Gold nano-particles supported on hematite and magnetite as highly selective catalysts for the hydrogenation of nitro-aromatics

The catalytic action of nano-sized Au particles supported on hematite (Fe2O3) and magnetite (Fe3O4) is compared in the continuous gas phase hydrogenation of p-chloronitrobenzene and m-dinitrobenzene. The catalysts were prepared by deposition-precipitation and have been characterised in terms of BET/pore volume, powder X-ray diffraction (XRD), temperature programmed reduction (TPR), H-2 chemisorption, high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) measurements. XRD confirmed the formation Fe2O3, which was transformed into Fe3O4 during TPR to 673 K with a concomitant decrease in BET area and pore volume. Post-TPR to 423 K, Au/Fe2O3 exhibited well dispersed pseudo-spherical Au particles with mean diameter = 2.0 nm. HRTEM and XPS demonstrate the encapsulation of Au in the Fe3O4 matrix after TPR to 423 K, which inhibited hydrogenation rate. Thermal treatment to 673 K resulted in the segregation of Au on the Fe3O4 surface and the formation of nano-scale particles with mean diameter = 4.0 nm. Similar activities were recorded over both Au/Fe2O3 and Au/Fe3O4 with exclusive nitro-group reduction to yield p-chloroaniline and m-nitroaniline, a response that is discussed in terms of Au electronic character.