Synthesis, spectroscopic and catalytic properties of FeO x /Al 2 O 3 nanopowders prepared by cw CO 2 laser vaporization

Abstract

Nanostructured powders xFe/nano-Al2O3 with the Fe loading of x = 0,0–5,0 wt% were obtained using laser vaporization by a cw CO2 laser. XRF, XRD, HRTEM, PL and UV-vis DRS techniques were employed to investigate physicochemical, structural and optical properties of the synthesized nanopowders with the average particle size of 9 nm. Nanopowders xFe/nano-Al2O3 as model catalysts were tested in isobutane dehydrogenation reaction. The results obtained were compared with similar data for the xFe/γPb-Al2O3 systems synthesized by the conventional sol-gel method. According to XRD and UV-vis DRS data, in the series of xFe/nano-Al2O3 samples a great part of Fe3+ ions is in the disordered environment of subsurface layers of Al2O3 nanocrystallites predominantly in tetrahedral coordination. In distinction to samples of the xFe/γPb-Al2O3 series, in the case of nanostructured xFe/nano-Al2O3 powders the formation of Fe2O3 phase does not occur at any concentrations of iron or conditions of testing. The analysis of the PL spectra of xFe/nano-Al2O3 powders also showed the presence of surface sites of Fe3+ ions, which were not detected for xFe/γPb-Al2O3. Catalytic testing of the xFe/nano-Al2O3 series samples in isobutane dehydrogenation revealed the formation of the iron active sites that ensure catalytic activity of the samples. Differences in the catalytic properties of FeOx/Al2O3 samples obtained by the sol-gel method and laser vaporization are related to different states of Fe3+ ions. Thus, the xFe/nano-Al2O3 nanopowders in contrast to xFe/γPb-Al2O3 contain a large amount of active Fe3+ sites. These sites involved in the dehydrogenation reaction are present predominantly on the surface of the nanopowders.