SURFACE REACTIVITY OF IRON-OXIDE PIGMENTARY POWDERS TOWARD ATMOSPHERIC COMPONENTS - XPS AND GRAVIMETRY OF OXYGEN AND WATER-VAPOR ADSORPTION

The adsorption of oxygen and water vapor on a number of specially prep ared alpha-Fe2O3 samples was measured gravimetrically at 25 degrees C. The samples themselves were prepared from a steel-pickling chemical w aste(97 wt% FeSO4 . 7H(2)O) by roasting the original material at 700 d egrees C for 5 h in air, oxygen, and nitrogen. Estimated surface cover ages by the adsorbed oxygen and water vapor were made on the basis of nitrogen-adsorption-based surface areas, while the nature of the sampl e surfaces was investigated by both X-ray photoelectron spectroscopy ( XPS) and field emission SEM (FESEM) techniques. In addition a depth pr ofiling study utilizing a sputtering argon beam and XPS was undertaken . Morphological studies using FESEM showed that, while the surface are as were essentially the same (27-29 m(2)/g) for all three samples, the sample prepared in nitrogen had a significantly larger particle size than the other two. These studies also indicated that neither oxygen n or water vapor adsorption caused any significant structural changes. T he differing sample preparations resulted in differing oxygenated surf aces for the alpha-Fe2O3 samples, with the degree of oxygenation decre asing in the order of preparatory gases: oxygen, (wet) air, nitrogen. The amounts of both oxygen and water vapor adsorbed were in inverse pr oportion to the original degree of surface oxygenation, though the amo unts of both represented fractional coverage at best, While the water vapor adsorption was always greater than that of oxygen, the former wa s more weakly adsorbed, as was indicated by the ease of desorption. De pth profiling failed to indicate any bulk diffusion of oxygen but coul d not be considered reliable since even the attenuated argon beam used here still brought about reduction of surface iron. Both oxygen and d issociative water adsorption are thought to involve surface sites of h igh coordination unsaturation. Oxygen is postulated to adsorb on such poorly oxygenated sites primarily as O-2(-); however, O-2(-) and possi bly O- or Fe = O are also thought to play a role. (C) 1996 Academic Pr ess, Inc.