Catalysts

Selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR) is central to stationary-source emission control, but catalyst deactivation and the management of spent materials remain closely connected challenges. This review examines catalyst development, deactivation diagnosis, regeneration, metal separation, and alternative applications within a common life-cycle framework. Recent studies demonstrate that acidity, redox behavior, interfacial structure, support morphology, and thermal stability must be balanced to obtain useful activity and selectivity under realistic gas compositions. Improvements in fresh-catalyst conversion do not necessarily establish resistance to combined poisoning or longer service life. For spent catalysts, the distinction between restoring an intact module, remanufacturing a support, and converting the material to a different use is essential. Selective detoxification and re-impregnation can retain catalytic value, whereas acid, alkaline, thermal, and mechanochemical routes provide different opportunities for recovering V, W, and Ti. Separation efficiency must be evaluated beyond leaching to include purification, product formation, and impurity accumulation during reagent recycling. Emerging applications in photocatalysis, adsorption, organic reforming, continuous-flow reduction, chlorobenzene oxidation, and metallurgy demonstrate that loss of SCR activity does not eliminate all material value. However, pollutant conversion is not equivalent to mineralization, and structural stabilization does not by itself establish environmental safety. A critical comparison of the available evidence supports source-specific classification, product-oriented impurity control, and complete elemental balances as the basis for route selection. Future progress requires long-term testing under complex feeds, continuous closed-loop recovery, and environmental and economic assessments that account for product quality, useful lifetime, and residual waste treatment.

The cycloaddition reaction of CO₂ with epoxides is an important way to realize the chemical fixation of CO₂. However, it often suffers from harsh reaction conditions, such as high pressure, high temperature and long reaction time. In order to explore an efficient catalyst for this reaction, 14 kinds of ionic liquid catalysts were designed, synthesized and used for the cycloaddition reactions of CO₂ with epoxides. The results showed that the dihydroxyl ionic liquid 3-(2-hydroxy-ethyl)-1-(3-hydroxy-propyl)-3H-imidazolium iodide (IL9) exhibits the best catalytic activity under mild reaction conditions: 60–100% yields of cyclic carbonates were obtained within 2 h at 100 °C and 1 atm CO₂ without any solvents and co-catalysts, thus alleviating the high-pressure-dependent limitation of reported hydroxyl-functionalized ionic liquid catalysts and significantly reducing the reaction time. The functional ionic liquid IL9 not only maintains its structure and activity unchanged after five runs; it also exhibits potential advantages in the conversion of CO₂ from simulated flue gas (15% CO₂/85% N₂). Further density functional theory (DFT) calculations revealed that the IL9 could activate epoxides and CO₂ by iodide anion, powerful hydrogen-bonding and the electrostatic force of the imidazole nucleus, consistent with the experimental results.

Geosmin (GSM) and 2-methylisoborneol (2-MIB) are dominant taste-and-odor (T&O) substances in drinking water and have extremely low human perceptual thresholds (0.004 and 0.01 µg/L, respectively). While nontoxic, their continued presence generates consumer rejection, damages utility credibility, and results in enormous economic losses. The management of GSM and 2-MIB throughout the drinking water supply chain, from source water to tap, is investigated here through an integrative review of representative T&O occurrences across multiple continents, GSM and 2-MIB-producing organisms, environmental influencing factors, and the inadequate removal achieved by strictly conventional treatment processes (coagulation/filtration), which improves only to 36–64% (GSM) and 38–59% (2-MIB) with the addition of low-dose pre-ozonation (0–1 mg/L). An analysis of pre-outbreak detection methods (remote sensing, qPCR, ML, and phycocyanin-based fluorescence detectors) and novel sensors (bio-electronic nose, electronic tongue, and odorant binding protein (OBP2a)-based biosensors) was conducted. Notably, advanced biosensors such as the bio-electronic nose can detect both GSM and 2-MIB below their respective human odor thresholds (<10 ng/L) without the need for sample pre-treatment. In contrast, OBP2a-based platforms show affinity for 2-MIB (but not GSM) and require signal amplification or preconcentration to reach trace-level detection, while the electronic tongue operates near these threshold levels (~20 ng/L). This review presents the first integrated comparison of conventional, biological, integrated, and advanced oxidation treatments in terms of removal efficiency, operating cost, energy consumption, toxicity of byproducts, and technology readiness levels, and concludes that no single current technology can simultaneously achieve >95% removal, maintain operating costs below US$ 0.50/m³, prevent the formation of toxic byproducts, and ensure long-term operational stability. Consequently, the implementation of multi-barrier treatment systems is required. Particular attention is given to scalable and economical treatment alternatives for small- and medium-sized plants in developing countries, such as biological aerated filtration coupled with ultrafiltration and permanganate/bisulfite processes. Future research priorities include life cycle assessment of treatment trains, harmonized ecotoxicological assessment of transformation products, real-time sensor-integrated adaptive treatment, and climate-resilient treatment strategies for intensified eutrophication.