Waste-derived hollow-shell SiO2 from red mud as a novel support for Pd Nanoparticles: Sustainable synthesis and catalytic hydrogenation

In this study, red mud (RM), an industrial byproduct of alumina production, was utilized as a sustainable Si precursor, and subsequently used to purely synthesize hollow-shell structured SiO2 (HS). The synthesized HS was further employed as a support for palladium nanoparticles to prepare a Pd-loaded catalyst (Pd@HS) for catalytic hydrogenation of p-nitrophenol (p-NP). Characterization results confirmed the uniform formation of Pd nanoparticles on the surface of HS. Pd@HS exhibited a specific surface area of 50.78 m2/g, which was approximately 7.4 times higher than that of non-porous Pd@SiO2 (6.85 m2/g). In the presence of NaBH4, a complete reduction of p-NP was observed by Pd@HS within 20 min, resulting in the catalytic conversion to p-aminophenol (p-AP). Parametric studies revealed that the catalytic kinetics were enhanced with increasing NaBH4 concentration, catalyst dosage, and Pd loading amount. Mechanistic study suggested that NaBH4 first reduces Pd(II)-O species to metallic Pd0 on HS surface, leading to the creation of bound hydrogen species on the Pd0 surface and further stepwise hydrogenation of -NO2 group to -NH2 for formation of p-AP. These findings highlight the potential of RM as feasible Si sources and provide a sustainable route for producing functional silica-based materials for environmental and chemical applications.