Enhancing catalytic activity of gold nanoparticles in a standard redox reaction by investigating the impact of AuNPs size, temperature and reductant concentrations

Abstract

In this work, the catalytic activity of three different sizes of gold nano particles (AuNPs) (12, 30, and 45 nm) synthesized by the citrate reduction process studied in the conventional redox reaction of K₃Fe (CN₆)⁻³ to K₄Fe (CN₆)⁻⁴ using NaBH₄(reductant) at four different temperatures (5 °C, 10 °C, 15 °C, and 20 °C) and measured by UV–visible spectrophotometry. Comparative kinetic analysis of different sizes of AuNPs including rate constant, activation energy, Entropy values and Frequency of collisions are reported for the first time. Transmission electron microscopy analysis is employed to investigate morphology and particle size. Spherical nanoparticles of size 12, 30, and 45 nm were observed. The UV–visible spectra were recorded at regular intervals, and it was seen that the peak of K₃Fe (CN₆)⁻³ decreased gradually with time, at the same time surface plasmon resonance of AuNPs remained constant. As reaction catalysts, AuNPs maintain their optical density which shows their stability during the course of reaction. The kinetic parameters i.e., rate constant, and activation energy (k, t_1/2, E_a) were determined for three distinct sizes of gold nanoparticles (AuNPs) using the reductant at various concentrations. The value of k increases by increasing reductant concentration. This rise was significant for the small AuNPs. Increasing gold nanoparticle size (12, 30, 45 nm) decreased rate constant. As the size of AuNPs decreased the E_a reduced as well, i.e. 17.325 k cal mol⁻¹ for 12 nm, 19 k cal mol⁻¹ for 30 nm and 21 k cal mol⁻¹ for 45 nm AuNPs. For 50 mM of NaBH₄, k for 45 nm AuNPs is 0.10728 s⁻¹, but for 12 nm AuNPs, the value of k is 0.145 s⁻¹, indicating that the 12 nm AuNPs have the greatest rate constant values. The rate of reaction rises with an increase in reductant concentration and temperature, but this increase is significant in the case of small-sized nanoparticles, i.e., 12 nm, which have a high surface area and low E_a. Besides being a model redox reaction, the reduction of K₃Fe (CN₆)⁻³ to K₄Fe (CN₆)⁻⁴ has industrial use in making fertilizers and paint industry, anti-coating agent in colour photography, in dot etching and in amperometric biosensors.