Physical characterization of [formula omitted] model supported catalysts : III. Combined electron microscopic and spectroscopic studies of reduction and oxidation behavior

Abstract

The combination of results from transmission electron microscopy, X-ray photoelectron spectroscopy, and conversion electron Mössbauer spectroscopy are used to summarize and discuss the behavior of Fe TiO 2 model supported catalytic specimens following hydrogen and oxygen treatments at progressively higher temperatures. During hydrogen treatment, initial iron overlayers (ca. 5 nm thick) on TiO 2 undergo reduction, nucleation, and growth to small metallic iron particles at temperatures from 608 to 707 K; these iron crystallites spread over (or wet) the titania support at 773 K, forming particles with a “thin-crystal” morphology; and at 875 K, iron facilitates reduction of titania, accompanied by the diffusion of iron into the support. Following this high-temperature reduction, samples were treated in oxygen at ca. 950 K. Some of the iron that had diffused into the support returned to the surface. This high-temperature oxidation does not, however, simply reverse the effect of high-temperature reduction. Instead, the iron is converted into large particles of FeTi 2O 5. The lack of reversibility during sequential hydrogen and oxygen treatments at high temperatures is attributed to strong interactions between iron and titanium, manifested by the formation of dispersed and strongly interacting iron (e.g., γ-Fe; Fe x Ti, 1 ≲ x ≲ 2) or FeTi 2O 5 under reducing or oxidizing conditions, respectively.