) in phenol degradation under artificial sunlight irradiations. The crystal structure, optical property, morphology, surface groups, recombination rate of electron/hole pairs, and thermal stability of CN-CS were investigated by a variety of characterization techniques. This study contributes to the further promising applications of carbon nitride in metal-free catalysis

Catalytic Removal of Aqueous Contaminants on N‑Doped Graphitic Biochars: Inherent Roles of Adsorption and Nonradical Mechanisms

Environmentally friendly and low-cost catalysts are important for the rapid mineralization of organic contaminants in powerful advanced oxidation processes (AOPs). In this study, we reported N-doped graphitic biochars (N-BCs) as low-cost and efficient catalysts for peroxydisulfate (PDS) activation and the degradation of diverse organic pollutants in water treatment, including Orange G, phenol, sulfamethoxazole, and bisphenol A. The biochars at high annealing temperatures (>700 °C) presented highly graphitic nanosheets, large specific surface areas (SSAs), and rich doped nitrogen. In particular, N-BC derived at 900 °C (N-BC900) exhibited the highest degradation rate, which was 39-fold and 6.5-fold of that on N-BC400 and pristine biochar, respectively, and the N-BC900 surpassed most popular metal or nanocarbon catalysts. Different from the radical-based oxidation in N-BC400/PDS via the persistent free radicals (PFRs), singlet oxygen and nonradical pathways (surface-confined activated persulfate–carbon complexes) were discovered to dominate the oxidation processes in N-BC900/PDS. Moreover, the adsorption of organics was determined to be the key step determining reaction rate, revealing that the pre-adsorption of reactants significantly accelerated the nonradical oxidation pathway. This study not only provides robust and cheap carbonaceous materials for environmental remediation but also enables the first insight into the graphitic biochar-based nonradical catalysis

Efficient Catalytic Ozonation over Reduced Graphene Oxide for <i>p</i>‑Hydroxylbenzoic Acid (PHBA) Destruction: Active Site and Mechanism

Nanocarbons have been demonstrated as promising environmentally benign catalysts for advanced oxidation processes (AOPs) upgrading metal-based materials. In this study, reduced graphene oxide (rGO) with a low level of structural defects was synthesized via a scalable method for catalytic ozonation of <i>p</i>-hydroxylbenzoic acid (PHBA). Metal-free rGO materials were found to exhibit a superior activity in activating ozone for catalytic oxidation of organic phenolics. The electron-rich carbonyl groups were identified as the active sites for the catalytic reaction. Electron spin resonance (ESR) and radical competition tests revealed that superoxide radical (^•O₂^–) and singlet oxygen (¹O₂) were the reactive oxygen species (ROS) for PHBA degradation. The intermediates and the degradation pathways were illustrated from mass spectroscopy. It was interesting to observe that addition of NaCl could enhance both ozonation and catalytic ozonation efficiencies and make ·O₂^– as the dominant ROS. Stability of the catalysts was also evaluated by the successive tests. Loss of specific surface area and changes in the surface chemistry were suggested to be responsible for catalyst deactivation

Dual Nonradical Catalytic Pathways Mediated by Nanodiamond-Derived sp²/sp³ Hybrids for Sustainable Peracetic Acid Activation and Water Decontamination

Peracetic acid (PAA) oxidation catalyzed by metal-free carbons is promising for advanced water decontamination. Nevertheless, developing reaction-oriented and high-performance carbocatalysts has been limited by the ambiguous understanding of the intrinsic relationship between carbon chemical/molecular structure and PAA transformation behavior. Herein, we comprehensively investigated the PAA activation using a family of well-defined sp2/sp3 carbon hybrids from annealed nanodiamonds (ANDs). The activity of ANDs displays a volcano-type trend, with respect to the sp2/sp3 ratio. Intriguingly, sp3-C-enriched AND exhibits the best catalytic activity for PAA activation and phenolic oxidation, which is different from persulfate chemistry in which the sp2 network normally outperforms sp3 hybridization. At the electron-rich sp2-C site, PAA undergoes a reduction reaction to generate a reactive complex (AND-PAA*) and induces an electron-transfer oxidation pathway. At the sp3-C site adjacent to CO, PAA is oxidized to surface-confined OH* and O* successively, which ultimately evolves into singlet oxygen (1O2) as the primary reactive species. Benefiting from the dual nonradical regimes on sp2/sp3 hybrids, AND mediates a sustainable redox recycle with PAA to continuously generate reactive species to attack water contaminants, meanwhile maintaining structural/chemical integrity and exceptional reusability in cyclic runs

Combined Spectroscopic and Theoretical Approach to Sulfur-Poisoning on Cu-Supported Ti–Zr Mixed Oxide Catalyst in the Selective Catalytic Reduction of NO_<i>x</i>

The SO₂-poisoning on a Cu-supported Ti–Zr mixed oxide catalyst (Cu/Ti_0.7Zr_0.3O_2−δ) in selective catalytic reduction (SCR) of NO_<i>x</i> with C₃H₆ was investigated, and the different effects of SO₂ at varying reaction temperatures were clarified by in situ Fourier transform infrared (FTIR) spectroscopy combined with density functional theory (DFT) calculations. In situ FTIR results of the catalyst at low temperatures (150–250 °C) implied that the formation of sulfates on the surface inhibited the activation of NO and C₃H₆ as well as the reactivity of nitrates and NO₂. The weakened capacity of the catalyst toward acetate formation is an important reason for the decline of catalytic activity at low temperatures. At high temperatures (above 275 °C), the negative effect of SO₂ on the C₃H₆ activation to acetate is quite weak. More importantly, the generation of −NCO species is enhanced significantly via the reaction −CN + SO₂/SO₄^2– → −NCO, which is confirmed by both in situ FTIR experimental observations and DFT calculations. The promotion in the generation of −NCO species is the primary reason for the elevation of SCR activity at high temperatures

Different Crystallographic One-dimensional MnO₂ Nanomaterials and Their Superior Performance in Catalytic Phenol Degradation

Three one-dimensional MnO₂ nanoparticles with different crystallographic phases, α-, β-, and γ-MnO₂, were synthesized, characterized, and tested in heterogeneous activation of Oxone for phenol degradation in aqueous solution. The α-, β-, and γ-MnO₂ nanostructured materials presented in morphologies of nanowires, nanorods, and nanofibers, respectively. They showed varying activities in activation of Oxone to generate sulfate radicals for phenol degradation depending on surface area and crystalline structure. α-MnO₂ nanowires exhibited the highest activity and could degrade phenol in 60 min at phenol concentrations ranging in 25–100 mg/L. It was found that phenol degradation on α-MnO₂ followed first order kinetics with an activation energy of 21.9 kJ/mol. The operational parameters, such as MnO₂ and Oxone loading, phenol concentration and temperature, were found to influence phenol degradation efficiency. It was also found that α-MnO₂ exhibited high stability in recycled tests without losing activity, demonstrating itself to be a superior heterogeneous catalyst to the toxic Co₃O₄ and Co²⁺

Reduced Graphene Oxide for Catalytic Oxidation of Aqueous Organic Pollutants

We discovered that chemically reduced graphene oxide, with an <i>I</i>_D/<i>I</i>_G >1.4 (defective to graphite) can effectively activate peroxymonosulfate (PMS) to produce active sulfate radicals. The produced sulfate radicals (SO₄^•) are powerful oxidizing species with a high oxidative potential (2.5–3.1 vs 2.7 V of hydroxyl radicals), and can effectively decompose various aqueous contaminants. Graphene demonstrated a higher activity than several carbon allotropes, such as activated carbon (AC), graphite powder (GP), graphene oxide (GO), and multiwall carbon nanotube (MWCNT). Kinetic study of graphene catalyzed activation of PMS was carried out. It was shown that graphene catalysis is superior to that on transition metal oxide (Co₃O₄) in degradation of phenol, 2,4-dichlorophenol (DCP) and a dye (methylene blue, MB) in water, therefore providing a novel strategy for environmental remediation

Synergistic Catalytic Ozonation Mediated by Dual Active Sites of Oxygen Vacancies and Defects in Biomass-Derived Composites for Long-Lasting Water Decontamination

Environmental decontamination relies on low-cost, sustainable materials to drive diverse catalytic redox reactions. In this work, we used low-cost biomass to develop nanocomposites of N-doped carbon-supported zinc oxide (ZNC400) at a low temperature (400 °C), which exhibited ultrahigh activity in heterogeneous catalytic ozonation (HCO) for organic water decontamination. Our experimental and computational studies revealed the collaborative functions of dual active centers of defective carbons and oxygen vacancies (OVs)-containing zinc oxide (ZnO) at the composite interface for successive ozone (O3) adsorption and catalytic decomposition, respectively. Inspiringly, OVs on ZnO will spontaneously dissociate water molecules (H2O) to form surface hydroxyl groups (–OH) as key intermediates to accelerate O3 decomposition. This synergistic interplay results in the continuous generation of hydroxyl radicals (•OH) and maintains over 90.2% atrazine (ATZ) removal over five successive cycles, endowing ZNC400 with substantial reusability. Furthermore, four ATZ degradation pathways were proposed, and the corresponding toxicity was evaluated by the embryonic development of zebrafish in the treated water. Overall, the engineered dual-function catalyst effectively addresses the long-standing issue of poor stability of carbon materials in HCO and offers high-performance, cheap, and green catalysts for advanced water purification

Nitrogen-Doped Graphene for Generation and Evolution of Reactive Radicals by Metal-Free Catalysis

N-Doped graphene (NG) nanomaterials were synthesized by directly annealing graphene oxide (GO) with a novel nitrogen precursor of melamine. A high N-doping level, 8–11 at. %, was achieved at a moderate temperature. The sample of NG-700, obtained at a calcination temperature of 700 °C, showed the highest efficiency in degradation of phenol solutions by metal-free catalytic activation of peroxymonosulfate (PMS). The catalytic activity of the N-doped rGO (NG-700) was about 80 times higher than that of undoped rGO in phenol degradation. Moreover, the activity of NG-700 was 18.5 times higher than that of the most popular metal-based catalyst of nanocrystalline Co₃O₄ in PMS activation. Theoretical calculations using spin–unrestricted density functional theory (DFT) were carried out to probe the active sites for PMS activation on N-doped graphene. In addition, experimental detection of generated radicals using electron paramagnetic resonance (EPR) and competitive radical reactions was performed to reveal the PMS activation processes and pathways of phenol degradation on nanocarbons. It was observed that both ^•OH and SO₄^•– existed in the oxidation processes and played critical roles for phenol oxidation

Few-Layered Trigonal WS₂ Nanosheet-Coated Graphite Foam as an Efficient Free-Standing Electrode for a Hydrogen Evolution Reaction

Few-layered tungsten disulfide (WS₂) with a controlled-phase ratio (the highest trigonal-phase ratio being 67%) was exfoliated via lithium insertion. The exfoliated WS₂ nanosheets were then anchored onto three-dimensional (3D) graphite foam (GF) to fabricate free-standing binder-free electrodes. The 3D GF can increase the interfacial contact between the WS₂ nanosheets and the electrolyte and facilitate ion transfer. Without the nonconductive binder, an intimate contact between the WS₂ and GF interface can be created, leading to the improvement of electrical conductivity. In comparison to the pure WS₂ nanosheets, the overpotential for a hydrogen evolution reaction is significantly decreased from 350 mV to 190 mV at 10 mA/cm², and no deactivation occurs after 1000 cycles. The density functional theory computations reveal that the efficient catalytic activity of the trigonal-phase WS₂/GF electrode is attributed to the lower Gibbs free energy for H* adsorption and higher electrical conductivity