Abstract:Gold nanoparticles with different shapes were prepared and used as catalysts in the reduction of p-nitrophenol (PNP) in the aqueous phase and in the presence of sodium borohydride (NaBH 4 ). Parameters such as the reaction temperature, substrate/NaBH 4 molar ratio, and substrate/gold molar ratio were tested and evaluated. In this paper, we compare the catalytic reactivities of gold nanorods (AuNRs) and gold nanospheres (AuNSs), both synthesized by the seed-mediated method in the presence of cetyltrimethyl ammonium bromide (CTAB). Physical-chemical parameters such as the apparent rate constant (k app ) and activation energy (E a ) of the reactions were obtained for both systems. We observed that the catalytic system based on AuNRs is the most active. These colloidal dispersions were investigated and fully characterized by ultraviolet-visible absorption spectroscopy (UV-Vis) and transmission electron microscopy (TEM).

It is known that the reactivity of the nanocatalytic systems is related to the particle size and shape and also to the features of the capping agents on the nanostructures. In this study, gold nanorods (AuNRs) were synthesized by the seed-mediated method using different tetraalkylammonium bromide salts as capping agents, that are, cetyltrimethylammonium (CTABr), N,N-dimethyl-N‑cetyl-N-(2-hydroxyethyl)ammonium (HEA16Br), and N,N-dimethyl-N-cetyl- N-(2-hydroxypropyl)ammonium (HPA16Br), and used as catalyst for the chemical reduction of p-nitrophenol (PNP) in the presence of NaBH4. The catalytic systems were characterized by ultraviolet-visible (UV-Vis) absorption spectroscopy and transmission electron microscopy (TEM). The effect of the ammonium bromide-based capping agent on the catalytic activity of AuNRs was evaluated by performing the chemical reduction of p-nitrophenol in the presence of excess NaBH4 in aqueous medium. Under the reaction conditions employed, the catalytic systems displayed detectable subtle differences in terms of induction times and apparent activation energy (Ea) values. These results show that slight changes carried out in the chemical structure of the capping agent are able to imprint even slightly modification of the kinetic parameter of the catalytic reaction.