Catalytic activity and related properties of sol–gel-derived manganese-doped gahnite

The aim of this study was to investigate catalytic activity and thermal stability of the manganese-doped gahnite powders prepared by sol–gel synthesis. Gahnite catalysts with 4, 8, 12 and 25% of Zn replaced with Mn were characterised using differential thermal and thermogravimetric analysis (DTA/TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N2 adsorption/desorption isotherms and diffuse reflectance UV-Vis spectroscopy (DRS). After thermal treatment at 700 °C for 2 h, regardless of the amount of the dopant, pure spinel was obtained. However, for higher thermal treatment temperatures, traces of corundum appeared in samples with greater doping levels. It was indicated that this is a consequence of displacing of some Al3+ in octahedral sites with trivalent manganese. The isotherms resemble to type IV with H2 hysteresis loops, suggesting that material is mesoporous. Specific surface area increases with manganese content from 32 to 52 m2 g−1. The crystallite size is below 20 nm for all samples annealed at 700 °C for 2 h; crystallites remain in nano-region after annealing temperature increase to 900 °C but when treated at 1100 °C, sizes range between 200 and 300 nm. Moderate doping (8% of manganese) was found as favourable for crystallite size stability, i.e. showed the slowest coarsening rate. The catalytic activity of (Zn, Mn)Al2O4 nanoparticles for toluene oxidation has been tested in an integral fixed bed reactor. Tests indicate that the manganese-doped gahnite exhibits good catalytic activity for the toluene oxidation under conditions employed in the present study.