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

Differently stabilised metal sols have been used as precursors in the prepa ration of heterogeneous gold catalysts for liquid phase oxidation in water solution. The methodology of sols generation appears to be fundamental to o btaining nanoparticles; the support, instead, plays an important role in ma intaining particle dimension and morphology. Three different materials (gamma-Al2O3, SiO2 and activated carbon) have bee n 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 immobilisation step, the maintenanc e of the particle dimension observed in solution depends on both the suppor t and the type of sol. The gold particle mean size of the colloidal suspens ion is more easily maintained on oxidic supports than on carbon, the latter apparently needing both steric and polar stabilisation of the gold particl e. Comparison of Au/gamma-Al2O3 and Au/C catalyst activity in the liquid phase oxidation of ethylene glycol to glycolate highlighted the peculiarity of g old on carbon catalysts; in fact, the normally observed trend of reactivity is partially reversed, medium sized gold particle being the most active. ( C) 2000 Elsevier Science B.V. All rights reserved.