Catalytic reduction of p-nitrophenol over precious metals/highly ordered mesoporous silica

Precious metals, Au, Pt, and Pd, were successfully deposited on highly ordered mesoporous SBA-15. Two different reduction routes were employed to deposit precious metals: (1) under H2 and (2) using sodium citrate. Samples prepared using sodium citrate exhibit a uniform particle size of 10 nm while samples synthesized under H2 show high dispersion with a particle size of 8 and 20 nm for Pt and Pd, respectively. The surface area and pore volume of the mesoporous SBA-15 were significantly reduced due to the impregnation of the precious metals. Detailed TEM and XPS analyses reveal a uniform distribution of particles with a metallic valence state and no evidence of metallic oxides. The prepared catalysts were used to reduce p-nitrophenol (PNP) into p-aminophenol (PAP) where two trends were observed. The catalytic reduction efficiency for PNP reduction using the catalyst prepared with sodium citrate as a reducing agent is in the order of Au > Pt > Pd with a rate constant of 3.24 × 10−1 s−1 for Au/SBA-15. On the other hand, the catalyst prepared with H2 as a reducing agent showed a reverse trend Pd > Pt > Au with a rate constant of 7.15 × 10−1 s−1 for Pd/SBA-15. The highest catalyst efficiency was observed for the case of Pd/SBA-15 synthesized via the H2 route with a rate constant of 7.15 × 10−1 s−1. Also the reaction rate of Pd/SBA-15 synthesized via the H2 route was 2.2 times higher than that of Au/SBA-15 prepared using the sodium citrate route.