Open Access
Review
03 September 2025Defect Engineering in Carbon-Based Metal-Free Catalysts: Active Sites, Reduction Mechanisms, and 3D Architectures for Sustainable 4-Nitrophenol Reduction
Nitrophenols (NPs), classified as priority pollutants due to their significant toxicity, persistence, and bioaccumulation potential, pose severe threats to ecosystems and human health. Catalytic reduction, particularly the conversion of NPs like 4-nitrophenol (4-NP) to less toxic aminophenols using sodium borohydride (NaBH4), represents a promising remediation strategy. While conventional metal-based catalysts face limitations including high cost, poor durability, and potential metal leaching, carbon-based metal-free catalysts (C-MFCs) have emerged as highly efficient, sustainable, and cost-effective alternatives. However, the precise reaction mechanisms governing NP reduction over C-MFCs remain ambiguous, and significant debate surrounds the nature of the active sites and the structure-activity relationships dictating performance. This review systematically elucidates the catalytic sites and associated reduction mechanisms in C-MFCs. We comprehensively summarize design principles centered on defect engineering strategies, encompassing single-atom (N, S, B, P, O), dual-atom (B,N; N,S; N,P), and tri-atom (B,N,F; N,P,F) doping, alongside non-doping defects such as edge and pore defects. The critical structure-performance relationships linking these engineered active sites to catalytic activity (e.g., turnover frequency, TOF) are analyzed, integrating experimental evidence and theoretical insights. Furthermore, strategies for constructing three-dimensional architectures to enhance active site accessibility and catalyst stability are highlighted. This work provides fundamental insights to guide the rational design of next-generation high-performance C-MFCs for sustainable nitrophenol pollution control.
Open Access
Article
05 September 2025A Green Way for the Synthesis of Ester Oil by an Ionic Liquid as Both a Catalyst and Lubricant Additive
A series of ionic liquids 1-alkyl-3-methylim idazole bis(2-ethylhexyl) phosphate, were prepared, and the catalytic performance of ionic liquids was evaluated through the esterification reaction of pentaerythrotol and hexanoic acid at a stoichiometric ratio as a model reaction. The results showed that the [BMIM][DEHP] and [HMIM][DEHP] exhibited good catalytic activity. The [HMIM][DEHP] was chosen as a lubricant additive to further investigate the tribological properties after the reaction, and the results for both COF and WSD and wear volume indicate that the introduction of [HMIM][DEHP] has improved the friction reducing and anti-wear properties of pentaerythrotol tetra-hexanoate.
Open Access
Article
28 September 2025Enhancing the Performance of Sr2Fe1.3Ni0.2Mo0.5O6−δ as Methane-Fueled SOFC Anode via In-Situ Exsolution of Ni-Fe Nano-Catalyst
The Sr2Fe1.5Mo0.5O6−δ (SFMO) perovskite exhibits promising performance as a solid oxide fuel cell (SOFC) anode for hydrogen fuel but demonstrates limited catalytic activity with hydrocarbon fuels. To address this limitation, a Sr2Fe1.3Ni0.2Mo0.5O6−δ (SFNMO) perovskite was developed via B-site Ni substitution, and its in-situ exsolution behavior and methane electrooxidation performance were systematically investigated. Combined XRD, SEM, and TEM-EDS analyses reveal the in-situ exsolution of Ni-rich Ni-Fe alloy nanoparticles from the SFNMO matrix under a hydrogen atmosphere. A symmetrical SOFC employing Gd0.1Ce0.9O2−δ (GDC) electrolyte and SFNMO electrodes achieved an initial maximum power density of 82 mW cm−2 in wet methane fuel at 800 °C, which represents an approximately 33% improvement over the symmetrical cell with SFMO electrode (61 mW cm−2). Remarkably, the cell maintained stable operation under constant current for 50 h in methane fuel, with the peak power density further increasing to 113 mW cm−2, demonstrating the excellent catalytic activity of the in-situ exsolved Ni-Fe nanoparticles for methane conversion.
Open Access
Article
11 October 2025Catalytic Potential of Green-Synthesized Iron Nanoparticles from Psidium guajava for 4-Nitrophenol Reduction
This study presents a sustainable approach for the green synthesis of iron nanoparticles (Fe(NPs)) using an aqueous extract of Psidium guajava (guava leaves) as a reducing and stabilizing agent. The FeNPs were applied in the catalytic reduction of 4-nitrophenol. To minimize the use of sodium borohydride (NaBH4), different volumetric ratios of plant extract and NaBH4 were tested. The influence of these ratios on the physicochemical and morphological properties of the FeNPs was evaluated using X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), high-resolution field-emission SEM (HR-FESEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and N₂ physisorption. Increasing the proportion of plant extract led to reduced crystallinity, larger particle sizes, and lower surface areas. Despite these changes, using up to 40% extract improved catalytic performance, achieving over 90% reduction of 4-nitrophenol. Ecotoxicological assessments confirmed the biocompatibility of the FeNPs, the effective neutralization of 4-nitrophenol toxicity post-reduction, and highlighted the inherent toxicity of NaBH4. These findings demonstrate the potential of Psidium guajava-mediated FeNPs as eco-friendly catalysts for pollutant reduction, combining efficiency with reduced environmental impact.
Open Access
Article
20 October 2025Divergent Aging Mechanisms of Calcium Arsenic Residue under Dry-Wet and Freeze-Thaw Cycles: Toxic Metal Mobility, Multiscale Physicochemical Characterization, and Escalated Ecological Risks
This study investigates the long-term mobility and ecological risks of As, Zn, and Cd in calcium arsenic residue (CAR) under simulated dry-wet (DW) and freeze-thaw (FT) cycles. Accelerated aging experiments, combined with multiscale characterization (XRD, XPS, SEM, FTIR), revealed distinct transformation mechanisms. DW cycles promoted carbonate-driven dissolution, As(III) oxidation to As(V) (resulting in an 18.4% increase in As(V) as shown by XPS), and sulfide oxidation (with reductions of 47.7% in ZnS and 15.08% in CdS). These processes increased the acid-soluble metal fractions (F1: As by 11.3%, Zn by 6.0%, and Cd by 8.7%) and metal release rates (52.39% for As, 42.63% for Zn, and 68.55% for Cd under DW conditions). In contrast, FT cycles induced mechanical fracturing and ice-mediated stabilization, which limited ion migration, partially amorphized ZnO, and promoted the precipitation of Cd(OH)2. Ecological risk assessments indicated rising risks, with integrated potential ecological risk indices (IPER) reaching 11,187.85 under DW conditions and 10,668.29 under FT conditions, with arsenic contributing over 80%. The Risk Assessment Code (RAC) reclassified all metals into moderate-risk categories (As: 11.9–19.7%, Zn: 9.4–15.2%, Cd: 12.1–18.6%). Weibull modeling (α = 6.98–10.98, R2 > 0.96) described the nonlinear kinetics, showing that cadmium aged the fastest (λ: Cd > As > Zn), with delayed but persistent risks under FT conditions. These results underscore the importance of developing climate-resilient stabilization strategies. The integrated framework combining mineral evolution, kinetics, and risk forecasting offers significant insights for managing legacy CAR pollution under changing climate conditions.
