A pH-Induced Size Controlled Deposition of Colloidal Ag Nanoparticles on Alumina Support for Catalytic Application

A rational synthetic method of supported metal catalysts has been proposed. Highly dispersed Ag nanoparticles (NPs) with a mean diameter of ca. 10 nm and a narrow size distribution have been prepared in the presence of 3-mercaptopropionic acid (3-MPA) as a protective reagent. The zeta electric potential of the Ag NPs was negatively charged in the region of pH higher than 5 due to the presence of dissociated carboxylate ion (−COO⁻), which led to the electric repulsion between Ag NPs. On the other hand, the electric charge gradually increased in the region of pH less than 5 owing to the formation of a protonated carboxyl group (−COOH), which induced the hydrogen bonding between Ag NPs. Such pH-triggered assembly dispersion control enables a strong protocol to deposit Ag NPs with different diameters on the positively charged Al₂O₃ support by electrostatic attraction. The obtained Ag/Al₂O₃ materials were characterized by means of UV−vis spectra and transmission electron microscopy (TEM), and the catalytic activities were evaluated in the reduction of 4-nitrophenol in water. The reaction rate increased with increasing pH of the prepared colloidal solution, which is correlated with the decreasing size of the Ag NPs. The reduction of 4-nitrophenol is considered as a structure-insensitive reaction, and all surface atoms of the Ag NPs act catalytically as the same active species in this size range by performing calculations on the Ag crystallites.