Structural and surface characterization of pure and sulfated iron oxide

Iron oxide catalysts, pure and surface-doped with variable amounts of sulfate groups, have been prepared and characterized by means of electron microscopy (HR-TEM), N2-adsorption at 77 K (BET area and porosity), X-ray diffraction (XRD), adsorption microcalorimetry, and various spectroscopic techniques (UV-Vis, Raman, and FTIR). The catalysts were prepared from aqueous solutions of ammonia and iron nitrate, with production of an amorphous precipitate that transformed into R-Fe2O3 upon calcination at 773 K. The sulfation of iron oxide was carried out by adding two dosed amounts of aqueous (NH4)2SO4 to the starting hydroxide, as normally done for the preparation of other sulfated oxidic catalysts. Nominal sulfate loadings of 2 and 8 wt % were obtained, corresponding to ca. 1.4 and ca. 4.1 sulfate groups per nm2, respectively. The use of various experimental techniques allowed the description of the morphological and structural aspects of the systems, whereas in-situ IR spectroscopy of adsorbed probe-molecules (e.g., CO, CO2, and 2,6-dimethylpyridine) and the combined use of IR spectroscopy and microcalorimetry of CO adsorption at room temperature were used to characterize surface acidity and basicity. TEM, XRD, and BET measurements indicate that the presence of sulfate groups does not modify the crystalline structure of alfa-Fe2O3, but decreases crystallites size and, consequently, increases specific surface area. The presence of sulfates increases the strength of surface Lewis acidity, but the overall amount of Lewis acid sites decreases, while few Brønsted acidic sites of medium-low strength are produced. Surface basicity of iron oxide, mainly tested by the adsorption of CO2 and formation of surface carbonate-like species, is gradually decreased by sulfates, but not suppressed. The vacuum reducibility of iron oxide (which can be spectroscopically evidenced both by a color change (UV-Vis spectra) and by the formation, upon CO adsorption, of surface carbonyl-like species with a pi-back-donation component (IR spectra)) turns out to be dramatically hindered by the presence of surface sulfates.