O efeito da presença de complexos de rutênio na cinética de redução do 4-nitrofenol utilizando nanopartículas de ouro como catalisadores.

Abstract

The goal of this work was studying the reduction reaction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) catalyzed by gold nanoparticles (AuNPsn–) to produce 4-aminophenol (4-AP). The value of observed rate constant (kobs) and the half-life time (t1/2) of the reduction reaction was determined using a pseudo-first order rate law in relation to the 4-nitrophenolate concentration. The influence of the temperature on the rate constant was determined using the Arrhenius equation, which provided the activation energy (Ea = 172 kJ mol-1) and the pre-exponential factor (A= 6.6 x 1026 s-1). The Eyring equation was used to afford the activation enthalpy variation (ΔH‡ = 170 kJ mol-1) and activation entropy variation (ΔS‡ = 31.27 e.u), showing that the reduction reaction of the 4-NP is highly entropic process in the presence of NaBH4 as reducing agent and AuNPsn- as catalytic species. Additionally, the interaction process of ruthenium cationic complexes (Ru+) with AuNPsn– to produce aggregates, which was labeled herein such as nRu+/AuNPsn–, was studied using three different models: the Langmuir isotherm (direct plot); Scatchard model and Benesi-Hildebrand model. These models provide thermodynamic parameters such as the dissociation constant (Kd), formation constant (Kf), Hill coefficient (n). These aggregates were then applied as catalysts in the 4-NP reduction reaction, where the kobs were determined and compared to the values found in the catalytic reaction for AuNPsn– without additives. Furthermore, the AuNPsn– were synthesized by the reduction of HAuCl4 with tribasic sodium citrate, which conferred them an anionic character, and they were characterized by ultraviolet/visible spectrometry (UV/vis), due to the formation of a typical plasmon band at 520 nm. The ruthenium complexes were synthesized following protocols described in the current literature. All catalytic reactions, kinetics measurements and aggregation processes were carried out in a spectrophotometer of UV/vis within 200 – 800 nm, in the temperature range of 28.5 – 50 °C +– 0.01 °C, using a temperature controller for the blank and sample cuvettes.