metal sols as useful tool for heterogeneous gold catalyst preparation: reinvestigation of a liquid phase oxidation

Differently stabilized metal sols have been used as precursors in the preparation of heterogeneous gold catalysts for liquid phase oxidation in water solution. The methodology of sols generation appears to be fundamental to obtaining nanoparticles; the support, instead, plays an important role in maintaining particle dimension and morphology. Three different materials (γ-Al2O3, SiO2 and activated carbon) have been used as the supporting agents for different gold sols that were obtained by reducing HAuCl4 with NaBH4 in the presence of polyvinylalcohol (PVA) or polyvinylpirrolidone (PVP) and with the tetrakis(hydroxymethyl)phosphonium chloride (THPC)/NaOH system. During the immobilization step, the maintenance of the particle dimension observed in solution depends on both the support and the type of sol. The gold particle mean size of the colloidal suspension is more easily maintained on oxidic supports than on carbon, the latter apparently needing both steric and polar stabilization of the gold particle. Comparison of Au/γ-Al2O3 and Au/C catalyst activity in the liquid phase oxidation of ethylene glycol to glycolate high-lighted the peculiarity of gold on carbon catalysts; in fact, the normally observed trend of reactivity is partially reversed, medium sized gold particle being the most active.