RESUMO
Highly stable gold nanoparticles immobilized on the surface of amine-functionalized nanocomposite microspheres possessing a magnetite (Fe3O4) nanoparticle core and a silica (SiO2) shell (Au/SiO2-shell/Fe3O4-core) were prepared. These gold nanocomposite catalysts were tested for 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) reduction in aqueous solution in the temperature range 293-323 K and in the presence of aqueous NaBH4 reducing agent. The magnetically recyclable gold catalyst showed high stability (â¼3 months), efficient recyclability (up to 10 cycles), and high activity (â¼100% conversion within 225 s, â¼700 ppm 4-NP or 2-NA). The pseudo-first-order apparent reaction rate constants (k) of 4-NP and 2-NA reduction were 7.5 × 10-3 and 4.1 × 10-3 s-1, respectively, and with an apparent catalytic activity of 4.48 × 10-8 kmol/(m3 s).
RESUMO
In recent years, there have been great experimental and theoretical advances in the understanding of the epoxidation of propylene by O(2) and H(2) over Au supported on titanium-containing oxidic supports; however, thus far spectroscopic evidence of reacting species for proposed mechanisms has been lacking. Hydroperoxide species have been postulated as an intermediate responsible for the epoxidation of propylene with O(2) and H(2). In order to obtain direct evidence for the different type of active oxygen species, in situ UV-vis and EPR measurements were carried out during the epoxidation of propylene with O(2) and H(2) over a Au/Ti-SiO(2) (Ti/Si = 3:100) catalyst. It was determined that the adsorbed species of oxygen (O(2)(-)) resided on Au, more likely at a perimeter site, and it led to the formation of titanium hydroperoxo species. These results support the possible mechanism of formation of these hydroperoxo species via H(2)O(2) produced from O(2) and H(2) adsorbed on the Au surfaces.