What is it about?

Mesoporous silicas are industrially relevant materials because of their high specific surface area and the large diameter of their pores, suitable for the absorption of large organic molecules. The pores form a regular pattern, but the silica walls separating the pores are amorphous; so, it is very difficult to have detailed information on the structure of these walls by experimental techiques. In this work, we used modeling techniques to simulate the amorphous silica walls of mesoporous materials. In particular, we considered different kinds of structural defects, calculated their electronic and vibrational spectra, and compared them with available experimental data. This strategy allowed us to gather insight on the type of defect centers on these important materials.

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Why is it important?

We built plausible models of the defect centers and systematically analyzed them in terms of their vibrational and electronic properties. For the first time, also radical species were considered. We found that all the model defect structures were compatible with experimental results. A rich, complex picture of the pores surface emerges: different kinds of defects are simultaneously present, and are largely responsible of the interesting properties of these materials, including their reactivity. In particular, we concluded that a very reactive defect, known as 'strained siloxane bridge', should actually have radical nature.


This was one of the first endeavours to provide, by first-principles modeling, a realistic picture of the kind of defects present on the surfaces of mesoporous silica materials. Probably, the greatest achievement of this study was to propose that both diamagnetic (strained rings) and paramagnetic defects (radical species, deriving from homolytically broken bonds) are present. Further studies on these materials supported such a conclusion. Hence, at the high temperatures adopted in the fabrication of mesoporous materials, strained rings and radical species are in dynamical equilibrium and could interconvert among each other. This idea has helped to interpret the behaviour of mesoporous materials and could provide a deeper understanding of their catalytic processes.

Gloria Tabacchi
university of insubria

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This page is a summary of: Ab Initio Study of Defect Sites at the Inner Surfaces of Mesoporous Silicas, The Journal of Physical Chemistry, October 2003, American Chemical Society (ACS),
DOI: 10.1021/jp036182b.
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