Direct hydrothermal synthesis of iron-containing mesoporous silica SBA-15: Potential as a support for gold nanoparticles

The preparation of mesoporous silica SBA-15 with high iron loadings (14-90 wt % Fe₂O₃) as a suitable support for gold nanoparticles to be used in CO oxidation catalysis has been investigated. The support materials were prepared by a direct hydrothermal two-step pH adjusting method which consisted of the formation of the silica mesophase at low pH and the inclusion of Fe³⁺ at varying pH in the range 2-7. The materials were characterized by XRD, SEM, and TEM/EDX, N₂ porosimetry, and ⁵⁷Fe Mössbauer spectroscopy. At relatively low Fe loading, the SBA-15 structure is maintained and iron is predominantly surface grafted to the silica surface. Such a mesoporous silica can accommodate up to 40 wt % Fe ₂O₃ with a surface area of 460 m²/g. With increasing Fe content, precipitation of iron hydroxides competes with the surface grafting process and the resulting materials are an intimate mixture of hematite particles embedded in an Fe/SBA-15 matrix. A too high Fe³⁺ content in the synthesis gel results in a high rate of precipitation and impedes the formation of the silica mesophase. The stabilization of the mesophase at pH 7 is proposed to involve interactions of the surfactant with a surface grafted Fe³⁺ silica phase. The use of an SBA-15 which contains mainly surface grafted Fe³⁺ as a support for gold nanoparticles results in a more active catalyst for CO oxidation than gold supported by SBA-15 or iron oxide particles.