A recyclable ZnS/syndiotactic polystyrene composite aerogel for sustainable and highly selective CO₂ photoreduction to methane

The development of robust, recyclable, and highly selective photocatalysts for CO₂ reduction remains a key challenge in the pursuit of sustainable solar fuel production. In this work, a novel ZnS/syndiotactic polystyrene (sPS) composite aerogel was developed and evaluated as a photocatalytic system for the selective reduction of CO₂ to CH₄ under UV irradiation. The hybrid material was prepared through a gelation process followed by supercritical CO₂ drying, enabling the homogeneous incorporation of ZnS nanoparticles within a threedimensional porous sPS network. Structural and morphological characterization confirmed the successful integration of the semiconductor while preserving the high specific surface area (335 m2 g− 1 ), hierarchical mesoporosity, and mechanical integrity of the aerogel. Under spontaneous reaction conditions (pH ≈ 6.5), the ZnS/ sPS composite achieved a liquid-phase CO₂ conversion of approximately 40%, with CH₄ selectivity exceeding 95%. Although slightly lower CO₂ conversion was observed compared with bare ZnS because of diffusional limitations imposed by the polymeric network, the composite exhibited markedly improved photostability and retained its photocatalytic performance over five consecutive reaction cycles, whereas unsupported ZnS underwent severe deactivation due to photocorrosion. The photocatalytic activity was further enhanced by simple modification of the reaction medium. Acidification of the aqueous solution to pH ≈ 3 increased the liquid-phase CO₂ conversion to approximately 65–70%, while ethanol-assisted CO₂ photoreduction in the presence of 1% (v/v) ethanol as a sacrificial hole scavenger increased the liquid-phase CO₂ conversion to approximately 54% and enhanced methane production under spontaneous pH conditions by suppressing electron–hole recombination. The enhanced performance is attributed to the synergistic interaction between the ZnS nanoparticles and the porous hydrophobic sPS matrix, which improves catalyst stability while preserving the high selectivity toward methane formation. Although these findings represent a preliminary proof of concept, they demonstrate that the combination of polymer-supported semiconductor photocatalysts with simple modifications of the reaction medium offers a promising strategy for developing more efficient and durable systems for the sustainable valorization of CO₂ into solar fuels