Life-Cycle Management of NH3-SCR Catalysts: From Active-Site Design to Spent-Catalyst Valorization

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.