Amphiphilic gold nanoparticles supported on carbon nanotubes: Catalysts for the oxidation of lipophilic compounds by wet peroxide in biphasic systems

Abstract

Amphiphilic catalyst composed of Au NPs supported on a hydrophobic CNT (Au/CNT) has been prepared. The attachment of Au NPs on CNTs sidewall was performed without previous functionalization of CNTs by “in situ” redox reaction between Au3+ ions and negatively charged CNTs. The prepared materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) showed that metallic Au NPs with mean diameter of 5.8nm were randomly deposited on CNT surface. The catalytic activity of the Au/CNT was firstly evaluated by the degradation of methylene blue (MB) in aqueous solution. The results from these experiments provide evidence that Au/CNT interacts with the H2O2 aqueous phase to produce highly reactive oxygen species, which mineralize MB molecules. Under biphasic water/cyclohexane mixture, Au/CNT showed amphiphilic behavior, encapsulating and stabilizing emulsion droplets. EPR measurements using PBN (Nt-butyl-α-phenylnitrone) in cyclohexane phase as spin trap revealed that hydroxyl radicals chemically generated by interaction of Au/CNT with H2O2 can be transferred to organic phase. Kinetics of hydroxyl radical transfer using DPPH (1,1-diphenyl-2-picrylhydrazyl) as model molecule at different H2O2 concentrations was evaluated. The catalyst was shown to have a relatively high S-tolerant in the oxidation reaction with H2O2, as well as high reusability. This work opens the way for applications of Au/CNT as catalyst in oxidation of a wide range of lipophilic molecules by hydrogen peroxide.