Síntese de nanopartículas anisotrópicas de ouro e sua avaliação como catalisadores na redução química do p-nitrofenol

Abstract

Gold nanoparticles (AuNPs) are capable of catalyzing the chemical reduction of nitro compounds in the presence of NaBH4 in aqueous solution. A great part of the reactivity of these catalytic systems are related to the size and shape of these nanoparticles, as well as the nature and characteristics of the agents that generally cover these nanostructures. In this context, this work proposes the use of gold nanorods (AuNRs) coated with different tetra-alkylammonium bromide salts, hydroxylated or not, as coating agents; cetyltrimethylammonium bromide (CTABr), N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium bromide (HEA16Br) and N,N-dimethyl-N-cetyl-N-(2-hydroxypropyl) bromide ammonium (HPA16Br) and test them as nanocatalysts in the chemical reduction of p-nitrophenol in the presence of NaBH4. The AuNRs produced were characterized by ultraviolet and visible absorption spectroscopy (UV-vis) and transmission electron microscopy (TEM), showing between them small differences in their UV-Vis spectra, size and length/width ratio (L/W). All AuNRs prepared and coated with the different tetra-alkylammonium bromide salts as coating agents (AuNRs@CTABr, AuNRs@HEA16Br and AuNRs@HPA16Br) were active in the p-nitrophenol chemical reduction when in the presence of excess NaBH4 in aqueous medium, because without it, the reaction was not observed under the conditions used. However, under the same reaction conditions, different kinetic data was obtained. For each catalytic system, different induction times were observed to detect the beginning of the reaction for all systems, the apparent velocity constants (kapp) of the reaction at different temperatures (20, 25, 30, 35, 40, 45, 50 and 55 °C) and the apparent activation energies (Ea) were determined as 41, 43 and 50 J mol-1 K-1 for the systems catalyzed by AuNRs@CTABr, AuNRs@HEA16Br and AuNRs@HPA16Br, respectively. These results show that small modifications in the chemical structure of the coating agent are able to modify the chemical/catalytic reactivity of the nanoparticle.