Direct Hydrothermal Synthesis of Iron-Containing Mesoporous Silica SBA-15: Potential as a Support for Gold Nanoparticles

Abstract

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 % Fe2O3 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.