The effect of magic number phosphine stabilised mono- and bimetallic Au, Cu and Au-Cu nanoparticles as catalysts in the reduction of 4-nitrophenol–A kinetic study

Phosphine stabilised mono- and bimetallic nanoparticles (NPs) of varying sizes were synthesised using a combined dendrimer encapsulated-monolayer protected cluster (DEN-MPC) method. According to this method, the NPs encapsulated within the dendrimer framework (DENs) are extracted using a ligand such as 1,3,5-triaza-7-phosphaadamantane (PTA), forming hydrophilic monolayer protected clusters (MPCs). The synthesis of MPCs with different average sizes (Aum, Cun, Aum–Cun and Au–Cu(m:n), where n = 55, 70, 95, 130 and 147 and m = 7, 13, 18, 31, 55) was achieved by varying the dendrimer to metal ratio. Various analytical techniques, including ultraviolet–visible (UV–Vis) spectroscopy, high-resolution transmission electron microscopy (HRTEM) and inductively coupled plasma optical electron spectroscopy (ICP-OES), were used to confirm the synthesis of the MPCs. The MPC were evaluated as catalysts in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-Amp), using excess sodium borohydride (NaBH 4) as the reducing agent. The catalytic reaction was studied by monitoring the absorbance of the 4-nitrophenolate ions at 400 nm using UV–Vis spectroscopy. The average particle sizes of the MPCs ranged from 5–10 nm and were found to be active catalysts for the reduction of 4-NP. The Cun-PTA NPs exhibited a higher activity compared to the Aum-PTA NPs. However, they were not considered ideal catalysts due to their structural instability. The bimetallic Aum-Cun PTA NPs were more stable and displayed higher catalytic activity than the monometallic Aum NPs due to a synergistic effect between the Au NPs and Cu NPs; hence, the bimetallic NPs have proven to be the best catalysts for the reduction of 4-NP. The effect of various parameters, such as the concentration of NaBH4 and 4-NP, catalyst concentration, temperature and the average particle size, on the reaction rate was studied. In addition, the thermodynamic parameters relating to the reaction rate (Kapp) were also calculated and the kinetic data showed that the reduction of 4-NP to 4-Amp using PTA NPs follows the Langmuir–Hinshelwood model.