Critical Assessment of Waste-Derived Reducing Agent and Support in Ag Nanoparticle Catalysts for p-Nitrophenol Reduction

The transition toward more sustainable catalytic processes has driven increasing interest in waste-derived reducing agents and biomass-based carbon supports. In this study, silver nanoparticles (Ag NPs) were synthesized via conventional NaBH4 reduction or through a bio-derived route using orange peel extract (OPE) and subsequently employed either as colloidal catalysts or immobilized on commercial activated carbon (AC) or coconut-derived carbon (CC). Catalytic activity was evaluated through the reduction of p-nitrophenol under pseudo-first-order conditions using UV–Vis spectroscopy. OPE-derived Ag NPs exhibited slightly higher activity than NaBH4-reduced nanoparticles, while immobilization on carbon supports generally enhanced reaction rates, with Ag/ACBH showing the highest kinetic constant. In contrast, CC-based systems displayed lower absolute activity but improved cost-normalized performance due to the lower cost of the support. A preliminary cost–performance analysis, based on direct material costs, suggested that catalytic efficiency trends can be significantly altered when economic factors are considered, highlighting that the most active system does not necessarily correspond to the most cost-effective one. Stability tests showed progressive deactivation over reuse cycles, mainly attributed to surface oxidation and/or poisoning phenomena. These results demonstrate that integrating waste-derived reagents with low-cost supports can provide competitive catalytic systems, although further optimization is required to improve their long-term operational robustness.