Synthesis, Spectroscopic and Catalytic Properties of FeOx/Al2O3 Nanopowders Prepared by CO2 Laser Vaporization

Nanostructured powders xFe/nano-Al₂O₃ with the Fe loading of x = 0.0 – 5.0 wt% were obtained using laser vaporization by 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/γ_Pb-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 the tetrahedral coordination. In distinction to samples of the xFe/γ_Pb-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/γ_Pb-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 FeO_x/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/γ_Pb-Al₂O₃, contain a large amount of active Fe³⁺ sites. These sites, being involved in the dehydrogenation reaction, are present predominantly on the surface of the nanopowders.