Strong oxide-oxide interactions in silica-supported magnetite catalysts. 2. The core/shell nature of the interaction

Water-gas shift activity measurements at ca. 650 K, nitric oxide chemisorption at 273 K for surface area determination, and Mössbauer spectroscopy were used to study the surface properties of magnetite (Fe₃O₄) supported on silica. A strong interaction between these two oxides takes place, due to the substitution of Si⁴⁺ for tetrahedral Fe³⁺ near the surface of magnetite. As a result, the octahedral sites of magnetite become enriched in Fe³⁺. This substitution can be described by a core and shell model in which a core of unperturbed Fe₃O₄ is surrounded by a Si⁴⁺-substituted shell that is ca. 0.5 nm thick. This shell is formed irreversibly during treatment of silica-supported magnetite in H₂O-containing environments at ca. 650 K and near atmospheric pressures. While the formation of this thin shell does not significantly diminish the capacity of silica-supported magnetite to chemisorb NO, the apparent turnover frequency for water-gas shift (based on rate measurements and the NO chemisorption uptake) can be reduced by approximately one order of magnitude.