Au-SnOx Hybrid Nanoparticles Encaged in Hollow Mesoporous Silica Nanoreactors for Catalytic Reduction of p-Nitrophenol

p-nitrophenol (p-NP) is a pollutant with environmental persistence, bioaccumulation potential, and significant health risks, and is widely dispersed in wastewater, so efficient removal of p-NP is imperative. Among the various methods, the catalytic reduction of p-NP to p-aminophenol (p-AP) using sodium borohydride (NaBH4) is a particularly promising one and, herein, catalysts play a crucial role. Among the various metals, Au shows unique catalytic activity for p-NP reduction. However, nanosized Au often exhibit limited activity and stability due to their high surface free energy. To address this challenge, we designed and synthesized Au-SnOx hybrid nanoparticles confined within hollow mesoporous silica nanoreactors (Au-SnOx@hm-SiO2) via a soft-template-assisted co-adsorption strategy. The resulting bimetallic Au-SnOx@hm-SiO2 nanoreactor showed significantly enhanced catalytic activity toward the NaBH4-mediated reduction of p-nitrophenol (p-NP) compared with its monometallic Au@hm-SiO2 counterpart, owing to the synergistic effect between Au and SnOx. Among various Au/Sn ratios, the catalyst with an Au/Sn molar ratio of 1:0.1 demonstrated the highest activity, achieving complete conversion of p-NP within 5 min at a p-NP/Au molar ratio of 529:1—a tenfold improvement over Au@hm-SiO2. Moreover, the catalyst maintained high efficiency over six consecutive cycles, with only slight deactivation, benefiting from the protective silica shell.